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Prof.Dr. Hedef Dhafir El-Yassin 2012

Biochemistry of Hormones

A case oriented approach

1. Lecture 1: Introduction to the Biochemistry of hormones and their mechanism of actions.

Thursday

16/2

2. Lecture 2: Introduction to the Biochemistry of hormones and their mechanism of actions.

Sunday

19/2

3. Lecture 3: Biochemistry and disorders of hormones of the hypothalamus and pituitary gland

(hypothalamus- pituitary axis).

Thursday

23/2

4. Lecture 4: Biochemistry and disorders of hormones of the hypothalamus and pituitary gland

(hypothalamus- pituitary axis).

Sunday

26/2

5. Lecture 5: Biochemistry and disorders of hormones of the thyroid and parathyroid gland

Sunday

4/3

6. Lecture 6: Biochemistry and disorders of hormones of the pancreas

Thursday

8/3

7. Lecture 7: Biochemistry and disorders of hormones of the adrenal gland

Sunday

11/3

8. Lecture 8: Biochemistry and disorders of hormones of the adipose tissues, heart and kidney

Sunday

18/3

Aim and objective of the above eight lectures is to understand:

1. The functions of hormones and the mechanisms involved in the regulation of their

secretion.

2. The types of receptor-hormone interactions and the specific effect that each type can

be produced in the cell.

3.  the hypothalamus- pituitary axis.
4.  the  biochemistry and disorders of hormones secretion from different glands.
5.  and to learn the major causes of endocrine disorders by discussing clinical cases.

References:

1. "Biochemistry" by Lubert Stryer

(textbook)

2. "Textbook of Biochemistry with Clinical Correlations" by T.M.Devlin

(additional reading)

3. "Lippincott's Illustrated Reviews in Biochemistry" by P.C.Champe, R.A.Harvey and

D.R.Ferrier

(additional reading)

4. "Harper's Biochemistry" by R.K.Murray, D.K.Granner, P.A. Mayes and V.W.Rodwell.

(additional reading)

5. "Clinical Laboratory Science Review" By Robert R. Harr

(additional reading)

Prof.Dr. Hedef Dhafir El-Yassin 2012

INTRODUCTION
TO THE

BIOCHEMISTRY

OF HORMONES
AND THEIR

MECHANISM

OF ACTIONS

2012

Prof. Dr. Hedef Dhafir El-Yassin

Prof.Dr. Hedef Dhafir El-Yassin 2012

Introduction to the Biochemistry of

Hormones and their Mechanism of Actions

Lecture 1

Thursday 16/2/2012

Lecture 2

Sunday 19/2/2012

The endocrine system is one of the two coordinating and integrating systems of the

body. It acts through chemical messengers - hormones –carried in the circulation.

Two systems control all physiologic processes:

•

The nervous system

exerts point-to-point control through nerves, similar to

sending messages by conventional telephone. Nervous control is electrical in

nature and fast.

•

The endocrine system

broadcasts its hormonal messages to essentially all cells

by secretion into blood and extracellular fluid. Like a radio broadcast, it

requires a receiver to get the message - in the case of endocrine messages, cells

must bear a receptor for the hormone being broadcast in order to respond.

Endocrinology is the study of hormones, their receptors and the intracellular

signaling pathways they invoke. Distinct endocrine organs are scattered throughout

the body. These are organs that are largely or at least famously devoted to secretion

of hormones. In addition to the classical endocrine organs, many other cells in the

body secrete hormones. Myocytes in the atria of the heart and scattered epithelial

cells in the stomach and small intestine are examples of what is sometimes called the

"diffuse" endocrine system. If the term hormone is defined broadly to include all

secreted chemical messengers, then virtually all cells can be considered part of the

endocrine system.

•

All pathophysiologic events are influenced by the endocrine milieu: There are

no cell types, organs or processes that are not influenced - often profoundly -

by hormone signaling.

•

All "large" physiologic effects a re mediated by multiple hormones acting in

concert: Normal growth from birth to adulthood, for example, is surely

dependent on growth hormone, but thyroid hormones, insulin-like growth

Prof.Dr. Hedef Dhafir El-Yassin 2012

factor-1, glucocorticoids and several other hormones are also critically

involved in this process.

•

There are many hormones known and little doubt that others remain to be

discovered.

Hormones, Receptors and Target Cells

1. Classes of Hormones

Knowing the basic structure of a hormone gives a considerable knowledge

about its receptor and mechanism of action.

Like all molecules, hormones are synthesized, exist in a biologically active

state for a time, and then degrade or are destroyed. Having an appreciation

for the "half-life" and mode of elimination of a hormone aids in understanding

its role in physiology and is critical when using hormones as drugs.

Most commonly, hormones are categorized into four structural groups,

with members of each group having many properties in common:

•

Peptides and proteins

•

Amino acid derivatives

•

Steroids

•

Fatty acid derivatives - Eicosanoids

Peptides and Proteins

Peptide and protein hormones are products of translation. They vary

considerably in size and post-translational modifications, ranging from

peptides as short as three amino acids to large, multisubunit glycoproteins.

Peptide hormones are synthesized in endoplasmic reticulum, transferred to

the Golgi and packaged into secretory vesicles for export.

Prof.Dr. Hedef Dhafir El-Yassin 2012

They can be secreted by one of two pathways:

•

Regulated secretion: The cell stores hormone in secretory granules

and releases them in "bursts" when stimulated. This is the most

commonly used pathway and allows cells to secrete a large amount of

hormone over a short period of time.

•

Constitutive secretion: The cell does not store hormone, but secretes

it from secretory vesicles as it is synthesized.

Most peptide hormones circulate unbound to other proteins, but exceptions exist; for

example, insulin-like growth factor-1 binds to one of several binding proteins. In general,

the half-life of circulating peptide hormones is only a few minutes.

Several important peptide hormones are secreted from the pituitary gland.

The anterior pituitary secretes:

•

Luteinizing hormone and follicle stimulating hormone, which act on the

gonads.

•

prolactin, which acts on the mammary gland,

•

adrenocortiotrpic hormone (ACTH), which acts on the adrenal cortex to

regulate the secretion of glucocorticoids, and

•

growth hormone, which acts on bone, muscle and liver.

The posterior pituitary gland secretes:

•

antidiuretic hormone, also called vasopressin, and

•

oxytocin.

Peptide hormones are produced by many different organs and tissues,

however, including:

•

the heart (atrial-natriuretic peptide (ANP) or atrial natriuretic factor

(ANF))

•

pancreas (insulin and somatostatin),

•

the gastrointestinal tract cholecystokinin, gastrin, and

•

fat stores (leptin)

Prof.Dr. Hedef Dhafir El-Yassin 2012

Many neurotransmitters are secreted and released in a similar fashion to peptide

hormones, and some 'neuropeptides' may be used as neurotransmitters in the nervous

system in addition to acting as hormones when released into the blood. When a peptide

hormone binds to receptors on the surface of the cell, a second messenger appears in the

cytoplasm.

Amino acid derivatives

There are two groups of hormones derived from the amino acid

tyrosine:

•

Thyroid hormones are basically a "double" tyrosine with the critical

incorporation of 3 or 4 iodine atoms.

•

Catecholamines include epinephrine and norepinephrine, which are

used as both hormones and neurotransmitters.

The circulating half-life of thyroid hormones is on the order of a few days.

Two other amino acids are used for synthesis of hormones:

•

Tryptophan is the precursor to serotonin and

the pineal hormone melatonin

•

Glutamic acid is converted to histamine

Steroids

Steroids are lipids and, more specifically,

derivatives of cholesterol. Examples include the

sex steroids such as testosterone and adrenal

steroids such as cortisol. The first and rate-limiting step in the synthesis of all

steroid hormones is conversion of cholesterol to pregnenolone, which

Prof.Dr. Hedef Dhafir El-Yassin 2012

demonstrate the system of numbering rings and carbons for identification of

different steroid hormones.

Pregnenolone is formed on the inner membrane of mitochondria then shuttled

back and forth between mitochondrion and the endoplasmic reticulum for

further enzymatic transformations involved in synthesis of derivative steroid

hormones. Newly synthesized steroid hormones are rapidly secreted from the

cell, with little if any storage. Increases in secretion reflect accelerated rates

of synthesis. Following secretion, all steroids bind to some extent to plasma

proteins.

Steroid hormones are typically eliminated by inactivating metabolic

transformations and excretion in urine or bile.

4. Fatty Acid Derivatives - Eicosanoids

Eicosanoids are a large group of molecules derived from

polyunsaturated fatty acids. The principal groups of

hormones of this class are prostaglandins, prostacyclins,

leukotrienes and thromboxanes.

Arachadonic acid is the most abundant precursor for these hormones. Stores

of arachadonic acid are present in membrane lipids and released through the

action of various lipases.

A great variety of cells produce prostaglandins , including those of the liver,

kidneys, heart, lungs, thymus gland, pancreas, brain, and reproductive

organs. In contrast to hormones, prostaglandins usually act locally, affecting

only adjacent cells or the very cell that secreted it.

Prostaglandins are potent and are presented in very small quantities. They

are not stored in cells but rather are synthesized just before release. These

hormones are rapidly inactivated by being metabolized, and are typically

active for only a few seconds

How are prostaglandins different from hormones?

They do not use the blood for transportation, they act locally and are synthesized in

cell membrane just before release

Prof.Dr. Hedef Dhafir El-Yassin 2012

Receptors and Target Cells

A given hormone usually affects only a limited number of cells, which

are called target cells. A target cell responds to a hormone because it

bears receptors for the hormone.

Hormone receptors are found either exposed on the surface of the cell or

within the cell, depending on the type of hormone. In very basic terms,

binding of hormone to receptor triggers a cascade of reactions within the cell

that affects function.

Hormone receptors have two essential qualities:

1. The receptor must be able to recognize a unique binding site within the

hormone in order to discriminate between the hormone and all other

proteins.

2. The receptor must be able to transmit the information gained from

binding to the hormone into a cellular response.

Hormones may be secreted into blood and affect cells at distant sites. Some

hormones known to act and affect neighboring cells or even have effects on

the same cells that secreted the hormone. . Three actions are defined:

•

Endocrine action

: the hormone is distributed in blood and binds to distant

target cells.

•

Paracrine action

: the hormone acts locally by diffusing from its source to

target cells in the neighborhood.

•

Autocrine action

: the hormone acts on the same cell that produced it.

Prof.Dr. Hedef Dhafir El-Yassin 2012

Types of hormones

Chemical messengers

Example

Description

Intracellular
chemical signal

Prostaglandins

Secreted by cells in a
local area and
influences the activity
of the same cell from
which it was secreted

Autocrine

Histamine,
Prostaglandins

Produced by a wide
variety of tissues and
secreted into tissue
spaces; has a
localized effect on
adjacent cells

Paracrine

Thyroxine,
Insulin

Secreted into the
blood by specialized
cells; travels by the
blood to target tissues

Hormone

Oxytocin,
Antidiuretic
hormone

Produced by neurons
and functions like
hormones

Neurohormone

Acetylcholine,
norepinephrine

Produced by neurones
and secreted into
extracellular spaces
by nerve terminals;
travels short
distances, influences
postsynaptic cells or
effector cells.

Neurotransmitter

Two important terms are used to refer to molecules that bind to the

hormone-binding sites of receptors:

•

Agonists

are molecules that bind the receptor and induce all the post-

receptor events that lead to a biologic effect. In other words, they act

like the "normal" hormone, although perhaps more or less potently.

Antagonists

are molecules that bind the receptor and block binding of

the agonist, but fail to trigger intracellular signaling events. Hormone

antagonists are widely used as drugs.

Prof.Dr. Hedef Dhafir El-Yassin 2012

Mechanism of Action of Hormone:

Understanding mechanism of action is not only of great interest to basic

science, but critical to understanding and treating diseases of the endocrine

system, and in using hormones as drugs.

There are two fundamental mechanisms by which a hormone can change its

target cell. These mechanisms are:

•

Activation of enzymes and other dynamic molecules: Most

enzymes fluctuate between conformational states that are catalytically

active versus inactive. Many hormones affect their target cells by

inducing such transitions, usually causing an activation of one of more

enzymes. Because enzymes are catalysts and often serve to activate

additional enzymes, a seemingly small change induced by hormone-

receptor binding can lead to widespread consequences within the cell.

•

Modulation of gene expression: Stimulating transcription of a group of

genes clearly can alter a cell's phenotype by leading to a burst of

synthesis of new proteins. Similarly, if transcription of a group of

previously active genes is shut off, the corresponding proteins will soon

disappear from the cell.

More specifically, when a receptor becomes bound to a hormone, it

undergoes a conformational change which allows it to interact

productively with other components of the cells, leading ultimately to an

alteration in the physiologic state of the cell.

Despite the molecular diversity of hormones, all hormone receptors can be

categorized into one of two types, based on their location within the cell:

Prof.Dr. Hedef Dhafir El-Yassin 2012

Location of Receptor

Classes of Hormones

Principle Mechanism

of Action

Cell surface receptors

(plasma membrane)

Proteins and peptides,

catecholamines and

eicosanoids

(water soluble)

Generation of second

messengers which alter

the activity of other

molecules - usually

enzymes - within the cell

Intracellular receptors

(cytoplasm and/or

nucleus)

Steroids and thyroid

hormones

(lipid soluble)

Alter transcriptional

activity of responsive

genes

Prof.Dr. Hedef Dhafir El-Yassin 2012

1. Hormones with Cell Surface Receptors

Protein and peptide hormones, catecholamines like epinephrine, and

eicosanoids such as prostaglandins find their receptors decorating the

plasma membrane of target cells. Binding of hormone to receptor initiates a

series of events which leads to generation of so-called second messengers

within the cell (the hormone is the first messenger). The second messengers

then trigger a series of molecular interactions that alter the physiologic state

of the cell. Another term used to describe this entire process is signal

transduction.

Structure of Cell Surface Receptors

Cell surface receptors are integral membrane proteins and, as such, have

regions that contribute to three basic domains:

•

Extracellular domains: Some of the residues exposed to the outside

of the cell interact with and bind the hormone - another term for these

regions is the ligand-binding domain.

•

Transmembrane domains: Hydrophobic stretches of amino acids are

"comfortable" in the lipid bilayer and serve to anchor the receptor in the

membrane.

•

Cytoplasmic or intracellular domains: Tails or loops of the receptor

that are within the cytoplasm react to hormone binding by interacting in

some way with other molecules, leading to generation of second

messengers.

As shown below, some receptors are simple, single-pass proteins; many

growth factor receptors take this form. Others, such as the receptor for

insulin, have more than one subunit. Another class, which includes the beta-

adrenergic receptor, is threaded through the membrane seven times.

Prof.Dr. Hedef Dhafir El-Yassin 2012

Interaction of the hormone-bound receptor with other membrane or

cytoplasmic proteins is the key to generation of second messengers and

transduction of the hormonal signal.

Second Messenger Systems

Nonsteroid hormones (water soluble) do not enter the cell but bind to plasma

membrane receptors, generating a chemical signal (second messenger)

inside the target cell. Second messengers activate other intracellular

chemicals to produce the target cell response.

