Ras oncogene induces β-galactoside α2,6-sialyltransferase (ST6Gal I) via a RalGEF-mediated signal to its housekeeping promoter

Aberrant Glycosylation in Pancreatic Ductal Adenocarcinoma 3D Organoids Is Mediated by KRAS Mutations

Aberrant glycosylation in tumor cells is a hallmark during carcinogenesis. KRAS gene mutations are the most well-known oncogenic abnormalities but their association with glycan alterations in pancreatic ductal adenocarcinoma (PDAC) is largely unknown. We employed patient-derived 3D organoids to culture pure live PDAC cells, excluding contamination by fibroblasts and immune cells, to gasp the comprehensive cancer cell surface glycan expression profile using lectin microarray and transcriptomic analyses. Surgical specimens from 24 PDAC patients were digested and embedded into a 3D culture system. Surface-bound glycans of 3D organoids were analyzed by high-density, 96-lectin microarrays. KRAS mutation status and expression of various glycosyltransferases were analyzed by RNA-seq. We successfully established 16 3D organoids: 14 PDAC, 1 intraductal papillary mucinous neoplasm (IPMN), and 1 normal pancreatic duct. KRAS was mutated in 13 (7 G12V, 5 G12D, 1 Q61L) and wild in 3 organoids (1 normal duct, 1 IPMN, 1 PDAC). Lectin reactivity of AAL (Aleuria aurantia) and AOL (Aspergillus oryzae) with binding activity to α1-3 fucose was higher in organoids with KRAS mutants than those with KRAS wild-type. FUT6 (α1-3fucosyltransferase 6) and FUT3 (α1-3/4 fucosyltransferase 3) expression was also higher in KRAS mutants than wild-type. Meanwhile, mannose-binding lectin (rRSL [Ralstonia solanacearum] and rBC2LA [Burkholderia cenocepacia]) signals were higher while those of galactose-binding lectins (rGal3C and rCGL2) were lower in the KRAS mutants. We demonstrated here that PDAC 3D-cultured organoids with KRAS mutations were dominantly covered in increased fucosylated glycans, pointing towards novel treatment targets and/or tumor markers.

Role of tumor cell sialylation in pancreatic cancer progression

Pancreatic ductal adenocarcinoma (PDAC) is one of the deadliest malignancies and is currently the third leading cause of cancer death. The aggressiveness of PDAC stems from late diagnosis, early metastasis, and poor efficacy of current chemotherapies. Thus, there is an urgent need for effective biomarkers for early detection of PDAC and development of new therapeutic strategies. It has long been known that cellular glycosylation is dysregulated in pancreatic cancer cells, however, tumor-associated glycans and their cognate glycosylating enzymes have received insufficient attention as potential clinical targets. Aberrant glycosylation affects a broad range of pathways that underpin tumor initiation, metastatic progression, and resistance to cancer treatment. One of the prevalent alterations in the cancer glycome is an enrichment in a select group of sialylated glycans including sialylated, branched N-glycans, sialyl Lewis antigens, and sialylated forms of truncated O-glycans such as the sialyl Tn antigen. These modifications affect the activity of numerous cell surface receptors, which collectively impart malignant characteristics typified by enhanced cell proliferation, migration, invasion and apoptosis-resistance. Additionally, sialic acids on tumor cells engage inhibitory Siglec receptors on immune cells to dampen anti-tumor immunity, further promoting cancer progression. The goal of this review is to summarize the predominant changes in sialylation occurring in pancreatic cancer, the biological functions of sialylated glycoproteins in cancer pathogenesis, and the emerging strategies for targeting sialoglycans and Siglec receptors in cancer therapeutics.

ST6Gal1: Oncogenic signaling pathways and targets

The Golgi-sialyltransferase ST6Gal1 (βgalactosidase α2,6 sialyltransferase 1), adds the negatively charged sugar, sialic acid, to the terminal galactose of N-glycosylated proteins. Upregulation of ST6Gal1 is observed in many malignancies, and a large body of research has determined that ST6Gal1-mediated α2,6 sialylation impacts cancer hallmarks. ST6Gal1 affects oncogenic behaviors including sustained proliferation, enhanced self-renewal, epithelial-to-mesenchymal transition, invasion, and chemoresistance. However, there are relatively few ST6GaL1 related signaling pathways that are well-established to mediate these biologies: greater delineation of specific targets and signaling mechanisms that are orchestrated by ST6Gal1 is needed. The aim of this review is to provide a summary of our current understanding of select oncogenic signaling pathways and targets affected by ST6Gal1.

Caveolin-1 knockout mice have altered serum N-glycan profile and sialyltransferase tissue expression

Caveolin-1 (Cav-1) is a constitutive protein within caveolar membranes. Previous studies from our group and others indicated that Cav-1 could mediate N-glycosylation, α2,6-sialylation, and fucosylation in mouse hepatocarcinoma cells in vitro. However, little is known about the effect of Cav-1 expression on glycosylation modifications in vivo. In this study, the N-glycan profiles in serum from Cav-1^-/- mice were investigated by lectin microarray and mass spectrometric analysis approaches. The results showed that levels of multi-antennary branched, α2,6-sialylated, and galactosylated N-glycans increased, while high-mannose typed and fucosylated N-glycans decreased in the serum of Cav-1^-/- mice, compared with that of wild-type mice. Furthermore, the real-time quantitative PCR analysis indicated that α2,6-sialyltransferase gene expression decreased significantly in Cav-1^-/- mouse organ tissues, but α2,3- and α2,8-sialyltransferase did not. Of them, both mRNA and protein expression levels of the β-galactoside α2,6-sialyltransferase 1 (ST6Gal-I) had dramatically reduced in Cav-1^-/- mice organ tissues, which was consistent with the α2,6-sialyl Gal/GalNAc level reduced significantly in tissues instead of serum from Cav-1^-/- mice. These results provide for the first time the N-glycans profile of Cav-1^-/- mice serum, which will facilitate understanding the function of Cav-1 from the perspective of glycosylation.

Aberrant Sialylation in Cancer: Biomarker and Potential Target for Therapeutic Intervention?

Simple Summary

Sialylation is a post-translational modification that consists in the addition of sialic acid to growing glycan chains on glycoproteins and glycolipids. Aberrant sialylation is an established hallmark of several types of cancer, including breast, ovarian, pancreatic, prostate, colorectal and lung cancers, melanoma and hepatocellular carcinoma. Hypersialylation can be the effect of increased activity of sialyltransferases and results in an excess of negatively charged sialic acid on the surface of cancer cells. Sialic acid accumulation contributes to tumor progression by several paths, including stimulation of tumor invasion and migration, and enhancing immune evasion and tumor cell survival. In this review we explore the mechanisms by which sialyltransferases promote cancer progression. In addition, we provide insights into the possible use of sialyltransferases as biomarkers for cancer and summarize findings on the development of sialyltransferase inhibitors as potential anti-cancer treatments.

