Glycodynamics of Mucin Biosynthesis in Gastrointestinal Tumor Cells

Deciphering cellular and molecular mechanism of MUC13 mucin involved in cancer cell plasticity and drug resistance

There has been a surge of interest in recent years in understanding the intricate mechanisms underlying cancer progression and treatment resistance. One molecule that has recently emerged in these mechanisms is MUC13 mucin, a transmembrane glycoprotein. Researchers have begun to unravel the molecular complexity of MUC13 and its impact on cancer biology. Studies have shown that MUC13 overexpression can disrupt normal cellular polarity, leading to the acquisition of malignant traits. Furthermore, MUC13 has been associated with increased cancer plasticity, allowing cells to undergo epithelial-mesenchymal transition (EMT) and metastasize. Notably, MUC13 has also been implicated in the development of chemoresistance, rendering cancer cells less responsive to traditional treatment options. Understanding the precise role of MUC13 in cellular plasticity, and chemoresistance could pave the way for the development of targeted therapies to combat cancer progression and enhance treatment efficacy.

Nitric Oxide-Scavenging, Anti-Migration Effects, and Glycosylation Changes after Hemin Treatment of Human Triple-Negative Breast Cancer Cells: A Mechanistic Study

The enhanced expression of nitric oxide (•NO) synthase predicts triple-negative breast cancer outcome and its resistance to different therapeutics. Our earlier work demonstrated the efficiency of hemin to scavenge the intra- and extracellular •NO, proposing its potency as a therapeutic agent for inhibiting cancer cell migration. In continuation, the present work evaluates the effects of •NO on the migration of MDA-MB-231 cells and how hemin modulates the accompanied cellular behavior, focusing on the corresponding expression of cellular glycoproteins, migration-associated markers, and mitochondrial functions. We demonstrated for the first time that while •NO induced cell migration, hemin contradicted that by •NO-scavenging. This was in combination with modulation of the •NO-enhanced glycosylation patterns of cellular proteins with inhibition of the expression of specific proteins involved in the epithelial-mesenchymal transition. These effects were in conjunction with changes in the mitochondrial functions related to both •NO, hemin, and its nitrosylated product. Together, these results suggest that hemin can be employed as a potential anti-migrating agent targeting •NO-scavenging and regulating the expression of migration-associated proteins.

Altered glycosylation of several metastasis-associated glycoproteins with terminal GalNAc defines the highly invasive cancer cell phenotype

Several distinct metastasis-associated glycosylation changes have been shown to promote cancer cell invasion and metastasis, the main cause of death of cancer patients. However, it is unclear whether their presence reflects cell- or tissue-specific variations for metastasis, or species needed to drive different phases of the metastatic cascade. To address this issue from a different perspective, we investigated here whether different cancer cell lines share any glycotopes that are common and important for their invasive phenotype. By using lectin microarray glycan profiling and an established myoma tissue-based 3D invasion assay, we identified a single glycotope recognized by Helix Pomatia agglutinin (HPA), whose expression level in different cancer cells correlated significantly with their invasive potential. Lectin pull-down assay and LC-MS/MS analysis in highly- (A431 and SW-48) and poorly invasive (HepG2 and RCC4) cancer cells revealed ~85 glycoproteins of which several metastasis-promoting members of the integrin family of cell adhesion receptors, the epidermal growth factor receptor (EGFR) and the matrix metalloproteinase-14 (MMP-14) were among the abundant ones. Moreover, we showed that the level of the GalNAc glycotope in MMP-14, EGFR, αV-, β1- and β4 integrin in highly and poorly invasive cancer cells correlated positively with their invasive potential. Collectively, our findings suggest that altered glycosylation of several metastasis-associated glycoproteins with terminal GalNAc drives the highly invasive cancer cell phenotype.

Glycosylation in Cervical Cancer: New Insights and Clinical Implications

Cervical cancer has become the most frequent female malignancy and presents as a general health challenge in many countries undergoing economic development. Various human papillomaviruses (HPV) types have appeared as one of the most critically identifiable causes of widespread cervical cancers. Conventional cervical cytological inspection has limitations of variable sensitivity according to cervical cytology. Glycobiology has been fundamental in related exploration in various gynecologic and reproductive fields and has contributed to our understanding of cervical cancer. It is associated with altered expression of N-linked glycan as well as abnormal expression of terminal glycan structures. The analytical approaches available to determine serum and tissue glycosylation, as well as potential underlying molecular mechanisms involved in the cellular glycosylation alterations, are monitored. Moreover, cellular glycosylation influences various aspects of cervical cancer biology, ranging from cell surface expressions, cell-cell adhesion, cancer signaling, cancer diagnosis, and management. In general, discoveries in glycan profiling make it technically reproducible and affordable to perform serum glycoproteomic analyses and build on previous work exploring an expanded variety of glycosylation markers in the majority of cervical cancer patients.

