• gastroenterology;
  • inflammation


  1. Top of page
  2. Abstract.
  3. Activation of NF-κB
  4. Cell-specific role of NF-κB in IBD
  5. Blockade of NF-κB activation as a therapeutic strategy in IBD
  6. Conflict of interest statement
  7. References

Apart from genetic and environmental factors, the mucosal immune system of the gut plays a central role in the pathogenesis of inflammatory bowel disease (IBD). In the healthy gut, the mucosal immune system ensures the balance between pro- and anti-inflammatory mediators and thereby allows an effective defence against luminal pathogens but at the same time prevents an overwhelming immune reaction directed against the huge amount of harmless luminal antigens (for example, components of food or nonpathological bacteria). In both entities of IBD (Crohn’s disease and ulcerative colitis) this immunological balance is severely impaired and shifted towards the pro-inflammatory side. The chronic mucosal inflammation in IBD is caused by hyperactivation of effector immune cells, which produce high levels of pro-inflammatory cytokines like tumour necrosis factor-α, interleukin-6 and interferon-γ, resulting in colonic tissue damage. The nuclear transcription factor kappaB (NF-κB) was identified as one of the key regulators in this immunological setting. Its activation is markedly induced in IBD patients and through its ability to promote the expression of various pro-inflammatory genes, NF-κB strongly influences the course of mucosal inflammation. Considering the different cell-type specific effects which are mediated by NF-κB, this review aims at describing the complex role of NF-κB in IBD and discusses existing pharmacological attempts to block the activation of NF-κB to develop new therapeutic strategies in IBD.

Activation of NF-κB

  1. Top of page
  2. Abstract.
  3. Activation of NF-κB
  4. Cell-specific role of NF-κB in IBD
  5. Blockade of NF-κB activation as a therapeutic strategy in IBD
  6. Conflict of interest statement
  7. References

In general, the family of nuclear transcription factor kappaB (NF-κB) proteins consists of five different members, which are namely p65 (RelA), c-Rel, RelB, p50 and p52. These proteins are all characterized by a structurally conserved N-terminal 300 amino acid region containing specific domains, which allow dimerization, nuclear localization and DNA-binding [1–3]. Amongst these members of the NF-κB family, only p65, c-Rel and RelB are directly able to activate the transcription of target genes. The transcriptional capacity of p50 and p52, which are initially synthesized as large precursors called p105 and p100, are dependent on dimerization with p65, c-Rel or RelB [2, 3]. In unstimulated cells, the majority of NF-κB dimers are inactivated and retained in the cytoplasm by association with small inhibitory molecules called IκBα, IκBβ or IκBε [4]. These inhibitors of NF-κB are able to mask the motif within the amino acid sequence of NF-κB, which is responsible for the nuclear localization of NF-κB. In addition, IκBα is also able to enter the nucleus by itself and subsequently mediates the blockade of DNA-binding of NF-κB and promotes the nuclear export of NF-κB [4]. To activate NF-κB, there exist two different intracellular pathways – the classic and the alternative pathway – where both result in the release of NF-κB from its inhibitors and in the nuclear localization of NF-κB [5–7]. Classic activation of NF-κB can be initiated by a broad panel of different stimuli including bacterial cell wall components like lipopolysaccharide, pro-inflammatory cytokines like tumour necrosis factor (TNF)-α or interleukin (IL)-1, viruses and DNA damaging agents [6]. These triggering substances are able to induce intracellular signalling cascades, resulting in a subsequent activation of the IκB kinase (IKK) complex. The IKK complex is composed of two catalytic subunits IKKα and IKKβ as well as a regulatory protein, named NFkappaB essential modulator (NEMO). Whilst NEMO serves as an essential adaptor molecule, both catalytic subunits are able to phosphorylate specific serine residues within the IκB molecules [1, 8, 9]. Phosphorylation of NF-κB-bound IκB, mainly mediated by IKKβ in case of classic NF-κB activation, subsequently initiates the proteosomal degradation of IκB and finally allows the nuclear localization of NF-κB [7, 10]. Some inducers of the classical NF-κB pathway (for example the TNF receptor family member CD40) are additionally able to trigger the alternative NF-κB pathway, which aims at post-translational processing of the p100 precursor to mature p52 [3, 7]. In contrast to the classical pathway, NEMO is not absolutely required for alternative NF-κB activation and instead of IKKβ, IKKα is of indispensable importance [3, 7]. Irrespective of the signalling cascade, which leads to its activation, activated and translocated NF-κB dimers in the nucleus are able to interact with regulatory NF-κB elements in promoters and enhancers, thereby inducing the expression of NF-κB target genes [11]. Generally, genes which are regulated by the transcriptional activity of NF-κB can be categorized into four functional groups: inflammatory and immunoregulatory genes, cell cycle regulating genes, anti-apoptotic genes and genes that encode negative regulators of NF-κB (autoinhibitory feedback loop) [5].

