Nuclear factor-kappa B in intestinal protection... : Current Opinion in Gastroenterology (original) (raw)

Introduction

The transcription factor, nuclear factor-kappa B (NF-κB), is a central regulator of inflammatory and immune responses and plays a key role in the pathophysiology of clinically important diseases in most organ systems. The NF-κB family of proteins consists of five members in mammals, c-Rel, RelA (p65), RelB, NF-κB1, and NF-κB2 (p100), which all share a conserved Rel-homology domain responsible for DNA-binding activity, protein dimerization, and nuclear translocation. These subunits form homodimers or heterodimers, which constitute the transcriptionally active or suppressive forms of NF-κB. Inactive NF-κB is located in the cytosol complexed with an inhibitory protein, IκB. Ligand binding to particular membrane or cytosolic receptors leads to activation of the key enzyme, IκB kinase (IKK), a complex composed of the regulatory IKKγ (NEMO) subunit and two enzymatically active subunits, IKKα and IKKβ. Canonical IKK activation involves IKKβ and results in phosphorylation of IκB, dissociation of IκB from NF-κB, ubiquitination, and ultimate proteasomal degradation of the inhibitor. Most commonly, this pathway leads to activation of the prototypic heterodimer, p50/RelA. In the alternative IKK signaling pathway, IKKα is activated and causes phosphorylation and processing of p100, leading to formation of p52/RelB dimers. Upon activation, NF-κB dimers are translocated into the nucleus where they bind to selective target gene promoters and mediate the transcriptional activation or, in some cases, suppression of numerous (>200) genes involved in innate and adaptive immune responses.

Important progress has been made over the last few years in defining the functions of NF-κB signaling in normal homeostasis and specific disease processes in the intestinal tract . Mucosal inflammation in patients with inflammatory bowel disease (IBD) and in experimental models of intestinal inflammation is accompanied by elevated levels of activated NF-κB, particularly p65, p50, and c-Rel, as determined by electrophoretic mobility shift assays and immunostaining . In parallel, expression of prototypic NF-κB target genes, including IL-1β, IL-6, TNFα, and IL-12 p40 is increased . Given that many of these gene products promote inflammation, it has been reasoned that inflammation in IBD and other conditions can be attenuated through inhibition of NF-κB activity. Initial data on murine inflammation models supported this contention .

However, recent findings in gene-targeted mice indicate that NF-κB has more diverse functions than initially anticipated, governing both protective and destructive responses, which depend on the cell types involved and the specific pathophysiological conditions. This review highlights new findings on the protective and inflammatory functions of NF-κB in different disease models and cell types in small and large intestine.

Protective intestinal functions of nuclear factor-kappa B

NF-κB has key protective functions in the intestinal tract in response to acute challenges that cause injury and inflammation. In addition, it can play an important role in maintaining normal homeostasis in the intestinal tract.

Nuclear factor-kappa B in normal intestinal homeostasis

Total ablation of several of the key components of the NF-κB signaling pathway is embryonically lethal in mice, precluding further analysis of their physiological importance in the fully developed intestinal tract. However, recent studies in conditional gene-targeted mice have revealed important functions of NF-κB in maintaining normal intestinal homeostasis. Deletion of IKKγ/NEMO, the common regulatory subunit of the IKK complex, in intestinal epithelial cells caused severe spontaneous colitis in mice at a young age . Intestinal epithelial apoptosis was extensive in these mice, and accordingly epithelial barrier integrity was compromised, leading to translocation of commensal bacteria into the mucosa . In addition, expression of the antimicrobial peptide, β-defensin 3, was attenuated in the absence of epithelial IKKγ, suggesting that a weakened constitutive epithelial defense may also contribute to increased bacterial influx into the mucosa (Fig. 1). Recognition of translocated bacteria appeared to be important for sustaining inflammation, as additional deletion of the central Toll-like receptor (TLR) signal adaptor, MyD88, abolished colitis in the epithelial IKKγ-deficient mice . A similar inflammation phenotype was observed in mice lacking both IKKα and IKKβ, whereas mice lacking either one of the two enzymatically active IKK subunits had no spontaneous colitis . These data suggest that complete loss of epithelial IKK signaling compromises normal intestinal homeostasis, whereas IKKα and IKKβ exhibit functional redundancy in this context.

