Platelet-derived growth factor-induced activation of sphingosine kinase requires phosphorylation of the PDGF receptor tyrosine residue responsible for binding of PLCgamma

Lysophosphatidate Promotes Sphingosine 1-Phosphate Metabolism and Signaling: Implications for Breast Cancer and Doxorubicin Resistance

Lysophosphatidate (LPA) and sphingosine 1-phosphate (S1P) promote vasculogenesis, angiogenesis, and wound healing by activating a plethora of overlapping signaling pathways that stimulate mitogenesis, cell survival, and migration. As such, maladaptive signaling by LPA and S1P have major effects in increasing tumor progression and producing poor patient outcomes after chemotherapy and radiotherapy. Many signaling actions of S1P and LPA are not redundant; each are vital in normal physiology and their metabolisms differ. In the present work, we studied how LPA signaling impacts S1P metabolism and signaling in MDA-MB-231 and MCF-7 breast cancer cells. LPA increased sphingosine kinase-1 (SphK1) synthesis and rapidly activated cytosolic SphK1 through association with membranes. Blocking phospholipase D activity attenuated the LPA-induced activation of SphK1 and the synthesis of ABCC1 and ABCG2 transporters that secrete S1P from cells. This effect was magnified in doxorubicin-resistant MCF-7 cells. LPA also facilitated S1P signaling by increasing mRNA expression for S1P1 receptors. Doxorubicin-resistant MCF-7 cells had increased S1P2 and S1P3 receptor expression and show increased LPA-induced SphK1 activation, increased expression of ABCC1, ABCG2 and greater S1P secretion. Thus, LPA itself and LPA-induced S1P signaling counteract doxorubicin-induced death of MCF-7 cells. We conclude from the present and previous studies that LPA promotes S1P metabolism and signaling to coordinately increase tumor growth and metastasis and decrease the effectiveness of chemotherapy and radiotherapy for breast cancer treatment.

Prognostic Impact of Sphingosine Kinase 1 in Nonsmall Cell Lung Cancer

Bioactive sphingolipid is clearly relevant to lung physiology. The relationship of the bioactive sphingolipid pathway to pulmonary disease has been studied in cellular, tissue, and animal model, including lung cancer models. The samples of 53 patients diagnosed with nonsmall cell lung carcinoma (NSCLC) between June 2009 and May 2014 at our hospital were analyzed. Immunohistochemical (IHC) analysis was performed. The degree of immunostaining was reviewed and scored. Using this method of assessment, we evaluated the IHC score of sphingosine kinase 1 (SPHK1), vimentin, E-cadherin, and Ki-67. Both invasive adenocarcinoma cell and squamous cell carcinoma cell were well stained by SPHK1, and fibroblasts were also well stained by SPHK1. Although the IHC score of SPHK1 was not significantly differed between invasive adenocarcinoma and squamous cell carcinoma, the IHC scores of fibroblast, vimentin, and Ki-67 were higher in squamous cell carcinoma than invasive adenocarcinoma. Correlation among IHC scores in each of invasive adenocarcinoma and squamous cell carcinoma was performed. SPHK1 had positive correlation with both fibroblast and Ki-67, and fibroblast and Ki-67 had also positive correlation in invasive adenocarcinoma. On the contrary, SPHK1 had no significant correlation with fibroblast, and had negative correlation with Ki-67 in squamous cell carcinoma. Although there was not significant prognostic difference in SPHK1 score (P = .09), IHC score high group tended to be worse on relapse-free survival. SPHK1 might be prognostic factor in lung-invasive adenocarcinoma and novel target for drug against lung-invasive adenocarcinoma.

Trans-2-enoyl-CoA reductase limits Ca2+ accumulation in the endoplasmic reticulum by inhibiting the Ca2+ pump SERCA2b

The endoplasmic reticulum (ER) contains various enzymes that metabolize fatty acids (FAs). Given that FAs are the components of membranes, FA metabolic enzymes might be associated with regulation of ER membrane functions. However, it remains unclear whether there is the interplay between FA metabolic enzymes and ER membrane proteins. Trans-2-enoyl-CoA reductase (TER) is an FA reductase present in the ER membrane and catalyzes the last step in the FA elongation cycle and sphingosine degradation pathway. Here we identify sarco(endo)plasmic reticulum Ca²⁺-ATPase 2b (SERCA2b), an ER Ca²⁺ pump responsible for Ca²⁺ accumulation in the ER, as a TER-binding protein by affinity purification from HEK293 cell lysates. We show that TER directly binds to SERCA2b by in vitro assays using recombinant proteins. Thapsigargin, a specific SERCA inhibitor, inhibits this binding. TER binds to SERCA2b through its conserved C-terminal region. TER overexpression suppresses SERCA2b ATPase activity in microsomal membranes of HEK293 cells. Depletion of TER increases Ca²⁺ storage in the ER and accelerates SERCA2b-dependent Ca²⁺ uptake to the ER after ligand-induced Ca²⁺ release. Moreover, depletion of TER reduces the Ca²⁺-dependent nuclear translocation of nuclear factor of activated T cells 4. These results demonstrate that TER is a negative regulator of SERCA2b, implying the direct linkage of FA metabolism and Ca²⁺ accumulation in the ER.

Sphingosine-1-phosphate signalling drives an angiogenic transcriptional programme in diffuse large B cell lymphoma

Although the over-expression of angiogenic factors is reported in diffuse large B-cell lymphoma (DLBCL), the poor response to anti-VEGF drugs observed in clinical trials suggests that angiogenesis in these tumours might be driven by VEGF-independent pathways. We show that sphingosine kinase-1 (SPHK1), which generates the potent bioactive sphingolipid sphingosine-1-phosphate (S1P), is over-expressed in DLBCL. A meta-analysis of over 2000 cases revealed that genes correlated with SPHK1 mRNA expression in DLBCL were significantly enriched for tumour angiogenesis meta-signature genes; an effect evident in both major cell of origin (COO) and stromal subtypes. Moreover, we found that S1P induces angiogenic signalling and a gene expression programme that is present within the tumour vasculature of SPHK1-expressing DLBCL. Importantly, S1PR1 functional antagonists, including Siponimod, and the S1P neutralising antibody, Sphingomab, inhibited S1P signalling in DLBCL cells in vitro. Furthermore, Siponimod, also reduced angiogenesis and tumour growth in an S1P-producing mouse model of angiogenic DLBCL. Our data define a potential role for S1P signalling in driving an angiogenic gene expression programme in the tumour vasculature of DLBCL and suggest novel opportunities to target S1P-mediated angiogenesis in patients with DLBCL.

Anti-cancer effect of pristimerin by inhibition of HIF-1α involves the SPHK-1 pathway in hypoxic prostate cancer cells

BACKGROUND

Hypoxia is a typical character of locally advanced solid tumours. The transcription factor hypoxia-inducible factor 1α (HIF-1α) is the main regulator under the hypoxic environment. HIF-1α regulates various genes to enhance tumour progression, angiogenesis, and metastasis. Sphingosine kinase 1 (SPHK-1) is a modulator of HIF-1α.

METHODS

To investigate the molecular mechanisms of pristimerin in association with SPHK-1 pathways in hypoxic PC-3 cancer cells. Vascular endothelial growth factor (VEGF) production, cell cycles, and SPHK-1 activity were measured, and western blotting, an MTT assay, and an RNA interference assay were performed.

RESULTS

Pristimerin inhibited HIF-1α accumulation in a concentration- and-time-dependent manner in hypoxic PC-3 cells. Pristimerin suppressed the expression of HIF-1α by inhibiting SPHK-1. Moreover, inhibiting SPHK-1 with a sphingosine kinase inhibitor enhanced the suppression of HIF-1α, phosphorylation AKT, and glycogen synthase kinase-3β (GSK-3β) by pristimerin under hypoxia. Furthermore, a reactive oxygen species (ROS) scavenger enhanced the inhibition of HIF-1α and SPHK-1 by pristimerin.

CONCLUSION

Taken together, these findings suggest that pristimerin can exert an anti-cancer activity by inhibiting HIF-1α through the SPHK-1 pathway.

