Identification of amino acid residues important for ligand binding to Fas

CD95/Fas stoichiometry in future precision medicine

CD95, also known as Fas, belongs to the tumor necrosis factor (TNF) receptor superfamily. The main biological function of this receptor is to orchestrate and control the immune response since mutations in CD95 or deregulation of its downstream signaling pathways lead to auto-immunity and inflammation. Interestingly, more than twenty years ago, pioneer studies highlighted that like TNFR1, TRAILR1 or CD40, CD95 pre-associates at the plasma membrane in a ligand-independent fashion. This self-association occurs through a domain designated pre-ligand assembly domain or PLAD. Although the disruption of this pre-association prevents CD95 signaling, no drugs targeting this region have been generated because many questions remain on the stoichiometry and conformation of this receptor. Despite more than 40.000 publications, no crystal structure of CD95 alone or in combination with its ligand, CD95L, exists. Based on other TNFR members, we herein discuss the predicted conformation of CD95 at the plasma membrane and how these putative structures might account for the induction of the cell signaling pathways.

Molecular Docking of Lactoferrin with Apoptosis-Related Proteins Insights into Its Anticancer Mechanism

Human Lactoferrin (hLf), a multifunctional glycoprotein, has been analyzed through molecular docking to evaluate its role in apoptosis regulation and its potential as an anticancer agent. The docking results highlight XIAP (X-linked Inhibitor of Apoptosis Protein) and Caspase-3 as the most reliable targets, where hLf disrupts XIAP's inhibition of Caspase-3 and Caspase-9, potentially restoring apoptotic signaling; hLf also stabilizes Caspase-3, enhancing its activation in intrinsic and extrinsic pathways. Weaker interactions were observed with Fas, Bcl-2, and Akt. hLf's role in Fas signaling is likely due to expression upregulation rather than direct binding. In contrast, its binding to Bcl-2 may disrupt anti-apoptotic function, and its interaction with Akt suggests interference with pro-survival signaling. These findings suggest that hLf may promote apoptosis by enhancing caspase activation and modulating key apoptotic regulators, supporting its potential use in cancer treatment. However, further experimental validation is needed to confirm these interactions and their therapeutic implications.

The Crosstalk of Apoptotic and Non-Apoptotic Signaling in CD95 System

The mechanisms of CD95 (Fas/APO-1)-mediated extrinsic apoptotic pathway in cancer cells have been extensively studied. The majority of human cells express CD95, but not all these cells can induce extrinsic apoptosis. Accumulating evidence has shown that CD95 is a multifunctional protein, and its stimulation can also elicit non-apoptotic or even survival signals. It has become clear that under certain cellular contexts, due to the various checkpoints, CD95 activation can trigger both apoptotic and non-apoptotic signals. The crosstalk of death and survival signals may occur at different levels of signal transduction. The strength of the CD95 stimulation, initial levels of anti-apoptotic proteins, and posttranslational modifications of the core DISC components have been proposed to be the most important factors in the life/death decisions at CD95. Successful therapeutic targeting of CD95 signaling pathways will require a better understanding of the crosstalk between CD95-induced apoptotic and cell survival pathways. In this review, in order to gain a systematic understanding of the crosstalk between CD95-mediated apoptosis and non-apoptotic signaling, we will discuss these issues in a step-by-step way.

Advanced multiparametric image spectroscopy and super-resolution microscopy reveal a minimal model of CD95 signal initiation

Unraveling the concentration-dependent spatiotemporal organization of receptors in the plasma membrane is crucial to understand cell signal initiation. A paradigm of this process is the oligomerization of CD95 during apoptosis signaling, with different oligomerization models being discussed. Here, we establish the molecular-sensitive approach cell lifetime Förster resonance energy transfer image spectroscopy to determine CD95 configurations in live cells. These data are corroborated by stimulated emission depletion microscopy, confocal photobleaching step analysis, and fluorescence correlation spectroscopy. We probed CD95 interactions for concentrations of ~10 to 1000 molecules per square micrometer, over nanoseconds to hours, and molecular to cellular scales. Quantitative benchmarking was achieved establishing high-fidelity monomer and dimer controls. While CD95 alone is primarily monomeric (~96%) and dimeric (4%), the addition of ligand induces oligomerization to dimers/trimers (~15%) leading to cell death. This study highlights molecular concentration effects and oligomerization dynamics. It reveals a minimal model, where small CD95 oligomers suffice to efficiently initiate signaling.

Role of metalloproteases in the CD95 signaling pathways

CD95L (also known as FasL or CD178) is a member of the tumor necrosis family (TNF) superfamily. Although this transmembrane ligand has been mainly considered as a potent apoptotic inducer in CD95 (Fas)-expressing cells, more recent studies pointed out its role in the implementation of non-apoptotic signals. Accordingly, this ligand has been associated with the aggravation of inflammation in different auto-immune disorders and in the metastatic occurrence in different cancers. Although it remains to decipher all key factors involved in the ambivalent role of this ligand, accumulating clues suggest that while the membrane bound CD95L triggers apoptosis, its soluble counterpart generated by metalloprotease-driven cleavage is responsible for its non-apoptotic functions. Nonetheless, the metalloproteases (MMPs and ADAMs) involved in the CD95L shedding, the cleavage sites and the different stoichiometries and functions of the soluble CD95L remain to be elucidated. To better understand how soluble CD95L triggers signaling pathways from apoptosis to inflammation or cell migration, we propose herein to summarize the different metalloproteases that have been described to be able to shed CD95L, their cleavage sites and the biological functions associated with the released ligands. Based on these new findings, the development of CD95/CD95L-targeting therapeutics is also discussed.

Secreted Fas Decoys Enhance the Antitumor Activity of Engineered and Bystander T Cells in Fas Ligand-Expressing Solid Tumors

T-cell immunotherapy has demonstrated remarkable clinical outcomes in certain hematologic malignancies. However, efficacy in solid tumors has been suboptimal, partially due to the hostile tumor microenvironment composed of immune-inhibitory molecules. One such suppressive agent abundantly expressed in solid tumors is Fas ligand (FasL), which can trigger apoptosis of Fas-expressing effector cells such as T cells and natural killer (NK) cells. To alleviate this FasL-induced suppression of tumor-specific immune cells in solid tumors, we describe here the development of a Fas decoy that is secreted by engineered cells upon activation and sequesters the ligand, preventing it from engaging with Fas on the surface of effector cells. We further improved the immune-stimulatory effects of this approach by creating a Fas decoy and IL15 cytokine fusion protein, which enhanced the persistence and antitumor activity of decoy-engineered as well as bystander chimeric-antigen receptor (CAR) T cells in xenograft models of pancreatic cancer. Our data indicate that secreted Fas decoys can augment the efficacy of both adoptively transferred and endogenous tumor-specific effector cells in FasL-expressing solid tumors.

