Inhibitors of glucosylceramide synthase

Gaucher Disease: A Glance from a Medicinal Chemistry Perspective

Rare diseases are particular pathological conditions affecting a limited number of people and few drugs are known to be effective as therapeutic treatment. Gaucher disease, caused by a deficiency of the lysosomal enzyme glucocerebrosidase, belongs to this class of disorders, and it is considered the most common among the Lysosomal Storage Diseases. The two main therapeutic approaches are the Enzyme Replacement Therapy (ERT) and the Substrate Reduction Therapy (SRT). ERT, consisting in replacing the defective enzyme by administering a recombinant enzyme, is effective in alleviating the visceral symptoms, hallmarks of the most common subtype of the disease whereas it has no effects when symptoms involve CNS, since the recombinant protein is unable to significantly cross the Blood Brain Barrier. The SRT strategy involves inhibiting glucosylceramide synthase (GCS), the enzyme responsible for the production of the associated storage molecule. The rational design of new inhibitors of GCS has been hampered by the lack of either the crystal structure of the enzyme or an in-silico model of the active site which could provide important information regarding the interactions of potential inhibitors with the target, but, despite this, interesting results have been obtained and are herein reviewed.

Role of Globotriaosylceramide in Physiology and Pathology

At first glance, the biological function of globoside (Gb) clusters appears to be that of glycosphingolipid (GSL) receptors for bacterial toxins that mediate host-pathogen interaction. Indeed, certain bacterial toxin families have been evolutionarily arranged so that they can enter eukaryotic cells through GSL receptors. A closer look reveals this molecular arrangement allocated on a variety of eukaryotic cell membranes, with its role revolving around physiological regulation and pathological processes. What makes Gb such a ubiquitous functional arrangement? Perhaps its peculiarity is underpinned by the molecular structure itself, the nature of Gb-bound ligands, or the intracellular trafficking unleashed by those ligands. Moreover, Gb biological conspicuousness may not lie on intrinsic properties or on its enzymatic synthesis/degradation pathways. The present review traverses these biological aspects, focusing mainly on globotriaosylceramide (Gb3), a GSL molecule present in cell membranes of distinct cell types, and proposes a wrap-up discussion with a phylogenetic view and the physiological and pathological functional alternatives.

The Glucosylceramide Synthase Inhibitor PDMP Causes Lysosomal Lipid Accumulation and mTOR Inactivation

For many years, the biology of glycosphingolipids was elucidated with the help of glucosylceramide synthase (GCS) inhibitors such as 1-phenyl-2-decanoylamino-3-morpholino-1-propanol (PDMP). Additionally, PDMP gained interest because of its chemosensitizing effects. Several studies have successfully combined PDMP and anti-cancer drugs in the context of cancer therapy. However, the mechanism of action of PDMP is not fully understood and seems to go beyond glycolipid inhibition. Here, we used a functionalized sphingosine analogue (pacSph) to investigate the acute effects of PDMP on cellular sphingolipid distribution and found that PDMP, but not other GCS inhibitors, such as ND-DNJ (also called Miglustat), induced sphingolipid accumulation in lysosomes. This effect could be connected to defective export from lysosome, as monitored by the prolonged lysosomal staining of sphingolipids as well as by a delay in the metabolic conversion of the pacSph precursor. Additionally, other lipids such as lysobisphosphatidic acid (LBPA) and cholesterol were enriched in lysosomes upon PDMP treatment in a time-dependent manner. We could further correlate early LBPA enrichment with dissociation of the mechanistic target of rapamycin (mTOR) from lysosomes followed by nuclear translocation of its downtream target, transcription factor EB (TFEB). Altogether, we report here a timeline of lysosomal lipid accumulation events and mTOR inactivation arising from PDMP treatment.

The sirtuin inhibitor cambinol reduces intracellular glucosylceramide with ceramide accumulation by inhibiting glucosylceramide synthase

The accumulation of glucosylceramide (GlcCer), which is synthesized by UDP-glucose ceramide glucosyltransferase (UGCG), is associated with several diseases, including Gaucher disease and Parkinson's disease. Since the inhibition of UGCG can be used to treat diseases caused by GlcCer accumulation, several UGCG inhibitors have been developed. In this study, we report on the inhibition of UGCG activity by cambinol, a sirtuin inhibitor. Unlike conventional UGCG inhibitors, cambinol has no structural similarity to GlcCer. LC-ESI MS/MS analysis revealed that the cellular GlcCer levels were reduced by cambinol with an increase in ceramide, the GlcCer precursor. Histidine 193 plays an important role in the inhibition of UGCG via a known UGCG inhibitor, D-PDMP. However, cambinol was found to inhibit UGCG activity in a histidine 193-independent manner. This study provides insights into the mechanism of inhibition of UGCG activity by cambinol, and provides a basis for the development of a cambinol-based novel UGCG inhibitor.

Targeting Glucosylceramide Synthesis in the Treatment of Rare and Common Renal Disease

Sphingolipids, including ceramides, glycosphingolipids, sphingomyelin, and sphingosine-1-phosphate, have been recognized as important molecules that regulate critical cellular functions. Although originally studied in the context of lysosomal storage diseases, the roles of these compounds in more common disorders involving metabolism, vascular disease, and aberrant growth has been the focus of recent studies, including in disorders that affect the kidneys. These efforts have led to new insights into Fabry disease, a classic disorder of lysosomal function that results in renal failure as well as in more common renal diseases including diabetic nephropathy and polycystic kidney disease. Pathways for glycosphingolipid synthesis can be targeted with orally available small-molecule inhibitors, creating new opportunities for the treatment of both rare and common kidney diseases.

Lysosomal phospholipase A2

Lysosomal phospholipase A2 (PLA2G15) is a ubiquitous enzyme uniquely characterized by a subcellular localization to the lysosome and late endosome. PLA2G15 has an acidic pH optimum, is calcium independent, and acts as a transacylase in the presence of N-acetyl-sphingosine as an acceptor. Recent studies aided by the delineation of the crystal structure of PLA2G15 have clarified further the catalytic mechanism, sn-1 versus sn-2 specificity, and the basis whereby cationic amphiphilic drugs inhibit its activity. PLA2G15 has recently been shown to hydrolyze short chain oxidized phospholipids which access the catalytic site directly based on their aqueous solubility. Studies on the PLA2G15 null mouse suggest a role for the enzyme in the catabolism of pulmonary surfactant. PLA2G15 may also have a role in host defense and in the processing of lipid antigens for presentation by CD1 proteins. This article is part of a Special Issue entitled Novel functions of phospholipase A2 Guest Editors: Makoto Murakami and Gerard Lambeau.

