The TNFR2-TRAF signaling complex contains two novel proteins related to baculoviral inhibitor of apoptosis proteins

Binding sites of cytoplasmic effectors TRAF1, 2, and 3 on CD30 and other members of the TNF receptor superfamily

CD30 is present on the surfaces of malignant cells from patients with Hodgkin's lymphoma, anaplastic large cell lymphoma, and other lymphomas. The yeast two hybrid genetic screen method was used to identify molecular effectors which mediate CD30 signalling events. Clones corresponding to genes coding for TRAF1, TRAF2, and TRAF3 molecules, postulated to be involved in signalling via the TNF and CD40 receptors, were isolated. In this report, we show that the CD30 intracellular tail contains two motifs that bind TRAFs. The more amino terminal motif, 558PHYPEQET565, binds TRAF2 and 3, while the more carboxyl terminal motif, 576MLSVEEEG583, binds TRAF1 and 2. We show that these amino acid motifs are conserved in TNFRp75 and CD40 and that sequences in these receptors homologous to TRAF-binding sequences found in CD30 can selectively bind the TRAFs in a predictable manner.

A BIR motif containing gene of African swine fever virus, 4CL, is nonessential for growth in vitro and viral virulence

An African swine fever virus (ASFV) gene with similarity to viral and cellular inhibitor of apoptosis genes (iap) has been described in the African isolate Malawi Lil-20/1 (ORF 4CL) and a cell-culture-adapted European virus, BA71V (ORF A224L). The similarity of the ASFV gene to genes involved in inhibiting cellular apoptosis suggested the gene may regulate apoptosis in ASFV-infected cells and thus may function in ASFV virulence and/or host range. Sequence analysis of additional African and European pathogenic isolates demonstrates that this gene is highly conserved among both pig and tick ASFV isolates and that its similarity to iap genes is limited to the presence of a single IAP repeat motif (BIR motif) in the ASFV gene. To study gene function, a 4CL gene deletion mutant, delta 4CL, was constructed from the pathogenic Malawi Lil-20/1 isolate. Growth characteristics of delta 4CL in swine macrophage cell cultures were indistinguishable from those of parental virus. Infected macrophage survival time and the induction and magnitude of apoptosis in virus-infected macrophages were comparable for cells infected with either delta 4CL or parental virus. In infected swine, delta 4CL exhibited an unaltered Malawi Lil-20/1 virulence phenotype. These data indicate that, although highly conserved among ASFV isolates, the 4CL gene is nonessential for growth in macrophage cell cultures in vitro and for pig virulence. Additionally, despite its limited similarity to JAP genes, 4CL exhibits no anti-apoptotic function in infected macrophage cell cultures. The high degree of gene conservation among ASFV isolates, together with the apparent lack of function in the swine host, suggests 4CL may be a host range gene involved in aspects of infection in the arthropod host, ticks of the genus Ornithodoros.

TRAF-interacting protein (TRIP): a novel component of the tumor necrosis factor receptor (TNFR)- and CD30-TRAF signaling complexes that inhibits TRAF2-mediated NF-kappaB activation

Through their interaction with the TNF receptor-associated factor (TRAF) family, members of the tumor necrosis factor receptor (TNFR) superfamily elicit a wide range of biological effects including differentiation, proliferation, activation, or cell death. We have identified and characterized a novel component of the receptor-TRAF signaling complex, designated TRIP (TRAF-interacting protein), which contains a RING finger motif and an extended coiled-coil domain. TRIP associates with the TNFR2 or CD30 signaling complex through its interaction with TRAF proteins. When associated, TRIP inhibits the TRAF2-mediated NF-kappaB activation that is required for cell activation and also for protection against apoptosis. Thus, TRIP acts as a receptor-proximal regulator that may influence signals responsible for cell activation/proliferation and cell death induced by members of the TNFR superfamily.

Bik and Bak induce apoptosis downstream of CrmA but upstream of inhibitor of apoptosis

Recent studies have identified a number of cell death pathway components. In this study, we describe the role that two such components, Bik and Bak, play in initiating the apoptotic program. These Bcl-2 family members engage the death pathway downstream of the block imposed by the serpin CrmA, but upstream of the block initiated by cellular inhibitors of apoptosis, which are a family of molecules characterized by a conserved baculovirus inhibitor of apoptosis repeat motif. Distal death pathway components activated by Bik and Bak are similar to those activated by the CD-95 (Fas/Apo1) and tumor necrosis factor death receptors.

