Sphingolipids differentially regulate mitogen-activated protein kinases and intracellular Ca2+ in vascular smooth muscle: effects on CREB activation

Proteomic-Based Approach Reveals the Involvement of Apolipoprotein A-I in Related Phenotypes of Autism Spectrum Disorder in the BTBR Mouse Model

Autism spectrum disorder (ASD) is a neurodevelopmental disorder. Abnormal lipid metabolism has been suggested to contribute to its pathogenesis. Further exploration of its underlying biochemical mechanisms is needed. In a search for reliable biomarkers for the pathophysiology of ASD, hippocampal tissues from the ASD model BTBR T+ Itpr3tf/J (BTBR) mice and C57BL/6J mice were analyzed, using four-dimensional (4D) label-free proteomic analysis and bioinformatics analysis. Differentially expressed proteins were significantly enriched in lipid metabolic pathways. Among them, apolipoprotein A-I (ApoA-I) is a hub protein and its expression was significantly higher in the BTBR mice. The investigation of protein levels (using Western blotting) also confirmed this observation. Furthermore, expressions of SphK2 and S1P in the ApoA-I pathway both increased. Using the SphK inhibitor (SKI-II), ASD core phenotype and phenotype-related protein levels of P-CREB, P-CaMKII, and GAD1 were improved, as shown via behavioral and molecular biology experiments. Moreover, by using SKI-II, we found proteins related to the development and function of neuron synapses, including ERK, caspase-3, Bax, Bcl-2, CDK5 and KCNQ2 in BTBR mice, whose levels were restored to protein levels comparable to those in the controls. Elucidating the possible mechanism of ApoA-I in ASD-associated phenotypes will provide new ideas for studies on the etiology of ASD.

The dynamics and role of sphingolipids in eukaryotic organisms upon thermal adaptation

All living beings have an optimal temperature for growth and survival. With the advancement of global warming, the search for understanding adaptive processes to climate changes has gained prominence. In this context, all living beings monitor the external temperature and develop adaptive responses to thermal variations. These responses ultimately change the functioning of the cell and affect the most diverse structures and processes. One of the first structures to detect thermal variations is the plasma membrane, whose constitution allows triggering of intracellular signals that assist in the response to temperature stress. Although studies on this topic have been conducted, the underlying mechanisms of recognizing thermal changes and modifying cellular functioning to adapt to this condition are not fully understood. Recently, many reports have indicated the participation of sphingolipids (SLs), major components of the plasma membrane, in the regulation of the thermal stress response. SLs can structurally reinforce the membrane or/and send signals intracellularly to control numerous cellular processes, such as apoptosis, cytoskeleton polarization, cell cycle arresting and fungal virulence. In this review, we discuss how SLs synthesis changes during both heat and cold stresses, focusing on fungi, plants, animals and human cells. The role of lysophospholipids is also discussed.

Comparative Proteomic Analysis of Pleurotus ostreatus Reveals Great Metabolic Differences in the Cap and Stipe Development and the Potential Role of Ca2+ in the Primordium Differentiation

Pleurotus ostreatus is a widely cultivated edible fungus around the world. At present, studies on the developmental process of the fruiting body are limited. In our study, we compared the differentially expressed proteins (DEPs) in the stipe and cap of the fruiting body by high-throughput proteomics. GO and pathway analysis revealed the great differences in the metabolic levels, including sucrose and starch metabolism, and sphingolipid signaling and metabolism, and the differences of 16 important DEPs were validated further by qPCR analysis in expression level. In order to control the cap and stipe development, several chemical inducers were applied to the primordium of the fruiting body according to the pathway enrichment results. We found that CaCl₂ can affect the primordium differentiation through inhibiting the stipe development. EGTA (ethyleneglycol bis (β-aminoethyl ether)-N,N,N′,N′-tetraacetic acid) treatment confirmed the inhibitory role of Ca²⁺ in the stipe development. Our study not only shows great metabolic differences during the cap and stipe development but also reveals the underlying mechanism directing the primordium differentiation in the early development of the fruiting body for the first time. Most importantly, we provide a reliable application strategy for the cultivation and improvement of the Pleurotus ostreatus, which can be an example and reference for a more edible fungus.

S1P mediates human amniotic cells proliferation induced by a 50-Hz magnetic field exposure via ERK1/2 signaling pathway

Extremely low frequency electromagnetic field (ELF-EMF) is a kind of physical stimulus in public and occupational environment. Numerous studies have indicated that exposure of cells to ELF-EMF could promote cell proliferation. But the detailed mechanisms implicated in these proliferative processes remain unclear. In the present experiment, the possible roles of sphingosine-1-phosphate (S1P) in 50-Hz magnetic field (MF)-induced cell proliferation were investigated. Results showed that exposure of human amniotic (FL) cells to a 50-Hz MF with an intensity of 0.4 mT significantly enhanced ceramide metabolism, increased S1P production, activated extracellular signal regulated kinase 1/2 (ERK1/2), and promoted cell proliferation. All of these effects induced by MF exposure could be inhibited by SKI II, an inhibitor of sphingosine kinase (SphK). In addition, both the cell proliferative response and the ERK1/2 activation induced by MF exposure were blocked completely by U0126, a specific inhibitor of MEK (ERK kinases 1 and 2). Taken together, the findings in present study suggested that S1P mediated 50-Hz MF-induced cell proliferation via triggering ERK1/2 signal pathway.

