Ca2+ signaling induced by sphingosylphosphorylcholine and sphingosine 1-phosphate via distinct mechanisms in rat glomerular mesangial cells

Sphingosylphosphorylcholine ameliorates experimental sjögren's syndrome by regulating salivary gland inflammation and hypofunction, and regulatory B cells

Sjögren syndrome (SS) is an autoimmune disease in which immune cells infiltrate the exocrine gland. Since SS is caused by a disorder of the immune system, treatments should regulate the immune response. Sphingosylphosphorylcholine (SPC) is a sphingolipid that mediates cellular signaling. In immune cells, SPC has several immunomodulatory functions. Accordingly, this study verifies the immunomodulatory ability and therapeutic effect of SPC in SS. To understand the function of SPC in SS, we treated SPC in female NOD/ShiJcl (NOD) mice. The mice were monitored for 10 weeks, and inflammation in the salivary glands was checked. After SPC treatment, we detected the expression of regulatory B (B_reg) cells in mouse splenocytes and the level of salivary secretion-related genes in human submandibular gland (HSG) cells. Salivary flow rate was maintained in the SPC-treated group compared to the vehicle-treated group, and inflammation in the salivary gland tissues was relieved by SPC. SPC treatment in mouse cells and HSG cells enhanced B_reg cells and salivary secretion markers, respectively. This study revealed that SPC can be considered as a new therapeutic agent against SS.

Sphingosine-1-Phosphate Metabolism and Signaling in Kidney Diseases

In the past few decades, sphingolipids and sphingolipid metabolites have gained attention because of their essential role in the pathogenesis and progression of kidney diseases. Studies in models of experimental and clinical nephropathies have described accumulation of sphingolipids and sphingolipid metabolites, and it has become clear that the intracellular sphingolipid composition of renal cells is an important determinant of renal function. Proper function of the glomerular filtration barrier depends heavily on the integrity of lipid rafts, which include sphingolipids as key components. In addition to contributing to the structural integrity of membranes, sphingolipid metabolites, such as sphingosine-1-phosphate (S1P), play important roles as second messengers regulating biologic processes, such as cell growth, differentiation, migration, and apoptosis. This review will focus on the role of S1P in renal cells and how aberrant extracellular and intracellular S1P signaling contributes to the pathogenesis and progression of kidney diseases.

Lipid Regulation of Sodium Channels

The lipid landscapes of cellular membranes are complex and dynamic, are tissue dependent, and can change with the age and the development of a variety of diseases. Researchers are now gaining new appreciation for the regulation of ion channel proteins by the membrane lipids in which they are embedded. Thus, as membrane lipids change, for example, during the development of disease, it is likely that the ionic currents that conduct through the ion channels embedded in these membranes will also be altered. This chapter provides an overview of the complex regulation of prokaryotic and eukaryotic voltage-dependent sodium (Nav) channels by fatty acids, sterols, glycerophospholipids, sphingolipids, and cannabinoids. The impact of lipid regulation on channel gating kinetics, voltage-dependence, trafficking, toxin binding, and structure are explored for Nav channels that have been examined in heterologous expression systems, native tissue, and reconstituted into artificial membranes. Putative mechanisms for Nav regulation by lipids are also discussed.

Sphingosine-1-phosphate regulates the expression of adherens junction protein E-cadherin and enhances intestinal epithelial cell barrier function

BACKGROUND

The regulation of intestinal barrier permeability is important in the maintenance of normal intestinal physiology. Sphingosine-1-phosphate (S1P) has been shown to play a pivotal role in enhancing barrier function in several non-intestinal tissues. The current study determined whether S1P regulated function of the intestinal epithelial barrier by altering expression of E-cadherin, an important protein in adherens junctions.

METHODS

Studies were performed upon cultured differentiated IECs (IEC-Cdx2L1 line) using standard techniques.

RESULTS

S1P treatment significantly increased levels of E-cadherin protein and mRNA in intestinal epithelial cells (IECs) and also led to E-cadherin localizing strongly to the cell-cell border. S1P also improved the barrier function as indicated by a decrease in 14C-mannitol paracellular permeability and an increase in transepithelial electrical resistance (TEER) in vitro.

