The zinc finger protein A20 inhibits TNF-induced NF-kappaB-dependent gene expression by interfering with an RIP- or TRAF2-mediated transactivation signal and directly binds to a novel NF-kappaB-inhibiting protein ABIN

Zinc regulates iNOS-derived nitric oxide formation in endothelial cells

Aberrant production of nitric oxide (NO) by inducible NO synthase (iNOS) has been implicated in the pathogenesis of endothelial dysfunction and vascular disease. Mechanisms responsible for the fine-tuning of iNOS activity in inflammation are still not fully understood. Zinc is an important structural element of NOS enzymes and is known to inhibit its catalytical activity. In this study we aimed to investigate the effects of zinc on iNOS activity and expression in endothelial cells. We found that zinc down-regulated the expression of iNOS (mRNA+protein) and decreased cytokine-mediated activation of the iNOS promoter. Zinc-mediated regulation of iNOS expression was due to inhibition of NF-κB transactivation activity, as determined by a decrease in both NF-κB-driven luciferase reporter activity and expression of NF-κB target genes, including cyclooxygenase 2 and IL-1β. However, zinc did not affect NF-κB translocation into the nucleus, as assessed by Western blot analysis of nuclear and cytoplasmic fractions. Taken together our results demonstrate that zinc limits iNOS-derived high output NO production in endothelial cells by inhibiting NF-κB-dependent iNOS expression, pointing to a role of zinc as a regulator of iNOS activity in inflammation.

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Zinc inhibits iNOS-dependent nitrite accumulation in endothelial cells.

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Zinc decreases cytokine-induced iNOS expression in endothelial cells.

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Zinc inhibits iNOS promoter activity.

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NF-kB silencing abolishes cytokine-induced iNOS expression.

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Zinc inhibits the transactivation activity of NF-κB.

Alternative expression pattern of MALT1-A20-NF-κB in patients with rheumatoid arthritis

Rheumatoid arthritis (RA) is an inflammatory autoimmune disorder; abnormal T cell immunity plays a critical role in the development of RA. Recently, A20 was identified as a key negative regulator for T cell activation and inflammatory signaling and may be involved in RA pathogenesis. In this study, we analyzed the expression level of A20, NF-κB, and A20 regulatory factor mucosa-associated lymphoid tissue lymphoma translocation gene 1 (MALT1) in patients with RA. Real-time PCR was used to determine the expression level of MALT1, MALT-V1, A20, and NF-κB genes in RA and healthy individuals (HI). Significantly lower A20 expression was found in RA patients compared with those in the healthy group, while NF-κB overexpression could be detected in patients with RA. Moreover, the MALT1 and MALT1-V1 expression level was downregulated in RA patients. A positive correlation between MALT1 and A20 and MALT1-V1 and A20 was found in patients with RA, and a tendency towards a negative correlation was found between MALT1 and NF-κB, MALT1-V1 and NF-κB, and A20 and NF-κB. In conclusion, we first characterized the alternative expression pattern of MALT1, A20, and NF-κB in RA, which may be related to abnormal T cell activation.

The TLR and IL-1 signalling network at a glance

Toll-like receptors (TLRs) and the receptors for interleukin (IL)-1, IL-18 and IL-33 are required for defence against microbial pathogens but, if hyper-activated or not switched off efficiently, can cause tissue damage and inflammatory and autoimmune diseases. Understanding how the checks and balances in the system are integrated to fight infection without the network operating out of control will be crucial for the development of improved drugs to treat these diseases in the future. In this Cell Science at a Glance article and the accompanying poster, I provide a brief overview of how one of these intricate networks is controlled by the interplay of protein phosphorylation and protein ubiquitylation events, and the mechanisms in myeloid cells that restrict and terminate its activation to prevent inflammatory and autoimmune diseases. Finally, I suggest a few protein kinases that have been neglected as drug targets, but whose therapeutic potential should be explored in the light of recent advances in our understanding of their roles in the innate immune system.

Emergence of the A20/ABIN-mediated inhibition of NF-κB signaling via modifying the ubiquitinated proteins in a basal chordate

In the past decade, ubiquitination has been well documented to have multifaceted roles in regulating NF-κB activation in mammals. However, its function, especially how deubiquitinating enzymes balance the NF-κB activation, remains largely elusive in invertebrates. Investigating bbtA20 and its binding proteins, bbt A20-binding inhibitor of NF-κB (bbtABIN1) and bbtABIN2, in Chinese amphioxus Branchiostoma belcheri tsingtauense, we found that bbtABIN2 can colocalize and compete with bbt TNF receptor-associated factor 6 to connect the K63-linked polyubiquitin chains, whereas bbtABIN1 physically links bbtA20 to bbt NF-κB essential modulator (bbtNEMO) to facilitate the K48-linked ubiquitination of bbtNEMO. Similar to human A20, bbtA20 is a dual enzyme that removes the K63-linked polyubiquitin chains and builds the K48-linked polyubiquitin chains on bbt receptor-interacting serine/threonine protein kinase 1b, leading to the inhibition of NF-κB signaling. Our study not only suggests that ubiquitination is an ancient strategy in regulating NF-κB activation but also provides the first evidence, to our knowledge, for ABINs/A20-mediated inhibition of NF-κB via modifying the ubiquitinated proteins in a basal chordate, adding information on the stepwise development of vertebrate innate immune signaling.

Regulatory roles of tumor necrosis factor alpha-induced proteins (TNFAIPs) 3 and 9 in arthritis

Tumor necrosis factor alpha (TNFα) and interleukin-6 (IL-6) have proved to be important in rheumatoid arthritis (RA) because the outcome of RA has greatly improved with the recent availability of biologics targeting them. It is well accepted that these cytokines are involved in the activation of the nuclear factor-κB (NF-κB) signaling pathway, but our understanding of the dependency of these pro-inflammatory cytokines and the link between them in RA is currently limited. Recently, we and others proved the importance of TNFα-induced protein (TNFAIP), due to the spontaneous development of arthritis in deficient animals that are dependent on IL-6. To date, nine TNFAIPs have been identified, and TNFAIP3 and TNFAIP9 were found to be clearly associated with mouse and human arthritis. In this review, we compare and discuss recent TNFAIP topics, especially focusing on TNFAIP3 and TNFAIP9 in autoimmune arthritis in mice and humans.

