Molecular cloning of a human cDNA encoding beta-1,4-galactosyltransferase with 37% identity to mammalian UDP-Gal:GlcNAc beta-1,4-galactosyltransferase

Decreased B4GALT1 promotes hepatocellular carcinoma cell invasiveness by regulating the laminin-integrin pathway

Beta1,4-galactosyltransferases (B4GALTs) play a crucial role in several diseases, including cancer. B4GALT1 is highly expressed in the liver, and patients with mutations in B4GALT1 exhibit hepatopathy. However, the role of B4GALT1 in liver cancer remains unclear. Here, we found that B4GALT1 was significantly downregulated in hepatocellular carcinoma (HCC) tissue compared with the adjacent liver tissue, and low B4GALT1 expression was associated with vascular invasion and poor overall survival in patients with HCC. Additionally, silencing or loss of B4GALT1 enhanced HCC cell migration and invasion in vitro and promoted lung metastasis of HCC in NOD/SCID mice. Moreover, B4GALT1 knockdown or knockout increased cell adhesion to laminin, whereas B4GALT1 overexpression decreased the adhesion. Through a mass spectrometry-based approach and Griffonia simplicifolia lectin II (GSL-II) pull-down assays, we identified integrins α6 and β1 as the main protein substrates of B4GALT1 and their N-glycans were modified by B4GALT1. Further, the increased cell migration and invasion induced by B4GALT1 knockdown or knockout were significantly reversed using a blocking antibody against integrin α6 or integrin β1. These results suggest that B4GALT1 downregulation alters N-glycosylation and enhances the laminin-binding activity of integrin α6 and integrin β1 to promote invasiveness of HCC cells. Our findings provide novel insights into the role of B4GALT1 in HCC metastasis and highlight targeting the laminin-integrin axis as a potential therapeutic strategy for HCC with low B4GALT1 expression.

Transcriptional Mechanism of the Mouse β4-Galactosyltransferase 6 Gene in Mouse Neuroblastoma Cell Line Neuro-2a

Lactosylceramide (Lac-Cer) constitutes the backbone structure of various gangliosides whose abnormal expression is associated with malignancy of neuroblastoma. The understanding of the regulatory mechanism of Lac-Cer contributes to the development of neuroblastoma therapy. In this study, the transcriptional mechanism of mouse β4-galactosyltransferase (β4GalT) 6, which is one of Lac-Cer synthase, was analyzed using mouse neuroblastoma cell line Neuro-2a. The -226 to -13 region relative to the most downstream transcriptional start site was determined to be the promoter region by luciferase assay using the 5'-deletion constructs. The mutation into the activating protein (AP) 4-binding site -110/-101 drastically decreased the promoter activity, indicating that this site is mainly implicated in the transcription. Furthermore, the mutation into the GATA-binding site -210/-201 or another AP4-binding site -202/-193 partially decreased the promoter activity. The study suggests that the mouse β4GalT6 gene is transcriptionally regulated by AP4 in cooperation with GATA family transcription factor in neuroblastoma.

Expression of β-1,4-galactosyltransferases during Aging in Caenorhabditis elegans

BACKGROUND

Altered plasma activity of β-1,4-galac-tosyl-transferases (B4GALTs) is a novel candidate biomarker of human aging. B4GALT1 is assumed to be largely responsible for this activity increase, but how it modulates the aging process is unclear at present.

OBJECTIVES

To determine how expression of B4GALT1 and other B4GALT enzymes changes during aging of an experimentally tractable model organism, Caenorhabditis elegans.

METHODS

Targeted analysis of mRNA levels of all 3 C. elegans B4GALT family members was performed by qPCR in wild-type and in long-lived daf-2 (insulin/IGF1-like receptor)-deficient or germline-deficient animals.

RESULTS

bre-4 (B4GALT1/2/3/4) is the only B4GALT whose expression increases during aging in wild-type worms. In addition, bre-4 levels also rise during aging in long-lived daf-2-deficient worms, but not in animals that are long-lived due to the lack of germline stem cells. On the other hand, expression of sqv-3 (B4GALT7) and of W02B12.11 (B4GALT5/6) appears decreased or constant, respectively, in all backgrounds during aging.

CONCLUSIONS

The age-dependent bre-4 mRNA increase in C. elegans parallels the age-dependent B4GALT activity increase in humans and is consistent with C. elegans being a suitable experimental organism to define potentially conserved roles of B4GALT1 during aging.

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.

Challenge to the suppression of tumor growth by the β4-galactosyltransferase genes

It has been well established that structural changes in glycans attached to proteins and lipids are associated with malignant transformation of cells. We focused on galactose residues among the sugars since they are involved in the galectin-mediated biology, and many carbohydrate antigens are frequently expressed on this sugar. We found changes in the expression of the β4-galactosyltransferase (β4GalT) 2 and 5 genes in cancer cells: decreased expression of the β4GalT2 gene and increased expression of the β4GalT5 gene. The growth of mouse melanoma cells showing enhanced expression of the β4GalT2 gene or reduced expression of the β4GalT5 gene is inhibited remarkably in syngeneic mice. Tumor growth inhibition is probably caused by the induction of apoptosis, inhibition of angiogenesis, and/or reduced MAPK signals. Direct transduction of human β4GalT2 cDNA together with the adenovirus vector into human hepatocellular carcinoma cells grown in SCID mice results in marked growth retardation of the tumors. β4GalT gene-transfer appears to be a potential tool for cancer therapy.

Deciphering the glycogenome of schistosomes

Schistosoma mansoni and other Schistosoma sp. are multicellular parasitic helminths (worms) that infect humans and mammals worldwide. Infection by these parasites, which results in developmental maturation and sexual differentiation of the worms over a period of 5–6 weeks, induces antibodies to glycan antigens expressed in surface and secreted glycoproteins and glycolipids. There is growing interest in defining these unusual parasite-synthesized glycan antigens and using them to understand immune responses, their roles in immunomodulation, and in using glycan antigens as potential vaccine targets. A key problem in this area, however, has been the lack of information about the enzymes involved in elaborating the complex repertoire of glycans represented by the schistosome glycome. Recent availability of the nuclear genome sequences for Schistosoma sp. has created the opportunity to define the glycogenome, which represents the specific genes and cognate enzymes that generate the glycome. Here we describe the current state of information in regard to the schistosome glycogenome and glycome and highlight the important classes of glycans and glycogenes that may be important in their generation.

[Regulation of human β-1,4-galactosyltransferase V gene expression in cancer cells]

β-1,4-Galactosyltransferase (β-1,4-GalT) V - whose human and mouse genes were cloned by us - has been suggested to be involved in the biosyntheses of N-glycans, O-glycans, and lactosylceramide by in vitro studies. Our recent study showed that β-1,4-GalT V-knockout mice are embryonic lethal, suggesting the importance of the glycans synthesized by β-1,4-GalT V for embryonic development. A subsequent study showed that murine β-1,4-GalT V is involved in the biosynthesis of lactosylceramide. It is well known that the glycosylation of cell surface glycoproteins and glycolipids changes dramatically upon the malignant transformation of cells. We found that among six β-1,4-GalTs the gene expression of only β-1,4-GalT V increases upon malignant transformation. The expression of the β-1,4-GalT V gene has been shown to be regulated by transcription factors Sp1 and Ets-1 in cancer cells. Both transcription factors regulate the gene expression levels of not only glycosyltransferases, but also key molecules involved in tumor growth, invasion and metastasis. Therefore, the abnormal glycosylation and malignant phenotypes of cancer cells are considered to be suppressed by regulating the expression levels of the transcription factor genes. This review gives a summary account of the gene discovery, in vivo function, and transcriptional mechanism of β-1,4-GalT V. Also, a perspective on applications of the manipulation of transcription factor genes to cancer therapy will be discussed.

