Characterization of the structural determinants required for the high affinity interaction of asparagine-linked oligosaccharides with immobilized Phaseolus vulgaris leukoagglutinating and erythroagglutinating lectins

Effect of staurosporine on N-glycosylation and cell adhesion to fibronectin of SW480 human colorectal adenocarcinoma cells

As N-glycosylation of tumor cell surface proteins affects metastasis of the cells, it was considered that the suppression of metastasis by staurosporine, a protein kinase C inhibitor, is partly caused by changes in N-glycosylation. To examine this possibility, we studied the glycosylation of membrane proteins of SW480 human colorectal adenocarcinoma cells before and after treatment with staurosporine by lectin blot analysis. The results showed that the reactivity of leuko-agglutinating phytohemagglutinin and Datura stramonium agglutinin, both of which bind to highly branched N-linked oligosaccharides characteristic of cancer cells, decreases significantly in the staurosporine-treated cells. In accordance with this, the gene expression of the N-acetylglucosaminyltransferase V, which synthesizes the GlcNAcbeta1-->6 branch of highly branched N-linked oligosaccharides decreased by 30-40% in the drug-treated cells. Since a decrease in the lectin binding was found in several glycoproteins including fibronectin (FN)-receptor, effect of the changes in N-glycosylation of the cells on cell adhesion to FN-matrix was examined. The results showed that the number of cells attached to FN-matrix increases upon treatment of the cells with staurosporine, indicating that the change of N-glycosylation of the FN-receptor promotes cell adhesion to the extracellular matrix, which may lead to the suppression of metastasis of cancer cells.

Detection of isoforms of recombinant human erythropoietin by various plant lectins after isoelectric focusing

A screening method to determine the binding behavior of lectins toward recombinant human erythropoietin (rHuEPO) was developed. Twenty-three different lectins were tested for this purpose. rHuEPO isoforms were separated by isoelectric focusing using the International Olympic Committee (IOC) and World Anti-Doping Agency (WADA) accredited method for the direct detection of the prohibited doping substance erythropoietin (EPO). For the visualization of the rHuEPO isoforms lectins were used instead of antibodies. Optimization of the screening protocol enabled the detection of a maximum number of rHuEPO isoforms. By means of this protocol information about the binding properties of a lectin toward each individual rHuEPO isoform was accessible. All evaluated lectins showed significant differences in their binding behavior. The most intense response was obtained with WGA, DSL, PHA-E, LEL, PSA, and LCA. While WGA, DSL, PHA-E, and LEL were able to bind all isoforms detected by the standard antibody, LCA and PSA demonstrated a clear preference for rHuEPO isoforms located in the more basic region of the electropherogram. Further lectins tested were ConA, succWGA, PHA-L, RCA, SNA, MAA, STL, ECL, GSL-II, SJA, SBA, UEA-I, Jacalin, PNA, DBA, GSL-I, and VVA. Compared to the lectins mentioned above, they showed reduced sensitivity. Endogenous and recombinant EPO only differ in the composition of their N- and O-glycan moieties. As lectins possess the unique ability to recognize subtle differences in glycan substructures, they represent an interesting approach for their structural characterization. Furthermore, they might be useful for affinity enrichment/purification of rHuEPO in doping control.

N-acetylglucosaminyltransferase V (Mgat5)-mediated N-glycosylation negatively regulates Th1 cytokine production by T cells

The differentiation of naive CD4(+) T cells into either proinflammatory Th1 or proallergic Th2 cells strongly influences autoimmunity, allergy, and tumor immune surveillance. We previously demonstrated that beta1,6GlcNAc-branched complex-type (N-acetylglucosaminyltransferase V (Mgat5)) N-glycans on TCR are bound to galectins, an interaction that reduces TCR signaling by opposing agonist-induced TCR clustering at the immune synapse. Mgat5(-/-) mice display late-onset spontaneous autoimmune disease and enhanced resistance to tumor progression and metastasis. In this study we examined the role of beta1,6GlcNAc N-glycan expression in Th1/Th2 cytokine production and differentiation. beta1,6GlcNAc N-glycan expression is enhanced by TCR stimulation independent of cell division and declines at the end of the stimulation cycle. Anti-CD3-activated splenocytes and naive T cells from Mgat5(-/-) mice produce more IFN-gamma and less IL-4 compared with wild-type cells, the latter resulting in the loss of IL-4-dependent down-regulation of IL-4Ralpha. Swainsonine, an inhibitor of Golgi alpha-mannosidase II, blocked beta1,6GlcNAc N-glycan expression and caused a similar increase in IFN-gamma production by T cells from humans and mice, but no additional enhancement in Mgat5(-/-) T cells. Mgat5 deficiency did not alter IFN-gamma/IL-4 production by polarized Th1 cells, but caused an approximately 10-fold increase in IFN-gamma production by polarized Th2 cells. These data indicate that negative regulation of TCR signaling by beta1,6GlcNAc N-glycans promotes development of Th2 over Th1 responses, enhances polarization of Th2 cells, and suggests a mechanism for the increased autoimmune disease susceptibility observed in Mgat5(-/-) mice.

Altered melanoma cell surface glycosylation mediates organ specific adhesion and metastasis via lectin receptors on the lung vascular endothelium

Adhesive interactions between the molecules on cancer cells and the target organ are one of the key determinants of the organ specific metastasis. In this communication we show that b1,6 branched N-oligosaccharides which are expressed in a metastasis-dependent manner on B16-melanoma metastatic cell lines, participate in the adhesion process. We demonstrate that high metastatic cells show significantly increased translocation of one of the major carriers of these oligosaccharides, lysosome associated membrane protein (LAMP1), to the cell surface. LAMP1 on high metastatic cells, carry very high levels of these oligosaccharides, which are further substituted with poly N-acetyl lactosamine (polylacNAc), resulting in the expression of high density of very high affinity ligands for galectin-3 on the cell surface. We show that galectin-3 is expressed in highest amount in the lungs as compared to other representative organs. Blocking galectin-3 by pre-incubating the frozen sections of the lungs with 100 mM lactose, substantially inhibited the adhesion of high metastatic cells, while pre-incubation with sucrose had no effect. Finally, by in situ labeling and immunoprecipitation experiment, we demonstrated that the lung vascular endothelial cells express galectin-3 constitutively on their surface. Galectin-3 on the organ endothelium could thus serve as the first anchor for the circulating cancer cells, expressing high density of very high affinity ligands on their surface, and facilitate organ specific metastasis.

Influenza virus entry and infection require host cell N-linked glycoprotein

A widely held view of influenza virus infection is that the viral receptor consists of cell surface carbohydrate sialic acid, which can be present as glycoprotein or glycolipid. Here, we examined influenza virus entry and infection in Lec1 cells, a mutant CHO cell line deficient in terminal N-linked glycosylation caused by a mutation in the N-acetylglucosaminyltransferase I (GnT1) gene. We show that influenza virus cannot infect Lec1 cells, despite having full capacity to undergo virus binding and fusion. Lec1 cells also show no virus replication defect, and infection was restored in Lec1 cells expressing wild-type GnT1. Viruses were apparently arrested at the level of internalization from the plasma membrane and were not endocytosed. Lec1 cells were refractory to infection by several strains of influenza virus, including H1 and H3 strains of influenza A, as well as influenza B virus. Finally, cleavage of N-glycans from wild-type CHO cells markedly reduced infection by influenza virus. We suggest that influenza virus specifically requires N-linked glycoprotein for entry into cells, and that sialic acid, although acting as an efficient attachment factor, is not sufficient as an influenza virus receptor in vivo.

Inactivation of the Mgat1 gene in oocytes impairs oogenesis, but embryos lacking complex and hybrid N-glycans develop and implant

Complex and hybrid N-glycans contain sugar residues that have been implicated in fertilization, compaction of the embryo, and implantation. Inactivation of the Mgat1 gene responsible for their synthesis is embryonic lethal, but homozygous mutant blastocysts are phenotypically normal due to the presence of maternal Mgat1 gene transcripts. To identify roles for complex and hybrid N-glycans in oogenesis and preimplantation development, the Mgat1 gene in oocytes was deleted by using a ZP3Cre recombinase transgene. All mutant oocytes had an altered zona pellucida (ZP) that was thinner than the control ZP, and they did not possess complex N-glycans but contained ZP1, ZP2, and ZP3 glycoproteins. Mutant eggs were fertilized, all embryos implanted, and heterozygotes developed to birth. However, mutant females had decreased fertility, yielded fewer eggs after stimulation with gonadotropins, and produced a reduced number of preimplantation embryos and less progeny than controls. About 25% of embryonic day 3.5 (E3.5) embryos derived from mutant eggs were severely retarded in development, even when they were heterozygous and expressed complex N-glycans. Thus, a proportion of Mgat1(-)(/)(-) oocytes were developmentally compromised. Surprisingly, mutant eggs also gave rise to Mgat1(-)(/)(-) embryos that developed normally, implanted, and progressed to E9.5. Therefore, complex or hybrid N-glycans are required at some stage of oogenesis for the generation of a developmentally competent oocyte, but fertilization, blastogenesis, and implantation may proceed in their absence.

