Transformation by myc prevents fusion but not biochemical differentiation of C2C12 myoblasts: mechanisms of phenotypic correction in mixed culture with normal cells

To study the effects of myc oncogene on muscle differentiation, we infected the murine skeletal muscle cell line C2C12 with retroviral vectors encoding various forms of avian c- or v-myc oncogene. myc expression induced cell transformation but, unlike many other oncogenes, prevented neither biochemical differentiation, nor commitment (irreversible withdrawal from the cell cycle). Yet, myotube formation by fusion of differentiated cells was strongly inhibited. Comparison of uninfected C2C12 myotubes with differentiated myc-expressing C2C12 did not reveal consistent differences in the expression of several muscle regulatory or structural genes. The present results lead us to conclude that transformation by myc is compatible with differentiation in C2C12 cells. myc expression induced cell death under growth restricting conditions. Differentiated cells escaped cell death despite continuing expression of myc, suggesting that the muscle differentiation programme interferes with the mechanism of myc-induced cell death. Cocultivation of v-myc-transformed C2C12 cells with normal fibroblasts or myoblasts restored fusion competence and revealed two distinguishable mechanisms that lead to correction of the fusion defect.