Receptor-mediated activation of phospholipase D by sphingosine 1-phosphate in skeletal muscle C2C12 cells. A role for protein kinase C

Irisin Is Target of Sphingosine-1-Phosphate/Sphingosine-1-Phosphate Receptor-Mediated Signaling in Skeletal Muscle Cells

Irisin is a hormone-like myokine produced in abundance by skeletal muscle (SkM) in response to exercise. This myokine, identical in humans and mice, is involved in many signaling pathways related to metabolic processes. Despite much evidence on the regulators of irisin and the relevance of sphingolipids for SkM cell biology, the contribution of these latter bioactive lipids to the modulation of the myokine in SkM is missing. In particular, we have examined the potential involvement in irisin formation/release of sphingosine-1-phosphate (S1P), an interesting bioactive molecule able to act as an intracellular lipid mediator as well as a ligand of specific G-protein-coupled receptors (S1PR). We demonstrate the existence of distinct intracellular pools of S1P able to affect the expression of the irisin precursor FNDC. In addition, we establish the crucial role of the S1P/S1PR axis in irisin formation/release as well as the autocrine/paracrine effects of irisin on myoblast proliferation and myogenic differentiation. Altogether, these findings provide the first evidence for a functional crosstalk between the S1P/S1PR axis and irisin signaling, which may open new windows for potential therapeutic treatment of SkM dysfunctions.

S1P/S1P Receptor Signaling in Neuromuscolar Disorders

The bioactive sphingolipid metabolite, sphingosine 1-phosphate (S1P), and the signaling pathways triggered by its binding to specific G protein-coupled receptors play a critical regulatory role in many pathophysiological processes, including skeletal muscle and nervous system degeneration. The signaling transduced by S1P binding appears to be much more complex than previously thought, with important implications for clinical applications and for personalized medicine. In particular, the understanding of S1P/S1P receptor signaling functions in specific compartmentalized locations of the cell is worthy of being better investigated, because in various circumstances it might be crucial for the development or/and the progression of neuromuscular diseases, such as Charcot-Marie-Tooth disease, myasthenia gravis, and Duchenne muscular dystrophy.

Involvement of released sphingosine 1-phosphate/sphingosine 1-phosphate receptor axis in skeletal muscle atrophy

Skeletal muscle (SkM) atrophy is caused by several and heterogeneous conditions, such as cancer, neuromuscular disorders and aging. In most types of SkM atrophy overall rates of protein synthesis are suppressed, protein degradation is consistently elevated and atrogenes, such as the ubiquitin ligase Atrogin-1/MAFbx, are up-regulated. The molecular regulators of SkM waste are multiple and only in part known. Sphingolipids represent a class of bioactive molecules capable of modulating the destiny of many cell types, including SkM cells. In particular, we and others have shown that sphingosine 1phosphate (S1P), formed by sphingosine kinase (SphK), is able to act as trophic and morphogenic factor in myoblasts. Here, we report the first evidence that the atrophic phenotype observed in both muscle obtained from mice bearing the C26 adenocarcinoma and C2C12 myotubes treated with dexamethasone was characterized by reduced levels of active phospho-SphK1. The importance of SphK1 activity is also confirmed by the specific pharmacological inhibition of SphK1 able to increase Atrogin-1/MAFbx expression and reduce myotube size and myonuclei number. Furthermore, we found that SkM atrophy was accomplished by significant increase of S1P transporter Spns2 and in changes in the pattern of S1P receptor (S1PRs) subtype expression paralleled by increased Atrogin-1/MAFbx expression, suggesting a role for the released S1P and of specific S1PR-mediated signaling pathways in the control of the ubiquitin ligase. Altogether, these findings provide the first evidence that SphK1/released S1P/S1PR axis acts as a molecular regulator of SkM atrophy, thereby representing a new possible target for therapy in many patho-physiological conditions.

Lysophosphatidic Acid Signaling Axis Mediates Ceramide 1-Phosphate-Induced Proliferation of C2C12 Myoblasts

Sphingolipids are not only crucial for membrane architecture but act as critical regulators of cell functions. The bioactive sphingolipid ceramide 1-phosphate (C1P), generated by the action of ceramide kinase, has been reported to stimulate cell proliferation, cell migration and to regulate inflammatory responses via activation of different signaling pathways. We have previously shown that skeletal muscle is a tissue target for C1P since the phosphosphingolipid plays a positive role in myoblast proliferation implying a role in muscle regeneration. Skeletal muscle displays strong capacity of regeneration thanks to the presence of quiescent adult stem cells called satellite cells that upon trauma enter into the cell cycle and start proliferating. However, at present, the exact molecular mechanism by which C1P triggers its mitogenic effect in myoblasts is lacking. Here, we report for the first time that C1P stimulates C2C12 myoblast proliferation via lysophosphatidic acid (LPA) signaling axis. Indeed, C1P subsequently to phospholipase A2 activation leads to LPA₁ and LPA₃ engagement, which in turn drive Akt (protein kinase B) and ERK1/2 (extracellular signal-regulated kinases 1/2) activation, thus stimulating DNA synthesis. The present findings shed new light on the key role of bioactive sphingolipids in skeletal muscle and provide further support to the notion that these pleiotropic molecules might be useful therapeutic targets for skeletal muscle regeneration.

Phospholipase D signaling pathways and phosphatidic acid as therapeutic targets in cancer

Phospholipase D is a ubiquitous class of enzymes that generates phosphatidic acid as an intracellular signaling species. The phospholipase D superfamily plays a central role in a variety of functions in prokaryotes, viruses, yeast, fungi, plants, and eukaryotic species. In mammalian cells, the pathways modulating catalytic activity involve a variety of cellular signaling components, including G protein-coupled receptors, receptor tyrosine kinases, polyphosphatidylinositol lipids, Ras/Rho/ADP-ribosylation factor GTPases, and conventional isoforms of protein kinase C, among others. Recent findings have shown that phosphatidic acid generated by phospholipase D plays roles in numerous essential cellular functions, such as vesicular trafficking, exocytosis, autophagy, regulation of cellular metabolism, and tumorigenesis. Many of these cellular events are modulated by the actions of phosphatidic acid, and identification of two targets (mammalian target of rapamycin and Akt kinase) has especially highlighted a role for phospholipase D in the regulation of cellular metabolism. Phospholipase D is a regulator of intercellular signaling and metabolic pathways, particularly in cells that are under stress conditions. This review provides a comprehensive overview of the regulation of phospholipase D activity and its modulation of cellular signaling pathways and functions.

