Age-, tissue-, and Ah genotype-dependent differences in the binding of 3-methylcholanthrene and its metabolite(s) to mouse DNA

The induction of transplacental carcinogenesis by 3-methylcholanthrene (MC) in mice is determined, in part, by the genotype at the Ah locus. The relationship of Ah genotype and MC-induced DNA adducts was tested by comparing the response of pregnant and fetal C57BL/6 mice (Ahb Ahb; responsive to the induction of MC metabolism) and DBA/2mice (Ahd Ahd; nonresponsive). On day 17 of gestation (day 1 = presence of vaginal plug), C57BL/6 mice were treated i.p. with 100 mg/kg MC and DBA/2 mice with 30 mg/kg. Mice were sacrificed 24 h later and the tissues were analyzed for the presence of DNA adducts using the P1 nuclease version of the 32P-postlabeling method. With a 3.3-fold difference in administered dose, the total adduct levels in fetal DNA were (a) similar in both strains with the exception of liver, for which C57BL/6 mice had more adducts; (b) higher in the lung than skin, liver, or thymus; and (c) only 1/4 to 1/14 of the adult levels. Maternal DBA/2DNA contained more adducts in the thoracic lymph nodes and liver but fewer in the placenta and lung, compared to maternal C57BL/6 DNA. More adducts were detected in lung DNA than liver DNA in C57BL/6 mice. In contrast, these levels were similar in DBA/2 mice. When the difference in dose administered was considered in conjunction with this, less MC bound to DNA of C57BL/6 than DBA/2 mice overall. To identify adducts, oxidized metabolites of MC, 1-hydroxy-, 2-hydroxy-, 9,10-dihydrodiol-, or 3-methoxymethyl-MC, were topically applied to the dorsal skin of both strains. All of these metabolites produced adducts. Approximately 14 different adduct spots were detected. The two most abundant adducts were produced by 1-hydroxy-, 2-hydroxy-, and 9,10-dihydrodiol-MC. One of these also contained a 3-hydroxymethyl group. Several adducts did not contain the 9,10-dihydroxy group. The adducts derived from 3-methoxymethyl-MC were consistently found in greater abundance in DNA from C57BL/6 tissues, compared with DBA/2. Thus, oxidation of the 3-methyl group may be enhanced by Ah-dependent induction of MC metabolism. Together, these results suggest that the individual and total adduct levels are influenced by the genotype at the Ah locus, the route of administration, and the metabolite(s) with tissue and age specificity.