Acid Sphingomyelinase Overexpression Enhances the Antineoplastic Effects of Irradiation In Vitro and In Vivo

Ceramides are fuel gauges on the drive to cardiometabolic disease

Ceramides are signals of fatty acid excess that accumulate when a cell's energetic needs have been met and its nutrient storage has reached capacity. As these sphingolipids accrue, they alter the metabolism and survival of cells throughout the body including in the heart, liver, blood vessels, skeletal muscle, brain, and kidney. These ceramide actions elicit the tissue dysfunction that underlies cardiometabolic diseases such as diabetes, coronary artery disease, metabolic-associated steatohepatitis, and heart failure. Here, we review the biosynthesis and degradation pathways that maintain ceramide levels in normal physiology and discuss how the loss of ceramide homeostasis drives cardiometabolic pathologies. We highlight signaling nodes that sense small changes in ceramides and in turn reprogram cellular metabolism and stimulate apoptosis. Finally, we evaluate the emerging therapeutic utility of these unique lipids as biomarkers that forecast disease risk and as targets of ceramide-lowering interventions that ameliorate disease.

The relevance of acid sphingomyelinase as a potential target for therapeutic intervention in hepatic disorders: current scenario and anticipated trends

Acid sphingomyelinase (ASMase) serves as one of the most remarkable enzymes in sphingolipid biology. ASMase facilitates the hydrolysis of sphingomyelin, yielding ceramide and phosphorylcholine via the phospholipase C signal transduction pathway. Owing to its prominent intervention in apoptosis, ASMase, and its product ceramide is now at the bleeding edge of lipid research due to the coalesced efforts of several research institutions over the past 40 years. ASMase-catalyzed ceramide synthesis profoundly alters the physiological properties of membrane structure in response to a broad range of stimulations, orchestrating signaling cascades for endoplasmic reticulum stress, autophagy, and lysosomal membrane permeabilization, which influences the development of hepatic disorders, such as steatohepatitis, hepatic fibrosis, drug-induced liver injury, and hepatocellular carcinoma. As a result, the potential to modulate the ASMase action with appropriate pharmaceutical antagonists has sparked a lot of curiosity. This article emphasizes the fundamental mechanisms of the systems that govern ASMase aberrations in various hepatic pathologies. Furthermore, we present an insight into the potential therapeutic agents used to mitigate ASMase irregularities and the paramountcy of such inhibitors in drug repurposing.

Engineered Lipidic Nanomaterials Inspired by Sphingomyelin Metabolism for Cancer Therapy

Sphingomyelin (SM) and its metabolites are crucial regulators of tumor cell growth, differentiation, senescence, and programmed cell death. With the rise in lipid-based nanomaterials, engineered lipidic nanomaterials inspired by SM metabolism, corresponding lipid targeting, and signaling activation have made fascinating advances in cancer therapeutic processes. In this review, we first described the specific pathways of SM metabolism and the roles of their associated bioactive molecules in mediating cell survival or death. We next summarized the advantages and specific applications of SM metabolism-based lipidic nanomaterials in specific cancer therapies. Finally, we discussed the challenges and perspectives of this emerging and promising SM metabolism-based nanomaterials research area.

The Role of Sphingolipids Metabolism in Cancer Drug Resistance

Drug resistance continues to be one of the major challenges to cure cancer. As research in this field evolves, it has been proposed that numerous bioactive molecules might be involved in the resistance of cancer cells to certain chemotherapeutics. One well-known group of lipids that play a major role in drug resistance are the sphingolipids. Sphingolipids are essential components of the lipid raft domains of the plasma membrane and this structural function is important for apoptosis and/or cell proliferation. Dysregulation of sphingolipids, including ceramide, sphingomyelin or sphingosine 1-phosphate, has been linked to drug resistance in different types of cancer, including breast, melanoma or colon cancer. Sphingolipid metabolism is complex, involving several lipid catabolism with the participation of key enzymes such as glucosylceramide synthase (GCS) and sphingosine kinase 1 (SPHK1). With an overview of the latest available data on this topic and its implications in cancer therapy, this review focuses on the main enzymes implicated in sphingolipids metabolism and their intermediate metabolites involved in cancer drug resistance.

