Transforming growth factor-β-induced inhibition of myogenesis is mediated through smad pathway and is modulated by microtubule dynamic stability

The expression of muscle- specific genes associated with myogenesis is controlled by several myogenic transcription factors, including myogenin and MEF2D. Transforming growth factor-beta ( TGF-beta) has been shown to inhibit myogenesis, yet the molecular mechanisms underlying such inhibition are not known. In the present study, TGF-beta was shown to inhibit myogenin and MEF2D expression and myotube formation in C2C12 myoblasts cultured in differentiation medium in a cell density - dependent manner. Transfection of C2C12 cells with Smad7, an antagonist for TGF-beta/Smad signaling, restored the capacity of these cells to differentiate in the presence of TGF-beta or when cultured in growth medium at low confluence, conditions that hinder muscle differentiation. Moreover, nocodazole, a microtubule- destabilizing agent, enhanced the inhibition of myogenesis exerted by TGF-beta, an effect that could be restored by tubulin- polymerizing agent taxol, both of which have been shown to affect Smad- microtubule interaction and regulate TGF-beta/ Smad signaling. Our results indicate that TGF-beta inhibits myogenesis, at least in part, via Smad pathway, and provide evidence that low- dose pharmacological agents taxol and nocodazole can be used as a means to modulate myogenesis without affecting cell survival.