Study of DNA repair synthesis induced by ultraviolet irradiation and carcinogenic chemicals

The rate of DNA repair synthesis was studied using [3H]-thymidine to pulse confluent, hydroxyurea-treated, human diploid fibroblasts (WI-38) at intervals up to 76 hr after treatment with either 45 erg/sq mm or 10 μM N-acetoxy-2-acetylaminofluorene. These doses induced equal repair incorporation during the first 4 hr after exposure when DNA's were prepared on CsCl buoyant density gradients. During this initial time period the uv-treated cells showed an almost-linear increase in repair synthesis while the chemically-treated cells showed a lag from 0 to 0.5 hr, followed by linear incorporation from 0.5 to 4 hr. After uv exposure, the rate of DNA repair synthesis was highest from 0 to 4 hr, and decreased considerably by 8 to 12 hr, so that by 24 to 28 hr the rate was only 20% of that from 0 to 4 hr, and by 72 to 76 hr incorporation was close to non-irradiated control levels. The rate of repair synthesis after continuous N-acetoxy-2-acetylaminofluorene administration decreased much more slowly, and by 72 to 76 hr was still about 50% of that from 0 to 4 hr as determined by CsCl gradients and autoradiography. Removal of N-acetoxy-2-acetylaminofluorene from the cultures at 4 hr reduced the 72 to 76 hr incorporation to 30% of the 0 to 4 value. Over a range of doses, three promotors, croton oil, 12-O-tetradecanoyl-phorbol-13-acetate (TPA) and anthralin were found to inhibit DNA repair synthesis while their non-promoting analogs phorbol and 1,8 dihydroxyanthraquinone had little effect. Another tumor promoter, phenol, inhibited DNA repair synthesis only at very high concentrations while an analog, 4-nitrophenol, produced inhibition of DNA repair synthesis at molar concentrations at which phenol had no effect