Cell-specific role of NF-κB in IBD

  1. Top of page
  2. Abstract.
  3. Activation of NF-κB
  4. Cell-specific role of NF-κB in IBD
  5. Blockade of NF-κB activation as a therapeutic strategy in IBD
  6. Conflict of interest statement
  7. References

Dysregulated cytokine production and signalling mechanisms by intestinal epithelial cells, lymphocytes and macrophages have been implicated in the pathogenesis of inflammatory bowel disease (IBD), and the transcription factor NF-κB turned out to be one of the major regulatory components in this complex scenario. Obviously the expression and activation of NF-κB is strongly induced in the inflamed gut of IBD patients. Especially macrophages and epithelial cells isolated from inflamed gut specimens from IBD patients showed augmented levels of NF-κB p65 [12]. Analysing the activation status of NF-κB in this setting, by immunofluorescence staining of activated p65 in biopsies, clearly demonstrated that NF-κB is not just expressed but is also in a state of activation in mucosal macrophages and epithelial cells in IBD patients. Interestingly, the amount of activated NF-κB correlated significantly with the severity of intestinal inflammation [13]. In addition to macrophages and epithelial cells, lamina propria fibroblasts are also assumed to play a NF-κB mediated pro-inflammatory role in IBD [14]. Because of the unequal role and function of macrophages, epithelial cells and fibroblasts within the mucosal immune system and very broad panel of genes which are controlled by NF-κB, it is necessary to carefully regard the different cell-specific effects of NF-κB which contribute to the pathogenesis of IBD.

In IBD patients, the increased NF-κB expression in mucosal macrophages is accompanied by an increased capacity of these cells to produce and secrete TNF-α, IL-1 and IL-6 [12]. This finding nicely reflects the central function of NF-κB in monocytes, which is the induction and control of pro-inflammatory cytokines. Beside TNF-α, IL-1 and IL-6, NF-κB is also able to regulate the expression of IL-12 and IL-23 [15, 16]. To some extent, these pro-inflammatory cytokines are directly involved in the mucosal tissue damage typically occuring in IBD. For example, TNF-α and IL-1 mediated upregulation of matrix metalloproteinase production results in severe damage of the extracellular matrix and mucosal degradation [17, 18]. But predominantly NF-κB-induced cytokines are responsible for further stimulation, activation and differentiation of lamina propria immune cells, resulting in the perpetuation of mucosal inflammation. Especially, the differentiation of T-helper (Th)1 cells which are of pivotal importance for the pathogenesis of IBD [17, 19] is strongly driven by the major Th1-inducing cytokine IL-12, but is also supported by IL-23 and TNF-α [17, 19–21]. As an additional feature, NF-κB-induced TNF-α is in turn able to potentiate the activation of NF-κB, thereby providing a kind of positive feedback [18].

