Racemization of the substrate and product by serine palmitoyltransferase from Sphingobacterium multivorum yields two enantiomers of the product from d-serine

Serine palmitoyltransferase (SPT) catalyzes the pyridoxal-5'-phosphate (PLP)-dependent decarboxylative condensation of l-serine and palmitoyl-CoA to form 3-ketodihydrosphingosine (KDS). Although SPT was shown to synthesize corresponding products from amino acids other than l-serine, it is still arguable whether SPT catalyzes the reaction with d-serine, which is a question of biological importance. Using high substrate and enzyme concentrations, KDS was detected after the incubation of SPT from Sphingobacterium multivorum with d-serine and palmitoyl-CoA. Furthermore, the KDS comprised equal amounts of 2S and 2R isomers. 1H-NMR study showed a slow hydrogen-deuterium exchange at Cα of serine mediated by SPT. We further confirmed that SPT catalyzed the racemization of serine. The rate of the KDS formation from d-serine was comparable to those for the α-hydrogen exchange and the racemization reaction. The structure of the d-serine-soaked crystal (1.65 Å resolution) showed a distinct electron density of the PLP-l-serine aldimine, interpreted as the racemized product trapped in the active site. The structure of the α-methyl-d-serine-soaked crystal (1.70 Å resolution) showed the PLP-α-methyl-d-serine aldimine, mimicking the d-serine-SPT complex prior to racemization. Based on these enzymological and structural analyses, the synthesis of KDS from d-serine was explained as the result of the slow racemization to l-serine, followed by the reaction with palmitoyl-CoA, and SPT would not catalyze the direct condensation between d-serine and palmitoyl-CoA. It was also shown that the S. multivorum SPT catalyzed the racemization of the product KDS, which would explain the presence of (2R)-KDS in the reaction products.

Krüppel-like factor 12 regulates aging ovarian granulosa cell apoptosis by repressing SPHK1 transcription and sphingosine-1-phosphate (S1P) production

Oxidative stress triggered by aging, radiation, or inflammation impairs ovarian function by inducing granulosa cell (GC) apoptosis. However, the mechanism inducing GC apoptosis has not been characterized. Here, we found that ovarian GCs from aging patients showed increased oxidative stress, enhanced reactive oxygen species activity, and significantly decreased expression of the known antiapoptotic factor sphingosine-1-phosphate/sphingosine kinase 1 (SPHK1) in GCs. Interestingly, the expression of Krüppel-like factor 12 (KLF12) was significantly increased in the ovarian GCs of aging patients. Furthermore, we determined that KLF12 was significantly upregulated in hydrogen peroxide-treated GCs and a 3-nitropropionic acid-induced in vivo model of ovarian oxidative stress. This phenotype was further confirmed to result from inhibition of SPHK1 by KLF12. Interestingly, when endogenous KLF12 was knocked down, it rescued oxidative stress-induced apoptosis. Meanwhile, supplementation with SPHK1 partially reversed oxidative stress-induced apoptosis. However, this function was lost in SPHK1 with deletion of the binding region to the KLF12 promoter. SPHK1 reversed apoptosis caused by hydrogen peroxide-KLF12 overexpression, a result further confirmed in an in vitro ovarian culture model and an in vivo 3-nitropropionic acid-induced ovarian oxidative stress model. Overall, our study reveals that KLF12 is involved in regulating apoptosis induced by oxidative stress in aging ovarian GCs and that sphingosine-1-phosphate/SPHK1 can rescue GC apoptosis by interacting with KLF12 in negative feedback.

Sphingosine Kinases as Druggable Targets

There is substantial evidence that the enzymes, sphingosine kinase 1 and 2, which catalyse the formation of the bioactive lipid sphingosine 1-phosphate, are involved in pathophysiological processes. In this chapter, we appraise the evidence that both enzymes are druggable and describe how isoform-specific inhibitors can be developed based on the plasticity of the sphingosine-binding site. This is contextualised with the effect of sphingosine kinase inhibitors in cancer, pulmonary hypertension, neurodegeneration, inflammation and sickling.

Ceramide and Sphingosine Regulation of Myelinogenesis: Targeting Serine Palmitoyltransferase Using microRNA in Multiple Sclerosis

Ceramide and sphingosine display a unique profile during brain development, indicating their critical role in myelinogenesis. Employing advanced technology such as gas chromatography-mass spectrometry, high performance liquid chromatography, and immunocytochemistry, along with cell culture and molecular biology, we have found an accumulation of sphingosine in brain tissues of patients with multiple sclerosis (MS) and in the spinal cord of rats induced with experimental autoimmune encephalomyelitis. The elevated sphingosine leads to oligodendrocyte death and fosters demyelination. Ceramide elevation by serine palmitoyltransferse (SPT) activation was the primary source of the sphingosine elevation as myriocin, an inhibitor of SPT, prevented sphingosine elevation and protected oligodendrocytes. Supporting this view, fingolimod, a drug used for MS therapy, reduced ceramide generation, thus offering partial protection to oligodendrocytes. Sphingolipid synthesis and degradation in normal development is regulated by a series of microRNAs (miRNAs), and hence, accumulation of sphingosine in MS may be prevented by employing miRNA technology. This review will discuss the current knowledge of ceramide and sphingosine metabolism (synthesis and breakdown), and how their biosynthesis can be regulated by miRNA, which can be used as a therapeutic approach for MS.

Light Stress-Induced Increase of Sphingosine 1-Phosphate in Photoreceptors and Its Relevance to Retinal Degeneration

Sphingosine 1-phosphate (S1P) is a potent lipid mediator that modulates inflammation and angiogenesis. In this study, we investigated the possible involvement of S1P in the pathology of light-induced retinal degeneration in vivo and in vitro. The intracellular S1P and sphingosine kinase (SphK) activity in a photoreceptor cell line (661W cells) was significantly increased by exposure to light. The enhancement of SphK1 expression was dependent on illumination, and all-trans-retinal significantly promoted SphK1 expression. S1P treatment reduced protein kinase B (Akt) phosphorylation and increased the protein expression of cleaved caspase-3, and induced photoreceptor cell apoptosis. In vivo, light exposure enhanced the expression of SphK1 in the outer segments of photoreceptors. Intravitreal injection of a SphK inhibitor significantly suppressed the thinning of the outer nuclear layer and ameliorated the attenuation of the amplitudes of a-waves and b-waves of electroretinograms during light-induced retinal degeneration. These findings imply that light exposure induces the synthesis of S1P in photoreceptors by upregulating SphK1, which is facilitated by all-trans-retinal, causing retinal degeneration. Inhibition of this enhancement may be a therapeutic target of outer retinal degeneration, including age-related macular degeneration.

