Effects of synthetic sphingosine-1-phosphate analogs on arachidonic acid metabolism and cell death

Regulation of phosphatidic Acid metabolism by sphingolipids in the central nervous system

This paper explores the way ceramide, sphingosine, ceramide 1-phosphate, and sphingosine 1-phosphate modulate the generation of second lipid messengers from phosphatidic acid in two experimental models of the central nervous system: in vertebrate rod outer segments prepared from dark-adapted retinas as well as in rod outer segments prepared from light-adapted retinas and in rat cerebral cortex synaptosomes under physiological aging conditions. Particular attention is paid to lipid phosphate phosphatase, diacylglycerol lipase, and monoacylglycerol lipase. Based on the findings reported in this paper, it can be concluded that proteins related to phototransduction phenomena are involved in the effects derived from sphingosine 1-phosphate/sphingosine or ceramide 1-phosphate/ceramide and that age-related changes occur in the metabolism of phosphatidic acid from cerebral cortex synaptosomes in the presence of either sphingosine 1-phosphate/sphingosine or ceramide 1-phosphate/ceramide. The present paper demonstrates, in two different models of central nervous system, how sphingolipids influence phosphatidic acid metabolism under different physiological conditions such as light and aging.

Vanadate-induced activation of cytosolic phospholipase A2α in L929 cells: Roles of tyrosine kinase, protein kinase C, and extracellular signal-regulated kinase

Orthovanadate (Na3VO4), which acts as an inhibitor of protein tyrosine phosphatases, has a various pharmacological effects including the release of arachidonic acid (AA) from cells. We investigated roles of alpha-type cytosolic phospholipase A2 (cPLA2alpha), Src family kinases (Src) and protein kinase C (PKC) in the release of AA induced by Na3VO4 from a murine fibroblast cell line, L929. C12 cells, a variant of L929 that lacks expression of cPLA2alpha, were used along with a clone of C12 cells that are stably expressing cPLA2alpha (C12-cPLA2alpha cells). In the presence of a Ca2+ ionophore (10 microM A23187), 5 and 10mM Na3VO4 synergistically stimulated AA release from L929 and C12-cPLA2alpha cells, and to a much lesser extent from control C12 cells. The release of AA by Na3VO4/A23187 was inhibited by a selective cPLA2alpha inhibitor (3 microM pyrrophenone). The release of AA by Na3VO4/A23187 was significantly inhibited by a PKC inhibitor (10 microM GF109203X), in PKC-depleted cells, by a Src inhibitor (2 microM PP2) and by an inhibitor of extracellular signal-regulated kinase 1/2 (ERK1/2) kinase (10 microM U0126). The phosphorylation of ERK1/2 was stimulated by Na3VO4, and the response was significantly decreased by inhibitors of Src, PKC and ERK1/2 kinase. Our data show that Na3VO4 stimulates AA release largely via cPLA2alpha activation in Ca2+-dependent manner, and the cross-talk between Src and PKC and the ERK-dependent pathways are involved in Na3VO4-induced AA release from L929 cells.

Up-regulation of cytosolic phospholipase A2α expression by N,N-diethyldithiocarbamate in PC12 cells; involvement of reactive oxygen species and nitric oxide

Disulfiram (an alcohol-aversive drug) and related compounds are known to provoke several side effects involving behavioral and neurological complications. N,N-diethyldithiocarbamate (DDC) is considered as one of the main toxic species of disulfiram and acts as an inhibitor of superoxide dismutase. Since arachidonic acid (AA) formation is regulated by reactive oxygen species (ROS) and related to toxicity in neuronal cells, we investigated the effects of DDC on AA release and expression of the alpha type of cytosolic phospholipase A(2) (cPLA(2)alpha) in PC12 cells. Treatment with 80-120 microM DDC that causes a moderate increase in ROS levels without cell toxicity stimulated cPLA(2)alpha mRNA and its protein expression. The expression was mediated by extracellular-signal-regulated kinase (ERK1/2), one of the mitogen-activated protein kinases. Treatment with N(G) nitro-L-arginine methyl ester (an inhibitor of nitric oxide synthase, 1 mM) and oxy-hemoglobin (a scavenger of nitric oxide, 2 mg/mL) abolished the DDC-induced responses (ERK1/2 phosphorylation and cPLA(2)alpha expression). We also showed DDC-induced up-regulation of the mRNA expression of lipocortin 1, an inhibitor of PLA(2). Furthermore, DDC treatment of the cells enhanced Ca(2+)-ionophore-induced AA release in 30 min, although the effect was limited. Changes in AA metabolism in DDC-treated cells may have a potential role in mediating neurotoxic actions of disulfiram. In this study, we show the first to demonstrate the up-regulation of cPLA(2)alpha expression by DDC treatment in neuronal cells.

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.

Involvement of p38 MAP kinase-mediated cytochrome c release on sphingosine-1-phosphate (S1P)- and N-monomethyl-S1P-induced cell death of PC12 cells

d-erythro-Sphingosine-1-phosphate (S1P), a sphingolipid metabolite, affects various neuronal functions including cell fate. S1P appears to have contradictory effects in PC12 cells, a neuronal model cell line; neurite retraction and cell survival/differentiation. In the present study, we examined whether S1P induces cell death in undifferentiated PC12 cells. Culture with S1P at 20 microM for 4 h caused lactate dehydrogenase leakage 24 h later. The response was reduced by an inhibitor of caspases and accompanied by the release of cytochrome c and DNA fragmentation. S1P caused the phosphorylation of p38 mitogen-activated protein kinase (p38 MAPK) within 10 min. An inhibitor of p38 MAPK (10 microM SB203580) inhibited both the release of cytochrome c and DNA fragmentation induced by S1P. Treatment with nerve growth factor or pertussis toxin (PTX) decreased S1P-induced phosphorylation of p38 MAPK and cell death. These findings suggest that S1P-activated p38 MAPK acts as a death signal upstream of the release of cytochrome c. N-Monomethyl-S1P (MM-S1P), a weak agonist in cells expressing S1P1 receptors, had marked effects (phosphorylation of p38 MAPK, release of cytochrome c and DNA fragmentation) at lower concentrations than S1P and in a PTX-sensitive manner. These findings show that the activation of S1P receptors by S1P and MM-S1P causes cell death accompanied by DNA fragmentation via the p38 MAPK pathway-mediated release of cytochrome c in PC12 cells. The potential of S1P and MM-S1P to act as agonists of S1P receptors and as intracellular messengers is discussed.