Sphingosine 1-phosphate modulates antigen capture by murine Langerhans cells via the S1P2 receptor subtype

Bioactive lipids in the skin barrier mediate its functionality in health and disease

Our skin protects us from external threats including ultraviolet radiation, pathogens and chemicals, and prevents excessive trans-epidermal water loss. These varied activities are reliant on a vast array of lipids, many of which are unique to skin, and that support physical, microbiological and immunological barriers. The cutaneous physical barrier is dependent on a specific lipid matrix that surrounds terminally-differentiated keratinocytes in the stratum corneum. Sebum- and keratinocyte-derived lipids cover the skin's surface and support and regulate the skin microbiota. Meanwhile, lipids signal between resident and infiltrating cutaneous immune cells, driving inflammation and its resolution in response to pathogens and other threats. Lipids of particular importance include ceramides, which are crucial for stratum corneum lipid matrix formation and therefore physical barrier functionality, fatty acids, which contribute to the acidic pH of the skin surface and regulate the microbiota, as well as the stratum corneum lipid matrix, and bioactive metabolites of these fatty acids, involved in cell signalling, inflammation, and numerous other cutaneous processes. These diverse and complex lipids maintain homeostasis in healthy skin, and are implicated in many cutaneous diseases, as well as unrelated systemic conditions with skin manifestations, and processes such as ageing. Lipids also contribute to the gut-skin axis, signalling between the two barrier sites. Therefore, skin lipids provide a valuable resource for exploration of healthy cutaneous processes, local and systemic disease development and progression, and accessible biomarker discovery for systemic disease, as well as an opportunity to fully understand the relationship between the host and the skin microbiota. Investigation of skin lipids could provide diagnostic and prognostic biomarkers, and help identify new targets for interventions. Development and improvement of existing in vitro and in silico approaches to explore the cutaneous lipidome, as well as advances in skin lipidomics technologies, will facilitate ongoing progress in skin lipid research.

Pathogenic sphingosine 1-phosphate pathway in psoriasis: a critical review of its pathogenic significance and potential as a therapeutic target

Sphingosine-1-phosphate (S1P) is a sphingolipid mediator that exerts a variety of biological functions, including immune, cardiovascular, and neurological regulation as well as tumor promotion, through high-affinity G protein-coupled receptors (S1P_1-5). It has been reported that circulating S1P levels remain higher in patients with psoriasis than in healthy individuals and that circulating S1P levels do not decrease after anti-TNF-α treatment in those patients. The S1P-S1PR signaling system plays an important role in inhibiting keratinocyte proliferation, regulating lymphocyte migration, and promoting angiogenesis, thus contributing to the regulation of psoriasis pathogenesis. Here, we review the mechanisms by which S1P-S1PR signaling affects the development of psoriasis and the available clinical/preclinical evidence for targeting S1P-S1PR in psoriasis. S1P-S1PR signaling mechanisms may partially explain the link between psoriasis and its comorbidities. Although the detailed mechanisms remain to be elucidated, S1P may be a new target for future psoriasis remission.

Sphingosine 1-Phosphate as Essential Signaling Molecule in Inflammatory Skin Diseases

Sphingolipids are crucial molecules of the mammalian epidermis. The formation of skin-specific ceramides contributes to the formation of lipid lamellae, which are important for the protection of the epidermis from excessive water loss and protect the skin from the invasion of pathogens and the penetration of xenobiotics. In addition to being structural constituents of the epidermal layer, sphingolipids are also key signaling molecules that participate in the regulation of epidermal cells and the immune cells of the skin. While the importance of ceramides with regard to the proliferation and differentiation of skin cells has been known for a long time, it has emerged in recent years that the sphingolipid sphingosine 1-phosphate (S1P) is also involved in processes such as the proliferation and differentiation of keratinocytes. In addition, the immunomodulatory role of this sphingolipid species is becoming increasingly apparent. This is significant as S1P mediates a variety of its actions via G-protein coupled receptors. It is, therefore, not surprising that dysregulation in the signaling pathways of S1P is involved in the pathophysiological conditions of skin diseases. In the present review, the importance of S1P in skin cells, as well as the immune cells of the skin, is elaborated. In particular, the role of the molecule in inflammatory skin diseases will be discussed. This is important because interfering with S1P signaling pathways may represent an innovative option for the treatment of inflammatory skin diseases.

Sphingosine 1-Phosphate Signaling at the Skin Barrier Interface

Sphingosine 1-phosphate (S1P) is a product of membrane sphingolipid metabolism. S1P is secreted and acts via G-protein-coupled receptors, S1PR1-5, and is involved in diverse cellular functions, including cell proliferation, immune suppression, and cardiovascular functions. Recent studies have shown that the effects of S1P signaling are extended further by coupling the different S1P receptors and their respective downstream signaling pathways. Our group has recently reported that S1P inhibits cell proliferation and induces differentiation in human keratinocytes. There is a growing understanding of the connection between S1P signaling, skin barrier function, and skin diseases. For example, the activation of S1PR1 and S1PR2 during bacterial invasion regulates the synthesis of inflammatory cytokines in human keratinocytes. Moreover, S1P-S1PR2 signaling is involved in the production of inflammatory cytokines and can be triggered by epidermal mechanical stress and bacterial invasion. This review highlights how S1P affects human keratinocyte proliferation, differentiation, immunoreaction, and mast cell immune response, in addition to its effects on the skin barrier interface. Finally, studies targeting S1P-S1PR signaling involved in inflammatory skin diseases are also presented.

Ceramides in Skin Health and Disease: An Update

Ceramides are a class of sphingolipid that is the backbone structure for all sphingolipids, such as glycosphingolipids and phosphosphingolipids. While being a minor constituent of cellular membranes, ceramides are the major lipid component (along with cholesterol, free fatty acid, and other minor components) of the intercellular spaces of stratum corneum that forms the epidermal permeability barrier. These stratum corneum ceramides consist of unique heterogenous molecular species that have only been identified in terrestrial mammals. Alterations of ceramide molecular profiles are characterized in skin diseases associated with compromised permeability barrier functions, such as atopic dermatitis, psoriasis and xerosis. In addition, hereditary abnormalities of some ichthyoses are associated with an epidermal unique ceramide species, omega-O-acylceramide. Ceramides also serve as lipid modulators to regulate cellular functions, including cell cycle arrest, differentiation, and apoptosis, and it has been demonstrated that changes in ceramide metabolism also cause certain diseases. In addition, ceramide metabolites, sphingoid bases, sphingoid base-1-phosphate and ceramide-1-phosphate are also lipid mediators that regulate cellular functions. In this review article, we describe diverse physiological and pathological roles of ceramides and their metabolites in epidermal permeability barrier function, epidermal cell proliferation and differentiation, immunity, and cutaneous diseases. Finally, we summarize the utilization of ceramides as therapy to treat cutaneous disease.