Prof.Dr. Hedef Dhafir El-Yassin 2012

The action of nonsteroid hormones. Images from Purves et al., Life: The Science of Biology, 4th Edition, by
Sinauer Associates

Currently, four second messenger systems are recognized in cells, as

summarized in the table below. Note that not only do multiple hormones

utilize the same second messenger system, but a single hormone can utilize

more than one system.

Prof.Dr. Hedef Dhafir El-Yassin 2012

Second Messenger

Examples of Hormones Which Utilize This

System

Cyclic AMP

Epinephrine and norepinephrine, glucagon,

luteinizing hormone, follicle stimulating

hormone, thyroid-stimulating hormone,

calcitonin, parathyroid hormone, antidiuretic

hormone

Protein kinase activity

Insulin, growth hormone, prolactin, oxytocin,

erythropoietin, several growth factors

Calcium and/or

phosphoinositides

Epinephrine and norepinephrine, antidiuretic

hormone, gonadotropin-releasing hormone,

thyroid-releasing hormone.

Cyclic GMP

Atrial naturetic hormone, nitric oxide

In all cases, the seemingly small signal generated by hormone binding

its receptor is amplified within the cell into a cascade of actions that

changes the cell's physiologic state. Presented below are two examples of

second messenger systems commonly used by hormones.

1. Cyclic AMP Second Messenger Systems

Cyclic adenosine monophosphate (cAMP) is a nucleotide generated from

ATP through the action of the enzyme adenylate cyclase. The intracellular

concentration of cAMP is increased or decreased by a variety of hormones

and such fluctuations affect a variety of cellular processes. One prominent

and important effect of elevated concentrations of cAMP is activation of a

cAMP-dependent protein kinase called protein kinase A.

Protein kinase A is nominally in a catalytically-inactive state, but becomes

active when it binds cAMP. Upon activation, protein kinase A phosphorylates

a number of other proteins, many of which are themselves enzymes that are

either activated or suppressed by being phosphorylated. Such changes in

enzymatic activity within the cell clearly alter its state.

Prof.Dr. Hedef Dhafir El-Yassin 2012

Simple Example: mechanism of action of glucagon:

•

Glucagon binds its receptor in the plasma membrane of target cells

(e.g. hepatocytes).

•

Bound receptor interacts with and, through a set of G proteins, turns on

adenylate cyclase, which is also an integral membrane protein.

•

Activated adenylate cyclase begins to convert ATP to cyclic AMP,

resulting in an elevated intracellular concentration of cAMP.

•

High levels of cAMP in the cytosol make it probable that protein kinase

A will be bound by cAMP and therefore catalytically active.

•

Active protein kinase A "runs around the cell" adding phosphates to

other enzymes, thereby changing their conformation and modulating

their catalytic activity .

•

Levels of cAMP decrease due to destruction by cAMP-

phosphodiesterase and the inactivation of adenylate cyclase.

In the above example, the hormone's action was to modify the activity of pre-

existing components in the cell. Elevations in cAMP also have important

effects on transcription of certain genes.

Prof.Dr. Hedef Dhafir El-Yassin 2012

2. Tyrosine Kinase Second Messenger Systems

The receptors for several protein hormones are themselves protein

kinases which are switched on by binding of hormone. The kinase

activity associated with such receptors results in phosphorylation of tyrosine

residues on other proteins. Insulin is an example of a hormone whose

receptor is a tyrosine kinase.

The hormone binds to domains exposed on the cell's surface, resulting in a

conformational change that activates kinase domains located in the

cytoplasmic regions of the receptor. In many cases, the receptor

phosphorylates itself as part of the kinase activation process. The activated

receptor phosphorylates a variety of intracellular targets, many of which are

enzymes that become activated or are inactivated upon phosphorylation.

As seen with cAMP second messenger systems, activation of receptor

tyrosine kinases leads to rapid modulation in a number of target proteins

within the cell. Some of the targets of receptor kinases are protein

phosphatases which, upon activation by receptor tyrosine kinase, become

competent to remove phosphates from other proteins and alter their activity.

Again, a seemingly small change due to hormone binding is amplified into a

multitude of effects within the cell.

Hormones with Intracellular Receptors

Receptors for steroid and thyroid hormones are located inside target cells, in

the cytoplasm or nucleus, and function as ligand-dependent transcription

factors. The hormone-receptor complex binds to promoter regions of

responsive genes and stimulate or sometimes inhibit transcription from those

genes. Thus, the mechanism of action of these hormones is to modulate

gene expression in target cells. By selectively affecting transcription from a

battery of genes, the concentration of those respective proteins are altered,

which clearly can change the phenotype of the cell.

Prof.Dr. Hedef Dhafir El-Yassin 2012

Structure of Intracellular Receptors

Steroid and thyroid hormone receptors are members of a large group of

transcription factors. All of these receptors are composed of a single

polypeptide chain that has, three distinct domains:

•

The amino-terminus

: In most cases, this region is involved in

activating or stimulating transcription by interacting with other

components of the transcriptional machinery. The sequence is highly

variable among different receptors.

•

DNA binding domain

: Amino acids in this region are responsible for

binding of the receptor to specific sequences of DNA.

•

The carboxy-terminus or ligand-binding domain

: This is the region

that binds hormone.

Prof.Dr. Hedef Dhafir El-Yassin 2012

In addition to these three core domains, two other important regions of the

receptor protein are a nuclear localization sequence, which targets the protein

to nucleus, and a dimerization domain, which is responsible for latching two

receptors together in a form capable of binding DNA.

Prof.Dr. Hedef Dhafir El-Yassin 2012

Hormone-Receptor Binding and Interactions with DNA

Being lipids, steroid hormones enter the cell by simple diffusion across the

plasma membrane. Thyroid hormones enter the cell by facilitated diffusion.

The receptors exist either in the cytoplasm or nucleus, which is where they

meet the hormone. When hormone binds to receptor, a characteristic

series of events occurs:

•

Receptor activation

is the term used to describe conformational

changes in the receptor induced by binding hormone. The major

consequence of activation is that the receptor becomes competent to

bind DNA.

•

Activated receptors bind to hormone response elements

, which are

short specific sequences of DNA which are located in promoters of

hormone-responsive genes.

•

Transcription from those genes to which the receptor is bound is

affected.

Most commonly, receptor binding stimulates transcription.

The hormone-receptor complex thus functions as a transcription factor.

Prof.Dr. Hedef Dhafir El-Yassin 2012

Steroid Hormones

The second mechanism involves steroid hormones, which pass through the

plasma membrane and act in a two step process. Steroid hormones bind,

once inside the cell, to the nuclear membrane receptors, producing an

activated hormone-receptor complex. The activated hormone-receptor

complex binds to DNA and activates specific genes,

increasing production of

proteins.

Prof.Dr. Hedef Dhafir El-Yassin 2012

Control of Endocrine Activity

The physiologic effects of hormones depend largely on their concentration in blood and

extracellular fluid.

The concentration of hormone as seen by target cells is determined by three factors:

1. Rate of production

Synthesis and secretion of hormones are the most highly

regulated aspect of endocrine control. Such control is mediated by positive and

negative feedback circuits.

2. Rate of delivery

An example of this effect is blood flow to a target organ or group

of target cells:

TRANSPORT OF HORMONES: hormones must be transported at least some distance to

their target organs. The primary transport medium is the plasma, although the lymphatic

system and the cerebrospinal fluid are also important. Since delivery of the hormone to its

target tissue is required before a hormone can exert its effects, the presence or absence of

specific transport mechanisms play a major role in mediating hormonal action.

A) The water-soluble hormones (peptide hormones, catecholamines) are transported

in plasma in solution and require no specific transport mechanism. Because of this,

the water-soluble hormones are generally short-lived. These properties allow for

rapid shifts in circulating hormone concentrations, which is necessary with the

pulsatile tropic hormones or the catecholamines. This is consistent with the

rapid onset of action of the water-soluble hormones.

B) The lipid-soluble hormones (thyroid hormone, steroids) circulate in the plasma

bound to specific carrier proteins. Many of the proteins have a high affinity for

specific hormone, such as thyroxine-binding globulin (TBG), sex hormone-

binding globulin (SHBG), and cortisol-binding globulin (CBG). Non-specific, low-

affinity] binding of these hormones to albumin also occurs. Carrier proteins

act as reservoirs of hormone. Since it is generally believed that only the free hor-

mone can enter cells, a dynamic equilibrium must exist between the bound and

free hormone. Thus, alterations in the amount of binding protein available, or in

the affinity of the hormone for the binding protein, can markedly alter the total

circulating pool of hormone without affecting the free concentration of hormone.

Carrier proteins act as buffers to both blunt sudden increases in hormone

concentration and to diminish degradation of the hormone once it is secreted. Thus, the

half-life of hormones that utilize carrier proteins is longer than those that are not protein-

Prof.Dr. Hedef Dhafir El-Yassin 2012

bound. Indeed, carrier proteins have a profound effect on the clearance rate of hormones;

the greater the capacity for high affinity binding of the hormone in the plasma, the slower

the clearance rate. Also, the carrier proteins allow slow, tonic delivery of the hormone to

its target tissue. This is consistent with the slower onset of action of the lipid-soluble

hormones.

3. Rate of degradation and elimination

Hormones, like all biomolecules, have

characteristic rates of decay, and are metabolized and excreted from the body

through several routes.

HORMONE METABOLISM: Clearance of hormones from the circulation plays a critical role

in the modulation of hormone levels in response to varied physiologic and pathologic

processes. The time required to reach a new steady-state concentration in response to

changes in hormone release is dependent upon the half-life of the hormone in the serum.

Thus, an increase in hormone release or administration will have a much more marked effect

if the hormone is cleared rapidly from the circulation as opposed to one that is cleared more

slowly.

Most peptide hormones have a plasma half-life measured in minutes, consistent with the

rapid actions and pulsatile nature of the secretion of these hormones. This rapid clearance

is achieved by the lack of protein binding in the plasma, degradation or internalization of the

hormone at its site of action, and ready clearance of the hormone by the kidney. Binding to

serum proteins markedly decreases hormone clearance, as is observed with the steroid

hormones and the iodothyronines. Metabolism of the steroid hormones occurs primarily in

the liver by reductions, conjugations, oxidations, and hydroxylations, which serve to

inactivate the hormone and increase their water-solubility, facilitating their excretion in the

urine and the bile. Metabolic transformation also may serve to activate an inactive hormone

precursor, such as the deiodination of thyroxine to form T

. Hormone metabolism is not as

tightly regulated as is hormone synthesis and release. However, alterations in the metabolic

pathways may be clinically important.

Feedback Control of Hormone Production

Feedback circuits are at the root of most control mechanisms in physiology, and are

particularly prominent(obvious) in the endocrine system. Instances of positive feedback

certainly occur, but negative feedback is much more common.

Feedback loops are used extensively to regulate secretion of hormones in the

hypothalamic-pituitary axis. An important example of a negative feedback loop is seen in

control of thyroid hormone secretion. The thyroid hormones thyroxine and triiodothyronine

Prof.Dr. Hedef Dhafir El-Yassin 2012

("T4 and T3") are synthesized and secreted by thyroid glands and affect metabolism

throughout the body. The basic mechanisms for control in this system are:

•

Neurons in the hypothalamus secrete thyroid releasing hormone (TRH), which

stimulates cells in the anterior pituitary to secrete thyroid-stimulating hormone

(TSH).

•

TSH binds to receptors on epithelial cells in the thyroid gland, stimulating synthesis

and secretion of thyroid hormones, which affect probably all cells in the body.

•

When blood concentrations of thyroid hormones increase above a certain threshold,

TRH-secreting neurons in the hypothalamus are inhibited and stop secreting TRH.

This is an example of "negative feedback".

Inhibition of TRH secretion leads to shut-off of TSH secretion, which leads to shut-off of

thyroid hormone secretion. As thyroid hormone levels decay below the threshold, negative

feedback is relieved, TRH secretion starts again, leading to TSH secretion...

Prof.Dr. Hedef Dhafir El-Yassin 2012

Biochemistry and

Disorders of

Hormones of the

Hypothalamic and

pituitary gland

(hypothalamus

and pituitary axis)

1. Hormones of the hypothalamus

Prof. Dr. Hedef Dhafir El-Yassin 2012

Prof.Dr. Hedef Dhafir El-Yassin 2012

Hypothalamic and Pituitary Hormones

Lecture 3

Thursday 23/2

1. Hormones of the hypothalamus

The hypothalamus is an integral part of the substance of the brain. A small cone-shaped

structure, it projects downward, ending in the pituitary stalk, a tubular connection to the

pituitary gland, which is a double lobed structure that produces the endocrine secretions

when stimulated by the hypothalamus.

Regulation of hormone secretion

The hypothalamus regulates homeostasis

It links the {nervous_system} to the {endocrine

system}. In addition to secreting neurotransmitters and neuromodulators, the

hypothalamus synthesizes and secretes a number of neurohormones. The neurons

secreting neurohormones are true endocrine cells.

Prof.Dr. Hedef Dhafir El-Yassin 2012

Hormonal cascade of signals from CNS to ultimate hormone.

The target "gland" is the last hormone-producing tissue in the cascade, which is stimulated
by an appropriate anterior pituitary hormone. Examples are thyroid gland, adrenal cortex,
ovary and testes. Ultimate hormone feedback negatively on sites producing intermediate
hormones in the cascade. Amounts (nanogram (ng), microgram (µg), and milligram (mg)
represent approximate quantities of hormone released.

Environmental or

internal signal

CNS

Limbic system

Electrical –chemical signal

Hypothalamus

Electrical –chemical signal

Target "gland"

Releasing hormones (ng)

Anterior pituitary tropic hormone (µg)

Ultimate Hormone (mg)

Systemic effects

Prof.Dr. Hedef Dhafir El-Yassin 2012

The hypothalamus controls each lobe

of the pituitary slightly differently.

1. control of Anterior lobe

a. The hypothalamus acts

as an endocrine gland.

b. Hormones are sent

from the hypothalamus

to the anterior pituitary

via a blood vessel

called the portal vein.

c. The target tissue is the

anterior lobe of the

pituitary e.g. LH, TSH,

and FSH.

2. control of the Posterior lobe

d. Neuro-hormones are synthesized in the hypothalamus neurons. They are

transported and stored in vesicles in the axon ending located in the posterior

pituitary.

e. Nerve impulses travel down the axon into the posterior pituitary. This causes

the release of the vesicles of hormones into the blood stream at the

posterior pituitary e.g. oxytocin , and ADH.

Prof.Dr. Hedef Dhafir El-Yassin 2012

Many hormonal systems involve hypothalamus.

Cascade of hormonal responses starting with an external or internal signal. This signal is
transmitted first to the CNS and may involve the limbic system, including the hippocampus
and amygdala. These components innervate the hypothalamus in a specific region, which
responds by secreting (nanogram amounts) a specific releasing hormone. Releasing
hormones are transported down a closed portal system to the anterior pituitary, where they
cause secretion of microgram amounts of specific anterior pituitary hormones. These
access the general circulation through fenestrated local capillaries and trigger release of
an ultimate hormone in microgram to milligram daily amounts, which generates its
response by binding to receptors in target tissues. Overall, this system is an amplifying
cascade. Consequently, the organism is in intimate association with the external
environment. Solid arrows indicate a secretory process. Long arrows studded with open or
closed circles indicate negative feedback pathways (ultra-short, short, and long feedback
loops).

