Sphingosine kinase signalling in immune cells

Lipidomic alterations in human saliva from cystic fibrosis patients

Cystic fibrosis is a hereditary metabolic disorder characterized by impaired traffic of chloride ions and water through membranes of the respiratory and gastrointestinal, that causes inadequate hydration of airway surfaces, dehydrated mucous secretions and a high-sodium chloride sweat. Although the classical presentation of the condition is well known, a better characterization of metabolic alterations related is need. In particular, the metabolic composition alterations of biological fluids may be influence by the disease state and could be captured as putative signature to set targeted therapeutic strategies. A targeted comprehensive mass spectrometry-based platform was employed to dissect the lipid content of saliva samples form CF patients, in order to investigate alterations in the lipid metabolic homeostasis related to the pathology, chronic obstructive pulmonary disease, Pseudomonas Aeruginosa infection, pancreatic insufficiency, liver disfunction and diabetes-related complications.

Association between host genetics of sheep and the rumen microbial composition

A synergy between the rumen microbiota and the host genetics has created a symbiotic relationship, beneficial to the host's health. In this study, the association between the host genetics and rumen microbiome of Damara and Meatmaster sheep was investigated. The composition of rumen microbiota was estimated through the analysis of the V3-V4 region of the 16S rRNA gene, while the sheep blood DNA was genotyped with Illumina OvineSNP50 BeadChip and the genome-wide association (GWA) was analyzed. Sixty significant SNPs dispersed in 21 regions across the Ovis aries genome were found to be associated with the relative abundance of seven genera: Acinetobacter, Bacillus, Clostridium, Flavobacterium, Prevotella, Pseudomonas, and Streptobacillus. A total of eighty-four candidate genes were identified, and their functional annotations were mainly associated with immunity responses and function, metabolism, and signal transduction. Our results propose that those candidate genes identified in the study may be modulating the composition of rumen microbiota and further indicating the significance of comprehending the interactions between the host and rumen microbiota to gain better insight into the health of sheep.

Current Understanding of the Immunomodulatory Activities of High-Density Lipoproteins

Lipoproteins interact with immune cells, macrophages and endothelial cells - key players of the innate and adaptive immune system. High-density lipoprotein (HDL) particles seem to have evolved as part of the innate immune system since certain HDL subspecies contain combinations of apolipoproteins with immune regulatory functions. HDL is enriched in anti-inflammatory lipids, such as sphingosine-1-phosphate and certain saturated lysophospholipids. HDL reduces inflammation and protects against infection by modulating immune cell function, vasodilation and endothelial barrier function. HDL suppresses immune cell activation at least in part by modulating the cholesterol content in cholesterol/sphingolipid-rich membrane domains (lipid rafts), which play a critical role in the compartmentalization of signaling pathways. Acute infections, inflammation or autoimmune diseases lower HDL cholesterol levels and significantly alter HDL metabolism, composition and function. Such alterations could have a major impact on disease progression and may affect the risk for infections and cardiovascular disease. This review article aims to provide a comprehensive overview of the immune cell modulatory activities of HDL. We focus on newly discovered activities of HDL-associated apolipoproteins, enzymes, lipids, and HDL mimetic peptides.

Impact of sphingosine and acetylsphingosines on the aggregation and toxicity of metal-free and metal-treated amyloid-β

Pathophysiological shifts in the cerebral levels of sphingolipids in Alzheimer's disease (AD) patients suggest a link between sphingolipid metabolism and the disease pathology. Sphingosine (SP), a structural backbone of sphingolipids, is an amphiphilic molecule that is able to undergo aggregation into micelles and micellar aggregates. Considering its structural properties and cellular localization, we hypothesized that SP potentially interacts with amyloid-β (Aβ) and metal ions that are found as pathological components in AD-affected brains, with manifesting its reactivity towards metal-free Aβ and metal-bound Aβ (metal–Aβ). Herein, we report, for the first time, that SP is capable of interacting with both Aβ and metal ions and consequently affects the aggregation of metal-free Aβ and metal–Aβ. Moreover, incubation of SP with Aβ in the absence and presence of metal ions results in the aggravation of toxicity induced by metal-free Aβ and metal–Aβ in living cells. As the simplest acyl derivatives of SP, N-acetylsphingosine and 3-O-acetylsphingosine also influence metal-free Aβ and metal–Aβ aggregation to different degrees, compared to SP. Such slight structural modifications of SP neutralize its ability to exacerbate the cytotoxicity triggered by metal-free Aβ and metal–Aβ. Notably, the reactivity of SP and the acetylsphingosines towards metal-free Aβ and metal–Aβ is determined to be dependent on their formation of micelles and micellar aggregates. Our overall studies demonstrate that SP and its derivatives could directly interact with pathological factors in AD and modify their pathogenic properties at concentrations below and above critical aggregation concentrations.

The reactivity of sphingosine and acetylsphingosines towards both metal-free and metal-treated amyloid-β is demonstrated showing a correlation of their micellization properties.