A Compendium of Information on the Lysosome

For a long time, lysosomes were considered as mere waste bags for cellular constituents. Thankfully, studies carried out in the past 15 years were brimming with elegant and crucial breakthroughs in lysosome research, uncovering their complex roles as nutrient sensors and characterizing them as crucial multifaceted signaling organelles. This review presents the scientific knowledge on lysosome physiology and functions, starting with their discovery and reviewing up to date ground-breaking discoveries highlighting their heterogeneous functions as well as pending questions that remain to be answered. We also review the roles of lysosomes in anti-cancer drug resistance and how they undergo a series of molecular and functional changes during malignant transformation which lead to tumor aggression, angiogenesis, and metastases. Finally, we discuss the strategy of targeting lysosomes in cancer which could lead to the development of new and effective targeted therapies.

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.

Physiological functions and therapeutic applications of neutral sphingomyelinase and acid sphingomyelinase

Sphingomyelin (SM) can be converted into ceramide (Cer) by neutral sphingomyelinase (NSM) and acid sphingomyelinase (ASM). Cer is a second messenger of lipids and can regulate cell growth and apoptosis. Increasing evidence shows that NSM and ASM play key roles in many processes, such as apoptosis, immune function and inflammation. Therefore, NSM and ASM have broad prospects in clinical treatments, especially in cancer, cardiovascular diseases (such as atherosclerosis), nervous system diseases (such as Alzheimer's disease), respiratory diseases (such as chronic obstructive pulmonary disease) and the phenotype of dwarfisms in adolescents, playing a complex regulatory role. This review focuses on the physiological functions of NSM and ASM and summarizes their roles in certain diseases and their potential applications in therapy.

Acid sphingomyelinase-dependent autophagic degradation of GPX4 is critical for the execution of ferroptosis

Ferroptosis is a type of regulated cell death characterized by ROS accumulation and devastating lipid peroxidation (LPO). The role of acid sphingomyelinase (ASM), a key enzyme in sphingolipid metabolism, in the induction of apoptosis has been studied; however, to date its role in ferroptosis is unclear. In this study, we report that ASM plays a hitherto unanticipated role in promoting ferroptosis. Mechanistically, Erastin (Era) treatment results in the activation of ASM and generation of ceramide, which are required for the Era-induced reactive oxygen species (ROS) generation and LPO. Inhibition of nicotinamide adenine dinucleotide phosphate oxidase (NADPH oxidase) or removal of intracellular ROS, significantly reduced Era-induced ASM activation, suggesting that NADPH oxidase-derived ROS regulated ASM-initiated redox signaling in a positive feedback manner. Moreover, ASM-mediated activation of autophagy plays a critical role in ferroptosis inducers (FINs)-induced glutathione peroxidase 4 (GPX4) degradation and ferroptosis activation. Genetic or pharmacological inhibition of ASM diminishes Era-induced features of autophagy, GPX4 degradation, LPO, and subsequent ferroptosis. Importantly, genetic activation of ASM increases ferroptosis in cancer cells induced by various FINs. Collectively, these findings reveal that ASM plays a novel role in ferroptosis that could be exploited to improve pathological conditions that link to ferroptosis.

Sphingolipid Metabolism in Glioblastoma and Metastatic Brain Tumors: A Review of Sphingomyelinases and Sphingosine-1-Phosphate

Glioblastoma (GBM) is a primary malignant brain tumor with a dismal prognosis, partially due to our inability to completely remove and kill all GBM cells. Rapid tumor recurrence contributes to a median survival of only 15 months with the current standard of care which includes maximal surgical resection, radiation, and temozolomide (TMZ), a blood-brain barrier (BBB) penetrant chemotherapy. Radiation and TMZ cause sphingomyelinases (SMase) to hydrolyze sphingomyelins to generate ceramides, which induce apoptosis. However, cells can evade apoptosis by converting ceramides to sphingosine-1-phosphate (S1P). S1P has been implicated in a wide range of cancers including GBM. Upregulation of S1P has been linked to the proliferation and invasion of GBM and other cancers that display a propensity for brain metastasis. To mediate their biological effects, SMases and S1P modulate signaling via phospholipase C (PLC) and phospholipase D (PLD). In addition, both SMase and S1P may alter the integrity of the BBB leading to infiltration of tumor-promoting immune populations. SMase activity has been associated with tumor evasion of the immune system, while S1P creates a gradient for trafficking of innate and adaptive immune cells. This review will explore the role of sphingolipid metabolism and pharmacological interventions in GBM and metastatic brain tumors with a focus on SMase and S1P.

