The phosphorylation of protein kinase C as a potential measure of activation.

As a means of determining the role of protein kinase C in the signal transduction from novel growth factors and hormones, we investigated the effects of well-characterized agents on the phosphorylation state of protein kinase C itself. These studies show that agents that stimulate protein kinase C either directly (phorbol esters) or indirectly through phosphatidylinositol breakdown (platelet-derived growth factor) induce an increase in the phosphorylation state of the kinase. By contrast, epidermal growth factor, which does not stimulate protein kinase C in fibroblasts, does not increase the phosphorylation state of protein kinase C, but leads to a decrease. The data suggest that the phosphorylation state of protein kinase C is dynamically controlled and can be used to provide evidence of protein kinase C activation.