TRAF5, a novel tumor necrosis factor receptor-associated factor family protein, mediates CD40 signaling

Differential requirements for tumor necrosis factor receptor-associated factor family proteins in CD40-mediated induction of NF-kappaB and Jun N-terminal kinase activation

CD40 is a member of the tumor necrosis factor receptor family that mediates a number of important signaling events in B-lymphocytes and some other types of cells through interaction of its cytoplasmic (ct) domain with tumor necrosis factor receptor-associated factor (TRAF) proteins. Alanine substitution and truncation mutants of the human CD40ct domain were generated, revealing residues critical for binding TRAF2, TRAF3, or both of these proteins. In contrast to TRAF2 and TRAF3, direct binding of TRAF1, TRAF4, TRAF5, or TRAF6 to CD40 was not detected. However, TRAF5 could be recruited to wild-type CD40 in a TRAF3-dependent manner but not to a CD40 mutant (Q263A) that selectively fails to bind TRAF3. CD40 mutants with impaired binding to TRAF2, TRAF3, or both of these proteins completely retained the ability to activate NF-kappaB and Jun N-terminal kinase (JNK), implying that CD40 can stimulate TRAF2- and TRAF3-independent pathways for NF-kappaB and JNK activation. A carboxyl-truncation mutant of CD40 lacking the last 32 amino acids required for TRAF2 and TRAF3 binding, CD40(Delta32), mediated NF-kappaB induction through a mechanism that was suppressible by co-expression of TRAF6(DeltaN), a dominant-negative version of TRAF6, but not by TRAF2(DeltaN), implying that while TRAF6 does not directly bind CD40, it can participate in CD40 signaling. In contrast, TRAF6(DeltaN) did not impair JNK activation by CD40(Delta32). Taken together, these findings reveal redundancy in the involvement of TRAF family proteins in CD40-mediated NF-kappaB induction and suggest that the membrane-proximal region of CD40 may stimulate the JNK pathway through a TRAF-independent mechanism.

NF-κB Is a Central Regulator of the Intestinal Epithelial Cell Innate Immune Response Induced by Infection with Enteroinvasive Bacteria

Human intestinal epithelial cells up-regulate the expression of an inflammatory gene program in response to infection with a spectrum of different strains of enteroinvasive bacteria. The conserved nature of this program suggested that diverse signals, which are activated by enteroinvasive bacteria, can be integrated into a common signaling pathway that activates a set of proinflammatory genes in infected host cells. Human intestinal epithelial cell lines, HT-29, Caco-2, and T84, were infected with invasive bacteria that use different strategies to induce their uptake and have different intracellular localizations (i.e., Salmonella dublin, enteroinvasive Escherichia coli, or Yersinia enterocolitica). Infection with each of these bacteria resulted in the activation of TNF receptor associated factors, two recently described serine kinases, IκB kinase (IKK) α and IKKβ, and increased NF-κB DNA binding activity. This was paralleled by partial degradation of IκBα and IκBε in bacteria-infected Caco-2 cells. Mutant proteins that act as superrepressors of IKKβ and IκBα inhibited the up-regulated transcription and expression of downstream targets genes of NF-κB that are key components of the epithelial inflammatory gene program (i.e., IL-8, growth-related oncogene-α, monocyte chemoattractant protein-1, TNF-α, cyclooxygenase-2, nitric oxide synthase-2, ICAM-1) activated by those enteroinvasive bacteria. These studies position NF-κB as a central regulator of the epithelial cell innate immune response to infection with enteroinvasive bacteria.

The p75 neurotrophin receptor (p75NTR) alters tumor necrosis factor-mediated NF-kappaB activity under physiological conditions, but direct p75NTR-mediated NF-kappaB activation requires cell stress

The p75 neurotrophin receptor (p75NTR) has been linked to activation of the NF-kappaB transcriptional complex in oligodendrocytes, Schwann cells, and PCNA cells. In this report, tumor necrosis factor (TNF)- and neurotrophin-mediated NF (nuclear factor)-kappaB activation were compared in several cell lines. All cell types showed TNF-mediated activation of NF-kappaB, but direct neurotrophin-dependent activation of NF-kappaB was never observed under normal growth conditions. In PCNA cells, a modest nerve growth factor (NGF)-dependent induction of NF-kappaB was detected but only after cells were subjected to severe stress. Although NGF binding did not directly activate NF-kappaB under normal conditions, NGF consistently altered TNF-dependent NF-kappaB activation in each cell type examined, and extended exposure to NGF and TNF always increased NF-kappaB activation over that achieved with TNF alone. The increase in NF-kappaB activity mediated by NGF correlated with reduced levels of IkappaBalpha; NGF added alone had no effect on IkappaBalpha levels, but when added with TNF, NGF treatment significantly reduced IkappaBalpha levels. We propose that modulation of cytokine receptor signaling is a significant physiological function of the p75 neurotrophin receptor and that previous reports of direct NF-kappaB activation through p75NTR reflect this modulatory activity.

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.

Mutant cells that do not respond to interleukin-1 (IL-1) reveal a novel role for IL-1 receptor-associated kinase

Mutagenized human 293 cells containing an interleukin-1 (IL-1)-regulated herpes thymidine kinase gene, selected in IL-1 and gancyclovir, have yielded many independent clones that are unresponsive to IL-1. The four clones analyzed here carry recessive mutations and represent three complementation groups. Mutant A in complementation group I1 lacks IL-1 receptor-associated kinase (IRAK), while the mutants in the other two groups are defective in unknown components that function upstream of IRAK. Expression of exogenous IRAK in I1A cells (I1A-IRAK) restores their responsiveness to IL-1. Neither NFkappaB nor Jun kinase is activated in IL-1-treated I1A cells, but these responses are restored in I1A-IRAK cells, indicating that IRAK is required for both. To address the role of the kinase activity of IRAK in IL-1 signaling, its ATP binding site was mutated (K239A), completely abolishing kinase activity. In transfected I1A cells, IRAK-K239A was still phosphorylated upon IL-1 stimulation and, surprisingly, still complemented all the defects in the mutant cells. Therefore, IRAK must be phosphorylated by a different kinase, and phospho-IRAK must play a role in IL-1-mediated signaling that does not require its kinase activity.

