The receptor for the cytotoxic ligand TRAIL

Herpesvirus entry mediator ligand (HVEM-L), a novel ligand for HVEM/TR2, stimulates proliferation of T cells and inhibits HT29 cell growth

Herpesvirus entry mediator (HVEM), a member of the tumor necrosis factor (TNF) receptor family, mediates herpesvirus entry into cells during infection. Upon overexpression, HVEM activates NF-kappaB and AP-1 through a TNF receptor-associated factor (TRAF)-mediated mechanism. Using an HVEM-Fc fusion protein, we screened soluble forms of novel TNF-related proteins derived from an expressed sequence tag data base. One of these, which we designated HVEM-L, specifically bound to HVEM-Fc with an affinity of 44 nM. This association was confirmed with soluble and membrane forms of both receptor and ligand. HVEM-L mRNA is expressed in spleen, lymph nodes, macrophages, and T cells and encodes a 240-amino acid protein. A soluble, secreted form of the protein stimulates proliferation of T lymphocytes during allogeneic responses, inhibits HT-29 cell growth, and weakly stimulates NF-kappaB-dependent transcription.

Thymic Expression of the Transcription Factor Nur77 Rescues the T Cell But Not the B Cell Abnormality of gld/gld Mice

Fas and Fas ligand are critical regulators of lymphocyte homeostasis. Disruption of this pathway in the spontaneous mouse mutant gld leads to autoimmunity characterized by the appearance of a population of CD4−8−B220+ T cells and the production of autoantibodies. Nur77 is a transcription factor that is induced upon TCR signaling. Constitutive thymic expression of Nur77 leads to apoptosis. We have previously shown that introduction of this Nur77 transgene can eliminate the accumulation of abnormal T cells in the periphery of gld/gld mice. In this report, we further characterized the effects of the Nur77 transgene on the gld phenotype. Nur77-mediated apoptosis is evident in the thymuses of mice with either a gld/gld homozygous or gld/+ heterozygous background. Consequently, few mature T cells are generated in these mice. In addition, mature T cells exhibit a diminished response to proliferative signals through CD3. Interestingly, the Nur77 transgene failed to reduce serum levels of Igs and anti-DNA Abs to wild-type levels. These data suggest that the rescue of the T cell lymphoproliferative syndrome in gld/gld mice by the Nur77 transgene is mediated by events in the thymus and that B cell autoimmune disease associated with the gld mutation can develop independently of the T cell abnormality.

p38 MAPK Is Required for CD40-Induced Gene Expression and Proliferation in B Lymphocytes

We have investigated the activation of the p38 MAPK pathway in response to CD40 engagement in multiple B cell lines and in human tonsillar B cells to define the role of p38 MAPK in proliferation, NF-κB activation and gene expression. Cross-linking CD40 rapidly stimulates both p38 MAPK and its downstream effector, MAPKAPK-2. Inhibition of p38 MAPK activity in vivo with the specific cell-permeable inhibitor, SB203580, under conditions that completely prevented MAPKAPK-2 activation, strongly perturbed CD40-induced tonsillar B cell proliferation while potentiating the B cell receptor (BCR)-driven proliferative response. SB203580 also significantly reduced expression of a reporter gene driven by a minimal promoter containing four NF-κB elements, indicating a requirement for the p38 MAPK pathway in CD40-induced NF-κB activation. However, CD40-mediated NF-κB binding was not affected by SB203580, suggesting that NF-κB may not be a direct target for the CD40-induced p38 MAPK pathway. In addition, SB203580 selectively reduced CD40-induced CD54/ICAM-1 expression, whereas CD40-dependent expression of CD40 and CD95/Fas and four newly defined CD40-responsive genes cIAP2, TRAF1, TRAF4/CART and DR3 were unaffected. Our observations show that the p38 MAPK pathway is required for CD40-induced proliferation and that CD40 induces gene expression via both p38 MAPK-dependent and -independent pathways.

Inhibition of fas death signals by FLIPs

The death receptor Fas is a member of the tumor necrosis factor receptor family; upon interaction with its ligand it efficiently activates caspases and induces apoptosis. Despite abundant Fas surface expression, however, Fas death-signals are frequently interrupted. Many viruses express antiapoptotic proteins, including caspase inhibitors, Bcl-2 homologues and death-effector-domain-containing proteins that are termed FLIPs (FLICE [Fas-associated death-domain-like IL-1beta-converting enzyme]-inhibitory proteins). Cellular homologues of these inhibitors have been identified. Cellular FLIPs structurally resemble caspase-8 except that they lack proteolytic activity. FLIPs are highly expressed in tumor cells, T lymphocytes and healthy, but not injured, myocytes; this suggests a critical role of FLIPs as endogenous modulators of apoptosis.

TRAIL: a molecule with multiple receptors and control mechanisms

Apoptosis research is benefiting from bioinformatic approaches to identify new components of the cell death machinery and novel cell death inducers/receptors. Over the past year, knowledge of the system involving TNF-related apoptosis-inducing ligand (TRAIL) and its receptors has increased via genomic database analysis to include four distinct receptors that interact with a single ligand. Currently, these molecules are of major interest due to their potential roles and application in cancer therapy.

