Translocation of TRAF proteins regulates apoptotic threshold of cells

A novel tumor necrosis factor receptor-associated factor 6 (TRAF6) gene from Macrobrachiumrosenbergii involved in antibacterial defense against Aeromonas hydrophila

Tumor necrosis factor receptor-associated factor 6 (TRAF6) is an adapter protein that triggers downstream cascades mediated by both TNFR and the interleukin-1 receptor/Toll-like receptor (IL-1R/TLR) superfamily. TRAF6 is involved in various biological processes, including innate and adaptive immunity. In the present study, a homolog of TRAF6 from Macrobrachium rosenbergii (MrTRAF6) was identified and characterized. The full-length cDNA of MrTRAF6 consisted of 2,114 nucleotides with an open reading frame (ORF) of 1,695 nucleotides encoding a 564-amino acid protein that contained a conserved TRAF family motif including two RING-type zinc fingers and a C-terminal meprin and TRAF homology (MATH) domain. The putative amino sequence of MrTRAF6 shared 45.5-97.3% identity with TRAF6s from other crustacean species with the highest identity to Macrobrachium nipponense TRAF6. Phylogenetic analysis revealed that MrTRAF6 was closely related to TRAF6 of invertebrates and clustered with crustaceans. According to gene expression analysis, the MrTRAF6 transcript demonstrated broad expression in all tissues tested, with the highest expression level in gill and the lowest in muscle tissues. Upon immune challenge with Aeromonas hydrophila, significant upregulation of MrTRAF6 expression was found in the gill, hepatopancreas, hemocyte, and muscle. Furthermore, an RNA interference assay showed that silencing MrTRAF6 by dsRNA could reduce the expression of mannose-binding lectin (MBL) and crustin, but no significant change was detected in anti-lipopolysaccharide factor 5 (ALF5) levels. In addition, the cumulative mortality rate of MrTRAF6-silenced M. rosenbergii was significantly increased after A. hydrophila infection. These findings indicated that MrTRAF6 is involved in antibacterial activity and plays a critical role in the innate immune response of M. rosenbergii.

Autophagy differentially regulates TNF receptor Fn14 by distinct mammalian Atg8 proteins

Autophagy, a conserved membrane trafficking process, sequesters cytoplasmic components into autophagosomes and targets them for lysosomal degradation. The TNF receptor Fn14 participates in multiple intracellular signaling pathways and is strongly induced upon tissue injury and solid tumorigenesis. While Fn14 is a short-lived protein, the regulation of its levels is largely obscure. Here we uncover a role for autophagy in Fn14 turnover, wherein specific core autophagy Atg8 proteins play distinct roles: Fn14 accumulates in the ERGIC in absence of GABARAP but within endosomes in the vicinity of autophagic membranes in absence of GATE-16. Moreover, GABARAP regulates overall cellular levels of Fn14, whereas GATE-16 regulates TWEAK signaling by Fn14 and thereby NF-κB activity. These findings not only implicate different Atg8 proteins in distinct roles within the mechanism of selective autophagic regulation of Fn14, but may also provide a more general view of their role in mediating autophagosome biogenesis from different membrane sources.

A TRAF2 binding independent region of TNFR2 is responsible for TRAF2 depletion and enhancement of cytotoxicity driven by TNFR1

Tumor Necrosis Factor (TNF) interacts with two receptors known as TNFR1 and TNFR2. TNFR1 activation may result in either cell proliferation or cell death. TNFR2 activates Nuclear Factor-kappaB (NF-kB) and c-Jun N-terminal kinase (JNK) which lead to transcriptional activation of genes related to cell proliferation and survival. This depends on the binding of TNF Receptor Associated Factor 2 (TRAF2) to the receptor. TNFR2 also induces TRAF2 degradation. In this work we have investigated the structural features of TNFR2 responsible for inducing TRAF2 degradation and have studied the biological consequences of this activity. We show that when TNFR1 and TNFR2 are co-expressed, TRAF2 depletion leads to an enhanced TNFR1 cytotoxicity which correlates with the inhibition of NF-kB. NF-kB activation and TRAF2 degradation depend of different regions of the receptor since TNFR2 mutants at amino acids 343-349 fail to induce TRAF2 degradation and have lost their ability to enhance TNFR1-mediated cell death but are still able to activate NF-kB. Moreover, whereas NF-kB activation requires TRAF2 binding to the receptor, TRAF2 degradation appears independent of TRAF2 binding. Thus, TNFR2 mutants unable to bind TRAF2 are still able to induce its degradation and to enhance TNFR1-mediated cytotoxicity. To test further this receptor crosstalk we have developed a system stably expressing in cells carrying only endogenous TNFR1 the chimeric receptor RANK-TNFR2, formed by the extracellular region of RANK (Receptor activator of NF-kB) and the intracellular region of TNFR2.This has made possible to study independently the signals triggered by TNFR1 and TNFR2. In these cells TNFR1 is selectively activated by soluble TNF (sTNF) while RANK-TNFR2 is selectively activated by RANKL. Treatment of these cells with sTNF and RANKL leads to an enhanced cytotoxicity.

Signal transduction by tumor necrosis factor receptors

Tumor necrosis factor (TNF) is a key mediator in the inflammatory response which is implicated in the onset of a number of diseases. Research on TNF led to the characterization of the largest family of cytokines known until now, the TNF superfamily, which exert their biological effects through the interaction with transmembrane receptors of the TNFR superfamily. TNF itself exerts its biological effects interacting with two different receptors: TNFR1 and TNFR2. TNFR1 presents a death domain on its intracellular region. In contrast to TNFR1, TNFR2 does not have a death domain. Activation of TNFR1 implies the consecutive formation of two different TNF receptor signalling complexes. Complex I controls the expression of antiapoptotic proteins that prevent the triggering of cell death processes, whereas Complex II triggers cell death processes. TNFR2 only signals for antiapoptotic reactions. However, recent evidence indicates that TNFR2 also signals to induce TRAF2 degradation. TRAF2 is a key mediator in signal transduction of both TNFR1 and TNFR2. Thus, this novel signalling pathway has two important implications: on one hand, it represents an auto regulatory loop for TNFR2; on the other hand, when this signal is triggered TNFR1 activity is modified so that antiapoptotic pathways are inhibited and apoptotic reactions are enhanced.

