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

Development of a Novel, Potent, and Selective Sialyltransferase Inhibitor for Suppressing Cancer Metastasis

Sialyltransferase-catalyzed membrane protein and lipid glycosylation plays a vital role as one of the most abundant post-translational modifications and diversification reactions in eukaryotes. However, aberrant sialylation has been associated with cancer malignancy and metastasis. Sialyltransferases thus represent emerging targets for the development of small molecule cancer drugs. Herein, we report the inhibitory effects of a recently discovered lithocholic acid derivative FCW393 on sialyltransferase catalytic activity, integrin sialyation, cancer-associated signal transduction, MDA-MB-231 and B16F10 cell migration and invasion, and in in vivo studies, on tumor growth, metastasis, and angiogenesis. FCW393 showed effective and selective inhibition of the sialyltransferases ST6GAL1 (IC50 = 7.8 μM) and ST3GAL3 (IC50 = 9.45 μM) relative to ST3GAL1 (IC50 > 400 μM) and ST8SIA4 (IC50 > 100 μM). FCW393 reduced integrin sialylation in breast cancer and melanoma cells dose-dependently and downregulated proteins associated with the integrin-regulated FAK/paxillin and GEF/Rho/ROCK pathways, and with the VEGF-regulated Akt/NFκB/HIF-1α pathway. FCW393 inhibited cell migration (IC50 = 2.6 μM) and invasion in in vitro experiments, and in in vivo studies of tumor-bearing mice, FCW393 reduced tumor size, angiogenesis, and metastatic potential. Based on its demonstrated selectivity, cell permeability, relatively low cytotoxicity (IC50 = 55 μM), and high efficacy, FCW393 shows promising potential as a small molecule experimental tool compound and a lead for further development of a novel cancer therapeutic.

Role of ST6GAL1 and ST6GAL2 in subversion of cellular signaling during enteroaggregative Escherichia coli infection of human intestinal epithelial cell lines

Emerging evidence have suggested that aberrant sialylation on cell-surface carbohydrate architecture may influence host-pathogen interactions. The α2,6-sialyltransferase (ST) enzymes were found to alter the glycosylation pattern of the pathogen-infected host cell-surface proteins, which could facilitate its invasion. In this study, we assessed the role of specific α2,6-ST enzymes in the regulation of enteroaggregative E. coli (EAEC)-induced cell signaling pathways in human intestinal epithelial cells. EAEC-induced expression of α2,6-ST family genes in HCT-15 and INT-407 cell lines was assessed at mRNA level by qRT-PCR. Specific esi-RNA was used to silence the target ST-gene in each of the EAEC-infected cell type. Subsequently, the role of these enzymes in regulation of EAEC-induced cell signaling pathways was unraveled by analyzing the expression of MAPkinases (ERK1/2, p38, JNK) and transcription factors (NFκB, cJun, cFos, STAT) at mRNA and protein levels by qRT-PCR and western immunoblotting, respectively, expression of selected sialoglycoproteins by western immunoblotting along with the secretory IL-8 response using sandwich ELISA. ST6GAL-1 and ST6GAL-2 were efficiently silenced in EAEC-infected HCT-15 and INT-407 cells, respectively. Significant reduction in EAEC-induced activation of MAPKs, transcription factors, sialoglycoproteins, and IL-8 secretion was noted in ST-silenced cells in comparison to the respective control cells. We propose that ST6GAL-1 and ST6GAL-2 are quintessential for EAEC-induced stimulation of MAPK-mediated pathways, resulting in activation of transcription factors, leading to an inflammatory response in the human intestinal epithelial cells. Our study may be helpful to design better therapeutic strategies to control EAEC- infection. KEY POINTS: • EAEC induces α2,6-sialyltransferase (ST) upregulation in intestinal epithelial cells • Target STs (ST6GAL-1 & ST6GAL-2) were efficiently silenced using specific esiRNAs • Expression of MAPKs, transcription factors & IL-8 was reduced in ST silenced cells.

Sialyltransferases and Neuraminidases: Potential Targets for Cancer Treatment

Cancers are the leading cause of death, causing around 10 million deaths annually by 2020. The most common cancers are those affecting the breast, lungs, colon, and rectum. However, it has been noted that cancer metastasis is more lethal than just cancer incidence and accounts for more than 90% of cancer deaths. Thus, early detection and prevention of cancer metastasis have the capability to save millions of lives. Finding novel biomarkers and targets for screening, determination of prognosis, targeted therapies, etc., are ways of doing so. In this review, we propose various sialyltransferases and neuraminidases as potential therapeutic targets for the treatment of the most common cancers, along with a few rare ones, on the basis of existing experimental and in silico data. This compilation of available cancer studies aiming at sialyltransferases and neuraminidases will serve as a guide for scientists and researchers working on possible targets for various cancers and will also provide data about the existing drugs which inhibit the action of these enzymes.

α2,6 Sialylation mediated by ST6GAL1 promotes glioblastoma growth

One of the least-investigated areas of brain pathology research is glycosylation, which is a critical regulator of cell surface protein structure and function. β-Galactoside α2,6-sialyltransferase (ST6GAL1) is the primary enzyme that α2,6 sialylates N-glycosylated proteins destined for the plasma membrane or secretion, thereby modulating cell signaling and behavior. We demonstrate a potentially novel, protumorigenic role for α2,6 sialylation and ST6GAL1 in the deadly brain tumor glioblastoma (GBM). GBM cells with high α2,6 sialylation exhibited increased in vitro growth and self-renewal capacity and decreased mouse survival when orthotopically injected. α2,6 Sialylation was regulated by ST6GAL1 in GBM, and ST6GAL1 was elevated in brain tumor-initiating cells (BTICs). Knockdown of ST6GAL1 in BTICs decreased in vitro growth, self-renewal capacity, and tumorigenic potential. ST6GAL1 regulates levels of the known BTIC regulators PDGF Receptor β (PDGFRB), Activated Leukocyte Cell Adhesion Molecule, and Neuropilin, which were confirmed to bind to a lectin-recognizing α2,6 sialic acid. Loss of ST6GAL1 was confirmed to decrease PDGFRB α2,6 sialylation, total protein levels, and the induction of phosphorylation by PDGF-BB. Thus, ST6GAL1-mediated α2,6 sialylation of a select subset of cell surface receptors, including PDGFRB, increases GBM growth.

