Hormones�By�Prof. Dr. Syed Touqeer Abbas

• Biochemical classification
• Mechanism of action
• Hierarchy
• Feedback loops
• Signal transduction

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Learning Outcomes

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At the end of the unit the learners will be able to:

Appraise the basic principles of Endocrinology along with the biochemical basis and related abnormalities

Learning Objectives

 Describe the general principles of endocrine system

 Classify the hormones according to their chemical nature & Mechanism of Action

 Explain Cell surface receptors with special emphasis on G protein coupled receptors

 Discuss Intracellular second messenger signaling cascade

 Describe the Intracellular ligand receptors

 Explain the Site of synthesis, stimulus for secretion, mechanism of action, receptors, intracellular effects, target cells, tissues and biochemical role & hypo/hyper secretion of Growth Hormone

 Explain the Site of synthesis, stimulus for secretion, mechanism of action, receptors, intracellular effects, target cells, tissues and biochemical role & hypo/hyper secretion of Thyroid hormone

 Explain the Site of synthesis, stimulus for secretion, mechanism of action, receptors, intracellular effects, target cells, tissues and biochemical role & hypo/hyper secretion adrenal hormones

 Explain the Site of synthesis, stimulus for secretion, mechanism of action, receptors, intracellular effects, target cells, tissues and biochemical role\ & hypo/hyper secretion Androgens & Estrogens.

 Describe the Site of synthesis, stimulus for secretion, mechanism of action, receptors, intracellular effects, target cells, tissues and biochemical role & hypo/hyper secretion pancreatic hormones

 Explain the Site of synthesis, stimulus for secretion, mechanism of action, receptors, intracellular effects, target cells, tissues and biochemical role& hypo/hyper secretion of parathyroid hormone

Classical definition of a hormone

1. Chemical messengers
2. Synthesized by living cells and
3. Secreted by a specific gland
4. Secreted directly into the blood stream
5. Carried by the blood
6. Acts on a specific target
7. At a site distant from site of secretion
8. Secreted in minute quantities
9. Acts via specific receptors to exert specific actions

New Definition of a Hormone

"Any substance released by a cell and which acts on another cell, near or far, regardless of the means of conveyance"

Similarities and Dissimilarities of Hormone and Enzyme

The hormones have several characteristics in common like they

act as body catalysts , required only in small quantities, not used up during the reaction.

They differ from enzymes in these ways:

• Produced in an organ other than that in which they ultimately perform their action.

• They are secreted in blood prior to use.

• The circulating levels of hormones can give some indication of endocrine gland activity and target organ exposure. Because of the small amounts of the hormones required, blood levels of the hormones are extremely low. In many cases it is ng/μg or mIU, etc.

• Structurally they are not always proteins. Few hormones

are protein in nature, few are small peptides.

Some hormones are derived from amino acids while

some are steroid in nature.

General functions of hormones

• Control Reproductive processes: differentiation, maturation, gametogenesis.
• Growth and development: stimulate or inhibit cellular proliferation
• Homeostasis: Maintenance of healthy internal environment in a continuously changing external and internal environments
• Metabolism: anabolic and catabolic processes, muscular activity, excretion, reabsorption of ions
• Energy production, utilization and storage
• Animal behavior: sexual, aggressive and maternal
• Other hormones (synthesis, secretion, permissive action)

The major hormone secreting glands are:

• Pituitary

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• Thyroid

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• Parathyroid

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• Adrenal

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• Pancreas

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• Ovaries

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• Testes.

Several other glandular tissues are considered to

secrete hormones, viz.:

• JG cells of kidney: May produce the hormone erythropoietin

which regulates erythrocyte maturation, erythropoiesis.

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• Thymus: This produces a hormone that circulates from

this organ to stem cells in lymphoid organ inducing

them to become immunologically competent lymphocytes.

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• Pineal gland: It produces a hormone that antagonises

the secretion or effects of ACTH. It also produces

factors called glomerulotrophins that regulates the

adrenal secretion of aldosterone.

