Polysialyltransferase: a new target in metastatic cancer

Site-specific immobilization of the endosialidase reveals QSOX2 is a novel polysialylated protein

Polysialic acid (polySia) is a linear polymer of α2,8-linked sialic acid residues that is of fundamental biological interest due to its pivotal roles in the regulation of the nervous, immune, and reproductive systems in healthy human adults. PolySia is also dysregulated in several chronic diseases, including cancers and mental health disorders. However, the mechanisms underpinning polySia biology in health and disease remain largely unknown. The polySia-specific hydrolase, endoneuraminidase NF (EndoN), and the catalytically inactive polySia lectin EndoNDM, have been extensively used for studying polySia. However, EndoN is heat stable and remains associated with cells after washing. When studying polySia in systems with multiple polysialylated species, the residual EndoN that cannot be removed confounds data interpretation. We developed a strategy for site-specific immobilization of EndoN on streptavidin-coated magnetic beads. We showed that immobilizing EndoN allows for effective removal of the enzyme from samples, while retaining hydrolase activity. We used the same strategy to immobilize the polySia lectin EndoNDM, which enabled the enrichment of polysialylated proteins from complex mixtures such as serum for their identification via mass spectrometry. We used this methodology to identify a novel polysialylated protein, QSOX2, which is secreted from the breast cancer cell line MCF-7. This method of site-specific immobilization can be utilized for other enzymes and lectins to yield insight into glycobiology.

The Capacity of Drug-Metabolising Enzymes in Modulating the Therapeutic Efficacy of Drugs to Treat Rhabdomyosarcoma

Rhabdomyosarcoma (RMS) is a rare soft tissue sarcoma (STS) that predominantly affects children and teenagers. It is the most common STS in children (40%) and accounts for 5-8% of total childhood malignancies. Apart from surgery and radiotherapy in eligible patients, standard chemotherapy is the only therapeutic option clinically available for RMS patients. While survival rates for this childhood cancer have considerably improved over the last few decades for low-risk and intermediate-risk cases, the mortality rate remains exceptionally high in high-risk RMS patients with recurrent and/or metastatic disease. The intensification of chemotherapeutic protocols in advanced-stage RMS has historically induced aggravated toxicity with only very modest therapeutic gain. In this review, we critically analyse what has been achieved so far in RMS therapy and provide insight into how a diverse group of drug-metabolising enzymes (DMEs) possess the capacity to modify the clinical efficacy of chemotherapy. We provide suggestions for new therapeutic strategies that exploit the presence of DMEs for prodrug activation, targeted chemotherapy that does not rely on DMEs, and RMS-molecular-subtype-targeted therapies that have the potential to enter clinical evaluation.

Glycation Interferes with the Expression of Sialyltransferases and Leads to Increased Polysialylation in Glioblastoma Cells

Glioblastoma (GBM) is a highly aggressive brain tumor that often utilizes aerobic glycolysis for energy production (Warburg effect), resulting in increased methylglyoxal (MGO) production. MGO, a reactive dicarbonyl compound, causes protein alterations and cellular dysfunction via glycation. In this study, we investigated the effect of glycation on sialylation, a common post-translational modification implicated in cancer. Our experiments using glioma cell lines, human astrocytes (hA), and primary glioma samples revealed different gene expressions of sialyltransferases among cells, highlighting the complexity of the system. Glycation has a differential effect on sialyltransferase expression, upregulating ST8SIA4 in the LN229 and U251 cell lines and decreasing the expression in normal hA. Subsequently, polysialylation increased in the LN229 and U251 cell lines and decreased in hA. This increase in polysialylation could lead to a more aggressive phenotype due to its involvement in cancer hallmark processes such as immune evasion, resistance to apoptosis, and enhancing invasion. Our findings provide insights into the mechanisms underlying GBM aggressiveness and suggest that targeting glycation and sialylation could be a potential therapeutic strategy.

The NMR studies of CMP inhibition of polysialylation

The overexpression of polysialic acid (polySia) on neural cell adhesion molecules (NCAM) promotes hypersialylation, and thus benefits cancer cell migration and invasion. It has been proposed that the binding between the polysialyltransferase domain (PSTD) and CMP-Sia needs to be inhibited in order to block the effects of hypersialylation. In this study, CMP was confirmed to be a competitive inhibitor of polysialyltransferases (polySTs) in the presence of CMP-Sia and triSia (oligosialic acid trimer) based on the interactional features between molecules. The further NMR analysis suggested that polysialylation could be partially inhibited when CMP-Sia and polySia co-exist in solution. In addition, an unexpecting finding is that CMP-Sia plays a role in reducing the gathering extent of polySia chains on the PSTD, and may benefit for the inhibition of polysialylation. The findings in this study may provide new insight into the optimal design of the drug and inhibitor for cancer treatment.