LIGHT, a novel ligand for lymphotoxin beta receptor and TR2/HVEM induces apoptosis and suppresses in vivo tumor formation via gene transfer

LIGHT is a new member of tumor necrosis factor (TNF) cytokine family derived from an activated T cell cDNA library. LIGHT mRNA is highly expressed in splenocytes, activated PBL, CD8(+) tumor infiltrating lymphocytes, granulocytes, and monocytes but not in the thymus and the tumor cells examined. Introduction of LIGHT cDNA into MDA-MB-231 human breast carcinoma caused complete tumor suppression in vivo. Histological examination showed marked neutrophil infiltration and necrosis in LIGHT expressing but not in the parental or the Neo-transfected MDA-MB-231 tumors. Interferon gamma (IFNgamma) dramatically enhances LIGHT-mediated apoptosis. LIGHT protein triggers apoptosis of various tumor cells expressing both lymphotoxin beta receptor (LTbetaR) and TR2/HVEM receptors, and its cytotoxicity can be blocked specifically by addition of a LTbetaR-Fc or a TR2/HVEM-Fc fusion protein. However, LIGHT was not cytolytic to the tumor cells that express only the LTbetaR or the TR2/HVEM or hematopoietic cells examined that express only the TR2/HVEM, such as PBL, Jurkat cells, or CD8(+) TIL cells. In contrast, treatment of the activated PBL with LIGHT resulted in release of IFNgamma. Our data suggest that LIGHT triggers distinct biological responses based on the expression patterns of its receptors on the target cells. Thus, LIGHT may play a role in the immune modulation and have a potential value in cancer therapy.

Intracellular Regulation of TRAIL-Induced Apoptosis in Human Melanoma Cells

The observation that TNF-related apoptosis-inducing ligand (TRAIL), a member of the TNF cytokine family, induces apoptosis in a number of different tumor cell types led us to compare the tumoricidal effects of TRAIL to those of other TNF family molecules on human melanoma cells. We found that a high proportion of the melanoma cell lines tested were killed by TRAIL, whereas all the melanoma lines were resistant to the other TNF family cytokines tested. TRAIL-induced death was characterized by caspase activation and cellular protein cleavage within minutes of TRAIL addition, and death could be completely inhibited by the caspase inhibitors Ile-Glu-Thr-Asp (IETD) and Val-Ala-Asp (VAD), indicating the presence of a TRAIL receptor signaling pathway similar to that identified for Fas and TNF receptors. Specific TRAIL receptor expression was determined by RT-PCR, and the presence of mRNA encoding the “protective” TRAIL receptors did not correspond to resistance or sensitivity to TRAIL-induced apoptosis. Addition of protein synthesis inhibitors to TRAIL-resistant melanomas rendered them sensitive to TRAIL, indicating that the presence or the absence of intracellular apoptosis inhibitors may mediate resistance or sensitivity to TRAIL-mediated apoptosis. Expression of one such inhibitor, FLICE-inhibitory protein (FLIP), was highest in the TRAIL-resistant melanomas, while being low or undetectable in the TRAIL-sensitive melanomas. Furthermore, addition of actinomycin D to TRAIL-resistant melanomas resulted in decreased intracellular concentrations of FLIP, which correlated with their acquisition of TRAIL sensitivity. Collectively, our results indicate that TRAIL-induced apoptosis occurs through a caspase signaling cascade and that resistance is controlled by intracellular regulators of apoptosis.

Dual signaling of the Fas receptor: initiation of both apoptotic and necrotic cell death pathways

Murine L929 fibrosarcoma cells were transfected with the human Fas (APO-1/CD95) receptor, and the role of various caspases in Fas-mediated cell death was assessed. Proteolytic activation of procaspase-3 and -7 was shown by Western analysis. Acetyl-Tyr-Val-Ala-Asp-chloromethylketone and benzyloxycarbonyl-Asp(OMe)-Glu(OMe)-Val-Asp(OMe)-fluoromethylketone++ +, tetrapeptide inhibitors of caspase-1- and caspase-3-like proteases, respectively, failed to block Fas-induced apoptosis. Unexpectedly, the broad-spectrum caspase inhibitors benzyloxycarbonyl-Val-Ala-Asp(OMe)-fluoromethylketone and benzyloxycarbonyl-Asp(OMe)-fluoromethylketone rendered the cells even more sensitive to Fas-mediated cell death, as measured after 18 h incubation. However, when the process was followed microscopically, it became clear that anti-Fas-induced apoptosis of Fas-transfected L929 cells was blocked during the first 3 h, and subsequently the cells died by necrosis. As in tumor necrosis factor (TNF)-induced necrosis, Fas treatment led to accumulation of reactive oxygen radicals, and Fas-mediated necrosis was inhibited by the oxygen radical scavenger butylated hydroxyanisole. However, in contrast to TNF, anti-Fas did not activate the nuclear factor kappaB under these necrotic conditions. These results demonstrate the existence of two different pathways originating from the Fas receptor, one rapidly leading to apoptosis, and, if this apoptotic pathway is blocked by caspase inhibitors, a second directing the cells to necrosis and involving oxygen radical production.

Mechanisms of hepatic toxicity. I. TNF-induced liver injury

Tumor necrosis factor-alpha (TNF-alpha) functions as a two-edged sword in the liver. TNF-alpha is required for normal hepatocyte proliferation during liver regeneration. It functions both as a comitogen and to induce the transcription factor nuclear factor-kappaB, which has antiapoptotic effects. On the other hand, TNF-alpha is the mediator of hepatotoxicity in many animal models, including those involving the toxins concanavalin A and lipopolysaccharide. TNF-alpha has also been implicated as an important pathogenic mediator in patients with alcoholic liver disease and viral hepatitis.

TNF-related ligands and their receptors

Multicellular organisms have the challenging task of coordinating the activities of many distinct cell types. This coordination is accomplished largely by cell-associated and soluble signalling molecules that act locally or distantly to alter target-cell physiology. The tumour necrosis factor family of cytokines are type II transmembrane proteins that are important regulators of homeostasis and have been implicated as mediators of disease. These molecules serve as ligands for a family of cell-surface receptors termed the tumour necrosis factor/nerve growth factor (TNF/NGF) receptor family. The receptors are type I transmembrane proteins capable of mediating a wide range of responses in vitro and in vivo. Signal transduction is mediated by several newly discovered cytoplasmic proteins that couple these receptors to downstream signalling events. The elucidation and use of spontaneously occurring mutants in TNF-related ligands and receptors in addition to gene-targeting experiments have begun to clarify the diverse biological effects mediated by this superfamily of cytokines.