Tamoxifen acts on Trypanosoma cruzi sphingolipid pathway triggering an apoptotic death process

This study shows the effects of tamoxifen, a known estrogen receptor antagonist used in the treatment of breast cancer, on the sphingolipid pathway of Trypanosoma cruzi, searching for potential chemotherapeutic targets. A dose-dependent epimastigote growth inhibition at increasing concentration of tamoxifen was determined. In blood trypomastigotes, treatment with 10 μM showed 90% lysis, while 86% inhibition of intracellular amastigote development was obtained using 50 μM. Lipid extracts from treated and non-treated metabolically labelled epimastigotes evidenced by thin layer chromatography different levels of sphingolipids and MALDI-TOF mass spectrometry analysis assured the identity of the labelled species. Comparison by HPLC-ESI mass spectrometry of lipids, notably exhibited a dramatic increase in the level of ceramide in tamoxifen-treated parasites and a restrained increase of ceramide-1P and sphingosine, indicating that the drug is acting on the enzymes involved in the final breakdown of ceramide. The ultrastructural analysis of treated parasites revealed characteristic morphology of cells undergoing an apoptotic-like death process. Flow cytometry confirmed cell death by an apoptotic-like machinery indicating that tamoxifen triggers this process by acting on the parasitic sphingolipid pathway.

Inhibiting Sphingosine Kinase 2 Derived-sphingosine-1-phosphate Ameliorates Psoriasis-like Skin Disease via Blocking Th17 Differentiation of Naïve CD4 T Lymphocytes in Mice

Sphingosine-1-phosphate (S1P) is a signalling sphingolipid metabolite that regulates important cell processes, including cell proliferation and apoptosis. Circulating S1P levels have been reported to be increased in patients with psoriasis relative to healthy patients. The aim of this study was to examine the potency of S1P inhibition using an imiquimod-induced psoriasis mouse model. Both topical ceramidase and sphingosine kinase 1/2 inhibition, which blocks S1P generation, alleviated imiquimod-induced skin lesions and reduced the serum interleukin 17-A levels induced by application of imiquimod. These treatments also normalized skin mRNA levels of genes associated with inflammation and keratinocyte differentiation. Inhibition of sphingosine kinase 2, but not sphingosine kinase 1, diminished levels of suppressor of cytokine signalling 1 and blocked T helper type 17 differentiation of naïve CD4+ T cells; imiquimod-induced psoriasis-like skin symptoms were also ameliorated. These results indicate the distinct effects of sphingosine kinase 1 and sphingosine kinase 2 inhibition on T helper type 17 generation and suggest molecules that inhibit S1P formation, including ceramidase and sphingosine kinase 2 inhibitors, as novel therapeutic candidates for psoriasis.

Chemokine Receptors, CXCR1 and CXCR2, Differentially Regulate Exosome Release in Hepatocytes

Exosomes are small membrane vesicles released by different cell types, including hepatocytes, that play important roles in intercellular communication. We have previously demonstrated that hepatocyte-derived exosomes contain the synthetic machinery to form sphingosine-1-phosphate (S1P) in target hepatocytes resulting in proliferation and liver regeneration after ischemia/reperfusion (I/R) injury. We also demonstrated that the chemokine receptors, CXCR1 and CXCR2, regulate liver recovery and regeneration after I/R injury. In the current study, we sought to determine if the regulatory effects of CXCR1 and CXCR2 on liver recovery and regeneration might occur via altered release of hepatocyte exosomes. We found that hepatocyte release of exosomes was dependent upon CXCR1 and CXCR2. CXCR1-deficient hepatocytes produced fewer exosomes, whereas CXCR2-deficient hepatocytes produced more exosomes compared to their wild-type controls. In CXCR2-deficient hepatocytes, there was increased activity of neutral sphingomyelinase (Nsm) and intracellular ceramide. CXCR1-deficient hepatocytes had no alterations in Nsm activity or ceramide production. Interestingly, exosomes from CXCR1-deficient hepatocytes had no effect on hepatocyte proliferation, due to a lack of neutral ceramidase and sphingosine kinase. The data demonstrate that CXCR1 and CXCR2 regulate hepatocyte exosome release. The mechanism utilized by CXCR1 remains elusive, but CXCR2 appears to modulate Nsm activity and resultant production of ceramide to control exosome release. CXCR1 is required for packaging of enzymes into exosomes that mediate their hepatocyte proliferative effect.

Ceramide Production Mediates Aldosterone-Induced Human Umbilical Vein Endothelial Cell (HUVEC) Damages

Here, we studied the underlying mechanism of aldosterone (Aldo)-induced vascular endothelial cell damages by focusing on ceramide. We confirmed that Aldo (at nmol/L) inhibited human umbilical vein endothelial cells (HUVEC) survival, and induced considerable cell apoptosis. We propose that ceramide (mainly C18) production might be responsible for Aldo-mediated damages in HUVECs. Sphingosine-1-phosphate (S1P), an anti-ceramide lipid, attenuated Aldo-induced ceramide production and following HUVEC damages. On the other hand, the glucosylceramide synthase (GCS) inhibitor PDMP or the ceramide (C6) potentiated Aldo-induced HUVEC apoptosis. Eplerenone, a mineralocorticoid receptor (MR) antagonist, almost completely blocked Aldo-induced C18 ceramide production and HUVEC damages. Molecularly, ceramide synthase 1 (CerS-1) is required for C18 ceramide production by Aldo. Knockdown of CerS-1 by targeted-shRNA inhibited Aldo-induced C18 ceramide production, and protected HUVECs from Aldo. Reversely, CerS-1 overexpression facilitated Aldo-induced C18 ceramide production, and potentiated HUVEC damages. Together, these results suggest that C18 ceramide production mediates Aldo-mediated HUVEC damages. MR and CerS-1 could be the two signaling molecule regulating C18 ceramide production by Aldo.

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.

Extracellular sphingosine-1-phosphate: a novel actor in human glioblastoma stem cell survival

Glioblastomas are the most frequent and aggressive intracranial neoplasms in humans, and despite advances and the introduction of the alkylating agent temozolomide in therapy have improved patient survival, resistance mechanisms limit benefits. Recent studies support that glioblastoma stem-like cells (GSCs), a cell subpopulation within the tumour, are involved in the aberrant expansion and therapy resistance properties of glioblastomas, through still unclear mechanisms. Emerging evidence suggests that sphingosine-1-phosphate (S1P) a potent onco-promoter able to act as extracellular signal, favours malignant and chemoresistance properties in GSCs. Notwithstanding, the origin of S1P in the GSC environment remains unknown. We investigated S1P metabolism, release, and role in cell survival properties of GSCs isolated from either U87-MG cell line or a primary culture of human glioblastoma. We show that both GSC models, grown as neurospheres and expressing GSC markers, are resistant to temozolomide, despite not expressing the DNA repair protein MGMT, a major contributor to temozolomide-resistance. Pulse experiments with labelled sphingosine revealed that both GSC types are able to rapidly phosphorylate the long-chain base, and that the newly produced S1P is efficiently degraded. Of relevance, we found that S1P was present in GSC extracellular medium, its level being significantly higher than in U87-MG cells, and that the extracellular/intracellular ratio of S1P was about ten-fold higher in GSCs. The activity of sphingosine kinases was undetectable in GSC media, suggesting that mechanisms of S1P transport to the extracellular environment are constitutive in GSCs. In addition we found that an inhibitor of S1P biosynthesis made GSCs sensitive to temozolomide (TMZ), and that exogenous S1P reverted this effect, thus involving extracellular S1P as a GSC survival signal in TMZ resistance. Altogether our data implicate for the first time GSCs as a pivotal source of extracellular S1P, which might act as an autocrine/paracrine signal contributing to their malignant properties.

