Activity of hypoxia-inducible factor 2α is regulated by association with the NF-κB essential modulator

The hypoxia- inducible factors 1 alpha ( HIF-1 alpha) and 2 alpha ( HIF- 2 alpha) 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- 2 alpha, but not HIF- 1 alpha, with the NF- kappa B 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- 2 alpha transcriptional activity, independently of the HIF- 2 alpha transactivation domain, consistent with a model by which NEMO aids CBP/ p300 recruitment to HIF- 2 alpha. In contrast, HIF- 2 alpha overexpression does not alter NF- kappa B signaling, suggesting that the functional consequence of the HIF- 2 alpha/ NEMO interaction is limited to the HIF pathway. The specificity of NEMO for HIF- 2 alpha represents one of the few known differential protein- protein interactions between the HIF- alpha proteins, which has important implications for the activity of HIF- 2 alpha and is also the first postulated NF- kappa B- independent role for NEMO.