A regulatory role for TRAF1 in antigen-induced apoptosis of T cells

Relationship between Mycobacterium avium subspecies paratuberculosis, IL-1alpha, and TRAF1 in primary bovine monocyte-derived macrophages

Mycobacterium avium subspecies paratuberculosis (MAP) is a facultative intracellular pathogen that resides in host macrophage cells. Presently, little is known about how MAP is able to subvert the normal bacteriocidal functions of infected macrophages. Previously, we reported that ileal tissues from MAP infected cattle contained high levels of interleukin-1 alpha (IL-1alpha) and tumor necrosis factor receptor-associated factor 1 (TRAF1), relative to ileal tissues from uninfected cattle. High-level expression of these two proteins could have profound effects on macrophage function, intracellular signaling, and apoptosis. We now demonstrate that high levels of TRAF1 protein are located primarily within macrophages infiltrating areas of MAP infection. We have also utilized cultured bovine monocyte-derived macrophage cells (MDM) either infected with live MAP or stimulated with recombinant IL-1alpha (rIL-1alpha) to determine if there is a relationship between IL-1alpha and TRAF1 expression. These studies have identified a dose dependent increase in TRAF1 protein levels in bovine MDM in response to infection with live MAP or following treatment with rIL-1alpha. Sustained TRAF1 protein expression was dependent upon interaction of rIL-1alpha with it's receptor and rIL-1beta was also able to enhance TRAF1 gene expression. Our results suggest that MAP may use the IL-1-TRAF1 system to enhance TRAF1 protein expression in infected bovine MDM. These novel results provide evidence for a new avenue of research on the effect of MAP and other intracellular pathogens on macrophage signaling and apoptosis.

A critical role for TNF receptor-associated factor 1 and Bim down-regulation in CD8 memory T cell survival

The mechanisms that allow the maintenance of immunological memory remain incompletely defined. Here we report that tumor necrosis factor receptor (TNFR)-associated factor (TRAF) 1, a protein recruited in response to several costimulatory TNFR family members, is required for maximal CD8 T cell responses to influenza virus in mice. Decreased recovery of CD8 T cells in vivo occurred under conditions where cell division was unimpaired. In vitro, TRAF1-deficient, antigen-activated T cells accumulated higher levels of the proapoptotic BH3-only family member Bim, particularly the most toxic isoform, Bim(S). In the presence of excess IL-15, memory phenotype T cells with similar surface phenotype and comparable levels of Bcl-2 family members could be generated from WT or TRAF1-deficient T cell receptor transgenic OT-I T cells. However, when the memory CD8 T cells were allowed to compete for survival signals in the absence of antigen in vivo, the TRAF1-deficient T cells showed decreased recovery compared with TRAF1-sufficient T cells. This defect in T cell recovery in vivo was alleviated by introduction of siRNA to down-modulate Bim in TRAF1-deficient memory T cells. These studies identify the TRAF1 signaling axis and Bim down-regulation as critical for CD8 memory T cell survival in vivo.

IL-15-dependent induction of 4-1BB promotes antigen-independent CD8 memory T cell survival

Mice lacking CD137L (4-1BBL) show normal primary expansion and contraction of the CD8+ T cell response to influenza virus, but exhibit a defect in Ag-specific CD8+ T cell numbers at 3-6 wk postinfection. Previous results showed that the decrease in CD8+ T cell numbers in this model is not due to a programming defect during primary expansion. Thus, it appears that 4-1BB/4-1BBL interactions control the number of surviving CD8+ effector memory cells, late in the primary response. In this report, we asked how 4-1BB on T cells could play a role after Ag has apparently been cleared from the host. We show that IL-15, a cytokine involved in regulation of CD8+ memory T cell survival, induces the expression of 4-1BB on CD8+CD44(high) memory phenotype T cells, but not on CD4+ T cells. The Ag-independent induction of 4-1BB by IL-15 was dependent on MAPK p38 and ERK activation. Transfer of in vitro-generated OT-I CD8+ memory T cells into unimmunized wild-type or 4-1BBL-deficient hosts revealed a 2- to 3-fold survival advantage when 4-1BBL was present, recapitulating the effect seen in the endogenous response to influenza in mice. Decreases in the overall number of memory CD8+ T cells were also observed in the bone marrow of unmanipulated 4-1BBL-deficient mice. These data suggest a model whereby 4-1BB expression on memory CD8+ T cells, perhaps due to encounter with IL-15 in the bone marrow, allows 4-1BB/4-1BBL interactions to maintain memory CD8 T cell survival in the absence of Ag.

Cooperation between TNF Receptor-Associated Factors 1 and 2 in CD40 Signaling

TNFR-associated factor 1 (TRAF1) is unique among the TRAF family, lacking most zinc-binding features, and showing marked up-regulation following activation signals. However, the biological roles that TRAF1 plays in immune cell signaling have been elusive, with many reports assigning contradictory roles to TRAF1. The overlapping binding site for TRAFs 1, 2, and 3 on many TNFR superfamily molecules, together with the early lethality of mice deficient in TRAFs 2 and 3, has complicated the quest for a clear understanding of the functions of TRAF1. Using a new method for gene targeting by homologous recombination in somatic cells, we produced and studied signaling by CD40 and its viral oncogenic mimic, latent membrane protein 1 (LMP1) in mouse B cell lines lacking TRAF1, TRAF2, or both TRAFs. Results indicate that TRAFs 1 and 2 cooperate in CD40-mediated activation of the B cell lines, with a dual deficiency leading to a markedly greater loss of function than that of either TRAF alone. In the absence of TRAF1, an increased amount of TRAF2 was recruited to lipid rafts, and subsequently, more robust degradation of TRAF2 and TRAF3 was induced in response to CD40 signaling. In contrast, LMP1 did not require either TRAFs 1 or 2 to induce activation. Taken together, our findings indicate that TRAF1 and TRAF2 cooperate in CD40 but not LMP1 signaling and suggest that cellular levels of TRAF1 may play an important role in modulating the degradation of TRAF2 and TRAF3 in response to signals from the TNFR superfamily.

Expression of the tumor necrosis factor receptor-associated factors 1 and 2 and regulation of the nuclear factor-kappaB antiapoptotic activity in human gliomas

OBJECT

Tumor necrosis factor receptor (TNFR)-associated factors (TRAFs) are a recently established group of proteins involved in the intracellular signaling of the TNFR superfamily members. The TRAFs have been implicated in promoting cell survival through the activation of transcription factor nuclear factor (NF)-kappaB. The authors investigated the expression of NF-kappaB, caspase 3, TRAF1, TRAF2, and TRAF-associated NF-kappaB activator/TRAF-interacting protein (TANK/I-TRAF), a regulator of TRAF activity, in human gliomas.

METHODS

Tumor samples were obtained in 27 adult patients harboring seven low-grade gliomas, nine anaplastic astrocytomas, and 11 glioblastomas multiforme. The NF-kappaB activation was analyzed using the electrophoresis mobility shift assay; TRAF1, TRAF2, TANK/I-TRAF, and caspase 3 expression were studied using Western blot analysis. Upregulated NF-kappaB DNA-binding activity, compared with that in normal brain tissue, was detected in all tumor samples (p = 0.002). The level of NF-kappaB activity showed some correlation with World Health Organization tumor grades (p = 0.01), even though variable activity levels were demonstrated in relation to tissue heterogeneity, which resulted in a substantial number of outliers in the quantitative analysis. Increased levels of TRAF1, TRAF2, and TANK/ I-TRAF were expressed in astrocytomas compared with levels in normal brain tissue (p = 0.02, 0.006, and 0.01, respectively).

CONCLUSIONS

Data in this study confirm the upregulation of NF-kappaB in gliomas and reveal a correlation between levels of this transcription factor and tumor grade. A constitutive expression of TRAF1, TRAF2, and TANK/I-TRAF in human gliomas was documented. These proteins are involved in the intracellular signal transduction of the TNFR superfamily and in the control of NF-kappaB expression and its antiapoptotic activity.

