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Dagmar Aimová, Jitka Poljaková, Věra Kotrbová, Michaela Moserová, Eva Frei, Volker Arlt and Marie Stiborová

Ellipticine and benzo(a)pyrene increase their own metabolic activation via modulation of expression and enzymatic activity of cytochromes P450 1A1 and 1A2

Two compounds known to covalently bind to DNA after their activation with cytochromes P450 (CYPs), carcinogenic benzo(a)pyrene (BaP) and an antineoplastic agent ellipticine, were investigated for their potential to induce CYP and NADPH:CYP reductase (POR) enzymes in rodent livers, the main target organ for DNA adduct formation. Two animal models were used in the study: (i) rats as animals mimicking the fate of ellipticine in humans and (ii) mice, especially wild-type (WT) and hepatic POR null (HRN™) mouse lines. Ellipticine and BaP induce expression of CYP1A enzymes in livers of experimental models, which leads to increase in their enzymatic activity. In addition, both compounds are capable of generating DNA adducts, predominantly in livers of studied organisms. As determined by 32P postlabelling analysis, levels of ellipticine-derived DNA adducts formed in vivo in the livers of HRN™ mice were reduced (by up to 65%) relative to levels in WT mice, indicating that POR mediated CYP enzyme activity is important for the activation of ellipticine. In contrast to these results, 6.4 fold higher DNA binding of BaP was observed in the livers of HRN™ mice than in WT mice. This finding suggests a detoxication role of CYP1A in BaP metabolism in vivo. In in vitro experiments, DNA adduct formation in calf thymus DNA was up to 25 fold higher in incubations of ellipticine or BaP with microsomes from pretreated animals than with controls. This stimulation effect was attributed to induction of CYP1A1/2 enzymes, which are responsible for oxidative activation of both compounds to the metabolites generating major DNA adducts in vitro. Taken together, these results demonstrate that by inducing CYP1A1/2, ellipticine and BaP modulate their own enzymatic metabolic activation and detoxication, thereby modulating their either pharmacological (ellipticine) and/or genotoxic potential (both compounds).

Open access

Jana Mizerovská, Helena Dračínská, Volker Arlt, Jiří Hudeček, Petr Hodek, Heinz Schmeiser, Eva Frei and Marie Stiborová

Rat cytochromes P450 oxidize 3-aminobenzanthrone, a human metabolite of the carcinogenic environmental pollutant 3-nitrobenzanthrone

3-Aminobenzanthrone (3-ABA) is a human metabolite of carcinogenic 3-nitrobenzanthrone (3-NBA), which occurs in diesel exhaust and air pollution. Understanding which cytochrome P450 (CYP) enzymes are involved in metabolic activation and/or detoxication of this toxicant is important in the assessment of an individual's susceptibility to this substance. The aim of this study was to evaluate the efficiency of rat hepatic CYPs to oxidize 3-ABA and to examine the metabolites formed during such an oxidation. The metabolites formed by CYPs in rat hepatic microsomes were separated by high performance liquid chromatography (HPLC). 3-ABA is oxidized by these enzymes to three metabolites, which were separated by HPLC as distinguish product peaks. Using co-chromatography with synthetic standards, two of them were identified to be oxidative metabolites of 3-ABA, N-hydroxy-3-ABA and 3-NBA. The structure of another 3-ABA metabolite remains to be characterized. To define the role of rat hepatic CYP enzymes in metabolism of 3-ABA, we investigated the modulation of its oxidation using different inducers of CYPs for treatment of rats to enrich the liver microsomes with individual CYPs. Based on these studies, we attribute most of 3-ABA oxidation in rat hepatic microsomes to CYP2B, followed by CYP1A, although a role of other hepatic CYPs cannot be ruled out. Inhibition of 3-ABA oxidation by selective inhibitors of individual CYPs, supported this finding.