T cells down-regulate macrophage TNF production by IRAK1-mediated IL-10 expression and control innate hyperinflammation

Endotoxemia is caused by excessive inflammation, but the immune system has various mechanisms to avoid collateral organ damage in endotoxemia. A handful of reports have shown that innate immune responses are suppressed by the adaptive immune system. However, the molecular mechanism by which adaptive immune cells suppress innate inflammatory responses is not clear. Here, we report that T cells are shown to interact with macrophages at the early stage of enodotoxemia and to prolong survival of mice through controlling TNF and IL-10 levels by macrophage CD40 stimulation. The cross-talk between CD40 and toll-like receptor (TLR4) signaling first mediates IL-1 receptor-associated kinase 1 (IRAK1) nuclear translocation and its binding to the IL-10 gene promoter in macrophages, without interfering with the NFκB pathway. IL-10 is then detected by macrophages in an autocrine fashion to destabilize Tnfa mRNA. To induce IRAK1-mediated IL-10 expression, signals from both CD40 and TLR4 are essential. CD40 signaling induces IRAK1 sumoylation in the presence of TNF receptor-associated factor 2 (TRAF2) and intracellular isoform of osteopontin (iOPN) whereas TLR4 signaling provides IFN regulatory factor 5 (IRF5) as a chaperone for sumoylated IRAK1 nuclear translocation. Interaction of T cells with macrophages was observed in the spleen in vivo after endotoxemia induction with LPS injection. Our study demonstrates a mechanistic basis for the immunosuppressive role of macrophage CD40 in LPS endotoxemia.

B-cell-intrinsic hepatitis C virus expression leads to B-cell-lymphomagenesis and induction of NF-κB signalling

Hepatitis C virus (HCV) infection leads to the development of hepatic diseases, as well as extrahepatic disorders such as B-cell non-Hodgkin's lymphoma (B-NHL). To reveal the molecular signalling pathways responsible for HCV-associated B-NHL development, we utilised transgenic (Tg) mice that express the full-length HCV genome specifically in B cells and develop non-Hodgkin type B-cell lymphomas (BCLs). The gene expression profiles in B cells from BCL-developing HCV-Tg mice, from BCL-non-developing HCV-Tg mice, and from BCL-non-developing HCV-negative mice were analysed by genome-wide microarray. In BCLs from HCV-Tg mice, the expression of various genes was modified, and for some genes, expression was influenced by the gender of the animals. Markedly modified genes such as Fos, C3, LTβR, A20, NF-κB and miR-26b in BCLs were further characterised using specific assays. We propose that activation of both canonical and alternative NF-κB signalling pathways and down-regulation of miR-26b contribute to the development of HCV-associated B-NHL.

Regulatory effect of zinc finger protein A20 on inflammation and apoptosis: Implications for inflammatory bowel disease

Zinc finger protein A20, also known as tumor necrosis factor alpha-induced protein 3 (TNFAIP3), is a protein with dual enzyme activities: ubiquitination and deubiquitination. It can negatively regulate inflammation and apoptosis mediated by a variety of signal transduction pathway, playing an important role in the occurrence, development and prognosis of inflammatory diseases, especially inflammatory bowel diseases. In recent years, the role of zinc finger protein A20 in inflammation and apoptosis has gained growing concern. This paper gives a brief overview of the biological effects of zinc finger protein A20 on inflammation and apoptosis mediated by tumor necrosis factor alpha/tumor necrosis factor receptor 1, nuclear factor kappa B, lipopolysaccharide/lipopolysaccharide - nuclear factor kappa B, oxidized low density lipoprotein and other pathways as well as the negative regulatory effect of zinc finger protein A20 on intestinal inflammation and epithelial apoptosis in inflammatory bowel disease.

Molecular control of the NEMO family of ubiquitin-binding proteins

Research over the past decade has revealed how NF-κB essential modulator (NEMO; also known as IKKγ) regulates the IKKα-IKKβ signalling axis in the innate immune system. The discovery that NEMO is a polyubiquitin-binding protein and that the IKK complex is modulated by other protein kinases that are themselves controlled by polyubiquitin chains has provided a deeper molecular understanding of the non-degradative roles of ubiquitylation. New mechanistic insights of NEMO and related polyubiquitin-binding proteins have become a paradigm for how the interplay between phosphorylation and ubiquitylation controls cell signalling networks in health and disease.

Single-nucleotide Polymorphism and Haplotypes of TNIP1 Associated with Systemic Lupus Erythematosus in a Chinese Han Population

Objective.

To determine the association of systemic lupus erythematosus (SLE) with single-nucleotide polymorphisms (SNP) in the TNIP1 gene and compare the expression of this gene in cases and controls from a Chinese Han population in this replication study.

Methods.

Matrix-assisted laser desorption ionization time-of-flight mass spectrometry was used to genotype 19 SNP in TNIP1 in Chinese Han patients with SLE (n = 341) and controls (n = 356). Genotypes were analyzed by codominant, dominant, and recessive models. Analysis of allele frequencies and linkage disequilibrium was also performed. Western blotting and qRT-PCR were used to measure the expression of these genes in peripheral blood mononuclear cells of SLE cases and controls.

Results.

Seven SNP loci were significantly associated with SLE in our population (p < 0.05 for all comparisons). Two TNIP1 gene haplotypes (ATTGCGC and GTCCTAT) were associated with SLE (p = 0.0246 and p = 0.0024, respectively). Western blotting and qRT-PCR results provide evidence that patients with SLE had significantly reduced expression of TNIP1/ABIN-1 relative to controls.

Conclusion.

Analysis of SNP in the TNIP1 gene and expression of this gene in peripheral blood lymphocytes indicated these SNP were associated with the occurrence of SLE in Han Chinese patients. Future studies should examine the roles of these SNP in the pathogenesis of SLE.

Cutting edge: ABIN-1 protects against psoriasis by restricting MyD88 signals in dendritic cells

Psoriasis is a chronic, inflammatory skin disease caused by a combination of environmental and genetic factors. The Tnip1 gene encodes A20 binding and inhibitor of NF-κB-1 (ABIN-1) protein and is strongly associated with susceptibility to psoriasis in humans. ABIN-1, a widely expressed ubiquitin-binding protein, restricts TNF- and TLR-induced signals. In this study, we report that mice lacking ABIN-1 specifically in dendritic cells (DCs), ABIN-1(fl) CD11c-Cre mice, exhibit perturbed immune homeostasis. ABIN-1-deficient DCs display exaggerated NF-κB and MAPK signaling and produce more IL-23 than do normal cells in response to TLR ligands. Challenge of ABIN-1(fl) CD11c-Cre mice with topical TLR7 ligand leads to greater numbers of Th17 and TCRγδ T cells and exacerbated development of psoriaform lesions. These phenotypes are reversed by DC-specific deletion of the TLR adaptor MyD88. These studies link ABIN-1 with IL-23 and IL-17, and they provide cellular and molecular mechanisms by which ABIN-1 regulates susceptibility to psoriasis.

TNF-α up-regulates cellular inhibitor of apoptosis protein 2 (c-IAP2) via c-Jun N-terminal kinase (JNK) pathway in nasopharyngeal carcinoma

Inhibitor of apoptosis proteins (IAPs) contribute to both tumor progression and tumor metastasis. Here, we show that pro-inflammatory cytokine TNF-α induced the up-regulation of c-IAP2 in the potential metastatic nasopharyngeal carcinoma (NPC) cells in a dose- and time-dependent manner. This up-regulation is tolerant, as the pre-treatment of NPC cells with TNF-α reversed the up-regulation of c-IAP2 induced by TNF-α re-stimulation. TNF-α activated MAKP signals, including ERK, JNK and p38, and NF-κB signal, but only inhibition of JNK signal transduction reversed the induction of c-IAP2, suggesting that JNK signaling contributed to the c-IAP2 induction. The results from in vitro scratch wound-healing assays showed that TNF-α promoted cell invasion, which was reversed by the inhibition of JNK signaling. Taken together, these studies suggested that pro-inflammation cytokine TNF-α may be a promoter for NPC metastasis, and the anti-inflammatory therapy may be of benefit to the prevention of NPC metastasis.

