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

Protective effect of TNIP2 on the inflammatory response of microglia after spinal cord injury in rats

BACKGROUND

Spinal cord injury (SCI) is a devastating disease that can lead to tissue loss and neurological dysfunction. TNIP2 is a negative regulator of NF-κB signaling due to its capacity to bind A20 and suppress inflammatory cytokines-induced NF-κB activation. However, the anti-inflammatory role of TNIP2 in SCI remains unclear. Our study's intention was to evaluate the effect of TNIP2 on the inflammatory response of microglia after spinal cord injury in rats.

METHODS

HE staining and Nissl staining were performed on day 3 following SCI to analyze the histological changes. To further investigate the functional changes of TNIP2 after SCI, we performed immunofluorescence staining experiments. The effect of LPS on TNIP2 expression in BV2 cells was examined by western blot. The levels of TNF-α, IL-1β, and IL-6 in spinal cord tissues of rats with SCI and in BV2 cells with LPS were measured by using qPCR.

RESULTS

TNIP2 expression was closely associated with the pathophysiology of SCI in rats, and TNIP2 was involved in regulating functional changes in microglia. TNIP2 expression was increased during SCI in rats and that overexpression of TNIP2 inhibited M1 polarization and pro-inflammatory cytokine production in microglia, which might ultimately protect against inflammatory responses through the MAPK and NF-κB signaling pathways.

CONCLUSIONS

The present study provides evidence for a role of TNIP2 in the regulation of inflammation in SCI and suggests that induction of TNIP2 expression alleviated the inflammatory response of microglia.

Unlocking the Untapped Potential of Endothelial Kinase and Phosphatase Involvement in Sepsis for Drug Treatment Design

Sepsis is a devastating clinical condition that can lead to multiple organ failure and death. Despite advancements in our understanding of molecular mechanisms underlying sepsis and sepsis-associated multiple organ failure, no effective therapeutic treatment to directly counteract it has yet been established. The endothelium is considered to play an important role in sepsis. This review highlights a number of signal transduction pathways involved in endothelial inflammatory activation and dysregulated endothelial barrier function in response to sepsis conditions. Within these pathways – NF-κB, Rac1/RhoA GTPases, AP-1, APC/S1P, Angpt/Tie2, and VEGF/VEGFR2 – we focus on the role of kinases and phosphatases as potential druggable targets for therapeutic intervention. Animal studies and clinical trials that have been conducted for this purpose are discussed, highlighting reasons why they might not have resulted in the expected outcomes, and which lessons can be learned from this. Lastly, opportunities and challenges that sepsis and sepsis-associated multiple organ failure research are currently facing are presented, including recommendations on improved experimental design to increase the translational power of preclinical research to the clinic.

Transcriptome Profiling of Bovine Macrophages Infected by Mycobacterium avium spp. paratuberculosis Depicts Foam Cell and Innate Immune Tolerance Phenotypes

