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

HTLV gene regulation: because size matters, transcription is not enough

Despite being discovered in animals in the early 20th century, the scientific interest in retroviruses was boosted with the discovery of human retroviruses (human T-leukemia/lymphoma virus [HTLV] and HIV), which are responsible for significant morbidity and mortality. HTLV was identified more than 25 years ago as the etiological agent of adult T-cell leukemia/lymphoma. It was then shown to be a complex retrovirus, given that it not only encodes the characteristic retroviral Gag, Pol and Env proteins, but also regulatory and accessory proteins. Since the first studies documenting the role of these proteins in viral expression, the picture has become increasingly more complex. Indeed, owing to the limited size of its genome that contains overlapping open-reading frames, HTLV has evolved unique ways to regulate its expression. Retroviral expression was originally thought to be mainly controlled through the regulation of transcription from the 5 long-terminal repeats, but we now know that the 3 long-terminal repeats also serve as promoters. Regulation of splicing and mRNA export, and post-translational modifications of viral protein also play a major role. This review discusses the latest insights gained into the field of HTLV gene expression.

Unraveling the genetics of complex diseases: susceptibility genes for rheumatoid arthritis and psoriasis

Talk of numerous genetic risk factors for rheumatoid arthritis (RA) and psoriasis has been percolating for years, but with the exception of the human leukocyte antigen (HLA) region, none have been definitively identified. Recently the results of multiple, well powered, genetic case-control studies have begun to appear providing convincing statistical evidence for at least ten non-HLA related risk genes or loci (C5/TRAF1, CD40, CTLA4, KIF5A/PIP4K2C, MMEL1/TNFRSF14, PADI4, PRKCQ, PTPN22, STAT4, and TNFAIP3/OLIG3) for RA and six (IL12B, IL13, IL23R, STAT2/IL23A, TNFAIP3, and TNIP1) for psoriasis. These initial, novel findings are beginning to shed light on the molecular pathways pertinent to the individual diseases and highlight the pleiotropic effects of several risk factors as well as the allelic heterogeneity underlying susceptibility to these and other autoimmune diseases.

Cellular expression of A20 and ABIN-3 in response to Toll-like receptor-4 stimulation

Although Toll-like receptor (TLR)-induced expression of several proinflammatory genes is required to provoke an efficient immune response, excessive or prolonged activation of TLR signaling can contribute to the development of septic shock and several inflammatory diseases. Given this inherent danger of unrestrained TLR signaling to the organism, it is not surprising that many negative feedback mechanisms have evolved to hold TLR signaling in check. In this context, TLR stimulation induces several negative regulators of TLR-induced signaling to nuclear factor (NF)-kappaB dependent gene expression. Here we describe the use of Western blotting and reverse transcriptase polymerase chain reaction (RT-PCR) to study respectively the cellular protein and mRNA expression levels of the NF-kappaB inhibitory proteins A20 and ABIN-3 in response to TLR4 stimulation by lipopolysaccharide (LPS).

A20 takes on tumors: tumor suppression by an ubiquitin-editing enzyme

Many B cell cancers are characterized in part by the dysregulation of the NF-kappaB signaling pathway. A new study identifies somatic mutations in TNFAIP3, the gene encoding the NF-kappaB inhibitor A20, in Hodgkin lymphomas and primary mediastinal lymphomas. These data reveal the role of A20 as a tumor suppressor protein.

A20: central gatekeeper in inflammation and immunity

Inappropriate functioning of the immune system is linked to immune deficiency, autoimmune disease, and cancer. It is therefore not surprising that intracellular immune signaling pathways are tightly controlled. One of the best studied transcription factors in immune signaling is NF-kappaB, which is activated by multiple receptors and regulates the expression of a wide variety of proteins that control innate and adaptive immunity. A20 is an early NF-kappaB-responsive gene that encodes a ubiquitin-editing protein that is involved in the negative feedback regulation of NF-kappaB signaling. Here, we discuss the mechanism of action of A20 and its role in the regulation of inflammation and immunity.

A large scale shRNA barcode screen identifies the circadian clock component ARNTL as putative regulator of the p53 tumor suppressor pathway

BACKGROUND

The p53 tumor suppressor gene is mutated in about half of human cancers, but the p53 pathway is thought to be functionally inactivated in the vast majority of cancer. Understanding how tumor cells can become insensitive to p53 activation is therefore of major importance. Using an RNAi-based genetic screen, we have identified three novel genes that regulate p53 function.

RESULTS

We have screened the NKI shRNA library targeting 8,000 human genes to identify modulators of p53 function. Using the shRNA barcode technique we were able to quickly identify active shRNA vectors from a complex mixture. Validation of the screening results indicates that the shRNA barcode technique can reliable identify active shRNA vectors from a complex pool. Using this approach we have identified three genes, ARNTL, RBCK1 and TNIP1, previously unknown to regulate p53 function. Importantly, ARNTL (BMAL1) is an established component of the circadian regulatory network. The latter finding adds to recent observations that link circadian rhythm to the cell cycle and cancer. We show that cells having suppressed ARNTL are unable to arrest upon p53 activation associated with an inability to activate the p53 target gene p21(CIP1).

CONCLUSIONS

We identified three new regulators of the p53 pathway through a functional genetic screen. The identification of the circadian core component ARNTL strengthens the link between circadian rhythm and cancer.

ABINs: A20 binding inhibitors of NF-kappa B and apoptosis signaling

ABINs have been described as three different proteins (ABIN-1, ABIN-2, ABIN-3) that bind the ubiquitin-editing nuclear factor-kappaB (NF-kappaB) inhibitor protein A20 and which show limited sequence homology. Overexpression of ABINs inhibits NF-kappaB activation by tumor necrosis factor (TNF) and several other stimuli. Similar to A20, ABIN-1 and ABIN-3 expression is NF-kappaB dependent, implicating a potential role for the A20/ABIN complex in the negative feedback regulation of NF-kappaB activation. Adenoviral gene transfer of ABIN-1 has been shown to reduce NF-kappaB activation in mouse liver and lungs. However, ABIN-1 as well as ABIN-2 deficient mice exhibit only slightly increased or normal NF-kappaB activation, respectively, possibly reflecting redundant NF-kappaB inhibitory activities of multiple ABINs. Other functions of ABINs might be non-redundant. For example, ABIN-1 shares with A20 the ability to inhibit TNF-induced apoptosis and as a result ABIN-1 deficient mice die during embryogenesis due to TNF-dependent fetal liver apoptosis. On the other hand, ABIN-2 is required for optimal TPL-2 dependent extracellularly regulated kinase activation in macrophages treated with TNF or Toll-like receptor ligands. ABINs have recently been shown to contain an ubiquitin-binding domain that is essential for their NF-kappaB inhibitory and anti-apoptotic activities. In this context, ABINs were proposed to function as adaptors between ubiquitinated proteins and other regulatory proteins. Alternatively, ABINs might disrupt signaling complexes by competing with other ubiquitin-binding proteins for the binding to specific ubiquitinated targets. Altogether, these findings implicate an important role for ABINs in the regulation of immunity and tissue homeostasis.

ABIN-1 is a ubiquitin sensor that restricts cell death and sustains embryonic development

Proteins that directly regulate TNFR signaling play critical roles in regulating cellular activation and survival. A20 Binding and Inhibitor of NFkB (ABIN-1) is a novel protein that is thought to inhibit NFkB signaling (1, 2). Here we show that mice deficient for ABIN-1 die during embryogenesis with fetal liver apoptosis, anemia and hypoplasia. ABIN-1 deficient cells are hypersensitive to TNF-induced PCD, and TNF deficiency rescues ABIN-1 deficient embryos. ABIN-1 inhibits caspase 8 recruitment to FADD in TNF-induced signaling complexes, preventing caspase 8 cleavage and PCD. Moreover, ABIN-1 directly binds polyubiquitin chains and this ubiquitin sensing activity is required for ABIN-1’s anti-apoptotic activity. These studies provide new insights into how ubiquitination and ubiquitin sensing proteins regulate cellular and organismal survival.

