Regulation of N-acetylglucosaminyltransferase V activity. Kinetic comparisons of parental, Rous sarcoma virus-transformed BHK, and L-phytohemagglutinin-resistant BHK cells using synthetic substrates and an inhibitory substrate analog

Structure and mechanism of cancer-associated N-acetylglucosaminyltransferase-V

N-acetylglucosaminyltransferase-V (GnT-V) alters the structure of specific N-glycans by modifying α1-6-linked mannose with a β1-6-linked N-acetylglucosamine branch. β1-6 branch formation on cell surface receptors accelerates cancer metastasis, making GnT-V a promising target for drug development. However, the molecular basis of GnT-V's catalytic mechanism and substrate specificity are not fully understood. Here, we report crystal structures of human GnT-V luminal domain with a substrate analog. GnT-V luminal domain is composed of a GT-B fold and two accessary domains. Interestingly, two aromatic rings sandwich the α1-6 branch of the acceptor N-glycan and restrain the global conformation, partly explaining the fine branch specificity of GnT-V. In addition, interaction of the substrate N-glycoprotein with GnT-V likely contributes to protein-selective and site-specific glycan modification. In summary, the acceptor-GnT-V complex structure suggests a catalytic mechanism, explains the previously observed inhibition of GnT-V by branching enzyme GnT-III, and provides a basis for the rational design of drugs targeting N-glycan branching.

Glycans and cancer: role of N-glycans in cancer biomarker, progression and metastasis, and therapeutics

Glycosylation is catalyzed by various glycosyltransferase enzymes which are mostly located in the Golgi apparatus in cells. These enzymes glycosylate various complex carbohydrates such as glycoproteins, glycolipids, and proteoglycans. The enzyme activity of glycosyltransferases and their gene expression are altered in various pathophysiological situations including cancer. Furthermore, the activity of glycosyltransferases is controlled by various factors such as the levels of nucleotide sugars, acceptor substrates, nucleotide sugar transporters, chaperons, and endogenous lectin in cancer cells. The glycosylation results in various functional changes of glycoproteins including cell surface receptors and adhesion molecules such as E-cadherin and integrins. These changes confer the unique characteristic phenotypes associated with cancer cells. Therefore, glycans play key roles in cancer progression and treatment. This review focuses on glycan structures, their biosynthetic glycosyltransferases, and their genes in relation to their biological significance and involvement in cancer, especially cancer biomarkers, epithelial-mesenchymal transition, cancer progression and metastasis, and therapeutics. Major N-glycan branching structures which are directly related to cancer are β1,6-GlcNAc branching, bisecting GlcNAc, and core fucose. These structures are enzymatic products of glycosyltransferases, GnT-V, GnT-III, and Fut8, respectively. The genes encoding these enzymes are designated as MGAT5 (Mgat5), MGAT3 (Mgat3), and FUT8 (Fut8) in humans (mice in parenthesis), respectively. GnT-V is highly associated with cancer metastasis, whereas GnT-III is associated with cancer suppression. Fut8 is involved in expression of cancer biomarker as well as in the treatment of cancer. In addition to these enzymes, GnT-IV and GnT-IX (GnT-Vb) will be also discussed in relation to cancer.

Reduction in Golgi apparatus dimension in the absence of a residential protein, N-acetylglucosaminyltransferase V

Various proteins are involved in the generation and maintenance of the membrane complex known as the Golgi apparatus. We have used mutant Chinese hamster ovary (CHO) cell lines Lec4 and Lec4A lacking N-acetylglucosaminyltransferase V (GlcNAcT-V, MGAT5) activity and protein in the Golgi apparatus to study the effects of the absence of a single glycosyltransferase on the Golgi apparatus dimension. Quantification of immunofluorescence in serial confocal sections for Golgi α-mannosidase II and electron microscopic morphometry revealed a reduction in Golgi volume density up to 49 % in CHO Lec4 and CHO Lec4A cells compared to parental CHO cells. This reduction in Golgi volume density could be reversed by stable transfection of Lec4 cells with a cDNA encoding Mgat5. Inhibition of the synthesis of β1,6-branched N-glycans by swainsonine had no effect on Golgi volume density. In addition, no effect on Golgi volume density was observed in CHO Lec1 cells that contain enzymatically active GlcNAcT-V, but cannot synthesize β1,6-branched glycans due to an inactive GlcNAcT-I in their Golgi apparatus. These results indicate that it may be the absence of the GlcNAcT-V protein that is the determining factor in reducing Golgi volume density. No dimensional differences existed in cross-sectioned cisternal stacks between Lec4 and control CHO cells, but significantly reduced Golgi stack hits were observed in cross-sectioned Lec4 cells. Therefore, the Golgi apparatus dimensional change in Lec4 and Lec4A cells may be due to a compaction of the organelle.

Concise syntheses of selective inhibitors against α-1,3-galactosyltransferase

Several iminosugar-based uridine diphosphate galactose (UDP-Gal) mimetics 1-4 including d- and l-epimers were designed and synthesized by concise routes, and these synthetic compounds were evaluated for the inhibition of α-1,3- and β-1,4-galactosyltransferases in vitro. The experimental data demonstrated that l-epimer 2 displayed the strongest inhibitory activity with moderate selectivity against α-1,3-galactosyltransferase.