Prof.Dr. Hedef Dhafir El-Yassin 2012

1) Thyrotropin-releasing hormone(TRH)

Is the simplest of the hypothalamic neuropeptides. It consists essentially of three amino

acids. Its basic sequence is glutamic acid-histidine-proline, although both ends of the

peptide are modified. The simplicity of this structure is deceiving for TRH is involved in an

extraordinary array of functions. Some of which are:

a. It stimulates the secretion of thyroid-stimulating hormone from the pituitary.

b. It also affects the secretion of prolactin from the pituitary.

The TRH-secreting cells are subject to stimulatory and inhibitory influences from higher

centers in the brain and they also are inhibited by circulating thyroid hormone.

2) Gonadotropin-releasing hormone (GnRH)

Also known as luteinizing hormone-releasing hormone (LHRH), is a peptide chain of 10

amino acids. It stimulates the synthesis and release of the two pituitary gonadotropins,

luteinizing hormone (LH) and follicle-stimulating hormone (FSH).

3) Corticotropin-releasing hormone (CRH)

Is a large peptide consisting of a single chain of 41 amino acids. It stimulates not only

secretion of corticotropin in the pituitary gland but also the synthesis of corticotropin in the

corticotropin-producing cells (corticotrophs) of the anterior lobe of the pituitary gland. Many

factors, both neurogenic and hormonal, regulate the secretion of CRH. Among the

hormones that play an important role in modulating the influence of CRH is cortisol, the

major hormone secreted by the adrenal cortex, which, as part of the negative feedback

mechanism. Vasopressin, the major regulator of the body's excretion of water, has an

additional ancillary role in stimulating the secretion of CRH.

Excessive secretion of CRH leads to an increase in the size and number of corticotrophs

in the pituitary gland, often resulting in a pituitary tumor. This, in turn, leads to excessive

stimulation of the adrenal cortex, resulting in high circulating levels of adrenocortical

hormones, the clinical manifestations of which are known as Cushing's syndrome.

Conversely, a deficiency of CRH-producing cells can, by a lack of stimulation of the

pituitary and adrenal cortical cells, result in adrenocortical deficiency.

4) Growth hormone-releasing hormone (GHRH or GRH)

Like CRH, growth hormone-releasing hormone (GHRH) is a large peptide. A number of

forms have been described that differ from one another only in minor details and in the

number of amino acids (varying from 37 to 49). It is stimulated by stresses, including

physical exercise, and secretion is blocked by a powerful inhibitor called somatostatin.

Negative feedback control of GHRH secretion is mediated largely through compounds

called somatomedins, growth-promoting hormones that are generated when tissues are

Prof.Dr. Hedef Dhafir El-Yassin 2012

exposed to growth hormone itself.

Isolated deficiency of GHRH (in which there is normal functioning of the hypothalamus

except for this deficiency) may be the cause of one form of dwarfism, a general term

applied to all individuals with abnormally small stature.

5) Prolactin release factor (PRF):

Appears to be released from the hypothalamus in a pulsatile fashion and it is the

fluctuation in PRF that regulates the circulating level of prolactin.

6) Somatostatin (Growth hormone release-inhibiting hormone; somatotropin

release-inhibiting hormone ( GHRIH or SRIH)

Somatostatin refers to a number of polypeptides consisting of chains of 14 to 28 amino

acids. Somatostatin is also a powerful inhibitor of pituitary TSH secretion. Somatostatin,

like TRH, is widely distributed in the central nervous system and in other tissues. It serves

an important paracrine function in the islets of Langerhans, by blocking the secretion of

both insulin and glucagon from adjacent cells. Somatostatin has emerged not only as a

powerful blocker of the secretion of GH, insulin, glucagon, and other hormones but also as

a potent inhibitor of many functions of the gastrointestinal tract, including the secretion of

stomach acid, the secretion of pancreatic enzymes, and the process of intestinal

absorption.

7) Prolactin release-inhibiting hormones (Dopamine and GAP)

GAP= GnRH-associated peptide

The hypothalamic regulation of prolactin secretion from the pituitary is different from the

hypothalamic regulation of other pituitary hormones in two respects:

1. First, the hypothalamus primarily inhibits rather than stimulates the release of

prolactin from the pituitary.

2. Second, this major inhibiting factor is not a neuropeptide, but rather the

neurotransmitter dopamine. Prolactin deficiency is known to occur, but only rarely.

Excessive prolactin production (hyperprolactinemia) is a common endocrine

abnormality.

Prof.Dr. Hedef Dhafir El-Yassin 2012

Table: Hypothalamic hypophysial-target gland hormones form integrated feedback loops

Hypothalamic hormones

No. of A.A

structure

Pituitary

Hormone

Affected

Target Gland

Hormone

Affected

Thyrotropin-releasing hormone

(TRH)

TSH (PRL)

, T

Gonadotropin-releasing

hormone (GnRH)

LH, FSH

Androgens,

estrogens,

progestins

Corticotropin-releasing

hormone (CRH)

ACTH

Cortisol

Growth hormone-releasing

hormone(GHRH or GRH)

IGF-1

Prolactin release factor

Not

established

PRL

neurohormones

Somatostatin (Growth hormone

release-inhibiting hormone;

somatotropin release-inhibiting

hormone (GHRIH or SRIH)

GH (TSH, FSH,

ACTH)

IGK-1; T

andT

Prolactin- release-inhibiting

hormones (Dopamine and GAP)

(PRIH or PIH)

PRL

neurohormones

The hypothalamic hormone has a secondary or lesser effect on the hormones in parentheses.

Prof.Dr. Hedef Dhafir El-Yassin 2012

Biochemistry and

Disorders of

Hormones of the

Hypothalamic and

pituitary gland

(hypothalamus

and pituitary axis

2. Hormones of the pituitary gland

Prof. Dr. Hedef Dhafir El-Yassin 2012

Prof.Dr. Hedef Dhafir El-Yassin 2012

Lecture 4

Sunday 26/2

2. Hormones of the Pituitary gland

The pituitary gland, also known

as the hypophysis, is a roundish

organ that lies immediately

beneath the hypothalamus.

Careful examination of the

pituitary gland reveals that it

composed of two distinctive

parts:

•

The

anterior pituitary

(adenohypophysis) is a

classical gland composed predominantly of cells that secrete protein hormones.

•

The

posterior pituitary

(neurohypophysis) is not really an organ, but an extension

of the hypothalamus. It is composed largely of the axons of hypothalamic neurons

which extend downward as a large bundle

behind the anterior pituitary.

The target cells for most of the hormones produced in

these tissues are themselves endocrine cells.

The pituitary gland is often called the "master

gland" of the body. The anterior and posterior pituitary

secretes a number of hormones that collectively

influence all cells and affect virtually all physiologic

processes.

Prof.Dr. Hedef Dhafir El-Yassin 2012

Table: The major hormones synthesized and secreted by the pituitary gland, along

with summary statements about their major target organs and physiologic effects.

Hormone

Major target

organ(s)

Major Physiologic Effects

Growth hormone

Liver, adipose
tissue

Promotes growth (indirectly),
control of protein, lipid and
carbohydrate metabolism

Thyroid-stimulating h.

Thyroid gland

Stimulates secretion of thyroid
hormones

Adrenocorticotropic h. Adrenal gland

(cortex)

Stimulates secretion of
glucocorticoids

Prolactin

Mammary gland

Milk production

Luteinizing hormone

Ovary and testis

Control of reproductive function

Follicle-stimulating h.

Ovary and testis

Control of reproductive function

Antidiuretic hormone

Kidney

Conservation of body water

Oxytocin

Ovary and testis

Stimulates milk ejection and uterine
contractions

As seen in the table above, the anterior pituitary synthesizes and secreted six major

hormones. Individual cells within the anterior pituitary secrete a single hormone (or

possibly two in some cases). Thus, the anterior pituitary contains at least six distinctive

endocrinocytes

The cells that secrete thyroid-stimulating hormone do not also secrete growth hormone,

and they have receptors for thyroid-releasing hormone, not growth hormone-releasing

hormone.

Prof.Dr. Hedef Dhafir El-Yassin 2012

Overview of anterior pituitary hormones with hypothalamic releasing

hormones and their actions

Prof.Dr. Hedef Dhafir El-Yassin 2012

Anterior Pituitary Hormones

1. Growth Hormone

Growth hormone, also known as somatotropin, is a

protein hormone of about 190 amino acids that is

synthesized and secreted by cells called somatotrophs in

the anterior pituitary. It is a major participant in control of

several complex physiologic processes, including growth

and metabolism. Growth hormone is also of considerable

interest as a drug used in both humans and animals.

Physiologic Effects of Growth Hormone

A critical concept in understanding growth hormone

activity is that it has two distinct types of effects:

•

Direct effects

are the result of growth hormone binding its receptor on target cells.

Fat cells (adipocytes), for example, have growth hormone receptors, and growth

hormone stimulates them to break down triglyceride and suppresses their ability to

take up and accumulate circulating lipids.

Indirect effects

are mediated primarily by a insulin-like growth factor-1 (IGF-1), a

hormone that is secreted from the liver and other tissues in response to growth hormone.

A majority of the growth promoting effects of growth hormone is actually due to IGF-1

acting on its target cells. IGF-1 also appears to be the key player in muscle growth. It

stimulates amino acid uptake and protein synthesis in muscle and other tissues.

Metabolic Effects

•

Protein metabolism: In general, growth hormone stimulates protein anabolism in

many tissues. This effect reflects increased amino acid uptake, increased protein

synthesis and decreased oxidation of proteins.

•

Fat metabolism: Growth hormone enhances the utilization of fat by stimulating

triglyceride breakdown and oxidation in adipocytes.

•

Carbohydrate metabolism: Growth hormone is one of a battery of hormones that

serves to maintain blood glucose within a normal range. Growth hormone is often

said to have anti-insulin activity, because it suppresses the abilities of insulin to

stimulate uptake of glucose in peripheral tissues and enhance glucose synthesis in

the liver.

•

Mineral metabolism: promotes a positive calcium, magnesium and phosphate

balance and causes the retention of Na

, K

and Cl

Prof.Dr. Hedef Dhafir El-Yassin 2012

Control of Growth Hormone Secretion

Production of growth hormone is modulated by many

factors, including stress, exercise, nutrition, sleep and

growth hormone itself. However, its primary controllers

are two hypothalamic hormones and one hormone from

the stomach:

•

Growth hormone-releasing hormone (GHRH)

is a hypothalamic peptide that stimulates both

the synthesis and secretion of growth hormone.

•

Somatostatin (SS) is a peptide produced by

several tissues in the body, including the

hypothalamus. Somatostatin inhibits growth

hormone release in response to GHRH and to

other stimulatory factors such as low blood glucose concentration.

•

Ghrelin is a peptide hormone secreted from the stomach. Ghrelin binds to

receptors on somatotrophs and potently stimulates secretion of growth hormone.

Growth hormone secretion is also part of a negative feedback loop involving IGF-1.

High blood levels of IGF-1 lead to decreased secretion of growth hormone not only by

directly suppressing the somatotroph, but by stimulating release of somatostatin from the

hypothalamus.

Growth hormone also feeds back to inhibit GHRH secretion and probably has a direct

(autocrine) inhibitory effect on secretion from the somatotroph.

Integration of all the factors that affect growth hormone synthesis and secretion lead to a

pulsatile pattern of release. In children and young adults, the most intense period of growth

hormone release is shortly after the onset of deep sleep.

Disease States

A deficiency state can result not only from a deficiency in production of the hormone, but in

the target cell's response to the hormone.

Clinically, deficiency in growth hormone or receptor defects are known as growth

retardation or dwarfism. The manifestation of growth hormone deficiency depends upon

the age of onset of the disorder and can result from either heritable or acquired disease.

The effect of excessive secretion of growth hormone is also very dependent on the age of

onset and is seen as two distinctive disorders:

Prof.Dr. Hedef Dhafir El-Yassin 2012

•

Gigantism is the result of excessive growth hormone secretion that begins in young

children or adolescents. It is a very rare disorder, usually resulting from a tumor of

somatotropes.

•

Acromegaly results from excessive secretion of growth hormone in adults. The

excessive growth hormone and IGF-1 also lead to metabolic derangements,

including glucose intolerance.

2. Thyroid Stimulating Hormone

Thyroid-stimulating hormone, also known as thyrotropin, is secreted from cells in

the anterior pituitary called thyrotrophs, finds its receptors on epithelial cells in the

thyroid gland, and stimulates that gland to synthesize and release thyroid

hormones.

TSH is a glycoprotein hormone composed of

two subunits, which are non-covalently bound

to one another. The alpha subunit of TSH is

also present in two other pituitary glycoprotein

hormones, follicle-stimulating hormone and

luteinizing hormone. In other words, TSH is composed of

alpha subunit bound to the TSH beta subunit, and TSH

associates only with its own receptor. Free alpha and beta

subunits have essentially no biological activity.

TSH has several acute effects on thyroid function. These

occur in minutes and involve increases of all phases of T

and T

biosynthesis. TSH also has several chronic effects

on the thyroid. These require several days and include

increases in the synthesis of proteins, phospholipids, and

nucleic acids and in the size of number of thyroid cells.

The most important controller of TSH secretion is thyroid-releasing hormone.

Secretion of thyroid-releasing hormone, and hence, TSH, is inhibited by high blood

levels of thyroid hormones in a classical negative feedback loop.

Prof.Dr. Hedef Dhafir El-Yassin 2012

3. Adrenocorticotropic Hormone

Adrenocorticotropic hormone, stimulates the adrenal cortex by enhancing the

conversion of cholesterol to pregnenolone. More specifically, it stimulates secretion

of glucocorticoids such as cortisol, and has little control over secretion of aldosterone,

the other major steroid hormone from the adrenal cortex. Another name for ACTH is

corticotropin.

ACTH is secreted from the anterior pituitary in response to corticotropin-releasing

hormone from the hypothalamus. Corticotropin-releasing hormone

is secreted in response to many types of stress, which makes sense

in view of the "stress management" functions of glucocorticoids.

Corticotropin-releasing hormone itself is inhibited by

glucocorticoids, making it part of a classical negative feedback

loop.

Within the pituitary gland, ACTH is produced in a process that

also generates several other hormones. A large precursor protein

named proopiomelanocortin (POMC) is synthesized and

proteolytically chopped into several fragments as depicted below.

The major attributes of the hormones other than ACTH that are produced in this process

are summarized as follows:

•

Lipotropin: Originally described as having weak lipolytic effects, its major

importance is as the precursor to beta-endorphin.

•

Beta-endorphin and Met-enkephalin: Opioid peptides with pain-alleviation and

euphoric effects.

•

Melanocyte-stimulating hormone (MSH): Known to control melanin pigmentation

in the skin of most vertebrates.

Prof.Dr. Hedef Dhafir El-Yassin 2012

4. Prolactin

Prolactin is a single-chain protein hormone closely related to growth hormone. It is

secreted by so-called lactotrophs in the anterior pituitary. It is also synthesized and

secreted by a broad range of other cells in the body.

