Inhibition of L- and P-selectin by a rationally synthesized novel core 2-like branched structure containing GalNAc-Lewisx and Neu5Acalpha2-3Galbeta1-3GalNAc sequences

Novel interactions of complex carbohydrates with peanut (PNA), Ricinus communis (RCA-I), Sambucus nigra (SNA-I) and wheat germ (WGA) agglutinins as revealed by the binding specificities of these lectins towards mucin core-2 O-linked and N-linked glycans and related structures

Plant lectins through their multivalent quaternary structures bind intrinsically flexible oligosaccharides. They recognize fine structural differences in carbohydrates and interact with different sequences in mucin core 2 or complex-type N-glycan chain and also in healthy and malignant tissues. They are used in characterizing cellular and extracellular glycoconjugates modified in pathological processes. We study here, the complex carbohydrate-lectin interactions by determining the effects of substituents in mucin core 2 tetrasaccharide Galβ1-4GlcNAcβ1-6(Galβ1-3)GalNAcα-O-R and fetuin glycopeptides on their binding to agarose-immobilized lectins PNA, RCA-I, SNA-I and WGA. Briefly, in mucin core 2 tetrasaccharide (i) structures modified by α2-3/6-Sialyl LacNAc, LewisX and α1-3-Galactosyl LacNAc resulted in regular binding to PNA whereas compounds with 6-sulfo LacNAc displayed no-binding; (ii) strucures bearing α2-6-sialyl 6-sulfo LacNAc, or 6-sialyl LacdiNAc carbohydrates displayed strong binding to SNA-I; (iii) structures with α2-3/6-sialyl, α1-3Gal LacNAc or LewisX were non-binder to RCA-I and compounds with 6-sulfo LacNAc only displayed weak binding; (iv) structures containing LewisX, 6-Sulfo LewisX, α2-3/6-sialyl LacNAc, α2-3/6-sialyl 6-sulfo LacNAc and GalNAc Lewis-a were non-binding to WGA, those with α1-2Fucosyl, α1-3-Galactosyl LacNAc, α2-3-sialyl T-hapten plus 3'/6'sulfo LacNAc displayed weak binding, and compounds with α2-3-sialyl T-hapten, α2.6-Sialyl LacdiNAc, α2-3-sialyl D-Fucβ1-3 GalNAc and Fucα-1-2 D-Fucβ-1-3GalNAc displaying regular binding and GalNAc LewisX and LacdiNAc plus D-Fuc β-1-3 GalNAcα resulting in tight binding. RCA-I binds Fetuin triantennary asialoglycopeptide 100 % after α-2-3 and 25 % after α-2-6 sialylation, 30 % after α-1-2 and 100 % after α-1-3 fucosylation, and 50 % after α-1-3 galactosylation. WGA binds 3-but not 6-Fucosyl chitobiose core. Thus, information on the influence of complex carbohydrate chain constituents on lectin binding is apparently essential for the potential application of lectins in glycoconjugate research.

Enhancing MS(n) mass spectrometry strategy for carbohydrate analysis: A b2 ion spectral library

Searchable mass spectral libraries for glycans may be enhanced using a B2 ion library. Using a quadrupole ion-trap mass spectrometer, successive fragmentations of sodiated oligosaccharides were carried out in the positive ion mode. In B,Y-type fragmentation, disaccharide B2 ions are generated which correspond to specific glycosidic linkages using progressive MS stages. Fragmentation of "B2 ions" corresponding to glycosidic linkages such as Hex-Fuc, Hex-Hex, Hex-HexNAc, HexNAc-Hex and HexNAc-HexNAc, were systematically studied in low energy CID and collected to form a "B2 library". Linkages produce characteristic fragmentation patterns in the absence of cross-ring fragmentation. Patterns of "B2 ions" rely on relative stability of glycosidic bonds and carbohydrate-metal complexes in the gas phase. MS(n) studies of linear, branched trisaccharides and tetrasaccharides show that isomers for which B2 ion information is not available are rarely a problem in practice by their absence in an isomeric sequence or by their scarcity in nature. This MS strategy for linkage determination of carbohydrates aided by a "B2 library" was developed with a scope for expansion, providing an improved tool for glycomics. We validated this method examining levels of expressed activities of two glycosyl transferases in cancer cell lines: β3(B3GALNT2) and β4GalNAcT(B4GALNT3&4) that generate GalNAcβ3GlcNAcβ and GalNAcβ4GlcNAcβ.