Emerging roles of sphingosylphosphorylcholine in modulating cardiovascular functions and diseases

Sphingosylphosphorylcholine (SPC) is a bioactive sphingolipid in blood plasma that is metabolized from the hydrolysis of the membrane sphingolipid. SPC maintains low levels in the circulation under normal conditions, which makes studying its origin and action difficult. In recent years, however, it has been revealed that SPC may act as a first messenger through G protein-coupled receptors (S1P_1-5, GPR12) or membrane lipid rafts, or as a second messenger mediating intracellular Ca²⁺ release in diverse human organ systems. SPC is a constituent of lipoproteins, and the activation of platelets promotes the release of SPC into blood, both implying a certain effect of SPC in modulating the pathological process of the heart and vessels. A line of evidence indeed confirms that SPC exerts a pronounced influence on the cardiovascular system through modulation of the functions of myocytes, vein endothelial cells, as well as vascular smooth muscle cells. In this review we summarize the current knowledge of the potential roles of SPC in the development of cardiovascular diseases and discuss the possible underlying mechanisms.

Ginsenoside Rg1 attenuates hypoxia and hypercapnia-induced vasoconstriction in isolated rat pulmonary arterial rings by reducing the expression of p38

BACKGROUND

Pulmonary arterial hypertension (PAH) is a fatal disease characterized by increased pulmonary arteriolar resistance. Pulmonary vasoconstriction has been proved to play a significant role in PAH. We previously reported that Panax notoginseng saponins (PNS) might attenuate hypoxia and hypercapnia-induced pulmonary vasoconstriction (HHPV).

METHODS

In the present study, our specific objective was to investigate the role of ginsenoside Rg1, a major component of PNS, in this process and the possible underlying mechanism. The second order pulmonary rings isolated from the Sprague-Dawley rats were treated with different dosage of ginsenoside Rg1 at 8, 40, or 100 mg/L respectively, both before and during the conditions of hypoxia and hypercapnia. Contractile force changes of the rings were detected. Furthermore, SB203580, the selective inhibitor for p38 activation was applied to the rings. Pulmonary arterial smooth muscle cells (PASMCs) were cultured under hypoxic and hypercapnic conditions, and ginsenoside Rg1 was administered to detect the changes induced by p38.

RESULTS

Under the hypoxic and hypercapnic conditions, we observed a biphasic pulmonary artery contractile response to the second pulmonary artery rings. It is hypothesized that the observed attenuation of vasoconstriction and the production of vasodilation could have been induced by ginsenoside Rg1. This effect was significantly reinforced by SB203580 (P<0.05 or P<0.01). The expression of p38 in the PASMCs under hypoxic and hypercapnic conditions was significantly activated (P<0.05 or P<0.01) and the observed activation was attenuated by ginsenoside Rg1 (P<0.05 or P<0.01).

CONCLUSIONS

Our findings strongly support the significant role of ginsenoside Rg1 in the inhibition of hypoxia and hypercapnia-induced vasoconstriction by the p38 pathway.

Sphingosylphosphorylcholine inhibits macrophage adhesion to vascular smooth muscle cells

Inflammation in de-endothelialised arteries contributes to the development of cardiovascular diseases. The process that initiates this inflammatory response is the adhesion of monocytes/macrophages to exposed vascular smooth muscle cells, typically stimulated by cytokines such as tumour necrosis factor-α (TNF). The aim of this study was to determine the effect of the sphingolipid sphingosylphosphorylcholine (SPC) on the interaction of monocytes/macrophages with vascular smooth muscle cells. Rat aortic smooth muscle cells and rat bone marrow-derived macrophages were co-cultured using an in vitro assay following incubation with sphingolipids to assess inter-cellular adhesion. We reveal that SPC inhibits the TNF-induced adhesion of macrophages to smooth muscle cells. This anti-adhesive effect was the result of SPC-induced changes to the smooth muscle cells (but not the macrophages) and was mediated, at least partly, via the sphingosine 1-phosphate receptor subtype 2. Lipid raft domains were also required. Although SPC did not alter expression or membrane distribution of the adhesion proteins intercellular adhesion molecule-1 and vascular cellular adhesion protein-1 in smooth muscle cells, SPC preincubation inhibited the TNF-induced increase in inducible nitric oxide synthase (NOS2) resulting in a subsequent decrease in nitric oxide production. Inhibiting NOS2 activation in smooth muscle cells led to a decrease in the adhesion of macrophages to smooth muscle cells. This study has therefore delineated a novel pathway which can inhibit the interaction between macrophages and vascular smooth muscle cells via SPC-induced repression of NOS2 expression. This mechanism could represent a potential drug target in vascular disease.