Novel functions of S1P in chronic itchy and inflammatory skin diseases

S1P is a pleotropic sphingolipid signalling molecule that acts through binding to five high-affinity G-protein coupled receptors. S1P-signaling affects cell fate in a multitude of ways, e.g. influencing cell differentiation, proliferation, and apoptosis, as well as playing an important role in immune cell trafficking. Though many effects of S1P-signaling in the human body have been discovered, the full range of functions is yet to be understood. For inflammatory skin diseases such as atopic dermatitis and psoriasis, evidence is emerging that dysfunction and imbalance of the S1P-axis is a contributing factor. Multiple studies investigating the efficacy of S1PR modulators in alleviating the severity and symptoms of skin conditions in various animal models and human clinical trials have shown promising results and validated the interest in the S1P-axis as a potential therapeutic target. Even though the involvement of S1P-signalling in inflammatory skin diseases still requires further clarification, the implications of the recent findings may prompt expansion of research to additional skin conditions and more S1P-axis modulatory pharmaceuticals.

Uncovering the 'sphinx' of sphingosine 1-phosphate signalling: from cellular events to organ morphogenesis

Sphingosine 1-phosphate (S1P) is a bioactive sphingolipid metabolite, functioning as a signalling molecule in diverse cellular processes. Over the past few decades, studies of S1P signalling have revealed that the physiological activity of S1P largely depends on S1P metabolizing enzymes, transporters and receptors on the plasma membrane, as well as on the intracellular proteins that S1P binds directly to. In addition to its roles in cancer signalling, immunity and inflammation, a large body of evidence has identified a close link of S1P signalling with organ morphogenesis. Here we discuss the vital role of S1P signalling in orchestrating various cellular events during organ morphogenesis through analysing each component along the extracellular and intracellular S1P signalling axes. For each component, we review advances in our understanding of S1P signalling and function from the upstream regulators to the downstream effectors and from cellular behaviours to tissue organization, primarily in the context of morphogenetic mechanisms. S1P-mediated vesicular trafficking is also discussed as a function independent of its signalling function. A picture emerges that reveals a multifaceted role of S1P-dependent pathways in the development and maintenance of organ structure and function.

Atopic dermatitis: an expanding therapeutic pipeline for a complex disease

Atopic dermatitis (AD) is a common chronic inflammatory skin disease with a complex pathophysiology that underlies a wide spectrum of clinical phenotypes. AD remains challenging to treat owing to the limited response to available therapies. However, recent advances in understanding of disease mechanisms have led to the discovery of novel potential therapeutic targets and drug candidates. In addition to regulatory approval for the IL-4Ra inhibitor dupilumab, the anti-IL-13 inhibitor tralokinumab and the JAK1/2 inhibitor baricitinib in Europe, there are now more than 70 new compounds in development. This Review assesses the various strategies and novel agents currently being investigated for AD and highlights the potential for a precision medicine approach to enable prevention and more effective long-term control of this complex disease.

Recent advances in understanding of the complex phenotype and mechanisms underlying atopic dermatitis (AD) have revealed multiple new potential targets for pharmacological intervention. Here, Bieber reviews therapeutic strategies and assesses the expanding pipeline for the therapy of AD, highlighting the potential for a precision medicine approach to the management of this complex disorder.

The Importance of Metabolism for Immune Homeostasis in Allergic Diseases

There is increasing evidence that the metabolic status of T cells and macrophages is associated with severe phenotypes of chronic inflammation, including allergic inflammation. Metabolic changes in immune cells have a crucial role in their inflammatory or regulatory responses. This notion is reinforced by metabolic diseases influencing global energy metabolism, such as diabetes or obesity, which are known risk factors of severity in inflammatory conditions, due to the metabolic-associated inflammation present in these patients. Since several metabolic pathways are closely tied to T cell and macrophage differentiation, a better understanding of metabolic alterations in immune disorders could help to restore and modulate immune cell functions. This link between energy metabolism and inflammation can be studied employing animal, human or cellular models. Analytical approaches rank from classic immunological studies to integrated analysis of metabolomics, transcriptomics, and proteomics. This review summarizes the main metabolic pathways of the cells involved in the allergic reaction with a focus on T cells and macrophages and describes different models and platforms of analysis used to study the immune system and its relationship with metabolism.

Blockage of sphingosine-1-phosphate receptor 2 attenuates 2,4-dinitrochlorobenzene-induced atopic dermatitis in mice

Sphingosine-1-phosphate (S1P) and its receptors have been implicated in functions of Langerhans cells and atopic dermatitis. In this study, we investigated the roles of S1P receptor type 2 (S1P₂) in a mouse model of atopic dermatitis, which was induced by topical application of 2,4-dinitrochlorobenzene (DNCB) on ventral skin on D0, followed by repeated DNCB challenge on both ears from D7 to D49. Wild-type mice with atopic dermatitis displayed severe inflammation and mast cell accumulation in ear tissues and elevated IgE levels in serum. Furthermore, the mice showed significantly increased sizes of draining lymph nodes, high levels of inflammatory cytokines (IL-4, IL-13, IL-17, and IFN-γ) in the ears and lymph nodes and high levels of chemokines CCL17 and CCL22 in ears. Administration of JTE-013, a selective antagonist of S1P₂ (3 mg/kg, i.p, from D19 to D49) before DNCB challenge significantly suppressed DNCB-induced atopic responses in ears and lymph nodes. JTE-013 administration also significantly decreased the lymph nodes sizes, the levels of inflammatory cytokines (IL-4, IL-13, IL-17, and IFN-γ) in the ears and lymph nodes, and the levels of chemokines CCL17 and CCL22 in ears. Furthermore, the inflammatory responses of atopic dermatitis were greatly ameliorated in S1pr2 gene-deficient mice. As CCL17 and CCL22 are CCR4 ligands, acting as Th2-attracting chemokines, we investigated CCL17 and CCL22 expression in bone marrow-derived dendritic cells (BMDCs) from wild-type and S1pr2 gene-deficient mice. Addition of IL-4 (10 ng/mL) markedly increased the levels of CCL17 and CCL22, but IL-4-induced CCL17 and CCL22 expression was significantly blunted in BMDCs from S1pr2 gene-deficient mice. Furthermore, pretreatment with JTE-013 (1-30 μM) dose-dependently suppressed this induction in BMDCs from wild-type mice. Our results demonstrate that blockage of S1P₂ ameliorates not only DNCB-induced atopic dermatitis symptoms but also Th2 cell-attracting capacity of dendritic cells, suggesting S1P₂ as a potential therapeutic target for atopic dermatitis.

