NF-kappaB activator Act1 associates with IL-1/Toll pathway adaptor molecule TRAF6

IL-23/IL-17 immune axis mediates the imiquimod-induced psoriatic inflammation by activating ACT1/TRAF6/TAK1/NF-κB pathway in macrophages and keratinocytes

The interleukin-23 (IL-23)/IL-17 immune axis has been linked to the pathology of psoriasis, but how this axis contributes to skin inflammation in this disease remains unclear. We measured inflammatory cytokines associated with the IL-23/IL-17 immune axis in the serum of patients with psoriasis using enzyme-linked immunosorbent assays. Psoriasis was induced in male C57BL/6J mice using imiquimod (IMQ) cream, and animals received intraperitoneal injections of recombinant mouse anti-IL-23A or anti-IL-17A antibodies for 7 days. The potential effects of the IL-23/IL-17 immune axis on skin inflammation were assessed based on pathology scoring, hematoxylin-eosin staining of skin samples, and quantitation of inflammatory cytokines. Western blotting was used to evaluate levels of the following factors in skin: ACT1, TRAF6, TAK1, NF-κB, and pNF-κB. The serum of psoriasis patients showed elevated levels of several cytokines involved in the IL-23/IL-17 immune axis: IL-2, IL-4, IL-8, IL-12, IL-17, IL-22, IL-23, and interferon-γ. Levels of IL-23p19 and IL-17 were increased in serum and skin of IMQ-treated mice, while ACT1, TRAF6, TAK1, NF-κB, and pNF-κB were upregulated in the skin. A large proportion of NF-κB p65 localized in nucleus of involucrin+ cells in the epidermis and in F4/80+ cells of the dermis of psoriatic lesional skin. Treating these animals with anti-IL-23 or anti-IL-17 antibodies improved pathological score and immune imbalance, mitigated skin inflammation and downregulated ACT1, TRAF6, TAK1, NF-κB, and pNF-κB in skin. Our results suggest that skin inflammation mediated by the IL-23/IL-17 immune axis in psoriasis involves activation of the ACT1/TRAF6/TAK1/NF-κB pathway in keratinocytes and macrophage.

Neddylation-Independent Activities of MLN4924

MLN4924, also known as pevonedistat, is a highly selective small-molecule inhibitor of NEDD8 (neuronal precursor cell-expressed developmentally downregulated protein 8)-activating enzyme (NAE) to block the entire neddylation modification cascade, leading to inactivation of cullin-RING ligases (CRLs), since activation of CRLs requires cullin neddylation. MLN4924 showed impressive anticancer activity in many preclinical studies and is currently in several Phase I/II clinical trials for anticancer therapy as a single agent or in combination with chemotherapeutic drugs.In addition to well-characterized anti-neddylation activity, recent studies showed that MLN4924 has several neddylation-independent activities. First, MLN4924 triggers EGFR dimerization to activate EGFR and its downstream RAS/MAPK and PI3K/AKT1 signals, leading to enhanced tumor sphere formation, accelerated EGF-mediated wound healing, and inhibited ciliogenesis. Second, MLN4924 induces PKM2 tetramerization to promote glycolysis, thus affecting energy metabolism. Third, MLN4924 inhibits the interaction between ACT1 (NF-κB activator 1) and TRAF6 (tumor necrosis factor receptor-associated factor 6) and attenuates IL-17A-mediated activation of NF-κB to reduce pulmonary inflammation. Fourth, MLN4924 inhibits IRF3 binding to the IFN-β promoter to inhibit IFN-β production. And finally, MLN4924 activates the JNK signaling pathway to reduce c-FLIP levels, thus enhancing TRAIL-induced apoptosis. This chapter will summarize these neddylation-independent activities of MLN4924 and discuss the underlying mechanisms and potential therapeutic applications.

IL-17 stimulates the expression of CCL2 in cardiac myocytes via Act1/TRAF6/p38MAPK-dependent AP-1 activation

IL-17 participates in the development of many autoimmune diseases by promoting the expression of some chemokines. Chemokine C-C motif ligand 2 (CCL2) is an important factor at the infiltration of mononuclear cells in the myocardial tissue of viral myocarditis (VMC). It was found that IL-17 could aggravate myocardial injury by upregulating CCL2. But the underlying mechanism involved in CCL2 secretion induced by IL-17 in cardiac myocytes remains unclear. This study investigated the role of transcription factor AP-1 in IL-17 induced CCL2 expression. The results showed that IL-17 mediated the activation of Act1, TRAF6, p38MAPK and c-Jun/AP-1 not Wnt or PI3K signalling pathway to upregulate CCL2 expression in cardiac myocytes. After blocking Act1/TRAF6/p38MAPK cascade and interfering AP-1 with Curcumin or c-Jun siRNA, CCL2 expression induced by IL-17 was significantly attenuated at both mRNA and protein levels. Furthermore, the phosphorylation of c-Jun was suppressed when cardiac myocytes were treated with Act1 siRNA, TRAF6 siRNA, SB203580 (p38MAPK inhibitor) or SP600125 (JNK inhibitor) in cardiac myocytes. In conclusion, IL-17 could stimulate the expression of CCL2 in cardiac myocytes via Act1/TRAF6/p38MAPK-dependent AP-1 activation, which may provide a new target for the diagnosis and treatment of VMC.

