Comparative study of glycosyltransferase activities in Caco-2 cells before and after enterocytic differentiation using lectin-affinity high-performance liquid chromatography.

Human colonic adenocarcinoma Caco-2 cells differentiate into enterocytes by induction with sodium butyrate after confluence. Our previous studies have shown that there are high levels of H type 1 blood group antigen and core 2 structure present in O-glycans of the glycoproteins from these differentiated cells and these O-glycans appear to be indispensable for the process of differentiation of the cells (J. Amano and M. Oshima, 1999, J. Biol. Chem. 274, 21209-21216). Here, we have determined the glycosyltransferase activities using lectin-affinity HPLC because the method enabled easy separation and identification of mixtures of isomeric oligosaccharide structures due to the high resolution and reproducibility. The activities of beta 3-galactosyltransferase, alpha 2-fucosyltransferase, which are responsible for H type 1 antigen biosynthesis, and core 2 beta 6-N-acetylglucosaminyltransferase in differentiated Caco-2 cells were higher than those in undifferentiated cells. These results demonstrate that an increase in specific glycosyltransferase activities brought on a change of the O-glycan structures during differentiation.     

The action of TNFalpha and TGFbeta include specific alterations of the glycosylation of bovine and human chondrocytes

Joint destruction in arthritis is often associated with high levels of inflammatory cytokines. Previous work has shown that inflammatory conditions can alter the activities of glycosyltransferases that synthesize the glycan chains of glycoproteins, and that these changes in turn can influence the functions of glycoproteins. We therefore examined glycosyltransferases involved in glycoprotein biosynthesis in primary cultures of bovine articular chondrocytes and human chondrocytes isolated from knee cartilage of osteoarthritis patients. Bovine chondrocytes exhibited enzyme activities involved in the synthesis of bi-antennary complex Asn-linked N-glycans, as well as the enzymes involved in the synthesis of GalNAc-Ser/Thr-linked O-glycans with the core 1 structure. Human chondrocytes, in addition, were able to synthesize more complex O-glycans with core 2 structures. TNFalpha was found to induce apoptosis in chondrocytes, and this process was associated with significant changes in lectin binding to chondrocyte cell surface glycans. TGFbeta increased cell proliferation, and had significant effects on cell surface glycosylation in bovine but not in human cells. These cytokine-specific effects were partially correlated with changes in glycosyltransferase activities. Thus, chondrocytes have many of the enzymes necessary for the synthesis of N- and O-glycan chains of glycoproteins. The O-glycosylation pathways and the effects of TNFalpha and TGFbeta on glycosylation differed between bovine and human chondrocytes. These alterations are of potential importance for the regulation of the functions of cell surface receptors on chondrocytes, and for an understanding of the pathophysiology of arthritis.     

Glycosyltransferases involved in type 1 chain and Lewis antigen biosynthesis exhibit glycan and core chain specificity

Sialyl Lewis A (SLe(a)), Lewis A (Le(a)), and Lewis B (Le(b)) have been studied in many different biological contexts, for example in microbial adhesion and cancer. Their biosynthesis is complex and involves beta1,3-galactosyltransferases (beta3Gal-Ts) and a combined action of alpha2- and/or alpha4-fucosyltransferases (Fuc-Ts). Further, O-glycans with different core structures have been identified, and the ability of beta3Gal-Ts and Fuc-Ts to use these as substrates has not been resolved. Therefore, to examine the in vivo specificity of enzymes involved in SLe(a), Le(a), and Le(b) synthesis, we have transiently transfected CHO-K1 cells with relevant human glycosyltransferases and, on secreted reporter proteins, detected the resulting Lewis antigens on N- and O-linked glycans using western blotting and Le-specific antibodies. beta3Gal-T1, -T2, and -T5 could synthesize type 1 chains on N-linked glycans, but only beta3Gal-T5 worked on O-linked glycans. The latter enzyme could use both core 2 and core 3 precursor structures. Furthermore, the specificity of FUT5 and FUT3 in Le(a) and Le(b) synthesis was different, with FUT5 fucosylating H type 1 only on core 2, but FUT3 fucosylating H type 1 much more efficient on core 3 than on core 2. Finally, FUT1 and FUT2 were both found to direct alpha2-fucosylation on type 1 chains on both N- and O-linked structures. This knowledge enables us to engineer recombinant glycoproteins with glycan- and core chain-specific Lewis antigen substitution. Such tools will be important for investigations on the fine carbohydrate specificity of Le(b)-binding lectins, such as Helicobacter pylori adhesins and DC-SIGN, and may also prove useful as therapeutics.     