Involvement of murine β-1,4-galactosyltransferase V in lactosylceramide biosynthesis

Human β-1,4-galactosyltransferase (β-1,4-GalT) V was shown to be involved in the biosynthesis of N-glycans, O-glycans and lactosylceramide (Lac-Cer) by in vitro studies. To determine its substrate specificity, enzymatic activity and its products were analyzed using mouse embryonic fibroblast (MEF) cells from β-1,4-GalT V (B4galt5)-mutant mice. Analysis of expression levels of the β-1,4-GalT I-VI genes revealed that the expression of the β-1,4-GalT V gene in B4galt5 ( +/- ) - and B4galt5 ( -/- ) -derived MEF cells are a half and null when compared to that of B4galt5 ( +/+ )-derived MEF cells without altering the expression levels of other β-1,4-GalT genes. These MEF cells showed no apparent difference in their growth. When β-1,4-GalT activities were determined towards GlcNAcβ-S-pNP, no significant difference in its specific activity was obtained among B4galt5 ( +/+ )-, B4galt5 ( +/- ) - and B4galt5 ( -/- ) -derived MEF cells. No significant differences were obtained in structures and amounts of N-glycans and lectin bindings to membrane glycoproteins among B4galt5 ( +/+ )-, B4galt5 ( +/- ) - and B4galt5 ( -/- ) -derived MEF cells. However, when cell homogenates were incubated with glucosylceramide in the presence of UDP-[(3)H]Gal, Lac-Cer synthase activity in B4galt5 ( +/- ) - and B4galt5 ( -/- ) -derived MEF cells decreased to 41% and 11% of that of B4galt5 ( +/+ )-derived MEF cells. Consistent with this, amounts of Lac-Cer and its derivative GM3 in B4galt5 ( -/- ) -derived MEF cells decreased remarkably when compared with those of B4galt5 ( +/+ )-derived MEF cells. These results indicate that murine β-1,4-GalT V is involved in Lac-Cer biosynthesis.

Beta4-galactosyltransferase-5 is a lactosylceramide synthase essential for mouse extra-embryonic development

Glycosphingolipids (GSLs) are important for various biological functions in the nervous system, the immune system, embryogenesis and in other tissues and processes. Lactosylceramide (LacCer), which is synthesized from glucosylceramide (GlcCer) by LacCer synthase, is a core structure of GSLs, including gangliosides. LacCer synthase was reported to be synthesized by the beta4-galactosyltransferase-6 (beta4GalT-6) gene in the rat brain. However, the existence of another LacCer synthase gene was shown in cultured cells lacking beta4GalT-6. Here, we report that LacCer synthase is mainly synthesized by the beta4GalT-5 gene during early mouse embryogenesis, and its disruption is embryonic lethal. beta4GalT-5-deficient embryos showed developmental retardation from E7.5 and died by E10.5 as reported previously. LacCer synthase activity was significantly reduced in beta4GalT-5-deficient embryos and extra-embryonic endoderm (XEN) cells derived from blastocysts, and it was recovered when beta4GalT-5 cDNA was introduced into beta4GalT-5-deficient XEN cells. The amounts of LacCer and GM3 ganglioside were drastically reduced, while GlcCer accumulated in the beta4GalT-5-deficient XEN cells. Hematoma and ectopically accumulated trophoblast giant cells were observed in the anti-mesometrial pole of the extra-embryonic tissues, although all three embryonic layers formed. beta4GalT-5-deficient embryos developed until E12.5 as chimeras with wild-type tetraploid cells, which formed the extra-embryonic membranes, indicating that extra-embryonic defects caused the early embryonic lethality. Our results suggest that beta4GalT-5 is essential for extra-embryonic development during early mouse embryogenesis.

Beta1,4-galactosyltransferase V regulates self-renewal of glioma-initiating cell

Glioma results from unregulated expansion of a self-renewing glioma-initiating cell population. The regulatory pathways which are essential for sustaining the self-renewal of glioma-initiating cells remain largely unknown. Cell surface N-linked oligosaccharides play functional roles in determining cell fate and are associated with glioma malignancy. Previously, we have reported that beta1,4-galactosyltransferase V (beta1,4GalT V) effectively galactosylates the GlcNAcbeta1-->6Man arm of the highly branched N-glycans and positively regulates glioma cell growth. Here, we show that decreasing the expression of beta1,4GalT V by RNA interference in glioma cells attenuated the formation of polylactosamine and inhibited the ability of tumor formation in vivo. Down-regulation of beta1,4GalT V depleted CD133-positive cells in glioma xenograft, and inhibited the self-renewal capacity and the tumorigenic potential of glioma-initiating cells. These data reveal a critical role of beta1,4GalT V in the self-renewal and tumorigenicity of glioma-initiating cells, and indicate that manipulating beta1,4GalT V expression may have therapeutic potential for the treatment of malignant glioma.

Beta1,4-galactosyltransferase V A growth regulator in glioma

One of the most prominent transformation-associated changes in the sugar chains of glycoproteins is an increase in the large N-glycans of cell surface glycoprotein. beta1,4-galactosyltransferase V (beta1,4GalT V) could effectively galactosylate the GlcNAcbeta1-->6 branch which is a marker of glioma. The expression of beta1,4GalT V is increased in the process of glioma development. beta1,4GalT V regulates the invasion, growth in vivo and in vitro of glioma cells. Downregulation of beta1,4GalT V expression increases the sensitivity of malignant glioma cells to DNA damage drugs. Furthermore, beta1,4GalT V regulates Ras and AKT signaling involving in glioma behaviors. Meanwhile, Ras/MAPK and PI3K/AKT signaling pathways are involved in the transcription regulation of beta1,4GalT V gene. E1AF transcription factor, a downstream target of Ras/MAPK and PI3K/AKT signaling pathways, regulates the transcription of beta1,4GalT V in cooperation with Sp1 transcription factor. The contribution of beta1,4GalT V in glioma development is further confirmed in glioma-initiation cells. beta1,4GalT V regulates the self-renewal of glioma-initiation cells. We now present evidence that beta1,4GalT V functions as a positive growth regulator in glioma and might represent a novel target in glioma therapy.

Enzymatic synthesis of lacto-N-difucohexaose I which binds to Helicobacter pylori

Helicobacter pylori is known to bind with sugar chains possessing Lewis b structure. We are trying to combine oligosaccharides containing Lewis b sugar chain to water insoluble polysaccharide through some linker. Lacto-N-difucohexaose I (LNDFH I; Fucalpha1-->2Galbeta1-->3[Fucalpha1-->4]GlcNAcbeta1-->3Galbeta1-->4Glc) fits for that purpose, since it consists of Lewis b tetrasaccharide and lactose whose d-glucose residue can be utilized as a linker. We thus developed a method to synthesize this hexaose enzymatically. First, beta-1,3-N-acetylglucosaminyltransferase (beta-1,3-GnT) was partially purified from bovine blood by an established method. Using this enzyme preparation, d-GlcNAc was attached to the d-galactose residue of lactose with a beta-1,3-linkage to produce lacto-N-triose II at 44% yield. The low yield was thought to be due to contaminating N-acetylglucosaminidase that would have hydrolyzed the product, lacto-N-triose II. Next, d-galactose was attached by transglycosylation using ortho-nitrophenyl beta-d-galactopyranoside as a donor with the aid of recombinant beta-1,3-galactosidase from Bacillus circulans to generate lacto-N-tetraose (LNT) at 22% yield. l-Fucose was then linked to the d-galactose residue of LNT via an alpha-1,2-linkage using recombinant human fucosyltransferase I (FUT1) expressed in a baculovirus system (71% yield). The obtained pentasaccharide was subsequently incubated with GDP-beta-l-fucose and commercial fucosyltransferase III (FUT3) to attach l-fucose to the d-GlcNAc residue of LNT with an alpha-1,4-linkage. After purification with an activated carbon column chromatography, 1.7 mg of LNDFH I was obtained (85% yield). We thus produced LNDFH I over four enzymatic steps with a yield of 6%.

Essential role of beta-1,4-galactosyltransferase 2 during medaka (Oryzias latipes) gastrulation

Glycans are known to play important roles in vertebrate development; however, it is difficult to analyze in mammals because it takes place in utero. Therefore, we used medaka (Oryzias latipes) to clarify the roles of glycans during vertebrate development. beta-1,4-Galactosyltransferase is one of the key enzymes in the biosynthesis of the lactosamine structures that are commonly found on glycoproteins and glycolipids. Here, we show the essential role of beta4GalT2 during medaka development. Depletion of beta4GalT2 by morpholino antisense oligonucleotide injection resulted in significant morphological defects, such as shortening of the anterior-posterior axis, cyclopia, impaired somite segmentation, and head hypoplasia. In situ hybridization analyses revealed that the loss of beta4GalT2 led to defective anterior-posterior axis elongation during gastrulation without affecting organizer formation. Furthermore, a cell tracing experiment demonstrated that beta4GalT2 knockdown mainly affects mediolateral cell intercalation, which contributes to anterior-posterior axis elongation. A cell transplantation experiment indicated that glycans are produced by beta4GalT2 cell-autonomously during gastrulation. beta4GalT2 depletion also led to enhanced apoptosis; however, this does not account for the phenotypic abnormalities, as blockade of apoptosis failed to compensate for the beta4GalT2 depletion. Our data suggest that beta4GalT2 activity is cell-autonomously required in cells undergoing mediolateral cell intercalation, which drives extension movements during medaka gastrulation.

