Preferential induction of the rat hepatic P450 I proteins by the food carcinogen 2‐amino‐3‐methyl‐imidazo[4,5‐f]quinoline

Abstract

1. Administration of the food carcinogen, 2-amino-3-methyl-imidazo[4,5-flquinoline (IQ) to rats gave rise to significant dose-dependent increases in the microsomal 0-deethylations of ethoxycoumarin and ethoxyresorufin but had no effect on the 0-dealkylation of pentoxyresorufin and the NADPH-dependent reduction of cytochrome c, and decreased the N-demethylation of dimethylnitrosamine. Microsomal cytochrome b5 and total cytochrome P-450 levels decreased following the administration of the carcinogen.2. Hepatic microsomal preparations from IQ-treated animals were much more efficient than control in activating the premutagen 2-amino-6-methyldipyrido[l,2-u: 3',2'-dlimidazole to mutagenic intermediates in the Ames test.3. Immunoquantification of two of the major families of cytochrome P-450, namely P450 I and P450 I1 B, using ELISA techniques showed that treatment with IQ induced the apoprotein levels of the P450 I family but4. Immunoblot analysis employing polyclonal antibodies against P450 I revealed that IQ induced both isoenzymes of this family, namely P450 I A1 and A2.5. It is concluded that IQ is an inducer of the rat hepatic monooxygenases, selectively inducing the P450 I family as predicted by a computer-graphic analysis of its dimensions which showed that it is a large, essentially planar, molecule.The carcinogenicity of most chemicals is mediated, not by the parent form, but by reactive metabolic intermediates that readily interact with DNA initiating the process of tumourigenesis. By far the most prominent enzymes in the metabolic activation of chemical carcinogens are the ubiquitous monooxygenases whose terminal oxidase, cytochrome P-450, exists as a superfamily of proteins each of which confers to the system its characteristic substrate specificity [l].One of these families, namely P450 I (cytochromes P-448) appears to direct the metdbOhn of chemical carcinogens towards pathways that yield reactive species rather than inactive metabolites [2, 31. This family of isoenzymes is also involved in the activation of the aminoimidazo-azaarenes [4,5], which have been isolated from broiled fish and meat [6, 71. They are mutagens in a number of in vitro systems [8-111, activate a variety of oncogenes [12] and are potent animal carcinogens [I 3 -151. The prototype is IQ (2-amino-3-methylimidazo[4,5-flquinoline) whose activation proceeds through an initial N-hydroxylation to yield 2-amino-3-methyl-Nhydroxyimidazo[4,5-j~quinoline 1161, the proximate carcinogen, which undergoes further metabolism to generate the ultimate carcinogen [5]. The P450 I concentration in untreated animals is very small, immunologically calculated to be less than 5% of the total cytochrome P-450 content [17, 181, so that the activation of IQ is likely to be a minor pathway. However, many chemical carcinogens, such as the polycyclic The present study was therefore undertaken to establish if (a) IQ induces the rat hepatic monooxygenases, and (b) selectively induces the P450 I family that facilitates its activation. Since chemicals that inte...

Contrasting effects of acute and chronic dietary exposure to 2-amino-3-methyl-imidazo[4,5-f]quinoline (IQ) on xenobiotic metabolising enzymes in the male Fischer 344 Rat: implications for chemoprevention studies

“…These data, however, contrast with the limited information on IQ-induced changes in XME that are currently described in the literature [28,29,49]. Such observations encouraged us to study further the effects of acute exposure in the same animal strain as our chronic studies.…”

“…From previous studies [28], we had anticipated that a primary effect of chronic IQ exposure would be upregulation of CYP1A and possibly other CYP isozymes. However, no significant changes were observed for CYP1A, CYP3A2 or CYP2E1.…”

“…IQ also undergoes 5-hydroxylation, catalysed by CYP, plus conjugation reactions catalysed by UGT and sulphotransferases to yield non-toxic metabolites in the rat and human [12,27]. There is evidence that IQ itself can modulate the expression of the CYP enzymes involved in its own metabolism in the rat [28,29] at doses (20 -130 mg · kg -1 ) and exposure regimes typically used in short-term chemoprevention studies [16,30]. As it is a planar molecule, IQ was proposed as a ligand for the Ah-receptor and a subsequent agonist for the xenobiotic response element (XRE) [28].…”

“…There is evidence that IQ itself can modulate the expression of the CYP enzymes involved in its own metabolism in the rat [28,29] at doses (20 -130 mg · kg -1 ) and exposure regimes typically used in short-term chemoprevention studies [16,30]. As it is a planar molecule, IQ was proposed as a ligand for the Ah-receptor and a subsequent agonist for the xenobiotic response element (XRE) [28]. XRE induction by the carcinogen being tested against could well confound interpretation of chemoprevention studies.…”

Tools and Strategies for the Assessment of Enzyme Induction in Drug Discovery and Development

“…It can provide accurate protein quantitation with a relatively small amount of sample with rapid turnaround time. In drug metabolism studies, EIA/ELISA has been applied to confi rm drug -induced changes in expression levels of drug metabolizing enzymes such as P450 isozymes, GSTs, and drug transporters (Rodrigues et al, 1989 ;Huang and Gibson, 1991 ;Makowska et al, 1992 ;Geng and Strobel, 1998 ;Kyokawa et al, 2001 ;Devi and Devaraj, 2006 ). Mass spectrometry (MS) can be used for both absolute quantitation and relative quantitation of proteins and peptides.…”