TRAF-3 mRNA splice-deletion variants encode isoforms that induce NF-kappaB activation

Although TRAF-3 gene products are required for signaling in T-B cell collaboration, full-length TRAF-3 appears to lack signaling function in transient transfection assays that measure NF-kappaB activation. However, the TRAF-3 gene also encodes at least three mRNA splice-deletion variants that predict protein isoforms (delta25aa, delta52aa and delta56aa) with altered zinc (Zn) finger domains and unknown functional capacities. To determine whether TRAF-3 splice-deletion variants may transmit activating receptor signals to the nucleus, cDNAs for five additional splice-variant isoforms (delta27aa, delta83aa, delta103aa, delta130aa and delta221aa) were cloned from a TRAF-3+ lymphoma and the expression and function of each of the eight TRAF-3 splice-deletion variants was analyzed. Among the splice-deletion variants, TRAF-3 delta130 mRNA is expressed by tonsillar B cells and by each of a panel of B and T cell lines. TRAF-3 delta221 protein is expressed by tonsillar B cells and by each of the lymphocytic lines. The functional effect of over-expressing each TRAF-3 splice-deletion variant on NF-kappaB activation was studied in 293 T cells. Seven of the TRAF-3 splice-deletion variants, such as TRAF-3 delta130, induce substantial NF-kappaB-driven luciferase activity (80-500 fold). In contrast, TRAF-3 delta221 (in which the complete Zn finger domain is absent) fails to induce NF-kappaB activation. Although full-length TRAF-3 alone is inactive, it augments the functional effects of the seven activating TRAF-3 splice-deletion variants (1.4-5 fold). These data indicate that alterations of the Zn finger domains render the TRAF-3 splice-deletion variants capable of inducing NF-kappaB activation and that full-length TRAF-3 augments their signaling.

The zinc finger protein A20 inhibits TNF-induced NF-kappaB-dependent gene expression by interfering with an RIP- or TRAF2-mediated transactivation signal and directly binds to a novel NF-kappaB-inhibiting protein ABIN

The zinc finger protein A20 is a tumor necrosis factor (TNF)- and interleukin 1 (IL-1)-inducible protein that negatively regulates nuclear factor-kappa B (NF-kappaB)-dependent gene expression. However, the molecular mechanism by which A20 exerts this effect is still unclear. We show that A20 does not inhibit TNF- induced nuclear translocation and DNA binding of NF-kappaB, although it completely prevents the TNF- induced activation of an NF-kappaB-dependent reporter gene, as well as TNF-induced IL-6 and granulocyte macrophage-colony stimulating factor gene expression. Moreover, NF-kappaB activation induced by overexpression of the TNF receptor-associated proteins TNF receptor-associated death domain protein (TRADD), receptor interacting protein (RIP), and TNF recep- tor-associated factor 2 (TRAF2) was also inhibited by expression of A20, whereas NF-kappaB activation induced by overexpression of NF-kappaB-inducing kinase (NIK) or the human T cell leukemia virus type 1 (HTLV-1) Tax was unaffected. These results demonstrate that A20 inhibits NF-kappaB-dependent gene expression by interfering with a novel TNF-induced and RIP- or TRAF2-mediated pathway that is different from the NIK-IkappaB kinase pathway and that is specifically involved in the transactivation of NF-kappaB. Via yeast two-hybrid screening, we found that A20 binds to a novel protein, ABIN, which mimics the NF-kappaB inhibiting effects of A20 upon overexpression, suggesting that the effect of A20 is mediated by its interaction with this NF-kappaB inhibiting protein, ABIN.

Gene targeting in the analysis of mammalian apoptosis and TNF receptor superfamily signaling

Apoptosis, or programmed cell death (PCD), is the subject of much current investigative interest. Developing embryos and many adult organ systems require the tight coupling of cellular proliferation and PCD to ensure proper organogenesis and optimal tissue function. Over the past decade, our knowledge of the genetic basis underlying the execution of apoptosis in mammals has progressed enormously, thanks largely to groundbreaking studies performed in the nematode Caenorhabditis elegans. In contrast, the components of the signaling apparatus that links the various death stimuli and the receptors they stimulate to the execution mechanism remain relatively unknown. It is only in the past 4 years that studies of signal transduction via members of the tumor necrosis factor (TNF) receptor superfamily have identified a plethora of novel signaling proteins, including molecules that are directly involved in apoptosis signaling, and others that regulate the induction of cell death. This two-part review focuses on the biology of apoptosis and signaling through members of the TNF receptor superfamily as revealed by the study of gene-targeted "knockout" mice. These genetic mutant animals are invaluable tools not only for confirming or refuting a proposed function of a particular gene in an in vivo setting, but also for uncovering novel functions for a gene that were not anticipated from conventional in vitro experiments. In the field of apoptosis, as for many other areas of biomedical research, knockout mice and cell lines can be used as models for studying human disease, with the ultimate goal of developing therapeutic strategies.

Severe osteopetrosis, defective interleukin-1 signalling and lymph node organogenesis in TRAF6-deficient mice

BACKGROUND

TRAF6, a member of the tumour necrosis factor receptor-associated factor family, was first identified as a transducer of CD40 and interleukin-1 receptor (IL-1R) signals based on the interaction of TRAF6 with the cytoplasmic tail of CD40 and with the IL-1R associated kinase in vitro. However, the functions of TRAF6 in vivo remain unidentified.

RESULTS

We show that TRAF6-/- mice exhibit severe osteopetrosis and are defective in osteoclast formation. In vitro culture experiments revealed that osteoclast precursor cells derived from TRAF6-/- mice are unable to differentiate to functional osteoclasts in response to osteoclast differentiation factor (ODF). In bone marrow of TRAF6-/- mice, the number of sIgM+B220+ immature B cells is markedly reduced while the ratio of proB to preB cells is not affected. In contrast, development of thymocytes is not affected. Furthermore, TRAF6-/- mice are defective in lymph node organogenesis and IL-1 signalling in thymocytes.

CONCLUSIONS

The results identify TRAF6 as an essential component of ODF signalling pathway, and also show that TRAF6 plays pivotal roles in immune and inflammatory systems in vivo.

Cutting Edge: Contrasting Roles of TNF Receptor-Associated Factor 2 (TRAF2) and TRAF3 in CD40-Activated B Lymphocyte Differentiation

In B lymphocytes, CD40 signals contribute to the activation of Ab secretion, isotype switching, T cell costimulation, and immunological memory. TRAF proteins appear to be important components of the CD40 signal transduction complex, but their roles in the activation of B cell effector functions are poorly understood. We examined the contributions of TNF receptor-associated factor 2 (TRAF2) and TRAF3 to CD40-activated differentiation in mouse B cells transfected with inducible TRAF and dominant-negative TRAF cDNAs. We find that binding of TRAF2 and TRAF3 to CD40 is not required for the induction of Ab secretion, but that both TRAF molecules can regulate the activation process. We demonstrate a negative regulatory role for TRAF3 and that this activity is dependent on the availability of an intact TRAF3-binding site in the cytoplasmic domain of CD40. In contrast, TRAF2 appears to play a positive role in B cell differentiation, and this activity is apparent even when its binding site on CD40 is disrupted.

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.

