Low absolute mutagenic efficiency but high cytotoxicity of a non-bay region diol epoxide derived from benzo[ a]pyrene

Abstract

Insights into the mechanisms of chemical carcinogenesis can sometimes be gained by comparing the effects of closely related chemicals which differ in carcinogenic potency. We have treated Chinese hamster ovary (CHO) cells with a non-carcinogenic metabolite of benzo[ a]pyrene, 9 r,10 t-dihydroxy-7 c,8 c-oxy-7,8,9,10-tetrahydrobenzo[ a]pyrene (BPDE-III), and measured the formation and persistence of DNA adducts. We have correlated this binding data with cytotoxicity and mutagenicity in a DNA-repair-proficient CHO cell line (AT3-2) and in two derived lines, UVL-1 and UVL-10, which are unable to repair bulky DNA adducts. These data are compared with similar studies of the effects of the carcinogenic metabolite, 7 r,8 t-dihydroxy-9 t,10 t-oxy-7,8,9,10-tetrahydrobenzo[ a]pyrene (BPDE-I). Synchronous fluorescence spectroscopy was used to measure the levels of BPDE-III-DNA adducts in treated cells. Adduct levels increased linearly with dose, but the absolute binding levels were about 30-fold lower than in comparable incubations with BPDE-I. Measurements of the removal of adducts derived from these two diol epoxides indicated no significant difference in the rate of repair measured 24 h post-treatment. When cells were treated with increasing doses of BPDE-III, survival curves were obtained which exhibited a shoulder region at low doses and an exponential decrease in plating efficiency at higher doses. By comparison of the D 0's, the DNA-repair-deficient cell lines were found to be 4–5 fold more sensitive to the killing effects of BPDE-III than were the repair-proficient AT3-2 cells. Mutants at the APRT and HPRT loci were selected 1 week after treatment with BPDE-III at doses which resulted in > 10% survival. Mutant frequencies at both loci were found to increase linearly with dose. In the DNA-repair-deficient cells, both the HPRT and APRT loci were hypermutable, giving 9–15-fold higher mutant fractions in UVL-1 and UVL-10 than in AT3-2 cells at equal initial doses. When we compared the mutational efficiency of BPDE-III at both loci in AT3-2 cells (the mutant frequency in mutants/10 6 survivors at a dose which results in 1 adduct per 10 6 base pairs) with out previous studies of BPDE-I, we found that BPDE-III was 4–5 times less efficient as a mutagen than BPDE-I. However, a similar comparison of cytotoxic efficiency on a per adduct formed basis, revealed that BPDE-III was 2–3-fold more effective than BPDE-I. Since we have found no difference in the rate of removal of BPDE-I and BPDE-III adducts, these data suggest that BPDE-III adducts are inherently less mutagenic but more cytotoxic than are BPDE-I adducts.