Roles of NF-κB in health and disease: mechanisms and therapeutic potential

ACTIVATION AND BIOLOGICAL FUNCTIONS OF THE CANONICAL PATHWAY

The canonical NF-κB signalling pathway in immune cells is typically activated by pro-inflammatory cytokines [such as TNF-α (tumour necrosis factor-α) and IL-1β (interleukin-1β)] [4], viruses, TLR (Toll-like receptor) ligands [8] and engagement of the TCR (T-cell receptor) [9] (Figure 1). As a result of this signalling, the serine/threonine-specific IKK (IκB kinase) is activated and phosphorylates IκBs, the cytosolic inhibitors of NF-κB. IκB is subsequently ubiquitinated and undergoes proteasomal degradation. Loss of IκB binding permits nuclear accumulation of NF-κB, as free NF-κB binds several NF-κB-binding sites across the genomic DNA [10].

In response to TNFR (TNF receptor) signalling, TRADD (TNFR1-associated death domain protein), TRAF2 (TNFR-associated factor 2), TRAF5 and RIP (receptor-interacting protein) are recruited to the receptor complex. Specific adaptor proteins that are recruited to the membrane proximal cytoplasmic TIR [Toll/IL-1R (IL-1 receptor)] domain of each TLR include MyD88 (myeloid differentiation factor 88), TRIF (TIR domain-containing adaptor protein inducing interferon-β), TIRAP (TIR-containing adaptor protein) and TRAM (TIR-domain-containing adaptor molecule). The adaptor proteins are capable of activating discrete downstream signalling pathways [8]. Engagement of IL-1R/TLR leads to the recruitment of TRAF6, IRAK1 (IL-1R-associated kinase 1) and IRAK4 to the receptor complex. Antigen-specific stimulation of TCR and CD28 co-receptors transduce the activating signal via PKCθ (protein kinase C θ) (T-lymphocytes) or PKCβ (B-lymphocytes) to the oligmerization of the CBM [CARMA1 (caspase recruitment domain-containing membrane-associated guanylate kinase protein 1)/Bcl-10/MALT1 (mucosa-associated lymphatic tissue 1)] complex [9] and the ubiquitination of NEMO (NF-κB essential modulator), the regulatory subunit of IKK [11].

Activation of the IKK complex requires the phosphorylation of T loop serine residues of either IKK2 or IKK1. There are two proposed mechanisms through which this phosphorylation can occur [12]. First, the induced oligomerization of TRAF, RIP, NEMO or CBM forms a multi-protein signalosome that provides a scaffold for IKK complex oligomerization, resulting in close proximity or conformational changes in IKKs that facilitates trans-autophosphorylation of the T loop serine residues. Secondly, the signalosome may be the platform for the putative IKK kinase to phosphorylate IKK. RIP or TRAF6 recruits the TAK1 [TGF (transforming growth factor)-β-activated kinase 1)] complex through the association with TAB2 (TAK1-associated protein 2) to phosphorylate IKK2 within the activation loop, thereby activating the IKK complex [13,14]. Whether TAK1 is an IKK kinase, or acts through an intermediary kinase {e.g. MEKK3 [MAPK (mitogen-activated protein kinase)/ERK (extracellular-signal-regulated kinase) kinase kinase 3]}, is not yet clear [15].