Immune Control by TRAF6-Mediated Pathways of Epithelial Cells in the EIME (Epithelial Immune Microenvironment)

In the protective responses of epithelial tissues, not only immune cells but also non-immune cells directly respond to external agents. Epithelial cells can be involved in the organization of immune responses through two phases. First, the exogenous harmful agents trigger the primary responses of the epithelial cells leading to various types of immune cell activation. Second, cytokines produced by the immune cells that are activated directly by the external agents and indirectly by the epithelial cell products elicit the secondary responses giving rise to further propagation of immune responses. TRAF6 is a ubiquitin E3 ligase, which intermediates between various types of receptors for exogenous agents or endogenous mediators and activation of subsequent transcriptional responses via NF-kappaB and MAPK pathways. TRAF6 ubiquitously participates in many protective responses in immune and non-immune cells. Particularly, epithelial TRAF6 has an essential role in the primary and secondary responses via driving type 17 response in psoriatic inflammation of the skin. Consistently, many psoriasis susceptibility genes encode the TRAF6 signaling players, such as ACT1 (TRAF3IP2), A20 (TNFAIP3), ABIN1 (TNIP1), IL-36Ra (IL36RN), IkappaBzeta (NFKBIZ), and CARD14. Herein, we describe the principal functions of TRAF6, especially in terms of positive and regulatory immune controls by interaction between immune cells and epithelial cells. In addition, we discuss how TRAF6 in the epithelial cells can organize the differentiation of immune responses and drive inflammatory loops in the epithelial immune microenvironment, which is termed EIME.

RECK suppresses interleukin-17/TRAF3IP2-mediated MMP-13 activation and human aortic smooth muscle cell migration and proliferation

Sustained inflammation and matrix metalloproteinase (MMP) activation contribute to vascular occlusive/proliferative disorders. Interleukin-17 (IL-17) is a proinflammatory cytokine that signals mainly via TRAF3 Interacting Protein 2 (TRAF3IP2), an upstream regulator of various critical transcription factors, including AP-1 and NF-κB. Reversion inducing cysteine rich protein with kazal motifs (RECK) is a membrane-anchored MMP inhibitor. Here we investigated whether IL-17A/TRAF3IP2 signaling promotes MMP-13-dependent human aortic smooth muscle cell (SMC) proliferation and migration, and determined whether RECK overexpression blunts these responses. Indeed, IL-17A treatment induced (a) JNK, p38 MAPK, AP-1, NF-κB, and CREB activation, (b) miR-21 induction, (c) miR-27b and miR-320 inhibition, (d) MMP-13 expression and activation, (e) RECK suppression, and (f) SMC migration and proliferation, all in a TRAF3IP2-dependent manner. In fact, gain of TRAG3IP2 function, by itself, induced MMP-13 expression and activation, and RECK suppression. Furthermore, treatment with recombinant MMP-13 stimulated SMC migration in part via ERK activation. Importantly, RECK gain-of-function attenuated MMP-13 activity without affecting its mRNA or protein levels, and inhibited IL-17A- and MMP-13-induced SMC migration. These results indicate that increased MMP-13 and decreased RECK contribute to IL-17A-induced TRAF3IP2-dependent SMC migration and proliferation, and suggest that TRAF3IP2 inhibitors or RECK inducers have the potential to block the progression of neointimal thickening in hyperplastic vascular diseases.

Molecular characterization and expression analysis of tumor necrosis factor receptor-associated factors 3 and 6 in large yellow croaker (Larimichthys crocea)

The large yellow croaker (Larimichthys crocea) has a well-developed innate immune system. To gain a better understanding of the defense mechanisms involved in this system, we studied tumor necrosis factor receptor-associated factors (TRAFs), which play important roles in the Toll-like receptor (TLR) pathway. We characterized the full-length open reading frames and protein structures of TRAF3 and TRAF6 to determine their identities, and conducted phylogenetic analysis to determine their evolutionary relationships. To assess the roles of TRAFs in innate immune responses in the large yellow croaker, we performed quantitative reverse-transcription PCR (qRT-PCR) to characterize expression profiles in a range of tissues at different stages after challenge with polyinosinic polycytidylic acid (poly I:C) and Vibrio anguillarum. Following poly I:C challenge, the expression levels of TRAF3 and TRAF6 were highest in the kidneys and lowest in the spleen, whereas after infection with V. anguillarum, TRAF6 expression was the highest in the kidneys and lowest in the liver.