Prolactin is synthesized as a prohormone. Following cleavage of the signal peptide, the

length of the mature hormone is between 194 and 199 amino acids, depending on

species. Hormone structure is stabilized by three intramolecular disulfide bonds.

Overall, several hundred different actions have been reported for prolactin in various

species. Some of its major effects are:

1. Mammary Gland Development, Milk Production and Reproduction

2. Effects on Immune Function

The prolactin receptor is widely expressed by immune cells, and some types of

lymphocytes synthesize and secrete prolactin. These observations suggest that prolactin

may act as an autocrine or paracrine modulator of immune activity..

Control of Prolactin Secretion

In contrast to what is seen with all the

other pituitary hormones, the

hypothalamus suppresses prolactin

secretion from the pituitary.

Dopamine serves as the major prolactin-

inhibiting factor or brake on prolactin

secretion. In addition to inhibition by

dopamine, prolactin secretion is positively

regulated by several hormones, including

thyroid-releasing hormone, gonadotropin-

releasing hormone and vasoactive

intestinal polypeptide.

Estrogens provide a well-studied

positive control over prolactin synthesis

and secretion.

Prof.Dr. Hedef Dhafir El-Yassin 2012

5. Gonadotropins: Luteinizing and Follicle Stimulating Hormones

Luteinizing hormone (LH) and follicle-stimulating hormone (FSH) are called

gonadotropins because stimulate the gonads - in males, the testes, and in females,

the ovaries. As described for thyroid-simulating hormone, LH and FSH are large

glycoproteins composed of alpha and beta subunits. The alpha subunit is identical in all

three of these anterior pituitary hormones, while the beta subunit is unique for each

hormone with the ability to bind its own receptor.

Luteinizing Hormone

In both sexes, LH stimulates secretion of sex steroids from the gonads. In the testes,

it stimulates the synthesis and secretion of testosterone. The ovary respond to LH

stimulation by secretion of testosterone, which is converted into estrogen by adjacent

granulosa cells.

LH is required for continued development and function of corpora lutea. The name

luteinizing hormone derives from this effect of inducing luteinization of ovarian follicles.

Follicle-Stimulating Hormone

As its name implies, FSH stimulates the maturation of ovarian follicles. FSH is also

critical for sperm production. It supports the function of Sertoli cells, which in turn

support many aspects of sperm cell maturation.

Control of Gonadotropin Secretion

The principle regulator of LH and FSH secretion is

gonadotropin-releasing hormone or GnRH (also

known as LH-releasing hormone). In a classical

negative feedback loop, sex steroids inhibit secretion of

GnRH and also appear to have direct negative effects

on gonadotrophs.

This regulatory loop leads to pulsatile secretion of LH

and, to a much lesser extent, FSH. Numerous

hormones influence GnRH secretion, and positive and

negative control over GnRH and gonadotropin secretion

is actually considerably more complex than described in

the figure. For example, the gonads secrete at least two additional hormones - inhibin and

activin , which selectively inhibit and activate FSH secretion from the pituitary.

Prof.Dr. Hedef Dhafir El-Yassin 2012

Posterior Pituitary Hormones

1. Antidiuretic Hormone (Vasopressin)

Roughly, 60% of the mass of the body is water, and despite wide

variation in the amount of water taken in each day, body water

content remains incredibly stable. Such precise control of body

water and solute concentrations is a function of several hormones

acting on both the kidneys and vascular system, but there is no doubt

that antidiuretic hormone is a key player in this process.

Antidiuretic hormone, also known as vasopressin, is a nine

amino acid peptide secreted from the posterior pituitary.

Physiologic Effects of Antidiuretic Hormone

The single most important effect of antidiuretic hormone is to conserve body water by

reducing the output of urine.

Antidiuretic hormone stimulates water reabsorbtion by stimulating insertion of "water

channels" or aquaporins into the membranes of kidney tubules. These channels transport

solute-free water through tubular cells and back into blood, leading to a decrease in

plasma osmolarity and an increase osmolarity of urine.

Control of Antidiuretic Hormone Secretion

1. The most important variable regulating antidiuretic hormone secretion is

plasma osmolarity, or the concentration of solutes in blood. When plasma

osmolarity is below a certain threshold, the

osmoreceptors are not activated and antidiuretic

hormone secretion is suppressed. When

osmolarity increases above the threshold, the

ever-alert osmoreceptors recognize this and

stimulate the neurons that secrete antidiuretic

hormone. As seen the figure, antidiuretic

hormone concentrations rise steeply and linearly

with increasing plasma osmolarity.

2. Secretion of antidiuretic hormone is also simulated by decreases in blood

pressure and volume, conditions sensed by stretch receptors in the heart and

large arteries. Changes in blood pressure and volume are not nearly as sensitive a

stimulator as increased osmolarity, but are nonetheless potent in severe conditions.

Prof.Dr. Hedef Dhafir El-Yassin 2012

For example, Loss of 15 or 20% of blood volume by hemorrhage results in massive

secretion of antidiuretic hormone.

Another potent stimulus of antidiuretic hormone is nausea and vomiting.

Disease States

The most common disease of man and animals related to antidiuretic hormone is diabetes

insipidus. This condition can arise from either of two situations:

•

Hypothalamic ("central") diabetes insipidus

results from a deficiency in

secretion of antidiuretic hormone from the posterior pituitary. Causes of this disease

include head trauma, and infections or tumors involving the hypothalamus.

•

Nephrogenic diabetes insipidus

occurs when the kidney is unable to respond to

antidiuretic hormone. Most commonly, this results from some type of renal disease,

but mutations in the ADH receptor gene or in the gene encoding aquaporin-2 have

also been demonstrated in affected humans.

The major sign of either type of diabetes insipidus is excessive urine production.

Some human patients produce as much as 16 liters of urine per day! If adequate water is

available for consumption, the disease is rarely life-threatening, but withholding water can

be very dangerous.

2. Oxytocin

Oxytocin in a nine amino acid peptide that is synthesized in hypothalamic neurons and

transported down axons of the posterior pituitary for secretion into

blood. Oxytocin differs from antidiuretic hormone in two of the nine

amino acids. Both hormones are packaged into granules and

secreted along with carrier proteins called neurophysins.

Control of Oxytocin Secretion

A number of factors can inhibit oxytocin release, among them acute

stress. For example, oxytocin neurons are repressed by

catecholamines, which are released from the adrenal gland in response to many types of

stress, including fright.

Prof.Dr. Hedef Dhafir El-Yassin 2012

Clinical correlation

Testing Activity of the Anterior Pituitary

Releasing hormones and chemical analogs, particularly of the smaller peptides, are now

routinely synthesized. The gonadotropin-releasing hormone, a decapeptide, is available for

use in assessing the function of the anterior pituitary. This is of importance when a disease

situation may involve either the hypothalamus, the anterior pituitary, or the end organ.

Infertility is an example of such a situation. What needs to be assessed in which organ is

at fault in the hormonal cascade. Initially, the end organ, in this case the gonads, must be

considered. This can be accomplished by injecting the anterior pituitary hormone LH or

FSH. If sex hormone secretion is elicited, then the ultimate gland would appear to be

functioning properly. Next, the anterior pituitary would need to be analyzed. This can be

done by i.v. administration of synthetic GnRH; by this route, GnRH can gain access to the

gonadotropic cells of the anterior pituitary and elicit secretion of LH and FSH. Routinely,

LH levels are measured in the blood as a function of time after the injection. These levels

are measured by radioimmunoassay (RIA) in which radioactive LH or hCG is displaced

from binding to an LH-binding protein by LH in the serum sample. The extent of the

competition is proportional to the amount of LH in the serum. In this way a progress of

response is measured that will be within normal limits or clearly deficient. If the response is

deficient, the anterior pituitary cells are not functioning normally and are the cause of the

syndrome. On the other hand, normal pituitary response to GnRH would indicate that the

hypothalamus was non-functional. Such a finding would prompt examination of the

hypothalamus for conditions leading to insufficient availability/production of releasing

hormones. Obviously, the knowledge of hormone structure and the ability to synthesize

specific hormones permits the diagnosis of these disease states.

Source: Marshall, J. C. and Barkan, A. L., "Disorders of the hypothalamus and anterior

pituitary. In: W. N. Kelley (Ed.), Internal Medicine." New York: Lippincott, 1989, p. 2759.

Conn, P. M. "The molecular basis of gonadotropin-releasing hormone action". Endocr.

Rev. 7:3, 1986.

Prof.Dr. Hedef Dhafir El-Yassin 2012

Clinical correlation

Hypopituitarism

The hypothalamus is connected to the anterior pituitary by a delicate stalk that contains

the portal system through which releasing hormones, secreted from the hypothalamus,

gain access to the anterior pituitary cells. In the cell membranes of these cells are specific

receptors for releasing hormones. In most cases, different cells express deferent releasing

hormone receptors. The connection between the hypothalamus and anterior pituitary can

be disrupted by trauma or tumors. Trauma can occur in automobile accidents or other local

damaging events that may result in severing of the stalk, thus preventing the releasing

hormones from reaching their target anterior pituitary cells. When this happens, the

anterior pituitary cells no longer have their signaling mechanism for the release of anterior

pituitary hormones. In the case of tumors of the pituitary gland, all of the anterior pituitary

hormones may not be shut off to the same degree or the secretion of some may disappear

sooner than others.

In any case, if the hypopituitarism occurs, this condition may result in a life-threatening

situation in which the clinician must determine the extent of loss of pituitary hormones,

especially ACTH. Posterior pituitary hormones (Oxytocin and vasopressin) may also be

lost, precipitating a problem of excessive urination (vasopressin deficiency) that must be

addressed. The usual therapy involves administration of the end-organ hormones, such

as, thyroid hormone, cortisol, sex hormones, and progestin; with female patients it is also

necessary to maintain the ovarian cycle. These hormones can be easily administered in

the oral form. Growth hormone deficiency is not a problem in the adult but would be an

important problem in a growing child.

The patient must learn to anticipate needed increases of cortisol in the face of stressful

situations. Fortunately, these patients are usually maintained in reasonably good condition.

Source: Marshall, J. C. and Barkan, A. L., "Disorders of the hypothalamus and anterior

pituitary. In: W. N. Kelley (Ed.), Internal Medicine." New York: Lippincott, 1989, p.

2159.Robinson,A.G."Disorders of the posterior pituitary". In: W. N. Kelley(Ed.), Internal

Medicine. New York:

Prof.Dr. Hedef Dhafir El-Yassin 2012

Question : Hypopituitarism may result from trauma, such as an automobile accident

severing the stalk connecting the hypothalamus and anterior pituitary, or from tumors of

the pituitary gland. In trauma, usually all of the releasing hormones from hypothalamus fail

to reach the anterior pituitary. With a tumor of the gland, some or all of the pituitary

hormones may be shut off. Posterior pituitary hormones may also be lost. Hypopituitarism

can be life threatening. Usual therapy is administration of end-organ hormones in oral

form.

1) If the stalk between the hypothalamus and anterior pituitary is severed, the pituitary

would fail to cause the ultimate release of all of the following hormones except :

a) ACTH.

b) estradiol.

c) oxytocin.

d) testosterone.

e) thyroxine.

Answers:

1) C Oxytocin is released from posterior pituitary. A, B, D, and E all require releasing

hormones from hypothalamus for anterior pituitary to release them.

Prof.Dr. Hedef Dhafir El-Yassin 2012

Lecture 5

Sunday 4/3

Biochemistry and Disorder of Hormones of the

Thyroid and Parathyroid Glands

The Thyroid Gland

The thyroid gland (Greek thyros “shield”) is shaped like a shield and lies just below

the Adam’s apple in the front of the neck.

The thyroid gland secretes thyroxine and smaller amounts of triiodothyronine (T3),

which stimulate oxidative respiration in most cells in the body and, in so doing, help

set the body’s basal metabolic rate. In children, these thyroid hormones also promote

growth and stimulate maturation of the central nervous system. Children with

underactive thyroid glands are therefore stunted in their growth and suffer severe

mental retardation, a condition called cretinism. This differs from pituitary dwarfism,

which results from inadequate GH and is not associated with abnormal intellectual

development.

People who are hypothyroid (whose secretion of thyroxine is too low) can take

thyroxine orally, as pills. Only thyroxine and the steroid hormones (as in

contraceptive pills), can be taken orally because they are nonpolar and can pass

through the plasma membranes of intestinal epithelial cells without being digested.

The thyroid gland also secretes calcitonin, a peptide hormone that plays a role in

maintaining proper levels of calcium (Ca++) in the blood. When the blood Ca++

concentration rises too high, calcitonin stimulates the uptake of Ca++ into bones, thus

lowering its level in the blood.

Prof.Dr. Hedef Dhafir El-Yassin 2012

Thyroid Hormones

1. Thyroxin (T4) and triiodotyonine T3

Biochemistry of Thyroid Hormones

Thyroid hormones are derivatives of the amino acid tyrosine bound covalently to

iodine. The two principal thyroid hormones are:

•

thyroxine (T

or L-3,5,3',5'-tetraiodothyronine)

•

triiodotyronine (T

or L-3,5,3'-triiodothyronine).

Thyroid hormones are basically two tyrosines linked together with the critical

addition of iodine at three or four positions on the aromatic rings. The number

and position of the iodines is important. Several other iodinated molecules are

generated that have little or no biological activity; so called "reverse T

A large majority of the thyroid hormone secreted from the thyroid gland is T

but T

is the considerably more active hormone. Although some T

is also

secreted, the bulk of the T

is derived by deiodination of T

in peripheral tissues,

especially liver and kidney. Deiodination of T

also yields reverse T

, a molecule

with no known metabolic activity.

Thyroid hormones are poorly soluble in water, and more than 99% of the T

and T

circulating in blood is bound to carrier proteins. The principle carrier of thyroid

hormones is thyroxine-binding globulin, a glycoprotein synthesized in the liver.

Another carrier is albumin.

Prof.Dr. Hedef Dhafir El-Yassin 2012

Synthesis and Secretion of Thyroid Hormones

The entire synthetic process occurs in three major steps:

•

Production and accumulation of the raw

materials

•

Fabrication or synthesis of the hormones on a

backbone or scaffold of precursor

•

Release of the free hormones from the

scaffold and secretion into blood

Raw materials:

•

Tyrosines are provided from a large

glycoprotein scaffold called thyroglobulin,.

A molecule of thyroglobulin contains 134 tyrosines, although only a handful of

these are actually used to synthesize T

and T

•

Iodine, or more accurately iodide (I

), is taken up from blood by thyroid epithelial

cells, which have on their outer plasma membrane an "iodine trap". Once inside

the cell, iodide is transported into the lumen of the follicle along with

thyroglobulin.

Fabrication of thyroid hormones is conducted by the enzyme thyroid peroxidase,

Thyroid peroxidase catalyzes two sequential reactions:

1. Iodination of tyrosines on thyroglobulin (also known as "organification of

iodide").

