Down-regulation of the detoxifying enzyme NAD(P)H:quinone oxidoreductase 1 by vanadium in Hepa 1c1c7 cells

Abstract

Recent data suggest that vanadium (V 5+) compounds exert protective effects against chemical-induced carcinogenesis, mainly through modifying various xenobiotic metabolizing enzymes. In fact, we have shown that V 5+ down-regulates the expression of Cyp1a1 at the transcriptional level through an ATP-dependent mechanism. However, incongruously, there is increasing evidence that V 5+ is found in higher amounts in cancer cells and tissues than in normal cells or tissues. Therefore, the current study aims to address the possible effect of this metal on the regulation of expression of an enzyme that helps maintain endogenous antioxidants used to protect tissues/cells from mutagens, carcinogens, and oxidative stress damage, NAD(P)H:quinone oxidoreductase 1 (Nqo1). In an attempt to examine these effects, Hepa 1c1c7 cells and its AhR-deficient version, c12, were treated with increasing concentrations of V 5+ in the presence of two distinct Nqo1 inducers, the 2,3,7,8-tetrachlorodibenzo- p-dioxin (TCDD) and isothiocyanate sulforaphane (SUL). Our results showed that V 5+ inhibits the TCDD- and SUL-mediated induction of Nqo1 at mRNA, protein, and catalytic activity levels. At transcriptional level, V 5+ was able to decrease the TCDD- and SUL-induced nuclear accumulation of Nrf2 and the subsequent binding to antioxidant responsive element (ARE) without affecting Nrf2 protein levels. Looking at post-transcriptional level; we found that V 5+ did not affect Nqo1 mRNA transcripts turn-over rates. However, at the post-translational level V 5+ increased Nqo1 protein half-life. In conclusion, the present study demonstrates that V 5+ down-regulates Nqo1 at the transcriptional level, possibly through inhibiting the ATP-dependent activation of Nrf2.