Sphingosylphosphorylcholine is a potent inducer of intercellular adhesion molecule-1 expression in human keratinocytes

Review of Eukaryote Cellular Membrane Lipid Composition, with Special Attention to the Fatty Acids

Biological membranes, primarily composed of lipids, envelop each living cell. The intricate composition and organization of membrane lipids, including the variety of fatty acids they encompass, serve a dynamic role in sustaining cellular structural integrity and functionality. Typically, modifications in lipid composition coincide with consequential alterations in universally significant signaling pathways. Exploring the various fatty acids, which serve as the foundational building blocks of membrane lipids, provides crucial insights into the underlying mechanisms governing a myriad of cellular processes, such as membrane fluidity, protein trafficking, signal transduction, intercellular communication, and the etiology of certain metabolic disorders. Furthermore, comprehending how alterations in the lipid composition, especially concerning the fatty acid profile, either contribute to or prevent the onset of pathological conditions stands as a compelling area of research. Hence, this review aims to meticulously introduce the intricacies of membrane lipids and their constituent fatty acids in a healthy organism, thereby illuminating their remarkable diversity and profound influence on cellular function. Furthermore, this review aspires to highlight some potential therapeutic targets for various pathological conditions that may be ameliorated through dietary fatty acid supplements. The initial section of this review expounds on the eukaryotic biomembranes and their complex lipids. Subsequent sections provide insights into the synthesis, membrane incorporation, and distribution of fatty acids across various fractions of membrane lipids. The last section highlights the functional significance of membrane-associated fatty acids and their innate capacity to shape the various cellular physiological responses.

Mammalian Epidermis: A Compendium of Lipid Functionality

Mammalian epidermis is a striking example of the role of lipids in tissue biology. In this stratified epithelium, highly specialized structures are formed that leverage the hydrophobic properties of lipids to form an impermeable barrier and protect the humid internal environment of the body from the dry outside. This is achieved through tightly regulated lipid synthesis that generates the molecular species unique to the tissue. Beyond their fundamental structural role, lipids are involved in the active protection of the body from external insults. Lipid species present on the surface of the body possess antimicrobial activity and directly contribute to shaping the commensal microbiota. Lipids belonging to a variety of classes are also involved in the signaling events that modulate the immune responses to environmental stress as well as differentiation of the epidermal keratinocytes themselves. Recently, high-resolution methods are beginning to provide evidence for the involvement of newly identified specific lipid molecules in the regulation of epidermal homeostasis. In this review we give an overview of the wide range of biological functions of mammalian epidermal lipids.

Cutting Edge of the Pathogenesis of Atopic Dermatitis: Sphingomyelin Deacylase, the Enzyme Involved in Its Ceramide Deficiency, Plays a Pivotal Role

Atopic dermatitis (AD) is characterized clinically by severe dry skin and functionally by both a cutaneous barrier disruption and an impaired water-holding capacity in the stratum corneum (SC) even in the nonlesional skin. The combination of the disrupted barrier and water-holding functions in nonlesional skin is closely linked to the disease severity of AD, which suggests that the barrier abnormality as well as the water deficiency are elicited as a result of the induced dermatitis and subsequently trigger the recurrence of dermatitis. These functional abnormalities of the SC are mainly attributable to significantly decreased levels of total ceramides and the altered ceramide profile in the SC. Clinical studies using a synthetic pseudo-ceramide (pCer) that can function as a natural ceramide have indicated the superior clinical efficacy of pCer and, more importantly, have shown that the ceramide deficiency rather than changes in the ceramide profile in the SC of AD patients plays a central role in the pathogenesis of AD. Clinical studies of infants with AD have shown that the barrier disruption due to the ceramide deficiency is not inherent and is essentially dependent on postinflammatory events in those infants. Consistently, the recovery of trans-epidermal water loss after tape-stripping occurs at a significantly slower rate only at 1 day post-tape-stripping in AD skin compared with healthy control (HC) skin. This resembles the recovery pattern observed in Niemann-Pick disease, which is caused by an acid sphingomyelinase (aSMase) deficiency. Further, comparison of ceramide levels in the SC between before and after tape-stripping revealed that whereas ceramide levels in HC skin are significantly upregulated at 4 days post-tape-stripping, their ceramide levels remain substantially unchanged at 4 days post-tape-stripping. Taken together, the sum of these findings strongly suggests that an impaired homeostasis of a ceramide-generating process may be associated with these abnormalities. We have discovered a novel enzyme, sphingomyelin (SM) deacylase, which cleaves the N-acyl linkage of SM and glucosylceramide (GCer). The activity of SM deacylase is significantly increased in AD lesional epidermis as well as in the involved and uninvolved SC of AD skin, but not in the skin of patients with contact dermatitis or chronic eczema, compared with HC skin. SM deacylase competes with aSMase and β-glucocerebrosidase (BGCase) to hydrolyze their common substrates, SM and GCer, to yield their lysoforms sphingosylphosphorylcholine (SPC) and glucosylsphingosine (GSP), respectively, instead of ceramide. Consistently, those reaction products (SPC and GSP) accumulate to a greater extent in the involved and uninvolved SC of AD skin compared with chronic eczema or contact dermatitis skin as well as HC skin. Successive chromatographies were used to purify SM deacylase to homogeneity with a single band of ≈43 kDa and with an enrichment of >14,000-fold. Analysis of a protein spot with SM deacylase activity separated by 2D-SDS-PAGE using MALDI-TOF MS/MS allowed its amino acid sequence to be determined and to identify it as the β-subunit of acid ceramidase (aCDase), an enzyme consisting of α- and β-subunits linked by amino-bonds and a single S-S bond. Western blotting of samples treated with 2-mercaptoethanol revealed that whereas recombinant human aCDase was recognized by antibodies to the α-subunit at ≈56 and ≈13 kDa and the β-subunit at ≈43 kDa, the purified SM deacylase was detectable only by the antibody to the β-subunit at ≈43 kDa. Breaking the S-S bond of recombinant human aCDase with dithiothreitol elicited the activity of SM deacylase with an apparent size of ≈40 kDa upon gel chromatography in contrast to aCDase activity with an apparent size of ≈50 kDa in untreated recombinant human aCDase. These results provide new insights into the essential role of SM deacylase as the β-subunit aCDase that causes the ceramide deficiency in AD skin.

