Extracellular signal-regulated kinase 1/2 mitogen-activated protein kinase is involved in myostatin-regulated differentiation repression

The cytokines of transforming growth factor beta (TGF-beta) and its superfamily members are potent regulators of tumorigenesis and multiple cellular events. Myostatin is a member of TGF-beta superfamily and plays a negative role in the control of cell proliferation and differentiation. We now show that myostatin rapidly activated the extracellular signal-regulated kinase 1/2 (Erk1/2) cascade in C2C12 myoblasts. A more remarkable Erk1/2 activation stimulated by myostatin was observed in differentiating cells than proliferating cells. The results also showed that Ras was the upstream regulator and participated in myostatin-induced Erk1/2 activation because the expression of a dominant-negative Ras prevented myostatin-mediated inhibition of Erk1/2 activation and proliferation. Importantly, the myostatin-suppressed myotube fusion and differentiation marker gene expression were attenuated by blockade of Erk1/2 mitogen-activated protein kinase (MAPK) pathway through pretreatment with MAPK/Erk kinase 1 (MEK1) inhibitor PD98059, indicating that myostatin-stimulated activation of ErkI/2 negatively regulates myogenic differentiation. Activin receptor type IIb (ActRIIb) was previously suggested as the only type II membrane receptor triggering myostatin signaling. In this study, by using synthesized small interfering RNAs and dominant-negative ActRIIb, we show that myostatin failed to stimulate Erk1/2 phosphorylation and could not inhibit myoblast differentiation in ActRIIb-knockdown C2C12 cells, indicating that ActRIIb was required for myostatin-stimulated differentiation suppression. Altogether, our findings in this report provide the first evidence to reveal functional role of the Erk1/2 MAPK pathway in myostatin action as a negative regulator of muscle cell growth.