Discrete signaling regions in the lymphotoxin-beta receptor for tumor necrosis factor receptor-associated factor binding, subcellular localization, and activation of cell death and NF-kappaB pathways

Different degree of loss-of-function among four missense mutations in the EDAR gene responsible for autosomal recessive hypohidrotic ectodermal dysplasia may be associated with the phenotypic severity

Hypohidrotic ectodermal dysplasia is a rare condition characterized by hypohidrosis, hypodontia, and hypotrichosis. The disease can show X-linked recessive, autosomal dominant or autosomal recessive inheritance trait. Of these, the autosomal forms are caused by mutations in either EDAR or EDARADD. To date, the underlying pathomechanisms or genotype-phenotype correlations for autosomal forms have not completely been disclosed. In this study, we performed a series of in vitro studies for four missense mutations in the death domain of EDAR protein: p.R358Q, p.G382S, p.I388T, and p.T403M. The results revealed that p.R358Q- and p.T403M-mutant EDAR showed different expression patterns from wild-type EDAR in both western blots and immunostainings. NF-κB reporter assays demonstrated that all the mutant EDAR showed reduced activation of NF-κB, but the reduction by p.G382S- and p.I388T-mutant EDAR was moderate. Co-immunoprecipitation assays showed that p.R358Q- and p.T403M-mutant EDAR did not bind with EDARADD at all, whereas p.G382S- and p.I388T-mutant EDAR maintained the affinity to some extent. Furthermore, we demonstrated that all the mutant EDAR proteins analyzed aberrantly bound with TRAF6. Sum of the data suggest that the degree of loss-of-function is different among the mutant EDAR proteins, which may be associated with the severity of the disease.

The E3 Ubiquitin Ligase TRAF6 Interacts with the Cellular Prion Protein and Modulates Its Solubility and Recruitment to Cytoplasmic p62/SQSTM1-Positive Aggresome-Like Structures

The cellular prion protein (PrP^C) is a ubiquitous glycoprotein highly expressed in the brain where it is involved in neurite outgrowth, copper homeostasis, NMDA receptor regulation, cell adhesion, and cell signaling. Conformational conversion of PrP^C into its insoluble and aggregation-prone scrapie form (PrP^Sc) is the trigger for several rare devastating neurodegenerative disorders, collectively referred to as prion diseases. Recent work indicates that the ubiquitin-proteasome system is involved in quality control of PrP^C. To better dissect the role of ubiquitination in PrP^C physiology, we focused on the E3 RING ubiquitin ligase tumor necrosis factor receptor (TNFR)-associated factor 6 (TRAF6). Here, we report that PrP^C interacts with TRAF6 both in vitro, in cells, and in vivo, in the mouse brain. Transient overexpression of TRAF6 indirectly modulates PrP^C ubiquitination and triggers redistribution of PrP^C into the insoluble fraction. Importantly, in the presence of wild-type TRAF6, but not a mutant lacking E3 ligase activity, PrP^C accumulates into cytoplasmic aggresome-like inclusions containing TRAF6 and p62/SQSTM1. Our results suggest that TRAF6 ligase activity could exert a role in the regulation of PrP^C redistribution in cells under physiological conditions. This novel interaction may uncover possible mechanisms of cell clearance/reorganization in prion diseases.

LTβR Signaling Controls Lymphatic Migration of Immune Cells

The pleiotropic functions of lymphotoxin (LT)β receptor (LTβR) signaling are linked to the control of secondary lymphoid organ development and structural maintenance, inflammatory or autoimmune disorders, and carcinogenesis. Recently, LTβR signaling in endothelial cells has been revealed to regulate immune cell migration. Signaling through LTβR is comprised of both the canonical and non-canonical-nuclear factor κB (NF-κB) pathways, which induce chemokines, cytokines, and cell adhesion molecules. Here, we focus on the novel functions of LTβR signaling in lymphatic endothelial cells for migration of regulatory T cells (Tregs), and specific targeting of LTβR signaling for potential therapeutics in transplantation and cancer patient survival.

Receptor Oligomerization and Its Relevance for Signaling by Receptors of the Tumor Necrosis Factor Receptor Superfamily

With the exception of a few signaling incompetent decoy receptors, the receptors of the tumor necrosis factor receptor superfamily (TNFRSF) are signaling competent and engage in signaling pathways resulting in inflammation, proliferation, differentiation, and cell migration and also in cell death induction. TNFRSF receptors (TNFRs) become activated by ligands of the TNF superfamily (TNFSF). TNFSF ligands (TNFLs) occur as trimeric type II transmembrane proteins but often also as soluble ligand trimers released from the membrane-bound form by proteolysis. The signaling competent TNFRs are efficiently activated by the membrane-bound TNFLs. The latter recruit three TNFR molecules, but there is growing evidence that this is not sufficient to trigger all aspects of TNFR signaling; rather, the formed trimeric TNFL–TNFR complexes have to cluster secondarily in the cell-to-cell contact zone for full TNFR activation. With respect to their response to soluble ligand trimers, the signaling competent TNFRs can be subdivided into two groups. TNFRs of one group, designated as category I TNFRs, are robustly activated by soluble ligand trimers. The receptors of a second group (category II TNFRs), however, failed to become properly activated by soluble ligand trimers despite high affinity binding. The limited responsiveness of category II TNFRs to soluble TNFLs can be overcome by physical linkage of two or more soluble ligand trimers or, alternatively, by anchoring the soluble ligand molecules to the cell surface or extracellular matrix. This suggests that category II TNFRs have a limited ability to promote clustering of trimeric TNFL–TNFR complexes outside the context of cell–cell contacts. In this review, we will focus on three aspects on the relevance of receptor oligomerization for TNFR signaling: (i) the structural factors which promote clustering of free and liganded TNFRs, (ii) the signaling pathway specificity of the receptor oligomerization requirement, and (iii) the consequences for the design and development of TNFR agonists.

Clathrin- and dynamin-dependent endocytosis limits canonical NF-κB signaling triggered by lymphotoxin β receptor

Background

Lymphotoxin β receptor (LTβR) is a member of tumor necrosis factor receptor (TNFR) superfamily which regulates the immune response. At the cellular level, upon ligand binding, the receptor activates the pro-inflammatory NF-κB and AP-1 pathways. Yet, the intracellular distribution of LTβR, the routes of its endocytosis and their connection to the signaling activation are not characterized. Here, we investigated the contribution of LTβR internalization to its signaling potential.

Methods

Intracellular localization of LTβR in unstimulated and stimulated cells was analyzed by confocal microscopy. Endocytosis impairment was achieved through siRNA- or CRISPR/Cas9-mediated depletion, or chemical inhibition of proteins regulating endocytic routes. The activation of LTβR-induced signaling was examined. The levels of effector proteins of the canonical and non-canonical branches of the NF-κB pathway, and the phosphorylation of JNK, Akt, ERK1/2, STAT1 and STAT3 involved in diverse signaling cascades, were measured by Western blotting. A transcriptional response to LTβR stimulation was assessed by qRT-PCR analysis.

Results

We demonstrated that LTβR was predominantly present on endocytic vesicles and the Golgi apparatus. The ligand-bound pool of the receptor localized to endosomes and was trafficked towards lysosomes for degradation. Depletion of regulators of different endocytic routes (clathrin-mediated, dynamin-dependent or clathrin-independent) resulted in the impairment of LTβR internalization, indicating that this receptor uses multiple entry pathways. Cells deprived of clathrin and dynamins exhibited enhanced activation of canonical NF-κB signaling represented by increased degradation of IκBα inhibitor and elevated expression of LTβR target genes. We also demonstrated that clathrin and dynamin deficiency reduced to some extent LTβR-triggered activation of the non-canonical branch of the NF-κB pathway.