Membrane lipid therapy: Modulation of the cell membrane composition and structure as a molecular base for drug discovery and new disease treatment

Nowadays we understand cell membranes not as a simple double lipid layer but as a collection of complex and dynamic protein-lipid structures and microdomains that serve as functional platforms for interacting signaling lipids and proteins. Membrane lipids and lipid structures participate directly as messengers or regulators of signal transduction. In addition, protein-lipid interactions participate in the localization of signaling protein partners to specific membrane microdomains. Thus, lipid alterations change cell signaling that are associated with a variety of diseases including cancer, obesity, neurodegenerative disorders, cardiovascular pathologies, etc. This article reviews the newly emerging field of membrane lipid therapy which involves the pharmacological regulation of membrane lipid composition and structure for the treatment of diseases. Membrane lipid therapy proposes the use of new molecules specifically designed to modify membrane lipid structures and microdomains as pharmaceutical disease-modifying agents by reversing the malfunction or altering the expression of disease-specific protein or lipid signal cascades. Here, we provide an in-depth analysis of this emerging field, especially its molecular bases and its relevance to the development of innovative therapeutic approaches.

Sphingosylphosphorylcholine potentiates vasoreactivity and voltage-gated Ca2+ entry via NOX1 and reactive oxygen species

Aims

Sphingosylphosphorylcholine (SPC) elicits vasoconstriction at micromolar concentrations. At lower concentrations (≤1 µmol/L), however, it does not constrict intrapulmonary arteries (IPAs), but strongly potentiates vasoreactivity. Our aim was to determine whether this also occurs in a systemic artery and to delineate the signalling pathway.

Methods and results

Rat mesenteric arteries and IPAs mounted on a myograph were challenged with ∼25 mmol/L [K⁺] to induce a small vasoconstriction. SPC (1 µmol/L) dramatically potentiated this constriction in all arteries by ∼400%. The potentiation was greatly suppressed or abolished by inhibition of phospholipase C (PLC; U73122), PKCε (inhibitory peptide), Src (PP2), and NADPH oxidase (VAS2870), and also by Tempol (superoxide scavenger), but not by inhibition of Rho kinase (Y27632). Potentiation was lost in mesenteric arteries from p47^phox–/–, but not NOX2^−/–, mice. The intracellular superoxide generator LY83583 mimicked the effect of SPC. SPC elevated reactive oxygen species (ROS) in vascular smooth muscle cells, and this was blocked by PP2, VAS2870, and siRNA knockdown of PKCε. SPC (1 µmol/L) significantly reduced the EC₅₀ for U46619-induced vasoconstriction, an action ablated by Tempol. In patch-clamped mesenteric artery cells, SPC (200 nmol/L) enhanced Ba²⁺ current through L-type Ca²⁺ channels, an action abolished by Tempol but mimicked by LY83583.

Conclusion

Our results suggest that low concentrations of SPC activate a PLC-coupled and NOX1-mediated increase in ROS, with consequent enhancement of voltage-gated Ca²⁺ entry and thus vasoreactivity. We speculate that this pathway is not specific for SPC, but may also contribute to vasoconstriction elicited by other G-protein coupled receptor and PLC-coupled agonists.

Plasma membranes as heat stress sensors: from lipid-controlled molecular switches to therapeutic applications

The classic heat shock (stress) response (HSR) was originally attributed to protein denaturation. However, heat shock protein (Hsp) induction occurs in many circumstances where no protein denaturation is observed. Recently considerable evidence has been accumulated to the favor of the "Membrane Sensor Hypothesis" which predicts that the level of Hsps can be changed as a result of alterations to the plasma membrane. This is especially pertinent to mild heat shock, such as occurs in fever. In this condition the sensitivity of many transient receptor potential (TRP) channels is particularly notable. Small temperature stresses can modulate TRP gating significantly and this is influenced by lipids. In addition, stress hormones often modify plasma membrane structure and function and thus initiate a cascade of events, which may affect HSR. The major transactivator heat shock factor-1 integrates the signals originating from the plasma membrane and orchestrates the expression of individual heat shock genes. We describe how these observations can be tested at the molecular level, for example, with the use of membrane perturbers and through computational calculations. An important fact which now starts to be addressed is that membranes are not homogeneous nor do all cells react identically. Lipidomics and cell profiling are beginning to address the above two points. Finally, we observe that a deregulated HSR is found in a large number of important diseases where more detailed knowledge of the molecular mechanisms involved may offer timely opportunities for clinical interventions and new, innovative drug treatments. This article is part of a Special Issue entitled: Membrane Structure and Function: Relevance in the Cell's Physiology, Pathology and Therapy.

Key role of lipids in heat stress management

Heat stress is a common and, therefore, an important environmental impact on cells and organisms. While much attention has been paid to severe heat stress, moderate temperature elevations are also important. Here we discuss temperature sensing and how responses to heat stress are not necessarily dependent on denatured proteins. Indeed, it is clear that membrane lipids have a pivotal function. Details of membrane lipid changes and the associated production of signalling metabolites are described and suggestions made as to how the interconnected signalling network could be modified for helpful intervention in disease.

