Sialosignaling: Sialyltransferases as engines of self-fueling loops in cancer progression

Reduced sialylation of airway mucin impairs mucus transport by altering the biophysical properties of mucin

Mucus stasis is a pathologic hallmark of muco-obstructive diseases, including cystic fibrosis (CF). Mucins, the principal component of mucus, are extensively modified with hydroxyl (O)-linked glycans, which are largely terminated by sialic acid. Sialic acid is a negatively charged monosaccharide and contributes to the biochemical/biophysical properties of mucins. Reports suggest that mucin sialylation may be altered in CF; however, the consequences of reduced sialylation on mucus clearance have not been fully determined. Here, we investigated the consequences of reduced sialylation on the charge state and conformation of the most prominent airway mucin, MUC5B, and defined the functional consequences of reduced sialylation on mucociliary transport (MCT). Reduced sialylation contributed to a lower charged MUC5B form and decreased polymer expansion. The inhibition of total mucin sialylation de novo impaired MCT in primary human bronchial epithelial cells and rat airways, and specific α-2,3 sialylation blockade was sufficient to recapitulate these findings. Finally, we show that ST3 beta-galactoside alpha-2,3-sialyltransferase (ST3Gal1) expression is downregulated in CF and partially restored by correcting CFTR via Elexacaftor/Tezacaftor/Ivacaftor treatment. Overall, this study demonstrates the importance of mucin sialylation in mucus clearance and identifies decreased sialylation by ST3Gal1 as a possible therapeutic target in CF and potentially other muco-obstructive diseases.

Specific sialylation of N-glycans and its novel regulatory mechanism

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

Reduced Sialylation of Airway Mucin Impairs Mucus Transport by Altering the Biophysical Properties of Mucin

Mucus stasis is a pathologic hallmark of muco-obstructive diseases, including cystic fibrosis (CF). Mucins, the principal component of mucus, are extensively modified with hydroxyl (O)-linked glycans, which are largely terminated by sialic acid. Sialic acid is a negatively charged monosaccharide and contributes to the biochemical/biophysical properties of mucins. Reports suggest that mucin sialylation may be altered in CF; however, the consequences of reduced sialylation on mucus clearance have not been fully determined. Here, we investigated the consequences of reduced sialylation on the charge state and conformation of the most prominent airway mucin, MUC5B, and defined the functional consequences of reduced sialylation on mucociliary transport (MCT). Reduced sialylation contributed to a lower charged MUC5B form and decreased polymer expansion. The inhibition of total mucin sialylation de novo impaired MCT in primary human bronchial epithelial cells and rat airways, and specific α-2,3 sialylation blockade was sufficient to recapitulate these findings. Finally, we show that ST3 beta-galactoside alpha-2,3-sialyltransferase (ST3Gal1) expression is downregulated in CF and partially restored by correcting CFTR via Elexacaftor/Tezacaftor/Ivacaftor treatment. Overall, this study demonstrates the importance of mucin sialylation in mucus clearance and identifies decreased sialylation by ST3Gal1 as a possible therapeutic target in CF and potentially other muco-obstructive diseases.

Sialyltransferase ST3GAL4 confers osimertinib resistance and offers strategies to overcome resistance in non-small cell lung cancer

The third-generation EGFR-TKI osimertinib is widely used in EGFR-mutated positive non-small cell lung cancer (NSCLC) patients, but drug resistance is inevitable. The currently known mechanisms only explain resistance in a small proportion of patients. For most patients, the mechanism of osimertinib resistance is still unclear, especially for EGFR-independent resistance. Herein, we thoroughly investigated the novel mechanism of osimertinib resistance and treatment strategies. We identified that ST3GAL4, a sialyltransferase, catalyzes terminal glycan sialylation of receptor protein tyrosine kinases, which induces acquired resistance to osimertinib in vitro and in vivo. In addition, ST3GAL4 is generally overexpressed in osimertinib-resistant patients with unknown resistance mechanisms. ST3GAL4 modifies MET glycosylation on N785 with sialylation, which antagonizes K48-related ubiquitin-dependent MET degradation and subsequently activates MET and its downstream proliferation signaling pathways. Meanwhile, ST3GAL4 knockdown or inhibition by brigatinib resensitizes resistant non-small cell lung cancer cells to osimertinib in vitro and in vivo This study suggests that ST3GAL4 can induce acquired resistance to osimertinib, which may be an important EGFR-independent resistance mechanism Furthermore, targeting ST3GAL4 with brigatinib provides new strategies to overcome osimertinib resistance.

Ceramide metabolism-related prognostic signature and immunosuppressive function of ST3GAL1 in osteosarcoma

Osteosarcoma is the most common primary malignant bone tumor with elevated disability and mortality rates in children and adolescents and the therapeutic effect for osteosarcoma has remained stagnant in the past 30 years. Emerging evidence has shown ceramide metabolism plays a vital role in tumor progression, but its mechanisms in osteosarcoma progression remain unknown. Through consensus clustering and LASSO regression analysis based on the osteosarcoma cohorts from TARGET database, we constructed a ceramide metabolism-related prognostic signature including ten genes for osteosarcoma, with ST3GAL1 exhibiting the highest hazard ratio. Biological signatures analysis demonstrated that ceramide metabolism was associated with immune-related pathways, immune cell infiltration and the expression of immune checkpoint genes. Single-cell profiling revealed that ceramide metabolism was enriched in myeloid, osteoblast and mesenchymal cells. The interaction between TAMs and CD8+ T cells played an essential role in osteosarcoma. ST3GAL1 regulated the SPP1-CD44 interaction between TAMs and CD8+ T cells and IL-10 secretion in TAMs through α2,3 sialic acid receptors, which inhibited CD8+ T cell function. IHC analysis showed that ST3GAL1 expression correlated with the prognosis of osteosarcoma patients. Co-culture assay revealed that upregulation of ST3GAL1 in tumor cells regulated the differentiation of TAMs and cytokine secretion. Collectively, our findings demonstrated that ceramide metabolism was associated with clinical outcome in osteosarcoma. ST3GAL1 facilitated tumor progression through regulating tumor immune microenvironment, providing a feasible therapeutic approach for patients with osteosarcoma.

Recent advances in the development of sialyltransferase inhibitors to control cancer metastasis: A comprehensive review

Metastasis accounts for the majority of cancer-associated mortalities, representing a huge health and economic burden. One of the mechanisms that enables metastasis is hypersialylation, characterized by an overabundance of sialylated glycans on the tumor surface, which leads to repulsion and detachment of cells from the original tumor. Once the tumor cells are mobilized, sialylated glycans hijack the natural killer T-cells through self-molecular mimicry and activatea downstream cascade of molecular events that result in inhibition of cytotoxicity and inflammatory responses against cancer cells, ultimately leading to immune evasion. Sialylation is mediated by a family of enzymes known as sialyltransferases (STs), which catalyse the transfer of sialic acid residue from the donor, CMP-sialic acid, onto the terminal end of an acceptor such as N-acetylgalactosamine on the cell-surface. Upregulation of STs increases tumor hypersialylation by up to 60% which is considered a distinctive hallmark of several types of cancers such as pancreatic, breast, and ovarian cancer. Therefore, inhibiting STs has emerged as a potential strategy to prevent metastasis. In this comprehensive review, we discuss the recent advances in designing novel sialyltransferase inhibitors using ligand-based drug design and high-throughput screening of natural and synthetic entities, emphasizing the most successful approaches. We analyse the limitations and challenges of designing selective, potent, and cell-permeable ST inhibitors that hindered further development of ST inhibitors into clinical trials. We conclude by analysing emerging opportunities, including advanced delivery methods which further increase the potential of these inhibitors to enrich the clinics with novel therapeutics to combat metastasis.

