Principles of bioactive lipid signalling: lessons from sphingolipids

It has become increasingly difficult to find an area of cell biology in which lipids do not have important, if not key, roles as signalling and regulatory molecules. The rapidly expanding field of bioactive lipids is exemplified by many sphingolipids, such as ceramide, sphingosine, sphingosine-1-phosphate (S1P), ceramide-1-phosphate and lyso-sphingomyelin, which have roles in the regulation of cell growth, death, senescence, adhesion, migration, inflammation, angiogenesis and intracellular trafficking. Deciphering the mechanisms of these varied cell functions necessitates an understanding of the complex pathways of sphingolipid metabolism and the mechanisms that regulate lipid generation and lipid action.     

Sphingolipids and membrane biology as determined from genetic models

The importance of sphingolipids in membrane biology was appreciated early in the twentieth century when several human inborn errors of metabolism were linked to defects in sphingolipid degradation. The past two decades have seen an explosion of information linking sphingolipids with cellular processes. Studies have unraveled mechanistic details of the sphingolipid metabolic pathways, and these findings are being exploited in the development of novel therapies, some now in clinical trials. Pioneering work in yeast has laid the foundation for identifying genes encoding the enzymes of the pathways. The advent of the era of genomics and bioinformatics has led to the identification of homologous genes in other species and the subsequent creation of animal knock-out lines for these genes. Discoveries from these efforts have re-kindled interest in the role of sphingolipids in membrane biology. This review highlights some of the recent advances in understanding sphingolipids' roles in membrane biology as determined from genetic models.     

Yeast sphingolipids: recent developments in understanding biosynthesis, regulation, and function

Sphingolipids function as required membrane components of virtually all eukaryotic cells. Data indicate that members of the sphingolipid family of lipids, including sphingoid bases, sphingoid base phosphates, ceramides, and complex sphingolipids, serve vital functions in cell biology by both direct mechanisms (e.g., binding to G-protein coupled receptors to transduce an extracellular signal) and indirect mechanisms (e.g., facilitating correct intracellular protein transport). Because of the diverse roles these lipids play in cell biology, it is important to understand not only their biosynthetic pathways and regulation of sphingolipid synthesis, but also the mechanisms by which some sphingolipid species with specific functions are modified or converted to other sphingolipid species with alternate functions. Due to many factors including ease of culture and genetic modification, and conservation of major sphingolipid metabolic pathways, Saccharomyces cerevisiae has served as an ideal model system with which to identify enzymes of sphingolipid biosynthesis and to dissect sphingolipid function. Recent exciting developments in sphingolipid synthesis, transport, signaling, and overall biology continue to fuel vigorous investigation and inspire investigations in mammalian sphingolipid biology.     

Yeast sphingolipid metabolism: clues and connections.

This review of sphingolipid metabolism in the budding yeast Saccharomyces cerevisiae contains information on the enzymes and the genes that encode them, as well as connections to other metabolic pathways. Particular attention is given to yeast homologs, domains, and motifs in the sequence, cellular localization of enzymes, and possible protein-protein interactions. Also included are genetic interactions of special interest that provide clues to the cellular biological roles of particular sphingolipid metabolic pathways and specific sphingolipids.     

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 post-genomic approach to understanding sphingolipid metabolism in Arabidopsis thaliana

AIMS: To highlight the importance of sphingolipids and their metabolites in plant biology. SCOPE: The completion of the arabidopsis genome provides a platform for the identification and functional characterization of genes involved in sphingolipid biosynthesis. Using the yeast Saccharomyces cerevisiae as an experimental model, this review annotates arabidopsis open reading frames likely to be involved in sphingolipid metabolism. A number of these open reading frames have already been subject to functional characterization, though the majority still awaits investigation. Plant-specific aspects of sphingolipid biology (such as enhanced long chain base heterogeneity) are considered in the context of the emerging roles for these lipids in plant form and function. CONCLUSIONS: Arabidopsis provides an excellent genetic and post-genomic model for the characterization of the roles of sphingolipids in higher plants.     

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.     

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.     

