ACTIVATION OF MITOGEN-ACTIVATED PROTEIN-KINASE BY PROTEIN-KINASE-C ISOTYPE-ALPHA, ISOTYPE-BETA(I) AND ISOTYPE-GAMMA, BUT NOT ISOTYPE-EPSILON

Treatment of CHO.T cells with either PMA or insulin led to the activat ion of MAP kinase by similar to 3-fold, and p90(rsk) by similar to 4-f old. Over-expression of the alpha, beta(I) or gamma isoforms of protei n kinase C caused a substantial enhancement of the effect of PMA on th e activation of MAP kinase and p90(rsk), however, the effect of insuli n was unchanged. Over-expression of the epsilon isoform of protein kin ase C did not alter the effect of either PMA or insulin on the activat ion of MAP kinase and p90(rsk), The results suggest that protein kinas e C isotypes alpha, beta(I) and gamma, but not epsilon, can mediate MA P kinase activation by PMA, and strongly support the hypothesis that p rotein kinase C isoforms can initiate distinct signalling pathways.