Whilst NF-κB in macrophages is regarded as an explicit pro-inflammatory player in the setting of IBD, its role in epithelial cells turned out to be more controversial. IL-6-induced NF-κB activation in colonic epithelial cells could be demonstrated to be associated with an increased epithelial expression of intercellular adhesion molecule-1 which plays a critical role in the recruitment of neutrophil granulocytes to the site of inflammation [22]. So obviously, NF-κB is clearly involved in some pro-inflammatory epithelial signalling cascades, but on the other hand, there exist recent data from different experimental models in knockout mice, which clearly demonstrate an anti-inflammatory overall function of NF-κB in colonic epithelial cells [23, 24]. For example, intestinal epithelial cell-specific inhibition of NF-κB through conditional ablation of NEMO caused a spontaneous development of severe chronic intestinal inflammation in mice. NEMO-deficient intestinal epithelial cells in this model revealed an increased rate of apoptosis and a decreased production of antimicrobial peptides, subsequently resulting in an impaired integrity of the epithelial barrier and in an enhanced mucosal immune response, triggered by invading bacteria [23]. Furthermore, mice with an intestinal epithelial cell-specific deletion of IKK-β showed a reduced expression of the epithelial cell restricted cytokine thymic stromal lymphopoetin, an impaired development of a pathogen specific Th2 response and therefore an exacerbated production of pro-inflammatory Th1 cytokines after parasite infection occurred [24]. Both these models of intestinal epithelial cell-specific NF-κB inhibition clearly demonstrated a critical regulatory role of NF-κB for epithelial integrity and for the intestinal immune homeostasis [23, 24]. In colonic lamina propria fibroblasts, NF-κB activation can be initiated via T cell expressed CD40L. CD40L was shown to interact with the CD40 receptor, expressed on the surface of colonic fibroblast, and thereby to induce the activation of NF-κB. Subsequently, NF-κB activation in fibroblasts resulted in an increased expression of cytokines like IL-8, IL-6 and the monocyte chemotactic protein [14]. In this way, colonic fibroblasts showed the capacity to participate in a NF-κB-dependent manner in the immunopathogenesis of IBD [14].

This broad panel of different cell-specific ways in which NF-κB influences numerous immunological processes within the intestinal mucosa, clearly demonstrates the central involvement of this transcription factor in the development, maintenance and chronification of IBD.

Blockade of NF-κB activation as a therapeutic strategy in IBD

  1. Top of page
  2. Abstract.
  3. Activation of NF-κB
  4. Cell-specific role of NF-κB in IBD
  5. Blockade of NF-κB activation as a therapeutic strategy in IBD
  6. Conflict of interest statement
  7. References

As early studies revealed that trinitrobenzene sulphonic acid (TNBS)-induced colitis could successfully be treated by local administration of p65 antisense oligonucleotides [12], the NF-κB pathway soon became an attractive target for therapeutic interventions in IBD.

Many of the already established immunosuppressive drugs in IBD like corticosteroids, sulfasalazine, methotrexate and anti-TNF-α antibodies are known to mediate their anti-inflammatory effects at least partly via inhibition of NF-κB activity [25–29]. For example, corticosteroids are able to induce an increased expression of IκBa, which in turn retains NF-κB in the cytoplasm and interact physically with p65, thereby preventing the transactivation of NF-κB [25, 26, 30]. In agreement with these data, colonic mononuclear-, epithelial- and endothelial cells from glucocorticoid-treated IBD patients showed significantly lower nuclear NF-κBp65 levels than cells from untreated patients [26].

