Role of membrane sphingomyelin and ceramide in platform formation for Fas-mediated apoptosis

Investigating the impact of 2-OHOA-embedded liposomes on biophysical properties of cancer cell membranes via Laurdan two-photon microscopy imaging

2-Hydroxyoleic acid (2-OHOA) has gained attention as a membrane lipid therapy (MLT) anti-cancer drug. However, in the viewpoint of anti-cancer drug, 2-OHOA shows poor water solubility and its effectiveness still has space for improvement. Thus, this study aimed to overcome the problems by formulating 2-OHOA into liposome dosage form. Furthermore, in the context of MLT reagents, the influence of 2-OHOA on the biophysical properties of the cytoplasmic membrane remains largely unexplored. To bridge this gap, our study specifically focused the alterations in cancer cell membrane fluidity and lipid packing characteristics before and after treatment. By using a two-photon microscope and the Laurdan fluorescence probe, we noted that liposomes incorporating 2-OHOA induced a more significant reduction in cancer cell membrane fluidity, accompanied by a heightened rate of cellular apoptosis when compared to the non-formulated 2-OHOA. Importantly, the enhanced efficacy of 2-OHOA within the liposomal formulation demonstrated a correlation with its endocytic uptake mechanism. In conclusion, our findings underscore the significant influence of 2-OHOA on the biophysical properties of cancer plasma membranes, emphasizing the potential of liposomes as an optimized delivery system for 2-OHOA in anti-cancer therapy.

Leveraging altered lipid metabolism in treating B cell malignancies

B cell malignancies, comprising over 80 heterogeneous blood cancers, pose significant prognostic challenges due to intricate oncogenic signaling. Emerging evidence emphasizes the pivotal role of disrupted lipid metabolism in the development of these malignancies. Variations in lipid species, such as phospholipids, cholesterol, sphingolipids, and fatty acids, are widespread across B cell malignancies, contributing to uncontrolled cell proliferation and survival. Phospholipids play a crucial role in initial signaling cascades leading to B cell activation and malignant transformation through constitutive B cell receptor (BCR) signaling. Dysregulated cholesterol and sphingolipid homeostasis support lipid raft integrity, crucial for propagating oncogenic signals. Sphingolipids impact malignant B cell stemness, proliferation, and survival, while glycosphingolipids in lipid rafts modulate BCR activation. Additionally, cancer cells enhance fatty acid-related processes to meet heightened metabolic demands. In obese individuals, the obesity-derived lipids and adipokines surrounding adipocytes rewire lipid metabolism in malignant B cells, evading cytotoxic therapies. Genetic drivers such as MYC translocations also intrinsically alter lipid metabolism in malignant B cells. In summary, intrinsic and extrinsic factors converge to reprogram lipid metabolism, fostering aggressive phenotypes in B cell malignancies. Therefore, targeting altered lipid metabolism has translational potential for improving risk stratification and clinical management of diverse B cell malignancy subtypes.

Sphingomyelin synthase 2 promotes the stemness of breast cancer cells via modulating NF-κB signaling pathway

OBJECTIVES

Multi-drug resistance (MDR) to chemotherapy is the main obstacle influencing the anti-tumor effect in breast cancer, which might lead to the metastasis and recurrence of cancer. Until now, there are still no effective methods that can overcome MDR. In this study, we aimed to investigate the role of sphingomyelin synthase 2 (SMS2) in breast cancer resistance.

METHODS

Quantitative RT-PCR analysis was performed to assess changes in mRNA expression. Western blot analysis was performed to detect protein expression. Inhibitory concentration value of adriamycin (ADR) was evaluated using CCK 8 assay. The stemness ability of breast cancer cells was assessed by spheroid-formation assay. Immunofluorescence staining was conducted to show the cellular distribution of proteins. Breast tumor masses were harvested from the xenograft tumor mouse model.

RESULTS

SMS2 overexpression increased the IC50 values of breast cancer cells. SMS2 decreased the CD24 transcription level but increased the transcription levels of stemness-related genes including CD44, ALDH, OCT 4 and SOX2 in breast cancer cells. SMS2 overexpression promoted the nuclear translocation of phosphorylated NF-κB, while suppression of SMS2 could inhibit the NF-κB pathway.

CONCLUSIONS

SMS2 increased the stemness of breast cancer cells via NF-κB signaling pathway, leading to resistance to the chemotherapeutic drug ADR. Thus, SMS2 might play a critical role in the development of breast cancer resistance, which is a previously unrecognized mechanism in breast cancer MDR development.

Hypoxia induced deregulation of sphingolipids in colon cancer is a prognostic marker for patient outcome

Sphingolipids are important for the physicochemical properties of cellular membranes and deregulated in tumors. In human colon cancer tissue ceramide synthase (CerS) 4 and CerS5 are reduced which correlates with a reduced survival probability of late-stage colon cancer patients. Both enzymes are reduced after hypoxia in advanced colorectal cancer (CRC) cells (HCT-116, SW620) but not in non-metastatic CRC cells (SW480, Caco-2). Downregulation of CerS4 or CerS5 in advanced CRC cells enhanced tumor formation in nude mice and organoid growth in vitro. This was accompanied by an enhanced proliferation rate and metabolic changes leading to a shift towards the Warburg effect. In contrast, CerS4 or CerS5 depletion in Caco-2 cells reduced tumor growth in vivo. Lipidomic and proteomic analysis of membrane fractions revealed significant changes in tumor-promoting cellular pathways and cellular transporters. This study identifies CerS4 and CerS5 as prognostic markers for advanced colon cancer patients and provides a comprehensive overview about the associated cellular metabolic changes. We propose that the expression level of CerS4 and CerS5 in colon tumors could serve as a basis for decision-making for personalized treatment of advanced colon cancer patients. Trial registration: The study was accredited by the study board of the Deutsche Krebsgesellschaft (Registration No: St-D203, 2017/06/30, retrospectively registered).