Alymphoplasia is caused by a point mutation in the mouse gene encoding Nf-kappa b-inducing kinase

The alymphoplasia (aly) mutation of mouse is autosomal recessive and characterized by the systemic absence of lymph nodes (LN) and Peyer's patches (PP) and disorganized splenic and thymic structures with immunodeficiency. Although recent reports have shown that the interaction between lymphotoxin (LT) and the LT beta-receptor (Ltbeta r, encoded by Ltbr) provides a critical signal for LN genesis in mice, the aly locus on chromosome 11 is distinct from those for LT and its receptor. We found that the aly allele carries a point mutation causing an amino acid substitution in the carboxy-terminal interaction domain of Nf-kappa b-inducing kinase (Nik, encoded by the gene Nik). Transgenic complementation with wild-type Nik restored the normal structures of LN, PP, spleen and thymus, and the normal immune response in aly/aly mice. In addition, the aly mutation in a kinase domain-truncated Nik abolished its dominant-negative effect on Nf-kappa b activation induced by an excess of Ltbeta r. Our observations agree with previous reports that Ltbeta r-deficient mice showed defects in LN genesis and that Nik is a common mediator of Nf-kappa b activation by the tumour necrosis factor (TNF) receptor family. Nik is able to interact with members of the TRAF family (Traf1, 2, 3, 5 and 6), suggesting it acts downstream of TRAF-associating receptor signalling pathways, including Tnfr, Cd40, Cd30 and Ltbeta r. The phenotypes of aly/aly mice are more severe than those of Ltbr-/- mice, however, indicating involvement of Nik in signal transduction mediated by other receptors.

TRAF6 deficiency results in osteopetrosis and defective interleukin-1, CD40, and LPS signaling

Bone resorption and remodeling is an intricately controlled, physiological process that requires the function of osteoclasts. The processes governing both the differentiation and activation of osteoclasts involve signals induced by osteoprotegerin ligand (OPGL), a member of tumor necrosis factor (TNF) superfamily, and its cognate receptor RANK. The molecular mechanisms of the intracellular signal transduction remain to be elucidated. Here we report that mice deficient in TNF receptor-associated factor 6 (TRAF6) are osteopetrotic with defects in bone remodeling and tooth eruption due to impaired osteoclast function. Using in vitro assays, we demonstrate that TRAF6 is crucial not only in IL-1 and CD40 signaling but also, surprisingly, in LPS signaling. Furthermore, like TRAF2 and TRAF3, TRAF6 is essential for perinatal and postnatal survival. These findings establish unexpectedly diverse and critical roles for TRAF6 in perinatal and postnatal survival, bone metabolism, LPS, and cytokine signaling.

An 11-Amino Acid Sequence in the Cytoplasmic Domain of CD40 Is Sufficient for Activation of c-Jun N-Terminal Kinase, Activation of MAPKAP Kinase-2, Phosphorylation of IκBα, and Protection of WEHI-231 Cells from Anti-IgM-Induced Growth Arrest

We have previously shown that CD40 causes strong activation of the c-Jun N-terminal kinase (JNK), the p38 mitogen-activated protein kinases (MAPK) and MAPKAP kinase-2, a downstream target of p38 MAPK. To identify signaling motifs in the CD40 cytoplasmic domain that are responsible for activation of these kinases, we have created a set of 11 chimeric receptors consisting of the extracellular and transmembrane domains of CD8 fused to portions of the murine CD40 cytoplasmic domain. These chimeric receptors were expressed in WEHI-231 B lymphoma cells. We found that amino acids 35–45 of the CD40 cytoplasmic domain constitute an independent signaling motif that is sufficient for activation of the JNK and p38 MAPK pathways, as well as for induction of IκBα phosphorylation and degradation. Amino acids 35–45 were also sufficient to protect WEHI-231 cells from anti-IgM-induced growth arrest. This is the same region of CD40 required for binding the TNF receptor-associated factor-2 (TRAF2), TRAF3, and TRAF5 adapter proteins. These data support the idea that one or more of these TRAF proteins couple CD40 to the kinase cascades that activate NF-κB, JNK, and p38 MAPK.

CD40 Ligand-CD40 Interaction Induces Chemokines in Cervical Carcinoma Cells in Synergism with IFN-γ

Cellular immunity plays a major role in controlling human papilloma virus infection and development of cervical carcinoma. Mononuclear cell infiltration possibly due to the action of chemokines becomes prominent in the tumor tissue. In fact, the macrophage chemoattractant protein-1, MCP-1, was detected in cervical squamous cell carcinoma in situ, whereas absent in cultured cells. From this, unknown environmental factors were postulated regulating chemokine expression in vivo. In this study, we show high CD40 expression on cervical carcinoma cells and CD40 ligand (CD40L) staining on attracted T cells in tumor tissue, suggesting a paracrine stimulation mechanism via CD40L-CD40 interactions. We therefore investigated chemokine synthesis in nonmalignant and malignant human papilloma virus-positive cell lines after CD40L exposure. Constitutive expression of MCP-1, MCP-3, RANTES, and IFN-γ-inducible protein-10 was almost undetectable in all cell lines tested. CD40L was able to induce MCP-1 production; however, despite much higher CD40 expression in malignant cells, MCP-1 induction was significantly lower compared with nontumorigenic cells. After sensitization with IFN-γ, another T cell-derived cytokine showing minimal effects on CD40 expression levels, CD40 ligation led to a more than 20-fold MCP-1 induction in carcinoma cell lines. An even stronger effect was observed for IFN-γ-inducible protein-10. Our study highlights the synergism of T cell-derived mediators such as CD40L and IFN-γ for chemokine responses in cervical carcinoma cells, helping to understand the chemokine expression patterns observed in vivo.

Phosphorylation of TRAF2 inhibits binding to the CD40 cytoplasmic domain

TRAF2 is a signal transducing adaptor molecule which binds to the CD40 cytoplasmic domain. We have found that it is phosphorylated, predominantly on serine residues, when transiently overexpressed in 293 cells. The phosphorylation appears to be related to the signaling events that are activated by TRAF2 under these circumstances, since two nonfunctional mutants were found to be phosphorylated significantly less than the wild-type protein. Furthermore, the phosphorylation status of TRAF2 had significant effects on the ability of the protein to bind to CD40, as evidenced by our observations that the CD40 cytoplasmic domain interacted preferentially with underphosphorylated TRAF2 and that phosphatase treatment significantly enhanced the binding of TRAF2 to CD40. We conclude from these studies that the phosphorylation of TRAF2 is likely to play an important role in regulating signaling by virtue of its ability to influence the CD40-TRAF2 interaction.

Differential Responses to CD40 Ligation Among Burkitt Lymphoma Lines That Are Uniformly Responsive to Epstein-Barr Virus Latent Membrane Protein 1

Ligation of CD40 on the surface of B cells induces multiple phenotypic effects, many of which are mimicked by the EBV latent membrane protein 1 (LMP1) through its interaction with downstream components of the CD40 signaling pathway. Because the effects of LMP1 have been most closely studied in human Burkitt Lymphoma (BL) cell lines retaining a tumor biopsy-like phenotype in vitro, we have examined the response of a panel of such lines to CD40 ligation. Two distinct patterns of response were observed that were unrelated to the surface level of CD40 or to the EBV genome status of the lines. Following exposure to either CD40-specific mAbs or the soluble trimeric ligand (sCD40L), high responder (HR) lines showed rapid aggregation, activation of NF-κB, up-regulation of cell surface markers ICAM-1/CD54 and Fas/CD95, and growth inhibition. Aggregation was seen at lower doses than those required to elicit the other effects. By contrast, low responder (LR) lines showed no detectable response to CD40 mAbs, while their responses to sCD40L were limited to activation of NF-κB and up-regulation of CD95 only. However, in transfection experiments, LMP1 uniformly induced the full spectrum of phenotypic effects in both HR and LR lines. We conclude that some BL cell lines show a highly restricted response to CD40 ligation but remain fully susceptible to LMP1.

