Class IA PI3K isoforms lead to differential signaling downstream of PKB/AKT

Abstract

PI3K pathway has been deregulated in one third of human cancers. All Class IA PI3Ks, which are composed of catalytic and regulatory subunits, catalyze convertion of PIP2 to PIP3 on plasma membrane. The catalytic subunits of Class IA – p110α, p110β, and p110δ –, are found to be mutated/amplified in cancers. As inhibiting all Class IA catalytic isoforms lead to widespread toxicity, identification of isoform specific targets of especially p110α and p110β have the potential to transform targeted therapy for PI3K deregulated cancers. In our studies, isogenic mouse embryonic fibroblasts (MEFs) were used as they constitute a genuine model for signaling experiments with their genetically stable, and nontransformed phenotype. MEFs were engineered to have their first exons of PIK3CA and PIK3CB floxed, enabling double knock out of p110alpha and beta in a temporally regulated manner, which allowed us to study their isoform specific targets. Myristoylation (Myr), a lipidation signal anchoring proteins to the plasma membrane, leads to constitutive activation of kinases. We tagged p110s with Myr signal and transfected MEFs with them to study their novel as well as redundant targets. Proliferation assays, pharmacological inhibition, Western Blots are used to elucidate the unique targets of p110 isoforms. Myristoylated p110s result in activation of unique as well as common Akt substrates. These unique targets were highlighted in proliferation experiments where MEFs were treated either with Doxorubicin or Cisplatin, chemotherapeutic agents to induce apoptosis. Cell cycle analysis of double knock-out overexpression constructs generated in MEFs have shown that p110α and p110β downstream signaling lead to different cell cycle kinetics. mTORC1 inhibition by Rapamycin, mTORC1 inhibition by Everolimus, and Rac1 inhibition by EHT1864 translate differentially to the corresponding downstream targets in p110s. We also tested a potential scaffold function p110β together with Rac1 to phosphorylate mTOR by using docking simulations. This study suggests differential regulation of translation, metabolism as well as survival signalling downstream of unique class IA p110 isoforms.