Involvement of APO2 ligand/TRAIL in activation-induced death of Jurkat and human peripheral blood T cells

The interaction of Fas with Fas ligand (FasL) mediates activation-induced cell death (AICD) of T hybridomas and of mature T lymphocytes. The TNF/TNF receptor system also plays a significant role in AICD of mature T cells and in the maintenance of peripheral tolerance. We previously demonstrated that in human Jurkat leukemia cells, AICD is triggered mainly by the rapid release of preformed FasL upon TCR stimulation. In the present work, we show that the cytotoxic cytokine APO2 ligand (APO2L; also known as TRAIL) is constitutively expressed as an intracytoplasmic protein in Jurkat T cells and derived sublines. APO2L is also detected in fresh human peripheral blood mononuclear cells (PBMC) from a significant number of donors, and the amount of both FasL and APO2L substantially increases upon blast generation. A neutralizing anti-APO2L monoclonal antibody (mAb) partially suppresses the cytotoxicity induced by supernatants of phytohemagglutinin (PHA)-prestimulated Jurkat or human PBMC on non-activated Jurkat cells, indicating that APO2L is released by these cells and contributes to AICD. A combination of neutralizing anti-APO2L and anti-Fas mAb blocks around 60 % of the toxicity associated with supernatants from PHA-activated human PBMC. These results show that FasL and APO2L account for the majority of cytotoxic activity released during AICD, and suggest that additional uncharacterized factors may also contribute to this process.

TNF-Related Apoptosis-Inducing Ligand (TRAIL) Induces Apoptosis in Fas Ligand-Resistant Melanoma Cells and Mediates CD4 T Cell Killing of Target Cells

We have previously shown that melanoma cells were resistant to apoptosis induced by TNF family members Fas ligand (FasL), TNF-α, and CD40L. FasL also was not involved in CD4 T cell-mediated killing of melanoma cells. In the present study, we have tested melanoma cells for their susceptibility to apoptosis induced by human TNF-related apoptosis-inducing ligand (TRAIL) and the ability of a mAb against TRAIL to inhibit apoptosis and CD4 CTL-mediated killing of melanoma and Jurkat target cells. The results show that TRAIL-induced apoptosis in cells from 7 of 10 melanoma cell lines tested as well as in Jurkat T cells. Susceptibility to apoptosis was increased in some of the cell lines by treatment with cyclohexamide or actinomycin D. The melanoma cells were resistant to apoptosis induced by FasL, TNF-α, and CD40L. mAb M180 against TRAIL inhibited apoptosis induced by TRAIL. It was also found to inhibit CD4 CTL-mediated killing of Jurkat T cells as well as autologous and allogeneic melanoma cells. The degree of inhibition produced by the mAb varied between different clones of CTL and according to the susceptibility of the target cells to TRAIL-induced apoptosis. These results suggest that TRAIL is an important mediator of cell death induced by CTL and may have an important therapeutic role against human melanoma.

Death receptors: signaling and modulation

Apoptosis is a cell suicide mechanism that enables metazoans to control cell number in tissues and to eliminate individual cells that threaten the animal's survival. Certain cells have unique sensors, termed death receptors, on their surface. Death receptors detect the presence of extracellular death signals and, in response, they rapidly ignite the cell's intrinsic apoptosis machinery.

Antibodies to TR2 (Herpesvirus Entry Mediator), a New Member of the TNF Receptor Superfamily, Block T Cell Proliferation, Expression of Activation Markers, and Production of Cytokines

TR2 (TNFR-related 2) is a recently identified member of the TNFR family with homology to TNFRII. We have demonstrated previously that TR2 mRNA is expressed in resting and activated human T cells and that TR2-Ig partially inhibits an allogeneic mixed leukocyte proliferation response. We now characterize TR2 further by the use of specific mAbs. Flow-cytometry analysis using TR2 mAbs confirmed that resting PBL express high levels of cell surface TR2, and that TR2 is widely expressed on all freshly isolated lymphocyte subpopulations. However, stimulation of purified T cells with either PHA or PHA plus PMA resulted in decreased surface expression within 48 h of activation before returning to resting levels at 72 h. TR2 mAbs inhibited CD4+ T cell proliferation in response to stimulation by immobilized CD3 or CD3 plus CD28 mAbs. Assay of culture supernatants by ELISA showed inhibition of TNF-α, IFN-γ, IL-2, and IL-4 production, which, for IL-2 and TNF-α was also confirmed by intracellular cytokine staining. Furthermore, expression of activation markers on CD4+ T cells, including CD25, CD30, CD69, CD71, and OX40 (CD134), was inhibited. TR2 mAbs inhibited proliferation in a three-way MLR, and a response to soluble recall Ag, tetanus toxoid. In conclusion, these results suggest that TR2 is involved in the activation cascade of T cell responses and TR2 mAbs prevent optimal T cell proliferation, cytokine production, and expression of activation markers.

Targeted disruption of the mouse Caspase 8 gene ablates cell death induction by the TNF receptors, Fas/Apo1, and DR3 and is lethal prenatally

Homozygous targeted disruption of the mouse Caspase 8 (Casp8) gene was found to be lethal in utero. The Caspase 8 null embryos exhibited impaired heart muscle development and congested accumulation of erythrocytes. Recovery of hematopoietic colony-forming cells from the embryos was very low. In fibroblast strains derived from these embryos, the TNF receptors, Fas/Apo1, and DR3 were able to activate the Jun N-terminal kinase and to trigger IkappaB alpha phosphorylation and degradation. They failed, however, to induce cell death, while doing so effectively in wild-type fibroblasts. These findings indicate that Caspase 8 plays a necessary and nonredundant role in death induction by several receptors of the TNF/NGF family and serves a vital role in embryonal development.