Abstract

Sialylation is an integral part of cellular function, governing many biological processes including cellular recognition, adhesion, molecular trafficking, signal transduction and endocytosis. Sialylation is controlled by the levels and the activities of sialyltransferases on glycoproteins and lipids. Altered gene expression of these enzymes in cancer yields to cancer-specific alterations of glycoprotein sialylation. Mounting evidence indicate that hypersialylation is closely associated with cancer progression and metastatic spread, and can be of prognostic significance in human cancer. Aberrant sialylation is not only a result of cancer, but also a driver of malignant phenotype, directly impacting key processes such as tumor cell dissociation and invasion, cell-cell and cell-matrix interactions, angiogenesis, resistance to apoptosis, and evasion of immune destruction. In this review we provide insights on the impact of sialylation in tumor progression, and outline the possible application of sialyltransferases as cancer biomarkers. We also summarize the most promising findings on the development of sialyltransferase inhibitors as potential anti-cancer treatments.

Glycosyltransferase ST6Gal-I promotes the epithelial to mesenchymal transition in pancreatic cancer cells

ST6Gal-I, an enzyme upregulated in numerous malignancies, adds α2-6-linked sialic acids to select membrane receptors, thereby modulating receptor signaling and cell phenotype. In this study, we investigated ST6Gal-I's role in epithelial to mesenchymal transition (EMT) using the Suit2 pancreatic cancer cell line, which has low endogenous ST6Gal-I and limited metastatic potential, along with two metastatic Suit2-derived subclones, S2-013 and S2-LM7AA, which have upregulated ST6Gal-I. RNA-Seq results suggested that the metastatic subclones had greater activation of EMT-related gene networks than parental Suit2 cells, and forced overexpression of ST6Gal-I in the Suit2 line was sufficient to activate EMT pathways. Accordingly, we evaluated expression of EMT markers and cell invasiveness (a key phenotypic feature of EMT) in Suit2 cells with or without ST6Gal-I overexpression, as well as S2-013 and S2-LM7AA cells with or without ST6Gal-I knockdown. Cells with high ST6Gal-I expression displayed enrichment in mesenchymal markers (N-cadherin, slug, snail, fibronectin) and cell invasiveness, relative to ST6Gal-I-low cells. Contrarily, epithelial markers (E-cadherin, occludin) were suppressed in ST6Gal-I-high cells. To gain mechanistic insight into ST6Gal-I's role in EMT, we examined the activity of epidermal growth factor receptor (EGFR), a known EMT driver. ST6Gal-I-high cells had greater α2-6 sialylation and activation of EGFR than ST6Gal-I-low cells. The EGFR inhibitor, erlotinib, neutralized ST6Gal-I-dependent differences in EGFR activation, mesenchymal marker expression, and invasiveness in Suit2 and S2-LM7AA, but not S2-013, lines. Collectively, these results advance our understanding of ST6Gal-I's tumor-promoting function by highlighting a role for ST6Gal-I in EMT, which may be mediated, at least in part, by α2-6-sialylated EGFR.

Regulation of ST6GAL1 sialyltransferase expression in cancer cells

The ST6GAL1 sialyltransferase, which adds α2-6 linked sialic acids to N-glycosylated proteins, is overexpressed in a wide range of human malignancies. Recent studies have established the importance of ST6GAL1 in promoting tumor cell behaviors such as invasion, resistance to cell stress and chemoresistance. Furthermore, ST6GAL1 activity has been implicated in imparting cancer stem cell characteristics. However, despite the burgeoning interest in the role of ST6GAL1 in the phenotypic features of tumor cells, insufficient attention has been paid to the molecular mechanisms responsible for ST6GAL1 upregulation during neoplastic transformation. Evidence suggests that these mechanisms are multifactorial, encompassing genetic, epigenetic, transcriptional and posttranslational regulation. The purpose of this review is to summarize current knowledge regarding the molecular events that drive enriched ST6GAL1 expression in cancer cells.

Ginsenosides, potent inhibitors of sialyltransferase

The overexpression of sialic acids and sialyltransferases (STs) during malignant transformation and progression could result in the aberrant sialylation of cancer cells. Therefore, interfering the sialic acid synthesis might be an effective pathway in cancer therapy. In this study, we assessed that the antitumor inhibitors of 20(S)-ginsenosides Rg3, 20(R)-ginsenosides Rg3, 20(S)-ginsenosides Rh2, and 20(R)-ginsenosides Rh2 could block the sialoglycans in liver cancer cells HepG2. The results showed that these four compounds could inhibit the expressions of the total and free sialic acid at different levels in HepG2, respectively; also, it showed dose dependence. In addition, the results of the enzyme-linked immunosorbent assay showed that the above four compounds can inhibit the expression of STs significantly. We also found that these compounds could mediate the block of sialylation of α2,3- and α2,6-linked sialic acids in HepG2 cells by flow cytometry. Meanwhile, the results of the molecular docking investigation showed that these compounds showed strong interaction with ST6GalI and ST3GalI. These results verified that the ginsenosides have a powerful inhibiting aberrant sialylation, and it laid a theoretical foundation for further research on the investigation of ginsenosides as the target inhibitors on STs.

Impact of sialyltransferase ST6GAL1 overexpression on different colon cancer cell types

Cancer-associated glycan structures can be both tumor markers and engines of disease progression. The structure Siaα2,6Galβ1,4GlcNAc (Sia6LacNAc), synthesized by sialyltransferase ST6GAL1, is a cancer-associated glycan. Although ST6GAL1/Sia6LacNAc are often overexpressed in colorectal cancer (CRC), their biological and clinical significance remains unclear. To get insights into the clinical relevance of ST6GAL1 expression in CRC, we interrogated The Cancer Genome Atlas with mRNA expression data of hundreds of clinically characterized CRC and normal samples. We found an association of low ST6GAL1 expression with microsatellite instability (MSI), BRAF mutations and mucinous phenotype but not with stage, response to therapy and survival. To investigate the impact of ST6GAL1 expression in experimental systems, we analyzed the transcriptome and the phenotype of the CRC cell lines SW948 and SW48 after retroviral transduction with ST6GAL1 cDNA. The two cell lines display the two main pathways of CRC transformation: chromosomal instability and MSI, respectively. Constitutive ST6GAL1 expression induced much deeper transcriptomic changes in SW948 than in SW48 and affected different genes in the two cell lines. ST6GAL1 expression affected differentially the tyrosine phosphorylation induced by hepatocyte growth factor, the ability to grow in soft agar, to heal a scratch wound and to invade Matrigel in the two cell lines. These results indicate that the altered expression of a cancer-associated glycosyltransferase impacts the gene expression profile, as well as the phenotype, although in a cancer subtype-specific manner.

Insight in Adhesion Protein Sialylation and Microgravity Dependent Cell Adhesion-An Omics Network Approach

The adhesion behavior of human tissue cells changes in vitro, when gravity forces affecting these cells are modified. To understand the mechanisms underlying these changes, proteins involved in cell-cell or cell-extracellular matrix adhesion, their expression, accumulation, localization, and posttranslational modification (PTM) regarding changes during exposure to microgravity were investigated. As the sialylation of adhesion proteins is influencing cell adhesion on Earth in vitro and in vivo, we analyzed the sialylation of cell adhesion molecules detected by omics studies on cells, which change their adhesion behavior when exposed to microgravity. Using a knowledge graph created from experimental omics data and semantic searches across several reference databases, we studied the sialylation of adhesion proteins glycosylated at their extracellular domains with regards to its sensitivity to microgravity. This way, experimental omics data networked with the current knowledge about the binding of sialic acids to cell adhesion proteins, its regulation, and interactions in between those proteins provided insights into the mechanisms behind our experimental findings, suggesting that balancing the sialylation against the de-sialylation of the terminal ends of the adhesion proteins' glycans influences their binding activity. This sheds light on the transition from two- to three-dimensional growth observed in microgravity, mirroring cell migration and cancer metastasis in vivo.

