Inhibition of N-linked oligosaccharide processing in tumor cells is associated with enhanced tissue inhibitor of metalloproteinases (TIMP) gene expression

Extrinsic sialylation is dynamically regulated by systemic triggers in vivo

Recent reports have documented that extracellular sialyltransferases can remodel both cell-surface and secreted glycans by a process other than the canonical cell-autonomous glycosylation that occurs within the intracellular secretory apparatus. Despite association of the abundance of these extracellular sialyltransferases, particularly ST6Gal-1, with disease states such as cancer and a variety of inflammatory conditions, the prevalence of this extrinsic glycosylation pathway in vivo remains unknown. Here we observed no significant extrinsic sialylation in resting mice, suggesting that extrinsic sialylation is not a constitutive process. However, extrinsic sialylation in the periphery could be triggered by inflammatory challenges, such as exposure to ionizing radiation or to bacterial lipopolysaccharides. Sialic acids from circulating platelets were used in vivo to remodel target cell surfaces. Platelet activation was minimally sufficient to elicit extrinsic sialylation, as demonstrated with the FeCl₃ model of mesenteric artery thrombosis. Although extracellular ST6Gal-1 supports extrinsic sialylation, other sialyltransferases are present in systemic circulation. We also observed in vivo extrinsic sialylation in animals deficient in ST6Gal-1, demonstrating that extrinsic sialylation is not mediated exclusively by ST6Gal-1. Together, these observations form an emerging picture of glycans biosynthesized by the canonical cell-autonomous glycosylation pathway, but subjected to remodeling by extracellular glycan-modifying enzymes.

Biosynthetic Machinery Involved in Aberrant Glycosylation: Promising Targets for Developing of Drugs Against Cancer

Cancer cells depend on altered metabolism and nutrient uptake to generate and keep the malignant phenotype. The hexosamine biosynthetic pathway is a branch of glucose metabolism that produces UDP-GlcNAc and its derivatives, UDP-GalNAc and CMP-Neu5Ac and donor substrates used in the production of glycoproteins and glycolipids. Growing evidence demonstrates that alteration of the pool of activated substrates might lead to different glycosylation and cell signaling. It is already well established that aberrant glycosylation can modulate tumor growth and malignant transformation in different cancer types. Therefore, biosynthetic machinery involved in the assembly of aberrant glycans are becoming prominent targets for anti-tumor drugs. This review describes three classes of glycosylation, O-GlcNAcylation, N-linked, and mucin type O-linked glycosylation, involved in tumor progression, their biosynthesis and highlights the available inhibitors as potential anti-tumor drugs.

Bitter sweetness of complexity

Glycosylation of proteins, lipids and mucins has gained increasing complexity in the course of evolution. Metazoans and mammals exhibit extensively exploited pathways of N-glycan biosynthesis, with unique features that are not found in plants or protozoans.Paralleling the complexity of N-glycan structure, their impact on regulatory processes has become very diverse and has evolved into a multidimensional lattice imprinting modes of cellular communication. Processes that are regulated by N-glycans are cellular adhesion and motility, growth factor and cytokine signalling, metabolic homeostasis, and binding of certain pathogens. Consequently, alterations in N-glycan biosynthesis interfere with cellular proliferation and differentiation and may produce disturbances in embryonic development, trigger inflammatory processes, favour tumour development and enhance the metastastic dissemination of primary tumours. Particular N-glycans that have been causally related to these pathological scenarios are the complex-type N-glycans, branching from oligomannosidic core structures into β-glycosidic linkages, connected to acetylated glucosamine and galactose, and yield extended lactosamine chains of variable length. These N-acetyllactosamines are preferred building blocks for further modification by fucosylation, sialylation, and sulphation, thus creating binding sites for different galectins or selectins. The focus of this review will be on the b1,6-N -acetylglucosaminyltransferase-V/GnT-V/MGAT5, a phylogenically conserved enzyme that is required for the synthesis of β1,6-branched complex-type oligosaccharides in the medial Golgi compartment, and its implications in metabolism and cancer progression.

