Command and Control: Physiology of Growth Hormone Secretion

Dr Fraser Gibb

Consultant Endocrinologist UNIVERS SIT . THE THE UNIVERSITY of EDINBURGH Edinburgh Medical School . OF G EDINBUR MBChB

Growth Hormone: Introduction

Command and control Physiology of growth hormone secretion

• 'Anabolic' hormone
• Single chain 191 amino acid polypeptide
• Genome locus on chromosome 17q22-24
• GH-N expressed in pituitary somatotrophs (2 splice variants
• Somatotrophs comprise 35 - 45% of anterior pituitary (Prop-1 -> Pit-1)
• Acromegaly is a disorder of excess GH production

MBChB Command and control Physiology of growth hormone secretion

GH Gene Cluster

Pituitary Placenta GH gene cluster GH-N CS-L CS-A GH-V CS-A 5 3 GH-N 1 2 3 4 5 B Aan Ley (Pro 22 KDa GH 186 TY - COOH 195 170 C Figure from Williams Textbook of Endocrinology, 14th Edition, Elsevier

GH Control: Overview

MBChB Command and control Physiology of growth hormone secretion

GH-IGF Axis Stimulation

Stimulation Deep sleep a-Adrenergic Fasting Acetylcholine Sex steroids Stress Amino acids Hypoglycemia Ghrelin

GH-IGF Axis Suppression

Suppression Hypothalamus GHRH SRIF Pituitary Obesity ß-Adrenergic Glucocorticoids FFA Glucose Hypothyroidism IGF1 GH GH GHR GH GHR Liver Fat GH GHBP IGF1 IGF1 IGFBP Catabolism Inhibition GH GHR Undernutrition Acute illness Chronic illness GH receptor deficiency GHR antibodies IGF1 receptor deficiency IGF1 IGER Bone and muscle IGF1 (auto/paracrine) Growth Figure from Williams Textbook of Endocrinology, 14th Edition, Elsevier

Control of GH Secretion: GHRH

MBChB Command and control Physiology of growth hormone secretion

• Growth hormone releasing hormone (GHRH) released from the arcuate area of the hypothalamus into the blood supply to the anterior pituitary
• GHRH receptor (GPCR): Gs Adenylate cyclase TCAMP TiCa2+ Release GH and ^GH mRNA
• Enhanced by: estradiol, ghrelin, glucocorticoids (acutely) and starvation
• Blunted by: somatostatin, obesity, insulin, glucose, rage

Hypothalamus Releasing Factors

Parvicellular Hypophysiotropic Neuron Location: PeVH, PVH (TRH, CRH, Somatostatin) Arc (GHRH, GnRH, Dopamine) POA GnRH Hypothalamus Releasing factors Tropic hormones (ACTH, TSH, GH, LH, FSH, Prolactin) Anterior Pituitary Gland Figure from Williams Textbook of Endocrinology, 14th Edition, Elsevier

Ghrelin

MBChB Command and control Physiology of growth hormone secretion

• Orexigenic (enhance food consumption)
• 28 amino acid peptide produced in the stomach - ^ fasting and pre-meal
• Natural ligand of the growth hormone secretagogue receptor (GHS-R) > increases GHRH release
• Enhances GH secretion
• Maintains blood glucose levels during starvation (via GH)

GHRH and Ghrelin Interaction

160 GHRH + ghrelin · Ghrelin 4 GHRH 120 GH (ug/L) 80 - 40 - 0 - -15 0 30 60 90 120 150 180 Time (min) Figure from Williams Textbook of Endocrinology, 14th Edition, Elsevier

Somatostatin

MBChB Command and control Physiology of growth hormone secretion

• SRIF: Somatotropin release-inhibiting factor
• SST-14 in PVN of hypothalamus (SST-28 in gut)
• Peptide hormone
• 5 GPCRs: SSR 2 and 5 dominant in pituitary
• Suppressive effect on multiple pituitary hormones (including GH and TSH) and insulin
• Somatostatin analogues are used in the treatment of GH excess and other neuroendocrine conditions

