Early activation of sphingosine kinase in mast cells and recruitment to FcepsilonRI are mediated by its interaction with Lyn kinase

Integrative roles of sphingosine kinase in liver pathophysiology

Bioactive sphingolipids and enzymes that metabolize sphingolipid-related substances have been considered as critical messengers in various signaling pathways. One such enzyme is the crucial lipid kinase, sphingosine kinase (SphK), which mediates the conversion of sphingosine to the potent signaling substance, sphingosine-1-phosphate. Several studies have demonstrated that SphK metabolism is strictly regulated to maintain the homeostatic balance of cells. Here, we summarize the role of SphK in the course of liver disease and illustrate its effects on both physiological and pathological conditions of the liver. SphK has been implicated in a variety of liver diseases, such as steatosis, liver fibrosis, hepatocellular carcinoma, and hepatic failure. This study may advance the understanding of the cellular and molecular foundations of liver disease and establish therapeutic approaches via SphK modulation.

The Crosstalk between FcεRI and Sphingosine Signaling in Allergic Inflammation

Sphingolipid molecules have recently attracted attention as signaling molecules in allergic inflammation diseases. Sphingosine-1-phosphate (S1P) is synthesized by two isoforms of sphingosine kinases (SPHK 1 and SPHK2) and is known to be involved in various cellular processes. S1P levels reportedly increase in allergic inflammatory diseases, such as asthma and anaphylaxis. FcεRI signaling is necessary for allergic inflammation as it can activate the SPHKs and increase the S1P level; once S1P is secreted, it can bind to the S1P receptors (S1PRs). The role of S1P signaling in various allergic diseases is discussed. Increased levels of S1P are positively associated with asthma and anaphylaxis. S1P can either induce or suppress allergic skin diseases in a context-dependent manner. The crosstalk between FcεRI and S1P/SPHK/S1PRs is discussed. The roles of the microRNAs that regulate the expression of the components of S1P signaling in allergic inflammatory diseases are also discussed. Various reports suggest the role of S1P in FcεRI-mediated mast cell (MC) activation. Thus, S1P/SPHK/S1PRs signaling can be the target for developing anti-allergy drugs.

Tyrosine-phosphorylation and activation of glucosylceramide synthase by v-Src: Its role in survival of HeLa cells against ceramide

Sphingolipids represent a family of cellular lipid-molecules that regulate physiological and pathophysiological processes. Glucosylceramide (GlcCer), the simplest glycosphingolipid (GSL), is synthesized from ceramide and UDP-glucose by GlcCer synthase (GCS). Both GlcCer (and resulting GSLs) and ceramide regulate various cellular functions including cell death and multiple drug resistance. Src family tyrosine kinases are up-regulated in various human cancer cells. We examined the effect of v-Src expression on GCS activity, the formation of 4-nitrobenzo-2-oxa-1,3-diazole (NBD)-labeled GlcCer from NBD-ceramide, and the effect of tyrosine¹³² mutation in GCS on ceramide-induced cytotoxicity in HeLa cells. Expression of v-Src increased the formation of NBD-GlcCer in both intact cells without marked changes in other sphingolipid metabolites and cell homogenates without changing affinities of NBD-ceramide and UDP-glucose. Expression of v-Src also increased tyrosine-phosphorylated levels in GCS proteins in HeLa and HEK293T cells. In HEK293T cells transiently expressing the GCS mutant, GCS-Y132F-HA, showing replacement of the tyrosine¹³² residue with phenylalanine, tyrosine-phosphorylated levels in GCS proteins were significantly lower than those in control cells expressing the GCS-wild-type-HA. The formation of NBD-GlcCer in HeLa cells stably expressing GCS-Y132F-HA was significantly lower than that in the control. Ceramide-induced cytotoxicity in HeLa-GCS-Y132F-HA cells was significantly greater than in the control. In this study, we showed for the first time that expression of v-Src up-regulated GCS activity via tyrosine phosphorylation of the enzyme in a post-translational manner. Mechanisms of Src-induced resistance to ceramide-induced cytotoxicity are discussed in relation to the Src-induced up-regulation of GCS activity.

Molecular mechanisms of regulation of sphingosine kinase 1

Within the last 3 decades, there has been intense study of bioactive sphingolipids and the enzymes which metabolize those lipids. One enzyme is the critical lipid kinase sphingosine kinase 1 (SK1), which produces the potent and pleiotropic signaling lipid, sphingosine 1-phosphate (S1P). SK1 and S1P have been implicated in a host of different diseases including cancer, chronic inflammation, and metabolic diseases. However, while there is ample knowledge about the importance of these molecules in the development and progression of disease there is a dearth of knowledge of the molecular mechanisms which regulate SK1 function. In this review, we will cover some of the more recent and exciting findings about the different ways SK1 function can be regulated, from transcriptional regulation to protein stability. Finally, we will delve into recent structural insights into SK1 and how they might relate to function at cell membranes.

The Emerging Picture of Mast Cell Activation: The Complex Regulatory Network of High-Affinity Receptor for Immunoglobulin E Signaling

It is now well known that immunoglobulin E (IgE) and mast cells (MCs) are important participants in allergic diseases. MCs contain electron-dense secretory granules which are filled with inflammatory mediators. The interaction of an allergen (antigen) with an antigen-specific IgE-bound high-affinity receptor for IgE (FcεRI) is an essential step in MC activation as well as subsequent downstream signaling events. What we know is that IgE and FcεRI activate a complex regulatory network (i.e., signaling molecules and messengers) that governs both the type of MC activation and the symptoms of allergic disease. This review focuses on recent discoveries that shed new light on FcεRI signaling networks, holding promise for the development of new therapeutic solutions in the treatment of allergic diseases.

Role of Sphingosine-1-Phosphate in Mast Cell Functions and Asthma and Its Regulation by Non-Coding RNA

Sphingolipid metabolites are emerging as important signaling molecules in allergic diseases specifically asthma. One of the sphingolipid metabolite, sphingosine-1-phosphate (S1P), is involved in cell differentiation, proliferation, survival, migration, and angiogenesis. In the allergic diseases, alteration of S1P levels influences the differentiation and responsiveness of mast cells (MCs). S1P is synthesized by two sphingosine kinases (SphKs), sphingosine kinase 1, and sphingosine kinase 2. Engagement of IgE to the FcεRI receptor induces the activation of both the SphKs and generates S1P. Furthermore, SphKs are also essential to FcεRI-mediated MC activation. Activated MCs export S1P into the extracellular space and causes inflammatory response and tissue remodeling. S1P signaling has dual role in allergic responses. Activation of SphKs and secretion of S1P are required for MC activation; however, S1P signaling plays a vital role in the recovery from anaphylaxis. Several non-coding RNAs have been shown to play a crucial role in controlling the MC-associated inflammatory and allergic responses. Thus, S1P signaling pathway and its regulation by non-coding RNA could be explored as an exciting potential therapeutic target for asthma and other MC-associated diseases.