Protein partners of deubiquitinating enzymes

Protein modification by ubiquitin and ubiquitin-like molecules is a critical regulatory process. Like most regulated protein modifications, ubiquitination is reversible. Deubiquitination, the reversal of ubiquitination, is quickly being recognized as an important regulatory strategy. Nearly one hundred human DUBs (deubiquitinating enzymes) in five different gene families oppose the action of several hundred ubiquitin ligases, suggesting that both ubiquitination and its reversal are highly regulated and specific processes. It has long been recognized that ubiquitin ligases are modular enzyme systems that often depend on scaffolds and adaptors to deliver substrates to the catalytically active macromolecular complex. Although many DUBs bind ubiquitin with reasonable affinities (in the nM to microM range), a larger number have little affinity but exhibit robust catalytic capability. Thus it is apparent that these DUBs must acquire their substrates by binding the target protein in a conjugate or by associating with other macromolecular complexes. We would then expect that a study of protein partners of DUBs would reveal a variety of substrates, scaffolds, adaptors and ubiquitin receptors. In the present review we suggest that, like ligases, much of the regulation and specificity of deubiquitination arises from the association of DUBs with these protein partners.

Beyond tumor necrosis factor receptor: TRADD signaling in toll-like receptors

Tumor necrosis factor receptor 1-associated death domain protein (TRADD) is the core adaptor recruited to TNF receptor 1 (TNFR1) upon TNFalpha stimulation. In cells from TRADD-deficient mice, TNFalpha-mediated apoptosis and TNFalpha-stimulated NF-kappaB, JNK, and ERK activation are defective. TRADD is also important for germinal center formation, DR3-mediated costimulation of T cells, and TNFalpha-mediated inflammatory responses in vivo. TRADD deficiency does not enhance IFNgamma-induced signaling. Importantly, TRADD has a novel role in TLR3 and TLR4 signaling. TRADD participates in the TLR4 complex formed upon LPS stimulation, and TRADD-deficient macrophages show impaired cytokine production in response to TLR ligands in vitro. Thus, TRADD is a multifunctional protein crucial both for TNFR1 signaling and other signaling pathways relevant to immune responses.

ABINs inhibit EGF receptor-mediated NF-kappaB activation and growth of EGF receptor-overexpressing tumour cells

The epidermal growth factor receptor (EGFR) is frequently overexpressed in various tumours of epidermal origin and is held responsible for tumourigenicity and tumour persistence. Increased nuclear factor (NF)-kappaB activity has been suggested to be involved in the malignant behaviour of EGFR-overexpressing cells. However, the mechanisms that regulate EGF-induced NF-kappaB activation are still largely unknown. Here we show that EGF can induce NF-kappaB-dependent gene expression independently from IkappaBalpha degradation or p100 processing in EGFR-overexpressing HEK293T cells. Moreover, EGF-induced NF-kappaB activation could be inhibited by overexpression of ABINs, which were previously identified as intracellular inhibitors of tumour necrosis factor, interleukin-1 and lipopolysaccharide-induced NF-kappaB activation. Knockdown of ABIN-1 by RNA interference boosted the NF-kappaB response upon EGF stimulation. The C-terminal ubiquitin-binding domain containing region of ABINs was crucial and sufficient for NF-kappaB inhibition. Adenoviral gene transfer of ABINs reduced constitutive NF-kappaB activity as well as the proliferation of EGFR-overexpressing A431 and DU145 human carcinoma cells. Altogether, these results demonstrate an important role for an ABIN-sensitive non-classical NF-kappaB signalling pathway in the proliferation of EGFR-overexpressing tumour cells, and indicate a potential use for ABIN gene therapy in the treatment of cancer.

Inhibition of proinflammatory and innate immune signaling pathways by a cytomegalovirus RIP1-interacting protein

TNFalpha is an important cytokine in antimicrobial immunity and inflammation. The receptor-interacting protein RIP1 is an essential component of the TNF receptor 1 signaling pathway that mediates the activation of NF-kappaB, MAPKs, and programmed cell death. It also transduces signals derived from Toll-like receptors and intracellular sensors of DNA damage and double-stranded RNA. Here, we show that the murine CMV M45 protein binds to RIP1 and inhibits TNFalpha-induced activation of NF-kappaB, p38 MAPK, and caspase-independent cell death. M45 also inhibited NF-kappaB activation upon stimulation of Toll-like receptor 3 and ubiquitination of RIP1, which is required for NF-kappaB activation. Hence, M45 functions as a viral inhibitor of RIP1-mediated signaling. The results presented here reveal a mechanism of viral immune subversion and demonstrate how a viral protein can simultaneously block proinflammatory and innate immune signaling pathways by interacting with a central mediator molecule.

Prostaglandin E receptor type 4-associated protein interacts directly with NF-kappaB1 and attenuates macrophage activation

Macrophage activation participates pivotally in the pathophysiology of chronic inflammatory diseases, including atherosclerosis. Through the receptor EP4, prostaglandin E(2) (PGE(2)) exerts an anti-inflammatory action in macrophages, suppressing stimulus-induced expression of certain proinflammatory genes, including chemokines. We recently identified a novel EP4 receptor-associated protein (EPRAP), whose function in PGE(2)-mediated anti-inflammation remains undefined. Here we demonstrate that PGE(2) pretreatment selectively inhibits lipopolysaccharide (LPS)-induced nuclear factor kappaB1 (NF-kappaB1) p105 phosphorylation and degradation in mouse bone marrow-derived macrophages through EP4-dependent mechanisms. Similarly, directed EPRAP expression in RAW264.7 cells suppresses LPS-induced p105 phosphorylation and degradation, and subsequent activation of mitogen-activated protein kinase kinase 1/2. Forced expression of EPRAP also inhibits NF-kappaB activation induced by various proinflammatory stimuli in a concentration-dependent manner. In co-transfected cells, EPRAP, which contains multiple ankyrin repeat motifs, directly interacts with NF-kappaB1 p105/p50 and forms a complex with EP4. In EP4-overexpressing cells, PGE(2) enhances the protective action of EPRAP against stimulus-induced p105 phosphorylation, whereas EPRAP silencing in RAW264.7 cells impairs the inhibitory effect of PGE(2)-EP4 signaling on LPS-induced p105 phosphorylation. Additionally, EPRAP knockdown as well as deficiency of NF-kappaB1 in macrophages attenuates the inhibitory effect of PGE(2) on LPS-induced MIP-1beta production. Thus, PGE(2)-EP4 signaling augments NF-kappaB1 p105 protein stability through EPRAP after proinflammatory stimulation, limiting macrophage activation.

Ubiquitin signals in the NF-kappaB pathway

The NF-kappaB (nuclear factor kappaB) transcription factors control cell survival, proliferation and innate and adaptive immune response. Post-translational modifications of key components of the NF-kappaB pathway provide the molecular basis for signal transmission from the cell membrane to the nucleus. Here, we describe the involvement of different types of ubiquitin modification in the regulation of the NF-kappaB signalling pathway.

Inflammatory cardiac valvulitis in TAX1BP1-deficient mice through selective NF-kappaB activation

Nuclear factor kappa B (NF-kappaB) is a key mediator of inflammation. Unchecked NF-kappaB signalling can engender autoimmune pathologies and cancers. Here, we show that Tax1-binding protein 1 (TAX1BP1) is a negative regulator of TNF-alpha- and IL-1beta-induced NF-kappaB activation and that binding to mono- and polyubiquitin by a ubiquitin-binding Zn finger domain in TAX1BP1 is needed for TRAF6 association and NF-kappaB 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-alpha and IL-1beta. TAX1BP1-/- cells are more highly activated for NF-kappaB than control cells when stimulated with TNF-alpha or IL-1beta. Mechanistically, TAX1BP1 acts in NF-kappaB signalling as an essential adaptor between A20 and its targets.