A Novel Nuclear Function for the Interleukin-17 Signaling Adaptor Protein Act1

In the context of the human airway, interleukin-17A (IL-17A) signaling is associated with severe inflammation, as well as protection against pathogenic infection, particularly at mucosal surfaces such as the airway. The intracellular molecule Act1 has been demonstrated to be an essential mediator of IL-17A signaling. In the cytoplasm, it serves as an adaptor protein, binding to both the intracellular domain of the IL-17 receptor as well as members of the canonical nuclear factor kappa B (NF-κB) pathway. It also has enzymatic activity, and serves as an E3 ubiquitin ligase. In the context of airway epithelial cells, we demonstrate for the first time that Act1 is also present in the nucleus, especially after IL-17A stimulation. Ectopic Act1 expression can also increase the nuclear localization of Act1. Act1 can up-regulate the expression and promoter activity of a subset of IL-17A target genes in the absence of IL-17A signaling in a manner that is dependent on its N- and C-terminal domains, but is NF-κB independent. Finally, we show that nuclear Act1 can bind to both distal and proximal promoter regions of DEFB4, one of the IL-17A responsive genes. This transcriptional regulatory activity represents a novel function for Act1. Taken together, this is the first report to describe a non-adaptor function of Act1 by directly binding to the promoter region of IL-17A responsive genes and directly regulate their transcription.

Cardiac-restricted Overexpression of TRAF3 Interacting Protein 2 (TRAF3IP2) Results in Spontaneous Development of Myocardial Hypertrophy, Fibrosis, and Dysfunction

TRAF3IP2 (TRAF3 interacting protein 2; previously known as CIKS or Act1) is a key intermediate in the normal inflammatory response and the pathogenesis of various autoimmune and inflammatory diseases. Induction of TRAF3IP2 activates IκB kinase (IKK)/NF-κB, JNK/AP-1, and c/EBPβ and stimulates the expression of various inflammatory mediators with negative myocardial inotropic effects. To investigate the role of TRAF3IP2 in heart disease, we generated a transgenic mouse model with cardiomyocyte-specific TRAF3IP2 overexpression (TRAF3IP2-Tg). Echocardiography, magnetic resonance imaging, and pressure-volume conductance catheterization revealed impaired cardiac function in 2-month-old male transgenic (Tg) mice as evidenced by decreased ejection fraction, stroke volume, cardiac output, and peak ejection rate. Moreover, the male Tg mice spontaneously developed myocardial hypertrophy (increased heart/body weight ratio, cardiomyocyte cross-sectional area, GATA4 induction, and fetal gene re-expression). Furthermore, TRAF3IP2 overexpression resulted in the activation of IKK/NF-κB, JNK/AP-1, c/EBPβ, and p38 MAPK and induction of proinflammatory cytokines, chemokines, and extracellular matrix proteins in the heart. Although myocardial hypertrophy decreased with age, cardiac fibrosis (increased number of myofibroblasts and enhanced expression and deposition of fibrillar collagens) increased progressively. Despite these adverse changes, TRAF3IP2 overexpression did not result in cell death at any time period. Interestingly, despite increased mRNA expression, TRAF3IP2 protein levels and activation of its downstream signaling intermediates remained unchanged in the hearts of female Tg mice. The female Tg mice also failed to develop myocardial hypertrophy. In summary, these results demonstrate that overexpression of TRAF3IP2 in male mice is sufficient to induce myocardial hypertrophy, cardiac fibrosis, and contractile dysfunction.

Tumor necrosis factor receptor- associated factor 6 (TRAF6) regulation of development, function, and homeostasis of the immune system

Tumor necrosis factor receptor (TNFR)-associated factor 6 (TRAF6) is an adapter protein that mediates a wide array of protein-protein interactions via its TRAF domain and a RING finger domain that possesses non-conventional E3 ubiquitin ligase activity. First identified nearly two decades ago as a mediator of interleukin-1 receptor (IL-1R)-mediated activation of NFκB, TRAF6 has since been identified as an actor downstream of multiple receptor families with immunoregulatory functions, including members of the TNFR superfamily, the Toll-like receptor (TLR) family, tumor growth factor-β receptors (TGFβR), and T-cell receptor (TCR). In addition to NFκB, TRAF6 may also direct activation of mitogen-activated protein kinase (MAPK), phosphoinositide 3-kinase (PI3K), and interferon regulatory factor pathways. In the context of the immune system, TRAF6-mediated signals have proven critical for the development, homeostasis, and/or activation of B cells, T cells, and myeloid cells, including macrophages, dendritic cells, and osteoclasts, as well as for organogenesis of thymic and secondary lymphoid tissues. In multiple cellular contexts, TRAF6 function is essential not only for proper activation of the immune system but also for maintaining immune tolerance, and more recent work has begun to identify mechanisms of contextual specificity for TRAF6, involving both regulatory protein interactions, and messenger RNA regulation by microRNAs.

Trans-system mechanisms against ischemic myocardial injury

A mammalian organism possesses a hierarchy of naturally evolved protective mechanisms against ischemic myocardial injury at the molecular, cellular, and organ levels. These mechanisms comprise regional protective processes, including upregulation and secretion of paracrine cell-survival factors, inflammation, angiogenesis, fibrosis, and resident stem cell-based cardiomyocyte regeneration. There are also interactive protective processes between the injured heart, circulation, and selected remote organs, defined as trans-system protective mechanisms, including upregulation and secretion of endocrine cell-survival factors from the liver and adipose tissue as well as mobilization of bone marrow, splenic, and hepatic cells to the injury site to mediate myocardial protection and repair. The injured heart and activated remote organs exploit molecular and cellular processes, including signal transduction, gene expression, cell proliferation, differentiation, migration, mobilization, and/or extracellular matrix production, to establish protective mechanisms. Both regional and trans-system cardioprotective mechanisms are mediated by paracrine and endocrine messengers and act in coordination and synergy to maximize the protective effect, minimize myocardial infarction, and improve myocardial function, ensuring the survival and timely repair of the injured heart. The concept of the trans-system protective mechanisms may be generalized to other organ systems-injury in one organ may initiate regional as well as trans-system protective responses, thereby minimizing injury and ensuring the survival of the entire organism. Selected trans-system processes may serve as core protective mechanisms that can be exploited by selected organs in injury. These naturally evolved protective mechanisms are the foundation for developing protective strategies for myocardial infarction and injury-induced disorders in other organ systems.

