The extracellular domains of FasL and Fas are sufficient for the formation of supramolecular FasL-Fas clusters of high stability

Super-Resolution Imaging of Fas/CD95 Reorganization Induced by Membrane-Bound Fas Ligand Reveals Nanoscale Clustering Upstream of FADD Recruitment

Signaling through the TNF-family receptor Fas/CD95 can trigger apoptosis or non-apoptotic cellular responses and is essential for protection from autoimmunity. Receptor clustering has been observed following interaction with Fas ligand (FasL), but the stoichiometry of Fas, particularly when triggered by membrane-bound FasL, the only form of FasL competent at inducing programmed cell death, is not known. Here we used super-resolution microscopy to study the behavior of single molecules of Fas/CD95 on the plasma membrane after interaction of Fas with FasL on planar lipid bilayers. We observed rapid formation of Fas protein superclusters containing more than 20 receptors after interactions with membrane-bound FasL. Fluorescence correlation imaging demonstrated recruitment of FADD dependent on an intact Fas death domain, with lipid raft association playing a secondary role. Flow-cytometric FRET analysis confirmed these results, and also showed that some Fas clustering can occur in the absence of FADD and caspase-8. Point mutations in the Fas death domain associated with autoimmune lymphoproliferative syndrome (ALPS) completely disrupted Fas reorganization and FADD recruitment, confirming structure-based predictions of the critical role that these residues play in Fas–Fas and Fas–FADD interactions. Finally, we showed that induction of apoptosis correlated with the ability to form superclusters and recruit FADD.

TNF receptor agonists induce distinct receptor clusters to mediate differential agonistic activity

Monoclonal antibodies (mAb) and natural ligands targeting costimulatory tumor necrosis factor receptors (TNFR) exhibit a wide range of agonistic activities and antitumor responses. The mechanisms underlying these differential agonistic activities remain poorly understood. Here, we employ a panel of experimental and clinically-relevant molecules targeting human CD40, 4-1BB and OX40 to examine this issue. Confocal and STORM microscopy reveal that strongly agonistic reagents induce clusters characterized by small area and high receptor density. Using antibody pairs differing only in isotype we show that hIgG2 confers significantly more receptor clustering than hIgG1 across all three receptors, explaining its greater agonistic activity, with receptor clustering shielding the receptor-agonist complex from further molecular access. Nevertheless, discrete receptor clustering patterns are observed with different hIgG2 mAb, with a unique rod-shaped assembly observed with the most agonistic mAb. These findings dispel the notion that larger receptor clusters elicit greater agonism, and instead point to receptor density and subsequent super-structure as key determinants.

A multiscale study on the mechanisms of spatial organization in ligand-receptor interactions on cell surfaces

The binding of cell surface receptors with extracellular ligands triggers distinctive signaling pathways, leading into the corresponding phenotypic variation of cells. It has been found that in many systems, these ligand-receptor complexes can further oligomerize into higher-order structures. This ligand-induced oligomerization of receptors on cell surfaces plays an important role in regulating the functions of cell signaling. The underlying mechanism, however, is not well understood. One typical example is proteins that belong to the tumor necrosis factor receptor (TNFR) superfamily. Using a generic multiscale simulation platform that spans from atomic to subcellular levels, we compared the detailed physical process of ligand-receptor oligomerization for two specific members in the TNFR superfamily: the complex formed between ligand TNFα and receptor TNFR1 versus the complex formed between ligand TNFβ and receptor TNFR2. Interestingly, although these two systems share high similarity on the tertiary and quaternary structural levels, our results indicate that their oligomers are formed with very different dynamic properties and spatial patterns. We demonstrated that the changes of receptor’s conformational fluctuations due to the membrane confinements are closely related to such difference. Consistent to previous experiments, our simulations also showed that TNFR can preassemble into dimers prior to ligand binding, while the introduction of TNF ligands induced higher-order oligomerization due to a multivalent effect. This study, therefore, provides the molecular basis to TNFR oligomerization and reveals new insights to TNFR-mediated signal transduction. Moreover, our multiscale simulation framework serves as a prototype that paves the way to study higher-order assembly of cell surface receptors in many other bio-systems.

Receptor Oligomerization and Its Relevance for Signaling by Receptors of the Tumor Necrosis Factor Receptor Superfamily

With the exception of a few signaling incompetent decoy receptors, the receptors of the tumor necrosis factor receptor superfamily (TNFRSF) are signaling competent and engage in signaling pathways resulting in inflammation, proliferation, differentiation, and cell migration and also in cell death induction. TNFRSF receptors (TNFRs) become activated by ligands of the TNF superfamily (TNFSF). TNFSF ligands (TNFLs) occur as trimeric type II transmembrane proteins but often also as soluble ligand trimers released from the membrane-bound form by proteolysis. The signaling competent TNFRs are efficiently activated by the membrane-bound TNFLs. The latter recruit three TNFR molecules, but there is growing evidence that this is not sufficient to trigger all aspects of TNFR signaling; rather, the formed trimeric TNFL–TNFR complexes have to cluster secondarily in the cell-to-cell contact zone for full TNFR activation. With respect to their response to soluble ligand trimers, the signaling competent TNFRs can be subdivided into two groups. TNFRs of one group, designated as category I TNFRs, are robustly activated by soluble ligand trimers. The receptors of a second group (category II TNFRs), however, failed to become properly activated by soluble ligand trimers despite high affinity binding. The limited responsiveness of category II TNFRs to soluble TNFLs can be overcome by physical linkage of two or more soluble ligand trimers or, alternatively, by anchoring the soluble ligand molecules to the cell surface or extracellular matrix. This suggests that category II TNFRs have a limited ability to promote clustering of trimeric TNFL–TNFR complexes outside the context of cell–cell contacts. In this review, we will focus on three aspects on the relevance of receptor oligomerization for TNFR signaling: (i) the structural factors which promote clustering of free and liganded TNFRs, (ii) the signaling pathway specificity of the receptor oligomerization requirement, and (iii) the consequences for the design and development of TNFR agonists.

The FasLane to ocular pathology-metalloproteinase cleavage of membrane-bound FasL determines FasL function

Fas ligand (FasL) is best known for its ability to induce cell death in a wide range of Fas-expressing targets and to limit inflammation in immunoprivileged sites such as the eye. In addition, the ability of FasL to induce a much more extensive list of outcomes is being increasingly explored and accepted. These outcomes include the induction of proinflammatory cytokine production, T cell activation, and cell motility. However, the distinct and opposing functions of membrane-associated FasL (mFasL) and the C-terminal soluble FasL fragment (sFasL) released by metalloproteinase cleavage is less well documented and understood. Both mFasL and sFasL can form trimers that engage the trimeric Fas receptor, but only mFasL can form a multimeric complex in lipid rafts to trigger apoptosis and inflammation. By contrast, a number of reports have now documented the anti-apoptotic and anti-inflammatory activity of sFasL, pointing to a critical regulatory function of the soluble molecule. The immunomodulatory activity of FasL is particularly evident in ocular pathology where elimination of the metalloproteinase cleavage site and the ensuing increased expression of mFasL can severely exacerbate the extent of inflammation and cell death. By contrast, both homeostatic and increased expression of sFasL can limit inflammation and cell death. The mechanism(s) responsible for the protective activity of sFasL are discussed but remain controversial. Nevertheless, it will be important to consider therapeutic applications of sFasL for the treatment of ocular diseases such as glaucoma.

