Carbohydrate-dependent cell adhesion

Recent Advances in Stereoselective Chemical O-Glycosylation Reactions

Carbohydrates involving glycoconjugates play a pivotal role in many life processes. Better understanding toward glycobiological events including the structure–function relationship of these biomolecules and for diagnostic and therapeutic purposes including tailor-made vaccine development and synthesis of structurally well-defined oligosaccharides (OS) become important. Efficient chemical glycosylation in high yield and stereoselectivity is however challenging and depends on the fine tuning of a protection profile to get matching glycosyl donor–acceptor reactivity along with proper use of other important external factors like catalyst, solvent, temperature, activator, and additive. So far, many glycosylation methods have been reported including several reviews also. In the present review, we will concentrate our discussion on the recent trend on α- and β-selective glycosylation reactions reported during the past decade.

Attenuation of neutrophil-mediated liver injury in mice by drug-free E-selectin binding polymer

Inflammation with neutrophils infiltration is a prominent feature of alcohol-related liver disease (ARLD) and contributes to the severity of liver injury. Although an array of potential treatments has been studied, the standard treatment regimen is controversial and can induce severe side effects and infection-related complications. E-selectin, a cytokine inducible cell adhesion molecule, mediates the initial interaction of leucocytes with endothelial cells, and facilitates their further adhesion and extravasation into inflamed tissues. Given the important role of E-selectin in leukocytes trafficking, we hypothesized that a synthetic polymer presenting multiple copies of E-selectin binding peptide in a polyvalent manner (P-Esbp) may block the "roads" that facilitate neutrophil infiltration, inhibit the recruitment of neutrophils to the inflamed sites and reduce the extent of liver injury. We now demonstrate in vitro that P-Esbp reduced the recruitment of neutrophils (collected from blood of donors) on activated human umbilical vein endothelial cells (HUVEC) under flow conditions. Pre-treatment of mice with P-Esbp prior to alcohol binge attenuated alcohol-induced serum transaminase (ALT, AST) elevation, reduced pro-inflammatory cytokines (TNFα and IL-1ẞ) and chemokines (MIP-2/CXCL2 and MCP-1/CCL2) in National Institute on Alcohol Abuse and Alcoholism (NIAAA) model. Also, the up-regulation of neutrophil marker Ly6G and the number of MPO positive cells in the injured tissue was significantly reduced by the treatment, indicating diminished neutrophil infiltration. Moreover, as a result of P-Esbp treatment, E-selectin expression in the liver (mRNA and protein level) was downregulated, suggesting a potential to decrease ongoing local inflammatory response. Overall, our findings highlight the anti-inflammatory properties of the "drug-free" P-Esbp and its therapeutic potential to attenuate an excessive inflammation where infiltrating neutrophils can damage tissues and organs.

Sequence-Defined Introduction of Hydrophobic Motifs and Effects in Lectin Binding of Precision Glycomacromolecules

This study investigates the influence of an increasingly hydrophobic backbone of multivalent glycomimetics based on sequence-defined oligo(amidoamines) on their resulting affinity toward bacterial lectins. Glycomacromolecules are obtained by stepwise assembly of tailor-made building blocks on solid support, using both hydrophobic aliphatic and aromatic building blocks to enable a gradual change in hydrophobicity of the backbone. Their binding behavior toward model lectin Concanavalin A (ConA) is evaluated using isothermal titration calorimetry (ITC) and surface plasmon resonance (SPR) showing higher affinities for glycomacromolecules with higher content of hydrophobic and aromatic moieties in the backbone. Finally, glycomacromolecules are tested in a bacterial adhesion inhibition study against Escherichia coli where more hydrophobic backbones yield higher inhibitory potentials most likely due to additional secondary interactions with hydrophobic regions of the protein receptor as well as a change in conformation exposing carbohydrate ligands for increased binding. Overall, the results highlight the influence and thereby importance of the polymer backbone itself on the resulting properties of polymeric biomimetics.

