Patterns of DNA adduct formation in liver and mammary epithelial cells of rats treated with 7,12-dimethylbenz(a)anthracene, and selective effects of chemopreventive agents

7,12-Dimethylbenz(a)anthracene (DMBA) is a prototype carcinogen that induce s a high yield of mammary tumors in rats after a single feeding. We investi gated the induction and chemoprevention of DNA adducts in female Sprague Da wley rats receiving DMBA by gavage according to a variety of treatment sche dules. The patterns of P-32-postlabeled DNA adducts in liver and mammary ep ithelial cells were similar to those produced by the ill vitro reaction of metabolically activated DMBA with calf thymus DNA. There was a high and sta tistically significant correlation between dose of DMBA administered to rat s (0, 0.6, 2.4, and 12 mg/kg body weight) and levels of DNA adducts in both types of cells. The regression lines relating DMBA doses to total DNA addu ct levels were significantly divergent and crossed at 1.5 mg/kg body weight , indicating that, at lower doses, the formation of DNA adducts is more int ense in target mammary cells, whereas at higher doses, DNA adduct levels ar e more elevated in liver cells, presumably due to the greater metabolic cap acity of this organ, When the rats were sacrificed 7 days rather than 2 day s after DMBA administration, DNA adduct levels were approximately halved in both liver and mammary cells, The observed patterns can be interpreted bas ed on toxicokinetic factors, local and distant metabolism, removal of DNA a dducts by excision repair, and cell proliferation rate. Of three chemopreve ntive agents given with the diet to rats treated with 12 mg of DMBA, 5,6-be nzoflavone (1650 ppm) was the most effective, inhibiting DNA adduct formati on in liver and mammary cells by 96.5 and 83.5%, respectively. Feeding of 1 ,2-dithiole-3-thione (600 ppm) inhibited this biomarker by 68.5 and 50.2%, whereas butyl hydroxyanisole (BHA; 5000 ppm) showed a significant inhibitio n in the liver (46.5%) but: was ineffective in mammary cells (29.0%, not si gnificant). These data correlate nicely with the results of a parallel stud y in which 5,6-benzoflavone, 1,2-dithiole-3-thione, and BHA inhibited forma tion of hemoglobin adducts by 80.0, 44.0, and 0%, respectively; the inciden ce of mammary tumors by 82.4, 47.1, and 5.9%, respectively; and their multi plicity by 92.6, 80.0, and 7.4%, respectively, Therefore, biomarkers of bio logically effective dose are highly predictive of the efficacy of chemoprev entive agents in the DMBA rat mammary model, The selective inhibition by BH A of DNA adducts in the liver but not in mammary cells is consistent with t he finding that this phenolic antioxidant stimulated phase II activities in the liver but not in the mammary gland (L. L. Song et at, manuscript in pr eparation). In any case, the broad-spectrum inducer 5,6-BF appears to be mo re effective than the two monofunctional phase II inducers, presumably beca use an enhanced activation of DMBA to reactive metabolites is coordinated w ith their blocking, detoxification, and excretion.