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

VLDL Biogenesis and Secretion: It Takes a Village

The production and secretion of VLDLs (very-low-density lipoproteins) by hepatocytes has a direct impact on liver fat content, as well as the concentrations of cholesterol and triglycerides in the circulation and thus affects both liver and cardiovascular health, respectively. Importantly, insulin resistance, excess caloric intake, and lack of physical activity are associated with overproduction of VLDL, hepatic steatosis, and increased plasma levels of atherogenic lipoproteins. Cholesterol and triglycerides in remnant particles generated by VLDL lipolysis are risk factors for atherosclerotic cardiovascular disease and have garnered increasing attention over the last few decades. Presently, however, increased risk of atherosclerosis is not the only concern when considering today's cardiometabolic patients, as they often also experience hepatic steatosis, a prevalent disorder that can progress to steatohepatitis and cirrhosis. This duality of metabolic risk highlights the importance of understanding the molecular regulation of the biogenesis of VLDL, the lipoprotein that transports triglycerides and cholesterol out of the liver. Fortunately, there has been a resurgence of interest in the intracellular assembly, trafficking, degradation, and secretion of VLDL by hepatocytes, which has led to many exciting new molecular insights that are the topic of this review. Increasing our understanding of the biology of this pathway will aid to the identification of novel therapeutic targets to improve both the cardiovascular and the hepatic health of cardiometabolic patients. This review focuses, for the first time, on this duality.

The role of protein glycosylation in the occurrence and outcome of acute ischemic stroke

Acute ischemic stroke (AIS) is a serious and life-threatening disease worldwide. Despite thrombolysis or endovascular thrombectomy, a sizeable fraction of patients with AIS have adverse clinical outcomes. In addition, existing secondary prevention strategies with antiplatelet and anticoagulant drugs therapy are not able to adequately decrease the risk of ischemic stroke recurrence. Thus, exploring novel mechanisms for doing so represents an urgent need for the prevention and treatment of AIS. Recent studies have discovered that protein glycosylation plays a critical role in the occurrence and outcome of AIS. As a common co- and post-translational modification, protein glycosylation participates in a wide variety of physiological and pathological processes by regulating the activity and function of proteins or enzymes. Protein glycosylation is involved in two causes of cerebral emboli in ischemic stroke: atherosclerosis and atrial fibrillation. Following ischemic stroke, the level of brain protein glycosylation becomes dynamically regulated, which significantly affects stroke outcome through influencing inflammatory response, excitotoxicity, neuronal apoptosis, and blood-brain barrier disruption. Drugs targeting glycosylation in the occurrence and progression of stroke may represent a novel therapeutic idea. In this review, we focus on possible perspectives about how glycosylation affects the occurrence and outcome of AIS. We then propose the potential of glycosylation as a therapeutic drug target and prognostic marker for AIS patients in the future.

The known unknowns of apolipoprotein glycosylation in health and disease

Apolipoproteins, the protein component of lipoproteins, play an important role in lipid transport, lipoprotein assembly, and receptor recognition. Apolipoproteins are glycosylated and the glycan moieties play an integral role in apolipoprotein function. Changes in apolipoprotein glycosylation correlate with several diseases manifesting in dyslipidemias. Despite their relevance in apolipoprotein function and diseases, the total glycan repertoire of most apolipoproteins remains undefined. This review summarizes the current knowledge and knowledge gaps regarding human apolipoprotein glycan composition, structure, glycosylation site, and functions. Given the relevance of glycosylation to apolipoprotein function, we expect that future studies of apolipoprotein glycosylation will contribute new understanding of disease processes and uncover relevant biomarkers and therapeutic targets. Considering these future efforts, we also provide a brief overview of current mass spectrometry based technologies that can be applied to define detailed glycan structures, site-specific compositions, and the role of emerging approaches for clinical applications in biomarker discovery and personalized medicine.

DPM1 expression as a potential prognostic tumor marker in hepatocellular carcinoma

Background

Altered glycosylation of proteins contributes to tumor progression. Dolichol phosphate mannose synthase (DPMS), an essential mannosyltransferase, plays a central role in post-translational modification of proteins, including N-linked glycoproteins, O-mannosylation, C-mannosylation and glycosylphosphatidylinositol anchors synthesis. Little is known about the function of DPMS in liver cancer.

Methods

The study explored the roles of DPMS in the prognosis of hepatocellular carcinoma using UALCAN, Human Protein Atlas, GEPIA, cBioPortal and Metascape databases. The mRNA expressions of DPM1/2/3 also were detected by quantitative real-time PCR experiments in vitro.

Results

The transcriptional and proteinic expressions of DPM1/2/3 were both over-expressed in patients with hepatocellular carcinoma. Over-expressions of DPMS were discovered to be dramatically associated with clinical cancer stages and pathological tumor grades in hepatocellular carcinoma patients. In addition, higher mRNA expressions of DPM1/2/3 were found to be significantly related to shorter overall survival in liver cancer patients. Futhermore, high genetic alteration rate of DPMS (41%) was also observed in patients with liver cancer, and genetic alteration in DPMS was associated with shorter overall survival in hepatocellular carcinoma patients. We also performed quantitative real-time PCR experiments in human normal hepatocytes and hepatoma cells to verify the expressions of DPM1/2/3 and results showed that the expression of DPM1 was significantly increased in hepatoma cells SMMC-7721 and HepG2.

Conclusions

Taken together, these results suggested that DPM1 could be a potential prognostic biomarker for survivals of hepatocellular carcinoma patients.

