Week 3 - L4

Introduction to Intracellular Pathways and Receptor Tyrosine Kinases

  • The video covers the intracellular pathways activated by receptor tyrosine kinases (RTKs).
  • Discussion on targeting RTKs to treat cancer.
  • Introduction to tyrosine kinase-associated receptors, a subgroup within enzyme-linked receptors.

Overview of the Ras Family and MAP Kinase Pathway

  • The Ras family of GTPases activates the MAP kinase pathway.
  • MAP = Mitogen Activated Protein, which consists of a series of kinases activated by RTKs and Ras.
  • Illustration of the MAP kinase pathway activation using insulin.

Activation Mechanisms of RTKs

  • The signaling molecule binds to the receptor tyrosine kinase.
  • Activation occurs through dimerization and autophosphorylation.
  • Results in phosphorylation of multiple tyrosine residues within the intracellular region.
  • Phosphorylated tyrosine residues are binding sites for the SH2 domain in the adaptor protein GRAB2.
  • GRAB2 contains two SH3 domains, binding proline-rich regions in the guanine exchange factor, SoS.
Role of SoS
  • SoS exchanges GDP bound to inactive Ras for GTP, activating Ras.
  • Active Ras propagates the signal and activates the MAP kinase pathway.

The MAP Kinase Cascade

  • The MAP kinase cascade involves three key kinases: RAF, MEK, and ERK.
  • Kinase cascade: one molecule activates the next sequentially.
  • Each kinase phosphorylates its downstream targets:
    • RAF activates MEK via phosphorylation.
    • MEK activates ERK also via phosphorylation.

Functional Effects of ERK Activation

  • ERK targets relate to transcription, gene expression, and changes in protein activity.
  • ERK is also referred to as MAP kinase; thus, it is also called MAPK.
  • MEK is classified as MAP kinase kinase (MAPKK), while RAF is MAP kinase kinase kinase (MAPKKK).

Specific Example: Insulin and MAP Kinase Pathway

  • Insulin receptor directly phosphorylates insulin receptor substrate 1 (IRS-1).
  • IRS-1 binds GRAB2, leading to activation of SoS and subsequently Ras.
  • Ras activates RAF, then MEK, and later ERK.
  • ERK enters the nucleus, activating transcription factors, which promote transcription and protein synthesis.
  • Primary outcome: stimulation of cell division (mitogenic activity).

Introduction to the PI3K-AKT Pathway

  • PI3K (phosphoinositide 3-kinase) pathway promotes cell survival by inhibiting apoptosis.
  • PI3K phosphorylates inositol phospholipids in the plasma membrane.
  • Phosphorylation of membrane phospholipids provides docking sites for signaling proteins like AKT (also known as PKB).

Activation of AKT

  • Activated by docking with phosphoinositide and phosphorylation by PDK1 and mTOR.
  • AKT promotes cell growth, survival, and inhibition of apoptosis.
Role of Phosphoinositides
  • Phosphoinositide bisphosphate (PIP2) phosphorylated to phosphoinositide triphosphate (PIP3).
  • PIP3 provides docking sites for PDK1 and AKT.
  • After phosphorylation, AKT dissociates, remaining active as it moves through the cytosol to other targets.

Specific Example: Insulin and PI3K Pathway

  • Insulin activates IRS-1, leading to PI3K binding.
  • PI3K converts PIP2 to PIP3, activating AKT through PDK phosphorylation.
  • AKT phosphorylates GSK3 (glycogen synthase kinase 3), leading to glycogen synthesis activation.
  • Furthermore, AKT stimulates GLUT4 transporter movement to the cell membrane for glucose uptake (not in liver cells).

Inhibition of Apoptosis by AKT

  • AKT inhibits apoptosis by inactivating proapoptotic factor BAD while activating antiapoptotic factor BCL2.
  • Balance between pro and antiapoptotic proteins determines cell fate.

Dual Pathway Activation by Receptor Tyrosine Kinases

  • RTKs can phosphorylate various residues, allowing multiple signaling pathways.
  • Simultaneous activation of MAPK and PI3K pathways results in cell growth and survival.

Connection Between RTKs and Cancer

  • Overactive or overexpressed RTKs contribute to cancer through uncontrolled growth and proliferation.
  • Proto-oncogenes can mutate into oncogenes, leading to malignant transformations.
  • Targeted therapies can inhibit RTK pathways or activate them if they are not functional.

Example of Uncontrolled Activation

  • Example of a truncated receptor leading to continuous activation without the signaling molecule.
  • This situation can lead to tumorigenesis.

Therapeutic Approaches Targeting RTKs

  • Various drugs exist to inhibit specific kinases within the MAPK pathway.
  • Examples:
    • Trastuzumab (Herceptin) targets HER2 receptor, which is overexpressed in some breast cancers.
    • Prevents downstream activation of pathways promoting cell growth and proliferation.

Overview of Tyrosine Kinase Associated Receptors

  • Tyrosine kinase associated receptors do not have intrinsic kinase activity but rely on associated kinases.
  • Examples include the cytokine receptors.
  • JAK family (Janus kinases) and SRC family are kinases associated with these receptors.

Mechanism of Cytokine Receptor Activation

  • Binding of a cytokine leads to receptor dimerization and cross-phosphorylation by JAKs.
  • Receptor phosphorylation provides binding sites for STAT proteins (transcription factors).
  • Activated STATs dimerize and translocate to the nucleus to regulate gene transcription.

Summary of Key Signaling Pathways

  • MAP kinase pathway: cell division & proliferation.
  • PI3K-AKT pathway: cell growth, survival, and inhibition of apoptosis.
  • Activation of these pathways can lead to various cellular responses depending on the context of the RTK activated.

Conclusion

  • Expounding on receptors, their mechanisms, pathways influenced by signaling, and their implications in diseases like cancer.
  • Importance of understanding these pathways for developing therapeutic strategies against cancer.
  • Upcoming content includes workshop questions and consolidation exercises to assess comprehension of the material discussed.