STUDIES ON FUNCTIONS OF THIOREDOXIN REDUCTASE IN DETOXIFICATION OF HEAVY METALS

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The mammalian thioredoxin reductase (TrxR) is a selenocysteine-containing flavoprotein that regulates the thioredoxin system, one of the major systems that maintain the intracellular redox balance. To date, three TrxR isozymes, TrxR1, TrxR2 and TrxR3, have been identified. We previously reported that TrxR1 was induced by heavy metals such as cadmium (Cd) and arsenite (As). In this study, the functions of TrxR1 in detoxification of heavy metals were investigated as follows :(1)Transcriptional regulation of Cd-induced TrxR1 expression-Treatment of bovine arterial endothelial cells with Cd enhanced the promoter activity of the 5'-flanking region of human TrxR1 gene (nucleotides -1692 to +49). Deletion and site-directed mutation of antioxidant responsive element (ARE) (nucleotide -62 to -48) in this region abolished the response to Cd. Overexpression of NF-E2-related factor-2 (Nrf2) augmented the TrxR1 promoter activity. These results indicated that Cd-induced TrxR1 gene expression is mediated by the activation of Nrf2 and its binding to ARE in the TrxR1 gene promoter. We further found that in addition to Cd, the activators of Nrf2, such as diethyl maleate (DEM) and As, induced both TrxR1 and Trx gene expression. Nrf2 might play an important role in the regulation of the cellular Trx system consisting of Trx and TrxR.(2)Involvement of TrxR1 in cellular defense against heavy metals-We silenced the expression of TrxR1 in HeLa cells by using short-interfering RNA and found that the gene silencing of TrxR1 had a dual effect on Cd- and As-induced celldeath, depending on the concentration of heavy metals. The TrxR1 silencing increased the sensitivity toward a low dose of heavy metals but decreased the sensitivity toward a high dose of heavy metals. These results suggested that TrxR1 might play an important role in the cellular defense system against heavy metals in two ways.