Brefeldin A induced inhibition of de novo globo- and neolacto-series glycolipid core chain biosynthesis in human cells. Evidence for an effect on beta 1–>4galactosyltransferase activity

Xyloside Derivatives as Molecular Tools to Selectively Inhibit Heparan Sulfate and Chondroitin Sulfate Proteoglycan Biosynthesis

Glycosaminoglycan (GAG) side chains of proteoglycans are involved in a wide variety of developmental and pathophysiological functions. Similar to a gene knockout, the ability to inhibit GAG biosynthesis would allow us to examine the function of endogenous GAG chains. However, ubiquitously and irreversibly knocking out all GAG biosynthesis would cause multiple effects, making it difficult to attribute a specific biological role to a specific GAG structure in spatiotemporal manner. Reversible and selective inhibition of GAG biosynthesis would allow us to examine the importance of endogenous GAGs to specific cellular, tissue, or organ systems. In this chapter, we describe the chemical synthesis and biological evaluation of xyloside derivatives as selective inhibitors of heparan sulfate and chondroitin/dermatan sulfate proteoglycan biosynthesis.

Sphingolipid metabolism and interorganellar transport: localization of sphingolipid enzymes and lipid transfer proteins

In recent decades, many sphingolipid enzymes, sphingolipid-metabolism regulators and sphingolipid transfer proteins have been isolated and characterized. This review will provide an overview of the intracellular localization and topology of sphingolipid enzymes in mammalian cells to highlight the locations where respective sphingolipid species are produced. Interestingly, three sphingolipids that reside or are synthesized in cytosolic leaflets of membranes (ceramide, glucosylceramide and ceramide-1-phosphate) all have cytosolic lipid transfer proteins (LTPs). These LTPs consist of ceramide transfer protein (CERT), four-phosphate adaptor protein 2 (FAPP2) and ceramide-1-phosphate transfer protein (CPTP), respectively. These LTPs execute functions that affect both the location and metabolism of the lipids they bind. Molecular details describing the mechanisms of regulation of LTPs continue to emerge and reveal a number of critical processes, including competing phosphorylation and dephosphorylation reactions and binding interactions with regulatory proteins and lipids that influence the transport, organelle distribution and metabolism of sphingolipids.

Expression of Neutral Glycosphingolipids in the Brain and Spleen of Mice Lacking TNF Receptor 1

We investigated the expression of neutral glycosphingolipids (GSLs) in the brain and spleen of mice lacking the gene for the tumor necrosis factor-alpha receptor p55 (TNFR1). Neutral GSLs of the ganglio-, globo-, and neolacto-series were determined in the tissues of homozygous (TNFR1-/-) and control heterozygous (TNFR1+/-) animals by high performance thin layer chromatography (HPTLC) overlay immunostaining with specific antibodies. The spleen of homozygous TNFR1 knockout mice lacked glucosylceramide substituted with palmitic acid, GlcCer(C16), and showed severe reduction in the expression of GlcCer(C24). In addition, gangliotetraosylceramide substituted with palmitic acid, Gg4Cer(C16), and globotetraosylceramide, Gb4Cer, were down-regulated in the TNFR1-/- spleen in comparison with the heterozygous control. The brain of both groups of animals (TNFR1-/- and TNFR1+/-) did not express detectable levels of Gg4Cer, Gb5Cer and Gb4Cer, but the brain of TNFR1 knockout mice expressed abundant globotriaosylceramide, Gb3Cer, compared to no expression in control heterozygous mice. nLcCer(C24) had slightly higher (1.4 fold) expression in the brain of TNFR1-/- mice compared with the control animals. This study provides in vivo evidence that TNF signaling via the TNFR1 is involved in the acquisition of a divergent GSL assembly in the brain, an immunologically privileged organ, and the spleen, typical secondary lymphoid organ.

Inhibition of heparan sulfate and chondroitin sulfate proteoglycan biosynthesis

Proteoglycans (PGs) are composed of a protein moiety and a complex glycosaminoglycan (GAG) polysaccharide moiety. GAG chains are responsible for various biological activities. GAG chains are covalently attached to serine residues of the core protein. The first step in PG biosynthesis is xylosylation of certain serine residues of the core protein. A specific linker tetrasaccharide is then assembled and serves as an acceptor for elongation of GAG chains. If the production of endogenous GAG chains is selectively inhibited, one could determine the role of these endogenous molecules in physiological and developmental functions in a spatiotemporal manner. Biosynthesis of PGs is often blocked with the aid of nonspecific agents such as chlorate, a bleaching agent, and brefeldin A, a fungal metabolite, to elucidate the biological roles of GAG chains. Unfortunately, these agents are highly lethal to model organisms. Xylosides are known to prime GAG chains. Therefore, we hypothesized that modified xylose analogs may able to inhibit the biosynthesis of PGs. To test this, we synthesized a library of novel 4-deoxy-4-fluoroxylosides with various aglycones using click chemistry and examined each for its ability to inhibit heparan sulfate and chondroitin sulfate using Chinese hamster ovary cells as a model cellular system.

