title: Effect of phorbol myristate acetate on the recovery of spontaneous and ultraviolet light-induced 6-thioguanine and ouabain-resistant Chinese hamster cells.
abstract: 12-0-Tetradecanoyl-phorbol-13-acetate (TPA) and phorbol were tested in Chinese hamster cells for their effects on mutagenesis (resistance to 6-thioguanine and to ouabain), DNA repair, and survival after ultraviolet (UV) irradiation. Recovery of 6-thioguanine- and ouabain-resistant colonies was significantly increased by TPA treatment and, to a lesser extent, by phorbol in UV-irradiated cells. Moreover, maximum enhancement of recoverable UV-induced 6-thioguanine- and ouabain-resistant mutants occurred when TPA was present after the mutation "expression" time and after the completion of DNA repair. This eenhancement effect, while persisting up to 18 days in the 6-thioguanine mutation system, was maximal when TPA was applied about 2 days after UV irradiation for the ouabain resistance mutation system. No significant decrease in cell survival was noted after post-UV treatment with TPA or phorbol, under conditions where there was a slight but nonspecific inhibition of unscheduled DNA repair synthesis. These results do not support the hypothesis that the tumor-promoting activity of TPA is due to its ability to inhibit "error-free" excision repair. The results are, however, consistent with a "two-stage" hypothesis of carcinogenesis which includes mutational and epigenetic mechanisms to explain the initiation and promotion phases.
mesh_terms: D000818:Animals; D002470:Cell Survival / Q000187:drug effects / Q000528:radiation effects; D002478:Cells, Cultured; D006224:Cricetinae; D004260:DNA Repair / Q000187:drug effects*; D004351:Drug Resistance; D009154:Mutation / Q000187:drug effects* / Q000528:radiation effects; D009374:Neoplasms, Experimental / Q000209:etiology; D010042:Ouabain / Q000494:pharmacology*; D010704:Phorbols / Q000494:pharmacology*; D013755:Tetradecanoylphorbol Acetate / Q000494:pharmacology*; D013866:Thioguanine / Q000494:pharmacology*; D013997:Time Factors; D014466:Ultraviolet Rays
chemical_list: D010704:Phorbols; D010042:Ouabain; D013866:Thioguanine; D013755:Tetradecanoylphorbol Acetate