Myostatin, a negative regulator of muscle growth, functions by inhibiting myoblast proliferation

Myostatin, a member of the transforming growth factor-beta (TGF-beta) super family, has been shown to be a negative regulator of myogenesis. Here we sh ow that myostatin functions by controlling the proliferation of muscle prec ursor cells. When C2C12 myoblasts were incubated with myostatin, proliferat ion of myoblasts decreased with increasing levels of myostatin. Fluorescenc e-activated cell sorting analysis revealed that myostatin prevented the pro gression of myoblasts from the G(1)- to S-phase of the cell cycle. Western analysis indicated that myostatin specifically up-regulated p21(Waf1,Cip1). a cyclin-dependent kinase inhibitor, and decreased the levels and activity of Cdk2 protein in myoblasts, Furthermore, we also observed that in myoblas ts treated with myostatin protein, Rb was predominately present in the hypo phosphorylated form. These results suggests that, in response to myostatin signaling, there is an increase in p21 expression and a decrease in Cdk2 pr otein and activity thus resulting in an accumulation of hypophosphorylated Rb protein. This, in turn, leads to the arrest of myoblasts in G(1)-phase o f cell cycle, Thus, we propose that the generalized muscular hyperplasia ph enotype observed in animals that lack functional myostatin could be as a re sult of deregulated myoblast proliferation.