Ceramide glycosylation catalyzed by glucosylceramide synthase and cancer drug resistance

Glucosylceramide synthase (GCS), converting ceramide to glucosylceramide, catalyzes the first reaction of ceramide glycosylation in sphingolipid metabolism. This glycosylation by GCS is a critical step regulating the modulation of cellular activities by controlling ceramide and glycosphingolipids (GSLs). An increase of ceramide in response to stresses, such as chemotherapy, drives cells to proliferation arrest and apoptosis or autophagy; however, ceramide glycosylation promptly eliminates ceramide and consequently, these induced processes, thus protecting cancer cells. Further, persistently enhanced ceramide glycosylation can increase GSLs, participating in selecting cancer cells to drug resistance. GCS is overexpressed in diverse drug-resistant cancer cells and in tumors of breast, colon, and leukemia that display poor response to chemotherapy. As ceramide glycosylation by GCS is a rate-limiting step in GSL synthesis, inhibition of GCS sensitizes cancer cells to anticancer drugs and eradicates cancer stem cells. Mechanistic studies indicate that uncoupling ceramide glycosylation can modulate gene expression, decreasing MDR1 through the cSrc/β-catenin pathway and restoring p53 expression via RNA splicing. These studies not only expand our knowledge in understanding how ceramide glycosylation affects cancer cells but also provide novel therapeutic approaches for targeting refractory tumors.

Glioma cell death induced by irradiation or alkylating agent chemotherapy is independent of the intrinsic ceramide pathway

BACKGROUND/AIMS

Resistance to genotoxic therapy is a characteristic feature of glioma cells. Acid sphingomyelinase (ASM) hydrolyzes sphingomyelin to ceramide and glucosylceramide synthase (GCS) catalyzes ceramide metabolism. Increased ceramide levels have been suggested to enhance chemotherapy-induced death of cancer cells.

METHODS

Microarray and clinical data for ASM and GCS in astrocytomas WHO grade II-IV were acquired from the Rembrandt database. Moreover, the glioblastoma database of the Cancer Genome Atlas network (TCGA) was used for survival data of glioblastoma patients. For in vitro studies, increases in ceramide levels were achieved either by ASM overexpression or by the GCS inhibitor DL-threo-1-phenyl-2-palmitoylamino-3-morpholino-1-propanol (PPMP) in human glioma cell lines. Combinations of alkylating chemotherapy or irradiation and ASM overexpression, PPMP or exogenous ceramide were applied in parental cells. The anti-glioma effects were investigated by assessing proliferation, metabolic activity, viability and clonogenicity. Finally, viability and clonogenicity were assessed in temozolomide (TMZ)-resistant cells upon treatment with PPMP, exogenous ceramide, alkylating chemotherapy, irradiation or their combinations.

RESULTS

Interrogations from the Rembrandt and TCGA database showed a better survival of glioblastoma patients with low expression of ASM or GCS. ASM overexpression or PPMP treatment alone led to ceramide accumulation but did not enhance the anti-glioma activity of alkylating chemotherapy or irradiation. PPMP or exogenous ceramide induced acute cytotoxicity in glioblastoma cells. Combined treatments with chemotherapy or irradiation led to additive, but not synergistic effects. Finally, no synergy was found when TMZ-resistant cells were treated with exogenous ceramide or PPMP alone or in combination with TMZ or irradiation.

CONCLUSION

Modulation of intrinsic glioma cell ceramide levels by ASM overexpression or GCS inhibition does not enhance the anti-glioma activity of alkylating chemotherapy or irradiation.

Attenuation of acetic acid-induced gastric ulcer formation in rats by glucosylceramide synthase inhibitors

INTRODUCTION

Ceramide has been suggested to play a role in apoptosis during gastric ulcerogenesis. The present study is designed to investigate whether accumulated ceramide could serve as the effector molecules of ulcer formation in a rat model of acetic acid-induced gastric ulcer.

METHODS

The effect of fumonisin B1, an inhibitor of ceramide synthase, and of d,l,-threo-1-phenyl-2-hexadecanoylamino-3-morpholino-1-propanol (PPMP) and N-butyldeoxynojirimycin (NB-DNJ), both inhibitors of glucosylceramide synthase, on the accumulation of ceramide and formation of gastric ulcer were examined in the rat model of acetic acid-induced gastric ulcer.

RESULTS

Fumonisin B1 attenuated acetic acid-induced gastric ulcer formation, associated with a decrease in the number of apoptotic cells. Our results showed that it is neither the C18- nor the C24-ceramide itself, but the respective metabolites that were ulcerogenic, because PPMP and NB-DNJ attenuated gastric mucosal apoptosis and the consequent mucosal damage in spite of their reducing the degradation of ceramide.

CONCLUSION

The ceramide pathway, in particular, the metabolites of ceramide, significantly contributes to acetic acid-induced gastric damage, possibly via enhancing apoptosis. On the other hand, PPMP and NB-DNJ treatment attenuated gastric mucosal apoptosis and ulcer formation despite increasing the ceramide accumulation, suggesting that it was not the ceramides themselves, but their metabolites that contributed to the ulcer formation in the acetic acid-induced gastric ulcer model.

Dissociation of ERK signalling inhibition from the anti-amyloidogenic action of synthetic ceramide analogues

Inhibition of GSL (glycosphingolipid) synthesis reduces Aβ (amyloid β-peptide) production in vitro. Previous studies indicate that GCS (glucosylceramide synthase) inhibitors modulate phosphorylation of ERK1/2 (extracellular-signal-regulated kinase 1/2) and that the ERK pathway may regulate some aspects of Aβ production. It is not clear whether there is a causative relationship linking GSL synthesis inhibition, ERK phosphorylation and Aβ production. In the present study, we treated CHO cells (Chinese-hamster ovary cells) and SH-SY5Y neuroblastoma cells, that both constitutively express human wild-type APP (amyloid precursor protein) and process this to produce Aβ, with GSL-modulating agents to explore this relationship. We found that three related ceramide analogue GSL inhibitors, based on the PDMP (D-threo-1-phenyl-2-decanoylamino-3-morpholino-1-propanol) structure, reduced cellular Aβ production and in all cases this was correlated with inhibition of pERK (phosphorylated ERK) formation. Importantly, the L-threo enantiomers of these compounds (that are inferior GSL synthesis inhibitors compared with the D-threo-enantiomers) also reduced ERK phosphorylation to a similar extent without altering Aβ production. Inhibition of ERK activation using either PD98059 [2-(2-amino-3-methoxyphenyl)-4H-1-benzopyran-4-one] or U0126 (1,4-diamino-2,3-dicyano-1,4-bis[2-aminophenylthio] butadiene) had no impact on Aβ production, and knockdown of endogenous GCS using small interfering RNA reduced cellular GSL levels without suppressing Aβ production or pERK formation. Our data suggest that the alteration in pERK levels following treatment with these ceramide analogues is not the principal mechanism involved in the inhibition of Aβ generation and that the ERK signalling pathway does not play a crucial role in processing APP through the amyloidogenic pathway.

Property-based design of a glucosylceramide synthase inhibitor that reduces glucosylceramide in the brain

Synthesis inhibition is the basis for the treatment of type 1 Gaucher disease by the glucosylceramide synthase (GCS) inhibitor eliglustat tartrate. However, the extended use of eliglustat and related compounds for the treatment of glycosphingolipid storage diseases with CNS manifestations is limited by the lack of brain penetration of this drug. Property modeling around the D-threo-1-phenyl-2-decanoylamino-3-morpholino-propanol (PDMP) pharmacophore was employed in a search for compounds of comparable activity against the GCS but lacking P-glycoprotein (MDR1) recognition. Modifications of the carboxamide N-acyl group were made to lower total polar surface area and rotatable bond number. Compounds were screened for inhibition of GCS in crude enzyme and whole cell assays and for MDR1 substrate recognition. One analog, 2-(2,3-dihydro-1H-inden-2-yl)-N-((1R,2R)-1-(2,3-dihydrobenzo[b][1,4]dioxin-6-yl)-1-hydroxy-3-(pyrrolidin-1-yl)propan-2-yl)acetamide (CCG-203586), was identified that inhibited GCS at low nanomolar concentrations with little to no apparent recognition by MDR1. Intraperitoneal administration of this compound to mice for 3 days resulted in a significant dose dependent decrease in brain glucosylceramide content, an effect not seen in mice dosed in parallel with eliglustat tartrate.

