Volume 396, Issue 6-7

The high-density lipoprotein (HDL) is one of the most important endogenous cardiovascular protective markers. HDL is an attractive target in the search for new pharmaceutical therapies and in the prevention of cardiovascular events. Some of HDL’s anti-atherogenic properties are related to the signaling molecule sphingosine-1-phosphate (S1P), which plays an important role in vascular homeostasis. However, for different patient populations it seems more complicated. Significant changes in HDL’s protective potency are reduced under pathologic conditions and HDL might even serve as a proatherogenic particle. Under uremic conditions especially there is a change in the compounds associated with HDL. S1P is reduced and acute phase proteins such as serum amyloid A (SAA) are found to be elevated in HDL. The conversion of HDL in inflammation changes the functional properties of HDL. High amounts of SAA are associated with the occurrence of cardiovascular diseases such as atherosclerosis. SAA has potent pro-atherogenic properties, which may have impact on HDL’s biological functions, including cholesterol efflux capacity, antioxidative and anti-inflammatory activities. This review focuses on two molecules that affect the functionality of HDL. The balance between functional and dysfunctional HDL is disturbed after the loss of the protective sphingolipid molecule S1P and the accumulation of the acute-phase protein SAA. This review also summarizes the biological activities of lipid-free and lipid-bound SAA and its impact on HDL function.

Viruses exploit membranes and their components such as sphingolipids in all steps of their life cycle including attachment and membrane fusion, intracellular transport, replication, protein sorting and budding. Examples for sphingolipid-dependent virus entry are found for: human immunodeficiency virus (HIV), which besides its protein receptors also interacts with glycosphingolipids (GSLs); rhinovirus, which promotes the formation of ceramide-enriched platforms and endocytosis; or measles virus (MV), which induces the surface expression of its own receptor CD150 via activation of sphingomyelinases (SMases). While SMase activation was implicated in Ebola virus (EBOV) attachment, the virus utilizes the cholesterol transporter Niemann-Pick C protein 1 (NPC1) as ‘intracellular’ entry receptor after uptake into endosomes. Differential activities of SMases also affect the intracellular milieu required for virus replication. Sindbis virus (SINV), for example, replicates better in cells lacking acid SMase (ASMase). Defined lipid compositions of viral assembly and budding sites influence virus release and infectivity, as found for hepatitis C virus (HCV) or HIV. And finally, viruses manipulate cellular signaling and the sphingolipid metabolism to their advantage, as for example influenza A virus (IAV), which activates sphingosine kinase 1 and the transcription factor NF-κB.

From the most simple sphingoid bases to their complex glycosylated derivatives, several sphingolipid species were shown to have a role in fundamental cellular events and/or disease. Increasing evidence places lipid-lipid interactions and membrane structural alterations as central mechanisms underlying the action of these lipids. Understanding how these molecules exert their biological roles by studying their impact in the physical properties and organization of membranes is currently one of the main challenges in sphingolipid research. Herein, we review the progress in the state-of-the-art on the biophysical properties of sphingolipid-containing membranes, focusing on sphingosine, ceramides, and glycosphingolipids.

Acid sphingomyelinase and ceramide have previously been shown to play a central role in infections with Neisseria gonorrhoeae, Staphylococcus aureus, Listeria monocytogenes, Pseudomonas aeruginosa, Salmonella typhimurium, Escherichia coli, and Mycobacterium avium . Recent studies have extended the role of sphingolipids in bacterial infections and have demonstrated that ceramide and sphingosine are central to the defense of lungs against bacterial pathogens. Ceramide accumulates in the airway epithelium of cystic fibrosis and ceramide synthase 2 (CerS2)-deficient mice, which respond to the lack of very long chain (C22-C24-) ceramides with a profound compensatory increase of long chain (mainly C16-) ceramides. In contrast, sphingosine is present in healthy airways and is almost completely absent from diseased or deficient epithelial cells. Both sphingolipids are crucially involved in the high susceptibility to infection of cystic fibrosis and CerS2-deficient mice, as indicated by findings showing that the normalization of ceramide and sphingosine levels rescue these mice from acute infection with P. aeruginosa .