Sphingosine 1-phosphate axis: a new leader actor in skeletal muscle biology

Sphingosine 1-phosphate (S1P) is a bioactive lipid involved in the regulation of biological processes such as proliferation, differentiation, motility, and survival. Here we review the role of S1P in the biology and homeostasis of skeletal muscle. S1P derives from the catabolism of sphingomyelin and is produced by sphingosine phosphorylation catalyzed by sphingosine kinase (SK). S1P can act either intracellularly or extracellularly through specific ligation to its five G protein-coupled receptors (GPCR) named S1P receptors (S1PR). Many experimental findings obtained in the last 20 years demonstrate that S1P and its metabolism play a multifaceted role in the regulation of skeletal muscle regeneration. Indeed, this lipid is known to activate muscle-resident satellite cells, regulating their proliferation and differentiation, as well as mesenchymal progenitors such as mesoangioblasts that originate outside skeletal muscle, both involved in tissue repair following an injury or disease. The molecular mechanism of action of S1P in skeletal muscle cell precursors is highly complex, especially because S1P axis is under the control of a number of growth factors and cytokines, canonical regulators of skeletal muscle biology. Moreover, this lipid is crucially involved in the regulation of skeletal muscle contractile properties, responsiveness to insulin, fatigue resistance and tropism. Overall, on the basis of these findings S1P signaling appears to be an appealing pharmacological target for improving skeletal muscle repair. Nevertheless, further understanding is required on the regulation of S1P downstream signaling pathways and the expression of S1PR. This article will resume our current knowledge on S1P signaling in skeletal muscle, hopefully stimulating further investigation in the field, aimed at individuating novel molecular targets for ameliorating skeletal muscle regeneration and reducing fibrosis of the tissue after a trauma or due to skeletal muscle diseases.

Sphingosine-1-phosphate receptor 3 influences cell cycle progression in muscle satellite cells

Skeletal muscle retains a resident stem cell population called satellite cells, which are mitotically quiescent in mature muscle, but can be activated to produce myoblast progeny for muscle homeostasis, hypertrophy and repair. We have previously shown that satellite cell activation is partially controlled by the bioactive phospholipid, sphingosine-1-phosphate, and that S1P biosynthesis is required for muscle regeneration. Here we investigate the role of sphingosine-1-phosphate receptor 3 (S1PR3) in regulating murine satellite cell function. S1PR3 levels were high in quiescent myogenic cells before falling during entry into cell cycle. Retrovirally-mediated constitutive expression of S1PR3 led to suppressed cell cycle progression in satellite cells, but did not overtly affect the myogenic program. Conversely, satellite cells isolated from S1PR3-null mice exhibited enhanced proliferation ex-vivo. In vivo, acute cardiotoxin-induced muscle regeneration was enhanced in S1PR3-null mice, with bigger muscle fibres compared to control mice. Importantly, genetically deleting S1PR3 in the mdx mouse model of Duchenne muscular dystrophy produced a less severe muscle dystrophic phenotype, than when signalling though S1PR3 was operational. In conclusion, signalling though S1PR3 suppresses cell cycle progression to regulate function in muscle satellite cells.

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Expression of S1PR3 is associated with non-cycling myoblasts.

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Constitutive expression of S1PR3 leads to reduced cell proliferation.

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Satellite cells lacking S1PR3 have enhanced proliferation.

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Muscle regeneration is improved in the absence of S1PR3.

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The dystrophic phenotype in mdx mice is improved by the absence of S1PR3.

Role of sphingosine 1-phosphate in skeletal muscle cell biology

Studies performed in the last fifteen years have clearly established that the bioactive sphingolipid sphingosine 1-phosphate (S1P) affects various different biological properties of myogenic precursor cells as well as physiological features of adult skeletal muscle. Noticeably, in myogenic precursor cells multiple growth factors and cytokines cross-communicate with S1P axis and the engagement of distinct S1P receptor subtypes appears to be crucially implicated in transmitting specific biological effects. This paper summarizes current research findings and discloses the potential for new therapeutics designed to alter S1P signaling with the aim of improving skeletal muscle repair.

S1P lyase in skeletal muscle regeneration and satellite cell activation: exposing the hidden lyase

Sphingosine-1-phosphate (S1P) is a bioactive sphingolipid whose actions are essential for many physiological processes including angiogenesis, lymphocyte trafficking and development. In addition, S1P serves as a muscle trophic factor that enables efficient muscle regeneration. This is due in part to S1P's ability to activate quiescent muscle stem cells called satellite cells (SCs) that are needed for muscle repair. However, the molecular mechanism by which S1P activates SCs has not been well understood. Further, strategies for harnessing S1P signaling to recruit SCs for therapeutic benefit have been lacking. S1P is irreversibly catabolized by S1P lyase (SPL), a highly conserved enzyme that catalyzes the cleavage of S1P at carbon bond C(2-3), resulting in formation of hexadecenal and ethanolamine-phosphate. SPL enhances apoptosis through substrate- and product-dependent events, thereby regulating cellular responses to chemotherapy, radiation and ischemia. SPL is undetectable in resting murine skeletal muscle. However, we recently found that SPL is dynamically upregulated in skeletal muscle after injury. SPL upregulation occurred in the context of a tightly orchestrated genetic program that resulted in a transient S1P signal in response to muscle injury. S1P activated quiescent SCs via a sphingosine-1-phosphate receptor 2 (S1P2)/signal transducer and activator of transcription 3 (STAT3)-dependent pathway, thereby facilitating skeletal muscle regeneration. Mdx mice, which serve as a model for muscular dystrophy (MD), exhibited skeletal muscle SPL upregulation and S1P deficiency. Pharmacological SPL inhibition raised skeletal muscle S1P levels, enhanced SC recruitment and improved mdx skeletal muscle regeneration. These findings reveal how S1P can activate SCs and indicate that SPL suppression may provide a therapeutic strategy for myopathies. This article is part of a Special Issue entitled Advances in Lysophospholipid Research.

G-protein coupled receptors in stem cell self-renewal and differentiation

Stem cells have great potential for understanding early development, treating human disease, tissue trauma and early phase drug discovery. The factors that control the regulation of stem cell survival, proliferation, migration and differentiation are still emerging. Some evidence now exists demonstrating the potent effects of various G-protein coupled receptor (GPCR) ligands on the biology of stem cells. This review aims to give an overview of the current knowledge of the regulation of embryonic and somatic stem cell maintenance and differentiation by GPCR ligands.

Sphingosine kinase-1/S1P1 signalling axis negatively regulates mitogenic response elicited by PDGF in mouse myoblasts

PDGF is known to be critically implicated in skeletal muscle repair; however its molecular mechanism of action has been only marginally investigated. In this study we show that in mouse myoblasts PDGF transactivates S1P(1) receptor via sphingosine kinase (SK)-1 activation and that this molecular event exerts a negative regulation of the mitogenic effect elicited by this growth factor. Indeed, pharmacological inhibition of S1P(1), or its specific silencing increased PDGF-dependent cell proliferation, whereas S1P(1) overexpression diminished the biological effect. Moreover, the mitogenic response to PDGF was enhanced by pharmacological inhibition of SK activity as well as specific silencing of SK1 but not SK2. Furthermore, ERK1/2 signalling pathway was found to be upstream of the observed attenuation of PDGF-induced cell proliferation. Interestingly, PDGF-directed engagement of S1P(1) exerted also a positive modulatory action of the growth factor-dependent cell motility. The here highlighted dual role of S1P(1)-mediated signalling in response to myoblast challenge with PDGF is likely important to guarantee the fine control of the biological response to this growth factor, finalized to efficient repopulation of skeletal muscle after damage, where a tight balance between proliferation and migration of tissue progenitor cells is required.

