A number of recent studies have implicated tissue hypoxia in both acute and chronic inflammatory diseases, particularly as they relate to mucosal surfaces involving epithelial cells. In this context, a protective role for the transcriptional regulator hypoxia-inducible factor (HIF) was demonstrated through conditional deletion of epithelial HIF-1α in a murine model of colitis (J. Clin. Invest. 2004; 114:1098−1106. Here, we hypothesized that pharmacologic activation of HIF would similarly provide a protective adaptation to murine colitic disease. For these purposes, we used a novel prolyl hydroxylase (PHD) inhibitor (FG-4497) which readily stabilizes HIF-1α and subsequently drives the expression downstream HIF target genes (e.g. erythropoietin). Our results show that the FG-4497-mediated induction of HIF-1α provides an overall beneficial influence on clinical symptoms (weight loss, colon length, tissue TNFα) in murine TNBS colitis, most likely due to their barrier protective function and wound healing during severe tissue hypoxia at the site of inflammation. Taken together these findings emphasize the role of epithelial HIF-1α during inflammatory diseases in the colon and may provide the basis for a therapeutic use of PHD inhibitors in inflammatory mucosal disease.

Extracellular adenosine (Ado) has been implicated as central signaling molecule during conditions of limited oxygen availability (hypoxia), regulating physiologic outcomes as diverse as vascular leak, leukocyte activation, and accumulation. Presently, the molecular mechanisms that elevate extracellular Ado during hypoxia are unclear. In the present study, we pursued the hypothesis that diminished uptake of Ado effectively enhances extracellular Ado signaling. Initial studies indicated that the half-life of Ado was increased by as much as fivefold after exposure of endothelia to hypoxia. Examination of expressional levels of the equilibrative nucleoside transporter (ENT)1 and ENT2 revealed a transcriptionally dependent decrease in mRNA, protein, and function in endothelia and epithelia. Examination of the ENT1 promoter identified a hypoxia inducible factor 1 (HIF-1)–dependent repression of ENT1 during hypoxia. Using in vitro and in vivo models of Ado signaling, we revealed that decreased Ado uptake promotes vascular barrier and dampens neutrophil tissue accumulation during hypoxia. Moreover, epithelial Hif1 α mutant animals displayed increased epithelial ENT1 expression. Together, these results identify transcriptional repression of ENT as an innate mechanism to elevate extracellular Ado during hypoxia.

Inflammatory diseases influence tissue metabolism, altering regulation of extracellular adenine nucleotides, with a resultant protective influence of adenosine. Ecto-5′-nucleotidase (CD73) is a central surface enzyme generating extracellular adenosine. Thus, we hypothesized that CD73 is protective in mucosal inflammation as modeled by trinitrobenzene sulfonate (TNBS) colitis. Initial studies revealed a >3-fold induction of CD73 mRNA levels after TNBS colitis. Additionally, the severity of colitis was increased, as determined by weight loss and colonic shortening, in cd73−/− mice relative to cd73+/+ controls. Likewise, enteral administration of the selective CD73 inhibitor α,β-methylene ADP to cd73+/+ mice resulted in a similar increase in severity of TNBS colitis. Gene array profiling of cytokine mRNA expression, verified by real-time PCR, revealed a >90% down-regulation of IFN-αA in cd73−/− mice and α,β-methylene ADP-treated cd73+/+ mice, compared with cd73+/+ mice. Exogenous administration of recombinant IFN-αA partially protected TNBS-treated cd73−/− mice. Cytokine profiling revealed similar increases in both IFN-γ and TNF-α mRNA in colitic animals, independent of genotype. However, IL-10 mRNA increased in wild-type mice on day 3 after TNBS administration, whereas cd73−/− mice mounted no IL-10 response. This IL-10 response was restored in the cd73−/− mice by exogenous IFN-αA. Further cytokine profiling revealed that this IL-10 induction is preceded by a transient IFN-αA induction on day 2 after TNBS exposure. Together, these studies indicate a critical regulatory role for CD73-modulated IFNαA in the acute inflammatory phase of TNBS colitis, thereby implicating IFN-αA as a protective element of adenosine signaling during mucosal inflammation.