Sphingomyelin Deacylase, the Enzyme Involved in the Pathogenesis of Atopic Dermatitis, Is Identical to the β-Subunit of Acid Ceramidase

A ceramide deficiency in the stratum corneum (SC) is an essential etiologic factor for the dry and barrier-disrupted skin of patients with atopic dermatitis (AD). Previously, we reported that sphingomyelin (SM) deacylase, which hydrolyzes SM and glucosylceramide at the acyl site to yield their lysoforms sphingosylphosphorylcholine (SPC) and glucosylsphingosine, respectively, instead of ceramide and/or acylceramide, is over-expressed in AD skin and results in a ceramide deficiency. Although the enzymatic properties of SM deacylase have been clarified, the enzyme itself remains unidentified. In this study, we purified and characterized SM deacylase from rat skin. The activities of SM deacylase and acid ceramidase (aCDase) were measured using SM and ceramide as substrates by tandem mass spectrometry by monitoring the production of SPC and sphingosine, respectively. Levels of SM deacylase activity from various rat organs were higher in the order of skin > lung > heart. By successive chromatography using Phenyl-5PW, Rotofor, SP-Sepharose, Superdex 200 and Shodex RP18-415, SM deacylase was purified to homogeneity with a single band of an apparent molecular mass of 43 kDa with an enrichment of > 14,000-fold. Analysis by MALDI-TOF MS/MS using a protein spot with SM deacylase activity separated by 2D-SDS-PAGE allowed its amino acid sequence to be determined and identified as the β-subunit of aCDase, which consists of α- and β-subunits linked by amino bonds and a single S-S bond. Western blotting of samples treated with 2-mercaptoethanol revealed that, whereas recombinant human aCDase was recognized by antibodies to the α-subunit at ~56 kDa and ~13 kDa and the β-subunit at ~43 kDa, the purified SM deacylase was detectable only by the antibody to the β-subunit at ~43 kDa. Breaking the S-S bond of recombinant human aCDase with dithiothreitol elicited the activity of SM deacylase with ~40 kDa upon gel chromatography. These results provide new insights into the essential role of SM deacylase expressed as an aCDase-degrading β-subunit that evokes the ceramide deficiency in AD skin.

A novel sphingosylphosphorylcholine and sphingosine-1-phosphate receptor 1 antagonist, KRO-105714, for alleviating atopic dermatitis

Background

Atopic dermatitis (eczema) is a type of inflammation of the skin, which presents with itchy, red, swollen, and cracked skin. The high global incidence of atopic dermatitis makes it one of the major skin diseases threatening public health. Sphingosylphosphorylcholine (SPC) and sphingosine-1-phosphate (S1P) act as pro-inflammatory mediators, as an angiogenesis factor and a mitogen in skin fibroblasts, respectively, both of which are important biological responses to atopic dermatitis. The SPC level is known to be elevated in atopic dermatitis, resulting from abnormal expression of sphingomyelin (SM) deacylase, accompanied by a deficiency in ceramide. Also, S1P and its receptor, sphingosine-1-phosphate receptor 1 (S1P1) are important targets in treating atopic dermatitis.

Results

In this study, we found a novel antagonist of SPC and S1P1, KRO-105714, by screening 10,000 compounds. To screen the compounds, we used an SPC-induced cell proliferation assay based on a high-throughput screening (HTS) system and a human S1P1 protein-based [³⁵S]-GTPγS binding assay. In addition, we confirmed the inhibitory effects of KRO-105714 on atopic dermatitis through related cell-based assays, including a tube formation assay, a cell migration assay, and an ELISA assay on inflammatory cytokines. Finally, we confirmed that KRO-105714 alleviates atopic dermatitis symptoms in a series of mouse models.

Conclusions

Taken together, our data suggest that SPC and S1P1 antagonist KRO-105714 has the potential to alleviate atopic dermatitis.

Role of Sphingosylphosphorylcholine in Tumor and Tumor Microenvironment

Sphingosylphosphorylcholine (SPC) is a unique type of lysosphingolipid found in some diseases, and has been studied in cardiovascular, neurological, and inflammatory phenomena. In particular, SPC’s studies on cancer have been conducted mainly in terms of effects on cancer cells, and relatively little consideration has been given to aspects of tumor microenvironment. This review summarizes the effects of SPC on cancer and tumor microenvironment, and presents the results and prospects of modulators that regulate the various actions of SPC.