Sphingosine 1-phosphate signaling is involved in skeletal muscle regeneration

Sphingosine 1-phosphate (S1P) is a bioactive lipid known to control cell growth that was recently shown to act as a trophic factor for skeletal muscle, reducing the progress of denervation atrophy. The aim of this work was to investigate whether S1P is involved in skeletal muscle fiber recovery (regeneration) after myotoxic injury induced by bupivacaine. The postnatal ability of skeletal muscle to grow and regenerate is dependent on resident stem cells called satellite cells. Immunofluorescence analysis demonstrated that S1P-specific receptors S1P(1) and S1P(3) are expressed by quiescent satellite cells. Soleus muscles undergoing regeneration following injury induced by intramuscular injection of bupivacaine exhibited enhanced expression of S1P(1) receptor, while S1P(3) expression progressively decreased to adult levels. S1P(2) receptor was absent in quiescent cells but was transiently expressed in the early regenerating phases only. Administration of S1P (50 microM) at the moment of myotoxic injury caused a significant increase of the mean cross-sectional area of regenerating fibers in both rat and mouse. In separate experiments designed to test the trophic effects of S1P, neutralization of endogenous circulating S1P by intraperitoneal administration of anti-S1P antibody attenuated fiber growth. Use of selective modulators of S1P receptors indicated that S1P(1) receptor negatively and S1P(3) receptor positively modulate the early phases of regeneration, whereas S1P(2) receptor appears to be less important. The present results show that S1P signaling participates in the regenerative processes of skeletal muscle.

Sphingosine 1-phosphate increases glucose uptake through trans-activation of insulin receptor

Sphingosine 1-phosphate (S1P) is a bioactive lipid that acts through a family of G-protein-coupled receptors. Herein, we report evidence of a novel redox-based cross-talk between S1P and insulin signaling pathways. In skeletal muscle cells S1P, through engagement of its S1P(2) receptor, is found to produce a transient burst of reactive oxygen species through a calcium-dependent activation of the small GTPase Rac1. S1P-induced redox-signaling is sensed by protein tyrosine phosphatase-1B, the main negative regulator of insulin receptor phosphorylation, which undergoes oxidation and enzymatic inhibition. This redox-based inhibition of the phosphatase provokes a ligand-independent trans-phosphorylation of insulin receptor and a strong increase in glucose uptake. Our results propose a new role of S1P, recognizing the lipid as an insulin-mimetic cue and pointing at reactive oxygen species as critical regulators of the cross-talk between S1P and insulin pathways. Any possible implication of S1P-directed insulin signaling in diabetes and insulin resistance remains to be established.

Regulation of transient receptor potential canonical channel 1 (TRPC1) by sphingosine 1-phosphate in C2C12 myoblasts and its relevance for a role of mechanotransduction in skeletal muscle differentiation

Transient receptor potential canonical (TRPC) channels provide cation and Ca2+ entry pathways, which have important regulatory roles in many physio-pathological processes, including muscle dystrophy. However, the mechanisms of activation of these channels remain poorly understood. Using siRNA, we provide the first experimental evidence that TRPC channel 1 (TRPC1), besides acting as a store-operated channel, represents an essential component of stretch-activated channels in C2C12 skeletal myoblasts, as assayed by whole-cell patch-clamp and atomic force microscopic pulling. The channel's activity and stretch-induced Ca2+ influx were modulated by sphingosine 1-phosphate (S1P), a bioactive lipid involved in satellite cell biology and tissue regeneration. We also found that TRPC1 was functionally assembled in lipid rafts, as shown by the fact that cholesterol depletion resulted in the reduction of transmembrane ion current and conductance. Association between TRPC1 and lipid rafts was increased by formation of stress fibres, which was elicited by S1P and abolished by treatment with the actin-disrupting dihydrocytochalasin B, suggesting a role for cytoskeleton in TRPC1 membrane recruitment. Moreover, TRPC1 expression was significantly upregulated during myogenesis, especially in the presence of S1P, implicating a crucial role for TRPC1 in myoblast differentiation. Collectively, these findings may offer new tools for understanding the role of TRPC1 and sphingolipid signalling in skeletal muscle regeneration and provide new therapeutic approaches for skeletal muscle disorders.

Sphingosine 1-Phosphate Mediates Proliferation and Survival of Mesoangioblasts

Mesoangioblasts are stem cells capable of differentiating in various mesodermal tissues and are presently regarded as suitable candidates for cell therapy of muscle degenerative diseases, as well as myocardial infarction. The enhancement of their proliferation and survival after injection in vivo could greatly improve their ability to repopulate damaged tissues. In this study, we show that the bioactive sphingolipid sphingosine 1-phosphate (S1P) regulates critical functions of mesoangioblast cell biology. S1P evoked a full mitogenic response in mesoangioblasts, measured by labeled thymidine incorporation and cell counting. Moreover, S1P strongly counteracted the apoptotic process triggered by stimuli as diverse as serum deprivation, C2-ceramide treatment, or staurosporine treatment, as assessed by cell counting, as well as histone-associated fragments and caspase-3 activity determinations. S1P acts both as an intracellular messenger and through specific membrane receptors. Real-time polymerase chain reaction analysis revealed that mesoangioblasts express the S1P-specific receptor S1P3 and, to a minor extent, S1P1 and S1P2. By using S1P receptor subtype-specific agonists and antagonists, we found that the proliferative response to S1P was mediated mainly by S1P2. By contrast, the antiapoptotic effect did not implicate S1P receptors. These findings demonstrate an important role of S1P in mesoangioblast proliferation and survival and indicate that targeting modulation of S1P-dependent signaling pathways may be used to improve the efficiency of muscle repair by these cells. Disclosure of potential conflicts of interest is found at the end of this article.

Sphingosine kinase activity is required for myogenic differentiation of C2C12 myoblasts

Sphingosine kinase (SphK) is a conserved lipid kinase that catalyzes the formation of sphingosine 1-phosphate (S1P), an important lipid mediator, which regulates fundamental biological processes. Here, we provide evidence that SphK is required for the achievement of cell growth arrest as well as myogenic differentiation of C2C12 myoblasts. Indeed, SphK activity, SphK1 protein content and S1P formation were found to be enhanced in myoblasts that became confluent as well as in differentiating cells. Enforced expression of SphK1 reduced the myoblast proliferation rate, enhanced the expression of myogenic differentiation markers and anticipated the onset of differentiated muscle phenotype. Conversely, down-regulation of SphK1 by specific silencing by RNA interference or overexpression of the catalytically inactive SphK1, significantly increased cell growth and delayed the beginning of myogenesis; noticeably, exogenous addition of S1P rescued the biological processes. Importantly, stimulation of myogenesis in SphK1-overexpressing myoblasts was abrogated by treatment with short interfering RNA specific for S1P(2) receptor. This is the first report of the role of endogenous SphK1 in myoblast growth arrest and stimulation of myogenesis through the formation of S1P that acts as morphogenic factor via the engagement of S1P(2).