Group XV phospholipase A₂, a lysosomal phospholipase A₂

A phospholipase A₂ was identified from MDCK cell homogenates with broad specificity toward glycerophospholipids including phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine, and phosphatidylglycerol. The phospholipase has the unique ability to transacylate short chain ceramides. This phospholipase is calcium-independent, localized to lysosomes, and has an acidic pH optimum. The enzyme was purified from bovine brain and found to be a water-soluble glycoprotein consisting of a single peptide chain with a molecular weight of 45 kDa. The primary structure deduced from the DNA sequences is highly conserved between chordates. The enzyme was named lysosomal phospholipase A₂ (LPLA₂) and subsequently designated group XV phospholipase A₂. LPLA₂ has 49% of amino acid sequence identity to lecithin-cholesterol acyltransferase and is a member of the αβ-hydrolase superfamily. LPLA₂ is highly expressed in alveolar macrophages. A marked accumulation of glycerophospholipids and extensive lamellar inclusion bodies, a hallmark of cellular phospholipidosis, is observed in alveolar macrophages in LPLA₂(-/-) mice. This defect can also be reproduced in macrophages that are exposed to cationic amphiphilic drugs such as amiodarone. In addition, older LPLA₂(-/-) mice develop a phenotype similar to human autoimmune disease. These observations indicate that LPLA₂ may play a primary role in phospholipid homeostasis, drug toxicity, and host defense.

VEGF recruits lactosylceramide to induce endothelial cell adhesion molecule expression and angiogenesis in vitro and in vivo

Angiogenesis is largely driven by vascular endothelial growth factor (VEGF). However, the role of lipid second messengers such as lactosylceramide (LacCer) and LacCer synthase in angiogenesis is not well understood. We have determined the distribution of various LacCer synthase mRNA transcripts using sequential analysis of gene expression (SAGE). Endothelial cells from colon cancer tissues had a 4.5-fold increase in a LacCer synthase transcript (beta1,4GalT-V) as compared to normal colon tissue endothelial cells. Consequently, our focus turned to understanding the role of this enzyme in regulating VEGF-induced angiogenesis in vitro and in vivo. Herein, we show that in human endothelial cells, VEGF-induced angiogenesis is mitigated by dimethylsphingosine and suramin; inhibitors of sphingosine kinase 1(SphK-1) and sphingosine1-phosphate receptor 1(S1P (1)), respectively, and this were bypassed by LacCer but not by S1P. VEGF and basic fibroblast growth factor-induced angiogenesis was mitigated by PDMP; an inhibitor of glucosylceramide synthase and LacCer synthase in human umbilical vein endothelial cells (HUVEC) and human aortic endothelial cells (HAEC). Likewise, GalT-V gene ablation using corresponding siRNA also mitigated VEGF-induced angiogenesis. In Matrigel plug angiogenesis assay in nude mice, angiogenesis was markedly inhibited by D-PDMP with concordantly diminished LacCer synthase activity. Mechanistic studies revealed that the use of LY294002, a PI3 kinase inhibitor, mitigated VEGF-induced expression of platelet-endothelial cell adhesion molecule (PECAM-1/CD31); the trans-endothelial migration of a monocyte cell line (U-937) and angiogenesis in HAEC cells. Since this enzyme is a target for VEGF action and LacCer serves as a lipid second messenger in inducing angiogenesis in vitro and in vivo, novel therapeutic approaches may be developed using our findings to mitigate colon cancer.

Regulation of lipid metabolism by sphingolipids

Sphingolipids, together with phospholipids and cholesterol are key components of membrane lipid bilayers, contribute to specialized membrane domains called rafts and function as signaling molecules. Sphingolipids have been recognized to exert a distinct role in the post-transcriptional regulation of the sterol-regulatory element binding proteins (SREBPs), key transcription factors of lipid synthesis. Sphingolipid synthesis is an obligate activator of SREBP. Inhibition of sphingolipid synthesis decreases SREBP on a post-transcriptional level. With the exception of enzymes that synthesize sphingolipids, SREBPs regulate the transcription of key enzymes that synthesize cholesterol, phospholipids and fatty acids. This observation suggests an exclusive role for sphingolipids in the regulation of lipid metabolism. Although exact mechanisms how sphingolipids regulate lipid metabolism are currently not known, this relationship has important implications with regard to cellular lipid homeostasis, composition of lipoproteins and development of atherosclerosis.

The acylation of lipophilic alcohols by lysosomal phospholipase A2

A novel lysosomal phospholipase A(2) (LPLA2) with specificity toward phosphatidylethanolamine and phosphatidylcholine was previously purified and cloned. LPLA2 transfers sn-1 or sn-2 acyl groups of phospholipids to the C1 hydroxyl of the short-chain ceramide N-acetylsphingosine (NAS) under acidic conditions. The common features of lipophilic alcohols serving as acceptor molecules in the transacylase reaction were examined. 1-O-Hexadecyl-2-acetyl-sn-glycerol (HAG) was acylated by LPLA2 similar to NAS. HAG competed with NAS and inhibited NAS acylation. The transacylation of 1-O-hexadecyl-glycerol (HG), 1-O-palmityl-2-O-methyl-sn-glycerol (PMG), and monoacylglycerols was also investigated. HG, PMG, 1- or 3-palmitoyl-sn-glycerol, and 2-palmitoylglycerol were converted to 1,3-alkylacylglycerol, 1,2-dialkyl-3-acylglycerol, 1,3-diacylglycerol, and 1,2- or 2,3-diacylglycerol, respectively. HG and monoacylglycerol inhibited the acylation of NAS by the enzyme with IC(50) values of 35 and 45 microM, respectively. Additionally, the enzyme acylated glycerol to produce 1- or 3-acyl-sn-glycerol but not 2-acylglycerol. Therefore, the preferred acceptor molecules for LPLA2 are primary alcohols with one long carbon chain and one small nonpolar residue linked to the C2 position of ethanol. The enzyme acylated other natural lipophilic alcohols, including anandamide and oleoylethanolamide. Thus, LPLA2 may function to remodel acyl groups and modulate the biological and pharmacological activities of some lipophilic alcohols.