N-glycoproteins bearing beta1-6 branched oligosaccharides from the A375 human melanoma cell line analysed by tandem mass spectrometry

Tumour-associated alterations of cell surface glycosylation play a crucial role in the adhesion and metastasis of cancer cells. It is well known that the metastatic potential is associated with increased GlcNAc beta1-6 branching in N-glycans of tumour cells specifically recognized by a lectin from Phaseolus vulgaris leukoagglutinin (PHA-L). We identified proteins bearing GlcNAc beta1-6 branched N-glycans in the A375 human melanoma cell line by affinity chromatography separation on a PHA-L agarose column, followed by immunoidentification and tandem mass spectrometry (MS/MS) analysis. Amongst the proteins identified were integrin subunits alpha2, alpha3, alpha5 and beta1, as well as N-cadherin and lysosome-associated membrane proteins (LAMP-1 and LAMP-2). In addition, L1, Mac-2 binding protein (Mac-2-BP), activated leukocyte cell adhesion molecule/CD166 (ALCAM) and melanotransferrin were shown to react with PHA-L. Some of these proteins are connected mainly with nervous tissues or the immune system and play a crucial role in cell adhesion processes. The presence of GlcNAc beta1-6 branched oligosaccharides in these proteins may influence their adhesion properties, reducing adhesion of the cells to the extracellular matrix (ECM) and thus facilitating tumour cell invasion.

Co-localization of beta1,6-branched oligosaccharides and coarse melanin in macrophage-melanoma fusion hybrids and human melanoma cells in vitro

Fusion hybrids between normal macrophages and Cloudman S91 melanoma cells were shown earlier to have increased metastatic potential, along with high expression of beta1,6-N-acetylglucosaminyltransferase V and beta1,6-branched oligosaccharides. Curiously, hybrids, but not parental melanoma cells, also produced 'coarse melanin'- autophagic vesicles with multiple melanosomes. As beta1,6-branched oligosaccharides were known to be associated with metastasis, and coarse melanin had been described in invasive human melanomas, we looked for potential relationships between the two. Using lectin- and immunohistochemistry, we analyzed cell lines producing coarse melanin for beta1,6-branched oligosaccharides: gp100/pmel-17 (a melanosomal structural component) and CD63 (a late endosome/lysosome component associated with melanoma and certain other human cancers). Cell lines used in this study were (i) hybrid 94-H48, a highly metastatic, macrophage-melanoma experimental fusion hybrid; (ii) 6(neo) mouse melanoma cells, the weakly metastatic, parental fusion partner; and (iii) SKmel-23, a human melanoma cell line derived from a metastasis. Coarse melanin granules were prominent both in hybrids and in SKmel-23 cells, and co-localized with stains for beta1,6-branched oligosaccharides, gp100/pmel 17, and CD63. This is the first report of this phenotype being expressed in vitro, although co-expression of beta1,6-branched oligosaccharides and coarse melanin was recently shown to be a common and pervasive characteristic in archival specimens of human melanomas, and was most prominent in metastases. The results suggest that pathways of melanogenesis in melanoma may differ significantly from those in normal melanocytes. In vitro expression of this phenotype provides new biological systems for more detailed analyses of its genesis and regulation at the molecular genetic level.

Purification, characterization, and sugar binding specificity of an N-Glycolylneuraminic acid-specific lectin from the mushroom Chlorophyllum molybdites

A carbohydrate-binding protein was isolated from the carpophores of the mushrooms and designated the Chlorophyllum molybdites lectin (CML) based on its origin. The molecular mass of CML was 32 kDa, and it was composed of two 16-kDa monomers with no disulfide bonds. Monosaccharide analysis indicated that 12% of the mass of CML was carbohydrate and consisted of GlcNAc:GalNAc:Gal:Man:l-Fuc in a molar ratio of 1.5:1.9: 4.4:4.8:1.0. In the hemagglutination inhibition assay, CML exhibited the strongest binding specificity toward N-glycolylneuraminic acid (NeuGc) among the monosaccharides tested, whereas NeuAc did not inhibit the hemagglutination at all. GalNAc and Mealpha-GalNAc were also inhibitory at much higher concentrations than NeuGc. Among the glycoproteins, asialobovine submaxillary mucin (BSM) and porcine stomach mucin (PSM) showed strong inhibitory effects. In surface plasmon resonance analysis, asialo-BSM and PSM exhibited the strongest binding affinity. After co-injection of CML and NeuGc or GalNAc onto the asialo-BSM- or PSM-immobilized chip, the dissociation of CML from the immobilized PSM was accelerated by NeuGc and GalNAc, but the dissociation of CML from the immobilized asialo-BSM was only promoted by GalNAc. These results and the other surface plasmon resonance experiments allowed us to conclude that the binding of asialo-BSM to CML was because of an interaction between the lectin and the GalNAc residues of asialo-BSM, and both the NeuGc and GalNAc residues were responsible for the binding of PSM to CML. The results also suggested that CML had two different carbohydrate binding domains, one specific for NeuGc and the other for GalNAc.

Transcriptional Regulation of the Human β-1,4-Galactosyltransferase V Gene in Cancer Cells

Beta-1,4-galactosyltransferase (beta-1,4-GalT) V is a constitutively expressed enzyme that can effectively galactosylate the GlcNAcbeta1-->6Man group of the highly branched N-glycans that are characteristic of tumor cells. Upon malignant transformation of cells, the expression of the beta-1,4-GalT V gene increases in accordance with the increase in the amounts of highly branched N-glycans. Lectin blot analysis showed that the galactosylation of highly branched N-glycans is inhibited significantly in SH-SY5Y human neuroblastoma cells by the transfection of the antisense beta-1,4-GalT V cDNA, indicating the biological importance of the beta-1,4-GalT V for the functions of highly branched N-glycans. We cloned the 2.3-kb 5'-flanking region of the human beta-1,4-GalT V gene, and we identified the region -116/-18 relative to the transcription start site as that having promoter activity. The region was found to contain several putative binding sites for transcription factors, including AP2, AP4, N-Myc, Sp1, and upstream stimulatory factor. Electrophoretic mobility shift assay showed that Sp1 binds to nucleotide positions -81/-69 of the promoter region. Mutations induced in the Sp1-binding site showed that the promoter activity of the beta-1,4-GalT V gene is impaired completely in cancer cells. In contrast, the promoter activity increased significantly by the transfection of the Sp1 cDNA into A549 human lung carcinoma cells. Mithramycin A, which inhibits the binding of Sp1 to its binding site, reduced the promoter activation and expression of the beta-1,4-GalT V gene in A549 cells. These results indicate that Sp1 plays an essential role in the transcriptional activity of the beta-1,4-GalT V gene in cancer cells.

Glycan arrays lead to the discovery of autoimmunogenic activity of SARS-CoV

Using carbohydrate microarrays, we characterized the carbohydrate binding activity of SARS-CoV neutralizing antibodies elicited by an inactivated SARS-CoV vaccine. In these antibodies, we detected undesired autoantibody reactivity specific for the carbohydrate moieties of an abundant human serum glycoprotein asialo-orosomucoid (ASOR). This observation provides important clues for the selection of specific immunologic probes to examine whether SARS-CoV expresses antigenic structures that mimic the host glycan. We found that lectin PHA-L (Phaseolus vulgaris L.), which is specific for a defined complex carbohydrate of ASOR, stained the SARS-CoV-infected cells specifically and intensively. Taken together, we present immunologic evidence that a carbohydrate structure of SARS-CoV shares antigenic similarity with host glycan complex carbohydrates. The experimental approaches we applied in this study are likely applicable for the identification of immunologic targets of other viral pathogens.

Links between CD147 function, glycosylation, and caveolin-1

Cell surface CD147 shows remarkable variations in size (31-65 kDa) because of heterogeneous N-glycosylation, with the most highly glycosylated forms functioning to induce matrix metalloproteinase (MMP) production. Here we show that all three CD147 N-glycosylation sites make similar contributions to both high and low glycoforms (HG- and LG-CD147). l-Phytohemagglutinin lectin binding and swainsonine inhibition experiments indicated that HG-CD147 contains N-acetylglucosaminyltransferase V-catalyzed, beta1,6-branched, polylactosamine-type sugars, which account for its excess size. Therefore, CD147, which is itself elevated on invasive tumor cells, may make a major contribution to the abundance of beta1,6-branched polylactosamine sugars that appear on invasive tumor cells. It was shown previously that caveolin-1 associates with CD147, thus inhibiting CD147 self-aggregation and MMP induction; now we show that caveolin-1 associates with LG-CD147 and restricts the biosynthetic conversion of LG-CD147 to HG-CD147. In addition, HG-CD147 (but not LG-CD147) was preferentially captured as a multimer after treatment of cells with a homobifunctional cross-linking agent and was exclusively recognized by monoclonal antibody AAA6, a reagent that selectively recognizes self-associated CD147 and inhibits CD147-mediated MMP induction. In conclusion, we have 1) determined the biochemical basis for the unusual size variation in CD147, 2) established that CD147 is a major carrier of beta1,6-branched polylactosamine sugars on tumor cells, and 3) determined that caveolin-1 can inhibit the conversion of LG-CD147 to HG-CD147. Because it is HG-CD147 that self-aggregates and stimulates MMP induction, we now have a mechanism to explain how caveolin-1 inhibits these processes. These results help explain the previously established tumor suppressor functions of caveolin-1.

