An Intact Canonical NF-κB Pathway Is Required for Inflammatory Gene Expression in Response to Hypoxia

Abstract

Hypoxia is a feature of the microenvironment in a number of chronic inflammatory conditions due to increased metabolic activity and disrupted perfusion at the inflamed site. Hypoxia contributes to inflammation through the regulation of gene expression via key oxygen-sensitive transcriptional regulators including the hypoxia-inducible factor (HIF) and NF-kB. Recent studies have revealed a high degree of interdependence between HIF and NF-kB signaling; however, the relative contribution of each to hypoxia-induced inflammatory gene expression remains unclear. In this study, we use transgenic mice expressing luciferase under the control of NF-kB to demonstrate that hypoxia activates NF-kB in the heart and lungs of mice in vivo. Using small interfering RNA targeted to the p65 subunit of NF-kB, we confirm a unidirectional dependence of hypoxic HIF-1a accumulation upon an intact canonical NF-kB pathway in cultured cells. Cyclooxygenase-2 and other key proinflammatory genes are transcriptionally induced by hypoxia in a manner that is both HIF-1 and NF-kB dependent, and in mouse embryonic fibroblasts lacking an intact canonical NF-kB pathway, there is a loss of hypoxia-induced inflammatory gene expression. Finally, under conditions of hypoxia, HIF-1a and the p65 subunit of NF-kB directly bind to the cyclooxygenase-2 promoter. These results implicate an essential role for NF-kB signaling in inflammatory gene expression in response to hypoxia both through the regulation of HIF-1 and through direct effects upon target gene expression. The Journal of Immunology, 2011, 186: 000–000.