GLT25D2 Is Critical for Inflammatory Immune Response to Promote Acetaminophen-Induced Hepatotoxicity by Autophagy Pathway

Acetaminophen (APAP) overdose induces hepatocyte necrosis and causes liver hepatotoxicity. Currently, the role of galactosyltransferase in APAP-induced liver injury is still unclear. This study assessed the contribution of the GLT25D2 gene, a kind of collagen galactosyltransferase, to the development of APAP-induced liver injury. This study found that the expression of GLT25D2 markedly increased first and then decreased in the liver of mice treated with APAP, however, it downregulated in the liver of APAP overdose-patients compared with normal controls. Knockout of GLT25D2 significantly ameliorated the liver injury, meanwhile, it downregulated the proinflammatory cytokines (IL-6, TNF-α) and chemokines (CXCL-10, MIG and CXCL-1) levels, however, and upregulated the anti-inflammatory cytokines (IL-22, IL-10) levels. Mechanistic explorations showed that (1) GLT25D2 knockout promoted autophagy pathway; and (2) the GLT25D2 knockout-induced autophagy selected to clear damaged mitochondria in APAP-induced liver injury by mitophagy; and (3) the autophagy intervention by Atg 7 siRNA cancelled liver protection by knockout of GLT25D2 through regulating liver inflammation. In conclusion, our study proves that the upregulated expression of GLT25D2 decreased autophagy contributing to APAP-induced hepatotoxicity by mediating the inflammatory immune regulatory mechanism.

Improving Immunotherapy Through Glycodesign

Immunotherapy is revolutionizing health care, with the majority of high impact "drugs" approved in the past decade falling into this category of therapy. Despite considerable success, glycosylation-a key design parameter that ensures safety, optimizes biological response, and influences the pharmacokinetic properties of an immunotherapeutic-has slowed the development of this class of drugs in the past and remains challenging at present. This article describes how optimizing glycosylation through a variety of glycoengineering strategies provides enticing opportunities to not only avoid past pitfalls, but also to substantially improve immunotherapies including antibodies and recombinant proteins, and cell-based therapies. We cover design principles important for early stage pre-clinical development and also discuss how various glycoengineering strategies can augment the biomanufacturing process to ensure the overall effectiveness of immunotherapeutics.

The glycomic effect of N-acetylglucosaminyltransferase III overexpression in metastatic melanoma cells. GnT-III modifies highly branched N-glycans

N-acetylglucosaminyltransferase III (GnT-III) is known to catalyze N-glycan "bisection" and thereby modulate the formation of highly branched complex structures within the Golgi apparatus. While active, it inhibits the action of other GlcNAc transferases such as GnT-IV and GnT-V. Moreover, GnT-III is considered as an inhibitor of the metastatic potential of cancer cells both in vitro and in vivo. However, the effects of GnT-III may be more diverse and depend on the cellular context. We describe the detailed glycomic analysis of the effect of GnT-III overexpression in WM266-4-GnT-III metastatic melanoma cells. We used MALDI-TOF and ESI-ion-trap-MS/MS together with HILIC-HPLC of 2-AA labeled N-glycans to study the N-glycome of membrane-attached and secreted proteins. We found that the overexpression of GnT-III in melanoma leads to the modification of a broad range of N-glycan types by the introduction of the "bisecting" GlcNAc residue with highly branched complex structures among them. The presence of these unusual complex N-glycans resulted in stronger interactions of cellular glycoproteins with the PHA-L. Based on the data presented here we conclude that elevated activity of GnT-III in cancer cells does not necessarily lead to a total abrogation of the formation of highly branched glycans. In addition, the modification of pre-existing N-glycans by the introduction of "bisecting" GlcNAc can modulate their capacity to interact with carbohydrate-binding proteins such as plant lectins. Our results suggest further studies on the biological function of "bisected" oligosaccharides in cancer cell biology and their interactions with carbohydrate-binding proteins.

Hepatic aberrant glycosylation by N-acetylglucosaminyltransferase V accelerates HDL assembly

Glycosylation is involved in various pathophysiological conditions. N-Acetylglucosaminyltransferase V (GnT-V), catalyzing β1-6 branching in asparagine-linked oligosaccharides, is one of the most important glycosyltransferases involved in cancer and the immune system. Recent findings indicate that aberrant N-glycan structure can modify lipid metabolism. In this study, we investigated the effects of aberrant glycosylation by GnT-V on high-density lipoprotein cholesterol (HDL) assembly. We used GnT-V transgenic (Tg) mice and GnT-V Hep3B cell (human hepatoma cell line) transfectants. The study also included 96 patients who underwent medical health check-ups. Total serum cholesterol levels, particularly HDL-cholesterol (HDL-C) levels, were significantly increased in Tg vs. wild-type (WT) mice. Hepatic expression of apolipoprotein AI (ApoAI) and ATP-binding cassette subfamily A member 1 (ABCA1), two important factors in HDL assembly, were higher in Tg mice compared with WT mice. ApoAI and ABCA1 were also significantly elevated in GnT-V transfectants compared with mock-transfected cells. Moreover, ApoAI protein in the cultured media of GnT-V transfectants was significantly increased. Finally, we found a strong correlation between serum GnT-V activity and HDL-C concentration in human subjects. Multivariate logistic analyses demonstrated that GnT-V activity was an independent and significant determinant for serum HDL-C levels even adjusted with age and gender differences. Further analyses represented that serum GnT-V activity had strong correlation especially with the large-size HDL particle concentration. These findings indicate that enhanced hepatic GnT-V activity accelerated HDL assembly and could be a novel mechanism for HDL synthesis.