Recycling of sphingosine is regulated by the concerted actions of sphingosine-1-phosphate phosphohydrolase 1 and sphingosine kinase 2

In yeast, the long-chain sphingoid base phosphate phosphohydrolase Lcb3p is required for efficient ceramide synthesis from exogenous sphingoid bases. Similarly, in this study, we found that incorporation of exogenous sphingosine into ceramide in mammalian cells was regulated by the homologue of Lcb3p, sphingosine-1-phosphate phosphohydrolase 1 (SPP-1), an endoplasmic reticulum resident protein. Sphingosine incorporation into endogenous long-chain ceramides was increased by SPP-1 overexpression, whereas recycling of C(6)-ceramide into long-chain ceramides was not altered. The increase in ceramide was inhibited by fumonisin B(1), an inhibitor of ceramide synthase, but not by ISP-1, an inhibitor of serine palmitoyltransferase, the rate-limiting step in the de novo biosynthesis of ceramide. Mass spectrometry analysis revealed that SPP-1 expression increased the incorporation of sphingosine into all ceramide acyl chain species, particularly enhancing C16:0, C18:0, and C20:0 long-chain ceramides. The increased recycling of sphingosine into ceramide was accompanied by increased hexosylceramides and, to a lesser extent, sphingomyelins. Sphingosine kinase 2, but not sphingosine kinase 1, acted in concert with SPP-1 to regulate recycling of sphingosine into ceramide. Collectively, our results suggest that an evolutionarily conserved cycle of phosphorylation-dephosphorylation regulates recycling and salvage of sphingosine to ceramide and more complex sphingolipids.

Novel therapeutic targets for the treatment of Fabry disease

Fabry disease is an X-linked lysosomal storage disorder resulting from deficient activity of alpha-galactosidase A. The traditional concept that is used to explain the complications of the disease involves progressive accumulation of globotriaosylceramide in endothelial and smooth muscle cells, resulting in vascular damage. Clinically, progressive renal insufficiency, cardiac involvement and brain pathology evolves. Two pharmaceutical companies have developed enzyme replacement therapy in Fabry disease. Although the first clinical trials showed great promise, it is clear that long-term effects are not as robust as was anticipated. Stabilisation of renal function and decreases in cardiac hypertrophy has been observed, but some patients may experience progressive complications. As there are recent indications that serum components contribute to the pathophysiology of Fabry disease, fundamental studies are needed to unravel the precise role and identity of these factors. Combination of these basic studies with clinical follow up may ultimately reveal when the 'point of no return' is reached. Advanced renal insufficiency seems to be a clinical indicator of lack of response, but other signs and symptoms are probably related to adverse outcome. It is anticipated that in the future controlled studies in early symptomatic or presymptomatic patients will be required. In addition, alternative strategies such as substrate reduction or chaperone therapy, either alone or in combination with enzyme replacement therapy, should be explored. Because Fabry disease is rare, collaborative efforts should be undertaken and openness of data should be strived for.

Inhibiting glycosphingolipid synthesis improves glycemic control and insulin sensitivity in animal models of type 2 diabetes

Previous reports have shown that glycosphingolipids can modulate the activity of the insulin receptor, and studies in transgenic mice suggest a link between altered levels of various gangliosides and the development of insulin resistance. Here, we show that an inhibitor of glycosphingolipid synthesis can improve glucose control and increase insulin sensitivity in two different diabetic animal models. In the Zucker diabetic fatty rat, the glucosylceramide synthase inhibitor (1R,2R)-nonanoic acid[2-(2',3'-dihydro-benzo [1, 4] dioxin-6'-yl)-2-hydroxy-1-pyrrolidin-1-ylmethyl-ethyl]- amide-l-tartaric acid salt (Genz-123346) lowered glucose and A1C levels and improved glucose tolerance. Drug treatment also prevented the loss of pancreatic beta-cell function normally observed in the Zucker diabetic fatty rat and preserved the ability of the animals to secrete insulin. In the diet-induced obese mouse, treatment with Genz-123346 normalized A1C levels and improved glucose tolerance. Analysis of the phosphorylation state of the insulin receptor and downstream effectors showed increased insulin signaling in the muscles of the treated Zucker diabetic fatty rats and diet-induced obese mice. These results suggest that inhibiting glycosphingolipid synthesis can significantly improve insulin sensitivity and glucose homeostasis and may therefore represent a novel therapeutic approach for the treatment of type 2 diabetes.

Sphingolipids: major regulators of lipid metabolism

PURPOSE OF REVIEW

Sphingolipids and their metabolites regulate a great variety of cellular processes. Recent findings implicate sphingolipids in the regulation of lipid synthesis, lipoprotein metabolism and the development of atherosclerosis.

RECENT FINDINGS

Sphingolipid synthesis correlates with the regulation of the sterol-regulatory element-binding proteins - key transcription factors of genes of lipid metabolism. Inhibition of sphingolipid synthesis decreases synthesis of genes regulated by sterol regulatory element-binding protein, such as the rate-limiting enzymes of fatty acid and cholesterol synthesis as well as fatty-acyl-CoA synthases, important in the synthesis of phospholipids. In animal models, inhibition of sphingolipid synthesis correlates with decreased atherosclerotic lesions and a decreased susceptibility of lipoproteins to aggregate--a key mechanism in the development of the atherosclerotic lesion. The demonstration that ceramide and glucosylceramide (metabolites of sphingolipid synthesis) affect cholesterol efflux and mechanisms that regulate plasma high-density lipoprotein concentrations is further evidence for a role of sphingolipids in the regulation of lipid homeostasis. Direct mechanisms of how sphingolipid synthesis regulates lipid synthesis are currently unknown. The recent identification of key proteins of synthesis and specific transport proteins that regulate sphingolipid synthesis, however, is expected to contribute to the understanding about the interdependent regulation of sphingolipid and lipid metabolism.

SUMMARY

Emerging data strongly suggest a role of sphingolipid synthesis in the regulation of transcription factors and regulatory proteins that control cellular lipid homeostasis.

Sphingosine-1-phosphate phosphohydrolase regulates endoplasmic reticulum-to-golgi trafficking of ceramide

Previous studies demonstrated that sphingosine-1-phosphate (S1P) phosphohydrolase 1 (SPP-1), which is located mainly in the endoplasmic reticulum (ER), regulates sphingolipid metabolism and apoptosis (H. Le Stunff et al., J. Cell Biol. 158:1039-1049, 2002). We show here that the treatment of SPP-1-overexpressing cells with S1P, but not with dihydro-S1P, increased all ceramide species, particularly the long-chain ceramides. This was not due to inhibition of ceramide metabolism to sphingomyelin or monohexosylceramides but rather to the inhibition of ER-to-Golgi trafficking, determined with the fluorescent ceramide analog N-(4,4-difluoro-5,7-dimethyl-4-bora-3a,4a-diaza-s-indacene-3-pentanoyl)-d-erythro-sphingosine (DMB-Cer). Fumonisin B1, an inhibitor of ceramide synthase, prevented S1P-induced elevation of all ceramide species and corrected the defect in ER transport of DMB-Cer, readily allowing its detection in the Golgi. In contrast, ceramide accumulation had no effect on either the trafficking or the metabolism of 6-([N-(7-nitrobenzo-2-oxa-1,3-diazol-4-yl)amino]hexanoyl)-sphingosine, which rapidly labels the Golgi even at 4 degrees C. Protein trafficking from the ER to the Golgi, determined with vesicular stomatitis virus ts045 G protein fused to green fluorescent protein, was also inhibited in SPP-1-overexpressing cells in the presence of S1P but not in the presence of dihydro-S1P. Our results suggest that SPP-1 regulates ceramide levels in the ER and thus influences the anterograde membrane transport of both ceramide and proteins from the ER to the Golgi apparatus.