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.

Role of a Disease-associated ST3Gal-4 in Non-small Cell Lung Cancer

Sialylation promotes tumorigenesis by affecting various cancer-related events, including apoptosis inhibition, cell growth, invasion, migration, metastasis, chemo-resistance, and immunomodulation in favor of tumor progression. An altered expression of sialyltransferase enzymes is responsible for synthesizing various tumor-associated sialylated structures. In the present study, our findings have revealed a significant up-regulation of ST3Gal-4 transcript in the two major subtypes of NSCLC cell lines [squamous cell carcinoma cell line (NCI-H520) and adenocarcinoma cell line (A549)]. Thus, the role of the ST3Gal-4 gene was assessed on cancer-associated signal transduction pathways in these cells in view of proliferation, invasion, and migration. ST3Gal-4 was silenced by transfection of both the cell lines with esi-ST3Gal-4-RNA, which RT-PCR and western immunoblotting confirmed. Silencing of ST3Gal-4 resulted in a decreased expression of MAL-I interacting membrane-HSP60, identified earlier as an α2,3-sialylated glycoprotein, thus pointing towards the possible role of ST3Gal-4 in its sialylation. The proliferation, invasion, and migration of both types of NSCLC cells were reduced significantly in the ST3Gal-4 silenced cells. Our findings were substantiated by the down-regulation of β-catenin and E-cadherin, a reduced expression of activated AKT1, ERK1/2, and NF-ƙB in these cells. We propose that ST3Gal-4 may be the disease-associated sialyltransferase involved in α2,3 sialylation of the membrane proteins, including HSP60 of the NSCLC cells. This may lead to the conformational alteration of these proteins, required for the activation of E-cadherin/β-catenin, AKT, and ERK/NF-ƙB mediated signal transduction pathways in these cells, resulting in their proliferation, invasion, and migration.

ST6Gal1: Oncogenic signaling pathways and targets

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

The incorrect use of CD75 as a synonym for ST6GAL1 has fostered the expansion of commercial "ST6GAL1" antibodies that do not recognize ST6GAL1

The ST6GAL1 Golgi sialyltransferase is upregulated in many human malignancies, however, detection of ST6GAL1 protein in cancer tissues has been hindered by the prior lack of antibodies. Recently, numerous commercial antibodies for ST6GAL1 have become available, however, many of these do not, in fact, recognize ST6GAL1. Decades ago, the CD75 cell-surface epitope was mistakenly suggested to be the same molecule as ST6GAL1. While this was rapidly disproven, the use of CD75 as a synonym for ST6GAL1 has persisted, particularly by companies selling "ST6GAL1" antibodies. CD75 is reportedly a sialylated epitope which appears to encompass a range of glycan structures and glycan carriers. In this study, we evaluated the LN1 and ZB55 monoclonal antibodies, which are advertised as ST6GAL1 antibodies but were initially developed as CD75-recognizing antibodies (neither was raised against ST6GAL1 as the immunogen). Importantly, the LN1 and ZB55 antibodies have been widely used by investigators, as well as the Human Protein Atlas database, to characterize ST6GAL1 expression. Herein, we used cell and mouse models with controlled expression of ST6GAL1 to compare LN1 and ZB55 with an extensively validated polyclonal antibody to ST6GAL1. We find that LN1 and ZB55 do not recognize ST6GAL1, and furthermore, these 2 antibodies recognize different targets. Additionally, we utilized the well-validated ST6GAL1 antibody to determine that ST6GAL1 is overexpressed in bladder cancer, a finding that contradicts prior studies which employed LN1 to suggest ST6GAL1 is downregulated in bladder cancer. Collectively, our studies underscore the need for careful validation of antibodies purported to recognize ST6GAL1.

Altered glycosylation in pancreatic cancer and beyond

Pancreatic ductal adenocarcinoma (PDA) is one of the deadliest cancers and is projected to soon be the second leading cause of cancer death. Median survival of PDA patients is 6-10 mo, with the majority of diagnoses occurring at later, metastatic stages that are refractory to treatment and accompanied by worsening prognoses. Glycosylation is one of the most common types of post-translational modifications. The complex landscape of glycosylation produces an extensive repertoire of glycan moieties, glycoproteins, and glycolipids, thus adding a dynamic and tunable level of intra- and intercellular signaling regulation. Aberrant glycosylation is a feature of cancer progression and influences a broad range of signaling pathways to promote disease onset and progression. However, despite being so common, the functional consequences of altered glycosylation and their potential as therapeutic targets remain poorly understood and vastly understudied in the context of PDA. In this review, the functionality of glycans as they contribute to hallmarks of PDA are highlighted as active regulators of disease onset, tumor progression, metastatic capability, therapeutic resistance, and remodeling of the tumor immune microenvironment. A deeper understanding of the functional consequences of altered glycosylation will facilitate future hypothesis-driven studies and identify novel therapeutic strategies in PDA.

