Redox-regulated signaling by lactosylceramide in the proliferation of human aortic smooth muscle cells

Lipid rafts keep NADPH oxidase in the inactive state in human renal proximal tubule cells

Recent studies have indicated the importance of cholesterol-rich membrane lipid rafts (LRs) in oxidative stress-induced signal transduction. Reduced nicotinamide-adenine dinucleotide phosphate (NADPH) oxidases, the major sources of reactive oxygen species, are implicated in cardiovascular diseases, including hypertension. We tested the hypothesis that NADPH oxidase subunits and activity are regulated by LRs in human renal proximal tubule cells. We report that a high proportion of p22(phox) and the small GTPase Rac1 are expressed in LRs in human renal proximal tubule cells. The D(1)-like receptor agonist, fenoldopam (1 micromol/L per 20 minutes) dispersed Nox subunits within LRs and non-LRs and decreased oxidase activity (30.7+/-3.3%). In contrast, cholesterol depletion (2% methyl-beta-cyclodextrin [beta CD]) translocated NADPH oxidase subunits out of LRs and increased oxidase activity (154.0+/-10.5% versus control, 103.1+/-3.4%), which was reversed by cholesterol repletion (118.9+/-9.9%). Moreover, NADPH oxidase activation by beta CD (145.5+/-9.0%; control: 98.6+/-1.6%) was also abrogated by the NADPH oxidase inhibitors apocynin (100.4+/-3.2%) and diphenylene iodonium (9.5+/-3.3%). Furthermore, beta CD-induced reactive oxygen species production was reversed by knocking down either Nox2 (81.0+/-5.1% versus beta CD: 162.0+/-2.0%) or Nox4 (108.0+/-10.8% versus beta CD: 152.0+/-9.8%). We have demonstrated for the first time that disruption of LRs results in NADPH oxidase activation that is abolished by antioxidants and silencing of Nox2 or Nox4. Therefore, in human renal proximal tubule cells, LRs maintain NADPH oxidase in an inactive state.

Involvement of very long fatty acid-containing lactosylceramide in lactosylceramide-mediated superoxide generation and migration in neutrophils

The neutral glycosphingolipid lactosylceramide (LacCer) forms lipid rafts (membrane microdomains) coupled with the Src family kinase Lyn on the plasma membranes of human neutrophils; ligand binding to LacCer activates Lyn, resulting in neutrophil functions, such as superoxide generation and migration (Iwabuchi and Nagaoka, Lactosylceramide-enriched glycosphingolipid signaling domain mediates superoxide generation from human neutrophils, Blood 100, 1454-1464, 2002 and Sato et al. Induction of human neutrophil chemotaxis by Candida albicans-derived beta-1,6-long glycoside side-chain-branched beta glycan, J. Leukoc. Biol. 84, 204-211, 2006). Neutrophilic differentiated HL-60 cells (D-HL-60 cells) express almost the same amount of LacCer as neutrophils. However, D-HL-60 cells do not have Lyn-associated LacCer-enriched lipid rafts and lack LacCer-mediated superoxide-generating and migrating abilities. Here, we examined the roles of LacCer molecular species of different fatty acid compositions in these processes. Liquid chromatography-mass spectrometry analyses revealed that the very long fatty acid C24:0 and C24:1 chains were the main components of LacCer (31.6% on the total fatty acid content) in the detergent-resistant membrane fraction (DRM) from neutrophil plasma membranes. In contrast, plasma membrane DRM of D-HL-60 cells included over 70% C16:0-LacCer, but only 13.6% C24-LacCer species. D-HL-60 cells loaded with C24:0 or C24:1-LacCer acquired LacCer-mediated migrating and superoxide-generating abilities, and allowed Lyn coimmunoprecipitation by anti-LacCer antibody. Lyn knockdown by siRNA completely abolished the effect of C24:1-LacCer loading on LacCer-mediated migration of D-HL-60 cells. Immunoelectron microscopy revealed that LacCer clusters were closely associated with Lyn molecules in neutrophils and C24:1-LacCer-loaded D-HL-60 cells, but not in D-HL-60 cells or C16:0-LacCer-loaded cells. Taken together, these observations suggest that LacCer species with very long fatty acids are specifically necessary for Lyn-coupled LacCer-enriched lipid raft-mediated neutrophil superoxide generation and migration.

Ceramide: a common pathway for atherosclerosis?

Plasma sphingomyelin concentration is correlated with the development of atherosclerosis. It has been found to exist in significantly higher concentrations in aortic plaque. This appears to have clinical relevance as well as it has been shown to be an independent predictor of coronary artery disease. Ceramide, the backbone of sphingolipids, is the key component which affects atherosclerotic changes through its important second-messenger role. This paper sheds light on some of the current literature supporting the significance of ceramide with respect to its interactions with lipids, inflammatory cytokines, homocysteine and matrix metalloproteinases. Furthermore, the potential therapeutic implications of modulating ceramide concentrations are also discussed.

Hydrogen peroxide mediates a transient vasorelaxation with tempol during oxidative stress

Tempol catalyzes the formation of H(2)O(2) from superoxide and relaxes blood vessels. We tested the hypothesis that the generation of H(2)O(2) by tempol in vascular smooth muscle cells during oxidative stress contributes to the vasorelaxation. Tempol and nitroblue tetrazolium (NBT) both metabolize superoxide in vascular smooth muscle cells, but only tempol generates H(2)O(2). Rat pressurized mesenteric arteries were exposed for 20 min to the thromboxane-prostanoid receptor agonist, U-46619, or norepinephrine. During U-46619, tempol caused a transient dilation (22 +/- 2%), whereas NBT was ineffective (2 +/- 1%), and neither dilated vessels constricted with norepinephrine, which does not cause vascular oxidative stress. Neither endothelium removal nor blockade of K(+) channels with 40 mM KCl affected the tempol-induced dilation, but catalase blunted the tempol dilation by 53 +/- 7%. Tempol, but not NBT, increased H(2)O(2) in rat mesenteric vessels detected with dichlorofluorescein. To test physiological relevance in vivo, topical application of tempol caused a transient dilation (184 +/- 20%) of mouse cremaster arterioles exposed to angiotensin II for 30 min, which was not seen with NBT (9 +/- 4%). The vasodilation to tempol was reduced by 68 +/- 6% by catalase. We conclude that the transient relaxation of blood vessels by tempol after prolonged exposure to U-46619 or angiotensin II is mediated in part via production of H(2)O(2) and is largely independent of the endothelium and potassium channels.

Diphenyleneiodonium suppresses apoptosis in cerulein-stimulated pancreatic acinar cells

NADPH oxidase has been considered a major source of reactive oxygen species in phagocytic and non-phagocytic cells. Apoptosis linked to oxidative stress has been implicated in pancreatitis. Recently, we demonstrated that NADPH oxidase subunits Nox1, p27phox, p47phox, and p67phox are constitutively expressed in pancreatic acinar cells, which are activated by cerulein, a cholecystokinin analogue. Cerulein induces an acute and edematous form of pancreatitis. We investigated whether inhibition of NADPH oxidase by diphenyleneiodonium suppresses the production of reactive oxygen species and apoptosis by determining viable cell numbers, DNA fragmentation, TUNEL staining, caspase-3 activity, and the expression of apoptosis-inducing factor in pancreatic acinar AR42J cells stimulated with cerulein. Inhibition on NADPH oxidase by diphenyleneiodonium was assessed by the alterations in NADPH oxidase activity and translocation of the cytosolic subunits p67phox and p47phox to the membrane. Intracellular Ca2+ level was monitored to investigate the relationship between NADPH oxidase and Ca2+ in cells stimulated with cerulein. As a result, cerulein induced the activation of NADPH, increased production of reactive oxygen species, and apoptotic indices determined by the expression of apoptosis-inducing factor, caspase-3 activation, TUNEL staining, DNA fragmentation, and cell viability. Treatment with DPI inhibited cerulein-induced activation of NADPH oxidase, the production of reactive oxygen species, and apoptosis, but not the increase of intracellular Ca2+ levels in pancreatic acinar cells. These results demonstrate that the cerulein-induced increase in intracellular Ca2+ level may be an upstream event of NADPH oxidase activation. Diphenyleneiodonium, an NADPH oxidase inhibitor, inhibits the expression of apoptosis-inducing factor and caspase-3 activation, and thus apoptosis in pancreatic acinar cells.