The effect of TNF-alpha on glycosylation pathways in bovine synoviocytes.

Synoviocytes are fibroblastic cells that line joint cavities. These cells synthesize numerous cell-surface and extracellular-matrix glycoproteins that are required for maintenance of the joint. Joint inflammation, such as occurs in arthritis, has been shown to have major effects on synoviocyte proliferation and on the biosynthesis of glycoproteins. The structures of the carbohydrate moieties of glycoproteins, however, and the enzymes involved in their synthesis have not yet been described for synoviocytes. Therefore, to characterize the cell-surface glycoconjugates, synoviocytes were isolated from bovine ankles, and the cells were grown in primary cultures. Lectin-binding assays were used to identify exposed N- and O-glycan carbohydrate determinants on synoviocytes, and specific enzyme assays were used to identify some of the glycosyltransferases involved in the synthesis of the glycan chains. A number of the enzymes that synthesize N- and O-linked oligosaccharides were found to be active in cell-free extracts of synoviocytes, including those that synthesize core-1-based O-glycans and the more complex bi-antennary N-glycans. To understand the molecular events underlying the inflammatory response in the synovium of arthritis patients, we examined the effect of the inflammatory cytokine tumour necrosis factor alpha (TNF-alpha) on synoviocytes and on glycosylation profiles. TNF-alpha treatment, which induces apoptosis in synoviocytes, was accompanied by changes in lectin-binding patterns, indicating alterations in the expression of cell-surface oligosaccharides. Concurrently, changes in specific enzyme activities were observed in treated cells. Two enzymes potentially important to the inflammatory process, core 2 beta6-GlcNAc-transferase and beta4-Gal-transferase, increased after TNF-alpha treatment. This is the first study of glycoprotein biosynthesis in synoviocytes, and it shows that synoviocytes have a characteristic glycosylation phenotype that is altered in the presence of inflammatory cytokines.     

Poly-N-acetyllactosaminyl O-glycans attached to leukosialin. The presence of sialyl Le(x) structures in O-glycans.

Poly-N-acetyllactosamine extension has been found in O-glycans in addition to N-glycans and glycosphingolipids. Attempts were made in HL-60 and K562 cells to determine the amount of poly-N-acetyllactosaminyl O-glycans in the major sialoglycoprotein, leukosialin. Leukosialin was immunoprecipitated from [3H]glucosamine-labeled HL-60 and K562 cells. Glycopeptides were prepared by Pronase digestion, and O-glycan-containing glycopeptides were isolated by affinity chromatography using Jacalin-agarose. The glycopeptides bound to Jacalin-agarose and those unbound were treated with alkaline borohydride, and the released O-glycans were fractionated by Bio-Gel P-4 filtration. Sequential glycosidase digestion of the O-glycans, with or without pretreatment by fucosidase or neuraminidase, revealed the following conclusions. 1) Leukosialin from HL-60 cells contains about 1-2 poly-N-acetyllactosaminyl O-glycan chains/molecule. 2) About 50% of these poly-N-acetyllactosaminyl O-glycans contain sialyl Le(x) termini, NeuNAc alpha 2-->3Gal beta 1-->4 (Fuc alpha 1-->3)GlcNAc beta 1-->R. The amount of sialyl Le(x) structure in leukosialin is roughly equivalent to that on cell surfaces of HL-60 cells. 3) Leukosialin from K562 cells, on the other hand, contains no detectable amount of poly-N-acetyllactosaminyl O-glycans. 4) The presence of poly-N-acetyllactosamine in O-glycans is dependent on the core 2 beta 1,6-N-acetylglucosaminyl transferase. 5) Jacalin-agarose binds to sialylated small oligosaccharides such as NeuNAc alpha 2-->3Gal beta 1-->3(NeuNAc alpha 2-->6) GalNAc but not the hexasaccharide NeuNAc alpha 2-->3Gal beta 1-->3(NeuNAc alpha 2-->3Gal beta 1-->4GlcNAc beta 1-->6) GalNAc. These results indicate that the formation of polylactosaminyl O-glycans and sialyl Le(x) structure in O-glycans is dependent on the core 2 formation.     