NF-kappaB signal triggering and termination by tumor necrosis factor receptor 2

Tumor necrosis factor receptor 2 (TNFR2) activates transcription factor κB (NF-κB) and c-Jun N-terminal kinase (JNK). The mechanisms mediating these activations are dependent on the recruitment of TNF receptor-associated factor 2 (TRAF2) to the intracellular region of the receptor. TNFR2 also induces TRAF2 degradation. We show that in addition to the well characterized TRAF2 binding motif 402-SKEE-405, the human receptor contains another sequence located at the C-terminal end (amino acids 425-439), which also recruits TRAF2 and activates NF-κB. In addition to that, human TNFR2 contains a conserved region (amino acids 338-379) which is responsible for TRAF2 degradation and therefore of terminating NF-κB signaling. TRAF2 degradation and the lack of NF-κB activation when both TNFR1 and TNFR2 are co-expressed results in an enhanced ability of TNFR1 to induce cell death, showing that the cross-talk between both receptors is of a great biological relevance. Induction of TRAF2 degradation appears to be independent of TRAF2 binding to the receptor. Amino acids 343-TGSSDSS-349 are essential for inducing TRAF2 degradation because deletion mutants of this region or point mutations at serine residues 345 and 346 or 348 and 349 obliterate the ability of TNFR2 to induce TRAF2 degradation.

TRAF2 must bind to cellular inhibitors of apoptosis for tumor necrosis factor (tnf) to efficiently activate nf-{kappa}b and to prevent tnf-induced apoptosis

Tumor necrosis factor (TNF) receptor-associated factor-2 (TRAF2) binds to cIAP1 and cIAP2 (cIAP1/2) and recruits them to the cytoplasmic domain of several members of the TNF receptor (TNFR) superfamily, including the TNF-TNFR1 ligand-receptor complex. Here, we define a cIAP1/2-interacting motif (CIM) within the TRAF-N domain of TRAF2, and we use TRAF2 CIM mutants to determine the role of TRAF2 and cIAP1/2 individually, and the TRAF2-cIAP1/2 interaction, in TNFR1-dependent signaling. We show that both the TRAF2 RING domain and the TRAF2 CIM are required to regulate NF-kappaB-inducing kinase stability and suppress constitutive noncanonical NF-kappaB activation. Conversely, following TNFR1 stimulation, cells bearing a CIM-mutated TRAF2 showed reduced canonical NF-kappaB activation and TNF-induced RIPK1 ubiquitylation. Remarkably, the RING domain of TRAF2 was dispensable for these functions. However, like the TRAF2 CIM, the RING domain of TRAF2 was required for protection against TNF-induced apoptosis. These results show that TRAF2 has anti-apoptotic signaling roles in addition to promoting NF-kappaB signaling and that efficient activation of NF-kappaB by TNFR1 requires the recruitment of cIAP1/2 by TRAF2.

The effect of aging on OX40 agonist-mediated cancer immunotherapy

Agents that enhance T cell co-stimulatory signaling have emerged as promising cancer immunotherapies. Our laboratory has been evaluating the TNF receptor co-stimulatory molecule, OX40, which has the capacity to augment critical aspects of T cell function and induce tumor regression in animal models. Effective stimulation of OX40 expressing T cells was accomplished with agonist antibodies to OX40 that were eventually translated into a clinical trial for cancer patients. A recent attempt to assess the affect of immune senescence on OX40 therapy, revealed a dramatic loss of efficacy of the agonist therapy in older tumor-bearing mice. The deficiency in OX40-enhanced anti-tumor responses in older mice correlated with a decrease in the number of differentiated effector T cells. Further investigation suggests that the underlying age-related decline in the agonist OX40-mediated T cell responses was not inherent to the T cells themselves, but related to the host environment. Thus, effective use of immunotherapies based on T cell co-stimulatory molecules may require additional modifications, such as immune stimulants to increase innate immunity, to address age-related defects that reside outside of the T cell and within the host environment.

TNF-like weak inducer of apoptosis inhibits proinflammatory TNF receptor-1 signaling

Soluble TNF-like weak inducer of apoptosis (TWEAK) trimers induce, in a variety of cell lines, translocation of cytosolic tumor necrosis factor (TNF) receptor-associated factor-2 (TRAF2) to a triton X-100-insoluble compartment without changes in the total cellular TRAF2 content. TWEAK-induced TRAF2 translocation is paralleled by a strong increase in nuclear factor kappaB 2 (NFkappaB2)/p100 processing to p52, indicating that TRAF2 redistribution is sufficient for activation of the alternative NFkappaB pathway. In accordance with the crucial role of TRAF2 in proinflammatory, anti-apoptotic TNF receptor-1 (TNFR1) signaling, we observed that TWEAK-primed cells have a reduced capacity to activate the classical NFkappaB pathway or JNK (cJun N-terminal kinase) in response to TNF. Furthermore, TWEAK-primed cells are sensitized for the TNFR1-mediated induction of apoptotic and necrotic cell death. Notably, the expression of the NFkappaB-regulated, TRAF2-interacting TRAF1 protein can attenuate TWEAK-induced depletion of the triton X-100-soluble TRAF2 fraction and improve TNFR1-induced NFkappaB signaling in TWEAK-primed cells. Taken together, we demonstrate that soluble TWEAK desensitizes cells for proinflammatory TNFR1 signaling and thus identify TWEAK as a modifier of TNF signaling.

