Glycosphingolipids of human plasma lipoproteins

Glycosphingolipids in Diabetes, Oxidative Stress, and Cardiovascular Disease: Prevention in Experimental Animal Models

Diabetes contributes to about 30% morbidity and mortality world-wide and has tidal wave increases in several countries in Asia. Diabetes is a multi-factorial disease compounded by inflammation, dyslipidemia, atherosclerosis, and is sometimes accompanied with gains in body weight. Sphingolipid pathways that interplay in the enhancement of the pathology of this disease may be potential therapeutic targets. Thus, the application of advanced sphingolipidomics may help predict the progression of this disease and therapeutic outcomes in man. Pre-clinical studies using various experimental animal models of diabetes provide valuable information on the role of sphingolipid signaling networks in diabetes and the efficacy of drugs to determine the translatability of innovative discoveries to man. In this review, we discuss three major concepts regarding sphingolipids and diabetes. First, we discuss a possible involvement of a monosialodihexosylceramide (GM3) in insulin-insulin receptor interactions. Second, a potential role for ceramide (Cer) and lactosylceramide (LacCer) in apoptosis and mitochondrial dysfunction is proposed. Third, a larger role of LacCer in antioxidant status and inflammation is discussed. We also discuss how inhibitors of glycosphingolipid synthesis can ameliorate diabetes in experimental animal models.

Management of metabolic syndrome and reduction in body weight in type II diabetic mice by inhibiting glycosphingolipid synthesis

Metabolic syndrome is defined by hyperlipidemia and cardiovascular complications. We have examined whether inhibition of glycosphingolipid synthesis can interfere with metabolic syndrome in a male mouse model of type II diabetes (db/db). The db/db and control mice (C57/BL6) (n = 6) fed chow for 30 weeks received vehicle (5% Tween-80 in PBS; 100 μl), or a biopolymer-encapsulated D-threo-1-phenyl-2-decanoylamino-3-morpholino-1-propanol (BPD) glycosphingolipid synthesis inhibitor daily via oral gavage for 6 weeks. Echocardiography revealed increased Ao-IMT in db/db mice compared to control. However, BPD decreased Ao-IMT, monohexosylceramide and dihexosylceramide, LDL, triglycerides, glucose, and raised HDL levels in db/db mice. This was due to increased gene expression of HMG-CoA reductase, LDLr, SREBP2, and bile acids: Cy7-a hydroxylase, LXR and FXR, lipoprotein lipase, VLDL receptor and PPAR. Treatment also increased the expression of superoxide dismutase-II to reduce the pro-oxidant status in these mice. We observed that decreased cholesterol levels correlated with decreased cholesterol sensing proteins e.g. NPC1 gene/protein expression and mammalian target of rapamycin (mTORC-1) and reduced body weight. Thus, glycosphingolipid synthesis inhibition is a novel approach to manage metabolic syndrome and reduce body weight in diabetic mice and with potential applications in humans.

Metabolic Pathway Genes Associated with Susceptibility Genes to Coronary Artery Disease

Coronary artery disease (CAD) is one of the leading threats to global health. Previous research has proven that metabolic pathway disorders, such as high blood lipids and diabetes, are one of the risk factors that mostly cause CAD. However, the crosstalk between metabolic pathways and CAD was mostly studied on physiology processes by analyzing a single gene function. A canonical correlation analysis was used to identify the metabolic pathways, which were integrated as a unit to coexpress with CAD susceptibility genes, and to resolve additional metabolic factors that are related to CAD. Seven pathways, including citrate cycle, ubiquinone, terpenoid quinone biosynthesis, and N-glycan biosynthesis, were identified as an integrated unit coexpressed with CAD genes. These pathways could not be revealed as a coexpressed pathway through traditional methods as each single gene has weak correlation. Furthermore, sets of genes in these pathways were candidate markers for diagnosis and detection from patients' serum.

Identification of Staphylococcal Enterotoxin B Domains Involved in Binding to Cultured Human Kidney Proximal Tubular Cells: Imparting Proliferation and Death

Studies suggest that staphylococcal enterotoxin B (SEB) is initially harbored in the kidney by binding to digalactosylceramide molecules in the proximal tubular cells. However, little is known in regard to the peptide motif within SEB that binds to these cells and imparts toxic effects. Herein, using human kidney proximal tubular cells (PTs) we have performed a systematic study on the binding of various peptides and peptide analogs of SEB and demonstrate a structure-functional relationship. Using [125I]labeled SEB peptides, we show a high affinity and displaceable binding of SEB 191–220 to human PT cells. Binding was mitigated by the use of antibody against SEB, by digalactosylceramide (the putative receptor), and by the use of endoglycoceramidase, which selectively removes the oligosaccharide backbones from glycosphingolipids. Our structure/ functional studies revealed that peptide 130–160 induces a concentration-dependent increase in programmed cell death/ apoptosis in human proximal tubular cells. Mechanistic studies further suggest that SEB/SEB peptide (130–160) impart apoptosis via the activation of neutral sphingomyelinase, which hydrolizes sphingomyelin to ceramide and phosphocholine. SEB 130–160 mediated apoptosis was mitigated by preincubation of cells with antibody against SEB and an SEB 130–160 antibody.

Inhibition of glycosphingolipid synthesis ameliorates atherosclerosis and arterial stiffness in apolipoprotein E-/- mice and rabbits fed a high-fat and -cholesterol diet

BACKGROUND

Glycosphingolipids, integral components of the cell membrane, have been shown to serve as messengers, transducing growth factor-initiated phenotypes. Here, we have examined whether inhibition of glycosphingolipid synthesis could ameliorate atherosclerosis and arterial stiffness in transgenic mice and rabbits.