Involvement of de novo ceramide biosynthesis in macrophage death induced by activation of ATP-sensitive P2X7 receptor

Macrophage ionotropic P2X7 receptors regulate cell-death through ill-defined signaling pathways. Here, we investigated the role of ceramide, an apoptogenic sphingolipid and showed that ATP stimulated ceramide accumulation in macrophages. Benzoylbenzoyl-ATP, a potent P2X7 agonist, was able to mimic the effects of ATP on ceramide accumulation while oxidized ATP had the opposite effect. Ceramide accumulation was blocked by de novo ceramide biosynthesis inhibitors. Interestingly, ATP-induced caspase-3/7 activation was dependent on ceramide generation. Finally, we showed that de novo ceramide biosynthesis is involved in ATP-induced macrophage death in a caspase-dependent manner. Our results indicate a novel role of ceramide in P2X7-regulated cell-death.

An in vitro model of Fabry disease

Fabry disease is an X-linked inherited loss of alpha-galactosidase A (alpha-Gal A). Affected patients experience complications that include neuropathy, renal failure, and cardiovascular disease. Although the genetic and biochemical basis of this sphingolipidosis is well studied, the basis for the vascular disease remains poorly understood. In an attempt to create a suitable in vitro model of this disease, conditions for the growth of primary cultures of aortic endothelial cells from wild-type and alpha-Gal A -/0 mice were established. The cultured cells demonstrated CD-31 expression by flow cytometry and LDL binding by immunofluorescence. The glycolipid expression patterns were compared between wild-type and alpha-Gal A null cells. Importantly, cells from alpha-Gal A -/0 mice but not alpha-Gal A +/0 mice expressed high levels of the globo-series glycosphingolipid globotriaosylceramide (Gb3). The age-dependent elevation in Gb3 was measured. By 4 mo of age, alpha-Gal A -/0 mouse aortic endothelial cells achieved their peak Gb3 levels. The ability to lower Gb3 levels pharmacologically was assessed next. The glucosylceramide synthase inhibitor ethylenedioxyphenyl-P4 significantly lowered but did not eliminate Gb3 levels by 96 h of treatment. Gb3 synthesis was completely blocked as measured by [14C]galactose labeling. Recombinant alpha-Gal A more significantly lowered Gb3 levels by 48 h but had a more limited effect on de novo synthesis. Together, both agents eliminated detectable Gb3. In summary, primary cultures of aortic endothelial cells from Fabry mice retain the phenotype of elevated globo-series glycosphingolipids. These cells provide a useful model for comparing pharmacologic agents used for glycolipid reduction.

Glucosylceramide synthase blockade down-regulates P-glycoprotein and resensitizes multidrug-resistant breast cancer cells to anticancer drugs

Overexpression of glucosylceramide synthase (GCS), a pivotal enzyme in glycolipid biosynthesis, contributes to cancer cell resistance to chemotherapy. We previously showed that transfection of doxorubicin-resistant MCF-7-AdrR cells with GCS antisense restored cell sensitivity to doxorubicin and greatly enhanced sensitivity to vinblastine and paclitaxel. In that study, doxorubicin promoted generation of ceramide in MCF-7-AdrR/GCS antisense cells; the present study implicates factors in addition to ceramide that augment sensitivity to chemotherapy. Although GCS antisense cells showed enhanced ceramide formation compared with MCF-7-AdrR when challenged with paclitaxel, GCS antisense cells also showed a 10-fold increase in levels of intracellular drug (paclitaxel and vinblastine). In addition, transfected cells had dramatically decreased expression (80%) of P-glycoprotein and a 4-fold decrease in the level of cellular gangliosides. Chemical inhibition of GCS produced the same effects as antisense transfection: exposure of MCF-7-AdrR cells to the GCS inhibitor 1-phenyl-2-palmitoylamino-3-morpholino-1-propanol (PPMP, 5.0 micromol/L, 4 days) decreased ganglioside levels, restored sensitivity to vinblastine, enhanced vinblastine uptake 3-fold, and diminished expression of MDR1 by 58%, compared with untreated controls. A similar effect was shown in vinblastin-resistant KB-V0.01 cells; after 7 days with PPMP (10 micromol/L), MDR1 expression fell by 84% and P-glycoprotein protein levels decreased by 50%. MCF-7-AdrR cells treated with small interfering RNAs to specifically block GCS also showed a dramatic decrease in MDR1 expression. This work shows that limiting GCS activity down-regulates the expression of MDR1, a phenomenon that may drive the chemosensitization associated with blocking ceramide metabolism. The data suggest that lipids play a role in the expression of multidrug resistance.

Gangliosides inhibit flagellin signaling in the absence of an effect on flagellin binding to toll-like receptor 5

A recent study (Ogushi, K., Wada, A., Niidome, T., Okuda, T., Llanes, R., Nakayama, M., Nishi, Y., Kurazono, H., Smith, K. D., Aderem, A., Moss, J., and Hirayama, T. (2004) J. Biol. Chem. 279, 12213-12219) concluded that gangliosides serve as co-receptors for flagellin signaling via toll-like receptor 5 (TLR5). In view of several findings in this study that were inconsistent with a role for gangliosides as co-receptors, we re-examined this important issue. Using TLR5-negative RAW 264.7 cells and a TLR5-enhanced yellow fluorescent protein chimera, we established an assay for specific binding of flagellin to cells. Inhibition of clatherin-mediated internalization of flagellin.TLR5-enhanced yellow fluorescent protein complexes did not impair flagellin activation of IRAK-1. Thus flagellin signal occurs at the cell surface and not intracellularly. Exogenous addition of mixed gangliosides (GM1, GD1a, and GT1b) as well as GD1a itself inhibited flagellin-induced interleukin-1 receptor-associated kinase activation as well as tumor necrosis factor alpha production in HeNC2, THP-1, and RAW 264.7 cells. Gangliosides inhibited flagellin signaling in the absence of an effect on flagellin binding to TLR5. Depletion of gangliosides in RAW 264.7 cells did not alter the concentration dependence or magnitude of flagellin signaling as measured by interleukin-1 receptor-associated kinase activation or tumor necrosis factor alpha production. Our findings are consistent with the conclusions that gangliosides are not essential co-receptors for flagellin and that the inhibitory effect of gangliosides is mediated by at least one mechanism that is distinct from any effect on the binding of flagellin to TLR5.

Anti-Plasmodium activity of ceramide analogs

Background

Sphingolipids are key molecules regulating many essential functions in eukaryotic cells and ceramide plays a central role in sphingolipid metabolism. A sphingolipid metabolism occurs in the intraerythrocytic stages of Plasmodium falciparum and is associated with essential biological processes. It constitutes an attractive and potential target for the development of new antimalarial drugs.

Methods

The anti-Plasmodium activity of a series of ceramide analogs containing different linkages (amide, methylene or thiourea linkages) between the fatty acid part of ceramide and the sphingoid core was investigated in culture and compared to the sphingolipid analog PPMP (d,1-threo-1-phenyl-2-palmitoylamino-3-morpholino-1-propanol). This analog is known to inhibit the parasite sphingomyelin synthase activity and block parasite development by preventing the formation of the tubovesicular network that extends from the parasitophorous vacuole to the red cell membrane and delivers essential extracellular nutrients to the parasite.