Glycan antagonists and inhibitors: a fount for drug discovery

Glycans, the carbohydrate chains of glycoproteins, proteoglycans, and glycolipids, represent a relatively unexploited area for drug development compared with other macromolecules. This review describes the major classes of glycans synthesized by animal cells, their mode of assembly, and available inhibitors for blocking their biosynthesis and function. Many of these agents have proven useful for studying the biological activities of glycans in isolated cells, during embryological development, and in physiology. Some are being used to develop drugs for treating metabolic disorders, cancer, and infection, suggesting that glycans are excellent targets for future drug development.

High-Throughput Screening for Enzyme Inhibitors Using Frontal Affinity Chromatography with Liquid Chromatography and Mass Spectrometry

This work presents new frontal affinity chromatography (FAC) methodologies for high-throughput screening of compound libraries, designed to increase screening rates and improve sensitivity and ruggedness in performance. A FAC column constructed around the enzyme N-acetylglucosaminyltransferase V (GnT-V) was implemented in the identification of potential enzyme inhibitors from two libraries of trisaccharides. Effluent from the FAC column was fractionated, sequentially processed via LC/MS, and referenced to a similar analysis through a control FAC column lacking the enzyme. The resulting multidimensional data sets were compared across corresponding sample and control fractions to identify binders, in a semiautomated approach. A strong binder in the protonated form at m/z 795 was identified from the first library of 81 compounds, exhibiting an estimated Kd value of 0.3 microM. Other binders yielded Kd values ranging from 0.35 to 3.35 microM. To demonstrate the improvement in performance of this FAC-LC/MS approach over the conventional online FAC/MS approach, 15 compounds from this library were blended with a second library of 1000 synthetic trisaccharides and screened against GnT-V. All ligands in the 15-compound set were identified in this larger screen, and no ligands of greater affinity than compound 1 were found. Our results show that FAC-LC/MS is a reliable method for screening large compound libraries directly and useful for large-scale ligand discovery initiatives.

Glycoscience finally comes of age

To take its place alongside genomics and proteomics, glycoscience needs recognition from scientists

UDP-GlcNAc concentration is an important factor in the biosynthesis of beta1,6-branched oligosaccharides: regulation based on the kinetic properties of N-acetylglucosaminyltransferase V

Human beta1,6-N-acetylglucosaminyltransferase V (GnT-V) was expressed by baculovirus-insect cell system, and the purified recombinant enzyme was kinetically characterized. The data obtained were used to establish the kinetic basis of the substrate specificity toward donor nucleotide sugars, and also revealed that K(m) values for the donors are much higher compared to those of other GlcNAc transferases, the kinetic properties of which have been reported. Because this exceptionally higher K(m) suggests that GnT-V is physiologically present at far from saturated conditions, it would appear that the production of beta1,6-branched oligosaccharide, which is formed by GnT-V, could be regulated in vivo by the concentration of the donor, UDP-GlcNAc, as well as the expression levels of the enzyme. When B16 melanoma cells, which express high levels of GnT-V, were incubated with GlcNAc, the beta1,6-branched oligosaccharide levels were increased, as judged by a lectin blot analysis, in conjunction with an increase in intracellular UDP-GlcNAc. These findings suggest that the level of UDP-GlcNAc can be a critical factor in the production of beta1,6-branched oligosaccharides, for example, by tumor cells, which have been thought to be closely associated with tumor progression and metastasis.

Frontal affinity chromatography coupled to mass spectrometry for screening mixtures of enzyme inhibitors

Frontal affinity chromatography coupled online to mass spectrometry (FAC/MS) has previously been used to estimate binding constants for individual protein ligands present in mixtures of compounds. In this study FAC/MS is used to determine enzyme substrate kinetic parameters and binding constants for enzyme inhibitors. Recombinant human N-acetylglucosaminyltransferase V was biotinylated and adsorbed onto immobilized streptavidin in a microcolumn (20 microL). The enzyme was shown to be catalytically competent transferring GlcNAc from the donor UDP-GlcNAc to beta-d-GlcpNAc-(1-->2)-alpha-d-Manp-(1-->6)-beta-d-Glcp-OR acceptor giving beta-d-GlcpNAc-(1-->2)-[beta-d-GlcpNAc-(1-->6)]-alpha-d-Manp-(1-->6)-beta-d-Glcp-OR as the reaction product. The kinetic parameters K(m) and V(max) for the immobilized enzyme could be determined by FAC/MS and were comparable to those measured in solution. Analysis of a mixture of eight trisaccharide analogs in a single run yielded K(d) values for each of the eight compounds ranging from 0.3 to 36 microM. These K(d) values were 2 to 10 times lower than the inhibition constants, K(I)'s, determined in solution using a standard radiochemical assay. However, the ranking order of K(d)'s was the same as the ranking of K(I) values. FAC/MS assays can therefore be employed for the rapid estimation of inhibitor K(d) values making it a valuable tool for enzyme inhibitor evaluations.