Elevation of plasma lysosphingomyelin-509 and urinary bile acid metabolite in Niemann-Pick disease type C-affected individuals

Niemann-Pick disease type C (NPC) is a neurovisceral disorder associated with the accumulation of lipids such as cholesterol and sphingolipids. NPC is caused by either NPC1 or NPC2, which encode lysosomal proteins located at membraneous and soluble fractions, respectively. For the past decade, the oxidation products of cholesterol, such as cholestane-3β,5α,6β-triol and 7-ketocholesterol, have been considered selective biomarkers for NPC. However, recent evidence has indicated numerous novel biomarkers for NPC, which raises the possibility that the diagnosis of NPC might be associated with the elevation of multiple lipid biomarkers, rather than a single biomarker. Sphingosylphosphorylcholine (SPC) has been suggested to be one such biomarker for NPC, in which elevated sphingomyelin is a potential precursor. Thus, we first performed a validation study of plasma SPC using LC-MS/MS. The results showed the following plasma concentrations in the NPC-affected and control individuals, respectively: 8.2 ± 2.8 nM (mean ± SD; median, 7.0 nM; max, 11.7 nM; min, 5.1 nM; n = 5) and 3.1 ± 1.4 nM (median, 2.9 nM; max, 4.8 nM; min, 1.5 nM; n = 7). We further extended the study to plasma lysophingomyelin-509 for NPC, a newly reported biomarker with uncharacterized chemical nature. Based on these result with plasma SPC as a surrogate marker, the value of mean of median of plasma lysophingomyelin-509 in NPC-affected individuals elevated at 65.2 (max, 73.2; min, 26.7; n = 5). Furthermore, the efficacy of plasma SPC and lysosphingomyelin-509 as promising biomarkers for this disorder was supported by the finding that the urinary concentration of 3β-sulfooxy-7β-N-acetylglucosaminyl-5-cholen-24-oic acid, an established biomarker for NPC, was also elevated in the NPC-affected individuals. These results suggest that a novel combination of plasma biomarkers, such as SPC and/or lysophingomyelin-509, and urinary bile acid metabolite could offer a promising platform for the diagnosis of NPC.

Glycyrrhizic acid prevents ultraviolet-B-induced photodamage: a role for mitogen-activated protein kinases, nuclear factor kappa B and mitochondrial apoptotic pathway

Glycyrrhizic acid (GA), a natural triterpene, has received attention as an agent that has protective effects against chronic diseases including ultraviolet UV-B-induced skin photodamage. However, the mechanism of its protective effect remains elusive. Here, we used an immortalized human keratinocyte cell line (HaCaT) and a small animal model (BALB/c mice), to investigate the protective effects of GA against UV-B-induced oxidative damage, and additionally, delineated the molecular mechanisms involved in the UV-B-mediated inflammatory and apoptotic response. In the HaCaT cells, GA inhibited the UV-B-mediated increase in intracellular reactive oxygen species (ROS) and down-regulated the release of pro-inflammatory cytokines interleukin (IL)-1α, -1β and -6, tumor necrosis factor (TNF)-α and prostaglandin E2 (PGE2). GA inhibited UV-B-mediated activation of p38 and JNK MAP kinases, COX-2 expression and nuclear translocation of NF-κB. Furthermore, GA inhibited UV-B-mediated apoptosis by attenuating translocation of Bax from the cytosol to mitochondria, thus preserving mitochondrial integrity. GA-treated HaCaT cells also exhibited elevated antiapoptotic Bcl-2 protein, concomitant with reduced caspase-3 cleavage and decreased PARP-1 protein. In BALB/c mice, topical application of GA on dorsal skin exposed to UV-B irradiation protected against epidermal hyperplasia, lymphocyte infiltration and expression of several inflammatory proteins, p38, JNK, COX-2, NF-κB and ICAM-1. Based on the above findings, we conclude that GA protects against UV-B-mediated photodamage by inhibiting the signalling cascades triggered by oxidative stress, including MAPK/NF-κB activation, as well as apoptosis. Thus, GA has strong potential to be used as a therapeutic/cosmeceutical agent against photodamage.

Critical Role of the Sphingolipid Pathway in Stroke: a Review of Current Utility and Potential Therapeutic Targets

Sphingolipids are a series of cell membrane-derived lipids which act as signaling molecules and play a critical role in cell death and survival, proliferation, recognition, and migration. Sphingosine-1-phosphate acts as a key signaling molecule and regulates lymphocyte trafficking, glial cell activation, vasoconstriction, endothelial barrier function, and neuronal death pathways which plays a critical role in numerous neurological conditions. Stroke is a second leading cause of death all over the world and effective therapies are still in great demand, including ischemic stroke and hemorrhagic stroke as well as poststroke repair. Significantly, sphingolipid activities change after stroke and correlate with stroke outcome, which has promoted efforts to testify whether the sphingolipid pathway could be a novel therapeutic target in stroke. The sphingolipid metabolic pathway, the connection between the pathway and stroke, as well as therapeutic interventions to manipulate the pathway to reduce stroke-induced brain injury are discussed in this review.