Study of O-glycan sialylation in C6 cultured glioma cells: evidence for post-translational regulation of a beta-galactoside alpha 2,3 sialyltransferase activity by N-glycosylation

We have studied the Gal beta 1-3GalNAc-R alpha 2,3 sialyltransferase from C6 glioma cells transferring Neu5Ac from CMP-Neu5Ac onto O-glycans of glycoproteins. Using synchronized C6 glioma cells, we showed that the alpha 2,3 sialyltransferase activity was inhibited by tunicamycin to a greater extend than DNA and protein biosynthesis suggesting inhibition of N-glycosylation of this enzyme. Additional demonstration of N-glycosylation of the alpha 2,3 sialytransferase was provided through ConA-Sepharose binding. Treatment of partially purified alpha 2,3 sialytransferase by peptide-N-glycosidase F showed a significative inhibition demonstrating that N-glycan moiety is required for complete activity of the C6 glioma cell alpha 2,3 sialyltransferase.     

The pattern of glycosyl- and sulfotransferase activities in cancer cell lines: a predictor of individual cancer-associated distinct carbohydrate structures for the structural identification of signature glycans

Carbohydrate chains of cancer glycoprotein antigens contain major outer changes dictated by tissue-specific regulation of glycosyltransferase genes, the availability of sugar nucleotides, and competition between enzymes for acceptor intermediates during glycan elongation. However, it is evident from recent studies with recombinant mucin probes that the final glycosylation profiles of mucin glycoproteins are mainly determined by the cellular repertoire of glycosyltransferases. Hence, we examined various cancer cell lines for the levels of fucosyl-, beta-galactosyl, beta-N-acetylgalactosaminyl-, sialyl-, and sulfotransferase activities that generate the outer ends of the oligosaccharide chains. We have identified glycosyltransferases activities at the levels that would give rise to O-glycan chains as reported by others in breast cancer cell lines, T47D, ZR75-1, MCF-7, and MDA-MB-231. Most breast cancer cells express Gal-3-O-sulfotransferase specific for T-hapten Gal beta1-->3GalNAc alpha-, whereas the enzyme from colon cancer cells exhibits a vast preference for the Gal beta1,4GlcNAc terminal unit in O-glycans. We also studied ovarian cancer cells SW626 and PA-1 and hepatic cancer cells HepG2. Our studies show that alpha1,2-L-fucosyl-T, alpha(2,3) sialyl-T, and 3-O-Sulfo-T capable of acting on the mucin core 2 tetrasaccharide, Gal beta1,4GlcNAc beta1,6(Gal beta1,3)GalNAc alpha-, can also act on the Globo H antigen backbone, Gal beta1,3GalNAc beta1,3Gal alpha-, suggesting the existence of unique carbohydrate moieties in certain cancer-associated glycolipids. Briefly, our study indicates the following: (i) 3'-Sulfo-T-hapten has an apparent relationship to the tumorigenic potential of breast cancer cells; (ii) the 3'-sulfo Lewis(x), the 3-O-sulfo-Globo unit, and the 3-fucosylchitobiose core could be uniquely associated with colon cancer cells; (iii) synthesis of a polylactosamine chain and T-hapten are favorable in ovarian cancer cells due to negligible sialyltransferase activities; and (iv) a 6'-sialyl LacNAc unit and 3'-sialyl T-hapten appear to be prevalent structures in hepatic cancer cell glycans. Thus, it is apparent that different cancer cells are expressing unique glycan epitopes, which could be novel targets for cancer diagnosis and treatment.     