DNA hypomethylation upregulates expression of the MGAT3 gene in HepG2 cells and leads to changes in N-glycosylation of secreted glycoproteins

Changes in N-glycosylation of plasma proteins are observed in many types of cancer, nevertheless, few studies suggest the exact mechanism involved in aberrant protein glycosylation. Here we studied the impact of DNA methylation on the N-glycome in the secretome of the HepG2 cell line derived from hepatocellular carcinoma (HCC). Since the majority of plasma glycoproteins originate from the liver, the HepG2 cells represent a good model for glycosylation changes in HCC that are detectable in blood, which is an easily accessible analytic material in a clinical setting. Two different concentrations of 5-aza-2′-deoxycytidine (5-aza-2dC) differentially affected global genome methylation and induced different glycan changes. Around twenty percent of 84 glyco-genes analysed changed expression level after the 5-aza-2dC treatment as a result of global genome hypomethylation. A correlation study between the changes in glyco-gene expression and the HepG2 glycosylation profile suggests that the MGAT3 gene might be responsible for the glycan changes consistently induced by both doses of 5-aza-2dC. Core-fucosylated tetra-antennary structures were decreased in quantity likely as a result of hypomethylated MGAT3 gene promoter followed by increased expression of this gene.

Ectopic expression of N-acetylglucosaminyltransferase V accelerates hepatic triglyceride synthesis

AIM

Glycosylation changes induce various types of biological phenomena in human diseases. N-Acetylglucosaminyltransferase V (GnT-V) is one of the most important glycosyltransferases involved in cancer biology. Recently, many researchers have challenged studies of lipid metabolism in cancer. To elucidate the relationships between cancer and lipid metabolism more precisely, we investigated the effects of GnT-V on lipid metabolism. In this study, we investigated the effects of aberrant glycosylation by GnT-V on hepatic triglyceride production.

METHODS

We compared lipid metabolism in GnT-V transgenic (Tg) mice with that of wild-type (WT) mice fed with normal chow or a choline-deficient amino acid-defined (CDAA) diet in vivo. HepG2 cells and GnT-V transfectants of Hep3B cells were used in an in vitro study.

RESULTS

Serum triglyceride levels and hepatic very low-density lipoprotein (VLDL) secretion in Tg mice were significantly elevated compared with that of WT mice. Hepatic lipogenic genes (Lxrα, Srebp1, Fas and Acc) and VLDL secretion-related gene (Mttp1) were significantly higher in Tg mice. Expression of these genes was also significantly higher in GnT-V transfectants than in mock cells. Knockdown of GnT-V decreased, while both epidermal growth factor and transforming growth factor-β1 stimulation increased LXRα gene expression in HepG2 cells. Finally, we found that the blockade of VLDL secretion by CDAA diet induced massive hepatic steatosis in Tg mice.

CONCLUSION

Our study demonstrates that enhancement of hepatic GnT-V activity accelerates triglyceride synthesis and VLDL secretion. Glycosylation modification by GnT-V regulation could be a novel target for a therapeutic approach to lipid metabolism.

Glycosyltransferases and non-alcoholic fatty liver disease

Non-alcoholic fatty liver disease (NAFLD) is the most common form of chronic liver disease and its incidence is increasing worldwide. However, the underlying mechanisms leading to the development of NAFLD are still not fully understood. Glycosyltransferases (GTs) are a diverse class of enzymes involved in catalyzing the transfer of one or multiple sugar residues to a wide range of acceptor molecules. GTs mediate a wide range of functions from structure and storage to signaling, and play a key role in many fundamental biological processes. Therefore, it is anticipated that GTs have a role in the pathogenesis of NAFLD. In this article, we present an overview of the basic information on NAFLD, particularly GTs and glycosylation modification of certain molecules and their association with NAFLD pathogenesis. In addition, the effects and mechanisms of some GTs in the development of NAFLD are summarized.

Mechanism of the effect of glycosyltransferase GLT8D2 on fatty liver

Background

Recent studies have shown that some glycosyltransferases are involved in the development of nonalcoholic fatty liver disease (NAFLD). The objective of this study was to explore the effect and mechanism of glycosyltransferase GLT8D2 on fatty liver.

Methods

Rat model of NAFLD was established by induction with high-fat-diet. The GLT8D2 expression in rat liver was examined using immunohistochemistry. Oil Red O staining and triglyceride assay were used to measure the effect of abnormal GLT8D2 expression on lipid accumulation in HepG2 cells. The expression levels of lipid metabolism-related key molecules, namely sterol regulatory element-binding protein-1c (SREBP-1c), stearoyl-coA desaturase (SCD), carnitine palmitoyltransferase-1 (CPT1) and microsomal triglyceride transfer protein (MTP), in HepG2 cells with abnormal GLT8D2 expression were determined by western blot analyses.

Results

The expression of GLT8D2 was higher in the liver of rats with NAFLD than in the control rats, and GLT8D2 was mainly located around lipid droplets in hepatocytes. GLT8D2 expression increased in steatosis HepG2 cells compared with that in normal HepG2 cells. GLT8D2 positively regulated lipid droplet accumulation and triglyceride content in HepG2 cells. Upregulation or knockdown of GLT8D2 had no effect on the expressions of SREBP-1c, SCD or CPT-1 proteins in HepG2 cells. However, GLT8D2 expression negatively regulated the expression of MTP protein in HepG2 cells.

Conclusion

GLT8D2 participated in NAFLD pathogenesis possibly by negatively regulating MTP expression. Specific inhibition of GLT8D2 via an antagonistic strategy could provide a potential candidate approach for treatment of NAFLD.

Glycosyltransferase GLT8D2 positively regulates ApoB100 protein expression in hepatocytes

Non-alcoholic fatty liver disease (NAFLD) is characterized by triglyceride (TG) accumulation in hepatocytes. Very low density lipoprotein (VLDL) is a major secretory product of the liver that transports endogenously synthesized TG. Disrupted VLDL secretion may contribute to the accumulation of TG in hepatocytes. ApoB100 (apolipoprotein B100) is a glycoprotein and an essential protein component of VLDL. Its glycosylation may affect VLDL assembly and secretion. However, which glycosyltransferase catalyzes apoB100 glycosylation is unknown. In this study, we cloned the GLT8D2 (glycosyltransferase 8 domain containing 2) gene from HepG2 cells and generated a series of plasmids for in vitro studies of its molecular functions. We discovered that GLT8D2 was localized in the ER, interacted with apoB100, and positively regulated the levels of apoB100 protein in HepG2 cells. Based on these results, we propose that GLT8D2 is a glycosyltransferase of apoB100 that regulates apoB100 levels in hepatocytes.