A20/TNFAIP3 inhibits NF-κB activation induced by the Kaposi's sarcoma-associated herpesvirus vFLIP oncoprotein

Kaposi's sarcoma-associated herpesvirus (KSHV) K13/vFLIP (viral Flice-inhibitory protein) induces transcription of numerous genes through NF-κB activation, including pro-inflammatory cytokines, which contribute to the pathogenesis of Kaposi's sarcoma (KS). In this study, we report that KSHV vFLIP induces the expression of the NF-κB regulatory proteins A20, ABIN-1 and ABIN-3 (A20-binding NF-κB inhibitors) in primary human endothelial cells, and that KS spindle cells express A20 in KS tissue. In reporter assays, A20 strongly impaired vFLIP-induced NF-κB activation in 293T cells, but ABIN-1 and ABIN-3 did not. Mutational analysis established that the C-terminal domain (residues 427-790) is critical for A20 modulation of NF-κB, but the ubiquitin-editing OTU (ovarian tumor) domain is not. In functional assays, A20 inhibited vFLIP-induced expression of the chemokine IP-10, reduced vFLIP-induced cell proliferation and increased IKK1 protein levels. Thus, we demonstrate that A20 negatively regulates NF-κB activation directly induced by KSHV vFLIP. By attenuating excessive and prolonged vFLIP-induced NF-κB activation that could be harmful to KSHV-infected cells, A20 likely has an important role in the pathogenesis of KSHV-associated diseases, in which vFLIP is expressed.

A functional genomics screen for microRNA regulators of NF-kappaB signaling

BACKGROUND

The nuclear factor-KappaB (NF-κB) pathway is conserved from fruit flies to humans and is a key mediator of inflammatory signaling. Aberrant regulation of NF-κB is associated with several disorders including autoimmune disease, chronic inflammation, and cancer, making the NF-κB pathway an attractive therapeutic target. Many regulatory components of the NF-κB pathway have been identified, including microRNAs (miRNAs). miRNAs are small non-coding RNAs and are common components of signal transduction pathways. Here we present a cell-based functional genomics screen to systematically identify miRNAs that regulate NF-κB signaling.

RESULTS

We screened a library of miRNA mimics using a NF-κB reporter cell line in the presence and absence of tumor necrosis factor (+/- TNF). There were 9 and 15 hits in the -TNF and +TNF screens, respectively. We identified putative functional targets of these hits by integrating computational predictions with NF-κB modulators identified in a previous genome-wide cDNA screen. miR-517a and miR-517c were the top hits, activating the reporter 86- and 126-fold, respectively. Consistent with these results, miR-517a/c induced the expression of endogenous NF-κB targets and promoted the nuclear localization of p65 and the degradation of IκB. We identified TNFAIP3 interacting protein1 (TNIP1) as a target and characterized a functional SNP in the miR-517a/c binding site. Lastly, miR-517a/c induced apoptosis in vitro, which was phenocopied by knockdown of TNIP1.

CONCLUSIONS

Our study suggests that miRNAs are common components of NF-κB signaling and miR-517a/c may play an important role in linking NF-κB signaling with cell survival through TNIP1.

Human TNFα-induced protein 3-interacting protein 1 (TNIP1) promoter activation is regulated by retinoic acid receptors

Coregulator proteins play key roles in transcriptional control by members of the nuclear receptor superfamily. Previously, we demonstrated that tumor necrosis factor α (TNFα)-induced protein 3-interacting protein 1 (TNIP1) is a corepressor of agonist-bound retinoic acid receptors (RARs). Additionally, TNIP1 has been shown to repress peroxisome proliferator-activated receptors (PPAR) and NF-κB activity and interact with HIV proteins nef and matrix. TNIP1 transcriptional regulation, however, is under studied. Here we show that under permissive epigenetic conditions, TNIP1 expression is induced by all trans retinoic acid (ATRA). Within a 6000 bp region of the human TNIP1 promoter we cloned, both proximal and distal promoter regions are RAR responsive with the latter having RA response elements (RAREs) recognizable by their sequence and functionality in native promoter and synthetic RARE luciferase constructs, EMSA, and ChIP assays. These findings suggest a feedback loop whereby RARs activate expression of TNIP1, which then attenuates their activity. Together with anticipated constitutive transcription factors and the previously described NF-κB-responsiveness of the proximal TNIP1 promoter, the expected combinatorial control of TNIP1 expression could likely modulate TNIP1's impact in any of its target pathways. The degree of control by RARs or other transcription factors would in turn depend on their cell-specific level of expression and/or activation from signals in the environment such as ATRA and TNFα.

The role of A20 in the pathogenesis of lymphocytic malignancy

Autoimmune phenomena were identified in many different cases of hematological diseases and solid tumors, which may be due to alterations in the expression and function of the NF-κB signaling pathway. Recently, a number of studies have shown that the deletion or mutation of A20, a negative regulator of NF-κB, is frequently found in lymphomas, suggesting that it may be a linker between the altered immune response and leukemogenesis. The aim of this review is to summarize current findings of the A20 biological functions and its molecular mechanism as a tumor suppressor and immune regulator. The identification of A20 mutations and deletions in lymphocytic malignancy and the predictive significance of these aberrations are also reviewed.

Association of two independent functional risk haplotypes in TNIP1 with systemic lupus erythematosus

OBJECTIVE

Systemic lupus erythematosus (SLE) is an autoimmune disease characterized by autoantibody production and altered type I interferon expression. Genetic surveys and genome-wide association studies have identified >30 SLE susceptibility genes. One of these genes, TNIP1, encodes the ABIN1 protein. ABIN1 functions in the immune system by restricting NF-κB signaling. The present study was undertaken to investigate the genetic factors that influence association with SLE in genes that regulate the NF-κB pathway.

METHODS

We analyzed a dense set of genetic markers spanning TNIP1 and TAX1BP1, as well as the TNIP1 homolog TNIP2, in case-control populations of diverse ethnic origins. TNIP1, TNIP2, and TAX1BP1 were fine-mapped in a total of 8,372 SLE cases and 7,492 healthy controls from European-ancestry, African American, Hispanic, East Asian, and African American Gullah populations. Levels of TNIP1 messenger RNA (mRNA) and ABIN1 protein in Epstein-Barr virus-transformed human B cell lines were analyzed by quantitative reverse transcription-polymerase chain reaction and Western blotting, respectively.