Identification and functional characterization of DR6, a novel death domain-containing TNF receptor

Tumor nectosis factor (TNF) receptors are key players in inflammation and immune regulation. A new member of this family, termed death receptor-6 (DR6), has been identified. Like other death receptors, DR6 is a type I transmembrane receptor, possesses four extracellular cysteine-rich motifs and a cytoplasmic death domain. DR6 is expressed in most human tissues and abundant transcript was detected in heart, brain, placenta, pancreas, thymus, lymph node and several non-lymphoid cancer cell lines. DR6 interacts with TRADD, which has previously been shown to associate with TNFR1. Furthermore, ectopic expression of DR6 in mammalian cells induces apoptosis and activation of both NF-kappaB and JNK.

Molecular cloning and characterization of the mouse tag7 gene encoding a novel cytokine

Cloning of the mouse tag7 gene encoding a novel cytokine is described. The Tag7 protein consists of 182 amino acids. Genomic organization of the tag7 gene and its promoter region remind those of the genes of the tumor necrosis factor locus, although the tag7 gene is not linked to this locus. The gene is located on chromosome 7 at the area that corresponds to band 7A3, which has genetic linkage with lupus-like disease in mouse models. tag7 transcription is essential for lymphoid organs. It is also detected in certain areas of lungs, brain, and intestine and in some tumors. Tag7 protein is detectable in both cell-associated and soluble forms. The soluble form of Tag7 triggers apoptosis in mouse L929 cells in vitro and does not involve NF-kappaB activation. The relationship between Tag7 and tumor necrosis factor family of ligands is discussed.

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

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

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

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

TR1, a new member of the tumor necrosis factor receptor superfamily, induces fibroblast proliferation and inhibits osteoclastogenesis and bone resorption

A newly identified member of the tumor necrosis factor receptor (TNFR) superfamily shows activities associated with osteoclastogenesis inhibition and fibroblast proliferation. This new member, called TR1, was identified from a search of an expressed sequence tag database, and encodes 401 amino acids with a 21-residue signal sequence. Unlike other members of TNFR, TR1 does not contain a transmembrane domain and is secreted as a 62 kDa glycoprotein. TR1 gene maps to chromosome 8q23-24.1 and its mRNA is abundantly expressed on primary osteoblasts, osteogenic sarcoma cell lines, and primary fibroblasts. The receptors for TR1 were detected on a monocytic cell line (THP-1) and in human fibroblasts. Scatchard analyses indicated two classes of high and medium-high affinity receptors with a kD of approximately 45 and 320 pM, respectively. Recombinant TR1 induced proliferation of human foreskin fibroblasts and potentiated TNF-induced proliferation in these cells. In a coculture system of osteoblasts and bone marrow cells, recombinant TR1 completely inhibited the differentiation of osteoclast-like multinucleated cell formation in the presence of several bone-resorbing factors. TR1 also strongly inhibited bone-resorbing function on dentine slices by mature osteoclasts and decreased 45Ca release in fetal long-bone organ cultures. Anti-TR1 monoclonal antibody promoted the formation of osteoclasts in mouse marrow culture assays. These results indicate that TR1 has broad biological activities in fibroblast growth and in osteoclast differentiation and its functions.

Death domain receptors and their role in cell demise

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

Fas and mutant estrogen receptor chimeric gene: a novel suicide vector for tamoxifen-inducible apoptosis

Several cancer gene therapy strategies involve suicide genes to kill the neoplasm, or to regulate effector cells such as lymphocytes. We have developed an inducible apoptosis system with a Fas-estrogen receptor fusion protein (MfasER) for rapid elimination of transduced cells. In the present study, we further improved this molecular switch for estrogen-inducible apoptosis to overcome concerns with the wild-type estrogen receptor and its natural ligand, 17beta-estradiol (E2). The ligand-binding domain of MfasER was replaced with that of a mutant estrogen receptor which is unable to bind estrogen yet retains affinity for a synthetic ligand, 4-hydroxytamoxifen (Tm). The resultant fusion protein (MfasTmR) and MfasER were expressed in L929 cells for examination of their ligand specificities. Tm induced apoptosis in MfasTmR-expressing cells (L929MfasTmR) at 10(-8) M or higher concentrations, but induced no apoptosis in MfasER-expressing cells (L929MfasER) at up to 10(-6) M. On the other hand, E2 induced apoptosis in L929MfasER at concentrations as low as 10(-10)-10(-9) M, while it did so partially in L929MfasTmR at concentrations greater than 10(-7) M. Thus, L929MfasTmR cells were highly susceptible to Tm, but refractory to E2, with 100-1,000 times more tolerance than L929MfasER. These results suggest that the MfasTmR/Tm system would induce apoptosis in the target cells more safely in vivo, working independently of endogenous estrogen.

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

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

Inhibition of Nuclear Factor κB Activation Attenuates Apoptosis Resistance in Lymphoid Cells

Death-inducing ligands (DILs) such as tumor necrosis factor α (TNFα) or the cytotoxic drug doxorubicin have been shown to activate a nuclear factor κB (NFκB)-dependent program that may rescue cells from apoptosis induction. We demonstrate here that TRAIL (TNF-related apoptosis-inducing ligand), a recently identified DIL, also activates NFκB in lymphoid cell lines in a kinetic similar to TNFα. NFκB activity is independent from FADD, caspases, and apoptosis induction. To study the influence of NFκB activity on apoptosis mediated by TRAIL, CD95, TNFα, or doxorubicin, NFκB activation was inhibited using the proteasome inhibitor N-acetyl-L-leucinyl-L-leucinyl-L-norleucinal or transient overexpression of mutant IκBα. Sensitivity for induction of apoptosis was markedly increased by these treatments in apoptosis sensitive cell lines. Moreover, both in cell lines and in primary leukemia cells that are resistant towards induction of apoptosis by DILs and doxorubicin, antagonization of NFκB activity partially restored apoptosis sensitivity. These data suggest that inhibition of NFκB activation may provide a molecular approach to increase apoptosis sensitivity in anticancer treatment.

