A biochemical mechanism for the oncogenic potential of the p110β catalytic subunit of phosphoinositide 3-kinase

Abstract

Class I PI3-kinases signal downstream of receptor tyrosine kinases and G protein-coupled receptors and have been implicated in tumorigenesis. Although the oncogenic potential of the PI3-kinase subunit p110α requires its mutational activation, other p110 isoforms can induce transformation when overexpressed in the wildtype state. In wild-type p110α, N345 in the C2 domain forms hydrogen bonds with D560 and N564 in the inter-SH2 (iSH2) domain of p85, and mutations of p110α or p85 that disrupt this interface lead to increased basal activity and transformation. Sequence analysis reveals that N345 in p110α aligns with K342 in p110β. This difference makes wild-type p110β analogous to a previously described oncogenic mutant, p110α-N345K. We now show that p110β is inhibited by p85 to a lesser extent than p110α and is not differentially inhibited by wild-type p85 versus p85 mutants that disrupt the C2-iSH2 domain interface. Similar results were seen in soft agar and focus-formation assays, where p110β was similar to p110α-N345K in transforming potential. Inhibition of p110β by p85 was enhanced by a K342N mutation in p110β, which led to decreased activity in vitro, decreased basal Akt and ribosomal protein S6 kinase (S6K1) activation, and decreased transformation in NIH 3T3 cells. Moreover, unlike wild-type p110β, p110β-K342N was differentially regulated by wild-type and mutant p85, suggesting that the inhibitory C2-iSH2 interface is functional in this mutant. This study shows that the enhanced transforming potential of p110β is the result of its decreased inhibition by p85, due to the disruption of an inhibitory C2-iSH2 domain interface. oncogenic mutation | oncogenic transformation | lipid kinase P I3-kinases are important regulators of cell growth and survival, and they mediate responses to extracellular stimuli through receptor tyrosine kinases (RTKs) and G protein-coupled receptors (GPCRs) (1). Disruption of normal PI3-kinase signaling is observed in cancer and many other diseases (1, 2). Class IA PI3-kinases are obligate heterodimeric proteins composed of a catalytic subunit (p110α, -β, -δ) and a regulatory subunit (p85α, p85β, p55α, p50α, and p55γ) (3). It had been suggested previously that class IA PI3-kinases function primarily downstream of RTKs, with the Src homology 2 (SH2) domains of p85 targeting the p85/p110 dimers to phosphorylated tyrosine residues in activated receptors or their adaptor molecules (1). However, recent data suggest that p110β is activated downstream of GPCRs, although it still is unclear whether p110β is activated only by GPCRs or by RTKs as well (4, 5).p85 and p110 are multidomain proteins that interact with each other and with multiple upstream regulators and downstream effectors (6). p85 consists of an N-terminal Src homology 3 (SH3) domain, two proline-rich regions surrounding a breakpoint cluster region (BCR) homology region that can bind to small GTPases, and two SH2 domains flanking the inter-SH2 (iSH2) antiparallel coiled coil. Class IA p110 subunits are composed of an adaptorbindin...

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The p110α and p110β isoforms of PI3K play divergent roles in mammary gland development and tumorigenesis

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