Target-specific gene silencing of layer-by-layer assembled gold-cysteamine/siRNA/PEI/HA nanocomplex

Target-specific intracellular delivery of small interfering RNA (siRNA) is regarded as one of the most important technologies for the development of siRNA therapeutics. In this work, a cysteamine modified gold nanoparticles (AuCM)/siRNA/polyethyleneimine (PEI)/hyaluronic acid (HA) complex was successfully developed using a layer-by-layer method for target-specific intracellular delivery of siRNA by HA receptor mediated endocytosis. Atomic force microscopic and zeta potential analyses confirmed the formation of a AuCM/siRNA/PEI/HA complex having a particle size of ca. 37.3 nm and a negative surface charge of ca. -12 mV. With a negligible cytotoxicity, AuCM/siRNA/PEI/HA complex showed an excellent target-specific gene silencing efficiency of ca. 70% in the presence of 50 vol % serum, which was statistically much higher than that of siRNA/Lipofectamine 2000 complex. In the competitive binding tests with free HA, dark-field bioimaging and inductively coupled plasma-atomic emission spectroscopy confirmed the target-specific intracellular delivery of AuCM/siRNA/PEI/HA complex to B16F1 cells with HA receptors. Moreover, the systemic delivery of AuCM/siRNA/PEI/HA complex using apolipoprotein B (ApoB) siRNA as a model drug resulted in a significantly reduced ApoB mRNA level in the liver tissue. Taken together, AuCM/siRNA/PEI/HA complex was thought to be developed as target-specific siRNA therapeutics for the systemic treatment of various liver diseases.

A mutual regulation between cell-cell adhesion and N-glycosylation: implication of the bisecting GlcNAc for biological functions

Changes in oligosaccharide structures are associated with numerous physiological and pathological events. E-cadherin-mediated cell-cell adhesion is believed to be both temporally and spatially regulated during development, and represents a key step in the acquisition of the invasive phenotype for many tumors. Here, we focus mainly on a mutual regulation between E-cadherin-mediated cell-cell adhesion and N-acetylglucosaminyltransferase III (GnT-III) expression, and discuss its implications for biological functions.

Alteration of protein glycosylation in liver diseases

Chronic liver diseases are a serious health problem worldwide. The current gold standard to assess structural liver damage is through a liver biopsy which has several disadvantages. A non-invasive, simple and non-expensive test to diagnose liver pathology would be highly desirable. Protein glycosylation has drawn the attention of many researchers in the search for an objective feature to achieve this goal. Glycosylation is a posttranslational modification of many secreted proteins and it has been known for decades that structural changes in the glycan structures of serum proteins are an indication for liver damage. The aim of this paper is to give an overview of this altered protein glycosylation in different etiologies of liver fibrosis / cirrhosis and hepatocellular carcinoma. Although individual liver diseases have their own specific markers, the same modifications seem to continuously reappear in all liver diseases: hyperfucosylation, increased branching and a bisecting N-acetylglucosamine. Analysis at mRNA and protein level of the corresponding glycosyltransferases confirm their altered status in liver pathology. The last part of this review deals with some recently developed glycomic techniques that could potentially be used in the diagnosis of liver pathology.

Branched N-glycans regulate the biological functions of integrins and cadherins

Glycosylation is one of the most common post-translational modifications, and approximately 50% of all proteins are presumed to be glycosylated in eukaryotes. Branched N-glycans, such as bisecting GlcNAc, beta-1,6-GlcNAc and core fucose (alpha-1,6-fucose), are enzymatic products of N-acetylglucosaminyltransferase III, N-acetylglucosaminyltransferase V and alpha-1,6-fucosyltransferase, respectively. These branched structures are highly associated with various biological functions of cell adhesion molecules, including cell adhesion and cancer metastasis. E-cadherin and integrins, bearing N-glycans, are representative adhesion molecules. Typically, both are glycosylated by N-acetylglucosaminyltransferase III, which inhibits cell migration. In contrast, integrins glycosylated by N-acetylglucosaminyltransferase V promote cell migration. Core fucosylation is essential for integrin-mediated cell migration and signal transduction. Collectively, N-glycans on adhesion molecules, especially those on E-cadherin and integrins, play key roles in cell-cell and cell-extracellular matrix interactions, thereby affecting cancer metastasis.

Mass spectrometric approach for screening modifications of total serum N-glycome in human diseases: application to cirrhosis

Congenital and acquired modifications of glycosylation in diseases are a rapidly growing field that demonstrates the importance of glycosylation in human biology. Unfortunately, in clinical biochemistry, very few tests are available to explore oligosaccharide metabolism on a large scale. Such an assay needs to be of high throughput, rapid, and preferentially noninvasive. In the present study, we describe a method to analyze qualitative variations of N-glycosylation of human serum proteins. The method is based on direct release of N-linked oligosaccharides from patient serum samples, a single-step purification, and a matrix-assisted laser desorption ionization time of flight mass spectrometric analysis. A complementary structural study of the released oligosaccharides was achieved by enzymatic digestions, linkage analysis, and electrospray ionization ion trap mass spectrometry (ESI-IT-MS) of the permethylated N-glycome. A total of 26 oligosaccharide structures were individualized, their presence in human serum being the result of the combination of the biosynthesis and catabolic pathways. Application of the protocol to the serum of patients with cirrhosis demonstrates the ability of this assay to identify acquired modifications of glycosylation. Furthermore, we have analyzed the N-glycans and showed the increase in bisecting N-acetylglucosamine residue, core fucosylation, and the presence of an important population of neutral oligosaccharides. The study of total serum N-glycome modifications is a preliminary for the discovery of new noninvasive diagnostic or prognostic biomarkers resulting from the variations of the N-glycan metabolism during diseases.