Designed ferritin nanocages displaying trimeric TRAIL and tumor-targeting peptides confer superior anti-tumor efficacy

TRAIL is considered a promising target for cancer therapy because it mediates activation of the extrinsic apoptosis pathway in a tumor-specific manner by binding to and trimerizing its functional receptors, DR4 or DR5. Although recombinant human TRAIL has shown high potency and specificity for killing cancer cells in preclinical studies, it has failed in multiple clinical trials for several reasons, including a very short half-life mainly caused by instability of the monomeric form of TRAIL and rapid renal clearance of the off-targeted TRAIL. To overcome such obstacles, we developed a TRAIL-active trimer nanocage (TRAIL-ATNC) that presents the TRAIL ligand in its trimer-like conformation by connecting it to a triple helix sequence that links to the threefold axis of the ferritin nanocage. We also ligated the tumor-targeting peptide, IL4rP, to TRAIL-ATNC to enhance tumor targeting. The developed TRAIL-ATNCIL4rP showed enhanced agonistic activity compared with monomeric TRAIL. The in vivo serum half-life of TRAIL-ATNCIL4rP was ~ 16-times longer than that of native TRAIL. As a consequence of these properties, TRAIL-ATNCIL4rP exhibited efficacy as an anti-tumor agent in vivo against xenograft breast cancer as well as orthotopic pancreatic cancer models, highlighting the promise of this system for development as novel therapeutics against cancer.

Principles and Applications of Biological Membrane Organization

Many critical biological events, including biochemical signaling, membrane traffic, and cell motility, originate at membrane surfaces. Each such event requires that members of a specific group of proteins and lipids rapidly assemble together at a specific site on the membrane surface. Understanding the biophysical mechanisms that stabilize these assemblies is critical to decoding and controlling cellular functions. In this article, we review progress toward a quantitative biophysical understanding of the mechanisms that drive membrane heterogeneity and organization. We begin from a physical perspective, reviewing the fundamental principles and key experimental evidence behind each proposed mechanism. We then shift to a biological perspective, presenting key examples of the role of heterogeneity in biology and asking which physical mechanisms may be responsible. We close with an applied perspective, noting that membrane heterogeneity provides a novel therapeutic target that is being exploited by a growing number of studies at the interface of biology, physics, and engineering.

A computational model for understanding the oligomerization mechanisms of TNF receptor superfamily

By recognizing members in the tumor necrosis factor (TNF) receptor superfamily, TNF ligand proteins function as extracellular cytokines to activate various signaling pathways involved in inflammation, proliferation, and apoptosis. Most ligands in TNF superfamily are trimeric and can simultaneously bind to three receptors on cell surfaces. It has been experimentally observed that the formation of these molecular complexes further triggers the oligomerization of TNF receptors, which in turn regulate the intracellular signaling processes by providing transient compartmentalization in the membrane proximal regions of cytoplasm. In order to decode the molecular mechanisms of oligomerization in TNF receptor superfamily, we developed a new computational method that can physically simulate the spatial-temporal process of binding between TNF ligands and their receptors. The simulations show that the TNF receptors can be organized into hexagonal oligomers. The formation of this spatial pattern is highly dependent not only on the molecular properties such as the affinities of trans and cis binding, but also on the cellular factors such as the concentration of TNF ligands in the extracellular area or the density of TNF receptors on cell surfaces. Moreover, our model suggests that if TNF receptors are pre-organized into dimers before ligand binding, these lateral interactions between receptor monomers can play a positive role in stabilizing the ligand-receptor interactions, as well as in regulating the kinetics of receptor oligomerization. Altogether, this method throws lights on the mechanisms of TNF ligand-receptor interactions in cellular environments.

Importance of TRAIL Molecular Anatomy in Receptor Oligomerization and Signaling. Implications for Cancer Therapy

(TNF)-related apoptosis-inducing ligand (TRAIL) is able to activate the extrinsic apoptotic pathway upon binding to DR4/TRAIL-R1 and/or DR5/TRAIL-R2 receptors. Structural data indicate that TRAIL functions as a trimer that can engage three receptor molecules simultaneously, resulting in receptor trimerization and leading to conformational changes in TRAIL receptors. However, receptor conformational changes induced by the binding of TRAIL depend on the molecular form of this death ligand, and not always properly trigger the apoptotic cascade. In fact, TRAIL exhibits a much stronger pro-apoptotic activity when is found as a transmembrane protein than when it occurs as a soluble form and this enhanced biological activity is directly linked to its ability to cluster TRAIL receptors in supra-molecular structures. In this regard, cells involved in tumor immunosurveillance, such as activated human T cells, secrete endogenous TRAIL as a transmembrane protein associated with lipid microvesicles called exosomes upon T-cell reactivation. Consequently, it seems clear that a proper oligomerization of TRAIL receptors, which leads to a strong apoptotic signaling, is crucial for inducing apoptosis in cancer cells upon TRAIL treatment. In this review, the current knowledge of oligomerization status of TRAIL receptors is discussed as well as the implications for cancer treatment when using TRAIL-based therapies.

TNFRSF receptor-specific antibody fusion proteins with targeting controlled FcγR-independent agonistic activity

Antibodies specific for TNFRSF receptors that bind soluble ligands without getting properly activated generally act as strong agonists upon FcγR binding. Systematic analyses revealed that the FcγR dependency of such antibodies to act as potent agonists is largely independent from isotype, FcγR type, and of the epitope recognized. This suggests that the sole cellular attachment, achieved by Fc domain-FcγR interaction, dominantly determines the agonistic activity of antibodies recognizing TNFRSF receptors poorly responsive to soluble ligands. In accordance with this hypothesis, we demonstrated that antibody fusion proteins harboring domains allowing FcγR-independent cell surface anchoring also act as strong agonist provided they have access to their target. This finding defines a general possibility to generate anti-TNFRSF receptor antibodies with FcγR-independent agonism. Moreover, anti-TNFRSF receptor antibody fusion proteins with an anchoring domain promise superior applicability to conventional systemically active agonists when an anchoring target with localized disease associated expression can be addressed.