Consistent with a key role of NF-κB in maintaining intestinal homeostasis, another recent study demonstrated that a significant fraction (35–40%) of mice lacking TLR5, a sensor of bacterial flagellins and strong activator of NF-κB, develop spontaneous colitis. Furthermore, TLR5 deficiency accelerated the course of spontaneous colitis that develops in the absence of IL-10 . These results indicate that TLR5-dependent recognition of flagellin-producing intestinal bacteria and presumably the accompanying NF-κB activation are important for maintaining normal intestinal homeostasis. On the contrary, intestinal bacteria also contribute to the development of colitis in this model, as antibiotic treatment of TLR5-deficient mice attenuated spontaneous inflammation . In contrast to TLR5 deficiency, loss of MyD88, TLR2, or TLR4, all of which play key roles in recognizing or transducing bacterial signals and in activating NF-κB, is not associated with spontaneous colitis, although mice lacking MyD88 or TLR4 are more susceptible to acute chemically induced colitis . Thus, despite the more limited bacterial recognition potential of TLR5 (only a modest number of commensal bacteria express flagellins) compared with TLR4 (all Gram-negative bacteria) or MyD88 (which transduces recognition signals for all bacteria), TLR5, and presumably TLR5-dependent NF-κB activation, plays a more important role in maintaining intestinal homeostasis. It remains to be established whether NF-κB, in fact, mediates the protective functions of TLR5, or whether other signaling pathways are responsible.

Nuclear factor-kappa B in radiation-induced intestinal injury

Whole-body irradiation damages fast-growing tissues, particularly bone marrow, skin, and intestine. Large doses kill actively proliferating cells in the intestinal crypts and can compromise stem cell functions, leading to loss of regenerative potential and secondarily to mucosal inflammation. Epithelial cell death upon ionizing radiation is not only determined directly by the cellular damage inflicted by the radiation, but is also actively governed by cellular responses to the damage. NF-κB has a protective function in this situation. Thus, mice that selectively lack the IKKβ kinase in the intestinal epithelium exhibit marked increases in radiation-induced epithelial cell apoptosis . Increased epithelial apoptosis is accompanied by decreased expression of antiapoptotic Bcl-2 family members, suggesting a mechanism of NF-κB-dependent cell protection in this model .

Furthermore, pretreatment of mice with sublethal doses of lipopolysaccharide (LPS) normally activates NF-κB in the intestinal epithelium and protects against radiation-induced damage, but the absence of epithelial IKKβ signaling completely abolishes this protection . A recent study provides further support for the protective function of NF-κB in radiation-induced intestinal damage. Administration of a polypeptide derived from Salmonella flagellin that binds to TLR5 and activates NF-κB, protected mice from acute intestinal radiation damage and resulted in improved survival through the inhibition of apoptosis in intestinal cells . Flagellin peptides that could not activate NF-κB did not provide radioprotection . TLR5 activation was accompanied by induction of gene products, such as superoxide dismutase 2 and granulocyte colony-stimulating factor, that can limit radiation damage and promote recovery from it . Although these studies did not directly address the responsible cell types, it is well known that intestinal epithelial cells express TLR5 and respond to TLR5 ligands with NF-κB activation and increased expression of NF-κB target genes .

Intestinal nuclear factor-kappa B in ischemic and thermal injury

Transient ischemia of the intestine followed by reperfusion leads to activation of NF-κB and rapid induction of inflammatory cytokines, most importantly TNFα . Upon entering the bloodstream, this cytokine activates chemokine expression and massive leukocyte influx in the lungs, which is a major cause of mortality in this condition. Genetic ablation of IKKβ in the intestinal epithelium or, as recently shown, pharmacologic inhibition of NF-κB prevents TNFα induction and the ensuing lung inflammation, and significantly improves mortality in murine models . However, these interventions also exacerbated delayed damage in the intestine , which was paralleled and probably caused by enhanced epithelial apoptosis . Thus, under conditions of intestinal ischemia–reperfusion, attenuation of systemic disease by NF-κB inhibition is obtained at the expense of increased local destruction, underlining that NF-κB has local protective functions. Interestingly, brief intentional exposure of tissues to ischemia protects against deleterious effects of prolonged ischemia–reperfusion . This ‘ischemic preconditioning’, when applied to the small intestine, was recently shown to be accompanied by enhanced intestinal NF-κB activation , suggesting that controlled NF-κB activation may provide a strategy for intestinal protection during intestinal transplantation surgery.

