Regulation of p110beta signaling and functions by interacting proteins

Abstract

Phosphatidylinositide 3-kinases (PI3Ks) are a family of lipid kinases with essential roles in cell growth and survival, and are therefore important in physiology and disease. The Class I PI3Ks mediate responses to extracellular ligands by signaling downstream of receptors. Of the class I PI3Ks, p110beta is unique in that it is activated downstream of both RTKs and GPCRs, and is also the only isoform to bind the active form of the small GTPase Rab5. p110beta is rarely mutated in cancers, but over-expression of the WT protein is transforming. Furthermore, tumors caused by loss of PTEN seem to depend on p110beta signaling. These data suggest that WT p110beta is regulated differently from the other class I PI3Ks. The aim of this work is to characterize the effects of three different interacting partners of p110beta - p85, Gbetagamma subunits, and Rab5 - on its oncogenic potential by identifying the regions of interaction and their input towards p110beta-dependent functions and transformation. We have identified the unique regulatory interfaces of p110beta that mediate its interactions with p85, Gbetagamma and Rab5. Our data has shown that p110beta displays a disrupted C2-iSH2 interface with p85, leading to a higher basal activity and contributing to the transforming potential of over-expressed p110beta. In addition, characterizing the Gbetagamma interaction has allowed us to design specific mutants and peptidomimetics that specifically disrupt this interaction. These tools have defined an essential role for GPCR-mediated Gbetagamma input to p110beta as essential in driving PTEN loss tumors. We have also mapped the p110beta-Rab5 interaction interface, and while loss of Rab5 binding did not affect the transforming potential of p110beta, it resulted in a loss of Rapamycin-induced macroautophagy in cells. These studies define the regions of p110beta that are required for its various unique interactions, in particular with Gbetagamma and Rab5-GTP, but also shed light on the differential regulation of p110beta by p85. We conclude that p110beta is unique among class I PI3Ks in the range of its interactions with upstream regulatory proteins. Characterizing these interactions may allow us to block p110beta-dependent tumorigenesis as well as other functions of p110beta.