Inhibition of plasmid reporter gene expression in CHO cells by DNA adducts of 2-amino-3-methylimidazo[4,5-f]quinoline and 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine

3-Methylcholanthrene elicits DNA adduct formation in the CYP1A1 promoter region and attenuates reporter gene expression in rat H4IIE cells

Cytochrome CYP1A (CYP1A) enzymes catalyze bioactivation of 3-methylcholanthrene (MC) to genotoxic metabolites. Here, we tested the hypothesis that CYP1A2 catalyzes formation of MC-DNA adducts that are preferentially formed in the promoter region of CYP1A1, resulting in modulation of CYP1A1 gene expression. MC bound covalently to plasmid DNA (50 micro g) containing human CYP1A1 promoter (pGL3-1A1), when incubated with wild-type (WT) liver microsomes (2 mg) and NAPPH 37 degrees C for 2h, giving rise to 9 adducts, as determined by (32)P-postlabeling. Eighty percent of adducts was located in the promoter region. Transient transfection of the adducted plasmids into rat hepatoma (H4IIE) cells for 16h, followed by MC (1 micro M) treatment for 24h inhibited reporter (luciferase) gene expression by 75%, compared to unadducted controls. Our results suggest that CYP1A2 plays a key role in sequence-specific MC-DNA adduct formation in the CYP1A1 promoter region, leading to attenuation of CYP1A1 gene expression.

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

The heterocyclic amines (HCAs) are a family of mutagenic/carcinogenic compounds produced during the pyrolysis of creatine, amino acids and proteins. The major subclass of HCAs found in the human diet comprise the aminoimidazoazaarenes (AIAs) 2-amino-3-methylimidazo[4,5-f]quinoline (IQ), 2-amino-3,4-dimethylimidazo[4,5-f]quinoline (MeIQ), 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQx), 2-amino-3,4,8-trimethylimidazo[4,5-f]quinoxaline (DiMeIQx) and 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP). All, except DiMeIQx, have been shown to be carcinogenic in animals. These compounds are present in cooked muscle meats at the p.p.b. level. Since the discovery of the HCAs in the late 1970s, many studies have examined the DNA adducts of these compounds. This review compiles the literature on AIA-DNA adducts including their identification and characterization, pathways of formation, mutagenesis in vitro and in vivo, and their association with carcinogenesis in animal models. It is now known that metabolic activation leading to the formation of DNA adducts is critical for mutagenicity and carcinogenicity of these compounds. All of the AIAs studied adduct to the guanine base, the major adduct being formed at the C8 position. Two AIAs, IQ and MeIQx, also form minor adducts at the N2 position of guanine. A growing body of literature has reported on the mutation spectra induced by AIA-guanine adducts. Studies of animal tumors induced by AIAs have begun to relate AIA-DNA adduct-induced mutagenic events with the mutations found in critical genes associated with oncogenesis. Several studies have demonstrated the feasibility of chemoprevention of AIA tumorigenesis. Only a few studies have reported on the detection of AIA-DNA adducts in human tissues; difficulties persist in the routine detection of AIA-DNA adducts in humans for the purpose of biomonitoring of exposure to AIAs. The AIAs are nevertheless regarded as possible human carcinogens, and future research on AIA-DNA adducts is likely to help address the role of AIAs in human cancer.