Hyperexpression of N-acetylglucosaminyltransferase-III in liver tissues of transgenic mice causes fatty body and obesity through severe accumulation of Apo A-I and Apo B

N-Acetylglucosaminyltransferase (GnT)-III catalyzes the attachment of an N-acetylglucosamine (GlcNAc) residue to mannose in beta(1-4) configuration in the region of N-glycans and forms a bisecting GlcNAc. To investigate the pathophysiological role of dysregulated glycosylation mediated by aberrantly expressed GnT-III, we generated transgenic mice hyperexpressing the human GnT-III in the liver by introducing human GnT-III cDNA under the control of mouse albumin enhancer/promoter. Total five transgenic founder mice (pGnTSVTpA-10, -14, -20, -25, and -51) expressed the human GnT-III in their livers and were characterized by molecular genetic means. The copy number of transgene integrated into the genome of these mice ranged between 1 and 3 copies per haploid genome. Northern and Western blot analyses showed that the transgene is specifically expressed in the liver but not in any other tissues tested. The triglyceride level in GnT-III transgenic mice was significantly decreased, however, no significant differences in the levels of glucose, cholesterol, or albumin were observed between transgenic and nontransgenic mice. Although glutamate oxaloacetic transaminase and glutamic pyruvic transaminase activities of transgenic mice were also higher than those of nontransgenic mice, no differences in total bililubin and total protein were observed between the two animal lines. Large amounts of apolipoprotein (Apo) A-I and Apo B were specifically detected in the intracellular liver of transgenic mice. The accumulation of Apo A-I in hepatocytes may be due to aberrant glycosylation, since glycosylated Apo A-I was not observed in transgenic mice. However, the accumulated Apo B was severely glycosylated. Therefore, it is suggested that highly expressed transgenic GnT-III allowed unknown target proteins to be glycosylated in large amounts, and the resulting target protein(s) disrupted in assembly formation of Apo A-I in the hepatocytes and cause a decrease in the release of lipoproteins and accumulations of Apo A-I and Apo B in the liver. The transgenic mice showed aberrant glycosylation by GnT-III, resulting in numerous lipid droplets in liver tissues and the obesity. These mice showed microvesicular fatty changes with abnormal lipid accumulation in the hepatocytes. Our study provides the basis for future analysis of the role of glycosylation in hepatic pathogenesis. In the transgenic mice, Apo A-I and Apo B were significantly increased compared with levels in nontransgenic liver tissues.

Expression ofN-Acetylglucosaminyltransferase V Is Associated with Prognosis and Histology in Non-Small Cell Lung Cancers

PURPOSE

N-Acetylglucosaminyltransferase V (GnT-V), a key enzyme in the formation of branching of asparagine-linked oligosaccharides, is strongly linked to tumor invasion and metastasis of colon and breast cancers. However, GnT-V is expressed in many tissues, including normal lung. GnT-V expression has not been examined previously in human lung cancers. The objective of this study is to examine GnT-V expression in non-small cell lung cancers (NSCLCs) and to determine its relationship to biological and clinicopathological characteristics and prognosis.

EXPERIMENTAL DESIGN

GnT-V expression was studied by immunohistochemistry in 217 surgically resected NSCLCs and analyzed statistically in relation to various characteristics.

RESULTS

High GnT-V expression was found in 113 (52.1%) NSCLCs, and low GnT-V expression was found in 104 (47.9%) NSCLCs. Multivariate logistic regression analysis revealed a significant association between low GnT-V expression and squamous cell carcinomas, as compared with nonsquamous cell carcinomas (P = 0.02). Among biological characteristics of tumors, Ki-67 labeling index was higher in tumors with low GnT-V expression than in those with high GnT-V expression, although this difference was not statistically significant (P = 0.09). Patients with tumors having low GnT-V expression had significantly shorter survival time than patients with tumors having high GnT-V expression in 103 patients with pStage I NSCLCs (5-year survival rates, 49% and 86%, respectively; P = 0.0009), as well as in 59 patients with pStage I non-squamous cell carcinomas (5-year survival rates, 54% and 89%, respectively; P = 0.007). Low GnT-V expression was a significant unfavorable prognostic factor in pStage I NSCLCs (hazard ratio, 2.86; P = 0.002) and in pStage I nonsquamous cell carcinomas (hazard ratio, 3.02; P = 0.02). Furthermore, beta1-6 branching of asparagine-linked oligosaccharides, which are products of GnT-V, were increased highly or moderately in 8 of 10 tumors with high GnT-V expression, as judged by leukoagglutinating phytohemagglutinin staining.

CONCLUSIONS

GnT-V expression is associated with histology in NSCLCs. Low GnT-V expression is associated with shorter survival and poor prognosis in pStage I overall NSCLCs and non-squamous cell carcinomas.

Determination of modulation of ligand properties of synthetic complex-type biantennary N-glycans by introduction of bisecting GlcNAcin silico,in vitroandin vivo

We have investigated the consequences of introducing a bisecting GlcNAc moiety into biantennary N-glycans. Computational analysis of glycan conformation with prolonged simulation periods in vacuo and in a solvent box revealed two main effects: backfolding of the alpha1-6 arm and stacking of the bisecting GlcNAc and the neighboring Man/GlcNAc residues of both antennae. Chemoenzymatic synthesis produced the bisecting biantennary decasaccharide N-glycan and its alpha2-3(6)-sialylated variants. They were conjugated to BSA to probe the ligand properties of N-glycans with bisecting GlcNAc. To assess affinity alterations in glycan binding to receptors, testing was performed with purified lectins, cultured cells, tissue sections and animals. The panel of lectins, including an adhesion/growth-regulatory galectin, revealed up to a sixfold difference in affinity constants for these neoglycoproteins relative to data on the unsubstituted glycans reported previously [André, S., Unverzagt, C., Kojima, S., Dong, X., Fink, C., Kayser, K. & Gabius, H.-J. (1997) Bioconjugate Chem. 8, 845-855]. The enhanced affinity for galectin-1 is in accord with the increased percentage of cell positivity in cytofluorimetric and histochemical analysis of carbohydrate-dependent binding of labeled neoglycoproteins to cultured tumor cells and routinely processed lung cancer sections. Intravenous injection of iodinated neoglycoproteins carrying galactose-terminated N-glycans into mice revealed the highest uptake in liver and spleen for the bisecting compound compared with the unsubstituted or core-fucosylated N-glycans. Thus, this substitution modulates ligand properties in interactions with lectins, a key finding of this report. Synthetic glycan tailoring provides a versatile approach to the preparation of newly substituted glycans with favorable ligand properties for medical applications.

N-Acetylglucosaminyltransferase V Expression Levels Regulate Cadherin-associated Homotypic Cell-Cell Adhesion and Intracellular Signaling Pathways

A common glycan alteration in transformed cells and human tumors is the highly elevated levels of N-linked beta(1,6)glycans caused by increased transcription of N-acetylglucosaminyltransferase V (GnT-V). Here, we define the involvement of GnT-V in modulation of homotypic cell-cell adhesion in human fibrosarcoma HT1080 and mouse NIH3T3 cells. Increased GnT-V expression resulted in a significant decrease in the rates of calcium-dependent cell-cell adhesion. Reduced cell-cell adhesion was blocked by function-blocking antibody against N-cadherin and abrogated by pre-treatment of cells with swainsonine, demonstrating the involvement of N-cadherin in the cell-cell adhesion and that changes in N-linked beta(1,6)glycan expression are responsible for the reduction in rates of adhesion, although this reduction could be mediated by the altered N-linked glycosylation of glycoproteins other than N-cadherin. Overexpression of GnT-V had no effect on the levels of cell surface expression of N-cadherin; however, it did cause a marked enhancement of both beta(1,6) branching and poly-N-acetyllactosamine expression on N-cadherin. GnT-V overexpression resulted in decreased N-cadherin clustering on the cell surface induced by anti-N-cadherin antibody and affected the outside-in signal transduction pathway of ERK mediated by N-cadherin. Overexpression of GnT-V sensitized stimulation of tyrosine phosphorylation of catenins by growth factors and expression of v-src, which is consistent with its reduction of cell-cell adhesion. In vitro, GnT-V-overexpressing cells showed increased motility concomitant with increased phosphorylation of catenins. Moreover, GnT-V-deficient embryo fibroblasts from GnT-V homozygous null mice (GnT-V(-/-)) express N-cadherin and showed significantly increased levels of N-cadherin-based cell-cell adhesion compared with those from GnT-V(+/-) mice. These levels of adhesion were inhibited significantly by transient expression of GnT-V, confirming the hypothesis that levels of GnT-V can regulate cadherin-associated homotypic cell-cell adhesion. Aberrant N-linked beta(1,6) branching that occurs during oncogenesis can, therefore, lessen cell-cell adhesion, contributing to increased cellular motility and invasiveness.

