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-induc ible, immediate early genes, the protooncogene c-jun, is maintained un der low-serum conditions during myogenic differentiation of L6 myoblas ts. In this report we have used agents that modulate protein kinases a nd 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 expressi on of c-jun and myogenin are regulated independent of each other. Furt hermore, down regulation of c-jun expression does not cause an increas e in myogenin expression, suggesting that c-jun does not suppress myog enin expression in these cells. Electrophoretic mobility shift and ult raviolet cross-linking analyses revealed that the AP-1 complexes of pr oliferating myoblasts and differentiating myotubes are formed of simil ar set of polypeptides, and the AP- 1 binding activity is probably mod ulated by posttranslational modifications in differentiating L6 cells.