The possible contribution of a general glycosphingolipid transporter, GM2 activator protein, to atherosclerosis

We previously found that oxidized low-density lipoprotein (LDL) elevated the expression of mRNA of GalNAcbeta1-4[NeuNAcalpha2-3]Galbeta1-4Glc-Cer (GM2) ganglioside activator protein, in human monocyte-derived macrophages. Recently, GM2 activator protein has become known as a general glycosphingolipid transporter as well as a specific cofactor for the hydrolysis of GM2 ganglioside by lysosomal beta-hexosaminidase A. Accumulation of glycosphingolipids has been observed in the serum or aorta of atherosclerotic model animals and humans. The proliferation of aortic smooth muscle cells, elevation of LDL uptake by macrophages, interfering LDL clearance by the liver, and enhancement of platelet adhesion to collagen have been proposed as the underlying mechanisms of glycosphingolipid-mediated atherogenesis. The GM2 activator protein can bind, solubilize and transport a broad spectrum of lipid molecules, indicating that GM2 activator protein may function as a general intra- and inter-cellular lipid transport protein. Collectively, elevated levels of GM2 activator protein in the aorta may be another feature of human atherosclerosis.

Altered intracellular redox status in Gaucher disease fibroblasts and impairment of adaptive response against oxidative stress

Gaucher disease (GD) is a lysosomal storage disorder, due to glucosylceramide (GlcCer) accumulation in several body tissues, which causes cellular failure by yet unidentified mechanisms. Several evidence indicates that GD pathogenesis is associated to an impairment in intracellular redox state. In fibroblast primary cultures, reactive oxygen species (ROS) levels and protein carbonyl content resulted significantly increased in GD patients compared to healthy donors, suggesting that GD cells, facing a condition of chronic oxidative stress, have evolved an adaptive response to survive. The ROS rise is probably due to NAD(P)H oxidase activity, being inhibited by the treatment with diphenylene iodonium chloride. Interestingly, GD cells are more sensitive to H(2)O(2) induced cell death, suggesting a dysregulation in the adaptive response to oxidative stress in which APE1/Ref-1 plays a central role. We found that the cytoplasmic amounts of APE1/Ref-1 protein were significantly higher in GD fibroblasts with respect to controls, and that GD cells failed to upregulate its expression upon H(2)O(2) treatment. Both ROS and APE1/Ref-1 increases are due to GlcCer accumulation, being prevented by treatment of GD fibroblasts with Cerezyme and induced in healthy fibroblasts treated with conduritol-beta-epoxide. These data, suggesting that GD cells display an impairment in the cellular redox state and in the adaptive cellular response to oxidative stress, may open new perspectives in the comprehension of GD pathogenesis.

Lactosylceramide is required in apoptosis induced by N-Smase

Lactosylceramide (LacCer) is a member of the glycosphingolipid family which has been recently recognized as a signaling intermediate in the regulation of cell proliferation and cell adhesion. In this paper, we present our studies pointing to a potential role of LacCer in inducing apoptosis. In our studies we employed a human osteosarcoma cell line MG-63 (wild type, WT) and a neutral sphingomyelinase (N-SMase) deficient cell line CC derived from MG-63 (mutant) cells. We observed that WT cells were highly sensitive to tumor necrosis factor-alpha (TNF-alpha), ceramide and LacCer-induced apoptosis. In contrast, the mutant cells were insensitive to TNF-alpha-induced apoptosis as they did not generate ceramide and LacCer. However, the exogenous supply of ceramide and/or LacCer rendered the mutant cells apoptotic. Interestingly, preincubation of cells with D-threo-1-phenyl-2-decanoylamino-3-morpholino-1-propanol (D-PDMP), an inhibitor of glucosylceramide synthase and lactosylceramide synthase, abrogated ceramide-induced apoptosis but not LacCer-induced apoptosis in both WT cells and the mutant cells. Moreover, TNF-alpha and LacCer-induced apoptosis required the generation of reactive oxygen species (ROS) in WT cells. However, since mutant cells did not produce significant amounts of LacCer and ROS in response to TNF-alpha treatment they are insensitive to TNF-alpha-induced apoptosis. In summary, our studies suggest that TNF-alpha-induced N-SMase activation and production of ceramide is required to activate the apoptosis pathway in human osteosarcoma cells. But it is not sufficient to induce apoptosis. Rather, the conversion of ceramide to LacCer and ROS generation are critical for apoptosis.

Local glutathione redox status does not regulate ileal mucosal growth after massive small bowel resection in rats

Glutathione (GSH) concentration affects cell proliferation and apoptosis in intestinal and other cell lines in vitro. However, in vivo data on gut mucosal GSH redox status and cell turnover are limited. We investigated the effect of altered GSH redox status on the ileal mucosa in a rat model of short bowel syndrome following massive small bowel resection (SBR). Rats underwent 80% mid-jejunoileal resection (RX) or small bowel transection (TX; as operative controls), with administration of either saline or D, L-buthionine-sulfoximine (BSO), a specific inhibitor of cellular GSH synthesis. Ileal mucosal redox, morphology, and indices of cell proliferation and apoptosis were determined at different days after surgery. Ileal GSH redox status was assessed by GSH and GSH disulfide (GSSG) concentrations and the redox potential of GSH/GSSG (Eh). Ileal lipid peroxidation [free malondialdehyde (MDA)] was measured as an index of lipid peroxidation. BSO markedly decreased ileal mucosal GSH, oxidized GSH/GSSG Eh, and increased MDA content without inducing morphological damage as assessed by light or electron microscopy. As expected, SBR stimulated adaptive growth of ileal villus height and total mucosal height at 7 d after surgery, but this response was unaffected by BSO treatment despite a modest increase in crypt cell apoptosis. Ileal cell proliferation (crypt cell bromodeoxyuridine incorporation) increased at 2 d after SBR but was unaffected by BSO. Collectively, our in vivo data show that marked depletion of ileal GSH and oxidation of the GSH redox pool does not alter indices of ileal epithelial proliferation or SBR-induced ileal mucosal adaptive growth.

Nitric oxide in blood. The nitrosative-oxidative disequilibrium hypothesis on the pathogenesis of cardiovascular disease

There is growing evidence that altered production and/or spatio-temporal distribution of reactive oxidant species and reactive nitrosative species in blood creates oxidative and/or nitrosative stresses in the failing myocardium and endothelium. This contributes to the abnormal cardiac and vascular phenotypes that characterize cardiovascular disease. These derangements at the system level can now be interpreted at the integrated cellular and molecular levels in terms of effects on signaling elements in the heart and vasculature. The end results of nitric oxide/redox disequilibrium have implications for cardiac and vascular homeostasis and may result in the development of atherosclerosis, myocardial tissue remodelling and hypertrophy. Reactive oxygen species/reactive nitrogen species generation is also attributed to the transit from hypertrophic to apoptotic phenotypes, a possible mechanism of myocardial failure. In this review, we highlight the possible roles of altered production and/or spatio-temporal distribution of reactive oxidant species and reactive nitrosative species in blood on the pathogenesis of the failing cardiovascular system.

Inhibition of atherosclerosis by the serine palmitoyl transferase inhibitor myriocin is associated with reduced plasma glycosphingolipid concentration

Glycosphingolipids (GSL) have been implicated as potential atherogenic lipids. Inhibition of hepatic serine palmitoyl transferase (SPT) reduces plasma sphingomyelin (SM) levels in the absence of changes in cholesterol or triglyceride (TG) concentration and this leads to a reduction of atherosclerosis in apolipoprotein-E gene knockout (apoE(-/-)) mice. The possibility that the reduced atherosclerosis resulting from SPT inhibition is associated with decreases in plasma GSL concentration has not been examined and was the primary aim of this investigation. We show that intraperitoneal delivery of the SPT inhibitor myriocin for 9 weeks inhibits atherosclerosis in apoE(-/-) mice fed a high fat diet. Lesion inhibition was most pronounced at the aortic arch and distal sites of the thoracic and abdominal aorta. There was also a trend towards a reduction in lesion area at the aortic root. Myriocin treatment resulted in significant reductions in both plasma SM and GSL concentration of 42% and 25%, as assessed by enzymatic and HPLC methods, respectively. Moreover, SM and GSL concentrations were significantly correlated, indicating that SPT inhibition suppresses the synthesis of both these sphingolipids concomitantly. The inhibition of atherosclerosis induced by myriocin was not associated with changes in plasma cholesterol or TG concentrations or lipoprotein profiles as determined by FPLC. These data indicate that therapeutic reduction of plasma SM and/or GSL concentrations may offer a novel treatment for atherosclerosis.