Lymphotoxin-beta receptor signaling complex: role of tumor necrosis factor receptor-associated factor 3 recruitment in cell death and activation of nuclear factor kappaB

The binding of heterotrimeric lymphotoxin, LT alpha1 beta2, to the LTbeta receptor (LTbeta R), a member of the tumor necrosis factor receptor (TNFR) superfamily, induces nuclear factor kappaB (NF-kappaB) activation and cell death in HT29 adenocarcinoma cells. We now show that treatment with LT alpha1 beta2 or agonistic LTbeta R antibodies causes rapid recruitment of TNFR-associated factor 3 (TRAF3) to the LTbeta R cytoplasmic domain. Further, stable overexpression of a TRAF3 mutant that lacks the RING and zinc finger domains inhibits LTbeta R-mediated cell death. The inhibition is specific for LTbeta R cell death signaling, since NF-kappaB activation by LT alpha1 beta2 and Fas-mediated apoptosis are not inhibited in the same cells. The mutant and endogenous TRAF3s are both recruited at equimolar amounts to the LTbeta R, suggesting that the mutant disrupts the function of the signaling complex. These results implicate TRAF3 as a critical component of the LTbeta R death signaling complex and indicate that at least two independent signaling pathways are initiated by LTbeta R ligation.

Induction of nuclear factor kappaB by the CD30 receptor is mediated by TRAF1 and TRAF2

CD30 is a lymphoid cell-specific surface receptor which was originally identified as an antigen expressed on Hodgkin's lymphoma cells. Activation of CD30 induces the nuclear factor kappaB (NF-kappaB) transcription factor. In this study, we define the domains in CD30 which are required for NF-kappaB activation. Two separate elements of the cytoplasmic domain which were capable of inducing NF-kappaB independently of one another were identified. The first domain (domain 1) mapped to a approximately 120-amino-acid sequence in the membrane-proximal region of the CD30 cytoplasmic tail, between residues 410 and 531. A second, more carboxy-terminal region (domain 2) was identified between residues 553 and 595. Domain 2 contains two 5- to 10-amino-acid elements which can mediate the binding of CD30 to members of the tumor necrosis factor receptor-associated factor (TRAF) family of signal transducing proteins. Coexpression of CD30 with TRAF1 or TRAF2 but not TRAF3 augmented NF-kappaB activation through domain 2 but not domain 1. NF-kappaB induction through domain 2 was inhibited by coexpression of either full-length TRAF3 or dominant negative forms of TRAF1 or TRAF2. In contrast, NF-kappaB induction by domain 1 was not affected by alterations in TRAF protein levels. Together, these data support a model in which CD30 can induce NF-kappaB by both TRAF-dependent and -independent mechanisms. TRAF-dependent induction of NF-kappaB appears to be regulated by the relative levels of individual TRAF proteins in the cell.

The Epstein-Barr virus LMP1 amino acid sequence that engages tumor necrosis factor receptor associated factors is critical for primary B lymphocyte growth transformation

Epstein-Barr virus (EBV) latent membrane protein 1 (LMP1) is essential for transforming primary B lymphocytes into lymphoblastoid cell lines. EBV recombinants with LMP1 genes truncated after the proximal 45 codons of the LMP1 carboxyl terminus are adequate for transformation. The proximal 45 residues include a domain that engages the tumor necrosis factor receptor associated factors (TRAFs). We investigated the importance of the TRAF binding domain by assaying the transforming ability of recombinant EBV genomes with a deletion of LMP1 codons 185-211. This mutation eliminates TRAF association in yeast and in lymphoblasts but does not affect LMP1 stability or localization. Specifically mutated recombinant EBV genomes were generated by transfecting P3HR-1 cells with overlapping EBV cosmids. Infection of primary B lymphocytes resulted in cell lines that were coinfected with an LMP1 delta185-211 EBV recombinant and P3HR-1 EBV, which has a wild-type LMP1 gene but is transformation defective due to another deletion. Despite the equimolar mixture of wild-type and mutated LMP1 genes in virus preparations from five coinfected cell lines, only the wild-type LMP1 gene was found in 412 cell lines obtained after transformation of primary B lymphocytes. No transformed cell line had only the LMP1 delta185-211 gene. An EBV recombinant with a Flag-tagged LMP1 gene passaged in parallel segregated from the coinfecting P3HR-1. These data indicate that the LMP1 TRAF binding domain is critical for primary B lymphocyte growth transformation.

CD30 induction of human immunodeficiency virus gene transcription is mediated by TRAF2

CD30 is a member of the tumor necrosis factor receptor (TNFR) superfamily expressed on activated T and B lymphocytes and natural killer cells. Ligation of CD30 was previously shown to induce NF-kappaB activation and HIV expression in chronically infected T lymphocytes. In this study, we report that two members of the TNFR-associated factor (TRAF) family of proteins, TRAF1 and TRAF2, independently bind to the intracellular domain of CD30 (CD30IC). Transient overexpression of TRAF2, but not TRAF1, induced NF-kappaB activation and HIV-1-long terminal repeat-driven transcription in the T cell line, KT3. Moreover, dominant negative mutants consisting of the TRAF domain of TRAF1 and TRAF2 inhibited CD30 induction of NF-kappaB activation and HIV-1 transcription. These results suggest that CD30 ligation may enhance the expression of HIV via TRAF-2-mediated activation of NF-kappaB.