Characterization of colorectal mucus using infrared spectroscopy: a potential target for bowel cancer screening and diagnosis

Biological materials presenting early signs of cancer would be beneficial for cancer screening/diagnosis. In this respect, the suitability of potentially exploiting mucus in colorectal cancer was tested using infrared spectroscopy in combination with statistical modeling. Twenty-six paraffinized colon tissue biopsy sections containing mucus regions from 20 individuals (10 normal and 16 cancerous) were measured using mid-infrared spectroscopic imaging. A digital de-paraffinization, followed by cluster analysis driven digital color-coded multi-staining segmented the infrared images into various histopathological features such as epithelium, connective tissue, stroma, and mucus regions within the tissue sections. Principal component analysis followed by supervised linear discriminant analysis was carried out on pure mucus and epithelial spectra from normal and cancerous regions of the tissue. For the mucus-based classification, a sensitivity of 96%, a specificity of 83%, and an area under the curve performance of 95% was obtained. For the epithelial tissue-based classification, a sensitivity of 72%, a specificity of 88%, and an area under the curve performance of 89% was obtained. The mucus spectral profiles further showed contributions indicative of glycans including that of sialic acid changes between these pathology groups. The study demonstrates that infrared spectroscopic analysis of mucus discriminates colorectal cancers with high sensitivity. This concept could be exploited to develop screening/diagnostic approaches complementary to histopathology.

Mucins and Pathogenic Mucin-Like Molecules Are Immunomodulators During Infection and Targets for Diagnostics and Vaccines

Mucins and mucin-like molecules are highly O-glycosylated proteins present on the cell surface of mammals and other organisms. These glycoproteins are highly diverse in the apoprotein and glycan cores and play a central role in many biological processes and diseases. Mucins are the most abundant macromolecules in mucus and are responsible for its biochemical and biophysical properties. Mucin-like molecules cover various protozoan parasites, fungi and viruses. In humans, modifications in mucin glycosylation are associated with tumors in epithelial tissue. These modifications allow the distinction between normal and abnormal cell conditions and represent important targets for vaccine development against some cancers. Mucins and mucin-like molecules derived from pathogens are potential diagnostic markers and targets for therapeutic agents. In this review, we summarize the distribution, structure, role as immunomodulators, and the correlation of human mucins with diseases and perform a comparative analysis of mucins with mucin-like molecules present in human pathogens. Furthermore, we review the methods to produce pathogenic and human mucins using chemical synthesis and expression systems. Finally, we present applications of mucin-like molecules in diagnosis and prevention of relevant human diseases.

Mucins: the Old, the New and the Promising Factors in Hepatobiliary Carcinogenesis

Mucins are large O-glycoproteins with high carbohydrate content and marked diversity in both the apoprotein and the oligosaccharide moieties. All three mucin types, trans-membrane (e.g., MUC1, MUC4, MUC16), secreted (gel-forming) (e.g., MUC2, MUC5AC, MUC6) and soluble (non-gel-forming) (e.g., MUC7, MUC8, MUC9, MUC20), are critical in maintaining cellular functions, particularly those of epithelial surfaces. Their aberrant expression and/or altered subcellular localization is a factor of tumour growth and apoptosis induced by oxidative stress and several anti-cancer agents. Abnormal expression of mucins was observed in human carcinomas that arise in various gastrointestinal organs. It was widely believed that hepatocellular carcinoma (HCC) does not produce mucins, whereas cholangiocarcinoma (CC) or combined HCC-CC may produce these glycoproteins. However, a growing number of reports shows that mucins can be produced by HCC cells that do not exhibit or are yet to undergo, morphological differentiation to biliary phenotypes. Evaluation of mucin expression levels in precursors and early lesions of CC, as well as other types of primary liver cancer (PLC), conducted in in vitro and in vivo models, allowed to discover the mechanisms of their action, as well as their participation in the most important signalling pathways of liver cystogenesis and carcinogenesis. Analysis of mucin expression in PLC has both basic research and clinical value. Mucins may act as oncogenes and tumour-promoting (e.g., MUC1, MUC13), and/or tumour-suppressing factors (e.g., MUC15). Given their role in promoting PLC progression, both classic (MUC1, MUC2, MUC4, MUC5AC, MUC6) and currently tested mucins (e.g., MUC13, MUC15, MUC16) have been proposed to be important diagnostic and prognostic markers. The purpose of this review was to summarize and update the role of classic and currently tested mucins in pathogenesis of PLC, with explaining the mechanisms of their action in HCC carcinogenesis. It also focuses on determination of the diagnostic and prognostic role of these glycoproteins in PLC, especially focusing on HCC, CC and other hepatic tumours with- and without biliary differentiation.

Mass Spectral Detection of Forward- and Reverse-Hydrogen/Deuterium Exchange Resulting from Residual Solvent Vapors in Electrospray Sources

Characterizing glycans is analytically challenging since glycans are heterogeneous, branched polymers with different three-dimensional conformations. Hydrogen/deuterium exchange-mass spectrometry (HDX-MS) has been used to analyze native conformations and dynamics of biomolecules by measuring the mass increase of analytes as labile protons are replaced with deuterium following exposure to deuterated solvents. The rate of exchange is dependent on the chemical functional group, the presence of hydrogen bonds, pH, temperature, charge, and solvent accessibility. HDX-MS of carbohydrates is challenging due to the rapid exchange rate of hydroxyls. Here, we describe an observed HDX reaction associated with residual solvent vapors saturating electrospray sources. When undeuterated melezitose was infused after infusing D₂O, samples with up to 73% deuterium exchange were detected. This residual solvent HDX was observed for both carbohydrates and peptides in multiple instruments and dependent on sample infusion rate, infusion time, and deuterium content of the solvent. This residual solvent HDX was observed over several minutes of sample analysis and persisted long enough to alter the measured deuterium labeling and possibly change the interpretation of the results. This work illustrates that residual solvent HDX competes with in-solution HDX for rapidly exchanging functional groups. Thus, we propose conditions to minimize this effect, specifically for top-down, in-electrospray ionization, and quench-flow HDX experiments. Graphical Abstract ᅟ.