The IKK-neutralizing compound Bay11 kills supereffector CD8 T cells by altering caspase-dependent activation-induced cell death

Antigen with dual costimulation through CD137 and CD134 induces powerful CD8 T cell responses. These effector T cells are endowed with an intrinsic survival program resulting in their accumulation in vivo, but the signaling components required for survival are unknown. We tested a cadre of pathway inhibitors and found one preclinical compound, Bay11-7082 (Bay11), which prevented survival. Even the gammac cytokine family members IL-2, -4, -7, and -15 could not block death, nor could pretreatment with IL-7. We found that dual costimulation caused loading of phosphorylated IkappaBalpha (p-IkappaBalpha) and high basal levels of NF-kappaB activity in the effector CD8 T cells. Bay11 trumped both events by reducing the presence of p-IkappaBalpha and ensuing NF-kappaB activity. Not all pathways were impacted to this degree, however, as mitogen-mediated ERK phosphorylation was evident during NF-kappaB inhibition. Nonetheless, Bay11 blocked TCR-stimulated cytokine synthesis by rapidly accentuating activation-induced cell death through elicitation of a caspase-independent pathway. Thus, in effector CD8 T cells, Bay11 forces a dominant caspase-independent death signal that cannot be overcome by an intrinsic survival program nor by survival-inducing cytokines. Therefore, Bay11 may be a useful tool to deliberately kill death-resistant effector T cells for therapeutic benefit.

cIAP1-dependent TRAF2 degradation regulates the differentiation of monocytes into macrophages and their response to CD40 ligand

Peripheral blood monocytes are plastic cells that migrate to tissues and differentiate into various cell types, including macrophages, dendritic cells, and osteoclasts. We have described the migration of cellular inhibitor of apoptosis protein 1 (cIAP1), a member of the IAP family of proteins, from the nucleus to the Golgi apparatus in monocytes undergoing differentiation into macrophages. Here we show that, once in the cytoplasm, cIAP1 is involved in the degradation of the adaptor protein tumor necrosis factor receptor-associated factor 2 (TRAF2) by the proteosomal machinery. Inhibition of cIAP1 prevents the decrease in TRAF2 expression that characterizes macrophage formation. We demonstrate that TRAF2 is initially required for macrophage differentiation as its silencing prevents Ikappa-Balpha degradation, nuclear factor-kappaB (NF-kappaB) p65 nuclear translocation, and the differentiation process. Then, we show that cIAP1-mediated degradation of TRAF2 allows the differentiation process to progress. This degradation is required for the macrophages to be fully functional as TRAF2 overexpression in differentiated cells decreases the c-Jun N-terminal kinase-mediated synthesis and the secretion of proinflammatory cytokines, such as interleukin-8 and monocyte chemoattractant protein 1 (MCP-1) in response to CD40 ligand. We conclude that TRAF2 expression and subsequent degradation are required for the differentiation of monocytes into fully functional macrophages.

Tumor necrosis factor-alpha can provoke cleavage and activation of sterol regulatory element-binding protein in ethanol-exposed cells via a caspase-dependent pathway that is cholesterol insensitive

Ethanol induces the development of hepatic steatosis, increasingly recognized as causing vulnerability to subsequent liver injury. Ethanol has been shown to activate SREBP-1 (sterol regulatory element-binding protein) processing through the conventional cholesterol-sensitive pathway (1). The present study demonstrates that ethanol can also bring about SREBP-1 cleavage and activation through a novel pathway dependent on the endoplasmic reticulum-localized caspases-4 and -12. Evidence is presented that tumor necrosis factor can stimulate caspase-4 and -12 activation in ethanol-exposed cells, which cleaves SREBP-1 to a transcriptionally active form to induce the synthesis of lipogenic enzymes and triglycerides. Moreover, the caspase-4 and -12-dependent activation of SREBP-1 is insensitive to the normal negative feedback exerted by cholesterol and is mediated by the translocation of the scaffolding protein, TRAF-2, to the endoplasmic reticulum.

Understanding signaling cascades in melanoma

Understanding regulatory pathways involved in melanoma development and progression has advanced significantly in recent years. It is now appreciated that melanoma is the result of complex changes in multiple signaling pathways that affect growth control, metabolism, motility and the ability to escape cell death programs. Here we review the major signaling pathways currently known to be deregulated in melanoma with an implication to its development and progression. Among these pathways are Ras, B-Raf, MEK, PTEN, phosphatidylinositol-3 kinase (PI3Ks) and Akt which are constitutively activated in a significant number of melanoma tumors, in most cases due to genomic change. Other pathways discussed in this review include the [Janus kinase/signal transducer and activator of transcription (JAK/STAT), transforming growth factor-beta pathways which are also activated in melanoma, although the underlying mechanism is not yet clear. As a paradigm for remodeled signaling pathways, melanoma also offers a unique opportunity for targeted drug development.

Resistance of the target islet tissue to autoimmune destruction contributes to genetic susceptibility in Type 1 diabetes

Type 1 diabetes occurs when self-reactive T lymphocytes destroy the insulin-producing islet beta cells of the pancreas. The defects causing this disease have often been assumed to occur exclusively in the immune system. We present evidence that genetic variation at the Idd9 diabetes susceptibility locus determines the resilience of the targets of autoimmunity, the islets, to destruction. Susceptible islets exhibit hyper-responsiveness to inflammatory cytokines resulting in enhanced cell death and increased expression of the death receptor Fas. Fas upregulation in beta cells is mediated by TNFR2, and colocalization of TNFR2 with the adaptor TRAF2 in NOD beta cells is altered. TNFR2 lies within the candidate Idd9 interval and the diabetes-associated variant contains a mutation adjacent to the TRAF2 binding site. A component of diabetes susceptibility may therefore be determined by the target of the autoimmune response, and protective TNFR2 signaling in islets inhibit early cytokine-induced damage required for the development of destructive autoimmunity.

REVIEWERS

This article was reviewed by Matthiasvon Herrath, HaraldVon Boehmer, and Ciriaco Piccirillo (nominated by Ethan Shevach).

Function of tumor necrosis factor receptor family members on regulatory T-cells

Effector cells play a crucial role in the immune system of higher vertebrates in eliminating invading pathogens and transformed cells that could cause disease or death of the individual. To be effective and specific, immune responses have to distinguish between self and nonself. Mechanisms of central and peripheral tolerance have evolved to control effector cells that could respond to autoantigens. Regulatory T-cells (Treg cells) are critical modulators of effector cells in the periphery that suppress autoreactive T-cells but are also involved in modulating immune responses against invading pathogens. Identification of surface markers of Treg cells and the development of in vitro systems to study the suppressive function of Treg cells have revealed distinct phenotypic and functional subsets of Treg cells. Several tumor necrosis factor receptor (TNFR) family members have been shown to play a role in the development, homeostasis, and suppressor function of Treg cells. Recent findings suggest that TNFRs and other cell-surface molecules of Treg cells can be explored for therapeutic strategies targeting autoimmune disorders, cancer, and immune responses against pathogens.

