Recent progress in the molecular biology of the cloned N-acetylglucosaminyltransferases

Glycomic analysis of gastric carcinoma cells discloses glycans as modulators of RON receptor tyrosine kinase activation in cancer

BACKGROUND

Terminal α2-3 and α2-6 sialylation of glycans precludes further chain elongation, leading to the biosynthesis of cancer relevant epitopes such as sialyl-Lewis X (SLe(X)). SLe(X) overexpression is associated with tumor aggressive phenotype and patients' poor prognosis.

METHODS

MKN45 gastric carcinoma cells transfected with the sialyltransferase ST3GAL4 were established as a model overexpressing sialylated terminal glycans. We have evaluated at the structural level the glycome and the sialoproteome of this gastric cancer cell line applying liquid chromatography and mass spectrometry. We further validated an identified target expression by proximity ligation assay in gastric tumors.

RESULTS

Our results showed that ST3GAL4 overexpression leads to several glycosylation alterations, including reduced O-glycan extension and decreased bisected and increased branched N-glycans. A shift from α2-6 towards α2-3 linked sialylated N-glycans was also observed. Sialoproteomic analysis further identified 47 proteins with significantly increased sialylated N-glycans. These included integrins, insulin receptor, carcinoembryonic antigens and RON receptor tyrosine kinase, which are proteins known to be key players in malignancy. Further analysis of RON confirmed its modification with SLe(X) and the concomitant activation. SLe(X) and RON co-expression was validated in gastric tumors.

CONCLUSION

The overexpression of ST3GAL4 interferes with the overall glycophenotype of cancer cells affecting a multitude of key proteins involved in malignancy. Aberrant glycosylation of the RON receptor was shown as an alternative mechanism of oncogenic activation.

GENERAL SIGNIFICANCE

This study provides novel targets and points to an integrative tumor glycomic/proteomic-profiling for gastric cancer patients' stratification. This article is part of a Special Issue entitled "Glycans in personalised medicine" Guest Editor: Professor Gordan Lauc.

Remodeling the oligosaccharides on β-glucocerebrosidase using hydrophobic interaction chromatography and applications of hydroxyl ethyl starch for improving remodeling and enhancing protein stability

In this article, we describe a hydrophobic interaction chromatography (HIC) method to remodel the carbohydrates on recombinant human β-glucocerebrosidase (GCR) and the use of hydroxyl ethyl starch (HES) an ethylated starch polymer, to improve this process. GCR is a therapeutic protein used in the treatment of Gaucher disease, a life threatening condition in which patients lack sufficient functional levels of this enzyme. Gaucher disease is the most common inherited lysosomal storage disorder resulting in hepatomegaly, splenomegaly, and bone and lung pathology due to the accumulation of glucosylceramide in the lysosomes of macrophages (Beutler and Grabowski, 2001). The oligosaccharide remodeling of GCR, performed on HIC using three enzymes that remove sugars, increases macrophage uptake through the mannose receptor and thereby lowers its therapeutic dose versus unmodified GCR (Furbish et al., 1981; Van Patten et al., 2007). In this article we describe findings that the addition of HES lowered the amounts of three deglycosylating enzymes needed for remodeling GCR. HES also stabilized the activity of α-glucosidase, α-galactosidase, and GCR under conditions in which these three enzymes rapidly lose activity in the absence of this polymer. Circular dichroism (CD) and second derivative UV spectroscopy revealed that the secondary and tertiary structure of α-glucosidase was unchanged while for GCR there was a slight compaction of the secondary structure but no apparent affect on the tertiary structure. The thermal stability of both GCR and α-glucosidase were enhanced by HES as both molecules showed an increased transition midpoint (T(m)).

