Loss of expression of N-acetylglucosaminyltransferase Va results in altered gene expression of glycosyltransferases and galectins

Specific sialylation of N-glycans and its novel regulatory mechanism

Altered glycosylation is a common feature of cancer cells. Some subsets of glycans are found to be frequently enriched on the tumor cell surface and implicated in different tumor phenotypes. Among these, changes in sialylation have long been associated with metastatic cell behaviors such as invasion and enhanced cell survival. Sialylation typically exists in three prominent linkages: α2,3, α2,6, and α2,8, catalyzed by a group of sialyltransferases. The aberrant expression of all three linkages has been related to cancer progression. The increased α2,6 sialylation on N-glycans catalyzed by β-galactoside α2,6 sialyltransferase 1 (ST6Gal1) is frequently observed in many cancers. In contrast, functions of α2,3 sialylation on N-glycans catalyzed by at least three β-galactoside α2,3-sialyltransferases, ST3Gal3, ST3Gal4, and ST3Gal6 remain elusive due to a possibility of compensating for one another. In this minireview, we briefly describe functions of sialylation and recent findings that different α2,3 sialyltransferases specifically modify target proteins, as well as sialylation regulatory mechanisms vis a complex formation among integrin α3β1, Golgi phosphoprotein 3 (GOLPH3), phosphatidylinositol 4-kinase IIα (PI4KIIα), focal adhesion kinase (FAK) and sialyltransferase, which suggests a new concept for the regulation of glycosylation in cell biology.

A Markov model of glycosylation elucidates isozyme specificity and glycosyltransferase interactions for glycoengineering

Glycosylated biopharmaceuticals are important in the global pharmaceutical market. Despite the importance of their glycan structures, our limited knowledge of the glycosylation machinery still hinders controllability of this critical quality attribute. To facilitate discovery of glycosyltransferase specificity and predict glycoengineering efforts, here we extend the approach to model N-linked protein glycosylation as a Markov process. Our model leverages putative glycosyltransferase (GT) specificity to define the biosynthetic pathways for all measured glycans, and the Markov chain modelling is used to learn glycosyltransferase isoform activities and predict glycosylation following glycosyltransferase knock-in/knockout. We apply our methodology to four different glycoengineered therapeutics (i.e., Rituximab, erythropoietin, Enbrel, and alpha-1 antitrypsin) produced in CHO cells. Our model accurately predicted N-linked glycosylation following glycoengineering and further quantified the impact of glycosyltransferase mutations on reactions catalyzed by other glycosyltransferases. By applying these learned GT-GT interaction rules identified from single glycosyltransferase mutants, our model further predicts the outcome of multi-gene glycosyltransferase mutations on the diverse biotherapeutics. Thus, this modeling approach enables rational glycoengineering and the elucidation of relationships between glycosyltransferases, thereby facilitating biopharmaceutical research and aiding the broader study of glycosylation to elucidate the genetic basis of complex changes in glycosylation.

Unique Binding Specificities of Proteins toward Isomeric Asparagine-Linked Glycans

The glycan ligands recognized by Siglecs, influenza viruses, and galectins, as well as many plant lectins, are not well defined. To explore their binding to asparagine (Asn)-linked N-glycans, we synthesized a library of isomeric multiantennary N-glycans that vary in terminal non-reducing sialic acid, galactose, and N-acetylglucosamine residues, as well as core fucose. We identified specific recognition of N-glycans by several plant lectins, human galectins, influenza viruses, and Siglecs, and explored the influence of sialic acid linkages and branching of the N-glycans. These results show the unique recognition of complex-type N-glycans by a wide variety of glycan-binding proteins and their abilities to distinguish isomeric structures, which provides new insights into the biological roles of these proteins and the uses of lectins in biological applications to identify glycans.