Increased tissue permeability is commonly associated with hypoxia of many origins. Since hypoxia-inducible factor (HIF) represents a predominant hypoxia signaling mechanism, we compared hypoxia-elicited changes in tissue barrier function in mice conditionally lacking intestinal epithelial hypoxia-inducible factor-1alpha (hif1a). Somewhat surprisingly, these studies revealed that mutant hif1a mice were protected from hypoxia-induced increases in intestinal permeability in vivo. Guided by microarray analysis of tissues derived from these mutant hif1a mice, we identified HIF-1-dependent repression of vasodilator-stimulated phosphoprotein (VASP), a molecule known to be important in the control of cytoskeletal dynamics, including barrier function. Studies at the mRNA and protein level confirmed hypoxia-elicited repression of VASP in murine tissue, cultured epithelia and endothelia, as well as human saphenous vein ex vivo. Targeted repression of VASP by siRNA recapitulated our findings with hypoxia and directed overexpression of VASP abolished hypoxia-induced barrier dysfunction. Studies in the cloned human VASP promoter revealed hypoxia-dependent transcriptional repression, and functional studies by chromatin immunoprecipitation (ChIP) and site-directed mutagenesis revealed hypoxia-dependent binding of HIF-1alpha to the human VASP promoter. These studies identify HIF-1-dependent repression of VASP as a control point for hypoxia-regulated barrier dysfunction.

Tissue edema is commonly associated with hypoxia. Generally, such episodes of fluid accumulation are self-limiting. At present, little is known about mechanisms to compensate excessive fluid transport. Here we describe an adaptive mechanism to dampen fluid loss during hypoxia. Initial studies confirmed previous observations of attenuated electrogenic Cl- secretion after epithelial hypoxia. A screen of known ion transporters in Cl- -secreting epithelia revealed selective downregulation of Na-K-2Cl cotransporter NKCC1 mRNA, protein, and function. Subsequent studies identified transcriptional repression of NKCC1 mediated by hypoxia-inducible factor (HIF). Chromatin immunoprecipitation analysis identified a functional HIF binding site oriented on the antisense strand of genomic DNA downstream of the transcription start site corresponding to the NKCC1 5'-untranslated region. Additional in vivo studies using conditional Hif1a-null mice revealed that the loss of HIF-1alpha in Cl- -secreting epithelia results in a loss of NKCC1 repression. These studies describe a novel regulatory pathway for NKCC1 transcriptional repression by hypoxia. These results suggest that HIF-dependent repression of epithelial NKCC1 may provide a compensatory mechanism to prevent excessive fluid loss during hypoxia.

Inflammatory diseases change tissue metabolism, resulting in alterations in extracellular adenine nucleotide regulation and a resultant protective influence of adenosine (Ado). We hypothesized that Ecto‐5′‐nucleotidase (CD73), a central surface enzyme in the generation of extracellular Ado, is protective in mucosal inflammation as modeled by TNBS colitis. Initial studies revealed a greater than 3‐fold increase in CD73 mRNA levels following induction of TNBS colitis. We further demonstrate more severe colitis, as determined by weight loss and colonic shortening, in cd73−/− mice relative to cd73+/+ controls. Likewise, systemic administration of the selective CD73 inhibitor APCP to cd73+/+ mice resulted in a similar increase in severity of TNBS colitis. Gene array profiling of cytokine‐associated mRNA expression revealed a greater than 90% down‐regulation of IFNαA in cd73−/− compared to cd73+/+ mice. Real‐time PCR verified this profound colonic IFNαA down‐regulation in both cd73−/− mice and APCP‐treated cd73+/+ mice. Finally, exogenous administration of recombinant IFNαA partially protected cd73−/− mice from disease severity in colitis. Together, these studies indicate a critical regulatory role for CD73‐modulated IFNαA in the acute inflammatory phase of TNBS colitis, and implicate IFNαA as a protective element of Ado signalling during mucosal inflammation.

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