Beta-galactoside alpha2,6 sialyltransferase in human colon cancer: contribution of multiple transcripts to regulation of enzyme activity and reactivity with Sambucus nigra agglutinin

Sialylated glycoproteins and sialyltransferases in digestive cancers: Mechanisms, diagnostic biomarkers, and therapeutic targets

Sialic acid (SA), as the ultimate epitope of polysaccharides, can act as a cap at the end of polysaccharide chains to prevent their overextension. Sialylation is the enzymatic process of transferring SA residues onto polysaccharides and is catalyzed by a group of enzymes known as sialyltransferases (SiaTs). It is noteworthy that the sialylation level of glycoproteins is significantly altered when digestive cancer occurs. And this alteration exhibits a close correlation with the progression of these cancers. In this review, from the perspective of altered SiaTs expression levels and changed glycoprotein sialylation patterns, we summarize the pathogenesis of gastric cancer (GC), colorectal cancer (CRC), pancreatic ductal adenocarcinoma (PDAC), and hepatocellular carcinoma (HCC). Furthermore, we propose potential early diagnostic biomarkers and prognostic indicators for different digestive cancers. Finally, we summarize the therapeutic value of sialylation in digestive system cancers.

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.

Investigation of acidic free-glycans in urine and their alteration in cancer

Alterations to glycans in cancer patients have been used to identify novel tumor biomarkers. Most of these studies have focused on protein glycosylation but less attention has been paid to free-glycans. Here, we analyzed acidic free-glycans in the urine of cancer patients to identify novel tumor marker candidates. Specifically, urine samples were collected from patients with gastric cancer, pancreatic cancer and cholangiocarcinoma as well as normal controls. The free-glycans were extracted from creatinine-adjusted urine and fluorescently labeled with 2-aminopyridine. Initially, we performed profiling of urinary free-glycans by high-performance liquid chromatography and mass spectrometry with enzymatic and chemical degradation. More than 100 glycans, including novel structures, were identified. The chromatographic peaks suggested some of these glycans were present at elevated levels in cancer patients. To verify cancer-associated alterations, we compared the glycan levels between cancer patients and normal controls by selected reaction monitoring. Representative structures of glycans with elevated levels in cancer patients included the following: small glycans related to sialyllactose; sialyl Lewis X; lactose- and N-acetyllactosamine (LacNAc) type-II-core glycans with LacNAc (type-I or II)-extensions and modifications of α1,3/4-fucose and/or 6-sulfate on the Glc/GlcNAc; free-N-glycans containing sialylation or β1,6-branch of 6-sulfo Lewis X; novel NeuAcα2-3Galβ1-4(+/−Fucα1-3) Xylα1-3Glc glycans. Our results provide further insight into urinary free-glycans and suggest the potential utility of these compounds as tumor markers.

Altered linkage pattern of N-glycan sialic acids in pseudomyxoma peritonei

Pseudomyxoma peritonei (PMP) is a highly mucinous adenocarcinoma growing in the peritoneal cavity and most commonly originating from the appendix. Glycans play an important role in carcinogenesis, and glycosylation is altered in malignant diseases, including PMP. We have previously demonstrated that fucosylation of N-glycans is increased in PMP, but we did not observe modulation of overall sialylation. As sialic acids can be attached to the rest of the glycan via α2,3- or α2,6-linkage, we have now analyzed the linkage patterns of sialic acids in tissue specimens of normal appendices, low-grade appendiceal mucinous neoplasms (LAMN), low-grade (LG) PMP and high-grade (HG) PMP. For the linkage analysis, the enzymatically released acidic N-glycans were first treated with ethyl esterification or α2,3-sialidase digestion followed by MALDI-TOF mass spectrometry. Significant increase in the relative abundance of α2,6-sialylated and decrease in α2,3-sialylated N-glycans was observed in PMP tumors as compared to the normal appendices (P < 0.025). More specifically, increased α2,6-sialylation (P < 0.05) and decreased α2,3-sialylation (P < 0.01) were detected in afucosylated and monofucosylated N-glycans of PMPs, whereas the less abundant multifucosylated glycans, containing terminal fucose, demonstrated increased α2,3-sialylation (P < 0.01). Importantly, the increase in α2,6-sialylation was also detected between PMP and the appendiceal precursor lesion LAMN (P < 0.01). The identified glycosylation alterations produce ligands for sialic acid-binding immunoglobulin-like lectins (Siglecs) and sialofucosylated glycans binding selectins, which play a role in the peritoneal dissemination and progression of the disease.

N‑glycosylation and receptor tyrosine kinase signaling affect claudin‑3 levels in colorectal cancer cells

Changes in protein levels in different components of the apical junctional complex occur in colorectal cancer (CRC). Claudin-3 is one of the main constituents of tight junctions, and its overexpression can increase the paracellular flux of macromolecules, as well as the malignant potential of CRC cells. The aim of this study was to investigate the molecular mechanisms involved in the regulation of claudin-3 and its prognostic value in CRC. In silico evaluation in each of the CRC consensus molecular subtypes (CMSs) revealed that high expression levels of CLDN3 (gene encoding claudin-3) in CMS2 and CMS3 worsened the patients' long-term survival, whereas a decrease in claudin-3 levels concomitant with a reduction in phosphorylation levels of epidermal growth factor receptor (EGFR) and insulin-like growth factor 1 receptor (IGF1R) could be achieved by inhibiting N-glycan biosynthesis in CRC cells. We also observed that specific inactivation of these receptor tyrosine kinases (RTKs) led to a decrease in claudin-3 levels, and this regulation seems to be mediated by phospholipase C (PLC) and signal transducer and activator of transcription 3 (STAT3) in CRC cells. RTKs are modulated by their N-linked glycans, and inhibition of N-glycan biosynthesis decreased the claudin-3 levels; therefore, we evaluated the correlation between N-glycogenes and CLDN3 expression levels in each of the CRC molecular subtypes. The CMS1 (MSI immune) subtype concomitantly exhibited low expression levels of CLDN3 and N-glycogenes (MGAT5, ST6GAL1, and B3GNT8), whereas CMS2 (canonical) exhibited high gene expression levels of CLDN3 and N-glycogenes (ST6GAL1 and B3GNT8). A robust positive correlation was also observed between CLDN3 and B3GNT8 expression levels in all CMSs. These results support the hypothesis of a mechanism integrating RTK signaling and N-glycosylation for the regulation of claudin-3 levels in CRC, and they suggest that CLDN3 expression can be used to predict the prognosis of patients identified as CMS2 or CMS3.

