Involvement of phospholipase D in sphingosine 1-phosphate-induced activation of phosphatidylinositol 3-kinase and Akt in Chinese hamster ovary cells overexpressing EDG3

Deciphering S1P downregulation and sphingolipid homeostasis disruption in fungal keratitis via multi-omics and MALDI-MSI analysis

PURPOSE

The absence of effective treatment strategies in Fungal Keratitis (FK) emphasizes the critical need to understand the pathogenic mechanisms to enhance therapeutic outcomes. Sphingolipids have been proved to play a pivotal role in the pathogenesis of fungal infections, but the specific alteration in sphingolipids and regulatory pathways remain elusive. Our aim is to gain insight into the pathophysiological mechanisms of sphingolipid homeostasis in FK through multi-omics analysis.

METHODS

Matrix-assisted laser desorption/ionization-mass spectrometry imaging (MALDI-MSI) was performed in FK patients and mouse model. Furthermore, time-course RNA-seq was performed and Weighted gene co-expression network analysis (WGCNA) was used to reveal the driver genes in FK. We further investigated the effect of FTY-720, a mimetic of sphingosine 1-phosphate (S1P), on the progression of FK.

RESULTS

MALDI-MSI analysis of FK patients revealed a downregulation of sphingolipids, with sphingolipid metabolism identified as the most prominently enriched pathway. These alterations were validated in mouse model, in which S1P, ceramide, ceramide 1-phosphate and sphingomyelin were found to be downregulated. Time-course transcriptomic analysis suggests that degradation of sphingolipids by specific enzymes drives the progression of FK, involving phospholipid degradation, downregulation of TOR pathway, and activation of innate immune response. Consequently, epithelial cell function was inhibited and cell death increased. Importantly, restoring sphingolipid homeostasis by FTY-720 reversed the level of S1P and relieved the progression of FK.

CONCLUSION

In summary, this study reveals that disruption of sphingolipid homeostasis promotes disease progression in FK. Furthermore, restoring sphingolipid homeostasis emerges as a promising strategy to mitigate the progression of FK.

The role of sphingosine-1-phosphate in autophagy and related disorders

S1P, also referred to as sphingosine-1-phosphate, is a lipid molecule with bioactive properties involved in numerous cellular processes such as cell growth, movement, programmed cell death, self-degradation, cell specialization, aging, and immune system reactions. Autophagy is a meticulously controlled mechanism in which cells repurpose their elements to maintain cellular balance. There are five stages in autophagy: initiation, nucleation, elongation and maturation, fusion, and degradation. New research has provided insight into the complex connection between S1P and autophagy, uncovering their interaction in both normal and abnormal circumstances. Gaining knowledge about the regulatory mechanism of S1P signaling on autophagy can offer a valuable understanding of its function in well-being and illness, potentially leading to innovative therapeutic concepts for diverse ailments. Hence, this review analyzes the essential stages in mammalian autophagy, with a specific emphasis on recent research exploring the control of each stage by S1P. Additionally, it sheds light on the roles of S1P-induced autophagy in various disorders.

Potential of a sphingosine 1‑phosphate receptor antagonist and sphingosine kinase inhibitors as targets for multiple myeloma treatment

Sphingosine 1-phosphate (S1P) is a bioactive lipid involved in cancer progression through its binding to S1P receptors (S1PRs). However, the association between multiple myeloma (MM) and S1P is unclear. The current study aimed to investigate the potential anti-cancer effects of fingolimod and sphingosine kinase (SK) inhibitors in myeloma cells and the effects of S1P-induced chemoresistance and neovascularization on MM cell proliferation. MM cell lines were treated with the S1PR1 antagonist fingolimod and the SK inhibitors ABC294640 and SK1-I, after which cell proliferation was measured. Protein expression was also assessed under each condition using immunoblotting. Serum S1P levels in patients with MM, monoclonal gammopathy of undetermined significance and healthy volunteers were assessed. Human umbilical vessel cells (HUVECs) were co-cultured with anti-S1P agents to assess the effect on cell migration. All treatments suppressed myeloma cell proliferation and caspase-3-mediated apoptosis by suppressing S1P activity. These findings suggest that S1P activation is associated with proliferation and survival for MM cells. S1P attenuated the proteosome inhibitor (PI) effect, while the anti-S1P agents recovered the effect. In addition, S1P promoted the migration and proliferation of HUVECs, whereas the S1P inhibitors reduced the influence of S1P. This study highlights the therapeutic potential of anti-S1P agents for MM treatment. Inhibition of S1P function may overcome resistance to PI developed by myeloma cells and inhibit the changes to the bone marrow microenvironment via neovascularization.

The Cross-Talk Between Sphingolipids and Insulin-Like Growth Factor Signaling: Significance for Aging and Neurodegeneration

Bioactive sphingolipids: sphingosine, sphingosine-1-phosphate (S1P), ceramide, and ceramide-1-phosphate (C1P) are increasingly implicated in cell survival, proliferation, differentiation, and in multiple aspects of stress response in the nervous system. The opposite roles of closely related sphingolipid species in cell survival/death signaling is reflected in the concept of tightly controlled sphingolipid rheostat. Aging has a complex influence on sphingolipid metabolism, disturbing signaling pathways and the properties of lipid membranes. A metabolic signature of stress resistance-associated sphingolipids correlates with longevity in humans. Moreover, accumulating evidence suggests extensive links between sphingolipid signaling and the insulin-like growth factor I (IGF-I)-Akt-mTOR pathway (IIS), which is involved in the modulation of aging process and longevity. IIS integrates a wide array of metabolic signals, cross-talks with p53, nuclear factor κB (NF-κB), or reactive oxygen species (ROS) and influences gene expression to shape the cellular metabolic profile and stress resistance. The multiple connections between sphingolipids and IIS signaling suggest possible engagement of these compounds in the aging process itself, which creates a vulnerable background for the majority of neurodegenerative disorders.

The Role of Ceramide and Sphingosine-1-Phosphate in Alzheimer's Disease and Other Neurodegenerative Disorders

Bioactive sphingolipids-ceramide, sphingosine, and their respective 1-phosphates (C1P and S1P)-are signaling molecules serving as intracellular second messengers. Moreover, S1P acts through G protein-coupled receptors in the plasma membrane. Accumulating evidence points to sphingolipids' engagement in brain aging and in neurodegenerative disorders such as Alzheimer's, Parkinson's, and Huntington's diseases and amyotrophic lateral sclerosis. Metabolic alterations observed in the course of neurodegeneration favor ceramide-dependent pro-apoptotic signaling, while the levels of the neuroprotective S1P are reduced. These trends are observed early in the diseases' development, suggesting causal relationship. Mechanistic evidence has shown links between altered ceramide/S1P rheostat and the production, secretion, and aggregation of amyloid β/α-synuclein as well as signaling pathways of critical importance for the pathomechanism of protein conformation diseases. Sphingolipids influence multiple aspects of Akt/protein kinase B signaling, a pathway that regulates metabolism, stress response, and Bcl-2 family proteins. The cross-talk between sphingolipids and transcription factors including NF-κB, FOXOs, and AP-1 may be also important for immune regulation and cell survival/death. Sphingolipids regulate exosomes and other secretion mechanisms that can contribute to either the spread of neurotoxic proteins between brain cells, or their clearance. Recent discoveries also suggest the importance of intracellular and exosomal pools of small regulatory RNAs in the creation of disturbed signaling environment in the diseased brain. The identified interactions of bioactive sphingolipids urge for their evaluation as potential therapeutic targets. Moreover, the early disturbances in sphingolipid metabolism may deliver easily accessible biomarkers of neurodegenerative disorders.

Roles of sphingosine-1-phosphate in reproduction

Sphingosine-1-phosphate (S1P) plays crucial roles in the regulation of cell growth, proliferation, differentiation, cell survival, migration, and angiogenesis. In the reproductive system, S1P protects mammalian germ cells from irradiation or chemotherapy-induced cell death in vivo and in vitro. Moreover, S1P could improve the survival rate of thawed ovary and transplanted ovary. Furthermore, S1P could improve the developmental potential of oocyte and preimplantation embryo. In conclusion, S1P plays important roles in reproduction.