RESULTS

We found significant associations between SLE and genetic variants within TNIP1, but not in TNIP2 or TAX1BP1. After resequencing and imputation, we identified 2 independent risk haplotypes within TNIP1 in individuals of European ancestry that were also present in African American and Hispanic populations. Levels of TNIP1 mRNA and ABIN1 protein were reduced among subjects with these haplotypes, suggesting that they harbor hypomorphic functional variants that influence susceptibility to SLE by restricting ABIN1 expression.

CONCLUSION

Our results confirm the association signals between SLE and TNIP1 variants in multiple populations and provide new insight into the mechanism by which TNIP1 variants may contribute to SLE pathogenesis.

A20: linking a complex regulator of ubiquitylation to immunity and human disease

A20 (also known as TNFAIP3) is a potent anti-inflammatory signalling molecule that restricts multiple intracellular signalling cascades. Recent studies in three general areas have converged to highlight the clinical and biological importance of A20. First, human genetic studies have strongly linked polymorphisms and mutations in the gene encoding A20 to inflammatory, autoimmune and malignant diseases. Second, studies in gene-targeted mice have revealed that A20 regulates multiple immune cell functions and prevents experimental diseases that closely mimic human conditions. Third, biochemical studies have unveiled complex mechanisms by which A20 regulates ubiquitin-dependent nuclear factor-κB and cell-survival signals. Taken together, these studies are revealing the importance of A20-mediated regulation of ubiquitin-dependent signalling in human disease.

A20: linking a complex regulator of ubiquitylation to immunity and human disease

A20 (also known as TNFAIP3) is a potent anti-inflammatory signalling molecule that restricts multiple intracellular signalling cascades. Recent studies in three general areas have converged to highlight the clinical and biological importance of A20. First, human genetic studies have strongly linked polymorphisms and mutations in the gene encoding A20 to inflammatory, autoimmune and malignant diseases. Second, studies in gene-targeted mice have revealed that A20 regulates multiple immune cell functions and prevents experimental diseases that closely mimic human conditions. Third, biochemical studies have unveiled complex mechanisms by which A20 regulates ubiquitin-dependent nuclear factor-κB and cell-survival signals. Taken together, these studies are revealing the importance of A20-mediated regulation of ubiquitin-dependent signalling in human disease.

Specific recognition of linear polyubiquitin by A20 zinc finger 7 is involved in NF-κB regulation

LUBAC (linear ubiquitin chain assembly complex) activates the canonical NF-κB pathway through linear polyubiquitination of NEMO (NF-κB essential modulator, also known as IKKγ) and RIP1. However, the regulatory mechanism of LUBAC-mediated NF-κB activation remains elusive. Here, we show that A20 suppresses LUBAC-mediated NF-κB activation by binding linear polyubiquitin via the C-terminal seventh zinc finger (ZF7), whereas CYLD suppresses it through deubiquitinase (DUB) activity. We determined the crystal structures of A20 ZF7 in complex with linear diubiquitin at 1.70-1.98 Å resolutions. The crystal structures revealed that A20 ZF7 simultaneously recognizes the Met1-linked proximal and distal ubiquitins, and that genetic mutations associated with B cell lymphomas map to the ubiquitin-binding sites. Our functional analysis indicated that the binding of A20 ZF7 to linear polyubiquitin contributes to the recruitment of A20 into a TNF receptor (TNFR) signalling complex containing LUBAC and IκB kinase (IKK), which results in NF-κB suppression. These findings provide new insight into the regulation of immune and inflammatory responses.

Regulation of NF-κB by deubiquitinases

The nuclear factor-κB (NF-κB) pathway is a critical regulator of innate and adaptive immunity. Noncanonical K63-linked polyubiquitination plays a key regulatory role in NF-κB signaling pathways by functioning as a scaffold to recruit kinase complexes containing ubiquitin-binding domains. Ubiquitination is balanced by deubiquitinases that cleave polyubiquitin chains and oppose the function of E3 ubiquitin ligases. Deubiquitinases therefore play an important role in the termination of NF-κB signaling and the resolution of inflammation. In this review, we focus on NF-κB regulation by deubiquitinases with an emphasis on A20 and CYLD. Deubiquitinases and the ubiquitin/proteasome components that regulate NF-κB may serve as novel therapeutic targets for inflammatory diseases and cancer.

Leishmania donovani exploits host deubiquitinating enzyme A20, a negative regulator of TLR signaling, to subvert host immune response

TLRs, which form an interface between mammalian host and microbe, play a key role in pathogen recognition and initiation of proinflammatory response thus stimulating antimicrobial activity and host survival. However, certain intracellular pathogens such as Leishmania can successfully manipulate the TLR signaling, thus hijacking the defensive strategies of the host. Despite the presence of lipophosphoglycan, a TLR2 ligand capable of eliciting host-defensive cytokine response, on the surface of Leishmania, the strategies adopted by the parasite to silence the TLR2-mediated proinflammatory response is not understood. In this study, we showed that Leishmania donovani modulates the TLR2-mediated pathway in macrophages through inhibition of the IKK-NF-κB cascade and suppression of IL-12 and TNF-α production. This may be due to impairment of the association of TRAF6 with the TAK-TAB complex, thus inhibiting the recruitment of TRAF6 in TLR2 signaling. L. donovani infection drastically reduced Lys 63-linked ubiquitination of TRAF6, and the deubiquitinating enzyme A20 was found to be significantly upregulated in infected macrophages. Small interfering RNA-mediated silencing of A20 restored the Lys 63-linked ubiquitination of TRAF6 as well as IL-12 and TNF-α levels with a concomitant decrease in IL-10 and TGF-β synthesis in infected macrophages. Knockdown of A20 led to lower parasite survival within macrophages. Moreover, in vivo silencing of A20 by short hairpin RNA in BALB/c mice led to increased NF-κB DNA binding and host-protective proinflammatory cytokine response resulting in effective parasite clearance. These results suggest that L. donovani might exploit host A20 to inhibit the TLR2-mediated proinflammatory gene expression, thus escaping the immune responses of the host.

Emerging roles for TNIP1 in regulating post-receptor signaling

A vast number of cellular processes and signaling pathways are regulated by various receptors, ranging from transmembrane to nuclear receptors. These receptor-mediated processes are modulated by a diverse set of regulatory proteins. TNFα-induced protein 3-interacting protein 1 is such a protein that inhibits both transduction by transmembrane receptors, such as TNFα-receptor, EGF-R, and TLR, and nuclear receptors' PPAR and RAR activity. These receptors play key roles in regulating inflammation and inflammatory diseases. A growing number of references have implicated TNIP1 through GWAS and expression studies in chronic inflammatory diseases such as psoriasis and rheumatoid arthritis, although TNIP1s exact role has yet been determined. In this review, we aim to integrate the current knowledge of TNIP1s functions with the diseases in which it has been associated to potentially elucidate the role this regulator has in promoting or alleviating these inflammatory diseases.

