TAK1 is a component of the Epstein-Barr virus LMP1 complex and is essential for activation of JNK but not of NF-kappaB

In Silico Modeling and Characterization of Epstein-Barr Virus Latent Membrane Protein 1 Protein

Latent membrane protein 1 (LMP1) plays a crucial role in Epstein-Barr virus (EBV)'s ability to establish latency and is involved in developing and progressing EBV-associated cancers. Additionally, EBV-infected cells affect the immune responses, making it challenging for the immune system to eliminate them. Due to the aforementioned reasons, it is crucial to understand the structural features of LMP1, which are essential for the development of novel cancer therapies that target its signaling pathways. To date, there is yet to be a complete LMP1 protein structure; therefore, in our work, we modeled the full-length LMP1 containing the short cytoplasmic N-terminus, six transmembrane domains (TMDs), and a long-simulated C-terminus. Our model showed good stability and protein compactness evaluated through accelerated-molecular dynamics, where the conformational ensemble exhibited compact folds, particularly in the TMDs. Our results suggest that specific domains or motifs, predominantly in the C-terminal domain of LMP1, show promise as potential drug targets. As a whole, our work provides insights into key structural features of LMP1 that will allow the development of novel LMP1 therapies.

Multiplexed, image-based pooled screens in primary cells and tissues with PerturbView

Optical pooled screening (OPS) is a scalable method for linking image-based phenotypes with cellular perturbations. However, it has thus far been restricted to relatively low-plex phenotypic readouts in cancer cell lines in culture due to limitations associated with in situ sequencing of perturbation barcodes. Here, we develop PerturbView, an OPS technology that leverages in vitro transcription to amplify barcodes before in situ sequencing, enabling screens with highly multiplexed phenotypic readouts across diverse systems, including primary cells and tissues. We demonstrate PerturbView in induced pluripotent stem cell-derived neurons, primary immune cells and tumor tissue sections from animal models. In a screen of immune signaling pathways in primary bone marrow-derived macrophages, PerturbView uncovered both known and novel regulators of NF-κB signaling. Furthermore, we combine PerturbView with spatial transcriptomics in tissue sections from a mouse xenograft model, paving the way to in situ screens with rich optical and transcriptomic phenotypes. PerturbView broadens the scope of OPS to a wide range of models and applications.

Duck Plague Virus Negatively Regulates IFN Signaling to Promote Virus Proliferation via JNK Signaling Pathway

Duck plague virus (DPV), a member of the alphaherpesvirus subfamily, can cause severe damage and immunosuppression in ducks and geese in China. Since lacking an available cell model, the antiviral signal transduction pathways induction and regulation mechanisms related to DPV infection in duck cells are still enigmatic. Our previous study developed a monocyte/macrophages cell model, which has been applied to study innate immunity with DPV. In the present study, we compared and analyzed transcriptome associated with the DPV infection of CHv (virulent strain) and CHa (avirulent strain) at 48hpi based on the duck monocyte/macrophages cell model and RNA-seq technology. Differentially expressed genes (DEGs) analysis showed 2,909 and 2,438 genes altered in CHv and CHa infected cells compared with control cells. Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analysis showed that the DEGs were mainly involved in biological processes such as metabolic pathways, viral infectious diseases, immune system, and signal transduction. The CHv and CHa virus differentially regulated MAPK, NF-κB, and IFN signaling pathways based on transcriptome sequencing data and RT-qPCR results. The JNK inhibitor SP600125 enhanced the IFN signaling, but potentially reduced the VSV and DPV titers in the cell culture supernatant, indicating that JNK negatively regulates the IFN pathway and the inflammatory pathway to promote virus proliferation. The research results may provide promising information to understand the pathogenesis of DPV and provide a novel mechanism by which DPV modulates antiviral signaling and facilitate virus proliferation through hijacking the JNK pathway, which provides a new means for the prevention and control of DPV infection.

H5N1 infection impairs the alveolar epithelial barrier through intercellular junction proteins via Itch-mediated proteasomal degradation

The H5N1 subtype of the avian influenza virus causes sporadic but fatal infections in humans. H5N1 virus infection leads to the disruption of the alveolar epithelial barrier, a pathologic change that often progresses into acute respiratory distress syndrome (ARDS) and pneumonia. The mechanisms underlying this remain poorly understood. Here we report that H5N1 viruses downregulate the expression of intercellular junction proteins (E-cadherin, occludin, claudin-1, and ZO-1) in several cell lines and the lungs of H5N1 virus-infected mice. H5N1 virus infection activates TGF-β-activated kinase 1 (TAK1), which then activates p38 and ERK to induce E3 ubiquitin ligase Itch expression and to promote occludin ubiquitination and degradation. Inhibition of the TAK1-Itch pathway restores the intercellular junction structure and function in vitro and in the lungs of H5N1 virus-infected mice. Our study suggests that H5N1 virus infection impairs the alveolar epithelial barrier by downregulating the expression of intercellular junction proteins at the posttranslational level.