Pro- and anti-apoptotic fate decisions induced by di- and trimeric synthetic cytokine receptors

Synthetic strategies to activate cytokine receptors so far only address standard dimeric cytokine receptor assemblies. The 19 ligands of the tumor necrosis factor superfamily (TNFSF), however, form noncovalent trimers and receptor trimerization is considered to be essential for receptor activation. Synthetic TNFR1, TNFR2, and Fas/CD95 receptors were activated by synthetic trimeric ligands which induced NF-κB signaling or Caspase-induced apoptosis. Albeit dimeric receptor activation did not induce synthetic TNFR1 and TNFR2 signaling, dimeric FasL induced extenuated apoptosis. Simultaneous integration of dimeric Interleukin (IL-)6 receptor gp130 and trimeric Fas as synthetic cytokine receptors in one cell enabled binary cell fate decisions, gp130-mediated proliferation or Fas-mediated apoptosis. In summary, our modular fully synthetic cytokine signaling system allows precisely orchestrated cellular responses to selectively induce pro- and anti-apoptotic signaling via canonical dimeric receptors of the IL-6 family and non-canonical trimeric receptor complexes of the TNF superfamily.

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SyCyRs induce TNFR1 or TNFR2 mediated NF-κB activation as trimers or oligomers.

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Fas-SyCyR induces Caspase-induced apoptosis as trimer and as dimer.

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Synthetic loss of function Fas-SyCyR fails to induce Caspase mediated apoptosis.

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gp130-and Fas-SyCyR in one cell enable proliferation via gp130 or apoptosis via Fas.

Fundamental Mechanisms of Regulated Cell Death and Implications for Heart Disease

Twelve regulated cell death programs have been described. We review in detail the basic biology of nine including death receptor-mediated apoptosis, death receptor-mediated necrosis (necroptosis), mitochondrial-mediated apoptosis, mitochondrial-mediated necrosis, autophagy-dependent cell death, ferroptosis, pyroptosis, parthanatos, and immunogenic cell death. This is followed by a dissection of the roles of these cell death programs in the major cardiac syndromes: myocardial infarction and heart failure. The most important conclusion relevant to heart disease is that regulated forms of cardiomyocyte death play important roles in both myocardial infarction with reperfusion (ischemia/reperfusion) and heart failure. While a role for apoptosis in ischemia/reperfusion cannot be excluded, regulated forms of necrosis, through both death receptor and mitochondrial pathways, are critical. Ferroptosis and parthanatos are also likely important in ischemia/reperfusion, although it is unclear if these entities are functioning as independent death programs or as amplification mechanisms for necrotic cell death. Pyroptosis may also contribute to ischemia/reperfusion injury, but potentially through effects in non-cardiomyocytes. Cardiomyocyte loss through apoptosis and necrosis is also an important component in the pathogenesis of heart failure and is mediated by both death receptor and mitochondrial signaling. Roles for immunogenic cell death in cardiac disease remain to be defined but merit study in this era of immune checkpoint cancer therapy. Biology-based approaches to inhibit cell death in the various cardiac syndromes are also discussed.

Platelet-RBC interaction mediated by FasL/FasR induces procoagulant activity important for thrombosis

Red blood cells (RBCs) influence rheology, and release ADP, ATP, and nitric oxide, suggesting a role for RBCs in hemostasis and thrombosis. Here, we provide evidence for a significant contribution of RBCs to thrombus formation. Anemic mice showed enhanced occlusion times upon injury of the carotid artery. A small population of RBCs was located to platelet thrombi and enhanced platelet activation by a direct cell contact via the FasL/FasR (CD95) pathway known to induce apoptosis. Activation of platelets in the presence of RBCs led to platelet FasL exposure that activated FasR on RBCs responsible for externalization of phosphatidylserine (PS) on the RBC membrane. Inhibition or genetic deletion of either FasL or FasR resulted in reduced PS exposure of RBCs and platelets, decreased thrombin generation, and reduced thrombus formation in vitro and protection against arterial thrombosis in vivo. Direct cell contacts between platelets and RBCs via FasL/FasR were shown after ligation of the inferior vena cava (IVC) and in surgical specimens of patients after thrombectomy. In a flow restriction model of the IVC, reduced thrombus formation was observed in FasL-/- mice. Taken together, our data reveal a significant contribution of RBCs to thrombosis by the FasL/FasR pathway.

Crystal Structure of the Complex of Human FasL and Its Decoy Receptor DcR3

The apoptotic effect of FasL:Fas signaling is disrupted by DcR3, a unique secreted member of the tumor necrosis factor receptor superfamily, which also binds and neutralizes TL1A and LIGHT. DcR3 is highly elevated in patients with various tumors and contributes to mechanisms by which tumor cells to evade host immune surveillance. Here we report the crystal structure of FasL in complex with DcR3. Comparison of FasL:DcR3 structure with our earlier TL1A:DcR3 and LIGHT:DcR3 structures supports a paradigm involving the recognition of invariant main-chain and conserved side-chain functionalities, which is responsible for the recognition of multiple TNF ligands exhibited by DcR3. The FasL:DcR3 structure also provides insight into the FasL:Fas recognition surface. We demonstrate that the ability of recombinant FasL to induce Jurkat cell apoptosis is significantly enhanced by native glycosylation or by structure-inspired mutations, both of which result in reduced tendency to aggregate. All of these activities are efficiently inhibited by recombinant DcR3.

Neutralization of the CD95 ligand by APG101 inhibits invasion of glioma cells in vitro

Supplemental Digital Content is available in the text.