Glycosylation profile of integrin α3β1 changes with melanoma progression

Glycosylation of integrins has been implicated in the modulation of their function. Characterisation of carbohydrate moieties of alpha(3) and beta(1) subunits from non-metastatic (WM35) and metastatic (A375) human melanoma cell lines was carried out on peptide-N-glycosidase F-released glycans using matrix-assisted laser desorption ionization mass spectrometry (MALDI-MS). beta(1) integrin subunit from both cell lines displayed tri- and tetraantennary oligosaccharides complex type glycans, but only in A375 cell line was the sialylated tetraantennary complex type glycan (Hex(7)HexNAc(6)FucSia(4)) present. In contrast, only alpha(3) subunit from metastatic cells possessed beta1-6 branched structures. Our data indicate that the beta(1) and alpha(3) subunits expressed by the metastatic A375 cell line carry beta1-6 branched structures, suggesting that these cancer-associated glycan chains may modulate tumor cell adhesion by affecting the ligand binding properties of alpha(3)beta(1) integrin. In direct ligand binding assays, alpha(3)beta(1) integrin from both cell lines binds strongly to fibronectin and to much lesser degree to placental laminin. No binding to collagen IV was observed. Enzymatic removal of sialic acid residues from purified alpha(3)beta(1) integrin stimulates its adhesion to all examined ECM proteins. Our data suggest that the glycosylation profile of alpha(3)beta(1) integrin in human melanoma cells correlates with the acquisition of invasive capacity during melanoma progression.

A novel β(1,6)-N-acetylglucosaminyltransferase V (GnT-VB)1

UDP-N-acetylglucosamine:alpha(1,6)-D-mannoside beta(1,6)-N-acetylglucosaminyltransferase (GnT-V, Mgat5) functions in the biosynthesis of N-linked glycans and is transcriptionally upregulated by oncogene signaling. We report here the cloning and characterization of a human cDNA encoding a distinct enzyme with related substrate specificity, termed GnT-VB, which is predicted to have 53% similarity to the original amino acid sequence of GnT-V(A). Transient expression of GnT-VB cDNA in COS7 cells yielded significant increases of activity toward GnT-VA acceptors, including synthetic saccharides and N-linked glycopeptides, with some differences in specificity. Unlike GnT-VA, GnT-VB required divalent cation for full activity. EST databases showed expression of a 6 bp (+) splice isoform of GnT-VB; when expressed, this enzyme showed significantly reduced activity. CHO Lec4 cells, which do not express GnT-VA or B activity, lack synthesis of the N-linked beta(1,6) branch, and do not bind L-phytohemagglutinin (L-PHA), were transfected with GnT-VB or GnT-VA; both then bound significant amounts of L-PHA, demonstrating that both enzymes synthesized N-linked beta(1,6) branched glycans in vivo. Real-time polymerase chain reaction results showed that GnT-VB mRNA was highly expressed in brain and testis, with lesser levels in other tissues, while human GnT-VA showed a more general expression, but with low levels in brain and no expression in skeletal muscle.

Tamm-Horsfall glycoprotein: biology and clinical relevance

Tamm-Horsfall glycoprotein (THP) is the most abundant urinary protein in mammals. Urinary excretion occurs by proteolytic cleavage of the large ectodomain of the glycosyl phosphatidylinositol-anchored counterpart exposed at the luminal cell surface of the thick ascending limb of Henle's loop. We describe the physical-chemical structure of human THP and its biosynthesis and interaction with other proteins and leukocytes. The clinical relevance of THP reported here includes: (1) involvement in the pathogenesis of cast nephropathy, urolithiasis, and tubulointerstitial nephritis; (2) abnormalities in urinary excretion in renal diseases; and (3) the recent finding that familial juvenile hyperuricemic nephropathy and autosomal dominant medullary cystic kidney disease 2 arise from mutations of the THP gene. We critically examine the literature on the physiological role and mechanism(s) that promote urinary excretion of THP. Some lines of research deal with the in vitro immunoregulatory activity of THP, termed uromodulin when isolated from urine of pregnant women. However, an immunoregulatory function in vivo has not yet been established. In the most recent literature, there is renewed interest in the capacity of urinary THP to compete efficiently with urothelial cell receptors, such as uroplakins, in adhering to type 1 fimbriated Escherichia coli. This property supports the notion that abundant THP excretion in urine is promoted in the host by selective pressure to obtain an efficient defense against urinary tract infections caused by uropathogenic bacteria.

Structurally distinct requirements for binding of P-selectin glycoprotein ligand-1 and sialyl Lewis x to Anaplasma phagocytophilum and P-selectin

Colonization of neutrophils by the bacterium Anaplasma phagocytophilum causes the disease human granulocytic ehrlichiosis. The pathogen also infects mice, its natural host. Like binding of P-selectin, binding of A. phagocytophilum to human neutrophils requires expression of P-selectin glycoprotein ligand-1 (PSGL-1) and alpha1-3-fucosyltransferases that construct the glycan determinant sialyl Lewis x (sLex). Binding of A. phagocytophilum to murine neutrophils, however, requires expression of alpha1-3-fucosyltransferases but not PSGL-1. To further characterize the molecular features that A. phagocytophilum recognizes, we measured bacterial binding to microspheres bearing specific glycoconjugates or to cells expressing human PSGL-1 and particular glycosyltransferases. Like P-selectin, A. phagocytophilum bound to purified human PSGL-1 and to glycopeptides modeled after the N terminus of human PSGL-1 that presented sLex on an O-glycan. Unlike P-selectin, A. phagocytophilum bound to glycopeptides that contained sLex but lacked tyrosine sulfation or a specific core-2 orientation of sLex on the O-glycan. A. phagocytophilum bound only to glycopeptides that contained a short amino acid sequence found in the N-terminal region of human but not murine PSGL-1. Unlike P-selectin, A. phagocytophilum bound to cells expressing PSGL-1 in cooperation with sLex on both N-and O-glycans. Moreover, bacteria bound to microspheres coupled independently with glycopeptide lacking sLex and with sLex lacking peptide. These results demonstrate that, unlike P-selectin, A. phagocytophilum binds cooperatively to a nonsulfated N-terminal peptide in human PSGL-1 and to sLex expressed on PSGL-1 or other glycoproteins. Distinct bacterial adhesins may mediate these cooperative interactions.

Glycosylation of the materno-foetal interface in the pregnant viviparous placentotrophic lizard Chalcides chalcides: a lectin histochemical study

Glycosylation of the foeto-maternal interface of the skink Chalcides chalcides has been examined at various stages of gestation using lectin histochemistry. Specimens of incubatory chamber or placenta from early, mid-, late- and near-term pregnancy were fixed and embedded in epoxy resin. Areas of foeto-maternal apposition were probed with a panel of biotinylated lectins followed by an avidin-peroxidase revealing system to identify various classes of glycan at the interface. Both the external epithelium of unspecialized bilaminar omphalopleure, which forms by early pregnancy, and chorioallantoic membrane which develops by mid-pregnancy, were composed of two phenotypes, one of which secreted a wide range of glycans including high mannose and complex N-glycan, N-acetyl glucosamine, lactosamine and galactosamine, which became less prominent from mid-pregnancy onwards. The uterine epithelium also contained a well-developed secretory apparatus producing a similar range of glycans and there were indications that glycosylated secretions were taken up by the overlying chorioallantois. Foetal vasculature was well developed while maternal vessels appeared more contracted, and both were richly sialylated like their therian equivalents. Our findings indicate that this reptile has evolved a true epitheliochorial placenta with many aspects in common with its therian counterparts but also with unique features of its own.

Asparagine-linked oligosaccharide-independent secretion of egg envelope glycoprotein ZPC of the Japanese quail (Coturnix japonica)

In avian species, a glycoprotein homologous to mammalian ZPC is synthesized in the granulosa cells of developing follicles. We have previously reported that the newly synthesized ZPC (proZPC) in the granulosa cells is cleaved at the consensus furin cleavage site to generate mature ZPC prior to secretion. In the present study, we examined the role of asparagine (N)-linked oligosaccharides in the proteolytic processing of proZPC and the subsequent secretion of ZPC by using site-directed mutagenesis of the consensus sequence for N-glycosylation, and tunicamycin, an inhibitor for N-glycosylation of glycoprotein. Western blot analysis demonstrated that tunicamycin did not block either proteolytic cleavage of proZPC or the subsequent ZPC secretion. Moreover, a site-directed mutant that possesses a mutated sequence for N-glycosylation was efficiently secreted from the cells. These results indicate that proteolytic cleavage of proZPC, and the subsequent ZPC secretion occur in the absence of N-linked oligosaccharides. Therefore, the addition of N-glycans to ZPC polypeptide is not required for quail ZPC secretion.

