DNA synthesis, mutagenesis, DNA damage, and cytotoxicity in cultured mammalian cells treated with alkylating agents.

The effects of N-methyl- and N-ethyl-N'-nitro-N-nitrosoguanidines and methyl and ethyl methanesulfonates on DNA synthesis in C3H/10T 1/2 mouse embryo fibroblasts and Chinese hamster V79 cells have been analyzed by measuring the incorporation of tritiated thymidine into the trichloroacetic acid-insoluble fraction and the purified DNA of the cells, and by measuring the fraction of parental DNA replicated by equilibrium density sedimentation in neutral cesium chloride gradients. The inhibition of DNA synthesis produced by the alkylating agents at less than or equal to 90% lethal dose was uniformly associated with the cytotoxicity of all four of the agents in both lines of cells. The molar concentrations of the compounds required to inhibit DNA synthesis or produce cytotoxicity also correlated well in a log-log plot, with their potencies in producing DNA lesions that are associated with mutagenesis. However, equitoxic doses of the alkylating agents that inhibited DNA synthesis to equal extents produced frequencies of mutations and of alkali-labile DNA lesions that differed by about an order of magnitude, which did not produce major departures from the linear correlation apparent in the above log-log plot. Therefore, the kinds of DNA lesions that inhibit DNA synthesis and are associated with cytotoxicity in cells treated with the above alkylating agents are not uniformly associated with alkali-labile DNA lesions and gene mutations. Thus, evaluations of mutagenic potency, by assays of DNA synthesis and cytotoxicity, will be falsely high for compounds that predominantly produce DNA lesions that inhibit DNA synthesis and are associated with cytotoxicity. Such evaluations will be falsely low for compounds that predominantly produce DNA lesions that are mutagenic.