ReviewSmall-molecule inhibitors of NF-κB for the treatment of inflammatory joint disease
Introduction
The two most common diseases affecting human articulating joints are rheumatoid arthritis (RA) and osteoarthritis (OA). The debilitating joint destruction associated with RA has long been attributed to ongoing chronic inflammation of the synovial lining. The infiltration of this tissue with immunocompetent cells and the proliferation of synovial fibroblasts leads to the formation of pannus tissue, which invades and degrades the articular cartilage and subchondral bone. In comparison, the major pathological feature of OA is the gradual destruction and loss of the articular cartilage, which has been linked to a combination of mechanical and biochemical factors. Synovial inflammation in OA is thought to be a secondary process driven by the degradation of articular cartilage and the release of potentially immunogenic molecules during this process [1].
Despite the obvious differences in their pathologies, both diseases share a number of molecular targets that have been considered key control points for therapeutic intervention. These include the pleiotropic proinflammatory cytokines interleukin (IL)-1β and tumor necrosis factor α (TNF-α); extracellular matrix degrading enzymes such as the matrix metalloproteinases (MMPs); prostaglandins and nitric oxide. Evidence for the pivotal roles of TNF-α and IL-1β in the pathogenesis of RA has been demonstrated both in preclinical [2] and, more importantly, clinical studies using biological agents such as etanercept (Enbrel®; Immunex Corporation, Seattle, WA, USA) and infliximab (Remicade®; Centocor Inc., Malvern, PA, USA) to block their activity 3., 4.. There is also compelling evidence implicating these molecules in the progressive destruction of the articular cartilage in OA 5., 6•.. The use of biological agents for the treatment of OA appears unlikely, given that the nature of the disease is not life threatening, coupled with the prohibitive cost of biological therapies. It is also becoming increasingly clear that inhibition of a single cytokine may be insufficient to arrest the disease process 3., 7.. An alternative approach would be to develop orally active small-molecule inhibitors of the signal transduction pathways that drive the production and activity of these cytokines. The activation of nuclear factor-κB (NF-κB) is known to be pivotal for the regulated synthesis and activity of inflammatory cytokines, including TNF-α and IL-1β, and also for several other mediators involved in the pathogenesis of OA and RA, including cyclooxygenase-2 (COX-2), inducible nitric oxide synthase (iNOS) and MMP-1. In this review, we will discuss the NF-κB activation pathway, its role in inflammation and the potential for therapeutic modulation in chronic joint diseases.
NF-κB was first described in 1986 as a DNA-binding protein bound to a decameric consensus motif in the immunoglobulin κ light-chain gene enhancer [8]. Since then, NF-κB proteins have been identified in the cytoplasm of every cell type examined. The NF-κB family comprises proteins belonging to the Rel family of transcription factors, which includes five distinct gene products in mammals: RelA (p65), RelB, c-Rel, p50/p105 (NF-κB1), and p52/p100 (NF-κB2) 9., 10., 11., 12.. Although the individual family members have the ability to form a variety of homodimers and heterodimers, RelA/p50 complexes are predominately observed in activated cells and are thus considered the classical NF-κB dimer. All NF-κB/Rel proteins possess a conserved amino-terminal Rel homology domain (RHD), which is required for dimer formation, interaction with inhibitor κB (IκB) proteins, nuclear translocation and sequence-specific DNA binding. In addition, unlike p50 and p52, which are processed from precursor proteins, RelA, c-Rel and RelB possess C-terminal transactivation domains and are therefore capable of directly inducing the transcription of target genes 9., 10., 11., 12.. Knockout studies of individual NF-κB proteins have clearly demonstrated specific biological functions for the various family members, thus providing additional insight into the complex nature of the Rel family of transcription factors 13., 14•..