2. Synthesis of thyroxine or triiodothyronine from two iodotyrosines.

Thyroid hormones are excised from their thyroglobulin scaffold by digestion in

lysosomes of thyroid epithelial cells. Free thyroid hormones apparently diffuse out

of lysosomes, through the basal plasma membrane of the cell, and into blood where

they quickly bind to carrier proteins for transport to target cells.

Prof.Dr. Hedef Dhafir El-Yassin 2012

Control of Thyroid Hormone Synthesis and Secretion

Each of the processes described above appears to be stimulated by

thyroid-

stimulating hormone

from the anterior pituitary gland. Binding of TSH to its

receptors on thyroid epithelial cells stimulates synthesis of the iodine transporter,

thyroid peroxidase and thyroglobulin. When TSH levels are low, rates of thyroid

hormone synthesis and release diminish.

The thyroid gland is part of the

hypothalamic-pituitary-thyroid axis, and

control of thyroid hormone secretion is

exerted by classical negative feedback.

Thyroid Hormone Receptors and

Mechanism of Action

Receptors for thyroid hormones are

intracellular DNA-binding proteins that

function as hormone-responsive

transcription factors, very similar

conceptually to the

receptors for steroid

hormones

Thyroid hormones enter cells through

membrane transporter proteins. A number of plasma membrane transporters have

been identified, some of which require ATP hydrolysis; the relative importance of

different carrier systems is not yet clear and may differ among tissues. Once inside

the nucleus, the hormone binds its receptor, and the hormone-receptor complex

interacts with specific sequences of DNA in the promoters of responsive genes.

The effect of receptor binding to DNA is to modulate gene expression, either by

stimulating or inhibiting transcription of specific genes.

Prof.Dr. Hedef Dhafir El-Yassin 2012

Metabolic Effects of Thyroid Hormones

Thyroid hormones have profound effects on many physiologic processes, such as

development, growth and metabolism. They stimulate diverse metabolic activities

most tissues, leading to an increase in basal metabolic rate. One consequence of

this activity is to increase body heat production, which seems to result, at least in

part, from increased oxygen consumption and rates of ATP hydrolysis. A few

examples of specific metabolic effects of thyroid hormones include:

•

Lipid metabolism: Increased thyroid hormone levels stimulate fat

mobilization, leading to increased concentrations of fatty acids in plasma. They

also enhance oxidation of fatty acids in many tissues. Finally, plasma

concentrations of cholesterol and triglycerides are inversely correlated with

thyroid hormone levels - one diagnostic indication of hypothyroidism is

increased blood cholesterol concentration.

•

Carbohydrate metabolism: Thyroid hormones stimulate almost all aspects of

carbohydrate metabolism, including enhancement of insulin-dependent entry of

glucose into cells and increased gluconeogenesis and glycogenolysis to

generate free glucose.

Other Effects: A few additional, effects of thyroid hormones include:

•

On muscle: T3 increases glucose uptake by muscle cells it also stimulate

protein synthesis and therefore growth of muscle through its stimulatory

actions on gene expression. Thyroid hormone sensitizes the muscle cell to the

glycogenolytic actions of epinephrine. Glycolysis in muscle is increased by

this action of T

•

On the pancreas: thyroid hormone increases the sensitivity of the

cells of

the pancreas to those stimuli that normally promote insulin release and is

required for optimal insulin secretion

•

On Cardiovascular system: Thyroid hormones increases heart rate, cardiac

contractility and cardiac output. They also promote vasodilation, which leads

to enhanced blood flow to many organs.

Prof.Dr. Hedef Dhafir El-Yassin 2012

•

On Central nervous system: Both decreased and increased concentrations of

thyroid hormones lead to alterations in mental state. Too little thyroid

hormone, and the individual tends to feel mentally sluggish, while too much

induces anxiety and nervousness.

•

On Reproductive system: Normal reproductive behavior and physiology is

dependent on having essentially normal levels of thyroid hormone.

Hypothyroidism in particular is commonly associated with infertility.

Thyroid Disease States

1. Hypothyroidism

2. Hyperthyroidism

a. . Graves's disease.

2. Calcitonin

Calcitonin is a hormone secreted from the parafolicular of C cells in the thyroid

gland, known to participate in calcium and phosphorus metabolism.

Calcitonin is a 32 amino acid peptide cleaved from a larger prohormone. It contains a

single disulfide bond, which causes the amino terminus to assume the shape of a ring.

Physiologic Effects of Calcitonin

Calcitonin plays a role in calcium and phosphorus metabolism. In particular,

calcitonin has the ability to decrease blood calcium levels at least in part by

effects on two well-studied target organs:

•

Bone: Calcitonin suppresses resorption of bone by inhibiting the activity of

osteoclasts, a cell type that "digests" bone matrix, releasing calcium and

phosphorus into blood.

•

Kidney: Calcium and phosphorus are prevented from being lost in urine by

reabsorption in the kidney tubules. Calcitonin inhibits tubular reabsorption of

these two ions, leading to increased rates of their loss in urine.

Prof.Dr. Hedef Dhafir El-Yassin 2012

Control of Calcitonin Secretion

The most prominent factor controlling calcitonin secretion is the extracellular

concentration of ionized calcium. Elevated blood calcium levels strongly stimulate

calcitonin secretion, and secretion is suppressed when calcium concentration falls

below normal.

Disease States

A large number of diseases are associated with abnormally increased or decreased

levels of calcitonin, but pathologic effects of abnormal calcitonin secretion per se are

not generally recognized.

The Parathyroid Glands and Calcium Homeostasis

The parathyroid glands are four small glands attached to the thyroid. Because of their

size, researchers ignored them until well into this century.

The hormone produced by the parathyroid glands is a peptide called parathyroid

hormone (PTH). It is one of only two hormones in humans that are absolutely

essential for survival (the other is aldosterone). PTH is synthesized and released in

response to falling levels of Ca++ in the blood.

Hormones that regulate calcium metabolism

Calcium ions regulate a number of important physiological and biochemical process.

These include:

1. neuromuscular excitability

2. blood coagulation

3. secretory processes

4. membrane integrity and plasma membrane transport

5. enzyme reactions

6. the release of hormones and neurotransmitters

7. and the intracellular action of a number of hormones.

Prof.Dr. Hedef Dhafir El-Yassin 2012

In addition the proper extracellular fluid and periosteal concentration of Ca

and

-3

are required for bone mineralization.

Prof.Dr. Hedef Dhafir El-Yassin 2012

To ensure that these processes operate normally, the plasma Ca

concentration is

maintained within very narrow limits by the actions of the following hormones:

1. Parathyroid Hormone (PTH)

Parathyroid hormone is the most important endocrine regulator of calcium and

phosphorus concentration in extracellular fluid. This hormone is secreted from

cells of the parathyroid glands and finds its major target cells in bone and kidney.

Like most other protein hormones, parathyroid hormone is synthesized as a

preprohormone. After intracellular processing, the mature hormone is packaged

within the Golgi into secretory vesicles, then secreted into blood by exocytosis.

Parathyroid hormone is secreted as a linear protein of 84 amino acids.

Physiologic Effects of Parathyroid Hormone

If calcium ion concentrations in extracellular fluid fall below normal, PTH

brings them back within the normal range. In conjunction with increasing calcium

concentration, the concentration of phosphate ion in blood is reduced. Parathyroid

hormone accomplishes its job by stimulating at least three processes:

•

Mobilization of calcium from bone: Although the mechanisms remain obscure, a

well-documented effect of parathyroid hormone is to stimulate osteoclasts to

reabsorb bone mineral, liberating calcium into blood.

•

Enhancing absorption of calcium from the small intestine: Facilitating calcium

absorption from the small intestine would clearly serve to elevate blood levels of

calcium. Parathyroid hormone stimulates this process, but indirectly by

stimulating production of the active form of vitamin D in the kidney. Vitamin D

induces synthesis of a calcium-binding protein in intestinal epithelial cells that

facilitates efficient absorption of calcium into blood.

Prof.Dr. Hedef Dhafir El-Yassin 2012

•

Suppression of calcium loss in urine: In addition to stimulating fluxes of

calcium into blood from bone and intestine, parathyroid hormone puts a brake on

excretion of calcium in urine, thus conserving calcium in blood.

This effect is mediated by stimulating tubular reabsorption of calcium. Another effect

of parathyroid hormone on the kidney is to stimulate loss of phosphate ions in urine.

Control of Parathyroid Hormone Secretion

Parathyroid hormone is released in response to low extracellular concentrations

of free calcium. Changes in blood phosphate concentration can be associated with

changes in parathyroid hormone secretion, but this appears to be an indirect effect

and phosphate per se is not a significant regulator of this hormone.

When calcium concentrations fall below the normal range, there is a steep increase in

secretion of parathyroid hormone. Low levels of the hormone are secreted even when

blood calcium levels are high.

Disease States

Excessive secretion of parathyroid hormone is seen in two forms:

•

Primary hyperparathyroidism is the result of parathyroid gland disease.

•

Secondary hyperparathyroidism is the situation where disease outside of the

parathyroid gland leads to excessive secretion of parathyroid hormone. A

common cause of this disorder is kidney disease. It can also result from

inadequate nutrition - for example, diets that are deficient in calcium or vitamin

D, or which contain excessive phosphorus.

Prof.Dr. Hedef Dhafir El-Yassin 2012

Inadequate production of parathyroid hormone - hypoparathyroidism -

typically results in decreased concentrations of calcium and increased

concentrations of phosphorus in blood. Common causes of this disorder include

surgical removal of the parathyroid glands and disease processes that lead to

destruction of parathyroid glands.

2. Vitamin D

Vitamin D is a fat-soluble steroid hormone precursor that contributes to the

maintenance of normal levels of calcium and phosphorus in the bloodstream. It is

also known as calciferol.

Vitamin D

is produced in the skin by conversion of 7-dehydrocholesterol by UV.

Calciferol travels in the blood to the liver where it is converted into 25[OH] Vitamin

. This compound travels to the kidney where it is converted into Calcitriol

(1,25[OH]

Vitamin D

). This final step is promoted by the PTH.

Although called a vitamin, calciferol and its products fully qualify as hormones

because they are:

•

Made in certain cells

•

Carried in the blood

•

Affect gene transcription in target cells

Diseases

Vitamin D deficiency is known to cause several bone diseases, due to insufficient

calcium or phosphate in the bones:

Rickets: a childhood disease characterized by failure of growth and deformity

of long bones.

Osteoporosis: a condition characterized by fragile bones.

Osteomalacia: a bone-thinning disorder in adults that is characterised by

proximal muscle weakness and bone fragility. Osteomalacia can only occur in

a mature skeleton.

3. Calcitonin (discussed earlier)

The following table summarizes body responses to conditions that would otherwise
lead to serious imbalances in calcium and phosphate levels in blood

Prof.Dr. Hedef Dhafir El-Yassin 2012

Calcium Deprivation

Calcium Loading

Parathyroid hormone

Secretion stimulated

Secretion inhibited

Vitamin D

Production stimulated by increased
parathyroid hormone secretion

Synthesis suppressed due to low parathyroid
hormone secretion

Calcitonin

Very low level secretion

Secretion stimulated high blood calcium

Intestinal absorption
of calcium

Enhanced due to activity of vitamin D on
intestinal epithelial cells

Low basal uptake

Release of calcium
and phosphate from
bone

Stimulated by increased parathyroid
hormone and vitamin D

Decreased due to low parathyroid hormone and
vitamin D

Renal excretion of
calcium

Decreased due to enhanced tubular
reabsorption stimulated by elevated
parathyroid hormone and vitamin D;
hypocalcemia also activates calcium
sensors in loop of Henle to directly
facilitate calcium reabsorption

Elevated due to decreased parathyroid hormone-
stimulated reabsorption.

Renal excretion of
phosphate

Strongly stimulated by parathyroid
hormone; this phosphaturic activity
prevents adverse effects of elevated
phosphate from bone resorption

Decreased due to hypoparathyroidism

General Response

Typically seen near normal serum
concentrations of calcium and phosphate
due to compensatory mechanisms. Long
term deprivation leads to bone thining
(osteopenia).

Low intestinal absorption and enhanced renal
excretion guard against development of
hypercalcemia.

Summary

Prof.Dr. Hedef Dhafir El-Yassin 2012

Clinical Correlations

A 48-year-old woman was admitted to the hospital because of weight loss, palpitation,

weakness, and exophthalmos. She stated that a goiter, which had been present for years,

had recently begun to enlarge. She was extremely irritable, could not tolerate heat and

was short of breath. Physical examination revealed bilateral eyelid lag. The thyroid gland

was diffusely enlarged, and a bruit was audible over the right lobe. Her heart was

enlarged, a prominent apical thrust was noted, and there was a soft systolic heart murmur

along the left sternal border. Laboratory examination revealed that the hemoglobin was 1.8

mmol/L and that the hematocrit was 38%. The basal metabolic rate was 145% of normal.

plasma T4 and T3 were grossly elevated, and the

131

I uptake by the thyroid gland was

very high (18% in 4 hr). A diagnosis of hyperthyroidism was made.

Biochemical questions:

1. What are T3 and T4, and how are they related to the thyroid gland?

2. How are TRH and TSH involved in the regulation of thyroid hormone production and

secretion?

3. What is the mechanism of increased

131

I uptake in hyperthyroidism?

T3 and T4, are the thyroid hormones, triiodothyronine and thyroxine, respectively. Both are

tyrosine derivatives. Although lesser amounts of T3 are released by the thyroid gland, it

has a more potent effect than T4 in producing the hyper metabolic effects of the thyroid

hormones. T4 is converted to T3 in the target cells, and it is likely that T3, actually is the

metabolically active form of the thyroid hormone. In this sense, T4 may be considered as a

prohormone.

TSH is released from-the anterior pituitary and stimulates T3 and T4 production and

release. In turn, TSH release is stimulated by TRH, which is made in the hypothalamus.

When the plasma T3 and T4 concentrations are elevated, TRH production and release are

inhibited. This leads to decreased T3 and T4 release from the thyroid gland.

T3 and T4 contain iodine atoms attached to their phenolic rings. Thyroglobulin, the protein

precursor of these hormones that is contained in the thyroid cells, has many iodinated

tyrosine residues. The iodine is obtained from iodide ions in the blood plasma, and the

thyroid-cells have the capacity to take up and concentrate iodide ions.

Prof.Dr. Hedef Dhafir El-Yassin 2012

In hyperthyroidism the thyroid gland is rnore active than normal. It synthesizes more

thyroglobulin, T3, and T4 and takes up much larger amounts of iodine than in the euthyroid

(normal) state. Therefore, when

131

I is administered to a hyperthyroid patient, a larger

fraction of the dose is concentrated within the thyroid gland than in a euthyroid subject.

This is useful clinically in two ways.

1. Small quantities of

131

I can be administered as a diagnostic test of thyroid function.

After administration, the radiation emanating from the thyroid grand can be

measured at various times by placing a scanning device over the neck. Greater

than normal uptakes occur in hyper thyroidism, and less than normal uptakes occur

if the thyroid gland is hypo functioning (hypothyroidism).

2. The enhanced iodine uptake can be used to treat hyper thyroidism. If larger

amounts of

131

I are administered, enough

131

I will concentrate in the thyroid to

provide intense but localized radiation to the glandular cells. This will destroy many

of the T3- and T4- producing cell, reducing the excessive function of the thyroid and

correcting the hyperthyroidism.