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.

Protocol to analyze lipid asymmetry in the plasma membrane

The plasma membrane containing cholesterol exhibits phospholipid asymmetry, with phosphatidylcholine and sphingomyelin enriched in its outer leaflet and phosphatidylserine (PtdSer) and phosphatidylethanolamine (PtdEtn) on the cytoplasmic side. We herein describe steps for bacterial expression of recombinant proteins that bind to membrane lipids, followed by affinity purification. Using fluorescence-labeled phospholipid analogs, we further detail the assay to detect flippase activity, which maintains the single-sided distribution of PtdSer and PtdEtn, in mammalian cells. For complete details on the use and execution of this protocol, please refer to Segawa et al. (2021).¹.

Plasma Ceramides Pathophysiology, Measurements, Challenges, and Opportunities

Ceramides are a family of lipid molecules, composed of sphingosine and a fatty acid, and transported by lipoproteins (primarily by low-density lipoproteins) in the bloodstream. They are not only structural lipids, but multifunctional and bioactive molecules with key roles in many important cellular pathways, such as inflammatory processes and apoptosis, representing potential biomarkers of cardiometabolic diseases as well as pharmacological targets. Recent data reported ceramide modulation by diet and aerobic exercise, suggesting nutrients and exercise-targeting sphingolipid pathways as a countermeasure, also in combination with other therapies, for risk and progression of chronic disease prevention and health maintenance. In this review, we focus on the available data regarding remarks on ceramide structure and metabolism, their pathophysiologic roles, and the effect of dietary habit and aerobic exercise on ceramide levels. Moreover, advancements and limitations of lipidomic techniques and simplification attempts to overcome difficulties of interpretation and to facilitate practical applications, such as the proposal of scores, are also discussed.

Role of ceramide/sphingomyelin (SM) balance regulated through "SM cycle" in cancer

Sphingomyelin synthase (SMS), which comprises of two isozymes, SMS1 and SMS2, is the only enzyme that generates sphingomyelin (SM) by transferring phosphocholine of phosphatidylcholine to ceramide in mammals. Conversely, ceramide is generated from SM hydrolysis via sphingomyelinases (SMases), ceramide de novo synthesis, and the salvage pathway. The biosynthetic pathway for SM and ceramide content by SMS and SMase, respectively, is called "SM cycle." SM forms a SM-rich microdomain on the cell membrane to regulate signal transduction, such as proliferation/survival, migration, and inflammation. On the other hand, ceramide acts as a lipid mediator by forming a ceramide-rich platform on the membrane, and ceramide exhibits physiological actions such as cell death, cell cycle arrest, and autophagy induction. Therefore, the regulation of ceramide/SM balance by SMS and SMase is responsible for diverse cell functions not only in physiological cells but also in cancer cells. This review outlines the implications of ceramide/SM balance through "SM cycle" in cancer progression and prevention. In addition, the possible involvement of "SM cycle" is introduced in anti-cancer tumor immunity, which has become a hot topic to innovate a more effective and safer way to conquer cancer in recent years.

Liver X receptors are required for thymic resilience and T cell output

The thymus is a primary lymphoid organ necessary for optimal T cell development. Here, we show that liver X receptors (LXRs)—a class of nuclear receptors and transcription factors with diverse functions in metabolism and immunity—critically contribute to thymic integrity and function. LXRαβ-deficient mice develop a fatty, rapidly involuting thymus and acquire a shrunken and prematurely immunoinhibitory peripheral T cell repertoire. LXRαβ’s functions are cell specific, and the resulting phenotypes are mutually independent. Although thymic macrophages require LXRαβ for cholesterol efflux, thymic epithelial cells (TECs) use LXRαβ for self-renewal and thymocytes for negative selection. Consequently, TEC-derived LXRαβ protects against homeostatic premature involution and orchestrates thymic regeneration following stress, while thymocyte-derived LXRαβ limits cell disposal during negative selection and confers heightened sensitivity to experimental autoimmune encephalomyelitis. These results identify three distinct but complementary mechanisms by which LXRαβ governs T lymphocyte education and illuminate LXRαβ’s indispensable roles in adaptive immunity.

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.

Regulation of human sphingomyelin synthase 1 translation through its 5'-untranslated region

Bcr‐abl1 oncogene causes a shift in the transcription start site of the SMS1 gene (SGMS1) encoding the sphingomyelin (SM) synthesizing enzyme, sphingomyelin synthase 1 (SMS1). This results in an mRNA with a significantly shorter 5′‐UTR, called 7‐SGMS1, which is translated more efficiently than another transcript (IIb‐SGMS1) with a longer 5′UTR in Bcr‐abl1‐positive cells. Here, we determine the effects of these alternative 5′UTRs on SMS1 translation and investigate the key features underlying such regulation. First, the presence of the longer IIb 5′UTR is sufficient to greatly impair translation of a reporter gene. Deletion of the upstream open reading frame (−164 nt) or of the predicted stem‐loops in the 5′UTR of IIb‐SGMS1 has minimal effects on SGMS1 translation. Conversely, deletion of nucleotides −310 to −132 enhanced transcription of IIb‐SGMS1 to reach that of 7‐SGMS1. We thus suggest that regulatory features within nucleotides −310 and −132 modulate IIb‐SGMS1 translation efficiency.