Sphingosine-1-phosphate rapidly increases cortisol biosynthesis and the expression of genes involved in cholesterol uptake and transport in H295R adrenocortical cells

In the acute phase of adrenocortical steroidogenesis, adrenocorticotrophin (ACTH) activates a cAMP/PKA-signaling pathway that promotes the transport of free cholesterol to the inner mitochondrial membrane. We have previously shown that ACTH rapidly stimulates the metabolism of sphingolipids and the secretion of sphingosine-1-phosphate (S1P) in H295R cells. In this study, we examined the effect of S1P on genes involved in the acute phase of steroidogenesis. We show that S1P increases the expression of steroidogenic acute regulatory protein (StAR), 18-kDa translocator protein (TSPO), low-density lipoprotein receptor (LDLR), and scavenger receptor class B type I (SR-BI). S1P-induced StAR mRNA expression requires Gα(i) signaling, phospholipase C (PLC), Ca(2+)/calmodulin-dependent kinase II (CamKII), and ERK1/2 activation. S1P also increases intracellular Ca(2+), the phosphorylation of hormone sensitive lipase (HSL) at Ser(563), and cortisol secretion. Collectively, these findings identify multiple roles for S1P in the regulation of glucocorticoid biosynthesis.

The impact of bioactive lipids on cardiovascular development

Lysophospholipids comprise a group of bioactive molecules with multiple biological functions. The cardinal members of this signalling molecule group are sphingosylphosphorylcholine (SPC), lysophosphatidic acid (LPA), and sphingosine 1-phosphate (S1P) which are, at least in part, homologous to each other. Bioactive lipids usually act via G-protein coupled receptors (GPCRs), but can also function as direct intracellular messengers. Recently, it became evident that bioactive lipids play a role during cellular differentiation development. SPC induces mesodermal differentiation of mouse ES cells and differentiation of promyelocytic leukemia cells, by a mechanism being critically dependent on MEK-ERK signalling. LPA stimulates the clonal expansion of neurospheres from neural stem/progenitor cells and induces c-fos via activation of mitogen- and stress-activated protein kinase 1 (MSK1) in ES cells. S1P acts on hematopoietic progenitor cells as a chemotactic factor and has also been found to be critical for cardiac and skeletal muscle regeneration. Furthermore, S1P promotes cardiogenesis and similarly activates Erk signalling in mouse ES cells. Interestingly, S1P may also act to maintain human stem cell pluripotency. Both LPA and S1P positively regulate the proliferative capacity of murine ES cells. In this paper we will focus on the differential and developmental impact of lysophospholipids on cardiovascular development.

Apoptotic cell-derived factors induce arginase II expression in murine macrophages by activating ERK5/CREB

Apoptotic cell (AC)-derived factors alter the physiology of macrophages (MΦs) towards a regulatory phenotype, characterized by reduced nitric oxide (NO) production. Impaired NO formation in response to AC-conditioned medium (CM) was facilitated by arginase II (ARG II) expression, which competes with inducible NO synthase for L-arginine. Here we explored signaling pathways allowing CM to upregulate ARG II in RAW264.7 MΦs. Sphingosine-1-phosphate (S1P) was required and acted synergistically with a so far unidentified factor to elicit high ARG II expression. S1P activated S1P(2), since S1P(2) knockdown prevented ARG II upregulation. Furthermore, ERK5 knockdown attenuated CM-mediated ARG II protein induction. CREB was implicated as shown by EMSA analysis and decoy-oligonucleotides scavenging CREB in RAW264.7 MΦs, which blocked ARG II expression. We conclude that AC-derived S1P binds to S1P(2) and acts synergistically with other factors to activate ERK5 and concomitantly CREB. This signaling cascade shapes an anti-inflammatory MΦ phenotype by ARG II induction.

S1P activates store-operated calcium entry via receptor- and non-receptor-mediated pathways in vascular smooth muscle cells

Sphingosine-1-phosphate (S1P) has been shown to modulate intracellular Ca(2+) through both G protein-coupled receptors and intracellular second messenger pathways. The precise mechanism by which S1P activates store-operated calcium entry (SOCE) in vascular smooth muscle cells (VSMCs) has not been fully characterized. Because sphingolipids and Ca(2+) modulate proliferation and constriction in VSMCs, characterizing the connection between S1P and SOCE may provide novel therapeutic targets for vascular diseases. We found that S1P triggered STIM1 puncta formation and SOCE in VSMCs. S1P-activated SOCE was inhibited by 2-aminoethoxydiphenyl borate (2-APB), diethylstilbestrol (DES), and gadolinium (Gd(3+)). SOCE was observed in VSMCs lacking either S1P(2) or S1P(3) receptors, suggesting that S1P acts via multiple signaling pathways. Indeed, both extracellular and intracellular S1P application increased the total internal reflection fluorescence signal in VSMCs cells transfected with STIM1-yellow fluorescent protein in a 2-APB-sensitive manner. These data, and the fact that 2-APB, DES, and Gd(3+) all inhibited S1P-induced cerebral artery constriction, suggest that SOCE modulates S1P-induced vasoconstriction in vivo. Finally, S1P-induced SOCE was larger in proliferative than in contractile VSMCs, correlating with increases in STIM1, Orai1, S1P(1), and S1P(3) receptor mRNA. These data demonstrate that S1P can act through both receptors and a novel intracellular pathway to activate SOCE. Because S1P-induced SOCE contributes to vessel constriction and is increased in proliferative VSMCs, it is likely that S1P/SOCE signaling in proliferative VSMCs may play a role in vascular dysfunction such as atherosclerosis and diabetes.