Factor-Xa-induced mitogenesis and migration require sphingosine kinase activity and S1P formation in human vascular smooth muscle cells

AIMS

Sphingosine-1-phosphate (S1P) is a cellular signalling lipid generated by sphingosine kinase-1 (SPHK1). The aim of the study was to investigate whether the activated coagulation factor-X (FXa) regulates SPHK1 transcription and the formation of S1P and subsequent mitogenesis and migration of human vascular smooth muscle cells (SMC).

METHODS AND RESULTS

FXa induced a time- (3-6 h) and concentration-dependent (3-30 nmol/L) increase of SPHK1 mRNA and protein expression in human aortic SMC, resulting in an increased synthesis of S1P. FXa-stimulated transcription of SPHK1 was mediated by the protease-activated receptor-1 (PAR-1) and PAR-2. In human carotid artery plaques, expression of SPHK1 was observed at SMC-rich sites and was co-localized with intraplaque FX/FXa content. FXa-induced SPHK1 transcription was attenuated by inhibitors of Rho kinase (Y27632) and by protein kinase C (PKC) isoforms (GF109203X). In addition, FXa rapidly induced the activation of the small GTPase Rho A. Inhibition of signalling pathways which regulate SPHK1 expression, inhibition of its activity or siRNA-mediated SPHK1 knockdown attenuated the mitogenic and chemotactic response of human SMC to FXa.

CONCLUSION

These data suggest that FXa induces SPHK1 expression and increases S1P formation independent of thrombin and that this involves the activation of Rho A and PKC signalling. In addition to its key function in coagulation, this direct effect of FXa on human SMC may increase cell proliferation and migration at sites of vessel injury and thereby contribute to the progression of vascular lesions.

A strategy for enrichment of the bioactive sphingoid base-1-phosphates produced by Hypericum perforatum L. in a balloon type airlift reactor

An efficient enrichment method using immobilized metal affinity chromatography (IMAC) was developed for selective extraction of bioactive sphingoid base-1-phosphates (SB1Ps) from adventitious roots of Hypericum perforatum cultured in bioreactor. The phosphate-selective IMAC enrichment coupled with LC-MS/MS enabled sensitive analysis of low-abundance SB1Ps present in the root biomass, which would not be feasible otherwise due to severe interferences from complex biological matrices. The time-dependent growth rate and production of SB1Ps from adventitious roots were investigated. The level of phytosphingosine-1-phosphate, which was found to be the major SB1Ps, reached a maximum amount of 635.6pmolpergram of dry weight after 3weeks of culture and decreased between 3 and 5weeks of culture subsequently. On the other hand, sphingosine-1-phosphate and sphinganine-1-phosphate were present at levels of 18.91 and 73.15pmolpergram of dry weight, respectively, after a week of culture and their level decreased thereafter.

Inhibition of sphingosine kinase-2 suppresses inflammation and attenuates graft injury after liver transplantation in rats

Inflammation mediates/promotes graft injury after liver transplantation (LT). This study investigated the roles of sphingosine kinase-2 (SK2) in inflammation after LT. Liver grafts were stored in UW solution with and without ABC294640 (100 µM), a selective inhibitor of SK2, before implantation. Hepatic sphingosine-1-phosphate (S1P) levels increased ∼4-fold after LT, which was blunted by 40% by ABC294640. Hepatic toll-like receptor-4 (TLR4) expression and nuclear factor-κB (NF-κB) p65 subunit phosphorylation elevated substantially after transplantation. The pro-inflammatory cytokines/chemokines tumor necrosis factor-α, interleukin-1β and C-X-C motif chemokine 10 mRNAs increased 5.9-fold, 6.1-fold and 16-fold, respectively following transplantation, while intrahepatic adhesion molecule-1 increased 5.7-fold and monocytes/macrophage and neutrophil infiltration and expansion of residential macrophage population increased 7.8-13.4 fold, indicating enhanced inflammation. CD4+ T cell infiltration and interferon-γ production also increased. ABC294640 blunted TLR4 expression by 60%, NF-κB activation by 84%, proinflammatory cytokine/chemokine production by 45-72%, adhesion molecule expression by 54% and infiltration of monocytes/macrophages and neutrophils by 62-67%. ABC294640 also largely blocked CD4+ T cell infiltration and interferon-γ production. Focal necrosis and apoptosis occurred after transplantation with serum alanine aminotransferase (ALT) reaching ∼6000 U/L and serum total bilirubin elevating to ∼1.5 mg/dL. Inhibition of SK2 by ABC294640 blunted necrosis by 57%, apoptosis by 74%, ALT release by ∼68%, and hyperbilirubinemia by 74%. Most importantly, ABC294640 also increased survival from ∼25% to ∼85%. In conclusion, SK2 plays an important role in hepatic inflammation responses and graft injury after cold storage/transplantation and represents a new therapeutic target for liver graft failure.

Terpene alcohols inhibit de novo sphingolipid biosynthesis

The terpene alcohols geranyllinalool, phytol (diterpene alcohol), and farnesol (sesquiterpene alcohol) were newly found to inhibit sphingolipid de novo biosynthesis in LLC-PK₁ cells (pig kidney epithelial cells). A simple chromatographic bioassay was established for the screening of inhibitory compounds able to reduce the amount of sphinganine, an intermediate metabolite of sphingolipid biosynthesis. The screening strategy was based on the degree of suppression of fumonisin B₁ (FB₁-induced sphinganine accumulation following co-treatment with selected terpene alcohols. L-cycloserine and ISP-1, specific serine palmitoyltransferase (SPT) inhibitors, were used as positive controls. Our results show that measuring reduced sphinganine levels after treatment with 2 µM FB₁ in combination with the putative inhibitory compounds provides a useful screening bioassay for evaluating compounds causing sphingolipid depletion. Intracellular sphinganine concentrations were analyzed using the fluorescent peak areas of the O-phthalaldehyde (OPA) derivatives of sphinganine eluted with 87 % acetonitrile on a reversed-phase column. Geranyllinalool, phytol, and farnesol were identified as novel SPT inhibitors that reduce FB₁-induced sphinganine accumulation and thus inhibit the first step of sphingolipid de novo synthesis.

Isolation and functional characterisation of the genes encoding Δ(8)-sphingolipid desaturase from Brassica rapa

Δ(8)-Sphingolipid desaturase is the key enzyme that catalyses desaturation at the C8 position of the long-chain base of sphingolipids in higher plants. There have been no previous studies on the genes encoding Δ(8)-sphingolipid desaturases in Brassica rapa. In this study, four genes encoding Δ(8)-sphingolipid desaturases from B. rapa were isolated and characterised. Phylogenetic analyses indicated that these genes could be divided into two groups: BrD8A, BrD8C and BrD8D in group I, and BrD8B in group II. The two groups of genes diverged before the separation of Arabidopsis and Brassica. Though the four genes shared a high sequence similarity, and their coding desaturases all located in endoplasmic reticulum, they exhibited distinct expression patterns. Heterologous expression in Saccharomyces cerevisiae revealed that BrD8A/B/C/D were functionally diverse Δ(8)-sphingolipid desaturases that catalyse different ratios of the two products 8(Z)- and 8(E)-C18-phytosphingenine. The aluminium tolerance of transgenic yeasts expressing BrD8A/B/C/D was enhanced compared with that of control cells. Expression of BrD8A in Arabidopsis changed the ratio of 8(Z):8(E)-C18-phytosphingenine in transgenic plants. The information reported here provides new insights into the biochemical functional diversity and evolutionary relationship of Δ(8)-sphingolipid desaturase in plants and lays a foundation for further investigation of the mechanism of 8(Z)- and 8(E)-C18-phytosphingenine biosynthesis.