The heterogeneity of mast cell tryptase from human lung and skin

There has long been conjecture over the degree to which there may be structural and functional heterogeneity in the tetrameric serine protease tryptase (EC 3.4.21.59), a major mediator of allergic inflammation. We have applied 2D gel electrophoresis to analyze the extent, nature, and variability of this heterogeneity in lysates of mast cells isolated from lung and skin, and in preparations of purified tryptase. Gels were silver stained, or the proteins transferred to nitrocellulose blots and probed with either tryptase-specific monoclonal antibodies or various lectins. Tryptase was the major protein constituent in mast cell lysates, and presented as an array of 9-12 diffuse immunoreactive spots with molecular masses ranging from 29 to 40 kDa, and pI values from 5.1 to 6.3. Although the patterns obtained for lung and skin tryptase were broadly similar, differences were observed between tissues and between individual donors. Lectin binding studies indicated the presence of mono-antennary or bi-antennary complex-type oligosaccharide with varying degrees of sialylation. Deglycosylation with protein-N-glycosidase F (PNGase F) reduced the size of both lung and skin tryptase, while incubation with PNGase F or neuraminidase narrowed the pI range, indicating variable degrees of glycosylation as a major contributor to the size and charge heterogeneity. Comparison of different purified preparations of lung and skin tryptase revealed no significant difference in pH profiles, but differences were seen in reactivity towards a range of chromogenic substrates, with substantial differences in Km, kcat and degree of cooperativity. Mathematical modeling indicated that the variety in kinetics parameters could not result solely from the sum of varying amounts of isoforms obeying Michaelis-Menten kinetics but with different values of Km and kcat. The heterogeneity demonstrated for tryptase in these studies suggests that there are important differences in tryptase function in different tissues.

UDP-N-acetylglucosamine:alpha-6-D-mannoside beta1,6 N-acetylglucosaminyltransferase V (Mgat5) deficient mice

Targeted gene mutations in mice that cause deficiencies in protein glycosylation have revealed functions for specific glycans structures in embryogenesis, immune cell regulation, fertility and cancer progression. UDP-N-acetylglucosamine:alpha-6-D-mannoside beta1,6 N-acetylglucosaminyltransferase V (GlcNAc-TV or Mgat5) produces N-glycan intermediates that are elongated with poly N-acetyllactosamine to create ligands for the galectin family of mammalian lectins. We generated Mgat5-deficient mice by gene targeting methods in embryonic stem cells, and observed a complex phenotype in adult mice including susceptibility to autoimmune disease, reduced cancer progression and a behavioral defect. We found that Mgat5-modified N-glycans on the T cell receptor (TCR) complex bind to galectin-3, sequestering TCR within a multivalent galectin-glycoprotein lattice that impedes antigen-dependent receptor clustering and signal transduction. Integrin receptor clustering and cell motility are also sensitive to changes in Mgat5-dependent N-glycosylation. These studies demonstrate that low affinity but high avidity interactions between N-glycans and galectins can regulate the distribution of cell surface receptors and their responsiveness to agonists.

Biological consequences of overexpressing or eliminating N-acetylglucosaminyltransferase-TIII in the mouse

N-acetylglucosaminyltransferase III (GlcNAc-TIII), a product of the human MGAT3 gene, was discovered as a glycosyltransferase activity in hen oviduct. GlcNAc-TIII transfers GlcNAc in beta4-linkage to the core Man of complex or hybrid N-glycans, and thereby alters not only the composition, but also the conformation of the N-glycan. The dramatic consequences of the addition of this bisecting GlcNAc residue are reflected in the altered binding of lectins that recognize Gal residues on N-glycans. Changes in GlcNAc-TIII expression correlate with hepatoma and leukemia in rodents and humans, and the bisecting GlcNAc on Asn 297 of human IgG antibodies enhances their effector functions. Overexpression of a cDNA encoding GlcNAc-TIII alters growth control and cell-cell interactions in cultured cells, and in transgenic mice. While mice lacking GlcNAc-TIII are viable and fertile, they exhibit retarded progression of diethylnitrosamine (DEN)-induced liver tumors. Further biological functions of GlcNAc-TIII are expected to be uncovered as mice with a null mutation in the Mgat3 gene are challenged.

Inhibition of proliferation and induction of differentiation of glioma cells with Datura stramonium agglutinin

We found that a lectin, Datura stramonium agglutinin, induced irreversible differentiation in C6 glioma cells. The differentiated cells had long processes, a low rate of proliferation and a high content of glial fibrillary acidic protein. When the medium was replaced with Datura stramonium agglutinin-free medium after 1 h, cell proliferation continued to be inhibited. Experiments with several other lectins indicated that both recognition of linear N-acetyllactosamine repeats and recognition of multiantennary units of cell-surface glycans were required for the inhibition of C6 proliferation. Proliferation of four human glial tumour cells was also inhibited by Datura stramonium agglutinin. Further, these differentiated human glial tumour cells had long processes and a high content of glial fibrillary acidic protein similar to differentiated C6 glioma cells. Taken together, these observations suggest that Datura stramonium agglutinin may be useful as a new therapy for treating glioma without side effects.

The high specificities of Phaseolus vulgaris erythro- and leukoagglutinating lectins for bisecting GlcNAc or beta 1-6-linked branch structures, respectively, are attributable to loop B

Despite very similar tertiary structures based upon a common framework, legume lectins exhibit an amazing variety of sugar binding specificities. While most of these lectins recognize rather discrete sugar linkages, Phaseolus vulgaris erythroagglutinating and leukoagglutinating lectins (E(4)- and L(4)-PHA) are unique in recognizing larger structures. E(4)- and L(4)-PHA are known to recognize complex type N-glycans containing bisecting GlcNAc or a beta1,6-linked branch, respectively. However, the detailed mechanisms of molecular recognition are poorly understood. In order to dissect the contributions of different portions of each lectin, we carried out region-swapping mutagenesis between E(4)- and L(4)-PHA. We prepared six chimeric lectins by exchanging different combinations of loop B and the central portion of loop C, two of four loops thought to be important for the recognition of monosaccharides (Sharma, V., and Surolia, A. (1997) J. Mol. Biol. 267, 433-445). The chimeric lectins' sugar binding activities were evaluated quantitatively by surface plasmon resonance. These comparisons indicate that the high specificities of E(4)- and L(4)-PHA toward bisecting GlcNAc and beta1,6-linked branch structures are almost solely attributable to loop B. The contribution of the central portion of loop C to the recognition of those structural motifs was found to be negligible. Instead, it modulates affinity toward LacNAc residues present at the nonreducing terminus. Moreover, some of the chimeric lectins prepared in this study showed even higher specificities/affinities than native E(4)- and L(4)-PHA toward complex sugar chains containing either a bisecting GlcNAc residue or a beta1,6-linked branch.

Prometastatic effect of N-acetylglucosaminyltransferase V is due to modification and stabilization of active matriptase by adding beta 1-6 GlcNAc branching

Oligosaccharide moieties of glycoproteins are structurally altered during development, carcinogenesis, and malignant transformations. It is well known that beta1-6 GlcNAc branching, a product of UDP-GlcNAc alpha-mannoside beta1-6-N-acetylglucosaminyltransferase (GnT-V), is associated with malignant transformation as the results of such alterations. However, the mechanism by which beta1-6 GlcNAc branching is linked to metastasis remains unclear, because the identification of specific glycoprotein(s) that are glycosylated by GnT-V and its biological function have not been examined. We herein report that matriptase, which activates both urokinase-type plasminogen activator and hepatocyte growth factor, is a target protein for GnT-V. The overexpression of GnT-V in gastric cancer cells leads to severe peritoneal dissemination in athymic mice, which can be attributed to the increased expression of matriptase. This increase was due to the acquired resistance of matriptase to degradation, since it is glycosylated by GnT-V and a corresponding increase in the active form. These results indicate that this process is a key element in malignant transformation, as the direct result of oligosaccharide modification.

Expression of bisecting N-acetylglucosaminyltransferase-III in human hepatocarcinoma tissues, fetal liver tissues, and hepatoma cell lines of Hep3B and HepG2

In this paper, uridine diphosphate (UDP)-N-acetylglucosamine/beta-D-mannoside beta-1,4 N-acetylglucosaminyltransferase III (GlcNAc-transferase-III C 2.4.1.144) activity was determined in human hepatoma cell lines of Hep3B and HepG2, and also compared with those of normal liver tissues and primary hepatocytes. GlcNAc-transferase-III enzymes of Hep3B and HepG2 were mainly detected in the membrane fraction. When GlcN,GlcN-biant-PA and UDP-GlcNAc were used as substrates, the Km values (4.7 mM for UDP-GlcNAc and 1.1 mM for GlcN, GlcN-biant-PA) of Hep3B GlcNAc-transferase-III were distinguishable from those of HepG2 GlcNAc-transferase-III (6.8 mM for UDP-GlcNAc and 3.4 mM for GlcN,GlcN-biant-PA). Furthermore, Hep3B enzyme in membrane fraction showed about 1.5-fold higher specific activity (1423 pmol/hr/mg) than that of HepG2 (1066 pmol/hr/mg). Normal liver cells and primary adult hepatocytes are characterized by a very low level of GlcNAc-transferase-III activity, whereas human hepatoma cells exhibited high activities. These data were supported by reverse transcription-polymerase chain reaction results, showing that expression of the GlcNAc-transferase-III mRNA increased in proportion to the enzymatic activities. Although the mechanism underlying the induction of this enzyme is unknown, lectin blot analysis showed that oligosaccharides in many glycoproteins were observed in hepatoma cells. By treating hepatocarcinoma cultures that express GlcNAc-transferase-III with inhibitors (tunicamycin, deoxymannojirimycin, and swainsonine) of different steps of the glycosylation, we provide evidence that expression of GlcNAc-transferase-III mRNA is dependent on glycosylation of cellular proteins.