METHODS AND RESULTS

Apolipoprotein E(-/-) mice (12 weeks of age; n=6) were fed regular chow or a Western diet (1.25% cholesterol, 2% fat). Mice were fed 5 or 10 mg/kg of an inhibitor of glycosphingolipid synthesis, D-threo-1-phenyl-2-decanoylamino-3-morpholino-1-propanol (D-PDMP), solubilized in vehicle (5% Tween-80 in PBS); the placebo group received vehicle only. At 20 and 36 weeks of age, serial echocardiography was performed to measure aortic intima-media thickening. Aortic pulse-wave velocity measured vascular stiffness. Feeding mice a Western diet markedly increased aortic pulse-wave velocity, intima-media thickening, oxidized low-density lipoprotein, Ca(2+) deposits, and glucosylceramide and lactosylceramide synthase activity. These were dose-dependently decreased by feeding D-PDMP. In liver, D-PDMP decreased cholesterol and triglyceride levels by raising the expression of SREBP2, low-density lipoprotein receptor, HMGCo-A reductase, and the cholesterol efflux genes (eg, ABCG5, ABCG8). D-PDMP affected very-low-density lipoprotein catabolism by increasing the gene expression for lipoprotein lipase and very-low-density lipoprotein receptor. Rabbits fed a Western diet for 90 days had extensive atherosclerosis accompanied by a 17.5-fold increase in total cholesterol levels and a 3-fold increase in lactosylceramide levels. This was completely prevented by feeding D-PDMP.

CONCLUSIONS

Inhibition of glycosphingolipid synthesis ameliorates atherosclerosis and arterial stiffness in apolipoprotein E(-/-) mice and rabbits. Thus, inhibition of glycosphingolipid synthesis may be a novel approach to ameliorate atherosclerosis and arterial stiffness.

Analysis of human plasma lipids and soybean lecithin by means of high-performance thin-layer chromatography and matrix-assisted laser desorption/ionization mass spectrometry

An improved analytical strategy for the analysis of complex lipid mixtures using matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) in combination with high-performance thin-layer chromatography (HPTLC) is reported. Positive ion MALDI RTOF MS was applied as a rapid screening tool for the various neutral (e.g. triacylglycerols) and polar (e.g. glycerophospholipids and -sphingolipids) lipid classes derived from crude lipid extracts of e.g. human plasma as well as soybean lecithin. Finally, MALDI seamless post-source decay (PSD) product ion analysis was performed in order to obtain further structural information (head- and acyl-group identification) of selected lipid species and structure verification. A Coomassie Brilliant Blue R-250 staining protocol for lipids on HPTLC plates was evaluated and was found to be fully compatible with subsequent MALDI-MS. Lipids were analyzed after elution from the HPTLC phase material of the selected band (corresponding to certain lipid classes) by using the proper organic solvent mixture or in few cases directly from the HPTLC plates (a type of on-line HPTLC/MALDI-MS coupling). More than 70 distinct lipid species from seven different lipid classes in the range between m/z 500 and 1500 could be identified from the lipid extracts of human plasma and soybean lecithin, respectively. The general high sensitivity of MALDI-MS detection allowed the analysis of even minor lipid classes from only very small volumes of human plasma (50 microL). The combination of HPTLC, Coomassie staining and positive ion MALDI curved field RTOF-MS represents a straightforward strategy during lipidomics studies of food and clinically relevant human lipid samples.

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.

Ganglioside from eel serum high density lipoprotein (HDL) and its role as a ligand for HDL binding protein

First, we attempted to isolate glycosphingolipids from eel serum HDL. A single ganglioside containing N-acetylneuraminic acid (NeuAc), which is positive with resorcinol and orcinol reactions, was purified. The mobilities of the purified ganglioside and its lyso-form on high performance TLC were similar as those of authentic GM4 and its lyso-form, respectively. The mass of the purified ganglioside was determined by TOF mass spectrometer, and the mass of its oligosaccharide was the same as that of authentic GM4 from human brain consisting of disaccharide of NeuAc and galactose. The ganglioside from eel HDL was not hydrolyzed by recombinant endoglycoceramidase II, which cannot hydrolyze between galactose and ceramide of gangliosides, but hydrolyzes between glucose and ceramide. We concluded from these results that the ganglioside purified from eel serum HDL is GM4. Second, we investigated the effects of the ganglioside on binding of HDL labeled with fluorescein isothiocyanate (FITC-HDL) to cultured eel hepatocytes and on FITC-HDL ligand blotting by using plasma membrane proteins of the hepatocytes. Stimulatory effect of GM4 on FITC-HDL binding to the hepatocytes and FITC-HDL ligand blotting suggests strongly that GM4 is a ligand for HDL binding protein of eel hepatocytes.

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.

