An essential role for membrane rafts in the initiation of Fas/CD95-triggered cell death in mouse thymocytes

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.

Strong activation of p53 by actinomycin D and nutlin-3a overcomes the resistance of cancer cells to the pro-apoptotic activity of the FAS ligand

The FAS ligand (FASLG) is expressed on lymphocytes, which employ it to activate death receptors on target cells. Cancer cells are generally resistant to apoptosis triggered by FASLG. In this work, we found a way to circumvent this resistance by treatment with actinomycin D (ActD) and nutlin-3a (Nut3a). We selected this drug combination based on our transcriptomic data showing strong activation of proapoptotic genes, including those for receptor-mediated apoptosis, in cells exposed to actinomycin D and nutlin-3a. To test our hypothesis, we pre-exposed cancer cell lines to this drug combination for 45 h and then treated them with recombinant FASLG. This almost instantaneously killed most cells. Actinomycin D and nutlin-3a strongly cooperated in the sensitization because the effect of the drugs acting solo was not as spectacular as the drug combination, which together with FASLG killed more than 99% of cells. Based on the caspase activation pattern (caspase-8, caspase-9, caspase-10), we conclude that both extrinsic and intrinsic pro-apoptotic pathways were engaged. In engineered p53-deficient cells, this pro-apoptotic effect was completely abrogated. Therefore, the combination of ActD + Nut3a activates p53 in an extraordinary way, which overcomes the resistance of cancer cells to apoptosis triggered by FASLG. Interestingly, other combinations of drugs, e.g., etoposide + nutlin-3a, actinomycin D + RG7112, and actinomycin D + idasanutlin had a similar effect. Moreover, normal human fibroblasts are less sensitive to death induced by ActD + Nut3a + FASLG. Our findings create the opportunity to revive the abandoned attempts of cancer immunotherapy employing the recombinant FAS ligand.

β-Sitosterol as a Promising Anticancer Agent for Chemoprevention and Chemotherapy: Mechanisms of Action and Future Prospects

Cancer is one of the primary causes of death worldwide, and its incidence continues to increase yearly. Despite significant advances in research, the search for effective and nontoxic preventive and therapeutic agents remains greatly important. Cancer is a multimodal disease, where various mechanisms play significant roles in its occurrence and progression. This highlights the need for multitargeted approaches that are not only safe and inexpensive but also provide effective alternatives for current therapeutic regimens. β-Sitosterol (SIT), the most abundant phytosterol found in various plant foods, represents such an option. Preclinical evidence over the past few decades has overwhelmingly shown that SIT exhibits multiple anticancer activities against varied cancers, such as liver, cervical, colon, stomach, breast, lung, pancreatic, and prostate cancers, in addition to leukemia, multiple myeloma, melanoma, and fibrosarcoma. In this article, we present the latest advances and perspectives on SIT-systematically summarizing its antitumor mechanisms of action into 7 main sections and combining current challenges and prospects-for its use as a promising agent for cancer prevention and treatment. In particular, SIT plays a role in cancer prevention and treatment mainly by enhancing apoptosis, inducing cell cycle arrest, bidirectionally regulating oxidative stress, improving metabolic reprogramming, inhibiting invasion and metastasis, modulating immunity and inflammation, and combating drug resistance. Although SIT holds such great promise, the poor aqueous solubility and bioavailability coupled with low targeting efficacy limit its therapeutic efficacy and clinical application. Further research on novel drug delivery systems may improve these deficiencies. Overall, through complex and pleiotropic mechanisms, SIT has good potential for tumor chemoprevention and chemotherapy. However, no clinical trials have yet proven this potential. This review provides theoretical basis and rationality for the further design and conduct of clinical trials to confirm the anticancer activity of SIT.

Advances in the Pathophysiology of Thrombosis in Antiphospholipid Syndrome: Molecular Mechanisms and Signaling through Lipid Rafts

The pathological features of antiphospholipid syndrome (APS) are related to the activity of circulating antiphospholipid antibodies (aPLs) associated with vascular thrombosis and obstetric complications. Indeed, aPLs are not only disease markers, but also play a determining pathogenetic role in APS and exert their effects through the activation of cells and coagulation factors and inflammatory mediators for the materialization of the thromboinflammatory pathogenetic mechanism. Cellular activation in APS necessarily involves the interaction of aPLs with target receptors on the cell membrane, capable of triggering the signal transduction pathway(s). This interaction occurs at specific microdomains of the cell plasma membrane called lipid rafts. In this review, we focus on the key role of lipid rafts as signaling platforms in the pathogenesis of APS, and propose this pathogenetic step as a strategic target of new therapies in order to improve classical anti-thrombotic approaches with "new" immunomodulatory drugs.

Rafting on the Plasma Membrane: Lipid Rafts in Signaling and Disease

The plasma membrane is not a uniform phospholipid bilayer; it has specialized membrane nano- or microdomains called lipid rafts. Lipid rafts are small cholesterol and sphingolipid-rich plasma membrane islands. Although their existence was long debated, their presence in the plasma membrane of living cells is now well accepted with the advent of super-resolution imaging techniques. It is interesting to note that lipid rafts function to compartmentalize receptors and their regulators and substantially modulate cellular signaling. In this review, we will examine the role of lipid rafts and caveolae-lipid raft-like microdomains with a distinct 3D morphology-in cellular signaling. Moreover, we will investigate how raft compartmentalized signaling regulates diverse physiological processes such as proliferation, apoptosis, immune signaling, and development. Also, the deregulation of lipid raft-mediated signaling during tumorigenesis and metastasis will be explored.

Alteration of cholesterol distribution at the plasma membrane of cancer cells: From evidence to pathophysiological implication and promising therapy strategy

Cholesterol-enriched domains are nowadays proposed to contribute to cancer cell proliferation, survival, death and invasion, with important implications in tumor progression. They could therefore represent promising targets for new anticancer treatment. However, although diverse strategies have been developed over the years from directly targeting cholesterol membrane content/distribution to adjusting sterol intake, all approaches present more or less substantial limitations. Those data emphasize the need to optimize current strategies, to develop new specific cholesterol-targeting anticancer drugs and/or to combine them with additional strategies targeting other lipids than cholesterol. Those objectives can only be achieved if we first decipher (i) the mechanisms that govern the formation and deformation of the different types of cholesterol-enriched domains and their interplay in healthy cells; (ii) the mechanisms behind domain deregulation in cancer; (iii) the potential generalization of observations in different types of cancer; and (iv) the specificity of some alterations in cancer vs. non-cancer cells as promising strategy for anticancer therapy. In this review, we will discuss the current knowledge on the homeostasis, roles and membrane distribution of cholesterol in non-tumorigenic cells. We will then integrate documented alterations of cholesterol distribution in domains at the surface of cancer cells and the mechanisms behind their contribution in cancer processes. We shall finally provide an overview on the potential strategies developed to target those cholesterol-enriched domains in cancer therapy.