Early lethality of beta-1,4-galactosyltransferase V-mutant mice by growth retardation

The beta-1,4-galactosyltransferase (beta-1,4-GalT) V whose human and mouse genes were cloned by us has been suggested to be involved in the biosynthesis of N-glycans and O-glycans, and lactosylceramide. To determine its biological function, beta-1,4-GalT V (B4galt5) mutant mice obtained by a gene trap method were analyzed. Analysis of pre- and post-implantation embryos revealed that the B4galt5(-/-) mice die by E10.5 while B4galt5(+/-) mice were born and grown normally. Histological study showed that most tissues are formed in B4galt5(-/-) embryos but their appearance at E10.5 is close to that of B4galt5(+/-) embryos at E9.0-9.5. The results indicate that the growth is delayed by one to one and half day in B4galt5(-/-) embryos when compared to B4galt5(+/-) embryos, which results in early death of the embryos by E10.5, probably due to hematopoietic and/or placental defects.

Bidirectional N-acetylglucosamine transfer mediated by beta-1,4-N-acetylglucosaminyltransferase III

beta-1,4-N-Acetylglucosaminyltransferase III (GnT-III) catalyzes the formation of the bisecting GlcNAc and plays a regulatory role in the biosynthesis of the N-linked oligosaccharide. In this study, we examined whether the glycosyl transfer catalyzed by GnT-III is reversible, and, in addition, investigated the equilibrium of the GnT-III-catalyzed reaction. Incubation of the agalactosyl-bisected biantennary oligosaccharide with GnT-III in the presence of the sufficiently high concentration of uridine diphosphate (UDP) resulted in conversion of the bisected oligosaccharide into the nonbisected one. This reaction was accompanied by the stoichiometric formation of UDP-GlcNAc, which appeared to result from the transfer of GlcNAc from the oligosaccharide to UDP. Thus, these results indicate that GnT-III is capable of perceivably catalyzing the reverse reaction in vitro, as found in some glycosyltransferases. When the equilibrium of the reaction was kinetically analyzed, it was found that the state of the equilibrium is greatly displaced toward the formation of the bisecting GlcNAc. In terms of free energy change, as estimated, the reaction by GnT-III can be comparable to the hydrolysis of ATP. Although GnT-III catalyzes bidirectional transfer of GlcNAc between the oligosaccharide and UDP, the removal of the bisecting GlcNAc is unlikely in vivo, due to the displacement of the equilibrium. It is known that equilibria of certain glycosyltransferase reactions are not biased as greatly as the case of GnT-III, and thus it seems likely that there are a variety of equilibrium states in glycosyltransferase reactions. In living cells, the assembly of oligosaccharides could be regulated by not only rate control but also equilibrium control.

N-Glycan fucosylation of epidermal growth factor receptor modulates receptor activity and sensitivity to epidermal growth factor receptor tyrosine kinase inhibitor

The glycosylation of cell surface proteins is important for cancer biology processes such as cellular proliferation or metastasis. alpha1,6-Fucosyltransferase (FUT8) transfers a fucose residue to n-linked oligosaccharides on glycoproteins. Herein, we study the effect of fucosylation on epidermal growth factor receptor (EGFR) activity and sensitivity to an EGFR-specific tyrosine kinase inhibitor (EGFR-TKI). The increased fucosylation of EGFR significantly promoted EGF-mediated cellular growth, and the decreased fucosylation by stable FUT8 knockdown weakened the growth response in HEK293 cells. The overexpression of FUT8 cells were more sensitive than the control cells to the EGFR-TKI gefitinib, and FUT8 knockdown decreased the sensitivity to gefitinib. Finally, to examine the effects in a human cancer cell line, we constructed stable FUT8 knockdown A549 cells, and found that these cells also decreased EGF-mediated cellular growth and were less sensitive than the control cells to gefitinib. In conclusion, we demonstrated that the modification of EGFR fucosylation affected EGF-mediated cellular growth and sensitivity to gefitinib. Our results provide a novel insight into how the glycosylation status of a receptor may affect the sensitivity of the cell to molecular target agents.

The expression patterns of beta1,4 galactosyltransferase I and V mRNAs, and Galbeta1-4GlcNAc group in rat gastrocnemius muscles post sciatic nerve injury

Glycosylation is one of the most important post-translational modifications. It is clear that the single step of beta1,4-galactosylation is performed by a family of beta1,4-galactosyltransferases (beta1,4-GalTs), and that each member of this family may play a distinct role in different tissues and cells. beta1,4-GalT I and V are involved in the biosynthesis of N-linked oligosaccharides and play roles in sciatic nerve regeneration after sciatic nerve injury. In the present study, the expression of beta1,4-galactosyltransferase (beta1,4-GalT) I, V mRNAs and Galbeta1-4GlcNAc group were examined in rat gastrocnemius muscles after sciatic nerve crush and transection. Real time PCR revealed that beta1,4-GalT I and V mRNAs expressed at a high level in normal gastrocnemius muscles and decreased gradually from 6 h, reached the lowest level at 2 weeks, then restored gradually to relatively normal level at 4 weeks after sciatic nerve crush. In contrast, in sciatic nerve transection model, beta1,4-GalT I and V mRNAs decreased gradually from 6 h, and remained on a low level at 4 weeks in gastrocnemius muscles after sciatic nerve transection. In situ hybridization indicated that beta1,4-GalT I and V mRNAs localized in numerous myocytes and muscle satellite cells under normal conditions and at 4 weeks after sciatic nerve crush, and in a few muscle satellite cells at 4 weeks after sciatic nerve transection. Furthermore, lectin blotting showed that the expression level of the Galbeta1-4GlcNAc group decreased from 6 h, reached the lowest level at 2 weeks, and restored to relatively normal level at 4 weeks after sciatic nerve crush. RCA-I lectin histochemistry demonstrated that Galbeta1-4GlcNAc group localized in numerous membranes of myocytes and muscle satellite cells in normal and at 4 weeks after sciatic nerve crush, and in a few muscle satellite cells at 2 and 4 weeks after sciatic nerve transection. These results indicated that the expressions of beta1,4-GalT I, V mRNAs and Galbeta1-4GlcNAc group were involved in the process of denervation and reinnervation, which suggests that beta1,4-GalT I, V mRNAs and Galbeta1-4GlcNAc group may play an important role in the muscle regeneration.

Protein N-glycosylation in the baculovirus-insect cell system

One of the major advantages of the baculovirus-insect cell system is that it is a eukaryotic system that can provide posttranslational modifications, such as protein N-glycosylation. However, this is a vastly oversimplified view, which reflects a poor understanding of insect glycobiology. In general, insect protein glycosylation pathways are far simpler than the corresponding pathways of higher eukaryotes. Paradoxically, it is increasingly clear that various insects encode and can express more elaborate protein glycosylation functions in restricted fashion. Thus, the information gathered in a wide variety of studies on insect protein N-glycosylation during the past 25 years has provided what now appears to be a reasonably detailed, comprehensive, and accurate understanding of the protein N-glycosylation capabilities of the baculovirus-insect cell system. In this chapter, we discuss the models of insect protein N-glycosylation that have emerged from these studies and how this impacts the use of baculovirus-insect cell systems for recombinant glycoprotein production. We also discuss the use of these models as baselines for metabolic engineering efforts leading to the development of new baculovirus-insect cell systems with humanized protein N-glycosylation pathways, which can be used to produce more authentic recombinant N-glycoproteins for drug development and other biomedical applications.