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.

Alginate oligosaccharide attenuates α2,6-sialylation modification to inhibit prostate cancer cell growth via the Hippo/YAP pathway

Chitosan oligosaccharides have been reported to inhibit various tumors. However, the water-soluble marine plant oligosaccharide alginate oligosaccharide (AOS) has only rarely been reported to have anti-cancer effects. Moreover, the inhibitory effect of AOS on prostate cancer and the underlying molecular mechanism remain unknown. This study shows that AOS inhibited cell growth, which was consistent with the attenuation of α2,6-sialylation modification. Furthermore, AOS inhibited ST6Gal-1 promoter activity and thus affected transcriptional processes. In addition, AOS could activate the Hippo/YAP pathway and block the recruitment of both the coactivator YAP and c-Jun. Furthermore, YAP interacted with the transcription factor c-Jun and regulated the transcriptional activity of the downstream target ST6Gal-1 gene. Consistent with in vitro data, AOS suppressed the tumorigenicity of prostate cancer cells via the Hippo/YAP pathway in vivo. In summary, these data indicate that AOS slows the proliferation of prostate cancer and provides a basis for the healthy function of kelp in traditional cognition.

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.

Silencing of ST6Gal I enhances colorectal cancer metastasis by down-regulating KAI1 via exosome-mediated exportation and thereby rescues integrin signaling

Aberrant sialylation has long been correlated with human cancer. Increased ST6 Gal I (β-galactoside α 2, 6 sialyltransferase) and consequently higher levels of cell-surface α 2, 6 sialylation has been associated with human colorectal cancer (CRC) metastasis. We have extensive circumstantial data that sialylation is connected to cancer metastasis, but we do not understand in detail how sialylation can switch on/off multiple steps in cancer metastasis. To investigate the molecular mechanism underlying the ST6Gal I-mediated metastasis of CRC, we silenced the ST6Gal I gene in a metastatic SW620 CRC cell line (SW620-shST6Gal I) and examined the metastatic behavior of the cells. We found that various hallmarks of metastatic ability were considerably enhanced in ST6Gal 1-depleted SW620 clones, as assessed both in vitro and in vivo . In particular, the metastasis suppressor, KAI1, was down-regulated in ST6Gal I-deficient SW620 clones. This reflected the increased exosome-mediated exportation of KAI1, and was associated with a decrease in the KAI1-mediated inhibition of integrin. These findings indicate that gene silencing of ST6Gal I could enhance metastasis of CRC by down-regulating KAI1 activity and rescuing its negative effects on integrin signaling.

Characterization of a high-affinity sialic acid-specific CBM40 from Clostridium perfringens and engineering of a divalent form

CBMs (carbohydrate-binding modules) are a class of polypeptides usually associated with carbohydrate-active enzymatic sites. We have characterized a new member of the CBM40 family, coded from a section of the gene NanI from Clostridium perfringens. Glycan arrays revealed its preference towards α(2,3)-linked sialosides, which was confirmed and quantified by calorimetric studies. The CBM40 binds to α(2,3)-sialyl-lactose with a Kd of ∼30 μM, the highest affinity value for this class of proteins. Inspired by lectins' structure and their arrangement as multimeric proteins, we have engineered a dimeric form of the CBM, and using SPR (surface plasmon resonance) we have observed 6–11-fold binding increases due to the avidity affect. The structures of the CBM, resolved by X-ray crystallography, in complex with α(2,3)- or α(2,6)-sialyl-lactose explain its binding specificity and unusually strong binding.

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.

An oncogenic protein Golgi phosphoprotein 3 up-regulates cell migration via sialylation

Recently, the Golgi phosphoprotein 3 (GOLPH3) and its yeast homolog Vps74p have been characterized as essential for the Golgi localization of glycosyltransferase in yeast. GOLPH3 has been identified as a new oncogene that is commonly amplified in human cancers to modulate mammalian target of rapamycin signaling. However, the molecular mechanisms of the carcinogenic signaling pathway remain largely unclear. To investigate whether the expression of GOLPH3 was involved in the glycosylation processes in mammalian cells, and whether it affected cell behavior, we performed a loss-of-function study. Cell migration was suppressed in GOLPH3 knockdown (KD) cells, and the suppression was restored by a re-introduction of the GOLPH3 gene. HPLC and LC/MS analysis showed that the sialylation of N-glycans was specifically decreased in KD cells. The specific interaction between sialyltransferases and GOLPH3 was important for the sialylation. Furthermore, overexpression of α2,6-sialyltransferase-I rescued cell migration and cellular signaling, both of which were blocked in GOLPH3 knockdown cells. These results are the first direct demonstration of the role of GOLPH3 in N-glycosylation to regulate cell biological functions.

Sialylation by β-galactoside α-2,6-sialyltransferase and N-glycans regulate cell adhesion and invasion in human anaplastic large cell lymphoma

The interaction between cell surface glycans and extracellular matrix (ECM) including galectins is known to be closely associated with tumor cell adhesion, invasion and metastasis. We analyzed the roles of cell surface sialylation or glycosylation in galectin or ECM-mediated cell adhesion and invasion of human malignant lymphoma cells. Neuraminidase from Arthrobacter ureafaciens (AU) treatment resulted in reduction of cell adhesion to galectin-8 in human anaplastic large cell lymphoma (H-ALCL) which was established in our laboratory. The knockdown of β-galactoside α-2,6-sialyltransferase (ST6Gal1) by siRNA showed inhibition of ST6Gal1 expression in the cytoplasm of H-ALCL cells on immunohistochemical findings, and showed dramatic enhancement of cell adhesion to galectin-8. On the other hand, α-2,3-specific neuraminidase treatment resulted in moderate enhancement of cell adhesion to galectin-8. We performed chemically artificial modification of cell surface O-glycans by treatment of benzyl 2-acetamido-2-deoxy-α-D-galactopyranoside (Bz-α-GalNAc) in H-ALCL. Cell adhesion to galectin-8 was enhanced by treatment of Bz-α-GalNAc suggesting that inhibition of elongation of O-glycans may enhance cell adhesion to galectin-8 in H-ALCL cells. On the other hand inhibition of elongation of N-glycosylation by tunicamycin (TM) resulted in inhibition of Phaseolus vulgaris-L (L-PHA) lectin-binding activity and inhibited cell adhesion to galectin-8,laminin and fibronectin. Neuraminidase treatment enhanced cell adhesion to laminin, and knockdown of ST6Gal1 resulted in enhancement of cell adhesion to laminin, but not to fibronectin, collagen type 1 and 4. Galectin-8 pre-treatment dramatically enhanced cell adhesion to laminin and neuraminidase treatment also enhanced cell adhesion to laminin in combination with galectin-8. Rho inhibitor, C3-transferase pre-treatment resulted in inhibition of cell invasion to galectin-8. Phosphatidylinositol 3-phosphate kinase (PI3K) inhibitor, wortmannin inhibits the cell invasive capacity to galectin-8. Neuraminidase treatment induces growth inhibition of lymphoma cells by galectin-8.