Epstein-Barr virus circRNAome as host miRNA sponge regulates virus infection, cell cycle, and oncogenesis

Epstein-Barr virus (EBV) is an oncogenic virus that infects more than 90% of the world’s population. The proteins and miRNAs encoded by EBV are involved in multiple human malignancies. Recently R-resistance RNA-seq demonstrated that EBV-encoded circular RNAs. The current research aims to explore their functions in EBV-associated malignancies. Total 56 miRNAs were sponged by circRNAome. 24 and 9 in EBV host B and epithelial cells out of 56 miRNAs were detectable by miRNA-seq. 18 and 5 miRNAs were down-regulated in both types of host cells, respectively, after EBV infection. The network between five miRNAs and their targets included 1414 genes, 1419 nodes, and 2423 edges. These targets were enriched in multiple categories, and most of them were up-regulated in EBV-infected cells. These data represented the first report that EBV circRNAs could sponge the miRNAs to promote the up-regulated expression of their targets, involving in malignancies associated with EBV.

A central role of IKK2 and TPL2 in JNK activation and viral B-cell transformation

IκB kinase 2 (IKK2) is well known for its pivotal role as a mediator of the canonical NF-κB pathway, which has important functions in inflammation and immunity, but also in cancer. Here we identify a novel and critical function of IKK2 and its co-factor NEMO in the activation of oncogenic c-Jun N-terminal kinase (JNK) signaling, induced by the latent membrane protein 1 (LMP1) of Epstein-Barr virus (EBV). Independent of its kinase activity, the TGFβ-activated kinase 1 (TAK1) mediates LMP1 signaling complex formation, NEMO ubiquitination and subsequent IKK2 activation. The tumor progression locus 2 (TPL2) kinase is induced by LMP1 via IKK2 and transmits JNK activation signals downstream of IKK2. The IKK2-TPL2-JNK axis is specific for LMP1 and differs from TNFα, Interleukin-1 and CD40 signaling. This pathway mediates essential LMP1 survival signals in EBV-transformed human B cells and post-transplant lymphoma, and thus qualifies as a target for treatment of EBV-induced cancer.

Role of TGF-β-activated kinase 1 (TAK1) activation in H5N1 influenza A virus-induced c-Jun terminal kinase activation and virus replication

Activation of c-Jun terminal kinase (JNK) by the nonstructural protein 1 (NS1) of the H5N1 subtype of influenza A virus (IAV) plays an important role in inducing autophagy and virus replication. However, the mechanisms of NS1-induced JNK activation remain elusive. Here we first confirmed the ability of H5N1 (A/mallard/Huadong/S/2005) to activate JNK and to induce autophagy in 293T cells, a human embryonic kidney cell line. We further showed that TAK1, MAP kinase kinase 4 (MKK4), and JNK were activated in 293T cells transfected with the NS1 gene of the H5N1 virus. JNK activation by the NS1 protein or by H5N1 virus was blocked by 5Z-7-Oxozeaenol (5Z), a TAK1-specific inhibitor, and by TAK1 siRNA. Further study showed that 5Z and TAK1 siRNA suppressed H5N1 virus-induced autophagy and inhibited virus replication. Our study unveiled a previously unrecognized role of TAK1 in IAV replication, IAV-induced JNK activation, and autophagy.

The MAP3K7-mTOR Axis Promotes the Proliferation and Malignancy of Hepatocellular Carcinoma Cells

Targeted therapy is currently limited for patients with hepatocellular carcinoma (HCC) due to the lack of suitable targets. Kinases play pivotal roles in many cellular biological processes, whereas dysregulation of kinases may lead to various diseases, particularly cancer. However, the role of kinases in HCC malignancy remains unclear. In this study, we employed a kinome small interfering RNA (siRNA) library, comprising 710 kinase-related genes, to screen whether any kinases were essential for cell proliferation in various HCC cell lines. Through a kinome siRNA library screening, we found that MAP3K7 was a crucial gene for HCC cell proliferation. Pharmacological or genetic ablation of MAP3K7 diminished the growth, migration, and invasion of HCC cells, including primary HCC cells. Stable knockdown of MAP3K7 attenuated tumor formation in a spheroid cell culture model and tumor xenograft mouse model. In addition, silencing MAP3K7 reduced the phosphorylation and expression of mammalian target of rapamycin (mTOR) in HCC cells. MAP3K7 expression was positively correlated with mTOR expression in tumors of patients with HCC. Higher co-expression of MAP3K7 and mTOR was significantly associated with poor prognosis of HCC. Taken together, our results revealed that the MAP3K7-mTOR axis might promote tumorigenesis and malignancy, which provides a potential marker or therapeutic target for HCC patients.

