Differences in the rate of DNA adduct removal and the efficiency of mutagenesis for two benzo[a]pyrene diol epoxides in CHO cells

Abstract

The initiation of carcinogenesis by carcinogens such as 7r,8t-dihydroxy-9,10t-oxy-7,8,9,10-tetrahydrobenzo[a]pyrene (BPDE-I) is thought to involve the formation of DNA adducts. However, the diastereomeric diol epoxide, 7r,8t-dihydroxy-9,10c-oxy-7,8,9,10-tetrahydrobenzo[a]pyrene (BPDE-II), also forms DNA adducts but is inactive in standard carcinogenesis models. We have measured the formation and loss of DNA adducts derived from BPDE-II in a DNA-repair-proficient line of Chinese hamster ovary (CHO) cells, AT3-2, and in two derived mutant cell lines, UVL-1 and UVL-10, which are unable to repair bulky DNA adducts. BPDE-II adducts were lost from cellular DNA in AT3-2 cells with a half-life of 13.8 h; this was about twice the rate found for BPDE-I adducts. BPDE-II adducts were also lost from DNA in UVL-1 and UVL-10 cells, but at a much slower rate. When purified DNA was modified in vitro with BPDE-II and then held at 37 degrees C, DNA adducts were removed at a rate identical to that seen in UVL-1 and UVL-10 cells, suggesting that the loss in these cells was not due to enzymatic DNA-repair processes but to chemical lability of the adducts. Mutant frequencies at the APRT and HPRT loci were measured at BPDE-II doses that resulted in greater than 20% survival, and were found to increase linearly with dose. In the DNA-repair-deficient cells, the HPRT locus was moderately hypermutable compared with AT3-2 cells (about 5-fold); the APRT locus was extremely hypermutable, giving about 25-fold higher mutant fractions in UVL-1 and UVL-10 than in AT3-2 cells at equal initial levels of binding. When we compared the mutational efficiency of BPDE-II at both loci in AT3-2 cells (the mutant frequency in mutants/10(6) survivors at a dose that resulted in one adduct per 10(6) base pairs) with our previous studies of BPDE-1, we found that BPDE-II was 4-5 times less efficient as a mutagen than BPDE-I. This difference in mutational efficiency could be explained in part by the increased rate of loss of BPDE-II adducts from the cellular DNA, part of which was due to an increased rate of enzymatic removal of these lesions compared with the removal of BPDE-I adducts.