Selective Hypoxia-Sensitive Oxomer Formation by FIH Prevents Binding of the NF-B Inhibitor IB to NF-B Subunits

Pharmacologic inhibitors of cellular hydroxylase oxygen sensors are protective in multiple preclinical 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 oxygen sensor factor inhibiting HIF (FIH) forms an oxomer with the NF-kappa B inhibitor beta (I kappa B beta). 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 kappa B protein family formed an oxomer with FIH, demonstrating that FIH-I kappa B beta oxomer formation was highly selective. In contrast to the known FIH-dependent oxomer formation with the deubiquitinase OTUB1, FIH-I kappa B beta oxomer formation did not occur via an I kappa B beta asparagine residue, but depended on the amino acid sequence VAERR contained within a loop between I kappa B beta ankyrin repeat domains 2 and 3. Oxomer formation prevented I kappa B beta from binding to its primary interaction partners p65 and c-Rel, subunits of NF-kappa B, the master regulator of the cellular transcriptional response to pro-inflammatory stimuli. We therefore propose that FIH-mediated oxomer formation with I kappa B beta contributes to the hypoxia-dependent regulation of inflammation.