Activated IKK complex phosphorylates IκBα at two conserved C-terminal serine residues (Ser³² and Ser³⁶) and IκBβ at Ser¹⁹ and Ser²³. The phosphorylated serine residues are the signals for binding by β-TrCP (transducin repeat-containing protein) containing E3 ubiquitin ligase [16,17]. An E2 ubiquitin ligase, Ubc5, is recruited to transfer Lys⁴⁸-linked ubiquitin chains from E1 ubiquitin ligase to the phosphorylated IκBs. Polyubiquitinated IκBs are then targeted for degradation by the proteasome, thus freeing NF-κB dimers to translocate into the nucleus to activate transcription. Besides activating NF-κB, TNFR engagement also leads to the activation of MAPK or SAPK (stress-activated protein kinase) cascades. Activation of MAPK signalling by TNFRs follows a two-stage process [18]. In stage 1, TRAF2, TRAF3, c-IAP1 (cellular inhibitor of apoptosis 1)/c-IAP2, Ubc13 (an E2 ubiquitin ligase), NEMO and MEKK1 are recruited to form a multi-protein complex at the cytosolic tail of the engaged receptor. In stage two, c-IAP1/c-IAP2 polyubiquitinates TRAF3 to promote TRAF3 degradation by the proteasome. The subsequent release of the multi-protein complex into the cytosol leads to MEKK1 activation and initiation of the MAPK/SAPK signalling cascade.

The canonical pathway plays key roles in the initiation of innate immunity and inflammation responses, as well as the development and maturation of innate and adaptive immune cells [4,19]. The canonical NF-κB signalling pathway is highly conserved between mammals and invertebrates [20]. Two independent immune signalling pathways operate in Drosophila to act as defence mechanisms against microbial infections. Functional homologues of IKK, NF-κB and IκB can also be found in zebrafish (Danio rerio) to regulate vertebrate developmental processes [21,22].

In response to foreign invaders, the canonical pathway may be initiated in macrophages, granulocytes, dendritic cells, B-cells, fibroblasts and endothelial cells. Antimicrobial products, such as defensins, RNS (reactive nitrogen species) and ROS (reactive oxygen species) produced by activated cells, have microbiocidal effects. The increased production of inflammatory cytokines, chemokines, MMPs (matrix metalloproteinases), enzymes (such as cyclo-oxygenases) and expression of cell-adhesion molecules enhanced the recruitment and activation of the recruited immune cells. Dendritic cells increase the expression of co-stimulatory molecules B7-1 (CD80) and B7-2 (CD86) to ensure the maximal activation of T-cells [23]. NF-κB-mediated expression of anti-apoptotic genes enhances the survival of developing and mature granulocytes [24,25], and both p50 and RelA are required for the development of CD8α⁺ and CD8α⁻ dendritic cells [4].

The canonical NF-κB signalling also plays important roles in the maturation and function of lymphocytes in the adaptive immune system. Antigen-specific stimulation of TCR and CD28 co-receptors acts through NF-κB to promote the survival and proliferation of the activated lymphocytes [19,26]. During development, NF-κB-dependent pro-survival signals counteract the death stimulus from TNF-α [27], and p50/RelA or RelA/c-Rel are required for the early development of B- and T-cells before the pre-antigen receptor stage [4].

Although it is generally accepted that IKK2, but not IKK1, mediates the activation of canonical NF-κB signalling, an exception occurs in the adult liver [28]. Both IKK1- and IKK2-dependent signalling are required to protect the liver from LPS (lipopolysaccharide)-induced toxicity, trigger the canonical NF-κB signalling in response to TNF-α and maintain the integrity of the hepatic bile duct.

ACTIVATION AND BIOLOGICAL FUNCTIONS OF THE NON-CANONICAL PATHWAY

The non-canonical NF-κB signalling pathway is activated by some members of the TNF cytokine family [LT-β (lymphotoxin-β), BAFF (B-cell-activating factor) and CD40] and RANKL [RANK (receptor activator of NF-κB) ligand]. NIK (NF-κB-inducing kinase) is a labile serine/threonine kinase central to the activation of non-canonical NF-κB signalling. NIK is negatively regulated by TRAF2, c-IAP1 and c-IAP2 via ubiquitination and proteasome-dependent degradation [29–31]. Degradation of TRAF2, c-IAP1 and c-IAP2 upon receptor engagement leads to the stabilization and activation of NIK. NIK directly phosphorylates and activates IKK1, which leads to NF-κB2 (p100) phosphorylation, proteolytic cleavage of p100 to produce p52 and subsequent RelB–p52 dimer translocation into the nucleus. The biological output by active p52 may also be the result of a hybrid pathway involving p52/RelA or p52/c-Rel [4,19,32]. The non-canonical pathway plays important roles in the development of secondary lymphoid organs, immune cell maturation and osteoclastogenesis [33].