Glycocalyx transduces membrane leak in brain tumor cells exposed to sharp magnetic pulsing

Mechanisms by which electric (E) or magnetic (B) fields might be harnessed to affect tumor cell behavior remain poorly defined, presenting a barrier to translation. We hypothesized in early studies that the glycocalyx of lung cancer cells might play a role in mediating plasma membrane leak by low-frequency pulsed magnetic fields (Lf-PMF) generated on a low-energy solenoid platform. In testing glioblastoma and neuroblastoma cells known to overexpress glycoproteins rich in modifications by the anionic glycan sialic acid (Sia), exposure of brain tumor cells on the same platform to a pulse train that included a 5 min 50Hz Lf-PMF (dB/dt ∼ 2 T/s at 10 ms pulse widths) induced a very modest but significant protease leak above that of control nonexposed cells (with modest but significant reductions in long-term tumor cell viability after the 5 min exposure). Using a markedly higher dB/dt system (80 T/s pulses, 70 μs pulse-width at 5.9 cm from a MagVenture coil source) induced markedly greater leak by the same cells, and eliminating Sia by treating cells with AUS sialidase immediately preexposure abrogated the effect entirely in SH-SY5Y neuroblastoma cells, and partially in T98G glioblastoma cells. The system demonstrated significant leak (including inward leak of propidium iodide), with reduced leak at lower dB/dt in a variety of tumor cells. The ability to abrogate Lf-PMF protease leak by pretreatment with sialidase in SH-SY5Y brain tumor cells or with heparin lyase in A549 lung tumor cells indicated the importance of heavy Sia or heparan sulfate glycosaminoglycan glycocalyx modifications as dominant glycan species mediating Lf-PMF membrane leak in respective tumor cells. This "first-physical" Lf-PMF tumor glycocalyx event, with downstream cell stress, may represent a critical and "tunable" transduction mechanism that depends on characteristic anionic glycans overexpressed by distinct malignant tumors.

Cancer-specific glycosylation of CD13 impacts its detection and activity in preclinical cancer tissues

Harnessing the differences between cancer and non-cancer tissues presents new opportunities for selective targeting by anti-cancer drugs. CD13, a heavily glycosylated protein, is one example with significant unmet clinical potential in cancer drug discovery. Despite its high expression and activity in cancers, CD13 is also expressed in many normal tissues. Here, we report differential tissue glycosylation of CD13 across tissues and demonstrate for the first time that the nature and pattern of glycosylation of CD13 in preclinical cancer tissues are distinct compared to normal tissues. We identify cancer-specific O-glycosylation of CD13, which selectively blocks its detection in cancer models but not in normal tissues. In addition, the metabolism activity of cancer-expressed CD13 was observed to be critically dependent on its unique glycosylation. Thus, our data demonstrate the existence of discrete cancer-specific CD13 glycoforms and propose cancer-specific CD13 glycoforms as a clinically useful target for effective cancer-targeted therapy.

The cancer-associated glycan polysialic acid is dysregulated in systemic sclerosis and is associated with fibrosis

OBJECTIVE

Systemic sclerosis (SSc) is a rare but deadly disease characterized by autoimmunity, vasculopathy, and fibrosis. Fibrotic complications associated with SSc correlate with severe morbidity and mortality. Previous studies in SSc have identified fibroblasts as the primary drivers of fibrosis; however, the mechanism(s) promoting this are not well understood. Aberrant glycosylation, particularly polysialylation (polySia), has been described as a prominent feature of aggressive cancers. Inspired by this observation, we aimed to determine if polySia is dysregulated in various forms of SSc.

METHODS

All patients with SSc met the 2013 ACR/EULAR. Patients were sub-classified into limited cutaneous (lSSc, N = 5 or 46 patients for polySia quantification in the dermis or serum; respectively), diffuse cutaneous (dSSc, N = 11 or 18 patients for polySia quantification in the dermis or serum; respectively), or patients with dSSc treated with an autologous stem cell transplantation (post-ASCT, N = 4 patients for quantification in the dermis). Dermal polySia levels were measured via immunofluorescence microscopy in 10 μm dermal sections, quantified in each group (healthy volunteers (HC), lSSc, dSSc, and post-ASCT) and correlated with skin fibrosis (via the modified Rodnan skin score (mRSS)). Similarly, serum polySia was quantified in each group, and correlated with the mRSS.

RESULTS

Dermal polySia levels were highest in patients with dSSc (compared to HC < 0.001), and correlated with the degree of fibrosis in all of the groups (P = 0.008). Serum polySia was higher in all SSc groups (p < 0.001) and correlated with the severity of mRSS (p < 0.0001).

CONCLUSION

Polysia is more abundant in the skin and sera from patients with SSc and correlates with the degree of skin fibrosis. The aberrant expression of polySia highlights its potential use as a biomarker in patients with progressive forms of SSc. Dysregulated polySia levels in SSc further emphasizes the cancer-like phenotype present in SSc, which may promote fibrosis and immune dysregulation.

