Abstract. Toll-like receptors (TLRs) as well as the receptors for tumor necrosis factor (TNF-R) and interleukin-1 (IL-1R) play an important role in innate immunity by regulating the activity of distinct transcription factors such as nuclear factor-kB (NF-kB). TLR, IL-1R and TNF-R signaling to NF-kB converge on a common IkB kinase complex that phosphorylates the NF-kB inhibitory protein IkBa. However, upstream signaling components are in large part receptor-specific. Nevertheless, the principles of signaling are similar, involving the recruitment of specific adaptor proteins and the activation of kinase cascades in which protein-protein interactions are controlled by poly-ubiquitination. In this review, we will discuss our current knowledge of NF-kB signaling in response to TLR-4, TNF-R and IL-1R stimulation, with a special focus on the similarities and dissimilarities among these pathways.Keywords. Toll-like receptor 4, interleukin-1, tumor necrosis factor, NF-kB, signal transduction. NF-kB, does it still need to be introduced?Nuclear factor kB (NF-kB) is the generic name of a family of transcription factors that regulate the expression of a large number of genes involved in immune and inflammatory responses, as well as in cell survival, cell proliferation and cell differentiation. NFkB transcription factors are activated in response to various stimuli, including cytokines, infectious agents, injury and other stressful conditions requiring rapid reprogramming of gene expression. Inappropriate activation of the NF-kB signaling pathway is implicated in the pathogenesis of chronic inflammation and autoimmunity, certain hereditary disorders and various cancers. In mammals, the NF-kB family consists of five proteins sharing a highly conserved Rel homology domain: c-Rel, RelB, p65 (= RelA), p105 (= NF-kB1) and p100 (=NF-kB2). The first three contain Cterminal transactivation domains, while the others share a long C-terminal domain with multiple copies of ankyrin repeats, which inhibit their activation.
Nuclear factor kappa B (NF‐κB) is a key mediator of inflammation. Unchecked NF‐κB signalling can engender autoimmune pathologies and cancers. Here, we show that Tax1‐binding protein 1 (TAX1BP1) is a negative regulator of TNF‐α‐ and IL‐1β‐induced NF‐κB activation and that binding to mono‐ and polyubiquitin by a ubiquitin‐binding Zn finger domain in TAX1BP1 is needed for TRAF6 association and NF‐κB inhibition. Mice genetically knocked out for TAX1BP1 are born normal, but develop age‐dependent inflammatory cardiac valvulitis, die prematurely, and are hypersensitive to low doses of TNF‐α and IL‐1β. TAX1BP1−/− cells are more highly activated for NF‐κB than control cells when stimulated with TNF‐α or IL‐1β. Mechanistically, TAX1BP1 acts in NF‐κB signalling as an essential adaptor between A20 and its targets.
A20 (also known as TNFAIP3) is a cytoplasmic protein that plays a key role in the negative regulation of inflammation and immunity. Polymorphisms in the A20 gene locus have been identified as risk alleles for multiple human autoimmune diseases, and A20 has also been proposed to function as a tumor suppressor in several human B-cell lymphomas. A20 expression is strongly induced by multiple stimuli, including the proinflammatory cytokines TNF and IL-1, and microbial products that trigger pathogen recognition receptors, such as Toll-like receptors. A20 functions in a negative feedback loop, which mediates its inhibitory functions by downregulating key proinflammatory signaling pathways, including those controlling NF-κB-and IRF3-dependent gene expression. Activation of these transcription factors is controlled by both K48-and K63-polyubiquitination of upstream signaling proteins, respectively triggering proteasome-mediated degradation or interaction with other signaling proteins. A20 turns off NF-B and IRF3 activation by modulating both types of ubiquitination. Induction of K48-polyubiquitination by A20 involves its C-terminal zincfinger ubiquitin-binding domain, which may promote interaction with E3 ligases, such as Itch and RNF11 that are involved in mediating A20 inhibitory functions. A20 is thought to promote de-ubiquitination of K63-polyubiquitin chains either directly, due to its N-terminal deubiquitinase domain, or by disrupting the interaction between E3 and E2 enzymes that catalyze K63-polyubiquitination. A20 is subject to different mechanisms of regulation, including phosphorylation, proteolytic processing, and association with ubiquitin binding proteins. Here we review the expression and biological activities of A20, as well as the underlying molecular mechanisms.
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