Regulation and functional roles of sphingosine kinases

Glial Sphingosine-Mediated Epigenetic Regulation Stabilizes Synaptic Function in Drosophila Models of Alzheimer's Disease

Destabilization of neural activity caused by failures of homeostatic regulation has been hypothesized to drive the progression of Alzheimer's Disease (AD). However, the underpinning mechanisms that connect synaptic homeostasis and the disease etiology are yet to be fully understood. Here, we demonstrated that neuronal overexpression of amyloid β (Aβ) causes abnormal histone acetylation in peripheral glia and completely blocks presynaptic homeostatic potentiation (PHP) at the neuromuscular junction in Drosophila The synaptic deficits caused by Aβ overexpression in motoneurons are associated with motor function impairment at the adult stage. Moreover, we found that a sphingosine analog drug, Fingolimod, ameliorates synaptic homeostatic plasticity impairment, abnormal glial histone acetylation, and motor behavior defects in the Aβ models. We further demonstrated that perineurial glial sphingosine kinase 2 (Sk2) is not only required for PHP, but also plays a beneficial role in modulating PHP in the Aβ models. Glial overexpression of Sk2 rescues PHP, glial histone acetylation, and motor function deficits that are associated with Aβ in Drosophila Finally, we showed that glial overexpression of Sk2 restores PHP and glial histone acetylation in a genetic loss-of-function mutant of the Spt-Ada-Gcn5 Acetyltransferase complex, strongly suggesting that Sk2 modulates PHP through epigenetic regulation. Both male and female animals were used in the experiments and analyses in this study. Collectively, we provided genetic evidence demonstrating that abnormal glial epigenetic alterations in Aβ models in Drosophila are associated with the impairment of PHP and that the sphingosine signaling pathway displays protective activities in stabilizing synaptic physiology.SIGNIFICANCE STATEMENT Fingolimod, an oral drug to treat multiple sclerosis, is phosphorylated by sphingosine kinases to generate its active form. It is known that Fingolimod enhances the cognitive function in mouse models of Alzheimer's disease (AD), but the role of sphingosine kinases in AD is not clear. We bridge this knowledge gap by demonstrating the relationship between impaired homeostatic plasticity and AD. We show that sphingosine kinase 2 (Sk2) in glial cells is necessary for homeostatic plasticity and that glial Sk2-mediated epigenetic signaling has a protective role in synapse stabilization. Our findings demonstrate the potential of the glial sphingosine signaling as a key player in glia-neuron interactions during homeostatic plasticity, suggesting it could be a promising target for sustaining synaptic function in AD.

Altered Smooth Muscle Cell Histone Acetylome by the SPHK2/S1P Axis Promotes Pulmonary Hypertension

BACKGROUND

Epigenetic regulation of vascular remodeling in pulmonary hypertension (PH) is poorly understood. Transcription regulating, histone acetylation code alters chromatin accessibility to promote transcriptional activation. Our goal was to identify upstream mechanisms that disrupt epigenetic equilibrium in PH.

METHODS

Human pulmonary artery smooth muscle cells (PASMCs), human idiopathic pulmonary arterial hypertension (iPAH):human PASMCs, iPAH lung tissue, failed donor lung tissue, human pulmonary microvascular endothelial cells, iPAH:PASMC and non-iPAH:PASMC RNA-seq databases, NanoString nCounter, and cleavage under targets and release using nuclease were utilized to investigate histone acetylation, hyperacetylation targets, protein and gene expression, sphingolipid activation, cell proliferation, and gene target identification. SPHK2 (sphingosine kinase 2) knockout was compared with control C57BL/6NJ mice after 3 weeks of hypoxia and assessed for indices of PH.

RESULTS

We identified that Human PASMCs are vulnerable to the transcription-promoting epigenetic mediator histone acetylation resulting in alterations in transcription machinery and confirmed its pathological existence in PH:PASMC cells. We report that SPHK2 is elevated as much as 20-fold in iPAH lung tissue and is elevated in iPAH:PASMC cells. During PH pathogenesis, nuclear SPHK2 activates nuclear bioactive lipid S1P (sphingosine 1-phosphate) catalyzing enzyme and mediates transcription regulating histone H3K9 acetylation (acetyl histone H3 lysine 9 [Ac-H3K9]) through EMAP (endothelial monocyte activating polypeptide) II. In iPAH lungs, we identified a 4-fold elevation of the reversible epigenetic transcription modulator Ac-H3K9:H3 ratio. Loss of SPHK2 inhibited hypoxic-induced PH and Ac-H3K9 in mice. We discovered that pulmonary vascular endothelial cells are a priming factor of the EMAP II/SPHK2/S1P axis that alters the acetylome with a specificity for PASMC, through hyperacetylation of histone H3K9. Using cleavage under targets and release using nuclease, we further show that EMAP II-mediated SPHK2 has the potential to modify the local transcription machinery of pluripotency factor KLF4 (Krüppel-like factor 4) by hyperacetylating KLF4 Cis-regulatory elements while deletion and targeted inhibition of SPHK2 rescues transcription altering Ac-H3K9.

CONCLUSIONS

SPHK2 expression and its activation of the reversible histone H3K9 acetylation in human pulmonary artery smooth muscle cell represent new therapeutic targets that could mitigate PH vascular remodeling.

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

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

The SphK1/S1P Axis Regulates Synaptic Vesicle Endocytosis via TRPC5 Channels

Sphingosine-1-phosphate (S1P), a bioactive sphingolipid concentrated in the brain, is essential for normal brain functions, such as learning and memory and feeding behaviors. Sphingosine kinase 1 (SphK1), the primary kinase responsible for S1P production in the brain, is abundant within presynaptic terminals, indicating a potential role of the SphK1/S1P axis in presynaptic physiology. Altered S1P levels have been highlighted in many neurologic diseases with endocytic malfunctions. However, it remains unknown whether the SphK1/S1P axis may regulate synaptic vesicle endocytosis in neurons. The present study evaluates potential functions of the SphK1/S1P axis in synaptic vesicle endocytosis by determining effects of a dominant negative catalytically inactive SphK1. Our data for the first time identify a critical role of the SphK1/S1P axis in endocytosis in both neuroendocrine chromaffin cells and neurons from mice of both sexes. Furthermore, our Ca2+ imaging data indicate that the SphK1/S1P axis may be important for presynaptic Ca2+ increases during prolonged stimulations by regulating the Ca2+ permeable TRPC5 channels, which per se regulate synaptic vesicle endocytosis. Collectively, our data point out a critical role of the regulation of TRPC5 by the SphK1/S1P axis in synaptic vesicle endocytosis.SIGNIFICANCE STATEMENT Sphingosine kinase 1 (SphK1), the primary kinase responsible for brain sphingosine-1-phosphate (S1P) production, is abundant within presynaptic terminals. Altered SphK1/S1P metabolisms has been highlighted in many neurologic disorders with defective synaptic vesicle endocytosis. However, whether the SphK1/S1P axis may regulate synaptic vesicle endocytosis is unknown. Here, we identify that the SphK1/S1P axis regulates the kinetics of synaptic vesicle endocytosis in neurons, in addition to controlling fission-pore duration during single vesicle endocytosis in neuroendocrine chromaffin cells. The regulation of the SphK1/S1P axis in synaptic vesicle endocytosis is specific since it has a distinguished signaling pathway, which involves regulation of Ca2+ influx via TRPC5 channels. This discovery may provide novel mechanistic implications for the SphK1/S1P axis in brain functions under physiological and pathologic conditions.

Cross-Regulation of the Cellular Redox System, Oxygen, and Sphingolipid Signalling

Redox-active mediators are now appreciated as powerful molecules to regulate cellular dynamics such as viability, proliferation, migration, cell contraction, and relaxation, as well as gene expression under physiological and pathophysiological conditions. These molecules include the various reactive oxygen species (ROS), and the gasotransmitters nitric oxide (NO∙), carbon monoxide (CO), and hydrogen sulfide (H2S). For each of these molecules, direct targets have been identified which transmit the signal from the cellular redox state to a cellular response. Besides these redox mediators, various sphingolipid species have turned out as highly bioactive with strong signalling potential. Recent data suggest that there is a cross-regulation existing between the redox mediators and sphingolipid molecules that have a fundamental impact on a cell’s fate and organ function. This review will summarize the effects of the different redox-active mediators on sphingolipid signalling and metabolism, and the impact of this cross-talk on pathophysiological processes. The relevance of therapeutic approaches will be highlighted.