Correlated gene expression between beta-1,4-galactosyltransferase V and N-acetylglucosaminyltransferase V in human cancer cell lines

Since our previous study showed that the gene expression level of beta-1,4-galactosyltransferase (beta-1,4-GalT) V is only increased in mouse NIH3T3 transformant and that beta-1,4-GalT V preferentially galactosylates the GlcNAcbeta1 --> 6Man branch of oligosaccharides [Shirane et al. (1999) Biochem. Biophys. Res. Commun. 265, 434-438], whether its gene expression is correlated with malignant transformation was investigated. Northern blot analysis of beta-1, 4-GalTs I, II, III, IV, V, and VI and N-acetylglucosaminyltransferase (GlcNAcT)V in human cancer cell lines showed that the gene expression levels of beta-1,4-GalT V but not other beta-1,4-GalTs are strongly correlated with those of GlcNAcT V whose activity was shown to increase by malignant transformation. These results indicate that beta-1,4-GalT V is involved in the galactosylation of highly branched oligosaccharides characteristic of malignantly transformed cells.

Minimal catalytic domain of N-acetylglucosaminyltransferase V

UDP-GlcNAc: Manalpha1-6Manbeta-R beta1-6 N-acetylglucosaminyltransferase V (EC 2.4.1.155, GlcNAc-TV) is a Golgi enzyme that substitutes the trimannosyl core in the biosynthetic pathway for complex-type N-linked glycans. GlcNAc-TV activity is regulated by oncogenes frequently activated in cancer cells ( ras, src, and her2/neu ) and by activators of T lymphocytes. Overexpression of GlcNAc-TV in epithelial cells results in morphological transformation, while tumor cell mutants selected for loss of GlcNAc-TV products show diminished malignant potential in mice. In this report, we have expressed and characterized a series of N- and C-terminal deletions of GlcNAc-TV. Portions of GlcNAc-TV sequence were fused at the N-terminal domain to IgG-binding domains of staphylococcal Protein A and expressed in CHOP cells. The secreted fusion proteins were purified by IgG Sepharose affinity chromatography and assayed for enzyme activities. The peptide sequence S(213-740)of GlcNAc-TV was determined to be essential for the catalytic activity, the remaining amino acids comprising a 183 amino acid stem region, a 17 amino acid transmembrane domain and a 12 amino acid cytosolic moiety. Further deletion of 5 amino acids to produce peptide R(218-740)reduced enzyme activity by 20-fold. Similar K(m)and V(max)values for donor and acceptor were observed for peptide S(213-740), the minimal catalytic domain, and peptide Q(39-740), which also included the stem region. Truncation of five amino acids from the C-terminus also resulted in a 20-fold loss of catalytic activity. Secondary structure predictions suggest a high frequency of turns in the stem region, and more contiguous stretches of alpha-helix found in the catalytic domain.

Involvement of beta-1,4-galactosyltransferase V in malignant transformation-associated changes in glycosylation

In spite of marked changes in the glycosylation upon malignant transformation of cells, no biological significance of beta-1, 4-galactosyltransferase (beta-1,4-GalT) activities has been elucidated. When beta-1,4-GalT activities toward 1 mM GlcNAcbeta-S-pNP were determined using homogenates of NIH3T3 and its transformant, MTAg, MTAg contained 1.3 times higher activities. Northern blot analysis, however, revealed that the beta-1,4-GalT V gene expression increases by three times with a decrease in that of beta-1,4-GalT II by one-fifth and without significant changes in those of other beta-1,4-GalTs in MTAg. Analysis of beta-1,4-GalT V acceptor-specificity showed that the GlcNAcbeta1-->6Man group of the GlcNAcbeta1-->6(GlcNAbeta1-->2)Manalpha1- branch is galactosylated. These results indicate that changes in beta-1,4-GalT II and V activities are important for the altered glycosylation.

Characterization of UDP-N-acetylglucosamine:alpha-6-d-mannoside beta-1,6-N-acetylglucosaminyltransferase V from a human hepatoma cell line Hep3B