Results

Analogs containing methylene linkage showed a considerably higher anti-Plasmodium activity (IC₅₀ in the low nanomolar range) than PPMP and their counterparts with a natural amide linkage (IC₅₀ in the micromolar range). The methylene analogs blocked irreversibly P. falciparum development leading to parasite eradication in contrast to PPMP whose effect is cytostatic. A high sensitivity of action towards the parasite was observed when compared to their effect on the human MRC-5 cell growth. The toxicity towards parasites did not correlate with the inhibition by methylene analogs of the parasite sphingomyelin synthase activity and the tubovesicular network formation, indicating that this enzyme is not their primary target.

Conclusions

It has been shown that ceramide analogs were potent inhibitors of P. falciparum growth in culture. Interestingly, the nature of the linkage between the fatty acid part and the sphingoid core considerably influences the antiplasmodial activity and the selectivity of analogs when compared to their cytotoxicity on mammalian cells. By comparison with their inhibitory effect on cancer cell growth, the ceramide analogs might inhibit P. falciparum growth through modulation of the endogenous ceramide level.

Killing tumours by ceramide-induced apoptosis: a critique of available drugs

Over 1000 research papers have described the production of programmed cell death (apoptosis) by interventions that elevate the cell content of ceramide (Cer). Other interventions, which lower cellular Cer, have been found to interfere with apoptosis induced by other agents. Some studies have shown that slowing the formation of proliferation-stimulating sphingolipids also induces apoptosis. These relationships are due to the two different aspects of Cer: Cer itself produces apoptosis, but metabolic conversion of Cer into either sphingosine 1-phosphate or glucosphingolipids leads to cell proliferation. The balance between these two aspects is missing in cancer cells, and yet intervention by stimulating or blocking only one or two of the pathways in Cer metabolism is very likely to fail. This results from two properties of cancer cells: their high mutation rate and the preferential survival of the most malignant cells. Tumours treated with only one or two drugs that elevate Cer can adjust the uncontrolled processes to either maintain or to 'aggravate' the excessive growth, angiogenesis and metastasis characteristics of tumours. These treatments might simply elevate the production of growth factors, receptors and other substances that reduce the effectiveness of Cer. Tumour cells that do not adapt in this way undergo apoptosis, leaving the adapted cells free to grow and, ultimately, to 'subdue' their host. Thus it is important to kill every type of cancer cell present in the tumour rapidly and simultaneously, using as many different agents to control as many pathways as possible. To aid this approach, this article catalogues many of the drugs that act on different aspects of Cer metabolism. The techniques described here may lead to the development of practical chemotherapy for cancer and other diseases of excess proliferation.

Phosphatidylcholine synthesis is elevated in neuronal models of Gaucher disease due to direct activation of CTP:phosphocholine cytidylyltransferase by glucosylceramide

Glucosylceramide (GlcCer) accumulates in the inherited metabolic disorder, Gaucher disease, because of the defective activity of lysosomal glucocerebrosidase. We previously demonstrated that upon GlcCer accumulation, cultured hippocampal neurons exhibit modified growth patterns, altered endoplasmic reticulum density, and altered calcium release from intracellular stores. We here examined the relationship between GlcCer accumulation and phospholipid synthesis. After treatment of neurons with an active site-directed inhibitor of glucocerebrosidase, or in neurons obtained from a mouse model of Gaucher disease, [14C]methyl choline incorporation into [14C]phosphatidylcholine ([14C]PC) and [14C]sphingomyelin was elevated, as were [14C]CDP-choline levels, suggesting that CTP:phosphocholine cytidylyltransferase (CCT) is activated. Indeed, CCT activity was elevated in neurons that had accumulated GlcCer. GlcCer, but not galactosylceramide (GalCer), stimulated CCT activity in rat brain homogenates, and significantly higher levels of CCT were membrane associated in cortical homogenates from a mouse model of Gaucher disease compared with wild-type mice. Because CCT mRNA and protein levels were unaltered in either neurons or brain tissue that had accumulated GlcCer, it appeared likely that GlcCer activates CCT by a post-translational mechanism. This was verified by examination of the effect of GlcCer on CCT purified about 1200-fold from rat brain. GlcCer stimulated CCT activity, with stimulation observed at levels as low as 2.5 mol% and with maximal activation reached at 10 mol%. In contrast, GalCer had no effect. Together, these data demonstrate that GlcCer directly activates CCT, which results in elevated PC synthesis, which may account for some of the changes in growth rates observed upon neuronal GlcCer accumulation.

Sphingosine-1-phosphate phosphohydrolase in regulation of sphingolipid metabolism and apoptosis

Sphingosine-1-phosphate (S1P) is a bioactive sphingolipid metabolite that regulates diverse biological processes by binding to a family of G protein-coupled receptors or as an intracellular second messenger. Mammalian S1P phosphatase (SPP-1), which degrades S1P to terminate its actions, was recently cloned based on homology to a lipid phosphohydrolase that regulates the levels of phosphorylated sphingoid bases in yeast. Confocal microscopy surprisingly revealed that epitope-tagged SPP-1 is intracellular and colocalized with the ER marker calnexin. Moreover, SPP-1 activity and protein appeared to be mainly enriched in the intracellular membranes with lower expression in the plasma membrane. Treatment of SPP-1 transfectants with S1P markedly increased ceramide levels, predominantly in the intracellular membranes, diminished survival, and enhanced apoptosis. Remarkably, dihydro-S1P, although a good substrate for SPP-1 in situ, did not cause significant ceramide accumulation or increase apoptosis. Ceramide accumulation induced by S1P was completely blocked by fumonisin B1, an inhibitor of ceramide synthase, but only partially reduced by myriocin, an inhibitor of serine palmitoyltransferase, the first committed step in de novo synthesis of ceramide. Furthermore, S1P, but not dihydro-S1P, stimulated incorporation of [3H]palmitate, a substrate for both serine palmitoyltransferase and ceramide synthase, into C16-ceramide. Collectively, our results suggest that SPP-1 functions in an unprecedented manner to regulate sphingolipid biosynthesis and is poised to influence cell fate.

Molecular basis for high renal cell sensitivity to the cytotoxic effects of shigatoxin-1: upregulation of globotriaosylceramide expression

Cellular injury in post-diarrheal hemolytic-uremic syndrome (D+HUS) is related to shigatoxin (Stx) binding to globotriaosylceramide (Gb3). High renal Gb3 expression may determine renal susceptibility in D+HUS; however, the molecular mechanism(s) responsible for such relatively abundant Gb3 levels are unknown. Consequently, kidney cells expressing high Gb3 (cultured human proximal tubule cells [HPT]) were compared with non-kidney cells with low Gb3 content (cultured human brain microvascular endothelial cells [HBEC]). HPT were much more sensitive to the cytotoxic and protein synthesis inhibitory effects of Stx-1; this correlated with Gb3 content and (125)I-Stx-1 binding. HPT had greater Gb3 synthase (GalT6) and lower alpha-galactosidase activities than HBEC, whereas lactosylceramide synthase (GalT2) activity was higher in HBEC. Ceramide glucosyltransferase (CGT) activity was similar between the two cell types. The higher HPT GalT6 activity was associated with increased GalT6 mRNA steady-state levels, but no difference in GalT6 mRNA half-life. The lower HPT alpha-galactosidase activity was associated with reduced alpha-galactosidase mRNA steady-state levels but no difference in alpha-galactosidase mRNA half-life. Higher HBEC GalT2 activity was associated with increased steady-state GalT2 mRNA levels. These studies suggest that high renal Gb3 expression is due to enhanced GalT6 gene transcription and reduced alpha-galactosidase gene transcription and occur despite relatively low GalT2 activity.

