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

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 p110 alpha requires its mutational activation, other p110 isoforms can induce transformation when overexpressed in the wildtype state. In wild-type p110 alpha, N345 in the C2 domain forms hydrogen bonds with D560 and N564 in the inter-SH2 (iSH2) domain of p85, and mutations of p110 alpha or p85 that disrupt this interface lead to increased basal activity and transformation. Sequence analysis reveals that N345 in p110 alpha aligns with K342 in p110 beta. This difference makes wild-type p110 beta analogous to a previously described oncogenic mutant, p110 alpha-N345K. We now show that p110 beta is inhibited by p85 to a lesser extent than p110 alpha 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 p110 beta was similar to p110 alpha-N345K in transforming potential. Inhibition of p110 beta by p85 was enhanced by a K342N mutation in p110 beta, 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 p110 beta, p110 beta-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 p110 beta is the result of its decreased inhibition by p85, due to the disruption of an inhibitory C2-iSH2 domain interface.