Serum, like phorbol esters, rapidly activates protein kinase C in intact quiescent fibroblasts

Addition of serum to quiescent cultures of Swiss 3T3 cells and mouse embryo fibroblasts causes a rapid increase in the phosphorylation of an 80 000 mol. wt. cellular protein (termed 80 K). The effect is dose- and time-dependent; enhancement in 80 K phosphorylation can be detected as early as 10-15 s after adding serum. In contrast, platelet-derived growth factor elicits the response after a lag of 1.5 min suggesting that this growth factor does not mediate the response to serum. Recently a rapid increase in the phosphorylation of an 80 K cellular protein following treatment with phorbol esters or diacylglycerol has been shown to reflect the activation of protein kinase C in intact fibroblasts. The 80 K phosphoproteins generated in response to serum and to phorbol dibutyrate (PBt2) co-migrated in one- and two-dimensional PAGE and produced identical phosphopeptide fragments when subjected to partial digestion with Staphyloccocus aureus V8 protease. These observations suggest that the same 80 K protein is generated in response to serum and PBt2. We conclude that activation of protein kinase C in intact cells is one of the earliest effects elicited by serum in quiescent fibroblasts.