Regulation of the PI3K pathway through a p85α monomer-homodimer equilibrium.

Lydia W T Cheung,Gordon B Mills,Katarzyna W Walkiewicz,Tabot M D Besong,Huifang Guo,Stefan T Arold,David H Hawke

doi:10.7554/elife.06866

Abstract

The canonical action of the p85α regulatory subunit of phosphatidylinositol 3-kinase (PI3K) is to associate with the p110α catalytic subunit to allow stimuli-dependent activation of the PI3K pathway. We elucidate a p110α-independent role of homodimerized p85α in the positive regulation of PTEN stability and activity. p110α-free p85α homodimerizes via two intermolecular interactions (SH3:proline-rich region and BH:BH) to selectively bind unphosphorylated activated PTEN. As a consequence, homodimeric but not monomeric p85α suppresses the PI3K pathway by protecting PTEN from E3 ligase WWP2-mediated proteasomal degradation. Further, the p85α homodimer enhances the lipid phosphatase activity and membrane association of PTEN. Strikingly, we identified cancer patient-derived oncogenic p85α mutations that target the homodimerization or PTEN interaction surface. Collectively, our data suggest the equilibrium of p85α monomer-dimers regulates the PI3K pathway and disrupting this equilibrium could lead to disease development.

Highlights

The phosphatidylinositol 3-kinase (PI3K) pathway is a master regulator of many cellular processes
The p85α PR1:Src homology 3 (SH3) domain interaction contributes to stabilization of the p85α homodimer and PTEN binding, whereas PR2 contributes to PTEN binding but not to homodimer formation
Using analytical ultracentrifugation (AUC) and microscale thermophoresis (MST), we demonstrated that purified recombinant full-length p85α homodimerized with a micromolar dissociation constant Kd in absence of other proteins under reducing conditions (Kd was 7 ± 0.7 μM and 3.9 ± 0.2 μM for AUC and MST, respectively) (Figure 1—figure supplements 1, 2)

Summary

Introduction

The phosphatidylinositol 3-kinase (PI3K) pathway is a master regulator of many cellular processes. Activation of the class 1A PI3K is gated, in part, by the p85α regulatory subunit, which contains a Src homology 3 (SH3) domain, two proline-rich (PR) regions (PR1 and PR2) separated by a Rho-GAP/ BCR-homology (BH) domain, and two Src homology 2 (SH2) domains (nSH2, cSH2) flanking an interSH2 (iSH2) domain. The interaction of p85α nSH2-iSH2-cSH2 fragment with the p110 catalytic subunit represses p110 activity allosterically and stabilizes p110 against degradation, creating a pool of stable but quiescent p110 (Yu et al, 1998). Somatic mutations of the p85α:p110α interface disrupt the inhibitory action of p85α on bound p110α leading to PI3K pathway activation in cancers (Miled et al, 2007; Jaiswal et al, 2009). PIK3R1 mutations outside the nSH2-iSH2-cSH2 fragment are relatively common in cancers, endometrial cancer. The mechanisms underlying the transforming activity of these mutations remain to be elucidated and may provide novel biomarkers or therapeutic opportunities

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