Selective activation of protein kinase C isoforms by v-Src

Protein kinase C (PKC) is a gene family consisting of no less than 11 distinct isoforms. In both murine and rat fibroblasts, we detected expression of four PKC isoforms: the conventional PKC α, the novel PKCs δ and ε, and the atypical PKC ζ. With the conventional and novel PKC isoforms, membrane association has been used to show PKC activation. In cells transformed by v- Src, there was a Ca²⁺-dependent increase in membrane association of the α isoform relative to the nontransformed parental cells. The ζ isoform had a slightly increased membrane association in murine fibroblasts transformed by v-Src but not in rat fibroblasts transformed by v-Src. However, since it is not clear whether cellular distribution of ζ isoform correlates with activation, the data are inconclusive with regard to this isoform. Interestingly, of the Ca²⁺-independent PKC isoforms δ and ε, only the δ isoform was preferentially associated with membrane fractions in v-Src- transformed cells. The lack of PKC ε activation was not due to lack of responsiveness to diacylglycerol (DG), since exogenously supplied DG and phorbol ester were both able to induce membrane association of PKC ε. Thus, the differential activation of the δ and ε isoforms by v-Src suggests a more complex mechanism for the activation of the novel Ca²⁺-independent PKC isoforms, involving more than simply elevating DG levels. Since PKC has been implicated in the intracellular signals activated by v-Src that lead to transformation, the selective activation of PKC α and δ suggests a role in mitogenesis and transformation for these PKC isoforms.