Growth factors and endogenous control of cell proliferation.

Incubation of resting (serum-deprived) NIH 3T3 mouse fibroblasts for 12 hours with PDGF1) stimulates the onset of DNA synthesis. A brief exposure (45 minutes) of resting cells to inhibitors of protein synthesis (cycloheximide or puromycin) exerts similar effect inducing by itself the entry of cells into the S period. Incubation with EGF1) following pretreatment with either PDGF or protein synthesis inhibitors does not enhance the number of cells synthesizing DNA. The results support the assumption that the acquirement, by resting cells, of competence for DNA replication includes, as a necessary step, the down-regulation of intracellular growth inhibitors whose formation depends on protein synthesis.