A signaling cascade mediated by ceramide, src and PDGFRβ coordinates the activation of the redox-sensitive neutral sphingomyelinase-2 and sphingosine kinase-1

Sphingolipids and Atherosclerosis: The Dual Role of Ceramide and Sphingosine-1-Phosphate

Sphingolipids are bioactive molecules that play either pro- and anti-atherogenic roles in the formation and maturation of atherosclerotic plaques. Among SLs, ceramide and sphingosine-1-phosphate showed antithetic properties in regulating various molecular mechanisms and have emerged as novel potential targets for regulating the development of atherosclerosis. In particular, maintaining the balance of the so-called ceramide/S1P rheostat is important to prevent the occurrence of endothelial dysfunction, which is the trigger for the entire atherosclerotic process and is strongly associated with increased oxidative stress. In addition, these two sphingolipids, together with many other sphingolipid mediators, are directly involved in the progression of atherogenesis and the formation of atherosclerotic plaques by promoting the oxidation of low-density lipoproteins (LDL) and influencing the vascular smooth muscle cell phenotype. The modulation of ceramide and S1P levels may therefore allow the development of new antioxidant therapies that can prevent or at least impair the onset of atherogenesis, which would ultimately improve the quality of life of patients with coronary artery disease and significantly reduce their mortality.

Natural flavonoids derived from herbal medicines are potential anti-atherogenic agents by inhibiting oxidative stress in endothelial cells

As the common pathological basis of various cardiovascular diseases, the morbidity and mortality of atherosclerosis (AS) have increased in recent years. Unfortunately, there are still many problems in the treatment of AS, and the prevention and treatment of the disease is not ideal. Up to now, the occurrence and development of AS can roughly include endothelial cell dysfunction, vascular smooth muscle cell proliferation, inflammation, foam cell production, and neoangiogenesis. Among them, endothelial dysfunction, as an early event of AS, plays a particularly important role in promoting the development of AS. In addition, oxidative stress occurs throughout the causes of endothelial dysfunction. Some previous studies have shown that flavonoids derived from herbal medicines are typical secondary metabolites. Due to its structural presence of multiple active hydroxyl groups, it is able to exert antioxidant activity in diseases. Therefore, in this review, we will search PubMed, Web of Science, Elesvier, Wliey, Springer for relevant literature, focusing on flavonoids extracted from herbal medicines, and summarizing how they can prevent endothelial dysfunction by inhibiting oxidative stress. Meanwhile, in our study, we found that flavonoid represented by quercetin and naringenin showed superior protective effects both in vivo and in vitro, suggesting the potential of flavonoid compounds in the treatment of AS.

Mechanisms underlying Nrf2 nuclear translocation by non-lethal levels of hydrogen peroxide: p38 MAPK-dependent neutral sphingomyelinase2 membrane trafficking and ceramide/PKCζ/CK2 signaling

Hydrogen peroxide is an aerobic metabolite playing a central role in redox signaling and oxidative stress. H₂O₂ could activate redox sensitive transcription factors, such as Nrf2, AP-1 and NF-κB by different manners. In some cells, treatment with non-lethal levels of H₂O₂ induces rapid activation of Nrf2, which upregulates expression of a set of genes involved in glutathione (GSH) synthesis and defenses against oxidative damage. It depends on two steps, the rapid translational activation of Nrf2 and facilitation of Nrf2 nuclear translocation. We review the molecular mechanisms by which H₂O₂ induces nuclear translocation of Nrf2 in cultured cells by highlighting the role of neutral sphingomyelinase 2 (nSMase2), a GSH sensor. H₂O₂ enters cells through aquaporin channels in the plasma membrane and is rapidly reduced to H₂O by GSH peroxidases to consume cellular GSH, resulting in nSMase2 activation to generate ceramide. H₂O₂ also activates p38 MAP kinase, which enhances transfer of nSMase2 from perinuclear regions to plasma membrane lipid rafts to accelerate ceramide generation. Low levels of ceramide activate PKCζ, which then activates casein kinase 2 (CK2). These protein kinases are able to phosphorylate Nrf2 to stabilize and activate it. Notably, Nrf2 also binds to caveolin-1 (Cav1), which protects Nrf2 from Keap1-mediated degradation and limits Nrf2 nuclear translocation. We propose that Cav1serves as a signaling hub for the control of H₂O₂-mediated phosphorylation of Nrf2 by kinases, which results in release of Nrf2 from Cav1 to facilitate nuclear translocation. In summary, H₂O₂ induces GSH depletion which is recovered by Nrf2 activation dependent on p38/nSMase2/ceramide signaling.