Ceramide/Sphingomyelin Rheostat Regulated by Sphingomyelin Synthases and Chronic Diseases in Murine Models

Ceramide and sphingomyelin (SM) are major components of the double membrane-bound sphingolipids. Ceramide is an essential bioactive lipid involved in numerous cell processes including apoptosis, necrosis, and autophagy-dependent cell death. Inversely, SM regulates opposite cellular processes such as proliferation and migration by changing receptor-mediated signal transduction in the lipid microdomain. SM is generated through a transfer of phosphocholine from phosphatidylcholine to ceramide by SM synthases (SMSs). Research during the past several decades has revealed that the ceramide/SM balance in cellular membranes regulated by SMSs is important to decide the cell fate, survival, and proliferation. In addition, recent experimental studies utilizing SMS knockout mice and murine disease models provide evidence that SMS-regulated ceramide/SM balance is involved in human diseases. Here, we review the basic structural and functional characteristics of SMSs and focus on their cellular functions through the regulation of ceramide/SM balance in membrane microdomains. In addition, we present the pathological or physiological implications of SMSs by analyzing their role in SMS-knockout mice and human disease models. This review finally presents evidence indicating that the regulation of ceramide/SM balance through SMS could be a therapeutic target for human disorders.

Molecular dissection of box jellyfish venom cytotoxicity highlights an effective venom antidote

The box jellyfish Chironex fleckeri is extremely venomous, and envenoming causes tissue necrosis, extreme pain and death within minutes after severe exposure. Despite rapid and potent venom action, basic mechanistic insight is lacking. Here we perform molecular dissection of a jellyfish venom-induced cell death pathway by screening for host components required for venom exposure-induced cell death using genome-scale lenti-CRISPR mutagenesis. We identify the peripheral membrane protein ATP2B1, a calcium transporting ATPase, as one host factor required for venom cytotoxicity. Targeting ATP2B1 prevents venom action and confers long lasting protection. Informatics analysis of host genes required for venom cytotoxicity reveal pathways not previously implicated in cell death. We also discover a venom antidote that functions up to 15 minutes after exposure and suppresses tissue necrosis and pain in mice. These results highlight the power of whole genome CRISPR screening to investigate venom mechanisms of action and to rapidly identify new medicines.

Box jellyfish venom causes tissue damage, pain, and death through unknown molecular mechanisms. Here, Lau et al. perform a CRISPR screen to identify genes required for venom action and use this information to develop an antidote that blocks venom-induced pain and tissue damage in vivo.

Role and Function of Sphingomyelin Biosynthesis in the Development of Cancer

Sphingomyelin (SM) biosynthesis represents a complex, finely regulated process, mostly occurring in vertebrates. It is intimately linked to lipid transport and it is ultimately carried out by two enzymes, SM synthase 1 and 2, selectively localized in the Golgi and plasma membrane. In the course of the SM biosynthetic reaction, various lipids are metabolized. Because these lipids have both structural and signaling functions, the SM biosynthetic process has the potential to affect diverse important cellular processes (such as cell proliferation, cell survival, and migration). Thus defects in SM biosynthesis might directly or indirectly impact the normal physiology of the cell and eventually of the organism. In this chapter, we will focus on evidence supporting a role for SM biosynthesis in specific cellular functions and how its dysregulation can affect neoplastic transformation.

γ-Tocotrienol induces apoptosis in pancreatic cancer cells by upregulation of ceramide synthesis and modulation of sphingolipid transport

Background

Ceramide synthesis and metabolism is a promising target in cancer drug development. γ-tocotrienol (GT3), a member of the vitamin E family, orchestrates multiple effects that ensure the induction of apoptosis in both, wild-type and RAS-mutated pancreatic cancer cells. Here, we investigated whether these effects involve changes in ceramide synthesis and transport.

Methods

The effects of GT3 on the synthesis of ceramide via the de novo pathway, and the hydrolysis of sphingomyelin were analyzed by the expression levels of the enzymes serine palmitoyl transferase, ceramide synthase-6, and dihydroceramide desaturase, and acid sphingomyelinase in wild-type RAS BxPC3, and RAS-mutated MIA PaCa-2 and Panc 1 pancreatic cancer cells. Quantitative changes in ceramides, dihydroceramides, and sphingomyelin at the cell membrane were detected by LCMS. Modulation of ceramide transport by GT3 was studied by immunochemistry of CERT and ARV-1, and the subsequent effects at the cell membrane was analyzed via immunofluorescence of ceramide, caveolin, and DR5.

Results

GT3 favors the upregulation of ceramide by stimulating synthesis at the ER and the plasma membrane. Additionally, the conversion of newly synthesized ceramide to sphingomyelin and glucosylceramide at the Golgi is prevented by the inhibition of CERT. Modulation ARV1 and previously observed inhibition of the HMG-CoA pathway, contribute to changes in membrane structure and signaling functions, allows the clustering of DR5, effectively initiating apoptosis.

Conclusions

Our results suggest that GT3 targets ceramide synthesis and transport, and that the upregulation of ceramide and modulation of transporters CERT and ARV1 are important contributors to the apoptotic properties demonstrated by GT3 in pancreatic cancer cells.

Discovery of N-hydroxy-3-alkoxybenzamides as direct acid sphingomyelinase inhibitors using a ligand-based pharmacophore model

Acid sphingomyelinase (ASM) has been shown to be involved in many physiological processes, emerging to be a promising drug target. In this study, we constructed a ligand-based pharmacophore model of ASM inhibitors and applied this model to optimize the lead compound α-mangostin, a known inhibitor of ASM. 23 compounds were designed and evaluated in vitro for ASM inhibition, of these, 10 compounds were found to be more potent than α-mangostin. This high hit ratio confirmed that the presented model is very effective and practical. The most potent hit, 1c, was found to selectively and competitively inhibit the enzyme and inhibit the generation of ceramide in a dose-dependent manner. Furthermore, 1c showed favorable anti-apoptosis and anti-inflammatory activity. Interactions with key residues and the Zn²⁺ cofactor of 1c were found by docking simulation. These results provide promising leads and important guidance for further development of efficient ASM inhibitors and drug candidates.

