Molecular and Cell Biology of the Endocrine System by Marc Imhotep Cray, M.D.

Molecular and Cell Biology of the Endocrine System Hormone-Receptor Interactions and Signal Transduction Mechanisms

Marc Imhotep Cray, M.D.

Learning Objectives 1. Describe the four classes of chemical messengers and how they signal cells. 2. Understand the similarities and differences between the autonomic nervous system and endocrine system in maintaining homeostasis and integrated cellular communication. 3. Describe the common characteristics of all hormones. 4. Describe the four major families of receptors. 5. Describe hormone-receptor interactions and signal transduction mechanisms. Marc Imhotep Cray, MD

Learning Objectives cont. 6. Describe the organization and functional anatomy of the endocrine system. 7. Describe the chemical nature and classification of hormones. 8. Explain the molecular-cellular mechanism of action of peptide hormones and how they exert their effects on target cells. 9. Explain the molecular-cellular mechanisms of action of steroid and thyroid hormones and how they exert their effects on target cells. 10. Define binding protein, bound hormone, and free hormone and discuss the effect of binding proteins on circulating hormone levels Marc Imhotep Cray, MD

Organization of Endocrine System ď ą Hormones are secreted into blood by endocrine organs throughout body, affecting physiological function at various target sites Illustration Ledger: ACTH, adrenocorticotropic hormone; ADH, antidiuretic hormone; CCK, cholecystokinin; CRH, corticotropin-releasing hormone; FSH, folliclestimulating hormone; GH, growth hormone; GHRH, growth hormoneâ&#x20AC;&#x201C;releasing hormone; GIP, gastric inhibitory peptide; GLP-1, glucagon-like peptide-1; GnRH, gonadotropin releasing hormone; LH, luteinizing hormone; MSH, melanocyte stimulating hormone; PRL, prolactin; TRH, thyrotropin-releasing hormone; TSH, thyroid-stimulating hormone Marc Imhotep Cray, MD

Mulroney SE & Myers AK. Netter's Essential Physiology 2nd Ed. Philadelphia: Elsevier, 2016.

Overview of Endocrine System  Endocrine system uses hormones to transfer information between different tissues  It is a finely regulated machine that uses feedback loops and sensors to ensure constant homeostasis within body  It plays some form of regulatory role in almost all physiologic processes  It has effects on development, growth, metabolism and reproduction and works with almost every organ system, including the nervous and immune system  Control is mediated by a combination of neural and endocrine systems located in hypothalamus and pituitary gland (“The Master Gland”)

 In contrast to neurotransmitters--which work in synapse between neuron endplate and receptors they act on-- hormones are secreted into circulation and can work on tissues far away from source of origin Marc Imhotep Cray, MD

Nervous system vs Endocrine system in homeostasis  Two major regulatory systems make important contributions to homeostasis:  the nervous system and  the endocrine system  Common properties:     

maintain homeostasis extensive use of negative feedback high-level integration in brain ability to influence processes in distant regions of body both systems use chemicals for transmission of information

Nervous

versus

Endocrine

Wired

Wireless

Neurotransmitters

Hormones

Short Distance

Long Distance

Closeness

Receptor Specificity

Rapid Onset

Delayed Onset

Short Duration

Prolonged Duration

Rapid Response

Regulation 6

NS vs ES in homeostasis: Differences Nervous system is hard-wired with signalling molecule delivered precisely to point where it is needed  This means that only a few different signalling molecules are required as they do not affect any cells except at their site of delivery

In nervous system specificity is conferred by hard-wiring: neurotransmitter is delivered directly and specifically to cell which has receptor and responds to transmitter Endocrine system is not hard-wired All cells in body are exposed to hormones so there is a wide range of signalling molecules needed In endocrine system, where hormones are delivered to all cells, specificity comes only from expression of the receptor for hormone.  Only cells with receptors for a hormone can respond to that particular hormonal signal Marc Imhotep Cray, MD

Overview of Endocrine System Classes of Hormones ď ą Hormones can be divided into five major classes: 1. 2. 3. 4. 5.

amino acid derivatives such as dopamine, catecholamine, and thyroid hormone small neuropeptides such as gonadotropin-releasing hormone (GnRH), thyrotropinreleasing hormone (TRH), somatostatin, and vasopressin large proteins such as insulin, luteinizing hormone (LH), and PTH produced by classic endocrine glands steroid hormones such as cortisol, estrogen, progesterone and testosterone that are synthesized from cholesterol-based precursors and vitamin derivatives such as retinoids (vitamin A) and vitamin D

As a rule ď ą amino acid derivatives and peptide hormones are water-soluble and interact with cell-surface membrane receptors ď ą Steroids, thyroid hormones, vitamin D, and retinoids are lipid-soluble and interact with intracellular--cytoplasmic & nuclear--receptors Marc Imhotep Cray, MD

Chemical Classes and Hormones Amino acid derivative

Proteins

Epinephrine (adrenaline) Thyroid hormones (T3, T4)

Insulin Insulin-like growth actors (IGFs ) Growth hormone (GH) Prolactin (PRL) Placental lactogen (PL) Parathyroid hormone (PTH)

Peptides Thyrotropin-releasing hormone (TRH) Gonadotropin-releasing hormone (GnRH) Vasopressin Oxytocin (OT) Vasoactive intes tinal peptide (VIP) Glucagon Adrenocorticotropic hormone (ACTH) Somatostatin

