Inka Brockhausen

Pathways of O-glycan biosynthesis in cancer cells

Glycoproteins with O-glycosidically linked carbohydrate chains of complex structures and functions are found in secretions and on the cell surfaces of cancer cells. The structures of O-glycans are often unusual or abnormal in cancer, and greatly contribute to the phenotype and biology of cancer cells. Some of the mechanisms of changes in O-glycosylation pathways have been determined in cancer model systems. However, O-glycan biosynthesis is a complex process that is still poorly understood. The glycosyltransferases and sulfotransferases that synthesize O-glycans appear to exist as families of related enzymes of which individual members are expressed in a tissue- and growth-specific fashion. Studies of their regulation in cancer may reveal the connection between cancerous transformation and glycosylation which may help to understand and control the abnormal biology of tumor cells. Cancer diagnosis may be based on the appearance of certain glycosylated epitopes, and therapeutic avenues have been designed to attack cancer cells via their glycans.

Mucin-type O-glycans in human colon and breast cancer: glycodynamics and functions.

The glycoproteins of tumour cells are often abnormal, both in structure and in quantity. In particular, the mucin‐type O‐glycans have several cancer‐associated structures, including the T and Tn antigens, and certain Lewis antigens. These structural changes can alter the function of the cell, and its antigenic and adhesive properties, as well as its potential to invade and metastasize. Cancer‐associated mucin antigens can be exploited in diagnosis and prognosis, and in the development of cancer vaccines. The activities and Golgi localization of glycosyltransferases are the basis for the glycodynamics of cancer cells, and determine the ranges and amounts of specific O‐glycans produced. This review focuses on the glycosyltransferases of colon and breast cancer cells that determine the pathways of mucin‐type O‐glycosylation, and the proposed functional and pathological consequences of altered O‐glycans.

Mucin glycoproteins in neoplasia

Mucins are high molecular weight glycoproteins that are heavily glycosylated with many oligosaccharide side chains linked O-glycosidically to the protein backbone. With the recent application of molecular biological methods, the structures of apomucins and regulation of mucin genes are beginning to be understood. At least nine human mucin genes have been identified to date. Although a complete protein sequence is known for only three human mucins (MUC1, MUC2, and MUC7), common motifs have been identified in many mucins. The pattern of tissue and cell-specific expression of these mucin genes are emerging, suggesting a distinct role for each member of this diverse mucin gene family. In epithelial cancers, many of the phenotypic markers for pre-malignant and malignant cells have been found on the carbohydrate and peptide moieties of mucin glycoproteins. The expression of carbohydrate antigens appears to be due to modification of peripheral carbohydrate structures and the exposure of inner core region carbohydrates. The expression of some of the sialylated carbohydrate antigens appears to correlate with poor prognosis and increased metastatic potential in some cancers. The exposure of peptide backbone structures of mucin glycoproteins in malignancies appears to be due to abnormal glycosylation during biosynthesis. Dysregulation of tissue and cell-specific expression of mucin genes also occurs in epithelial cancers. At present, the role of mucin glycoproteins in various stages of epithelial cell carcinogenesis (including the preneoplastic state and metastasis), in cancer diagnosis and immunotherapy is under investigation.

Glycoproteins and Their Relationship to Human Disease

Glycoproteins are proteins that carry N- and O-glycosidically-linked carbohydrate chains of complex structures and functions. N-glycan chains are assembled in the endoplasmic reticulum and the Golgi by a controlled sequence of glycosyltransferase and glycosidase processing reactions involving dolichol intermediates. The assembly of O-glycans occurs in the Golgi and does not involve dolichol. For most reactions, families of glycosyltransferases exist; the expression of the individual enzymes within a family is often subject to complex regulation. The biosynthesis of N- and O-glycan is controlled at the level of gene expression, mRNA, enzyme protein activity and localization, and through substrate and cofactor concentrations at the site of synthesis. This complex regulation results in many hundreds of structures, the range of which varies in different species, cell types, tissue types, states of development and differentiation. In diseased cells, the relative proportions of these structures are often characteristically different from normal, and may be useful for the assessment of the stage of the disease and for diagnosis. Knowledge of disease-specific glycoprotein structures and their functions may be used therapeutically, in immunotherapy, in blocking cell adhesion or interfering with other binding or biological processes. Recently, some of the mechanisms underlying glycoprotein alterations in disease have been elucidated. This opens the possibility of an active interference in the disease process. The functions of glycans in diseased cells will become more clear with the tools of molecular biology and transgenic animal models.