Conclusions

Our work shows that the impairment of clathrin- and dynamin-dependent internalization amplifies a cellular response to LTβR stimulation. We postulate that receptor internalization restricts responsiveness of the cell to subthreshold stimuli.

Expression of Immune System-Related Membrane Receptors CD40, RANK, BAFFR and LTβR is Associated with Clinical Outcome of Operated Non-Small-Cell Lung Cancer Patients

An increasing number of studies implicates the NF-κB (Nuclear Factor of kappa light chain gene enhancer in B cells) alternative pathway in non-small-cell lung cancer (NSCLC). We assessed the clinical significance of CD40 (Tumor necrosis factor receptor superfamily member 5, TNFRSF5), BAFFR (B-cell activating factor receptor), RANK (Receptor activator of NF-κB) and LTβR (lymphotoxin β receptor) receptors, which activate the alternative pathway of NF-κB, in NSCLC. Evaluation of CD40, BAFFR, RANK and LTβR expression was performed based on the Cancer Genome Atlas (TCGA) and the Genotype-Tissue Expression (GTEx) datasets, while protein expression was assessed by immunohistochemistry in specimens from 119 operated NSCLC patients. CD40 gene overexpression was correlated with improved five-year overall survival (OS) (p < 0.001), while increased BAFFR and LTβR mRNA levels were associated with worse OS in patients with adenocarcinomas (p < 0.001 and p < 0.001, respectively). Similarly, patients with adenocarcinomas exhibited a negative correlation between membranous BAFFR protein expression in carcinoma cells and three- and five-year survival (p = 0.021; HR, 4.977 and p = 0.030; HR, 3.358, respectively) as well as between BAFFR protein overexpression in cancer-associated fibroblasts (CAFs) and two-year survival (p = 0.036; HR, 1.983). Patients with increased LTβR nuclear protein staining or stage II patients with lower cytoplasmic LTβR protein expression had worse five-year OS (p = 0.039 and p = 0.008, respectively). Moreover, CD40 protein expression in tumor infiltrating lymphocytes (TILs) and CAFs was positively associated with metastatic spread while BAFFR protein expression in CAFs was negatively associated with bone metastasis (p = 0.041). Our data suggests that CD40, BAFFR, RANK and LTβR play an important role in NSCLC and further supports the role of NF-κB alternative pathway in NSCLC.

Reverse Signaling of Tumor Necrosis Factor Superfamily Proteins in Macrophages and Microglia: Superfamily Portrait in the Neuroimmune Interface

The tumor necrosis factor (TNF) superfamily (TNFSF) is a protein superfamily of type II transmembrane proteins commonly containing the TNF homology domain. The superfamily contains more than 20 protein members, which can be released from the cell membrane by proteolytic cleavage. Members of the TNFSF function as cytokines and regulate diverse biological processes, including immune responses, proliferation, differentiation, apoptosis, and embryogenesis, by binding to TNFSF receptors. Many TNFSF proteins are also known to be responsible for the regulation of innate immunity and inflammation. Both receptor-mediated forward signaling and ligand-mediated reverse signaling play important roles in these processes. In this review, we discuss the functional expression and roles of various reverse signaling molecules and pathways of TNFSF members in macrophages and microglia in the central nervous system (CNS). A thorough understanding of the roles of TNFSF ligands and receptors in the activation of macrophages and microglia may improve the treatment of inflammatory diseases in the brain and periphery. In particular, TNFSF reverse signaling in microglia can be exploited to gain further insights into the functions of the neuroimmune interface in physiological and pathological processes in the CNS.

The Tumor Necrosis Factor Family: Family Conventions and Private Idiosyncrasies

The tumor necrosis factor (TNF) cytokine family and the TNF/nerve growth factor (NGF) family of their cognate receptors together control numerous immune functions, as well as tissue-homeostatic and embryonic-development processes. These diverse functions are dictated by both shared and distinct features of family members, and by interactions of some members with nonfamily ligands and coreceptors. The spectra of their activities are further expanded by the occurrence of the ligands and receptors in both membrane-anchored and soluble forms, by "re-anchoring" of soluble forms to extracellular matrix components, and by signaling initiation via intracellular domains (IDs) of both receptors and ligands. Much has been learned about shared features of the receptors as well as of the ligands; however, we still have only limited knowledge of the mechanistic basis for their functional heterogeneity and for the differences between their functions and those of similarly acting cytokines of other families.

Regulation of T cell afferent lymphatic migration by targeting LTβR-mediated non-classical NFκB signaling

Lymphotoxin-beta receptor (LTβR) signaling in lymphatic endothelial cells (LEC) regulates leukocyte afferent lymphatic transendothelial migration (TEM). The function of individual signaling pathways for different leukocyte subsets is currently unknown. Here, we show that LTβR signals predominantly via the constitutive and ligand-driven non-classical NIK pathway. Targeting LTβR-NIK by an LTβR-derived decoy peptide (nciLT) suppresses the production of chemokines CCL21 and CXCL12, and enhances the expression of classical NFκB-driven VCAM-1 and integrin β4 to retain T cells on LEC and precludes T cell and dendritic cell TEM. nciLT inhibits contact hypersensitivity (CHS) at both the sensitization and elicitation stages, likely by inhibiting leukocyte migration. By contrast, targeting LTβR-classical NFκB signaling during the elicitation and resolution stages attenuates CHS, possibly by promoting leukocyte egress. These findings demonstrate the importance of LTβR signaling in leukocyte migration and LEC and lymphatic vessel function, and show that antagonist peptides may serve as lead compounds for therapeutic applications.

Lymphotoxin beta receptor (LTβR) signalling regulates leukocyte migration through the lymphatic endothelial layers. Here, the authors show that treatment of an LTβR-derived decoy peptide can target the non-classical NFκB pathway to inhibit T cell and dendritic cell migration and ameliorate contact hypersensitivity in mouse models.

Original Ligand for LTβR Is LIGHT: Insight into Evolution of the LT/LTβR System

The lymphotoxin (LT)/LTβ receptor (LTβR) axis is crucial for the regulation of immune responses and development of lymphoid tissues in mammals. Despite the importance of this pathway, the existence and function of LT and LTβR remain obscure for nonmammalian species. In this study, we report a nonmammalian LTβR and its ligand. We demonstrate that TNF-New (TNFN), which has been considered orthologous to mammalian LT, was expressed on the cell surface as a homomer in vitro. This different protein structure indicates that TNFN is not orthologous to mammalian LTα and LTβ. Additionally, we found that LTβR was conserved in teleosts, but the soluble form of recombinant fugu LTβR did not bind to membrane TNFN under the circumstance tested. Conversely, the LTβR recombinant bound to another ligand, LIGHT, similar to that of mammals. These findings indicate that teleost LTβR is originally a LIGHT receptor. In the cytoplasmic region of fugu LTβR, recombinant fugu LTβR bound to the adaptor protein TNFR-associated factor (TRAF) 2, but little to TRAF3. This difference suggests that teleost LTβR could potentially activate the classical NF-κB pathway with a novel binding domain, but would have little ability to activate an alternative one. Collectively, our results suggested that LIGHT was the original ligand for LTβR, and that the teleost immune system lacked the LT/LTβR pathway. Acquisition of the LT ligand and TRAF binding domain after lobe-finned fish may have facilitated the sophistication of the immune system and lymphoid tissues.