The ral exchange factor rgl2 promotes cardiomyocyte survival and inhibits cardiac fibrosis

Cardiomyocytes compensate to acute cardiac stress by increasing in size and contractile function. However, prolonged stress leads to a decompensated response characterized by cardiomyocyte death, tissue fibrosis and loss of cardiac function. Identifying approaches to inhibit this transition to a decompensated response may reveal important targets for treating heart failure. The Ral guanine nucleotide disassociation (RalGDS) proteins are Ras-interacting proteins that are upregulated by hypertrophic stimuli. The Ral guanine nucleotide dissociation stimulator-like 2 (Rgl2) is a member of the RalGDS family that modulates expression of hypertrophic genes in cardiomyocytes. However, the pathophysiologic consequence of increased Rgl2 expression in cardiomyoctyes remains unclear. To evaluate the effect of increasing Rgl2 activity in the heart, transgenic mice with cardiac-targeted over-expression of Rgl2 were generated. Although Ral activation was increased, there were no apparent morphologic or histological differences between the hearts of Rgl2 transgenic and nontransgenic mice indicating that increased Rgl2 expression had no effect on basal cardiac phenotype. To determine if Rgl2 modulates the cardiac response to stress, mice were infused with the ß-adrenergic receptor agonist, isoproterenol. Isoproterenol infusion increased heart mass in both Rgl2 transgenic and nontransgenic mice. However, unlike nontransgenic mice, Rgl2 transgenic mice showed no morphologic evidence of cardiomyocyte damage or increased cardiac fibrosis following isoproterenol infusion. Increased Rgl2 expression in cultured cardiomyocytes stimulated Ral activation and inhibited staurosporine-induced apoptosis via increased activation of PI3-kinase. Activation of the PI3-kinase signaling pathway was confirmed in hearts isolated from Rgl2 transgenic mice. Increased expression and function of Rgl2 in cardiomyocytes promotes activation of the PI3-kinase signaling cascade and protects from carciomyocyte death and pathologic cardiac fibrosis. Taken further, these results suggest that Rgl2 upregulation in hypertrophic hearts may be a protetive mechanism, and that Rgl2 may be a novel therapeutic target in treating heart disease.

Dissecting the PI3K Signaling Axis in Pediatric Solid Tumors: Novel Targets for Clinical Integration

Children with solid tumors represent a unique population. Recent improvements in pediatric solid tumor survival rates have been confined to low- and moderate-risk cancers, whereas minimal to no notable improvement in survival have been observed in high-risk and advanced-stage childhood tumors. Treatments for patients with advanced disease are rarely curative, and responses to therapy are often followed by relapse, which highlights the large unmet need for novel therapies. Recent advances in cancer treatment have focused on personalized therapy, whereby patients are treated with agents that best target the molecular drivers of their disease. Thus, a better understanding of the pathways that drive cancer or drug resistance is of critical importance. One such example is the phosphatidylinositol 3-kinase (PI3K)/Akt/mammalian target of rapamycin (mTOR) pathway, which is activated in many solid cancer patients and represents a target for therapy. PI3K/Akt/mTOR pathway activation has also been observed in tumors resistant to agents targeting upstream receptor tyrosine kinases (RTKs). Agents that target this pathway have the potential to shut down survival pathways, and are being explored both in the setting of pathway-activating mutations and for their ability to restore sensitivity to upstream signaling targeted agents. Here, we examine the role of the PI3K/Akt/mTOR pathway in pediatric solid tumors, review the novel agents being explored to target this pathway, and explore the potential role of the inhibition of this pathway in the clinical development of these agents in children.

Fer kinase limits neutrophil chemotaxis toward end target chemoattractants

Neutrophil recruitment and directional movement toward chemotactic stimuli are important processes in innate immune responses. This study examines the role of Fer kinase in neutrophil recruitment and chemotaxis to various chemoattractants in vitro and in vivo. Mice targeted with a kinase-inactivating mutation (Fer(DR/DR)) or wild type (WT) were studied using time-lapse intravital microscopy to examine leukocyte recruitment and chemotaxis in vivo. In response to keratinocyte-derived cytokine, no difference in leukocyte chemotaxis was observed between WT and Fer(DR/DR) mice. However, in response to the chemotactic peptide WKYMVm, a selective agonist of the formyl peptide receptor, a 2-fold increase in leukocyte emigration was noted in Fer(DR/DR) mice (p < 0.05). To determine whether these defects were due to Fer signaling in the endothelium or other nonhematopoietic cells, bone marrow chimeras were generated. WKYMVm-induced leukocyte recruitment in chimeric mice (WT bone marrow to Fer(DR/DR) recipients or vice versa) was similar to WT mice, suggesting that Fer kinase signaling in both leukocytes and endothelial cells serves to limit chemotaxis. Purified Fer(DR/DR) neutrophils demonstrated enhanced chemotaxis toward end target chemoattractants (WKYMVm and C5a) compared with WT using an under-agarose gel chemotaxis assay. These defects were not observed in response to intermediate chemoattractants (keratinocyte-derived cytokine, MIP-2, or LTB(4)). Increased WKYMVm-induced chemotaxis of Fer(DR/DR) neutrophils correlated with sustained PI3K activity and reduced reliance on the p38 MAPK pathway compared with WT neutrophils. Together, these data identify Fer as a novel inhibitory kinase for neutrophil chemotaxis toward end target chemoattractants through modulation of PI3K activity.

Regulation of autophagy and its associated cell death by "sphingolipid rheostat": reciprocal role of ceramide and sphingosine 1-phosphate in the mammalian target of rapamycin pathway

The role of "sphingolipid rheostat" by ceramide and sphingosine 1-phosphate (S1P) in the regulation of autophagy remains unclear. In human leukemia HL-60 cells, amino acid deprivation (AA(-)) caused autophagy with an increase in acid sphingomyleinase (SMase) activity and ceramide, which serves as an autophagy inducing lipid. Knockdown of acid SMase significantly suppressed the autophagy induction. S1P treatment counteracted autophagy induction by AA(-) or C(2)-ceramide. AA(-) treatment promoted mammalian target of rapamycin (mTOR) dephosphorylation/inactivation, inducing autophagy. S1P treatment suppressed mTOR inactivation and autophagy induction by AA(-). S1P exerts biological actions via cell surface receptors, and S1P(3) among five S1P receptors was predominantly expressed in HL-60 cells. We evaluated the involvement of S1P(3) in suppressing autophagy induction. S1P treatment of CHO cells had no effects on mTOR inactivation and autophagy induction by AA(-) or C(2)-ceramide. Whereas S1P treatment of S1P(3) overexpressing CHO cells resulted in activation of the mTOR pathway, preventing cells from undergoing autophagy induced by AA(-) or C(2)-ceramide. These results indicate that S1P-S1P(3) plays a role in counteracting ceramide signals that mediate mTOR-controlled autophagy. In addition, we evaluated the involvement of ceramide-activated protein phosphatases (CAPPs) in ceramide-dependent inactivation of the mTOR pathway. Inhibition of CAPP by okadaic acid in AA(-)- or C(2)-ceramide-treated cells suppressed dephosphorylation/inactivation of mTOR, autophagy induction, and autophagy-associated cell death, indicating a novel role of ceramide-CAPPs in autophagy induction. Moreover, S1P(3) engagement by S1P counteracted cell death. Taken together, these results indicated that sphingolipid rheostat in ceramide-CAPPs and S1P-S1P(3) signaling modulates autophagy and its associated cell death through regulation of the mTOR pathway.

New insights into p53 signaling and cancer cell response to DNA damage: implications for cancer therapy

Activation of the p53 signaling pathway by DNA-damaging agents was originally proposed to result either in cell cycle checkpoint activation to promote survival or in apoptotic cell death. This model provided the impetus for numerous studies focusing on the development of p53-based cancer therapies. According to recent evidence, however, most p53 wild-type human cell types respond to ionizing radiation by undergoing stress-induced premature senescence (SIPS) and not apoptosis. SIPS is a sustained growth-arrested state in which cells remain viable and secrete factors that may promote cancer growth and progression. The p21(WAF1) (hereafter p21) protein has emerged as a key player in the p53 pathway. In addition to its well-studied role in cell cycle checkpoints, p21 regulates p53 and its upstream kinase (ATM), controls gene expression, suppresses apoptosis, and induces SIPS. Herein, we review these and related findings with human solid tumor-derived cell lines, report new data demonstrating dynamic behaviors of p53 and p21 in the DNA damage response, and examine the gain-of-function properties of cancer-associated p53 mutations. We point out obstacles in cancer-therapeutic strategies that are aimed at reactivating the wild-type p53 function and highlight some alternative approaches that target the apoptotic threshold in cancer cells with differing p53 status.

Neutral sphingomyelinase 2 deficiency increases hyaluronan synthesis by up-regulation of Hyaluronan synthase 2 through decreased ceramide production and activation of Akt

Fibroblasts from the fro/fro mouse, with a deletion in the Smpd3 gene coding for the active site of neutral sphingomyelinase 2 (NSMase2), secreted increased amounts of hyaluronan (HA). This was reversed by transfection with the Smpd3 gene, suggesting a connection between sphingolipid and glycosaminoglycan metabolism. The deficiency of NSMase2 resulted in storage of sphingomyelin (SM) and cholesterol with a 50% reduction in ceramides (Cer). RT-PCR and Western blot analysis showed that increased HA secretion resulted from increased hyaluronan synthase 2 (HAS2) activity localized to sphingolipid-enriched lipid rafts. Although cholesterol levels were also elevated in lipid rafts from mouse fibroblasts deficient in lysosomal acid SMase activity (deletion of the Smpd1(-/-) gene), there was no increase in HA secretion. We then showed that in fro/fro fibroblasts, the reduced ceramide was associated with decreased phosphorylation of protein phosphatase 2A (PP2A) and increased phosphorylation of its substrate Akt-p, together with PI3K, PDK1, mTOR (mammalian target of rapamycin), and p70S6K, although PTEN was unaffected. Exogenous ceramide, as well as inhibitors of Akt (Akt inhibitor VIII), PI 3-kinase (LY294002 and wortmannin), and mTOR (rapamycin) reduced secretion of HA, whereas the NSMase2 inhibitor GW4869 increased HA synthesis and secretion. We propose that NSMase2/Cer are the key mediators of the regulation of HA synthesis, via microdomains and the Akt/mTOR pathway.