VISA is an adapter protein required for virus-triggered IFN-beta signaling

Viral infection or stimulation of TLR3 triggers signaling cascades, leading to activation of the transcription factors IRF-3 and NF-kappaB, which collaborate to induce transcription of type I interferon (IFN) genes. In this study, we identified a protein termed VISA (for virus-induced signaling adaptor) as a critical component in the IFN-beta signaling pathways. VISA recruits IRF-3 to the cytoplasmic viral dsRNA sensor RIG-I. Depletion of VISA inhibits virus-triggered and RIG-I-mediated activation of IRF-3, NF-kappaB, and the IFN-beta promoter, suggesting that VISA plays a central role in virus-triggered TLR3-independent IFN-beta signaling. Our data also indicate that VISA interacts with TRIF and TRAF6 and mediates bifurcation of the TLR3-triggered NF-kappaB and IRF-3 activation pathways. These findings suggest that VISA is critically involved in both virus-triggered TLR3-independent and TLR3-mediated antiviral IFN signaling.

Glucocorticoid-Induced TNF Receptor, a Costimulatory Receptor on Naive and Activated T Cells, Uses TNF Receptor-Associated Factor 2 in a Novel Fashion as an Inhibitor of NF-κB Activation

Glucocorticoid-induced TNFR (GITR) has been implicated as an essential regulator of immune responses to self tissues and pathogens. We have recently shown that GITR-induced cellular events promote survival of naive T cells, but are insufficient to protect against activation-induced cell death. However, the molecular mechanisms of GITR-induced signal transduction that influence physiologic and pathologic immune responses are not well understood. TNFR-associated factors (TRAFs) are pivotal adapter proteins involved in signal transduction pathways of TNFR-related proteins. Yeast two-hybrid assays and studies in HEK293 cells and primary lymphocytes indicated interactions between TRAF2 and GITR mediated by acidic residues in the cytoplasmic domain of the receptor. GITR-induced activation of NF-kappaB is blocked by A20, an NF-kappaB-inducible protein that interacts with TRAFs and functions in a negative feedback mechanism downstream of other TNFRs. Interestingly, in contrast with its effects on signaling triggered by other TNFRs, our functional studies revealed that TRAF2 plays a novel inhibitory role in GITR-triggered NF-kappaB activation.

TNF-alpha induced c-IAP1/TRAF2 complex translocation to a Ubc6-containing compartment and TRAF2 ubiquitination

Signaling through tumor necrosis factor receptor 2 (TNF-R2) results in ubiquitination of TRAF2 by the E3 c-IAP1. In this report, we confirm that TRAF2 translocates to a Triton X-100 (TX)-insoluble compartment upon TNF-R2 engagement. Moreover, TRAF2 ubiquitination occurs in this compartment, from which TRAF2 is degraded in a proteasome-dependent manner. Confocal microscopy demonstrated that the TX-insoluble compartment is perinuclear and co-localizes with endoplasmic reticulum (ER) markers. The ER transmembrane Ubc6 bound to c-IAP1 and served as a cognate E2 for c-IAP1's E3 activity in vitro. Furthermore, Ubc6 co-localized with translocated TRAF2/c-IAP1 in the ER-associated compartment in vivo, and a catalytically inactive Ubc6 mutant inhibited TNF-alpha-induced, TNF-R2-dependent TRAF2 degradation. These results indicate that upon TNF-R2 signaling, translocation of TRAF2 and c-IAP1 to an ER-associated, Ubc6-containing perinuclear compartment is required for the ubiquitination of TRAF2 by c-IAP1. Therefore, the ER plays a key role in the TNF-R-mediated signal transduction cascade by acting as a site of assembly for E2/E3/substrate complexes.

Sequestration of TRAF2 into stress granules interrupts tumor necrosis factor signaling under stress conditions

The cellular stress response (SR) is a phylogenetically conserved protection mechanism that involves inhibition of protein synthesis through recruitment of translation factors such as eIF4G into insoluble stress granules (SGs) and blockade of proinflammatory responses by interruption of the signaling pathway from tumor necrosis factor alpha (TNF-alpha) to nuclear factor-kappaB (NF-kappaB) activation. However, the link between these two physiological phenomena has not been clearly elucidated. Here we report that eIF4GI, which is a scaffold protein interacting with many translation factors, interacts with TRAF2, a signaling molecule that plays a key role in activation of NF-kappaB through TNF-alpha. These two proteins colocalize in SGs during cellular exposure to stress conditions. Moreover, TRAF2 is absent from TNFR1 complexes under stress conditions even after TNF-alpha treatment. This suggests that stressed cells lower their biological activities by sequestration of translation factors and TRAF2 into SGs through a protein-protein interaction.

Novel shift of Jak/Stat signalling characterizes the protective effect of aurintricarboxylic acid (ATA) from tumor necrosis factor-alpha toxicity in human B lymphocytes

Previous results demonstrated that the occurrence of death in human peripheral B lymphocytes by TNF-alpha was paralleled by the activation of the cytoplasmic Jak1 and Tyk2 protein kinases, along with the recruitment of transcription factors Stat3 and Stat5b. In this study we demonstrate that the balance of survival signals in the presence of TNF-alpha was altered by the addition of a salicylate compound, the endonuclease inhibitor aurintricarboxylic acid (ATA). Apoptosis effected by TNF-alpha alone was suppressed by ATA and this event was paralleled by phosphorylation and nuclear translocation of Jak2, Stat2, Stat4 and NF-kB, along with inhibition of caspase activation. These results confirm that among the different cellular responses evoked by TNF-alpha in human B cells, recruitment of Jak/Stat proteins and possible related gene modulation represent contributing factors and address the issue of the development of potential therapeutic strategies aimed at the control of systemic or local effects produced by TNF-alpha.