Glycoproteins Thyroid-stimulating hormone (TSH) Follicle-stimulating hormone (FSH) Luteinizing hormone (LH) Chorionic gonadotropin (CG) Marc Imhotep Cray, MD

Steroid Estrogens (e.g. estradiol) Androgens (e.g. testosterone) Progesterone Cortisol Aldosterone

Vitamin derivatives vitamin A vitamin D 9

Overview of Endocrine System cont. Hypothalamus  One of most important function of hypothalamus is to link the nervous system to the endocrine system via pituitary gland (hypophysis)  Hypothalamus is also responsible for certain metabolic processes and other activities of autonomic nervous system (See Lect. 2)  Hypothalamus synthesizes and secretes neurohormones, often called “hypothalamic-releasing hormones” (next slide)which in turn stimulate or inhibit secretion of pituitary hormones Marc Imhotep Cray, MD

Overview of Endocrine System cont. Chemical nature of hypothalamic factors Hypothalamus, which is a part of CNS and not a gland, produces many releasing and inhibitory hormones which control secretion of anterior pituitary hormones Hypothalamic hormone/factor

Chemical nature

1. Thyrotropin releasing hormone (TRH) 2. Corticotropin releasing hormone (CRH) 3. Gonadotropin releasing hormone (GnRH), (LH-RH/FSH-RH) 4. Prolactin release inhibitory hormone (PRIH) 5. Growth hormone releasing hormone (GHRH) 6. Somatostatin (Growth hormone release inhibitory hormone)

Tripeptide Peptide (41 AAs) Decapeptide

Marc Imhotep Cray, MD

Dopamine Peptide (40, 44 AAs) Peptide (14 AA)

Overview of Endocrine System cont. Functional classification of hormones Tropic hormone vs Non-tropic hormone vs Trophic hormone  Tropic hormones are hormones that have other endocrine glands as their target (endocrine target tissues)  Most tropic hormones are produced and secreted by anterior pituitary  For example: Hypothalamus secretes tropic hormones that target anterior pituitary, and thyroid gland secretes thyroxine, which targets hypothalamus and therefore can be considered a tropic hormone (Other examples: TSH, FSH, LH, ACTH)

 Non-tropic hormones are hormones that directly stimulate target cells to induce effects (nonendocrine target tissues)  Non-tropic hormones are those that act directly on targeted tissues or cells, and not on other endocrine gland to stimulate release of other hormones (Ex. GH, PTH, prolactin, oxytocin, vasopressin, aldosterone and MSH)

 Trophic hormones are hormones that have a growth effect, hyperplasia or on tissue they are stimulating. (Ex. TSH, GH, ACTH) Marc hypertrophy, Imhotep Cray, MD

Schematic Overview of Endocrine System

Marc Imhotep Cray, MD

Costanzo LS. BRS Physiology. 5th ed. (Board review series). New York: Elsevier; 2009.

Overview of Endocrine System cont. Overall Function  A hormone is a substance secreted into bloodstream by one tissue but has actions at remote tissues  Hormones maintain homeostasis by regulating processes such as growth & development, metabolism, and reproduction  Hormones: maintain homeostasis through feedback loops  Hormones: act slowly relative to nervous system Nervous System chemical mediator (neurotransmitter) Endocrine system chemical mediator (hormone) Basic physiologic, biochemical and pharmacologic principles are same. Marc Imhotep Cray, MD

Overview of Endocrine System cont. Routes by which chemical signals are delivered to cells 1. Autocrine chemical messengers stimulates the cell that originally secreted it (e.g. WBCs) 2. Paracrine chemical messengers act locally on nearby cells (e.g. cytokines) 3. Neurotransmitters secreted by neurons that activate an adjacent cellď&#x192; another neuron, a muscle cell, or a glandular cell (e.g. acetylcholine) 4. Endocrine chemical messengers are hormones secreted into bloodstream by certain glands and cellsâ&#x20AC;&#x201C; and act at a distant site (e.g. insulin) Marc Imhotep Cray, MD

Modified from: Brown TA, Brown D. USMLE Step 1 Secrets, 3rd Ed. Saunders, 2013

Overview of Endocrine System cont.  Classically, hormones are released into bloodstream and act on tissues distant from site of hormone production an endocrine effect however,  Some hormones act locally within tissue where they are produced called “local hormones” or paracrine effects  Some hormones have both local and systemic effects  act in a paracrine and endocrine manner  Example is testosterone, has local actions in testes and hormonal effects on muscle

 Some hormones, particularly growth factors, exert their actions on cells which secrete them called autocrine effects Marc Imhotep Cray, MD

Overview of Endocrine System cont. Hormone Action  True hormones (endocrine secretions) are released by “ductless glands” and are carried by bloodstream to their sites of action  Hormones are part of a larger group of substances that includes autocrine, paracrine, and neuroendocrine secretions

Marc Imhotep Cray, MD

Mulroney SE & Myers AK. Netter's Essential Physiology 2nd Ed. Philadelphia: Elsevier, 2016.

