NF-κB has been implicated in the pathogenesis of a number of inflammatory diseases, such as rheumatoid arthritis (RA), inflammatory bowel disease (IBD), multiple sclerosis, atherosclerosis, systemic lupus erythematosus, type I diabetes, chronic obstructive pulmonary disease and asthma.73 In response to different cellular stimuli, NF-κB plays a complex role in different cell types and in different diseases states.
Rheumatoid arthritis
RA is an autoimmune and inflammatory disease characterized by immune cell infiltration into the synovium, associated with chronic inflammation and destruction of cartilage and bone.74 A major inflammatory mediator of RA is NF-κB, which has been demonstrated in studies using both animal models and human patients. For example, several early studies have detected NF-κB activation in synovial tissue of RA patients.75–78 In mouse collagen-induced arthritis, NF-κB activation in synovial tissue precedes the development of clinical symptoms and increases along with disease progression.79 NF-κB activation has also been associated with rat arthritis induced by different agents, such as pristine and streptococcal cell wall.80,81 Similarly, NF-κB activation in rats by intra-articular transfer of an adenoviral vector encoding IKKβ induces synovial inflammation and clinical signs of arthritis, whereas intra-articular transfer of a dominant-negative IKKβ mutant suppresses adjuvant-induced arthritis.82 In line with these findings, mice with myeloid cell-specific deficiency of A20, a deubiquitinase negatively regulating NF-κB signaling, spontaneously develop polyarthritis with typical features of RA.83 Finally, NF-κB inhibition by decoy oligonucleotides or the IKK inhibitor BMS-345541 ameliorates adjuvant-induced arthritis.16,84
The pathogenesis of RA involves a variety of cell types, including innate immune cells such as monocytes/macrophages, T cells, B cells and synovial fibroblasts.85 NF-κB mediates the induction of pro-inflammatory cytokines, such as TNF-α, IL-1 and IL-6, in monocytes/macrophages.86 Many of these cytokines are capable of activating NF-κB in innate immune cells and fibroblasts, thereby inducing the expression of additional inflammatory cytokines and chemokines, leading to further recruitment of inflammatory immune cells and dissemination of inflammation.84 The canonical and noncanonical NF-κB pathways also mediate RANK ligand-induced differentiation of monocytes/macrophages into the bone-resorbing osteoclasts, whose deregulation contributes to inflammatory bone loss associated with RA.87–89 Among the different subsets of T cells, Th17 cells are particularly important for the pathogenesis of RA.90 As described above, NF-κB promotes Th17 differentiation both indirectly through induction of inflammatory cytokines, IL-1, IL-6 and IL-23, in innate immune cells and directly regulates Th17 lineage transcription factors in T cells.2,91,92 Deregulated activation of NF-κB also contributes to aberrant survival of self-reactive B cells and production of auto-antibodies that contribute to the pathogenesis of RA.93 In particular, RA patients often display elevated serum levels of B-cell activating factor belonging to TNF family associated with deregulated activation of the noncanonical NF-κB. Therefore, NF-κB mediates the pathogenesis of RA by functioning in different cell types.
Inflammatory bowel disease
Inflammatory bowel diseases, including Crohn’s disease and ulcerative colitis, are chronic inflammatory disorders of the gastrointestinal tract thought to result from inappropriate inflammatory responses to intestinal microbes.94 The pathogenesis of IBD involves multiple cell types of the mucosal immune system, including intestinal epithelial cells, innate immune cells such as macrophages and neutrophils, T cells and innate lymphoid cells.95 Strong evidence suggests the involvement of NF-κB in the pathogenesis of IBD. Constitutive NF-κB activation has been found in inflamed colonic tissue of IBD patients.96,97 Furthermore, genetic mutations in NF-κB-stimulating immune receptors, such as NOD2, and NF-κB target genes, such as IL-12 and IL-23, are associated with human IBD.94 Polymorphisms and mutations in the NFKB1 gene, which encodes the IκB-like molecule p105 and its processing product p50, have also been associated with IBD.98–100 These genetic alterations appear to inhibit NFKB1 gene expression or alter the stability and function of the protein products. Consistently, mice carrying a knockin mutation in the NFKB1 gene to block generation of p105 spontaneously develop intestinal inflammation with IBD-like features.43 A number of other animal model studies have also demonstrated that genetic deficiency in negative regulators of the canonical NF-κB pathway, such as the deubiquitinases CYLD and A20, promotes colonic inflammation.101–103 In line with these findings, decoy oligonucleotides that target the DNA-binding activity of NF-κB proteins ameliorate colitis induced by trinitrobenzene sulfonic acid and Dextran sulfate sodium.104,105 Deletion of IKKβ in myeloid cells also inhibits experimental colitis and colitis-associated cancer.106 These findings are consistent with the role of NF-κB in mediating induction of pro-inflammatory cytokines in innate immune cells and the differentiation Th1 and Th17 subsets of inflammatory T cells.