Cloning and expression analysis of cDNAs corresponding to genes activated in cucumber showing systemic acquired resistance after BTH treatment

BACKGROUND

Infection of plants by necrotizing pathogens can lead to the rapid and localized induction of a complex set of defense responses resulting in a restriction of pathogen growth and spread. Subsequently, an increase of plant resistance against a broad spectrum of pathogens is observed systemically. This plant immunity is known as Systemic Acquired Resistance. To identify components of the transduction pathway, we cloned and analysed the expression pattern of several mRNAs accumulating in cucumber plants after induction of Systemic Acquired Resistance.

RESULTS

We tested on cucumber different compounds known to induce systemic acquired resistance. Among these, BTH (benzo(1,2,3)thiadiazole-7-carbothioic acid S-methyl ester) proved to be very effective. mRNA RT-PCR differential display was used to identify mRNA sequences induced 24 hours after the application of 10 microM BTH to cucumber plants. A cDNA library constructed from cucumber plants sprayed with 10 microM BTH was screened to get corresponding full length cDNAs. Among the identified cDNAs were those coding for a putative ras-related GTP-binding protein, a putative beta-1,4-N-Acetylglucosaminyltranferase III and a putative pathogenesis related protein. The time course of accumulation of the three corresponding mRNAs was analysed by northern blotting in plants treated by BTH or in plants infected by Colletotrichum lagenarium.

CONCLUSIONS

The mRNA RT-PCR differential display technique allowed the identification of three genes possibly involved in Systemic Acquired Resistance in cucumber. Pathogenesis-related proteins are known to be involved in plant defence against pathogens. GTP-binding protein and N-acetylglucosaminyltranferase III have been reported to be components of signal transduction pathways in mammals and plants.

Hyperexpression of N-acetylglucosaminyltransferase-III in liver tissues of transgenic mice causes fatty body and obesity through severe accumulation of Apo A-I and Apo B

N-Acetylglucosaminyltransferase (GnT)-III catalyzes the attachment of an N-acetylglucosamine (GlcNAc) residue to mannose in beta(1-4) configuration in the region of N-glycans and forms a bisecting GlcNAc. To investigate the pathophysiological role of dysregulated glycosylation mediated by aberrantly expressed GnT-III, we generated transgenic mice hyperexpressing the human GnT-III in the liver by introducing human GnT-III cDNA under the control of mouse albumin enhancer/promoter. Total five transgenic founder mice (pGnTSVTpA-10, -14, -20, -25, and -51) expressed the human GnT-III in their livers and were characterized by molecular genetic means. The copy number of transgene integrated into the genome of these mice ranged between 1 and 3 copies per haploid genome. Northern and Western blot analyses showed that the transgene is specifically expressed in the liver but not in any other tissues tested. The triglyceride level in GnT-III transgenic mice was significantly decreased, however, no significant differences in the levels of glucose, cholesterol, or albumin were observed between transgenic and nontransgenic mice. Although glutamate oxaloacetic transaminase and glutamic pyruvic transaminase activities of transgenic mice were also higher than those of nontransgenic mice, no differences in total bililubin and total protein were observed between the two animal lines. Large amounts of apolipoprotein (Apo) A-I and Apo B were specifically detected in the intracellular liver of transgenic mice. The accumulation of Apo A-I in hepatocytes may be due to aberrant glycosylation, since glycosylated Apo A-I was not observed in transgenic mice. However, the accumulated Apo B was severely glycosylated. Therefore, it is suggested that highly expressed transgenic GnT-III allowed unknown target proteins to be glycosylated in large amounts, and the resulting target protein(s) disrupted in assembly formation of Apo A-I in the hepatocytes and cause a decrease in the release of lipoproteins and accumulations of Apo A-I and Apo B in the liver. The transgenic mice showed aberrant glycosylation by GnT-III, resulting in numerous lipid droplets in liver tissues and the obesity. These mice showed microvesicular fatty changes with abnormal lipid accumulation in the hepatocytes. Our study provides the basis for future analysis of the role of glycosylation in hepatic pathogenesis. In the transgenic mice, Apo A-I and Apo B were significantly increased compared with levels in nontransgenic liver tissues.

The hepatitis B virus X protein inhibits secretion of apolipoprotein B by enhancing the expression of N-acetylglucosaminyltransferase III

The X protein of hepatitis B virus (HBx) plays a major role on hepatocellular carcinoma (HCC). Apolipoprotein B (apoB) in the liver is an important glycoprotein for transportation of very low density lipoproteins and low density lipoproteins. Although lipid accumulation in the liver is known as one of the factors for the HCC, the relationship between HBx and apoB during the HCC development is poorly understood. To better understand the biological significance of HBx in HCC, liver Chang cells that specifically express HBx were established and characterized. In this study we demonstrate that overexpression of HBx significantly up-regulates the expression of UDP-N-acetylglucosamine:beta-d-mannoside-1,4-N-acetylglucosaminyltransferase-III (GnT-III), an enzyme that functions as a bisecting-N-acetylglucosamine (GlcNAc) transferase in apoB, and increases GnT-III promoter activity in a chloramphenicol acetyltransferase assay. GnT-III expression levels of HBx-transfected cells appeared to be higher than that of hepatocarcinoma cells as well as GnT-III-transfected cells, indicating that HBx may has a strong GnT-III promotor-enhancing activity. Intracellular levels of apoBs, which contained the increased bisecting GlcNAc, were accumulated in HBx-transfected liver cells. These cells as well as GnT-III-transfected liver cells revealed the inhibition of apoB secretion and the increased accumulation of intracellular triglyceride and cholesterol compared with vector-transfected cells. Moreover, overexpression of GnT-III and HBx in liver cells was shown to down-regulate the transcriptional level of microsomal triglyceride transfer protein, which regulates the assembly and secretion of apoB. Therefore, our study strongly suggested that the HBx increase in intracellular accumulation of aberrantly glycosylated apoB resulted in inhibition of secretion of apoB as well as intracellular lipid accumulation by elevating the expression of GnT-III.