Stimulation of colonic mucosal growth associated with oxidized redox status in rats

Limited data in animal models suggest that colonic mucosa undergoes adaptive growth following massive small bowel resection (SBR). In vitro data suggest that intestinal cell growth is regulated by reactive oxygen species and redox couples [e.g., glutathione (GSH)/glutathione disulfide (GSSG) and cysteine (Cys)/cystine (CySS) redox]. We investigated the effects of SBR and alterations in redox on colonic growth indexes in rats after either small bowel transection (TX) or 80% midjejunoileal resection (RX). Rats were pair fed +/- blockade of endogenous GSH synthesis with buthionine sulfoximine (BSO). Indexes of colonic growth, proliferation, and apoptosis and GSH/GSSG and Cys/CySS redox potentials (E(h)) were determined. RX significantly increased colonic crypt depth, number of cells per crypt, and epithelial cell proliferation [crypt cell bromodeoxyuridine (BrdU) incorporation]. Administration of BSO markedly decreased colonic mucosal GSH, GSSG, and Cys concentrations in both TX and RX groups, with a resultant oxidation of GSH/GSSG and Cys/CySS E(h). BSO did not alter colonic crypt cell apoptosis but significantly increased all colonic mucosal growth indexes (crypt depth, cells/crypt, and BrdU incorporation) in both TX and RX groups in a time- and dose-dependent manner. BSO significantly decreased plasma GSH and GSSG, oxidized GSH/GSSG E(h), and increased plasma Cys and CySS concentrations. Collectively, these data provide in vivo evidence indicating that oxidized colonic mucosal redox status stimulates colonic mucosal growth in rats. The data also suggest that GSH is required to maintain normal colonic and plasma Cys/CySS homeostasis in these animal models.

Recent advances in the immunobiology of ceramide

Ceramide, a sphingosine-based lipid molecule, has emerged as a key regulator of a wide spectrum of biological processes such as cellular differentiation, proliferation, apoptosis and senescence. Sphingomyelinase-dependent hydrolysis of sphingomyelin and de novo synthesis involving the coordinated action of serinepalmitoyl transferase and ceramide synthase are the two major pathways involved in ceramide synthesis. Clustering of plasma membrane rafts into ceramide-enriched platforms serves as an important transmembrane signaling mechanism for cell surface receptors. Ceramides have been implicated in apoptosis, stress signaling cascades as well as ion channels. There is accumulating evidence that targeted manipulation of ceramide metabolism pathway has immense therapeutic potential and may eventually prove to be a boon in the design of novel strategies and development of innovative treatments for diverse conditions including cardiovascular diseases, cancer and Alzheimer's disease. As yet uncharacterized natural ceramide analogs and novel inhibitors of ceramide metabolism might prove to have potent effects in the drugs. In this review, we discuss significant advances that continue to provide intriguing insights into the complex cellular and molecular mechanisms underlying ceramide-mediated signaling cascades.

Reactive oxygen species mediates disialoganglioside GD3-induced inhibition of ERK1/2 and matrix metalloproteinase-9 expression in vascular smooth muscle cells

Sialic acid containing glycosphingolipids (gangliosides) are thought to play important roles in the function of various biological phenomena such as atherosclerosis. We have previously shown that the overexpression of the disialoganglioside (GD3) synthase gene effectively suppresses cell proliferation, cell cycle progression, and MMP-9 expression in vascular smooth muscle cells (VSMC). However, the issue of how the overexpression of GD3 synthase gene results in the inhibition of cellular responses in VSMC remains unclear. The findings herein demonstrate that overexpression of the GD3 synthase gene suppresses VSMC responses through the generation of reactive oxygen species (ROS). Superoxide and hydrogen peroxide were generated at increased levels in GD3 synthase gene transfectants in comparison with empty vector (EV) -transfected VSMC. This phenomenon was blocked by antioxidants such as N-acetyl-L-cysteine (NAC) and pyrrolidine dithiocarbamate (PDTC). Increased ROS generation was associated with a decreased endogenous antioxidant activity, increased lipid peroxidation, and mitochondrial DNA damage. Further studies revealed that the blockade of ROS function with antioxidants reversed the effect of GD3 synthase gene overexpression on VSMC proliferation and cell cycle regulation in response to platelet-derived growth factor (PDGF). In addition, we found that treatment with antioxidants reversed the decreased matrix metalloproteinase-9 (MMP-9) expression in response to TNF-alpha as determined by zymography and immunoblot in GD3 synthase gene transfectants. This recovery effect was characterized by the up-regulation of MMP-9 promoter activity, which was transcriptionally regulated at NF-kappaB and activation protein-1 (activating protein (AP) -1) sites in the MMP-9 promoter. These findings suggest that ROS may play a role in GD3 synthase gene-mediated VSMC phenotypic changes that may contribute to plaque instability in atherosclerosis.

Sphingolipid signaling and redox regulation

Sphingolipids including ceramide and its derivatives such as ceramide-1-phosphate, glycosyl-ceramide, and sphinogosine (-1-phosphate) are now recognized as novel intracellular signal mediators for regulation of inflammation, apoptosis, proliferation, and differentiation. One of the important and regulated steps in these events is the generation of these sphingolipids via hydrolysis of sphingomyelin through the action of sphingomyelinases (SMase). Several lines of evidence suggest that reactive oxygen species (ROS; O2-, H2O2, and OH-,) and reactive nitrogen species (RNS; NO, and ONOO-) and cellular redox potential, which is mainly regulated by cellular glutathione (GSH), are tightly linked to the regulation of SMase activation. On the other hand, sphingolipids are also known to play an important role in maintaining cellular redox homeostasis through regulation of NADPH oxidase, mitochondrial integrity, and antioxidant enzymes. Therefore, this paper reviews the relationship between cellular redox and sphingolipid metabolism and its biological significance.

Lipid-lowering drugs

Although a change in life-style is often the method of first choice for lipid lowering, lipid-lowering drugs, in general, help to control elevated levels of different forms of lipids in patients with hyperlipidemia. While one group of drugs, statins, lowers cholesterol, the other group, fibrates, is known to take care of fatty acids and triglycerides. In addition, other drugs, such as ezetimibe, colesevelam, torcetrapib, avasimibe, implitapide, and niacin are also being considered to manage hyperlipidemia. As lipids are very critical for cardiovascular diseases, these drugs reduce fatal and nonfatal cardiovascular abnormalities in the general population. However, a number of recent studies indicate that apart from their lipid-lowering activities, statins and fibrates exhibit multiple functions to modulate intracellular signaling pathways, inhibit inflammation, suppress the production of reactive oxygen species, and modulate T cell activity. Therefore, nowadays, these drugs are being considered as possible therapeutics for several forms of human disorders including cancer, autoimmunity, inflammation, and neurodegeneration. Here I discuss these applications in the light of newly discovered modes of action of these drugs.

Physiological and pathophysiological aspects of ceramide

Activation of cells by receptor- and nonreceptor-mediated stimuli not only requires a change in the activity of signaling proteins but also requires a reorganization of the topology of the signalosom in the cell. The cell membrane contains distinct domains, rafts that serve the spatial organization of signaling molecules in the cell. Many receptors or stress stimuli transform rafts by the generation of ceramide. These stimuli activate the acid sphingomyelinase and induce a translocation of this enzyme onto the extracellular leaflet of the cell membrane. Surface acid sphingomyelinase generates ceramide that serves to fuse small rafts and to form large ceramide-enriched membrane platforms. These platforms cluster receptor molecules, recruit intracellular signaling molecules to aggregated receptors, and seem to exclude inhibitory signaling factors. Thus ceramide-enriched membrane platforms do not seem to be part of a specific signaling pathway but may facilitate and amplify the specific signaling elicited by the cognate stimulus. This general function may enable these membrane domains to be critically involved in the induction of apoptosis by death receptors and stress stimuli, bacterial and viral infections of mammalian cells, and the regulation of cardiovascular functions.