Acid Sphingomyelinase Downregulation Enhances Mitochondrial Fusion and Promotes Oxidative Metabolism in a Mouse Model of Melanoma

Melanoma is the most severe type of skin cancer. Its unique and heterogeneous metabolism, relying on both glycolysis and oxidative phosphorylation, allows it to adapt to disparate conditions. Mitochondrial function is strictly interconnected with mitochondrial dynamics and both are fundamental in tumour progression and metastasis. The malignant phenotype of melanoma is also regulated by the expression levels of the enzyme acid sphingomyelinase (A-SMase). By modulating at transcriptional level A-SMase in the melanoma cell line B16-F1 cells, we assessed the effect of enzyme downregulation on mitochondrial dynamics and function. Our results demonstrate that A-SMase influences mitochondrial morphology by affecting the expression of mitofusin 1 and OPA1. The enhanced expression of the two mitochondrial fusion proteins, observed when A-SMase is expressed at low levels, correlates with the increase of mitochondrial function via the stimulation of the genes PGC-1alpha and TFAM, two genes that preside over mitochondrial biogenesis. Thus, the reduction of A-SMase expression, observed in malignant melanomas, may determine their metastatic behaviour through the stimulation of mitochondrial fusion, activity and biogenesis, conferring a metabolic advantage to melanoma cells.

Lysosome as the Black Hole for Checkpoint Molecules

Lysosomes, as digestive organelles full of hydrolases, have complex functions and play an important role in cellular physiological and pathological processes. In normal physiological conditions, lysosomes can sense the nutritional state and be responsible for recycling raw materials to provide nutrients, affecting cell signaling pathways and regulating cell proliferation. Lysosomes are related to many diseases and associated with metastasis and drug resistance of tumors. In recent years, much attention has been paid to the tumor immunotherapy especially immune checkpoint blockade therapy. Accumulating data suggest that lysosomes may serve as a major destruction for immune checkpoint molecules, and secretory lysosomes can temporarily store immune checkpoint proteins. Once activated, the compounds contained in secretory lysosomes are released to the surface of cell membrane rapidly. Inhibitions of lysosomes can overcome the chemoresistance of some tumors and enhance the efficacy of immunotherapy.

Sphingolipids in inflammatory hypoxia

Hypoxia due to rapid tumor growth with impaired neovascularization and inflammation resulting from immune cell activation are hallmarks of cancer. Hypoxia-inducible factors control transcriptional adaptation in response to low oxygen conditions, both in tumor and immune cells. In addition, sphingolipids become increasingly recognized as important cell mediators in tumor and inflammatory hypoxia. Recent studies have identified acid sphingomyelinase (ASM), a central enzyme in the sphingolipid metabolism, as a regulator of several types of stress stimuli pathways and an important player in the tumor microenvironment. Therefore, this review will address the connection between the hypoxic response and the ASM/ceramide system in the context of inflammatory hypoxia.

Essential role for acid sphingomyelinase-inhibited autophagy in melanoma response to cisplatin

The sphingolipid metabolising enzyme Acid Sphingomyelinase (A-SMase) has been recently shown to inhibit melanoma progression and correlate inversely to tumour grade. In this study we have investigated the role of A-SMase in the chemo-resistance to anticancer treatmentusing mice with melanoma allografts and melanoma cells differing in terms of expression/activity of A-SMase. Since autophagy is emerging as a key mechanism in tumour growth and chemo-resistance, we have also investigated whether an action of A-SMase in autophagy can explain its role. Melanoma sensitivity to chemotherapeutic agent cisplatin in terms of cell viability/apoptosis, tumour growth, and animal survival depended directly on the A-SMase levels in tumoural cells. A-SMase action was due to inhibition of autophagy through activation of Akt/mammalian target of rapamycin (mTOR) pathway. Treatment of melanoma-bearing mice with the autophagy inhibitor chloroquine restored sensitivity to cisplatin of tumours expressing low levels of A-SMase while no additive effects were observed in tumours characterised by sustained A-SMase levels. The fact that A-SMase in melanomas affects mTOR-regulated autophagy and plays a central role in cisplatin efficacy encourages pre-clinical testing on the modulation of A-SMase levels/activity as possible novel anti-neoplastic strategy.

Lysosomal Biology in Cancer

Cells depend on the lysosome for sequestration and degradation of macromolecules in order to maintain metabolic homeostasis. These membrane-enclosed organelles can receive intracellular and extracellular cargo through endocytosis, phagocytosis, and autophagy. Lysosomes establish acidic environments to activate enzymes that are able to break down biomolecules engulfed through these various pathways. Recent advances in methods to study the lysosome have allowed the discovery of extended roles for the lysosome in various diseases, including cancer, making it an attractive and targetable node for therapeutic intervention. This review focuses on key aspects of lysosomal biology in the context of cancer and how these properties can be exploited for the development of new therapeutic strategies. This will provide a contextual framework for how advances in methodology could be applied in future translational research.

