Received for publication February 21, 2008.
Revised June 3, 2008.
Accepted for publication June 4, 2008.

Down-regulation of the carcinogen metabolizing enzyme cytochrome P450 1a1 by vanadium

Abstract

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 TCDD-mediated induction of Cyp1a1 mRNA, protein, and activity levels after V⁵⁺ treatments in a dose-dependent manner. Investigating the effect of co-exposure 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 the subsequent formation of AhR/Arnt/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 effect of V⁵⁺ on existing Cyp1a1 mRNA and protein levels, we showed that V⁵⁺ did not affect the 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.

Key words: Ah receptor, anticancer agents, CYP gene regulation, CYP inhibition, CYP1A, heavy metals