Sphingolipid homeostasis in the web of metabolic routes

Sphingolipids play a key role in cells as structural components of membrane lipid bilayers and signaling molecules implicated in important physiological and pathological processes. Their metabolism is tightly regulated. Mechanisms controlling sphingolipid metabolism are far from being completely understood. However, they already reveal the integration of sphingolipids in the whole metabolic network as signaling devices that coordinate different metabolic pathways. A picture of sphingolipids integrated into metabolic networks might help to understand sphingolipid homeostasis. This review describes recent advances in the regulation of de novo sphingolipid synthesis with a focus on the bridges that exist with other metabolic pathways and the importance of this crosstalk in the control of sphingolipid homeostasis. This article is part of a Special Issue entitled New Frontiers in Sphingolipid Biology.     

Autophagy regulates sphingolipid levels in the liver

Sphingolipid levels are tightly regulated to maintain cellular homeostasis. During pathologic conditions such as in aging, inflammation, and metabolic and neurodegenerative diseases, levels of some sphingolipids, including the bioactive metabolite ceramide, are elevated. Sphingolipid metabolism has been linked to autophagy, a critical catabolic process in both normal cell function and disease; however, the in vivo relevance of the interaction is not well-understood. Here, we show that blocking autophagy in the liver by deletion of the Atg7 gene, which is essential for autophagosome formation, causes an increase in sphingolipid metabolites including ceramide. We also show that overexpression of serine palmitoyltransferase to elevate de novo sphingolipid biosynthesis induces autophagy in the liver. The results reveal autophagy as a process that limits excessive ceramide levels and that is induced by excessive elevation of de novo sphingolipid synthesis in the liver. Dysfunctional autophagy may be an underlying mechanism causing elevations in ceramide that may contribute to pathogenesis in diseases.     

Unraveling the role of the Target of Rapamycin signaling in sphingolipid metabolism

Sphingolipids are important bioactive molecules that regulate basic aspects of cellular metabolism and physiology, including cell growth, adhesion, migration, senescence, apoptosis, endocytosis, and autophagy in yeast and higher eukaryotes. Since they have the ability to modulate the activation of several proteins and signaling pathways, variations in the relative levels of different sphingolipid species result in important changes in overall cellular functions and fate.Sphingolipid metabolism and their route of synthesis are highly conserved from yeast to mammalian cells. Studies using the budding yeast Saccharomyces cerevisiae have served in many ways to foster our understanding of sphingolipid dynamics and their role in the regulation of cellular processes. In the past decade, studies in S. cerevisiae have unraveled a functional association between the Target of Rapamycin (TOR) pathway and sphingolipids, showing that both TOR Complex 1 (TORC1) and TOR Complex 2 (TORC2) branches control temporal and spatial aspects of sphingolipid metabolism in response to physiological and environmental cues. In this review, we report recent findings in this emerging and exciting link between the TOR pathway and sphingolipids and implications in human health and disease.     

Cancer and sphingolipid storage disease therapy using novel synthetic analogs of sphingolipids

Sphingolipid metabolites have become recognized for their participation in cell functions and signaling events that control a wide array of cellular activities. Two main sphingolipids, ceramide and sphingosine-1-phosphate, are involved in signaling pathways that regulate cell proliferation, apoptosis, motility, differentiation, angiogenesis, stress responses, protein synthesis, carbohydrate metabolism, and intracellular trafficking. Ceramide and S1P often exert opposing effects on cell survival, ceramide being pro-apoptotic and S1P generally promoting cell survival. Therefore, the conversion of one of these metabolites to the other by sphingolipid enzymes provides a vast network of regulation and provides a useful therapeutic target. Here we provide a survey of the current knowledge of the roles of sphingolipid metabolites in cancer and in lipid storage disease. We review our attempts to interfere with this network of regulation and so provide new treatments for a range of diseases. We synthesized novel analogs of sphingolipids which inhibit the hydrolysis of ceramide or its conversion to more complex sphingolipids. These analogs caused elevation of ceramide levels, leading to apoptosis of a variety of cancer cells. Administration of a synthetic analog to tumor-bearing mice resulted in reduction and even disappearance of the tumors. Therapies for sphingolipid storage diseases, such as Niemann-Pick and Gaucher diseases were achieved by two different strategies: inhibition of the biosynthesis of the substrate (substrate reduction therapy) and protection of the mutated enzyme (chaperone therapy). Sphingolipid metabolism was monitored by the use of novel fluorescent sphingolipid analogs. The results described in this review indicate that our synthetic analogs could be developed both as anticancer drugs and for the treatment of sphingolipid storage diseases.     

