Rac1-mediated membrane raft localization of PI3K/p110β is required for its activation by GPCRs or PTEN loss

Abstract

We aimed to understand how spatial compartmentalization in the plasma membrane might contribute to the functions of the ubiquitous class IA phosphoinositide 3-kinase (PI3K) isoforms, p110α and p110β. We found that p110β localizes to membrane rafts in a Rac1-dependent manner. This localization potentiates Akt activation by G-protein-coupled receptors (GPCRs). Thus genetic targeting of a Rac1 binding-deficient allele of p110β to rafts alleviated the requirement for p110β-Rac1 association for GPCR signaling, cell growth and migration. In contrast, p110α, which does not play a physiological role in GPCR signaling, is found to reside in nonraft regions of the plasma membrane. Raft targeting of p110α allowed its EGFR-mediated activation by GPCRs. Notably, p110β dependent, PTEN null tumor cells critically rely upon raft-associated PI3K activity. Collectively, our findings provide a mechanistic account of how membrane raft localization regulates differential activation of distinct PI3K isoforms and offer insight into why PTEN-deficient cancers depend on p110β.DOI: http://dx.doi.org/10.7554/eLife.17635.001

Rac1-stimulated macropinocytosis enhances Gβγ activation of PI3Kβ

“…Unlike previous studies in MEFs [15,16], we do not see a requirement for Rac1 binding to p110β during GPCR-mediated activation of PI3Kβ. However, we have uncovered a novel mechanism for the effects of Rac1 on PI3Kβ.…”

“…The role of macropinosomes (or macropinocytotic cups) in enhancing Gβγ signaling to PI3Kβ that we observe in breast cancer cells has some parallels to the requirement for lipid raft targeting of PI3Kβ described in MEFs [16]. In both cases, a membrane subdomain enhances the interaction between PI3Kβ and Gβγ.…”

“…Studies in MEFs have revealed dual pathways for PI3Kβ activation by GPCRs, involving either direct binding of Gβγ to PI3Kβ [10] or Gβγ-stimulation of the ELMO/DOCK/Rac1 pathway, which leads to the binding of activated Rac1 to PI3Kβ [15] and targeting of PI3Kβ to lipid rafts [16]. For unclear reasons, we cannot detect a requirement for Rac1 binding during LPA-stimulated activation of PI3Kβ in MDA-MB-231 cells.…”

“…Previous studies in MEFs have focused on the coupling between Rac- and Gβγ-mediated activation at the level of p110β itself, and have suggested the existence of a feed–forward loop involving Rac1-mediated activation of PI3Kβ- and PI3Kβ-mediated activation of Rac1 [25], as well as the enhancement of GPCR signaling to PI3Kβ by Rac1-mediated targeting of PI3Kβ to lipid rafts [16]. The present study identifies another layer of regulation, in which Rac1-stimulated macropinosomes enhance Gβγ coupling to PI3Kβ independently of Rac1 binding to p110β.…”

CRKL Mediates p110β-Dependent PI3K Signaling in PTEN-Deficient Cancer Cells

“…We have recently published data showing that RAC activation is necessary to localize p110β to lipid rafts, an essential component of cell growth signaling in PTEN null cells that rely on p110β for PI3K signaling (Cizmecioglu et al, 2016). We showed that a second signal was also required for full p110β activation.…”