Characterization of human apolipoprotein B100 oligosaccharides in LDL subfractions derived from normal and hyperlipidemic plasma: deficiency of alpha-N-acetylneuraminyllactosyl-ceramide in light and small dense LDL particles

The carbohydrate composition of apolipoprotein (apo) B100, particularly its degree of sialylation, may contribute to the atherogenic properties of low-density lipoprotein (LDL). We analyzed LDL apoB100 glycans derived from normolipidemic, hypercholesterolemic, and hypertriglyceridemic diabetic subjects. Using exoglycosidase carbohydrate sequencing and matrix-assisted laser desorption/ionization mass spectrometry to analyze fluorescently labeled oligosaccharides, we report evidence for several carbohydrates not previously identified on apoB100, including truncated complex biantennary N-glycans and hybrid N-glycans. The distribution and diversity of the apoB100 glycans isolated from all individuals was highly conserved. The N-glycan composition of apoB100 derived from five LDL subpopulations (LDL1, d = 1.018-1.023; LDL2, d = 1.023-1.030; LDL3, d = 1.030-1.040; LDL4, d = 1.040-1.051; LDL5, d = 1.051-1.065 g/ml) did not vary in normolipidemic or hypercholesterolemic subjects. Furthermore, we found no evidence for "desialylated" apoB100 glycans in any of the samples analyzed. Analysis of the most abundant LDL ganglioside, alpha-N-acetylneuraminyllactosyl-ceramide, revealed a deficiency in small dense LDL and in the most buoyant subpopulation. These data provide a novel explanation for the apparent deficiency of sialic acid in small dense LDL and indicate that the global apoB100 N-glycan composition is invariable in the patient groups studied.

Lipopolysaccharide-induced ischemic tolerance is associated with increased levels of ceramide in brain and in plasma

Intravenous administration of lipopolysaccharide (LPS) (0.9 mg/kg) has been shown to induce ischemic tolerance in spontaneously hypertensive rats (SHR). TNF-alpha is believed to play a crucial role in preconditioning as its inhibition with TNF-alpha-binding protein abolished tolerance. Our recent studies (Liu et al., Am. J. Physiol. 278 C144, 2000) have demonstrated that ceramide, a downstream messenger in TNF-alpha signaling, is a mediator of hypoxia-induced tolerance in neuronal cells. To test the hypothesis that ceramide contributes to LPS-induced tolerance in vivo, SHR were injected intravenously with either LPS or saline and the levels of ceramide in brain and in plasma were determined by reversed phase HPLC. LPS injection resulted in a significant increase of ceramide in plasma with a maximum at 24 h (8.32+/-1.14 pmol/microl (LPS) vs. 2.65+/-0.62 pmol/microl (saline)). LPS also induced ceramide upregulation in brain cortex, which started between 6 and 12 h and remained elevated up to 48 h after LPS injection. Fluorescent NBD-C6 ceramide was able to cross blood-brain barrier and was found in brain vessels, perivascular cells and in brain parenchyma 30 min after intravenous injection. These findings demonstrate that LPS preconditioning leads to elevation of ceramide in brain and plasma and, in conjunction with previous work, suggests that ceramide plays a role in LPS-induced protection against brain ischemic injury in vivo.

Immune defects observed in patients with primary malignant brain tumors

Malignant glioblastomas (gliomas) account for approximately one third of all diagnosed brain tumors. Yet, a decade of research has made little progress in advancing the treatment of these tumors. In part this lack of progress is linked to the challenge of discovering how glial tumors are capable of both modulating host immune function and neutralizing immune-based therapies. Patients with gliomas exhibit a broad suppression of cell-mediated immunity. The impaired cell-mediated immunity observed in patients with gliomas appears to result from immunosuppressive factor(s) secreted by the tumor. This article reviews what has been elucidated about the immune defects of patients harboring glioma and the glioma-derived factors which mediate this immunosuppression. A model involving systemic cytokine dysregulation is presented to suggest how the immune defects arise in these individuals.

Regulation of glycosphingolipid metabolism in liver during the acute phase response

The host response to infection is associated with multiple alterations in lipid and lipoprotein metabolism. We have shown recently that endotoxin (lipopolysaccharide (LPS)) and cytokines enhance hepatic sphingolipid synthesis, increase the activity and mRNA levels of serine palmitoyltransferase, the first committed step in sphingolipid synthesis, and increase the content of sphingomyelin, ceramide, and glucosylceramide (GlcCer) in circulating lipoproteins in Syrian hamsters. Since the LPS-induced increase in GlcCer content of lipoproteins was far greater than that of ceramide or sphingomyelin, we have now examined the effect of LPS and cytokines on glycosphingolipid metabolism. LPS markedly increased the mRNA level of hepatic GlcCer synthase, the enzyme that catalyzes the first glycosylation step of glycosphingolipid synthesis. The LPS-induced increase in GlcCer synthase mRNA levels was seen within 2 h, sustained for 8 h, and declined to base line by 24 h. LPS-induced increase in GlcCer synthase mRNA was partly accounted for by an increase in its transcription rate. LPS produced a 3-4-fold increase in hepatic GlcCer synthase activity and significantly increased the content of GlcCer (the immediate product of GlcCer synthase reaction) as well as ceramide trihexoside and ganglioside GM3 (products distal to the GlcCer synthase step) in the liver. Moreover, both tumor necrosis factor-alpha and interleukin-1beta, cytokines that mediate many of the metabolic effects of LPS, increased hepatic GlcCer synthase mRNA levels in vivo as well as in HepG2 cells in vitro, suggesting that these cytokines can directly stimulate glycosphingolipid metabolism. These results indicate that LPS and cytokines up-regulate glycosphingolipid metabolism in vivo and in vitro. An increase in GlcCer synthase mRNA levels and activity leads to the increase in hepatic GlcCer content and may account for the increased GlcCer content in circulating lipoproteins during the acute phase response.