Mouse Sphingosine Kinase 1a Is Negatively Regulated through Conventional PKC-Dependent Phosphorylation at S373 Residue

Sphingosine kinase is a lipid kinase that converts sphingosine into sphingosine-1-phosphate, an important signaling molecule with intracellular and extracellular functions. Although diverse extracellular stimuli influence cellular sphingosine kinase activity, the molecular mechanisms underlying its regulation remain to be clarified. In this study, we investigated the phosphorylation-dependent regulation of mouse sphingosine kinase (mSK) isoforms 1 and 2. mSK1a was robustly phosphorylated in response to extracellular stimuli such as phorbol ester, whereas mSK2 exhibited a high basal level of phosphorylation in quiescent cells regardless of agonist stimulation. Interestingly, phorbol ester-induced phosphorylation of mSK1a correlated with suppression of its activity. Chemical inhibition of conventional PKCs (cPKCs) abolished mSK1a phosphorylation, while overexpression of PKCα, a cPKC isoform, potentiated the phosphorylation, in response to phorbol ester. Furthermore, an in vitro kinase assay showed that PKCα directly phosphorylated mSK1a. In addition, phosphopeptide mapping analysis determined that the S373 residue of mSK1a was the only site phosphorylated by cPKC. Interestingly, alanine substitution of S373 made mSK1a refractory to the inhibitory effect of phorbol esters, whereas glutamate substitution of the same residue resulted in a significant reduction in mSK1a activity, suggesting the significant role of this phosphorylation event. Taken together, we propose that mSK1a is negatively regulated through cPKC-dependent phosphorylation at S373 residue.

Contribution of phosphoproteomics in understanding SRC signaling in normal and tumor cells

The membrane-anchored, non-receptor tyrosine kinase (non-RTK) SRC is a critical regulator of signal transduction induced by a large variety of cell-surface receptors, including RTKs that bind to growth factors to control cell growth and migration. When deregulated, SRC shows strong oncogenic activity, probably because of its capacity to promote RTK-mediated downstream signaling even in the absence of extracellular stimuli. Accordingly, SRC is frequently deregulated in human cancer and is thought to play important roles during tumorigenesis. However, our knowledge on the molecular mechanism by which SRC controls signaling is incomplete due to the limited number of key substrates identified so far. Here, we review how phosphoproteomic methods have changed our understanding of the mechanisms underlying SRC signaling in normal and tumor cells and discuss how these novel findings can be used to improve therapeutic strategies aimed at targeting SRC signaling in human cancer.

High expression of sphingosine kinase 1 is associated with poor prognosis in nasopharyngeal carcinoma

It has been reported that sphingosine kinase 1 (SPHK1), an oncogenic enzyme, was involved in the development and progression of a number of human cancers. However, the role of SPHK1 in nasopharyngeal carcinoma (NPC) is largely unknown. The present study aimed to characterize the expression of SPHK1 in human NPC and evaluate its clinical significance. Real-time quantitative reverse transcriptase-polymerase chain reaction (qRT-PCR) and Western blot analyses showed that the expression of SPHK1 mRNA and protein in NPC specimens was significantly higher than that in non-tumorous nasopharyngeal mucosa biopsies. Immunohistochemistry (IHC) was conducted to characterize the expression pattern of SPHK1 in 142 archived paraffin-embedded NPC specimens. Statistical analyses revealed that high levels of SPHK1 expression were associated with the clinical stages, locoregional recurrence and distant metastasis of NPC. NPC patients with high levels of SPHK1 expression had shorter survival time, whereas those with lower levels of SPHK1 expression survived longer. Moreover, multivariate analysis suggested that SPHK1 up-regulation was an independent prognostic factor for NPC. Our results suggest for the first time that SPHK1 is involved in the development and progression of NPC, which can be used as a useful prognostic marker for NPC patients and may be an effective target for treating NPC.

Ceramide-induced apoptosis in renal tubular cells: a role of mitochondria and sphingosine-1-phoshate

Ceramide is synthesized upon stimuli, and induces apoptosis in renal tubular cells (RTCs). Sphingosine-1 phosphate (S1P) functions as a survival factor. Thus, the balance of ceramide/S1P determines ceramide-induced apoptosis. Mitochondria play a key role for ceramide-induced apoptosis by altered mitochondrial outer membrane permeability (MOMP). Ceramide enhances oligomerization of pro-apoptotic Bcl-2 family proteins, ceramide channel, and reduces anti-apoptotic Bcl-2 proteins in the MOM. This process alters MOMP, resulting in generation of reactive oxygen species (ROS), cytochrome C release into the cytosol, caspase activation, and apoptosis. Ceramide regulates apoptosis through mitogen-activated protein kinases (MAPKs)-dependent and -independent pathways. Conversely, MAPKs alter ceramide generation by regulating the enzymes involving ceramide metabolism, affecting ceramide-induced apoptosis. Crosstalk between Bcl-2 family proteins, ROS, and many signaling pathways regulates ceramide-induced apoptosis. Growth factors rescue ceramide-induced apoptosis by regulating the enzymes involving ceramide metabolism, S1P, and signaling pathways including MAPKs. This article reviews evidence supporting a role of ceramide for apoptosis and discusses a role of mitochondria, including MOMP, Bcl-2 family proteins, ROS, and signaling pathways, and crosstalk between these factors in the regulation of ceramide-induced apoptosis of RTCs. A balancing role between ceramide and S1P and the strategy for preventing ceramide-induced apoptosis by growth factors are also discussed.

Sphingosine kinase 1 is regulated by peroxisome proliferator-activated receptor α in response to free fatty acids and is essential for skeletal muscle interleukin-6 production and signaling in diet-induced obesity

We previously demonstrated that sphingosine kinase 1 (Sphk1) expression and activity are up-regulated by exogenous palmitate (PAL) in a skeletal muscle model system and in diet-induced obesity in mice; however, potential functions and in vivo relevance of this have not been addressed. Here, we aimed to determine the mechanism by which PAL regulates SphK1 in muscle, and to determine potential roles for its product, sphingosine-1-phosphate (S1P), in muscle biology in the context of obesity. Cloning and analysis of the mouse Sphk1 promoter revealed a peroxisome proliferator-activated receptor (PPAR) α cis-element that mediated activation of a reporter under control of the Sphk1 promoter; direct interaction of PPARα was demonstrated by chromatin immunoprecipitation. PAL treatment induced the proinflammatory cytokine interleukin (IL)-6 in a manner dependent on SphK1, and this was attenuated by inhibition of the sphingosine-1-phosphate receptor 3 (S1PR3). Diet-induced obesity in mice demonstrated that IL-6 expression in muscle, but not adipose tissue, increased in obesity, but this was attenuated in Sphk1(-/-) mice. Moreover, plasma IL-6 levels were significantly decreased in obese Sphk1(-/-) mice relative to obese wild type mice, and muscle, but not adipose tissue IL-6 signaling was activated. These data indicate that PPARα regulates Sphk1 expression in the context of fatty acid oversupply and links PAL to muscle IL-6 production. Moreover, this function of SphK1 in diet-induced obesity suggests a potential role for SphK1 in obesity-associated pathological outcomes.

Soluble vascular endothelial growth factor receptor-2 and inflammatory factors in macular edema with branch retinal vein occlusion

PURPOSE

To investigate relationships among vitreous fluid levels of soluble vascular endothelial growth factor receptor-2 (sVEGFR-2), vascular endothelial growth factor (VEGF), and soluble intercellular adhesion molecule 1 (sICAM-1) in patients with branch retinal vein occlusion (BRVO) and macular edema or patients with idiopathic macular hole.

DESIGN

Retrospective case-control study.

METHODS

SETTING

Tokyo Women's Medical University and Eguchi Eye Hospital.

PATIENT POPULATION

Forty-nine Japanese patients who underwent unilateral vitrectomy (27 with BRVO and 22 with macular hole).

OBSERVATION PROCEDURES

Vitreous fluid samples were obtained during vitreoretinal surgery to measure the levels of sVEGFR-2, VEGF, and sICAM-1. Retinal ischemia was evaluated from capillary nonperfusion on fluorescein angiography. Macular edema was examined by optical coherence tomography.

MAIN OUTCOME MEASURES

Vitreous fluid levels of the 3 molecules and severity of macular edema.

RESULTS

BRVO patients had a significantly higher vitreous fluid level of sVEGFR-2 (median, 1670 pg/mL; interquartile range [IQL], 1205 to 2225 pg/mL) than macular hole patients (median, 1265 pg/mL; IQR, 731 to 1800 pg/mL; P = .017), as was the case for VEGF (median, 237 pg/mL; IQR, 42.2 to 1305 pg/mL; vs median, 15.6 pg/mL; IQR, 15.6 to 15.6 pg/mL; P < .001) and sICAM-1 (median, 10.1 ng/mL; IQR, 6.3 to 22.5 ng/mL; vs median, 4.1 ng/mL; IQR, 3.3 to 6.0 ng/mL; P < .001). In BRVO patients, there was a significant positive correlation between vitreous fluid levels of sVEGFR-2 or VEGF and sICAM-1, but not between sVEGFR-2 and VEGF. Vitreous fluid levels of all 3 molecules were correlated significantly the with severity of macular edema in BRVO patients.