The vertebrate sialylation machinery: structure-function and molecular evolution of GT-29 sialyltransferases

Every eukaryotic cell is covered with a thick layer of complex carbohydrates with essential roles in their social life. In Deuterostoma, sialic acids present at the outermost positions of glycans of glycoconjugates are known to be key players in cellular interactions including host-pathogen interactions. Their negative charge and hydrophilic properties enable their roles in various normal and pathological states and their expression is altered in many diseases including cancers. Sialylation of glycoproteins and glycolipids is orchestrated by the regulated expression of twenty sialyltransferases in human tissues with distinct enzymatic characteristics and preferences for substrates and linkages formed. However, still very little is known on the functional organization of sialyltransferases in the Golgi apparatus and how the sialylation machinery is finely regulated to provide the ad hoc sialome to the cell. This review summarizes current knowledge on sialyltransferases, their structure-function relationships, molecular evolution, and their implications in human biology.

Attenuation of Polysialic Acid Biosynthesis in Cells by the Small Molecule Inhibitor 8-Keto-sialic acid

Sialic acids are key mediators of cell function, particularly with regard to cellular interactions with the surrounding environment. Reagents that modulate the display of specific sialyl glycoforms at the cell surface would be useful biochemical tools and potentially allow for therapeutic intervention in numerous challenging chronic diseases. While multiple strategies are being explored for the control of cell surface sialosides, none that shows high selectivity between sialyltransferases or that targets a specific sialyl glycoform has yet to emerge. Here, we describe a strategy to block the formation of α2,8-linked sialic acid chains (oligo- and polysialic acid) through the use of 8-keto-sialic acid as a chain-terminating metabolic inhibitor that, if incorporated, cannot be elongated. 8-Keto-sialic acid is nontoxic at effective concentrations and serves to block polysialic acid synthesis in cancer cell lines and primary immune cells, with minimal effects on other sialyl glycoforms.

Milk Polysialic Acid Levels Rapidly Decrease in Line with the N-Acetylneuraminic Acid Concentrations during Early Lactation in Dairy Cows

Sialylated milk oligosaccharides and glycoconjugates have several positive effects on the mucosal barrier, the gut microbiome, and an effective immune system. For this reason, they are important biomolecules for mammary gland health and optimal development of offspring. In milk, the major sialic acid, N-acetylneuraminic acid (Neu5Ac), can be attached as monosialyl-residues or as polymers. To investigate the sialylation processes during lactation of German Holstein cows, we analyzed udder tissue in addition to milk at different time points of lactation. The analysis of the milk samples revealed that both the levels of Neu5Ac and its polymer, polysialic acid (polySia), rapidly decreased during the first three days of lactation, and a high interindividual variance was observed. In mature milk, however, the sialylation status remains relatively constant. The results indicate that mammary gland epithelial cells are one source for milk polySia, since immunohistochemistry of udder tissue exhibited strong polySia staining in these cells. Furthermore, both polysialyltransferases, ST8SiaII and ST8SiaIV, are expressed. Based on known functions of monosialyl residues and polySia, we discuss the potential impact of these biomolecules and the consequences of the heterogeneous sialylation status of milk in relation to udder health and offspring health.

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.

No Impact of PolySia-NCAM Expression on Treatment Response in Neuroendocrine Neoplasms of the Lung

Simple Summary

Polysialic acids (polySia) are localized on the neuronal cell adhesion molecule (NCAM). They are expressed on numerous tumors of neural crest origin. These include lung neuroendocrine tumors such as atypical carcinoid, large cell neuroendocrine and small cell carcinomas. Interfering with polySia is considered a potential approach in the development of tumor therapies. In this study, we investigated whether polySia expression has an impact on disease progression, treatment response, and prognosis. To this end, tissue samples from 28 patients were analyzed by immunohistochemistry for polySia-NCAM presence. In conclusion, NCAM-polySia is not very useful as a prognostic factor for poor disease outcome. However, it is still interesting as a therarpeutic target for individual tumor therapy, as a majority of patients (78.6%) showed a strong staining signal for NCAM-polySia.

Abstract

Background: Polysialic acids (abbr. polySia) are found on numerous tumors, including neuroendocrine lung tumors. They have previously been shown to impact metastatic potential, as they can influence the signaling and adhesion properties of neuronal cell adhesion molecules (abbr. NCAM) and other cell adhesion molecules. Therefore, the aim of this small pilot study was to analyze whether there was a correlation between polySia-NCAM expression and specific clinical or histopathologic characteristics, and if polySia-NCAM expression had an impact on treatment response, disease progression and prognosis of lung neuroendocrine neoplasms. Methods: This work was based on an analysis of 28 digitized patient records and corresponding patient samples. The response to therapy was radiologically determined at the time of diagnosis and at certain intervals during therapy following the current RECIST1.1 and volumetric sphere calculation. To analyze whether polySia-NCAM expression had prognostic relevance, polySia-NCAM-positive and -negative cases were compared in a Kaplan-Meier survival analysis. Findings: A majority of 78.6% lung neuroendocrine neoplasms showed a strong staining signal for polySia-NCAM. There was a significant correlation between expression and histopathological grade (p = 0.0140), since carcinoids were less likely polySia-NCAM-positive compared to small cell lung carcinoma (abbr. SCLC) and large cell neuroendocrine carcinomas of the lung (abbr. LCNEC). There was no significant association between polySia-NCAM expression and clinical characteristics (age: p = 0.3405; gender: p = 0.6730; smoking history: p = 0.1145; ECOG: p = 0.1756, UICC8 stage: p = 0.1182) or radiologically determined disease progression, regardless of the criteria used to categorize response (RECIST 1.1: p = 0.0759; sphere: p = 0.0580). Furthermore, polySia-NCAM expression did not affect progression-free survival (p = 0.4198) or overall survival (p = 0.6918). Interpretation: PolySia-NCAM expression was more common in high-grade compared to low-grade neuroendocrine neoplasms of the lung; however, this small pilot study failed to show an association between polySia-NCAM expression and response to therapy.