Orphan receptor IL-17RD regulates Toll-like receptor signalling via SEFIR/TIR interactions

Receptor families of the innate immune response engage in 'cross-talk' to tailor optimal immune responses against invading pathogens. However, these responses are subject to multiple levels of regulation to keep in check aberrant inflammatory signals. Here, we describe a role for the orphan receptor interleukin-17 receptor D (IL-17RD) in negatively regulating Toll-like receptor (TLR)-induced responses. Deficiency of IL-17RD expression in cells leads to enhanced pro-inflammatory signalling and gene expression in response to TLR stimulation, and Il17rd(-/-) mice are more susceptible to TLR-induced septic shock. We demonstrate that the intracellular Sef/IL-17R (SEFIR) domain of IL-17RD targets TIR adaptor proteins to inhibit TLR downstream signalling thus revealing a paradigm involving cross-regulation of members of the IL-17R and TLR families.

IL-17 cytokines in immunity and inflammation

Interleukin 17 (IL-17) and its closest relative, IL-17F, have recently drawn much attention in the field of immunology. IL-17 and IL-17F are expressed by a distinct type of T cells, T helper 17 cells and certain other lymphocytes. These cytokines play key regulatory roles in host defense and inflammatory diseases. In this review, we summarize the recent findings in IL-17 biology and the progress towards understanding the regulatory mechanisms of IL-17 expression and signaling mechanisms. This knowledge will benefit the development of novel immune modulators that enhance immunity to various infections and reduce inflammatory damage in infected patients.

IL-17 receptor adaptor protein Act1/CIKS plays an evolutionarily conserved role in antiviral signaling

Double-stranded RNA-induced antiviral gene expression in mammalian cells requires activation of IFN regulatory factor 3 (IRF3). In this study, we show that the IL-17R adaptor protein Act1/CIKS is involved in this process. Small interfering RNA-mediated knockdown of Act1 in primary human skin fibroblasts specifically attenuates expression of IFN-β and IFN-stimulated antiviral genes induced by a synthetic viral mimic, polyinosinic-polycytidylic acid. Ectopic expression of Act1 potentiates the IRF3-driven expression of a synthetic reporter construct as well as the induction of antiviral genes. We demonstrate that this effect is dependent on the ability of Act1 to functionally and physically interact with IκB kinase ε (IKKε), a known IRF3 kinase, and IRF3: 1) Act1 binds IKKε and IRF3; 2) Act1-induced IRF3 activation can be blocked specifically by coexpression of a catalytically inactive mutant of IKKε; and 3) mutants of IRF3, either lacking the C terminus or mutated at the key phosphorylation sites, important for its activation by IKKε, do not support Act1-dependent IRF3 activation. We also show that a zebrafish Act1 protein is able to trigger antiviral gene expression in human cells, which suggests an evolutionarily conserved function of vertebrate Act1 in the host defense against viruses. On the whole, our study demonstrates that Act1 is a component of antiviral signaling.

Function of Act1 in IL-17 family signaling and autoimmunity

The maintenance of immune homeostasis requires the delicate balance between response to foreign antigens and tolerance to self. As such, when this balance is disrupted, immunodeficiency or autoimmunity may manifest. The adaptor molecule known as Act1 is a critical mediator of IL-17 receptor receptor family signaling. This chapter will detail the current understanding of Act1 's role in signal transduction as well as address the fundamental role of Act1 in autoimmunity. At the molecular level Act1 interacts with IL-17 R through the conserved SEFIR domain, binds TRAF proteins and exerts E3 ubiquitin ligase activity. In in vivo models, Act1 deficiency provides protection against experimental autoimmune diseases, such as colitis and EAE. Yet mice lacking in Act1 develop spontaneous autoimmune diseases. Indeed, the utility of Act1 seems to rely on the specific cell type expression that may determine the pathway that Act1 mediates.

Cell type- and stimulus-specific mechanisms for post-transcriptional control of neutrophil chemokine gene expression

mRNAs encoding inflammatory chemokines that recruit neutrophils frequently exhibit short half-lives that serve to limit their expression under inappropriate conditions but are often prolonged to ensure adequate levels during inflammatory response. Extracellular stimuli that modulate the stability of such mRNAs may be the same as the transcriptional activator, as is the case with TLR ligands, or may cooperate with independent transcriptional stimuli, as with IL-17, which extends the half-life of TNF-induced transcripts. These different stimuli engage independent signaling pathways that target different instability mechanisms distinguished by dependence on different regulatory nucleotide sequence motifs within the 3'UTRs, which involve that action of different mRNA-binding proteins. The selective use of these pathways by different stimuli and in distinct cell populations provides the potential for tailoring of chemokine expression patterns to meet specific needs in different pathophysiologic circumstances.

