BL1 Basics of Hormonal Action

• Hormones


  Simple biochemical messengers, Natural organic substances that regulate growth and other functions in organisms, Classified based on chemical composition (peptide hormones, steroid hormones) and target organ (reproductive hormones, autocrine and paracrine)

  
• Characteristics of Hormones

  • Exerts effect in biocatalytic amounts, Amount secreted is not at uniform rate, Turnover is varied and usually rapid, Exert multiple actions, Exhibits high degree of specificity, Response varies according to the target cell
• Precursor

  Small amount required for catalytic action, Eliminated immediately after done, New hormone comes
• Hormone action

  Applied in various regions, e.g. estrogen metabolism
• Synthesis of Steroid Hormones

  1. Progesterone (Progestin)

  2. Cortisol (glucocorticoids)

  3. Aldosterone (Mineralocorticoids)

  4. Dehydroepiandrosterone (DHEA) and Dehydroepiandrosterone sulfate (DHEA-S)

  5. Testosterone (Androgen)

  6. Estradiol (Oestrogen)

  
• Biological effects of steroid hormones

  • Progesterone = prepare uterus lining for implantation of ovum and mammary glands for lactation
  • Cortisol (glucocorticoids) = Promoted gluconeogenesis
  • Aldosterone = Helps regulate body's water and electrolyte balance
  • DHEA & DHEA-S = source for testosterone & estrogen in peripheral tissue
  • testosterone (androgen) = male secondary sex characteristics
  • Estradiol (Estrogen) = female secondary sex characteristics

  
• Synthesis of Thyroid Hormones

  1. Uptake of iodine

  2. Oxidation of iodine (thyroperoxidase, NADPH)

  3. Iodination (Thyroglobulin)

  4. Coupling

  5. Storage

  6. Utilisation

  7. Hydrolysis (Proteases)

  8. Release (deiodinase to form T3)

  9. Salvaging of iodine

  10. Transport of thyroid hormones

  11. Catabolism of thyroid hormones

  
• Metabolic effects of thyroid hormones

  • Calorigenic effect or thermogenesis
  • Basal metabolic rate (BMR) is increased
  • Stimulation of RNA synthesis and protein synthesis
  • Loss of body weight as a feature of hyperthyroidism
  • Increased gluconeogenesis and carbohydrate oxidation
  • Increased fatty acid metabolism and cholesterol degradation
• General principles of hormone action

  Trophic hormones, Synergism, Permissiveness, Antagonism
• Tissue responses to hormone action

  • Determined by the presence of specific receptors, Hormones can activate multiple receptor isoforms

  
• Plasma Membrane Receptors


  Present in or on the surface of the cell membrane, Eg. Ion channel linked receptor, G-protein coupled receptor, Enzyme linked receptor
• Classes of Protein Receptors

  • Class I: G-protein-coupled receptors, Class II: Receptors that are also enzymes, Class III: Receptors associated with enzymes, Class IV: Receptors coupled to ion channels
• Cyclic AMP mediated hormone action

  cAMP level in the cell is regulated by its rate of production by adenylate cyclase (AC) and hydrolysis to 5'-AMP by Phosphodiesterase (PDE)
• Membrane receptor signal transduction

  Receptor is attached to G-protein, which has α, β, and γ subunits, When hormone attaches, α subunit detaches, GTP is bound, Gα-GTP activates adenyl cyclase, cAMP is generated, Protein kinase contains two catalytic units and two regulatory units, cAMP binds with regulatory units, Catalytic units are free, Kinase is now active, Active protein kinase phosphorylates enzyme proteins
• Adenylate cyclase (AC)


  Catalysis of cAMP from ATP
• Phosphodiesterase (PDE)


  Hydrolysis of cAMP to 5'-AMP
• Membrane receptor signal transduction

  Receptor-G-protein-mediated signal transduction
• PTX (pertussis toxin)

  • Blocks the catalysis of GTP exchange by the receptor
• CTX (cholera toxin)

  • Inhibitor of GTPase
• Receptor-G-protein-mediated signal transduction


  1. Receptor is attached to G-protein, which has α, β, and γ subunits. It is bound with GDP, and is inactive. These are membrane-bound.

  2. When hormone attaches, α subunit detaches, GTP is bound; Gα-GTP activates adenyl cyclase, cAMP is generated.

  3. Protein kinase contains two catalytic units; but these are attached to two regulatory units, and are inactive.

  4. cAMP binds with regulatory units; now catalytic units are free; kinase is now active.

  5. Active protein kinase phosphorylates enzyme proteins.

  
• 1. receptor
  2. G-protein, AC-adenyl cyclase, H-hormone, C-catalytic unit, R-regulatory unit, cAMP-cyclic AMP
• Calcium transport systems

  • Voltage gated calcium channels
  • Sodium/calcium antiport transporter
  • Calcium transporting ATPase
• Calcium transporting ATPase transporter

  Accumulates calcium within the lumen of ER (sarcoplasmic reticulum) in muscle. These calcium ions can be released into the cytoplasm by an inositol triphosphate (IP3) gated calcium channel or by a ligand gated calcium release channel (ryanodine receptor).
• Hormones can increase the cytosolic calcium level

  By altering the permeability of the membrane<|>The action of Ca-H+-ATPase pump which extrudes calcium in exchange for H+<|>By releasing the intracellular calcium stores
• Calmodulin


  The calcium dependent regulatory protein within the cell has four calcium binding sites
• Signal transduction mechanism for the oxytocin receptor in the myometrium

  1. a classic 7-transmembrane domain receptor linked through a G protein to phospholipase C