Treatment of unresectable stage IV metastatic melanoma with aviscumine after anti-neoplastic treatment failure: a phase II, multi-centre study

Background

Aviscumine, a recombinant plant protein, is an immune modulator that induces ribotoxic stress at the 28S ribosomal RNA subunit. In this way cytokine release and T-cell responses are enhanced. This phase II trial was conducted to test the efficacy and safety of aviscumine in patients with systemically pre-treated metastatic melanoma stage IV.

Methods

A total of 32 patients with progressive stage IV melanoma after failure of standard therapy were enrolled onto a single-arm, multi-centre, open-label, phase II trial. All patients had an ECOG performance status of 0 or 1. Patients received 350 ng aviscumine twice weekly by subcutaneous injection until progression. The primary end points were progression-free survival (PFS) and overall survival (OS). Safety was assessed as adverse events (AEs). Tumor response was assessed every eight weeks and survival of patients was followed up to one year after the end of therapy. Thirty one patients (intent-to-treat population (ITT)) were assessed for efficacy; safety was assessed in the whole population.

Results

One patient achieved a partial response (PR) and 10 patients showed stable disease/no change (SD). The median progression-free survival (mPFS) was 63 days (95% CI 57–85) and median overall survival (mOS) was 335 days (95% CI 210–604). In total 210 treatment-emergent adverse events were recorded. Grade 1 or 2 AEs occurred in 72% of patients and were mostly application-site effects such as pruritus Grade 3–4 treatment-emergent drug-related adverse events occurred in 9% of patients.

Conclusion

These results suggest that aviscumine may have a clinical impact in patients with previously treated metastatic melanoma and provide rationale for further clinical evaluation of this agent. In the light of effective new immune checkpoint blockers it might be a candidate for combinations with these agents.

Trial registration

ClinicalTrials.gov: NCT00658437

Multilectin-assisted fractionation for improved single-dot tissue glycome profiling in clinical glycoproteomics

To survey the glycome shift in cancer, single-dot tissue glycome profiling was improved by incorporating a lectin-assisted fractionation. The differential analysis of tissue specimens from colorectal cancer patients (n = 45) revealed that unfucosylated/α2,6-sialylated glycoproteins significantly increased in patients with poor prognoses. The detailed annotation will be an indispensable supplement for cancer-related glyco-biomarker discovery.

Galectin-1-mediated cell adhesion, invasion and cell death in human anaplastic large cell lymphoma: regulatory roles of cell surface glycans

Galectin-1 is known to be one of the extracellular matrix proteins. To elucidate the biological roles of galectin-1 in cell adhesion and invasion of human anaplastic large cell lymphoma, we performed cell adhesion and invasion assays using the anaplastic large cell lymphoma cell line H-ALCL, which was previously established in our laboratory. From the cell surface lectin array, treatment with neuraminidase from Arthrobacter ureafaciens which cleaves all linkage types of cell surface sialic acid enhanced Arachis hypogaea (PNA), Helix pomatia (HPA) and Phaseolus vulgaris-L (L-PHA) lectin binding reactivity to cell surface of lymphoma cells suggesting that neuraminidase removes cell surface sialic acid. In cell adhesion and invasion assays treatment with neuraminidase markedly enhanced cell adhesion to galectin-1 and decreased cell invasive capacity through galectin-1. α2,6-linked sialic acid may be involved in masking the effect of the interaction between galectin-1 and cell surface glycans. H-ALCL cells expressed the β-galactoside-α2,6-sialyltransferase ST6Gal1. On resialylation assay by recombinant ST6Gal1 with CMP-Neu5Ac, α2,6-resialylation of L-PHA reactive oligosaccharide by ST6Gal1 resulted in inhibition of H-ALCL cell adhesion to galectin-1 compared to the desialylated H-ALCL cells. On knockdown experiments, knockdown of ST6Gal1 dramatically enhanced cell adhesion to galectin-1. N-glycosylation inhibitor swainsonine treatment resulted in enhancement of cell adhesion to galectin-1. In glycomic analysis using the lectin blocking assay treatment with PNA, Artocarpus integrifolia (Jacalin), Glycine max (SBA), Helix pomatia (HPA), Vicia villosa (VVA), Ulex europaeus (UEA-1), Triticum vulgaris (WGA), Canavalia ensiformis (ConA), Phaseolus vulgaris-L (L-PHA), Phaseolus vulgaris-E4 (E-PHA), Datura stramonium (DSA) lectins resulted in modulation of lymphoma cell to galectin-1 suggesting that several types of glycans may regulate cell adhesion to galectin-1 by steric hindrance. The adhesive capacity of H-ALCL cells is regulated by phosphatidylinositol 3 phosphate kinase (PI3K) and actin cytoskeleton, and the invasive capacity of H-ALCL cells is regulated by PI3K, mitogen-activated protein kinase (MAPK), Rho and actin cytoskeleton. Furthermore, galectin-1-induced cell death in H-ALCL cells was accompanied by inhibition of CD45 protein tyrosine phosphatase (PTP) activity. In conclusion, cell adhesion and invasion to galectin-1 appeared to be regulated by cell surface sialylation and N-glycosylation, and galectin-1 regulates cell death through inhibition of CD45 PTP activity of H-ALCL.

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.

