Modulation of NAD(P)H:quinone oxidoreductase by vanadium in human hepatoma HepG2 cells

Recent studies demonstrated the carcinogenicity and the mutagenicity of vanadium compounds. In addition, vanadium (V5+) was found to enhance the effects of other genotoxic agents. However, the mechanism by which V5+ induce toxicity remain unknown. In the current study we examined the effect of V5+ (as ammonium metavanadate, NH4VO3) on the expression of NAD(P)H: quinone oxidoreductase 1 (NQO1) in human hepatoma HepG2 cells. Therefore, HepG2 cells were treated with increasing concentrations of V5+ in the presence of two NQO1 inducers, the 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and isothiocyanate sulforaphane (SUL). Our results showed that V5+ inhibited the TCDD- and SUL-mediated induction of NQO1 at mRNA, protein and activity levels. Investigating the effect of V5+ at transcriptional levels revealed that V5+ significantly inhibited the TCDD- and SUL-mediated induction of antioxidant responsive element (ARE)-dependent luciferase reporter gene expression. In addition, V5+ was able to decrease the TCDD- and SUL-induced nuclear accumulation of nuclear factor erythroid 2-related factor-2 (Nrf2) without affecting Nrf2 mRNA or protein levels. Looking at the post-transcriptional level, V5+ did not affect NQO1 mRNA stability, thus eliminating the possible role of V5+ in decreasing NQO1 mRNA levels through this mechanism. In contrast, at post-translational level, V5+ was able to significantly decrease NQO1 protein half-life. The present study demonstrates for the first time that V5+ down-regulates NQO1 at the transcriptional and post-translational levels in the human hepatoma HepG2 cells via AhR- and Nrf2-dependent mechanisms.