Context-dependent roles for lymphotoxin-β receptor signaling in cancer development

The LTα1β2 and LIGHT TNF superfamily cytokines exert pleiotropic physiological functions through the activation of their cognate lymphotoxin-β receptor (LTβR). Interestingly, since the discovery of these proteins, accumulating evidence has pinpointed a role for LTβR signaling in carcinogenesis. Early studies have shown a potential anti-tumoral role in a subset of solid cancers either by triggering apoptosis in malignant cells or by eliciting an anti-tumor immune response. However, more recent studies provided robust evidence that LTβR signaling is also involved in diverse cell-intrinsic and microenvironment-dependent pro-oncogenic mechanisms, affecting several solid and hematological malignancies. Consequently, the usefulness of LTβR signaling axis blockade has been investigated as a potential therapeutic approach for cancer. Considering the seemingly opposite roles of LTβR signaling in diverse cancer types and their key implications for therapy, we here extensively review the different mechanisms by which LTβR activation affects carcinogenesis, focusing on the diverse contexts and different models assessed.

ESCRT proteins restrict constitutive NF-κB signaling by trafficking cytokine receptors

Because signaling mediated by the transcription factor nuclear factor κB (NF-κB) is initiated by ligands and receptors that can undergo internalization, we investigated how endocytic trafficking regulated this key physiological pathway. We depleted all of the ESCRT (endosomal sorting complexes required for transport) subunits, which mediate receptor trafficking and degradation, and found that the components Tsg101, Vps28, UBAP1, and CHMP4B were essential to restrict constitutive NF-κB signaling in human embryonic kidney 293 cells. In the absence of exogenous cytokines, depletion of these proteins led to the activation of both canonical and noncanonical NF-κB signaling, as well as the induction of NF-κB-dependent transcriptional responses in cultured human cells, zebrafish embryos, and fat bodies in flies. These effects depended on cytokine receptors, such as the lymphotoxin β receptor (LTβR) and tumor necrosis factor receptor 1 (TNFR1). Upon depletion of ESCRT subunits, both receptors became concentrated on and signaled from endosomes. Endosomal accumulation of LTβR induced its ligand-independent oligomerization and signaling through the adaptors TNFR-associated factor 2 (TRAF2) and TRAF3. These data suggest that ESCRTs constitutively control the distribution of cytokine receptors in their ligand-free state to restrict their signaling, which may represent a general mechanism to prevent spurious activation of NF-κB.

Activation of lymphotoxin-beta receptor enhances the LPS-induced expression of IL-8 through NF-κB and IRF-1

Lymphotoxin-beta receptor (LTβR), a receptor for LIGHT and LTα1β2, is expressed on the epithelial, stromal, and myeloid cells. LTβR is known to affect the lymphoid organ development and immune homeostasis. However, its role in macrophage function has not been sufficiently elucidated. The effect of LTβR stimulation in the inflammatory activation of macrophages was investigated by treating the human macrophage-like cell line THP-1 with LTβR-specific monoclonal antibody. Interestingly, combined treatment with anti-LTβR antibody and LPS caused the synergistic induction of IL-8 expression at the transcriptional level. Analysis indicated that nuclear factor (NF)-κB activity was enhanced via the mitogen-activated protein kinase (MAPK) and glycogen synthase kinase (GSK)-3β/cAMP response element binding protein (CREB) pathways. In addition, LTβR stimulation induced the expression of interferon regulatory factor (IRF)-1, one of the major transcription factors of IL-8 gene. Down-regulation of IRF-1 expression reduced the enhancing effect caused by LTβR stimulation. This indicates that the LTβR stimulation enhances the LPS-induced expression of IL-8 via the combined action of NF-κB and IRF-1.

The alteration of SHARPIN expression in the mouse brainstem during herpes simplex virus 1-induced facial palsy

Bell's palsy presents a unilateral weakness or paralysis of the face due to acute dysfunction of the peripheral facial nerve with no readily identifiable cause. Although data show that herpes simplex virus type 1 (HSV-1) may be the possible causative agent of Bell's palsy, the precise mechanism of the paralysis is still unknown. SHANK-associated RH domain-interacting protein (SHARPIN) is thought to play a role in the control of inflammatory responses. In order to clarify the molecular pathway of SHARPIN involved in the facial palsy caused by HSV-1 in mice and the inhibitory effect of corticosteroids, we used 4-week-old Balb/c mice inoculated with HSV-1 for experiments. The expression and location of SHARPIN in the facial nucleus of brainstem were detected respectively by quantitative real-time polymerase chain reaction, western blot and immunofluorescence. Expression level of SHARPIN increased and peaked at 2 days and then decreased in the facial nucleus of brainstem after the manifestation of the facial paralysis. After the administration of MPSS, the protein expression of SHARPIN at the peak point was down-regulated. Our results suggest that SHRPIN were activated during the inflammatory reaction in the HSV-1-induced facial paralysis. MPSS can effectively inhibit the expression of SHARPIN that may contribute to attenuate HSV-1-mediated nervous system damage.

Intermolecular binding between TIFA-FHA and TIFA-pT mediates tumor necrosis factor alpha stimulation and NF-κB activation

The forkhead-associated (FHA) domain recognizes phosphothreonine (pT) with high specificity and functional diversity. TIFA (TRAF-interacting protein with an FHA domain) is the smallest FHA-containing human protein. Its overexpression was previously suggested to provoke NF-κB activation, yet its exact roles in this signaling pathway and the underlying molecular mechanism remain unclear. Here we identify a novel threonine phosphorylation site on TIFA and show that this phosphorylated threonine (pT) binds with the FHA domain of TIFA, leading to TIFA oligomerization and TIFA-mediated NF-κB activation. Detailed analysis indicated that unphosphorylated TIFA exists as an intrinsic dimer and that the FHA-pT9 binding occurs between different dimers of TIFA. In addition, silencing of endogenous TIFA resulted in attenuation of tumor necrosis factor alpha (TNF-α)-mediated downstream signaling. We therefore propose that the TIFA FHA-pT9 binding provides a previously unidentified link between TNF-α stimulation and NF-κB activation. The intermolecular FHA-pT9 binding between dimers also represents a new mechanism for the FHA domain.

Induction of the alternative NF-κB pathway by lymphotoxin αβ (LTαβ) relies on internalization of LTβ receptor

Several tumor necrosis factor receptor (TNFR) family members activate both the classical and the alternative NF-κB pathways. However, how a single receptor engages these two distinct pathways is still poorly understood. Using lymphotoxin β receptor (LTβR) as a prototype, we showed that activation of the alternative, but not the classical, NF-κB pathway relied on internalization of the receptor. Further molecular analyses revealed a specific cytosolic region of LTβR essential for its internalization, TRAF3 recruitment, and p100 processing. Interestingly, we found that dynamin-dependent, but clathrin-independent, internalization of LTβR appeared to be required for the activation of the alternative, but not the classical, NF-κB pathway. In vivo, ligand-induced internalization of LTβR in mesenteric lymph node stromal cells correlated with induction of alternative NF-κB target genes. Thus, our data shed light on LTβR cellular trafficking as a process required for specific biological functions of NF-κB.

How do pleiotropic kinase hubs mediate specific signaling by TNFR superfamily members?