Gene expression profile analysis in response to α1,2-fucosyl transferase (FUT1) gene transfection in epithelial ovarian carcinoma cells

The aim of this study was to identify differentially expressed genes (DEGs) in response to α1,2-fucosyl transferase (FUT1) gene transfection in epithelial ovarian cancer cells. Human whole-genome oligonucleotide microarrays were used to determine whether gene expression profile may differentiate the epithelial ovarian cell line Caov-3 transfected with FUT1 from the empty plasmid-transfected cells. Quantitative real-time PCR and immunohistochemical staining validated the microarray results. Gene expression profile identified 215 DEGs according to the selection criteria, in which 122 genes were upregulated and 93 genes were downregulated. Gene Ontology (GO) and canonical pathway enrichment analysis were applied, and we found that these DEGs are involved in BioCarta mammalian target of rapamycin (mTOR) pathway, BioCarta eukaryotic translation initiation factor 4 (EIF4) pathway, and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways in cancer. Interaction network analysis predicted genes participating in the regulatory connection. Highly differential expression of TRIM46, PCF11, BCL6, PTEN, and FUT1 genes was validated by quantitative real-time PCR in two cell line samples. Finally, BCL6 and Lewis Y antigen were validated at the protein level by immunohistochemistry in 103 paraffin-embedded ovarian cancer tissues. The identification of genes in response to FUT1 may provide a theoretical basis for the investigations of the molecular mechanism of ovarian cancer.

Total synthesis of a sialyl Lewis(x) derivative for the diagnosis of cancer

The total synthesis of aminoethyl glycoside of sialyl Lewis(x) (sLe(x)) is described. A galactose donor was condensed with a diol of glucosamine to afford regioselectively a β1,4 linked disaccharide, which was further stereoselectively fucosylated to provide a protected Lewis(x) trisaccharide. After chemical modification, the trisaccharide was sialylated to give regio- and stereoselectively an azidoethyl glycoside of sLe(x). Finally, deprotection and azide reduction afforded the target compound. This compound will be coupled with protein and then be used to conduct further preclinical studies for the diagnosis of cancer.

Cancer vaccines and carbohydrate epitopes

Tumor-associated carbohydrate antigens (TACA) result from the aberrant glycosylation that is seen with transformation to a tumor cell. The carbohydrate antigens that have been found to be tumor-associated include the mucin related Tn, Sialyl Tn, and Thomsen-Friedenreich antigens, the blood group Lewis related Lewis(Y), Sialyl Lewis(X) and Sialyl Lewis(A), and Lewis(X) (also known as stage-specific embryonic antigen-1, SSEA-1), the glycosphingolipids Globo H and stage-specific embryonic antigen-3 (SSEA-3), the sialic acid containing glycosphingolipids, the gangliosides GD2, GD3, GM2, fucosyl GM1, and Neu5GcGM3, and polysialic acid. Recent developments have furthered our understanding of the T-independent type II response that is seen in response to carbohydrate antigens. The selection of a vaccine target antigen is based on not only the presence of the antigen in a variety of tumor tissues but also on the role this antigen plays in tumor growth and metastasis. These roles for TACAs are being elucidated. Newly acquired knowledge in understanding the T-independent immune response and in understanding the key roles that carbohydrates play in metastasis are being applied in attempts to develop an effective vaccine response to TACAs. The role of each of the above mentioned carbohydrate antigens in cancer growth and metastasis and vaccine attempts using these antigens will be described.

Synthetic vaccines based on N- and O-glycopeptides–molecular tools for immunotherapy and diagnostics

This chapter summarizes available methods for the preparation of synthetic vaccines based on glycopeptides and recent advances in this field. It further includes results of their immunological evaluation. Syntheses of glycopeptides of defined chemical structure and conjugation of these compounds to a carrier protein or an immunostimulant are of interest for the development of new immunotherapeutics and/or antibody-based diagnostics. Since a number of years, the aberrant glycosylation of the tumorassociated mucin MUC1 forming tumor specific epitopes on the epithelial cell surface has been considered an attractive research target for the preparation of such vaccines. Examples of synthetic vaccines directed against the O-glycosylated MUC1 tandem repeats will here be given including synthetic MUC1 glycopeptides conjugated to a T-cell epitope peptide, to a carrier protein, to a lipid immunostimulant or the multimeric presentation of glycopeptides on dendrimers. Other attractive targets for immunotherapy are the viral envelope proteins HIV gp120 and HIV gp41 , which are highly glycosylated with high-mannose and complex type N-glycans. Examples will be given, which illustrate syntheses of high-mannose HIV gp120 or gp41 glycopeptides with the natural peptide backbone or with a non-natural cyclic backbone to mimic the high-mannose cluster domain of HIV gp120. In addition the synthesis and immunological evaluation of a vaccine will be described, which contains the high-mannose cluster mimotope glycopeptide conjugated to an outer membrane protein complex (OMPC) as the carrier.