Essential roles of sphingosine 1-phosphate receptor types 1 and 3 in human hepatic stellate cells motility and activation

The biological roles of sphingosine 1-phosphate (S1P) and S1P receptors (S1PRs) have been broadly investigated. However, at present pathophysiological roles of S1P/S1PRs axis in liver fibrosis are not well defined. Here, we investigated the functions of S1P/S1PRs axis in human hepatic stellate cells (HSC) line, LX-2 cells. We found that S1PR types 1, 2 and 3 (S1PR1-3) are clearly detected in LX-2 cells, as determined by RT-PCR, Western blot and immunocytochemistry analysis. S1P exerted a powerful migratory action on LX-2 cells, as determined in Boyden chambers, and stimulated fibrogenic activity of LX-2 cells, as demonstrated by increase of expression of smooth muscle α-actin, procollagen α1(I) and α1(III) and total hydroxyproline content. Moreover, the effects of S1P were mimicked by S1PR1 agonist SEW2871, and abrogated by W146 (S1PR1 antagonist) and/or silencing S1PR1, three expression with small interfering RNA, suggesting the main roles of S1PR1 and 3. However, studies with S1PR2 antagonist JTE-013 and silencing S1PR2 expression indicated that S1PR2 negatively regulated S1P-induced cell migration. Interestingly, exogenously added S1P induced significant up-regulation of sphingosine kinase-1 and the synthesis of additional S1P, and expression of S1PR1,3, but not S1PR2. In conclusion, our data have identified an additional function regulated by S1P/S1PR1,3 axis involving migration and fibrogenic activation of HSCs. These results suggest that selective modulation of S1PR activity may represent a new antifibrotic strategy.

Sphingosine 1-phosphate (S1P)/S1P receptors are involved in human liver fibrosis by action on hepatic myofibroblasts motility

BACKGROUND & AIMS

Directed migration of hepatic myofibroblasts (hMFs) contributes to the development of liver fibrosis. However, the signals regulating the motility of these cells are incompletely understood. We have recently shown that sphingosine 1-phosphate (S1P) and S1P receptors (S1PRs) are involved in mouse liver fibrogenesis. Here, we investigated the role of S1P/S1PRs signals in human liver fibrosis involving motility of human hMFs.

METHODS

S1P level in the liver was examined by high-performance liquid chromatography. Expression of S1PRs was characterized, in biopsy specimens of human liver and cultured hMFs, by immunofluorescence and real-time RT-PCR or Western blot analysis. Cell migration was determined in Boyden chambers, by using the selective S1P receptor agonist or antagonist and silencing of S1PRs expression with small interfering RNA.

RESULTS

S1P level in the human fibrotic liver was increased through up-regulation of sphingosine kinase (SphK), irrespective of the etiology of fibrosis. S1P receptors type 1, 2, and 3 (S1P(1,2,3)) were expressed in human hMFs in vivo and in vitro. Interestingly, S1P(1,3) were strongly induced in human fibrotic samples, whereas expression of S1P(2) was massively decreased. S1P exerted a powerful migratory action on human hMFs. Furthermore, the effect of S1P was mimicked by SEW2871 (an S1P(1) agonist), and blocked by suramin (an S1P(3) antagonist) and by silencing S1P(1,3) expression. In contrast, JTE-013 (an S1P(2) antagonist) and silencing of S1P(2) expression enhanced S1P-induced migration.

CONCLUSIONS

SphK/S1P/S1PRs signaling axis plays an important role in human liver fibrosis and is involved in the directed migration of human hMFs into the damaged areas.

Activation of cytosolic phospholipase A2 downstream of the Src-phospholipase D1 (PLD1)-protein kinase C γ (PKCγ) signaling axis is required for hypoxia-induced pathological retinal angiogenesis

In view of understanding the mechanisms of retinal neovascularization, we had reported previously that vascular endothelial growth factor (VEGF)-induced pathological retinal angiogenesis requires the activation of Src-PLD1-PKCγ signaling. In the present work, we have identified cytosolic phospholipase A(2) (cPLA(2)) as an effector molecule of Src-PLD1-PKCγ signaling in the mediation of VEGF-induced pathological retinal angiogenesis based on the following observations. VEGF induced cPLA(2) phosphorylation in a time-dependent manner in human retinal microvascular endothelial cells (HRMVECs). VEGF also induced arachidonic acid (AA) release in a dose-, time-, and cPLA(2)-dependent manner. Depletion of cPLA(2) levels inhibited VEGF-induced HRMVEC DNA synthesis, migration, and tube formation. In addition, the exogenous addition of AA rescued VEGF-induced HRMVEC DNA synthesis, migration, and tube formation from inhibition by down-regulation of cPLA(2). Inhibition of Src, PLD1, or PKCγ attenuated VEGF-induced cPLA(2) phosphorylation and AA release. Consistent with these findings, hypoxia induced cPLA(2) phosphorylation and activity in VEGF-Src-PLD1-PKCγ-dependent manner in a mouse model of oxygen-induced retinopathy. In addition, siRNA-mediated down-regulation of cPLA(2) levels in the retina abrogated hypoxia-induced retinal endothelial cell proliferation and neovascularization. These observations suggest that cPLA(2)-dependent AA release is required for VEGF-induced Src-PLD1-PKCγ-mediated pathological retinal angiogenesis.

PLD1-dependent PKCgamma activation downstream to Src is essential for the development of pathologic retinal neovascularization

Vascular endothelial growth factor (VEGF) appears to be an important mediator of pathologic retinal angiogenesis. In understanding the mechanisms of pathologic retinal neovascularization, we found that VEGF activates PLD1 in human retinal microvascular endothelial cells, and this event is dependent on Src. In addition, VEGF activates protein kinase C-gamma (PKCgamma) via Src-dependent PLD1 stimulation. Inhibition of Src, PLD1, or PKCgamma via pharmacologic, dominant negative mutant, or siRNA approaches significantly attenuated VEGF-induced human retinal microvascular endothelial cell migration, proliferation, and tube formation. Hypoxia also induced Src-PLD1-PKCgamma signaling in retina, leading to retinal neovascularization. Furthermore, siRNA-mediated down-regulation of VEGF inhibited hypoxia-induced Src-PLD1-PKCgamma activation and neovascularization. Blockade of Src-PLD1-PKCgamma signaling via the siRNA approach also suppressed hypoxia-induced retinal neovascularization. Thus, these observations demonstrate, for the first time, that Src-dependent PLD1-PKCgamma activation plays an important role in pathologic retinal angiogenesis.

Cancer treatment strategies targeting sphingolipid metabolism

Ceramide and sphingosine-1-phosphate are related sphingolipid metabolites that can be generated through a de novo biosynthetic route or derived from the recycling of membrane sphingomyelin. Both these lipids regulate cellular responses to stress, with generally opposing effects. Sphingosine-1-phosphate functions as a growth and survival factor, acting as a ligand for a family of G protein-coupled receptors, whereas ceramide activates intrinsic and extrinsic apoptotic pathways through receptor-independent mechanisms. A growing body of evidence has implicated ceramide, sphingosine-1-phosphate and the genes involved in their synthesis, catabolism and signaling in various aspects of oncogenesis, cancer progression and drug- and radiation resistance. This may be explained in part by the finding that both lipids impinge upon the PI3K/ AKT pathway, which represses apoptosis and autophagy. In addition, sphingolipids influence cell cycle progression, telomerase function, cell migration and stem cell biology. Considering the central role of ceramide in mediating physiological as well as pharmacologically stimulated apoptosis, ceramide can be considered a tumor-suppressor lipid. In contrast, sphingosine-1-phosphate can be considered a tumor-promoting lipid, and the enzyme responsible for its synthesis functions as an oncogene. Not surprisingly, genetic mutations that result in reduced ceramide generation, increased sphingosine-1-phosphate synthesis or which reduce steady state ceramide levels and increase sphingosine-1-phosphate levels have been identified as mechanisms of tumor progression and drug resistance in cancer cells. Pharmacological tools for modulating sphingolipid pathways are being developed and represent novel therapeutic strategies for the treatment of cancer.