Topical Application of S1P2 Antagonist JTE-013 Attenuates 2,4-Dinitrochlorobenzene-Induced Atopic Dermatitis in Mice

Sphingosine-1-phosphate (S1P) and its receptors have been implicated in atopic dermatitis. S1P₂ was found to function as a pro-allergic receptor, while its antagonist JTE-013 was found to suppress allergic asthma in mice. Topical application of JTE-013 has not been investigated in an in vivo model of atopic dermatitis. Therefore, the therapeutic potential of JTE-013 topical application was evaluated by the use of a 2,4-dinitrochlorobenzene (DNCB)-induced atopic dermatitis mouse model. DNCB-induced inflammation and mast cell accumulation in skin tissues were significantly suppressed by topical JTE-013 treatment in BALB/c mice. DNCB-induced increase of lymph nodes sizes and elevated inflammatory cytokines (IL-4, IL-13, IL-17, and IFN-γ) in lymph nodes were also significantly reduced by the JTE-013 treatment. Elevated serum levels of IgE were significantly suppressed by the topical treatment of JTE-013. In summary, the topical treatment of JTE-013 S1P₂ antagonist suppressed DNCB-induced atopic dermatitis symptoms and immune responses. These results suggested JTE-013 as a potential therapeutic agent for atopic dermatitis.

Plasma Levels of the Bioactive Sphingolipid Metabolite S1P in Adult Cystic Fibrosis Patients: Potential Target for Immunonutrition?

Recent research has linked sphingolipid (SL) metabolism with cystic fibrosis transmembrane conductance regulator (CFTR) activity, affecting bioactive lipid mediator sphingosine-1-phosphate (S1P). We hypothesize that loss of CFTR function in cystic fibrosis (CF) patients influenced plasma S1P levels. Total and unbound plasma S1P levels were measured in 20 lung-transplanted adult CF patients and 20 healthy controls by mass spectrometry and enzyme-linked immunosorbent assay (ELISA). S1P levels were correlated with CFTR genotype, routine laboratory parameters, lung function and pathogen colonization, and clinical symptoms. Compared to controls, CF patients showed lower unbound plasma S1P, whereas total S1P levels did not differ. A positive correlation of total and unbound S1P levels was found in healthy controls, but not in CF patients. Higher unbound S1P levels were measured in ΔF508-homozygous compared to ΔF508-heterozygous CF patients (p = 0.038), accompanied by higher levels of HDL in ΔF508-heterozygous patients. Gastrointestinal symptoms were more common in ΔF508 heterozygotes compared to ΔF508 homozygotes. This is the first clinical study linking plasma S1P levels with CFTR function and clinical presentation in adult CF patients. Given the emerging role of immunonutrition in CF, our study might pave the way for using S1P as a novel biomarker and nutritional target in CF.

S1P Signaling in the Tumor Microenvironment

Sphingosine-1-phosphate (S1P), together with other phosphosphingolipids, has been found to regulate complex cellular function in the tumor microenvironment (TME) where it acts as a signaling molecule that participates in cell-cell communication. S1P, through intracellular and extracellular signaling, was found to promote tumor growth, angiogenesis, chemoresistance, and metastasis; it also regulates anticancer immune response, modulates inflammation, and promotes angiogenesis. Interestingly, cancer cells are capable of releasing S1P and thus modifying the behavior of the TME components in a way that contributes to tumor growth and progression. Therefore, S1P is considered an important therapeutic target, and several anticancer therapies targeting S1P signaling are being developed and tested in clinics.

Novel Insights into the Role of HDL-Associated Sphingosine-1-Phosphate in Cardiometabolic Diseases

Sphingolipids are key signaling molecules involved in the regulation of cell physiology. These species are found in tissues and in circulation. Although they only constitute a small fraction in lipid composition of circulating lipoproteins, their concentration in plasma and distribution among plasma lipoproteins appears distorted under adverse cardiometabolic conditions such as diabetes mellitus. Sphingosine-1-phosphate (S1P), one of their main representatives, is involved in regulating cardiomyocyte homeostasis in different models of experimental cardiomyopathy. Cardiomyopathy is a common complication of diabetes mellitus and represents a main risk factor for heart failure. Notably, plasma concentration of S1P, particularly high-density lipoprotein (HDL)-bound S1P, may be decreased in patients with diabetes mellitus, and hence, inversely related to cardiac alterations. Despite this, little attention has been given to the circulating levels of either total S1P or HDL-bound S1P as potential biomarkers of diabetic cardiomyopathy. Thus, this review will focus on the potential role of HDL-bound S1P as a circulating biomarker in the diagnosis of main cardiometabolic complications frequently associated with systemic metabolic syndromes with impaired insulin signaling. Given the bioactive nature of these molecules, we also evaluated its potential of HDL-bound S1P-raising strategies for the treatment of cardiometabolic disease.