Therapeutic Strategies and Pharmacological Tools Influencing S1P Signaling and Metabolism

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

Sphingosine 1-Phosphate Produced by Sphingosine Kinase 2 Intrinsically Controls Platelet Aggregation In Vitro and In Vivo

RATIONALE

Platelets are known to play a crucial role in hemostasis. Sphingosine kinases (Sphk) 1 and 2 catalyze the conversion of sphingosine to the bioactive metabolite sphingosine 1-phosphate (S1P). Although platelets are able to secrete S1P on activation, little is known about a potential intrinsic effect of S1P on platelet function.

OBJECTIVE

To investigate the role of Sphk1- and Sphk2-derived S1P in the regulation of platelet function.

METHODS AND RESULTS

We found a 100-fold reduction in intracellular S1P levels in platelets derived from Sphk2(-/-) mutants compared with Sphk1(-/-) or wild-type mice, as analyzed by mass spectrometry. Sphk2(-/-) platelets also failed to secrete S1P on stimulation. Blood from Sphk2-deficient mice showed decreased aggregation after protease-activated receptor 4-peptide and adenosine diphosphate stimulation in vitro, as assessed by whole blood impedance aggregometry. We revealed that S1P controls platelet aggregation via the sphingosine 1-phosphate receptor 1 through modulation of protease-activated receptor 4-peptide and adenosine diphosphate-induced platelet activation. Finally, we show by intravital microscopy that defective platelet aggregation in Sphk2-deficient mice translates into reduced arterial thrombus stability in vivo.

CONCLUSIONS

We demonstrate that Sphk2 is the major Sphk isoform responsible for the generation of S1P in platelets and plays a pivotal intrinsic role in the control of platelet activation. Correspondingly, Sphk2-deficient mice are protected from arterial thrombosis after vascular injury, but have normal bleeding times. Targeting this pathway could therefore present a new therapeutic strategy to prevent thrombosis.

Function of cancer associated genes revealed by modern univariate and multivariate association tests

Copy number variation (CNV) plays a role in pathogenesis of many human diseases, especially cancer. Several whole genome CNV association studies have been performed for the purpose of identifying cancer associated CNVs. Here we undertook a novel approach to whole genome CNV analysis, with the goal being identification of associations between CNV of different genes (CNV-CNV) across 60 human cancer cell lines. We hypothesize that these associations point to the roles of the associated genes in cancer, and can be indicators of their position in gene networks of cancer-driving processes. Recent studies show that gene associations are often non-linear and non-monotone. In order to obtain a more complete picture of all CNV associations, we performed omnibus univariate analysis by utilizing dCov, MIC, and HHG association tests, which are capable of detecting any type of association, including non-monotone relationships. For comparison we used Spearman and Pearson association tests, which detect only linear or monotone relationships. Application of dCov, MIC and HHG tests resulted in identification of twice as many associations compared to those found by Spearman and Pearson alone. Interestingly, most of the new associations were detected by the HHG test. Next, we utilized dCov's and HHG's ability to perform multivariate analysis. We tested for association between genes of unknown function and known cancer-related pathways. Our results indicate that multivariate analysis is much more effective than univariate analysis for the purpose of ascribing biological roles to genes of unknown function. We conclude that a combination of multivariate and univariate omnibus association tests can reveal significant information about gene networks of disease-driving processes. These methods can be applied to any large gene or pathway dataset, allowing more comprehensive analysis of biological processes.

Signal transduction and chemotaxis in mast cells

Mast cells play crucial roles in both innate and adaptive arms of the immune system. Along with basophils, mast cells are essential effector cells for allergic inflammation that causes asthma, allergic rhinitis, food allergy and atopic dermatitis. Mast cells are usually increased in inflammatory sites of allergy and, upon activation, release various chemical, lipid, peptide and protein mediators of allergic reactions. Since antigen/immunoglobulin E (IgE)-mediated activation of these cells is a central event to trigger allergic reactions, innumerable studies have been conducted on how these cells are activated through cross-linking of the high-affinity IgE receptor (FcεRI). Development of mature mast cells from their progenitor cells is under the influence of several growth factors, of which the stem cell factor (SCF) seems to be the most important. Therefore, how SCF induces mast cell development and activation via its receptor, KIT, has been studied extensively, including a cross-talk between KIT and FcεRI signaling pathways. Although our understanding of the signaling mechanisms of the FcεRI and KIT pathways is far from complete, pharmaceutical applications of the knowledge about these pathways are underway. This review will focus on recent progresses in FcεRI and KIT signaling and chemotaxis.

Sphingosine-1-phosphate and other lipid mediators generated by mast cells as critical players in allergy and mast cell function

Sphingosine-1-phosphate (S1P), platelet activating factor (PAF) and eicosanoids are bioactive lipid mediators abundantly produced by antigen-stimulated mast cells that exert their function mostly through specific cell surface receptors. Although it has long been recognized that some of these bioactive lipids are potent regulators of allergic diseases, their exact contributions to disease pathology have been obscured by the complexity of their mode of action and the regulation of their metabolism. Indeed, the effects of such lipids are usually mediated by multiple receptor subtypes that may differ in their signaling mechanisms and functions. In addition, their actions may be elicited by cell surface receptor-independent mechanisms. Furthermore, these lipids may be converted into metabolites that exhibit different functionalities, adding another layer of complexity to their overall biological responses. In some instances, a second wave of lipid mediator synthesis by both mast cell and non-mast cell sources may occur late during inflammation, bringing about additional roles in the altered environment. New evidence also suggests that bioactive lipids in the local environment can fine-tune mast cell maturation and phenotype, and thus their responsiveness. A better understanding of the subtleties of the spatiotemporal regulation of these lipid mediators, their receptors and functions may aid in the pursuit of pharmacological applications for allergy treatments.