The EBV Transforming Protein, Latent Membrane Protein 1, Mimics and Cooperates with CD40 Signaling in B Lymphocytes

Latent membrane protein 1 (LMP1) is required for EBV-induced immortalization of human B cells, and expression of the protein in the absence of other viral proteins leads to an activated phenotype in B cells. It has been well documented that LMP1 causes B cells to up-regulate adhesion molecules, such as LFA-1 and ICAM-1, and coactivation molecules, such as B7-1 and CD23, as well as to activate NF-κB. Ligation of the endogenous B cell CD40 molecule also induces these and other activated phenotypic changes. Here, we report that expression of LMP1 also activates B cells to secrete Ig and IL-6 and rescues them from B cell receptor-mediated growth arrest analogous to CD40 signaling. Furthermore, an HLA-A2LMP1 chimeric construct demonstrates that the oligomerization of the carboxyl-terminal 200 amino acids of LMP1 is sufficient for B cell signaling. Finally, we demonstrate that LMP1 and CD40 signaling pathways interact cooperatively in inducing B cell effector functions.

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.

Specificities of CD40 signaling: involvement of TRAF2 in CD40-induced NF-kappaB activation and intercellular adhesion molecule-1 up-regulation

Several tumor necrosis factor receptor-associated factor (TRAF) family proteins including TRAF2, TRAF3, TRAF5, and TRAF6, as well as Jak3, have been implicated as potential mediators of CD40 signaling. An extensive in vitro binding study indicated that TRAF2 and TRAF3 bind to the CD40 cytoplasmic tail (CD40ct) with much higher affinity than TRAF5 and TRAF6 and that TRAF2 and TRAF3 bind to different residues of the CD40ct. Using CD40 mutants incapable of binding TRAF2, TRAF3, or Jak3, we found that the TRAF2-binding site of the CD40ct is critical for NF-kappaB and stress-activated protein kinase activation, as well as the up-regulation of the intercellular adhesion molecule-1 (ICAM-1) gene, whereas binding of TRAF3 and Jak3 is dispensable for all of these functions. Overexpression of a dominantly active IkappaBalpha strongly inhibited CD40-induced NF-kappaB activation, ICAM-1 promoter activity, and cell-surface ICAM-1 up-regulation. These studies suggest a potential signal transduction pathway from the CD40 receptor to the transcriptional activation of the ICAM-1 gene.

Two differently regulated nuclear factor kappaB activation pathways triggered by the cytoplasmic tail of CD40

CD40 signaling modulates the immune response at least in part by activation of nuclear factor kappaB (NFkappaB). It has been shown that two distinct domains in the CD40 cytoplasmic tail (cyt), namely cyt-N and cyt-C, independently activate NFkappaB. Although four members of the tumor necrosis factor receptor-associated factor (TRAF) family, including TRAF2, TRAF3, TRAF5, and TRAF6, bind to the CD40 cyt, how each TRAF protein contributes to the NFkappaB activation by CD40 is not clear. Here we report that TRAF2, TRAF3, and TRAF5 bind cyt-C, whereas TRAF6 binds cyt-N. cyt-N is conserved poorly between human and mouse CD40, while cyt-C is highly conserved. However, single aa substitution of Glu-235 in cyt-N of human CD40 with Ala abolishes the binding of TRAF6 to cyt-N and NFkappaB activation by cyt-N. Conservation of this Glu between mouse and human CD40 strongly suggests that TRAF6 could link cyt-N to signals essential for CD40-mediated immune response. Furthermore, NFkappaB activation by cyt-C is inhibited by a kinase-negative form of NFkappaB-inducing kinase more efficiently than that by cyt-N, consistent with the result that NFkappaB activation by TRAF2 and TRAF5 is inhibited by a kinase-negative form of NFkappaB-inducing kinase more efficiently than that by TRAF6. These results indicate that NFkappaB activating signals emanating from cyt-N and cyt-C are mediated by the different members of the TRAF family and could be regulated in a distinct manner.

Signalling by CD95 and TNF receptors: not only life and death

Members of the TNF family of receptors play important roles in normal physiology and in defence. The recent rapid progress in the understanding of the mechanisms of apoptosis has been accompanied by assumptions that TNF family receptors such as CD95(Fas/APO-1) only have a role in regulating cell survival. While regulation of cell death is one important function of TNF family receptors, they are capable of activating signal transduction pathways that have many other effects. The present review will focus on signalling of some TNF family receptors in the immune system, not only for apoptosis, but also for survival or activation.

Receptor activator of NF-kappaB recruits multiple TRAF family adaptors and activates c-Jun N-terminal kinase

Receptor activator of NF-kappaB (RANK) is a recently cloned member of the tumor necrosis factor receptor (TNFR) superfamily, and its function has been implicated in osteoclast differentiation and dendritic cell survival. Many of the TNFR family receptors recruit various members of the TNF receptor-associated factor (TRAF) family for transduction of their signals to NF-kappaB and c-Jun N-terminal kinase. In this study, the involvement of TRAF family members and the activation of the JNK pathway in signal transduction by RANK were investigated. TRAF1, 2, 3, 5, and 6 were found to bind RANK in vitro. Association of RANK with each of these TRAF proteins was also detected in vivo. Expression of RANK in cultured cells also induced the activation of JNK, which was blocked by a dominant-negative form of JNK. Furthermore, by employing various C-terminal deletion mutants of RANK, the regions responsible for TRAF interaction and JNK activation were identified. TRAF5 was determined to bind to the C-terminal 11 amino acids and the other TRAF members to a region N-terminal to the TRAF5 binding site. The domain responsible for JNK activation was localized to the same region where TRAF1, 2, 3, and 6 bound, which suggests that these TRAF molecules might mediate the RANK-induced JNK activation.

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.

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.

The Fanconi anemia group C gene product modulates apoptotic responses to tumor necrosis factor-alpha and Fas ligand but does not suppress expression of receptors of the tumor necrosis factor receptor superfamily

Exposure of hematopoietic progenitor cells (HPC) from mice and humans with Fanconi anemia group C (FAC) to interferon-gamma (IFN-gamma) or tumor necrosis factor-alpha (TNF-alpha) at doses too low to inhibit growth of normal HPC induces profound apoptotic responses. Because the IFN-gamma hypersensitivity of cells lacking the FAC protein is mediated, in part, through priming of the Fas pathway, and because several other members of this family are capable of inducing apoptosis either alone or in concert with each other, we tested the hypothesis that IFN-gamma induces increased expression of members of the TNF receptor (TNFR) superfamily in cells nullizygous for the FAC gene. Using isogenic human Epstein-Barr virus-transformed lymphoblast cell lines and c-kit+ bone marrow cells from mice with inactivating mutations of the FAC locus, we quantified mRNA levels by reverse transcriptase polymerase chain reaction and surface expression of the gene products by flow cytometry of TNFR1, TNFR2, Fas, CD30, CD40, and nerve growth factor receptor. We found that neither constitutive nor IFN-gamma-induced expression of these receptors was influenced by the absence of a functional FAC gene product, and expression of these receptors was not suppressed in nullizygous cells complemented with the normal FAC cDNA. We conclude that, although exaggerated apoptotic responses in FAC-deficient cells are at least partially mediated through activation of members of the TNFR superfamily, the normal FAC protein does not function as a direct suppressor of this family of molecules and inactivation of FAC does not augment expression of these proteins.