Structure and function of beta -1,4-galactosyltransferase

Beta-1,4-galactosylransferase (beta4Gal-T1) participates in the synthesis of Galbeta1-4-GlcNAc-disaccharide unit of glycoconjugates. It is a trans-Golgi glycosyltransferase (Glyco-T) with a type II membrane protein topology, a short N-terminal cytoplasmic domain, a membrane-spanning region, as well as a stem and a C-terminal catalytic domain facing the trans-Golgi-lumen. Its hydrophobic membrane-spanning region, like that of other Glyco-T, has a shorter length compared to plasma membrane proteins, an important feature for its retention in the trans-Golgi. The catalytic domain has two flexible loops, a long and a small one. The primary metal binding site is located at the N-terminal hinge region of the long flexible loop. Upon binding of metal ion and sugar-nucleotide, the flexible loops undergo a marked conformational change, from an open to a closed conformation. Conformational change simultaneously creates at the C-terminal region of the flexible loop an oligosaccharide acceptor binding site that did not exist before. The loop acts as a lid covering the bound donor substrate. After completion of the transfer of the glycosyl unit to the acceptor, the saccharide product is ejected; the loop reverts to its native conformation to release the remaining nucleotide moiety. The conformational change in beta4Gal-T1 also creates the binding site for a mammary gland-specific protein, alpha-lactalbumin (LA), which changes the acceptor specificity of the enzyme toward glucose to synthesize lactose during lactation. The specificity of the sugar donor is generally determined by a few residues in the sugar-nucleotide binding pocket of Glyco-T, conserved among the family members from different species. Mutation of these residues has allowed us to design new and novel glycosyltransferases, with broader or requisite donor and acceptor specificities, and to synthesize specific complex carbohydrates as well as specific inhibitors for these enzymes.

Molecular cloning of pigGnT-I and I.2: An application to xenotransplantation

Xenotransplantation is one of the most attractive solutions for the current worldwide shortage of organs. The knocking out of alpha1,3-galactosyltransferase in pigs resulted in a drastic reduction in xenoantigenicity. However, more recent studies indicate that other xeno-antigens, so-called non-Gal antigens, will also need to be downregulated. In this study, pig N-acetylglucosaminyltransferase I (GnT-I), a key enzyme that initiates the biosynthesis of hybrid- and complex-type N-linked sugar chains, was isolated and the pigGnT-I.2 specific for the O-linked sugar chain was also isolated. Point mutants, pigGnT-I(123) and pigGnT-I(320), were subsequently constructed. While pigGnT-I(123) shows an indistinct dominant negative effect for endogenous GnT-I in pig cells, pigGnT-I(320) had a drastic effect. In addition, in the case of pig cell transfectants with pigGnT-I(320), cell surface carbohydrate structures were significantly altered and its antigenicity to human serum was reduced. Consequently, pigGnT-I(320) appears to be potentially useful in xenotransplantation by remodeling the carbohydrate structures on pig cells.

Substrate-induced conformational changes in glycosyltransferases

Oligosaccharide chains of glycoproteins, glycolipids and glycosaminoglycans are synthesized by glycosyltransferases by the transfer of specific glycosyl moieties from activated sugar-nucleotide donors to specific acceptors. Structural studies on several of these enzymes have shown that one or two flexible loops at the substrate-binding site of the enzymes undergo a marked conformational change from an open to a closed conformation on binding the donor substrate. This conformational change, in which the loop acts as a lid covering the bound donor substrate, creates an acceptor-binding site. After the glycosyl unit is transferred from the donor to the acceptor, the saccharide product is ejected and the loop reverts to its native conformation, thereby releasing the remaining nucleotide moiety. The specificity of the sugar donor is determined by a few residues in the sugar-nucleotide-binding pocket of the enzyme that are conserved among the family members from different species.

