Transforming Growth Factor-β–Induced Inhibition of Myogenesis Is Mediated Through Smad Pathway and Is Modulated by Microtubule Dynamic Stability

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Abstract
The expression of muscle-specific genes associated with myogenesis is controlled by several myogenic transcription factors, including myogenin and MEF2D. Transforming growth factor-β (TGF-β) has been shown to inhibit myogenesis, yet the molecular mechanisms underlying such inhibition are not known. In the present study, TGF-β 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-β/Smad signaling, restored the capacity of these cells to differentiate in the presence of TGF-β 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-β, 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-β/Smad signaling. Our results indicate that TGF-β 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.

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