The hypoxia-inducible factor (HIF) is a key regulator of the transcriptional response to hypoxia. While the mechanism underpinning HIF activation is well understood, little is known about its resolution. Both the protein and the mRNA levels of HIF-1α (but not HIF-2α) were decreased in intestinal epithelial cells exposed to prolonged hypoxia. Coincident with this, microRNA (miRNA) array analysis revealed multiple hypoxia-inducible miRNAs. Among these was miRNA-155 (miR-155), which is predicted to target HIF-1α mRNA. We confirmed the hypoxic upregulation of miR-155 in cultured cells and intestinal tissue from mice exposed to hypoxia. Furthermore, a role for HIF-1α in the induction of miR-155 in hypoxia was suggested by the identification of hypoxia response elements in the miR-155 promoter and confirmed experimentally. Application of miR-155 decreased the HIF-1α mRNA, protein, and transcriptional activity in hypoxia, and neutralization of endogenous miR-155 reversed the resolution of HIF-1α stabilization and activity. Based on these data and a mathematical model of HIF-1α suppression by miR-155, we propose that miR-155 induction contributes to an isoform-specific negative-feedback loop for the resolution of HIF-1α activity in cells exposed to prolonged hypoxia, leading to oscillatory behavior of HIF-1α-dependent transcription.

“What is the O₂ concentration in a normoxic cell culture incubator?” This and other frequently asked questions in hypoxia research will be answered in this review. Our intention is to give a simple introduction to the physics of gases that would be helpful for newcomers to the field of hypoxia research. We will provide background knowledge about questions often asked, but without straightforward answers. What is O₂ concentration, and what is O₂ partial pressure? What is normoxia, and what is hypoxia? How much O₂ is experienced by a cell residing in a culture dish in vitro vs in a tissue in vivo? By the way, the O₂ concentration in a normoxic incubator is 18.6%, rather than 20.9% or 20%, as commonly stated in research publications. And this is strictly only valid for incubators at sea level.

Significance: Oxygen-sensing hydroxylases are a family of enzymes that control the cellular adaptive response to hypoxia. Hydroxylase inhibitors reduce inflammation in vivo; however, the anti-inflammatory mechanism of action remains unclear. IL-1β is a cytokine that potently promotes inflammation through activation of the transcription factor NF-κB. Here, we demonstrate that hydroxylase inhibition leads to a suppression of IL-1β–induced NF-κB activity and provide insight into the underlying mechanism involved. This work develops our understanding of how hydroxylase inhibition regulates IL-1β–induced inflammation and sheds light on our understanding of the association between hypoxic and inflammatory signaling pathways, underscoring the potential use of hydroxylase inhibitors for the treatment of inflammatory disease.