Higher-Order Clustering of the Transmembrane Anchor of DR5 Drives Signaling

Receptor clustering on the cell membrane is critical in the signaling of many immunoreceptors, and this mechanism has previously been attributed to the extracellular and/or the intracellular interactions. Here, we report an unexpected finding that for death receptor 5 (DR5), a receptor in the tumor necrosis factor receptor superfamily, the transmembrane helix (TMH) alone in the receptor directly assembles a higher-order structure to drive signaling and that this structure is inhibited by the unliganded ectodomain. Nuclear magnetic resonance structure of the TMH in bicelles shows distinct trimerization and dimerization faces, allowing formation of dimer-trimer interaction networks. Single-TMH mutations that disrupt either trimerization or dimerization abolish ligand-induced receptor activation. Surprisingly, proteolytic removal of the DR5 ectodomain can fully activate downstream signaling in the absence of ligand. Our data suggest a receptor activation mechanism in which binding of ligand or antibodies to overcome the pre-ligand autoinhibition allows TMH clustering and thus signaling.

Early activation of CD95 is limited and localized to the cytotoxic synapse

The cytotoxic synapse formed between cytotoxic T lymphocytes or natural killer cells expressing CD95L and target cells with CD95 on their surface is a key pathway for apoptosis induction by the immune system. Despite similarities with the immune synapse in antigen presenting cells, little is known about the role of the spatiotemporal organization of agonistic proteins/receptor interactions for CD95 signaling. Here, we have developed an artificial cytotoxic synapse to examine how mobility and geometry of an anti-CD95 agonistic antibody affect receptor aggregation and mobility, ie the first step of receptor activation. By measuring the distribution, diffusion coefficient, and fraction of immobile CD95 receptor in living cells, we show that at short times, the initial activation of CD95 occurs locally and is limited to the contact region of the cytotoxic synapse. This anisotropic activation of apoptotic signaling supports a role for confined interactions on the efficiency of signal transduction that may have implications for biomedical applications of extrinsic apoptosis induction.

Use of antigen-primed dendritic cells for inducing antitumor immune responses in vitro in patients with non-small cell lung cancer

Cancer is associated with a reduction in immature and mature circulating dendritic cells (DCs), and with an impaired migratory capacity, compared with healthy donors. Therefore, modern approaches to the in vitro generation of DCs loaded with tumor antigens and their use for inducing antitumor immune responses in vivo are being investigated. The purpose of the present study was to investigate the phenotypic and functional characteristics of peripheral blood DC subsets in patients with non-small cell lung cancer (NSCLC), and the development of an antitumor cytotoxic response by mononuclear cells (MNCs) from patients using in vitro generated antigen-primed DCs. Heparinized peripheral venous blood samples were obtained from 10 healthy donors and 20 patients with a histologically verified diagnosis of NSCLC. The ability of antigen-activated DCs to stimulate the activity of MNCs against autologous tumor cells was evaluated using a cytotoxic test. Peripheral blood DC subsets from patients with NSCLC were identified to be decreased and to exhibit an impaired ability to mature, compared with healthy donors. Furthermore, DCs generated from MNCs from patients with NSCLC were able to stimulate a specific cytotoxic response when loaded with autologous tumor lysates or RNA and matured, in vitro. A perforin and granzyme B-dependent mode of cytotoxicity was primarily induced. The ability of DCs loaded with tumor antigens to increase the cytotoxic activity of MNCs against NSCLC cells in vitro indicates the effective induction and co-stimulation of T lymphocytes by the generated DCs.

Novel strategies to mimic transmembrane tumor necrosis factor-dependent activation of tumor necrosis factor receptor 2

Tumor necrosis factor receptor 2 (TNFR2) is known to mediate immune suppression and tissue regeneration. Interestingly, the transmembrane form of tumor necrosis factor (tmTNF) is necessary to robustly activate TNFR2. To characterize the stoichiometry and composition of tmTNF during TNFR2 activation, we constructed differently oligomerized single chain TNF ligands (scTNF) comprised of three TNF homology domain (THD) protomers that mimic tmTNF. Using a variety of cellular and in vivo assays, we can show that higher oligomerization of the scTNF trimers results in more efficient TNF/TNFR2 clustering and subsequent signal transduction. Importantly, the three-dimensional orientation of the scTNF trimers impacts the bioactivity of the oligomerized scTNF ligands. Our data unravel the organization of tmTNF-mimetic scTNF ligands capable of robustly activating TNFR2 and introduce novel TNFR2 agonists that hold promise as therapeutics to treat a variety of diseases.

Nrf2 Inhibits Periodontal Ligament Stem Cell Apoptosis under Excessive Oxidative Stress

The present study aimed to analyze novel mechanisms underlying Nrf2-mediated anti-apoptosis in periodontal ligament stem cells (PDLSCs) in the periodontitis oxidative microenvironment. We created an oxidative stress model with H₂O₂-treated PDLSCs. We used real-time PCR, Western blotting, TUNEL staining, fluorogenic assay and transfer genetics to confirm the degree of oxidative stress and apoptosis as well as the function of nuclear factor-erythroid 2-related factor 2 (Nrf2). We demonstrated that with upregulated levels of reactive oxygen species (ROS) and malondialdehyde (MDA), the effect of oxidative stress was obvious under H₂O₂ treatment. Oxidative molecules were altered after the H₂O₂ exposure, whereby the signaling of Nrf2 was activated with an increase in its downstream effectors, heme oxygenase-1 (HO-1), NAD(P)H:quinone oxidoreductase 1 (NQO1) and γ-glutamyl cysteine synthetase (γ-GCS). Additionally, the apoptosis levels gradually increased with oxidative stress by the upregulation of caspase-9, caspase-3, Bax and c-Fos levels in addition to the downregulation of Bcl-2. However, there was no alterations in levels of caspase-8. The enhanced antioxidant effect could not mitigate the occurrence of apoptosis. Furthermore, Nrf2 overexpression effectively improved the anti-oxidative levels and increased cell proliferation. At the same time, overexpression effectively restrained TUNEL staining and decreased the molecular levels of caspase-9, caspase-3, Bax and c-Fos, but not that of caspase-8. In contrast, silencing the expression of Nrf2 levels had the opposite effect. Collectively, Nrf2 alleviates PDLSCs via its effects on regulating oxidative stress and anti-intrinsic apoptosis by the activation of oxidative enzymes.

Caspases rule the intracellular trafficking cartel

During apoptosis, caspases feast on several hundreds of cellular proteins to orchestrate rapid cellular demise. Indeed, caspases are known to get a taste of every cellular process in one way or another, activating some, but most often shutting them down. Thus, it is not surprising that caspases proteolyze proteins involved in intracellular trafficking with particularly devastating consequences for this important process. This review article focuses on how caspases target the machinery responsible for smuggling goods within and outside the cell.