Entry of muscle satellite cells into the cell cycle requires sphingolipid signaling

Adult skeletal muscle is able to repeatedly regenerate because of the presence of satellite cells, a population of stem cells resident beneath the basal lamina that surrounds each myofiber. Little is known, however, of the signaling pathways involved in the activation of satellite cells from quiescence to proliferation, a crucial step in muscle regeneration. We show that sphingosine-1-phosphate induces satellite cells to enter the cell cycle. Indeed, inhibiting the sphingolipid-signaling cascade that generates sphingosine-1-phosphate significantly reduces the number of satellite cells able to proliferate in response to mitogen stimulation in vitro and perturbs muscle regeneration in vivo. In addition, metabolism of sphingomyelin located in the inner leaflet of the plasma membrane is probably the main source of sphingosine-1-phosphate used to mediate the mitogenic signal. Together, our observations show that sphingolipid signaling is involved in the induction of proliferation in an adult stem cell and a key component of muscle regeneration.

Sphingosine 1-phosphate regulates cytoskeleton dynamics: implications in its biological response

The bioactive sphingolipid sphingosine 1-phosphate (S1P) elicits robust cytoskeletal rearrangement in a large variety of cell systems, mainly acting through a panel of specific cell surface receptors, named S1P receptors. Recent studies have begun to delineate the molecular mechanisms involved in the complex process responsible for cytoskeletal rearrangement following S1P ligation to its receptors. Notably, changes of cell shape and/or motility induced by S1P via cytoskeletal remodelling are functional to the biological action exerted by S1P which appears to be highly cell-specific. This review focuses on the current knowledge of the regulatory mechanisms of cytoskeleton dynamics elicited by S1P, with special emphasis on the relationship between cytoskeletal remodelling and the biological effects evoked by the sphingolipid in various cell types.

Sphingosine 1-phosphate inhibits cell migration in C2C12 myoblasts

This study shows that sphingosine 1-phosphate (S1P) exerts an anti-migratory action in C2C12 myoblasts by reducing directional cell motility and fully abrogating the chemotactic response to insulin-like growth factor-1. The anti-migratory response to S1P required ligation to S1P(2), being attenuated in myoblasts where the receptor was down-regulated by specific antisense oligodeoxyribonucleotides or small interfering RNA (siRNA) and conversely potentiated in S1P(2)-overexpressing myoblasts. The investigation of RhoA and Rac GTPases, critically implicated in cell motility regulation, demonstrated that RhoA was rapidly activated by S1P, while Rac1 was unaffected within the first 5 min but stimulated thereafter. RhoA, but not Rac activation, was identified as a S1P(2)-dependent pathway in experiments in which receptor expression was attenuated by siRNA treatment or up-regulated by S1P(2)-encoding plasmid transfection. Finally, by expression of the dominant negative mutant of RhoA, the GTPase was found implicated in the anti-migratory action of S1P, whereas modulation of Rac1 functionality unaffected the anti-chemotactic effect of S1P, ruling out a role for this protein in the biological response. Since S1P was previously shown to inhibit myoblast proliferation and stimulate myogenesis, the here identified novel biological activity is in favour of a complex physiological role of the sphingolipid in the process of muscle repair.

Peptide and non-peptide G-protein coupled receptors (GPCRs) in skeletal muscle

G-protein coupled receptors (GPCRs) represent a large class of cell surface receptors that mediate a multitude of functions. Over the years, a number of GPCRs and ancillary proteins have been shown to be expressed in skeletal muscle. Unlike the case with other muscle tissues like cardiac and vascular smooth muscle cells, there has been little attempt at systematically analyzing GPCRs in skeletal muscle. Here we have compiled all the GPCRs that are expressed in skeletal muscle. In addition, we review the known function of these receptors in both skeletal muscle tissue and in cultured skeletal muscle cells.

Sphingosine 1-phosphate induces cytoskeletal reorganization in C2C12 myoblasts: physiological relevance for stress fibres in the modulation of ion current through stretch-activated channels

Sphingosine 1-phosphate (S1P) is a bioactive lipid that is abundantly present in the serum and mediates multiple biological responses. With the aim of extending our knowledge on the role played by S1P in the regulation of cytoskeletal reorganization, native as well as C2C12 myoblasts stably transfected with green fluorescent protein (GFP)-tagged α- and β-actin constructs were stimulated with S1P (1 μM) and observed under confocal and multiphoton microscopes. The addition of S1P induced the appearance of actin stress fibres and focal adhesions through Rho- and phospholipase D (PLD)-mediated pathways. The cytoskeletal response was dependent on the extracellular action of S1P through its specific surface receptors, since the intracellular delivery of the sphingolipid by microinjection was unable to modify the actin cytoskeletal assembly. Interestingly, it was revealed by whole-cell patch-clamp that S1P-induced stress fibre formation was associated with increased ion currents and conductance through stretch-activated channels (SACs), thereby suggesting a possible regulatory role for organized actin in channel sensitivity. Experiments aimed at stretching the plasma membrane of C2C12 cells, using the cantilever of an atomic force microscope, indicated that there was a Ca2+ influx through putative SACs. In conclusion, the present data suggest novel mechanisms of S1P signalling involving actin cytoskeletal reorganization and Ca2+ elevation through SACs that might influence myoblastic functions.

Sphingosine 1-phosphate regulates myogenic differentiation: a major role for S1P2 receptor

In this study a novel biological activity of sphingosine 1-phosphate (S1P) in C2C12 myoblasts was identified. In these cells the bioactive lipid profoundly regulated myogenesis exerting an antimitogenic activity, by reducing serum-induced cell proliferation, and acting as powerful prodifferentiating agent by enhancing the expression of myogenic differentiation markers such as myogenin, myosin heavy chain, and caveolin-3. The S1P-dependent diminution of serum-induced labeled thymidine incorporation was abrogated by antisense oligodeoxyribonucleotides (ODN) to S1P2, but not to S1P1 or S1P3 receptor, also expressed in C2C12 cells, implicating S1P2 in the biological response. Using antisense ODN and short interfering RNA treatment, we highlighted the key role played by S1P2 in the S1P-dependent induction of muscle-specific gene products. Notably, S1P2 overexpression increased the content of myogenic markers and hastened the onset of differentiated muscle phenotype in comparison with control cells. Cell treatment with pertussis toxin did not affect the biological responses to S1P, ruling out the involvement of Gi-mediated events in the signaling promoted by the sphingolipid. Among the various signaling pathways activated by S1P, the activation of ERK1/ERK2 and p38 MAPK, both identified as downstream effectors of S1P2, was required for the inhibition of cell proliferation and the stimulation of myogenic differentiation, respectively.