A metabolic labeling approach for glycoproteomic analysis reveals altered glycoprotein expression upon GALNT3 knockdown in ovarian cancer cells

Epithelial ovarian cancer (EOC) is a disease responsible for more deaths among women in the Western world than all other gynecologic malignancies. There is urgent need for new therapeutic targets and a better understanding of EOC initiation and progression. We have previously identified the polypeptide N-acetylgalactosaminyltransferase 3 (GALNT3) gene, a member of the GalNAc-transferases (GalNAc-Ts) gene family, as hypomethylated and overexpressed in high-grade serous EOC tumors, compared to low malignant potential EOC tumors and normal ovarian tissues. This data also suggested for a role of GALNT3 in aberrant EOC glycosylation, possibly implicated in disease progression. To evaluate differential glycosylation in EOC caused by modulations in GALNT3 expression, we used a metabolic labeling strategy for enrichment and mass spectrometry-based characterization of glycoproteins following GALNT3 gene knockdown (KD) in A2780s EOC cells. A total of 589 differentially expressed glycoproteins were identified upon GALNT3 KD. Most identified proteins were involved in mechanisms of cellular metabolic functions, post-translational modifications, and some have been reported to be implicated in EOC etiology. The GALNT3-dependent glycoproteins identified by this metabolic labeling approach support the oncogenic role of GALNT3 in EOC dissemination and may be pursued as novel EOC biomarkers and/or therapeutic targets.

BIOLOGICAL SIGNIFICANCE

Knowledge of the O-glycoproteome has been relatively elusive, and the functions of the individual polypeptide GalNAc-Ts have been poorly characterized. Alterations in GalNAc-Ts expression were shown to provide huge variability in the O-glycoproteome in various pathologies, including cancer. The application of a chemical biology approach for the metabolic labeling and subsequent characterization of O-glycoproteins in EOC using the Ac4GalNAz metabolite has provided a strategy allowing for proteomic discovery of GalNAc-Ts specific functions. Our study supports an essential role of one of the GalNAc-Ts - GALNT3, in EOC dissemination, including its implication in modulating PTMs and EOC metabolism. Our approach validates the use of the applied metabolic strategy to identify important functions of GalNAc-Ts in normal and pathological conditions.

The emerging role of acid sphingomyelinase in autophagy

Autophagy, the main intracellular process of cytoplasmic material degradation, is involved in cell survival and death. Autophagy is regulated at various levels and novel modulators of its function are being continuously identified. An intriguing recent observation is that among these modulators is the sphingolipid metabolising enzyme, Acid Sphingomyelinase (A-SMase), already known to play a fundamental role in apoptotic cell death participating in several pathophysiological conditions. In this review we analyse and discuss the relationship between autophagy and A-SMase describing how A-SMase may regulate it and defining, for the first time, the existence of an A-SMase-autophagy axis. The imbalance of this axis plays a role in cancer, nervous system, cardiovascular, and hepatic disorders.

Targeting the lysosome in cancer

Lysosomes are membrane-bound intracellular organelles that receive macromolecules delivered by endocytosis, phagocytosis, and autophagy for degradation and recycling. Over the last decade, advances in lysosome research have established a broad role for the lysosome in the pathophysiology of disease. In this review, we highlight the recent discoveries in lysosome biology, with an emphasis on their implications for cancer therapy. We focus on targeting the lysosome in cancer by exploring lysosomal biogenesis and its role in the crosstalk between apoptosis and autophagy. We also discuss how lysosomal inhibition could emerge as a new therapeutic strategy to overcome drug resistance in cancer.

Modulation of Acid Sphingomyelinase in Melanoma Reprogrammes the Tumour Immune Microenvironment

The inflammatory microenvironment induces tumours to acquire an aggressive and immunosuppressive behaviour. Since acid sphingomyelinase (A-SMase) downregulation in melanoma was shown to determine a malignant phenotype, we aimed here to elucidate the role of A-SMase in the regulation of tumour immunogenic microenvironment using in vivo melanoma models in which A-SMase was either downregulated or maintained at constitutively high levels. We found high levels of inflammatory factors in low A-SMase expressing tumours, which also displayed an immunosuppressive/protumoural microenvironment: high levels of myeloid-derived suppressor cells (MDSCs) and regulatory T lymphocytes (Tregs), as well as low levels of dendritic cells (DCs). In contrast, the restoration of A-SMase in melanoma cells not only reduced tumour growth and immunosuppression, but also induced a high recruitment at tumour site of effector immune cells with an antitumoural function. Indeed, we observed a poor homing of MDSCs and Tregs and the increased recruitment of CD8⁺ and CD4⁺ T lymphocytes as well as the infiltration of DCs and CD8⁺/CD44^high T lymphocytes. This study demonstrates that change of A-SMase expression in cancer cells is sufficient per se to tune in vivo melanoma growth and that A-SMase levels modulate immune cells at tumour site. This may be taken into consideration in the setting of therapeutic strategies.