Mycobacterium avium spp. paratuberculosis (MAP) is the causative agent of Johne's disease (JD), also known as paratuberculosis, in ruminants. The mechanisms of JD pathogenesis are not fully understood, but it is known that MAP subverts the host immune system by using macrophages as its primary reservoir. MAP infection in macrophages is often studied in healthy cows or experimentally infected calves, but reports on macrophages from naturally infected cows are lacking. In our study, primary monocyte-derived macrophages (MDMs) from cows diagnosed as positive (+) or negative (–) for JD were challenged in vitro with live MAP. Analysis using next-generation RNA sequencing revealed that macrophages from JD(+) cows did not present a definite pattern of response to MAP infection. Interestingly, a considerable number of genes, up to 1436, were differentially expressed in JD(–) macrophages. The signatures of the infection time course of 1, 4, 8, and 24 h revealed differential expression of ARG2, COL1A1, CCL2, CSF3, IL1A, IL6, IL10, PTGS2, PTX3, SOCS3, TNF, and TNFAIP6 among other genes, with major effects on host signaling pathways. While several immune pathways were affected by MAP, other pathways related to hepatic fibrosis/hepatic stellate cell activation, lipid homeostasis, such as LXR/RXR (liver X receptor/retinoid X receptor) activation pathways, and autoimmune diseases (rheumatoid arthritis or atherosclerosis) also responded to the presence of live MAP. Comparison of the profiles of the unchallenged MDMs from JD(+) vs. JD(–) cows showed that 868 genes were differentially expressed, suggesting that these genes were already affected before monocytes differentiated into macrophages. The downregulated genes predominantly modified the general cell metabolism by downregulating amino acid synthesis and affecting cholesterol biosynthesis and other energy production pathways while introducing a pro-fibrotic pattern associated with foam cells. The upregulated genes indicated that lipid homeostasis was already supporting fat storage in uninfected JD(+) MDMs. For JD(+) MDMs, differential gene expression expounds long-term mechanisms established during disease progression of paratuberculosis. Therefore, MAP could further promote disease persistence by influencing long-term macrophage behavior by using both tolerance and fat-storage states. This report contributes to a better understanding of MAP's controls over the immune cell response and mechanisms of MAP survival.

Regulation of NF-κB by the p105-ABIN2-TPL2 complex and RelAp43 during rabies virus infection

At the crossroad between the NF-κB and the MAPK pathways, the ternary complex composed of p105, ABIN2 and TPL2 is essential for the host cell response to pathogens. The matrix protein (M) of field isolates of rabies virus was previously shown to disturb the signaling induced by RelAp43, a NF-κB protein close to RelA/p65. Here, we investigated how the M protein disturbs the NF-κB pathway in a RelAp43-dependant manner and the potential involvement of the ternary complex in this mechanism. Using a tandem affinity purification coupled with mass spectrometry approach, we show that RelAp43 interacts with the p105-ABIN2-TPL2 complex and we observe a strong perturbation of this complex in presence of M protein. M protein interaction with RelAp43 is associated with a wide disturbance of NF-κB signaling, involving a modulation of IκBα-, IκBβ-, and IκBε-RelAp43 interaction and a favored interaction of RelAp43 with the non-canonical pathway (RelB and p100/p52). Monitoring the interactions between host and viral proteins using protein-fragment complementation assay and bioluminescent resonance energy transfer, we further show that RelAp43 is associated to the p105-ABIN2-TPL2 complex as RelAp43-p105 interaction stabilizes the formation of a complex with ABIN2 and TPL2. Interestingly, the M protein interacts not only with RelAp43 but also with TPL2 and ABIN2. Upon interaction with this complex, M protein promotes the release of ABIN2, which ultimately favors the production of RelAp43-p50 NF-κB dimers. The use of recombinant rabies viruses further indicates that this mechanism leads to the control of IFNβ, TNF and CXCL2 expression during the infection and a high pathogenicity profile in rabies virus infected mice. All together, our results demonstrate the important role of RelAp43 and M protein in the regulation of NF-κB signaling.

Rabies virus is a recurring zoonosis responsible for about 60,000 deaths per year. A key feature of rabies virus is its stealth, allowing it to spread within the host and escape the immune response. To do so, rabies virus developed several mechanisms, including a thorough interference with cell signaling pathways. Here, we focused our attention on the molecular aspects of rabies virus escape to the NF-κB pathway through the interaction between the M protein and the NF-κB protein RelAp43. Monitoring close range interactions, we found that RelAp43 plays an important role in the stabilization of the p105-ABIN2-TPL2 complex, which is essential in the regulation of both NF-κB and MAPK pathways, and we brought a new insight on the dynamics within the host protein complex. These results were confirmed in living cells and in mice. Overall, our data suggest that rabies virus interference with the p105-ABIN2-TPL2 complex is a cornerstone of its stealth strategy to escape the immune response.