Modulation of life and death by the tumor necrosis factor receptor-associated factors (TRAFs)

The TNF receptor-associated factor (TRAF) family is a group of adapter proteins that link a wide variety of cell surface receptors. Including the TNF and IL-1 receptor superfamily to diverse signaling cascades, which lead to the activation of NF-kappaB and mitogen-activated protein kinases. In addition, TRAFs interact with a variety of proteins that regulate receptor-induced cell death or survival. Thus, TRAF-mediated signals may directly induce cell survival or interfere with the death receptor-induced apoptosis.

The expression and the regulatory role of OX40 and 4-1BB heterodimer in activated human T cells

OX40 and 4-1BB are members of the tumor necrosis factor (TNF) family of costimulatory receptors whose signaling is important for differential immune responses mediated by CD4+ or CD8+ T cells. Although activated T cells may acquire OX40/4-1BB double-positive phenotype and signaling from each receptor is expected to influence cell functions, the relevance between OX40 and 4-1BB has never been investigated before. While we were investigating the expression of OX40 and 4-1BB on activated human T cells, we found that they colocalize. The study of receptor gene–transfected cells showed that both receptors coendocytose and the complex of OX40 and 4-1BB was detected by specific ligands or antibodies (Abs). The heterodimer of OX40 and 4-1BB was identified by sodium dodecyl sulfate–polyacrylamide gel electrophoresis (SDS-PAGE) under nonreduced conditions and was associated with the tumor receptor–associated factor (TRAF) family proteins in a unique manner. Furthermore, the stimulation of OX40/4-1BB rendered cells sensitive to apoptosis induced by TNF-α that accompanied reduced activation of nuclear factor-κB (NF-κB). Finally, the OX40/4-1BB stimulation repressed the mitogen response in activated CD25+CD4+ T cells and preactivated CD8+ T cells. Thus, the OX40/4-1BB heterodimer appears to represent a unique regulatory receptor in activated T cells.

Acute tumor necrosis factor-alpha-induced liver injury in the absence of tumor necrosis factor receptor-associated factor 1 gene expression

Pulmonary and serum levels of tumor necrosis factor-alpha (TNF-alpha), are elevated in many lung diseases, causing local inflammation, fever, and multiorgan, including hepatic, dysfunction. Cellular responses to TNF-alpha are determined by recruitment of specific proteins to intracellular receptor signaling complexes. One of these proteins, TNF receptor-associated factor 1 (TRAF1), is highly regulated in pulmonary cells. To determine the effect of reduced pulmonary TRAF1 expression, TRAF1-null (-/-) and control, BALB/c (wild-type), mice were treated intratracheally, intraperitoneally, or intravenously, with TNF-alpha. Despite relatively mild lung injury, intratracheal TNF-alpha-treated TRAF1-/- mice exhibited marked liver injury with an approximate fivefold increase in serum liver enzyme levels as compared to wild-type mice. In addition, serum TNF-alpha levels were strikingly elevated in TRAF1-/- mice. Pretreatment with neutralizing anti-TNFRI antibody significantly reduced liver injury and serum TNF-alpha. Cells isolated by bronchoalveolar lavage from intratracheally treated TRAF1-/- mice produced more TNF-alpha than cells from treated wild-type mice, suggesting that lung cells contributed to elevated serum TNF-alpha. These studies suggest that TRAF1 provides negative feedback for TNF-alpha synthesis and limits TNFRI-mediated systemic effects of TNF-alpha originating in the lung.

Control of T cell viability

The factors affecting T cell viability vary depending on the type and status of the T cell involved. Naive T cells die via a Bcl-2/Bim dependent route. Their deaths are prevented in animals by IL-7 and contact with MHC. Activated T cells die in many different ways. Among these is a pathway involving signals that come from outside the T cell and affect it via surface receptors such as Fas. Activated T cells also die through a pathway driven by signals generated within the T cell itself, a cell autonomous route. This pathway involves members of the Bcl-2 family, in particular Bcl-2, Bcl-xl, Bim, and probably Bak. The viability of CD8+ and CD4+ memory T cells is controlled in different ways. CD8+ memory T cells are maintained by IL-15 and IL-7. The control of CD4+ memory T cells is more mysterious, with roles reported for IL-7 and/or contact via the TCR.

Receptor-mediated choreography of life and death

The cytokine tumor necrosis factor was originally identified as a protein that kills tumor cells. So far, 18 distinct members of this family have been identified. All of them regulate cell survival, proliferation, differentiation, and cell death, also called apoptosis. The apoptosis induced by TNF, and other members of the family, for example, FasL, VEGI, and TRAIL is mediated through death receptors. The apoptotic signals by these cytokines are transduced by eight different death domain- (DD) containing receptors (TNFR1, also called DR1; Fas, also called DR2; DR3, DR4, DR5, DR6, NGFR, and EDAR). The intracellular portion of all these receptors contains a region approximately 80 amino acids long referred to as the "death domain." Upon activation by its ligand, the DD recruits various proteins that mediate both death and proliferation of the cells. These proteins in turn recruit other proteins via their DDs or death effector domains. The actual destruction of the cell, however, is accomplished by serial activation of a family of proteases referred to as caspases. Cell death is negatively regulated by a family of proteins that includes decoy receptors, silencer of DD, sentrin, cellular FLICE inhibitory protein, cellular inhibitors of apoptosis, and survivin. This review is an attempt to describe how these negative and positive players of cell death perform a harmonious dance with each other.

TNF receptor-associated factor 6 deficiency during hemopoiesis induces Th2-polarized inflammatory disease

Toll-like receptors (TLR) initiate rapid innate immune responses by recognizing microbial products. These events in turn lead to the development of an efficient adaptive immune response through the up-regulation of a number of costimulatory molecules, including members of the TNF/TNFR superfamily, on the surface of an APC. TNFR-associated factor 6 (TRAF6) is a common signaling adapter used by members of both the TNFR and the TLR/IL-1R superfamilies, and as such plays a critical role in the development of immune responses. As TRAF6-deficient mice die prematurely, we generated chimeras reconstituted with TRAF6-deficient fetal liver cells to analyze functions of TRAF6 in vivo in the hemopoietic compartment. We found that TRAF6-deficient chimeras develop a progressive lethal inflammatory disease associated with massive organ infiltration and activation of CD4(+) T cells in a Th2-polarized phenotype, and a defect in IL-18 responsiveness. When recombination-activating gene 2(-/-) blastocysts were complemented with TRAF6-deficient embryonic stem cells, a marked elevation of activated CD4(+) T cells and progressive inflammatory disease were also observed. Moreover, T cell activation and lethal inflammation were not reversed in mixed chimeric mice generated from normal and TRAF6-deficient fetal liver cells. These results suggest that deletion of TRAF6 induces a dominant Th2-type polarized autoimmune response. Therefore, in addition to playing a critical role in innate and adaptive immunity, TRAF6 is likely to play a previously unrecognized role in the maintenance of self-tolerance.

Enhanced expression of interleukin-1alpha and tumor necrosis factor receptor-associated protein 1 in ileal tissues of cattle infected with Mycobacterium avium subsp. paratuberculosis

Infection with Mycobacterium avium subsp. paratuberculosis is associated with high levels of morbidity, decreased production, and early culling in dairy cattle. Clinical symptoms of Johne's disease include persistent diarrhea, inappetence, and resultant weight loss due to chronic inflammation of the small intestine. Although the presence or absence of intestinal lesions cannot be used as a definitive indicator of M. avium subsp. paratuberculosis infection, most infected cattle exhibit significant changes to intestinal mucosa, with the focus of pathology surrounding the ileal cecal junction. Typical pathology of M. avium subsp. paratuberculosis infection includes inflammation, thickening of the lumenal wall, and hyperplasia in draining lymph nodes. To further understand the pathology of Johne's disease, we compared the gene expression profiles of ileal tissues from Johne's disease-positive (n = 6), and Johne's disease-negative (n = 5) Holstein cattle. Gene expression profiles were compared with a bovine total leukocyte (BOTL-3) cDNA microarray. Genes that were expressed at significantly higher levels (>1.5-fold; P < 0.05) in tissues from Johne's disease-infected animals relative to noninfected animals included those encoding tumor necrosis factor receptor-associated protein 1 (TRAF1), interleukin-1alpha (IL-1alpha), MCP-2, N-cadherin, and beta1 integrin (CD29). Dramatic upregulation of IL-1alpha (21.5-fold) and TRAF1 (27.5-fold) gene expression in tissues of Johne's disease-positive cows relative to tissues from control cows was confirmed by quantitative real-time PCR. Western blot analysis confirmed that IL-1alpha and TRAF1 mRNA levels resulted in increased protein expression in tissues of Johne's disease-positive cattle relative to tissues from control cattle. High levels of IL-1alpha can produce symptoms similar to those found in clinical Johne's disease. Taken together, the data presented in this report suggest that many outward symptoms of Johne's disease may be due to IL-1alpha toxicity. In addition, enhanced levels of TRAF1 could result in cells within the lesions of Johne's disease-positive cattle that are highly resistant to TNF-alpha-induced signaling.