Changes in mRNA expression of members of TGFB1-associated pathways in human leukocytes during EBV infection

Transforming growth factor β 1 (TGFB1) likely contributes to the pathogenesis of Epstein-Barr virus (EBV)-mediated cancer. A microarray containing 59 probes for detecting mRNA of members of TGFB1-associated pathways was developed. mRNA expression of TGFB1 receptors and members of connected pathways were examined in peripheral blood leukocytes of patients during acute EBV infection and after recovery. TGFB1 and TGFBR2 mRNA expression was increased in patients with EBV infection. Similarly, mRNA expression of protein kinase C (PRKCB), MAP3K7, PDLIM7, and other members of TGFB1 and NF-κB signaling pathways increased. A shift of mRNA transcript variant expression of some key members (TGFBR2, PRKCB, and NFKBIB) of involved signaling pathways was detected. After the patients' recovery, most of the altered mRNA expression has been normalized. We speculate that in patients with EBV infection, members of TGFB1-associated pathways contribute to the suppression of proapoptotic and induction of pro-survival factors in leukocytes. The modulation of TGFB1-associated pathways may be considered as a potential risk factor in the development of EBV-associated tumors in patients with acute EBV infection.

The interaction between mitochondria and oncoviruses

Mitochondria play important roles in multiple aspects of viral tumorigenesis. Mitochondrial genomes contribute to the host's genetic background. After viruses enter the cell, they modulate mitochondrial function and thus alter bioenergetics and retrograde signaling pathways. At the same time, mitochondria also regulate and mediate viral oncogenesis. In this context, oncogenesis by oncoviruses like Hepatitis B virus (HBV), Hepatitis C virus (HCV), Human papilloma virus (HPV), Human Immunodeficiency virus (HIV) and Epstein-Barr virus (EBV) will be discussed.

Role of the JNK Pathway in Varicella-Zoster Virus Lytic Infection and Reactivation

Mechanisms of neuronal infection by varicella-zoster virus (VZV) have been challenging to study due to the relatively strict human tropism of the virus and the paucity of tractable experimental models. Cellular mitogen-activated protein kinases (MAPKs) have been shown to play a role in VZV infection of nonneuronal cells, with distinct consequences for infectivity in different cell types. Here, we utilize several human neuronal culture systems to investigate the role of one such MAPK, the c-Jun N-terminal kinase (JNK), in VZV lytic infection and reactivation. We find that the JNK pathway is specifically activated following infection of human embryonic stem cell-derived neurons and that this activation of JNK is essential for efficient viral protein expression and replication. Inhibition of the JNK pathway blocked viral replication in a manner distinct from that of acyclovir, and an acyclovir-resistant VZV isolate was as sensitive to the effects of JNK inhibition as an acyclovir-sensitive VZV isolate in neurons. Moreover, in a microfluidic-based human neuronal model of viral latency and reactivation, we found that inhibition of the JNK pathway resulted in a marked reduction in reactivation of VZV. Finally, we utilized a novel technique to efficiently generate cells expressing markers of human sensory neurons from neural crest cells and established a critical role for the JNK pathway in infection of these cells. In summary, the JNK pathway plays an important role in lytic infection and reactivation of VZV in physiologically relevant cell types and may provide an alternative target for antiviral therapy.IMPORTANCE Varicella-zoster virus (VZV) has infected over 90% of people worldwide. While primary infection leads to the typically self-limiting condition of chickenpox, the virus can remain dormant in the nervous system and may reactivate later in life, leading to shingles or inflammatory diseases of the nervous system and eye with potentially severe consequences. Here, we take advantage of newer stem cell-based technologies to study the mechanisms by which VZV infects human neurons. We find that the c-Jun N-terminal kinase (JNK) pathway is activated by VZV infection and that blockade of this pathway limits lytic replication (as occurs during primary infection). In addition, JNK inhibition limits viral reactivation, exhibiting parallels with herpes simplex virus reactivation. The identification of the role of the JNK pathway in VZV infection of neurons reveals potential avenues for the development of alternate antiviral drugs.

TAK1 regulates hepatic lipid homeostasis through SREBP

Sterol regulatory element-binding proteins (SREBPs) are key transcription factors regulating cholesterol and fatty acid biosynthesis. SREBP activity is tightly regulated to maintain lipid homeostasis, and is modulated upon extracellular stimuli such as growth factors. While the homeostatic SREBP regulation is well studied, stimuli-dependent regulatory mechanisms are still elusive. Here we demonstrate that SREBPs are regulated by a previously uncharacterized mechanism through TGF-β activated kinase 1 (TAK1), a signaling molecule of inflammation. We found that TAK1 binds to and inhibits mature forms of SREBPs. In an in vivo setting, hepatocyte-specific Tak1 deletion upregulates liver lipid deposition and lipogenic enzymes in the mouse model. Furthermore, hepatic Tak1 deficiency causes steatosis pathologies including elevated blood triglyceride and cholesterol levels, which are established risk factors for the development of hepatocellular carcinoma (HCC) and are indeed correlated with Tak1-deficiency-induced HCC development. Pharmacological inhibition of SREBPs alleviated the steatosis and reduced the expression level of the HCC marker gene in the Tak1-deficient liver. Thus, TAK1 regulation of SREBP critically contributes to the maintenance of liver homeostasis to prevent steatosis, which is a potentially important mechanism to prevent HCC development.

