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Down-Regulation of the Carcinogen-Metabolizing Enzyme Cytochrome P450 1a1 by Vanadium

Faculty of Pharmacy and Pharmaceutical Sciences, University of Alberta, Edmonton, Alberta, Canada

Vanadium (V⁵⁺), a heavy metal contaminant with important toxicological consequences, has received considerable attention as an anticancer agent, although the mechanisms remain unknown. As a first step to investigate these mechanisms, we examined the effect of V⁵⁺ (as ammonium metavanadate, NH₄VO₃) on the expression of the aryl hydrocarbon receptor (AhR)-regulated gene: cytochrome P450 1a1 (Cyp1a1) at each step of the AhR signal transduction pathway, using Hepa 1c1c7 cells. Our results showed a significant reduction in 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-mediated induction of Cyp1a1 mRNA, protein and activity levels after V⁵⁺ treatments in a dose-dependent manner. Investigation of the effect of coexposure to V⁵⁺ and TCDD at transcriptional levels revealed that V⁵⁺ significantly inhibited TCDD-mediated induction of AhR-dependent luciferase reporter gene expression. Furthermore, despite not affecting the direct activation of the cytosolic AhR by TCDD and subsequently transforming it to a DNA-binding form, V⁵⁺ inhibited the nuclear accumulation of liganded AhR and subsequent formation of the AhR/aryl hydrocarbon nuclear translocator (Arnt)/xenobiotic responsive element (XRE) complex. Importantly, the V⁵⁺-mediated inhibition of AhR/Arnt/XRE complex formation coincided with a significant decrease in ecto-ATPase activity. Looking at the post-transcriptional and post-translational effects of V⁵⁺ on existing Cyp1a1 mRNA and protein levels, we showed that V⁵⁺ did not affect Cyp1a1 mRNA or protein stability, thus eliminating possible role of V⁵⁺ in modifying Cyp1a1 gene expression through these mechanisms. This study provides the first evidence that V⁵⁺ down-regulates the expression of Cyp1a1 at the transcriptional level through an ATP-dependent mechanism.