Inhibition of Nuclear Factor κB Activation Attenuates Apoptosis Resistance in Lymphoid Cells

Death-inducing ligands (DILs) such as tumor necrosis factor α (TNFα) or the cytotoxic drug doxorubicin have been shown to activate a nuclear factor κB (NFκB)-dependent program that may rescue cells from apoptosis induction. We demonstrate here that TRAIL (TNF-related apoptosis-inducing ligand), a recently identified DIL, also activates NFκB in lymphoid cell lines in a kinetic similar to TNFα. NFκB activity is independent from FADD, caspases, and apoptosis induction. To study the influence of NFκB activity on apoptosis mediated by TRAIL, CD95, TNFα, or doxorubicin, NFκB activation was inhibited using the proteasome inhibitor N-acetyl-L-leucinyl-L-leucinyl-L-norleucinal or transient overexpression of mutant IκBα. Sensitivity for induction of apoptosis was markedly increased by these treatments in apoptosis sensitive cell lines. Moreover, both in cell lines and in primary leukemia cells that are resistant towards induction of apoptosis by DILs and doxorubicin, antagonization of NFκB activity partially restored apoptosis sensitivity. These data suggest that inhibition of NFκB activation may provide a molecular approach to increase apoptosis sensitivity in anticancer treatment.

Osteoprotegerin is a receptor for the cytotoxic ligand TRAIL

TRAIL is a tumor necrosis factor-related ligand that induces apoptosis upon binding to its death domain-containing receptors, DR4 and DR5. Two additional TRAIL receptors, TRID/DcR1 and DcR2, lack functional death domains and function as decoy receptors for TRAIL. We have identified a fifth TRAIL receptor, namely osteoprotegerin (OPG), a secreted tumor necrosis factor receptor homologue that inhibits osteoclastogenesis and increases bone density in vivo. OPG-Fc binds TRAIL with an affinity of 3.0 nM, which is slightly weaker than the interaction of TRID-Fc or DR5-Fc with TRAIL. OPG inhibits TRAIL-induced apoptosis of Jurkat cells. Conversely, TRAIL blocks the anti-osteoclastogenic activity of OPG. These data suggest potential cross-regulatory mechanisms by OPG and TRAIL.

Caspase Activation Is Required for Nitric Oxide–Mediated, CD95(APO-1/Fas)–Dependent and Independent Apoptosis in Human Neoplastic Lymphoid Cells

Nitric oxide (NO), an important effector molecule involved in immune regulation and host defense, was shown to induce apoptosis in lymphoma cells. In the present report the NO donor glycerol trinitrate was found to induce apoptosis in Jurkat cells that are sensitive to CD95-mediated kill. In contrast, a CD95-resistant Jurkat subclone showed substantial protection from apoptosis after exposure to NO. NO induced mRNA expression of CD95 (APO-1/Fas) and TRAIL/APO-2 ligands. Moreover, NO triggered apoptosis in freshly isolated human leukemic lymphocytes which were also sensitive to anti-CD95 treatment. The ability of NO to induce apoptosis was completely blocked by a broad-spectrum ICE (interleukin-1β converting enzyme)-protease/caspase inhibitor and correlated with FLICE/caspase-8 activation. This activation was abrogated in some neoplastic lymphoid cells but not in others by the inhibitor of protein synthesis cycloheximide. Our results were confirmed using an in vitro experimental model of coculture of human lymphoid target cells with activated bovine endothelial cells generating NO as effectors. Furthermore, the inhibition of endogenous NO production with the inducible NO synthase inhibitor NG-monomethyl-L-arginine caused a complete abrogation of the apoptotic effect. Our data provide evidence that NO-induced apoptosis in human neoplastic lymphoid cells strictly requires activation of caspases, in particular FLICE, the most CD95 receptor-proximal caspase. Depending on the cell line tested this activation required or was independent of the CD95 receptor/ligand system.

Apoptosis and thyroiditis

The origin of the various forms of autoimmune thyroiditis remains unclear. Most investigations into the pathogenesis of these disorders have focused on immune abnormalities that might lead to an autoimmune response. However, no unique immune response to thyroid autoantigens has been identified that either is limited to patients with thyroiditis or is absolutely correlated with clinical disease expression. CD8 T-cell-mediated cytotoxicity is thought to be a major cause of thyroid follicular cell damage in thyroiditis. This damage is produced in part through the induction of apoptosis in thyroid cells. Recent studies have demonstrated that programmed cell death is regulated in thyroid cells and that a major pathway for immune-mediated apoptosis, the Fas pathway, is blocked by labile inhibitors in a manner that could prevent cytotoxicity. This review also examines several other types of regulation of apoptotic pathways in thyrocytes. We hypothesize that the regulation of programmed cell death pathways in the thyroid may alter the expression of autoimmune thyroid diseases by modifying the susceptibility of thyroid cells to immune-mediated apoptosis.

Caspase Activation Is Required for Nitric Oxide–Mediated, CD95(APO-1/Fas)–Dependent and Independent Apoptosis in Human Neoplastic Lymphoid Cells

Nitric oxide (NO), an important effector molecule involved in immune regulation and host defense, was shown to induce apoptosis in lymphoma cells. In the present report the NO donor glycerol trinitrate was found to induce apoptosis in Jurkat cells that are sensitive to CD95-mediated kill. In contrast, a CD95-resistant Jurkat subclone showed substantial protection from apoptosis after exposure to NO. NO induced mRNA expression of CD95 (APO-1/Fas) and TRAIL/APO-2 ligands. Moreover, NO triggered apoptosis in freshly isolated human leukemic lymphocytes which were also sensitive to anti-CD95 treatment. The ability of NO to induce apoptosis was completely blocked by a broad-spectrum ICE (interleukin-1β converting enzyme)-protease/caspase inhibitor and correlated with FLICE/caspase-8 activation. This activation was abrogated in some neoplastic lymphoid cells but not in others by the inhibitor of protein synthesis cycloheximide. Our results were confirmed using an in vitro experimental model of coculture of human lymphoid target cells with activated bovine endothelial cells generating NO as effectors. Furthermore, the inhibition of endogenous NO production with the inducible NO synthase inhibitor NG-monomethyl-L-arginine caused a complete abrogation of the apoptotic effect. Our data provide evidence that NO-induced apoptosis in human neoplastic lymphoid cells strictly requires activation of caspases, in particular FLICE, the most CD95 receptor-proximal caspase. Depending on the cell line tested this activation required or was independent of the CD95 receptor/ligand system.