The role of sphingosine 1-phosphate metabolism in brain health and disease

Lipids are essential structural and functional components of the central nervous system (CNS). Sphingolipids are ubiquitous membrane components which were discovered in the brain in the late 19^th century. In mammals, the brain contains the highest concentration of sphingolipids in the body. Sphingosine 1-phosphate (S1P) derived from membrane sphingolipids evokes multiple cellular responses which, depending on its concentration and localization, make S1P a double-edged sword in the brain. In the present review we highlight the role of S1P in brain development and focus on the often contrasting findings regarding its contributions to the initiation, progression and potential recovery of different brain pathologies, including neurodegeneration, multiple sclerosis (MS), brain cancers, and psychiatric illnesses. A detailed understanding of the critical implications of S1P in brain health and disease may open the door for new therapeutic options. Thus, targeting S1P-metabolizing enzymes and/or signaling pathways might help overcome, or at least ameliorate, several brain illnesses.

Loss of S1P Lyase Expression in Human Podocytes Causes a Reduction in Nephrin Expression That Involves PKCδ Activation

Sphingosine 1-phosphate (S1P) lyase (SPL, Sgpl1) is an ER-associated enzyme that irreversibly degrades the bioactive lipid, S1P, and thereby regulates multiple cellular functions attributed to S1P. Biallelic mutations in the human Sglp1 gene lead to a severe form of a particular steroid-resistant nephrotic syndrome, suggesting that the SPL is critically involved in maintaining the glomerular ultrafiltration barrier, which is mainly built by glomerular podocytes. In this study, we have investigated the molecular effects of SPL knockdown (kd) in human podocytes to better understand the mechanism underlying nephrotic syndrome in patients. A stable SPL-kd cell line of human podocytes was generated by the lentiviral shRNA transduction method and was characterized for reduced SPL mRNA and protein levels and increased S1P levels. This cell line was further studied for changes in those podocyte-specific proteins that are known to regulate the ultrafiltration barrier. We show here that SPL-kd leads to the downregulation of the nephrin protein and mRNA expression, as well as the Wilms tumor suppressor gene 1 (WT1), which is a key transcription factor regulating nephrin expression. Mechanistically, SPL-kd resulted in increased total cellular protein kinase C (PKC) activity, while the stable downregulation of PKCδ revealed increased nephrin expression. Furthermore, the pro-inflammatory cytokine, interleukin 6 (IL-6), also reduced WT1 and nephrin expression. In addition, IL-6 caused increased PKCδ Thr⁵⁰⁵ phosphorylation, suggesting enzyme activation. Altogether, these data demonstrate that nephrin is a critical factor downregulated by the loss of SPL, which may directly cause podocyte foot process effacement as observed in mice and humans, leading to albuminuria, a hallmark of nephrotic syndrome. Furthermore, our in vitro data suggest that PKCδ could represent a new possible pharmacological target for the treatment of a nephrotic syndrome induced by SPL mutations.

Mebendazole Impedes the Proliferation and Migration of Pancreatic Cancer Cells through SK1 Inhibition Dependent Pathway

Pancreatic ductal adenocarcinoma (PDAC) has one of the highest mortality rates and requires the development of highly efficacious medications that can improve the efficiency of existing treatment methods. In particular, in PDAC, resistance to conventional chemotherapy reduces the effectiveness of anticancer drugs, decreasing the therapeutic efficiency. Sphingosine 1-phosphate (S1P), produced by sphingosine kinase (SK), plays a vital role in cancer growth, metastasis, chemotherapy, and drug resistance. Focusing on the structural characteristics of mebendazole (MBZ), we studied whether MBZ would affect metastasis, invasion, and drug resistance in cancer by lowering S1P production through inhibition of SK activity. MBZ selectively inhibited SK1 more than SK2 and regulated the levels of sphingolipids. MBZ inhibited the proliferation and migration of cancer cells in other PDAC cell lines. To determine whether the effect of MBZ on cancer cell growth and migration is S1P-mediated, S1P was treated, and the growth and migration of cancer cells were observed. It was found that MBZ inhibited S1P-induced cancer cell growth, and MBZ showed a growth inhibitory effect by regulating the JAK2/STAT3/Bcl-2 pathway. The phosphorylation of focal adhesion kinase (FAK), a transcription factor that regulates migration, was inhibited by MBZ, so it was found that the effect of MBZ regulates the migration of cancer cells through the S1P/FAK/vimentin pathway. In conclusion, our study suggests that the anthelmintic MBZ can be used as a potential therapeutic agent for treating PDAC and for structural synthesis studies of its analogs.

SphK-produced S1P in somatic cells is indispensable for LH-EGFR signaling-induced mouse oocyte maturation

Germ cell division and differentiation require intimate contact and interaction with the surrounding somatic cells. Luteinizing hormone (LH) triggers epidermal growth factor (EGF)-like growth factors to promote oocyte maturation and developmental competence by activating EGF receptor (EGFR) in somatic cells. Here, we showed that LH-EGFR signaling-activated sphingosine kinases (SphK) in somatic cells. The activation of EGFR by EGF increased S1P and calcium levels in cumulus-oocyte complexes (COCs), and decreased the binding affinity of natriuretic peptide receptor 2 (NPR2) for natriuretic peptide type C (NPPC) to release the cGMP-mediated meiotic arrest. These functions of EGF were blocked by the SphK inhibitor SKI-II, which could be reversed by the addition of S1P. S1P also activated the Akt/mTOR cascade reaction in oocytes and promoted targeting protein for Xklp2 (TPX2) accumulation and oocyte developmental competence. Specifically depleting Sphk1/2 in somatic cells reduced S1P levels and impaired oocyte meiotic maturation and developmental competence, resulting in complete female infertility. Collectively, SphK-produced S1P in somatic cells serves as a functional transmitter of LH-EGFR signaling from somatic cells to oocytes: acting on somatic cells to induce oocyte meiotic maturation, and acting on oocytes to improve oocyte developmental competence.

O-Cyclic Phytosphingosine-1-Phosphate Protects against Motor Dysfunctions and Glial Cell Mediated Neuroinflammation in the Parkinson's Disease Mouse Models

O-cyclic phytosphingosine-1-phosphate (cPS1P) is a novel and chemically synthesized sphingosine metabolite derived from phytosphingosine-1-phosphate (S1P). This study was undertaken to unveil the potential neuroprotective effects of cPS1P on two different mouse models of Parkinson's disease (PD). The study used 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and neuron specific enolase promoter human alpha-synuclein (NSE-hαSyn) Korl transgenic mice. MPTP was injected for five consecutive days and cPS1P was injected for alternate days for six weeks intraperitoneally. We performed behavioral tests and analyzed the immunohistochemistry and immunofluorescence staining in the substantia nigra pars compacta (SNpc) and the striatum. The behavior tests showed a significant reduction in the motor functions in the PD models, which was reversed with the administration of cPS1P. In addition, both PD-models showed reduced expression of the sphingosine-1-phosphate receptor 1 (S1PR1), and α-Syn which was restored with cPS1P treatment. In addition, administration of cPS1P restored dopamine-related proteins such as tyrosine hydroxylase (TH), vesicular monoamine transporter 2 (VMAT2), and dopamine transporter (DAT). Lastly, neuroinflammatory related markers such as glial fibrillary acidic protein (GFAP), ionized calcium-binding adapter protein-1 (Iba-1), c-Jun N-terminal kinases (JNK), nuclear factor kappa-light-chain-enhancer of activated B cells (NF-kB), tumor necrosis factor-alpha (TNF-α), and interleukin 1 beta (IL-1β) were all reduced after cPS1P administration. The overall findings supported the notion that cPS1P protects against dopamine depletion, neuroinflammation, and PD-associated symptoms.

Sphingosine 1-Phosphate Metabolism and Signaling

Sphingosine 1-phosphate (S1P) is a well-defined bioactive lipid molecule derived from membrane sphingolipid metabolism. In the past decades, a series of key enzymes involved in generation of S1P have been identified and characterized in detail, as well as enzymes degrading S1P. S1P requires transporter to cross the plasma membrane and carrier to deliver to its cognate receptors and therefore transduces signaling in autocrine, paracrine, or endocrine fashions. The essential roles in regulation of development, metabolism, inflammation, and many other aspects of life are mainly executed when S1P binds to receptors provoking the downstream signaling cascades in distinct cells. This chapter will review the synthesis, degradation, transportation, and signaling of S1P and try to provide a comprehensive view of the biology of S1P, evoking new enthusiasms and ideas into the field of the fascinating S1P.