A Rheostat of Ceramide and Sphingosine-1-Phosphate as a Determinant of Oxidative Stress-Mediated Kidney Injury

Reactive oxygen species (ROS) modulate sphingolipid metabolism, including enzymes that generate ceramide and sphingosine-1-phosphate (S1P), and a ROS-antioxidant rheostat determines the metabolism of ceramide-S1P. ROS induce ceramide production by activating ceramide-producing enzymes, leading to apoptosis, while they inhibit S1P production, which promotes survival by suppressing sphingosine kinases (SphKs). A ceramide-S1P rheostat regulates ROS-induced mitochondrial dysfunction, apoptotic/anti-apoptotic Bcl-2 family proteins and signaling pathways, leading to apoptosis, survival, cell proliferation, inflammation and fibrosis in the kidney. Ceramide inhibits the mitochondrial respiration chain and induces ceramide channel formation and the closure of voltage-dependent anion channels, leading to mitochondrial dysfunction, altered Bcl-2 family protein expression, ROS generation and disturbed calcium homeostasis. This activates ceramide-induced signaling pathways, leading to apoptosis. These events are mitigated by S1P/S1P receptors (S1PRs) that restore mitochondrial function and activate signaling pathways. SphK1 promotes survival and cell proliferation and inhibits inflammation, while SphK2 has the opposite effect. However, both SphK1 and SphK2 promote fibrosis. Thus, a ceramide-SphKs/S1P rheostat modulates oxidant-induced kidney injury by affecting mitochondrial function, ROS production, Bcl-2 family proteins, calcium homeostasis and their downstream signaling pathways. This review will summarize the current evidence for a role of interaction between ROS-antioxidants and ceramide-SphKs/S1P and of a ceramide-SphKs/S1P rheostat in the regulation of oxidative stress-mediated kidney diseases.

Oxidative stress increases 1-deoxysphingolipid levels in chronic kidney disease

Chronic kidney disease (CKD) leads to deep changes in lipid metabolism and obvious dyslipidemia. The dysregulation of lipid metabolism in turn results in CKD progression and the complications of cardiovascular diseases. To obtain a profound insight into the associated dyslipidemia in CKD, we performed lipidomic analysis to measure lipid metabolites in the serum from a rat 5/6 nephrectomy (5/6 Nx) model of CKD as well as in the serum from CKD patients. HK-2 cells were also used to examine oxidative stress-induced sphingolipid changes. Totally 182 lipid species were identified in 5/6 Nx rats. We found glycerolipids, total free fatty acids, and sphingolipids levels were significantly upregulated in 5/6 Nx rats. The atypical sphingolipids, 1-deoxysphingolipids, were significantly altered in both CKD animals and human CKD patients. The levels of 1-deoxysphingolipids directly relevant to the level of oxidative stress in vivo and in vitro. These results demonstrate that 1-deoxysphingolipid levels are increased in CKD and this increase directly correlates with increased kidney oxidative stress.

A Novel Combination of Fruits and Vegetables Prevents Diet-Induced Hepatic Steatosis and Metabolic Dysfunction in Mice

BACKGROUND

Epidemiological studies suggest that higher fruits and vegetables (F&V) consumption correlates with reduced risk of hepatic steatosis, yet evidence for causality and the underlying mechanisms is lacking.

OBJECTIVES

We aimed to determine the causal relation between F&V consumption and improved metabolic disorders in mice fed high-fat (HF) (Experiment-1) or normal-fat (Experiment-2) diets and its underlying mechanisms.

METHODS

Six-week-old male C57BL/6J mice were randomly grouped and fed diets supplemented at 0%-15% (wt:wt) with a freeze-dried powder composed of 24 commonly consumed F&V (human equivalent of 0-9 servings/d) for 20 wk. In Experiment-1, mice were fed an HF (45% kcal fat) diet with 0% (HF0), 5%, 10%, or 15% (HF15) F&V or a matched low-fat control diet (10% kcal fat). In Experiment-2, mice were fed an AIN-93 diet (basal) (B, 16% kcal fat) with 0% (B0), 5%, 10%, or 15% (B15) F&V supplementation. Body weight and composition, food intake, hepatic steatosis, inflammation, ceramide levels, sphingomyelinase activity, and gut microbiota were assessed.

RESULTS

In Experiment-1, mice fed the HF15 diet had lower weight gain (17.9%), hepatic steatosis (48.4%), adipose tissue inflammation, blood (24.6%) and liver (33.9%) ceramide concentrations, and sphingomyelinase activity (38.8%) than HF0 mice (P < 0.05 for all). In Experiment-2, mice fed the B15 diet had no significant changes in weight gain but showed less hepatic steatosis (28.5%), blood and adipose tissue inflammation, and lower blood (30.0%) ceramide concentrations than B0 mice (P < 0.05 for all). These F&V effects were associated with favorable microbiota changes.

CONCLUSIONS

These findings represent the first evidence for a causal role of high F&V intake in mitigating hepatic steatosis in mice. These beneficial effects may be mediated through changes in ceramide and/or gut microbiota, and suggest that higher than currently recommended servings of F&V may be needed to achieve maximum health benefits.

