Gastroenterology
Volume 134, Issue 1 , Pages 346-348, January 2008

Life in the Gut Without Oxygen: Adaptive Mechanisms and Inflammatory Bowel Disease

  • Anthony T. Blikslager

      Affiliations

    • Corresponding Author InformationAddress requests for reprints to: Anthony T. Blikslager, MD, Department of Clinical Sciences, North Carolina State University, 4700 Hillsborough Street, Raleigh, North Carolina 27606.

Article Outline

 

See “Mucosal protection by hypoxia-inducible factor prolyl hydroxylase inhibition” by Robinson A, Keely S, Karhausen J, et al on page 145; and “The hydroxylase inhibitor dimethyloxalylglycine is protective in a murine model of colitis” by Cummins EP, Seeballuck F, Keely SJ, et al on page 156.

The epithelium that lines the intestine exists in a constant state of hypoxia. In the small intestine, this has classically been explained by a countercurrent exchange mechanism, wherein oxygen from arterial blood entering the villi diffuses across to neighboring venules traveling from the tip down toward the base of the villus.1 The crucial components of countercurrent exchange are in close proximity of a tubular or vascular supply with components traveling in the opposite direction at a low velocity, a semipermeable membrane, and differing concentrations of a solute. All of these components are present in the small intestine, with resultant countercurrent exchange of oxygen and relatively hypoxic villous tips. However, there appear to be other reasons for intestinal epithelial hypoxia because it is also present in the colon where there are no villi. The easiest explanation for this is the distance of the epithelium from the vascular supply, so that most of the oxygen has been taken up by subepithelial tissues before oxygenated blood reaches the level of the epithelium. Regardless of the mechanism, a steep oxygen gradient has been documented in the gut.1 This is important, because, without oxygen, transport mechanisms and barrier function become disrupted. In fact, the intestine is one of the most energy-demanding organ systems because of the Na+/K+ ATPase, an epithelial basolateral transporter that is critical to intestinal function. Additional energy is required to fuel maintenance of barrier function, including interepithelial tight junctions, which are heavily laden with signaling molecules. Fortunately, the apical epithelium has developed adaptive mechanisms that allow it to function in an environment without optimal oxygenation.

Aside from reduced oxygen availability, the intestinal mucosa has to respond to microorganisms in its lumen in an appropriate way to avoid developing an inflammatory response. Some patients may not respond appropriately to luminal bacteria, possibly resulting in inflammatory bowel disease (IBD), depending on other factors involved in development of disease.2 Currently, investigators are attempting to determine which bacteria are likely candidates for triggering IBD under specific circumstances.3 Additional factors may influence the development of IBD, including genetic influences and lifestyle.4 Furthermore, Robinson et al point out in this issue of Gastroenterology that it has recently been recognized that hypoxia is a feature of IBD, and may in some way influence inflammation.5, 6 Until these mechanisms can be better understood, a number of investigators use simplified murine models of IBD in which injurious agents such as detergents are administered. The most common detergents used are DSS and TNBS, which cause epithelial injury and subsequent inflammation.

A critical component of the adaptive response to hypoxia is the regulation of molecules that protect the mucosal epithelium, including hypoxia inducible factor (HIF). However, this transcription factor is degraded by hydroxylation under normal conditions, presumably because of overlapping protective and adaptive mechanisms that allow the gut to continue to transport and maintain a barrier under conditions of “physiologic hypoxia” (a term adopted by Robinson et al5). When the gut is subjected to pathophysiologic levels of hypoxia, HIF is stabilized and helps to protect the intestine from injury.7 In this issue of Gastroenterology, two groups show that HIF can be used to protect the intestine from detergent-induced IBD in rodents (Figure 1). In the study by Robinson et al and the study by Cummins et al,8 HIF degradation was prevented by pharmacologic inhibitors of hydroxylases, the enzymes that result in inactivation of HIF. Although the hydroxylase inhibitors used were different, the results were similar: an increase in stable HIF-1 levels. This in turn protected mice against colonic injury, inflammation, and clinical signs of IBD. Recent studies have shown that HIF is also protective against other diseases that may be associated with hypoxia. For example, investigators showed that HIF protected the intestine from injury during necrotizing enterocolitis,9 a disease of infants in which a combination of hypoxic and dietary factors seems to play a role in causing loss of epithelial barrier function and subsequent sepsis.

  • View full-size image.
  • Figure 1. 

    Protective and reparative mechanisms of HIF in model IBD intestine. Note in upper panel infiltration of neutrophils following a period of hypoxia and repair (arrows). These neutrophils traverse the repaired mucosa, and may lead to recurrence of epithelial loss. Note in the lower panel restitution of ileum subjected to ischemia/reperfusion in which tissues have been allowed to recover. Evidence of restitution is noted in the form of flattened epithelial cells crawling across denuded mucosa. However, further repair is required in some sections of tissue (arrows).