Sphingosine 1-phosphate-mediated activation of ezrin-radixin-moesin proteins contributes to cytoskeletal remodeling and changes of membrane properties in epithelial otic vesicle progenitors

Hearing loss is among the most prevalent sensory impairments in humans. Cochlear implantable devices represent the current therapies for hearing loss but have various shortcomings. ERM (ezrin- radixin -moesin) are a family of adaptor proteins that link plasma membrane with actin cytoskeleton, playing a crucial role in cell morphology and in the formation of membrane protrusions. Recently, bioactive sphingolipids have emerged as regulators of ERM proteins. Sphingosine 1-phosphate (S1P) is a pleiotropic sphingolipid which regulates fundamental cellular functions such as proliferation, survival, migration as well as processes such as development and inflammation mainly via ligation to its specific receptors S1PR (S1P_1-5). Experimental findings demonstrate a key role for S1P signaling axis in the maintenance of auditory function. Preservation of cellular junctions is a fundamental function both for S1P and ERM proteins, crucial for the maintenance of cochlear integrity. In the present work, S1P was found to activate ERM in a S1P₂-dependent manner in murine auditory epithelial progenitors US/VOT-E36. S1P-induced ERM activation potently contributed to actin cytoskeletal remodeling and to the appearance of ionic currents and membrane passive properties changes typical of more differentiated cells. Moreover, PKC and Akt activation was found to mediate S1P-induced ERM phosphorylation. The obtained findings contribute to demonstrate the role of S1P signaling pathway in inner ear biology and to disclose potential innovative therapeutical approaches in the field of hearing loss prevention and treatment.

Crosstalk between core-multishell nanocarriers for cutaneous drug delivery and antigen-presenting cells of the skin

Owing their unique chemical and physical properties core-multishell (CMS) nanocarriers are thought to underlie their exploitable biomedical use for a topical treatment of skin diseases. This highlights the need to consider not only the efficacy of CMS nanocarriers but also the potentially unpredictable and adverse consequences of their exposure thereto. As CMS nanocarriers are able to penetrate into viable layers of normal and stripped human skin ex vivo as well as in in vitro skin disease models the understanding of nanoparticle crosstalk with components of the immune system requires thorough investigation. Our studies highlight the biocompatible properties of CMS nanocarriers on Langerhans cells of the skin as they did neither induce cytotoxicity and genotoxicity nor cause reactive oxygen species (ROS) or an immunological response. Nevertheless, CMS nanocarriers were efficiently taken up by Langerhans cells via divergent endocytic pathways. Bioimaging of CMS nanocarriers by fluorescence lifetime imaging microscopy (FLIM) and flow cytometry indicated not only a localization within the lysosomes but also an energy-dependent exocytosis of unmodified CMS nanocarriers into the extracellular environment.

Mast cells and sphingosine-1-phosphate underlie prelesional remodeling in a mouse model of eczema

BACKGROUND

Atopic dermatitis (AD) is a chronic skin inflammation that affects children and adults worldwide, but its pathogenesis remains ill-understood.

METHODS

We show that a single application of OVA to mouse skin initiates remodeling and cellular infiltration of the hypodermis measured by a newly developed computer-aided method.

RESULTS

Importantly, we demonstrate that skin mast cell (MC) activation and local sphingosine-1-phosphate (S1P) are significantly augmented after OVA treatment in mice. Deficiency in sphingosine kinase (SphK)1, the S1P-producing enzyme, or in MC, remarkably mitigates all signs of OVA-mediated remodeling and MC activation. Furthermore, skin S1P levels remain unchanged in MC-deficient mice exposed to OVA. LPS-free OVA does not recapitulate any of the precursor signs of AD, supporting a triggering contribution of LPS in AD that, per se, suffice to activate local MC and elevate skin S1P.

CONCLUSION

We describe MC and S1P as novel pathogenic effectors that initiate remodeling in AD prior to any skin lesions and reveal the significance of LPS in OVA used in most studies, thus mimicking natural antigen (Ag) exposure.

Nuclear Translocation of SGPP-1 and Decrease of SGPL-1 Activity Contribute to Sphingolipid Rheostat Regulation of Inflammatory Dendritic Cells

A balanced sphingolipid rheostat is indispensable for dendritic cell function and survival and thus initiation of an immune response. Sphingolipid levels are dynamically maintained by the action of sphingolipid enzymes of which sphingosine kinases, S1P phosphatases (SGPP-1/2) and S1P lyase (SGPL-1), are pivotal in the balance of S1P and sphingosine levels. In this study, we present that SGPP-1 and SGPL-1 are regulated in inflammatory dendritic cells and contribute to S1P fate. TLR-dependent activation caused SGPL-1 protein downregulation with subsequent decrease of enzymatic activity by two-thirds. In parallel, confocal fluorescence microscopy revealed that endogenous SGPP-1 was expressed in nuclei of naive dendritic cells and was translocated into the cytoplasmatic compartment upon inflammatory stimulation resulting in dephosphorylation of S1P. Mass spectrometric determination showed that a part of the resulting sphingosine was released from the cell, increasing extracellular levels. Another route of diminishing intracellular S1P was possibly taken by its export via ATP-binding cassette transporter C1 which was upregulated in array analysis, while the S1P transporter, spinster homolog 2, was not relevant in dendritic cells. These investigations newly describe the sequential expression and localization of the endogenous S1P regulators SGPP-1 and SGPL-1 and highlight their contribution to the sphingolipid rheostat in inflammation.

Sphingosine metabolism as a therapeutic target in cutaneous melanoma

Melanoma is by far the most aggressive type of skin cancer with a poor prognosis in its advanced stages. Understanding the mechanisms involved in melanoma pathogenesis, response, and resistance to treatment has gained a lot of attention worldwide. Recently, the role of sphingolipid metabolism has been studied in cutaneous melanoma. Sphingolipids are bioactive lipid effector molecules involved in the regulation of various cellular signaling pathways such as inflammation, cancer cell proliferation, death, senescence, and metastasis. Recent studies suggest that sphingolipid metabolism impacts melanoma pathogenesis and is a potential therapeutic target. This review focuses on defining the role of sphingolipid metabolism in melanoma carcinogenesis, discussing sphingolipid-based therapeutic approaches, and highlighting the areas that require more extensive research.

Sphingosine-1-phosphate modulators in inflammatory skin diseases - lining up for clinical translation

The bioactive lysophospholipid sphingosine-1-phosphate (S1P) is best known for its activity as T-cell-active chemoattractant regulating the egress of T cells from the lymph node and, consequently, the availability of T cells for migration into peripheral tissues. This physiological role of S1P is exploited by the drug fingolimod, a first-line therapy for multiple sclerosis, which "detains" T cells in the lymph nodes. In recent year, it has been elucidated that S1P exerts regulatory functions far beyond T-cell egress from the lymph node. Thus, it additionally regulates, among others, homing of several immune cell populations into peripheral tissues under inflammatory conditions. In addition, evidence, mostly derived from mouse models, has accumulated that S1P may be involved in the pathogenesis of several inflammatory skin disorder and that S1P receptor modulators applied topically are effective in treating skin diseases. These recent developments highlight the pharmacological modulation of the S1P/S1P receptor system as a potential new therapeutic strategy for a plethora of inflammatory skin diseases. The impact of S1P receptor modulation on inflammatory skin diseases next requires testing in human patients.