Proteins and saccharides of the sea urchin organic matrix of mineralization: characterization and localization in the spine skeleton

Properties of the echinoderm skeleton are under biological control, which is exerted in part by the organic matrix embedded in the mineralized part of the skeleton. This organic matrix consists of proteins and glycoproteins whose carbohydrate component is specifically involved in the control mechanisms. The saccharide moiety of the organic matrix of the spines of the echinoid Paracentrotus lividus was characterized using enzyme-linked lectin assays (ELLAs). O-glycoproteins, different types of complex N-glycoproteins, and terminal sialic acids were detected. Sialic acids are known to interact with Ca ions and could play an important role in the mineralization process. Some of the carbohydrate components detected by ELLAs as well as two organic matrix proteins (SM30 and SM50) were localized within different subregions of the spine skeleton using field-emission scanning electron microscopy. The mappings show that some of these components are not homogeneously distributed in the different skeletal subregions. For example, some N-glycoproteins were preferentially located in the putative amorphous subregion of the skeleton, whereas some O-glycoproteins were localized in the subregion where skeletal growth is inhibited. These results suggest that the biological control exerted on the skeletal properties can be partly modulated by local differences in the organic matrix composition.

Comparison of sucrase-free isomaltase with sucrase-isomaltase purified from the house musk shrew Suncus murinus

We purified sucrase-isomaltase and sucrase-free isomaltase from a normal and a sucrase-deficient line, respectively, of the house musk shrew Suncus murinus and examined the effects of mutation on enzyme structure and activities. Recent cDNA cloning studies have predicted that sucrase-free mutant isomaltase lacks the C-terminal 69 amino acids of normal isomaltase, as well as the entire sucrase. On SDS-polyacrylamide gel electrophoresis purified sucrase-free isomaltase gave a single protein band of 103 kDa, while sucrase-isomaltase gave two major protein bands of 106 and 115 kDa. The 115, but not 106, kDa band was quite similar to the 103 kDa band on Western blotting with Aleuria aurantia lectin and antibody against shrew sucrase-isomaltase, suggesting that the 115 and 103 kDa bands are due to normal and mutant isomaltases, respectively, in accordance with the above prediction. Purified isomaltase and sucrase-isomaltase were similar in Km and Vmax (based on isomaltase mass) values for isomaltose hydrolysis and in inhibition of isomaltase activity by antibody against rabbit sucrase-isomaltase, suggesting that the enzymatic properties of isomaltase are mostly unaffected by mutation.

Differential gene expression of beta-1,4-galactosyltransferases I, II and V during mouse brain development

Since most brain glycoproteins from beta-1,4-galactosyltransferase (beta-1,4-GalT) I knockout mice were galactosylated without apparent reduction the gene expression of novel beta-1,4-GalTs II and V which are involved in N-linked oligosaccharide biosynthesis in addition to beta-1,4-GalT I was studied during mouse brain development. Isolation and characterization of beta-1,4-GalT II and V cDNAs from mouse brains indicates that they are also functioning in the brain. Northern blot analysis revealed that the beta-1,4-GalT I gene is expressed mainly in mid-embryonic stages, while the expression level of beta-1,4-GalT II transcript remains constant and of beta-1,4-GalT V transcript increases during mouse brain development after birth. In situ hybridization revealed that beta-1,4-GalT II and V signals are present in most neural cells, with a marked difference between them in the hippocampus of adult mouse brain tissue. The differential gene expression of beta-1,4-GalTs I, II and V during mouse brain development could affect the differential galactosylation of brain glycoproteins, as revealed by lectin blot analysis.

Tumor-associated carbohydrate antigens defining tumor malignancy: basis for development of anti-cancer vaccines

Tumors expressing a high level of certain types of tumor-associated carbohydrate antigens (TACAs) exhibit greater metastasis and progression than those expressing low level of TACAs, as reflected in decreased patient survival rate. Well-documented examples of such TACAs are: (i) H/Le(y)/Le(a) in primary non-small cell lung carcinoma; (ii) sialyl-Le(x) (SLe(x)) and sialyl-Le(a) (SLe(a)) in various types of cancer; (iii) Tn and sialyl-Tn in colorectal, lung, breast, and many other cancers; (iv) GM2, GD2, and GD3 gangliosides in neuroectodermal tumors (melanoma and neuroblastoma); (v) globo-H in breast, ovarian, and prostate cancer; (vi) disialylgalactosylgloboside in renal cell carcinoma. Some glycosylations and TACAs suppress invasiveness and metastatic potential. Well-documented examples are: (i) blood group A antigen in primary lung carcinoma; (ii) bisecting beta1 --> 4GlcNAc of N-linked structure in melanoma and other cancers; (iii) galactosylgloboside (GalGb4) in seminoma. The biochemical mechanisms by which the above glycosylation changes promote or suppress tumor metastasis and invasion are mostly unknown. A few exceptional cases in which we have some knowledge are: (i) SLe(x) and SLe(a) function as E-selectin epitopes promoting tumor cell interaction with endothelial cells; (ii) some tumor cells interact through binding of TACA to specific proteins other than selectin, or to specific carbohydrate expressed on endothelial cells or other target cells (carbohydrate-carbohydrate interaction); (iii) functional modification of adhesive receptor (integrin, cadherin, CD44) by glycosylation. So far, a few successful cases of anti-cancer vaccine in clinical trials have been reported, employing TACAs whose expression enhances malignancy. Examples are STn for suppression of breast cancer, GM2 and GD3 for melanoma, and globo-H for prostate cancer. Vaccine development canbe extended using other TACAs, with the following criteria for success: (i) the antigen is expressed highly on tumor cells; (ii) high antibody production depending on two factors: (a) clustering of antigen used in vaccine; (b) choice of appropriate carrier protein or lipid; (iii) high T cell response depending on choice of appropriate carrier protein or lipid; (iv) expression of the same antigen in normal epithelial tissues (e.g., renal, intestinal, colorectal) may not pose a major obstacle, i.e., these tissues are not damaged during immune response. Idiotypic anti-carbohydrate antibodies that mimic the surface profile of carbohydrate antigens, when administered to patients, elicit anti-carbohydrate antibody response, thus providing an effect similar to that of TACAs for suppression of tumor progression. An extension of this idea is the use of peptide mimetics of TACAs, based on phage display random peptide library. Although examples are so far highly limited, use of such "mimotopes" as immunogens may overcome the weak immunogenicity of TACAs in general.

A novel carbohydrate binding activity of annexin V toward a bisecting N-acetylglucosamine

A bisecting GlcNAc-binding protein was purified from a Triton X-100 extract of a porcine spleen microsomal fraction using affinity chromatography, in conjunction with an agalacto bisected biantennary sugar chain-immobilized Sepharose. Since the erythroagglutinating phytohemagglutinin (E-PHA) lectin preferentially binds to sugar chains which contain the bisecting GlcNAc, during purification the binding activity of the protein was evaluated by monitoring the inhibition of lectin binding to the N-acetylglucosaminyltransferase III (GnT-III)-transfected K562 cells which express high levels of the bisecting GlcNAc. The molecular mass of the purified protein was found to be 33 kDa, as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. By sequencing analysis, the isolated protein was identified as annexin V. Flow cytometric analysis showed that fluorescein-labeled annexin V binds to the GnT-III-transfected cells but not to mock cells, and that the binding was not affected by the addition of phospholipids. Furthermore, surface plasmon resonance measurements indicated that annexin V binds to the agalacto bisected biantennary sugar chain with a K(d) of 200 microM while essentially no binding was observed in the case of the corresponding non-bisected sample. These results suggest that annexin V has a novel carbohydrate binding activity and may serve as an endogenous lectin for mediating possible signals of bisecting GlcNAc, which have been implicated in a variety of biological functions.

Both cell-surface carbohydrates and protein tyrosine phosphatase are involved in the differentiation of astrocytes in vitro

Astrocytes are important in the development and maintenance of functions of the CNS, acting in cooperation with neurons and other glial cells. The glycans on astrocyte membrane are believed to play important roles in cell-cell communication. Plant lectins are useful probes, because the lectins can bind to certain cell surface receptors and elicit cellular responses that are normally activated by endogenous ligands for those receptors. In the present study, we investigated the effect of Datura stramonium agglutinin (DSA) on astrocytes and characterized several molecular events. The addition of DSA to a culture of flat, polygonal, immature astrocytes derived from the neonatal rat cerebellum caused the cells to become stellate in shape, similar to astrocytes observed in vivo, concomitant with an increase in expression of astrocyte-specific intermediate filament (glial fibrillary acidic protein [GFAP]) and inhibition of proliferation. These results indicate that DSA binds to astrocytes and triggers differentiation. We also found a decrease in the extent of tyrosine-phosphorylation of a 38-kDa protein. To elucidate the molecular events during astrocyte differentiation, we examined the effects of various signal transduction inhibitors on the transformation from the polygonal to stellate shape (stellation). Interestingly, only tyrosine phosphatase inhibitors, orthovanadate and phenylarsine oxide, showed an inhibitory effect. Our results suggest that DSA induced astrocyte differentiation acts via tyrosine dephosphorylation.