In contrast to its pro-inflammatory role in myeloid cells, NF-κB has a protective role in intestinal epithelial cells, where it is required for maintaining epithelial integrity and intestinal immune homeostasis.107,108 Conditional deletion of NEMO, IKKβ or both IKKα and IKKβ in intestinal epithelial cells causes spontaneous development of chronic intestinal inflammation in mice.107,108 Thus, aberrant activation of NF-κB or its genetic deficiency may both contribute to the pathogenesis of IBD, with its functions differing between innate immune cells and epithelial cells.
Multiple sclerosis
Multiple sclerosis is an inflammatory disease of the central nervous system (CNS) generally considered to be an autoimmune disease involving the pathogenic action of CNS-specific CD4+ T cells, particularly Th1 and Th17 cells.109 The involvement of NF-κB signaling pathway in multiple sclerosis has been suggested by genome-wide association studies. These studies have identified a number of NF-κB-related factors as susceptibility candidates, such as RelA, IκBα, IκBz, NIK, Bcl10 and MALT1.110–112 Consistently, both the canonical and noncanonical NF-κB pathways play an important role in the pathogenesis of EAE, a widely used animal model of multiple sclerosis involving immunization of mice with peptides derived from CNS proteins, such as myelin oligodendrocyte glycoprotein.2,113. T-cell-specific deletion of IKKβ or oral administration of an IKKβ inhibitor, PS1145, renders mice refractory to EAE induction.44 Genetic deficiency in IKK upstream signaling factors of the TCR pathway, including CARMA1 and MALT1, also ameliorate EAE induction.114–116 The canonical NF-κB members RelA and c-Rel mediate expression of the Th17 lineage transcription factor RORγt and, thereby, promote Th17 differentiation.117,118 An important role of noncanonical NF-κB pathway in EAE regulation has been demonstrated using mutant mice lacking the kinase NIK or expressing a processing-defective p100 mutant.52,53,119,120 Noncanonical NF-κB regulates both recall responses and encephalitogenic function of Th17 cells.52,53 Regarding the latter, the noncanonical NF-κB member p52, in synergy with c-Rel, mediates expression of the inflammatory cytokine GM-CSF in Th17 cells.53
In addition to its function in regulating the differentiation and effector function of T cells, NF-κB regulates EAE through action of innate immune cells. Constitutive activation of NF-κB in myeloid cells, as a result of IκBα deletion using the LysM-Cre system, causes more severe CNS inflammation and clinical scores in myelin oligodendrocyte glycoprotein-induced EAE,121 whereas myeloid cell-specific deletion of IKKβ inhibits EAE induction associated with impaired generation of inflammatory Th1 and Th17 cells.122 NF-κB also functions in the CNS to regulate neuroinflammation, since conditional deletion of NEMO or IKKβ using Nes-Cre, which is specific for neuronal cells including neurons, astrocytes and oligodendrocytes, partially inhibits EAE induction.123 Furthermore, NF-κB inhibition in astrocytes via transgenic expression of a degradation-resistant form of IκBα (IκBα-dn) inhibits inflammatory cytokine expression and reduces the disease severity in EAE.124,125
Atherosclerosis
Atherosclerosis is a progressive and inflammatory disorder of the arterial wall, characterized by the accumulation of low-density lipoprotein (LDL) particles and immune cells in the subendothelial space.126 The pathogenesis of atherosclerosis involves different cell types, including endothelial cells, monocytes and T cells.126 It is generally thought that the disease initiation involves activation of endothelial cells to express chemotactic factors and cell adhesion molecules that mediate recruitment of blood monocytes into the arterial intima, where they differentiate into macrophages and, following uptake of LDL particles, eventually become lipid-laden foam cells involved in atherosclerotic plaque formation. NF-κB regulates the expression of a large array of genes involved in different aspects of atherosclerotic pathogenesis.127 In vascular endothelial cells, NF-κB mediates induction of pro-inflammatory cytokines, chemotactic factors and adhesion molecules, thereby promoting monocyte recruitment and disease progression.127–131 Conditional deletion of NEMO or transgenic expression of a degradation-resistant IκBα in endothelial cells inhibits chemokine expression and monocyte recruitment, coupled with reduced disease severity of atherosclerosis, in ApoE-deficient mice fed with a cholesterol-rich diet.129 NF-κB also functions in myeloid cells to promote inflammatory gene expression and conversion of macrophages into foam cells.127 Transgenic expression of a non-degradable IκBα in macrophages reduces lipid loading and foam-cell formation, whereas myeloid cell-specific IκBα deletion sensitizes atherosclerosis development in LDL receptor-deficient mice.132,133 In line with these findings, myeloid cell-specific deletion of IKKβ reduces atherosclerotic lesion areas in LDL receptor-deficient mice fed with high-fat diet, which is associated with attenuated activities of macrophages in inflammatory gene expression, adhesion, migration and lipid uptake.134 Surprisingly, however, an earlier study suggests that deletion of IKKβ in myeloid cells increases atherosclerotic lesion sizes in the LDL receptor-deficient mice.134,135 The reason for such a discrepancy is unclear, although it could be due to the differential experimental approaches used in these two different studies.