Caspase-mediated cleavage converts the tumor necrosis factor (TNF) receptor-associated factor (TRAF)-1 from a selective modulator of TNF receptor signaling to a general inhibitor of NF-kappaB activation

The role of tumor necrosis factor (TNF) receptor-associated factor (TRAF)-1 in NF-kappaB activation by various members of the TNF receptor family is not well understood, and conflicting data have been published. Here, we show that TRAF1 differentially affects TRAF2 recruitment and activation of NF-kappaB by members of the TNF receptor family. Interestingly, a naturally occurring caspase-derived cleavage product of TRAF1 solely comprising its TRAF domain (TRAF1-(164-416)) acted as a general inhibitor of NF-kappaB activation. In contrast, a corresponding fragment generated by cleavage of TRAF3 showed no effect in this regard. In accordance with these functional data, TRAF1, but not TRAF3, interacted with the IKK complex via its N-TRAF domain. Endogenous TRAF1 and the overexpressed TRAF domain of TRAF1 were found to be constitutively associated with the IKK complex, whereas endogenous receptor interacting protein was only transiently associated with the IKK complex upon TNF stimulation. Importantly, the caspase-generated TRAF1-fragment, but not TRAF1 itself inhibited IKK activation. Our results suggest that TRAF1 and TRAF1-(164-416) exert their regulatory effects on receptor-induced NF-kappaB activation not only by modulation of TRAF2 receptor interaction but especially TRAF1-(164-416) also by directly targeting the IKK complex.

Differential expression and function of A20 and TRAF1 in Hodgkin lymphoma and anaplastic large cell lymphoma and their induction by CD30 stimulation

A20 and TRAF1 are two anti-apoptotic components of the intracellular signalling pathway of the tumour necrosis factor receptor (TNFR) family. Induction of apoptosis seems to be a main function of these receptors. It is astonishing that a member of this family, CD30, is overexpressed by highly proliferating tumours such as Hodgkin lymphoma (HL) and anaplastic large cell lymphoma (ALCL). It is known that CD30 stimulation leads to the apoptosis of ALCL tumour cells but not of Hodgkin-Reed-Sternberg (HRS) cells. We have already established the overexpression of TRAF1 in HRS cells. In this study we demonstrate that A20 is highly expressed in the HRS cells in 20/22 of cases of classical HL, in 4/4 cases of nodular lymphocyte-predominant HL (NLPHL), and in 2/2 cases of the anaplastic variant of diffuse large B cell lymphoma. In contrast, all other non-Hodgkin lymphomas, including ALCL, revealed either no A20 reactivity, or reactivity in less than 1% of all tumour cells. CD30 stimulation induced A20 and TRAF1 expression. This effect was most prominent in HL and ALCL cell lines with low basal expression levels of these molecules. Immunohistological studies of reactive lymphoid blasts in tonsillar tissue demonstrated that co-expression of CD30, A20, and TRAF1 also occurs in vivo. Cell lines with high basal A20 and TRAF1 expression were resistant to CD30-mediated apoptosis. The sensitivity to CD30-induced apoptosis was increased by inhibition of protein synthesis. TRAF1 transfection decreased CD30-induced apoptosis. Our data suggest that A20 and TRAF1 contribute to apoptosis resistance and, therefore, play an important role in the pathogenesis of classical HL.

The biologic role for nuclear factor-kappaB in disease and its potential involvement in mucosal injury associated with anti-neoplastic therapy

Oral mucosal barrier injury (mucositis) is a frequent, painful, serious, dose-limiting toxicity associated with many anti-neoplastic drugs and radiation to the head and neck. Results of recent studies suggest that mucositis is the result of a complex series of interactive biological events that take place in the submucosa and epithelium. The nuclear transcription factor NF-kappaB has been implicated in the control of a broad range of biological responses, the activation of a large number of specific cellular genes, and the determination of the fate of cells exposed to ionizing radiation and anti-neoplastic drugs. Of particular importance to mucositis is the fact that NF-kappaB regulates key elements in the apparent sequence that leads to normal tissue toxicity. Not the least of these is the effect that NF-kappaB activation has on apoptosis. In particular, a paradox exists between the potential pro-apoptotic effect NF-kappaB exerts on normal cells, and the anti-apoptotic and cytoprotective effect it causes in tumor cells. This paper provides a review of the structure and function of NF-kappaB and speculates how its apparent enigmatic effect on normal and tumor cells may occur.

TRAF1 is a critical regulator of JNK signaling by the TRAF-binding domain of the Epstein-Barr virus-encoded latent infection membrane protein 1 but not CD40

The oncogenic Epstein-Barr virus (EBV)-encoded latent infection membrane protein 1 (LMP1) mimics a constitutive active tumor necrosis factor (TNF) family receptor in its ability to recruit TNF receptor-associated factors (TRAFs) and TNF receptor-associated death domain protein (TRADD) in a ligand-independent manner. As a result, LMP1 constitutively engages signaling pathways, such as the JNK and p38 mitogen-activated protein kinases (MAPK), the transcription factor NF-kappaB, and the JAK/STAT cascade, and these activities may explain many of its pleiotropic effects on cell phenotype, growth, and transformation. In this study we demonstrate the ability of the TRAF-binding domain of LMP1 to signal on the JNK/AP-1 axis in a cell type- dependent manner that critically involves TRAF1 and TRAF2. Thus, expression of this LMP1 domain in TRAF1-positive lymphoma cells promotes significant JNK activation, which is blocked by dominant-negative TRAF2 but not TRAF5. However, TRAF1 is absent in many established epithelial cell lines and primary nasopharyngeal carcinoma (NPC) biopsy specimens. In these cells, JNK activation by the TRAF-binding domain of LMP1 depends on the reconstitution of TRAF1 expression. The critical role of TRAF1 in the regulation of TRAF2-dependent JNK signaling is particular to the TRAF-binding domain of LMP1, since a homologous region in the cytoplasmic tail of CD40 or the TRADD-interacting domain of LMP1 signal on the JNK axis independently of TRAF1 status. These data further dissect the signaling components used by LMP1 and identify a novel role for TRAF1 as a modulator of oncogenic signals.

Tumor necrosis factor receptor-associated factor 1 gene overexpression in B-cell chronic lymphocytic leukemia: analysis of NF-kappa B/Rel-regulated inhibitors of apoptosis

B-cell chronic lymphocytic leukemia (B-CLL) is characterized by a resistance toward apoptosis-inducing agents. Nuclear factor-kappaB (NF-kappaB)/Rel has been shown to regulate the expression of antiapoptotic genes, such as members of the inhibitor of apoptosis protein (IAP) and tumor necrosis factor receptor-associated factor (TRAF) gene families. Expression and regulation of NF-kappaB/Rel-dependent inhibitors of apoptosis have not been collectively studied in B-CLL. We examined expression of known NF-kappaB/Rel-regulated antiapoptotic genes by RNAse protection assay, real-time polymerase chain reaction, and immunoblotting in patients with B-CLL. TRAF1 and to a lesser extent TRAF2 were overexpressed in B-CLL lymphocytes as compared with normal CD19(+) B cells. TRAF1 overexpression did not correlate with markers of disease progression or overall survival. Furthermore, we found high constitutive expression of the IAP genes c-IAP-1, c-IAP-2, and XIAP both in normal and B-CLL lymphocytes. Focusing on the regulation of TRAF1, NF-kappaB/Rel activity in B-CLL nuclear extracts was shown to bind to TRAF1 promoter elements. However, IkappaB kinase (IKK) activity was not increased in CLL lymphocytes as compared with normal CD19(+) B cells. The known IKK inhibitor sulfasalazine did not compromise TRAF1 expression. Thus, although our study revealed a common expression pattern of NF-kappaB/Rel-regulated inhibitors of apoptosis, our findings indicate an IKK-independent regulation of TRAF1 in B-CLL.