Aquaporin-3 deficiency slows cyst enlargement in experimental mouse models of autosomal dominant polycystic kidney disease

Human autosomal dominant polycystic kidney disease (ADPKD) is characterized by bilateral renal cysts that lead to a decline in kidney function. Previous studies reported aquaporin (AQP)-3 expression in cysts derived from collecting ducts in ADPKD. To study the role of AQP3 in cyst development, we generated 2 polycystic kidney disease (PKD) mouse models: kidney-specific Pkd1 knockout mice and inducible Pkd1 knockout mice, each without and with AQP3 deletion. In both models, kidney sizes and cyst indexes were significantly reduced in AQP3-null PKD mice compared with AQP3-expressing PKD mice, with the difference seen mainly in collecting duct cysts. AQP3-deficient kidneys showed significantly reduced ATP content, increased phosphorylated (p)-AMPK, and decreased p-ERK and p-mammalian target of rapamycin (mTOR). In a matrix-grown Madin-Darby canine kidney cyst model, AQP3 expression promoted cyst enlargement and was associated with increased expression of hypoxia-inducible factor 1-α and glucose transporter 1 and increased glucose uptake. Our data suggest that the slowed renal cyst enlargement in AQP3 deficiency involves impaired energy metabolism in the kidney through AMPK and mTOR signaling and impaired cellular glucose uptake. These findings implicate AQP3 as a novel determinant of renal cyst enlargement and hence a potential drug target in ADPKD.-Wang, W., Geng, X., Lei, L., Jia, Y., Li, Y., Zhou, H., Verkman, A. S., Yang, B. Aquaporin-3 deficiency slows cyst enlargement in experimental mouse models of autosomal dominant polycystic kidney disease.

Ceramide and Regulation of Vascular Tone

In addition to playing a role as a structural component of cellular membranes, ceramide is now clearly recognized as a bioactive lipid implicated in a variety of physiological functions. This review aims to provide updated information on the role of ceramide in the regulation of vascular tone. Ceramide may induce vasodilator or vasoconstrictor effects by interacting with several signaling pathways in endothelial and smooth muscle cells. There is a clear, albeit complex, interaction between ceramide and redox signaling. In fact, reactive oxygen species (ROS) activate different ceramide generating pathways and, conversely, ceramide is known to increase ROS production. In recent years, ceramide has emerged as a novel key player in oxygen sensing in vascular cells and mediating vascular responses of crucial physiological relevance such as hypoxic pulmonary vasoconstriction (HPV) or normoxic ductus arteriosus constriction. Likewise, a growing body of evidence over the last years suggests that exaggerated production of vascular ceramide may have detrimental effects in a number of pathological processes including cardiovascular and lung diseases.

nSMase2 (Type 2-Neutral Sphingomyelinase) Deficiency or Inhibition by GW4869 Reduces Inflammation and Atherosclerosis in Apoe-/- Mice

OBJECTIVE

Atherosclerosis is a chronic multifactorial and inflammatory disease of large and medium arteries and the leading cause of cardiovascular diseases worldwide. The aim of this study was to investigate whether and how the nSMase2 (type 2-neutral sphingomyelinase), a key enzyme of sphingolipid metabolism, may contribute to the development of atherosclerotic lesions.

APPROACH AND RESULTS

The role of nSMase2 in atherosclerosis was investigated in Apoe^-/-;Smpd3^fro/fro mice, mutant for nSMase2, and in Apoe^-/-;Smpd3^+/+ mice intraperitoneally injected with GW4869, a pharmacological nSMase2 inhibitor. The defect or inhibition of nSMase2 resulted in a reduction of atherosclerotic lesions and a decrease in macrophage infiltration and lipid deposition, although cholesterolemia remained unchanged. nSMase2 inhibition decreased the inflammatory response of murine endothelial cells to oxLDL (oxidized low-density lipoprotein), as assessed by the significant reduction of MCP-1 (monocyte chemoattractant protein 1), ICAM-1 (intercellular adhesion molecule-1), and VCAM-1 (vascular cell adhesion molecule-1) mRNA expressions and macrophage recruitment. Likewise, in RAW264.7 or in macrophages isolated from Apoe^-/-/Smpd3^fro/fro or Apoe^-/-/Smpd3^+/+ mice stimulated by lipopolysaccharides, nSMase2 inhibition resulted in a decrease in the expression of inflammatory molecules. Mechanistically, the anti-inflammatory response resulting from nSMase2 inhibition involves Nrf2 (nuclear factor [erythroid-derived 2]-like 2 or NF-E2-related factor-2) activation in both endothelial cells and macrophages, as assessed by the lack of protective effect of GW4869 in endothelial cells silenced for Nrf2 by small interfering RNAs, and in lipopolysaccharide-stimulated macrophages issued from Nrf2-KO mice.

CONCLUSIONS

The genetic deficiency or inhibition of nSMase2 strongly decreases the development of atherosclerotic lesions in Apoe^-/- mice, by reducing inflammatory responses through a mechanism involving the Nrf2 pathway. Inhibitors of nSMase2 may, therefore, constitute a novel approach to slow down atherosclerosis progression.

