Fas ligand, Bcl-2, granulocyte colony-stimulating factor, and p38 mitogen-activated protein kinase: Regulators of distinct cell death and survival pathways in granulocytes.

The short life span of granulocytes, which limits many inflammatory responses, is thought to be influenced by the Bcl-2 protein family, death receptors such as CD95 (Fas/APO-1), stress-activated protein kinases such as p38 mitogen-activated protein kinase (MAPK), and proinflammatory cytokines like granulocyte colony-stimulating factor (G-CSF). To clarify the roles of these various regulators in granulocyte survival, we have investigated the spontaneous apoptosis of granulocytes in culture and that induced by Fas ligand or chemotherapeutic drugs, using cells from normal, CD95-deficient lpr, or vav-bcl-2 transgenic mice. CD95-induced apoptosis, which required receptor aggregation by recombinant Fas ligand or the membrane-bound ligand, was unaffected by G-CSF treatment or Bcl-2 overexpression. Conversely, spontaneous and drug-induced apoptosis occurred normally in lpr granulocytes but were suppressed by G-CSF treatment or Bcl-2 overexpression. Although activation of p38 MAPK has been implicated in granulocyte death, their apoptosis actually was markedly accelerated by specific inhibitors of this kinase. These results suggest that G-CSF promotes granulocyte survival largely through the Bcl-2-controlled pathway, whereas CD95 regulates a distinct pathway to apoptosis that is not required for either their spontaneous or drug-induced death. Moreover, p38 MAPK signaling contributes to granulocyte survival rather than their apoptosis.

Conversion of membrane-bound Fas(CD95) ligand to its soluble form is associated with downregulation of its proapoptotic activity and loss of liver toxicity.

Human Fas ligand (L) (CD95L) and tumor necrosis factor (TNF)-alpha undergo metalloproteinase-mediated proteolytic processing in their extracellular domains resulting in the release of soluble trimeric ligands (soluble [s]FasL, sTNF-alpha) which, in the case of sFasL, is thought to be implicated in diseases such as hepatitis and AIDS. Here we show that the processing of sFasL occurs between Ser126 and Leu127. The apoptotic-inducing capacity of naturally processed sFasL was reduced by >1,000-fold compared with membrane-bound FasL, and injection of high doses of recombinant sFasL in mice did not induce liver failure. However, soluble FasL retained its capacity to interact with Fas, and restoration of its cytotoxic activity was achieved both in vitro and in vivo with the addition of cross-linking antibodies. Similarly, the marginal apoptotic activity of recombinant soluble TNF-related apoptosis-inducing ligand (sTRAIL), another member of the TNF ligand family, was greatly increased upon cross-linking. These results indicate that the mere trimerization of the Fas and TRAIL receptors may not be sufficient to trigger death signals. Thus, the observation that sFasL is less cytotoxic than membrane-bound FasL may explain why in certain types of cancer, systemic tissue damage is not detected, even though the levels of circulating sFasL are high.

APRIL, a new ligand of the tumor necrosis factor family, stimulates tumor cell growth.

Members of the tumor necrosis factor (TNF) family induce pleiotropic biological responses, including cell growth, differentiation, and even death. Here we describe a novel member of the TNF family designated APRIL (for a proliferation-inducing ligand). Although transcripts of APRIL are of low abundance in normal tissues, high levels of mRNA are detected in transformed cell lines, and in human cancers of colon, thyroid, and lymphoid tissues in vivo. The addition of recombinant APRIL to various tumor cells stimulates their proliferation. Moreover, APRIL-transfected NIH-3T3 cells show an increased rate of tumor growth in nude mice compared with the parental cell line. These findings suggest that APRIL may be implicated in the regulation of tumor cell growth.

A soluble form of B cell maturation antigen, a receptor for the tumor necrosis factor family member APRIL, inhibits tumor cell growth.

A proliferation-inducing ligand (APRIL) is a ligand of the tumor necrosis factor (TNF) family that stimulates tumor cell growth in vitro and in vivo. Expression of APRIL is highly upregulated in many tumors including colon and prostate carcinomas. Here we identify B cell maturation antigen (BCMA) and transmembrane activator and calcium modulator and cyclophilin ligand (CAML) interactor (TACI), two predicted members of the TNF receptor family, as receptors for APRIL. APRIL binds BCMA with higher affinity than TACI. A soluble form of BCMA, which inhibits the proliferative activity of APRIL in vitro, decreases tumor cell proliferation in nude mice. Growth of HT29 colon carcinoma cells is blocked when mice are treated once per week with the soluble receptor. These results suggest an important role for APRIL in tumorigenesis and point towards a novel anticancer strategy.

BAFF, a novel ligand of the tumor necrosis factor family, stimulates B cell growth.

