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 exp ression of four PKC isoforms: the conventional PKC alpha, the novel PK Cs delta and epsilon, and the atypical PKC zeta. With the conventional and novel PKC isoforms, membrane association has been used to show PK C activation. In cells transformed by v-Src, there was a Ca2+-dependen t increase in membrane association of the cu isoform relative to the n ontransformed parental cells. The zeta isoform had a slightly increase d 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 zeta isoform correlates with ac tivation, the data are inconclusive with regard to this isoform. Inter estingly, of the Ca2+-independent PKC isoforms delta and epsilon, only the delta isoform was preferentially associated with membrane fractio ns in v-Src-transformed cells. The lack of PKC epsilon activation was not due to lack of responsiveness to diacylglycerol (DG), since exogen ously supplied DG and phorbol ester were both able to induce membrane association of PKC epsilon. Thus, the differential activation of the d elta and epsilon isoforms by v-Src suggests a more complex mechanism f or the activation of the novel Ca2+-independent PKC isoforms, involvin g more than simply elevating DC levels. Since PKC has been implicated in the intracellular signals activated by v-Src that lead to transform ation, the selective activation of PKC alpha and delta suggests a role in mitogenesis and transformation for these PKC isoforms.