Anemia and hemorrhagic shock are leading causes of morbidity and mortality worldwide, and transfusion of human blood products is the ideal treatment for these conditions. As human erythrocytes age during storage in blood banks they undergo many biochemical and structural changes, termed the red blood cell ‘storage lesion’. Specifically, ATP and pH levels decrease as metabolic end products, oxidative stress, cytokines, and cell-free hemoglobin increase. Also, membrane proteins and lipids undergo conformational and organizational changes that result in membrane loss, viscoelastic changes and microparticle formation. As a result, transfusion of aged blood is associated with a host of adverse consequences such as decreased tissue perfusion, increased risk of infection, and increased mortality. This review summarizes current research detailing the known parts of the erythrocyte storage lesion and their physiologic consequences.

Sphingolipids are not only essential components of cellular membranes but also function as intracellular and extracellular mediators that regulate important physiological cellular processes including cell survival, proliferation, apoptosis, differentiation, migration and immune responses. The liver possesses the unique ability to regenerate after injury in a complex manner that involves numerous mediators, including sphingolipids such as ceramide and sphingosine 1-phosphate. Here we present the current understanding of the involvement of the sphingolipid pathway and the role this pathway plays in regulating liver injury, repair and regeneration. The regulation of sphingolipids and their enzymes may have a great impact in the development of novel therapeutic modalities for a variety of liver injuries and diseases.

Cancer therapies result in the killing of cancer cells but remain largely ineffective, with most patients dying of their disease. The methodology described here is a new image-guided cancer treatment under development that relies on physical methods to alter tumour biology. It enhances tumour responses to radiation significantly by synergistically destroying tumour blood vessels using microbubbles. It achieves tumour specificity by confining the ultrasonic fields that stimulate microbubbles to tumour location only. By perturbing tumour vasculature and activating specific genetic pathways in endothelial cells, the technique has been demonstrated to sensitise the targeted tissues to subsequent therapeutic application of radiation, resulting in significantly enhanced cell killing through a ceramide-dependent pathway initiated at the cell membrane. The treatment reviewed here destroys blood vessels, significantly enhancing the anti-vascular effect of radiation and improving tumour cure. The significant enhancement of localised tumour cell kill observed with this method means that radiation-based treatments can be potentially made more potent and lower doses of radiation utilised. The technique has the potential to have a profound impact on the practice of radiation oncology by offering a novel and safe means of reducing normal tissue toxicity while at the same time significantly increasing treatment effectiveness.

Lysosomal storage diseases (LSDs) are mainly caused by the defective activity of lysosomal hydrolases. A sub-class of LSDs are the sphingolipidoses, in which sphingolipids accumulate intra-cellularly. We here discuss the role of innate immunity in the sphingolipidoses, and compare the pathways of activation in two classical sphingolipidoses, namely Gaucher disease and Sandhoff disease, and in Niemann-Pick C disease, in which the main storage material is cholesterol but sphingolipids also accumulate. We discuss the mechanisms leading to neuroinflammation, and the different pathways of neuroinflammation in the different diseases, and suggest that intervention in these pathways may be a useful therapeutic approach to address these devastating human diseases.

The prevailing mechanisms of action of traditional chemotherapeutic agents have been challenged by sphingolipid cancer research. Many studies have shown that ceramide generation in response to cytotoxic agents is central to tumor cell death. Ceramide can be generated either via hydrolysis of cell-membrane sphingomyelin by sphingomyelinases, hydrolysis of cerebrosides, or via de novo synthesis by ceramide synthases. Ceramide can act as a second messenger for apoptosis, senescence or autophagy. Inherent or acquired alterations in the sphingolipid pathway can account for resistance to the classic chemotherapeutic agents. In particular, it has been shown that activation of the acid ceramidase can lead to the formation of sphingosine 1-phosphate, which then antagonizes ceramide signaling by initiating a pro-survival signaling pathway. Furthermore, ceramide glycosylation catalyzed by glucosylceramide synthase converts ceramide to glucosylceramide, thus eliminating ceramide and consequently protecting cancer cells from apoptosis. In this review, we describe the effects of some of the most commonly used chemotherapeutic agents on ceramide generation, with a particular emphasis on strategies used to enhance the efficacy of these agents.

