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The aryl hydrocarbon receptor (AhR) mediates a variety of biological responses to ubiquitous environmental pollutants. AhR is ligand activated transcription factor with high affinities for aromatic planar compounds such as β-naphthoflavone (BNF), 3-methylcholanthrene (3-MC), benzo[a]pyrene (BaP) or dioxin (TCDD). After binding appropriate ligand, AhR trigger induction of expression of some phase I and phase II drug metabolizing genes together with numerous other genes. One of such gene appear to be polymerase (DNA directed) kappa (Polκ). Polκ gene encodes newly identified low fidelity DNA polymerase. The enzyme bypasses benzo[a]pyrene-N2-dG lesions in a mostly error free manner by incorporating predominantly dC opposite the bulky lesions. It was demonstrated that AhR activation increases expression of the mouse Polκ gene and probably human POLK gene. In this study we examined the effect of i.p. administration of different AhR ligands on the expression of Polκ, RNA polymerase II polypeptide A (PolR2a) and cytochrome P450 1B1 (CYP1B1), the genes controlled by AhR in Sprague-Dawley rat liver. Quantitative real-time RT-PCR analysis revealed significant induction in the mRNA expression levels of Polκ and PolR2a following BNF treatment. Time courses of mRNA expression after treatment with BNF were similar in both genes, with maximal increases at 8h after treatment. The maximal induction of CYP1B1 and CYP1A1 expression was observed after 24 and 8h after BNF injection, respectively. TCDD treatment caused the significant increase in the mRNA level of CYP1B1 at 72h after administration of the ligand but no effect on Polκ and PolR2a mRNA expression was observed. These results confirm connection between AhR and Polκ, and strongly suggest that AhR up-regulates the mRNA transcription of PolR2a as well. However physiological importance of AhR dependent regulation of PolR2a expression must be further elucidated.

The aryl hydrocarbon receptor (AhR) mediates a variety of biological responses to ubiquitous environmental pollutants. AhR together with ARNT, AhRR, HIF1alpha represent a novel basic helix-loop-helix/PAS family of transcriptional regulators. Their interplay may affect the xenobiotic response. In this study, the effect of i.p. administration of different AhR ligands on the expression of AhR, AhRR, ARNT, HIF1alpha and CYP1A1 and NAD(P)H: quinone oxidoreductase (NQO1), the enzymes controlled by AhR were examined in Sprague-Dawley rat liver. Quantitative real-time RT-PCR analysis revealed no changes in the mRNA expression of ARNT and HIF1alpha following 3-methylcholanthrene (3-MC), 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) or beta-naphthoflavone (BNF) treatment. AhRR expression was affected by TCDD but not by BNF and 3-MC. Expression of AhR mRNA and of the markers of its activation, CYP1A1 and NQO1, was significantly increased by administration of TCDD, 3-MC and, to lower extent, BNF. These results indicate that binding of the ligands to AhR up-regulates the mRNA transcription not only of CYP1A1 and NQO1, but also of AhR itself. The level of AhR induction depends on the potency of xenobiotic metabolizing enzymes inducer.

The effects of the route of benzo[a]pyrene administration on sister chromatid exchange (SCE) and B[a]P diol epoxide (B[a]PDE)-DNA adducts formation in bone marrow cells of Ah responsive (C57BL/6; B6) and Ah non-responsive (DBA/2; D2) mice were determined. Animals were treated intraperitoneally (i.p.), intragastrically (i.g.) or topically with two 100 mg/kg doses of benzo[a]pyrene 24 h apart and killed 96 h after the first treatment. Significant increase in the frequencies of SCE and the level of B[a]PDE-DNA adducts as measured by synchronous fluorescence spectrophotometry were detected in D2 mice as compared to B6 mice. The route of administration had little effect on SCE levels in bone marrow cells in D2 mice. In B6 mice higher levels of SCE were observed following i.p. administration as compared to i.g. or topical administration. In both strains the highest level of B[a]PDE-DNA adducts was formed after i.p. administration of B[a]P. We conclude that the i.p. route of B[a]P administration is the most effective in inducing SCE and B[a]P-DNA adducts formation. SCE induction does not correlate linearly with the amount of B[a]PDE-DNA adducts formed in these cells after administration of the above dose of B[a]P.

1. Institute of Human Genetic, Polish Academy of Sciences, Poznan, Poland 2. Department of Otolaryngology, Poznan University of Medical Sciences, Poland 3. Institute of Human Genetics, University Hospital Schleswig-Holstein Campus Kiel, Germany 4. Department of OtorhinolaryngologyeHead and Neck Surgery and Department of Medical Biochemistry, Turku University Central Hospital and Turku University, Finland