Regulation of the subcellular localization of tumor necrosis factor receptor-associated factor (TRAF)2 by TRAF1 reveals mechanisms of TRAF2 signaling

Tumor necrosis factor receptor-associated factor (TRAF)2 is a critical adaptor molecule for tumor necrosis factor (TNF) receptors in inflammatory and immune signaling. Upon receptor engagement, TRAF2 is recruited to CD40 and translocates to lipid rafts in a RING finger-dependent process, which enables the activation of downstream signaling cascades including c-Jun NH(2)-terminal kinase (JNK) and nuclear factor (NF)-kappaB. Although TRAF1 can displace TRAF2 and CD40 from raft fractions, it promotes the ability of TRAF2 activate signaling over a sustained period of time. Removal of the RING finger of TRAF2 prevents its translocation into detergent-insoluble complexes and renders it dominant negative for signaling. TRAF1(-/-) dendritic cells show attenuated responses to secondary stimulation by TRAF2-dependent factors and increased stimulus-dependent TRAF2 degradation. Replacement of the RING finger of TRAF2 with a raft-targeting signal restores JNK activation and association with the cyto-skeletal protein Filamin, but not NF-kappaB activation. These findings offer insights into the mechanism of TRAF2 signaling and identify a physiological role for TRAF1 as a regulator of the subcellular localization of TRAF2.

TRAF1: lord without a RING

Tumor necrosis factor (TNF) receptor-associated factors (TRAFs) constitute a family of adaptor proteins that associate with the cytosolic tail of particular TNF-family receptors (TNFR) and regulate cytokine signaling by linking TNFRs with downstream protein kinases, ubiquitin ligases, and other effector proteins. A total of six members of this family (TRAF1-6) have been identified in mammals. TRAF1 is unique among TRAFs because it lacks a RING finger domain present in TRAF2-6 that has been shown to be required for TRAF2- and TRAF6-mediated activities. TRAF1 also has the most restricted expression among TRAFs, and is found almost exclusively in activated lymphocytes, dendritic cells, and certain epithelia. Recent evidence obtained from TRAF1(-/-) mice shows that TRAF1-deficient T cells are hyper-responsive to TNF-alpha, having increased T cell receptor (TCR)-dependent T cell proliferation rates in vitro. Also, these TRAF1(-/-) mice had increased sensitivity to TNF-alpha-induced skin necrosis in vivo. These results support a role for TRAF1 as a negative regulator of signaling by certain TNF-family receptors. This review summarizes current knowledge about TRAF1, focusing on the new information provided by these TRAF1-deficient mice. Also, the pros and cons of TRAFs as potential targets for drug discovery are discussed.

Stable inhibition of NF-kappa B in salivary gland cells does not enhance sensitivity to TNF-alpha-induced apoptosis due to upregulation of TRAF-1 expression

The transcription factor NF-kappa B inhibits the apoptotic response induced by TNF-alpha. However, in salivary gland cell clones (ACMT-6 and ACMT-7) in which NF-kappa B activation was suppressed by introduction of a super-repressor form of I kappa B-alpha cDNA, TNF-alpha did not cause apoptosis. Thus, to investigate the molecular mechanism involved in the unresponsiveness of these cell clones to TNF-alpha-induced apoptosis, we examined the effect of TNF-alpha on the expression of antiapoptotic proteins, including TNF receptor-associated factor (TRAF)-1, TRAF-2, cellular inhibitor of apoptosis protein (cIAP)-1, and cIAP-2. Here we show that expression of TRAF-1 was commonly detected by treatment with TNF-alpha in ACMT-6, ACMT-7, and an empty vector-transfected cell clone (ACpRc-1) and that downregulation of TRAF-1 protein by either treatment with an antisense oligonucleotide or introduction of an antisense plasmid resulted in the induction of apoptosis in these cell clones. Our results, therefore, suggest that one of the mechanisms by which cells acquire resistance to TNF-alpha-induced apoptosis is a TNF-alpha induction of TRAF-1.

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.

TRAF1 is a negative regulator of TNF signaling. enhanced TNF signaling in TRAF1-deficient mice

TNF receptor-associated factor 1 (TRAF1) is a unique TRAF protein because it lacks a RING finger domain and is predominantly expressed in activated lymphocytes. To elucidate the function of TRAF1, we generated TRAF1-deficient mice. TRAF1(-/-) mice are viable and have normal lymphocyte development. TRAF1(-/-) T cells exhibit stronger than wild-type (WT) T cell proliferation to anti-CD3 mAb, which persisted in the presence of IL-2 or anti-CD28 antibodies. Activated TRAF1(-/-) T cells, but not TRAF1(+/+) T cells, responded to TNF by proliferation and activation of the NF-kappa B and AP-1 signaling pathways. This TNF effect was mediated by TNFR2 (p75) but not by TNFR1 (p55). Furthermore, skin from TRAF1(-/-) mice was hypersensitive to TNF-induced necrosis. These findings suggest that TRAF1 is a negative regulator of TNF signaling.

Crosstalk between NF-kappaB-activating and apoptosis-inducing proteins of the TNF-receptor complex

The cytokine tumor necrosis factor (TNF) elicits a wide range of biological responses, including inflammation, cell proliferation, differentiation, and apoptosis. Although the molecular mechanisms of TNF signaling have been largely elucidated, the principle that regulates the balance of life and death is still unknown. This review will focus on the crosstalk that exists between proteins of the TNF receptor (TNF-R) signalosome, and which are involved in the initiation of nuclear factor kappa B (NF-kappaB) activation or apoptosis. At least four different mechanisms of regulation can be distinguished: (i) NF-kappaB-mediated induction of proteins of the TNF-R complex; (ii) NF-kappaB-independent protection against apoptosis by the TNF-R-associating factor 2 (TRAF2)-mediated recruitment of antiapoptotic proteins; (iii) dual activation of apoptosis and NF-kappaB by a single molecule; and (iv) amplification of the death signal by proteolytic inactivation of signaling proteins that are involved in NF-kappaB activation or cell survival.

Diphosphoryl lipid A from Rhodobacter sphaeroides blocks the binding and internalization of lipopolysaccharide in RAW 264.7 cells

Diphosphoryl lipid A derived from the nontoxic LPS of Rhodobacter sphaeroides (RsDPLA) has been shown to be a powerful LPS antagonist in both human and murine cell lines. In addition, RsDPLA also can protect mice against the lethal effects of toxic LPS. In this study, we complexed both the deep rough LPS from Escherichia coli D31 m4 (ReLPS) and RsDPLA with 5- and 30-nm colloidal gold and compared their binding to the RAW 264.7 cell line by electron microscopy. Both ReLPS and RsDPLA bound to the cells with the following observations. First, binding studies revealed that pretreatment with RsDPLA completely blocked the binding and thus internalization of ReLPS-gold conjugates to these cells at both 37 degrees C and 4 degrees C. Second, ReLPS was internalized via micropinocytosis (noncoated plasma membrane invaginations) involving formation of caveolae-like structures and leading to the formation of micropinocytotic vesicles, macropinocytosis (or phagocytosis), formation of clathrin-coated pits (receptor mediated), and penetration through plasma membrane into cytoplasm. Third, in contrast, RsDPLA was internalized predominantly via macropinocytosis. These studies show for the first time that RsDPLA blocks the binding and thus internalization of LPS as observed by scanning and transmission electron microscopy.