Intravenous sphingosylphosphorylcholine protects ischemic and postischemic myocardial tissue in a mouse model of myocardial ischemia/reperfusion injury

HDL, through sphingosine-1-phosphate (S1P), exerts direct cardioprotective effects on ischemic myocardium. It remains unclear whether other HDL-associated sphingophospholipids have similar effects. We therefore examined if HDL-associated sphingosylphosphorylcholine (SPC) reduces infarct size in a mouse model of transient myocardial ischemia/reperfusion. Intravenously administered SPC dose-dependently reduced infarct size after 30 minutes of myocardial ischemia and 24 hours reperfusion compared to controls. Infarct size was also reduced by postischemic, therapeutical administration of SPC. Immunohistochemistry revealed reduced polymorphonuclear neutrophil recruitment to the infarcted area after SPC treatment, and apoptosis was attenuated as measured by TUNEL. In vitro, SPC inhibited leukocyte adhesion to TNFα-activated endothelial cells and protected rat neonatal cardiomyocytes from apoptosis. S1P₃ was identified as the lysophospholipid receptor mediating the cardioprotection by SPC, since its effect was completely absent in S1P₃-deficient mice. We conclude that HDL-associated SPC directly protects against myocardial reperfusion injury in vivo via the S1P₃ receptor.

Sphingosylphosphorylcholine is a proinflammatory mediator in cerebral arteries

Inflammation has an important function in the development of cerebral vasospasm after subarachnoid hemorrhage (SAH); however, the mediators of this inflammatory response have not been clearly identified. In this study, we have investigated the potential function of two sphingolipids, which occur naturally in plasma and serum, sphingosylphosphorylcholine (SPC) and sphingosine 1-phosphate (S1P), to act as proinflammatory mediators in cerebral artery vascular smooth muscle (VSM) cells. In rat cerebral arteries, SPC but not S1P activated p38 mitogen-activated protein kinase (MAPK). Using transcription factor arrays, two proinflammatory transcription factors activated by SPC in cerebral arteries were identified--nuclear factor-κB and CCAAT-enhancer-binding protein. Both these transcription factors were activated by SPC in a p38MAPK-dependent manner. To determine whether this contributed to vascular inflammation, an inflammatory protein array was performed, which showed that SPC increased release of the chemokine monocyte chemoattractant protein-1 (MCP-1) in cultured rat VSM cells. This increase in MCP-1 expression was confirmed in cerebral arteries. The S1P did not increase MCP-1 release. Taken together, our results suggest that SPC, but not S1P, can act as a proinflammatory mediator in cerebral arteries. This may contribute to inflammation observed after SAH and may be part of the initiating event in vasospasm.

Sphingosine-1-phosphate signaling in vasculogenesis and angiogenesis

Blood vessels either form de novo through the process of vasculogenesis or through angiogenesis that involves the sprouting and proliferation of endothelial cells in pre-existing blood vessels. A complex interactive network of signaling cascades downstream from at least three of the nine known G-protein-coupled sphingosine-1-phosphate (S1P) receptors act as a prime effector of neovascularization that occurs in embryonic development and in association with various pathologies. This review focuses on the current knowledge of the roles of S1P signaling in vasculogenesis and angiogenesis, with particular emphasis on vascular cell adhesion and motility responses.

Development of a sphingosylphosphorylcholine detection system using RNA aptamers

Sphingosylphosphorylcholine (SPC) is a lysosphingolipid that exerts multiple functions, including acting as a spasmogen, as a mitogenic factor for various types of cells, and sometimes as an inflammatory mediator. Currently, liquid chromatography/tandem mass spectrometry (LC/MS/MS) is used for the quantitation of SPC. However, because of the complicated procedures required it may not be cost effective, hampering its regular usage in a routine practical SPC monitoring. In this report, we have generated RNA aptamers that bind to SPC with high affinity using an in vitro selection procedure and developed an enzyme-linked aptamer assay system using the minimized SPC aptamer that can successfully distinguish SPC from the structurally related sphingosine 1-phosphate (S1P). This is the first case of the Systematic Evolution of Ligands by EXponential enrichment (SELEX) process being performed with a lysosphingolipid. The SPC aptamers would be valuable tools for the development of aptamer-based medical diagnosis and for elucidating the biological role of SPC.

