REGULATION OF C-JUN AP-1 EXPRESSION IN RAT L6 MYOBLASTS

Myogenic differentiation of skeletal myoblasts in culture is triggered by withdrawal of serum mitogens. Expression of one of the serum-inducible, immediate early genes, the protooncogene c-jun, is maintained under low-serum conditions during myogenic differentiation of L6 myoblasts. In this report we have used agents that modulate protein kinases and Ca2+ levels to investigate how the expression of c-jun and myogenin mRNA and also the activator protein 1 (AP-1) DNA-binding activity are regulated in differentiating L6 cells. Our results show that expression of c-jun and myogenin are regulated independent of each other. Furthermore, down regulation of c-jun expression does not cause an increase in myogenin expression, suggesting that c-jun does not suppress myogenin expression in these cells. Electrophoretic mobility shift and ultraviolet cross-linking analyses revealed that the AP-1 complexes of proliferating myoblasts and differentiating myotubes are formed of similar set of polypeptides, and the AP- 1 binding activity is probably modulated by posttranslational modifications in differentiating L6 cells.