Time-dependent apoptosis of alveolar macrophages from rats exposed to bleomycin: involvement of tnf receptor 2

Tumor necrosis factor-alpha (TNF-a) is produced by alveolar macrophages (AM) in response to bleomycin (BLM) exposure. This cytokine has been linked to BLM-induced pulmonary inflammation, an early drug effect, and to lung fibrosis, the ultimate toxic effect of BLM. The present study was carried out to study the time dependence of apoptotic signaling pathways and the potential roles of TNF receptors in BLM-induced AM apoptosis. Male Sprague-Dawley rats were exposed to saline or BLM (1 mg/kg) by intratracheal instillation. At 1, 3, or 7 d postexposure, AM were isolated by bronchoalveolar (BAL) lavage and evaluated for apoptosis by ELISA. The release of cytochrome c from mitochrondria, the activation of caspase-3, -8, and -9, the cleavage of nuclear poly(ADP-ribose) polymerase (PARP), and the expression of TNF receptors (TNF-R1/p55 and TNF-R2/p75), TNF-R-associated factor 2 (TRAF2), and cellular inhibitor of apoptosis 1 (c-IAP1) were determined by immunoblotting. The results showed that BLM exposure induced AM apoptosis, with the highest apoptotic effect occurring at 1 d after exposure and gradually decreasing at 3 and 7 d postexposure, but still remaining significantly above the control level. The maximal translocation of cytochromec from mitochondria into the cytosol was observed at 1 d postexposure, whereas the activation of caspase-9 and caspase-3 and caspase-3-dependent cleavage of PARP was found to reach a peak level at 3 d postexposure. BLM exposure had no marked effect on AM expression of TNF-R1 or caspase-8 activation, but significantly increased the expression of TNF-R2 that was accompanied by a rise in c-IAP1 and a decrease in TRAF2. This induction of TNF-R2 by BLM was significant on d 1 and increased with greater exposure time. In vitro studies showed that pretreatment of naive AM with a TNF-R2 antibody significantly inhibited BLM-induced caspase-3 activity and apoptosis. These results suggest that BLM-induced apoptosis involves multiple pathways in a time-dependent manner. Since maximal BLM-induced AM apoptosis (1 d postexposure) preceded maximal changes in caspase-9 and -3 (3 d postexposure), it is possible that a caspase-independent mechanism is involved in this initial response. These results indicate that the sustained expression of TNF-R2 in AM by BLM exposure may sensitize these cells to TNF-a-mediated toxicity.

Apoptosis defects and chemotherapy resistance: molecular interaction maps and networks

Intrinsic (innate) and acquired (adaptive) resistance to chemotherapy critically limits the outcome of cancer treatments. For many years, it was assumed that the interaction of a drug with its molecular target would yield a lethal lesion, and that determinants of intrinsic drug resistance should therefore be sought either at the target level (quantitative changes or/and mutations) or upstream of this interaction, in drug metabolism or drug transport mechanisms. It is now apparent that independent of the factors above, cellular responses to a molecular lesion can determine the outcome of therapy. This review will focus on programmed cell death (apoptosis) and on survival pathways (Bcl-2, Apaf-1, AKT, NF-kappaB) involved in multidrug resistance. We will present our molecular interaction mapping conventions to summarize the AKT and IkappaB/NF-kappaB networks. They complement the p53, Chk2 and c-Abl maps published recently. We will also introduce the 'permissive apoptosis-resistance' model for the selection of multidrug-resistant cells.

Protein kinase PKN1 associates with TRAF2 and is involved in TRAF2-NF-κB signaling pathway

PKN1 is a fatty acid and Rho-activated serine/threonine protein kinase whose catalytic domain is highly homologous to protein kinase C (PKC) family. In yeast two-hybrid screening for PKN1 binding proteins, we identified tumor necrosis factor alpha (TNFalpha) receptor-associated factor 2 (TRAF2). TRAF2 is one of the major mediators of TNF receptor superfamily transducing TNF signal to various functional targets, including activation of NF-kappaB, JNK, and apoptosis. FLAG-tagged PKN1 was co-immunoprecipitated with endogenous TRAF2 from HEK293 cell lysate, and in vitro binding assay using the deletion mutants of TRAF2 showed that PKN1 directly binds to the TRAF domain of TRAF2. PKN1 has the TRAF2-binding consensus sequences PXQX (S/T) at amino acid residues 580-584 (PIQES), and P580AQ582A mutant was not co-immunoprecipitated with TRAF2. Furthermore, the reduced expression of PKN1 by RNA interference (RNAi) down-regulated TRAF2-induced NF-kappaB activation in HEK293T cells. These results suggest that PKN1 is involved in TRAF2-NF-kappaB signaling pathway.

Ubiquitination and translocation of TRAF2 is required for activation of JNK but not of p38 or NF-kappaB

TRAF2 is a RING finger protein that regulates the cellular response to stress and cytokines by controlling JNK, p38 and NF-kappaB signaling cascades. Here, we demonstrate that TRAF2 ubiquitination is required for TNFalpha-induced activation of JNK but not of p38 or NF-kappaB. Intact RING and zinc finger domains are required for TNFalpha-induced TRAF2 ubiquitination, which is also dependent on Ubc13. TRAF2 ubiquitination coincides with its translocation to the insoluble cellular fraction, resulting in selective activation of JNK. Inhibition of Ubc13 expression by RNAi resulted in inhibition of TNFalpha-induced TRAF2 translocation and impaired activation of JNK but not of IKK or p38. TRAF2 aggregates in the cytoplasm, as seen in Hodgkin-Reed-Sternberg lymphoma cells, resulting in constitutive NF-kappaB activity but failure to activate JNK. These findings demonstrate that the TRAF2 RING is required for Ubc13-dependent ubiquitination, resulting in translocation of TRAF2 to an insoluble fraction and activation of JNK, but not of p38 or NF-kappaB. Altogether, our findings highlight a novel mechanism of TRAF2-dependent activation of diverse signaling cascades that is impaired in Hodgkin-Reed-Sternberg cells.