As compared with the thyroid, other tissues take up very little iodine. Consequently, most

of the

131

I that is not taken up by the thyroid is rapidly excreted in the urine, and there is

comparatively little radiation exposure in other tissues. In some respects this is a safer

form of treatment than surgical removal of a large portion of he hyperactive grand. It is not

without some danger, however, for

131

I treatment can lean in some cases to either

hypothyroidism or even thyroid cancer.

Prof.Dr. Hedef Dhafir El-Yassin 2012

Lecture

Thursday

/2012

Biochemistry and Disorders of Hormones of the Pancreas

The bulk of the pancreas is an exocrine gland secreting pancreatic fluid into the duodenum

after a meal.

However, scattered through the pancreas are several hundred thousand clusters of cells

called islets of Langerhans. The islets are endocrine tissue containing four types of cells.

In order of abundance, they are the:

beta cells, which secrete insulin and amylin.

alpha cells, which secrete glucagon;

delta cells, which secrete somatostatin, and

gamma cells, which secrete pancreatic polypeptide (PP).

Beta Cells

Insulin is a small protein consisting of

•

an alpha chain of 21 amino acids linked by two disulfide (S

—S) bridges to a

•

beta chain of 30 amino acids.

Beta cells have channels in their plasma membrane that serve as glucose detectors. Beta

cells secrete insulin in response to a rising level of circulating glucose ("blood sugar").

Prof.Dr. Hedef Dhafir El-Yassin 2012

Gluconeogenesis

Glucogenolysis

Lipolysis

Ketogenesis

Proteolysis

Uptake of ions (especially

and PO

-3

Protein synthesis

Glycogen synthesis

Glycolysis

Glucose uptake in muscle

and adipose tissue

Insulin

Insulin affects many organs.

It stimulates skeletal muscle fibers to

•

take up glucose and convert it into glycogen;

•

Take up amino acids from the blood and convert them into protein.

acts on liver cells

•

stimulating them to take up glucose from the blood and convert it into glycogen

•

inhibiting production of the enzymes involved in breaking glycogen back down

("glycogenolysis") and

•

inhibiting "gluconeogenesis"; that is, the conversion of fats and proteins into glucose.

acts on fat (adipose) cells to stimulate the uptake of glucose and the synthesis of fat.

acts on cells in the hypothalamus to reduce appetite.

Actions of Insulin

Metabolic process

Reaction

consequence

glycogenesis

Glucose to Glycogen

(-) Blood glucose

glycogenolysis

Glycogen to Glucose

(+) Blood glucose

gluconeogenesis

Amino acids to Glucose

(+) Blood glucose

Protein synthesis

Amino acids to protein

(-) Blood amino acids

Protein degradation

Protein to Amino acids

(+) Blood amino acids

Fat synthesis (lipogenesis or

triglyceride synthesis

Fatty acids and glycerol to

triglycerides

(-) Blood fatty acids

Fat breakdown (lipolysis or

triglycerides degradation

Triglycerides to Fatty acids and

glycerol

(+) Blood fatty acids

(-) decrease in

(+) increase in

Prof.Dr. Hedef Dhafir El-Yassin 2012

Amylin

Amylin is a peptide of 37 amino acids, which is also secreted by the beta cells of the

pancreas.

Some of its actions:

•

inhibits the secretion of glucagon;

•

slows the emptying of the stomach;

•

sends a satiety signal to the brain.

Amylin (IAPP) was identified independently by two groups as the major component of

diabetes-associated islet amyloid deposits in 1987

Alpha Cells

The alpha cells of the islets secrete glucagon, a polypeptide of 29 amino acids.

Glucagon acts principally on the liver where it stimulates the conversion of

•

glycogen into glucose ("glycogenolysis") and

•

fat and protein into intermediate metabolites that are ultimately converted into

glucose ("gluconeogenesis")

In both cases, the glucose is deposited in the blood.

Glucagon secretion is

•

stimulated by low levels of glucose in the blood;

•

inhibited by high levels of glucose in the blood, and

•

inhibited by amylin.

The physiological significance of this is that glucagon functions to maintain a steady level

of blood sugar level between meals.

Delta Cells

The delta cells secrete somatostatin. This consists of two polypeptides, one of 14 amino

acids and one of 28.

Somatostatin has a variety of functions. Taken together, they work to reduce the rate at

which food is absorbed from the contents of the intestine.

Somatostatin is also secreted by the hypothalamus and by the intestine.

Gamma Cells

The gamma cells of the islets secrete a 36-amino-acid pancreatic polypeptide. Its function

is to self regulate the pancreas secretion activities . it also has effects on hepatic glycogen

levels and gastrointestinal secretions.

Its secretion human is increased after a protein meal, fasting, exercise and acute

hypoglycemia and is decreased by somatostatin.,.

Prof.Dr. Hedef Dhafir El-Yassin 2012

Synthesis and release of insulin and glucagon

Insulin and glucagon are synthesized in different cell types of the endocrine pancreas, which

consists of microscopic clusters of small glands (the islets of Langerhans) . The

α cells secrete

glucagon, and the

β cells secrete insulin into the hepatic portal vein via the pancreatic veins.

•

Synthesis and secretion of Insulin

Insulin is a polypeptide hormone. The active form of ins ulin is composed of two polypeptide

chains (the A-chain and the B-chain) linked by two interchain disulfide bonds. The A-chain has

an additional intrachain disulfide bond.

Insulin, like many other polypeptide hormones, is synthesized as a preprohormone that is

converted in the rough endoplasmic reticulum (RER) to proinsulin. The "pre" sequence, a short

hydrophobic signal sequence at the N-terminal end, is cleaved as it enters the lumen of the

RER. Proinsulin folds into the proper conformation and disulfide bonds are formed between

the cysteine residues. It is then transported in microvesicles to the Golgi complex. It leaves

the Golgi complex in storage vesicles, where a protease removes the C-peptide (a fragment

with no hormonal activity) and a few small remnants, resulting in the formation of biologically

active insulin. Zinc ions are also transported in these storage vesicles. Cleavage of the C-

peptide decreases the solubility of the resulting insulin, which then coprecipitates with zinc.

Exocytosis of the insulin storage vesicles from the cytosol of the

β cell into the blood is

stimulated by rising levels of glucose in the blood bathing the

β cells.

Glucose enters the

β cell via specific glucose transporter proteins known as GLUT2.

Glucose is phosphorylated through the action of glucokinase to form glucose 6-phosphate,

which is metabolized through glycolysis, the TCA cycle, and oxidative phosphorylation.

Prof.Dr. Hedef Dhafir El-Yassin 2012

These reactions result in an increase

in ATP levels within the

β cell. As the β

cell [ATP|/ [ADP] ratio increases, the

activity of a membrane-bound, ATP-

dependent K

channel is inhibited (i.e.,

the channel is closed). The closing of

this channel leads to a membrane

depolarization, which activates a

voltage-gated

channel

that

allows Ca

to enter the

β cell such that

intracellular Ca

levels increase

significantly. The increase in intra-

cellular Ca

stimulates the fusion of

insulin containing exocytotic vesicles with the plasma membrane, resulting in insulin

secretion. Thus, an increase in glucose levels within the

β cells initiates insulin release.

•

Stimulation and inhibition of insulin release

The release of insulin occurs within minutes after the pancreas is exposed to a high glucose

concentration. The threshold for insulin release is approximately 80 mg/glucose /dL. Above

80 mg/dL, the rate of insulin release is not an all-or-nothing -response but is proportional to

the glucose concentration up to approximately 300 mg/dL glucose. As insulin is secreted,

the synthesis of new insulin molecules is stimulated, so that secretion is maintained until

blood glucose levele fall. Insulin is rapidly removed from the circulation and degraded by the

liver and to a lesser extent by kidney and skeletal muscle) so that blood insulin levels decrease

rapidly

A number of factors other than the blood glucose concentration can modulate insulin such

as:

1. neural signals

2. certain amino acids

3. gastric inhibitory polypeptide (GIP, a gut hormone released after the ingestion of

food)

4. epinephrine secreted in response to fasting, stress, trauma and vigorous exercise

decrease the release of insulin

Prof.Dr. Hedef Dhafir El-Yassin 2012

•

Synthesis and secretion of Glucagon

Glucagon a polypeptide hormone, is synthesized in the

α cells of the pancreas by cleavage

of the much larger preproglucagon, a 160-amino acid peptide. Like insulin preproglucagon

is produced on the rough endoplasmic reticulum and is converted to proglucagon as it enters

the ER lumen. Proteolytic cleavage at various sites produce the mature 29-amino acid

glucagon and larger glucagon-containing fragments (named glucagon-like peptides I and

2). Glucagon is rapidly metabolized, primarily in the liver and kidneys. Its plasma half-life is only

about 3 to 5 minutes.

Insulin

Glucagon Epinephrine

Glucagon secretion is regulated principally by circulating levels of glucose and insulin.

Increasing levels of each inhibit glucagon release. Glucose probably has both a direct

suppressive effect on secretion of glucagon from the

α cell as well as an indirect effect, the latter

being mediated by its ability to stimulate the release of insulin.

Certain hormones stimulate glucagon secretion. :

1) catecholamines (epinephrine)

2) cortisol

3) gut hormones

4) Many amino acids also stimulate glucagon release.

Metabolic effects of glucagon

1. Effects on carbohydrate metabolism: The intravenous administration of glucagon leads to an immediate

rise in blood glucose. This results from an increase in the breakdown of liver (not muscle) glycogen and

an increase in gluconeogenesis.

2. Effects on lipid metabolism: Glucagon favors hepatic oxidation

fatty acids and the

subsequent formation of ketone bodies acetyl CoA. The lipolytic effect of glucagon in adipose

tissue is minimal in humans.

Glycogenolysis

Gluconeogenesis

Glycogenolysis

Gluconeogenesis

Prof.Dr. Hedef Dhafir El-Yassin 2012

3. Effects on protein metabolism: Glucagon increases uptake of amino acids by the liver,

resulting in increased availability of carbon skeletons for gluconeogenesis. As a

consequence plasma levels of amino acids are decreased.

DIABETES MELLITUS:

This is a metabolic disorder occurring as a result of an insulin lack or a surplus of insulin

antagonists leading to a relative insulin lack. It is characterized by hyperglycemia and

glucosuria. Three types of the disease are described-

1. Juvenile type (type 1)

in children and is due to an absolute deficiency of insulin.

2. Maturity onset type

: It is usually associated with obesity. A form of diabetes known as

MODY (maturity onset diabetes of the young) results from mutations in either

pancreatic glucokinase or specific nuclear transcription factors. Thus, although these

patients can release insulin, they do so at higher than normal glucose levels, and are

therefore almost always in a hyperglycemic state. Interestingly, however, these

patients are somewhat resistant to the long-term complications of chronic

hyperglycemia.

3. Secondary diabetes:

There is hyperfunction of one or other of insulin anatgonists leading to

a relative insufficiency of insulin eg: acromegaly (excess of growth hormone); Cushing's

disease (excess of glucocorticoids); hyperthyroidism etc.

Glucagon is one of the contributory factors in the etiology of diabetes mellitus. Its blood levels

are elevated in severe diabetes with ketoacidosis. The

α cells seem to be insensitive to the high

blood glucose levels in diabetes and continue to secrete large amounts of glucagon.

Somatostatin, a hypothalamic factor inhibiting the release of growth hormone, also inhibits the

release of glucagon and is in experimental use as an adjunct to insulin in the control of

severe diabetes mellitus.

4. Type 2 diabetes:

a combination of insulin resistance and dysfunctional

cells.

Insulin resistance:

Is the decreased ability of target tissues, such as liver adipose and

muscle to respond properly to normal circulating concentrations of insulin. For example ,

insulin resistance is characterized by uncontrolled hepatic glucose production and decreased

glucose uptake by muscle and adipose tissue.

In a moderately severe early diabetes mellitus the following features are present-

1. Hyperglycemia.

2. Glycosuria.

3. Loss of weight due to increased breakdown of fat and tissue protein.

4. Increased production of ketone bodies by the liver and their incomplete utilization by

tissues leading to their accumulation in blood (ketosis) and elimination in urine (ketonuria).

Prof.Dr. Hedef Dhafir El-Yassin 2012

5. Lowering of the pH of blood due to circulating keto acids (acidosis).

6. Dehydration due to elimination of large amounts of water with glucose in urine.

7. Negative nitrogen balance due to conversion of more amino acids into glucose

(increased gluconeogensis).

8. Increased levels of lipid, fatty, acid and cholesterol in blood (lipemia).

9. Increased tendency to develop cataract in the eye and arteriosclerotic lesions of blood

vessels.

Hyperinsulinism; This may occur on account of islet-cell tumors involving the

β-cells. There is

overproduction of insulin resulting in spontaneous attacks of hypoglycemia associated with
sweating, and fainting attacks which are relieved by ingestion of glucose or a lump of sugar.

Prof.Dr. Hedef Dhafir El-Yassin 2012

Insulin and Glucagon receptors

1. Insulin Receptor

Insulin Binding to it's Receptor Followed by Activation of the Receptor:

Insulin binds switching ON the receptor. Once the receptor is ON (catalytically active)

insulin may disaccociate and be degraded.

The Tyr Kinase domain is phosphorylated.

A "cascade" of events takes place (see below).

Biochemical / Physiological Responses:

Prof.Dr. Hedef Dhafir El-Yassin 2012

2. Glucagon Receptor

Representative pathway for the activation of cAMP-dependent protein kinase, PKA. In this

example glucagon binds to its' cell-surface receptor, thereby activating the receptor.

Activation of the receptor is coupled to the activation of a receptor-coupled G-protein

(GTP-binding and hydrolyzing protein) composed of 3 subunits. Upon activation the

α-

subunit dissociates and binds to and activates adenylate cyclase. Adenylate cylcase then

converts ATP to cyclic-AMP (cAMP). The cAMP thus produced then binds to the

regulatory subunits of PKA leading to dissociation of the associated catalytic subunits. The

catalytic subunits are inactive until dissociated from the regulatory subunits. Once released

the catalytic subunits of PKA phosphorylate numerous substrate using ATP as the

phosphate donor

Prof.Dr. Hedef Dhafir El-Yassin 2012

Clinical cases and correlations

Insulinoma

A 36-year-old woman was referred to a university hospital for evaluation of spells of

dizziness and weakness. These spells typically lasted for 10 min and were occurring with

increasing frequency. The spells usually came on after a large meal and could be

terminated by her eating candy or drinking fruit juice. After each episode the patient was

hungry and tired, and her memory was blurred. The patient's physical examination was

within normal limits except for mild obesity. She claimed to have gained 20 kg during the

preceding 2 yr. After a 13-hr fast her blood glucose concentration was 2.1 mmollL. After a

5-hr glucose tolerance test, her blood glucose was 2.6 mmol/L.

Celiac angiography revealed an abnormality in the body and tail of the pancreas. The

patient developed one of her spells while a medical student was in her room, and he was

able to obtain a blood sample during the episode. This sample contained 1.1mmol/L of

glucose. The patient was transferred to the surgical service, and an insulin-secreting

pancreatic adenoma (tumor) was removed, requiring resection of 90% of the pancreas.