Somatostatin Structure

-300 UE-A CRE TATA 60 TSE !! TSE 50 bp Intron Exon 1 Exon 2 200 bp SST-28 5'UTR 3'UTR Poly A Signal Peptide SST-14 100 bp PC1/PC2 CPE SST-28 Ser - Ala - Asn - Ser - Asn - Pro - Ala - Met - Ala - Pro - Arg - Glu - Arg - Lys - Ala - Gly - Cys - Lys - Asn- Phe - Phe - Trp - Lys - Thr - Phe - Thr - Ser - Cys SST-28(1-12) Ser - Ala - Asn - Ser - Asn - Pro - Ala - Met - Ala - Pro - Arg - Glu SST-14 Ala - Gly - Cys - Lys - Asn - Phe -Phe). (Trp S S Lys Cys - Ser - Thr - Phe - Thr Figure from Williams Textbook of Endocrinology, 14th Edition, Elsevier

GH and its Receptor

MBChB Command and control Physiology of growth hormone secretion

• GH binds to a specific receptor homodimer
• GHR - 70kd protein in cytokine/haematopoietin superfamily
• GHR mostly expressed in liver
• Intracellular signalling by phosphorylation cascade involving JAK/STAT pathway
• Predominant action is stimulation of hepatic IGF-1 synthesis and secretion

GHR Intracellular Signaling

GH Extracellular GHR GHP Intracellular JAK2 JAK2 ® P ® ® ® TRENDS in Endocrinology & Metabolism

Insulin-like Growth Factor 1 (IGF-1)

MBChB Command and control Physiology of growth hormone secretion

• Small peptide
• Responsible for most of the linear growth-promoting activities of GH
• 40% of IGF-1 variation is heritable and relates to final height
• 99% protein-bound (IGFBPs 1-6 - IGFBP-3 + by GH)
• Liver (75% of plasma IGF-1) / autocrine and paracrine - both under GH control
• IGF-1 receptor - 2 alpha and 2 beta subunits - present across multiple tissues
• Ligand binding -> TK activity -> IRS-1 (insulin receptor substrate)

IGF-1 Receptor Signaling

Ligand Affinities: Ins >> ¡GF.I > IGF-II IGF-I > IGF-II >> Ins IGF-I > IGF-II > Ins R insulin-binding 4- Cysteine-rich domain + determinants Plasma Membrane Juxtamcm arend Mation Phosphorylation Sites 4= Tyrosine kingas Regulatory 999 Phosphorylation + Sites C-ermi nal phosphorylation Insulin Receptor IGF-I Receptor IGF/Insulin Hybrid IGE-R GF-R Ras PIPZ GTR (GDP ® P p110 À ADP ADP Sos MEK-K Raf Grb2 Shc ATF P85 ADP Sos MEK Grb2 P IRS-1 AKT P P Shc MAP-K Mitogenic and metabolic responses Figure from UpToDate® ATE - ATP PIP3 P 1020

Overview of GH Receptors

A GH receptors Raphe nucleus GHS receptors Periventricular nucleus O GHRH receptors SST neuron 5-HT + CRH neuron + + + Brainstem catecholaminergic inputs ACh + * GHRH neuron + + NPY neuron SST neuron DA Arcuate nucleus Galanin Hypothalamus + to CNS + GHRH Leptin + SST + Ghrelin Fat Pituitary FFA A *= O A + IGF-1 GH Stomach A Liver Ghrelin Figure from Williams Textbook of Endocrinology, 14th Edition, Elsevier MBChB Command and control Physiology of growth hormone secretion Basal forebrain * SST receptors

Stimulation and Inhibition of GH

MBChB Command and control Physiology of growth hormone secretion

Stimulatory Factors of GH

Physiologic Factors Hormones and Neurotransmitters Pathologic Factors Stimulatory Factors Episodic, spontaneous release Insulin hypoglycemia 2-Deoxyglucose Acromegaly TRH GnRH Glucose Arginine Interleukins 1, 2, 6 Ghrelin Galanin Opioids (u-receptors) Melatonin Classic neurotransmitters @2-Adrenergic agonists ß-Adrenergic antagonists M1 cholinergic agonists 5-HT1D receptor agonists H1 histamine agonists GABA (basal levels) Dopamine (? D2 receptor) Estrogen Testosterone Glucocorticoids (acute)

