Lipid rafts, death receptors and CASMERs: new insights for cancer therapy

Piezo1 Mechano-Activation Is Augmented by Resveratrol and Differs between Colorectal Cancer Cells of Primary and Metastatic Origin

Cancer cells must survive aberrant fluid shear stress (FSS) in the circulation to metastasize. Herein, we investigate the role that FSS has on colorectal cancer cell apoptosis, proliferation, membrane damage, calcium influx, and therapeutic sensitization. We tested this using SW480 (primary tumor) and SW620 cells (lymph node metastasis) derived from the same patient. The cells were exposed to either shear pulses, modeling millisecond intervals of high FSS seen in regions of turbulent flow, or sustained shear to model average magnitudes experienced by circulating tumor cells. SW480 cells were significantly more sensitive to FSS-induced death than their metastatic counterparts. Shear pulses caused significant cell membrane damage, while constant shear decreased cell proliferation and increased the expression of CD133. To investigate the role of mechanosensitive ion channels, we treated cells with the Piezo1 agonist Yoda1, which increased intracellular calcium. Pretreatment with resveratrol further increased the calcium influx via the lipid-raft colocalization of Piezo1. However, minimal changes in apoptosis were observed due to calcium saturation, as predicted via a computational model of apoptosis. Furthermore, SW480 cells had increased levels of Piezo1, calcium influx, and TRAIL-mediated apoptosis compared to SW620 cells, highlighting differences in the mechano-activation of metastatic cells, which may be a necessary element for successful dissemination in vivo.

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.

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.

Multiplicity of Glycosphingolipid-Enriched Microdomain-Driven Immune Signaling

Glycosphingolipids (GSLs), together with cholesterol, sphingomyelin (SM), and glycosylphosphatidylinositol (GPI)-anchored and membrane-associated signal transduction molecules, form GSL-enriched microdomains. These specialized microdomains interact in a cis manner with various immune receptors, affecting immune receptor-mediated signaling. This, in turn, results in the regulation of a broad range of immunological functions, including phagocytosis, cytokine production, antigen presentation and apoptosis. In addition, GSLs alone can regulate immunological functions by acting as ligands for immune receptors, and exogenous GSLs can alter the organization of microdomains and microdomain-associated signaling. Many pathogens, including viruses, bacteria and fungi, enter host cells by binding to GSL-enriched microdomains. Intracellular pathogens survive inside phagocytes by manipulating intracellular microdomain-driven signaling and/or sphingolipid metabolism pathways. This review describes the mechanisms by which GSL-enriched microdomains regulate immune signaling.

Direct Endoplasmic Reticulum Targeting by the Selective Alkylphospholipid Analog and Antitumor Ether Lipid Edelfosine as a Therapeutic Approach in Pancreatic Cancer

Pancreatic ductal adenocarcinoma (PDAC), the most common malignancy of the pancreas, shows a dismal and grim overall prognosis and survival rate, which have remained virtually unchanged for over half a century. PDAC is the most lethal of all cancers, with the highest mortality-to-incidence ratio. PDAC responds poorly to current therapies and remains an incurable malignancy. Therefore, novel therapeutic targets and drugs are urgently needed for pancreatic cancer treatment. Selective induction of apoptosis in cancer cells is an appealing approach in cancer therapy. Apoptotic cell death is highly regulated by different signaling routes that involve a variety of subcellular organelles. Endoplasmic reticulum (ER) stress acts as a double-edged sword at the interface of cell survival and death. Pancreatic cells exhibit high hormone and enzyme secretory functions, and thereby show a highly developed ER. Thus, pancreatic cancer cells display a prominent ER. Solid tumors have to cope with adverse situations in which hypoxia, lack of certain nutrients, and the action of certain antitumor agents lead to a complex interplay and crosstalk between ER stress and autophagy-the latter acting as an adaptive survival response. ER stress also mediates cell death induced by a number of anticancer drugs and experimental conditions, highlighting the pivotal role of ER stress in modulating cell fate. The alkylphospholipid analog prototype edelfosine is selectively taken up by tumor cells, accumulates in the ER of a number of human solid tumor cells-including pancreatic cancer cells-and promotes apoptosis through a persistent ER-stress-mediated mechanism both in vitro and in vivo. Here, we discuss and propose that direct ER targeting may be a promising approach in the therapy of pancreatic cancer, opening up a new avenue for the treatment of this currently incurable and deadly cancer. Furthermore, because autophagy acts as a cytoprotective response to ER stress, potentiation of the triggering of a persistent ER response by combination therapy, together with the use of autophagy blockers, could improve the current gloomy expectations for finding a cure for this type of cancer.