Isoflurane protects human kidney proximal tubule cells against necrosis via sphingosine kinase and sphingosine-1-phosphate generation

BACKGROUND/AIMS

We previously showed that the inhalational anesthetic isoflurane protects against renal ischemia reperfusion injury in part via sphingosine kinase (SK)-mediated synthesis of sphingosine-1-phosphate (S1P). In this study, we tested the hypothesis that isoflurane directly targets renal proximal tubule cells via SK activation, S1P synthesis and activation of S1P receptors to initiate cytoprotective signaling.

METHODS AND RESULTS

Isoflurane-mediated phosphorylation of extracellular signal-regulated kinase (ERK) and Akt and induction of HSP70 in human kidney proximal tubule (HK-2) cells were inhibited by dimethylsphingosine (DMS), an SK inhibitor, and VPC23019, an S1P(1/3) receptor selective antagonist, in HK-2 cells. A selective S1P(1) receptor agonist, SEW2781, mimicked isoflurane-induced phosphorylation of ERK and Akt and induction of HSP70. Moreover, isoflurane-mediated protection against H(2)O(2)-induced necrosis of HK-2 cells was significantly attenuated by an S1P(1/3) receptor antagonist, VPC23019, and by SK inhibitors DMS or 4-[[4- (4-chlorophenyl)-2-thiazolyl]amino]phenol. Finally, overexpression of the SK1 enzyme in HK-2 cells protected against H(2)O(2)-induced necrosis.

CONCLUSIONS

Collectively, our study demonstrates that S1P released via isoflurane-mediated SK1 stimulation produces direct anti-necrotic effects probably via S1P(1) receptor-mediated cytoprotective signaling (ERK/Akt phosphorylation and HSP70 induction) in HK-2 cells. Our findings may help to unravel the cellular signaling pathways of volatile anesthetic-mediated renal protection and lead to new therapeutic applications of volatile anesthetics during the perioperative period.

Role of sphingosine kinase 1 in allergen-induced pulmonary vascular remodeling and hyperresponsiveness

BACKGROUND

Immunologic processes might contribute to the pathogenesis of pulmonary arterial hypertension (PAH), a fatal condition characterized by progressive pulmonary arterial remodeling, increased pulmonary vascular resistance, and right ventricular failure. Experimental allergen-driven lung inflammation evoked morphologic and functional vascular changes that resembled those observed in patients with PAH. Sphingosine kinase 1 (SphK1) is the main pulmonary contributor to sphingosine-1-phosphate (S1P) synthesis, a modulator of immune and vascular functions.

OBJECTIVE

We sought to investigate the role of SphK1 in allergen-induced lung inflammation.

METHODS

SphK1-deficient mice and C57Bl/6 littermates (wild-type [WT] animals) were subjected to acute or chronic allergen exposure.

RESULTS

After 4 weeks of systemic ovalbumin sensitization and local airway challenge, airway responsiveness increased less in SphK1(-/-) compared with WT mice, whereas pulmonary vascular responsiveness was greatly increased and did not differ between strains. Acute lung inflammation led to an increase in eosinophils and mRNA expression for S1P phosphatase 2 and S1P lyase in lungs of WT but not SphK1(-/-) mice. After repetitive allergen exposure for 8 weeks, airway responsiveness was not augmented in SphK1(-/-) or WT mice, but pulmonary vascular responsiveness was increased in both strains, with significantly higher vascular responsiveness in SphK1(-/-) mice compared with that seen in WT mice. Increased vascular responsiveness was accompanied by remodeling of the small and intra-acinar arteries.

CONCLUSION

: The data support a role for SphK1 and S1P in allergen-induced airway inflammation. However, SphK1 deficiency increased pulmonary vascular hyperresponsiveness, which is a component of PAH pathobiology. Moreover, we show for the first time the dissociation between inflammation-induced remodeling of the airways and pulmonary vasculature.

Ceramide inhibits LPS-induced production of IL-5, IL-10, and IL-13 from mast cells

Mast cells are central regulators of allergic inflammation through production of various chemical mediators and cytokines. Bacterial infection occasionally worsens allergic inflammation. Although the exact mechanism of this phenomenon remains unclear, we have previously reported that LPS stimulates mast cells to produce not only pro-inflammatory cytokines, such as IL-6 and TNF-alpha, but also Th2-type cytokines, such as IL-5 and IL-13, and a regulatory cytokine, IL-10. In the present study, we have studied the effect of ceramide on LPS-mediated cytokine production from mast cells, as ceramide modulates various cellular functions in many cell types. Administration of cell-permeable C8 ceramide reduced production of IL-5, IL-10, and IL-13 from LPS-stimulated mouse bone marrow-derived mast cells (BMMCs) apparently through transcriptional inhibition, but did not affect IL-6 or TNF-alpha production. Consistently, LPS-stimulated production of IL-5, IL-10, and IL-13 from BMMCs is significantly enhanced in the presence of fumonisin B1, a de novo ceramide synthesis inhibitor. Interestingly, the same C8 ceramide treatment showed opposite effects on cytokine production from LPS-stimulated macrophages, reducing IL-6 and TNF-alpha while not affecting IL-10 production. C8 ceramide pretreatment significantly reduced LPS-induced Akt phosphorylation in BMMCs, but not in macrophages. Furthermore, pretreatment of BMMCs by wortmannin, a specific inhibitor of PI3 kinase, inhibited LPS-stimulated expression of IL-5, IL-10, and IL-13, but not that of TNF-alpha or IL-6. Thus, ceramide appeared to down-regulate LPS-stimulated production of IL-5, IL-10, and IL-13 from mast cells by inhibiting PI3 kinase-Akt pathway in a cell type-specific manner.

Maternal alcohol consumption increases sphingosine levels in the brains of progeny mice

The effect of 'binge' alcohol upon sphingolipid metabolism in the fetal alcohol syndrome (FAS) was examined in pregnant mice (C57BL/6J) by administering a single dose of alcohol during the third trimester (gestational day 15-16). The control mice were administered a sucrose solution of equal caloric value. Brains from progeny at postnatal days 5, 15, 21 and 30 were dissected into three regions, and sphingolipid concentrations of the brain regions were determined including assay of monoglycosylceramide, ceramide, sphingosine and sphingomyelin. We found that a single dose of ethanol induces an elevation of sphingosine (2-3.5-fold) in the brain of progeny. The level of brain ceramide at a dose of 1.5 g/kg was significantly higher than control. Alcohol consumption during pregnancy induces neuronal loss in progeny brains. Our result suggests that the elevation of sphingosine in progeny brain induced by maternal alcohol consumption may be responsible for observed neuronal loss in FAS.