Galectin-1 binds different CD43 glycoforms to cluster CD43 and regulate T cell death

Galectin-1 kills immature thymocytes and activated peripheral T cells by binding to glycans on T cell glycoproteins including CD7, CD45, and CD43. Although roles for CD7 and CD45 in regulating galectin-1-induced death have been described, the requirement for CD43 remains unknown. We describe a novel role for CD43 in galectin-1-induced death, and the effects of O-glycan modification on galectin-1 binding to CD43. Loss of CD43 expression reduced galectin-1 death of murine thymocytes and human T lymphoblastoid cells, indicating that CD43 is required for maximal T cell susceptibility to galectin-1. CD43, which is heavily O-glycosylated, contributes a significant fraction of galectin-1 binding sites on T cells, as T cells lacking CD43 bound approximately 50% less galectin-1 than T cells expressing CD43. Although core 2 modification of O-glycans on other glycoprotein receptors is critical for galectin-1-induced cross-linking and T cell death, galectin-1 bound to CD43 fusion proteins modified with either unbranched core 1 or branched core 2 O-glycans and expression of core 2 O-glycans did not enhance galectin-1 binding to CD43 on T cells. Moreover, galectin-1 binding clustered CD43 modified with either core 1 or core 2 O-glycans on the T cell surface. Thus, CD43 bearing either core 1 or core 2 O-glycans can positively regulate T cell susceptibility to galectin-1, identifying a novel function for CD43 in controlling cell death. In addition, these studies demonstrate that different T cell glycoproteins on the same cell have distinct requirements for glycan modifications that allow recognition and cross-linking by galectin-1.     

Pathways of mucin O-glycosylation in normal and malignant rat colonic epithelial cells reveal a mechanism for cancer-associated Sialyl-Tn antigen expression

The Sialyl-Tn antigen (Sialyl alpha-Ser/Thr) is expressed as a cancer-associated antigen on the surface of cancer cells. Its presence is associated with a poor prognosis in patients with colorectal and other cancers. We previously reported that Sialyl-Tn expression in LSC human colon cancer cells could be explained by a specific lack of the activity of core 1 beta3-Gal-transferase (Brockhausen et al., Glycoconjugate J. 15, 595-603, 1998) and an inability to synthesize the common O-glycan core structures. To support this mechanism, or find other mechanisms to explain Sialyl-Tn antigen expression, we investigated the O-glycosylation pathways in clonal rat colon cancer cell lines that were selected for positive or negative expression of Sialyl-Tn antigen, and compared these pathways to those in normal rat colonic mucosa. Normal rat colonic mucosa had very active glycosyltransferases synthesizing O-glycan core structures 1 to 4. Several sialyl-, sulfo- and fucosyltransferases were also active. An M type core 2 beta6-GlcNAc-transferase was found to be present in rat colon mucosa and all of the rat colon cancer cells. O-glycosylation pathways in rat colon cancer cells were significantly different from normal rat colonic mucosa; for example, rat colon cancer cells lost the ability to synthesize O-glycan core 3. All rat colon cancer cell lines, regardless of the Sialyl-Tn phenotype, expressed glycosyltransferases assembling complex O-glycans of core 1 and core 2 structures (unlike human LSC colon cancer cells which lack core 1 beta3-Gal-transferase activity). It was the activity of CMP-sialic acid:GalNAc-mucin alpha6-sialyltransferase that coincided with Sialyl-Tn expression. Sialyl-Tn negative cells had a several fold higher activity of core 2 beta6-GlcNAc-transferase which synthesizes complex O-glycans that may mask adjacent Sialyl-Tn epitopes. The results suggest a new mechanism controlling Sialyl-Tn expression in cancer cells.     

Roles of O-linked oligosaccharides in immune responses

Many functional glycoproteins are expressed on the lymphocyte cell surface. Some of them carry O-linked oligosaccharides (O-glycans), which are conjugated through serine or threonine residues. During various biological processes, including T-cell activation, a tetrasaccharide on the T-cell surface is dramatically converted to a branched hexasaccharide, called core2 O-glycan. The same structural change in O-glycans is also found on the lymphocytes from patients with immunodeficiency conditions such as Wiskott-Aldrich syndrome and AIDS. Several studies revealing the roles of core2 O-glycans in immune responses show that this is a biologically significant change. In particular, core2 O-glycans expressed on the cell surface reduce cell-cell interactions, thereby regulating immune responses. Furthermore, core2 O-glycan is a key backbone structure in forming selectin ligands. Thus, O-linked oligosaccharides, in particular those containing core2 branches, play vital roles in immune responses and may play dual roles in certain situations. This review will summarize the results obtained from various studies investigating the roles of O-glycans in immunological processes. BioEssays 23:46-53, 2001.     