Changes in plasma lipidome following initiation of antiretroviral therapy

INTRODUCTION

HIV and antiretroviral therapy (ART) have been associated with increased cardiovascular disease and important changes in lipid metabolism. Advances in mass-spectrometry technology allow for the detailed assessment of individual lipid species which may illuminate the mechanisms underlying increased cardiovascular risk. We describe the change in plasma lipidome with initiation of antiretroviral therapy and compare these by regimen.

METHODS

Plasma lipid profiling (by electrospray isonisation-tandem mass spectrometry) was performed on ARV-naive HIV positive participants randomised to one of three regimens; tenofovir/emtricitabine with efavirenz, ritonavir-boosted atazanavir (atazanavir/r) or zidovudine/abacavir. Participants (n = 115) who remained on their randomised regimen with complete samples available at baseline, week 12 and 48 were included. 306 lipid species from 22 lipid classes were analysed.

RESULTS

Initiation of ART led to significant changes in lipidome which were partly dependent on the randomised regimen received. This led to significant differences in 72 lipid species and 7 classes (cholesterol ester, free cholesterol, phosphatidylcholine, GM3 ganglioside, trihexosylceramide, monohexosylceramide, and ceramides) by arm at week 48. Consistently higher lipid concentrations were seen with efavirenz compared with atazanavir/r or zidovudine/abacavir. Twelve of the lipid species and two lipid classes (cholesterol esters and ceramides) that were significantly increased in the efavirenz arm compared with the atazanavir/r or zidovudine/abacavir arms have previously been associated with future cardiovascular events in HIV positive patients. Change in HIV viral load was predictive of change in 3 lipid species.

CONCLUSIONS

Initiation of ART lead to significant changes in the plasma lipidome that were greatest in those receiving efavirenz.

Sphingosine 1-Phosphate Signaling as a Target in Hepatic Fibrosis Therapy

Liver fibrosis is an excess production of extracellular matrix proteins as a result of chronic liver disease which leads to cell death and organ dysfunction. The key cells involved in fibrogenesis are resident hepatic stellate cells (HSCs) which are termed myofibroblasts after activation, acquiring contractile, proliferative, migratory and secretory capability. Sphingosine 1-phosphate (S1P) is a bioactive sphingolipid with well-established effects on angiogenesis, carcinogenesis and immunity. Accumulating evidence demonstrates that this metabolite is involved in the profibrotic inflammatory process through the regulation of pleiotropic cell responses, such as vascular permeability, leukocyte infiltration, cell survival, migration, proliferation and HSCs differentiation to myofibroblasts. S1P is synthesized by sphingosine kinases (SphKs) and many of its actions are mediated by S1P specific cell surface receptors (S1P_1-5), although different intracellular targets of S1P have been identified. Modulation of SphKs/S1P/S1P receptors signaling is known to result in beneficial effects on various in vivo and in vitro models of liver fibrosis. Thus, a better knowledge of the molecular mechanisms involved in the modulation of the S1P pathway could help to improve liver fibrosis therapy. In this review, we analyze the effects of the S1P axis on the fibrogenic process, and the involvement of a range of inhibitors or approaches targeting enzymes related to S1P in the abrogation of pathological fibrogenesis. All in all, targeting this pathway offers therapeutic potential in the treatment of hepatic fibrosis.

Biofluid lipidome: a source for potential diagnostic biomarkers

Lipidomics is the identification and quantitation of changes in the lipidome of a cell, tissue, organ or biofluid in health and disease using high resolution mass spectrometry. Lipidome of a particular organism has relevance to the disease manifestation as it reflects the metabolic changes which can be a consequence of the disease. Hence these changes in the molecules can be considered as potential markers for screening and early detection of the disease. Biological fluids as blood/serum/plasma, urine, saliva, tear and cerebrospinal fluid, due to their accessibility, offer ease of collection with minimal or no discomfort to the patient and provide a ready footprint of the metabolic changes occurring during disease. This review provides a brief introduction to lipidomics and its role in understanding the metabolic changes in health and disease followed by discussion on the chemical diversity of the lipid species and their biological role, mammalian lipids and their metabolism and role of lipids in pathogens and the immune response before dwelling further into importance of studying lipidome in various biological fluids. The challenges in performing a lipidomic analysis at the experimental and data analysis stages are discussed.

Therapeutic Strategies and Pharmacological Tools Influencing S1P Signaling and Metabolism

During the last two decades the study of the sphingolipid anabolic, catabolic, and signaling pathways has attracted enormous interest. Especially the introduction of fingolimod into market as first p.o. therapeutic for the treatment of multiple sclerosis has boosted this effect. Although the complex regulation of sphingosine-1-phosphate (S1P) and other catabolic and anabolic sphingosine-related compounds is not fully understood, the influence on different (patho)physiological states from inflammation to cytotoxicity as well as the availability of versatile pharmacological tools that represent new approaches to study these states are described. Here, we have summarized various aspects concerning the many faces of sphingolipid function modulation by different pharmacological tools up to clinical candidates. Due to the immense heterogeneity of physiological or pharmacological actions and complex cross regulations, it is difficult to predict their role in upcoming therapeutic approaches. Currently, inflammatory, immunological, and/or antitumor aspects are discussed.

