Class IA PI3K isoforms lead to differential signalling downstream of PKB/Akt

Abstract

Objectives The catalytic subunits of Class IA PI3K, p110 alpha, p110 beta, and p110 delta, phosphorylates phosphatidylinositol 4,5-bisphosphate (PIP2) into phosphatidylinositol 3,4,5-trisphosphate (PIP3) on the plasma membrane. In cancer, these catalytic subunits are usually found to be altered or amplified. Because pan-PI3K inhibition results in systemic toxicities, finding specific targets for the ubiquitous PI3K isoforms offers considerable potential for enhancing the effectiveness of PI3K-targeted therapy. Methods We aim to delineate the isoform-specific druggable targets of the PI3K by deleting PIK3CA (encoding p110 alpha) and PIK3CB (encoding p110 beta) by Cre mediated excision and ectopically expressing p110 alpha, p110 beta, or p110 delta with or without myristoylation (Myr) tag in mouse embryonic fibroblasts (MEFs). Myr is a lipidation signal that translocates proteins to plasma membrane permanently. This translocation renders p110s constitutively activated as they remain in close proximity to PIP(2 )on the membrane. Results Unique and redundant Akt targets are identified downstream of different PI3K isoforms. mTORC1, one of the targets of fully-activated Akt, has been observed to be differentially regulated in MEFs upon expression of p110 alpha or p110 beta. The varying dependencies on mTORC1 and Rac1 led us to analyse a potential scaffolding function of p110 beta with Rac1 to mediate phosphorylation and activation of mTOR using platforms for the modeling of biomolecular complexes. We also documented that p110 alpha and p110 beta support cell cycle kinetics differentially. Conclusions This study suggests differential regulation of protein translation, metabolism, cell cycle, and survival signaling downstream of unique p110 targets, underlying the importance of cancer treatment according to the deregulated p110 isoform.