Sphingolipids and Redox Signaling in Renal Regulation and Chronic Kidney Diseases

Significance: Sphingolipids play critical roles in the membrane biology and intracellular signaling events that influence cellular behavior and function. Our review focuses on the cellular mechanisms and functional relevance of the cross talk between sphingolipids and redox signaling, which may be critically implicated in the pathogenesis of different renal diseases. Recent Advances: Reactive oxygen species (ROS) and sphingolipids can regulate cellular redox homeostasis through the regulation of NADPH oxidase, mitochondrial integrity, nitric oxide synthase (NOS), and antioxidant enzymes. Over the last two decades, there have been significant advancements in the field of sphingolipid research, and it was in 2010 for the first time that sphingolipid receptor modulator was exploited as a therapeutic in humans. The cross talk of sphingolipids with redox signaling pathways becomes an important mechanism in the development of many different diseases such as renal diseases. Critical Issues: The critical issues to be addressed in this review are how sphingolipids interact with the redox signaling pathway to regulate renal function and even result in chronic kidney diseases. Ceramide, sphingosine, and sphingosine-1-phosphate (S1P) as main signaling sphingolipids are discussed in more detail. Future Directions: Although sphingolipids and ROS may mediate or modulate cellular responses to physiological and pathological stimuli, more translational studies and mechanistic pursuit in a tissue- or cell-specific way are needed to enhance our understanding of this important topic and to develop effective therapeutic strategies to treat diseases associated with redox signaling and sphingolipid cross talk. Antioxid. Redox Signal. 28, 1008-1026.

Proatherogenic effects of 4-hydroxynonenal

4-hydroxy-2-nonenal (HNE) is a α,β-unsaturated hydroxyalkenal generated by peroxidation of n-6 polyunsaturated fatty acid. This reactive carbonyl compound exhibits a huge number of biological properties that result mainly from the formation of HNE-adducts on free amino groups and thiol groups in proteins. In the vascular system, HNE adduct accumulation progressively leads to cellular dysfunction and tissue damages that are involved in the progression of atherosclerosis and related diseases. HNE contributes to the atherogenicity of oxidized LDL, by forming HNE-apoB adducts that deviate the LDL metabolism to the scavenger receptor pathway of macrophagic cells, and lead to the formation of foam cells. HNE activates transcription factors (Nrf2, NF-kappaB) that (dys)regulate various cellular responses ranging from hormetic and survival signaling at very low concentrations, to inflammatory and apoptotic effects at higher concentrations. Among a variety of cellular targets, HNE can modify signaling proteins involved in atherosclerotic plaque remodeling, particularly growth factor receptors (PDGFR, EGFR), cell cycle proteins, mitochondrial and endoplasmic reticulum components or extracellular matrix proteins, which progressively alters smooth muscle cell proliferation, angiogenesis and induces apoptosis. HNE adducts accumulate in the lipidic necrotic core of advanced atherosclerotic lesions, and may locally contribute to macrophage and smooth muscle cell apoptosis, which may induce plaque destabilization and rupture, thereby increasing the risk of athero-thrombotic events.

Dual signaling evoked by oxidized LDLs in vascular cells

The oxidative theory of atherosclerosis relies on the modification of low density lipoproteins (LDLs) in the vascular wall by reactive oxygen species. Modified LDLs, such as oxidized LDLs, are thought to participate in the formation of early atherosclerotic lesions (accumulation of foam cells and fatty streaks), whereas their role in advanced lesions and atherothrombotic events is more debated, because antioxidant supplementation failed to prevent coronary disease events and mortality in intervention randomized trials. As oxidized LDLs and oxidized lipids are present in atherosclerotic lesions and are able to trigger cell signaling on cultured vascular cells and macrophages, it has been proposed that they could play a role in atherogenesis and atherosclerotic vascular remodeling. Oxidized LDLs exhibit dual biological effects, which are dependent on extent of lipid peroxidation, nature of oxidized lipids (oxidized phospholipids, oxysterols, malondialdehyde, α,β-unsaturated hydroxyalkenals), concentration of oxidized LDLs and uptake by scavenger receptors (e.g. CD36, LOX-1, SRA) that signal through different transduction pathways. Moderate concentrations of mildly oxidized LDLs are proinflammatory and trigger cell migration and proliferation, whereas higher concentrations induce cell growth arrest and apoptosis. The balance between survival and apoptotic responses evoked by oxidized LDLs depends on cellular systems that regulate the cell fate, such as ceramide/sphingosine-1-phosphate rheostat, endoplasmic reticulum stress, autophagy and expression of pro/antiapoptotic proteins. In vivo, the intimal concentration of oxidized LDLs depends on the influx (hypercholesterolemia, endothelial permeability), residence time and lipid composition of LDLs, oxidative stress intensity, induction of defense mechanisms (antioxidant systems, heat shock proteins). As a consequence, the local cellular responses to oxidized LDLs may stimulate inflammatory or anti-inflammatory pathways, angiogenic or antiangiogenic responses, survival or apoptosis, thereby contributing to plaque growth, instability, complication (intraplaque hemorrhage, proteolysis, calcification, apoptosis) and rupture. Finally, these dual properties suggest that oxLDLs could be implicated at each step of atherosclerosis development, from early fatty streaks to advanced lesions, depending on the nature and concentration of their oxidized lipid content.