Inhibitory Effect of Resveratrol on LPS-induced Glomerular Mesangial Cells Proliferation and TGF-β1 Expression via Sphingosine Kinase 1 Pathway

OBJECTIVE

To elucidate the renoprotective effect of resveratrol (RSV) on sphingosine kinase 1 (SphK1) signaling pathway and expression of its downstream molecules including activator protein 1 (AP-1) and transformation growth factor-β1 (TGF-β1) in lipopolysaccharide (LPS)-induced glomerular mesangial cells (GMCs).

METHODS

The rat GMCs line (HBZY-1) were cultured and randomly divided into 5 groups, including control, LPS (100 ng/mL), and 5, 10, 20 µmol/L RSV-treated groups. In addition, SphK1 inhibitor (SK-II) was used as positive control. GMCs were pretreated with RSV for 2 h and treated with LPS for another 24 h. GMCs proliferation was determined by 3-(4, 5-dimethylthiazol-2-yl)-2, 5-diphenyl tetrazolium bromide (MTT) assay. The proteins expression of SphK1, p-c-Jun and TGF-β1 in GMCs were detected by Western blot, and DNA-binding activity of AP-1 was performed by electrophoretic mobility shift assay (EMSA). The binding activity between RSV and SphK1 protein was detected by AutoDock Vina and visualized by Discovery Studio 2016.

RESULTS

LPS could obviously stimulate GMCs proliferation, elevate SphK1, p-c-Jun and TGF-β1 expression levels and increase the DNA-binding activity of AP-1 (P<0.05 or P<0.01), whereas these effects were significantly blocked by RSV pretreatment. It was also suggested that the effect of RSV was similar to SK-II (P>0.05). Moreover, RSV exhibited good binding affinity towards SphK1, with docking scores of -8.1 kcal/moL and formed hydrogen bonds with ASP-178 and LEU-268 in SphK1.

CONCLUSION

RSV inhibited LPS-induced GMCs proliferation and TGF-β1 expression, which may be independent of its hypoglycemic effect on preventing the development of mesangial cell fibrosis and closely related to the direct inhibition of SphK1 pathway.

S1P Signalling Axis Is Necessary for Adiponectin-Directed Regulation of Electrophysiological Properties and Oxidative Metabolism in C2C12 Myotubes

Background: Adiponectin (Adn), released by adipocytes and other cell types such as skeletal muscle, has insulin-sensitizing and anti-inflammatory properties. Sphingosine 1-phosphate (S1P) is reported to act as effector of diverse biological actions of Adn in different tissues. S1P is a bioactive sphingolipid synthesized by the phosphorylation of sphingosine catalyzed by sphingosine kinase (SK) 1 and 2. Consolidated findings support the key role of S1P in the biology of skeletal muscle. Methods and Results: Here we provide experimental evidence that S1P signalling is modulated by globular Adn treatment being able to increase the phosphorylation of SK1/2 as well as the mRNA expression levels of S1P₄ in C2C12 myotubes. These findings were confirmed by LC-MS/MS that showed an increase of S1P levels after Adn treatment. Notably, the involvement of S1P axis in Adn action was highlighted since, when SK1 and 2 were inhibited by PF543 and ABC294640 inhibitors, respectively, not only the electrophysiological changes but also the increase of oxygen consumption and of aminoacid levels induced by the hormone, were significantly inhibited. Conclusion: Altogether, these findings show that S1P biosynthesis is necessary for the electrophysiological properties and oxidative metabolism of Adn in skeletal muscle cells.

The anti-osteosarcoma cell activity by the sphingosine kinase 1 inhibitor SKI-V

Sphingosine kinase 1 (SphK1) expression and activity are elevated in human osteosarcoma (OS) and is a promising target of therapy. SKI-V is a non-competitive and highly-efficient non-lipid SphK1 inhibitor. The potential anti-OS cell activity by the SphK1 inhibitor was studied here. In primary OS cells and immortalized cell lines, SKI-V robustly suppressed cell survival, growth and proliferation as well as cell mobility, and inducing profound OS cell death and apoptosis. The SphK1 inhibitor was however non-cytotoxic nor pro-apoptotic in human osteoblasts. SKI-V robustly inhibited SphK1 activation and induced accumulation of ceramides, without affecting SphK1 expression in primary OS cells. The SphK1 activator K6PC-5 or sphingosine-1-phosphate partially inhibited SKI-V-induced OS cell death. We showed that SKI-V concurrently blocked Akt-mTOR activation in primary OS cells. A constitutively-active Akt1 (ca-Akt1, S473D) construct restored Akt-mTOR activation and mitigated SKI-V-mediated cytotoxicity in primary OS cells. In vivo, daily injection of SKI-V potently suppressed OS xenograft tumor growth in nude mice. In SKI-V-administrated OS xenograft tissues, SphK1 inhibition, ceramide increase and Akt-mTOR inhibition were detected. Together, SKI-V exerts significant anti-OS activity by inhibiting SphK1 and Akt-mTOR cascades in OS cells.

A Comprehensive Review on the Interplay between Neisseria spp. and Host Sphingolipid Metabolites

Sphingolipids represent a class of structural related lipids involved in membrane biology and various cellular processes including cell growth, apoptosis, inflammation and migration. Over the past decade, sphingolipids have become the focus of intensive studies regarding their involvement in infectious diseases. Pathogens can manipulate the sphingolipid metabolism resulting in cell membrane reorganization and receptor recruitment to facilitate their entry. They may recruit specific host sphingolipid metabolites to establish a favorable niche for intracellular survival and proliferation. In contrast, some sphingolipid metabolites can also act as a first line defense against bacteria based on their antimicrobial activity. In this review, we will focus on the strategies employed by pathogenic Neisseria spp. to modulate the sphingolipid metabolism and hijack the sphingolipid balance in the host to promote cellular colonization, invasion and intracellular survival. Novel techniques and innovative approaches will be highlighted that allow imaging of sphingolipid derivatives in the host cell as well as in the pathogen.

The Role of Sphingomyelin Metabolism in the Protection of Rat Brain Microvascular Endothelial Cells by Mild Hypothermia

BACKGROUND

Sphingomyelin, composed of ceramide (CER), sphingosine (Sph), and sphingosine-1-phosphate (S1P), is an essential structural component of cellular membranes and plays an important role in the signal transduction regulating cell proliferation, differentiation, and apoptosis. CER is mainly metabolized to Sph, and under the action of sphingosine kinases (SphKs), Sph produces S1P, which can be converted back to Sph by S1P phosphatase. It is suggested that the fate of cells is controlled partly by the interconversion of CER and intracellular S1P. SphK2 is considered the main kinase of S1P synthesis in the central nervous system. The objective of this study was to explore the hypothesis that SphK2 and sphingomyelin metabolism participated in the process of cell apoptosis and the protection of mild hypothermia.

METHODS

Rat brain microvascular endothelial cells were divided into groups for intervention of SphK2 inhibitor, SphK2 small interfering RNA (SiRNA) transfection, ischemia-reperfusion, and mild hypothermia. After interventions, cell apoptosis was detected by 4,6-diamino-2-phenyl indole (DAPI) and flow cytometry, the expression of apoptosis-related protein was detected by Western Blot, and SphK2 enzyme activity and the content of sphingomyelin were determined.

RESULTS

ABC294640 and transfection of SphK2 SiRNA could increase apoptosis, accompanied by the increase of the expression of proapoptotic genes Caspase3 and Bax and the decrease of the expression of BCL-2. This effect could be partially reversed with mild hypothermia. Ischemia-reperfusion injury, transfection of SphK2 SiRNA, and the addition of ABC294640 could significantly inhibit the activity of SphK2, accompanied by the increase of CERs and the decrease of S1P. Mild hypothermia could reverse the changes of sphingolipids but have no significant effect on the activity of sphk2.

CONCLUSIONS

Mild hypothermia can inhibit the occurrence of apoptosis and reverse the changes of apoptosis-related genes and sphingomyelin content induced by ischemia-reperfusion injury, but the effect on sphk2 enzyme activity was not significant.