Density functional theory based probe of the affinity interaction of saccharide ligands with extra-cellular sialic acid residues

Changes in glycosylation pattern leads to malignant transformations among the cells. In combination with upregulated actions of sialyltransferases, it ultimately leads to differential expression of sialic acid (SA) at cell surface. Given its negative charge and localization to extracellular domain, SA has been exploited for the development of targeted theranostics using approaches, such as, cationization and appending recognition saccharides on carrier surface. In this study, we have performed quantum mechanical calculations based on density functional theory (DFT) to study the interaction of saccharides with extracellular SA. Gradient-corrected DFT with the three parameter function (B3) was utilized for the calculation of Lee-Yang-Parr (LYP) correlation function. Atomic charge, vibrational frequencies and energy of the optimized structures were calculated through B3LYP. Our calculations demonstrate a stronger galactose-sialic acid interaction at tumour-relevant low pH and hyperthermic condition. These results support the application of pH responsive delivery vehicles and targeted hyperthermic chemotherapy for eradicating solid tumour deposits. These studies, conducted a priori, can guide the formulation scientists over appropriate choice of ligands and their applications in the design of 'smart' theranostic tools.

Disease-associated glycans on cell surface proteins

Most of membrane molecules including cell surface receptors and secreted proteins including ligands are glycoproteins and glycolipids. Therefore, identifying the functional significance of glycans is crucial for developing an understanding of cell signaling and subsequent physiological and pathological cellular events. In particular, the function of N-glycans associated with cell surface receptors has been extensively studied since they are directly involved in controlling cellular functions. In this review, we focus on the roles of glycosyltransferases that are involved in the modification of N-glycans and their target proteins such as epidermal growth factor receptor (EGFR), ErbB3, transforming growth factor β (TGF-β) receptor, T-cell receptors (TCR), β-site APP cleaving enzyme (BACE1), glucose transporter 2 (GLUT2), E-cadherin, and α5β1 integrin in relation to diseases and epithelial-mesenchymal transition (EMT) process. Above of those proteins are subjected to being modified by several glycosyltransferases such as N-acetylglucosaminyltransferase III (GnT-III), N-acetylglucosaminyltransferase IV (GnT-IV), N-acetylglucosaminyltransferase V (GnT-V), α2,6 sialyltransferase 1 (ST6GAL1), and α1,6 fucosyltransferase (Fut8), which are typical N-glycan branching enzymes and play pivotal roles in regulating the function of cell surface receptors in pathological cell signaling.

Expression of N-Acetylglucosaminyltransferase III Suppresses α2,3-Sialylation, and Its Distinctive Functions in Cell Migration Are Attributed to α2,6-Sialylation Levels

N-Acetylglucosaminyltransferase III (GnT-III), which catalyzes the addition of the bisecting GlcNAc branch on N-glycans, is usually described as a metastasis suppressor. Overexpression of GnT-III inhibited migration in multiple types of tumor cells. However, these results seem controversial to the clinical observations for the increased expression of GnT-III in human hepatomas, glioma, and ovarian cancers. Here, we present evidence that these inconsistencies are mainly attributed to the different expression pattern of cell sialylation. In detail, we show that overexpression of GnT-III significantly inhibits α2,3-sialylation but not α2,6-sialylation. The migratory ability of cells without or with a low level of α2,6-sialylation is consistently suppressed after GnT-III overexpression. In contrast, the effects of GnT-III overexpression are variable in tumor cells that are highly α2,6-sialylated. Overexpression of GnT-III promotes the cell migration in glioma cells U-251 and hepatoma cells HepG2, although it has little influence in human breast cancer cell MDA-MB-231 and gastric cancer cell MKN-45. Interestingly, up-regulation of α2,6-sialylation by overexpressing β-galactoside α2,6-sialyltranferase 1 in the α2,6-hyposialylated HeLa-S3 cells abolishes the anti-migratory effects of GnT-III. Conversely, depletion of α2,6-sialylation by knock-out of β-galactoside α2,6-sialyltranferase 1 in α2,6-hypersialylated HepG2 cells endows GnT-III with the anti-migratory ability. Taken together, our data clearly demonstrate that high expression of α2,6-sialylation on the cell surface could affect the anti-migratory role of GnT-III, which provides an insight into the mechanistic roles of GnT-III in tumor metastasis.

Prognostic significance of ST3GAL-1 expression in patients with clear cell renal cell carcinoma

BACKGROUND

Aberrant sialylated carbohydrate synthesis is frequently noted in various cancers. Sialyltransferase ST3GAL-1, which adds a sialic acid in an α-2,3 linkage to Gal β1,3 GalNAc, preforms an important role in modulating cellular behaviors. However, little is known about prognostic significance of ST3GAL-1 in clear cell renal cell carcinoma (ccRCC). In this study, we aimed to investigate the prognostic significance of sialyltransferase ST3GAL-1 and its correlation with clinical outcomes in patients with ccRCC.