The secondary lymphoid organs, including the lymph nodes, Peyer's patches, MALT and spleen, are sites where organized arrangement of cells allows proximal interaction between the stromal and haemopoietic cells to promote the development and activation of lymphocytes. LT-β, RANKL and TNF-α contribute to NF-κB activation, leading to the production of organotypic chemokines and anti-apoptotic factors to promote and maintain the architecture of these secondary lymphoid organs [33].

B-lymphocyte development is dependent on non-canonical NF-κB signalling, which is initiated from pre-BCR (B-cell receptor) and BAFF receptors in early, late and mature B-cells. Both canonical and non-canonical NF-κB pathways are required to promote the proliferation, survival and maturation of B-lymphocytes [4,19]. CD40 ligand, on the other hand, activates both canonical and non-canonical signalling to promote isotype switching in mature B-cells.

Osteoclastogenesis, or the formation of bone matrix, involves the deposition of organic matrix within the hollow lumen of the bone. It is the result of a balanced activity between the bone-forming osteoblasts and boneresorption osteoclasts [33,34]. Both TNF-α- and RANKL-induced NF-κB signalling regulates osteoclast formation and function. Osteoclasts fail to form in mice deficient in RANK, RANKL or p50/p52, resulting in enhanced bone matrix deposition in a condition called osteopetrosis [33]. p50 and p52 are likely to pair with RelB in the regulation of osteoclast differentiation in vivo [35].

OTHER PROPOSED FUNCTIONS OF NF-κB AND IKK COMPLEX

Recently, the role of NF-κB in development is extended to that of ES (embryonic stem) cells [36]. NF-κB activity is inhibited in undifferentiated but activated in differentiating ES cells. The overexpression of NF-κB proteins promoted differentiation, whereas suppression of NF-κB signalling in NEMO-knockout (ikbkg^−/−) or IκBαM-expressing ES cells enhances the expression of pluripotent markers. The roles played by NF-κB to promote tumour growth may also be accounted for by its ability to promote aerobic glycolysis through enhancing GLUT4 (glucose transporter 4) expression [37]. RelA is activated in cells null for p53 and/or overexpressing Ras, and could account in part for GLUT4 expression and the promotion of aerobic glycolysis to provide energy for tumour growth. NF-κB is also reported to regulate HIF-1α (hypoxia-inducible factor-1α) transcription and to trigger a hypoxic response in the low-oxygen environment experienced during rapid tumour growth, infection and inflammation [38].

Other than phosphorylating IκBs and NF-κB subunits, more NF-κB- and IκB-independent targets of IKK1 and IKK2 are being reported [39], suggesting that the impact of IKK activation on cell signalling could be more widespread. Recent additions to the list of IKK2 targets include the SNARE [SNAP (soluble N-ethylmaleimide-sensitive fusion protein-attachment protein) receptor] family protein SNAP-23 and Aurora kinase A. IKK2 promotes pro-inflammatory cytokine production by mast cells, either by activating the transcription of pro-inflammatory cytokines through NF-κB or by promoting mast cell degranulation through SNAP-23 phosphorylation by IKK2 in an NF-κB-independent manner [40]. IKK2 phosphorylates Aurora kinase A to negatively regulate Aurora kinase A protein stability and kinase activity [41]. The timely inactivation of Aurora kinase A is necessary for normal mitotic progression and the maintenance of mitotic spindle polarity.