Tumour suppressor p16(INK4a) - anoikis-favouring decrease in N/O-glycan/cell surface sialylation by down-regulation of enzymes in sialic acid biosynthesis in tandem in a pancreatic carcinoma model

Tumour suppressor p16(INK4a) is known to exert cell-cycle control via cyclin-dependent kinases. An emerging aspect of its functionality is the orchestrated modulation of N/O-glycosylation and galectin expression to induce anoikis in human Capan-1 pancreatic carcinoma cells. Using chemoselective N/O-glycan enrichment technology (glycoblotting) and product characterization, we first verified a substantial decrease in sialylation. Tests combining genetic (i.e. transfection with α2,6-sialyltransferase-specific cDNA) or metabolic (i.e. medium supplementation with N-acetylmannosamine to track down a bottleneck in sialic acid biosynthesis) engineering with cytofluorometric analysis of lectin binding indicated a role of limited substrate availability, especially for α2,6-sialylation, which switches off reactivity for anoikis-triggering homodimeric galectin-1. Quantitative MS analysis of protein level changes confirmed an enhanced galectin-1 presence along with an influence on glycosyltransferases (β1,4-galactosyltransferase-IV, α2,3-sialyltransferase-I) and detected p16(INK4a) -dependent down-regulation of two enzymes in the biosynthesis pathway for sialic acid [i.e. the bifunctional UDP-N-acetylglucosamine 2-epimerase/N-acetylmannosamine kinase (GNE) and N-acetylneuraminic acid 9-phosphate synthase] (P < 0.001). By contrast, quantitative assessment for the presence of nuclear CMP-N-acetylneuraminic acid synthase (which is responsible for providing the donor for enzymatic sialylation that also acts as feedback inhibitor of the epimerase activity of GNE) revealed a trend for an increase. Partial restoration of sialylation in GNE-transfected cells supports the implied role of sialic acid availability for the glycophenotype. Fittingly, the extent of anoikis was reduced in double-transfected (p16(INK4a) /GNE) cells. Thus, a second means of modulating cell reactivity to the growth effector galectin-1 is established in addition to the common route of altering α2,6-sialyltransferase expression: regulating enzymes of the pathway for sialic acid biosynthesis.

Characterization of P1 promoter activity of the beta-galactoside alpha2,6-sialyltransferase I gene (siat 1) in cervical and hepatic cancer cell lines

The level of beta-galactoside alpha2,6-sialyltransferase I (ST6Gal I) mRNA, encoded by the gene siat1, is increased in malignant tissues. Expression is regulated by different promoters - P1, P2 and P3 - generating three mRNA isoforms H, X and YZ. In cervical cancer tissue the mRNA isoform H, which results from P1 promoter activity, is increased. To study the regulation of P1 promoter, different constructs from P1 promoter were evaluated by luciferase assays in cervical and hepatic cell lines. Deletion of a fragment of 1048 bp (-89 to +24 bp) increased 5- and 3-fold the promoter activity in C33A and HepG2 cell lines, respectively. The minimal region with promoter activity was a 37 bp fragment in C33A cells. The activity of this region does not require the presence of an initiator sequence. In HepG2 cells the minimal promoter activity was detected in the 66 bp fragment. Sp1 (-32) mutation increased the promoter activity only in HepG2 cells. HNF1 mutation decreased promoter activity in HepG2 cell line but not in C33A cells. We identified a large region that plays a negative regulation role. The regulation of promoter activity is cell type specific. Our study provides new insights into the complex transcriptional regulation of siat1 gene.

Cleavage of ST6Gal I by radiation-induced BACE1 inhibits golgi-anchored ST6Gal I-mediated sialylation of integrin β1 and migration in colon cancer cells

Background

Previously, we found that β-galactoside α2,6-sialyltransferase (ST6Gal I), an enzyme that adds sialic acids to N-linked oligosaccharides of glycoproteins and is frequently overexpressed in cancer cells, is up-regulated by ionizing radiation (IR) and cleaved to a form possessing catalytic activity comparable to that of the Golgi-localized enzyme. Moreover, this soluble form is secreted into the culture media. Induction of ST6Gal I significantly increased the migration of colon cancer cells via sialylation of integrin β1. Here, we further investigated the mechanisms underlying ST6Gal I cleavage, solubilization and release from cells, and addressed its functions, focusing primarily on cancer cell migration.

Methods

We performed immunoblotting and lectin affinity assay to analyze the expression of ST6 Gal I and level of sialylated integrin β1. After ionizing radiation, migration of cells was measured by in vitro migration assay. α2, 6 sialylation level of cell surface was analyzed by flow cytometry. Cell culture media were concentrated and then analyzed for soluble ST6Gal I levels using an α2, 6 sialyltransferase sandwich ELISA.

Result

We found that ST6Gal I was cleaved by BACE1 (β-site amyloid precursor protein-cleaving enzyme), which was specifically overexpressed in response to IR. The soluble form of ST6Gal I, which also has sialyltransferase enzymatic activity, was cleaved from the Golgi membrane and then released into the culture media. Both non-cleaved and cleaved forms of ST6Gal I significantly increased colon cancer cell migration in a sialylation-dependent manner. The pro-migratory effect of the non-cleaved form of ST6Gal I was dependent on integrin β1 sialylation, whereas that of the cleaved form of ST6Gal I was not, suggesting that other intracellular sialylated molecules apart from cell surface molecules such as integrin β1 might be involved in mediating the pro-migratory effects of the soluble form of ST6Gal I. Moreover, production of soluble form ST6Gal I by BACE 1 inhibited integrin β1 sialylation and migration by Golgi-anchored form of ST6Gal I.

Conclusions

Our results suggest that soluble ST6Gal I, possibly in cooperation with the Golgi-bound form, may participate in cancer progression and metastasis prior to being secreted from cancer cells.