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• GI tract: Few hormones are also produced by certain

specialised cells of GI tract and they are called GI

Hormones

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• CLASSIFICATION ACCORDING TO THE MECHANISM OF ACTION OF HORMONES
• Hormones are classified in several ways, one of which is according to their mechanism of action. The basis of this classification is dependent on their solubility either in lipid or water. They are classified as follows:
• I. Lipophilic (lipid-soluble)
• II. Hydrophilic (water-soluble)
• Lipophilic hormones get transported to their target cell by associating with carrier proteins in the plasma since they cannot dissolve in plasma. This process circumvents the process of solubility while it also prolong the half-life of the hormones. Their lipophilic nature makes it easy for them to diffuse across the plasma membrane of the target cell to bind with their specific receptors in the cytoplasm or nucleus.

Lipophilic hormones, which include steroid hormones, iodothyronines, retinoids and calcitriols are relatively small molecules (300-800 Da). With the exception of the iodothyronines, they are not stored by hormone-forming cells, but are released immediately after being synthesized. Via intracellular receptors, they mainly act on transcription. They diffuse across the cell membrane and bind with their receptor in the cytoplasm (steroids) or in the nucleus (thyroid hormones)

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• Hydrophilic hormones get transported to their target site freely in the plasma since they are soluble in it. On getting to the target cell, they are unable to pass through the plasma membrane because of their hydrophilic nature which contrasts with the hydrophobic nature of the membrane interior. As a result of this, there is need for a second messenger that will relate the message carried by the hormone to the cell.

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• The second messenger system is of many types:
•  Adenylyl cyclase-cAMP second messenger system
•  Phospholipid second messenger system
•  Calcium-calmodulin second messenger system.
• All these systems are utilized by hydrophilic hormones. It should be noted that a specific hormone can utilize more than just one of these systems in its action.

Hormonal Receptors

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Hormone Receptors have Two Mandatory characteristics

1. The Receptor Must Identify A Unique Binding Site Within The Hormone In Order To Distinguish The Hormone From All Other molecules

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2. The receptor must be able to Transmit The Signal Caused By Binding With The Hormone Into A Cellular response

Classification of Hormonal receptors

1. Intracellular Receptors:

• These 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 stimulates or sometimes inhibits transcription from those genes.

• Intracellular Receptors are composed of a single polypeptide chain that has three distinct domains : The amino - terminus, DNA binding domain, and the carboxy – terminus or ligand – binding domain.

• Steroid and thyroid hormones act on these

receptors.

Classification of Hormonal receptors

2. Cell surface receptors:

• These are located on the plasma membrane of target cells.

• Binding of hormone to receptor initiates a series of events which leads to generation of second messengers within the cell.

• The second messengers then trigger a series of molecular interactions that alter the physiologic state of the cell (signal transduction).

• Cell surface receptors are integral membrane proteins; they are composed of three parts: Extracellular domains, Transmembrane domains and Cytoplasmic or intracellular domains.

• Protein and peptide hormones and catecholamines act on these receptors.

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Second Messenger Systems

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•Water soluble hormones act through binding to cell surface receptors and activation of one of the second messenger systems.

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• Multiple hormones utilize the same second messenger system. Also a single hormone can utilize more than one system.

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•The small signal generated by hormone binding to its receptor is amplified within the cell into a cascade of actions that changes the cell's physiologic state.

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•Examples are: Cyclic AMP, Protein kinase activity, Cyclic GMP, and Calcium and/or Phosphatidylinositide

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The process of increasing the response to a stimulus specifically : increase in a cellular response to a molecular stimulus due to increase in the number of receptors on the cell surface.

Humoral Stimuli

The term “humoral” is derived from the term “humor,” which refers to bodily fluids such as blood. A humoral stimulus refers to the control of hormone release in response to changes in extracellular fluids such as blood or the ion concentration in the blood. For example, a rise in blood glucose levels triggers the pancreatic release of insulin. Insulin causes blood glucose levels to drop, which signals the pancreas to stop producing insulin in a negative feedback loop.