Dynamic evolution of CIKS (TRAF3IP2/Act1) in metazoans

CIKS (TRAF3IP2/Act1) is important for inflammatory responses and autoimmunity control through its dual functions in CD40L/BAFF and IL17 signaling in mammalians. In this study, we performed comparative and evolutionary analyses of CIKSs from metazoans. Although nematode (Caenorabditis elegans) and sea urchin (Strongylocentrotus purpuratus) have IL17 and IL17 receptors, we found no CIKS in their genomes. The ancient CIKS-like (CIKSL) genes from the invertebrates lottia (Lottia gigantea) and amphioxus (Branchiostoma floridae) have an additional DEATH domain compared with other CIKSLs/CIKSs. Our data suggest that the ancient CIKSL evolved into early chordate CIKS possibly through gene tandem duplication and gene fission. Based on phylogenetic and synteny analyses, vertebrate CIKS genes are divided into two groups, one of which is orthologous to human CIKS and the other is paralogous. Expression analysis indicated that cephalochordata amphioxus IL17 together with CIKS might play an ancient and conserved role in host defense against bacterial infections. During the evolutionary process, the CIKS genes have obtained more and more functions through cooperation with other genes.

E3 ubiquitin ligase CHIP facilitates Toll-like receptor signaling by recruiting and polyubiquitinating Src and atypical PKC{zeta}

In mouse macrophages and dendritic cells, the CHIP E3 ubiquitin ligase is needed for transduction of signals initiated by TLR4 and TLR9 stimulation.

The carboxyl terminus of constitutive heat shock cognate 70 (HSC70)–interacting protein (CHIP, also known as Stub1) is a U box–containing E3 ubiquitin ligase that is important for protein quality control. The role of CHIP in innate immunity is not known. Here, we report that CHIP knockdown inhibits Toll-like receptor (TLR) 4– and TLR9-driven signaling, but not TLR3-driven signaling; proinflammatory cytokine and type 1 interferon (IFN) production; and maturation of antigen-presenting cells, including macrophages and dendritic cells. We demonstrate that CHIP can recruit the tyrosine kinase Src and atypical protein kinase C ζ (PKCζ) to the TLR complex, thereby leading to activation of IL-1 receptor–associated kinase 1, TANK-binding kinase 1, and IFN regulatory factors 3 and 7. CHIP acts as an E3 ligase for Src and PKCζ during TLR signaling. CHIP-mediated enhancement of TLR signaling is inhibited by IFNAR deficiency or expression of ubiquitination resistant mutant forms of Src or PKCζ. These findings suggest that CHIP facilitates the formation of a TLR signaling complex by recruiting, ubiquitinating, and activating Src and PKCζ.

Molecular mechanisms of TNFR-associated factor 6 (TRAF6) utilization by the oncogenic viral mimic of CD40, latent membrane protein 1 (LMP1)

Latent membrane protein 1 (LMP1), encoded by Epstein-Barr virus, is required for EBV-mediated B cell transformation and plays a significant role in the development of posttransplant B cell lymphomas. LMP1 has also been implicated in exacerbation of autoimmune diseases such as systemic lupus erythematosus. LMP1 is a constitutively active functional mimic of the tumor necrosis factor receptor superfamily member CD40, utilizing tumor necrosis factor receptor-associated factor (TRAF) adaptor proteins to induce signaling. However, LMP1-mediated B cell activation is amplified and sustained compared with CD40. We have previously shown that LMP1 and CD40 use TRAFs 1, 2, 3, and 5 differently. TRAF6 is important for CD40 signaling, but the role of TRAF6 in LMP1 signaling in B cells is not clear. Although TRAF6 binds directly to CD40, TRAF6 interaction with LMP1 in B cells has not been characterized. Here we tested the hypothesis that TRAF6 is a critical regulator of LMP1 signaling in B cells, either as part of a receptor-associated complex and/or as a cytoplasmic adaptor protein. Using TRAF6-deficient B cells, we determined that TRAF6 was critical for LMP1-mediated B cell activation. Although CD40-mediated TRAF6-dependent signaling does not require the TRAF6 receptor-binding domain, we found that LMP1 signaling required the presence of this domain. Furthermore, TRAF6 was recruited to the LMP1 signaling complex via the TRAF1/2/3/5 binding site within the cytoplasmic domain of LMP1.

IL-17-induced NF-kappaB activation via CIKS/Act1: physiologic significance and signaling mechanisms

Interleukin-17 (IL-17) is essential in host defense against extracellular bacteria and fungi, especially at mucosal sites, but it also contributes significantly to inflammatory and autoimmune disease pathologies. Binding of IL-17 to its receptor leads to recruitment of adaptor protein CIKS/Act1 via heterotypic association of their respective SEFIR domains and activation of transcription factor NF-κB; it is not known whether CIKS and/or NF-κB are required for all gene induction events. Here we report that CIKS is essential for all IL-17-induced immediate-early genes in primary mouse embryo fibroblasts, whereas NF-κB is profoundly involved. We also identify a novel subdomain in the N terminus of CIKS that is essential for IL-17-mediated NF-κB activation. This domain is both necessary and sufficient for interaction between CIKS and TRAF6, an adaptor required for NF-κB activation. The ability of decoy peptides to block this interaction may provide a new therapeutic strategy for intervention in IL-17-driven autoimmune and inflammatory diseases.