Sialilated β1,6 branched N-oligosaccharides modulate adhesion, chemotaxis and motility of melanoma cells: Effect on invasion and spontaneous metastasis properties

B16BL6 cells, selected specifically for invasive characteristics from B16F10 mouse melanoma cells, displayed greater ability to metastasize to lungs and produced larger colonies than the parent cells, when injected intravenously. When the two cell lines were compared for surface beta1,6-branched N-oligosaccharides by flow cytometry using Leuco-Phyto-Heam-Agglutinin, B16BL6 were found to express significantly higher levels. Inhibition of the oligosaccharide expression, by treatment of the cells with swainsonine or antisense-N-acetyl glucosaminyl-transferase-V, significantly reduced metastasis and invasion (>50%). Further, inhibition of oligosaccharides on the molecules like beta1 integrin (one of the major carriers) caused 30-45% reduction in their adherence to extra-cellular-matrix components especially collagen IV and laminin, and chemotaxis towards fibronectin and matrigel. The inhibition also decreased haptotaxis by approximately 50% to fibronectin but surprisingly was enhanced towards laminin by approximately 75%. The cells on which the expression of these oligosaccharides was inhibited failed to exhibit the characteristic spontaneous metastasis and adhesion properties of B16BL6 cells. In none of the cases, however, the secretion of matrix-metallo-proteases correlated with oligosaccharide expression. Sialylation of surface oligosaccharides was found to be accompanied by even higher motility and adherence to the substrates. These results strongly support an important role of cell surface beta1,6-linked N-oligosaccharides, especially the sialylated derivatives, in the processes that influence invasion and metastasis.

GnT-V, macrophage and cancer metastasis: a common link

GnT-V generated, beta1,6-branched polylactosamines are a common feature shared by normal granulocytes, monocytes, and a variety of malignant cells. Furthermore, activation of GnT-V in oncogenic transformation induces invasiveness and metastatic potential in mice as well as in humans. In view of the common expression of lymphocytic/monocytic trait, motility, and GnT-V by metastatic cancer cells, macrophage fusion hybrids were generated in vitro with Cloudman S91 mouse melanoma cells to test whether the parental traits are co-expressed in hybrids and how those are related to altered phenotypes in relation to metastasis. In fact, the fusion hybrids are highly metastatic in vivo, motile in vitro, and express macrophage-associated traits of increased GnT-V activity, beta1,6 branching, and polylactosamine content. A Spontaneously formed lung melanoma metastases have been identified and characterized as host x tumor hybrid containing higher DNA content than parental cells and increased GnT-V activity. The results, taken together, could reflect prior fusion of tumor-associated macrophages with cells of the primary tumor, and therefore establish a possible common link between elevated expression of GnT-V and malignant transformation, a well-known report. Moreover, the fusion hybrids with metastatic potential ranging from high to low offer a genetically matched model system, for identification and characterization of differentially expressed genes in association with metastasis, since the fusion partners are derived from the same species of mouse (DBA/2J).

Golgi alpha-mannosidase II deficiency in vertebrate systems: implications for asparagine-linked oligosaccharide processing in mammals

The maturation of N-glycans to complex type structures on cellular and secreted proteins is essential for the roles that these structures play in cell adhesion and recognition events in metazoan organisms. Critical steps in the biosynthetic pathway leading from high mannose to complex structures include the trimming of mannose residues by processing mannosidases in the endoplasmic reticulum (ER) and Golgi complex. These exo-mannosidases comprise two separate families of enzymes that are distinguished by enzymatic characteristics and sequence similarity. Members of the Class 2 mannosidase family (glycosylhydrolase family 38) include enzymes involved in trimming reactions in N-glycan maturation in the Golgi complex (Golgi mannosidase II) as well as catabolic enzymes in lysosomes and cytosol. Studies on the biological roles of complex type N-glycans have employed a variety of strategies including the treatment of cells with glycosidase inhibitors, characterization of human patients with enzymatic defects in processing enzymes, and generation of mouse models for the enzyme deficiency by selective gene disruption approaches. Corresponding studies on Golgi mannosidase II have employed swainsonine, an alkaloid natural plant product that causes "locoism", a phenocopy of the lysosomal storage disease, alpha-mannosidosis, as a result of the additional targeting of the broad-specificity lysosomal mannosidase by this compound. The human deficiency in Golgi mannosidase II is characterized by congenital dyserythropoietic anemia with splenomegaly and various additional abnormalities and complications. Mouse models for Golgi mannosidase II deficiency recapitulate many of the pathological features of the human disease and confirm that the unexpectedly mild effects of the enzyme deficiency result from a tissue-specific and glycoprotein substrate-specific alternate pathway for synthesis of complex N-glycans. In addition, the mutant mice develop symptoms of a systemic autoimmune disorder as a consequence of the altered glycosylation. This review will discuss the biochemical features of Golgi mannosidase II and the consequences of its deficiency in mammalian systems as a model for the effects of alterations in vertebrate N-glycan maturation during development.

