PI3K/Akt Growth Signaling

The phosphoinositide 3-kinase/Akt pathway is the most frequently activated signaling cascade in cancer. Controls cell survival, growth, metabolism, and resistance to apoptosis.

Overview

Receptor tyrosine kinases (EGFR, HER2, IGFR) activate PI3K, which phosphorylates PIP2 to PIP3 at the membrane. PIP3 recruits Akt (PKB) and PDK1, leading to Akt phosphorylation. Active Akt phosphorylates >100 substrates promoting survival (BAD inactivation), growth (TSC2 → mTORC1 activation), metabolism (GSK3β inactivation → glycogen synthesis), and proliferation (p27 inactivation). PTEN is the key tumor suppressor that dephosphorylates PIP3.

Key Steps

1. Growth factor binds RTK → receptor dimerization and autophosphorylation
2. PI3K regulatory subunit (p85) binds phospho-tyrosines; catalytic subunit (p110) generates PIP3
3. PIP3 recruits Akt and PDK1 to membrane via PH domains
4. PDK1 phosphorylates Akt-Thr308; mTORC2 phosphorylates Akt-Ser473 for full activation
5. Active Akt phosphorylates TSC2 → mTORC1 activation, BAD → anti-apoptosis, GSK3β → metabolism
6. PTEN tumor suppressor dephosphorylates PIP3 → pathway termination

Disease Relevance

• Cancer: PIK3CA mutations occur in ~30% of breast cancers. PTEN loss is among the most common tumor suppressor events. PI3K inhibitors (alpelisib) are FDA-approved for PIK3CA-mutant breast cancer.
• Alzheimer's: PI3K/Akt signaling is neuroprotective. Insulin resistance (impaired PI3K/Akt in brain) is a risk factor for AD — sometimes called 'type 3 diabetes.'

Therapeutic Targets

• Quercetin: PI3K inhibition via direct kinase binding
• Curcumin: Inhibits PI3K/Akt pathway at multiple nodes
• Berberine: Modulates PI3K/Akt via AMPK-mediated crosstalk