Sphingolipid pathway enzymes modulate cell fate and immune responses

Sphingolipids (SLs) are a class of essential, bioactive lipids. The SL family includes over 4000 distinct molecules, characterized by their sphingoid base (long-chain aliphatic amine) backbone. SLs are key components of cell membranes, yet their roles go well beyond structure. SLs are involved in many cellular processes including cell differentiation, apoptosis, growth arrest and senescence. As cancer cells routinely display increased growth properties and escape from cell death, it has been suggested that enzymes involved in SL synthesis or catabolism may be altered in cancer cells. In this review, we discuss the role of SL pathway enzymes in cancer, and in acquired resistance to therapy. The use of inhibitors and gene silencing approaches targeting these SL pathways is also explored. Finally, we elaborate on the role of SL pathway enzymes in the tumor microenvironment and their effect on immune cell function.

Raft-based sphingomyelin interactions revealed by new fluorescent sphingomyelin analogs

Sphingomyelin (SM) has been proposed to form cholesterol-dependent raft domains and sphingolipid domains in the plasma membrane (PM). How SM contributes to the formation and function of these domains remains unknown, primarily because of the scarcity of suitable fluorescent SM analogs. We developed new fluorescent SM analogs by conjugating a hydrophilic fluorophore to the SM choline headgroup without eliminating its positive charge, via a hydrophilic nonaethylene glycol linker. The new analogs behaved similarly to the native SM in terms of their partitioning behaviors in artificial liquid order-disorder phase-separated membranes and detergent-resistant PM preparations. Single fluorescent molecule tracking in the live-cell PM revealed that they indirectly interact with each other in cholesterol- and sphingosine backbone-dependent manners, and that, for ∼10-50 ms, they undergo transient colocalization-codiffusion with a glycosylphosphatidylinositol (GPI)-anchored protein, CD59 (in monomers, transient-dimer rafts, and clusters), in CD59-oligomer size-, cholesterol-, and GPI anchoring-dependent manners. These results suggest that SM continually and rapidly exchanges between CD59-associated raft domains and the bulk PM.

ER residency of the ceramide phosphoethanolamine synthase SMSr relies on homotypic oligomerization mediated by its SAM domain

SMSr/SAMD8 is an ER-resident ceramide phosphoethanolamine synthase with a critical role in controlling ER ceramides and suppressing ceramide-induced apoptosis in cultured cells. SMSr-mediated ceramide homeostasis relies on the enzyme's catalytic activity as well as on its N-terminal sterile α-motif or SAM domain. Here we report that SMSr-SAM is structurally and functionally related to the SAM domain of diacylglycerol kinase DGKδ, a central regulator of lipid signaling at the plasma membrane. Native gel electrophoresis indicates that both SAM domains form homotypic oligomers. Chemical crosslinking studies show that SMSr self-associates into ER-resident trimers and hexamers that resemble the helical oligomers formed by DGKδ-SAM. Residues critical for DGKδ-SAM oligomerization are conserved in SMSr-SAM and their substitution causes a dissociation of SMSr oligomers as well as a partial redistribution of the enzyme to the Golgi. Conversely, treatment of cells with curcumin, a drug disrupting ceramide and Ca2+ homeostasis in the ER, stabilizes SMSr oligomers and promotes retention of the enzyme in the ER. Our data provide first demonstration of a multi-pass membrane protein that undergoes homotypic oligomerization via its SAM domain and indicate that SAM-mediated self-assembly of SMSr is required for efficient retention of the enzyme in the ER.

Antitumor activity of a novel and orally available inhibitor of serine palmitoyltransferase

Metabolic reprogramming is an essential hallmark of neoplasia. Therefore, targeting cancer metabolism, including lipid synthesis, has attracted much interest in recent years. Serine palmitoyltransferase (SPT) plays a key role in the initial and rate-limiting step of de novo sphingolipid biosynthesis, and inhibiting SPT activity prevents the proliferation of certain cancer cells. Here, we identified a novel and orally available SPT inhibitor, compound-2. Compound-2 showed an anti-proliferative effect in several cancer cell models, reducing the levels of the sphingolipids ceramide and sphingomyelin. In the presence of compound-2, exogenously added S1P partially compensated the intracellular sphingolipid levels through the salvage pathway by partially rescuing compound-2-induced cytotoxicity. This suggested that the mechanism underlying the anti-proliferative effect of compound-2 involved the reduction of sphingolipid levels. Indeed, compound-2 promoted multinuclear formation with reduced endogenous sphingomyelin levels specifically in a compound-2-sensitive cell line, indicating that the effect was induced by sphingolipid reduction. Furthermore, compound-2 showed potent antitumor activity without causing significant body weight loss in the PL-21 acute myeloid leukemia mouse xenograft model. Therefore, SPT may be an attractive therapeutic anti-cancer drug target for which compound-2 may be a promising new drug.

A Journey of Cytolethal Distending Toxins through Cell Membranes

The multifunctional role of lipids as structural components of membranes, signaling molecules, and metabolic substrates makes them an ideal partner for pathogens to hijack host cell processes for their own survival. The properties and composition of unique membrane micro-domains such as membrane rafts make these regions a natural target for pathogens as it affords them an opportunity to hijack cell signaling and intracellular trafficking pathways. Cytolethal distending toxins (Cdts), members of the AB2 family of toxins are comprised of three subunits, the active, CdtB unit, and the binding, CdtA-CdtC unit. Cdts are cyclomodulins leading to cell cycle arrest and apoptosis in a wide variety of cell types. Cdts from several species share a requirement for membrane rafts, and often cholesterol specifically for cell binding and CdtB mediated cytotoxicity. In this review we focus on how host–cell membrane bilayer organization contributes to the cell surface association, internalization, and action of bacteria derived cytolethal distending toxins (Cdts), with an emphasis on Aggregatibacter actinomycetemcomitans Cdt.