The activity of NF-κB is regulated through its association with an IκB protein. This class of inhibitory proteins includes IκBα, IκBβ, IκBγ, IκBε, p100, p102, Bcl-3 and the recently described inducible-IκBζ 9., 10., 11., 15•.. Each IκB protein possesses a conserved ankyrin-like repeat domain, which mediates the protein–protein interaction with NF-κB. In addition, IκBα, IκBβ and IκBε contain N-terminal regulatory domains that are required for their signal-induced degradation. To date, IκBα is the best-characterized member of this family.
Section snippets
Regulation of NF-κB activation by IκB and the IκB kinases
In most unstimulated cells, NF-κB is found in an inactive state complexed to an IκB protein 9., 10., 11.. Binding of IκB to NF-κB effectively masks the nuclear localization sequence present in the RHD, resulting in the cytoplasmic sequestration of the NF-κB dimers. However, upon cell activation by stimuli such as cytokines (including TNF-α and IL-1β), endotoxin (lipopolysaccharide, LPS), reactive oxygen intermediates, viral proteins and physical stress, IκB undergoes phosphorylation on two
NF-κB activation and joint inflammation
Constitutive activation of NF-κB has been reported in a number of inflammatory disorders, including asthma [37], inflammatory bowel disease [38] and RA 39., 40.. Immunohistochemical staining for active NF-κB has been demonstrated in the synovium of RA patients 39., 40. and is associated with cells of the intimal lining and vascular endothelium [40]. Activated NF-κB has also been observed in the synovium of OA patients, albeit to a lesser extent. Furthermore, constitutive activation of NF-κB has
Potential points of therapeutic intervention
In light of its important role as a central mediator of inflammation, selective inhibitors of NF-κB should prove to be efficacious anti-inflammatory agents. Interestingly, anti-inflammatory drugs such as glucocorticoids, and high-dose aspirin and sulfasalazine have been reported to decrease NF-κB activation through various mechanisms [48]. As such, some of the anti-inflammatory action of these compounds may be mediated through the inhibition of NF-κB activation. In addition, other small
Conclusions
NF-κB is a master coordinator of inflammatory processes and its role in inflammatory diseases is well established. It is widely believed that the identification of safe, selective inhibitors of NF-κB activation will result in powerful new agents for the treatment of chronic inflammatory disorders. Recent advances in our understanding of the complex molecular mechanisms regulating this signal transduction pathway have yielded numerous potential targets for therapeutic intervention. As compounds
Acknowledgements
The authors acknowledge Elizabeth Capper and Sanjay Kumar for critical reading of 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
References (64)
Cytokines in rheumatoid arthritis: trials and tribulations
Mol Med Today
(2000)- et al.
Multiple nuclear factors interact with the immunoglobulin enhancer sequences
Cell
(1986) - et al.
NF-κB: ten years after
Cell
(1996) - et al.
A novel IκB protein, IκB-ζ induced by proinflammatory stimuli, negatively regulates NF-κB in the nuclei
J Biol Chem
(2001) - et al.
Identification and characterization of an IκB kinase
Cell
(1997) - et al.
The IκB kinase complex (IKK) contains two kinase subunits, IKKα and IKKβ, necessary for IκB phosphorylation and NF-κB activation
Cell
(1997) - et al.
Complementation cloning of NEMO, acomponent of the IκB kinase complex essential for NF-κB activation
Cell
(1998) - et al.
IKKε is part of a novel PMA-inducible IκB kinase complex
Mol Cell
(2000) - et al.
Embryonic lethality, liver degeneration, and impaired NF-κB activation in IKK-β deficient mice
Immunity
(1999) - et al.