Another condition in which local tissue damage can cause systemic disease is thermal skin injury. Extensive and severe skin burns can cause intestinal damage secondary to TNFα release . Loss of intestinal epithelial IKKβ exacerbated the mucosal damage, which was associated with elevated active caspase-3 and thus presumably epithelial apoptosis . In parallel, expression levels of the genes encoding the antiapoptotic proteins, Bcl-xL and c-FLIP, were attenuated in the absence of epithelial IKKβ . These data suggest that anti-inflammatory therapy of severe skin burns that inhibits NF-κB systemically runs the risk of aggravating intestinal injury.

Nuclear factor-kappa B and acute intestinal inflammation

Oral or rectal challenge of mice with a number of injurious agents, including dextran sulfate sodium (DSS) or acetic acid, causes acute but transient inflammation in the intestine. Despite the predominance of inflammation-associated cytokines in these situations, many of which are targets of NF-κB, recent studies have revealed that NF-κB has protective functions under these conditions. Selective loss of RelA in intestinal epithelial cells enhanced acute colitis and increased mortality induced by DSS feeding . The RelA-deficient mice exhibited increased epithelial apoptosis and ulcerations after DSS challenge, which was probably related to attenuated expression of key antiapoptotic gene products such as Bcl-xL that are known targets of NF-κB . In another recent study , a similar phenotype was observed in mice lacking intestinal epithelial IKKβ. These mice experienced more pronounced acute inflammation upon DSS administration, and showed delayed recovery after DSS discontinuation . Importantly, pharmacologic inhibition of IKKβ in wild-type mice also exacerbated inflammation in the DSS-induced colitis model, indicating that the protective functions of NF-κB were dominant even if the functional loss was not confined to the epithelium .

Epithelial deficiency in p65 or IKKβ leads to increased apoptosis upon DSS challenge , which can explain the greater ulcerations in the respective mice (Fig. 1). In addition, an indirect mechanism of epithelial protection appears to be at work (Fig. 1), as microarray studies of epithelial cells in mice treated with an IKKβ inhibitor not only revealed the expected suppression of NF-κB target genes, but also of target genes for signal transducer and activator of transcription (STAT) and interferon regulatory factor (IRF) transcription factors . In particular, heat shock protein (HSP) 70 failed to get induced normally in IKKβ-inhibited mice . This factor, whose expression is not controlled by NF-κB, is known to protect epithelial cells against different stresses and the mucosa against acute inflammation when overexpressed as a transgene . Epithelial HSP70 production was likely induced by cytokines, such as IL-11 and IL-22, which are primarily products of macrophages and activate STAT transcription factors. Epithelial NF-κB intersected with these pathways by contributing to the recruitment of macrophages, which exert protective functions in this context , through the induction of chemokines (Fig. 1). Irrespective of the specific mechanisms of protection, loss of RelA and IKKβ signaling compromised epithelial barrier functions during an acute inflammatory challenge, which led to greater ulceration and presumably secondary influx of commensal bacteria and their products into the mucosa.

Bacterial products such as LPS activate NF-κB through TLR-dependent pathways. Systemic administration of LPS causes endotoxic shock characterized by the release of inflammatory cytokines, particularly TNFα and IL-1, which are critical mediators of the pathophysiological sequelae of the shock. Apart from involvement in systemic cytokine release, NF-κB also has protective intestinal functions in models of endotoxic shock. Thus, mice that lack NF-κB p50 and are heterozygous for p65 are highly susceptible to LPS, because they develop an enteropathy related to increased epithelial cell death and loss of barrier function . Epithelial destruction was abolished in the knockout animals by neutralizing TNFα, whereas direct systemic TNFα administration led to enhanced epithelial apoptosis and barrier loss, indicating that the cytokine was necessary and sufficient to mediate the LPS-induced enteropathy in the absence of p50/p65 .

Nuclear factor-kappa B and intestinal bacterial infection

Recognition of conserved microbial products by TLRs and subsequent activation of NF-κB not only activates host defense against pathogenic bacteria, but can also provide mucosal protection. A recent study demonstrated that mice lacking TLR5, which recognizes bacterial flagellin, exhibit enhanced acute intestinal inflammation upon infection with Salmonella enterica. These studies only employed those TLR5 knockout mice that had not developed spontaneous colitis (see above). Consistent with a role of flagellin–TLR5 interaction in host protection, intestinal infection of wild-type mice with a flagellin mutant of Salmonella also caused greater acute intestinal inflammation and damage, a phenotype that could be reversed by systemic administration of purified flagellin . Increased destruction was probably related to higher levels of epithelial apoptosis, caused by a failure to NF-κB-dependent antiapoptotic genes such as cIAP-2, in the intestine . A previous study had shown that flagellin is an activator of NF-κB and its target genes in cultured intestinal epithelial cells infected with Salmonella.