Methods for Testing Immunological Factors

Hypersensitivity reactions can be elicited by various factors: either immunologically induced, i.e., allergic reactions to natural or synthetic compounds mediated by IgE, or non-immunologically induced, i.e., activation of mediator release from cells through direct contact, without the induction of, or the mediation through immune responses. Mediators responsible for hypersensitivity reactions are released from mast cells. An important preformed mediator of allergic reactions found in these cells is histamine. Specific allergens or the calcium ionophore 48/80 induce release of histamine from mast cells. The histamine concentration can be determined with the o-phthalaldehyde reaction.

Sphingosine-1-phosphate: a Janus-faced mediator of fibrotic diseases

Sphingosine-1-phosphate (S1P) is a pleiotropic lipid mediator that acts either on G protein-coupled S1P receptors on the cell surface or via intracellular target sites. In addition to the well established effects of S1P in angiogenesis, carcinogenesis and immunity, evidence is now continuously accumulating which demonstrates that S1P is an important regulator of fibrosis. The contribution of S1P to fibrosis is of a Janus-faced nature as S1P exhibits both pro- and anti-fibrotic effects depending on its site of action. Extracellular S1P promotes fibrotic processes in a S1P receptor-dependent manner, whereas intracellular S1P has an opposite effect and dampens a fibrotic reaction by yet unidentified mechanisms. Fibrosis is a result of chronic irritation by various factors and is defined by an excess production of extracellular matrix leading to tissue scarring and organ dysfunction. In this review, we highlight the general effects of extracellular and intracellular S1P on the multistep cascade of pathological fibrogenesis including tissue injury, inflammation and the action of pro-fibrotic cytokines that stimulate ECM production and deposition. In a second part we summarize the current knowledge about the involvement of S1P signaling in the development of organ fibrosis of the lung, kidney, liver, heart and skin. Altogether, it is becoming clear that targeting the sphingosine kinase-1/S1P signaling pathway offers therapeutic potential in the treatment of various fibrotic processes. This article is part of a Special Issue entitled Advances in Lysophospholipid Research.

The role of sphingosine kinase 1 in patients with severe acute pancreatitis

OBJECTIVE

To investigate the role of sphingosine kinase 1 (SphK1)/sphingosine 1-phosphate (S1P) signaling in inflammatory response in severe acute pancreatitis (SAP).

BACKGROUND

SAP is an acute inflammatory process of the pancreas, which may lead to systemic inflammatory response syndrome and multiorgan dysfunction syndrome. SphK1 and its product S1P have been implicated in inflammatory response and various immune cell functions. However, the potential role for SphK1/S1P in inflammatory response in SAP is still unclear.

METHODS

Twenty-two patients with SAP were enrolled in this study. SphK1 expression on peripheral neutrophils, monocytes, and lymphocytes was evaluated by flow cytometry. SphK enzymatic activity in neutrophils and lymphocytes was measured using a radiometric assay. The expression of S1P1 and S1P3 mRNA was determined by reverse transcriptase-polymerase chain reaction (RT-PCR). The serum levels of tumor necrosis factor-α (TNF-α), interleukin-1 (IL-1β), and IL-6 were measured by ELISA.

RESULTS

The expression of SphK1 and SphK activity were markedly increased in peripheral immune cells in the early stage of SAP and then reduced in the restoration stage in the patients. Moreover, we found that the level of S1P3 mRNA in peripheral neutrophils and lymphocytes of SAP patients was significantly elevated in the early stage as compared with the healthy volunteers, and it reduced in the restoration period. SphK1 expression on human peripheral neutrophils, monocytes, and CD4(+) T lymphocytes were positively correlated with the APACHE (Acute Physiological and Chronic Health Evaluation) II scores in patients with SAP. The levels of serum proinflammatory cytokines including TNF-α, IL-1β, and IL-6 showed similar shifts with intracellular SphK1 expression in SAP patients.

CONCLUSIONS

The authors identified a link between the SphK1 expression on peripheral immune cells and the severity of SAP. Observations showed a possible immunomodulating role for SphK1/S1P signaling in inflammatory response in SAP, suggesting that regulation of SphK1/S1P pathway may represent novel targets in the treatment of SAP.

Glucocorticoid receptor and breast cancer

Stress enhances glucocorticoid (GC) synthesis, which alters inflammation and immune responses, as well as cellular proliferation and apoptosis in a number of tissues. Increasingly, stress has been associated with cancer progression, and in particular in breast cancer. Consequently, an operational glucocorticoid receptor system in breast tissue influences breast cancer development. In this review, we summarize the data on the GC/GR system in normal and tumoral breast tissue. We also review the molecular mechanisms by which GCs control apoptosis and proliferation in breast cancer models and how GCs alter the chemotherapy of breast cancer treatment when used in combination. Finally, we discuss the participation of GR in breast tumorigenesis under hormone replacement therapy.