Sequential action of Ets-1 and Sp1 in the activation of the human beta-1,4-galactosyltransferase V gene involved in abnormal glycosylation characteristic of cancer cells

Malignant transformation is associated with increased gene expression of beta-1,4-galactosyltransferase (beta-1,4-GalT) V, which contributes to the biosynthesis of highly branched N-linked oligosaccharides characteristic of cancer cells. Our previous study showed that expression of the human beta-1,4-GalT V gene is regulated by Sp1 (Sato, T., and Furukawa, K. (2004) J. Biol. Chem. 279, 39574-39583), and a subsequent study showed that the gene expression is also activated by Ets-1, a product of the oncogene (Sato, T., and Furukawa, K. (2005) Glycoconj. J. 22, 365). Herein we report the mechanism of beta-1,4-GalT V gene activation by these transcription factors. The gene expression and promoter activity of beta-1,4-GalT V increased when the ets-1 cDNA was transfected into A549 cells, which contain a small amount of Ets-1, but decreased dramatically when the dominant-negative ets-1 cDNA was transfected into HepG2 cells, which contain a large amount of Ets-1. Luciferase assays using deletion constructs of the beta-1,4-GalT V gene promoter showed that promoter region -116 to +22 is critical for the transcriptional activation of the gene by Ets-1. Despite the presence of one Ets-1-binding site, which overlapped the Sp1-binding site, electrophoretic mobility shift assays showed that the region bound preferentially to Sp1 rather than to Ets-1. To solve this problem, we examined the transcriptional regulation of the human Sp1 gene by Ets-1 and found that the gene expression and promoter activity of Sp1 are regulated by Ets-1 in cancer cells. Functional analyses of two Ets-1-binding sites in the Sp1 gene promoter showed that only Ets-1-binding site -413 to -404 is involved in the activation of the gene by Ets-1. These results indicate that Ets-1 enhances expression of the beta-1,4-GalT V gene through activation of the Sp1 gene in cancer cells.

Down-regulation of beta1,4-galactosyltransferase V is a critical part of etoposide-induced apoptotic process and could be mediated by decreasing Sp1 levels in human glioma cells

beta1,4-Galactosyltransferase V (beta1,4GalT V; EC 2.4.1.38) is considered to be very important in glioma for expressing transformation-related highly branched N-glycans. Recently, we have characterized beta1,4GalT V as a positive growth regulator in several glioma cell lines. However, the role of beta1,4GalT V in glioma therapy has not been clearly reported. In this study, interfering with the expression of beta1,4GalT V by its antisense cDNA in SHG44 human glioma cells markedly promoted apoptosis induced by etoposide and the activation of caspases as well as processing of Bid and expression of Bax and Bak. Conversely, the ectopic expression of beta1,4GalT V attenuated the apoptotic effect of etoposide on SHG44 cells. In addition, both the beta1,4GalT V transcription and the binding of total or membrane glycoprotein with Ricinus communis agglutinin-I (RCA-I) were partially reduced in etoposide-treated SHG44 cells, correlated well with a decreased level of Sp1 that has been identified as an activator of beta1,4GalT V transcription. Collectively, our results suggest that the down-regulation of beta1,4GalT V expression plays an important role in etoposide-induced apoptosis and could be mediated by a decreasing level of Sp1 in SHG44 cells, indicating that inhibitors of beta1,4GalT V may enhance the therapeutic efficiency of etoposide for malignant glioma.

A beta1,4-galactosyltransferase is required for convergent extension movements in zebrafish

Our understanding of how complex carbohydrates function during embryonic development is still very limited, primarily due to the large number of glycosyltransferases now known to be involved in their synthesis. To overcome these limitations, we have taken advantage of the zebrafish system to analyze the function of complex carbohydrates during development by down-regulating the expression of specific glycosyltransferases. Herein, we report the identification of the zebrafish ortholog of mammalian beta1,4-galactosyltransferase I, beta4GalT1, and its requirement for proper convergent extension movements during gastrulation. beta4GalT1 is expressed in the oocyte and throughout the embryo during the first 24 h of development. Knockdown of zebrafish beta4GalT1 by two independent morpholino oligonucleotides results in embryos with a truncated anterior-posterior axis, as well as elongated somites and moderate defects in the patterning of the head mesenchyme. Co-injection of zebrafish beta4GalT1 mRNA returns galactosyltransferase activity to control levels and rescues the defects produced by morpholino oligonucleotides. In situ hybridizations of various molecular markers reveal that the axial mesoderm of epiboly stage embryos is abnormally widened in beta4GalT1 morphants, indicative of abnormal convergent extension. Consistent with this, the rate of anterior-posterior axis elongation is reduced relative to control-injected embryos, similar to that seen in known convergent extension mutants. Among the many potential substrates for beta4GalT1 is laminin, a principle component of the extracellular matrix that supports cell movements such as those that occur during convergent extension. Previous in vitro studies have shown that the galactosylation status of laminin directly influences its ability to support cell spreading and migration. In this regard, laminin isolated from beta4GalT1 morphant embryos is poorly galactosylated, which may contribute to defective cell migration during convergent extension movements. This work demonstrates that zebrafish can be used to identify critical developmental roles for specific glycosyltransferases that would not be obvious otherwise, such as an absolute requirement for beta4GalT1 during convergent extension movements.

Beta1,4-galactosyltransferase V functions as a positive growth regulator in glioma

beta1,4-galactosyltransferase V (GalT V; EC 2.4.1.38) can effectively galactosylate the GlcNAcbeta1-->6Man arm of the highly branched N-glycans that are characteristic of glioma. Previously, we have reported that the expression of GalT V is increased in the process of glioma. However, currently little is known about the role of GalT V in this process. In this study, the ectopic expression of GalT V could promote the invasion and survival of glioma cells and transformed astrocytes. Furthermore, decreasing the expression of GalT V in glioma cells promoted apoptosis, inhibited the invasion and migration and the ability of tumor formation in vivo, and reduced the activation of AKT. In addition, the activity of GalT V promoter could be induced by epidermal growth factor, dominant active Ras, ERK1, JNK1, and constitutively active AKT. Taken together, our results suggest that GalT V functioned as a novel glioma growth activator and might represent a novel target in glioma therapy.

β4GalT-II is a key regulator of glycosylation of the proteins involved in neuronal development

Seven members of the human beta1,4-galactosyltransferase (beta4GalTs) have been identified and characterized by many groups. beta4GalTs play important roles in the extension of N- and O-linked glycans involved in several biological events. However, it has not been clear which beta4GalTs can act on glycoproteins, such as alpha-dystroglycan and Notch receptors, involved in neuronal development. To clarify which beta4GalTs can function, we determined the enzyme activities toward such motifs and the transcript levels in human normal tissues. Among human beta4GalTs, both beta4GalT-I and beta4GalT-II could act efficiently on all substrates, but the relative activity of beta4GalT-II was higher than that of beta4GalT-I. Transcript of beta4GalT-I was widely expressed except for brain, and on the other hand, that of beta4GalT-II was expressed at high levels in the brain. Thus, these results suggest that among human beta4GalTs, beta4GalT-II is a major regulator of the synthesis of glycans involved in neuronal development.

Transcriptional Regulation of the Human β-1,4-Galactosyltransferase V Gene in Cancer Cells

Beta-1,4-galactosyltransferase (beta-1,4-GalT) V is a constitutively expressed enzyme that can effectively galactosylate the GlcNAcbeta1-->6Man group of the highly branched N-glycans that are characteristic of tumor cells. Upon malignant transformation of cells, the expression of the beta-1,4-GalT V gene increases in accordance with the increase in the amounts of highly branched N-glycans. Lectin blot analysis showed that the galactosylation of highly branched N-glycans is inhibited significantly in SH-SY5Y human neuroblastoma cells by the transfection of the antisense beta-1,4-GalT V cDNA, indicating the biological importance of the beta-1,4-GalT V for the functions of highly branched N-glycans. We cloned the 2.3-kb 5'-flanking region of the human beta-1,4-GalT V gene, and we identified the region -116/-18 relative to the transcription start site as that having promoter activity. The region was found to contain several putative binding sites for transcription factors, including AP2, AP4, N-Myc, Sp1, and upstream stimulatory factor. Electrophoretic mobility shift assay showed that Sp1 binds to nucleotide positions -81/-69 of the promoter region. Mutations induced in the Sp1-binding site showed that the promoter activity of the beta-1,4-GalT V gene is impaired completely in cancer cells. In contrast, the promoter activity increased significantly by the transfection of the Sp1 cDNA into A549 human lung carcinoma cells. Mithramycin A, which inhibits the binding of Sp1 to its binding site, reduced the promoter activation and expression of the beta-1,4-GalT V gene in A549 cells. These results indicate that Sp1 plays an essential role in the transcriptional activity of the beta-1,4-GalT V gene in cancer cells.