Clusters of apoptotic signaling molecule-enriched rafts, CASMERs: membrane platforms for protein assembly in Fas/CD95 signaling and targets in cancer therapy

Mammalian cells show the ability to commit suicide through the activation of death receptors at the cell surface. Death receptors, among which Fas/CD95 is one of their most representative members, lack enzymatic activity, and depend on protein-protein interactions to signal apoptosis. Fas/CD95 death receptor-mediated apoptosis requires the formation of the so-called death-inducing signaling complex (DISC), bringing together Fas/CD95, Fas-associated death domain-containing protein and procaspase-8. In the last two decades, cholesterol-rich lipid raft platforms have emerged as scaffolds where Fas/CD95 can be recruited and clustered. The co-clustering of Fas/CD95 and rafts facilitates DISC formation, bringing procaspase-8 molecules to be bunched together in a limited membrane region, and leading to their autoproteolytic activation by oligomerization. Lipid raft platforms serve as a specific region for the clustering of Fas/CD95 and DISC, as well as for the recruitment of additional downstream signaling molecules, thus forming the so-called cluster of apoptotic signaling molecule-enriched rafts, or CASMER. These raft/CASMER structures float in the membrane like icebergs, in which the larger portion lies inside the cell and communicates with other subcellular structures to facilitate apoptotic signal transmission. This allows an efficient spatiotemporal compartmentalization of apoptosis signaling machinery during the triggering of cell death. This concept of proapoptotic raft platforms as a basic chemical-biological structure in the regulation of cell death has wide-ranging implications in human biology and disease, as well as in cancer therapy. Here, we discuss how these raft-centered proapoptotic hubs operate as a major linchpin for apoptosis signaling and as a promising target in cancer therapy.

Suppression of the ABCA1 Cholesterol Transporter Impairs the Growth and Migration of Epithelial Ovarian Cancer

Simple Summary

Epithelial ovarian cancer (EOC) is the most lethal gynaecological cancer. Over 80% of cases have already spread at diagnosis, and these patients face a five-year survival rate of 35%. EOC cells often spread to the greater omentum, an abdominal fat pad. Here, EOC cells take-up cholesterols. Excessive amounts of cholesterol are lethal; thus, we proposed that the ABCA1 cholesterol transporter exports cholesterol from serous EOC cells to maintain cholesterol balance. Indeed, we found that reducing the level of ABCA1 could suppress serous EOC growth in two-dimensional as well as three-dimensional cell culture and also hindered their migration, a key process required for cancer spread. We also identified drugs that impair EOC cell growth by inhibiting cholesterol export. Our data demonstrate that disrupting the cholesterol balance by targeting ABCA1 may be an effective treatment strategy for EOC patients.

Abstract

Background: Epithelial ovarian cancer (EOC) is the most lethal gynaecological malignancy with over 80% of cases already disseminated at diagnosis and facing a dismal five-year survival rate of 35%. EOC cells often spread to the greater omentum where they take-up cholesterol. Excessive amounts of cholesterol can be cytocidal, suggesting that cholesterol efflux through transporters may be important to maintain homeostasis, and this may explain the observation that high expression of the ATP-binding cassette A1 (ABCA1) cholesterol transporter has been associated with poor outcome in EOC patients. Methods: ABCA1 expression was silenced in EOC cells to investigate the effect of inhibiting cholesterol efflux on EOC biology through growth and migration assays, three-dimensional spheroid culture and cholesterol quantification. Results: ABCA1 suppression significantly reduced the growth, motility and colony formation of EOC cell lines as well as the size of EOC spheroids, whilst stimulating expression of ABCA1 reversed these effects. In serous EOC cells, ABCA1 suppression induced accumulation of cholesterol. Lowering cholesterol levels using methyl-B-cyclodextrin rescued the effect of ABCA1 suppression, restoring EOC growth. Furthermore, we identified FDA-approved agents that induced cholesterol accumulation and elicited cytocidal effects in EOC cells. Conclusions: Our data demonstrate the importance of ABCA1 in maintaining cholesterol balance and malignant properties in EOC cells, highlighting its potential as a therapeutic target for this disease.

Lipid raft involvement in signal transduction in cancer cell survival, cell death and metastasis

Lipid rafts are cholesterol- and sphingolipid-enriched specialized membrane domains within the plasma membrane. Lipid rafts regulate the density and activity of signal receptors by compartmentalizing them, promoting signalling cascades that play important roles in the survival, death and metastasis of cancer cells. In this review, we emphasize the current concept initially postulated by F. Mollinedo and C. Gajate on the importance of lipid rafts in cancer survival, death and metastasis by describing representative signalling pathways, including the IGF system and the PI3K/AKT, Fas/CD95, VEGF/VEGFR2 and CD44 signalling pathways, and we also discuss the concept of CASMER (cluster of apoptotic signalling molecule-enriched rafts), coined, originally introduced and further advanced by F. Mollinedo and C. Gajate in the period 2005-2010. Then, we summarize relevant research progress and suggest that lipid rafts play important roles in the survival, death and metastasis of cancer cells, making them promising targets for cancer therapy.