Cellular signaling roles of TGF beta, TNF alpha and beta APP in brain injury responses and Alzheimer's disease

beta-Amyloid precursor protein (beta APP), transforming growth factor beta (TGF beta), and tumor necrosis factor-alpha (TNF alpha) are remarkably pleiotropic neural cytokines/neurotrophic factors that orchestrate intricate injury-related cellular and molecular interactions. The links between these three factors include: their responses to injury; their interactive effects on astrocytes, microglia and neurons; their ability to induce cytoprotective responses in neurons; and their association with cytopathological alterations in Alzheimer's disease. Astrocytes and microglia each produce and respond to TGF beta and TNF alpha in characteristic ways when the brain is injured. TGF beta, TNF alpha and secreted forms of beta APP (sAPP) can protect neurons against excitotoxic, metabolic and oxidative insults and may thereby serve neuroprotective roles. On the other hand, under certain conditions TNF alpha and the fibrillogenic amyloid beta-peptide (A beta) derivative of beta APP can promote damage of neuronal and glial cells, and may play roles in neurodegenerative disorders. Studies of genetically manipulated mice in which TGF beta, TNF alpha or beta APP ligand or receptor levels are altered suggest important roles for each factor in cellular responses to brain injury and indicate that mediators of neural injury responses also have the potential to enhance amyloidogenesis and/or to interfere with neuroregeneration if expressed at abnormal levels or modified by strategic point mutations. Recent studies have elucidated signal transduction pathways of TGF beta (serine/threonine kinase cascades), TNF alpha (p55 receptor linked to a sphingomyelin-ceramide-NF kappa B pathway), and secreted forms of beta APP (sAPP; receptor guanylate cyclase-cGMP-cGMP-dependent kinase-K+ channel activation). Knowledge of these signaling pathways is revealing novel molecular targets on which to focus neuroprotective therapeutic strategies in disorders ranging from stroke to Alzheimer's disease.

Characterization of reaper- and FADD-induced apoptosis in a lepidopteran cell line

Expression of the reaper gene (rpr) correlates with the initiation of apoptosis in Drosophila melanogaster. Transient expression of rpr in the lepidopteran SF-21 cell line induced apoptosis displaying nuclear condensation and fragmentation, oligonucleosomal ladder formation, cell surface blebbing, and apoptotic body formation. Inhibitors of ICE-family proteases p35 and crmA, as well as members of the iap class of genes, Op-iap and D-iap2, but not bcl-2 family members, blocked rpr-induced apoptosis. Mutational analysis of rpr provided no support for the proposed sequence similarity of Reaper and death domain proteins. Mutations in the N-terminal region of Reaper, which displays sequence similarity to Hid and Grim, other Drosophila gene products correlated with the initiation of apoptosis, suggested that these residues might be functionally important. The mammalian cDNA encoding FADD (Fas-associating protein with a death domain) also induced cell death in SF-21 cells, but death progressed more slowly and with features which distinguished it from rpr-induced apoptosis. Several bcl-2 family members delayed or blocked FADD-induced cellular death. Thus, apoptosis initiated by Reaper progressed by a faster path which appeared to differ from that of FADD-induced apoptosis.

Spodoptera frugiperda caspase-1, a novel insect death protease that cleaves the nuclear immunophilin FKBP46, is the target of the baculovirus antiapoptotic protein p35

Employing the degenerate primer-dependent polymerase chain reaction approach used recently to clone human Mch2, we have identified and cloned the insect Spodoptera frugiperda target of the baculovirus antiapoptotic protein p35. This protein named Sf caspase-1 belongs to the family of caspases and is highly related to human Mch3 and CPP32 in sequence and specific activity. The proenzyme of Sf caspase-1 is 299 amino acids in length and can undergo autocatalytic processing in Escherichia coli to an active enzyme heterocomplex. Autoprocessing occurs at Asp-28, Asp-184, and Asp-195 to generate the large p19/p18 and small p12 subunits. Sf caspase-1 is able to induce apoptosis in Sf9 cells and is capable of cleaving p35 to similar sized fragments as observed with extracts from p35 null mutant baculovirus-infected Sf9 cells. Sf caspase-1 activity is potently inhibited by p35, suggesting that it is an important target of this antiapoptotic protein. Finally, the Sf9 nuclear immunophilin FKBP46 was identified as a death-associated substrate for Sf caspase-1.

RIP mediates tumor necrosis factor receptor 1 activation of NF-kappaB but not Fas/APO-1-initiated apoptosis

The CD95 (Fas/APO-1) and tumor necrosis factor (TNF) receptor pathways share many similarities, including a common reliance on proteins containing 'death domains' for elements of the membrane-proximal signal relay. We have created mutant cell lines that are unable to activate NF-kappaB in response to TNF. One of the mutant lines lacks RIP, a 74 kDa Ser/Thr kinase originally identified by its ability to associate with Fas/APO-1 and induce cell death. Reconstitution of the line with RIP restores responsiveness to TNF. The RIP-deficient cell line is susceptible to apoptosis initiated by anti-CD95 antibodies. An analysis of cells reconstituted with mutant forms of RIP reveals similarities between the action of RIP and FADD/MORT-1, a Fas-associated death domain protein.

