The NF-κB pathway is composed of receptor and relative adaptor molecules (TNFR, IL-1R, TLR, LTβR, BR3, CD40, and RANK), IKK complex (IκB kinase: IKKα, IKKβ, and NEMO/IKKγ), IκB proteins (IκBα, IκBβ, IκBε, Bcl-3, IκBζ, IκBns, p100 and p105), and NF-κB dimers (RelA, RelB, cRel, p50 and p52). The NF-κB pathway is characterized by two fundamentally distinct activating mechanisms referred to as the canonical (classical) and the non-canonical (alternative) NF-κB pathway. Canonical signaling relies upon IKK mediated degradation of IκB, while the non-canonical signaling is critically depends on NF-κB inducing kinase (NIK) mediated processing of p100 into p52. Moreover, the rapid and reversible inflammatory immune response primarily occurs through the canonical pathway, while the slower and irreversible developmental response typically occurs through the non-canonical pathway. [1][2]
In the canonical NF-κB pathway, NF-κB proteins (primarily containing RelA:RelA, RelA:p50, cRel:cRel, and cRel:p50) are bound and inhibited in the cytoplasm by the classical IκB proteins (IκBα, IκBβ and IκBε), which can be activated by NF-κB to generate a negative feedback loop. The key step for controlling NF-κB activity is the regulation of the IκB and NF-κB interaction. The canonical NF-κB pathway can be activated by a variety of extracellular stimuli, including pro-inflammatory cytokines, pathogen-associated molecular patterns (PAMPs), and specific danger-associated molecular patterns (DAMPs) including: tumor necrosis factor alpha (TNF-α), interleukin 1 (IL-1), bacterial lipopolysaccharide (LPS), and viral proteins. When receiving these signals, the relative receptors (TNFR, TRADD, TRAF, IL-1R and TLR) mediate NEMO-dependent IKK activation through TAK1 or trans-autophosphorylation which in turn phosphorylates IκB proteins that causes the β-TrCP mediated ubiquitination and proteasomal degradation of IκB proteins. In addition, NF-κB becomes available for activation through various post-translational modifications, following which it is translocated to the nucleus where it acts alone or in combination with other transcription factor families (i.e. AP-1, Ets, and Stat) to activate the expression of specific target genes responsible for inflammation and survival. The NF-κB dimer specificity that is responsible for regulating NF-κB target genes is mediated by alternate conformations of dimers triggered by different κB site sequences. The canonical pathway must be transient and properly controlled via the mechanisms of post-induction attenuation or termination of signaling since prolonged NF-κB activation can induce aberrant gene expression, leading to chronic inflammation and cancer. Unlike IκBs, A20 inhibits signaling upstream of IKK via its enzymatic function as a protease of signaling-associated K63-linked ubiquitin chains. The promoter-bound RelA and cRel can be negatively regulated by the E3 ligase PIAS1, and can be phosphorylated by IKK activity thereby catalyzing their degradation. The distinct degradation pathways between the free and bound IκBs result in highly dynamic homeostatic control of the NF-κB signaling pathway, which imparts functional robustness to this signaling pathway. [1][3]
In the non-canonical NF-κB pathway, RelB:p100 complexes are inactive in the cytoplasm. A select set of cell-differentiating or developmental stimuli, such as lymphotoxin β (LTβ), B cells activating factor (BAFF), CD40 ligand and receptor activator of NF-κB ligand (RANKL) can activate the non-canonical NF-κB pathway. Upon stimulation, receptors such as LTβR, BR3, CD40 and receptor activator of nuclear factor kappa B (RANK) activate the NF-κB inducing kinase (NIK) leading to the activation of IKKα homo-dimer (lacking IKKγ). Both NIK and IKKα phosphorylate p100 at Ser866, Ser870, Ser99, Ser108, Ser115, Ser123 and Ser872. Phosphorylation of p100 leads to its ubiquitination mediated by β-TrCP and partial proteasomal processing into mature p52 subunits, resulting in transcriptionally competent RelB:p52 complexes that translocate to the nucleus and regulate a distinct class of genes. Additionally, IκBδ within the p100-containing IκBsome is inactivated through degradation of the C-terminal ARD of p100, leading to the release