The Latent Membrane Protein 1 (LMP1)

Almost exactly twenty years after the discovery of Epstein-Barr virus (EBV), the latent membrane protein 1 (LMP1) entered the EBV stage, and soon thereafter, it was recognized as the primary transforming gene product of the virus. LMP1 is expressed in most EBV-associated lymphoproliferative diseases and malignancies, and it critically contributes to pathogenesis and disease phenotypes. Thirty years of LMP1 research revealed its high potential as a deregulator of cellular signal transduction pathways leading to target cell proliferation and the simultaneous subversion of cell death programs. However, LMP1 has multiple roles beyond cell transformation and immortalization, ranging from cytokine and chemokine induction, immune modulation, the global alteration of gene and microRNA expression patterns to the regulation of tumor angiogenesis, cell-cell contact, cell migration, and invasive growth of tumor cells. By acting like a constitutively active receptor, LMP1 recruits cellular signaling molecules associated with tumor necrosis factor receptors such as tumor necrosis factor receptor-associated factor (TRAF) proteins and TRADD to mimic signals of the costimulatory CD40 receptor in the EBV-infected B lymphocyte. LMP1 activates NF-κB, mitogen-activated protein kinase (MAPK), phosphatidylinositol 3-kinase (PI3-K), IRF7, and STAT pathways. Here, we review LMP1's molecular and biological functions, highlighting the interface between LMP1 and the cellular signal transduction network as an important factor of virus-host interaction and a potential therapeutic target.

Mimicry of cytokine pathways by human herpesviruses

ABSTRACT 

Viruses have evolved to subvert host cell pathways to enable their replication and persistence. In particular, virus-encoded gene products target the host's immune system to evade elimination by antiviral immune defenses. Cytokines are soluble, secreted proteins, which regulate many aspects of immune responses, by providing signals through cell surface receptors on target cells. Cytokine pathways are therefore attractive targets for modulation by viruses during their replication cycle. This review deals with modulation of cytokine pathways by the human herpesvirus, a family of viruses that are capable of life-long persistence in the host and cause severe disease particularly in immunocompromised individuals.

Poxviral protein A52 stimulates p38 mitogen-activated protein kinase (MAPK) activation by causing tumor necrosis factor receptor-associated factor 6 (TRAF6) self-association leading to transforming growth factor β-activated kinase 1 (TAK1) recruitment

Vaccinia virus encodes a number of proteins that inhibit and manipulate innate immune signaling pathways that also have a role in virulence. These include A52, a protein shown to inhibit IL-1- and Toll-like receptor-stimulated NFκB activation, via interaction with interleukin-1 receptor-associated kinase 2 (IRAK2). Interestingly, A52 was also found to activate p38 MAPK and thus enhance Toll-like receptor-dependent IL-10 induction, which was TRAF6-dependent, but the manner in which A52 manipulates TRAF6 to stimulate p38 activation was unclear. Here, we show that A52 has a non-canonical TRAF6-binding motif that is essential for TRAF6 binding and p38 activation but dispensable for NFκB inhibition and IRAK2 interaction. Wild-type A52, but not a mutant defective in p38 activation and TRAF6 binding (F154A), caused TRAF6 oligomerization and subsequent TRAF6-TAK1 association. The crystal structure of A52 shows that it adopts a Bcl2-like fold and exists as a dimer in solution. Residue Met-65 was identified as being located in the A52 dimer interface, and consistent with that, A52-M65E was impaired in its ability to dimerize. A52-M65E although capable of interacting with TRAF6, was unable to cause either TRAF6 self-association, induce the TRAF6-TAK1 association, or activate p38 MAPK. The results suggest that an A52 dimer causes TRAF6 self-association, leading to TAK1 recruitment and p38 activation. This reveals a molecular mechanism whereby poxviruses manipulate TRAF6 to activate MAPKs (which can be proviral) without stimulating antiviral NFκB activation.

The germinal center kinase TNIK is required for canonical NF-κB and JNK signaling in B-cells by the EBV oncoprotein LMP1 and the CD40 receptor

TNIK has an important function in physiological activation and viral transformation of human B-cells by interacting with the TRAF6 adapter complex and mediating NF-κB and JNK signal transduction.

