A function for phosphatidylinositol 3-kinase β (p85α-p110β) in fibroblasts during mitogenesis:: Requirement for insulin- and lysophosphatidic acid-mediated signal transduction

We have previously shown that phosphatidylinositol 3-kinase alpha (PI 3-K alpha) (p85 alpha-p110 alpha) is required for DNA synthesis induced by various growth factors (S. Roche, M. Koegl, and S. A. Courtneidge, Proc. Natl. Acad, Sci. USA 91:9185-9189, 1994) in fibroblasts. In the present study, we have investigated the function of PI 3-K beta (p85 alpha-p110 beta) during mitogenesis. By using antibodies specific to p110 beta we showed that PI 3-K beta is expressed in NIH 3T3 cells. PI 3-K beta and PI 3-K alpha have common features: PI 3-K beta is tightly associated with a protein serine kinase that phosphorylates p85 alpha, it interacts with the Src-middle T antigen complex and the activated platelet-derived growth factor (PDGF) receptor in fibroblasts in vivo, and it becomes tyrosine phosphorylated after PDGF stimulation. PI 3-K beta was also activated in Swiss 3T3 and Cos7 cells stimulated with lysophosphatidic acid (LPA), a mitogen that interacts with a heterotrimeric G protein-coupled receptor. In contrast PI 3-K alpha was activated to a lesser extent in these cells. Microinjection of neutralizing antibodies specific for p110 beta into quiescent fibroblasts inhibited DNA synthesis induced by both insulin and LPA but poorly affected PDGF receptor signaling. Therefore, PI 3-K beta plays an important role in transmitting the mitogenic response induced by some, but not all, growth factors. Finally, we show that while oncogenic V12Ras interacts with type I PI 3-Ks, it could induce DNA synthesis in the absence of active PI 3-K alpha and PI 3-K beta, suggesting that Ras uses other effecters for DNA synthesis.