ST6Gal-I-mediated sialylation of the epidermal growth factor receptor modulates cell mechanics and enhances invasion

Heterogeneity within the glycocalyx influences cell adhesion mechanics and signaling. However, the role of specific glycosylation subtypes in influencing cell mechanics via alterations of receptor function remains unexplored. It has been shown that the addition of sialic acid to terminal glycans impacts growth, development, and cancer progression. In addition, the sialyltransferase ST6Gal-I promotes epidermal growth factor receptor (EGFR) activity, and we have shown EGFR is an 'allosteric mechano-organizer' of integrin tension. Here, we investigated the impact of ST6Gal-I on cell mechanics. Using DNA-based tension gauge tether probes of variable thresholds, we found that high ST6Gal-I activity promotes increased integrin forces and spreading in Cos-7 and OVCAR3, OVCAR5, and OV4 cancer cells. Further, employing inhibitors and function-blocking antibodies against β1, β3, and β5 integrins and ST6Gal-I targets EGFR, tumor necrosis factor receptor, and Fas cell surface death receptor, we validated that the observed phenotypes are EGFR-specific. We found that while tension, contractility, and adhesion are extracellular-signal-regulated kinase pathway-dependent, spreading, proliferation, and invasion are phosphoinositide 3-kinase-Akt serine/threonine kinase dependent. Using total internal reflection fluorescence microscopy and flow cytometry, we also show that high ST6Gal-I activity leads to sustained EGFR membrane retention, making it a key regulator of cell mechanics. Our findings suggest a novel sialylation-dependent mechanism orchestrating cellular mechanics and enhancing cell motility via EGFR signaling.

Bacterial colonization and TH17 immunity are shaped by intestinal sialylation in neonatal mice

Interactions between the neonate host and its gut microbiome are central to the development of a healthy immune system. However, the mechanisms by which animals alter early colonization of microbiota for their benefit remain unclear. Here, we investigated the role of early-life expression of the α2,6-sialyltransferase ST6GAL1 in microbiome phylogeny and mucosal immunity. Fecal, upper respiratory, and oral microbiomes of pups expressing or lacking St6gal1 were analyzed by 16S rRNA sequencing. At weaning, the fecal microbiome of St6gal1-KO mice had reduced Clostridiodes, Coprobacillus, and Adlercreutzia, but increased Helicobacter and Bilophila. Pooled fecal microbiomes from syngeneic donors were transferred to antibiotic-treated wild-type mice, before analysis of recipient mucosal immune responses by flow cytometry, RT-qPCR, microscopy, and ELISA. Transfer of St6gal1-KO microbiome induced a mucosal Th17 response, with expression of T-bet and IL-17, and IL-22-dependent gut lengthening. Early life intestinal sialylation was characterized by RT-qPCR, immunoblot, microscopy, and sialyltransferase enzyme assays in genetic mouse models at rest or with glucocorticoid receptor modulators. St6gal1 expression was greatest in the duodenum, where it was mediated by the P1 promoter and efficiently inhibited by dexamethasone. Our data show that the inability to produce α2,6-sialyl ligands contributes to microbiome-dependent Th17 inflammation, highlighting a pathway by which the intestinal glycosylation regulates mucosal immunity.

Ninety-six-hour starved peripheral blood mononuclear cell supernatant inhibited LA7 breast cancer stem cells induced tumor via reduction in angiogenesis and alternations in Gch1 and Spr expressions

Introduction

The microenvironment of solid tumors such as breast cancer is heterogeneous and complex, containing different types of cell, namely, cancer stem cells and immune cells. We previously reported the immunoregulatory behavior of the human immune cell in a solid tumor microenvironment-like culture under serum starvation stress for 96 h. Here, we examined the effect of this culture-derived solution on breast cancer development in rats.

Method

Ninety-six-hour starved PBMCs supernatant (96 h-SPS) was collected after culturing human PBMCs for 96 h under serum starvation condition. Breast cancer stem cells, LA7 cell line, was used for in vitro study by analyzing gene expression status and performing cytotoxicity, proliferation, scratch wound healing assays, followed by in vivo tumor induction in three groups of mature female Sprague Dawley rats. Animals were treated with 96 h-SPS or RPMI and normal saline as control, n = 6 for each group. After biochemical analysis of iron, lactate, and pH levels in the dissected tumors, Ki67 antigen expression, angiogenesis, and necrosis evaluation were carried out. Metabolic-related gene expression was assessed using RT-qPCR. Moreover, 96 h-SPS composition was discovered by Nano-LC-ESI-MS/MS.

Results

96 h-SPS solution reduced the LA7 cell viability, proliferation, and migration and Gch1 and Spr genes expression in vitro (p< 0.05), whereas stemness gene Oct4 was upregulated (p< 0.01). The intracellular lactate was significantly decreased in the 96 h-SPS treated group (p = 0.007). In this group, Gch1 and Spr were significantly downregulated (p< 0.05), whereas the Sox2 and Oct4 expression was not changed significantly. The number of vessels and mitosis (Ki67⁺ cells) in the 96 h-SPS-treated group was significantly reduced (p = 0.024). The increased rate of necrosis in this group was statistically significant (p = 0.04). Last, proteomics analysis revealed candidate effectors' components of 96 h-SPS solution.

Conclusion

96 h-SPS solution may help to prevent cancer stem cell mediated tumor development. This phenomenon could be mediated through direct cytotoxic effects, inhibition of cell proliferation and migration in association with reduction in Gch1 and Spr genes expression, angiogenesis and mitosis rate, and necrosis augmentation. The preliminary data obtained from the present study need to be investigated on a larger scale and can be used as a pilot for further studies on the biology of cancer development.