The Mutual Relationship between Glycosylation and Non-Coding RNAs in Cancer and Other Physio-Pathological Conditions

Glycosylation, which consists of the enzymatic addition of sugars to proteins and lipids, is one of the most important post-co-synthetic modifications of these molecules, profoundly affecting their activity. Although the presence of carbohydrate chains is crucial for fine-tuning the interactions between cells and molecules, glycosylation is an intrinsically stochastic process regulated by the relative abundance of biosynthetic (glycosyltransferases) and catabolic (glycosidases) enzymes, as well as sugar carriers and other molecules. Non-coding RNAs, which include microRNAs, long non-coding RNAs and circRNAs, establish a complex network of reciprocally interacting molecules whose final goal is the regulation of mRNA expression. Likewise, these interactions are stochastically regulated by ncRNA abundance. Thus, while protein sequence is deterministically dictated by the DNA/RNA/protein axis, protein abundance and activity are regulated by two stochastic processes acting, respectively, before and after the biosynthesis of the protein axis. Consequently, the worlds of glycosylation and ncRNA are closely interconnected and mutually interacting. In this paper, we will extensively review the many faces of the ncRNA-glycosylation interplay in cancer and other physio-pathological conditions.

Photothermal Therapy of Neuroblastoma via Polysialic Acid-Targeting Nanomissiles

Exploring new targets and developing novel targeted therapies are urgently needed for neuroblastoma therapy. Polysialic acid (polySia), a linear homopolymer of sialic acid units that correlates well with tumor progression and poor prognosis, has emerged as a potential target for neuroblastoma. However, the lack of polySia-specific binding reagents has severely limited the development of polySia-targeting therapeutics for neuroblastoma. Herein, the construction of polySia-targeting nanomissiles via molecular imprinting for the photothermal therapy of neuroblastoma is reported. Oligosialic acid (oligoSia) containing 3-4 units is considered as a characteristic structure for the recognition of polySia, while oligoSia containing 4-7 units digested from polySia is employed as the template. Via boronate-affinity controllable oriented surface imprinting, oligoSia-imprinted nanoparticles (oSia-MIP) are prepared. The oSia-MIP allows for specifically recognizing polySia and targeting polySia overexpressed neuroblastoma cells in vitro and in vivo. oSia-MIP loaded with indocyanine green is prepared and experimentally demonstrated to be a potent targeted photothermal therapeutic for neuroblastoma. Equipping the core substrate with functional entities, the developed polySia targeting nanoplatform can be accommodated to various therapeutic modalities, holding great promise for neuroblastoma targeted therapy.

alpha2,3 sialic acid processing enzymes expression in gastric cancer tissues reveals that ST3Gal3 but not Neu3 are associated with Lauren's classification, angiolymphatic invasion and histological grade

Gastric cancer (GC) is one of the leading causes of cancer-related deaths worldwide. Despite progress in the last decades, there are still no reliable biomarkers for the diagnosis of and prognosis for GC. Aberrant sialylation is a widespread critical event in the development of GC. Neuraminidases (Neu) and sialyltransferases (STs) regulate the ablation and addition of sialic acid during glycoconjugates biosynthesis, and they are a considerable source of biomarkers in various cancers. This study retrospectively characterized Neu3 and ST3Gal3 expression by immunohistochemistry in 71 paraffin-embedded GC tissue specimens and analyzed the relationship between their expression and the clinicopathological parameters. Neu3 expression was markedly increased in GC tissues compared with non-tumoral tissues (p<0.0001). Intratumoral ST3Gal3 staining was significantly associated with intestinal subtype (p=0.0042) and was negatively associated with angiolymphatic invasion (p=0.0002) and higher histological grade G3 (p=0.0066). Multivariate analysis revealed that ST3Gal3 positivity is able to predict Lauren's classification. No associations were found between Neu3 staining and clinical parameters. The in silico analysis of mRNA expression in GC validation cohorts corroborates the significant ST3Gal3 association with higher histological grade observed in our study. These findings suggest that ST3Gal3 expression may be an indicator for aggressiveness of primary GC.

Role of tumor cell sialylation in pancreatic cancer progression

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

Site-Specific Intact N-Linked Glycopeptide Characterization of Prostate-Specific Membrane Antigen from Metastatic Prostate Cancer Cells

The composition of N-linked glycans that are conjugated to the prostate-specific membrane antigen (PSMA) and their functional significance in prostate cancer progression have not been fully characterized. PSMA was isolated from two metastatic prostate cancer cell lines, LNCaP and MDAPCa2b, which have different tissue tropism and localization. Isolated PSMA was trypsin-digested, and intact glycopeptides were subjected to LC-HCD-EThcD-MS/MS analysis on a Tribrid Orbitrap Fusion Lumos mass spectrometer. Differential qualitative and quantitative analysis of site-specific N-glycopeptides was performed using Byonic and Byologic software. Comparative quantitative analysis demonstrates that multiple glycopeptides at asparagine residues 51, 76, 121, 195, 336, 459, 476, and 638 were in significantly different abundance in the two cell lines (p < 0.05). Biochemical analysis using endoglycosidase treatment and lectin capture confirm the MS and site occupancy data. The data demonstrate the effectiveness of the strategy for comprehensive analysis of PSMA glycopeptides. This approach will form the basis of ongoing experiments to identify site-specific glycan changes in PSMA isolated from disease-stratified clinical samples to uncover targets that may be associated with disease progression and metastatic phenotypes.

Comprehensive landscape of the ST3GAL family reveals the significance of ST3GAL6-AS1/ST3GAL6 axis on EGFR signaling in lung adenocarcinoma cell invasion

Sialylation aberration has been implicated in lung cancer development by altering signaling pathways. Hence, it is urgent to identify key sialyltransferases in the development of lung adenocarcinoma (LUAD), which is a common malignant subtype of non-small cell lung cancer. Herein, by systematically investigating the expression levels of ST3GAL family members in several public databases, we consistently found the frequent downregulation of ST3GAL6 in LUAD samples. Its downregulation is significantly negatively associated with stage, and significantly reduced in proximal-proliferative molecular subtype and predicts poor clinical outcomes. By protein–protein interaction network analysis and validation, we found that ST3GAL6 deficiency promotes LUAD cell invasiveness with the activated EGFR/MAPK signaling, accompanied by the elevated expression levels of matrix metalloproteinases 2 and 9, which can be partially reversed by EGFR inhibitor, gefitinib. Additionally, the ST3GAL6 level was positively regulated by ST3GAL6-AS1, an antisense long non-coding RNA to its host gene. The downregulation of ST3GAL6-AS1 also heralds a worse prognosis in LUAD patients and promotes LUAD cell invasiveness, recapitulating the function of its host gene, ST3GAL6. Altogether, ST3GAL6-AS1-regulated ST3GAL6 is a frequently downregulated sialyltransferase in LUAD patients and negatively regulates EGFR signaling, which can serve as a promising independent prognostic marker in LUAD patients.