Polyhydroxylated alkaloids -- natural occurrence and therapeutic applications

Over one hundred polyhydroxylated alkaloids have been isolated from plants and micro-organisms. These alkaloids can be potent and highly selective glycosidase inhibitors and are arousing great interest as tools to study cellular recognition and as potential therapeutic agents. However, only three of the natural products so far have been widely studied for therapeutic potential due largely to the limited commercial availability of the other compounds.

Evaluation of the effects of swainsonine, captopril, tangeretin and nobiletin on the biological behaviour of brain tumour cells in vitro

Although intrinsic tumours of the brain seldom metastasize to distant sites, their diffuse, infiltrative-invasive growth within the brain generally precludes successful surgical and adjuvant therapy. Hence, attention has now focused on novel therapeutic approaches to combat brain tumours that include the use of anti-invasive and anti-proliferative agents. The effect of four anti-invasive agents, swainsonine (a locoweed alkaloid), captopril (an anti-hypertensive drug), tangeretin and nobiletin (both citrus flavonoids), were investigated on various parameters of brain tumour invasion such as matrix metalloproteinase (MMP) secretion, migration, invasion and adhesion. A standard cytotoxicity assay was used to optimize working concentrations of the drugs on seven human brain tumour-derived cell lines of various histological type and grade of malignancy. A qualitative assessment by gelatin zymography revealed that the effect of these agents varied between the seven cell lines such that the low grade pilocytic astrocytoma was unaffected by three of the agents. In contrast, downregulation of the two gelatinases, MMP-2 and MMP-9 was seen in the grade 3 astrocytoma irrespective of which agent was used. Generally, swainsonine was the least effective whereas the citrus flavonoids, particularly nobiletin, showed the greatest downregulation of secretion of the MMPs. Furthermore, captopril and nobiletin were most efficient at inhibiting invasion, migration and adhesion in four representative cell lines (an ependymoma, a grade II oligoastrocytoma, an anaplastic astrocytoma and a glioblastoma multiforme). Yet again, the effects of the four agents varied between the four cell lines. Nobiletin was, nevertheless, the most effective agent used in these assays. In conclusion, the differential effects seen on the various parameters studied by these putative anti-invasive agents may be the result of interference with MMPs and other mechanisms underlying the invasive phenotype. From these pilot studies, it is possible that these agents, especially the citrus flavonoids, could be of future therapeutic value. However, further work is needed to validate this in a larger study.

Regulation of the GnT-V promoter by transcription factor Ets-1 in various cancer cell lines

Although the precise role of oligosaccharides in metastasis is presently unknown, numerous studies suggest that the beta1-6 branching structure of N-linked oligosaccharides plays a role in tumor metastasis. N-Acetylglucosaminyltransferase V (GnT-V), which catalyzes the formation of the beta1-6 branch, therefore appears to play a crucial role in tumor metastasis. Recently, we demonstrated that the expression of the GnT-V gene is regulated by a transcriptional factor, Ets-1 (Kang, R., Saito, H., Ihara, Y., Miyoshi, E., Koyama, N., Sheng, Y., and Taniguchi, N. (1996) J. Biol. Chem. 271, 26706-26712). In this study, we report an investigation of the general requirement for Ets-1 in the expression of GnT-V in cancer cell lines. In 16 cancer cell lines, the levels of GnT-V mRNA were closely correlated with ets-1 expression (r = 0.97; p < 0.0001). An increase in ets-1 levels by transfection of its cDNA led to an enhancement in GnT-V expression in cells that normally expressed low levels of ets-1. In contrast, the transfection of dominant negative ets-1 into cells that express high levels of ets-1 resulted in a decrease in GnT-V expression. Although Ets-1 cooperates with c-Jun in certain gene expressions, this was not the case in the regulation of the GnT-V gene. These results suggest that Ets-1 plays a significant role in regulating the expression of GnT-V in a variety of cancers and might be involved in the potential for malignancy via the action of GnT-V.

