Activity of hypoxia-inducible factor 2alpha is regulated by association with the NF-kappaB essential modulator.

The hypoxia-inducible factors 1alpha (HIF-1alpha) and 2alpha (HIF-2alpha) are key regulators of the transcriptional response to low oxygen and are closely related in domain architecture, DNA binding, and activation mechanisms. Despite these similarities, targeted disruption of the HIF-alpha genes in mice results in distinctly different phenotypes demonstrating nonredundancy of function, although the underlying mechanisms remain unclear. Here we report on the novel and specific interaction of HIF-2alpha, but not HIF-1alpha, with the NF-kappaB essential modulator (NEMO) using immunoprecipitation, mammalian two-hybrid, and in vitro protein interaction assays. Reporter gene assays demonstrate that this interaction specifically enhances normoxic HIF-2alpha transcriptional activity, independently of the HIF-2alpha transactivation domain, consistent with a model by which NEMO aids CBP/p300 recruitment to HIF-2alpha. In contrast, HIF-2alpha overexpression does not alter NF-kappaB signaling, suggesting that the functional consequence of the HIF-2alpha/NEMO interaction is limited to the HIF pathway. The specificity of NEMO for HIF-2alpha represents one of the few known differential protein-protein interactions between the HIF-alpha proteins, which has important implications for the activity of HIF-2alpha and is also the first postulated NF-kappaB-independent role for NEMO.