POSITIVE REGULATORS OF NF-κB

It is of great interest as to how NF-κB is able to activate a specific transcription programme and produce a specific biological effect in response to a myriad of stimuli. Several positive and negative mechanisms appear to operate at different levels, within specific context and cell types, to contribute to the final biological outcome. Regulators may act upstream of the IKK complex, at the IKK complex or at the NF-κB subunits. The NF-κB pathway does not function in isolation, but cross-talks with other pathways and co-operates with other transcription factors/co-regulators through protein–protein interactions and post-translational modifications to shape the final transcription programme [39,42,43].

The activation profiles of IKK and NF-κB dimers differ between canonical and non-canonical pathways and between different activators of the canonical pathways. Studies from knockout mouse models indicate that NF-κB subunits have distinct and non-overlapping functions [44]. The different combination of NF-κB dimers that can potentially be formed adds to the diversity in transcription programmes that can be turned on [45]. Although the different dimers have broad sequence recognition specificities that largely overlap, NF-κB–DNA interactions play an important role in determining transcriptional specificity. The affinity differences in NF-κB–κB site interactions may contribute to transcriptional specificity in some instances (e.g. c-Rel compared with RelA affinity for the κB site on the IL-12 promoter) but not in many others [1]. Instead, transcriptional specificity is affected by the recruitment of different transcription factors and co-activators on to different promoters [46]. p38 MAPK is recruited to the promoters of selective cytokine and chemokine genes [e.g. IL-8 and MCP-1 (monocyte chemoattractant protein-1)] in response to inflammatory stimuli to promote histone H3 Ser¹⁰ phosphorylation, which marks the promoter sites for NF-κB recruitment and transactivation [47]. Histone H3 Ser¹⁰ phosphorylation and enhancement of gene expression on the IκBα and IL-6 promoter by NF-κB on the other hand is IKK1-dependent [48,49]. The co-operative binding by several transcriptional factors and co-activators, in combination with the induced bending and structural changes in DNA within an enhanceosome, add another level of control to the transcription [50].

Post-translational modifications of NF-κB subunits and their regulators can influence their activity. This has been extensively reviewed recently by Perkins [42], and will only be briefly discussed here. Post-translational modifications include phosphorylation, ubiquitination, acetylation, sumoylation and nitrosylation and are reported to affect the activity of the IKK complex, IκB proteins and NF-κB subunits. For example, the phosphorylation of IKK2 within the activation loop at Ser¹⁷⁷ and Ser¹⁸¹ is necessary for its activation [12]. The TRAF proteins were proposed to function as ubiquitin ligases that promote Lys⁶³-linked self-ubiquitination and polyubiquitination of RIP and NEMO [51,52]. Lys⁶³-linked polyubiquitinated proteins may serve as docking sites for the assembly of signalling complexes. Phosphorylation of IκBα at Ser³²/Ser³⁶ is a signal for Lys⁴⁸-linked ubiquitination and subsequent proteolytic degradation by the proteasome [16,17]. Phosphorylation of IκBα at Ser²⁸³, Ser²⁸⁹, Ser²⁹³, Thr²⁹¹ and Thr²⁹⁹ within the PEST domain in response to short wavelength UV light or Her2/Neu oncogene is also a signal for protein degradation [42]. Phosphorylation of RelA at Ser²⁷⁶, Ser⁵²⁹ and Ser⁵³⁶ enhances transcriptional activation, whereas phosphorylation at Thr⁴³⁵ and Thr⁵⁰⁵ is implicated in the negative regulation of RelA transactivation [42]. Phosphorylation of RelA at Ser²⁷⁶ and Ser⁵³⁶ promotes p300/CBP [CREB (cAMP-response-element-binding protein)-binding protein] binding and RelA acetylation at Lys³¹⁰ promotes gene expression [53–56]. p300 and CBP also acetylate RelA at Lys²¹⁸ and Lys²²¹; acetylated Lys²²¹ enhances RelA DNA binding and, together with acetylated Lys²¹⁸, inhibits the association of RelA with newly synthesized IκBα [56].