Homing of bone marrow mesenchymal stem cells mediated by sphingosine 1-phosphate contributes to liver fibrosis

BACKGROUND/AIMS

Myofibroblasts play a central role in the pathogenesis of liver fibrosis. Myofibroblasts of bone marrow (BM) origin have recently been identified in fibrotic liver. However, little is known about the mechanism that controls their mobilization in vivo. Here we confirmed that BM mesenchymal stem cells (BMSCs) can migrate to the damaged liver and differentiate into myofibroblasts. We also investigated the mechanism underlying the homing of BMSCs after liver injury.

METHODS

ICR mice were lethally irradiated and received BM transplants from enhanced green fluorescent protein transgenic mice. Carbon tetrachloride or bile duct ligation was used to induce liver fibrosis. The fibrotic liver tissue was examined by immunofluorescent staining to identify BM-derived myofibroblasts.

RESULTS

BMSCs contributed significantly to myofibroblast population in fibrotic liver. Moreover, analysis in vivo and in vitro suggested that homing of BMSCs to the damaged liver was in response to sphingosine 1-phosphate (S1P) gradient between liver and BM. Furthermore, S1P receptor type 3 (S1P3) was required for migration of BMSCs triggered by S1P.

CONCLUSIONS

S1P mediates liver fibrogenesis through homing of BMSCs via S1P3 receptor, which may represent a novel therapeutic target in liver fibrosis through inhibiting S1P formation and/or receptor activation.

Simultaneous inhibition of mitogen-activated protein kinase and phosphatidylinositol 3-kinase pathways augment the sensitivity to actinomycin D in Ewing sarcoma

PURPOSE

Ewing sarcoma cells, of which over 85% retain chimeric fusion gene EWS/Fli-1, are by and large more resistant to chemotherapeutics compared to nonneoplastic cells. The purpose of this study is to determine the role of EWS/Fli-1 fusion and its downstream targets regarding the cells' resistance against actinomycin D (ActD), which is one of the most commonly used antitumor agents in combination chemotherapy of Ewing sarcomas.

METHODS

Cytotoxicity was measured by WST-8 assay. Caspase-dependent and -independent cell death was examined by fluorescence microscope. Protein expression was analyzed by western blotting. Caspase activity was determined by Caspase-Glo assay.

RESULTS

ActD-induced caspase-dependent apoptotic cell death to Ewing sarcoma TC-135 cells in a dose- and time- dependent manner. Knockdown of EWS/Fli-1 fusion by siRNA resulted in enhancement of ActD-induced apoptosis. ActD treatment activated both mitogen-activated protein kinase kinase (MEK)/extracellular signal-regulated kinase (ERK) and phosphatidylinositol 3-kinase (PI3K)/Akt pathways although in a distinctive manner. Combined administration of U0126 (MEK inhibitor) and LY294002 (PI3K inhibitor) significantly enhanced ActD-induced apoptosis in vitro and suppressed xenograft tumor growth in vivo.

CONCLUSIONS

The present study demonstrated for the first time that combination of U0126 and LY294002 can augment the cytotoxicity of ActD against Ewing sarcoma cells in vitro and in vivo. Our results indicate that further study on combination of conventional chemotherapies with MEK and PI3K inhibitors may be considered for innovative treatments of Ewing sarcoma patients.

Physiological role for phosphatidic acid in the translocation of the novel protein kinase C Apl II in Aplysia neurons

In Aplysia californica, the serotonin-mediated translocation of protein kinase C (PKC) Apl II to neuronal membranes is important for synaptic plasticity. The orthologue of PKC Apl II, PKCepsilon, has been reported to require phosphatidic acid (PA) in conjunction with diacylglycerol (DAG) for translocation. We find that PKC Apl II can be synergistically translocated to membranes by the combination of DAG and PA. We identify a mutation in the C1b domain (arginine 273 to histidine; PKC Apl II-R273H) that removes the effects of exogenous PA. In Aplysia neurons, the inhibition of endogenous PA production by 1-butanol inhibited the physiological translocation of PKC Apl II by serotonin in the cell body and at the synapse but not the translocation of PKC Apl II-R273H. The translocation of PKC Apl II-R273H in the absence of PA was explained by two additional effects of this mutation: (i) the mutation removed C2 domain-mediated inhibition, and (ii) the mutation decreased the concentration of DAG required for PKC Apl II translocation. We present a model in which, under physiological conditions, PA is important to activate the novel PKC Apl II both by synergizing with DAG and removing C2 domain-mediated inhibition.

Signal transduction underlying the control of urinary bladder smooth muscle tone by muscarinic receptors and beta-adrenoceptors

The normal physiological contraction of the urinary bladder, which is required for voiding, is predominantly mediated by muscarinic receptors, primarily the M3 subtype, with the M2 subtype providing a secondary backup role. Bladder relaxation, which is required for urine storage, is mediated by beta-adrenoceptors, in most species involving a strong beta3-component. An excessive stimulation of contraction or a reduced relaxation of the detrusor smooth muscle during the storage phase of the micturition cycle may contribute to bladder dysfunction known as the overactive bladder. Therefore, interference with the signal transduction of these receptors may be a viable approach to develop drugs for the treatment of overactive bladder. The prototypical signaling pathway of M3 receptors is activation of phospholipase C (PLC), and this pathway is also activated in the bladder. Nevertheless, PLC apparently contributes only in a very minor way to bladder contraction. Rather, muscarinic-receptor-mediated bladder contraction involves voltage-operated Ca2+ channels and Rho kinase. The prototypical signaling pathway of beta-adrenoceptors is an activation of adenylyl cyclase with the subsequent formation of cAMP. Nevertheless, cAMP apparently contributes in a minor way only to beta-adrenoceptor-mediated bladder relaxation. BKCa channels may play a greater role in beta-adrenoceptor-mediated bladder relaxation. We conclude that apart from muscarinic receptor antagonists and beta-adrenoceptor agonists, inhibitors of Rho kinase and activators of BKCa channels may have potential to treat an overactive bladder.

Protein kinase C-epsilon regulates sphingosine 1-phosphate-mediated migration of human lung endothelial cells through activation of phospholipase D2, protein kinase C-zeta, and Rac1

The signaling pathways by which sphingosine 1-phosphate (S1P) potently stimulates endothelial cell migration and angiogenesis are not yet fully defined. We, therefore, investigated the role of protein kinase C (PKC) isoforms, phospholipase D (PLD), and Rac in S1P-induced migration of human pulmonary artery endothelial cells (HPAECs). S1P-induced migration was sensitive to S1P(1) small interfering RNA (siRNA) and pertussis toxin, demonstrating coupling of S1P(1) to G(i). Overexpression of dominant negative (dn) PKC-epsilon or -zeta, but not PKC-alpha or -delta, blocked S1P-induced migration. Although S1P activated both PLD1 and PLD2, S1P-induced migration was attenuated by knocking down PLD2 or expressing dnPLD2 but not PLD1. Blocking PKC-epsilon, but not PKC-zeta, activity attenuated S1P-mediated PLD stimulation, demonstrating that PKC-epsilon, but not PKC-zeta, was upstream of PLD. Transfection of HPAECs with dnRac1 or Rac1 siRNA attenuated S1P-induced migration. Furthermore, transfection with PLD2 siRNA, infection of HPAECs with dnPKC-zeta, or treatment with myristoylated PKC-zeta peptide inhibitor abrogated S1P-induced Rac1 activation. These results establish that S1P signals through S1P(1) and G(i) to activate PKC-epsilon and, subsequently, a PLD2-PKC-zeta-Rac1 cascade. Activation of this pathway is necessary to stimulate the migration of lung endothelial cells, a key component of the angiogenic process.

Mutation of Y179 on phospholipase D2 (PLD2) upregulates DNA synthesis in a PI3K-and Akt-dependent manner

Phospholipase D2 (PLD2), one of the two mammalian members of the PLD family, has been implicated in cell proliferation, transformation, tumor progression and survival. However, as precise mechanistic details are still unknown, we investigated here if the PLD2 isoform would signal through the PI3K/AKT pathway. Transient expression of PLD2 in COS7 cells with either the WT or with a Y179F mutant, resulted in an increased basal phosphorylation of AKT in residues T308 and S473, in a PI3K-dependent manner. Transfection of PLD2-Y179F (but not the wild type) caused an increased (>2-fold) DNA synthesis even in the absence of extracellular stimuli. Other signaling mechanisms downstream such PLD/PI3K dependence (that might lead to DNA synthesis regulation) were further studied. PLD2-Y179F caused an increase in phosphorylation of p42/p44 ERK and in the expression of G0/G1 phase transition markers (p21 CIP, PCNA), and these effects, too, were dependent on PI3K. Interestingly, Akt, once activated induced the phosphorylation of PLD2 on residue T175, an effect that was inhibited by LY296004. Lastly, if PLD2-Y179F is further mutated in residue K758 (PLD2 Y179F-K758R), which renders inactive a catalytic site, DNA synthesis is then abrogated, indicating that the activity of the enzyme (i.e. synthesis of PA) is necessary for the observed effects. In conclusion, the unavailability of residue Y179 on PLD2 to become phosphorylated leads to an augmentation of DNA synthesis concomitantly with MEK and AKT phosphorylation, in a process that is dependent on PI3K and independent of any extracellular stimuli. This might be critical for the maintenance of the PLD2-regulated proliferative status.