CONCLUSIONS

These results indicate that S1P increases levels of E-cadherin, both in cellular amounts and at the cell-cell junctions, and leads to improved barrier integrity in cultured intestinal epithelial cells.

Effect of direct albumin binding to sphingosylphosphorylcholine in Jurkat T cells

We investigated the effects of serum on lysophospholipid-induced cytotoxicity in Jurkat T cells. We found that sphingosylphosphorylcholine (SPC, also known as lysosphingomyelin) induced cytotoxicity and that albumin in serum could protect cells by binding directly to SPC. Furthermore, we also found that SPC induced ROS generation, increased [Ca(2+)](i), and decreased MMP. However, those effects were only observed at concentrations higher than 10 microM and were only induced in albumin-free media. Therefore, SPC may be trapped by albumin in plasma and unable to exert its effects under normal conditions, although at high concentrations, SPC could induce several responses such as ROS generation, increased [Ca(2+)](i), and decreased MMP in Jurkat T cells.

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.

Signal transduction underlying the vascular effects of sphingosine 1-phosphate and sphingosylphosphorylcholine

Two related lysosphingolipids, sphingosine 1-phosphate (S1P) and sphingosylphosphorylcholine (SPC) mediate diverse cellular responses through signals transduced by either activation of G-protein coupled receptors or possibly by acting intracellularly. Vascular responses to S1P and SPC measured both in vivo and in dissected vessels show predominantly vasoconstriction with some evidence for vasodilation. Although stimulation with S1P or SPC generally leads to similar vascular responses, the signalling pathways stimulated to produce these responses are often distinct. Nevertheless, mobilization of Ca2+ from intracellular stores and influx of extracellular Ca2+, which both increase [Ca2+]i, occur in response to S1P and SPC. Both mobilization of Ca2+ from intracellular stores and influx of extracellular Ca2+ occur in response to S1P and SPC. As well, both S1P and SPC induce Ca2+-sensitization in vascular smooth muscle which is mediated through Rho kinase activation. In the endothelium, S1P and SPC stimulate the production of the vasodilator, nitric oxide through activation of endothelial nitric oxide synthase. This activation occurs through phosphorylation by Akt and through binding of Ca2+-calmodulin upon increased [Ca2+]i. These lysosphingolipids also activate cyclooxygenase-2 which produces prostaglandins with both vasoconstrictor and vasodilator properties. A balance between the signals inducing vasodilation versus the signals inducing vasoconstriction will determine the vascular outcome. Thus, perturbations in S1P and SPC concentrations, relative expression of receptors or downstream signalling pathways may provide a mechanism for pathophysiological conditions such as hypertension. Given this background, recent studies examining a potential role for S1P and SPC in hypertension and vascular dysfunction in aging are discussed.

Indomethacin differentiates the renal effects of sphingosine-1-phosphate and sphingosylphosphorylcholine

The sphingomyelin breakdown products sphingosine-1-phosphate (S1P) and sphingosylphosphorylcholine (SPC) constrict intrarenal microvessels in vitro in a pertussis toxin (PTX) sensitive manner, and S1P also reduces renal blood flow in vivo. Nevertheless, both S1P and SPC have been reported to enhance diuresis and natriuresis. This pattern is similar to that of neuropeptide Y, which also reduces renal blood flow and enhances diuresis and natriuresis. The latter effects are inhibited by the cyclooxygenase inhibitor indomethacin, and various S1P and SPC responses have also been linked to the cyclooxygenase pathway. Therefore, we have investigated whether indomethacin can alter the renal effects of S1P and SPC in anaesthetised rats in vivo. In line with earlier experiments S1P bolus injections dose-dependently reduced renal blood flow (by up to 4.8 +/- 0.5 ml min(-1)), and this was not significantly affected by indomethacin treatment (5 mg kg(-1) i.p.). Infusion of S1P but not of SPC (30 microg kg(-1) min(-1) each) for 60 min reduced renal blood flow by up to 0.8 +/- 0.2 ml min(-1), and this was not markedly altered by indomethacin. Despite the differential renovascular effect, both S1P and SPC enhanced diuresis by up to 215 +/- 65 and 201 +/- 58 microl 15 min(-1) respectively, and natriuresis by up to 25 +/- 9 and 29 +/- 11 micromol 15 min(-1) respectively. While indomethacin abolished the SPC-induced diuresis and natriuresis, it, if anything, slightly enhanced the diuretic and natriuretic effect of S1P. To determine whether tubular SPC effects are receptor-mediated, PTX experiments were performed. SPC-induced enhancements of diuresis and natriuresis were abolished by PTX. We conclude that S1P, SPC and neuropeptide Y exhibit distinct patterns of modulation of renal function and that indomethacin allows such effects to be differentiated.

