Activation of Nrf2 by Electrophiles Is Largely Independent of the Selenium Status of HepG2 Cells

Selenoenzymes, whose activity depends on adequate selenium (Se) supply, and phase II enzymes, encoded by target genes of nuclear factor erythroid 2-related factor 2 (Nrf2), take part in governing cellular redox homeostasis. Their interplay is still not entirely understood. Here, we exposed HepG2 hepatoma cells cultured under Se-deficient, Se-adequate, or Se-supranutritional conditions to the Nrf2 activators sulforaphane, cardamonin, or diethyl maleate. Nrf2 protein levels and intracellular localization were determined by immunoblotting, and mRNA levels of Nrf2 target genes and selenoproteins were assessed by qRT-PCR. Exposure to electrophiles resulted in rapid induction of Nrf2 and its enrichment in the nucleus, independent of the cellular Se status. All three electrophilic compounds caused an enhanced expression of Nrf2 target genes, although with differences regarding extent and time course of their induction. Whereas Se status did not significantly affect mRNA levels of the Nrf2 target genes, gene expression of selenoproteins with a low position in the cellular "selenoprotein hierarchy", such as glutathione peroxidase 1 (GPX1) or selenoprotein W (SELENOW), was elevated under Se-supplemented conditions, as compared to cells held in Se-deficient media. In conclusion, no major effect of Se status on Nrf2 signalling was observed in HepG2 cells.