Tyrosine kinase receptors

RTK Fundamentals - The Dimer's Dance

• Structure: Monomeric transmembrane proteins with an extracellular ligand-binding domain and an intracellular tyrosine kinase domain.
• Activation: Triggered by the binding of a specific ligand (e.g., insulin, EGF, PDGF).
• Dimerization: Ligand binding causes two receptor monomers to form a dimer.
• Autophosphorylation: The kinase domain of one monomer phosphorylates tyrosine residues on the tail of the other, and vice-versa (cross-phosphorylation).
• Docking & Signaling: Phosphorylated tyrosines serve as docking sites for signaling proteins with SH2 domains.

⭐ Many oncogenes, like HER2, are RTKs. Overexpression or mutation can lead to ligand-independent dimerization and constant activation, driving uncontrolled cell growth.

MAP Kinase Pathway - Growth Signal Freeway

• Initiation: Growth factor (e.g., EGF, FGF) binds to its specific Receptor Tyrosine Kinase (RTK), triggering receptor dimerization and autophosphorylation.
• Transduction: The adaptor protein GRB2 and Guanine nucleotide Exchange Factor SOS are recruited to the phosphorylated RTK.
• Activation: SOS promotes the exchange of GDP for GTP on Ras, a small G-protein. This activates Ras.
• Cascade: Activated Ras-GTP initiates a phosphorylation cascade.

⭐ Ras is a key proto-oncogene. Activating mutations, common in pancreatic, lung, and colon cancers, lock Ras in a GTP-bound (active) state, driving uncontrolled proliferation even without growth factors.

PI3K/Akt Pathway - Survival & Growth Pro

• Activation: Ligand binding to RTK recruits PI3K (Phosphoinositide 3-kinase).
• Mechanism: PI3K phosphorylates membrane lipid PIP2 to form PIP3.
  • PIP3 acts as a docking site, leading to the activation of Akt (Protein Kinase B).
• Downstream Effects: Akt activation promotes:
  • Cell Survival: ↓ Apoptosis by inhibiting proteins like Bad and FOXO.
  • Cell Growth: Activates mTOR, boosting protein synthesis.
  • Metabolism: ↑ Glucose uptake (GLUT4) & glycogen synthesis.

⭐ PTEN (Phosphatase and Tensin Homolog) is a key tumor suppressor that dephosphorylates PIP3, turning off this pro-survival pathway. Loss-of-function mutations are common in cancers like endometrial, prostate, and glioblastoma.

Insulin Receptor - The Glucose Gatekeeper

• Structure: Pre-formed dimer with intrinsic tyrosine kinase activity (RTK).
• Mechanism: Insulin binding → autophosphorylation of tyrosine residues.
  • Recruits Insulin Receptor Substrate (IRS-1).
• Dual Pathways Activated by IRS-1:
  • PI3K Pathway (Metabolic): IRS-1 → PI3K → PIP3 → Akt (PKB) activation.
    • Promotes translocation of GLUT4 transporters to the cell surface in muscle & adipose tissue.
    • ↑ Glucose uptake.
  • MAPK Pathway (Mitogenic): IRS-1 → GRB2 → Ras → MAPK activation.
    • Regulates gene expression for cell growth & proliferation.

⭐ High-Yield: Insulin signaling has two arms. The PI3K/Akt pathway controls metabolic actions (glucose uptake), while the RAS/MAPK pathway controls mitogenic effects (cell growth). This separation is key in understanding insulin resistance pathologies.

High‑Yield Points - ⚡ Biggest Takeaways

• Ligand binding triggers receptor dimerization and subsequent autophosphorylation of tyrosine residues.
• Phosphorylated tyrosines serve as docking sites for signaling proteins containing SH2 domains.
• Key downstream cascades are the MAP kinase pathway (RAS-RAF-MEK-ERK) and the PI3K/AKT pathway.
• The insulin receptor is a classic example, though it exists as a pre-formed dimer.
• Primarily bind growth factors like EGF, PDGF, and FGF, regulating cell growth and proliferation.
• Gain-of-function mutations are a hallmark of many cancers, making them key oncologic drug targets.