Genome wide CRISPR screening reveals a role for sialylation in the tumorigenesis and chemoresistance of acute myeloid leukemia cells

Aberrant activation of cytokine and growth factor signal transduction pathways confers enhanced survival and proliferation properties to acute myeloid leukemia (AML) cells. However, the mechanisms underlying the deregulation of signaling pathways in leukemia cells are unclear. To identify genes capable of independently supporting cytokine-independent growth, we employed a genome-wide CRISPR/Cas9-mediated loss-of-function screen in GM-CSF-dependent human AML TF-1 cells. More than 182 genes (p < 0.01) were found to suppress the cytokine-independent growth of TF-1 cells. Among the top hits, genes encoding key factors involved in sialylation biosynthesis were identified; these included CMAS, SLC35A1, NANS, and GNE. Knockout of either CMAS or SLC35A1 enabled cytokine-independent proliferation and survival of AML cells. Furthermore, NSG (NOD/SCID/IL2Rγ^-/-) mice injected with CMAS or SLC35A1-knockout TF-1 cells exhibited a shorter survival than mice injected with wild-type cells. Mechanistically, abrogation of sialylation biosynthesis in TF-1 cells induced a strong activation of ERK signaling, which sensitized cells to MEK inhibitors but conferred resistance to JAK inhibitors. Further, the surface level of α2,3-linked sialic acids was negatively correlated with the sensitivity of AML cell lines to MEK/ERK inhibitors. We also found that sialylation modulated the expression and stability of the CSF2 receptor. Together, these results demonstrate a novel role of sialylation in regulating oncogenic transformation and drug resistance development in leukemia. We propose that altered sialylation could serve as a biomarker for targeted anti-leukemic therapy.

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.

Association of ST6GAL1 and CYP19A1 polymorphisms in the 3'-UTR with astrocytoma risk and prognosis in a Chinese Han population

Background

Astrocytoma is a common type of central nervous system tumor. In this study, we investigated the correlation between ST6GAL1 and CYP19A1 polymorphisms and the risk and prognosis of astrocytoma.

Methods

A total of 365 astrocytoma patients and 379 healthy controls were genotyped using the Agena MassARRAY system. The correlation between ST6GAL1 and CYP19A1 variants and astrocytoma risk was calculated using logistic regression. The survival rate of patients with astrocytoma was analyzed to evaluate prognosis.

Results

We found that the ST6GAL1-rs2239611 significantly decreased the risk of astrocytoma in the codominant model (p = 0.044) and dominant model (p = 0.049). In stratified analyses, CYP19A1-rs2255192 might be associated with a higher risk of astrocytoma among the low-grade subgroup under recessive (p = 0.034) and additive (p = 0.030) models. However, CYP19A1-rs4646 had a risk-decreasing effect on the high-grade subgroup in the codominant model (p = 0.044). The results of Cox regression analysis showed that the CYP19A1-rs2239611 and -rs1042757 polymorphisms were significantly correlated with the prognosis of astrocytoma.

Conclusion

Our results suggest that ST6GAL1 and CYP19A1 genes may be a potential biomarker of genetic susceptibility and prognosis to astrocytoma in the Chinese Han population.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12885-021-08110-1.

Downregulation of Stemness Genes and Induction of Necrosis in Rat LA7 Cancer Stem Cells Induced Tumors Treated with Starved Fibroblasts Culture Supernatant

BACKGROUND

Stem cell differentiation therapy is a promising strategy in cancer treatment. we show that protein cocktail prepared from serum starved fibroblasts has therapeutic potential based on this strategy.

METHODS

The condition medium was prepared from foreskin isolated fibroblasts and analyzed by Liquid chromatography electrospray ionization mass spectrometry-mass spectrometry (LC-ESI-MS/MS). LA7 mammary gland cancer stem cells originated tumors were induced in Sprague Dawley rats. The rats treated subcutaneously with DMEM (group A), condition medium (group B), or normal saline (group C) once daily for 7 days. Then the tumors were removed and divided into the two parts, one part was used to quantify gene expression by stem-loop RT-qPCR assay and the other part was used for Hematoxylin & Eosin (H & E), Giemsa, and immunohistochemistry (IHC) staining.

RESULTS

All induced tumors appeared as sarcomatoid carcinoma (SC). Immunohistochemistry staining confirmed this conclusion by recognizing the tumor as Ki67+, cytokeratin+, vimentine+, and estrogen receptor negative SC. RT-qPCR analysis revealed that Oct4-, Sox-2, Nanog- gene expression was much reduced in the condition medium treated tumors versus proper controls (p< 0.05). Tissue necrosis was more prevalent in this group while tumors volume was diminished almost by 40%. The LC-ESI-MS/MS analysis unrevealed the stemness reducing and the cell death inducing proteins such as, pigment epithelium-derived factor (PEDF), insulin like growth factor binding protein-5 (IGFBP-5) and -7 (IGFBP-7) in the condition medium.

CONCLUSION

This study showed that the substances released from starved human fibroblasts were able to down-regulate the stemness-related genes and induce necrosis in LA7 derived tumors.

The role of O-glycosylation in human disease

O-glycosylation is a highly frequent post-translation modification of proteins, with important functional implications in both physiological and disease contexts. The biosynthesis of O-glycans depends on several layers of regulation of the cellular glycosylation machinery, being organ-, tissue- and cell-specific. This review provides insights on the molecular mechanism underlying O-glycan biosynthesis and modification, and highlights illustrative examples of diseases that are triggered or modulated by aberrant cellular O-glycosylation. Particular relevance is given to genetic disorders of glycosylation, infectious diseases and cancer. Finally, we address the potential of O-glycans and their biosynthetic pathways as targets for novel therapeutic strategies.

Heterogeneity in 2,6-Linked Sialic Acids Potentiates Invasion of Breast Cancer Epithelia

Heterogeneity in phenotypes of malignantly transformed cells and aberrant glycan expression on their surface are two prominent hallmarks of cancers that have hitherto not been linked to each other. In this paper, we identify differential levels of a specific glycan linkage: α2,6-linked sialic acids within breast cancer cells in vivo and in culture. Upon sorting out two populations with moderate, and relatively higher, cell surface α2,6-linked sialic acid levels from the triple-negative breast cancer cell line MDA-MB-231, both populations (denoted as medium and high 2,6-Sial cells, respectively) stably retained their levels in early passages. Upon continuous culturing, medium 2,6-Sial cells recapitulated the heterogeneity of the unsorted line whereas high 2,6-Sial cells showed no such tendency. Compared with high 2,6-Sial cells, the medium 2,6-Sial counterparts showed greater adhesion to reconstituted extracellular matrices (ECMs) and invaded faster as single cells. The level of α2,6-linked sialic acids in the two sublines was found to be consistent with the expression of a specific glycosyl transferase, ST6GAL1. Stably knocking down ST6GAL1 in the high 2,6-Sial cells enhanced their invasiveness. When cultured together, medium 2,6-Sial cells differentially migrated to the edge of growing tumoroid-like cocultures, whereas high 2,6-Sial cells formed the central bulk. Multiscale simulations in a Cellular Potts model-based computational environment calibrated to our experimental findings suggest that differential levels of cell-ECM adhesion, likely regulated by α2,6-linked sialic acids, facilitate niches of highly invasive cells to efficiently migrate centrifugally as the invasive front of a malignant breast tumor.