Mitochondrial Targeting Involving Cholesterol-Rich Lipid Rafts in the Mechanism of Action of the Antitumor Ether Lipid and Alkylphospholipid Analog Edelfosine

The ether lipid edelfosine induces apoptosis selectively in tumor cells and is the prototypic molecule of a family of synthetic antitumor compounds collectively known as alkylphospholipid analogs. Cumulative evidence shows that edelfosine interacts with cholesterol-rich lipid rafts, endoplasmic reticulum (ER) and mitochondria. Edelfosine induces apoptosis in a number of hematological cancer cells by recruiting death receptors and downstream apoptotic signaling into lipid rafts, whereas it promotes apoptosis in solid tumor cells through an ER stress response. Edelfosine-induced apoptosis, mediated by lipid rafts and/or ER, requires the involvement of a mitochondrial-dependent step to eventually elicit cell death, leading to the loss of mitochondrial membrane potential, cytochrome c release and the triggering of cell death. The overexpression of Bcl-2 or Bcl-xL blocks edelfosine-induced apoptosis. Edelfosine induces the redistribution of lipid rafts from the plasma membrane to the mitochondria. The pro-apoptotic action of edelfosine on cancer cells is associated with the recruitment of F1FO-ATP synthase into cholesterol-rich lipid rafts. Specific inhibition of the FO sector of the F1FO-ATP synthase, which contains the membrane-embedded c-subunit ring that constitutes the mitochondrial permeability transcription pore, hinders edelfosine-induced cell death. Taking together, the evidence shown here suggests that the ether lipid edelfosine could modulate cell death in cancer cells by direct interaction with mitochondria, and the reorganization of raft-located mitochondrial proteins that critically modulate cell death or survival. Here, we summarize and discuss the involvement of mitochondria in the antitumor action of the ether lipid edelfosine, pointing out the mitochondrial targeting of this drug as a major therapeutic approach, which can be extrapolated to other alkylphospholipid analogs. We also discuss the involvement of cholesterol transport and cholesterol-rich lipid rafts in the interactions between the organelles as well as in the role of mitochondria in the regulation of apoptosis in cancer cells and cancer therapy.

Rafting Down the Metastatic Cascade: The Role of Lipid Rafts in Cancer Metastasis, Cell Death, and Clinical Outcomes

Lipid rafts are tightly packed, cholesterol- and sphingolipid-enriched microdomains within the plasma membrane that play important roles in many pathophysiologic processes. Rafts have been strongly implicated as master regulators of signal transduction in cancer, where raft compartmentalization can promote transmembrane receptor oligomerization, shield proteins from enzymatic degradation, and act as scaffolds to enhance intracellular signaling cascades. Cancer cells have been found to exploit these mechanisms to initiate oncogenic signaling and promote tumor progression. This review highlights the roles of lipid rafts within the metastatic cascade, specifically within tumor angiogenesis, cell adhesion, migration, epithelial-to-mesenchymal transition, and transendothelial migration. In addition, the interplay between lipid rafts and different modes of cancer cell death, including necrosis, apoptosis, and anoikis, will be described. The clinical role of lipid raft-specific proteins, caveolin and flotillin, in assessing patient prognosis and evaluating metastatic potential of various cancers will be presented. Collectively, elucidation of the complex roles of lipid rafts and raft components within the metastatic cascade may be instrumental for therapeutic discovery to curb prometastatic processes.