TNIP2 is a Hub Protein in the NF-κB Network with Both Protein and RNA Mediated Interactions

The NF-κB family of transcription factors is pivotal in controlling cellular responses to environmental stresses; abnormal nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) signaling features in many autoimmune diseases and cancers. Several components of the NF-κB signaling pathway have been reported to interact with the protein TNIP2 (also known as ABIN2), and TNIP2 can both positively and negatively regulate NF-κB- dependent transcription of target genes. However, the function of TNIP2 remains elusive and the cellular machinery associating with TNIP2 has not been systematically defined. Here we first used a broad MudPIT/Halo Affinity Purification Mass Spectrometry (AP-MS) approach to map the network of proteins associated with the NF-κB transcription factors, and establish TNIP2 as an NF-κB network hub protein. We then combined AP-MS with biochemical approaches in a more focused study of truncated and mutated forms of TNIP2 to map protein associations with distinct regions of TNIP2. NF-κB interacted with the N-terminal region of TNIP2. A central region of TNIP2 interacted with the endosomal sorting complex ESCRT-I via its TSG101 subunit, a protein essential for HIV-1 budding, and a single point mutant in TNIP2 disrupted this interaction. The major gene ontology category for TNIP2 associated proteins was mRNA metabolism, and several of these associations, like KHDRBS1, were lost upon depletion of RNA. Given the major association of TNIP2 with mRNA metabolism proteins, we analyzed the RNA content of affinity purified TNIP2 using RNA-Seq. Surprisingly, a specific limited number of mRNAs was associated with TNIP2. These RNAs were enriched for transcription factor binding, transcription factor cofactor activity, and transcription regulator activity. They included mRNAs of genes in the Sin3A complex, the Mediator complex, JUN, HOXC6, and GATA2. Taken together, our findings suggest an expanded role for TNIP2, establishing a link between TNIP2, cellular transport machinery, and RNA transcript processing.

USP35 activated by miR let-7a inhibits cell proliferation and NF-κB activation through stabilization of ABIN-2

Ubiquitin specific protease 35 (USP35) is a member of deubiquitylases (DUBs). It remains largely unknown about the biological role and the regulation mechanism of USP35. Here, we first identified miR let-7a as a positive regulator of USP35 expression and showed that USP35 expression positively correlates with miR let-7a expression in different cancer cell lines and tissues. Then, we showed that USP35 expression was decreased dramatically in the tumor tissues compared with the adjacent non-cancerous tissues. USP35 overexpression inhibited cell proliferation in vitro and inhibited xenograft tumor growth in vivo. Furthermore, we revealed that USP35 acts as a functional DUB and stabilizes TNFAIP3 interacting protein 2 (ABIN-2) by promoting its deubiquitination. Functionally, both ABIN-2 and USP35 could inhibit TNFα-induced NF-κB activation and overexpression of ABIN-2 alleviated USP35-loss induced activation of NF-κB. Collectively, our data indicated that miR let-7a-regulated USP35 can inhibit NF-κB activation by deubiquitination and stabilization of ABIN-2 protein and eventually inhibit cell proliferation. Overall, our study provides a novel rationale of targeting miR let-7a-USP35-ABIN-2 pathway for the therapy of cancer patients.

The biology of A20-binding inhibitors of NF-kappaB activation (ABINs)

The family of A20-Binding Inhibitors of NF-kappaB (ABINs) consists of three proteins, ABIN-1, ABIN-2 and ABIN-3, which were originally identified as A20-binding proteins and inhibitors of cytokines and Lipopolysaccharide (LPS) induced NF-kappaB activation. ABIN family members have limited sequence homology in a number of short regions that mediate A20-binding, ubiquitin-binding, and NF-kappaB inhibition. The functional role of A20 binding to ABINs remains unclear, although an adaptor function has been suggested. ABIN-1 and ABIN-3 expression is upregulated when cells are triggered by NF-kappaB-activating stimuli, suggesting a role for these ABINs in a negative feedback regulation of NF-kappaB signaling. Additional ABIN functions have been reported such as inhibition of TNF-induced hepatocyte apoptosis, regulation of HIV-1 replication for ABIN-1, and Tumor Progression Locus 2 (TPL-2)-mediated Extracellular signal-Regulated Kinase (ERK) activation for ABIN-2. In mice, ABIN-1 overexpression reduces allergic airway inflammation and TNF-mediated liver injury, ABIN-2 overexpression delays liver regeneration, and ABIN-3 overexpression partially protects against LPS-induced acute liver failure. Analysis of mice deficient in ABIN-1 or ABIN-2 demonstrates the important immune regulatory function of ABINs. Future studies should clarify the functional implication of the A20-ABIN interaction in supporting ABINs' mechanisms of action.