Foreign body response to subcutaneous biomaterial implants in a mast cell-deficient Kit(w-Sh) murine model

Mast cells (MCs)_are recognized for their functional role in wound-healing and allergic and inflammatory responses - host responses that are frequently detrimental to implanted biomaterials if extended beyond acute reactivity. These tissue reactions impact especially on the performance of sensing implants such as continuous glucose monitoring (CGM) devices. Our hypothesis that effective blockade of MC activity around implants could alter the host foreign body response (FBR) and enhance the in vivo lifetime of these implantable devices motivated this study. Stem cell factor and its ligand c-KIT receptor are critically important for MC survival, differentiation and degranulation. Therefore, an MC-deficient sash mouse model was used to assess MC relationships to the in vivo performance of CGM implants. Additionally, local delivery of a tyrosine kinase inhibitor (TKI) that inhibits c-KIT activity was also used to evaluate the role of MCs in modulating the FBR. Model sensor implants comprising polyester fibers coated with a rapidly dissolving polymer coating containing drug-releasing degradable microspheres were implanted subcutaneously in sash mice for various time points, and the FBR was evaluated for chronic inflammation and fibrous capsule formation around the implants. No significant differences were observed in the foreign body capsule formation between control and drug-releasing implant groups in MC-deficient mice. However, fibrous encapsulation was significantly greater around the drug-releasing implants in sash mice compared to drug-releasing implants in wild-type (e.g. MC-competent) mice. These results provide insights into the role of MCs in the FBR, suggesting that MC deficiency provides alternative pathways for host inflammatory responses to implanted biomaterials.

Usage of sphingosine kinase isoforms in mast cells is species and/or cell type determined

FcεRI engagement in mast cells (MCs) induces the activation of two distinct sphingosine kinase isoforms (SphK1 and SphK2) to produce sphingosine-1-phosphate, a mediator essential for MC responses. Whereas embryonic-derived SphK2-null MCs showed impaired responses to Ag, RNA silencing studies on other MC types indicated a dominant role for SphK1. Given the known functional heterogeneity of MCs, we explored whether the reported differences in SphK1 or SphK2 usage could be reflective of phenotypic differences between MC populations. Using lentiviral-based short hairpin RNA to silence SphK1 or SphK2, we found that SphK2 is required for murine MC degranulation, calcium mobilization, and cytokine and leukotriene production, irrespective of the tissue from which the MC progenitors were derived, the stage of MC granule maturity, or the conditions used for differentiation. This finding was consistent with the lack of a full allergic response in SphK2-null mice challenged to undergo passive cutaneous anaphylaxis. A redundant role for both SphKs was uncovered, however, in chemotaxis toward Ag in all MC types tested and in TNF-α production in certain MC types. In contrast, human MC responses were dependent only on SphK1, associating with a more robust expression of this isoform and a more varied representation of SphK variants relative to murine MCs. The findings show that the function of SphK1 and SphK2 can be interchangeable in MCs; however, an important determinant of SphK isoform usage is the species of origin and an influencing factor, the tissue from which MCs may be derived and/or their differentiation state.

Recruitment of sphingosine kinase to presynaptic terminals by a conserved muscarinic signaling pathway promotes neurotransmitter release

Sphingolipids are potent lipid second messengers that regulate cell differentiation, migration, survival, and secretion, and alterations in sphingolipid signaling have been implicated in a variety of diseases. However, how sphingolipid levels are regulated, particularly in the nervous system, remains poorly understood. Here, we show that the generation of sphingosine-1-phosphate by sphingosine kinase (SphK) promotes neurotransmitter release. Electrophysiological, imaging, and behavioral analyses of Caenorhabditis elegans mutants lacking sphingosine kinase sphk-1 indicate that neuronal development is normal, but there is a significant defect in neurotransmitter release from neuromuscular junctions. SPHK-1 localizes to discrete, nonvesicular regions within presynaptic terminals, and this localization is critical for synaptic function. Muscarinic agonists cause a rapid increase in presynaptic SPHK-1 abundance, whereas reduction of endogenous acetylcholine production results in a rapid decrease in presynaptic SPHK-1 abundance. Muscarinic regulation of presynaptic SPHK-1 abundance is mediated by a conserved presynaptic signaling pathway composed of the muscarinic acetylcholine receptor GAR-3, the heterotrimeric G protein Gαq, and its effector, Trio RhoGEF. SPHK-1 activity is required for the effects of muscarinic signaling on synaptic transmission. This study shows that SPHK-1 promotes neurotransmitter release in vivo and identifies a novel muscarinic pathway that regulates SphK abundance at presynaptic terminals.

Deciphering the structure and function of FcεRI/mast cell axis in the regulation of allergy and anaphylaxis: a functional genomics paradigm

Allergy and anaphylaxis are inflammatory disorders caused by immune reactions mainly induced by immunoglobulin-E that signal through the high-affinity FcεRI receptor to release the inflammatory mediators from innate immune cells. The FcεRI/mast cell axis is potently involved in triggering various intracellular signaling molecules to induce calcium release from the internal stores, induction of transcription factors such as NF-kB, secretion of various cytokines as well as lipid mediators, and degranulation, resulting in the induction of allergy and anaphylaxis. In this review, we discuss various cellular and molecular mechanisms triggered through FcεRI/mast cell axis in allergy and anaphylaxis with a special emphasis on the functional genomics paradigm.

Mast cell chemotaxis - chemoattractants and signaling pathways

Migration of mast cells is essential for their recruitment within target tissues where they play an important role in innate and adaptive immune responses. These processes rely on the ability of mast cells to recognize appropriate chemotactic stimuli and react to them by a chemotactic response. Another level of intercellular communication is attained by production of chemoattractants by activated mast cells, which results in accumulation of mast cells and other hematopoietic cells at the sites of inflammation. Mast cells express numerous surface receptors for various ligands with properties of potent chemoattractants. They include the stem cell factor (SCF) recognized by c-Kit, antigen, which binds to immunoglobulin E (IgE) anchored to the high affinity IgE receptor (FcεRI), highly cytokinergic (HC) IgE recognized by FcεRI, lipid mediator sphingosine-1-phosphate (S1P), which binds to G protein-coupled receptors (GPCRs). Other large groups of chemoattractants are eicosanoids [prostaglandin E₂ and D₂, leukotriene (LT) B₄, LTD₄, and LTC₄, and others] and chemokines (CC, CXC, C, and CX3C), which also bind to various GPCRs. Further noteworthy chemoattractants are isoforms of transforming growth factor (TGF) β1–3, which are sensitively recognized by TGF-β serine/threonine type I and II β receptors, adenosine, C1q, C3a, and C5a components of the complement, 5-hydroxytryptamine, neuroendocrine peptide catestatin, tumor necrosis factor-α, and others. Here we discuss the major types of chemoattractants recognized by mast cells, their target receptors, as well as signaling pathways they utilize. We also briefly deal with methods used for studies of mast cell chemotaxis and with ways of how these studies profited from the results obtained in other cellular systems.