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

Analysis of glycans from erythrocyte membrane glycoproteins from beta1,4-galactosyltransferase-1 (beta4GalT-1)-deficient mice revealed moderately decreased galactosylation but comparable polylactosamine content compared to control beta4GalT-1(+/-) mice. The increased expression of more branched N-glycans was observed in beta4GalT-1(-/-) mice, and its extent was more remarkable in elder beta4GalT-1(-/-) mice (28 weeks old) than in younger beta4GalT-1(-/-) mice (6-9 weeks old). In relation to this issue, the less galactosylation of biantennary glycans was observed in the elder group, suggesting that beta4GalTs actually compete with N-acetylglucosaminyltransferases IV and V in erythroid cells. In contrast, approximately 80% of core 2 O-glycans were not beta1,4-galactosylated regardless of age of the knockout mice. These results suggest that beta4GalT-1 expressed in erythroid cells may regulate a constant branch formation of N-glycans and plays a predominant role in beta1,4-galactosylation of core 2 O-glycan.

Molecular cloning and functional characterization of a Lepidopteran insect beta4-N-acetylgalactosaminyltransferase with broad substrate specificity, a functional role in glycoprotein biosynthesis, and a potential functional role in glycolipid biosynthesis

A degenerate PCR approach was used to isolate a lepidopteran insect cDNA encoding a beta4-galactosyl-transferase family member. The isolation and initial identification of this cDNA was based on bioinformatics, but its identification as a beta4-galactosyltransferase family member was experimentally confirmed. The newly identified beta4-galactosyltransferase family member had unusually broad donor and acceptor substrate specificities in vitro, as transferred galactose, N-acetylglucosamine, and N-acetylgalactosamine to carbohydrate, glycoprotein, and glycolipid acceptors. However, the enzyme preferentially utilized N-acetylgalactosamine as the donor for all three acceptors, and its derived amino acid sequence was closely related to a known N-acetylgalactosaminyltransferase. These data suggested that the newly isolated cDNA encodes a beta4-N-acetylgalactosaminyltransferase that functions in insect cell glycoprotein biosynthesis, glycolipid biosynthesis, or both. The remainder of this study focused on the role of this enzyme in N-glycoprotein biosynthesis. The results showed that the purified enzyme transferred N-acetylgalactosamine, but no detectable galactose or N-acetylglucosamine, to a synthetic N-glycan in vitro. The structure of the reaction product was confirmed by chromatographic, mass spectroscopic, and nuclear magnetic resonance analyses. Co-expression of the new cDNA product in insect cells with an N-glycoprotein reporter showed that it transferred N-acetylgalactosamine, but no detectable galactose or N-acetylglucosamine, to this N-glycoprotein in vivo. Confocal microscopy showed that a GFP-tagged version of the enzyme was localized in the insect cell Golgi apparatus. In summary, this study demonstrated that lepidopteran insect cells encode and express a beta4-N-acetylgalactosaminyltransferase that functions in N-glycoprotein biosynthesis and perhaps in glycolipid biosynthesis, as well. The isolation and characterization of this gene and its product contribute to our basic understanding of insect protein N-glycosylation pathways and to the growing body of evidence that insects can produce glycoproteins with complex N-glycans.

Rapid cell senescence-associated changes in galactosylation of N-linked oligosaccharides in human lung adenocarcinoma A549 cells

Rapid senescence was induced into human lung adenocarcinoma A549 cells by transforming growth factor-beta1. Lectin blot analysis of membrane glycoprotein samples showed that the binding of Ricinus communis agglutinin-I to protein bands increased markedly while those of other lectins together with protein components did not change significantly with senescence. This indicates that the beta-1,4-galactosylation of N-linked oligosaccharides is stimulated by rapid senescence. Analysis of the enzymatic background of senescence showed 1.5 times higher beta-1,4-galactosyltransferase (beta-1,4-GalT) activity and 2-5 times higher expression levels of beta-1,4-GalT II, III, V, and VI genes are associated with rapid senescence. Incubation of the cells on RCA-I-coated plates in the absence of fetal calf serum showed that the viability of the senescent cells is half that of the control cells. Therefore, it is hypothesized that galactose residues expressed by rapid senescent can induce a lethal signal in cells if they interact with appropriate receptors.

Characterization of a novel ZP3-independent sperm-binding ligand that facilitates sperm adhesion to the egg coat

During mammalian fertilization, sperm adhere to the extracellular coat of the egg, or zona pellucida, in a species-specific manner. In mouse, evidence suggests that sperm recognize and bind to specific oligosaccharide ligands within the zona pellucida glycoprotein, ZP3, viaβ1,4-galactosyltransferase I (GalT I), a lectin-like receptor on the sperm surface. Although in vitro experiments using isolated gametes lend support to this model, recent in vivo studies of genetically altered mice question whether ZP3 and/or GalT I are solely responsible for sperm-egg binding. In this regard, sperm from GalT I-null mice bind poorly to ZP3 and fail to undergo a zona-induced acrosome reaction; however, they still bind to the ovulated egg coat in vitro.

In this report, we characterize a novel ZP3- and GalT I-independent mechanism for sperm adhesion to the egg coat. Results show that the ovulated zona pellucida contains at least two distinct ligands for sperm binding: a ZP3-independent ligand that is peripherally associated with the egg coat and facilitates gamete adhesion; and a ZP3-dependent ligand that is present in the insoluble zona matrix and is recognized by sperm GalT I to facilitate acrosomal exocytosis. The ZP3-independent ligand is not a result of contamination by egg cortical granules, nor is it the mouse homolog of oviduct-specific glycoprotein. It behaves as a 250 kDa, WGA-reactive glycoprotein with a basic isoelectric point, distinguishing it from the acidic glycoproteins that form the insoluble matrix of the egg coat. When eluted from isoelectric focusing gels, the acidic matrix glycoproteins possess sperm-binding activity for wild-type sperm, but not for GalT I-null sperm,whereas the basic glycoprotein retains sperm-binding activity for both wild-type and GalT I-null sperm. Thus, GalT I-null sperm are able to resolve gamete recognition into at least two distinct binding events, leading to the characterization of a novel, peripherally associated, sperm-binding ligand on the ovulated zona pellucida.

Suppression of proliferation and neurite extension of human neuroblastoma SH-SY5Y cells on immobilizedPsathyrella velutina lectin

Glycoproteins from mammalian brain tissues contain unique N-linked oligosaccharides terminating with beta-N-acetylglucosamine residues. Lectin blot analysis of membrane glycoprotein samples from human neuroblastoma SH-SY5Y cells showed that several protein bands bind to Psathylera velutina lectin (PVL), which interacts with beta-N-acetylglucosamine-terminating oligosaccharides. No lectin positive bands were detected by digestion with jack bean beta-N-acetyl-hexosaminidase or N-glycanase before incubation with the lectin, indicating that the cells contain beta-N-acetylglucosamine-terminating N-linked oligosaccharides. When cells were cultured in dishes with different concentrations of PVL, the cell proliferation was inhibited in a dose-dependent manner. Similarly, the neurite extension, which was stimulated with nerve growth factor, was also inhibited in a manner dependent on the lectin dose. Cell proliferation and neurite extension were recovered by the addition of 10 mM N-acetylglucosamine into the medium. Immunoblot analysis of the activation of mitogen-activated protein (MAP) kinases and protein kinase C revealed that phosphorylation of 42-kDa and 44-kDa MAP kinases and 80-kDa protein kinase C are inhibited when SH-SY5Y cells are cultured in PVL-coated dishes, but are restored by the addition of the haptenic sugar into the medium, indicating that MAP kinase and protein kinase C pathways are inhibited by interaction with immobilized PVL. These results indicate that beta-N-acetylglucosamine-terminating N-linked oligosaccharides expressed on neural cells can induce intracellular signals upon binding to extracellular receptors, and are important for growth regulation of neural cells.

