Effects of the food mutagens MeIQx and PhIP on the expression of cytochrome P450IA proteins in various tissues of male and female rats.

The promutagenic 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQx) and 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) found in cooked food are converted to their active forms mainly by cytochrome P450 forms IA1 and IA2. By induction of these isoenzymes the food mutagens could thus influence their own rate of activation. Male and female Wistar rats were given MeIQx, PhIP, beta-naphthoflavone (BNF) or saline i.p. at 50 mg/kg body wt on three consecutive days. On the fourth day the rats were killed and lungs, kidneys, liver and intestines taken. The microsomal fraction from each organ was prepared as well as 9000 g supernatant from the liver. The induction of cytochrome P450IA was measured at the protein level by enzymatic assays (ethoxyresorufin-O-deethylation, Ames' mutagenicity test) and immunoassays (Western blot) and at the pretranslational level by RNA hybridization (Northern blot). The binding affinities of MeIQx, PhIP and 2-amino-3-methylimidazo[4,5-f]quinoline (IQ) for the 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-receptor were studied by evaluation of the competition with 3H-labelled TCDD for specific binding. Ethoxyresorufin-O-deethylase (EROD) activity was significantly increased in the liver (males 2.1-fold, females 3.3-fold), kidneys (males 2.1-fold, females 1.8-fold) and lungs (males 4.3-fold, females 3-fold) of the MeIQx-treated rats. Furthermore, the levels of cytochrome P450IA proteins were increased in these animals. It was not possible, however, to detect the corresponding mRNA. In the case of the PhIP-treated animals a significantly increased EROD activity (2.7-fold) and an increased cytochrome P450IA protein level were seen only in the male lungs. Only a very weak TCDD-receptor affinity was observed for PhIP, whereas MeIQx or IQ did not appear to compete significantly with [3H]TCDD for binding to the TCDD-receptor. It is concluded that MeIQx is a weak inducer of cytochrome P450IA in several organs of the rat, while PhIP induced these isoenzymes only in the male lungs. More work is needed to clarify the mechanism(s) whereby this induction occurs.