Mechanism of Siglec-8-induced human eosinophil apoptosis: role of caspases and mitochondrial injury

Sialic acid binding immunoglobulin like lectin (Siglec)-8 crosslinking with specific antibodies causes human eosinophil apoptosis. Mechanisms by which Siglec-8 crosslinking induces apoptosis are not known. Peripheral blood eosinophils were examined for caspase, mitochondria and reactive oxygen species (ROS) involvement after incubating the cells with anti-Siglec-8 crosslinking Abs or control Abs, in the presence or absence of selective inhibitors. Siglec-8 crosslinking induced rapid cleavage of caspase-3, caspase-8, and caspase-9 in eosinophils. Selective caspase-8 and/or caspase-9 inhibitors inhibited this apoptosis. Siglec-8 crosslinking on eosinophils increased dissipation of mitochondrial membrane potential upstream of caspase activation. Rotenone and antimycin, inhibitors of mitochondrial respiratory chain components, completely inhibited apoptosis. Additional experiments with an inhibitor of ROS, diphenyleneiodonium, demonstrated that ROS was also essential for Siglec-8-mediated apoptosis and preceded Siglec-8-mediated mitochondrial dissipation. These experiments show that Siglec-8-induced apoptosis occurs through the sequential production of ROS, followed by induction of mitochondrial injury and caspase cleavage.

Inhibitory effect of trilinolein on endothelin-1-induced c-fos gene expression in cultured neonatal rat cardiomyocytes

Trilinolein, isolated from the traditional Chinese herb Sanchi (Panax notoginseng), has been shown to have myocardial protective effects via its antioxidant ability. However, the cellular and molecular mechanisms of the protective effect of trilinolein in the heart remain to be elucidated. Oxidative mechanisms have been implicated in neonatal cardiomyocyte hypertrophy. We previously reported that ET-1 induces ROS generation via the ET(A) receptor and ROS modulates c-fos gene expression. We have therefore examined whether trilinolein attenuates ROS production and ET-1-induced c-fos gene expression in cardiomyocytes. Cultured neonatal rat cardiomyocytes were stimulated with ET-1 (10 nM), and c-fos gene expression was examined. Trilinolein (1 and 10 microM) inhibited ET-1-induced c-fos gene expression in cardiomyocytes. We also examined the effects of trilinolein on ET-1-increased NADPH oxidase activity and superoxide formation. Trilinolein inhibited ET-1-increased NADPH oxidase activity and superoxide formation in a concentration-dependent manner. This increase in superoxide production by ET-1 was significantly inhibited by trilinolein, diphenyleneiodonium, or N-acetylcysteine. Trilinolein also decreased ET-1- or H2O2-induced extracellular signal-regulated kinase (ERK) phosphorylation, c-Jun NH2-terminal kinase (JNK) phosphorylation, and activator protein-1 activation. These data indicate that trilinolein inhibits ET-1-induced ERK phosphorylation, JNK phosphorylation, and c-fos gene expression via attenuating superoxide production in cardiomyocytes.

PrP(106-126) activates neuronal intracellular kinases and Egr1 synthesis through activation of NADPH-oxidase independently of PrPc

Prion diseases are characterised by severe neural lesions linked to the presence of an abnormal protease-resistant isoform of cellular prion protein (PrPc). The peptide PrP(106-126) is widely used as a model of neurotoxicity in prion diseases. Here, we examine in detail the intracellular signalling cascades induced by PrP(106-126) in cortical neurons and the participation of PrPc. We show that PrP(106-126) induces the activation of subsets of intracellular kinases (e.g., ERK1/2), early growth response 1 synthesis and induces caspase-3 activity, all of which are mediated by nicotinamide adenine dinucleotide phosphate hydrogen-oxidase activity and oxidative stress. However, cells lacking PrPc are similarly affected after peptide exposure, and this questions the involvement of PrPc in these effects.

Identification of a biologically active component in minimally oxidized low density lipoprotein (MM-LDL) responsible for aortic smooth muscle cell proliferation

Although low concentrations (10 microg/ml) of oxidized LDL density lipoproteins (Ox-LDL) and minimally modified LDL (MM-LDL) can stimulate the proliferation of aortic smooth muscle cells the biologically active component responsible for this phenomena has not been identified. Here we report that the 1-palmitoyl-2-(5-oxovaleroyl)-sn-glycero-4-phosphocholine (m/e594.3) (POVPC) present in MM-LDL but not 1-palmitoyl-2-glutaryl-sn-glycero-3-phophochline (m/e610.2)(PGPC) can stimulate the activity of UDP-galactose:glucosylceramide (beta 1-->4) galactosyltransferase (GalT-2) and produce lactosyceramide (LacCer). LacCer, in turn, generated superoxide radicals (O(2)(.-)). This is accompanied by the phosphorylation/activation of a cytosolic transcriptional factor p(44) MAPK and the subsequent proliferation of human aortic smooth muscle cells. D-threo-1-phenyl-2-decanoylamino-3-morpholino-1-propanol (D-PDMP), an inhibitor of GalT-2, impaired the induction of GalT-2 activity, O(2)(.-)generation, and cell proliferation. Thus POVPC may serve as a surrogate in MM-LDL mediated induction of aortic smooth muscle cells (A-SMC) proliferation via GalT-2 activation. The LacCer produced as a consequence of GalT-2 activation may serve as a lipid second messenger in the activation of an oxidant sensitive transcriptional pahtway that ultimately leads to cell proliferation and may contribute to the pathophysiology of atherosclerosis.

A novel role of lactosylceramide in the regulation of tumor necrosis factor alpha-mediated proliferation of rat primary astrocytes. Implications for astrogliosis following neurotrauma

The present study describes the role of glycosphingolipids in neuroinflammatory disease and investigates tumor necrosis factor alpha (TNFalpha)-induced astrogliosis following spinal cord injury. Astrogliosis is the hallmark of neuroinflammation and is characterized by proliferation of astrocytes and increased glial fibrillary acidic protein (GFAP) gene expression. In primary astrocytes, TNFalpha stimulation increased the intracellular levels of lactosylceramide (LacCer) and induced GFAP expression and astrocyte proliferation. D-threo-1-phenyl-2-decanoylamino-3-morpholino-1-propanol.HCl (PDMP), a glucosylceramide synthase and LacCer synthase (GalT-2) inhibitor, inhibited astrocyte proliferation and GFAP expression, which were reversed by exogenous supplementation of LacCer but not by other glycosphingolipids. TNFalpha caused a rapid increase in the activity of GalT-2 and synthesis of LacCer. Silencing of GalT-2 gene using antisense oligonucleotides also attenuated the proliferation of astrocytes and GFAP expression. The PDMP and antisense-mediated inhibition of proliferation and GFAP expression was well correlated with decreased Ras/ERK1/2 pathway activation. Furthermore, TNFalpha-mediated astrocyte proliferation and GFAP expression was also inhibited by LY294002, a phosphatidylinositol 3-kinase inhibitor, which was reversed by exogenous LacCer. LY294002 also inhibited TNFalpha-induced GalT-2 activation and LacCer synthesis, suggesting a phosphatidylinositol 3-kinase-mediated regulation of GalT-2. In vivo, PDMP treatment attenuated chronic ERK1/2 activation and spinal cord injury (SCI)-induced astrocyte proliferation with improved functional recovery post-SCI. Therefore, the in vivo studies support the conclusions drawn from cell culture studies and provide evidence for the role of LacCer in TNFalpha-induced astrogliosis in a rat model of SCI. To our knowledge, this is the first report demonstrating the role of LacCer in the regulation of TNFalpha-induced proliferation and reactivity of primary astrocytes.

Ceramide-Induced Impairment of Endothelial Function Is Prevented by CuZn Superoxide Dismutase Overexpression

Objective— Ceramide is an important intracellular second messenger that may also increase superoxide. The goal of this study was to determine whether overexpression of CuZn superoxide dismutase (SOD) protects against ceramide-induced increases in vascular superoxide and endothelial dysfunction.