Differential TRAF3 utilization by a variant human CD40 receptor with enhanced signaling

CD40 is required for T cell-dependent humoral immunity, but it can also contribute to the pathogenesis of autoimmunity and B cell malignancy. The TNFR-associated factor (TRAF)2 and TRAF6 adaptor proteins are positive regulators of CD40 signaling required to activate downstream kinase cascades and transcription factors. In contrast, TRAF3 can serve as a negative regulator of CD40 signaling, and CD40 signals are amplified in TRAF3(-/-) B cells. We previously reported a gain-of-function polymorphism of the human CD40 receptor, hCD40-P227A, which signals in an amplified manner to B lymphocytes. In this study, we show that hCD40-P227A binds more TRAF3 and TRAF5, as well as certain associated proteins, than wild-type-CD40. Studies in TRAF-deficient B cell lines revealed that hCD40-P227A uses TRAF3 as a positive rather than negative regulator. Although located outside of any known TRAF binding sites, the P227A polymorphism can alter TRAF binding and dramatically changes the role played by TRAF3 in CD40 signaling.

An atopic dermatitis-like skin disease with hyper-IgE-emia develops in mice carrying a spontaneous recessive point mutation in the Traf3ip2 (Act1/CIKS) gene

Spontaneous mutant mice that showed high levels of serum IgE and an atopic dermatitis (AD)-like skin disease were found in a colony of the KOR inbred strain that was derived from Japanese wild mice. No segregation was observed between hyper-IgE-emia and dermatitis in (BALB/c x KOR mutant) N(2) mice, suggesting that the mutation can be attributed to a single recessive locus, which we designated adjm (atopic dermatitis from Japanese mice). All four adjm congenic strains in different genetic backgrounds showed both hyper-IgE-emia and dermatitis, although the disease severity varied among strains. Linkage analysis using (BALB/c x KOR-adjm/adjm) N(2) mice restricted the potential adjm locus to the 940 kb between D10Stm216 and D10Stm238 on chromosome 10. Sequence analysis of genes located in this region revealed that the gene AI429613, which encodes the mouse homologue of the human TNFR-associated factor 3-interacting protein 2 (TRAF3IP2) protein (formerly known as NF-kappaB activator 1/connection to IkappaB kinase and stress-activated protein kinase/Jun kinase), carried a single point mutation leading to the substitution of a stop codon for glutamine at amino acid position 214. TRAF3IP2 has been shown to function as an adaptor protein in signaling pathways mediated by the TNFR superfamily members CD40 and B cell-activating factor in epithelial cells and B cells as well as in the IL-17-mediated signaling pathway. Our results suggest that malfunction of the TRAF3IP2 protein causes hyper-IgE-emia through the CD40- and B cell-activating factor-mediated pathway in B cells and causes skin inflammation through the IL-17-mediated pathway. This study demonstrates that the TRAF3IP2 protein plays an important role in AD and suggests the protein as a therapeutic target to treat AD.

Diversity in post-transcriptional control of neutrophil chemoattractant cytokine gene expression

Regulation of neutrophil chemokine gene expression represents an important feature in tissue inflammation. While chemokine gene transcription through the action of NFkappaB is recognized as an essential component of this process, it is now clear that post-transcriptional mechanisms, particularly the rates of decay of mature cytoplasmic mRNA, provides an essential component of this control. Chemokine and other cytokine mRNA half life is known to be controlled via adenine-uridine rich sequence motifs localized within 3' untranslated regions (UTRs), the most common of which contains one or more copies of the pentameric AUUUA sequence. In myeloid cells AUUUA sequences confer instability through the action of RNA binding proteins such as tristetraprolin (TTP). The resulting instability can be regulated in response to extra-cellular stimuli including Toll like receptor ligands that signal to control the function of TTP through pathways involving the activation of p38 MAP kinases. Recent findings indicate that substantial mechanistic diversity is operative in non-myeloid cells in response to alternate pro-inflammatory stimuli such as IL-17. These pathways target distinct instability sequences that do not contain the AUUUA pentamer motif, do not signal through p38 MAPK, and function independently of TTP.

IL-17F: regulation, signaling and function in inflammation

The IL-17 cytokine family is composed of six members. IL-17F, discovered in 2001, recently has drawn increasing attention due to its greatest similarity to IL-17, a widely recognized inflammatory cytokine. The genes encoding IL-17 and IL-17F are localized in the same chromosomal region and are co-expressed by CD4+ and gammadelta T cells. IL-17F can be secreted as homodimers or heterodimers with IL-17. Similar to IL-17, IL-17F utilizes IL-17RA and IL-17RC as its receptor and employs Act1 and TRAF6 as its signal transducers to induce the expression of pro-inflammatory cytokines and chemokines in many different cell types. However, mice lacking either IL-17 or IL-17F exhibit distinct defects in experimental models of asthma and colitis. These results have laid the basis to understand the role of IL-17F in the pathogenesis of human diseases.