T cell antigen receptor stimulation induces MALT1 paracaspase-mediated cleavage of the NF-kappaB inhibitor A20

The paracaspase MALT1 mediates T cell antigen receptor-induced signaling to the transcription factor NF-kappaB and is indispensable for T cell activation and proliferation. Enhanced expression of MALT1 or aberrant expression of a fusion protein of the apoptosis inhibitor API2 and MALT1 has been linked to mucosa-associated lymphoid tissue lymphoma. Despite the presence of a caspase-like domain, MALT1 proteolytic activity has not yet been demonstrated. Here we show that T cell antigen receptor stimulation induced recruitment of the NF-kappaB inhibitor A20 into a complex of MALT1 and the adaptor protein Bcl-10, leading to MALT1-mediated processing of A20. API2-MALT1 expression likewise resulted in cleavage of A20. MALT1 cleaved human A20 after arginine 439 and impaired its NF-kappaB-inhibitory function. Our studies identify A20 as a substrate of MALT1 and emphasize the importance of MALT1 proteolytic activity in the 'fine tuning' of T cell antigen receptor signaling.

NF-kappaB suppression by the deubiquitinating enzyme Cezanne: a novel negative feedback loop in pro-inflammatory signaling

Transcription factors belonging to the NF-kappaB family regulate inflammation by inducing pro-inflammatory molecules (e.g. interleukin (IL)-8) in response to cytokines (e.g. tumor necrosis factor (TNF) alpha, IL-1) or other stimuli. Several negative regulators of NF-kappaB, including the ubiquitin-editing enzyme A20, participate in the resolution of inflammatory responses. We report that Cezanne, a member of the A20 family of the deubiquitinating cysteine proteases, can be induced by TNFalpha in cultured cells. Silencing of endogenous Cezanne using small interfering RNA led to elevated NF-kappaB luciferase reporter gene activity and enhanced expression of IL-8 transcripts in TNFalpha-treated cells. Thus we conclude that endogenous Cezanne can attenuate NF-kappaB activation and the induction of pro-inflammatory transcripts in response to TNF receptor (TNFR) signaling. Overexpression studies revealed that Cezanne suppressed NF-kappaB nuclear translocation and transcriptional activity by targeting the TNFR signaling pathway at the level of the IkappaB kinase complex or upstream from it. These effects were not observed in a form of Cezanne that was mutated at the catalytic cysteine residue (Cys209), indicating that the deubiquitinating activity of Cezanne is essential for NF-kappaB regulation. Finally, we demonstrate that Cezanne can be recruited to activated TNFRs where it suppresses the build-up of polyubiquitinated RIP1 signal adapter proteins. Thus we conclude that Cezanne forms a novel negative feedback loop in pro-inflammatory signaling and that it suppresses NF-kappaB activation by targeting RIP1 signaling intermediaries for deubiquitination.

Expression of the NF-kappaB inhibitor ABIN-3 in response to TNF and toll-like receptor 4 stimulation is itself regulated by NF-kappaB

Although the nuclear factor-kappaB (NF-kappaB)-dependent gene expression is critical to the induction of an efficient immune response to infection or tissue injury, excessive or prolonged NF-kappaB signalling can contribute to the development of several inflammatory diseases. Therefore, the NF-kappaB signal transduction pathway is tightly regulated by several intracellular proteins. We have previously identified A20-binding inhibitor of NF-kappaB activation (ABIN)-3 as an lipopolysaccharide (LPS)-inducible protein in monocytes that negatively regulates NF-B activation in response to tumour necrosis factor (TNF) and LPS. Here we report that ABIN-3 expression is also up-regulated upon TNF treatment of monocytes and other non-myeloid cell types. We also found a significantly enhanced expression of ABIN-3 in monocytes of sepsis patients, which is restored to control levels by corticotherapy. To further understand the transcriptional regulation of ABIN-3 expression, we isolated the human ABIN-3 promoter and investigated its activation in response to TNF and LPS. This revealed that the LPS- and TNF-inducible expression of ABIN-3 is dependent on the binding of NF-kappaB to a specific B site in the ABIN-3 promoter. Altogether, these data indicate an important role for NF-kappaB-dependent gene expression of ABIN-3 in the negative feedback regulation of TNF receptor and toll-like receptor 4 induced NF-kappaB activation.

Molecular basis for the unique deubiquitinating activity of the NF-kappaB inhibitor A20

Nuclear factor kappaB (NF-kappaB) activation in tumor necrosis factor, interleukin-1, and Toll-like receptor pathways requires Lys63-linked nondegradative polyubiquitination. A20 is a specific feedback inhibitor of NF-kappaB activation in these pathways that possesses dual ubiquitin-editing functions. While the N-terminal domain of A20 is a deubiquitinating enzyme (DUB) for Lys63-linked polyubiquitinated signaling mediators such as TRAF6 and RIP, its C-terminal domain is a ubiquitin ligase (E3) for Lys48-linked degradative polyubiquitination of the same substrates. To elucidate the molecular basis for the DUB activity of A20, we determined its crystal structure and performed a series of biochemical and cell biological studies. The structure reveals the potential catalytic mechanism of A20, which may be significantly different from papain-like cysteine proteases. Ubiquitin can be docked onto a conserved A20 surface; this interaction exhibits charge complementarity and no steric clash. Surprisingly, A20 does not have specificity for Lys63-linked polyubiquitin chains. Instead, it effectively removes Lys63-linked polyubiquitin chains from TRAF6 without dissembling the chains themselves. Our studies suggest that A20 does not act as a general DUB but has the specificity for particular polyubiquitinated substrates to assure its fidelity in regulating NF-kappaB activation in the tumor necrosis factor, interleukin-1, and Toll-like receptor pathways.

Involvement of Ymer in suppression of NF-kappaB activation by regulated interaction with lysine-63-linked polyubiquitin chain

It is known that the cytoplasmic zinc finger protein A20 functionally dampens inflammatory signals and apoptosis via inhibition of NF-kappaB activation and biochemically acts as a unique ubiquitin-modifying protein with deubiquitinating activity and ubiquitin ligase activity. However, the molecular mechanisms of A20-modulated signal transduction that influence normal immune responses or tumor immunity have not been fully elucidated. Using a yeast two-hybrid system to search for proteins interacting with A20, we identified one novel binding protein, Ymer. Ymer, which has been reported to be highly phosphorylated on tyrosine residues via EGF stimulation, bound to lysine (K)-63-linked polyubiquitin chain on receptor-interacting serine/threonine-protein kinase 1 (RIP1), which is essential for NF-kappaB signaling in collaboration with A20. A luciferase assay showed that NF-kappaB signaling was down-regulated by overexpression of Ymer, whereas knock-down of Ymer up-regulated NF-kappaB signaling even without stimulation. These findings demonstrate that Ymer is likely to be a negative regulator for the NF-kappaB signaling pathway.

Relationship of toll-like receptor 4, nuclear factor kappa B and acute pancreatitis

Acute pancreatitis (AP) is a serious commonly-occurring disease. Toll-like receptor 4 (TLR4) and nuclear factor kappa B (NF-κB) are closely related to the activation of many cytokines that have important roles in the occurrence and development of AP. It is already acknowledged that TLR4 and NF-κB have roles in the pathogenesis of acute necrotizing pancreatitis (ANP) because they are essential in the inducing and mediating of inflammation. It is thought that TLR4 induces LPS signaling, which leads to the activation and translocation of NF-κB, and then stimulates the production of proinflammatory cytokines that result in the occurrence of inflammation. Recently, however, new concepts about the specific signaling pathway of TLR4/NF-κB and the factors participating in it have been proposed. This review summarizes the TLR4/NF-κB signaling pathway and outlines the factors that can down- or up-regulate TLR4/NF-κB expression and other factors that activate NF-κB.