Imaging Mass Spectrometry Reveals Acyl-Chain- and Region-Specific Sphingolipid Metabolism in the Kidneys of Sphingomyelin Synthase 2-Deficient Mice

Obesity was reported to cause kidney injury by excessive accumulation of sphingolipids such as sphingomyelin and ceramide. Sphingomyelin synthase 2 (SMS2) is an important enzyme for hepatic sphingolipid homeostasis and its dysfunction is considered to result in fatty liver disease. The expression of SMS2 is also high in the kidneys. However, the contribution of SMS2 on renal sphingolipid metabolism remains unclear. Imaging mass spectrometry is a powerful tool to visualize the distribution and provide quantitative data on lipids in tissue sections. Thus, in this study, we analyzed the effects of SMS2 deficiency on the distribution and concentration of sphingomyelins in the liver and kidneys of mice fed with a normal-diet or a high-fat-diet using imaging mass spectrometry and liquid chromatography/electrospray ionization-tandem mass spectrometry. Our study revealed that high-fat-diet increased C18–C22 sphingomyelins, but decreased C24-sphingomyelins, in the liver and kidneys of wild-type mice. By contrast, SMS2 deficiency decreased C18–C24 sphingomyelins in the liver. Although a similar trend was observed in the whole-kidneys, the effects were minor. Interestingly, imaging mass spectrometry revealed that sphingomyelin localization was specific to each acyl-chain length in the kidneys. Further, SMS2 deficiency mainly decreased C22-sphingomyelin in the renal medulla and C24-sphingomyelins in the renal cortex. Thus, imaging mass spectrometry can provide visual assessment of the contribution of SMS2 on acyl-chain- and region-specific sphingomyelin metabolism in the kidneys.

Detectors for evaluating the cellular landscape of sphingomyelin- and cholesterol-rich membrane domains

Although sphingomyelin and cholesterol are major lipids of mammalian cells, the detailed distribution of these lipids in cellular membranes remains still obscure. However, the recent development of protein probes that specifically bind sphingomyelin and/or cholesterol provides new information about the landscape of the lipid domains that are enriched with sphingomyelin or cholesterol or both. Here, we critically summarize the tools to study distribution and dynamics of sphingomyelin and cholesterol. This article is part of a Special Issue entitled: The cellular lipid landscape edited by Tim P. Levine and Anant K. Menon.

Pore-forming toxins: Properties, diversity, and uses as tools to image sphingomyelin and ceramide phosphoethanolamine

Pore-forming toxins (PFTs) represent a unique class of highly specific lipid-binding proteins. The cytotoxicity of these compounds has been overcome through crystallographic structure and mutation studies, facilitating the development of non-toxic lipid probes. As a consequence, non-toxic PFTs have been utilized as highly specific probes to visualize the diversity and dynamics of lipid nanostructures in living and fixed cells. This review is focused on the application of PFTs and their non-toxic analogs as tools to visualize sphingomyelin and ceramide phosphoethanolamine, two major phosphosphingolipids in mammalian and insect cells, respectively. This article is part of a Special Issue entitled: Pore-Forming Toxins edited by Mauro Dalla Serra and Franco Gambale.

Role of Sphingolipids and Metabolizing Enzymes in Hematological Malignancies

Sphingolipids such as ceramide, sphingosine-1-phosphate and sphingomyelin have been emerging as bioactive lipids since ceramide was reported to play a role in human leukemia HL-60 cell differentiation and death. Recently, it is well-known that ceramide acts as an inducer of cell death, that sphingomyelin works as a regulator for microdomain function of the cell membrane, and that sphingosine-1-phosphate plays a role in cell survival/proliferation. The lipids are metabolized by the specific enzymes, and each metabolite could be again returned to the original form by the reverse action of the different enzyme or after a long journey of many metabolizing/synthesizing pathways. In addition, the metabolites may serve as reciprocal bio-modulators like the rheostat between ceramide and sphingosine-1-phosphate. Therefore, the change of lipid amount in the cells, the subcellular localization and the downstream signal in a specific subcellular organelle should be clarified to understand the pathobiological significance of sphingolipids when extracellular stimulation induces a diverse of cell functions such as cell death, proliferation and migration. In this review, we focus on how sphingolipids and their metabolizing enzymes cooperatively exert their function in proliferation, migration, autophagy and death of hematopoetic cells, and discuss the way developing a novel therapeutic device through the regulation of sphingolipids for effectively inhibiting cell proliferation and inducing cell death in hematological malignancies such as leukemia, malignant lymphoma and multiple myeloma.

Functional characterization of enzymes catalyzing ceramide phosphoethanolamine biosynthesis in mice

Besides bulk amounts of SM, mammalian cells produce small quantities of the SM analog ceramide phosphoethanolamine (CPE). Little is known about the biological role of CPE or enzymes responsible for CPE production. Heterologous expression studies revealed that SM synthase (SMS)2 is a bifunctional enzyme producing both SM and CPE, whereas SMS-related protein (SMSr) serves as monofunctional CPE synthase. Acute disruption of SMSr catalytic activity in cultured cells causes a rise in endoplasmic reticulum (ER) ceramides, fragmentation of ER exit sites, and induction of mitochondrial apoptosis. To address the relevance of CPE biosynthesis in vivo, we analyzed the tissue-specific distribution of CPE in mice and generated mouse lines lacking SMSr and SMS2 catalytic activity. We found that CPE levels were >300-fold lower than SM in all tissues examined. Unexpectedly, combined inactivation of SMSr and SMS2 significantly reduced, but did not eliminate, tissue-specific CPE pools and had no obvious impact on mouse development or fertility. While SMSr is widely expressed and serves as the principal CPE synthase in the brain, blocking its catalytic activity did not affect ceramide levels or secretory pathway integrity in the brain or any other tissue. Our data provide a first inventory of CPE species and CPE-biosynthetic enzymes in mammals.