Specific uptake mechanisms of well-tolerated thermoresponsive polyglycerol-based nanogels in antigen-presenting cells of the skin

Engineered nanogels are of high value for a targeted and controlled transport of compounds due to the ability to change their chemical properties by external stimuli. As it has been indicated that nanogels possess a high ability to penetrate the stratum corneum, it cannot be excluded that nanogels interact with dermal dendritic cells, especially in diseased skin. In this study the potential crosstalk of the thermoresponsive nanogels (tNGs) with the dendritic cells of the skin was investigated with the aim to determine the immunotoxicological properties of the nanogels. The investigated tNGs were made of dendritic polyglycerol (dPG) and poly(glycidyl methyl ether-co-ethyl glycidyl ether) (p(GME-co-EGE)), as polymer conferring thermoresponsive properties. Although the tNGs were taken up, they displayed neither cytotoxic and genotoxic effects nor any induction of reactive oxygen species in the tested cells. Interestingly, specific uptake mechanisms of the tNGs by the dendritic cells were depending on the nanogels cloud point temperature (Tcp), which determines the phase transition of the nanoparticle. The study points to caveolae-mediated endocytosis as being the major tNGs uptake mechanism at 37°C, which is above the Tcp of the tNGs. Remarkably, an additional uptake mechanism, beside caveolae-mediated endocytosis, was observed at 29°C, which is the Tcp of the tNGs. At this temperature, which is characterized by two different states of the tNGs, macropinocytosis was involved as well. In summary, our study highlights the impact of thermoresponsivity on the cellular uptake mechanisms which has to be taken into account if the tNGs are used as a drug delivery system.

Sphingosine-1-Phosphate Receptor-2 Antagonists: Therapeutic Potential and Potential Risks

The sphingosine-1-phosphate (S1P) signaling system with its specific G-protein-coupled S1P receptors, the enzymes of S1P metabolism and the S1P transporters, offers a multitude of promising targets for drug development. Until today, drug development in this area has nearly exclusively focused on (functional) antagonists at the S1P₁ receptor, which cause a unique phenotype of immunomodulation. Accordingly, the first-in class S1P₁ receptor modulator, fingolimod, has been approved for the treatment of relapsing-remitting multiple sclerosis, and novel S1P₁ receptor (functional) antagonists are being developed for autoimmune and inflammatory diseases such as psoriasis, inflammatory bowel disease, lupus erythematodes, or polymyositis. Besides the S1P₁ receptor, also S1P₂ and S1P₃ are widely expressed and regulate many diverse functions throughout the body. The S1P₂ receptor, in particular, often exerts cellular functions which are opposed to the functions of the S1P₁ receptor. As a consequence, antagonists at the S1P₂ receptor have the potential to be useful in a contrasting context and different areas of indication compared to S1P₁ antagonists. The present review will focus on the therapeutic potential of S1P₂ receptor antagonists and discuss their opportunities as well as their potential risks. Open questions and areas which require further investigations will be emphasized in particular.

Activation-Induced Cell Death of Dendritic Cells Is Dependent on Sphingosine Kinase 1

Sphingosine 1-phosphate (S1P) is an immune modulatory lipid mediator and has been implicated in numerous pathophysiological processes. S1P is produced by sphingosine kinase 1 (Sphk1) and Sphk2. Dendritic cells (DCs) are central for the direction of immune responses and crucially involved in autoimmunity and cancerogenesis. In this study we examined the function and survival of bone marrow-derived DCs under long-term inflammatory stimulation. We observed that differentiated cells undergo activation-induced cell death (AICD) upon LPS stimulation with an increased metabolic activity shortly after stimulation, followed by a rapid activation of caspase 3 and subsequent augmented apoptosis. Importantly, we highlight a profound role of Sphk1 in secretion of inflammatory cytokines and survival of dendritic cells that might be mediated by a change in sphingolipid levels as well as by a change in STAT3 expression. Cell growth during differentiation of Sphk1-deficient cells treated with the functional S1P receptor antagonist FTYP was reduced. Importantly, in dendritic cells we did not observe a compensatory regulation of Sphk2 mRNA in Sphk1-deficient cells. Instead, we discovered a massive increase in Sphk1 mRNA concentration upon long-term stimulation with LPS in wild type cells that might function as an attempt to rescue from inflammation-caused cell death. Taken together, in this investigation we describe details of a crucial involvement of sphingolipids and Sphk1 in AICD during long-term immunogenic activity of DCs that might play an important role in autoimmunity and might explain the differences in immune response observed in in vivo studies of Sphk1 modulation.

Fingolimod targeting protein phosphatase 2A differently affects IL-33 induced IL-2 and IFN-γ production in CD8(+) lymphocytes

Multiple sclerosis patients are treated with fingolimod (FTY720), a prodrug that acts as an immune modulator. FTY720 is first phosphorylated to FTY720-P and then internalizes sphingosine-1-phosphate receptors, preventing lymphocyte sequestration. IL-33 is released from necrotic endothelial cells and contributes to MS severity by coactivating T cells. Herein we analyzed the influence of FTY720, FTY720-P, and S1P on IL-33 induced formation of IL-2 and IFN-γ, by using IL-33 receptor overexpressing EL4 cells, primary CD8(+) T cells, and splenocytes. EL4-ST2 cells released IL-2 after IL-33 stimulation that was inhibited dose-dependently by FTY720-P but not FTY720. In this system, S1P increased IL-2, and accordingly, inhibition of S1P producing sphingosine kinases diminished IL-2 release. In primary CD8(+) T cells and splenocytes IL-33/IL-12 stimulation induced IFN-γ, which was prevented by FTY720 but not FTY720-P, independently from intracellular phosphorylation. The inhibition of IFN-γ by nonphosphorylated FTY720 was mediated via the SET/protein phosphatase 2A (PP2A) pathway, since a SET peptide antagonist also prevented IFN-γ formation and the inhibition of IFN-γ by FTY720 was reversible by a PP2A inhibitor. While our findings directly improve the understanding of FTY720 therapy in MS, they could also contribute to side effects of FTY720 treatment, like progressive multifocal leukoencephalopathy, caused by an insufficient immune response to a viral infection.

