Regulation of phosphoinositide hydrolysis in cultured astrocytes by sphingosine and psychosine

Inhibition of the IGF-1-PI3K-Akt-mTORC2 pathway in lipid rafts increases neuronal vulnerability in a genetic lysosomal glycosphingolipidosis

Glycosphingolipid (GSL) accumulation is implicated in the neuropathology of several lysosomal conditions, such as Krabbe disease, and may also contribute to neuronal and glial dysfunction in adult-onset conditions such as Parkinson's disease, Alzheimer's disease and multiple sclerosis. GSLs accumulate in cellular membranes and disrupt their structure; however, how membrane disruption leads to cellular dysfunction remains unknown. Using authentic cellular and animal models for Krabbe disease, we provide a mechanism explaining the inactivation of lipid raft (LR)-associated IGF-1-PI3K-Akt-mTORC2, a pathway of crucial importance for neuronal function and survival. We show that psychosine, the GSL that accumulates in Krabbe disease, leads to a dose-dependent LR-mediated inhibition of this pathway by uncoupling IGF-1 receptor phosphorylation from downstream Akt activation. This occurs by interfering with the recruitment of PI3K and mTORC2 to LRs. Akt inhibition can be reversed by sustained IGF-1 stimulation, but only during a time window before psychosine accumulation reaches a threshold level. Our study shows a previously unknown connection between LR-dependent regulation of mTORC2 activity at the cell surface and a genetic neurodegenerative disease. Our results show that LR disruption by psychosine desensitizes cells to extracellular growth factors by inhibiting signal transmission from the plasma membrane to intracellular compartments. This mechanism serves also as a mechanistic model to understand how alterations of the membrane architecture by the progressive accumulation of lipids undermines cell function, with potential implications in other genetic sphingolipidoses and adult neurodegenerative conditions. This article has an associated First Person interview with the first author of the paper.

Lysosphingomyelin prevents behavioral aberrations and hippocampal neuron loss induced by the metabotropic glutamate receptor agonist quisqualate

1. Excessive excitation of brain neurons by the excitatory neurotransmitter, glutamate, induces a cascade of events leading to increased intracellular Ca++, neuronal degeneration and death. 2. Recent in vitro research has demonstrated that a natural cationic amphiphile in the brain, lysosphingomyelin, may be able to prevent neuronal degeneration by repressing phosphosinositidase-C overactivation induced by excessive excitation of the metabotropic glutamate receptor. 3. This research tested the latter finding in vivo in a rat model of glutamate excitotoxicity. Intracerebroventricular (i.c.v.) administration of the Group 1 metabotropic glutamate receptor (mGluR) agonist, quisqualate, produced seizures, akinesia, destruction of hippocampal pyramidal cell dendritic microtubule-associated protein-2, and major loss of hippocampal CA sector neurons. 4. Prophylactic i.c.v. infusion of lysosphingomyelin powerfully attenuates these quisqualate-induced behaviors and prevents neuronal degeneration. 5. Lysosphingomyelin may be of clinical use in allaying progressive Group 1 mGluR-induced hippocampal cognitive and motor disorders including Alzheimer's disease, brain seizure, and stroke.

Psychosine blocks quisqualate-induced glutamate excitotoxicity in hippocampal CA sector neurons

Quisqualate is a potent specific agonist for Group 1 metabotropic glutamate receptors (mGluR's), that activate G protein-coupled phospholipase C (PLC) in a molecular signal-transduction mechanism that raises cytoplasmic Ca2+ and, when excessive, damages hippocampal neurons. Psychosine (beta-galactosylsphingosine), a cationic lysosphingolipid occurring naturally in nervous tissues, dose-dependently inhibited PLC activation induced by metabotropic alpha 1-adrenergic receptor signaling in cultured rat brain astrocytes in vitro. In the present study, we have tested neuroprotective efficacy of psychosine in vivo, in a rat model of glutamate excitotoxicity induced by intracerebroventricular (i.c.v.) administration of quisqualate. A sublethal i.c.v. dose of quisqualate caused episodes of prolonged akinesia and convulsions, and major damage to pyramidal neurons of the hippocampal CA1 and CA3 sector, but not to granule cell neurons of the dentate gyrus. Prior infusion of psychosine greatly attenuated quisqualate-induced behaviors, and fully prevented destruction by quisqualate of vulnerable hippocampal neurons. Psychosine may prove useful in prophylaxis of neurodegenerative disorders that arise from intensive hippocampal Group 1 mGluR stimulation.