CEA is the major PHA-L-reactive glycoprotein in colon carcinoma cell lines and tumors: relationship between K-ras activation and beta1-6 branching of N-linked carbohydrate on CEA

Previously we have shown that a positive correlation existed between the presence of beta1-6 branching of N-linked carbohydrate (detected as PHA-L reactivity) and the level of Ras activation in colon carcinoma cell lines. In these cell lines the major PHA-L-reactive species was found to be 180 kDa. Here we identified this species to be carcinoembryonic antigen (CEA) by demonstrating that: (a) CEA immunoreactivity and PHA-L reactivity colocalized on blots of crude cellular membranes from these cell lines, and that (b) immunoprecipitation of CEA resulted in quantitative coprecipitation of PHA-L reactivity at 180 kDa. Metabolic labeling of cell line HTB39 with [(3)H]mannose revealed that CEA was the predominantly labeled glycoprotein. This indicated that CEA was the major PHA-L-reactive species due its high level of expression. The amount of PHA-L reactivity present on CEA, expressed as the PHA-L/CEA ratio, was found to vary between cell lines. This ratio was found to correlate closely with the level of Ras activation in these cells. In cellular membrane isolated from primary colon carcinoma, the major PHA-L-reactive species was also 180 kDa. This reactivity colocalized with CEA immunoreactivity, indicating that the major beta1-6-branching glycoprotein in membranes from primary colon carcinoma was CEA. Similar to that seen in cell lines, the amount of PHA-L reactivity on CEA in human tumor samples varied, suggesting that a similar paradigm of Ras-induced expression of beta1-6 branching may occur in human colon carcinoma.

Unusual N-glycosylation of a recombinant human erythropoietin expressed in a human lymphoblastoid cell line does not alter its biological properties

Erythropoietin (Epo) is a 166 amino acids protein containing three N-glycosylation sites (Asn-24, Asn-38, and Asn-83) and 1 O- glycosylation site (Ser-126) and involved in the regulation of the level of red blood cells. Today, only one recombinant human Epo (rHuEpo), produced in CHO cell line, is extensively used in therapy to cure severe anemia. The structure of the glycan chains of this rHuEpo slightly differ of those of the urinary human Epo (uHuEpo), considered as the natural Epo molecule. In an attempt to produce a rHuEpo as close as possible to the uHuEpo, Epo gene was expressed in a human lymphoblastoid cell line, named RPMI 1788. In order to fully characterize the Epo-RPMI, structural characterizations of the protein skeleton as well as glycan chains were undergone. As expected, the amino acid sequence of the Epo-RPMI conformed to that of uHuEpo. Surprisingly, the structure of some N-glycan chains, as mainly determined by ESI-MS, revealed some unusual characteristics. Thus, 80% of N-glycans possess a bisecting GlcNAc residue, 25% bear a second fucose residue which is present, in a large part, in a sialyl Le(x)motif, and 13% contain more than three LacNAc repeats (up to five per molecule). Despite these unusual structural characteristics, the data concerning the in vitro and in vivo biological activities were not impaired when compared to Epo-CHO and uHuEpo.

A homogeneous cell-based assay to identify N-linked carbohydrate processing inhibitors

Malignant transformation is accompanied by altered cell surface glycosylation. N-Linked oligosaccharides carrying beta1-6GlcNAc branches are associated with tumor invasion and metastasis. Therefore, compounds that can enter cells and block biosynthesis of beta1-6GlcNAc-branched glycans without overt cytotoxicity are potential anticancer agents. We have developed a homogeneous cell-based assay for detection of such compounds. The method enables identification of agents that block beta1-6GlcNAc-branched glycan expression after incubation for 16-20 h with MDAY-D2 tumor cells, thereby protecting the cells from the subsequent addition of leukoagglutinin, a cytotoxic plant lectin. We observed that MDAY-D2 cell number is directly proportional to the level of endogenous alkaline phosphatase activity measured spectrophotometrically in cultures after the addition of substrate. The alkaline phosphatase assay was capable of detecting as few as 1,500 cells. The assay was readily adapted for high-throughput screening as reagent costs are low and no cell harvesting and washing steps are required. Under high-throughput operating conditions, the coefficient of variation for controls was found to be 4.2%. The results suggest that measurement of alkaline phosphatase in this cell assay format may be adapted for wider applications in high-throughput screenings for compounds that relieve cells from other growth inhibitors.

Suppression of tumor growth and metastasis in Mgat5-deficient mice

Golgi beta1,6N-acetylglucosaminyltransferase V (MGAT5) is required in the biosynthesis of beta1,6GlcNAc-branched N-linked glycans attached to cell surface and secreted glycoproteins. Amounts of MGAT5 glycan products are commonly increased in malignancies, and correlate with disease progression. To study the functions of these N-glycans in development and disease, we generated mice deficient in Mgat5 by targeted gene mutation. These Mgat5-/- mice lacked Mgat5 products and appeared normal, but differed in their responses to certain extrinsic conditions. Mammary tumor growth and metastases induced by the polyomavirus middle T oncogene was considerably less in Mgat5-/- mice than in transgenic littermates expressing Mgat5. Furthermore, Mgat5 glycan products stimulated membrane ruffling and phosphatidylinositol 3 kinase-protein kinase B activation, fueling a positive feedback loop that amplified oncogene signaling and tumor growth in vivo. Our results indicate that inhibitors of MGAT5 might be useful in the treatment of malignancies by targeting their dependency on focal adhesion signaling for growth and metastasis.

Isolation and characterization of glycophorin from nucleated (chicken) erythrocytes

A sialoglycoprotein fraction was isolated from chicken erythrocytes by two methods based on the phenol extraction or chloroform/2-propanol extraction of differently prepared erythrocyte membranes. Both preparations gave in SDS-PAGE two major PAS-stained bands (GP2 and GP3), which migrated as 60- and 33-kDa species, respectively, compared to reference proteins, or as 44- and 23-kDa molecules, compared to human glycophorins. Some less abundant slower migrating PAS-stained components, antigenically related to GP2 and GP3, also were detected. No evidence for the presence of antigenically distinct glycoproteins of leukosialin type was obtained. Interconversion in SDS-PAGE, similar carbohydrate composition, and similar antigenic properties of GP2 and GP3 indicated that they are a dimer and monomer, respectively, of the same glycoprotein which shows properties that allow it to be classified as a glycophorin. Lectin binding studies and methylation analysis of beta-elimination products of chicken glycophorin preparation showed the presence of O-glycans and N-glycans. The major O-glycans include sialylated Galbeta1-3GalNAc units and more complex GlcNAc-containing chains. Among the N-glycans, there are complex-type biantennary structures with a bisecting GlcNAc residue, accompanied by chains with additional antennas linked to alpha-mannose residues. A characteristic feature of the chicken glycophorin is a relatively high proportion of N-glycans to O-glycans, compared to the glycophorin A from human erythrocytes.

Characterization of progressive changes in ZPC of the vitelline membrane of quail oocyte following oviductal transport

The inner layer of the vitelline membrane of avian oocyte is equivalent to the zona pellucida of mammalian oocytes or to the vitelline envelope of amphibian oocytes. One of the two major glycoproteins in the inner layer of quail vitelline membrane, formerly called 33-kDa glycoprotein, is homologous to mammalian ZPC, one of the components of zona pellucida. Quail ZPC is found to have different mobilities on SDS-polyacrylamide gel electrophoresis depending on whether it is obtained from the preovulatory follicle or from the laid eggs. In order to characterize the progressive changes in the molecular size of quail ZPC during the oviductal transport, the inner layer isolated from the follicle was incubated in vivo in various regions of the oviduct and subjected to Western blot analysis with anti-quail ZPC antiserum. The quail ZPC of the inner layer incubated in infundibulum reduced its apparent molecular weight, exhibiting the same electrophoretic mobility as that of laid eggs. The similar reduction in molecular weight was observed after the in vitro incubation of the inner layer with the extracts of infundibulum. From the comparison of the N-terminal amino acid sequences, it was found that the first 26 residues of the quail ZPC in follicular oocytes are missing from the ZPC of laid eggs. In addition, lectin blot analysis suggested the modification of oligosaccharide chains during the oviductal transport. These results represent the first description in the avian oviduct of the presence of protease, which is similar to oviductin, a trypsin-like protease involved in the hydrolysis of a major component of the vitelline envelope of amphibian oocytes. Mol. Reprod. Dev. 55:175-181, 2000.