The potential for phospholipase D as a new therapeutic target

Mammalian phospholipase D (PLD), a signal transduction-activated enzyme, hydrolyzes phosphatidylcholine to generate the lipid second messenger phosphatidic acid (PA) and choline. Genetic and pharmacological methods have implicated PLD and its product PA in a wide variety of cellular processes including vesicle trafficking, receptor signaling, cell proliferation and survival. Dysregulation of these cell biologic processes occurs in a diverse range of illnesses including cancer. This review summarizes PLD regulation and function and highlights its potential as a therapeutic target in disease settings.

Phospholipase D2‐derived phosphatidic acid binds to and activates ribosomal p70 S6 kinase independently of mTOR

The product of phospholipase D (PLD) enzymatic action in cell membranes, phosphatidic acid (PA), regulates kinases implicated in NADPH oxidase activation, as well as the mammalian target of rapamycin (mTOR) kinase. However, other protein targets for this lipid second messenger must exist in order to explain other key PA-mediated cellular functions. In this study, PA was found to specifically and saturably bind to and activate recombinant and immunoprecipitated endogenous ribosomal S6 kinase (S6K) with a stoichiometry of 94:1 lipid/protein. Polyphosphoinositides PI4-P and PI4,5P2 and cardiolipin could also bind to and activate S6K, albeit with different kinetics. Conversely, PA with at least one acyl side chain saturated (10:0) was ineffective in binding or activating the enzyme. Transfection of COS-7 cells with a wild-type myc-(pcDNA)-PLD2 construct resulted in high PLD activity, concomitantly with an increase in ribosomal p70S6K enzyme activity and phosphorylation in T389 and T421/S424 as well as phosphorylation of p70S6K's natural substrate S6 protein in S235/S236. Overexpression of a lipase inactive mutant (K758R), however, failed to induce an increase in both PLD and S6K activity or phosphorylation, indicating that the enzymatic activity of PLD2 (i.e., synthesis of PA) must be present to affect S6K. Neither inhibiting mTOR kinase activity with rapamycin nor silencing mTOR gene expression altered the augmentative effect of PLD2 exerted on p70S6K activity. This finding indicates that PA binds to and activates p70S6K, even in the absence of mTOR. Lastly, COS-7 transfection with PLD2 changed the pattern of subcellular expression, and a colocalization of S6K and PLD2 was observed by immunofluorescence microscopy. These results show for the first time a direct (mTOR-independent) participation of PLD in the p70S6K pathway and implicate PA as a nexus that brings together cell phospholipases and kinases.

Critical pathways in heart function: bis(2-chloroethoxy)methane-induced heart gene transcript change in F344 rats

Gene transcript changes after exposure to the heart toxin, bis(2-chloroethoxy)methane (CEM), were analyzed to elucidate mechanisms in cardiotoxicity and recovery. CEM was administered to 5-week-old male F344/N rats at 0, 200, 400, or 600 mg/kg by dermal exposure, 5 days per week, for a total of 12 doses by study day 16. Heart toxicity occurred after 2 days of dosing in all 3 regions of the heart (atrium, ventricle, interventricular septum) and was characterized by myofiber vacuolation, necrosis, mononuclear-cell infiltration, and atrial thrombosis. Ultrastructural analysis revealed that the primary site of damage was the mitochondrion. By day 5, even though dosing was continued, the toxic lesions in the heart began to resolve, and by study day 16, the heart appeared histologically normal. RNA was extracted from whole hearts after 2 or 5 days of CEM dosing. After a screen for transcript change by microarray analysis, dose-response trends for selected transcripts were analyzed by qRT-PCR. The selected transcripts code for proteins involved in energy production, control of calcium levels, and maintenance of heart function. The down-regulation of ATP subunit transcripts (Atp5j, ATP5k), which reside in the mitochondrial membranes, indicated a decrease in energy supply at day 2 and day 5. This was accompanied by down-regulation of transcripts involved in high-energy consumption processes such as membrane transport and ion channel transcripts (e.g., abc1a, kcnj12). The up-regulation of transcripts encoding for temperature regulation and calcium binding proteins (ucp1 and calb3) only at the 2 low exposure levels, suggest that these adaptive processes cannot occur in association with severe cardiotoxicity as seen in hearts at the high exposure level. Transcript expression changes occurred within 2 days of CEM exposure, and were dose-and time-dependent. The heart transcript changes suggest that CEM cardiotoxicity activates protective processes associated energy conservation and maintenance of heart function.

Text mining of full-text journal articles combined with gene expression analysis reveals a relationship between sphingosine-1-phosphate and invasiveness of a glioblastoma cell line

Background

Sphingosine 1-phosphate (S1P), a lysophospholipid, is involved in various cellular processes such as migration, proliferation, and survival. To date, the impact of S1P on human glioblastoma is not fully understood. Particularly, the concerted role played by matrix metalloproteinases (MMP) and S1P in aggressive tumor behavior and angiogenesis remains to be elucidated.

Results

To gain new insights in the effect of S1P on angiogenesis and invasion of this type of malignant tumor, we used microarrays to investigate the gene expression in glioblastoma as a response to S1P administration in vitro. We compared the expression profiles for the same cell lines under the influence of epidermal growth factor (EGF), an important growth factor. We found a set of 72 genes that are significantly differentially expressed as a unique response to S1P. Based on the result of mining full-text articles from 20 scientific journals in the field of cancer research published over a period of five years, we inferred gene-gene interaction networks for these 72 differentially expressed genes. Among the generated networks, we identified a particularly interesting one. It describes a cascading event, triggered by S1P, leading to the transactivation of MMP-9 via neuregulin-1 (NRG-1), vascular endothelial growth factor (VEGF), and the urokinase-type plasminogen activator (uPA). This interaction network has the potential to shed new light on our understanding of the role played by MMP-9 in invasive glioblastomas.

Conclusion

Automated extraction of information from biological literature promises to play an increasingly important role in biological knowledge discovery. This is particularly true for high-throughput approaches, such as microarrays, and for combining and integrating data from different sources. Text mining may hold the key to unraveling previously unknown relationships between biological entities and could develop into an indispensable instrument in the process of formulating novel and potentially promising hypotheses.

Antitumor activity of sphingosine kinase inhibitors

Sphingosine kinase (SK) is an oncogenic sphingolipid-metabolizing enzyme that catalyzes the formation of the mitogenic second messenger sphingosine-1-phosphate (S1P) at the expense of proapoptotic ceramide. Thus, SK is an attractive target for cancer therapy because blockage of S1P formation leads to inhibition of proliferation, as well as the induction of apoptosis in cancer cells. We have recently identified novel SK inhibitors with nanomolar to low micromolar potencies toward recombinant human SK. This study describes the continuing analysis of these inhibitors through in vitro and in vivo experiments. All three structurally diverse SK inhibitors tested showed antitumor activity in mice without exhibiting toxicity. Blood and tumor inhibitor concentrations exceeded in vitro potency levels. Cell signaling analyses in vitro revealed mixed inhibition of mitogen-activated protein kinase kinase and Akt phosphorylation by the SK inhibitors. Importantly, 4-[4-(4-chloro-phenyl)-thiazol-2-ylamino]-phenol (SKI-II) is orally bioavailable, detected in the blood for at least 8 h, and showed a significant inhibition of tumor growth in mice. These compounds are the first examples of nonlipid selective inhibitors of SK with in vivo antitumor activity and provide leads for further development of inhibitors of this important molecular target.

Membrane trafficking factors are involved in the hypotonic activation of the taurine channel in the little skate (Raja erinacea) red blood cell

In response to volume expansion, red blood cells of the little skate (Raja erinacea) initially swell and then release small organic compounds and osmotically obligated water in what is called a regulatory volume decrease (RVD) to restore cell volume. One of the major intracellular solutes lost during this process is the non-metabolized beta amino acid taurine. This hypoosmotic-induced increase in cell taurine permeability requires the anion exchanger, skAE1. The abundance of this transporter increases on the surface plasma membrane by a process of exocytosis. The second-messenger pathways involved in exocytosis of skAE1 were investigated with the use of inhibitors which affect membrane trafficking. Hypoosmotic-stimulated taurine uptake was significantly decreased by 42% with wortmannin, a phosphatidylinositol 3-kinase (PI3 kinase) inhibitor. Additional evidence for the involvement of PI3K was obtained with a second inhibitor, LY294002, which decreased the hypoosmotic-stimulated taurine uptake by 28%. The state of actin is also involved, as the actin filament depolymerizer latrunculin B decreased hypoosmotic-stimulated taurine uptake by approximately 40%. Although hypoosmotic conditions did not stimulate changes in the distribution of actin between filamentous and globular forms, latrunculin stimulated a decrease in filamentous actin and increase in globular actin in both isoosmotic and hypoosmotic conditions. Disruptors of other potential cytoskeletal factors (myosin, kinesin, dynein, and microtubules) did not affect taurine uptake. The present results suggest that the exocytosis of skAE1 stimulated by hyposmotic-induced cell volume expansion requires activation of PI3 kinase and is regulated by the state of actin filaments.

