Inhibitor of nuclear factor-kappaB alpha derepresses hypoxia-inducible factor-1 during moderate hypoxia by sequestering factor inhibiting hypoxia-inducible factor from hypo xia-inducible factor 1α

Abstract

Hypoxia and inflammation often develop concurrently in numerous diseases, and both hypoxia-inducible factor (HIF)-1α and nuclear factorkappaB (NF-κB) are key transcription factors of stress response genes. An NF-κB inhibitor, inhibitor of NF-κBα (IκBα), was found to interact with factor inhibiting HIF (FIH) and to be hydroxylated by FIH. However, FIH did not functionally regulate IκBα, and the consequence of the FIH.IκBα interaction thus remains uncertain. In the present study, we tested the possibility that IκBα regulates FIH. FIH.IκBα binding was confirmed by yeast two-hybrid and coimmunoprecipitation analyses. Functionally, IκBα expression further enhanced the transcriptional activity of HIF-1α under hypoxic conditions. Furthermore, IκBα knockdown repressed HIF-1α activity. Mechanistically, IκBα derepressed HIF-1α activity by inhibiting the FIH-mediated Asn803 hydroxylation of HIF-1α. It was also found that IκBα activated HIF-1α by sequestering FIH from HIF-1α. However, the effect of IκBαon HIF-1α activity was only observed in atmospheres containing 1% or more of oxygen. After tumor necrosis factor-a treatment, IκBα downregulation, Asn803 hydroxylation and HIF-1α inactivation all occurred up to 8 h, but subsided later. On the basis of these results, we propose that IκBα plays a positive regulatory role during HIF-1-mediated gene expression. Therefore, IκBα, owing to its interactions with NF-κB and HIF-1α may play a pivotal role in the crosstalk between the molecular events that underlie inflammatory and hypoxic responses.