Sphingosine kinase and sphingosine 1-phosphate in asthma

Sphingolipids are amphiphatic molecules ubiquitously expressed in all eukaryotic cell membranes. Initially characterized as structural components of cell membranes, sphingolipids have emerged as sources of important signalling molecules over the past decade. Sphingolipid metabolites, such as ceramide and S1P (sphingosine 1-phosphate), have been demonstrated to have roles as potent bioactive messengers involved in cell differentiation, proliferation, apoptosis, migration and angiogenesis. The importance of SphK (sphingosine kinase) and S1P in inflammation has been demonstrated extensively. The prevalence of asthma is increasing in many developed nations. Consequently, there is an urgent need for the development of new agents for the treatment of asthma, especially for patients who respond poorly to conventional therapy. Recent studies have demonstrated the important role of SphK and S1P in the development of asthma by regulating pro-inflammatory responses. These novel pathways represent exciting potential therapeutic targets in the treatment of asthma and are described in the present review.

An obligate role for membrane-associated neutral sphingomyelinase activity in orienting chemotactic migration of human neutrophils

For polymorphonuclear neutrophils (PMNs) to orient migration to chemotactic gradients, weak external asymmetries must be amplified into larger internal signaling gradients. Lipid mediators, associated with the plasma membrane and within the cell, participate in generating these gradients. This study examined the role in PMN chemotaxis of neutral sphingomyelinase (N-SMase), a plasma membrane-associated enzyme that converts sphingomyelin to ceramide. A noncompetitive N-SMase inhibitor, GW4869 (5 mM, 5 minutes), did not inhibit PMN motility (as percentage of motile cells, or mean cell velocity), but it abrogated any orientation of movement toward the source of the chemotaxin, formylmethionylleucylphenylanaline (FMLP) (net displacement along the gradient axis in micrometers, or as percentage of total migration distance). This defect could be completely reversed by treatment with lignoceric ceramide (5 μg/ml, 15 minutes). Immunolocalization studies demonstrated that N-SMase (1) distributes preferentially toward the leading edge of some elongated cells, (2) is associated with the plasma membrane, (3) is more than 99.5% localized to the cytofacial aspect of the plasma membrane, (4) is excluded from pseudopodial extensions, and (5) increases rapidly in response to FMLP. Morphologically, the inhibition of N-SMase limited cellular spreading and the extension of sheet-like pseudopods. Elongated PMNs demonstrated a polarized distribution of GTPases, with Rac 1/2 accumulated at, and RhoA excluded from, the front of the cell. This polarity was negated by N-SMase inhibition and restored by lignoceric ceramide. We conclude that N-SMase at the cytofacial plasma membrane is an essential element for the proper orientation of PMNs in FMLP gradients, at least in part by polarizing the distribution of Rac 1/2 and RhoA GTPases.

New developments on the TNFα-mediated signalling pathways

TNFα (tumour necrosis factor α) is an extensively studied pleiotropic cytokine associated with the pathogenesis of a variety of inflammatory diseases. It elicits a wide spectrum of cellular responses which mediates and regulates inflammation, immune response, cell survival, proliferation and apoptosis. TNFα initiates its responses by binding to its receptors. TNFα-induced effector responses are mediated by the actions and interactions among the various intracellular signalling mediators in the cell. TNFα induces both survival and apoptotic signal in a TRADD (TNF receptor-associated DD)-dependent and -independent way. The signals are further transduced via a variety of signalling mediators, including caspases, MAPKs (mitogen-activated protein kinases), phospholipid mediators and miRNA/miR (microRNA), whose roles in specific functional responses is not fully understood. Elucidating the complexity and cross talks among signalling mediators involved in the TNFα-mediated responses will certainly aid in the identification of molecular targets, which can potentially lead to the development of novel therapeutics to treat TNFα-associated disorders and in dampening inflammation.

Attenuation of arthritis in rodents by a novel orally-available inhibitor of sphingosine kinase

Pro-inflammatory cytokines like TNF-α activate sphingosine kinase (SK). Therefore, inhibition of SK is a potential molecular target for the treatment of rheumatoid arthritis.

AIMS

The primary goal of this study was to assess the efficacy of ABC249640 (a selective SK-2 inhibitor) in two models of rodent arthritis. A secondary goal was to evaluate the pharmacological profile of ABC294640, when given in combination with methotrexate.

METHODS

The efficacy of ABC294640 was determined by paw diameter/volume measurements, histological evaluations, and micro-CT analyses.

RESULTS

ABC294640 attenuated both collagen-induced arthritis in mice, as well as adjuvant-induced arthritis in rats. With the adjuvant arthritis model, the prophylactic efficacy profile of ABC294640 was similar to indomethacin. Of note, ABC294640 reduced the bone and cartilage degradation, associated with adjuvant-induced arthritis. Rats treated with a suboptimal dose of MTX (50 μg/kg/day) in combination with ABC249640 (100 mg/kg/day) had better anti-arthritis effects in the adjuvant model, than treatment with either agent alone.

CONCLUSION

Our results suggest that ABC249640 is an orally available drug candidate with a good pre-clinical efficacy profile for the prevention and/or treatment of RA.