BIOLOGICAL SIGNIFICANCE

Glycosylation is an important class of the "postranslationome", which includes manifold aspects of post-translational protein modification, affecting protein conformation, providing ligands for protein receptors [1-5], and encoding unique haptenic [6,7] or antigenic markers for oncology [8-11] and other applications. Identification of individual monomeric units, linkages, ring size, branching and anomerity has posed significant challenges to mass spectrometrists. MS(n) is a growing key instrumental method to differentiate among isomers [12]. While the potential isomers in oligosaccharides are impossibly large [12], likely possibilities can be limited by the biological system, including the expressed glycosyl transferases [13-20]. Mass spectra from sequential stages of collision activation (MS(n)) can supply structural details for precise characterization of linkage, monomer ID, substitutions, anomerity and branching [21-25]. There is a fundamental need for high throughput tools in glycomics to complement proteome studies. In that regard, nothing could be more important than searchable spectral library files for structural confirmation. The National Academy of Science (NAS) report (http://glyco.nas.edu) recommends the need of more than 10,000 synthetic structures of carbohydrates to advance the field of glycomics. This study demonstrates that the general reproducibility of ion trap spectra, and energy independence from modes of ionization and collisional activation, make compiling an MS(n) library for carbohydrate identification an achievable research target [26]. We intend to use the new B2 library for carbohydrate differences found on cancers, where we profile the glycosyltransferases to predict classes of potential structures, and use the library for MS identification of the expected cohort of altered structures.

Synthesis of Carbohydrate-based Natural Products from Leonurus japonicus and their Biological Evaluation as Anti-oxidants

Natural products are important materials that have found a wide variety of uses, especially in medicine. Traditional Chinese medicine (TCM) has especially taken advantage of natural products and compounds found in Leonurus, a species of herb used extensively in TCM to treat various ailments. Herein we describe the synthesis of three natural products from Leonurus japonicus and our investigation of their hepatoprotective properties.

Mammalian sialyltransferase ST3Gal-II: its exchange sialylation catalytic properties allow labeling of sialyl residues in mucin-type sialylated glycoproteins and specific gangliosides

While glycosyltransferases are known to display unidirectional enzymatic activity, recent studies suggest that some can also catalyze readily reversible reactions. Recently, we found that mammalian sialyltransferase ST3Gal-II can catalyze the formation of CMP-NeuAc from 5'-CMP in the presence of a donor containing the NeuAcα2,3Galβ1,3GalNAc unit [Chandrasekaran, E. V., et al. (2008) Biochemistry 47, 320-330]. This study shows by using [9-(3)H]- or [(14)C]sialyl mucin core 2 compounds that ST3Gal-II exchanges sialyl residues between CMP-NeuAc and the NeuAcα2,3Galβ1,3GalNAc unit and also radiolabels sialyl residues in gangliosides GD1a and GT1b, but not GM1. Exchange sialylation proceeds with relative ease, which is evident from the following. (a) Radiolabeleling of fetuin was ~2-fold stronger than that of asialo fetuin when CMP- [9-(3)H]NeuAc was generated in situ from 5'-CMP and [9-(3)H]NeuAcα2,3Galβ1,3GalNAcβ1,3Galα-O-Me by ST3Gal-II. (b) ST3Gal-II exchanged radiolabels between [(14)C]sialyl fetuin and [9-(3)H]NeuAcα2,3Galβ1,3GalNAcβ1,3Galα-O-Me by generating CMP-[(14)C]- and -[9-(3)H]NeuAc through 5'-CMP; only 20.3% (14)C and 28.0% (3)H remained with the parent compounds after the sialyl exchange. The [9-(3)H]sialyl-tagged MN glycophorin A, human chorionic gonadotropin β subunit, GlyCAM-1, CD43, fetuin, porcine Cowper's gland mucin, bovine casein macroglycopeptide, human placental glycoproteins, and haptoglobin were analyzed by using Pronase digestion, mild alkaline borohydride treatment, Biogel P6, lectin agarose, and silica gel thin layer chromatography. Sulfated and sialylated O-glycans were found in GlyCAM-1 and human placental glycoproteins. This technique has the potential to serve as an important tool as it provides a natural tag for the chemical and functional characterization of O-glycan-bearing glycoproteins.