Lipid Raft Isolation by Sucrose Gradient Centrifugation and Visualization of Raft-Located Proteins by Fluorescence Microscopy: The Use of Combined Techniques to Assess Fas/CD95 Location in Rafts During Apoptosis Triggering

Lipid rafts are heterogeneous membrane domains enriched in cholesterol, sphingolipids, and gangliosides that serve as sorting platforms to compartmentalize and modulate signaling pathways. Death receptors and downstream signaling molecules have been reported to be recruited into these raft domains during the triggering of apoptosis. Here, we provide two protocols that support the presence of Fas/CD95 in lipid rafts during apoptosis, involving lipid raft isolation and confocal microscopy techniques. A detailed protocol is provided for the isolation of lipid rafts, by taking advantage of their resistance to Triton X-100 solubilization at 4 °C, followed by subsequent sucrose gradient centrifugation and analysis of the protein composition of the different gradient fractions by Western blotting. In addition, we also provide a detailed protocol for the visualization of the coclustering of Fas/CD95 death receptor and lipid rafts, as assessed by using anti-Fas/CD95 antibodies and fluorescent dye-conjugated cholera toxin B subunit that binds to ganglioside GM1, a main component of lipid rafts, by immunofluorescence and confocal microscopy. These protocols can be extended to any protein of interest to be analyzed for its association to lipid rafts.

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.

Proteome and Acetylome Analysis Identifies Novel Pathways and Targets Regulated by Perifosine in Neuroblastoma

Perifosine, an Akt inhibitor, has been shown to be effective in controlling neuroblastoma tumor growth. However, studies indicate that in addition to the ability to inhibit Akt, other mechanisms contribute to perifosine’s anti-tumor activity. To gain insight into perifosine anti-tumor activity in neuroblastoma we have studied changes in the proteome and acetylome after perifosine treatment in SK-N-AS neuroblastoma cells using SILAC labeling, affinity enrichment, high-resolution and LC-MS/MS analysis. Bioinformatic analysis indicates that, a total of 5,880 proteins and 3,415 lysine acetylation sites were quantified in SK-N-AS cells and 216 differentially expressed proteins and 115 differentially expressed lysine acetylation sites were obtained. These differentially expressed proteins and lysine acetylated proteins were involved in a number of different biological functions, metabolic pathways and pathophysiological processes. This study details the impact of perifosine on proteome and lysine acetylome in SK-N-AS cells and expands our understanding of the mechanisms of perifosine action in neuroblastoma.

Isolation of Lipid Rafts Through Discontinuous Sucrose Gradient Centrifugation and Fas/CD95 Death Receptor Localization in Raft Fractions

Lipid raft domains, enriched in sphingolipids and cholesterol, serve as sorting platforms and hubs for signal transduction proteins, and show resistance to detergent solubilization. Despite rafts have been involved in survival processes, these membrane domains have also been shown to play a major role in the modulation of death receptor signaling. Here, we describe a detailed protocol for isolating lipid rafts from whole cells by taking advantage of the lipid raft resistance to Triton X-100 solubilization at 4 °C, followed by sucrose gradient centrifugation, with subsequent analysis of Fas/CD95 death receptor localization in the raft fractions by immunoblotting. This method is also useful to localize additional proteins in membrane rafts.

Comparison of biological effects of modulated electro-hyperthermia and conventional heat treatment in human lymphoma U937 cells

Loco-regional hyperthermia treatment has long history in oncology. Modulated electro-hyperthermia (mEHT, trade name: oncothermia) is an emerging curative treatment method in this field due to its highly selective actions. The impedance-matched, capacitive-coupled modulated radiofrequency (RF) current is selectively focused in the malignant cell membrane of the cancer cells. Our objective is studying the cell-death process and comparing the cellular effects of conventional water-bath hyperthermia treatment to mEHT. The U937 human histiocytic lymphoma cell line was used for the experiments. In the case of conventional hyperthermia treatment, cells were immersed in a thermoregulated water bath, whereas in the case of mEHT, the cells were treated using a special RF generator (LabEHY, Oncotherm) and an applicator. The heating dynamics, the maximum temperature reached (42 °C) and the treatment duration (30 min) were exactly the same in both cases. Cell samples were analysed using different flow cytometric methods as well as microarray gene expression assay and western blot analysis was also used to reveal the molecular basis of the induced effects. Definite difference was observed in the biological response to different heat treatments. At 42 °C, only mEHT induced significant apoptotic cell death. The GeneChip analysis revealed a whole cluster of genes, which are highly up-regulated in case of only RF heating, but not in conventional heating. The Fas, c-Jun N-terminal kinases (JNK) and ERK signalling pathway was the dominant factor to induce apoptotic cell death in mEHT, whereas the cell-protective mechanisms dominated in case of conventional heating. This study has clearly shown that conventional hyperthermia and RF mEHT can result in different biological responses at the same temperature. The reason for the difference is the distinct, non-homogenous energy distribution on the cell membrane, which activates cell death-related signalling pathways in mEHT treatment but not in conventional heat treatment.