Down-regulation of hydrogen peroxide-induced PKC delta activation in N-acetylglucosaminyltransferase III-transfected HeLaS3 cells

N-acetylglucosaminyltransferase III (GnT-III) is a key enzyme that inhibits the extension of N-glycans by introducing a bisecting N-acetylglucosamine residue. Our previous studies have shown that modification of N-glycans by GnT-III affects a number of intracellular signaling pathways. In this study, the effects of GnT-III on the cellular response to reactive oxygen species (ROS) were examined. We found that an overexpression of GnT-III suppresses H(2)O(2)-induced apoptosis in HeLaS3 cells. In the case of GnT-III transfectants, activation of Jun N-terminal kinase (JNK) following H(2)O(2) treatment was markedly reduced compared with control cells. Either the depletion of protein kinase C (PKC) by prolonged treatment with phorbol 12-myristate 13-acetate or the inhibition of PKC by the specific inhibitor H7 attenuated the H(2)O(2)-induced activation of JNK1 and apoptosis in control cells but not in the GnT-III transfectants. Furthermore, we found that H(2)O(2)-induced phosphorylation of PKC delta was markedly suppressed in GnT-III transfectants. Rottlerin, a specific inhibitor of PKC delta, significantly inhibited H(2)O(2)-induced activation of JNK1 in control cells, indicating that PKC delta is involved in the pathway. These findings suggest that the overexpression of GnT-III suppresses H(2)O(2)-induced activation of PKC delta-JNK1 pathway, resulting in inhibition of apoptosis.

Overexpression of N-acetylglucosaminyltransferase III enhances the epidermal growth factor-induced phosphorylation of ERK in HeLaS3 cells by up-regulation of the internalization rate of the receptors

N-Acetylglucosaminyltransferase III (GnT-III) is a key enzyme that inhibits the extension of N-glycans by introducing a bisecting N-acetylglucosamine residue. In this study we investigated the effect of GnT-III on epidermal growth factor (EGF) signaling in HeLaS3 cells. Although the binding of EGF to the epidermal growth factor receptor (EGFR) was decreased in GnT-III transfectants to a level of about 60% of control cells, the EGF-induced activation of extracellular signal-regulated kinase (ERK) in GnT-III transfectants was enhanced to approximately 1.4-fold that of the control cells. A binding analysis revealed that only low affinity binding of EGF was decreased in the GnT-III transfectants, whereas high affinity binding, which is considered to be responsible for the downstream signaling, was not altered. EGF-induced autophosphorylation and dimerization of the EGFR in the GnT-III transfectants were the same levels as found in the controls. The internalization rate of EGFR was, however, enhanced in the GnT-III transfectants as judged by the uptake of (125)I-EGF and Oregon Green-labeled EGF. When the EGFR internalization was delayed by dansylcadaverine, the up-regulation of ERK phosphorylation in GnT-III transfectants was completely suppressed to the same level as control cells. These results suggest that GnT-III overexpression in HeLaS3 cells resulted in an enhancement of EGF-induced ERK phosphorylation at least in part by the up-regulation of the endocytosis of EGFR.

Remodeling of sugar chain structures of human interferon-gamma

Natural human interferon (IFN)-gamma has mainly biantennary complex-type sugar chains and scarcely has multiantennary structures. We attempted to remodel the sugar chain structures using IFN-gamma as a model glycoprotein. To obtain the branching glycoforms of IFN-gamma, we introduced the genes for GnT-IV (UDP-N-acetylglucosamine:alpha-1,3-D-mannoside beta-1, 4-N-acetylglucosaminyltransferase) and/or GnT-V (UDP-N-acetylglucosamine:alpha-1,6-D-mannoside beta-1, 6-N-acetylglucosaminyltransferase) into Chinese hamster ovary (CHO) cells producing human IFN-gamma. The parental CHO cells produced IFN-gamma with biantennary sugar chains mainly. When the GnT-IV activity was increased, triantennary sugar chains with a branch produced by GnT-IV increased up to 66.9% of the total sugar chains. When the GnT-V activity was increased, triantennary sugar chains with a corresponding branch increased up to 55.7% of the total sugar chains. When the GnT-IV and -V activities were increased at a time, tetraantennary sugar chains increased up to 56.2% of the total sugar chains. The proportion of these multiantennary sugar chains corresponded to the intracellular activities of GnT-IV and -V. What is more, lectin blot and flow cytometric analysis indicated that the multi-branch structure of the sugar chains was increased not only on IFN-gamma, one of the secretory glycoproteins, but also on almost CHO cellular proteins by introducing either or both of the GnT genes. The results suggest that the branching structure of sugar chains of glycoproteins could be controlled by cellular GnT-IV and GnT-V activities. This technology can produce glycoforms out of natural occurrence, which should enlarge the potency of glycoprotein therapeutics.