Sphingosine 1-phosphate: a novel stimulator of aldosterone secretion

Sphingosine-1-phosphate (S1P) is a bioactive sphingolipid capable of regulating critical physiological and pathological functions. Here, we report for the first time that S1P stimulates aldosterone secretion in cells of the zona glomerulosa of the adrenal gland. Regulation of aldosterone secretion is important because this hormone controls electrolyte and fluid balance and is implicated in cardiovascular homeostasis. S1P-stimulated aldosterone secretion was dependent upon the protein kinase C (PKC) isoforms alpha and delta and extracellular Ca2+, and it was inhibited by pertussis toxin (PTX). S1P activated phospholipase D (PLD) through a PTX-sensitive mechanism, also involving PKC alpha and delta and extracellular Ca2+. Primary alcohols, which attenuate the formation of phosphatidic acid (the product of PLD), and cell-permeable ceramides, which inhibit PLD activity, blocked S1P-stimulated aldosterone secretion. Furthermore, propranolol, chlorpromazine, and sphingosine, which are potent inhibitors of phosphatidate phosphohydrolase (PAP) (the enzyme that produces diacylglycerol from phosphatidate), also blocked aldosterone secretion. These data suggest that the PLD/PAP pathway plays a crucial role in the regulation of aldosterone secretion by S1P and that Gi protein-coupled receptors, extracellular Ca2+, and the PKC isoforms alpha and delta are all important components in the cascade of events controlling this process.

PEA-15 Modulates TNFα Intracellular Signaling in Astrocytes

PEA-15 is a small protein (15 kDa) that was first identified as an abundant phosphoprotein in brain astrocytes and subsequently shown to be widely expressed in different tissues and highly conserved among mammals. It is composed of an N-terminal death effector domain (DED) and a C-terminal tail of irregular structure. PEA-15 is regulated by multiple calcium-dependent phosphorylation pathways. PEA-15 is ideally positioned to play a major role in signal integration. Accordingly, it has been demonstrated that PEA-15 diverts astrocytes from TNFalpha-triggered apoptosis and regulates the actions of the ERK MAP kinase cascade by binding to ERK and altering its subcellular localization. Expression of PEA-15 directs TNFalpha outcomes toward survival, whereas its absence allows the development of the cytokine-induced cell death.

Lysophosphatidic acid acyltransferase-beta: a novel target for induction of tumour cell apoptosis

Phosphatidic acid (PA) is a component of cellular membranes that is also a mediator of certain cell signalling functions associated with oncogenesis. These include ras/raf/Erk and Akt/mTor [1-3]. The authors have investigated whether it would be possible to interrupt these known oncogenic pathways through the inhibition of lysophosphatidic acid acyltransferase (LPAAT), an enzyme that catalyses the biosynthesis of PA. The expression and activity of the LPAAT-beta isoform are elevated in human tumours, and the respective gene displays transforming capacity when overexpressed in vitro. Inhibition by either genetic means or by isoform-specific small molecules results in a block to cell signalling pathways and apoptosis. Furthermore, the small-molecule inhibitors of LPAAT-beta are not cytotoxic to a number of normal cell types, including primary bone marrow progenitors, indicating a differential dependence of tumour cells on LPAAT-beta function. These discoveries indicate that LPAAT-beta represents a potential novel cancer therapy target.

G-protein-coupled OX1 orexin/hcrtr-1 hypocretin receptors induce caspase-dependent and -independent cell death through p38 mitogen-/stress-activated protein kinase

We have investigated the signaling of OX(1) receptors to cell death using Chinese hamster ovary cells as a model system. OX(1) receptor stimulation with orexin-A caused a delayed cell death independently of cytosolic Ca(2+) elevation. The classical mitogen-activated protein kinase (MAPK) pathways, ERK and p38, were strongly activated by orexin-A. p38 was essential for induction of cell death, whereas the ERK pathway appeared protective. A pathway often implicated in the p38-mediated cell death, activation of p53, did not mediate the cell death, as there was no stabilization of p53 or increase in p53-dependent transcriptional activity, and dominant-negative p53 constructs did not inhibit cell demise. Under basal conditions, orexin-A-induced cell death was associated with compact chromatin condensation and it required de novo gene transcription and protein synthesis, the classical hallmarks of programmed (apoptotic) cell death. However, though the pan-caspase inhibitor N-benzyloxycarbonyl-Val-Ala-Asp-(O-methyl)fluoromethyl ketone (Z-VAD-fmk) fully inhibited the caspase activity, it did not rescue the cells from orexin-A-induced death. In the presence of Z-VAD-fmk, orexin-A-induced cell death was still dependent on p38 and de novo protein synthesis, but it no longer required gene transcription. Thus, caspase inhibition causes activation of alternative, gene transcription-independent death pathway. In summary, the present study points out mechanisms for orexin receptor-mediated cell death and adds to our general understanding of the role of G-protein-coupled receptor signaling in cell death by suggesting a pathway from G-protein-coupled receptors to cell death via p38 mitogen-/stress-activated protein kinase independent of p53 and caspase activation.

Roles for C16-ceramide and sphingosine 1-phosphate in regulating hepatocyte apoptosis in response to tumor necrosis factor-alpha

Tumor necrosis factor (TNF)-alpha signals cell death and simultaneously induces the generation of ceramide, which is metabolized to sphingosine and sphingosine 1-phosphate (S1P) by ceramidase (CDase) and sphingosine kinase. Because the dynamic balance between the intracellular levels of ceramide and S1P (the "ceramide/S1P rheostat") may determine cell survival, we investigated these sphingolipid signaling pathways in TNF-alpha-induced apoptosis of primary hepatocytes. Endogenous C16-ceramide was elevated during TNF-alpha-induced apoptosis in both rat and mouse primary hepatocytes. The putative acid sphingomyelinase (ASMase) inhibitor imipramine inhibited TNF-alpha-induced apoptosis and C16-ceramide increase as did the knock out of ASMase. Overexpression of neutral CDase (NCDase) inhibited the TNF-alpha-induced increase of C16-ceramide and apoptosis in rat primary hepatocytes. Moreover, NCDase inhibited liver injury and hepatocyte apoptosis in mice treated with D-galactosamine plus TNF-alpha. This protective effect was abrogated by the sphingosine kinase inhibitor N,N-demethylsphingosine, suggesting that the survival effect of NCDase is due to not only C16-ceramide reduction but also S1P formation. Administration of S1P or overexpression of NCDase activated the pro-survival kinase AKT, and overexpression of dominant negative AKT blocked the survival effect of NCDase. In conclusion, activation of ASMase and generation of C16-ceramide contributed to TNF-alpha-induced hepatocyte apoptosis. NCDase prevented apoptosis both by reducing C16-ceramide and by activation of AKT through S1P formation. Therefore, the cross-talk between sphingolipids and AKT pathway may determine hepatocyte apoptosis by TNF-alpha.

Inhibition of platelet-derived growth factor-induced cell growth signaling by a short interfering RNA for EWS-Fli1 via down-regulation of phospholipase D2 in Ewing sarcoma cells

EWS-Fli1, a fusion gene resulting from a chromosomal translocation t(11;22, q24;q12) and found in Ewing sarcoma and primitive neuroectodermal tumors, encodes a transcriptional activator and promotes cellular transformation. However, the precise biological functions of its products remain unknown. To investigate the role of EWS-Fli1 in cell growth signaling, we transfected Ewing sarcoma TC-135 cells with short interfering RNAs for EWS-Fli1. EWS-Fli1 knockdown reduced cell growth and platelet-derived growth factor (PDGF)-BB-induced activation of the growth signaling enzymes. Interestingly, phospholipase D2 (but not the PDGF-BB receptor) showed marked down-regulation in the EWS-Fli1-knocked down TC-135 cells compared with the control cells. In Ewing sarcoma TC-135 cells, the PDGF-BB-induced phosphorylation of growth signaling involving extracellular signal-regulated kinase, Akt, p70S6K, and the expression of cyclin D3 were markedly inhibited by transfection with short interfering RNA phospholipase (PL)-D2. The PDGF-BB-induced activation of growth signaling was also suppressed by 1-butanol, which prevents the production of phosphatidic acid by phospholipase D (but not by t-butyl alcohol), thereby implicating PLD2 in PDGF-BB-mediated signaling in TC-135 cells. These results suggest that EWS-Fli1 may play a role in the regulation of tumor proliferation-signaling enzymes via PLD2 expression in Ewing sarcoma cells.

