Polylactosamine synthesis and branch formation of N-glycans in β1,4-galactosyltransferase-1-deficient mice

High expression of β3GnT8 is associated with the metastatic potential of human glioma

Changes in glycosylation due to specific alterations of glycosyltransferase activity have been shown in various tumor cells, including human glioma cells. β1,3-N‑acetylglucosaminyltransferase-8 (β3GnT8) catalyzes the formation of polylactosamine on β1-6 branched N-glycans. Upregulated expression of β3GnT8 was described in some tumors, but its precise role in regulating glioma invasion and metastasis remains unclear. In this study, we report on an investigation of the expression of β3GnT8 in human glioma by immunohistochemical analysis. Out of 42 glioma tissues, 37 (88.1%) showed positive β3GnT8 expression, which was significantly higher than that in normal brain tissues (P<0.001). Additionally, the level of β3GnT8 increased with increased pathological grade of gliomas. Silencing of β3GnT8 in U251 glioma cells attenuated the formation of polylactosamine, and decreased cell proliferation, migration and metastatic ability in vitro and in vivo. By contrast, the overexpression of β3GnT8 in U251 cells exhibited enhanced metastatic potential. A positive correlation between β3GnT8 and matrix metalloproteinase-2 (MMP-2) expression in U251 cells was also observed. The results demonstrated a critical role of β3GnT8 in the metastatic potential of glioma cells, indicating that manipulating β3GnT8 expression may have therapeutic potential for the treatment of malignant glioma.

N-linked polylactosamine glycan synthesis is regulated by co-expression of β3GnT2 and GCNT2

Poly-N-acetyllactosamine (PLN) is a unique glycan composed of repeating units of the common disaccharide (Galβ1,4-GlcNAcβ1,3)n . The expression of PLN on glycoprotein core structures minimally requires enzyme activities for β1,4-galactosyltransferase (β4GalT) and β1,3-N-acetylglucosminyltransferase (β3GnT). Because β4GalTs are ubiquitous in most cells, PLN expression is generally ascribed to the tissue-specific transcription of eight known β3GnT genes in mice. In the olfactory epithelium (OE), β3GnT2 regulates expression of extended PLN chains that are essential for axon guidance and neuronal survival. N-glycan branching and core composition, however, can also modulate the extent of PLN modification. Here, we show for the first time that the β1,6-branching glycosyltransferase GCNT2 (formerly known as IGnT) is expressed at high levels specifically in the OE and other sensory ganglia. Postnatally, GCNT2 is maintained in mature olfactory neurons that co-express β3GnT2 and PLN. This highly specific co-expression suggests that GCNT2 and β3GnT2 function cooperatively in PLN synthesis. In support of this, β3GnT2 and GCNT2 co-transfection in HEK293T cells results in high levels of PLN expression on the cell surface and on adenylyl cyclase 3, a major carrier of PLN glycans in the OE. These data clearly suggest that GCNT2 functions in vivo together with β3GnT2 to determine PLN levels in olfactory neurons by regulating β1,6-branches that promote PLN extension.

Altered oligosaccharide structures reduce colitis induction in mice defective in β-1,4-galactosyltransferase

BACKGROUND & AIMS

Oligosaccharide modifications induce various functional changes in immune cells. The galactose-deficient fraction of fucosylated IgG oligosaccharides is increased, whereas that of β-1,4-galactosyltransferase I (B4GalTI) is reduced, in patients with Crohn's disease. We investigated the role of oligosaccharide modification in the pathophysiology of colitis using B4galt1-deficient mice.

METHODS

Colitis severity was compared between B4galt1(+/-) and B4galt1(+/+) mice. B cells isolated from B4galt1(+/-) and B4galt1(+/+) mice were adoptively transferred to recombination activating gene 2(-/-) mice, in which colitis was induced by administration of CD4(+)CD62L(+) T cells. Cell-surface glycan profiles were determined by lectin microarray analysis. Cytokine production was determined in a coculture of various types of cells isolated from either B4galt1(+/-) or B4galt1(+/+) mice.

RESULTS

Colitis induction by dextran sodium sulfate or trinitrobenzene sulfonic acid was significantly reduced in B4galt1(+/-) mice, which had galactose deficiency in IgG oligosaccharides (similar to patients with Crohn's disease) compared with B4galt1(+/+) mice. Amelioration of colitis was associated with increased production of interleukin-10 by macrophages in B4galt1(+/-) mice. Colitis induction in recombination activating gene 2(-/-) mice by administration of CD4(+)CD62L(+) T cells was reduced by cotransfer of B cells isolated from B4galt1(+/-), but not from B4galt1(+/+) mice. Lectin microarray analysis revealed increased expression of polylactosamines on B4galt1(+/-) B cells and macrophages, compared with B4galt1(+/+) cells. The production of interleukin-10 from macrophages was induced via their direct interaction with B4galt1(+/-) B cells.