Methods and Results— Carotid arteries from CuZnSOD-transgenic (CuZnSOD-Tg) and nontransgenic littermates were examined in vitro. Immunohistochemistry confirmed that CuZnSOD protein was greater in carotid artery from CuZnSOD-Tg compared with nontransgenic mice. Ceramide ( N -acetyl- d -sphingosine; 1 and 10 μmol/L) produced concentration-dependent impairment ( P <0.05) of vasorelaxation in response to the endothelium-dependent agonist acetylcholine (ACh) in nontransgenic mice. For example, 100 μmol/L ACh relaxed arteries from nontransgenic mice by 96±4% and 52±5% in the presence of vehicle and 10 μmol/L ceramide, respectively. In contrast, ceramide (1 or 10 μmol/L) had no effect ( P >0.05) on responses of carotid artery to ACh in CuZnSOD-Tg mice. Ceramide had no effect on nitroprusside- or papaverine-induced relaxation in CuZnSOD-Tg or nontransgenic mice. Ceramide increased superoxide in arteries from nontransgenic vessels, and this effect was prevented by polyethyleneglycol-SOD (50 U/mL) or overexpression of CuZnSOD.

Conclusions— These results suggest that ceramide-induced increases in superoxide impair endothelium-dependent relaxation, and that select overexpression of the CuZn isoform of SOD prevents ceramide-induced oxidative stress in vessels.

A novel role of lactosylceramide in the regulation of lipopolysaccharide/interferon-gamma-mediated inducible nitric oxide synthase gene expression: implications for neuroinflammatory diseases

In the present study a possible role of glycosphingolipids (GSLs) in inducible nitric oxide synthase (iNOS) gene expression and nitric oxide (NO) production after spinal cord injury (SCI) in rats has been established. In primary rat astrocytes lipopolysaccharide (LPS) and interferon-gamma (IFN-gamma) treatment increased the intracellular levels of lactosylceramide (LacCer) and induced iNOS gene expression. d-Threo-1-phenyl-2-decanoylamino-3-morpholino-1-propanol.HCI (PDMP), a glucosylceramide synthase and LacCer synthase (galactosyltransferase, GalT-2) inhibitor, inhibited LPS/IFN-gamma induced iNOS expression, which was reversed by exogenously supplied LacCer, but not by other glycosphingolipids. LPS/IFN-gamma caused a rapid increase in the activity of GalT-2 and synthesis of LacCer. Silencing of GalT-2 gene with the use of antisense oligonucleotides resulted in decreased LPS/IFN-gamma-induced iNOS, TNF-alpha, and IL-1beta gene expression. The PDMP-mediated reduction in LacCer production and inhibition of iNOS expression correlated with decreased Ras and ERK1/2 activation along with decreased IkappaB phosphorylation, NF-kappaB DNA binding activity, and NF-kappaB-luciferase reporter activity. LacCer-mediated Ras activation was redox-mediated and was attenuated by antioxidants N-acetyl cysteine (NAC) and pyrrolidine dithiocarbamate (PDTC). In vivo administration of PDMP after SCI resulted in improved functional outcome (Basso, Beattie, Bresnahan score); inhibition of iNOS, TNF-alpha, and IL-1beta expression; decreased neuronal apoptosis; and decreased tissue necrosis and demyelination. The in vivo studies supported the conclusions drawn from cell culture studies and provided evidence for the possible role of GalT-2 and LacCer in SCI-induced inflammation and pathology. To our knowledge this is the first report of a role of LacCer in iNOS expression and the advantage of GSL depletion in attenuating post-SCI inflammation to improve the outcome of SCI.

Nuclear factor kappa B activation by NADPH oxidases

Reactive oxygen species (ROS) initiate activation of the transcription factor NF-kappaB in a variety of cell systems. Perhaps the most potent biological source of ROS is the NADPH oxidase of phagocytic cells, a multi-component system that catalyzes the formation of superoxide anion. Although phagocytes use this oxidase to kill ingested microorganisms, the products also mediate a broad range of biological oxidation reactions and some evidence exists for activation of NF-kappaB through this mechanism. Moreover, the components of the phagocyte NADPH oxidase are present in certain non-phagocytic cells and recently discovered homologues of the catalytic component gp91(phox) are expressed in a number of tissues. We explored the hypothesis that the products of NADPH oxidases cause the activation of NF-kappaB. K562 human erythrokeukemia cells transfected with constructs for expression of gp91(phox), plus other essential NADPH oxidase components generated substantial amounts of superoxide when activated with phorbol ester, lesser amounts with arachidonic acid exposure, and none with TNFalpha. Gel shift assays demonstrated induction of NF-kappaB in K562 cells exposed to TNFalpha and specificity was shown by oligonucleotide competition. Supershift assays demonstrated the presence in nuclear complexes of the NF-kappaB components p65/RelA and p50. Nuclear complexes of identical electrophoretic mobility were induced in phorbol ester-stimulated K562 cells that expressed the complete NADPH oxidase system, but not in cells lacking one of the essential oxidase components. K562 cells were relatively resistant to NF-kappaB induction by exogenous peroxide, but certain other cell types (HEK293 and HeLaS3) demonstrated such induction upon exposure to reagent hydrogen peroxide or glucose oxidase plus glucose and this was blocked by catalase. Finally, we found a biphasic pattern of gp91(phox) expression in rat liver during aging. High levels observed in young animals decreased in middle age, but increased again in old age. Collectively, these studies demonstrate the potential for NADPH-dependent induction of NF-kappaB and raise the possibility of a role for this pathway in the biology of aging.

Angioplasty-induced superoxide anions and neointimal hyperplasia in the rabbit carotid artery: suppression by the isoflavone trans-tetrahydrodaidzein

Reactive oxygen species (ROS) may contribute to the development of stenosis in balloon catheter injured arteries. As isoflavones exhibit effects on ROS and cell proliferation In vitro that appear useful in preventing such stenosis, we examined the effects of the isoflavone trans-tetrahydrodaidzein (trans-THD) on development of neointimal lesions in relation to elevations in ROS in balloon catheter injured arteries. Carotid arteries of rabbits treated with either vehicle or trans-THD were injured with an inflated balloon catheter and cell proliferation, collagen content, ROS and vessel structure determined over the ensuing 28 days. Seven days after injury neointimal smooth muscle cell proliferation was reduced by 50% (p < 0.05) whilst medial cell proliferation was largely unaffected (p > 0.10). At this time ROS levels in vehicle-treated rabbits were elevated 3-fold compared to uninjured arteries (p < 0.05). Treatment with trans-THD reduced ROS levels to those seen in uninjured arteries (p > 0.05). The antiproliferative effects of trans-THD on intimal cell proliferation persisted 14 days after the injury, and twenty eight days after injury the size of the lumen in trans-THD-treated animals was 27% greater (p < 0.05) and the intima area: vessel area reduced by 40% (p < 0.05). The small effects of trans-THD on collagen accumulation was not statistically significant, indicating that effects on neointimal cell proliferation was the major mechanism by which this isoflavone attenuated development of the neointima. Intimal smooth muscle cells and ROS represent potentially important targets for the antiproliferative actions of trans-THD in injured arteries. Strategies using such isoflavones may be useful for preventing restenosis after vascular manipulations in humans.

Ceramide Triggers Weibel–Palade Body Exocytosis

The sphingolipid ceramide mediates a variety of stress responses, including vascular inflammation and thrombosis. Activated endothelial cells release Weibel-Palade bodies, granules containing von Willebrand factor (vWF) and P-selectin, which induce leukocyte rolling and platelet adhesion and aggregation. We hypothesized that ceramide induces vascular inflammation and thrombosis in part by triggering Weibel-Palade body exocytosis. We added ceramide to human aortic endothelial cells and assayed Weibel-Palade body exocytosis by measuring the concentration of vWF released into the media. Exogenous ceramide induces vWF release from endothelial cells in a dose-dependent manner. Activators of endogenous ceramide production, neutral sphingomyelinase, or tumor necrosis factor-α also induce Weibel-Palade body exocytosis. We next studied NO effects on ceramide-induced Weibel-Palade body exocytosis because NO can inhibit vascular inflammation. The NO donor S -nitroso- N -acetylpenicillamine decreases ceramide-induced vWF release in a dose-dependent manner, whereas the NO synthase inhibitor N G -nitro- l -arginine methyl ester increases ceramide-induced vWF release. In summary, our findings show that endogenous ceramide triggers Weibel-Palade body exocytosis, and that endogenous NO inhibits ceramide-induced exocytosis. These data suggest a novel mechanism by which ceramide induces vascular inflammation and thrombosis.