Members of the tumor necrosis factor (TNF) family induce pleiotropic biological responses, including cell growth, differentiation, and even death. Here we describe a novel member of the TNF family, designated BAFF (for B cell activating factor belonging to the TNF family), which is expressed by T cells and dendritic cells. Human BAFF was mapped to chromosome 13q32-34. Membrane-bound BAFF was processed and secreted through the action of a protease whose specificity matches that of the furin family of proprotein convertases. The expression of BAFF receptor appeared to be restricted to B cells. Both membrane-bound and soluble BAFF induced proliferation of anti-immunoglobulin M-stimulated peripheral blood B lymphocytes. Moreover, increased amounts of immunoglobulins were found in supernatants of germinal center-like B cells costimulated with BAFF. These results suggest that BAFF plays an important role as costimulator of B cell proliferation and function.

Inhibition of toxic epidermal necrolysis by blockade of CD95 with human intravenous immunoglobulin.

Toxic epidermal necrolysis (TEN, Lyell's syndrome) is a severe adverse drug reaction in which keratinocytes die and large sections of epidermis separate from the dermis. Keratinocytes normally express the death receptor Fas (CD95); those from TEN patients were found to express lytically active Fas ligand (FasL). Antibodies present in pooled human intravenous immunoglobulins (IVIG) blocked Fas-mediated keratinocyte death in vitro. In a pilot study, 10 consecutive individuals with clinically and histologically confirmed TEN were treated with IVIG; disease progression was rapidly reversed and the outcome was favorable in all cases. Thus, Fas-FasL interactions are directly involved in the epidermal necrolysis of TEN, and IVIG may be an effective treatment.

The caspase-8 inhibitor FLIP promotes activation of NF-kappaB and Erk signaling pathways.

BACKGROUND: Activation of Fas (CD95) by its ligand (FasL) rapidly induces cell death through recruitment and activation of caspase-8 via the adaptor protein Fas-associated death domain protein (FADD). However, Fas signals do not always result in apoptosis but can also trigger a pathway that leads to proliferation. We investigated the level at which the two conflicting Fas signals diverge and the protein(s) that are implicated in switching the response. RESULTS: Under conditions in which proliferation of CD3-activated human T lymphocytes is increased by recombinant FasL, there was activation of the transcription factors NF-kappaB and AP-1 and recruitment of the caspase-8 inhibitor and FADD-interacting protein FLIP (FLICE-like inhibitory protein). Fas-recruited FLIP interacts with TNF-receptor associated factors 1 and 2, as well as with the kinases RIP and Raf-1, resulting in the activation of the NF-kappaB and extracellular signal regulated kinase (Erk) signaling pathways. In T cells these two signal pathways are critical for interleukin-2 production. Increased expression of FLIP in T cells resulted in increased production of interleukin-2. CONCLUSIONS: We provide evidence that FLIP is not simply an inhibitor of death-receptor-induced apoptosis but that it also mediates the activation of NF-kappaB and Erk by virtue of its capacity to recruit adaptor proteins involved in these signaling pathways.

Activation of a pro-apoptotic amplification loop through inhibition of NF-kappaB-dependent survival signals by caspase-mediated inactivation of RIP.

Death domain containing members of the tumor necrosis factor receptor (TNFR) superfamily can induce apoptosis or cell activation. However, the mechanisms by which these opposing programs are selected remain unclear. Frequently, NF-kappaB activation conveys protection against cell death. We show that the serine/threonine kinase RIP that is required for TNF-induced NF-kappaB activation is processed by caspase-8 into a dominant-negative (DN) fragment during death receptor-induced apoptosis, thereby leading to a blockade of NF-kappaB-mediated anti-apoptotic signals. Our results suggest that cleavage of RIP is part of an amplification loop which is triggered by Fas and most likely by other death receptors.

Characterization of two receptors for TRAIL.

Two receptors for TRAIL, designated TRAIL-R2 and TRAIL-R3, have been identified. Both are members of the tumor necrosis factor receptor family. TRAIL-R2 is structurally similar to the death-domain-containing receptor TRAIL-R1 (DR-4), and is capable of inducing apoptosis. In contrast, TRAIL-R3 does not promote cell death. TRAIL-R3 is highly glycosylated and is membrane bound via a putative phosphatidylinositol anchor. The extended structure of TRAIL-R3 is due to the presence of multiple threonine-, alanine-, proline- and glutamine-rich repeats (TAPE repeats). TRAIL-R2 shows a broad tissue distribution, whereas the expression of TRAIL-R3 is restricted to peripheral blood lymphocytes (PBLs) and skeletal muscle. All three TRAIL receptors bind TRAIL with similar affinity, suggesting a complex regulation of TRAIL-mediated signals.

Development of improved soluble inhibitors of FasL and CD40L based on oligomerized receptors.