Nonclinical safety assessment of recombinant human acid sphingomyelinase (rhASM) for the treatment of acid sphingomyelinase deficiency:the utility of animal models of disease in the toxicological evaluation of potential therapeutics

Recombinant human acid sphingomyelinase (rhASM) is being developed as an enzyme replacement therapy for patients with acid sphingomyelinase deficiency (Niemann-Pick disease types A and B), which causes sphingomyelin to accumulate in lysosomes. In the acid sphingomyelinase knock-out (ASMKO) mouse, intravenously administered rhASM reduced tissue sphingomyelin levels in a dose-dependent manner. When rhASM was administered to normal rats, mice, and dogs, no toxicity was observed up to a dose of 30mg/kg. However, high doses of rhASM≥10mg/kg administered to ASMKO mice resulted in unexpected toxicity characterized by cardiovascular shock, hepatic inflammation, adrenal hemorrhage, elevations in ceramide and cytokines (especially IL-6, G-CSF, and keratinocyte chemoattractant [KC]), and death. The toxicity could be completely prevented by the administration of several low doses (3mg/kg) of rhASM prior to single or repeated high doses (≥20mg/kg). These results suggest that the observed toxicity involves the rapid breakdown of large amounts of sphingomyelin into ceramide and/or other toxic downstream metabolites, which are known signaling molecules with cardiovascular and pro-inflammatory effects. Our results suggest that the nonclinical safety assessment of novel therapeutics should include the use of specific animal models of disease whenever feasible.

All members in the sphingomyelin synthase gene family have ceramide phosphoethanolamine synthase activity

Sphingomyelin synthase-related protein (SMSr) synthesizes the sphingomyelin analog ceramide phosphoethanolamine (CPE) in cells. Previous cell studies indicated that SMSr is involved in ceramide homeostasis and is crucial for cell function. To further examine SMSr function in vivo, we generated Smsr KO mice that were fertile and had no obvious phenotypic alterations. Quantitative MS analyses of plasma, liver, and macrophages from the KO mice revealed only marginal changes in CPE and ceramide as well as other sphingolipid levels. Because SMS2 also has CPE synthase activity, we prepared Smsr/Sms2 double KO mice. We found that CPE levels were not significantly changed in macrophages, suggesting that CPE levels are not exclusively dependent on SMSr and SMS2 activities. We then measured CPE levels in Sms1 KO mice and found that Sms1 deficiency also reduced plasma CPE levels. Importantly, we found that expression of Sms1 or Sms2 in SF9 insect cells significantly increased not only SM but also CPE formation, indicating that SMS1 also has CPE synthase activity. Moreover, we measured CPE synthase Km and Vmax for SMS1, SMS2, and SMSr using different NBD ceramides. Our study reveals that all mouse SMS family members (SMSr, SMS1, and SMS2) have CPE synthase activity. However, neither CPE nor SMSr appears to be a critical regulator of ceramide levels in vivo.

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.

Ceramide kinase promotes tumor cell survival and mammary tumor recurrence

Recurrent breast cancer is typically an incurable disease and, as such, is disproportionately responsible for deaths from this disease. Recurrent breast cancers arise from the pool of disseminated tumor cells (DTC) that survive adjuvant or neoadjuvant therapy, and patients with detectable DTCs following therapy are at substantially increased risk for recurrence. Consequently, the identification of pathways that contribute to the survival of breast cancer cells following therapy could aid in the development of more effective therapies that decrease the burden of residual disease and thereby reduce the risk of breast cancer recurrence. We now report that ceramide kinase (Cerk) is required for mammary tumor recurrence following HER2/neu pathway inhibition and is spontaneously upregulated during tumor recurrence in multiple genetically engineered mouse models for breast cancer. We find that Cerk is rapidly upregulated in tumor cells following HER2/neu downregulation or treatment with Adriamycin and that Cerk is required for tumor cell survival following HER2/neu downregulation. Consistent with our observations in mouse models, analysis of gene expression profiles from more than 2,200 patients revealed that elevated CERK expression is associated with an increased risk of recurrence in women with breast cancer. In addition, although CERK expression is associated with aggressive subtypes of breast cancer, including those that are estrogen receptor-negative, HER2(+), basal-like, or high grade, its association with poor clinical outcome is independent of these clinicopathologic variables. Together, our findings identify a functional role for Cerk in breast cancer recurrence and suggest the clinical utility of agents targeted against this prosurvival pathway.

Ephrin-A3 and ephrin-A4 contribute to microglia-induced angiogenesis in brain endothelial cells

The association of microglia with brain vasculature during development and the reduced brain vascular complexity in microglia-deficient mice suggest the role of microglia in cerebrovascular angiogenesis. However, the underlying molecular mechanism remains unclear. Here, using an in vitro angiogenesis model, we found the culture supernatant of BV2 microglial cells significantly enhanced capillary-like tube formation and migration of brain microvascular endothelial cells (BMECs). The expression of angiogenic factors, ephrin-A3 and ephrin-A4, were specifically upregulated in BMECs exposed to BV2-derived culture supernatant. Knockdown of ephrin-A3 and ephrin-A4 in BMECs by siRNA significantly attenuated the enhanced angiogenesis and migration of BMECs induced by BV2 supernatant. Our further results indicated that the ability of BV2 supernatant to promote endothelial angiogenesis was caused by the soluble tumor necrosis factor α (TNF-α) released from BV2 microglial cells. Moreover, the upregulations of ephrin-A3 and ephrin-A4 in BMECs in response to BV2 supernatant were effectively abolished by neutralization antibody against TNF-α and TNF receptor 1, respectively. The present study provides evidence that microglia upregulates endothelial ephrin-A3 and ephrin-A4 to facilitate in vitro angiogenesis of brain endothelial cells, which is mediated by microglia-released TNF-α.