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

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

Regulation of ST6GAL1 sialyltransferase expression in cancer cells

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

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.

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.

Bovine serum albumin affects N-glycoforms of murine IgG monoclonal antibody purified from hybridoma supernatants

Immunoglobulin G (IgG) is a class of monoclonal antibodies (mAbs) commonly produced in mammalian cell lines. These cell lines are grown in finely adjusted culture media, which contain components that may impact glycoforms. As variation of N-glycoforms can impact the biological properties of IgGs, medium composition should be controlled. Here, we studied the effects on IgG N-glycoforms of different components in hybridoma culture media, specifically compared bovine serum albumin (BSA) with other small molecules, using a matrix-assisted laser desorption/ionization quadrupole ion trap time-of-flight multistage mass spectrometry (MALDI-QIT-TOF MSⁿ)-based approach. We show that small molecular additives caused little change in glycan species, though a number of these reagents, especially glutamine, affected levels of glycosylation. In comparison, the addition of macromolecular protein BSA significantly changed IgG N-glycan patterns, not only in species but also in glycosylation levels. Together, our finding suggests that BSA increases the complexity of IgG N-glycoforms, thus raising the difficulty in maintaining glycoforms consistency during antibody production. Therefore, the effect of BSA on IgG N-glycans should be considered when designing optimal medium formulations for IgG production. KEY POINTS: • Small molecular medium additives only affect glycosylation levels of IgG N-glycans. • BSA significantly changes IgG N-glycoforms as a medium additive. • BSA's skewing of IgG N-glycoforms should be considered in IgG production.

B cells suppress medullary granulopoiesis by an extracellular glycosylation-dependent mechanism

The immune response relies on the integration of cell-intrinsic processes with cell-extrinsic cues. During infection, B cells vacate the marrow during emergency granulopoiesis but return upon restoration of homeostasis. Here we report a novel glycosylation-mediated crosstalk between marrow B cells and hematopoietic progenitors. Human B cells secrete active ST6GAL1 sialyltransferase that remodels progenitor cell surface glycans to suppress granulopoiesis. In mouse models, ST6GAL1 from B cells alters the sialylation profile of bone marrow populations, and mature IgD+ B cells were enriched in sialylated bone marrow niches. In clinical multiple myeloma, ST6GAL1 abundance in the multiple myeloma cells negatively correlated with neutrophil abundance. These observations highlight not only the ability of medullary B cells to influence blood cell production, but also the disruption to normal granulopoiesis by excessive ST6GAL1 in malignancy.

I-branched carbohydrates as emerging effectors of malignant progression

Cell surface carbohydrates, termed "glycans," are ubiquitous posttranslational effectors that can tune cancer progression. Often aberrantly displayed or found at atypical levels on cancer cells, glycans can impact essentially all progressive steps, from malignant transformation to metastases formation. Glycans are structural entities that can directly bind promalignant glycan-binding proteins and help elicit optimal receptor-ligand activity of growth factor receptors, integrins, integrin ligands, lectins, and other type-1 transmembrane proteins. Because glycans play an integral role in a cancer cell's malignant activity and are frequently uniquely expressed, preclinical studies on the suitability of glycans as anticancer therapeutic targets and their promise as biomarkers of disease progression continue to intensify. While sialylation and fucosylation have predominated the focus of cancer-associated glycan modifications, the emergence of blood group I antigens (or I-branched glycans) as key cell surface moieties capable of modulating cancer virulence has reenergized investigations into the role of the glycome in malignant progression. I-branched glycans catalyzed principally by the I-branching enzyme GCNT2 are now indicated in several malignancies. In this Perspective, the putative role of GCNT2/I-branching in cancer progression is discussed, including exciting insights on how I-branches can potentially antagonize the cancer-promoting activity of β-galactose-binding galectins.

The regulatory ZFAS1/miR-150/ST6GAL1 crosstalk modulates sialylation of EGFR via PI3K/Akt pathway in T-cell acute lymphoblastic leukemia

BACKGROUND

Noncoding RNAs, including microRNAs (miRNAs) and long non-coding RNAs (lncRNAs) are becoming key parts in the development of multidrug resistance (MDR) in T-cell acute lymphoblastic leukemia (T-ALL). Abnormal expression in sialyated N-glycans have been observed in MDR leukemia. However, the role of sialylation regulated MDR remains poorly understood. The aim of this work is to analyze the alternation of N-glycans in T-ALL MDR.

METHODS

Here, mass spectrometry (MS) is analyzed to screen the N-glycan profiles from ALL cell line CR and adriamycin (ADR)-resistant CR (CR/A) cells. The expression of sialyltransferase (ST) genes in T-ALL cell lines and bone marrow mononuclear cells (BMMCs) of T-ALL patients were analyzed using qRT-PCR. Functionally, T-ALL cell proliferation and MDR are detected through CCK8 assay, colony formation assay, western blot and flow cytometry. RIP assay and Dual-luciferase reporter gene assay confirm the binding association between ZFAS1 and miR-150. Xenograft nude mice models are used to determine the role of ST6GAL1 in vivo.