Sphingosine 1-phospate differentially modulates maturation and function of human Langerhans-like cells

BACKGROUND

As mediators between innate and adaptive immune responses, Langerhans cells (LCs) are in the focus of recent investigations to determine their role in allergic inflammatory diseases like allergic contact dermatitis and atopic dermatitis. Sphingosine 1-phosphate (S1P) is a crucial lipid mediator in the skin and potentially interferes with LC homeostasis but also functional properties, such as cytokine release, migration and antigen-uptake which are considered to be key events in the initiation and maintenance of pathological disorders.

OBJECTIVE

Here, we used human Langerhans-like cells to study the influence of S1P-mediated signalling on LC maturation, cytokine release, migration and endocytosis.

METHODS

Immature Langerhans-like cells were generated from the human acute myeloid leukaemia cell line MUTZ-3 (MUTZ-LCs) and human primary monocytes (MoLCs). S1P receptor expression was determined by quantitative RT-PCR and western blotting. Expression of maturation markers were investigated by flow cytometry. The influence of S1P signalling on cytokine release was quantified by ELISA. Migration assays and FITC-dextran uptake in the presence of S1P, specific S1 P receptor agonists and antagonists as well as fingolimod (FTY720) were analysed through fluorescence microscopy and flow cytometry.

RESULTS

S1P receptor protein expression was confirmed for S1P1, S1P2 and S1P4 in MUTZ-LCs and S1P1 and S1P2 in MoLCs. In mature cells S1P receptors were downregulated. S1P did not induce maturation in MUTZ-LCs, whereas in MoLCs CD83 and CD86 were slightly upregulated. IL-8 release of MUTZ-LCs matured in the presence of S1P was not altered, however, reduced IL-6 and IL-12p70 levels were observed in mature MoLCs. Interestingly, immature MUTZ-LCs revealed a significantly increased S1P-dependent migratory capacity, whereas CCL20 induced migration was significantly decreased in the presence of S1P. Furthermore, migratory capacity towards CCL21 in mature MUTZ-LCs but not MoLCs was significantly lower when cells were stimulated with S1P. S1P, FTY720 and specific S1P receptor agonists did not modulate the endocytotic capacity of immature MUTZ-LCs and MoLCs. These findings were further supported by testing specific antagonists of S1P1-4 in the absence or presence of S1P.

CONCLUSION

Our data demonstrate that S1P regulates key events of human LC maturation including cytokine release and migration. These findings are of particular importance when considering the potential use of S1P in inflammatory skin disorders.

Sphingosine 1-phosphate Receptor 2 Signaling Suppresses Macrophage Phagocytosis and Impairs Host Defense against Sepsis

BACKGROUND

Sepsis is characterized by an inappropriate systemic inflammatory response and bacteremia that promote multiorgan failure and mortality. Sphingosine 1-phosphate receptor 2 (S1PR2) modulates endotoxin-induced inflammation in endothelium. However, as a highly expressed S1P receptor in macrophages, its role in regulating macrophage response to bacterial infection remains unclear.

METHODS

Cecal ligation and puncture or intratracheal instillation of Escherichia coli was induced in wild-type or S1pr2-deficient mice. The antibacterial ability of cell-specific S1PR2 was tested in bone marrow reconstitution mice or mice with macrophage-specific deletion. Signaling molecules responsible for S1PR2-mediated phagocytosis were also measured in the bone marrow-derived macrophages. In addition, S1PR2 expression levels and its correlation with severity of sepsis were determined in critically ill patients (n = 25).

RESULTS

Both genetic deletion and pharmaceutical inhibition of S1PR2 significantly limited bacterial burden, reduced lung damage, and improved survival (genetic deletion, 0% in S1pr2 vs. 78.6% in S1pr2, P < 0.001; pharmaceutical inhibition, 9.1% in vehicle vs. 22.2% in S1PR2 antagonist, P < 0.05). This protection was attributed to the enhanced phagocytic function of S1PR2-deficient macrophages (mean fluorescent intensity, 2035.2 ± 202.1 vs. 407.8 ± 71.6, P < 0.001). Absence of S1PR2 in macrophage inhibits RhoA-dependent cell contraction and promotes IQGAP1-Rac1-dependent lamellipodial protrusion, whose signaling pathways depend on extracellular stimulators. In septic patients, increased S1PR2 levels in peripheral blood mononuclear cells were positively correlated with the severity of sepsis (r = 0.845, P < 0.001).

CONCLUSIONS

This study implies that S1PR2, as a critical receptor in macrophage, impairs phagocytosis and antimicrobial defense in the pathogenesis of sepsis. Interventions targeting S1PR2 signaling may serve as promising therapeutic approaches for sepsis.

Decreased plasma levels of the endothelial protective sphingosine-1-phosphate are associated with dengue-induced plasma leakage

BACKGROUND

A transient endothelial hyperpermeability is a hallmark of severe dengue infections. Sphingosine-1-phosphate (S1P) maintains vascular integrity and protects against plasma leakage. We related plasma S1P levels to dengue-induced plasma leakage and studied mechanisms that may underlie the decrease in S1P levels in dengue.

METHODS

We determined circulating levels of S1P in 44 Indonesian adults with acute dengue and related levels to plasma leakage, as determined by daily ultrasonography, and to levels of its chaperone apolipoprotein M, other lipoproteins and platelets.

RESULTS

Plasma S1P levels were decreased during dengue and patients with plasma leakage had lower median levels compared to those without (638 vs. 745 nM; p < 0.01). ApoM and other lipoprotein levels were also decreased during dengue, but did not correlate to S1P levels. Platelet counts correlated positively with S1P levels, but S1P levels were not higher in frozen-thawed platelet rich plasma, arguing against platelets as an important cellular source of S1P in dengue.

CONCLUSIONS

Decreased plasma S1P levels during dengue are associated with plasma leakage. We speculate that decreased levels of ApoM underlies the lower S1P levels. Modulation of S1P levels and its receptors may be a novel therapeutic intervention to prevent plasma leakage in dengue.