Identification and characterization of a sphingolipid delta 4-desaturase family

Sphingolipids desaturated at the Delta4-position are important signaling molecules in many eukaryotic organisms, including mammals. In a bioinformatics approach, we now identified a new family of protein sequences from animals, plants, and fungi and characterized these sequences biochemically by expression in Saccharomyces cerevisiae. This resulted in the identification of the enzyme sphingolipid Delta4-desaturase (dihydroceramide desaturase) from Homo sapiens, Mus musculus, Drosophila melanogaster, and Candida albicans, in addition to a bifunctional sphingolipid Delta4-desaturase/C-4-hydroxylase from M. musculus. Among the sequences investigated are the Homo sapiens membrane lipid desaturase, the M. musculus degenerative spermatocyte, and the Drosophila melanogaster degenerative spermatocyte proteins. During spermatogenesis, but not oogenesis of des mutant flies, both cell cycle and spermatid differentiation are specifically blocked at the entry into the first meiotic division, leading to male sterility. This mutant phenotype can be restored to wild-type by complementation with a functional copy of the des gene (Endo, K., Akiyama, T., Kobayashi S., and Okada, M. (1996) Mol. Gen. Genet. 253, 157-165). These results suggest that Delta4-desaturated sphingolipids provide an early signal necessary to trigger the entry into both meiotic and spermatid differentiation pathways during Drosophila spermatogenesis.     

Sphingosine-1-phosphate metabolism: A structural perspective

Abstract

Sphingolipids represent an important class of bioactive signaling lipids which have key roles in numerous cellular processes. Over the last few decades, the levels of bioactive sphingolipids and/or their metabolizing enzymes have been realized to be important factors involved in disease development and progression, most notably in cancer. Targeting sphingolipid-metabolizing enzymes in disease states has been the focus of many studies and has resulted in a number of pharmacological inhibitors, with some making it into the clinic as therapeutics. In order to better understand the regulation of sphingolipid-metabolizing enzymes as well as to develop much more potent and specific inhibitors, the field of sphingolipids has recently taken a turn toward structural biology. The last decade has seen the structural determination of a number of sphingolipid enzymes and effector proteins. In these terms, one of the most complete arms of the sphingolipid pathway is the sphingosine-1-phosphate (S1P) arm. The structures of proteins involved in the function and regulation of S1P are being used to investigate further the regulation of said proteins as well as in the design and development of inhibitors as potential therapeutics.     

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 Metabolism and Interorganellar Transport: Localization of Sphingolipid Enzymes and Lipid Transfer Proteins

In recent decades, many sphingolipid enzymes, sphingolipid‐metabolism regulators and sphingolipid transfer proteins have been isolated and characterized. This review will provide an overview of the intracellular localization and topology of sphingolipid enzymes in mammalian cells to highlight the locations where respective sphingolipid species are produced. Interestingly, three sphingolipids that reside or are synthesized in cytosolic leaflets of membranes (ceramide, glucosylceramide and ceramide‐1‐phosphate) all have cytosolic lipid transfer proteins (LTPs). These LTPs consist of ceramide transfer protein (CERT), four‐phosphate adaptor protein 2 (FAPP2) and ceramide‐1‐phosphate transfer protein (CPTP), respectively. These LTPs execute functions that affect both the location and metabolism of the lipids they bind. Molecular details describing the mechanisms of regulation of LTPs continue to emerge and reveal a number of critical processes, including competing phosphorylation and dephosphorylation reactions and binding interactions with regulatory proteins and lipids that influence the transport, organelle distribution and metabolism of sphingolipids.      

p53 and regulation of bioactive sphingolipids

Both the sphingolipid and p53 pathways are important regulators- and apparent collaborators-of cell-fate decisions. Whereas some investigations have suggested that ceramide and more complex sphingolipids function upstream of p53 or in a p53-independent manner, other studies propose that p53-dependent alterations in these sphingolipids can also contribute to apoptosis. Further studies focusing on sphingolipid metabolizing enzymes have revealed that they function similarly both upstream and downstream of p53 activation. However, whereas various components of the sphingolipid and p53 pathways may simultaneously function to elicit apoptosis and/or growth inhibition, SMase and SK1 may undergo explicit regulation by p53 that could contribute to ceramide-induced senescence in cells. Thus, we propose that regulation of bioactive sphingolipid signaling molecules could be of therapeutic benefit in the treatment of p53-dependent cancers.     

