Veterinary Physiology-III

Mechanism of protein (Hydrophilic) hormone action

MECHANISM OF PROTEIN HORMONE ACTION

• The first step in protein or peptide hormone action, the hormone functions as the first messenger and binds with specific cell surface receptor sites in a target organ; the receptors on the target cell selectively "bind" or"trap" or take up a particular hormone on the cell membrane of target organ.
• In the second step the hormone forms a receptor hormone complex which induces receptor activation. The activated receptor initiates a chain of intracellular events
• The membrane receptors can be grouped into the following types based on how the protein hormone signal is transferred from the receptor into the cell
  • receptors that activate G-proteins
  • receptors functioning as ion channels
  • receptors that activate protein kinases
  • receptors that activate guanylyl cyclase

Receptors that activate G-proteins

• The receptors are coupled to G-proteins (consists of α-,β-, and γ- subunits) which are membrane proteins and which can bind with GTP or GDP. G-protein coupled receptors are involved in signal transduction of protein/ peptide hormones, acetylcholine, serotonin, some receptors of epinephrine and norepinephrine.
• Binding of the hormone to the receptor brings about conformational change in receptor. The activated receptor can now bind with G-protein which undergo conformational change and become activated. The activated G-protein releases GDP from α-subunit and binds with GTP. This binding with GTP dissociates α-subunit from β-γ subunits of G-protein
• The released α-subunit functions (1) by activating or inhibiting many intracellular enzymes or (2) by opening ion channels and modifying ionic movement across the membrane
• Generally, the G-protein activates an enzyme that increases or decreases formation of intracellular regulatory molecules called second messenger.
• The second messenger activates enzymes – protein kinases and phosphorylates cellular enzymes by transferring phosphate from ATP to proteins
• The phosphorylated enzymes increase or decrease intracellular enzyme activity
• Some of the important second messengers formed by the G-protein coupled receptors are
  • cyclic adenosine monophosphate (cAMP) 
  • diacylglycerol (DAG) 
  • inositol triphosphate (IP3) 
  • eicosanoids
• Adenyl cyclase is a membrane bound enzyme whichconverts cytoplasmic ATP to cylic 3, 5-adenosine monophosphate (cAMP).Adenyl cyclase is activated by activated-G-protein.
• The second messenger, cAMP is released into the cytoplasm where it activates cAMP-dependent protein kinase called protein kinase-A. (phosphodiesterase, an intracellular enzyme inactivates cAMP to AMP)
• Intracellular accumulation of cAMP
  • modifies the enzyme activity - phosphorylation ofprotein kinase which is responsible for biological response 
  • alters membrane permeability or 
  • may stimulate hormone release.
• The protein kinase A phosphorylates key intracellular proteins and alters their activity and produces various biological effects ofthe hormone.
• DAG:The cell membrane enzyme phospholipase-C (PLC) acts on a membrane phospholipid called phosphotidylinositol bisphosphate (PIP3) and cleaves it into AG andIP3. PLC is activated by G-protein.
• The membrane-bound DAG binds to a protein kinase called protein kinase C which phosphorylates intracellular proteins similar to protein kinase A and brings about biological effects
• The IP3 formed along with DAG enters the cytosol, opens Ca 2+ channels in the smooth endoplasmic reticulum (important Ca 2+ storage place) and the Ca 2+ diffuses out to increase the cytoplasmic Ca 2+ concentration.This Ca 2+ binds to calmodulin (calcium binding protein) and the calcium-calmodulin complex activates calcium-dependent protein kinase which then phosphorylates intracellular proteins to alter cellular functions.
• G-protein can also activate a membrane bound enzyme known as phospholipase A2 and the activated enzyme breaks down phosphotidylcholine and releases the long-chain fatty acid arachidonic acid from the membrane phospholipid.
• An intracellular enzyme cyclooxygenase converts the arachidonic acid to prostaglandins and thromboxanes while another intracellular enzyme lipoxygenase converts arachidonic acid to leukotrienes. The prostaglandins, thromboxanes and leukotrienes are the eicosanoids which are produced in most cell s of the body. The eicosanoids function as intracellular or extracellular signalling molecules and bring about autocrine / paracrine effects

Receptors functioning as ion channels

• This type of receptor function is important in neurons and muscles. Many of the ion channels on the cell membrane also act as receptors for the first messenger; binding of hormone will open or close the ion channels.
• Post-synaptic membrane of neurons and their target cells contain ligand-gated channels which bind with neurotransmitters. In some cases the first messenger activate G-protein second messenger which in turn regulate ion channel opening/ closing.
  

Receptors that activate tyrosine kinase

• Tyrosine-kinase receptor activation is important for the action insulin, growth factors like IGF-I, epidermal growth factor, and platelet-derived growth factor.
• The hormone binds to the membrane receptor and the activated receptor itself acquires tyrosine kinase activity. The tyrosine kinase phosphorylates the amino acid tyrosine present in intracellular proteins which then alters the functions of cytosolic enzymes producing the biological effects of the hormone.
• For hormones like PRL, GH, EPO and cytokines, the binding of the hormone with receptor activates intracellular tyrosine kinases which then phosphorylates tyrosine to bring about response

Receptors that activate guanylyl cyclase

• cGMP formed by the action of guanylyl cyclase on GTPact as second messenger similar to cAMP. cGMP activates cGMP-dependent protein kinase called protein kinase-G which phosphorylates cellular proteins and brings about biological effects
• Atrial natriuretic peptide produces its effect through cGMP
• In all cases, a small signal generated by a hormone binding to its receptor is amplified many thousand times (signal amplification) within the cell and brings about cascade of actions that change the cell's physiologic state.