Endocrinology - Reproduction Introduction Mohammed Y. Kalimi, Ph.D. Objectives: After studying this material, the student will: 1. Identify the chemical nature of thyroid hormones, TRH, TSH, GH, somatoatatin, prolactin, insulin, glucagon and growth factors. 2. Recognize the concept of hormonal biosynthesis, storage, release, plasma half life, degradation and control mechanisms. 3. Understand the physiological effects and mechanisms of hormonal actions. 4. Understand the concept of endocrine diseases in terms of hyper (over secretion) and hypo (deficiency) functioning of the endocrine glands. I. Basic Concepts Endocrine Gland: A ductless gland whose secretions (hormones) are released into the vascular system for an action on distant cells (target cells). Hormones: Chemical agents synthesized by ductless glands and released into the blood stream where they evoke a physiological response by acting on specific tissues/organs by way of specific receptors. Paracrine secretion: Secretory product is released into intercellular space to influence immediately neighboring cells. Neurocrine: A hormone secreted by a nerve cell, released from nerve endings into the blood stream, and carried to another area where it exerts specific effects on target cells. The chemical substance released from nerve endings which exert effects on the adjacent nerve cells across a short synapse is called a neurotransmitter. Autocrine: Secretory product (hormone) acts back on the cell of origin or adjacent identical cells.
Target organ: The organ or cells on which a particular hormone elicits a response. May be another endocrine organ. Feedback: The response of a particular target organ may feedback to an endocrine gland and may modify the output of that gland. Feedback may be either stimulatory (positive) or inhibitory (negative). II. Hormones A. Classification of Hormones: The chemical nature of hormones varies widely but three major categories are recognized. 1. Peptide hormone: Ranging from simple dipeptides (two amino acids) to large proteins containing over 200 amino acids. Examples: Hypothalamic, anterior pituitary, posterior pituitary and pancreatic hormones. 2. Biogenic amines - Amino acid derivatives. Examples: Thyroid hormones and catecholamines (both derived from modification of the amino acid tyrosine). 3. Steroid hormones: A large family of molecules based on the 17 carbon aromatic cyclopentanoperhydrophenanthrene nucleus, among which relatively minor chemical differences are associated with marked differences in biological activity. Examples: Adrenal cortex, reproductive gland hormones and the active metabolites of vitamin D. B. Nomenclature and acronyms: Hormones are named for either their gland of origin (thyroxine, testosterone), function (calcitonin, progesterone, prolactin), or chemical structure (triiodothyronine, aldosterone). Abbreviations are often used in preference to the complete name for a hormone (follicle-stimulating hormone = FSH; luteinizing hormone releasing hormone = LHRH ; parathyroid hormone = PTH; estradiol = E 2 ).
C. Biosynthesis of hormones: 1. Biosynthesis of peptide hormones: The peptide hormones are synthesized by rough endoplasmic reticulum as a pre-pro-hormone. 2. Biosynthesis of steroid and amine hormones: The steroid and amine hormones are synthesized from cholesterol and tyrosine respectively through a series of enzymatic reactions by smooth endoplasmic reticulum, and mitochondria. D. Storage of hormones: In comparison with exocrine glands, storage is minimal with the exception of the thyroid gland. E. Release of hormones: Excitation-secretion coupling and release Intracellular calcium camp Activation of microtubular and microfilament system Fusion of membrane of the secretory granule with that of the cell Secretion of hormone by exocytosis 1. Hormone concentration in the various effluents from a particular endocrine gland should exceed that of arterial blood supplying the gland. 2. Secretion or production rate of a hormone, nmoles/minute, ng/minute. 3. Plasma concentration of a hormone, nmoles/ml, ng/ml. F. Transport of hormones in blood: 1. Bound to carrier proteins: Steroid and thyroid hormones circulate bound to specific globulins. In general there is a good correlation between the amount of bound hormone in circulation and the plasma half-life of a hormone; the more bound the longer the half-life. 2. Unbound or free: With few exceptions (IGF-I), peptides and protein hormones circulate unbound (also catecholamines). G. Clearance of hormones from the circulation: 1. Biological half life (t ½)