RELT, a new member of the tumor necrosis factor receptor superfamily, is selectively expressed in hematopoietic tissues and activates transcription factor NF-kappaB

The members of tumor necrosis factor receptor (TNFR) superfamily have been designated as the "guardians of the immune system" due to their roles in immune cell proliferation, differentiation, activation, and death (apoptosis). This study reports the cloning of a new member of the TNFR superfamily, RELT (Receptor Expressed in Lymphoid Tissues). RELT is a type I transmembrane glycoprotein with a cysteine-rich extracellular domain, possessing significant homology to other members of the TNFR superfamily, especially TNFRSF19, DR3, OX40, and LTbeta receptor. The messenger RNA of RELT is especially abundant in hematologic tissues such as spleen, lymph node, and peripheral blood leukocytes as well as in leukemias and lymphomas. RELT is able to activate the NF-kappaB pathway and selectively binds tumor necrosis factor receptor-associated factor 1. Although the soluble form of RELT fusion protein does not inhibit the one-way mixed lymphocyte reaction, immobilized RELT is capable of costimulating T-cell proliferation in the presence of CD3 signaling. These results define a new member of the TNFR superfamily that may be a potential regulator of immune responses.

Constitutive NF-kappaB maintains high expression of a characteristic gene network, including CD40, CD86, and a set of antiapoptotic genes in Hodgkin/Reed-Sternberg cells

Constitutively activated nuclear factor (NF)-kappaB is observed in a variety of neoplastic diseases and is a hallmark of the malignant Hodgkin and Reed-Sternberg cells (H/RS) in Hodgkin lymphoma. Given the distinctive role of constitutive NF-kappaB for H/RS cell viability, NF-kappaB-dependent target genes were searched for by using adenoviral expression of the super-repressor IkappaBDeltaN. A surprisingly small but characteristic set of genes, including the cell-cycle regulatory protein cyclin D2, the antiapoptotic proteins Bfl-1/A1, c-IAP2, TRAF1, and Bcl-x(L), and the cell surface receptors CD86 and CD40 were identified. Thus, constitutive NF-kappaB activity maintains expression of a network of genes, which are known for frequent, marker-like expression in primary or cultured H/RS cells. Intriguingly, CD40, which is able to activate CD86 or Bcl-x(L) via NF-kappaB, is itself transcriptionally regulated by NF-kappaB through a promoter proximal binding site. NF-kappaB inhibition resulted in massive spontaneous and p53-independent apoptosis, which could be rescued by ectopic expression of Bcl-x(L), underscoring its dominant role in survival of H/RS cells. Hence, NF-kappaB controls a signaling network in H/RS cells, which promotes tumor cell growth and confers resistance to apoptosis.

5-Fluorouracil suppression of NF-KappaB is mediated by the inhibition of IKappab kinase activity in human salivary gland cancer cells

We have recently shown that 5-Fluorouracil (5-FU) suppresses the transcription factor NF-kappaB in human salivary gland cancer cells (cl-1) by mediating upregulation of IkappaB-alpha expression. However, the precise mechanism involved in this action has not yet been elucidated. IkappaB kinases (IKK-alpha and IKK-beta) are the key components of the IKK complex that mediates activation of NF-kappaB in response to external stimuli such as cytokines. In addition, NF-kappaB-inducing kinase (NIK) and mitogen-activated protein kinase kinase kinase 1 (MEKK-1), both of which are the upstream kinases for the IKKs, interact with and activate the IKKs. Thus, we investigated the molecular mechanisms involved in the suppression of NF-kappaB by 5-FU. Although 5-FU did not affect the expression levels of IKKs, NIK, or MEKK-1, IKK activity in cl-1 cells was suppressed at both 6 h and 12 h after treatment with 2 microgram/ml 5-FU. Moreover, when cells were treated with various concentrations of 5-FU for 12 h, the concentration of 2 microgram/ml efficiently inhibited the IKK activity as compared to 1, 5, or 10 microgram/ml. The expression of Fas-associated death domain-like interleukin 1-converting enzyme-inhibitory protein (FLIP), which acts as an inhibitor of an initiator caspase (caspase-8), was down-regulated by 5-FU treatment in cl-1 cells. Apoptosis, as evidenced by cleavage of poly(ADP-ribose) polymerase through the action of an executioner caspase (caspase-3), was also clearly observed. Thus, these results suggest that 5-FU induction of apoptosis in cl-1 cells may be mediated by suppression of NF-kappaB via inhibition of IKK activity.

TRAF1 is a substrate of caspases activated during tumor necrosis factor receptor-alpha-induced apoptosis

TRAF family proteins are signal-transducing adapter proteins that interact with the cytosolic domains of tumor necrosis factor (TNF) family receptors. Here we show that TRAF1 (but not TRAF2-6) is cleaved by certain caspases in vitro and during TNF-alpha- and Fas-induced apoptosis in vivo. (160)LEVD(163) was identified as the caspase cleavage site within TRAF1, generating two distinct fragments. Significant enhancement of TNF receptor-1 (CD120a)- and, to a lesser extent, Fas (CD95)-mediated apoptosis was observed when overexpressing the C-terminal TRAF1 fragment in HEK293T and HT1080 cells. The same fragment was capable of potently suppressing TNF receptor-1- and TRAF2-mediated nuclear factor-kappaB activation in reporter gene assays, providing a potential mechanism for the enhancement of TNF-mediated apoptosis. Cell death induced by other death receptor-independent stimuli such as cisplatin, staurosporine, and UV irradiation did not result in cleavage of TRAF1, and overexpression of the C-terminal TRAF1 fragment did not enhance cell death in these cases. TRAF1 cleavage was markedly reduced in cells that contain little procaspase-8 protein, suggesting that this apical protease in the TNF/Fas death receptor pathway is largely responsible. These data identify TRAF1 as a specific target of caspases activated during TNF- and Fas-induced apoptosis and illustrate differences in the repertoire of protease substrates cleaved during activation of different apoptotic pathways.

CD40-mediated signaling in monocytic cells: up-regulation of tumor necrosis factor receptor-associated factor mRNAs and activation of mitogen-activated protein kinase signaling pathways

The biochemical pathways involved in CD40 signaling have been extensively studied in B cells and B cell lines, and appear to be primarily initiated by recruitment of the tumor necrosis factor (TNF) receptor-associated factor (TRAF) signaling proteins to the CD40 cytoplasmic domain. Signaling pathways activated through CD40 in monocytes/macrophages have not been characterized as well as in B cells. Using human monocytes and the human monocytic cell line THP1, we examined signal transduction events induced by CD40 engagement with its ligand, CD154. In human monocytes, all TRAF mRNAs were expressed constitutively and CD40 ligation resulted in a strong up-regulation of TRAF1 mRNA. In THP1 cells, CD40 ligation induced expression of TRAF1 and TRAF5 mRNAs. Engagement of CD40 in both monocytes and THP1 cells led to the rapid and transient activation of the extracellular signal-regulated kinases (ERK) 1 and 2, and to low levels of JNK activation. No CD40-dependent activation of p38 mitogen-activated protein kinase (MAPK) was found. In CD154-stimulated monocytes and THP1 cells the upstream ERK1/2 activator, MAPK kinase (MEK) 1/2, and downstream substrate, c-Myc, were activated. By blocking activation of ERK1/2 with a MEK-specific inhibitor, PD98059, CD40-dependent secretion of the pro-inflammatory cytokines, TNF-alpha, IL-6 and IL-8, was demonstrated to be linked to the ERK1/2 pathway. The ERK1/2 pathway did not appear to be involved in up-regulating TRAF1 and TRAF5 mRNAs in THP1 cells. Collectively, these results suggest distinct differences between B cells and monocytic cells in CD40-dependent activation of MAPK pathways.

