Critical activities of Rac1 and Cdc42Hs in skeletal myogenesis: Antagonistic effects of JNK and p38 pathways

The Rho family of GTT-binding proteins plays a critical role in a variety o f cellular processes, including cytoskeletal reorganization and activation of kinases such as p38 and C-jun N-terminal kinase (JNK) MAPKs. We report h ere that dominant negative forms of Rac1 and Cdc42Hs inhibit the expression of the muscle-specific genes myogenin, troponin T, and myosin heavy chain in L6 and C2 myoblasts. Such inhibition correlates with decreased p38 activ ity. Active RhoA, RhoG, Rac1, and Cdc42Hs also prevent myoblast-to-myotube transition but affect distinct stages: RhoG, Rac1, and Cdc42Hs inhibit the expression of all muscle-specific genes analyzed, whereas active RhoA poten tiates their expression but prevents the myoblast fusion process. We furthe r show by two different approaches that the inhibitory effects of active Ra c1 and Cdc42Hs are independent of their morphogenic activities. Rather, myo genesis inhibition is mediated by the JNK pathway, which also leads to a cy toplasmic redistribution of Myf5. We propose that although Rho proteins are required for the commitment of myogenesis, they differentially influence t his process, positively for RhoA and Rac1/Cdc42Hs through the activation of the SRF and p38 pathways, respectively, and negatively for Rac1/Cdc42Hs th rough the activation of the JNK pathway.