S1P Stimulates Erythropoietin Production in Mouse Renal Interstitial Fibroblasts by S1P1 and S1P3 Receptor Activation and HIF-2α Stabilization

Erythropoietin (Epo) is the critical hormone for erythropoiesis. In adults, Epo is mainly produced by a subset of interstitial fibroblasts in the kidney, with minor amounts being produced in the liver and the brain. In this study, we used the immortalized renal interstitial fibroblast cell line FAIK F3-5 to investigate the ability of the bioactive sphingolipid sphingosine 1-phosphate (S1P) to stimulate Epo production and to reveal the mechanism involved. Stimulation of cells with exogenous S1P under normoxic conditions (21% O₂) led to a dose-dependent increase in Epo mRNA and protein levels and subsequent release of Epo into the medium. S1P also enhanced the stabilization of HIF-2α, a key transcription factor for Epo expression. S1P-stimulated Epo mRNA and protein expression was abolished by HIF-2α mRNA knockdown or by the HIF-2 inhibitor compound 2. Furthermore, the approved S1P receptor modulator FTY720, and its active form FTY720-phosphate, both exerted a similar effect on Epo expression as S1P. The effect of S1P on Epo was antagonized by the selective S1P₁ and S1P₃ antagonists NIBR-0213 and TY-52156, but not by the S1P₂ antagonist JTE-013. Moreover, inhibitors of the classical MAPK/ERK, the p38-MAPK, and inhibitors of protein kinase (PK) C and D all blocked the effect of S1P on Epo expression. Finally, the S1P and FTY720 effects were recapitulated in the Epo-producing human neuroblastoma cell line Kelly, suggesting that S1P receptor-dependent Epo synthesis is of general relevance and not species-specific. In summary, these data suggest that, in renal interstitial fibroblasts, which are the primary source of plasma Epo, S1P_{1 and 3} receptor activation upregulates Epo under normoxic conditions. This may have a therapeutic impact on disease situations such as chronic kidney disease, where Epo production is impaired, causing anemia, but it may also have therapeutic value as Epo can mediate additional tissue-protective effects in various organs.

miR-19a/b-3p promotes inflammation during cerebral ischemia/reperfusion injury via SIRT1/FoxO3/SPHK1 pathway

Background

Stroke affects 3–4% of adults and kills numerous people each year. Recovering blood flow with minimal reperfusion-induced injury is crucial. However, the mechanisms underlying reperfusion-induced injury, particularly inflammation, are not well understood. Here, we investigated the function of miR-19a/b-3p/SIRT1/FoxO3/SPHK1 axis in ischemia/reperfusion (I/R).

Methods

MCAO (middle cerebral artery occlusion) reperfusion rat model was used as the in vivo model of I/R. Cultured neuronal cells subjected to OGD/R (oxygen glucose deprivation/reperfusion) were used as the in vitro model of I/R. MTT assay was used to assess cell viability and TUNEL staining was used to measure cell apoptosis. H&E staining was employed to examine cell morphology. qRT-PCR and western blot were performed to determine levels of miR-19a/b-3p, SIRT1, FoxO3, SPHK1, NF-κB p65, and cytokines like TNF-α, IL-6, and IL-1β. EMSA and ChIP were performed to validate the interaction of FoxO3 with SPHK1 promoter. Dual luciferase assay and RIP were used to verify the binding of miR-19a/b-3p with SIRT1 mRNA.

Results

miR-19a/b-3p, FoxO3, SPHK1, NF-κB p65, and cytokines were elevated while SIRT1 was reduced in brain tissues following MCAO/reperfusion or in cells upon OGD/R. Knockdown of SPHK1 or FoxO3 suppressed I/R-induced inflammation and cell death. Furthermore, knockdown of FoxO3 reversed the effects of SIRT1 knockdown. Inhibition of the miR-19a/b-3p suppressed inflammation and this suppression was blocked by SIRT1 knockdown. FoxO3 bound SPHK1 promoter and activated its transcription. miR-19a/b-3p directly targeted SIRT1 mRNA.

Conclusion

miR-19a/b-3p promotes inflammatory responses during I/R via targeting SIRT1/FoxO3/SPHK1 axis.

A2 B Adenosine Receptors and Sphingosine 1-Phosphate Signaling Cross-Talk in Oligodendrogliogenesis

Oligodendrocyte-formed myelin sheaths allow fast synaptic transmission in the brain. Impairments in the process of myelination, or demyelinating insults, might cause chronic diseases such as multiple sclerosis (MS). Under physiological conditions, remyelination is an ongoing process throughout adult life consisting in the differentiation of oligodendrocyte progenitor cells (OPCs) into mature oligodendrocytes (OLs). During pathological events, this process fails due to unfavorable environment. Adenosine and sphingosine kinase/sphingosine 1-phosphate signaling axes (SphK/S1P) play important roles in remyelination processes. Remarkably, fingolimod (FTY720), a sphingosine analog recently approved for MS treatment, plays important roles in OPC maturation. We recently demonstrated that the selective stimulation of A_{2 B} adenosine receptors (A_{2 B} Rs) inhibit OPC differentiation in vitro and reduce voltage-dependent outward K⁺ currents (I _K ) necessary to OPC maturation, whereas specific SphK1 or SphK2 inhibition exerts the opposite effect. During OPC differentiation A_{2 B} R expression increases, this effect being prevented by SphK1/2 blockade. Furthermore, selective silencing of A_{2 B} R in OPC cultures prompts maturation and, intriguingly, enhances the expression of S1P lyase, the enzyme responsible for irreversible S1P catabolism. Finally, the existence of an interplay between SphK1/S1P pathway and A_{2 B} Rs in OPCs was confirmed since acute stimulation of A_{2 B} Rs activates SphK1 by increasing its phosphorylation. Here the role of A_{2 B} R and SphK/S1P signaling during oligodendrogenesis is reviewed in detail, with the purpose to shed new light on the interaction between A_{2 B} Rs and S1P signaling, as eventual innovative targets for the treatment of demyelinating disorders.

Loss of sphingosine kinase 2 enhances Wilm's tumor suppressor gene 1 and nephrin expression in podocytes and protects from streptozotocin-induced podocytopathy and albuminuria in mice

The sphingosine 1-phosphate (S1P) is a bioactive sphingolipid that is now appreciated as key regulatory factor for various cellular functions in the kidney, including matrix remodeling. It is generated by two sphingosine kinases (Sphk), Sphk1 and Sphk2, which are ubiquitously expressed, but have distinct enzymatic activities and subcellular localizations. In this study, we have investigated the role of Sphk2 in podocyte function and its contribution to diabetic nephropathy. We show that streptozotocin (STZ)-induced nephropathy and albuminuria in mice is prevented by genetic depletion of Sphk2. This protection correlated with an increased protein expression of the transcription factor Wilm's tumor suppressor gene 1 (WT1) and its target gene nephrin, and a reduced macrophage infiltration in immunohistochemical renal sections of STZ-treated Sphk2^-/- mice compared to STZ-treated wildtype mice. To investigate changes on the cellular level, we used an immortalized human podocyte cell line and generated a stable knockdown of Sphk2 (Sphk2-kd) by a lentiviral transduction method. These Sphk2-kd cells accumulated sphingosine as a consequence of the knockdown, and showed enhanced nephrin and WT1 mRNA and protein expressions similar to the finding in Sphk2 knockout mice. Treatment of wildtype podocytes with the highly selective Sphk2 inhibitor SLM6031434 caused a similar upregulation of nephrin and WT1 expression. Furthermore, exposing cells to the profibrotic mediator transforming growth factor β (TGFβ) resulted on the one side in reduced nephrin and WT1 expression, but on the other side, in upregulation of various profibrotic marker proteins, including connective tissue growth factor (CTGF), fibronectin (FN) and plasminogen activator inhibitor (PAI) 1. All these effects were reverted by Sphk2-kd and SLM6031434. Mechanistically, the protection by Sphk2-kd may depend on accumulated sphingosine and inhibited PKC activity, since treatment of cells with exogenous sphingosine not only reduced the phosphorylation pattern of PKC substrates, but also increased WT1 protein expression. Moreover, the selective stable knockdown of PKCδ increased WT1 expression, suggesting the involvement of this PKC isoenzyme in WT1 regulation. The glucocorticoid dexamethasone, which is a treatment option in many glomerular diseases and is known to mediate a nephroprotection, not only downregulated Sphk2 and enhanced cellular sphingosine, but also enhanced WT1 and nephrin expressions, thus, suggesting that parts of the nephroprotective effect of dexamethasone is mediated by Sphk2 downregulation. Altogether, our data demonstrated that loss of Sphk2 is protective in diabetes-induced podocytopathy and can prevent proteinuria, which is a hallmark of many glomerular diseases. Thus, Sphk2 could serve as a new attractive pharmacological target to treat proteinuric kidney diseases.