The acute intestinal functions of NF-κB appear to differ from those important in systemic bacterial infection and other factors beyond NF-κB play a role. A recent study on a bacterial deubiquitinating enzyme, Salmonella secreted factor L (SseL), demonstrated that the enzyme suppressed NF-κB activation in cells exposed to Salmonella by removing ubiquitins from phosphorylated IκB, thereby preventing its proteasomal degradation necessary for nuclear NF-κB translocation. Infection of mice with an SseL mutant of Salmonella caused greater liver inflammation, presumably as a consequence of more NF-κB activation and higher expression of NF-κB-dependent inflammatory cytokines . In this situation, the potential protective functions of NF-κB appeared to be outweighed by the inflammatory functions.

Furthermore, another Salmonella factor, AvrA, was also shown to inhibit NF-κB activation by deubiquitination , as well as the Jun N-terminal kinase (JNK) signaling pathway , yet infection with AvrA-deficient bacteria increased epithelial apoptosis upon intestinal infection . Here, the role of NF-κB in mucosal protection was either overcome by the increased induction of inflammatory, and perhaps apoptosis-inducing cytokines, or the proapoptotic functions of JNK were dominant .

Inflammatory functions of nuclear factor-kappa B in the intestine

In addition to its protective roles under certain conditions, NF-κB is also a central regulator of mucosal inflammation and damage in other situations, particularly those of a chronic nature.

Nuclear factor-kappa B in chronic intestinal inflammation

In contrast to its protective effects under conditions of acute injury and inflammation, NF-κB promotes inflammation under many chronic conditions in the intestinal tract. This was first demonstrated in a chemically induced [by 2,4,6-trinitrobenzene sulfonic acid (TNBS)] model of T cell-dependent colitis and in spontaneous colitis in IL-10-deficient mice, in which rectal administration of antisense oligonucleotides against NF-κB p65 attenuated inflammation . Later, intrarectal and intraperitoneal administration of an NF-κB decoy oligonucleotide encapsulated in hemagglutinating virus of Japan envelope vesicles, which promote high uptake efficiency into cells in vitro and in vivo, was also shown to ameliorate colitis induced by TNBS as well as oxazolone . TNBS-induced colitis is dominated by T helper 1 (Th1)-type cytokines [IL-12 and IFNγ], whereas oxazolone-induced colitis is driven by Th2-type cytokines (IL-4 and IL-13), suggesting that the therapeutic effects of NF-κB inhibition were independent of the underlying immune T-cell deviation. Furthermore, the NF-κB decoy oligonucleotide was not only effective when given early in colitis induction, but also as treatment in fully established inflammation , which provides hope for the utility of this intervention strategy in the treatment of Crohn's disease.

Recent studies have confirmed and expanded these important concepts on the role of NF-κB in promoting and sustaining chronic intestinal inflammation. The IκB phosphorylation inhibitor, BAY 11-7085, attenuated colitis in IL-10-deficient mice selectively associated with Enterococcus faecalis and Escherichia coli. Furthermore, a short peptide mimicking a critical domain in IKKα and IKKβ that binds to the regulatory IKKγ/NEMO subunit blocks NF-κB activation effectively in vitro and in vivo. This NEMO-binding domain peptide ameliorated spontaneous colitis in IL-10-deficient mice and TNBS-induced colitis in wild-type mice . Thus, combined blockade of the canonical and alternative pathway of NF-κB activation was highly effective in attenuating colitis.

Studies in gene-targeted mice have revealed some of the NF-κB subunits and critical cell types that mediate the overall effects of NF-κB inhibition in chronic colitis. In a model of innate colitis in RAG-deficient mice infected with the enteric pathogen, Helicobacter hepaticus, the NF-κB subunit, c-Rel, was required for induction of inflammation . In contrast, loss of NF-κB p50 had no impact on colitis. The failure to induce colitis was associated with decreased expression of IL-12 p40 and IL-23 p19, proinflammatory cytokine subunits (of IL-12 and IL-23, respectively) whose production by macrophages is critically dependent on c-Rel . Furthermore, transfer of colitogenic CD4+ CD45RBhigh T cells to recombinase-activating gene (RAG)-deficient recipients resulted in diminished colitis when mice were also deficient in c-Rel, indicating that c-Rel is required within innate immune cells for mediating T cell-dependent colitis . In another recent study , loss of IKKβ in macrophages/neutrophils in conditional knockout mice delayed the onset of spontaneous colitis in IL-10-deficient mice, whereas selective IKKβ deficiency in intestinal epithelial cells had no effect on colitis. These data further support the concept that NF-κB in innate immune cells promotes chronic intestinal inflammation.