Upregulation of mRNA for the melanocortin-1 receptor but not for melanogenic proteins in macrophage x melanoma fusion hybrids exhibiting increased melanogenic and metastatic potential

Fusion of mouse peritoneal macrophages or human blood monocytes with weakly metastatic mouse Cloudman S91 melanoma cells resulted in hybrids with enhanced metastatic potential (Rachkovsky et al., 1998. Clin. Exp. Metastasis, 16: 299-312). With few exceptions, such hybrids also showed increased basal- and MSH-induced pigmentation, at least in part through increased N-glycosylation of melanogenic proteins (Sodi et al., 1998. Pigment Cell Res., 11: 299-309). Here we report analyses regarding expression of the melanocyte-stimulating hormone (MSH) receptor (melanocortin-1 receptor, MC1-R) and the melanogenic proteins, tyrosinase (E.C. 1.14.18.1), tyrosinase-related protein 1 (TRP-1), and the tyrosinase-related protein 2 (TRP-2, E.C. 5.3.2.3), by a panel of cell lines consisting of parental Cloudman S91 melanoma cells, macrophages from DBA/2J mice, artificially derived macrophage x melanoma hybrids of high and low metastatic potential, and a naturally occurring highly metastatic hybrid between a Cloudman S91 tumor cell and a DBA/2J tumor-infiltrating cell. We show that incubation of cells with MSH/isobutylmethylxanthine (IBMX) resulted in strong melanogenic and morphologic responses in high metastatic hybrids compared to parental cells and the low metastatic hybrid, and that high metastatic hybrids exhibit increased mRNA expression for MC1-R accompanied by increased 125I-alphaMSH binding. Although tyrosinase activity and the protein level for tyrosinase and TRP-2, but not for TRP-1, were increased in the high metastatic hybrids versus the other cells, no significant changes in mRNA either for tyrosinase or for TRPs were observed in them. Furthermore, unlike tyrosinase, the abundance and gel mobility pattern of TRP-2 did not correlate with changes in activity in all hybrids and parental melanoma cells. The results suggest that although the activity MC1-R and tyrosinase correlate with enhanced basal as well as MSH-induced melanogenesis in metastatic/melanotic hybrids, their expression is differentially regulated, i.e., regulation of MC1-R while at transcriptional level, the TRPs are primarily regulated via post-transcriptional mechanisms in high metastatic hybrids.

Implication of N-acetylglucosaminyltransferases III and V in cancer: gene regulation and signaling mechanism

N-acetylglucosaminyltransferases III (GnT-III) and V (GnT-V) play a pivotal role in the processing of N-linked glycoproteins, and are highly involved in cancer progression and metastasis. Expression of GnT-III and GnT-V in the liver is enhanced during hepatocarcinogenesis, although they are not expressed in the normal liver. Gene expression of GnT-V is regulated by a transcriptional factor, ets-1, which is involved in angiogenesis and invasion of tumor cells. When the formation of the product of GnT-V, GlcNAc-beta1-6 branches, is inhibited by overexpression of GnT-III, lung metastasis of melanoma cells is suppressed. Modification of glycoprotein receptors such as the receptors for epidermal growth factor and nerve growth factor by GnT-III sense transfection changes an intracellular signaling pathway, which may lead to a variety of biological alterations in tumor cells. In this review, we focus on cancer progression and metastasis in relation to GnT-III and GnT-V.