Sphingolipids in the DNA damage response

Recently, sphingolipid metabolizing enzymes have emerged as important targets of many chemotherapeutics and DNA damaging agents and therefore play significant roles in mediating the physiological response of the cell to DNA damage. In this review we will highlight points of connection between the DNA damage response (DDR) and sphingolipid metabolism; specifically how certain sphingolipid enzymes are regulated in response to DNA damage and how the bioactive lipids produced by these enzymes affect cell fate.

Altering the sphingolipid acyl chain composition prevents LPS/GLN-mediated hepatic failure in mice by disrupting TNFR1 internalization

The involvement of ceramide in death receptor-mediated apoptosis has been widely examined with most studies focusing on the role of ceramide generated from sphingomyelin hydrolysis. We now analyze the effect of the ceramide acyl chain length by studying tumor necrosis factor α receptor-1 (TNFR1)-mediated apoptosis in a ceramide synthase 2 (CerS2) null mouse, which cannot synthesize very-long acyl chain ceramides. CerS2 null mice were resistant to lipopolysaccharide/galactosamine-mediated fulminant hepatic failure even though TNFα secretion from macrophages was unaffected. Cultured hepatocytes were also insensitive to TNFα-mediated apoptosis. In addition, in both liver and in hepatocytes, caspase activities were not elevated, consistent with inhibition of TNFR1 pro-apoptotic signaling. In contrast, Fas receptor activation resulted in the death of CerS2 null mice. Caspase activation was blocked because of the inability of CerS2 null mice to internalize the TNFR1; whereas Fc-TNFα was internalized to a perinuclear region in hepatocytes from wild-type mice, no internalization was detected in CerS2 null mice. Our results indicate that altering the acyl chain composition of sphingolipids inhibits TNFR1 internalization and inhibits selective pro-apoptotic downstream signaling for apoptosis.

Adenoviral transduction of human acid sphingomyelinase into neo-angiogenic endothelium radiosensitizes tumor cure

These studies define a new mechanism-based approach to radiosensitize tumor cure by single dose radiotherapy (SDRT). Published evidence indicates that SDRT induces acute microvascular endothelial apoptosis initiated via acid sphingomyelinase (ASMase) translocation to the external plasma membrane. Ensuing microvascular damage regulates radiation lethality of tumor stem cell clonogens to effect tumor cure. Based on this biology, we engineered an ASMase-producing vector consisting of a modified pre-proendothelin-1 promoter, PPE1(3x), and a hypoxia-inducible dual-binding HIF-2α-Ets-1 enhancer element upstream of the asmase gene, inserted into a replication-deficient adenovirus yielding the vector Ad5H2E-PPE1(3x)-ASMase. This vector confers ASMase over-expression in cycling angiogenic endothelium in vitro and within tumors in vivo, with no detectable enhancement in endothelium of normal tissues that exhibit a minute fraction of cycling cells or in non-endothelial tumor or normal tissue cells. Intravenous pretreatment with Ad5H2E-PPE1(3x)-ASMase markedly increases SDRT cure of inherently radiosensitive MCA/129 fibrosarcomas, and converts radiation-incurable B16 melanomas into biopsy-proven tumor cures. In contrast, Ad5H2E-PPE1(3x)-ASMase treatment did not impact radiation damage to small intestinal crypts as non-dividing small intestinal microvessels did not overexpress ASMase and were not radiosensitized. We posit that combination of genetic up-regulation of tumor microvascular ASMase and SDRT provides therapeutic options for currently radiation-incurable human tumors.

Recombinant human acid sphingomyelinase as an adjuvant to sorafenib treatment of experimental liver cancer

BACKGROUND

Hepatocellular carcinoma (HCC) is the most common form of liver cancer and the third leading cause of cancer death worldwide. The only approved systemic treatment for unresectable HCC is the oral kinase inhibitor, sorafenib. Recombinant human acid sphingomyelinase (rhASM), which hydrolyzes sphingomyelin to ceramide, is an orphan drug under development for the treatment of Type B Niemann-Pick disease (NPD). Due to the hepatotropic nature of rhASM and its ability to generate pro-apoptotic ceramide, this study evaluated the use of rhASM as an adjuvant treatment with sorafenib in experimental models of HCC.