Apoptosis and disease: regulation and clinical relevance of programmed cell death

Regulation of the homeostatic balance between cell proliferation and cell death is essential for development and maintenance of multicellular organisms. Physiologic, or programmed, cell death is dependent on a genetically encoded and evolutionarily conserved pathway that induces a form of cellular suicide known as apoptosis. In the past decade, it has become clear that the regulatory mechanisms controlling programmed cell death are as fundamental, and as complex, as those regulating cell proliferation. Perturbation of the signaling cascades regulating apoptosis, whether by extracellular triggers, acquired or germline genetic mutations, or viral mimicry of signaling molecules, can result in a wide variety of human diseases. Analysis of these regulatory pathways has led to a better understanding of the etiology and pathogenesis of many human diseases, notably cancers, infectious diseases including AIDS, autoimmune diseases, and neurodegenerative/neurodevelopmental diseases. Our understanding of the regulation of programmed cell death in health and disease is far from complete, and the challenge of converting that understanding into new therapeutic modalities has only begun to be approached.

Association of TRAF1, TRAF2, and TRAF3 with an Epstein-Barr virus LMP1 domain important for B-lymphocyte transformation: role in NF-kappaB activation

The Epstein-Barr virus (EBV) transforming protein LMP1 appears to be a constitutively activated tumor necrosis factor receptor (TNFR) on the basis of an intrinsic ability to aggregate in the plasma membrane and an association of its cytoplasmic carboxyl terminus (CT) with TNFR-associated factors (TRAFs). We now show that in EBV-transformed B lymphocytes most of TRAF1 or TRAF3 and 5% of TRAF2 are associated with LMP1 and that most of LMP1 is associated with TRAF1 or TRAF3. TRAF1, TRAF2, and TRAF3 bind to a single site in the LMP1 CT corresponding to amino acids (aa) 199 to 214, within a domain which is important for B-lymphocyte growth transformation (aa 187 to 231). Further deletional and alanine mutagenesis analyses and comparison with TRAF binding sequences in CD40, in CD30, and in the LMP1 of other lymphycryptoviruses provide the first evidence that PXQXT/S is a core TRAF binding motif. The negative effects of point mutations in the LMP1(1-231) core TRAF binding motif on TRAF binding and NF-kappaB activation genetically link the TRAFs to LMP1(1-231)-mediated NF-kappaB activation. NF-kappaB activation by LMP1(1-231) is likely to be mediated by TRAF1/TRAF2 heteroaggregates since TRAF1 is unique among the TRAFs in coactivating NF-kappaB with LMP1(1-231), a TRAF2 dominant-negative mutant can block LMP1(1-231)-mediated NF-kappaB activation as well as TRAF1 coactivation, and 30% of TRAF2 is associated with TRAF1 in EBV-transformed B cells. TRAF3 is a negative modulator of LMP1(1-231)-mediated NF-kappaB activation. Surprisingly, TRAF1, -2, or -3 does not interact with the terminal LMP1 CT aa 333 to 386 which can independently mediate NF-kappaB activation. The constitutive association of TRAFs with LMP1 through the aa 187 to 231 domain which is important in NF-kappaB activation and primary B-lymphocyte growth transformation implicates TRAF aggregation in LMP1 signaling.

The tumor necrosis factor receptor 2 signal transducers TRAF2 and c-IAP1 are components of the tumor necrosis factor receptor 1 signaling complex

The two cell surface receptors for tumor necrosis factor (TNF) interact with a number of intracellular signal transducing proteins. The association of TRADD, a 34-kDa cytoplasmic protein containing a C-terminal death domain, with aggregated TNF receptor 1 (TNF-R1) through their respective death domains leads to NF-kappa B activation and programmed cell death. In contrast, TNF receptor 2 (TNF-R2) interacts with the TNF receptor associated factors 2/1 (TRAF2/TRAF1) heterocomplex, which mediates the recruitment of two cellular inhibitor of apoptosis proteins (c-IAP1 and c-IAP2) to TNF-R2. Here we show that the TNF-R2 signal transducers TRAF2 and c-IAP1 are a part of the TNF-R1 signaling complex. The recruitment of TRAF2 and c-IAP1 to TNF-R1 is TNF-dependent, is mediated by TRADD, and is independent of TNF-R2. These data establish the physiological involvement of TRAF2 and c-IAP1 in TNF-R1 signaling and help provide a molecular explanation for both the overlapping and distinct signals generated by the two TNF receptors.

Inhibition of interleukin 1 beta-converting enzyme-mediated apoptosis of mammalian cells by baculovirus IAP

Apoptosis can be a potent weapon against viral infection and consequently has selected for viruses carrying antiapoptosis genes. Two baculovirus proteins, IAP and p35, can prevent insect cells from dying in response to infection. p35, which interferes with members of the Ced-3 family of cysteine proteases, can also function in mammalian cells. We investigated the ability of IAP from Orgyia pseudotsugata nuclear polyhedrosis virus to prevent death of mammalian cells. IAP was transiently expressed in mammalian cells and its ability to block cell death caused by expression of interleukin-1 beta converting enzyme (ICE), FADD, or the ICE homologues ICH-1 and ICE-Lap3, was investigated. IAP strongly inhibited ICE- and ICH-1-induced cell death but protected only partially against death by overexpression of FADD and not at all against death due to enforced ICE-Lap3 expression. These results demonstrate that a baculoviral IAP protein can functionally interact with conserved components of the apoptosis machinery in mammalian cells.

