AbstractAddition of fetal calf serum (FCS) to serum‐deprived L6J1 rat myoblasts increases fos‐like immunoreactivity. The nuclear immunoreactivity reached a maximum 2 h after serum addition. Effects of the c‐fosprotein on myoblast proliferation were analyzed in L6J1 rat myoblasts transfected with the murine c‐fosgene under control of a metallothionein promoter. L6J1 myoblasts with elevated expression of transfected c‐fosreached higher cell densities thanneotransfected control myoblasts when approaching a stationary phase in normal culture conditions (5 FCS). The differences in cell densities were even more pronounced at low serum concentrations (0.5 FCS). c‐fostransfected cells also had a faster growth rate than did control cells in serum‐free medium supplemented with calcium chloride, lithium chloride, sodium selenite, hydrocortisone, and insulin. The cell morphology of c‐fostransfected L6J1 myoblasts was not affected compared to control myoblasts.These results suggest that c‐fosprotein expression in L6J1 myoblasts is activated by serum and that mitogenic stimulation of L6J1 myoblasts is facilitated by the presence of elevated amount
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