A role for the cytoplasmic adaptor protein Act1 in mediating IL-17 signaling

Interleukin (IL)-17 (also known as IL-17A) plays an important role in host defense and inflammatory disorders, in part by linking the activation of a subset of T lymphocytes to the mobilization of neutrophils and macrophages. IL-17 exerts its effects both directly and indirectly; the latter by stimulating the production of various chemokines, IL-6, and growth factors from resident cells in the affected tissue. As a result, IL-17 coordinates the innate immune response to extracellular bacteria, which is interesting because IL-17 is produced by several types of T cells that are traditionally regarded as key players in adaptive immunity. Studies have uncovered the function and relevance of a unique subset of CD4(+) T helper (Th) cells that produce IL-17 (Th17 cells), but our understanding of the function of IL-17 receptors (IL-17Rs) and their downstream signaling pathways remains poor. This Review discusses studies that suggest that the cytoplasmic adaptor protein Act1 [nuclear factor-kappaB (NF-kappaB) activator 1] is essential for linking stimulation of IL-17Rs to downstream signaling pathways, and, therefore, that Act1 might play a role in local inflammatory responses. Act1 mediates activation of NF-kappaB and the subsequent production of IL-6 and chemokines that are chemotactic for neutrophils and macrophages. These findings have increased our understanding of host defense against bacteria and indicated a role for Act1 in mediating in chronic inflammatory disease. Future studies on Act1 and IL-17 signaling should contribute to the identification and improved understanding of the mechanisms behind aberrant innate immune responses in chronic inflammatory disease.

Act1 adaptor protein is an immediate and essential signaling component of interleukin-17 receptor

Interleukin (IL)-17, the founding member of the IL-17 cytokine family, is the hallmark of a novel subset of CD4+ T cells that is regulated by TGFbeta, IL-6, and IL-23. IL-17 plays an important role in promoting tissue inflammation in host defense against infection and in autoimmune diseases. Although IL-17 has been reported to regulate the expression of proinflammatory cytokines, chemokines, and matrix metalloproteinases, the signaling mechanism of IL-17 receptor has not been understood. An earlier study found that IL-17 activates NF-kappaB and MAPK pathways and requires TRAF6 to induce IL-6. However, it is unknown what molecule(s) directly associates with IL-17 receptor to initiate the signaling. We demonstrate here that IL-17 receptor family shares sequence homology in their intracellular region with Toll-IL-1 receptor (TIR) domains and with Act1, a novel adaptor previously reported as an NF-kappaB activator. MyD88 and IRAK4, downstream signaling components of TIR, are not required for IL-17 signaling. On the other hand, Act1 and IL-17 receptor directly associate likely via homotypic interaction. Deficiency of Act1 in fibroblast abrogates IL-17-induced cytokine and chemokine expression, as well as the induction of C/EBPbeta, C/EBPdelta, and IkappaBzeta. Also, absence of Act1 results in a selective defect in IL-17-induced activation of NF-kappaB pathway. These results thus indicate Act1 as a membrane-proximal adaptor of IL-17 receptor with an essential role in induction of inflammatory genes. Our study not only for the first time reveals an immediate signaling mechanism downstream of an IL-17 family receptor but also has implications in therapeutic treatment of various immune diseases.

Discoidin domain receptor 1: a new class of receptor regulating leukocyte-collagen interaction

Previous studies demonstrated that type I collagen, a major component of the extracellular matrix, could influence the differentiation and function of leukocytes; however, it is not clear whether those effects of collagen were based on its interaction with the classic collagen receptors, alpha1beta1 and alpha2beta1 integrins. We recently detected significant upregulation of discoidin domain receptor 1 (DDR1), a new class of collagen receptor, in human leukocytes, including neutrophils, monocytes, and lymphocytes, in vitro, leading to the hypothesis that the leukocyte-activating effects of collagen might be owing to its interaction with DDR1. In this review, we summarize our recent findings demonstrating that DDR1-collagen interaction facilitates the adhesion, migration, differentiation/maturation, and cytokine/chemokine production of leukocytes. We also describe the intracellular signaling pathways activated by DDR1 interaction with collagen.

Signaling to NF-kappaB

The transcription factor NF-kappaB has been the focus of intense investigation for nearly two decades. Over this period, considerable progress has been made in determining the function and regulation of NF-kappaB, although there are nuances in this important signaling pathway that still remain to be understood. The challenge now is to reconcile the regulatory complexity in this pathway with the complexity of responses in which NF-kappaB family members play important roles. In this review, we provide an overview of established NF-kappaB signaling pathways with focus on the current state of research into the mechanisms that regulate IKK activation and NF-kappaB transcriptional activity.

Multiple gene differential expression patterns in human ischemic liver: Safe limit of warm ischemic time

AIM: To investigate the multiple gene differential expression patterns in human ischemic liver and to produce the evidence about the hepatic ischemic safety time.

