Characterization of a UV-resistant strain, UVr-10, established from a human clonal cell line, RSb, with high sensitivity to UV, 4NQO, MNNG and interferon.

Characterization was performed of a UV-resistant variant strain. UVr-10, derived from a human clonal cell line, RSb, with high sensitivity not only to the lethal effect of 254-nm far-ultraviolet (UV) irradiation but also to the effects of 4-nitroquinoline 1-oxide (4NQO) and N-methyl-N'-nitro-N-nitrosoguanidine (MNNG), and to the cell proliferation inhibition (CPI) effect of human leukocyte interferon (HuIFN-alpha) preparations. Colony-formation assays confirmed the increased resistance of UVr-10 cells to both UV and 4NQO, but no increased resistance to MNNG. The marked recovery from the inhibition of the total cellular DNA synthesis of UVr-10 cells, estimated by [methyl-3H]thymidine ([3H]dThd) uptake into the cellular DNA materials, was seen during 6 h after UV irradiation or 4NQO treatment even under the conditions without the recovery uptake into those of the parent RSb cells, but not during 6 h after MNNG treatment. Comparative studies on the activity of DNA repair synthesis between UVr-10 and RSb cells, by measuring the extent of UV-, 4NQO- or MNNG-induced unscheduled DNA synthesis (UDS) and DNA repair replication, revealed an increased activity of UVr-10 cells to UV and 4NQO but no significant increase of the activity to MNNG. These results suggest that increased DNA repair activities of a UVr-10 cell line may account for its becoming resistant to the lethal effect of UV and 4NQO. Concerning the CPI effect of HuIFN-alpha, UVr-10 cells showed increased resistance. Further, the DNA synthesis activity of UVr-10 cells was not so inhibited by HuIFN-alpha exposure as that of RSb cells. However, HuIFN-alpha-exposed UVr-10 cells showed more enhanced levels of activity of pppA(2'p5'A)n synthetase (2-5A synthetase) than the exposed RSb, thus suggesting that HuIFN-alpha could exert enough intracellular effect even in UVr-10 cells. The implication of the increased resistance of UVr-10 cells to the effects of UV, 4NQO and HuIFN-alpha, but not to those of MNNG, is discussed.