Lipid rafts as signaling hubs in cancer cell survival/death and invasion: implications in tumor progression and therapy: Thematic Review Series: Biology of Lipid Rafts

Cholesterol/sphingolipid-rich membrane domains, known as lipid rafts or membrane rafts, play a critical role in the compartmentalization of signaling pathways. Physical segregation of proteins in lipid rafts may modulate the accessibility of proteins to regulatory or effector molecules. Thus, lipid rafts serve as sorting platforms and hubs for signal transduction proteins. Cancer cells contain higher levels of intracellular cholesterol and lipid rafts than their normal non-tumorigenic counterparts. Many signal transduction processes involved in cancer development (insulin-like growth factor system and phosphatidylinositol 3-kinase-AKT) and metastasis [cluster of differentiation (CD)44] are dependent on or modulated by lipid rafts. Additional proteins playing an important role in several malignant cancers (e.g., transmembrane glycoprotein mucin 1) are also being detected in association with lipid rafts, suggesting a major role of lipid rafts in tumor progression. Conversely, lipid rafts also serve as scaffolds for the recruitment and clustering of Fas/CD95 death receptors and downstream signaling molecules leading to cell death-promoting raft platforms. The partition of death receptors and downstream signaling molecules in aggregated lipid rafts has led to the formation of the so-called cluster of apoptotic signaling molecule-enriched rafts, or CASMER, which leads to apoptosis amplification and can be pharmacologically modulated. These death-promoting rafts can be viewed as a linchpin from which apoptotic signals are launched. In this review, we discuss the involvement of lipid rafts in major signaling processes in cancer cells, including cell survival, cell death, and metastasis, and we consider the potential of lipid raft modulation as a promising target in cancer therapy.

Ordered Lipid Domains Assemble via Concerted Recruitment of Constituents from Both Membrane Leaflets

Lipid rafts serve as anchoring platforms for membrane proteins. Thus far they escaped direct observation by light microscopy due to their small size. Here we used differently colored dyes as reporters for the registration of both ordered and disordered lipids from the two leaves of a freestanding bilayer. Photoswitchable lipids dissolved or reformed the domains. Measurements of domain mobility indicated the presence of 120 nm wide ordered and 40 nm wide disordered domains. These sizes are in line with the predicted roles of line tension and membrane undulation as driving forces for alignment.

The anticonvulsive Phenhydan® suppresses extrinsic cell death

Different forms of regulated cell death-like apoptosis and necroptosis contribute to the pathophysiology of clinical conditions including ischemia-reperfusion injury, myocardial infarction, sepsis, and multiple sclerosis. In particular, the kinase activity of the receptor-interacting serine/threonine protein kinase 1 (RIPK1) is crucial for cell fate in inflammation and cell death. However, despite its involvement in pathological conditions, no pharmacologic inhibitor of RIPK1-mediated cell death is currently in clinical use. Herein, we screened a collection of clinical compounds to assess their ability to modulate RIPK1-mediated cell death. Our small-scale screen identified the anti-epilepsy drug Phenhydan® as a potent inhibitor of death receptor-induced necroptosis and apoptosis. Accordingly, Phenhydan® blocked activation of necrosome formation/activation as well as death receptor-induced NF-κB signaling by influencing the membrane function of cells, such as lipid raft formation, thus exerting an inhibitory effect on pathophysiologic cell death processes. By targeting death receptor signaling, the already FDA-approved Phenhydan® may provide new therapeutic strategies for inflammation-driven diseases caused by aberrant cell death.

Role of Bioactive Sphingolipids in Inflammation and Eye Diseases

Inflammation is a common underlying factor in a diversity of ocular diseases, ranging from macular degeneration, autoimmune uveitis, glaucoma, diabetic retinopathy and microbial infection. In addition to the variety of known cellular mediators of inflammation, such as cytokines, chemokines and lipid mediators, there is now considerable evidence that sphingolipid metabolites also play a central role in the regulation of inflammatory pathways. Various sphingolipid metabolites, such as ceramide (Cer), ceramide-1-phosphate (C1P), sphingosine-1-phosphate (S1P), and lactosylceramide (LacCer) can contribute to ocular inflammatory diseases through multiple pathways. For example, inflammation generates Cer from sphingomyelins (SM) in the plasma membrane, which induces death receptor ligand formation and leads to apoptosis of retinal pigment epithelial (RPE) and photoreceptor cells. Inflammatory stress by reactive oxygen species leads to LacCer accumulation and S1P secretion and induces proliferation of retinal endothelial cells and eventual formation of new vessels. In sphingolipid/lysosomal storage disorders, sphingolipid metabolites accumulate in lysosomes and can cause ocular disorders that have an inflammatory etiology. Sphingolipid metabolites activate complement factors in the immune-response mediated pathogenesis of macular degeneration. These examples highlight the integral association between sphingolipids and inflammation in ocular diseases.

Molecular Targets of Epigallocatechin-Gallate (EGCG): A Special Focus on Signal Transduction and Cancer

Green tea is a beverage that is widely consumed worldwide and is believed to exert effects on different diseases, including cancer. The major components of green tea are catechins, a family of polyphenols. Among them, epigallocatechin-gallate (EGCG) is the most abundant and biologically active. EGCG is widely studied for its anti-cancer properties. However, the cellular and molecular mechanisms explaining its action have not been completely understood, yet. EGCG is effective in vivo at micromolar concentrations, suggesting that its action is mediated by interaction with specific targets that are involved in the regulation of crucial steps of cell proliferation, survival, and metastatic spread. Recently, several proteins have been identified as EGCG direct interactors. Among them, the trans-membrane receptor 67LR has been identified as a high affinity EGCG receptor. 67LR is a master regulator of many pathways affecting cell proliferation or apoptosis, also regulating cancer stem cells (CSCs) activity. EGCG was also found to be interacting directly with Pin1, TGFR-II, and metalloproteinases (MMPs) (mainly MMP2 and MMP9), which respectively regulate EGCG-dependent inhibition of NF-kB, epithelial-mesenchimal transaction (EMT) and cellular invasion. EGCG interacts with DNA methyltransferases (DNMTs) and histone deacetylases (HDACs), which modulates epigenetic changes. The bulk of this novel knowledge provides information about the mechanisms of action of EGCG and may explain its onco-suppressive function. The identification of crucial signalling pathways that are related to cancer onset and progression whose master regulators interacts with EGCG may disclose intriguing pharmacological targets, and eventually lead to novel combined treatments in which EGCG acts synergistically with known drugs.

Lipid rafts in immune signalling: current progress and future perspective

Lipid rafts are dynamic assemblies of proteins and lipids that harbour many receptors and regulatory molecules and so act as a platform for signal transduction. They float freely within the liquid-disordered bilayer of cellular membranes and can cluster to form larger ordered domains. Alterations in lipid rafts are commonly found to be associated with the pathogenesis of several human diseases and recent reports have shown that the raft domains can also be perturbed by targeting raft proteins through microRNAs. Over the last few years, the importance of lipid rafts in modulating both innate and acquired immune responses has been elucidated. Various receptors present on immune cells like B cells, T cells, basophils and mast cells associate with lipid rafts on ligand binding and initiate signalling cascades leading to inflammation. Furthermore, disrupting lipid raft integrity alters lipopolysaccharide-induced cytokine secretion, IgE signalling, and B-cell and T-cell activation. The objective of this review is to summarize the recent progress in understanding the role of lipid rafts in the modulation of immune signalling and its related therapeutic potential for autoimmune diseases and inflammatory disorders.