Sox2 promotes expression of the ST6Gal-I glycosyltransferase in ovarian cancer cells

Background

The ST6Gal-I glycosyltransferase, which adds α2–6-linked sialic acids to N-glycosylated proteins is upregulated in a wide range of malignancies including ovarian cancer. Prior studies have shown that ST6Gal-I-mediated sialylation of select surface receptors remodels intracellular signaling to impart cancer stem cell (CSC) characteristics. However, the mechanisms that contribute to ST6Gal-I expression in stem-like cancer cells are poorly understood.

Results

Herein, we identify the master stem cell transcription factor, Sox2, as a novel regulator of ST6Gal-I expression. Interestingly, SOX2 and ST6GAL1 are located within the same tumor-associated amplicon, 3q26, and these two genes exhibit coordinate gains in copy number across multiple cancers including ~ 25% of ovarian serious adenocarcinomas. In conjunction with genetic co-amplification, our studies suggest that Sox2 directly binds the ST6GAL1 promoter to drive transcription. ST6Gal-I expression is directed by at least four distinct promoters, and we identified the P3 promoter as the predominant promoter utilized by ovarian cancer cells. Chromatin Immunoprecipitation (ChIP) assays revealed that Sox2 binds regions proximal to the P3 promoter. To confirm that Sox2 regulates ST6Gal-I expression, Sox2 was either overexpressed or knocked-down in various ovarian cancer cell lines. Sox2 overexpression induced an increase in ST6Gal-I mRNA and protein, as well as surface α2–6 sialylation, whereas Sox2 knock-down suppressed levels of ST6Gal-I mRNA, protein and surface α2–6 sialylation.

Conclusions

These data suggest a process whereby SOX2 and ST6GAL1 are coordinately amplified in cancer cells, with the Sox2 protein then binding the ST6GAL1 promoter to further augment ST6Gal-I expression. Our collective results provide new insight into mechanisms that upregulate ST6Gal-I expression in ovarian cancer cells, and also point to the possibility that some of the CSC characteristics commonly attributed to Sox2 may, in part, be mediated through the sialyltransferase activity of ST6Gal-I.

A complex between phosphatidylinositol 4-kinase IIα and integrin α3β1 is required for N-glycan sialylation in cancer cells

Aberrant N-glycan sialylation of glycoproteins is closely associated with malignant phenotypes of cancer cells and metastatic potential, which includes cell adhesion, migration, and growth. Recently, phosphatidylinositol 4-kinase IIα (PI4KIIα), which is localized to the trans-Golgi network, was identified as a regulator of Golgi phosphoprotein 3 (GOLPH3) and of vesicle transport in the Golgi apparatus. GOLPH3 is a target of PI4KIIα and helps anchor sialyltransferases and thereby regulates sialylation of cell surface receptors. However, how PI4KIIα-mediated sialyation of cell surface proteins is regulated remains unclear. In this study, using several cell lines, CRISPR/Cas9-based gene knockout and short hairpin RNA-mediated silencing, RT-PCR, lentivirus-mediated overexpression, and immunoblotting methods, we confirmed that PI4KIIα knockdown suppresses the sialylation of N-glycans on the cell surface, in Akt phosphorylation and activation, and integrin α3-mediated cell migration of MDA-MB-231 breast cancer cells. Interestingly, both integrin α3β1 and PI4KIIα co-localized to the trans-Golgi network, where they physically interacted with each other, and PI4KIIα specifically associated with integrin α3 but not α5. Furthermore, overexpression of both integrin α3β1 and PI4KIIα induced hypersialylation. Conversely, integrin α3 knockout significantly inhibited the sialylation of membrane proteins, such as the epidermal growth factor receptor, as well as in total cell lysates. These findings suggest that the malignant phenotype of cancer cells is affected by a sialylation mechanism that is regulated by a complex between PI4KIIα and integrin α3β1.

ST6Gal-I modulates docetaxel sensitivity in human hepatocarcinoma cells via the p38 MAPK/caspase pathway

The β-galactoside α2-6-sialyltransferase 1 (ST6Gal-I) is the principal sialyltransferase responsible for the addition of α2-6-sialic acid to the termini N-glycans on cell surface. Although ST6Gal-I in cancer cell resistance to chemotherapeutics agents has been previously reported, the role of ST6Gal-I in clinical drug resistance of hepatocellular carcinoma (HCC) is not fully understood. In this study, we found that knockdown of ST6Gal-I increased the sensitivity of hepatocarcinoma MHCC97-H cells to docetaxel treatment by instigating the process of apoptosis. Silencing ST6Gal-I expression decreased the survival rate of MHCC97-H cells after docetaxel treatment. Importantly, ST6Gal-I silencing resulted in an increasing of phospho-p38, Bax, Bad, cytochrome c and the cleaved caspase-9, 3 and PARP, while a decreasing of the anti-apoptotic protein Bcl-2. In addition, we found that p38 MAPK and caspase-3 inhibitors can reduce the enhanced apoptosis levels of MHCC97-H cells resulted by either ST6Gal-I silencing or docetaxel treatment. Conversely, exogenous expression of ST6Gal-I in hepatocarcinoma Huh7 cells inhibited apoptotic cell death and prevented docetaxel-induced apoptosis by inhibiting p38 MAPK mediated mitochondrial-dependent pathway. Taken together, these results indicate that ST6Gal-I might play a positive role in mediating the survival of human hepatocarcinoma cells and could be a potential target for gene and antitumor drugs therapy.

The role of N-glycans in colorectal cancer progression: potential biomarkers and therapeutic applications

Changes in glycosylation, which is one of the most common protein post-translational modifications, are considered to be a hallmark of cancer. N-glycans can modulate cell migration, cell-cell adhesion, cell signaling, growth and metastasis. The colorectal cancer (CRC) is a leading cause of cancer-related mortality and the correlation between CRC progression and changes in the pattern of expression of N-glycans is being considered in the search for new biomarkers. Here, we review the role of N-glycans in CRC cell biology. The perspectives on emerging N-glycan-related anticancer therapies, along with new insights and challenges, are also discussed.