Lysophospholipid Receptors, as Novel Conditional Danger Receptors and Homeostatic Receptors Modulate Inflammation-Novel Paradigm and Therapeutic Potential

There are limitations in the current classification of danger-associated molecular patterns (DAMP) receptors. To overcome these limitations, we propose a new paradigm by using endogenous metabolites lysophospholipids (LPLs) as a prototype. By utilizing a data mining method we pioneered, we made the following findings: (1) endogenous metabolites such as LPLs at basal level have physiological functions; (2) under sterile inflammation, expression of some LPLs is elevated. These LPLs act as conditional DAMPs or anti-inflammatory homeostasis-associated molecular pattern molecules (HAMPs) for regulating the progression of inflammation or inhibition of inflammation, respectively; (3) receptors for conditional DAMPs and HAMPs are differentially expressed in human and mouse tissues; and (4) complex signaling mechanism exists between pro-inflammatory mediators and classical DAMPs that regulate the expression of conditional DAMPs and HAMPs. This novel insight will facilitate identification of novel conditional DAMPs and HAMPs, thus promote development of new therapeutic targets to treat inflammatory disorders.

Sphingolipids and inflammatory diseases of the skin

Mammalian skin protects our body against external assaults due to a well-organized skin barrier. The formation of the skin barrier is a complex process, in which basal keratinocytes lose their mitotic activity and differentiate to corneocytes. These corneocytes are embedded in intercellular lipid lamellae composed of ceramides, cholesterol, fatty acids, and cholesterol esters. Ceramides are the dominant lipid molecules and their reduction is connected with a transepidermal water loss and an epidermal barrier dysfunction resulting in inflammatory skin diseases. Moreover, bioactive sphingolipid metabolites like ceramide-1-phosphate, sphingosylphosphorylcholine, and sphingosine-1-phosphate are also involved in the biological modulation of keratinocytes and immune cells of the skin. Therefore, it is not astonishing that a dysregulation of sphingolipid metabolism has been identified in inflammatory skin diseases such as atopic dermatitis and psoriasis vulgaris. This chapter will describe not only the specific sphingolipid species and their skin functions but also the dysregulation of sphingolipid metabolism in inflammatory skin diseases.

Sphingolipids and atherosclerosis

The atherosclerotic lesion contains a high amount of sphingolipids, a large group of structurally diverse lipids that regulate distinct biological functions beyond their role as structural membrane components. Assessment of their role in atherogenesis has been enabled after genes that regulate their metabolism had been identified and facilitated by the more wide availability of mass spectrometry. Here we discuss recent mechanistic insights obtained in animal and epidemiological studies that have greatly enhanced our understanding of mechanisms how sphingolipids affect the atherosclerotic process.

Development of a sphingosylphosphorylcholine detection system using RNA aptamers

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

Sphingosylphosphorylcholine down-regulates filaggrin gene transcription through NOX5-based NADPH oxidase and cyclooxygenase-2 in human keratinocytes

Sphingosylphosphorylcholine (SPC) mediates various inflammatory and behavioral responses in atopic dermatitis. Recent studies have shown that dysfunction of the epidermal permeability barrier itself plays a primary role in the etiology of atopic dermatitis. However, the effects of SPC on major proteins essential to the development of the epidermal permeability barrier such as filaggrin, loricrin, involucrin, keratin 1, keratin 10 and small proline-rich proteins are still unclear. In this study, we demonstrated that SPC significantly reduces filaggrin gene transcription, implying that SPC plays a pivotal role in impairment of the epidermal permeability barrier in atopic dermatitis lesional skin. In cultured normal human keratinocytes (NHKs), SPC increases the intracellular level of reactive oxygen species (ROS) and up-regulates NADPH oxidase 5 (NOX5) gene transcription. SPC also stimulates prostaglandin (PG) E(2) production by increasing cyclooxygenase (COX)-2 expression in NHK. The effects of the prostanoid EP receptor agonists, limaprost, butaprost, and sulprostone on filaggrin gene expression in NHK suggest that the prostanoid EP2 receptor plays a significant role in the PGE(2)-mediated filaggrin down-regulation. In contrast, limaprost and butaprost do not affect NOX5 expression in NHK, implying that the NOX5-regulated ROS pathway stimulated by SPC may be upstream of the COX-2 pathway. We propose that the increase in SPC levels further aggravates dermatological symptoms of atopic dermatitis through SPC-induced down-regulation of filaggrin in NHK.

Sphingolipids in inflammation: pathological implications and potential therapeutic targets

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

Sphingosylphosphorylcholine as a novel calmodulin inhibitor

S1P (sphingosine 1-phosphate) and SPC (sphingosylphosphorylcholine) have been recently recognized as important mediators of cell signalling, regulating basic cellular processes such as growth,differentiation, apoptosis, motility and Ca2+ homoeostasis.Interestingly, they can also act as first and second messengers. Although their activation of cell-surface G-protein-coupled receptors has been studied extensively, not much is known about heir intracellular mechanism of action, and their target proteins are yet to be identified. We hypothesized that these sphingolipids might bind to CaM (calmodulin), the ubiquitous intracellular Ca2+sensor. Binding assays utilizing intrinsic tyrosine fluorescence of the protein, dansyl-labelled CaM and surface plasmon resonance revealed that SPC binds to both apo- and Ca2+-saturated CaM selectively, when compared with the related lysophospholipid mediators S1P, LPA (lysophosphatidic acid) and LPC (lysophosphatidylcholine). Experiments carried out with the model CaM-binding domain melittin showed that SPC dissociates the CaM-target peptide complex, suggesting an inhibitory role. The functional effect of the interaction was examined on two target enzymes, phosphodiesterase and calcineurin, and SPC inhibited the Ca2+/CaM-dependent activity of both. Thus we propose that CaM might be an intracellular receptor for SPC, and raise the possibility of a novel endogenous regulation of CaM.