Tumor necrosis factor receptor (TNFR) superfamily members mediate the cellular response to a wide variety of biological inputs. The responses range from cell death, survival, differentiation, proliferation, to the regulation of immunity. All these physiological responses are regulated by a limited number of highly pleiotropic kinases. The fact that the same signaling molecules are involved in transducing signals from TNFR superfamily members that regulate different and even opposing processes raises the question of how their specificity is determined. Regulatory strategies that can contribute to signaling specificity include scaffolding to control kinase specificity, combinatorial use of several signal transducers, and temporal control of signaling. In this review, we discuss these strategies in the context of TNFR superfamily member signaling.

TNF Superfamily Networks: bidirectional and interference pathways of the herpesvirus entry mediator (TNFSF14)

The herpesvirus entry mediator (HVEM; TNFRSF14) can activate either proinflammatory or inhibitory signaling pathways. HVEM engages two distinct types of ligands, the canonical TNF-related cytokines, LIGHT and Lymphotoxin-α, and the Ig-related membrane proteins, BTLA (B and T lymphocyte attenuator) and CD160. Recent evidence indicates that the signal generated by HVEM depends on the context of its ligands expressed in trans or in cis. HVEM engagement by all of its ligands in trans initiates bidirectional signaling. In contrast, naïve T cells coexpress BTLA and HVEM forming a cis-complex that interferes with the activation of HVEM by extraneous ligands in the surrounding microenvironment. The HVEM Network is emerging as a key survival system for effector and memory T cells in mucosal tissues.

TRAF3 controls activation of the canonical and alternative NFkappaB by the lymphotoxin beta receptor

Components of lymphotoxin beta receptor (LTBR)-associated signaling complexes, including TRAF2, TRAF3, NIK, IKK1, and IKK2 have been shown to participate in the coupling of LTBR to NFkappaB. Here, we report that TRAF3 functions as a negative regulator of LTBR signaling via both canonical and non-canonical NFkappaB pathways by two distinct mechanisms. Analysis of NFkappaB signaling in cell lines with functionally intact NFkappaB pathway but lacking LTBR-mediated induction of NFkappaB target genes revealed an inverse association of cellular TRAF3 levels with LTBR-specific defect in canonical NFkappaB activation. Increased expression of TRAF3 correlated with its increased recruitment to LTBR-induced signaling complexes, decreased recruitment of TRAF2, and attenuated phosphorylation of IkappaB alpha and RelA. In contrast, activation of NFkappaB by TNF did not depend on TRAF3 levels. siRNA-mediated depletion of TRAF3 promoted recruitment of TRAF2 and IKK1 to activated LTBR, enabling LTBR-inducible canonical NFkappaB signaling and NFkappaB target gene expression. TRAF3 knock-down also increased mRNA and protein expression of several non-canonical NFkappaB components, including NFkappaB2/p100, RelB, and NIK, accompanied by processing of NFkappaB2/p100 into p52. These effects of TRAF3 depletion did not require LTBR signaling and were consistent with autonomous activation of the non-canonical NFkappaB pathway. Our data illustrate the function of TRAF3 as a dual-mode repressor of LTBR signaling that controls activation of canonical NFkappaB, and de-repression of the intrinsic activity of non-canonical NFkappaB. Modulation of cellular TRAF3 levels may thus contribute to regulation of NFkappaB-dependent gene expression by LTBR by affecting the balance of LTBR-dependent activation of canonical and non-canonical NFkappaB pathways.

LTbetaR signaling induces cytokine expression and up-regulates lymphangiogenic factors in lymph node anlagen

The formation of lymph nodes is a complex process crucially controlled through triggering of LTbetaR on mesenchymal cells by LTalpha(1)beta(2) expressing lymphoid tissue inducer (LTi) cells. This leads to the induction of chemokines to attract more hematopoietic cells and adhesion molecules to retain them. In this study, we show that the extravasation of the first hematopoietic cells at future lymph node locations occurs independently of LTalpha and that these cells, expressing TNF-related activation-induced cytokine (TRANCE), are the earliest LTi cells. By paracrine signaling the first expression of LTalpha(1)beta(2) is induced. Subsequent LTbetaR triggering on mesenchymal cells leads to their differentiation to stromal organizers, which now also start to express TRANCE, IL-7, as well as VEGF-C, in addition to the induced adhesion molecules and chemokines. Both TRANCE and IL-7 will further induce the expression of LTalpha(1)beta(2) on newly arrived immature LTi cells, resulting in more LTbetaR triggering, generating a positive feedback loop. Thus, LTbetaR triggering by LTi cells during lymph node development creates a local environment to which hematopoietic precursors are attracted and where they locally differentiate into fully mature, LTalpha(1)beta(2) expressing, LTi cells. Furthermore, the same signals may regulate lymphangiogenesis to the lymph node through induction of VEGF-C.

Genes involved in radiation therapy response in head and neck cancers

OBJECTIVES

This is a pilot study designed to identify gene expression profiles able to stratify head and neck squamous cell carcinoma (HNSCC) tumors that may or may not respond to chemoradiation or radiation therapy.

STUDY DESIGN

We prospectively evaluated 14 HNSCC specimens, arising from patients undergoing chemoradiotherapy or radiotherapy alone with curative intent. A complete response was assessed by clinical evaluation with no evidence of gross tumor after a 2-year follow-up period.

METHODS

Residual biopsy samples from eight complete responders (CR) and six nonresponders (NR) were evaluated by genome-wide gene expression profiling using HG-U133A 2.0 arrays. Univariate parametric t-tests with proportion of false discoveries controlled by multivariate permutation tests were used to identify genes with significantly different gene expression levels between CR and NR cases. Six different prediction algorithms were used to build gene predictor lists. Three representative genes showing 100% crossvalidation support after leave-one-out crossvalidation (LOOCV) were further validated using real-time QRT-PCR.

RESULTS

We identified 167 significant probe sets that discriminate between the two classes, which were used to build gene predictor lists. Thus, 142 probe sets showed an accuracy of prediction ranging from 93% to 100% across all six prediction algorithms. The genes represented by these 142 probe sets were further classified into different functional networks that included cellular development, cellular movement, and cancer.

CONCLUSIONS

The results presented herein offer encouraging preliminary data that may provide a basis for a more precise prognosis of HNSCC, as well as a molecular-based therapy decision for the management of these cancers.

Protein-protein interactions in TRAF3

TNF-receptor-associated factors (TRAFs) are intracellular proteins that bind to the cytoplasmic portion of TNF receptors and mediate downstream signaling. The six known TRAF proteins play overlapping yet distinct roles in controlling immune responses as well as cellular processes such as activation of NF-kappaB and JNK signaling pathways. For example, CD40 binds to TRAF2, TRAF3 and TRAF6 to control B cell differentiation, proliferation and growth. In contrast, binding of lymphotoxin-beta receptor (LTbetaR) to TRAF2 and TRAF5 propagates signals leading to activation of NF-kappaB, while binding to TRAF3 induces negative regulation of this pathway and leads to apoptosis in tumor cells. Binding recognition is mediated by specific contacts of a consensus recognition sequence in the partner with residues in a hydrophobic crevice on the TRAF molecule. Since each of these protein-protein interactions occurs within this same binding crevice, it appears that TRAF-mediated cellular mechanisms may be regulated, in part, by the level of expression or recruitment of the adaptor proteins or receptors that are competing for the crevice. The specific contacts of CD40, LTbetaR and BAFF-R have been defined in crystal structures of the complex with TRAF3. In addition, the downstream regulator TANK and the viral oncogenic protein LMP1 from the Epstein Barr virus also bind to the same TRAF crevice and these contacts have also been described crystallographically. Comparison of these five crystal structures has revealed that the recognition motifs in each of these proteins are accommodated in one TRAF3 binding crevice and that the binding interface is structurally and functionally adaptive. In this chapter, the molecular details of the interactions will be described and correlated with the functional implications for multiple TRAF3 roles in cellular regulation.

