HYDROLYSIS OF PHOSPHATIDYLCHOLINE COUPLES RAS TO ACTIVATION OF RAF PROTEIN-KINASE DURING MITOGENIC SIGNAL-TRANSDUCTION

We have investigated the relationship between hydrolysis of phosphatid ylcholine (PC) and activation of the Raf-I protein kinase in Ras-media ted transduction of mitogenic signals. As previously reported, cotrans fection of a PC-specific phospholipase C (PC-PLC) expression plasmid b ypassed the block to cell proliferation resulting from expression of t he dominant inhibitory mutant Ras N-17. In contrast, PC-PLC failed to bypass the inhibitory effect of dominant negative Raf mutants, suggest ing that PC-PLC functions downstream of Ras but upstream of Raf. Consi stent with this hypothesis, treatment of quiescent cells with exogenou s PC-PLC induced Raf activation, even when normal Ras function was blo cked by Ras N-17 expression. Further, activation of Raf in response to mitogenic growth factors was blocked by inhibition of endogenous PC-P LC. Taken together, these results indicate that hydrolysis of PC media tes Raf activation in response to mitogenic growth factors.