Sphingosylphosphorylcholine acts in an anti-inflammatory manner in renal mesangial cells by reducing interleukin-1β-induced prostaglandin E2 formation

Sphingosylphosphorylcholine (SPC) is a bioactive lipid that binds to G protein-coupled-receptors and activates various signaling cascades. Here, we show that in renal mesangial cells, SPC not only activates various protein kinase cascades but also activates Smad proteins, which are classical members of the transforming growth factor-beta (TGFbeta) signaling pathway. Consequently, SPC is able to mimic TGFbeta-mediated cell responses, such as an anti-inflammatory and a profibrotic response. Interleukin-1beta-stimulated prostaglandin E(2) formation is dose-dependently suppressed by SPC, which is paralleled by reduced secretory phospholipase A(2) (sPLA(2)) protein expression and activity. This effect is due to a reduction of sPLA(2) mRNA expression caused by inhibited sPLA(2) promoter activity. Furthermore, SPC upregulates the profibrotic connective tissue growth factor (CTGF) protein and mRNA expression. Blocking TGFbeta signaling by a TGFbeta receptor kinase inhibitor causes an inhibition of SPC-stimulated Smad activation and reverses both the negative effect of SPC on sPLA(2) expression and the positive effect on CTGF expression. In summary, our data show that SPC, by mimicking TGFbeta, leads to a suppression of proinflammatory mediator production and stimulates a profibrotic cell response that is often the end point of an anti-inflammatory reaction. Thus, targeting SPC receptors may represent a novel therapeutic strategy to cope with inflammatory diseases.

Sphingosylphosphocholine effects on cultured astrocytes reveal mechanisms potentially involved in neurotoxicity in Niemann-Pick type A disease

Niemann-Pick type A is a disease characterized by the absence of a functional SMPD1 (acidic sphingomyelinase) gene and the abnormal accumulation of sphingomyelin. Under these conditions, also sphingosylphosphocholine (SPC, a sphingomyelin metabolite) accumulates in various tissues, including the brain, where it might act as a toxic stimulus, contributing to the appearance of the neurological symptoms. We studied the effects of SPC on astrocytic and neuronal cultures from rat. In particular, we investigated the possibility that SPC acts on astrocytes and that this represents the first step leading to neurodegeneration. Our results show that acute administration of SPC to astrocytes in culture promotes Ca2+ responses and a release of glutamate that, in turn, leads to cytosolic [Ca2+] elevation in neurons. We also show that chronic stimulation by SPC leads astrocytes to proliferate, but can also change their phenotype towards an activated state that might contribute to the inflammatory responses. Interestingly, upon acute SPC stimulation, activated astrocytes release more glutamate. In conclusion, we show that both chronic and acute exposure to SPC can constitute harmful signals that may have a role in the sequence of events leading to neurodegeneration.

Sphingosylphosphorylcholine activates dendritic cells, stimulating the production of interleukin-12

Compared with other lysophospholipid mediators such as sphingosine-1-phosphate and lysophosphatidic acid, little is known about the physiological significance of the related bioactive lysosphingolipid sphingosylphosphorylcholine (SPC), which is present in high-density lipoprotein particles. The present study was undertaken to evaluate the effect of SPC on human immature dendritic cells (DCs). Reverse transcription-polymerase chain reaction and flow cytometry assays revealed that DCs express two putative receptors for SPC, ovarian cancer G-protein-coupled receptor 1 and G-protein-coupled receptor 4. Exposure to SPC induced a rapid and transient increase in intracellular free calcium concentrations but did not stimulate endocytosis or chemotaxis of DCs. SPC increased the expression of HLA-DR, CD86 and CD83 and improved the T-cell priming ability of DCs, as well as the ability of DCs to stimulate the production of interferon-gamma by allogeneic peripheral blood mononuclear cells during the mixed lymphocyte reaction. Consistent with these results, we also observed that SPC stimulated the production of interleukin (IL)-12 and IL-18 by DCs. Taken together, our results support the notion that the accumulation of SPC in peripheral tissues during the course of inflammatory processes may favour the development of T helper type 1 immunity.

A novel lysophospholipid- and pH-sensitive receptor, GPR4, in brain endothelial cells regulates monocyte transmigration

Abundant evidence documents the highly proinflammatory actions of lysophosphatidylcholine (LPC). Further, LPC, found in high amounts in oxidized low-density lipoprotein (LDL), is implicated as an atherogenic factor. In endothelial cells, LPC impairs endothelial barrier function through GPR4, a novel receptor hypothesized to be sensitive to LPC and protons. The authors investigated the stimulation by LPC or low pH of GPR4 in human brain microvascular endothelial cells (HBMECs) and whether the activated GPR4 regulates in vitro monocyte transmigration. The results indicated that HBMECs stimulated by LPC (5 microM), but not low pH, showed a twofold increase in monocyte transmigration. Using retroviruses containing siRNA to GPR4, a > 60% reduction of GPR4 expression resulted in blockade of the LPC-stimulated transmigration. The inhibited response was restored by co-expression with an small interference RNA (siRNA)-resistant, but functional, GPR4 mutant construct. To investigate potential signaling mechanisms, the siRNA-mediated knockdown of GPR4 also prevented LPC-induced RhoA activation. C3 transferase, a Rho inhibitor, prevented approximately approximately 65% of the LPC-stimulated transmigration. LPC also increased MLC phosphorylation by 5 min, which was inhibited by the Rho kinase inhibitor, Y-27632 (10 microM) or ML-7 (myosin light chain kinase (MLCK) inhibitor). The findings indicate that the proinflammatory and atherogenic LPC stimulated endothelial GPR4, which promoted monocyte transmigration through a RhoA-dependent pathway.

