Hypothalamopituitary Hormones: Biosynthesis and their Mechanism of Action Deeba S. Jairajpuri, Ph.D. Dept. of Medical Biochemistry

Pituitary gland Located in the Sella turcica of the skull. Has anterior and posterior parts, which secret tropic and effector hormones. Its hormonal secretion is controlled by peptides secreted by the hypothalamus, The hypothalamus is controlled by stimuli from CNS and by feed back mechanisms

RECEPTORS FOR PEPTIDE HORMONES The receptors for peptide hormones are located in the plasma membrane. Receptor structure: -A single polypeptide chain with an extracellular, transmembrane, and intracellular domains, e.g. IGF -A single polypeptide chain with multiple intracellular, transmembrane, and extracellular domains, e.g. CRH -Other receptors are composed of multiple polypeptides (e.g. the insulin receptor).

Binding of hormones to cell surface receptors leads to cascade of reactions, release of second messengers, change of proteins /enzymes activity, and performance of biochemical task. Second messenger, cAMP Activation of G-protein coupled receptors (GPCRs)  association with G-proteins  activates adenylate cyclase  conversion of ATP to cAMP  cAMP-dependent protein kinase (PKA) activation RECEPTORS FOR PEPTIDE HORMONES

Ca 2+ / inositol triphosphate (IP 3 )/diacylglycerol (DAG) Hormone binding  interaction with a stimulatory G-protein  activation of membrane- localized PLCγ  hydrolysis of phosphatidylinositol 4,5-bisphosphate (PIP2)  2 messengers: IP3 (cytosol-soluble) + DAG (remains in the membrane phase) – Cytosolic IP3  opening Ca2+ channels on endoplasmic reticulum  release of Ca2+ in cytosol  activation of many enzymes (activates calmodulin). – DAG  it binds and activates protein kinase C (PKC)  opens voltage-gated Ca2+ channels in the plasma membrane  serine and threonine phosphorylation on many proteins (modulating their catalytic activity. RECEPTORS FOR PEPTIDE HORMONES

POSTERIOR PITUITARY HORMONES Both vasopressin (antidiuretic hormone [ADH]) and oxytocin are synthesized as prohormones in neural cell bodies in the hypothalamus, stored in the posterior pituitary and released in systemic circulation when needed. Both are nonapeptides (9 amino acid ) differ in 2 amino acids. OXYTOCIN The principal action of oxytocin is ejection of milk from the lactating mammary gland as well has a role in parturition. The mechanism of oxytocin action involve regulation by increase intracellular Ca 2+ The oxytocin receptor belongs to a seven-membrane-spanning receptor family.

VASOPRESSIN (ADH: anti-diuretic hormone ) Have 2 receptors V1 &V2 ADH acts on V2 receptors in distal convoluted tubules of kidneys, result in H2O retention ADH binding to V2 stimulates adenylate cyclase, increase cAMP, promotes water reabsorption. ADH binding to V1 activates phospholipase C, increase IP3 & DAG increase of intracellular Ca ++, activation of protein kinase C, the major effect of V1 receptors is vasoconstriction and increase in peripheral vascular resistance, hence the name vasopressin

ANTERIOR PITUITARY HORMONES Pituitary hormones are either tropic (actions on other endocrine gland e.g. TSH), or direct effectors (act on target tissues e.g. GH). Three categories of hormones: 1.The growth hormone-prolactin group, includes Growth hormone, prolactin and chorionic somatomammotropin, which are protein with similar structure. 2.The glycoprotein hormones: are TSH, FSH, LH & human chorionic gonadotropin (HCG). 3.The pro-opiomelanocortin peptides Hormones; ACTH, lipotropin and melanocyte stimulating hormone, are derived from a single gene.

GROWTH HORMONE (GH) 191 amino acid residues contains 2 disulfide bridges. It is a globular protein. GH is the most abundant hormone in the human pituitary gland. Growth hormone secretion is pulsatile nature (10 pulses per day). About 40% of the hormone is bound to GH-binding protein (GHBP), a extracellular fragment of the GH receptor. REGULATION OF GH SECRETION GRH stimulates whereas GRIH or somatostatin from the hypothalamus inhibits GH secretion from the anterior pituitary. Hypoglycemia stimulates GH release, whereas hyperglycemia inhibits release. GH release also occurs in response to certain amino acids (L-arginine). Estrogen promotes an increase in number GH mRNA GH production is influenced by many factors such as sleep, stress (pain, cold, surgery), exercise, food intake etc.

MECHANISM OF ACTION OF GH The GH have 2 binding sites, each site binds single-chain membrane-bound protein receptor in target cells. That is followed by dimerization of the receptors and activation of a tyrosine kinase enzyme known as Janus kinase 2 (Jak-2) (attached to the intracellular domains of the dimer). Two Jak-2 kinases phosphorylate each other and tyrosine residues of the receptor. This leads to a cascade of phosphorylation reactions result in activation of; – Proteins transcription (STAT), – Mitogen-activated protein (MAP) kinase – Phosphoinositide-3-kinase (PI kinase) Overall result is on linear growth and metabolism of carbohydrate, lipid, mineral, protein.