CONCLUSIONS

sVEGFR-2 may induce an increase of vascular permeability together with or via sICAM-1, or both with and via sICAM-1, in BRVO patients with macular edema.

Sphingosine kinase 1 and sphingosine 1-phosphate receptor 3 are functionally upregulated on astrocytes under pro-inflammatory conditions

BACKGROUND

Reactive astrocytes are implicated in the development and maintenance of neuroinflammation in the demyelinating disease multiple sclerosis (MS). The sphingosine kinase 1 (SphK1)/sphingosine1-phosphate (S1P) receptor signaling pathway is involved in modulation of the inflammatory response in many cell types, but the role of S1P receptor subtype 3 (S1P(3)) signaling and SphK1 in activated rat astrocytes has not been defined.

METHODOLOGY/PRINCIPAL FINDINGS

Using immunohistochemistry we observed the upregulation of S1P(3) and SphK1 expression on reactive astrocytes and SphK1 on macrophages in MS lesions. Increased mRNA and protein expression of S1P(3) and SphK1, as measured by qPCR and Western blotting respectively, was observed after treatment of rat primary astrocyte cultures with the pro-inflammatory stimulus lipopolysaccharide (LPS). Activation of SphK by LPS stimulation was confirmed by SphK activity assay and was blocked by the use of the SphK inhibitor SKI (2-(p-hydroxyanilino)-4-(p-chlorphenyl) thiazole. Treatment of astrocytes with a selective S1P(3) agonist led to increased phosphorylation of extracellular signal-regulated kinase (ERK)-1/2), which was further elevated with a LPS pre-challenge, suggesting that S1P(3) upregulation can lead to increased functionality. Moreover, astrocyte migration in a scratch assay was induced by S1P and LPS and this LPS-induced migration was sensitive to inhibition of SphK1, and independent of cell proliferation. In addition, S1P induced secretion of the potentially neuroprotective chemokine CXCL1, which was increased when astrocytes were pre-challenged with LPS. A more prominent role of S1P(3) signaling compared to S1P(1) signaling was demonstrated by the use of selective S1P(3) or S1P(1) agonists.

CONCLUSION/SIGNIFICANCE

In summary, our data demonstrate that the SphK1/S1P(3) signaling axis is upregulated when astrocytes are activated by LPS. This signaling pathway appears to play a role in the establishment and maintenance of astrocyte activation. Upregulation of the pathway in MS may be detrimental, e.g. through enhancing astrogliosis, or beneficial through increased remyelination via CXCL1.

Redox regulation of ERK1/2 activation induced by sphingosine 1-phosphate in fibroblasts: involvement of NADPH oxidase and platelet-derived growth factor receptor

BACKGROUND

Sphingosine 1-phosphate (S1P) is a sphingolipid metabolite synthesized after stimulation with growth factors or cytokines. S1P extracellular effects are mediated through specific Gi-protein coupled receptors (GPCRs). Recently, we demonstrated in NIH3T3 fibroblasts stimulated by platelet-derived growth factor (PDGF) or S1P the NADPH oxidase activation and the H(2)O(2) intracellular level increase trough the Gi protein involvement.

METHODS

NIH3T3 fibroblast cell cultures were used. Western blot and quantitative analyses by Chemidoc-Quantity-One software were performed. H(2)O(2) level was assayed by fluorescence spectrophotometric analysis, and cell proliferation by counted manually or ELISA kit.

RESULTS

This study demonstrates, in NIH 3T3 fibroblasts, a novel redox regulated mechanism of S1P-induced activation of ERK 1/2 related to NADPH oxidase activity and intracellular H(2)O(2) level increase with PDGF receptor tyrosine kinase involvement through a transactivation mechanism. This event is mediated by S1P(1) and S1P(3) receptors by Gi proteins and can contribute to S1P mitogenic signaling.

CONCLUSION

These results can be related to mechanisms of cross-talk previously identified between receptor tyrosine kinase, including PDGFreceptor, and several GPCR ligands.

GENERAL SIGNIFICANCE

The redox-sensitive ERK1/2 and PDGFr tyrosine kinase activity could be targets for therapies in diseases in which deregulation of intracellular oxidative status and the consequent alteration of S1P and/or PDGF signaling pathway are involved.

Allergy therapy: the therapeutic potential of targeting sphingosine kinase signalling in mast cells

Mast cell activation is a central event in allergic diseases, and investigating the signalling pathways triggered during mast cell activation may lead to the discovery of novel therapeutic targets. Mast cells can be activated by a multitude of stimuli including antibodies/antigen, cytokines/chemokines and neuropeptides, resulting in a variety of responses including the immediate release of potent inflammatory mediators. Moreover, recent data suggest that mast cell-mediated responses are also influenced by the differential sphingolipids/sphingosine to sphingosine-1-phosphate ratio. The importance of sphingolipids as potent biological mediators of both intracellular and extracellular responses is being increasingly recognized and accepted; it is now appreciated that activation of mast cells, via the high-affinity IgE-receptor (FcepsilonRI) leads to the activation of sphingosine kinases (SphK), resulting in increased formation of sphingosine-1-phosphate. Furthermore, FcepsilonRI activates SphK-dependent calcium mobilization in mast cells, leading to degranulation, cytokine, and eicosanoid production, and chemotaxis. In the past two years a critical role for SphK in allergic responses in vivo has emerged. In this review, I focus on the current understanding of the role of sphingosine kinases during mast cell signalling in vitro and their role during hypersensitivity responses in vivo, and discuss the potential of these enzymes as novel therapeutic targets to treat allergic diseases.

Clinical significance of sphingosine kinase-1 expression in human astrocytomas progression and overall patient survival

PURPOSE

To characterize the expression of sphingosine kinase-1 (SPHK1) in human astrocytomas and to investigate the association between SPHK1 expression and progression of astrocytomas.

EXPERIMENTAL DESIGN

The expression of SPHK1 in normal human astrocytes, astrocytoma cell lines, and four pairs of matched astrocytoma tissues and their adjacent normal brain tissues were detected by quantitative reverse transcription-PCR and Western blot. In addition, SPHK1 protein expression was examined in 243 cases of histologically characterized astrocytomas by immunohistochemistry. Statistical analyses were applied to test for prognostic and diagnostic associations.

RESULTS

SPHK1 in astrocytoma cell lines was elevated at both mRNA and protein levels, and the SPHK1 mRNA and protein were significantly up-regulated by up to 6.8- and 40-fold, respectively, in primary astrocytomas compared with those in the adjacent noncancerous brain tissues. Immunohistochemical analysis showed that 100 of 243 (41.2%) paraffin-embedded archival astrocytoma biopsies exhibited high expression of SPHK1. Statistical analysis suggested that the up-regulation of SPHK1 was significantly correlated with the histologic grade of astrocytoma (P=0.000) and that patients with high SPHK1 level exhibited shorter survival time (P<0.001). Multivariate analysis revealed that SPHK1 up-regulation might be an independent prognostic indicator for the survival of patients with astrocytoma.

CONCLUSIONS

SPHK1 might represent a novel and useful prognostic marker for astrocytoma and play a role during the development and progression of the disease.

Cytoplasmic signalling by the c-Abl tyrosine kinase in normal and cancer cells

c-Abl is a non-receptor tyrosine kinase which is localized both in the nucleus and cytoplasm, and is involved in the regulation of cell growth, survival and morphogenesis. Although c-Abl nuclear function has been extensively studied, recent data also indicate an important role in cytoplasmic signalling through mitogenic and adhesive receptors. Here, we review the mechanisms by which growth factors promote cytoplasmic c-Abl activation and signalling and its function in the induction of DNA synthesis, changes in cell morphology and receptor endocytosis. The importance of de-regulated c-Abl cytoplasmic signalling in solid tumours is also discussed.

Sphingosine kinase/sphingosine 1-phosphate signalling in central nervous system

Sphingolipids were once regarded as inert structural components of cell membranes. Now these metabolites are generally believed to be important bioactive molecules that control a wide repertoire of cellular processes such as proliferation and survival of cells. Along with these ubiquitous cell functions observed in many peripheral tissues sphingolipid metabolites, especially sphingosine 1-phosphate, exert important neuron-specific functions such as regulation of neurotransmitter release. This review summarizes physiological and pathological roles of sphingolipid metabolites emphasizing the role of sphingosine 1-phosphate in the central nervous system.