Lyn kinase controls TLR4-dependent IKK and MAPK activation modulating the activity of TRAF-6/TAK-1 protein complex in mast cells

Mast cells (MCs) control allergic reactions and contribute to protective innate immune responses through TLR4 activation. The tyrosine kinase Lyn is important to the high affinity IgE receptor (FcεRI) signal transduction system in MCs, but its role on the TLR4 signalling cascade is still elusive. Here, we characterized several TLR4-triggered responses in bone marrow-derived mast cells (BMMCs) from wild-type (WT) and Lyn(-/-) mice. We found that Lyn(-/-) MCs secreted lower amounts of TNF-α after LPS challenge when compared with WT cells. Lyn(-/-) BMMCs showed less MAPK, IκB phosphorylation and NF-κB nuclear translocation after TLR-4 triggering than WT cells. LPS-induced MAPK and inhibitor of IκB kinase (IKK) phosphorylation were importantly reduced in the absence of Lyn. A constitutive interaction between TNF receptor associated factor 6 (TRAF-6) and phosphorylated TGF-β-activated kinase (TAK-1) was observed in Lyn(-/-) BMMCs and this complex was insensitive to LPS addition. Lyn kinase was activated and associated to TRAF-6 shortly after LPS addition in WT MCs. Analyzing two local MC-dependent innate immune responses in vivo, we found that Lyn positively controls early TNF-α production and immune cell recruitment after an intraperitoneal injection of LPS. Our results indicate that Lyn plays a positive role in TLR4-induced production of TNF-α in MCs controlling the activity of the TRAF-6/TAK-1 protein complex.

An emerging role for the lipid mediator sphingosine-1-phosphate in mast cell effector function and allergic disease

Sphingosine-1-phosphate (S1P) plays important roles regulating functions of diverse biological systems, including the immune system. S1P affects immune cell function mostly by acting through its receptors at the cell membrane but it can also induce S1P receptor-independent responses in the cells where it is generated. S1P produced in allergically-stimulated mast cells mediates degranulation, cytokine and lipid mediator production and migration of mast cells towards antigen by mechanisms that are both S1P receptor-dependent and independent. Even in the absence of an antigen challenge, the differentiation and responsiveness of mast cells can be affected by chronic exposure to elevated S1P from a nonmast cell source, whichmay occur under pathophysiological conditions, potentially leading to the hyper-responsiveness of mast cells. The role of S1P extends beyond the regulation of the function of mast cells to the regulation of the surrounding or distal environment. S1P is exported out of antigen-stimulated mast cells and into the extracellular space and the resulting S1P gradient within the tissue may influence diverse surrounding tissue cells and several aspects of the allergic disease, such as inflammation or tissue remodeling. Furthermore, recent findings indicate that vasoactive mediators released systemically by mast cells induce the production of S1P in nonhematopoietic compartments, where it plays a role in regulating the vascular tone and reducing the hypotension characteristic of the anaphy lactic shock and thus helping the recovery. The dual actions of S1P, promoting the immediate response of mast cells, while controlling the systemic consequences of mast cell activity will be discussed in detail.

Sphingosine kinase 1 and sphingosine-1-phosphate receptor 2 are vital to recovery from anaphylactic shock in mice

Sphingosine kinase 1 (SphK1) and SphK2 are ubiquitous enzymes that generate sphingosine-1-phosphate (S1P), a ligand for a family of G protein-coupled receptors (S1PR1-S1PR5) with important functions in the vascular and immune systems. Here we explore the role of these kinases and receptors in recovery from anaphylaxis in mice. We found that Sphk2-/- mice had a rapid recovery from anaphylaxis. In contrast, Sphk1-/- mice showed poor recovery from anaphylaxis and delayed histamine clearance. Injection of S1P into Sphk1-/- mice increased histamine clearance and promoted recovery from anaphylaxis. Adoptive cell transfer experiments demonstrated that SphK1 activity was required in both the hematopoietic and nonhematopoietic compartments for recovery from anaphylaxis. Mice lacking the S1P receptor S1PR2 also showed a delay in plasma histamine clearance and a poor recovery from anaphylaxis. However, S1P did not promote the recovery of S1pr2-/- mice from anaphylaxis, whereas S1pr2+/- mice showed partial recovery. Unlike Sphk2-/- mice, Sphk1-/- and S1pr2-/- mice had severe hypotension during anaphylaxis. Thus, SphK1-produced S1P regulates blood pressure, histamine clearance, and recovery from anaphylaxis in a manner that involves S1PR2. This suggests that specific S1PR2 agonists may serve to counteract the vasodilation associated with anaphylactic shock.

Essential roles of sphingosine-1-phosphate receptor 2 in human mast cell activation, anaphylaxis, and pulmonary edema

Systemic exacerbation of allergic responses, in which mast cells play a critical role, results in life-threatening anaphylactic shock. Sphingosine-1–phosphate (S1P), a ligand for a family of G protein–coupled receptors, is a new addition to the repertoire of bioactive lipids secreted by activated mast cells. Yet little is known of its role in human mast cell functions and in anaphylaxis. We show that S1P₂ receptors play a critical role in regulating human mast cell functions, including degranulation and cytokine and chemokine release. Immunoglobulin E–triggered anaphylactic responses, including elevation of circulating histamine and associated pulmonary edema in mice, were significantly attenuated by the S1P₂ antagonist JTE-013 and in S1P₂-deficient mice, in contrast to anaphylaxis induced by administration of histamine or platelet-activating factor. Hence, S1P and S1P₂ on mast cells are determinants of systemic anaphylaxis and associated pulmonary edema and might be beneficial targets for anaphylaxis attenuation and prophylaxis.

Involvement of Fc receptors in disorders of the central nervous system

Immunoglobulins are proteins with a highly variable antigen-binding domain and a constant region (Fc domain) that binds to a cell surface receptor (FcR). Activation of FcRs in immune cells (lymphocytes, macrophages, and mast cells) triggers effector responses including cytokine production, phagocytosis, and degranulation. In addition to their roles in normal responses to infection or tissue injury, and in immune-related diseases, FcRs are increasingly recognized for their involvement in neurological disorders. One or more FcRs are expressed in microglia, astrocytes, oligodendrocytes, and neurons. Aberrant activation of FcRs in such neural cells may contribute to the pathogenesis of major neurodegenerative conditions including Alzheimer's disease, Parkinson's disease, ischemic stroke, and multiple sclerosis. On the other hand, FcRs may play beneficial roles in counteracting pathological processes; for e.g., FcRs may facilitate removal of amyloid peptides from the brain and so protect against Alzheimer's disease. Knowledge of the functions of FcRs in the nervous system in health and disease is leading to novel preventative and therapeutic strategies for stroke, Alzheimer's disease, and other neurological disorders.