Identification and characterization of small molecules that inhibit intracellular toxin transport

Shiga toxin (Stx), cholera toxin (Ctx), and the plant toxin ricin are among several toxins that reach their intracellular destinations via a complex route. Following endocytosis, these toxins travel in a retrograde direction through the endosomal system to the trans-Golgi network, Golgi apparatus, and endoplasmic reticulum (ER). There the toxins are transported across the ER membrane to the cytosol, where they carry out their toxic effects. Transport via the ER from the cell surface to the cytosol is apparently unique to pathogenic toxins, raising the possibility that various stages in the transport pathway can be therapeutically targeted. We have applied a luciferase-based high-throughput screen to a chemical library of small-molecule compounds in order to identify inhibitors of Stx. We report two novel compounds that protect against Stx and ricin inhibition of protein synthesis, and we demonstrate that these compounds reversibly inhibit bacterial transport at various stages in the endocytic pathway. One compound (compound 75) inhibited transport at an early stage of Stx and Ctx transport and also provided protection against diphtheria toxin, which enters the cytosol from early endosomes. In contrast, compound 134 inhibited transport from recycling endosomes through the Golgi apparatus and protected only against toxins that access the ER. Small-molecule compounds such as these will provide insight into the mechanism of toxin transport and lead to the identification of compounds with therapeutic potential against toxins routed through the ER.

Expression of glycosphingolipids in lymph nodes of mice lacking TNF receptor 1: biochemical and flow cytometry analysis

The expression of gangliosides and neutral glycosphingolipids (GSLs) in the lymph nodes of mice lacking the gene for the tumour necrosis factor-alpha receptor p55 (TNFR1) has been investigated. GSL expression in the tissues of mice homozygous (TNFR1-/-) or heterozygous (TNFR1+/-) for the gene deletion was analysed by flow cytometry and high-performance thin-layer chromatography (HPTLC) followed by immunostaining with specific antibodies. HPTLC immunostaining revealed that lymph nodes from TNFR1-/- mice had reduced expression of ganglioside GM1b and GalNAc-GM1b, neolacto-series gangliosides, as well as the globo- (Gb3, Gb4 and Gb5) and ganglio-series (Gg3 and Gg4) neutral GSLs. Flow cytometry of freshly isolated lymph node cells showed no significant differences in GSL expression, except for the GalNAc-GM1b ganglioside, which was less abundant on T lymphocytes from TNFR1-/- lymph nodes. In TNFR1-/- mice, GalNAc-GM1b+/CD4+ T cells were twofold less abundant (3.8% vs 7.6% in the control mice), whereas GalNAc-GM1b+/CD8+ T cells were fourfold less abundant (5.0% vs 20.2% in the control mice). This study provides in vivo evidence that TNF signalling via the TNFR1 is important for the activation of GM1b-type ganglioside biosynthetic pathway in CD8 T lymphocytes, suggesting its possible role in the effector T lymphocyte function.

Glycotherapy for cancer: remodeling of ganglioside pattern as an effective approach for cancer therapy

We have found that an increase in the ganglioside GM3 is a prerequisite for the induction of terminal differentiation, cuhninating in death by apoptosis, of human colonic carcinoma cells in vitro. To evaluate the therapeutic effect of increasing GM3 in human colonic carcinoma cells, we examined whether treated cells lose their tumorigenic activity and whether this approach is effective against cancer cells growing in vivo. Cells of the human colonic carcinoma cell line HCT 116 not only differentiated but also lost their tumorigenic activity by an artificial increase in GM3. When HCT 116 tumors growing in nude mice were treated with a drug that increases GM3, an appreciable increase in GM3 and induction of apoptosis were clearly observed. The growth of treated tumors was greatly suppressed. These results suggest that the modulation of ganglioside expression to introduce gangliosides with biological activity into cancer cells could be a novel effective approach for cancer therapy.