The tumor necrosis factor-receptor-associated factor 2 (TRAF2)- and Nck-interacting kinase (TNIK) is a ubiquitously expressed member of the germinal center kinase family. The TNIK functions in hematopoietic cells and the role of TNIK-TRAF interaction remain largely unknown. By functional proteomics we identified TNIK as interaction partner of the latent membrane protein 1 (LMP1) signalosome in primary human B-cells infected with the Epstein-Barr tumor virus (EBV). RNAi-mediated knockdown proved a critical role for TNIK in canonical NF-κB and c-Jun N-terminal kinase (JNK) activation by the major EBV oncoprotein LMP1 and its cellular counterpart, the B-cell co-stimulatory receptor CD40. Accordingly, TNIK is mandatory for proliferation and survival of EBV-transformed B-cells. TNIK forms an activation-induced complex with the critical signaling mediators TRAF6, TAK1/TAB2, and IKKβ, and mediates signalosome formation at LMP1. TNIK directly binds TRAF6, which bridges TNIK's interaction with the C-terminus of LMP1. Separate TNIK domains are involved in NF-κB and JNK signaling, the N-terminal TNIK kinase domain being essential for IKKβ/NF-κB and the C-terminus for JNK activation. We therefore suggest that TNIK orchestrates the bifurcation of both pathways at the level of the TRAF6-TAK1/TAB2-IKK complex. Our data establish TNIK as a novel key player in TRAF6-dependent JNK and NF-κB signaling and a transducer of activating and transforming signals in human B-cells.

The germinal center kinase family member TNIK was discovered in a yeast-two-hybrid screen for interaction partners of the adapter proteins TRAF2 and Nck, and here we show it is one of the missing molecular players in two key signaling pathways in B-lymphocytes. We found that TNIK is crucial for the activities of the CD40 receptor on Bcells and its viral mimic, the latent membrane protein 1 (LMP1) of Epstein-Barr virus (EBV). EBV is a human DNA tumor virus that is associated with various malignancies. It targets and transforms B-cells by hijacking the cellular signaling machinery via its oncogene LMP1. In normal Bcell physiology, the CD40 receptor is central to the immune response by mediating B-cell activation and proliferation. TNIK turns out to be an organizer of the LMP1- and CD40-induced signaling complexes by interacting with the TRAF6 adapter protein, well known for its role in linking distinct signaling pathways. Through this mechanism the two receptors depend on TNIK to activate the canonical NF-κB and JNK signal transduction pathways, which are important for the physiological activation of B-cells (a process that enables antibody production), as well as for their transformation into tumor cells. TNIK thus constitutes a key player in the transmission of physiological and pathological signals in human B-cells that might serve as a future therapeutic target against B-cell malignancies.

Roles of the kinase TAK1 in TRAF6-dependent signaling by CD40 and its oncogenic viral mimic, LMP1

The Epstein-Barr virus (EBV)-encoded protein latent membrane protein 1 (LMP1) is essential for EBV-mediated B cell transformation and plays a critical role in the development of post-transplant B cell lymphomas. LMP1 also contributes to the exacerbation of autoimmune diseases such as systemic lupus erythematosus (SLE). LMP1 is a functional mimic of the tumor necrosis factor receptor (TNFR) superfamily member CD40, and relies on TNFR-associated factor (TRAF) adaptor proteins to mediate signaling. However, LMP1 activation signals to the B cell are amplified and sustained compared to CD40 signals. We previously demonstrated that LMP1 and CD40 use TRAF molecules differently. Although associating with CD40 and LMP1 via separate mechanisms, TRAF6 plays a significant role in signal transduction by both. It is unknown whether TRAF6 mediates CD40 versus LMP1 functions via distinct or shared pathways. In this study, we tested the hypothesis that TRAF6 uses the kinase TAK1 to trigger important signaling pathways following both CD40 and LMP1 stimulation. We determined that TAK1 was required for JNK activation and interleukin-6 (IL-6) production mediated by CD40 and LMP1, in both mouse and human B cells. Additionally, TRAF3 negatively regulated TRAF6-dependent, CD40-mediated TAK1 activation by limiting TRAF6 recruitment. This mode of regulation was not observed for LMP1 and may contribute to the dysregulation of LMP1 compared to CD40 signals.

Modulation of autophagy-like processes by tumor viruses

Autophagy is an intracellular degradation pathway for long-lived proteins and organelles. This process is activated above basal levels upon cell intrinsic or environmental stress and dysregulation of autophagy has been linked to various human diseases, including those caused by viral infection. Many viruses have evolved strategies to directly interfere with autophagy, presumably to facilitate their replication or to escape immune detection. However, in some cases, modulation of autophagy appears to be a consequence of the virus disturbing the cell’s metabolic signaling networks. Here, we summarize recent advances in research at the interface of autophagy and viral infection, paying special attention to strategies that human tumor viruses have evolved.