Sphingosine kinase 1 localized to the plasma membrane lipid raft microdomain overcomes serum deprivation induced growth inhibition

Several studies have demonstrated that sphingosine kinase 1 (SphK1) translocates to the plasma membrane (PM) upon its activation and further suggested the plasma membrane lipid raft microdomain (PMLRM) as a target for SphK1 relocalization. To date, however, direct evidence of SphK1 localization to the PMLRM has been lacking. In this report, using multiple biochemical and subcellular fractionation techniques we demonstrate that endogenous SphK1 protein and its substrate, D-erythro-sphingosine, are present within the PMLRM. Additionally, we demonstrate that the PMA stimulation of SphK1 localized to the PMLRM results in production of sphingosine-1-phosphate as well as induction of cell growth under serum deprivation conditions. We further report that Ser225Ala and Thr54Cys mutations, reported to abrogate phosphatidylserine binding, block SphK1 targeting to the PMLRM and SphK1 induced cell growth. Together these findings provide direct evidence that the PMLRM is the major site of action for SphK1 to overcome serum-deprived cell growth inhibition.

FcepsilonRI-mediated mast cell migration: signaling pathways and dependence on cytosolic free Ca2+ concentration

IgE-sensitized rat basophilic leukemia (RBL)-2H3 mast cells have been shown to migrate towards antigen. In the present study we tried to identify the mechanism by which antigen causes mast cell migration. Antigen caused migration of RBL-2H3 cells at the concentration ranges of 1000-fold lower than those required for degranulation and the dose response was biphasic. This suggests that mast cells can detect very low concentration gradients of antigen (pg/ml ranges), which initiate migration until they degranulate near the origin of antigen, of which concentration is in the ng/ml ranges. Similar phenomenon was observed in human mast cells (HMCs) derived from CD34(+) progenitors. As one mechanism of mast cell migration, we tested the involvement of sphingosine 1-phosphate (S1P). Fc epsilon RI-mediated cell migration was dependent on the production of S1P but independent of a S1P receptor or its signaling pathways as determined with S1P receptor antagonist VPC23019 and Gi protein inhibitor pertussis toxin (PTX). This indicated that the site of action of S1P produced by antigen stimulation was intracellular. However, S1P-induced mast cell migration was dependent on S1P receptor activation and inhibited by both VPC23019 and PTX. Cell migration towards antigen or extracellular S1P was dependent on the activation of the phosphatidylinositol 3-kinase (PI3K) and mitogen-activated protein kinase (MAPK) pathways, while only migration towards antigen was inhibited by the inhibitors of sphingosine kinase and phospholipase C (PLC) and intracellular calcium chelator BAPTA. In summary, our data suggest that the high affinity receptor for IgE (Fc epsilon RI)-mediated mast cell migration is dependent on the production of S1P but independent of S1P receptors. Cell migration mediated by either Fc epsilon RI or S1P receptors involves activation of both PI3K and MAPK.

Sphingosine-1-phosphate induces development of functionally mature chymase-expressing human mast cells from hematopoietic progenitors

Mast cells (MCs) play a critical role in both acute and chronic inflammation and mature in peripheral tissues from bone marrow-derived progenitors that circulate in the blood as immature precursors. MCs developed from cord blood-derived progenitors cultured with stem cell factor (SCF) alone express intragranular tryptase (MC(T)s), the phenotype predominant in the lung. MC progenitors are likely to encounter the serum-borne bioactive sphingolipid metabolite, sphingosine-1-phosphate (S1P), during migration to target tissues. S1P accelerated the development of cord blood-derived MCs (CB-MCs) and strikingly increased the numbers of MC-expressing chymase. These MCs have functional Fc epsilonRIs, and similar to skin MC(TC)s that express both tryptase and chymase, also express CD88 and are activated by anaphylatoxin C5a and the secretagogue compound 48/80. S1P induced release of IL-6, a cytokine known to promote development of functionally mature MC(TC)s, from cord blood cultures containing adherent macrophages, and from highly purified macrophages, but not from macrophage-depleted CB-MCs. In contrast, S1P stimulated secretion of the chemokine, monocyte chemoattractant protein 1 (MCP-1/CCL2), from these macrophage-depleted and purified CB-MCs. These results suggest crucial roles for S1P in regulating development of human MCs and their functions and reveal a complex interplay between macrophages and MC progenitors in the development of mature human MCs.

The CCT/TRiC chaperonin is required for maturation of sphingosine kinase 1

Sphingosine kinase 1 (SK1) catalyses the generation of sphingosine 1-phosphate (S1P), a bioactive phospholipid that influences a diverse range of cellular processes, including proliferation, survival, adhesion, migration, morphogenesis and differentiation. SK1 is controlled by various mechanisms, including transcriptional regulation, and post-translational activation by phosphorylation and protein-protein interactions which can regulate both the activity and localisation of this enzyme. To gain a better understanding of the regulatory mechanisms controlling SK1 activity and function we performed a yeast two-hybrid screen to identify SK1-interacting proteins. Using this approach we identified that SK1 interacts with subunit 7 (eta) of cytosolic chaperonin CCT (chaperonin containing t-complex polypeptide, also called TRiC for TCP-1 ring complex), a hexadecameric chaperonin that binds unfolded polypeptides and mediates their folding and release in an ATP-dependent manner. Further analysis of the SK1-CCTeta interaction demonstrated that other CCT/TRiC subunits also associated with SK1 in HEK293T cell lysates in an ATP-sensitive manner, suggesting that the intact, functional, multimeric CCT/TRiC complex associated with SK1. Furthermore, pulse-chase studies indicated that CCT/TRiC binds specifically to newly translated SK1. Finally, depletion of functional CCT/TRiC through the use of RNA interference in HeLa cells or temperature sensitive CCT yeast mutants reduced cellular SK1 activity. Thus, combined this data suggests that SK1 is a CCT/TRiC substrate, and that this chaperonin facilitates folding of newly translated SK1 into its mature active form.

Sphingosine kinase/sphingosine 1-phosphate signalling in central nervous system

Sphingolipids were once regarded as inert structural components of cell membranes. Now these metabolites are generally believed to be important bioactive molecules that control a wide repertoire of cellular processes such as proliferation and survival of cells. Along with these ubiquitous cell functions observed in many peripheral tissues sphingolipid metabolites, especially sphingosine 1-phosphate, exert important neuron-specific functions such as regulation of neurotransmitter release. This review summarizes physiological and pathological roles of sphingolipid metabolites emphasizing the role of sphingosine 1-phosphate in the central nervous system.