The negative regulation of Toll‐like receptor and associated pathways

Toll-like receptors (TLRs) are essential mediators of both innate and adaptive immunity by recognizing and eliciting responses upon invasion of pathogens. The response of TLRs must be stringently regulated as exaggerated expression of signalling components as well as pro-inflammatory cytokines can have devastating effects on the host, resulting in chronic inflammatory diseases, autoimmune disorders and aid in the pathogenesis of TLR-associated human diseases. Therefore, it is essential that negative regulators act at multiple levels within TLR signalling cascades, as well as through eliciting negative-feedback mechanisms in order to synchronize the positive activation and negative regulation of signal transduction to avert potentially harmful immunological consequences. This review explores the various mechanisms employed by negative regulators to ensure the appropriate modulation of both immune and inflammatory responses.

ABIN-3: a molecular basis for species divergence in interleukin-10-induced anti-inflammatory actions

Whereas interleukin-10 (IL-10) is an anti-inflammatory cytokine known to regulate macrophage activation, its full mechanism of action remains incompletely defined. In a screen to identify novel IL-10-induced genes, we cloned the mouse ortholog of human ABIN-3 (also termed LIND). ABIN-3 expression was induced selectively by IL-10 in both mouse and human mononuclear phagocytes coordinately undergoing proinflammatory responses. In contrast to the previously characterized ABINs, mouse ABIN-3 was incapable of inhibiting NF-kappaB activation by proinflammatory stimuli. Generation and analysis of ABIN-3-null mice demonstrated that ABIN-3 is unnecessary for the anti-inflammatory effects of IL-10 as well as for proper negative regulation of NF-kappaB. Conversely, human ABIN-3 was capable of inhibiting NF-kappaB activation in response to signaling from Toll-like receptor, IL-1, and tumor necrosis factor. Enforced expression of human ABIN-3 in human monocytic cells suppressed the cytoplasmic degradation of IkappaBalpha, the activation of NF-kappaB, and the induction of proinflammatory genes. Comparative sequence analyses revealed that mouse ABIN-3 lacks a complete ABIN homology domain, which was required for the functional activity of human ABIN-3. ABIN-3 is, thus, an IL-10-induced gene product capable of attenuating NF-kappaB in human macrophages yet is inoperative in mice and represents a basis for species-specific differences in IL-10 actions.

Elevated amount of Toll-like receptor 4 mRNA in bronchial epithelial cells is associated with airway inflammation in horses with recurrent airway obstruction

Recurrent airway obstruction (RAO) is characterized by neutrophilic airway inflammation and obstruction, and stabling of susceptible horses triggers acute disease exacerbations. Stable dust is rich in endotoxin, which is recognized by Toll-like receptor (TLR) 4. In human bronchial epithelium, TLR4 stimulation leads to elevation of interleukin (IL)-8 mRNA expression. The zinc finger protein A20 negatively regulates this pathway. We hypothesized that TLR4 and IL-8 mRNA and neutrophil numbers are elevated and that A20 mRNA is not increased in RAOs during stabling compared with controls and with RAOs on pasture. We measured the maximal change in pleural pressure (DeltaPpl(max)), determined inflammatory cell counts in bronchoalveolar lavage fluid (BAL), and quantified TLR4, IL-8, and A20 mRNA in bronchial epithelium by quantitative RT-PCR. We studied six horse pairs, each pair consisting of one RAO and one control horse. Each pair was studied when the RAO-affected horse had airway obstruction induced by stabling and after 7, 14, and 28 days on pasture. Stabling increased BAL neutrophils, DeltaPpl(max), and TLR4 (4.14-fold change) significantly in RAOs compared with controls and with RAOs on pasture. TLR4 correlated with IL-8 (R2 = 0.75). Whereas stabling increased IL-8 in all horses, A20 was unaffected. IL-8 was positively correlated with BAL neutrophils (R2 = 0.43) and negatively with A20 (R2 = 0.44) only in RAO-affected horses. Elevated TLR4 expression and lack of A20 upregulation in bronchial epithelial cells from RAO-affected horses may contribute to elevated IL-8 production, leading to exaggerated neutrophilic airway inflammation in response to inhalation of stable dust.

LIND/ABIN-3 is a novel lipopolysaccharide-inducible inhibitor of NF-kappaB activation

Recognition of lipopolysaccharide (LPS) by Toll-like receptor (TLR)4 initiates an intracellular signaling pathway leading to the activation of nuclear factor-kappaB (NF-kappaB). Although LPS-induced activation of NF-kappaB is critical to the induction of an efficient immune response, excessive or prolonged signaling from TLR4 can be harmful to the host. Therefore, the NF-kappaB signal transduction pathway demands tight regulation. In the present study, we describe the human protein Listeria INDuced (LIND) as a novel A20-binding inhibitor of NF-kappaB activation (ABIN) that is related to ABIN-1 and -2 and, therefore, is further referred to as ABIN-3. Similar to the other ABINs, ABIN-3 binds to A20 and inhibits NF-kappaB activation induced by tumor necrosis factor, interleukin-1, and 12-O-tetradecanoylphorbol-13-acetate. However, unlike the other ABINs, constitutive expression of ABIN-3 could not be detected in different human cells. Treatment of human monocytic cells with LPS strongly induced ABIN-3 mRNA and protein expression, suggesting a role for ABIN-3 in the LPS/TLR4 pathway. Indeed, ABIN-3 overexpression was found to inhibit NF-kappaB-dependent gene expression in response to LPS/TLR4 at a level downstream of TRAF6 and upstream of IKKbeta. NF-kappaB inhibition was mediated by the ABIN-homology domain 2 and was independent of A20 binding. Moreover, in vivo adenoviral gene transfer of ABIN-3 in mice reduced LPS-induced NF-kappaB activity in the liver, thereby partially protecting mice against LPS/D-(+)-galactosamine-induced mortality. Taken together, these results implicate ABIN-3 as a novel negative feedback regulator of LPS-induced NF-kappaB activation.

NF-κB, AP-1, Zinc-deficiency and aging

Zinc (Zn) deficiency, a frequent condition in human population especially in aged persons, induces oxidative stress and subsequently activates/inhibits oxidant-sensitive transcription factors that can affect cell function, proliferation and survival leading to disease. Zn deficiency-triggered oxidative stress could affect cell signalling, including transcription factors containing Zn finger motifs and other oxidant-sensitive transcription factors such as nuclear factor kappa B (NF-kappaB) and activator protein-1 (AP-1). AP-1 can be activated in Zn deficiency that can occur secondary to an increase in cellular H(2)O(2), followed by activation of MAPKs p38 and JNK. Similarly, the cytosolic steps of the NF-kappaB cascade are activated by oxidants in Zn deficiency. However, an impaired nuclear transport of the active transcription factor leads to a low expression of NF-kappaB-dependent genes that could be involved in multiple steps of Zn deficiency associated pathology. We present here evidence that, following experimental depletion of Zn, both NF-kappaB and AP-1 signallings are altered in primary T cells isolated from young and elderly healthy individuals under CD3/CD28 costimulation. A supplementation of Zn restored both NF-kappaB and AP-1 activation in CD3/CD28 costimulated T cells from young, but not from elderly, healthy individuals, indicating that the Zn deficiency is only one component of a more complex mechanism involved in immunosenescence. In this review we summarize our present knowledge on NF-kappaB and AP-1 activation and underline the role of Zn in this process, especially in the context of Zn deficiency observed in aged persons leading to immunosenescence.