An assay system for measuring the acute production of sphingosine 1-phosphate in intact monolayers

Sphingosine kinase (SK) is a signaling enzyme that phosphorylates sphingosine to produce sphingosine 1-phosphate. Sphingosine and sphingosine 1-phosphate (S1P) belong to a class of bioactive sphingolipid metabolites that are critical in a number of cellular processes, yet often have opposing biological functions. The intracellular localization of sphingosine kinase has been demonstrated in multiple studies to be a critical aspect of its signaling function. To date, assays of sphingosine kinase activity have been developed for measuring activity in lysates, where the effects of localization are lost. Here we outline a system in which the rate of production of S1P can be measured in intact cells using exogenously added radiolabeled ATP instead of tritiated sphingosine. The surprising ability of ATP to enter unpermeabilized monolayers is one aspect that makes this assay simple, efficient, and inexpensive, yet sensitive enough to measure endogenous enzyme activity. The assay is well behaved in terms of kinetics and substrate dependence. Overall, this assay is ideal for future studies to identify changes in S1P production in intact cells such as those that result from the differential intracellular targeting of sphingosine kinase.

Sphingosine 1-Phosphate-Mediated Trafficking of Pathogenic Th2 and Mast Cells for the Control of Food Allergy

Sphingosine 1-phosphate (S1P) has been proposed as a regulator of lymphocyte trafficking, but its role in mucosa-associated diseases, such as in food allergies, remains to be elucidated. To examine the role of S1P in allergic diseases in the intestine, we used a Th2 cell-mediated Ag-specific allergic diarrhea model and demonstrated that type 1 S1P receptor (S1P(1)) expression was preferentially associated with pathogenic CD4(+) T cells for the development of allergic reactions. Consistent with this demonstration, treatment with FTY720, a modulator of the S1P(1), prevented allergic diarrhea by inhibiting the migration of systemically primed pathogenic CD4(+) T cells induced by oral challenge with allergen into the large intestine. In addition, FTY720 hampered mast cell infiltration into the large intestine, whereas eosinophil infiltration into the large intestine and total and allergen-specific serum IgE production were comparable between mock- and FTY720-treated groups. These results suggest that modulation of the S1P-mediated pathway to inhibit the migration of pathogenic CD4(+) T cells and mast cells into the large intestine could be a novel strategy for preventing allergic diarrhea.

Th17 augmentation in OTII TCR plus T cell-selective type 1 sphingosine 1-phosphate receptor double transgenic mice

Sphingosine 1-phosphate (S1P) in blood and lymph controls lymphoid traffic and tissue migration of T cells through signals from the type 1 S1PR (S1P(1)), but less is known of effects of the S1P-S1P(1) axis on nonmigration functions of T cells. CD4 T cells from a double transgenic (DTG) mouse express OTII TCRs specific for OVA peptide 323-339 (OVA) and a high level of transgenic S1P(1), resistant to suppression by T cell activation. OVA-activated DTG CD4 T cells respond as expected to S1P by chemotactic migration and reduction in secretion of IFN-gamma. In addition, DTG CD4 T cells stimulated by OVA secrete a mean of 2.5-fold more IL-17 than those from OTII single transgenic mice with concomitantly higher levels of mRNA encoding IL-17 by real-time PCR and of CD4 T cells with intracellular IL-17 detected by ELISPOT assays. OVA challenge of s.c. air pockets elicited influx of more OTII TCR-positive T cells producing a higher level of IL-17 in DTG mice than OTII control mice. Augmentation of the number and activity of Th17 cells by the S1P-S1P(1) axis may thus enhance host defense against microbes and in other settings increase host susceptibility to autoimmune diseases.

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

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

Sphingomyelin synthase 1 suppresses ceramide production and apoptosis post-photodamage

The role of sphingomyelin synthase 1 (SMS1), the Golgi membrane isoform of the enzyme, in ceramide metabolism and apoptosis after photodamage with the photosensitizer Pc 4 (PDT) is unclear. In the present study, using electrospray ionization/double mass spectrometry, we show that in Jurkat cells overexpressing SMS1, increases in ceramides were lower than in empty-vector transfectants post-PDT. Similarly, the responses of dihydroceramides and dihydrosphingosine, precursors of ceramide in the de novo synthetic pathway, were attenuated in SMS1-overexpressor after photodamage, suggesting the involvement of the de novo pathway. Overexpression of SMS1 was associated with differential regulation of sphingomyelin levels, as well as with the reduced inhibition of the enzyme post-treatment. Concomitant with the suppressed ceramide response, PDT-induced DEVDase activation was substantially reduced in SMS1-overexpressors. The data show that overexpression of SMS1 is associated with suppressed ceramide response and apoptotic resistance after photodamage.

basA regulates cell wall organization and asexual/sexual sporulation ratio in Aspergillus nidulans

Sphingolipid C4 hydroxylase catalyzes the conversion of dihydrosphingosine to phytosphingosine. In Saccharomyces cerevisiae, Sur2 is essential for sphingolipid C4 hydroxylation activity but not essential for normal growth. Here we demonstrate that the Aspergillus nidulans Sur2 homolog BasA is also required for phytosphingosine biosynthesis but is also essential for viability. We previously reported that a point missense mutation in basA resulted in aberrant cell wall thickening. Here our data suggest that accumulation of dihydrosphingosine is responsible for this phenotype. In addition, two different mutations in basA consistently accelerated the transition from asexual development to sexual development compared to the wild-type strain. The phenotype could be suppressed by exogenous addition of phytosphingosine. Northern analysis suggests that faster sexual development in the basA mutant might be due to a higher transcription level of ppoA and steA, genes demonstrated to coordinate a balance between asexual and sexual development in A. nidulans. Consistent with these findings, mutations in the ceramide-synthase-encoding genes barA and lagA also caused faster transition from asexual to sexual development, supporting the involvement of sphingolipid metabolism in fungal morphogenesis.

Promotion of lymphocyte egress into blood and lymph by distinct sources of sphingosine-1-phosphate

Lymphocytes require sphingosine-1-phosphate (S1P) receptor-1 to exit lymphoid organs, but the source(s) of extracellular S1P and whether S1P directly promotes egress are unknown. By using mice in which the two kinases that generate S1P were conditionally ablated, we find that plasma S1P is mainly hematopoietic in origin, with erythrocytes a major contributor, whereas lymph S1P is from a distinct radiation-resistant source. Lymphocyte egress from thymus and secondary lymphoid organs was markedly reduced in kinase-deficient mice. Restoration of S1P to plasma rescued egress to blood but not lymph, and the rescue required lymphocyte expression of S1P-receptor-1. Thus, separate sources provide S1P to plasma and lymph to help lymphocytes exit the low-S1P environment of lymphoid organs. Disruption of compartmentalized S1P signaling is a plausible mechanism by which S1P-receptor-1 agonists function as immunosuppressives.

Gene expression alterations in the sphingolipid metabolism pathways during progression of dementia and Alzheimer's disease: a shift toward ceramide accumulation at the earliest recognizable stages of Alzheimer's disease?