Sialylation of epidermal growth factor receptor regulates receptor activity and chemosensitivity to gefitinib in colon cancer cells

β-Galactoside α2,6-sialyltransferase (ST6Gal-I) has been shown to catalyze α2,6 sialylation of N-glycan, an action that is highly correlated with colon cancer progression and metastasis. We have recently demonstrated that ST6Gal-I-induced α2,6 sialylation is critical for adhesion and migration of colon cancer cells. Increase of α2,6 sialylation also contributes to radioresistance of colon cancer. A number of studies have focused on the involvement of sialylation in tumorigenesis, but the mechanism underlying ST6Gal-I-induced cancer progression and the identity of enzyme substrates has received scant research attention. To provide further support for the relevance of ST6Gal-I in the malignancy of colon cancer, we prepared and characterized a ST6Gal-I-knockdown SW480 colorectal carcinoma cell line. We found that inhibition of ST6Gal-I expression increased cell proliferation and tumor growth in vitro and in vivo. An examination of the effect of sialylation on epidermal growth factor receptor (EGFR) activity and downstream signaling, which are highly correlated with cell proliferation, showed that the loss of ST6Gal-I augmented EGF-induced EGFR phosphorylation and activation of extracellular signal-regulated kinase (ERK) in colon cancer cells. Moreover, ST6Gal-I induced sialylation of both wild type and mutant EGFR. These studies provide the first demonstration that ST6Gal-I induces EGFR sialylation in human colon cancer cell lines. Importantly, the anticancer effect of the EGFR kinase inhibitor, gefitinib, was increased in ST6Gal-I-deficient colon cancer cells. In contrast, overexpression of ST6Gal I decreased the cytotoxic effect of gefitinib. These results suggest that sialylation of the EGFR affects EGF-mediated cell growth and induces chemoresistance to gefitinib in colon cancer cells.

Structure and binding analysis of Polyporus squamosus lectin in complex with the Neu5Ac{alpha}2-6Gal{beta}1-4GlcNAc human-type influenza receptor

Glycan chains that terminate in sialic acid (Neu5Ac) are frequently the receptors targeted by pathogens for initial adhesion. Carbohydrate-binding proteins (lectins) with specificity for Neu5Ac are particularly useful in the detection and isolation of sialylated glycoconjugates, such as those associated with pathogen adhesion as well as those characteristic of several diseases including cancer. Structural studies of lectins are essential in order to understand the origin of their specificity, which is particularly important when employing such reagents as diagnostic tools. Here, we report a crystallographic and molecular dynamics (MD) analysis of a lectin from Polyporus squamosus (PSL) that is specific for glycans terminating with the sequence Neu5Acα2-6Galβ. Because of its importance as a histological reagent, the PSL structure was solved (to 1.7 Å) in complex with a trisaccharide, whose sequence (Neu5Acα2-6Galβ1-4GlcNAc) is exploited by influenza A hemagglutinin for viral adhesion to human tissue. The structural data illuminate the origin of the high specificity of PSL for the Neu5Acα2-6Gal sequence. Theoretical binding free energies derived from the MD data confirm the key interactions identified crystallographically and provide additional insight into the relative contributions from each amino acid, as well as estimates of the importance of entropic and enthalpic contributions to binding.

Molecular phylogeny and functional genomics of beta-galactoside alpha2,6-sialyltransferases that explain ubiquitous expression of st6gal1 gene in amniotes

Sialyltransferases are key enzymes in the biosynthesis of sialoglycoconjugates that catalyze the transfer of sialic residue from its activated form to an oligosaccharidic acceptor. β-Galactoside α2,6-sialyltransferases ST6Gal I and ST6Gal II are the two unique members of the ST6Gal family described in higher vertebrates. The availability of genome sequences enabled the identification of more distantly related invertebrates' st6gal gene sequences and allowed us to propose a scenario of their evolution. Using a phylogenomic approach, we present further evidence of an accelerated evolution of the st6gal1 genes both in their genomic regulatory sequences and in their coding sequence in reptiles, birds, and mammals known as amniotes, whereas st6gal2 genes conserve an ancestral profile of expression throughout vertebrate evolution.

Involvement of ST6Gal I in the biosynthesis of a unique human colon cancer biomarker candidate, alpha2,6-sialylated blood group type 2H (ST2H) antigen

The alpha2,6-sialylated blood group type 2H (ST2H) antigen (Fucalpha1-2(NeuAcalpha2-6)Galbeta1-4GlcNAcbeta1-3Galbeta1-4Glc-Cer) is a fucoganglioside found in human colon cancer tissues. To elucidate an enzyme responsible for the ST2H antigen formation, we screened some partially purified candidate enzymes, alpha2,6-sialyltransferases, ST6Gal I and ST6Gal II, and alpha1,2-fucosyltransferases, FUT1 and FUT2 for their activities towards pyridylaminated type 2H (Fucalpha1-2Galbeta1-4GlcNAcbeta1-3Galbeta1-4Glc-PA) or LS-tetrasaccharide c (LST-c: NeuAcalpha2-6Galbeta1-4GlcNAcbeta1-3Galbeta1-4Glc-PA) as acceptor substrates. Here we show the ST6Gal I transfers NeuAc from the donor CMP-NeuAc to the terminal Gal of PA-type 2H, which formed the ST2H antigen, but the others could not synthesize it. Using a recombinant ST6Gal I, enzymatic reactions with two types of acceptors, PA-type 2H and PA-lacto-N-neotetraose (LNnT), were kinetically analysed. On the basis of catalytic efficiency (V(max)/K(m)), the specificity of ST6Gal I towards the PA-type 2H was estimated to be 42 times lower than that for PA-LNnT. The overexpression of ST6Gal I in human colon cancer DLD-1 cells effectively resulted in the ST2H antigen formation, as judged by LC-ESI-IT-MS. Many lines of evidence suggest the up-regulation of ST6Gal I in human colon cancer specimens. Collectively, these findings indicate that ST6Gal I is responsible for ST2H antigen biosynthesis in human colon cancer cells.

Sialylation of integrin beta1 is involved in radiation-induced adhesion and migration in human colon cancer cells

PURPOSE

Previously, we reported that radiation-induced ST6 Gal I gene expression was responsible for an increase of integrin beta1 sialylation. In this study, we have further investigated the function of radiation-mediated integrin beta1 sialylation in colon cancer cells.

METHODS AND MATERIALS

We performed Western blotting and lectin affinity assay to analyze the expression and level of sialylated integrin beta1. After exposure to ionizing radiation (IR), adhesion and migration of cells were measured by in vitro adhesion and migration assay.