Selective expression of different fucosylated epitopes on two distinct sets of Schistosoma mansoni cercarial O-glycans: identification of a novel core type and Lewis X structure.

The glycobiology of Schistosoma mansoni is dominated by developmentally regulated expression of various fucosylated structures, most notably the Lewis X epitope and a multifucosylated sequence, Fuc alpha1-->2Fuc alpha1-->, in its various forms. For the infective cercarial stage, Lewis X has been structurally identified on glycosphingolipids and N-glycans of total glycoprotein extracts, and a population of multifucosylated glycoproteins were found to carry a unique terminal sequence, +/-Fuc alpha1-->2Fuc alpha1-->[3GalNAc beta1-->4(Fuc alpha1-->2Fuc alpha1--> 2Fuc alpha1-->3) GlcNAc beta1-->3Gal alpha1-->](n), on their O-glycans. Using a mass spectrometry approach coupled with chromatographic separation, sequential exoglycosidase digestion, periodate oxidation, and other chemical derivatization, we demonstrate that Lewis X could also be carried on the cercarial O-glycans, but the two distinctive sets of fucosylated epitopes were conjugated to two different core structures. Lewis X, lacNAc, or single GlcNAc was found to attach directly to the -->3Gal beta1-->3GalNAc core and indirectly via another beta-Gal residue branching off from C6 of the reducing end GalNAc to give a biantennary-like structure. The -->3(+/-Gal beta1-->6)Gal beta1-->3(-->3Gal beta1-->6)GalNAc core thus characterized represents a novel core type for O-glycans. In contrast, the previously characterized multifucosylated terminal sequences were carried on conventional type 1 and 2 cores. The smallest structures of the reductively released O-glycans were defined as GalNAc beta1-->4GlcNAc beta1-->3Gal beta1-->3GalNAcitol with a total of two to four fucoses attached to the terminal lacdiNAc. alpha-Galactosylation of the nonreducing terminal beta-GalNAc instead of fucose capping leads to further elongation with another lacdiNAc unit that could also extend directly from C6 of the reducing end GalNAc and similarly elongated or terminated.     

Modulation of O-glycans and N-glycans on murine CD8 T cells fails to alter annexin V ligand induction by galectin 1

Thymic negative selection and contraction of responding T cell oligoclones after infection represent important cell ablation processes required for maintaining T cell homeostasis. It has been proposed that galectin 1 contributes to these processes through interaction with lactosyl sequences principally on cell surface glycoproteins bearing core 2 (C2GnT1)-branched O-glycans. According to this model, specific T cell surface proteins cross-linked by galectin 1 induce signaling, ligand redistribution, and apoptosis in both immature thymocytes and activated T cells. The influence of lactosyl residues contained in branched O-glycans or complex N-glycans on galectin 1 binding and induction of annexin V ligand in murine CD8 T cells was assessed. Neither galectin binding nor galectin-induced expression of annexin V ligand was perturbed under conditions in which: 1) C2GnT1 activity was differentially induced by CD8 T cell activation/culture with IL-2 vs IL-4; 2) activated CD8(+) T cells lacked C2GnT1 expression; or 3) complex N-glycan formation was blocked by swainsonine. The maintenance of galectin 1 binding and induced annexin V expression under conditions that alter lactosamine abundance on O- or complex N-glycans suggest that galectin 1-mediated apoptosis is neither a simple function of fluctuating C2GnT1 activity nor a general C2GnT1-dependent mechanism underlying contraction of CD8 T cells subsequent to activation.     