Biological consequences of overexpressing or eliminating N-acetylglucosaminyltransferase-TIII in the mouse

N-acetylglucosaminyltransferase III (GlcNAc-TIII), a product of the human MGAT3 gene, was discovered as a glycosyltransferase activity in hen oviduct. GlcNAc-TIII transfers GlcNAc in beta4-linkage to the core Man of complex or hybrid N-glycans, and thereby alters not only the composition, but also the conformation of the N-glycan. The dramatic consequences of the addition of this bisecting GlcNAc residue are reflected in the altered binding of lectins that recognize Gal residues on N-glycans. Changes in GlcNAc-TIII expression correlate with hepatoma and leukemia in rodents and humans, and the bisecting GlcNAc on Asn 297 of human IgG antibodies enhances their effector functions. Overexpression of a cDNA encoding GlcNAc-TIII alters growth control and cell-cell interactions in cultured cells, and in transgenic mice. While mice lacking GlcNAc-TIII are viable and fertile, they exhibit retarded progression of diethylnitrosamine (DEN)-induced liver tumors. Further biological functions of GlcNAc-TIII are expected to be uncovered as mice with a null mutation in the Mgat3 gene are challenged.

Truncated, inactive N-acetylglucosaminyltransferase III (GlcNAc-TIII) induces neurological and other traits absent in mice that lack GlcNAc-TIII

N-Acetylglucosaminyltransferase III (GlcNAc-TIII), the product of the Mgat3 gene, transfers the bisecting GlcNAc to the core mannose of complex N-glycans. The addition of this residue is regulated during development and has functional consequences for receptor signaling, cell adhesion, and tumor progression. Mice homozygous for a null mutation at the Mgat3 locus (Mgat3(Delta)) or for a targeted mutation in the Mgat3 gene (previously called Mgat3(neo), but herein renamed Mgat3(T37) because the allele generates inactive GlcNAc-TIII of approximately 37 kDa) were found to exhibit retarded progression of liver tumors. Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry of neutral N-glycans from kidneys revealed no significant differences, and both mutants showed the expected lack of N-glycan species with an additional GlcNAc. However, the two mutants differed in several biological traits. Mgat3(T37/T37) homozygotes in a mixed or 129(SvJ) background were retarded in growth rate and exhibited an altered leg clasp reflex, an altered gait, and defective nursing behavior. Pups abandoned by Mgat3(T37/T37) mothers were rescued by wild-type foster mothers. None of these Mgat3(T37/T37) traits were exhibited by Mgat3(Delta/Delta) mice or by heterozygous mice carrying the Mgat3(T37) mutation. Similarly, no dominant-negative effect was observed in Chinese hamster ovary cells expressing truncated GlcNAc-TIII in the presence of wild-type GlcNAc-TIII. However, compound heterozygotes carrying both the Mgat3(T37) and Mgat3(Delta) mutations exhibited a marked leg clasp reflex, indicating that in the absence of wild-type GlcNAc-TIII, truncated GlcNAc-TIII causes this phenotype. The Mgat3 gene was expressed in brain at embryonic day 10.5 and thereafter and in neurons of adult cerebellum. The mutant Mgat3 gene was also highly expressed in Mgat3(T37/T37) brain. This may be the basis of the unexpected neurological phenotype induced by truncated, inactive GlcNAc-TIII in the mouse.

Antibodies that recognize bisected complex N-glycans on cell surface glycoproteins can be made in mice lacking N-acetylglucosaminyltransferase III

The bisecting GlcNAc is transferred to complex or hybrid N-glycans by the action of N-acetylglucosaminyltransferase III (GlcNAc-TIII) encoded by the Mgat3 gene. CHO cells expressing mouse GlcNAc-TIII were shown by matrix-assisted laser desorption ionization (MALDI) mass spectrometry to produce mainly complex N-glycans with the predicted extra (bisecting) GlcNAc. In order to probe biological functions of the bisecting GlcNAc, antibodies that recognize this residue in the context of complex cell surface glycoconjugates were sought. The LEC10 gain-of-function Chinese hamster ovary (CHO) cell mutant that expresses GlcNAc-TIII and complex N-glycans with the bisecting GlcNAc was used to immunize Mgat3(+/+) and Mgat3(-/-) mice. ELISA of whole sera showed that polyclonal antibodies that bound specifically to LEC10 cells were obtained solely from Mgat3(-/-) mice. Fluorescence-activated cell cytometry of different CHO glycosylation mutants and western blotting after glycosidase treatments were used to show that anti-LEC10 cell antisera from Mgat3(-/-) mice recognize cellular glycoproteins with complex N-glycans containing both a bisecting GlcNAc and Gal residues. The polyclonal antibody specificity was similar to that of the lectin E-PHA. IgM-depleted serum containing IgG and IgA antibodies retained full binding activity. Therefore Mgat3(-/-) mice but not wild type mice can be used effectively to produce polyclonal antibodies that specifically recognize glycoproteins bearing complex N-glycans with a bisecting GlcNAc.