TNF receptor family members fused to the constant domain of immunoglobulin G have been widely used as immunoadhesins in basic in vitro and in vivo research and in some clinical applications. In this study, we assemble soluble, high avidity chimeric receptors on a pentameric scaffold derived from the coiled-coil domain of cartilage oligomeric matrix protein (COMP). The affinity of Fas and CD40 (but not TNFR-1 and TRAIL-R2) to their ligands is increased by fusion to COMP, when compared to the respective Fc chimeras. In functional assays, Fas:COMP was at least 20-fold more active than Fas:Fc at inhibiting the action of sFasL, and CD40:COMP could block CD40L-mediated proliferation of B cells, whereas CD40:Fc could not. In conclusion, members of the TNF receptor family can display high specificity and excellent avidity for their ligands if they are adequately multimerized.

Bcl-rambo, a novel Bcl-2 homologue that induces apoptosis via its unique C-terminal extension.

The Bcl-2 family of proteins plays a central regulatory role in apoptosis. We have identified a novel, widely expressed Bcl-2 member which we have named Bcl-rambo. Bcl-rambo shows overall structural homology to the anti-apoptotic Bcl-2 members containing conserved Bcl-2 homology (BH) motifs 1, 2, 3, and 4. Unlike Bcl-2, however, the C-terminal membrane anchor region is preceded by a unique 250 amino acid insertion containing two tandem repeats. No interaction of Bcl-rambo with either anti-apoptotic (Bcl-2, Bcl-x(L), Bcl-w, A1, MCL-1, E1B-19K, and BHRF1) or pro-apoptotic (Bax, Bak, Bik, Bid, Bim, and Bad) members of the Bcl-2 family was observed. In mammalian cells, Bcl-rambo was localized to mitochondria, and its overexpression induces apoptosis that is specifically blocked by the caspase inhibitors, IAPs, whereas inhibitors controlling upstream events of either the 'death receptor' (FLIP, FADD-DN) or the 'mitochondrial' pro-apoptotic pathway (Bcl-x(L)) had no effect. Surprisingly, the Bcl-rambo cell death activity was induced by its membrane-anchored C-terminal domain and not by the Bcl-2 homology region. Thus, Bcl-rambo constitutes a novel type of pro-apoptotic Bcl-2 member that triggers cell death independently of its BH motifs.

Fas ligand-induced c-Jun kinase activation in lymphoid cells requires extensive receptor aggregation but is independent of DAXX, and Fas-mediated cell death does not involve DAXX, RIP, or RAIDD.

Jun kinase signaling can be elicited by death receptor activation, but the mechanism and significance of this event are still unclear. It has been reported that cross-linking Abs to Fas trigger c-Jun N-terminal kinase (JNK) signaling via caspase-mediated activation of MEKK1 (JNK kinase kinase), elevation of ceramide levels or by recruitment of death domain associated protein (DAXX) to Fas. The effect of physiological ligand for Fas on JNK signaling was never investigated, although evidence is accumulating that Fas ligand is able to induce cellular responses distinct from those evoked by Ab-mediated cross-linking of Fas. Therefore, we investigated the effect of Fas ligand on JNK signaling. Like its ability to induce cell death, Fas ligand reliably activated JNK only upon extensive aggregation of the receptor. Although this was partially dependent on caspase activation, DAXX was not required. DAXX and other death receptor-associated proteins, which have been reported to bind directly or indirectly to Fas, such as receptor interacting protein (RIP) and RIP-associated ICH-1/CED-3-homologous protein with a death domain (RAIDD), were shown to be dispensable for Fas ligand-induced apoptosis.

TRAIL receptors 1 (DR4) and 2 (DR5) signal FADD-dependent apoptosis and activate NF-kappaB.

TRAIL induces apoptosis through two closely related receptors, TRAIL-R1 (DR4) and TRAIL-R2 (DR5). Here we show that TRAIL-R1 can associate with TRAIL-R2, suggesting that TRAIL may signal through heteroreceptor signaling complexes. Both TRAIL receptors bind the adaptor molecules FADD and TRADD, and both death signals are interrupted by a dominant negative form of FADD and by the FLICE-inhibitory protein FLIP. The recruitment of TRADD may explain the potent activation of NF-kappaB observed by TRAIL receptors. Thus, TRAIL receptors can signal both death and gene transcription, functions reminiscent of those of TNFR1 and TRAMP, two other members of the death receptor family.

Characterization of Fas (Apo-1, CD95)-Fas ligand interaction.