Nuclear factor-kappa B in necrotizing enterocolitis

Necrotizing enterocolitis is a major cause of morbidity and mortality in newborns, particularly premature infants. The pathogenesis is poorly understood, but elements of tissue necrosis and inflammation are dominant features. Recent studies in rodent models, in which a similar disease can be induced in prematurely born pups by a combination of hypoxia, cold stress, and feeding of a modified infant formula, show that NF-κB was persistently activated in the whole intestine, and particularly in the epithelium, after induction of necrotizing enterocolitis. Consistent with this, an expression of NF-κB target genes such as TNFα and chemokine (C–X–C motif) ligand 2 (CXCL2) is increased in newborns with necrotizing enterocolitis and in animal models of the condition . Importantly, a blockade of NF-κB activation with a NEMO-binding domain peptide markedly attenuated inflammation and reduced mortality in the rodent model , indicating that genes controlled by NF-κB, such as TNFα, are critical drivers, or at least mediators, of the disease processes that underlie necrotizing enterocolitis.

Conclusion

The results from different models of intestinal infection, inflammation, and injury indicate that NF-κB has multiple, and often opposing, functions in the pathogenesis of intestinal diseases. Thus, NF-κB and its target genes provide protection under many conditions, including in normal homeostasis and settings of acute colitis. Yet, it is also clear that NF-κB contributes to inflammation and tissue destruction under other conditions, particularly those associated with chronic T cell-dependent colitis. What explains these seemingly contradictory results? Three major concepts are likely to be relevant and provide at least partial explanations for these observations.

First, NF-κB governs the expression of numerous genes, whose products can be roughly divided into those with classical proinflammatory functions, for example, many chemokines, and those with cell-protective functions such as the antiapoptotic proteins cIAP-2 and Bcl-xL. Accordingly, inhibition of NF-κB target genes can have divergent effects, depending on which group of genes is dominant in the pathogenesis of the respective disease. For example, colitis induced by DSS feeding is dominated by epithelial cell apoptosis and destruction. Diminished expression of NF-κB target genes with antiapoptotic and cell-protective functions in epithelial cells would be expected to exacerbate disease, as was indeed observed in acute DSS colitis models (Fig. 1). Additional loss of NF-κB-controlled inflammatory cytokines in immune cells is not sufficient to counteract the intensified inflammatory stimulation that derives from greater epithelial barrier loss and enhanced exposure to products of the luminal microbiota . On the contrary, NF-κB is a key proinflammatory regulator under conditions in which inflammatory cytokine production dominates the pathogenesis, such as in liver granulomas induced by Salmonella infection , or in intestinal inflammation induced by colitogenic T cells (Fig. 1).

Second, it matters which cell types drive the pathogenesis of a particular condition and what role NF-κB plays in that cell type (Fig. 1). For instance, irradiation damage is mostly targeted to the epithelium in the intestine, as actively proliferating cells are vulnerable to DNA-damaging effects of ionizing radiation. Protective NF-κB functions in the epithelium are more important than any other NF-κB functions under these conditions. Consequently, loss of epithelial NF-κB exacerbates radiation injury . By contrast, chronic intestinal inflammation driven by aberrant T cells is primarily dependent on the NF-κB functions in those cells. Protection against cell death by NF-κB target-gene products promotes survival and functions of these cells, and thereby acts in a proinflammatory manner . Conversely, inhibition of NF-κB under these conditions attenuates intestinal inflammation .

Third, NF-κB is not a single transcription factor with defined transcriptional activity, but rather consists of various homodimers and heterodimers that show overlap and differences in their target genes and transcriptional activation or suppression profiles. Although this concept has not yet been widely applied to intestinal biology, future studies are likely to find subtle but relevant differences between the functions of the different NF-κB subunits. For example, c-Rel was important for controlling expression of IL-12 p40 and IL-23 p19 in macrophages, which mediated intestinal inflammation upon transfer of colitogenic T cells . Yet, c-Rel did not govern the expression of IL-6, TNFα, or IP-10 , cytokines that are known NF-κB target genes with inflammatory functions. Furthermore, the two major pathways of NF-κB activation, canonical activation through IKKβ and alternative activation through IKKα, are likely to have both overlapping and unique roles in intestinal biology. For example, deficiency of IKKβ or IKKα in the intestinal epithelium alone had no apparent phenotype, but loss of both, or of the common regulatory subunit IKKγ/NEMO, resulted in spontaneous inflammation .