Selectin Ligands Sialyl-Lewis a and Sialyl-Lewis x in Gastrointestinal Cancers

The tetrasaccharide structures Siaα2,3Galβ1,3(Fucα1,4)GlcNAc and Siaα2,3Galβ1,4(Fucα1,3)GlcNAc constitute the epitopes of the carbohydrate antigens sialyl-Lewis a (sLe^a) and sialyl-Lewis x (sLe^x), respectively, and are the minimal requirement for selectin binding to their counter-receptors. Interaction of sLe^x expressed on the cell surface of leucocytes with E-selectin on endothelial cells allows their arrest and promotes their extravasation. Similarly, the rolling of cancer cells ectopically expressing the selectin ligands on endothelial cells is potentially a crucial step favoring the metastatic process. In this review, we focus on the biosynthetic steps giving rise to selectin ligand expression in cell lines and native tissues of gastrointestinal origin, trying to understand whether and how they are deregulated in cancer. We also discuss the use of such molecules in the diagnosis of gastrointestinal cancers, particularly in light of recent data questioning the ability of colon cancers to express sLe^a and the possible use of circulating sLe^x in the early detection of pancreatic cancer. Finally, we reviewed the data dealing with the mechanisms that link selectin ligand expression in gastrointestinal cells to cancer malignancy. This promising research field seems to require additional data on native patient tissues to reach more definitive conclusions.

The Glycosyltransferase ST6Gal-I Protects Tumor Cells against Serum Growth Factor Withdrawal by Enhancing Survival Signaling and Proliferative Potential

A hallmark of cancer cells is the ability to survive and proliferate when challenged with stressors such as growth factor insufficiency. In this study, we report a novel glycosylation-dependent mechanism that protects tumor cells from serum growth factor withdrawal. Our results suggest that the β-galactoside α-2,6-sialyltransferase 1 (ST6Gal-I) sialyltransferase, which is up-regulated in numerous cancers, promotes the survival of serum-starved cells. Using ovarian and pancreatic cancer cell models with ST6Gal-I overexpression or knockdown, we find that serum-starved cells with high ST6Gal-I levels exhibit increased activation of prosurvival signaling molecules, including pAkt, p-p70S6K, and pNFκB. Correspondingly, ST6Gal-I activity augments the expression of tumor-promoting pNFκB transcriptional targets such as IL-6, IL-8, and the apoptosis inhibitor cIAP2. ST6Gal-I also potentiates expression of the cell cycle regulator cyclin D2, leading to increased phosphorylation and inactivation of the cell cycle inhibitor pRb. Consistent with these results, serum-starved cells with high ST6Gal-I expression maintain a greater number of S phase cells compared with low ST6Gal-I expressors, reflecting enhanced proliferation. Finally, selective enrichment in clonal variants with high ST6Gal-I expression is observed upon prolonged serum deprivation, supporting the concept that ST6Gal-I confers a survival advantage. Collectively, these results implicate a functional role for ST6Gal-I in fostering tumor cell survival within the serum-depleted tumor microenvironment.

The Tumor-Associated Glycosyltransferase ST6Gal-I Regulates Stem Cell Transcription Factors and Confers a Cancer Stem Cell Phenotype

The glycosyltransferase ST6Gal-I, which adds α2-6-linked sialic acids to substrate glycoproteins, has been implicated in carcinogenesis; however, the nature of its pathogenic role remains poorly understood. Here we show that ST6Gal-I is upregulated in ovarian and pancreatic carcinomas, enriched in metastatic tumors, and associated with reduced patient survival. Notably, ST6Gal-I upregulation in cancer cells conferred hallmark cancer stem-like cell (CSC) characteristics. Modulating ST6Gal-I expression in pancreatic and ovarian cancer cells directly altered CSC spheroid growth, and clonal variants with high ST6Gal-I activity preferentially survived in CSC culture. Primary ovarian cancer cells from patient ascites or solid tumors sorted for α2-6 sialylation grew as spheroids, while cells lacking α2-6 sialylation remained as single cells and lost viability. ST6Gal-I also promoted resistance to gemcitabine and enabled the formation of stably resistant colonies. Gemcitabine treatment of patient-derived xenograft tumors enriched for ST6Gal-I-expressing cells relative to pair-matched untreated tumors. ST6Gal-I also augmented tumor-initiating potential. In limiting dilution assays, subcutaneous tumor formation was inhibited by ST6Gal-I knockdown, whereas in a chemically induced tumor initiation model, mice with conditional ST6Gal-I overexpression exhibited enhanced tumorigenesis. Finally, we found that ST6Gal-I induced expression of the key tumor-promoting transcription factors, Sox9 and Slug. Collectively, this work highlighted a previously unrecognized role for a specific glycosyltransferase in driving a CSC state. Cancer Res; 76(13); 3978-88. ©2016 AACR.

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.

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.

A guide into glycosciences: How chemistry, biochemistry and biology cooperate to crack the sugar code

BACKGROUND

The most demanding challenge in research on molecular aspects within the flow of biological information is posed by the complex carbohydrates (glycan part of cellular glycoconjugates). How the 'message' encoded in carbohydrate 'letters' is 'read' and 'translated' can only be unraveled by interdisciplinary efforts.

SCOPE OF REVIEW

This review provides a didactic step-by-step survey of the concept of the sugar code and the way strategic combination of experimental approaches characterizes structure-function relationships, with resources for teaching.

MAJOR CONCLUSIONS

The unsurpassed coding capacity of glycans is an ideal platform for generating a broad range of molecular 'messages'. Structural and functional analyses of complex carbohydrates have been made possible by advances in chemical synthesis, rendering production of oligosaccharides, glycoclusters and neoglycoconjugates possible. This availability facilitates to test the glycans as ligands for natural sugar receptors (lectins). Their interaction is a means to turn sugar-encoded information into cellular effects. Glycan/lectin structures and their spatial modes of presentation underlie the exquisite specificity of the endogenous lectins in counterreceptor selection, that is, to home in on certain cellular glycoproteins or glycolipids.

GENERAL SIGNIFICANCE

Understanding how sugar-encoded 'messages' are 'read' and 'translated' by lectins provides insights into fundamental mechanisms of life, with potential for medical applications.

Reversal effect of ST6GAL 1 on multidrug resistance in human leukemia by regulating the PI3K/Akt pathway and the expression of P-gp and MRP1

β-galactoside α2, 6-sialyltransferse gene (ST6GAL) family has two members, which encode corresponding enzymes ST6Gal I and ST6Gal II. The present atudy was to investigate whether and how ST6GAL family involved in multidrug resistance (MDR) in human leukemia cell lines and bone marrow mononuclear cells (BMMC) of leukemia patients. Real-time PCR showed a high expression level of ST6GAL1 gene in both MDR cells and BMMCs (*P<0.05). Alternation of ST6GAL1 levels had a significant impact on drug-resistant phenotype changing of K562 and K562/ADR cells both in vitro and in vivo. However, no significant changes were observed of ST6GAL2 gene. Further data revealed that manipulation of ST6GAL1 modulated the activity of phosphoinositide 3 kinase (PI3K)/Akt signaling and consequently regulated the expression of P-glycoprotein (P-gp, *P<0.05) and multidrug resistance related protein 1 (MRP1, *P<0.05), which are both known to be associated with MDR. Therefore we postulate that ST6GAL1 is responsible for the development of MDR in human leukemia cells probably through medicating the activity of PI3K/Akt signaling and the expression of P-gp and MRP1.