The role of sphingosine-1-phosphate and its receptors in asthma

Sphingosine-1-phosphate (S1P) is a bioactive sphingolipid metabolite that plays important roles in allergic responses, including asthma and anaphylaxis, the incidence of which is rising worldwide especially in industrialized urban populations. In this review, we will discuss how S1P is formed and released, and how it acts at many cellular levels, including mast cells, the airway epithelium, airway smooth muscle and many immune cells. Since the actions of S1P on all of these cells could exacerbate allergic responses, the proteins that synthesize, release and respond to S1P offer plausible targets for a new generation of antiinflammatory therapeutics.

A current understanding of Fc epsilon RI-dependent mast cell activation

Mast cell activation via the high-affinity immunoglobulin (Ig) E receptor Fc epsilon RI is a topic of extensive investigation with therapeutic potential in allergic disease. The protein tyrosine kinases Fyn, Lyn, and Syk are intimately linked with the early events initiated by allergen-mediated aggregation of IgE-occupied Fc epsilon RI. Fyn and Lyn initiate signaling events that are organized by adaptor molecules, which compartmentalize and coordinate the activity of activated protein and lipid kinases and phospholipases to generate lipid products essential for signal amplification and mast cell function. Fyn and Lyn counter-regulate phosphatidylinositol 3-OH kinase (PI3K), controlling the produced amount of phosphatidylinositol (3,4,5)-trisphosphate (PIP3), a key regulator of mast cell degranulation. Fyn and Lyn also activate sphingosine kinases (SphK), which generate sphingosine-1-phosphate (S1P), thus contributing to mast cell chemotaxis and degranulation. Here, we summarize the current knowledge and future challenges and directions.

Regulation and functions of sphingosine kinases in the brain

It has long been known that sphingolipids, especially sphingomyelin, a principal component of myelin, are highly enriched in the central nervous system and are structural components of all eukaryotic cell membranes. In the last few years, substantial evidence has accumulated from studies of many types of cells demonstrating that in addition to their structural roles, their breakdown products form a new class of signaling molecules with potent and myriad regulatory effects on essentially every cell in the body. While the sphingolipid metabolites sphingosine and its precursor ceramide have been associated with cell growth arrest and apoptosis, sphingosine-1-phosphate (S1P) enhances proliferation, differentiation, and cell survival as well as regulates many physiological and pathological processes. The relative levels of these three interconvertible sphingolipid metabolites, and thus cell fate, are strongly influenced by the activity of sphingosine kinases, of which there are two isoforms, designated SphK1 and SphK2, the enzymes that phosphorylate sphingosine to produce S1P. Not much is yet known of the importance of S1P in the central nervous system. Therefore, this review is focused on current knowledge of regulation of SphK1 and SphK2 on both transcriptional and post-translational levels and the functions of these isozymes and their product S1P and its receptors in the central nervous system.

Eukaryotic elongation factor 1A interacts with sphingosine kinase and directly enhances its catalytic activity

Sphingosine 1-phosphate (S1P) has many important roles in mammalian cells, including contributing to the control of cell survival and proliferation. S1P is generated by sphingosine kinases (SKs), of which two mammalian isoforms have been identified (SK1 and SK2). To gain a better understanding of SK regulation, we have used a yeast two-hybrid screen to identify SK1-interacting proteins and established elongation factor 1A (eEF1A) as one such protein that associates with both SK1 and SK2. We show the direct interaction of eEF1A with the SKs in vitro, whereas the physiological relevance of this association was demonstrated by co-immunoprecipitation of the endogenous proteins from cell lysates. Although the canonical role of eEF1A resides in protein synthesis, it has also been implicated in other roles, including regulating the activity of some signaling enzymes. Thus, we examined the potential role of eEF1A in regulation of the SKs and show that eEF1A is able to directly increase the activity of SK1 and SK2 approximately 3-fold in vitro. Substrate kinetics demonstrated that eEF1A increased the catalytic rate of both SKs, while having no observable effect on substrate affinities of these enzymes for either ATP or sphingosine. Overexpression of eEF1A in quiescent Chinese hamster ovary cells increased cellular SK activity, whereas a small interfering RNA-mediated decrease in eEF1A levels in MCF7 cells substantially reduced cellular SK activity and S1P levels, supporting the in vivo physiological relevance of this interaction. Thus, this study has established a novel mechanism of regulation of both SK1 and SK2 that is mediated by their interaction with eEF1A.

Unraveling the complexities of sphingosine-1-phosphate function: the mast cell model

Sphingosine-1-phosphate (S1P) is a lipid mediator involved in diverse biological processes, from vascular and neural development to the regulation of lymphocyte trafficking. Many of its functions are regulated by five widely expressed S1P G-protein-coupled receptors (S1P(1-5)). S1P is produced mostly intracellularly, thus, much of its potential as an autocrine and paracrine mediator depends on how, when, and where it is generated or secreted out of the cells. However, S1P can also have intracellular activity independent of its receptors, adding to the complexity of S1P function. The mast cell, a major effector cell during an allergic response, has proven instrumental towards understanding the complex regulation and function of S1P. Antigen (Ag) engagement of the IgE receptor in mast cells stimulates sphingosine kinases, which generate S1P and are involved in the activation of calcium fluxes critical for mast cell responses. In addition, mast cells secrete considerable amounts of S1P upon activation, thus affecting the surrounding tissues and recruiting inflammatory cells. Export of S1P is also involved in the autocrine transactivation of S1P receptors present in mast cells. The in vivo response of mast cells, however, is not strictly dependent on their ability to generate S1P, but they are also affected by changes in S1P in the environment previous to Ag challenge. This review will discuss the recent advances towards understanding the intricacies of S1P generation, secretion and regulation in mast cells. In addition, how S1P receptors are activated and their involvement in mast cell functions will also be covered, including new insights on the role of S1P in the mast cell-mediated allergic response of systemic anaphylaxis.

Distinct roles of sphingosine kinases 1 and 2 in human mast-cell functions

Sphingosine-1-phosphate (S1P) is now emerging as a potent lipid mediator produced by mast cells that contributes to inflammatory and allergic responses. In contrast to its weak effect on degranulation of murine mast cells, S1P potently induced degranulation of the human LAD2 mast-cell line and cord blood-derived human mast cells (hMCs). S1P also stimulated production and secretion of cytokines, TNF-alpha and IL-6, and markedly enhanced secretion of a chemokine, CCL2/MCP-1, important modulators of inflammation. S1P is produced in mast cells by the 2 sphingosine kinases, SphK1 and SphK2. SphK1 but not SphK2 plays a critical role in IgE/Ag-induced degranulation, migration toward antigen, and CCL2 secretion from hMCs, as determined by specifically down-regulating their expression. However, both isoenzymes were required for efficient TNF-alpha secretion. Taken together, our data suggest that differential formation of S1P by SphK1 and SphK2 has distinct and important actions in hMCs.

