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-κB. Recent studies have revealed a high degree of interdependence between HIF and NF-κB 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-κB to demonstrate that hypoxia activates NF-κB in the heart and lungs of mice in vivo. Using small interfering RNA targeted to the p65 subunit of NF-κB, we confirm a unidirectional dependence of hypoxic HIF-1α accumulation upon an intact canonical NF-κB 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-κB dependent, and in mouse embryonic fibroblasts lacking an intact canonical NF-κB pathway, there is a loss of hypoxia-induced inflammatory gene expression. Finally, under conditions of hypoxia, HIF-1α and the p65 subunit of NF-κB directly bind to the cyclooxygenase-2 promoter. These results implicate an essential role for NF-κB 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 Inflammatory Tumor Microenvironment, Epithelial Mesenchymal Transition and Lung Carcinogenesis

“…Chronic inflammation also contributes to the local deficiency of oxygen due to the combination of reduced circulation at inflammatory sites and increased metabolic demand from infiltrating immune cells. In a feed-forward manner, hypoxia can promote chronic inflammation and thus, itself, in the developing TME through activation of NF-κB signaling in macrophages, neutrophils and non-immune cells, a finding recently confirmed in vivo in the lungs of mice [65,66].…”

Understanding complexity in the HIF signaling pathway using systems biology and mathematical modeling

“…IL-1), inhibition of the prolyl-4-hydroxylases blocks NF-B activity suggesting a stimulus and/or cell type-specific 14 interaction between HIF and NF-B pathway [92]. Once active, NF-B induces HIF-1 via evolutionary conserved consensus binding sites identified in the HIF-1 promoter [91,[93][94][95]. Since NF-B activity is not sufficient for the accumulation of the HIF-1 protein in the absence of hypoxia, current data suggest that the canonical NF-B pathway contributes to the maintenance of the HIF-1mRNA level, a pre-requisite of the PHD-mediated posttranslational regulation of the HIF system [91].…”

NFκB and HIF display synergistic behaviour during hypoxic inflammation

“…This creates a situation whereby both the hypoxic signalling pathway, through the Hypoxia Inducible factor (HIF), and the inflammatory signalling pathway, through Nuclear factor-!B (NF!B), are activated. Recent studies have shown that hypoxia influences the NF!B pathway and that HIF may play an important role in inflammation [4][5][6][7][8]. However, the relative contributions of HIF and NF!B into creating transcriptional activation profiles leading to a coordinated regulation of hypoxia-induced inflammatory gene expression remain unclear.…”

“…Due to its role as a master regulator of immunity and inflammation, its transcriptional activity and regulatory pathway have been an area of intense research [12]. Substantial evidence now exists that hypoxia can activate NF!B in vivo [8,13] and in vitro [5,7,8,14]. While the exact mechanism involved in the activation of NF!B remains to be fully elucidated, recent evidence has suggested that the same oxygen sensing enzymes, which confer oxygen sensitivity to the HIF pathway also play a role in activation of NF!B in response to hypoxia [5,14].…”

“…In addition, a group of proinflammatory genes, including cyclooxygenase (COX-2) and inducible nitric oxide synthase (iNOS) contains functional response elements for both HIF and NF!B in their promoter regions. We have previously shown that NF!B both directly but also indirectly, through its regulation of HIF-1#, regulates COX-2 expression in response to hypoxia [8].…”