Sepsis-induced suppression of lung innate immunity is mediated by IRAK-M

Sepsis results in a state of relative immunosuppression, rendering critically ill patients susceptible to secondary infections and increased mortality. Monocytes isolated from septic patients and experimental animals display a "deactivated" phenotype, characterized by impaired inflammatory and antimicrobial responses, including hyporesponsiveness to LPS. We investigated the role of the LPS/TLR4 axis and its inhibitor, IL-1 receptor-associated kinase-M (IRAK-M), in modulating the immunosuppression of sepsis using a murine model of peritonitis-induced sepsis followed by secondary challenge by intratracheal Pseudomonasaeruginosa. Septic mice demonstrated impaired alveolar macrophage function and increased mortality when challenged with intratracheal Pseudomonas as compared with nonseptic controls. TLR2 and TLR4 expression was unchanged in the lung following sepsis, whereas levels of IRAK-M were upregulated. Macrophages from IRAK-M-deficient septic mice produced higher levels of proinflammatory cytokines ex vivo and greater costimulatory molecule expression in vivo as compared with those of their WT counterparts. Following sepsis and secondary intrapulmonary bacterial challenge, IRAK-M(-/-) animals had higher survival rates and improved bacterial clearance from lung and blood compared with WT mice. In addition, increased pulmonary chemokine and inflammatory cytokine production was observed in IRAK-M(-/-) animals, leading to enhanced neutrophil recruitment to airspaces. Collectively, these findings indicate that IRAK-M mediates critical aspects of innate immunity that result in an immunocompromised state during sepsis.

Hepatitis C virus core protein induces expression of genes regulating immune evasion and anti-apoptosis in hepatocytes

Hepatitis C virus (HCV) Core protein is implicated in the development of hepatocellular carcinoma (HCC). We utilized a HepG2 human hepatocyte cell line with inducible expression of HCV Core protein (HCV-1b) to investigate the early effects of Core protein on hepatocyte gene expression and to identify molecular processes modulated by the Core protein. A significant change was observed in the expression of 407 genes, which included genes regulating apoptosis, immune response, and cell cycle. Some of these genes were previously known to be tumor markers. The decreased expression of chemo-attractants such as TNFSF10, CCL20, and osteopontin was observed, which suggested that HCV Core expression could lead to suppression of inflammatory response as well as trafficking of macrophages and neutrophils to the site of HCV infection. An increased expression of anti-apoptosis factors including PAK2, API5, BH1, Tax1BP1, DAXX, and TNFAIP3/A20 was observed. Some of these genes were also linked to the regulation of NFKB activation and that the alteration of their expression levels, by HCV Core, might lead to the suppression NFKB activation of inflammatory responses. Our data suggested that Core expression may contribute to the viral persistence by protecting infected hepatocytes from cell death by the suppressing apoptosis and inflammatory reaction to HCV viral infection.

Selective expansion of marginal zone B cells in Emicro-API2-MALT1 mice is linked to enhanced IkappaB kinase gamma polyubiquitination

The translocation t(11;18)(q21;q21) that generates an API2-MALT1 fusion protein is the most common structural abnormality among the genetic defects reported in mucosa-associated lymphoid tissue (MALT)-type lymphomas, and its presence correlates with the apparent lack of further genetic instability or chromosomal imbalances. Hence, constitutive nuclear factor-kappaB (NF-kappaB) activation induced by the API2-MALT1 fusion protein is considered essential for B-cell transformation. To examine its role in B-cell development and lymphomagenesis, Emu-API2-MALT1 transgenic mice were produced. Our data show that expression of the API2-MALT1 fusion protein alone is not sufficient for the development of lymphoma masses within 50 weeks. Nevertheless, API2-MALT1 expression affected B-cell maturation in the bone marrow and triggered the specific expansion of splenic marginal zone B cells. Polyubiquitination of IkappaB kinase gamma (IKKgamma), indicative for enhanced NF-kappaB activation, was increased in splenic lymphocytes and promoted the survival of B cells ex vivo. In addition, we show that the API2-MALT1 fusion resided in the cholesterol- and sphingolipid-enriched membrane microdomains, termed lipid rafts. We provide evidence that association of the MALT1 COOH terminal with the lipid rafts, which is mediated by the API2 portion, is sufficient to trigger NF-kappaB activation via enhanced polyubiquitination of IKKgamma. Taken together, these data support the hypothesis that the API2-MALT1 fusion protein can contribute to MALT lymphoma formation via increased NF-kappaB activation.

ABIN-1 Binds to NEMO/IKKγ and Co-operates with A20 in Inhibiting NF-κB

Nuclear factor kappaB (NF-kappaB) plays a pivotal role in inflammation, immunity, stress responses, and protection from apoptosis. Canonical activation of NF-kappaB is dependent on the phosphorylation of the inhibitory subunit IkappaBalpha that is mediated by a multimeric, high molecular weight complex, called IkappaB kinase (IKK) complex. This is composed of two catalytic subunits, IKKalpha and IKKbeta, and a regulatory subunit, NEMO/IKKgamma. The latter protein is essential for the activation of IKKs and NF-kappaB, but its mechanism of action is not well understood. Here we identified ABIN-1 (A20 binding inhibitor of NF-kappaB) as a NEMO/IKKgamma-interacting protein. ABIN-1 has been previously identified as an A20-binding protein and it has been proposed to mediate the NF-kappaB inhibiting effects of A20. We find that both ABIN-1 and A20 inhibit NF-kappaB at the level of the IKK complex and that A20 inhibits activation of NF-kappaB by de-ubiquitination of NEMO/IKKgamma. Importantly, small interfering RNA targeting ABIN-1 abrogates A20-dependent de-ubiquitination of NEMO/IKKgamma and RNA interference of A20 impairs the ability of ABIN-1 to inhibit NF-kappaB activation. Altogether our data indicate that ABIN-1 physically links A20 to NEMO/IKKgamma and facilitates A20-mediated de-ubiquitination of NEMO/IKKgamma, thus resulting in inhibition of NF-kappaB.

ABIN-2 is required for optimal activation of Erk MAP kinase in innate immune responses

The TPL-2 MEK kinase is essential for activation of the Erk MAP kinase pathway during innate immune responses. TPL-2 is found in complex with ABIN-2 (A20-binding inhibitor of NF-kappaB 2). Here, using antigen-presenting cells from ABIN-2-deficient mice, we show that ABIN-2 was required for optimal activation of Erk induced by receptors that signal via TPL-2, including Toll-like receptor 4 and tumor necrosis factor receptor 1 in macrophages, and CD40 in B cells. ABIN-2 was necessary for the maintenance of TPL-2 protein stability. In contrast, ABIN-2 deficiency did not affect agonist-induced regulation of transcription factor NF-kappaB. Stimulation of ABIN-2-deficient macrophages via Toll-like receptor 4 showed that different thresholds of Erk signaling were required for optimal induction of tumor necrosis factor and interleukin 1beta. Thus, ABIN-2 acts to positively regulate the Erk signaling potential by stabilizing TPL-2.

A comprehensive map of the toll-like receptor signaling network

Recognition of pathogen-associated molecular signatures is critically important in proper activation of the immune system. The toll-like receptor (TLR) signaling network is responsible for innate immune response. In mammalians, there are 11 TLRs that recognize a variety of ligands from pathogens to trigger immunological responses. In this paper, we present a comprehensive map of TLRs and interleukin 1 receptor signaling networks based on papers published so far. The map illustrates the possible existence of a main network subsystem that has a bow-tie structure in which myeloid differentiation primary response gene 88 (MyD88) is a nonredundant core element, two collateral subsystems with small GTPase and phosphatidylinositol signaling, and MyD88-independent pathway. There is extensive crosstalk between the main bow-tie network and subsystems, as well as feedback and feedforward controls. One obvious feature of this network is the fragility against removal of the nonredundant core element, which is MyD88, and involvement of collateral subsystems for generating different reactions and gene expressions for different stimuli.