Tumor necrosis factor receptors (Tnfr) in mouse fibroblasts deficient in Tnfr1 or Tnfr2 are signaling competent and activate the mitogen-activated protein kinase pathway with differential kinetics

To dissect tumor necrosis factor receptor (Tnfr)-1 (CD120a) and Tnfr2 (CD120b)-dependent signal transduction pathways, primary fibroblasts isolated from inguinal adipose tissue of wild type (wt), tnfr1(o), tnfr2(o), and tnfr1(o)/tnfr2(o) mice were studied. The mitogen-activated protein kinases Erk1 and Erk2 were found to be tyrosine-phosphorylated and activated by Tnf treatment in all wt, tnfr1(o), and tnfr2(o) fibroblasts; the activation was down-regulated 60 min after the start of steady state Tnf treatment. Distinct kinetics of Erk1 and Erk2 activation were detected; the Tnfr1-mediated activation of Erk1 and Erk2 started more slowly and persisted for more prolonged times as compared with Tnfr2 activation. Raf-1, Raf-B, Mek-1, Mek kinase, and p90(rsk) kinases were also shown to be activated independently in a distinct time-dependent pattern through the two Tnf receptors. In addition, both Tnfr1 and Tnfr2 mediated independently the activation of the transcription factor Ap-1 albeit with parallel activation kinetics. In contrast, Tnfr1 exclusively mediated activation of NF-kappaB and fibroblast proliferation; however, Tnfr2 enhanced proliferation triggered through Tnfr1. These findings indicate distinct but also overlapping roles of Tnfr1 and Tnfr2 in primary mouse fibroblasts and suggest different regulation mechanisms of signal transduction pathways under the control of both Tnf receptors.

Dissection of TNF receptor 1 effector functions: JNK activation is not linked to apoptosis while NF-kappaB activation prevents cell death

Through its type 1 receptor (TNFR1), the cytokine TNF elicits an unusually wide range of biological responses, including inflammation, tumor necrosis, cell proliferation, differentiation, and apoptosis. We investigated how TNFR1 activates different effector functions; the protein kinase JNK, transcription factor NF-kappaB, and apoptosis. We found that the three responses are mediated through separate pathways. Recruitment of the signal transducer FADD to the TNFR1 complex mediates apoptosis but not NF-kappaB or JNK activation. Two other signal transducers, RIP and TRAF2, mediate both JNK and NF-kappaB activation. These two responses, however, diverge downstream to TRAF2. Most importantly, JNK activation is not involved in induction of apoptosis, while activation of NF-kappaB protects against TNF-induced apoptosis.

TRAF6 is a signal transducer for interleukin-1

Many cytokines signal through different cell-surface receptors to activate the transcription factor NF-kappaB. Members of the TRAF protein family have been implicated in the activation of NF-kappaB by the tumour-necrosis factor (TNF)-receptor superfamily. Here we report the identification of a new TRAF family member, designated TRAF6. When overexpressed in human 293 cells, TRAF6 activates NF-kappaB. A dominant-negative mutant of TRAF6 inhibits NF-kappaB activation signalled by interleukin-1 (IL-1) but not by TNF. IL-1 treatment of 293 cells induces the association of TRAF6 with IRAK, a serine/threonine kinase that is rapidly recruited to the IL-1 receptor after IL-1 induction. These findings indicate that TRAF proteins may function as signal transducers for distinct receptor families and that TRAF6 participates in IL-1 signalling.

Suicide by order: some open questions about the cell-killing activities of the TNF ligand and receptor families

That leukocytes can produce proteins with cell-killing activities has been known for almost 30 years (Granger GA, Kolb EP. J Immunol 1968, 101; Ruddle NH, Waksman BH. J Exp Med 1968, 128, 1267-1279; Carswell EA, Old LJ, Kassel S, Green S, Fiore N, Williamson B. Proc Natl Acad Sci USA 1975, 72, 3666-3670). However, it is only recently that the nature of this cell killing activity has become clear. What appeared initially to be merely a toxic effect of the leukocyte-produced proteins (hence their initial name, 'lymphotoxins') has turned out to represent a new kind of biological mechanism, whose understanding required a radical change of concepts concerning the ways in which the life and death of the cell are controlled. The leukocyte-produced 'toxic' proteins turned out to act, not through any toxic feature of the proteins themselves, but by activating destructive mechanisms that pre-exist within the target cell. Their action thus represents a way by which one cell can dictate suicide to another. Within the last few years more has been learnt about their mechanisms of action than was learnt throughout the two preceding decades. Nevertheless, many questions still remain unresolved. The purpose of this exposition is to spell out some of these open questions.