Sphingosine kinase activity is required for sphingosine-mediated phospholipase D activation in C2C12 myoblasts

Sphingosine (Sph) has been implicated as a modulator of membrane signal transduction systems and as a regulatory element of cardiac and skeletal muscle physiology, but little information is presently available on its precise mechanism of action. Recent studies have shown that sphingosine 1-phosphate (S1P), generated by the action of sphingosine kinase (SphK) on Sph, also possesses biological activity, acting as an intracellular messenger, as well as an extracellular ligand for specific membrane receptors. At present, however, it is not clear whether the biological effects elicited by Sph are attributable to its conversion into S1P. In the present study, we show that Sph significantly stimulated phospholipase D (PLD) activity in mouse C2C12 myoblasts via a previously unrecognized mechanism that requires the conversion of Sph into S1P and its subsequent action as extracellular ligand. Indeed, Sph-induced activation of PLD was inhibited by N,N-dimethyl-D-erythro-sphingosine (DMS), at concentrations capable of specifically inhibiting SphK. Moreover, the crucial role of SphK-derived S1P in the activation of PLD by Sph was confirmed by the observed potentiated effect of Sph in myoblasts where SphK1 was overexpressed, and the attenuated response in cells transfected with the dominant negative form of SphK1. Notably, the measurement of S1P formation in vivo by employing labelled ATP revealed that cell-associated SphK activity in the extracellular compartment largely contributed to the transformation of Sph into S1P, with the amount of SphK released into the medium being negligible. It will be important to establish whether the mechanism of action identified in the present study is implicated in the multiple biological effects elicited by Sph in muscle cells.

Phospholipase D

Phospholipase D catalyses the hydrolysis of the phosphodiester bond of glycerophospholipids to generate phosphatidic acid and a free headgroup. Phospholipase D activities have been detected in simple to complex organisms from viruses and bacteria to yeast, plants, and mammals. Although enzymes with broader selectivity are found in some of the lower organisms, the plant, yeast, and mammalian enzymes are selective for phosphatidylcholine. The two mammalian phospholipase D isoforms are regulated by protein kinases and GTP binding proteins of the ADP-ribosylation and Rho families. Mammalian and yeast phospholipases D are also potently stimulated by phosphatidylinositol 4,5-bisphosphate. This review discusses the identification, characterization, structure, and regulation of phospholipase D. Genetic and pharmacological approaches implicate phospholipase D in a diverse range of cellular processes that include receptor signaling, control of intracellular membrane transport, and reorganization of the actin cytoskeleton. Most ideas about phospholipase D function consider that the phosphatidic acid product is an intracellular lipid messenger. Candidate targets for phospholipase-D-generated phosphatidic acid include phosphatidylinositol 4-phosphate 5-kinases and the raf protein kinase. Phosphatidic acid can also be converted to two other lipid mediators, diacylglycerol and lyso phosphatidic acid. Coordinated activation of these phospholipase-D-dependent pathways likely accounts for the pleitropic roles for these enzymes in many aspects of cell regulation.

Activation of phospholipase D by bradykinin and sphingosine 1-phosphate in A549 human lung adenocarcinoma cells via different GTP-binding proteins and protein kinase C delta signaling pathways

Phospholipase D (PLD) is involved in the signaling by many extracellular ligands, and its regulation appears to be quite complex. We investigated the signaling pathways initiated by bradykinin (BK) or sphingosine 1-phosphate (S1P) in A549 cells to define molecular mechanisms responsible for their additive effects on PLD activity. BK and S1P each elicited a sustained increase in phosphatidic acid content through a rapid and transient activation of PLD. The two pathways demonstrated rapid homologous downregulation, but heterologous desensitization was not observed. Action of both agonists required protein kinase C (PKC) activation and Ca(2+) influx but was mediated by different heterotrimeric G proteins. In membranes, inhibition of PKCdelta by rottlerin enhanced BK activation of PLD but inhibited that by S1P. Rottlerin inhibited activation of PLD in nuclei by both BK and S1P. By in situ immunofluorescence or cell fractionation followed by immunoblotting, PLD1 was concentrated primarily in nuclei, whereas the membrane fraction contained PLD2 and PLD1. Moreover, PKCdelta specifically phosphorylated recombinant PLD2, but not PLD1. BK and S1P similarly enhanced RhoA translocation to nuclei, whereas BK was less efficacious than S1P on RhoA relocalization to membranes. Effects of both agonists on the nuclear fraction, which contains only PLD1, are compatible with a RhoA- and PKCdelta-dependent process. In membranes, which contain both PLD1 and PLD2, the stimulatory effect of S1P on PLD activity can best be explained by RhoA- and PKCdelta-dependent activation of PLD1; in contrast, the effects of BK on RhoA translocation and enhancement of BK-stimulated PLD activity by PKC inhibition are both consistent with PLD2 serving as its primary target.

Sphingosine 1-phosphate induces cell contraction via calcium-independent/Rho-dependent pathways in undifferentiated skeletal muscle cells

We have previously shown that sphingosine 1-phosphate (S1P) can induce intracellular Ca(2+) mobilization and cell contraction in C2C12 myoblasts and that the two phenomena are temporally unrelated. Although Ca(2+)-independent mechanisms of cell contraction have been the focus of numerous studies on Ca(2+) sensitization of smooth muscle, comparatively less studies have focused on the role that these mechanisms play in the regulation of skeletal muscle contractility. Phosphorylation and activation of myosin by Rho-dependent kinase mediate most of Ca(2+)-independent contractile responses. In the present study, we examined the potential role of Rho/Rho-kinase cascade activation in S1P-induced C2C12 cell contraction. First, we showed that depletion of Ca(2+), by pre-treatment with BAPTA, did not affect S1P-induced myoblastic contractility, whereas it abolished S1P-induced Ca(2+) transients. These results correlated with the absence of troponin C and with the immature cytoskeletal organization of these cells. Experimental evidence demonstrating the involvement of Rho pathway in S1P-stimulated myoblast contraction included: the activation/translocation of RhoA to the membrane in response to agonist-stimulation in cells depleted of Ca(2+) and the inhibition of dynamic changes of the actin cytoskeleton in cells where Rho functions had been inhibited either by overexpression of RhoGDI, a physiological inhibitor of GDP dissociation from Rho proteins, or by pretreatment with Y-27632, a specific Rho kinase inhibitor. Contribution of protein kinase C in this cytoskeletal rearrangement was also evaluated. However, the pretreatment with Gö6976 or rottlerin, specific inhibitors of PKC alpha and PKC delta, respectively, failed to inhibit the agonist-induced myoblastic contraction. Single particle tracking of G-actin fluorescent probe was performed to statistically evaluate actin cytoskeletal dynamics in response to S1P. Stimulation with S1P was also able to increase the phosphorylation level of myosin light chain II. In conclusion, our results strongly suggest that Ca(2+)-independent/Rho-Rho kinase-dependent pathways may exert an important role in S1P-induced myoblastic cell contraction.

Roles of phospholipase D in apoptosis and pro-survival

Accumulating evidence has recognized phospholipase D (PLD) as an important element in signal transduction of cell responses, including proliferation and differentiation, However, its role in pro-apoptotic, anti-apoptotic or pro-survival signaling is not well-understood. Involvement of PLD in these signaling mechanisms is considered to differ depending on the cell type and the extracellular stimulus.