Innate immune responses in NF-kappaB-repressing factor-deficient mice

NF-kappaB-repressing factor (NRF) is a transcriptional silencer protein that specifically counteracts the basal activity of several NF-kappaB-dependent promoters by direct binding to specific neighboring DNA sequences. In cell culture experiments, the reduction of NRF mRNA leads to a derepression of beta interferon, interleukin-8, and inducible nitric oxide synthase transcription. The X chromosome-located single-copy NRF gene is ubiquitously expressed and encodes a protein of 690 amino acids. The N-terminal part contains a nuclear localization signal, the DNA-binding domain, and the NF-kappaB-repressing domain, while the C-terminal part is responsible for double-stranded RNA binding and nucleolar localization. To study the function of NRF in a systemic context, transgenic mice lacking the NRF gene were created. Against predictions from in vitro experiments, mice with a deletion of the NRF gene are viable and have a phenotype that is indistinguishable from wild-type mice, even after challenge with different pathogens. The data hint towards an unexpected functional redundancy of NRF.

Production and characterization of a monoclonal antibody specific to Nef-associated factor 1 (Naf1)/A20-binding inhibitor of NF-kappaB activation (ABIN-1)

Cellular protein Naf1 (Nef-associated factor 1) or ABIN-1 (A20-binding inhibitor of NF-kappaB activation) is an important cellular protein, expressed in various human tissues and T-cell lines. Naf1 protein has two isoforms (Naf1alpha and Naf1beta) with different C-termini, produced by alternative splicing. Naf1alpha and Naf1beta have approximately 2800 and 2600 nucleotides, with an open reading frame of 1941 and 1781 nucleotides, encoding the 72-kDa Naf1alpha and 68-kDa Naf1beta proteins, respectively. In the present study, we generated a monoclonal antibody (MAb) against human Naf1, which recognizes full-length, endogenous Naf1 of both isotypes. For this purpose, recombinant 6xHis and myc-tagged N-terminal Naf1(38135), Naf1(N) protein was produced by using the baculovirus expression system. Recombinant Naf1(N) protein was used to immunize Balb/c mice, and a hybridoma cell line producing stable and highly specific MAb with strong affinity to Naf1 was established. We further characterized this antibody by immunofluorescent assay and Western blot analysis to confirm effectiveness in detecting recombinant and endogenous Naf1. By Western blot analysis of recombinant Naf1-N fusion proteins with overlapping N-terminal sequences, the epitope targeted by anti-Naf1 MAb was determined as the 81-88-amino acid region of human Naf1.

Adenoviral gene transfer of ABIN-1 protects mice from TNF/galactosamine-induced acute liver failure and lethality

Tumor necrosis factor (TNF) is a proinflammatory cytokine that plays a central role in acute and chronic hepatitis B and C infection and alcoholic liver disease as well as fulminant liver failure. TNF-induced liver failure is characterized by parenchymal cell apoptosis and inflammation leading to liver cell necrosis. The transcription factor NF-kappaB is believed to mediate at least part of the proinflammatory effects of TNF, and is therefore a favorite drug target. However, NF-kappaB also suppresses TNF-mediated hepatocyte apoptosis, implicating a potential cytotoxic effect of NF-kappaB inhibitors in the liver. This dual function of NF-kappaB emphasizes the need for therapeutics that can inhibit both TNF-induced NF-kappaB activation and cell death. Here we describe that adenoviral expression of the NF-kappaB inhibitory protein ABIN-1, but not an IkappaBalpha superrepressor (IkappaBalpha(s)), completely prevents lethality in the TNF/D-(+)-galactosamine-induced model of liver failure. Protection was associated with a significant decrease in TNF-induced leukocyte infiltration as well as hepatocyte apoptosis. The differential effects of ABIN-1 and IkappaBalpha(s) suggest a role for an NF-kappaB independent function of ABIN-1. Indeed, ABIN-1 was found to prevent not only NF-kappaB activation, but also apoptosis of cultured hepatocytes in response to TNF, explaining its protective effect against TNF-induced liver failure. In conclusion, ABIN-1 has a dual NF-kappaB inhibitory and anti-apoptotic activity in the liver, which might be of considerable interest for the treatment of inflammatory liver diseases.

Functional dichotomy of A20 in apoptotic and necrotic cell death

ROS (reactive oxygen species) play important roles in the progression of a number of human pathologies. ROS promote cell death, but can also induce gene transcription. The transcription factor NF-kappaB (nuclear factor kappaB) plays a critical role in oxidative stress responses. One of the proteins regulated by NF-kappaB is the zinc-finger protein A20. In TNF (tumour necrosis factor)-alpha signalling, NF-kappaB induction of A20 leads to increased cell survival. In the present paper, we show that in response to oxidative stress, A20 actually enhances cell death by necrosis, but not by apoptosis. Exposure of cells to ROS leads to the up-regulation of A20 which acts via a negative-feedback loop to block NF-kappaB activation and cellular survival. Silencing of A20 by RNAi (RNA interference) increases both the induction of NF-kappaB and the subsequent survival of cells exposed to high doses of oxidative stress, which, in untreated cells, promotes death by necrosis. Cells which express high basal levels of A20 are less protected from oxidative-stress-induced cell death when compared with cells with lower A20 expression. We also show that A20 regulates NF-kappaB by blocking the degradation of IkappaB (inhibitory protein kappaB) alpha. These data highlight a novel role for A20 in oxidative stress responses by terminating NF-kappaB-dependent survival signalling and thus sensitizing cells to death by necrosis.

Novel markers of the human follicle-associated epithelium identified by genomic profiling and microdissection

BACKGROUND & AIMS

Regulation of gene expression in the follicle-associated epithelium (FAE) over Peyer's patches is largely unknown. CCL20, a chemokine that recruits immature dendritic cells, is one of the few FAE-specific markers described so far. Lymphotoxin beta (LTalpha1beta2) expressed on the membrane of immune cells triggers CCL20 expression in enterocytes. In this study, we measured expression profiles of LTalpha1beta2-treated intestinal epithelial cells and selected CCL20 -coregulated genes to identify new FAE markers.

METHODS

Genomic profiles of T84 and Caco-2 cell lines treated with either LTalpha1beta2, flagellin, or tumor necrosis factor alpha were measured using the Affymetrix GeneChip U133A. Clustering analysis was used to select CCL20 -coregulated genes, and laser dissection microscopy and real-time polymerase chain reaction on human biopsy specimens was used to assess the expression of the selected markers.

RESULTS

Applying a 2-way analysis of variance, we identified regulated genes upon the different treatments. A subset of genes involved in inflammation and related to the nuclear factor kappaB pathway was coregulated with CCL20 . Among these genes, the antiapoptotic factor TNFAIP3 was highly expressed in the FAE. CCL23 , which was not coregulated in vitro with CCL20 , was also specifically expressed in the FAE.

CONCLUSIONS

We have identified 2 novel human FAE specifically expressed genes. Most of the CCL20 -coregulated genes did not show FAE-specific expression, suggesting that other signaling pathways are critical to modulate FAE-specific gene expression.