Sphingolipids in atherosclerosis and vascular biology

Sphingolipids and their metabolic products are now known to have second-messenger functions in a variety of cellular signaling pathways. Lactosylceramide (LacCer), a glycosphingolipid (GSL) present in vascular cells such as endothelial cells, smooth muscle cells, macrophages, neutrophils, platelets, and monocytes, contributes to atherosclerosis. Large amounts of LacCer accumulate in fatty streaks, intimal plaque, and calcified intimal plaque, along with oxidized low density lipoproteins (Ox-LDLs), growth factors, and proinflammatory cytokines. A possible role for LacCer in vascular cell biology was suggested when this GSL was found to stimulate the proliferation in vitro of aortic smooth muscle cells (ASMCs). A further link of LacCer in atherosclerosis was uncovered by the finding that Ox-LDLs stimulated specifically the biosynthesis of LacCer. Ox-LDL-stimulated endogenous synthesis of LacCer by activation of UDP-Gal:GlcCer,beta1-4galtransferase (GalT-2) is an early step in this signaling pathway. In turn, LacCer serves as a lipid second messenger that orchestrates a signal transduction pathway, ultimately leading to cell proliferation. This signaling pathway includes LacCer-mediated activation of NADPH oxidase that produces superoxide. Such superoxide molecules stimulate the GTP loading of p21(ras). Subsequently, the kinase cascade (Raf-1, Mek2, and p44MAPK [mitogen-activated protein kinase]) is activated. The phosphorylated form of p44MAPK translocates from the cytoplasm to the nucleus and engages in c-fos expression, proliferating cell nuclear antigen (PCNA) such as cyclin activation, and cell proliferation takes place. Interestingly, D-threo-1-phenyl-2-decanoylamino-3-morpholino-1-propanol (D-PDMP), an inhibitor of GalT-2, can abrogate the Ox-LDL-mediated activation of GalT-2, the signal kinase cascade noted above, as well as cell proliferation. Additional studies have revealed that LacCer mediates the tumor necrosis factor-alpha (TNF-alpha)-induced nuclear factor-kappaB expression and intercellular adhesion molecule (ICAM-1) expression in vascular endothelial cells via the redox-dependent transcriptional pathway. LacCer also stimulates the expression of CD11/CD8, or Mac-1, on the surface of human neutrophils. Collectively, this phenomenon may contribute to the adhesion of neutrophils or monocytes to the endothelial cell surface and thus initiate the process of atherosclerosis. In addition, the LacCer-mediated proliferation of ASMCs may contribute to the progression of atherosclerosis. On the other hand, programmed cell death (apoptosis) by proinflammatory cytokines such as TNF-alpha, interleukin-1, and high concentrations of Ox-LDL occur via activation of a cell membrane-associated neutral sphingomyelinase (N-SMase). N-SMase hydrolyzes sphingomyelin into ceramide and phosphocholine. In turn, ceramide or a homologue serves as an important stress-signaling molecule. Interestingly, an antibody against N-SMase can abrogate Ox-LDL- and TNF-alpha-induced apoptosis and therefore may be useful for in vivo studies of apoptosis in experimental animals. Because plaque stability is an integral aspect of atherosclerosis management, activation of N-SMase and subsequent apoptosis may be vital events in the onset of plaque rupture, stroke, or heart failure. Interestingly, in human liver cells, N-SMase action mediates the TNF-alpha-induced maturation of the sterol regulatory-element binding protein. Moreover, a cell-permeable ceramide can reconstitute the phenomenon above in a sterol-independent fashion. Such findings may provide new avenues for therapy for patients with atherosclerosis. The findings described here indicate an important role for sphingolipids in vascular biology and provide an exciting opportunity for further research in vascular disease and atherosclerosis.

Endotoxin and cytokines increase hepatic sphingolipid biosynthesis and produce lipoproteins enriched in ceramides and sphingomyelin

Alterations in triglyceride and cholesterol metabolism often accompany inflammatory diseases and infections. We studied the effects of endotoxin (lipopolysaccharide [LPS]) and cytokines on hepatic sphingolipid synthesis, activity of serine palmitoyltransferase (SPT), the first and rate-limiting enzyme in sphingolipid synthesis, and lipoprotein sphingolipid content in Syrian hamsters. Administration of LPS induced a 2-fold increase in hepatic SPT activity. The increase in activity first occurred at 16 hours, peaked at 24 hours, and was sustained for at least 48 hours. Low doses of LPS produced maximal increases in SPT activity, with half-maximal effect seen at approximately 0.3 microg LPS/100 g body weight. LPS increased hepatic SPT mRNA levels 2-fold, suggesting that the increase in SPT activity was due to an increase in SPT mRNA. LPS treatment also produced 75% and 2.5-fold increases in hepatic sphingomyelin and ceramide synthesis, respectively. Many of the metabolic effects of LPS are mediated by cytokines. Interleukin 1 (IL-1), but not tumor necrosis factor, increased both SPT activity and mRNA levels in the liver of intact animals, whereas both IL-1 and tumor necrosis factor increased SPT mRNA levels in HepG2 cells. IL- produced a 3-fold increase in SPT mRNA in HepG2 cells, and the half-maximal dose was 2 ng/mL. IL-1 also increased the secretion of sphingolipids into the medium. Analysis of serum lipoprotein fractions demonstrated that very low density lipoprotein, intermediate density lipoprotein, and low density lipoprotein isolated from animals treated with LPS contained significantly higher amounts of ceramide, glucosylceramide, and sphingomyelin. Taken together, these results indicate that LPS and cytokines stimulate hepatic sphingolipid synthesis, which results in an altered structure of circulating lipoproteins and may promote atherogenesis.

