Protein synthesis inhibitors, like growth factors, may render resting 3T3 cells competent for DNA synthesis: a radioautographic and cell fusion study

. Serum‐deprived (0.1–0.2%) resting NIH 3T3 mouse fibroblasts pre‐incu‐bated with cycloheximide (7.5 μ/ml), or puromycin (10 μ/ml), were fused with stimulated cells taken 10 h after changing the medium to one containing 10% serum, and DNA synthesis was investigated in the nuclei of monokaryons, homodikaryons and heterodikaryons using radioautography with the double‐labelling technique. Pre‐incubation of resting cells with inhibitors of protein synthesis for 1–4 h abolished their ability to suppress DNA synthesis in stimulated nuclei in heterokaryons. Three hours after the removal of cycloheximide from the medium, the resting cells acquired once again the inhibitory capacity for entry of stimulated nuclei into the S period. This inhibitory influence disappeared also in the case of post‐fusion cycloheximide application as well as following an 8–12 h pre‐treatment of resting cells with actinomycin D (1 μ/ml) prior to fusion. Pre‐incubation of resting cells for 12 h with PDGF (1 u/ml ‐ ) followed by an 8–48 h incubation in serum‐free medium stimulated the onset of DNA synthesis. A brief exposure (45 min) of resting cells to cycloheximide (7.5 μ/ml), or puromycin (7.5 μ/ml), exerted a similar effect, inducing by itself the entry of cells into the S period. The results support the assumption that 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.