Endogenous activation of c-myc expression and DNA synthesis in serum-starved neonatal rat smooth muscle cells

Earlier studies have shown that smooth muscle cells (SMC) from arteries of neonatal and adult rats differ markedly in their in vitro growth characteristics. Since some of these differences may be relevant to the proliferation of SMC in atherosclerotic plaques we examined the expression of three proto-oncogenes ( c-fos, c-jun, and c-myc) and an SMC-specific differentiation marker (α-actin) in cultured SMC. In presence of serum cultured adult SMC contained lower levels of α-actin mRNA than neonatal cells. In neonatal cells serum-starvation resulted in a distinct increase in both c-myc and α-actin mRNA levels, whereas the expression of these genes appeared to be unaffected in adult cells. Transfer of adult SMC proliferating in the presence of fetal calf serum to serum-free medium for 48 h almost completely inhibited DNA synthesis, whereas transfer of neonatal SMC to serum-free medium reduced DNA synthesis only to about 50%. Serum-starved adult and neonatal SMC did not contain c-fos or c-jun transcripts, but in both cell types serum-stimulation resulted in a comparable increase in the expression of both genes. The present results demonstrate clear differences in the mechanisms regulating gene expression in adult and neonatal SMC.