Expression of bisecting N-acetylglucosaminyltransferase-III in human hepatocarcinoma tissues, fetal liver tissues, and hepatoma cell lines of Hep3B and HepG2

In this paper, uridine diphosphate (UDP)-N-acetylglucosamine/beta-D-mannoside beta-1,4 N-acetylglucosaminyltransferase III (GlcNAc-transferase-III C 2.4.1.144) activity was determined in human hepatoma cell lines of Hep3B and HepG2, and also compared with those of normal liver tissues and primary hepatocytes. GlcNAc-transferase-III enzymes of Hep3B and HepG2 were mainly detected in the membrane fraction. When GlcN,GlcN-biant-PA and UDP-GlcNAc were used as substrates, the Km values (4.7 mM for UDP-GlcNAc and 1.1 mM for GlcN, GlcN-biant-PA) of Hep3B GlcNAc-transferase-III were distinguishable from those of HepG2 GlcNAc-transferase-III (6.8 mM for UDP-GlcNAc and 3.4 mM for GlcN,GlcN-biant-PA). Furthermore, Hep3B enzyme in membrane fraction showed about 1.5-fold higher specific activity (1423 pmol/hr/mg) than that of HepG2 (1066 pmol/hr/mg). Normal liver cells and primary adult hepatocytes are characterized by a very low level of GlcNAc-transferase-III activity, whereas human hepatoma cells exhibited high activities. These data were supported by reverse transcription-polymerase chain reaction results, showing that expression of the GlcNAc-transferase-III mRNA increased in proportion to the enzymatic activities. Although the mechanism underlying the induction of this enzyme is unknown, lectin blot analysis showed that oligosaccharides in many glycoproteins were observed in hepatoma cells. By treating hepatocarcinoma cultures that express GlcNAc-transferase-III with inhibitors (tunicamycin, deoxymannojirimycin, and swainsonine) of different steps of the glycosylation, we provide evidence that expression of GlcNAc-transferase-III mRNA is dependent on glycosylation of cellular proteins.

A catalytically inactive beta 1,4-N-acetylglucosaminyltransferase III (GnT-III) behaves as a dominant negative GnT-III inhibitor

beta 1,4-N-Acetylglucosaminyltransferase III (GnT-III) plays a regulatory role in the biosynthesis of N-glycans, and it has been suggested that its product, a bisecting GlcNAc, is involved in a variety of biological events as well as in regulating the biosynthesis of the oligosaccharides. In this study, it was found, on the basis of sequence homology, that GnT-III contains a small region that is significantly homologous to both snail beta 1,4GlcNAc transferase and beta1,4Gal transferase-1. Subsequent mutational analysis demonstrated an absolute requirement for two conserved Asp residues (Asp321 and Asp323), which are located in the most homologous region of rat GnT-III, for enzymatic activity. The overexpression of Asp323-substituted, catalytically inactive GnT-III in Huh6 cells led to the suppression of the activity of endogenous GnT-III, but no significant decrease in its expression, and led to a specific inhibition of the formation of bisected sugar chains, as shown by structural analysis of the total N-glycans from the cells. These findings indicate that the mutant serves a dominant negative effect on a specific step in N-glycan biosynthesis. This type of 'dominant negative glycosyltransferase', identified has potential value as a powerful tool for defining the precise biological roles of the bisecting GlcNAc structure.

Characterization of human apolipoprotein B100 oligosaccharides in LDL subfractions derived from normal and hyperlipidemic plasma: deficiency of alpha-N-acetylneuraminyllactosyl-ceramide in light and small dense LDL particles

The carbohydrate composition of apolipoprotein (apo) B100, particularly its degree of sialylation, may contribute to the atherogenic properties of low-density lipoprotein (LDL). We analyzed LDL apoB100 glycans derived from normolipidemic, hypercholesterolemic, and hypertriglyceridemic diabetic subjects. Using exoglycosidase carbohydrate sequencing and matrix-assisted laser desorption/ionization mass spectrometry to analyze fluorescently labeled oligosaccharides, we report evidence for several carbohydrates not previously identified on apoB100, including truncated complex biantennary N-glycans and hybrid N-glycans. The distribution and diversity of the apoB100 glycans isolated from all individuals was highly conserved. The N-glycan composition of apoB100 derived from five LDL subpopulations (LDL1, d = 1.018-1.023; LDL2, d = 1.023-1.030; LDL3, d = 1.030-1.040; LDL4, d = 1.040-1.051; LDL5, d = 1.051-1.065 g/ml) did not vary in normolipidemic or hypercholesterolemic subjects. Furthermore, we found no evidence for "desialylated" apoB100 glycans in any of the samples analyzed. Analysis of the most abundant LDL ganglioside, alpha-N-acetylneuraminyllactosyl-ceramide, revealed a deficiency in small dense LDL and in the most buoyant subpopulation. These data provide a novel explanation for the apparent deficiency of sialic acid in small dense LDL and indicate that the global apoB100 N-glycan composition is invariable in the patient groups studied.

Ectopic expression of alpha1,6 fucosyltransferase in mice causes steatosis in the liver and kidney accompanied by a modification of lysosomal acid lipase

The alpha1,6 fucosyltransferase (alpha1,6 FucT) catalyzes the transfer of a fucose from GDP-fucose to the innermost GlcNAc residue of N-linked glycans via an alpha1,6 linkage. alpha1,6 FucT was overexpressed in transgenic mice under the control of a combined cytomegalovirus and chicken beta-actin promoter. Histologically numerous small vacuoles, in which lipid droplets had accumulated, were observed in hepatocytes and proximal renal tubular cells. Electron microscopic studies showed that the lipid droplets were membrane-bound and apparently localized within the lysosomes. Cholesterol esters and triglycerides were significantly increased in liver and kidney of the transgenic mice. Liver lysosomal acid lipase (LAL) activity was significantly lower in the transgenic mice compared to the wild mice, whereas LAL protein level, which was detected immunochemically, was increased, indicating that the specific activity of LAL was much lower in the transgenic mice. In all of the transgenic and nontransgenic mice examined, the activity of liver LAL was negatively correlated with the level of alpha1,6 FucT activity. As evidenced by lectin and immunoblot analysis, LAL was found to be more fucosylated in the transgenic mice, suggesting that the aberrant fucosylation of LAL causes an accumulation of inactive LAL in the lysosomes. Such an accumulation of inactive LAL could be a likely cause for a steatosis in the lysosomes of the liver and kidney in the case of the alpha1,6 FucT transgenic mice.