Sphingolipid metabolism affects the anticancer effect of cisplatin

Cisplatin, a DNA crosslinking agent, is widely used for the treatment of a variety of solid tumors. Numerous studies have demonstrated that sphingolipid metabolism, which acts as a target for cisplatin treatment, is a highly complex network that consists of sphingolipid signaling molecules and related catalytic enzymes. Ceramide (Cer), which is the central molecule of this network, has been established to induce apoptosis. However, another molecule, sphingosine-1-phosphate (S1P), exerts the opposite function, i.e., serves as a regulator of pro-survival. Other sphingolipid molecules, including dihydroceramide, ceramide-1-phosphate, glucosylceramide (GluCer), and sphingosine (Sph), or sphingolipid catalytic enzymes such as Sph kinase (SphK), Cer synthase (CerS), and S1P lyase, have also attracted considerable attention, particularly Cer, GluCer, SphK, CerS, and S1P lyase, which have been implicated in cisplatin resistance. This review summarizes specific molecules involved in sphingolipid metabolism and related catalytic enzymes affecting the anticancer effect of cisplatin, particularly in relation to induction of apoptosis and drug resistance.

Metabolic Precursor of Cholesterol Causes Formation of Chained Aggregates of Liquid-Ordered Domains

7-Dehydrocholesterol, an immediate metabolic predecessor of cholesterol, can accumulate in tissues due to some metabolic abnormalities, causing an array of symptoms known as Smith-Lemli-Opitz syndrome. Enrichment of cellular membranes with 7-dehydrocholesterol interferes with normal cell-signaling processes, which involve interaction between rafts and formation of the so-called signaling platforms. In model membranes, cholesterol-based ordered domains usually merge upon contact. According to our experimental data, ordered domains in the model systems where cholesterol is substituted for 7-dehydrocholesterol never merge on the time scale of the experiment, but clusterize into necklace-like aggregates. We attribute such different dynamical behavior to altered properties of the domain boundary. In the framework of thickness mismatch model, we analyzed changes of interaction energy profiles of two approaching domains caused by substitution of cholesterol by 7-dehydrocholesterol. The energy barrier for domain merger is shown to increase notably, with simultaneous appearance of another distinct local energy minimum. Such energy profile is in perfect qualitative agreement with the experimental observations. The observed change of domain dynamics can impair proper interaction between cellular rafts underlying pathologies associated with deviations in cholesterol metabolism.

Role of mitochondrial raft-like microdomains in the regulation of cell apoptosis

Lipid rafts are envisaged as lateral assemblies of specific lipids and proteins that dissociate and associate rapidly and form functional clusters in cell membranes. These structural platforms are not confined to the plasma membrane; indeed lipid microdomains are similarly formed at subcellular organelles, which include endoplasmic reticulum, Golgi and mitochondria, named raft-like microdomains. In addition, some components of raft-like microdomains are present within ER-mitochondria associated membranes. This review is focused on the role of mitochondrial raft-like microdomains in the regulation of cell apoptosis, since these microdomains may represent preferential sites where key reactions take place, regulating mitochondria hyperpolarization, fission-associated changes, megapore formation and release of apoptogenic factors. These structural platforms appear to modulate cytoplasmic pathways switching cell fate towards cell survival or death. Main insights on this issue derive from some pathological conditions in which alterations of microdomains structure or function can lead to severe alterations of cell activity and life span. In the light of the role played by raft-like microdomains to integrate apoptotic signals and in regulating mitochondrial dynamics, it is conceivable that these membrane structures may play a role in the mitochondrial alterations observed in some of the most common human neurodegenerative diseases, such as Amyotrophic lateral sclerosis, Huntington's chorea and prion-related diseases. These findings introduce an additional task for identifying new molecular target(s) of pharmacological agents in these pathologies.

Absence of Activation of DNA Repair Genes and Excellent Efficacy of Phosphaplatins against Human Ovarian Cancers: Implications To Treat Resistant Cancers

Phosphaplatins, platinum(II) and platinum(IV) complexes coordinated to a pyrophosphate moiety, exhibit excellent antitumor activities against a variety of cancers. To determine whether phosphaplatins trigger resistance to treatment by engaging DNA damage repair genes, a yeast genome-wide fitness assay was used. Treatment of yeast cells with pyrodach-2 (D2) or pyrodach-4 (D4) revealed no particular sensitivity to nucleotide excision repair, homologous recombination repair, or postreplication repair when compared with platin control compounds. Also, TNF receptor superfamily member 6 (FAS) protein was overexpressed in phosphaplatin-treated ovarian tumor cells, and platinum colocalized with FAS protein in lipid rafts. An overactivation of sphingomyelinase (ASMase) was noted in the treated cells, indicating participation of an extrinsic apoptotic mechanism due to increased ceramide release. Our results indicate that DNA is not the target of phosphaplatins and accordingly, that phosphaplatins might not cause resistance to treatment. Activation of ASMase and FAS along with the colocalization of platinum with FAS in lipid rafts support an extrinsic apoptotic signaling mechanism that is mediated by phosphaplatins.

Agonist mobility on supported lipid bilayers affects Fas mediated death response

Extrinsic apoptosis is initiated by recognition and clustering of the single-pass transmembrane proteins Fas ligand and Fas expressed at the surface of closely apposed lymphocytes and target cells, respectively. Since Fas-mediated death response was mainly studied with soluble antibodies, the mobility constraints for receptor activation by a membrane embedded agonist is not well understood. We explored this influence by stimulating apoptosis on functionalized supported lipid bilayers, where we quantified agonist mobility by z-scan fluorescence correlation spectroscopy. Using different lipid compositions, we show that the apoptotic response correlates with increased lateral mobility of the agonist in the lipid bilayer.