Increasing the α 2, 6 sialylation of glycoproteins may contribute to metastatic spread and therapeutic resistance in colorectal cancer

Abnormal glycosylation due to dysregulated glycosyltransferases and glycosidases is a key phenomenon of many malignancies, including colorectal cancer (CRC). In particular, increased ST6 Gal I (β-galactoside α 2, 6 sialyltransferase) and subsequently elevated levels of cell-surface α 2, 6-linked sialic acids have been associated with metastasis and therapeutic failure in CRC. As many CRC patients experience metastasis to the liver or lung and fail to respond to curative therapies, intensive research efforts have sought to identify the molecular changes underlying CRC metastasis. ST6 Gal I has been shown to facilitate CRC metastasis, and we believe that additional investigations into the involvement of ST6 Gal I in CRC could facilitate the development of new diagnostic and therapeutic targets. This review summarizes how ST6 Gal I has been implicated in the altered expression of sialylated glycoproteins, which have been linked to CRC metastasis, radioresistance, and chemoresistance.

Targeting aberrant sialylation in cancer cells using a fluorinated sialic acid analog impairs adhesion, migration, and in vivo tumor growth

Cancer cells decorate their surface with a dense layer of sialylated glycans by upregulating the expression of sialyltransferases and other glycogenes. Although sialic acids play a vital role in many biologic processes, hypersialylation in particular has been shown to contribute to cancer cell progression and metastasis. Accordingly, selective strategies to interfere with sialic acid synthesis might offer a powerful approach in cancer therapy. In the present study, we assessed the potential of a recently developed fluorinated sialic acid analogue (P-3F(ax)-Neu5Ac) to block the synthesis of sialoglycans in murine melanoma cells and the consequences on cell adhesion, migration, and in vivo growth. The results showed that P-3F(ax)-Neu5Ac readily caused depletion of α2,3-/α2,6-linked sialic acids in B16F10 cells for several days. Long-term inhibition of sialylation for 28 days was feasible without affecting cell viability or proliferation. Moreover, P-3F(ax)-Neu5Ac proved to be a highly potent inhibitor of sialylation even at high concentrations of competing sialyltransferase substrates. P-3F(ax)-Neu5Ac-treated cancer cells exhibited impaired binding to poly-l-lysine, type I collagen, and fibronectin and diminished migratory capacity. Finally, blocking sialylation of B16F10 tumor cells with this novel sialic acid analogue reduced their growth in vivo. These results indicate that P-3F(ax)-Neu5Ac is a powerful glycomimetic capable of inhibiting aberrant sialylation that can potentially be used for anticancer therapy.

Regulation of the metastatic cell phenotype by sialylated glycans

Tumor cells exhibit striking changes in cell surface glycosylation as a consequence of dysregulated glycosyltransferases and glycosidases. In particular, an increase in the expression of certain sialylated glycans is a prominent feature of many transformed cells. Altered sialylation has long been associated with metastatic cell behaviors including invasion and enhanced cell survival; however, there is limited information regarding the molecular details of how distinct sialylated structures or sialylated carrier proteins regulate cell signaling to control responses such as adhesion/migration or resistance to specific apoptotic pathways. The goal of this review is to highlight selected examples of sialylated glycans for which there is some knowledge of molecular mechanisms linking aberrant sialylation to critical processes involved in metastasis.

ST6Gal-I sialyltransferase confers cisplatin resistance in ovarian tumor cells

Background

Platinum drugs, including cisplatin, are a frontline therapeutic in ovarian cancer treatment and acquired resistance to these agents is a major contributor to ovarian cancer morbidity and mortality. In this study a novel glycosylation-dependent mechanism for cisplatin resistance is described. Specifically, cisplatin-induced cell death is blocked by the activity of the ST6Gal-I sialyltransferase. ST6Gal-I modifies specific receptors by adding a negatively charged sialic acid sugar which influences diverse receptor functions. Overexpression of ST6Gal-I is a hallmark of ovarian and other cancers and its expression has been correlated to metastasis and poor prognosis.

Methods

Tumor cell viability and apoptotic induction were determined in cell lines with ST6Gal-I overexpression and knockdown. In addition, cell populations with acquired resistance to cisplatin were assayed for endogenous ST6Gal-I expression.

Results

We show that forced expression of ST6Gal-I in OV4 ovarian cancer cells that lack endogenous ST6Gal-I causes reduced activation of caspase 3 and increased cell viability following cisplatin treatment. Conversely, forced ST6Gal-I knockdown in Pa-1 cells with high endogenous ST6Gal-I increases cisplatin-induced caspase activation and cell death. A2780 ovarian cancer cells selected for stable cisplatin resistance display upregulated endogenous ST6Gal-I when compared with parental, cisplatin-sensitive, A2780 cells. Similarly, extended low dose cisplatin treatment of a Pa-1 polyclonal ST6Gal-I shRNA knockdown population led to selection for subclones with elevated ST6Gal-I expression.

Conclusions

Receptor sialylation by ST6Gal-I confers a survival advantage for tumor cells in the presence of cisplatin. These collective findings support a role for ST6Gal-I in chemoresistance and highlight ST6Gal-I as a potential therapeutic target for platinum resistant tumors.

Caveolin-1 up-regulates integrin α2,6-sialylation to promote integrin α5β1-dependent hepatocarcinoma cell adhesion

The alterations of integrin glycosylation play a crucial role in tumor metastasis. Our previous studies indicated that caveolin-1 promoted the expression of the key α2,6-sialytransferase ST6Gal-I and fibronectin-mediated adhesion of mouse hepatocarcinoma cell. Herein, we investigated the role of α2,6-sialylated α5-integrin in the adhesion of mouse hepatocarcinoma H22 cell. We demonstrated that caveolin-1 up-regulated cell surface α2,6-linked sialic acid via stimulating ST6Gal-I transcription. Cell surface α2,6-sialylation was required for integrin α5β1-dependent cell adhesion to fibronectin, and an increase in α2,6-linked sialic acid on α5-subunit facilitated fibronectin-mediated focal adhesion kinase phosphorylations, suggesting that α2,6-sialylated α5-subunit promoted integrin α5β1-dependent cell adhesion.

Sialylation of the Fas death receptor by ST6Gal-I provides protection against Fas-mediated apoptosis in colon carcinoma cells

The glycosyltransferase, ST6Gal-I, adds sialic acid in an α2-6 linkage to the N-glycans of membrane and secreted glycoproteins. Up-regulation of ST6Gal-I occurs in many cancers, including colon carcinoma, and correlates with metastasis and poor prognosis. However, mechanisms by which ST6Gal-I facilitates tumor progression remain poorly understood due to limited knowledge of enzyme substrates. Herein we identify the death receptor, Fas (CD95), as an ST6Gal-I substrate, and show that α2-6 sialylation of Fas confers protection against Fas-mediated apoptosis. Intriguingly, differences in ST6Gal-I activity do not affect the function of DR4 or DR5 death receptors upon treatment with TRAIL, implicating a selective effect of ST6Gal-I on the Fas receptor. Using ST6Gal-I knockdown and forced overexpression colon carcinoma cell models, we find that α2-6 sialylation of Fas prevents apoptosis stimulated by FasL as well as the Fas-activating antibody, CH11, as evidenced by decreased activation of caspases 8 and 3. We also show that α2-6 sialylation of Fas does not alter the binding of CH11, but rather inhibits the capacity of Fas to induce apoptosis by blocking the association of FADD with Fas cytoplasmic tails, an event that initiates death-inducing signaling complex formation. Furthermore, α2-6 sialylation of Fas inhibits Fas internalization, which is required for apoptotic signaling. Although dysregulated Fas activity is a well known mechanism through which tumors evade apoptosis, the current study is the first to link Fas insensitivity to the actions of a specific sialyltransferase. This finding establishes a new paradigm by which death receptor function is impaired for the self-protection of tumors against apoptosis.