A cell surface receptor defined by a mAb mediates a unique type of cell death similar to oncosis

Cell death is mediated by distinct pathways including apoptosis and oncosis in response to various death signals. To characterize molecules involved in cell death, a panel of mAbs was raised by immunizing mice with apoptotic cells. One of these antibodies, designated anti-Porimin (for pro-oncosis receptor inducing membrane injury), was found to directly induce a unique type of cell death in Jurkat cells. Anti-Porimin defines a 110-kDa cell surface receptor on Jurkat cells. Functionally, anti-Porimin alone rapidly mediates pore formation on the plasma membrane and induces cell death without participation of complement. Both the cellular expression and functional characteristics of the Porimin antigen indicate that it is distinct from the CD95 (Fas/Apo-1) and other cell receptors known to induce apoptosis. Anti-Porimin-mediated cell death was preceded by cell aggregation, formation of plasma membrane pores, and the appearance of membrane blebs. More important, these cells show neither DNA fragmentation nor apoptotic bodies, but display lethal damage of the cell membrane. Cell death by anti-Porimin is distinct from complement-dependent cytolysis or complement-independent apoptosis but is similar to that described for oncosis, a form of cell death accompanied by the membrane damage followed by karyolysis. The induction of cell death by anti-Porimin may represent a unique cell surface receptor-mediated pathway of cell death in the human lymphoid system.

Phosphorylation of FADD/MORT1 and Fas by Kinases That Associate with the Membrane-Proximal Cytoplasmic Domain of Fas

Fas (Apo-1, CD95), a member of the TNFR family, is expressed on a variety of cell types and transduces an apoptotic signal. Since Fas does not possess known enzymatic activities, proteins that interact with the cytoplasmic domain of Fas regulate the death signal. Several proteins have been identified, primarily using the yeast two-hybrid system, that associate with the death domain of Fas. One of these proteins, FADD/MORT1, can be phosphorylated, although the kinase that is responsible has not been identified. Furthermore, direct signaling connections between Fas and its known activation of sphingomyelinase or NF-κB have not been made, suggesting that other proteins may associate with Fas. In this study, a series of glutathione S-transferase fusion proteins was constructed that contained the cytoplasmic domain of murine Fas. These proteins were used to search for additional proteins that associate with Fas. Novel proteins, including kinases, were identified that associated specifically with the membrane-proximal, cytoplasmic tail of Fas but not with the death domain. One of these kinases phosphorylates FADD/MORT1. Moreover, the membrane-proximal region of Fas itself was phosphorylated by one of the associating kinases. These findings suggest that, similar to the Fas-related p55 TNFR, the membrane-proximal region of Fas likely participates in signaling.

Duplication of the DR3 gene on human chromosome 1p36 and its deletion in human neuroblastoma

The human DR3 gene, whose product is also known as Wsl-1/APO-3/TRAMP/LARD, encodes a tumor necrosis factor-related receptor that is expressed primarily on the surface of thymocytes and lymphocytes. DR3 is capable of inducing both NF-kappa B activation and apoptosis when overexpressed in mammalian cells, although its ligand has not yet been identified. We report here that the DR3 gene locus is tandemly duplicated on human chromosome band 1p36.2-p36.3 and that these genes are hemizygously deleted and/or translocated to another chromosome in neuroblastoma (NB) cell lines with amplified MYCN. Duplication of at least a portion of the DR3 gene, including the extracellular and transmembrane regions but not the cytoplasmic domain, was demonstrated by both fluorescence in situ hybridization and genomic Southern blotting. In most NB cell lines, both the DR3 and the DR3L sequences are simultaneously deleted and/or translocated to another chromosome. Finally, DR3/ Wsl-1 protein expression is quite variable among these NB cell lines, with very low or undetectable levels in 7 of 17 NB cell lines.

APO2 ligand: a novel lethal weapon against malignant glioma?

APO2L (TRAIL) is a novel CD95L (Fas/APO-1-L) homologous cytotoxic cytokine that interacts with various receptors which transmit (DR4, DR5) or inhibit (DcR1, DcR2) an apoptotic signal. Here, we report that human glioma cell lines preferentially express mRNAs for agonistic death receptors DR4 (8/12) and DR5 (11/12) rather than the death-inhibitory decoy receptors DcR1 (4/12) and DcR2 (2/12). Ten of 12 cell lines are susceptible to APO2L-induced apoptosis. The resistant cell lines, U138MG and U373MG, are cross-resistant to CD95L-induced apoptosis. Similar to CD95L-induced apoptosis, APO2L-induced apoptosis is inhibited by ectopic expression of the caspase inhibitor, crm-A, or of bcl-2, or by coexposure to the corticosteroid, dexamethasone, or the lipoxygenase inhibitor, nordihydroguaretic acid. There is no correlation between p53 genetic status of the cell lines and their susceptibility to APO2L-induced apoptosis, but the latter is moderately enhanced by ectopic expression of wild-type p53. APO2L targeting may be a promising approach for selectively targeting apoptosis to human malignant glioma cells.