METHODOLOGY/PRINCIPAL FINDINGS

In vitro, rhASM/sorafenib treatment reduced the viability of Huh7 liver cancer cells more than sorafenib. In vivo, using a subcutaneous Huh7 tumor model, mouse survival was increased and proliferation in the tumors decreased to a similar extent in both sorafenib and rhASM/sorafenib treatment groups. However, combined rhASM/sorafenib treatment significantly lowered tumor volume, increased tumor necrosis, and decreased tumor blood vessel density compared to sorafenib. These results were obtained despite poor delivery of rhASM to the tumors. A second (orthotopic) model of Huh7 tumors also was established, but modest ASM activity was similarly detected in these tumors compared to healthy mouse livers. Importantly, no chronic liver toxicity or weight loss was observed from rhASM therapy in either model.

CONCLUSIONS/SIGNIFICANCE

The rhASM/sorafenib combination exhibited a synergistic effect on reducing the tumor volume and blood vessel density in Huh7 xenografts, despite modest activity of rhASM in these tumors. No significant increases in survival were observed from the rhASM/sorafenib treatment. The poor delivery of rhASM to Huh7 tumors may be due, at least in part, to low expression of mannose receptors. The safety and efficacy of this approach, together with the novel findings regarding enzyme targeting, merits further investigation.

Use of acid sphingomyelinase for cancer therapy

Acid sphingomyelinase (ASM) is a lipid hydrolase that cleaves the sphingolipid, sphingomyelin, into ceramide. Mutations in the ASM gene (SMPD1) result in the rare lysosomal storage disorder, Niemann-Pick disease (NPD). In addition to its role in NPD, over the past two decades, the importance of sphingolipids, and ASM in particular, in normal physiology and the pathophysiology of numerous common diseases also has become known. For example, altered sphingolipid metabolism occurs in many cancers, generally reducing the levels of the pro-apoptotic lipid, ceramide, and/or elevating the levels of the proliferative lipid, sphingosine-1-phosphate (S1P). These changes likely contribute to the tumorigenicity and/or metastatic capacity of the cancer. In addition, many cancer therapies induce ceramide-mediated death, and cancer cells have evolved novel mechanisms to overcome this effect. In the present review, we discuss sphingolipid metabolism in cancer, and specifically the potential for pharmacological modulation using ASM. Of note, recombinant human ASM (rhASM) has been produced for human use and is being evaluated as a treatment for NPD. Thus, its use for cancer therapy could be rapidly evaluated in the clinic after appropriate animal model studies have been completed. As this enzyme was initially studied in the context of NPD, we start with a brief overview of the history of ASM and NPD, followed by a discussion of the role of ASM in cancer biology, and then summarize emerging preclinical efficacy studies using rhASM as an adjunct in the treatment of solid tumors.

Natural products as platforms for the design of sphingolipid-related anticancer agents

Modulation of sphingolipid metabolism is a promising strategy for cancer therapy that has already opened innovative approaches for the development of pharmacological tools and rationally designed new drugs. On the other hand, natural products represent a classical and well-established source of chemical diversity that has guided medicinal chemists on the development of new chemical entities with potential therapeutic use. Based on these premises, the aim of this chapter is to provide the reader with a general overview of some of the most representative families of sphingolipid-related natural products that have been described in the recent literature as lead compounds for the design of new modulators of sphingolipid metabolism. Special emphasis is placed on the structural aspects of natural sphingoids and synthetic analogs that have found application as anticancer agents. In addition, their cellular targets and/or their mode of action are also considered.

Active autophagy in the tumor microenvironment: A novel mechanism for cancer metastasis

Autophagy is a lysosomal degradation process which is key for the regulation of the turnover of long-lived or damaged proteins and organelles and which promotes cell survival during nutrient deprivation or other microenvironmental stresses. Current evidence supports the hypothesis that autophagy suppresses tumorigenesis, particularly during the early stages of tumor initiation. However, in established tumors, autophagy promotes survival under stressful conditions during cancer progression and in response to chemotherapy; however, the mechanism by which autophagy influences cancer metastasis remains unknown. In this review, we discuss the capacity of an abnormal tumor environment to induce autophagy and consider how this relates to tumor metastasis and the attractive prospect of manipulating autophagic signaling pathways as potential targets for the treatment of cancer metastasis.

