A FUNCTION FOR PHOSPHATIDYLINOSITOL 3-KINASE-BETA (P85-ALPHA-P110-BETA) IN FIBROBLASTS DURING MITOGENESIS - REQUIREMENT FOR INSULIN-MEDIATED 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 induce d by various growth factors (S. Roche, M. Koegl, and S. A. Courtneidge , Proc. Natl. Acad, Sci. USA 91:9185-9189, 1994) in fibroblasts. In th e 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 tightl y associated with a protein serine kinase that phosphorylates p85 alph a, it interacts with the Src-middle T antigen complex and the activate d platelet-derived growth factor (PDGF) receptor in fibroblasts in viv o, 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 heterotr imeric G protein-coupled receptor. In contrast PI 3-K alpha was activa ted 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 PDG F receptor signaling. Therefore, PI 3-K beta plays an important role i n transmitting the mitogenic response induced by some, but not all, gr owth factors. Finally, we show that while oncogenic V12Ras interacts w ith type I PI 3-Ks, it could induce DNA synthesis in the absence of ac tive PI 3-K alpha and PI 3-K beta, suggesting that Ras uses other effe cters for DNA synthesis.