A novel missense mutation in the galactosyltransferase-I (B4GALT7) gene in a family exhibiting facioskeletal anomalies and Ehlers-Danlos syndrome resembling the progeroid type

The Ehlers-Danlos syndrome (EDS) is a heterogeneous group of heritable connective tissue disorders characterized by skin hyperextensibility, joint hypermobility, and tissue fragility. Several genes have been implicated to result in EDS phenotypes. The progeroid type of EDS is characterized by wrinkled, loose skin on the face, curly fine hair, scanty eyebrows and eyelashes, in addition to the classical features of EDS. Here we describe two similarly affected individuals in two sibships of a large consanguineous family from Qatar. DNA samples from affected and unaffected members of the family were analyzed for homozygosity of polymorphic markers associated with genes that have been implicated in EDS. Among 28 markers analyzed, homozygosity was only observed for D5S469 and D5S2111, which were markers for galactosyltransferase-I (B4GALT7) located on chromosome 5q35.2, where the previously reported progeroid-like variant of EDS has been mapped. Exons harboring the coding regions and exon-intron junctions of B4GALT7 were amplified by PCR and examined for mutations. A homozygous misssense C to T substitution at nucleotide 808 in the coding region was discovered in both affected individuals. The carrier parents were heterozygous for this mutation, which was not found among 76 DNA samples from control individuals of the same ethnicity. Segregation of this novel mutation in the family further confirmed the allelic variant and its recessive mode of inheritance in this type of EDS. The C to T substitution results in an arginine to cysteine change at amino acid residue 270 that is located in the catalytically active extracellular C-terminal domain. This change could result in abnormal protein folding and/or aberrant interactions of mutated galactosyltransferase-I with other extracellular matrix proteins leading to the development of a progeroid-like phenotype in affected individuals.

Distinct patterns of expression of the beta-1,4-galactosyltransferases during testicular development in the mouse

Glycosylation is one of the most important post-translational modifications and it is clear that the single step of beta-1,4-galactosylation is performed by a family of beta-1,4-galactosyltransferases (beta4-GalTs) and that each member of this family may play a distinct role in different tissues and cells. In this study, we characterized the gene expression of six beta4-GalTs in mouse testis and analyzed the changes of galactosylation of testis glycoproteins during postnatal development. Northern blot analysis revealed that beta4-GalT-I and beta4-GalT-IV were expressed mainly in newborn mouse testis and that the expression of beta4-GalT-II increased markedly and persisted at the highest levels in adult mouse testis. The expression of beta4-GalT-III and beta4-GalT-V, however, remained relatively at low levels during mouse testicular development. In contrast, the expression of beta4-GalT-VI was undetectable in mouse testis. The gene expression of beta4-GalT-II in mouse testis was further analyzed by in situ hybridization due to its unique expression pattern. Strong hybridization signals were detected in the seminiferous tubules and the expression varied among the different stages of spermatogenic differentiation. The distinct gene expression patterns of beta4-GalTs in mouse testis could affect the differential galactosylation of testis glycoproteins, as revealed by lectin histochemistry analysis.

Identification and characterization of a Drosophila melanogaster ortholog of human beta1,4-galactosyltransferase VII

Drosophila melanogaster is widely considered to be an attractive model organism for studying the functions of the carbohydrate moieties of glycoconjugates produced by higher eukaryotes. However, the pathways of glycoconjugate biosynthesis are not as well defined in insects as they are in higher eukaryotes. One way to address this problem is to identify genes in the Drosophila genome that might encode relevant functions, express them, and determine the functions of the gene products by direct biochemical assays. In this study, we used this approach to identify a putative Drosophila beta4-galactosyltransferase gene and determine the enzymatic activity of its product. Biochemical assays demonstrated that this gene product could transfer galactose from UDP-galactose to a beta-xylosyl acceptor, but not to other acceptors in vitro. The apparent K(m) values for the donor and acceptor substrates indicated that this gene product is a functional galactosyltransferase. Additional assays showed that the enzyme is activated by manganese, has a slightly acidic pH optimum, and is localized in the insect cell Golgi apparatus. These results showed that Drosophila encodes an ortholog of human beta4-galactosyltransferase-VII, also known as galactosyltransferase I, which participates in proteoglycan biosynthesis by transferring the first galactose to xylose in the linkage tetrasaccharide of glycosaminoglycan side chains.

Beta 1,4-galactosyltransferase (beta 4GalT)-IV is specific for GlcNAc 6-O-sulfate. Beta 4GalT-IV acts on keratan sulfate-related glycans and a precursor glycan of 6-sulfosialyl-Lewis X

The Galbeta1-->4(SO(3)(-)-->6)GlcNAc moiety is present in various N-linked and O-linked glycans including keratan sulfate and 6-sulfosialyl-Lewis X, an L-selectin ligand. We previously found beta1,4-galactosyltransferase (beta4GalT) activity in human colonic mucosa, which prefers GlcNAc 6-O-sulfate (6SGN) as an acceptor to non-substituted GlcNAc (Seko, A., Hara-Kuge, S., Nagata, K., Yonezawa, S., and Yamashita, K. (1998) FEBS Lett. 440, 307-310). To identify the gene for this enzyme, we purified the enzyme from porcine colonic mucosa. The purified enzyme had the characteristic requirement of basic lipids for catalytic activity. Analysis of the partial amino acid sequence of the enzyme revealed that the purified beta4GalT has a similar sequence to human beta4GalT-IV. To confirm this result, we prepared cDNA for each of the seven beta4GalTs cloned to date and examined substrate specificities using the membrane fractions derived from beta4GalT-transfected COS-7 cells. When using several N-linked and O-linked glycans with or without 6SGN residues as acceptor substrates, only beta4GalT-IV efficiently recognized 6SGN, keratan sulfate-related oligosaccharides, and Galbeta1-->3(SO(3)(-)-->6GlcNAcbeta1-->6) GalNAcalpha1-O-pNP, a precursor for 6-sulfosialyl-Lewis X. These results suggested that beta4GalT-IV is a 6SGN-specific beta4GalT and may be involved in the biosynthesis of various glycoproteins carrying a 6-O-sulfated N-acetyllactosamine moiety.

Targeted mutations in beta1,4-galactosyltransferase I reveal its multiple cellular functions

Beta1,4-galactosyltransferase I (GalT I) is one of the most extensively studied glycosyltransferases. It is localized in the trans-Golgi compartment of most eukaryotic cells, where it participates in the elongation of oligosaccharide chains on glycoproteins and glycolipids. GalT I has also been reported in non-Golgi locations, most notably the cell surface, where it has been suggested to function non-biosynthetically as a receptor for extracellular glycoside substrates. Cloning of the GalT I cDNAs revealed that the gene encodes two similar proteins that differ only in the length of their cytoplasmic domains. Whether these different GalT I proteins, or isoforms, have similar or different biological roles is a matter of active investigation. The functions of the GalT I proteins have been addressed by targeted mutations that eliminate either both GalT I isoforms or just the long GalT I isoform. Eliminating both GalT I proteins abolishes most, but not all, GalT activity, an observation that led to the realization that other GalT family members must exist. The loss of both GalT I isoforms leads to neonatal lethality due to a wide range of phenotypic abnormalities that are most likely the result of decreased galactosylation. When the long isoform of GalT I is eliminated, galactosylation proceeds grossly normal via the short GalT I isoform, but specific defects in cell interactions occur that are thought to depend upon a non-biosynthetic function of the long GalT I isoform.

Identification of a Drosophila gene encoding xylosylprotein beta4-galactosyltransferase that is essential for the synthesis of glycosaminoglycans and for morphogenesis

In mammals, the xylosylprotein beta4-galactosyltransferase termed beta4GalT7 (XgalT-1, EC ) participates in proteoglycan biosynthesis through the transfer of galactose to the xylose that initiates each glycosaminoglycan chain. A Drosophila cDNA homologous to mammalian beta4-galactosyltransferases was identified using a human beta4GalT7 cDNA as a probe in a BLAST analysis of expressed sequence tags. The Drosophila cDNA encodes a type II membrane protein with 322 amino acids and shows 49% identity to human beta4GalT7. Extracts from L cells transfected with the cDNA exhibited marked galactosyltransferase activity specific for a xylopyranoside acceptor. Moreover, transfection with the cloned cDNA restored glycosaminoglycan synthesis in beta4GalT7-deficient Chinese hamster ovary cells. In transfectant lysates the properties of Drosophila and human beta4GalT7 resembled each other, except that Drosophila beta4GalT7 showed a less restricted specificity and was active at a wider range of temperatures. Drosophila beta4GalT7 is expressed throughout development, with higher expression levels in adults. Reduction of Drosophila beta4GalT7 levels using expressed RNA interference (RNAi) in imaginal discs resulted in an abnormal wing and leg morphology similar to that of flies with defective Hedgehog and Decapentaplegic signaling, which are known to depend on intact proteoglycan biosynthesis. Immunohistochemical analysis of tissues confirmed that both heparan sulfate and chondroitin sulfate biosynthesis were impaired. Our results demonstrate that Drosophila beta4GalT7 has the in vitro and in vivo properties predicted for an ortholog of human beta4GalT7 and is essential for normal animal development through its role in proteoglycan biosynthesis.