Mechanisms of neuroblastoma cell growth inhibition by CARP-1 functional mimetics

Neuroblastomas (NBs) are a clinically heterogeneous group of extra cranial pediatric tumors. Patients with high-risk, metastatic NBs have a long-term survival rate of below 40%, and are often resistant to current therapeutic modalities. Due to toxic side effects associated with radiation and chemotherapies, development of new agents is warranted to overcome resistance and effectively treat this disease in clinic. CARP-1 functional mimetics (CFMs) are an emerging class of small molecule compounds that inhibit growth of diverse cancer cell types. Here we investigated NB inhibitory potential of CFMs and the molecular mechanisms involved. CFM-1, -4, and -5 inhibited NB cell growth, in vitro, independent of their p53 and MYCN status. CFM-4 and -5 induced apoptosis in NB cells in part by activating pro-apoptotic stress-activated kinases (SAPKs) p38 and JNK, stimulating CARP-1 expression and cleavage of PARP1, while promoting loss of the oncogenes C and N-myc as well as mitotic cyclin B1. Treatments of NB cells with CFM-4 or -5 also resulted in loss of Inhibitory κB (IκB) α and β proteins. Micro-RNA profiling revealed upregulation of XIAP-targeting miR513a-3p in CFM-4-treated NB, mesothelioma, and breast cancer cells. Moreover, exposure of NB and breast cancer cells to CFM-4 or -5 resulted in diminished expression of anti-apoptotic XIAP1, cIAP1, and Survivin proteins. Expression of anti-miR513a-5p or miR513a-5p mimic, however, interfered with or enhanced, respectively, the breast cancer cell growth inhibition by CFM-4. CFMs also impacted biological properties of the NB cells by blocking their abilities to migrate, form colonies in suspension, and invade through the matrix-coated membranes. Our studies indicate anti-NB properties of CFM-4 and 5, and suggest that these CFMs and/or their future analogs have potential as anti-NB agents.

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

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

CARP-1 functional mimetics are a novel class of small molecule inhibitors of malignant pleural mesothelioma cells

Malignant pleural mesothelioma (MPM) is an asbestos-related thoracic malignancy that is characterized by late metastases, and resistance to therapeutic modalities. The toxic side-effects of MPM therapies often limit their clinical effectiveness, thus necessitating development of new agents to effectively treat and manage this disease in clinic. CARP-1 functional mimetics (CFMs) are a novel class of compounds that inhibit growth of diverse cancer cell types. Here we investigated MPM cell growth suppression by the CFMs and the molecular mechanisms involved. CFM-1, -4, and -5 inhibited MPM cell growth, in vitro, in part by stimulating apoptosis. Apoptosis by CFM-4 involved activation of pro-apoptotic stress-activated protein kinases (SAPKs) p38 and JNK, elevated CARP-1 expression, cleavage of PARP1, and loss of the oncogene c-myc as well as mitotic cyclin B1. Treatments of MPM cells with CFM-4 resulted in depletion of NF-κB signaling inhibitor ABIN1 and Inhibitory κB (IκB)α and β, while increasing expression of pro-apoptotic death receptor (DR) 4 protein. CFM-4 enhanced expression of serine-phosphorylated podoplanin and cleavage of vimetin. CFMs also attenuated biological properties of the MPM cells by blocking their abilities to migrate, form colonies in suspension, and invade through the matrix-coated membranes. Both podoplanin and vimentin regulate processes of cell motility and invasion, and their expression often correlates with metastatic disease, and poor prognosis. The fact that phosphorylation of serines in the cytoplasmic domain of podoplanin interferes with processes of cellular motility, CFM-4-dependent elevated phosphorylated podoplanin and cleavage of vimentin underscore a metastasis inhibitory property of these compounds, and suggest that CFMs and/or their future analogs have potential as anti-MPM agents.