Sphingolipids are a ubiquitous family of essential lipids with an increasingly understood role as biologically active mediators in numerous physiologic and pathologic processes. Two particular sphingolipid species, sphingosine-1-phosphate and ceramide, and their metabolites interact both directly and indirectly with endothelial cells to regulate vascular permeability. Sphingosine-1-phosphate generally augments endothelial integrity while ceramide tends to promote vascular leak, and a tight balance between the two is necessary to maintain normal physiologic function. The mechanisms by which sphingolipids regulate endothelial barrier function are complex and occur through multiple different pathways, and disruptions or imbalances in these pathways have been implicated in a number of specific disease processes. With improved understanding of sphingolipid biology, endothelial function, and the interactions between the two, several targets for therapeutic intervention have emerged and there is immense potential for further advancement in this field.

Sphingolipids have emerged as an important lipid mediator in intracellular signalling and metabolism. Ceramide, which is central to sphingolipid metabolism, is generated either via a de novo pathway, by attaching fatty acyl CoA to a long-chain base, or via a salvage pathway, by degrading pre-existing sphingolipids. As a ‘sphingolipid rheostat’ has been proposed, the balance between ceramide and sphingosine-1-phosphate has been the object of considerable attention. Ceramide has recently been reported to have a different function depending on its acyl chain length: six ceramide synthases (CerS) determine the specific ceramide acyl chain length in mammals. All CerS-deficient mice generated to date show that sphingolipids with defined acyl chain lengths play distinct pathophysiological roles in disease models. This review describes recent advances in understanding the associations of CerS with various diseases and includes clinical case reports.

Acid sphingomyelinase (ASM), a key enzyme in sphingolipid metabolism, hydrolyzes sphingomyelin to ceramide and phosphorylcholine. In mammals, the expression of a single gene, SMPD1, results in two forms of the enzyme that differ in several characteristics. Lysosomal ASM (L-ASM) is located within the lysosome, requires no additional Zn 2+ ions for activation and is glycosylated mainly with high-mannose oligosaccharides. By contrast, the secretory ASM (S-ASM) is located extracellularly, requires Zn 2+ ions for activation, has a complex glycosylation pattern and has a longer in vivo half-life. In this review, we summarize current knowledge regarding the physiology and pathophysiology of S-ASM, including its sources and distribution, molecular and cellular mechanisms of generation and regulation and relevant in vitro and in vivo studies. Polymorphisms or mutations of SMPD1 lead to decreased S-ASM activity, as detected in patients with Niemann-Pick disease B. Thus, lower serum/plasma activities of S-ASM are trait markers. No genetic causes of increased S-ASM activity have been identified. Instead, elevated activity is the result of enhanced release (e.g., induced by lipopolysaccharide and cytokine stimulation) or increased enzyme activation (e.g., induced by oxidative stress). Increased S-ASM activity in serum or plasma is a state marker of a wide range of diseases. In particular, high S-ASM activity occurs in inflammation of the endothelium and liver. Several studies have demonstrated a correlation between S-ASM activity and mortality induced by severe inflammatory diseases. Serial measurements of S-ASM reveal prolonged activation and, therefore, the measurement of this enzyme may also provide information on past inflammatory processes. Thus, S-ASM may be both a promising clinical chemistry marker and a therapeutic target.

Despite the therapeutic potential of sphingolipids, the ability to develop this class of compounds as active pharmaceutical ingredients has been hampered by issues of solubility and delivery. Beyond these technical hurdles, significant challenges in completing the necessary preclinical studies to support regulatory review are necessary for commercialization. This review seeks to identify the obstacles and potential solutions in the translation of a novel liposomal technology from the academic bench to investigational new drug (IND) stage by discussing the preclinical development of the Ceramide NanoLiposome (CNL), which is currently being developed as an anticancer drug for the initial indication of hepatocellular carcinoma (HCC).

Host T cell activation, a key step in obtaining adaptive immunity against pathogens, is initiated by the binding of the T cell receptor to a foreign antigenic peptide presented by the major histocompatibility complex on the surface of an antigen-presenting cell and, consequently, formation of an immunological synapse. Within the immunological synapse, the engagement of the T cell receptor in cooperation with simultaneous ligation of co-stimulatory molecules induces a precisely organized cascade of signaling events and pathways that regulate clonal expansion and differentiation of naïve T cells into effector T cells contributing to pathogen clearance. The biochemical changes that underlie T cell activation and differentiation, however, not only involve proteins but also lipids. In particular, catabolic cleavage of sphingomyelin generating ceramide can substantially influence functional responses in cells of the immune system. Changes in sphingomyelin and ceramide content have been reported to directly impact on membrane physiology, thus modifying signal transmission and interfering with diverse aspects of T cell activity. In this review we will focus on sphingomyelin breakdown/ceramide generation in T cells with regard to their function and development of T cell-mediated immunity.