Inhibitory Factors of GH

Physiologic Factors Hormones and Neurotransmitters Pathologic Factors Inhibitory Factorsª Postprandial Glucose infusion Neuropeptides hyperglycemia Elevated free fatty acids Elevated GH levels Elevated IGF-1 Somatostatin Calcitonin Neuropeptide Y (NPYb) CRHb Obesity Hypothyroidism Hyperthyroidism (pituitary) REM sleep Senescence, aging Classic neurotransmitters @1/2-Adrenergic antagonists B2-Adrenergic agonists H1 histamine antagonists Serotonin antagonist Nicotinic cholinergic agonists Glucocorticoids (chronic) Acromegaly L-Dopa D2 receptor DA agonists Phentolamine Galanin Psychologic Slow-wave sleep Postprandial glucose decline Fasting Physical Amino acid infusions Arginine, lysine Neuropeptides Exercise Stress GHRH Protein depletion Starvation Anorexia nervosa Renal failure Liver cirrhosis Type 1 diabetes mellitus Figure from Williams Textbook of Endocrinology, 14th Edition, Elsevier

Pattern of GH Secretion

MBChB Command and control Physiology of growth hormone secretion

GH Secretion Over Time

Control day Fasting day 20 Meals 15- 1 1 1 GH (µg/L) 10- 5- 0- 0800 2000 Time (hours) 0800 0800 2000 0800 Time (hours)

GH Secretory Bursts

Interval Young Adult Fasting Obesity Middle Age 24-h secretion (ug/24 h) 540 ± 44 2171 ± 77 ±20 196 ± 65 333 Secretory bursts (number in 24 h) 12± 1 32 ± 2 3 ± 0.5 10± 1 GH burst (µg) 45 ± 4 64 ±9 24 ± 5 10±6

• Pulsatile: reductions in tonic inhibition by somatostatin
• Secretion is diurnal with 2/3 secreted at night with onset at slow wave sleep
• GH is typically undetectable for 50% of the day
• GH secretion peaks at puberty and falls with ageing (~50% every 7 years)

Figures from Williams Textbook of Endocrinology, 14th Edition, Elsevier

IGF-1 Concentration Across the Lifespan

MBChB Command and control Physiology of growth hormone secretion

IGF-1 in Males

Males 800 2.5% 700 25% - 50% 600 - 75% 97.5% 500 IGFI (ng/ml) 400 300 200 100 0 0 10 20 30 40 50 60 70 80 Age (years)

IGF-1 in Females

Females 800 2.5% 700 25% - 50% 600 75% 97.5% 500 IGFI (ng/ml) 400 300 200 100 0 0 10 20 30 40 50 60 70 80 Age (years) A B Figures from Williams Textbook of Endocrinology, 14th Edition, Elsevier

What Does GH Do?

MBChB Command and control Physiology of growth hormone secretion

GH Actions

IGF1 GH + Immune function Lipolysis Insulin resistance Skeletal muscle Renal function Chondrocyte proliferation hypertrophy Bone, tooth Insulin synthesis Cardiovascular endothelium Gastrointestinal Reproduction Neurogenesis Hepatic metabolism Figure from Williams Textbook of Endocrinology, 14th Edition, Elsevier

What Does GH Do? (Continued)

MBChB Command and control Physiology of growth hormone secretion

• Acts directly and indirectly (via IGF-1) to stimulate linear growth - action on epiphyseal growth plates
• Increases lipolysis and lipid oxidation (mobilization of stored TG)
• Stimulates protein synthesis
• Antagonism of insulin action
• Phosphate, water and sodium retention

Figure from Williams Textbook of Endocrinology, 14th Edition, Elsevier

What Does IGF-1 Do?

MBChB Command and control Physiology of growth hormone secretion

• Decrease blood glucose / improve insulin sensitivity
• Stimulate whole body protein synthesis and inhibit proteolysis
• Reversal of catabolic effects of glucocorticoids on protein synthesis
• Anabolic effect on bone (^ markers of bone formation)

IGF-1 Functions and Feedback

Hypothalamus GHRH - Thymus development and function, Hematopoiesis. Pituitary GH Ghrelin GH + IGF-1 Glomerular development. function and integrity. Cardiovascular development and protection, Vasodilator. Hepatocyte proliferation. Liver regeneration, · Fetal grow and differentiation IGF-1 Muscle growth and development. . Placental growth and development Normal bone growth. Ovarian folliculogenesis. Testicular function and integrity. . Negative feedback - v GH release Somatostatin Neuronal development, myelinization and protection Amyloid-B clearance. Antiinflammatory. - Immune cell function.