FTY720 impairs necrosis development after ischemia-reperfusion injury

Ischemia-reperfusion (IR) injury is a common early feature that contributes to graft damage by impairing resident cell function. Our previous results showed that IR injury impaired renal function, by causing extensive tubular necrosis and increasing MHC class II and ICAM-1 molecule expression by mesangial cells (MC). MCs are likely candidates to come into close contact with immune cells such as monocytes or lymphocytes. It has been suggested that under inflammatory circumstances, there is increased MC expression of MHC class II, of adhesion molecules (such as ICAM-1), of cytokines receptors, and of molecules associated with cellular death (apoptosis). The immunosuppressive properties of FTY720 have been shown in clinical and experimental situations. It has also been shown to be protective against IR injury in rats. We sought to evaluate the role of FTY720 in a murine IR model by measuring renal function, tubular necrosis, and surface molecule expression by cultured mesangial cells. Intravenous administration of FTY720 (1 mg/kg) immediately before IR induction did not improve the short-term (24 hours) outcome of renal function or reduced MHC class II and ICAM-1 surface molecule expression. However, there was a decreased percentage of tubular necrosis in mice treated with FTY720 (51.3% +/- 1.6%) compared with vehicle-treated mice (66% +/- 5.5%). These results suggest a protective role of FTY720 in an IR injury model. More studies are required to identify the mechanisms involved in the protective activity of FTY720 in the IR injury model.

Transcriptional profiling of gene expression patterns during sphingosine 1-phosphate-induced mesangial cell proliferation

Sphingosine 1-phosphate (S1P) is known to regulate cell proliferation, apoptosis, and motility. Recently, we have reported that S1P and its analogue dihydro-S1P (DHS1P) promote proliferation of rat cultured mesangial cells. To investigate the signaling mechanisms underlying S1P- and DHS1P-induced mesangial cell proliferation, we performed cDNA microarray analysis of gene expression during mesangial cell proliferation. In terms of the overall pattern, gene expression waves induced by S1P and DHS1P were similar to those induced by a potent mesangial mitogen platelet-derived growth factor (PDGF), whereas we found several genes, such as two growth factors, connective tissue growth factor (CTGF) and heparin-binding EGF-like growth factor (HB-EGF), which were induced by the sphingolipids, but not by PDGF. Cluster analysis also identified calcium-dependent molecules highly expressed in DHS1P-stimulated cells compared to S1P-stimulated cells. Calcium mobilization analysis showed that DHS1P had higher magnitudes of intracellular calcium mobilization than S1P, suggesting that S1P and DHS1P differentially regulate intracellular calcium mobilization, possibly leading to different gene expression in mesangial cells. The large-scale monitoring of gene expression performed here allows us to identify S1P-induced transcriptional properties during mesangial cell proliferation.

Sphingosylphosphorylcholine-induced contraction of feline ileal smooth muscle cells is mediated by Galphai3 protein and MAPK

We studied the mechanism of sphingosylphosphorylcholine (SPC)-induced contraction in feline ileal smooth muscle cells. Western blotting revealed that G protein subtypes of Galpha(i1), Galpha(i3) and Galpha(o) existed in feline ileum. Galpha(i3) antibody penetration into permeabilized cells decreased SPC-induced contraction. In addition, incubation of [35S]guanosine 5'-O-(3-thiotriphosphate) ([35S]GTPgammaS) with membrane fraction increased its binding to Galpha(i3) subtype after SPC treatment, suggesting that the signalling pathways invoked by SPC were mediated by Galpha(i3) protein. MAPK kinase (MEK) inhibitor PD98059 blocked the contraction significantly, but p38 mitogen-activated protein kinase (MAPK) inhibitor SB202190 did not. Chelerythrine and neomycin also inhibited the contraction. However, cotreatment of PD98059 and chelerythrine showed no significant difference. Phosphorylation of p44/42 MAPK was increased by SPC treatment, which was reversed by pretreatment of inhibitors of signalling molecules that decreased SPC-induced contraction previously. The same result was obtained in the assay of MAPK activity.

