Inhibition of myogenesis by okadaic acid, an inhibitor of protein phosphatases, 1 and 2A, correlates with the induction of AP1

Recently, we demonstrated that okadaic acid, an inhibitor of protein phosphatases 1 and 2 A, inhibits myogenesis by extinguishing the expression of MyoD1 and inducing the expression of Id. Since it has been reported that transformation by c-fos also inhibits myogenesis through inhibition of MyoDl expression, we examined the effects of okadaic acid on the activation of the c-fos and jun family of proto-oncogenes in an attempt to understand the mechanism by which okadaic acid inhibits the myogenic differentiation. Treatment of C2C12 cells in growth medium with okadaic acid increased expression of the mRNAs for the c-fos family continuously and for the jun family to a lesser extent. In contrast, in differentiation medium, the induction of c-fos, c-jun, and fos B mRNAs by okadaic acid was transient, whereas fra-1, jun D, and jun B mRNAs were induced continuously, suggesting that okadaic acid regulates the expression of the c-fos and jun family through complex regulatory mechanisms depending on the state of differentiation of the cells. Transfection of c-jun and c-fos promoter-chloramphenicol acetyltransferase constructs demonstrated that the effects of okadaic acid on the induction of cfos and c-jun are mediated through the activation of promoter elements. These results suggest that some of the targets of protein phosphatases 1 and 2A may include transcription factors capable of forming API complexes and that these factors may play an important role during myogenic differentiation.