UDP-N-acetylglucosamine:alpha-6-d-mannoside beta-1, 6-N-acetylglucosaminyltransferase V (GlcNAcT-V) has been purified from cell extracts of the human hepatoma cell line, Hep3B, with 8.7% recovery. The purified enzymes had molecular masses of about 67 and 65 kDa on denaturated and natural conditions, respectively. The values of pI was 5.9. The GlcNAcT-V, when resolved by SDS-PAGE, was positive for Schiff staining, suggesting that the enzyme is glycoprotein. When GlcN,GlcN-biant-PA and UDP-GlcNAc were used as substrates, the enzyme displayed a temperature optimum of around 50 degrees C and optimum an pH of 6.5. The enzyme was stable in response to incubation from pH 4.5 to pH 10.5 at 4 degrees C for 24 h. The presence of UDP-GlcNAc and GlcN,GlcN-bi-PA protected the enzyme from heat inactivation, the extent depending upon the substrate concentration. The activity of the enzyme was stimulated by Mn2+ ion; however, it was inhibited by Fe3+. The enzyme activity was inhibited by another series of NDP-sugars including ADP-, CDP-, GDP-, and TDP-GlcNAc. Studies on the activity of the enzyme toward a variety of pyridylaminated sugars showed that the enzyme is most active toward biantennary (GlcN,GlcN-bi-PA) sugars. The enzymes had apparent Km values of 1.28 and 5.8 mM for GlcN,GlcN-bi-PA and UDP-GlcNAc, respectively. In order to isolate the GlcNAcT-V gene, PCR primers of GNN-1 and GNN-8 were designed and the amplified PCR product carrying the gene was cloned and sequenced. Nucleotide sequence analysis showed a 2220-bp open reading frame encoding a 740-amino-acid protein. This was almost same as the previously reported human sequences, except for some sequence differences in three amino acids. The three amino acid changes were as follows: 375V --> L, 555T --> R, and 592A --> G. These studies represent the detailed characterization of a purified GlcNAcT-V from human hepatoma cell Hep3B.

Synthesis and evaluation of eight aminodeoxy trisaccharide inhibitors for N-acetylglucosaminyltransferase-V

N-Acetylglucosaminyltransferase-V is an important enzyme controlling the branching pattern of N-linked oligosaccharides. This enzyme recognizes the trisaccharide octyl 2-acetamido-2-deoxy-beta-D-glucopyranosyl-(1-->2)-alpha-D-mannopyranosyl -(1-->6)-beta-D-glucopyranoside (5) as a substrate and adds a beta-linked GlcNAc residue to OH-6 of the central alpha-Man unit. Eight analogs of 5 were chemically synthesized where C-6 of the alpha-Man residue in 5 was deoxygenated, and structurally diverse modifications were introduced at C-4 of the same residue. The key intermediate prepared for this purpose was octyl 2-acetamido-2-deoxy-beta-D- glucopyranosyl-(1-->2)-4-amino-4,6-dideoxy-alpha-D-mannopyranosyl- (1-->6)-beta-D-glucopyranoside (7a) where the original 4'-amino group was readily derivatized on the unprotected sugar. The eight analogs 7a-7h were evaluated as inhibitors for GlcNAcT-V, both isolated (from hamster kidney) and cloned (from rat kidney). All of the compounds were found to be competitive inhibitors with Ki in the range of 3-106 microM. The conclusion of this work is that recognition of acceptor 5 does not involve contact of the C-6--C-4 end of the alpha-Man residue with the protein in the E-I (or E-S) complex.

Transcriptional regulation of N-acetylglucosaminyltransferase V by the src oncogene

Transformation of baby hamster kidney fibroblasts by the Rous sarcoma virus causes a significant increase in the GlcNAcbeta(1, 6)Man-branched oligosaccharides by elevating the activity and mRNA transcript levels encoding N-acetylglucosaminyltransferase V (GlcNAc-T V). Elevated activity and mRNA levels could be inhibited by blocking cell proliferation with herbimycin A, demonstrating that Src kinase activity can regulate GlcNAc-T V expression. 5' RACE analysis was used to identify a 3-kilobase 5'-untranslated region from GlcNAc-T V mRNA and locate a transcriptional start site in a 25-kilobase pair GlcNAc-T V human genomic clone. A 6-kilobase pair fragment of the 5' region of the gene contained AP-1 and PEA3/Ets binding elements and, when co-transfected with a src expression plasmid into HepG2 cells, conferred src-stimulated transcriptional enhancement upon a luciferase reporter gene. This stimulation by src could be antagonized by co-transfection with a dominant-negative mutant of the Raf kinase, suggesting the involvement of Ets transcription factors in the regulation of GlcNAc-T V gene expression. The src-responsive element was localized by 5' deletion analysis to a 250-base pair region containing two overlapping Ets sites. src stimulation of transcription from this region was inhibited by co-transfection with a dominant-negative mutant of Ets-2, demonstrating that the effects of the src kinase on GlcNAc-T V expression are dependent on Ets.

A search for pyrophosphate mimics for the development of substrates and inhibitors of glycosyltransferases

The design and synthesis of several beta-1,4-galactosyltransferase inhibitors are reported. Mimics of the pyrophosphate-Mn2+ complex were the focus of the design. Malonic, tartaric, and monosaccharide moieties were used as replacements of the pyrophosphate moiety, and galactose or azasugars with potent galactosidase inhibitory activity were used as the 'donor' component. Compound 6, in which glucose was used as the pyrophosphate-Mn2+ complex mimic and galactose as the 'donor' component, showed the best inhibitory activity towards the transferase with a Ki of 119.6 microM.