Enzymatic basis for sialyl-Tn expression in human colon cancer cells

Sialyl-Tn antigen (SAα2-6 GalNAcα-Ser/Thr) is expressed as a cancer-associated antigen on the surface of cancer cells and its expression correlates with a poor prognosis in patients with colorectal and other adenocarcinomas. To understand the enzymatic basis of sialyl-Tn (STn) antigen expression, we used two clonal cell lines, LSB and LSC, derived from LS174T human colonic cancer cells. LSC cells express only the truncated carbohydrate antigen Tn (GalNAcα-Ser/Thr) and sialyl-Tn on their mucin molecules, whereas LSB cells express elongated oligosaccharide chains. Both cell lines demonstrated similar activities of glycosyltransferases involved in the biosynthesis of elongated and terminal structures of complex O-glycans. However, LSC cells were unable to synthesize core 1 (Galβ1-3GalNAc-) because the ubiquitous enzyme activity of UDP-Gal:GalNAc-R β3-Gal-transferase (core 1 β3-Gal-transferase) was lacking. Core 1 β3-Gal-transferase could not be reactivated in LSC cells by treatment with sodium butyrate or by in vivo growth of LSC cells in nude mice. In contrast, LSB cells were able to synthesize and process core 1 and core 2 (GlcNAcβ1-6 (Galβ1-3) GalNAc-). LSC cells represent the first example of a non-hematopoietic cell line which lacks core 1 β3-Gal-transferase activity. The lack of core 1 β3-Gal-transferase in LSC cells explains why they are incapable of forming the common mucin O-glycan core structures and are committed to synthesizing the short Tn and STn oligosaccharides. These findings suggest that the activity of core 1 β3-Gal-transferase is an important determinant of the STn phenotype of colon cancer cells.

Over-expression of ST3Gal-I promotes mammary tumorigenesis

Changes in glycosylation are common in malignancy, and as almost all surface proteins are glycosylated, this can dramatically affect the behavior of tumor cells. In breast carcinomas, the O-linked glycans are frequently truncated, often as a result of premature sialylation. The sialyltransferase ST3Gal-I adds sialic acid to the galactose residue of core 1 (Galβ1,3GalNAc) O-glycans and this enzyme is over-expressed in breast cancer resulting in the expression of sialylated core 1 glycans. In order to study the role of ST3Gal-I in mammary tumor development, we developed transgenic mice that over-express the sialyltransferase under the control of the human membrane-bound mucin 1 promoter. These mice were then crossed with PyMT mice that spontaneously develop mammary tumors. As expected, ST3Gal-I transgenic mice showed increased activity and expression of the enzyme in the pregnant and lactating mammary glands, the stomach, lungs and intestine. Although no obvious defects were observed in the fully developed mammary gland, when these mice were crossed with PyMT mice, a highly significant decrease in tumor latency was observed compared to the PyMT mice on an identical background. These results indicate that ST3Gal-I is acting as a tumor promoter in this model of breast cancer. This, we believe, is the first demonstration that over-expression of a glycosyltransferase involved in mucin-type O-linked glycosylation can promote tumorigenesis.