Biochemical questions

1. An insulinoma is an insulin-secreting tumor. How did the presence of such a tumor

explain the patient's symptoms?

2. Proinsulin was found in large quantities in this patient's plasma. What is the

relationship of proinsulin to insulin? What is preproinsulin?

3. What effects of increased insulin secretion might have predisposed this woman to

obesity?

4. What digestive problems might result from excision of 90% of the pancreas?

The insulinoma was producing insulin. Because of the excessive amount of insulin-

secreting tissue, too much insulin was released after dietary carbohydrate intake. This

caused hypoglycemia during the 5-hr glucose tolerance test and after meals, producing

the spells of weakness and dizziness. In addition, to this normal insulin release when

carbohydrate was ingested, the tumor also was secreting some insulin continuously.

This inappropriate insulin release caused the low blood glucose concentration during

prolonged fasting.

Proinsulin is the prohormone form of insulin that is made in the

-cells of the pancreatic

islets. It has no insulin-like action. After synthesis on the ribosomes, the initial precursor,

preproinsulin, penetrates through the endoplasmic reticulum into the lumen of this

Prof.Dr. Hedef Dhafir El-Yassin 2012

organelle. The leader sequence is removed in this process, forming proinsulin, which is

transported to the Golgi apparatus and stored in granules.

Proinsulin is converted to insulin in these granules by proteolytic cleavage, but the

conversion is incomplete. When insulin is discharged from the

-cell, some proinsulin that

remains in the granule also is released. Likewise, C-peptide that is split out in the

conversion of proinsulin to insulin is released during insulin secretion, but it too, has no

insulin-like activity.

Insulin acts on adipocytes, enhancing fatty acid storage as triglyceride. It binds to specific

receptors on the cell surface and facilitates glucose entry into the adipocyte, increasing the

availability of the triose backbone, glycerol 3-phosphate, needed for triglyceride synthesis.

This also provide glucose carbon atoms for fatty acid syntesis.

In addition, it increases the content of lipoprotein lipase in the adipose tissue. This enzyme

catalyzes the hydrolysis of chyromicron and VLDL triglycerides, a step that is required to

transfer their fatty acids into the adipocytes for resynthesis into triglyceride. Much of the

fatty acid stored in the adipose tissue is delivered to the adipocytes in the form of

lipoprotein triglycerides, either VLDL from the liver or chylomicrons from the intestine.

Therefore the elevated lipoprotein lipase activity also favors triglyceride formation in the

adipose tissue. Those adipose tissue effects that were mediated by the excessive insulin

production could have contributed to he recent weight gain noted by this patient.

In addition to polypeptide hormones, the pancreas making many digestive enzymes.

These include amylase for dietary starches, lipase for triglycerides, chymotrypsin and

trypsin for proteins, as well as several others. Since 90%, of the pancrease was excised,

the remaining 10% may not produce sufficient amounts of these enzymes to adequately

digest large meals. This might lead to malnutrition and weight toss in spite of an adequate

diet. Because of this possibility, six or more smaller meals rather than three regular meals

each day might be recommended.

Question: Binding of insulin to its receptor:

a) Occurs on the

ß-subunit.

b)  Induces autophosphorylation.
c)  Reduces binding of cytosolic substrate proteins.
d)  Leads only to phosphorylation of proteins.
e)  Does not lead to release of a second messenger.

Answer:

B This occurs on tyrosine residues of the

ß-subunit. A: Binding is to the α-

subunit. C: Autophosphorylation facilitates binding. D: Some proteins are
dephosphorylated. E: A second messenger may account for short-term metabolic effects.

Prof.Dr. Hedef Dhafir El-Yassin 2012

Hormones of the

adrenal gland

[Prof. Dr. H.D.El-Yassin 2012

Hormones of the adrenal cortex

Prof.Dr. Hedef Dhafir El-Yassin 2012

Lecture 7

Sunday 11/3/2012

Hormones of the adrenal gland

The structure of the adrenal gland

The two adrenal glands (also called the suprarenal glands) are situated in the abdomen,

on either side of the vertebral column, above the kidneys and below the diaphragm

When cut in half each gland consists of

An outer cortex, yellow in color and

An inner medulla, which is dark red, or grey

Prof.Dr. Hedef Dhafir El-Yassin 2012

The cortex consists of three distinct zones

Zona glomerulosa

Zona fasciculata

Zona reticularis

Each zone has a characteristic histology and secretes different types of hormones

Layer

Name

Primary product

Most
superficial
cortical
layer

Zona
glomerulosa

mineralocorticoid (aldosterone) which is responsible for the
regulation of salt and water balance in the body

Middle
cortical
layer

Zona
fasciculata

glucocorticoid (cortisol) which regulates the level of
carbohydrate in the body

Deepest
cortical
layer

Zona
reticularis

sex hormones (progesterone, oestrogen precursors and
androgens) which have a role in the development of sexual
characteristics

Prof.Dr. Hedef Dhafir El-Yassin 2012

The medulla consists of many large columnar cells called "chromaffin cells". These
synthesize and secrete catecholamines

when stimulated by the sympathetic nervous

system.

Hormone synthesis

All adrenocortical hormones are synthesized from cholesterol. Cholesterol is transported

into the adrenal gland. Subsequent steps to generate aldosterone and cortisol, primarily

occur in the adrenal cortex:

•

Progesterone

→ (hydroxylation at C21) → 11-Deoxycorticosterone → (two further

hydroxylations at C11 and C18)

→ Aldosterone

•

Progesterone → (hydroxylation at C17) → 17-alpha-hydroxyprogesterone

→

(hydroxylation at C21)

→ 11-Deoxycortisol → (hydroxylation at C11) → Cortisol

Cholesterol is converted to pregnenolone (P5) by cytochrome P450 cholesterol side

chain cleavage . P5 is the precursor of all the other steroids and stands at the first

branch point in the adrenal steroidogenic network

Steroidogenic defects can cause congenital adrenal hyperplasia (CAH). This condition

may cause symptoms ranging from mild acne to salt wasting, depending on the nature of

the genetic mutation.

In hypoandrenocorticism (Addison's disease) and CAH the error involves the two

enzymes cytochrome P450 17a-hydroxylase/17-20 lyase and cytochrome P450 21-

hydroxylase respectively.

Because of a lack of the glucocorticoids and mineralocorticoids the brain signals the

adrenal gland with adrenocorticotrophic hormone (ACTH) to produce more of the deficient

steroids. Consequently, there is an over production of P5 that in turn leads to an over

production of DHEA, which is converted to androgens and estrogens outside of the

adrenal gland due to the DHEA in circulation diffusing into other steroidogenic tissues with

the appropriate activities.

Hormones secreted by the Adrenal Cortex

1. Mineralocorticoids

The primary mineralocorticoids aldosterone is

aldosterone

. Its

secretion is regulated by the

oligopeptide

angiotensin II

(angiotensin II is regulated by

angiotensin I

, which in turn is

regulated by

renin

). Aldosterone is secreted in response to high

extracellular

potassium

levels, low extracellular

sodium

levels,

Prof.Dr. Hedef Dhafir El-Yassin 2012

and low fluid levels and blood volume. Aldosterone affects

metabolism

in different ways:

a. It increases

urinary

excretion of potassium

ions

b. It increases interstitial levels of sodium ions

c. It increases water retention and blood volume

Removal of the adrenal glands leads to death within just a few days. Due to:

the concentration of potassium in extracelluar fluid becomes dramatically elevated

urinary excretion of sodium is high and the concentration of sodium in extracellular

fluid decreases significantly

volume of extracellular fluid and blood decrease

the heart begins to function poorly, cardiac output declines and shock ensues

Clearly mineralocorticoids are acutely critical for maintenance of life!

Prof.Dr. Hedef Dhafir El-Yassin 2012

Aldosterone and Mineralocorticoid
Receptors
Cortisol, have "weak mineralocorticoid

activity", which is of some importance

because cortisol is secreted very much

more abundantly than aldosterone. i.e. a

small fraction of the mineralocorticoid response in the body is due to cortisol rather than

aldosterone.

The mineralocorticoid receptor binds both aldosterone and cortisol with equal affinity.

Moreover, the same DNA sequence serves as a

hormone response element

for the

activated (steroid-bound) forms of both mineralocorticoid and glucocorticoid receptors.

How can aldosterone stimulate specific biological effects in this kind of

system, particularly when blood concentrations of cortisol are something like 2000-

fold higher than aldosterone?

In aldosterone-responsive cells, cortisol is effectively destroyed, allowing

aldosterone to bind its receptor without competition. Target cells for aldosterone express

the enzyme 11-beta-hydroxysteroid dehydrogenase, which has no effect on aldosterone,

but converts cortisol to cortisone, which has only a very weak affinity for the

mineralocorticoid receptor. In essence, this enzyme "protects" the cell from cortisol and

allows aldosterone to act appropriately.

An interesting demonstration of this enzyme protection system is seen in chronic licorice

intoxication. The major components of licorice are glycyrrhizic acid and its hydrolytic

product, glycyrrhetinic acid (GE). GE is a potent inhibitor of 11

ß-hydroxysteroid

dehydrogenase. By blocking the activity of this inactivating enzyme. GE facilitates the

binding of cortisol to renal mineralocorticoid receptors and hence induces AME syndrome

(Apparent Mineralocorticoid Excess Syndrome).

Control of Aldosterone Secretion

The two most significant regulators of aldosterone secretion are:

•

Concentration of potassium ions in extracellular fluid: Small increases in blood

levels of potassium strongly stimulate aldosterone secretion.

•

Angiotensin II: Activation of the renin-angiotensin system as a result of decreased

renal blood flow (usually due to decreased vascular volume) results in release of

angiotensin II, which stimulates aldosterone secretion.

Prof.Dr. Hedef Dhafir El-Yassin 2012

Factors which suppress aldosterone secretion include atrial naturetic hormone, high

sodium concentration and potassium deficiency.

Disease States: A deficiency in aldosterone can occur by itself or, more commonly, in

conjunction with a glucocorticoid deficiency, and is known as hypoadrenocorticism or

Addison's disease

2. Glucocorticoids

Cortisol and Glucocorticoid Receptors

Cortisol binds to the glucocorticoid receptor in the

cytoplasm and the hormone-receptor complex is then

translocated into the nucleus, where it binds to its DNA

response element and modulates transcription from a

battery of genes, leading to changes in the cell's

phenotype.

Only about 10% of circulating cortisol is free. The remaining majority circulates bound to

plasma proteins, particularly corticosteroid-binding globulin (transcortin).

Metaboilc Effects of Glucocorticoids

There seem to be no cells that lack glucocorticoid receptors and as a consequence, these

steroid hormones have a huge number of effects on physiologic systems.

The name glucocorticoid derives from early observations that these hormones were

involved in glucose metabolism.

Cortisol stimulates several processes that collectively serve to increase and maintain

normal concentrations of glucose in blood. These effects include:

•

Stimulation of gluconeogenesis, particularly in the liver: This pathway results in

the synthesis of glucose from non-hexose substrates such as amino acids and

lipids . Enhancing the expression of enzymes involved in gluconeogenesis is

probably the best known metabolic function of glucocorticoids.

•

Mobilization of amino acids from extrahepatic tissues: These serve as

substrates for gluconeogenesis.

•

Inhibition of glucose uptake in muscle and adipose tissue: A mechanism to

conserve glucose.

•

Stimulation of fat breakdown in adipose tissue: The fatty acids released by

lipolysis are used for production of energy in tissues like muscle, and the released

glycerol provide another substrate for gluconeogenesis.

Prof.Dr. Hedef Dhafir El-Yassin 2012

Control of Cortisol Secretion

Cortisol and other glucocorticoids are secreted in response to a single stimulator:

adrenocorticotropic hormone (ACTH) from the anterior pituitary. ACTH is itself

secreted under control of the hypothalamic peptide corticotropin-releasing hormone

(CRH).

Virtually any type of physical or mental stress

results in elevation of cortisol concentrations in

blood due to enhanced secretion of CRH in the

hypothalamus. This fact sometimes makes it very

difficult to assess glucocorticoid levels, especially

being restrained for blood sampling, is enough

stress to artificially elevate cortisol levels several

fold!

Cortisol secretion is suppressed by classical

negative feedback loops. When blood

concentrations rise above a certain threshold,

cortisol inhibits CRH secretion from the

hypothalamus, which turns off ACTH secretion,

which leads to a turning off of cortisol secretion from

the adrenal. The combination of positive and negative control on CRH secretion results in

pulsatile secretion of cortisol. Typically, pulse amplitude and frequency are highest in the

morning and lowest at night.

ACTH, also known as corticotropin, binds to receptors in the plasma membrane of cells in

the adrenal. Hormone-receptor engagement activates adenyl cyclase, leading to elevated

intracellular levels of cyclic AMP which leads ultimately to activation of the enzyme

systems involved in biosynthesis of cortisol from cholesterol.

Disease States

1. Cushings disease or hyperadrenocorticism.

2. Insufficient production of cortisol, often accompanied by an aldosterone deficiency,

is called Addison's disease or hypoadrenocorticism.

Prof.Dr. Hedef Dhafir El-Yassin 2012

3. Androgens

The most important androgens include:

1. Testosterone: a hormone with a wide variety of effects, ranging from enhancing

muscle mass and stimulation of cell growth to the development of the secondary

sex characteristics.

2. Dihydrotestosterone (DHT): a metabolite of testosterone, and a more potent

androgen than testosterone in that it binds more strongly to androgen receptors.

3. Androstenedione (Andro): an androgenic steroid produced by the testes, adrenal

cortex, and ovaries. While androstenediones are converted metabolically to

testosterone and other androgens, they are also the parent structure of estrone.

4. Dehydroepiandrosterone (DHEA): It is the primary precursor of natural estrogens.

DHEA is also called dehydroisoandrosterone or dehydroandrosterone.

Prof.Dr. Hedef Dhafir El-Yassin 2012

[Hormones of the

Adrenal Gland]

[Prof.Dr.H.D.El-Yassin]

2. Hormones of the adrenal Medulla

Prof.Dr. Hedef Dhafir El-Yassin 2012

Hormones secreted by the Adrenal Medulla

Cells in the adrenal medulla synthesize and secrete epinephrine and norepinephrine.

Following release into blood, these hormones bind adrenergic receptors on target cells,

where they induce essentially the same effects as direct sympathetic nervous stimulation.

Synthesis Catecholamines

Synthesis of catecholamines begins with the amino acid tyrosine, which is taken up

by chromaffin cells in the medulla and converted to norepinephrine and epinephrine

through the following steps:

Norepinephine and epinephrine are stored in electron-dense granules which also contain

ATP and several neuropeptides. Secretion of these hormones is stimulated by

acetylcholine release. Many types of "stresses" stimulate such secretion, including

exercise, hypoglycemia and trauma. Following secretion into blood, the catecholamines

bind loosely to and are carried in the circulation (50%) by albumin and other serum

proteins. Once secreted their half life in the circulation is short (approximately 12 min) but

Prof.Dr. Hedef Dhafir El-Yassin 2012

they have a large effect on heart, vessels, metabolism, brain, muscles etc. all as part of

stress responses.