Stability studies of C-4',6' acetal benzylmaltosides synthesized as inhibitors of smooth muscle cell proliferation

In our investigations to synthesize inhibitors of smooth muscle cell (SMC) proliferation, compound 6a displayed submicromolar activity in in vitro antiproliferative assays and reduced intimal thickening using a rat balloon angioplasty model via i.v. administration. In order to identify analogs that could be administered orally, the chemical instability of the C-4',6' acetal of compound 6a was addressed. Several novel variants with increased acid stability and comparable in vitro activity were prepared.

Complement-independent Ab-induced peroxide lysis of platelets requires 12-lipoxygenase and a platelet NADPH oxidase pathway

Antiplatelet GPIIIa49-66 Ab of HIV-related thrombocytopenic patients induces thrombocytopenia and platelet fragmentation by the generation of peroxide and other reactive oxygen species (ROS). Here we report the presence of a functional platelet NADPH oxidase pathway that requires activation by the platelet 12-lipoxygenase (12-LO) pathway to fragment platelets. A new Ab-mediated mechanism is described in which the platelet 12-LO product, 12(S)-HETE activates the NADPH oxidase pathway to generate ROS.

Lactosylceramide recruits PKCalpha/epsilon and phospholipase A2 to stimulate PECAM-1 expression in human monocytes and adhesion to endothelial cells

Despite the importance of platelet/endothelial cell adhesion molecule-1 (PECAM-1, CD31) in the adhesion and diapedesis of monocytes/lymphocytes, little is known about the mechanisms by which it is regulated. We explored the role of a glycosphingolipid, lactosylceramide (LacCer), in modulating PECAM-1 expression and cell adhesion in human monocytes. We observed that LacCer specifically exerted a time-dependent increase in PECAM-1 expression in U-937 cells. Maximal increase in PECAM-1 protein occurred after incubation with LacCer for 60 min. LacCer activated PKCalpha and -epsilon by translocating them from cytosol to membrane. This was accompanied by the activation of phospholipase A(2) (PLA(2)) and the increase of cell adhesion, which were abrogated by chelerythrine chloride, 2-[1-(3-dimethylaminopropyl)-1H-indol-3-yl]-3-(1H-indol-3-yl)-maleimide and 12-(2-cyanoethyl)-6,7,12,13-tetrahydro-13-methyl-5-oxo-5H-indolo(2,3-a)pyrrolo(3,4-c)-carbazole (GO 6976) (PKC inhibitors). Similarly, bromoenol lactone (a Ca(2+)-independent PLA(2) inhibitor) and methyl arachidonyl fluorophosphonate (an inhibitor of cytosolic PLA(2) and Ca(2+)-independent PLA(2)) inhibited LacCer-induced PLA(2) activity. Bromophenacyl bromide (a PLA(2) inhibitor) abrogated LacCer-induced PECAM-1 expression, and this was bypassed by arachidonic acid. Furthermore, the arachidonate-induced up-regulation of PECAM-1 was abrogated by indomethacin [a cyclooxygenase (COX)-1 and -2 inhibitor] or N-[2-(cyclohexyloxy)-4-nitrophenyl]-methanesulfonamide (a COX-2 inhibitor) but not nordihydroguaiaretic acid (a lipoxygenase inhibitor). In sum, PKCalpha/epsilon are the primary targets for the activation of LacCer. Downstream activation of intracellular Ca(2+)-independent PLA(2) and/or cytosolic PLA(2) results in the production of arachidonic acid, which in turn serves as a precursor for prostaglandins that subsequently stimulate PECAM-1 expression and cell adhesion. These findings may be relevant in explaining the role of LacCer in the regulation of PECAM-1 and related pathophysiology.

EGF mediates protection against Fas-induced apoptosis by depleting and oxidizing intracellular GSH stocks

Several pieces of evidence have demonstrated the importance of reduction/oxidation (redox) signaling in biological processes, including sensitivity toward apoptosis. In parallel, it was recently reported that growth factors induce the generation of reactive oxygen species (ROS). Therefore, we tested the hypothesis that the anti-apoptotic effect of epidermal growth factor (EGF) was mediated by changes in the redox state of hepatocytes through changes in GSH stocks. Isolated mouse hepatocytes were cultured and exposed to anti-Fas stimulation in order to induce apoptosis. Cell death by apoptosis was assessed by Hoechst 33258 staining and by measuring caspase-3 proteolysis activity. Cell treatment with EGF significantly decreased total (GSx) and reduced (GSH) glutathione levels in the presence and the absence of anti-Fas. Furthermore, glutathione reductase activity was lower in EGF-treated cultures (by 28%) as compared to untreated cultures which lead to a significant decline in GSH/GSx ratio. These effects were found to be EGF-receptor tyrosine kinase activity dependent. Co-stimulation of cells with anti-Fas and EGF attenuated caspase-3 activation and cell death by apoptosis by 70%. GSH monoethylester (GSHmee) significantly attenuated the effect of EGF on GSH and GSH/GSx ratio. It caused an increase in caspase-3 activation and in the percentage of apoptotic cells in anti-Fas + EGF-treated cells, thus resulting in a 53% decline in the protective effect of EGF. In conclusion, EGF induces a significant and specific depletion and oxidization of intracellular GSH, paralleled by a protection against Fas-induced apoptosis. GSH repenishment partly counteracted these effects suggesting that GSH depletion contributed to the protective effect of EGF against caspase-3 activation and cell death by apoptosis.

The role of endogenous NADPH oxidases in airway and pulmonary vascular smooth muscle function

Reactive oxygen species generated from NADPH oxidase(s) in airway smooth muscle cells and pulmonary artery smooth muscle cells are important signaling intermediates. Nox4 appears to be the predominant gp91 homologue in these cells. However, expression of NADPH oxidase components is dependent on phenotype, and different homologues may be expressed during different functional states of the cell. NADPH oxidase(s) appear to be important not only for mitogenesis by these cells, but also for O(2) sensing. The regulation of NADPH oxidase(s) in airway and pulmonary artery smooth muscle cells has important implications for the pathobiochemistry of asthma and pulmonary vascular diseases.

Mechanisms of ouabain toxicity

The suggested involvement of ouabain in hypertension raised the need for a better understanding of its cellular action, but the mechanisms of ouabain toxicity are only now being uncovered. In the present study, we show that reduced glutathione (GSH) protected ouabain-sensitive (OS) cells from ouabain-induced toxicity and that the inhibition of GSH synthesis by D, L-buthionine-(S,R)-sulfoximine (BSO) sensitized ouabain-resistant (OR) cells. We could not observe formation of *OH or H2O2, but there was an increase in O2*-only in OS cells. Unexpectedly, an increased number of OR cells depolarized after treatment with ouabain, and BSO blocked this depolarization. Moreover, GSH increased ouabain-induced depolarization in OS cells. A sustained increase in tyrosine phosphorylation (P-Tyr) and Ras expression was observed after treatment of OS cells, and GSH prevented it. Conversely, BSO induced P-Tyr and Ras expression in ouabain-treated OR cells. The results obtained have three major implications: There is no direct correlation between membrane depolarization and ouabain-induced cell death; ouabain toxicity is not directly related to its classical action as a Na+, K+-ATPase inhibitor but seems to be associated to signal transduction, and GSH plays a major role in preventing ouabain-induced cell death.

Interactions of angiotensin II with NAD(P)H oxidase, oxidant stress and cardiovascular disease

An elevation in angiotensin II (Ang II) levels is a common occurrence in a diverse number of cardiovascular diseases including hypertension, hypercholesterolaemia, atherosclerotic coronary artery disease, left ventricular hypertrophy (LVH), heart failure and diabetes. An important effect of Ang II is activation of the NAD(P)H oxidase, a major source of reactive oxygen species (ROS) production by vascular cells. This increase in cellular ROS contributes to the pathogenesis of vascular disease by altering endothelial cell function, enhancing smooth muscle cell growth and proliferation, stimulating inflammatory proteins, including macrophage chemoattractant agents, growth factors and cytokines, and modulating matrix remodelling. Studies of genetically-altered mice have unequivocally shown that activation of the NAD(P)H oxidase by Ang II contributes to hypertension, LVH and atherosclerosis. Furthermore, increasing evidence suggest that the NAD(P)H oxidase contributes to human disease, suggesting that it is a potential target for future therapeutic intervention.