NF-κB signalling can also be regulated by the tyrosine phosphorylation status of its regulators. For example, tyrosine phosphorylation of IκBα leads to the release of NF-κB from the IκBα–NF-κB complex to serve as a means of activating NF-κB in response to re-oxygenation of hypoxic cells [57]. Less is known about the tyrosine kinases that are involved in NF-κB regulation. The tyrosine phosphatase SHP-2 (Src homology 2 domain-containing protein tyrosine phosphatase-2) is reported to positively regulate NF-κB activation [58]. SHP-2-deficient cells fail to activate NF-κB and produce IL-6 in response to IL-1β and TNF-α stimulation. The exact substrate of SHP-2 in this connection is not known, but the phosphatase may function through its association with the IKK complex and IL-1R to activate NF-κB.

CELLULAR INHIBITORS OF NF-κB

Activation of TNF-α and TLR signalling is essential for the elimination of invading pathogens; however, excessive activation of TLR signalling and inflammation may lead to immunopathological conditions, such as sepsis and autoimmune diseases. These immune signalling pathways must be precisely controlled so that they are switched on at the right time and only for an appropriate length of time. There are several built-in negative regulators to switch NF-κB signalling off when the stimuli are no longer present.

HUMAN DISEASES RESULTING FROM LOSS- OF-FUNCTION IN NF-κB SIGNALLING

The study of inherited human diseases (Table 2) resulting from mutations in the genes involved in the NF-κB signalling pathway has provided valuable information to help understand the signalling and function of NF-κB in vivo, confirm the already known functions of NF-κB and provide new insights for the role of NF-κB in different cell types and organ systems [89].

IP (incontinentia pigmenti) is an X-linked disorder, where chromosomal re-arrangement or mutations leads to premature termination of the IKBKG gene (encoding NEMO), resulting in truncated NEMO that is incapable of NF-κB activation [90]. The disease mainly affects female heterozygotes, as homozygotes and affected males typically die in utero. The affected individuals have inflammatory skin phenotypes with blistering, hyperkeratinization, hyperpigmentation and dermal scarring. Owing to an imbalance in X-linked gene inactivation, the skin contains a mosaic of cells with normal or mutated NEMO. The initial trigger for this inflammatory skin disorder is not known. Conditional knockout of the Ikbkb gene (encoding IKK2) in the epidermis indicated that IKK2-mediated NF-κB activity is essential for regulating mechanisms that maintain immune homoeostasis in the skin [91]. A disruption of immune homoeostasis may trigger IL-1 and TNF-α production by wild-type skin cells. Wild-type skin cells undergo proliferation and trigger more inflammatory responses upon stimulation; however, due to the lack of NF-κB activation, the Ikbkg-mutant cells undergo apoptosis and are cleared from the system.

Other forms of inherited Ikbkg gene mutation lead to smaller deletions in NEMO. The mutant NEMO retains the ability to weakly activate NF-κB. The diseased phenotype ranges from severe immune deficiency in affected males to mild IP in females (Table 2). Other phenotypes displayed by patients with defects in NF-κB activation point to the role of NF-κB signalling in the proper morphogenesis of epidermis adnexes, lymphatic vessels development, osteoclastogenesis and neural function. The use of tissue-specific knockout animal models will be useful in addressing the mechanisms and roles of NF-κB signalling in these organ systems.

Several inherited immunodeficiencies arise due to defects in TLR-induced signalling [92]. Autosomal recessive mutations in IRAK4 resulted in specific impairment in Toll and IL-1R signalling, which result in an increased susceptibility to pyogenic bacterial infections including IPD (invasive pneumococcal disease). Mutations in TLR3 and UNC93B1 impair TLR3 responses and patients are predisposed to herpes simplex encephalitis. MyD88 is a key adaptor downstream of most TLRs and IL-1Rs. Patients with autosomal-recessive MyD88 deficiency suffer from life-threatening and recurrent pyogenic bacterial infections including IPD [93].

