PI3K Isoforms Explained: p110-alpha, -beta, -gamma and -delta

Four Catalytic Isoforms

The class I PI3K catalytic subunits are p110-alpha (PIK3CA), p110-beta (PIK3CB), p110-gamma (PIK3CG) and p110-delta (PIK3CD). Alpha and beta are expressed broadly; gamma and delta are concentrated in immune cells.

All generate the same lipid product, PIP3, but they are recruited and regulated differently, so blocking one does not necessarily block the others.

p110-alpha: The Cancer Workhorse

p110-alpha carries the recurrent activating mutations in cancer and mediates most growth-factor-driven PI3K signalling downstream of receptor tyrosine kinases. It is the target of alpelisib in PIK3CA-mutant breast cancer.

A characteristic on-target effect of p110-alpha inhibition is hyperglycaemia, because insulin signalling runs through this isoform.

p110-beta and PTEN-Null Tumours

p110-beta is regulated partly through G-protein-coupled receptors and is the isoform that many PTEN-deficient tumours rely on for continued PI3K signalling.

This led to development of p110-beta-selective and alpha/beta inhibitors aimed specifically at PTEN-null cancers such as some prostate cancers, though clinical results have been modest.

p110-delta and -gamma: Immune Isoforms

p110-delta is critical for B-cell signalling, and the delta-selective inhibitor idelalisib and dual delta/gamma inhibitor duvelisib are used in certain B-cell malignancies, with immune-mediated toxicities such as colitis and pneumonitis.

p110-gamma shapes myeloid-cell behaviour in the tumour microenvironment, and gamma-selective inhibition is being tested as an immunotherapy combination rather than a direct anti-tumour agent.

Interpretation Notes

A PIK3CA mutation is an alpha-isoform event; it does not imply dependence on beta, gamma or delta, and it is the alpha isoform that current breast-cancer biomarker testing concerns.

Isoform-selective inhibitors have distinct toxicity profiles that follow directly from where each isoform normally acts.

Key Takeaways

• ·Class I PI3K has four catalytic isoforms: alpha, beta (broad) and gamma, delta (immune).
• ·p110-alpha carries cancer hotspot mutations and is targeted by alpelisib; hyperglycaemia is on-target.
• ·PTEN-null tumours often depend on p110-beta.
• ·p110-delta and -gamma inhibitors are used in B-cell cancers and studied as immunotherapy partners.