Induction of apoptosis by baculovirus transactivator IE1

Apoptosis is induced upon infection of SF-21 cells by mutants of the baculovirus Autographa californica nuclear polyhedrosis virus (AcMNPV) lacking a functional p35 gene which encodes a stoichiometric inhibitor of members of the interleukin-1beta converting enzyme family of cysteine proteases (N.J. Bump et al, Science 269:1885-1888, 1995; R.J. Clem, M. Fechheimer, and L.K. Miller, Science 254:1388-1390, 1991). We found that transfection of SF-21 cells with the AcMNPV ie-1 gene was sufficient to induce apoptosis, which was characterized by fragmentation of cellular DNA into oligonucleosomal fragments and apoptotic body formation. No signs of apoptosis were observed in Trichoplusia ni TN-368 cells transfected with ie-1, a result which is consistent with the observation that p35 mutants of AcMNPV do not induce apoptosis in this cell line. Cotransfection of SF-21 cells with p35 blocked ie-1-induced apoptosis, indicating that expression of ie-1 activates apoptosis through a p35-inhibitable cysteine protease pathway. Cotransfection with Cp-iap, an active member of another family of antiapoptotic inhibitors of apoptosis (iaps), also inhibited IE1-induced apoptosis. Thus, ie-1 may participate in inducing apoptosis in AcMNPV-infected cells, although the dependence of induction on DNA replication suggests that ie-1 is not the direct apoptotic signal during infection. The ie-1 gene product, IE1, is known to be a potent transactivator of baculovirus gene expression that interacts with specific palindromic sequences which can act as transcriptional enhancers and as origins of DNA replication in transient assays.

Tumor necrosis factor receptor associated factor 2 is a mediator of NF-kappa B activation by latent infection membrane protein 1, the Epstein-Barr virus transforming protein

Latent infection membrane protein 1 (LMP1), the Epstein-Barr virus transforming protein, associates with tumor necrosis factor receptor (TNFR) associated factor 1 (TRAF1) and TRAF3. Since TRAF2 has been implicated in TNFR-mediated NF-kappa B activation, we have evaluated the role of TRAF2 in LMP1-mediated NF-kappa B activation. TRAF2 binds in vitro to the LMP1 carboxyl-terminal cytoplasmic domain (CT), coprecipitates with LMP1 in B lymphoblasts, and relocalizes to LMP1 plasma membrane patches. A dominant negative TRAF2 deletion mutant that lacks amino acids 6-86 (TRAF/ delta 6-86) inhibits NF-kappa B activation from the LMP1 CT and competes with TRAF2 for LMP1 binding. TRAF2 delta 6-86 inhibits NF-kappa B activation mediated by the first 45 amino acids of the LMP1 CT by more than 75% but inhibits NF-kappa B activation through the last 55 amino acids of the CT by less than 40%. A TRAF interacting protein, TANK, inhibits NF-kappa B activation by more than 70% from both LMP1 CT domains. These data implicate TRAF2 aggregation in NF-kappa B activation by the first 45 amino acids of the LMP1 CT and suggest that a different TRAF-related pathway may be involved in NF-kappa B activation by the last 55 amino acids of the LMP1 CT.

CD30/TNF receptor-associated factor interaction: NF-kappa B activation and binding specificity

CD30 is a member of the tumor necrosis factor (TNF) receptor superfamily. CD30 is expressed on normal activated lymphocytes, on several virally transformed T- or B-cell lines and on neoplastic cells of Hodgkin's lymphoma. The interaction of CD30 with its ligand induces pleiotropic effects on cells resulting in proliferation, differentiation, or death. The CD30 cytoplasmic tail interacts with TNF receptor-associated factors (TRAFs), which have been shown to transduce signals mediated by TNF-R2 and CD40. We demonstrate here that TRAF2 also plays an important role in CD30-induced NF-kappa B activation. We also show that TRAF2-mediated activation of NF-kappa B plays a role in the activation of HIV transcription induced by CD30 cross-linking. Detailed site-directed mutagenesis of the CD30 cytoplasmic tail reveals that there are two independent binding sites for TRAF, each interacting with a different domain of TRAF. Furthermore, we localized the TRAF-C binding site in CD30 to a 5-7 amino acid stretch.

Anatomy of TRAF2. Distinct domains for nuclear factor-kappaB activation and association with tumor necrosis factor signaling proteins

The tumor necrosis factor (TNF) receptor-associated factor (TRAF) family of proteins interact with and transduce signals for members of the TNF receptor superfamily. TRAF1, TRAF2, and TRAF3 share a conserved C-terminal TRAF domain. TRAF2 plays a key role in transducing signals for activation of the transcription factor nuclear factor-kappaB (NF-kappaB). We have performed extensive mutational analysis on TRAF2, examining the requirements for NF-kappaB activation, self-association, and interaction with other molecules involved in TNF signaling. Examination of point mutants and TRAF2-TRAF3 chimeric proteins indicates that the N-terminal RING finger and two adjacent zinc fingers of TRAF2 are required for NF-kappaB activation. The two distinct TRAF-N and TRAF-C subdomains of the TRAF domain appear to independently mediate self-association and interaction with TRAF1. Interaction of TRAF2 with TNF-R2 and TRADD requires sequences at the C terminus of the TRAF-C domain, whereas interaction with the protein kinase receptor-interacting protein V(RIP) occurs via sequences at the N terminus of the TRAF-C domain. Thus, distinct domains of TRAF2 are involved in recruitment and signaling functions.