Gene expression profiling in conjunction with physiological rescues of IKKalpha-null cells with wild type or mutant IKKalpha reveals distinct classes of IKKalpha/NF-kappaB-dependent genes

Cellular responses to stress-like stimuli require the IkappaB kinase (IKK) signalsome (IKKalpha, IKKbeta, and NEMO/IKKgamma) to activate NF-kappaB-dependent genes. IKKbeta and NEMO/IKKgamma are required to release NF-kappaB p65/p50 heterodimers from IkappaBalpha, resulting in their nuclear migration and sequence-specific DNA binding; but IKKalpha was found to be dispensable for this initial phase of canonical NF-kappaB activation. Nevertheless, IKKalpha-/- mouse embryonic fibroblasts (MEFs) fail to express NF-kappaB targets in response to proinflammatory stimuli, uncovering a nuclear role for IKKalpha in NF-kappaB activation. However, it remains unknown whether the global defect in NF-kappaB-dependent gene expression of IKKalpha-/- cells is caused by the absence of IKKalpha kinase activity. We show by gene expression profiling that rescue of near physiological levels of wild type IKKalpha in IKKalpha-/- MEFs globally restores expression of their canonical NF-kappaB target genes. To prove that the kinase activity of IKKalpha was required on a genomic scale, the same physiological rescue was performed with a kinase-dead, ATP binding domain IKKalpha mutant (IKKalpha(K44M)). Remarkably, the IKKalpha(K44M) protein rescued approximately 28% of these genes, albeit in a largely stimulus-independent manner with the notable exception of several genes that also acquired tumor necrosis factor-alpha responsiveness. Thus the IKKalpha-containing signalsome unexpectedly functions in the presence and absence of extracellular signals in both kinase-dependent and -independent modes to differentially modulate the expression of five distinct classes of IKKalpha/NF-kappaB-dependent genes.

A20 is a negative regulator of IFN regulatory factor 3 signaling

IFN regulatory factor 3 (IRF-3) is a critical transcription factor that regulates an establishment of innate immune status following detection of viral pathogens. Recent studies have revealed that two IkappaB kinase (IKK)-like kinases, NF-kappaB-activating kinase/Traf family member-associated NF-kappaB activator-binding kinase 1 and IKK-i/IKKepsilon, are responsible for activation of IRF-3, but the regulatory mechanism of the IRF-3 signaling pathway has not been fully understood. In this study, we report that IRF-3 activation is suppressed by A20, which was initially identified as an inhibitor of apoptosis and inducibly expressed by dsRNA. A20 physically interacts with NF-kappaB-activating kinase/Traf family member-associated NF-kappaB activator-binding kinase 1 and IKK-i/IKKepsilon, and inhibits dimerization of IRF-3 following engagement of TLR3 by dsRNA or Newcastle disease virus infection, leading to suppression of the IFN stimulation response element- and IFN-beta promoter-dependent transcription. Importantly, knocking down of A20 expression by RNA interference results in enhanced IRF-3-dependent transcription triggered by the stimulation of TLR3 or virus infection. Our study thus demonstrates that A20 is a candidate negative regulator of the signaling cascade to IRF-3 activation in the innate antiviral response.

Adenoviral gene transfer of the NF-kappa B inhibitory protein ABIN-1 decreases allergic airway inflammation in a murine asthma model

Airway inflammation is a characteristic of many lung disorders, including asthma and chronic obstructive pulmonary disease. Using a murine model of allergen-induced asthma, we have demonstrated that adenovirus-mediated delivery of the nuclear factor-kappaB (NF-kappaB) inhibitory protein ABIN-1 to the lung epithelium results in a considerable reduction of allergen-induced eosinophil infiltration into the lungs. This is associated with an ABIN-1-induced decrease in allergen-specific immunoglobulin E levels in serum, as well as a significant reduction of eotaxin, interleukin-4, and interleukin-1beta in bronchoalveolar lavage fluid. These findings not only prove that NF-kappaB plays a critical role in the pathogenesis of allergic inflammation but also illustrate that inhibiting NF-kappaB could have therapeutic value in the treatment of asthma and potentially other chronic inflammatory lung diseases.

A20 inhibits NF-κB activation by dual ubiquitin-editing functions

Deregulation of the transcription factor NF-kappaB can mediate several inflammatory diseases in addition to cancer. Therefore, several proteins, including the zinc finger protein A20, tightly control its activation. Recently, the underlying mechanism by which A20 downregulates NF-kappaB activation in response to the pro-inflammatory cytokine tumor necrosis factor (TNF) has been described. A20 was shown to exert two opposing activities: sequential de-ubiquitination and ubiquitination of the TNF receptor-interacting protein (RIP), thereby targeting RIP to proteasomal degradation.

A novel caspase-2 complex containing TRAF2 and RIP1

The enzymatic activity of caspases is implicated in the execution of apoptosis and inflammation. Here we demonstrate a novel nonenzymatic function for caspase-2 other than its reported proteolytic role in apoptosis. Caspase-2, unlike caspase-3, -6, -7, -9, -11, -12, and -14, is a potent inducer of NF-kappaB and p38 MAPK activation in a TRAF2-mediated way. Caspase-2 interacts with TRAF1, TRAF2, and RIP1. Furthermore, we demonstrate that endogenous caspase-2 is recruited into a large and inducible protein complex, together with TRAF2 and RIP1. Structure-function analysis shows that NF-kappaB activation occurs independent of enzymatic activity of the protease and that the caspase recruitment domain of caspase-2 is sufficient for the activation of NF-kappaB and p38 MAPK. These results demonstrate the inducible assembly of a novel protein complex consisting of caspase-2, TRAF2, and RIP1 that activates NF-kappaB and p38 MAPK through the caspase recruitment domain of caspase-2 independently of its proteolytic activity.

Gene expression in IFN-gamma-activated murine macrophages

Macrophages are critical for natural immunity and play a central role in specific acquired immunity. The IFN-gamma activation of macrophages derived from A/J or BALB/c mice yielded two different patterns of antiviral state in murine hepatitis virus 3 infection, which were related to a down-regulation of the main virus receptor. Using cDNA hybridization to evaluate mRNA accumulation in the cells, we were able to identify several genes that are differently up- or down-regulated by IFN-gamma in A/J (267 and 266 genes, respectively, up- and down-regulated) or BALB/c (297 and 58 genes, respectively, up- and down-regulated) mouse macrophages. Macrophages from mice with different genetic backgrounds behave differently at the molecular level and comparison of the patterns of non-activated and IFN-gamma-activated A/J or BALB/c mouse macrophages revealed, for instance, an up-regulation and a down-regulation of genes coding for biological functions such as enzymatic reactions, nucleic acid synthesis and transport, protein synthesis, transport and metabolism, cytoskeleton arrangement and extracellular matrix, phagocytosis, resistance and susceptibility to infection and tumors, inflammation, and cell differentiation or activation. The present data are reported in order to facilitate future correlation of proteomic/transcriptomic findings as well as of results obtained from a classical approach for the understanding of biological phenomena. The possible implication of the role of some of the gene products relevant to macrophage biology can now be further scrutinized. In this respect, a down-regulation of the main murine hepatitis virus 3 receptor gene was detected only in IFN-gamma-activated macrophages of resistant mice.

Linking JNK signaling to NF-kappaB: a key to survival

In addition to marshalling immune and inflammatory responses, transcription factors of the NF-kappaB family control cell survival. This control is crucial to a wide range of biological processes, including B and T lymphopoiesis, adaptive immunity, oncogenesis and cancer chemoresistance. During an inflammatory response, NF-kappaB activation antagonizes apoptosis induced by tumor necrosis factor (TNF)-alpha, a protective activity that involves suppression of the Jun N-terminal kinase (JNK) cascade. This suppression can involve upregulation of the Gadd45-family member Gadd45beta/Myd118, which associates with the JNK kinase MKK7/JNKK2 and blocks its catalytic activity. Upregulation of XIAP, A20 and blockers of reactive oxygen species (ROS) appear to be important additional means by which NF-kappaB blunts JNK signaling. These recent findings might open up entirely new avenues for therapeutic intervention in chronic inflammatory diseases and certain cancers; indeed, the Gadd45beta-MKK7 interaction might be a key target for such intervention.