Transcriptional regulation of the vascular endothelial glycome by angiogenic and inflammatory signalling

Vascular endothelial cells undergo many molecular changes during pathological processes such as inflammation and tumour development. Tumours such as malignant lymphomas affecting bone marrow are dependent on interactions with endothelial cells for (1) site-specific homing and (2) tumour-induced angiogenesis. Modifications in glycosylation are responsible for fine-tuning of distinct endothelial surface receptors. In order to gain a comprehensive insight into the regulation of the endothelial glycome, comprising genes encoding for sugar transporters (sugar s/t), glycosyltransferases (GT), glycan-degrading enzymes (GD) and lectins (GBP), we performed gene profiling analysis of the human bone marrow-derived microvascular endothelial cell line HBMEC-60 that resembles closely in its biological behaviour primary bone marrow endothelial cells. HBMEC were activated by either angiogenic VEGF or the inflammatory cytokine TNF. Approximately 48% (207 genes) of the 432 glycome genes tested were found to be expressed in HBMEC-60 cells. Inflammatory and angiogenic signals produce different profiles of up- or down-regulated glycome genes, most prominent changes were seen under TNF stimulation in terms of signal intensity and number of alterations. Stimulation by VEGF and TNF affected primarily genes encoding for glycosyltransferases and in particular those important for terminal modulation. For instance, an enhanced alpha2,6 sialylation was observed after TNF stimulation at the transcriptional and glycan expression level whereas transcription of ST3Gal1 sialylating in alpha2,3 position was enhanced after VEGF stimulation. Transcriptional analysis of the glycome gives insights into the differential regulation of glycosylation pathways and may help to understand the functional impact of endothelial glycosylation.

Structural Basis of the Catalytic Reaction Mechanism of Novel 1,2-α-L-Fucosidase from Bifidobacterium bifidum

1,2-alpha-L-fucosidase (AfcA), which hydrolyzes the glycosidic linkage of Fucalpha1-2Gal via an inverting mechanism, was recently isolated from Bifidobacterium bifidum and classified as the first member of the novel glycoside hydrolase family 95. To better understand the molecular mechanism of this enzyme, we determined the x-ray crystal structures of the AfcA catalytic (Fuc) domain in unliganded and complexed forms with deoxyfuconojirimycin (inhibitor), 2'-fucosyllactose (substrate), and L-fucose and lactose (products) at 1.12-2.10 A resolution. The AfcA Fuc domain is composed of four regions, an N-terminal beta region, a helical linker, an (alpha/alpha)6 helical barrel domain, and a C-terminal beta region, and this arrangement is similar to bacterial phosphorylases. In the complex structures, the ligands were buried in the central cavity of the helical barrel domain. Structural analyses in combination with mutational experiments revealed that the highly conserved Glu566 probably acts as a general acid catalyst. However, no carboxylic acid residue is found at the appropriate position for a general base catalyst. Instead, a water molecule stabilized by Asn423 in the substrate-bound complex is suitably located to perform a nucleophilic attack on the C1 atom of L-fucose moiety in 2'-fucosyllactose, and its location is nearly identical near the O1 atom of beta-L-fucose in the products-bound complex. Based on these data, we propose and discuss a novel catalytic reaction mechanism of AfcA.