Sphingoid bases and phospholipase D activation

There is increased interest in physiological functions and mechanisms of action of sphingolipids metabolites, ceramide, sphingosine, and sphingosine-1-phosphate (SPP), members of a new class of lipid second messengers. This review summarizes current knowledge regarding the role of these sphingolipids metabolites in the actions of growth factors and focuses on the second messenger roles of sphingosine and its metabolite, SPP, in the regulation of cell growth. We also discuss possible interactions with intermediates of the well known glycerophospholipid cycle. Sphingosine and SPP generally provide positive mitogenic signals whereas ceramide has been reported to induce apoptosis and cell arrest in several mammalian cell lines. Stimulation of phospholipase D leading to an increase in phosphatidic acid, a positive regulator of cell growth, by sphingosine and SPP, and its inhibition by ceramide, might be related to their opposite effects on cell growth. This also indicates that sphingolipid turnover could regulate the diacylglycerol cycle. Cross-talk between sphingolipid turnover pathways and the diacylglycerol cycle increases complexity of signaling pathways leading to cellular proliferation and adds additional sites of regulation.

Ca2+ mobilizing action of sphingosine in Jurkat human leukemia T cells. Evidence that sphingosine releases Ca2+ from inositol trisphosphate- and phosphatidic acid-sensitive intracellular stores through a mechanism independent of inositol trisphosphate

Effects of sphingosine on Ca2+ mobilization in the human Jurkat T cell line were examined. Sphingosine increased the cytoplasmic Ca2+ concentration ([Ca2+]i) in a dose-dependent manner with an ED50 of around 8 microM. Sphingosine and OKT3, a CD3 monoclonal antibody, transiently increased [Ca2+]i, which declined to the resting level in the absence of extracellular Ca2+. Under the same conditions, pretreatment with sphingosine inhibited but did not abolish an increase in [Ca2+]i induced by the subsequent addition of OKT3 and vice versa. However, pretreatment with sphingosine did not affect an increase in [Ca2+]i induced by OKT3 in the presence of Ca2+. OKT3 increased IP3 formation, but sphingosine did not affect the level of IP3 by itself nor did it cause IP3 formation induced by OKT3. In permeabilized Jurkat cells, the addition of IP3 released Ca2+ from nonmitochondrial intracellular stores, but the addition of sphingosine did not. Sphingosine, stearylamine, and psychosine increased [Ca2+]i and diacylglycerol (DG) kinase activation; however, ceramide did not, whereas sphingosine 1-phosphate slightly activated DG kinase without elevation of [Ca2+]i. Pretreatment with R59022, a DG kinase inhibitor, abolished the peak but did not affect the sustained response to [Ca2+]i to sphingosine. Phosphatidic acid (PA) elevated [Ca2+]i, after which it declined to a resting level even in the presence of extracellular Ca2+. In accordance with this, PA did not stimulate 45Ca2+ uptake into cells, but sphingosine and OKT3 did. Pretreatment with PA partially inhibited a rise in [Ca2+]i induced by the subsequent addition of sphingosine and vice versa in the absence of extracellular Ca2+. Under similar conditions, pretreatment with PA affected an elevation of [Ca2+]i induced by OKT3 less, after which the subsequent addition of sphingosine did not increase [Ca2+]i. In permeabilized Jurkat cells, the addition of IP3 did not release Ca2+, but PA did in the presence of heparin. Pretreatment with thapsigargin, a microsomal Ca2+-ATPase inhibitor, abolished the rises of [Ca2+]i induced by the subsequent addition of sphingosine, OKT3, and PA in the absence of extracellular Ca2+. The present results suggest that at least two kinds of intracellular Ca2+ stores exist in Jurkat cells, both of which are IP3- and PA-sensitive, and that sphingosine mobilizes Ca2+ from both stores in an IP3-independent manner. Furthermore, the IP3- but not the PA-sensitive intracellular Ca2+ store seems to regulate Ca2+ entry induced by sphingosine.

Pertussis toxin inhibits phospholipase C activation and Ca2+ mobilization by sphingosylphosphorylcholine and galactosylsphingosine in HL60 leukemia cells. Implications of GTP-binding protein-coupled receptors for lysosphingolipids

