Inhibition of myogenesis by the H-ras oncogene: implication of a role for protein kinase C

Expression of the oncogenic form of H-ras p21 in the mouse myogenic cell line, 23A2, blocks myogenesis and inhibits expression of the myogenic regulatory factor gene, MyoD1. Previous studies from a number of laboratories have demonstrated that the activation of ras p21 is associated with changes in phospholipid metabolism that directly, or indirectly, lead to elevated levels of intracellular diacylglycerol and the subsequent activation of protein kinase C (PKC). To assess the importance of PKC activity to the ras-induced inhibition of skeletal myogenesis, we examined the levels of PKC activity associated with the terminal differentiation of wild-type myoblasts and with the differentiation-defective phenotype of 23A2 ras cells. We demonstrate that there is a 50% reduction in PKC activity during normal myogenesis and that PKC activity is required for myoblast fusion, but not for the transcriptional activation of muscle-specific genes. In contrast, we found that the differentiation-defective 23A2 ras cells possess two- to threefold more PKC activity than wild-type myofibers and that reducing the PKC activity in these cultures does not reverse their non-myogenic phenotype. On the other hand, if PKC activity is downregulated in 23A2 cells before the expression of activated ras p21, myogenesis is not inhibited. These results suggest that activated ras p21 relies on a PKC- dependent signal transduction pathway to initiate, but not to sustain, its negative effects on 23A2 skeletal myogenesis and underscore the potential importance of PKC activity to the proper control of skeletal muscle differentiation.