Vesicle size and stability of biomimetic liposomes from 3'-sulfo-Lewis a (SuLea) containing glycolipids

We report on the use of a natural Lewis type saccharide ligand, 3'-sulfo-Lewis a (SuLe(a)) for glycocalyx-mimetic surface modification of liposomes. Two SuLe(a)-containing glycolipids, monovalent SuLe(a)-lipid and trivalent SuLe(a) (TSuLe(a))-lipid, were synthesized, and used with 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC) and cholesterol to prepare unilaminar vesicles (ULVs) by a freeze-thaw and extrusion method. The effects of the glycolipid concentrations and the pore sizes of extrusion membranes on vesicle size and stability were investigated by photon correlation spectroscopy (PCS). Glycoliposomes, with 5% SuLe(a)- or TSuLe(a)-lipids obtained by 50 nm extrusion, had 25-30% more vesicles less than 100 nm in diameter compared with the 100 nm extrusion. TSuLe(a)-liposomes always produced larger vesicle size than SuLe(a)-liposomes, which we attribute to the larger TSuLe(a) headgroup. Both SuLe(a)- and TSuLe(a)-liposomes increased their vesicle size with increasing glycolipid concentration from 5% to 15%, and demonstrated good stability at room temperature for over 1 month. Further increasing the glycolipid concentration to 20% resulted in large vesicle aggregation after 5 days for TSuLe(a)-liposomes, while the SuLe(a)-liposomes remained stable for 10 days. SuLe(a)- and TSuLe(a)-liposomes with 15% glycolipids demonstrated better stability due to the electrostatic effect from the negatively charged SuLe(a) and TSuLe(a) headgroups. The results indicate that the biomimetic liposomes with SuLe(a)- and TSuLe(a)-lipids with 5 to 15% incorporation are sufficiently stable for the potential applications in targeted drug delivery.

Facile synthesis of sulfonium ion derivatives of 1,5-anhydro-5-thio-l-fucitol as potential α-l-fucosidase inhibitors

Five sulfonium ion derivatives with 1,5-anhydro-5-thio-L-fucitol as a core structure were efficiently synthesized as potential alpha-L-fucosidase inhibitors. The key unit, the tri-O-benzyl derivative of L-fucitol, was readily synthesized from methyl alpha-D-mannopyranoside. Alkylation with methyl iodide or 5-methoxycarbonyl-1-pentyl iodide in acetonitrile containing AgBF4 afforded the corresponding alkylated sulfonium tetrafluoroborates. Alternatively, ring opening of three 1,3-cyclic sulfates in 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) containing K2CO3 afforded the corresponding zwitterionic sulfonium sulfates.

Asymmetric synthesis of the l-fuco-nojirimycin, a nanomolar α-l-fucosidase inhibitor

We describe the asymmetric synthesis of the 5-amino-5-deoxy-l-fucose (l-fuco-nojirimycin) which appears as a very potent fucosidase inhibitor with a K(i) value of 1 nM.

Identification of the catalytic nucleophile of the family 29 alpha-L-fucosidase from Thermotoga maritima through trapping of a covalent glycosyl-enzyme intermediate and mutagenesis

Fucose-containing glycoconjugates are key antigenic determinants in many biological processes. A change in expression levels of the enzymes responsible for tailoring these glycoconjugates has been associated with many pathological conditions and it is therefore surprising that little information is known regarding the mechanism of action of these important catabolic enzymes. Thermotoga maritima, a thermophilic bacterium, produces a wide range of carbohydrate-processing enzymes including a 52-kDa alpha-L-fucosidase that has 38% sequence identity and 56% similarity to human fucosidases. The catalytic nucleophile of this enzyme was identified to be Asp-224 within the peptide sequence 222WNDMGWPEKGKEDL235 using the mechanism-based covalent inactivator 2-deoxy-2-fluoro-alpha-L-fucosyl fluoride. The 10(4)-fold lower activity (kcat/Km) of the site-directed mutant D224A, and the subsequent rescue of activity upon addition of exogenous nucleophiles, conclusively confirms this assignment. This article presents the first direct identification of the catalytic nucleophile of an alpha-L-fucosidase, a key step in the understanding of these important enzymes.