Endothelial Glycocalyx

The glycocalyx is a polysaccharide structure that protrudes from the body of a cell. It is primarily conformed of glycoproteins and proteoglycans, which provide communication, electrostatic charge, ionic buffering, permeability, and mechanosensation-mechanotransduction capabilities to cells. In blood vessels, the endothelial glycocalyx that projects into the vascular lumen separates the vascular wall from the circulating blood. Such a physical location allows a number of its components, including sialic acid, glypican-1, heparan sulfate, and hyaluronan, to participate in the mechanosensation-mechanotransduction of blood flow-dependent shear stress, which results in the synthesis of nitric oxide and flow-mediated vasodilation. The endothelial glycocalyx also participates in the regulation of vascular permeability and the modulation of inflammatory responses, including the processes of leukocyte rolling and extravasation. Its structural architecture and negative charge work to prevent macromolecules greater than approximately 70 kDa and cationic molecules from binding and flowing out of the vasculature. This also prevents the extravasation of pathogens such as bacteria and virus, as well as that of tumor cells. Due to its constant exposure to shear and circulating enzymes such as neuraminidase, heparanase, hyaluronidase, and matrix metalloproteinases, the endothelial glycocalyx is in a continuous process of degradation and renovation. A balance favoring degradation is associated with a variety of pathologies including atherosclerosis, hypertension, vascular aging, metastatic cancer, and diabetic vasculopathies. Consequently, ongoing research efforts are focused on deciphering the mechanisms that promote glycocalyx degradation or limit its syntheses, as well as on therapeutic approaches to improve glycocalyx integrity with the goal of reducing vascular disease. © 2022 American Physiological Society. Compr Physiol 12: 1-31, 2022.

De-sialylation of glycopeptides by acid treatment: enhancing sialic acid removal without reducing the identification

Sialic acid, a common terminal monosaccharide on many glycoconjugates, plays essential roles in many biological processes such as immune responses, pathogen recognition, and cancer development. For various purposes, sialic acids may need to be removed from glycopeptides or glycans, mainly using enzymatical or chemical approaches. In this study, we found that most commonly used chemical methods couldn't completely remove sialic acids from glycopeptides. Although the de-sialylation efficiency could be further enhanced by increasing the treatment time or acid concentration, the undesirable side reactions on the peptide portion would decrease glycopeptide identification. By adding the deamidation on carbamidomethyl-cysteine (C), asparagine (N), and glutamine (Q) residues as a variable modification during database search, most of the unidentified spectra could be recovered. This optional acid-treatment and database search method for the complete removal of sialic acids without losing much spectral identification should be quite useful for many glycomic and glycoproteomic studies.

Glycosyltransferases in Cancer: Prognostic Biomarkers of Survival in Patient Cohorts and Impact on Malignancy in Experimental Models

Background: Glycosylation changes are a main feature of cancer. Some carbohydrate epitopes and expression levels of glycosyltransferases have been used or proposed as prognostic markers, while many experimental works have investigated the role of glycosyltransferases in malignancy. Using the transcriptomic data of the 21 TCGA cohorts, we correlated the expression level of 114 glycosyltransferases with the overall survival of patients. Methods: Using the Oncolnc website, we determined the Kaplan−Meier survival curves for the patients falling in the 15% upper or lower percentile of mRNA expression of each glycosyltransferase. Results: Seventeen glycosyltransferases involved in initial steps of N- or O-glycosylation and of glycolipid biosynthesis, in chain extension and sialylation were unequivocally associated with bad prognosis in a majority of cohorts. Four glycosyltransferases were associated with good prognosis. Other glycosyltransferases displayed an extremely high predictive value in only one or a few cohorts. The top were GALNT3, ALG6 and B3GNT7, which displayed a p < 1 × 10−9 in the low-grade glioma (LGG) cohort. Comparison with published experimental data points to ALG3, GALNT2, B4GALNT1, POFUT1, B4GALT5, B3GNT5 and ST3GAL2 as the most consistently malignancy-associated enzymes. Conclusions: We identified several cancer-associated glycosyltransferases as potential prognostic markers and therapeutic targets.

Extracellular sialyltransferase st6gal1 in breast tumor cell growth and invasiveness

The sialyltransferase ST6GAL1 that adds α2-6 linked sialic acids to N-glycans of cell surface and secreted glycoproteins is prominently associated with many human cancers. Tumor-native ST6GAL1 promotes tumor cell behaviors such as invasion and resistance to cell stress and chemo- and radio-treatments. Canonically, ST6GAL1 resides in the intracellular secretory apparatus and glycosylates nascent glycoproteins in biosynthetic transit. However, ST6GAL1 is also released into the extracellular milieu and extracellularly remodels cell surface and secreted glycans. The impact of this non-canonical extrinsic mechanism of ST6GAL1 on tumor cell pathobiology is not known. We hypothesize that ST6GAL1 action is the combined effect of natively expressed sialyltransferase acting cell-autonomously within the ER-Golgi complex and sialyltransferase from extracellular origins acting extrinsically to remodel cell-surface glycans. We found that shRNA knockdown of intrinsic ST6GAL1 expression resulted in decreased ST6GAL1 cargo in the exosome-like vesicles as well as decreased breast tumor cell growth and invasive behavior in 3D in vitro cultures. Extracellular ST6GAL1, present in cancer exosomes or the freely soluble recombinant sialyltransferase, compensates for insufficient intrinsic ST6GAL1 by boosting cancer cell proliferation and increasing invasiveness. Moreover, we present evidence supporting the existence novel but yet uncharacterized cofactors in the exosome-like particles that potently amplify extrinsic ST6GAL1 action, highlighting a previously unknown mechanism linking this enzyme and cancer pathobiology. Our data indicate that extracellular ST6GAL1 from remote sources can compensate for cellular ST6GAL1-mediated aggressive tumor cell proliferation and invasive behavior and has great clinical potential for extracellular ST6GAL1 as these molecules are in the extracellular space should be easily accessible targets.

The Distinct Roles of Sialyltransferases in Cancer Biology and Onco-Immunology

Aberrant glycosylation is a key feature of malignant transformation. Hypersialylation, the enhanced expression of sialic acid-terminated glycoconjugates on the cell surface, has been linked to immune evasion and metastatic spread, eventually by interaction with sialoglycan-binding lectins, including Siglecs and selectins. The biosynthesis of tumor-associated sialoglycans involves sialyltransferases, which are differentially expressed in cancer cells. In this review article, we provide an overview of the twenty human sialyltransferases and their roles in cancer biology and immunity. A better understanding of the individual contribution of select sialyltransferases to the tumor sialome may lead to more personalized strategies for the treatment of cancer.

The Cancer-Associated Antigens Sialyl Lewisa/x and Sda: Two Opposite Faces of Terminal Glycosylation

Simple Summary

The glycosyltransferase β1,4-N-acetylgalactosaminyltransferae 2 (B4GALNT2), product of the B4GALNT2 gene is responsible for the biosynthesis of the carbohydrate antigen Sd^a. Both the enzyme and its cognate antigen display a restricted pattern of tissue expression and modulation in colorectal, gastric, and mammary cancers. In colorectal cancer, B4GALNT2 is generally downregulated, but patients displaying higher expression survive longer. The sialyl Lewis^a and sialyl Lewis^x antigens are associated with malignancy. Their biosynthesis and that of Sd^a are mutually exclusive. Forced expression of B4GALNT2 in colorectal cancer cell lines modulates the transcriptome towards lower malignancy, reducing stemness. These effects are independent of B4GALNT2-induced sLe^a/sLe^x inhibition. Thus, B4GALNT2 is a marker of better prognosis and a cancer-restraining enzyme in colorectal cancer, with a therapeutic potential.

