Human exposure to the class of nitropolynuclear
aromatic hydrocarbons is via inhalation and/or ingestion. Therefore, one of the goals of this study was to determine the propensity of the environmental contaminant
6-nitrochrysene (6-NC) for inducing
mammary cancer following its
oral administration to female CD rats. 2-Amino-1-methyl-6-phenylimidazo[4,5-
b]pyridine (
PhIP), an established mammary
carcinogen in the same animal model, was used as a positive control and
trioctanoin as a negative control. Thirty-day-old female CD rats were gavaged once weekly for 8 weeks with 6-NC at 50, 25, or 12.5 micromol/rat or
PhIP at 50 micromol/rat in 500 microL of
trioctanoin. Twenty-three weeks after the last
carcinogen administration, rats were decapitated, necropsied, and evaluated histologically. The most common mammary
tumors were
adenocarcinomas, followed by
adenomas and
fibroadenomas. The incidence and multiplicity (mean +/- standard deviation) of mammary
adenocarcinomas induced by these two
carcinogens at the highest dose (6-NC: 90%, 3.73 +/- 2.74;
PhIP: 83%, 2.62 +/- 2.58) were significantly higher than those in control rats (10%, 0.10 +/- 0.31). However, there were no statistically significant differences between groups treated with 6-NC and
PhIP or among groups receiving various doses of 6-NC. Following its metabolic activation, 6-NC is known to bind covalently to
DNA; however, it remains to be determined whether it can also induce
DNA base oxidation. Thus, employing the same route of administration, our studies revealed no effect of 6-NC on the basal level of
8-hydroxy-2'-deoxyguanosine (8-OHdG) in the mammary gland in tests at 6, 24, and 48 h after 6-NC treatment and at termination of the
carcinogenesis assay in the normal, noninvolved tissue and in mammary
tumors. This result suggests that covalent
DNA binding of 6-NC metabolites is important in the induction of
mammary cancer in rats. Therefore, the other goal of this study was to compare the tumorigenic activities of 6-NC and its metabolites in the rat mammary gland by intramammary administration. This route has also been used in our laboratory to induce
mammary cancer in the rat by 6-NC and is employed here to avoid systemic effects and to determine the role of the mammary gland in the metabolic activation of 6-NC and its metabolites. Toward this end, a new method was developed to obtain ample materials of trans-1,2-dihydroxy-1,2-dihydro-6-aminochrysene (1,2-DHD-6-AC); other metabolites were synthesized as reported previously. On the basis of the results, the carcinogenic potency toward the mammary gland is ranked in the following order: 6-NC > 1,2-DHD-6-NC > 6-AC > 6-NCDE > 1,2-DHD-6-AC. Among the metabolites tested, 1,2-DHD-6-NC was the most potent
carcinogen. It was significantly more active than its reduced product 1,2-DHD-6-AC. However, the potency of 1,2-DHD-6-NC was not significantly different from 6-AC, a metabolite derived from simple nitroreduction, or from 6-NCDE. Collectively, these results suggest that metabolites derived from both ring-oxidation and nitroreduction contribute to the overall carcinogenicity of 6-NC in the rat mammary gland.