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Learning Objectives

  • Understand the biochemical activation of Ras.
  • Identify the three Src Homology domains (SH1, SH2, SH3) and their functions.
  • Explain how ligand-receptor binding leads to diverse cell responses.
  • Discuss Ras-regulated and Wnt-regulated signaling pathways.
  • Examine the role these pathways play in cancer.

Overview of Content

  • Ras Upstream Stimulator (Sos)
  • Src Homology Domains (SH)
  • SH2 Domain Containing Proteins
  • Ras-Regulated Signaling Pathways
  • Ras-Rak-Mapk Pathway
  • PI3 Kinase Pathway
  • PI3K-AKT in Cancers
  • Wnt–β-catenin Pathway
  • Wnt–β-catenin Pathway in Cancers

Role of Ligand-Receptor Binding

  • Ligand binding activates receptors, resulting in a cascade of intracellular events leading to changes in cell behavior.
  • Mechanisms include activation of tyrosine kinase domains leading to transphosphorylation of receptor's cytoplasmic domains, generating signatures of phosphotyrosine residues that recruit downstream signaling partners.

Ras Protein Signaling

  • Ras acts as a binary switch and is key in a complex signaling cascade.
  • Son of Sevenless (Sos):
    • Functions as a guanine nucleotide exchange factor (GEF), activating Ras by promoting the exchange of GDP for GTP.
    • Links receptor tyrosine kinases to Ras via adaptor proteins Shc and Grb2.

Src Homology Domains

  • Src Homology Domains (SH1, SH2, SH3):
    • SH1: Catalytic domain, present in all receptor tyrosine kinases.
    • SH2: Binds phosphorylated tyrosines, enabling proteins to localize to specific sites in the cell.
    • SH3: Binds proline-rich sequences in partner proteins.

Signaling Pathways Regulated by Ras

  • Raf-MAPK Pathway:

    • Activated by Ras-GTP, leading to activation of downstream kinases that affect transcription and translation.
    • B-Raf Mutations: In ~50% of melanomas, mutations increase kinase activity significantly.

  • PI3 Kinase Pathway:

    • Predominantly involved in suppressing apoptosis and stimulating cell growth via Akt.
    • PI3K Activation: Converts PIP2 to PIP3, recruiting proteins with pleckstrin homology (PH) domains.
    • Loss of PTEN, a tumor suppressor, enhances PI3K signaling leading to cancer progression.

Wnt–β-Catenin Pathway

  • Regulates cell proliferation and maintains undifferentiated states in cells.
  • β-Catenin: Exists in cytoplasm, associated with cell adhesion receptors or free in the cytosol with rapid turnover.
  • Pathway Activation in Cancer:
    • Increased Wnt expression observed in breast cancers.
    • Mutations leading to dysregulation of β-catenin degradation are common in colorectal and other cancers, impacting tumor development.

Key Insights on Cancer Signaling

  • Signaling pathways, particularly Ras and Wnt, are frequently mutated or dysregulated in a variety of cancers, emphasizing their role in tumor biology.
  • Understanding these pathways can offer insights into potential therapeutic targets for cancer treatment.

Questions?

  • Ensure clarity on key points regarding Ras and Wnt pathways in cancer biology and their implications in research and clinical practice.