Exploring the link between ceramide and ionizing radiation

The aim of radiotherapy is to eradicate cancer cells with ionizing radiation; tumor cell death following irradiation can be induced by several signaling pathways, most of which are triggered as a consequence of DNA damage, the primary and major relevant cell response to radiation. Several lines of evidence demonstrated that ceramide, a crucial sensor and/or effector of different signalling pathways promoting cell cycle arrest, death and differentiation, is directly involved in the molecular mechanisms underlying cellular response to irradiation. Most of the studies strongly support a direct relationship between ceramide accumulation and radiation-induced cell death, mainly apoptosis; for this reason, defining the contribution of the multiple metabolic pathways leading to ceramide formation and the causes of its dysregulated metabolism represent the main goal in order to elucidate the ceramide-mediated signaling in radiotherapy. In this review, we summarize the current knowledge concerning the different routes leading to ceramide accumulation in radiation-induced cell response with particular regard to the role of the enzymes involved in both ceramide neogenesis and catabolism. Emphasis is placed on sphingolipid breakdown as mechanism of ceramide generation activated following cell irradiation; the functional relevance of this pathway, and the role of glycosphingolipid glycohydrolases as direct targets of ionizing radiation are also discussed. These new findings add a further attractive point of investigation to better define the complex interplay between sphingolipid metabolism and radiation therapy.

Subcellular localization and activation of ADAM proteases in the context of FasL shedding in T lymphocytes

The "A Disintegrin And Metalloproteinases" (ADAMs) form a subgroup of the metzincin endopeptidases. Proteolytically active members of this protein family act as sheddases and govern key processes in development and inflammation by regulating cell surface expression and release of cytokines, growth factors, adhesion molecules and their receptors. In T lymphocytes, ADAM10 sheds the death factor Fas Ligand (FasL) and thereby regulates T cell activation, death and effector function. Although FasL shedding by ADAM10 was confirmed in several studies, its regulation is still poorly defined. We recently reported that ADAM10 is highly abundant on T cells whereas its close relative ADAM17 is expressed at low levels and transiently appears at the cell surface upon stimulation. Since FasL is also stored intracellularly and brought to the plasma membrane upon stimulation, we addressed where the death factor gets exposed to ADAM proteases. We report for the first time that both ADAM10 and ADAM17 are associated with FasL-containing secretory lysosomes. Moreover, we demonstrate that TCR/CD3/CD28-stimulation induces a partial positioning of both proteases and FasL to lipid rafts and only the activation-induced raft-positioning results in FasL processing. TCR/CD3/CD28-induced FasL proteolysis is markedly affected by reducing both ADAM10 and ADAM17 protein levels, indicating that in human T cells also ADAM17 is implicated in FasL processing. Since FasL shedding is affected by cholesterol depletion and by inhibition of Src kinases or palmitoylation, we conclude that it requires mobilization and co-positioning of ADAM proteases in lipid raft-like platforms associated with an activation of raft-associated Src-family kinases.

Lipid rafts as major platforms for signaling regulation in cancer

Cell signaling does not apparently occur randomly over the cell surface, but it seems to be integrated very often into cholesterol-rich membrane domains, termed lipid rafts. Membrane lipid rafts are highly ordered membrane domains that are enriched in cholesterol, sphingolipids and gangliosides, and behave as major modulators of membrane geometry, lateral movement of molecules, traffic and signal transduction. Because the lipid and protein composition of membrane rafts differs from that of the surrounding membrane, they provide an additional level of compartmentalization, serving as sorting platforms and hubs for signal transduction proteins. A wide number of signal transduction processes related to cell adhesion, migration, as well as to cell survival and proliferation, which play major roles in cancer development and progression, are dependent on lipid rafts. Despite lipid rafts harbor mainly critical survival signaling pathways, including insulin-like growth factor I (IGF-I)/phosphatidylinositol 3-kinase (PI3K)/Akt signaling, recent evidence suggests that these membrane domains can also house death receptor-mediated apoptotic signaling. Recruitment of this death receptor signaling pathway in membrane rafts can be pharmacologically modulated, thus opening up the possibility to regulate cell demise with a therapeutic use. The synthetic ether phospholipid edelfosine shows a high affinity for cholesterol and accumulates in lipid rafts in a number of malignant hematological cells, leading to an efficient in vitro and in vivo antitumor activity by inducing translocation of death receptors and downstream signaling molecules to these membrane domains. Additional antitumor drugs have also been shown to act, at least in part, by recruiting death receptors in lipid rafts. The partition of death receptors together with downstream apoptotic signaling molecules in membrane rafts has led us to postulate the concept of a special liquid-ordered membrane platform coined as "cluster of apoptotic signaling molecule-enriched rafts" (CASMER), referring to raft platforms enriched in apoptotic molecules. CASMERs act as scaffolds for apoptosis signaling compartmentalization, facilitating and stabilizing protein-protein interactions by local assembly of cross-interacting molecules, which leads to apoptosis amplification and a decrease in apoptotic signal threshold. Edelfosine also displaced survival PI3K/Akt signaling from lipid rafts, leading to Akt inhibition, in mantle cell lymphoma cells. Thus, membrane rafts could act as scaffold structures where segregation of pro- from anti-apoptotic molecules could take place. In this review, we summarize our view of how reorganization of the protein composition of lipid raft membrane domains regulates cell death and therefore it might be envisaged as a novel target in the treatment of cancer.

The nanoscale organization of the B lymphocyte membrane

The fluid mosaic model of Singer and Nicolson correctly predicted that the plasma membrane (PM) forms a lipid bi-layer containing many integral trans-membrane proteins. This model also suggested that most of these proteins were randomly dispersed and freely diffusing moieties. Initially, this view of a dynamic and rather unorganized membrane was supported by early observations of the cell surfaces using the light microscope. However, recent studies on the PM below the diffraction limit of visible light (~250nm) revealed that, at nanoscale dimensions, membranes are highly organized and compartmentalized structures. Lymphocytes are particularly useful to study this nanoscale membrane organization because they grow as single cells and are not permanently engaged in cell:cell contacts within a tissue that can influence membrane organization. In this review, we describe the methods that can be used to better study the protein:protein interaction and nanoscale organization of lymphocyte membrane proteins, with a focus on the B cell antigen receptor (BCR). Furthermore, we discuss the factors that may generate and maintain these membrane structures.

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.