Tissue specific expression and chromosomal mapping of a human UDP-N-acetylglucosamine: alpha1,3-d-mannoside beta1, 4-N-acetylglucosaminyltransferase

A human cDNA for UDP- N -acetylglucosamine:alpha1,3-d-mannoside beta1,4- N- acetylglucosaminyltransferase (GnT-IV) was isolated from a liver cDNA library using a probe based on a partial cDNA sequence of the bovine GnT-IV. The cDNA encoded a complete sequence of a type II membrane protein of 535 amino acids which is 96% identical to the bovine GnT-IV. Transient expression of the human cDNA in COS7 cells increased total cellular GnT-IV activity 25-fold, demonstrating that this cDNA encodes a functional human GnT-IV. Northern blot analysis of normal tissues indicated that at least five different sizes of mRNA (9.7, 7.6, 5.1, 3.8, and 2.4 kb) forGnT-IV are expressed in vivo. Furthermore, these mRNAs are expressed at different levels between tissues. Large amounts of mRNA were detected in tissues harboring T lineage cells. Also, the promyelocytic leukemia cell line HL-60 and the lymphoblastic leukemia cell line MOLT-4 revealed abundant mRNA. Lastly, the gene was mapped at the locus on human chromosome 2, band q12 by fluorescent in situ hybridization.

The yeast expression system for recombinant glycosyltransferases

Glycosyltransferases are increasingly being used for in vitro synthesis of oligosaccharides. Since these enzymes are difficult to purify from natural sources, expression systems for soluble forms of the recombinant enzymes have been developed. This review focuses on the current state of development of yeast expression systems. Two yeast species have mainly been used, i.e. Saccharomyces cerevisiae and Pichia pastoris. Safety and ease of fermentation are well recognized for S. cerevisiae as a biotechnological expression system; however, even soluble forms of recombinant glycosyltransferases are not secreted. In some cases, hyperglycosylation may occur. P. pastoris, by contrast, secrete soluble orthoglycosylated forms to the supernatant where they can be recovered in a highly purified form. The review also covers some basic features of yeast fermentation and describes in some detail those glycosyltransferases that have successfully been expressed in yeasts. These include beta1,4galactosyltransferase, alpha2,6sialyltransferase, alpha2,3sialyltransferase, alpha1,3fucosyltransferase III and VI and alpha1,2mannosyltransferase. Current efforts in introducing glycosylation systems of higher eukaryotes into yeasts are briefly addressed.

A novel second isoenzyme of the human UDP-N-acetylglucosamine:alpha1,3-D-mannoside beta1,4-N-acetylglucosaminyltransferase family: cDNA cloning, expression, and chromosomal assignment

We isolated a novel cDNA encoding a second isoenzyme of UDP-N-acetylglucosamine:alpha1,3-D-mannoside beta1,4-N-acetylglucosaminyltransferase (GnT-IV; EC 2.4.1.145). The nucleotide and deduced amino acid sequences of the cDNA were homologous to those of the previously cloned human GnT-IV cDNA (63% and 62% identity, respectively). The new cDNA is also confirmed to express GnT-IV activity, suggesting that two isoenzymes of human GnT-IV exist. Although genomic Southern analysis suggested that both genes exist in many mammalian species and the chicken, northern analysis revealed that both genes are expressed in different ways in human tissues. This is the first report concerning the gene family of an N-acetylglucosaminyltransferase in mammals.

Ectopic expression of N-acetylglucosaminyltransferase III in transgenic hepatocytes disrupts apolipoprotein B secretion and induces aberrant cellular morphology with lipid storage

N-Acetylglucosaminyltransferase III (GnT-III) produces "bisecting-GlcNAc" and regulates the branching of N-glycans. GnT-III activity is elevated during hepatocarcinogenesis, which is in contrast to the undetectable level found in normal hepatocytes. To determine the biological significance of GnT-III in hepatocytes, transgenic mice that specifically express GnT-III in the liver were established and characterized. The transgenic hepatocytes had a swollen oval-like morphology, with many lipid droplets. Apolipoprotein B, which contained increased level of bisecting-GlcNAc accumulated in the transgenic hepatocytes. In the transgenic serum, triglycerides, the beta- and pre-beta-lipoprotein fractions, and apolipoprotein B100 were significantly decreased, compared with levels in nontransgenic serum. These abnormal phenotypes were more prominent in the mice with more copies of the transgene and a resulting high GnT-III activity. We demonstrate that aberrant glycosylation, as the direct result of the formation of bisecting-GlcNAc, disrupts the function of apolipoprotein B, leading to the generation of fatty liver. This observation suggests a novel mechanism for the pathogenesis of fatty liver.