Abstract

Terminal carbohydrate structures are particularly relevant in oncology because they can serve as cancer markers and alter the phenotype of cancer cells. The Sd^a antigen and the sialyl Lewis^x and sialyl Lewis^a (sLe^x and sLe^a) antigens are terminal structures whose biosynthesis is mutually exclusive. In this review, we describe the main features of the Sd^a antigen in cancer and its relationship with sLe^x/a antigens. Information was obtained from an extensive literature search and from The Cancer Genome Atlas (TCGA) public database. The Sd^a biosynthetic enzyme B4GALNT2 undergoes downregulation in colorectal (CRC) and stomach cancer, while it is ectopically expressed by a minority of breast cancer (BRCA) patients. High expression of B4GALNT2 is associated with better prognosis and a less malignant gene expression profile in CRC, while the opposite occurs in BRCA. The regulation of B4GALNT2 expression in CRC is multifactorial, involving gene methylation and miRNA expression. Forced expression of B4GALNT2 inhibited sLe^a/sLe^x and reduced malignancy and stemness in cells constitutively expressing sLe^x/a antigens. However, consistent effects were observed upon B4GALNT2 forced expression and in cells not expressing sLe^x/a antigens. Thus, B4GALNT2 and the Sd^a antigen exert a tumor-restraining activity in CRC and probably other gastrointestinal cancers, independently of sLe^x/a antigens.

Re-Expression of Poly/Oligo-Sialylated Adhesion Molecules on the Surface of Tumor Cells Disrupts Their Interaction with Immune-Effector Cells and Contributes to Pathophysiological Immune Escape

Glycans linked to surface proteins are the most complex biological macromolecules that play an active role in various cellular mechanisms. This diversity is the basis of cell-cell interaction and communication, cell growth, cell migration, as well as co-stimulatory or inhibitory signaling. Our review describes the importance of neuraminic acid and its derivatives as recognition elements, which are located at the outermost positions of carbohydrate chains linked to specific glycoproteins or glycolipids. Tumor cells, especially from solid tumors, mask themselves by re-expression of hypersialylated neural cell adhesion molecule (NCAM), neuropilin-2 (NRP-2), or synaptic cell adhesion molecule 1 (SynCAM 1) in order to protect themselves against the cytotoxic attack of the also highly sialylated immune effector cells. More particularly, we focus on α-2,8-linked polysialic acid chains, which characterize carrier glycoproteins such as NCAM, NRP-2, or SynCam-1. This characteristic property correlates with an aggressive clinical phenotype and endows them with multiple roles in biological processes that underlie all steps of cancer progression, including regulation of cell-cell and/or cell-extracellular matrix interactions, as well as increased proliferation, migration, reduced apoptosis rate of tumor cells, angiogenesis, and metastasis. Specifically, re-expression of poly/oligo-sialylated adhesion molecules on the surface of tumor cells disrupts their interaction with immune-effector cells and contributes to pathophysiological immune escape. Further, sialylated glycoproteins induce immunoregulatory cytokines and growth factors through interactions with sialic acid-binding immunoglobulin-like lectins. We describe the processes, which modulate the interaction between sialylated carrier glycoproteins and their ligands, and illustrate that sialic acids could be targets of novel therapeutic strategies for treatment of cancer and immune diseases.

Serum Sialylation Changes in Actinic Keratosis and Cutaneous Squamous Cell Carcinoma Patients

Cutaneous squamous cell carcinoma (cSCC), a malignant proliferation of the cutaneous epithelium, is the second most common skin cancer after basal cell carcinoma (BCC). Unlike BCC, cSCC exhibits a greater aggressiveness and the ability to metastasize to any organ in the body. Chronic inflammation and immunosuppression are important processes linked to the development of cSCC. The tumor can occur de novo or from the histological transformation of preexisting actinic keratoses (AK). Malignant cells exhibit a higher amount of sialic acid in their membranes than normal cells, and changes in the amount, type, or linkage of sialic acid in malignant cell glycoconjugates are related to tumor progression and metastasis. The aim of our study was to investigate the sialyation in patients with cSCC and patients with AK. We have determined the serum levels of total sialic acid (TSA), lipid-bound sialic acid (LSA), beta-galactoside 2,6-sialyltransferase I (ST6GalI), and neuraminidase 3 (NEU3) in 40 patients with cSCC, 28 patients with AK, and 40 healthy subjects. Data analysis indicated a significant increase in serum levels of TSA (p < 0.001), LSA (p < 0.001), ST6GalI (p < 0.001), and NEU3 (p < 0.001) in the cSCC group compared to the control group, whereas in patients with AK only the serum level of TSA was significantly higher compared to the control group (p < 0.001). When the cSCC and AK groups were compared, significant differences between the serum levels of TSA (p < 0.001), LSA (p < 0.001), ST6GalI (p < 0.001) and NEU3 (p < 0.001) were found. The rate of synthesis of sialoglycoconjugates and their rate of enzymatic degradation, expressed by the ST6GalI/NEU3 ratio, is 1.64 times lower in the cSCC group compared to the control group (p < 0.01) and 1.53 times lower compared to the AK group (p < 0.01). The tumor diameter, depth of invasion, and Ki67 were associated with higher levels of TSA and LSA. These results indicate an aberrant sialylation in cSCC that correlates with tumor aggressiveness.

Structural Insights in Mammalian Sialyltransferases and Fucosyltransferases: We Have Come a Long Way, but It Is Still a Long Way Down

Mammalian cell surfaces are modified with complex arrays of glycans that play major roles in health and disease. Abnormal glycosylation is a hallmark of cancer; terminal sialic acid and fucose in particular have high levels in tumor cells, with positive implications for malignancy. Increased sialylation and fucosylation are due to the upregulation of a set of sialyltransferases (STs) and fucosyltransferases (FUTs), which are potential drug targets in cancer. In the past, several advances in glycostructural biology have been made with the determination of crystal structures of several important STs and FUTs in mammals. Additionally, how the independent evolution of STs and FUTs occurred with a limited set of global folds and the diverse modular ability of catalytic domains toward substrates has been elucidated. This review highlights advances in the understanding of the structural architecture, substrate binding interactions, and catalysis of STs and FUTs in mammals. While this general understanding is emerging, use of this information to design inhibitors of STs and FUTs will be helpful in providing further insights into their role in the manifestation of cancer and developing targeted therapeutics in cancer.

Glycobiology of the Epithelial to Mesenchymal Transition

Glycosylation consists in the covalent, enzyme mediated, attachment of sugar chains to proteins and lipids. A large proportion of membrane and secreted proteins are indeed glycoproteins, while glycolipids are fundamental component of cell membranes. The biosynthesis of sugar chains is mediated by glycosyltransferases, whose level of expression represents a major factor of regulation of the glycosylation process. In cancer, glycosylation undergoes profound changes, which often contribute to invasion and metastasis. Epithelial to mesenchymal transition (EMT) is a key step in metastasis formation and is intimately associated with glycosylation changes. Numerous carbohydrate structures undergo up- or down-regulation during EMT and often regulate the process. In this review, we will discuss the relationship with EMT of the N-glycans, of the different types of O-glycans, including the classical mucin-type, O-GlcNAc, O-linked fucose, O-linked mannose and of glycolipids. Finally, we will discuss the role in EMT of galectins, a major class of mammalian galactoside-binding lectins. While the expression of specific carbohydrate structures can be used as a marker of EMT and of the propensity to migrate, the manipulation of the glycosylation machinery offers new perspectives for cancer treatment through inhibition of EMT.

Genetic Alterations in Gliomas Remodel the Tumor Immune Microenvironment and Impact Immune-Mediated Therapies

High grade gliomas are malignant brain tumors that arise in the central nervous system, in patients of all ages. Currently, the standard of care, entailing surgery and chemo radiation, exhibits a survival rate of 14-17 months. Thus, there is an urgent need to develop new therapeutic strategies for these malignant brain tumors. Currently, immunotherapies represent an appealing approach to treat malignant gliomas, as the pre-clinical data has been encouraging. However, the translation of the discoveries from the bench to the bedside has not been as successful as with other types of cancer, and no long-lasting clinical benefits have been observed for glioma patients treated with immune-mediated therapies so far. This review aims to discuss our current knowledge about gliomas, their molecular particularities and the impact on the tumor immune microenvironment. Also, we discuss several murine models used to study these therapies pre-clinically and how the model selection can impact the outcomes of the approaches to be tested. Finally, we present different immunotherapy strategies being employed in clinical trials for glioma and the newest developments intended to harness the immune system against these incurable brain tumors.

