Early stimulation by EGF plus insulin of rRNA, c-fos, and actin mRNA expression: inhibition by cytochalasin D.

Stimulation of membrane ruffling is one of the first events induced by addition of growth factors to quiescent cultures. In order to assess the importance of intact cytoskeleton in induction, by EGF + insulin, of early events such as stimulation of rRNA, c-fos, and actin mRNA expression, we studied the effect of cytochalasin D (CD) on these metabolisms. We observed that CD slightly increased rRNA synthesis in nonstimulated cells; conversely, it decreased rRNA synthesis in cells stimulated by EGF + insulin. The maximal inhibition observed was 60%. The c-fos mRNA was not expressed in control cells and was accumulated in cells stimulated by the mixture of EGF + insulin; this accumulation was inhibited by CD. Actin mRNA was expressed in control cells and its expression was stimulated by EGF + insulin. Addition of CD decreased actin mRNA accumulation in stimulated cells but increased this accumulation in unstimulated cells. Our results, taken together, show that CD specifically affected the stimulation of rRNA and mRNA expression induced by growth factors and suggest that intact cytoskeleton and possibly membrane ruffling favored this stimulation.