4-Hydroxynonenal Contributes to Angiogenesis through a Redox-Dependent Sphingolipid Pathway: Prevention by Hydralazine Derivatives

The neovascularization of atherosclerotic lesions is involved in plaque development and may contribute to intraplaque hemorrhage and plaque fragilization and rupture. Among the various proangiogenic agents involved in the neovascularization process, proatherogenic oxidized LDLs (oxLDLs) contribute to the formation of tubes via the generation of sphingosine 1-phosphate (S1P), a major mitogenic and proangiogenic sphingolipid mediator. In this study, we investigated whether 4-hydroxynonenal (4-HNE), an aldehydic lipid oxidation product abundantly present in oxLDLs, contributes to their proangiogenic properties. Immunofluorescence analysis of human atherosclerotic lesions from carotid endarterectomy showed the colocalization of HNE-adducts with CD31, a marker of endothelial cells, suggesting a close relationship between 4-HNE and neovessel formation. In vitro, low 4-HNE concentration (0.5-1 µM) elicited the formation of tubes by human microvascular endothelial cells (HMEC-1), whereas higher concentrations were not angiogenic. The formation of tubes by 4-HNE involved the generation of reactive oxygen species and the activation of the sphingolipid pathway, namely, the neutral type 2 sphingomyelinase and sphingosine kinase-1 (nSMase2/SK-1) pathway, indicating a role for S1P in the angiogenic signaling of 4-HNE. Carbonyl scavengers hydralazine and bisvanillyl-hydralazone inhibited the nSMase2/SK1 pathway activation and the formation of tubes on Matrigel® evoked by 4-HNE. Altogether, these results emphasize the role of 4-HNE in the angiogenic effect of oxLDLs and point out the potential interest of pharmacological carbonyl scavengers to prevent the neovascularization process.

Angiogenesis in the atherosclerotic plaque

Atherosclerosis is a multifocal alteration of the vascular wall of medium and large arteries characterized by a local accumulation of cholesterol and non-resolving inflammation. Atherothrombotic complications are the leading cause of disability and mortality in western countries. Neovascularization in atherosclerotic lesions plays a major role in plaque growth and instability. The angiogenic process is mediated by classical angiogenic factors and by additional factors specific to atherosclerotic angiogenesis. In addition to its role in plaque progression, neovascularization may take part in plaque destabilization and thromboembolic events. Anti-angiogenic agents are effective to reduce atherosclerosis progression in various animal models. However, clinical trials with anti-angiogenic drugs, mainly anti-VEGF/VEGFR, used in anti-cancer therapy show cardiovascular adverse effects, and require additional investigations.

The neutral sphingomyelinase-2 is involved in angiogenic signaling triggered by oxidized LDL

Capillaries of the external part of the normal arterial wall constitute the vasa vasorum network. In atherosclerotic lesions, neovascularization occurs in areas of intimal hyperplasia where it may promote plaque expansion, and intraplaque hemorrhage. Oxidized LDL that are present in atherosclerotic areas activate various angiogenic signaling pathways, including reactive oxygen species and the sphingosine kinase/sphingosine-1-phosphate pathway. We aimed to investigate whether oxidized LDL-induced angiogenesis requires neutral sphingomyelinase-2 activation and the neutral sphingomyelinase-2/sphingosine kinase-1 pathway. The role of neutral sphingomyelinase-2 in angiogenic signaling was investigated in Human Microvascular Endothelial Cells (HMEC-1) forming capillary tube on Matrigel and in vivo in the Matrigel plug assay in C57BL/6 mice and in the chicken chorioallantoic membrane model. Low concentration of human oxidized LDL elicits HMEC-1 capillary tube formation and neutral sphingomyelinase-2 activation, which were blocked by neutral sphingomyelinase-2 inhibitors, GW4869 and specific siRNA. This angiogenic effect was mimicked by low concentration of C6-Ceramide and was inhibited by sphingosine kinase-1 inhibitors. Upstream of neutral sphingomyelinase-2, oxidized LDL-induced activation required LOX-1, reactive oxygen species generation by NADPH oxidase and p38-MAPK activation. Inhibition of sphingosine kinase-1 blocked the angiogenic response and triggered HMEC-1 apoptosis. Low concentration of oxidized LDL was angiogenic in vivo, both in the Matrigel plug assay in mice and in the chorioallantoic membrane model, and was blocked by GW4869. In conclusion, low oxLDL concentration triggers sprouting angiogenesis that involves ROS-induced activation of the neutral sphingomyelinase-2/sphingosine kinase-1 pathway, and is effectively inhibited by GW4869.