Sphingolipids in inflammation: pathological implications and potential therapeutic targets

Sphingolipids are formed via the metabolism of sphingomyelin, a constituent of the plasma membrane, or by de novo synthesis. Enzymatic pathways result in the formation of several different lipid mediators, which are known to have important roles in many cellular processes, including proliferation, apoptosis and migration. Several studies now suggest that these sphingolipid mediators, including ceramide, ceramide 1-phosphate and sphingosine 1-phosphate (S1P), are likely to have an integral role in inflammation. This can involve, for example, activation of pro-inflammatory transcription factors in different cell types and induction of cyclooxygenase-2, leading to production of pro-inflammatory prostaglandins. The mode of action of each sphingolipid is different. Increased ceramide production leads to the formation of ceramide-rich areas of the membrane, which may assemble signalling complexes, whereas S1P acts via high-affinity G-protein-coupled S1P receptors on the plasma membrane. Recent studies have demonstrated that in vitro effects of sphingolipids on inflammation can translate into in vivo models. This review will highlight the areas of research where sphingolipids are involved in inflammation and the mechanisms of action of each mediator. In addition, the therapeutic potential of drugs that alter sphingolipid actions will be examined with reference to disease states, such as asthma and inflammatory bowel disease, which involve important inflammatory components. A significant body of research now indicates that sphingolipids are intimately involved in the inflammatory process and recent studies have demonstrated that these lipids, together with associated enzymes and receptors, can provide effective drug targets for the treatment of pathological inflammation.

Sphingosine kinase as an oncogene: autocrine sphingosine 1-phosphate modulates ML-1 thyroid carcinoma cell migration by a mechanism dependent on protein kinase C-alpha and ERK1/2

Sphingosine 1-phosphate (S1P) induces migration of the human thyroid follicular carcinoma cell line ML-1 by activation of S1P(1) and S1P(3) receptors, G(i) proteins, and the phosphatidylinositol 3-kinase-Akt pathway. Because sphingosine kinase isoform 1 (SK) recently has been implicated as an oncogene in various cancer cell systems, we investigated the functions of SK in the migration, proliferation and adhesion of the ML-1 cell line. SK overexpressing ML-1 cells show an enhanced secretion of S1P, which can be attenuated, by inhibiting SK activity and a multidrug-resistant transport protein (ATP-binding cassette transporter). Furthermore, overexpression of SK enhances serum-induced migration of ML-1 cells, which can be attenuated by blocking ATP-binding cassette transporter and SK, suggesting that the migration is mediated by autocrine signaling through secretion of S1P. Inhibition of protein kinase C alpha, with both small interfering RNA (siRNA) and small molecular inhibitors attenuates migration in SK overexpressing cells. In addition, SK-overexpressing cells show an impaired adhesion, slower cell growth, and an up-regulation of ERK1/2 phosphorylation, as compared with cells expressing a dominant-negative SK. Taken together, we present evidence suggesting that SK enhances migration of ML-1 cells by an autocrine mechanism and that the S1P-evoked migration is dependent on protein kinase C alpha, ERK1/2, and SK.

Nonkinase activity of MLCK in elongated filopodia formation and chemotaxis of vascular smooth muscle cells toward sphingosylphosphorylcholine

The actin-myosin interaction of vascular smooth muscle cells (VSMCs) is regulated by myosin light chain kinase (MLCK), which is a fusion protein of the central catalytic domain with the N-terminal actin-binding and C-terminal myosin-binding domains. In addition to the regulatory role of kinase activity mediated by the catalytic domain, nonkinase activity that derives from both terminals is able to exert a regulatory role as reviewed by Nakamura et al. (32). We previously showed that nonkinase activity mediated the filopodia upon the stimulation by sphingosylphosphorylcholine (SPC) (25). To explore the regulatory role of nonkinase activity in chemotaxis, we constructed VSMCs where the expression of MLCK was totally abolished by using a lentivirus-mediated RNAi system. We hypothesized that the MLCK-downregulated VSMCs were unable to form filopodia and to migrate upon SPC stimulation and confirmed the hypothesis. We further constructed a kinase-inactive mutant from bovine cDNA coding wild-type (WT) MLCK by mutating the ATP-binding sites located in the catalytic domain, followed by confirming the presence (absence) of the kinase activity of WT (kinase-inactive mutant). We transfected WT and the mutant into MLCK-downregulated VSMCs. We expected that the transfected VSMCs will recover the ability to induce filopodia and chemotaxis toward SPC and found both constructs rescued the ability. Because they share the actin- and myosin-binding domains, we concluded nonkinase activity plays a major role for SPC-induced migration.

Human neural progenitors express functional lysophospholipid receptors that regulate cell growth and morphology

Background

Lysophospholipids regulate the morphology and growth of neurons, neural cell lines, and neural progenitors. A stable human neural progenitor cell line is not currently available in which to study the role of lysophospholipids in human neural development. We recently established a stable, adherent human embryonic stem cell-derived neuroepithelial (hES-NEP) cell line which recapitulates morphological and phenotypic features of neural progenitor cells isolated from fetal tissue. The goal of this study was to determine if hES-NEP cells express functional lysophospholipid receptors, and if activation of these receptors mediates cellular responses critical for neural development.

