The relationship between DNA damage and mutation frequency in mammalian cell lines treated with N-nitroso- N-2-fluorenylacetamide

Abstract

The induction of DNA single-strand breaks in C3H10T1/2 mouse fibroblasts and Chinese hamster ovary (CHO) cells by N-nitroso- N-2-fluorenylacetamide (N-NO-2-FAA) was demonstrated by the alkaline elution technique. Without metabolic activating system (i.e., rat liver S9 fraction), N-NO-2-FAA exhibits more direct and strong damaging effects on DNA than its parent compound, 2-FAA, at equal concentration in both cell lines. To compare the DNA-damaging potency of N-NO-2-FAA with other well-known carcinogens, such as benzo[ a]pyrene, 2-nitrofluorene, and N-methyl- N′-nitro- N-nitrosoguanidine (MNNG), the order of potency is as follows: MNNG (5 μM) > N-NO-2-FAA (150 μM) > benzo[ a]pyrene (20 μM) at equitoxic concentrations, LD 37, in the same cell system. Another parallel experiment indicated that N-NO-2-FAA could disrupt the superhelicity of circular plasmid DNA (pBR 332) at a dose range of 0.1–50 mM; however, a complete conversion to form III linear DNA was found at the highest concentration (50 mM). After treatment with various concentrations of N-NO-2-FAA, ouabain resistance (oua r) was induced in C3H10T1/2 cells, while both oua r and 6-thioguanine resistance (6-TG r) were induced in CHO cells. The mutation frequency in the Na +/K +-ATPase locus in CHO cells (1.5 × 10 −6 mutants/ μM) is higher than that in C3H10T1/2 cells (1.0 × 10 −6 mutants/ μM). The maximal mutation frequency at the Na +/K +-ATPase gene locus was attained with 30 min of exposure in C3H10T1/2 cells, whereas the mutation frequency in CHO cells continued to increase up to 80 min of treatment. Similarly, the maximal mutation frequency at the HPRT locus also continued to increase up to 80 min of treatment. Finally, a linear plot of alkali-labile lesions versus 6-TG r mutations was obtained; but the same relationship was not observed in the case of oua r mutation.