Adrenergic Receptors and Mechanism of Action

The physiologic effects of epinephrine and norepinephrine are initiated by their binding to

adrenergic receptors on the surface of target cells. These receptors are prototypical

examples of seven-pass transmembrane proteins that are coupled to G proteins which

stimulate or inhibit intracellular signaling pathways.

There are two major classes of adrenergic receptors these are:

α adrenergic receptor (epinephrine and norepinephrine)

β adrenergic receptor (epinephrine )

Control of catecholamine release

The release of the catecholinamines is controlled from nerve cells within the posterior

hypothalamus which can ultimately stimulate acetylcholine release from nerve terminals of

the sympathetic nerves. This induces depolarization of the chromaffin cells and exocytosis

of the catecholamine containing granules following a rise in intracellular calcium

concentration.

Metabolic Effects of catecholamines Hormones

•

In general, circulating epinephrine and norepinephrine released from the

adrenal medulla have the same effects on target organs as direct stimulation

by sympathetic nerves, although their effect is longer lasting.

•

glycogenolysis to provide extra sources of glucose

•

Stimulation of lipolysis in fat cells to provided fatty acids for energy production in

many tissues and aids in conservation of dwindling reserves of blood glucose.

Prof.Dr. Hedef Dhafir El-Yassin 2012

•

Increased metabolic rate due to increased oxygen consumption and heat

production increase throughout the body in response to epinephrine binding beta

receptors.

•

Increased breakdown

of glycogen in skeletal

muscle to provide

glucose for energy

production.

Water and electrolytes

metabolism

•

Decreased sodium

excretion and glomerular

filtration due to direct effects on the kidney

•

effects on renin secretion leads to increased aldosterone production with effects on

distal sodium handling

•

Serum potassium may be increased

Catecholamine Degradation
All catecholamines are rapidly eliminated from target cells and the circulation by three
mechanisms:

1.  reuptake into secretory vesicles
2.  uptake in non-neural cells (mostly liver)
3.  degradation.

Degradation relies on two enzymes:

1. catechol O-methyltransferase (COMT) in non-neuronal tissues
2. and monoamine oxidase (MAO) within neurons.

to produce metabolites (metanephrines and vanillylmandelic acid (VMA)) from free
catecholamines.

Metabolites and free catecholamine are eliminated by direct filtration into the urine and
excreted as:

1)  free norepinephrine,
2)  conjugated norepinephrine
3)  metanephrines and
4)  VMA

Urine epinephrine (50%) is converted from norepinephrine by renal (not adrenal)
phenylethanolamine N-methyltransferase (PNMT) before excretion.

Prof.Dr. Hedef Dhafir El-Yassin 2012

Clinical cases and Correlations:

Cushing's syndrome

A 35-year-old man was admitted to the hospital because of irritability and emotional lability

together with muscle weakness and easy fatigability. Physical examination revealed that

his trunk was obese but his arms and legs were quite lean. He had a rounded facial

appearance and a small, nontender hump at the junction of his neck and back. Fullness

was noted in the supraclavicular regions, and purple striae were present in the subaxillary

areas. Laboratory examination revealed that the plasma cortisol concentration was

elevated (0.55 mmol/L), the fasting blood glucose was 8.3 mmol/L, and the urinary

excretion of 11-hydroxyandrosterone and 11-hydroxyetiocholanolone were greatly

increased. A diagnosis of Cushing's syndrome was made.

Biochemical questions

1. From what substance is cortisol synthesized?

2. How is cortisol synthesis regulated?

3. How is cortisol transported in the blood plasma?

4. What are the metabolic effects of cortisol in humans?

5. How does cortisol function at the molecular level?

6. How are 11-hydroxyandrosterone and 11-hydroxyetiocholanolone related to cortisol

and why were they excreted in increased amounts in this patient's urine?

7. Would you expect that excessive quantities of vanillylmandelic acid (VMA) also

would be excreted in this patient's urine?

Case discussion:

Cortisol, the main glucocorticoid hormone in humans, is synthesized in the zona

fasciculata of the adrenal cortex. Cholesterol is the precursor of cortisol as well as of the

approximately 50 other steroids that can be isolated from the adrenal cortex. Most of the

cholesterol used for steroid hormone synthesis is taken up from plasma lipoproteins and

stored in the adrenal cortical cells as cholesteryl esters. Progesterone is the common

intermediate in steroid hormone synthesis.

Regulation of synthesis cortisol synthesis is regulated by ACTH, a polypeptide hormone

secreted by the anterior pituitary gland. ACTH release, in turn, is stimulated by CRH,

produced in the hypothalamus. Cortisol production is decreased by high concentrations of

plasma cortisol or ACTH, both of which inhibit CRH release. ACTH stimulates cortisol

production by activating the hydrolysis of cholesteryl esters through a cAMp-dependent

mechanism.

Prof.Dr. Hedef Dhafir El-Yassin 2012

Transport Like the other steroid hormones, cortisol is transported in the blood by a

specific carrier protein, transcortin. This protein is a plasma globulin hat is synthesized in

the liver. It also transports corticosterone, the other major glucocoricoiod produce by the

adrenal cortex.

Metabolic effects One of the main metabolic effects of the glucocorticoid hormones is to

raise the blood glucose concentration by stimulating gluconeogenesis. Body proteins are

catabolized to provide the amino acid substrates for gluconeogenesis, leading to

decreases in muscle mass and weakness. Glucocorticoids operate through a

transcriptional mechanism. This involves binding of the hormone to cytoplasmic protein

receptors and transport of the steroid-receptor complex into the cell nucleus.

Once inside the nucleus, it activates the transcription of mRNA, which is specific for the

synthesis of these amino acid catabolic enzymes. The glucocorticoids have additional

actions, such as indirectly facilitating the stimulation of lipolysis, and destruction of

lymphocytes.

Catabolism and excretion Cortisol is metabolized and the l7-keto metabolites of cortisol

are 1l-hydroxyandrosterone and 11-hydroxyetiocholanolone. Almost all the cortisol

metabolites are excreted in the urine. Since cortisol is overproduced in Cushing's

syndrome, it is understandable that these two cortisol metabolites are excreted in

excessive amounts. .

VMA is the main urinary excretion product of the catecholamines. It is elevated in diseases

associated with excessive catecholamine production, such as pheochromcytoma. If a

tumor of the adrenal medulla or other chromaffin tissue is present, VMA excretion would

be increased. Cushing's disease results from overproduction of glucocorticoids by the

adrenal cortex and, of itself, does not involve the adrenal medulla or other catecholamine-

producing tissues. Thus there is no reason to suspect overproduction of either epinephrine

or norepinephrine in this case, and one would not expect VMA excretion to be affected if

the diagnosis of Cushing's syndrome is correct.

Prof.Dr. Hedef Dhafir El-Yassin 2012

Primary aldosteronism

A 44-year-old man who complained of headaches was discovered to be hypertensive.

Hypokalemia was noted; the plasma potassium concentration remained between 2.5 and 3

mmol/L during 3 wk of observation. Peripheral vein renin concentrations were consistently

less than 1 µlL, and they remained low even when the patient was placed on a low-sodium

diet. Urinary aldosterone excretion was consistently high, even when a high-sodium diet

was given. A venogram showed a questionable lesion in the left adrenal gland. A left

adrenalectomy was performed. On follow-up examination 2 months later, the patient was

found to be normotensive and normokalemic.

Biochemical questions

1. Explain the hypokalemia in this case. Would you expect any abnormality in total

body sodium content?

Apparent Mineralocorticoid Excess Syndrome

Some patients (usually children) exhibit the hypertension, hypokalemia, and suppression

of the renin-angiotensin-aldosterone system that would be expected if they were hyper-

secreting aldosterone. Since assays of plasma and urine may fail to identify, excess

mineralocorticoids, these patients are said to have the apparent mineralocorticoid excess

(AME) syndrome. This syndrome results from the failure of inactivation of cortisol by 11

ß-

hydroxysteroid dehydrogenase. Since the plasma levels of cortisol are about 100 times

higher than the levels for aldosterone, cortisol saturates the renal mineralocorticoid

receptor, causing sodium retention and suppression of the renin-angiotensin-aldosterone

axis.

Although this syndrome can result from a congenital defect in renal 11

ß-hydroxysteroid

dehydrogenase isoform, it can also be caused by ingesting excessive amounts of licorice.

Prof.Dr. Hedef Dhafir El-Yassin 2012

Questions

2) Glucocorticoid receptors are in the cytoplasm. AII of the following statements about the

process by which the hormone influences transcription arc correct except:

a) The hormone must be in the free state to cross the cell membrane.

b) Cytoplasmic receptors may be associated with heat shock proteins.

c) The receptor-hormone complex is not activated/transformed until it is

translocated to the nucleus.

d) In the nucleus, the activated/transformed receptor-hormone complex searches for

specific sequences on DNA called HREs (hormone receptor elements).

e) The activated receptor-hormone complex may either activate or repress

transcription of specific genes (only one activity per gene).

Dissociation of the heat shock protein from the receptor- hormone complex in the

cytosol activates the complex. A: Steroid hormones travel bound to plasma proteins, but

some is always free. D: These are consensus sequences in DNA. E: Activation is more

common, but glucocorticoids repress transcription of the proopiomelanocortin gene.

3) All of the following are normal events leading to secretion of aldosterone from the

adrenal gland except:

a) Renin is released by the kidney in hypovolemia.

b) Angiotensinogen binds to membrane receptors.

c) Ca

levels in the cell rise.

d) aldosterone is secreted into the blood.

Angiotensinogen is cleaved by renin to angiotensin I, which must further be cleaved

by converting enzyme to active angiotensin II. A: This is a major signal. C: This lead to

increased Ca

and activation of protein kinase C.

Prof.Dr. Hedef Dhafir El-Yassin 2012

Biochemistry and Disorders of Hormones of the

Kidney, Heart and Adipose tissue

Lecture 8

Sunday 8/3/2012

 

The human kidney secretes two hormones

•

Erythropoietin

•

Calcitriol (1,25[OH]

Vitamin D

)

•

Erythropoietin

(EPO) is a glycoprotein hormone that is a growth factor for erythrocyte (red blood cell)

precursors in the bone marrow.

•

In adults primarily by peritubular cells in the kidneys, where its production is stimulated

by low oxygen levels in the blood.

•

Some EPO is also produced by the liver, which is the primary source in the fetus.

Actions

EPO acts by binding to a specific erythropoietin receptor (EpoR) on the surface of red cell

precursors in the bone marrow, stimulating them to transform into mature red blood cells.

As a result the oxygen level in blood reaching the kidney rises and the amount of EPO

produced decreases.

People with failing kidneys can be kept alive by dialysis. But dialysis only cleanses the

blood from wastes. Without a source of EPO, these patients suffer from anemia.

 

In response to a rise in blood pressure, the heart releases two peptides

•

A-type Natriuretic Peptide (BNP)

This hormone of 28 amino acids is released from stretched atria (hence the "A").

•

B-type Natriuretic Peptide (BNP)

This hormone (29 amino acids) is released from the ventricles. (It was first discovered in

brain tissue; hence the "B")

Both hormones lower blood pressure by:

•

Relaxing arterioles

•

Inhibiting the secretion of renin and aldosterone

•

Inhibiting the reabsorption of sodium ion by the kidneys.

Prof.Dr. Hedef Dhafir El-Yassin 2012

The latter two effects reduce the reabsorption of water by the kidneys. So the volume of

urine increases as does the amount of sodium excreted in it. Te net effect of these actions

is to reduce blood pressure by reducing the volume of blood in the circulating system.

These effect give ANP and BNP their name (natrium= sodium; uresis= urinate)

Hormones of adipose tissue

Although the adipocyte's primary role is to store fat, it also functions as an endocrine cell that

releases numerous regulatory molecules, such as leptin, adiponectin, and resistin

1. Leptin: Studies of the molecular genetics of mouse obesity have led to the isolation of at

least six genes associated with obesity. The most well-known mouse gene, named Ob (for

obesity), leads to severe hereditary obesity in mice. It has been identified and cloned. In

one strain of fat mice, the gene

was completely absent, indicating

that the gene's protein product is

required to keep the animals'

weight under control. The

product of the Ob gene is a

hormone called leptin.

Leptin is produced proportionally to the

adipose mass and, thus, informs the

brain of the fat store level. It is secreted

by fat cells, and acts on the

hypothalamus of the brain to regulate

the amount of body fat through the

control of appetite and energy

expenditure. Leptin's secretion is

suppressed by depletion of fat stores

(starvation) and enhanced by expansion

of fat stores (well-fed state). Daily

injection of leptin causes overweight mice to lose weight and maintain weight loss. The protein also

causes weight loss in mice that are not obese. In humans, leptin increases the metabolic rate and

decreases appetite.

Prof.Dr. Hedef Dhafir El-Yassin 2012

However, plasma leptin in obese humans is usually normal for their fat mass, suggesting that

resistance to leptin, rather than its deficiency, occurs in human obesity. Other hormones

released by adipose tissue, such as adiponectin and resistin, may mediate insulin resistance

observed in obesity

2. resistin

The hormone resistin is one amongst a novel family of three proteins , known as resistin-

like molecules (RELMs). They are cysteine-rich secreted proteins associated with

pulmonary inflammation (also known as FIZZ3, found in inflammatory zone). It has 11

cysteine-residues synthesized as a propeptide of 108 amino acids and secreted as a

dimmer, build by a disulfide bridge of cysteine residues. Beside this intermolecular

disulfide bridge, 5 additional intramolecular ones exist.

Source of reistin

In humans, resistin expression in adiposite can be detected at a low level. it is higher in

abdominal fat stores than in thigh adipose tissue, this suggest a potential role in linking

central obesity to type 2 diabetes and/or cardiovascular disease. Human resistin is

expressed mainly in pancreatic islet, preadiposites, macrophages and bone marrow. So

resistin is of relevance for inflammation processes as well as for lipid metabolism.

In mice a correlation between adiposity, insulin resistance and resistin expression was

found empirically. In humans respective studies are not clear. Several show an association

of resistin serum concentration and adiposity or insulin resistance.

Prof.Dr. Hedef Dhafir El-Yassin 2012

Resistin putative role(s):

Relevance of resistin in physiological processes other than energy metabolism was

investigated. Experiments with endothelial cells gave interesting results, in which resistin

shown to be potentially able to influence endothelial inflammation and thereby

atherosclerosis.

Resistin shares some qualities with another protein secreted by fat cells and associated

with obesity, the hormone leptin. This hormone, discovered in 1995, seems to regulate

food intake.

There is still much to learn about resistin. But with each new piece fitted into the diabetes

puzzle, new possibilities arise.

There are two putative roles of resistin:

a. To directly cause insulin resistance

b. To block adipocyte differentiation

The latter might lead to ectopic fat storage (increased amounts of fat in skeletal

muscle and liver.

Future work….

Future research in this area aims to establish the role of resistin in human disease.

Measurement of resistin in a simple blood test might then be useful in detecting insulin

resistance and prediabetic conditions. Looking forward, counteracting resistin's affects on

the body might be a new approach to preventing and treating diabetes.