N-linked oligosaccharides and metastatic propensity in in vivo selected mouse mammary adenocarcinoma cells

Studies using metastatic variant selected in vivo from a cloned parental cell line demonstrate that the expression of beta1-6 branched, N-linked carbohydrates and sialic acid were positively associated with in vitro invasiveness and inversely associated with metastatic potential, adherence, and in vivo growth rate. These results suggest that at least within one tumor model, a negative association occurs between metastatic potential and 1-6 branched oligosaccharide expression. In these studies two metastatic variants, Cl-66M1 and Cl-66M2, were selected following serial in vivo passage of Cl-66, a clonal cell line obtained from a mouse mammary adenocarcinoma cell line. The parent cell line and the two metastatic variants were approximately equal in their adherence to fibronectin, laminin, and collagen type IV coated plastic. In contrast, both Cl-66M1 and Cl-66M2 had a significantly increased ability to invade through matrigel invasion chambers and expressed significantly increased levels of beta1-6 branched, N-linked carbohydrates, and sialic acid compared to the clonal parental cell line, Cl-66. Furthermore, the in vivo tumor growth rates of these selected variants were decreased compared to Cl-66 with the longest tumor volume doubling time observed with Cl-66M2.

Control of metastasis by Asn-linked, beta1-6 branched oligosaccharides in mouse mammary cancer cells

Studies in cell lines and malignant human tissues have shown that increased cell-surface Asn-linked beta1-6(GlcNAcbeta1-6Man) branching is associated with increased tumorigenic and metastatic properties. In this study, three mouse mammary cancer cell lines were transfected with an expression vector containing the mouse cDNA for N-acetylglucosaminyltransferase V (GlcNAcT-V EC 2.4.1.155), the glycosyltransferase responsible for initiating beta1-6 branching on Asn-linked carbohydrates. The cell lines were screened for increased cytotoxicity to L-PHA, a lectin specific for beta1-6 branching structures. Cell lines exhibiting increased L-PHA cytotoxicity expressed increased levels of beta1-6 branching structures. Northern blots detected the presence of GlcNAcT-V transcribed from the expression vector in the L-PHA sensitive cell lines. After injection into the tail veins of mice, transfected cell lines with increased beta1-6 branching on the cell surface formed elevated levels of lung tumors relative to control transfected cell lines (P < 0.002). Western blots of membrane proteins from GlcNAcT-V transfected and control cells probed with the lectins DSA and WGA did not show an increase in polyN-acetyllactosamine and sialic acid content in the transfected cell lines. These results demonstrate that a specific increase in beta1-6 branching due to an elevation in GlcNAcT-V expression increases metastatic potential.

Transfection of N-acetylglucosaminyltransferase III gene suppresses expression of hepatitis B virus in a human hepatoma cell line, HB611

beta-D-mannoside beta-1,4-N-acetylglucosaminyltransferase III (GnT-III) catalyzes the addition of N-acetylglucosamine in beta 1-4 linkage to the beta-linked mannose of the trimannosyl core of N-linked oligosaccharides and forms a bisecting GlcNAc structure. Although the biological meaning of the bisecting GlcNAc structure remains unclear, it is known that the attachment of a bisecting GlcNAc inhibits further processing of oligosaccharides by other glycosyltransferases. To investigate whether or not structural changes of oligosaccharides affect secretion and gene expression of hepatitis B virus (HBV), we introduced the GnT-III gene into a human hepatoma cell line, HB611, which secreted HBV-related proteins into the medium. Positive transfectants were cloned by hygromycin resistant selection. Three clones have high activities of GnT-III and secreted lower levels of HBV-related proteins into the medium in comparison with other clones. These clones showed marked suppression of HBV-related mRNAs and an increased binding with E-PHA as judged by lectin blot. Expression of beta actin, alpha fetoprotein, albumin, and prealubmin was not correlated with GnT-III activity in all the seven clones. Treatment of these cells with tunicamycin or swainsonine resulted in enhanced expression of HBV-related mRNA. These results indicate that some glycoproteins whose oligosaccharide structures are changed by over-expression of GnT-III suppress HBV gene expression.

Branching N-linked oligosaccharides in breast cancer

Tumor progression in rodent and human tumors is commonly associated with changes in glycoprotein glycosylation, in particular increased beta 1-6GlcNAc-branching, a regulatory step in expression of polylactosamine and extended-chain Lewis antigens. Loss of the branched oligosaccharides in murine tumor cells either due to somatic mutation, or treatment of the cells with the oligosaccharide processing inhibitor swainsonine, blocks tumor cells invasion in vitro and reduces solid tumor growth in vivo. Swainsonine and other inhibitors of N-linked oligosaccharide processing may be useful anti-cancer drugs, a premise which has begun to be tested in humans.