Solving the enigma: Mass spectrometry and small molecule probes to study sphingolipid function

Sphingolipids are highly bioactive lipids. Sphingolipid metabolism produces key membrane components (e.g. sphingomyelin) and a variety of signaling lipids with different biological functions (e.g. ceramide, sphingosine-1-phosphate). The coordinated activity of tens of different enzymes maintains proper levels and localization of these lipids with key roles in cellular processes. In this review, we highlight the signaling roles of sphingolipids in cell death and survival. We discuss recent findings on the role of specific sphingolipids during these processes, enabled by the use of lipidomics to study compositional and spatial regulation of these lipids and synthetic sphingolipid probes to study subcellular localization and interaction partners of sphingolipids to understand the function of these lipids.     

Sphingosine kinases, sphingosine 1-phosphate, apoptosis and diseases

Sphingolipids are ubiquitous components of cell membranes and their metabolites ceramide (Cer), sphingosine (Sph), and sphingosine-1-phosphate (S1P) have important physiological functions, including regulation of cell growth and survival. Cer and Sph are associated with growth arrest and apoptosis. Many stress stimuli increase levels of Cer and Sph, whereas suppression of apoptosis is associated with increased intracellular levels of S1P. In addition, extracellular/secreted S1P regulates cellular processes by binding to five specific G protein coupled-receptors (GPCRs). S1P is generated by phosphorylation of Sph catalyzed by two isoforms of sphingosine kinases (SphK), type 1 and type 2, which are critical regulators of the “sphingolipid rheostat”, producing pro-survival S1P and decreasing levels of pro-apoptotic Sph. Since sphingolipid metabolism is often dysregulated in many diseases, targeting SphKs is potentially clinically relevant. Here we review the growing recent literature on the regulation and the roles of SphKs and S1P in apoptosis and diseases.     

Sphingolipids: regulators of crosstalk between apoptosis and autophagy

Apoptosis and autophagy are two evolutionarily conserved processes that maintain homeostasis during stress. Although the two pathways utilize fundamentally distinct machinery, apoptosis and autophagy are highly interconnected and share many key regulators. The crosstalk between apoptosis and autophagy is complex, as autophagy can function to promote cell survival or cell death under various cellular conditions. The molecular mechanisms of crosstalk are beginning to be elucidated and have critical implications for the treatment of various diseases, such as cancer. Sphingolipids are a class of bioactive lipids that mediate many key cellular processes, including apoptosis and autophagy. By targeting several of the shared regulators, sphingolipid metabolites differentially regulate the induction of apoptosis and autophagy. Importantly, individual sphingolipid species appear to “switch” autophagy toward cell survival (e.g., sphingosine-1-phosphate) or cell death (e.g., ceramide, gangliosides). This review assesses the current understanding of sphingolipid-induced apoptosis and autophagy to address how sphingolipids mediate the “switch” between the cell survival and cell death. As sphingolipid metabolism is frequently dysregulated in cancer, sphingolipid-modulating agents, or sphingomimetics, have emerged as a novel chemotherapeutic strategy. Ultimately, a greater understanding of sphingolipid-mediated crosstalk between apoptosis and autophagy may be critical for enhancing the chemotherapeutic efficacy of these agents.     