ACTIONS OF GH EFFECT ON PROTEIN METABOLISM: -promotes the transport and incorporation of amino acids (aa)s into skeletal muscle, cardiac muscle, adipose tissue, and liver. - exerts a “protein-sparing” effect by mobilizing the body’s energy substrates, namely FFAs, in tissues where it stimulates protein synthesis. With overall positive nitrogen balance that leads to increase in body weight. EFFECT ON CARBOHYDRATE (CHO) METABOLISM: -GH antagonize insulin and causes hyperglycemia by: increases gluconeogenesis, decreases tissue glucose uptake and utilization, reduce glycolysis. EFFECT ON LIPID METABOLISM: -GH promotes lipolysis in the adipose tissue and increases the circulatory levels of FFAs and their oxidation. EFFECT ON MINERAL METABOLISM: -GH promotes bone mineralization. -GH through IGF-1 causes +ve balance of mineral Ca, phosphate and Mg. It also promotes retention of Na, K and chloride ion in the body.

INSULIN-LIKE GROWTH FACTORS (IGFS) GH-dependent polypeptide hormones that promote cell replication in most mesenchymally derived tissues. Responsible for the growth-promoting effects of GH, does not mediate the effects of GH on intermediary metabolism. Two IGFs, I and II, resemble proinsulin in structure. Exert insulin-like biological effects. Six serum proteins, produced mainly in the liver, bind IGFs in the circulation (IGF-binding proteins -IGFBPs 1 through 6). IGFs are released as they are produced (no storage).

PROLACTIN (PRL) Contains 199 (aa)s and 3 intramolecular disulfide bridges. Prolactin receptor is similar in type and size to GH receptor. Also known as lactogenic hormone, luteotropic hormone, mammotropin. PRL produced by lactotropes (acidophilic), its release increases during pregnancy / lactation, enhanced by estrogens and oxytocin. In mammary tissue, it stimulates the synthesis of milk-specific proteins (casein, lactalbumin, and lactoglobulin) by increasing the levels of several enzymes involved in carbohydrate (HMP shunt) and lipid (biosynthesis) metabolism and stimulates lactose production in mammary glands.

GLYCOPROTEIN HORMONES A family of highly glycosylated peptides (FSH, LH, hCG & TSH). Each of the glycoprotein hormones is an (α:β) heterodimer, with the α-subunit being identical in all members of the family. The biological activity of the hormone is determined by the β-subunit, which is not active in the absence of the α-subunit. All members of the glycoprotein family transduce their intracellular effects via their respective receptors associated with G-protein via cAMP.

PRO-OPIOMELANOCORTIN PEPTIDE GROUP (POMC) POMC gene is expressed in both the anterior and intermediate lobes of the pituitary gland. Protein product of the POMC gene is a 285 (aa)s precursor, it is differential processed to yield at least 8 peptides, based on site of synthesis and stimulus Processing by: glycosylations, acetylations, cleavage by tissues specific enzymes & etc.. POMC consists of 3 peptide groups: 1.ACTH; can give α-MSH and corticotropin like intermediate lobe peptide (CLIP). 2.β-Lipotropin; can produce γ-LPH, β-MSH. 3.N-terminal peptide, forms γ-MSH.

ADRENOCORTICOTROPIC HORMONE (ACTH) The ACTH acts on the cells of the zona fasciculate/reticularis of the adrenal cortex, simulate synthesis and release of cortisol and adrenal androgens. The mechanism of action of ACTH involves activation of adenylate cyclase, elevation of cAMP, and increased PKA activity of adrenal cortex tissue. The main effect is increase the steroid hormone synthesis. ACTH is a polypeptide of 39 residues, the first 24 of which are required for corticotropic activity. Because it contains the MSH sequence in residues 6–9 (His– Phe–Arg–Trp), ACTH has intrinsic melanocyte-stimulating activity (cause skin darkening if present in high concentrations).

β-LIPOTROPIN and MELANOCYTE- STIMULATING HORMONE Βeta-Lipotropin (beta-LPH)is found only in the pituitary and not in other tissues since it is rapidly degraded. It promotes lipolysis and increases mobilization of fatty acids. The beta-LPH is a precursor of beta-endorphin & enkephalins, which produce opiate-like effects on CNS, hence called opoid- peptides. MSH: 3 types of MSH (α, β and  ) are present in precursor POMC molecule of which α -MSH is important only in human (skin pigment). MSH binds melanocortin-1 receptor on melanocytes membrane, stimulates melanin production, then packaged in melanosomes.