title: 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.
authors.affiliation: Science Park-Research Division, University of Texas M.D. Anderson Cancer Center, Smithville 78957.
authors.forename: M C
authors.initials: MC
authors.lastname: MacLeod
authors.forename: A
authors.initials: A
authors.lastname: Daylong
authors.forename: G
authors.initials: G
authors.lastname: Adair
authors.forename: R M
authors.initials: RM
authors.lastname: Humphrey
mesh_terms: D015123:7,8-Dihydro-7,8-dihydroxybenzo(a)pyrene 9,10-oxide / Q000378:metabolism*; D000228:Adenine Phosphoribosyltransferase / Q000235:genetics; D000818:Animals; D001580:Benzopyrenes / Q000633:toxicity*; D016466:CHO Cells; D015197:Carcinogenicity Tests; D006224:Cricetinae; D004247:DNA / Q000378:metabolism*; D018736:DNA Adducts; D004260:DNA Repair / Q000187:drug effects*; D004305:Dose-Response Relationship, Drug; D004852:Epoxy Compounds / Q000378:metabolism / Q000633:toxicity*; D007041:Hypoxanthine Phosphoribosyltransferase / Q000235:genetics; D007700:Kinetics; D016296:Mutagenesis / Q000235:genetics*; D009152:Mutagenicity Tests; D009153:Mutagens / Q000633:toxicity; D009154:Mutation / Q000235:genetics; D013237:Stereoisomerism
chemical_list: C057268:7,8-dihydroxy-9,10-oxy-7,8,9,10-tetrahydrobenzo(a)pyrene-DNA adduct; D001580:Benzopyrenes; D018736:DNA Adducts; D004852:Epoxy Compounds; D009153:Mutagens; D015123:7,8-Dihydro-7,8-dihydroxybenzo(a)pyrene 9,10-oxide; C072019:benzo(a)pyrene diolepoxide I; C072116:BPDE-II; D004247:DNA; D000228:Adenine Phosphoribosyltransferase; D007041:Hypoxanthine Phosphoribosyltransferase