Aberrant Sialylation in Cancer: Therapeutic Opportunities

The surface of every eukaryotic cell is coated in a thick layer of glycans that acts as a key interface with the extracellular environment. Cancer cells have a different ‘glycan coat’ to healthy cells and aberrant glycosylation is a universal feature of cancer cells linked to all of the cancer hallmarks. This means glycans hold huge potential for the development of new diagnostic and therapeutic strategies. One key change in tumour glycosylation is increased sialylation, both on N-glycans and O-glycans, which leads to a dense forest of sialylated structures covering the cell surface. This hypersialylation has far-reaching consequences for cancer cells, and sialylated glycans are fundamental in tumour growth, metastasis, immune evasion and drug resistance. The development of strategies to inhibit aberrant sialylation in cancer represents an important opportunity to develop new therapeutics. Here, I summarise recent advances to target aberrant sialylation in cancer, including the development of sialyltransferase inhibitors and strategies to inhibit Siglecs and Selectins, and discuss opportunities for the future.

Polysialic Acid Sustains the Hypoxia-Induced Migration and Undifferentiated State of Human Glioblastoma Cells

Gliomas are the most common primary malignant brain tumors. Glioblastoma, IDH-wildtype (GBM, CNS WHO grade 4) is the most aggressive form of glioma and is characterized by extensive hypoxic areas that strongly correlate with tumor malignancy. Hypoxia promotes several processes, including stemness, migration, invasion, angiogenesis, and radio- and chemoresistance, that have direct impacts on treatment failure. Thus, there is still an increasing need to identify novel targets to limit GBM relapse. Polysialic acid (PSA) is a carbohydrate composed of a linear polymer of α2,8-linked sialic acids, primarily attached to the Neural Cell Adhesion Molecule (NCAM). It is considered an oncodevelopmental antigen that is re-expressed in various tumors. High levels of PSA-NCAM are associated with high-grade and poorly differentiated tumors. Here, we investigated the effect of PSA inhibition in GBM cells under low oxygen concentrations. Our main results highlight the way in which hypoxia stimulates polysialylation in U87-MG cells and in a GBM primary culture. By lowering PSA levels with the sialic acid analog, F-NANA, we also inhibited GBM cell migration and interfered with their differentiation influenced by the hypoxic microenvironment. Our findings suggest that PSA may represent a possible molecular target for the development of alternative pharmacological strategies to manage a devastating tumor like GBM.

Polymeric nanoparticles surface-complexed with boric acid actively target solid tumors overexpressing sialic acid

Sialic acid is a fundamental component of the tumor microenvironment, modulates cell-cell and cell-extracellular matrix interactions and is associated with bad prognosis and clinical outcomes in different cancers. Capitalizing on the ability of boric acid to form cyclic esters with diols, in this work, we design self-assembled multi-micellar colloidal systems of an amphiphilic poly(vinyl alcohol)-g-poly(methyl methacrylate) copolymer surface-modified with boric acid for the active targeting of solid tumors that overexpress sialic acid. Nanoparticles display sizes in the 100-200 nm range and a spherical morphology, as determined by dynamic light scattering and high resolution-scanning electron microscopy, respectively. The uptake and anti-proliferative activity are assessed in 2D and 3D models of rhabdomyosarcoma in vitro. Surface boration increases the nanoparticle permeability and uptake, especially in rhabdomyosarcoma spheroids that overexpress sialic acid to a greater extent than 2D cultures. The biodistribution of non-borated and borated nanoparticles upon intravenous injection to a subcutaneous rhabdomyosarcoma murine xenograft model confirm a statistically significant increase in the intertumoral accumulation of the modified nanocarriers with respect to the unmodified counterparts and a sharp decrease in major clearance organs such as the liver. Overall, our results highlight the promise of these borated nanomaterials to actively target hypersialylated solid tumors.

