Elevated c‐fos expression inhibits differentiation of L6 rat myoblasts

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Abstract
Expression of c‐fos is induced by a number of signals in several cell systems. Although the exact function of the c‐fos product is unknown, it has been implicated to be of importance for both cell growth and differentiation (Verma and Sassone‐Corsi, 1987). To analyze how c‐fos expression relates to in vitro myogenic differentiation, the kinetics of c‐fos mRNA expression during spontaneous in vitro differentiation of L6J1 myoblasts was examined; c‐fos transcripts were most abundant at day 4 of the differentiation process. Multinucleated myotubes and expression of α‐actin and myosin heavy chain (MHC) mRNA appeared later, at day 6 or 7, and increased to maximal levels after 10 days in culture. To analyze further the relation between c‐fos expression and L6J1 myogenic differentiation, L6J1 myoblasts were transfected with expression vectors containing the murine c‐fos gene driven by a metallothionein promoter. The growth rate of c‐fos‐transfected L6J1 cells did not differ from that of control cells. However, formation of myotubes was significantly reduced in c‐fos‐transfected L6J1 cultures compared with neo‐transfected controls. Myotube formation and expression of the myogenic markers α‐actin and MHC were reduced in subclones expressing high levels of c‐fos, but not in subclones with lower levels of c‐fos expression. These results indicate that a marked elevation of c‐fos expression at least partially inhibits L6J1 myogenic differentiation.