DNA microarray analysis of gene expression in alveolar epithelial cells in response to TNFalpha, LPS, and cyclic stretch

Recent evidence suggests that alveolar epithelial cells (AECs) may contribute to the development, propagation, and resolution of acute lung injury (ALI)/acute respiratory distress syndrome (ARDS). Proinflammatory cytokines, pathogen products, and injurious mechanical ventilation are important contributors of excessive inflammatory responses in the lung. In the present study, we used cDNA microarrays to define the gene expression patterns of A549 cells (an AEC line) in the early stages of three models of pulmonary parenchymal cell activation: cells treated with tumor necrosis factor-alpha (TNFalpha) (20 ng/ml), lipopolysaccharide (LPS, 1 microg/ml), or cyclic stretch (20% elongation) for either 1 h or 4 h. Differential gene expression profiles were determined by gene array analysis. TNFalpha induced an inflammatory response pattern, including induction of genes for chemokines, inflammatory mediators, and cell surface membrane proteins. TNFalpha also increased genes related to pro- and anti-apoptotic proteins, signal transduction proteins, and transcriptional factors. TNFalpha further induced a group of genes that may form a negative feedback loop to silence the NFkappaB pathway. Stimulation of AECs with mechanical stretch changed cell morphology and activated Src protein tyrosine kinase. The combination of TNFalpha plus stretch enhanced or attenuated expression of multiple genes. LPS decreased microfilament polymerization but had less impact on NFkappaB translocation and gene expression. Results from this study indicate that AECs can tailor their response to different stimuli or/and combination of stimuli and subsequently play an important role in acute inflammatory responses in the lung.

Angiotensin II early regulated genes in H295R human adrenocortical cells

Evidence for the dysregulation of aldosterone synthesis in cardiovascular pathophysiology has renewed interest in the control of its production. Cellular mechanisms by which angiotensin II (ANG II) stimulates aldosterone synthesis in the adrenal zona glomerulosa are incompletely understood. To elucidate the mechanism of intracellular signaling by ANG II stimulation in the adrenal, we have studied immediate-early regulated genes in human adrenal H295R cells using cDNA microarrays. H295R cells were stimulated with ANG II for 3 h. Gene expression was analyzed by microarray technology and validated by real-time RT-PCR. Eleven genes were found to be upregulated by ANG II. These encode the proteins for ferredoxin, Nor1, Nurr1, c6orf37, CAT-1, A20, MBLL, M-Ras, RhoB, GADD45α, and a novel protein designated FLJ45273 . Maximum expression levels for all genes occurred 3–6 h after ANG II stimulation. This increase was dose dependent and preceded maximal aldosterone production. Other aldosterone secretagogues, K+and endothelin-1 (ET-1), also induced the expression of these genes with variable efficiency depending on the gene and with lower potency than ANG II. ACTH had negligible effect on gene expression except for the CAT-1 and Nurr1 genes. These ANG II-stimulated genes are involved in several cellular functions and are good candidate effectors and regulators of ANG II-mediated effects in adrenal zona glomerulosa.

A20 is a potent inhibitor of TLR3- and Sendai virus-induced activation of NF-κB and ISRE and IFN-β promoter

Toll-like receptor 3 (TLR3) recognizes dsRNA generated during viral infection and activation of TLR3 results in induction of type I interferons (IFNs) and cellular anti-viral response. TLR3 is associated with a TIR domain-containing adapter protein TRIF, which activates distinct downstream pathways leading to activation of NF-kappaB and ISRE sites in the promoters of type I IFNs. We show here that A20, a NF-kappaB-inducible zinc finger protein that has been demonstrated to be an inhibitor of TNF-induced NF-kappaB activation and a physiological suppressor of inflammatory response, potently inhibited TLR3- and Sendai virus-mediated activation of ISRE and NF-kappaB and IFN-beta promoter in reporter gene assays. A20 also inhibited TRIF-, but not its downstream signaling components TBK1-, IKKbeta-, and IKKepsilon-mediated activation of ISRE and NF-kappaB and IFN-beta promoter. Moreover, A20 interacted with TRIF in co-immunoprecipitation experiments. Finally, expression of A20 could be induced at protein level by Sendai virus infection. These data suggest that A20 targets TRIF to inhibit TLR3-mediated induction of IFN-beta transcription and functions as a feedback negative regulator for TLR3 signaling and cellular anti-viral response.

A20 protects endothelial cells from TNF-, Fas-, and NK-mediated cell death by inhibiting caspase 8 activation

A20 is a stress response gene in endothelial cells (ECs). A20 serves a dual cytoprotective function, protecting from tumor necrosis factor (TNF)-mediated apoptosis and inhibiting inflammation via blockade of the transcription factor nuclear factor-kappaB (NF-kappaB). In this study, we evaluated the molecular basis of the cytoprotective function of A20 in EC cultures and questioned whether its protective effect extends beyond TNF to other apoptotic and necrotic stimuli. Our data demonstrate that A20 targets the TNF apoptotic pathway by inhibiting proteolytic cleavage of apical caspases 8 and 2, executioner caspases 3 and 6, Bid cleavage, and release of cytochrome c, thus preserving mitochondrion integrity. A20 also protects from Fas/CD95 and significantly blunts natural killer cell-mediated EC apoptosis by inhibiting caspase 8 activation. In addition to protecting ECs from apoptotic stimuli, A20 safeguards ECs from complement-mediated necrosis. These data demonstrate, for the first time, that the cytoprotective effect of A20 in ECs is not limited to TNF-triggered apoptosis. Rather, A20 affords broad EC protective functions by effectively shutting down cell death pathways initiated by inflammatory and immune offenders.

The inhibitor ABIN-2 disrupts the interaction of receptor-interacting protein with the kinase subunit IKKgamma to block activation of the transcription factor NF-kappaB and potentiate apoptosis

NF-kappaB (nuclear factor kappaB) proteins are key transcription factors that regulate gene expression in response to various extracellular stimuli. The pathway leading to the activation of NF-kappaB involves a complicated network that includes a number of signalling molecules. The recent identification of a wide range of negative regulators of NF-kappaB has given another layer of complexity in NF-kappaB activation. We and others have previously identified the protein ABIN-2 (A20 binding inhibitor of NF-kappaB 2) as an inhibitor of NF-kappaB activation. In the present paper, we demonstrate that ABIN-2 exerts its inhibitory function by blocking the interaction of RIP (receptor-interacting protein) with the downstream effector IKKgamma, a non-kinase component of the IkappaB (inhibitory kappaB) kinase complex. When overexpressed in cells, ABIN-2 bound to IKKgamma and prevented the association of IKKgamma with RIP. By a deletion mapping, a stretch of 50 amino acids on ABIN-2 is found to be essential for its interaction with IKKgamma. The ABIN-2 mutant that lacked these 50 amino acids did not interact with IKKgamma and, consequently, failed to inhibit NF-kappaB activation. Strikingly, a portion of RIP, which is similar to this 50-residue domain of ABIN-2, is also essential for RIP interaction with IKKgamma. The RIP mutant with deletion of this similar region did not associate with IKKgamma and had substantial reduction of its ability to mediate NF-kappaB activation. Taken together, these conserved 50 residues of ABIN-2 and RIP define a novel structural domain in mediating a key step in the NF-kappaB signalling pathway through the interaction with IKKgamma. Finally, the signalling pathway of NF-kappaB activation is known to promote survival in many cellular events. The mechanism for decision between cell death and survival is under fine regulation. In the present paper, we demonstrated further that the expression of ABIN-2 could promote the RIP-mediated apoptosis by presumably suppressing the anti-apoptotic effect of NF-kappaB.