The Interaction of the Gut Microbiota with the Mucus Barrier in Health and Disease in Human

Glycoproteins are major players in the mucus protective barrier in the gastrointestinal and other mucosal surfaces. In particular the mucus glycoproteins, or mucins, are responsible for the protective gel barrier. They are characterized by their high carbohydrate content, present in their variable number, tandem repeat domains. Throughout evolution the mucins have been maintained as integral components of the mucosal barrier, emphasizing their essential biological status. The glycosylation of the mucins is achieved through a series of biosynthetic pathways processes, which generate the wide range of glycans found in these molecules. Thus mucins are decorated with molecules having information in the form of a glycocode. The enteric microbiota interacts with the mucosal mucus barrier in a variety of ways in order to fulfill its many normal processes. How bacteria read the glycocode and link to normal and pathological processes is outlined in the review.

Eukaryotic protein glycosylation: a primer for histochemists and cell biologists

Proteins undergo co- and posttranslational modifications, and their glycosylation is the most frequent and structurally variegated type. Histochemically, the detection of glycan presence has first been performed by stains. The availability of carbohydrate-specific tools (lectins, monoclonal antibodies) has revolutionized glycophenotyping, allowing monitoring of distinct structures. The different types of protein glycosylation in Eukaryotes are described. Following this educational survey, examples where known biological function is related to the glycan structures carried by proteins are given. In particular, mucins and their glycosylation patterns are considered as instructive proof-of-principle case. The tissue and cellular location of glycoprotein biosynthesis and metabolism is reviewed, with attention to new findings in goblet cells. Finally, protein glycosylation in disease is documented, with selected examples, where aberrant glycan expression impacts on normal function to let disease pathology become manifest. The histological applications adopted in these studies are emphasized throughout the text.

Structural Characterization of Mucin O-Glycosylation May Provide Important Information to Help Prevent Colorectal Tumor Recurrence

Although colorectal cancer is a preventable and curable disease if early stage tumors are removed, it still represents the second cause of cancer-related death worldwide. Surgical resection is the only curative treatment but once operated the patient is either subjected to adjuvant chemotherapy or not, depending on the invasiveness of the cancer and risks of recurrence. In this context, we investigated, by mass spectrometry (MS), alterations in the repertoire of glycosylation of mucins from colorectal tumors of various stages, grades, and recurrence status. Tumors were also compared with their counterparts in resection margins from the same patients and with healthy controls. The obtained data showed an important decrease in the level of expression of sialylated core 3-based O-glycans in tumors correlated with an increase in sialylated core 1 structures. No correlation was established between stages of the tumor samples and mucin O-glycosylation. However, with the notable exception of sialyl Tn antigens, tumors with recurrence presented a milder alteration of glycosylation profile than tumors without recurrence. These results suggest that mucin O-glycans from tumors with recurrence might mimic a healthier physiological situation, hence deceiving the immune defense system.

A rapid and highly sensitive microchip electrophoresis of mono- and mucin-type oligosaccharides labeled with 7-amino-4-methylcoumarin

A selective separation method using a poly(methylmethacrylate) microchip was developed for 7-amino-4-methylcoumarin-labeled saccharides in a crude reaction mixture. In an alkaline borate buffer, saccharide derivatives formed strong anionic borate complexes. These complexes moved from the cathode to the anode in an electric field and were detected near the anode. Excess labeling reagents and other foreign substances remained at the inlet reservoir. A confocal fluorimetric detection system enabled the determination of monosaccharide derivatives with good linearity between at least 5 and 100 nM, corresponding to 50 fmol to 1 pmol per injection. The lower limit of detection (signal-to-noise = 5) was 2 nM. The sensitivity and linear quantitation range were comparable to reported values using fluorometric detection, capillary electrophoresis, or liquid chromatography. Application of the method showed excellent resolution in the analysis of O-linked glycans chemically released from glycoproteins.

Analysis of O-glycans as 9-fluorenylmethyl derivatives and its application to the studies on glycan array

A method is proposed for the analysis of O-glycans as 9-fluorenylmethyl (Fmoc) derivatives. After releasing the O-glycans from the protein backbone in the presence of ammonia-based media, the glycosylamines thus formed are conveniently labeled with Fmoc-Cl and analyzed by HPLC and MALDI-TOF MS after easy purification. Fmoc labeled O-glycans showed 3.5 times higher sensitivities than those labeled with 2-aminobenzoic acid in fluorescent detection. Various types of O-glycans having sialic acids, fucose, and/or sulfate residues were successfully labeled with Fmoc and analyzed by HPLC and MALDI-TOF MS. The method was applied to the comprehensive analysis of O-glycans expressed on MKN45 cells (human gastric adenocarcinoma). In addition, Fmoc-derivatized O-glycans were easily converted to free hemiacetal or glycosylamine-form glycans that are available for fabrication of glycan array and neoglycoproteins. To demonstrate the availability of our methods, we fabricate the glycan array with Fmoc labeled glycans derived from mucin samples and cancer cells. The model studies using the glycan array showed clear interactions between immobilized glycans and some lectins.

