Hg(II) Activates Thioredoxin Redox Status at Nanomolar Concentrations

Objectives: We have previously reported that 5-10 nM of mercury (Hg(II)) disrupts the redox status of human monocytes. In the current study, we tested the hypothesis that Hg(II) shifts the redox balance of thioredoxin (Trx), an important redox-based regulator of transcription factor function. If true, nM concentrations of Hg(II) may alter monocytic transcriptional activity and secretory function. Methods: We measured Trx reductase activity (rat liver), a direct regulator of Trx redox balance, using the DTNB-NADPH method (n=4). We then exposed THP1 monocytes to 0, 10 or 75 nM of Hg(II) for 6-72 h and measured activation of Nrf2 (immunoblots, n = 4), a transcription factor that regulates redox response and Trx levels. Redox-western analysis was used to assess Trx redox balance (IAA-conjugation method, n=2). Secretion of cytokines from monocytes was measured using ELISA (n=3). ANOVA and Tukey post-hoc analyses (a = 0.05) were used to detect differences in monocytic status. Results: TrxR activity in vitro was inhibited 100% by ≥10 nM Hg(II). Redox-western analysis confirmed that exposure to 10 nM of Hg(II) for 3 h markedly oxidized Trx redox balance in THP1 monocytes. At 6 h, both 10 and 75 nM Hg(II) significantly (2-3 fold over controls, p < 0.05) increased total levels of Nrf2 and its nuclear translocation. However, by 72 h, Nrf2 levels returned to baseline, suggesting cellular accommodation to Hg(II) exposures. When Hg(II)-exposed (72 h) monocytes were activated for 6 h with lipopolysaccharide, monocytic secretion of IL-a, TNFa, and IL6 (ELISA) were unchanged from controls (p > 0.05), suggesting that redox accommodation allowed normal activation of secretory pathways. Conclusions: Our results suggest that common blood levels of Hg(II) initially alter Trx redox balance and activate redox rescue pathways. However, over 72 h, monocytes appear to adjust to permit activation of normal inflammatory responses. (MCG Dental Research Center)