Endocrine System II Lecture 16 PDF

ENDOCRINE SYSTEM II PRINCIPLES OF HORMONE ACTION ANSC 3080 G. Bedecarrats  Learning Objectives  Describe and understand the mechanisms of hormone action Describe the mode of action of lipophilic hormones and their nuclear hormone receptors Describe the mode of...

ENDOCRINE SYSTEM II PRINCIPLES OF HORMONE ACTION ANSC 3080 G. Bedecarrats  Learning Objectives  Describe and understand the mechanisms of hormone action Describe the mode of action of lipophilic hormones and their nuclear hormone receptors Describe the mode of action of water-soluble hormones and cell their surface receptors  Hormone Actions  Hormones are secreted in the blood and act at a distance from the release site. They need to travel in the blood (solubility), survive long enough, and be active at the target site  Hormones trigger specific actions in specific target cells. Requirement for specific recognition: RECEPTORS  Principle of action is based on hormone biochemical structure and properties. Receptors = Key to Hormone Action  Hormones binding to receptors initiate the effects  Lipophilic hormones Diffuse out of producing cells Circulate mainly bound to carriers in the blood Diffuse in target cells  intracellular receptors  Water-soluble hormones Secreted (exocytosis) Circulate free in the blood Stay out of target cell  surface (extracellular) receptor  Lipophilic Hormones (Steroid and Thyroid Hormones)  Insoluble in water, circulate associated to carrier proteins  Some carriers are specific = globulins (CBG, DBG, SHBG, TBG)  Some are non-specific (albumin and prealbumin)  Carrier proteins are big: keep hormones in vessel, prevent hormone degradation Hormone + Carrier  Hormone-Carrier  A small portion remains free and diffuses to the tissues  Free hormone is the active portion, but is also susceptible to degradation  Free form (active) is involved in feedback loops  Carrier: serves as “hormone reservoir” “hormone buffer” “hormone protection” Lipophilic Hormones Carrier Proteins Play a Major Role in Controling Lipophilic Hormones a) Base line levels b) Binding protein doubles: free hormone decreases , bound hormone . c) Lack of negative feedback: hormone secretion , both free and bound forms. d) Concentration of free hormone back to normal. Hormone secretion stops. However total hormonal concentration still elevated.  Action of Lipophilic Hormones  Free hormones diffuse through plasma membrane of target  Binds to a specific intracellular receptor = nuclear hormone receptor (NHR)  Hormone-receptor complex translocates to the nucleus and binds to specific DNA sequence (response element)  Stimulates gene expression  de novo protein synthesis  NHR are considered “transcription factors”  In the case of Thyroid hormones, NHR is already in the nucleus and hormones diffuse all the way there Action of Lipophilic Hormones Steroids Thyroid hormones  Nuclear Hormone Receptor  Ligand binding domain: binds hormone  DNA binding domain: binds DNA of target gene  Activation domain: stimulates gene transcription  Orphan receptor: NHR cloned but no ligand found (yet)  Action of Lipophilic Hormones  The action affects the synthesis of new proteins by gene regulation = slow-acting hormones  F.Y.I.: NHR also have non-genomic effect  Case of Thyroid hormones: Major circulating form is T4 (thyroxine). When it enters target cell  converted to T3 (tri-iodo- thyronine) the cellular active form. Binds to its specific receptor after entering the nucleus.  After acting, hormones dissociate from the receptor, can be partly degraded in target cells (thyroid hormones), then goes back to circulation and is degraded in the liver.  