Reduced contact-inhibition and substratum adhesion in epithelial cells expressing GlcNAc-transferase V

Malignant transformation of fibroblast and epithelial cells is accompanied by increased beta 1-6 N-acetylglucosaminyltransferase V (GlcNAc-TV) activity, a Golgi N-linked oligosaccharide processing enzyme. Herein, we report that expression of GlcNAc-TV in Mv1Lu cells, an immortalized lung epithelial cell line results in loss of contact- inhibition of cell growth, an effect that was blocked by swainsonine, an inhibitor of Golgi processing enzyme alpha-mannosidase II. In serum- deprived and high density monolayer cultures, the GlcNAc-TV transfectants formed foci, maintained microfilaments characteristic of proliferating cells, and also experienced accelerated cell death by apoptosis. Injection of the GlcNAc-TV transfectants into nude mice produced a 50% incidence of benign tumors, and progressively growing tumors in 2:12 mice with a latency of 6 mo, while no growth was observed in mice injected with control cells. In short term adhesion assays, the GlcNAc-TV expressing cells were less adhesive on surfaces coated with fibronectin and collagen type IV, but no changes were observed in levels of cell surface alpha 5 beta 1 or alpha v beta 3 integrins. The larger apparent molecular weights of the LAMP-2 glycoprotein and integrin glycoproteins alpha 5, alpha v and beta 1 in the transfected cells indicates that their oligosaccharide chains are substrates for GlcNAc-TV. The results suggest that beta 1-6GlcNAc branching of N-linked oligosaccharides contributes directly to relaxed growth controls and reduce substratum adhesion in premalignant epithelial cells.     

A novel strategy for evasion of NK cell immunity by tumours expressing core2 O-glycans

The O-glycan branching enzyme, core2 β-1,6-N-acetylglucosaminyltransferase (C2GnT), forms O-glycans containing an N-acetylglucosamine branch connected to N-acetylgalactosamine (core2 O-glycans) on cell-surface glycoproteins. Here, we report that upregulation of C2GnT is closely correlated with progression of bladder tumours and that C2GnT-expressing bladder tumours use a novel strategy to increase their metastatic potential. Our results showed that C2GnT-expressing bladder tumour cells are highly metastatic due to their high ability to evade NK cell immunity and revealed the molecular mechanism of the immune evasion by C2GnT expression. Engagement of an NK-activating receptor, NKG2D, by its tumour-associated ligand, Major histocompatibility complex class I-related chain A (MICA), is critical to tumour rejection by NK cells. In C2GnT-expressing bladder tumour cells, poly-N-acetyllactosamine was present on core2 O-glycans on MICA, and galectin-3 bound the NKG2D-binding site of MICA through this poly-N-acetyllactosamine. Galectin-3 reduced the affinity of MICA for NKG2D, thereby severely impairing NK cell activation and silencing the NK cells. This new mode of NK cell silencing promotes immune evasion of C2GnT-expressing bladder tumour cells, resulting in tumour metastasis.     

Expression of the H type 1 blood group antigen during enterocytic differentiation of Caco-2 cells.

We made a comparative study of the structures of the oligosaccharides on the glycoproteins from Caco-2 human colonic adenocarcinoma cells, before and after differentiation. Enterocytic differentiated Caco-2 cells highly express H type 1 blood group antigen on the cell surface as well as activities of brush border membrane hydrolases, such as dipeptidyl peptidase IV and alkaline phosphatase. A strong correlation was observed between the amounts of H type 1 blood group antigen and the degrees of differentiation. Structural analysis with use of lectin affinity high performance liquid chromatography revealed that typical mucin-type sugar chains of the glycoproteins from undifferentiated cells have H type 2 group, linear polylactosamines, and core 1 structure. On the other hand, differentiated cells newly contain H type 1 and Le(b) groups and core 2 structure. Mucins with H type 1 group make contact with brush border membrane enzymes on differentiated cells. Furthermore benzyl 2-acetamide-2-deoxy-alpha-D-galactopyranoside inhibited both expression of H type 1 group on the cell surface and enhancement of brush border membrane enzyme activities even in the presence of a differentiating inducer. These results suggest that the mucin-type sugar chains with H type 1 group have important functions regarding differentiation of Caco-2 cells.     

