Effect of mercury(II) on Nrf2, thioredoxin reductase-1 and thioredoxin-1 in human monocytes

Objectives: Human blood levels of mercury are commonly 10 nM, but may transiently reach 50-75 nM after dental amalgam placement or removal. Controversy persists about the use of mercury because the effects of these 'trace' levels of mercury are not clear. Concentrations of mercury ≥5000 nM unequivocally alter redox balance in blood cells including monocytes. In the current study, we tested a hypothesis that concentrations of mercury <100 nM altered levels and activities of key proteins that maintain monocytic redox balance. Methods: Human THP1 monocytes were exposed to 10-75 nM of Hg(II) for 6-72 h, with or without activation by lipopolysaccharide (LPS). The redox management proteins Nrf2 and thioredoxin-1 (Trx1) were separated by electrophoresis, then quantified by immunoblotting. The activity of the seleno-enzyme thioredoxin reductase (TrxR1), important in maintaining Trx1 redox balance, was measured by cell-free and cell-dependent assays. Results: Concentrations of Hg(II) between 10-75 nM increased Nrf2 levels (3.5-4.5 fold) and decreased Trx1 levels (2-3 fold), but these changes persisted <24 h. Hg(II) potently inhibited (at concentrations of 5-50 nM) TrxR1 activity in both cell-free and intracellular assays. Furthermore, Hg(II) transiently amplified LPS-induced Nrf2 levels by 2-3 fold and limited LPS-induced decreases in Trx1. All effects of Hg(II) were mitigated by pre-adding N-acetyl-cysteine (NAC) or sodium selenide (Na₂SeO₃), supplements of cellular thiols and selenols, respectively. Significance: Our results suggest that nanomolar concentrations of Hg(II) transiently alter cellular redox balance in monocytes that trigger changes in Nrf2 and Trx1 levels. These changes indicate that monocytes have a capacity to adapt to trace concentrations of Hg(II) that are introduced into the bloodstream after dental amalgam procedures or fish consumption. The ability of monocytes to adapt suggests that low levels of mercury exposure from dental amalgam may not overtly compromise monocyte function.