There is mounting evidence linking Abeta42 generation in Alzheimer's disease (AD) with sphingomyelin catabolism. Using microarray technology to study 17 brain regions from subjects with varying severity of AD and dementia we detected multiple gene expression abnormalities of the key enzymes that control sphingolipid metabolism. These changes were correlated with the progression of clinical dementia. The upregulation of gene expression of the enzymes controlling synthesis de novo of Cer and the downregulation of the enzymes involved in glycosphingolipid synthesis was evident as early in disease progression as in mild dementia. Together these changes suggest a shift in sphingolipid metabolism towards accumulation of Cer, depletion of glycosphingolipids and the reduction of synthesis of the anti-apoptosis signaling lipid-sphingosine 1-phosphate as a function of disease progression. This disrupted balance within the sphingolipid metabolism may trigger signaling events promoting neurodegeneration across cortical regions. This potential mechanism may provide a link between lipid metabolism disturbance and AD.

Sphingosine kinase activity is required for myogenic differentiation of C2C12 myoblasts

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

Plasma cell S1P1 expression determines secondary lymphoid organ retention versus bone marrow tropism

After induction in secondary lymphoid organs, a subset of antibody-secreting cells (ASCs) homes to the bone marrow (BM) and contributes to long-term antibody production. The factors determining secondary lymphoid organ residence versus BM tropism have been unclear. Here we demonstrate that in mice treated with FTY720 or that lack sphingosine-1-phosphate (S1P) receptor-1 (S1P1) in B cells, IgG ASCs are induced and localize normally in secondary lymphoid organs but they are reduced in numbers in blood and BM. Many IgG ASCs home to BM on day 3 of the secondary response and day 3 splenic ASCs exhibit S1P responsiveness, whereas the cells remaining at day 5 are unable to respond. S1P1 mRNA abundance is higher in ASCs isolated from blood compared to spleen, whereas CXCR4 expression is lower. Blood ASCs also express higher amounts of Kruppel-like factor (KLF)2, a regulator of S1P1 gene expression. These findings establish an essential role for S1P1 in IgG plasma cell homing and they suggest that differential regulation of S1P1 expression in differentiating plasma cells may determine whether they remain in secondary lymphoid organs or home to BM.

Ceramide/sphingosine/sphingosine 1-phosphate metabolism on the cell surface and in the extracellular space

Sphingolipid metabolites, ceramide, sphingosine, and sphingosine 1-phosphate, have emerged as a new class of lipid biomodulators of various cell functions. These metabolites are known to function not only as intracellular second messengers, but also in the extracellular space. Sphingosine 1-phosphate especially has numerous functions as an important extracellular mediator that binds to cell surface S1P receptors. Recent studies have also shown that sphingolipid-metabolizing enzymes function not only in intracellular organelles but also in the extracellular spaces, including the outer leaflet of the plasma membrane. This review focuses on the metabolic enzymes (acid and alkaline sphingomyelinases, neutral ceramidase, and sphingosine kinase) that are involved in the production of the sphingolipid metabolites in these extracellular spaces, and on the metabolic pathway itself.

Extracellular export of sphingosine kinase-1a contributes to the vascular S1P gradient

Sphingosine 1-phosphate (S1P), produced by Sphks (sphingosine kinases), is a multifunctional lipid mediator that regulates immune cell trafficking and vascular development. Mammals maintain a large concentration gradient of S1P between vascular and extravascular compartments. Mechanisms by which S1P is released from cells and concentrated in the plasma are poorly understood. We recently demonstrated [Ancellin, Colmont, Su, Li, Mittereder, Chae, Stefansson, Liau and Hla (2002) J. Biol. Chem. 277, 6667-6675] that Sphk1 activity is constitutively secreted by vascular endothelial cells. In the present study, we show that among the five Sphk isoforms expressed in endothelial cells, the Sphk-1a isoform is selectively secreted in HEK-293 cells (human embryonic kidney cells) and human umbilical-vein endothelial cells. In sharp contrast, Sphk2 is not secreted. The exported Sphk-1a isoform is enzymatically active and produced sufficient S1P to induce S1P receptor internalization. Wild-type mouse plasma contains significant Sphk activity (179 pmol x min(-1) x g(-1)). In contrast, Sphk1-/- mouse plasma has undetectable Sphk activity and approx. 65% reduction in S1P levels. Moreover, human plasma contains enzymatically active Sphk1 (46 pmol x min(-1) x g(-1)). These results suggest that export of Sphk-1a occurs under physiological conditions and may contribute to the establishment of the vascular S1P gradient.

Sphingosine kinases, sphingosine 1-phosphate, apoptosis and diseases

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

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

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

The role of the lysophospholipid sphingosine 1-phosphate in immune cell biology

Sphingosine 1-phosphate (S1P) has been shown to be a bioactive lipid mediator intimately involved in mediating a variety of immunological processes. In particular, S1P regulates lymphocyte cell trafficking between the lymphatic system and the blood. The lysophospholipid signals mainly through five related G protein-coupled receptor subtypes, termed S1P(1) to S1P(5). S1P(1) seems to play an essential role in cell trafficking, as this receptor subtype promotes the egress of T and B cells from secondary lymphatic organs. This S1P(1)-mediated migratory response is a consequence of different S1P levels in the serum and lymphatic organs. In addition to its direct effects on lymphocyte motility, S1P strengthens cell barrier integrity in sinus-lining endothelial cells, thereby reducing lymphocyte egress out of lymph nodes. Furthermore, S1P modulates cytokine profiles in T and dendritic cells, resulting in an elevated differentiation of T helper-2 cells during the T cell activation process. It is of interest that the mode of molecular action of the novel immunomodulator FTY720 interferes with the signaling of S1P. After phosphorylation, FTY720 shares structural similarity with S1P, but in contrast to the natural ligand, phosphorylated FTY720 induces a prolonged internalization of S1P(1), resulting in an impaired S1P-mediated migration of lymphocytes.

Sphingosine kinase: biochemical and cellular regulation and role in disease

Sphingolipids have emerged as molecules whose metabolism is regulated leading to generation of bioactive products including ceramide, sphingosine, and sphingosine-1-phosphate. The balance between cellular levels of these bioactive products is increasingly recognized to be critical to cell regulation; whereby, ceramide and sphingosine cause apoptosis and growth arrest phenotypes, and sphingosine-1-phosphate mediates proliferative and angiogenic responses. Sphingosine kinase is a key enzyme in modulating the levels of these lipids and is emerging as an important and regulated enzyme. This review is geared at mechanisms of regulation of sphingosine kinase and the coming to light of its role in disease.

Lysophospholipids elevate [Ca2+]i and trigger exocytosis in bovine chromaffin cells

Sphingosine 1-phosphate (S1P) and lysophosphatidic acid (LPA) are responsible for many physiological functions, including angiogenesis, neuronal survival, and immunity. However, little is known about their effects in modulating the stimulus-secretion coupling in bovine chromaffin cells. The result of PCR showed that at least two receptors (S1P(3) and LPA(1)) were expressed in bovine chromaffin cells. The elevation of [Ca(2+)](i) by S1P was fast and sustaining; but the elevation by LPA was slow and transient. The EC(50) for S1P and LPA in elevating the [Ca(2+)](i) were 0.55+/-0.01 and 0.54+/-0.40microM, respectively. This elevation could be totally blocked by thapsigargin, 2-APB, and U73122. Pertussis toxin pretreatment inhibited about half of the elevation in [Ca(2+)](i) suggesting the involvement of G(i) and other G-proteins. Repetitive [Ca(2+)](i) elevations elicited by S1P, but not LPA, were inhibited by ryanodine. S1P was more effective than LPA in triggering exocytosis as measured by the changes in membrane capacitance. The whole-cell Ca(2+) current was inhibited by both lysophospholipids but Na(+) current was inhibited by S1P only. These results suggest the differential effects of LPA and S1P in releasing Ca(2+) from the intracellular Ca(2+) stores and modulating the stimulus-secretion coupling in bovine chromaffin cells.