Targeting the lymphotoxin-beta receptor with agonist antibodies as a potential cancer therapy

The lymphotoxin-beta receptor (LT beta R) is a tumor necrosis factor receptor family member critical for the development and maintenance of various lymphoid microenvironments. Herein, we show that agonistic anti-LT beta R monoclonal antibody (mAb) CBE11 inhibited tumor growth in xenograft models and potentiated tumor responses to chemotherapeutic agents. In a syngeneic colon carcinoma tumor model, treatment of the tumor-bearing mice with an agonistic antibody against murine LT beta R caused increased lymphocyte infiltration and necrosis of the tumor. A pattern of differential gene expression predictive of cellular and xenograft response to LT beta R activation was identified in a panel of colon carcinoma cell lines and when applied to a panel of clinical colorectal tumor samples indicated 35% likelihood a tumor response to CBE11. Consistent with this estimate, CBE11 decreased tumor size and/or improved long-term animal survival with two of six independent orthotopic xenografts prepared from surgical colorectal carcinoma samples. Targeting of LT beta R with agonistic mAbs offers a novel approach to the treatment of colorectal and potentially other types of cancers.

The LT beta R signaling pathway

The lymphotoxin-beta receptor (LTbetaR, TNFRSF3) signaling pathway activates gene transcription programs and cell death important in immune development and host defense. The TNF receptor associated factors (TRAF)-2, 3 and 5 function as adaptors linking LTbetaR signaling targets. Interestingly, TRAF deficient mice do not phenocopy mice deficient in components of the LTbetaR pathway, presenting a conundrum. Here, an update of our understanding and models of the LTbetaR signaling pathway are reviewed, with a focus on this conundrum.

Gene expression changes in long-term culture of T-cell clones: genomic effects of chronic antigenic stress in aging and immunosenescence

The adaptive immune response requires waves of T-cell clonal expansion on contact with altered self and contraction after elimination of antigen. In the case of persisting antigen, as occurs for example in cytomegalovirus or Epstein–Barr virus infection, this critical process can become dysregulated and responding T-cells enter into a dysfunctional senescent state. Longitudinal studies suggest that the presence of increased numbers of such T-cells is a poor prognostic factor for survival in the very elderly. Understanding the nature of the defects in these T-cells might facilitate intervention to improve immunity in the elderly. The process of clonal expansion under chronic antigenic stress can be modelled in vitro using continuously cultured T-cells. Here, we have used cDNA array technology to investigate differences in gene expression in a set of five different T-cell clones at early, middle and late passage in culture. Differentially expressed genes were confirmed by real-time polymerase chain reaction, and relationships between these assessed using Ingenuity Systems evidence-based association analysis. Several genes and chemokines related to induction of apoptosis and signal transduction pathways regulated by transforming growth factor β (TGFβ), epidermal growth factor (EGF), fos and β-catenin were altered in late compared to early passage cells. These pathways and affected genes may play a significant role in driving the cellular senescent phenotype and warrant further investigation as potential biomarkers of aging and senescence. These genes may additionally provide targets for intervention.

Targeting BAFF: Immunomodulation for autoimmune diseases and lymphomas

In an effort to develop more effective treatments for inflammatory diseases, immunologists have targeted numerous molecular pathways, but with limited success. Notable exceptions are anti-TNF agents, which have proved efficacious in a proportion of rheumatoid arthritis (RA) patients. Another TNF family member, termed BAFF ("B cell-activating factor belonging to the TNF family"), plays a central role in autoimmune diseases, as well as in B cell maturation, survival, and T cell activation. Agents that block BAFF have proven to be highly effective in the treatment of certain autoimmune conditions in mice. In addition, phase II data in human clinical trials for RA appear very promising. BAFF is also a survival factor for certain B cell lymphomas. Despite the relatively recent identification of BAFF, this molecule has provided considerable new insight into B cell homeostasis and immune function, and represents an important new molecular target for treatment of autoimmune diseases and lymphomas.

TRAF6 activation of PI 3-kinase-dependent cytoskeletal changes is cooperative with Ras and is mediated by an interaction with cytoplasmic Src

Interleukin 1 (IL-1) has been implicated in the reorganization of the actin cytoskeleton. An expression vector encoding a PKB/Akt pleckstrin-homology domain fused to a fluorescent protein was used to detect phosphoinositide 3-kinase (PI 3-kinase) products. It was observed that PI 3-kinase was activated either by treatment with IL-1 or by expression of either TRAF6, Src, MyD88 or dominant-positive PI 3-kinase, and resulted in the formation of long filopodia-like cellular protrusions that appeared to branch at membrane sites consisting of clusters of phosphoinositide. This depended upon a TRAF6 polyproline motif and Src catalytic activity, and was blocked by inhibitors of PI 3-kinase, Src and Ras. Using both conventional and split fluorescent protein probes fused to expressed TRAF6 and Src in living cells, the polyproline sequence of TRAF6 and the Src-homology 3 (SH3) domain of Src were shown to be required for interaction between these two proteins. Interaction occurred within the cytoplasm, and not at either the cell membrane or cytoplasmic sequestosomes. In addition, co-transfection of vectors expressing fluorescent-protein-fused TRAF6 and non-fluorescent MyD88, IRAK1 and IRAK2 revealed an inverse correlation between increased sequestosome formation and activation of both PI 3-kinase and NF-kappaB. Although a key factor in TRAF6-dependent activation of PI 3-kinase, ectopic expression of Src was insufficient for NF-kappaB activation and, in contrast to NF-kappaB, was not inhibited by IRAK2.

Essential role of TNF family molecule LIGHT as a cytokine in the pathogenesis of hepatitis

LIGHT is an important costimulatory molecule for T cell immunity. Recent studies have further implicated its role in innate immunity and inflammatory diseases, but its cellular and molecular mechanisms remain elusive. We report here that LIGHT is upregulated and functions as a proinflammatory cytokine in 2 independent experimental hepatitis models, induced by concanavalin A and Listeria monocytogenes. Molecular mutagenesis studies suggest that soluble LIGHT protein produced by cleavage from the cell membrane plays an important role in this effect through the interaction with the lymphotoxin-beta receptor (LTbetaR) but not herpes virus entry mediator. NK1.1+ T cells contribute to the production, but not the cleavage or effector functions, of soluble LIGHT. Importantly, treatment with a mAb that specifically interferes with the LIGHT-LTbetaR interaction protects mice from lethal hepatitis. Our studies thus identify a what we believe to be a novel function of soluble LIGHT in vivo and offer a potential target for therapeutic interventions in hepatic inflammatory diseases.

TRAF2 plays a key, nonredundant role in LIGHT-lymphotoxin beta receptor signaling

LIGHT is a member of the tumor necrosis factor (TNF) superfamily, and its function is mediated by at least two receptors, including lymphotoxin beta receptor (LTbetaR) and herpes simplex virus entry mediator. However, the molecular mechanism of LIGHT signaling mediated by LTbetaR has not been clearly defined. In this report, we demonstrate that TRAF2 is critical for LIGHT- and LTbetaR-mediated activation of both the transcription factor NF-kappaB and the mitogen-activated protein kinase JNK. In HeLa cells, LIGHT induces NF-kappaB and JNK activation, which can be blocked by the dominant negative mutant of TRAF2. In these cells, LIGHT causes the recruitment of TRAF2, TRAF3, and IkappaB kinase into the LTbetaR complex. Importantly, while both NF-kappaB and JNK are activated by LIGHT in wild-type mouse embryonic fibroblasts, no activation of either of these two pathways is observed in TRAF2 null fibroblasts. However, LIGHT-induced NF-kappaB and JNK activation can be restored by ectopic expression of TRAF2 in TRAF2-/- cells. Interestingly, in contrast to TNF signaling, the activation of both NF-kappaB and JNK by LIGHT was normal in RIP-/- and TRAF5-/- cells. Taken together, our data demonstrate that TRAF2, an important effector molecule of TNF signaling, plays a critical, nonredundant role in LIGHT-LTbetaR signaling.

