Signalling in Metabolic Regulation Flashcards
What are examples of enzyme-linked receptors?
- Tyrosine kinase receptors
- Insulin
- Epidermal growth factor (EGF) or Platelet derived growth factors (PDGF)
- JAK/STAT
- Growth hormone
- Interferon
- Serine Threonine receptor kinase
- TGF-B
Describe the major features of enzyme-linked receptors
- Predominantly single transmembrane domain receptors
- Activation leads to activation of receptor kinases
- Activation leads to the ativation of multiple signalling pathways
What are the general functions of enzyme-linked receptors?
Involved in the regulation of:
- Cell growth
- Division
- Differentiation
- Survival
- Migration
Inappropriate receptor activity is associated with diseases including cancer
How are enzyme-linked receptors activated?
Signalling proteins cross-link receptor chains
What does phosphorylation do?
Phosphorylation forms docking domains:
- Dimerisation brings 2 receptor molecules allowing auto-phosphorylation of specific tyrosine residues
- Not all tyrosine residues can be phosphorylated
- Phospho-tyrosine together with surrounding amino acids are recognised by SH2 domains of other proteins allowing them to bind and undergo activation
What does phosphorylated tyrosine act as?
A docking site for SH2 domains on other proteins
Activation of tyrosine kinase receptors activates multiple pathways:
- PI3 kinase binds to the phosphorylated receptor through SH2 domains and upon activation phosphorylates PIP2 to form PIP3
- PIP3 binds both PKB and PKD-1. Activated PKD-1 phosphorylates and activates PKB which then dissociates and activates downstream factors
- GRB is an adaptor protein. Ras is important in the regulation of cellular activation and growth. Mutations in ras are common in many tumour
- Grb-2 – growth receptor bound protein 2
GEF – guanine nucleotide exchange factor
What do kinases do?
Add phosphates
What do phosphatases do?
Remove phosphates
How is enzyme-linked receptor signalling stopped?
Activation of phosphotases
- De-phosphorylation leads to inactivation
- Phosphatases are activated as a result of the receptor activation
- Therefore, the signalling process sets in motion events leads to signal termination
Describe the inactivation of Ras
- When Ras is bound to GDP it is inactive
- Guanine nucleotide exchange factor (GEF) allows for GTP to displace GDP and bind to RAS as well as Raf. This is active Ras.
- Ras has intrinsic GTPase activity
- GTPase activating protein (GAP) binds to the GTP on Ras and Raf, enhances the action of GTPase activity, causing the hydrolysis of GTP into GDP
- Mutations in RAS leading to loss of activity are found in 30% of human tumours.
Describe the action of PTEN
Phosphatase that dephosphorylates PIP3
How may mutations in key regulatory mechanisms lead to cancer?
-
- Loss of phosphatase activity
- Loss of GTPase activity
Describe the action of the insulin receptor
- Insulin binds to the alpha-chain of the receptor
- Phosphorylates B-chains, IRS receptor substrate binds, activats PI3-kinase
- This phosphorylates PIP2 into PIP3, which then activates PKD-1.
- PKD-1 phosphorylates and activates PKB, which can then induce glycogen synthesis and protein synthesis, hence regulating several functions in a single cell
Describe the Jak-STAT pathway that is activated by GH
- Binding of GH leads to conformational changes in receptor structure, permits a single GH molecule to bind 2 receptor molecules. Co-operative effect, intracellular domains of the receptor also brought together
- JAK2 is anchored to the membrane, has 2 kinase domains. Dimerisation leads to phosphorylation and activation of JAK
- Activation of Jak2 by cross phosphorylation leads to phosphorylation of other proteins
- Phosphotyrosine acts as docking site for proteins that contain SH2 domain, specific residues in STAT allows dimerisation via SH2 domain. This stable dimer has a strong affinity for specific DNA binding sites and regulates gene expression.
- STATs dock on phospho-tyrosines, JAKs phosphrylate them
- STATs dissociate from receptor and dimerise via SH2 domain, can enter nucleus as gene regulatory proteins, act at target gene to alter transcription
How does the activation of TGFB differ to GH receptor signalling?
- There are 2 different single transmembrane receptors, type I and type II. They have different functions, but both have serine kinase domains.
- TGF-B binds to type II. The activated type II receptor recruits the type I receptor resulting in phosphorylation of the serine residues and forming an activated receptor complex.
- The activated type I receptor then binds a receptor specific Smad protein which it phsosphorylates on a serine residue
- Phosphorylated Smad then dissociates from the receptor and complexes with Smad4. This complex then translocated to the nucleus