Endothelial sphingosine kinase/SPNS2 axis is critical for vessel-like formation by human mesoangioblasts

The interaction between endothelial cells and pericytes is crucial for the stabilization of newly formed vessels in angiogenesis. The comprehension of the mechanisms regulating pericyte recruitment might open therapeutical perspectives on vascular-related pathologies. Sphingosine 1-phosphate (S1P) is a bioactive sphingolipid that derives from sphingomyelin catabolism and regulates biological functions in cell survival, proliferation, and differentiation. In this study, we aimed to identify the role of S1P axis in the intercellular communication between human mesenchymal progenitor mesoangioblasts (MAB) and endothelial cells (human microvascular endothelial cells (H-MVEC)) in the formation of capillary-like structures. We demonstrated that the S1P biosynthetic pathway brought about by sphingosine kinases (SK) SK1 and SK2 as well as spinster homolog 2 (SPNS2) transporter in H-MVEC is crucial for MAB migration measured by Boyden chambers and for the formation and stabilization of capillary-like structures in a 3D Matrigel culture. Moreover, the conditioned medium (CM) harvested from H-MVEC, where SK1, SK2, and SPNS2 were down-regulated, exerted a significantly diminished effect on MAB capillary morphogenesis and migration. Notably, we demonstrated that S1P1 and S1P3 receptors were positively involved in CM-induced capillary-like formation and migration, while S1P2 exerted a negative role on CM-induced migratory action of MAB. Finally, SK inhibition as well as MAB S1P1 and S1P3 down-regulation impaired H-MVEC-MAB cross-talk significantly reducing in vivo angiogenesis evaluated by Matrigel plug assay. These findings individuate novel targets for the employment of MAB in vascular-related pathologic conditions.

KEY MESSAGE

• Down-regulation of SK1/2 in H-MVEC impaired vessel formation when cultured with MAB. • H-MVEC SPNS2 is critical for morphogenesis and migration induced by H-MVEC CM of MAB. • CM from SK1- and SK2-siRNA H-MVEC impaired morphogenesis and migration of MAB. • S1P1/3 were involved on CM-induced morphogenesis and migration of MAB. • Matrigel plug assay showed the role of S1P axis in MAB-endothelial cell interaction.

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.

Sphingosine 1-phosphate counteracts insulin signaling in pancreatic β-cells via the sphingosine 1-phosphate receptor subtype 2

Glucolipotoxic stress has been identified as a key player in the progression of pancreatic β-cell dysfunction contributing to insulin resistance and the development of type 2 diabetes mellitus (T2D). It has been suggested that bioactive lipid intermediates, formed under lipotoxic conditions, are involved in these processes. Here, we show that sphingosine 1-phosphate (S1P) levels are not only increased in palmitate-stimulated pancreatic β-cells but also regulate β-cell homeostasis in a divergent manner. Although S1P possesses a prosurvival effect in β-cells, an enhanced level of the sphingolipid antagonizes insulin-mediated cell growth and survival via the sphingosine 1-phosphate receptor subtype 2 (S1P2) followed by an inhibition of Akt-signaling. In an attempt to investigate the role of the S1P/S1P2 axis in vivo, the New Zealand obese (NZO) diabetic mouse model, characterized by β-cell loss under high-fat diet (HFD) conditions, was used. The occurrence of T2D was accompanied by an increase of plasma S1P levels. To examine whether S1P contributes to the morphologic changes of islets via S1P2, the receptor antagonist JTE-013 was administered. Most interestingly, JTE-013 rescued β-cell damage clearly indicating an important role of the S1P2 in β-cell homeostasis. Therefore, the present study provides a new therapeutic strategy to diminish β-cell dysfunction and the development of T2D.

The effects of glucose and lipids in steatotic and non-steatotic livers in conditions of partial hepatectomy under ischaemia-reperfusion

BACKGROUND

Steatosis is a risk factor in partial hepatectomy (PH) under ischaemia-reperfusion (I/R), which is commonly applied in clinical practice to reduce bleeding. Nutritional support strategies, as well as the role of peripheral adipose tissue as energy source for liver regeneration, remain poorly investigated.

AIMS

To investigate whether the administration of either glucose or a lipid emulsion could protect steatotic and non-steatotic livers against damage and regenerative failure in an experimental model of PH under I/R. The relevance of peripheral adipose tissue in liver regeneration following surgery is studied.

METHODS

Steatotic and non-steatotic rat livers were subjected to surgery and the effects of either glucose or lipid treatment on damage and regeneration, and part of the underlying mechanisms, were investigated.

RESULTS

In non-steatotic livers, treatment with lipids or glucose provided the same protection against damage, regeneration failure and ATP drop. Adipose tissue was not required to regenerate non-steatotic livers. In the presence of hepatic steatosis, lipid treatment, but not glucose, protected against damage and regenerative failure by induction of cell cycle, maintenance of ATP levels and elevation of sphingosine-1-phosphate/ceramide ratio and phospholipid levels. Peripheral adipose tissue was required for regenerating the steatotic liver but it was not used as an energy source.

CONCLUSION

Lipid treatment in non-steatotic livers provides the same protection as that afforded by glucose in conditions of PH under I/R, whereas the treatment with lipids is preferable to reduce the injurious effects of liver surgery in the presence of steatosis.

Atheroprotective role of high-density lipoprotein (HDL)-associated sphingosine-1-phosphate (S1P)

Numerous epidemiological studies documented an inverse relationship between plasma high-density lipoprotein (HDL) cholesterol levels and the extent of atherosclerotic disease. However, clinical interventions targeting HDL cholesterol failed to show clinical benefits with respect to cardiovascular risk reduction, suggesting that HDL components distinct from cholesterol may account for anti-atherogenic effects attributed to this lipoprotein. Sphingosine-1-phosphate (S1P)-a lysosphingolipid exerting its biological activity via binding to specific G protein-coupled receptors and regulating a wide array of biological responses in a variety of different organs and tissues including the cardiovascular system-has been identified as an integral constituent of HDL particles. In the present review, we discuss current evidence from epidemiological studies, experimental approaches in vitro, and animal models of atherosclerosis, suggesting that S1P contributes to atheroprotective effects exerted by HDL particles.