Death Receptor 5 Networks Require Membrane Cholesterol for Proper Structure and Function

Death receptor 5 (DR5) is an apoptosis-inducing member of the tumor necrosis factor receptor superfamily, whose activity has been linked to membrane cholesterol content. Upon ligand binding, DR5 forms large clusters within the plasma membrane that have often been assumed to be manifestations of receptor co-localization in cholesterol-rich membrane domains. However, we have recently shown that DR5 clusters are more than just randomly aggregated receptors. Instead, these are highly structured networks held together by receptor dimers. These dimers are stabilized by specific transmembrane helix-helix interactions, including a disulfide bond in the long isoform of the receptor. The complex relationships among DR5 network formation, transmembrane helix dimerization, membrane cholesterol, and receptor activity has not been established. It is unknown whether the membrane itself plays an active role in driving DR5 transmembrane helix interactions or in the formation of the networks. We show that cholesterol depletion in cells does not inhibit the formation of DR5 networks. However, the networks that form in cholesterol-depleted cells fail to induce caspase cleavage. These results suggest a potential structural difference between active and inactive networks. As evidence, we show that cholesterol is necessary for the covalent dimerization of DR5 transmembrane domains. Molecular simulations and experiments in synthetic vesicles on the DR5 transmembrane dimer suggest that dimerization is facilitated by increased helicity in a thicker bilayer.

Liquid-liquid phase separation in cellular signaling systems

Liquid-liquid demixing or phase separation of protein with RNA is now recognized to be a key part of the mechanism for assembly of ribonucleoprotein granules. Cellular signaling also appears to employ phase separation as a mechanism for amplification or control of signal transduction both within the cytoplasm and at the membrane. The concept of receptor clustering, identified more than 3 decades ago, is now being examined through the lens of phase separation leading to new insights. Intrinsically disordered proteins or regions are central to these processes owing to their flexibility and accessibility for dynamic protein-protein interactions and post-translational modifications. We review some recent examples, examine the mechanisms driving phase separation and delineate the implications for signal transduction systems.

Correlating RANK ligand/RANK binding kinetics with osteoclast formation and function

The interaction between Receptor Activator of NF-κB Ligand (RANKL) and its receptor RANK is essential for the differentiation and bone resorbing capacity of the osteoclast. Osteoprotegerin (OPG), a soluble homodimer, acts as a decoy receptor for RANKL and thus inhibits osteoclastogenesis. An imbalance in the RANKL/RANK/OPG axis, with decreased OPG and/or increased RANKL, is associated with diseases that favor bone loss, including osteoporosis. Recently, we established a yeast surface display system and screened libraries of randomly mutated RANKL proteins to identify mutations that abolish binding to OPG while preserving recognition of RANK. These efforts yielded several RANKL variants possessing substantially higher affinity for RANK compared to their wild-type (WT) counterpart. Using recombinant RANKL mutant proteins, we find those with increased affinity for RANK produce more robust signaling in osteoclast lineage cells and have greater osteoclastogenic potential. Our results are the first to document gain of function RANKL mutations. They indicate that the physiological RANKL/RANK interaction is not optimized for maximal signaling and function, perhaps reflecting the need to maintain receptor specificity within the tumor necrosis factor superfamily (TNFSF). Instead, we find, a biphasic relationship exists between RANKL/RANK affinity and osteoclastogenic capacity. In our panel of RANKL variants, this relationship is driven entirely by manipulation of the kinetic off-rate. Our structure-based and yeast surface display-derived insights into manipulating this critical signaling axis may aid in the design of novel anti-resorptive therapies as well as provide a paradigm for design of other receptor-specific TNF superfamily ligand variants.

S-Nitrosylation in TNF superfamily signaling pathway: Implication in cancer

One of the key features of tumor cells is the acquisition of resistance to apoptosis. Thus, novel therapeutic strategies that circumvent apoptotic resistance and result in tumor elimination are needed. One strategy to induce apoptosis is to activate death receptor signaling pathways. In the tumor microenvironment, stimulation of Fas, Death receptor 4 (DR4) and tumor necrosis factor receptor 1 (TNFR1) can initiate multiple signaling pathways driving either tumor promotion or elimination. Nitric oxide (NO) is an important signaling molecule now understood to play a dual role in cancer biology. More and more attention is directed toward the role displayed by S-nitrosylation, the incorporation of an NO moiety to a cysteine thiol group, in promoting cell death in tumor cells. Protein post-translation modification by S-nitrosylation has decisive roles in regulating signal-transduction pathways. In this review, we summarize several examples of protein modification by S-nitrosylation that regulate signaling pathways engaged by members of the TNF superfamily (Fas ligand (FasL), Tumor-necrosis-factor-related apoptosis inducing ligand (TRAIL) and TNFalpha (TNFα)) and the way it influences cell fate decisions.

•

An overview of NO in regulating signaling pathways engaged by FasL, TRAIL and TNFα.

•

S-nitrosylation regulates protein activity and cancer cell death.

•

Exploiting NO for cancer therapy.

Principles of antibody-mediated TNF receptor activation

From the beginning of research on receptors of the tumor necrosis factor (TNF) receptor superfamily (TNFRSF), agonistic antibodies have been used to stimulate TNFRSF receptors in vitro and in vivo. Indeed, CD95, one of the first cloned TNFRSF receptors, was solely identified as the target of cell death-inducing antibodies. Early on, it became evident from in vitro studies that valency and Fcγ receptor (FcγR) binding of antibodies targeting TNFRSF receptors can be of crucial relevance for agonistic activity. TNFRSF receptor-specific antibodies of the IgM subclass and secondary cross-linked or aggregation prone dimeric antibodies typically display superior agonistic activity compared with dimeric antibodies. Likewise, anchoring of antibodies to cell surface-expressed FcγRs potentiate their ability to trigger TNFRSF receptor signaling. However, only recently has the relevance of oligomerization and FcγR binding for the in vivo activity of antibody-induced TNFRSF receptor activation been straightforwardly demonstrated in vivo. This review discusses the crucial role of oligomerization and/or FcγR binding for antibody-mediated TNFRSF receptor stimulation in light of current models of TNFRSF receptor activation and especially the overwhelming relevance of these issues for the rational development of therapeutic TNFRSF receptor-targeting antibodies.