Phenotype of ST3GAL3 deficient patients: A case and review of the literature

ST3GAL3 deficiency is an extremely rare autosomal recessive disorder caused by pathogenic mutations in the ST3GAL3 gene. Epilepsy, motor development delay, severe intellectual disability, and behavioral disorders have been reported to be associated with ST3GAL3 deficiency. In the present study, ST3GAL3 deficiency was caused by a homozygous splice-site mutation (NM_174964.4: c.936+1delG) in ST3GAL3. The patient described in this study was clinically similar to previously reported cases; nevertheless, we were able to detect repetitive behavior, previously not reported manifestations.

Glycosyltransferase B4GALNT2 as a Predictor of Good Prognosis in Colon Cancer: Lessons from Databases

BACKGROUND

glycosyltransferase B4GALNT2 and its cognate carbohydrate antigen Sd^a are highly expressed in normal colon but strongly downregulated in colorectal carcinoma (CRC). We previously showed that CRC patients expressing higher B4GALNT2 mRNA levels displayed longer survival. Forced B4GALNT2 expression reduced the malignancy and stemness of colon cancer cells.

METHODS

Kaplan-Meier survival curves were determined in "The Cancer Genome Atlas" (TCGA) COAD cohort for several glycosyltransferases, oncogenes, and tumor suppressor genes. Whole expression data of coding genes as well as miRNA and methylation data for B4GALNT2 were downloaded from TCGA.

RESULTS

the prognostic potential of B4GALNT2 was the best among the glycosyltransferases tested and better than that of many oncogenes and tumor suppressor genes; high B4GALNT2 expression was associated with a lower malignancy gene expression profile; differential methylation of an intronic B4GALNT2 gene position and miR-204-5p expression play major roles in B4GALNT2 regulation.

CONCLUSIONS

high B4GALNT2 expression is a strong predictor of good prognosis in CRC as a part of a wider molecular signature that includes ZG16, ITLN1, BEST2, and GUCA2B. Differential DNA methylation and miRNA expression contribute to regulating B4GALNT2 expression during colorectal carcinogenesis.

Terminal α2,6-sialylation of epidermal growth factor receptor modulates antibody therapy response of colorectal cancer cells

BACKGROUND

The epidermal growth factor receptor (EGFR) is a key protein involved in cancer development. Monoclonal antibodies targeting EGFR are approved for the treatment of metastatic colorectal cancer (CRC). Despite the beneficial clinical effects observed in subgroups of patients, the acquisition of resistance to treatment remains a major concern. Protein N-glycosylation of cellular receptors is known to regulate physiological processes leading to activation of downstream signaling pathways. In the present study, the role of EGFR-specific terminal ⍺2,6-sialylation was analyzed in modulation of the malignant phenotype of CRC cells and their resistance to monoclonal antibody Cetuximab-based therapy.

METHODS

Glycoengineered CRC cell models with specific sialyltransferase ST6GAL1 expression levels were applied to evaluate EGFR activation, cell surface glycosylation and therapeutic response to Cetuximab.

RESULTS

Glycoproteomic analysis revealed EGFR as a major target of ST6Gal1-mediated ⍺2,6-sialylation in a glycosite-specific manner. Mechanistically, CRC cells with increased ST6Gal1 expression and displaying terminal ⍺2,6-sialylation showed a marked resistance to Cetuximab-induced cytotoxicity. Moreover, we found that this resistance was accompanied by downregulation of EGFR expression and its activation.

CONCLUSIONS

Our data indicate that EGFR ⍺2,6-sialylation is a key factor in modulating the susceptibility of CRC cells to antibody targeted therapy, thereby disclosing a potential novel biomarker and providing key molecular information for tailor made anti-cancer strategies.

Targeting glyco-immune checkpoints for cancer therapy

Introduction: Targeting immune checkpoints with antibodies has significantly improved the outcome of cancer patients, but only few patients have long-term benefits from currently used PD-1/PD-L1 and CTLA-4 inhibitors. New approaches are needed to increase the number of patients going into long-term remission after cancer immunotherapy. Glyco-immune checkpoints are new targets for cancer immunotherapy. They are defined as immune-modulatory pathways including interactions of glycans with glycan-binding proteins or lectins. The most prominent pathway is the sialoglycan-Siglec axis and inhibitors of this axis are already successfully tested in early clinical trials.Area covered: Here, we summarize the current knowledge on glyco-immune checkpoints with a focus on the sialoglycan-Siglec axis. We also provide an overview on current approaches to clinically target glyco-immune checkpoints and give an outlook for the further clinical development of glyco-immune checkpoint targeting agents.Expert opinion: Glyco-immune checkpoints are interesting new targets to improve cancer immunotherapy. Antibodies targeting the sialoglycan-Siglec axis are already in clinical development. Other approaches with higher risk of toxicity including tumor-targeted sialidases are in late stage pre-clinical development. Despite the challenges, targeting of glyco-immune checkpoints could lead to the development of a new class of drugs providing improved anti-cancer immunity and eventually benefit cancer patients.

Recent advances in understanding the roles of sialyltransferases in tumor angiogenesis and metastasis

Abnormal glycosylation is a common characteristic of cancer cells and there is a lot of evidence that glycans can regulate the biological behavior of tumor cells. Sialylation modification, a form of glycosylation modification, plays an important role in cell recognition, cell adhesion and cell signal transduction. Abnormal sialylation on the surface of tumor cells is related to tumor migration and invasion, with abnormal expression of sialyltransferases being one of the main causes of abnormal sialylation. Recent studies provide a better understanding of the importance of the sialyltransferases, and how they influences cancer cell angiogenesis, adhesion and Epithelial-Mesenchymal Transition (EMT). The present review will provide a direction for future studies in determining the roles of sialyltransferases in cancer metastasis, and abnormal sialyltransferases are likely to be potential biomarkers for cancer.

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

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

Mucin-Type O-GalNAc Glycosylation in Health and Disease

Mucin-type GalNAc O-glycosylation is one of the most abundant and unique post-translational modifications. The combination of proteome-wide mapping of GalNAc O-glycosylation sites and genetic studies with knockout animals and genome-wide analyses in humans have been instrumental in our understanding of GalNAc O-glycosylation. Combined, such studies have revealed well-defined functions of O-glycans at single sites in proteins, including the regulation of pro-protein processing and proteolytic cleavage, as well as modulation of receptor functions and ligand binding. In addition to isolated O-glycans, multiple clustered O-glycans have an important function in mammalian biology by providing structural support and stability of mucins essential for protecting our inner epithelial surfaces, especially in the airways and gastrointestinal tract. Here the many O-glycans also provide binding sites for both endogenous and pathogen-derived carbohydrate-binding proteins regulating critical developmental programs and helping maintain epithelial homeostasis with commensal organisms. Finally, O-glycan changes have been identified in several diseases, most notably in cancer and inflammation, where the disease-specific changes can be used for glycan-targeted therapies. This chapter will review the biosynthesis, the biology, and the translational perspectives of GalNAc O-glycans.

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.