I-TRAF is a novel TRAF-interacting protein that regulates TRAF-mediated signal transduction

Tumor necrosis factor (TNF) receptor-associated factor (TRAF) proteins associate with and transduce signals from TNF receptor 2, CD40, and presumably other members of the TNF receptor superfamily. TRAF2 is required for CD40- and TNF-mediated activation of the transcription factor NF-kappa B. Here we describe the isolation and characterization of a novel TRAF-interacting protein, I-TRAF, that binds to the conserved TRAF-C domain of the three known TRAFs. Overexpression of I-TRAF inhibits TRAF2-mediated NF-kappa B activation signaled by CD40 and both TNF receptors. Thus, I-TRAF appears as a natural regulator of TRAF function that may act by maintaining TRAFs in a latent state.

Inhibition of insulin receptor signaling by TNF: potential role in obesity and non-insulin-dependent diabetes mellitus

Adipocytes produce a variety of molecules that are capable of functioning in both a paracrine and autocrine fashion. Tumor necrosis factor (TNF) is one of the proteins produced by adipocytes that has been shown to regulate adipocyte function. Interestingly, adipocyte expression of TNF increases with increasing adipocyte mass and expression of TNF is increased in adipocytes isolated from several genetic models of rodent obesity and from obese humans. This finding has led to the idea that TNF produced by adipocytes functions as a local "adipostat" to limit fat accumulation. Increased production of TNF by adipocytes, however, may contribute to insulin resistance in obesity and in non-insulin-dependent diabetes mellitus (NIDDM). TNF has been shown to inhibit insulin-simulated tyrosine phosphorylation of both the insulin receptor (IR) and insulin receptor substrate (IRS)-1 and to stimulate downregulation of the insulin-sensitive glucose transporter, GLUT4, in adipocytes. These findings raise the possibility that pharmacological inhibition of TNF may provide a novel therapeutic target to treat patients with NIDDM.

Altered neuronal and microglial responses to excitotoxic and ischemic brain injury in mice lacking TNF receptors

Brain injury, as occurs in stroke or head trauma, induces a dramatic increase in levels of tumor necrosis factor-alpha (TNF), but its role in brain injury response is unknown. We generated mice genetically deficient in TNF receptors (TNFR-KO) to determine the role of TNF in brain cell injury responses. Damage to neurons caused by focal cerebral ischemia and epileptic seizures was exacerbated in TNFR-KO mice, indicating that TNF serves a neuroprotective function. Oxidative stress was increased and levels of an antioxidant enzyme reduced in brain cells of TNFR-KO mice, indicating that TNF protects neurons by stimulating antioxidant pathways. Injury-induced microglial activation was suppressed in TNFR-KO mice, demonstrating a key role for TNF in injury-induced immune response. Drugs that target TNF signaling pathways may prove beneficial in treating stroke and traumatic brain injury.

Activation and homologous desensitization of human endothelial cells by CD40 ligand, tumor necrosis factor, and interleukin 1

We have reported previously that activation of human umbilical vein endothelial cells (HUVECs) through CD40, using a recombinant soluble form of trimerized CD40 ligand, leads to induction of E-selectin, vascular cell adhesion molecule-1 (VCAM-1), and intercellular adhesion molecule-1 (ICAM-1). Here, we compare the effects of CD40 ligand with those of tumor necrosis factor (TNF) and interleukin 1 (IL-1). All three ligands induce transient increases in E-selectin (peak 4 h) and VCAM-1 (peak 8-24 h), as well as sustained increases in ICAM-1 (plateau 24 h). Quantitatively, TNF is more potent than IL-1, which is much more potent than CD40 ligand. The same hierarchy is observed for transcriptional activation of an E-selectin promoter reporter gene construct in transiently transfected HUVECs. TNF and CD40 ligand each induced activation of the transcription factors NF-kappa B, IRF-1, and ATF-2/c-Jun, measured by electrophoretic mobility shift assays, but this response appeared quantitatively similar. All three agents transiently (peak 30 min) activated Jun NH2-terminal kinase (JNK), which has been implicated in transcription of E-selectin through its actions on ATF-2/c-Jun. Activation of JNK again showed a hierarchy of potency (TNF > IL-1 >> CD40 ligand), although the time course of induction was similar for all three agents. After 44 h of pretreatment, TNF, IL-1, and CD40 ligand each display homologous desensitization for reinduction of surface expression of E-selectin. A similar pattern of homologous desensitization for reactivation of JNK was observed. We conclude that TNF, IL-1, and CD40 ligand all activate similar responses in ECs, and that homologous desensitization of JNK may explain the inability of individual cytokines to reinduce E-selectin expression.