De novo biosynthesis of dihydrosphingosine-1-phosphate by sphingosine kinase 1 in mammalian cells

Sphingosine kinase 1 (SK1) is one of the two known kinases, which generates sphingosine-1-phosphate (S1P), a potent endogenous lipid mediator involved in cell survival, proliferation, and cell-cell interactions. Activation of SK1 and intracellular generation of S1P were suggested to be part of the growth and survival factor-induced signaling, and overexpression of SK1 provoked cell tumorigenic transformation. Using a highly selective and sensitive LC-MS/MS approach, here we show that SK1 overexpression, but not SK2, in different primary cells and cultured cell lines results in predominant upregulation of the synthesis of dihydrosphingosine-1-phosphate (DHS1P) compared to S1P. Stable isotope pulse-labeling experiments in conjunction with LC-MS/MS quantitation of different sphingolipids demonstrated strong interference of overexpressed SK1 with the de novo sphingolipid biosynthesis by deviating metabolic flow of newly formed sphingoid bases from ceramide formation toward the synthesis of DHS1P. On the contrary, S1P biosynthesis was not directly linked to the de novo sphingoid bases transformations and was dependent on catabolic generation of sphingosine from complex sphingolipids. As a result of SK1 overexpression, migration and Ca2+-response of human pulmonary artery endothelial cells (HPAEC) to stimulation with external S1P, but not thrombin, was strongly impaired. In contrast, selective increase in intracellular content of DHS1P or S1P through the uptake and phosphorylation of corresponding sphingoid bases had no effect on S1P-induced signaling or facilitation of wound healing. Furthermore, infection of human bronchial epithelial cells (HBEpC) with RSV A-2 virus increased SK1-mediated synthesis of DHS1P and S1P, whereas TNF-alpha enhanced only S1P production in HPAEC. These findings uncover a new functional role for SK1, which can control survival/death (DHS1P-S1P/ceramides) balance by targeting sphingolipid de novo biosynthesis and selectively generating DHS1P at a metabolic step preceding ceramide formation.

Biological effects of lysophospholipids

Lysophosphatidic acid (LPA) and sphingosine 1-phosphate (S1P) are potent biologically active lipid mediators that exert a wide range of cellular effects through specific G protein-coupled receptors. To date, four LPA receptors and five S1P receptors have been identified. These receptors are expressed in a large number of tissues and cell types, allowing for a wide variety of cellular responses to lysophospholipid signaling, including cell adhesion, cell motility, cytoskeletal changes, proliferation, angiogenesis, process retraction, and cell survival. In addition, recent studies in mice show that specific lysophospholipid receptors are required for proper cardiovascular, immune, respiratory, and reproductive system development and function. Lysophospholipid receptors may also have specific roles in cancer and other diseases. This review will cover identification and expression of the lysophospholipid receptors, as well as receptor signaling properties and function. Additionally, phenotypes of mice deficient for specific lysophospholipid receptors will be discussed to demonstrate how these animals have furthered our understanding of the role lysophospholipids play in normal biology and disease.

IgE-dependent activation of sphingosine kinases 1 and 2 and secretion of sphingosine 1-phosphate requires Fyn kinase and contributes to mast cell responses

Engagement of the high affinity receptor for IgE (FcepsilonRI) on mast cells results in the production and secretion of sphingosine 1-phosphate (S1P), a lipid metabolite present in the lungs of allergen-challenged asthmatics. Herein we report that two isoforms of sphingosine kinase (SphK1 and SphK2) are expressed and activated upon FcepsilonRI engagement of bone marrow-derived mast cells (BMMC). Fyn kinase is required for FcepsilonRI coupling to SphK1 and -2 and for subsequent S1P production. Normal activation of SphK1 and -2 was restored by expression of wild type Fyn but only partly with a kinase-defective Fyn, indicating that induction of SphK1 and SphK2 depended on both catalytic and noncatalytic properties of Fyn. Downstream of Fyn, the requirements for SphK1 activation differed from that of SphK2. Whereas SphK1 was considerably dependent on the adapter Grb2-associated binder 2 and phosphatidylinositol 3-OH kinase, SphK2 showed minimal dependence on these molecules. Fyn-deficient BMMC were defective in chemotaxis and, as previously reported, in degranulation. These functional responses were partly reconstituted by the addition of exogenous S1P to FcepsilonRI-stimulated cells. Taken together with our previous study, which demonstrated delayed SphK activation in Lyn-deficient BMMC, we propose a cooperative role between Fyn and Lyn kinases in the activation of SphKs, which contributes to mast cell responses.

Production profile of the major allergen Alt a 1 in Alternaria alternata cultures

BACKGROUND

The fungus Alternaria is strongly associated with asthma, but the importance of fungal allergen products is frequently underestimated. The profile of allergen release from fungal material is poorly understood.

OBJECTIVE

To investigate expression of the major allergen of Alternaria alternata, Alt a 1, during its growth in culture conditions for allergen extract production.

METHODS

Allergen expression was examined by Alt a 1-specific 2-site monoclonal antibody enzyme-linked immunosorbent assay, immunoblotting, and potency assays. The release of Alt a 1 was studied by transmission electron microscopy in conjunction with immunogold staining by using antibodies with specificity for Alt a 1.

RESULTS

A maximum amount of Alt a 1 was obtained after 4.5 weeks of growing, and it was found predominantly in the spent culture medium. In the same way, total IgE binding activity showed 15-fold more activity in the spent culture medium than in the buffer-extractable antigen fraction. Immunogold electron microscopy provided evidence that Alt a 1 is released from spores and mycelia.

CONCLUSIONS

Alternaria alternata allergenic proteins were constantly released into the culture medium, where they accumulated. Alt a 1 was a good marker for checking optimal culture conditions for A alternata extract production intended for clinical use.

Enhancement of intracellular sphingosine-1-phosphate production by inositol 1,4,5-trisphosphate-evoked calcium mobilisation in HEK-293 cells: endogenous sphingosine-1-phosphate as a modulator of the calcium response

Sphingosine-1-phosphate (S1P) regulates many cellular functions, such as migration, differentiation and growth. The effects of S1P are thought to be primarily mediated by G-protein coupled receptors, but an intracellular function as a calcium releasing second messenger has also been proposed. Here we show that in HEK-293 cells, exogenous S1P mobilises sequestered calcium by a mechanism primarily dependent on the phospholipase C (PLC)/inositol 1,4,5-trisphosphate (IP3) pathway, and secondarily on the subsequent synthesis of intracellular S1P. Stimulating HEK-293 cells exogenously with S1P increased the production of both inositol phosphates and intracellular S1P. The calcium response was inhibited in cells treated with 2-APB, caffeine or U73122, showing that the PLC/IP3 pathway for calcium release is activated in response to exogenous S1P. The calcium response was partially inhibited in cells treated with the sphingosine kinase inhibitor DMS and in cells expressing a catalytically inactive sphingosine kinase, showing that endogenously produced S1P is also involved. Importantly, 2-APB and U73122 inhibited the S1P-evoked production of intracellular S1P. S1P is therefore not likely a major calcium releasing second messenger in HEK-293 cells, but rather a secondary regulator of calcium mobilisation.