Results

Our results demonstrate that Lysophosphatidic Acid (LPA) and Sphingosine-1-phosphate (S1P) receptors are functionally expressed in hES-NEP cells and are coupled to multiple cellular signaling pathways. We have shown that transcript levels for S1P1 receptor increased significantly in the transition from embryonic stem cell to hES-NEP. hES-NEP cells express LPA and S1P receptors coupled to G_i/o G-proteins that inhibit adenylyl cyclase and to G_q-like phospholipase C activity. LPA and S1P also induce p44/42 ERK MAP kinase phosphorylation in these cells and stimulate cell proliferation via G_i/o coupled receptors in an Epidermal Growth Factor Receptor (EGFR)- and ERK-dependent pathway. In contrast, LPA and S1P stimulate transient cell rounding and aggregation that is independent of EGFR and ERK, but dependent on the Rho effector p160 ROCK.

Conclusion

Thus, lysophospholipids regulate neural progenitor growth and morphology through distinct mechanisms. These findings establish human ES cell-derived NEP cells as a model system for studying the role of lysophospholipids in neural progenitors.

Membranes: a meeting point for lipids, proteins and therapies

Membranes constitute a meeting point for lipids and proteins. Not only do they define the entity of cells and cytosolic organelles but they also display a wide variety of important functions previously ascribed to the activity of proteins alone. Indeed, lipids have commonly been considered a mere support for the transient or permanent association of membrane proteins, while acting as a selective cell/organelle barrier. However, mounting evidence demonstrates that lipids themselves regulate the location and activity of many membrane proteins, as well as defining membrane microdomains that serve as spatio-temporal platforms for interacting signalling proteins. Membrane lipids are crucial in the fission and fusion of lipid bilayers and they also act as sensors to control environmental or physiological conditions. Lipids and lipid structures participate directly as messengers or regulators of signal transduction. Moreover, their alteration has been associated with the development of numerous diseases. Proteins can interact with membranes through lipid co-/post-translational modifications, and electrostatic and hydrophobic interactions, van der Waals forces and hydrogen bonding are all involved in the associations among membrane proteins and lipids. The present study reviews these interactions from the molecular and biomedical point of view, and the effects of their modulation on the physiological activity of cells, the aetiology of human diseases and the design of clinical drugs. In fact, the influence of lipids on protein function is reflected in the possibility to use these molecular species as targets for therapies against cancer, obesity, neurodegenerative disorders, cardiovascular pathologies and other diseases, using a new approach called membrane-lipid therapy.

The potential roles of sphingolipids in vascular smooth-muscle function

Increasing experimental evidence has demonstrated that sphingolipids are likely to have an important regulatory function in the cardiovascular system. Two sphingolipids released from activated platelets, and therefore of particular relevance, are S1P (sphingosine 1-phosphate) and SPC (sphingosylphosphocholine). Both S1P and SPC can act as vasoconstrictors and may modulate VSMC (vascular smooth muscle cell) phenotype, as observed during the pathogenesis of vascular disease. Recent research has suggested that SPC may act as a pro-inflammatory mediator in VSMCs and, in some circumstances, may also contribute to the development of vascular disease.

Vascular effects of sphingolipids

The sphingomyelin metabolites ceramide, sphingosine 1-phosphate (S1P) and sphingosylphosphorylcholine (SPC) are emerging modulators of vascular tone. While ceramide appears to act primarily intracellularly, S1P and SPC appear to mainly work via specific receptors, although those for SPC have not yet been defined unequivocally. Each of the sphingomyelin metabolites can induce both vasoconstriction and vasodilatation and, in some cases--ceramide on the one hand, and S1P and SPC on the other hand--have opposite effects on vascular tone. The differences in effects between vessels may relate to the relative roles of endothelial and smooth muscle cells in mediating them, as well as to the distinct expression patterns of S1P receptors among vascular beds and among endothelial and smooth muscle cells. Recent evidence suggests that vascular tone is not only modulated by sphingomyelin metabolites which are exogenously added or reach the vessel wall via the bloodstream but also by those formed locally by cells in the vessel wall. Such local formation can be induced by known vasoactive agents such as angiotensin II and may serve a signalling function.

CONCLUSION

We conclude that sphingomyelin metabolites are important endogenous modulators of vascular function, which may contribute to the pathophysiology of some diseases and be targets for therapeutic interventions.

Ceramides and other bioactive sphingolipid backbones in health and disease: lipidomic analysis, metabolism and roles in membrane structure, dynamics, signaling and autophagy