TAK1 regulates NF-ΚB and AP-1 activation in airway epithelial cells following RSV infection

Respiratory syncytial virus (RSV) is the most common cause of epidemic respiratory diseases in infants and young children. RSV infection of airway epithelial cells induces the expression of immune/inflammatory genes through the activation of a subset of transcription factors, including Nuclear Factor-κB (NF-κB) and AP-1. In this study, we have investigated the signaling pathway leading to activation of these two transcription factors in response to RSV infection. Our results show that IKKβ plays a key role in viral-induced NF-κB activation, while JNK regulates AP-1-dependent gene transcription, as demonstrated by using kinase inactive proteins and chemical inhibitors of the two kinases. Inhibition of TAK1 activation, by overexpression of kinase inactive TAK1 or using cells lacking TAK1 expression, significantly reduced RSV-induced NF-κB and AP-1 nuclear translocation and DNA-binding activity, as well as NF-κB-dependent gene expression, identifying TAK1 as an important upstream signaling molecule regulating RSV-induced NF-κB and AP-1 activation.

Signal integration, crosstalk mechanisms and networks in the function of inflammatory cytokines

Infection or cell damage triggers the release of pro-inflammatory cytokines such as interleukin(IL)-1α or β and tumor necrosis factor (TNF)α which are key mediators of the host immune response. Following their identification and the elucidation of central signaling pathways, recent results show a highly complex crosstalk between various cytokines and their signaling effectors. The molecular mechanisms controlling signaling thresholds, signal integration and the function of feed-forward and feedback loops are currently revealed by combining methods from biochemistry, genetics and in silico analysis. Increasing evidence is mounted that defects in information processing circuits or their components can be causative for chronic or overshooting inflammation. As progress in biosciences has always benefitted from the use of well-studied model systems, research on inflammatory cytokines may function as a paradigm to reveal general principles of signal integration, crosstalk mechanisms and signaling networks.

Phosphorylation and polyubiquitination of transforming growth factor beta-activated kinase 1 are necessary for activation of NF-kappaB by the Kaposi's sarcoma-associated herpesvirus G protein-coupled receptor

Kaposi's sarcoma-associated herpesvirus (KSHV) G protein-coupled receptor (vGPCR) protein has been shown to induce several signaling pathways leading to the modulation of host gene expression. The hijacking of these pathways facilitates the viral life cycle and leads to tumorigenesis. In the present work, we show that transforming growth factor β (TGF-β)-activated kinase 1 (TAK1) is an important player in NF-κB activation induced by vGPCR. We observed that the expression of an inactive TAK1 kinase mutant (TAK1M) reduces vGPCR-induced NF-κB nuclear translocation and transcriptional activity. Consequently, the expression of several NF-κB target genes normally induced by vGPCR was blocked by TAK1M expression, including interleukin 8 (IL-8), Gro1, IκBα, COX-2, cIAP2, and Bcl2 genes. Similar results were obtained after downregulation of TAK1 by small interfering RNA (siRNA) technology. The expression of vGPCR recruited TAK1 to the plasma membrane, and vGPCR interacts with TAK1. vGPCR expression also induced TAK1 phosphorylation and lysine 63-linked polyubiquitination, the two markers of the kinase's activation. Finally, inhibition of TAK1 by celastrol inhibited vGPCR-induced NF-κB activation, indicating this natural compound could be used as a potential therapeutic drug against KSHV malignancies involving vGPCR.

Comprehending the complex connection between PKCbeta, TAK1, and IKK in BCR signaling

The transcription factor nuclear factor-kappaB (NF-kappaB) contributes to many events in the immune system. Characterization of NF-kappaB has facilitated our understanding of immune cell differentiation, survival, proliferation, and effector functions. Intense research continues to elucidate the role of NF-kappaB, which is shared in several receptor signaling pathways, such as Toll-like receptors, the tumor necrosis factor receptor, and antigen receptors. The specificity of cellular responses emanating from stimulation of these receptors is determined by post-translational modification, or 'fine tuning', which regulates spatiotemporal dynamics of downstream signaling. Understanding the fine tuning mechanisms of NF-kappaB activation is crucial for insights into biological regulation and for understanding how cellular signaling pathways are tightly regulated to guide different cell fates. In this review, we focus on recent advances that illuminate the fine tuning mechanisms of NF-kappaB activation by BCR signaling and have increased our comprehension of complex signal systems.

Transforming growth factor beta-activated kinase 1 (TAK1) kinase adaptor, TAK1-binding protein 2, plays dual roles in TAK1 signaling by recruiting both an activator and an inhibitor of TAK1 kinase in tumor necrosis factor signaling pathway