Synthesis and evaluation of sphingosine analogues as inhibitors of sphingosine kinases

Sphingolipid-metabolizing enzymes control the critical balance of the cellular levels of sphingolipids, including the apoptotic inducing ceramide (Cer) and the proliferative inducing sphingosine 1-phosphate (S1P). The production of S1P, catalyzed by the action of sphingosine kinases (SPHKs), is known to be critical for many cellular processes. However, it is suggested that SPHK, and/or its catalytic product S1P, plays critical roles in various diseases including autoimmune diseases, cancer, and allergies. However, there is a great limitation of specific pharmacological inhibitors for SPHKs. In this paper, we describe a novel and stereoselective method of synthesizing SPHKs inhibitors. We generated a number of novel compounds and identified a number of specific inhibitors of human SPHKs. These compounds demonstrated inhibition of SPHKs at micromolar concentrations, making them more potent than dimethylsphingosine (DMS), a well-known inhibitor of SPHKs. In particular, one of the inhibitors was found to be selective toward a particular isoform of SPHK.

Coordinate regulation of sphingosine kinase and actin dynamics

The activation of lipid-modifying enzymes generally involves a physical change in their interactions with the membrane substrate. For sphingosine kinase, a predominantly cytosolic enzyme in resting cells, activation is accompanied by translocation to specific subsets of cellular membranes where catalysis occurs. As all eukaryotic membranes have a tightly associated filamentous actin skeleton, we investigated potential regulatory interactions between sphingosine kinase and actin. Sphingosine kinase 1 (SK1) exhibited constitutive- and stimulus-enhanced association with actin filaments and F-actin-enriched membrane fractions in both intact macrophages and an in vitro reconstitution assay, whereas SK1 bound G-actin only under stimulated conditions. Actin inhibitors disrupted SK1 localization and increased its enzymatic activity. Both the localization and the activity of SK1 were coordinately regulated with the actin cytoskeleton. The association of enzymes with the actin cytoskeleton and how this regulates their activities and functions are subjects of intense interest. Here, we describe the approach we used to investigate regulation of SK1. This provides general methods that can be used to examine the role of actin in regulating enzyme activity in macrophages and other myeloid cells.

Anti-sphingosine-1-phosphate monoclonal antibodies inhibit angiogenesis and sub-retinal fibrosis in a murine model of laser-induced choroidal neovascularization

The efficacy of novel monoclonal antibodies that neutralize the pro-angiogenic mediator, sphingosine-1-phosphate (S1P), were tested using in vitro and in vivo angiogenesis models, including choroidal neovascularization (CNV) induced by laser disruption of Bruch's membrane. S1P receptor levels in human brain choroid plexus endothelial cells (CPEC), human lung microvascular endothelial cells, human retinal vascular endothelial cells, and circulating endothelial progenitor cells were examined by semi-quantitative PCR. The ability of murine or humanized anti-S1P monoclonal antibodies (mAbs) to inhibit S1P-mediated microvessel tube formation by CPEC on Matrigel was evaluated and capillary density in subcutaneous growth factor-loaded Matrigel plugs was determined following anti-S1P treatment. S1P promoted in vitro capillary tube formation in CPEC consistent with the presence of cognate S1P(1-5) receptor expression by these cells and the S1P antibody induced a dose-dependent reduction in microvessel tube formation. In a murine model of laser-induced rupture of Bruch's membrane, S1P was detected in posterior cups of mice receiving laser injury, but not in uninjured controls. Intravitreous injection of anti-S1P mAbs dramatically inhibited CNV formation and sub-retinal collagen deposition in all treatment groups (p<0.05 compared to controls), thereby identifying S1P as a previously unrecognized mediator of angiogenesis and subretinal fibrosis in this model. These findings suggest that neutralizing S1P with anti-S1P mAbs may be a novel method of treating patients with exudative age-related macular degeneration by reducing angiogenesis and sub-retinal fibrosis, which are responsible for visual acuity loss in this disease.

Effect of probiotic Lactobacillus rhamnosus GG intervention on global serum lipidomic profiles in healthy adults

AIM: To investigate the effect of three weeks’ intervention with a probiotic Lactobacillus rhamnosus GG (LGG) bacteria on global serum lipidomic profiles and evaluate whether the changes in inflammatory variables (CRP, TNF-α and IL-6) are reflected in the global lipidomic profiles of healthy adults.

METHODS: We performed UPLC/MS-based global lipidomic platform analysis of serum samples (n = 26) in a substudy of a randomised, double-blind, placebo-controlled 3-wk clinical intervention trial investigating the immunomodulatory effects of probiotics in healthy adults.

RESULTS: A total of 407 lipids were identified, corresponding to 13 different lipid classes. Serum samples showed decreases in the levels of lysophosphatidylcholines (LysoGPCho), sphingomyelins (SM) and several glycerophosphatidylcholines (GPCho), while triacylglycerols (TAG) were mainly increased in the probiotic LGG group during the intervention. Among the inflammatory variables, IL-6 was moderately associated by changes in global lipidomic profiles, with the top-ranked lipid associated with IL-6 being the proinflammatory LysoGPCho (20:4). There was a weak association between the lipidomic profiles and the two other inflammatory markers, TNF-α and CRP.