NAD(P)H oxidase activation by angiotensin II is dependent on p42/44 ERK-MAPK pathway activation in rat's vascular smooth muscle cells

OBJECTIVE

To determine whether the activation of nicotinamide adenine dinucleotide phosphate (NAD(P)H) oxidase and the increase of superoxide anion production by angiotensin II is dependent upon the activation of the ERK-MAPK pathway.

METHODS

Hypertension was induced in Sprague-Dawley rats by infusing angiotensin II (200 ng/kg per min) through osmotic pumps for 12 days. The effects of treatments including an angiotensin II type 1 (AT(1)) blocker losartan (20 mg/kg per day), a tyrosine kinase inhibitor genistein (1.6 microg/kg per min), a specific ERK-MAPK inhibitor, PD98059 (2 mg/kg per day) and an antioxidant alpha-lipoic acid (500 mg/kg of chow) were evaluated during angiotensin infusion. The aortic superoxide anion production, the ERK-MAPK pathway activity and the systolic blood pressure (SBP), were measured following those treatments.

RESULTS

Increases in the concentration of the superoxide anion (1622 to 3719 cpm), in NAD(P)H activity (107%) and in the ERK-MAPK activity (3.6-fold) in the aorta as well as a rise in the arterial pressure (136 to 184 mmHg) were observed 12 days after initiating the treatments (P < 0.05). When the angiotensin-treated rats were treated either with losartan, genistein, PD98059 or alpha-lipoic acid, increases in superoxide anion production, in NAD(P)H oxidase activity, in ERK-MAPK activity and in blood pressure were attenuated. A correlation between the superoxide anion production and the ERK-MAPK activity was also observed.

CONCLUSIONS

The present study suggests that the NAD(P)H-dependent increase of the superoxide anion production in the vascular tissue following a treatment with angiotensin II is dependent on the activation of the ERK-MAPK pathway.

Pepstatin A attenuates the inhibitory effect of N-acetyl-L-cysteine on proliferation of hepatic myofibroblasts (stellate cells)

The pharmacological interaction between pepstatin A, a specific inhibitor of cathepsin D, and N-acetyl-L-cysteine was analyzed using hepatic stellate cells in primary culture. Isolated rat stellate cells were cultured on plastic dishes in Dulbecco's modified Eagle's medium (DMEM). Proteins and phospho-proteins were detected by Western blot. DNA synthesis was determined by [3H]thymidine uptake. Pepstatin A restored DNA synthesis of stellate cells stimulated by either platelet-derived growth factor-BB (PDGF-BB) or insulin-like growth factor-I (IGF-I), an effect that was attenuated by N-acetyl-L-cysteine. This agent induced the recovery of both the expression of PDGF receptor beta and IGF-I receptor beta and the phosphorylation of p42/44 mitogen-activated protein kinase (MAPK) and Akt under stimulation with either PDGF-BB or IGF-I, which were downregulated by N-acetyl-L-cysteine. Expression of cathepsin D was induced in activated stellate cells. These results indicate that pepstatin A hampers the inhibitory effect of N-acetyl-L-cysteine on stellate cell growth by ameliorating growth factor receptor downregulation.

The platelet-endothelium interaction mediated by lectin-like oxidized low-density lipoprotein receptor-1 reduces the intracellular concentration of nitric oxide in endothelial cells

OBJECTIVES

To address the potential role of the endothelial lectin-like oxidized low-density lipoprotein receptor-1 (LOX-1) in the thrombotic system, in this study we first examined whether platelet interaction with LOX-1 generated reactive oxygen species (ROS) and superoxide (O2.-) and then investigated the relationship between the intracellular production of O2.- and the availability of nitric oxide (NO).

BACKGROUND

Oxidative inactivation of NO is regarded as an important cause of its decreased biologic activity which may favor platelet-dependent arterial thrombosis.

METHODS

Bovine aortic endothelial cells (BAECs) and Chinese hamster ovary-K1 cells stably expressing bovine LOX-1 (BLOX-1-CHO) were incubated at different times with human platelets. The ROS, O2.-, and NO were measured in cells by flow cytometry.

RESULTS

The incubation of BAECs and BLOX-1-CHO cells with human platelets induced a sharp and dose-dependent increase in intracellular concentration of ROS and O2.- (p from <0.01 to <0.001). The increase in intracellular concentration of O2.- was followed by a dose-dependent reduction in basal and bradykinin-induced intracellular NO concentration (p from <0.01 to <0.001). The increase in O2.- and the reduction of NO were inhibited by the presence of vitamin C and anti-LOX-1 monoclonal antibody (p < 0.001).

CONCLUSIONS

The results of this study show that one of the pathophysiologic consequences of platelet binding to LOX-1 may be the inactivation of NO through an increased cellular production of O2.-.

Glycated high-density lipoprotein regulates reactive oxygen species and reactive nitrogen species in endothelial cells

Nonenzymatic glycosylation of plasma proteins may contribute to the excess risk of developing atherosclerosis in patients with diabetes mellitus. Although it is believed that high-density lipoprotein (HDL) is glycosylated at an increased level in diabetic individuals, little is known about a possible linkage between glycated HDL and endothelial dysfunction in diabetes. To clarify whether glucose-modified HDL affects the function of endothelial cells, we first examined herein the level of H(2)O(2) generation from cultured human aortic endothelial cells (HAECs) exposed to a glycated oxidized HDL (gly-ox-HDL) prepared in vitro. Incubation for 48 hours with 100 microg/mL of gly-ox-HDL induced significant release of H(2)O(2) from cells and gly-ox-HDL-induced H(2)O(2) formation was inhibited in the presence of diphenyleneiodonium, an inhibitor of NADPH oxidase. In addition, stimulation of HAECs with gly-ox-HDL for 48 hours elicited a marked downregulation of catalase and Cu(2+), Zn(2+)-superoxide dismutase (CuZn-SOD), suggesting H(2)O(2) formation by gly-ox-HDL to be due to a disturbance involving oxidant and antioxidant enzymes in the cells. Treatment of HAECs with gly-ox-HDL attenuated the expression of endothelial nitric oxide synthase (eNOS), but not inducible nitric oxide synthase (iNOS), and this was followed by decreased production of nitric oxide (NO) by the cells. Furthermore, in vitro experiments with glycated HDL (gly-HDL) in the presence of 2 mmol/L EDTA and Cu(2+)-oxidized HDL suggested the effect of gly-HDL on endothelial function to be possibly potentiated by additional oxidative modification. Taking all of the above findings together, gly-ox-HDL may lead to the deterioration of vascular function through altered production of reactive oxygen species and reactive nitrogen species in endothelial cells.

Regulatory role of sphingomyelin metabolites in hypoxia-induced vascular smooth muscle cell proliferation

Vascular cell adaptive response to hypoxic stress includes enhanced production of sphingomyelin metabolites that regulate cell growth. Here, we examined the vascular smooth muscle (VSM) cell adaptive response to hypoxia (2 and 5% O(2)) and demonstrated that acute (</=16h) hypoxic stress significantly stimulated VSM cell growth compared to cells grown under normoxic (21% O(2)) conditions. This stimulatory effect of hypoxia on VSM cell growth was significantly inhibited by pretreatment of cells with D-erythro-N,N-dimethylsphingosine, an inhibitor of sphingosine kinase. These results suggest a mechanism by which sphingosine 1-phosphate (S-1-P), a promitogenic sphingolipid-derived second messenger, may play a key role in hypoxia-induced VSM cell growth. Supporting this, S-1-P formation was significantly increased in VSM cells subjected to hypoxia. The hypoxia-induced increase in S-1-P level correlated with the decrease in total cellular ceramide content, a sphingolipid metabolite associated with inhibition of cell growth. The activity of sphingomyelinase was also significantly inhibited in hypoxia-treated VSM cells, likely further contributing to a decrease in total intracellular content of ceramide. As a decrease in ceramide content may play a role in hypoxia-induced VSM growth, we next examined the effects of ceramide in VSM cell growth. Elevating intracellular ceramide content through exogenous (C(6)-ceramide) or endogenous (ceramidase inhibition) manipulations led to a decrease in hypoxia-induced VSM cell growth. In contrast, hypoxia-induced VSM cell growth was further enhanced by S-1-P treatment. Together, our study indicates that hypoxia-induced VSM cell growth may be modulated by sphingomyelin metabolism that results in reduction of total intracellular ceramide level with concomitant increase in S-1-P formation.