Enzymatic synthesis in vitro of the disulfated disaccharide unit of corneal keratan sulfate

Among the enzymes of the carbohydrate sulfotransferase family, human corneal GlcNAc 6-O-sulfotransferase (hCGn6ST, also known as human GlcNAc6ST-5/GST4beta) and human intestinal GlcNAc 6-O-sulfotransferase (hIGn6ST or human GlcNAc6ST-3/GST4alpha) are highly homologous. In the mouse, intestinal GlcNAc 6-O-sulfotransferase (mIGn6ST or mouse GlcNAc6ST-3/GST4) is the only orthologue of hCGn6ST and hIGn6ST. In the previous study, we found that hCGn6ST and mIGn6ST, but not hIGn6ST, have sulfotransferase activity to produce keratan sulfate (Akama, T. O., Nakayama, J., Nishida, K., Hiraoka, N., Suzuki, M., McAuliffe, J., Hindsgaul, O., Fukuda, M., and Fukuda, M. N. (2001) J. Biol. Chem. 276, 16271-16278). In this study, we analyzed the substrate specificities of these sulfotransferases in vitro using synthetic carbohydrate substrates. We found that all three sulfotransferases can transfer sulfate to the nonreducing terminal GlcNAc of short carbohydrate substrates. Both hCGn6ST and mIGn6ST, but not hIGn6ST, transfer sulfate to longer carbohydrate substrates that have poly-N-acetyllactosamine structures, suggesting the involvement of hCGn6ST and mIGn6ST in production of keratan sulfate. To clarify further the involvement of hCGn6ST in biosynthesis of keratan sulfate, we reconstituted the biosynthetic pathway in vitro by sequential enzymatic treatment of a synthetic carbohydrate substrate. Using four enzymes, beta1,4-galactosyltransferase-I, beta1,3-N-acetylglucosaminyltransferase-2, hCGn6ST, and keratan sulfate Gal 6-O-sulfotransferase, we were able to synthesize in vitro a product that conformed to the basic structural unit of keratan sulfate. Based on these results, we propose a biosynthetic pathway for N-linked keratan sulfate on corneal proteoglycans.

Molecular cloning and characterization of a novel chondroitin sulfate glucuronyltransferase that transfers glucuronic acid to N-acetylgalactosamine

We found a novel human gene (GenBank accession number, Kazusa DNA Research Institute KIAA1402) that possesses homology with chondroitin synthase. The full-length open reading frame consists of 772 amino acids and encodes a typical type II membrane protein. This enzyme had a domain containing beta 3-glycosyltransferase motifs, which might be a beta3-glucuronyltransferase domain, but no domain with beta 4-glycosyltransferase motifs, although both are found in chondroitin synthase. The putative catalytic domain was expressed in COS-7 cells as a soluble enzyme. Its glucuronyltransferase activity was observed when chondroitin and chondroitin sulfate polysaccharides and oligosaccharides were used as acceptor substrates. However, it was not detected when dermatan sulfate, hyaluronan, heparan sulfate, heparin, N-acetylheparosan, lactosamine tetrasaccharide, and linkage tri- and tetrasaccharide acceptors were employed. The reaction product, which was speculated to exhibit a GlcA beta 1-3GalNAc linkage structure at its non-reducing terminus, showed the following characteristics. 1) It was catabolized by beta-glucuronidase. 2) It was an acceptor for Escherichia coli K4 chondroitin polymerase (K4 chondroitin polymerase). 3) The product of K4 chondroitin polymerase was cleaved by chondroitinase ACII. On the other hand, no N-acetylgalactosaminyltransferase activity was detected toward any acceptors. Quantitative real time PCR analysis revealed that its transcripts were highly expressed in the placenta, small intestine, and pancreas, although they were ubiquitously expressed in various tissues and cell lines. This enzyme could play a role in the synthesis of chondroitin sulfate as a glucuronyltransferase.

Molecular cloning and enzymatic characterization of a UDP-GalNAc:GlcNAc(beta)-R beta1,4-N-acetylgalactosaminyltransferase from Caenorhabditis elegans

A common terminal structure in glycans from animal glycoproteins and glycolipids is the lactosamine sequence Gal(beta)4GlcNAc-R (LacNAc or LN). An alternative sequence that occurs in vertebrate as well as in invertebrate glycoconjugates is GalNAc(beta)4GlcNAc-R (LacdiNAc or LDN). Whereas genes encoding beta4GalTs responsible for LN synthesis have been reported, the beta4GalNAcT(s) responsible for LDN synthesis has not been identified. Here we report the identification of a gene from Caenorhabditis elegans encoding a UDP-GalNAc:GlcNAc(beta)-R beta1,4-N-acetylgalactosaminyltransferase (Ce(beta)4GalNAcT) that synthesizes the LDN structure. Ce(beta)4GalNAcT is a member of the beta4GalT family, and its cDNA is predicted to encode a 383-amino acid type 2 membrane glycoprotein. A soluble, epitope-tagged recombinant form of Ce(beta)4GalNAcT expressed in CHO-Lec8 cells was active using UDP-GalNAc, but not UDP-Gal, as a donor toward a variety of acceptor substrates containing terminal beta-linked GlcNAc in both N- and O-glycan type structures. The LDN structure of the product was verified by co-chromatography with authentic standards and (1)H NMR spectroscopy. Moreover, Chinese hamster ovary CHO-Lec8 and CHO-Lec2 cells expressing Ce(beta)4GalNAcT acquired LDN determinants on endogenous glycoprotein N-glycans, demonstrating that the enzyme is active in mammalian cells as an authentic beta4GalNAcT. The identification and availability of this novel enzyme should enhance our understanding of the structure and function of LDN-containing glycoconjugates.

Studies on the metal binding sites in the catalytic domain of beta1,4-galactosyltransferase

The catalytic domain of bovine beta1,4-galactosyltransferase (beta4Gal-T1) has been shown to have two metal binding sites, each with a distinct binding affinity. Site I binds Mn(2+) with high affinity and does not bind Ca(2+), whereas site II binds a variety of metal ions, including Ca(2+). The catalytic region of beta4Gal-T1 has DXD motifs, associated with metal binding in glycosyltransferases, in two separate sequences: D(242)YDYNCFVFSDVD(254) (region I) and W(312)GWGGEDDD(320) (region II). Recently, the crystal structure of beta4Gal-T1 bound with UDP, Mn(2+), and alpha-lactalbumin was determined in our laboratory. It shows that in the primary metal binding site of beta4Gal-T1, the Mn(2+) ion, is coordinated to five ligands, two supplied by the phosphates of the sugar nucleotide and the other three by Asp254, His347, and Met344. The residue Asp254 in the D(252)VD(254) sequence in region I is the only residue that is coordinated to the Mn(2+) ion. Region II forms a loop structure and contains the E(317)DDD(320) sequence in which residues Asp318 and Asp319 are directly involved in GlcNAc binding. This study, using site-directed mutagenesis, kinetic, and binding affinity analysis, shows that Asp254 and His347 are strong metal ligands, whereas Met344, which coordinates less strongly, can be substituted by alanine or glutamine. Specifically, substitution of Met344 to Gln has a less severe effect on the catalysis driven by Co(2+). Glu317 and Asp320 mutants, when partially activated by Mn(2+) binding to the primary site, can be further activated by Co(2+) or inhibited by Ca(2+), an effect that is the opposite of what is observed with the wild-type enzyme.