The ubiquitin code

The posttranslational modification with ubiquitin, a process referred to as ubiquitylation, controls almost every process in cells. Ubiquitin can be attached to substrate proteins as a single moiety or in the form of polymeric chains in which successive ubiquitin molecules are connected through specific isopeptide bonds. Reminiscent of a code, the various ubiquitin modifications adopt distinct conformations and lead to different outcomes in cells. Here, we discuss the structure, assembly, and function of this ubiquitin code.

Regulation of NF-κB signaling by the A20 deubiquitinase

The NF-κB transcription factor is a central mediator of inflammatory and innate immune signaling pathways. Activation of NF-κB is achieved by K63-linked polyubiquitination of key signaling molecules which recruit kinase complexes that in turn activate the IκB kinase (IKK). Ubiquitination is a highly dynamic process and is balanced by deubiquitinases that cleave polyubiquitin chains and terminate downstream signaling events. The A20 deubiquitinase is a critical negative regulator of NF-κB and inflammation, since A20-deficient mice develop uncontrolled and spontaneous multi-organ inflammation. Furthermore, specific polymorphisms in the A20 genomic locus predispose humans to autoimmune disease. Recent studies also indicate that A20 is an important tumor suppressor that is inactivated in B-cell lymphomas. Therefore, targeting A20 may form the basis of novel therapies for autoimmune disease and lymphomas.

No one can whistle a symphony alone – how different ubiquitin linkages cooperate to orchestrate NF-κB activity

Although it has been known for a long time that ubiquitylation has a major role in the activation and regulation of the nuclear factor kappa B (NF-κB) pathway, recent studies have revealed that the picture is a lot more complex than originally thought. NF-κB and ubiquitylation initially became linked when it was recognised that lysine (K)48-linked ubiquitin chains are involved in the processing of NF-κB precursors and the degradation of inhibitor of kappa B (IκB) proteins. Soon thereafter, it was reported that K63-linked chains were involved in the assembly of IκB kinase (IKK)-activating complexes and required for activation of the NF-κB signalling pathway. Recently, the discovery that atypical ubiquitin linkages, including linear and K11 linkages, are also involved in the activation of NF-κB has led to the need to re-evaluate existing models of how activation of this transcription factor is initiated and regulated. It is now becoming apparent that not only the canonical types of ubiquitin chains but possibly all linkage types have to be investigated in order to fully comprehend NF-κB activation. This can be considered a turning point in our view of the regulation of one of the most important pathways of gene induction. Hence, in this Commentary, we summarise the information that is currently available and incorporate it into a new model of NF-κB activation, thereby highlighting the emerging new challenges in understanding the role of ubiquitylation in NF-κB activation.

TNIP1, a retinoic acid receptor corepressor and A20-binding inhibitor of NF-κB, distributes to both nuclear and cytoplasmic locations

An increasingly wide range of functions, from repression of NF-κB signaling to protection from apoptosis, is being recognized for tumor necrosis factor α-induced protein 3-interacting protein 1 (TNIP1). The authors recently demonstrated TNIP1 interaction with and repression of liganded retinoic acid receptors, distinguishing it from the more typical NCoR and SMRT corepressors, which function only in the absence of ligand. To improve their understanding of TNIP1's roles in physiologic and pathologic events, the authors examined its distribution in normal and malignant human tissues and cultured cells. They found cytoplasmic and nuclear TNIP1 in normal skin keratinocytes as it colocalized with retinoic acid receptor α, one of the nuclear receptors it corepresses. Nuclear and cytoplasmic TNIP1 was also found in the malignant keratinocytes of squamous cell carcinomas. Compared to adjacent normal tissues of other organs, TNIP1 staining and distribution varied with increased levels in esophageal cancer and marked decreases in prostate cancer. The varying levels and distribution of TNIP1 in normal and disease state tissues could be expected to affect processes in which TNIP1 is involved, such as NF-κB and nuclear receptor signaling, possibly contributing to the disease course or response to therapies targeting these key players of cell growth and differentiation.

PPARγ and NF-κB regulate the gene promoter activity of their shared repressor, TNIP1

Human TNFAIP3 interacting protein 1 (TNIP1) has diverse functions including support of HIV replication through its interaction with viral Nef and matrix proteins, reduction of TNFα-induced signaling through its interaction with NF-κB pathway proteins, and corepression of agonist-bound retinoic acid receptors and peroxisome proliferator-activated receptors (PPAR). The wide tissue distribution of TNIP1 provides the opportunity to influence numerous cellular responses in these roles and defining control of TNIP1 expression would be central to improved understanding of its impact on cell function. We cloned 6kb of the human TNIP1 promoter and performed predictive and functional analyses to identify regulatory elements. The promoter region proximal to the transcription start site is GC-rich without a recognizable TATA box. In contrast to this proximal ~500bp region, 6kb of the promoter increased reporter construct constitutive activity over five-fold. Throughout the 6kb length, in silico analysis identified several potential binding sites for both constitutive and inducible transcription factors; among the latter were candidate NF-κB binding sequences and peroxisome proliferator response elements (PPREs). We tested NF-κB and PPAR regulation of the endogenous TNIP1 gene and cloned promoter by expression studies, electrophoretic mobility shift assays, and chromatin immunoprecipitations. We validated NF-κB sites in the TNIP1 promoter proximal and distal regions as well as one PPRE in the distal region. The ultimate control of the TNIP1 promoter is likely to be a combination of constitutive transcription factors and those subject to activation such as NF-κB and PPAR.

A20 and ABIN-3 possibly promote regression of trehalose 6,6'-dimycolate (TDM)-induced granuloma by interacting with an NF-kappa B signaling protein, TAK-1

OBJECTIVE

The objective of this paper is to examine the role of NF-kappa B inhibitors A20 and ABIN-family proteins in the trehalose 6,6'-dimycolate (TDM)-induced model of tuberculous granulomatous lesions.

MATERIALS AND METHODS

BALB/c mice were twice injected i.p. with w/o/w emulsions that contain TDM at a 1 week-interval. The mice were killed at days 0, 3, 7, 14, or 21 after the last injection. The mRNA and protein levels of A20 and ABIN-family proteins were measured by real-time PCR using mRNA or protein extract from the lesions. The activation status of NF-kappa B was analyzed by Western blotting and immunohistochemistry. Finally, the protein extracts were immunoprecipitated by anti-ABIN-3 antibody to identify the protein that potentially interacts with ABIN-3.

RESULTS

The activation of NF-kappa B pathway coincided with granuloma development, while A20 and ABIN-3 increased in accordance with granuloma regression. TAK-1 protein was co-precipitated with ABIN-3 by immunoprecipitation using anti-ABIN-3 antibody.

CONCLUSION

The results suggest that ABIN-3 contributed to granuloma regression by interacting with TAK-1 and, as a consequence, inhibiting activation of NF-kappa B pathway.

