The oncogenic protein p60v-src has competence activity but does not activate phosphatidylinositol turnover or protein kinase C in Balb/c 3T3 cells.

A Balb/c 3T3 cell-line infected with the temperature-sensitive Rous sarcoma virus LA90 has been used to investigate the mitogenic activity of p60v-src and the possible role of phosphatidylinositol turnover and protein kinase C activation in this process. Activation of the p60v-src tyrosine kinase alone was only weakly mitogenic to serum-deprived cells. However, transient activation of the kinase dramatically increased the mitogenic response to epidermal growth factor. Mitogenicity was not additive with phorbol esters. Down-regulation of protein kinase C by chronic administration of phorbol ester did not block the mitogenic effect of p60v-src. Following short-term activation of p60v-src, analysis of protein phosphorylations by giant two-dimensional electrophoresis revealed only very minor changes in any of the 1000 or so detectable phosphoproteins, whereas the activation of protein kinase C by a sub-optimal dose of phorbol ester significantly increased the phosphorylation of at least 10 proteins. Short-term activation of p60v-src did not cause any detectable increase in phosphatidylinositol turnover, or in diacylglycerol level. These results indicate that p60v-src can act, like platelet-derived growth factor, to induce competence in Balb/c 3T3 fibroblasts. They also suggest that even though p60v-src may activate phosphatidylinositol turnover in some other cell-lines, this effect is probably indirect and is not essential for induction of the transformed phenotype or for p60v-src-dependent mitogenesis.