New Modes of GPCR Signalling

Phosphoinositide-3-Kinase Pathway Activation in PTEN Deficient
Prostate Cancer Cells is Independent of Receptor Tyrosine Kinases and
Mediated by the p110 or p110 Catalytic Subunits
Xinnong Jiang1,2, Sen Chen1, John Asara1, Steven P. Balk1,*
1Division of Hematology-Oncology, Beth Israel Deaconess Medical Center/Harvard
Medical School, Boston, MA, USA
2 Huazhong University of Science and Technology, College of Life Science and
Technology 1037 Luoyu Road, Wuhan, Hubei
Class IA phosphoinositide-3-kinase (PI3K) p110 catalytic subunits are activated upon
SH2 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 PTEN loss in the majority of 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 observed in PTEN deficient PC3 PCa cells. LC/MS/MS and immunoblotting
showed that p85 was associated primarily with p110b and p110d. An interaction with
ErbB3 was also detected in serum starved cells LNCaP cells, but was independent of
ErbB3 tyrosine phosphoryation and was not required for basal PI3K activity. Basal
tyrosine phosphorylation of p110 and p110 could be blcoked by c-Src inhibitors,
but this did not suppress basal PI3K activity, which was similarly independent of Ras.
Using siRNA specific for each p110 isoform we found that basal PI3K activity was
mediated by p110 in PC3 cells, and by both p110 �and p110 �in LNCaP cells, while
p110 was required for PI3K activation in response to RTK stimulation by
heregulin- 1. Taken together, these findings indicate that basal PI3K activity in PTEN
deficient PCa cells is RTK independent and mediated by p110 and p110.