Whereas all these established drugs do not target NF-κB specifically, many efforts have been made during the last years to develop selective inhibitors of NF-κB. Beside the direct inhibition of NF-κB expression, driven for example by specific antisense oligonucleotides [12], there are further steps within the NF-κB activating signalling cascade, which represent promising targets for pharmacological inhibition of NF-κB. One possible strategy which has already been tested successfully in experimental mouse models of colitis is the direct targeting of the DNA-binding activity of specific NF-κB proteins by decoy oligonucleotides. It could be demonstrated by Fichtner-Feigl et al. [31] that local treatment with NF-κB specific decoy oligonucleotides resulted in an amelioration of chronic TNBS colitis in mice accompanied by reduced production of pro-inflammatory cytokines and a diminished development of fibrosis. Alternative attempts are aimed at the specific inhibition of nuclear import systems to prevent the translocation of NF-κB into the nucleus, target the IKK complex or try to stabilize IκB proteins by developing ubiquitylation or proteasome inhibitors [1]. The importance of proteasomal degradation within the NF-κB activating pathway for the pathogenesis of Crohn’s disease was underlined by the finding that the subunit composition and the proteolytic function of proteasomes differ between patients with Crohn’s disease, ulcerative colitis and healthy controls. Proteasomes isolated from patients with Crohn’s disease showed an enhanced capacity to process the inactive p105 precursor towards active p50 [32]. So obviously, the distinct proteasome subunits critically influence the severity of NF-κB-mediated inflammation in IBD. Correspondingly, MG-341, a selective inhibitor of 26S proteasomes, which can be administered orally has been proven to attenuate colonic inflammation in vivo [33].

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Although NF-κB is one of the key regulators in the immunological setting of IBD and therefore appears as a very promising target for therapeutic intervention in IBD, it is nevertheless important to remember that NF-κB is also involved in normal cell physiology [1]. For example, NF-κB activation is essential for physiological development of lymphocytes [4] and is critically involved in the mediation of effective host defence against bacterial infections [34]. With regard to possible systemic effects of therapeutic NF-κB inhibition, it should also be mentioned that blockade of NF-κB activation in murine hepatocytes was associated with a spontaneous development of hepatocellular carcinoma [35]. Therefore to avoid severe side effects, special attention should be paid to attempts which allow a local inhibition of NF-κB restricted to the immune cells within the inflamed colonic mucosa. NF-κB antisense oligonucleotides which were administered directly into the colon resulted in a significant reduction of TNBS-colitis in mice, without mediating any detectable side effects in other organs [12]. By carefully targeting specific NF-κB subunits or signalling components that are particularly involved in the pathogenesis of IBD, it might be possible to further minimize systemic toxic effects [1]. Considering these principles of local administration and high specificity, the therapeutical targeting of NF-κB activation probably represents a promising tool for future therapy of IBD.