Emerging role of alpha2,6-sialic acid as a negative regulator of galectin binding and function

Galectins are β-galactoside-binding lectins that regulate diverse cell behaviors, including adhesion, migration, proliferation, and apoptosis. Galectins can be expressed both intracellularly and extracellularly, and extracellular galectins mediate their effects by associating with cell-surface oligosaccharides. Despite intensive current interest in galectins, strikingly few studies have focused on a key enzyme that acts to inhibit galectin signaling, namely β-galactoside α2,6-sialyltransferase (ST6Gal-I). ST6Gal-I adds an α2,6-linked sialic acid to the terminal galactose of N-linked glycans, and this modification blocks galectin binding to β-galactosides. This minireview summarizes the evidence suggesting that ST6Gal-I activity serves as an "off switch" for galectin function.

Synthesis and expression of CDw75 antigen in human colorectal cancer

Background

Increased ST6Gal I activity has been associated with the α(2,6)sialylation enhancement of membrane glycoconjugates observed in metastatic colorectal carcinomas (CRC). Siaα(2,6)Galβ(1,4)GlcNAc sequence, known as CDw75, is a sialylated carbohydrate determinant generated by the ST6Gal I. This epitope has been reported to be associated with the progression of gastric and colorectal tumours, hence there are only a few conclusive studies to date.

Methods

By radioisotopic techniques we evaluated the ST6Gal I activity in healthy, transitional and tumour tissues from 43 patients with CRC. By immunohistochemistry we assessed the CDw75 expression in 25 colorectal adenomas, 43 tumours, 13 transitional and 28 healthy tissues of CRC patients.

Results

ST6Gal I activity was likewise found to be statistically higher in tumour tissue respect to healthy tissue from CRC patients. CDw75 expression was positive in 20% of colorectal adenomas. Furthermore, 70% of tumour specimens and 8.3% of transitional specimens were positive for CDw75 expression, whereas none of the healthy ones showed the presence of the epitope.

Conclusion

The major contribution of this study is the inclusion of data from transitional tissue and the analysis of CDw75 antigen expression in CRC and in colorectal adenomas, little known so far. ST6Gal I activity and CDw75 antigen expression were increased in CRC. Although their comparison did not reach the statistical significance, a great extent of patients showed both, an enhanced tumour ST6Gal I activity and an increased CDw75 expression in the tumour tissue. So, these two variables may play a role in malignant transformation. The expression of CDw75 in colorectal adenomas suggests that this antigen may be a tumour marker in CRC.

Compensation of loss of protein function in microsatellite-unstable colon cancer cells (HCT116): a gene-dependent effect on the cell surface glycan profile

Tumors that display a high level of microsatellite instability (MSI-H) accumulate somatic frameshift mutations in several genes. The compensation of this loss of function by transfection represents a suitable approach to tie respective gene deficiency to alterations in cellular characteristics. In view of the emerging significance of cell surface glycans as biochemical signals for presentation/activity of various receptors/integrins and for susceptibility to adhesion/growth-regulatory tissue lectins, we examined the glycophenotype in the MSI-H colon cancer cell line HCT116 for activin type 2 receptor (ACVR2), absent in melanoma 2 (AIM2), and transforming growth factor beta-type 2 receptor (TGFBR2) known to be associated with MSI colorectal carcinogenesis. A panel of probes specific for functional carbohydrate epitopes including human lectins was used to trace changes in cell surface levels, thereby initiating glycan analysis related to MSI. In particular, the presence of core substitutions and branching in N-glycans, the sialylation status of N- and O-glycans, and the presence of Le(a/x)-epitopes were profiled. Transient transfection affected the glycophenotype, depending on the nature of the gene and the probe. The TGFBR2 presence reduced binding of probes specific for a core substitution and increased branch length in N-glycosylation, even reaching a P-value of 0.0016. ACVR2/AIM2 influenced core 1 mucin-type O-glycosylation differentially, upregulation by ACVR2, and downregulation by AIM2. These alterations of cell surface glycosylation by gene products that are not directly associated with the machinery for glycan generation direct attention to pursue analysis of glycosylation in MSI tumor cells on the level of target glycoproteins and open the way for functional studies.

The p38 pathway inhibitor SB202190 activates MEK/MAPK to stimulate the growth of leukemia cells

In this study, the biological effects of signal transduction inhibitors on leukemia cells were examined. We found that the p38 inhibitor SB202190 enhanced the growth of THP-1 and MV4-11 cells. To determine the pathway affected by SB202190, we examined the 50% effective dose (ED(50)) values for THP-1 cell growth in combination with several inhibitors. In the presence of SB202190, the ED(50) values for the farnesyltransferase inhibitor FPT inhibitor II and MEK inhibitor U0126 were significantly decreased. Western blot analysis revealed that SB202190 increased the phosphorylation of C-Raf and extracellular regulated kinase (ERK), suggesting that Ras-Raf-MEK-mitogen-activated protein kinase (MAPK) pathway activation is involved in the leukemia cell growth induced by SB202190.

ST6Gal-I expression in ovarian cancer cells promotes an invasive phenotype by altering integrin glycosylation and function

Background

Ovarian adenocarcinoma is not generally discovered in patients until there has been widespread intraperitoneal dissemination, which is why ovarian cancer is the deadliest gynecologic malignancy. Though incompletely understood, the mechanism of peritoneal metastasis relies on primary tumor cells being able to detach themselves from the tumor, escape normal apoptotic pathways while free floating, and adhere to, and eventually invade through, the peritoneal surface. Our laboratory has previously shown that the Golgi glycosyltransferase, ST6Gal-I, mediates the hypersialylation of β₁ integrins in colon adenocarcinoma, which leads to a more metastatic tumor cell phenotype. Interestingly, ST6Gal-I mRNA is known to be upregulated in metastatic ovarian cancer, therefore the goal of the present study was to determine whether ST6Gal-I confers a similarly aggressive phenotype to ovarian tumor cells.

Methods

Three ovarian carcinoma cell lines were screened for ST6Gal-I expression, and two of these, PA-1 and SKOV3, were found to produce ST6Gal-I protein. The third cell line, OV4, lacked endogenous ST6Gal-I. In order to understand the effects of ST6Gal-I on cell behavior, OV4 cells were stably-transduced with ST6Gal-I using a lentiviral vector, and integrin-mediated responses were compared in parental and ST6Gal-I-expressing cells.

Results

Forced expression of ST6Gal-I in OV4 cells, resulting in sialylation of β1 integrins, induced greater cell adhesion to, and migration toward, collagen I. Similarly, ST6Gal-I expressing cells were more invasive through Matrigel.

