THE ROLE OF SIGNAL-TRANSDUCING EVENTS IN THE PROLIFERATIVE RESPONSE OF CELLS TO A MITOGENIC VIRAL K-RAS PROTEIN

Activated oncogenic ras proteins are powerful mitogenic agents which by themselved can initiate and maintain the proliferation of quiescent cells in the absence of any exogenous growth factors. In an attempt to understand how ras proteins induce proliferation we examined the early events in the G0 to G1 transition caused by the activation of a thermolabile K-ras protein in quiescent, serum-starfed tsKSV-transformed NRK cells. We show that ras reactivation, in the absence of exogenous growth factors, triggered a rapid surge in free cytosolic Ca2+ and diacylglycerol production, which led to a transient increase in membrane-associated protein kinase C (PKC) activity which was necessary for G1 transit. Unlike TPA-stimulated PKC activity, the ras-induced increase in PKC was rapidly extracted from membranes by EGTA. These signal transducing events occured despite the fact that ras activation did not induced the tyrosine phosphorylation of any known surface receptor. The results indicate that the K-ras protein triggers the G0 to G1 transition by an intracellular mechanism and not indirectly via autocrine stimulation. © 1990.