05 — PI3K-Akt Pathway and Signal Termination

Core Explanation

PI3K Activation and PIP3 Generation

PI3K (phosphoinositide 3-kinase) is recruited to activated RTKs (or GPCRs) via its regulatory subunit (p85), which binds phosphotyrosine residues through SH2 domains. PI3K then phosphorylates the membrane phospholipid PIP2 (phosphatidylinositol 4,5-bisphosphate) at the 3′ position of the inositol ring, generating PIP3 (phosphatidylinositol 3,4,5-trisphosphate).

Key concept: PIP3 is a lipid second messenger that remains in the membrane. It does not diffuse through the cytoplasm — instead, it creates localized membrane docking sites.

Akt Activation

Proteins containing PH domains (pleckstrin homology domains) bind PIP3 with high specificity. Akt (also called PKB — protein kinase B) is recruited to the plasma membrane via its PH domain. At the membrane, Akt is activated by two phosphorylation events:

1. PDK1 (phosphoinositide-dependent kinase 1) — also recruited by PIP3 via its PH domain — phosphorylates Akt at Thr308 in the activation loop. This provides partial activation.
2. mTORC2 (mammalian target of rapamycin complex 2) phosphorylates Akt at Ser473 in the hydrophobic motif. This is required for full Akt activity.

Once fully activated, Akt dissociates from the membrane and phosphorylates numerous cytosolic and nuclear targets to promote:

• Cell survival: Akt phosphorylates and inactivates pro-apoptotic proteins (Bad — prevents it from inhibiting Bcl-2; caspase-9; FOXO transcription factors — excludes them from the nucleus)
• Cell growth: Akt activates mTORC1 (indirectly, via TSC2 inhibition) → increased protein synthesis, ribosome biogenesis, and cell size
• Metabolism: Akt promotes glucose uptake (GLUT4 translocation to the membrane) and glycolysis
• Proliferation: Akt phosphorylates and inhibits p21 and p27 (Cdk inhibitors), releasing cell-cycle progression

PTEN — The Critical Brake

PTEN (phosphatase and tensin homolog deleted on chromosome 10) is a lipid phosphatase that removes the 3′ phosphate from PIP3, converting it back to PIP2. This directly terminates PI3K signaling at its source.

PTEN as a tumor suppressor:

• PTEN is the second most frequently mutated tumor suppressor in human cancer (after p53)
• Loss of PTEN → PIP3 accumulates → Akt becomes constitutively active → uncontrolled cell survival, growth, and proliferation
• PTEN mutations are found in prostate cancer, glioblastoma, endometrial cancer, melanoma, and many others
• Even heterozygous PTEN loss (haploinsufficiency) increases cancer risk

PTEN is NOT a protein phosphatase (primarily) — its tumor suppressor activity comes from its lipid phosphatase activity on PIP3.

Signal Termination — Why and How

Uncontrolled signaling drives cancer, chronic inflammation, and cell death. Cells use multiple redundant mechanisms to ensure signals are transient:

1. GTP hydrolysis: Gα subunits and small GTPases (Ras, Ran, Rab) have intrinsic GTPase activity. GAPs accelerate GTP hydrolysis, turning molecular switches OFF.

1. Phosphatases: Remove phosphate groups added by kinases.
   • Protein phosphatases: PP1, PP2A remove phosphates from serine/threonine
   • Lipid phosphatases: PTEN (PIP3→PIP2), SHIP (PIP3→PI(3,4)P2)
   • Tyrosine phosphatases: PTP1B, SHP2 remove phosphates from tyrosine

1. Phosphodiesterases (PDEs): Degrade cAMP and cGMP second messengers to 5′-AMP and 5′-GMP.

1. Receptor internalization (endocytosis): Activated receptors are recognized by adaptor proteins (β-arrestin for GPCRs) and internalized via clathrin-coated pits. Receptors can be:
   • Recycled back to the plasma membrane (resensitization)
   • Sorted to lysosomes for degradation (long-term desensitization)

1. Receptor degradation: Ubiquitination (by E3 ligases like c-Cbl for RTKs) tags receptors for lysosomal sorting.

1. Desensitization: GPCR kinases (GRKs) phosphorylate ligand-bound GPCRs, promoting β-arrestin binding. β-arrestin sterically blocks G protein coupling AND serves as an endocytic adaptor.

1. Negative feedback loops: Downstream effectors inhibit upstream components:
   • ERK phosphorylates and inhibits Sos (reduces Ras activation)
   • S6K (downstream of mTORC1) phosphorylates and inhibits IRS-1 (reduces PI3K activation)
   • These feedback loops create self-limiting signals

Clinical Connections

• PTEN loss → PI3K/Akt pathway hyperactivation → numerous cancers
• PIK3CA mutations (encoding the p110α catalytic subunit of PI3K) → constitutive PI3K activity — common in breast, colon, and endometrial cancers
• Akt inhibitors (e.g., ipatasertib, capivasertib) are in clinical trials for PTEN-deficient tumors
• Trastuzumab (anti-HER2) blocks RTK activation upstream of both Ras-MAPK and PI3K-Akt pathways in HER2+ breast cancer

Common Misconceptions

• Wrong: "PTEN is a protein phosphatase." Correct: PTEN's tumor suppressor function comes primarily from its lipid phosphatase activity on PIP3, though it has weak protein phosphatase activity.
• Wrong: "Signals just fade away over time." Correct: Signal termination requires active, energy-dependent mechanisms — phosphatases, GTPases, PDEs, and endocytic machinery.
• Wrong: "PI3K signaling is a simple on/off switch." Correct: PIP3 levels are dynamically regulated by the balance of PI3K (makes it) and PTEN/SHIP (removes it). The system functions more like a rheostat than a switch.