Mitogen withdrawal triggers myogenic differentiation in skeletal myoblasts in culture. We have examined the expression of the proto-oncogene c-jun during this process in mouse C2C12 myoblasts. c-jun belongs to a family of immediate early genes whose expression is activated in cultured cells in response to the addition of serum growth factors. Interestingly, expression of c-jun was maintained in mouse C2C12 and rat L6 myoblasts undergoing myogenic differentiation under low-serum conditions. Previously it has been reported that expression of c-jun is downregulated during differentiation of C2 cells. However, our results using C2C12 cells, a subclone of the C2 line, show that c-jun mRNA, protein and the activator-protein 1 (AP-1) DNA-binding activity were easily detected in proliferating myoblasts and differentiated myotubes. Although overexpression of c-jun has been shown to block myogenic differentiation in C2 cells, results presented here suggest that expression of c-jun at physiological levels may not interfere with skeletal myogenesis.