2. Metabolic clearance rate (MCR) H. Inactivation of hormones: 1. By specific target tissues (internalization and lysosomal degradation). 2. By liver and kidneys. 3. By both 1 and 2. I. Measurement of hormones: 1. Radioimmunoassay 2. Localization of hormones in tissues of origin and action: Immunocytochemistry. J. Control of hormonal secretion: 1. Feedback (positive or negative) 2. Integration between endocrine and nervous system 3. Neural control (dopaminergic, adrenergic, cholinergic etc.). 4. Others such as sleep-wake cycle, menstrual cycle, diurnal rhythm. K. Mechanism of action (covered separately): L. Major function of hormones: 1. Endocrine system helps initiate, mediate and regulate the processes of growth, differentiation, development, maturation and senescence. 2. Maintenance of homeostasis, fluid and electrolyte balance (Na +, K +, Ca ++, glucose, water ) 3. Regulation of cellular metabolism (fats, carbohydrates, proteins) 4. Sexual development and function, lactation and behavior. M. Malfunctioning of the endocrine system: Primarily caused by: 1. Overproduction of a hormone (hyperfunction) 2. Underproduction of a hormone (hypofunction) 3. Unresponsiveness of target organ (lack of receptor, etc.) = down-regulation 4. Production of abnormal hormone N. Neuroendocrinology: Control systems of the body
III. The Nervous System and the Endocrine System A. Similarities between the two: 1. Each synthesizes and releases specific chemical agents which are capable of influencing other cells by interacting with specific receptors. 2. Both neurons and endocrine cells generate electrical potentials and can be depolarized. B. Differences between the two: 1. Nervous system: a. Specific chemical agents released are disseminated only a very short distance. b. System is fast acting. c. Actions are relatively short-lived. d. Operates with point to point precision. e. Affects only glandular secretions and muscular contractions. 2. Endocrine system: a. Specific chemical agents are released and carried via the blood stream throughout the whole body. b. System is slow acting. c. Actions are relatively long-lived. d. Theoretically, has the potential of affecting every cell in the body. e. Affects a whole variety of cell types. C. More generalized definition is descriptive of cells that release a hormone in response to a neural stimulation. 1. Adrenal medulla under the influence of the sympathetic nervous system. 2. Some cells of the islets of Langerhans are affected in part by the autonomic nervous system. 3. Some cells in the CNS synthesize "hormones" that act as neurotransmitters.
Figure 1 IV. Hormones Origin A. Hypothalamus: Thyroid-stimulating-hormone-releasing-hormone (TRH) Corticotrophin-releasing-hormone (CRH) Luteinizing-hormone-releasing-hormone (LHRH) Growth-hormone-releasing-hormone (GHRH) Somatostatin Dopamine
B. Anterior Pituitary: Growth Hormone (GH) Prolactin Thyroid-Stimulating-Hormone (TSH) Adrenocorticotrophic-Hormone (ACTH) Luteinizing Hormone (LH) Follicle-Stimulating-Hormone (FSH) Melanocyte- Stimulating-Hormone (MSH) C. Posterior Pituitary: Vasopressin or Antidiuretic Hormone (ADH) Oxytocin D. Thyroid Gland: Thyroid Hormones (T3 and T4) : Follicular cells Calcitonin: Parafollicular cells E. Parathyroid gland: Parathyroid Hormone F. Adrenal Cortex: Glucocorticoids Aldosterone G. Adrenal Medulla: Epinephrine and Norepinephrine H. Pancreas: Insulin Glucagon Somatostatin I. Gonads (testes-male; ovary-female) Androgens Estrogens Progestins
J. Skin, Liver, and Kidney: Vitamin D K. Placenta: Human Chorionic Gonadotropin (hcg) Human Placental Lactogen (hpl) V. Mechanism of Hormone Action A. Mechanism of Steroid Hormone Action (Fig. 2): Used by steroid hormones such as estrogens, androgens, progesterone, aldosterone, glucocorticoids and vitamin D. In addition, thyroid hormones and vitamin A have a similar mechanism. The estrogen, progesterone, androgen, vitamin D and vitamin A receptors are primarily localized in the cell nucleus. Steps: 1. Endocrine gland + stimulus steroid release. 2. Steroid diffuses through the cell membrane into the cytoplasm of target cells. 3. Steroid binds to cytoplasmic and /or nuclear receptors forming a steroidreceptor complex. Steroid binding results in conformational changes called activation. The subsequent dimerization of the liganded receptor enables the steroid-receptor complex dimer to bind tightly to specific DNA sequences, called Steroid Responsive Elements(SRE), and interact with coregulator proteins thus activating or suppressing transcription (mrna) of genes under the control of steroid hormones.
Figure 2.
Figure 3. In general, agonist ligands of receptors promote binding of coactivator proteins that promote transcription initiation while binding of antagonists promotes interaction with corepressor proteins that facilitate transcription repression.the modular nature of receptors allows ligand, tissue and promoter specific interaction with select subsets of coregulators capable of elaborating distinct transcriptional and hence physiological responses to steroid signal. 4. mrna is translated into specific proteins (such as metabolic enzymes). 5. specific proteins, physiological responses. B. Cyclic AMP (camp) Mechanism (Fig. 3): The camp mechanism appears to be used by most (not all) peptide hormones (such as LH, FSH, TSH, ACTH, ADH via V 2 receptor, hcg, MSH, GHRH, CRH, catecholamines [ß 1, ß 2 receptors], calcitonin, glucagon and PTH).