Circular dichroic spectroscopy of N-acetylglucosaminyltransferase V and its substrate interactions

beta-1,6-N-Acetylglucosaminyltransferase V (EC 2.4.1.155) catalyzes the transfer of N-acetylglucosamine (GlcNAc) from UDP-GlcNAc in beta(1,6)-linkage to the alpha(1,6)-linked mannose of N-linked oligosaccharides. Circular dichroism (CD) was used to investigate the secondary structure of a recombinant, soluble form of the enzyme and its interaction with UDP-GlcNAc and an inhibitory substrate analog. The CD spectrum of the apoenzyme indicated the presence of small amounts of beta-structure and substantial amounts (>50%) of alpha-helicity. The CD spectra of solutions containing UDP-GlcNAc and different ratios of UDP-GlcNAc:enzyme were measured. Interestingly, the spectrum of each mixture could not be accounted for by simple additivity of the two individual spectra, indicating a change in environment of the chromophores and/or a conformational change of the substrate or protein concomitant with binding. Similar results were obtained with mixtures of UDP and the enzyme. Analysis of the CD difference spectra at three wavelengths yielded an estimated average Kd of 4.4 mM for UDP-GlcNAc and 3.8 mM for UDP. By contrast, addition of the CD spectrum of an inhibitory substrate analog of its oligosaccharide acceptor substrate and the CD spectrum of the enzyme could account for that observed of an inhibitor-enzyme mixture; moreover, addition of the inhibitor to a mixture of UDP-GlcNAc and enzyme did not alter the Kd associated with UDP-GlcNAc binding to the enzyme. These results and kinetic studies reported herein suggest an ordered reaction in which UDP-GlcNAc binds first to the enzyme, followed by the sequential binding of the trisaccharide substrate.

Recognition of beta-D-Gal p-(1-->3)-beta-D-Glc pNAc-OR acceptor analogues by the Lewis alpha-(1-->3/4)-fucosyltransferase from human milk

The Lewis alpha-(1-->3/4)-fucosyltransferase (E.C. 2.4.1.65) transfers L-fucose from GDP-fucose to OH-4 of the Glc pNAc residue in the disaccharide beta-D-Galp-(1-->3)-beta-D-Glc pNAc-OR [R = (CH2)8COOMe] (1) to give the Lewis-A blood group determinant beta-D-Gal p-(1-->3)-[alpha-L-Fuc p-(1-->4)]-beta-D-Glc pNAc-OR. Five deoxy analogous of 1, as well as its N-propionyl derivative, were chemically synthesized and kinetically evaluated as both substrates and inhibitors for the enzyme from human milk. The unmodified acceptor 1 had Km = 640 microM with Vmax set arbitrarily to 100. The 6-deoxy (Km = 400 microM, V(max) = 90) and N-propionyl compounds (Km = 330 microM, V(max) = 170) remained excellent substrates while the 4-deoxy compound was a very weak competitive inhibitor with Ki = 9 mM. Deoxygenation of OH-2' and OH-4'(of the Gal residue) in 1 had little effect on the activity. The OH-6 group of the Gal residue proved to be critical for recognition by the enzyme since substitution of this group with hydrogen led to an inactive compound.

Preparation of antisera to recombinant, soluble N-acetylglucosaminyltransferase V and its visualization in situ

N-Acetylglucosaminyltransferase V (GlcNAc-T V) is a glycosyltransferase which transfers N-acetylglucosamine in beta(1,6) linkage to the alpha(1,6)-linked mannose residue of Asn-linked oligosaccharides. This enzyme is characterized by several unusual properties: GlcNAc-T V is the largest lumenal Golgi glycosyltransferase described thus far, and its multiple mRNA transcripts range from 4.5 to about 9.5 kb; GlcNAc-T V mRNA and activity are regulated by the src tyrosine kinase signalling pathway; in brain tissue, large levels of GlcNAc-T V mRNA are present, but only relatively low levels of catalytic activity can be detected; a lectin-resistant cell line, Lec4A, expresses active GlcNAc-T V which is mislocalized intracellularly. In addition, the cell surface oligosaccharide products of this enzyme have been hypothesized to regulate intercellular adhesion. In order to devise specific inhibitors of this enzyme it is necessary to understand its physical structure and how structural changes can influence its activity and localization. We have expressed milligram amounts of a soluble form of recombinant rat GlcNAc-T V, purified it from CHO cell-conditioned media, and used it to prepare specific antisera. This antisera binds selectively to GlcNAc-T V and has been used to visualize B-16 mouse melanoma cell GlcNAc-T V on immunoblots after SDS-PAGE. When the antisera was used in immunofluorescence microscopy experiments on permeabilized B-16 and baby hamster kidney cells, intense, specific staining was observed in intracellular structures which appear to correspond to the Golgi apparatus.

Methyl 3-amino-3-deoxy-beta-D-galactopyranosyl-(1-->4)-2-acetamido-2- deoxy-beta-D-glucopyranoside: an inhibitor of UDP-D-galactose: beta-D-galactopyranosyl-(1-->4)-2-acetamido-2-deoxy-D-glucose (1-->3)-alpha-D-galactopyranosyltransferase

UDP-D-galactose:beta-D-galactopyranosyl-(1-->4)-2-acetamido-2-deoxy-D- glucose alpha-(1-->3)-D-galactopyranosyltransferase [E.C. 2.4.1.151] transfers D-galactosyl-residues from the sugar nucleotide with retention of configuration. We report here that synthetic methyl 3'-amino-3'-deoxy-N-acetyllactosaminide (9), where the hydroxyl group normally undergoing galactosylation has been replaced by amino group, is an inhibitor for this enzyme with Ki = 104 microM. The mode of inhibition is not competitive, but appears to be specific, since other glycosyltransferases were not affected by 9.

