The RhoA effector mDiaphanous regulates MyoD expression and cell cycle progression via SRF-dependent and SRF-independent pathways

Expression of the key muscle transcription factor MyoD is regulated by RhoA GTPase, which is an important regulator of adhesion-dependent signaling. We show that mDiaphanous (mDia) - an adaptor protein that mediates the effects of RhoA on cell motility and the cytoskeleton - is an upstream regulator of MyoD in C2C12 mouse myoblasts. Knockdown of mDia1 reduced MyoD expression and proliferation via a serum-response factor (SRF)-dependent pathway. Surprisingly, overexpression of a Rho-independent form of mDia1 (mDiaN3), despite activating SRF, also suppressed MyoD and the cell cycle, suggesting the presence of a second pathway downstream of mDia1. We present evidence that the alternative pathway by which mDia1 regulates MyoD involves T-cell factor (TCF)/lymphoid enhancer factor (LEF) and its co-activator, -catenin. TCF activity was suppressed by mDiaN3 and induced by silencing mDia. mDiaN3 disrupted the signal-dependent nuclear localization of -catenin and suppressed MyoD expression. Co-expression of a degradation-resistant form of -catenin with mDiaN3 restored MyoD expression, suggesting a mechanistic link between the two signaling proteins. We also implicate a region encompassing the FH1 domain of mDia1 in -catenin-TCF regulation. Taken together, our results suggest that a balance between two pathways downstream of mDia regulates MyoD expression and cell cycle progression.