Carboxyl ester lipase: structure-function relationship and physiological role in lipoprotein metabolism and atherosclerosis

Carboxyl ester lipase (CEL), previously named cholesterol esterase or bile salt-stimulated (or dependent) lipase, is a lipolytic enzyme capable of hydrolyzing cholesteryl esters, tri-, di-, and mono-acylglycerols, phospholipids, lysophospholipids, and ceramide. The active site catalytic triad of serine-histidine-aspartate is centrally located within the enzyme structure and is partially covered by a surface loop. The carboxyl terminus of the protein regulates enzymatic activity by forming hydrogen bonds with the surface loop to partially shield the active site. Bile salt binding to the loop domain frees the active site for accessibility by water-insoluble substrates. CEL is synthesized primarily in the pancreas and lactating mammary gland, but the enzyme is also expressed in liver, macrophages, and in the vessel wall. In the gastrointestinal tract, CEL serves as a compensatory protein to other lipolytic enzymes for complete digestion and absorption of lipid nutrients. Importantly, CEL also participates in chylomicron assembly and secretion, in a mechanism mediated through its ceramide hydrolytic activity. Cell culture studies suggest a role for CEL in lipoprotein metabolism and oxidized LDL-induced atherosclerosis. Thus, this enzyme, which has a wide substrate reactivity and diffuse anatomic distribution, may have multiple functions in lipid and lipoprotein metabolism, and atherosclerosis.

Role of ceramide in TNF-alpha-induced impairment of endothelium-dependent vasorelaxation in coronary arteries

The present study tested the hypothesis that ceramide, a sphingomylinase metabolite, serves as an second messenger for tumor necrosis factor-alpha (TNF-alpha) to stimulate superoxide production, thereby decreasing endothelium-dependent vasorelaxation in coronary arteries. In isolated bovine small coronary arteries, TNF-alpha (1 ng/ml) markedly attenuated vasodilator responses to bradykinin and A-23187. In the presence of N(G)-nitro-L-arginine methyl ester, TNF-alpha produced no further inhibition on the vasorelaxation induced by these vasodilators. With the use of 4,5-diaminofluorescein diacetate fluorescence imaging analysis, bradykinin was found to increase nitric oxide (NO) concentrations in the endothelium of isolated bovine small coronary arteries, which was inhibited by TNF-alpha. Pretreatment of the arteries with desipramine (10 microM), an inhibitor of acidic sphingomyelinase, tiron (1 mM), a superoxide scavenger, and polyethylene glycol-superoxide dismutase (100 U/ml) largely restored the inhibitory effect of TNF-alpha on bradykinin- and A-23187-induced vasorelaxation. In addition, TNF-alpha activated acidic sphingomyelinase and increased ceramide levels in coronary endothelial cells. We conclude that TNF-alpha inhibits NO-mediated endothelium-dependent vasorelaxation in small coronary arteries via sphingomyelinase activation and consequent superoxide production in endothelial cells.

Dual effect of ceramide on human endothelial cells: induction of oxidative stress and transcriptional upregulation of endothelial nitric oxide synthase

BACKGROUND

Generation of the second-messenger molecule ceramide by stimulated sphingomyelinase activity has been implicated in the inflammatory processes contributing to the pathogenesis of atherosclerosis. However, reports of stimulatory effects of ceramide on endothelial NO production in animal models suggest antiatherosclerotic effects of the molecule. Therefore, we investigated long-term effects of ceramide on NO generation in human endothelial cells.

METHODS AND RESULTS

In human umbilical vein endothelial cells (HUVECs) and in HUVEC-derived EA.hy 926 endothelial cells, C6-ceramide (N-hexanoyl-D-erythro-sphingosine) reduced the generation of bioactive NO (RFL-6 reporter-cell assay). At the same time, the signaling molecule increased endothelial NO synthase (eNOS) mRNA (RNase protection assay) and protein expression (Western blot). C6-ceramide stimulated eNOS transcription by a signaling mechanism involving protein phosphatase PP2A but did not modify the stability of the eNOS mRNA. Endothelial generation of reactive oxygen species (ROS) was increased by C6-ceramide [5-(and-6)-chloromethyl-2', 7'-dichlorodihydrofluorescein diacetate (CM-H(2)DCFDA) oxidation-based fluorescence assay], and this effect was partially reversed by the NOS inhibitor N(G)-nitro-L-arginine methyl ester (L-NAME). On the other hand, (6R)-5,6,7,8-tetrahydro-L-biopterin (BH(4)) normalized in part the ceramide-induced reduction in bioactive NO.

CONCLUSIONS

Ceramide produces oxidative stress in human endothelial cells, thereby reducing bioactive NO. The partial reversal of this reduction by BH(4) and the diminution of ROS generation by L-NAME suggest that ceramide promotes NADPH oxidase activity of eNOS, leading to ROS formation at the expense of NO synthesis. The ceramide-induced upregulation of eNOS gene transcription can be considered an ineffective compensatory mechanism. The decreased bioavailability of NO is likely to favor a proatherogenic role of ceramide.

Does the coronary risk factor low density lipoprotein alter growth and signaling in vascular smooth muscle cells?

There is increasing evidence that hypertension promotes low density lipoprotein (LDL) transportation into the subendothelial space of the vascular wall. Vascular smooth muscle cell (VSMC) proliferation plays an important role in the development and progression of cardiovascular diseases. Recently, several studies have demonstrated that LDL acts as a classic growth factor promoting VSMC growth via mitogenic signals normally elicited by classic growth factors. The present work summarizes current nontraditional concepts regarding possible cellular mechanisms through which hypertension and LDL may promote the development of atherosclerosis. Especially addressed are the possible effects of an elevated blood pressure in combination with LDL on VSMC growth. The new research concept concerning LDL as a growth factor and carrier for biological active phospholipids such as sphingosine-1-phosphate and sphingosylphosphorylcholine may contribute to an understanding of the pathogenesis of atherosclerosis by elevated high blood pressure.

Lactosylceramide-enriched glycosphingolipid signaling domain mediates superoxide generation from human neutrophils

This study is focused on the functional significance of neutrophil lactosylceramide (LacCer)–enriched microdomains, which are involved in the initiation of a signal transduction pathway leading to superoxide generation. Treatment of neutrophils with anti-LacCer antibody, T5A7 or Huly-m13, induced superoxide generation from the cells, which was blocked by PP1, a Src kinase inhibitor; wortmannin, a phosphatidylinositol-3 kinase inhibitor; SB203580, a p38 mitogen-activated protein kinase (MAPK) inhibitor; and H7, an inhibitor for protein kinase C. When promyelocytic leukemia HL-60 cells were differentiated into neutrophilic lineage by dimethyl sulfoxide (DMSO) treatment, they acquired superoxide-generating activity but did not respond to anti-LacCer antibodies. Density gradient centrifugation revealed that LacCer and Lyn were recovered in detergent-insoluble membrane (DIM) of neutrophils and DMSO-treated HL-60 cells. However, immunoprecipitation experiments indicated that LacCer was associated with Lyn in neutrophils but not in DMSO-treated HL-60 cells. Interestingly, T5A7 induced the phosphorylation of Lyn in neutrophils but not in DMSO-treated HL-60 cells. Moreover, T5A7 induced the phosphorylation of p38 MAPK in neutrophils. T5A7-induced Lyn phosphorylation in neutrophil DIM fraction was significantly enhanced by cholesterol depletion or sequestration with methyl-β-cyclodextrin or nystatin. Collectively, these data suggest that neutrophils are characterized by the presence of cell surface LacCer-enriched glycosphingolipid signaling domain coupled with Lyn and that the ligand binding to LacCer induces the activation of Lyn, which may be suppressibly regulated by cholesterol, leading to superoxide generation through the phosphatidylinositol-3 kinase–, p38 MAPK–, and protein kinase C–dependent signal transduction pathway.