Involvement of phospholipases D1 and D2 in sphingosine 1-phosphate-induced ERK (extracellular-signal-regulated kinase) activation and interleukin-8 secretion in human bronchial epithelial cells

Sphingosine 1-phosphate (S1P), a metabolite of sphingomyelin degradation, stimulates interleukin-8 (IL-8) secretion in human bronchial epithelial (Beas-2B) cells. The molecular mechanisms regulating S1P-mediated IL-8 secretion are yet to be completely defined. Here we provide evidence that activation of phospholipases D1 and D2 (PLD1 and PLD2) by S1P regulates the phosphorylation of extracellular-signal-regulated kinase (ERK) and IL-8 secretion in Beas-2B cells. S1P, in a time- and dose-dependent manner, enhanced the threonine/tyrosine phosphorylation of ERK. The inhibition of S1P-induced ERK phosphorylation by pertussis toxin and PD 98059 indicated coupling of S1P receptors to G(i) and the ERK signalling cascade respectively. Treatment of Beas-2B cells with butan-1-ol, but not butan-3-ol, abrogated the S1P-induced phosphorylation of Raf-1 and ERK, suggesting that PLD is involved in this activation. The roles of PLD1 and PLD2 in ERK activation and IL-8 secretion activated by S1P were investigated by infecting cells with adenoviral constructs of wild-type and catalytically inactive mutants of PLD1 and PLD2. Infection of Beas-2B cells with the wild-type constructs resulted in the activation of PLD1 and PLD2 by S1P and PMA. Also, the enhanced production of [(32)P]phosphatidic acid and [(32)P]phosphatidylbutanol in the presence of butan-1-ol and the increased phosphorylation of ERK by S1P were blocked by the catalytically inactive mutants hPLD1-K898R and mPLD2-K758R. Transient transfection of Beas-2B cells with human PLD1 and mouse PLD2 cDNAs potentiated S1P-mediated IL-8 secretion compared with vector controls. In addition, PD 98059 attenuated IL-8 secretion induced by S1P in a dose-dependent fashion. These results demonstrate that both PLD1 and PLD2 participate in S1P stimulation of ERK phosphorylation and IL-8 secretion in bronchial epithelial cells.

Phospholipase D1 is threonine-phosphorylated in human-airway epithelial cells stimulated by sphingosine-1-phosphate by a mechanism involving Src tyrosine kinase and protein kinase Cdelta

The regulatory role of protein kinase C (PKC) delta isoform in the stimulation of phospholipase D (PLD) by sphingosine-1-phosphate (SPP) in a human-airway epithelial cell line (CFNPE9o(-)) was revealed by using antisense oligodeoxynucleotide to PKCdelta, in combination with the specific inhibitor rottlerin. Cell treatment with antisense oligodeoxynucleotide, but not with sense oligodeoxynucleotide, completely eliminated PKCdelta expression and resulted in the strong inhibition of SPP-stimulated phosphatidic acid formation. Indeed, among the PKCalpha, beta, delta, epsilon and zeta isoforms expressed in these cells, only PKCdelta was activated on cell stimulation with SPP, as indicated by translocation into the membrane fraction. Furthermore, pertussis toxin and genistein eliminated both PKCdelta translocation and PLD activation. In particular, a significant reduction in phosphatidylbutanol formation by SPP was observed in the presence of 4-amino-5-(4-methylphenyl)-7-(t-butyl) pyrazolo [3,4-d] pyrimidine (PP1), an inhibitor of Src tyrosine kinase. Furthermore, the activity of Src kinase was slightly increased by SPP and inhibited by PP1. However, the level of PKCdelta tyrosine phosphorylation was not increased in SPP-stimulated cells, suggesting that Src did not directly phosphorylate PKCdelta. Finally, the level of serine phosphorylation of PLD1 and PLD2 isoforms was not changed, whereas the PLD1 isoform alone was threonine-phosphorylated in SPP-treated cells. PLD1 threonine phosphorylation was strongly inhibited by rottlerin, by anti-PKCdelta oligodeoxynucleotide and by PP1. In conclusion, in CFNPE9o(-) cells, SPP interacts with a membrane receptor linked to a G(i) type of G-protein, leading to activation of PLD, probably the PLD1 isoform, by a signalling pathway involving Src and PKCdelta.

A role for calcium in sphingosine 1-phosphate-induced phospholipase D activity in C2C12 myoblasts

Receptor-regulated phospholipase D (PLD) is a key signaling pathway implicated in the control of fundamental biological processes. Here evidence is presented that in addition to protein kinase C (PKC) and Rho GTPases, Ca(2+) response evoked by sphingosine 1-phosphate (S1P) also participates to the enzyme regulation. Ca(2+) was found critical for PKC(alpha)-mediated PLD activation. Moreover, S1P-induced PLD activity resulted diminished by calmodulin inhibitors such as W-7 and CGS9343B implicating its involvement in the process. A plausible candidate for Ca(2+)-dependent PLD regulation by S1P was represented by calcineurin, in view of the observed reduction of the stimulatory effect by cyclosporin A. In contrast, monomeric GTP-binding protein Ral was translocated to membranes by S1P in a Ca(2+)-independent manner, ruling out its possible role in agonist-mediated regulation of PLD.

Sphingosine 1-phosphate induces Ca2+ transients and cytoskeletal rearrangement in C2C12 myoblastic cells

In many cell systems, sphingosine 1-phosphate (SPP) increases cytosolic Ca2+ concentration ([Ca2+]i) by acting as intracellular mediator and extracellular ligand. We recently demonstrated (Meacci E, Cencetti F, Formigli L, Squecco R, Donati C, Tiribilli B, Quercioli F, Zecchi-Orlandini S, Francini F, and Bruni P. Biochem J 362: 349-357, 2002) involvement of endothelial differentiation gene (Edg) receptors (Rs) specific for SPP in agonist-mediated Ca2+ response of a mouse skeletal myoblastic (C2C12) cell line. Here, we investigated the Ca2+ sources of SPP-mediated Ca2+ transients in C2C12 cells and the possible correlation of ion response to cytoskeletal rearrangement. Confocal fluorescence imaging of C2C12 cells preloaded with Ca2+ dye fluo 3 revealed that SPP elicited a transient Ca2+ increase propagating as a wave throughout the cell. This response required extracellular and intracellular Ca2+ pool mobilization. Indeed, it was significantly reduced by removal of external Ca2+, pretreatment with nifedipine (blocker of L-type plasma membrane Ca2+ channels), and inositol 1,4,5-trisphosphate [Ins(1,4,5)P3]-mediated Ca2+ pathway inhibitors. Involvement of EdgRs was tested with suramin (specific inhibitor of Edg-3). Fluorescence associated with Ins(1,4,5)P3Rs and L-type Ca2+ channels was evident in C2C12 cells. SPP also induced C2C12 cell contraction. This event, however, was unrelated to [Ca2+]i increase, because the two phenomena were temporally shifted. We propose that SPP may promote C2C12 cell contraction through Ca2+-independent mechanisms.