RESULTS

IR increased sialylation of integrin beta1 responsible for its increased protein stability and adhesion and migration of colon cancer cells. However, for cells with an N-glycosylation site mutant of integrin beta1 located on the I-like domain (Mu3), these effects were dramatically inhibited. In addition, integrin beta1-mediated radioresistance was not observed in cells containing this mutant. When sialylation of integrin beta1 was targeted with a sulfonamide chalcone compound, inhibition of radiation-induced sialylation of integrin beta1 and inhibition of radiation-induced adhesion and migration occurred.

CONCLUSION

The increase of integrin beta1 sialylation by ST6 Gal I is critically involved in radiation-mediated adhesion and migration of colon cancer cells. From these findings, integrin beta1 sialylation may be a novel target for overcoming radiation-induced survival, especially radiation-induced adhesion and migration.

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.

The Potentials of Glycomics in Biomarker Discovery

Introduction

Glycans have unique characteristics that are significantly different from nucleic acids and proteins in terms of biosynthesis, structures, and functions. Moreover, their isomeric nature and the complex linkages between residues have made glycan analysis a challenging task. Disease development and progression are usually associated with alternations in glycosylation on tissue proteins and/or blood proteins. Glycans released from tissue/blood proteins hence provide a valuable source of biomarkers. In this postgenome era, glycomics is an emerging research field. Glycome refers to a repertoire of glycans in a tissue/cell type, while glycomics is the study of glycome. In the past few years, attempts have been made to develop novel methodologies for quantitative glycomic profiling and to identify potential glycobiomarkers. It can be foreseen that glycomics holds the promise for biomarker discovery. This review provides an overview of the unique features of glycans and the historical applications of such features to biomarker discovery.

Future Prospective

The concept of glycomics and its recent advancement and future prospective in biomarker research are reviewed. Above all, there is no doubt that glycomics is gaining momentum in biomarker research.

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.

Analysis of cell surface carbohydrate expression patterns in normal and tumorigenic human breast cell lines using lectin arrays

Cell surface carbohydrates play important roles in a wide variety of biological processes including cell adhesion, fertilization, differentiation, development, and tumor cell metastasis. Lectins are proteins of nonimmune origin which recognize and bind to specific carbohydrate structural epitopes. We have recently described the development and use of lectin arrays as tools for the elucidation of the carbohydrate structures expressed on cell surfaces. In the present work this technology is employed for the characterization of differences in carbohydrate expression patterns on normal and tumorigenic human breast cell lines, as well as on sublines differing in their tendency to "home" to different tissues during metastasis. Significant differences were observed, including changes that correlate with metastatic potential as well as with tissue-specific homing of metastatic cells.

Lectins as Bioactive Plant Proteins: A Potential in Cancer Treatment

Plant lectins, a unique group of proteins and glycoproteins with potent biological activity, occur in foods like wheat, corn, tomato, peanut, kidney bean, banana, pea, lentil, soybean, mushroom, rice, and potato. Thus, dietary intakes by humans can be significant. Many lectins resist digestion, survive gut passage, and bind to gastrointestinal cells and/or enter the circulation intact, maintaining full biological activity. Several lectins have been found to possess anticancer properties in vitro, in vivo, and in human case studies; they are used as therapeutic agents, preferentially binding to cancer cell membranes or their receptors, causing cytotoxicity, apoptosis, and inhibition of tumor growth. These compounds can become internalized into cells, causing cancer cell agglutination and/or aggregation. Ingestion of lectins also sequesters the available body pool of polyamines, thereby thwarting cancer cell growth. They also affect the immune system by altering the production of various interleukins, or by activating certain protein kinases. Lectins can bind to ribosomes and inhibit protein synthesis. They also modify the cell cycle by inducing non-apoptotic G1-phase accumulation mechanisms, G2/M phase cell cycle arrest and apoptosis, and can activate the caspase cascade. Lectins can also downregulate telomerase activity and inhibit angiogenesis. Although lectins seem to have great potential as anticancer agents, further research is still needed and should include a genomic and proteomic approach.

Cloning, expression in Escherichia coli and characterization of the recombinant Neu5Acalpha2,6Galbeta1,4GlcNAc-specific high-affinity lectin and its mutants from the mushroom Polyporus squamosus

Lectin from the mushroom Polyporus squamosus (PSL) has a unique carbohydrate-binding specificity for sialylated glycoconjugates containing Neu5Acalpha2,6Galbeta1,4Glc/GlcNAc trisaccharide sequences of asparagine-linked glycoproteins. In the present study, we elucidate the molecular basis for its binding specificity as well as establish a consistent source of this useful lectin using a bacterial expression system. cDNA cloning revealed that PSL contains a ricin B chain-like (QXW)(3) domain at its N-terminus that is composed of three homologous subdomains (alpha, beta and gamma). A recombinant lectin was expressed in Escherichia coli as a fully active, soluble form. It agglutinated rabbit erythrocytes and showed the highest affinity for Neu5Acalpha2,6Galbeta1,4GlcNAc, but not for the sialyl alpha2,3-linked isomer. We also investigated the structure-function relationship of PSL. A monomeric C-terminal deletion mutant lacking 40% of the lectin's molecular mass retained sugar-binding activity, indicating that the carbohydrate-binding sites are situated in the N-terminal portion of the lectin, whereas the C-terminal portion probably functions in oligomerization and structural stabilization. Mutant constructs that have single amino acid substitutions in the putative sugar-binding sites, based on sequence alignment with the ricin B-chain, indicate that the beta and gamma subdomains are most probably sugar-binding sites. The recombinantly expressed lectin will be a valuable reagent for the detection of the Neu5Acalpha2,6Galbeta1,4GlcNAc sequence of asparagine-linked glycans.

