[DNA synthesis in heterodikaryons obtained by the fusion of long-term stimulated cells of the NIH 3T3 line with short-term stimulated cells].

Serum-deprived (0.2%) resting NIH 3T3 mouse fibroblasts, stimulated for 0.5-4.0 h by serum (10%) or PDGF (beta-beta homodimer) (early cells), were fused with stimulated cells taken 10 h after changing the medium to one containing 10% serum DNA synthesis was investigated in the nuclei of monokaryons, homodikaryons and heterodikaryons, using radioautography with the double-labeling technique. Preincubation of resting cells with serum for 3 h or with the competence factor PDGF for 1.0-1.5 h abolished their ability to suppress DNA synthesis in late stimulated nuclei in heterodikaryons. Actomycin D restores the inhibitory capacity of resting cells stimulated by PDGF but not by serum. The results give evidence that the acquirement by resting cells of competence for DNA replication includes as a necessary step the down-regulation of intracellular growth inhibitors, and thus serum can overcome the influence of endogenous growth inhibitors, at any rate, by two ways.