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.

Impact of the mitogen-activated protein kinase pathway on the subproteome of detergent-resistant microdomains of colon carcinoma cells

Lipid rafts play a key role in the regulation of fundamentally important cellular processes, including cell proliferation, differentiation, and survival. The composition of such detergent-resistant microdomains (DRMs) is altered under pathologic conditions, including cancer. Although DRMs have been analyzed in colorectal carcinoma little information exists about their composition upon treatment with targeted drugs. Hence, a quantitative proteomic profiling approach was performed to define alterations within the DRM fraction of colorectal carcinoma cells upon treatment with the drug U0126, an inhibitor of the mitogen-activated protein kinase pathway. Comparative expression profilings resulted in the identification of 300 proteins, which could partially be linked to key oncogenic signaling pathways and tumor-related cellular features, such as cell proliferation, adhesion, motility, invasion, and apoptosis resistance. Most of these proteins were downregulated upon inhibitor treatment. In addition, quantitative proteomic profilings of cholesterol-depleted versus intact lipid rafts were performed to define, which U0126-regulated target structures represent bona fide raft proteins. Selected differentially abundant raft proteins were validated at the mRNA and/or protein level using U0126- or Trametinib-treated cells. The presented data provide insights into the molecular mechanisms associated with the response to the treatment with MEK inhibitors and might also lead to novel candidates for therapeutic interventions.

Cholesterol as a factor regulating the influence of natural (PAF and lysoPAF) vs synthetic (ED) ether lipids on model lipid membranes

In this work we have performed a comparative study on the effect of antineoplastic ether lipid-edelfosine (ED), its natural analogs - Platelet Activating Factor (PAF) and its precursor (lyso-PAF), both lacking anticancer properties, on cholesterol/phosphatidylcholine (Chol/PC) monolayers, serving as model membranes. Since all the above ether lipids are membrane active, it can be expected that their effect on membranes may differentiate their biological activity. Our investigations were aimed at studying potential relationship of the effect of ED, PAF and lyso-PAF on model membranes, differing in condensation. We have modified molecular packing of Chol/PC model systems either by increasing the level of sterol in the system or changing the structure of PC, while keeping the same sterol content. Additionally, we have performed a detailed comparison of the miscibility of ED, PAF and lyso-PAF with various membrane lipids. The collected data evidenced that all the investigated ether lipids influence Chol/PC films in the same way; however, in a different magnitude. Moreover, the interactions of ED, PAF and lyso-PAF with model membranes were the strongest at the highest level of sterol in the system. A thorough analysis of the obtained results has proved that the effect of the investigated ether lipids on membranes is not dependent on the condensation of the system, but it is strongly determined by the concentration of cholesterol. Since ED was found to interact with model membranes stronger than PAF and lyso-PAF, we have suggested that this fact may contribute to differences in cytotoxicity of these compounds.

Lipid raft-mediated Akt signaling as a therapeutic target in mantle cell lymphoma

Recent evidence shows that lipid raft membrane domains modulate both cell survival and death. Here, we have found that the phosphatidylinositol-3-kinase (PI3K)/Akt signaling pathway is present in the lipid rafts of mantle cell lymphoma (MCL) cells, and this location seems to be critical for full activation and MCL cell survival. The antitumor lipids (ATLs) edelfosine and perifosine target rafts, and we found that ATLs exerted in vitro and in vivo antitumor activity against MCL cells by displacing Akt as well as key regulatory kinases p-PDK1 (phosphatidylinositol-dependent protein kinase 1), PI3K and mTOR (mammalian TOR) from lipid rafts. This raft reorganization led to Akt dephosphorylation, while proapoptotic Fas/CD95 death receptor was recruited into rafts. Raft integrity was critical for Ser473 Akt phosphorylation. ATL-induced apoptosis appeared to correlate with the basal Akt phosphorylation status in MCL cell lines and primary cultures, and could be potentiated by the PI3K inhibitor wortmannin, or inhibited by the Akt activator pervanadate. Classical Akt inhibitors induced apoptosis in MCL cells. Microenvironmental stimuli, such as CD40 ligation or stromal cell contact, did not prevent ATL-induced apoptosis in MCL cell lines and patient-derived cells. These results highlight the role of raft-mediated PI3K/Akt signaling in MCL cell survival and chemotherapy, thus becoming a new target for MCL treatment.