New insights on mast cell activation via the high affinity receptor for IgE

Mast cells are innate immune cells that function as regulatory or effector cells and serve to amplify adaptive immunity. In adaptive immunity these cells function primarily through cell surface Fc receptors that bind immunoglobulin antibodies. The dysregulation of their adaptive role makes them central players in allergy and asthma. Upon encountering an allergen (antigen), which is recognized by immunoglobulin E (IgE) antibodies bound to the high affinity IgE receptor (FcepsilonRI) expressed on their cell surface, mast cells secrete both preformed and newly synthesized mediators of the allergic response. Blocking of these responses is an objective in therapeutic intervention of allergic diseases. Thus, understanding the mechanisms by which antigens elicit mast cell activation (via FcepsilonRI) holds promise toward identifying therapeutic targets. Here we review the most recent advances in understanding antigen-dependent mast cell activation. Specifically, we focus on the requirements for FcepsilonRI activation, the regulation of calcium responses, co-stimulatory signals in FcepsilonRI-mediated mast cell activation and function, and how genetics influences mast cell signaling and responses. These recent discoveries open new avenues of investigation with therapeutic potential.

Autocrine and paracrine roles of sphingosine-1-phosphate

Sphingosine-1-phosphate (S1P) is a bioactive sphingolipid metabolite that has been implicated in many biological processes, including cell migration, survival, proliferation, angiogenesis and immune and allergic responses. S1P levels inside cells are regulated tightly by the balance between its synthesis by sphingosine kinases and degradation by S1P lyases and S1P phosphatases. Activation of sphingosine kinase by any of a variety of agonists increases S1P levels, which in turn can function intracellularly as a second messenger or in an autocrine and/or paracrine fashion to activate and signal through S1P receptors present on the surface of the cell. This review summarizes recent findings on the roles of S1P as a mediator of the actions of cytokines, growth factors and hormones.

Src family kinases and lipid mediators in control of allergic inflammation

The Src family kinases Fyn and Lyn are important modulators of the molecular events initiated by engagement of the high-affinity IgE receptor (Fc epsilon RI). These kinases control many of the early signaling events and initiate the production of several lipid metabolites that have an important role in regulating mast cell responses. The intracellular level of phosphatidylinositol (3,4,5)-trisphosphate (PIP(3)), which is produced by phosphatidylinositol 3-OH kinase, plays an important role in determining the extent of a mast cells response to a stimulus. Enhanced levels lead to a hyperdegranulating phenotype (as seen in SHIP-1(-/-) and Lyn(-/-) mast cells), whereas decreased levels cause hypodegranulation (as seen in Fyn(-/-) mast cells). Downregulation of mast cell phosphatase and tensin homologue deleted on chromosone 10 expression, a phosphatase that reduces cellular levels of PIP(3), caused constitutive cytokine production, demonstrating that this response is particularly sensitive to PIP(3) levels. Lyn and Fyn are also intimately linked to other lipid kinases, like sphingosine kinases (SphK). By producing sphingosine-1-phosphate (S1P), SphKs contribute to mast cell chemotaxis and degranulation. In vivo studies now reveal that circulating S1P as well as that found within the mast cell is important in determining mast cell responsiveness. These studies demonstrate the connection between Src protein tyrosine kinases and lipid second messengers that control mast cell function and allergic responses.

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

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

New developments in FcepsilonRI regulation, function and inhibition

The high-affinity Fc receptor for IgE (FcepsilonRI), a multimeric immune receptor, is a crucial structure for IgE-mediated allergic reactions. In recent years, advances have been made in several important areas of the study of FcepsilonRI. The first area relates to FcepsilonRI-mediated biological responses that are antigen independent. The second area encompasses the biological relevance of the distinct signalling pathways that are activated by FcepsilonRI; and the third area relates to the accumulated evidence for the tight control of FcepsilonRI signalling through a broad array of inhibitory mechanisms, which are being developed into promising therapeutic approaches.

Metabolism and biological functions of two phosphorylated sphingolipids, sphingosine 1-phosphate and ceramide 1-phosphate

Sphingolipids are major lipid constituents of the eukaryotic plasma membrane. Without certain sphingolipids, cells and/or embryos cannot survive, indicating that sphingolipids possess important physiological functions that are not substituted for by other lipids. One such role may be signaling. Recent studies have revealed that some sphingolipid metabolites, such as long-chain bases (LCBs; sphingosine (Sph) in mammals), long-chain base 1-phosphates (LCBPs; sphingosine 1-phosphate (S1P) in mammals), ceramide (Cer), and ceramide 1-phosphate (C1P), act as signaling molecules. The addition of phosphate groups to LCB/Sph and Cer generates LCBP/S1P and C1P, respectively. These phospholipids exhibit completely different functions than those of their precursors. In this review, we describe recent advances in understanding the functions of LCBP/S1P and C1P in mammals and in the yeast Saccharomyces cerevisiae. Since LCB/Sph, LCBP/S1P, Cer, and C1P are mutually convertible, regulation of not only the total amount of the each lipid but also of the overall balance in cellular levels is important. Therefore, we describe in detail their metabolic pathways, as well as the genes involved in each reaction.

The sphingosine kinase-sphingosine-1-phosphate axis is a determinant of mast cell function and anaphylaxis

Sphingosine-1-phosphate, a key mediator in immune cell trafficking, is elevated in the lungs of asthmatic patients and regulates pulmonary epithelium permeability. Stimulation of mast cells by allergens induces two mammalian sphingosine kinases (Sphk1 and Sphk2) to produce sphingosine-1-phosphate (S1P). Little is known about the individual role of these kinases in regulating immune cell function. Here we show that in mast cells, Sphk2 is required for production of S1P, for calcium influx, for activation of protein kinase C, and for cytokine production and degranulation. However, susceptibility to in vivo anaphylaxis is determined both by S1P within the mast cell compartment and by circulating S1P generated by Sphk1 predominantly from a non-mast cell source(s). Thus, sphingosine kinases are determinants of mast cell responsiveness, demonstrating a previously unrecognized relationship with anaphylaxis.