Regulation and functions of sphingosine kinases in the brain

It has long been known that sphingolipids, especially sphingomyelin, a principal component of myelin, are highly enriched in the central nervous system and are structural components of all eukaryotic cell membranes. In the last few years, substantial evidence has accumulated from studies of many types of cells demonstrating that in addition to their structural roles, their breakdown products form a new class of signaling molecules with potent and myriad regulatory effects on essentially every cell in the body. While the sphingolipid metabolites sphingosine and its precursor ceramide have been associated with cell growth arrest and apoptosis, sphingosine-1-phosphate (S1P) enhances proliferation, differentiation, and cell survival as well as regulates many physiological and pathological processes. The relative levels of these three interconvertible sphingolipid metabolites, and thus cell fate, are strongly influenced by the activity of sphingosine kinases, of which there are two isoforms, designated SphK1 and SphK2, the enzymes that phosphorylate sphingosine to produce S1P. Not much is yet known of the importance of S1P in the central nervous system. Therefore, this review is focused on current knowledge of regulation of SphK1 and SphK2 on both transcriptional and post-translational levels and the functions of these isozymes and their product S1P and its receptors in the central nervous system.

Sphingosine kinase 1 is up-regulated during hypoxia in U87MG glioma cells. Role of hypoxia-inducible factors 1 and 2

Sphingosine 1-phosphate (S1P), a sphingolipid metabolite that plays an important role in the regulation of cell survival, growth, migration, and angiogenesis, acts both inside the cells and as an extracellular mediator through binding to five G protein-coupled receptors (S1P(1-5)). Sphingosine kinase 1 (SK1), the enzyme responsible for S1P production, is overexpressed in many solid tumors, including gliomas. One common feature of these tumors is the presence of "hypoxic regions," characterized by cells expressing high levels of hypoxia-inducible factors HIF-1alpha and HIF-2alpha, two transcription regulators that modulate the levels of proteins with crucial roles in tumor progression. So far, nothing is known about the role and the regulation of SK1 during tumor-induced hypoxia or about SK1 regulation and HIFs. Here we investigated the role of HIF-1alpha and HIF-2alpha in the regulation of SK1 during hypoxic stress in glioma-derived U87MG cells. We report that hypoxia increases SK1 mRNA levels, protein expression, and enzyme activity, followed by intracellular S1P production and S1P release. Interestingly, knockdown of HIF-2alpha by small interfering RNA abolished the induction of SK1 and the production of extracellular S1P after CoCl(2) treatment, whereas HIF-1alpha small interfering RNA resulted in an increase of HIF-2alpha and of SK1 protein levels. Moreover, using chromatin immunoprecipitation analysis, we demonstrate that HIF-2alpha binds the SK1 promoter. Functionally, we demonstrate that conditioned medium from hypoxia-treated tumor cells results in neoangiogenesis in human umbilical vein endothelial cells in a S1P receptor-dependent manner. These studies provide evidence of a link between S1P production as a potent angiogenic agent and the hypoxic phenotype observed in many tumors.

Dual and distinct roles for sphingosine kinase 1 and sphingosine 1 phosphate in the response to inflammatory stimuli in RAW macrophages

Sphingosine kinase 1 (SK1) and its product sphingosine-1-phosphate (S1P) have been implicated in the regulation of many cellular processes including growth regulation, protection from apoptosis, stimulation of angiogenesis, and most recently as mediators of the TNF-alpha inflammatory response. In this study we set out to examine the role of SK1/S1P in the RAW macrophage response to the potent inflammatory stimulus lipopolysaccharide (LPS). We show that LPS increases cellular levels of SK1 message and protein. This increase is at the transcriptional level and is accompanied by increased SK activity and generation of S1P. S1P is able to cause increases in COX-2 and PGE2 levels in RAW cells. Knockdown of SK1 using siRNA is able to inhibit the TNF but not the LPS inflammatory response. Moreover, knockdown of SK1 enhances both TNF- and LPS-induced apoptosis. These data indicate that there is a dual and distinct role for SK1 and S1P in the TNF and the LPS inflammatory pathways.

Activation of sphingosine kinase mediates suppressive effect of interleukin-6 on human multiple myeloma cell apoptosis

Interleukin 6 (IL-6) influences the growth and survival of multiple myeloma (MM) cells via the activation of multiple signalling cascades. Although sphingosine kinase (SPHK) signalling is known to play important roles in the regulation of cell proliferation and apoptosis, the role of SPHK activation in IL-6 signalling and in the pathology of MM remains unclear. This study found that IL-6 activated SPHK in MM cells, which mediates the suppressive effects of IL-6 on MM cell apoptosis. Both MM cell lines and primary MM cells constitutively expressed SPHK, and treatment of MM cells with IL-6 resulted in activation of SPHK in a concentration-dependent manner. Specific inhibitors of the phosphatidylinositol-3 kinase and extracellular signal-regulated kinase/mitogen-activated protein kinase pathways blocked the IL-6-induced activation of SPHK. It was further demonstrated that IL-6-induced activation of SPHK inhibited dexamethasone-induced apoptosis of MM cells. IL-6 stimulation or retroviral-mediated overexpression of SPHK1 in MM cells resulted in increased intracellular SPHK activity and upregulation of myeloid cell leukaemia-1 (Mcl-1), leading to increased cell proliferation and survival. Conversely, inhibition of SPHK1 by small interfering RNA reduced IL-6-induced upregulation of Mcl-1 and blocked the suppressive effect of IL-6 on MM cell apoptosis. Taken together, these results delineate a key role for SPHK activation in IL-6-induced proliferation and survival of MM cells, and suggest that SPHK may be a potential new therapeutic target in MM.

Distribution of sphingosine kinase activity and mRNA in rodent brain

Sphingosine-1-phosphate (S1P) is a lipid mediator that exerts multiple cellular functions through activation of a subfamily of G-protein-coupled receptors. Although there is evidence that S1P plays a role in the developing and adult CNS, little is known about the ability of brain parenchyma to synthesize this lipid. We have therefore analyzed the brain distribution of the enzymatic activity of the S1P synthesizing enzyme, sphingosine kinase (SPHK) [EC:2.7.1.91], as well as mRNA distribution for one of the two isoforms of this enzyme, sphingosine kinase 2. SPHK activity, measured by the conversion of [(3)H]sphingosine to [(3)H]S1P, is highest in cerebellum, followed by cortex and brainstem. Lowest activities were found in striatum and hippocampus. Sensitivity to 0.1% Triton-X suggests that this activity is accounted for by SPHK2. RT-PCR and in situ hybridization studies show that mRNA for this isoform has a distribution similar to that of SPHK activity. In vivo and in vitro ischemia increase SPHK activity and SPHK2 mRNA levels. These results indicate that SPHK2 is the predominant S1P-synthesizing isoform in normal brain parenchyma. Its heterogeneous distribution, in particular laminar distribution in cortex, suggests a neuronal localization and a possible role in cortical and cerebellar functions, in normal as well as ischemic brain.

Sphingosine kinase signalling in immune cells: potential as novel therapeutic targets

During the last few years, it has become clear that sphingolipids are sources of important signalling molecules. Particularly, the sphingolipid metabolites, ceramide and S1P, have emerged as a new class of potent bioactive molecules, implicated in a variety of cellular processes such as cell differentiation, apoptosis, and proliferation. Sphingomyelin (SM) is the major membrane sphingolipid and is the precursor for the bioactive products. Ceramide is formed from SM by the action of sphingomyelinases (SMase), however, ceramide can be very rapidly hydrolysed, by ceramidases to yield sphingosine, and sphingosine can be phosphorylated by sphingosine kinase (SphK) to yield S1P. In immune cells, the sphingolipid metabolism is tightly related to the main stages of immune cell development, differentiation, activation, and proliferation, transduced into physiological responses such as survival, calcium mobilization, cytoskeletal reorganization and chemotaxis. Several biological effectors have been shown to promote the synthesis of S1P, including growth factors, cytokines, and antigen and G-protein-coupled receptor agonists. Interest in S1P focused recently on two distinct cellular actions of this lipid, namely its function as an intracellular second messenger, capable of triggering calcium release from internal stores, and as an extracellular ligand activating specific G protein-coupled receptors. Inhibition of SphK stimulation strongly reduced or even prevented cellular events triggered by several proinflammatory agonists, such as receptor-stimulated DNA synthesis, Ca(2+) mobilization, degranulation, chemotaxis and cytokine production. Another very important observation is the direct role played by S1P in chemotaxis, and cellular escape from apoptosis. As an extracellular mediator, several studies have now shown that S1P binds a number of G-protein-coupled receptors (GPCR) encoded by endothelial differentiation genes (EDG), collectively known as the S1P-receptors. Binding of S1P to these receptors trigger an wide range of cellular responses including proliferation, enhanced extracellular matrix assembly, stimulation of adherent junctions, formation of actin stress fibres, and inhibition of apoptosis induced by either ceramide or growth factor withdrawal. Moreover, blocking S1P1-receptor inhibits lymphocyte egress from lymphatic organs. This review summarises the evidence linking SphK signalling pathway to immune-cell activation and based on these data discuss the potential for targeting SphKs to suppress inflammation and other pathological conditions.