Reduction of the major xenoantigen on glycosphingolipids of swine endothelial cells by various glycosyltransferases

The effect of the various glycosyltransferases on glycosphingolipids was examined, using transfected swine endothelial cell (SEC) lines. The reactivity of parental SEC to normal human serum (NHS) and Griffonia simplicifolia IB(4) (GSIB4) lectin, which binds to the Gal alpha1-3 Gal beta 1-4 GlcNAc-R (alpha-galactosyl epitope), was reduced by approximately 20% by the treatment with D-PDMP (D-threo-1-phenyl-2-decan- oylamino-3-morpholino-1-propanol), suggesting that glycosphingolipids contained by SEC have a considerable amount of the alpha-galactosyl epitope. The overexpression of two different types of glycosyltransferase, N-acetylglucosaminyl transferase III (GnT-III), as well as alpha2, 6-sialyltransferase (ST6Gal I), alpha2,3-sialyltransferase (ST3Gal III), and alpha1,2-fucosyltransferase (alpha1,2FT), suppresses the total antigenicity of SEC significantly. However, the reduction in reactivities toward NHS and GSIB4 lectin in the case of GnT-III transfectants was milder than those in other transfectants. Western blot analysis indicated that the glycoproteins in all transfectants had diminished reactivity to NHS and GSIB4 lectin to approximately the same extent. Therefore, the neutral glycosphingolipids of these transfectants were separated by thin layer chromatography, followed by immunostaining with NHS and GSIB4 lectin. The levels of the alpha-galactosyl epitope in glycosphingolipids were not decreased in the GnT-III transfectants but were in the ST6Gal I, ST3Gal III, and alpha1,2FT transfectants. These data indicate that ST6Gal I, ST3Gal III, and alpha1,2FT reduced the alpha-galactosyl epitope in both glycoproteins and glycosphingolipids, while GnT-III reduced them only in glycoproteins.

The alpha1-6-fucosyltransferase gene and its biological significance

GDP-L-Fuc:N-acetyl-beta-D-glucosaminide alpha1-6-fucosyltransferase (alpha1-6FucT) catalyzes the transfer of fucose from GDP-Fuc to N-linked type complex glycoproteins. This enzyme was purified from a human fibroblast cell line, porcine brain, a human gastric cancer cell line and human blood platelets. cDNA cloning of porcine and human alpha1-6FucT was performed from a porcine brain and gastric cancer cell cDNA libraries, respectively. Their homology is 92.2% at the nucleotide level and 95.7% at the amino acid level. No putative N-glycosylation sites were found in the predicted amino acid sequence. No homology to other fucosyltransferases such as alpha1-2FucT, alpha1-3FucT and alpha1-4FucT was found except for a region consisting of nine amino acids. The alpha1-6FucT gene is located at chromosome 14q24.3, which is also a different location from other fucosyltransferases reported to date. The alpha1-6FucT gene is the oldest gene family in the phylogenic trees among the nine cloned fucosyltransferase genes. alpha1-6FucT is widely expressed in various rat tissues and the expression of alpha1-6FucT in the liver is enhanced during hepatocarcinogenesis of LEC rats which develop hereditary hepatitis and hepatomas. In cases of human liver diseases, alpha1-6FucT is expressed in both hepatoma tissues and their surrounding tissues with chronic liver disease, but not in the case of normal liver. Serum alpha1-6-fucosylated alpha-fetoprotein (AFP) has been employed for an early diagnosis of patients with hepatoma. The mechanisms by which alpha1-6 fucosylation of AFP occurs in the hepatoma is not due to the up-regulation of alpha1-6FucT alone. Interestingly, when the alpha1-6FucT gene is transfected into Hep3B, a human hepatoma cell line, tumor formation in the liver of nude mice after splenic injection is dramatically suppressed. In this review, we focus on alpha1-6FucT and summarize its properties, gene expression and biological significance.

Masking and reduction of the Galactose-alpha1,3-Galactose (alpha-Gal) epitope, the major xenoantigen in swine, by the glycosyltransferase gene transfection

The alpha-Gal epitope (Gal-alpha1-3Gal-beta1-4-GlcNAc-R), which is biosynthesized by the UDP-Gal:alpha1-3-galactosyltransferase (alpha1, 3GT), is highly associated with hyperacute rejection in swine to human xenotransplantation. A variety of strategies have been pursued to reduce or eliminate this epitope from swine tissues. Since swine ES cells are not available at present, the targeted knock out of the alpha1,3GT is restricted. Other strategies, such as enzyme competition of the alpha1,3GT with other glycosyltransferases and/or control of sugar processing by the glycosyltransferases, provide a new insight into the downregulation of the alpha-Gal epitope. This review will focus on this type of strategy, which involves a gene transfection of variety of glycosyltransferases as competitors against alpha1,3GT.

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

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

Mvwf, a dominant modifier of murine von Willebrand factor, results from altered lineage-specific expression of a glycosyltransferase

We have identified altered lineage-specific expression of an N-acetylgalactosaminyltransferase gene, Galgt2, as the gain-of-function mechanism responsible for the action of the Mvwf locus, a major modifier of plasma von Willebrand factor (VWF) level in RIIIS/J mice. A switch of Galgt2 gene expression from intestinal epithelial cell-specific to a pattern restricted to the vascular endothelial cell bed leads to aberrant posttranslational modification and rapid clearance of VWF from plasma. Transgenic expression of Galgt2 directed to vascular endothelial cells reproduces the low VWF phenotype, confirming this switch in lineage-specific gene expression as the likely molecular mechanism for Mvwf. These findings identify alterations in glycosyltransferase function as a potential general mechanism for the genetic modification of plasma protein levels.