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

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

Skin Lipid Synthesis Inhibition: A Possible Means for Enhancing Percutaneous Delivery of Levodopa

Skin perturbation with ethanol followed by application of beta-chloroalanine (beta-CA) or atorvastatin (AVN) was employed for delaying the recovery of sphingosine (a precursor of ceramide) and cholesterol, respectively in epidermis of rats. Dose optimization studies revealed 600 microg of beta-CA and 750 microg of AVN significantly (p<0.05) inhibited the synthesis of sphingosine and cholesterol, respectively and prevented their replenishment to normal levels till 48 hr in viable rat skin. Co-application of calcium chloride (0.1 mM) inhibited the synthesis of both micro constituents of epidermis to a greater magnitude, whereas verapamil reduced this effect. The in vitro permeation of levodopa across treated skin portions was directly correlated with percentage of sphingosine and cholesterol inhibited by the treatments. The in vitro permeation of levodopa across skin excised after treatment with beta-CA or AVN was enhanced 3-fold. Effective plasma concentration (1.58 microg/ml) of levodopa in rats was achieved within 2 hr and maintained till 12 hr after AVN treatment, and increased to 36 hr with the co-application of calcium chloride. However, when the skin was treated with beta-CA, Ceff was achieved after 4 hr and was maintained till 36 hr. The inclusion of calcium chloride maintained Ceff for 48 hr. Hence, synthesis inhibition of skin lipids seems to offer a feasible means to enhance the systemic delivery of polar drugs like levodopa.

Sphingosine Contributes to Glucocorticoid-Induced Apoptosis of Thymocytes Independently of the Mitochondrial Pathway

During the selection process in the thymus, most thymocytes are eliminated by apoptosis through signaling via TCR or glucocorticoids. The involvement of ceramide (Cer) and sphingosine (SP), important apoptotic mediators, remains poorly defined in glucocorticoid-induced apoptosis. We report that, in mouse thymocytes, apoptosis triggered by 10(-6) M dexamethasone (DX) was preceded by a caspase-dependent Cer and SP generation, together with activation of acidic and neutral ceramidases. Apoptosis was drastically reduced by blocking either sphingolipid production (by acid sphingomyelinase inhibitor) or SP production (by ceramidase inhibitors), but not by inhibition of de novo Cer synthesis. Thus, SP generated through acid sphingomyelinase and ceramidase activity would contribute to the apoptotic effect of DX. Consistent with this hypothesis, SP addition or inhibition of SP kinase induced thymocyte apoptosis. DX induced a proteasome-dependent loss of mitochondrial membrane potential (Deltapsim) and caspase-8, -3, and -9 processing. Apoptosis was abolished by inhibition of Deltapsim loss or caspase-8 or -3, but not caspase-9. Deltapsim loss was independent of SP production and caspase-8, -3, and -9 activation. However, inhibition of SP production reduced caspase-8 and -3, but not caspase-9 processing. Proteasome inhibition impaired activation of the three caspases, whereas inhibition of Deltapsim loss solely blocked caspase-9 activation. These data indicate that DX-induced apoptosis is mediated in part by SP, which contributes, together with proteasome activity, to caspase-8-3 processing independently of mitochondria, and in part by the proteasome/mitochondria pathway, although independently of caspase-9 activation.

Transactivation of sphingosine-1-phosphate receptors by FcepsilonRI triggering is required for normal mast cell degranulation and chemotaxis

Mast cells secrete various substances that initiate and perpetuate allergic responses. Cross-linking of the high-affinity receptor for IgE (FcɛRI) in RBL-2H3 and bone marrow–derived mast cells activates sphingosine kinase (SphK), which leads to generation and secretion of the potent sphingolipid mediator, sphingosine-1–phosphate (S1P). In turn, S1P activates its receptors S1P₁ and S1P₂ that are present in mast cells. Moreover, inhibition of SphK blocks FcɛRI-mediated internalization of these receptors and markedly reduces degranulation and chemotaxis. Although transactivation of S1P₁ and Gi signaling are important for cytoskeletal rearrangements and migration of mast cells toward antigen, they are dispensable for FcɛRI-triggered degranulation. However, S1P₂, whose expression is up-regulated by FcɛRI cross-linking, was required for degranulation and inhibited migration toward antigen. Together, our results suggest that activation of SphKs and consequently S1PRs by FcɛRI triggering plays a crucial role in mast cell functions and might be involved in the movement of mast cells to sites of inflammation.

SPTLC1 mutation in twin sisters with hereditary sensory neuropathy type I

Hereditary sensory neuropathy type I (HSN I) is an autosomal dominant ulceromutilating disorder of the peripheral nervous system characterized by progressive sensory loss. HSN I locus maps to chromosome 9q22.1-22.3 and is caused by mutations in the gene coding for serine palmitoyltransferase long-chain base subunit 1 (SPTLC1). A novel missense mutation in exon 13 of the SPTLC1 gene (c.1160G-->C; p.G387A) in twin sisters with a severe HSN I phenotype is reported.

Disruption of sphingolipid biosynthesis in hepatocyte nodules: selective proliferative stimulus induced by fumonisin B1

In order to investigate the role of sphingolipid disruption in the cancer promoting potential of fumonisin B(1) (FB(1)) in the development of hepatocyte nodules, male Fischer 344 rats were subjected to cancer initiation (FB(1) containing diet or diethylnitrosamine (DEN) by i.p. injection) and promotion (2-acetylaminofluorene with partial hepatectomy, 2-AAF/PH) treatments followed by a secondary FB(1) dietary regimen. Sphinganine (Sa) and sphingosine (So) levels were measured by high performance liquid chromatography in control, surrounding and nodular liver tissues of the rats. The disruption of sphingolipid biosynthesis by the secondary FB(1) treatment in the control rats was significantly (P < 0.05) enhanced by the 2-AAF/PH cancer promotion treatment. The nodular and surrounding Sa levels returned to baseline following FB(1) initiation and 2-AAF/PH promotion. When comparing the groups subjected to the secondary FB(1) treatment, the initiation effected by FB(1) was less (P < 0.01) sensitive to the accumulation of Sa in the nodular and surrounding tissues than DEN initiation and the 2-AAF/PH control treatment. In contrast, the So level of FB(1) initiation was marginally increased in the nodules compared to the surrounding liver after 2-AAF/PH promotion and significantly (P < 0.05) higher with the secondary FB(1) treatment. Although, the FB(1)-induced hepatocyte nodules were not resistant to the disruption of sphingolipid biosynthesis, the nodular So levels were increased and might provide a selective growth stimulus possibly induced by bio-active sphingoid intermediates such as sphingosine 1-phosphate (S1P).