Glioblastoma is a disease characterized by rapid invasive tumour growth. Studies on the proapoptotic CD95/CD95L signalling pathway recently suggested a significant contribution of CD95 signalling towards the high degree of motility in glioma cells. Apogenix has developed APG101, a clinical phase II compound designed to bind and neutralize CD95L, and thus to interfere with CD95/CD95L-based signalling. APG101 has shown clinical efficacy in a controlled randomized phase II trial in patients with recurrent glioma. Because APG101 is not cytotoxic to tumour cells in vitro, we postulated that the anti-invasive function of APG101 is the main mechanism of action for this compound. Using three-dimensional spheroid invasion assays in vitro and in murine brain tissue cultures, we found that knockdown of endogenous CD95L reduced the invasive phenotype in our two glioblastoma model cell lines U87-MG and U251-MG. Invasion was restored in CD95L knockdown cells upon the addition of soluble recombinant CD95L and this effect was inhibited by APG101. We conclude that CD95L from autocrine and paracrine sources contributes towards the invasive phenotype of glioblastoma cells and that APG101 acts as a suppressor of proinvasive signalling by the CD95/CD95L pathway in glioblastoma.

Membrane trafficking of death receptors: implications on signalling

Death receptors were initially recognised as potent inducers of apoptotic cell death and soon ambitious attempts were made to exploit selective ignition of controlled cellular suicide as therapeutic strategy in malignant diseases. However, the complexity of death receptor signalling has increased substantially during recent years. Beyond activation of the apoptotic cascade, involvement in a variety of cellular processes including inflammation, proliferation and immune response was recognised. Mechanistically, these findings raised the question how multipurpose receptors can ensure selective activation of a particular pathway. A growing body of evidence points to an elegant spatiotemporal regulation of composition and assembly of the receptor-associated signalling complex. Upon ligand binding, receptor recruitment in specialized membrane compartments, formation of receptor-ligand clusters and internalisation processes constitute key regulatory elements. In this review, we will summarise the current concepts of death receptor trafficking and its implications on receptor-associated signalling events.

Ganoderma lucidum polysaccharides counteract inhibition on CD71 and FasL expression by culture supernatant of B16F10 cells upon lymphocyte activation

Immune responses to tumor-associated antigens are often detectable in tumor-bearing hosts, but they fail to eliminate malignant cells or prevent development of metastases. Tumor cells produce factors such as interleukin-10, transforming growth factor-β1 and vascular endothelial growth factor (VEGF) that suppress the function of immune cells or induce apoptosis of immune cells. Culture supernatant of tumor cells may contain these immunosuppressive factors which suppress lymphocyte activation. CD71 and FasL are two important molecules that are expressed upon lymphocyte activation. Counteraction against suppression CD71 and FasL expression upon lymphocyte activation may benefit tumor control. A potential component with this effect is Ganoderma lucidum polysaccharides (Gl-PS). In this study, Gl-PS was used on lymphocytes incubating with culture supernatant of B16F10 melanoma cells (B16F10-CS) in the presence of phytohemagglutinin. Following induction with phytohemagglutinin, B16F10-CS suppressed CD71 expression in lymphocytes (as detected by immunofluorescence and flow cytometry), proliferation in lymphocytes (as detected by MTT assay), and FasL expression in lymphocytes (as detected by immunocytochemistry and western blot analysis), while Gl-PS fully or partially counteracted these suppressions. Gl-PS showed counteractive effects against suppression induced by B16F10-CS on CD71 and FasL expression upon lymphocyte activation, suggesting the potential of Gl-PS to facilitate cancer immunotherapy.

A series of Fas receptor agonist antibodies that demonstrate an inverse correlation between affinity and potency

Receptor agonism remains poorly understood at the molecular and mechanistic level. In this study, we identified a fully human anti-Fas antibody that could efficiently trigger apoptosis and therefore function as a potent agonist. Protein engineering and crystallography were used to mechanistically understand the agonistic activity of the antibody. The crystal structure of the complex was determined at 1.9 Å resolution and provided insights into epitope recognition and comparisons with the natural ligand FasL (Fas ligand). When we affinity-matured the agonist antibody, we observed that, surprisingly, the higher-affinity antibodies demonstrated a significant reduction, rather than an increase, in agonist activity at the Fas receptor. We propose and experimentally demonstrate a model to explain this non-intuitive impact of affinity on agonist antibody signalling and explore the implications for the discovery of therapeutic agonists in general.

The role of FasL and Fas in health and disease

The FS7-associated cell surface antigen (Fas, also named CD95, APO-1 or TNFRSF6) attracted considerable interest in the field of apoptosis research since its discovery in 1989. The groups of Shin Yonehara and Peter Krammer were the first reporting extensive apoptotic cell death induction upon treating cells with Fas-specific monoclonal antibodies.1,2 Cloning of Fas3 and its ligand,4,5 FasL (also known as CD178, CD95L or TNFSF6), laid the cornerstone in establishing this receptor-ligand system as a central regulator of apoptosis in mammals. Therapeutic exploitation of FasL-Fas-mediated cytotoxicity was soon an ambitous goal and during the last decade numerous strategies have been developed for its realization. In this chapter, we will briefly introduce essential general aspects of the FasL-Fas system before reviewing its physiological and pathophysiological relevance. Finally, FasL-Fas-related therapeutic tools and concepts will be addressed.

Lack of Fas antagonism by Met in human fatty liver disease

Hepatocytes in fatty livers are hypersensitive to apoptosis and undergo escalated apoptotic activity via death receptor-mediated pathways, particularly that of Fas-FasL, causing hepatic injury that can eventually proceed to cirrhosis and end-stage liver disease. Here we report that the hepatocyte growth factor receptor, Met, plays an important part in preventing Fas-mediated apoptosis of hepatocytes by sequestering Fas. We also show that Fas antagonism by Met is abrogated in human fatty liver disease (FLD). Through structure-function studies, we found that a YLGA amino-acid motif located near the extracellular N terminus of the Met alpha-subunit is necessary and sufficient to specifically bind the extracellular portion of Fas and to act as a potent FasL antagonist and inhibitor of Fas trimerization. Using mouse models of FLD, we show that synthetic YLGA peptide tempers hepatocyte apoptosis and liver damage and therefore has therapeutic potential.