Sphingosine-1-phosphate inhibits nuclear factor kappaB activation and germ cell apoptosis in the human testis independently of its receptors

Early apoptosis-inducing events are potentially important targets for preventing germ cell loss caused by external stress. The sphingolipid sphingosine-1-phosphate (S1P) is an important regulator of stress-induced apoptosis. It affects the cell as an intracellular signaling molecule or as a ligand to its cell membrane-bound S1P(1-5) receptors. We previously demonstrated that S1P inhibits stress-induced male germ cell death in vitro and in vivo. Here, we further define the mechanisms of S1P-mediated inhibition of male germ cell death. Using immunohistochemistry, we detected expression of the S1P(1) and S1P(2) receptors in the somatic Sertoli cells of the human testis. In a culture of human seminiferous tubules, S1P inhibited germ cell apoptosis, suppressed both nuclear factor kappaB (NF-kappaB) DNA-binding activity and expression of phosphorylated Akt, but did not affect activator protein-1 (AP-1) DNA-binding activity. Dihydro-S1P, which binds to and activates S1P receptors but has no direct intracellular effect, suppressed neither apoptosis nor NF-kappaB activity. These results suggest that S1P inhibits male germ cell apoptosis independently of its receptors, possibly by inhibiting the transcription factor NF-kappaB and Akt phosphorylation.

Angiotensin II-induced Akt activation through the epidermal growth factor receptor in vascular smooth muscle cells is mediated by phospholipid metabolites derived by activation of phospholipase D

Angiotensin II (Ang II) activates cytosolic Ca(2+)-dependent phospholipase A(2) (cPLA(2)), phospholipase D (PLD), p38 mitogen-activated protein kinase (MAPK), epidermal growth factor receptor (EGFR) and Akt in vascular smooth muscle cells (VSMC). This study was conducted to investigate the relationship between Akt activation by Ang II and other signaling molecules in rat VSMC. Ang II-induced Akt phosphorylation was significantly reduced by the PLD inhibitor 1-butanol, but not by its inactive analog 2-butanol, and by brefeldin A, an inhibitor of the PLD cofactor ADP-ribosylation factor, and in cells infected with retrovirus containing PLD(2) siRNA or transfected with PLD(2) antisense but not control LacZ or sense oligonucleotide. Diacylglycerol kinase inhibitor II diminished Ang II-induced and diC8-phosphatidic acid (PA)-increased Akt phosphorylation, suggesting that PLD-dependent Akt activation is mediated by PA. Ang II-induced EGFR phosphorylation was inhibited by 1-butanol and PLD(2) siRNA and also by cPLA(2) siRNA. In addition, the inhibitor of arachidonic acid (AA) metabolism 5,8,11,14-eicosatetraynoic acid (ETYA) reduced both Ang II- and AA-induced EGFR transactivation. Furthermore, ETYA, cPLA(2) antisense, and cPLA(2) siRNA attenuated Ang II-elicited PLD activation. p38 MAPK inhibitor SB202190 [4-(4-flurophenyl)-2-(4-hydroxyphenyl)-5-(4-pyridyl)1H-imidazole] reduced PLD activity and EGFR and Akt phosphorylation elicited by Ang II. Pyrrolidine-1, a cPLA(2) inhibitor, and cPLA(2) siRNA decreased p38 MAPK activity. These data indicate that Ang II-stimulated Akt activity is mediated by cPLA(2)-dependent, p38 MAPK regulated PLD(2) activation and EGFR transactivation. We propose the following scheme of the sequence of events leading to activation of Akt in VSMC by Ang II: Ang II-->cPLA(2)-->AA-->p38 MAPK-->PLD(2)-->PA-->EGFR-->Akt.

Implication of phospholipase D2 in oxidant-induced phosphoinositide 3-kinase signaling via Pyk2 activation in PC12 cells

The role of phospholipase D (PLD) activation in hydrogen peroxide (H(2)O(2))-induced signal transduction and cellular responses is not completely understood. Here we present evidence that Ca(2+)-dependent tyrosine kinase, Pyk2, requires PLD activation to mediate survival pathways in rat pheochromocytoma PC12 cells under oxidative stress. The H(2)O(2)-induced phosphorylation of two Pyk2 sites (Tyr(580), and Tyr(881)) was suppressed by 1-butanol, an inhibitor of transphosphatidylation by PLD, and also by transfection of catalytically negative mouse PLD2K758R (PLD2KR). Furthermore, we found that PLD2 was associated with Pyk2 and Src, and that activation of PLD2 was required for H(2)O(2)-enhanced association of Src with Pyk2 leading to full activation of Pyk2. H(2)O(2)-induced phosphorylation of Akt and p70S6K was dependent on phosphatidylinositol 3-kinase (PI3K) activity and was abolished by 1-butanol but not t-butanol. Furthermore, the PI3K/Akt activation in response to H(2)O(2) was reduced by transfection of either PLD2KR or the dominant negative Pyk2DN. This study is the first demonstration that PLD2 activation is implicated in Src-dependent phosphorylation of Pyk2 (Tyr(580) and Tyr(881)) by promoting the complex formation between Pyk2 and activated Src in PC12 cells exposed to H(2)O(2), thereby resulting in activation of the survival signaling pathway PI3K/Akt/p70S6K.

Phospholipase D

Phospholipase D catalyses the hydrolysis of the phosphodiester bond of glycerophospholipids to generate phosphatidic acid and a free headgroup. Phospholipase D activities have been detected in simple to complex organisms from viruses and bacteria to yeast, plants, and mammals. Although enzymes with broader selectivity are found in some of the lower organisms, the plant, yeast, and mammalian enzymes are selective for phosphatidylcholine. The two mammalian phospholipase D isoforms are regulated by protein kinases and GTP binding proteins of the ADP-ribosylation and Rho families. Mammalian and yeast phospholipases D are also potently stimulated by phosphatidylinositol 4,5-bisphosphate. This review discusses the identification, characterization, structure, and regulation of phospholipase D. Genetic and pharmacological approaches implicate phospholipase D in a diverse range of cellular processes that include receptor signaling, control of intracellular membrane transport, and reorganization of the actin cytoskeleton. Most ideas about phospholipase D function consider that the phosphatidic acid product is an intracellular lipid messenger. Candidate targets for phospholipase-D-generated phosphatidic acid include phosphatidylinositol 4-phosphate 5-kinases and the raf protein kinase. Phosphatidic acid can also be converted to two other lipid mediators, diacylglycerol and lyso phosphatidic acid. Coordinated activation of these phospholipase-D-dependent pathways likely accounts for the pleitropic roles for these enzymes in many aspects of cell regulation.

Sphingolipid metabolites in neural signalling and function

Sphingolipid metabolites, such as ceramide, sphingosine, sphingosine-1-phosphate (S1P) and complex sphingolipids (gangliosides), are recognized as molecules capable of regulating a variety of cellular processes. The role of sphingolipid metabolites has been studied mainly in non-neuronal tissues. These studies have underscored their importance as signals transducers, involved in control of proliferation, survival, differentiation and apoptosis. In this review, we will focus on studies performed over the last years in the nervous system, discussing the recent developments and the current perspectives in sphingolipid metabolism and functions.

Phospholipase D elevates the level of MDM2 and suppresses DNA damage-induced increases in p53

Phospholipase D (PLD) has been reported to generate survival signals that prevent apoptosis induced by serum withdrawal. We have now found that elevated expression of PLD also suppresses DNA damage-induced apoptosis. Since DNA damage-induced apoptosis is often mediated by p53, we examined the effect of elevated PLD expression on the regulation of p53 stabilization. We report here that PLD suppresses DNA damage-induced increases in p53 stabilization in cells where PLD has been shown to provide a survival signal. Elevated expression of PLD also led to increased expression of the p53 E3 ubiquitin ligase MDM2 and increased turnover of p53. PLD1-stimulated increases in MDM2 expression and suppression of p53 activation were blocked by inhibition of mTOR and the mitogen-activated protein kinase pathway. Although PLD did not activate the phosphatidylinositol 3-kinase (PI3K)/Akt survival pathway activate the basal levels of PI3K activity were partially required for PLD1-induced increases in MDM2. These data provide evidence that survival signals generated by PLD involve suppression of the p53 response pathway.

Angiotensin II-stimulated cortisol secretion is mediated by phospholipase D

Angiotensin II (Ang-II) regulates a variety of cellular functions including cortisol secretion. In the present report, we demonstrate that Ang-II activates phospholipase D (PLD) in zona fasciculata (ZF) cells of bovine adrenal glands, and that this effect is associated to the stimulation of cortisol secretion by this hormone. PLD activation was dependent upon extracellular Ca2+, and was blocked by inhibition of protein kinase C (PKC). Using the reverse transcription-polymerase chain reaction technique, we demonstrated that ZF cells express both PLD-1 and PLD-2 isozymes. Primary alcohols, which attenuate the formation of phosphatidate (the product of PLD), and cell-permeable ceramides, which inhibit PLD potently, blocked Ang-II-stimulated cortisol secretion. Furthermore, propranolol or chlorpromazine, which are potent inhibitors of phosphatidate phosphohydrolase (PAP) (the enzyme that produces diacylglycerol from phosphatidate), also blocked cortisol secretion. These data suggest that the PLD/PAP pathway plays an important role in the regulation of cortisol secretion by Ang-II in ZF cells.