Alpha-lactalbumin induces bovine milk beta 1,4-galactosyltransferase to utilize UDP-GalNAc

We now report that alpha-lactalbumin (alpha-LA) has a novel effect on bovine milk UDP-Gal:GlcNAc-beta 1,4-galactosyltransferase (beta 1,4-GT) and induces the enzyme to efficiently utilize UDP-GalNAc as a donor. In the presence of alpha-LA the enzyme transfers GalNAc to free GlcNAc to produce GalNAc beta 1-4GlcNAc at a rate 55% of that compared to the rate when UDP-Gal is the donor in the absence of alpha-LA. The stimulation by alpha-LA is dependent on the concentrations of alpha-LA, acceptor, and sugar nucleotide. Interestingly, beta 1,4-GT is unable to transfer Gal-NAc to Glc with or without alpha-LA. alpha-LA also stimulates the transfer of GalNAc from UDP-GalNAc to various chitin oligomers, although the degree of stimulation decreases as the acceptor size increases. Thus, bovine milk beta 1,4-GT has an inherent ability to utilize two different sugar nucleotides and the sugar nucleotide preference is regulatable by alpha-LA.

Synthesis of beta-D-GlcpNAc-(1-->2)-5a-carba-alpha-D-Man p-(1-->6)-beta-D- Glcp-O(CH2)7CH3: a reactive acceptor analog for N-acetylglucosaminyltransferase-V

The branching enzyme N-acetylglucosaminyltransferase-V (GlcNAcT-V) recognizes the trisaccharide beta-D-GlcpNAc-(1-->2)-alpha-D-Man p-(1-->6)-beta-D-Glcp-O(CH2)7CH3 (1) as its minimum substrate. We report here the chemical synthesis of beta-D-GlcpNAc-(1-->2)-5a- carba-alpha-D-Manp-(1-->6)-beta-D-Glcp-O(CH2)7CH3 (2), a carbocyclic analog of 1 where the ring oxygen of the alpha-D-Manp residue is replaced by a methylene group. Trisaccharide 2 was found to be fully active as an acceptor for GlcNAcT-V, both with the enzyme isolated from hamster kidney and the one cloned from rat kidney. The kinetic parameters Km and Vmax for 1 and 2 were functionally equivalent. A preparative glycosylation reaction was performed using 2 as the acceptor with the cloned rat kidney enzyme. A tetrasaccharide formed by the addition of a Glc pNAc residue was the sole product as detected by 1H NMR spectroscopy and FAB mass spectrometry and was assigned the structure beta-D-GlcpNAc-(1-->2)-[beta-D-GlcpNAc-(1-->6)]-5a- carba-alpha-D-Manp-(1-->6)-beta-D-Glc p-O(CH2)7CH3 (13).

Substrate specificity and inhibition of UDP-GlcNAc:GlcNAc beta 1-2Man alpha 1-6R beta 1,6-N-acetylglucosaminyltransferase V using synthetic substrate analogues

UDP-GlcNAc:GlcNAc beta 1-2Man alpha 1-6R (GlcNAc to Man) beta 1,6- N-acetylglucosaminyltransferase V (GlcNAc-T V) adds a GlcNAc beta 1-6 branch to bi- and triantennary N-glycans. An increase in this activity has been associated with cellular transformation, metastasis and differentiation. We have used synthetic substrate analogues to study the substrate specificity and inhibition of the partially purified enzyme from hamster kidney and of extracts from hen oviduct membranes and acute myeloid leukaemia leukocytes. All compounds with the minimum structure GlcNAc beta 1-2Man alpha 1-6Glc/Man beta-R were good substrates for GlcNAc-T V. The presence of structural elements other than the minimum trisaccharide structure affected GlcNAc-T V activity without being an absolute requirement for activity. Substrates with a biantennary structure were preferred over linear fragments of biantennary structures. Kinetic analysis showed that the 3-hydroxyl of the Man alpha 1-3 residue and the 4-hydroxyl of the Man beta- residue of the Man alpha 1-6(Man alpha 1-3)Man beta-R N-glycan core are not essential for catalysis but influence substrate binding. GlcNAc beta 1-2(4,6-di-O-methyl-)Man alpha 1-6Glc beta-pnp was found to be an inhibitor of GlcNAc-T V from hamster kidney, hen oviduct microsomes and acute and chronic myeloid leukaemia leukocytes.