Purification and characterization of UDP-N-acetylglucosamine: alpha1,3-D-mannoside beta1,4-N-acetylglucosaminyltransferase (N-acetylglucosaminyltransferase-IV) from bovine small intestine

A new beta1,4-N-acetylglucosaminyltransferase (GnT) which involves in branch formation of Asn-linked complex-type sugar chains has been purified 224,000-fold from bovine small intestine. This enzyme requires divalent cations, such as Mn2+, and catalyzes the transfer of GlcNAc from UDP-GlcNAc to biantennary oligosaccharide and produces triantennary oligosaccharide with the beta1-4-linked GlcNAc residue on the Manalpha1-3 arm. The purified enzyme shows a single band of Mr 58,000 and behaves as a monomer. The substrate specificity demonstrated that the beta1-2-linked GlcNAc residue on the Manalpha1-3 arm (GnT-I product) is essential for the enzyme activity. beta1-4-Galactosylaion to this essential beta1-2-linked GlcNAc residue or N-acetylglucosaminylation to the beta-linked Man residue (bisecting GlcNAc, GnT-III product) blocks the enzyme action, while beta1-6-N-acetylglucosaminylation to the Manalpha1-6 arm (GnT-V product) increases the transfer. Based on these findings, we conclude that the purified enzyme is UDP-N-acetylglucosamine:alpha-3-D-mannoside beta-1,4-N-acetylglucosaminyltransferase IV (GnT-IV), that has been a missing link on biosynthesis of complex-type sugar chains.

Significant downregulation of the major swine xenoantigen by N-acetylglucosaminyltransferase III gene transfection

Introduction of the beta-D-mannoside beta-1,4-N-acetylglucosaminyltransferase III (GnT-III) gene into swine endothelial cells (SEC) reduced their susceptibility to normal human serum (NHS) in complement-mediated cell lysis and also suppressed the antigenicity to human natural antibodies as evidenced by flow cytometric analysis, as well as Griffonia simplicifolia 1 isolectin (1B4 lectin) binding to the Gal alpha1-3 Gal beta 1-4 GlcNAc-R (the alpha-galactosyl epitope). Western blot analysis indicated that proteins smaller than 66 kDa had diminished reactivity to NHS and 1B4 lectin. GnT-III, a key enzyme involved in branch formation of N-linked sugars, was found to downregulate the expression of xenoantigen, suggesting that this approach may be of value in clinical xenotransplantation in the future.

Expression of beta 1,6 branched asparagine-linked oligosaccharides in non-mitotic and non-migratory cells of normal human and rat tissues

Malignant transformation of cells leads to the synthesis of large asparagine-linked oligosaccharides that exhibit a higher degree of beta 1,6 branching. In rodent and human tumor cell lines and certain human tumors, increased beta 1,6 branching of oligosaccharides has been shown to be associated with metastasis. In addition, this structural change occurs in glycoproteins of stimulated normal human lymphocytes. The leukoagglutinating Phaseolus vulgaris lectin (L-PHA) has a high affinity for tri- and tetraantennary beta 1,6 branches carrying oligosaccharides and has been widely used for the detection of such structures by histochemistry and blotting. We have analyzed a spectrum of normal human and rat tissues using a sensitive silver-intensified lectin-gold technique. Staining by L-PHA was detected in undifferentiated cells of germinative layers of the gastrointestinal and respiratory tract as well as testis. However, differentiated and non-mitotic epithelia in most organs showed strong lectin staining as well. Notable exceptions were the epithelium of the colon and resting mammary gland, which were unreactive with L-PHA. The histochemical studies were supplemented by lectin blotting, which showed the presence of diverse L-PHA-reactive glycoproteins in rat tissues. Our data may be of importance for the use of L-PHA in studies on human malignant tumors.