RESULTS

Elevated expression of α2, 6-sialyltransferase 1 (ST6GAL1) has been detected. The altered level of ST6GAL1 was corresponding to the drug-resistant phenotype of T-ALL cell lines both in vitro and in vivo. ZFAS1/miR-150/ST6GAL1 axis was existed in T-ALL cell lines. MiR-150 was downregulated and inversely correlated to ST6GAL1 expression. ZFAS1 was a direct target of miR-150 and positively modulated ST6GAL1 level by binding miR-150. ZFAS1/miR-150/ST6GAL1 axis functioned to regulate ADR-resistant cell growth and apoptosis. Besides, EGFR was demonstrated to be a substrate of ST6GAL1, and the sialylated EGFR had an impact on the PI3K/Akt pathway.

CONCLUSION

Results suggested that ZFAS1/miR-150/ST6GAL1 axis involves in the progression of T-ALL/MDR further mediates sialylated EGFR via PI3K/Akt pathway. This work might have an application against T-ALL MDR.

Hypoxia and serum deprivation induces glycan alterations in triple negative breast cancer cells

Triple negative breast cancer (TNBC) is a major global public health problem. The lack of targeted therapy and the elevated mortality evidence the need for better knowledge of the tumor biology. Hypoxia and aberrant glycosylation are associated with advanced stages of malignancy, tumor progression and treatment resistance. Importantly, serum deprivation regulates the invasive phenotype and favors TNBC cell survival. However, in TNBC, the role of hypoxia and serum deprivation in the regulation of glycosylation remains largely unknown. The effects of hypoxia and serum deprivation on the expression of glycosyltransferases and glycan profile were evaluated in the MDA-MB-231 cell line. We showed that the overexpression of HIF-1α was accompanied by acquisition of epithelial-mesenchimal transition features. Significant upregulation of fucosyl- and sialyltransferases involved in the synthesis of tumor-associated carbohydrate antigens was observed together with changes in fucosylation and sialylation detected by Aleuria aurantia lectin and Sambucus nigra agglutinin lectin blots. Bioinformatic analysis further indicated a mechanism by which HIF-1α can regulate ST3GAL6 expression and the relationship within the intrinsic characteristics of TNBC tumors. In conclusion, our results showed the involvement of hypoxia and serum deprivation in glycosylation profile regulation of TNBC cells triggering breast cancer aggressive features and suggesting glycosylation as a potential diagnostic and therapeutic target.

Human PBMCs fight or flight response to starvation stress: Increased T-reg, FOXP3, and TGF-β1 with decreased miR-21 and Constant miR-181c levels

Regulatory T-lymphocytes play a prominent role in autoimmunity, allergy, and cancer. In some conditions such as inflammation and tumor, immune cells are encountered with metabolic stress. Emerging evidence indicates the contribution of microRNAs in both metabolism and immune regulation. Herewith, we have examined the in vitro effects of serum starvation for 16, 48, 72 and 96 h on the expression of T-reg differentiation markers (CD4, CD25, CD127, and FOXP3) as well as on the Transforming Growth Factor-β1 (TGF-β1) and some microRNAs (miR-21,-29a,-31,146a,-155,-181a and -181c) levels in human Peripheral Blood Mononuclear Cells (PBMCs). The percentage of CD4⁺CD25⁺CD127low/-FOXP3⁺ T-regs, as well as FOXP3 expression, was increased in starved lymphocytes (p < 0.01). 96 h-starved PBMCs had the lowest T-eff/T-reg ratio (p < 0.05). All the studied miRNAs except miR-181c were significantly down-regulated in those cells (p < 0.05), in particular, miR-29a and miR-155 were sharply declined in 48h-starved PBMCs (p < 0.01). There was a negative correlation between time of starvation and microRNAs expression, except for miR-181c (r-value = -0. 61 to -0.9 and p-value = 0.037 to 0). The percentage of T-reg was inversely correlated with all miRNAs levels except for miR-31 and miR-181c (r-value = -0.68 to -0.78 and p-value = 0.015 to 0.003). FOXP3 expression exhibited a same degree of negative correlation with miR-31 and miR-155 expression levels (r = -0.57 and p =  0.05, for both). Increasing starvation duration led to a rise inTGF-β1 protein levels (p<0.01), especially its active form (P<0.001). This study introduced the serum starvation as a tool for immunoregulation which acts probably through increasing TGF-β1 production and inducing some alterations in microRNAs expression.

The ST6Gal-I sialyltransferase protects tumor cells against hypoxia by enhancing HIF-1α signaling

Aberrant cell surface glycosylation is prevalent in tumor cells, and there is ample evidence that glycans have functional roles in carcinogenesis. Nonetheless, many molecular details remain unclear. Tumor cells frequently exhibit increased α2-6 sialylation on N-glycans, a modification that is added by the ST6Gal-I sialyltransferase, and emerging evidence suggests that ST6Gal-I-mediated sialylation promotes the survival of tumor cells exposed to various cell stressors. Here we report that ST6Gal-I protects cancer cells from hypoxic stress. It is well known that hypoxia-inducible factor 1α (HIF-1α) is stabilized in hypoxic cells, and, in turn, HIF-1α directs the transcription of genes important for cell survival. To investigate a putative role for ST6Gal-I in the hypoxic response, we examined HIF-1α accumulation in ovarian and pancreatic cancer cells in ST6Gal-I overexpression or knockdown experiments. We found that ST6Gal-I activity augmented HIF-1α accumulation in cells grown in a hypoxic environment or treated with two chemical hypoxia mimetics, deferoxamine and dimethyloxalylglycine. Correspondingly, hypoxic cells with high ST6Gal-I expression had increased mRNA levels of HIF-1α transcriptional targets, including the glucose transporter genes GLUT1 and GLUT3 and the glycolytic enzyme gene PDHK1 Interestingly, high ST6Gal-I-expressing cells also had an increased pool of HIF-1α mRNA, suggesting that ST6Gal-I may influence HIF-1α expression. Finally, cells grown in hypoxia for several weeks displayed enriched ST6Gal-I expression, consistent with a pro-survival function. Taken together, these findings unravel a glycosylation-dependent mechanism that facilitates tumor cell adaptation to a hypoxic milieu.

