02. Hormone Action

• fast response by a cell = receptor altering a protein, e.g., via phosphorylation
• slow response by a cell = when a receptor causes transcription of genes, causing synthesis of a new molecule
• hormone receptors must:

  • bind specifically to the hormone
  • bind to hormone with high affinity
  • receptor must be on specific tissues
  • receptor must be saturable and have a limited number of binding sites
  • must have reversible binding
  • must elicit biological response
• cell surface receptors are split into receptors linked to tyrosine kinase and receptors linked to G proteins
• receptors linked to tyrosine kinase can either be intrinsic tyrosine kinase and recruited tyrosine kinase
• tyrosine kinase transfers a phosphate group from ATP to a tyrosine residue. phosphorylation induces conformational changes.

  • can be intrinsic or recruited.
  • EGF and insulin receptors are both intrinsic TK receptors
• EGF receptor:

  1. EGF binds to hormone receptor causing conformational changes
  2. dimerisation occurs between the two receptors
  3. causes further conformational changes, exposing the kinase domain
  4. phosphorylation occurs, recruiting factors that activate Ras and active more complicated signalling pathways
• Ras is a small GTPase. this is inactive when bound to GDP and active when bound to GTP
• insulin receptor:

  • already a dimer
  • insulin binding creates intrinsic TK activity
  • PI3-kinase pathway is activated
• recruited TK activity

  • e.g. growth hormone receptors and prolactin
  • when hormone binds, conformational changes occur that expose domains that bind JAK (kinase)
  • phosphorylation occurs, allowing STAT to be recruited which gets phosphorylated and heads to the nucleus
  • binds to DNA and allows protein synthesis
• GPCRs:

  • 7 transmembrane domains
  • act via a second messenger
  • resting GPCR alpha subunit associated with GDP
  • hormone binds and causing conformational change allowing for exchange of GDP for GTP
  • alpha subunit released and activates 2nd messenger
• two mechanisms of GPCRs:

  1. alpha subunit activates a kinase to phosphorylate a protein to activate a second messenger.
  2. phospholipase C can be metabolised into IP3 which impacts the ER to increase Ca2+ concentration in the cytoplasm. calmodulin is calcium sensitive protein which gets activated and can activate kinases
• steroid hormones enter the cell and bind to internal receptors which bind to DNA and function as transcription factors

  • steroids have to be bound to a protein carrier in blood.
  • cannot enter fat with the carrier, so can only enter cells once they have left the protein carrier
• type I steroid receptors

  • homodimers that bind to corticosteroids
  • kept inactive by being complexed to heat shock proteins
  • ligand binding results in dissociation of HSPs and homodimerisation
  • translocate from cytoplasm to nucleus
  • receptor/DNA complex binds to hormone response elements
  • recruits other proteins which transcribes DNA downstream from HREs
• type II steroid receptors

  • heterodimers with RXR
  • always in nucleus bound to DNA in heterodimer complex, have protein that prevents them from transcribing
  • ligand binding to nuclear receptor causing dissociation of co-repressor and recruitment of co-activator proteins
• intracellular receptors generally have 3 domains: transactivation, DNA binding and hormone binding domains

  • once a hormone binds, within that domain AF-2 works with AF-1 to upregulate the transcription of DNA
• promoter = region of DNA where RNA polymerase attaches and initiates transcription
• hormone response elements where type I steroid receptors bind are usually found in the regulatory regions of the target gene
• monomeric HREs:

  • A/T HRE
• dimeric HREs:

  • Pal HRE - palindrome or inverted
  • DR HRE - direct repeat
  • IP HRE - inverted palindrome