Solid-phase synthesis for the identification of high-affinity bivalent lectin ligands

The development of carbohydrate-based therapeutics has been frustrated by the low affinities that characterize protein-carbohydrate complexation. Because of the oligomeric nature of most lectins, the use of multivalency may offer a successful strategy for the creation of high-affinity ligands. The solid-phase evaluation of libraries of peptide-linked multivalent ligands facilitates rapid examination of a large fraction of linker structure space. If such solid-phase assays are to replicate solution binding behavior, the potential for intermolecular bivalent binding on bead surfaces must be eliminated. Here we report the solid-phase synthesis and analysis of peptide-linked, spatially segregated mono- and bivalent ligands for the legume lectin concanavalin A. Bead shaving protocols were used for the creation of beads displaying spatially segregated binding sequences on the surface of Tentagel resins. The same ligands were also synthesized on PEGA resin to determine the effect of ligand presentation on solid-phase binding. While we set out to determine the lower limit of assay sensitivity, the unexpected observation that intermolecular bivalent ligand binding is enhanced for bivalent ligands relative to monovalent ligands allowed direct observation of the level of surface blocking required to prevent intermolecular bivalent ligand binding. For a protein with binding sites separated by 65 A, approximately 99.9% of Tentagel(1) surface sites and 99.99% of the total sites on a PEGA bead must be blocked to prevent intermolecular bivalent binding. We also report agglutination and calorimetric solution-phase binding studies of mono- and bivalent peptide-linked ligands.

New human microvascular endothelial cell lines with specific adhesion molecules phenotypes

Vascular endothelial cells recognize blood-borne circulating cells and allow them to extravasate in a tissue-specific manner. Because this property determines the selectivity of lymphocyte homing, it is fundamental in physiological as well as pathological processes (inflammation, autoimmune diseases, metastasis). As a tool to assess the molecular basis of endothelium selectivity, microvascular endothelial cell lines of distinct tissue origin were established. Endothelial cells, isolated from lymphoid tissues (lymph nodes and appendix) and from nonlymphoid immune sites--intestine, lung, and skin--were immortalized in vitro. Their general endothelial characteristics, such as the presence of von Willebrand factor (wWf), angiotensin-converting enzyme (ACE), VE-cadherin, and the intracellular E-selectin, were preserved. This article shows that these cell lines display phenotypic characteristics related to their tissue origin. Hence, endothelial cells from lymph nodes expressed peripheral lymph node addressins (PNAds). Endothelial cells from nonlymphoid tissues were ICAM-1 (intercellular adhesion molecule-1) and CD49e positive, whereas P-selectin was not equally distributed among the cell lines. Endothelial cells from mucosal sites reacted with antibody against human MAdCAM-1 (mucosal addressin cell adhesion molecule). In the adhesion test, lymphoid and myeloid cells adhere to endothelial cell lines in a distinct manner. These lines could be useful to study molecular mechanisms involved in tissue-specific cell-cell interaction.

Binding sites for Lewis antigens are expressed by human colon cancer cells and negatively affect their migration

In colon cancer, endothelial cell selectins can promote tumor cell attachment via interactions with sialylated Lewis antigens present at the surface of tumor cells, thereby facilitating tumor cell arrest and transmigration into the extravascular space. However, it is not known whether Lewis antigens interact with colon tumor cells and modify their migration. Our aim was to detect the presence of binding sites on human tumor cells for Lewis(a/x) antigens and their sialylated derivatives in vitro and in vivo and to analyze their influence on migration of colon cancer cells. The immunocytochemical and histochemical levels of expression of the four Lewis antigens were quantitatively determined in four human colon cancer cell lines and in in vivo nude mice xenografts. The levels of expression of specific binding sites for these sugar epitopes were determined by synthetic neoglycoconjugates. The influence of binding of these carbohydrate ligands on cancer cell migration was quantitatively evaluated by computer-assisted phase-contrast videomicroscopy performed on Matrigel culture supports either left uncoated or coated with neoglycoconjugate presenting synthetic Lewis(a), sialyl Lewis(a), Lewis(x), or sialyl Lewis(x) antigens. The influence of the calcium concentration in the culture medium on the Lewis antigen-mediated effects was checked. Human colon cancer cells expressed significant amounts of specific binding sites detected by the synthetic probes in addition to the oligosaccharide epitopes. The expression levels differed considerably between the four cell lines and between in vitro and in vivo specimens. Cell migration analysis revealed that the four Lewis antigens markedly decreased the levels of migration of the HCT-15 and LoVo cancer cells. This effect depends on the calcium concentration in the culture medium. Binding sites for Lewis epitopes are present on colon cancer cells. The functional relevance of these sites is indicated by the negative influence on cell migration of a matrix containing the oligosaccharides as ligand parts.