The role of lysosphingolipids in the regulation of biological processes

This review summarizes data on the role of lysosphingolipids (glucosyl- and galactosylsphingosines, sphingosine-1-phosphate, sphingosine-1-phosphocholine) in the regulation of various biological processes in normal and pathological states.

Sphingosylphosphorylcholine-induced interleukin-6 production is mediated by protein kinase C and p42/44 extracellular signal-regulated kinase in human dermal fibroblasts

BACKGROUND

Sphingosylphosphorylcholine (SPC) has been reported as a novel lipid mediator that exerts various actions on wound healing process.

OBJECTIVE

The aim of this study is to evaluate the involvement of interleukin-6 (IL-6) in SPC-induced wound healing acceleration.

METHODS

We performed immunohistochemical analysis to demonstrate the IL-6 induction by SPC. To analyze the signaling events, skin fibroblasts were treated with SPC, and then RT-PCR, ELISA and Western blot analyses were carried out.

RESULTS

SPC markedly induced interleukin-6 (IL-6) expression in rabbit ear wound. SPC also induced IL-6 expression at both the mRNA and protein levels in human dermal fibroblasts cultured in vitro. SPC rapidly phosphorylated p42/44 extracellular signal-regulated kinase (ERK). Pretreatment with PD 98059, a specific MAPK kinase 1/2 inhibitor, markedly suppressed SPC-induced IL-6 expression in a dose-dependent manner. Protein kinase C (PKC) activation by phorbol myristate acetate (PMA) potentiated IL-6 mRNA expression, whereas PKC inhibition by bisindolylmaleimide blocked SPC-induced p42/44 ERK phosphorylation and IL-6 expression. Over-expression of PKCalpha markedly induced the IL-6 expression and p42/44 ERK activation.

CONCLUSION

These results suggest that SPC-induced IL-6 production is mediated by PKC-dependent p42/44 ERK activation in human dermal fibroblasts cultured in vitro.

Sphingosylphosphorylcholine stimulates human monocyte-derived dendritic cell chemotaxis

AIM

To investigate the effects of sphingosylphosphorylcholine (SPC) on human monocyte-derived dendritic cell (DC) chemotaxis.

METHODS

Human DC were generated from peripheral blood monocytes by culturing them with granulocyte macrophage-colony stimulating factor and interleukin-4. The effect of SPC on the DC chemotactic migration was measured by chemotaxis assay. Intracellular signaling event involved in the SPC-induced DC chemotaxis was investigated with several inhibitors for specific kinase. The expression of the SPC receptors was examined by reverse transcription polymerase chain reaction.

RESULTS

We found that SPC induced chemotactic migration in immature DC (iDC) and mature DC (mDC). In terms of SPC-induced signaling events, mitogen activated protein kinase activation and Akt activation in iDC and mDC were stimulated. SPC-induced chemotaxis was mediated by extracellular signal-regulated protein kinase and phosphoino-sitide-3-kinase, but not by calcium in both iDC and mDC. Although mDC express ovarian cancer G protein-coupled receptor 1, but not G protein-coupled receptor 4, iDC do not express any of these receptors. To examine the involvement of sphingosine-1-phosphate (S1P) receptors, we checked the effect of an S1P receptor antagonist (VPC23019) on SPC-induced DC chemotaxis. VPC23019 did not affect SPC-induced DC chemotaxis.

CONCLUSION

The results suggest that SPC may play a role in regulating DC trafficking during phagocytosis and the T cell-stimulating phase, and the unique SPC receptor, which is different from S1P receptors, is involved in SPC-induced chemotaxis.

Epidermal sphingolipids: Metabolism, function, and roles in skin disorders

Mammalian epidermis produces and delivers large quantities of glucosylceramide and sphingomyelin precursors to stratum corneum extracellular domains, where they are hydrolyzed to corresponding ceramide species. This cycle of lipid precursor formation and subsequent hydrolysis represents a mechanism that protects the epidermis against potentially harmful effects of ceramide accumulation within nucleated cell layers. Prominent skin disorders, such as psoriasis and atopic dermatitis, have diminished epidermal ceramide levels, reflecting altered sphingolipid metabolism, that may contribute to disease severity/progression. Enzymatic processes in the hydrolysis of glucosylceramide and sphingomyelin, and the roles of sphingolipids in skin diseases, are the focus of this review.