The different prognostic significance of polysialic acid and CD56 expression in tumor cells and lymphocytes identified in breast cancer

Protein glycosylation, the attachment of carbohydrates onto proteins, is a fundamental process that alters the biological activity of proteins. Changes to glycosylation states are associated with many forms of cancer including breast cancer. Through immunohistological analysis of breast cancer patient tumors, we have discovered the expression of an atypical glycan-polysialic acid (polySia)-in breast cancer. Notably, we have identified polySia expression in not only tumor cells but also on tumor-infiltrating lymphocytes (TILs) and our study reveals ST8Sia4 as the predominant polysialyltransferase expressed. Evaluation of ST8Sia4 expression in tumor cells identified an association between high expression levels and poor patient outcomes whereas ST8Sia4 expression in infiltrating stromal cells was associated with good patient outcomes. Investigation into CD56, a protein known to be polysialylated, found CD56 and polySia expression on breast tumor cells and TILs. CD56 expression did not positively correlate with polySia expression except in patient tumors which expressed HER2. In these HER2 expressing tumors, CD56 expression was significantly associated with HER2 expression score. Evaluation of CD56 tumor cell expression identified a significant association between CD56 expression and poor patient outcomes. By contrast, CD56 expression on TILs was significantly associated with good clinical outcomes. Tumors with CD56+ TILs were also consistently polySia TIL positive. Interestingly, in tumors where TILs were CD56 low-to-negative, a polySia+ lymphocyte population was still identified and the presence of these lymphocytes was a poor prognostic indicator. Overall, this study provides the first detailed report of polySia and CD56 in breast cancer and demonstrates that the prognostic significance is dependent on the cell type expression within the tumor.

Comprehensive Analysis of Oligo/Polysialylglycoconjugates in Cancer Cell Lines

In cancer cells, cell-surface sialylation is altered, including a change in oligo/polysialic acid (oligo/polySia) structures. Since they are unique and rarely expressed in normal cells, oligo/polySia structures may serve as promising novel biomarkers and targets for therapies. For the diagnosis and treatment of the disease, a precise understanding of the oligo/polySia structures in cancer cells is necessary. In this study, flow cytometric analysis and gene expression datasets were obtained from sixteen different cancer cell lines. These datasets demonstrated the ability to predict glycan structures and their sialylation status. Our results also revealed that sialylation patterns are unique to each cancer cell line. Thus, we can suggest promising combinations of antibody and cancer cell for glycan prediction. However, the precise prediction of minor glycans need to be further explored.

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.

A cancer-unique glycan: de-N-acetyl polysialic acid (dPSA) linked to cell surface nucleolin depends on re-expression of the fetal polysialyltransferase ST8SIA2 gene

BACKGROUND

Polysialic acid (polySia) modifies six cell surface proteins in humans mainly during fetal development and some blood cells in adults. Two genes in humans, ST8SIA2 and ST8SIA4, code for polysialyltransferases that synthesize polySia. ST8SIA2 is highly expressed during fetal development and in cancer but not in adult normal human cells. ST8SIA4 is expressed in fetal and adult brain, spleen, thymus, and peripheral blood leukocytes and in cancer. We identified a derivative of polySia containing de-N-acetyl neuraminic acid residues (dPSA), which is expressed on the cell surface of human cancer cell lines and tumors but not normal cells.

METHODS

dPSA-modified proteins in several human cancer cell lines and normal blood cells were identified using co-immunoprecipitation with anti-dPSA antibodies, mass spectroscopy and Western blot. RNAi and CRISPR were used to knockdown and knockout, respectively, the polysialyltransferase genes in human melanoma SK-MEL-28 and neuroblastoma CHP-134 cell lines, respectively, to determine the effect on production of cell surface dPSA measured by flow cytometry and fluorescence microscopy.

RESULTS

We found that dPSA is linked to or associated with nucleolin, a nuclear protein reported to be on the cell surface of cancer but not normal cells. Knocking down expression of ST8SIA2 with RNAi or knocking out each gene individually and in combination using CRISPR showed that cell surface dPSA depended on expression of ST8SIA2.

CONCLUSIONS

The presence of dPSA specifically in a broad range of human cancers but not human adult normal cells offers novel possibilities for diagnosis, prevention and treatment targeting the dPSA antigen that appears to be cancer-specific, consistent across not only human cancers but also species, and may be an unrecognized mechanism of immune shielding.

Sialyltransferase Inhibitors for the Treatment of Cancer Metastasis: Current Challenges and Future Perspectives

Potent, cell-permeable, and subtype-selective sialyltransferase inhibitors represent an attractive family of substances that can potentially be used for the clinical treatment of cancer metastasis. These substances operate by specifically inhibiting sialyltransferase-mediated hypersialylation of cell surface glycoproteins or glycolipids, which then blocks the sialic acid recognition pathway and leads to deterioration of cell motility and invasion. A vast amount of evidence for the in vitro and in vivo effects of sialyltransferase inhibition or knockdown on tumor progression and tumor cell metastasis or colonization has been accumulated over the past decades. In this regard, this review comprehensively discusses the results of studies that have led to the recent discovery and development of sialyltransferase inhibitors, their potential biomedical applications in the treatment of cancer metastasis, and their current limitations and future opportunities.