Overall, current evidence suggests that NF-κB exerts diverse functions in the intestinal tract, which can be protective or destructive depending on the specific challenges and pathophysiological conditions, and the major cell types involved. These results caution that inhibitors of the NF-κB pathway need to be evaluated carefully for their potential use as anti-inflammatory agents in IBD therapy, as they have the potential to activate inflammation under certain conditions in a paradoxical fashion.

Acknowledgements

This work was supported by NIH grants DK70867 and DK35108, and a fellowship from the MFG Educative Science. We thank John J. Quinlan III for help in preparing the manuscript.

References and recommended reading

Papers of particular interest, published within the annual period of review, have been highlighted as:

• of special interest

•• of outstanding interest

Additional references related to this topic can also be found in the Current World Literature section in this issue (pp. 161–162).

1 Karrasch T, Jobin C. NF-κB and the intestine: friend or foe? Inflamm Bowel Dis 2008; 14:114–124.
2 Schreiber S, Nikolaus S, Hampe J. Activation of nuclear factor κB inflammatory bowel disease. Gut 1998; 42:477–484.
3 Rogler G, Brand K, Vogl D, et al. Nuclear factor κB is activated in macrophages and epithelial cells of inflamed intestinal mucosa. Gastroenterology 1998; 115:357–369.
4 Sanchez-Munoz F, Dominguez-Lopez A, Yamamoto-Furusho JK. Role of cytokines in inflammatory bowel disease. World J Gastroenterol 2008; 14:4280–4288.
5 Neurath MF, Pettersson S, Meyer zum Buschenfelde KH, et al. Local administration of antisense phosphorothioate oligonucleotides to the p65 subunit of NF-κB abrogates established experimental colitis in mice. Nat Med 1996; 2:998–1004.
6•• Nenci A, Becker C, Wullaert A, et al. Epithelial NEMO links innate immunity to chronic intestinal inflammation. Nature 2007; 446:557–561. This provocative study demonstrates that loss of the regulatory IKK subunit, IKKγ, in intestinal epithelial cells causes spontaneous colitis in mice, documenting that epithelial NF-κB plays a crucial role in maintenance of normal homeostasis.
7 Egan LJ, Eckmann L, Greten FR, et al. IκB-kinase β-dependent NF-κB activation provides radioprotection to the intestinal epithelium. Proc Natl Acad Sci U S A 2004; 101:2452–2457.
8•• Vijay-Kumar M, Sanders CJ, Taylor RT, et al. Deletion of TLR5 results in spontaneous colitis in mice. J Clin Invest 2007; 117:3909–3921. This study demonstrates that loss of TLR5 causes spontaneous intestinal inflammation in 35–40% of mice, and accelerates colitis development in IL-10-deficient mice. These data indicate that host recognition of flagellin-producing bacteria is important for controlling normal homeostasis in the intestine.
9 Rakoff-Nahoum S, Paglino J, Eslami-Varzaneh F, et al. Recognition of commensal microflora by toll-like receptors is required for intestinal homeostasis. Cell 2004; 118:229–241.
10 Fukata M, Michelsen KS, Eri R, et al. Toll-like receptor-4 is required for intestinal response to epithelial injury and limiting bacterial translocation in a murine model of acute colitis. Am J Physiol Gastrointest Liver Physiol 2005; 288:G1055–G1065.
11•• Burdelya LG, Krivokrysenko VI, Tallant TC, et al. An agonist of toll-like receptor 5 has radioprotective activity in mouse and primate models. Science 2008; 320:226–230. In this study, the authors have identified a flagellin peptide that activates NF-κB and protects against intestinal damage and mortality upon whole-body ionizing radiation, but did not decrease tumor radiosensitivity in mouse models. The peptide may be beneficial in improving the therapeutic index of cancer radiotherapy.
12 Bambou JC, Giraud A, Menard S, et al. In vitro and ex vivo activation of the TLR5 signaling pathway in intestinal epithelial cells by a commensal Escherichia coli strain. J Biol Chem 2004; 279:42984–42992.