Overexpression of phospholipase D prevents actinomycin D-induced apoptosis through potentiation of phosphoinositide 3-kinase signalling pathways in Chinese-hamster ovary cells

To examine the roles of PLD (phospholipase D) in the regulation of the apoptotic process, PLD1 and PLD2 were stably overexpressed in S1P3-CHO cells [CHO (Chinese-hamster ovary) cells expressing the S1P (sphingosine 1-phosphate) receptor S1P3]. Treatment of S1P3-CHO cells with ActD (actinomycin D) induced apoptosis, as shown by the occurrence of nuclear fragmentation and the caspase-dependent proteolytic cleavage of PARP [poly(ADP-ribose) polymerase] and protein kinase Cd. Overexpression of either PLD1 or PLD2 protected S1P3-CHO cells from ActD-induced apoptosis, as demonstrated by an increased number of viable cells and inhibition of PARP and protein kinase Cd cleavage. However, in the early phase of apoptosis, ActD induced an increase in PLD activity and activation of key factors in the cell-survival signalling pathways, such as PI3K (phosphoinositide 3-kinase), Akt, p70S6K (p70 S6 kinase) and ERK (extracellular-signal-regulated kinase). Furthermore, the ActD-induced activation of these survival signalling enzymes was potentiated by overexpression of either PLD1 or PLD2. The PI3K inhibitor LY294002 inhibited the ActD-induced activation of Akt and p70S6K, and completely abolished the effects of PLD1 or PLD2, whereas inhibition of ERK activity by the MEK inhibitor U0126 had a milder effect. The ActD-induced activation of p70S6K and ERKs was blocked by 1-butanol, but not by t-butanol; similar to S1P, exogenous PLD suppressed the ActD-induced events in the apoptosis signalling pathways. These results show that, in S1P3-CHO cells, increased expression of PLDs prevents ActD-induced apoptosis by enhanced activation of the PI3K signalling pathways.

Role of phospholipase D1 in the regulation of mTOR activity by lysophosphatidic acid

Mitogens activate protein translation through phosphorylation of p7S6 kinase (p70(S6K)) and eIF4E binding protein 1 (4E-BP1) mediated by the mammalian target of rapamycin (mTOR) or phosphoinositide 3-kinase (PI3K). A recent report (Science 294, 1942, 2001) has implicated phospholipase D (PLD) in mTOR signaling. We studied the role of PLD in the phosphorylation of p70(S6K) and 4E-BP1 induced by lysophosphatidic acid (LPA) and platelet-derived growth factor (PDGF) using fibroblasts deficient in PLD activity and also 1-butanol, which inhibits phosphatidic acid production by PLD. The reduction in PLD activity in both situations impaired the effect of LPA on mTOR signaling but did not inhibit the effect of PDGF. PDGF induced marked phosphorylation of Akt (a PI3K target) but this was not affected by PLD deficiency. LPA caused much less phosphorylation of Akt and this was dependent on PLD activity. Toxin B, which inactivates Rho GTPases, markedly impaired PLD1 activation and phosphorylation of Akt, p70(S6K), and 4E-BP1 induced by LPA but had a minimal or no effect on the actions of PDGF. These results support the hypothesis that LPA activates protein translation through the action of PLD1-generated PA on mTOR and the PI3K/Akt pathway whereas PDGF acts through P13K/Akt independent of PLD1.

Differential role for phospholipase D1 and phospholipase D2 in 12-O-tetradecanoyl-13-phorbol acetate-stimulated MAPK activation, Cox-2 and IL-8 expression

Phospholipase D (PLD) is expressed in many tissues and stimulated by growth factors and cytokines. However, the role of PLD in signal transduction is still not well-understood. Human embryonic kidney (HEK-293) cells exhibit low levels of both PLD1 and PLD2 mRNA, however, only PLD1 protein was detected by Western blot. When either isoform of PLD was stably expressed in HEK-293 cells, we observed an increased PLD activity in a cell-free system and a 12-O-tetradecanoyl-13-phorbol acetate (TPA)-stimulated increase in PLD activity in intact cells. This system was then used to elucidate the effects of PLD activity on TPA-stimulated signaling pathways. Two such pathways, the mitogen-activated protein kinases (MAPK), extracellular regulated protein kinase (ERK) and p38 are activated by growth factors and cellular stress, respectively. We found that TPA stimulated ERK phosphorylation regardless of the expression status of PLD. In contrast to ERK kinase, HEK-293 cells were unable to induce p38 phosphorylation by TPA stimulation. When HEK-293 cells expressed either PLD1 or PLD2, we observed elevated p38 phosphorylation in response to TPA stimulation. The ERK and p38 MAPKs can also stimulate the expression of both cyclooxygenase-2 (Cox-2) and interleukin-8 (IL-8). We used this system to differentiate the effect of PLD1 or PLD2 activity on the expression of Cox-2 and IL-8. Increased Cox-2 and IL-8 expression was found only in HEK-293 cells expressing PLD1. These data identify a novel role for the PLD1 isoform in the induction of gene expression and provide new insight into the differential role of PLD1 and PLD2 in cells.

Signal transduction underlying carbachol-induced contraction of rat urinary bladder. I. Phospholipases and Ca2+ sources

We have reexamined the muscarinic receptor subtype mediating carbachol-induced contraction of rat urinary bladder and investigated the role of phospholipase (PL)C, D, and A2 and of intra- and extracellular Ca2+ sources in this effect. Based on the nonsubtype-selective tolterodine, the highly M2 receptor-selective (R)-4-[2-[3-(4-methoxy-benzoylamino)-benzyl]-piperidin-1-ylmethyl]-piperidine-1-carboxylic acid amide (Ro-320-6206), and the highly M3 receptor-selective darifenacin and 3-(1-carbamoyl-1,1-diphenylmethyl)-1-(4-methoxyphenylethyl)pyrrolidine (APP), contraction occurs via M3 receptors. Carbachol stimulated inositol phosphate formation in rat bladder slices, and this was abolished by the phospholipase C inhibitor 1-(6-[([17beta]-3-methoxyestra-1,3,5[10]-trien-17-yl)-amino]hexyl)-1H-pyrrole-2,5-dione (U 73,122; 10 microM). Nevertheless, U 73,122 (1-10 microM) did not significantly affect carbachol-stimulated bladder contraction. Carbachol had only little effect on PLD activity in bladder slices, but the PLD inhibitor butan-1-ol, relative to its negative control butan-2-ol (0.3% each), caused detectable inhibition of carbachol-induced bladder contraction. The cytosolic PLA2 inhibitor arachidonyltrifluoromethyl ketone weakly inhibited carbachol-induced contraction at a concentration of 300 microM, but the cyclooxygenase inhibitor indomethacin (1-10 microM) remained without effect. The Ca2+ entry blocker nifedipine (10-100 nM) almost completely inhibited carbachol-induced bladder contraction. In contrast, 1-[beta-[3-(4-methoxyphenyl)propoxy]-4-methoxyphenethyl]-1H-imidazole HCl (SKF 96,365; 10 microM), an inhibitor of store-operated Ca2+ channels, caused little inhibition. We conclude that carbachol-induced contraction of rat bladder largely depends on Ca2+ entry through nifedipine-sensitive channels and, perhaps, PLD, PLA2, and store-operated Ca2+ channels, whereas cyclooxygenase and, surprisingly, also PLC are not involved to a relevant extent.

S1P 3 ‐mediated Akt activation and crosstalk with platelet‐derived growth factor receptor (PDGFR)

Akt plays a pivotal role in cell survival and tumorigenesis. We investigated the potential interaction between sphingosine-1-phosphate (S1P) and platelet-derived growth factor (PDGF) in the Akt signaling pathway. Using mouse embryonic fibroblasts (MEFs) from S1P receptor knockout mice, we show here that S1P3 was required for S473 phosphorylation of Akt by S1P. In addition, S1P-stimulated activation of Akt, but not ERK, was blocked by a PDGF receptor (PDGFR)-specific inhibitor, AG1296, suggesting a S1P3-mediated specific crosstalk between the Akt signaling pathways of S1P and PDGFR in MEFs. We investigated this crosstalk under different conditions and found that both Akt and ERK activation induced by S1P, but not lysophosphatidic acid (LPA), in HEY ovarian cancer cells required PDGFR but not epidermal growth factor receptor (EGFR) or insulin-like growth factor-I receptor (IGFR). Importantly, S1P induced a Gi-dependent tyrosine phosphorylation of PDGFR in HEY cells. This dependence on PDGFR in S1P-induced Akt activation was also observed in A2780, T47D, and HMEC-1 cells (which express S1P3), but not in PC-3 or GI-101A cells (which do not express S1P3), further supporting that S1P3 mediates the crosstalk between S1P and PDGFR. This is the first report demonstrating a unique interaction between S1P3 and PDGFR, in addition to demonstrating a specific role for S1P3 in S1P-induced Akt activation.