Effects of gastrokine‑2 expression on gastric cancer cell apoptosis by activation of extrinsic apoptotic pathways

Gastrokine‑2 is a putative gastric cancer‑specific tumor suppressor gene, the loss of which is known to be involved in the development and progression of gastric cancer, and restoration of gastrokine‑2 expression inhibits growth of gastric cancer cells in vitro. However, the underlying mechanism of these effects requires elucidation. In the present study, expression patterns of gastrokine‑2 protein were examined in gastric cancer tissues and cell lines. Expression of gastrokine‑2 was restored in gastric cancer cells in order to assess its effect on cell viability, apoptosis and gene expression. A total of 76 gastric cancer tissues with corresponding normal mucosae samples, and two gastric cancer cell lines (SGC‑7901 and AGS) were subjected to western blot analysis of gastrokine‑2 expression. SGC‑7901 cells were transiently transfected with gastrokine‑2 cDNA and then treated with anti‑CD95 and/or anti‑Fas antibodies prior to analysis of cell viability, apoptosis and gene expression levels. Expression of gastrokine‑2 protein was reduced or absent in gastric cancer tissues and gastric cancer cell lines. Following restoration of gastrokine‑2 expression, the protein expression level of Fas was significantly increased, but no marked change was observed in the levels of bcl‑2 and Bax proteins. Expression of gastrokine‑2 protein reduced gastric cancer cell viability and induced apoptosis. Activity of caspase‑3 and caspase‑8 was increased, but caspase‑9 activity remained unchanged in the SGC‑7901 cells. Reduction or knockout of gastrokine‑2 protein expression may contribute to gastric cancer development or progression, as the current study demonstrated that restoration of gastrokine‑2 expression induces apoptosis of gastric cancer cells through the extrinsic apoptosis pathway.

Phase transitions of multivalent proteins can promote clustering of membrane receptors

Clustering of proteins into micrometer-sized structures at membranes is observed in many signaling pathways. Most models of clustering are specific to particular systems, and relationships between physical properties of the clusters and their molecular components are not well understood. We report biochemical reconstitution on supported lipid bilayers of protein clusters containing the adhesion receptor Nephrin and its cytoplasmic partners, Nck and N-WASP. With Nephrin attached to the bilayer, multivalent interactions enable these proteins to polymerize on the membrane surface and undergo two-dimensional phase separation, producing micrometer-sized clusters. Dynamics and thermodynamics of the clusters are modulated by the valencies and affinities of the interacting species. In the presence of the Arp2/3 complex, the clusters assemble actin filaments, suggesting that clustering of regulatory factors could promote local actin assembly at membranes. Interactions between multivalent proteins could be a general mechanism for cytoplasmic adaptor proteins to organize membrane receptors into micrometer-scale signaling zones.

DOI:http://dx.doi.org/10.7554/eLife.04123.001

The membrane that surrounds a cell is made up of a mixture of lipid molecules and proteins. Membrane proteins perform a wide range of roles, including transmitting signals into, and out of, cells and helping neighboring cells to stick together. To perform these tasks, these proteins commonly need to bind to other molecules—collectively known as ligands—that are found either inside or outside the cell.

Membrane proteins are able to move around within the membrane, and in many systems, ligand binding causes the membrane proteins to cluster together. Although this clustering has been seen in many different systems, no general principles that describe how clustering occurs had been found.

Now, Banjade and Rosen have constructed an artificial cell membrane to investigate the clustering of a membrane protein called Nephrin, which is essential for kidneys to function correctly. When it is activated, Nephrin interacts with protein ligands called Nck and N-WASP that are found inside cells and helps filaments of a protein called actin to form. These filaments perform a number of roles including enabling cells to adhere to each other and to move.

In Banjade and Rosen's artificial system, when a critical concentration of ligands was exceeded, clusters of Nephrin, Nck and N-WASP suddenly formed. This suggests that the clusters form through a physical process known as ‘phase separation’. Banjade and Rosen found that this critical concentration depends on how strongly the proteins interact and the number of sites they possess to bind each other.

Within the clusters, the three proteins formed large polymer chains. The clusters were mobile and, over time, small clusters coalesced into larger clusters. Even though the clusters persisted for hours, individual proteins did not stay in a given cluster for long and instead continuously exchanged back-and-forth between the cluster and its surroundings.

When actin and another protein complex that interacts with N-WASP were added to the artificial membrane system, actin filaments began to form at the protein clusters. Banjade and Rosen suggest that such clusters act as ‘signaling zones’ that coordinate the construction of the actin filaments. Regions that are also found in many other signaling proteins mediate the interactions between Nephrin, Nck and N-WASP. Banjade and Rosen therefore suggest that phase separation and protein polymer formation could explain how many different types of membrane proteins form clusters.

DOI:http://dx.doi.org/10.7554/eLife.04123.002

Fas palmitoylation by the palmitoyl acyltransferase DHHC7 regulates Fas stability

The death receptor Fas undergoes a variety of post-translational modifications including S-palmitoylation. This protein acylation has been reported essential for an optimal cell death signaling by allowing both a proper Fas localization in cholesterol and sphingolipid-enriched membrane nanodomains, as well as Fas high-molecular weight complexes. In human, S-palmitoylation is controlled by 23 members of the DHHC family through their palmitoyl acyltransferase activity. In order to better understand the role of this post-translational modification in the regulation of the Fas-mediated apoptosis pathway, we performed a screen that allowed the identification of DHHC7 as a Fas-palmitoylating enzyme. Indeed, modifying DHHC7 expression by specific silencing or overexpression, respectively, reduces or enhances Fas palmitoylation and DHHC7 co-immunoprecipitates with Fas. At a functional level, DHHC7-mediated palmitoylation of Fas allows a proper Fas expression level by preventing its degradation through the lysosomes. Indeed, the decrease of Fas expression obtained upon loss of Fas palmitoylation can be restored by inhibiting the lysosomal degradation pathway. We describe the modification of Fas by palmitoylation as a novel mechanism for the regulation of Fas expression through its ability to circumvent its degradation by lysosomal proteolysis.

TWEAK inhibits TRAF2-mediated CD40 signaling by destabilization of CD40 signaling complexes

We found recently that TNF-like weak inducer of apoptosis (TWEAK) and fibroblast growth factor-inducible-14 (Fn14) by virtue of their strong capability to reduce the freely available cytoplasmic pool of TNFR-associated factor (TRAF)2 and cellular inhibitors of apoptosis (cIAPs) antagonize the functions of these molecules in TNFR1 signaling, resulting in sensitization for apoptosis and inhibition of classical NF-κB signaling. In this study, we demonstrate that priming of cells with TWEAK also interferes with activation of the classical NF-κB pathway by CD40. Likewise, there was strong inhibition of CD40 ligand (CD40L)-induced activation of MAPKs in TWEAK-primed cells. FACS analysis and CD40L binding studies revealed unchanged CD40 expression and normal CD40L-CD40 interaction in TWEAK-primed cells. CD40L immunoprecipitates, however, showed severely reduced amounts of CD40 and CD40-associated proteins, indicating impaired formation or reduced stability of CD40L-CD40 signaling complexes. The previously described inhibitory effect of TWEAK on TNFR1 signaling has been traced back to reduced activity of the TNFR1-associated TRAF2-cIAP1/2 ubiquitinase complex and did not affect the stability of the immunoprecipitable TNFR1 receptor complex. Thus, the inhibitory effect of TWEAK on CD40 signaling must be based at least partly on other mechanisms. In line with this, signaling by the CD40-related TRAF2-interacting receptor TNFR2 was also attenuated but still immunoprecipitable in TWEAK-primed cells. Collectively, we show that Fn14 activation by soluble TWEAK impairs CD40L-CD40 signaling complex formation and inhibits CD40 signaling and thus identify the Fn14-TWEAK system as a potential novel regulator of CD40-related cellular functions.