Still benched on its way to the bedside: sphingosine kinase 1 as an emerging target in cancer chemotherapy

For several decades, lipid biologists have investigated how sphingolipids contribute to physiology, cell biology, and cell fate. Foremost among these discoveries is the finding that the bioactive sphingolipids ceramide, sphingosine, and sphingosine-1-phosphate (S1P) have diverse and often opposing effects on cell fate. Interestingly, these bioactive sphingolipids can be interconverted by just a few enzymatic reactions. Therefore, much attention has been paid to the enzymes which govern these reactions with a disproportionate amount of focus on the enzyme sphingosine kinase 1 (SK1). Several studies have found that tissue expression of SK1 correlates with cancer stage, chemotherapy response, and tumor aggressiveness. In addition, overexpression of SK1 in multiple cancer cell lines increases their resistance to chemotherapy, promotes proliferation, allows for anchorage independent growth, and increases local angiogenesis. Inhibition of SK1 using either pharmacological inhibitors or by crossing SK1 null mice has shown promise in many xenograft models of cancer, as well as several genetic and chemically induced mouse models of carcinogenesis. Here, we review the majority of the evidence that suggests SK1 is a promising target for the prevention and/or treatment of various cancers. Also, we strongly advocate for further research into basic mechanisms of bioactive sphingolipid signaling, and an increased focus on the efficacy of SK inhibitors in non-xenograft models of cancer progression.     

Still benched on its way to the bedside: sphingosine kinase 1 as an emerging target in cancer chemotherapy

For several decades, lipid biologists have investigated how sphingolipids contribute to physiology, cell biology, and cell fate. Foremost among these discoveries is the finding that the bioactive sphingolipids ceramide, sphingosine, and sphingosine-1-phosphate (S1P) have diverse and often opposing effects on cell fate. Interestingly, these bioactive sphingolipids can be interconverted by just a few enzymatic reactions. Therefore, much attention has been paid to the enzymes which govern these reactions with a disproportionate amount of focus on the enzyme sphingosine kinase 1 (SK1). Several studies have found that tissue expression of SK1 correlates with cancer stage, chemotherapy response, and tumor aggressiveness. In addition, overexpression of SK1 in multiple cancer cell lines increases their resistance to chemotherapy, promotes proliferation, allows for anchorage independent growth, and increases local angiogenesis. Inhibition of SK1 using either pharmacological inhibitors or by crossing SK1 null mice has shown promise in many xenograft models of cancer, as well as several genetic and chemically induced mouse models of carcinogenesis. Here, we review the majority of the evidence that suggests SK1 is a promising target for the prevention and/or treatment of various cancers. Also, we strongly advocate for further research into basic mechanisms of bioactive sphingolipid signaling, and an increased focus on the efficacy of SK inhibitors in non-xenograft models of cancer progression.     

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.     

Human cytomegalovirus regulates bioactive sphingolipids

Sphingolipids are present in membranes of all eukaryotic cells. Bioactive sphingolipids also function as signaling molecules that regulate cellular processes such as proliferation, migration, and apoptosis. Human cytomegalovirus (HCMV) exploits a variety of cellular signaling pathways to promote its own replication. However, whether HCMV modulates lipid signaling pathways is an essentially unexplored area of research in virus-host cell interactions. In this study, we examined the accumulation of the bioactive sphingolipids and the enzymes responsible for the biosynthesis and degradation of these lipids. HCMV infection results in increased accumulation and activity of sphingosine kinase (SphK), the enzyme that generates sphingosine 1-phosphate (S1P) and dihydrosphingosine 1-phosphate (dhS1P). We also utilized a mass spectrometry approach to generate a sphingolipidomic profile of HCMV-infected cells. We show that HCMV infection results in increased levels of dhS1P and ceramide at 24 h, suggesting an enhancement of de novo sphingolipid synthesis. Subsequently dihydrosphingosine and dhS1P decrease at 48 h consistent with attenuation of de novo sphingolipid synthesis. Finally, we present evidence that de novo sphingolipid synthesis and sphingosine kinase activity directly impact virus gene expression and virus growth. Together, these findings demonstrate that host cell sphingolipids are dynamically regulated upon infection with a herpes virus in a manner that impacts virus replication.     

Membranes in balance: mechanisms of sphingolipid homeostasis

Sphingolipids and their metabolites play key cellular roles both as structural components of membranes and as signaling molecules that mediate responses to physiologic cues and stresses. Despite progress during the last two decades in defining the enzymatic machinery responsible for synthesizing and degrading sphingolipids, comparatively little is known about how these enzymes are regulated to ensure sphingolipid homeostasis. Here, we review new insights into how cells sense and control sphingolipid biosynthesis and transport. We also discuss emerging evidence that sphingolipid metabolism is closely coordinated with that of sterols and glycerolipids and with other processes that occur in the secretory pathway. An improved understanding of sphingolipid homeostasis promises to shed light on basic processes in cell biology and disease, including how cells establish and maintain the complex membrane composition and architecture that is a defining feature of eukaryotic cell biology.