HUMAN DISEASES RESULTING FROM GAIN- OF-FUNCTION IN NF-κB SIGNALLING

NF-κB signalling play key roles in several cellular and developmental processes, and the timely activation and inactivation of NF-κB signalling is essential for NF-κB to function in a controlled manner. Uncontrolled NF-κB signalling due to hyperactivation of NF-κB or loss-of-function of the negative regulators has been linked to several neoplasic disorders, chronic inflammatory disorders, insulin resistance, cachexia, Alzheimer's disease, transplant intolerance and organ ischaemia/reperfusion injury. The roles of NF-κB in cancers and chronic inflammatory disorders are briefly discussed below.

NF-κB SIGNALLING AS THERAPEUTIC TARGETS

As discussed above, there is ample evidence to demonstrate the roles of NF-κB in cancers and chronic inflammatory disorders. Hence blocking NF-κB signalling by specific inhibitors may be useful in the treatment of these disorders. The ability of more than 200 different stimuli to activate NF-κB and the large number of target genes turned on by NF-κB in many cell types make the design of pathway-specific inhibitors an arduous task. More than 700 compounds have been reported to inhibit NF-κB activation [128]. These NF-κB inhibitors may be classified into those that block signalling upstream of IKK, at the IKK step, at the IκB degradation step and the nuclear function of NF-κB subunits.

Agents that act upstream of the IKK complex include inhibitors of receptor activation by the natural ligand, inhibitors of adaptor protein recruitment to the receptor and inhibitors of IKK-activating kinases. Inhibitors of cytokine binding to its receptor are particularly relevant for certain chronic inflammatory disorders and inflammation-associated cancers. For example, the inhibition of TNF-α-induced NF-κB activation by anti-TNF-α antibodies is used in the control of IBD and arthritis [129,130]. The restricted use of adaptor proteins and kinases by different activators of NF-κB may provide suitable targets for pathway-specific inhibition. For example, TRAF2 is mainly used by TNFR and CD40, TRAF6 in IL-1/TLR signalling, CBM and PKC in BCR and TCR signalling, and NIK in non-canonical signalling pathways.

More than 150 compounds have been reported to inhibit NF-κB activation at the IKK step [128,131]. The commonly used anti-inflammatory drugs aspirin and sodium salicylate exert their effects in part by acting as competitive inhibitors of the ATP-binding site of IKK2. Thalidomide and its analogues are immunomodulatory drugs that have entered clinical trials for the treatment of MM and prostate cancers [132]. Among several hypothesis, the inhibition of IKK2 has been proposed to explain the therapeutic activity of thalidomide [133]. A small-molecule inhibitor of IKK (PS-1145) is selectively toxic towards the ABC-subtype of DLBCL [134]. NEMO is also an attractive target for IKK complex inhibition. A cell-permeant ten-amino-acid peptide corresponding to the NBD (NEMO-binding domain) of IKK2 can block the binding of NEMO to IKKs and the induction of the NF-κB canonical pathway by TNF-α [135,136]. The NBD peptide can effectively ameliorate inflammatory responses and block osteoclastogenesis in several animal models of inflammation [137].

Agents that suppress NF-κB activation by stabilizing IκB or blocking IκB degradation have been reported [128]. These include blockers of proteasome function and IκB ubiquitination. Bortezomib, a proteasome inhibitor, has entered clinical development for the treatment of MM [138]. The ubiquitination of IκB may be blocked by YopJ, IκBα phosphopeptides, RNAi to β-TrCP and dominant-negative mutants of β-TrCP. The efficacy of this group of NF-κB inhibitors is unlikely to result from the specific inhibition of NF-κB as they also block the degradation of other polyubiquitinated proteins.

Last, but not least, the nuclear functions of NF-κB may be blocked by the inhibition of nuclear translocation, DNA binding or transactivation function of NF-κB dimers. Cell-permeant peptides that contain the nuclear-localizing sequence of p50 can be used to saturate the nuclear import machineries to block the nuclear uptake of p50-containing NF-κB dimers. Inhibitors of NF-κB DNA binding include sesquiterpene lactones and decoy oligonucleotides that contain κB sites. Decoy oligonucleotides are not favoured for drug development due to their large size and polar nature, which are likely to hinder cellular uptake and bioavailability [131].