Expression of CD30 and Ox40 on T lymphocyte subsets is controlled by distinct regulatory mechanisms

Members of the TNF receptor (TNFR) superfamily are cell-surface proteins that can be found on most cell types including lymphocytes. Although some TNFR-related molecules are constitutively expressed, others, such as CD30 and Ox40, are induced upon activation of lymphocytes. CD30 and Ox40 are predominantly expressed on activated T helper (T(h))2 cells. Both receptors can activate c-Jun N-terminal kinase (JNK) and nuclear factor-kappaB (NF-kappaB) and have been suggested to play costimulatory roles in lymphocyte activation. To gain further insight into events triggered by both TNFR-related molecules, a detailed analysis of their expression patterns has been performed. We found that CD30 and Ox40 were coexpressed on T(h)2 cells. However, in contrast to CD30, Ox40 was also expressed on T(h)1 cells. Although expression of both receptors is augmented by interleukin-4, only CD30 expression is dependent on signal transducer and activator of transcription (STAT)-6-mediated signaling. Differences in the regulatory pathways controlling expression of CD30 and Ox40 suggest distinct, functional effects triggered by the two TNFR-related molecules during lymphocyte activation.

The signaling adaptors and pathways activated by TNF superfamily

Members of the TNF receptor superfamily play pivotal roles in numerous biological events in metazoan organisms. Ligand-mediated trimerization by corresponding homo- or heterotrimeric ligands, the TNF family ligands, causes recruitment of several intracellular adaptors, which activate multiple signal transduction pathways. While recruitment of death domain (DD) containing adaptors such as Fas associated death domain (FADD) and TNFR associated DD (TRADD) can lead to the activation of a signal transduction pathway that induces apoptosis, recruitment of TRAF family proteins can lead to the activation of transcription factors such as, NF-kappaB and JNK thereby promoting cell survival and differentiation as well as immune and inflammatory responses. Individual TNF receptors are expressed in different cell types and have a range of affinities for various intracellular adaptors, which provide tremendous signaling and biological specificities. In addition, numerous signaling modulators are involved in regulating activities of signal transduction pathways downstream of receptors in this superfamily. Most of the TNF receptor superfamily members as well as many of their signaling mediators, have been uncovered in the last two decades. However, much remains unknown about how individual signal transduction pathways are regulated upon activation by any particular TNF receptor, under physiological conditions.

NF-kappaB in cancer: a marked target

The imbalance between proliferation and programmed cell death (apoptosis) is one of the critical cellular events that lead to oncogenesis. While there is no doubt that uncontrolled cell proliferation is essential for the development of cancer, deregulation of apoptosis may play an equally important role in this process. Inhibition of apoptosis prevents the death of tumor cells with DNA damage either associated with carcinogenic initiation or cancer therapy. The transcription factor NF-kappaB is a key regulator in oncogenesis. By promoting proliferation and inhibiting apoptosis, NF-kappaB tips the balance between proliferation and apoptosis toward malignant growth in tumor cells.

Apoptosis and melanoma: molecular mechanisms

Melanoma cells can undergo self-destruction via programmed cell death, i.e. apoptosis. In these tumours, the molecular components of apoptosis include positive (apoptotic) and negative (anti-apoptotic) regulators. The former include p53, Bid, Noxa, PUMA, Bax, TNF, TRAIL, Fas/FasL, PITSLRE, interferons, and c-KIT/SCF. The latter include Bcl-2, Bcl-X(L), Mcl-1, NF-(K)B, survivin, livin, and ML-IAP. Alternatively, some molecules such as TRAF-2, c-Myc, endothelins, and integrins may have either pro- or anti-apoptotic effects. Some of these molecules are of potential therapeutic use, such as: (1) p53, which influences resistance to chemotherapy; (2) Mcl-1 and Bcl-X(L), which can override apoptosis; (3) TRAIL, which has selective fatal effects on tumour cells; (4) NF-(K)B, which when downregulated sensitizes cells to TRAIL and TNF; (5) the PITSLRE kinases, whose alteration appears to result in Fas resistance; (6) interferons, which sensitize cells to other factors; and (7) survivin and other IAPs that inhibit apoptosis. This review summarizes the state of current knowledge about the key molecular components and mechanisms of apoptosis in melanoma, discusses potential therapeutic ramifications, and provides directions for future research.

Hodgkin's lymphoma and CD30 signal transduction

Advances in molecular biology have shed light on the biological basis of Hodgkin's lymphoma (HL). Knowledge of the biological basis has enabled us to understand that most Hodgkin and Reed-Sternberg (H-RS) cells are derived from germinal center B-cells and constitutive nuclear factor kappaB (NF-kappaB) activation is a common molecular feature. Molecular mechanisms responsible for constitutive NF-kappaB activation, Epstein Barr virus latent membrane protein 1, and defective IkappaBalpha and IkappaB kinase activation have been clarified in the past several years. A recent study revealed the biological link between 2 characteristic features of H-RS cells: CD30 overexpression and constitutive NF-kappaB activation. Ligand-independent signaling by overexpressed CD3O was shown to be a common mechanism that induced constitutive NF-kappaB activation in these cells. These results suggest the self-growth-promoting potential of H-RS cells and redefine the biology of HL composed of H-RS cells and lymphocytes.

CD30-governor of memory T cells?

CD30 is well recognized as a marker expressed by a heterogeneous group of lymphomas and in several immune and autoimmune disorders. However, the function of CD30 in theses diseases or in the normal immune response has remained unclear. Studying gene expression patterns induced by stimulating CD30 signals on a large granular lymphoma cell line, YT, with an agonistic anti-CD30 antibody, we found that CD30 signals affected proapoptotic and antiapoptotic genes, regulated cytotoxicity, and controlled molecules regulating T cell traffic. Creating CD30-L deficient mice and studying CD8 CTL activation and memory responses, it was observed that the absence of CD30-L resulted in diminished primary clonal expansion of CD8 cells. In addition the absence of CD30-L abolished clonal contraction after primary expansion and interfered with secondary expansion upon boosting. The studies suggest that CD30 regulates CD8 CTL function and survival during memory responses and is important for clonal contraction.

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.