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

Several oncogenic proteins are known to influence cellular glycosylation. In particular, transfection of codon 12 point mutated H-Ras increases CMP-Neu5Ac: Galbeta1,4GlcNAc alpha2,6-sialyltransferase I (ST6Gal I) activity in rodent fibroblasts. Given that Ras mediates its effects through at least three secondary effector pathways (Raf, RalGEFs and PI3K) and that transcriptional control of mouse ST6Gal I is achieved by the selective use of multiple promoters, we attempted to identify which of these parameters are involved in linking the Ras signal to ST6Gal I gene transcription in mouse fibroblasts. Transformation by human K-Ras or H-Ras (S12 and V12 point mutations, respectively) results in a 10-fold increase in ST6Gal I mRNA, but no alteration in the expression of related sialyltransferases. Using an inducible H-RasV12 expression system, a direct causal link between activated H-Ras expression and elevated ST6Gal I mRNA was demonstrated. The accumulation of the ST6Gal I transcript in response to activated Ras was accompanied by an increase of alpha2,6-sialyltransferase activity and of Neu5Acalpha2,6Gal at the cell surface. Results obtained with H-RasV12 partial loss of function mutants H-RasV12S35 (Raf signal only), H-RasV12C40 (PI3-kinase signal only) and H-RasV12G37 (RalGEFs signal only) suggest that the H-Ras induction of the mouse ST6Gal I gene (Siat1) transcription is primarily routed through RalGEFs. 5'-Rapid amplification of cDNA ends analysis demonstrated that the increase in ST6Gal I mRNA upon H-RasV12 or K-RasS12 transfection is mediated by the Siat1 housekeeping promoter P3-associated 5' untranslated exons.

Transcriptional regulation of human beta-galactoside alpha2,6-sialyltransferase (hST6Gal I) gene in colon adenocarcinoma cell line

Previous studies have shown that hST6Gal I mRNA is overexpressed in colorectal cancer tissues compared with non-malignant or benign tissue. Moreover, Form 1 (hepatic form) mRNA isoform had a marked tendency to accumulate in colon cancer [Int. J. Cancer 88 (2000) 58-65]. These findings suggest that the transcriptional regulation of Form 1 is altered during malignant transformation. We report here transcriptional regulation of the hST6Gal I gene in colon adenocarcinoma cell lines. We characterized P1 promoter region, which regulates Form 1 mRNA expression, using luciferase assays. The result indicates that the nt-156 to -1 region is important for transcriptional activity of hST6Gal I gene in colon adenocarcinoma cell lines. The nt-156 to -1 region contains HNF1 recognition element. Mutation of the HNF1 site reduced luciferase activity by about 80% compared with the wild-type construct, suggesting that HNF1 site is involved in the transcription of Form 1 mRNA in colon cancer cells.

Identification and functional expression of a second human beta-galactoside alpha2,6-sialyltransferase, ST6Gal II

BLAST analysis of the human and mouse genome sequence databases using the sequence of the human CMP-sialic acid:beta-galactoside alpha-2,6-sialyltransferase cDNA (hST6Gal I, EC2.4.99.1) as a probe allowed us to identify a putative sialyltransferase gene on chromosome 2. The sequence of the corresponding cDNA was also found as an expressed sequence tag of human brain. This gene contained a 1590 bp open reading frame divided in five exons and the deduced amino-acid sequence didn't correspond to any sialyltransferase already known in other species. Multiple sequence alignment and subsequent phylogenic analysis showed that this new enzyme belonged to the ST6Gal subfamily and shared 48% identity with hST6Gal-I. Consequently, we named this new sialyltransferase ST6Gal II. A construction in pFlag vector transfected in COS-7 cells gave raise to a soluble active form of ST6Gal II. Enzymatic assays indicate that the best acceptor substrate of ST6Gal II was the free disaccharide Galbeta1-4GlcNAc structure whereas ST6Gal I preferred Galbeta1-4GlcNAc-R disaccharide sequence linked to a protein. The alpha2,6-linkage was confirmed by the increase of Sambucus nigra agglutinin-lectin binding to the cell surface of CHO transfected with the cDNA encoding ST6Gal II and by specific sialidases treatment. In addition, the ST6Gal II gene showed a very tissue specific pattern of expression because it was found essentially in brain whereas ST6Gal I gene is ubiquitously expressed.

Glycodynamics of Mucin Biosynthesis in Gastrointestinal Tumor Cells

Glycoproteins found in the secretions and on the surfaces of cancer cells include mucins and mucin-like glycoproteins. These molecules have been shown to carry antigens that are characteristically expressed on cancer cells, including Tn and T antigens and Lewis epitopes. The structures of O-glycans are often abnormal in gastrointestinal tumors, or else are present in abnormal amounts, and these structures greatly contribute to the phenotype and biology of cancer cells. It has been shown that glycans of cancer cells have functional importance in cell adhesion, invasion and metastasis. The possible mechanisms leading to these cancer-specific changes in carbohydrate structures (termed glycodynamics) involve altered mRNA expression and catalytic activities of glycosyltransferases and sulfotransferases found in tissues and cells of gastrointestinal tumors. In a number of cases it has been possible to correlate enzyme changes with oligosaccharide structures. Different mechanisms have been suggested leading to the synthesis of cancer-specific Lewis, T and Tn antigens, but the regulation of cancer mucin antigens generally appears to be very complex and is poorly understood. The expression levels of specific mucin antigens and enzymes in gastro-intestinal tumors have diagnostic as well as prognostic value. These antigens also have potential for cancer immunotherapy. However, we first need to unravel the complexity of the control of glycosylation in cancer cells. Most importantly, studies of the functional implications of the glycodynamics in cancer cells, as related to cell adhesion and impact on the immune system will provide promising directions for future research.

Differential expression of beta-galactoside alpha2,6 sialyltransferase and sialoglycans in normal and cirrhotic liver and hepatocellular carcinoma

SUMMARY

Sialyltransferases sialylate plasma glycoproteins in hepatocytes and may (as hepatic key enzymes) constitute markers for liver diseases. We examined expression of the prevalent alpha2,6 sialyltransferase (ST6Gal I) and sialoglycans in normal liver, cirrhotic liver, and hepatocellular carcinoma (HCC) using a new ST6Gal I-specific mAb and recombinant fusion proteins of CD22 and sialoadhesin recognizing alpha2,6- or alpha2,3-sialylated glycans in immunohistology and flow cytometry. In normal and cirrhotic liver, ST6Gal I and sialoglycans were localized in the Golgi region of hepatocytes surrounding the bile canaliculi and along the bile canaliculi, respectively. Sialoglycans were additionally recognized in Kupffer cells, bile ducts, endothelial cells, and oval cells. Well-differentiated and moderately differentiated HCC showed Golgi and diffuse cytoplasmic staining of ST6Gal I and sialoglycans, whereas the cytoplasmic staining for ST6Gal I and sialoglycans was decreased or even absent in poorly differentiated HCC. Detection of sialoglycans by the recombinant fusion proteins in Western blots of cell lysates derived from cell lines revealed two major double bands of sialoglycoproteins at 65 and 120 kDa for hepatocytes, three major bands at 54, 49, and 44 kDa for colonic epithelial cells, and one band at 60 kDa for endothelial cells. Our results describe the expression patterns of ST6Gal I and sialoglycans in various liver tissues and demonstrate an altered expression of these structures between benign and malignant hepatocellular lesions.

