p57(Kip2) Stabilizes the MyoD Protein by Inhibiting Cyclin E-Cdk2 Kinase Activity in Growing Myoblasts

We show that expression of p57(Kip2), a potent tight-binding inhibitor of several G(1) cyclin–cyclin-dependent kinase (Cdk) complexes, increases markedly during C2C12 myoblast differentiation. We examined the effect of p57(Kip2) on the activity of the transcription factor MyoD. In transient transfection assays, transcriptional transactivation of the mouse muscle creatine kinase promoter by MyoD was enhanced by the Cdk inhibitors. In addition, p57(Kip2), p21(Cip1), and p27(Kip1) but not p16(Ink4a) induced an increased level of MyoD protein, and we show that MyoD, an unstable nuclear protein, was stabilized by p57(Kip2). Forced expression of p57(Kip2) correlated with hypophosphorylation of MyoD in C2C12 myoblasts. A dominant-negative Cdk2 mutant arrested cells at the G(1) phase transition and induced hypophosphorylation of MyoD. Furthermore, phosphorylation of MyoD by purified cyclin E-Cdk2 complexes was inhibited by p57(Kip2). In addition, the NH2 domain of p57(Kip2) necessary for inhibition of cyclin E-Cdk2 activity was sufficient to inhibit MyoD phosphorylation and to stabilize it, leading to its accumulation in proliferative myoblasts. Taken together, our data suggest that repression of cyclin E-Cdk2-mediated phosphorylation of MyoD by p57(Kip2) could play an important role in the accumulation of MyoD at the onset of myoblast differentiation.