Mass spectrometric analysis of benzoylated sialooligosaccharides and differentiation of terminal alpha 2-->3 and alpha 2-->6 sialogalactosylated linkages at subpicomole levels

Perbenzoylated sialooligosaccharides were found to be stable derivatives, giving intense signals during the matrix-assisted laser desorption/ionization (MALDI) mass spectrometric analysis in the positive-ion mode. Terminal Neu5NAc alpha 2-->3 and alpha 2-->6Gal units of oligosaccharides undergo characteristic structural changes during benzoylation, yielding easily recognizable mass spectral patterns. Subpicomole carbohydrate samples were successfully benzoylated and analyzed through MALDI mass spectrometry.

Novel lipoamino acid- and liposaccharide-based system for peptide delivery: application for oral administration of tumor-selective somatostatin analogues

Lipoamino acid and liposaccharide conjugates of somatostatin analogue TT-232 were synthesized to modify the physicochemical properties of the parent peptide. The relative position, the number, and the nature of the lipid and/or saccharide moieties were varied. Experiments in vitro clearly showed that many compounds modified at the N- and/or C-terminus with lipid or sugar moieties retained the biological activity of the parent compound. An interesting construct was synthesized containing lipid and sugar units at opposite ends of the somatostatin analogue, so that the entire molecule could be considered as an amphipathic surfactant.

Thio-sugars. IV: Design and synthesis of S-linked fucoside analogs as a new class of alpha-L-fucosidase inhibitors

alpha-1-Thio-L-fucose derivative 4 and 5 as new alpha-fucosidase inhibitors (K1 = 4.6, and 5.9 microM) have been synthesized in three steps by base catalyzed coupling with bromonitromethane followed by reduction of the nitro group with sodium borohydride/cobalt chloride complex and acetylation.

Glycosyl-transferase catalyzed assemblage of sialyl-Lewis(x)-saccharopeptides

A series of glycohexopyranuronic acids are coupled to glucosamines to give 'disaccharides' which have the natural N-acetyl group of the glcNAc-moiety replaced by various sugar acids (-->saccharopeptides). These saccharopeptides are surprisingly good substrates for beta(1-4)galactosyl-transferase, alpha(2-3)sialyl-transferase, and fucosyl-transferase VI. The enzymes transfer successively galactose, sialic acid, and fucose from the corresponding donors onto these acceptor substrates--despite the far reaching alterations--regio- and stereospecifically in the expected manner to yield a new class of compounds, the sialyl-Lewis(x)-saccharopeptides.

In situ generated O-glycan core 1 structure as substrate for Gal(beta 1-3)GalNAc beta-1,6-GlcNAc transferase

beta-Galactosidase from bovine testes was used in a one pot reaction together with a recombinant beta-1,6-GlcNAc transferase for the synthesis of GlcNAc(beta 1-6)GalNAc(alpha 1-OBn) (core 6-Bn). The galactosidase, which reversibly links galactose via a (beta 1-3) linkage to N-acetylgalactosamine, provides the substrate for the GlcNAc transferase in situ. The synthesis was carried out with a yield > 90%.

On the preparative use of recombinant pig alpha(1-3)galactosyl-transferase

A series of non-natural N-acyl derivatives of lactosamine is incubated with recombinant alpha(1-3)galactosyl-transferase and UDP-galactose. The enzyme shows a high promiscuity towards the non-natural acceptors. It selectively transfers a galactose unit onto the 3-OH group of the terminal beta-linked galactose in an alpha-mode to give an array of linear-B trisaccharides.