Design and synthesis of multifunctional gold nanoparticles bearing tumor-associated glycopeptide antigens as potential cancer vaccines

The development of vaccines against specific types of cancers will offer new modalities for therapeutic intervention. Here, we describe the synthesis of a novel vaccine construction prepared from spherical gold nanoparticles of 3-5 nm core diameters. The particles were coated with both the tumor-associated glycopeptides antigens containing the cell-surface mucin MUC4 with Thomsen Friedenreich (TF) antigen attached at different sites and a 28-residue peptide from the complement derived protein C3d to act as a B-cell activating "molecular adjuvant". The synthesis entailed solid-phase glycopeptide synthesis, design of appropriate linkers, and attachment chemistry of the various molecules to the particles. Attachment to the gold surface was mediated by a novel thiol-containing 33 atom linker which was further modified to be included as a third "spacer" component in the synthesis of several three-component vaccine platforms. Groups of mice were vaccinated either with one of the nanoplatform constructs or with control particles without antigen coating. Evaluation of sera from the immunized animals in enzyme immunoassays (EIA) against each glycopeptide antigen showed a small but statistically significant immune response with production of both IgM and IgG isotypes. Vaccines with one carbohydrate antigen (B, C, and E) gave more robust responses than the one with two contiguous disaccharides (D), and vaccine E with a TF antigen attached to threonine at the 10th position of the peptide was selected for IgG over IgM suggesting isotype switching. The data suggested that this platform may be a viable delivery system for tumor-associated glycopeptide antigens.

The sweet side of tumor immunotherapy

Carbohydrate signatures on tumor cells have functional implications in tumor growth and metastasis and constitute valuable tools in cancer diagnosis and immunotherapy. Increasing data regarding the mechanisms by which they are recognized by the immune system are facilitating the design of more efficient immunotherapeutic protocols based on cancer-associated glycan structures. Recent molecular and proteomic studies revealed that carbohydrates are recognized, not only by B cells and antibodies, but also by cells from the innate arm of immunity, as well as by T cells, and are able to induce specific T-cell immunity and cytotoxicity. In this review, we discuss and update the different strategies targeting tumor-associated carbohydrate antigens that are being evaluated for antitumor immunotherapy, an approach that will be highly relevant, especially when combined with other strategies, in the future fight against cancer.

One-pot characterization of cancer cells by the analysis of mucin-type glycans and glycosaminoglycans

We developed an automated apparatus for rapid releasing of O-glycans from mucin-type glycoproteins [Anal. Biochem. 371 (2007) 52-61; Anal. Chem. 82 (2010) 7436-7443] and applied the device to analyze them in some cancer cell lines [J. Proteome Res. 8 (2009) 521-537]. We also found that the device is useful to release glycosaminoglycans from proteoglycans [Anal. Biochem. 362 (2007) 245-251]. Based on these studies, we developed a method for one-pot analysis of mucin-type glycans and glycosaminoglycans after releasing them from total protein pool obtained from some cancer cell lines. Mucin-type glycans were analyzed by a combination of high-performance liquid chromatography and mass spectrometry techniques, and glycosaminoglycans were analyzed by capillary electrophoresis as fluorescent-labeled unsaturated disaccharides after digestion with specific eliminases followed by fluorescent labeling. Ten cancer cell lines, including blood cancer cells as well as epithelial cancer cells, were used to assess the method. The results clearly revealed that both mucin-type glycans and glycosaminoglycans showed quite interesting profiles. Thus, the current technique will be a powerful tool for discovery of glycan markers of diseases.

Inflammatory cues modulate the expression of secretory product genes, Golgi sulfotransferases and sulfomucin production in LS174T cells

The signals that mediate goblet cell expression of specific mucin chemotypes are poorly defined. Animal and in vitro studies show that acidomucin chemotypes may be altered by inflammation and changes in intestinal microbiota. To examine factors that may elicit this response, human adenocarcinoma-derived LS174T cells, which have a goblet cell-like phenotype and produce both sulfo- and sialomucins, were used to examine the effects of selected microbial and host factors on expression of goblet cell secretory product genes, sulfotransferases and sulfomucin production. Expression of genes encoding mucin 2 (MUC2), resistin-like molecule β (RETNLB), and trefoil factor 3 (TFF3) and Golgi sulfotransferases, carbohydrate (N-acetylglucosamine 6-O) sulfotransferase 5 (CHST5) and galactose-3-O-sulfotransferase 2 (GAL3ST2), was measured by quantitative reverse transcriptase-polymerase chain reaction following treatment with bacterial flagellin, tumor necrosis factor α (TNF-α) or the mucogenic cytokine interleukin-13 (IL-13). Expression of the toll-like receptor 5 (TLR5) gene was also analysed. Sulfomucin expression was examined via high-iron diamide/alcian blue (HID/AB) histochemistry and immunofluorescent staining for the Sulfo Le(a) antigen, which is synthesized in part by GAL3ST2. Flagellin, IL-13 and TNF-α all significantly increased GAL3ST2, MUC2, TFF3 and TLR5 expression, while only IL-13 increased RETNLB and CHST5 expression. Based on HID/AB histochemistry, mucin sulfation was significantly increased in response to both flagellin and IL-13 but not TNF-α. Only treatment with flagellin increased the expression of the Sulfo Le(a) antigen. Collectively, these results indicate that bacterial flagellin, IL-13 and TNF-α differentially modulate the expression of goblet cell secretory product genes, sulfotransferases and sulfomucin production.