Requirement of N-glycosylation for the secretion of recombinant extracellular domain of human Fas in HeLa cells

Apoptosis has been shown to be associated with altered glycosylation patterns and biosynthesis of glycoproteins. A major cell surface receptor involved in the induction of apoptosis is Fas that is activated by binding Fas ligand but can also be activated by binding anti-Fas antibody. In order to determine whether the Fas receptor is glycosylated, the extracellular domain of human Fas (shFas) was expressed as a cleavable fusion protein (shFas-Fc) in HeLa cells. These cells were shown to express activities of glycosyltransferases involved in N- and O-glycan biosynthesis. The secreted shFas-Fc was shown to be a glycoprotein with heterogeneous glycan chains. MALDI mass spectrometry revealed a disperse molecular weight of shFas with an average of 23.4kDa. Western blots of shFas-Fc secreted from tunicamycin treated transfected HeLa cells showed that only N-glycosylated glycoforms were secreted, while the unglycosylated shFas-Fc remained intracellular. The results suggest that both N-glycosylation sites of the extracellular domain of Fas are occupied with large N-glycans that play a role in the expression of the glycoprotein.

Autoimmune lymphoproliferative syndrome: molecular basis of disease and clinical phenotype

Autoimmune lymphoproliferative syndrome (ALPS) is a variable clinical condition manifest by lymphoproliferative disease, autoimmune cytopenias and susceptibility to malignancy. Central to the cellular pathogenesis is defective FAS-induced apoptosis, which in turn leads to dysregulation of lymphocyte homeostasis. The majority of patients have heterozygous mutations in the FAS (TNFRSF6) gene, but the condition is genetically heterogeneous and mutations in FAS ligand and caspase-8 and caspase-10, all of which are involved in Fas mediated signalling, have also been identified. This review provides a detailed insight into the pathophysiology of lymphocyte apoptosis and how this relates to the variable and complex clinical manifestations of ALPS.

High-throughput screening of cell death inducible short peptides from TNF-related apoptosis-inducing ligand sequence

Therapeutic peptides and small molecules, rationally designed to trigger cell death have attracted strong attention. Cell death inducible peptides were screened from amino acid sequence of tumor necrosis factor-related apoptosis-inducing ligand (TRAIL). Using Fmoc solid phase synthesis, cellulose membrane-bound octameric peptide library of TRAIL scan was prepared and cell viability assay was directly performed on peptide disk with Jurkat cells. Six peptide sequences that could induce cell death were found. Peptide sequence with RNSCWSKD (TRAIL(227-234)) that exist in the zinc-binding site revealed high cell death inducible activity. Apoptotic cell death was observed when cells were treated with soluble synthesized peptide.

CD7-restricted activation of Fas-mediated apoptosis: a novel therapeutic approach for acute T-cell leukemia

Agonistic anti-Fas antibodies and multimeric recombinant Fas ligand (FasL) preparations show high tumoricidal activity against leukemic cells, but are unsuitable for clinical application due to unacceptable systemic toxicity. Consequently, new antileukemia strategies based on Fas activation have to meet the criterion of strictly localized action at the tumor-cell surface. Recent insight into the FasL/Fas system has revealed that soluble homotrimeric FasL (sFasL) is in fact nontoxic to normal cells, but also lacks tumoricidal activity. We report on a novel fusion protein, designated scFvCD7:sFasL, that is designed to have leukemia-restricted activity. ScFvCD7:sFasL consists of sFasL genetically linked to a high-affinity single-chain fragment of variable regions (scFv) antibody fragment specific for the T-cell leukemia-associated antigen CD7. Soluble homotrimeric scFvCD7:sFasL is inactive and acquires tumoricidal activity only after specific binding to tumor cell-surface-expressed CD7. Treatment of T-cell acute lymphoblastic leukemia (T-ALL) cell lines and patient-derived T-ALL, peripheral T-cell lymphoma (PTCL), and CD7-positive acute myeloid leukemia (AML) cells with homotrimeric scFvCD7:sFasL revealed potent CD7-restricted induction of apoptosis that was augmented by conventional drugs, farnesyl transferase inhibitor L-744832, and the proteasome inhibitor bortezomib (Velcade; Millenium, Cambridge, MA). Importantly, identical treatment did not affect normal human peripheral-blood lymphocytes (PBLs) and endothelial cells, with only moderate apoptosis in interleukin-2 (IL-2)/CD3-activated T cells. CD7-restricted activation of Fas in T-cell leukemic cells by scFvCD7:sFasL revitalizes interest in the applicability of Fas signaling in leukemia therapy.

Detection, epitope-mapping and function of anti-Fas autoantibody in patients with silicosis

Dysregulation of apoptosis through the Fas-Fas ligand pathway is associated with the onset of autoimmune disease. Since autoantibodies directed against unknown antigens are present in the sera of these patients, sera samples were examined for the presence of autoantibodies directed against the Fas molecule. Using Western blotting and a ProteinChip analysis, autoantibodies against Fas were detected in patients with silicosis, systemic lupus erythematosus (SLE) and systemic sclerosis (SSc), and weakly detected in healthy individuals. Using epitope mapping employing 12-amino-acid polypeptides with the SPOTs system, a minimum of four epitopes and a maximum of 10 epitopes were found. Several amino acid residues involved in binding FasL, such as C66, R87, L90, E93 and H126, were presented within the epitopes. Serum containing a large amount of anti-Fas autoantibody from silicosis patients inhibited the growth of a Fas-expressing human cell line, but did not inhibit the growth of a low Fas-expresser nor a Fas-expresser in which the Fas gene had been silenced by small interference RNA. All epitopes in the intracellular region of Fas were located in the death domain. The possible roles of anti-Fas autoantibody detected in healthy volunteers and patients with silicosis or autoimmune diseases are discussed here.