Late outgrowth endothelial progenitor cells engineered for improved survival and maintenance of function in transplant-related injury

Chronic allograft vasculopathy (CAV) is a major cause of organ transplant failure that responds poorly to treatment. Endothelial activation, dysfunction and apoptosis contribute to CAV, whereas strategies for protecting endothelium and maximizing endothelial repair may diminish it. Late outgrowth endothelial progenitor cells (LO-EPC) can home to areas of injury and integrate into damaged vessels, implying a role in vascular repair; however, in an allograft, LO-EPC would be exposed to the hazardous microenvironment associated with transplant-related ischaemia reperfusion (I/R) injury and persistent inflammation. We evaluated the in vitro effect of I/R injury and the proinflammatory cytokine tumour necrosis factor (TNF)-α on LO-EPC phenotype and function. We show that LO-EPC are intrinsically more tolerant than mature EC to I/R injury induced apoptosis, maintaining their proliferative, migratory and network formation capacity. Under inflammatory conditions, LO-EPC were activated and released higher levels of inflammatory cytokines, upregulated adhesion molecule expression, and were more susceptible to apoptosis. Lentiviral vector-mediated overexpression of the protective gene A20 in LO-EPC maintained their angiogenic phenotype and function, and protected them against TNF-α-mediated apoptosis, reducing ICAM-1 expression and inflammatory cytokine secretion. Administration of ex vivo modified LO-EPC overexpressing A20 might effect vascular repair of damaged allografts and protect from CAV.

TNFAIP3 gene polymorphisms are associated with response to TNF blockade in psoriasis

The tumor necrosis factor-alpha-induced protein 3 (TNFAIP3) gene has been associated with psoriasis, rheumatoid arthritis, type 1 diabetes mellitus, systemic lupus erythematosus, and celiac disease. TNFAIP3 encodes A20, a tumor necrosis factor (TNF)-α-inducible zinc finger protein thought to limit NF-κB-mediated immune responses. In this study, we report association of response of psoriasis to TNF blockers with two TNFAIP3 single-nucleotide polymorphisms (rs2230926 in exon 7 and rs610604 in intron 3) and their haplotypes. Treatment response was self-evaluated using a 0-5 visual analog scale in 433 psoriasis patients who received TNF blockers. Confirmation was sought in 199 psoriasis and psoriatic arthritis patients from Toronto who were followed up prospectively. Response variables were dichotomized separately in the two cohorts, yielding similar proportions of good responses. Whereas significant associations were observed only for the Michigan cohort, fixed-effects meta-analysis retained significant association between dosage of the G allele of rs610604 and good combined response to all TNF blockers (odds ratio (OR) = 1.50, P(corr) = 0.050) and etanercept (OR = 1.64, P(corr) = 0.016). The rs2230926 T-rs610604 G haplotype was similarly associated. By demonstrating an association with therapeutic response, these results provide a clinically relevant functional correlate to the recently described genetic association between psoriasis and TNFAIP3.

Neurodegenerative and inflammatory pathway components linked to TNF-α/TNFR1 signaling in the glaucomatous human retina

PURPOSE

This study aimed to determine retinal proteomic alterations in human glaucoma, with particular focus on links to TNF-α/TNFR1 signaling.

METHODS

Human retinal protein samples were obtained from 20 donors with (n = 10) or without (n = 10) glaucoma. Alterations in protein expression were individually analyzed by quantitative LC-MS/MS. Quantitative Western blot analysis with cleavage or phosphorylation site-specific antibodies was used for data validation, and cellular localization of selected proteins was determined by immunohistochemical analysis of the retina in an additional group of glaucomatous human donor eyes (n = 38) and nonglaucomatous controls (n = 30).

RESULTS

Upregulated retinal proteins in human glaucoma included a number of downstream adaptor/interacting proteins and protein kinases involved in TNF-α/TNFR1 signaling. Bioinformatic analysis of the high-throughput data established extended networks of diverse functional interactions with death-promoting and survival-promoting pathways and mediation of immune response. Upregulated pathways included death receptor-mediated caspase cascade, mitochondrial dysfunction, endoplasmic reticulum stress, calpains leading to apoptotic cell death, NF-κB and JAK/STAT pathways, and inflammasome-assembly mediating inflammation. Interestingly, retinal expression pattern of a regulator molecule, TNFAIP3, exhibited prominent variability between individual samples, and methylation of cytosine nucleotides in the TNFAIP3 promoter was found to be correlated with this variability among glaucomatous donors.

CONCLUSIONS

Findings of this study reveal a number of proteins upregulated in the glaucomatous human retina that exhibit many links to TNF-α/TNFR1 signaling. By highlighting various signaling molecules and regulators involved in cell death and immune response pathways and by correlating proteomic findings with epigenetic alterations, these findings provide a framework motivating further research.

ABIN1 protein cooperates with TAX1BP1 and A20 proteins to inhibit antiviral signaling

Upon virus infection, the innate immune response provides the first line of protection and rapidly induces type I interferons (IFNα/β), which mediate potent antiviral effects. To maintain homeostasis and prevent autoimmunity, IFN production is tightly regulated; however, the mechanisms of negative regulation are poorly understood. Herein, we demonstrate that the A20 binding inhibitor of NF-κB 1 (ABIN1) is a novel negative regulator of antiviral signaling. Overexpression of ABIN1 inhibited IFN-β promoter activation in response to virus infection or poly(I:C) transfection, whereas siRNA-mediated knockdown of ABIN1 enhanced IFN-β production upon virus infection. ABIN1 interacted with the A20 regulatory molecule TAX1BP1 and was essential for the recruitment of TAX1BP1 and A20 to the noncanonical IκB kinases TBK1 and IKKi in response to poly(I:C) transfection. ABIN1 and TAX1BP1 together disrupted the interactions between the E3 ubiquitin ligase TRAF3 and TBK1/IKKi to attenuate lysine 63-linked polyubiquitination of TBK1/IKKi. Finally, an intact ubiquitin binding domain of ABIN1 was essential for ABIN1 to interact with TBK1/IKKi and inhibit IFN-β production upon poly(I:C) transfection or virus infection. Together, these results suggest that ABIN1 requires its ubiquitin binding domain and cooperates with TAX1BP1 and A20 to restrict antiviral signaling.