CARP-1 functional mimetics: a novel class of small molecule inhibitors of medulloblastoma cell growth

Medulloblastomas (MBs) constitute an aggressive class of intracranial pediatric tumors. Current multimodality treatments for MBs include surgery, ionizing radiation, and chemotherapy. Toxic side effects of therapies coupled with high incidence of recurrence and the metastatic spread warrant development of more effective, less toxic therapies for this disease. CARP-1/CCAR1 is a peri-nuclear phospho-protein that is a co-activator of the cell cycle regulatory anaphase promoting complex/cyclosome (APC/C) E3 ligase. CARP-1 functional mimetics (CFMs) are a novel class of small molecule compounds that interfere with CARP-1 binding with APC/C subunit APC-2, and suppress growth of a variety of cancer cells in part by promoting apoptosis. Here we investigated MB growth inhibitory potential of the CFMs and found that CFM-4 inhibits growth of MB cells in part by inducing CARP-1 expression, promoting PARP cleavage, activating pro-apoptotic stress-activated protein kinases (SAPK) p38 and JNK, and apoptosis. Gene-array-based analysis of the CFM-4-treated Daoy MB cells indicated down-regulation of a number of key cell growth and metastasis-promoting genes including cell motility regulating small GTP binding protein p21Rac1, and extracellular matrix metallopeptidase (MMP)-10. Moreover, CFM-4 treatment stimulated expression of a number of molecules such as neurotrophin (NTF)3, and NF-κB signaling inhibitors ABIN1 and 2 proteins. Overexpression of NTF3 resulted in reduced MB cell viability while knock-down of NTF3 interfered with CFM-4-dependent loss of viability. CFMs also attenuated biological properties of the MB cells by blocking their abilities to migrate, form colonies in suspension, and invade through the matrix-coated membranes. Together our data support anti-MB properties of CFM-4, and provide a proof-of-concept basis for further development of CFMs as potential anti-cancer agents for MBs.

A20-binding inhibitor of nuclear factor-kappaB (NF-kappaB)-2 (ABIN-2) is an activator of inhibitor of NF-kappaB (IkappaB) kinase alpha (IKKalpha)-mediated NF-kappaB transcriptional activity

NF-κB transcription factors are pivotal players in controlling inflammatory and immune responses, as well as cell proliferation and apoptosis. Aberrant regulation of NF-κB and the signaling pathways that regulate its activity have been involved in various pathologies, particularly cancers, as well as inflammatory and autoimmune diseases. NF-κB activation is tightly regulated by the IκB kinase (IKK) complex, which is composed of two catalytic subunits IKKα and IKKβ, and a regulatory subunit IKKγ/NEMO. Although IKKα and IKKβ share structural similarities, IKKα has been shown to have distinct biological functions. However, the molecular mechanisms that modulate IKKα activity have not yet been fully elucidated. To understand better the regulation of IKKα activity, we purified IKKα-associated proteins and identified ABIN-2. Here, we demonstrate that IKKα and IKKβ both interact with ABIN-2 and impair its constitutive degradation by the proteasome. Nonetheless, ABIN-2 enhances IKKα- but not IKKβ-mediated NF-κB activation by specifically inducing IKKα autophosphorylation and kinase activity. Furthermore, we found that ABIN-2 serine 146 is critical for the ABIN-2-dependent IKKα transcriptional up-regulation of specific NF-κB target genes. These results imply that ABIN-2 acts as a positive regulator of NF-κB-dependent transcription by activating IKKα.