Structure-based design of an agonistic peptide targeting Fas

A small agonistic peptide FRAP-4 (WEWT, Fas reactive peptide-4) that binds to the human Fas molecule was discovered using our computer screening strategy named the Amino acid Complement Wave (ACW) method, which is based on the complementarities of interacting amino acids between comprehensive testing peptides and a target protein surface pocket. In silico docking studies demonstrated the specific interaction of FRAP-4 with the main Fas ligand (FasL) binding domain in the Fas molecule. An octamer of this peptide produced by carboxyl terminal linkages of polylysine branches (MAP), (FRAP-4)8-MAP, effectively induced apoptosis in human ovarian cancer cell line NOS4 cells that was associated with the activation of caspases-8, -9 and -3, and the cleavage of PARP. Alanine substitution of the N-terminal W in FRAP-4 resulted in complete loss of FasL-mimetic action of (FRAP-4)8-MAP, suggesting that the aromatic functionality at the N-terminal position W appears to play an essentially important role in Fas binding ability. These observations indicate that the FasL-mimetic peptide should serve as an excellent starting point for the design of effective compounds with FasL-mimetic activity. Furthermore, the ACW method for the structure-based design of optimized small peptides against receptor molecules such as Fas could open new avenues for the development of peptide mimetic and nonpeptidic organic forms to generate novel effective pharmaceuticals.

Genomic drug discovery for apoptosis regulation using a new computer screening amino acid complement wave method

A novel caspase-3-specific inhibitory peptide and an agonistic peptide that binds to the Fas molecule were discovered using our computer screening strategy called the amino acid complement wave (ACW) method, which is based on the complementarities of interacting amino acids between comprehensive testing peptides and a target protein surface pocket. The precise binding configurations of the designed peptides on the three-dimensional (3D) structure of the target protein and the prediction of binding affinities (DeltaG) are determined by the molecular docking program. A designed novel tetrapeptide inhibitor of caspase-3, Ac-DNLD-CHO, was revealed to have potent and specific inhibitory activity. When a designed Fas ligand mimic peptide (Fas reactive peptide-4, FRAP-4) was multimerized by carboxyl terminal linkages of polylysine branches (MAP), the octamer (FRAP-4)(8)-MAP effectively induced apoptosis of human ovarian cancer cell line NOS4 cells. Thus the ACW method for structure-based design of optimized small peptides can be used to further develop small peptidomimetic and nonpeptidic organic forms into a new generation of effective pharmaceuticals.

Selectivity of BAFF/BLyS and APRIL for Binding to the TNF Family Receptors BAFFR/BR3 and BCMA

BAFF (B cell activating factor of the TNF family, also known as BlyS and TALL-1), a TNF family cytokine critical for the development and function of B cells, has been reported to bind to three receptors, BCMA (B cell maturation protein), TACI (transmembrane activator and CAML [calcium-modulator and cyclophilin ligand] interactor), and BAFFR (BAFF receptor), but with widely conflicting values for the affinity and selectivity of binding. BCMA and TACI additionally bind APRIL (a proliferation-inducing ligand), the TNF family ligand most homologous to BAFF. Using soluble, monomeric forms of the receptors, we demonstrate that BAFFR binds BAFF with K(D) approximately 16 nM, while BCMA binds with K(D) approximately 1.6 microM, indicating a approximately 100-fold selectivity for binding to BAFFR over BCMA. APRIL shows the opposite selectivity, binding to BCMA with K(D) approximately 16 nM while showing no detectable affinity for BAFFR (K(D) > 3 microM). The binding of BAFF or APRIL to these receptors is highly sensitive to assay-dependent avidity effects, likely explaining the widely ranging affinity values reported in the literature. Binding of BAFF to BCMA-Fc, a bivalent fusion protein consisting of the extracellular domain of BCMA fused to the hinge and CH1 and CH2 domains of human IgG1, in solution or coated onto an ELISA plate gave apparent binding affinities of approximately 0.63 and approximately 0.15 nM, respectively, compared to values of K(D(app)) <or= 30 and approximately 100 pM for the corresponding BAFFR/IgG1 fusion protein, BAFFR-Fc. The high selectivity of BAFF for BAFFR versus BCMA is thus partly obscured in these multivalent assays. The intrinsically high selectivity inferred from the measurements with monomeric receptor correlates well with in vivo data from knockout mice, providing a possible explanation for the observations that interruption of the BAFFR gene in the A/WySnJ mouse produces a phenotype similar to the BAFF knockout mouse, while the BCMA knockout mouse has no discernible B cell phenotype.

Reactive nitrogen species-induced cell death requires Fas-dependent activation of c-Jun N-terminal kinase

Nitrogen dioxide is a highly toxic reactive nitrogen species (RNS) recently discovered as an inflammatory oxidant with great potential to damage tissues. We demonstrate here that cell death by RNS was caused by c-Jun N-terminal kinase (JNK). Activation of JNK by RNS was density dependent and caused mitochondrial depolarization and nuclear condensation. JNK activation by RNS was abolished in cells lacking functional Fas or following expression of a truncated version of Fas lacking the intracellular death domain. In contrast, RNS induced JNK potently in cells expressing a truncated version of tumor necrosis factor receptor 1 or cells lacking tumor necrosis factor receptor 1 (TNF-R1), illustrating a dependence of Fas but not TNF-R1 in RNS-induced signaling to JNK. Furthermore, Fas was oxidized, redistributed, and colocalized with Fas-associated death domain (FADD) in RNS-exposed cells, illustrating that RNS directly targeted Fas. JNK activation and cell death by RNS occurred in a Fas ligand- and caspase-independent manner. While the activation of JNK by RNS or FasL required FADD, the cysteine-rich domain 1 containing preligand assembly domain required for FasL signaling was not involved in JNK activation by RNS. These findings illustrate that RNS cause cell death in a Fas- and JNK-dependent manner and that this occurs through a pathway distinct from FasL. Thus, avenues aimed at preventing the interaction of RNS with Fas may attenuate tissue damage characteristic of chronic inflammatory diseases that are accompanied by high levels of RNS.

Fas-disabling small exocyclic peptide mimetics limit apoptosis by an unexpected mechanism

Fas ligand- (FasL) mediated apoptosis is an important element of tissue-specific organ damage. We have developed biologically active small exocyclic peptide mimetics that disable apoptotic functions of Fas. The most effective mimetic binds to both its receptor and FasL with comparable affinity. In vitro, the most effective antagonist blocked FasL-induced cytotoxicity completely and specifically. In vivo, the antagonistic mimetic also prevented Concanavilin A (Con A) induced hepatitis, a CD4(+) T cell-mediated animal model of liver injury. Although current approaches prevent Fas receptor signaling by excluding FasL binding to Fas, the small molecule mimetics reported here disable Fas by promoting a defective Fas-FasL receptor complex. This event desensitizes FasL-mediated apoptosis by inhibiting extracellular signal regulated kinase activity and up-regulating NF-kappaB.