Targeting selectins and selectin ligands in inflammation and cancer

Inflammation and cancer metastasis are associated with extravasation of leukocytes or tumor cells from blood into tissue. Such movement is believed to follow a coordinated and sequential molecular cascade initiated, in part, by the three members of the selectin family of carbohydrate-binding proteins: E-selectin (CD62E), L-selectin (CD62L) and P-selectin (CD62P). E-selectin is particularly noteworthy in disease by virtue of its expression on activated endothelium and on bone-skin microvascular linings and for its role in cell rolling, cell signaling and chemotaxis. E-selectin, along with L- or P-selectin, mediates cell tethering and rolling interactions through the recognition of sialo-fucosylated Lewis carbohydrates expressed on structurally diverse protein-lipid ligands on circulating leukocytes or tumor cells. Major advances in understanding the role of E-selectin in inflammation and cancer have been advanced by experiments assaying E-selectin-mediated rolling of leukocytes and tumor cells under hydrodynamic shear flow, by clinical models of E-selectin-dependent inflammation, by mice deficient in E-selectin and by mice deficient in glycosyltransferases that regulate the binding activity of E-selectin ligands. Here, the authors elaborate on how E-selectin and its ligands may facilitate leukocyte or tumor cell recruitment in inflammatory and metastatic settings. Antagonists that target cellular interactions with E-selectin and other members of the selectin family, including neutralizing monoclonal antibodies, competitive ligand inhibitors or metabolic carbohydrate mimetics, exemplify a growing arsenal of potentially effective therapeutics in controlling inflammation and the metastatic behavior of cancer.

Reversible sialylation: synthesis of cytidine 5'-monophospho-N-acetylneuraminic acid from cytidine 5'-monophosphate with alpha2,3-sialyl O-glycan-, glycolipid-, and macromolecule-based donors yields diverse sialylated products

Sialyltransferases transfer sialic acid from cytidine 5'-monophospho-N-acetylneuraminic acid (CMP-NeuAc) to an acceptor molecule. Trans-sialidases of parasites transfer alpha2,3-linked sialic acid from one molecule to another without the involvement of CMP-NeuAc. Here we report another type of sialylation, termed reverse sialylation, catalyzed by mammalian sialyltransferase ST3Gal-II. This enzyme synthesizes CMP-NeuAc by transferring NeuAc from the NeuAcalpha2,3Galbeta1,3GalNAcalpha unit of O-glycans, 3-sialyl globo unit of glycolipids, and sialylated macromolecules to 5'-CMP. CMP-NeuAc produced in situ is utilized by the same enzyme to sialylate other O-glycans and by other sialyltransferases such as ST6Gal-I and ST6GalNAc-I, forming alpha2,6-sialylated compounds. ST3Gal-II also catalyzed the conversion of 5'-uridine monophosphate (UMP) to UMP-NeuAc, which was found to be an inactive sialyl donor. Reverse sialylation proceeded without the need for free sialic acid, divalent metal ions, or energy. Direct sialylation with CMP-NeuAc as well as the formation of CMP-NeuAc from 5'-CMP had a wide optimum range (pH 5.2-7.2 and 4.8-6.4, respectively), whereas the entire reaction comprising in situ production of CMP-NeuAc and sialylation of acceptor had a sharp optimum at pH 5.6 (activity level 50% at pH 5.2 and 6.8, 25% at pH 4.8 and 7.2). Several properties distinguish forward/conventional versus reverse sialylation: (i) sodium citrate inhibited forward sialylation but not reverse sialylation; (ii) 5'-CDP, a potent forward sialyltransferase inhibitor, did not inhibit the conversion of 5'-CMP to CMP-NeuAc; and (iii) the mucin core 2 compound 3-O-sulfoGalbeta1,4GlcNAcbeta1,6(Galbeta1,3)GalNAcalpha-O-benzyl, an efficient acceptor for ST3Gal-II, inhibited the conversion of 5'-CMP to CMP-NeuAc. A significant level of reverse sialylation activity is noted in human prostate cancer cell lines LNCaP and PC3. Overall, the study demonstrates that the sialyltransferase reaction is readily reversible in the case of ST3Gal-II and can be exploited for the enzymatic synthesis of diverse sialyl products.

