Selective hypoxia-sensitive oxomer formation by FIH prevents binding of the NF-kB inhibitor IkBß to NF-?B subunits

Pharmacologic inhibitors of cellular hydroxylase oxygen sensors are protective in multiple pre-clinical in vivo models of inflammation. However, the molecular mechanisms underlying this regulation are only partly understood, preventing clinical translation. We previously proposed a new mechanism for cellular oxygen sensing: oxygen-dependent, (likely) covalent protein oligomer (oxomer) formation. Here, we report that the cellular oxygen sensor factor inhibiting HIF (FIH) forms an oxomer with the NF-?B inhibitor ß (I?Bß). The formation of this protein complex required FIH enzymatic activity and was prevented by pharmacologic inhibitors. Oxomer formation was highly hypoxia-sensitive and very stable. No other member of the I?B protein family formed an oxomer with FIH, demonstrating that FIH-I?Bß oxomer formation was highly selective. In contrast to FIH-dependent oxomer formation with the deubiquitinase OTUB1, FIH-I?Bß oxomer formation did not occur via an I?Bß asparagine residue, but through the amino acid sequence VAERR contained within a loop between I?Bß ankyrin repeat domains 2 and 3. Oxomer formation prevented I?Bß from binding to its primary interaction partners p65 and c-Rel, subunits of NF-?B, the master regulator of the cellular transcriptional response to pro-inflammatory stimuli. We therefore propose that FIH-mediated oxomer formation with I?Bß contributes to the hypoxia-dependent regulation of inflammation.