Sphingosine 1-phosphate evokes calcium signals in C2C12 myoblasts via Edg3 and Edg5 receptors

Sphingosine 1-phosphate (SPP) is a bioactive lipid that exerts multiple biological effects in a large variety of cell types, acting as either an intracellular messenger or an extracellular ligand coupled to Edg-family receptors (where Edg stands for endothelial differentiation gene). Here we report that in C(2)C(12) myoblasts SPP elicited significant Ca(2+) mobilization. Analysis of the process using a confocal laser-scanning microscope showed that the Ca(2+) response occurred in a high percentage of cells, despite variations in amplitude and kinetics. Quantitative analysis of SPP-induced Ca(2+) transients performed with a spectrophotofluorimeter showed that the rise in Ca(2+) was strictly dependent on availability of extracellular Ca(2+). Cell treatment with pertussis toxin partially prevented the Ca(2+) response induced by SPP, indicating that G(i)-coupled-receptors were involved. Indeed, SPP action was shown to be mediated by agonist-specific Edg receptors. In particular, suramin, an antagonist of the SPP-specific receptor Edg3, as well as down-regulation of Edg3 by cell transfection with antisense oligodeoxyribonucleotides (ODN), significantly reduced agonist-mediated Ca(2+) mobilization. Moreover, an antisense ODN designed to inhibit Edg5 expression also decreased the SPP-induced rise in Ca(2+), although to a lesser extent than that observed by inhibiting Edg3. On the contrary, the SPP response was unaffected in myoblasts loaded with antisense ODN specific for Edg1. Remarkably, the concomitant inhibition of Edg3 and Edg5 receptors abolished the SPP-induced Ca(2+) increase, supporting the notion that Ca(2+) mobilization in C(2)C(12) cells induced by SPP is a receptor-mediated process that involves Edg3 and Edg5, but not Edg1.

Protein kinase Calpha is a calpain target in cultured embryonic muscle cells

Previously we isolated a micro-calpain/PKCalpha complex from skeletal muscle which suggested tight interactions between the Ca2+-dependent protease and the kinase in this tissue. Our previous studies also underlined the involvement of ubiquitous calpains in muscular fusion and differentiation. In order to precise the relationships between PKCalpha and ubiquitous calpains in muscle cells, the expression of these two enzymes was first examined during myogenesis of embryonic myoblasts in culture. Our results show that calpains and PKCalpha are both present in myotubes and essentially localized in the cytosolic compartment. Moreover, calpains were mainly present after 40 h of cell differentiation concomitantly with a depletion of PKCalpha content in the particulate fraction and the appearance of PKMalpha fragment. These results suggest a possible calpain dependent down-regulation process of PKCalpha in our model at the time of intense fusion. In our experimental conditions phorbol myristate acetate (PMA) induced a rapid depletion of PKCalpha in the cytosolic fraction and its translocation toward the particulate fraction. Long term exposure of myotubes in the presence of PMA induced down-regulation of PKCalpha, this process being partially blocked by calpain inhibitors (CS peptide and inhibitor II) and antisense oligonucleotides for the two major ubiquitous calpain isoforms (m- and micro-calpains). Taken together, our findings argue for an involvement of calpains in the differentiation of embryonic myoblasts by limited proteolytic cleavage of PKCalpha.

Phospholipase D stimulation is required for sphingosine-1-phosphate activation of actin stress fibre assembly in human airway epithelial cells

In human airway epithelial cells, sphingosine-1-phosphate (SPP) and lysophosphatidic acid (LPA) stimulated the production of phosphatidic acid (PA), which was inhibited by the primary alcohol butan-1-ol, but not by the inactive butan-2-ol, clearly indicating phospholipase D (PLD) involvement. Both SPP and LPA stimulated actin stress fibre formation, which was also butan-2-ol-insensitive and inhibited by butan-1-ol. SPP-induced PLD activation and cytoskeletal remodelling were insensitive to brefeldin A and toxin B from Clostridium difficile, which conversely blocked the effect of LPA, suggesting that the monomeric GTPases ADP ribosylation factor (ARF) and Rho are involved in LPA, but not in SPP responses. Pertussis toxin inhibited SPP- but not LPA-induced effects. PLD activation and stress fibre formation by both lysolipids were abolished by the tyrosine kinase inhibitor genistein. Addition of PA to cells caused a massive stress fibre assembly. In conclusion, PLD is one of the signalling components linking SPP-receptor activation to assembly of actin stress fibres.

The lysophospholipids sphingosine-1-phosphate and lysophosphatidic acid enhance survival during hypoxia in neonatal rat cardiac myocytes

The lysophospholipids sphingosine-1-phosphate (S1P) and lysophosphatidic acid (LPA) stimulate cellular proliferation and affect numerous cellular functions by signaling through G protein-coupled endothelial differentiation gene-encoded (Edg) receptors. S1P and LPA also act as survival factors in many cell types, but have not previously been studied in cardiac myocytes. We incubated neonatal rat cardiac myocytes either in room air/1% CO2 (normoxia) or in an atmosphere of 99% N2/1%CO2 (hypoxia) at 37 degrees C for 18-20 h in the absence of glucose. Cell viability was measured using a calcein ester green fluorescence assay. Under normoxic conditions 88.7+/-1.0% of the cells were viable after 18-20 h. Severe hypoxia reduced viability to 61.3+/-4.3% (n=6, P<0.05). In myocytes preincubated with either 10 microM S1P or 1 microM LPA for 2 h, the effects of severe hypoxia on cell viability were prevented resulting in survival equivalent to normoxia. Neither the protein kinase C inhibitor chelethyrine (1 microM) nor the mitochondrial K(ATP) channel antagonist 5-hydroxydecanoic acid, (5-HD, 100 microM) had any effect on myocyte survival during severe hypoxia, but both agents completely abolished the ability of S1P to rescue cardiac myocytes from hypoxic cell death. We also tested the effects of dimethylsphingosine (DMS), which inhibits sphingosine kinase synthesis of S1P. Incubation of neonatal rat cardiac myocytes with 10 microM DMS for 2 h in the presence of serum resulted in 25-30% cell death during 18-20 h of normoxia. DMS-induced cell death was prevented by concurrent preincubation with either S1P or GM-1, a ganglioside that activates sphingosine kinase to increase intracellular levels of S1P. We conclude that both S1P and LPA are cardioprotective for hypoxic neonatal rat ventricular myocytes. S1P acts through cellular membrane receptors by signaling mechanisms involving protein kinase C and mitochondrial K(ATP) channels. Both endogenous and exogenously applied S1P are effective in preventing cell death induced by inhibition of sphingosine kinase.