CONCLUSIONS

Altered oligosaccharide structures on immune cells modulate mucosal inflammation. Oligosaccharides in immune cells might be a therapeutic target for inflammatory bowel diseases.

Analysis of carbohydrates and glycoconjugates by matrix-assisted laser desorption/ionization mass spectrometry: An update for 2003-2004

This review is the third update of the original review, published in 1999, on the application of matrix-assisted laser desorption/ionization (MALDI) mass spectrometry to the analysis of carbohydrates and glycoconjugates and brings the topic to the end of 2004. Both fundamental studies and applications are covered. The main topics include methodological developments, matrices, fragmentation of carbohydrates and applications to large polymeric carbohydrates from plants, glycans from glycoproteins and those from various glycolipids. Other topics include the use of MALDI MS to study enzymes related to carbohydrate biosynthesis and degradation, its use in industrial processes, particularly biopharmaceuticals and its use to monitor products of chemical synthesis where glycodendrimers and carbohydrate-protein complexes are highlighted.

Tumor suppressor p16INK4a--modulator of glycomic profile and galectin-1 expression to increase susceptibility to carbohydrate-dependent induction of anoikis in pancreatic carcinoma cells

Expression of the tumor suppressor p16(INK4a) after stable transfection can restore the susceptibility of epithelial tumor cells to anoikis. This property is linked to increases in the expression and cell-surface presence of the fibronectin receptor. Considering its glycan chains as pivotal signals, we assumed an effect of p16(INK4a) on glycosylation. To test this hypothesis for human Capan-1 pancreatic carcinoma cells, we combined microarray for selected glycosyltransferase genes with 2D chromatographic glycan profiling and plant lectin binding. Major differences between p16-positive and control cells were detected. They concerned expression of beta1,4-galactosyltransferases (down-regulation of beta1,4-galactosyltransferases-I/V and up-regulation of beta1,4-galactosyltransferase-IV) as well as decreased alpha2,3-sialylation of O-glycans and alpha2,6-sialylation of N-glycans. The changes are compatible with increased beta(1)-integrin maturation, subunit assembly and binding activity of the alpha(5)beta(1)-integrin. Of further functional relevance in line with our hypothesis, we revealed differential reactivity towards endogenous lectins, especially galectin-1. As a result of reduced sialylation, the cells' capacity to bind galectin-1 was enhanced. In parallel, the level of transcription of the galectin-1 gene increased conspicuously in p16(INK4a)-positive cells, and even figured prominently in a microarray on 1996 tumor-associated genes and in proteomic analysis. The cells therefore gain optimal responsiveness. The correlation between genetically modulated galectin-1 levels and anoikis rates in engineered transfectants inferred functional significance. To connect these findings to the fibronectin receptor, galectin-1 was shown to be co-immunoprecipitated. We conclude that p16(INK4a) orchestrates distinct aspects of glycosylation that are relevant for integrin maturation and reactivity to an endogenous effector as well as the effector's expression. This mechanism establishes a new aspect of p16(INK4a) functionality.

Kinetic properties and substrate specificities of two recombinant human N-acetylglucosaminyltransferase-IV isozymes

N-acetylglucosaminyltransferase (GnT)-IV catalyzes the formation of the GlcNAcbeta1-4 branch on the GlcNAcbeta1-2Manalpha1-3 arm of the core structure of N-glycans. Two human GnT-IV isozymes (GnT-IVa and GnT-IVb) had been identified, which exhibit different expression profiles among human tissues and cancer cell lines. To clarify the enzymatic properties of the respective enzymes, their kinetic parameters were determined using recombinant full-length enzymes expressed in COS7 cells. The K (m) of human GnT-IVb for UDP-GlcNAc was estimated to be 0.24 mM, which is 2-fold higher than that of human GnT-IVa. The K (m) values of GnT-IVb for pyridylaminated (PA) acceptor sugar chains with different branch numbers were 3- to 6-fold higher than those of GnT-IVa. To compare substrate specificities more precisely, we generated recombinant soluble enzymes of human GnT-IVa and GnT-IVb with N-terminal flag tags. Both enzymes showed similar substrate specificities as determined using fourteen PA-sugar chains. They preferred complex-type N-glycans over hybrid-types. Among the complex-type N-glycans tested, the relative activities of both enzymes were increased in proportion to the number of GlcNAc branches on the Man alpha1-6 arm. The Man alpha1-6 arm of the acceptors was not essential for their activities because a linear pentasaccharide lacking this arm, GlcNAcbeta1-2Manalpha1-3Manbeta1-4GlcNAcbeta1-4 GlcNAc-PA, was a substrate for both enzymes. These results indicate that human GnT-IVb exhibits the same acceptor substrate specificities as human GnT-IVa, although GnT-IVb has lower affinities for donors or acceptors than GnT-IVa. This suggests that GnT-IVa is more active than GnT-IVb under physiological conditions and that it primarily contributes to the biosynthesis of N-glycans.