Disruption of the glucosylceramide biosynthetic pathway in Aspergillus nidulans and Aspergillus fumigatus by inhibitors of UDP-Glc:ceramide glucosyltransferase strongly affects spore germination, cell cycle, and hyphal growth

The opportunistic mycopathogen Aspergillus fumigatus expresses both glucosylceramide and galactosylceramide (GlcCer and GalCer), but their functional significance in Aspergillus species is unknown. We here identified and characterized a GlcCer from Aspergillus nidulans, a non-pathogenic model fungus. Involvement of GlcCer in fungal development was tested on both species using a family of compounds known to inhibit GlcCer synthase in mammals. Two analogs, D-threo-1-phenyl-2-palmitoyl-3-pyrrolidinopropanol (P4) and D-threo-3',4'-ethylenedioxy-P4, strongly inhibited germination and hyphal growth. Neutral lipids from A. fumigatus cultured in the presence of these inhibitors displayed a significantly reduced GlcCer/GalCer ratio. These results suggest that synthesis of GlcCer is essential for normal development of A. fumigatus and A. nidulans.

Regulation of phospholipase C-gamma activity by glycosphingolipids

Glycosphingolipid-enriched domains are hot spots for cell signaling within plasma membranes and are characterized by the enrichment of glycosphingolipids. A role for glucosylceramide-based glycosphingolipids in phospholipase C-mediated inositol 1,4,5-trisphosphate formation has been previously documented. These earlier studies utilized a first generation glucosylceramide synthase inhibitor to deplete cells of their glycosphingolipids. Recently, more active and specific glucosylceramide synthase inhibitors, including d-threo-ethylendioxyphenyl-2-palmitoylamino-3-pyrrolidinopropanol (d-t-EtDO-P4), have been designed. d-t-EtDO-P4 has the advantage of blocking glucosylceramide synthase at low nanomolar concentrations but does not cause secondary elevations in cell ceramide levels. In the present study, d-t-EtDO-P4 depleted cellular glucosylceramide and lactosylceramide in cultured ECV304 cells at nanomolar concentrations without obvious cellular toxicity. The expression of several signaling proteins was evaluated in glycosphingolipid-depleted ECV304 cells to study the role of glycosphingolipids in phospholipase C-mediated signaling. No difference was observed in the cellular expression of phospholipase C-gamma between controls and glycolipid-depleted cells. Western blot analysis, however, revealed that depletion of endogenous glycosphingolipids in cultured ECV304 cells with d-t-EtDO-P4 induced tyrosine phosphorylation of phospholipase C-gamma in a concentration-dependent manner with maximum induction at 100 nm. The phosphorylation of phospholipase C-gamma induced by d-t-EtDO-P4 was abolished by exogenously added glucosylceramide, consistent with a specific glycosphingolipid-phospholipase C-gamma interaction. The phospholipase C-gamma phosphorylation was maximally enhanced by bradykinin when cells were exposed to 100 nm d-t-EtDO-P4. The measurement of cellular activity of phospholipase C-gamma, by myo-inositol 1,4,5-trisphosphate radioreceptor assay, demonstrated that depletion of glucosylceramide-based glycosphingolipids in cultured ECV304 cells with d-t-EtDO-P4 resulted in significantly increased formation of inositol 1,4,5-trisphosphate above base line, and an increased sensitivity of phospholipase C-gamma to bradykinin stimulation. Thus, the activation of phospholipase C-gamma is negatively regulated by membrane glycosphingolipids in ECV304 cells.

N-(4-hydroxyphenyl)retinamide increases ceramide and is cytotoxic to acute lymphoblastic leukemia cell lines, but not to non-malignant lymphocytes

The retinoid, N-(4-hydroxyphenyl)retinamide (4-HPR), mediates p53-independent cytotoxicity and can increase reactive oxygen species and ceramide in solid tumor cell lines. We determined changes in ceramide and cytotoxicity upon treatment with 4-HPR (3-12 microM) in six human acute lymphoblastic leukemia (ALL) cell lines: T cell (MOLT-3, MOLT-4, CEM), pre-B-cell (NALM-6, SMS-SB), and null cell (NALL-1). Exposure to 4-HPR (12 microM) for 96 h caused 4.7 (MOLT-3), 3.5 (MOLT-4), 3.9 (CEM), 2.9 (NALM-6), 4.7 (SMS-SB), AND 4.5 (NALL-1) logs of cell kill. The average 4-HPR concentration that killed 99% of cells (LC(99)) for all six lines was 4.8 microM (range: 1.5-8.9 microM). Treatment with 4-HPR (9 microM) for 24 h resulted in an 8.9 +/- 1.0-fold (range: 4.9-15.7-fold) increase of ceramide. Ceramide increase was time- and dose-dependent and abrogated by inhibitors of de novo ceramide synthesis. Concurrent inhibition of ceramide glycosylation/acylation by d,l-threo-(1-phenyl-2-hexadecanoylamino-3-morpholino-1-propanol) (PPMP) further increased ceramide levels, and synergistically increased 4-HPR cytotoxicity in four of six ALL cell lines. 4-HPR was minimally cytotoxic to peripheral blood mononuclear cells and a lymphoblastoid cell line, and increased ceramide <2-fold. Thus, 4-HPR was cytotoxic and increased ceramide in ALL cell lines, but not in non-malignant lymphoid cell types.

Differential chemosensitizing effect of two glucosylceramide synthase inhibitors in hepatoma cells

It has been proposed that ceramide mediates anthracyclin-induced apoptosis and that drug resistance may arise due to upregulated removal of this active lipid through glucosylation. We report that HepG2 hepatoma cells displayed only a modest apoptotic response to doxorubicin treatment, accompanied by a substantial elevation of ceramide levels only at toxic drug concentrations. D,L-threo-1-phenyl-2-decanoylamino-3-morpholino-1-propanol (PDMP) and D,L-threo-1-phenyl-2-hexadecanoylamino-3-pyrrolidino-1-propanol (PPPP), used at concentrations causing a 90% inhibition of ceramide glucosylation, enhanced doxorubicin-elicited ceramide elevation, but only PDMP potentiated apoptosis. Exogenously administered ceramide had only a marginal apoptotic effect on HepG2 cells; moreover, even in this case, apoptosis was propagated by PDMP but not by PPPP. PDMP moderately inhibited P-glycoprotein activity only at the highest concentration tested, but its chemosensitizing effect was still outstanding at lower concentrations, at which P-gp inhibition was no longer observed. These results demonstrate that the chemosensitizing effect of PDMP is, at least partly, independent from its activity as a glucosylceramide synthase inhibitor. Moreover, P-glycoprotein inhibition is not central to the phenomenon.