The mechanism whereby HIF prevents induction of disease is less clear. In the study by Robinson et al,5 increased HIF levels were associated with both reduction of changes in barrier function and inflammation. The authors demonstrate a decrease in the expression of critical proinflammatory cytokines such as tumor necrosis factor (TNF)-α and interferon (IFN)-γ in the presence of increased HIF levels. This may in part explain the beneficial effects of HIF on the intestinal barrier; cytokines such as TNF-α and IFN-γ have been shown to reduce barrier function in cell models.10 Additional effects of HIF were shown by Cummins et al,8 including decreased levels of a number of proinflammatory interleukins, and evidence for reduced neutrophil infiltration, assessed by myeloperoxidase levels, an enzyme expressed predominantly by neutrophils.

In addition to anti-inflammatory mechanisms, the hydroxylase inhibitor that was studied most extensively by Robinson et al (FG-4497)5 was also shown to induce marked contraction of collagen in vitro, suggesting that HIF plays a role in wound healing. Marked damage and inflammation, as was produced in both studies in this issue, would require initial wound healing, followed by contraction of the wound and epithelial restitution. Although restoration of barrier function to reduce continued influx of proinflammatory products from the lumen is vital, the role of HIF and fibrosis may be a future area for this research. This might require chronic models of IBD, most likely more subtle in nature than detergent-induced IBD. Alternate rodent models of a more chronic nature include interleukin-10 knockout mice that are susceptible to select luminal bacteria. Typically, these models involve gnotobiotic techniques so that individual bacteria, or a specific group of bacteria, can be introduced to colonize the gut.11, 12

The study by Cummins et al8 suggests that inhibition of hydroxylases with dimethyloxalylglycine (DMOG) has a more broad-spectrum effect than stabilization of HIF, as nuclear factor (NF)-κB levels were increased. Although this signaling pathway is typically associated with inflammation, elevated NF-κB levels decrease apoptosis, thereby potentially preserving the intestinal epithelial barrier in DSS-induced colitis. Apoptosis is likely applicable to loss of cells from the luminal surface of the intestine under normal and pathophysiologic conditions. The intestinal epithelium is characterized by rapid turnover of the epithelium, with stem cells located in the base of the crypts, that generated new epithelial cells, which mature as they escalate up the crypt and on to intestinal villi in the small intestinal mucosa and intercrypt epithelium in the colon. Finally, cells undergo apoptosis, and are removed from the intestinal epithelial monolayer in an orderly fashion to limit alteration of barrier function.13, 14 In other words, the apoptotic cell undergoes cell death in situ, allowing neighboring cells to fill the gap and limit disruption of the epithelial barrier.

The work shown in these studies provides evidence for the importance of HIF-1 in protection of and repair of IBD lesions induced in acute and subacute detergent models in rodents. Future directions include the applicability of inhibition of hydroxylases by novel pharmaceutical agents in patients with IBD. In addition, it is difficult to know from these studies which are the most critical mechanisms of HIF-1 activity in IBD: protection from loss of epithelial cells via inhibition of apoptosis, increased wound healing, or inhibition of inflammatory pathways. Conclusive studies, using a range of models that begin to incorporate factors thought to be involved in IBD in patients (the luminal microbial environment and the genetic background of the subject) may be an area for future study; targeting of HIF-1 signaling pathways may provide novel therapeutic regimens. Further clinical trials are required to determine applicability of hydroxylase inhibitors in humans, because rodents do not reflect all of the factors involved in IBD. Staging of disease is also likely to be very important. For example, inhibition of hydroxylases might be critical very early in the onset of IBD, but may have to be used more selectively during chronic disease once inflammation has been established. Importantly, translation of basic science studies like these to patients with differing courses and severity of disease is difficult. However, the findings provided in the studies in this issue of Gastroenterology are striking, particularly given the severity of detergent-induced rodent IBD, and the number of similarities between the findings of two entirely separate groups of investigators.

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References 

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PII: S0016-5085(07)02126-9

doi:10.1053/j.gastro.2007.11.049

Refers to article:

  • Mucosal Protection by Hypoxia-Inducible Factor Prolyl Hydroxylase Inhibition , 28 September 2007

    Andreas Robinson, Simon Keely, Jörn Karhausen, Mark E. Gerich, Glenn T. Furuta, Sean P. Colgan
    Gastroenterology January 2008 (Vol. 134, Issue 1, Pages 145-155)

  • The Hydroxylase Inhibitor Dimethyloxalylglycine Is Protective in a Murine Model of Colitis , 11 October 2007

    Eoin P. Cummins, Fergal Seeballuck, Stephen J. Keely, Niamh E. Mangan, John J. Callanan, Padraic G. Fallon, Cormac T. Taylor
    Gastroenterology January 2008 (Vol. 134, Issue 1, Pages 156-165.e1)

Gastroenterology
Volume 134, Issue 1 , Pages 346-348, January 2008