Regulation of lymphocyte clustering by CD30-mediated ICAM-1 up-regulation

CD30 is expressed transiently on activated B and T lymphocytes and constitutively on several B- and T cell lymphomas. CD30 functions include participation in negative selection of thymocytes, costimulation of activated T cells, isotype switching of B cells, and regulation of the effector activity of cytotoxic lymphocytes. Although CD30 is not a marker for T helper 2 (TH2) cells, it may participate in the polarization of TH1 and TH2 cells. The pleiotropic functions of CD30 are initiated by interaction of CD30-expressing cells with other immune competent cells expressing CD30-L and providing the signals for modulation of effector cell activity. Here, we report that CD30 signals generated by anti-CD30 on activated, normal murine T cells strongly up-regulate the expression of intercellular adhesion molecule 1 (ICAM-1, CD54), and to a lesser extent, ICAM-2 (CD102). CD30 signals moreover delay the subsequent decline of ICAM expression. CD30 cross-linking did not alter the expression of CD11a/CD18 (LFA-1), the counter receptor for ICAM abundant on T cells. CD30-mediated ICAM-1 up-regulation is independent of cytokine secretion and appears to be transmitted directly through NF-kappaB activation. CD30-mediated up-regulation of ICAM-1 expression led to a significant increase in cluster formation of lymph node cells. Increased lymphocyte self-aggregation mediated by CD30 may set the stage for fraternal signaling to modulate lymphocyte function.

A decision between life and death during TNF-alpha-induced signaling

Tumor necrosis factor-alpha (TNF-alpha), a proinflammatory cytokine, exerts its biological activity by signaling via its two receptors, TNF-RI and TNF-RII, and by activating NF-kappaB. NF-kappaB is essential for survival of many cell types; however, TNF-alpha also induces cell death. In this article, both the survival and cell death signaling by TNF-alpha and the role of caspases in turning off NF-kappaB survival signal are reviewed. Furthermore, a role of DAP kinase in TNF-induced apoptosis is discussed. Finally, the molecular basis of the effect of age on TNF-alpha-induced apoptosis in human T cells is reviewed.

Apoptotic crosstalk of TNF receptors: TNF-R2-induces depletion of TRAF2 and IAP proteins and accelerates TNF-R1-dependent activation of caspase-8

We have recently shown that stimulation of TNF-R2 selectively enhances apoptosis induction by the death receptor TNF-R1. Here, we demonstrate that stimulation of CD30 or CD40 also leads to selective enhancement of TNF-R1-induced cell death. Enhancement of apoptosis was correlated with the depletion of endogenous TRAF2 within 1 to 6 hours. Selective prestimulation of TNF-R2 for several hours inhibited TNF-R2-induced activation of the anti-apoptotic NF-κB pathway up to 90% and dramatically enhanced apoptosis induction by this receptor. When both TNF-receptors were stimulated simultaneously, TNF-R1-induced NF-κB activation remained unaffected but TNF-R1-induced apoptosis was still significantly enhanced. Compared with FasL-induced cell death TNF-R1-induced activation of caspase-8 was significantly weaker and delayed. Costimulation or prestimulation of TNF-R2 enhanced caspase-8 processing. Life cell imaging and confocal microscopy revealed that both TNF-R1 and TNF-R2 recruited the anti-apoptotic factor cIAP1 in a TRAF2-dependent manner. Thus, TNF-R2 may compete with TNF-R1 for the recruitment of newly synthesized TRAF2-bound anti-apoptotic factors, thereby promoting the formation of a caspase-8-activating TNF-R1 complex. Hence,TNF-R2 triggering can interfere with TNF-R1-induced apoptosis by inhibition of NF-κB-dependent production of anti-apoptotic factors and by blocking the action of anti-apoptotic factors at the post-transcriptional level.

Cytoplasmic aggregation of TRAF2 and TRAF5 proteins in the Hodgkin-Reed-Sternberg cells

We previously reported that ligand-independent signaling by highly expressed CD30 in Hodgkin-Reed-Sternberg (H-RS) cells is responsible for constitutive activation of NF-kappa B. In the present study, we characterize the intracellular localization of tumor necrosis factor (TNF) receptor associated factor (TRAF) proteins in H-RS cells. Confocal immunofluorescence microscopy of cell lines derived from H-RS cells and HEK293 transformants highly expressing CD30 revealed aggregation of TRAF2 and TRAF5 in the cytoplasm as well as clustering near the cell membrane. In contrast, TRAF proteins were diffusely distributed in the cytoplasm in cell lines unrelated to Hodgkin's disease (HD) and control HEK293 cells. Furthermore, the same intracellular distribution of TRAF proteins was demonstrated in H-RS cells of lymph nodes of HD, but not in lymphoma cells in lymph nodes of non-Hodgkin's lymphoma. Dominant-negative TRAF2 and TRAF5 suppressed cytoplasmic aggregation along with constitutive NF-kappa B activation in H-RS cell lines. Confocal immunofluorescence microscopy also revealed co-localization of IKK alpha, NIK, and I kappa B alpha with aggregated TRAF proteins in H-RS cell lines. These results suggest involvement of TRAF protein aggregation in the signaling process of highly expressed CD30 and suggest they function as scaffolding proteins. Thus, cytoplasmic aggregation of TRAF proteins appears to reflect constitutive CD30 signaling which is characteristic of H-RS cells.

TNF-RII and c-IAP1 mediate ubiquitination and degradation of TRAF2

Tumour necrosis factor-alpha (TNF-alpha) is a proinflammatory mediator that exerts its biological functions by binding two TNF receptors (TNF-RI and TNF-RII), which initiate biological responses by interacting with adaptor and signalling proteins. Among the signalling components that associate with TNF receptors are members of the TNF-R-associated factor (TRAF) family. TRAF2 is required for TNF-alpha-mediated activation of c-Jun N-terminal kinase (JNK), contributes to activation of NF-kappaB, and mediates anti-apoptotic signals,. TNF-RI and TNF-RII signalling complexes also contain the anti-apoptotic ('inhibitor of apoptosis') molecules c-IAP1 and c-IAP2 (refs 5, 6), which also have RING domain-dependent ubiquitin protein ligase (E3) activity. The function of IAPs in TNF-R signalling is unknown. Here we show that binding of TNF-alpha to TNF-RII induces ubiquitination and proteasomal degradation of TRAF2. Although c-IAP1 bound TRAF2 and TRAF1 in vitro, it ubiquitinated only TRAF2. Expression of wild-type c-IAP1, but not an E3-defective mutant, resulted in TRAF2 ubiquitination and degradation. Moreover, E3-defective c-IAP1 prevented TNF-alpha-induced TRAF2 degradation and inhibited apoptosis. These findings identify a physiologic role for c-IAP1 and define a mechanism by which TNF-RII-regulated ubiquitin protein ligase activity can potentiate TNF-induced apoptosis.

