Transcriptional modulation of the NAD(P)H:quinone oxidoreductase 1 by mercury in human hepatoma HepG2 cells

Abstract

NAD(P)H:quinone oxidoreductase (NQO1)-mediated detoxification of quinones plays a critical role in cancer prevention. Heavy metals such as mercury (Hg2+) alter the carcinogenicity of aryl hydrocarbon receptor ligands, mainly by modifying various xenobiotic-metabolizing enzymes such as NQO1. Therefore, we examined the effect of Hg2+ on the expression of NQO1 in human hepatoma HepG2 cells. For this purpose HepG2 cells were incubated with various concentrations of Hg2+ (2.5, 5, and 10μM) in the presence and absence of two NQO1 inducers, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and isothiocyanate sulforaphane (SUL), as bifunctional and monofunctional inducers, respectively. Analysis of the time-dependent effect of Hg2+ revealed that Hg2+ increased the expression of NQO1 mRNA in a time-dependent manner. In addition, Hg2+ increased NQO1 at the mRNA, protein, and activity levels in the presence and absence of both NQO1 inducers, TCDD and SUL, which coincided with increased nuclear accumulation of Nrf2 protein. Investigating the effect of Hg2+ at the transcriptional level revealed that Hg2+ significantly induced the antioxidant-responsive element-dependent luciferase reporter gene expression in the absence and the presence of both NQO1 inducers. NQO1 mRNA and protein decay experiments revealed a lack of posttranscriptional and posttranslational mechanisms. Transfecting HepG2 cells with siRNA for Nrf2 significantly decreased the Hg2+-mediated induction of NQO1 mRNA and catalytic activity by approximately 90%. In conclusion, we demonstrated that Hg2+ regulates the expression of the NQO1 gene through a transcriptional mechanism in human hepatoma HepG2 cells. In addition, Nrf2 is involved in the modulation of NQO1 by Hg2+.