Inhibitory effect of dietary 4-ipomeanol on DNA adduct formation by the food mutagen 2-amino-3-methylimidazo [4,5-f]quinoline (IQ)in male CDF₁ mice

Abstract

The food mutagen 2-amino-3-methylimidazo[4,5-f]quinoline (IQ) is carcinogenic in the CDF1 mouse liver, lungs and stomach. IQ is activated to its ultimate carcinogenic form by N-hydroxylation, catalyzed principally by hepatic microsomal cytochrome P450IA2, and further esterification, resulting in the formation of N-(deoxyguanosin-8-yl)-IQ and other adducts. The furanoterpenoid 4-ipomeanol (IPO) is a naturally occurring pneumotoxin which exerts its specific toxicity in Clara cells of the lung after activation by microsomal cytochrome P450. Because IPO is activated in the liver by a cytochrome P450IA2 enzyme, we evaluated IPO as a possible chemopreventive agent by assessing its ability to inhibit IQ-DNA adduct formation in the CDF1 mouse. Mice were put on an AIN-76A diet with or without 0.075% IPO from day 0 to 54. IQ (0.01%) was added to the diets from day 22 to 41 and animals were killed (four animals/time point) on days 42, 44, 46, 48, 50 and 54. Blood (for white blood cell isolation), liver, lungs, stomach, small intestine, cecum, colon, kidneys, spleen and heart were collected for analysis of IQ-DNA adducts by 32P-post-labeling. During the 12 day period after cessation of IQ exposure (days 42-54) IQ-DNA adduct formation was significantly inhibited in the liver (33.6-46.4%), lungs (29.9-58.6%), stomach (33.2-51.5%) and white blood cells (24.5-63.7%), but not in the other organs. Except in the colon, adduct removal from organs during days 42-54 was relatively slow (36.0-81.9% of day 42 levels remaining on day 54, 9.4-16.7% in the colon), but the presence of IPO in the diet did not influence the rate of adduct removal. Measurement of hepatic microsomal ethoxyresorufin deethylase, an activity specific for cytochrome P450IA isozymes, showed that the enzyme could be inhibited (14.1-68.1%) by IPO (0.05-10.0 mM) in vitro. It is concluded that IPO inhibits IQ-DNA adduct formation in target organs of the CDF1 mouse and that IPO may act by inhibiting N-hydroxylation of IQ. It is therefore possible that IPO may be a candidate chemopreventive agent against IQ-induced carcinogenesis.

DNA adducts of heterocyclic amine food mutagens: implications for mutagenesis and carcinogenesis

“…With one exception (256), the dietary concentrations of the inhibitors listed in Table III were not toxic to the animals. In certain organs, or with certain protocols, however, both CHL and I3C may act as tumor promoters (257,(259)(260)(261).…”

“…In in vivo studies in rodents, a number of agents have been shown to inhibit AIA-DNA adduct formation in various organs, usually including the target organ (Table III) (242)(243)(244)(245)(246)(247)(248)(249)(250)(251)(252)(253)(254)(255)(256). The dietary agents studied most widely include CHL, conjugated linoleic acid (CLA) and indole-3-carbinol (I3C) (242)(243)(244)(245)(246)(247).…”

Chemopreventive properties of Indole-3-Carbinol (I3C): Inhibition of DNA adduct formation of the dietary carcinogen, 2-Amino-1-Methyl-6-Phenylimidazo [4,5-b]Pyridine (PhIP), in female F344 rats

“…I3C was purchased from Sigma Chemical Company (St. Louis, MO). Materials for the 32 P-postlabeling assay were obtained from the same sources as before [32]. Powdered AIN-76A diet was prepared in the laboratory as described before [33], except that the antioxidants were omitted.…”

The Small Intestine as a Xenobiotic-Metabolizing Organ

“…7,12-Dimethyl(a)anthracene hydroxylation (Gentil and Sims, 1971), benzo(a)pyrene hydroxylation (Hietanen and Vainio, 1973;Wiebel et al, 1973), phenobarbital-inducible coumarin dealkylation (Lehrmann et al, 1973), aniline and biphenyl hydroxylations, and ethylmorphine N-demethylation (Chhabra et al, 1974) were all attributed to P450 activity in mouse enterocytes. Later studies identified P450 expression more specifically in mouse enterocytes: CYP1A1 was identified following its induction by BNF (Torronen et al, 1994) and by a polychlorinated biphenyl mixture, but only in Ah receptor-positive mice (Cummings and Schut, 1995); CYP3A was detected in mouse small intestine by erythromycin and cyclosporine activities and by immunoblot analysis (Berg-Candolfi et al, 1996); CYP1A, CYP2B, CYP2C, and CYP3A proteins were demonstrated to be induced in mouse small intestine by a food contaminant, imazalil (Muto et al, 1997); and CYP24 mRNA was induced in mouse small intestine by 1,25-dihydroxyvitamin D3 (Yoshimura et al, 1998). In one recent report, antibodies to rat CYP1A, 2C, 2D, 2E1, and 3A were used to probe P450 protein expression in the small intestine of untreated mice.…”