The Functional Role of Sphingosine Kinase 2

Sphingosine-1-phosphate (S1P) is a bioactive lipid molecule that is present in all eukaryotic cells and plays key roles in various extracellular, cytosolic, and nuclear signaling pathways. Two sphingosine kinase isoforms, sphingosine kinase 1 (SPHK1) and sphingosine kinase 2 (SPHK2), synthesize S1P by phosphorylating sphingosine. While SPHK1 is a cytoplasmic kinase, SPHK2 is localized to the nucleus, endoplasmic reticulum, and mitochondria. The SPHK2/S1P pathway regulates transcription, telomere maintenance, mitochondrial respiration, among many other processes. SPHK2 is under investigation as a target for treating many age-associated conditions, such as cancer, stroke, and neurodegeneration. In this review, we will focus on the role of SPHK2 in health and disease.

Targeting the Sphingosine Kinase/Sphingosine-1-Phosphate Signaling Axis in Drug Discovery for Cancer Therapy

Sphingolipid metabolites have emerged as critical players in the regulation of various physiological processes. Ceramide and sphingosine induce cell growth arrest and apoptosis, whereas sphingosine-1-phosphate (S1P) promotes cell proliferation and survival. Here, we present an overview of sphingolipid metabolism and the compartmentalization of various sphingolipid metabolites. In addition, the sphingolipid rheostat, a fine metabolic balance between ceramide and S1P, is discussed. Sphingosine kinase (SphK) catalyzes the synthesis of S1P from sphingosine and modulates several cellular processes and is found to be essentially involved in various pathophysiological conditions. The regulation and biological functions of SphK isoforms are discussed. The functions of S1P, along with its receptors, are further highlighted. The up-regulation of SphK is observed in various cancer types and is also linked to radio- and chemoresistance and poor prognosis in cancer patients. Implications of the SphK/S1P signaling axis in human pathologies and its inhibition are discussed in detail. Overall, this review highlights current findings on the SphK/S1P signaling axis from multiple angles, including their functional role, mechanism of activation, involvement in various human malignancies, and inhibitor molecules that may be used in cancer therapy.

Sphingosine kinase 2 restricts T cell immunopathology but permits viral persistence

Chronic viral infections are often established by the exploitation of immune-regulatory mechanisms that result in nonfunctional T cell responses. Viruses that establish persistent infections remain a serious threat to human health. Sphingosine kinase 2 (SphK2) generates sphingosine 1-phosphate, which is a molecule known to regulate multiple cellular processes. However, little is known about SphK2's role during the host immune responses to viral infection. Here, we demonstrate that SphK2 functions during lymphocytic choriomeningitis virus Cl 13 (LCMV Cl 13) infection to limit T cell immune pathology, which subsequently aids in the establishment of virus-induced immunosuppression and the resultant viral persistence. The infection of Sphk2-deficient (Sphk2-/-) mice with LCMV Cl 13 led to the development of nephropathy and mortality via T cell-mediated immunopathology. Following LCMV infection, Sphk2-/- CD4+ T cells displayed increased activity and proliferation, and these cells promoted overactive LCMV Cl 13-specific CD8+ T cell responses. Notably, oral instillation of an SphK2-selective inhibitor promoted protective T cell responses and accelerated the termination of LCMV Cl 13 persistence in mice. Thus, SphK2 is indicated as an immunotherapeutic target for the control of persistent viral infections.

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.

Altered sphingolipid function in Alzheimer's disease; a gene regulatory network approach

Sphingolipids (SLs) are bioactive lipids involved in various important physiological functions. The SL pathway has been shown to be affected in several brain-related disorders, including Alzheimer's disease (AD). Recent evidence suggests that epigenetic dysregulation plays an important role in the pathogenesis of AD as well. Here, we use an integrative approach to better understand the relationship between epigenetic and transcriptomic processes in regulating SL function in the middle temporal gyrus of AD patients. Transcriptomic analysis of 252 SL-related genes, selected based on GO term annotations, from 46 AD patients and 32 healthy age-matched controls, revealed 103 differentially expressed SL-related genes in AD patients. Additionally, methylomic analysis of the same subjects revealed parallel hydroxymethylation changes in PTGIS, GBA, and ITGB2 in AD. Subsequent gene regulatory network-based analysis identified 3 candidate genes, that is, SELPLG, SPHK1 and CAV1 whose alteration holds the potential to revert the gene expression program from a diseased towards a healthy state. Together, this epigenomic and transcriptomic approach highlights the importance of SL-related genes in AD, and may provide novel biomarkers and therapeutic alternatives to traditionally investigated biological pathways in AD.

SphK1-targeted miR-6784 inhibits functions of skin squamous cell carcinoma cells

Sphingosine kinase 1 (SphK1) is overexpressed in skin squamous cell carcinoma (SCC). It has emerged as a novel therapeutic oncotarget. The current study identified a novel SphK1-targeting microRNA, microRNA-6784 (miR-6784). Here, we show that miR-6784 is located at the cytoplasm of A431 skin SCC cells. It directly binds to SphK1 mRNA. Ectopic overexpression of miR-6784 inhibited SphK1 3’-untranslated region (UTR) luciferase activity and downregulated its expression. Moreover, miR-6784 overexpression caused ceramide accumulation in skin SCC cells. Functional studies in established (A431 and SCC9) and primary skin SCC cells revealed that miR-6784 overexpression inhibited cell viability, proliferation, migration, and invasion. It also simultaneously provoked apoptosis activation. Conversely, miR-6784 silencing by antagomiR-6784 induced SphK1 elevation and augmented A431 cell proliferation, migration, and invasion. miR-6784 overexpression-induced anti-A431 cell activity was inhibited by the expression of an UTR-null SphK1 construct. CRISPR/Cas9-induced SphK1 knockout inhibited A431 cell growth. Importantly, miR-6784 was completely ineffective when treating SphK1-knockout A431 cells. Collectively, miR-6784 silences SphK1 and inhibits skin SCC cell progression.

Apoptotic cell extrusion depends on single-cell synthesis of sphingosine-1-phosphate by sphingosine kinase 2

Collecting duct cells are physiologically subject to the hypertonic environment of the kidney. This condition is necessary for kidney maturation and function but represents a stress condition that requires active strategies to ensure epithelial integrity. Madin-Darby Canine Kidney (MDCK) cells develop the differentiated phenotype of collecting duct cells when subject to hypertonicity, serving as a model to study epithelial preservation and homeostasis in this particular environment. The integrity of epithelia is essential to achieve the required functional barrier. One of the mechanisms that ensure integrity is cell extrusion, a process initiated by sphingosine-1-phosphate (S1P) to remove dying or surplus cells while maintaining the epithelium barrier. Both types start with the activation of S1P receptor type 2, located in neighboring cells. In this work, we studied the effect of cell differentiation induced by hypertonicity on cell extrusion in MDCK cells, and we provide new insights into the associated molecular mechanism. We found that the different stages of differentiation influence the rate of apoptotic cell extrusion. Besides, we used a novel methodology to demonstrate that S1P increase in extruding cells of differentiated monolayers. These results show for first time that cell extrusion is triggered by the single-cell synthesis of S1P by sphingosine kinase 2 (SphK2), but not SphK1, of the extruding cell itself. Moreover, the inhibition or knockdown of SphK2 prevents cell extrusion and cell-cell junction protein degradation, but not apoptotic nuclear fragmentation. Thus, we propose SphK2 as the biochemical key to ensure the preservation of the epithelial barrier under hypertonic stress.

Sphingosine Kinase 1 Signaling in Breast Cancer: A Potential Target to Tackle Breast Cancer Stem Cells

Sphingosine kinases (SPHKs) are conserved lipid enzymes that catalyze the formation of sphingosine-1-phosphate (S1P) through ATP-dependent phosphorylation of sphingosine. Two distinct SPHK isoforms, namely SPHK1 and SPHK2, have been identified to date, and the former has been implicated for its oncogenic roles in cancer development and progression. While SPHK1 signaling axis has been extensively studied in non-stem breast cancer cells, recent evidence has emerged to suggest a role of SPHK1 in regulating cancer stem cells (CSCs). With the clinical implications of CSCs in disease relapse and metastasis, it is believed that therapeutic approaches that can eradicate both non-stem cancer cells and CSCs could be a key to cancer cure. In this review, we first explore the oncogenic functions of sphingosine kinase 1 in human cancers and summarize current research findings of SPHK1 signaling with a focus on breast cancer. We also discuss the therapeutic potentials and perspectives of targeting SPHK1 signaling in breast cancer and cancer stem cells. We aim to offer new insights and inspire future studies looking further into the regulatory functions of SPHK1 in CSC-driven tumorigenesis, uncovering novel therapeutic avenues of using SPHK1-targeted therapy in the treatment of CSC-enriched refractory cancers.