Surface expression of TRAIL/Apo-2 ligand in activated mouse T and B cells

Like other members of the TNF family, TRAIL/Apo-2 ligand induces apoptosis in sensitive target cells in a caspase-dependent fashion. We recently found that TRAIL may be constitutively expressed on the surface of mouse and human tumor cells of T and B origin. To define the pattern of TRAIL expression in normal immune cells, freshly isolated splenocytes, Concanavalin A/IL-2-activated T cells and lipopolysaccharide-activated B cells were analyzed by surface staining with or without secondary stimulation. Activated, but not resting, CD3+ cells expressed TRAIL in an activation-dependent fashion. Conversely, freshly isolated B220+ cells displayed surface TRAIL and CD95L that were retained following activation. Restimulation with the protein kinase C activator phorbol 12-myristate 13-acetate and the calcium ionophore ionomycin or an agonistic anti-CD3 monoclonal antibody induced significant up-regulation of surface TRAIL and CD95L in CD3+, TCRalphabeta cells with CD4+ or CD8+ phenotype. Similarly to CD95L, TRAIL up-regulation was protein synthesis dependent and cyclosporin A sensitive. These results indicate that both TRAIL and CD95L are displayed on the cell surface of activated immune cells and may thus represent complementary effector pathways in the regulatory functions of T and B cells.

Proteolytic activities that mediate apoptosis

Since the discovery that cells can activate their own suicide program, investigators have attempted to determine whether the events that are associated with this form of cell death are genetically determined. The discovery that the ced-3 gene of Caenorhabditis elegans encodes a cysteine protease essential for developmentally regulated apoptosis ignited interest in this area of research. As a result, we now know that cell death is specified by a number of genes and that this biologic process contributes significantly to development, tumorigenesis, and autoimmune disease. In this review I summarize what is currently known about signaling pathways involved in apoptosis, with particular emphasis on the function of the cysteine proteases known as caspases. However, there is also evidence that protease-independent cell death pathways exist. Is there a relationship between these two distinct mechanisms? If so, how do they communicate? Finally, even though the involvement of tumor necrosis factor/nerve growth factor family of receptors and cysteine proteases has been elegantly established as a component of many apoptotic signaling pathways, what happens downstream of these initial events? Why are only a selected group of cellular proteins--many nuclear--the targets of these proteases? Are nuclear events essential for apoptosis in vivo? Are the cellular genes that encode products involved in apoptotic signaling frequent targets of mutation/alteration during tumorigenesis? These are only a few questions that may be answered in the next ten years.

Apoptosis, graft rejection, and transplantation tolerance

Transplantation tolerance can be achieved through several mechanisms, including the action of suppressor cells, the induction of anergy, or the deletion of graft-reactive donor T cells. In this article, the possible involvement of programmed cell death (apoptosis) in allograft rejection and transplantation tolerance is discussed. The recent characterization of members of the tumor necrosis factor-alpha gene family has identified ligands (Fas ligand and TRAIL) and corresponding death receptors (DR). In rejected organ transplants, apoptotic cells are frequently encountered. Conversely, high-level expression of Fas ligand on the allograft correlates with graft acceptance in some models of organ transplantation. Furthermore, some of the immunosuppressive drugs currently in clinical use might exert their activity at least in part through effects on apoptotic pathways. From the available data, it can be inferred that apoptosis contributes to the outcome after organ transplantation, being involved both in graft rejection and in transplantation tolerance.

A role for FADD in T cell activation and development

FADD is a cytoplasmic adapter molecule that links the family of death receptors to the activation of caspases during apoptosis. We have produced transgenic mice expressing a dominantly interfering mutant of FADD, lacking the caspase-dimerizing death effector domain, as well as mice overexpressing the poxvirus serpin, CrmA, an inhibitor of caspases downstream of FADD. While thymocytes from either line of mice were completely protected from CD95-dependent cytotoxicity, neither transgene afforded protection from apoptosis induced during thymocyte selection and neither led to the lymphoproliferative disorders associated with deficiencies in CD95. However, in FADD dominant negative (FADDdd) mice, early thymocyte development was retarded and peripheral lymphocyte pools were devoid of normal populations of T cells. We show that thymocytes and peripheral T cells from FADDdd display signaling anomalies, implying that FADD plays a previously uncharacterized role in T cell development and activation.

Herpesvirus saimiri transforms human T-cell clones to stable growth without inducing resistance to apoptosis

Herpesvirus saimiri (HVS) transforms human T cells to stable growth in vitro. Since HVS codes for two different antiapoptotic proteins, growth transformation by HVS might be expected to confer resistance to apoptosis. We found that the expression of both viral antiapoptotic genes was restricted to cultures with viral replication and absent in growth-transformed human T cells. A comparative examination of HVS-transformed T-cell clones and their native parental clones revealed that the expression of Bcl-2, Bcl-X(L), Bax, and members of the tumor necrosis factor receptor (TNF-R) superfamily with a death domain, namely, TNF-RI, CD95, and TRAMP, were not modulated by HVS. Expression of CD30 was induced in HVS-transformed T cells, and these cells also expressed the CD30 ligand. Uninfected and transformed T cells were sensitive to CD95 ligation but resistant to apoptosis mediated by TRAIL or soluble TNF-alpha. CD95 ligand was constitutively expressed on transformed but not uninfected parental T cells. Both cell types showed similar sensitivity to cell death induction or inhibition of T-cell activation mediated by irradiation, oxygen radicals, dexamethasone, cyclosporine, and prostaglandin E2. Altogether, this study strongly suggests that growth transformation by HVS is based not on resistance to apoptosis but, rather, on utilization of normal cellular activation pathways.

Synthetic activation of caspases: artificial death switches

The development of safe vectors for gene therapy requires fail-safe mechanisms to terminate therapy or remove genetically altered cells. The ideal "suicide switch" would be nonimmunogenic and nontoxic when uninduced and able to trigger cell death independent of tissue type or cell cycle stage. By using chemically induced dimerization, we have developed powerful death switches based on the cysteine proteases, caspase-1 ICE (interleukin-1beta converting enzyme) and caspase-3 YAMA. In both cases, aggregation of the target protein is achieved by a nontoxic lipid-permeable dimeric FK506 analog that binds to the attached FK506-binding proteins, FKBPs. We find that intracellular cross-linking of caspase-1 or caspase-3 is sufficient to trigger rapid apoptosis in a Bcl-xL-independent manner, suggesting that these conditional proapoptotic molecules can bypass intracellular checkpoint genes, such as Bcl-xL, that limit apoptosis. Because these chimeric molecules are derived from autologous proteins, they should be nonimmunogenic and thus ideal for long-lived gene therapy vectors. These properties should also make chemically induced apoptosis useful for developmental studies, for treating hyperproliferative disorders, and for developing animal models to a wide variety of diseases.