The death-inducing receptor Fas is activated when cross-linked by the type II membrane protein Fas ligand (FasL). When human soluble FasL (sFasL, containing the extracellular portion) was expressed in human embryo kidney 293 cells, the three N-linked glycans of each FasL monomer were found to be essential for efficient secretion. Based on the structure of the closely related lymphotoxin alpha-tumor necrosis factor receptor I complex, a molecular model of the FasL homotrimer bound to three Fas molecules was generated using knowledge-based protein modeling methods. Point mutations of amino acid residues predicted to affect the receptor-ligand interaction were introduced at three sites. The F275L mutant, mimicking the loss of function murine gld mutation, exhibited a high propensity for aggregation and was unable to bind to Fas. Mutants P206R, P206D, and P206F displayed reduced cytotoxicity toward Fas-positive cells with a concomitant decrease in the binding affinity for the recombinant Fas-immunoglobulin Fc fusion proteins. Although the cytotoxic activity of mutant Y218D was unaltered, mutant Y218R was inactive, correlating with the prediction that Tyr-218 of FasL interacts with a cluster of three basic amino acid side chains of Fas. Interestingly, mutant Y218F could induce apoptosis in murine, but not human cells.

Ceramide enables fas to cap and kill.

Recent studies suggest that trimerization of Fas is insufficient for apoptosis induction and indicate that super-aggregation of trimerized Fas might be prerequisite. For many cell surface receptors, cross-linking by multivalent ligands or antibodies induces their lateral segregation within the plasma membrane and co-localization into "caps" on one pole of the cell. In this study, we show that capping of Fas is essential for optimal function and that capping is ceramide-dependent. In Jurkat T lymphocytes and in primary cultures of hepatocytes, ceramide elevation was detected as early as 15-30 s and peaked at 1 min after CH-11 and Jo2 anti-Fas antibody treatment, respectively. Capping was detected 30 s after Fas ligation, peaked at 2 min, and was maintained at a lower level for as long as 30 min in both cell types. Ceramide generation appeared essential for capping. Acid sphingomyelinase -/- hepatocytes were defective in Jo2-induced ceramide generation, capping, and apoptosis, and nanomolar concentrations of C(16)-ceramide restored these events. To further explore the role of ceramide in capping of Fas, we employed FLAG-tagged soluble Fas ligand (sFasL), which binds trimerized Fas but is unable to induce capping or apoptosis in Jurkat cells. Cross-linking of sFasL with M2 anti-FLAG antibody induced both events. Pretreatment of cells with natural C(16)-ceramide bypassed the necessity for forced antibody cross-linking and enabled sFasL to cap and kill. The presence of intact sphingolipid-enriched membrane domains may be essential for Fas capping since their disruption with cholesterol-depleting agents abrogated capping and prevented apoptosis. These data suggest that capping is a ceramide-dependent event required for optimal Fas signaling in some cells.

IgG subclass switch capacity is low in switched and in IgM-only, but high in IgD+IgM+, post-germinal center (CD27+) human B cells.

Recent studies have shown that in humans the germinal center reactions produce three types of V(D)J mutated B cells in similar proportions, i.e. Ig-switched, IgD-IgM+ (IgM-only) and IgD+IgM+ cells, and that together they form the CD27+ compartment of recirculating B cells. We investigated the Ig isotype switch capacity of these cells. Peripheral blood B subsets were sorted and IgG subclass secretion in presence or absence of IL-4 was compared in B cell assays which lead to Ig secretion in all (coculture with EL-4 thymoma cells) or only in CD27+ (CD40L stimulation) B cells. Already switched IgG+ B cells showed no significant sequential switch and IgM-only cells also had a low switch capacity, but IgD+CD27+ switched as much as IgD+CD27- B cells to all IgG subclasses. Thus, in switched B cells some alterations compromising further switch options occur frequently; IgM-only cells may result from aborted switch. However, IgD+CD27+ human B cells, extensively V(D)J mutated and "naive" regarding switch, build up a repertoire of B cells combining (1) novel cross-reactive specificities, (2) increased differentiation capacity (including after T-independent stimulation by Staphylococcus aureus Cowan I) and (3) the capacity to produce appropriate isotypes when they respond to novel pathogens.

Fas triggers an alternative, caspase-8-independent cell death pathway using the kinase RIP as effector molecule.

Cell death is achieved by two fundamentally different mechanisms: apoptosis and necrosis. Apoptosis is dependent on caspase activation, whereas the caspase-independent necrotic signaling pathway remains largely uncharacterized. We show here that Fas kills activated primary T cells efficiently in the absence of active caspases, which results in necrotic morphological changes and late mitochondrial damage but no cytochrome c release. This Fas ligand-induced caspase-independent death is absent in T cells that are deficient in either Fas-associated death domain (FADD) or receptor-interacting protein (RIP). RIP is also required for necrotic death induced by tumor necrosis factor (TNF) and TNF-related apoptosis-inducing ligand (TRAIL). In contrast to its role in nuclear factor kappa B activation, RIP requires its own kinase activity for death signaling. Thus, Fas, TRAIL and TNF receptors can initiate cell death by two alternative pathways, one relying on caspase-8 and the other dependent on the kinase RIP.