CONCLUSION: This was the first study to investigate the effects of probiotic intervention on global lipidomic profiles in humans. There are indications that probiotic LGG intervention may lead to changes in serum global lipid profiles, as reflected in decreased GPCho, LysoGPCho and SM as well as mainly increased TAG.

Sphingosine kinase signalling in immune cells: potential as novel therapeutic targets

During the last few years, it has become clear that sphingolipids are sources of important signalling molecules. Particularly, the sphingolipid metabolites, ceramide and S1P, have emerged as a new class of potent bioactive molecules, implicated in a variety of cellular processes such as cell differentiation, apoptosis, and proliferation. Sphingomyelin (SM) is the major membrane sphingolipid and is the precursor for the bioactive products. Ceramide is formed from SM by the action of sphingomyelinases (SMase), however, ceramide can be very rapidly hydrolysed, by ceramidases to yield sphingosine, and sphingosine can be phosphorylated by sphingosine kinase (SphK) to yield S1P. In immune cells, the sphingolipid metabolism is tightly related to the main stages of immune cell development, differentiation, activation, and proliferation, transduced into physiological responses such as survival, calcium mobilization, cytoskeletal reorganization and chemotaxis. Several biological effectors have been shown to promote the synthesis of S1P, including growth factors, cytokines, and antigen and G-protein-coupled receptor agonists. Interest in S1P focused recently on two distinct cellular actions of this lipid, namely its function as an intracellular second messenger, capable of triggering calcium release from internal stores, and as an extracellular ligand activating specific G protein-coupled receptors. Inhibition of SphK stimulation strongly reduced or even prevented cellular events triggered by several proinflammatory agonists, such as receptor-stimulated DNA synthesis, Ca(2+) mobilization, degranulation, chemotaxis and cytokine production. Another very important observation is the direct role played by S1P in chemotaxis, and cellular escape from apoptosis. As an extracellular mediator, several studies have now shown that S1P binds a number of G-protein-coupled receptors (GPCR) encoded by endothelial differentiation genes (EDG), collectively known as the S1P-receptors. Binding of S1P to these receptors trigger an wide range of cellular responses including proliferation, enhanced extracellular matrix assembly, stimulation of adherent junctions, formation of actin stress fibres, and inhibition of apoptosis induced by either ceramide or growth factor withdrawal. Moreover, blocking S1P1-receptor inhibits lymphocyte egress from lymphatic organs. This review summarises the evidence linking SphK signalling pathway to immune-cell activation and based on these data discuss the potential for targeting SphKs to suppress inflammation and other pathological conditions.

The Localization and Activity of Sphingosine Kinase 1 Are Coordinately Regulated with Actin Cytoskeletal Dynamics in Macrophages

The physiologic and pathologic functions of sphingosine kinase (SK) require translocation to specific membrane compartments. We tested the hypothesis that interactions with actin filaments regulate the localization of SK1 to membrane surfaces, including the plasma membrane and phagosome. Macrophage activation is accompanied by a marked increase in association of SK1 with actin filaments. Catalytically-inactive (CI)- and phosphorylation-defective (PD)-SK1 mutants exhibited reductions in plasma membrane translocation, colocalization with cortical actin filaments, membrane ruffling, and lamellipodia formation, compared with wild-type (WT)-SK1. However, translocation of CI- and PD-SK1 to phagosomes were equivalent to WT-SK1. SK1 exhibited constitutive- and stimulus-enhanced association with actin filaments and F-actin-enriched membrane fractions in both intact macrophages and a novel in vitro assay. In contrast, SK1 bound G-actin only under stimulated conditions. Actin inhibitors disrupted SK1 localization and modulated its activity. Conversely, reduction of SK1 levels or activity via RNA interference or specific chemical inhibition resulted in dysregulation of actin filaments. Thus, the localization and activity of SK1 are coordinately regulated with actin dynamics during macrophage activation.

Sphingosine-1-phosphate in allergic responses, asthma and anaphylaxis

Sphingosine-1-phosphate (S1P) is a bioactive sphingolipid metabolite involved in many cellular processes, acting not only as an extracellular ligand to its specific G protein-coupled receptors, but also as a putative intracellular messenger with yet unidentified targets. Mast cells are tissue-dwelling pivotal early effectors of allergic responses, which produce and secrete S1P that can bind to its receptors present on mast cells to influence their activation and functions. In this review, we will first discuss the current knowledge of S1P production by two isozymes of sphingosine kinase (SphK). Mechanisms of SphK activation will be discussed, with an emphasis on experimental approaches developed to study their differential activation and biological roles in the context of mast cells. The relevance of mast cells in the etiology of allergic disorders, asthma and anaphylaxis is well established. In this review, this concept will be revisited, focusing on the contribution of S1P production and secretion to the symptoms associated with dysregulated inflammatory responses. To conclude, counteracting the proinflammatory effects of S1P could be envisioned as a therapeutic strategy to treat allergic disorders, exacerbated airway inflammation, and anaphylactic reactions, and various options will be discussed, such as the development of pharmacological tools to inhibit SphKs, S1P neutralizing monoclonal antibody, and S1P receptor antagonists.