Lipid raft involvement in yeast cell growth and death

The notion that cellular membranes contain distinct microdomains, acting as scaffolds for signal transduction processes, has gained considerable momentum. In particular, a class of such domains that is rich in sphingolipids and cholesterol, termed as lipid rafts, is thought to compartmentalize the plasma membrane, and to have important roles in survival and cell death signaling in mammalian cells. Likewise, yeast lipid rafts are membrane domains enriched in sphingolipids and ergosterol, the yeast counterpart of mammalian cholesterol. Sterol-rich membrane domains have been identified in several fungal species, including the budding yeast Saccharomyces cerevisiae, the fission yeast Schizosaccharomyces pombe as well as the pathogens Candida albicans and Cryptococcus neoformans. Yeast rafts have been mainly involved in membrane trafficking, but increasing evidence implicates rafts in a wide range of additional cellular processes. Yeast lipid rafts house biologically important proteins involved in the proper function of yeast, such as proteins that control Na⁺, K⁺, and pH homeostasis, which influence many cellular processes, including cell growth and death. Membrane raft constituents affect drug susceptibility, and drugs interacting with sterols alter raft composition and membrane integrity, leading to yeast cell death. Because of the genetic tractability of yeast, analysis of yeast rafts could be an excellent model to approach unanswered questions of mammalian raft biology, and to understand the role of lipid rafts in the regulation of cell death and survival in human cells. A better insight in raft biology might lead to envisage new raft-mediated approaches to the treatment of human diseases where regulation of cell death and survival is critical, such as cancer and neurodegenerative diseases.

From mosaic to patchwork: matching lipids and proteins in membrane organization

Biological membranes encompass and compartmentalize cells and organelles and are a prerequisite to life as we know it. One defining feature of membranes is an astonishing diversity of building blocks. The mechanisms and principles organizing the thousands of proteins and lipids that make up membrane bilayers in cells are still under debate. Many terms and mechanisms have been introduced over the years to account for certain phenomena and aspects of membrane organization and function. Recently, the different viewpoints - focusing on lipids vs. proteins or physical vs. molecular driving forces for membrane organization - are increasingly converging. Here we review the basic properties of biological membranes and the most common theories for lateral segregation of membrane components before discussing an emerging model of a self-organized, multi-domain membrane or 'patchwork membrane'.

Anandamide exerts its antiproliferative actions on cholangiocarcinoma by activation of the GPR55 receptor

Cholangiocarcinomas are devastating cancers of biliary origin with limited treatment options. It has previously been shown that the endocannabinoid anandamide exerts antiproliferative effects on cholangiocarcinoma independent of any known cannabinoid receptors, and by the stabilization of lipid rafts, thereby allowing the recruitment and activation of the Fas death receptor complex. Recently, GPR55 was identified as a putative cannabinoid receptor; therefore, the role of GPR55 in the antiproliferative effects of anandamide was evaluated. GPR55 is expressed in all cholangiocarcinoma cells and liver biopsy samples to a similar level as in non-malignant cholangiocytes. Treatment with either anandamide or the GPR55 agonist, O-1602, reduced cholangiocarcinoma cell proliferation in vitro and in vivo. Furthermore, knocking down the expression of GPR55 prevented the antiproliferative effects of anandamide. Coupled to these effects was an increase in JNK activity. The antiproliferative effects of anandamide could be blocked by pretreatment with a JNK inhibitor and the lipid raft disruptors β-methylcyclodextrin and fillipin III. Activation of GPR55 by anandamide or O-1602 increased the amount of Fas in the lipid raft fractions, which could be blocked by pretreatment with the JNK inhibitor. These data represent the first evidence that GPR55 activation by anandamide can lead to the recruitment and activation of the Fas death receptor complex and that targeting GPR55 activation may be a viable option for the development of therapeutic strategies to treat cholangiocarcinoma.