The glycosyltransferase ST6Gal-I is enriched in cancer stem-like cells in colorectal carcinoma and contributes to their chemo-resistance

PURPOSE

Presence of cancer stem cells (CSCs) contributes to tumor outgrowth, chemo-resistance and relapse in some cancers including colorectal carcinoma (CRC). The current characterization methods of CSCs in CRC only allows enrichment of CSCs but not their purification. Recent reports showed that ST6 beta-galactoside alpha-2,6-sialyltransferase 1 (ST6Gal-I) plays an essential role in protecting tumor cells against harsh environment like oxidative stress and nutrient deprivation. Therefore, whether ST6Gal-I may be highly expressed in CSCs or whether it may enhance resistance of tumor cells to chemotherapy deserves exploration.

METHOD

ST6Gal-I levels were determined in CRC specimens, compared to paired normal colorectal tissue, and examined in CD133+ vs CD133- CRC cells, and CD44+ vs CD44- CRC cells. ST6Gal-I levels and their association with patient survival were examined. In vivo, 2 CRC cell lines Caco-2 and SW48 were transduced with two lentiviruses, one lentivirus carrying a green fluorescent protein reporter and a luciferase reporter under a cytomegalovirus promoter to allow tracing tumor cells by both fluorescence and luciferase activity, and one lentivirus carrying a nuclear red fluorescent protein under the control of ST6Gal-I promoter to allow separation of ST6Gal-I+ vs ST6Gal-I- CRC cells. Tumor sphere formation, resistance to fluorouracil-induced apoptosis, and frequency of tumor formation after serial adoptive transplantation were done on ST6Gal-I+ vs ST6Gal-I- CRC cells.

RESULT

ST6Gal-I levels were significantly upregulated in clinically obtained CRC specimens, compared to paired normal colorectal tissue. Poorer patient survival was detected in ST6Gal-I-high CRC, compared to ST6Gal-I-low subjects. Higher levels of ST6Gal-I were detected in CD133+ CRC cells than CD133- CRC cells, and in CD44+ CRC cells than in CD44- CRC cells. Compared to ST6Gal-I- CRC cells, ST6Gal-I+ CRC cells generated significantly more tumor spheres in culture, were more resistant to fluorouracil-induced apoptosis likely through upregulating cell autophagy, and generated tumor more frequently after serial adoptive transplantation.

CONCLUSION

ST6Gal-I may be highly expressed in the cancer stem-like cells in CRC and enhances cancer cell resistance to chemotherapy.

Sialylation of EGFR by the ST6Gal-I sialyltransferase promotes EGFR activation and resistance to gefitinib-mediated cell death

BACKGROUND

The ST6Gal-I sialyltransferase is upregulated in numerous cancers, and high expression of this enzyme correlates with poor patient prognosis in various malignancies, including ovarian cancer. Through its sialylation of a select cohort of cell surface receptors, ST6Gal-I modulates cell signaling to promote tumor cell survival. The goal of the present study was to investigate the influence of ST6Gal-I on another important receptor that controls cancer cell behavior, EGFR. Additionally, the effect of ST6Gal-I on cancer cells treated with the common EGFR inhibitor, gefitinib, was evaluated.

RESULTS

Using the OV4 ovarian cancer cell line, which lacks endogenous ST6Gal-I expression, a kinomics assay revealed that cells with forced overexpression of ST6Gal-I exhibited increased global tyrosine kinase activity, a finding confirmed by immunoblotting whole cell lysates with an anti-phosphotyrosine antibody. Interestingly, the kinomics assay suggested that one of the most highly activated tyrosine kinases in ST6Gal-I-overexpressing OV4 cells was EGFR. Based on these findings, additional analyses were performed to investigate the effect of ST6Gal-I on EGFR activation. To this end, we utilized, in addition to OV4 cells, the SKOV3 ovarian cancer cell line, engineered with both ST6Gal-I overexpression and knockdown, as well as the BxPC3 pancreatic cancer cell line with knockdown of ST6Gal-I. In all three cell lines, we determined that EGFR is a substrate of ST6Gal-I, and that the sialylation status of EGFR directly correlates with ST6Gal-I expression. Cells with differential ST6Gal-I expression were subsequently evaluated for EGFR tyrosine phosphorylation. Cells with high ST6Gal-I expression were found to have elevated levels of basal and EGF-induced EGFR activation. Conversely, knockdown of ST6Gal-I greatly attenuated EGFR activation, both basally and post EGF treatment. Finally, to illustrate the functional importance of ST6Gal-I in regulating EGFR-dependent survival, cells were treated with gefitinib, an EGFR inhibitor widely used for cancer therapy. These studies showed that ST6Gal-I promotes resistance to gefitinib-mediated apoptosis, as measured by caspase activity assays.

CONCLUSION

Results herein indicate that ST6Gal-I promotes EGFR activation and protects against gefitinib-mediated cell death. Establishing the tumor-associated ST6Gal-I sialyltransferase as a regulator of EGFR provides novel insight into the role of glycosylation in growth factor signaling and chemoresistance.