Characteristic structural features of schistosome cercarial N-glycans: expression of Lewis X and core xylosylation

Schistosomal egg N-glycans are the only examples in nature that have been structurally shown to contain beta2-xylosylation, alpha6-fucosylation, and alpha3-fucosylation on the N,N'-diacetyl chitobiose core. We present evidence that core difucosylated and xylosylated N-glycans are characteristics of Schistosoma japonicum eggs but not of the cercariae and adults, for which neither core xylosylation nor alpha3-fucosylation could be readily detected. In contrast, a majority of the N-glycans from Schistosoma mansoni cercariae but not the adults are core xylosylated. Tandem mass spectrometry analysis coupled with chromatographic mapping, sequential exoglycosidase digestion, and methylation analysis were employed to unambiguously define the structures of core beta2-xylosylated, alpha6-fucosylated N-glycans from S. mansoni cercariae. Unexpectedly, a majority of these N-glycans were found to carry Lewis X determinant, Galbeta1-->4(Fucalpha1-->3)GlcNAcbeta1-->, on the nonreducing termini of mono- and biantennary structures. The Lewis X-containing glycoproteins were found to be distinct from those carrying the complex, multifucosylated glycocalyx O-glycans reported previously. The corresponding N-glycans from S. japonicum cercariae are likewise dominated by Lewis X termini but without the core xylosylation. We concluded that the invading cercariae present an important and abundant source of Lewis X antigens, which may contribute to the induced humoral response upon infection. Following transformation and development into the adults, the N-glycans synthesized comprise a significantly larger amount of high mannose and fucosylated pauci-mannose structures in comparison with the cercarial N-glycans. A portion of the mono- and biantennary complex types were identified to carry Lewis X and fucosylated LacdiNAc termini, which could also be detected by mass spectrometry analysis on larger, complex-type structures.     

Stable surface expression of invariant chain prevents peptide presentation by HLA-DR.

Class II major histocompatibility complex (MHC) molecules are cell surface glycoproteins that bind and present immunogenic peptides to T cells. Intracellularly, class II molecules associate with a polypeptide referred to as the invariant (Ii) chain. Ii is proteolytically degraded and dissociates from the class II complex prior to cell surface expression of the mature class II alpha beta heterodimer. Using human fibroblasts transfected with HLA-DR1 and Ii cDNAs, we now demonstrate that truncation of the cytoplasmic domain of Ii results in the failure of Ii to dissociate from the alpha beta Ii complex and leads to stable expression of class II alpha beta Ii complexes on the cell surface. Furthermore, biochemical analysis and peptide presentation assays demonstrated that transfectants with stable surface alpha beta Ii complexes expressed very few free alpha beta heterodimers at the surface and were very inefficient in their ability to present immunogenic peptides to T cells. These results support the hypothesis that the cytoplasmic domain of Ii is responsible for endosomal targeting of alpha beta Ii and directly demonstrate that association with Ii interferes with the antigen presentation function of class II molecules.     

Detection of glycosyltransferases in the golden hamster (Mesocricetus auratus) oviduct and evidence for the regulation of O-glycan biosynthesis during the estrous cycle

Recently, we provided evidence that the glycosylation of hamster oviductin, a member of the mucin family of glycoproteins, is regulated during the estrous cycle. In order to further elucidate the glycosylation process of oviductal glycoproteins, we identified biosynthetic pathways involved in the assembly of mucin-type O-linked oligosaccharide (O-glycan) chains in the hamster oviduct. Our results demonstrated that the hamster oviduct has high activities of glycosyltransferases that synthesize O-glycans with core 1, 2, 3 and 4 structures as well as elongated structures. Oviduct therefore represents a typical mucin-secreting tissue. Our results also showed that specific glycosyltransferase activities are regulated during the estrous cycle. Mucin-type core 2 beta6-GlcNAc-transferase (C2GnT2) is responsible for synthesizing core 2 and core 4 structures in the oviduct. Specific assays for C2GnT2 revealed a cyclical pattern throughout the estrous cycle with high activity at the stages of proestrus and estrus and low activity at diestrus 1. Using semiquantitative RT-PCR, the mRNA levels for C2GnT2 in the estrous cycle stages could be correlated with the enzyme activities. An increase in glycosyltransferase activity in the hamster oviduct at the time of ovulation suggests that glycosylation of oviductal glycoproteins may be necessary for these proteins to exert their functions during the process of fertilization.