TNF receptor (TNFR)-associated factor (TRAF) 3 serves as an inhibitor of TRAF2/5-mediated activation of the noncanonical NF-kappaB pathway by TRAF-binding TNFRs

TNF family members and their receptors contribute to increased gene expression for inflammatory processes and intracellular cascades leading to programmed cell death, both via activation of NF-kappaB. TNF receptor (TNFR)-associated factors (TRAFs) are cytoplasmic adaptor proteins binding to various receptors of the TNFR family. In an attempt to delineate the role of individual TRAFs, we compared NF-kappaB activation by CD40(wt) and CD40 mutants with different TRAF recruitment patterns. Recognized only recently, NF-kappaB signaling occurs at least via two different pathways. Each pathway results in nuclear translocation of two different Reldimers, the canonical p50/RelA and the noncanonical p52/RelB. Here, we show that via TRAF6, CD40 mediates only the activation of the canonical NF-kappaB pathway. Via TRAF2/5, CD40 activates both the canonical and the noncanonical NF-kappaB pathways. We observed that TRAF3 specifically blocked the NF-kappaB activation via TRAF2/5. This inhibitory effect of TRAF3 depends on the presence of an intact zinc finger domain. Paradoxically, suppression of TRAF2/5-mediated NF-kappaB activation by TRAF3 resulted in enhanced transcriptional activity of TRAF6-mediated canonical NF-kappaB emanating from CD40. We also observed that 12 TNFR family members (p75TNFR, LTbetaR, RANK, HVEM, CD40, CD30, CD27, 4-1BB, GITR, BCMA, OX40, and TACI) are each capable of activating the alternative NF-kappaB pathway and conclude that TRAF3 serves as a negative regulator of this pathway for all tested receptors.

TRAF3 forms heterotrimers with TRAF2 and modulates its ability to mediate NF-{kappa}B activation

FRET experiments utilizing confocal microscopy or flow cytometry assessed homo- and heterotrimeric association of human tumor necrosis factor receptor-associated factors (TRAF) in living cells. Following transfection of HeLa cells with plasmids expressing CFP- or YFP-TRAF fusion proteins, constitutive homotypic association of TRAF2, -3, and -5 was observed, as well as heterotypic association of TRAF1-TRAF2 and TRAF3-TRAF5. A novel heterotypic association between TRAF2 and -3 was detected and confirmed by immunoprecipitation in Ramos B cells that constitutively express both TRAF2 and -3. Experiments employing deletion mutants of TRAF2 and TRAF3 revealed that this heterotypic interaction minimally involved the TRAF-C domain of TRAF3 as well as the TRAF-N domain and zinc fingers 4 and 5 of TRAF2. A novel flow cytometric FRET analysis utilizing a two-step approach to achieve linked FRET from CFP to YFP to HcRed established that TRAF2 and -3 constitutively form homo- and heterotrimers. The functional importance of TRAF2-TRAF3 heterotrimerization was demonstrated by the finding that TRAF3 inhibited spontaneous NF-kappaB, but not AP-1, activation induced by TRAF2. Ligation of CD40 on Ramos B cells by recombinant CD154 caused TRAF2 and TRAF3 to dissociate, whereas overexpression of TRAF3 in Ramos B cells inhibited CD154-induced TRAF2-mediated activation of NF-kappaB. Together, these results reveal a novel association between TRAF2 and TRAF3 that is mediated by unique portions of each protein and that specifically regulates activation of NF-kappaB, but not AP-1.

Structurally distinct recognition motifs in lymphotoxin-beta receptor and CD40 for tumor necrosis factor receptor-associated factor (TRAF)-mediated signaling

Lymphotoxin-beta receptor (LTbetaR) and CD40 are members of the tumor necrosis factor family of signaling receptors that regulate cell survival or death through activation of NF-kappaB. These receptors transmit signals through downstream adaptor proteins called tumor necrosis factor receptor-associated factors (TRAFs). In this study, the crystal structure of a region of the cytoplasmic domain of LTbetaR bound to TRAF3 has revealed an unexpected new recognition motif, 388IPEEGD393, for TRAF3 binding. Although this motif is distinct in sequence and structure from the PVQET motif in CD40 and PIQCT in the regulator TRAF-associated NF-kappaB activator (TANK), recognition is mediated in the same binding crevice on the surface of TRAF3. The results reveal structurally adaptive "hot spots" in the TRAF3-binding crevice that promote molecular interactions driving specific signaling after contact with LTbetaR, CD40, or the downstream regulator TANK.

TWEAK induces NF-kappaB2 p100 processing and long lasting NF-kappaB activation

Tumor necrosis factor (TNF)-like weak inducer of apoptosis (TWEAK) is a member of the TNF superfamily that has been shown to induce angiogenesis, apoptosis in tumor cells, and NF-kappaB activation through binding to its receptor, fibroblast growth factor-inducible 14. We have identified TWEAK as an inducer of constitutive NF-kappaB activation by expression cloning, and we report here sequential regulation by TWEAK of two separate signaling cascades for NF-kappaB activation, the NF-kappaB essential modulator-dependent and -independent signaling pathways. Upon TWEAK stimulation, IkappaBalpha is rapidly phosphorylated, generating NF-kappaB DNA-binding complexes containing p50 and RelA in a manner dependent on the canonical IkappaB kinase complex. Unlike TNF-alpha, TWEAK stimulation results in prolonged NF-kappaB activation with a transition of the DNA-binding NF-kappaB components from RelA- to RelB-containing complexes by 8 h, and the latter remained active in binding at least until 24 h post-stimulation. This long lasting activation is accompanied by the proteasome-mediated processing of NF-kappaB2/p100, which does not depend on the NF-kappaB essential modulator but requires IkappaB kinase 1 and functional NF-kappaB-inducing kinase activity. Finally, we show that fibroblast growth factor-inducible 14 with a mutation at its TNF receptor-associated factor (TRAF)-binding site cannot activate NF-kappaB and that TWEAK fails to induce the p100 processing and IkappaBalpha phosphorylation in cells deficient for TRAF2 and TRAF5. Our results thus identify TWEAK as a novel physiological regulator of the non-canonical pathway for NF-kappaB activation.