Steps: 1. Endocrine gland + stimulus Release of Hormone (1st messenger). 2. Hormone + target cell membrane receptor Hormone - Receptor Complex. Peptide hormone receptors are membrane bound, mobile, glycosylated, large peptides. 3. Hormone-Receptor complex, activation of G-protein, adenylyl cyclase activity. Hormone binding to receptor releases guanine diphosphate (GDP) from a binding site on the heterotrimeric G-protein permitting guanine triphosphate (GTP) to bind to the G- protein (GDP-GTP exchange). Heterotrimeric G-proteins are made up of three subunits α, β and γ, in order of decreasing mass. The binding of GTP to α subunit results in dissociation of Gα from βγ. The GTP bound - Gα-protein interacts with the adenylyl cyclase catalytic unit. This results in the activation of enzyme adenylyl cyclase, and production of camp from intracellular ATP. ATP + adenylyl cyclase camp + Pi The Gα-subunit serves as a coupling protein between receptor and adenyl cyclase, facilitating transmission of the hormonal signal. There are two types of G-proteins (i) stimulatory (G s ) and (ii) inhibitory (G i ). camp inhibitory hormones (somatostatin, dopamine) bind to Gi-protein and suppress adenylyl cyclase activity while camp dependent hormones (LH, FSH, TSH, ACTH, PTH, ADH via V2 receptor, GHRH, CRH, MSH, calcitonin, glucagon, epinephrine, hcg ) bind to Gs-protein and activate adenylyl cyclase. 4. ATP + adenylyl cyclase camp + Pi 5. Specific protein kinase(s) are activated by camp Protein Kinase (inactive)+ camp Protein Kinase (active) 6. The activated protein kinases catalyze phosphorylation of enzymes by ATP resulting in physiological responses. Protein +ATP + Activated protein kinase = Phosphoprotein + ADP
Inactivation of camp and phosphoproteins 1. camp is inactivated by the enzyme phosphodiesterase camp + phosphodiestrase 5' AMP (inactive). 2. Phosphoproteins are inactivated by the enzyme phosphatase Phosphoprotein + phosphatase = Protein + Pi C. The Calcium-phospholipid mechanism (Fig.4): Figure 4: Calcium-Phospholipid Pathway The calcium-phospholipid mediated mechanism is used by GnRH (LHRH), TRH, Angiotensin II, and ADH via the V 1 receptor. 1. Hormone + membrane receptor, Hormone - receptor complex. Formation of high affinity hormone - receptor complex results in transmission of the signal through a G-protein (Gq) to the enzyme phospholipase c'. 2. Activation of the cell membrane enzyme phospholipase 'c.' 3. Conversion of plasma membrane phospholipid, phosphatidyl inositol 4,5, biphosphate(pip 2 ) by activated enzyme phospholipase 'c' to inositol 1,4 5 triphosphate (IP 3 ) and 1,2 diacylglycerol. Inositol - 1,4,5 triphosphate (IP 3 ) enhances intracellular calcium and calciumcalmodulin processes. 1,2 diacylglycerol activates the enzyme protein kinase 'c' (PKC) which catalyzes the phosphorylation of proteins by ATP.
Figure 5: Internalization of Hormones The overall effects of protein kinase 'c' activation and elevated calcium ion concentrations include the opening and closing of ion channels, and increased gene transcription by phosphorylating gene regulatory proteins directly or indirectly by cascade. It is quite common for hormone actions (TSH, GHRH, etc.) to depend upon the camp mechanism together with the phospholipid - calcium mechanism (crosstalk). D. Internalization of Peptide Hormones (Fig.5). Many peptide hormones are known to be internalized. Steps of internalization mechanism: 1. Endocrine gland + stimulus hormone 2. Hormone binds to cell membrane receptor 3. Hormone-receptor complexes are clustered on the membrane 4. Membrane containing aggregated hormone-receptor complexes begin to fold inward forming a coated pit. 5. Coated pits are internalized into the cell forming endocytic vesicles called endosomes.
6. Endosomes (by an ATP-dependent process) may facilitate the release of ligand from receptor. Ligand and receptor are sorted, and the ligand is degraded by lysosomal enzymes (hydrolases). The internalized receptor may be recycled to the cell surface or degraded by lysosomal enzymes. VI. Hormone Action: Summary A. Steroid hormone (estrogen, androgen, progesterone, aldosterone, glucocorticoids, thyroid hormones, vitamin D, vitamin A) Mechanism: Binding of steroid to the cytoplasmic and/or nuclear receptors, dimerization of the hormone-receptor complexes, transcription (mrna synthesis),translation (enzyme or protein synthesis), and physiological responses. B. camp mechanism: 1. activation of adenylyl cyclase, camp, activation of phosphokinasea LH, FSH, TSH, ACTH, hcg, PTH, Calcitonin, Glucagon, CRH, GHRH, Epinephrine, ADH via V 2 receptor. 2. adenylyl cyclase, camp Dopamine, Somatostatin C. Calcium-phospholipid mechanism: activation of phospholipase C, conversion of PIP2 to IP3 and diacylglycerol. 1. IP3, intracellular calcium, 2. diacylglycerol, PKC. LHRH, TRH, Angiotensin II, ADH via V1 receptor. D. Associated tyrosine kinase-linked hormone action: Prolactin, GH E. Intrinsic tyrosine kinase-linked hormone action: Insulin, IGF-I