  1. Top of page
  2. Abstract.
  3. Activation of NF-κB
  4. Cell-specific role of NF-κB in IBD
  5. Blockade of NF-κB activation as a therapeutic strategy in IBD
  6. Conflict of interest statement
  7. References
  • 1
    Li Q, Verma IM. NF-kappaB regulation in the immune system. Nat Rev Immunol 2002; 2: 72534.
  • 2
    Siebenlist U, Franzoso G, Brown K. Structure, regulation and function of NF-kappa B. Annu Rev Cell Biol 1994; 10: 40555.
  • 3
    Dejardin E. The alternative NF-kappaB pathway from biochemistry to biology: pitfalls and promises for future drug development. Biochem Pharmacol 2006; 72: 116179.
  • 4
    Siebenlist U, Brown K, Claudio E. Control of lymphocyte development by nuclear factor-kappaB. Nat Rev Immunol 2005; 5: 43545.
  • 5
    Greten FR, Karin M. The IKK/NF-kappaB activation pathway – a target for prevention and treatment of cancer. Cancer Lett 2004; 206: 1939.
  • 6
    Karin M, Greten FR. NF-kappaB: linking inflammation and immunity to cancer development and progression. Nat Rev Immunol 2005; 5: 74959.
  • 7
    Bonizzi G, Karin M. The two NF-kappaB activation pathways and their role in innate and adaptive immunity. Trends Immunol 2004; 25: 2808.
  • 8
    Li XH, Fang X, Gaynor RB. Role of IKKgamma/nemo in assembly of the Ikappa B kinase complex. J Biol Chem 2001; 276: 4494500.
  • 9
    Zandi E, Rothwarf DM, Delhase M, Hayakawa M, Karin M. The IkappaB kinase complex (IKK) contains two kinase subunits, IKKalpha and IKKbeta necessary for IkappaB phosphorylation and NF-kappaB activation. Cell 1997; 91: 24352.
  • 10
    Greten FR, Arkan MC, Bollrath J, Hsu LC, Goode J, Miething C, Goktuna SI, Neuenhahn M, Fierer J, Paxian S, Van Rooijen N, Xu Y, O’Cain T, Jaffee BB, Busch DH, Duyster J, Schmid RM, Eckmann L, Karin M. NF-kappaB is a negative regulator of IL-1beta secretion as revealed by genetic and pharmacological inhibition of IKKbeta. Cell 2007; 130: 91831.
  • 11
    Baeuerle PA, Henkel T. Function and activation of NF-kappa B in the immune system. Annu Rev Immunol 1994; 12: 14179.
  • 12
    Neurath MF, Pettersson S, Meyer zum Buschenfelde KH, Strober W. Local administration of antisense phosphorothioate oligonucleotides to the p65 subunit of NF-kappa B abrogates established experimental colitis in mice. Nat Med 1996; 2: 9981004.
  • 13
    Rogler G, Brand K, Vogl D, Page S, Hofmeister R, Andus T, Knuechel R, Baeuerle PA, Scholmerich J, Gross V. Nuclear factor kappaB is activated in macrophages and epithelial cells of inflamed intestinal mucosa. Gastroenterology 1998; 115: 35769.
  • 14
    Gelbmann CM, Leeb SN, Vogl D, Maendel M, Herfarth H, Scholmerich J, Falk W, Rogler G. Inducible CD40 expression mediates NFkappaB activation and cytokine secretion in human colonic fibroblasts. Gut 2003; 52: 144856.
  • 15
    Becker C, Wirtz S, Blessing M, Pirhonen J, Strand D, Bechthold O, Frick J, Galle PR, Autenrieth I, Neurath MF. Constitutive p40 promoter activation and IL-23 production in the terminal ileum mediated by dendritic cells. J Clin Invest 2003; 112: 693706.
  • 16
    Becker C, Wirtz S, Ma X, Blessing M, Galle PR, Neurath MF. Regulation of IL-12 p40 promoter activity in primary human monocytes: roles of NF-kappaB, CCAAT/enhancer-binding protein beta, and PU.1 and identification of a novel repressor element (GA-12) that responds to IL-4 and prostaglandin E(2). J Immunol 2001; 167: 260818.
  • 17
    Pallone F, Monteleone G. Mechanisms of tissue damage in inflammatory bowel disease. Curr Opin Gastroenterol 2001; 17: 30712.
  • 18
    Holtmann MH, Neurath MF. Differential TNF-signaling in chronic inflammatory disorders. Curr Mol Med 2004; 4: 43944.
  • 19
    Neurath MF, Finotto S, Glimcher LH. The role of Th1/Th2 polarization in mucosal immunity. Nat Med 2002; 8: 56773.
  • 20