Manipulation of distinct NF-κB proteins alters interleukin-1β-induced human rheumatoid synovial fibroblast prostaglandin E2 formation
J Biol Chem
(1996)
The effect of dexamethasone on the expression of activated NF-kB in adjuvant arthritis
Clin Immunol Immunopathol
Transcription factors
Best Prac Res Clin Rheumatol
Inhibition of nuclear translocation of transcription factor NF-κB by a synthetic peptide containing a cell membrane-permeable motif and nuclear localization sequence
J Biol Chem
Inhibition of NF-kappa B activation in vitro and in vivo: role of 26S proteasome
Methods Enzymol
IκB kinases α and β show a random sequential kinetic mechanism and are inhibited by staurosporine and quercetin
J Biol Chem
Assay for IκB kinases using an in vivo biotinylated IκB protein substrate
Anal Biochem
The deposition of immunoglobulins and complement in osteoarthritic cartilage
Int Orthop
Combination benefit of treatment with the cytokine inhibitors interleukin-1 receptor antagonist and PEGylated soluble tumor necrosis factor receptor type I in animal models of rheumatoid arthritis
Arthritis Rheum
Theeffects of treatment with interleukin-1 receptor antagonist on the inflamed synovial membrane in rheumatoid arthritis
Rheumatology
The role of cytokines as inflammatory mediators in osteoarthritis: lessons from animal models
Connect Tissue Res
Anticytokine therapy for osteoarthritis
Expert Opin Biol Ther
Arguments for interleukin 1 as a target in chronic arthritis
Ann Rheum Dis
Structure, regulation and function of NF-κB
Annu Rev Cell Biol
REL/NF-κB/IκB story
Adv Cancer Res
The NF-κB and IκB proteins: new discoveries and insights
Annu Rev Immunol
NF-κB and rel proteins: evolutionarily conserved mediators of immune responses
Annu Rev Immunol
NF-κB: a key role in inflammatory diseases
J Clin Invest
Phosphorylation meets ubiquitination: the control of NF-κB activity
Annu Rev Immunol
Control of IκB-α proteolysis by site-specific signal-induced phosphorylation
Science
Coupling of a signal response domain in IκBα to multiple pathways for NF-κB activation
Mol Cell Biol
N- and C-terminal sequences control degradation of MAD3/IκBα in response to inducers of NF-κB activity
Mol Cell Biol
Acytokine-responsive IκB kinase that activates the transcription factor NF-κB
Nature
Cited by (62)
An overview on immunoregulatory and anti-inflammatory properties of chrysin and flavonoids substances
2017, Biomedicine and PharmacotherapyCitation Excerpt :Generally, NF-κB consisting of Rel family (p50 and p65 subunits) normally kept to an inactive cytoplasmic complex through binding to I-κB that acts as an inhibitory protein in unstimulated conditions. The activation of NF-κB causes phosphorylation of I-κB by IKK and free NF-κB translocated into the nucleus and bind to the κB binding sites in the promoter of the targets genes (Fig. 3) but the phosphorylated I-κB are targeted for ubiquitination and followed by proteosomal degradation [82]. Anti-inflammatory effects of several flavonoids have been related to the suppression of the NF-κB signal transduction pathway [68].
Carotenoid derivatives inhibit nuclear factor kappa B activity in bone and cancer cells by targeting key thiol groups
2014, Free Radical Biology and MedicineCitation Excerpt :Aberrant constitutive activation of the nuclear factor kappa B (NFkB) transcription system plays a key role in inflammatory processes and therefore affects chronic diseases such as osteoporosis and cancer [1–6].
Anti-inflammatory effects of EPA and DHA are dependent upon time and dose-response elements associated with LPS stimulation in THP-1-derived macrophages
2010, Journal of Nutritional BiochemistryBiological actions of curcumin on articular chondrocytes
2010, Osteoarthritis and CartilageCitation Excerpt :Inflammation, cartilage degradation, cell proliferation, angiogenesis and pannus formation are processes in which the role of NF-κB is prominent55. Consequently, the identification of inhibitors of NF-κB signalling should pave the way for the development of novel therapeutics for the treatment of chronic joint diseases55,56. However, the AP-1 and NF-κB transcription factors also play crucial homeostatic roles in the normal physiology of joints which justifies further research on these signal transduction pathways.
NF-κB modulators in osteolytic bone diseases
2009, Cytokine and Growth Factor ReviewsTheaflavin-3,3 ′ -Digallate Protects Cartilage from Degradation by Modulating Inflammation and Antioxidant Pathways
2022, Oxidative Medicine and Cellular Longevity