Oxysterols and mechanisms of survival signaling

Oxysterols, a family of oxidation products of cholesterol, are increasingly drawing attention of scientists to their multifaceted biochemical properties, several of them of clear relevance to human pathophysiology. Taken up by cells through both vesicular and non-vesicular ways or often generated intracellularly, oxysterols contribute to modulate not only the inflammatory and immunological response but also cell viability, metabolism and function by modulating several signaling pathways. Moreover, they have been recognized as elective ligands for the most important nuclear receptors. The outcome of such a complex network of intracellular reactions promoted by these cholesterol oxidation products appears to be largely dependent not only on the type of cells, the dynamic conditions of the cellular and tissue environment but also on the concentration of the oxysterols. Here focus has been given to the cascade of molecular events exerted by relatively low concentrations of certain oxysterols that elicit survival and functional signals in the cells, with the aim to contribute to further expand the knowledge about the biological and physiological potential of the biochemical reactions triggered and modulated by oxysterols.

Src regulates cigarette smoke-induced ceramide generation via neutral sphingomyelinase 2 in the airway epithelium

We previously demonstrated that the neutral sphingomyelinase (nSMase) 2 is the sole sphingomyelinase activated during cigarette smoke (CS)-induced oxidative stress of human airway epithelial cells, leading to ceramide generation and subsequent apoptosis of affected cells. Since then, we reported that nSMase2 is a phosphoprotein, the degree of enzymatic activity and stability of which are dictated by its degree of phosphorylation. Simultaneously, the non-receptor tyrosine kinase and proto-oncogene Src has increasingly become a target of interest in both smoking-related lung injury, such as chronic obstructive pulmonary disease, and lung cancer. Within this context, we tested and now present Src as a regulator of ceramide generation via modulation of nSMase2 phosphorylation and activity during CS-induced oxidative stress. Specifically, we provide evidence that Src activity is necessary for both CS-induced ceramide accumulation in vivo (129/Sv mice) and in vitro (human airway epithelial cells) and for nSMase2 activity during CS-induced oxidative stress. Moreover, because nSMase2 is exclusively phosphorylated on serines, we show that this occurs through Src-dependent activation of the serine/threonine kinase p38 mitogen-activated protein kinase during oxidative stress. Finally, we provide evidence that Src and p38 mitogen-activated protein kinase activities are critical for regulating nSMase2 phosphorylation. This study provides insights into a molecular target involved in smoking-related lung injury, represented here as nSMase2, and its modulation by the oncogene Src.

Role of Sphingosine-1-Phosphate in Transplant Vasculopathy Evoked by Anti-HLA Antibody

Transplant vasculopathy (TV) represents the main cause of late graft failure and limits the long-term success of organ transplantation. Cellular and humoral immune responses contribute to the pathogenesis of the concentric and diffuse intimal hyperplasia of arteries of the grafted organ. We recently reported that the mitogenic signaling, evoked in human vascular smooth muscle cells (hmSMC) by the anti-HLA class I monoclonal antibody W6/32, implicates neutral sphingomyelinase-2, suggesting a role for sphingolipids in intimal hyperplasia of TV. Here, we investigated whether the mitogenic sphingolipid, sphingosine-1-phosphate (S1P), is involved in intimal hyperplasia elicited by W6/32. Studies were done on cultured hmSMC and on an in vivo model of TV, consisting of human mesenteric arteries grafted into SCID/beige mice, injected weekly with W6/32. hmSMC migration and DNA synthesis elicited by W6/32 were inhibited by the sphingosine kinase-1 (SK1) inhibitor dimethylsphingosine, the anti-S1P antibody Sphingomab and the S1PR1/R3 inhibitor VPC23019. W6/32 stimulated SK1 activity, while siRNA silencing SK1, S1PR1 and S1PR3 inhibited hmSMC migration. In vivo, Sphingomab significantly reduced the intimal thickening induced by W6/32. These data emphasize the role of S1P in intimal hyperplasia elicited by the humoral immune response, and open perspectives for preventing TV with S1P inhibitors.

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

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

Phospholipase C and D regulation of Src, calcium release and membrane fusion during Xenopus laevis development