The role of apoptosis signal-regulating kinase 1 in lymphotoxin-beta receptor-mediated cell death

LIGHT (homologous to lymphotoxins, shows inducible expression, and competes with herpes simplex virus glycoprotein D for herpesvirus entry mediator, a receptor expressed by T lymphocytes) is a member of the tumor necrosis factor superfamily that can interact with lymphotoxin-beta receptor (LTbetaR), herpes virus entry mediator, and decoy receptor (DcR3). In our previous study, we showed that LIGHT is able to induce cell death via the non-death domain containing receptor LTbetaR to activate both caspase-dependent and caspase-independent pathway. In this study, a LIGHT mutein, LIGHT-R228E, was shown to exhibit similar binding specificity as wild type LIGHT to LTbetaR, but lose the ability to interact with herpes virus entry mediator. By using both LIGHT-R228E and agonistic anti-LTbetaR monoclonal antibody, we found that signaling triggered by LTbetaR alone is sufficient to activate both caspase-dependent and caspase-independent pathways. Cross-linking of LTbetaR is able to recruit TRAF3 and TRAF5 to activate ASK1, whereas its activity is inhibited by free radical scavenger carboxyfullerenes. The activation of ASK1 is independent of caspase-3 activation, and kinase-inactive ASK1-KE mutant can inhibit LTbetaR-mediated cell death. This suggests that ASK1 is one of the factors involved in the caspase-independent pathway of LTbetaR-induced cell death.

Loss of TACI causes fatal lymphoproliferation and autoimmunity, establishing TACI as an inhibitory BLyS receptor

BLys , a key cytokine that sustains B cell maturation and tolerance, binds three receptors: BR3, BCMA, and TACI. Results from knockout mice implicate a major functional role for BR3 and a redundant one for BCMA in B cell function. TACI's role is controversial based on defects in TI antibody responses accompanied by B cell hyperplasia in knockout mice. We have presently characterized a precise role for TACI in vivo. TACI(-/-) mice develop fatal autoimmune glomerulonephritis, proteinurea, and elevated levels of circulating autoantibodies. Treatment of B cells with TACI agonistic antibodies inhibits proliferation in vitro and activation of a chimeric receptor containing the TACI intracellular domain induces apoptosis. These results demonstrate the critical requirement for TACI in regulating B cell homeostasis.

TNFR-associated factor-3 is associated with BAFF-R and negatively regulates BAFF-R-mediated NF-kappa B activation and IL-10 production

TALL-1 is a member of the TNF family that is critically involved in B cell survival, maturation, and progression of lupus-like autoimmune diseases. TALL-1 has three receptors, including BCMA, TACI, and BAFF-R, which are mostly expressed by B lymphocytes. Gene knockout studies have indicated that BAFF-R is the major stimulatory receptor for TALL-1 signaling and is required for normal B cell development. The intracellular signaling mechanisms of BAFF-R are not known. In this report, we attempted to identify BAFF-R-associated downstream proteins by yeast two-hybrid screening. This effort identified TNFR-associated factor (TRAF)3 as a protein specifically interacting with BAFF-R in yeast two-hybrid assays. Coimmunoprecipitation experiments indicated that BAFF-R interacts with TRAF3 in B lymphoma cells and this interaction is stimulated by TALL-1 treatment. Domain mapping experiments indicated that both a 6-aa membrane proximal region and the C-terminal 35 aa of BAFF-R are required for its interaction with TRAF3. Moreover, overexpression of TRAF3 inhibits BAFF-R-mediated NF-kappaB activation and IL-10 production. Taken together, our findings suggest that TRAF3 is a negative regulator of BAFF-R-mediated NF-kappaB activation and IL-10 production.

Lymphotoxin-beta receptor mediates NEMO-independent NF-kappaB activation

Lymphotoxin-beta receptor (LTbetaR) is a member of the tumor necrosis factor receptor (TNFR) superfamily that activates nuclear factor-kappaB (NF-kappaB) through the IkappaB kinase (IKK) complex, the core of which is comprised of IKK1, IKK2 and NF-kappaB essential modulator (NEMO). We demonstrate here that the LTbetaR signaling to NF-kappaB activation does not necessarily require NEMO, which is essential for TNFR signaling. In the absence of NEMO, the p50 and RelB, but not RelA subunits of NF-kappaB are found in the nuclear DNA binding complexes induced by the LTbetaR signaling. Our results thus disclose NEMO-independent NF-kappaB activation by LTbetaR.

The lymphotoxin-beta receptor induces different patterns of gene expression via two NF-kappaB pathways

The lymphotoxin-beta receptor (LTbetaR) plays critical roles in inflammation and lymphoid organogenesis through activation of NF-kappaB. In addition to activation of the classical NF-kappaB, ligation of this receptor induces the processing of the cytosolic NF-kappaB2/p100 precursor to yield the mature p52 subunit, followed by translocation of p52 to the nucleus. This activation of NF-kappaB2 requires NIK and IKKalpha, while NEMO/IKKgamma is dispensable for p100 processing. IKKbeta-dependent activation of canonical NF-kappaB is required for the expression but not processing of p100 and for the expression of proinflammatory molecules including VCAM-1, MIP-1beta, and MIP-2 in response to LTbetaR ligation. In contrast, IKKalpha controls the induction by LTbetaR ligation of chemokines and cytokines involved in lymphoid organogenesis, including SLC, BLC, ELC, SDF1, and BAFF.

Structurally adaptive hot spots at a protein interaction interface on TRAF3

Tumor necrosis factor (TNF) signaling is controlled by receptors and intracellular signaling pathways that activate the NF-kappaB transcription factor. The resulting signals elicit immune responses and have important implications for disorders such as autoimmunity or allergic reactions. TNF-receptor-associated factors (TRAFs) bind to the cytoplasmic portion of TNFRs as well as downstream regulators and thus are co-inducers of the signal transduction. TRAF3 binds to diverse receptors and regulators by accomodating a conserved motif that is embedded in completely different structural frameworks. Thus, the protein-protein contact region on TRAF3 represents a binding interface that is structurally and functionally adaptive. In this report, three 'hot spots' at the TRAF3 protein-interaction interface are defined that provide the principal contact regions for different binding partners. The side-chains of residues at these 'hot spots' are flexible and undergo movements on binding the different partners. These side chain rearrangements provide a structural adaptability that promotes interaction with a variety of distinct proteins. It is proposed that similar adaptive 'hot spots' are also present on the binding surfaces of TRAF1, TRAF2 and TRAF5.

Essential role of NF-kappa B-inducing kinase in T cell activation through the TCR/CD3 pathway

NF-kappa B-inducing kinase (NIK) is involved in lymphoid organogenesis in mice through lymphotoxin-beta receptor signaling. To clarify the roles of NIK in T cell activation through TCR/CD3 and costimulation pathways, we have studied the function of T cells from aly mice, a strain with mutant NIK. NIK mutant T cells showed impaired proliferation and IL-2 production in response to anti-CD3 stimulation, and these effects were caused by impaired NF-kappa B activity in both mature and immature T cells; the impaired NF-kappa B activity in mature T cells was also associated with the failure of maintenance of activated NF-kappa B. In contrast, responses to costimulatory signals were largely retained in aly mice, suggesting that NIK is not uniquely coupled to the costimulatory pathways. When NIK mutant T cells were stimulated in the presence of a protein kinase C (PKC) inhibitor, proliferative responses were abrogated more severely than in control mice, suggesting that both NIK and PKC control T cell activation in a cooperative manner. We also demonstrated that NIK and PKC are involved in distinct NF-kappa B activation pathways downstream of TCR/CD3. These results suggest critical roles for NIK in setting the threshold for T cell activation, and partly account for the immunodeficiency in aly mice.