NF-kappaB at the crossroads of life and death

The choice between life and death is one of the major events in regulation of the immune system. T cells that specifically recognize viral or bacterial antigens are selected to survive and proliferate in response to infection, whereas those that are self-reactive are eliminated via apoptosis. Even the survival of alloreactive T cells requires their proper costimulation and, when infection subsides, the activated T cells are eliminated. A major regulator of such life or death decisions is the transcription factor NF-kappaB. However, NF-kappaB cannot function alone. A variety of mechanisms exist to modulate its activity and thereby affect the ultimate outcome of a cell's fate.

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.

T2BP, a novel TRAF2 binding protein, can activate NF-kappaB and AP-1 without TNF stimulation

TRAF2 is a key molecule involved in TNF signaling, which is crucial for the regulation of inflammatory processes. We have identified a novel TRAF2 binding protein, designated as T2BP (TRAF2 binding protein), by a mammalian two-hybrid screening approach. T2BP is a relatively small protein of 184 amino acids, which includes a forkhead-associated domain, the phosphopeptide binding motif. The interaction domain search showed that the TRAF domain in TRAF2 is required for the binding to T2BP whereas almost the entire protein in T2BP binds to TRAF2. The interaction was further confirmed by co-immunoprecipitation. Expression profiling for T2BP and TRAF2 revealed an ubiquitous expression in adult mouse tissues. Overexpression of T2BP in HEK293 cells activated NF-kappaB and AP-1 in a dose dependent manner as well as seen in the TNF-treated control cells. Our results suggest that T2BP is involved in the TNF-mediated signaling by its interaction with TRAF2.

Reactive oxygen species are downstream products of TRAF-mediated signal transduction

Members of the TNFR (tumor necrosis factor receptor) superfamily are involved in regulating activation and differentiation of cells as well as cell survival and programmed cell death/apoptosis. Multimerization of TNFRs can lead to recruitment of TRAFs (TNFR-associated factors) by the receptors resulting in activation of kinases and transcription factors, such as c-Jun N-terminal kinase and nuclear factor kappaB (NF-kappaB). Signal transduction triggered by TNF-alpha also induces an increase in intracellular reactive oxygen species (ROS). ROS have been suggested to play a role in NF-kappaB activation, which is thought to promote cell survival. However, oxidation of proteins and lipids by ROS can also result in apoptosis. The processes generating intracellular ROS and the mechanism(s) regulating the cellular redox status have not been fully elucidated. We investigated whether TRAFs play a role in controlling intracellular ROS levels. Our results indicate that recruitment of TRAFs to the plasma membrane of human embryonic kidney (HEK) 293 cells is crucial for activation of signaling pathways, which regulate ROS production in mitochondria. TRAF-mediated changes in ROS levels enhanced NF-kappaB activation but were not dependent on NF-kappaB-inducing kinase. Consistent with its anti-apoptotic function, Bcl-x(L) interfered with TRAF-mediated ROS generation but not NF-kappaB activation. Taken together, our results suggest a novel role of TRAFs in signal transduction pathways triggered by TNFR-related proteins, which balance cell survival and apoptosis by regulating the electron transport in mitochondria.

Sequence analysis identifies TTRAP, a protein that associates with CD40 and TNF receptor-associated factors, as a member of a superfamily of divalent cation-dependent phosphodiesterases

CD40 is a member of the tumor necrosis factor (TNF) receptor family. CD40-mediated signal transduction involves the recruitment of several cytoplasmic proteins and induces expression of a large number of genes. TTRAP, a novel protein that interacts with the cytoplasmic domain of CD40 and with TNF-receptor associated factors (TRAFs), has been cloned and shown to inhibit nuclear factor-kappaB activation (NF-kappaB). By using various bioinformatics-based sequence and structure analyses of proteins involved in signaling by the TNF receptor family, we found that TTRAP is a member of a superfamily of Mg(2+)/Mn(2+)-dependent phosphodiesterases. More precisely, our results suggest that TTRAP is related to the human APE1, a Mg(2+)-dependent endonuclease. This potential novel function of TTRAP raises the intriguing possibility for a role of APE1-like DNA-repair endonucleases in TNF receptor family-mediated signaling and functions.

FGF induces a switch in death receptor pathways in neuronal cells

Basic fibroblast growth factor (FGF2) has many roles in neuronal development and maintenance including effects on mitogenesis, survival, fate determination, differentiation, and migration. Using a conditionally immortalized rat hippocampal cell line, H19-7, and primary hippocampal cultures, we now demonstrate that FGF2 treatment differentially regulates members of the tumor necrosis factor (TNF) superfamily of death domain receptors and their ligands. H19-7 cells transferred from serum to defined (N2) medium undergo apoptosis by a Fas-dependent mechanism similar to primary neurons. In contrast, H19-7 cells treated with FGF undergo apoptosis by a Fas-independent mechanism. FGF suppresses the Fas death pathway but also induces apoptosis by activation of a TNFalpha death pathway in both H19-7 and hippocampal progenitor cells. Expression of the TNF receptor 1 (TNFR1) or TNFR2 in H19-7 cells is sufficient to sensitize the cells to TNFalpha, similar to the effects of FGF. Because TNFalpha can be either proapoptotic or antiapoptotic, these results provide an explanation for the divergent trophic effects of FGF2 treatment and the observation that multiple trophic inputs are required for the survival of specific neurons.