Role of caspase-8 in thymus function

The thymus is the primary organ responsible for de novo generation of immunocompetent T cells that have a diverse repertoire of antigen recognition. During the developmental process, 98% of thymocytes die by apoptosis. Thus apoptosis is a dominant process in the thymus and occurs through either death by neglect or negative selection or through induction by stress/aging. Caspase activation is an essential part of the general apoptosis mechanism, and data suggest that caspases may have a role in negative selection; however, it seems more probable that caspase-8 activation is involved in death by neglect, particularly in glucocorticoid-induced thymocyte apoptosis. Caspase-8 is active in double-positive (DP) thymocytes in vivo and can be activated in vitro in DP thymocytes by T-cell receptor (TCR) crosslinking to induce apoptosis. Caspase-8 is a proapoptotic member of the caspase family and is considered an initiator caspase, which is activated upon stimulation of a death receptor (e.g., Fas), recruitment of the adaptor molecule FADD, and recruitment and subsequent processing of procaspase-8. The main role of caspase-8 seems to be pro-apoptotic and, in this review, we will discuss about the involvement of caspase-8 in (1) TCR-triggered thymic apoptosis; (2) death receptor-mediated thymic apoptosis; and (3) glucocorticoid-induced thymic apoptosis. Regarding TCR triggering, caspase-8 is active in medullary, semi-mature heat-stable antigen(hi) (HAS(hi) SP) thymocytes as a consequence of strong TCR stimulation. The death receptors Fas, FADD, and FLIP are involved upstream of caspase-8 activation in apoptosis; whereas, Bid and HDAC7 are involved downstream of caspase-8. Finally, caspase-8 is involved in glucocortocoid-induced thymocyte apoptosis through an activation loop with the protein GILZ. GILZ activates caspase-8, promoting GILZ sumoylation and its protection from proteasomal degradation.

High molecular weight complex analysis of Epstein-Barr virus Latent Membrane Protein 1 (LMP-1): structural insights into LMP-1's homo-oligomerization and lipid raft association

LMP-1 is a constitutively active Tumor Necrosis Factor Receptor analog encoded by Epstein-Barr virus. LMP-1 activation correlates with oligomerization and raft localization, but direct evidence of LMP-1 oligomers is limited. We report that LMP-1 forms multiple high molecular weight native LMP-1 complexes when analyzed by BN-PAGE, the largest of which are enriched in detergent resistant membranes. The largest of these high molecular weight complexes are not formed by purified LMP-1 or by loss of function LMP-1 mutants. Consistent with these results we find a dimeric form of LMP-1 that can be stabilized by disulfide crosslinking. We identify cysteine 238 in the C-terminus of LMP-1 as the crosslinked cysteine. Disulfide crosslinking occurs post-lysis but the dimer can be crosslinked in intact cells with membrane permeable crosslinkers. LMP-1/C238A retains wild type LMP-1 NF-κB activity. LMP-1's TRAF binding, raft association and oligomerization are associated with the dimeric form of LMP-1. Our results suggest the possibility that the observed dimeric species results from inter-oligomeric crosslinking of LMP-1 molecules in adjacent core LMP-1 oligomers.

Role of ceramide in diabetes mellitus: evidence and mechanisms

Diabetes mellitus is a metabolic disease with multiple complications that causes serious diseases over the years. The condition leads to severe economic consequences and is reaching pandemic level globally. Much research is being carried out to address this disease and its underlying molecular mechanism. This review focuses on the diverse role and mechanism of ceramide, a prime sphingolipid signaling molecule, in the pathogenesis of type 1 and type 2 diabetes and its complications. Studies using cultured cells, animal models, and human subjects demonstrate that ceramide is a key player in the induction of β-cell apoptosis, insulin resistance, and reduction of insulin gene expression. Ceramide induces β-cell apoptosis by multiple mechanisms namely; activation of extrinsic apoptotic pathway, increasing cytochrome c release, free radical generation, induction of endoplasmic reticulum stress and inhibition of Akt. Ceramide also modulates many of the insulin signaling intermediates such as insulin receptor substrate, Akt, Glut-4, and it causes insulin resistance. Ceramide reduces the synthesis of insulin hormone by attenuation of insulin gene expression. Better understanding of this area will increase our understanding of the contribution of ceramide to the pathogenesis of diabetes, and further help in identifying potential therapeutic targets for the management of diabetes mellitus and its complications.

Intrinsic and extrinsic apoptotic pathway signaling as determinants of histone deacetylase inhibitor antitumor activity

Histone deacetylase inhibitors (HDACi) can elicit a range of biological responses that impede the growth and/or survival of tumor cells. Depending on the physiological context, HDACi can induce apoptosis via two well-defined apoptotic pathways; the intrinsic/mitochondrial pathway and the death receptor (DR)/extrinsic pathway. A number of groups have demonstrated that overexpression of prosurvival Bcl-2 family members significantly reduces HDACi-mediated tumor cell death and therapeutic efficacy in preclinical models. In many cases, HDACi activate the intrinsic pathway via upregulation of a number of proapoptotic BH3-only Bcl-2 family genes including Bim, Bid, and Bmf. Additionally, HDACi can engage the extrinsic pathway through upregulation of DR expression, reductions in c-FLIP, and upregulation of ligands such as TRAIL. Overall, it appears that activation of the intrinsic apoptotic pathway is the predominant mechanism of HDACi-induced tumor cell death; however, the DR pathway may also be engaged, either to amplify the apoptotic signal through the intrinsic pathway or to directly induce cell death.

Sphingolipids and response to chemotherapy

Chemotherapy is frequently used to treat primary or metastatic cancers, but intrinsic or acquired drug resistance limits its efficiency. Sphingolipids are important regulators of various cellular processes including proliferation, apoptosis, differentiation, angiogenesis, stress, and inflammatory responses which are linked to various aspects of cancer, like tumor growth, neoangiogenesis, and response to chemotherapy. Ceramide, the central molecule of sphingolipid metabolism, generally mediates antiproliferative and proapoptotic functions, whereas sphingosine-1-phosphate and other derivatives have opposing effects. Among the variety of enzymes that control ceramide generation, acid or neutral sphingomyelinases and ceramide synthases are important targets to allow killing of cancer cells by chemotherapeutic drugs. On the contrary, glucosylceramide synthase, ceramidase, and sphingosine kinase are other targets driving cancer cell resistance to chemotherapy. This chapter focuses on ceramide-based mechanisms leading to cancer therapy sensitization or resistance which could have some impacts on the development of novel cancer therapeutic strategies.

