Phosphoinositide 3-Kinase Pathway Activation in Phosphate and Tensin Homolog (PTEN)-deficient Prostate Cancer Cells Is Independent of Receptor Tyrosine Kinases and Mediated by the p110β and p110δ Catalytic Subunits

Class IA phosphoinositide 3-kinase (PI3K) p110 catalytic subunits are activated upon Src homology 2 domain-mediated binding of their p85 regulatory subunits to tyrosine-phosphorylated pYXXM motifs in receptor tyrosine kinases (RTKs) or adaptor proteins. The PI3K pathway is activated by phosphate and tensin homolog (PTEN) loss in most prostate cancers (PCa), but the contribution of upstream RTKs that may be targeted therapeutically has not been assessed. Immunoblotting of p85-associated proteins in serum-starved PTEN-deficient LNCaP and C4-2 PCa cells showed a small set of discrete tyrosine-phosphorylated proteins, but these proteins were not recognized by an anti-pYXXM motif antibody and were not found in PTEN-deficient PC3 PCa cells. LC/MS/MS using label-free proteomics and immunoblotting showed that p85 was associated primarily with p110 beta and p110 delta. An interaction with ErbB3 was also detected but was independent of ErbB3 tyrosine phosphorylation and was not required for basal PI3K activity. Basal tyrosine phosphorylation of p110 beta and p110 delta could be blocked by c-Src inhibitors, but this did not suppress PI3K activity, which was similarly independent of Ras. Basal PI3K activity was mediated by p110 beta in PC3 cells and by both p110 beta and p110 delta in LNCaP cells, whereas p110 alpha was required for PI3K activation in response to RTK stimulation by heregulin-beta 1. These findings show that basal PI3K activity in PTEN-deficient PCa cells is RTK-independent and can be mediated by p110 beta and p110 delta. Increased p110 beta expression in PCa may be required for RTK-independent PI3K pathway activation in adult prostate epithelium with genetic or epigenetic PTEN down-regulation.