Acid ceramidase (N-acylsphingosine deacylase, EC 3.5.1.23; AC) is the lipid hydrolase responsible for the degradation of ceramide into sphingosine and free fatty acids within lysosomes. The enzymatic activity was first identified over four decades ago and is deficient in two rare inherited disorders, Farber lipogranulomatosis (Farber disease) and spinal muscular atrophy with myoclonic epilepsy (SMA-PME). Importantly, AC not only hydrolyzes ceramide into sphingosine within acidic compartments, but also can synthesize ceramide from sphingosine at neutral pH, suggesting that the enzyme may have diverse functions depending on its subcellular location and the local pH. Within cells, AC exists in a complex with other lipid hydrolases and requires a polypeptide cofactor (saposin D) for full hydrolytic activity. Recent studies also have shown that AC is overexpressed in several human cancers, and that inhibition of this enzyme may be a useful cancer drug target. Aberrant AC activity has also been described in several other common diseases. The cDNA and gene ( ASAH1 ) encoding AC have been isolated, several mouse models of AC deficiency have been constructed, and the recombinant enzyme is currently being manufactured for the treatment of Farber disease and SMA-PME. Current information concerning the biology of this enzyme and its role in human disease is reviewed within.

Sphingolipid signaling in Caenorhabditis elegans is vital for sensing environmental change and effecting appropriate cellular response. Many molecular components in sphingolipid intermediary metabolism are conserved throughout evolution. Here we review use of C. elegans as a model system for conducting sphingolipid-based scientific investigation, which has helped us better understand vital roles these remarkable lipids play in human metabolism and disease.

The sphingolipid sphingosine-1-phosphate (S1P) has various functions in immune cell biology, regulating survival, proliferation, and, most prominently, migration. S1P couples to five G protein-coupled receptors (S1PR1–5) to transduce its effects on immune cell function. Expression of S1PR4 is restricted to immune cells. However, its impact on immune cell biology is largely elusive. In the current study, we intended to answer the question of whether S1P might affect plasmacytoid dendritic cell (pDC) migration, which dominantly express S1PR4. pDC are highly specialized cells producing large amounts of type I interferon in response to TLR7/9 ligands after viral infection or during autoimmunity. Surprisingly, we noticed a reduced abundance of pDC, particularly CD4 - pDC, in all organs of S1PR4-deficient vs. wildtype mice. This effect was not caused by altered migration of mature pDC, but rather a reduced potential of pDC progenitors, especially common DC progenitors, to differentiate into pDCs. In vitro studies suggested that reduced S1PR4-deficient pDC progenitor differentiation into mature pDC might be explained by both migration and differentiation of pDC progenitors in the bone marrow. As S1PR4 also affected the differentiation of CD34+ human hematopoietic stem cells into pDC, interfering with S1PR4 might be useful to reduce pDC numbers during autoimmunity.

FTY720 (fingolimod) is, after its phosphorylation by sphingosine kinase (SPHK) 2, a potent, non-selective sphingosine-1-phosphate (S1P) receptor agonist. FTY720 has been shown to reduce the nociceptive behavior in the paclitaxel model for chemotherapy-induced neuropathic pain through downregulation of S1P receptor 1 (S1P1) in microglia of the spinal cord. Here, we investigated the mechanisms underlying the antinociceptive effects of FTY720 in a model for trauma-induced neuropathic pain. We found that intrathecal administration of phosphorylated FTY720 (FTY720-P) decreased trauma-induced pain behavior in mice, while intraplantar administered FTY720-P had no effect. FTY720-P, but not FTY720, reduced the nociceptive behavior in SPHK2-deficient mice, suggesting the involvement of S1P receptors. Fittingly, intrathecal administration of antagonists for S1P1 or S1P3, W146 and Cay10444 respectively, abolished the antinociceptive effects of systemically administered FTY720, demonstrating that activation of both receptors in the spinal cord is necessary to induce antinociceptive effects by FTY720. Accordingly, intrathecal administration of S1P1 receptor agonists was not sufficient to evoke an antinociceptive effect. Taken together, the data show that, in contrast to its effects on chemotherapy-induced neuropathy, FTY720 reduces trauma-induced neuropathic pain by simultaneous activation of spinal S1P1 and S1P3 receptor subtypes.