    Oppmann B, Lesley R, Blom B, Timans JC, Xu Y, Hunte B, Vega F, Yu N, Wang J, Singh K, Zonin F, Vaisberg E, Churakova T, Liu M, Gorman D, Wagner J, Zurawski S, Liu Y, Abrams JS, Moore KW, Rennick D, De Waal-Malefyt R, Hannum C, Bazan JF, Kastelein RA. Novel p19 protein engages IL-12p40 to form a cytokine, IL-23, with biological activities similar as well as distinct from IL-12. Immunity 2000; 13: 71525.
  • 21
    Plevy SE, Landers CJ, Prehn J, Carramanzana NM, Deem RL, Shealy D, Targan SR. A role for TNF-alpha and mucosal T helper-1 cytokines in the pathogenesis of Crohn’s disease. J Immunol 1997; 159: 627682.
  • 22
    Wang L, Walia B, Evans J, Gewirtz AT, Merlin D, Sitaraman SV. IL-6 induces NF-kappa B activation in the intestinal epithelia. J Immunol 2003; 171: 3194201.
  • 23
    Nenci A, Becker C, Wullaert A, Gareus R, Van Loo G, Danese S, Huth M, Nikolaev A, Neufert C, Madison B, Gumucio D, Neurath MF, Pasparakis M. Epithelial NEMO links innate immunity to chronic intestinal inflammation. Nature 2007; 446: 55761.
  • 24
    Zaph C, Troy AE, Taylor BC, Berman-Booty LD, Guild KJ, Du Y, Yost EA, Gruber AD, May MJ, Greten FR, Eckmann L, Karin M, Artis D. Epithelial-cell-intrinsic IKK-beta expression regulates intestinal immune homeostasis. Nature 2007; 446: 5526.
  • 25
    Auphan N, DiDonato JA, Rosette C, Helmberg A, Karin M. Immunosuppression by glucocorticoids: inhibition of NF-kappa B activity through induction of I kappa B synthesis. Science 1995; 270: 28690.
  • 26
    Thiele K, Bierhaus A, Autschbach F, Hofmann M, Stremmel W, Thiele H, Ziegler R, Nawroth PP. Cell specific effects of glucocorticoid treatment on the NF-kappaBp65/IkappaBalpha system in patients with Crohn’s disease. Gut 1999; 45: 693704.
  • 27
    Majumdar S, Aggarwal BB. Methotrexate suppresses NF-kappaB activation through inhibition of IkappaBalpha phosphorylation and degradation. J Immunol 2001; 167: 291120.
  • 28
    Weber CK, Liptay S, Wirth T, Adler G, Schmid RM. Suppression of NF-kappaB activity by sulfasalazine is mediated by direct inhibition of IkappaB kinases alpha and beta. Gastroenterology 2000; 119: 120918.
  • 29
    Guidi L, Costanzo M, Ciarniello M, De Vitis I, Pioli C, Gatta L, Pace L, Tricerri A, Bartoloni C, Coppola L, Balistreri P, Doria G, Fedeli G, Gasbarrini GB. Increased levels of NF-kappaB inhibitors (IkappaBalpha and IkappaBgamma) in the intestinal mucosa of Crohn’s disease patients during infliximab treatment. Int J Immunopathol Pharmacol 2005; 18: 15564.
  • 30
    De Bosscher K, Schmitz ML, Vanden Berghe W, Plaisance S, Fiers W, Haegeman G. Glucocorticoid-mediated repression of nuclear factor-kappaB-dependent transcription involves direct interference with transactivation. Proc Natl Acad Sci USA 1997; 94: 135049.
  • 31
    Fichtner-Feigl S, Fuss IJ, Preiss JC, Strober W, Kitani A. Treatment of murine Th1- and Th2-mediated inflammatory bowel disease with NF-kappa B decoy oligonucleotides. J Clin Invest 2005; 115: 305771.
  • 32
    Visekruna A, Joeris T, Seidel D, Kroesen A, Loddenkemper C, Zeitz M, Kaufmann SH, Schmidt-Ullrich R, Steinhoff U. Proteasome-mediated degradation of IkappaBalpha and processing of p105 in Crohn’s disease and ulcerative colitis. J Clin Invest 2006; 116: 3195203.
  • 33
    Conner EM, Brand S, Davis JM, Laroux FS, Palombella VJ, Fuseler JW, Kang DY, Wolf RE, Grisham MB. Proteasome inhibition attenuates nitric oxide synthase expression, VCAM-1 transcription and the development of chronic colitis. J Pharmacol Exp Ther 1997; 282: 161522.
  • 34
    Naumann M. Nuclear factor-kappa B activation and innate immune response in microbial pathogen infection. Biochem Pharmacol 2000; 60: 110914.
  • 35
    Luedde T, Beraza N, Kotsikoris V, Van Loo G, Nenci A, De Vos R, Roskams T, Trautwein C, Pasparakis M. Deletion of NEMO/IKKgamma in liver parenchymal cells causes steatohepatitis and hepatocellular carcinoma. Cancer Cell 2007; 11: 11932.