Anticancer mechanisms and clinical application of alkylphospholipids

Synthetic alkylphospholipids (ALPs), such as edelfosine, miltefosine, perifosine, erucylphosphocholine and erufosine, represent a relatively new class of structurally related antitumor agents that act on cell membranes rather than on DNA. They selectively target proliferating (tumor) cells, inducing growth arrest and apoptosis, and are potent sensitizers of conventional chemo- and radiotherapy. ALPs easily insert in the outer leaflet of the plasma membrane and cross the membrane via an ATP-dependent CDC50a-containing 'flippase' complex (in carcinoma cells), or are internalized by lipid raft-dependent endocytosis (in lymphoma/leukemic cells). ALPs resist catabolic degradation, therefore accumulate in the cell and interfere with lipid-dependent survival signaling pathways, notably PI3K-Akt and Raf-Erk1/2, and de novo phospholipid biosynthesis. At the same time, stress pathways (e.g. stress-activated protein kinase/JNK) are activated to promote apoptosis. In many preclinical and clinical studies, perifosine was the most effective ALP, mainly because it inhibits Akt activity potently and consistently, also in vivo. This property is successfully exploited clinically in highly malignant tumors, such as multiple myeloma and neuroblastoma, in which a tyrosine kinase receptor/Akt pathway is amplified. In such cases, perifosine therapy is most effective in combination with conventional anticancer regimens or with rapamycin-type mTOR inhibitors, and may overcome resistance to these agents. This article is part of a Special Issue entitled Phospholipids and Phospholipid Metabolism.

Cilastatin protects against cisplatin-induced nephrotoxicity without compromising its anticancer efficiency in rats

Cisplatin is an anticancer agent marred by nephrotoxicity; however, limiting this adverse effect may allow the use of higher doses to improve its efficacy. Cilastatin, a small molecule inhibitor of renal dehydropeptidase I, prevents proximal tubular cells from undergoing cisplatin-induced apoptosis in vitro. Here, we explored the in vivo relevance of these findings and the specificity of protection for kidney cells in cisplatin-treated rats. Cisplatin increased serum blood urea nitrogen and creatinine levels, and the fractional excretion of sodium. Cisplatin decreased the glomerular filtration rate, promoted histological renal injury and the expression of many pro-apoptotic proteins in the renal cortex, increased the Bax/Bcl2 ratio, and oxidative stress in kidney tissue and urine. All these features were decreased by cilastatin, which preserved renal function but did not modify the pharmacokinetics of cisplatin area under the curve. The cisplatin-induced death of cervical, colon, breast, and bladder-derived cancer cell lines was not prevented by cilastatin. Thus, cilastatin has the potential to prevent cisplatin nephrotoxicity without compromising its anticancer efficacy.

Amphotericin B up-regulates lipid A-induced IL-6 production via caspase-8

Amphotericin B, an antifungal drug used to treat candidiasis, has been reported to induce pro-inflammatory cytokine production in cultured cells. This study investigated the effects of amphotericin B on pro-inflammatory cytokine production in response to lipid A, the bioactive component of lipopolysaccharide (LPS) in the cell walls of Gram-negative bacteria. Amphotericin B alone elicited a slight increase in interleukin (IL)-6 and IL-8 production by human gingival fibroblasts. However, amphotericin B synergistically up-regulated lipid A-induced production of IL-6 and IL-8. While amphotericin B minimally activated nuclear factor (NF)-κB, it synergistically increased lipid A-induced NF-κB activation. Pre-treatment with methyl-β-cyclodextrin (MβCD), a cholesterol-binding agent, reduced IL-6 and IL-8 production in human gingival fibroblasts. Cholesterol-saturated MβCD also reversed cytokine production, suggesting that the synergistic production of cytokines by amphotericin B and lipid A is dependent on cholesterol-rich microdomains. Amphotericin B activated caspase-8. In addition, a caspase-8 inhibitor inhibited IL-6 production by amphotericin B and lipid A. This suggests that caspase-8 is required for the synergistic production of IL-6 by amphotericin B and lipid A. Collectively, our results suggest that periodontal treatment carried out before amphotericin B treatment may protect against lipid A-induced pro-inflammatory cytokine production.

CD95 signaling in colorectal cancer

CD95 and its ligand (CD95L) are widely expressed in colorectal tumors, but their role in shaping tumor behavior is unclear. CD95 activation on tumor cells can lead to apoptosis, while CD95L attracts neutrophils, suggesting a function in tumor suppression. However, CD95 can also promote tumorigenesis, at least in part by activating non-apoptotic signaling pathways that stimulate tumor cell proliferation, invasion and survival. In addition, CD95 signaling in stromal cells and tumor-infiltrating inflammatory cells has to be taken into account when addressing the function of CD95 and its ligand in colorectal tumor biology. We present a model in which the tumor-suppressing and tumor-promoting activities of CD95/CD95L together determine colorectal tumor behavior. We also discuss how these multiple activities are changing our view of CD95 and CD95L as potential therapeutic targets in the treatment of colorectal cancer. We conclude that locking CD95 in apoptosis-mode may be a more promising anti-cancer strategy than simply inhibiting or stimulating CD95.

Organization and dynamics of Fas transmembrane domain in raft membranes and modulation by ceramide

To comprehend the molecular processes that lead to the Fas death receptor clustering in lipid rafts, a 21-mer peptide corresponding to its single transmembrane domain (TMD) was reconstituted into mammalian raft model membranes composed of an unsaturated glycerophospholipid, sphingomyelin, and cholesterol. The peptide membrane lateral organization and dynamics, and its influence on membrane properties, were studied by steady-state and time-resolved fluorescence techniques and by attenuated total reflection Fourier transformed infrared spectroscopy. Our results show that Fas TMD is preferentially localized in liquid-disordered membrane regions and undergoes a strong reorganization as the membrane composition is changed toward the liquid-ordered phase. This results from the strong hydrophobic mismatch between the length of the peptide hydrophobic stretch and the hydrophobic thickness of liquid-ordered membranes. The stability of nonclustered Fas TMD in liquid-disordered domains suggests that its sequence may have a protective function against nonligand-induced Fas clustering in lipid rafts. It has been reported that ceramide induces Fas oligomerization in lipid rafts. Here, it is shown that neither Fas TMD membrane organization nor its conformation is affected by ceramide. These results are discussed within the framework of Fas membrane signaling events.

Recruitment of cellular prion protein to mitochondrial raft-like microdomains contributes to apoptosis execution

PrP^C is identified as a new component of mitochondrial raft-like microdomains in T cells undergoing CD95/Fas–mediated apoptosis, and microtubular network integrity and function could play a role in the redistribution of PrP^C from the plasma membrane to the mitochondria.