A20-binding inhibitor of NF-κB (ABIN1) controls Toll-like receptor-mediated CCAAT/enhancer-binding protein β activation and protects from inflammatory disease

Toll-like receptors (TLRs) are expressed on innate immune cells and trigger inflammation upon detection of pathogens and host tissue injury. TLR-mediated proinflammatory-signaling pathways are counteracted by partially characterized anti-inflammatory mechanisms that prevent exaggerated inflammation and host tissue damage as manifested in inflammatory diseases. We biochemically identified a component of TLR-signaling pathways, A20-binding inhibitor of NF-κB (ABIN1), which recently has been linked by genome-wide association studies to the inflammatory diseases systemic lupus erythematosus and psoriasis. We generated ABIN1-deficient mice to study the function of ABIN1 in vivo and during TLR activation. Here we show that ABIN1-deficient mice develop a progressive, lupus-like inflammatory disease characterized by expansion of myeloid cells, leukocyte infiltrations in different parenchymatous organs, activated T and B lymphocytes, elevated serum Ig levels, and the appearance of autoreactive antibodies. Kidneys develop glomerulonephritis and proteinuria, reflecting tissue injury. Surprisingly, ABIN1-deficient macrophages exhibit normal regulation of major proinflammatory signaling pathways and mediators but show selective deregulation of the transcription factor CCAAT/enhancer binding protein β (C/EBPβ) and its target genes, such as colony-stimulating factor 3 (Csf3), nitric oxide synthase, inducible (Nos2), and S100 calcium-binding protein A8 (S100a8). Their gene products, which are intimately linked to innate immune cell expansion (granulocyte colony-stimulating factor), cytotoxicity (inducible nitric oxide synthase), and host factor-derived inflammation (S100A8), may explain, at least in part, the inflammatory phenotype observed. Together, our data reveal ABIN1 as an essential anti-inflammatory component of TLR-signaling pathways that controls C/EBPβ activity.

TNIP1 is a corepressor of agonist-bound PPARs

Nuclear receptor (NR) coregulators include coactivators, contributing to holoreceptor transcriptional activity, and corepressors, mediating NR target gene silencing in the absence of hormone. We identified an atypical NR coregulator, TNFα-induced protein 3-interacting protein 1 (TNIP1), from a peroxisome proliferator activated receptor (PPAR) α screen of a human keratinocyte cDNA library. TNIP1's complex nomenclature parallels its additional function as an NF-κB inhibitor. Here we show TNIP1 is an atypical NR corepressor using two-hybrid systems, biochemical studies, and receptor activity assays. The requirements for TNIP1-PPAR interaction are characteristic for coactivators; however, TNIP1 partially decreases PPAR activity. TNIP1 has separable transcriptional activation and repression domains suggesting a modular nature to its overall effect. It may provide a means of lowering receptor activity in the presence of ligand without total loss of receptor function. TNIP1's multiple roles and expression in several cell types suggest its regulatory effect depends on its expression level and the expression of other regulators in NR and/or NF-κB signaling pathways. As a NR coregulator, TNIP1 targeting agonist-bound PPAR and reducing transcriptional activity offers control of receptor signaling not available from typical corepressors and may contribute to combinatorial regulation of transcription.

Polyubiquitin binding to ABIN1 is required to prevent autoimmunity

The polyubiquitin-binding domain of ABIN1 limits TLR-induced MyD88 signaling to prevent spontaneous autoimmunity in mice.

The protein ABIN1 possesses a polyubiquitin-binding domain homologous to that present in nuclear factor κB (NF-κB) essential modulator (NEMO), a component of the inhibitor of NF-κB (IκB) kinase (IKK) complex. To address the physiological significance of polyubiquitin binding, we generated knockin mice expressing the ABIN1[D485N] mutant instead of the wild-type (WT) protein. These mice developed all the hallmarks of autoimmunity, including spontaneous formation of germinal centers, isotype switching, and production of autoreactive antibodies. Autoimmunity was suppressed by crossing to MyD88^−/− mice, demonstrating that toll-like receptor (TLR)–MyD88 signaling pathways are needed for the phenotype to develop. The B cells and myeloid cells of the ABIN1[D485N] mice showed enhanced activation of the protein kinases TAK, IKK-α/β, c-Jun N-terminal kinases, and p38α mitogen-activated protein kinase and produced more IL-6 and IL-12 than WT. The mutant B cells also proliferated more rapidly in response to TLR ligands. Our results indicate that the interaction of ABIN1 with polyubiquitin is required to limit the activation of TLR–MyD88 pathways and prevent autoimmunity.

A20 is induced by Kaposi sarcoma-associated herpesvirus-encoded viral FLICE inhibitory protein (vFLIP) K13 and blocks K13-induced nuclear factor-kappaB in a negative feedback manner

Expression of A20, a negative regulator of the NF-κB pathway, is frequently lost in several subtypes of Hodgkin and non-Hodgkin lymphoma. We report that A20 is expressed in Kaposi sarcoma-associated herpesvirus (KSHV)-infected primary effusion lymphoma cell lines, and its expression correlates closely with the expression of KSHV-encoded viral FLICE inhibitory protein K13. Ectopic expression of K13 induced A20 expression through NF-κB-mediated activation of A20 promoter. In turn, A20 blocked K13-induced NF-κB activity and up-regulation of proinflammatory cytokines CCL20 and IL-8 in a negative feedback fashion. Both the N-terminal deubiquitinating domain and the C-terminal zinc finger domain of A20 were involved in the inhibition of K13-induced NF-κB activity. Overexpression of A20 blocked K13-induced IκBα phosphorylation, NF-κB nuclear translocation, and cellular transformation. Consistent with the above, K13-induced IκBα phosphorylation and NF-κB transcriptional activation were enhanced in A20-deficient cells. Finally, A20 was found to interact physically with K13. Taken collectively, these results demonstrate that K13 is a key determinant of A20 expression in KSHV-infected cells, and A20 is a key negative regulator of K13-induced NF-κB activity. A20 might serve to control the inflammatory response to KSHV infection and protect KSHV-infected cells from apoptosis.

Arabidopsis SAP5 functions as a positive regulator of stress responses and exhibits E3 ubiquitin ligase activity

AtSAP5, one of approximately 14 members of the Stress Associated Protein gene family in Arabidopsis, was identified by its expression in response to salinity, osmotic, drought and cold stress. AtSAP5 shows strong homology to OSISAP1, an A20/AN1-type zinc finger protein implicated in stress tolerance in rice. To evaluate the function of AtSAP5 in the regulation of abiotic stress responses, transgenic Arabidopsis plants that over-express AtSAP5 (35S::AtSAP5) were characterized, along with wild-type and T-DNA knock-down plants. Plants that over-express AtSAP5 showed increased tolerance to environmental challenges including salt stress, osmotic stress and water deficit. Comparison of gene expression patterns between 35S::AtSAP5 transgenic plants and wild-type plants under normal conditions and water deficit stress indicated that over-expression of AtSAP5 correlates with up-regulation of drought stress responsive gene expression. Analysis of transgenic plants that express GFP-AtSAP5 showed that it is localized primarily in nuclei of root cells and recombinant AtSAP5 has E3 ubiquitin ligase activity in vitro. These results indicate that AtSAP5 has E3 ligase activity and acts as a positive regulator of stress responses in Arabidopsis.

A20 regulation of nuclear factor-κB: perspectives for inflammatory lung disease

Persistent activation of NF-κB is central to the pathogenesis of many inflammatory lung disorders, including cystic fibrosis, asthma, and chronic obstructive pulmonary disease. A20 is an endogenous negative regulator of NF-κB signaling, which has been widely described in autoimmune and inflammatory disorders, including diabetes and Crohn's disease, but which has received little attention in terms of chronic lung disorders. This review examines the existing body of research on A20 regulation of NF-κB signaling and details the mechanism and regulation of A20 action focusing, where possible, on pulmonary inflammation. A20 and its associated signaling molecules are highlighted as being of potential therapeutic interest for the treatment of inflammatory disorders, and a proposed model of A20 activity in inflammatory lung disease is provided.