State of the art in tumor antigen and biomarker discovery

Our knowledge of tumor immunology has resulted in multiple approaches for the treatment of cancer. However, a gap between research of new tumors markers and development of immunotherapy has been established and very few markers exist that can be used for treatment. The challenge is now to discover new targets for active and passive immunotherapy. This review aims at describing recent advances in biomarkers and tumor antigen discovery in terms of antigen nature and localization, and is highlighting the most recent approaches used for their discovery including “omics” technology.

In-site interaction evaluation of Tn density by inhibition/competition assays

The tumor-associated structure N-acetyl-galactosamine-O-Ser/Thr (Tn antigen), which is overexpressed in various tumor cell types, notably of the breast, ovary and colon, is an interesting determinant that is useful for cancer diagnosis and follow-up. The aim of this research was to study different assay strategies in order to determine the most sensitive system for further application in epitope characterization and binding assessment. The tetrameric isolectin obtained from Vicia villosa seeds (VVLB(4)) shows high affinity for the tumor-associated structure. A monoclonal antibody against VVLB(4), MabVV(34), was generated, and the interaction between MabVV(34) and VVLB(4) was studied by means of binding and inhibition assays. Several synthetic peptides (10 amino acid sequences) designed from the amino acid sequence of VVLB(4) and obtained from trypsin digestion were tested to determine which amino acids were involved in the interaction between MabVV(34) and VVLB(4). The further unraveling of this epitope was investigated by inhibition using designed synthetic peptides as well as mixtures mimicking variable density effect. Under the experimental circumstances, MabVV(34) was able to inhibit the binding of VVLB(4) to Tn. Two of the four peptide sequences assayed showed better inhibition properties. Finally, mixtures containing these selected sequences allowed the evaluation of binding and inhibition as a function of Tn density. We conclude that the present study facilitates the further development of a specific Tn marker and may contribute to the development of Tn-like radiolabelled peptides or Tn-specific radiolabelled fragments providing a highly selective tool for cancer diagnosis and treatment. This strategy may contribute to characterize the new generation of radiopharmaceuticals for diagnosis and therapy based on biomolecules like antibodies, fragments or peptides, whose application is directly guided by their specific molecular recognition.

Management of the human mucosal defensive barrier: evidence for glycan legislation

The human gastrointestinal barrier comprises several layers which enable protection against the external environment. The mucosal epithelium, lamina propria, glycocalyx and secreted mucus each make a contribution to barrier protection. Glycocalyx and secreted mucins constitute a glycosylated environment which interacts with the enteric microflora. Turnover of the mucus layer and the creation of binding ligands for bacteria are significant factors in gut homeostasis. The gut microbiota is composed of many bacterial species, but improved technology has allowed detection of populations present at different stages of development and in disease. Interaction of the microflora with the gut occurs from birth onwards and enables maturation of gut angiogenesis and glycosylation as demonstrated in mouse models. Glycan legislation regulates the ongoing interaction between the microflora and the host mucosa. This accounts for host glycosylation mechanisms providing a dynamic response to fluctuations in the gut microflora. Evidence for glycan legislation is based on a surgical model where intact mucosa can be compared with and without contact to the faecal microflora. In addition, mucosal cell glycosylation is assessed using inhibitors of O-glycan synthesis. These inhibitors lead to growth arrest in cultured colorectal cancer cell lines through the induction of apoptosis and downregulation of proliferation.

Expression of a core 3 disialyl-Le(x) hexasaccharide in human colorectal cancers: a potential marker of malignant transformation in colon

Cancer-associated alterations in cell surface and secreted glycoproteins have been catalogued for many years but many of the studies of alterations in mucin carbohydrate have relied on histochemical or immunohistochemical methods, with little direct chemical analysis. In this study, we analyzed the O-glycosylation pattern of MUC2 glycoprotein isolated from colorectal carcinomas, transitional mucosa and resection margins from three patients with blood group A, B and O, respectively. After alkaline borohydride treatment, the released oligosaccharides were structurally characterized by nanoESI Q-TOF tandem mass spectrometry without prior fractionation or derivatization. As expected, we found an increased expression of sialyl-Tn antigen in the colonic cancer mucins. A more interesting feature was the increased expression of a core 3 sialyl-Le(x) hexasaccharide, NeuAcalpha2-3Galbeta1-4(Fucalpha1-3)GlcNAcbeta1-3(NeuAcalpha2-6)GalNAc in tumor, which appeared to compete with its sulfo-Le(x) counterpart in normal tissue, SO3-3Galbeta1-4(Fucalpha1-3)GlcNAcbeta1-3(NeuAcalpha2-6)GalNAc. This antigen, whose structure was confirmed by NMR experiments, is based on a core 3 glycan and may be a potential marker for the malignant transformation of colonic cells. Unexpectedly, most of the glycans recovered in normal and carcinomas extracts were based on a sialylated core 3, GlcNAcbeta1-3(NeuAcalpha2-6)GalNAcol. Moreover, the pattern of glycosylation was very similar between mucins isolated from each sample, the main differences related to the level of expression of the major oligosaccharides. The data obtained in this investigation may have value for future screening studies on colorectal cancer.