Neuronal membrane dynamics as fine regulator of sphingolipid composition

Sphingolipid metabolism is an intricate network of several interdependent and co-regulated pathways. In addition to the mainstream biosynthetic and catabolic pathways, several processes, even if less important in contributing to the final tissue sphingolipid composition from the quantitative point of view, might become relevant when sphingolipid metabolism is for any reason dysregulated and concur to the onset of neuronal pathologies. The main subcellular sites involved in the mainstream metabolic pathway are represented by the Golgi apparatus (for the biosynthesis) and by the lysosomes (for catabolism). On the other hand, the minor collateral pathways are associated with the plasma membrane and membranes of other organelles, and likely play important roles in the local regulation of membrane dynamics and contribute to maintain a perfect membrane organization functional to the physiology of the cell. In this review, we will consider few aspects of the sphingolipid metabolic pathway depending by the dynamic of the membranes that seems to become relevant in neurodegenerative diseases.     

SVF1 regulates cell survival by affecting sphingolipid metabolism in Saccharomyces cerevisiae

Sphingolipid signaling plays an important role in the regulation of central cellular processes, including cell growth, survival, and differentiation. Many of the essential pathways responsible for sphingolipid biogenesis, and key cellular responses to changes in sphingolipid balance, are conserved between mammalian and yeast cells. Here we demonstrate a novel function for the survival factor Svf1p in the yeast sphingolipid pathway and provide evidence that Svf1p regulates the generation of a specific subset of phytosphingosine. Genetic analyses suggest that Svf1p acts in concert with Lcb4p and Lcb3p to generate a localized pool of phytosphingosine distinct from phytosphingosine generated by Sur2p. This subset is implicated in cellular responses to stress, as loss of SVF1 is associated with defects in the diauxic shift and the oxidative stress response. A genetic interaction between SVF1 and SUR2 demonstrates that both factors are required for optimal growth and survival, and phenotypic similarities between svf1delta sur2delta and ypk1delta suggest that pathways controlled by Svf1p and Sur2p converge on a signaling cascade regulated by Ypk1p. Loss of YPK1 together with disruption of either SVF1 or SUR2 is lethal. Together, these data suggest that compartmentalized generation of distinct intracellular subsets of sphingoid bases may be critical for activation of signaling pathways that control cell growth and survival.     

A matter of balance between life and death: Targeting reactive oxygen species (ROS)-induced autophagy for cancer therapy

Reactive oxygen species (ROS) have been implicated in many biological functions and diseases. Often their role is counterintuitive, where ROS can either promote cell survival or cell death depending on the cellular context. Similarly, autophagy is involved in many biological functions and diseases where it can either promote cell survival or cell death. There is now a growing consensus that ROS controls autophagy in multiple contexts and cell types. Furthermore, alterations in ROS and autophagy regulation contribute to cancer initiation and progression. However, how ROS and autophagy contribute to cancer and how to target either for cancer treatment is controversial. Blocking ROS generation could prevent cancer initiation, whereas blockage of autophagy seems to be required for initiation of cancer. In cancer progression, high levels of ROS correspond with increased metabolism, and under metabolic stress autophagy is required to maintain cellular integrity. In cancer treatment, therapeutic drugs that increase ROS and autophagy have been implicated in their mechanism for cell death, such as 2-methoxyestrodial (2-ME) and arsenic trioxide (As₂O₃), whereas other therapeutic drugs that induce ROS and autophagy seem to have a protective effect. This has led to different approaches to treat cancer patients where autophagy is either activated or inhibited. Both views of ROS and autophagy are valid and reflect the balance within a cell to either survive or die. Understanding this balancing act within a cell is essential to determine whether to block or activate ROS-controlled autophagy for cancer therapy.     

Pulsatilla chinensis saponins cause liver injury through interfering ceramide/sphingomyelin balance that promotes lipid metabolism dysregulation and apoptosis