Key involvement of all three GlcNAc hydroxyl groups in the recognition of beta-D-GlcpNAc-(1-->2)-alpha-D-Manp-(1-->6)-beta-D-Glcp-OR by N-acetylglucosaminyltransferase-V

N-Acetylglucosaminyltransferase-V (GlcNAc T-V) transfers a beta-linked GlcNAc residue from UDP-GlcNAc to OH-6' (of the alpha Man residue) in oligosaccharides terminating in the sequence beta-D-GlcpNAc-(1-->2)-alpha-D-Manp-(1-->6)-beta-D-Glcp(or Manp)-OR (3, R = (CH2)7CH3). It was previously found that OH-4" (of the GlcNAc residue) in 3 was a critical element for substrate recognition by this enzyme. We show here that OH-3" and OH-6" are also key recognition elements. Four analogs of trisaccharide 3 where OH-3" and OH-6" were replaced, independently, by NH2 and NHAc groups, were prepared by multi-step chemical synthesis and kinetically evaluated as substrates for GlcNAc T-V from hamster kidney. These substitutions were selected since they replaced the OH groups with groups probing both hydrogen bonding and steric bulk. The 3"-modified compounds were found to be very poor substrates with Km values more than 50-fold elevated over that for 3 (26 microM) while the 6"-modified compounds were completely inactive. An intact 3,4,6 triol system in the terminal GlcNAc residue therefore appears to be the key polar group system that is recognized by this enzyme.

Tumor cell surface beta 1-6 branched oligosaccharides and lung metastasis

NIH3T3 cells transfected with an activated Ha-ras oncogene were treated with L-PHA, the leukoagglutinin from red kidney beans. Cell lines resistant to L-PHA-mediated cytotoxicity were isolated and found to contain reduced levels of L-PHA-binding oligosaccharides. The levels of N-acetylglucosaminyltransferase V, the enzyme responsible for the initiation of the beta 1-6 branch, were reduced in L-PHA-resistant cells. Tumorigenicity in nude mice was unchanged by the change in oligosaccharide expression, but the ability to form lung tumors after intravenous injection was significantly reduced. These results demonstrate that the ability of NIH3T3 cells transfected with an activated Ha-ras oncogene to form lung tumors after intravenous injection into nude mice is reduced in all six L-PHA selected cell lines containing a reduction in beta 1-6 branched Asn-linked oligosaccharides.

Recognition of the acceptor beta-D-GlcpNAc-(1-->2)-alpha-D-Manp- (1-->6)-beta-D-Glcp-OR by N-acetyl-glucosaminyltransferase-V: none of the hydroxyl groups on the Glc-residue are important

The enzyme, N-acetylglucosaminyltransferase-V (GlcNAcT-V, E.C. 2.4.1.155), transfer a beta-D-GlcpNAc residue, from UDP-GlcNAc, to the OH-6 group of the Man residue in the synthetic acceptor beta-D-GlcpNAc-(1-->2)-alpha-D-Manp-(1-->6)-beta-D-Glcp-O(CH2)7 CH3 (3). Trisaccharide 3 is an excellent substrate for the enzyme from hamster kidney with a Km value of 26 microM. In this paper we examine the contribution of the Glc residue in 3 to acceptor recognition by this enzyme. beta-D-GlcpNAc-(1-->2)-alpha-D-Manp-O(CH2)7CH3 where the Glc residue in 3 has been deleted, was synthesized and found to be a very poor substrate with a Km value elevated to almost 2 mM. Two other analogues of 3, where the Glc residue was O-trimethylated (6) or O-tribenzylated (7), respectively, possessed Km values very near to those of 3. The Glc residue in 3 is thereby shown to present an important recognition element for GlcNAcT-V, but none of the free hydroxyl groups are required. This observation should facilitate the design of more hydrophobic and membrane-permeable analogues of 3 that are expected to function as specific glycosylation inhibitors.

Acceptor-substrate recognition by N-acetylglucosaminyltransferase-V: critical role of the 4"-hydroxyl group in beta-D-GlcpNAc-(1-->2)-alpha-D-Manp(1-->6)-beta-D-Glcp-OR

The enzyme N-acetylglucosaminyltransferase-V (GlcNAcT-V) transfers GlcNAc from UDP-GlcNAc to the OH-6' group of oligosaccharides terminating in the sequence beta-D-GlcpNAc-(1-->2)-alpha-D-Manp-(1-->6)-beta-D-Glcp (or Manp)-OR (5, R = (CH2)7CH3) to yield the sequence beta-D-GlcpNAc-(1-->2)-[beta-D-GlcpNAc-(1-->6)]-alpha-D-Manp-(1--> 6)- beta-D-Glcp (or Manp)-OR. Biosynthetically, if beta-(1-->4)-galactosyltransferase acts first on 5, the product beta-D-Galp-(1-->4)-beta-D-GlcpNAc-(1-->2)-alpha-D-Manp-(1-->6)-be ta-D-Glcp (or Manp)-OR (7) is no longer a substrate for GlcNAcT-V even though it retains the active OH-6' group. The reason for this loss in activity is examined in this paper. Six analogues of the acceptor trisaccharide 5, all with the reducing-end D-gluco configuration, were chemically synthesized. A key feature of the synthetic scheme is the use of 1,2-diaminoethane for the efficient removal of N-phthalimdo protecting groups. In these analogues OH-4 of the terminal sugar unit, the site of galactosylation by GalT in the normal GlcNAc-terminating trisaccharide 5, was systematically replaced by OMe, F, NH2, NHAc, and H, as well as inverted to the galacto configuration. The interactions of the resulting trisaccharide analogues with GlcNAcT-V from hamster kidney were then evaluated kinetically. All six compounds were found to be essentially inactive either as acceptors or as inhibitors of GlcNAcT-V. The conclusion is reached that galactosylation of natural acceptors for GlcNAcT-V destroys acceptor activity, not by introduction of the steric bulk of an added sugar residue, but by destroying an important hydrogen-bonding interaction of terminal OH-4 of the GlcNAc residues with the enzyme. This OH-4 group is therefore designated as a key polar group for GlcNAcT-V.