Design of a Cytotoxic Neuroblastoma-Targeting Agent Using an Enzyme Acting on Polysialic Acid Fused to a Toxin

Polysialic acid, an abundant cell surface component of the developing nervous system, which declines rapidly postnatally to virtual absence in the majority of adult tissues, is highly expressed in some malignant tumors including neuroblastoma. We found that the binding of a noncatalytic endosialidase to polysialic acid causes internalization of the complex from the surface of neuroblastoma kSK-N-SH cells, a subline of SK-N-SH, and leads to a complete relocalization of polysialic acid to the intracellular compartment. The binding and uptake of the endosialidase is polysialic acid-dependent as it is inhibited by free excess ligand or removal of polysialic acid by active endosialidase, and does not happen if catalytic endosialidase is used in place of inactive endosialidase. A fusion protein composed of the noncatalytic endosialidase and the cytotoxic portion of diphtheria toxin was prepared to investigate whether the cellular uptake observed could be used for the specific elimination of polysialic acid-containing cells. The conjugate toxin was found to be toxic to polysialic acid-positive kSK-N-SH with an IC₅₀ of 1.0 nmol/L. Replacing the noncatalytic endosialidase with active endosialidase decreased the activity to the level of nonconjugated toxin. Normal nonmalignant cells were selectively resistant to the toxin conjugate. The results demonstrate that noncatalytic endosialidase induces a quantitative removal and cellular uptake of polysialic acid from the cell surface which, by conjugation with diphtheria toxin fragment, can be exploited for the selective elimination of polysialic acid-containing tumor cells.

Glycosylation: Rising Potential for Prostate Cancer Evaluation

Simple Summary

Aberrant protein glycosylation is a well-known hallmark of cancer and is associated with differential expression of enzymes such as glycosyltransferases and glycosidases. The altered expression of the enzymes triggers cancer cells to produce glycoproteins with specific cancer-related aberrations in glycan structures. Increasing number of data indicate that glycosylation patterns of PSA and other prostate-originated proteins exert a potential to distinguish between benign prostate disease and cancer as well as among different stages of prostate cancer development and aggressiveness. This review summarizes the alterations in glycan sialylation, fucosylation, truncated O-glycans, and LacdiNAc groups outlining their potential applications in non-invasive diagnostic procedures of prostate diseases. Further research is desired to develop more general algorithms exploiting glycobiology data for the improvement of prostate diseases evaluation.

Abstract

Prostate cancer is the second most commonly diagnosed cancer among men. Alterations in protein glycosylation are confirmed to be a reliable hallmark of cancer. Prostate-specific antigen is the biomarker that is used most frequently for prostate cancer detection, although its lack of sensitivity and specificity results in many unnecessary biopsies. A wide range of glycosylation alterations in prostate cancer cells, including increased sialylation and fucosylation, can modify protein function and play a crucial role in many important biological processes in cancer, including cell signalling, adhesion, migration, and cellular metabolism. In this review, we summarize studies evaluating the prostate cancer associated glycosylation related alterations in sialylation, mainly α2,3-sialylation, core fucosylation, branched N-glycans, LacdiNAc group and presence of truncated O-glycans (sTn, sT antigen). Finally, we discuss the great potential to make use of glycans as diagnostic and prognostic biomarkers for prostate cancer.

Recent advances on smart glycoconjugate vaccines in infections and cancer

Vaccination is one of the greatest achievements in biomedical research preventing death and morbidity in many infectious diseases through the induction of pathogen-specific humoral and cellular immune responses. Currently, no effective vaccines are available for pathogens with a highly variable antigenic load, such as the human immunodeficiency virus or to induce cellular T-cell immunity in the fight against cancer. The recent SARS-CoV-2 outbreak has reinforced the relevance of designing smart therapeutic vaccine modalities to ensure public health. Indeed, academic and private companies have ongoing joint efforts to develop novel vaccine prototypes for this virus. Many pathogens are covered by a dense glycan-coat, which form an attractive target for vaccine development. Moreover, many tumor types are characterized by altered glycosylation profiles that are known as "tumor-associated carbohydrate antigens". Unfortunately, glycans do not provoke a vigorous immune response and generally serve as T-cell-independent antigens, not eliciting protective immunoglobulin G responses nor inducing immunological memory. A close and continuous crosstalk between glycochemists and glycoimmunologists is essential for the successful development of efficient immune modulators. It is clear that this is a key point for the discovery of novel approaches, which could significantly improve our understanding of the immune system. In this review, we discuss the latest advancements in development of vaccines against glycan epitopes to gain selective immune responses and to provide an overview on the role of different immunogenic constructs in improving glycovaccine efficacy.

An assay for quantitative analysis of polysialic acid expression in cancer cells

Polysialic acid (polySia) is a linear polysaccharide comprised of N-acetylneuraminic acid residues and its over-expression in cancer cells has been correlated with poor clinical prognosis. An assay has been developed for quantitative analysis of cellular polySia expression. This was achieved by extracting and purifying released polySia from glycoproteins by mild acid hydrolysis and optimised organic extraction. The polySia was further hydrolysed into Sia monomers, followed by fluorescent labelling and quantitative analysis. The assay was qualified utilising endoneuraminidase-NF to remove polySia from the surface of C6-ST8SiaII cancer cells (EC₅₀ = 2.13 ng/mL). The result was comparable to that obtained in a polySia-specific cellular ELISA assay. Furthermore, the assay proved suitable for evaluation of changes in polySia expression following treatment with a small molecule inhibitor of polysialylation. Given the importance of polySia in multiple disease states, notably cancer, this is a potentially vital tool with applications in the fields of drug discovery and glycobiology.