FUTURE DIRECTIONS IN NF-κB-BASED THERAPEUTICS

The use of animal models of inflammatory disorders and cancers has significantly improved our knowledge concerning the role of NF-κB in the pathogenesis of these disorders, as well as to test the efficacies of NF-κB-inhibitory drugs in the control of disease development and progression. Genetic ablation of IKBKG (NEMO), IKBKB (IKK2) and CHUK (IKK1) suggested that NF-κB plays both positive and negative roles in different cell types in the pathogenesis of inflammation-linked cancer models for different organ systems [105]. The availability of suitable cell-type/tissue-specific promoters should allow the study of the role of NF-κB signalling in the pathogenesis of other cancers such as that of the gastrointestinal tract and lung, as well as inflammatory disorders including rheumatoid arthritis and atherosclerosis. The cell-type-specific roles played by NF-κB also suggest that targeted delivery of NF-κB inhibitors is required for efficient therapy of the disease.

One caveat to the chronic use of NF-κB inhibitors is the suppression of immune activation in the normal immune cells, which can lead to severe immunodeficiency. Hence the prolonged use of NF-κB inhibitors is not desirable in cancer prevention. Strategies directed at the initial causes of persistent NF-κB activation (microbial, viral infections or chemicals) may be more effective. When used in cancer therapy, NF-κB inhibitors should be used intermittently for short durations [105].

Conventional inhibitors used to block inflammation and/or NF-κB activity, including proteasome inhibitors, glucocorticoids, NSAIDs (non-steroidal anti-inflammatory drugs) and antioxidants, are not ideal, as they target multiple pathways and produce undesirable side effects, hence highlighting the importance of pathway-specific inhibitors [77]. Even though NF-κB can be activated by many stimuli, there are few convergence points or nodes in the signalling cascades which are common to many upstream stimuli. Efficient targeting of proteins at these signalling nodes can be used to shut down the activity of discrete pathways while leaving others intact [139]. The distinction between the IKK2- and/or IKK1-dependent pathways had led to the design of specific IKK2 inhibitors for the treatment of chronic inflammatory diseases and cancers, such as IBD and MM, without affecting the developmental processes regulated by IKK1 [131,140]. The involvement of either MyD88- or TRIF-dependent inflammatory processes in the pathogenesis of different chronic inflammatory disorders may provide an opportunity to minimize ALI via TRIF inhibition, while leaving the MyD88-dependent innate immune functions intact [113]. Polymorphisms in certain immune signalling genes are associated with elevated risks for cancer development. Such polymorphic genes may provide promising clues and directions to pathway-, disease- and patient-specific therapy.

Cancer and chronic inflammation are complex diseases involving defects in multiple pathways. A combinatorial therapy to block several key mediators in one or more cell types is likely to be a more effective strategy. Beside NF-κB, other signalling pathways involved in inflammatory cytokine production that are potential therapeutic targets are AP-1 (activator protein-1), STAT3 and JNK1 [106]. Similarly, the mere inhibition of NF-κB is insufficient for a pronounced apoptotic response to induce tumour regression. Instead, NF-κB inhibitors can be used in combination with chemotherapeutic drugs or radiation to enhance tumour cell apoptosis [105].

Other than strategies to block positive NF-κB-activating signals, enhancing the actions of negative-acting regulators, are also useful to curb hyperactivated NF-κB signalling. Improving the efficacies of anti-inflammatory mediators and receptors [141], immune tolerance mechanisms [142,143] and antitumour immunity [106,144] hold promises for the therapy of cancers and chronic inflammatory disorders. It could be reasonably expected that, in the years to come, there will be a steady growth in more specific and efficacious therapeutic interventions based on NF-κB signalling.