Overview of Endocrine System cont. Sites and mechanisms of hormone action  Body releases a wide range of endogenous substances, including: neurotransmitters from neuronal cells (e.g. acetylcholine), hormones (e.g. insulin) or cytokines (e.g. interferon), that alter function of target cells  Hormones act on their specific receptors located on or within their target cells  Receptor activation by hormones is translated into response in a variety of ways 1. At cell membrane receptors 2. At cytoplasmic receptors 3. At nuclear receptor Marc Imhotep Cray, MD

Overview of Endocrine System cont. Sites and mechanisms of hormone action (2)  Binding of a hormone to its receptor initiates intracellular events that direct hormone’s action  Ultimately, all hormones produce their effects by altering intracellular protein activity  mechanism by which this occurs depends on location of hormone receptor  Receptors are typically located on cell surface or in cell nucleus  As a result, most hormones carry out their effects by means of two general mechanisms: 1. Signal transduction and second messenger systems Marc Imhotep2. Cray, Gene MD activation

Overview of Endocrine System cont. Receptors A receptor is a cell macromolecule either on surface cell or within cytoplasm or nucleus of a cell that is recognized by endogenous or exogenous substances (ligands) with specificity Receptors account for a majority of chemical signalling that occurs within body and are fundamental to ability of chemical messengers to alter function of living cells There are four major families of receptors:  (LGICs) ligand-gated ion channels (e.g. nicotinic ion channel)  (GPCRs) G-protein-coupled receptors (e.g. β-adrenoceptor)  (RTKs) tyrosine kinase receptors (e.g. insulin receptor)  (NHRs) Marc Imhotep Cray, MD intracellular receptors (e.g. glucocorticosteroid receptor)

Receptors

Raff RB, Rawls SM, Beyzarov EP. Netter's Illustrated Pharmacology, Updated Edition. Philadelphia: Sanders, 2014 Marc Imhotep Cray, MD

Functional anatomy of endocrine and metabolic systems  Endocrine and metabolic systems regulate seven major bodily functions (detail slides follow)  For each target tissue effect, endocrine glands release hormones in response to regulating factors, which include  physiologic (e.g. sleep and stress),  biochemical (e.g. glucose and Ca2+) and  hormonal (e.g. hypothalamic and enteric hormones) stimuli

Marc Imhotep Cray, MD

Functional anatomy of endocrine and metabolic systems (2)  Endocrine and metabolic system consists of a variety of organs (glands) that secrete substances (hormones) into blood which affect function of target tissues elsewhere in body  Glands include hypothalamus, pituitary, thyroid, adrenals, gonads, pancreatic islets of Langerhans and parathyroids  Endocrine system regulates seven major physiologic functions: 1) Availability of metabolic energy (fuel), 2) Metabolic rate, 3) Circulatory volume, 4) Somatic growth, 5) Calcium homeostasis , 6) Reproductive function 7) Adaptation to stress

 A cardinal feature of drug therapy of endocrine diseases is the interaction between exogenously administered drugs and “endogenous biochemistry and physiology” of hormones Marc Imhotep Cray, MD

Functional anatomy of endocrine and metabolic systems (3) Endocrine function 1. Availability of metabolic energy (fuel) Regulatory factors Serum glucose, amino acids, enteric hormones (somatostatin, cholecystokinin, gastrin, secretin), vagal reflex, sympathetic nervous system Endocrine organ / hormone Pancreatic islets of Langerhans/insulin, glucagon Target tissues All tissues, especially liver, skeletal muscle, adipose tissue, indirect Marc Imhotep Cray, MD effects on brain and red blood cells

Functional anatomy of endocrine and metabolic systems (4) Endocrine function 2. Metabolic rate Regulatory factors Hypothalamic thyrotropin-releasing hormone (TRH), pituitary thyrotropin (TSH) Endocrine organ / hormone Thyroid gland/triiodothyronine (T3) Target tissues All tissues Marc Imhotep Cray, MD

Functional anatomy of endocrine and metabolic systems (5) Endocrine function 3. Circulatory volume Regulatory factors Renin, angiotensin II, hypothalamic osmoreceptors Endocrine organ / hormone Adrenals /aldosterone, Pituitary/vasopressin Target tissues Kidney, blood vessels, CNS

Marc Imhotep Cray, MD

Functional anatomy of endocrine and metabolic systems (6) Endocrine function 4. Somatic growth Regulatory factors Hypothalamic growth hormone-releasing hormone (GHRH), somatostatin, sleep, exercise, stress, hypoglycemia Endocrine organ / hormone Pituitary/growth hormone, Liver/insulin-like growth factors (IGFs) Target tissues All tissues Marc Imhotep Cray, MD

Functional anatomy of endocrine and metabolic systems (7) Endocrine function 5. Calcium homeostasis Regulatory factors Serum Ca+ + and Mg++ concentration Endocrine organ / hormone Parathyroid glands/parathyroid hormone, calcitonin, vitamin D Target tissues Kidney, intestines, bone Marc Imhotep Cray, MD

Functional anatomy of endocrine and metabolic systems (8) Endocrine function 6. Reproductive function Regulatory factors Hypothalamic gonadotropin- releasing hormone (GnRH), pituitary, follicle stimulating hormone (FSH) and luteinizing hormone (LH), inhibins Endocrine organ / hormone Gonads / sex steroids, Adrenals/ androgens Target tissues Reproductive organs, CNS, various tissues Marc Imhotep Cray, MD