A single gene for human TRAF-3 at chromosome 14q32.3 encodes a variety of mRNA species by alternative polyadenylation, mRNA splicing and transcription initiation

Human TRAF-3 is a signaling molecule that interacts with the cytoplasmic tails of CD40 and other TNF-receptor family members. TRAF-3 mRNA is expressed as two major classes of approximately 2 and 8 kb and a number of TRAF-3 encoding cDNA clones differ in discrete gene segments. Because this variety of mRNA species could result from mRNA processing events and/or multiple genes, the structure and localization of TRAF-3 encoding gene elements were determined. FISH and radiation hybrid mapping demonstrated that TRAF-3 is located at chromosome 14q32.3, approximately 1 Mb centromeric to the Ig heavy chain gene complex. Physical mapping of four overlapping genomic PAC clones established that TRAF-3 transcripts are encoded by a single gene, comprised of 13 exons and spanning 130 kb. Alternative polyadenylation in the mRNA segment encoded by exon 12 accounts for the difference between the 2 kb and the 8 kb classes of transcripts. Alternative mRNA splicing in the coding region (encoded by exons 3-12) generates transcripts which delete exons 8 (75 nt), 7+8 (156 nt) or 8+9 (168 nt) and that encode distinct protein isoforms (delta25, delta52 and delta56 aa, respectively). Alternative splicing of exon 2 (139 nt) and alternative transcriptional initiation result in mRNA species with distinct 5'UTRs. Together, these data indicate that a single TRAF-3 gene encodes a variety of mRNA species by a combination of alternative polyadenylation, alternative mRNA splicing and/or alternative initiation.

OCA-B integrates B cell antigen receptor-, CD40L- and IL 4-mediated signals for the germinal center pathway of B cell development

Many of the key decisions in lymphocyte differentiation and activation are dependent on integration of antigen receptor and co-receptor signals. Although there is significant understanding of these receptors and their signaling pathways, little is known about the molecular requirements for signal integration at the level of activation of gene expression. Here we show that in primary B cells, expression of the B-cell specific transcription coactivator OCA-B (also known as OBF-1 or Bob-1) is regulated synergistically by the B-cell antigen receptor, CD40L and interleukin signaling pathways. Consistent with the requirement for multiple T cell-dependent signals to induce OCA-B, we find that OCA-B protein is highly expressed in germinal center B cells. Accordingly, germinal center formation is blocked completely in the absence of OCA-B expression in B cells, whereas the helper functions of OCA-B-deficient T cells are indistinguishable from controls. The requirement for OCA-B expression in B cells is germinal center specific since the development of primary B cell follicles, the marginal zone and plasma cells are all intact. Thus, OCA-B is the first example of a transcriptional coactivator that is both synergistically induced by and required for integration of signals that mediate cell fate decisions.

NF-κB Activation in CD27 Signaling: Involvement of TNF Receptor-Associated Factors in Its Signaling and Identification of Functional Region of CD27

CD27 belongs to TNF receptor family, and it is unique in this family for its disulfide-linked homodimerization of 55-kDa monomers. In the present study we demonstrate that overexpression of CD27 in 293 cells induces a low level of NF-κB activation, and the ligation of the receptor by its corresponding ligand (CD70) augments this signal dramatically. Either TNF receptor-associated factor-2 (TRAF2) or TRAF3 binds to the CD27 molecule from the coimmunoprecipitation experiment. This NF-κB activation signal is inhibited by dominant negative TRAF2 or intact TRAF3, indicating that TRAF2 and TRAF3 works as a mediator and an inhibitor, respectively. The activated NF-κB complex contains at least two components, p50 and p65, but not p52. All these phenomena have also been observed in the TNF receptor type II, CD30 and CD40 signaling system, indicating that this receptor family uses the common or similar molecules for this signal. Finally, we identified the 13-amino acid alignment in the cytoplasmic region of the CD27 molecule (residues 238–250 amino acids), which is critical for the NF-κB activation signal and also for its association with TRAFs. This amino acid alignment contains the EEEG sequence, which is essential for interaction of CD30 or CD40 with TRAFs (TRAF1 and TRAF2, but not TRAF3), and also contains the PIQED sequence, which is similar to PXQXT that is known to be necessary for interaction of TNF receptor II and CD30 with TRAFs (TRAF1, 2, and 3).

CD40 induces resistance to TNF-mediated apoptosis in a fibroblast cell line

CD40, a member of the TNF receptor family, has been characterized as an important T-B cell interaction molecule. In B cells it co-stimulates isotype switching, proliferation, adhesion and is involved in cell death regulation. In addition to B cells, CD40 expression was found on transformed cells and carcinomas. However, little is known about its functions in these cell types. Recent studies show that CD40 mediates the production of pro-inflammatory cytokines in non-hematopoietic cells, inhibits proliferation or induces cell death. In some cell types the apoptotic program triggered by CD40 is only executed when protein synthesis is blocked, suggesting the existence of constitutively expressed resistance proteins. Here we demonstrate that CD40, similar to the 55-kDa TNF receptor (p55TNFR), has a dual role in the regulation of apoptosis in such cells. In the fibroblast cell line SV80 both CD40 and the p55TNFR trigger apoptosis when protein synthesis is blocked with cycloheximide (CHX). Simultaneous activation of both receptors results in markedly enhanced cell death. However, CD40 activation more than 4 h prior to a challenge with TNF/CHX paradoxically conferred resistance to TNF-induced cell death. Protection correlated with NF-kappaB induction and up-regulation of the anti-apoptotic zinc finger protein A20. Overexpression of A20 in turn rendered SV80 cells resistant to TNF cytotoxicity. In conclusion, our data provide evidence that CD40 may regulate cell death in non-hematopoietic cells in a dual fashion: the decision upon apoptosis or survival of a CD40-activated cell seems to depend on its ability to up-regulate resistance factors.