Identification of Small Molecule Ceramide Kinase Inhibitors Using A Homogeneous Chemiluminescence High Throughput Assay

Lipid kinases have emerged as potentially important therapeutic targets in oncology and inflammation. Ceramide kinase (CERK) is a lipid kinase that catalyzes the formation of ceramide-1-phosphate from ceramide, a sphingolipid that is a key mediator of cellular apoptosis. Ceramide-1-phosphate has been shown to enhance the production of pro-inflammatory eicosonoids, to promote cell proliferation, and potentially to reduce intracellular ceramide levels by inhibition of acidic sphingomyelinases. Here we describe a homogeneous chemiluminescence assay that directly measures the ceramide-dependent ATP depletion by recombinant full-length human CERK. As compared to reported CERK assays that have limitations on compound throughput, the chemiluminescence assay has been miniaturized to a 1,536-well microtiter plate format and utilized to screen an ultra-large compound library (>4 million compounds). Multiple chemical scaffolds have been identified as CERK kinase inhibitors and characterized mechanistically, which to our knowledge represent the first known small molecule CERK inhibitors with nanomolar activities. These compounds can serve as tools to further elucidate the CERK pathway and its role in ceramide metabolism and human diseases.

Regulation and functional roles of sphingosine kinases

Sphingosine kinases (SphKs) catalyze the phosphorylation of sphingosine to sphingosine-1-phosphate (S1P). Together with other sphingolipid metabolizing enzymes, SphKs regulate the balance of the lipid mediators, ceramide, sphingosine, and S1P. The ubiquitous mediator S1P regulates cellular functions such as proliferation and survival, cytoskeleton architecture and Ca(2+) homoeostasis, migration, and adhesion by activating specific high-affinity G-protein-coupled receptors or by acting intracellularly. In mammals, two isoforms of SphK have been identified. They are activated by G-protein-coupled receptors, receptor tyrosine kinases, immunoglobulin receptors, cytokines, and other stimuli. The molecular mechanisms by which SphK1 and SphK2 are specifically regulated are complex and only partially understood. Although SphK1 and SphK2 appear to have opposing roles, promoting cell growth and apoptosis, respectively, they can obviously also substitute for each other, as mice deficient in either SphK1 or SphK2 had no obvious abnormalities, whereas double-knockout animals were embryonic lethal. In this review, our understanding of structure, regulation, and functional roles of SphKs is updated and discussed with regard to their implication in pathophysiological and disease states.

Of Spiders and Crabs: The Emergence of Lysophospholipids and Their Metabolic Pathways as Targets for Therapy in Cancer: Fig. 1

Lysophosphatidic acid (LPA) and sphingosine 1-phosphate (S1P), two small lysophospholipids, are potent inducers of many of the hallmarks of cancer including cell proliferation, survival, migration, invasion, and neovascularization in in vitro and in vivo tumor models. Furthermore, the enzymes metabolizing LPA and S1P and their receptors are aberrant in multiple cancer lineages and exhibit transforming activity altering patterns and targets for metastasis. Several recent studies show the remarkable activity of new chemical genomics and/or potential novel drugs in preclinical models. Combined with the physiologic and pathophysiologic activities of LPA and S1P, these studies suggest the implementation of preclinical and clinical evaluation of LPA and S1P as therapeutic targets.

Lysophospholipid receptors: signalling, pharmacology and regulation by lysophospholipid metabolism

The lysophospholipids, sphingosine-1-phosphate (S1P), lysophosphatidic acid (LPA), sphingosylphosphorylcholine (SPC) and lysophosphatidylcholine (LPC), activate diverse groups of G-protein-coupled receptors that are widely expressed and regulate decisive cellular functions. Receptors of the endothelial differentiation gene family are activated by S1P (S1P(1-5)) or LPA (LPA(1-3)); two more distantly related receptors are activated by LPA (LPA(4/5)); the GPR(3/6/12) receptors have a high constitutive activity but are further activated by S1P and/or SPC; and receptors of the OGR1 cluster (OGR1, GPR4, G2A, TDAG8) appear to be activated by SPC, LPC, psychosine and/or protons. G-protein-coupled lysophospholipid receptors regulate cellular Ca(2+) homoeostasis and the cytoskeleton, proliferation and survival, migration and adhesion. They have been implicated in development, regulation of the cardiovascular, immune and nervous systems, inflammation, arteriosclerosis and cancer. The availability of S1P and LPA at their G-protein-coupled receptors is regulated by enzymes that generate or metabolize these lysophospholipids, and localization plays an important role in this process. Besides FTY720, which is phosphorylated by sphingosine kinase-2 and then acts on four of the five S1P receptors of the endothelial differentiation gene family, other compounds have been identified that interact with more ore less selectivity with lysophospholipid receptors.