Background: P. chinensis saponins (PRS) are pentacyclic triterpenoid bioactive constituents from Pulsatilla chinensis (Bunge) Regel. In our previous study, PRS caused chronic liver injury (CLI) with the significant changes of lipid metabolites including sphingomyelin (SM) in serum after long-term administration. The SM in the hepatocytes membrane plays an indispensable role in maintaining cell membrane stability and regulating the extracellular and intracellular signal transduction. However, it is still unknown the pathway related to SM and the mechanism of CLI on hepatocyte.Purpose: The purpose of this study was to explore the hepatotoxicity mechanism of PRS in vivo and in vitro, to reveal the action of mechanism of SM and the pathway related to liver injury.Methods: SD rats were orally administered with PRS for 240 days and liver injury was evaluated by histological examinations. Metabolomics analysis was used to explore the liver metabolic pathway affected by PRS, and the expressions of related proteins were evaluated by western blots. To discover and elucidate the underlying mechanisms of metabolites changes induced by PRS at the cellular level, cellular morphology, MTT assays, western blots and cell membrane potential measurements were carried out using LO2 cells. Furthermore, the roles of SM and cholesterol (Chol) in hepatocyte injury were investigated individually in overload Chol and SM groups. Sphingolipid metabolic pathway related with ceramide/sphingomyelin (Cer/SM) balance was explored using cellular lipidomics and RT-PCR.Results: PRS gradually damaged the rat's liver in a time-dependent manner. The analysis of liver metabolism profiles showed that lipids metabolites were changed, including sphingolipid, bile acid, linoleic acid and fatty acid. We found that PRS induced apoptosis by interfering with bile acid-mediated sphingolipid metabolic pathway and Cer/SM balance in CLI. In in vitro experiments, PRS led to the increase of LDH leakage, depolarized cell membrane potential and caused cell membrane toxicity. Furthermore, PRS inducedG0/G1 phase cell cycle arrest in LO2 cells, simultaneously activated cellular extrinsic and intrinsic apoptosis pathways. PRS acted on SM and interfered with Cer/SM balance, which promote lipid metabolism dysregulation and apoptosis.Conclusion: PRS acted on SM to interfere Cer/SM balance on LO2 cell. Both in vivo and in vitro, PRS induced Cer/SM imbalance which promoted lipid metabolism disorder and apoptosis. Apoptosis and lipids changes gradually damaged the rats liver, and ultimately developed into CLI.     

Autophagy in cancer biology and therapy

The role of macroautophagy （hereafter autophagy） in cancer biology and response to clinical intervention is complex. It is clear that autophagy is dysregulated in a wide variety of tumor settings, both during tumor initiation and progression, and in response to therapy. However, the pleiotropic mechanistic roles of autophagy in controlling cell behavior make it difficult to predict in a given tumor setting what the role of autophagy, and, by extension, the therapeutic outcome of targeting autophagy, might be. In this review we summarize the evidence in the literature supporting pro- and anti-tumorigenic and -therapeutic roles of autophagy in cancer. This overview encompasses roles of autophagy in nutrient management, cell death, cell senescence, regulation of proteotoxic stress and cellular homeostasis, regulation of tumor-host interactions and participation in changes in metabolism. We also try to understand, where possible, the mechanistic bases of these roles for autophagy. We specifically expand on the emerging role of genetically- engineered mouse models of cancer in shedding light on these issues in vivo. We also consider how any or all of the above functions of autophagy proteins might be targetable by extant or future classes of pharmacologic agents. We conclude by briefly exploring non-canonical roles for subsets of the key autophagy proteins in cellular processes, and how these might impact upon cancer.     

A house divided: Ceramide, sphingosine, and sphingosine-1-phosphate in programmed cell death

Programmed cell death is an important physiological response to many forms of cellular stress. The signaling cascades that result in programmed cell death are as elaborate as those that promote cell survival, and it is clear that coordination of both protein- and lipid-mediated signals is crucial for proper cell execution. Sphingolipids are a large class of lipids whose diverse members share the common feature of a long-chain sphingoid base, e.g., sphingosine. Many sphingolipids have been shown to play essential roles in both death signaling and survival. Ceramide, an N-acylsphingosine, has been implicated in cell death following a myriad of cellular stresses. Sphingosine itself can induce cell death but via pathways both similar and dissimilar to those of ceramide. Sphingosine-1-phosphate, on the other hand, is an anti-apoptotic molecule that mediates a host of cellular effects antagonistic to those of its pro-apoptotic sphingolipid siblings. Extraordinarily, these lipid mediators are metabolically juxtaposed, suggesting that the regulation of their metabolism is of the utmost importance in determining cell fate. In this review, we briefly examine the role of ceramide, sphingosine, and sphingosine-1-phosphate in programmed cell death and highlight the potential roles that these lipids play in the pathway to apoptosis.     