Glycomic mapping of pseudomucinous human ovarian cyst glycoproteins: Identification of Lewis and sialyl Lewis glycotopes

Expression of sialyl Lewis x (sLe(x)) and sialyl Lewis a (sLe(a)) on cell-surface glycoproteins endows cells with the ability to adhere to E-, P-, and L-selectins present on endothelia, platelets, or leukocytes. Special arrangements of these glycotopes in cancers are thought to play a key role in metastasis. Previous studies have mostly described membrane-bound sLe(x) and sLe(a) activities. In this report, the major O-glycans of the secreted human ovarian cyst sialoglycoproteins from a Le(a+) nonsecretor individual (human ovarian cyst sample 350) were characterized by MS/MS analyses and immuno-/lectin-chemical assays. The results showed that HOC 350 carries a large number of epitopes for sLe(x), sLe(a), and Le(a) reactive antibodies. Advanced MS/MS sequencing coupled with mild periodate oxidation and exoglycosidase digestions further revealed that the O-glycans from HOC 350 are mostly of core 1 and 2 structures, extended and branched on the 3-arm with both type I and type II chains, complete with variable degrees of terminal sialylation and/or fucosylation to yield the sLe(x) or sLe(a) epitopes. Thus, the underlying core and peripheral backbone structures are similar to that of a previously proposed composite structural model for nonsialylated human ovarian cysts O-glycans, but with some notable distinguishing structural features in addition to sialylation.

Transport features, reaction kinetics and receptor biomechanics controlling selectin and integrin mediated cell adhesion

The distinct and overlapping roles of adhesion molecules belonging to the selectin and integrin families control the rate of leukocyte adhesion to stimulated vascular endothelial cells under hydrodynamic shear flow. Crystal structures have appeared for some of these interactions which complement molecular biology experiments, and clarify the molecular mechanism of the receptor-ligand binding interactions. Binding affinity data have also appeared using surface plasmon resonance and single-molecule biophysics experiments. These studies confirm and extend the predictions of previous experiments carried out in parallel-plate flow chambers, and cone and plate viscometers. This review discusses the current state of understanding on how molecular bond formation rates coupled with cellular and hydrodynamic features regulate leukocyte binding to endothelial cells.

Identification of Physiologically Relevant Substrates for Cloned Gal: 3-O-Sulfotransferases (Gal3STs)

Sulfated glycoconjugates regulate biological processes such as cell adhesion and cancer metastasis. We examined the acceptor specificities and kinetic properties of three cloned Gal:3-O-sulfotransferases (Gal3STs) ST-2, ST-3, and ST-4 along with a purified Gal3ST from colon carcinoma LS180 cells. Gal3ST-2 was the dominant Gal3ST in LS180. While the mucin core-2 structure Galbeta1,4GlcNAcbeta1,6(3-O-MeGalbeta1,3)GalNAcalpha-O-Bn (where Bn is benzyl) and the disaccharide Galbeta1,4GlcNAc served as high affinity acceptors for Gal3ST-2 and Gal3ST-3, 3-O-MeGalbeta1,4GlcNAcbeta1,-6(Galbeta1,3)GalNAcalpha-O-Bn and Galbeta1,3GalNAcalpha-O-Al (where Al is allyl) were efficient acceptors for Gal3ST-4. The activities of Gal3ST-2 and Gal3ST-3 could be distinguished with the Globo H precursor (Galbeta1,3GalNAcbeta1,3Galalpha-O-Me) and fetuin triantennary asialoglycopeptide. Gal3ST-2 acted efficiently on the former, while Gal3ST-3 showed preference for the latter. Gal3ST-4 also acted on the Globo H precursor but not the glycopeptide. In support of the specificity, Gal3ST-2 activity toward the Galbeta1,4GlcNAcbeta unit on mucin core-2 as well as the Globo H precursor could be inhibited competitively by Galbeta1,4GlcNAcbeta1,6(3-O-sulfoGalbeta1,3)GalNAcalpha-O-Bn but not 3-O-sulfoGalbeta1,-4GlcNAcbeta1,6(Galbeta1,3)GalNAcalpha-O-Bn. Remarkably these sulfotransferases were uniquely specific for sulfated substrates: Gal3ST-3 utilized Galbeta1,4(6-O-sulfo)-GlcNAcbeta-O-Al as acceptor, Gal3ST-2 acted efficiently on Galbeta1,3(6-O-sulfo)GlcNAcbeta-O-Al, and Gal3ST-4 acted efficiently on Galbeta1,3(6-O-sulfo)GalNAcalpha-O-Al. Mg(2+), Mn(2+), and Ca(2+) stimulated the activities of Gal3ST-2, whereas only Mg(2+) augmented Gal3ST-3 activity. Divalent cations did not stimulate Gal3ST-4, although inhibition was noted at high Mn(2+) concentrations. The fine substrate specificities of Gal3STs indicate a distinct physiological role for each enzyme.