Serine palmitoyl-CoA transferase (SPT) deficiency and sphingolipid levels in mice

Sphingolipids play a very important role in cell membrane formation, signal transduction, and plasma lipoprotein metabolism, and all these functions may have an impact on atherosclerotic development. Serine palmitoyl-CoA transferase (SPT) is the key enzyme in sphingolipid biosynthesis. To evaluate in vivo SPT activity and its role in sphingolipid metabolism, we applied homologous recombination to embryonic stem cells, producing mice with long chain base 1 (Sptlc1) and long chain base 2 (Sptlc2), two subunits of SPT, gene deficiency. Homozygous Sptlc11 and Sptlc2 mice are embryonic lethal, whereas heterozygous versions of both animals (Sptlc1(+/-), Sptlc2(+/-)) are healthy. Analysis showed that, compared with WT mice, Sptlc1(+/-) and Sptlc2(+/-) mice had: (1) decreased liver Sptlc1 and Sptlc2 mRNA by 44% and 57% (P<0.01 and P<0.0001, respectively); (2) decreased liver Sptlc1 mass by 50% and Sptlc2 mass by 70% (P<0.01 and P<0.01, respectively), moreover, Sptlc1 mass decreased by 70% in Sptlc2(+/-) mouse liver, while Sptlc2 mass decreased by 53% in Sptlc1(+/-) mouse liver (P<0.001 and P<0.01, respectively); (3) decreased liver SPT activity by 45% and 60% (P<0.01, respectively); (4) decreased liver ceramide (22% and 39%, P<0.05 and P<0.01, respectively) and sphingosine levels (22% and 31%, P<0.05 and P<0.01, respectively); (5) decreased plasma ceramide (45% and 39%, P<0.01, respectively), sphingosine-1-phosphate (31% and 32%, P<0.01, respectively) and sphingosine levels (22.5% and 25%, P<0.01, respectively); (6) dramatically decreased plasma lysosphingomyelin (17-fold and 16-fold, P<0.0001, respectively); and (7) no change of plasma sphingomyelin, triglyceride, total cholesterol, phospholipids, and liver sphingomyelin levels. These results indicated that both Sptlc1 and Sptlc2 interactions are necessary for SPT activity in vivo, and that SPT activity directly influences plasma sphingolipid levels. Furthermore, manipulation of SPT activity might well influence the course of such diseases as atherosclerosis.

Characterization of Surfactant-Induced Skin Damage through Barrier Recovery Induced by Pseudoacylceramides

BACKGROUND AND OBJECTIVE

To determine how much barrier-reinforcing effects of ceramides contribute to prevent the surfactant-induced cutaneous deterioration.

METHODS

We compared the effects of topical application of two types of pseudoceramides on cutaneous deterioration induced by sodium dodecyl sulfate (SDS) treatment for 10 days in association with alterations of barrier function.

RESULTS

Daily application of pseudoacylceramide immediately after each SDS treatment significantly prevented the marked elevation of transepidermal water loss, which was accompanied by a marked abrogation of the increased expression in intercellular adhesion molecule 1 by epidermal cells as well as by suppressed epidermal hyperplasia. In contrast, topical application of a nonacylated pseudoceramide with poor barrier-reinforcing potential showed less preventive effects on cutaneous deterioration.

CONCLUSION

These results strongly suggest that perturbation of the skin barrier is a causative factor in surfactant-induced cutaneous changes and that reinforcing the barrier function by ceramide application is effective in preventing the surfactant-induced skin deterioration.

Autocrine and paracrine regulation of melanocytes in human skin and in pigmentary disorders

Recently melanogenic paracrine or autocrine cytokine networks have been discovered in vitro between melanocytes and other types of skin cells. These include endothelin (ET)-1, granulocyte macrophage colony stimulating factor, membrane-type stem cell factor (SCF) and growth-related oncogene-alpha for interactions between keratinocytes and melanocytes, and hepatocyte growth factor and soluble type SCF for interactions between fibroblasts and melanocytes. These networks are also associated with corresponding receptors expressed on melanocytes, including ET B receptor and the SCF receptor, c-KIT. Consistent with in vitro findings on the melanogenic paracrine or autocrine cytokine networks, we have found that the up- or down-regulation of such networks is intrinsically involved in vivo in the stimulation of melanocyte functions in several epidermal hyper- or hypo-pigmentary disorders. These are ET-1/ET B receptor as well as membrane type SCF/c-KIT for ultraviolet B-melanosis, granulocyte macrophage colony stimulating factor for ultraviolet A-melanosis, ET-1/ET B receptor as well as membrane type SCF for lentigo senilis, growth related oncogene-alpha for Riehl's melanosis, sphingosylphosphorylcholine for hyperpigmentation in atopic dermatitis, ET-1 for seborrhoeic keratosis, soluble type SCF as well as hepatocyte growth factor for dermatofibroma and café-au-lait macules, and c-KIT for vitiligo vulgaris. These unveiled regulatory mechanisms involved in the abnormal up- or down-regulated levels of lesional melanocyte function provide new insights into therapeutic tools utilizing blockage of responsible cytokine networks.

Sphingosylphosphorylcholine is a Melanogenic Stimulator for Human Melanocytes

As lysosphingolipids have multiple bio-modulator functions in various types of cells, we measured the biological effects of sphingosylphosphorylcholine (SPC) on cultured human melanocytes to determine whether these lysosphingolipids have the potential to activate these cells. The addition of SPC to cultured human melanocytes significantly stimulated DNA synthesis assessed by [3H]thymidine and melanogenesis assessed by the release of [3H]H2O (tyrosinase activity), the incorporation of [14C]thiouracil (melanin synthesis) and dopa-oxidase activity. Reverse transcription-polymerase chain reaction of RNA isolated from human melanocytes exposed to SPC revealed an upregulation of mRNA transcripts for tyrosinase, microphthalmia-associated transcription factor-M, endothelin B receptor and the stem cell factor receptor, c-kit. An increase in expression of tyrosinase and c-kit proteins was also demonstrated by Western blot analysis. This stimulation of melanogenesis by SPC was associated with a marked increase in the phosphorylation of extracellular signal-regulated kinase 1/2. These results suggest that SPC may be a melanogenic stimulator of human melanocytes inducing the coordinated upregulated expression of various melanogenic molecules, including c-kit.