13 Miyamoto Y, Iimura M, Kaper JB, et al. Role of Shiga toxin versus H7 flagellin in enterohaemorrhagic Escherichia coli signalling of human colon epithelium in vivo. Cell Microbiol 2006; 8:869–879.
14 Chen LW, Egan L, Li ZW, et al. The two faces of IKK and NF-κB inhibition: prevention of systemic inflammation but increased local injury following intestinal ischemia-reperfusion. Nat Med 2003; 9:575–581.
15 Suzuki T, Yamashita K, Jomen W, et al. The novel NF-κB inhibitor, dehydroxymethylepoxyquinomicin, prevents local and remote organ injury following intestinal ischemia/reperfusion in rats. J Surg Res 2008; 149:69–75.
16 Zou L, Attuwaybi B, Kone BC. Effects of NF-κB inhibition on mesenteric ischemia-reperfusion injury. Am J Physiol Gastrointest Liver Physiol 2003; 284:G713–G721.
17 Moore-Olufemi SD, Kozar RA, Moore FA, et al. Ischemic preconditioning protects against gut dysfunction and mucosal injury after ischemia/reperfusion injury. Shock 2005; 23:258–263.
18 Ferencz A, Racz B, Gasz B, et al. Threshold level of NF-κB activation in small bowel ischemic preconditioning procedure. Transplant Proc 2006; 38:1800–1802.
19 Spies M, Chappell VL, Dasu MR, et al. Role of TNFα in gut mucosal changes after severe burn. Am J Physiol Gastrointest Liver Physiol 2002; 283:G703–G708.
20 Chen LW, Chen PH, Chang WJ, et al. IκB-kinase/nuclear factor-κB signaling prevents thermal injury-induced gut damage by inhibiting c-Jun NH2-terminal kinase activation. Crit Care Med 2007; 35:1332–1340.
21• Steinbrecher KA, Harmel-Laws E, Sitcheran R, et al. Loss of epithelial RelA results in deregulated intestinal proliferative/apoptotic homeostasis and susceptibility to inflammation. J Immunol 2008; 180:2588–2599. The data in this study show that loss of the NF-κB subunit p65 selectively in the intestinal epithelium sensitizes mice to colitis induced by feeding the irritant DSS, thus providing strong support for the concept that epithelial NF-κB has predominantly protective functions in acute intestinal inflammation.
22•• Eckmann L, Nebelsiek T, Fingerle AA, et al. Opposing functions of IKKβ during acute and chronic intestinal inflammation. Proc Natl Acad Sci U S A 2008; 105:15058–15063. This interesting study has employed a combination of genetic and pharmacological strategies to demonstrate that IKKβ/NF-κB can have protective or destructive functions in intestinal inflammation, depending on the underlying pathophysiological mechanisms and the primary cell types involved. Furthermore, a new indirect mechanism is proposed by which epithelial NF-κB can protect the epithelium against apoptosis by the actions of macrophage-derived cytoprotective cytokines and induction of epithelial heat shock proteins.
23 Greten FR, Eckmann L, Greten TF, et al. IKKβ links inflammation and tumorigenesis in a mouse model of colitis-associated cancer. Cell 2004; 118:285–296.
24 Musch MW, Ciancio MJ, Sarge K, et al. Induction of heat shock protein 70 protects intestinal epithelial IEC-18 cells from oxidant and thermal injury. Am J Physiol 1996; 270:C429–C436.
25• Tanaka K, Namba T, Arai Y, et al. Genetic evidence for a protective role for heat shock factor 1 and heat shock protein 70 against colitis. J Biol Chem 2007; 282:23240–23252. The authors have generated a new transgenic mouse model of epithelial HSP expression and demonstrate that these mice have attenuated acute inflammatory damage associated with DSS feeding, providing strong physiological evidence for the role of HSPs in protection of the intestinal epithelium.
26 Qualls JE, Kaplan AM, van Rooijen N, et al. Suppression of experimental colitis by intestinal mononuclear phagocytes. J Leukoc Biol 2006; 80:802–815.
27 Gadjeva M, Wang Y, Horwitz BH. NF-κB p50 and p65 subunits control intestinal homeostasis. Eur J Immunol 2007; 37:2509–2517.
28 Vijay-Kumar M, Aitken JD, Kumar A, et al. Toll-like receptor 5-deficient mice have dysregulated intestinal gene expression and nonspecific resistance to _Salmonella_-induced typhoid-like disease. Infect Immun 2008; 76:1276–1281.