MicroRNA-6862 inhibition elevates sphingosine kinase 1 and protects neuronal cells from MPP+-induced apoptosis

MPP⁺ (1-methyl-4-phenylpyridinium)-induced dopaminergic neuronal cell apoptosis is associated with sphingosine kinase 1 (SphK1) inhibition. We here tested the potential effect of microRNA-6862 (miR-6862), a novel SphK1-targeting miRNA, on MPP⁺-induced cytotoxicity in neuronal cells. MiR-6862 locates in the cytoplasm of SH-SY5Y neuronal cells. It directly binds to SphK1 mRNA. In SH-SY5Y cells and HCN-2 cells, ectopic overexpression of miR-6862 decreased SphK13’-untranslated region luciferase reporter activity and downregulated its expression. miR-6862 inhibition exerted opposite activity and elevated SphK1 expression. In neuronal cells, MPP⁺-induced cell death was significantly inhibited through miR-6862 inhibition. Conversely, ectopic overexpression of miR-6862 or CRISPR/Cas9-induced SphK1 knockout augmented MPP⁺-induced apoptosis in the neuronal cells. Importantly, antagomiR-6862 failed to inhibit MPP⁺-induced apoptosis in SphK1-knockout SH-SY5Y cells. These results suggest that inhibition of miR-6862 induces SphK1 elevation and protects neuronal cells from MPP⁺-induced cell death.

The Tumorigenic Effect of Sphingosine Kinase 1 and Its Potential Therapeutic Target

Sphingosine kinase 1 (SPHK1) regulates cell proliferation and survival by converting sphingosine to the signaling mediator sphingosine 1-phosphate (S1P). SPHK1 is widely overexpressed in most cancers, promoting tumor progression and is associated with clinical prognosis. Numerous studies have explored SPHK1 as a promising target for cancer therapy. However, due to insufficient knowledge of SPHK1 oncogenic mechanisms, its inhibitors’ therapeutic potential in preventing and treating cancer still needs further investigation. In this review, we summarized the metabolic balance regulated by the SPHK1/S1P signaling pathway and highlighted the oncogenic mechanisms of SPHK1 via the upregulation of autophagy, proliferation, and survival, migration, angiogenesis and inflammation, and inhibition of apoptosis. Drug candidates targeting SPHK1 were also discussed at the end. This review provides new insights into the oncogenic effect of SPHK1 and sheds light on the future direction for targeting SPHK1 as cancer therapy.

Validation of highly selective sphingosine kinase 2 inhibitors SLM6031434 and HWG-35D as effective anti-fibrotic treatment options in a mouse model of tubulointerstitial fibrosis

Renal fibrosis is characterized by chronic inflammation and excessive accumulation of extracellular matrix and progressively leads to functional insufficiency and even total loss of kidney function. In this study we investigated the anti-fibrotic potential of two highly selective and potent SK2 inhibitors, SLM6031434 and HWG-35D, in unilateral ureter obstruction (UUO), a model for progressive renal fibrosis, in mice. In both cases, treatment with SLM6031434 or HWG-35D resulted in an attenuated fibrotic response to UUO in comparison to vehicle-treated mice as demonstrated by reduced collagen accumulation and a decreased expression of collagen-1 (Col1), fibronectin-1 (FN-1), connective tissue growth factor (CTGF), and α-smooth muscle actin (α-SMA). Similar to our previous study in Sphk2^-/- mice, we found an increased protein expression of Smad7, a negative regulator of the pro-fibrotic TGFβ/Smad signalling cascade, accompanied by a strong accumulation of sphingosine in SK2 inhibitor-treated kidneys. Treatment of primary renal fibroblasts with SLM6031434 or HWG-35D dose-dependently increased Smad7 expression and ameliorated the expression of Col1, FN-1 and CTGF. In summary, these data prove the anti-fibrotic potential of SK2 inhibition in a mouse model of renal fibrosis, thereby validating SK2 as pharmacological target for the treatment of fibrosis in chronic kidney disease.

Dissecting Gq/11-Mediated Plasma Membrane Translocation of Sphingosine Kinase-1

Diverse extracellular signals induce plasma membrane translocation of sphingosine kinase-1 (SphK1), thereby enabling inside-out signaling of sphingosine-1-phosphate. We have shown before that G_q-coupled receptors and constitutively active Gα_q/11 specifically induced a rapid and long-lasting SphK1 translocation, independently of canonical G_q/phospholipase C (PLC) signaling. Here, we further characterized G_q/11 regulation of SphK1. SphK1 translocation by the M₃ receptor in HEK-293 cells was delayed by expression of catalytically inactive G-protein-coupled receptor kinase-2, p63Rho guanine nucleotide exchange factor (p63RhoGEF), and catalytically inactive PLCβ₃, but accelerated by wild-type PLCβ₃ and the PLCδ PH domain. Both wild-type SphK1 and catalytically inactive SphK1-G82D reduced M₃ receptor-stimulated inositol phosphate production, suggesting competition at Gα_q. Embryonic fibroblasts from Gα_q/11 double-deficient mice were used to show that amino acids W263 and T257 of Gα_q, which interact directly with PLCβ₃ and p63RhoGEF, were important for bradykinin B₂ receptor-induced SphK1 translocation. Finally, an AIXXPL motif was identified in vertebrate SphK1 (positions 100–105 in human SphK1a), which resembles the Gα_q binding motif, ALXXPI, in PLCβ and p63RhoGEF. After M₃ receptor stimulation, SphK1-A100E-I101E and SphK1-P104A-L105A translocated in only 25% and 56% of cells, respectively, and translocation efficiency was significantly reduced. The data suggest that both the AIXXPL motif and currently unknown consequences of PLCβ/PLCδ(PH) expression are important for regulation of SphK1 by G_q/11.

Sphingosine-1-Phosphate Metabolism in the Regulation of Obesity/Type 2 Diabetes

Obesity is a pathophysiological condition where excess free fatty acids (FFA) target and promote the dysfunctioning of insulin sensitive tissues and of pancreatic β cells. This leads to the dysregulation of glucose homeostasis, which culminates in the onset of type 2 diabetes (T2D). FFA, which accumulate in these tissues, are metabolized as lipid derivatives such as ceramide, and the ectopic accumulation of the latter has been shown to lead to lipotoxicity. Ceramide is an active lipid that inhibits the insulin signaling pathway as well as inducing pancreatic β cell death. In mammals, ceramide is a key lipid intermediate for sphingolipid metabolism as is sphingosine-1-phosphate (S1P). S1P levels have also been associated with the development of obesity and T2D. In this review, the current knowledge on S1P metabolism in regulating insulin signaling in pancreatic β cell fate and in the regulation of feeding by the hypothalamus in the context of obesity and T2D is summarized. It demonstrates that S1P can display opposite effects on insulin sensitive tissues and pancreatic β cells, which depends on its origin or its degradation pathway.

Modulation of DNA Damage Response by Sphingolipid Signaling: An Interplay that Shapes Cell Fate

Although once considered as structural components of eukaryotic biological membranes, research in the past few decades hints at a major role of bioactive sphingolipids in mediating an array of physiological processes including cell survival, proliferation, inflammation, senescence, and death. A large body of evidence points to a fundamental role for the sphingolipid metabolic pathway in modulating the DNA damage response (DDR). The interplay between these two elements of cell signaling determines cell fate when cells are exposed to metabolic stress or ionizing radiation among other genotoxic agents. In this review, we aim to dissect the mediators of the DDR and how these interact with the different sphingolipid metabolites to mount various cellular responses.

Design and Development of Novel Urea, Sulfonyltriurea, and Sulfonamide Derivatives as Potential Inhibitors of phingosine Kinase 1

Sphingosine kinase 1 (SphK1) is one of the well-studied drug targets for cancer and inflammatory diseases. Recently discovered small-molecule inhibitors of SphK1 have been recommended in cancer therapeutics; however, selectivity and potency of first-generation inhibitors are great challenge. In search of effective SphK1 inhibitors, a set of small molecules have been designed and synthesized bearing urea, sulfonylurea, sulfonamide, and sulfonyltriurea groups. The binding affinity of these inhibitors was measured by fluorescence-binding assay and isothermal titration calorimetry. Compounds 1, 5, 6, and 7 showed an admirable binding affinity to the SphK1 in the sub-micromolar range and significantly inhibited SphK1 activity with admirable IC50 values. Molecular docking studies revealed that these compounds fit well into the sphingosine binding pocket of SphK1 and formed significant number of hydrogen bonds and van der Waals interactions. These molecules may be exploited as potent and selective inhibitors of SphK1 that could be implicated in cancer therapeutics after the required in vivo validation.