Hormonal Stimuli

Hormonal stimuli refers to the release of a hormone in response to another hormone. A number of endocrine glands release hormones when stimulated by hormones released by other endocrine glands. For example, the hypothalamus produces hormones that stimulate the anterior portion of the pituitary gland. The anterior pituitary in turn releases hormones that regulate hormone production by other endocrine glands. The anterior pituitary releases the thyroid-stimulating hormone, which then stimulates the thyroid gland to produce the hormones T₃ and T₄. As blood concentrations of T₃ and T₄ rise, they inhibit both the pituitary and the hypothalamus in a negative feedback loop.

Neural Stimuli

In some cases, the nervous system directly stimulates endocrine glands to release hormones, which is referred to as neural stimuli. Recall that in a short-term stress response, the hormones epinephrine and norepinephrine are important for providing the bursts of energy required for the body to respond. Here, neuronal signaling from the sympathetic nervous system directly stimulates the adrenal medulla to release the hormones epinephrine and norepinephrine in response to stress.

General Mechanisms of Hormone Action • Hormone binds to cell surface or receptor inside target cell

• Cell may then – synthesize new molecules

– change permeability of membrane

– alter rates of reactions

• Each target cell responds to hormone differently

– liver cells---insulin stimulates glycogen synthesis – adipose---insulin stimulates triglyceride synthesis

The endocrine system is a collection of glands that secrete chemical messages we call hormones.

• These signals are passed through the blood to arrive at a target organ, which has cells possessing the appropriate receptor.

What Does the Endocrine System Do? Once a hormone is secreted from the endocrine gland that produced it; It travels, through the bloodstream to the cells designed to receive its message. These cells are called target cells.

 Along the way to the target cells, special proteins bind to some of the hormones. These proteins act as carriers that control the amount of hormone that is available for the cells to use.

 The target cells have receptors that latch onto only specific hormones, and each hormone has its own receptor, so that each hormone will communicate only with specific target cells that have receptors for that hormone.

 When the hormone reaches its target cell, it locks onto the cell's specific receptors and these hormone-receptor combinations transmit chemical instructions to the inner workings of the cell.

 When hormone levels reach a certain normal amount, the endocrine system helps the body to keep that level of hormone in the blood.

 An example of this process is : Parathyroid hormone. Parathyroid hormone increases the level of calcium in the blood. When the blood calcium level rises, the parathyroid glands sense the change and reduce their secretion of parathyroid hormone. This turnoff process is called a negative feedback system.

Mechanism of Hormone Action

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• General Mechanisms of Hormone Action

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• Hormone binds to cell surface or receptor inside target cell

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• Cell may then – synthesize new molecules – change permeability of membrane – alter rates of reactions

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• Each target cell responds to hormone differently – liver cells---insulin stimulates glycogen synthesis – adipose---insulin stimulates triglyceride synthesis

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Control of Hormone Secretion

• Regulated by signals from nervous system, chemical changes in the blood or by other hormones

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• Negative feedback control (most common) – decrease/increase in blood level is reversed

• Positive feedback control – the change produced by the hormone causes more hormone to be released

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• Disorders involve either hyposecretion or hypersecretion of a hormone

Endocrine-related Problems

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 Overproduction of a hormone

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 Underproduction of a hormone

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 Nonfunctional receptors that cause target cells to become insensitive to hormones

The seven principal glands and their hormones

Some hormones secreted from tissues

Gastrointestinal (luminal) Hormones

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Classification according to Chemical classes of hormones

• Protein and polypeptides
• Steroids
• Tyrosine-derived
• Eicosanoids
• Vitamins
• Miscellaneous group: Gaseous molecules (NO, CO), metabolic substances (glucose, lactic acid), chalones, lumones, pheromones

Polypeptides

Steroids

Amino acid derivatives

Insulin

glucagon

somatotropin

FSH

LH

vasopressin

Oxytocin

thyrotropin

ACTH

Estrogen

testosterone

cortisol

Aldosterone

corticosterone

Progesterone

Epinephrine

norepinephrine

dopamine

Thyroxine, T3 and T4

Melatonin

Serotonin

Polypeptide Hormone

Peptides

Cys

Try

Ile

Gln

Asn

Cys

Pro

Leu

Gly

S

S

Pregnenolone:

Progesterone

Aldosterone

Testosterone

Estradiol

Cortisol

Steroid Hormones

Tyrosine-derived hormones

HYPOTHALAMUS

• The hypothalamus contains neurons that control releases from the anterior pituitary.

