NF-κB activation contributes to indoleamine dioxygenase transcriptional synergy induced by IFN-γ and tumor necrosis factor-α

Interferon (IFN)-γ-induced expression of indoleamine 2,3-dioxygenase (IDO), an enzyme that inhibits some pathogens by limiting tryptophan availability, is transcriptionally enhanced by tumor necrosis factor (TNF)-α. The expression of interferon responsive factor (IRF)-1, an IFN-γ-induced transcriptional activator critical to IDO regulation, is also enhanced synergistically in response to IFN-γ and TNF-α. The IRF-1 regulatory region contains an IFN-γ-activated sequence (GAS) and a κB site, which bind STAT-1 and NF-κB, respectively. The TNF-α-mediated increase in STAT-1 activation in IFN-γ-treated cells enhances IRF-1 transcription; however, the contribution of TNF-α-mediated increases in nuclear NF-κB is uncertain. To identify whether binding of NF-κB upstream of the IRF-1 gene is rate-limiting in IRF-1 expression in response to IFN-γ and TNF-α, a proteasome inhibitor was utilized to maintain nuclear translocation of NF-κB at constitutive levels; its effect on IRF-1 expression and IDO-specific transcription was evaluated. By limiting NF-κB nuclear translocation, IRF-1 expression in IFN-γ and TNF-α treated cells was maintained at a level comparable to that achieved in response to IFN-γ alone, and the synergistic increase IDO transcription was blocked, suggesting that increases in NF-κB translocation are required for synergistic IDO expression in response to IFN-γ and TNF-α.