The role of the sphingolipid pathway in liver fibrosis: an emerging new potential target for novel therapies

Sphingolipids (SL) are a family of bioactive lipids and a major cellular membrane structural component. SLs include three main compounds: ceramide (Cer), sphingosine (Sp), and sphingosine-1-phosphate (S-1P), all of which have emerging roles in biological functions in cells, especially in the liver. They are under investigation in various liver diseases, including cirrhosis and end-stage liver disease. In this review, we provide an overview on the role of SLs in liver pathobiology and focus on their potential role in the development of hepatic fibrosis. We describe recent evidence and suggest SLs are a promising potential therapeutic target for the treatment of liver disease and fibrosis.

Downregulation of S1P Lyase Improves Barrier Function in Human Cerebral Microvascular Endothelial Cells Following an Inflammatory Challenge

Sphingosine 1-phosphate (S1P) is a key bioactive lipid that regulates a myriad of physiological and pathophysiological processes, including endothelial barrier function, vascular tone, vascular inflammation, and angiogenesis. Various S1P receptor subtypes have been suggested to be involved in the regulation of these processes, whereas the contribution of intracellular S1P (iS1P) through intracellular targets is little explored. In this study, we used the human cerebral microvascular endothelial cell line HCMEC/D3 to stably downregulate the S1P lyase (SPL-kd) and evaluate the consequences on endothelial barrier function and on the molecular factors that regulate barrier tightness under normal and inflammatory conditions. The results show that in SPL-kd cells, transendothelial electrical resistance, as a measure of barrier integrity, was regulated in a dual manner. SPL-kd cells had a delayed barrier build up, a shorter interval of a stable barrier, and, thereafter, a continuous breakdown. Contrariwise, a protection was seen from the rapid proinflammatory cytokine-mediated barrier breakdown. On the molecular level, SPL-kd caused an increased basal protein expression of the adherens junction molecules PECAM-1, VE-cadherin, and β-catenin, increased activity of the signaling kinases protein kinase C, AMP-dependent kinase, and p38-MAPK, but reduced protein expression of the transcription factor c-Jun. However, the only factors that were significantly reduced in TNFα/SPL-kd compared to TNFα/control cells, which could explain the observed protection, were VCAM-1, IL-6, MCP-1, and c-Jun. Furthermore, lipid profiling revealed that dihydro-S1P and S1P were strongly enhanced in TNFα-treated SPL-kd cells. In summary, our data suggest that SPL inhibition is a valid approach to dampenan inflammatory response and augmente barrier integrity during an inflammatory challenge.

Functional implications of pH-induced conformational changes in the Sphingosine kinase 1

Sphingosine kinase 1 (SphK1) catalyzes the conversion of sphingosine to sphingosine-1-phosphate that acts as a bioactive signalling molecule, and regulates various cellular processes including lymphocyte trafficking, angiogenesis and response to apoptotic stimuli. Abnormal expression of SphK1 has been observed in a wide range of cancers highlighting their role in tumour growth and metastasis. This enzyme also plays a critical role in metabolic and inflammatory diseases, including pulmonary fibrosis, diabetic neuropathy and Alzheimer's disease. In the present study, we have investigated the structural and conformational changes in SphK1 at varying pH using various spectroscopic techniques. Consistent results were observed with the function of SphK1 at corresponding pH values. SphK1 maintains its secondary and tertiary structure in the pH range of 7.5-10.0. However, protein aggregation was observed in the acidic pH range (4.0-6.5). At pH 2.0, the SphK1 exists in the molten-globule state. Kinase assay also shows that SphK1 activity was optimal in the pH range of 7.5-8.5. To complement in vitro results, we have performed 100 ns molecular dynamics simulation to examine the effect of pH on the structural stability of SphK1 at molecular level. SphK1 maintains its native conformation in the alkaline pH range with some residual fluctuations detected at acidic pH. A considerable correlation was noticed between spectroscopic, enzymatic activity and MD simulation studies. pH dependent structural changes can be further implicated to understand its association with disease condition, and cellular homeostasis with respect to protein function under variable pH conditions.

Sphingosine Kinase Activates the Mitochondrial Unfolded Protein Response and Is Targeted to Mitochondria by Stress

The mitochondrial unfolded protein response (UPR^mt) is critical for maintaining mitochondrial protein homeostasis in response to mitochondrial stress, but early steps in UPR^mt activation are not well understood. Here, we report a function for SPHK-1 sphingosine kinase in activating the UPR^mt in C. elegans. Genetic deficiency of sphk-1 in the intestine inhibits UPR^mt activation, whereas selective SPHK-1 intestinal overexpression is sufficient to activate the UPR^mt. Acute mitochondrial stress leads to rapid, reversible localization of SPHK-1::GFP fusion proteins with mitochondrial membranes before UPR^mt activation. SPHK-1 variants lacking kinase activity or mitochondrial targeting fail to rescue the stress-induced UPR^mt activation defects of sphk-1 mutants. Activation of the UPR^mt by the nervous system requires sphk-1 and elicits SPHK-1 mitochondrial association in the intestine. We propose that stress-regulated mitochondrial recruitment of SPHK-1 and subsequent S1P production are critical early events for both cell autonomous and cell non-autonomous UPR^mt activation.

The mitochondrial unfolded protein response (UPR^mt) maintains mitochondrial protein homeostasis in response to stress. Kim and Sieburth identify SPHK-1/sphingosine kinase as a positive regulator of the UPR^mt that promotes UPR^mt activation in response to a variety of mitochondrial stressors. SPHK-1 associates with mitochondria and SPHK-1 mitochondrial association is stress dependent, reversible, and necessary for the UPR^mt, indicating that SPHK-1 mitochondrial targeting is an early step in UPR^mt activation.

Expression of sphingosine kinase 1 and sphingosine 1-phosphate receptor 3 in malaria-associated acute lung injury/acute respiratory distress syndrome in a mouse model

This study aimed to investigate the expression of sphingosine kinase 1 (SphK-1) and sphingosine 1-phosphate receptor 3 (S1PR-3) in a mouse model of malaria-associated acute lung injury/acute respiratory distress syndrome (ALI/ARDS). DBA/2 mice were infected with Plasmodium berghei ANKA to generate an experimental model of malaria-associated ALI/ARDS. The infected mice were divided into 2 groups based on the histopathological study of lung tissues: those with and those without ALI/ARDS. The expression of the SphK-1 and S1PR-3 proteins in the lung tissues was investigated using immunohistochemical staining and Western blot analysis. In addition, the S1P level was quantified in plasma and lung tissues using an enzyme-linked immunosorbent assay (ELISA). The results demonstrated that the cellular expression of the SphK-1 and S1PR-3 proteins was significantly upregulated in endothelial cells, alveolar epithelial cells and alveolar macrophages in the lung tissues of malaria-infected mice with ALI/ARDS compared with those in the control groups. The increased expression of the SphK-1 and S1PR-3 proteins was confirmed using Western blot analysis. The concentration of S1P in plasma and lung tissues was significantly decreased in malaria-infected mice with ALI/ARDS compared with non-ALI/ARDS and control mice. Furthermore, increased expression of the SphK-1 and S1PR-3 proteins significantly correlated with lung injury scores and S1P concentrations in malaria-infected mice with ALI/ARDS. These findings highlight increased expression of SphK-1 and S1PR-3 in the lung tissues of malaria-infected mice with ALI/ARDS.

Sphingosine-1-phosphate signaling in blood pressure regulation

Sphingolipids were originally believed to play a role only as a backbone of mammalian cell membranes. However, sphingolipid metabolites, especially sphingosine-1-phosphate (S1P), are now recognized as new bioactive signaling molecules that are critically involved in numerous cellular functions of multiple systems including the immune system, central nervous system, and cardiovascular system. S1P research has accelerated in the last decade as new therapeutic drugs have emerged that target the S1P signaling axis to treat diseases of the immune and central nervous systems. There is limited knowledge of the specific effects on cardiovascular disease. This review discusses the current state of knowledge regarding the role of S1P on the regulation of blood pressure, vascular tone, and renal functions.

