Transactivation of gene expression by NF-κB is dependent on thioredoxin reductase activity

The redox-sensitive transcription factor NF-κB mediates the expression of genes involved in inflammation and cell survival. Thioredoxin reductase-1 (TR1) and its substrate thioredoxin-1 act together to reduce oxidized cysteine residues within the DNA-binding domain of NF-κB and promote maximal DNA-binding activity in vitro. It is not clear, however, if NF-κB is regulated via this mechanism within living cells. The purpose of this study was to determine the mechanism of NF-κB modulation by TR1 in cells stimulated with the inflammatory cytokine tumor necrosis factor-α (TNF). In both control cells and cells depleted of TR1 activity through chemical inhibition or siRNA knockdown, TNF stimulation resulted in degradation of the cytoplasmic NF-κB inhibitor IκB-α and translocation of NF-κB to the nucleus. Similarly, the DNA-binding activity and redox state of NF-κB were unaffected by TR1 depletion. In contrast, NF-κB-mediated gene expression was markedly inhibited in cells lacking TR1 activity, suggesting that the transactivation potential of NF-κB is sensitive to changes in TR1 activity. Consistent with this concept, phosphorylation of the transactivation domain of NF-κB was inhibited in the presence of curcumin. Surprisingly, another TR1 inhibitor, 1-chloro-2,4-dinitrobenzene, had no effect, and siRNA knockdown of TR1 actually increased phosphorylation at this site. These results demonstrate that TR1 activity controls the transactivation potential of NF-κB and that more than one mechanism may mediate this effect.