FADD: essential for embryo development and signaling from some, but not all, inducers of apoptosis

FADD (also known as Mort-1) is a signal transducer downstream of cell death receptor CD95 (also called Fas). CD95, tumor necrosis factor receptor type 1 (TNFR-1), and death receptor 3 (DR3) did not induce apoptosis in FADD-deficient embryonic fibroblasts, whereas DR4, oncogenes E1A and c-myc, and chemotherapeutic agent adriamycin did. Mice with a deletion in the FADD gene did not survive beyond day 11.5 of embryogenesis; these mice showed signs of cardiac failure and abdominal hemorrhage. Chimeric embryos showing a high contribution of FADD null mutant cells to the heart reproduce the phenotype of FADD-deficient mutants. Thus, not only death receptors, but also receptors that couple to developmental programs, may use FADD for signaling.

TRUNDD, a new member of the TRAIL receptor family that antagonizes TRAIL signalling

TRAIL/Apo-2L induces rapid apoptosis of a variety of tumor cell lines. A family of tumor necrosis factor receptor-related molecules have been identified as receptors for TRAIL. Herein, we report the identification of another member of the TRAIL receptor family, TRUNDD (TRAIL receptor with a truncated death domain). The TRUNDD transcript was detected in multiple human tissues. TRUNDD is highly homologous to all known TRAIL receptors and has an extracellular TRAIL-binding domain but lacks a functional intracellular death domain and does not induce apoptosis. Consistent with an inhibitory role, ectopic expression of TRUNDD attenuated TRAIL-induced apoptosis in mammalian cells.

Caspase-9, Bcl-XL, and Apaf-1 form a ternary complex

Genetic analysis of apoptosis in the nematode Caenorhabditis elegans has revealed the cell death machine to be composed of three core interacting components. CED-4 (equivalent to mammalian Apaf-1) is a nucleotide binding molecule that complexes with the zymogen form of the death protease CED-3, leading to its autoactivation and cell death. CED-9 blocks death by complexing with CED-4 and attenuating its ability to promote CED-3 activation. An equivalent ternary complex was found to be present in mammalian cells involving Apaf-1, the mammalian death protease caspase-9, and Bcl-XL, an anti-apoptotic member of the Bcl-2 family. Consistent with a central role for caspase-9, a dominant negative form effectively inhibited cell death initiated by a wide variety of inducers.

Tumor necrosis factor in the heart

The heart is a tumor necrosis factor (TNF)-producing organ. Both myocardial macrophages and cardiac myocytes themselves synthesize TNF. Accumulating evidence indicates that myocardial TNF is an autocrine contributor to myocardial dysfunction and cardiomyocyte death in ischemia-reperfusion injury, sepsis, chronic heart failure, viral myocarditis, and cardiac allograft rejection. Indeed, locally (vs. systemically) produced TNF contributes to postischemic myocardial dysfunction via direct depression of contractility and induction of myocyte apoptosis. Lipopolysaccharide or ischemia-reperfusion activates myocardial P38 mitogen-activated protein (MAP) kinase and nuclear factor kappa B, which lead to TNF production. TNF depresses myocardial function by nitric oxide (NO)-dependent and NO-independent (sphingosine dependent) mechanisms. TNF activation of TNF receptor 1 or Fas may induce cardiac myocyte apoptosis. MAP kinases and TNF transcription factors are feasible targets for anti-TNF (i.e., cardioprotective) strategies. Endogenous anti-inflammatory ligands, which trigger the gp130 signaling cascade, heat shock proteins, and TNF-binding proteins, also control TNF production and activity. Thus modulation of TNF in cardiovascular disease represents a realistic goal for clinical medicine.

Fas- and perforin-independent mechanism of cytotoxic T lymphocyte

Cytotoxic T lymphocytes (CTLs) play an important role in elimination of virus-infected cells (1). Recent studies revealed at least two distinct mechanisms that CTLs utilize to destroy their target cells. Both mechanisms induce target cell apoptosis specifically and directionally, but these processes are totally different. One is pore formation on target cell membrane by perforin secreted from CTLs (perforin-granzyme pathway), and the other is ligation of Fas, which is expressed on the surface of target cells and Fas ligand, on the surface of CTLs (Fas-FasL pathway) (2). Here we review our work and describe CTL clones that have novel lytic mechanisms derived from CD4-CD8- lymph node cells of gld mice.

Characterization of structural domains of human osteoclastogenesis inhibitory factor

Osteoclastogenesis inhibitory factor (OCIF) is a heparin-binding secretory glycoprotein that belongs to the tumor necrosis factor receptor (TNFR) family. OCIF is present both as a approximately 60-kDa monomer and a disulfide-linked homodimer. We attempted to characterize the seven structural domains of OCIF by determining the capabilities of various OCIF mutants to inhibit osteoclastogenesis, to interact with heparin, and to form dimers. We also examined a potential of domains 5 and 6, death domain homologous regions (DDHs), for inducing cell death by expressing OCIF/Fas fusion proteins. Our results show that: (i) the N-terminal portion of OCIF containing domains 1-4, which have structural similarity to the extracellular domains of the TNFR family proteins, is sufficient to inhibit osteoclastogenesis; (ii) a heparin-binding site is located in domain 7, and affinity for heparin does not correlate with the inhibitory activity; (iii) Cys-400 in domain 7 is the residue responsible for dimer formation; and (iv) the C-terminal portion containing domains 5 and 6, DDHs, has a high potential for mediating a cytotoxic signal when it is expressed in cells as an OCIF/Fas fusion protein in which the transmembrane region of Fas is inserted in front of DDHs.