AQP1-Driven Migration Is Independent of Other Known Adverse Factors but Requires a Hypoxic Undifferentiated Cell Profile in Neuroblastoma

Neuroblastoma is a biologically very heterogeneous tumor with its clinical manifestation ranging from spontaneous regression to highly aggressive metastatic disease. Several adverse factors have been linked to oncogenesis, tumor progression and metastases of neuroblastoma including NMYC amplification, the neural adhesion molecule NCAM, as well as CXCR4 as a promoter of metastases. In this study, we investigate to what extent the expression of AQP1 in neuroblastoma correlates with changing cellular factors such as the hypoxic status, differentiation, expression of known adverse factors such as NMYC and NCAM, and CXCR4-related metastatic spread. Our results show that while AQP1 expression leads to an increased migratory behavior of neuroblastoma cells under hypoxic conditions, we find that hypoxia is associated with a reduction of NMYC in the same cells. A similar effect can be observed when using the tetracycline driven mechanism of SH-EP/Tet cells. When NMYC is not expressed, the expression of AQP1 is increased together with an increased expression of HIF-1α and HIF-2α. We furthermore show that when growing cells in different cell densities, they express AQP1, HIF-1α, HIF-2α, NMYC and NCAM to different degrees. AQP1 expression correlates with a hypoxic profile of these cells with increased HIF-1α and HIF-2α expression, as well as with NMYC and NCAM expression in two out of three neuroblastoma cell lines. When investigating cell properties of the cells that actually migrate, we find that the increased APQ1 expression in the migrated cells correlates with an increased NMYC and NCAM expression again in two out of three cell lines. Expression of the tumor cell homing marker CXCR4 varies between different tumor areas and between cell lines. While some migrated tumor cells highly express CXCR4, cells of other origin do not. In the initial phase of migration, we determined a dominant role of AQP1 expression of migrating cells in the scratch assay.

Genome-Wide DNA Methylation Pattern of Cancer Stem Cells in Esophageal Cancer

BACKGROUND

Cancer stem cells (CSCs) are considered the main cause of cancer recurrence and metastasis, and DNA methylation is involved in the maintenance of CSCs. However, the methylation profile of esophageal CSCs remains unknown.

METHODS

Side population (SP) cells were isolated from esophageal squamous cell carcinoma (ESCC) cell lines KYSE150 and EC109. Sphere-forming cells were collected from human primary esophageal cancer cells. SP cells and sphere-forming cells were used as substitutes for cancer stem-like cells. We investigated the genome-wide DNA methylation profile in esophageal cancer stem-like cells using reduced representation bisulfite sequencing (RRBS).

RESULTS

Methylated cytosine (mC) was found mostly in CpG dinucleotides, located mostly in the intronic, intergenic, and exonic regions. Forty intersected differentially methylated regions (DMRs) were identified in these 3 groups of samples. Thirteen differentially methylated genes with the same alteration trend were detected; these included OTX1, SPACA1, CD163L1, ST8SIA2, TECR, CADM3, GRM1, LRRK1, CHSY1, PROKR2, LINC00658, LOC100506688, and NKD2. DMRs covering ST8SIA2 and GRM1 were located in exons. These differentially methylated genes were involved in 10 categories of biological processes and 3 cell signaling pathways.

CONCLUSIONS

When compared to non-CSCs, cancer stem-like cells have a differential methylation status, which provides an important biological base for understanding esophageal CSCs and developing therapeutic targets for esophageal cancer.

An overview and future prospects of sialic acids

Sialic acids (Sias) are negatively charged functional monosaccharides present in a wide variety of natural sources (plants, animals and microorganisms). Sias play an important role in many life processes, which are widely applied in the medical and food industries as intestinal antibacterials, antivirals, anti-oxidative agents, food ingredients, and detoxification agents. Most Sias are composed of N-acetylneuraminic acid (Neu5Ac, >99%), and Sia is its most commonly used name. In this article, we review Sias in terms of their structures, applications, determination methods, metabolism, and production strategies. In particular, we summarise and compare different production strategies, including extraction from natural sources, chemical synthesis, polymer decomposition, enzymatic synthesis, whole-cell catalysis, and de novo biosynthesis via microorganism fermentation. We also discuss research on their physiological functions and applications, barriers to efficient production, and strategies for overcoming these challenges. We focus on efficient de novo biosynthesis strategies for Neu5Ac via microbial fermentation using novel synthetic biology tools and methods that may be applied in future. This work provides a comprehensive overview of recent advances on Sias, and addresses future challenges regarding their functions, applications, and production.