Caspase-cleaved TRAF1 negatively regulates the antiapoptotic signals of TRAF2 during TNF-induced cell death

Tumor necrosis factor (TNF) signaling leads to pleiotropic responses in a wide range of cell types, in part by activating antiapoptotic and proapoptotic pathways. Previous studies have suggested that TNF receptor-associated factor (TRAF) 2 can mediate crucial antiapoptotic signals during TNF stimulation. However, it is unclear how the antiapoptotic signals via TRAF2 in TNF-R1 signaling is regulated. Here we show that TRAF1 is cleaved by caspase-8 into two fragments during apoptosis induced by TNF. Overexpression of the C-terminal cleavage product, TRAF1-c, increased TNF-induced cell death of hybridoma T cells. Importantly, we demonstrate that the cleavage product of TRAF1 coimmunoprecipitates with TRAF2 that is released from the TNF-R1 complex in response to prolonged TNF treatment. These results indicate that caspase-dependent cleavage of TRAF1 generates TRAF1-c fragments that are able to bind TRAF2, and then sequester TRAF2 from the TNF-R1 complex, rendering cells, at least in part, sensitive to TNF.

Identification and characterization of two CD40-inducible enhancers in the mouse TRAF1 gene locus

We have shown that CD40 engagement induces TRAF1 gene expression in B lymphocytes. Here we report that CD40-dependent TRAF1 gene transcription in murine B cells is controlled by two enhancer regions. One region is located approximately 2 kb upstream of the transcription start site and the other lies in the intron between exons 5 and 6. The upstream enhancer contains a single NF-kappaB site in addition to sites that bind constitutive transcription factors. Mutation of this NF-kappaB site completely abrogates CD40-driven TRAFl transcription. The intronic enhancer contains two sites that strongly bind the CD40-inducible factors NF-kappaB and AP-1. Simultaneous mutation of the AP-1 site and of the NF-kappaB site abolishes transcription driven by this enhancer. When cloned together into reporter constructs, the two TRAF1 enhancers do not synergize, suggesting that each enhancer may separately participate in the induction of TRAF1 transcription in B cells following CD40 activation.

TNFR-associated factor family protein expression in normal tissues and lymphoid malignancies

TNFR-associated factors (TRAFs) constitute a family of adapter proteins that associate with particular TNF family receptors. Humans and mice contain six TRAF genes, but little is known about their in vivo expression at the single cell level. The in vivo locations of TRAF1, TRAF2, TRAF5, and TRAF6 were determined in human and mouse tissues by immunohistochemistry. Striking diversity was observed in the patterns of immunostaining obtained for each TRAF family protein, suggesting their expression is independently regulated in a cell type-specific manner. Dynamic regulation of TRAFs was observed in cultured PBLs, where anti-CD3 Abs, mitogenic lectins, and ILs induced marked increases in the steady-state levels of TRAF1, TRAF2, TRAF5, and TRAF6. TRAF1 was also highly inducible by CD40 ligand in cultured germinal center B cells, whereas TRAF2, TRAF3, TRAF5, and TRAF6 were relatively unchanged. Analysis of 83 established human tumor cell lines by semiquantitative immunoblotting methods revealed tendencies of certain cancer types to express particular TRAFs. For example, expression of TRAF1 was highly restricted, with B cell lymphomas consistently expressing this TRAF family member. Consistent with results from tumor cell lines, immunohistochemical analysis of 232 non-Hodgkin lymphomas revealed TRAF1 overexpression in 112 (48%) cases. TRAF1 protein levels were also elevated in circulating B cell chronic lymphocytic leukemia specimens (n = 49) compared with normal peripheral blood B cells (p = 0.01), as determined by immunoblotting. These findings contribute to an improved understanding of the cell-specific roles of TRAFs in normal tissues and provide evidence of altered TRAF1 expression in lymphoid malignancies.

Suppression of tumor necrosis factor alpha-induced matrix metalloproteinase 9 production by the introduction of a super-repressor form of inhibitor of nuclear factor kappaBalpha complementary DNA into immortalized human salivary gland acinar cells. Prevention of the destruction of the acinar structure in Sjögren's syndrome salivary glands

OBJECTIVE

We have previously shown that specific enhancement in acinar cells of proteolytic activity induced by tumor necrosis factor alpha (TNFalpha) may be responsible for the destruction of the acinar structure in the salivary glands of patients with Sjögren's syndrome. Because matrix metalloproteinase 9 (MMP-9) is regulated by nuclear factor kappaB (NF-kappaB), we investigated the effect of a super-repressor form of inhibitor of nuclear factor kappaBalpha (srIkappaBalpha) on the suppression of TNFalpha-induced MMP-9 production in acinar cells.

METHODS

Two srIkappaBalpha complementary DNA (cDNA)-transfected acinar cell clones (ACMT-6 and ACMT-7) and 1 empty vector-transfected cell clone (ACpRc-1) were established. After treatment of cell clones with TNFalpha, the expression of MMP-9 was examined. In addition, the effect of TNFalpha on cell growth and the morphogenetic behavior of cell clones cultured on type IV collagen-coated dishes were examined.

RESULTS

TNFalpha induced the production of MMP-9 in the ACpRc-1 cell clone, but greatly suppressed MMP-9 production in ACMT-6 and ACMT-7 clones. No apparent cytotoxic effect of TNFalpha treatment was observed in these cell clones. When ACpRc-1 cells were seeded on type IV collagen-coated dishes in the presence of both TNFalpha and plasmin, type IV collagen interaction with the cells was lost and the cells entered apoptosis. However, even when ACMT-6 and ACMT-7 cells were cultured under the same culture conditions as those for ACpRc-1, these cell clones attached to the substrate and grew consistently without showing apoptosis. Conclusion. These observations indicate that suppression of TNFalpha-induced MMP-9 production by the introduction of srIkappaBalpha cDNA corrected the aberrant in vitro morphogenesis of acinar cells grown on type IV collagen.

TRAF4 deficiency leads to tracheal malformation with resulting alterations in air flow to the lungs

TRAF4 is one of six identified members of the family of TNFR-associated factors. While the other family members have been found to play important roles in the development and maintenance of a normal immune system, the importance of TRAF4 has remained unclear. To address this issue, we have generated TRAF4-deficient mice. Despite widespread expression of TRAF4 in the developing embryo, as well as in the adult, lack of TRAF4 expression results in a localized, developmental defect of the upper respiratory tract. TRAF4-deficient mice are born with a constricted upper trachea at the site of the tracheal junction with the larynx. This narrowing of the proximal end of the trachea results in respiratory air flow abnormalities and increases rates of pulmonary inflammation. These data demonstrate that TRAF4 is required to regulate the anastomosis of the upper and lower respiratory systems during development.

5-Fluorouracil induces apoptosis through the suppression of NF-kappaB activity in human salivary gland cancer cells

Activation of the transcription factor NF-kappaB results in protection against apoptosis, and the chemotherapeutic agent 5-Fluorouracil (5-FU) exerts its cytotoxic effect through the induction of apoptosis. Thus, we examined whether 5-FU could induce apoptosis through the suppression of NF-kappaB activity. We found that upon treatment of a human salivary gland cancer cell line (cl-1) with 5-FU, the NF-kappaB activity was suppressed in a time-dependent manner. This inhibition was mediated by a prevention of the degradation of the inhibitory IkappaB-alpha protein. In addition, the expression of TRAF-2 and cIAP-1, which are transcriptionally regulated by NF-kappaB and function as anti-apoptotic molecules through the interruption of caspase pathway, was also inhibited by 5-FU. Finally, the activity of caspase-8 and caspase-3 showed a significant increase in response to 5-FU. By flow cytometric analysis, 5-FU did not affect the expression level of Fas on the cell surface. Thus, our results suggest that one of the molecular mechanisms involved in 5-FU-induced apoptosis in cl-1 cells may be due to the suppression of NF-kappaB activity, resulting in the activation of the pro-apoptotic pathway.