Comparative studies on the structural features of O-glycans between leukemia and epithelial cell lines

Recently, we developed an automated apparatus for rapid releasing of O-glycans from mucin-type glycoproteins and proteoglycans ( Anal. Biochem. 2007 , 362 , 245 - 251 ; 2007 , 371 , 52 - 61 ). In the present paper, we released O-glycans from some leukemia and epithelial cells using the apparatus, and compared the profiles of O-glycans among these cells after fluorescent labeling of the released glycans with 2-aminobenzoic acid. The fluorescent labeled glycans were analyzed using a combination of HPLC and off-line MALDI-(QIT)TOF mass spectrometry We found that leukemia cells generally showed simple glycan profiles and commonly contained sialyl-T (NeuAcalpha2-3Galbeta1-3GalNAc) and disialyl-T (NeuAcalpha2-3Galbeta1-3(NeuAcalpha2-6)GalNAc) antigens as major O-glycans. In contrast, epithelial cancer cell lines usually showed extremely complex profiles. We found that polylactosamine-type O-glycans were abundantly present in MKN45 cells. Especially, we found characteristic glycans, of which Galbeta1-3 residue of core1 structure is modified with biantennary polylactosamine units. In contrast, this cell line did not contain polylactosamine-type N-glycans ( J. Proteome Res. 2006 , 5 , 88 - 97 ). These results suggest that the different biosynthetic pathways for N- and O-glycans are proposed. The method presented here will accelerate the speed for comprehensive analysis of O-glycans in biological samples and will be a powerful tool for clinical/biochemical analysis in cancer biology.

Gastric mucus alterations associated with murine Helicobacter infection

The C57BL/6 mouse has been shown to develop gastric adenocarcinoma after Helicobacter felis infection. This model was used to determine whether mucin and trefoil factor (TFF) expression after infection was altered in a similar fashion to the changes seen in the protective gastric mucus layer of the human stomach after H. pylori infection. Our results indicate that this mouse model mimics many of the changes seen after human H. pylori infection, including increased expression of muc4 and muc5b and loss of muc5ac. These alterations in mucin expression occurred as early as 4 weeks postinfection, before the development of significant mucous metaplasia or gastric dysplasia. The decrease in muc5ac expression occurred only in the body of the stomach and was not secondary to the adaptive immune response to infection, because a similar decrease in expression was seen after infection of B6.Rag-1(-/-) mice, which lack B and T cells. Intriguingly, the increased expression of Muc4 and Muc5b in infected C57BL/6 mice was not seen in the infected B6.Rag-1(-/-) mice. Because B6.Rag-1(-/-) mice do not develop gastric pathology after H. felis infection, these findings point to the potential role of Muc4 and Muc5b in disease progression. This manuscript contains online supplemental material at http://www.jhc.org. Please visit this article online to view these materials.

Specificity in cancer immunotherapy

From the earliest days in the field of tumor immunology three questions have been asked: do cancer cells express tumor-specific antigens, does the immune system recognize these antigens and if so, what is their biochemical nature? We now know that truly tumor-specific antigens exist, that they are caused by somatic mutations, and that these antigens can induce both humoral and cell-mediated immune responses. Because tumor-specific antigens are exclusively expressed by the cancer cell and are often crucial for tumorigenicity, they are ideal targets for anti-cancer immunotherapy. Nevertheless, the antigens that are targeted today by anti-tumor immunotherapy are not tumor-specific antigens, but antigens that are normal molecules also expressed by normal tissues (so-called "tumor-associated" antigens). If tumor-specific antigens exist and are ideal targets for immunotherapy, why are they not being targeted? In this review, we summarize current knowledge of tumor-specific antigens: their identification, immunological relevance and clinical use. We discuss novel tumor-specific epitopes and propose new approaches that could improve the success of cancer immunotherapy, especially for the treatment of solid tumors.

Rapid and sensitive analysis of mucin-type glycans using an in-line flow glycan-releasing apparatus

Rapid and sensitive analysis of glycans is essential for glycomics. We previously reported an apparatus, the AutoGlycoCutter (AGC), for rapid release of O-linked glycans under alkaline conditions and its application to rapid analysis of glycans in proteoglycans. We now report an application of the AGC to obtain mucin-type glycans with reducing end (i.e., hemiacetal group) within only 3 min. The released oligosaccharides could be labeled with fluorescent 2-aminobenzoic acid for analysis by normal-phase high-performance liquid chromatography (NP-HPLC) and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS). We could detect O-glycans from as low as 5 pmol of bovine caseino glycomacropeptide (CGMP) by the proposed procedures. The validity of the current method was shown by the analyses of the released O-glycans from some standard glycoproteins: bovine submaxillary mucin, bovine fetuin, porcine stomach mucin, and human colostrum immunoglobulin A. The advantage of the current method was also demonstrated in comparative analysis of mucin-type glycans in CGMP derived from three different animal species.