To die or not to die: that is the autophagic question

Macroautophagy (commonly referred to as autophagy) is the process by which intact organelles and/or large portions of the cytoplasm are engulfed within double-membraned autophagic vacuoles for degradation. Whereas basal levels of autophagy ensure the physiological turnover of old and damaged organelles, the massive accumulation of autophagic vacuoles may represent either an alternative pathway of cell death or an ultimate attempt for cells to survive by adapting to stress. The activation of the autophagic pathway beyond a certain threshold may promote cell death directly, by causing the collapse of cellular functions as a result of cellular atrophy (autophagic, or type II, cell death). Alternatively, autophagy can lead to the execution of apoptotic (type I) or necrotic (type III) cell death programs, presumably via common regulators such as proteins from the Bcl-2 family. On the other hand, limited self-eating can provide cells with metabolic substrates to meet their energetic demands under stressful conditions, such as nutrient deprivation, or favor the selective elimination of damaged (and potentially dangerous) organelles. In these instances, autophagy operates as a pro-survival mechanism. The coordinate regulation of these opposite effects of autophagy relies upon a complex network of signal transducers, most of which also participate in non-autophagic signaling cascades. Thus, autophagy occupies a crucial position within the cell's metabolism, and its modulation may represent an alternative therapeutic strategy in several pathological settings including cancer and neurodegeneration. Here, we present a general outline of autophagy followed by a detailed analysis of organelle-specific autophagic pathways and of their intimate connections with cell death.     

Yeast sphingolipids: metabolism and biology

Sphingolipids have recently emerged as important bioactive molecules in addition to being critical structural components of cellular membranes. These molecules have been implicated in regulating cell growth, differentiation, angiogenesis, apoptosis, and senescene. To study sphingolipid mediated biology, it is necessary to investigate sphingolipid metabolism and its regulation. The yeast Saccharomyces cerevisiae has allowed such studies to take place as the sphingolipid metabolic and regulatory pathways appear conserved across species. Using yeast genetic approaches most enzymes of sphingolipid metabolism have been identified and cloned which has led to identification of their mammalian homologues. Many of the yeast enzymes are targets of fungal toxins thus underscoring the importance of this pathway in yeast cell regulation. This review focuses on the yeast sphingolipid metabolic pathway and its role in regulation of yeast biology. Implication of the insights gained from yeast to mammalian cell regulation are discussed.     

A house divided: ceramide, sphingosine, and sphingosine-1-phosphate in programmed cell death

Programmed cell death is an important physiological response to many forms of cellular stress. The signaling cascades that result in programmed cell death are as elaborate as those that promote cell survival, and it is clear that coordination of both protein- and lipid-mediated signals is crucial for proper cell execution. Sphingolipids are a large class of lipids whose diverse members share the common feature of a long-chain sphingoid base, e.g., sphingosine. Many sphingolipids have been shown to play essential roles in both death signaling and survival. Ceramide, an N-acylsphingosine, has been implicated in cell death following a myriad of cellular stresses. Sphingosine itself can induce cell death but via pathways both similar and dissimilar to those of ceramide. Sphingosine-1-phosphate, on the other hand, is an anti-apoptotic molecule that mediates a host of cellular effects antagonistic to those of its pro-apoptotic sphingolipid siblings. Extraordinarily, these lipid mediators are metabolically juxtaposed, suggesting that the regulation of their metabolism is of the utmost importance in determining cell fate. In this review, we briefly examine the role of ceramide, sphingosine, and sphingosine-1-phosphate in programmed cell death and highlight the potential roles that these lipids play in the pathway to apoptosis.     