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• Seven hypothalamic hormones are released into a portal system connecting the hypothalamus and pituitary, and cause targets in the pituitary to release eight hormones

Insulin

• Insulin is a protein hormone, secreted by β-cells of islets of Langerhans of pancreas. It plays an important role in
• metabolism causing increased carbohydrate metabolism,
• glycogenesis and glycogen storage; FA synthesis/TG
• storage and amino acid uptake/protein synthesis.
• Thus insulin is an important anabolic hormone which act on
• variety of tissues. Major target tissues of insulin are the
• muscles, liver, adipose tissue and heart.
• RB cells, GI tract epithelial cells and renal tubular
• epithelial cells are rather generally unresponsive to insulin.

Eicosanoids

Vitamin D

Synthesis of Protein Hormones

1. Transcription of a gene in the nucleus 🡪 mRNA
2. Translation of mRNA by ribosomes on RER 🡪 pre-prohormone in ER
3. Post-translational modification:
1. Pre-prohormone in ER 🡪 prohormone by losing signal peptide sequence
2. Prohormone migrates to Golgi complex 🡪 incorporated into a vesicle
3. prohormone in vesicle + protease enzymes 🡪 hormone

Synthesis of Steroid Hormones

Activation of specific enzymes: 🡪 acetate 🡪 cholesterol 🡪 pregnenolone 🡪 to the diff hormones.

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The SER, mitochondria and cytoplasm contain the enzymes required for the transformations

Steroids of the Adrenal Cortex

Synthesis of the male sex hormones

Synthesis of Vitamin D

They are sterol hormones and have much in common to other steroid hormones

Its precursor, cholecalciferol, is obtained from diet or synthesized by the ultraviolet irradiation of provitamin D in the skin

Cholecalciferol, by a series of enzymes in the liver and kidney, is hydroxylated to the active hormone, calcitriol

Vitamin D

Synthesis of Tyrosine-derived hormones�

I.Thyroid hormones: is a unique process

• Thyroid cells concentrate iodine
• Thyroid cells synthesize a glycoprotein, thyroglobulin
• Iodine is oxidized
• Iodine is oxidized, then coupled to iodotyrosine within thyroglobulin (organification process) by thyroid peroxidase enzyme
• Reuptake of thyroglobulin by endocytosis
• Proteolytic digestion by lysosomal enzymes (hydrolyases) 🡪 T­3+ T4 (iodothyronines) and MIT+ DIT (iodotyrosines)

II. Catecholamines: They are synthesized from tyrosine by a number of enzymes in the cytoplasm and chromaffin granules

Synthesis of Catecholamines

Thyroid Hormone Synthesis

Synthesis of Eicosanoids

From fatty acid (arachidonic acid) released from phospholipids in cell membrane by means of a number of enzymes.

Pathways of Eicosanoids Synthesis

Storage

• Protein hormones: In secretory granules within the cytoplasm
• Steroid hormones: Are not stored. The hormones precursor, cholesterol esters, is the storage form
• Tyrosine-derived hormones
• Thyroid hormones: in the thyroglobulin
• Catecholamines: in secretory chromaffin granules in the cytoplasm + ATP + chromogranin
• Eicosanoids: Are not stored.
• Vitamin D: Cholecalciferol is stored in adipose tissue. Liver stores its metabolite

