Lipopolysaccharide Inhibits Thioredoxin Activity in Vitro and in Vivo

Abstract

Exogenous administration or overexpression of thioredoxin (Trx) has been shown to reduce endotoxic tissue injury. However, the role of endogenous Trx in endotoxin-induced tissue injury remains unknown. The aim of this study was to determine the effect of endotoxin on Trx activity. Human Trx (hTrx, 0.5 μg/ml) was incubated with lipopolysaccharide (LPS) in vitro and Trx activity was determined by insulin reduction assay. Our data showed that LPS inhibited Trx activity in a dose- (0.001-2.5 mg/ml) and time-dependent (1 to 4 h) fashion with the minimal significant inhibitory concentration of 10 μg/ml. To determine the effect of LPS on Trx activity in vivo, adult mice were treated with LPS and Trx activity was determined. Treatment with LPS almost completely abolished Trx activity in leukocytes and markedly reduced Trx activity in liver in a time-dependent fashion (80.6%, 71.1%, 61.4% and 51.6% at 6, 9, 16 and 24 hour post-LPS). Trx expression in both leukocytes and liver was significantly increased, rather than decreased in LPS-treated mice, indicating that the inhibitory effect cannot be attributed to its protein expression. LPS had no significant effect on Trx activity in heart and kidney. Collectively, our current results demonstrated for the first time that activity of Trx, a potent cytoprotective protein, can be significantly inhibited by LPS, suggesting that Trx inhibition may contribute to endotoxic tissue injury.