This review emphasizes how lipids regulate membrane fusion and the proteins involved in three developmental stages: oocyte maturation to the fertilizable egg, fertilization and during first cleavage. Decades of work show that phosphatidic acid (PA) releases intracellular calcium, and recent work shows that the lipid can activate Src tyrosine kinase or phospholipase C during Xenopus fertilization. Numerous reports are summarized to show three levels of increase in lipid second messengers inositol 1,4,5-trisphosphate and sn 1,2-diacylglycerol (DAG) during the three different developmental stages. In addition, possible roles for PA, ceramide, lysophosphatidylcholine, plasmalogens, phosphatidylinositol 4-phosphate, phosphatidylinositol 5-phosphate, phosphatidylinositol 4,5-bisphosphate, membrane microdomains (rafts) and phosphatidylinositol 3,4,5-trisphosphate in regulation of membrane fusion (acrosome reaction, sperm-egg fusion, cortical granule exocytosis), inositol 1,4,5-trisphosphate receptors, and calcium release are discussed. The role of six lipases involved in generating putative lipid second messengers during fertilization is also discussed: phospholipase D, autotaxin, lipin1, sphingomyelinase, phospholipase C, and phospholipase A2. More specifically, proteins involved in developmental events and their regulation through lipid binding to SH3, SH4, PH, PX, or C2 protein domains is emphasized. New models are presented for PA activation of Src (through SH3, SH4 and a unique domain), that this may be why the SH2 domain of PLCγ is not required for Xenopus fertilization, PA activation of phospholipase C, a role for PA during the calcium wave after fertilization, and that calcium/calmodulin may be responsible for the loss of Src from rafts after fertilization. Also discussed is that the large DAG increase during fertilization derives from phospholipase D production of PA and lipin dephosphorylation to DAG.

2013 Lush Science Prize

The annual Lush Science Prize is designed to reward outstanding contributions to 21st Century Toxicology Research. A Background Paper is prepared each year prior to the judging process, in order to provide the judging panel with a brief overview of current developments in the field of Replacement alternatives, particularly those relevant to the concept of toxicity pathways. The Background Paper includes information on some key institutional developments in the area--such as the OECD's Adverse Outcome Pathway Project, the Hamner Institute's work, and the Human Toxome Project, and on the phenomenon of collaborative computer systems relevant to the field. From the literature review that was also performed as part of the background research, the two papers receiving the highest score were recommended for consideration by the judges for the 2013 Science Prize.

Hyaluronan synthase-2 upregulation protects smpd3-deficient fibroblasts against cell death induced by nutrient deprivation, but not against apoptosis evoked by oxidized LDL

The neutral type 2 sphingomyelinase (nSMase2) hydrolyzes sphingomyelin and generates ceramide, a major bioactive sphingolipid mediator, involved in growth arrest and apoptosis. The role of nSMase2 in apoptosis is debated, and apparently contradictory results have been observed on fibroblasts isolated from nSMase2-deficient fragilitas ossium (homozygous fro/fro) mice. These mice exhibit a severe neonatal dysplasia, a lack of long bone mineralization and delayed apoptosis patterns of hypertrophic chondrocytes in the growth plate. We hypothesized that apoptosis induced by nutrient deprivation, which mimics the environmental modifications of the growth plate, requires nSMase2 activation. In this study, we have compared the resistance of fro/fro fibroblasts to different death inducers (oxidized LDL, hydrogen peroxide and nutrient starvation). The data show that nSMase2-deficient fro/fro cells resist to apoptosis evoked by nutrient starvation (fetal calf serum/glucose/pyruvate-free DMEM), whereas wt fibroblasts die after 48 h incubation in this medium. In contrast, oxidized LDL and hydrogen peroxide are similarly toxic to fro/fro and wt fibroblasts, indicating that nSMase2 is not involved in the mechanism of toxicity evoked by these agents. Interestingly, wt fibroblasts treated with the SMase inhibitor GW4869 were more resistant to starvation-induced apoptosis.

The resistance of fro/fro cells to starvation-induced apoptosis is associated with an increased expression of hyaluronan synthase 2 (HAS2) mRNAs and protein, which is inhibited by ceramide. In wt fibroblasts, this HAS2 rise and its protective effect did not occur, but exogenously added HA exhibited a protective effect against starvation-induced apoptosis.

The protective mechanism of HAS2 involves an increased expression of the heat-shock protein Hsp72, a chaperone with antiapoptotic activity. Taken together, these results highlight the role of nSMase2 in apoptosis evoked by nutrient starvation that could contribute to the delayed apoptosis of hypertrophic chondrocytes in the growth plate, and emphasize the antiapoptotic properties of HAS2.

•

Apoptosis evoked by oxidized LDL and H₂O₂ is comparable in fro/fro and wt fibroblasts.

•

fro/fro fibroblasts resist to apoptosis evoked by nutrient starvation.

•

HAS2 increased expression protects fro/fro fibroblasts against apoptosis.

•

HAS2 regulates the expression of the antiapoptotic heat-shock protein HsP72.

Annexin II-dependent actin remodelling evoked by hydrogen peroxide requires the metalloproteinase/sphingolipid pathway

Actin remodeling is a dynamic process associated with cell shape modification occurring during cell cycle and proliferation. Oxidative stress plays a role in actin reorganization via various systems including p38MAPK. Beside, the mitogenic response evoked by hydrogen peroxide (H₂O₂) in fibroblasts and smooth muscle cells (SMC) involves the metalloproteinase (MMPs)/sphingomyelinase 2 (nSMase2) signaling pathway. The aim of this work was to investigate whether this system plays a role in actin remodeling induced by H₂O₂.