METHODS

A total of 286 patients who underwent nephrectomy between 2005 and 2007 in a single academic center were recruited. Immunohistochemical staining was performed on tissue microarrays to assess the expression level. Kaplan-Meier method and Cox proportional hazard model were applied to assess the prognostic value of ST3GAL-1. Nomograms were generated as prediction model for overall survival and disease free survival at 5 and 8 years after nephrectomy.

RESULTS

The present results show high expression of ST3GAL-1 is associated with reduced overall survival (p = 0.013) and disease free survival (p = 0.004). In multivariate cox analyses, ST3GAL-1 was defined as an independent prognostic factor for overall survival (p = 0.006) and disease free survival (p = 0.001). After incorporation into the University of California Integrated Staging System (UISS) intermediate/high risk group for non-metastatic ccRCC, ST3GAL-1 could further distinguish patient with dismal prognosis (p = 0.015 and 0.002 for OS and DFS respectively). The nomograms revealed better predictive accuracy in predicting 5- and 8- year overall survival and disease free survival than the TNM stage alone.

CONCLUSIONS

ST3GAL-1 is an independent adverse prognostic factor for recurrence and survival of patients with ccRCC.

Significance of β-Galactoside α2,6 Sialyltranferase 1 in Cancers

Altered glycosylation is a common feature of cancer cells. It takes a variety of forms, which includes loss of expression or excessive expression of some structures, the accumulation of precursors, the appearance of novel structures, etc. Notably, these changes in glycan structure do not occur as a random consequence of disorder biology. Only a limited subset of oligosaccharides is found frequently enriched on the tumor cell surface and implicated in different tumor phenotypes. Among these, altered sialylation has long been associated with metastatic cell behaviors such as invasion and enhanced cell survival and accumulating evidence points to the alteration occurring in the sialic acid linkage to other sugars, which normally exists in three main configurations: α2,3, α2,6, and α2,8, catalyzed by a group of sialyltransferases. The aberrant expression of all three configurations has been described in cancer progression. However, the increased α2,6 sialylation catalyzed by β-galactoside α2,6 sialyltranferase 1 (ST6Gal I) is frequently observed in many types of the cancers. In this review, we describe the findings on the role of ST6Gal I in cancer progression, and highlight in particular the knowledge of how ST6Gal I-mediated α2,6 sialylated glycans or sialylated carrier proteins regulate cell signaling to promote the malignant phenotype of human carcinoma.

N-glycans and metastasis in galectin-3 transgenic mice

Poly-N-acetyl-lactosamine (polyLacNAc) on N-glycans facilitate lung specific metastasis of melanoma cells by serving as high affinity ligands for galectin-3, expressed in highest amounts in the lungs, on almost all its tissue compartments including on the surface of vascular endothelium. PolyLacNAc not only aids in initial arrest on the organ endothelium but in all the events of extravasation. Inhibition of polyLacNAc synthesis, or competitive inhibition of its interaction with galectin-3 all inhibited these processes and experimental metastasis. Transgenic galectin-3 mice, viz., gal-3(+/+) (wild type), gal-3(+/-) (hemizygous) and gal-3(-/-) (null) have been used to prove that galectin-3/polyLacNAc interactions are indeed critical for lung specific metastasis. Gal-3(+/-) mice which showed <50% expression of galectin-3 on the lungs also showed proportionate decrease in the number of B16F10 melanoma metastatic colonies affirming that galectin-3 and polyLacNAc interactions are indeed key determinants of lung metastasis. However, surprisingly, the number and size of metastatic colonies in gal-3(-/-) mice was very similar as that seen in gal-3(+/+) mice. The levels of lactose binding lectins on the lungs and the transcripts of other galectins (galectin-1, -8 and -9) which are expressed on lungs and have similar sugar binding specificities as galectins-3, remain unchanged in gal-3(+/+) and gal-3(-/-) mice. Further, inhibition of N-glycosylation with Swainsonine (SW) which drastically reduces metastasis of B16F10 cells in gal-3(+/+) mice, did not affect lung metastasis when assessed in gal-3(-/-) mice. Together, these results rule out the possibility of some other galectin taking over the function of galectin-3 in gal-3(-/-) mice. Chimeric mice generated to assess if absence of any effect on metastasis is due to compromised tumor immunity by replacing bone marrow of gal-3(-/-) mice with that from gal-3(+/+) mice, also failed to impact melanoma metastasis. As galectin-3 regulates several immune functions including maturation of different immune cells, compromised tumor immunity could be the major determinant of melanoma metastasis in gal-3(-/-) mice and warrants thorough investigation.

