Interactions between nitric oxide and hypoxia-inducible factor signaling pathways in inflammatory disease

Abstract

Induction and activation of nitric oxide (NO) synthases (NOS) and excessive production of NO are common features of almost all diseases associated with infection and acute or chronic inflammation, although the contribution of NO to the pathophysiology of these diseases is highly multifactorial and often still a matter of controversy. Because of its direct impact on tissue oxygenation and cellular oxygen (O(2)) consumption and re-distribution, the ability of NO to regulate various aspects of hypoxia-induced signaling has received widespread attention. Conditions of tissue hypoxia and the activation of hypoxia-inducible factors (HIF) have been implicated in hypoxia or in cancer biology, but are also being increasingly recognized as important features of acute and chronic inflammation. Thus, the activation of HIF transcription factors has been increasingly implicated in inflammatory diseases, and recent studies have indicated its critical importance in regulating phagocyte function, inflammatory mediator production, and regulation of epithelial integrity and repair processes. Finally, HIF also appears to contribute to important features of tissue fibrosis and epithelial-to-mesenchymal transition, processes that are associated with tissue remodeling in various non-malignant chronic inflammatory disorders. In this review, we briefly summarize the current state of knowledge with respect to the general mechanisms involved in HIF regulation and the impact of NO on HIF activation. Secondly, we will summarize the major recent findings demonstrating a role for HIF signaling in infection, inflammation, and tissue repair and remodeling, and will address the involvement of NO. The growing interest in hypoxia-induced signaling and its relation with NO biology is expected to lead to further insights into the complex roles of NO in acute or chronic inflammatory diseases and may point to the importance of HIF signaling as key feature of NO-mediated events during these disorders.

Figure 1. Schematic representation of the mechanisms by which NO impacts on HIF activation during normoxic or hypoxic conditions

Under normoxia (left panel) NO can inhibit prolyl hydroxylase (PHD) and factor inhibiting HIF (FIH) activity by interacting with enzyme bound Fe²⁺, preventing hydroxylation of HIF-1α proline and asparagine residues that target HIF-1α for degradation and block interaction with the transcriptional co-activator p300-CBP. NO can also increase HIF-1α expression through the activation of PI3K/AKT and MAPK pathways and stabilize HIF-1α by S-nitrosylation. S-nitrosylation of von Hippel Lindau (VHL) can also prevent HIF-1α ubiquitination and degradation. During hypoxic conditions (right panel), competitive binding of NO to mitochondrial cytochrome c oxidase results in re-distribution of O₂ to restore PHD activity. NO can also promote increases in intracellular free iron that enhance PHD activity. p300-CBP, p300-CREB-binding protein; FIH, factor inhibiting HIF; HIF-1α, hypoxia inducible factor-1α; HRE; hypoxic response element, PHD, prolyl hydroxylase; PI3K, phosphatidylinositol 3-kinase; VHL, von Hippel Lindau.

Figure 2. Schematic model of proposed actions of NO in airway epithelial regeneration following injury, and the involvement of HIF

Initial stages of epithelial repair may be promoted by low concentrations of NO from NOS2, which promote epithelial cell migration in part by induction of repair genes such as matrix metalloproteinase-9 (top panel). At later stages, elevated concentrations of NO may attenuate cell migration by suppressing MMP-9 expression and activation (through induction of PAI-1) and by activating p53 (lower panel). As indicated, some of these responses are mediated by the stabilization and activation of HIF-1 [87]. This biphasic control of cell migration by NO may be critical for a coordinated wound response and appropriate re-epithelialization. HIF-1α hypoxia inducible factor-1α; MMP-9 matrix metalloproteinase 9; NOS2, nitric oxide synthase2, PAI-1; plasminogen activator inhibitor-1.