We examined the possibility that cellular prion protein (PrP^C) plays a role in the receptor-mediated apoptotic pathway. We first found that CD95/Fas triggering induced a redistribution of PrP^C to the mitochondria of T lymphoblastoid CEM cells via a mechanism that brings into play microtubular network integrity and function. In particular, we demonstrated that PrP^C was redistributed to raft-like microdomains at the mitochondrial membrane, as well as at endoplasmic reticulum-mitochondria–associated membranes. Our in vitro experiments also demonstrated that, although PrP^C had such an effect on mitochondria, it induced the loss of mitochondrial membrane potential and cytochrome c release only after a contained rise of calcium concentration. Finally, the involvement of PrP^C in apoptosis execution was also analyzed in PrP^C-small interfering RNA–transfected cells, which were found to be significantly less susceptible to CD95/Fas–induced apoptosis. Taken together, these results suggest that PrP^C might play a role in the complex multimolecular signaling associated with CD95/Fas receptor–mediated apoptosis.

Does calcium contribute to the CD95 signaling pathway?

Death receptors play a crucial role in immune surveillance and cellular homeostasis, two processes circumvented by tumor cells. CD95 (also termed Fas or APO1) is a transmembrane receptor, which belongs to the tumor necrosis factor receptor superfamily, and induces a potent apoptotic signal. Initial steps of the CD95 signal take place through protein/protein interactions that bring zymogens such as caspase-8 and caspase-10 closer. Aggregation of these procaspases leads to their autoprocessing, to the release of activated caspases in the cytosol, which causes a caspase cascade, and to the transmission of the apoptotic signal. In parallel, CD95 engagement drives an increase in the intracellular calcium concentration (Ca(2+))i whose origin and functions remain controversial. Although Ca(2+) ions play a central role in apoptosis/necrosis induction, recent studies have highlighted a protective role of Ca(2+) in death receptor signaling. In the light of these findings, we discuss the role of Ca(2+) ions as modulators of CD95 signaling.

Actin-independent exclusion of CD95 by PI3K/AKT signalling: implications for apoptosis

The immune system eliminates infected or transformed cells through the activation of the death receptor CD95. CD95 engagement drives the recruitment of the adaptor protein Fas-associated death domain protein (FADD), which in turn aggregates and activates initiator caspases-8 and -10. The CD95-mediated apoptotic signal relies on the capacity to form the CD95/FADD/caspases complex termed the death-inducing signalling complex (DISC). Cells are classified according to the magnitude of DISC formation as either type I (efficient DISC formation) or type II (inefficient). CD95 localised to lipid rafts in type I cells, whereas the death receptor was excluded from these domains in type II cells. Here, we show that inhibition of both PI3K class IA and serine-threonine kinase Akt in type II cells promoted the redistribution of CD95 into lipid rafts, DISC formation and the initiation of the apoptotic signal. Strikingly, these molecular events took place independently of CD95L and the actin cytoskeleton. Overall, these findings highlight that the oncogenic PI3K/Akt signalling pathway participates in maintaining cells in a type II phenotype by excluding CD95 from lipid rafts.

S-nitrosylation of the death receptor fas promotes fas ligand-mediated apoptosis in cancer cells

BACKGROUND & AIMS

Fas belongs to the family of tumor necrosis factor receptors which induce apoptosis. Many cancer cells express Fas but do not undergo Fas-mediated apoptosis. Nitric oxide reverses this resistance by increasing levels of Fas at the plasma membrane. We studied the mechanisms by which NO affects Fas function.

METHODS

Colon and mammary cancer cell lines were incubated with the NO donor glyceryl trinitrate or lipid A; S-nitrosylation of Fas was monitored using the biotin switch assay. Fas constructs that contained mutations at cysteine residues that prevent S-nitrosylation were used to investigate the involvement of S-nitrosylation in Fas-mediated cell death. Apoptosis was monitored according to morphologic criteria.

RESULTS

NO induced S-nitrosylation of cysteine residues 199 and 304 in the cytoplasmic part of Fas. In cancer cells that overexpressed wild-type Fas, S-nitrosylation induced Fas recruitment to lipid rafts and sensitized the cells to Fas ligand. In cells that expressed a mutant form of Fas in which cysteine 304 was replaced by valine residue, NO-mediated translocation of Fas to lipid rafts was affected and the death-inducing signal complex and synergistic effect of glyceryl trinitrate-Fas ligand were inhibited significantly. These effects were not observed in cells that expressed Fas with a mutation at cysteine 199.

CONCLUSIONS

We identified post-translational modifications (S-nitrosylation of cysteine residues 199 and 304) in the cytoplasmic domain of Fas. S-nitrosylation at cysteine 304 promotes redistribution of Fas to lipid rafts, formation of the death-inducing signal complex, and induction of cell death.

The naturally processed CD95L elicits a c-yes/calcium/PI3K-driven cell migration pathway

Patients affected by chronic inflammatory disorders display high amounts of soluble CD95L. This homotrimeric ligand arises from the cleavage by metalloproteases of its membrane-bound counterpart, a strong apoptotic inducer. In contrast, the naturally processed CD95L is viewed as an apoptotic antagonist competing with its membrane counterpart for binding to CD95. Recent reports pinpointed that activation of CD95 may attract myeloid and tumoral cells, which display resistance to the CD95-mediated apoptotic signal. However, all these studies were performed using chimeric CD95Ls (oligomerized forms), which behave as the membrane-bound ligand and not as the naturally processed CD95L. Herein, we examine the biological effects of the metalloprotease-cleaved CD95L on CD95-sensitive activated T-lymphocytes. We demonstrate that cleaved CD95L (cl-CD95L), found increased in sera of systemic lupus erythematosus (SLE) patients as compared to that of healthy individuals, promotes the formation of migrating pseudopods at the leading edge of which the death receptor CD95 is capped (confocal microscopy). Using different migration assays (wound healing/Boyden Chamber/endothelial transmigration), we uncover that cl-CD95L promotes cell migration through a c-yes/Ca²⁺/PI3K-driven signaling pathway, which relies on the formation of a CD95-containing complex designated the MISC for Motility-Inducing Signaling Complex. These findings revisit the role of the metalloprotease-cleaved CD95L and emphasize that the increase in cl-CD95L observed in patients affected by chronic inflammatory disorders may fuel the local or systemic tissue damage by promoting tissue-filtration of immune cells.