Sphingosine kinase 1 expression is regulated by signaling through PI3K, AKT2, and mTOR in human coronary artery smooth muscle cells

Sphingosine kinase 1 (SphK1) is a lipid kinase implicated in mitogenic signaling pathways in vascular smooth muscle cells. We demonstrate that human coronary artery smooth muscle (HCASM) cells require SphK1 for growth and that SphK1 mRNA and protein levels are elevated in PDGF stimulated HCASM cells. To determine the mechanism of PDGF-induced SphK1 expression, we used pharmacological inhibitors of the PI3K/AKT/mTOR signaling pathway. Wortmannin, SH-5, and rapamycin significantly blocked PDGF-stimulated induction of SphK1 mRNA and protein expression, indicating a regulatory role of the PI3K/AKT/mTOR pathway in SphK1 expression. To determine which isoform of AKT regulates SphK1 mRNA and protein levels, siRNAs specific for AKT1, AKT2, and AKT3 were used. We show that AKT2 siRNA significantly blocked PDGF-stimulated increases in SphK1 mRNA and protein expression levels as well as SphK1 enzymatic activity levels. In contrast, AKT1 or AKT3 siRNA did not have an effect. Together, these results demonstrate that the PI3K/AKT/mTOR signaling pathway is involved in regulation of SphK1, with AKT2 playing a key role in PDGF-induced SphK1 expression in HCASM cells.

Tumor necrosis factor-alpha-stimulated cell proliferation is mediated through sphingosine kinase-dependent Akt activation and cyclin D expression

Tumor necrosis factor-alpha (TNF-alpha) has been shown to activate sphingosine kinase (SphK) in a variety of cell types. The extent to which SphK signaling mediates the pleiotropic effects of TNF-alpha is not entirely clear. The current study examined the role of SphK activity in TNF-alpha-stimulated cell proliferation in 1321N1 glioblastoma cells. We first demonstrated that pharmacological inhibitors of SphK markedly decrease TNF-alpha-stimulated DNA synthesis. Signaling mechanisms through which SphK mediated the effect of TNF-alpha on DNA synthesis were then examined. Inhibition of Rho proteins with C3 exoenzyme or of Rho kinase with Y27632 attenuated TNF-alpha-stimulated DNA synthesis. However, RhoA activation by TNF-alpha was not blocked by SphK inhibition. ERK activation was also required for TNF-alpha-stimulated DNA synthesis but likewise TNF-alpha-induced ERK activation was not blocked by inhibition of SphK. Thus, neither RhoA nor ERK activation are the SphK-dependent transducers of TNF-alpha-induced proliferation. In contrast, TNF-alpha-stimulated Akt phosphorylation, which was also required for DNA synthesis, was attenuated by SphK inhibition or SphK1 knockdown by small interfering RNA. Furthermore, cyclin D expression was increased by TNF-alpha in a SphK- and Akt-dependent manner. Additional studies demonstrated that TNF-alpha effects on DNA synthesis, ERK, and Akt phosphorylation are not mediated through cell surface Gi -coupled S1P receptors, because none of these responses were inhibited by pertussis toxin. We conclude that SphK-dependent Akt activation plays a significant role in TNF-alpha-induced cyclin D expression and cell proliferation.

Shp-2 tyrosine phosphatase is required for hepatocyte growth factor-induced activation of sphingosine kinase and migration in embryonic fibroblasts

Shp-2, a ubiquitously expressed protein tyrosine phosphatase containing two Src homology 2 domains, plays an important role in integrating signaling from the cell surface receptors to intracellular signaling mechanisms. Previous studies have demonstrated that the Shp-2 is involved in hepatocyte growth factor (HGF)-induced cell scattering. Here we report that Shp-2 is required for the HGF-induced activation of sphingosine kinase-1 (SPK1), a highly conserved lipid kinase that plays an important role in cell migration. Loss-of-function mutation of Shp-2 did not affect the expression of SPK1, but resulted in its inactivation and the blockage of HGF-induced migration in embryonic fibroblasts. Reintroduction of functional wild type (WT) Shp-2 into the mutant cells partially restored SPK1 activation, and overexpression of SPK1 in these mutant cells enhanced HGF-induced cell migration. Inhibition of expression or activity of SPK1 in WT cells markedly decreased intracellular S1P levels and HGF-induced cell migration. Furthermore, we found that Shp-2 co-immunoprecipitated with SPK1 and c-Met in embryonic fibroblasts. These studies suggest that Shp-2 is an SPK1-interacting protein and that it plays an indispensable role in HGF-induced SPK1 activation.

Two distinct pools of Src family tyrosine kinases regulate PDGF-induced DNA synthesis and actin dorsal ruffles

The mechanism by which the Src family of protein-tyrosine kinases (SFKs) regulate mitogenesis and morphological changes induced by platelet-derived growth factor (PDGF) is not well known. The cholesterol-enriched membrane microdomains, caveolae, regulate PDGF receptor signalling in fibroblasts and we examined their role in SFK functions. Here we show that caveolae disruption by membrane cholesterol depletion or expression of the dominant-negative caveolin-3 DGV mutant impaired Src mitogenic signalling including kinase activation, Myc gene induction and DNA synthesis. The impact of caveolae on SFK function was underscored by the capacity of Myc to overcome mitogenic inhibition as a result of caveolae disruption. Using biochemical fractionation we show that caveolae-enriched subcellular membranes regulate the formation of PDGF-receptor-SFK complexes. An additional pool of PDGF-activated SFKs that was insensitive to membrane cholesterol depletion was characterised in non-caveolae fractions. SFK activation outside caveolae was linked to the capacity of PDGF to induce F-actin rearrangements leading to dorsal ruffle formation. Inhibition of phospholipase C gamma (PLCgamma), sphingosine kinase and heterotrimeric Gi proteins implicates a PLC gamma-sphingosine-1-phosphate-Gi pathway for PDGF-induced SFK activation outside caveolae and actin assembly. In addition, the cytoplasmic tyrosine kinase Abl was identified as an important effector of this signalling cascade. We conclude that PDGF may stimulate two spatially distinct pools of SFKs leading to two different biological outcomes: DNA synthesis and dorsal ruffle formation.

Antigen-induced Ca2+ mobilization in RBL-2H3 cells: Role of I(1,4,5)P3 and S1P and necessity of I(1,4,5)P3 production

Inositol 1,4,5-trisphosphate (IP3) has long been recognized as a second messenger for intracellular Ca2+ mobilization. Recently, sphingosine 1-phosphate (S1P) has been shown to be involved in Ca2+ release from the endoplasmic reticulum (ER). Here, we investigated the role of S1P and IP3 in antigen (Ag)-induced intracellular Ca2+ mobilization in RBL-2H3 mast cells. Antigen-induced intracellular Ca2+ mobilization was only partially inhibited by the sphingosine kinase inhibitor dl-threo-dihydrosphingosine (DHS) or the IP3 receptor inhibitor 2-aminoethoxydiphenyl borate (2-APB), whereas preincubation with both inhibitors led to complete inhibition. In contrast, stimulation of A3 adenosine receptors with N5-ethylcarboxamidoadenosine (NECA) caused intracellular Ca2+ mobilization that was completely abolished by 2-APB but not by DHS, suggesting that NECA required only the IP3 pathway, while antigen used both the IP3 and S1P pathways. Interestingly, however, inhibition of IP3 production with the phospholipase C inhibitor U73122 completely abolished Ca2+ release from the ER induced by either stimulant. This suggested that S1P alone, without concomitant production of IP3, would not cause intracellular Ca2+ mobilization. This was further demonstrated in some clones of RBL-2H3 cells excessively overexpressing a beta isoform of Class II phosphatidylinositol 3-kinase (PI3KC2beta). In such clones including clone 5A4C, PI3KC2beta was overexpressed throughout the cell, although endogenous PI3KC2beta was normally expressed only in the ER. Overexpression of PI3KC2beta in the cytosol and the PM led to depletion of phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2), resulting in a marked reduction in IP3 production. This could explain the abolishment of intracellular Ca2+ mobilization in clone 5A4C. Supporting this hypothesis, the Ca2+ mobilization was reconstituted by the addition of exogenous PI(4,5)P2 in these cells. Our results suggest that both IP3 and S1P contribute to FcvarepsilonRI-induced Ca2+ release from the ER and production of IP3 is necessary for S1P to cause Ca2+ mobilization from the ER.