Involvement of ceramide in cell death responses in the pulmonary circulation

Ceramides are signaling sphingolipids involved in cellular homeostasis but also in pathological processes such as unwanted apoptosis, growth arrest, oxidative stress, or senescence. Several enzymatic pathways are responsible for the synthesis of ceramides, which can be activated in response to exogenous stimuli such as cytokines, radiation, or oxidative stress. Endothelial cells are particularly rich in acid sphingomyelinases, which can be rapidly activated to produce ceramides, both intracellular and at the plasma membrane. In addition, neutral sphingomyelinases, the de novo pathway and the ceramide recycling pathway, may generate excessive ceramides involved in endothelial cell responses. When up-regulated, ceramides trigger signaling pathways that culminate in endothelial cell death, which in murine lungs has been linked to the development of emphysema-like disease. Furthermore, ceramides may be released paracellularly where they are believed to exert paracrine activities. Such effects, along with ceramides released by inflammatory mediators, may contribute to lung inflammation and pulmonary edema, because ceramide-challenged pulmonary endothelial cells exhibit decreased barrier function, independent of apoptosis. Reestablishing the sphingolipid homeostasis, either by modulating ceramide synthesis or by opposing its biological effects through augmentation of the prosurvival sphingosine-1 phosphate, may alleviate acute or chronic pulmonary conditions characterized by vascular endothelial cell death or dysfunction.

Sphingolipids and cancer: ceramide and sphingosine-1-phosphate in the regulation of cell death and drug resistance

Sphingolipids have emerged as bioeffector molecules, controlling various aspects of cell growth and proliferation in cancer, which is becoming the deadliest disease in the world. These lipid molecules have also been implicated in the mechanism of action of cancer chemotherapeutics. Ceramide, the central molecule of sphingolipid metabolism, generally mediates antiproliferative responses, such as cell growth inhibition, apoptosis induction, senescence modulation, endoplasmic reticulum stress responses and/or autophagy. Interestingly, recent studies suggest de novo-generated ceramides may have distinct and opposing roles in the promotion/suppression of tumors, and that these activities are based on their fatty acid chain lengths, subcellular localization and/or direct downstream targets. For example, in head and neck cancer cells, ceramide synthase 6/C(16)-ceramide addiction was revealed, and this was associated with increased tumor growth, whereas downregulation of its synthesis resulted in ER stress-induced apoptosis. By contrast, ceramide synthase 1-generated C(18)-ceramide has been shown to suppress tumor growth in various cancer models, both in situ and in vivo. In addition, ceramide metabolism to generate sphingosine-1-phosphate (S1P) by sphingosine kinases 1 and 2 mediates, with or without the involvement of G-protein-coupled S1P receptor signaling, prosurvival, angiogenesis, metastasis and/or resistance to drug-induced apoptosis. Importantly, recent findings regarding the mechanisms by which sphingolipid metabolism and signaling regulate tumor growth and progression, such as identifying direct intracellular protein targets of sphingolipids, have been key for the development of new chemotherapeutic strategies. Thus, in this article, we will present conclusions of recent studies that describe opposing roles of de novo-generated ceramides by ceramide synthases and/or S1P in the regulation of cancer pathogenesis, as well as the development of sphingolipid-based cancer therapeutics and drug resistance.

Acidic sphingomyelinase controls hepatic stellate cell activation and in vivo liver fibrogenesis

The mechanisms linking hepatocellular death, hepatic stellate cell (HSC) activation, and liver fibrosis are largely unknown. Here, we investigate whether acidic sphingomyelinase (ASMase), a known regulator of death receptor and stress-induced hepatocyte apoptosis, plays a role in liver fibrogenesis. We show that selective stimulation of ASMase (up to sixfold), but not neutral sphingomyelinase, occurs during the transdifferentiation/activation of primary mouse HSCs into myofibroblast-like cells, coinciding with cathepsin B (CtsB) and D (CtsD) processing. ASMase inhibition or genetic down-regulation by small interfering RNA blunted CtsB/D processing, preventing the activation and proliferation of mouse and human HSCs (LX2 cells). In accordance, HSCs from heterozygous ASMase mice exhibited decreased CtsB/D processing, as well as lower levels of alpha-smooth muscle actin expression and proliferation. Moreover, pharmacological CtsB inhibition reproduced the antagonism of ASMase in preventing the fibrogenic properties of HSCs, without affecting ASMase activity. Interestingly, liver fibrosis induced by bile duct ligation or carbon tetrachloride administration was reduced in heterozygous ASMase mice compared with that in wild-type animals, regardless of their sensitivity to liver injury in either model. To provide further evidence for the ASMase-CtsB pathway in hepatic fibrosis, liver samples from patients with nonalcoholic steatohepatitis were studied. CtsB and ASMase mRNA levels increased eight- and threefold, respectively, in patients compared with healthy controls. These findings illustrate a novel role of ASMase in HSC biology and liver fibrogenesis by regulating its downstream effectors CtsB/D.