Normal CD40-mediated activation of monocytes and dendritic cells from patients with hyper-IgM syndrome due to a CD40 pathway defect in B cells

Patients with X-linked hyper-IgM syndrome [CD40 ligand (CD40L) deficiency] are prone to infections by intracellular parasites. It has been suggested that this susceptibility is caused by defective macrophage activation through the CD40L-CD40 pathway. We studied the CD40-mediated activation of monocytes and dendritic cells from patients affected with a CD40L+ hyper-IgM syndrome characterized by a defect of B lymphocyte responses to CD40 agonists. We show that the CD40-induced production of IL-6, IL-8 and TNF-alpha by monocytes, and IL-12 by dendritic cells, and expression of the activation markers CD83, the costimulatory molecules CD86 and CD80, and HLA-DR antigens were all similar in patient and control cells. This observation is consistent with the clinical characteristics of the syndrome: a defect of immunoglobulin switch but no susceptibility to opportunistic infections, as observed in CD40L-deficient patients. These observations suggest that CD40-mediated activation pathways could be, at least in part, different in B and monocytic/dendritic cell lineages.

Molecular determinants of NF-kappaB-inducing kinase action

NF-kappaB corresponds to an inducible eukaryotic transcription factor complex that is negatively regulated in resting cells by its physical assembly with a family of cytoplasmic ankyrin-rich inhibitors termed IkappaB. Stimulation of cells with various proinflammatory cytokines, including tumor necrosis factor alpha (TNF-alpha), induces nuclear NF-kappaB expression. TNF-alpha signaling involves the recruitment of at least three proteins (TRADD, RIP, and TRAF2) to the type 1 TNF-alpha receptor tail, leading to the sequential activation of the downstream NF-kappaB-inducing kinase (NIK) and IkappaB-specific kinases (IKKalpha and IKKbeta). When activated, IKKalpha and IKKbeta directly phosphorylate the two N-terminal regulatory serines within IkappaB alpha, triggering ubiquitination and rapid degradation of this inhibitor in the 26S proteasome. This process liberates the NF-kappaB complex, allowing it to translocate to the nucleus. In studies of NIK, we found that Thr-559 located within the activation loop of its kinase domain regulates NIK action. Alanine substitution of Thr-559 but not other serine or threonine residues within the activation loop abolishes its activity and its ability to phosphorylate and activate IKKalpha. Such a NIK-T559A mutant also dominantly interferes with TNF-alpha induction of NF-kappaB. We also found that ectopically expressed NIK both spontaneously forms oligomers and displays a high level of constitutive activity. Analysis of a series of NIK deletion mutants indicates that multiple subregions of the kinase participate in the formation of these NIK-NIK oligomers. NIK also physically assembles with downstream IKKalpha; however, this interaction is mediated through a discrete C-terminal domain within NIK located between amino acids 735 and 947. When expressed alone, this C-terminal NIK fragment functions as a potent inhibitor of TNF-alpha-mediated induction of NF-kappaB and alone is sufficient to disrupt the physical association of NIK and IKKalpha. Together, these findings provide new insights into the molecular basis for TNF-alpha signaling, suggesting an important role for heterotypic and possibly homotypic interactions of NIK in this response.

Role of the TRAF binding site and NF-kappaB activation in Epstein-Barr virus latent membrane protein 1-induced cell gene expression

In this study, we investigated the induction of cellular gene expression by the Epstein-Barr Virus (EBV) latent membrane protein 1 (LMP1). Previously, LMP1 was shown to induce the expression of ICAM-1, LFA-3, CD40, and EBI3 in EBV-negative Burkitt lymphoma (BL) cells and of the epidermal growth factor receptor (EGF-R) in epithelial cells. We now show that LMP1 expression also increased Fas and tumor necrosis factor receptor-associated factor 1 (TRAF1) in BL cells. LMP1 mediates NF-kappaB activation via two independent domains located in its C-terminal cytoplasmic tail, a TRAF-interacting site that associates with TRAF1, -2, -3, and -5 through a PXQXT/S core motif and a TRADD-interacting site. In EBV-transformed B cells or transiently transfected BL cells, significant amounts of TRAF1, -2, -3, and -5 are associated with LMP1. In epithelial cells, very little TRAF1 is expressed, and only TRAF2, -3, and -5, are significantly complexed with LMP1. The importance of TRAF binding to the PXQXT/S motif in LMP1-mediated gene induction was studied by using an LMP1 mutant that contains alanine point mutations in this motif and fails to associate with TRAFs. This mutant, LMP1(P204A/Q206A), induced 60% of wild-type LMP1 NF-kappaB activation and had approximately 60% of wild-type LMP1 effect on Fas, ICAM-1, CD40, and LFA-3 induction. In contrast, LMP1(P204A/Q206A) was substantially more impaired in TRAF1, EBI3, and EGF-R induction. Thus, TRAF binding to the PXQXT/S motif has a nonessential role in up-regulating Fas, ICAM-1, CD40, and LFA-3 expression and a critical role in up-regulating TRAF1, EBI3, and EGF-R expression. Further, D1 LMP1, an LMP1 mutant that does not aggregate failed to induce TRAF1, EBI3, Fas, ICAM-1, CD40, and LFA-3 expression confirming the essential role for aggregation in LMP1 signaling. Overexpression of a dominant form of IkappaBalpha blocked LMP1-mediated TRAF1, EBI3, Fas, ICAM-1, CD40, and LFA-3 up-regulation, indicating that NF-kappaB is an important component of LMP1-mediated gene induction from both the TRAF- and TRADD-interacting sites.

An intact zinc ring finger is required for tumor necrosis factor receptor-associated factor-mediated nuclear factor-kappaB activation but is dispensable for c-Jun N-terminal kinase signaling

The diverse biological effects of the tumor necrosis factor (TNF) receptor superfamily are believed to be mediated in part through TNF receptor-associated factors (TRAFs), a family of cytoplasmic adaptor proteins which can activate intracellular signaling pathways, including the nuclear factor-kappaB (NF-kappaB) and c-Jun N-terminal kinase (JNK) pathways. TRAFs 2, 5, and 6 strongly activate both pathways when overexpressed; however, TRAF 3 (a close homologue of TRAF 5) does not significantly activate either pathway. The current study addresses the structural basis for this difference by substituting corresponding domains of TRAF 5 into TRAF 3 and testing activation of both pathways. A small region of TRAF 5 (the first zinc finger and 10 residues of the second zinc finger) is sufficient to convert TRAF 3 into an activator of both pathways. Also, an intact zinc ring finger is required for NF-kappaB activation but not JNK activation. In agreement with this finding, TRAF 2A, a TRAF 2 splice variant with an altered ring finger, is a specific activator of JNK. These findings suggest that different domains of TRAFs may be involved in NF-kappaB and JNK signaling. Also, alternative splicing of TRAFs may represent a novel mechanism whereby TNF family receptors can mediate distinct downstream effects in different tissues.