Dual regulation of sphingosine 1-phosphate-induced phospholipase D activity through RhoA and protein kinase C-alpha in C2C12 myoblasts

Previous studies showed that in C2C12 cells, phospholipase D (PLD) and its known regulators, RhoA and protein kinase Calpha (PKCalpha), were downstream effectors in sphingosine 1-phosphate (SPP) signalling. Moreover, the role of PKC for SPP-mediated PLD activation and the requirement of PKCalpha for RhoA translocation were reported. The present results demonstrated that inactivation of RhoA, by overexpression of RhoGDP dissociation inhibitor (RhoGDI) as well as treatment with C3 exotoxin, attenuated SPP-stimulated PLD activity, supporting the involvement of RhoA in the stimulation of PLD activity by the bioactive lipid in C2C12 myoblasts. In addition, the effect of PKCalpha inhibitor Gö6976 on the SPP-induced PLD activation in myoblasts, where RhoA function was inactivated, was consistent with a dual regulation of the enzyme through RhoA and PKCalpha. Interestingly, the subcellular distribution of PLD isoforms, RhoA and PKCalpha, in SPP-stimulated cells supported the view that the functional relationship between the two PLD regulators, demonstrated to occur in SPP signalling, represents a novel mechanism of regulation of specifically localized PLD.

Sphingosine 1-phosphate signalling via the endothelial differentiation gene family of G-protein-coupled receptors

Sphingosine 1-phosphate (S1P) is stored in and released from platelets in response to cell activation. However, recent studies show that it is also released from a number of cell types, where it can function as a paracrine/autocrine signal to regulate cell proliferation, differentiation, survival, and motility. This review discusses the role of S1P in cellular regulation, both at the molecular level and in terms of health and disease. The main biochemical routes for S1P synthesis (sphingosine kinase) and degradation (S1P lyase and S1P phosphatase) are described. The major focus is on the ability of S1P to bind to a novel family of G-protein-coupled receptors (endothelial differentiation gene [EDG]-1, -3, -5, -6, and -8) to elicit signal transduction (via G(q)-, G(i)-, G(12)-, G(13)-, and Rho-dependent routes). Effector pathways regulated by S1P are divergent, such as extracellular signal-regulated kinase, p38 mitogen-activated protein kinase, phospholipases C and D, adenylyl cyclase, and focal adhesion kinase, and occur in multiple cell types, such as immune cells, neurones, smooth muscle, etc. This provides a molecular basis for the ability of S1P to act as a pleiotropic bioactive lipid with an important role in cellular regulation. We also give an account of the expanding role for S1P in health and disease; in particular, with regard to its role in atherosclerosis, angiogenesis, cancer, and inflammation. Finally, we describe future directions for S1P research and novel approaches whereby S1P signalling can be manipulated for therapeutic intervention in disease.

Permissive role of protein kinase C alpha but not protein kinase C delta in sphingosine 1-phosphate-induced Rho A activation in C2C12 myoblasts

Rho GTPases participate in various important signaling pathways and have been implicated in myogenic differentiation. Here the first evidence is provided that in C2C12 myoblasts sphingosine 1-phosphate (SPP) rapidly and transiently induced membrane association of Rho A in a pertussis toxin-insensitive manner. The bioactive lipid preferentially relocalized the GTPase to Golgi-enriched membrane. Translocation of Rho A was abolished by inhibition or down-regulation of protein kinase C (PKC). Notably, treatment with Gö6976, an inhibitor of conventional PKCs, which selectively blocked PKC alpha in these cells, prevented SPP-induced Rho A translocation. Conversely rottlerin, a selective inhibitor of PKC delta, was without effect, demonstrating that SPP signaling to Rho A involves PKC alpha but not PKC delta activation. This novel functional relationship between the two proteins may have a role in SPP-mediated regulation of downstream effectors.

Expression and characterization of Edg-1 receptors in rat cardiomyocytes: calcium deregulation in response to sphingosine 1-phosphate

Recent evidence indicates that sphingolipids are produced by the heart during hypoxic stress and by blood platelets during thrombus formation. It is therefore possible that sphingolipids may influence heart cell function by interacting with G-protein-coupled receptors of the Edg family. In the present study, it was found that sphingosine 1-phosphate (Sph1P), the prototypical ligand for Edg receptors, produced calcium overload in rat cardiomyocytes. The cDNA for Edg-1 was cloned from rat cardiomyocytes and, when transfected in an antisense orientation, effectively blocked Edg-1 protein expression and reduced the Sph1P-mediated calcium deregulation. Taken together, these results demonstrate that cardiomyocytes express an extracellular lipid-sensitive receptorsystem that can respond to sphingolipid mediators. Because the major source of Sph1P is from blood platelets, we speculate that Edg-mediated Sph1P negative inotropic and cardiotoxic effects may play important roles in acute myocardial ischemia where Sph1P levels are probably elevated in response to thrombus.

Receptor-activated phospholipase D is present in caveolin-3-enriched light membranes of C2C12 myotubes

Caveolin-3 (cav-3) is a key structural component of caveolar membrane in skeletal muscle. Cav-3-enriched light membrane (CELM) fractions obtained from C2C12 myotubes contain phospholipase D1 (PLD1) and its major regulators, RhoA and protein kinase Calpha (PKCalpha). All these proteins were found bound to cav-3. An in vivo assay of PLD activity, which allows to localize the reaction product in CELMs, indicated that the enzyme associated to this membrane microdomain was active. Moreover, bradykinin (BK), thrombin and phorbol 12-myristate 13-acetate induced rapid stimulation of PLD activity in CELMs. The cav-3-PLD1 complex was not affected by BK treatment, whereas the agonist induced a marked increase of RhoA association with cav-3. Furthermore, BK-induced PLD activation in CELMs was dependent, at least in part, on PKCalpha.

Sphingosine-1-phosphate activates phospholipase D in human airway epithelial cells via a G protein-coupled receptor

Sphingosine-1-phosphate (SPP) acts as a first messenger in immortalized human airway epithelial cells (CFNPE9o(-)), possibly interacting with an Edg family receptor. Expression of the SPP receptors Edg-1 and Edg-3, as well as a low level of Edg-5/H218, was detected in these cells, in agreement with their ability to specifically bind SPP. The related lipids, lysophosphatidic acid and sphingosylphosphorylcholine, were unable to displace SPP from its high affinity binding sites, suggesting that the biological responses to these different lysolipids are mediated by distinct receptors. SPP markedly inhibited forskolin-stimulated cAMP accumulation in a dose-dependent manner and caused a remarkable elevation of intracellular calcium, both effects being sensitive to pertussis toxin treatment. Most importantly, SPP stimulated phosphatidic acid formation, which was maximal after 2 min and decreased within 8-10 min. In the presence of butan-1-ol, suppression of SPP-induced phosphatidic acid formation and production of phosphatidylbutanol were found, clearly indicating activation of phospholipase D (PLD). This finding was also confirmed by analysis of the fatty acid composition of phosphatidic acid, showing an increase in the monounsaturated oleic acid only. The decrease of phosphatidic acid level after 8-10 min incubation with SPP was accompanied by a parallel increase of diacylglycerol production, which was abolished in the presence of butan-1-ol. This result indicates that activation of phospholipase D is followed by stimulation of phosphatidate phosphohydrolase activity. Phosphatidic acid formation was insensitive to protein kinase C inhibitors and almost completely inhibited by pertussis toxin treatment, suggesting that SPP activates phospholipase D via a G(i/o) protein-coupled receptor.