Overexpression of N-acetylglucosaminyltransferase III disrupts the tyrosine phosphorylation of Trk with resultant signaling dysfunction in PC12 cells treated with nerve growth factor

beta-1,4-N-Acetylglucosaminyltransferase III (GnT-III: EC 2.4.1.144) is a pivotal glycosyltransferase which participates in branch formation by catalysis of the synthesis of a bisecting GlcNAc structure in N-glycans. These structures are thought to be one of the unique features of the N-glycans of neural tissues. To examine the intracellullar role of GnT-III expression and its product in neural cells, its gene was overexpressed in rat pheochromocytoma PC12 cells which normally express a low level of GnT-III. In the GnT-III gene-transfected cells, lectin blot analysis showed that some glycoproteins showed increased levels of bisecting GlcNAc structures. Following treatment with nerve growth factor (NGF) the control cells showed neurite outgrowth for differentiation whereas the transfectants showed no morphological response or change in the rate of cell growth. Transient tyrosine phosphorylation of the Trk/NGF receptor was detected at 5-15 min after NGF treatment in control cells, but not detected in the GnT-III gene-transfected cells despite the intact binding of NGF to the cells. Moreover the dimerization of Trk with NGF treatment was not induced in the GnT-III transfectant as compared with the dimerization seen in control cells. These results indicate that overexpression of GnT-III gene in PC12 cells affects some functions of glycoprotein receptors such as Trk by alteration of N-glycan structures, and results in changes in the intracellular signaling pathway of tyrosine phosphorylation modified by NGF.

Bisecting GlcNAc structures act as negative sorting signals for cell surface glycoproteins in forskolin-treated rat hepatoma cells

The bisecting N-acetylglucosamine residue is formed by UDP-N-acetylglucosamine:beta-D-mannoside-beta-1, 4-N-acetylglucosaminyltransferase III (GnT-III), a key branching enzyme for N-glycans. We found that forskolin, an adenylyl cyclase activator, markedly enhanced GnT-III at the transcriptional level in various hepatoma cells and hepatocytes, resulting in an increase of bisecting GlcNAc residues in various glycoproteins, as judged from the lectin binding to erythroagglutinating phytohemagglutinin (E-PHA). In whole cell lysates, the E-PHA binding was increased, and leukoagglutinating phytohemagglutinin (L-PHA) binding was decreased at 12 h after forskolin treatment, by time, both GnT-III activity and mRNA had reached the maximum levels. In contrast, the binding capacity as to E-PHA, determined by fluorescence-activated cell sorting on the cell surface, was decreased, suggesting that bisecting GlcNAc structures in certain glycoproteins changed the expression levels of glycoproteins and decreased their sorting on the cell surface. Fractionated organelles of M31 cells showed that the binding capacity as to E-PHA was mainly localized in Golgi membranes and lysosomes. This was also supported by a fluorescence microscopy. In order to determine whether or not the bisecting GlcNAc residue acts as a sorting signal for glycoproteins, N-oligosaccharide structures of lysosomal-associated membrane glycoprotein 1 and beta-glucuronidase, gamma-glutamyltranspeptidase, and secretory glycoproteins such as ceruloplasmin and alpha-fetoprotein were measured by E-PHA and L-PHA blotting after immunoprecipitation. The expression levels of lysosomal membrane glycoprotein 1 and gamma-glutamyltranspeptidase on the cell surface were decreased at 12 h after forskolin treatment, indicating that the bisecting GlcNAc structure may act as a negative sorting signal for the cell surface glycoproteins and may alter the characteristics of hepatoma cells. This is the first report on glycoprotein sorting related to a specific structure of oligosaccharides, bisecting GlcNAc.