TNFR80-Dependent Enhancement of TNFR60-Induced Cell Death Is Mediated by TNFR-Associated Factor 2 and Is Specific for TNFR60

Costimulation of TNFR80 can strongly enhance TNFR60-induced cell death. In this study, we show that this enhancement is TNFR60 selective, as neither TNF-related apoptosis-inducing ligand/Apo2 ligand-, Apo1/Fas-, ceramide-, nor daunorubicin-mediated cell death was affected by costimulation of TNFR80. We further demonstrate that TNFR-associated factor 2 (TRAF2) is critically involved in both negative and positive regulation of TNF-induced cell death. Overexpression of TRAF2 and of a TRAF2 mutant, deficient in nuclear factor-κB activation, selectively desensitized and enhanced, respectively, TNFR60-induced cell death in HeLa cells. However, upon costimulation of TNFR80, which mediates activation of nuclear factor-κB and the c-Jun amino-terminal kinase via TRAF2, TNF-induced cell death is drastically enhanced in parental and TRAF2-transfected, but not in TRAF2 (87–501)-transfected cells. These data point to a critical role of TRAF2 in the apoptotic TNFR cross talk, whereby the TNFR80-dependent enhancement of TNFR60-induced cell death is due to TNFR80-mediated negative regulation of TRAF2 function(s). An interference with TRAF2 function was confirmed independently by analysis of c-Jun amino-terminal kinase activation via TNFR60 upon prestimulation of TNFR80. We propose that the apoptotic TNFR cross talk is based on TNFR80-mediated abrogation of antiapoptotic TRAF2-dependent signaling pathways initiated by TNFR60, but not Apo1/Fas or the apoptotic TNF-related apoptosis-inducing ligand receptors.

OCA-B integrates B cell antigen receptor-, CD40L- and IL 4-mediated signals for the germinal center pathway of B cell development

Many of the key decisions in lymphocyte differentiation and activation are dependent on integration of antigen receptor and co-receptor signals. Although there is significant understanding of these receptors and their signaling pathways, little is known about the molecular requirements for signal integration at the level of activation of gene expression. Here we show that in primary B cells, expression of the B-cell specific transcription coactivator OCA-B (also known as OBF-1 or Bob-1) is regulated synergistically by the B-cell antigen receptor, CD40L and interleukin signaling pathways. Consistent with the requirement for multiple T cell-dependent signals to induce OCA-B, we find that OCA-B protein is highly expressed in germinal center B cells. Accordingly, germinal center formation is blocked completely in the absence of OCA-B expression in B cells, whereas the helper functions of OCA-B-deficient T cells are indistinguishable from controls. The requirement for OCA-B expression in B cells is germinal center specific since the development of primary B cell follicles, the marginal zone and plasma cells are all intact. Thus, OCA-B is the first example of a transcriptional coactivator that is both synergistically induced by and required for integration of signals that mediate cell fate decisions.

Characterization of the intracellular domain of receptor activator of NF-kappaB (RANK). Interaction with tumor necrosis factor receptor-associated factors and activation of NF-kappab and c-Jun N-terminal kinase

Various members of the tumor necrosis factor (TNF) receptor superfamily interact directly with signaling molecules of the TNF receptor-associated factor (TRAF) family to activate nuclear factor kappaB (NF-kappaB) and the c-Jun N-terminal kinase (JNK) pathway. The receptor activator of NF-kappaB (RANK), a recently described TNF receptor family member, and its ligand, RANKL, promote survival of dendritic cells and differentiation of osteoclasts. RANK contains 383 amino acids in its intracellular domain (residues 234-616), which contain three putative TRAF-binding domains (termed I, II, and III). In this study, we set out to identify the region of RANK needed for interaction with TRAF molecules and for stimulation of NF-kappaB and JNK activity. We constructed epitope-tagged RANK (F-RANK616) and three C-terminal truncations, F-RANK330, F-RANK427, and F-RANK530, lacking 85, 188, and 285 amino acids, respectively. From this deletion analysis, we determined that TRAF2, TRAF5, and TRAF6 interact with RANK at its C-terminal 85-amino acid tail; the binding affinity appeared to be in the order of TRAF2 > TRAF5 > TRAF6. Furthermore, overexpression of RANK stimulated JNK and NF-kappaB activation. When the C-terminal tail, which is necessary for TRAF binding, was deleted, the truncated RANK receptor was still capable of stimulating JNK activity but not NF-kappaB, suggesting that interaction with TRAFs is necessary for NF-kappaB activation but not necessary for activation of the JNK pathway.

TNF Receptor-Associated Factor-3 Signaling Mediates Activation of p38 and Jun N-Terminal Kinase, Cytokine Secretion, and Ig Production Following Ligation of CD40 on Human B Cells

CD40 engagement induces a variety of functional outcomes following association with adaptor molecules of the TNF receptor-associated factor (TRAF) family. Whereas TRAF2, -5, and -6 initiate NF-κB activation, the outcomes of TRAF3-initiated signaling are less characterized. To delineate CD40-induced TRAF3-dependent events, Ramos B cells stably transfected with a dominant negative TRAF3 were stimulated with membranes expressing recombinant CD154/CD40 ligand. In the absence of TRAF3 signaling, activation of p38 and control of Ig production were abrogated, whereas Jun N-terminal kinase activation and secretion of IL-10, lymphotoxin-α, and TNF-α were partially blocked. By contrast, induction of apoptosis, activation of NF-κB, generation of granulocyte-macrophage CSF, and up-regulation of CD54, MHC class II, and CD95 were unaffected by the TRAF3 dominant negative. Together, these results indicate that TRAF3 initiates independent signaling pathways via p38 and JNK that are associated with specific functional outcomes.

RIP2 is a novel NF-kappaB-activating and cell death-inducing kinase

Through specific interactions with members of the tumor necrosis receptor (TNFR) family, adapter molecules such as the serine/threonine (Ser/Thr) kinase RIP mediate divergent signaling pathways including NF-kappaB activation and cell death. In this study, we have identified and characterized a novel 61-kDa protein kinase related to RIP that is a component of both the TNFR-1 and the CD40 signaling complexes. Receptor interacting protein-2 (RIP2) contains an N-terminal domain with homology to Ser/Thr kinases and a C-terminal caspase activation and recruitment domain (CARD), a homophilic interaction motif that mediates the recruitment of caspase death proteases. Overexpression of RIP2 signaled both NF-kappaB activation and cell death. Mutational analysis revealed the pro-apoptotic function of RIP2 to be restricted to its C-terminal CARD domain, whereas the intact molecule was necessary for NF-kappaB activation. RIP2 interacted with other members of the TNFR-1 signaling complex, including inhibitor of apoptosis protein cIAP1 and with members of the TNFR-associated factor (TRAF) family, specifically TRAF1, TRAF5, and TRAF6, but not with TRAF2, TRAF3, or TRAF4. These TRAF interactions mediate the recruitment of RIP2 to receptor signaling complexes.