The S1P2 receptor negatively regulates platelet-derived growth factor-induced motility and proliferation

Sphingosine-1-phosphate (S1P), a bioactive sphingolipid metabolite, is the ligand for five specific G protein-coupled receptors, named S1P(1) to S1P(5). In this study, we found that cross-communication between platelet-derived growth factor receptor and S1P(2) serves as a negative damper of PDGF functions. Deletion of the S1P(2) receptor dramatically increased migration of mouse embryonic fibroblasts toward S1P, serum, and PDGF but not fibronectin. This enhanced migration was dependent on expression of S1P(1) and sphingosine kinase 1 (SphK1), the enzyme that produces S1P, as revealed by downregulation of their expression with antisense RNA and small interfering RNA, respectively. Although S1P(2) deletion had no significant effect on tyrosine phosphorylation of the PDGF receptors or activation of extracellular signal-regulated kinase 1/2 or Akt induced by PDGF, it reduced sustained PDGF-dependent p38 phosphorylation and markedly enhanced Rac activation. Surprisingly, S1P(2)-null cells not only exhibited enhanced proliferation but also markedly increased SphK1 expression and activity. Conversely, reintroduction of S1P(2) reduced DNA synthesis and expression of SphK1. Thus, S1P(2) serves as a negative regulator of PDGF-induced migration and proliferation as well as SphK1 expression. Our results suggest that a complex interplay between PDGFR and S1P receptors determines their functions.

Sphingosine kinase signalling in immune cells

1. Sphingolipids are potent second messengers modulating biochemical intracellular events and acting as ligands to mediate extracellular systems. Sphingosine kinase (SPHK) is the enzyme that phosphorylates sphingosine into sphingosine-1-phosphate (S1P), a potent bioactive sphingolipid. 2. The fact that SPHK is highly conserved from protozoa to mammals and is ubiquitous in living tissues reveals important roles of the SPHK pathway for the maintenance of health maintenance. This is also supported by comprehensive reviews on features of its main product, S1P, as having intracellular as well as extracellular roles, inducing a wide range of physiological responses from triggering Ca2+ release from internal stores to promoting growth and cell motility. 3. Immune cell activities have been shown to be modulated by the dynamic balance between ceramide, sphingosine and S1P, conceptualized as a rheostat. Cell proliferation, differentiation, motility and survival have been attributed to the regulatory actions of S1P. The properties of SPHK activity in immune cells are linked to the functions of triggered growth and survival factors, phorbol esters, hormones, cytokines and chemokines, as well as antigen receptors, such as FcgammaRI and FcepsilonRI. 4. Mechanisms of the SPHK signalling pathway are explored as new targets for drug development to suppress inflammation and other pathological conditions.

Asp177 in C4 domain of mouse sphingosine kinase 1a is important for the sphingosine recognition

Sphingosine kinase (SK) is the enzyme that catalyzes the formation of sphingosine 1-phosphate (S1P). Although diverse biological functions have been reported for SK, its recognition site for its substrate sphingosine (Sph) is still unclear. We constructed various mutants of mouse sphingosine kinase 1a (mSK1a), carrying mutations in the C4 domain, which we had expected to encompass the Sph-binding site. We analyzed the influence of these mutations on the SK activity and substrate kinetics. One mutation, Asp177-->Asn177, caused a dramatic decrease in SK activity (to approximately 6% of wild type) and an increase in the Km value for Sph (10.1-->108 microM), with no change in the affinity for ATP. This result suggests that the C4 domain, especially the Asp177, is involved in the specific recognition of Sph. In this report, we are able, for the first time, to provide an account of the Sph-binding site of SK.

Sphingosine kinase activity counteracts ceramide-mediated cell death in human melanoma cells: role of Bcl-2 expression

While most of the pharmacological therapies for melanoma utilize the apoptotic machinery of the cells, the available therapeutic options are limited due to the ability of melanoma cells to resist programmed cell death. Human melanoma cell lines A-375 and M186 are sensitive to ceramide- and Fas-induced cell death, while Mel-2a and M221 are resistant. We have now found that Mel-2a and M221 cells have a significantly higher ceramide/sphingosine-1-phosphate (S1P) ratio than A-375 and M186 cells. As sphingosine kinase (SphK) type 1 plays a critical role in determining the dynamic balance between the proapoptotic sphingolipid metabolite ceramide and the prosurvival S1P, we examined its role in apoptosis of melanoma cells. Increasing SphK1 expression reduced the sensitivity of A-375 melanoma cells to Fas- and ceramide-mediated apoptosis. Conversely, downregulation of SphK1 with small interfering RNA decreased the resistance of Mel-2a cells to apoptosis. Importantly, overexpression of the prosurvival protein Bcl-2 in A-375 cells markedly stimulated SphK1 expression and activity, while downregulation of Bcl-2 reduced SphK1 expression. This link between Bcl-2 and SphK1 might be an additional clue to chemotherapy resistance of malignant melanoma.

The intracellular action of sphingosine 1-phosphate in GPVI-mediated Ca2+ mobilization in platelets

We analyzed the intracellular action of sphingosine 1-phosphate (Sph-1-P), formed from sphingosine (Sph) by sphingosine kinase (SPHK), in platelets. When sphingosine kinase activity was inhibited by N,N-dimethylsphingosine (DMS), Ca2+ mobilization induced by convulxin, an agonist of the collagen receptor glycoprotein VI (GPVI), was moderately but specifically abolished; that induced via G protein-coupled receptors was not affected. Under the same conditions, however, tyrosine phosphorylation of Syk and phospholipase Cgamma2, which is essential for the GPVI-mediated signaling, was not inhibited. Sphingosine kinase activity of the platelet membrane fraction increased specifically upon stimulation with convulxin or collagen. Our results suggest that intracellular sphingosine 1-phosphate is related to Ca2+ mobilization in GPVI-mediated signaling pathways.

Sphingosine kinase activation regulates hepatocyte growth factor induced migration of endothelial cells

Hepatocyte growth factor (HGF)-induced migration of endothelial cells is critical for angiogenesis. Sphingosine kinase (SPK) is a key enzyme catalyzing the formation of sphingosine-1-phosphate (S1P), a lipid messenger that is implicated in the regulation of a wide variety of important cellular events through both intracellular and extracellular mechanisms. The aim of this study was to investigate whether activation of SPK is involved in the migration of endothelial cells induced by HGF. The biological functions of HGF are mediated through the activation of its high-affinity tyrosine kinase receptor, c-met protooncogene. In the present study, Treatment of ECV304 endothelial cells with HGF resulted in tyrosine phosphorylation of c-Met and activation of SPK in a concentration-dependent manner. Either Ly294002 or PD98059, specific inhibitor of the PI3K and ERK/MAPK pathways, respectively, blocked the HGF-induced activation of SPK. HGF stimulation significantly increased intracellular S1P level, but no detectable secretion of S1P into the cell culture medium was observed. Treatment of ECV304 cells with pertussis toxin (PTX) has no effect on the HGF-induced migration, indicating extracellular S1P is dispensable for this process. Overexpression of wild-type SPK gene in ECV 304 cells increased the intracellular S1P and enhanced the HGF-induced migration, whereas inhibition of cellular SPK activity by N,N-dimethylsphingosine (DMS), a potent inhibitor of SPK, or by expression of a dominant-negative SPK (DN-SK) blocked the HGF-induced migration of ECV 304 cells. It is suggested that PI3K and ERK/MAPK mediated the activation of SPK and would be involved in the HGF-induced migration of endothelial cells. These results elucidate a novel mechanism by which intracellularly generated S1P mediates signaling from HGF/c-Met to the endothelial cell migration.

