PROLIFERATION, DEVELOPMENT AND DNA ADDUCT LEVELS IN THE MAMMARY-GLANDOF RATS GIVEN 2-AMINO-1-METHYL-6-PHENYLIMIDAZO[4,5-B]PYRIDINE AND A HIGH-FAT DIET

2-Amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) is a heterocycl ic amine derived from cooked meat that is a mammary gland carcinogen i n 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 ra ts, and the rats were then placed on a defined high fat (23.5% corn oi l) or low fat (5% corn oil) diet for up to 6 weeks. At various times a fter carcinogen and diet, and prior to carcinogenesis, we examined the percentage of proliferating cells in terminal end bud (TEB) epithelia l structures of the rat mammary gland by proliferating cell nuclear an tigen staining, mammary gland architecture by whole mounting, and PhIP -DNA adduct levels in mammary epithelial cells by the P-32-post-labeli ng assay. Immediately after dosing, the percentage of proliferating ep ithelial cells in TEBs was significantly higher in PhIP-treated rats t han in control rats receiving vehicle only [7.5 +/- 0.9% (n = 99) vers us 4.2 +/- 0.6% (n = 127), respectively], The mammary glands of PhIP-t reated 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 r ats 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 wh en 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 la st dose of PhIP, DNA adduct levels [relative adduct labeling (RAL) x 1 0(7), mean +/- SE] were 10.5 +/- 1.7 (n = 8) and 0.9 +/- 0.2 (n = 7) i n epithelial cells isolated from mammary gland and in the liver, respe ctively, DNA adduct removal rates from the mammary gland were not diff erent between rats on the high fat and low fat diets. Adducts were sti ll detected after 6 weeks on either diet. Thus, events that occurred p rior to neoplasia in the mammary glands of PhIP-treated rats include f ormation of PhIP-DNA adducts at relatively high levels, and enhanced p roliferation in TEBs (putative sites of origin of mammary gland carcin omas) and partial inhibition of TEE 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 t ime when PhIP-DNA adducts are still detectable. These early events may contribute to the targeting and carcinogenicity of PhIP to the mammar y gland of rats.