Phosphorylation-dependent association of the G4-1/G5PR regulatory subunit with IKKβ negatively modulates NF-κB activation through recruitment of protein phosphatase 5

The transcription factor NF-κB (nuclear factor κB) co-ordinates various gene expressions in response to diverse signals and is a critical regulator of inflammation and innate immunity. Several negative regulators of NF-κB have been identified as downstream targets of NF-κB and function as a feedback control of NF-κB activation. A few protein phosphatases have also been shown to inactivate NF-κB activation. However, little is known about how protein phosphatases detect and respond to NF-κB activation. In the present study, we report a regulatory subunit of PP5 (protein phosphatase 5), G4-1, that physically interacts with IKKβ [IκB (inhibitor of NF-κB) kinase β] and negatively regulates NF-κB activation. The association of G4-1 with IKKβ depends on the kinase activity of IKKβ. Mapping of the G4-1-binding domain of IKKβ reveals that the serine-rich domain in the C-terminus of IKKβ is required for G4-1 binding. When seven autophosphorylated serine residues in this domain were mutated to alanine, the mutant form of IKKβ lost its ability to bind G4-1 and was more potent than the wild-type kinase to activate NF-κB. Knockdown of G4-1 enhanced TNFα (tumour necrosis factor α)-induced NF-κB activity, and knockdown of PP5 totally abolished the inhibitory activity of G4-1 on NF-κB activation. The results of the present study suggest that G4-1 functions as an adaptor to recruit PP5 to the phosphorylated C-terminus of activated IKKβ and to down-regulate the activation of IKKβ.

Proteins that bind to IKKgamma (NEMO) and down-regulate the activation of NF-kappaB

Inhibitor of kappaB kinase (IKK) gamma (IKKgamma), also referred to as nuclear factor kappaB (NF-kappaB) essential modulator (NEMO), is an important component of the IKK complex. Following the exposure of cells to NF-kappaB-inducing stimuli, the IKK complex catalyzes the phosphorylation of inhibitor of kappaB (IkappaB) proteins, which is a critical step that leads to the activation of NF-kappaB via the canonical pathway. The exact functions of IKKgamma as part of the IKK complex have not been fully elucidated. A number of proteins have been identified as directly interacting with IKKgamma and modulating the activity of the IKK complex. This mini review covers eight proteins that have been reported to bind to IKKgamma and lead to the suppression of the activities of the IKK complex and hence result in the down-regulation of the activation of NF-kappaB. The reported mechanisms by which these interactions suppress the activation of the IKK complex include the deubiquitination of IKKgamma and competition with upstream activators for binding to IKKgamma.

miR-146a induces differentiation of periodontal ligament cells

Differentiation of periodontal ligament (PDL) cells occurs under specific induction; furthermore, NF-kappaB signaling is important for regulation of bone differentiation. MicroRNAs are small non-coding RNAs that repress the translation of target genes and modulate cellular processes. miR-146a has been reported to modulate NF-kappaB signaling. This study hypothesized that miR-146a has a regulatory role in PDL differentiation by affecting NF-kappaB signaling. Immortalized PDL (I-PDL) cell lines were established by exogenous telomerase expression. The genesis of alkaline phosphatase and the up-regulation of miR-146a were induced by ascorbic acid in the I-PDL cells and primary PDL cells. I-PDL cells with exogenous miR-146a expression showed attenuation of NF-kappaB activity and exhibited higher differentiation relative to the controls. Exogenous NF-kappaB expression decreased the expression of differentiation markers, while the inactivation of endogenous NF-kappaB increased alkaline phosphatase in I-PDL cells. This study concludes that miR-146a promotes the differentiation in PDL cells through the down-regulation of NF-kappaB signaling.