Signaling active CD95 receptor molecules trigger co-translocation of inactive CD95 molecules into lipid rafts

The capability of soluble CD95L trimers to trigger CD95-associated signaling pathways is drastically increased by oligomerization. The latter can be achieved, for example, by antibodies recognizing a N-terminal epitope tag in recombinant CD95L variants or by genetic engineering-enforced formation of hexamers. Using highly sensitive and accurate binding studies with recombinant CD95L variants equipped with a Gaussia princeps luciferase reporter domain, we found that oligomerization of CD95L has no major effect on CD95 occupancy. This indicates that the higher activity of oligomerized CD95L trimers is not related to an avidity-related increase in apparent affinity and points instead to a crucial role of aggregation of initially formed trimeric CD95L-CD95 complexes in CD95 activation. Furthermore, binding of soluble CD95L trimers was found to be insufficient to increase the association of CD95 with the lipid raft-containing membrane fraction. However, when Gaussia princeps luciferase-CD95L trimers were used as tracers to "mark" inactive CD95 molecules, increased association of these inactive receptors was observed upon activation of the remaining CD95 molecules by help of highly active hexameric Fc-CD95L or membrane CD95L. Moreover, in cells expressing endogenous CD95 and chimeric CD40-CD95 receptors, triggering of CD95 signaling via endogenous CD95 resulted in co-translocation of CD40-CD95 to the lipid raft fraction, whereas vice versa activation of CD95-associated pathways with Fc-CD40L via CD40-CD95 resulted in co-translocation of endogenous CD95. In sum, this shows that signaling-active CD95 molecules not only enhance their own association with the lipid raft-containing membrane fraction but also those of inactive CD95 molecules.

A minimal mathematical model for the initial molecular interactions of death receptor signalling

Tumor necrosis factor (TNF) is the name giving member of a large cytokine family mirrored by a respective cell membrane receptor super family. TNF itself is a strong proinflammatory regulator of the innate immune system, but has been also recognized as a major factor in progression of autoimmune diseases. A subgroup of the TNF ligand family, including TNF, signals via so-called death receptors, capable to induce a major form of programmed cell death, called apoptosis. Typical for most members of the whole family, death ligands form homotrimeric proteins, capable to bind up to three of their respective receptor molecules. But also unligated receptors occur on the cell surface as homomultimers due to a homophilic interaction domain. Based on these two interaction motivs (ligand/receptor and receptor/receptor) formation of large ligand/receptor clusters can be postulated which have been also observed experimentally. We use here a mass action kinetics approach to establish an ordinary differential equations model describing the dynamics of primary ligand/receptor complex formation as a basis for further clustering on the cell membrane. Based on available experimental data we develop our model in a way that not only ligand/receptor, but also homophilic receptor interaction is encompassed. The model allows formation of two distict primary ligand/receptor complexes in a ligand concentration dependent manner. At extremely high ligand concentrations the system is dominated by ligated receptor homodimers.

The tumor necrosis factor receptor stalk regions define responsiveness to soluble versus membrane-bound ligand

The family of tumor necrosis factor receptors (TNFRs) and their ligands form a regulatory signaling network that controls immune responses. Various members of this receptor family respond differently to the soluble and membrane-bound forms of their respective ligands. However, the determining factors and underlying molecular mechanisms of this diversity are not yet understood. Using an established system of chimeric TNFRs and novel ligand variants mimicking the bioactivity of membrane-bound TNF (mTNF), we demonstrate that the membrane-proximal extracellular stalk regions of TNFR1 and TNFR2 are crucial in controlling responsiveness to soluble TNF (sTNF). We show that the stalk region of TNFR2, in contrast to the corresponding part of TNFR1, efficiently inhibits both the receptor's enrichment/clustering in particular cell membrane regions and ligand-independent homotypic receptor preassembly, thereby preventing sTNF-induced, but not mTNF-induced, signaling. Thus, the stalk regions of the two TNFRs not only have implications for additional TNFR family members, but also provide potential targets for therapeutic intervention.

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) induces death receptor 5 networks that are highly organized

Recent evidence suggests that TNF-related apoptosis-inducing ligand (TRAIL), a death-inducing cytokine with anti-tumor potential, initiates apoptosis by re-organizing TRAIL receptors into large clusters, although the structure of these clusters and the mechanism by which they assemble are unknown. Here, we demonstrate that TRAIL receptor 2 (DR5) forms receptor dimers in a ligand-dependent manner at endogenous receptor levels, and these receptor dimers exist within high molecular weight networks. Using mutational analysis, FRET, fluorescence microscopy, synthetic biochemistry, and molecular modeling, we find that receptor dimerization relies upon covalent and noncovalent interactions between membrane-proximal residues. Additionally, by using FRET, we show that the oligomeric structure of two functional isoforms of DR5 is indistinguishable. The resulting model of DR5 activation should revise the accepted architecture of the functioning units of DR5 and the structurally homologous TNF receptor superfamily members.

The emerging role of matrix metalloproteases of the ADAM family in male germ cell apoptosis

Constitutive germ cell apoptosis during mammalian spermatogenesis is a key process for controlling sperm output and to eliminate damaged or unwanted cells. An increase or decrease in the apoptosis rate has deleterious consequences and leads to low sperm production. Apoptosis in spermatogenesis has been widely studied, but the mechanism by which it is induced under physiological or pathological conditions has not been clarified. We have recently identified the metalloprotease ADAM17 (TACE) as a putative physiological inducer of germ cell apoptosis. The mechanisms involved in regulating the shedding of the ADAM17 extracellular domain are still far from being understood, although they are important in order to understand cell-cell communications. Here, we review the available data regarding apoptosis during mammalian spermatogenesis and the localization of ADAM proteins in the male reproductive tract. We propose an integrative working model where ADAM17, p38 MAPK, protein kinase C (PKC) and the tyrosine kinase c-Abl participate in the physiological signalling cascade inducing apoptosis in germ cells. In our model, we also propose a role for the Sertoli cell in regulating the Fas/FasL system in order to induce the extrinsic pathway of apoptosis in germ cells. This working model could be applied to further understand constitutive apoptosis in spermatogenesis and in pathological conditions (e.g., varicocele) or following environmental toxicants exposure (e.g., genotoxicity or xenoestrogens).

Aberrant expression of Fas and FasL pro-apoptotic proteins in basal cell and squamous cell carcinomas

BACKGROUND

Fas and FasL are cell-surface receptors involved in apoptosis. Aim.  To investigate Fas and FasL expression levels in basal cell carcinoma (BCC) and squamous cell carcinoma (SCC) to gain an insight into the mechanisms of Fas/FasL apoptosis and immune evasion in epidermal tumours.