The biosynthesis of highly branched N-glycans: studies on the sequential pathway and functional role of N-actylglucosaminyltransferases I, II, III, IV, V and VI ☆

At least 6 N-acetylglucosaminyltransferases (GlcNAc-T I, II, III, IV, V and VI) are involved in initiating the synthesis of the various branches found in complex asparagine-linked oligosaccharides (N-glycans), as indicated below: GlcNAc beta 1-6 GlcNAc-T V GlcNAc beta 1-4 GlcNAc-T VI GlcNAc beta 1-2Man alpha 1-6 GlcNAc-T II GlcNAc beta 1-4Man beta 1-4-R GlcNAc T III GlcNAc beta 1-4Man alpha 1-3 GlcNAc-T IV GlcNAc beta 1-2 GlcNAc-T I where R is GlcNAc beta 1-4(+/- Fuc alpha 1-6)GlcNAcAsn-X. HPLC was used to study the substrate specificities of these GlcNAc-T and the sequential pathways involved in the biosynthesis of highly branched N-glycans in hen oviduct (I. Brockhausen, J.P. Carver and H. Schachter (1988) Biochem. Cell Biol. 66, 1134-1151). The following sequential rules have been established: GlcNAc-T I must act before GlcNAc-T II, III and IV; GlcNAc-T II, IV and V cannot act after GlcNAc-T III, i.e., on bisected substrates; GlcNAc-T VI can act on both bisected and non-bisected substrates; both Glc-NAc-T I and II must act before GlcNAc-T V and VI; GlcNAc-T V cannot act after GlcNAc-T VI. GlcNAc-T V is the only enzyme among the 6 transferases cited above which can be essayed in the absence of Mn2+. In studies on the possible functional role of N-glycan branching, we have measured GlcNAc-T III in pre-neoplastic rat liver nodules (S. Narasimhan, H. Schachter and S. Rajalakshmi (1988) J. Biol. Chem. 263, 1273-1281). The nodules were initiated by administration of a single dose of carcinogen 1,2-dimethyl-hydrazine.2 HCl 18 h after partial hepatectomy and promoted by feeding a diet supplemented with 1% orotic acid for 32-40 weeks. The nodules had significant GlcNAc-T III activity (1.2-2.2 nmol/h/mg), whereas the surrounding liver, regenerating liver 24 h after partial hepatectomy and control liver from normal rats had negligible activity (0.02-0.03 nmol/h/mg). These results suggest that GlcNAc-T III is induced at the pre-neoplastic stage in liver carcinogenesis and are consistent with the reported presence of bisecting GlcNAc residues in N-glycans from rat and human hepatoma gamma-glutamyl transpeptidase and their absence in enzyme from normal liver of rats and humans (A. Kobata and K. Yamashita (1984) Pure Appl. Chem. 56, 821-832).

Chapter 5 Biosynthesis 3. Biosynthesis of O-Glycans of the N-Acetylgalactosamine-α-Ser/Thr Linkage Type

Publisher Summary This chapter discusses the biosynthesis of Oligosaccharides (O-glycans) of the N-acetylgalactosamine- α-Ser/Thr linkage type. The synthesis of the frame-work of oligosaccharide chains regulates the expression of functional terminal carbohydrate structures. Thus, a control on the early steps of the biosynthetic pathways proves to have a great impact on the structures, properties, and biological functions of O-glycans. O-Glycosylation commonly changes during development, differentiation, growth, and in disease. In addition, various tissues and species express characteristic O-glycans associated with various biological functions. Genetic defects in the biosynthetic pathways of O-glycans are rare; possibly because the development of a multicellular organism is dependent on interactions with cellular carbohydrate.The sugars commonly found in O-glycans are GalNAc, Gal, GlcNAc, sialic acid, and fucose. Gal, GlcNAc and GalNAc may be sulfated and contribute with sialic acid to the acidity of O-glycans. O-glycans are found on many mammalian and non-mammalian soluble and membrane-bound glycoproteins, and on proteoglycans.

