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

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