Enhanced sphingosine-1-phosphate receptor 2 expression underlies female CNS autoimmunity susceptibility

Multiple sclerosis (MS) is an inflammatory disease of the CNS that is characterized by BBB dysfunction and has a much higher incidence in females. Compared with other strains of mice, EAE in the SJL mouse strain models multiple features of MS, including an enhanced sensitivity of female mice to disease; however, the molecular mechanisms that underlie the sex- and strain-dependent differences in disease susceptibility have not been described. We identified sphingosine-1-phosphate receptor 2 (S1PR2) as a sex- and strain-specific, disease-modifying molecule that regulates BBB permeability by destabilizing adherens junctions. S1PR2 expression was increased in disease-susceptible regions of the CNS of both female SJL EAE mice and female patients with MS compared with their male counterparts. Pharmacological blockade or lack of S1PR2 signaling decreased EAE disease severity as the result of enhanced endothelial barrier function. Enhanced S1PR2 signaling in an in vitro BBB model altered adherens junction formation via activation of Rho/ROCK, CDC42, and caveolin endocytosis-dependent pathways, resulting in loss of apicobasal polarity and relocation of abluminal CXCL12 to vessel lumina. Furthermore, S1PR2-dependent BBB disruption and CXCL12 relocation were observed in vivo. These results identify a link between S1PR2 signaling and BBB polarity and implicate S1PR2 in sex-specific patterns of disease during CNS autoimmunity.

Sphingosine-1-phosphate as signaling molecule in the skin: Relevance in atopic dermatitis

Sphingolipids are essential molecules of the mammalian epidermis. Keratinocytes generate and secrete huge amounts of ceramide-precursors to the extracellular domain of the stratum corneum, where they are further metabolized to specific ceramide species. The arrangement of ceramides to well-organized lipid lamellae is essential to form the epidermal barrier. Besides their role as structural components sphingolipids are also critical molecules involved in the modulation of epidermal cells. Sphingosine-1-phosphate (S1P) has been identified as a prominent signaling molecule which regulates fundamental functions of keratinocytes and skin dendritic cells. Thus, S1P inhibits proliferation of keratinocytes and induces their differentiation. Moreover, antigen uptake, migration and cytokine production of dendritic cells are under the control of this sphingolipid. A dysregulation of the sphingolipid metabolism has been discussed in inflammatory skin disorders like atopic dermatitis. Animal models of contact dermatitis provide evidence that topical treatment with S1P is connected with an anti-inflammatory action suggesting a novel approach for the treatment of atopic dermatitis.

Pseudomonas-derived ceramidase induces production of inflammatory mediators from human keratinocytes via sphingosine-1-phosphate

Ceramide is important for water retention and permeability barrier functions in the stratum corneum, and plays a key role in the pathogenesis of atopic dermatitis (AD). A Pseudomonas aeruginosa-derived neutral ceramidase (PaCDase) isolated from a patient with AD was shown to effectively degrade ceramide in the presence of Staphylococcus aureus-derived lipids or neutral detergents. However, the effect of ceramide metabolites on the functions of differentiating keratinocytes is poorly understood. We found that the ceramide metabolite sphingosine-1-phosphate (S1P) stimulated the production of inflammatory mediators such as TNF-α and IL-8 from three-dimensionally cultured human primary keratinocytes (termed "3D keratinocytes"), which form a stratum corneum. PaCDase alone did not affect TNF-α gene expression in 3D keratinocytes. In the presence of the detergent Triton X-100, which damages stratum corneum structure, PaCDase, but not heat-inactivated PaCDase or PaCDase-inactive mutant, induced the production of TNF-α, endothelin-1, and IL-8, indicating that this production was dependent on ceramidase activity. Among various ceramide metabolites, sphingosine and S1P enhanced the gene expression of TNF-α, endothelin-1, and IL-8. The PaCDase-enhanced expression of these genes was inhibited by a sphingosine kinase inhibitor and by an S1P receptor antagonist VPC 23019. The TNF-α-binding antibody infliximab suppressed the PaCDase-induced upregulation of IL-8, but not TNF-α, mRNA. PaCDase induced NF-κB p65 phosphorylation. The NF-κB inhibitor curcumin significantly inhibited PaCDase-induced expression of IL-8 and endothelin-1. VPC 23019 and infliximab inhibited PaCDase-induced NF-κB p65 phosphorylation and reduction in the protein level of the NF-κB inhibitor IκBα. Collectively, these findings suggest that (i) 3D keratinocytes produce S1P from sphingosine, which is produced through the hydrolysis of ceramide by PaCDase, (ii) S1P induces the production of TNF-α via S1P receptors, and (iii) released TNF-α stimulates the production of inflammatory mediators such as IL-8.

Involvement of sphingosine 1-phosphate in palmitate-induced insulin resistance of hepatocytes via the S1P2 receptor subtype

AIMS/HYPOTHESIS

Enhanced plasma levels of NEFA have been shown to induce hepatic insulin resistance, which contributes to the development of type 2 diabetes. Indeed, sphingolipids can be formed via a de novo pathway from the saturated fatty acid palmitate and the amino acid serine. Besides ceramides, sphingosine 1-phosphate (S1P) has been identified as a major bioactive lipid mediator. Therefore, our aim was to investigate the generation and function of S1P in hepatic insulin resistance.

METHODS

The incorporation of palmitate into sphingolipids was performed by rapid-resolution liquid chromatography-MS/MS in primary human and rat hepatocytes. The influence of S1P and the involvement of S1P receptors in hepatic insulin resistance was examined in human and rat hepatocytes, as well as in New Zealand obese (NZO) mice.

RESULTS

Palmitate induced an impressive formation of extra- and intracellular S1P in rat and human hepatocytes. An elevation of hepatic S1P levels was observed in NZO mice fed a high-fat diet. Once generated, S1P was able, similarly to palmitate, to counteract insulin signalling. The inhibitory effect of S1P was abolished in the presence of the S1P2 receptor antagonist JTE-013 both in vitro and in vivo. In agreement with this, the immunomodulator FTY720-phosphate, which binds to all S1P receptors except S1P2, was not able to inhibit insulin signalling.

CONCLUSIONS/INTERPRETATION

These data indicate that palmitate is metabolised by hepatocytes to S1P, which acts via stimulation of the S1P2 receptor to impair insulin signalling. In particular, S1P2 inhibition could be considered as a novel therapeutic target for the treatment of insulin resistance.