title: Influence of confluent holding time on UV light mutagenesis in human diploid fibroblasts.
abstract: We have investigated the induction of mutants resistant to 6-thioguanine (6TG) following 254 nm ultraviolet light exposure of density-inhibited cultures of human diploid fibroblasts. Phenotypic expression of 6TG resistance was maximal within 9 days and remained stable through 19 days after irradiation. In reconstruction studies, complete recovery of 6TG-resistant mutants occurred at cell densities of up to 35 000 cells per 100-mm petri dish. The induced mutation frequency increased linearly with dose over the range of 3-9 J/m2; the D0 of the survival curve was 4.2 J/m2. Delaying subculture to low density for 1.5-24 h after irradiation produced unexpected alterations in induced mutation frequencies. An increase in UV-induced mutations of approximately 3-fold was observed in cultures maintained in confluence for 3 h. This trend was reversed with longer holding times: the mutation frequency declined sharply in cultures held for 6 h as compared to the 3-h value, and thereafter showed a steady and gradual diminution to background levels. These data suggest that the repair of potentially mutagenic damage is a complex phenomenon which can lead to an increase or decrease in mutation frequency as a function of holding time. Although the decline in mutation frequency observed following longer holding intervals is consistent with the notion of an error-free process, we hypothesize that the increased mutation frequency produced by a short holding period reflects the existence of a cell-mediated process which enhances the mutagenic potential of at least some UV-induced DNA photoproducts.
mesh_terms: D004260:DNA Repair; D004351:Drug Resistance; D005347:Fibroblasts / Q000187:drug effects / Q000528:radiation effects*; D006801:Humans; D010641:Phenotype; D013866:Thioguanine / Q000494:pharmacology; D013997:Time Factors; D014466:Ultraviolet Rays
chemical_list: D013866:Thioguanine