Transforming growth factor beta-activated kinase 1 (TAK1) kinase is an indispensable signaling intermediate in tumor necrosis factor (TNF), interleukin 1, and Toll-like receptor signaling pathways. TAK1-binding protein 2 (TAB2) and its closely related protein, TAB3, are binding partners of TAK1 and have previously been identified as adaptors of TAK1 that recruit TAK1 to a TNF receptor signaling complex. TAB2 and TAB3 redundantly mediate activation of TAK1. In this study, we investigated the role of TAB2 by analyzing fibroblasts having targeted deletion of the tab2 gene. In TAB2-deficient fibroblasts, TAK1 was associated with TAB3 and was activated following TNF stimulation. However, TAB2-deficient fibroblasts displayed a significantly prolonged activation of TAK1 compared with wild type control cells. This suggests that TAB2 mediates deactivation of TAK1. We found that a TAK1-negative regulator, protein phosphatase 6 (PP6), was recruited to the TAK1 complex in wild type but not in TAB2-deficient fibroblasts. Furthermore, we demonstrated that both PP6 and TAB2 interacted with the polyubiquitin chains and this interaction mediated the assembly with TAK1. Our results indicate that TAB2 not only activates TAK1 but also plays an essential role in the deactivation of TAK1 by recruiting PP6 through a polyubiquitin chain-dependent mechanism.

TRAF5 is a critical mediator of in vitro signals and in vivo functions of LMP1, the viral oncogenic mimic of CD40

The cytoplasmic signaling protein TNF receptor-associated factor 5 (TRAF5) has been implicated in several biological roles in T-lymphocyte responses. However, a clear connection between in vivo TRAF5 immune cell functions and specific signaling pathways has not been made. This study shows that TRAF5 associated strongly with the viral oncogenic CD40 mimic latent membrane protein 1 (LMP1), in contrast to weaker association with CD40, for which it has been shown to play a modest role. LMP1 uses specific TRAFs differently than CD40, resulting in amplified and dysregulated CD40-like activation of B lymphocytes. When the cytoplasmic domain of LMP1 is expressed as a transgenic replacement for CD40 in mouse B cells, the resulting mouse exhibits measures of B-cell hyperactivity such as splenomegaly, lymphadenopathy, elevated serum IL-6, elevated serum autoantibodies, and abnormal splenic architecture. Thus, in contrast to CD40, TRAF5 may have an important nonredundant role as a positive mediator of LMP1 signaling and functions in B cells. To test this hypothesis, mice were created that express mCD40LMP1 in place of CD40, and are either sufficient or deficient in TRAF5. Results revealed that TRAF5 plays a critical role in LMP1-mediated c-Jun kinase signaling and is required for much of the abnormal phenotype observed in mCD40LMP1 transgenic mice. This is the first report showing a major requirement for TRAF5 in signaling by a specific receptor both in vitro and in vivo, as well as playing an important role in biological function in B lymphocytes.

Cross interference with TNF-alpha-induced TAK1 activation via EGFR-mediated p38 phosphorylation of TAK1-binding protein 1

Transforming growth factor-alpha-activated kinase 1 (TAK1) has been widely recognized as a kinase that regulates multiple intracellular signaling pathways evoked by cytokines and immune receptor activation. We have recently reported that tumor necrosis factor-alpha (TNF-alpha) triggers internalization of epidermal growth factor receptor (EGFR) through a TAK1-p38alpha signaling pathway, which results in a transient suppression of the EGFR. In the present study, we investigated the pathway of intracellular signaling in the opposite direction. Ligand-induced activation of EGFR caused phosphorylation of the TAK1-binding proteins TAB1 and TAB2 in a TAK1-independent manner. EGFR-mediated phosphorylation of TAB1 was completely inhibited by a chemical inhibitor and siRNA of p38alpha. The phosphorylation of TAB1 was occurred at Ser-423 and Thr-431, the residues underlying the p38-mediated feedback inhibition of TAK1. In contrast, phosphorylation of TAB2 was sustained, and largely resistant to p38 inhibition. The inducible phosphorylation of TAB1 interfered with a response of EGF-treated cells to TNF-alpha-induced TAK1 activation, which led to the reduction of NF-kappaB activation. Collectively, these results demonstrated that EGFR activation interfered with TNF-alpha-induced TAK1 activation via p38-mediated phosphorylation of TAB1.

Identification of TRIM23 as a cofactor involved in the regulation of NF-kappaB by human cytomegalovirus

Human cytomegalovirus (HCMV) regulates NF-kappaB during infection by a variety of mechanisms. For example, the HCMV gene product, UL144, is known to activate NF-kappaB in a tumor necrosis factor receptor (TNFR)-associated factor 6 (TRAF6)-dependent manner, causing the upregulation of the chemokine CCL22 (MDC). Viral UL144 is expressed from the UL/b' region of the HCMV genome at early times postinfection and is a TNFR1-like homologue. Despite this homology to the TNFR1 receptor superfamily, UL144 does not bind to members of the TNF ligand superfamily. We show here that the upregulation of NF-kappaB by UL144 is dependent upon cellular tripartite motif 23 (TRIM23) protein. We propose a mechanism by which UL144 activates NF-kappaB through a direct interaction with the cellular protein TRIM23 in a complex containing TRAF6. In contrast, TRIM23 is not involved in conventional double-stranded RNA signaling via NF-kappaB. Therefore, we present a novel role for TRIM23 that is specific to UL144-mediated activation of NF-kappaB during the course of virus infection.