Functional anatomy of endocrine and metabolic systems (9) Endocrine function 7. Adaptation to stress Regulatory factors Hypothalamic corticotropin- releasing hormone (CRH), pituitary adrenocorticotropic hormone (ACTH), hypoglycemia, stress Endocrine organ / hormone Adrenals/glucocorticosteroids, epinephrine Target tissues Many tissues: CNS, liver, skeletal muscle, adipose tissue, lymphocytes, fibroblasts, cardiovascular system Marc Imhotep Cray, MD

Endocrinology Basic Concepts Endocrine system’s target organ are usually located far from site of release of chemical mediator (hormone) of the signal  A hormone is a substance secreted by one tissue or gland that is transported via circulation to a site where it exerts its effects on different tissues (target cells)

Signaling mechanisms which use enzymes, neurotransmitters, hormones, and receptors are similar (aside from distance)  Hence, basic pharmacologic principles of therapy are same

Marc Imhotep Cray, MD

Endocrinology Basic Concepts (2) Feed-forward and feed-back mechanisms Key to understanding endocrine pharmacology are feed-forward and feed-back mechanisms that govern how “releasing” factors in hypothalamus control release of hormones in pituitary (regulatory hormones) that in turn cause release of second-tier hormones that target multiple organs within body For example,  Anterior pituitary hormones are transported to their target organs via systemic circulation In target organs, they stimulate growth, development, and secretion of other hormones, which both  activate specific functions in various organs and  exert negative feedback inhibition of the corresponding hypothalamic and pituitary hormones Marc Imhotep Cray, MD

Endocrinology Basic Concepts (3) Hormone-Receptor Interactions  There are three major biochemical classes of hormones: 1. proteins, peptides 2. modified amino acids (catecholamines and TH) 3. steroids

 All known hormones, and drugs that mimic hormones, act via one of two basic receptor systems:  

membrane-associated receptors and intracellular receptors

1. Membrane-associated receptors: (peptide & protein hormones) Membrane-associated receptors bind hydrophilic hormones (which penetrate plasma membrane poorly), such as  Insulin  Adrenocorticotropic hormone (ACTH), and Marc Imhotep MD  Cray,Epinephrine, outside the cell

Endocrinology Basic Concepts (4) Hormone-Receptor Interactions cont. 1. Membrane-associated receptors transmit signals into cell by a variety of “second messenger” mechanisms, including:  Changes in cyclic adenosine monophosphate (cAMP) or cyclic guanosine monophosphate (cGMP) caused by changes in activity of cyclases  Increased phosphoinositide turnover via increased phospholipase activity  Increased intracellular Ca2+ by action on Ca2+ channels  Increased tyrosine phosphorylation on specific proteins by action of tyrosine kinases (TKR) Marc Imhotep Cray, MD

What are the four primary classes of membranespanning receptors to which peptide hormones bind?  The four primary classes of membrane-spanning receptors to which peptide hormones bind are (Illust. next slide): 1) tyrosine and serine kinase receptors 2) receptor-linked kinases 3) G protein–coupled receptors, and 4) ligand-gated ion channels  “Prototypical” agonists (respectively) for the above receptor types: 1) insulin, growth factors (IGF-1, PDGF, EPO etc. ) 2) growth hormones (GHs), prolactin, cytokines (activate receptors of JAK/STAT superfamily) 3) peptide hormones, neurotransmitters and prostaglandins 4) neurotransmitters, amino acids Marc Imhotep Cray, MD

The four major classes of membrane receptors for peptide hormones and neurotransmitters

Marc Imhotep Cray, MD

Brown TA, Brown D. USMLE Step 1 Secrets, 3rd Ed. Saunders, 2013 36

Hormone 2nd -messenger systems (signal transduction)

McInnis M., Mehta S. Step-up to USMLE Step 1 2015 Edition. Wolters Kluwer, 2015 37 *Mechanism using a G protein, as shown in D (Next slide)

Hormone 2nd messenger systems, GPCRs G-protein mechanism 1. Messenger system before hormone binding 2. After hormone binding, GTP replaces GDP on G protein 3. GTP, attached to α subunit, dissociates from β−γ complex and converts ATP to cAMP 4. Hormone is released from binding site and complex returns to inactive state when GTPase cleaves GTP to GDP Marc Imhotep Cray, MD

McInnis M., Mehta S. Step-up to USMLE Step 1 2015 Edition. Wolters Kluwer, 2015 38

Hormone 2nd messenger system: G-Protein Classes G Protein Class

Action

Examples

ATP, adenosine triphosphate; cAMP, cyclic adenosine monophosphate; DAG, diacylglycerol; IP3, inositol triphosphate; PIP2, phosphatidylinositol 4,5-bisphosphate.

Basic Concepts (4) Hormone Receptors cont. 2. Intracellular receptors: (steroid hormones, TH, retinol & vitamin D) Intracellular receptors bind hydrophobic (lipophilic) hormones (which penetrate plasma membrane easily) such as        

Cortisol Aldosterone Estrogen Progesterone Testosterone T3/T4 Retinol vitamin D

Note: Nuclear Receptors T3, T4, Estrogen, Progesterone, Testosterone Cytoplasmic Receptors Glucocorticoids & Mineralocorticoids

inside cell-either in cytoplasm or nucleus  Intracellular receptors modulate transcription rate of specific Marc Imhoteptarget Cray, MD genes to change levels of cellular proteins