Sphingolipids are comprised of a backbone sphingoid base that may be phosphorylated, acylated, glycosylated, bridged to various headgroups through phosphodiester linkages, or otherwise modified. Organisms usually contain large numbers of sphingolipid subspecies and knowledge about the types and amounts is imperative because they influence membrane structure, interactions with the extracellular matrix and neighboring cells, vesicular traffic and the formation of specialized structures such as phagosomes and autophagosomes, as well as participate in intracellular and extracellular signaling. Fortunately, "sphingolipidomic" analysis is becoming feasible (at least for important subsets such as all of the backbone "signaling" subspecies: ceramides, ceramide 1-phosphates, sphingoid bases, sphingoid base 1-phosphates, inter alia) using mass spectrometry, and these profiles are revealing many surprises, such as that under certain conditions cells contain significant amounts of "unusual" species: N-mono-, di-, and tri-methyl-sphingoid bases (including N,N-dimethylsphingosine); 3-ketodihydroceramides; N-acetyl-sphingoid bases (C2-ceramides); and dihydroceramides, in the latter case, in very high proportions when cells are treated with the anticancer drug fenretinide (4-hydroxyphenylretinamide). The elevation of DHceramides by fenretinide is befuddling because the 4,5-trans-double bond of ceramide has been thought to be required for biological activity; however, DHceramides induce autophagy and may be important in the regulation of this important cellular process. The complexity of the sphingolipidome is hard to imagine, but one hopes that, when partnered with other systems biology approaches, the causes and consequences of the complexity will explain how these intriguing compounds are involved in almost every aspect of cell behavior and the malfunctions of many diseases.

Sphingosine-1-phosphate and sphingosylphosphorylcholine: two of a kind?

Sphingosine-1-phosphate and sphingosylphosphorylcholine are structurally related signalling molecules. Although they share some biological effects, it is debated whether this involves the same receptors. In this issue, Mathieson and Nixon report that these two lipids activate the same transcription factor but do so via distinct signalling pathways. Against this background, we discuss some of the potential pitfalls in studies comparing the effects of the two sphingolipids.

Signal transduction underlying the vascular effects of sphingosine 1-phosphate and sphingosylphosphorylcholine

Two related lysosphingolipids, sphingosine 1-phosphate (S1P) and sphingosylphosphorylcholine (SPC) mediate diverse cellular responses through signals transduced by either activation of G-protein coupled receptors or possibly by acting intracellularly. Vascular responses to S1P and SPC measured both in vivo and in dissected vessels show predominantly vasoconstriction with some evidence for vasodilation. Although stimulation with S1P or SPC generally leads to similar vascular responses, the signalling pathways stimulated to produce these responses are often distinct. Nevertheless, mobilization of Ca2+ from intracellular stores and influx of extracellular Ca2+, which both increase [Ca2+]i, occur in response to S1P and SPC. Both mobilization of Ca2+ from intracellular stores and influx of extracellular Ca2+ occur in response to S1P and SPC. As well, both S1P and SPC induce Ca2+-sensitization in vascular smooth muscle which is mediated through Rho kinase activation. In the endothelium, S1P and SPC stimulate the production of the vasodilator, nitric oxide through activation of endothelial nitric oxide synthase. This activation occurs through phosphorylation by Akt and through binding of Ca2+-calmodulin upon increased [Ca2+]i. These lysosphingolipids also activate cyclooxygenase-2 which produces prostaglandins with both vasoconstrictor and vasodilator properties. A balance between the signals inducing vasodilation versus the signals inducing vasoconstriction will determine the vascular outcome. Thus, perturbations in S1P and SPC concentrations, relative expression of receptors or downstream signalling pathways may provide a mechanism for pathophysiological conditions such as hypertension. Given this background, recent studies examining a potential role for S1P and SPC in hypertension and vascular dysfunction in aging are discussed.

Indomethacin differentiates the renal effects of sphingosine-1-phosphate and sphingosylphosphorylcholine

The sphingomyelin breakdown products sphingosine-1-phosphate (S1P) and sphingosylphosphorylcholine (SPC) constrict intrarenal microvessels in vitro in a pertussis toxin (PTX) sensitive manner, and S1P also reduces renal blood flow in vivo. Nevertheless, both S1P and SPC have been reported to enhance diuresis and natriuresis. This pattern is similar to that of neuropeptide Y, which also reduces renal blood flow and enhances diuresis and natriuresis. The latter effects are inhibited by the cyclooxygenase inhibitor indomethacin, and various S1P and SPC responses have also been linked to the cyclooxygenase pathway. Therefore, we have investigated whether indomethacin can alter the renal effects of S1P and SPC in anaesthetised rats in vivo. In line with earlier experiments S1P bolus injections dose-dependently reduced renal blood flow (by up to 4.8 +/- 0.5 ml min(-1)), and this was not significantly affected by indomethacin treatment (5 mg kg(-1) i.p.). Infusion of S1P but not of SPC (30 microg kg(-1) min(-1) each) for 60 min reduced renal blood flow by up to 0.8 +/- 0.2 ml min(-1), and this was not markedly altered by indomethacin. Despite the differential renovascular effect, both S1P and SPC enhanced diuresis by up to 215 +/- 65 and 201 +/- 58 microl 15 min(-1) respectively, and natriuresis by up to 25 +/- 9 and 29 +/- 11 micromol 15 min(-1) respectively. While indomethacin abolished the SPC-induced diuresis and natriuresis, it, if anything, slightly enhanced the diuretic and natriuretic effect of S1P. To determine whether tubular SPC effects are receptor-mediated, PTX experiments were performed. SPC-induced enhancements of diuresis and natriuresis were abolished by PTX. We conclude that S1P, SPC and neuropeptide Y exhibit distinct patterns of modulation of renal function and that indomethacin allows such effects to be differentiated.