Cell surface alpha 2,6 sialylation affects adhesion of breast carcinoma cells

Tumor-associated alterations of cell surface glycosylation play a crucial role in the adhesion and metastasis of carcinoma cells. The aim of this study was to examine the effect of alpha 2,6-sialylation on the adhesion properties of breast carcinoma cells. To this end mammary carcinoma cells, MDA-MB-435, were sense-transfected with sialyltransferase ST6Gal-I cDNA or antisense-transfected with a part of the ST6Gal-I sequence. Sense transfectants showed an enhanced ST6Gal-I mRNA expression and enzyme activity and an increased binding of the lectin Sambucus nigra agglutinin (SNA), specific for alpha 2,6-linked sialic acid. Transfection with ST6Gal-I in the antisense direction resulted in less enzyme activity and SNA reactivity. A sense-transfected clone carrying increased amounts of alpha 2,6-linked sialic acid adhered preferentially to collagen IV and showed reduced cell-cell adhesion and enhanced invasion capacity. In contrast, antisense transfection led to less collagen IV adhesion but enhanced homotypic cell-cell adhesion. In another approach, inhibition of ST6Gal-I enzyme activity by application of soluble antisense-oligodeoxynucleotides was studied. Antisense treatment resulted in reduced ST6 mRNA expression and cell surface 2,6-sialylation and significantly decreased collagen IV adhesion. Our results suggest that cell surface alpha 2,6-sialylation contributes to cell-cell and cell-extracellular matrix adhesion of tumor cells. Inhibition of sialytransferase ST6Gal-I by antisense-oligodeoxynucleotides might be a way to reduce the metastatic capacity of carcinoma cells.

Sialyltransferases in cancer

It has long been known that cancer cells often express more heavily sialylated glycans on their surface and that this feature sometimes correlates with invasion. It is now well established that specific sialylated structures, such as the Thomsen-Friedenreich-related antigens, the sialyl Lewis antigens, the sialyl alpha2-6 lactosaminyl structure, the polysialic acid or some gangliosides, can mediate cellular interactions and are altered in cancer cells. This review summarizes the current knowledge on the cancer-associated alterations in sialyltransferase expression which are often at the basis of the deranged expression of sialylated structures.

Biosynthesis of the cancer-related sialyl-alpha 2,6-lactosaminyl epitope in colon cancer cell lines expressing beta-galactoside alpha 2,6-sialyltransferase under a constitutive promoter

An elevation of beta-galactoside alpha 2,6-sialyltransferase (ST6Gal.I) enzyme activity and an increased alpha 2,6-sialylation of cell membranes are among the most prominent glycosylation changes associated with colon cancer; both modifications correlate with a worse prognosis. In our previous studies, we have frequently observed a discrepancy between the ST6Gal.I level within a colon cancer sample or cell line and the respective level of reactivity with the alpha 2,6-sialyl-specific lectin from Sambucus nigra (SNA). In this study, we have investigated quantitatively the biosynthesis of the sialyl-alpha 2,6-lactosaminyl epitope in two colon cancer cell types expressing the ST6Gal.I cDNA under the control of a constitutive promoter. By measuring the amount of ST6Gal.I mRNA using competitive RT-PCR, the expression of alpha 2,6-sialylated lactosaminic structures with SNA and anti-CDw75 Ig, and the presence of unsubstituted lactosaminic termini by Erythrina cristagalli lectin, we reached the following conclusions: (a) a high proportion of the cell surface lactosaminic termini remains unsubstituted, even in the presence of a very high ST6Gal.I activity. This proportion is strongly dependent on the cell type; (b) ST6Gal.I-transfected colon cancer cells do not express the CDw75 epitope; (c) the level of ST6Gal.I enzyme activity only partially correlates with the mRNA level; (d) despite the control by a constitutive promoter, the ST6Gal.I mRNA is not constantly expressed over time; and (e) a very large portion of the enzyme molecules is secreted in the extracellular milieu. These results indicate that post-transcriptional and post-translational mechanisms play a pivotal role in the control of alpha 2,6-sialylation in colon cancer cells.

The human sialyltransferase family

The human genome encodes probably more than 20 different sialyltransferases involved in the biosynthesis of sialylated glycoproteins and glycolipids but to date only 15 different human sialyltransferase cDNAs have been cloned and characterized. Each of the sialyltransferase genes is differentially expressed in a tissue-, cell type-, and stage-specific manner to regulate the sialylation pattern of cells. These enzymes differ in their substrate specificity, tissue distribution and various biochemical parameters. However, enzymatic analysis conducted in vitro with recombinant enzyme revealed that one linkage can be synthesized by multiple enzymes. We present here an overview of these human genes and enzymes, the regulation of their occurrence and their involvement in several physiological and pathological processes.

The sialyl-alpha2,6-lactosaminyl-structure: biosynthesis and functional role

Sialylation represents one of the most frequently occurring terminations of the oligosaccharide chains of glycoproteins and glycolipids. Sialic acid is commonly found alpha2,3- or alpha2,6-linked to galactose (Gal), alpha2,6-linked to N-acetylgalactosamine (GalNAc) or alpha2,8-linked to another sialic acid. The biosynthesis of the various linkages is mediated by the different members of the sialyltransferase family. The addition of sialic acid in alpha2,6-linkage to the galactose residue of lactosamine (type 2 chains) is catalyzed by beta-galactoside alpha2,6-sialyltransferase (ST6Gal.I). Although expressed by a single gene, this enzyme shows a complex pattern of regulation which allows its tissue- and stage-specific modulation. The cognate oligosaccharide structure, NeuAcalpha2,6Galbeta1,4GlcNAc, is widely distributed among tissues and is involved in biological processes such as the regulation of the immune response and the progression of colon cancer. This review summarizes the current knowledge on the biochemistry of ST6Gal.I and on the functional role of the sialyl-alpha2,6-lactosaminyl structure.