Sphingosylphosphorylcholine is upregulated in the stratum corneum of patients with atopic dermatitis

To clarify the functional relevance of sphingomyelin (SM) deacylase to the ceramide deficiency seen in atopic dermatitis (AD), we developed a new highly sensitive method and measured the metabolic intermediate sphingosylphosphorylcholine (SPC) that accumulates in the stratum corneum. SPC in intercellular lipids extracted from stratum corneum was reacted with [(14)C]acetic anhydride to yield [(14)C-C(2)]SM, which was then analyzed by TLC. In both the lesional and non-lesional stratum corneum obtained from patients with AD, there was a significant increase in the content of SPC over that of healthy control subjects. There was a reciprocal relationship between increases in SPC and decreases in ceramide levels of stratum corneum obtained from healthy controls, and from lesional and non-lesional skin from patients with AD. Comparison with other sphingolipids present in the stratum corneum demonstrated that there is a significant positive correlation between SPC and glucosylsphingosine, another lysosphingolipid derived from glucosylceramide by another novel epidermal enzyme, termed glucosylceramide deacylase. In contrast, there was no correlation between SPC and sphingosine, a degradative product generated from ceramide by ceramidase. These findings strongly suggest the physiological relevance of SM deacylase function in vivo to the ceramide deficiency found in the skin of patients with AD.

High-expression of sphingomyelin deacylase is an important determinant of ceramide deficiency leading to barrier disruption in atopic dermatitis

We have previously demonstrated that there is abnormal expression of sphingomyelin (SM) deacylase-like enzyme in the epidermis of patients with atopic dermatitis (AD), which results in decreased levels of ceramides in their involved and uninvolved stratum corneum. For quantitation of the expression of SM deacylase in AD, we synthesized 16-(9-anthroyloxy) hexadecanoylsphingosylphosphorylcholine or [palmitic acid-14C] SM and used them as substrates to directly measure the activity of SM deacylase by detecting the release of labeled free fatty acid. Direct enzymatic measurements demonstrated that stratum corneum from lesional forearm skin (volar side) of AD patients has an extremely high SM deacylase activity that is at least five times higher than in the stratum corneum from healthy controls. In stratum corneum from nonlesional skin of AD patients, SM deacylase activity is still at least three times higher than in healthy controls. In contrast, stratum corneum from contact dermatitis patients shows levels of SM deacylase similar to healthy controls. In extracts of whole epidermis biopsies from AD patients, SM deacylase activities are significantly (3-fold) increased over healthy controls in the particulate fraction, whereas there is no significant difference in the activity of sphingomyelinase between AD and healthy control. In peripheral blood lymphocytes of AD patients, there is no increase in activity compared with healthy controls, indicating a possibility that the high expression of SM deacylase is highly associated with the skin of AD patients. These findings suggest that, in contrast to changes in sphingolipid metabolism due to aging, the hitherto undiscovered enzyme SM deacylase, is highly expressed in the epidermis of AD patients, and competes with sphingomyelinase or beta-glucocerebrosidase for the common substrate SM or glucosylceramide, which leads to the ceramide deficiency of the stratum corneum in AD.

Sphingosylphosphorylcholine induces endothelial cell migration and morphogenesis

Sphingosylphosphorylcholine (SPC) is one of the biologically active phospholipids that may act as extracellular messengers. Particularly important is the role of these lipids in the angiogenic response, a complex process involving endothelial cell migration, proliferation, and morphologic differentiation. Here we demonstrate that SPC and its hydrolytic product, sphingosine, induce chemotactic migration of human and bovine endothelial cells. The response is approximately equal to that elicited by vascular endothelial cell growth factor. The effect of SPC and sphingosine was associated with a rapid down-regulation of Edg1, a sphingosine 1-phosphate (SPP)-specific receptor involved in endothelial cell chemotaxis. Both SPC and sphingosine induced differentiation of endothelial cells into capillary-like structures in vitro. Thus, SPC and sphingosine join SPP among the biologically active lipids with angiogenic potential. Since neuronal abnormalities accompany pathological accumulation of SPC in brain tissue, it is possible that SPC is a modulator of angiogenesis in neural tissue upon its release from brain cells following trauma or neoplastic growth.

Sphingosylphosphorylcholine is a ligand for ovarian cancer G-protein-coupled receptor 1

Sphingosylphosphorylcholine (SPC) is a bioactive lipid that acts as an intracellular and extracellular signalling molecule in numerous biological processes. Many of the cellular actions of SPC are believed to be mediated by the activation of unidentified G-protein-coupled receptors. Here we show that SPC is a high-affinity ligand for an orphan receptor, ovarian cancer G-protein-coupled receptor 1 (OGR1). In OGR1-transfected cells, SPC binds to OGR1 with high affinity (Kd = 33.3 nM) and high specificity and transiently increases intracellular calcium. The specific binding of SPC to OGR1 also activates p42/44 mitogen-activated protein kinases (MAP kinases) and inhibits cell proliferation. In addition, SPC causes internalization of OGR1 in a structurally specific manner.