Generation and metabolism of bioactive sphingosine-1-phosphate

Sphingosine-1-phosphate (S1P) is a bioactive lysosphingophospholipid that has been implicated in the regulation of vital biological processes. Abundant evidence indicates that S1P acts as both an intracellular messenger and an extracellular ligand for a family of five specific G protein-coupled S1P receptors (S1PRs). Cellular levels of S1P are tightly regulated in a spatio-temporal manner through its synthesis catalyzed by sphingosine kinases (SphKs) and degradation by S1P lyase (SPL) and specific S1P phosphohydrolases. Over the past decade, the identification and cloning of genes encoding S1P metabolizing enzymes has increased rapidly. Overexpression and deletion of these enzymes has provided important insights into the intracellular and the "inside-out" functions of S1P. The purpose of this review is to summarize the current knowledge of S1P metabolizing enzymes, their enzymatic properties, and their roles in the control of cellular functions by S1P.

Lysophospholipid receptor-dependent and -independent calcium signaling

Changes in cellular Ca(2+) concentrations form a ubiquitous signal regulating numerous processes such as fertilization, differentiation, proliferation, contraction, and secretion. The Ca(2+) signal, highly organized in space and time, is generated by the cellular Ca(2+) signaling toolkit. Lysophospholipids, such as sphingosine-1-phosphate (S1P), sphingosylphosphorylcholine (SPC), or lysophosphatidic acid (LPA) use this toolkit in a specific manner to initiate their cellular responses. Acting as agonists at G protein-coupled receptors, S1P, SPC, and LPA increase the intracellular free Ca(2+) concentration ([Ca(2+)](i)) by using the classical, phospholipase C (PLC)-dependent pathway as well as PLC-independent pathways such as sphingosine kinase (SphK)/S1P. The S1P(1) receptor, via protein kinase C, inhibits the [Ca(2+)](i) transients caused by other receptors. Both S1P and SPC also act intracellularly to regulate [Ca(2+)](i). Intracellular S1P mobilizes Ca(2+) in intact cells independently of G protein-coupled S1P receptors, and Ca(2+) signaling by many agonists requires SphK-mediated S1P production. As shown for the FcepsilonRI receptor, PLC and SphK may contribute specific components to the overall [Ca(2+)](i) transient. Of the many open questions, identification of the intracellular S1P target site(s) appears to be of particular importance.

Ca2+/calmodulin-dependent translocation of sphingosine kinase: role in plasma membrane relocation but not activation

Activation of sphingosine kinase (SPHK), thereby increasing cellular levels of sphingosine 1-phosphate (S1P), may be involved in a variety of intracellular responses including Ca(2+) signaling. This study uses mammalian SPHK1a, tagged with enhanced green fluorescent protein (eGFP), to examine whether translocation of this enzyme is linked with Ca(2+)-mobilizing responses. Real-time confocal imaging of SPHK1a-eGFP in human SH-SY5Y neuroblastoma cells visualized a relocation of the enzyme from the cytosol to the plasma membrane in response to Ca(2+)-mobilizing stimuli (muscarinic M(3)- or lysophosphatidic acid receptor activation, and thapsigargin-mediated store release). This redistribution was preceded by a transient increase in cytosolic SPHK1a-eGFP levels due to liberation of SPHK from localized higher intensity regions. Translocation was dependent on Ca(2+) mobilization from intracellular stores, and was prevented by pretreatment with the Ca(2+)/calmodulin inhibitor W-7, but not W-5 or KN-62. In functional studies, pretreatment with W-7 lowered basal and M(3)-receptor-mediated cellular S1P production. However, this pretreatment did not alter agonist-mediated Ca(2+) responses, and SPHK1a-eGFP activity itself appeared insensitive to Ca(2+)/calmodulin and W-7. These data suggest a role for Ca(2+)/calmodulin in controlling the subcellular distribution but not the activity of SPHK1a.

Sphingosine kinases: a novel family of lipid kinases

Sphingosine kinase (SPHK) catalyzes the formation of sphingosine-1-phosphate (S11). S1P plays an important role in regulation of a variety of biological processes through intracellular and extracellular actions. S1P has recently been shown to be the ligand for the EDG-1 family of G-protein-coupled receptors. To date, seven cloned SPHKs have been reported with confirmed SPHK activity, including human, mouse, yeast, and plant. A computer search of various databases suggests that a new SPHK family is emerging. The cloning and manipulation of SPHK genes will no doubt provide us with important information about the functions of S1P in a wide range of organisms.

PDGF upregulates delayed rectifier via Src family kinases and sphingosine kinase in oligodendroglial progenitors

An increase in the expression of the delayed rectifier current (I(K)) has been shown to correlate with mitogenesis in many cell types. However, pathways involved in the upregulation of I(K) by growth factors in oligodendroglial progenitors (OPs) have not been well-elucidated. In this study, we found that treatment with platelet-derived growth factor (PDGF) and basic fibroblast growth factor but not ciliary neurotrophic factor resulted in increased I(K) density and upregulation of Kv1.5 and Kv1.6 mRNA transcripts. The effect of PDGF on I(K) was blocked by mimosine, a cell cycle inhibitor, and by genistein, a tyrosine kinase inhibitor. Using inhibitors of PDGF-activated pathways, we found that PDGF-induced upregulation of Kv1.5 and I(K) density involves Src family tyrosine kinases, sphingosine kinase, and intracellular Ca(2+) but not ERK1/2 or phosphatidylinositol 3-kinase pathways. Furthermore, agents that were effective inhibitors of PDGF-induced I(K) upregulation also attenuated OP proliferation, supporting the concept that I(K) is an important link between PDGF-activated signaling cascades and cell cycle progression.

Sphingosine kinase, sphingosine-1-phosphate, and apoptosis

The sphingolipid metabolites ceramide (Cer), sphingosine (Sph), and sphingosine-1-phosphate (S1P) play an important role in the regulation of cell proliferation, survival, and cell death. Cer and Sph usually inhibit proliferation and promote apoptosis, while the further metabolite S1P stimulates growth and suppresses apoptosis. Because these metabolites are interconvertible, it has been proposed that it is not the absolute amounts of these metabolites but rather their relative levels that determines cell fate. The relevance of this "sphingolipid rheostat" and its role in regulating cell fate has been borne out by work in many labs using many different cell types and experimental manipulations. A central finding of these studies is that Sph kinase (SphK), the enzyme that phosphorylates Sph to form S1P, is a critical regulator of the sphingolipid rheostat, as it not only produces the pro-growth, anti-apoptotic messenger S1P, but also decreases levels of pro-apoptotic Cer and Sph. Given the role of the sphingolipid rheostat in regulating growth and apoptosis, it is not surprising that sphingolipid metabolism is often found to be disregulated in cancer, a disease characterized by enhanced cell growth, diminished cell death, or both. Anticancer therapeutics targeting SphK are potentially clinically relevant. Indeed, inhibition of SphK has been shown to suppress gastric tumor growth [Cancer Res. 51 (1991) 1613] and conversely, overexpression of SphK increases tumorigenicity [Curr. Biol. 10 (2000) 1527]. Moreover, S1P has also been shown to regulate angiogenesis, or new blood vessel formation [Cell 99 (1999) 301], which is critical for tumor progression. Furthermore, there is intriguing new evidence that S1P can act in an autocrine and/or paracrine fashion [Science 291 (2001) 1800] to regulate blood vessel formation [J. Clin. Invest. 106 (2000) 951]. Thus, SphK may not only protect tumors from apoptosis, it may also increase their vascularization, further enhancing growth. The cytoprotective effects of SphK/S1P may also be important for clinical benefit, as S1P has been shown to protect oocytes from radiation-induced cell death in vivo [Nat. Med. 6 (2000) 1109]. Here we review the growing literature on the regulation of SphK and the role of SphK and its product, S1P, in apoptosis.

Sphingosine 1-phosphate: a Ca2+ release mediator in the balance

Sphingosine 1-phosphate (S1P) is a lipid signalling molecule with Ca(2+) mobilising properties. Importantly for a role as a Ca(2+) release messenger, intracellular levels of S1P can be regulated by a variety of extracellular stimuli, via the enzyme sphingosine kinase. However, neither the mechanism underlying S1P generation, nor its actions at the endoplasmic reticulum are clear. Thus, the role of S1P as an intracellular mediator of Ca(2+) release remains in the balance.