METHODS

Real-time PCR and streptavidin-biotin immunohistochemistry techniques were used to detect Fas and FasL expression in BCC, SCC and normal skin specimens.

RESULTS

Fas and FasL mRNA expression and immunostaining levels were lower in the 21 BCC specimens than in normal skin, whereas they were higher in the 15 SCC specimens than in normal skin.

CONCLUSION

FasL immunostaining correlated with the capability of the tumour to invade and metastasize; the absent or weak immunostaining in the BCC specimens correlated with low capability, and the strong staining in the SCC specimens correlated with high capability.

The biological activity of FasL in human and mouse lungs is determined by the structure of its stalk region

Acute lung injury (ALI) is a life-threatening condition in critically ill patients. Injury to the alveolar epithelium is a critical event in ALI, and accumulating evidence suggests that it is linked to proapoptotic Fas/FasL signals. Active soluble FasL (sFasL) is detectable in the bronchoalveolar lavage (BAL) fluid of patients with ALI, but the mechanisms controlling its bioactivity are unclear. We therefore investigated how the structure of sFasL influences cellular activation in human and mouse lungs and the role of oxidants and proteases in modifying sFasL activity. The sFasL in BAL fluid from patients with ALI was bioactive and present in high molecular weight multimers and aggregates. Oxidants generated from neutrophil myeloperoxidase in BAL fluid promoted aggregation of sFasL in vitro and in vivo. Oxidation increased the biological activity of sFasL at low concentrations but degraded sFasL at high concentrations. The amino-terminal extracellular stalk region of human sFasL was required to induce lung injury in mice, and proteolytic cleavage of the stalk region by MMP-7 reduced the bioactivity of sFasL in human cells in vitro. The sFasL recovered from the lungs of patients with ALI contained both oxidized methionine residues and the stalk region. These data provide what we believe to be new insights into the structural determinants of sFasL bioactivity in the lungs of patients with ALI.

Insights into the molecular regulation of FasL (CD178) biology

Fas ligand (FasL, CD95L, APO-1L, CD178, TNFSF6, APT1LG1) is the key death factor of receptor-triggered programmed cell death in immune cells. FasL/Fas-dependent apoptosis plays a pivotal role in activation-induced cell death, termination of immune responses, elimination of autoreactive cells, cytotoxic effector function of T and NK cells, and the establishment of immune privilege. Deregulation or functional impairment of FasL threatens the maintenance of immune homeostasis and defense and results in severe autoimmunity. In addition, FasL has been implicated as an accessory or costimulatory receptor in T cell activation. The molecular mechanisms underlying this reverse signaling capacity are, however, poorly understood and still controversially discussed. Many aspects of FasL biology have been ascribed to selective protein-protein interactions mediated by a unique polyproline region located in the membrane-proximal intracellular part of FasL. Over the past decade, we and others identified a large number of putative FasL-interacting molecules that bind to this polyproline stretch via Src homology 3 or WW domains. Individual interactions were analyzed in more detail and turned out to be crucial for the lysosomal storage, the transport and the surface appearance of the death factor and potentially also for reverse signaling. This review summarizes the work in the framework of the Collaborative Research Consortium 415 (CRC 415) and provides facts and hypotheses about FasL-interacting proteins and their potential role in FasL biology.

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.

Control of death receptor ligand activity by posttranslational modifications

The death receptor ligands are involved in many physiological and pathological processes involving triggering of apoptosis, inflammation, proliferation, and activation. The expression of these molecules is reported to be tightly regulated at the transcriptional level. However, over the last few years, an increasing number of data demonstrated that the control of transcription is only one of the mechanisms that manage the expression of the death receptor ligands. Thus, this review is focused on posttranslational regulation of the three main members of this family, namely FasL, TNF-alpha, and TRAIL. We discuss here the importance of distribution, storage, and degranulation of these molecules, as well as their shedding by proteases on the control of death receptor ligands expression and activity.

Spermatocyte apoptosis, which involves both intrinsic and extrinsic pathways, explains the sterility of Graomys griseoflavus x Graomys centralis male hybrids

Spermatogenic impairment and the apoptotic pathways involved in establishing sterility of male hybrids obtained from crossing Graomys griseoflavus females with Graomys centralis males were studied. Testes from G. centralis, G. griseoflavus and hybrids were compared at different ages. Terminal transferase-mediated dUTP nick-end labelling assay (TUNEL), Fas, Bax and cytochrome c labelling were used for apoptosis evaluation, and calbindin D(28k) staining as an anti-apoptotic molecule. In 1-month-old animals, spermatocytes were positive for all apoptotic markers, but moderate TUNEL (+) spermatocyte frequency was only found in G. centralis. At subsequent ages, the apoptotic markers were downregulated in testes from parental cytotypes, but not in hybrid testes. TUNEL (+) spermatocytes were present at 78% and 44% per tubule cross-section in 2- and 3-month-old hybrid animals, respectively. Pachytene spermatocyte death in adult hybrids occurs via apoptosis, as revealed by high caspase-3 expression. Calbindin was highly expressed in spermatocytes of adult hybrids, in which massive cell death occurs via apoptosis. Calbindin co-localisation with TUNEL or Fas, Bax and cytochrome c was very limited, suggesting an inverse regulation of calbindin and apoptotic markers. Hybrid sterility is due to breakdown of spermatogenesis at the pachytene spermatocyte stage. Both extrinsic and intrinsic pathways are involved in apoptosis of spermatocytes, which are the most sensitive cell type to apoptotic stimuli.

Acrolein induces apoptosis through the death receptor pathway in A549 lung cells: role of p53

Acrolein, a highly reactive alpha,beta-unsaturated aldehyde, is an omnipresent environmental pollutant. Chronic and acute human exposures occur through exogenous and endogenous sources, including food, vapors of overheated cooking oil, house and forest fires, cigarette smoke, and automobile exhaust. Acrolein is a toxic byproduct of lipid peroxidation, which has been implicated in pulmonary, cardiac, and neurodegenerative diseases. This study shows that p53 is an initiating factor in acrolein-induced death receptor activation during apoptosis in A549 human lung cells. Exposure of cells to acrolein (0-50 micromol/L) mainly caused apoptosis, which was manifested by execution phase events such as condensation of nuclear chromatin, phosphatidylserine externalization, and poly(ADP-ribose) polymerase (PARP) cleavage. Levels of necrosis (approximately 5%) were low. Acrolein triggered the death receptor pathway of apoptosis, causing elevation of Fas ligand (FasL) and translocation of adaptor protein Fas-associated death domain to the plasma membrane. Acrolein caused activation of caspase-8, caspase-2, caspase-7, and the cross-talk pathway mediated by Bid cleavage. Activation of p53 and increased expression of p53-upregulated modulator of apoptosis (PUMA) occurred in response to acrolein. FasL upregulation and caspase-8 activation were decreased by p53 inhibitor pifithrin-alpha and antioxidant polyethylene glycol catalase. These findings increase our knowledge about the induction of cell death pathways by acrolein, which has important implications for human health.

