Distribution of covalent DNA adducts in mouse epidermal subpopulations after topical application of benzo(a)pyrene and 7,12-dimethylbenz(a)anthracene.

The distribution of benzo(a)pyrene [B(a)P] and 7,12-dimethylbenz(a)anthracene (DMBA):DNA adducts was examined in five different subpopulations of SENCAR mouse epidermal cells separated based on buoyant density in continuous gradients of 61.5% Percoll. Three fractions consisted of primarily basal cells (Fractions 3 to 5), while two less dense fractions (Fractions 1 and 2) consisted of primarily differentiating keratinocytes. The levels of B(a)P and DMBA:DNA adducts were examined at 1 h, 6 h, 24 h, 72 h (except DMBA), and 28 days after a single topical application of an initiating dose. Among the basal cell subpopulations, the level of covalent B(a)P:DNA adducts in Fraction 5 cells was significantly higher (P less than 0.05) than Fractions 3 and 4 at every time point examined. On the other hand, B(a)P:DNA adduct levels in Fraction 5 were only significantly higher than Fraction 2 at 6 h and 72 h and not significantly different from Fraction 1 at any time point. With DMBA, no significant differences were initially observed in the levels of covalent DNA adducts among the various Percoll fractions at 1 h and 6 h after treatment. However, at 24 h and at 28 days. Fraction 5 cells had significantly higher (P less than 0.05) levels of covalent DMBA:DNA adducts than Fractions 1 to 4. To explore whether the observed differences in DNA adduct levels were due to differences in metabolic activation, we examined the levels of covalent adducts among epidermal subpopulations after topical application of (+/-)-anti-benzo(a)pyrene-7,8-diol-9,10-epoxide (anti-BPDE). Interestingly, 3 h after treatment with anti-BPDE, significantly higher (P less than 0.05) levels of binding were found in Fraction 5 compared with Fractions 1 to 4. High-pressure liquid chromatographic analyses of B(a)P and DMBA:DNA adducts 6 h and 24 h after treatment did not show any significant differences in adduct profiles among the various subpopulations. These results demonstrate the presence and persistence of hydrocarbon:DNA adducts in all epidermal subpopulations isolated on continuous Percoll gradients for at least 28 days after treatment. Furthermore, of the three basal cell subpopulations, the most dense cells (Fraction 5) developed the highest DNA adduct levels within 24 h and retained these higher levels over 28 days. Finally, differences in DNA adduct levels among epidermal subpopulations do not appear to result from different metabolic capabilities of the cells. The potential significance of these results is discussed in terms of the process of skin tumor initiation.