Lactosylceramide stimulates human neutrophils to upregulate Mac-1, adhere to endothelium, and generate reactive oxygen metabolites in vitro

Glycosphingolipids (GSLs) and their metabolites play important roles in a variety of biological processes. We have previously reported that lactosylceramide (LacCer), a ubiquitous GSL, stimulates NADPH oxidase-dependent superoxide generation by aortic smooth muscle cells and their consequent proliferation. We postulated that LacCer may upregulate adhesion molecules on human polymorphonuclear leukocytes (hPMNs), perhaps also via NADPH oxidase-dependent reactive oxygen metabolite (ROM) generation. Incubation of hPMNs with LacCer upregulated CD11b/CD18 (Mac-1) and CD11c/CD18, as determined by fluorescence-automated cell sorting. LacCer also stimulated these hPMNs to generate superoxide via NADPH oxidase, as determined by lucigenin-enhanced chemiluminescence. However, the upregulation of Mac-1 by LacCer did not itself appear to be mediated by ROMs, since neither an antioxidant nor an NADPH oxidase inhibitor substantially inhibited the Mac-1 upregulation. However, this Mac-1 upregulation was significantly inhibited by two disparate phospholipase A2 (PLA2) inhibitors. Moreover, LacCer induced arachidonic acid metabolism, which was inhibited by the PLA2 inhibitors, but not by an NADPH oxidase inhibitor. To evaluate the effect of LacCer on hPMN adhesion to endothelium, hPMNs stimulated with LacCer were allowed to adhere to unstimulated human endothelial cell monolayers. LacCer stimulated hPMN adhesion to endothelial cells, which was blocked by anti-CD18 and by the PLA2 inhibitors. We conclude that LacCer stimulates both Mac-1 upregulation and superoxide generation in hPMNs but that ROMs are not the upstream signal for Mac-1 upregulation. This mechanism may well be relevant to acute endothelial injury in inflammation and other pathological conditions.

Intercellular transfer of shed tumor cell gangliosides

Three distinct steps underlie immumosuppression by tumor gangliosides: (i) their shedding by the tumor cell, (ii) binding to target leukocytes in the tumor microenvironment, and (iii) action upon the target cell. While shedding is well documented, cell to cell transfer of shed gangliosides is not. To address this, we employed a dual chamber culture system. In this system, metabolically radiolabeled lymphoma cells shed gangliosides into the conditioned medium of the contralateral chamber, which contained normal fibroblasts as the target cell. The shed lymphoma cell gangliosides bound avidly to the target fibroblasts in a trypsin-resistant manner (1-2 x 10(6) and 7 x 10(6) molecules/fibroblast in 24 and 48 h). Significantly higher than binding rates of purified lymphoma gangliosides added exogenously, these binding rates in a system which models the in vivo microenvironment suggest that cell to cell ganglioside transfer is a highly efficient process.

Regulation of synthesis of lactosylceramide in normal and tumor proximal tubular cells

We have measured the binding and degradation of low-density lipoprotein (LDL) and LDL-mediated effects on cholesteryl ester (CE) synthesis in cultured normal and tumor proximal tubular (PT) cells. The effects of LDL on the regulation of glycosphingolipid metabolism in cells was pursued employing radioactive precursors, e.g., [3H]serine, and [3H]glucose and by measuring the activity of UDP-galactose: glucosylceramide: B1-4 galactosyltransferase (GalT-2). In normal PT cells, there was a saturable and displaceable binding and degradation of 125I-LDL and a LDL mediated 14-fold stimulation of cholesteryl ester (CE) synthesis. This was accompanied by a suppression (70-80%) of incorporation of [3H]glucose and [3H]serine into GlcCer, LacCer, GbOse3Cer and GbOse4Cer and suppression (70-80%) of GalT-2 activity. In tumor PT cells, displaceable binding and degradation of 125I-LDL was not observed and LDL failed to stimulate CE synthesis. In such cells, LDL exerted a concentration-dependent stimulation of [3H]glucose and [3H]serine incorporation into GSL. Maximum stimulation (250%) of GalT-2 activity in tumor PT cells occurred with 25 micrograms LDL/ml medium. We conclude that LDL taken up via receptor mediated pathway decreases GaIT-2 activity in normal PT cells. In contrast, LDL not taken up via the LDL receptor pathway in tumor PT cells failed to suppress the incorporation of [3H]glucose and [3H]serine into glycosphingolipids and GalT-2 activity leading to a stimulation of lactosylceramide synthesis.

Immunosuppression by human gangliosides: I. Relationship of carbohydrate structure to the inhibition of T cell responses

Causes of cellular immunodeficiency frequently associated with cancer remain poorly understood. One possible mechanism is tumor cell membrane shedding of immunosuppressive molecules, such as the sialic acid-containing glycosphingolipids, gangliosides. To explore this interesting hypothesis and establish structure-activity relationships, we examined the effects of a series of highly purified human gangliosides on T cell function. In all, ten individual molecular species of two major biosynthetic pathways were compared for their ability to inhibit human T cell proliferative responses. They include GM1, GD1a, GD1b, and GT1b (the predominant normal brain species), and GM4, GM3, GM2, GD3, GD2 and GQ1b. Strikingly, each HPLC-purified molecule, from the simplest monosialoganglioside to the most complex polysialoganglioside, had potent inhibitory activity; even the ganglioside with the most elemental carbohydrate structure (GM4, one sialic acid linked to a monosaccharide) strongly inhibits T cell proliferative responses to tetanus toxoid (ID90 = 1.5 microM). The data also reveal a complex interplay between elements of oligosaccharide structure in determining immunosuppressive activity. Sialic acid is critical to maximal activity, and (i) immunosuppression is most potent in gangliosides containing a terminal sialic acid. (ii) Total desialylation almost abolishes activity and (iii) partial alteration (lactone formation) reduces activity. (iv) Activity is generally but not always higher with higher numbers of sialic acid residues/molecule, and (v) some larger neutral glycosphingolipids retain measurable immunosuppressive activity. Overall, the potent inhibition by gangliosides supports the hypothesis that shedding of these molecules by tumors creates a highly immunosuppressive microenvironment around the tumor, thereby inhibiting the function of infiltrating host leukocytes and contributing to diminished T cell responses in cancer.