Deubiquitinases in the regulation of NF-κB signaling

Nuclear factor-kappa B (NF-κB) is a critical regulator of multiple biological functions including innate and adaptive immunity and cell survival. Activation of NF-κB is tightly regulated to preclude chronic signaling that may lead to persistent inflammation and cancer. Ubiquitination of key signaling molecules by E3 ubiquitin ligases has emerged as an important regulatory mechanism for NF-κB signaling. Deubiquitinases (DUBs) counteract E3 ligases and therefore play a prominent role in the downregulation of NF-κB signaling and homeostasis. Understanding the mechanisms of NF-κB downregulation by specific DUBs such as A20 and CYLD may provide therapeutic opportunities for the treatment of chronic inflammatory diseases and cancer.

Recent advances in the genetics of systemic lupus erythematosus

Systemic lupus erythematosus (SLE) is a systemic autoimmune disease characterized by the production of antinuclear autoantibodies and the inflammatory infiltration of many organ systems. SLE is a complex disorder in which multiple genetic variants, together with environmental and hormonal factors, contribute to disease risk. In this article, we summarize our current understanding of the genetic contribution to SLE in light of recent genome-wide association studies, which have brought the total number of confirmed SLE susceptibility loci to 29. In the second section, we explore the functional implications of these risk loci and, in particular, highlight the role that many of these genes play in the Toll-like receptor and type I interferon signaling pathways. Finally, we discuss the genetic overlap between SLE and other autoimmune and inflammatory conditions as several risk loci are shared among multiple disorders, suggesting common underlying pathogenic mechanisms.

Expression, biological activities and mechanisms of action of A20 (TNFAIP3)

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-κB and IRF3 activation by modulating both types of ubiquitination. Induction of K48-polyubiquitination by A20 involves its C-terminal zinc-finger 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.

A20: from ubiquitin editing to tumour suppression

Clinicians have suspected for hundreds of years that chronic activation of the immune system contributes to the development of cancer. However, the molecular mechanisms that mediate this precarious interplay are only now being elucidated. Recent reports have identified A20 as a crucial tumour suppressor in various lymphomas. A20 is a ubiquitin-editing enzyme that attenuates the activity of proximal signalling complexes at pro-inflammatory receptors. In this Review we summarize the evidence linking chronic inflammation with tumorigenesis and consider how A20 modulates inflammatory signalling cascades, thereby providing a mechanism to explain how deregulation of ubiquitylation can promote tumorigenesis.

Ubiquitination and TNFR1 signaling

Death receptors are a subset of the tumor necrosis factor receptor (TNFR) family of proteins and share a characteristic cytoplasmic motif called the "death domain". In addition to mediating cell death, these receptors regulate cell proliferation, inflammatory responses, and tumor progression. Receptor occupancy triggers the assembly of several cytoplasmic molecules into distinct complexes, each initiating separate signaling events leading to different biological responses. Post-translational modifications involving ubiquitin, a peptide of 76 amino acids, regulate events at nearly all stages of signaling. All ubiquitin chains function as docking platforms for molecules with specific recognition motifs that either propagate the signal or target the protein for proteasomal degradation. Moreover, enzymes with ubiquitin thioesterase activity (deubiquitinating enzymes, or DUBs) reverse modifications by removing the ubiquitin chains, allowing ubiquitin editing at the molecular level. Ubiquitin protein ligases (E3s), DUBs, and signaling molecules with ubiquitin recognition motifs control TNFR1 mediated cell death and activation of NF-kappaB and JNK. Here, we discuss the current understanding of how these proteins regulate TNFR1 signaling.

A bacterial E3 ubiquitin ligase IpaH9.8 targets NEMO/IKKgamma to dampen the host NF-kappaB-mediated inflammatory response

NF-kappaB (nuclear factor kappaB) has a pivotal role in many cellular processes, including the inflammatory and immune responses and, therefore, its activation is tightly regulated by the IKK (IkappaB kinase) complex and by IkappaBalpha degradation. When Shigella bacteria multiply within epithelial cells they release peptidoglycans, which are recognized by Nod1 and stimulate the NF-kappaB pathway, thus leading to a severe inflammatory response. Here, we show that IpaH9.8, a Shigella effector possessing E3 ligase activity, dampens the NF-kappaB-mediated inflammatory response to the bacterial infection in a unique way. IpaH9.8 interacts with NEMO/IKKgamma and ABIN-1, a ubiquitin-binding adaptor protein, promoting ABIN-1-dependent polyubiquitylation of NEMO. Consequently, polyubiquitylated NEMO undergoes proteasome-dependent degradation, which perturbs NF-kappaB activation. As NEMO is essential for NF-kappaB activation, we propose that the polyubiquitylation and degradation of NEMO during Shigella infection is a new bacterial strategy to modulate host inflammatory responses.

TRAF-mediated TNFR-family signaling

The tumor necrosis factor (TNF) superfamily consists of a wide variety of cell-bound and secreted proteins that regulate numerous cellular processes. In particular, TNF-family proteins regulate the proliferation and death of tumor cells, as well as activated immune cells. This overview discusses the mammalian TNF receptor-associated factors (TRAFs), of which TRAF1, 2, 3, 5, and 6 have been shown to interact directly or indirectly with members of the TNF receptor superfamily. Structural features of TRAF proteins are described along with a discussion of TRAF-interacting proteins and the signaling pathways activated by the TRAF proteins. Finally, we examine the phenotypes observed in TRAF-knockout mice.

A20 is overexpressed in glioma cells and may serve as a potential therapeutic target

OBJECTIVE

A20 is a TNF-inducible primary response gene, which has been found to have antiapoptotic function in several cancer cells. This study investigates A20 expression in human glioma tissues and four glioma cell lines, and its effect on tumorigenesis of glioma cells and a mouse tumor model.

METHODS

Human glioma tissue samples and cells were subject to reverse transcription-PCR (RT-PCR), western blotting and immunohistochemistry. Glioma cells was tested by flow cytometry. A xenograft tumor model in mice was utilized to examine the knock-down effect of specific A20 siRNAs on tumorigenesis.

RESULTS

A20 was overexpressed in clinical glioma tissue samples (63.9%) and correlated with clinical staging. All four human glioma cell lines expressed A20, among which U87 displayed the strongest expression signals. Inhibiting A20 expression by siRNAs in vitro reduced the growth rates of glioma cells and resulted in G1/S arrest and increased apoptosis. In a mouse tumor model, local administration of siRNA significantly suppressed solid tumor growth.

CONCLUSIONS

A20 was overexpressed both in human glioma tissues and cell lines, and inhibiting A20 expression greatly slowed tumor cell growth in culture and in mice. These findings indicated that A20 is involved in tumorigenesis of human glioma, and may serve as a future therapeutic target.