Characterization of lectin resistant cell populations derived from human colon carcinoma: correlation of K-Ras with beta1-6 branching of N-linked carbohydrate and CEA production

Previous studies of cell lines derived from human colon carcinoma showed that the extent of beta1-6 branching on N-linked carbohydrate was associated with the presence of K-ras mutation and Ras-activation. We observed that the extent of Ras-activation in these cell lines depends not only upon the presence of an activating mutation in K-ras, but also on the amount of total K-Ras protein produced. Here we examined whether negative selective pressure by PHA-L against beta1-6 branching could select for cells having a lower level of K-Ras protein and Ras-activation. PHA-L binds specifically to the beta1-6 branch in N-linked carbohydrate. We utilized a K-ras mutant colon carcinoma cell line, HTB39, which had abundant beta1-6 branching and high levels of K-Ras mutant protein. Lectin resistant cell populations of HTB39 were generated and found to have less beta1-6 branching and less K-Ras protein than their parental counterpart. The lectin resistant cell populations produced lower levels of highly glycosylated CEA, which contributed to the lower level of beta1-6 branching in these cells. PHA-L resistant cell populations were two-fold less sensitive than the parental line to an inhibitor of farnesyl transferase (an enzyme essential for Ras processing and function). This suggested a decrease in dependence on K-ras mediated signaling. Collectively, the data indicated that beta1-6 branching of N-linked carbohydrate and CEA production were linked to K-Ras protein synthesis and activation of the Ras-signaling pathway.

Simplified procedure for fractionation and structural characterisation of complex mixtures of N-linked glycans, released from HIV-1 gp120 and other highly glycosylated viral proteins

HIV-1 gp120 is heavily glycosylated containing 24 N-glycosylation sites, and this makes elucidation of the significance of glycans at individual glycosylation sites a difficult task. A procedure is described where a complex mixture of biologically radiolabelled glycans of gp120, derived from a relatively small number of virus-infected cells may be characterized by a combination of N-glycanase release, single lectin separation, and normal phase HPLC (NP-HPLC). The method was applied in analysis of three N-linked glycosylation sites essential for the in vivo priming of T-cells, specific for an epitope in their vicinity (Sjölander, S., Bolmstedt, A., Akerblom, 1996. Virology 215, 124-133.). The carbohydrate compositions of wild type gp120 and of mutant variants gp120 lacking one, two, or all of these three active N-linked glycans were analysed. Cells were infected with r-vaccinia virus expressing wild-type gp120 or mutated gp120, or were infected with HIV-1BRU (wild type) or mutant virus variants. HIV-1 glycoproteins were purified by immunosorbent affinity chromatography and released glycans were separated on lectins, then analysed with NP-HPLC. Our data showed that the structural composition of glycans occupying two of the three glycosylation sites was heterogeneous but the site located adjacent to the T-cell epitope was equipped with one large, high mannose-type structure (> 11 units) with the capacity to cover a substantial part of the gp120 surface.

A novel evaluation system of metastatic potential of oral squamous cell carcinoma according to the histopathological and histochemical grading

We established a new evaluation system for metastatic potential of oral squamous cell carcinoma (SCC), utilizing a combined examination of histopathological grades of the carcinomas based on cell differentiation and invasive mode according to Yamamoto's criteria, and the cellular expressions of CD44, E-cadherin (E-cad), heparan sulfate glycosaminoglycan (HS-GAG) and Phaseolus vulgaris leukoagglutinin (L-PHA)-binding oligosaccharides on the carcinomas. Histochemical patterns of expression of these markers were classified into positive (+2), weakly positive (+), and negative (-). The histopathological grades and the histochemical patterns of the SCC were estimated on a 0-2 point scale, i.e. point 2 for poorly differentiated, mode 4D, CD44++, E-cad-, HS-GAG++, or L-PHA++; point 1 for moderately differentiated, mode 4C, CD44+, E-cad+, HS-GAG+, or L-PHA+; and point 0 for well differentiated, mode 1, mode 2, mode 3, CD44-, E-cad++, HS-GAG-, or L-PHA-. As a result, incidence of metastasis in the cases with a total score of more than 6 (62.8%) was significantly higher than that with a total score of less than 5 (9.3%). This evaluation system will yield useful information concerning the prognosis of patients with oral SCC.

Differential and cooperative polysialylation of the neural cell adhesion molecule by two polysialyltransferases, PST and STX

PST and STX are polysialyltransferases that form polysialic acid in the neural cell adhesion molecule (NCAM), and these two polysialyltransferases often exist together in the same tissues. To determine the individual and combined roles of PST and STX in polysialic acid synthesis, in the present study we asked if PST and STX differ in the acceptor requirement and if PST and STX act together in polysialylation of NCAM. We first examined whether PST and STX differ in the requirement of sialic acid and core structures of N-glycans attached to NCAM. Polysialic acid was formed well on Lec4 and Lec13 cells, which are defective in N-acetylglucosaminyltransferase V and GDP-fucose synthesis, respectively, demonstrating that a side chain elongating from GlcNAcbeta1-->6Manalpha1-->6R and alpha-1,6-linked fucose are not required. PST and STX were found to add polysialic acid on NCAM.Fc molecules sialylated by alpha-2,3- or alpha-2,6-linkage in vitro, but not on NCAM.Fc lacking either sialic acid. These results indicate that both PST and STX have relatively broad specificity on N-glycan core structures in NCAM and no remarkable difference exists between PST and STX for the requirement of core structures and sialic acid attached to the N-glycans of NCAM. We then, using various N-glycosylation site mutants of NCAM, discovered that PST strongly prefer the sixth N-glycosylation site, which is the closest to the transmembrane domain, over the fifth site. STX slightly prefer the sixth N-glycosylation site over the fifth N-glycosylation site. The results also demonstrated that polysialic acid synthesized by PST is larger than that synthesized by STX in vitro. Moreover, a mixture of PST and STX more efficiently synthesized polysialic acid on NCAM than PST or STX alone. These results suggest that polysialylation of NCAM is influenced by the difference between PST and STX in their preference for N-glycosylation sites on NCAM. The results also suggest that PST and STX form polysialylated NCAM in a synergistic manner.

Environmental pollutants as aetiological agents in female reproductive pathology: placental glycan expression in normal and polychlorinated biphenyl (PCB)-exposed mink (Mustela vison)

Polychlorinated biphenyls (PCB) may cause growth retardation or fetal death in mink. Pathological changes in endotheliochorial mink placentae were examined following exposure to PCB during gestation. Placentae from six animals with average fetal crown-rump (C-R) lengths between 16 and 53 mm given 0.65 mg/day Clophen A50 (low dose), and one from five animals with an average fetal C-R length of 14 mm given 1.3 mg/day (high dose), were examined. Mink were treated from 9 to 24 days before mating until killed at day 53. Placentae were formalin-fixed with four size-matched controls and embedded in resin. Sections were stained with five biotinylated lectins to detect specific glycans. Both control and treated (low dose) mink showed degenerative changes in maternal endothelium from 13-16 mm, revealed by increased lectin binding, caused probably by high cell turnover during tissue remodelling. Controls of 47 and 50 mm exhibited fewer degenerate maternal endothelial cells. The 31-mm PCB-treated tissue showed separation of the trophoblast from the interstitial layer and, at 53 mm, loss of its normal architecture, increased damage to maternal endothelium and infarction. High-dose PCB was extremely toxic, producing fetal death or extensive placental infarction by 14 mm C-R length. Lectin staining thus revealed the effects of PCB toxicity, shown by increased injury to maternal endothelium and severe trophoblastic damage.

Characterization of glycoproteins in the epithelial cells of human and other mammalian gallbladder. A review

The mammalian gallbladder mucosa is lined by a simple columnar epithelium. Typical surface epithelial cells (principal cells) contain short microvilli, secretory granules, dense bodies, mitochondria and Golgi apparatus. Dense bodies are thought to be lysosomes. Secretory granules contain mucous glycoproteins which are released to the lumen by exocytosis. Oligosaccharide side chains of mucous glycoproteins may provide a favorable environment for nucleation of cholesterol in gallstone formation; therefore they have been studied during the past decades. Histochemical techniques allow the in situ identification of carbohydrates at both the cellular and subcellular levels. The oligosaccharide chains of principal cell mucous glycoproteins have been studied by classical histochemical techniques (PAS, alcian blue, HID, etc). These techniques indicate that mammalian gallbladder mucous glycoproteins are heavily sulphated, whereas sialic acid residues are scarce. Neutral mucins have not been described in the mammalian gallbladder. Electron microscopic studies have located the oligosaccharide chains in secretory granules and Golgi apparatus. More recently, lectins (molecules which specifically recognize and bind with different saccharides or saccharide sequences) have been applied for the intracellular localization of carbohydrate residues. Lectin histochemistry has detected fucose, galactose, N-acetylglucosamine, N-acetylgalactosamine and N-acetylneuraminic acid residues in mucous granules, Golgi apparatus and apical membrane of human principal cells. Mannose residues were observed only in dense bodies. The combined use of deglycosylation procedures and lectin histochemistry has revealed a variety of terminal sequences in oligosaccharide chains of gallbladder mucous glycoproteins: Neu5Ac(alpha 2-3)Gal(beta 1-3)GalNAc, Neu5Ac(alpha 2-3)Gal(beta 1-4)GlcNAc and Gal(beta 1-4)GlcNAc. This technology also suggested the occurrence of N-linked oligosaccharides in the dense bodies of principal cells. Mucous granules mainly contained mucin-type O-linked oligosaccharides although some N-linked chains have also been detected. Gallstone formation is probably a complex process depending on multiple factors. Mucous glycoproteins are one of the factors involved in this process. Histochemical methods offer an excellent research tool for the characterization of glycoproteins in the epithelial cells of the gallbladder, thus contributing to the elucidation of the pathophysiology of gallstone formation.