PI3K–Akt Pathway

Core Concept

The phosphoinositide 3-kinase (PI3K)–Akt pathway is the cell's central survival, growth, and metabolic signaling axis. On growth-factor stimulation, PI3K is recruited to the membrane, where it phosphorylates phosphatidylinositol 4,5-bisphosphate (PIP2) on the 3-position of the inositol ring to produce phosphatidylinositol 3,4,5-trisphosphate (PIP3). PIP3 is a lipid second messenger that recruits the Ser/Thr kinase Akt (protein kinase B) to the membrane, where it is activated by phosphorylation. Akt then phosphorylates a broad set of substrates that promote glucose uptake, protein synthesis, cell survival, and proliferation. The pathway is kept in check by the lipid phosphatase PTEN, which removes the 3-phosphate from PIP3, and by the phosphoinositide phosphatases SHIP1/2.

Key Components

• PI3K: Class I heterodimer of a regulatory subunit (p85) and catalytic subunit (p110). p85 SH2 domains bind receptor phosphotyrosines.
• PIP2: Membrane lipid substrate.
• PIP3: Lipid second messenger generated by PI3K.
• PTEN: Phosphatase that dephosphorylates PIP3 back to PIP2 — the key negative regulator.
• Akt (PKB): Ser/Thr kinase recruited to PIP3 via its pleckstrin homology (PH) domain.
• PDK1: Activates Akt by phosphorylating Thr308 (also PH-domain-containing, recruited by PIP3).
• mTORC2: Phosphorylates Akt at Ser473 for full activation.
• Downstream targets: TSC2, GSK3β, FOXO, BAD, AS160.

Mechanism / How It Works

1. Recruitment of PI3K: The p85 regulatory subunit's SH2 domains bind phosphotyrosines on an activated RTK (or adaptor proteins such as IRS1), relieving inhibition of p110 and localizing it to the membrane where its substrate resides.
2. PIP3 generation: p110 phosphorylates PIP2 to PIP3.
3. Akt recruitment: Akt's PH domain binds PIP3 (and PIP2), translocating Akt to the plasma membrane.
4. Akt activation: PDK1 (also PH-domain-recruited) phosphorylates Akt on Thr308 in the activation loop; mTORC2 phosphorylates Ser473 in the hydrophobic motif. Doubly phosphorylated Akt is fully active.
5. Substrate phosphorylation: Active Akt phosphorylates targets at the consensus motif RxRxx(S/T):
   • TSC2 (inhibits it) → relieves TSC-mediated repression of mTORC1 → protein synthesis and cell growth.
   • GSK3β (inhibits it) → promotes glycogen synthesis and stabilizes pro-growth transcription factors.
   • FOXO (inhibits it, causing nuclear exclusion) → reduces pro-apoptotic and cell-cycle-inhibitory gene expression.
   • BAD (inhibits it) → anti-apoptotic.
   • AS160 → promotes GLUT4 translocation and glucose uptake.
6. Termination: PTEN dephosphorylates PIP3 to PIP2; SHIP dephosphorylates PIP3 to PIP2 (at the 5-position); PI3K signaling thus decays and Akt returns to the cytosol.

Energy and Directionality

The pathway is lipid-chemistry-driven. PIP3 is made by an ATP-consuming kinase (PI3K) and destroyed by phosphatases (PTEN, SHIP); its concentration is a tightly regulated, transient pool. Every Akt phosphorylation consumes ATP, and Akt's downstream kinase activity consumes more. Directionality is enforced because PIP3 production and degradation are separate, opposing enzyme reactions — the cell pays energy both to build the signal and to erase it, allowing rapid, reversible control of survival and growth decisions.

Experimental Evidence / Technique

• Wortmannin and LY294002: PI3K inhibitors that block PIP3 production and Akt activation, used to define the pathway.
• PH-domain GFP reporters (e.g., GFP–Akt-PH) translocate to the membrane when PIP3 rises, allowing live visualization of PI3K activity.
• PTEN knockout/tumor suppressors: Loss of PTEN elevates PIP3 and Akt activity and causes tumors; PTEN is among the most frequently mutated tumor suppressors in human cancer.
• Phospho-specific antibodies (anti-pAkt-Ser473/Thr308) are clinical biomarkers of pathway activation.
• Oncogenic PIK3CA mutations (encoding p110α) hyperactivate PI3K in breast, colon, and other cancers.

How it works

1. Recruitment of PI3K: The p85 regulatory subunit's SH2 domains bind phosphotyrosines on an activated RTK (or adaptor proteins such as IRS1), relieving inhibition of p110 and localizing it to the membrane where its substrate resides.
2. PIP3 generation: p110 phosphorylates PIP2 to PIP3.
3. Akt recruitment: Akt's PH domain binds PIP3 (and PIP2), translocating Akt to the plasma membrane.
4. Akt activation: PDK1 (also PH-domain-recruited) phosphorylates Akt on Thr308 in the activation loop; mTORC2 phosphorylates Ser473 in the hydrophobic motif. Doubly phosphorylated Akt is fully active.
5. Substrate phosphorylation: Active Akt phosphorylates targets at the consensus motif RxRxx(S/T):
   • TSC2 (inhibits it) → relieves TSC-mediated repression of mTORC1 → protein synthesis and cell growth.
   • GSK3β (inhibits it) → promotes glycogen synthesis and stabilizes pro-growth transcription factors.
   • FOXO (inhibits it, causing nuclear exclusion) → reduces pro-apoptotic and cell-cycle-inhibitory gene expression.
   • BAD (inhibits it) → anti-apoptotic.
   • AS160 → promotes GLUT4 translocation and glucose uptake.
6. Termination: PTEN dephosphorylates PIP3 to PIP2; SHIP dephosphorylates PIP3 to PIP2 (at the 5-position); PI3K signaling thus decays and Akt returns to the cytosol.

Common confusions

• PIP2 vs. PIP3: PIP2 is both a substrate for PI3K and the source of IP3/DAG via PLCβ — the same lipid feeds two different second-messenger systems.
• PTEN vs. PI3K: PTEN is a lipid phosphatase (removes the 3-phosphate from PIP3), not a kinase; it opposes PI3K directly.
• PDK1 vs. mTORC2: Both activate Akt, but at different sites (Thr308 vs. Ser473); full activity needs both.
• Akt vs. PKC: Both are recruited by membrane lipids and are Ser/Thr kinases, but Akt binds PIP3 via a PH domain and PKC binds DAG (and often Ca2+).

Quick review

• RTK-pTyr → PI3K (p85/p110) → PIP2 → PIP3.
• PIP3 → Akt (PH domain) → PDK1 (Thr308) + mTORC2 (Ser473) → active Akt.
• Akt → inhibit TSC2/GSK3β/FOXO/BAD, activate AS160 → growth, survival, metabolism.
• PTEN dephosphorylates PIP3 → off.