Low H₂O₂ dose (5 µM) rapidly triggered a signaling cascade leading to nSMase2 activation, src and annexin 2 (AnxA2) phosphorylation, and actin remodeling, in fibroblasts and SMC. These events were blocked by pharmacological inhibitors of MMPs (Ro28-2653) and p38MAPK (SB203580), and were lacking in MMP2^−/− and in nSMase2-mutant (fro) fibroblasts. Likewise, H₂O₂ was unable to induce actin remodeling in fro and MMP2^−/− fibroblasts or in cells pretreated with p38MAPK, or MMP inhibitors. Finally we show that nSMase2 activation by H₂O₂, depends on MMP2 and p38MAPK, and is required for the src-dependent phosphorylation of AnxA2, and actin remodeling.

Taken together, these findings indicate for the first time that AnxA2 phosphorylation and actin remodeling evoked by oxidative stress depend on the sphingolipid pathway, via MMP2 and p38MAPK.

•

Low concentration of H₂O₂ activates matrix metalloproteinases MMP-2.

•

MMP-2 activates p38MAPK, type 2 neutral sphingomyelinase.

•

This signaling pathway induces annexin II phosphorylation via src.

•

This pathway is involved in actin remodeling due to H₂O₂ stimulation.

Oxidized LDL-induced angiogenesis involves sphingosine 1-phosphate: prevention by anti-S1P antibody

BACKGROUND AND PURPOSE

Neovascularization occurring in atherosclerotic lesions may promote plaque expansion, intraplaque haemorrhage and rupture. Oxidized LDL (oxLDL) are atherogenic, but their angiogenic effect is controversial; both angiogenic and anti-angiogenic effects have been reported. The angiogenic mechanism of oxLDL is partly understood, but the role of the angiogenic sphingolipid, sphingosine 1-phosphate (S1P), in this process is not known. Thus, we investigated whether S1P is involved in the oxLDL-induced angiogenesis and whether an anti-S1P monoclonal antibody can prevent this effect.

EXPERIMENTAL APPROACH

Angiogenesis was assessed by capillary tube formation by human microvascular endothelial cells (HMEC-1) cultured on Matrigel and in vivo by the Matrigel plug assay in C57BL/6 mice.

KEY RESULTS

Human oxLDL exhibited a biphasic angiogenic effect on HMEC-1; low concentrations were angiogenic, higher concentrations were cytotoxic. The angiogenic response to oxLDL was blocked by the sphingosine kinase (SPHK) inhibitor, dimethylsphingosine, by SPHK1-siRNA and by an anti-S1P monoclonal antibody. Moreover, inhibition of oxLDL uptake and subsequent redox signalling by anti-CD36 and anti-LOX-1 receptor antibodies and by N-acetylcysteine, respectively, blocked SPHK1 activation and tube formation. In vivo, in the Matrigel plug assay, low concentrations of human oxLDL or murine oxVLDL also triggered angiogenesis, which was prevented by i.p. injection of the anti-S1P antibody.

CONCLUSION AND IMPLICATIONS

These data highlight the role of S1P in angiogenesis induced by oxLDL both in HMEC-1 cultured on Matrigel and in vivo in the Matrigel plug model in mice, and demonstrate that the anti-S1P antibody effectively blocks the angiogenic effect of oxLDL.

Lung injury and lung cancer caused by cigarette smoke-induced oxidative stress: Molecular mechanisms and therapeutic opportunities involving the ceramide-generating machinery and epidermal growth factor receptor

Chronic obstructive pulmonary disease (COPD) and lung cancer are frequently caused by tobacco smoking. However, these diseases present opposite phenotypes involving redox signaling at the cellular level. While COPD is characterized by excessive airway epithelial cell death and lung injury, lung cancer is caused by uncontrolled epithelial cell proliferation. Notably, epidemiological studies have demonstrated that lung cancer incidence is significantly higher in patients who have preexisting emphysema/lung injury. However, the molecular link and common cell signaling events underlying lung injury diseases and lung cancer are poorly understood. This review focuses on studies of molecular mechanism(s) underlying smoking-related lung injury (COPD) and lung cancer. Specifically, the role of the ceramide-generating machinery during cigarette smoke-induced oxidative stress leading to both apoptosis and proliferation of lung epithelial cells is emphasized. Over recent years, it has been established that ceramide is a sphingolipid playing a major role in lung epithelia structure/function leading to lung injury in chronic pulmonary diseases. However, new and unexpected findings draw attention to its potential role in lung development, cell proliferation, and tumorigenesis. To address this dichotomy in detail, evidence is presented regarding several protein targets, including Src, p38 mitogen-activated protein kinase, and neutral sphingomyelinase 2, the major sphingomyelinase that controls ceramide generation during oxidative stress. Furthermore, their roles are presented not only in apoptosis and lung injury but also in enhancing cell proliferation, lung cancer development, and resistance to epidermal growth factor receptor-targeted therapy for treating lung cancer.