Apoptotic effect of sphingosine 1-phosphate and increased sphingosine 1-phosphate hydrolysis on mesangial cells cultured at low cell density

The lipid mediator sphingosine 1-phosphate (S1P) may alter the proliferation of mesangial cells during pathophysiological processes. Here, S1P stimulated proliferation of rat mesangial cells and phosphorylation of MAPKs at subconfluent cell density. Both effects were inhibited by pertussis toxin treatment. Mesangial cells expressed several S1P receptors of the endothelial differentiation gene family: EDG-1, -3, -5, and -8. Conversely, S1P induced apoptosis at low cell density (2 x 10(4) cells/cm(2)), which was demonstrated by flow cytometry and Hoechst staining. Apoptosis was observed also in quiescent or growing cells and was not reverted by lysophosphatidic acid or platelet-derived growth factor. S1P enhanced phosphorylation of SAPKs. Incubation with [(33)P]S1P, [(3)H]S1P, and [(3)H]sphingosine demonstrated increased S1P hydrolysis, resulting in enhanced intracellular sphingosine levels and decreased S1P levels. A rise in total ceramide levels was also observed; however, ceramide did not originate from [(3)H]sphingosine, and S1P-induced apoptosis was not inhibited by fumonisin B, precluding involvement of de novo ceramide synthesis in apoptosis. Therefore, we suggest that sphingosine accumulation and decreased S1P are primarily responsible for S1P-induced apoptosis. In conclusion, incubation of low-density mesangial cells with S1P results in apoptosis, presumably due to increased S1P hydrolysis.

Sphingosine 1-phosphate stimulates rat mesangial cell proliferation from outside the cells

BACKGROUND

Proliferation of mesangial cells (MCs) is the initial step in glomerulonephritis, and platelet-derived mediators have been shown to play a significant role in this proliferation. Sphingosine 1-phosphate (S1P), one of the sphingolipids, is abundantly stored in platelets and is released upon stimulation. We examined the effects of S1P and related sphingolipids on the cell fate of cultured MCs in order to elucidate potential roles of these lipid mediators in glomerulonephritis.

METHODS

Cell proliferation was evaluated by bromodeoxy uridine (BrdU) incorporation together with MTS assay. Apoptosis of MCs was evaluated by examining annexin V staining and typical morphological changes in nuclei. We also examined the metabolism of [(3)H]sphingosine in MCs in either the presence or absence of platelet-derived growth factor (PDGF). The expression of endothelial differentiation genes (edg), which are the cell surface receptors for S1P in MCs, was examined by RT-PCR.

RESULTS

S1P, but not the other sphingolipids, stimulated MC proliferation. In contrast, dimethylsphingosine (DMS) induced apoptosis in the MCs. The amount of sphingosine (Sph) converted into S1P was small and was not affected by PDGF. This observation suggested that Sph kinase activity producing S1P from Sph was low in the MCs. Furthermore, expression of edg-1, -2 and -5 in MCs was confirmed by RT-PCR.

CONCLUSIONS

Our observations suggest that S1P stimulates MC proliferation from outside the cells, and not as a second messenger for PDGF. The modulation of MC fate with sphingolipids may provide possible strategies for the treatment of glomerulonephritis.