O-glycan regulation of apoptosis and proliferation in colorectal cancer cell lines

Cell growth pathways are mediated through protein–glycan interactions including O-glycosylation. Investigation of these growth pathways can be carried out using appropriate inhibitors to identify stage-specific events. We have adopted this approach to study a group of benzyl-O-N-acetyl-D-galactosamine analogues in human colorectal cancer cell lines. Exposure to O-glycan inhibitors resulted in the induction of apoptosis, a block in proliferation, accumulation of intracellular aryl-glycans and changes in related genes as detected by gene array. Colorectal cancer cell lines susceptible to the inhibitors showed growth arrest with all compounds. However, a differential action of each inhibitor was detected in the pattern of genes affected and in the structure of aryl-glycans formed.

Siglec-15: an immune system Siglec conserved throughout vertebrate evolution

Siglecs are vertebrate cell-surface receptors that recognize sialylated glycans. Here we have identified and characterized a novel Siglec, named Siglec-15. Siglec-15 is a type-I transmembrane protein consisting of: (i) two immunoglobulin (Ig)-like domains, (ii) a transmembrane domain containing a lysine residue, and (iii) a short cytoplasmic tail. Siglec-15 is expressed on macrophages and/or dendritic cells of human spleen and lymph nodes. We show that the extracellular domain of Siglec-15 preferentially recognizes the Neu5Acalpha2-6GalNAcalpha- structure. Siglec-15 associates with the activating adaptor proteins DNAX activation protein (DAP)12 and DAP10 via its lysine residue in the transmembrane domain, implying that it functions as an activating signaling molecule. Siglec-15 is the second human Siglec identified to have an activating signaling potential; unlike Siglec-14, however, it does not have an inhibitory counterpart. Orthologs of Siglec-15 are present not only in mammals but also in other branches of vertebrates; in contrast, no other known Siglec expressed in the immune system has been conserved throughout vertebrate evolution. Thus, Siglec-15 probably plays a conserved, regulatory role in the immune system of vertebrates.

Mucin-type O-glycans in human colon and breast cancer: glycodynamics and functions

The glycoproteins of tumour cells are often abnormal, both in structure and in quantity. In particular, the mucin-type O-glycans have several cancer-associated structures, including the T and Tn antigens, and certain Lewis antigens. These structural changes can alter the function of the cell, and its antigenic and adhesive properties, as well as its potential to invade and metastasize. Cancer-associated mucin antigens can be exploited in diagnosis and prognosis, and in the development of cancer vaccines. The activities and Golgi localization of glycosyltransferases are the basis for the glycodynamics of cancer cells, and determine the ranges and amounts of specific O-glycans produced. This review focuses on the glycosyltransferases of colon and breast cancer cells that determine the pathways of mucin-type O-glycosylation, and the proposed functional and pathological consequences of altered O-glycans.

Multiple hepatic receptors cooperate to eliminate secretory mucins aberrantly entering the bloodstream: are circulating cancer mucins the "tip of the iceberg"?

Hollow organs lined by columnar epithelial cells normally secrete mucins and their proteolytic fragments vectorially into the lumen. These heterogeneously O-glycosylated molecules are known to aberrantly enter the bloodstream in the setting of epithelial carcinomas and possibly during injury or inflammation. We have recently shown that carcinoma mucin fragments can trigger the rapid formation of platelet-rich microthrombi in vivo. Thus, mechanisms to clear such aberrantly secreted mucins must exist. Indeed, we found that i.v. injected carcinoma mucin fragments had an approximately 1 minute half-life in mice, which was primarily due to rapid clearance by hepatic reticuloendothelial cells. Inhibition of known glycan-recognizing hepatic clearance receptors showed involvement of multiple partially overlapping clearance systems. Studies of genetically deficient mice and incomplete competition between different mucins confirmed this result. Thus, multiple hepatic clearance receptors cooperate to eliminate secretory mucins entering the circulation, limiting potential pathology. This may also explain why mucin-type clustered O-glycosylation is rare on plasma proteins. Notably, small subsets of injected carcinoma mucins remained unrecognized by clearance systems, had a much longer half-life, and carried highly sialylated O-glycans. Similar circulating mucins were found in tumor-bearing mice despite lack of saturation of hepatic clearance mechanisms. Thus, circulating cancer mucins currently used as clinical diagnostic markers likely represent only the clearance-resistant "tip of the iceberg." Such aberrantly circulating mucins could play pathologic roles not only in cancer but also during injury or inflammation of hollow organs and in liver disease.

DMBT1 expression and glycosylation during the adenoma-carcinoma sequence in colorectal cancer

The gene DMBT1 (deleted in malignant brain tumour-1) has been proposed to play a role in brain and epithelial cancer, but shows unusual features for a classical tumour-suppressor gene. On the one hand, DMBT1 has been linked to mucosal protection, whereas, on the other, it potentially plays a role in epithelial differentiation. Thus its function in a particular tissue is of mechanistic importance for its role in cancer. Because the former function requires secretion to the lumen and the latter function may depend on its presence in the extracellular matrix, we decided to investigate DMBT1 expression, location and its mode of secretion during malignant transformation in colorectal cancer. Using human colorectal PC/AA cell lines and tissue sections from individual patients, we have examined the expression of DMBT1 and its glycosylation in the adenoma-carcinoma sequence leading to the adenocarcinoma phenotype.