Galectin-3 expressed on different lung compartments promotes organ specific metastasis by facilitating arrest, extravasation and organ colonization via high affinity ligands on melanoma cells

Interactions between molecules on the surface of tumor cells and those on the target organ endothelium play an important role in their arrest in an organ. Galectin-3 on the lung endothelium and high affinity ligands poly-N-acetyllactosamine (polyLacNAc) on N-oligosaccharides on melanoma cells facilitate such interactions. However, to extravasate and colonize an organ the cells must stabilize these interactions by spreading to retract endothelium, degrade exposed basement membrane (BM) and move into parenchyma and proliferate. Here, we show that galectin-3 is expressed on all the major compartments of the lungs and participates in not just promoting adhesion but also in spreading. We for the first time demonstrate that both soluble and immobilized galectin-3 induce secretion of MMP-9 required to breach vascular BM. Further, we show that immobilized galectin-3 is used as traction for the movement of cells. Downregulation of galactosyltransferases-I and -V resulted in significant loss in expression of polyLacNAc and thus reduced binding of galectin-3. This was accompanied with a loss in adhesion, spreading, MMP-9 secretion and motility of the cells on galectin-3 and thus their metastasis to lungs. Metastasis could also be inhibited by blocking surface polyLacNAc by pre-incubating cells with truncated galectin-3 (which lacked oligomerization domain) or by feeding mice with modified citrus pectin in drinking water. Overall, these results unequivocally show that polyLacNAc on melanoma cells and galectin-3 on the lungs play a critical role in arrest and extravasation of cells in the lungs and strategies that target these interactions inhibit lung metastasis.

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.

Over-expression of ST3Gal-I promotes mammary tumorigenesis

Changes in glycosylation are common in malignancy, and as almost all surface proteins are glycosylated, this can dramatically affect the behavior of tumor cells. In breast carcinomas, the O-linked glycans are frequently truncated, often as a result of premature sialylation. The sialyltransferase ST3Gal-I adds sialic acid to the galactose residue of core 1 (Galβ1,3GalNAc) O-glycans and this enzyme is over-expressed in breast cancer resulting in the expression of sialylated core 1 glycans. In order to study the role of ST3Gal-I in mammary tumor development, we developed transgenic mice that over-express the sialyltransferase under the control of the human membrane-bound mucin 1 promoter. These mice were then crossed with PyMT mice that spontaneously develop mammary tumors. As expected, ST3Gal-I transgenic mice showed increased activity and expression of the enzyme in the pregnant and lactating mammary glands, the stomach, lungs and intestine. Although no obvious defects were observed in the fully developed mammary gland, when these mice were crossed with PyMT mice, a highly significant decrease in tumor latency was observed compared to the PyMT mice on an identical background. These results indicate that ST3Gal-I is acting as a tumor promoter in this model of breast cancer. This, we believe, is the first demonstration that over-expression of a glycosyltransferase involved in mucin-type O-linked glycosylation can promote tumorigenesis.

Detection of differentially expressed wound-healing-related glycogenes in galectin-3-deficient mice

PURPOSE

A prior study showed that exogenous galectin-3 (Gal-3) stimulates re-epithelialization of corneal wounds in wild-type (Gal-3(+/+)) mice but, surprisingly, not in galectin-3-deficient (Gal-3(-/-)) mice. In an effort to understand why the injured corneas of Gal-3(-/-) mice are unresponsive to exogenous Gal-3, the present study was designed to determine whether genes encoding the enzymes that regulate the synthesis of glycan ligands of Gal-3 are differentially expressed in Gal-3(-/-) corneas compared with the Gal-3(+/+) corneas.

