SV40 LARGE-T INHIBITS MYOGENIC DIFFERENTIATION PARTIALLY THROUGH INDUCING C-JUN

Terminal differentiation of skeletal muscle cells is obligatorily accompanied by the expression of a battery of muscle-specific genes and cell fusion to form multinucleated myotubes. The transcription of some of the muscle-specific genes is activated by the products of myoD gene family including MyoD and myogenin. The mouse skeletal muscle cell line C2SVTts11, which is a clone of C2 cells transfected with SV40 T antigen genes (temperature-sensitive large T and wild-type small t) fused to metallothionein gene promoter, is prevented from differentiation when the large T is induced. If the large T is induced in the myotubes, which are preformed in the absence of large T expression, the terminally differentiated cells reenter the cell cycle. In good accordance with the induction of large T, endogenous c-jun but not c-fos or c-myc mRNA was induced, whereas the expression of myoD and myogenin was suppressed. Treatment of quiescent C2 cells with a tumor promoter, 12-O-tetradecanoylphorbol 13-acetate, transiently induced c-jun and c-fos mRNAs, and temporarily deinduced myoD and myogenin mRNAs just after the expression of the protooncogenes. To ascertain whether c-jun induced by large T is sufficient to inhibit myogenic differentiation, c-jun cDNA was transfected into C2 cells. As the levels of exogenous c-jun expression were higher in the transfected clones, the cells expressed lower levels of myoD gene family and they formed fewer myotubes. Even the cells expressing the highest levels of exogenous c-jun, however, still formed small myotubes containing a few nuclei under differentiation conditions. These results suggest that large T inhibits myogenic differentiation by suppressing the expression of the members of myoD gene family, partly through inducing c-jun. In addition to this, other mechanisms seem to be required to achieve complete inhibition.