A novel nonsense and inactivating variant of ST3GAL3 in two infant siblings suffering severe epilepsy and expressing circulating CA19.9

Three missense variants of ST3GAL3 are known to be responsible for a congenital disorder of glycosylation determining a neurodevelopmental disorder (intellectual disability/epileptic encephalopathy). Here we report a novel nonsense variant, p.Y220*, in two dichorionic infant twins presenting a picture of epileptic encephalopathy with impaired neuromotor development. Upon expression in HEK-293T cells, the variant appears totally devoid of enzymatic activity in vitro, apparently accumulated with respect to the wild-type or the missense variants, as detected by western blot, and in large part properly localized in the Golgi apparatus, as assessed by confocal microscopy. Both patients were found to efficiently express the CA19.9 antigen in the serum despite the total loss of ST3GAL3 activity, which thus appears replaceable from other ST3GALs in the synthesis of the sialyl-Lewis a epitope. Kinetic studies of ST3GAL3 revealed a strong preference for lactotetraosylceramide as acceptor and gangliotetraosylceramide was also efficiently utilized in vitro. Moreover, the p.A13D missense variant, the one maintaining residual sialyltransferase activity, was found to have much lower affinity for all suitable substrates than the wild-type enzyme with an overall catalytic efficiency almost negligible. Altogether the present data suggest that the apparent redundancy of ST3GALs deduced from knock-out mouse models only partially exists in humans. In fact, our patients lacking ST3GAL3 activity synthesize the CA19.9 epitope sialyl-Lewis a, but not all glycans necessary for fine brain functions, where the role of minor gangliosides deserves further attention.

Microglia-Centered Combinatorial Strategies Against Glioblastoma

Tumor-associated microglia (MG) and macrophages (MΦ) are important components of the glioblastoma (GBM) immune tumor microenvironment (iTME). From the recent advances in understanding how MG and GBM cells evolve and interact during tumorigenesis, we emphasize the cooperation of MG with other immune cell types of the GBM-iTME, mainly MΦ and T cells. We provide a comprehensive overview of current immunotherapeutic clinical trials and approaches for the treatment of GBM, which in general, underestimate the counteracting contribution of immunosuppressive MG as a main factor for treatment failure. Furthermore, we summarize new developments and strategies in MG reprogramming/re-education in the GBM context, with a focus on ways to boost MG-mediated tumor cell phagocytosis and associated experimental models and methods. This ultimately converges in our proposal of novel combinatorial regimens that locally modulate MG as a central paradigm, and therefore may lead to additional, long-lasting, and effective tumoricidal responses.

A novel variant of ST3GAL3 causes non-syndromic autosomal recessive intellectual disability in Iranian patients

BACKGROUND

The number of reported genes causing non-syndromic autosomal recessive intellectual disability (NS-ARID) is increasing. For example, mutations in the ST3GAL3 gene have been reported to be associated with NS-ARID. In the present study, we aimed to determine the genetic cause of the NS-ARID in a five-generation consanguineous Iranian family.

METHODS

We subjected four patients with an initial diagnosis of NS-ID in an Iranian family. To identify the possible genetic cause(s), whole-exome sequencing was performed on the proband and Sanger sequencing was applied to investigate co-segregation analysis. Using in silico predictive tools, the possible impacts of the variant on the structure and function of ST3Gal-III were predicted.

RESULTS

The common clinical features were detected in all affected members who were suffering from a severe ID. Using whole-exome sequencing, a novel variant, c.704C>T or p.(Thr235Met), in exon 9 of the ST3GAL3 gene (NM_001270461.2, OMIM# 606494) was identified and verified by Sanger sequencing. This variant is located next to the VS motif of ST3Gal-III, which is a vital part of the catalytical domains.

CONCLUSIONS

In the present study, we identified a novel missense variant, c.704C>T or p.(Thr235Met), in the ST3GAL3. To our knowledge, is the third variant in this gene to be associated with NS-ARID. Our findings highlight the need for further investigations into the mechanisms by which variants in ST3GAL3 contribute to neurological dysfunction.

Impact of sialyltransferase ST6GAL1 overexpression on different colon cancer cell types

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

Characterization and distribution of sialic acids in human testicular seminoma

Aberrant content of sialic acids (Sias) has been observed in various human cancer types in different organs. Sias have been implicated in cancerous transformation, invasiveness and metastasis, and in the escaping of cancer cells from immune surveillance. Indeed, Sias are commonly regarded as important biomarkers to distinguish cancer cells from their healthy counterparts. However, scarce and not exhaustive investigations have been performed on Sia content in testicular cancers and, in particular, in seminoma, one of the most common malignant testicular tumors. Hence, the aim of this study was to investigate the content and distribution of Sias with different glycosidic linkage, namely α2,3 and α2,6 galactose- or N-acetyl-galactosamine-linked Sias and polymeric Sia (polySia), in the germinal and stromal components of human testes affected by seminoma compared to normal testicular tissue. Structural changes in seminoma tissue were examined using hematoxylin-eosin staining. α2,3 and α2,6 linked Sias were evaluated by lectin histochemistry (Maackia amurensis agglutinin (MAA) and Sambucus nigra agglutinin (SNA)), while confocal immunofluorescence was used for polySia detection. Histopathological findings in seminoma tissue included loss of seminiferous tubules replaced by clusters of uniform polygonal cells with a clear cytoplasm, bundles of fibrotic tissue, numerous microvessels and some atrophic tubules. The content of α2,3 and α2,6 linked Sias was lost in almost all seminoma components respect to normal tissue, with the exception of microvessels in which it was higher. On the contrary, polySia level was increased in all the seminoma components compared to normal testicular tissue. Our findings suggest that an aberrant content of different Sias might have important and differential roles in seminoma development and progression. In particular, polySia might be implicated in seminoma progression by promoting cancer invasiveness and regulating the cross-talk between cancer cells, reactive stroma and vessels. Thus, the possibility that polySia might represent an important biomarker for seminoma deserves further investigation.

Aberrant sialylation in ovarian cancers

Sialylation (the covalent addition of sialic acid to the terminal end of glycoproteins or glycans), tightly regulated cell- and microenvironment-specific process and orchestrated by sialyltransferases and sialidases (neuraminidases) family, is one of the posttranslational modifications, which plays an important biological role in the maintenance of normal physiology and involves many pathological dysfunctions. Glycans have roles in all the cancer hallmarks, referring to capabilities acquired during all steps of cancer development to initiate malignant transformation (a driver of a malignant genotype), enable cancer cells to survive, proliferate, and metastasize (a consequence of a malignant phenotype), which includes sustaining proliferative signaling, evading growth suppressor, resisting cell apoptosis, enabling replicative immortality, inducing angiogenesis, reprogramming of energy metabolism, evading tumor destruction, accumulating inflammatory microenvironment, and activating invasion and accelerating metastases. Regarding the important role of altered sialylation of cancers, further knowledge about the initiation and the consequences of altered sialylation pattern in tumor cells is needed, because all may offer a better chance for developing novel therapeutic strategy. In this review, we would like to update alteration of sialylation in ovarian cancers.

Sialylation of FGFR1 by ST6Gal‑I overexpression contributes to ovarian cancer cell migration and chemoresistance

Fibroblast growth factor receptors (FGFRs) have been implicated in the malignant transformation and chemoresistance of epithelial ovarian cancer; however, the underlying molecular mechanisms are poorly understood. Increased sialyltransferase activity that enhances protein sialylation is an important post‑translational process promoting cancer progression and malignancy. In the present study, α2,6‑sialyltransferase (ST6Gal‑I) overexpression or knockdown cell lines were developed, and FGFR1 was examined to understand the effect of sialylation on migration and drug resistance, and the underlying mechanisms. It was identified that cells with ST6Gal‑I overexpression had increased cell viability and migratory ability upon serum deprivation. Moreover, ST6Gal‑I overexpression cells had strong resistance to paclitaxel, as demonstrated by low growth inhibition rate and cell apoptosis level. A mechanistic study showed that ST6Gal‑I overexpression induced high α2,6‑sialylation of FGFR1 and increased the expression of phospho‑ERK1/2 and phospho‑focal adhesion kinase. Further study demonstrated that the FGFR1 inhibitor PD173047 reduced cell viability and induced apoptosis; however, ST6Gal‑I overexpression decreased the anticancer effect of PD173047. In addition, ST6Gal‑I overexpression attenuated the effect of Adriamycin on cancer cells. Collectively, these results suggested that FGFR1 sialylation plays an important role in cell migration and drug chemoresistance in ovarian cancer cells.

