SV40 Large T Antigen Reinduces the Cell Cycle in Terminally Differentiated Myotubes through Inducing Cdk2, Cdc2, and Their Partner Cyclins

Terminally differentiated skeletal muscle myotubes are arrested in GO phase of the cell cycle and are unable to be released from this arrest by stimulation with mitogens including serum and growth factors. To inspect a possibility of reversing the quiescence at the GO phase, we have exploited the mouse skeletal muscle cell line C2SVTts11, which is a clone of C2 cells transfected with the SV40 T antigen gene (encoding thermolabile large T and wild-type small t) fused to an inducible promoter. When the large T is induced in the myotubes, the terminally differentiated cells reenter the cell cycle and proceed to S and M phases. To elucidate how large T forces the myotubes to traverse each phase of the cell cycle, we examined the expression and activity of Cdk2 and Cdc2, which in complex with cyclin A and cyclin B are essential for S and M phases, respectively in undifferentiated cells. The levels of their mRNAs and proteins and histone H1 kinase activity, which was ascribed to Cdc2-cyclin B, were high in the proliferating myoblasts but gradually decreased during terminal differentiation. In contrast, they were reinduced in the myotubes reentering the cell cycle. Stimulation of the myotubes with serum failed to evoke these factors. These results indicate that large T, but not mitogens, is able to drive terminally differentiated myotubes to pass each phase of the cell cycle through eliciting these factors as do mitogens on proliferating undifferentiated cells. Since large T is a nuclear protein, signals generated by the protein in the nucleus are likely to be sufficient to induce each phase of the cell cycle in the terminally differentiated cells.