Involvement of two NF-kappa B binding elements in tumor necrosis factor alpha -, CD40-, and epstein-barr virus latent membrane protein 1-mediated induction of the cellular inhibitor of apoptosis protein 2 gene

The antiapoptotic function of NF-kappaB is believed to be mediated through the induction of antiapoptotic genes. Among the antiapoptotic genes, cellular inhibitor of apoptosis protein 2 (c-IAP2/HIAP-1/MIHC) is originally identified as a molecule recruited to the tumor necrosis factor (TNF) receptor complex, and its expression is preferentially up-regulated by TNF and other stimuli activating NF-kappaB. However, direct evidence of transcriptional regulation of NF-kappaB on the c-IAP2 gene is still missing. Here, we have cloned and characterized the promoter region required for NF-kappaB-dependent transcription of the c-IAP2 gene. Sequencing of a 3.5-kilobase fragment of the 5'-flanking region of the c-IAP2 gene has identified a TATA-like sequence and potential binding sites for nuclear factor of activated T cells, interferon regulatory factor 1, activator protein 1, glucocorticoid response element, and three putative NF-kappaB binding elements. Deletion and mutational analysis of the 5'-flanking region linked to the luciferase gene revealed that transcriptional activation by TNF or interleukin 1 is mediated cooperatively by two NF-kappaB binding sites. Electrophoretic mobility shift assays characterized that the two NF-kappaB sites can be recognized and bound by the NF-kappaB p50/p65 heterodimer. In addition, the transcription of c-IAP2 promoter was strongly up-regulated when CD40 or Epstein-Barr virus latent membrane protein 1 was overexpressed.

Caspase-induced inactivation of the anti-apoptotic TRAF1 during Fas ligand-mediated apoptosis

The activation of the transcription factor NF-kappaB often results in protection against apoptosis. In particular, pro-apoptotic tumor necrosis factor (TNF) signals are blocked by proteins that are induced by NF-kappaB such as TNFR-associated factor 1 (TRAF1). Here we show that TRAF1 is cleaved after Asp-163 when cells are induced to undergo apoptosis by Fas ligand (FasL). The C-terminal cleavage product blocks the induction of NF-kappaB by TNF and therefore functions as a dominant negative (DN) form of TRAF1. Our results suggest that the generation of DN-TRAF1 is part of a pro-apoptotic amplification system to assure rapid cell death.

Tumor necrosis factor-alpha-induced lung cell expression of antiapoptotic genes TRAF1 and cIAP2

Tumor necrosis factor (TNF) receptor (TNFR)-associated factors 1 and 2 (TRAF1 and TRAF2) and inhibitor of apoptosis proteins cIAP1 (MIHB) and cIAP2 (MIHC) were recently identified as proteins that associate with the TNF-alpha receptors TNFRI (p55) and TNFRII (p75) and inhibit TNF-alpha-induced programmed cell death or apoptosis. In the original reports, TRAF1 expression, unlike the ubiquitous TRAF2, was restricted to specific tissues in the lung, spleen, and testis. TNF-alpha is increased in the lung in many forms of pulmonary disease. In the current study, Western analysis, immunohistochemistry, and ribonuclease protection assays were used to determine whether TNF-alpha regulates the expression of these TNFR-associated proteins in lung cells. We demonstrate for the first time TNF-alpha dose-dependent induction of TRAF1 protein and messenger RNA (mRNA) in human H441 and A549 pulmonary adenocarcinoma cell lines, as well as in lung cells of C57BL/6J mice after intratracheal administration of TNF-alpha. In contrast to the epithelial cells, TRAF1 was not induced by TNF-alpha in U937 cells, a human monocytic cell line, suggesting cell type-specific regulation. Similarly, cIAP2 mRNA was induced by TNF-alpha in both H441 and A549 pulmonary epithelial cells but not in U937 cells. TNF-alpha is a primary mediator of acute pulmonary inflammation and contributes to the pathophysiology of chronic lung diseases such as bronchopulmonary dysplasia (BPD), a fibrotic disease of prematurely born infants. Immunohistochemical staining of human neonatal lung tissue demonstrated increased TRAF1 in lungs of infants dying of pneumonia or BPD in comparison with those dying of congenital malformation. These studies support the hypothesis that the TRAF1 and cIAP2 genes are highly regulated in pulmonary cells and may play a role in human lung disease.

A survey of expressed genes in the leukocytes of Japanese flounder, Paralichthys olivaceus, infected with Hirame rhabdovirus

We constructed a cDNA library of Japanese flounder, Paralichthys olivaceus, leukocytes that were infected with Hirame rhabdovirus (HRV) in order to analyze some of the genes that are induced and expressed by virus infection in the immune system. Four hundred and fifty-two partial sequences representing 300 cDNA clones were obtained from the 5' and/or 3' ends of inserts derived from the Japanese flounder leukocyte cDNA library. About three-quarters of the 300 cDNA clones (217 clones, 72.3%) represented known genes in the public databases, whereas the remaining 83 (27.7%) of the clones did not show any significant homology with the sequences in the public databases. Clones matching known genes were classified into 12 categories according to their function or distribution. Only 40 (18.4%) of the 217 known genes showed homology with fish genes deposited in the database. Thirty (10%) of the clones, encoding 21 different sequences, and representing several categories, were identified as putative biodefense genes or genes associated with the immune response. Nineteen of the 21 putative biodefense or immune response-related cDNAs have not been previously reported in fish genes or cDNAs.

TRAF1 is a TNF inducible regulator of NF-kappaB activation

Tumor necrosis factor receptor (TNFR)-associated factor TRAF1 was first identified as a component of the TNFR2 signalling complex. Unlike the other members of the TRAF family, TRAF1 lacks the N-terminal ring finger motif and has a tissue specific expression. Here we demonstrate that expression of TRAF1 is induced by TNF and the protein kinase C (PKC) activator PMA, but not by interleukin-1 (IL-1). TNF-induced upregulation of TRAF1 could be prevented by pretreatment of the cells with the proteasome inhibitor MG-132, whereas the PKC inhibitor Ro31-8220 was without effect. Interestingly, overexpression of TRAF1 in HEK293T completely prevented NF-kappaB activation induced by TNF, IL-1, or overexpression of TRAF2 or TRAF6. These data suggest that inducible expression of TRAF1 may serve a negative regulatory function in NF-kappaB signalling pathways.

High-affinity interactions of tumor necrosis factor receptor-associated factors (TRAFs) and CD40 require TRAF trimerization and CD40 multimerization

Signaling by some TNF receptor family members, including CD40, is mediated by TNF receptor-associated factors (TRAFs) that interact with receptor cytoplasmic domains following ligand-induced receptor oligomerization. Here we have defined the oligomeric structure of recombinant TRAF domains that directly interact with CD40 and quantitated the affinities of TRAF2 and TRAF3 for CD40. Biochemical and biophysical analyses demonstrated that TRAF domains of TRAF1, TRAF2, TRAF3, and TRAF6 formed homo-trimers in solution. N-terminal deletions of TRAF2 and TRAF3 defined minimal amino acid sequences necessary for trimer formation and indicated that the coiled coil TRAF-N region is required for trimerization. Consistent with the idea that TRAF trimerization is required for high-affinity interactions with CD40, monomeric TRAF-C domains bound to CD40 significantly weaker than trimeric TRAFs. In surface plasmon resonance studies, a hierarchy of affinity of trimeric TRAFs for trimeric CD40 was found to be TRAF2 > TRAF3 >> TRAF1 and TRAF6. CD40 trimerization was demonstrated to be sufficient for optimal NF-kappaB and p38 mitogen activated protein kinase activation through wild-type CD40. In contrast, a higher degree of CD40 multimerization was necessary for maximal signaling in a cell line expressing a mutated CD40 (T254A) that signaled only through TRAF6. The affinities of TRAF proteins for oligomerized receptors as well as different requirements for degree of receptor multimerization appear to contribute to the selectivity of TRAF recruitment to receptor cytoplasmic domains.