Cooperation of Both TNF Receptors in Inducing Apoptosis: Involvement of the TNF Receptor-Associated Factor Binding Domain of the TNF Receptor 75

TNF-R55 is the main receptor mediating TNF-induced cytotoxicity. However, in some cells TNF-R75 also signals cell death. In PC60 cells, the presence of both receptor types is required to induce apoptosis following either specific TNF-R55 or TNF-R75 triggering, pointing to a mechanism of receptor cooperation. In this study, we extend previous observations and show that TNF-R55 and TNF-R75 cooperation in the case of apoptosis in PC60 cells is bidirectional. We also demonstrate ligand-independent TNF-R55-mediated cooperation in TNF-R75-induced granulocyte/macrophage-CSF secretion, but not vice versa. To determine which part of the intracellular TNF-R75 sequence was responsible for the observed receptor cooperation in apoptosis, we introduced different TNF-R75 mutant constructs in PC60 cells already expressing TNF-R55. Our data indicate that an intact TNF-R-associated factors 1 and 2 (TRAF1/TRAF2)-binding domain is required for receptor cooperation. These findings suggest a role for the TRAF complex in TNF-R cooperation in the induction of cell death in PC60 cells. Nevertheless, introduction of a dominant negative (DN) TRAF2 molecule was not able to affect receptor cooperation. Remarkably, TRAF2-DN overexpression, which was found to inhibit the TNF-dependent recruitment of endogenous wild-type TRAF2 to the TNF-R75 signaling complex, could neither block TNF-R55- or TNF-R75-induced NF-κB activation nor granulocyte/macrophage-CSF secretion. Possibly, additional factors different from TRAF2 are involved in TNF-mediated NF-κB activation.

Death domain receptors and their role in cell demise

Apoptotic signals are transduced by five death domain-containing receptors--TNFR1, Fas, DR3, DR4, and DR5--by binding to their ligands. The intracellular portion of all these receptors contains a region, approximately 80 amino acids long, referred to as the "death domain" (DD). On activation by its ligand, the DD recruits various proteins that mediate cell death. These proteins, in turn, recruit other proteins via their DDs or death effector domains (DED). The actual destruction of the cell, however, is accomplished by serial activation of a family of proteases referred to as caspases. Cell death is, in part, regulated by transmembrane decoy receptors that contain either none of or only part of the DD. This article briefly reviews what is known about the receptors and other proteins involved in apoptosis. In addition, because numerous proteins that mediate apoptosis have been discovered independently and simultaneously and thus are known by many different names, a comprehensive cross-referenced list of these proteins is provided.

The N-Terminal Domains Target TNF Receptor-Associated Factor-2 to the Nucleus and Display Transcriptional Regulatory Activity

The subcellular localization of the TNF receptor-associated factor-2 (TRAF2) adaptor protein in human endothelial cells, which mediates proinflammatory responses of TNF, has been analyzed by confocal immunofluorescence microscopy and by Western blotting of fractionated cell extracts. Rabbit antisera reactive with either amino- or carboxyl-terminal TRAF2 peptides frequently but not uniformly stain nuclei of cultured HUVEC or the established human endothelial cell line, ECV304. However, Western blotting reveals significant heterogeneity in the reactivities of these polyclonal Abs. Transiently transfected HUVEC expressing FLAG epitope-tagged TRAF2 consistently show prominent nuclear localization, and deletion mutants of TRAF2 identify the portion of the molecule responsible for nuclear localization as the amino-terminal ring finger domain. TNF treatment does not appear to influence the localization of endogenous or transfected TRAF2 protein. Transfection of the amino-terminal half of the TRAF2 molecule, containing the ring and zinc finger domains, which localizes to the nucleus, results in activation of E-selectin but not of NF-κB promoter-reporter gene transcription or of c-Jun N-terminal kinase activation. These observations suggest that TRAF2 may reside in the nucleus and directly regulate transcription, independent of its role in cytoplasmic signal transduction.

The TNF receptor family member CD27 signals to Jun N-terminal kinase via Traf-2

CD27 is a lymphocyte-specific member of the TNF receptor (TNFR) family. It is a costimulatory molecule for peripheral T cells, as defined by its ability to enhance the TCR-induced proliferative response. We show here that CD27 augments TCR-induced Jun N-terminal kinase (JNK) activity in primary murine lymph node T cells. To investigate how CD27 couples to JNK, we performed a yeast two hybrid screen with the CD27 cytoplasmic tail. This revealed that CD27 directly associates with Traf-2. Transfection experiments using dominant negative Traf-2 indicated that CD27 communicates with JNK via Traf-2. These findings group CD27 together with other members of the TNFR family, TNFR-1, -2, CD30 and CD40, which have all been shown to couple to Traf proteins. Since Traf proteins have been reported to initiate an anti-apoptotic signaling pathway, our data suggest that CD27 not only regulates proliferation, but also survival of T lymphocytes.

The regulation and roles of Rel/NF-kappa B transcription factors during lymphocyte activation

The activation of B and T cells by a wide range of stimuli can rapidly induce specific gene expression via a mechanism that promotes the nuclear translocation of different Rel/nuclear factor-kappa B (NF-kappa B) transcription factors which are normally resident in the cytoplasm. Recent findings highlight the crucial roles of specific Rel/NF-kappa B family members in the processes of cell division, apoptosis and differentiation that accompany lymphocyte activation.

TNF, apoptosis and autoimmunity: a common thread?

A subset of cytokine mediators belonging to the tumor necrosis factor (TNF) family cause apoptosis, acting through receptors and signaling pathways that have recently come to light. Further, at least one autoimmune disease results from a defined defect of apoptosis (mutations of the Fas ligand or its receptor). It is offered that many, and perhaps most autoimmune diseases may result from primary defects of apoptosis. Such defects may cause reflexive overproduction of TNF and other pro-apoptotic cytokines. The collateral damage produced by these mediators may be of pathogenetic importance in complex autoimmune disorders such as rheumatoid arthritis and Crohn disease, wherein TNF blockade is known to have ameliorative effects.

CD27, a member of the tumor necrosis factor receptor superfamily, activates NF-kappaB and stress-activated protein kinase/c-Jun N-terminal kinase via TRAF2, TRAF5, and NF-kappaB-inducing kinase

CD27 is a member of the tumor necrosis factor (TNF) receptor superfamily and is expressed on T, B, and NK cells. The signal via CD27 plays pivotal roles in T-T and T-B cell interactions. Here we demonstrate that overexpression of CD27 activates NF-kappaB and stress-activated protein kinase (SAPK)/c-Jun N-terminal kinase (JNK). Deletion analysis of the cytoplasmic domain of CD27 revealed that the C-terminal PIQEDYR motif was indispensable for both NF-kappaB and SAPK/JNK activation and was also required for the interaction with TNF receptor-associated factor (TRAF) 2 and TRAF5, both of which have been implicated in NF-kappaB activation by members of the TNF-R superfamily. Co-transfection of a dominant negative TRAF2 or TRAF5 blocked NF-kappaB and SAPK/JNK activation induced by CD27. Recently, a TRAF2-interacting kinase has been identified, termed NF-kappaB-inducing kinase (NIK). A kinase-inactive mutant NIK blocked CD27-, TRAF2-, and TRAF5-mediated NF-kappaB and SAPK/JNK activation. These results indicate that TRAF2 and TRAF5 are involved in NF-kappaB and SAPK/JNK activation by CD27, and NIK is a common downstream kinase of TRAF2 and TRAF5 for NF-kappaB and SAPK/JNK activation.