Sphingosine kinase mediates vascular endothelial growth factor-induced activation of ras and mitogen-activated protein kinases

Vascular endothelial growth factor (VEGF) signaling is critical to the processes of angiogenesis and tumor growth. Here, evidence is presented for VEGF stimulation of sphingosine kinase (SPK) that affects not only endothelial cell signaling but also tumor cells expressing VEGF receptors. VEGF or phorbol 12-myristate 13-acetate treatment of the T24 bladder tumor cell line resulted in a time- and dose-dependent stimulation of SPK activity. In T24 cells, VEGF treatment reduced cellular sphingosine levels while raising that of sphingosine-1-phosphate. VEGF stimulation of T24 cells caused a slow and sustained accumulation of Ras-GTP and phosphorylated extracellular signal-regulated kinase (phospho-ERK) compared with that after EGF treatment. Small interfering RNA (siRNA) that targets SPK1, but not SPK2, blocks VEGF-induced accumulation of Ras-GTP and phospho-ERK in T24 cells. In contrast to EGF stimulation, VEGF stimulation of ERK1/2 phosphorylation was unaffected by dominant-negative Ras-N17. Raf kinase inhibition blocked both VEGF- and EGF-stimulated accumulation of phospho-ERK1/2. Inhibition of SPK by pharmacological inhibitors, a dominant-negative SPK mutant, or siRNA that targets SPK blocked VEGF, but not EGF, induction of phospho-ERK1/2. We conclude that VEGF induces DNA synthesis in a pathway which sequentially involves protein kinase C (PKC), SPK, Ras, Raf, and ERK1/2. These data highlight a novel mechanism by which SPK mediates signaling from PKC to Ras in a manner independent of Ras-guanine nucleotide exchange factor.

Cloning and characterization of a protein kinase A anchoring protein (AKAP)-related protein that interacts with and regulates sphingosine kinase 1 activity

Sphingosine-1-phosphate (S1P) is a bioactive sphingolipid metabolite that has novel dual actions. S1P is the ligand for a family of G protein-coupled receptors known as S1PRs that mediate various physiological functions. Growth factors rapidly activate sphingosine kinase type 1 (SPHK1) resulting in phosphorylation of sphingosine to form S1P, which plays important roles in cell growth regulation and protection from apoptosis. However, little is known of the mechanism(s) by which SPHK activity is regulated. Using a yeast two-hybrid screening approach, we cloned a 3-kb cDNA encoding a SPHK1-interacting protein (SKIP). BLAST analysis revealed that SKIP corresponded to the C-terminal region of a larger ( approximately 7 kb) cDNA that encoded a protein with a high degree of similarity to a family of protein kinase A anchor proteins (AKAP). In confirmation of the yeast two-hybrid assay, glutathione S-transferase (GST)-SPHK1 specifically pulled down SKIP, whereas GST did not. Moreover, immunoprecipitation of in vitro translated SPHK1 and SKIP revealed that SKIP and SPHK1 are tightly associated. Furthermore, SKIP overexpression in NIH 3T3 fibroblasts reduced SPHK1 activity and interfered with its biological functions. The apoptotic-sparing effect of SPHK1 against serum deprivation was reduced when co-transfected with SKIP. In addition, SPHK1-enhanced cell proliferation was also abolished by SKIP, with a corresponding decrease in activation of ERK. Taken together, these results indicate that SKIP is a novel protein likely to play a regulatory role in the modulation of SPHK1 activity.

Involvement of sphingosine kinase in TNF-alpha-stimulated tetrahydrobiopterin biosynthesis in C6 glioma cells

In C6 glioma cells, the sphingolipid second messenger ceramide potentiates expression of inducible nitric-oxide synthase (iNOS) induced by tumor necrosis factor alpha (TNF-alpha) without affecting GTP cyclohydrolase I (GTPCH), the rate-limiting enzyme in the biosynthesis of 6(R)-5,6,7,8-tetrahydrobiopterin (BH(4)), a cofactor required for iNOS activity. TNF-alpha also stimulates sphingosine kinase, the enzyme that phosphorylates sphingosine to form sphingosine-1-phosphate (SPP), a further metabolite of ceramide. Several clones of C6 cells, expressing widely varying levels of sphingosine kinase, were used to examine the role of SPP in regulation of GTPCH and BH(4) biosynthesis. Overexpression of sphingosine kinase, with concomitant increased endogenous SPP levels, potentiated the effect of TNF-alpha on GTPCH expression and activity and BH(4) biosynthesis. In contrast, enforced expression of sphingosine kinase had no effect on iNOS expression or NO formation. Furthermore, N,N-dimethylsphingosine, a potent sphingosine kinase inhibitor, completely eliminated the increased GTPCH activity and expression induced by TNF-alpha. Surprisingly, we found that, although C6 cells can secrete SPP, which is enhanced by TNF-alpha, treatment of C6 cells with exogenous SPP or dihydro-SPP had no affect on BH(4) biosynthesis. However, both SPP and dihydro-SPP markedly stimulated ERK 1/2 in C6 cells, which express cell surface SPP receptors. Interestingly, although this ERK activation was blocked by PD98059, which also reduced cellular proliferation induced by enforced expression of sphingosine kinase, PD98059 had no effect on GTPCH activity. Collectively, these results suggest that only intracellularly generated SPP plays a role in regulation of GTPCH and BH(4) levels.

Depolarisation induces rapid and transient formation of intracellular sphingosine-1-phosphate

Formation of sphingosine-1-phosphate (SPP) by sphingosine kinase serves as a signalling pathway for various membrane receptors. Here, we show that membrane depolarisation is another mechanism by which this pathway can be activated. Formation of [(3)H]SPP as well as levels of endogenous SPP were rapidly and transiently increased in PC12 pheochromocytoma cells depolarised with high KCl. Time course and maximum were similar to those induced by bradykinin. Depolarisation-induced SPP production was also observed in RINm5F insulinoma cells, dependent on extracellular Ca(2+) and fully suppressed by verapamil, thus apparently caused by Ca(2+) influx via voltage-gated Ca(2+) channels. Studies with sphingosine kinase inhibitors and overexpression of sphingosine kinase revealed a partial contribution of this pathway to depolarisation-induced noradrenaline release and Ca(2+) increase.

Inositol 1,4,5-trisphosphate-independent calcium signalling by platelet-derived growth factor in the human SH-SY5Y neuroblastoma cell

In adherent SH-SY5Y human neuroblastoma cells, activation of G-protein-coupled muscarinic M3 receptors evoked a biphasic elevation of both intracellular [Ca(2+)] ([Ca(2+)]i) and inositol-1,4,5-trisphosphate (D-Ins(1,4,5)P3) mass. In both cases, temporal profiles consisted of rapid transient elevations followed by a decline to a lower, yet sustained level. In contrast, platelet-derived growth factor (PDGF), a receptor tyrosine kinase agonist acting via PDGF receptor b chains in these cells, elicited a slow and transient elevation of [Ca(2+)]i that returned to basal levels within 5 to 10 min with no evidence of inositol phosphate generation. Full responses for either receptor type required intracellular and extracellular Ca(2+) and mobilization of a shared thapsigargin-sensitive intracellular Ca(2+) store. Strategies that affected the ability of D-Ins(1,4,5)P3 to interact with the Ins(1,4,5)P3-receptor demonstrated an Ins(1,4,5)P3-dependency of the muscarinic receptor-mediated elevation of [Ca(2+)]i but showed that PDGF-mediated elevations of [Ca(2+)]i are Ins(1,4,5)P3-independent in these cells.

Cell type-specific localization of sphingosine kinase 1a in human tissues

Cell type-specific localization of sphingosine kinase 1a (SPHK1a) in tissues was analyzed with a rabbit polyclonal antibody against the 16 C-terminal amino acids derived from the recently reported mouse cDNA sequence of SPHK1a. This antibody (anti-SPHK1a antibody) can react specifically with SPHK1a of mouse, rat, and human tissues. Utilizing its crossreactivity to human SPHK1a, the cell-specific localization of SPHK1a in human tissues was histochemically examined. Strong positive staining for SPHK1a was observed in the white matter in the cerebrum and cerebellum, the red nucleus and cerebral peduncle in the midbrain, the uriniferous tubules in the kidney, the endothelial cells in vessels of various organs, and in megakaryocytes and platelets. The lining cells of sinusoids in the liver and splenic cords in the spleen showed moderate staining. Columnar epithelia in the intestine and Leydig's cells in the testis showed weak staining patterns. In addition, TPA-treated HEL cells, a human leukemia cell line, showed a megakaryocytic phenotype accompanied with increases in immunostaining of both SPHK1a and SPHK enzyme activity, suggesting that SPHK1a may be a novel marker of megakaryocytic differentiation and that this antibody is also useful for in vitro study of differentiation models.(J Histochem Cytochem 49:845-855, 2001)