2-Amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) retards mammary gland involution in lactating Sprague-Dawley rats.

2-Amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP), a compound from cooked meat, is an established mammary gland carcinogen in female rats. Four doses of PhIP (150 mg/kg, p.o., once per day) were given to lactating Sprague-Dawley rats separated from their 10-day-old pups to initiate involution of the gland. Twenty-four hours after the last dose, apoptotic index in the mammary gland, as measured by the TUNEL assay, was significantly higher in the gland from control rats than in the PhIP-treated rats (4.757 +/- 1.066 versus 1.905 +/- 0.248%; P < 0.05). In comparison with controls, alveoli in the mammary gland of PhIP-treated rats were also visibly larger and contained more secretory epithelial cells. The expression of Bax, a stimulator of apoptosis, and Bcl-2, an inhibitor of apoptosis, were quantitated by western blotting. Accordingly, Bax expression was 2.7-fold higher in control rats, whereas Bcl-2 expression was 3.1-fold higher in PhIP-treated rats, both changes being statistically different (Student's t-test, P < 0.05). Immunohistochemistry further confirmed a lower expression of Bax and higher expression of Bcl-2 in secretory alveolar epithelial cells of the PhIP-treated mammary gland. The findings are consistent with the notion that exposure to PhIP retarded involution via partial inhibition of programmed cell death. To investigate possible mechanisms for the inhibitory effects of PhIP on mammary gland involution, serum levels of prolactin, an important hormone for the maintenance of lactation, were measured in virgin rats with regular estrous cycles given PhIP (150 mg/kg, p.o.) on the morning of diestrous. After one estrous cycle, on proestrous morning, serum prolactin levels were 1.3-fold higher after PhIP than after control vehicle (one-way ANOVA, Fisher LSD multiple comparison test, P < 0. 05). PhIP exposure during involution was associated with the induction of benign mammary tumors. Seven out of 12 rats developed fibroadenomas, and one developed a tubulopapillary carcinoma within 1 year of receiving PhIP administration during involution (150 mg/kg, p.o., once per day for 5 days), and a high-fat diet (23.5% corn oil). An increase in serum prolactin level and the effects on mammary gland apoptosis seen with PhIP may have implications for the mechanisms of carcinogenic targeting of PhIP to the mammary gland.