Role of tyrosine kinase in the regulation of myogenin expression.

Using an affinity-purified anti-myogenin antibody, three stages of mouse myoblast C2C12 cells during myogenesis could be identified: proliferating myoblasts as myogenin-negative mononucleated cells, differentiating myoblasts as myogenin-positive mononucleated cells, and myotubes as myogenin-positive multinucleated cells. We found differential effects of genistein, an inhibitor of protein-tyrosine kinase, on myogenic cells during these three stages. Genistein severely inhibited myotube formation and myogenin production in differentiating myoblasts by inhibiting the transcription of the myogenin gene in a dose-dependent manner. We also found that genistein inactivated mitogen-activated protein kinase (MAP kinase) accompanied by suppression of myogenin expression. In contrast, genistein failed to inactivate MAP kinase and eliminate myogenin from myotubes. The results suggest that protein-tyrosine kinase plays a role in the transcriptional regulation of myogenin through the MAP kinase cascade during myogenesis. Furthermore, genistein inhibited the transactivation of the myosin heavy chain gene by constitutively expressed myogenin. Therefore, it is suggested that protein-tyrosine kinase is involved in the post-translational regulation of myogenin as well as in transcriptional regulation during myogenesis.