The tumor necrosis factor-inducible zinc finger protein A20 interacts with TRAF1/TRAF2 and inhibits NF-kappaB activation

TRAF1 and TRAF2 form an oligomeric complex that associates with the cytoplasmic domains of various members of the tumor necrosis factor (TNF) receptor superfamily. TRAF2 action is required for activation of the transcription factor NF-kappaB triggered by TNF and the CD40 ligand. Here we show that TRAF1 and TRAF2 interact with A20, a zinc finger protein, whose expression is induced by agents that activate NF-kappaB. Mutational analysis revealed that the N-terminal half of A20 interacts with the conserved C-terminal TRAF domain of TRAF1 and TRAF2. In cotransfection experiments, A20 blocked TRAF2-mediated NF-kappaB activation. A20 also inhibited TNF and IL-1-induced NF-kappaB activation, suggesting that it may inhibit NF-kappaB activation signaled by diverse stimuli. The ability of A20 to block NF-kappaB activation was mapped to its C-terminal zinc finger domain. Thus, A20 is composed of two functionally distinct domains, an N-terminal TRAF binding domain that recruits A20 to the TRAF2-TRAF1 complex and a C-terminal domain that mediates inhibition of NF-kappaB activation. Our findings suggest a possible molecular mechanism that could explain A20's ability to negatively regulate its own TNF-inducible expression.

Protein sequence motifs

Protein sequence motifs are signatures of protein families and can often be used as tools for the prediction of protein function. The generalization and modification of already known motifs are becoming major trends in the literature, even though new motifs are still being discovered at an approximately linear rate. The emphasis of motif analysis appears to be shifting from metabolic enzymes, in which motifs are associated with catalytic functions and thus often readily recognizable, to structural and regulatory proteins, which contain more divergent motifs. The consideration of structural information increasingly contributes to the identification of motifs and their sensitivity. Genome sequencing provides the basis for a systematic analysis of all motifs that are present in a particular organism. A systematically derived motif database is therefore feasible, allowing the classification of the majority of the newly appearing protein sequences into known families.

Signals and susceptibility to programmed death in b cells

Programmed death in B cells is a highly regulated process. During the past year it has become increasingly apparent that specific receptor signals influence B cell apoptosis in distinct ways as a function of developmental stage and/or apoptotic trigger. Studies making use of opposing signals for programmed death have begun to reveal molecular correlates of sensitivity and resistance to apoptosis.

CD30 contains two binding sites with different specificities for members of the tumor necrosis factor receptor-associated factor family of signal transducing proteins

CD30 is a member of the tumor necrosis factor (TNF) receptor family of proteins. CD30 can regulate proliferation of lymphocytes and may also play an important role in human immunodeficiency virus replication. However, little is known about CD30 signal transduction. We performed a yeast two-hybrid library screen with the cytoplasmic domain of CD30 and isolated multiple independent cDNAs encoding human tumor necrosis factor receptor-associated factor (TRAF) 1, TRAF2, and CRAF1 (TRAF3). The ability of TRAF1, TRAF2, and CRAF1 to associate with CD30 was confirmed using an in vitro coprecipitation assay, further demonstrating that the interaction was specific and direct. The TRAF-binding domain of CD30 was mapped to the COOH-terminal 36 amino acid residues, which contained two independent binding sites. CRAF1 bound only a single site, which contained the sequence PEQET, whereas TRAF1 and TRAF2 were capable of binding to either the PEQET site or an additional downstream domain. These data indicate that the TRAF protein binding pattern of CD30 differs from other TNF receptor family members and suggest that signaling specificity through TNF receptor family proteins may be achieved through differences in their abilities to bind TRAF proteins.

The cell-death machine

Apoptosis, or programmed cell death, is the physiological process whereby individual cells are deliberately eliminated to achieve homeostasis and proper metazoan development. Numerous genes have recently been identified that are involved in apoptosis: some are believed to encode death effectors, whereas others encode positive or negative regulators of the cell-death machine. Precisely how these various proteins interact in the molecular mechanism of apoptosis remains to be discovered.

Molecular and clinical aspects of apoptosis

Unwanted cells are removed by physiological cell death processes that are highly conserved throughout the animal kingdom. Physiological cell death plays an important role in development, tissue homeostasis and defence against viral infection and mutation. This review describes the molecular components that implement this process, the relevance of these to a variety of human diseases, and discusses the potential for novel therapies based on our understanding of them.

Drosophila homologs of baculovirus inhibitor of apoptosis proteins function to block cell death

Apoptotic cell death is a mechanism by which organisms eliminate superfluous or harmful cells. Expression of the cell death regulatory protein REAPER (RPR) in the developing Drosophila eye results in a small eye owing to excess cell death. We show that mutations in thread (th) are dominant enhancers of RPR-induced cell death and that th encodes a protein homologous to baculovirus inhibitors of apoptosis (IAPs), which we call Drosophila IAP1 (DIAP1). Overexpression of DIAP1 or a related protein, DIAP2, in the eye suppresses normally occurring cell death as well as death due to overexpression of rpr or head involution defective. IAP death-preventing activity localizes to the N-terminal baculovirus IAP repeats, a motif found in both viral and cellular proteins associated with death prevention.