Pharmacologic inhibition of sphingomyelin synthase (SMS) activity reduces apolipoprotein-B secretion from hepatocytes and attenuates endotoxin-mediated macrophage inflammation

Sphingomyelin synthase (SMS) plays an important role in plasma atherogenic lipoprotein metabolism, inflammation, and the development of atherosclerosis. To understand whether the impaired apoB secretion and inflammation response is a direct result from lack of SMS activity, in this study, we prepared a series of compounds that inhibit SMS activity. Further, we characterized Dy105, the most potent inhibitor. We found that Dy105 treatment significantly reduces SM levels in SM-rich microdomain on cell membranes. Moreover, we found that SMS inhibition reduces apoB secretion in a human hepatoma cell line and reduces the activation of NFκB and p38, a MAP kinase, in bone marrow derived macrophages. These studies provided further evidence that SMS activity regulates atherogenic lipoprotein metabolism and inflammatory responses. Pharmacologic inhibition of SMS may be a new therapy for atherosclerosis by reducing apoB secretion, and reducing inflammation.

Recruitment of the adaptor protein Grb2 to EGFR tetramers

Adaptor protein Grb2 binds phosphotyrosines in the epidermal growth factor (EGF) receptor (EGFR) and thereby links receptor activation to intracellular signaling cascades. Here, we investigated how recruitment of Grb2 to EGFR is affected by the spatial organization and quaternary state of activated EGFR. We used the techniques of image correlation spectroscopy (ICS) and lifetime-detected Förster resonance energy transfer (also known as FLIM-based FRET or FLIM–FRET) to measure ligand-induced receptor clustering and Grb2 binding to activated EGFR in BaF/3 cells. BaF/3 cells were stably transfected with fluorescently labeled forms of Grb2 (Grb2–mRFP) and EGFR (EGFR–eGFP). Following stimulation of the cells with EGF, we detected nanometer-scale association of Grb2–mRFP with EGFR–eGFP clusters, which contained, on average, 4 ± 1 copies of EGFR–eGFP per cluster. In contrast, the pool of EGFR–eGFP without Grb2–mRFP had an average cluster size of 1 ± 0.3 EGFR molecules per punctum. In the absence of EGF, there was no association between EGFR–eGFP and Grb2–mRFP. To interpret these data, we extended our recently developed model for EGFR activation, which considers EGFR oligomerization up to tetramers, to include recruitment of Grb2 to phosphorylated EGFR. The extended model, with adjustment of one new parameter (the ratio of the Grb2 and EGFR copy numbers), is consistent with a cluster size distribution where 2% of EGFR monomers, 5% of EGFR dimers, <1% of EGFR trimers, and 94% of EGFR tetramers are associated with Grb2. Together, our experimental and modeling results further implicate tetrameric EGFR as the key signaling unit and call into question the widely held view that dimeric EGFR is the predominant signaling unit.

Regulation of the cancer cell membrane lipid composition by NaCHOleate: effects on cell signaling and therapeutical relevance in glioma

This review summarizes the cellular bases of the effects of NaCHOleate (2-hydroxyoleic acid; 2OHOA; Minerval) against glioma and other types of tumors. NaCHOleate, activates sphingomyelin synthase (SGMS) increasing the levels of cell membrane sphingomyelin (SM) and diacylglycerol (DAG) together with reductions of phosphatidylethanolamine (PE) and phosphatidylcholine (PC). The increases in the membrane levels of NaCHOleate itself and of DAG induce a translocation and overexpression of protein kinase C (PKC) and subsequent reductions of Cyclin D, cyclin-dependent kinases 4 and 6 (CDKs 4 and 6), hypophosphorylation of the retinoblastoma protein, inhibition of E2F1 and knockdown of dihydrofolate reductase (DHFR) impairing DNA synthesis. In addition in some cancer cells, the increases in SM are associated with Fas receptor (FasR) capping and ligand-free induction of apoptosis. In glioma cell lines, the increases in SM are associated with the inhibition of the Ras/MAPK and PI3K/Akt pathways, in association with p27Kip1 overexpression. Finally, an analysis of the Repository of Molecular Brain Neoplasia Data (REMBRANDT) database for glioma patient survival shows that the weight of SM-related metabolism gene expression in glioma patients' survival is similar to glioma-related genes. Due to its low toxicity and anti-tumoral effect in cell and animal models its status as an orphan drug for glioma treatment by the European Medicines Agency (EMA) was recently acknowledged and a phase 1/2A open label, non-randomized study was started in patients with advanced solid tumors including malignant glioma. This article is part of a Special Issue entitled: Membrane Structure and Function: Relevance in the Cell's Physiology, Pathology and Therapy.

Evolving concepts in cancer therapy through targeting sphingolipid metabolism

Traditional methods of cancer treatment are limited in their efficacy due to both inherent and acquired factors. Many different studies have shown that the generation of ceramide in response to cytotoxic therapy is generally an important step leading to cell death. Cancer cells employ different methods to both limit ceramide generation and to remove ceramide in order to become resistant to treatment. Furthermore, sphingosine kinase activity, which phosphorylates sphingosine the product of ceramide hydrolysis, has been linked to multidrug resistance, and can act as a strong survival factor. This review will examine several of the most frequently used cancer therapies and their effect on both ceramide generation and the mechanisms employed to remove it. The development and use of inhibitors of sphingosine kinase will be focused upon as an example of how targeting sphingolipid metabolism may provide an effective means to improve treatment response rates and reduce associated treatment toxicity. This article is part of a Special Issue entitled Tools to study lipid functions.

