The PI3K/AKT/mTOR Pathway in Cancer

PI3K Activation and PIP3 Generation

The PI3K family comprises three classes; in cancer, class IA PI3Ks — heterodimers of a p110 catalytic subunit (α, β, or δ) and a p85 regulatory subunit — are the most important. PI3Kα (p110α, encoded by PIK3CA) is the isoform most frequently mutated in cancer. In normal signalling, activated RTKs recruit PI3Kα to phosphotyrosine motifs via p85 SH2 domains, relieving p85-mediated inhibition of p110α. The released p110α catalyses phosphorylation of PIP2 to PIP3 at the plasma membrane inner leaflet.

PTEN (phosphatase and tensin homolog) is the counteracting lipid phosphatase, dephosphorylating PIP3 back to PIP2. The dynamic equilibrium between PI3K and PTEN activity determines cellular PIP3 levels and therefore AKT activation amplitude. PIK3CA gain-of-function mutations (H1047R in the kinase domain, E545K/E542K in the helical domain) or PTEN loss-of-function mutations both shift this balance irreversibly toward high PIP3 and constitutive AKT activity.

AKT Activation and Its 100+ Substrates

PIP3 recruits AKT to the plasma membrane via its pleckstrin homology (PH) domain. Membrane co-localised PDK1 phosphorylates AKT Thr308 in the activation loop, achieving ~20% of maximal activity. mTORC2 (the RICTOR-containing complex) then phosphorylates AKT Ser473 in the hydrophobic motif, achieving full kinase activation. This two-step phosphorylation requirement creates a robust coincidence detection mechanism that normally requires both PI3K activity and mTORC2-mediated amplification.

Active AKT phosphorylates over 100 substrates regulating virtually every cancer hallmark: BAD (Ser136, apoptosis resistance), MDM2 (Ser166/186, p53 degradation), TSC2 (Thr1462, mTORC1 activation), GSK3β (Ser9, cyclin D1 and MYC stabilisation), FOXO1/3a (cytoplasmic sequestration, BIM/p27 suppression), and eNOS (NO-mediated vasodilation and vascular permeability).

mTOR Complexes and Translational Control

mTOR forms two functionally distinct complexes: mTORC1 (with RAPTOR, mLST8, PRAS40, DEPTOR) drives anabolic biosynthesis, and mTORC2 (with RICTOR, mSin1, mLST8) activates AKT and regulates the cytoskeleton. mTORC1 integrates signals from growth factors (via AKT-TSC2-RHEB), amino acids (RAG GTPase-Ragulator lysosomal pathway), and cellular energy (AMPK opposition). When all conditions are met, mTORC1 phosphorylates S6K1 (Thr389) and 4EBP1 at multiple sites, releasing eIF4E from inhibition and driving cap-dependent translation of growth-promoting mRNAs.

The IRS-1 negative feedback loop — where mTORC1-activated S6K1 phosphorylates IRS-1 at inhibitory serines — critically limits PI3K re-activation under normal conditions. However, when mTOR is inhibited by rapamycin/everolimus, this feedback is relieved, causing paradoxical AKT hyperactivation through sustained PI3K→AKT Thr308 phosphorylation. This compensatory AKT activation limits the clinical efficacy of mTORC1-only inhibitors and motivates dual PI3K/mTOR inhibitors.

Approved Inhibitors and Biomarker Selection

The PI3K/AKT/mTOR pathway has generated three approved drug classes across multiple tumour types. Alpelisib (PI3Kα-selective) targets the PIK3CA-mutant isoform specifically, approved for PIK3CA-mutant HR+/HER2− breast cancer (SOLAR-1 trial: 11.0 vs 5.7 months median PFS, HR 0.65). Capivasertib (pan-AKT inhibitor) is approved for AKT1/PIK3CA/PTEN-altered HR+ breast cancer (CAPItello-291 trial: 7.3 vs 3.1 months median PFS, HR 0.60). Everolimus (mTORC1 inhibitor) is approved for RCC, HR+ breast, and pancreatic NETs.

Key Takeaways

• ·PI3Kα (PIK3CA) phosphorylates PIP2→PIP3 at the plasma membrane in response to RTK activation; PTEN is the opposing lipid phosphatase — their dynamic equilibrium determines AKT activation amplitude. PIK3CA hotspot mutations (H1047R, E545K) and PTEN loss both shift this balance toward constitutive AKT activation.
• ·AKT is fully activated by sequential PDK1 (Thr308) and mTORC2 (Ser473) phosphorylation, enabling phosphorylation of >100 substrates including MDM2 (p53 destabilisation), TSC2 (mTORC1 activation), BAD (apoptosis resistance), and FOXO1/3a (BIM/p27 suppression).
• ·The S6K1–IRS-1 negative feedback loop, relieved by mTORC1 inhibition, causes paradoxical AKT hyperactivation — the dominant mechanism limiting everolimus/temsirolimus efficacy and motivating dual PI3K/mTOR inhibitor development.
• ·Alpelisib (PI3Kα-selective, PIK3CA-mutant HR+ breast: SOLAR-1) and capivasertib (pan-AKT, PIK3CA/AKT1/PTEN-altered HR+ breast: CAPItello-291) represent the two approved biomarker-stratified PI3K/AKT pathway inhibitor strategies.
• ·The PI3K/AKT/mTOR pathway is altered in >70% of human cancers and intersects with virtually every major oncogenic driver — EGFR→RAS→PI3K, HER2→PI3K, and KRAS-independent PI3K activation — making it the most broadly activated oncogenic signalling axis in human cancer.