PI3K-Akt pathway

PI3K-Akt Pathway - The Survival Signal

• Trigger: Growth factors (insulin, IGF-1) bind to receptor tyrosine kinases (RTKs).
• Mechanism: Activated RTK recruits & activates PI3K.
  • PI3K phosphorylates membrane lipid $PIP_2$ to form $PIP_3$.
  • $PIP_3$ acts as a docking site, recruiting Akt (Protein Kinase B) and PDK1.
• Downstream Effects: Akt activation promotes cell survival and proliferation.
  • Inhibits apoptosis (by phosphorylating Bad).
  • Stimulates cell growth (via mTORC1).

⭐ PTEN is a critical tumor suppressor that turns this pathway OFF by dephosphorylating $PIP_3$ to $PIP_2$.

The Cascade - How It Fires Up

• Receptor Binding: Growth factors (e.g., insulin, IGF-1) bind to and activate Receptor Tyrosine Kinases (RTKs).
• PI3K Recruitment: The activated RTK recruits PI3K to the plasma membrane.
• PIP3 Generation: PI3K phosphorylates $PIP_2$ (Phosphatidylinositol 4,5-bisphosphate) to form the second messenger $PIP_3$ (Phosphatidylinositol 3,4,5-trisphosphate).
• Akt/PDK1 Docking: $PIP_3$ acts as a docking site, recruiting Akt (Protein Kinase B) and PDK1 to the membrane.

• Akt Activation: PDK1 and mTORC2 phosphorylate Akt, leading to its full activation. Activated Akt then moves to phosphorylate downstream targets, promoting cell survival and growth.

⭐ The tumor suppressor PTEN (Phosphatase and Tensin Homolog) directly opposes this pathway by dephosphorylating $PIP_3$ back to $PIP_2$. Loss-of-function mutations in PTEN are a common driver of many cancers, including endometrial, prostate, and glioblastoma.

Downstream Effects - Master of Cell Fate

Activated Akt orchestrates a pro-survival and pro-growth cellular program by phosphorylating a range of downstream targets.

• Inhibits Apoptosis (Promotes Survival):

  • Akt phosphorylates and inactivates Bad, a pro-apoptotic protein.
  • Inactive Bad releases anti-apoptotic proteins like Bcl-2 and Bcl-xL, blocking programmed cell death.

• Stimulates Cell Growth & Proliferation:

  • Activates mTOR: Akt activates mTORC1, a master regulator of cell growth, leading to ↑ protein and lipid synthesis.
  • Inactivates GSK-3β: Phosphorylation inhibits Glycogen Synthase Kinase-3β, promoting glycogen synthesis and cell proliferation.

• Negative Regulation: PTEN

  • The pathway is terminated by PTEN (Phosphatase and Tensin Homolog).
  • This tumor suppressor dephosphorylates PIP₃ back to PIP₂, shutting down Akt signaling.

⭐ Loss-of-function mutations in the PTEN tumor suppressor gene are common in many cancers (e.g., endometrial, glioblastoma), leading to unchecked cell proliferation.

Clinical Correlations - When Good Pathways Go Bad

• Cancer Hallmark: Uncontrolled ↑ PI3K/Akt signaling is a key driver in many cancers, promoting cell proliferation and survival.
  • Commonly seen in breast, prostate, lung, and endometrial cancers.
• Genetic Drivers:
  • Activating mutations in the PIK3CA gene.
  • Loss-of-function mutations in the PTEN tumor suppressor gene. PTEN normally shuts off the PI3K pathway.
• Targeted Therapies:
  • PI3K Inhibitors: Alpelisib, Idelalisib.
  • mTOR Inhibitors: Sirolimus (Rapamycin), Everolimus act downstream of Akt to block cell growth.

⭐ Loss of the PTEN tumor suppressor is a frequent cause of pathway overactivation, particularly prominent in endometrial cancer.

High‑Yield Points - ⚡ Biggest Takeaways

• Activated by growth factors like insulin, this pathway is crucial for cell growth and proliferation.
• PI3K phosphorylates PIP2 to generate the second messenger PIP3.
• PIP3 recruits Akt (Protein Kinase B) and PDK1 to the plasma membrane, leading to Akt activation.
• Activated Akt promotes cell survival by inhibiting apoptosis (e.g., by phosphorylating Bad).
• Akt stimulates cell growth and protein synthesis through activation of mTORC1.
• The tumor suppressor PTEN is a key negative regulator, dephosphorylating PIP3 back to PIP2.