METHODS

Glycogene microarray technology was used to identify differentially expressed glycosyltransferases in healing Gal-3(+/+) and Gal-3(-/-) corneas.

RESULTS

Of approximately 2000 glycogenes on the array, the expression of 8 was upregulated and that of 14 was downregulated more than 1.3-fold in healing Gal-3(-/-) corneas. A galactosyltransferase, beta3GalT5, which has the ability to synthesize Gal-3 ligands was markedly downregulated in healing Gal-3(-/-) corneas. The genes for polypeptide galactosaminyltransferases (ppGalNAcT-3 and -7) that are known to initiate O-linked glycosylation and N-aspartyl-beta-glucosaminidase, which participates in the removal of N-glycans, were found to be upregulated in healing Gal-3(-/-) corneas. Microarray data were validated by qRT-PCR.

CONCLUSIONS

Based on the known functions of the differentially expressed glycogenes, it appears that the glycan structures on glycoproteins and glycolipids, synthesized as a result of the differential glycogene expression pattern in healing Gal-3(-/-) corneas may lead to the downregulation of specific counterreceptors for Gal-3. This may explain, at least in part, why, unlike healing Gal-3(+/+) corneas, the healing Gal-3(-/-) corneas are unresponsive to the stimulatory effect of exogenous Gal-3 on re-epithelialization of corneal wounds.

Novel fluorescent glycan microarray strategy reveals ligands for galectins

Galectin-1 (Gal-1) and galectin-3 (Gal-3) are widely expressed galectins with immunoregulatory functions in animals. To explore their glycan specificity, we developed microarrays of naturally occurring glycans using a bifunctional fluorescent linker, 2-amino-N-(2-aminoethyl)-benzamide (AEAB), directly conjugated through its arylamine group by reductive amination to free glycans to form glycan-AEABs (GAEABs). Glycans from natural sources were used to prepare over 200 GAEABs, which were purified by multidimensional high-pressure liquid chromatography and covalently immobilized onto N-hydroxysuccinimide-activated glass slides via their free alkylamine. Fluorescence-based screening demonstrated that Gal-1 recognizes a wide variety of complex N-glycans, whereas Gal-3 primarily recognizes poly-N-acetyllactosamine-containing glycans independent of N-glycan presentation. GAEABs provide a general solution to glycan microarray preparation from natural sources for defining the specificity of glycan-binding proteins.

Knockdown of GnT-Va expression inhibits ligand-induced downregulation of the epidermal growth factor receptor and intracellular signaling by inhibiting receptor endocytosis

Changes in the expression of N-glycan branching glycosyltransferases can alter cell surface receptor functions, involving their levels of cell surface retention, rates of internalization into the endosomal compartment, and subsequent intracellular signaling. To study in detail the regulation of signaling of the EGF receptor (EGFR) by GlcNAcbeta(1,6)Man branching, we utilized specific siRNA to selectively knockdown GnT-Va expression in the highly invasive human breast carcinoma line MDA-MB231, which resulted in the attenuation of its invasiveness-related phenotypes. Compared to control cells, ligand-induced downregulation of EGFR was significantly inhibited in GnT-Va-suppressed cells. This effect could be reversed by re-expression of GnT-Va, indicating that changes in ligand-induced receptor downregulation were dependent on GnT-Va activity. Knockdown of GnT-Va had no significant effect on c-Cbl mediated receptor ubiquitination and degradation, but did cause the inhibition of receptor internalization, showing that altered signaling and delayed ligand-induced downregulation of EGFR expression resulted from decreased EGFR endocytosis. Similar results were obtained with HT1080 fibrosarcoma cells treated with GnT-Va siRNA. Inhibited receptor internalization caused by the expression of GnT-Va siRNA appeared to be independent of galectin binding since decreased EGFR internalization in the knockdown cells was not affected by the treatment of the cells with lactose, a galectin inhibitor. Our results show that decreased GnT-Va activity due to siRNA expression in human carcinoma cells inhibits ligand-induced EGFR internalization, consequently resulting in delayed downstream signal transduction and inhibition of the EGF-induced, invasiveness-related phenotypes.