Sphingosine kinase 1 regulates pro-inflammatory responses triggered by TNFalpha in primary human monocytes

Monocytes play an important role in inflammation, angiogenesis, and atherosclerosis. During these processes monocytes release pre-formed proinflammatory mediators from granules, and synthesize de novo cytokines and chemokines important in the amplification of the inflammatory response. One of the most prominent triggers of inflammatory responses is the cytokine TNFalpha. However, the intracellular signaling cascades triggered by TNFalpha are not fully understood. In this study we investigated the roles of SPHK on the TNFalpha-triggered responses on human primary monocytes. We show that TNFalpha rapidly triggers S1P generation and activation of SPHK. Moreover, our data shows that SPHK1 is the isoform activated by TNFalpha, and plays an essential role on the TNFalpha-triggered intracellular Ca2+ signals, degranulation, cytokine production, and activation of NFkappaB, thus suggesting a pivotal role for SPHK1 on the proinflammatory responses triggered by TNFalpha.

Macrophage’s Proinflammatory Response to a Mycobacterial Infection Is Dependent on Sphingosine Kinase-Mediated Activation of Phosphatidylinositol Phospholipase C, Protein Kinase C, ERK1/2, and Phosphatidylinositol 3-Kinase

Previous studies have shown that the ability of Mycobacterium tuberculosis to block a Ca(2+) flux is an important step in its capacity to halt phagosome maturation. This affect on Ca(2+) release results from M. tuberculosis inhibition of sphingosine kinase (SPK) activity. However, these studies did not address the potential role of SPK and Ca(2+) in other aspects of macrophage activation including production of proinflammatory mediators. We previously showed that nonpathogenic Mycobacterium smegmatis and to a lesser extent pathogenic Mycobacterium avium, activate Ca(2+)-dependent calmodulin/calmodulin kinase and MAPK pathways in murine macrophages leading to TNF-alpha production. However, whether SPK functions in promoting MAPK activation upon mycobacterial infection was not defined in these studies. In the present work we found that SPK is required for ERK1/2 activation in murine macrophages infected with either M. avium or M. smegmatis. Phosphoinositide-specific phospholipase C (PI-PLC) and conventional protein kinase C (cPKC) were also important for ERK1/2 activation. Moreover, there was increased activation of cPKC and PI3K in macrophages infected with M. smegmatis compared with M. avium. This cPKC and PI3K activation was dependent on SPK and PI-PLC. Finally, in macrophages infected with M. smegmatis compared with M. avium, we observed enhanced secretion of TNF-alpha, IL-6, RANTES, and G-CSF and found production of these inflammatory mediators to be dependent on SPK, PI-PLC, cPKC, and PI3K. These studies are the first to show that the macrophage proinflammatory response following a mycobacterial infection is regulated by SPK/PI-PLC/PKC activation of ERK1/2 and PI3K pathways.

De novo biosynthesis of dihydrosphingosine-1-phosphate by sphingosine kinase 1 in mammalian cells

Sphingosine kinase 1 (SK1) is one of the two known kinases, which generates sphingosine-1-phosphate (S1P), a potent endogenous lipid mediator involved in cell survival, proliferation, and cell-cell interactions. Activation of SK1 and intracellular generation of S1P were suggested to be part of the growth and survival factor-induced signaling, and overexpression of SK1 provoked cell tumorigenic transformation. Using a highly selective and sensitive LC-MS/MS approach, here we show that SK1 overexpression, but not SK2, in different primary cells and cultured cell lines results in predominant upregulation of the synthesis of dihydrosphingosine-1-phosphate (DHS1P) compared to S1P. Stable isotope pulse-labeling experiments in conjunction with LC-MS/MS quantitation of different sphingolipids demonstrated strong interference of overexpressed SK1 with the de novo sphingolipid biosynthesis by deviating metabolic flow of newly formed sphingoid bases from ceramide formation toward the synthesis of DHS1P. On the contrary, S1P biosynthesis was not directly linked to the de novo sphingoid bases transformations and was dependent on catabolic generation of sphingosine from complex sphingolipids. As a result of SK1 overexpression, migration and Ca2+-response of human pulmonary artery endothelial cells (HPAEC) to stimulation with external S1P, but not thrombin, was strongly impaired. In contrast, selective increase in intracellular content of DHS1P or S1P through the uptake and phosphorylation of corresponding sphingoid bases had no effect on S1P-induced signaling or facilitation of wound healing. Furthermore, infection of human bronchial epithelial cells (HBEpC) with RSV A-2 virus increased SK1-mediated synthesis of DHS1P and S1P, whereas TNF-alpha enhanced only S1P production in HPAEC. These findings uncover a new functional role for SK1, which can control survival/death (DHS1P-S1P/ceramides) balance by targeting sphingolipid de novo biosynthesis and selectively generating DHS1P at a metabolic step preceding ceramide formation.

The emerging field of lipidomics

The crucial role of lipids in cell, tissue and organ physiology is demonstrated by a large number of genetic studies and by many human diseases that involve the disruption of lipid metabolic enzymes and pathways. Examples of such diseases include cancer, diabetes, as well as neurodegenerative and infectious diseases. So far, the explosion of information in the fields of genomics and proteomics has not been matched by a corresponding advancement of knowledge in the field of lipids, which is largely due to the complexity of lipids and the lack of powerful tools for their analysis. Novel analytical approaches--in particular, liquid chromatography and mass spectrometry--for systems-level analysis of lipids and their interacting partners (lipidomics) now make this field a promising area of biomedical research, with a variety of applications in drug and biomarker development.