Summary: molecular-cellular mechanisms of hormone action (illustrations next 6 slides) ď ą Hormones act on their specific receptors located on or within their target cells ď ą Receptor activation by hormones is translated into response in a variety of ways 1. At cell membrane receptors (proteins and peptides hormones) (steroids, T4/T3, Vit. D, retinol) 2. At cytoplasmic receptors 3. At nuclear receptor Marc Imhotep Cray, MD

Summary: Sites and mechanisms of hormone action 1. At cell membrane receptors

a. Through alteration of intracellular cAMP concentration alteration of protein kinase A regulation of cell function: Ca2+ acting as third messenger in some situations 

Epinephrine, Glucagon, TSH, FSH, LH , PTH, Calcitonin, ACTH, some hypothalamic releasing hormones, Vasopressin (V2) Raff RB, Rawls SM, Beyzarov EP. Netter's Illustrated Pharmacology, Updated Edition. Saunders, 2014

Marc Imhotep Cray, MD

Summary: Sites and mechanisms of hormone action 1. At cell membrane receptors b. Through IP3 DAG generation: release of intracellular Ca2+ and protein kinase C activation ď&#x201A;§ Vasopressin (V1) ,Oxytocin Raff RB, Rawls SM, Beyzarov EP. Netter's Illustrated Pharmacology, Updated Edition. Saunders, 2014

Marc Imhotep Cray, MD

Summary: Sites and mechanisms of hormone action 1. At cell membrane receptors c. Direct transmembrane activation of tyrosine protein kinase phosphorylation cascade regulation of various enzymes  Insulin, Growth hormone , Prolactin Raff RB, Rawls SM, Beyzarov EP. Netter's Illustrated Pharmacology, Updated Edition. Saunders, 2014

Marc Imhotep Cray, MD

Summary: Sites and mechanisms of hormone action 2. At cytoplasmic receptors Penetrating cell membrane, hormone combines with a cytoplasmic receptor  exposes its DNA binding domain migrates to nucleus and binds to specific genes DNA mediated mRNA synthesis synthesis of functional proteins  Steroidal hormones: Glucocorticoids, Mineralocorticoid, Androgens, Estrogens , Progestins  Calcitriol (also called 1,25dihydroxycholecalciferol or 1,25dihydroxyvitamin D) Marc Imhotep Cray, MD

Brunton LL, Chabner BA , Knollmann BC (Eds.). Goodman and Gilman’s The Pharmacological Basis of Therapeutics. 12th ed. McGraw-Hill, 2011

Summary: Sites and mechanisms of hormone action 3. At nuclear receptor

Marc Imhotep Cray, MD

Raff RB, Rawls SM, Beyzarov EP. Netter's Illustrated Pharmacology, Updated Edition. Saunders, 2014

Hormone penetrates nucleus, combines with its receptor alters DNA- RNA mediated protein synthesis  Thyroid hormones: Triiodothyronine, Thyroxine  Estrogen, testosterone, glucocorticoids, vitamin D, aldosterone, progesterone

Receptor Families and Signaling Pathways: Class of Receptors Used by Various Hormones Receptor Class

Hormones and Related Substances

cAMP

LH, FSH, ACTH, TSH, PTH, hCG, CRH, glucagon, ADH (V2)

cGMP

NO, ANP

IP3

GnRH, GHRH, oxytocin, TRH, ADH (V1)

Steroid receptor (intracellular)

Estrogen, testosterone, glucocorticoids, vitamin D, aldosterone, progesterone, T3/T4

Tyrosine kinase

Insulin, growth factors (e.g., IGF, PDGF), GH, prolactin

Redrawn after: Brown TA, Brown D. USMLE Step 1 Secrets, 3rd Ed. Saunders, 2013

Ledger: ACTH, adrenocorticotropic hormone; ANP, atrial natriuretic peptide; cAMP, cyclic adenosine monophosphate; cGMP, cyclic guanosine monophosphate; CRH, corticotropin-releasing hormone; FSH, follicle-stimulating hormone; GH, growth hormone; GHRH, growth hormoneâ&#x20AC;&#x201C;releasing hormone; GnRH, gonadotropin-releasing hormone; hCG, human chorionic gonadotropin; IGF, insulin-like growth factor; IP3, inositol triphosphate; NO, nitric oxide; PDGF, platelet-derived growth factor; PTH, parathyroid hormone; T3, triiodothyronine; T4, thyroxine; TRH, thyrotropin-releasing hormone; TSH, thyroid stimulating hormone. Marc Imhotep Cray, MD

Hypothalamic-pituitary signaling pathways Response of an anterior pituitary gland cell to a hypothalamic factor (neurohormone) is initiated when hypothalamic factor (a peptide) binds to specific G protein-coupled receptors located on plasma membrane of appropriate anterior pituitary cell type  Most of these receptors alter levels of intracellular cAMP or IP3 and calcium  Molecular details of receptor signaling provide a biochemical basis for understanding hypothalamic factor action (example in next slide) Marc Imhotep Cray, MD

Hypothalamic-pituitary signaling pathways (2) For example:  Growth Hormone-Releasing Hormone (GHRH) binding to its receptors on somatotrophs increases intracellular cAMP and Ca2+ levels, whereas  Somatostatin (Somatotropin ReleaseInhibiting Hormone, SRIH) binding to its receptors on somatotrophs decreases intracellular cAMP and Ca+2  These signaling pathways provide a biochemical explanation for opposing activities GHRH and somatostatin on pituitary somatotroph release of GH

Costanzo LS. Physiology (Basic Review Series), 5th ed. New York: Elsevier, 2009. 49