The human tumor necrosis factor (TNF) receptor-associated factor 1 gene (TRAF1) is up-regulated by cytokines of the TNF ligand family and modulates TNF-induced activation of NF-kappaB and c-Jun N-terminal kinase

To understand how the TNF receptor-associated factor 1 (TRAF1) is transcriptionally regulated, in vitro DNA binding assays, promoter-reporter gene assays, and RNase protection assays were performed with the human TRAF1 gene. Binding of NF-kappaB to three of five putative binding sites within the human TRAF1 promoter was found in electrophoretic mobility shift assay studies, and analysis of TRAF1 gene promoter luciferase constructs confirmed the functional importance of these elements. Moreover, triggering of TNF-R1, CD40, and the interleukin-1 receptor resulted in transcription of the TRAF1 gene, whereas receptors that are not activators or only poor activators of NF-kappaB in HeLa cells failed to show a significant TRAF1 induction. Because it has been shown that members of the TRAF family are involved in activation of NF-kappaB and the c-Jun N-terminal kinase (JNK) by the interleukin-1 receptor and members of the TNF receptor superfamily, a role of TRAF1 in receptor cross-talk and/or feedback regulation of activated receptor signaling complexes can be suggested. In fact, we found that TNF-induced activation of JNK is prolonged in transfectants overexpressing TRAF1, whereas overexpression of a deletion mutant of TRAF1 in which the N-terminal part had been replaced by the green fluorescent protein interfered with TNF-induced activation of NF-kappaB and JNK.

Mature T lymphocyte apoptosis--immune regulation in a dynamic and unpredictable antigenic environment

Apoptosis of mature T lymphocytes preserves peripheral homeostasis and tolerance by countering the profound changes in the number and types of T cells stimulated by diverse antigens. T cell apoptosis occurs in at least two major forms: antigen-driven and lymphokine withdrawal. These forms of death are controlled in response to local levels of IL-2 and antigen in a feedback mechanism termed propriocidal regulation. Active antigen-driven death is mediated by the expression of death cytokines such as FasL and TNF. These death cytokines engage specific receptors that assemble caspase-activating protein complexes. These signaling complexes tightly regulate cell death but are vulnerable to inherited defects. Passive lymphokine withdrawal death may result from the cytoplasmic activation of caspases that is regulated by mitochondria and the Bcl-2 protein. The human disease, Autoimmune Lymphoproliferative Syndrome (ALPS) is due to dominant-interfering mutations in the Fas/APO-1/CD95 receptor and other components of the death pathway. The study of ALPS patients reveals the necessity of apoptosis for preventing autoimmunity and allows the genetic investigation of apoptosis in humans. Immunological, cellular, and molecular evidence indicates that throughout the life of a T cell, apoptosis may be evoked in excessive, harmful, or useless clonotypes to preserve a healthy and balanced immune system.

CD40 signaling through tumor necrosis factor receptor-associated factors (TRAFs). Binding site specificity and activation of downstream pathways by distinct TRAFs

Tumor necrosis factor receptor-associated factors (TRAFs) associate with the CD40 cytoplasmic domain and initiate signaling after CD40 receptor multimerization by its ligand. We used saturating peptide-based mutational analyses of the TRAF1/TRAF2/TRAF3 and TRAF6 binding sequences in CD40 to finely map residues involved in CD40-TRAF interactions. The core binding site for TRAF1, TRAF2, and TRAF3 in CD40 could be minimally substituted. The TRAF6 binding site demonstrated more amino acid sequence flexibility and could be optimized. Point mutations that eliminated or enhanced binding of TRAFs to one or both sites were made in CD40 and tested in quantitative CD40-TRAF binding assays. Sequences flanking the core TRAF binding sites were found to modulate TRAF binding, and the two TRAF binding sites were not independent. Cloned stable transfectants of human embryonic kidney 293 cells that expressed wild type CD40 or individual CD40 mutations were used to demonstrate that both TRAF binding sites were required for optimal NF-kappaB and c-Jun N-terminal kinase activation. In contrast, p38 mitogen-activated protein kinase activation was primarily dependent upon TRAF6 binding. These studies suggest a role in CD40 signaling for competitive TRAF binding and imply that CD40 responses reflect an integration of signals from individual TRAFs.

The ubiquitin-homology protein, DAP-1, associates with tumor necrosis factor receptor (p60) death domain and induces apoptosis

The tumor necrosis factor receptor, p60 (TNF-R1), transduces death signals via the association of its cytoplasmic domain with several intracellular proteins. By screening a mammalian cDNA library using the yeast two-hybrid cloning technique, we isolated a ubiquitin-homology protein, DAP-1, which specifically interacts with the cytoplasmic death domain of TNF-R1. Sequence analysis reveals that DAP-1 shares striking sequence homology with the yeast SMT3 protein that is essential for the maintenance of chromosome integrity during mitosis (Meluh, P. B., and Koshland, D. (1995) Mol. Biol. Cell 6, 793-807). DAP-1 is nearly identical to PIC1, a protein that interacts with the PML tumor suppressor implicated in acute promyelocytic leukemia (Boddy, M. N., Howe, K., Etkin, L. D., Solomon, E., and Freemont, P. S. (1996) Oncogene 13, 971-982), and the sentrin protein, which associates with the Fas death receptor (Okura, T., Gong, L., Kamitani, T., Wada, T., Okura, I., Wei, C. F., Chang, H. M., and Yeh, E. T. (1996) J. Immunol. 157, 4277-4281). The in vivo interaction between DAP-1 and TNF-R1 was further confirmed in mammalian cells. In transient transfection assays, overexpression of DAP-1 suppresses NF-kappaB/Rel activity in 293T cells, a human kidney embryonic carcinoma cell line. Overexpression of either DAP-1 or sentrin causes apoptosis of TNF-sensitive L929 fibroblast cell line, as well as TNF-resistant osteosarcoma cell line, U2OS. Furthermore, the dominant negative Fas-associated death domain protein (FADD) protein blocks the cell death induced by either DAP-1 or FADD. Collectively, these observations highly suggest a role for DAP-1 in mediating TNF-induced cell death signaling pathways, presumably through the recruitment of FADD death effector.

TNF receptor associated factors in cytokine signaling

Just four years ago the first two members of a new family of molecules involved in signal transduction by members of the TNF receptor superfamily were described and designated TNF Receptor Associated Factors (TRAFs). In the meantime six human and murine TRAFs as well as a TRAF protein from C. elegans have been molecularly cloned. From our current point of view, TRAF proteins appear to represent multifunctional signal adaptors, tightly embedded in a network of signals culminating in the activation of kinase cascades that finally lead to the activation of c-Jun N-terminal kinase. p38 mitogen activated protein kinase, and the transcription factor NF-kappaB, thereby also affecting the balance between survival and cell death. Some of the activities of the individual TRAF family members may be redundant although transgenic knockout animal models have already shown that crucial signaling pathways for single TRAF molecules in vivo can be defined.

Tumor Necrosis Factor Receptor-Associated Factor 1 Is Overexpressed in Reed-Sternberg Cells of Hodgkin’s Disease and Epstein-Barr Virus-Transformed Lymphoid Cells

The tumor necrosis factor (TNF) receptor-associated factor 1 (TRAF1) is a member of the recently defined TRAF family. It takes part in the signal transduction of the TNF receptor 2 (TNFR2), the lymphotoxin-β receptor (LT-βR), CD40, CD30, and LMP1; is induced by LMP1 in vitro; and protects lymphoid cells from apoptosis. To identify the cells in which TRAF1 is active in vivo, we studied TRAF1 transcripts in normal lymphoid tissue, in Epstein-Barr virus (EBV)-induced lymphoproliferations, and in malignant lymphomas with special reference to those that overexpress the cytokine receptor CD30 and CD40 of the TNF receptor family at the single-cell level using a radioactive in situ hybridization. In normal lymphoid tissue, TRAF1 message proved to be absent from all resting B and T cells as well as from macrophages and accessory cells (follicular dendritic cells and interdigitating cells) and present in few perifollicular and intrafollicular lymphoid blasts. In contrast, there was a high and consistent TRAF1 overexpression in EBV-induced lymphoproliferations and Hodgkin’s disease. Nearly all non-Hodgkin’s lymphoma show low or no TRAF1 expression. Only some cases of diffuse large B-cell lymphoma showed a moderate to high TRAF1 signal. Several of the latter cases were EBV+. These data confirm that TRAF1 is an inducible molecule and indicates its deregulation in the mentioned disorders with the potential of a blockage of the apoptotic pathway.