Complex oligosaccharide investigations: synthesis of an octasaccharide incorporating the dimeric Le(x) structure of PSGL-1

The synthesis of an octasaccharide containing the dimeric Le(x) oligosaccharide structure found in PSGL-1 carbohydrate chains is reported. Several approaches were investigated employing regioselective and stereoselective glycosylation procedures, and a novel Lewis(x) trisaccharide donor, 7, was prepared and utilized as a key intermediate building block in the scheme developed for the construction of octasaccharide 3. Toward the preparation of 7, investigations into the influence of different protecting groups upon the relative reactivities of disaccharide acceptor moieties, 25 or 26, and the fucosyl donors, 10 and 11, were conducted using similar glycosylating conditions. Dramatic differences were noted between the effects of electron-donating and electron-withdrawing groups upon the reactivity of the acceptor hydroxyl. A similar effect upon the glycosylating capability of the donor molecule was, likewise, observed. The repeat use of donor 7 was instrumental in the synthesis of the desired dimeric octasaccharide structure 3. The structure and purity of 3 and important intermediates were fully characterized by DQF-COSY, TOCSY, ROESY, and ESI mass spectroscopy.

A new method for assaying adhesion of cancer cells to the greater omentum and its application for evaluating anti-adhesion activities of chemically synthesized oligosaccharides

A new ex vivo method for assaying adhesion of cancer cells to the greater omentum has been developed using mouse greater omentum and [3H]labelled human gastric and mouse colorectal cancer cells. Since the adhesion rates were found to increase up to 18 h and labelled cells seemed to be stable during the period, the present method could be useful for investigating adhesion of cancer cells to the greater omentum, which must occur at the first step of the peritoneal dissemination. The adhesion of cancer cells to the greater omentum was inhibited by a series of chemically synthesized oligosaccharides and Gal beta1,3[3OMeGal beta1,4GlcNAc beta1,6]alphaBn was found to be the best inhibitor. The anti-tumor effect of this novel tetrasaccharide in vivo was shown in preliminary experiments using Balb/c mice and colon26 cells.

Binding of glycosulfopeptides to P-selectin requires stereospecific contributions of individual tyrosine sulfate and sugar residues