真菌鞘脂代谢及其与抗真菌药物关系研究进展

鞘脂是真核细胞中普遍存在的成分,它在真核细胞的胞吞、胞饮、信号转导、细胞的生长、凋亡、分化、衰老等过程中起着非常重要的作用.该文论述了真菌鞘脂代谢的途径及参与其合成的相关酶和基因,并比较了真菌与哺乳动物鞘脂代谢途径的不同,旨在为研究新型无公害抗真菌药物提供理论依据.     

Ceramide synthases at the centre of sphingolipid metabolism and biology

Sphingolipid metabolism in metazoan cells consists of a complex interconnected web of numerous enzymes, metabolites and modes of regulation. At the centre of sphingolipid metabolism reside CerSs (ceramide synthases), a group of enzymes that catalyse the formation of ceramides from sphingoid base and acyl-CoA substrates. From a metabolic perspective, these enzymes occupy a unique niche in that they simultaneously regulate de novo sphingolipid synthesis and the recycling of free sphingosine produced from the degradation of pre-formed sphingolipids (salvage pathway). Six mammalian CerSs (CerS1-CerS6) have been identified. Unique characteristics have been described for each of these enzymes, but perhaps the most notable is the ability of individual CerS isoforms to produce ceramides with characteristic acyl-chain distributions. Through this control of acyl-chain length and perhaps in a compartment-specific manner, CerSs appear to regulate multiple aspects of sphingolipid-mediated cell and organismal biology. In the present review, we discuss the function of CerSs as critical regulators of sphingolipid metabolism, highlight their unique characteristics and explore the emerging roles of CerSs in regulating programmed cell death, cancer and many other aspects of biology.     

Sphingolipid Metabolic Disorders in Trembler Mouse Peripheral Nerves In Vivo Result from an Abnormal Substrate Supply

Abstract: Sphingolipid metabolic pathways in the peripheral nerves of dysmyelinating Trembler mice were studied in vivo, using intraneurally injected [³H]palmitate as the exogenous substrate. The kinetic analysis of the experimental data obtained for the mutant revealed that, as in normal nerves, two metabolically and kinetically independent pathways are implicated in the biosynthesis of the major peripheral nerve sphingolipids: the ceramide pathway and another pathway in which there is no detectable labeled intermediate ("direct amidification"). The results also show that, in the Trembler mouse sciatic nerves: (a) The severely deficient sphingolipid biosynthesis results from the constitution of a qualitatively and quantitatively abnormal fatty acid substrate pool destined for metabolism via the ceramide pathway, which ensures the totality of the galactocerebroside labeling and two-thirds of that of sphingomyelin. The ceramide intermediates of this pathway are labeled only on their fatty acyl moiety, which contains only 16-carbon atom chains. (b) "Direct amidification" events implicated in sphingolipid labeling are decreased compared with normal and account for the remaining sphingomyelin formation.     

Molecular facets of sphingolipids: Mediators of diseases

Sphingolipids constitute a biologically active lipid class that is significantly important from both structural and regulatory aspects. The manipulation of sphingolipid metabolism is currently being studied as a novel strategy for cancer therapy. The basics of this therapeutic approach lie in the regulation property of sphingolipids on cellular processes, which are important in a cell's fate, such as cell proliferation, apoptosis, cell cycle arrest, senescence, and inflammation. Furthermore, the mutations in the enzymes catalyzing some specific reactions in the sphingolipid metabolism cause mortal lysosomal storage diseases like Fabry, Gaucher, Niemann-Pick, Farber, Krabbe, and Metachromatic Leukodystrophy. Therefore, the alteration of the sphingolipid metabolic pathway determines the choice between life and death. Understanding the sphingolipid metabolism and regulation is significant for the development of new therapeutic approaches for all sphingolipid-related diseases, as well as for cancer. An important feature of the sphingolipid metabolic pathway is the compartmentalization into endoplasmic reticulum, the Golgi apparatus, lysosome and plasma membrane, and this compartmentalization makes the transport of sphingolipids critical for proper functioning. This paper focuses on the structures, metabolic pathways, localization, transport mechanisms, and diseases of sphingolipids in Saccharomyces cerevisiae and humans, and provides the latest comprehensive information on sphingolipid research.