Lactosylceramide stimulates aortic smooth muscle cell proliferation

We have investigated the effects of various sphingolipids on aortic smooth muscle cell proliferation employing viable cell counting, [3H] thymidine incorporation into DNA and the release of lactate dehydrogenase. Assays for UDP Gal: GlcCer Bl-4 galactosyltransferase (GalT-2) in control and treated cells were pursued simultaneously. Lactosylceramide stimulated cell proliferation in the order of 5 fold. Antibody against LacCer but not GbOse3Cer blocked the proliferative effects of LacCer in these cells. This phenomena was specific for aortic smooth muscle cells as LacCer decreased cell viability of aortic endothelial cells and had no effect on pulmonary endothelial cells. D-PDMP inhibited the activity of GalT-2 in smooth muscle cells and markedly prevented cell proliferation. In contrast, L-PDMP stimulated the activity of GalT-2 in smooth muscle cells and stimulated cell proliferation. Antibody against GalT-2 inhibited cell proliferation. Our findings suggest that the activation of GalT-2 leads to increased LacCer levels, which in turn, may be involved in aortic smooth muscle cell proliferation.

A study of in vitro and in vivo morphological changes of ependymal cells induced by galactocerebrosides

Ependymal cells in culture and in vivo were treated with mixture of galactocerebrosides. Galactocerebroside is the major glycolipid of myelin and in demyelinating diseases is found in cerebrospinal fluid. Morphological changes induced by this treatment were examined by microscopy at both optical and ultrastructural levels. In vitro, cilia, microvilli, and junctions between the cells disappeared, processes containing intermediate filaments developed, and the cells lost characteristics typical of ependymal cells and became more astrocyte-like. As shown by vital staining with a fluorescent compound and by nuclear incorporation of bromodeoxyuridine, cells did not proliferate during the period of galactocerebroside treatment and the morphological transformation was restricted to the ependymal cells. In contrast, asialoganglioside-GM1 and sulfatides had no effect on ependymal cell morphology. Some of the in vitro observations could be reproduced in vivo. Junctions between ependymal cells disappeared and intercellular spaces appeared between these cells and the cerebral parenchyma at the basolateral side of the ependymal layer. At the apical side, morphological modifications of junctions and cilia were less evident. As these experimental conditions resemble those existing during demyelination the morphological changes described may account for perturbations of the physiological functions of the ependymal cell.

Acylation-stimulatory activity in hyperapobetalipoproteinemic fibroblasts: enhanced cholesterol esterification with another serum basic protein, BP II

Cultured fibroblasts from patients with familial hyperapobetalipoproteinemia (hyperapoB) were used to determine if a defect in lipid metabolism was present. Three basic proteins (BP I, BP II, and BP III) were isolated from normal human serum by preparative isoelectric focusing, preparative SDS/PAGE, and reversed-phase HPLC. The Mr and pI values of these proteins were 14,000 and 9.10 for BP I, 27,500 and 8.48 for BP II, and 55,000 and 8.73 for BP III. These proteins differed significantly in their content of arginine, cysteine, proline, histidine, serine, and methionine. BP I appears to be the same protein as acylation-stimulating protein, but BP II and BP III appeared different from acylation-stimulating protein and other lipid carrier proteins. BP I, BP II, and BP III stimulated the incorporation of [14C]oleate into lipid esters in normal fibroblasts, an effect that was time and concentration dependent. In hyperapoB cells, BP II markedly increased (up to 9-fold) the incorporation of [14C]oleate into cholesteryl ester compared with that in normal cells; in addition, there was a 50% decrease in the stimulation of triglyceride acylation and cholesterol esterification with BP I. No difference between normal and hyperapoB cells was observed with BP III. In summary, the identification of another serum basic protein, BP II, led to the elucidation of another cellular defect in hyperapoB fibroblasts, enhanced cholesterol esterification, which may be related to the precocious atherosclerosis and abnormal lipoprotein metabolism seen in hyperapoB.

Immunological determination of galactosylceramide level in blood as a serum index of active demyelination

An enzyme-linked immunosorbent assay (ELISA) to determine the level of galactosylceramide (GalC) in biological fluids is described. The assay uses GalC-coated plastic microtiter plates, with binding of an antibody to GalC detected by a peroxidase-labeled second antibody. The GalC level was directly estimated in the biological samples, without prior extraction, by competition with the coated hapten. This method allows the detection of 62 pmol of GalC (1.2 nmol/ml). Results using this procedure revealed positive sera only among patients suffering a myelin-destructive process: either primary, as in multiple sclerosis, or secondary to brain damage, as during ischemic strokes.