Sialylation of FGFR1 by ST6Gal‑I overexpression contributes to ovarian cancer cell migration and chemoresistance

Fibroblast growth factor receptors (FGFRs) have been implicated in the malignant transformation and chemoresistance of epithelial ovarian cancer; however, the underlying molecular mechanisms are poorly understood. Increased sialyltransferase activity that enhances protein sialylation is an important post‑translational process promoting cancer progression and malignancy. In the present study, α2,6‑sialyltransferase (ST6Gal‑I) overexpression or knockdown cell lines were developed, and FGFR1 was examined to understand the effect of sialylation on migration and drug resistance, and the underlying mechanisms. It was identified that cells with ST6Gal‑I overexpression had increased cell viability and migratory ability upon serum deprivation. Moreover, ST6Gal‑I overexpression cells had strong resistance to paclitaxel, as demonstrated by low growth inhibition rate and cell apoptosis level. A mechanistic study showed that ST6Gal‑I overexpression induced high α2,6‑sialylation of FGFR1 and increased the expression of phospho‑ERK1/2 and phospho‑focal adhesion kinase. Further study demonstrated that the FGFR1 inhibitor PD173047 reduced cell viability and induced apoptosis; however, ST6Gal‑I overexpression decreased the anticancer effect of PD173047. In addition, ST6Gal‑I overexpression attenuated the effect of Adriamycin on cancer cells. Collectively, these results suggested that FGFR1 sialylation plays an important role in cell migration and drug chemoresistance in ovarian cancer cells.

Sialic acid-binding immunoglobulin-like lectins (Siglecs) detect self-associated molecular patterns to regulate immune responses

The mammalian immune system evolved to tightly regulate the elimination of pathogenic microbes and neoplastic transformed cells while tolerating our own healthy cells. Here, we summarize experimental evidence for the role of Siglecs-in particular CD33-related Siglecs-as self-receptors and their sialoglycan ligands in regulating this balance between recognition of self and non-self. Sialoglycans are found in the glycocalyx and extracellular fluids and matrices of all mammalian cells and can be considered as self-associated molecular patterns (SAMPs). We also provide an overview of the known interactions of Siglec receptors and sialoglycan-SAMPs. Manipulation of the Siglec-SAMP axis offers new therapeutic opportunities for the treatment of inflammatory conditions, autoimmune diseases and also cancer immunotherapy.

Analysis of the Effect of Increased α2,3-Sialylation on RTK Activation in MKN45 Gastric Cancer Spheroids Treated with Crizotinib

In the scenario of personalized medicine, targeted therapies are currently the focus of cancer drug development. These drugs can block the growth and spread of tumor cells by interfering with key molecules involved in malignancy, such as receptor tyrosine kinases (RTKs). MET and Recepteur d'Origine Nantais (RON), which are RTKs frequently overactivated in gastric cancer, are glycoprotein receptors whose activation have been shown to be modulated by the cellular glycosylation. In this work, we address the role of sialylation in gastric cancer therapy using an innovative 3D high-throughput cell culture methodology that mimics better the in vivo tumor features. We evaluate the response to targeted treatment of glycoengineered gastric cancer cell models overexpressing the sialyltransferases ST3GAL4 or ST3GAL6 by subjecting 3D spheroids to the tyrosine kinase inhibitor crizotinib. We show here that 3D spheroids of ST3GAL4 or ST3GAL6 overexpressing MKN45 gastric cancer cells are less affected by the inhibitor. In addition, we disclose a potential compensatory pathway via activation of the Insulin Receptor upon crizotinib treatment. Our results suggest that cell sialylation, in addition of being involved in tumor progression, could play a critical role in the response to tyrosine kinase inhibitors in gastric cancer.

Resveratrol Inhibits the Tumorigenesis of Follicular Thyroid Cancer via ST6GAL2-Regulated Activation of the Hippo Signaling Pathway

Follicular thyroid carcinoma (FTC) is a common endocrine malignancy with highly aggressive features. In this study, next-generation sequencing technology was used to identify aberrant expression of sialyltransferase (ST) family members in FTC. Aberrant high expression of alpha-2,6-sialyltransferase 2 (ST6GAL2) was demonstrated to promote tumorigenesis of FTC in vitro and in vivo. Furthermore, ST6GAL2 promoted tumorigenesis by inactivating the Hippo signaling pathway. Resveratrol is a native compound extracted from Vitis species, and many studies have confirmed its protective cardiovascular and antineoplastic effects. Here we found that resveratrol can inhibit the tumorigenesis of FTC by suppressing the expression of ST6GAL2, further activating the Hippo pathway. In summary, this study revealed the role of the ST6GAL2-Hippo signaling pathway in FTC tumorigenesis and indicated that resveratrol, a commonly found antineoplastic compound, could inhibit tumorigenesis of FTC by regulating the abovementioned pathways.

Monoclonal Antibody Targeting Sialyl-di-Lewisa-Containing Internalizing and Noninternalizing Glycoproteins with Cancer Immunotherapy Development Potential

Tumor glycans constitute attractive targets for therapeutic antibodies. The sialylated glycocalyx plays a prominent role in cancer progression and immune evasion. Here, we describe the characterization of the mAb, FG129, which targets tumor-associated sialylated glycan, and demonstrate its potential for multimodal cancer therapy. FG129, obtained through BALB/c mouse immunizations with liposomes containing membrane glycan extracts from the colorectal cancer cell line LS180, is an mIgG1κ that targets sialyl-di-Lewis^a-containing glycoproteins. FG129, as well as its chimeric human IgG1 variant, CH129, binds with nanomolar functional affinity to a range of colorectal, pancreatic, and gastric cancer cell lines. FG129 targets 74% (135/182) of pancreatic, 50% (46/92) of gastric, 36% (100/281) of colorectal, 27% (89/327) of ovarian, and 21% (42/201) of non-small cell lung cancers, by IHC. In our pancreatic cancer cohort, high FG129 glyco-epitope expression was significantly associated with poor prognosis (P = 0.004). Crucially, the glyco-epitope displays limited normal tissue distribution, with FG129 binding weakly to a small percentage of cells within gallbladder, ileum, liver, esophagus, pancreas, and thyroid tissues. Owing to glyco-epitope internalization, we validated payload delivery by CH129 through monomethyl auristatin E (MMAE) or maytansinoid (DM1 and DM4) conjugation. All three CH129 drug conjugates killed high-binding colorectal and pancreatic cancer cell lines with (sub)nanomolar potency, coinciding with significant in vivo xenograft tumor control by CH129-vcMMAE. CH129, with its restricted normal tissue distribution, avid tumor binding, and efficient payload delivery, is a promising candidate for the treatment of sialyl-di-Lewis^a-expressing solid tumors, as an antibody-drug conjugate or as an alternative cancer immunotherapy modality.