Extracellular sphingosylphosphorylcholine (SPC) and galactosylsphingosine (psychosine) induced Ca2+ mobilization in a dose-dependent manner in HL60 leukemia cells. The rapid and transient increase in intracellular Ca2+ concentration ([Ca2+]i) elicited by SPC and psychosine at concentrations lower than 30 microM was inhibited by treatment of the cells with pertussis toxin (PTX) and U73122, a phospholipase C inhibitor, as was the case for UTP, a P2-purinergic agonist. The increase in [Ca2+]i induced by these lysosphingolipids was associated with inositol phosphate production, which was also sensitive to PTX and U73122. The inositol phosphate response is not secondary to the increase in [Ca2+]i as evidenced by the observation that thapsigargin and ionomycin, Ca2+ mobilizing agents, never induced inositol phosphate production and, unlike lysosphingolipids, the [Ca2+]i rise by these agents was totally insensitive to PTX and U73122. When HL60 cells were differentiated into neutrophil-like cells by dibutyryl cyclic AMP, inositol phosphate and Ca2+ responses to AlF4- were enhanced, probably reflecting an increase in the amount of Gi2 and Gi3 compared with undifferentiated cells. In the neutrophil-like cells, however, the responses to SPC and psychosine were markedly attenuated. This may exclude the possibility that the lysosphingolipids activate rather directly PTX-sensitive GTP-binding proteins or the phospholipase C itself. Other lysosphingolipids including glucosylsphingosine (glucopsychosine) and sphingosylgalactosyl sulfate (lysosulfatides) at 30 microM or lower concentrations also showed PTX- and U73122-sensitive Ca2+ mobilization and inositol phosphate response in a way similar to SPC and psychosine. However, platelet-activating factor and lysoglycerophospholipids such as lysophosphatidylcholine and lysophosphatidic acid were less effective than these lysosphingolipids in the induction of Ca2+ mobilization. Taken together, the results indicate that a group of lysosphingolipids at appropriate doses induces Ca2+ mobilization through inositol phosphate production by phospholipase C activation. The lysosphingolipids-induced enzyme activation may be mediated by PTX-sensitive GTP-binding protein-coupled receptors, which may be different from previously identified platelet-activating factor receptor or lysophosphatidic acid receptor.

Involvement of a pertussis toxin-sensitive G protein in the mitogenic signaling pathways of sphingosine 1-phosphate

Sphingosine 1-phosphate, a sphingolipid metabolite, was previously reported to increase DNA synthesis in quiescent Swiss 3T3 fibroblasts and to induce transient increases in intracellular free calcium (Zhang, H., Desai, N. N., Olivera, A., Seki, T., Brooker, G., and Spiegel, S. (1991) J. Cell Biol. 114, 155-167). In the present study, pretreatment of Swiss 3T3 fibroblasts with pertussis toxin reduced sphingosine 1-phosphate-induced DNA synthesis. Sphingosine 1-phosphate decreased cellular cAMP levels and also caused a drastic decrease in isoproterenol- and forskolin-stimulated cAMP accumulation. Pertussis toxin treatment prevented the inhibitory effect of sphingosine 1-phosphate on cAMP accumulation, suggesting that a pertussis toxin-sensitive Gi or Gi-like protein may be involved in sphingosine 1-phosphate-mediated inhibition of cAMP accumulation. Mitogenic concentrations of sphingosine 1-phosphate stimulated production of inositol phosphates which was inhibited by pertussis toxin, while the response to bradykinin was not affected. Furthermore, calcium release induced by sphingosine 1-phosphate, but not by bradykinin, was also attenuated by pertussis toxin treatment. However, sphingosine 1-phosphate-induced phosphatidic acid accumulation was unaffected by pertussis toxin. The increase in specific DNA binding activity of activator protein-1, which was induced by treatment of quiescent Swiss 3T3 fibroblasts with sphingosine 1-phosphate, was also inhibited by pertussis toxin. These results suggest that some of the sphingosine 1-phosphate-induced signaling pathways are mediated by G proteins that are substrates for pertussis toxin.

Sphingosine-1-phosphate, a novel second messenger involved in cell growth regulation and signal transduction, affects growth and invasiveness of human breast cancer cells

This review will focus on the role of sphingosine and its phosphorylated derivative sphingosine-1-phosphate (SPP) in cell growth regulation and signal transduction. We will show that many of the effects attributed to sphingosine in quiescent Swiss 3T3 fibroblasts are mediated via its conversion to SPP. We propose that SPP has appropriate properties to function as an intracellular second messenger based on the following: it elicits diverse cellular responses; it is rapidly produced from sphingosine by a specific kinase and rapidly degraded by a specific lyase; its concentration is low in quiescent cells but increases rapidly and transiently in response to the growth factors, fetal calf serum (FCS) and platelet derived growth factor (PDGF); it releases Ca2+ from internal sources in an InsP3-independent manner; and finally, it may link sphingolipid signaling pathways to cellular ras-mediated signaling pathways by elevating phosphatidic acid levels. The effects of this novel second messenger on growth, differentiation and invasion of human breast cancer cells will be discussed.