  2. After receptor stimulation with oxytocin (OT), the α-subunit of the heterotrimeric G protein hydrolyses GTP to GDP and releases the βγ-subunit to stimulate phospholipase C

  3. This enzyme converts phosphatidyl inositides to diacylglycerol and inositol-1,4,5-trisphosphate (IP3)

  4. Diacylglycerol stimulates protein kinase C activity causing phosphorylation of substrates which will characterize the response of the specific cell type

  5. The IP 3 stimulates flux of Ca 2+ into the cytoplasm through calcium channels, principally from the sarcoplasmic reticulum but also from the extracellular space

  6. The increased Ca 2+ will combine with calmodulin to stimulate myosin light chain kinase (MLCK) to produce myometrial contraction

  7. A characteristic response would be synthesis and release of prostaglandins

  
• Insulin receptor


  Tyrosine kinase receptor which is a heterotetrameric glycoprotein, consisting of 2 extracellular α and 2 transmembrane β subunits linked together by disulfide bonds<|>The α subunits carry insulin binding sites, while β subunits have tyrosine kinase activity, involved in intracellular signaling
• Cyclic guanosine monophosphate (cGMP)


  It is derived from GTP (guanosine triphosphate)<|>There are two major pathways of its synthesis, one via a membrane-bound guanylyl cyclase bound to a natriuretic peptide receptor, and the other a soluble guanylyl cyclase which is activated by nitric oxide<|>Like cyclic AMP, cGMP is degraded by phosphodiesterases. Some phosphodiesterases only affect cGMP (eg. PDE-5A, the target of sildenafil) whereas others (PDE-2 and PDE-3) can hydrolyse both cAMP and cGMP<|>Visual cycle, Smooth muscle relaxation and vasodilation are examples of this pathway

  
• Inhibitors of cAMP- and cGMP- dependent phosphodiesterase activity

  • Increases cAMP levels by inhibiting PDE
  • Caffeine, theophylline, and theobromine are methylxanthines derived from coffee, tea, and cocoa, respectively
  • Theophylline: the most potent of the three
  • Adenylate is always active

  
• Phosphoprotein phosphatases

  Dephosphorylate the phosphoprotein → returned to basal level

  
• Insulin activates PDE


  Decreasing the cellular level of cAMP
• Nuclear hormone receptors

  Ligand-regulated transcription factor that control gene expression by binding to target genes usually in the region near their promoters<|>Class I: steroid hormone. Are present in either the cytosol or the nucleus. Ligand binding promotes dissociation of certain proteins and formation of receptor homodimers that bind to specific DNA element (HREs)<|>Class II: thyroid hormone, retinoid, vitamin D, PPAR. Receptors already present in the nucleus in the unliganded state. They are commonly active in the absence of hormone

  
• Cytoplasmic Receptors (Intracellular receptors)

  Present in cell cytoplasm<|>Steroid hormone receptor proteins have a molecular weight of about 80-100 KD. Each monomer binds to a single steroid molecule at a hydrophobic site, but on binding to genes they dimerize<|>The steroid hormone diffuses through the plasma membrane<|>Steroid hormone binds an intracellular receptor (in the cytoplasm). The hormone receptor (HR) complex is formed in the cytoplasm<|>The HR complex is translocated to the nucleus<|>In the nucleus, the HR binds to the hormone response elements (HRE) or steroid response elements (SRE)<|>Binding initiates transcription (DNA to mRNA)<|>SRE acts as an enhancer element and when stimulated by the hormone, increases transcriptional activity<|>The newly formed mRNA is translated to a specific protein, which brings about the metabolic effects

  
• Hormone-responsive elements (HRE)

  Estrogen-response element (ERE)<|>Glucocorticoid-responsive element (GRE)<|>Thyroid hormone-responsive element (T3RE)<|>Vitamin D-responsive element (VDRE)

  
• Mechanism of hormone action

  Based on the solubility in water, there are two types<|>Hydrophilic hormones: Water-soluble. Reaches the target cell through blood. Primarily acts through second messenger system. Eg. Catecholamines, peptide hormones<|>Lipophilic hormones: Poorly soluble in water. Binds to plasma protein to reach the target cell. Activates genes on binding with receptors in the nucleus. Eg. Steroids and Thyroid hormones

  
• Mechanism of action of water-soluble hormones

  1. Fixed-receptor hypothesis

  2. Water-soluble hormones diffuses freely in blood

  3. Binds to the receptor in the plasma membrane

  4. Hormone receptor complex activates G-protein

  5. Activates adenylate cyclase

  6. ATP converted to cAMP

  7. cAMP activates one or more protein kinases

  8. Protein kinases phosphorylates one or more cellular proteins

  9. Phosphorylated proteins produces the physiological response

  10. Phosphodiesterase inactivates cAMP

  
• Mechanism of action of lipid-soluble hormones

  1. Mobile-receptor hypothesis

  2. Binds to specific receptor in the cell membrane

  3. Forms hormone cell receptor complex

  4. Diffuses to the nucleus

  5. Receptor released for reuse

  6. Released steroid activates specific gene to produce mRNA

  7. mRNA moves to the cytoplasm and initiates enzyme synthesis

  8. Hormone is not attached to the plasma membrane but moves freely in nucleus

  
• Receptor regulation

  Negative or "down" regulation: decrease numbers of receptors<|>Positive or "up" regulation: increase numbers of receptors
• Interaction of insulin and its receptor

  Heterodimer: α and β subunits
• Regulation of Hormone Secretion

  1. Nervous: Direct innervation via autonomic nervous system, Neurosecretory neurons control of adenohypophysis, Neurosecretory neurons control of neurohypophysis

  2. Hormonal control: Negative feedback mechanisms used to control the release of various hormones

  3. Humoral control