The organizing potential of sphingolipids in intracellular membrane transport

Eukaryotes are characterized by endomembranes that are connected by vesicular transport along secretory and endocytic pathways. The compositional differences between the various cellular membranes are maintained by sorting events, and it has long been believed that sorting is based solely on protein-protein interactions. However, the central sorting station along the secretory pathway is the Golgi apparatus, and this is the site of synthesis of the sphingolipids. Sphingolipids are essential for eukaryotic life, and this review ascribes the sorting power of the Golgi to its capability to act as a distillation apparatus for sphingolipids and cholesterol. As Golgi cisternae mature, ongoing sphingolipid synthesis attracts endoplasmic reticulum-derived cholesterol and drives a fluid-fluid lipid phase separation that segregates sphingolipids and sterols from unsaturated glycerolipids into lateral domains. While sphingolipid domains move forward, unsaturated glycerolipids are retrieved by recycling vesicles budding from the sphingolipid-poor environment. We hypothesize that by this mechanism, the composition of the sphingolipid domains, and the surrounding membrane changes along the cis-trans axis. At the same time the membrane thickens. These features are recognized by a number of membrane proteins that as a consequence of partitioning between domain and environment follow the domains but can enter recycling vesicles at any stage of the pathway. The interplay between protein- and lipid-mediated sorting is discussed.

Cell type specific localization of sphingomyelin biosynthesis

We have studied the incorporation of [(14)C]serine and of [(3)H]sphingosine into sphingomyelin in the presence or absence of brefeldin A (BFA) in three different cell types. Administration of BFA (1 microgram/ml) to fibroblasts for 24 h increased the incorporation of label into sphingomyelin 1.5-3 fold compared with untreated controls. In contrast, BFA strongly decreased sphingomyelin biosynthesis (4-5 fold) in cerebellar neurons as well as in neuroblastoma cells. The effect of BFA on glycosphingolipid formation, however, was similar in all three cell types studied: an increased labeling of the precursor glycolipids GlcCer, LacCer, GM3 and GD3 was paralleled by a decreased formation of complex gangliosides, GM1, GD1a, GT1b and GQ1b. Our data therefore suggest that in neuronal cells sphingomyelin synthesis, like the formation of complex gangliosides, is localized primarily distal to the BFA block, in a post-Golgi compartment, most probably the trans-Golgi network, whereas in fibroblasts sphingomyelin biosynthesis is mainly localized prior to the BFA block, in the Golgi apparatus, as has been shown for LacCer, GlcCer, GM3 and GD3 synthases.

Sphingolipid transport in eukaryotic cells

Sphingolipids constitute a sizeable fraction of the membrane lipids in all eukaryotes and are indispensable for eukaryotic life. First of all, the involvement of sphingolipids in organizing the lateral domain structure of membranes appears essential for processes like protein sorting and membrane signaling. In addition, recognition events between complex glycosphingolipids and glycoproteins are thought to be required for tissue differentiation in higher eukaryotes and for other specific cell interactions. Finally, upon certain stimuli like stress or receptor activation, sphingolipids give rise to a variety of second messengers with effects on cellular homeostasis. All sphingolipid actions are governed by their local concentration. The intricate control of their intracellular topology by the proteins responsible for their synthesis, hydrolysis and intracellular transport is the topic of this review.

Inverse relationship of expression between GM3 and globo-series ganglioside in human renal cell carcinoma

Renal cell carcinoma (RCC) is highly metastatic. We previously showed that expression of globo-series ganglioside is associated with the metastatic potential of RCC. However, the mechanism of metastasis remains largely unknown, and there is no effective therapy for metastasis. It was recently shown that induction of differentiation of colon cancer cells by brefeldin A was accompanied by an increase of GM3 with a concomitant decrease of neolacto-series gangliosides. To get a clue to a new method of therapy for RCC, we investigated whether the similar changes occur in RCC cells expressing globo-series ganglioside. Growth suppression and an increase of GM3 simultaneous with a decrease of monosialosyl galactosyl globoside, a member of globo-series gangliosides, were observed in human RCC cell line ACHN following brefeldin A treatment. The resultant change of the ganglioside profile is inversely related to the ganglioside pattern associated with the malignant potential of RCC and almost coincided with that representative of RCC cases showing favorable prognoses. It is suggested that the inverse relationship of expression between GM3 and globo-series ganglioside is reflected on the degree of malignancy of RCC, and may be useful as one of the indicators for exploiting treatment methods of RCC.

Induction of terminal differentiation and apoptosis in human colonic carcinoma cells by brefeldin A, a drug affecting ganglioside biosynthesis

An appreciable increase in G(M3) with a concomitant decrease in some neolacto-series gangliosides was observed during differentiation of human colonic carcinoma HCT 116 cells induced by a differentiating agent. When the cells were treated with brefeldin A (BFA), a striking increase in de novo biosynthesis of G(M3) and a decrease in biosynthesis of neolactoseries gangliosides were observed after 6 h. Clear morphological changes to differentiated epithelial cells and an arrest of cells in the G0/G1 phase of the cell cycle were observed after 1 day of treatment. Then the cells were led to apoptosis. This activity was not affected by forskolin, which antagonizes the effects of BFA on protein transport and the Golgi apparatus. These results suggest that the differentiation-inducing activity of BFA might be due to its modulatory effect on ganglioside biosynthesis, and that a specific change in ganglioside pattern is an essential prerequisite for induction of differentiation, providing a novel target for differentiation therapy of cancer.

