2-Amino-1-methyl-6-phenylimidazo[4,5-
b]pyridine (
PhIP) is a heterocyclic
amine derived from cooked meat that is a mammary gland
carcinogen in rats. A carcinogenic dose-regimen of
PhIP (75 mg/kg, p.o., 10 doses, once per day) was administered to 43-day old female Sprague-Dawley rats, and the rats were then placed on a defined high fat (23.5%
corn oil) or low fat (5%
corn oil) diet for up to 6 weeks. At various times after
carcinogen and diet, and prior to
carcinogenesis, we examined the percentage of proliferating cells in terminal end bud (TEB) epithelial structures of the rat mammary gland by
proliferating cell nuclear antigen staining, mammary gland architecture by whole mounting, and
PhIP-DNA adduct levels in mammary epithelial cells by the 32P-post-labeling assay. Immediately after dosing, the percentage of proliferating epithelial cells in TEBs was significantly higher in
PhIP-treated rats than in control rats receiving vehicle only [7.5 +/- 0.9% (n = 99) versus 4.2 +/- 0.6% (n = 127), respectively]. The mammary glands of
PhIP-treated rats showed a significantly lower density of alveolar buds (ABs) and a higher density of TEBs than control rats, which suggests that
PhIP exposure partially inhibited the normal glandular differentiation of TEBs to ABs. After 6 weeks on the diet, proliferation in TEBs was statistically higher in rats given
PhIP plus a high fat diet than in rats given vehicle plus a
low fat diet. The mammary glands from rats on a high fat diet also showed a statistically higher density of TEBs when compared with rats on a
low fat diet [2.08 +/- 0.34% versus 1.04 +/- 0.20%, respectively (n = 6)].
PhIP-DNA adduct levels were relatively high in mammary epithelial cells of treated rats. At 3 h after the last dose of
PhIP, DNA adduct levels [relative adduct labeling (RAL) x 10(7), mean +/- SE] were 10.5 +/- 1.7 (n = 8) and 0.9 +/- 0.2 (n = 7) in epithelial cells isolated from mammary gland and in the liver, respectively.
DNA adduct removal rates from the mammary gland were not different between rats on the high fat and
low fat diets. Adducts were still detected after 6 weeks on either diet. Thus, events that occurred prior to
neoplasia in the mammary glands of
PhIP-treated rats include formation of
PhIP-
DNA adducts at relatively high levels, and enhanced proliferation in TEBs (putative sites of origin of mammary gland
carcinomas) and partial inhibition of TEB differentiation. The high fat diet, a promoter of
PhIP-induced mammary gland
carcinogenesis, appeared to sustain the proliferative effect of
PhIP in mammary gland TEBs at a time when
PhIP-
DNA adducts are still detectable. These early events may contribute to the targeting and carcinogenicity of
PhIP to the mammary gland of rats.