Distinct effects of different calcium-mobilizing agents on cell death in NG108-15 neuroblastoma X glioma cells

The effects of different calcium-mobilizing agents on cell death were characterized in NG108-15 neuroblastoma x glioma hybrid cells. Carbonyl cyanide p-trifluoromethoxyphenylhydrazone (FCCP) increased the cytosolic Ca(2+) concentration ([Ca(2+)](i)) and caused cell death. Thapsigargin (TG) not only increased the [Ca(2+)](i) and caused cell death but also induced neurite outgrowth via activation of phospholipase A(2) and cytochrome P450 epoxygenase. In contrast, bradykinin increased the [Ca(2+)](i), but had no effect on cell morphology or cell death. Cell death occurred by two different mechanisms, one of which was caspase-3-dependent and the other caspase-3-independent. Caspase-3 activation was Ca(2+)-dependent, whereas neurite outgrowth was Ca(2+)-independent. TG- or FCCP-induced caspase-3 activation occurred at the same time, but the cell death induced by TG was delayed. TG treatment did not enhance the generation of nitric oxide or cAMP or secretion of glial-derived neurotrophic factor or neurotrophin-3, but activated sphingosine kinase. Furthermore, inhibition of sphingosine kinase accelerated TG-induced cell death, and exogenous sphingosine 1-phosphate (S1P) protected cells from FCCP-induced cell death by about 60%. These results indicate that, in these cells, depletion of intracellular nonmitochondrial or mitochondrial Ca(2+) stores causes cell death, that TG activates phospholipase A(2) and sphingosine kinase, and that arachidonic acid induces neurite outgrowth, whereas S1P delays cell death.

Antiproliferative effect of nitric oxide on rat glomerular mesangial cells via inhibition of mitogen-activated protein kinase

The effect of nitric oxide (NO) donors and lipopolysaccharide (LPS) on the proliferation of rat glomerular mesangial cells was characterized. Exogenous application of a NO donor inhibited serum-induced proliferation in a time- and dose-dependent manner. S-Nitrosoglutathione (GSNO) also increased cGMP generation and arachidonic acid release, but it did not cause any measurable increase in the cytosolic Ca2+ concentration. Chelation of cytosolic Ca2+ or inhibition of mitogen-activated protein kinase (MAPK) kinase had an inhibitory effect on proliferation, but neither enhanced the antiproliferative effect of GSNO. In contrast, inhibition of guanylate cyclase or phospholipase A2 had no effect on proliferation, but partially reversed GSNO-induced antiproliferation by approximately 98 and 65%, respectively. GSNO did not cause cell death. Incubation of cells with LPS induced endogenous NO generation and had an antiproliferative effect. LPS-induced antiproliferation was reversed completely by inhibition of nitric oxide synthase and partially by inhibition of guanylate cyclase or phospholipase A2. GSNO or LPS inhibited serum-induced MAPK activation, and both effects were partially reversed by inhibition of guanylate cyclase or phospholipase A2. Inclusion of 8-bromo-cGMP or arachidonic acid in the growth medium resulted in a similar antiproliferative effect. In conclusion, in rat glomerular mesangial cells, MAPK inhibition and an antiproliferative effect could be induced by either an increase in the cellular concentration of NO or exposure of the cells to LPS. Part of the effect of NO was attributable to the increased cellular cGMP generation and arachidonic acid release.

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.

Changes in sphingolipid levels induced by epidermal growth factor in osteoblastic cells. Effects of these metabolites on cytosolic calcium levels

Sphingolipids mediate a number of cellular functions in a variety of cell systems. The role they play in osteoblast signaling is yet unknown. This study investigated the effects of epidermal growth factor (EGF) on the levels of ceramide, sphingosine (SPH), and sphingosine-1-phosphate (S1P) in rat calvariae osteoblastic cells, and whether these metabolites mediated cytosolic calcium ([Ca2+]i) mobilization in these cells. EGF significantly (P<0.05) increased the levels of all three sphingolipids, and the phorbol ester PMA partially inhibited these effects. SPH and S1P markedly increased [Ca2+]i levels, with thapsigargin (depletes [Ca2+]i pools) decreasing the response by 60%. Verapamil (calcium channel blocker) only inhibited ceramide's effects on [Ca2+]i. Furthermore, SPH enhanced the EGF' induced increase in [Ca2+]i. This study demonstrates that ceramide, SPH and S1P mediate [Ca2+]i mobilization in rat calvarial osteoblastic cells, and that EGF induces changes in the levels of these metabolites with PKC playing an important role in the mechanisms regulating these events.