Formation and removal of 7,12-dimethylbenz[a]anthracene -- nucleic acid adducts in rat mammary epithelial cells with different susceptibility to carcinogenesis

Abstract

The susceptibility of the rat mammary gland to 7,12-dimethylbenz[a]anthracene (DMBA)-induced carcinogenesis is maximal when DMBA is fed to young virgin rats, diminishing with increasing age and becoming almost nil when pregnancy and lactation occur prior to carcinogen administration. This present study was carried out to determine whether these differences in susceptibility to DMBA are related to the ability of the gland to form and/or remove DMBA—nucleic acid adducts. Primary mammary epithelial cell cultures derived from young virgin (YV), old virgin (OV) and parous (P) Sprague-Dawley rats were treated in vitro with [ ³ H]-DMBA for 24 h. Hydrolysates of DNA isolated from these cells were chromatographed on Sephadex LH-20 columns. In cells from the three groups of animals, the chromatographic profiles were qualitatively similar with three 3H-labeled products eluting in the region expected for hydrocarbon — nucleoside adducts. This similarity in profiles suggested that identical DMBA—nucleic acid adducts are generated in each group. The chromatographic data are also consistent with the hypothesis that the hydrocarbon — DNA adducts of DMBA were generated through reactions of a “bay-region” diol-epoxide of DMBA with DNA. Excision of DMBA—DNA adducts was investigated by measuring the time-dependent decrease in specific activity of the less dense parental DNA peak following centrifugation on alkaline CsCI gradients. Within 48 h post-DMBA treatment, P cells removed ∼36% of the adducts from DNA while YV and OV cells removed only 24% and 12% of the adducts respectively. In addition, P cells excised the adducts at a rate 1.5 and 5 times faster than either YV or OV cells. These results suggest that the lower susceptibility of the parous rat towards DMBA-induced carcinogenesis may be due not to the generation of different DMBA — DNA adducts, but to a more efficient DNA repair mechanism that is the result of increased gland differentiation through the process of pregnancy and lactation.