Spontaneous and plasma factor-mediated transfer of pyrenyl cerebrosides between model and native lipoproteins

A series of pyrenyl glucocerebrosides was synthesized by reacylation of psychosine with pyrene-labeled fatty acids having 3-11 methylene units. When incorporated into model high-density lipoproteins consisting of dimyristoylphosphatidylcholine-apolipoprotein A-II complexes and incubated with unlabeled complexes, these lipids exhibited spontaneous transfer. Half times of transfer varied from 1.5 min to 365 min at 37 degrees C. The logarithm of the rate of transfer was linearly related to the number of fatty acyl methylene units and HPLC retention time. Transfer occurred by passage of lipid monomers through the aqueous phase. Spontaneous transfer of the glycolipids also occurred when they were incorporated into native high-density lipoproteins. Rates of transfer between native high-density lipoprotein particles were higher than those observed between model high-density lipoprotein particles. A partially purified lipid exchange protein from plasma, as well as unfractionated lipoprotein-deficient serum, stimulated the transfer of fluorescent glycolipid between model high-density lipoprotein or native high-density lipoprotein and low-density lipoprotein 2-24 fold. The protein also stimulated the transfer of tritiated ganglioside GM3 between native low-density lipoprotein and high-density lipoprotein. This protein may play a role in glycolipid exchange in vivo.

Glycosphingolipids of human plasma

A number of glycosphingolipids, including 10 gangliosides, not previously identified in human plasma have been characterized. The plasma contains 2 micrograms of lipid-bound sialic acid/ml plasma and 54% of the gangliosides are monosialo, 30% disialo, 10% trisialo, and 6% tetrasialo. Individual glycosphingolipids were purified by high-performance liquid chromatography and thin-layer chromatography, and were characterized on the basis of their chromatographic mobility, carbohydrate composition, hydrolysis by glycosidases, methylation analysis, and immunostaining with anti-glycosphingolipid antibodies. The monosialogangliosides were identified as GM3, GM2, sialosyl(2-3)- and sialosyl(2-6)lactoneotetraosylceramides, sialosyllacto-N-nor-hexaosylceramide, and sialosyllacto-N-isooctaosylceramide. The major gangliosides in the polysialo fractions contained a ganglio-N-tetraose backbone and were identified as GD3, GD1a, GD1b, and GQ1b. The most abundant neutral glycosphingolipids were glucosyl, lactosyl, globotriaosyl, globotetraosyl and lactoneotetraosylceramides. The other neutral glycosphingolipids, tentatively identified by immunostaining with monoclonal antibodies, contained H1, Lea, Leb, and lacto-N-fucopentose III (X hapten) structures.

A review of the immunogenic and immuno-modulatory properties of glycosphingolipids

This review summarizes recent data concerning the immunogenicity and immunomodulatory properties of glycosphingolipids. Many murine monoclonal antibodies that react with glycosphingolipids have been described recently. Most of these antibodies have been elicited by immunization with tumor cells and they may also bind to glycoproteins that contain similar carbohydrate sequences. Immunization with a variety of tissues, murine teratocarcinomas, myeloid leukemia, and carcinomas of the human lung, colon and stomach, has elicited antibodies that react with the sugar sequence Gal beta 1-4[Fuc alpha 1-3]GlcNAc beta 1-3Gal----. The suppression of lymphocyte responses to mitogens and antigens by gangliosides in vitro has led to suggestions that these glycolipids possess immunodulatory properties in vivo. The in vitro studies were performed by incubating mononuclear cells with either dispersions of pure gangliosides or ganglioside-containing liposomes. In vivo gangliosides are found only in cell membranes or in lipoproteins, where they represent a small mole percent of total lipids, and there is little information about the transfer of gangliosides from lipoproteins to cells in vivo. A role for gangliosides as modulators of the immune response is an interesting possibility that is not supported by physiologically relevant data at present.

Distribution of antithrombin III and glucosylceramide in human plasma lipoproteins and lipoprotein deficient plasma

We have investigated the distribution of antithrombin-III and glucosylceramide (Glc-Cer) in human plasma, plasma lipoproteins and lipoprotein-deficient plasma. Antithrombin III activity was measured employing immunochemical and biological assays. Glc-Cer was quantified by gas liquid chromatography (GLC). Whole plasma contained 145 micrograms antithrombin III/ml plasma, all of which was associated with the lipoprotein-deficient plasma (d greater than 1.25 g/ml). Whereas, most if not all the plasma GlcCer was associated with plasma low density lipoproteins (LDL) (d-1.022-1.055 g/ml) and high density lipoproteins (HDL) (d-1.063-1.25). GlcCer was not found in the lipoprotein-deficient plasma. We conclude that GlcCer on lipoproteins does not contribute to antithrombin III activity. Moreover, the absence of GlcCer in lipoprotein-deficient plasma does not impair antithrombin-III activity.

Localization of urinary lactosylceramide in cytoplasmic vesicles of renal tubular cells in homozygous familial hypercholesterolemia

An average 15-fold increase in lactosylceramide (LacCer) in the sediment of receptor-negative, familial hypercholesterolemic (FH) homozygotes has been reported [Chatterjee, S., Sekerke, C.S. & Kwiterovich, P.O., Jr. (1982) J. Lipid Res. 23, 513-522]. We report here the abnormal urinary excretion of significant numbers of renal tubular cells in eight FH homozygotes. The mean activity of gamma-glutamyltransferase, a marker for renal tubular cells, was twice as high in urinary sediment of FH homozygotes as in normals. Membrane-enclosed cytoplasmic vesicles that stained strongly positive with a fluorescein-labeled antibody against LacCer were found in the renal tubular cells of all homozygotes except two who had undergone a portacaval shunt. These two had normal urinary levels of LacCer, and the cytoplasmic vesicles were vacuolated. In the other six, most of the fluorescent antibody label was intracellular and perinuclear. The cytoplasmic vesicles stained strongly with polychromatic Papanicolaou stain, periodic acid/Schiff reagent, and oil red O. Electron microscopy revealed perinuclear membrane-enclosed lipid and free lipid droplets. When two FH homozygotes, who excreted increased LacCer, underwent plasma exchange, the cytoplasmic vesicles became empty, and the urinary LacCer level decreased into the normal range. We conclude that the increased urinary excretion of LacCer in FH homozygotes occurs in renal tubular cells and that the intracellular locatin of LacCer is within cytoplasmic vesicles. The presence of LacCer within these vesicles can be modulated by treatment with plasma exchange.