ST6Gal-I sialyltransferase promotes chemoresistance in pancreatic ductal adenocarcinoma by abrogating gemcitabine-mediated DNA damage

Pancreatic ductal adenocarcinoma (PDAC) is an aggressive malignancy with a poor prognosis. Gemcitabine, as a single agent or in combination therapy, remains the frontline chemotherapy despite its limited efficacy due to de novo or acquired chemoresistance. There is an acute need to decipher mechanisms underlying chemoresistance and identify new targets to improve patient outcomes. Here, we report a novel role for the ST6Gal-I sialyltransferase in gemcitabine resistance. Utilizing MiaPaCa-2 and BxPC-3 PDAC cells, we found that knockdown (KD) of ST6Gal-I expression, as well as removal of surface α2-6 sialic acids by neuraminidase, enhances gemcitabine-mediated cell death assessed via clonogenic assays and cleaved caspase 3 expression. Additionally, KD of ST6Gal-I potentiates gemcitabine-induced DNA damage as measured by comet assays and quantification of γH2AX foci. ST6Gal-I KD also alters mRNA expression of key gemcitabine metabolic genes, RRM1, RRM2, hENT1, and DCK, leading to an increased gemcitabine sensitivity ratio, an indicator of gemcitabine toxicity. Gemcitabine-resistant MiaPaCa-2 cells display higher ST6Gal-I levels than treatment-naïve cells along with a reduced gemcitabine sensitivity ratio, suggesting that chronic chemotherapy selects for clonal variants with more abundant ST6Gal-I. Finally, we examined Suit2 PDAC cells and Suit2 derivatives with enhanced metastatic potential. Intriguingly, three metastatic and chemoresistant subclones, S2-CP9, S2-LM7AA, and S2-013, exhibit up-regulated ST6Gal-I relative to parental Suit2 cells. ST6Gal-I KD in S2-013 cells increases gemcitabine-mediated DNA damage, indicating that suppressing ST6Gal-I activity sensitizes inherently resistant cells to gemcitabine. Together, these findings place ST6Gal-I as a critical player in imparting gemcitabine resistance and as a potential target to restore PDAC chemoresponse.

ST6Gal-I sialyltransferase promotes tumor necrosis factor (TNF)-mediated cancer cell survival via sialylation of the TNF receptor 1 (TNFR1) death receptor

Activation of the tumor necrosis factor receptor 1 (TNFR1) death receptor by TNF induces either cell survival or cell death. However, the mechanisms mediating these distinct outcomes remain poorly understood. In this study, we report that the ST6Gal-I sialyltransferase, an enzyme up-regulated in numerous cancers, sialylates TNFR1 and thereby protects tumor cells from TNF-induced apoptosis. Using pancreatic and ovarian cancer cells with ST6Gal-I knockdown or overexpression, we determined that α2-6 sialylation of TNFR1 had no effect on early TNF-induced signaling events, including the rapid activation of NF-κB, c-Jun N-terminal kinase (JNK), extracellular signal-regulated kinase (ERK), and Akt (occurring within 15 min). However, upon extended TNF treatment (6-24 h), cells with high ST6Gal-I levels exhibited resistance to TNF-induced apoptosis, as indicated by morphological evidence of cell death and decreased activation of caspases 8 and 3. Correspondingly, at these later time points, high ST6Gal-I expressers displayed sustained activation of the survival molecules Akt and NF-κB. Additionally, extended TNF treatment resulted in the selective enrichment of clonal variants with high ST6Gal-I expression, further substantiating a role for ST6Gal-I in cell survival. Given that TNFR1 internalization is known to be essential for apoptosis induction, whereas survival signaling is initiated by TNFR1 at the plasma membrane, we examined TNFR1 localization. The α2-6 sialylation of TNFR1 was found to inhibit TNF-induced TNFR1 internalization. Thus, by restraining TNFR1 at the cell surface via sialylation, ST6Gal-I acts as a functional switch to divert signaling toward survival. These collective findings point to a novel glycosylation-dependent mechanism that regulates the cellular response to TNF and may promote cancer cell survival within TNF-rich tumor microenvironments.

Transition state-based ST6Gal I inhibitors: Mimicking the phosphodiester linkage with a triazole or carbamate through an enthalpy-entropy compensation

Human β-galactoside α-2,6-sialyltransferase I (ST6Gal I) catalyses the synthesis of sialylated glycoconjugates. Overexpression of ST6Gal I is observed in many cancers, where it promotes metastasis through altered cell surface sialylation. A wide range of sialyltransferase inhibitors have been developed, with analogues structurally similar to the transition state exhibiting the highest inhibitory activity. To improve synthetic accessibility and pharmacokinetics of previously reported inhibitors, the replacement of the charged phosphodiester linker with a potential neutral isostere such as a carbamate or a 1,2,3-triazole has been investigated. Extensive molecular dynamics simulations have demonstrated that compounds with the alternate linkers could maintain key interactions with the human ST6Gal I active site, demonstrating the potential of a carbamate or a 1,2,3-triazole as a phosphodiester isostere. Free energy perturbation calculations provided energetic evidence suggesting that the carbamate and 1,2,3-triazole were slightly more favourable than the phosphodiester. Further exploration with free energy component, quasi-harmonic and cluster analysis suggested that there is an enthalpy-entropy compensation accounting for the replacement of the flexible charged phosphodiester with a neutral and rigid isostere. Overall, these simulations provide a strong rationale for the use of a carbamate or 1,2,3-triazole as a phosphodiester isostere in the development of novel inhibitors of human ST6Gal I.

Differential expression of ST6GAL1 in the tumor progression of colorectal cancer

Elevated expression of β-galactoside α2,6-sialyltranferase 1 (ST6GAL1) has been observed in colorectal cancer (CRC) and demonstrated to be important for its tumorigenesis. Here, we found that ST6GAL1 expression was significantly higher in non-metastatic tumors (stage I and II) than that in metastatic tumors (stage III and IV) using 62 pair-matched tumor/normal tissues. To elucidate the molecular mechanisms of how ST6GAL1 affected the CRC progression, we performed a global identification of the substrates of ST6GAL1 in the colon adenocarcinoma cell line SW480. A total of 318 membrane proteins were identified differentially affected by ST6GAL1 overexpression using metabolic labeling and proteomic analysis. Subsequent bioinformatic analysis revealed a list of potential substrates that might mediate the different functions of ST6GAL1 in CRC including cell movement, cell death and survival. Taken together, these results indicate a dynamic change in the expression of ST6GAL1 during the CRC progression and provide a list of sialylated proteins potentially relevant to the different functions of ST6GAL1 in CRC.