Sphingosine kinase type 2 activation by ERK-mediated phosphorylation

Sphingosine 1-phosphate (S1P), a potent lipid mediator, is a ligand for a family of five G protein-coupled receptors (S1P(1-5)) that have been shown to regulate a variety of biological responses important for cancer progression. The cellular level of S1P is low and tightly regulated in a spatio-temporal manner through its synthesis catalyzed by two sphingosine kinases, denoted SphK1 and SphK2. Many stimuli activate and translocate SphK1 to the plasma membrane by mechanisms that are dependent on its phosphorylation. Much less is known about activation of SphK2. Here we demonstrate that epidermal growth factor (EGF) as well as the protein kinase C activator, phorbol ester, induce rapid phosphorylation of hSphK2 which was markedly reduced by inhibition of MEK1/ERK pathway. Down-regulation of ERK1 blocked EGF-induced phosphorylation of SphK2. Recombinant ERK1 phosphorylated hSphK2 in vitro and increased its enzymatic activity. ERK1 also was found to be in a complex with hSphK2 in vivo. Site-directed mutagenesis indicated that hSphK2 is phosphorylated on Ser-351 and Thr-578 by ERK1 and that phosphorylation of these residues is important for EGF-stimulated migration of MDA-MB-453 cells. These studies provide the first clues to the mechanism of agonist-mediated SphK2 activation and enhance understanding of the regulation of SphK2 activity by phosphorylation and its role in movement of human breast cancer cells toward EGF.

Adaptive and innate immune reactions regulating mast cell activation: from receptor-mediated signaling to responses

In this article, we have described studies that have demonstrated that mast cells can be activated as a consequence of adaptive and innate immune reactions and that these responses can be modified by ligands for other receptors expressed on the surface of mast cells. These various stimuli differentially activate multiple signaling pathways within the mast cells required for the generation and/or release of inflammatory mediators. Thus, the composition of the suite of mediators released and the physiologic ramifications of these responses are dependent on the stimuli and the microenvironment in which the mast cells are activated. Knowledge of the different signaling molecules used by cell surface receptors may allow selective pharmacologic targeting such that inhibiting the adverse effects of mast cell activation can be achieved without influencing the beneficial effects of mast cell activation. The exact interconnections between the signaling pathways initiated by the surface receptors described in this article remain to be completely worked out; thus, this remains a topic for future investigation.

Role of ABCC1 in export of sphingosine-1-phosphate from mast cells

Mast cells play a pivotal role in inflammatory and immediate-type allergic reactions by secreting a variety of potent inflammatory mediators, including sphingosine-1-phosphate (S1P). However, it is not known how S1P is released from cells. Here, we report that S1P is exported from mast cells independently of their degranulation and demonstrate that it is mediated by ATP binding cassette (ABC) transporters. Constitutive and antigen-stimulated S1P release was inhibited by MK571, an inhibitor of ABCC1 (MRP1), but not by inhibitors of ABCB1 (MDR-1, P-glycoprotein). Moreover, down-regulation of ABCC1 with small interfering RNA, which decreased its cell surface expression, markedly reduced S1P export from both rat RBL-2H3 and human LAD2 mast cells. Transport of S1P by ABCC1 influenced migration of mast cells toward antigen but not degranulation. These findings have important implications for S1P functions in mast cell-mediated immune responses.

Sphingosine 1-phosphate phosphatase 2 is induced during inflammatory responses

Sphingosine 1-phosphate (S1P) levels in cells and, consequently, its bioactivity as a signalling molecule are controlled by the action of enzymes responsible for its synthesis and degradation. In the present report, we examined alterations in expression patterns of enzymes involved in S1P-metabolism (sphingosine kinases including their splice variants, sphingosine 1-phosphate phosphatases, and sphingosine 1-phosphate lyase) under certain inflammatory conditions. We found that sphingosine kinase type 1 (SPHK1) mRNA could be triggered in a cell type-specific manner; individual SPHK1 splice variants were induced with similar kinetics. Remarkably, expression and activity of S1P phosphatase 2 (SPP2) was found to be highly upregulated by inflammatory stimuli in a variety of cells (e.g., neutrophils, endothelial cells). Bandshift analysis using oligonucleotides spanning predicted NFkappaB sites within the SPP2 promoter and silencing of NFkappaB/RelA via RelA-directed siRNA demonstrated that SPP2 is an NFkappaB-dependent gene. Silencing of SPP2 expression in endothelial cells, in turn, led to a marked reduction of TNF-alpha-induced IL-1beta mRNA and protein and to a partial reduction of induced IL-8, suggesting a pro-inflammatory role of SPP2. Notably, up-regulation of SPP2 was detected in samples of lesional skin of patients with psoriasis, an inflammatory skin disease. This study provides detailed insights into the regulation of SPP2 gene expression and suggests that SPP2 might be a novel player in pro-inflammatory signalling.

Sphingosine kinases, sphingosine 1-phosphate, apoptosis and diseases

Sphingolipids are ubiquitous components of cell membranes and their metabolites ceramide (Cer), sphingosine (Sph), and sphingosine-1-phosphate (S1P) have important physiological functions, including regulation of cell growth and survival. Cer and Sph are associated with growth arrest and apoptosis. Many stress stimuli increase levels of Cer and Sph, whereas suppression of apoptosis is associated with increased intracellular levels of S1P. In addition, extracellular/secreted S1P regulates cellular processes by binding to five specific G protein coupled-receptors (GPCRs). S1P is generated by phosphorylation of Sph catalyzed by two isoforms of sphingosine kinases (SphK), type 1 and type 2, which are critical regulators of the "sphingolipid rheostat", producing pro-survival S1P and decreasing levels of pro-apoptotic Sph. Since sphingolipid metabolism is often dysregulated in many diseases, targeting SphKs is potentially clinically relevant. Here we review the growing recent literature on the regulation and the roles of SphKs and S1P in apoptosis and diseases.

Development, migration, and survival of mast cells

Mast cells play a pivotal role in immediate hypersensitivity and chronic allergic reactions that can contribute to asthma, atopic dermatitis, and other allergic diseases. Because mast cell numbers are increased at sites of inflammation in allergic diseases, pharmacologic intervention into the proliferation, migration, and survival (or apoptosis) of mast cells could be a promising strategy for the management of allergic diseases. Mast cells differentiate from multipotent hematopoietic progenitors in the bone marrow. Stem cell factor (SCF) is a major chemotactic factor for mast cells and their progenitors. SCF also elicits cell-cell and cell-substratum adhesion, facilitates the proliferation, and sustains the survival, differentiation, and maturation, of mast cells. Therefore, many aspects of mast cell biology can be understood as interactions of mast cells and their precursors with SCF and factors that modulate their responses to SCF and its signaling pathways. Numerous factors known to have such a capacity include cytokines that are secreted from activated T cells and other immune cells including mast cells themselves. Recent studies also demonstrated that monomeric IgE binding to FcepsilonRI can enhance mast-cell survival. In this review we discuss the factors that regulate mast cell development, migration, and survival.