[Lipoproteins associated with lipoprotein B in human serum low density lipoproteins]

Small amounts of lipoprotein C and lipoprotein D could be observed in low density lipoproteins (1.030-1.055 g/ml), using electroimmunomigration and polyacrylamide gel electrophoresis. Lipoprotein structures containing several apolipoproteins such as lipoprotein (B+C) or (B+D) were not detected in these low density lipoproteins. Lipoproteins C and D could not be separated from lipoprotein B by using gel filtration and affinity chromatography on heparin-Sepharose. Apolipoproteins C-III and D measured by electroimmunoassay are 3.2 +/- 1.2% and 1.15 +/- 0.6%, respectively, of the proteins found in the density range 1.030-1.055 g/ml, so there is, therefore, about 1 mol of apolipoprotein C-III and 0.1 mol of apolipoprotein D per mol of apolipoprotein B.

Hyperapobetalipoproteinaemia in two families with xanthomas and phytosterolaemia

The death of a 13-year-old boy from coronary atherosclerosis prompted the study of an Amish family. Five of his twelve sibs had tendon and tuberous xanthomas, and increased plasma plant sterols, particularly beta-sitosterol. The plasma level of the major apoprotein of low density lipoprotein (LDL), the B protein, was very high (mean 173 mg/dl) in these five sibs, while the LDL cholesterol level was moderately increased (209 mg/dl). Four other sibs and both parents had an increased LDL B protein level with a normal or mildly raised plasma total and LDL cholesterol level (hyperapobeta-lipoproteinaemia). Evidence for coronary artery disease was found in both parents and three xanthomatous sibs. The original family with beta-sitosterolaemia and xanthomatosis, described in 1974, was re-examined. The proband and her sister had persistent phytosterolaemia and normocholesterolaemia but increased LDL B protein levels. Both parents, two uncles, and three of four grandparents had increased LDL B protein levels and normal total and LDL cholesterol levels. The proband's father had atypical angina pectoris. People with the full syndrome (phytosterolaemia, xanthomas, and hyperapobetalipoproteinaemia) are most probably homozygous for a mutant allele. An increased LDL B protein level permits the identification of heterozygotes in these families, even though in the fasting state they show no phytosterolaemia. The homozygote and probably the heterozygote are at increased risk for cardiovascular atherosclerotic disease.

Stability of plasma low density lipoprotein with abnormal glycolipid composition from patients with Fabry's disease

Fabry's disease is a glycosphingolipid storage disease associated with premature generalized arteriosclerosis. Plasma low density lipoprotein (d 1.019--1.055 g/ml; LDL) was isolated from 4 healthy male volunteers (LDL-N) and from 4 patients with Fabry's disease (LDL-F), in whom plasma globotriaosylceramide (GbOse3) levels were 2--4 times above normal. LDL-N was labeled with 131I and LDL-F with 125I by the iodine monochloride method; both were then injected together intravenously into 4 mongreal dogs. The rates at which they were metabolized were determined by measuring plasma radioiodine levels daily for 7--8 days. No significant difference (Fisher's F-test) was observed between LDL-N and LDL-F in terms of the size of the intravascular compartments (LDL-N, 86.3 +/- 4.2%;; LDL-F, 94.2 +/- 7.7%), biological half-lives (LDL-N, 24.1 +/- 4.2 h; LDL-F, 23.5 +/- 3.7 h), or fractional catabolic rates (LDL-N, 0.849 +/- 0.066; LDL-F, 0.796 +/- 0.070). The results indicate that significant abnormalities of the neutral glycosphingolipid composition of LDL, such as occur in Fabry's disease, do not affect the metabolism of the lipoprotein apoprotein in dogs. The arteriosclerosis in patients with the disease is probably due to damage to vessel walls occurring as a result of defective GbOse3 metabolism and accumulation of glycosphingolipid in the tissue, rather than to abnormal LDL metabolism.

Properties of immobilized fig alpha-galactosidase and effect on ceramide-3 content of plasma from patients with Fabry's disease

The possibility of lowering the level of ceramide-3 (galactosyl-alpha(1 leads to 4)-galactosyl-beta(1 leads to 4)-glucosyl-beta(1 leads to 1)-ceramide) in the plasma of patients with Fabry's disease was investigated. An immobilized alpha-galactosidase (alpha-D-galactoside galactohydrolase, EC 3.2.1.22) was prepared by coupling purified fig alpha-galactosidase to Sepharose 4B. The pH optimum for the hydrolysis of the artificial substrate p-nitro-phenyl-alpha-D-galactopyranoside was shifted by approx. 0.5--1.0 pH unit to higher pH values upon coupling of the enzyme to Sepharose 4B. The immobilized enzyme was more stable than the native enzyme to incubation at 60 degrees C. The immobilized enzyme was able to hydrolyse ceramide-3 either at pH 4.5 or at pH 7.4 in an artificial system in which sodium taurocholate was used to solubilize the substrate. In contrast, when the immobilized enzyme was incubated with normal plasma or plasma from a patient with Fabry's disease, in which elevated levels of ceramide-3 occur, no hydrolysis of the glycosphingo-lipid could be detected. The results suggest that lowering of level of ceramide-3 in plasma from patients with Fabry's disease by enzymic means is not feasible.