Release

• Protein hormones: By exocytosis
• Steroid hormones: by diffusion immediately upon synthesis
• Vitamin D: by diffusion immediately upon synthesis
• Tyrosine – derived hormones:
• Thyroid hormones: fusion of lysosomes with colloid droplets, the hormones are released by exocytosis from the basement membrane
• Catecholamines: stimulus-secretion coupling requiring Ca, vesicular exocytosis
• Eicosanoids: by diffusion

Transport

• Protein hormones: mostly unbound, free in the blood
• Steroid hormones: Bound to a plasma protein (high- affinity binding to globulin and low-affinity to albumin). Cortisol to transcortin, sex hormones to sex-hormone-binding globulin (SHBG).
• Vitamin D: Bound to a globulin (transcalciferin)
• Tyrosine-derived hormone:
• Thyroid hormones: Mostly bound to thyronine-binding globulin (TBG) or prealbumin (transthyretin)
• Catecholamines: Bound to albumin.
• Eicosanoids: Are not transported. They act as autocrine or paracrine hormones

Purpose of binding of hormones to proteins:

1. The hormone is protected from the inactivating systems present in the blood.
2. The hormone is maintained in a “stored” circulating form to be readily available to its target tissues.
3. Ensure ubiquitous distribution of the water-insoluble hormones.

Relation between bound and unbound hormone

A dynamic equilibrium exists between the concentrations of free (unbound) hormone, plasma protein, and the hormone-protein complex:

[H]x[P] [HP]

[H]x[P]

Where K is the dissociation constant

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[HP]

K

Peripheral Conversion

Some biologically active hormones are converted to other equally active hormones in peripheral tissues such as liver, breast adipose tissue, brain etc

Example:

Testosterone dihydrotestosterone

Thyroxine (T₄) Triiodothyronine (T₃)

• Single hormone, different effects.

Example:

Estradiol acts on ovarian follicles to promote granulosa cell differentiation, on uterus to stimulate its growth and maintain the cyclic change of uterine mucosa, on mammary gland to stimulate ductal growth, on bone to promote linear growth and closure of epiphyseal plates, on HPA to regulate secretion of gonadotropins and prolactin, on metabolic processes to affect adipose tissue distribution, volume of ECF, etc

• Several hormones, single function.

Example:

Release of fatty acids (lipolysis) from adipose tissue stimulated by catecholamines, glucagon, secretin, prolactin and B-lipotropin

Permissive effect of Hormones

• It is the effect that some hormones exhibit, these hormones have little effect by themselves, but when they are present they affect other hormones to become fully manifested.
• Example:

Development of mammary gland, under infleunce of prolactin, estradiol & progesterone and the permissive influence of glucocorticoids, thyroid hormones and insulin