NFkappaB-dependent signaling pathways

The transcription factor NFkappaB is activated by numerous stimuli. Once NFkappaB is fully activated, it participates in the regulation of various target genes in different cells to exert its biological functions. NFkappaB has often been referred to as a central mediator of the immune response, since a large variety of bacteria and viruses can lead to the activation of NFkappaB, which in turn controls the expression of many inflammatory cytokines, chemokines, immune receptors, and cell surface adhesion molecules. Recent studies have shown that NFkappaB may function more generally as a central regulator of stress responses, since different stressful conditions, including physical stress, oxidative stress, and exposure to certain chemicals, also lead to NFkappaB activation. Furthermore, NFkappaB blocks cell apoptosis in several cell types. Taken together, these findings make it clear that NFkappaB plays an important role in cell proliferation and differentiation. It is the intention of this review to cover the various NFkappaB-dependent signaling pathways, thereby to achieve a better understanding of the mechanisms of NFkappaB activation and the physiological functions of activated NFkappaB.

All TRAFs are not created equal: common and distinct molecular mechanisms of TRAF-mediated signal transduction

The tumor necrosis factor (TNF) receptor associated factors (TRAFs) have emerged as the major signal transducers for the TNF receptor superfamily and the interleukin-1 receptor/Toll-like receptor (IL-1R/TLR) superfamily. TRAFs collectively play important functions in both adaptive and innate immunity. Recent functional and structural studies have revealed the individuality of each of the mammalian TRAFs and advanced our understanding of the underlying molecular mechanisms. Here, we examine this functional divergence among TRAFs from a perspective of both upstream and downstream TRAF signal transduction pathways and of signaling-dependent regulation of TRAF trafficking. We raise additional questions and propose hypotheses regarding the molecular basis of TRAF signaling specificity.

Signaling and subcellular localization of the TNF receptor Edar

Tabby and downless mutant mice have identical phenotypes characterized by deficient development of several ectodermally derived organs such as teeth, hair, and sweat glands. Edar, encoded by the mouse downless gene and defective in human dominant and recessive forms of autosomal hypohidrotic ectodermal dysplasia (EDA) syndrome, is a new member of the tumor necrosis factor (TNF) receptor superfamily. The ligand of Edar is ectodysplasin, a TNF-like molecule mutated in the X-linked form of EDA and in the spontaneous mouse mutant Tabby. We have analyzed the response of Edar signaling in transfected cells and show that it activates nuclear factor-kappaB (NF-kappaB) in a dose-dependent manner. When Edar was expressed at low levels, the NF-kappaB response was enhanced by coexpression of ectodysplasin. The activation of NF-kappaB was greatly reduced in cells expressing mutant forms of Edar associated with the downless phenotype. Overexpression of Edar did not activate SAPK/JNK nor p38 kinase. Even though Edar harbors a death domain its overexpression did not induce apoptosis in any of the four cell lines analyzed, nor was there any difference in apoptosis in developing teeth of wild-type and Tabby mice. Additionally, we show that the subcellular localization of dominant negative alleles of downless is dramatically different from that of recessive or wild-type alleles. This together with differences in NF-kappaB responses suggests an explanation for the different mode of inheritance of the different downless alleles.

Lymphotoxins and cytomegalovirus cooperatively induce interferon-beta, establishing host-virus détente

Tumor necrosis factor (TNF)-related cytokines regulate cell death and survival and provide strong selective pressures for viruses, such as cytomegalovirus (CMV), to evolve counterstrategies in order to persist in immune-competent hosts. Signaling by the lymphotoxin (LT)-beta receptor or TNF receptor-1, but not Fas or TRAIL receptors, inhibits the cytopathicity and replication of human CMV by a nonapoptotic, reversible process that requires nuclear factor kappa B (NF-kappa B)-dependent induction of interferon-beta (IFN-beta). Efficient induction of IFN-beta requires virus infection and LT signaling, demonstrating the need for both host and viral factors in the curtailment of viral replication without cellular elimination. LT alpha-deficient mice and LT beta R-Fc transgenic mice were profoundly susceptible to murine CMV infection. Together, these results reveal an essential and conserved role for LTs in establishing host defense to CMV.

Essential role of nuclear factor (NF)-kappaB-inducing kinase and inhibitor of kappaB (IkappaB) kinase alpha in NF-kappaB activation through lymphotoxin beta receptor, but not through tumor necrosis factor receptor I

Both nuclear factor (NF)-kappaB-inducing kinase (NIK) and inhibitor of kappaB (IkappaB) kinase (IKK) have been implicated as essential components for NF-kappaB activation in response to many external stimuli. However, the exact roles of NIK and IKKalpha in cytokine signaling still remain controversial. With the use of in vivo mouse models, rather than with enforced gene-expression systems, we have investigated the role of NIK and IKKalpha in signaling through the type I tumor necrosis factor (TNF) receptor (TNFR-I) and the lymphotoxin beta receptor (LTbetaR), a receptor essential for lymphoid organogenesis. TNF stimulation induced similar levels of phosphorylation and degradation of IkappaBalpha in embryonic fibroblasts from either wild-type or NIK-mutant mice. In contrast, LTbetaR stimulation induced NF-kappaB activation in wild-type mice, but the response was impaired in embryonic fibroblasts from NIK-mutant and IKKalpha-deficient mice. Consistent with the essential role of IKKalpha in LTbetaR signaling, we found that development of Peyer's patches was defective in IKKalpha-deficient mice. These results demonstrate that both NIK and IKKalpha are essential for the induction of NF-kappaB through LTbetaR, whereas the NIK-IKKalpha pathway is dispensable in TNFR-I signaling.

Three adenovirus E3 proteins cooperate to evade apoptosis by tumor necrosis factor-related apoptosis-inducing ligand receptor-1 and -2

Adenovirus encodes multiple gene products that regulate proapoptotic cellular responses to viral infection mediated by both the innate and adaptive immune systems. The E3-10.4K and 14.5K gene products are known to modulate the death receptor Fas. In this study, we demonstrate that an additional viral E3 protein, 6.7K, functions in the specific modulation of the two death receptors for tumor necrosis factor-related apoptosis-inducing ligand (TRAIL). The 6.7K protein is expressed on the cell surface and forms a complex with the 10.4K and 14.5K proteins, and this complex is sufficient to induce down-modulation of TRAIL receptor-1 and -2 from the cell surface and reverse the sensitivity of infected cells to TRAIL-mediated apoptosis. Down-modulation of TRAIL-R2 by the E3 complex is dependent on the cytoplasmic tail of the receptor, but the death domain alone is not sufficient. These results identify a mechanism for viral modulation of TRAIL receptor-mediated apoptosis and suggest the E3 protein complex has evolved to regulate the signaling of selected cytokine receptors.

The lymphotoxin-beta receptor is necessary and sufficient for LIGHT-mediated apoptosis of tumor cells

LIGHT is a tumor necrosis factor (TNF) ligand superfamily member, which binds two known cellular receptors, lymphotoxin-beta receptor (LTbetaR) and the herpesvirus entry mediator (HveA). LIGHT is a homotrimer that activates proapoptotic and integrin-inducing pathways. Receptor binding residues via LIGHT were identified by introducing point mutations in the A' --> A" and D --> E loops of LIGHT, which altered binding to LTbetaR and HveA. One mutant of LIGHT exhibits selective binding to HveA and is inactive triggering cell death in HT29.14s cells or induction of ICAM-1 in fibroblasts. Studies with HveA- or LTbetaR-specific antibodies further indicated that HveA does not contribute, either cooperatively or by direct signaling, to the death pathway activated by LIGHT. LTbetaR, not HveA, recruits TNF receptor-associated factor-3 (TRAF3), and LIGHT-induced death is blocked by a dominant negative TRAF3 mutant. Together, these results indicate that TRAF3 recruitment propagates death signals initiated by LIGHT-LTbetaR interaction and implicates a distinct biological role for LIGHT-HveA system.