29 Vijay-Kumar M, Wu H, Jones R, et al. Flagellin suppresses epithelial apoptosis and limits disease during enteric infection. Am J Pathol 2006; 169:1686–1700.
30 Gewirtz AT, Simon PO Jr, Schmitt CK, et al. Salmonella typhimurium translocates flagellin across intestinal epithelia, inducing a proinflammatory response. J Clin Invest 2001; 107:99–109.
31• Le Negrate G, Faustin B, Welsh K, et al. Salmonella secreted factor L deubiquitinase of Salmonella typhimurium inhibits NF-κB, suppresses IκBα ubiquitination and modulates innate immune responses. J Immunol 2008; 180:5045–5056. The authors found that the Salmonella virulence factor, SseL, prevents NF-κB activation by deubiquitinating IκB. Bacterial SseL mutants caused greater liver inflammation upon systemic infection in mice, presumably because of higher production of proinflammatory cytokines due to loss of NF-κB inhibition.
32 Ye Z, Petrof EO, Boone D, et al. Salmonella effector AvrA regulation of colonic epithelial cell inflammation by deubiquitination. Am J Pathol 2007; 171:882–892.
33• Jones RM, Wu H, Wentworth C, et al. Salmonella AvrA coordinates suppression of host immune and apoptotic defenses via JNK pathway blockade. Cell Host Microbe 2008; 3:233–244. In this interesting study, it is shown that the Salmonella factor AvrA inhibits NF-κB activation by deubiquitination of IκB, as well as JNK signaling. Bacterial mutants caused more apoptosis in the intestinal epithelium after intestinal infection, indicating that the epithelium-protective function of NF-κB was not important or was overcome by the proapoptotic function of JNK in this model.
34 Fichtner-Feigl S, Fuss IJ, Preiss JC, et al. Treatment of murine Th1- and Th2-mediated inflammatory bowel disease with NF-κB decoy oligonucleotides. J Clin Invest 2005; 115:3057–3071.
35• Karrasch T, Kim JS, Muhlbauer M, et al. Gnotobiotic IL-10−/−; NF-κB(EGFP) mice reveal the critical role of TLR/NF-κB signaling in commensal bacteria-induced colitis. J Immunol 2007; 178:6522–6532. This study employs a sophisticated genetic model for examining NF-κB activation in vivo. The model used to demonstrate that the development of colitis in IL-10-deficient mice associated with two defined bacterial species is dependent on IKKβ/NF-κB activation.
36 Dave SH, Tilstra JS, Matsuoka K, et al. Amelioration of chronic murine colitis by peptide-mediated transduction of the IκB kinase inhibitor NEMO binding domain peptide. J Immunol 2007; 179:7852–7859.
37 Shibata W, Maeda S, Hikiba Y, et al. Cutting edge: The IκB kinase (IKK) inhibitor, NEMO-binding domain peptide, blocks inflammatory injury in murine colitis. J Immunol 2007; 179:2681–2685.
38•• Wang Y, Rickman BH, Poutahidis T, et al. c-Rel is essential for the development of innate and T cell-induced colitis. J Immunol 2008; 180:8118–8125. This study demonstrates that c-Rel is critical for controlling IL-12 and IL-23 expression in macrophages, but is not required for induction of other prototypic NF-κB target genes such as IL-6 and TNFα. In parallel, c-Rel in innate immune cells played a key role in mediating T cell-dependent colitis in murine models.
39 Le Mandat Schultz A, Bonnard A, Barreau F, et al. Expression of TLR-2, TLR-4, NOD2 and pNF-κB in a neonatal rat model of necrotizing enterocolitis. PLoS ONE 2007; 2:e1102.
40 De Plaen IG, Liu SX, Tian R, et al. Inhibition of nuclear factor-κB ameliorates bowel injury and prolongs survival in a neonatal rat model of necrotizing enterocolitis. Pediatr Res 2007; 61:716–721.
41 Viscardi RM, Lyon NH, Sun CC, et al. Inflammatory cytokine mRNAs in surgical specimens of necrotizing enterocolitis and normal newborn intestine. Pediatr Pathol Lab Med 1997; 17:547–559.
42 Halpern MD, Clark JA, Saunders TA, et al. Reduction of experimental necrotizing enterocolitis with anti-TNF-α. Am J Physiol Gastrointest Liver Physiol 2006; 290:G757–G764.

Keywords:

apoptosis; inflammation; innate immunity; intestine; mucosal immunology