Hormone-Receptor Interactions cont. ď ą We have just completed a discussion of hormonereceptor interactions from the vantage point of the receptor ď ą In the next 5 slides we will view the interaction from the hormone side

Marc Imhotep Cray, MD

Cellular MOA of steroid & TH hormones  Steroid hormones are lipophilic therefore, they diffuse across plasma membrane and form complexes with cytosolic or nuclear receptors bound complexes then activate transcription of various genes  B/C steroid hormones rely on the intermediary process of gene expression and protein translation it can take hours to days for their effects to manifest  Examples of steroid hormones are testosterone, estrogen, progesterone, cortisol, and aldosterone  Cholesterol is precursor to all steroid hormones  Although thyroid hormone is not a steroid hormone TH uses same cellular mechanism as steroids Marc Imhotep Cray, MD

Transport of steroid and thyroid hormones Why are total serum steroid hormone & TH levels not an accurate reflection of hormone activity?

 Most of steroid hormones & TH in serum are inactive because they are attached to serum binding proteins  Only free hormone is biologically active Free hormone is in equilibrium with bound hormone: [Free hormone] + [Binding protein] [Hormone-binding protein complex] For example: In circulation, T3/T4 exist in both active free and inactive protein-bound forms  T4 is 99.98% bound, with only 0.02% circulating free.  T3 is slightly less protein bound (99.8%), resulting in a considerably higher circulating free fraction (0.2%)

 Another important factor that affects hormone activity is concentration (represented as []) of cellular hormone receptors available Marc Imhotep Cray, MD for binding a specific hormone and mediating its action

Transport of steroid & thyroid hormones (2) Binding of steroid and thyroid hormones to plasma proteins has several beneficial effects, including:  Facilitation of transport  Prolonged half-life  Hormone reservoir  Steroid and thyroid hormones are minimally soluble in blood binding to plasma proteins renders them water soluble and facilitates their transport  Protein binding prolongs circulating half-life of these hormones e.g., not filtered/excreted by kidney

 Protein-bound form of hormone serves as a “reservoir” of hormone that minimizes changes in free hormone concentration when hormone secretion from Cray, its endocrine gland changes abruptly Marc Imhotep MD

MOA of peptide hormones & catecholamines  Peptide hormones and catecholamines are not highly lipid-diffusible and thus cannot cross plasma membrane  They bind to cell surface membrane receptors (next slide), which initiate a variety of biochemical events, including: o activation or inhibition of enzymes o alteration of membrane proteins, and o mediation of cellular trafficking

 These processes can occur within seconds to minutes o Nevertheless, peptide hormones can stimulate gene expression as well, and this effect can be delayed as it is with steroid hormones  Examples of peptide hormones are insulin, parathyroid hormone (PTH), vasopressin (antidiuretic hormone), and oxytocin

Marc Imhotep Cray, MD

N.B. intact peptides and proteins are not absorbed across the intestinal lumen; local proteases digest them into their constituent amino acids. Thus, therapeutic administration of a peptide hormone or hormone antagonist must be accomplished by a non-oral routes (IM, SQ, IV ).

To summarize: Mechanisms by which peptide and steroid hormones signal Remember: ď ą amino acid derivatives and peptide/protein hormones are water-soluble and interact with cell-surface membrane receptors ď ą Steroids, thyroid hormones, vitamin D, and retinoids are lipidsoluble and interact with intracellular(cytoplasmic & nuclear) receptors Marc Imhotep Cray, MD

Brown TA, Brown D. USMLE Step 1 Secrets, 3rd Ed. Saunders, 2013

Summary of distinguishing features of steroid, protein/peptide, and amine hormones

Kelly LJ. Essentials of Human Physiology for Pharmacy. Boca Raton: CRC Press, 2004.

Pathophysiologic & Pharmacologic Concepts Hypopituitarism may be partial or complete and may result from hypothalamic disease (leading to deficiency of hypothalamic-releasing hormones) or intrinsic pituitary disease(causing pituitary hormone deficiency)

Hypopituitarism may affect any of these pituitary hormones:  thyrotropin (TSH)  growth hormone(GH)  luteinizing hormone (LH)  follicle stimulating hormone (FSH) and  corticotropin (ACTH) Marc Imhotep Cray, MD

Pathophysiologic & Pharmacologic Concepts (2)  In targeting one of these hormones of hypopituitarism  therapy for GH deficiency aims to restore normal body composition, as well as, in children, to promote linear growth  Therapy for acromegaly, caused by excessive GH secretion, includes  surgery and (or) radiation, or  use of a GH inhibitor o Octreotide o Lanreotide o Pegvisomant Marc Imhotep Cray, MD

Pathophysiologic & Pharmacologic Concepts (3) Hypothyroidism can result from either thyroid (high TSH, low T3 &T4) or hypothalamic (or) pituitary dysfunction (low T3, T4, TSH)  Treatment of choice is hormone substitution by using a synthetic hormone Hyperthyroidism (thyrotoxicosis) is characterized by increased metabolism, and primary treatment options include  surgery  radioactive iodine or  drugs that inhibit formation of T3 &T4 by blocking utilization of iodine Marc Imhotep Cray, MD

