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

Abstract

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 mDial reduced MyoD expression and proliferation via a serum-response factor (SRF)-dependent pathway. Surprisingly, overexpression of a Rho-independent form of mDia1 (mDiaΔN3), 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, β-catenin. TCF activity was suppressed by mDiaΔN3 and induced by silencing mDia. mDiaΔN3 disrupted the signal-dependent nuclear localization of β-catenin and suppressed MyoD expression. Co-expression of a degradation-resistant form of β-catenin with mDiaΔN3 restored MyoD expression, suggesting a mechanistic link between the two signaling proteins. We also implicate a region encompassing the FH1 domain of mDia1 in β-catenin-TCF regulation. Taken together, our results suggest that a balance between two pathways downstream of mDia regulates MyoD expression and cell cycle progression.