Mutagenicity of 2-amino-3-methylimidazo[4,5-f]quinoline in colon and liver of Big Blue rats: role of DNA adducts, strand breaks, DNA repair and oxidative stress

Abstract

The contribution of oxidative stress, different types of DNA damage and expression of DNA repair enzymes in colon and liver mutagenesis induced by 2-amino-3-methylimidazo [4,5-f]quinoline (IQ) was investigated in four groups of six Big Blue rats fed diets with 0, 20, 70, and 200 mg IQ/kg for 3 weeks. There were dose-response relationships of DNA adducts ((32)P-postlabeling) and DNA strand breaks (comet assay) in colon and liver tissues, with the highest levels of DNA adducts and strand breaks in the colon. There was dose-dependent induction of mutations in both the colon and the liver, and the same IQ dose produced two-fold more cII mutations in the liver compared with the colon. The IQ-induced mutation spectrum in the colon was not significantly different to that of control rats. The expression of ERCC1 and OGG1 was higher in the colon than liver, and was unaffected by the IQ diet. Investigations of oxidative stress biomarkers produced inconclusive results. Oxidative DNA damage detected by the endonuclease III enzyme and 7-hydro-8-oxo-2'-deoxyguanosine in colon, liver and/or urine was unaltered by IQ. However, there was increased level of gamma-glutamyl semialdehyde in liver proteins, indicating a higher rate of protein oxidation in the liver following IQ administration. In plasma and erythrocytes there were unaltered levels of oxidized protein, malondialdehyde, and antioxidant enzyme activities (superoxide dismutase, glutathione peroxidase, catalase, glutathione reductase) indicating no systemic oxidative stress. However, the level of total vitamin C was increased in plasma, with the largest fraction being in the reduced form. In conclusion, our results indicate that DNA adducts rather than oxidative stress are responsible for the initiation of IQ-induced carcinogenesis of the liver and colon. A lower frequency of mutations in the colon than in the liver could be related to higher expression of DNA repair enzymes in the former.

Lack of large intestinal carcinogenicity of 2‐amino‐1‐methyl‐6‐phenylimidazo[4,5‐b]pyridine at low doses in rats initiated with azoxymethane

“…39 However, in a case of 2-amino-3-methylimidazo[4,5-f]quinoline (IQ)-induced rat colon and liver carcinogenesis, DNA adducts were not found to correlate directly with mutation frequency. 40 Similar to this, there was no correlation between adduct levels and number of ACF induced by PhIP between different strains of rats. 41 Murata et al 42 suggested that DNA adduct formation is a prerequisite for carcinogenicity, but DNA adducts themselves may not be sufficient.…”

“…For carcinogenesis with genotoxic chemicals such as HCAs, there has been an extensive focus on initiation or other biologic reactions, with roles for DNA adducts, mutations and oxidative stress, [39][40][41][42][43][44] but correlations with HCA-induced carcinogenesis remain uncertain. For example, DNA adducts formation is necessary for cancer induction by genotoxic carcinogens and the levels of PhIP-DNA adducts largely correlate with tumor yield.…”

Sucrose and IQ induced mutations in rat colon by independent mechanism

“…The level of oxidative DNA damage in terms of 7-hydro-8-oxo-2 -deoxyguanosine (8-oxodG) was determined by HPLC-EC in liver and colon mucosa cells [18] and by HPLC MS/MS in urine [19]. In liver and colon mucosa cells DNA strand breaks were determined by the comet assay [11].…”

“…Big Blue rats have been used to determine mutational events in several organs. After oral administration of IQ a trend towards increased G:C → T:A transversions and decreased G:C → A:T transitions in the colon has been observed [10,11].…”

“…The mechanism includes metabolic activation followed by adduct formation with DNA [10]. In rats IQ is mutagenic in liver, colon and other organs [10,11]. Big Blue rats have been used to determine mutational events in several organs.…”