32P-postlabeling analysis of the formation and persistence of DNA adducts in mammary glands of parous and nulliparous mice treated with benzo[a]pyrene.

Abstract

The susceptibility of the rodent mammary gland to known chemical carcinogens can vary with the stage of gland development. Full-term pregnancy (parity) can confer permanent structural and functional changes in the gland that are associated with decreased breast cancer risk in humans and protection from mammary carcinogenesis in rodents. In this study, the formation and persistence of benzo[a]pyrene (B[a]P)-derived DNA adducts in vivo was determined in the abdominal mammary organs of adult nulliparous and parous BALB/c mice treated orally with the carcinogen. Mammary DNA isolated from animals in both groups revealed only one major adduct on TLC maps by P1-nuclease 32P-postlabeling analysis. The major adduct co-migrated with the (+)-enantiomer of anti-BPDE-dGp. Much lower levels of the (-)-enantiomer were detected. Most of the adduct quantitated was probably contributed by cells of the stromal compartment, since the gland-free organ (cleared fat pad) generated essentially equivalent profiles and level of adduct by 32P-postlabeling. Comparable levels of the B[a]P-derived adduct were observed in the intact mammary organ of both nulliparous and parous mice during a time course from 1 to 28 days after treatment. In both cases, adduct removal occurred exponentially with a half-life of approximately 16 days. The capacity for de novo formation of reactive metabolites by the mammary organ was demonstrated in vitro: digests of DNA from mammary mince exposed to B[a]P generated an adduct on TLC maps that also co-migrated with the (+)-anti-BPDE-dGp standard. Thus, our cumulative findings confirm the capacity of mammary cells to form potentially carcinogenic DNA adducts; however, the functional changes that occur in the mouse mammary gland as a result of parity did not influence the profile, level or persistence of adduct following exposure to a known carcinogen. Other factors, such as changes in mammary cell cycle kinetics or responsiveness to promotional stimuli may be more causally related to reduction in incidence of neoplasia observed in parous animals.