P-selectin glycoprotein ligand-1 (PSGL-1) is a mucin on leukocytes that binds to selectins. P-selectin binds to an N-terminal region of PSGL-1 that requires sulfation of at least one of three clustered tyrosines (TyrSO(3)) and an adjacent core-2-based O-glycan expressing sialyl Lewis x (C2-O-sLe(x)). We synthesized glycosulfopeptides (GSPs) modeled after this region of PSGL-1 to explore the roles of individual TyrSO(3) residues, the placement of C2-O-sLe(x) relative to TyrSO(3), the relative contributions of fucose and sialic acid on C2-O-sLe(x), and the function of the peptide sequence for binding to P-selectin. Binding of GSPs to P-selectin was measured by affinity chromatography and equilibrium gel filtration. 2-GSP-6, which has C2-O-sLe(x) at Thr-57 and TyrSO(3) at residues 46, 48, and 51, bound to P-selectin with high affinity (K(d) approximately 650 nm), whereas an isomeric trisulfated GSP containing C2-O-sLe(x) at Thr-44 bound much less well. Non-sulfated glycopeptide (2-GP-6) containing C2-O-sLe(x) at Thr-57 bound to P-selectin with approximately 40-fold lower affinity (K(d) approximately 25 microm). Proteolysis of 2-GP-6 abolished detectable binding of the residual C2-O-sLe(x)-Thr to P-selectin, demonstrating that the peptide backbone contributes to binding. Monosulfated and disulfated GSPs bound significantly better than non-sulfated 2-GP-6, but sulfation of Tyr-48 enhanced affinity (K(d) approximately 6 microm) more than sulfation of Tyr-46 or Tyr-51. 2-GSP-6 lacking sialic acid bound to P-selectin at approximately 10% that of the level of the parent 2-GSP-6, whereas 2-GSP-6 lacking fucose did not detectably bind; thus, fucose contributes more than sialic acid to binding. Reducing NaCl from 150 to 50 mm markedly enhanced binding of 2-GSP-6 to P-selectin (K(d) approximately 75 nm), demonstrating the charge dependence of the interaction. These results reveal a stereospecific interaction of P-selectin with PSGL-1 that includes distinct contributions of each of the three TyrSO(3) residues, adjacent peptide determinants, and fucose/sialic acid on an optimally positioned core-2 O-glycan.

Chemical synthesis of sulfated oligosaccharides with a beta-D-Gal-(1-->3)

The syntheses of two sulfated pentasaccharides: beta-D-Gal6SO3Na-(1-->3)-[beta-D-Gal-(1-->4)-alpha-L-Fuc-(1-->3)-beta-D-Glc-NAc-(1-->6)]-alpha-D-GalNAc-->OMe (1) and beta-D-Gal6SO3Na-(1-->3)-[beta-D-Gal-(1-->4)-alpha-L-Fuc-(1-->3)-beta-D-Glc-NAc6SO3Na-(1-->6)]-alpha-D-GalNAc-->OMe (2) by using Lewisx trisaccharides 12 and 16 as glycosyl donors are described. Sulfated oligosaccharides 1-2 and intermediate compounds are fully characterized by 2D 1H-1H DQF-COSY and 2D ROESY experiments.

Chemical synthesis of a core 2 branched pentasaccharide containing a carboxylate group

Design and synthesis of a carboxylate-containing pentasaccahride 1 with the Galbeta(1-4) (Fucalpha1-3)GlcNAcbeta(1-6)[3-[1-carboxymethyl]-Galbeta+ ++(1-3)]GalNAcalpha-OMe sequence, which is obtained through regioselective coupling of the 6-OH of a novel acceptor 9 with Lewis(x) donor 10 catalyzed by NIS-TfOH are described.

N-glycan structures of matrix metalloproteinase-1 derived from human fibroblasts and from HT-1080 fibrosarcoma cells

Matrix metalloproteinase-1 (MMP-1) is a collagenolytic metalloproteinase capable of cleaving native triple-helical forms of several collagen subtypes, as well as a number of non-collagenous substrates. The role of MMP-1 in various diseases affecting the connective tissue is well characterized. MMP-1 is secreted as both glycosylated and unglycosylated species, and the two forms have been shown to be identical with respect to substrate specificity, specific activity and inhibitory profile. No function for the glycan moiety of the enzyme has been ascribed to date. In the present study, we report on the detailed characterization of MMP-1-derived oligosaccharides. Using strategies based on sequential exoglycosidase digestion combined with matrix-assisted laser desorption ionization-time of flight MS and electrospray tandem MS, we have characterized the N-glycan structures of MMP-1, derived from human dermal fibroblasts and from the HT-1080 fibrosarcoma cell line. MMP-1 derived from fibroblasts was found to carry mainly alpha 2,3-sialylated complex-type diantennary glycans. On the other hand, HT-1080 cells produce MMP-1 that has a heterogeneous glycosylation pattern, comprising diantennary glycans carrying Lewis X, LacdiNAc, sialylated LacdiNAc and GalNAc beta 1,4 (Fuc alpha 1,3)GlcNAc (LacdiNAc analogue of Lewis X) as terminal elements. We also show that, of the two potential glycosylation sites in the MMP-1 sequence, only Asn120 is used.