Ceramide-rich platforms in transmembrane signaling

Recent evidence suggests that ceramide regulates stress signaling via reorganization of the plasma membrane. The focus of this review will be to discuss the mechanism by which acid sphingomyelinase (ASMase)-generated ceramide initiates transmembrane signaling in the plasma membrane exoplasmic leaflet. In particular, we review the unique biophysical properties of ceramide that render it proficient in formation of signaling domains termed ceramide-rich platforms (CRPs), and the role of CRPs in the pathophysiology of various diseases. The biomedical significance of CRPs makes these structures an attractive therapeutic target.

Acid sphingomyelinase, cell membranes and human disease: lessons from Niemann-Pick disease

Acid sphingomyelinase (ASM) plays an important role in normal membrane turnover through the hydrolysis of sphingomyelin, and is one of the key enzymes responsible for the production of ceramide. ASM activity is deficient in the genetic disorder Types A and B Niemann-Pick disease (NPD). ASM knockout (ASMKO) mice were originally constructed to study this disorder, and numerous defects in ceramide-related signaling have been shown. Studies in these mice have further suggested that ASM may be involved in the pathogenesis of several common diseases through the reorganization of membrane microdomains. This review will focus on the role of ASM in membrane biology, with a specific emphasis on what a rare genetic disorder (NPD) has taught us about more common events.

Promotion of autophagy as a mechanism for radiation sensitization of breast tumor cells

Radiation has long been a useful component of the treatment regimen for solid tumors. However, some malignancies are relatively resistant to radiation treatment while even tumors that may initially respond (to both radiation and chemotherapy) may eventually recover proliferative capacity. A variety of approaches have been utilized in the efforts to enhance radiation sensitivity. Recent studies have identified autophagy as a cell death pathway that may mediate the radiosensitizing effects of selected treatments. Studies in our laboratory support the premise that radiosensitization of breast tumor cells by vitamin D or vitamin D analogs is mediated through autophagy. In addition, promotion of autophagic cell death by a vitamin D analog in irradiated breast tumor cells delays and attenuates the proliferative recovery that may be a preclinical indicator of disease recurrence.

Roles and regulation of secretory and lysosomal acid sphingomyelinase

Acid sphingomyelinase occupies a prominent position in sphingolipid catabolism, catalyzing the hydrolysis of sphingomyelin to ceramide and phosphorylcholine. Enzymatic dysfunction of acid sphingomyelinase results in Niemann-Pick disease, a lysosomal storage disorder characterized at the cellular level by accumulation of sphingomyelin within the endo-lysosomal compartment. Over the past decade interest in the role of acid sphingomyelinase has moved beyond its "housekeeping" function in constitutive turnover of sphingomyelin in the lysosome to include study of regulated ceramide generation. Ceramide functions as a bioactive sphingolipid with pleiotropic signaling properties, and has been implicated in diverse cellular processes of physiologic and pathophysiologic importance. Though many cellular enzymes have the capacity to generate ceramide,there is growing appreciation that "all ceramides are not created equal." Ceramides likely exert distinct effects in different cellular/subcellular compartments by virtue of access to other sphingolipid enzymes (e.g.ceramidases), effector molecules (e.g. ceramide-activated protein phosphatases), and neighboring lipids and proteins (e.g. cholesterol, ion channels). One of the unique features of acid sphingomyelinase is that it has been implicated in the hydrolysis of sphingomyelin in three different settings--the endo-lysosomal compartment,the outer leaflet of the plasma membrane, and lipoproteins. How a single gene product has the capacity to function in these diverse settings, and the subsequent impact on downstream ceramide-mediated biology is the subject of this review.

Ceramide-enriched membrane domains--structure and function

Membrane lipids seem to be organized and not randomly distributed in the cell membrane. In particular, sphingolipids seem to interact with cholesterol in the outer leaflet of the cell membrane resulting in the formation of distinct membrane domains, i.e. rafts. The generation of ceramide within rafts alters their biophysical properties and results in the formation of large ceramide-enriched membrane platforms. These platforms serve to cluster receptor molecules and to organize intracellular signalling molecules to facilitate signal transduction via a receptor upon stimulation. Thus, ceramide-enriched membrane domains amplify not only receptor-, but also stress-mediated signalling events. Although many receptors cluster, the molecular mechanisms mediating this important and general event in signal transduction need to be identified.