Targeting Aberrant Sialylation to Treat Cancer

Cell surface carbohydrates (known as glycans) are often aberrantly expressed or found at atypical levels in cancer. Glycans can impact all steps in tumour progression, from malignant transformation to metastasis, and have roles in all the cancer hallmarks. An increased understanding of glycans in the metastatic cascade offers exciting new therapeutic opportunities. Glycan-based targeting strategies are currently being tested in clinical trials and are a rich and untapped frontier for development. As we learn more about cancer glycobiology, new targets will continue to emerge for drug design. One key change in tumour glycosylation is the upregulation of cancer-associated sialylated glycans. Abnormal sialylation is integral to tumour growth, metastasis and immune evasion; therefore, targeting sialic acid moieties in cancer could be of high therapeutic value. Here, we summarise the changes to sialic acid biology in cancer and discuss recent advances and technologies bringing sialic-acid targeting treatments to the forefront of cancer therapeutics.

Carcinoembryonic antigen is a sialyl Lewis x/a carrier and an E‑selectin ligand in non‑small cell lung cancer

The formation of distant metastasis resulting from vascular dissemination is one of the leading causes of mortality in non-small cell lung cancer (NSCLC). This metastatic dissemination initiates with the adhesion of circulating cancer cells to the endothelium. The minimal requirement for the binding of leukocytes to endothelial E-selectins and subsequent transmigration is the epitope of the fucosylated glycan, sialyl Lewis x (sLe^x), attached to specific cell surface glycoproteins. sLe^x and its isomer sialyl Lewis a (sLe^a) have been described in NSCLC, but their functional role in cancer cell adhesion to endothelium is still poorly understood. In this study, it was hypothesised that, similarly to leukocytes, sLe glycans play a role in NSCLC cell adhesion to E-selectins. To assess this, paired tumour and normal lung tissue samples from 18 NSCLC patients were analyzed. Immunoblotting and immunohisto-chemistry assays demonstrated that tumour tissues exhibited significantly stronger reactivity with anti-sLe^x/sLe^a antibody and E-selectin chimera than normal tissues (2.2- and 1.8-fold higher, respectively), as well as a higher immunoreactive score. High sLe^x/sLe^a expression was associated with bone metastasis. The overall α1,3-fucosyltransferase (FUT) activity was increased in tumour tissues, along with the mRNA levels of FUT3, FUT6 and FUT7, whereas FUT4 mRNA expression was decreased. The expression of E-selectin ligands exhibited a weak but significant correlation with the FUT3/FUT4 and FUT7/FUT4 ratios. Additionally, carcinoembryonic antigen (CEA) was identified in only 8 of the 18 tumour tissues; CEA-positive tissues exhibited significantly increased sLe^x/sLe^a expression. Tumour tissue areas expressing CEA also expressed sLe^x/sLe^a and showed reactivity to E-selectin. Blot rolling assays further demonstrated that CEA immunoprecipitates exhibited sustained adhesive interactions with E-selectin-expressing cells, suggesting CEA acts as a functional protein scaffold for E-selectin ligands in NSCLC. In conclusion, this work provides the first demonstration that sLe^x/sLe^a are increased in primary NSCLC due to increased α1,3-FUT activity. sLe^x/sLe^a is carried by CEA and confers the ability for NSCLC cells to bind E-selectins, and is potentially associated with bone metastasis. This study contributes to identifying potential future diagnostic/prognostic biomarkers and therapeutic targets for lung cancer.

ST3Gal3 confers paclitaxel‑mediated chemoresistance in ovarian cancer cells by attenuating caspase‑8/3 signaling

The aberrant expression of sialyltransferase has a role in cell differentiation, neoplastic transformation and the progression of various types of cancer. Our previous studies have shown that high expression of β-galactoside-α2,3-sialyltransferase III (ST3Gal3) in the metastatic ovarian cancer cell line HO8910PM attenuated cisplatin-induced apoptosis. The present study demonstrated that paclitaxel-induced chemoresistance in ovarian cancer cells upregulated the expression of ST3Gal3 and reduced the activity of caspase-8/3. The results of the present study revealed that the endogenous levels of ST3Gal3 mRNA and protein were significantly higher in HO8910PM cells compared with SKOV3 cells. A higher expression of ST3Gal3 was correlated with an increased resistance to paclitaxel, while the downregulation of ST3Gal3 resulted in paclitaxel-induced apoptosis. Paclitaxel upregulated ST3Gal3 expression at the mRNA and protein levels in HO8910PM cells, but not in SKOV3 cells. Silencing of ST3Gal3 by small interfering RNA reversed these effects and increased the protein levels of caspase-8/3, which may contribute to paclitaxel-induced apoptosis. The results of the present study suggested that ST3Gal3 was a target for paclitaxel-related resistance during ovarian cancer chemotherapy.

Contribution of sialic acids to integrin α5β1 functioning in melanoma cells

PURPOSE

To establish the relationship between sialylation of integrin α5β1 and possible alteration in the function of α5β1 receptor in melanoma cells.

MATERIALS AND METHODS

Integrin α5β1 was isolated from primary WM115 (RGP/VGP-like phenotype) and metastatic WM266-4 (lymph node metastasis) cells via affinity chromatography. Integrin α5β1 sialylation and the shift in relative masses of the enzymatically desialylated subunits were confirmed by confocal microscopy and SDS-PAGE, respectively. The ELISA assay was performed to evaluate sialic acid (SA) influence on integrin α5β1 binding to fibronectin (FN). Cell invasion was investigated by the Transwell invasion assay. The effect of neuraminidases treatment on melanoma cells was assessed by flow cytometry using Maackia amurensis and Sambucus nigra lectins.

RESULTS

Both subunits of integrin α5β1 were found to be more abundantly sialylated in primary than in metastatic cells. The removal of SA had no effect on the purified integrin α5β1 binding to FN. Although metastatic cells underwent more pronounced desialylation than primary cells, invasion of primary WM115 cells was more dependent on the presence of α2-3 linked SA than it was in the case of metastatic WM266-4 cells. In both melanoma cell lines not only integrin α5β1 was involved in invasion, however simultaneous desialylation and usage of anti-integrin α5β1 antibodies resulted in lower invasion abilities of primary WM115 cells.

CONCLUSIONS

Our data suggest that in primary melanoma cells integrin α5β1 action is more likely dependent on its glycosylation profile, i.e. the presence of SA residues, which influence (decreased) their invasion properties and may facilitate malignant melanoma progression.

Regular alteration of protein glycosylation in skeletal muscles of hibernating Daurian ground squirrels (Spermophilus dauricus)

Glycosylation is one of the most common post-translational protein modifications and is closely associated with muscle atrophy. This study aims to investigate the changes in glycan profiles in the fast-twitch extensor digitorum longus (EDL) muscles of Daurian ground squirrels (Spermophilus dauricus) during hibernation as well as the correlation between protein glycosylation and muscle atrophy prevention in hibernating animals. The results showed that there was no significant change in the muscle-to-body mass ratio, muscle fiber cross-sectional area (CSA), fiber distribution and ultrastructures in the EDL muscles of ground squirrels during hibernation. Alterations of six glycans comprising sialic acid α2-3 galactose (Sia2-3Gal) and Fucα1-2Galβ1-4Glc(NAc) in the EDL muscles were observed. In addition, the observed downregulation of sialyltransferase (ST3Gals) mRNA levels and upregulation of fucosyltransferase (FUT1 and FUT2) mRNA levels during hibernation and the subsequent restoration to normal levels during periodic interbout arousal were consistent with the changes in sialic acid and fucose modifications. Our results indicate that changes in ST3Gals and FUTs in the EDL muscles of Daurian ground squirrels during hibernation can alter sialylation and fucosylation of muscle glycoproteins, which may protect the skeletal muscles of hibernating Daurian ground squirrels from disuse atrophy.

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

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