Conclusion

ST6Gal-I mediated sialylation of β1 integrins in ovarian cancer cells may contribute to peritoneal metastasis by altering tumor cell adhesion and migration through extracellular matrix.

Sialylation of beta1 integrins blocks cell adhesion to galectin-3 and protects cells against galectin-3-induced apoptosis

In previous studies, we determined that beta1 integrins from human colon tumors have elevated levels of alpha2-6 sialylation, a modification added by beta-galactosamide alpha-2,6-sialyltranferase I (ST6Gal-I). Intriguingly, the beta1 integrin is thought to be a ligand for galectin-3 (gal-3), a tumor-associated lectin. The effects of gal-3 are complex; intracellular forms typically protect cells against apoptosis through carbohydrate-independent mechanisms, whereas secreted forms bind to cell surface oligosaccharides and induce apoptosis. In the current study, we tested whether alpha2-6 sialylation of the beta1 integrin modulates binding to extracellular gal-3. Herein we report that SW48 colonocytes lacking alpha2-6 sialylation exhibit beta1 integrin-dependent binding to gal-3-coated tissue culture plates; however, binding is attenuated upon forced expression of ST6Gal-I. Removal of alpha2-6 sialic acids from ST6Gal-I expressors by neuraminidase treatment restores gal-3 binding. Additionally, using a blot overlay approach, we determined that gal-3 binds directly and preferentially to unsialylated, as compared with alpha2-6-sialylated, beta1 integrins. To understand the physiologic consequences of gal-3 binding, cells were treated with gal-3 and monitored for apoptosis. Galectin-3 was found to induce apoptosis in parental SW48 colonocytes (unsialylated), whereas ST6Gal-I expressors were protected. Importantly, gal-3-induced apoptosis was inhibited by function blocking antibodies against the beta1 subunit, suggesting that beta1 integrins are critical transducers of gal-3-mediated effects on cell survival. Collectively, our results suggest that the coordinate up-regulation of gal-3 and ST6Gal-I, a feature that is characteristic of colon carcinoma, may confer tumor cells with a selective advantage by providing a mechanism for blockade of the pro-apoptotic effects of secreted gal-3.

Tumor cell migration and invasion are regulated by expression of variant integrin glycoforms

The ST6Gal-I glycosyltransferase, which adds alpha2-6-linked sialic acids to glycoproteins, is overexpressed in colon adenocarcinoma, and enzyme activity is correlated with tumor cell invasiveness. Previously we reported that forced expression of oncogenic ras in HD3 colonocytes causes upregulation of ST6Gal-I, leading to increased alpha2-6 sialylation of beta1 integrins. To determine whether ras-induced sialylation is involved in promoting the tumor cell phenotype, we used shRNA to downregulate ST6Gal-I in ras-expressors, and then monitored integrin-dependent responses. Here we show that forced ST6Gal-I downregulation, leading to diminished alpha2-6 sialylation of integrins, inhibits cell adhesion to collagen I, a beta1 ligand. Correspondingly, collagen binding is reduced by enzymatic removal of cell surface sialic acids from ras-expressors with high ST6Gal-I levels (i.e., no shRNA). Cells with forced ST6Gal-I downregulation also exhibit decreased migration on collagen I and diminished invasion through Matrigel. Importantly, GD25 cells, which lack beta1 integrins (and ST6Gal-I), do not demonstrate differential invasiveness when forced to express ST6Gal-I, suggesting that the effects of variant sialylation are mediated specifically by beta1 integrins. The observation that cell migration and invasion can be blocked in oncogenic ras-expressing cells by forcing ST6Gal-I downregulation implicates differential sialylation as an important ras effector, and also suggests that ST6Gal-I is a promising therapeutic target.

Tumor suppressor p16INK4a--modulator of glycomic profile and galectin-1 expression to increase susceptibility to carbohydrate-dependent induction of anoikis in pancreatic carcinoma cells

Expression of the tumor suppressor p16(INK4a) after stable transfection can restore the susceptibility of epithelial tumor cells to anoikis. This property is linked to increases in the expression and cell-surface presence of the fibronectin receptor. Considering its glycan chains as pivotal signals, we assumed an effect of p16(INK4a) on glycosylation. To test this hypothesis for human Capan-1 pancreatic carcinoma cells, we combined microarray for selected glycosyltransferase genes with 2D chromatographic glycan profiling and plant lectin binding. Major differences between p16-positive and control cells were detected. They concerned expression of beta1,4-galactosyltransferases (down-regulation of beta1,4-galactosyltransferases-I/V and up-regulation of beta1,4-galactosyltransferase-IV) as well as decreased alpha2,3-sialylation of O-glycans and alpha2,6-sialylation of N-glycans. The changes are compatible with increased beta(1)-integrin maturation, subunit assembly and binding activity of the alpha(5)beta(1)-integrin. Of further functional relevance in line with our hypothesis, we revealed differential reactivity towards endogenous lectins, especially galectin-1. As a result of reduced sialylation, the cells' capacity to bind galectin-1 was enhanced. In parallel, the level of transcription of the galectin-1 gene increased conspicuously in p16(INK4a)-positive cells, and even figured prominently in a microarray on 1996 tumor-associated genes and in proteomic analysis. The cells therefore gain optimal responsiveness. The correlation between genetically modulated galectin-1 levels and anoikis rates in engineered transfectants inferred functional significance. To connect these findings to the fibronectin receptor, galectin-1 was shown to be co-immunoprecipitated. We conclude that p16(INK4a) orchestrates distinct aspects of glycosylation that are relevant for integrin maturation and reactivity to an endogenous effector as well as the effector's expression. This mechanism establishes a new aspect of p16(INK4a) functionality.

Preferential reduction of the α-2-6-sialylation from cell surface N-glycans of human diploid fibroblastic cells by in vitro aging

Human diploid fibroblastic cell line, TIG-3, has a finite life span of about 80 population doubling levels (PDL), and is used for in vitro aging studies. Young cells (PDL 23) grew to higher cell densities at a higher growth rate than aged cells (PDL 77). When the electrophoretic mobility of cells was determined, the negative surface charge of the aged cells decreased significantly when compared to that of young cells. Lectin blot analysis of membrane glycoproteins showed that the alpha-2-6-sialylation but not the alpha-2-3-sialylation of N-glycans decreases markedly in the aged cells when compared to the young cells. In support of this observation, the cDNA microarray assay and reverse transcription-polymerase chain reaction (RT-PCR) analysis showed that the gene expression of the alpha-2,6-sialyltransferase I (ST6Gal I), which transfers sialic acid to galactose residues of N-glycans, decreases in the aged cells. These results indicate that the concordant decrease of the alpha-2,6-sialylation of N-glycans with the ST6Gal I gene expression is induced in TIG-3 cells by in vitro aging.