Immune response to Thomsen-Friedenreich disaccharide and glycan engineering

Cancer-associated mucins show frequent alterations of their oligosaccharide chain profile, with a switch to unmask normally cryptic O-glycan backbone and core regions. Epithelial tumour cells typically show overexpression of the uncovered Gal(beta)1-3GalNAc(alpha)-O-Ser/Thr (Core 1) structure, known as the T antigen or the Thomsen-Friedenreich antigen, the oligosaccharide chain of which is called the Thomsen-Friedenreich disaccharide (TFD). T antigen expression has been associated with immunosuppression, metastasis dissemination, and the proliferation of cancer cells. Several different strategies have been used to trigger a specific immune response to TFD. Natural T antigen and synthetic TFD residues have low immunodominance. In the T antigen, flexibility of the glycosidic bond reduces the immunogenicity of the sugar residue. Enhanced rigidity should favour certain glycan conformations and thereby improve TFD immunotargeting. We propose the term 'glycan engineering' for this approach. Such engineering of TFD should reduce the flexibility of its glycan moiety and thereby enhance its stability, rigidity and immunogenicity.

Action of a library of O-glycosylation inhibitors on the growth of human colorectal cancer cells in culture

O-glycosylation is thought to play a significant role in the regulation of cell growth. However, only limited information is available, and few specific and selective inhibitors have been found. We have synthesized a library of O-glycosylation inhibitors based on benzyl-O-N-acetyl-D-galactosamine. These inhibitors were tested with an established series of human colorectal cancer cell lines, which model the adenoma-carcinoma sequence. Cancer cells were incubated with the inhibitors, and examined for cell growth patterns, and cellular and subcellular glycosylation using a range of lectins with confocal microscopy. The specificity of O-glycan inhibition was confirmed for the library, relative to other forms of glycosylation. All inhibitors tested resulted in smaller cell yields. However, a differential effect on O-glycosylation was detected using the lectins showing variation of localization at a subcellular level in the various cell lines. Further differential action of the inhibitor library was observed for apoptosis and on the cell cycle with the cell lines tested. This work demonstrates that O-glycosylation is closely involved in the regulation of cell growth in colorectal cancer cells and that the generation of a library of low-molecular-mass inhibitors offers a valuable means of examining this regulation at the molecular level.

Intestinal candyfloss

In this issue of the Biochemical Journal, Robbe et al. have employed a state-of-the-art MS technique to re-examine the sweet and sticky sugar covering of human intestinal mucosa. The availability of highly sensitive MS methods has been instrumental in determining the complex structures of mucin oligosaccharides, and in demonstrating that there are variations in structure along the sections of the gut. In contrast to previous studies, these results described by Robbe et al. show some correlation with the activities of enzymes synthesizing mucin oligosaccharides in the human colon. Many questions still remain to be answered: for example, regarding the regulation of the relative amounts of sugar chains in individual mucins, and their roles in the homoeostasis of the intestinal mucosa. Structural studies are a solid basis for understanding the functions of sugar chains, and the mechanisms and significance of changes during the development of intestinal disease.

Coming of age: carbohydrates and immunity

Adaptive immune responses have long been considered the "territory" of antigenic proteins, whereas carbohydrates are characterized as T-cell-independent antigens that are not typically recognized by the complete adaptive machinery. The current modus operandi when searching for dominant epitopes is the use of synthetic peptides designed from the primary structure of interesting target proteins; however, there is growing evidence that sugars can also play a critical role in immune recognition. Findings reported in this issue of the European Journal of Immunology begin to shed light on the differences in protein glycosylation that can occur in association with disorders like rheumatoid arthritis and the effect these changes have on collagen recognition by the immune system. Other recent studies have shown that immunodominant glycopeptide "remnant" epitopes as well as glycosylation changes on self-proteins can generate autoimmunity. Finally, some types of carbohydrates are now known to be processed and presented to T cells by class II MHC. Taken together, these advances illustrate a clear importance for carbohydrate recognition in foreign and self antigens by the adaptive immune system. With the common presence of carbohydrate molecules on eukaryotic, prokaryotic, and viral surfaces, the impact of carbohydrates on adaptive immunity is now indisputable.

Thomsen-Friedenreich antigen expression in gastric carcinomas is associated with MUC1 mucin VNTR polymorphism

Aberrant glycosylation of mucins is a common phenomenon associated with oncogenic transformation. We investigated the association between expression of the tumor-associated antigens T, Tn, and sialyl-Tn and polymorphism in the length of the MUC1 mucin tandem repeat in a series of gastric carcinomas. We further evaluated the relevance of MUC1 tandem repeat length on the expression of these tumor-associated carbohydrate antigens (TACAs) using a gastric carcinoma cell line model expressing recombinant MUC1 constructs carrying 0, 3, 9, and 42 repeats. Gastric carcinomas showed a high prevalence of Tn and sialyl-Tn antigens, whereas T antigen was less frequently expressed. The expression of T antigen was significantly higher in gastric carcinomas from patients homozygous for MUC1 large tandem repeat alleles. No significant associations were found for Tn and sialyl-Tn antigens. This novel association was reinforced by the gastric carcinoma cell line model experiments, where de novo expression of T antigen was detected in clones transfected with larger VNTR regions. Our results indicate that polymorphism in the MUC1 tandem repeat influences the expression of TACAs in gastric cancer cells and may therefore allow the identification of subgroups of patients that develop more aggressive tumors expressing T antigen.