Angiopoietin-1/Tie2 signaling pathway inhibits lipopolysaccharide-induced activation of RAW264.7 macrophage cells

Angiopoietin-1 (Ang1) is a ligand for the endothelial-specific tyrosine kinase receptor Tie2 and has been shown to play an essential role in embryonic vasculature development. There have been many studies about the anti-inflammatory effects of Ang1, most of which focus on endothelium cells. In the present study, we explore the role of Ang1-Tie2 signaling in the activation of macrophages upon lipopolysaccharide (LPS) stimulation. We found that Tie2 receptor is expressed on macrophages and Ang1 could inhibit LPS-induced activation of macrophages, as evidenced by cell migration and TNF-alpha production, specifically through Tie2 receptor. We further investigated the mechanism and found that Ang1-Tie2 could block LPS-induced activation of NF-kappaB which has been shown to be necessary for macrophage activation with LPS treatment. Thus, we described, for the first time, the role of Ang1-Tie2 signaling in macrophage activation and the possible mechanisms in response to immune stimulation.

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.

Inhibition of NF-kappaB signaling via tyrosine phosphorylation of Ymer

Cytoplasmic zinc finger protein A20 functionally dampens inflammatory signals and apoptosis via inhibition of NF-kappaB activation. We have reported that Ymer interacts with A20 and lysine (K)-63-linked polyubiquitin chain and that Ymer inhibits NF-kappaB signaling in collaboration with A20. It has also been reported that Ymer is phosphorylated by EGF stimulation. We found that Ymer was considerably phosphorylated on tyrosine residues also via Src family kinases such as Lck. A luciferase reporter assay showed that mutation of tyrosines on Ymer (YmerY217/279/304F) results in loss of the inhibitory activity for NF-kappaB signaling. Furthermore, a soft agar colony formation assay showed that the combination of SrcY527F and YmerY217/279/304F has no ability for anchorage-independent growth, suggesting that tyrosine phosphorylation of Ymer is important for inhibition of the NF-kappaB-mediated apoptotic pathway. These findings demonstrate that Ymer is likely to be a negative regulator for the NF-kappaB signaling pathway.

Positive and negative signaling components involved in TNFalpha-induced NF-kappaB activation

Tumor Necrosis Factor alpha (TNFalpha) is a pro-inflammatory cytokine that plays important roles in different biological processes, including the induction of other cytokines. One of the most important downstream signaling targets activated by TNFalpha is the NF-kappaB transcription factor, which has been identified to be involved in inflammatory, anti-apoptotic, and immune responses. Stimulation of cells with TNFalpha triggers activation of NF-kappaB through various signaling molecules, including TRAF2, RIP, MAP3K, and the IKK complex. Recently, numerous studies have been performed to explore the detailed mechanism by which NF-kappaB is activated upon TNFalpha stimulation. Current understanding of this pathway has been focused on the identification of signaling components, the role of post-translational modification and the sub-cellular translocation of those components. Additionally, more negative regulators in the TNF-IKK pathway are emerging.

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.

RIP1, a kinase on the crossroads of a cell's decision to live or die

Binding of inflammatory cytokines to their receptors, stimulation of pathogen recognition receptors by pathogen-associated molecular patterns, and DNA damage induce specific signalling events. A cell that is exposed to these signals can respond by activation of NF-kappaB, mitogen-activated protein kinases and interferon regulatory factors, resulting in the upregulation of antiapoptotic proteins and of several cytokines. The consequent survival may or may not be accompanied by an inflammatory response. Alternatively, a cell can also activate death-signalling pathways, resulting in apoptosis or alternative cell death such as necrosis or autophagic cell death. Interplay between survival and death-promoting complexes continues as they compete with each other until one eventually dominates and determines the cell's fate. RIP1 is a crucial adaptor kinase on the crossroad of these stress-induced signalling pathways and a cell's decision to live or die. Following different upstream signals, particular RIP1-containing complexes are formed; these initiate only a limited number of cellular responses. In this review, we describe how RIP1 acts as a key integrator of signalling pathways initiated by stimulation of death receptors, bacterial or viral infection, genotoxic stress and T-cell homeostasis.

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.