Cross-regulation between herpesviruses and the TNF superfamily members

Herpesviruses have evolved numerous strategies to subvert host immune responses so they can coexist with their host species. These viruses 'co-opt' host genes for entry into host cells and then express immunomodulatory genes, including mimics of members of the tumour-necrosis factor (TNF) superfamily, that initiate and alter host-cell signalling pathways. TNF superfamily members have crucial roles in controlling herpesvirus infection by mediating the direct killing of infected cells and by enhancing immune responses. Despite these strong immune responses, herpesviruses persist in a latent form, which suggests a dynamic relationship between the host immune system and the virus that results in a balance between host survival and viral control.

TAK1-binding protein 1, TAB1, mediates osmotic stress-induced TAK1 activation but is dispensable for TAK1-mediated cytokine signaling

TAK1 kinase is an indispensable intermediate in several cytokine signaling pathways including tumor necrosis factor, interleukin-1, and transforming growth factor-beta signaling pathways. TAK1 also participates in stress-activated intracellular signaling pathways such as osmotic stress signaling pathway. TAK1-binding protein 1 (TAB1) is constitutively associated with TAK1 through its C-terminal region. Although TAB1 is known to augment TAK1 catalytic activity when it is overexpressed, the role of TAB1 under physiological conditions has not yet been identified. In this study, we determined the role of TAB1 in TAK1 signaling by analyzing TAB1-deficient mouse embryonic fibroblasts (MEFs). Tumor necrosis factor- and interleukin-1-induced activation of TAK1 was entirely normal in Tab1-deficient MEFs and could activate both mitogen-activated protein kinases and NF-kappaB. In contrast, we found that osmotic stress-induced activation of TAK1 was largely impaired in Tab1-deficient MEFs. Furthermore, we showed that the C-terminal 68 amino acids of TAB1 were sufficient to mediate osmotic stress-induced TAK1 activation. Finally, we attempted to determine the mechanism by which TAB1 activates TAK1. We found that TAK1 is spontaneously activated when the concentration is increased and that it is totally dependent on TAB1. Cell shrinkage under the osmotic stress condition increases the concentration of TAB1-TAK1 and may oligomerize and activate TAK1 in a TAB1-dependent manner. These results demonstrate that TAB1 mediates TAK1 activation only in a subset of TAK1 pathways that are mediated through spontaneous oligomerization of TAB1-TAK1.

Constitutive activation of TAK1 by HTLV-1 tax-dependent overexpression of TAB2 induces activation of JNK-ATF2 but not IKK-NF-kappaB

HTLV-1 Tax oncoprotein induces persistent activation of the transcription factor NF-kappaB and CREB (cAMP-response element-binding protein)/ATF. Transforming growth factor-beta-activated kinase 1 (TAK1) has been shown to play a critical role in these transcription factors. Here, we found that TAK1 was constitutively activated in Tax-positive HTLV-1-transformed T cells. Tax induced persistent overexpression of TAK1-binding protein 2 (TAB2), but not TAB3, which is essential for TAK1 activation. Surprisingly, TAK1 was not involved in the activation of NF-kappaB. On the other hand, JNK and p38 mitogen-activated protein kinases were activated by TAK1. In addition, ATF2, but not CREB, was a target for the TAK1-JNK pathway, and p38 negatively regulated TAK1 activity through TAB1 phosphorylation. These results indicate that Tax-mediated TAK1 activation is important for the activation of ATF2 rather than NF-kappaB.

Osmotic stress activates the TAK1-JNK pathway while blocking TAK1-mediated NF-kappaB activation: TAO2 regulates TAK1 pathways

Osmotic stress activates MAPKs, including JNK and p38, which play important roles in cellular stress responses. Transforming growth factor-beta-activated kinase 1 (TAK1) is a member of the MAPK kinase kinase (MAPKKK) family and can activate JNK and p38. TAK1 can also activate IkappaB kinase (IKK) that leads to degradation of IkappaB and subsequent NF-kappaB activation. We found that TAK1 is essential for osmotic stress-induced activation of JNK but is not an exclusive mediator of p38 activation. Furthermore, we found that although TAK1 was highly activated upon osmotic stress, it could not induce degradation of IkappaB or activation of NF-kappaB. These results suggest that TAK1 activity is somehow modulated to function specifically in osmotic stress signaling, leading to the activation of JNK but not of IKK. To elucidate the mechanism underlying this modulation, we screened for potential TAK1-binding proteins. We found that TAO2 (thousand-and-one amino acid kinase 2) associates with TAK1 and can inhibit TAK1-mediated activation of NF-kappaB but not of JNK. We observed that TAO2 can interfere with the interaction between TAK1 and IKK and thus may regulate TAK1 function. TAK1 is activated by many distinct stimuli, including cytokines and stresses, and regulation by TAO2 may be important to activate specific intracellular signaling pathways that are unique to osmotic stress.