Water-soluble Hormones (Proteins and catecholamines)  Cannot pass the phospholipid membrane barrier  Secreted in vesicles by exocytosis, circulate free Exception: IGF-1 circulates bound to carrier  Bind to specific receptors on surface of target cell  Do not enter the cell to act  need second messengers  Receptor is the key mediator between hormone and second messengers  CELL SURFACE RECEPTOR Cell membrane hormone receptor Intracellular signaling ECD TMD ICD Hydrophobic region “hiding” in the membrane’s phospholipids (transmembrane domain: TMD) Hydrophilic regions located outside (extracellular domain: ECD) and inside (intracellular domain: ICD) the cell Hormone-Receptor  activation of intracellular second messengers  Major Types of Cell Surface Receptors Receptor type Messenger G-proteins coupled Adenylate Cyclase; cAMP Phospholipase C; Ca2+ Tyrosine kinase Auto-phosphorylation Interleukin/cytokine family JAK-STAT Serine kinase (TGFß) SMADs family  G-proteins coupled Receptors a) Hormone binds to a receptor coupled to G-proteins ( subunits) b) Change in receptor conformation = exchange of GDP with GTP on G SU c) G SU dissociates from  SUs and activates a membrane protein d) Activated membrane protein stimulates a cascade of second messengers e) The second messengers elicit the biological response in the cell There are 3 major types of G-proteins differing by their  SU (sub unit)  Adenylate cyclase-cAMP-PKA pathway  Receptor coupled to G-proteins s or i (-adrenergic, LH)  Membrane associated enzyme is Adenylate Cyclase (AC)  Gs stimulates AC; Gi inhibits AC  AC hydrolyses ATP into cyclic AMP (cAMP)  FOR YOUR INFORMATION 1. Adenylate Cyclase increases the intracellular levels of cAMP 2. cAMP stimulates protein kinase A by phosphorylation 3. Activated PKA Phosphorylates intracellular proteins Stimulates gene transcription 4. Biological action 5. cAMP degraded by phosphodiesterase, PKA inactivated by a phosphoprotein phosphatase FOR YOUR INFORMATION  Phospholipase C pathway (PLC)  Receptor coupled to G-protein q  Membrane associated protein is PLC  Gq stimulates PLC  FYI: PLC hydrolyses phosphoinositol diphosphate (PIP2) into inositol triphosphate (IP3) and diacylglycerol (DAG)  Results is the activation of:  Ca2+/calmodulin = release of Ca2+  Protein kinase C = phosphorylation cascade  Biological action  FOR YOUR INFORMATION Gq Ca2+  Tyrosine Kinase Receptor  Binding of hormone  activation of receptor = autophosphorylation  becomes a kinase  phosphorylates tyrosines on target proteins  No need for second messenger like G-coupled proteins, directly phosphorylates target proteins  Consists of 3 domains: Transmembrane domain Extracellular domain for ligand recognition Cytoplasmic domain with autophosphorylation site that transmits regulatory signals and contains ATP binding sites Tyrosine Kinase Receptor (insulin)  Cytokine Receptors  Receptors for cytokines (GH, prolactin, erythropoietin, interferons and interleukins) do not have intrinsic kinase activity  Receptor exists as monomer. Binding of hormone causes dimerization and binding of JAK tyrosine kinase which phosphorylates the receptor  Phosphotyrosines act as docking sites for intracellular signalling molecules – STATs (signal transducers and activators of transcription) which activate various genes ECD TMD Y Y P-Y Y-P P-Y Y-P ICD STAT Y Y P-Y Y-P P-Y Y-P JAK PP PP transcription  Receptor Serine Kinase  TGF family (activin, inhibin, MIS) mainly involved in control of cell proliferation and differentiation  Binding of hormone results in heterodimer formation: Receptor I + Receptor II.  RII is specific to the hormone. After binding, H-RII complex recruits RI  Same RI can be recruited by different H-RII complexes  Serine residues on RI get phosphorylated by RII  Activated receptor phosphorylates Smads proteins that will dimerize, translocate in the nucleus and modulate gene transcription (J. Teixeira et al., 2001, Endocr. Rev.)  Action of Cell Surface Receptors  Cascade of intracellular messengers amplifies the signal several thousand times  Specific effects on target cells depend on the type and amount of messenger activated: Immediate effects: enzyme activation, exocytosis Slow effect: stimulation of gene transcription, de novo protein synthesis  After signalling, receptor-hormone complex is internalised: Fuses to lysosome and is degraded Dissociates and receptor is recycled to the cell surface FYI: Cross-talk, case of the GnRHR Ruf F, Fink MY, Sealfon SC (2003) Front Neuroendocrinol. 24:181-99