FTY720 (Fingolimod; Gilenya ® ) is an immune-modulatory prodrug which, after intracellular phosphorylation by sphingosine kinase 2 (SphK2) and export, mimics effects of the endogenous lipid mediator sphingosine-1-phosphate. Fingolimod has been introduced to treat relapsing-remitting multiple sclerosis. However, little has been published about the immune cell membrane penetration and subcellular distribution of FTY720 and FTY720-P. Thus, we applied a newly established LC-MS/MS method to analyze the subcellular distribution of FTY720 and FTY720-P in subcellular compartments of spleen cells of wild type, SphK1- and SphK2-deficient mice. These studies demonstrated that, when normalized to the original cell volume and calculated on molar basis, FTY720 and FTY720-P dramatically accumulated several hundredfold within immune cells reaching micromolar concentrations. The amount and distribution of FTY720 was differentially affected by SphK1- and SphK2-deficiency. On the background of recently described relevant intracellular FTY720 effects in the nanomolar range and the prolonged application in multiple sclerosis, this data showing a substantial intracellular accumulation of FTY720, has to be considered for benefit/risk ratio estimates.

Sphingosine 1-phosphate (S1P) is generated by sphingosine kinase (SK)-1 and -2 and acts mainly as an extracellular ligand at five specific receptors, denoted S1P 1–5 . After activation, S1P receptors regulate important processes in the progression of renal diseases, such as mesangial cell migration and survival. Previously, we showed that dexamethasone enhances SK-1 activity and S1P formation, which protected mesangial cells from stress-induced apoptosis. Here we demonstrate that dexamethasone treatment lowered S1P 1 mRNA and protein expression levels in rat mesangial cells. This effect was abolished in the presence of the glucocorticoid receptor antagonist RU-486. In addition, in vivo studies showed that dexamethasone downregulated S1P 1 expression in glomeruli isolated from mice treated with dexamethasone (10 mg/kg body weight). Functionally, we identified S1P 1 as a key player mediating S1P-induced mesangial cell migration. We show that dexamethasone treatment significantly lowered S1P-induced migration of mesangial cells, which was again reversed in the presence of RU-486. In summary, we suggest that dexamethasone inhibits S1P-induced mesangial cell migration via downregulation of S1P 1 . Overall, these results demonstrate that dexamethasone has functional important effects on sphingolipid metabolism and action in renal mesangial cells.

Both of the sphingosine kinase (SK) subtypes SK-1 and SK-2 catalyze the production of the bioactive lipid molecule sphingosine 1-phosphate (S1P). However, the subtype-specific cellular functions are largely unknown. In this study, we investigated the cellular function of SK-2 in primary mouse renal mesangial cells (mMC) and embryonic fibroblasts (MEF) from wild-type C57BL/6 or SK-2 knockout (SK2ko) mice. We found that SK2ko cells displayed a significantly higher proliferative and migratory activity when compared to wild-type cells, with concomitant increased cellular activities of the classical extracellular signal regulated kinase (ERK) and PI3K/Akt cascades, and of the small G protein RhoA. Furthermore, we detected an upregulation of SK-1 protein and S1P 3 receptor mRNA expression in SK-2ko cells. The MEK inhibitor U0126 and the S1P 1/3 receptor antagonist VPC23019 blocked the increased migration of SK-2ko cells. Additionally, S1P 3 ko mesangial cells showed a reduced proliferative behavior and reduced migration rate upon S1P stimulation, suggesting a crucial involvement of the S1P 3 receptor. In summary, our data demonstrate that SK-2 exerts suppressive effects on cell growth and migration in renal mesangial cells and fibroblasts, and that therapeutic targeting of SKs for treating proliferative diseases requires subtype-selective inhibitors.