Mitomycin C induces bystander killing in homogeneous and heterogeneous hepatoma cellular models

Background

Hepatocellular carcinoma (HCC) is one of the most common cancers worldwide that is particularly refractory to chemotherapy. Several studies have proposed combination chemotherapy regimen for HCC treatment. However, these therapies are not effective in regressing tumor and prolonging survival of patient's suffering from HCC. Therefore, the development of more effective therapeutic tools and new strategies for the treatment of HCC are urgently needed. Over the last decade much attention has been focused on "bystander effect" as a possible therapeutic strategy for the treatment of certain human tumors. Interest in this therapeutic approach originated from numerous reports describing the radiation induced bystander effect. However, the knowledge about chemotherapy induced bystander effect is still limited. Hence, chemotherapy induced bystander phenomenon in hepatoma cells was explored by utilizing Mitomycin C (MMC).

Results

MMC induced bystander killing was observed only in hepatoma cells and it did not occur in cervical cancer cells. MMC induced bystander killing was transferable via medium. It occurred in co-cultured cells indicating the involvement of secreted as well as membrane bound factors. FasL and TRAIL were detected in the conditioned medium from treated cells. In medium transfer experiment, pre-treatment with EDTA (a broad range protease inhibitor) diminished MMC induced bystander killing. Following drug exposure, expression of Fas and TRAIL receptors increased and treatment with neutralizing antibodies against FasL and TRAIL inhibited bystander killing.

Conclusion

Our results highlight the therapeutic importance of MMC in the treatment of HCC and implicate role of membrane bound and secreted forms of FasL and TRAIL in MMC induced bystander killing.

Membrane-bound Fas ligand requires RIP1 for efficient activation of caspase-8 within the death-inducing signaling complex

The serine-threonine kinase RIP1 was originally identified through its ability to bind to the death domain of Fas (CD95). RIP1 has been shown to be recruited to the Fas death-inducing signaling complex (DISC) and is required for the induction of necrotic cell death. In this study, we show that in Jurkat T lymphocytes, RIP1 is also necessary for the most efficient activation of downstream caspases by Fas when treated with membrane-bound Fas ligand, but not with agonistic Abs or cross-linked soluble Fas ligand. RIP1 participates in the Fas-associated death domain protein-mediated recruitment of caspase-8 to the Fas receptor complex in a manner that promotes caspase-8 activation. Cross-linking Abs, such as CH11, bypass the requirement for RIP1 in caspase activation by initiating larger, though less efficient, DISC complexes, while membrane-bound Fas ligand initiates a smaller but more efficient DISC that functions, in part, by effectively incorporating more RIP1 into the complex. Consequently, RIP1 is likely a more integral part of physiological signaling through the Fas/CD95 receptor complex than previously recognized; at least when the signal is mediated by full-length membrane-bound FasL. Cross-linked soluble FasL, which also occurs physiologically, behaves similarly to the CH11 Ab, and may therefore be more likely to initiate nonapoptotic Fas signaling due to less RIP1 in the receptor complex. Thus, agonists that bind the same Fas receptor initiate mechanistically distinct pathways resulting in differential cytotoxicity.

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

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

Acitretin induces apoptosis through CD95 signalling pathway in human cutaneous squamous cell carcinoma cell line SCL-1

Skin cancers are by far the most common human malignancies. Retinoids have shown promising preventive and therapeutic effects against a variety of human malignancies. The aim of this study was to investigate the apoptosis-inducing effect of acitretin on human skin squamous cell carcinoma (SCC) SCL-1 cells. We found that acitretin preferentially inhibited the growth of SCL-1 cells in a dose- and time-dependent manner, but not of non-malignant keratinocyte HaCaT cells. This inhibition appeared to be due to induction of apoptosis as revealed by enzyme-linked immunosorbent assay. AnnexinV/propidium iodide assay and morphological observation confirmed the pro-apoptotic effect of acitretin on SCL-1 cells. We further demonstrated that apoptosis was induced within 1-2 days and involved activation of caspases-8, -9, -3 and poly (ADP-ribose) polymerase (PARP). Caspase-8 inhibitor effectively suppressed acitretin-induced apoptosis whereas caspase-9 inhibitor did not. Acitretin increased the levels of CD95 (Fas), CD95-ligand and Fas-associated death domain. Neutralizing ZB4 anti-Fas antibody significantly inhibited the apoptosis in SCL-1 cells induced by acitretin. These results suggest that acitretin is able to induce apoptosis in skin cancer cells possibly via death receptor CD95 apoptosis pathway without affecting the viability of normal keratinocyte.

Palmitoylation of the TRAIL receptor DR4 confers an efficient TRAIL-induced cell death signalling

S-palmitoylation is a lipid modification that regulates membrane–protein association and influences protein trafficking, stability or aggregation, thus playing an important role in protein signalling. We previously demonstrated that the palmitoylation of Fas, one of the DD (death domain)-containing members of the TNFR [TNF (tumour necrosis factor) receptor] superfamily, is essential for the redistribution of this receptor into lipid rafts, an obligatory step for the death signal transmission. Here we investigate the requirement of protein palmitoylation in the activities of other DD-containing death receptors. We show that DR4 is palmitoylated, whereas DR5 and TNFR1 are not. Furthermore, DR4 palmitoylation is required for its raft localization and its ability to oligomerize, two essential features in TRAIL (TNF-related apoptosis-inducing ligand)-induced death signal transmission.

Enforced covalent trimerisation of soluble feline CD134 (OX40)-ligand generates a functional antagonist of feline immunodeficiency virus

The feline immunodeficiency virus (FIV) targets activated CD4-positive helper T cells preferentially, inducing an AIDS-like immunodeficiency in its natural host species, the domestic cat. The primary receptor for FIV is CD134, a member of the tumour necrosis factor receptor superfamily (TNFRSF) and all primary viral strains tested to date use CD134 for infection. To investigate the effect of the natural ligand for CD134 on FIV infection, feline CD134L was cloned and expressed in soluble forms. However, in contrast to murine or human CD134L, soluble feline CD134L (sCD134L) did not bind to CD134. Receptor-binding activity was restored by enforced covalent trimerisation following the introduction of a synthetic trimerisation domain from tenascin (TNC). Feline and human TNC-CD134Ls retained the species-specificity of the membrane-bound forms of the ligand while murine TNC-CD134L displayed promiscuous binding to feline, human or murine CD134. Feline and murine TNC-CD134Ls were antagonists of FIV infection; however, potency was both strain-specific and substrate-dependent, indicating that the modulatory effects of endogenous sCD134L, or exogenous CD134Lbased therapeutics, may vary depending on the viral strain.