The T lymphocyte structure CD60 contains a sialylated carbohydrate epitope that is expressed on both gangliosides and glycoproteins

The CD60 antigen is expressed on a majority of T cells in autoimmune lesions, and anti-CD60 can activate T lymphocytes. CD60 has been defined as the GD3 ganglioside, and subsequently as the 9-O-acetylated form of GD3. However, other evidence suggests that anti-CD60 recognizes a glycoprotein or family of glycoproteins expressed by T lymphocytes. The current studies were undertaken to better define the identity of the CD60 antigen on both T cells and non-T cells. Treatment of intact cells with neuraminidases of various specificities confirmed that detection of the CD60 epitope depends on expression of an alpha2, 8-disialic acid carbohydrate linkage, as is found in GD3 and related gangliosides. However, the sialicacid polymer colominic acid inhibited anti-GD2 and anti-GD3, but not anti-CD60 from binding to cell surfaces. Expression of CD60 did not correlate with expression of GD3 on a variety of cell lines and T cell populations. Expression of CD60 and 9-O-acetyl-GD3 was roughly parallel on some non-T cell lines such as melanoma cells, but on T cells expression of CD60 was consistently greater. Antibodies to GD2, GD3 and 9-O-acetyl-GD3 were ineffective at inhibiting binding of anti-CD60 to CD60+ cells. Activation responses of T cells to anti-CD60 were inducible in either the presence or absence of a response to anti-GD3. A novel inhibitor of glucosyl ceramide synthesis, D-threo-1-phenyl-2-palmitoylamino-3-pyrrolidino-1-propanol (D-t-P4) reduced expression of GD3 much more than CD60 on activated T lymphocytes. Following biotinylation of HUT78 T cells, anti-CD60 immunoprecipitated a 70 kDa antigen. Taken together, the present data and previous findings suggest that anti-CD60 can recognize both a modified form of the GD3 ganglioside and a carbohydrate-dependent complex epitope present on one or more glycoproteins. This glycoprotein epitope may be the more abundant and functionally significant CD60 antigen on T lymphocytes, while 9-O-acetyl-GD3 is likely to be the principal structure recognized by anti-CD60 on melanoma cells. These findings emphasize the complexity of understanding the functional roles of carbohydrate epitopes in cell activation.

Potentiation of okadaic acid-induced ceramide elevation but not apoptosis by inhibition of glucosylceramide synthase in human neuroepithelioma cells

Caspase-dependent apoptosis induced by okadaic acid (OA) in CHP-100 neuroepithelioma cells has previously been shown to associate with a rapid and sustained elevation in intracellular ceramide concentration. We now report that treatment of CHP-100 cells with OA also evoked a rapid elevation in glucosylceramide levels that was maintained at steady state as cells underwent apoptosis; moreover, as observed for ceramide, OA-induced glucosylceramide accumulation was not blocked by fumonisin B1. Remarkably, when cell death was prevented by caspase inhibition, glucosylceramide accumulation was potentiated and ceramide elevation reduced, thus suggesting that, during apoptosis completion, accumulation of ceramide was partly driven by impairment of its glucosylation through a caspase-dependent mechanism. We studied whether ceramide glucosylation provided a mechanism for negative modulation of OA-induced apoptosis. We observed that the blocking of glucosylceramide synthesis markedly potentiated OA-induced ceramide elevation, but neither accelerated apoptosis onset nor potentiated the apoptotic response. These results indicate that modulation of ceramide glucosylation does not affect the apoptotic response to okadaic acid and suggest that caution must be exercised concerning the possibility that ceramide plays a key role in apoptosis induction.

Sphingolipids

A significant corpus of work over the last decade has firmly established an important role for sphingolipids in a variety of important biological processes. Such processes include signaling events related to cell growth, differentiation, programmed cell death, and stress responses. These processes not only involve those sphingolipids that accumulate as a result of a variety of inherited lysosomal storage disorders, but, in addition, sphingolipids associated with long-chain base metabolism. This article reviews the chemical properties, pathways, regulated metabolism, and signaling function of sphingolipids. In addition, the potential roles of sphingolipids in renal-specific processes are considered. While a variety of cellular functions have been ascribed to sphingolipids, in many cases proof of the concept has yet to be well established. Thus, a number of critical questions can be posed in interpreting these studies. Several of these questions are considered.

Reduction of globotriaosylceramide in Fabry disease mice by substrate deprivation

We used a potent inhibitor of glucosylceramide synthase to test whether substrate deprivation could lower globotriaosylceramide levels in alpha-galactosidase A (alpha-gal A) knockout mice, a model of Fabry disease. C57BL/6 mice treated twice daily for 3 days with D-threo-1-ethylendioxyphenyl-2-palmitoylamino-3-pyrrolidi no-propanol (D-t-EtDO-P4) showed a concentration-dependent decrement in glucosylceramide levels in kidney, liver, and spleen. A single intraperitoneal injection of D-t-EtDO-P4 resulted in a 55% reduction in renal glucosylceramide, consistent with rapid renal glucosylceramide metabolism. A concentration-dependent decrement in renal and hepatic globotriaosylceramide levels was observed in alpha-Gal A(-) males treated for 4 weeks with D-t-EtDO-P4. When 8-week-old alpha-Gal A(-) males were treated for 8 weeks with 10 mg/kg twice daily, renal globotriaosylceramide fell to below starting levels, consistent with an alpha-galactosidase A-independent salvage pathway for globotriaosylceramide degradation. Complications observed with another glucosylceramide synthase inhibitor, N-butyldeoxynojirimycin, including weight loss and acellularity of lymphatic organs, were not observed with D-t-EtDO-P4. These data suggest that Fabry disease may be amenable to substrate deprivation therapy.

Glycosphingolipid depletion in fabry disease lymphoblasts with potent inhibitors of glucosylceramide synthase

BACKGROUND

Fabry disease is an inherited X-linked disorder resulting in the loss of activity of the lysosomal hydrolase alpha-galactosidase A and causing the clinical manifestations of renal failure, cerebral vascular disease, and myocardial infarction. The phenotypic expression of this disorder is manifest by the accumulation of glycosphingolipids containing alpha-galactosyl linkages, most prominently globotriaosylceramide.

METHODS

Based on quantitative structure activity studies, we recently reported two newly designed glucosylceramide synthase inhibitors based on 1-phenyl-2-palmitoylamino-3-pyrrolidino-1-propanol (P4). These inhibitors, 4'-hydroxy-P4 and ethylenedioxy-P4, were evaluated for their ability to deplete globotriaosylceramide and other glucosylceramide-based lipids in Fabry lymphocytes and were compared with N-butyldeoxynojirimycin, another reported glucosylceramide synthase inhibitor.

RESULTS

Concentrations as low as 10 nmol/L of 4'-hydroxy-P4 and ethylenedioxy-P4 resulted in 70 and 80% depletion, respectively, of globotriaosylceramide, with maximal depletion occurring at three days of treatment. There was no impairment of cell growth. In contrast, N-butyldeoxynojirimycin only minimally lowered globotriaosylceramide levels, even at concentrations as high as 10 micromol/L. Globotriaosylceramide depletion was confirmed by the loss of binding of FITC-conjugated verotoxin B subunit to the lymphoblasts.

CONCLUSIONS

These findings suggest that selective glucosylceramide synthase inhibitors are highly effective in the depletion of globotriaosylceramide from Fabry cell lines. We suggest that these compounds have potential therapeutic utility in the treatment of Fabry disease.