Pathophysiologic & Pharmacologic Concepts (4) Principal functions of glucocorticoids involve regulation of carbohydrate metabolism and a variety of other physiologic actions Synthetic corticosteroids (eg, hydrocortisone, prednisone, and dexamethasone) are widely used as therapeutic agents in Tx of cancer and autoimmune or inflammatory-type disorders Pharmacologic treatment is also available for  insufficient adrenal function manifested as Addison disease  excess glucocorticoid exposure Cushing syndrome Marc Imhotep Cray, MD

Pathophysiologic & Pharmacologic Concepts (5) Diabetes mellitus (DM) is a syndrome caused by a relative or absolute deficiency of insulin, with hyperglycemia being hallmark medical finding  DM can occur as either an early onset form (type 1) or a gradual-onset form (type 2)  In T1DM, insulin-producing β cells of pancreas are destroyed or insufficiently active, and patients require lifelong treatment with exogenous insulin  In T2DM, adequate control of disease may be achieved by means of diet and exercise if these methods fail, patients take oral hypoglycemic agents, which cause o lower plasma glucose levels o improve insulin resistance, and o reduce long-term complications (macrovascular and microvascular problems such as neuropathy, nephropathy, retinopathy and CVD) Marc Imhotep Cray, MD

Pathophysiologic & Pharmacologic Concepts (6)  For type 1 DM Insulin is sole treatment and is sometimes also used for type 2 DM  For type 2 DM, drugs include  sulfonylureas, which stimulate insulin secretion from pancreatic β cells  metformin, a biguanide that decreases blood glucose levels by reducing hepatic glucose production and glycogen metabolism in liver and improving insulin resistance  meglitinides, which increase insulin secretion from pancreatic β cells  α-glucosidase inhibitors, which delay carbohydrate digestion and glucose absorption and  thiazolidinedione (TZD) derivatives (eg, rosiglitazone and pioglitazone), which reduce insulin resistance Marc Imhotep Cray, MD

Hormones of hypothalamic-pituitary axis Individual Axes: (Hormonal Feedback Regulatory Systems) Anterior Pituitary Gland Hypothalamic-Pituitary–Growth Hormone Axis Hypothalamic-Pituitary–Prolactin Axis Hypothalamic-Pituitary–Thyroid Axis Hypothalamic-Pituitary–Adrenal Axis Hypothalamic-Pituitary–Gonadal Axis Posterior Pituitary Gland Antidiuretic Hormone (ADH) Oxytocin

Marc Imhotep Cray, MD

McInnis M., Mehta S. Step-up to USMLE Step 1 2015 Edition. Wolters Kluwer, 2015. 63

Hypothalamus Hypothalamus, which is a part of CNS and not a gland, produces many releasing and inhibitory hormones which control secretion of anterior pituitary hormones Hypothalamic hormone/factor

Chemical nature

Tripeptide Peptide (41 AAs) Decapeptide

Marc Imhotep Cray, MD

Dopamine Peptide (40, 44 AAs) Peptide (14 AA)

Pituitary gland  Pituitary gland is composed of two morphologically and functionally distinct components:  Anterior lobe (adenohypophysis) and  Posterior lobe (neurohypophysis)  Anterior pituitary constitutes about 80% of gland  Production of most pituitary hormones is controlled by positively and negatively acting factors from hypothalamus carried to anterior pituitary by a portal vascular system (hypothlamic-hypophesal portal system)

Marc Imhotep Cray, MD

Hormones released by anterior pituitary

Kumar V, Abbas AK. Robbins and Cotran Pathologic Basis of Disease 9th Ed. Philadelphia: Saunders, 2015.

The adenohypophysis releases five hormones that are in turn under the control of various stimulatory and inhibitory hypothalamic releasing factors. TSH, Thyroid-stimulating hormone (thyrotropin); PRL, prolactin; ACTH, adrenocorticotropic hormone (corticotropin); GH, growth hormone (somatotropin); FSH, follicle-stimulating hormone; LH, luteinizing hormone. The stimulatory releasing factors are TRH (thyrotropin-releasing hormone), CRH (corticotropin-releasing hormone), GHRH (growth hormone-releasing hormone), GnRH (gonadotropin-releasing hormone). The inhibitory hypothalamic influences comprise PIF (prolactin inhibitory factor or dopamine) and growth hormone inhibitory factor (GIH or somatostatin). 66

Hormonal Feedback Regulatory Systems  Feedback control , both negative and positive, is a fundamental feature of endocrine systems  Each of major hypothalamic-pituitary- hormone axes is governed by negative feedback, a process that maintains hormone levels within a relatively narrow range

Examples of hypothalamic-pituitary negative feedback include (1) thyroid hormones on TRH-TSH axis (2) cortisol on CRH-ACTH axis (3) gonadal steroids on GnRH-LH/FSH axis, and (4) IGF-I on growth hormone-releasing hormone (GHRH)-GH axis  These regulatory loops include both positive (e.g., TRH, TSH) and negative (e.g., T 4 , T 3 ) components, allowing for precise control of hormone levels Marc Imhotep Cray, MD

Feed-forward and Feed-back Mechanisms  Key to understanding endocrinology are feed-forward and feed-back mechanisms that govern how “releasing” factors in hypothalamus control release of hormones in pituitary (regulatory/tropic hormones) that in turn cause release of second-tier hormones that target multiple organs within body Marc Imhotep Cray, MD