Synthesis of 4-nitrophenyl O-(2-acetamido-2-deoxy-beta-D-glucopyranosyl)- (1-->2)-O-(6-O-methyl-alpha-D-mannopyranosyl)-(1-->6)-beta-D- glucopyranoside and its 4',6'-di-O-methyl analog. Potential inhibitors of N-acetylglucosaminyl-transferase V (GnT-V)

O-(2-Acetamido-3,4,6-tri-O-acetyl-2-deoxy-beta-D-glucopyranosyl)-(1-->2) -3,4 - di-O-acetyl-6-O-methyl-alpha-D-mannopyranosyl bromide and O-(2-acetamido-3,4,6-tri-O-acetyl-2-deoxy-beta-D-glucopyranosyl)- (1-->2)-3-O-acetyl-4,6-di-O-methyl-alpha-D-mannopyranosyl bromide were each condensed with 4-nitrophenyl 2,3-di-O-acetyl-beta-D-glucopyranoside, and the products were deprotected to yield, respectively, beta-D-Glc p NAc-(1-->2)-6-O-Me-alpha-D-Man p-(1-->6)-beta-D-Glc p and beta-D-Glc p NAc-(1-->2)-4,6-di-O-Me-alpha-D-Man p-(1-->6)-beta-D-Glc p, as their 4-nitrophenyl glycosides. These trisaccharides are expected to function as inhibitors for N-acetylglucosaminyltransferase V.

[Synthesis of modified oligosaccharides of the N-glycoprotein as substrate for N-acetylglucosaminyltransferase I]

In the synthesis of modified derivatives of octyl O-(alpha-D-mannopyranosyl)-(1-->3)-O-[(alpha-D-mannopyranosyl)-(1-->6)]- beta-D-mannopyranoside, 4.,5-epoxypentyl, a 4-diazirinopentyl, and a 5-(iodoacetamido)pentyl group were attached to the 3''-OH of the trisaccharide. The diazirino derivative may be especially suitable for photolabeling of the active site of N-acetylglucosaminyltransferase I (GlcNAcT-I). In addition, the 2'-OH group of the above-mentioned trisaccharide was reduced to a 2'-deoxy group and substituted 2'-O-methyl group.

Novel glycosylation-defective baby hamster kidney cells

The plant lectin wheat germ agglutinin (WGA) has previously been used to select more than ten different glycosylation-defective phenotypes in a variety of mammalian somatic cells. Three WGA-resistant phenotypes have now been obtained spontaneously from baby hamster kidney (BHK) cells. These mutant BHK cells exhibit a pattern of cross resistance and sensitivity to multiple plant lectins, suggesting that the cell surface carbohydrates of these cells are altered. Two WGA-resistant BHK phenotypes appear similar to WGA-resistant CHO cells that lack terminal sialic acid and galactose residues on their cell surface carbohydrates. The third WGA-resistant BHK cell phenotype has not previously been seen in WGA-resistant mammalian cells.

Combined chemical-enzymic synthesis of a dideoxypentasaccharide for use in a study of the specificity of N-acetyl-glucosaminyltransferase-III

The biantennary oligosaccharide glycoside beta-D-GlcpNAc-(1----2)-alpha-D- Manp-(1----3)- [beta-D-GlcpNAc-(1----2)-alpha-D-Manp-(1----6)]-beta-D-Manp- OR is a potential substrate for N-acetylglucosaminyltransferases (GlcNAcTs) III-V. The dideoxypentasaccharide glycoside beta-D-GlcpNAc-(1----2)-4- deoxy-alpha-D-lyxo-Hexp-(1----3)- [beta-DGlcpNAc-(1----2)-6-deoxy-alpha-D-Manp-(1----6)] beta-D-Manp-O(CH2)7CH3 (5), where the hydroxyl groups that would be acted on by GlcNAcTs IV and V have been removed, was prepared as a possible specific acceptor for GlcNAcT-III. The strategy involved the chemical synthesis of beta-D-GlcpNAc-(1----2)-4-deoxy-alpha-D-lyxo-Hexp-(1----3)-] 6- deoxy-alpha-D-Manp-(1----6)]-beta-D-Manp-O)CH2)7CH3 and then addition of the last GlcpNAc residue using partially purified GlcNAcT-II from rabbit liver. Preliminary results, using detergent extracts from rat kidney, indicate that 5 is an acceptor for a GlcNAcT whose identity remains to be established.