Sphingosine kinase 2 is a negative regulator of inflammatory macrophage activation

Sphingosine kinases (SPHK) generate the sphingolipid sphingosine-1-phosphate, which, among other functions, is a potent regulator of inflammation. While SPHK1 produces S1P to promote inflammatory signaling, the role of SPHK2 is unclear due to divergent findings in studies utilizing gene depletion versus inhibition of catalytic activity. We sought to clarify how SPHK2 affects inflammatory signaling in human macrophages, which are main regulators of inflammation. SPHK2 expression and activity were rapidly decreased within 6 h upon stimulating primary human macrophages with lipopolysaccharide (LPS), but was upregulated after 24 h. At 24 h following LPS stimulation, targeting SPHK2 with the inhibitor ABC294640, a specific siRNA or by using Sphk2^-/- mouse peritoneal macrophages increased inflammatory cytokine production. Downregulation of SPHK2 in primary human macrophages within 6 h of LPS treatment was blocked by inhibiting autophagy. SPHK2 overexpression or inhibiting autophagy 6 h after human macrophage activation with LPS suppressed inflammatory cytokine release. Mechanistically, SPHK2 suppressed LPS-triggered NF-κB activation independent of its catalytic activity and prevented increased mitochondrial ROS formation downstream of LPS. In conclusion, SPHK2 is an anti-inflammatory protein in human macrophages that is inversely coupled to inflammatory cytokine production. This needs consideration when targeting SPHK2 with specific inhibitors.

Sphingosine kinase and sphingosine-1-phosphate regulate epithelial cell architecture by the modulation of de novo sphingolipid synthesis

Sphingolipids regulate several aspects of cell behavior and it has been demonstrated that cells adjust their sphingolipid metabolism in response to metabolic needs. Particularly, sphingosine-1-phosphate (S1P), a final product of sphingolipid metabolism, is a potent bioactive lipid involved in the regulation of various cellular processes, including cell proliferation, cell migration, actin cytoskeletal reorganization and cell adhesion. In previous work in rat renal papillae, we showed that sphingosine kinase (SK) expression and S1P levels are developmentally regulated and control de novo sphingolipid synthesis. The aim of the present study was to evaluate the participation of SK/S1P pathway in the triggering of cell differentiation by external hypertonicity. We found that hypertonicity evoked a sharp decrease in SK expression, thus activating the de novo sphingolipid synthesis pathway. Furthermore, the inhibition of SK activity evoked a relaxation of cell-cell adherens junction (AJ) with accumulation of the AJ complex (E-cadherin/β-catenin/α-catenin) in the Golgi complex, preventing the acquisition of the differentiated cell phenotype. This phenotype alteration was a consequence of a sphingolipid misbalance with an increase in ceramide levels. Moreover, we found that SNAI1 and SNAI2 were located in the cell nucleus with impairment of cell differentiation induced by SK inhibition, a fact that is considered a biochemical marker of epithelial to mesenchymal transition. So, we suggest that the expression and activity of SK1, but not SK2, act as a control system, allowing epithelial cells to synchronize the various branches of sphingolipid metabolism for an adequate cell differentiation program.

S1P mediates human amniotic cells proliferation induced by a 50-Hz magnetic field exposure via ERK1/2 signaling pathway

Extremely low frequency electromagnetic field (ELF-EMF) is a kind of physical stimulus in public and occupational environment. Numerous studies have indicated that exposure of cells to ELF-EMF could promote cell proliferation. But the detailed mechanisms implicated in these proliferative processes remain unclear. In the present experiment, the possible roles of sphingosine-1-phosphate (S1P) in 50-Hz magnetic field (MF)-induced cell proliferation were investigated. Results showed that exposure of human amniotic (FL) cells to a 50-Hz MF with an intensity of 0.4 mT significantly enhanced ceramide metabolism, increased S1P production, activated extracellular signal regulated kinase 1/2 (ERK1/2), and promoted cell proliferation. All of these effects induced by MF exposure could be inhibited by SKI II, an inhibitor of sphingosine kinase (SphK). In addition, both the cell proliferative response and the ERK1/2 activation induced by MF exposure were blocked completely by U0126, a specific inhibitor of MEK (ERK kinases 1 and 2). Taken together, the findings in present study suggested that S1P mediated 50-Hz MF-induced cell proliferation via triggering ERK1/2 signal pathway.

The Role of S1P and the Related Signaling Pathway in the Development of Tissue Fibrosis

Tissue fibrosis, including pulmonary fibrosis, hepatic fibrosis, and cardiac fibrosis, is an important stage in the development of many diseases. It can lead to structural damage and dysfunction and even severe carcinogenesis or death. There is currently no effective method for the treatment of fibrosis. At present, the molecular mechanism of tissue fibrosis has not yet been fully elucidated, but many studies have demonstrated that it is involved in conveying the complex messages between fibroblasts and various cytokines. Sphingosine 1-phosphate (S1P) is a naturally bioactive sphingolipid. S1P and the related signaling pathways are important intracellular metabolic pathways involved in many life activities, including cell proliferation, differentiation, apoptosis, and cellular signal transduction. Increasing evidence suggests that S1P and its signaling pathways play an important role in the development of tissue fibrosis; however, the mechanisms of these effects have not yet been fully elucidated, and even the role of S1P and its signaling pathways are still controversial. This article focuses on the role of S1P and the related signaling pathways in the development of fibrosis of lung, liver, heart, and other tissues, with emphasis on the application of inhibitors of some of molecules in the pathway in clinical treatment of fibrosis diseases.

Quercetin ameliorates pulmonary fibrosis by inhibiting SphK1/S1P signaling

Idiopathic pulmonary fibrosis is an agnogenic chronic disorder with high morbidity and low survival rate. Quercetin is a flavonoid found in a variety of herbs with anti-fibrosis function. In this study, bleomycin was employed to induce a pulmonary fibrosis mouse model. The quercetin administration ameliorated bleomycin-induced pulmonary fibrosis, evidenced by the expression level changes of hydroxyproline, fibronectin, α-smooth muscle actin, Collagen I, and Collagen III. Similar results were observed in transforming growth factor (TGF)-β-treated human embryonic lung fibroblast (HELF). The bleomycin or TGF-β administration caused the increase of sphingosine-1-phosphate (S1P) level in pulmonary tissue and HELF cells, as well as its activation-required kinase, sphingosine kinase 1 (SphK1), and its degradation enzyme, sphinogosine-1-phosphate lyase (S1PL). However, the increase of S1P, SphK1, and S1PL was attenuated by application of quercetin. In addition, the effect of quercetin on fibrosis was abolished by the ectopic expression of SphK1. The colocalization of SphK1/S1PL and fibroblast specific protein 1 (FSP1) suggested the roles of fibroblasts in pulmonary fibrosis. In summary, we demonstrated that quercetin ameliorated pulmonary fibrosis in vivo and in vitro by inhibiting SphK1/S1P signaling.

Involvement of released sphingosine 1-phosphate/sphingosine 1-phosphate receptor axis in skeletal muscle atrophy

Skeletal muscle (SkM) atrophy is caused by several and heterogeneous conditions, such as cancer, neuromuscular disorders and aging. In most types of SkM atrophy overall rates of protein synthesis are suppressed, protein degradation is consistently elevated and atrogenes, such as the ubiquitin ligase Atrogin-1/MAFbx, are up-regulated. The molecular regulators of SkM waste are multiple and only in part known. Sphingolipids represent a class of bioactive molecules capable of modulating the destiny of many cell types, including SkM cells. In particular, we and others have shown that sphingosine 1phosphate (S1P), formed by sphingosine kinase (SphK), is able to act as trophic and morphogenic factor in myoblasts. Here, we report the first evidence that the atrophic phenotype observed in both muscle obtained from mice bearing the C26 adenocarcinoma and C2C12 myotubes treated with dexamethasone was characterized by reduced levels of active phospho-SphK1. The importance of SphK1 activity is also confirmed by the specific pharmacological inhibition of SphK1 able to increase Atrogin-1/MAFbx expression and reduce myotube size and myonuclei number. Furthermore, we found that SkM atrophy was accomplished by significant increase of S1P transporter Spns2 and in changes in the pattern of S1P receptor (S1PRs) subtype expression paralleled by increased Atrogin-1/MAFbx expression, suggesting a role for the released S1P and of specific S1PR-mediated signaling pathways in the control of the ubiquitin ligase. Altogether, these findings provide the first evidence that SphK1/released S1P/S1PR axis acts as a molecular regulator of SkM atrophy, thereby representing a new possible target for therapy in many patho-physiological conditions.