Enhanced branching morphogenesis in mammary glands of mice lacking cell surface beta1,4-galactosyltransferase

Development of the mammary gland is influenced both by the systemic hormonal environment and locally through cell-cell and cell-extracellular matrix (ECM) interactions. We have previously demonstrated aberrant mammary gland morphogenesis in transgenic mice with elevated levels of the long isoform of beta1,4-galactosyltransferase 1 (GalT), a proportion of which is targeted to the plasma membrane, where it plays a role in cell-ECM interactions. Here, we show that mammary glands of mice lacking the long GalT isoform exhibit a complementary phenotype. Cell-surface GalT activity was reduced by over 60%, but because the short GalT isoform is intact, total GalT activity was reduced only slightly relative to wild type. Mammary glands from long GalT-null mice were characterized by excess branching, and this phenotype was accompanied by altered expression of laminin chains. Laminin alpha1 and alpha3 were reduced 2.4- and 3.0-fold, respectively, while expression of laminin gamma2 was elevated 2.3-fold. The expression and cleavage of laminin gamma2 have been correlated with branching and cell migration, and Western blotting revealed an altered pattern in gamma2 cleavage products in long GalT-null mammary glands. We then examined the expression of metalloproteases that cleave laminins or that have been shown to play a role in mammary gland morphogenesis. Expression of MT1-MMP, a membrane-bound protease that can cleave laminin gamma2, was elevated 5.5-fold in the long GalT-nulls. MMP 7 was also elevated 5.1-fold. Our results suggest that expression of surface GalT is important for the proper regulation of matrix expression and deposition, which in turn regulates the proper branching morphogenesis of the mammary epithelial ductal system.

Chemoenzymatic synthesis of biotinylated nucleotide sugars as substrates for glycosyltransferases

The enzymatic oxidation of uridine 5'-diphospho-alpha-D-galactose (UDP-Gal) and uridine 5'-diphospho-N-acetyl-alpha-D-galactosamine (UDP-GalNAc) with galactose oxidase was combined with a chemical biotinylation step involving biotin-epsilon-amidocaproylhydrazide in a one-pot synthesis. The novel nucleotide sugar derivatives uridine 5'-diphospho-6-biotin-epsilon-amidocaproylhydrazino-alpha-D-galactose (UDP-6-biotinyl-Gal) and uridine 5'-diphospho-6-biotin-epsilon-amidocaproylhydrazino-N-acetyl-alpha-D-galactosamine (UDP-6-biotinyl-GalNAc) were synthesized on a 100-mg scale and characterized by mass spectrometry (fast atom bombardment and matrix-assisted laser desorption/ionization time of flight) and one/two dimensional NMR spectroscopy. It could be demonstrated for the first time, by use of UDP-6-biotinyl-Gal as a donor substrate, that the human recombinant galactosyltransferases beta3Gal-T5, beta4Gal-T1, and beta4Gal-T4 mediate biotinylation of the neoglycoconjugate bovine serum albumin-p-aminophenyl N-acetyl-beta-D-glucosaminide (BSA-(GlcNAc)17) and ovalbumin. The detection of the biotin tag transferred by beta3Gal-T5 onto BSA-(GlcNAc)17 with streptavidin-enzyme conjugates gave detection limits of 150 pmol of tagged GlcNAc in a Western blot analysis and 1 pmol of tagged GlcNAc in a microtiter plate assay. The degree of Gal-biotin tag transfer onto agalactosylated hybrid N-glycans present at the single glycosylation site of ovalbumin was dependent on the Gal-T used (either beta3Gal-T5, beta4Gal-T4, or beta4Gal-T1), which indicates that the acceptor specificity may direct the transfer of the Gal-biotin tag. The potential of this biotinylated UDP-Gal as a novel donor substrate for human galactosyltransferases lies in the targeting of distinct acceptor structures, for example, under-galactosylated glycoconjugates, which are related to diseases, or in the quality control of glycosylation of recombinant and native glycoproteins.

alpha-Lactalbumin (LA) stimulates milk beta-1,4-galactosyltransferase I (beta 4Gal-T1) to transfer glucose from UDP-glucose to N-acetylglucosamine. Crystal structure of beta 4Gal-T1 x LA complex with UDP-Glc

beta-1,4-Galactosyltransferase 1 (Gal-T1) transfers galactose (Gal) from UDP-Gal to N-acetylglucosamine (GlcNAc), which constitutes its normal galactosyltransferase (Gal-T) activity. In the presence of alpha-lactalbumin (LA), it transfers Gal to Glc, which is its lactose synthase (LS) activity. It also transfers glucose (Glc) from UDP-Glc to GlcNAc, constituting the glucosyltransferase (Glc-T) activity, albeit at an efficiency of only 0.3-0.4% of Gal-T activity. In the present study, we show that LA increases this activity almost 30-fold. It also enhances the Glc-T activity toward various N-acyl substituted glucosamine acceptors. Steady state kinetic studies of Glc-T reaction show that the K(m) for the donor and acceptor substrates are high in the absence of LA. In the presence of LA, the K(m) for the acceptor substrate is reduced 30-fold, whereas for UDP-Glc it is reduced only 5-fold. In order to understand this property, we have determined the crystal structures of the Gal-T1.LA complex with UDP-Glc x Mn(2+) and with N-butanoyl-glucosamine (N-butanoyl-GlcN), a preferred sugar acceptor in the Glc-T activity. The crystal structures reveal that although the binding of UDP-Glc is quite similar to UDP-Gal, there are few significant differences observed in the hydrogen bonding interactions between UDP-Glc and Gal-T1. Based on the present kinetic and crystal structural studies, a possible explanation for the role of LA in the Glc-T activity has been proposed.

Stage- and tissue-specific expression of a beta-1,4-galactosyltransferase in the embryonic epidermis

Changes in oligosaccharide structures of glycoconjugates have been observed, and are postulated to have key roles in embryonic development and differentiation. N-Acetylglucosamine (GlcNAc) beta-1,4-galactosyltransferase (beta4GalT) AKI showed different expression patterns in time and space, and different enzymatic activity from the other known family members. The epidermis of mouse embryo included a high level of AKI activities, which transferred galactose (Gal) to endogenous glycoprotein (molecular weight 130 kDa) (GP130). The maximum activity was for 13.5-d postcoitum embryos. Specific antibody against AKI inhibited 81% of GlcNAc betaGalT activities, which indicates that AKI represents the major part of the embryonic epidermis enzymes. AKI shows 2.2 times higher galactosyltransferase activity toward Gal-acceptor glucose with alpha-lactalbumin (alpha-LA) than toward GlcNAc without alpha-LA. AKI is also expressed in mouse melanoma and leukemia cell lines and in human basal cell carcinoma specimens. The GP130 Gal acceptor once galactosylated by AKI may be directly involved in epidermal differentiation and oncogenesis.

Galactosylation of N-linked oligosaccharides by human beta-1,4-galactosyltransferases I, II, III, IV, V, and VI expressed in Sf-9 cells

Several studies showed that Sf-9 cells can synthesize the galactosylated N-linked oligosaccharides if beta-1,4-galactosyltransferase (beta-1,4-GalT) is supplied. The full-length human beta-1,4-GalT I, II, III, IV, V, and VI cDNAs were independently transfected into Sf-9 cells, and the galactosylation of endogenous membrane glycoproteins was examined by lectin blot analysis using Ricinus communis agglutinin-I (RCA-I), which preferentially interacts with oligosaccharides terminated with Galbeta1-->4GlcNAc group. Several RCA-I-reactive bands appeared in all of the gene-transfected cells, and disappeared on treatment of blots with beta-1,4-galactosidase or N-glycanase prior to incubation with lectin. Introduction of the antisense beta-1,4-GalT II and V cDNAs separately into human colorectal adenocarcinoma SW480 cells, in which beta-1,4-GalT I, II, and V genes were expressed, resulted in the reduction of RCA-I binding toward N-linked oligosaccharides of the membrane glycoproteins. Differences were found in their K(m) values toward UDP-Gal and GlcNAcbeta-S-pNP and in their acceptor specificities toward oligosaccharides with the GlcNAcbeta1-->4(GlcNAcbeta1-->2)Man branch and with the GlcNAcbeta1-->6(GlcNAcbeta1-->2)Man branch. These results indicate that beta-1,4-GalTs II, III, IV, V, and VI are involved in the N-linked oligosaccharide biosynthesis cooperatively but not in a redundant manner with beta-1,4-GalT I within cells.