A Protein Kinase C/Ras/ERK Signaling Pathway Activates Myeloid Fibronectin Receptors by Altering β1 Integrin Sialylation

Here we report that myeloid cells differentiating along the monocyte/macrophage lineage down-regulate the ST6Gal-I sialyltransferase via a protein kinase C/Ras/ERK signaling cascade. In consequence, the beta1 integrin subunit becomes hyposialylated, which stimulates the ligand binding activity of alpha5beta1 fibronectin receptors. Pharmacologic inhibitors of protein kinase C, Ras, and MEK, but not phosphoinositide 3-kinase, block ST6Gal-I down-regulation, integrin hyposialylation, and fibronectin binding. In contrast, constitutively active MEK stimulates these same events, indicating that ERK is both a necessary and sufficient activator of hyposialylation-dependent integrin activation. Consistent with the enhanced activity of hyposialylated cell surface integrins, purified alpha5beta1 receptors bind fibronectin more strongly upon enzymatic desialylation, an effect completely reversed by resialylation of these integrins with recombinant ST6Gal-I. Finally, we have mapped the N-glycosylation sites on the beta1 integrin to better understand the potential effects of differential sialylation on integrin structure/function. Notably, there are three N-glycosylated sites within the beta1 I-like domain, a region that plays a crucial role in ligand binding. Our collective results suggest that variant sialylation, induced by a specific signaling cascade, mediates the sustained increase in cell adhesiveness associated with monocytic differentiation.

Phenotypic changes induced by expression of beta-galactoside alpha2,6 sialyltransferase I in the human colon cancer cell line SW948

Beta-galactoside alpha2,6 sialyltransferase (ST6Gal.I), the enzyme which adds sialic acid in alpha2,6-linkage on lactosaminic termini of glycoproteins, is frequently overexpressed in cancer, but its relationship with malignancy remains unclear. In this study, we have investigated the phenotypic changes induced by the expression of alpha2,6-sialylated lactosaminic chains in the human colon cancer cell line SW948 which was originally devoid of ST6Gal.I. Clones derived from transfection with the ST6Gal.I cDNA were compared with untransfected cells and mock transfectants. The ST6Gal.I-expressing clones show (1) increased adherence to fibronectin and collagen IV but not to hyaluronic acid. Treatment with Clostridium perfrigens neuraminidase reduces the binding to fibronectin and collagen IV of ST6Gal.I-expressing cells but not that of ST6Gal.I-negative cells; (2) accumulation and more focal distribution of beta1 integrins on the cell surface; (3) different distribution of actin fibers; (4) flatter morphology and reduced tendency to multilayer growth; (5) improved ability to heal a scratch wound; (6) reduced ability to grow at the subcutaneous site of injection in nude mice. Our data suggest that the presence of alpha2,6-linked sialic acid on membrane glycoconjugates increases the binding to extracellular matrix components, resulting in a membrane stabilization of beta1 integrins, further strengthening the binding. This mechanism can provide a basis for the flatter morphology and the reduced tendency to multilayer growth, resulting in a more ordered tissue organization. These data indicate that in the cell line SW948, the effect of ST6Gal.I expression is consistent with the attenuation of the neoplastic phenotype.

Hypersialylation of beta1 integrins, observed in colon adenocarcinoma, may contribute to cancer progression by up-regulating cell motility

Colon adenocarcinomas are known to express elevated levels of alpha2-6 sialylation and increased activity of ST6Gal-I, the Golgi glycosyltransferase that creates alpha2-6 linkages. Elevated ST6Gal-I positively correlates with metastasis and poor survival, and therefore ST6Gal-I-mediated hypersialylation likely plays a role in colorectal tumor invasion. Previously we found that oncogenic ras (present in roughly 50% of colon adenocarcinomas) up-regulates ST6Gal-I and, in turn, increases sialylation of beta1 integrin adhesion receptors in colon epithelial cells. However, we wanted to know if this pattern held true in vivo and, if so, how beta1 hypersialylation might contribute to colon tumor progression. In the present study, we find that beta1 integrins from colon adenocarcinomas consistently carry higher levels of alpha2-6 sialic acid. To explore the effects of increased alpha2-6 sialylation on beta1-integrin function, we stably expressed ST6Gal-I in a colon epithelial cell line lacking endogenous ST6Gal-I. ST6Gal-I expressors (with alpha2-6 sialylated beta1 integrins) exhibited up-regulated attachment to collagen I and laminin and increased haptotactic migration toward collagen I, relative to parental cells (with completely unsialylated beta1 integrins). Blockade of ST6Gal-I expression with short interfering RNA reversed collagen binding back to the level of ST6Gal-I nonexpressors, confirming that alpha2-6 sialylation regulates beta1 integrin function. Finally, we show that beta1 integrins from ST6Gal-I expressors have increased association with talin, a marker for integrin activation. Collectively, these findings suggest that beta1 hypersialylation may augment colon tumor progression by altering cell preference for certain extracellular matrix milieus, as well as by stimulating cell migration.

Isolation of transcriptomal changes attributable to LHON mutations and the cybridization process

Leber's hereditary optic neuropathy (LHON) is thought to be the most common disease resulting from mitochondrial DNA (mtDNA) point mutations, and transmitochondrial cytoplasmic hybrid (cybrid) cell lines are the most frequently used model for understanding the pathogenesis of mitochondrial disorders. We have used oligonucleotide microarrays and a novel study design based on shared transcripts to allocate transcriptomal changes into rho-zero-dependent, cybridization-dependent and LHON-dependent categories in these cells. The analysis indicates that the rho-zero process has the largest transcriptomal impact, followed by the cybridization process, and finally the LHON mutations. The transcriptomal impacts of the rho-zero and cybridization processes preferentially and significantly affect the mitochondrial compartment, causing upregulation of many transcripts involved in oxidative phosphorylation, presumably in response to the mtDNA depletion that occurs at the rho-zero step. Nine LHON-specific transcriptional alterations were shared among osteosarcoma cybrids and lymphoblasts bearing LHON mutations. Notably, the aldose reductase transcript was overexpressed in LHON cybrids and lymphoblasts. Aldose reductase is also overexpressed in diabetic retinopathy, leading to optic nerve and retinal complications. The LHON-specific increase in transcript level was confirmed by quantitative reverse transcription-polymerase chain reaction (RT-PCR), and a western blot confirmed a higher level of aldose reductase in mutant mitochondria. One product of aldose reductase is sorbitol, which has been linked to osmotic stress, oxidative stress and optic neuropathy, and sorbitol levels were increased in LHON cybrids. If these results are confirmed in patient tissues, aldose reductase inhibitors could have some therapeutic value for LHON.

RalA interacts with ZONAB in a cell density-dependent manner and regulates its transcriptional activity

Ral proteins are members of the Ras superfamily of small GTPases and are involved in signalling pathways for actin cytoskeleton remodelling, cell cycle control, cellular transformation and vesicle transport. To identify novel RalA effector proteins, we used the reverse Ras recruitment system and found that RalA interacts with a Y-box transcription factor, ZO-1-associated nucleic acid-binding protein (ZONAB), in a GTP-dependent manner. The amount of the RalA-ZONAB complex increases as epithelial cells become more dense and increase cell contacts. The RalA-ZONAB interaction results in a relief of transcriptional repression of a ZONAB-regulated promoter. Additionally, expression of oncogenic Ras alleviates transcriptional repression by ZONAB in a RalA-dependent manner. The data presented here implicate the RalA/ZONAB interaction in the regulation of ZONAB function.