of associated RelB:p50 and RelA:p50 dimers. Since the NF-κB pathway is instrumental to developmental processes that require sustained signaling, the non-canonical pathway is critical due to its ability to exhibit slow build-up and constant activity. RANKL normally works by enabling the differentiation of osteoclasts from monocytes. Osteoprotegerin (OPG) is a decoy receptor homolog for the RANK ligand, inhibiting RANK activity by binding to RANKL, and is closely involved in regulating NF-κB activation. NF-κB induced expression of TRAF3 impedes RelB activation by degrading NIK, playing a negative role in non-canonical signaling. IKKα phosphorylates NIK at Ser809, Ser812 and Ser815, which leads to the destabilization of NIK, providing a way to fine-tune non-canonical signaling. MicroRNAs such as miR-223, miR-15a, and miR-16 have been noted to target IKKα resulting in modulating the strength of the non-canonical signaling. [1][4]
Recent analysis studies reveal that the synthesis of the constituents of the non-canonical pathway such as RelB and p52 is controlled by the canonical IKK-IκB-RelA:p50 signaling. Moreover, the generation of the canonical and non-canonical dimers, such as RelA:p50 and RelB:p52, are also mechanistically interlinked. The IκBδ protein is capable of inhibiting RelA and is responsive to non-canonical stimuli to serve as a mediator of crosstalk between the canonical and non-canonical NF-κB pathways. The observations suggest that an integrated NF-κB system network underlies activation of both RelA and RelB containing dimer and that a malfunctioning canonical pathway will lead to an aberrant cellular response in the non-canonical pathway as well. [1]
Aberrant activation of NF-κB has been linked to inflammatory and autoimmune diseases, septic shock, viral infection, improper immune-functioning, and plays a role in cancer development. In the case of HIV-1, activation of NF-κB may be involved in the activation of the virus from its inactive state. Yersinia outer protein J (YopJ) secreted by Yersinia pestis is the causative agent of plague that prevents the ubiquitination of IκB causing the pathogen to effectively inhibit the NF-κB pathway and thus block the immune response induced by Yersinia infection. NF-κB pathway is of great interest in cancer given its constitutive activation and contribution to cell survival and resistance to therapy in this disease. As NF-κB regulates genes that control cell proliferation and cell survival, suppression of NF-κB limits the proliferation of cancer cells and induces apoptosis. Hence, methods of inhibiting NF-κB signaling also have the potential therapeutic application in cancer and inflammatory diseases. Currently, there are hundreds of identified compounds that have been shown to inhibit the NF-κB pathway. Clinical trials have consistently shown that NF-κB decoys (synthetic oligodeoxynucleotide, termed ODN, specifically blocks NF-κB) are particularly effective for the treatment of dermatitis and rheumatoid arthritis. Blocking NF-κB by NF-κB decoy is effective for the treatment of acute myocarditis and prevents ischemia reperfusion injury in the heart. Many natural products, including antioxidants, have also been shown to inhibit NF-κB and have been promoted to exhibit anti-cancer and anti-inflammatory activity. By inhibiting RANKL, DDenosumab functions to increase bone mineral density and reduce fracture rates in many patients. Furthermore, Olmesartan and Dithiocarbamates can also inhibit the NF-κB signaling cascade. Concerning the protein inhibitors of NF-κB pathway, IFRD1 represses the activity of RelA by enhancing the HDAC-mediated deacetylation of the RelA subunit at lysine 310 by favoring the recruitment of HDAC3 to RelA. As a new inhibitor of ubiquitin-proteasome-system, MLN4924 inhibits the NEDD8-activating enzyme and blocks NF-κB signaling in primary diffuse large B cell lymphoma, resulting in tumor regression. Despite the rational for inhibiting NF-κB in cancer with constitutive or chemotherapy-induced NF-κB activation, attention should be taken with other types of cancer in which NF-κB activation could be a homeostatic switch, possibly limiting genotoxic damage or toning down an inflated innate immune response. [2][5][6]