Infrequent Fas mutations but no Bax or p53 mutations in early mycosis fungoides: a possible mechanism for the accumulation of malignant T lymphocytes in the skin

Mycosis fungoides (MF) is the most frequent manifestation of cutaneous T cell lymphoma but its cause and pathophysiology remain unclear. Because progression of lesions is characteristically slow, we hypothesized that mycosis fungoides originates from an accumulation of lymphocytes due to defective apoptosis of skin homing T lymphocytes. In this study, we investigate possible alterations of three molecules regulating apoptosis, i.e., Fas antigen, Bax, and p53, at the genomic level in skin lesions from 44 patients with MF, as Fas mediates one of two major pathways for apoptosis of activated T cells. Fas mutations were found in six patients using a polymerase chain reaction and single-strand conformational polymorphism method followed by cloning and sequencing of abnormal polymerase chain reaction products. The mutations predict for defective transmission of the death signal in three cases. Immunohistochemistry demonstrated the lack of Fas protein expression on dermal lymphocytes in one case with Fas gene mutation predicting for a truncated death domain, whereas Fas protein was expressed by dermal lymphocytes in the other investigated cases. By contrast, no mutations of Bax or p53 were found, whereas immunohistochemistry demonstrated increased p53 expression in the nucleus of basal keratinocytes above the neoplastic infiltrate in some MF cases. These results support the hypothesis that Fas defects may play a role in the pathogenesis of MF.

Differentiation of promyelocytic leukaemia: alterations in Fas (CD95/Apo-1) and Fas ligand (CD178) expression

The survival of leukaemic blasts contributes to the pathological mechanism of acute promyelocytic leukaemia (APL). While treatment of APL using retinoic acid (RA) is a model of differentiation therapy, little is known about possible effects of this treatment on the Fas/FasL system. Investigation of APL cells from patients undergoing differentiation therapy with RA and of promyelocytic HL-60 and monoblastic U-937 cells cultured with RA revealed a reduction of surface expression of both Fas and its ligand. Accordingly, the sensitivity of the cells to anti-Fas-induced apoptosis decreased proportionally and the reduced expression of FasL resulted in a decreased ability of the leukaemic cells to induce apoptosis in T cells. Our findings demonstrate that there are significant changes in Fas and FasL expression during RA treatment of APL, which probably have consequences for the interaction between host immune and leukaemia cells, and thus may be involved in the beneficial effects of differentiation therapy.

Retinoic acid and vitamin E modulate expression and release of CD178 in carcinoma cells: consequences for induction of apoptosis in CD95-sensitive cells

CD178 (CD95-ligand) is expressed on several tumor cells and likely influences the interaction of the tumor with the host immune system. However, little is known about the mechanisms that regulate its expression on the cell surface. We have evaluated the ability of various compounds and cytokines to regulate cell surface expression and release of soluble CD178 in various carcinoma cell lines. Vitamin E succinate (VES) and retinoic acid (RA) were found to reduce CD178 surface expression, whereas interferon-gamma stimulated a slight upregulation. At 48 h, the regulation of surface CD178 by VES and RA arose from a small decrease in CD178 mRNA and to a greater extent due to an increase in the release of soluble CD178; the latter was blocked by addition of a metalloproteinase inhibitor. Accordingly, VES and RA treatment diminished the ability of tumor cells to kill CD95-sensitive cells and this effect was markedly reduced by the presence of a metalloproteinase inhibitor. Our results indicate that, in vitro, CD178 expression on the cell surface of tumor cells can be regulated by agents that alter both expression and release of the ligand. In vivo, such treatments may play an important role in the outcome of tumor sensitivity or resistance to host immune mechanisms.

Soluble CD137 (4-1BB) ligand is released following leukocyte activation and is found in sera of patients with hematological malignancies

Expression of CD137 ligand (4-1BBL), a member of the TNF family of proteins, has been reported on several types of APCs, various carcinoma cells, and can be induced on activated T cells. In this study, we report that the soluble ligand was released constitutively at low levels from leukocytes and at higher levels following cellular activation. Release from cells was blocked by addition of a metalloproteinase inhibitor which concomitantly caused the accumulation of 4-1BBL on the cell surface. In addition, we show that a soluble form of 4-1BBL was present at high levels in the sera of some patients with various hematological diseases, but only at low levels in healthy donors. Soluble 4-1BBL was active in that it competed with recombinant 4-1BBL for binding to the 4-1BB receptor and was able to costimulate IL-2 and IFN-gamma release from peripheral T cells. These results indicate that the release of soluble 4-1BBL from the cell surface is mediated by one or more sheddases and likely regulates 4-1BB-4-1BBL interactions between cells in vivo. Cleavage of 4-1BBL to an active soluble form would alter both proximal and distal cellular responses, including cell survival and costimulatory or inflammatory responses, that are mediated through the 4-1BB pathway. This, in turn, would likely alter disease progression or outcome.

Humanization and epitope mapping of neutralizing anti-human Fas ligand monoclonal antibodies: structural insights into Fas/Fas ligand interaction

Fas ligand (L)/CD95L, a proapoptotic member of the TNF family, is a potential target for clinical intervention in various diseases. In the present study, we generated a humanized anti-human FasL mAb and characterized the epitopes of neutralizing mAbs by extensive alanine-scanning mutagenesis of human FasL. The predicted molecular model of FasL trimer revealed that the mAbs recognize largely overlapped conformational epitopes that are composed of two clusters, one around the outer tip-forming D-E loop and another near the top of FasL. Both of these sites on FasL are critically involved in the direct interaction with the corresponding receptor, Fas. These results suggest that the mAbs efficiently neutralize FasL cytotoxicity by masking both of these FasL/Fas contact sites.