Purification and cDNA cloning of porcine brain GDP-L-Fuc:N-acetyl-beta-D-glucosaminide alpha1-->6fucosyltransferase

GDP-L-Fuc:N-acetyl-beta-D-glucosaminide alpha1-->6fucosyltransferase (alpha1-6FucT; EC 2.4.1.68), which catalyzes the transfer of fucose from GDP-Fuc to N-linked type complex glycopeptides, was purified from a Triton X-100 extract of porcine brain microsomes. The purification procedures included sequential affinity chromatographies on GlcNAcbeta1-2Manalpha1-6(GlcNAcbeta1-2Manalpha1- 2)Manbeta1-4GlcNAcbet a1-4GlcNAc-Asn-Sepharose 4B and synthetic GDP-hexanolamine-Sepharose 4B columns. The enzyme was recovered in a 12% final yield with a 440, 000-fold increase in specific activity. SDS-polyacrylamide gel electrophoresis of the purified enzyme gave a major band corresponding to an apparent molecular mass of 58 kDa. The alpha1-6FucT has 575 amino acids and no putative N-glycosylation sites. The cDNA was cloned in to pSVK3 and was then transiently transfected into COS-1 cells. alpha1-6FucT activity was found to be high in the transfected cells, as compared with non- or mock-transfected cells. Northern blotting analyses of rat adult tissues showed that alpha1-6FucT was highly expressed in brain. No sequence homology was found with other previously cloned fucosyltransferases, but the enzyme appears to be a type II transmembrane protein like the other glycosyltransferases.

Transcriptional regulation of the N-acetylglucosaminyltransferase V gene in human bile duct carcinoma cells (HuCC-T1) is mediated by Ets-1

N-Acetylglucosaminyltransferase V (GnT-V) catalyzes the transfer of N-acetylglucosamine from UDP-N-acetylglucosamine to alpha-6-D-mannoside to produce the beta1-6 linked branching of N-glycan oligosaccharides, which controls the polylactosamine content. The expression of N-acetylglucosaminyltransferase V, which contains 17 exons and spans 155 kilobase pairs, is expressed in a tissue- and cell type-specific manner and is regulated at the level of transcription by multiple promoters (Saito, H., Gu, J., Nishikawa, A., Ihara, Y., Fujii, J., Kohgo, Y., and Taniguchi, N. (1995) Eur. J. Biochem. 233, 18-26). To elucidate the mechanism by which the GnT-V gene is expressed in a cell- and tissue-specific manner, cell-restricted expression was analyzed using the 5'-upstream regions of the human GnT-V gene spanning base pairs -2760 to +23 in a human bile duct carcinoma cell line, HuCC-T1. We characterized two cis-acting elements that are potentially important in HuCC-T1 cell-specific expression. The two elements each contain an Ets-1 binding site, 5'-GGA-3'. Specific binding of Ets-1 to the respective elements was demonstrated by competition analysis as well as by antibody supershift experiments. Cotransfection of an Ets-1 expression plasmid along with a GnT-V promoter-luciferase reporter plasmid revealed the participation of Ets-1 in the regulation of the GnT-V gene transcription. These data indicated that the transcriptional regulation of the GnT-V gene was mediated by transcription factor Ets-1.

Aberrant glycosylation of E-cadherin enhances cell-cell binding to suppress metastasis

Introduction of the beta1-4 N-acetylglucosaminyltransferase (GnT-III) gene was reported to suppress metastasis in highly metastatic B16-hm murine melanoma cells (Yoshimura, M., Nishikawa, A. , Ihara, Y., Taniguchi, S., and Taniguchi, N.(1995) Proc. Natl. Acad. Sci. U. S. A. 92, 8754-8758). In this study, the effect of GnT-III gene transfer on E-cadherin was studied, since E-cadherin acts as a suppressor of metastasis. E-cadherin expression at cell-cell contacts of B16-hm cells expressing high GnT-III activity was greater than controls without affecting transcription. Lectin blotting showed that E-cadherin from GnT-III transfectants was glycosylated by ectopically expressed GnT-III. The glycosylated E-cadherin exhibited the delayed turnover and the decreased release from cell surface, as compared with the native E-cadherin, resulting in the elevated expression at the cell-cell border of GnT-III transfectants. Furthermore, cell-cell aggregation was enhanced in GnT-III transfectants, indicating that the glycosylated E-cadherin is biologically functional. These results suggest that the glycosylated E-cadherin contributes to the suppression of metastasis by the introduction of GnT-III gene into melanoma cells.