Hypothalamic Releasing Hormone

Pituitary Tropic (Signal) Hormone

Target Glands

Second-tier Hormone Negative Feedback

Organ-System Effect 68

Negative and Positive Feedback Regulation  In most cases, a hypothalamic– pituitary–target gland axis is regulated by negative feedback, whereby tropic hormone of anterior pituitary gland has negative feedback effects on hypothalamus and target gland hormone has negative feedback effects on both hypothalamus and anterior pituitary  By way of these mechanisms levels of target gland hormone are maintained within normal physiological range N.B. Positive Feedback Although negative feedback is primary homeostatic mechanism in endocrine system, rare examples of positive feedback exist (e.g., menstrual cycle ). These positive feedback mechanisms are, by nature, self-limited, as dictated by need for homeostasis in physiological systems. Marc Imhotep Cray, MD

Example of positive feedback  Prime example of positive feedback occurs during menstrual cycle  In late follicular phase of cycle, estradiol levels rise above a critical point, above which positive feedback occurs

 High estradiol concentration results in a surge in hypothalamic secretion of GnRH and pituitary secretion of LH and FSH, inducing ovulation  Ovulation and transformation of ovarian follicular cells into corpus luteum signals end of positive feedback Marc Imhotep Cray, MD

Concept of Feedback Loop What is a feedback loop? Hormone synthesis and release are governed at multiple levels  Hormone synthesis and release (secretion) from an organ of interest typically involves regulation by a pituitary hormone, which itself is regulated by a hypothalamic hormone This general pathway structure is commonly referred to as a hypothalamic-pituitary-(organ) axis  e.g., HPO axis refers to ovary, HPA axis refers to adrenal gland These relationships are often depicted using feedback loops (next 2 slide)

Marc Imhotep Cray, MD

Regulation of hormone synthesis and secretion cont.

 A negative feedback mechanism is an example of a negative effect

Solid lines = positive effect Dashed lines = negative effect

 It is essential to understand “the negative feedback principle” of hypothalamic /pituitary/ target organ axis

 Negative feedback occurs when a product downstream of an axis inhibits production of a reactant by which it is regulated  for example, thyroid hormone inhibition of thyroid-stimulating hormone (TSH) Marc Imhotep Cray, MD

Pazdernik TL, Kerecsen L. Rapid Review Pharmacology, 3rd Ed. Mosby, 2010

Example, thyroid hormone feedback loop ď ąA small reduction of thyroid hormone triggers a rapid increase of TRH and TSH secretion, resulting in thyroid gland stimulation and increased thyroid hormone production ď ąWhen thyroid hormone reaches a normal level, it feeds back to suppress TRH and TSH, and a new steady state is attained Marc Imhotep Cray, MD

Brown TA, Brown D. USMLE Step 1 Secrets, 3rd Ed. Saunders, 2013

Anterior Pituitary Gland Cell Types, Hypothalamic Control Factors, and Hormonal Targets

Golan DE et.al. Principles of Pharmacology: The Pathophysiologic Basis of Drug Therapy 3rd Ed. Lippincott Williams & Wilkins, 2012 Marc Imhotep Cray, MD

Relationships Among Hypothalamic, Pituitary, and Target Gland Hormones HYPOTHALAMIC

PITUITARY

TARGET ORGAN

TARGET ORGAN HORMONES

GHRH (+), SRIH (–)

GH (+)

Liver

Somatomedins

CRH (+)

ACTH (+)

Adrenal cortex

Glucocorticoids Mineralocorticoids Androgens

TRH (+)

TSH (+)

Thyroid

T4, T3

GnRH or LHRH (+)

FSH (+), LH (+)

Gonads

Estrogen Progesterone Testosterone

Breast

—

Dopamine (–), PRH (+), TRH (+) Prolactin (+)

Redrawn after: Pazdernik TL, Kerecsen L. Rapid Review Pharmacology, 3rd Ed. Mosby, 2010

+, stimulant; –, inhibitor; ACTH, adrenocorticotropic hormone; CRH, corticotropin-releasing hormone; FSH, folliclestimulating hormone; GH, growth hormone; GHRH, growth hormone–releasing hormone; GnRH, gonadotropinreleasing hormone; LHRH, luteinizing hormone-releasing hormone; LH, luteinizing hormone; PRH, prolactin-releasing hormone; SRIH, somatotropin-releasing inhibiting hormone; TRH, thyrotropin releasing hormone; TSH, thyroidstimulating Marc Imhotephormone. Cray, MD

Some disorders often requiring applications of endocrine and metabolic medications:

Kibble J , Cannarozzi ML. Pathophysiology Flash Cards. New York: McGraw-Hill, 2013 Marc Imhotep Cray, MD

THE END

See next slide for hypermedia to further study tools and resources. 77

Further study tools and resources: Inside the Endocrine System BMS Cloud Folder:  Endocrine System Pathology Outline  Endocrine System Pathology Ppt.  Endocrine Pathology Case 1 SDL Tutorial  Endocrine Pathology Case 2 SDL Tutorial  Endocrinology Tutorial 1 Postpartum Necrosis  Endocrinology Tutorial 2 MEN Syndromes  Endocrinology Tutorial 3 Anterior Pituitary  Diabetes mellitus Type 1 SDL Tutorial  Diabetes mellitus Type 2 SDL Tutorial  Endocrine Pathology Clinical Vignettes  Endocrine Pathology Rapid Review Notes Also see Medical Pathology Cloud Folder Marc Imhotep Cray, MD