UDP-Gal: GlcNAc-R β1,4-galactosyltransferase—a target enzyme for drug design. Acceptor specificity and inhibition of the enzyme

Galactosyltransferases are important enzymes for the extension of the glycan chains of glycoproteins and glycolipids, and play critical roles in cell surface functions and in the immune system. In this work, the acceptor specificity and several inhibitors of bovine β1,4-Gal-transferase T1 (β4GalT, EC 2.4.1.90) were studied. Series of analogs of N-acetylglucosamine (GlcNAc) and GlcNAc-carrying glycopeptides were synthesized as acceptor substrates. Modifications were made at the 3-, 4- and 6-positions of the sugar ring of the acceptor, in the nature of the glycosidic linkage, in the aglycone moiety and in the 2-acetamido group. The acceptor specificity studies showed that the 4-hydroxyl group of the sugar ring was essential for β4GalT activity, but that the 3-hydroxyl could be replaced by an electronegative group. Compounds having the anomeric β-configuration were more active than those having the α-configuration, and O-, S- and C-glycosyl compounds were all active as substrates. The aglycone was a major determinant for the rate of Gal-transfer. Derivatives containing a 2-naphthyl aglycone were inactive as substrates although quinolinyl groups supported activity. Several compounds having a bicyclic structure as the aglycone were found to bind to the enzyme and inhibited the transfer of Gal to control substrates. The best small hydrophobic GlcNAc-analog inhibitor was found to be 1-thio-N-butyrylGlcNβ-(2-naphthyl) with a Ki of 0.01 mM. These studies help to delineate β4GalT–substrate interactions and will aid in the development of biologically applicable inhibitors of the enzyme.

ProcessingO-glycan core 1, Galβ1-3GalNAcα-R. Specificities of core 2, UDP-GlcNAc: Galβ1-3GalNAc-R(GlcNAc to GalNAc) β6-N-acetylglucosaminyltransferase and CMP-sialic acid:Galβ1-3GalNAc-R α3-sialyltransferase

To elucidate control mechanisms ofO-glycan biosynthesis in leukemia and to develop biosynthetic inhibitors we have characterized core 2 UDP-GlcNAc:Galβ1-3GalNAc-R(GlcNAc to GalNAc) β6-N-acetylglucosaminyl-transferase (EC 2.4.1.102; core 2 β6-GlcNAc-T) and CMP-sialic acid: Galβ1-3GalNAc-R α3-sialyltransferase (EC 2.4.99.4; α3-SA-T), two enzymes that are significantly increased in patients with chronic myelogenous leukemia (CML) and acute myeloid leukemia (AML). We observed distinct tissue-specific kinetic differences for the core 2 β6-GlcNAc-T activity; core 2 β6-GlcNAc-T from mucin secreting tissue (named core 2 β6-GlcNAc-T M) is accompanied by activities that synthesize core 4 [GlcNAcβ1-6(GlcNAcβ1-3)GalNAc-R] and blood group I [GlcNAcβ1-6(GlcNAcβ1-3)Galβ-R] branches; core 2 β6-GlcNAc-T in leukemic cells (named core 2 β-GlcNAc-T L) is not accompanied by these two activities and has a more restricted specificity. Core 2 β6-GlcNAc-T M and L both have an absolute requirement for the 4- and 6-hydroxyls ofN-acetylgalactosamine and the 6-hydroxyl of galactose of the Galβ1-3GalNAcα-benzyl substrate but the recognition of other substituents of the sugar rings varies, depending on the tissue. α3-sialytransferase from human placenta and from AML cells also showed distinct specificity differences, although the enzymes from both tissues have an absolute requirement for the 3-hydroxyl of the galactose residue of Galβ1-3GalNAcα-Bn. Galβ1-3(6-deoxy)GalNAcα-Bn and 3-deoxy-Galβ1-3GalNAcα-Bn competitively inhibited core 2 β6-GlcNAc-T and α3-sialyltransferase activities, respectively.