Inhibition of sphingolipid metabolism enhances resveratrol chemotherapy in human gastric cancer cells

Resveratrol, a chemopreventive agent, is rapidly metabolized in the intestine and liver via glucuronidation. Thus, the pharmacokinetics of resveratrol limits its efficacy. To improve efficacy, the activity of resveratrol was investigated in the context of sphingolipid metabolism in human gastric cancer cells. Diverse sphingolipid metabolites, including dihydroceramides (DHCer), were tested for their ability to induce resveratrol cytotoxicity. Exposure to resveratrol (100 μM) for 24 hr induced cell death and cell cycle arrest in gastric cancer cells. Exposure to the combination of resveratrol and dimethylsphingosine (DMS) increased cytotoxicity, demonstrating that sphingolipid metabolites intensify resveratrol activity. Specifically, DHCer accumulated in a resveratrol concentration-dependent manner in SNU-1 and HT-29 cells, but not in SNU-668 cells. LC-MS/MS analysis showed that specific DHCer species containing C24:0, C16:0, C24:1, and C22:0 fatty acids chain were increased by up to 30-fold by resveratrol, indicating that resveratrol may partially inhibit DHCer desaturase. Indeed, resveratrol mildly inhibited DHCer desaturase activity compared to the specific inhibitor GT-11 or to retinamide (4-HPR); however, in SNU-1 cells resveratrol alone exhibited a typical cell cycle arrest pattern, which GT-11 did not alter, indicating that inhibition of DHCer desaturase is not essential to the cytotoxicity induced by the combination of resveratrol and sphingolipid metabolites. Resveratrol-induced p53 expression strongly correlated with the enhancement of cytotoxicity observed upon combination of resveratrol with DMS or 4-HPR. Taken together, these results show that DHCer accumulation is a novel lipid biomarker of resveratrol-induced cytotoxicity in human gastric cancer cells.

The involvement of sphingolipids in chronic obstructive pulmonary diseases

Chronic obstructive pulmonary disease (COPD) includes a spectrum of conditions that have in common varying degrees of airflow obstruction, such as chronic bronchitis and emphysema. There is an increasing evidence of involvement of sphingolipids as key molecular mediators or biomarkers of disease in emphysema, chronic bronchitis, and more recently in asthma, another disease characterized by (reversible) airflow obstruction. Given the recognized central role of oxidative stress and inflammatory stimuli along with involvement of immune responses, apoptosis, and tissue remodeling in the development of chronic obstructive lung diseases, it is not surprising that sphingolipids have been shown to play important role in their pathobiology. In particular the pro-apoptotic effects of ceramide were suspected as events in the lung destruction that occurs as a result of apoptotic loss of structural cells comprising the alveolar walls, such as microvascular endothelial cells and alveolar epithelial cells. In addition, the role of ceramide was investigated in models of larger airway epithelial cell stress responses to cigarette smoke, in the context of ensuing airway remodeling and inflammation. This chapter discusses current evidence of sphingolipid perturbations in experimental models of COPD and relevant links to human disease based on translational and epidemiological data.

Imaging local sphingomyelin-rich domains in the plasma membrane using specific probes and advanced microscopy

Sphingomyelin (SM) is one of the major lipids in the mammalian plasma membrane. Multiple lines of evidence suggest that SM plays at least two functional roles in the cell, as a reservoir of lipid second messengers and as a platform for signaling molecules. To understand the molecular organization and dynamics of the SM-rich membrane domains, new approaches have been developed utilizing newly characterized specific SM-binding probes and state-of-the-art microscopy techniques. The toxic protein from the sea anemone, equinatoxin II, has been characterized as a specific probe for SM. The cytolytic protein from the earthworm, lysenin, has also been used as a SM-specific probe for the analysis of the heterogeneity of SM-rich membrane domains. Recently, using a non-toxic form of lysenin, we showed the spatial and temporal localization of SM in the plasma membrane by confocal and super-resolution microscopy. New microscopy techniques have also been introduced by other groups to help visualize membrane lipid domains. Here we review the most recent studies on imaging the SM-rich domains in biological membranes. This article is part of a Special Issue entitled New Frontiers in Sphingolipid Biology.

Sustained activation of sphingomyelin synthase by 2-hydroxyoleic acid induces sphingolipidosis in tumor cells

The mechanism of action of 2-hydroxyoleic acid (2OHOA), a potent antitumor drug, involves the rapid and specific activation of sphingomyelin synthase (SMS), leading to a 4-fold increase in SM mass in tumor cells. In the present study, we investigated the source of the ceramides required to sustain this dramatic increase in SM. Through radioactive and fluorescent labeling, we demonstrated that sphingolipid metabolism was altered by a 24 h exposure to 2OHOA, and we observed a consistent increase in the number of lysosomes and the presence of unidentified storage materials in treated cells. Mass spectroscopy revealed that different sphingolipid classes accumulated in human glioma U118 cells after exposure to 2OHOA, demonstrating a specific effect on C16-, C20-, and C22-containing sphingolipids. Based on these findings, we propose that the demand for ceramides required to sustain the SMS activation (ca. 200-fold higher than the basal level) profoundly modifies both sphingolipid and phospholipid metabolism. As the treatment is prolonged, tumor cells fail to adequately metabolize sphingolipids, leading to a situation resembling sphingolipidosis, whereby cell viability is compromised.

Increased oxidative stress impairs adipose tissue function in sphingomyelin synthase 1 null mice

Sphingomyelin synthase 1 (SMS1) catalyzes the conversion of ceramide to sphingomyelin. Here, we found that SMS1 null mice showed lipodystrophic phenotype. Mutant mice showed up-regulation of plasma triglyceride concentrations accompanied by reduction of white adipose tissue (WAT) as they aged. Lipoprotein lipase (LPL) activity was severely reduced in mutant mice. In vivo analysis indicated that fatty acid uptake in WAT but not in liver decreased in SMS1 null compared to wild-type mice. In vitro analysis using cultured cell revealed that SMS1 depletion reduced fatty acid uptake. Proteins extracted from WAT of mutant mice were severely modified by oxidative stress, and up-regulation of mRNAs related to apoptosis, redox adjustment, mitochondrial stress response and mitochondrial biogenesis was observed. ATP content of WAT was reduced in SMS1 null mice. Blue native gel analysis indicated that accumulation of mitochondrial respiratory chain complexes was reduced. These results suggest that WAT of SMS1 null mice is severely damaged by oxidative stress and barely functional. Indeed, mutant mice treated with the anti-oxidant N-acetyl cysteine (NAC) showed partial recovery of lipodystrophic phenotypes together with normalized plasma triglyceride concentrations. Altogether, our data suggest that SMS1 is crucial to control oxidative stress in order to maintain WAT function.