METHODS: The responses of cells to hepatic ischemia and hypoxia at hepatic ischemia were analyzed by cDNA microarrary representing 4000 different human genes containing 200 apoptotic correlative genes.

RESULTS: There were lower or normal expression levels of apoptotic correlative genes during the periods of hepatic ischemia for 0-15 min, the maintenance homostatic genes were expressed significantly higher at the same time. But at the hepatic ischemia for 30 min, the expression levels of maintenance homeostatic genes were down-regulated, the expressions of many apoptotic correlative genes and nuclear transcription factors were activated and up-regulated.

CONCLUSION: HIF-1, APAF-1, PCDC10, FBX5, DFF40, DFFA XIAP, survivin may be regarded as the signal genes to judge the degree of hepatic ischemic-hypoxic injure, and the apoptotic liver cell injury due to ischemia in different time limits. The safe limit of human hepatic warm ischemic time appears to be generally less then 30 min.

Activation of discoidin domain receptor 1 isoform b with collagen up-regulates chemokine production in human macrophages: role of p38 mitogen-activated protein kinase and NF-kappa B

Macrophages produce an array of proinflammatory mediators at sites of inflammation and contribute to the development of inflammatory responses. Important roles for cytokines, such as IL-1 or TNF-alpha, and bacterial products, such as LPS, in this process have been well documented; however, the role for the extracellular matrix proteins, such as collagen, remains unclear. We previously reported that discoidin domain receptor 1 (DDR1), a nonintegrin collagen receptor, is expressed during differentiation of human monocytes into macrophages, and the interaction of the DDR1b isoform with collagen facilitates their differentiation via the p38 mitogen-activated protein kinase (MAPK) pathway. In this study, we report that the interaction of DDR1b with collagen up-regulates the production of IL-8, macrophage inflammatory protein-1alpha, and monocyte chemoattractant protein-1 in human macrophages in a p38 MAPK- and NF-kappaB-dependent manner. p38 MAPK was critical for DDR1b-mediated, increased NF-kappaB trans-activity, but not for IkappaB degradation or NF-kappaB nuclear translocation, suggesting a role for p38 MAPK in the modification of NF-kappaB. DDR1b-mediated IkappaB degradation was mediated through the recruitment of the adaptor protein Shc to the LXNPXY motif of the receptor and the downstream TNFR-associated factor 6/NF-kappaB activator 1 signaling cascade. Taken together, our study has identified NF-kappaB as a novel target of DDR1b signaling and provided a novel mechanism by which tissue-infiltrating macrophages produce large amounts of chemokines during the development of inflammatory diseases. Intervention of DDR1b signaling may be useful to control inflammatory diseases in which these proteins play an important role.

The PDZ protein tax-interacting protein-1 inhibits beta-catenin transcriptional activity and growth of colorectal cancer cells

Wnt signaling is essential during development while deregulation of this pathway frequently leads to the formation of various tumors including colorectal carcinomas. A key component of the pathway is beta-catenin that, in association with TCF-4, directly regulates the expression of Wnt-responsive genes. To identify novel binding partners of beta-catenin that may control its transcriptional activity, we performed a mammalian two-hybrid screen and isolated the Tax-interacting protein (TIP-1). The in vivo complex formation between beta-catenin and TIP-1 was verified by coimmunoprecipitation, and a direct physical association was revealed by glutathione S-transferase pull-down experiments in vitro. By using a panel of deletion mutants of both proteins, we demonstrate that the interaction is mediated by the PDZ (PSD-95/DLG/ZO-1 homology) domain of TIP-1 and requires primarily the last four amino acids of beta-catenin. TIP-1 overexpression resulted in a dose-dependent decrease in the transcriptional activity of beta-catenin when tested on the TOP/FOPFLASH reporter system. Conversely, siRNA-mediated knock-down of endogenous TIP-1 slightly increased endogenous beta-catenin transactivation function. Moreover, we show that overexpression of TIP-1 reduced the proliferation and anchorage-independent growth of colorectal cancer cells. These data suggest that TIP-1 may represent a novel regulatory element in the Wnt/beta-catenin signaling pathway.

Role of the adaptor protein CIKS in the activation of the IKK complex

Nuclear factor kappaB (NF-kappaB) plays a pivotal role in numerous cellular processes, including stress response, inflammation, and protection from apoptosis. Therefore, the activity of NF-kappaB needs to be tightly regulated. We have previously identified a novel gene, named CIKS (connection to IkappaB-kinase and SAPK), able to bind the regulatory sub-unit NEMO/IKKgamma and to activate NF-kappaB. Here, we demonstrate that CIKS forms homo-oligomers, interacts with NEMO/IKKgamma, and is recruited to the IKK-complex upon cell stimulation. In addition, we identified the regions of CIKS responsible for these functions. We found that the ability of CIKS to oligomerize, and to be recruited to the IKK-complex is not sufficient to activate the NF-kappaB. In fact, a deletion mutant of CIKS able to oligomerize, to interact with NEMO/IKKgamma, and to be recruited to the IKK-complex does not activate NF-kappaB, suggesting that CIKS needs a second level of regulation to efficiently activate NF-kappaB.