Overexpression of ABIN-2, a negative regulator of NF-kappaB, delays liver regeneration in the ABIN-2 transgenic mice

Activation of NF-kappaB is one of the earliest responses at the start of liver regeneration, and is required for hepatocyte cell cycle progression. The A20-binding inhibitor of NF-kappaB activation-2, ABIN-2, is an inhibitor of NF-kappaB. However, its effects on hepatocyte cell cycle progression are not known and its involvement in liver regeneration has not been explored. In this study, the temporal expression pattern of the mouse ABIN-2 was studied during liver regeneration induced by partial hepatectomy. We demonstrate that ABIN-2 is rapidly and transiently induced, and expression peaked at around 8h post-hepatectomy. To test that the inducible expression of ABIN-2 serves to regulate NF-kappaB during liver regeneration, transgenic mice overexpressing human ABIN-2 protein in the liver were generated. Our transgenic data demonstrated that overexpression of ABIN-2 inhibited NF-kappaB nuclear translocation, which peaked at around 2-4h post-hepatectomy, and this led to an impairment of the G1/S transition as well as a delay in hepatocyte cell cycle progression of the regenerating liver. In addition, overexpression of ABIN-2 specifically inhibited endogenous ABIN-2 mRNA induction, suggesting a negative feedback mechanism for ABIN-2 expression. In conclusion, ABIN-2 may function as a negative regulator that downregulates NF-kappaB activation during liver regeneration.

Automated yeast two-hybrid screening for nuclear receptor-interacting proteins

High throughput analysis of protein-protein interactions is an important sector of hypothesis-generating research. Using an improved and automated version of the yeast two-hybrid system, we completed a large interaction screening project with a focus on nuclear receptors and their cofactors. A total of 425 independent yeast two-hybrid cDNA library screens resulted in 6425 potential interacting protein fragments involved in 1613 different interaction pairs. We show that simple statistical parameters can be used to narrow down the data set to a high confidence set of 377 interaction pairs where validated interactions are enriched to 61% of all pairs. Within the high confidence set, there are 64 novel proteins potentially binding to nuclear receptors or their cofactors. We discuss several examples of high interest, and we expect that communication of this huge data set will help to complement our knowledge of the protein interaction repertoire of this family of transcription factors and instigate the characterization of the various novel candidate interactors.

Inhibition of NF-κB Activation by Peptides Targeting NF-κB Essential Modulator (NEMO) Oligomerization

NF-kappa B essential modulator/IKK-gamma (NEMO/IKK-gamma) plays a key role in the activation of the NF-kappa B pathway in response to proinflammatory stimuli. Previous studies suggested that the signal-dependent activation of the IKK complex involves the trimerization of NEMO. The minimal oligomerization domain of this protein consists of two coiled-coil subdomains named Coiled-coil 2 (CC2) and leucine zipper (LZ) (Agou, F., Traincard, F., Vinolo, E., Courtois, G., Yamaoka, S., Israel, A., and Veron, M. (2004) J. Biol. Chem. 279, 27861-27869). To search for drugs inhibiting NF-kappa B activation, we have rationally designed cell-permeable peptides corresponding to the CC2 and LZ subdomains that mimic the contact areas between NEMO subunits. The peptides were tagged with the Antennapedia/Penetratin motif and delivered to cells prior to stimulation with lipopolysaccharide. Peptide transduction was monitored by fluorescence-activated cell sorter, and their effect on lipopolysaccharide-induced NF-kappa B activation was quantified using an NF-kappa B-dependent beta-galactosidase assay in stably transfected pre-B 70Z/3 lymphocytes. We show that the peptides corresponding to the LZ and CC2 subdomains inhibit NF-kappa B activation with an IC(50) in the mum range. Control peptides, including mutated CC2 and LZ peptides and a heterologous coiled-coil peptide, had no inhibitory effect. The designed peptides are able to induce cell death in human retinoblastoma Y79 cells exhibiting constitutive NF-kappa B activity. Our results provide the "proof of concept" for a new and promising strategy for the inhibition of NF-kappa B pathway activation through targeting the oligomerization state of the NEMO protein.