Rule: All hormones interact with target cells by

first binding to specific receptors located either on

the plasma membrane or as a cytosolic protein

Rule: The receptor for hormones must be linked

to a component that is able to respond to the

binding of hormone with its receptor

Rule: Substances that fool the responder into

thinking a hormone has bound are call agonists

Rule: Substances that prevent the binding of the

natural hormone and do not elicit a response from

the receptor are called antagonists

Cyclic AMP

Cyclic GMP

Ca²⁺

Diacylgycerol

Protein

substrates

PK-A

PK-G

Calmodulin

PK-C

Protein Ser/Thr

kinases

Protein substrates

Protein substrates

Protein substrates

Multifunctional

kinases

Other

phospholipases

1

2

3

4

5

Tyrosine

kinase

IP₃

G

G

G

G

Insulin

Glucagon

T-cell

Activation

Nitric

oxide

G protein

End result is

phosphorylation of

one or more proteins

Hypothalamus

Anterior pituitary

Posterior pituitary

Thyrotropin

ACTH

Somatotropin

LH

FSH

Prolactin

Vasopressin

Oxytocin

Thyroid

Adrenal

Cortex

Adrenal

Medulla

Pancreas

Ovary

Testis

Muscles

liver

Tissues

Liver,

muscles

Estradiol

Testosterone

Insulin,

glucagon,

somatostatin

T3

Cortisol

aldosterone

Mammary

glands

Reproductive

organs

Epinephrine

Releasing

hormones

Nervous

Feedback Loops

Rule: Hormones elicit their own shut off mechanism

Hypothalamus

Corticotropin

releasing factor

Anterior

Pituitary

β-Corticotropin

Cortisol

Adrenal

Cortex

Rule: All peptide hormones are synthesized as

inactive “pre-pro” precursors

Rule: A signal peptide must be cleaved off to

activate the mature form of the hormone

Signal Transduction

Definition: The series events and components that

take part in transmitting a hormonal signal to a

the interior of the cell

Membrane or cytosolic Receptor

Signal Initiator

Target molecule

Signal mediator

Cyclic AMP System

Receptor

Adenylate cyclase

G-protein

Protein kinases

c-AMP

Stimulate (Gs) and

inhibit (Gi)

G-Proteins

So-named because they bind GTP, displacing GDP

Work with many receptors

Both Stimulate and inhibit hormone signals

A family of membrane proteins that exist in an inactive

(GDP) and an active (GTP) state

GTP is a time-bomb slowly ticking

When GTP is hydrolyzed to GDP, stimulation is stopped

Resting

Active

Inactive

Resting

ATP

cAMP

PO₄

GTP

GDP

α

GDP

GTP

α

4 ATP

4 cAMP

Cell response

AT

Protein

kinase

Inactive

protein

Active

protein

hormone

Adenylate cyclase

Signaling System

AC

R_S

Inhibitor

R_i

γ

β

Tyrosine Kinase Receptors

Ligand

N

C

Cross

phosphorylation

Cell membrane (lipid bilayer)

Growth hormone

Extracellular domain

of Growth Hormone Receptor

Intracellular

Extracellular

Growth Hormone Receptor

Binding to receptor forces dimerization of receptor subunits for cross phosphorylation

-OPO₃⁼

⁼O₃PO-

Tyrosines

Cell Signaling

via RTK and

Ras

Kinases

Challenge to Students

• Many of the proteins that you just saw are coded by genes referred to as “oncogenes”, meaning they are capable of transforming a normal cell into a cancer cell. Src, Ras, ErbB, affect cell growth and differentiation.
• The viral forms of these genes lack regulation, and the mammalian form (proto-oncogenes) are subject to mutation.
• If you want to learn what causes a normal cell to become a cancer cell (malignant transformation), this is a good place to start.

What is Behind the Biochemistry of Cancer?

1. An alteration of genes/proteins involved in:

a. Cell proliferation

b. Apoptosis (programmed cell death)

c. Differentiation

2. Acquisition of a phenotype that allows cells to:

a. Proliferate without limits

b. Evade apoptosis

c. Generate its own mitogenic signals

d. Ignore growth inhibitory signals

e. Acquire vasculature (angiogenesis) – solid tumors

f. Invade and colonize (metastasize) other tissue

Genes Mutated

1. ras protein (25% of cancers)

2. p53 tumor suppressor (50% of cancers)

a. controls DNA repair

b. controls apoptosis

3. Tyrosine kinase receptor (HER2/neu)

a. controls ras (overexpression)

We Know

1. Biochemical pathways from ras to p53

2. Role of p53 in apoptosis and DNA repair

We Don’t Know

1. Molecular circuitry for enhancing secretion of angiogenic factors from cancer cells

2. The regulation of elements controlling the migration and extravastion capabilities of cancer cells

Take Home

• Most hormones never penetrate cells
• All hormones have receptors
• Internal responses are initiated by the receptor
• Receptors work with G proteins
• G proteins stimulate protein kinases
• Protein kinases comprise a cell signaling cascade
• G proteins turn off when GTP is hydrolyzed to GDP, canceling the hormone action

Take Home (Part 2)

• Some receptors are protein tyrosine kinases
• Kinase activity is initiated by dimerization
• Kinase autophosphorylate receptors
• Phosphotyrosines bind to SH-2 domains
• Activation starts a kinase cascade
• Phosphorylated proteins enter nucleus
• DNA transcription turns on specific genes