Functional organization of the Golgi apparatus in glycosphingolipid biosynthesis. Lactosylceramide and subsequent glycosphingolipids are formed in the lumen of the late Golgi

Biosynthesis of plasma membrane sphingolipids involves the coordinate action of enzymes localized to individual compartments of the biosynthetic secretory pathway of proteins. These stations include the endoplasmic reticulum and the Golgi apparatus. Although a precise localization of all the enzymes that synthesize glycosphingolipids has not been achieved to date, it is assumed that the sequence of events in glycosphingolipid biosynthesis resembles that in glycoprotein biosynthesis, i.e. that early reactions occur in early stations (endoplasmic reticulum and cis/medial Golgi) of the pathway, and late reactions occur in late stations (trans Golgi/trans Golgi network). Using truncated analogues of ceramide and glucosylceramide that allow measurement of enzyme activities in intact membrane fractions, we have reinvestigated the localization of individual enzymes involved in glycosphingolipid biosynthesis and for the first time studied the localization of lactosylceramide synthase after partial separation of Golgi membranes as previously described (Trinchera, M., and Ghidoni, R. (1989) J. Biol. Chem. 264, 15766-15769). Here, we show that the reactions involved in higher glycosphingolipid biosynthesis, including lactosylceramide synthesis, all reside in the lumen of the late Golgi compartments from rat liver.

Endoscopic observation for detection and monitoring of N-butyl-N-(4-hydroxybutyl)nitrosamine--induced bladder tumor in rats

Recent advances in tumor carbohydrate biochemistry have demonstrated antitumor effects of locally administered GM3 ganglioside on mouse MBT-2 tumor. When intravesical therapy in N-butyl-N(4-hydroxybutyl)nitrosamine (BBN)-induced rat bladder tumor is attempted, it is essential to identify the tumor, to classify its size before therapy and to monitor the effect of the therapy. To establish a more reliable experimental therapeutic system, we assessed the development of BBN-induced rat-bladder tumor by endoscopic observation. BBN-induced bladder tumors in rats were observed serially using a 4.2-F flexible fiberscope. The endoscopic findings were compared with the histopathological findings. Intravesical tumor growth varied greatly between individual rats. The smallest change detected by endoscopy was a small edematous lesion histologically proved to be papilloma. The largest nodular lesion was determined to be a papillary, transitional cell carcinoma. This noninvasive method makes the BBN rat experimental system more reliable by allowing confirmation of tumor formation and classification of the tumor volume prior to therapy.

Alpha 1,4galactosyltransferase activity and Gb3Cer expression in human leukaemia/lymphoma cell lines

We have used two methods to evaluate the level of expression of Gb3Cer in several human leukaemia/lymphoma cell lines representative of the myeloid (K562, KG-1, HL-60, and lymphoid (Reh, Daudi, Raji, RPMI 8226, CCRF-CEM, MOLT-4) lineages blocked at varied stages of differentiation. TLC immunostaining of glycolipid extracts with a monoclonal antibody, 12-101, and FACS analysis with the same antibody were used to demonstrate that the expression of Gb3Cer in neoplastic myeloid and lymphoid cells is both lineage and differentiation dependent. As a possible control point in the regulated expression of Gb3Cer we have investigated the first committed step in the synthesis of globo series glycosphingolipids that involves UDP-Gal:LacCer alpha (1,4)-galactosyltransferase (alpha 1,4GalT). We present the first characterization of this enzyme in a human myeloid cell line using an ELISA-based assay, which was subsequently used to measure alpha 1,4GalT activity in the human leukaemia/lymphoma cell lines. In general, there is a positive correlation between the levels of endogenous Gb3Cer and the level of the alpha 1,4 GalT activity. However, in two cases (KG-1 and CCRF-CEM) the level of enzyme activity did not correspond to the level of Gb3Cer expression.

Transport and sorting of membrane lipids

The lipid composition of cellular membranes may seem unnecessarily complex. However, the lipid composition of each membrane is carefully regulated by local metabolism and specificity in transport, marking the functional significance for the cell. Recent research has revealed unexpected discoveries concerning the topology of lipid synthesis, specificity in lipid transport, and the function of lipid and protein microdomains in sorting.