Cell cycle phase-specific survival of CD95 ligand-challenged Jurkat cells: upregulation of heat-shock response

An important means of regulating T-cell function occurs via physical deletion (cytolysis) of unnecessary/unwanted T cells. Among cytolytic pathways, CD95 (Fas)-based killing plays a prominent role. Although activation of T cells results in rapid upregulation of surface CD95 expression, sensitivity to CD95-based killing lags behind. To assess determinants of resistance to CD95-based killing, we used Jurkat cells as a model. Analysis of the 10% survivors of a LD(90) dose of CD95 ligand (CD95L) at 24 h demonstrated them to arise preferentially from the S + G2/M phases of the cell cycle and to remain clustered in S + G2/M without undergoing cell division. Protein immunoblot, immunocytochemistry, and RT-PCR analyses demonstrated that hsp72 was markedly upregulated in CD95L survivors within hours of CD95L challenge, indicative of a heat-shock response. Indeed, exposure of Jurkat cells to bona fide heat shock did markedly upregulate hsp72 and, upon subsequent CD95L challenge, did greatly enhance cell survival with persistent clustering to S + G2/M. These findings collectively suggest that in response to a CD95L insult, development of a heat-shock response above some critical threshold level can protect against lethality. This raises the possibility that exaggerated and/or protracted heat-shock responses under in vivo conditions may favor the survival of T cells (including autoaggressive T cells) that otherwise would be destined to die via a CD95-based pathway.

A docking model of key components of the DISC complex: death domain superfamily interactions redefined

Apoptosis is mediated by a highly regulated signal transduction cascade that eventually leads to precisely directed cell death. The death-inducing signaling complex (DISC), composed of Fas, FADD, and caspase-8, is an apical signaling complex that mediates receptor-induced apoptosis. We have docked the experimentally determined structures of the Fas and FADD death domains into a model of a partial DISC signaling complex. The arrangement of Fas and FADD was determined using the interaction modes of the two heterodimer crystal structures determined to date, Pelle/Tube and Apaf-1/procaspase-9. The proposed model reveals that both interactions can be accommodated in a single multimeric complex. Importantly, the model is consistent with reported site-directed mutagenesis data indicating residues throughout the domain are critical for function. These results imply that members of the death domain superfamily have the potential for multivalent interactions, offering novel possibilities for regulation of apoptotic signaling.

Differential susceptibility to CD95 (Apo-1/Fas) and MHC class II-induced apoptosis during murine dendritic cell development

Disappearance of antigen presenting cells (APC) from the lymph node occurs following antigen specific interactions with T cells. We have investigated the regulation of CD95 (Apo-1/Fas) induced apoptosis during murine dendritic cell (DC) development. Consistent with the moderate levels of CD95 surface expression and low, or absent, MHC class II expression, immature DC in bone marrow cultures were highly sensitive to CD95 induced apoptosis, but insensitive to class II mediated apoptosis. In contrast, mature splenic, epidermal and bone marrow derived DC were fully resistant to CD95 induced cell death, but sensitive to class II induced apoptosis. Although caspase 3 and 8 activation was detected in immature DC undergoing CD95L-induced apoptosis, the pan-caspase inhibitor zVAD-fmk did not inhibit the early events of CD95-induced mitochondrial depolarisation or phosphatidyl serine exposure and only partially inhibited the killing of immature DC. In contrast, zVAD-fmk was completely effective in preventing CD95L mediated death of murine thymocytes. Collectively, these data do not support a major role of CD95: CD95L ligation in apoptosis of mature DC, but rather emphasise the existence of distinct pathways for the elimination of DC at different stages of maturation.

Promotion of activated human B cell apoptosis and inhibition of Ig production by soluble CD95 ligand: CD95-based downregulation of Ig production need not culminate in activated B cell death

CD95/CD95L interactions are vital to normal lymphoid homeostasis and in the protection against autoimmunity. To directly assess the effects of CD95L on activated B cell survival and Ig responses, purified human peripheral blood B cells, activated in vitro with SAC + rIL2, were incubated with a soluble CD95L fusion protein (fp) and assayed for apoptosis and IgG/IgM production. CD95L fp reproducibly increased apoptosis of these activated B cells and inhibited their Ig production. However, CD95L fp-mediated effects on activated B cell survival could be uncoupled from those on Ig production in that a soluble CD40L fp was incapable of reversing CD95L fp-mediated downregulation of Ig responses despite inhibiting CD95L fp-mediated apoptosis. Moreover, despite the specific caspase-8 inhibitor z-IETD-fmk substantially protecting transformed CL-01 B cells from CD95L fp-mediated apoptosis and permitting their ongoing proliferation, caspase-8 inhibition had no protective effects on CD95L fp-mediated inhibition of constitutive IgM production by CL-01 B cells. Collectively, these results point to a CD95-based downregulatory pathway in activated B cells that need not necessarily culminate in their death.

Fas preassociation required for apoptosis signaling and dominant inhibition by pathogenic mutations

Heterozygous mutations encoding abnormal forms of the death receptor Fas dominantly interfere with Fas-induced lymphocyte apoptosis in human autoimmune lymphoproliferative syndrome. This effect, rather than depending on ligand-induced receptor oligomerization, was found to stem from ligand- independent interaction of wild-type and mutant Fas receptors through a specific region in the extracellular domain. Preassociated Fas complexes were found in living cells by means of fluorescence resonance energy transfer between variants of green fluorescent protein. These results show that formation of preassociated receptor complexes is necessary for Fas signaling and dominant interference in human disease.

Molecular cloning and characterization of cynomolgus monkey Fas

The Fas-FasL system plays a crucial role in the maintenance of homeostasis in the immune system. To characterize the Fas/FasL system in macaque monkeys that are commonly used as experimental primates, we cloned and sequenced Fas cDNA derived from the cynomolgus monkey. The predicted amino acid sequence consists of 331 amino acids with a calculated molecular weight of 35,800. The extracellular cysteine-rich motif of cynomolgus Fas is highly homologous to that of humans (96%), whereas the intracellular death domain has a relatively low similarity to that of humans (86%). An agonistic Fas antibody (CH11) or cynomolgus FasL induced apoptosis in human Fas-transfected K562 cells in the presence of CHX but not in the cynomolgus Fas transfectant. CH11 and FasL failed to trigger apoptosis in the transfectant expressing human-cynomolgus chimera Fas consisting mostly of human-derived extracellular region and cynomolgus-derived intracellular portion. On the other hand, the transfectant expressing cynomolgus-human chimera Fas with human-derived intracellular region underwent apoptosis upon exposure to FasL. In addition, the virus-transformed, Fas-positive cynomolgus monkey cell line was highly sensitive to FasL. These findings suggest that the lack of apoptotic activity in the cynomolgus Fas transfectant in the human cell line might be related to the species-specific structure of Fas, especially of the death domain.

Structure and function of Fas/Fas ligand

Fas is a member of the TNF receptor family, that contain 2-6 cysteine-rich domains (CRDs) in their extracellular regions, a single transmembrane domain and variably sized intracytoplasmic domains. Fas belongs to a subgroup of family members that have a "death domain" near the carboxy-terminal region of the molecule. This domain binds to adaptor molecules that transmit a death signal to the cell. Signal transduction is complex and involves caspases, ceramides and stress pathways. Fas ligand is biologically active as a homotrimer. Receptor binding has been localized to the C-terminus and a self-association motif to the N-terminus of the ligand extracellular domain. Expression of ligand in a functionally active form is highly regulated at the transcriptional level as well as by cleavage by metalloproteinases. Since Fas/Fas ligand delete activated cells in the peripheral immune system, defects in this pathway predispose to autoimmune disorders.

Identification and characterization of a ligand-independent oligomerization domain in the extracellular region of the CD95 death receptor

The CD95 death receptor plays an important role in several physiological and pathological apoptotic processes involving in particular the immune system. CD95 ligation leads to clustering of the receptor cytoplasmic "death domains" and recruitment of the zymogen form of caspase-8 to the cell surface. Activation of this protease through self-cleavage, followed by activation of downstream effector caspases, culminates in cleavage of a set of cellular proteins resulting in apoptosis with disassembly of the cell. It is very well known that the extracellular region of the CD95 receptor is required for CD95L interaction and that the death domain is necessary for the induction of the apoptotic signaling. Here, we identified and characterized a novel CD95 ligand- and death domain-independent oligomerization domain mapping to the NH(2)-terminal extracellular region of the CD95 receptor. In vitro and in vivo studies indicated that this domain, conserved among all soluble CD95 variants, mediates homo-oligomerization of the CD95 receptor and of the soluble CD95 proteins, as well as hetero-oligomerization of the receptor with the soluble variants. These results offer new insight into the mechanism of apoptosis inhibition mediated by the soluble CD95 proteins and suggest a role of the extracellular oligomerization domain in the regulation of the non-signaling state of the CD95 receptor.

Epitopes and functional responses defined by a panel of anti-Fas (CD95) monoclonal antibodies

Fas (CD95) is a cell surface glycoprotein that mediates apoptotic cell death when cross-linked with agonistic anti-Fas monoclonal antibodies (MAbs) or the endogenous Fas ligand. In this study, we investigated the in vitro biological properties of a panel of anti-human Fas MAbs. We found that five anti-Fas MAbs of IgG1 subclass (B.E28, B.G30, B.L25, DX2, and B.G34) induced marked apoptotic cell death in Fas-expressing leukemia cells, although this killing was delayed when compared to the cytolytic effect mediated by the prototypic anti-Fas MAb of IgM subclass (clone CH-11). On the other hand, four clones (ZB4, B.G27, B.D29, and B.K14) efficiently blocked apoptotic cell death induced by the CH-11 MAb or Fas ligand. The ability of these MAbs to inhibit cell death appeared to correlate with their relative affinity for the Fas molecule. Furthermore, different clones recognized the same epitope and elicited different effects (induction or inhibition of cell killing); conversely, different clones elicited the same effect but recognized different epitopes. These results suggest that the different biological effects of anti-Fas MAbs would not be mediated in an epitope-restricted manner. The relative binding affinity might correlate to some extent with the biological properties of the MAb.

Analysis of Fas-ligand interactions using a molecular model of the receptor-ligand interface

A molecular model of the complex between Fas and its ligand was generated to better understand the location and putative effects of site-specific mutations, analyze interactions at the Fas-FasL interface, and identify contact residues. The modeling study was conservative in the sense that regions in Fas and its ligand which could not be predicted with confidence were omitted from the model to ensure accuracy of the analysis. Using the model, it was possible to map four of five N-linked glycosylation sites in Fas and FasL and to study 10 of 11 residues previously identified by mutagenesis as important for binding. Interactions involving six of these residues could be analyzed in detail and their importance for binding was rationalized based on the model. The predicted structure of the Fas-FasL interface was consistent with the experimentally established importance of these residues for binding. In addition, five previously not targeted residues were identified and predicted to contribute to binding via electrostatic interactions. Despite its limitations, the study provided a much improved basis to understand the role of Fas and FasL residues for binding compared to previous residue mapping studies using only a molecular model of Fas.

Superantigen-Driven, CD8+ T Cell-Mediated Down-Regulation: CD95 (Fas)-Dependent Down-Regulation of Human Ig Responses Despite CD95-Independent Killing of Activated B Cells

Staphylococcal superantigens, including staphylococcal enterotoxin B (SEB), promote vigorous T cell-dependent Ig responses at low dose (0.01 ng/ml). In contrast, more mitogenic high dose SEB (100 ng/ml) profoundly inhibits the Ig responses. To assess the contribution of CD8+ T cells to this inhibition, high dose SEB-dependent killing of activated B cells and down-regulation of Ig responses were determined. Rapid killing (4 h) of activated B cells was effected by high dose SEB-activated CD8+ T cells (CD8*), but not by high-dose SEB-activated CD4+ T cells (CD4*), and required the presence of high dose SEB during the cytotoxicity assay. This killing was abrogated by chelation of extracellular calcium or by treatment with concanamycin A but was only modestly affected by treatment with brefeldin A, suggesting a perforin-based pathway of killing. Despite their widely disparate abilities to rapidly kill activated B cells, CD8* and CD4* demonstrated similar quantitative abilities to effect high dose SEB-dependent down-regulation of Ig responses. Antagonist anti-CD95 mAb substantially reversed high dose SEB-dependent down-regulation effected by CD8* but had no appreciable effects on high dose SEB-dependent killing of activated B cells. These observations strongly suggest that the small fraction of activated B cells that secrete Ig are selectively sensitive to CD95-based killing but resistant to CD95-independent killing. This finding may help explain why clinical autoimmunity associated with increased titers of autoantibodies is a predominant feature of defects in CD95 or CD95 ligand.