An efficient assay for identification and quantitative evaluation of potential polysialyltransferase inhibitors

The polysialyltransferases (polySTs) catalyse the polymerisation of polysialic acid, which plays an important role in tumour metastasis. While assays are available to assess polyST enzyme activity, there is no methodology available specifically optimised for identification and quantitative evaluation of potential polyST inhibitors. The development of an HPLC-fluorescence-based enzyme assay described within includes a comprehensive investigation of assay conditions, including evaluation of metal ion composition, enzyme, substrate and acceptor concentrations, temperature, pH, and tolerance to DMSO, followed by validation using known polyST inhibitors. Thorough analysis of each of the assay components provided a set of optimised conditions. Under these optimised conditions, the experimentally observed K_i value for CMP, a competitive polyST inhibitor, was strongly correlated with the predicted K_i value, based on the classical Cheng-Prusoff equation [average fold error (AFE) = 1.043]. These results indicate that this assay can provide medium-throughput analysis for enzyme inhibitors with high accuracy, through determining the corresponding IC₅₀ values with substrate concentration at the K_M, without the need to perform extensive kinetic studies for each compound. In conclusion, an in vitro cell-free assay for accurate assessment of polyST inhibition is described. The utility of the assay for routine identification of potential polyST inhibitors is demonstrated, allowing quantitative measurement of inhibition to be achieved, and exemplified through assessment of full competitive inhibition. Given the considerable and growing interest in the polySTs as important anti-metastatic targets in cancer drug discovery, this is a vital tool to enable preclinical identification and evaluation of novel polyST inhibitors.

Exosome-associated polysialic acid modulates membrane potentials, membrane thermotropic properties, and raft-dependent interactions between vesicles

In mammals, polysialic acid (polySia) attached to a small number of transmembrane protein carriers occurs on the surface of plasma membranes of neural, cancer, immune, and placental trophoblast cells. Here, our goal was to demonstrate the presence of polySia on exosomes and its effect on membrane properties. We isolated exosomes and found that polysialylated exosomes in fetal bovine serum originate mostly from placental trophoblasts, while in calf bovine serum, they originate from immune cells. Enzymatic removal of polySia chains from the exosomal surface makes the membrane surface potential more positive, transmembrane potential more negative, and reduces the activation energy for membrane anisotropy changes. We demonstrate for the first time that exosomes could interact through polySia-raft interactions. We suggest that polysialylation of exosomal membrane can have a thermo-protecting effect and can modulate exosome-plasma membrane interactions.

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.

Sialidase NEU1 suppresses progression of human bladder cancer cells by inhibiting fibronectin-integrin α5β1 interaction and Akt signaling pathway

Background

Sialic acids are widely distributed in animal tissues, and aberrantly expressed in a variety of cancer types. High expression of sialic acid contributes to tumor aggressiveness by promoting cell proliferation, migration, angiogenesis, and metastasis. Sialidases are responsible for removal of sialic acids from glycoproteins and glycolipids.

Methods

N-glycomics of bladder cancer cells were detected by MALDI-TOF mass spectrometry. Sialic acid modification in bladder cancer tissue was determined by lectin blot. The down-regulation of NEU1 in bladder cancer cells was determined by high resolution liquid chromatography mass spectrometry (HR LC-MS). The effects of sialidase NEU1 expression on proliferation and apoptosis of human bladder cancer cells were examined by western blot, RT-PCR, confocal imaging and flow cytometry. Moreover, the function of sialic acids on fibronectin-integrin α5β1 interaction were assayed by immunoprecipitation and ELISA. The importance of NEU1 in tumor formation in vivo was performed using BALB/c-nu mice. Expression of NEU1 in primary human bladder cancer tissue samples was estimated using bladder cancer tissue microarray.

Results

(1) Downregulation of NEU1 was primarily responsible for aberrant expression of sialic acids in bladder cancer cells. (2) Decreased NEU1 expression was correlated with bladder cancer progression. (3) NEU1 overexpression enhanced apoptosis and reduced proliferation of bladder cancer cells. (4) NEU1 disrupted FN-integrin α5β1 interaction and deactivated the Akt signaling pathway. (5) NEU1 significantly suppressed in vivo tumor formation in BALB/c-nu mice.

Conclusions

Our data showed that NEU1 inhibited cancer cell proliferation, induced apoptosis, and suppressed tumor formation both in vitro and in vivo, by disrupting interaction of FN and integrin β1 and inhibiting the Akt signaling pathway. Our observations indicate that NEU1 is an important modulator of the malignant properties of bladder cancer cells, and is a potential therapeutic target for prognosis and treatment of bladder cancer.

The Cooperative Effect between Polybasic Region (PBR) and Polysialyltransferase Domain (PSTD) within Tumor-Target Polysialyltranseferase ST8Sia II

ST8Sia II (STX) is a highly homologous mammalian polysialyltransferase (polyST), which is a validated tumor-target in the treatment of cancer metastasis reliant on tumor cell polysialylation. PolyST catalyzes the synthesis of α2,8-polysialic acid (polySia) glycans by carrying out the activated CMP-Neu5Ac (Sia) to N- and O-linked oligosaccharide chains on acceptor glycoproteins. In this review article, we summarized the recent studies about intrinsic correlation of two polybasic domains, Polysialyltransferase domain (PSTD) and Polybasic region (PBR) within ST8Sia II molecule, and suggested that the critical amino acid residues within the PSTD and PBR motifs of ST8Sia II for polysialylation of Neural cell adhesion molecules (NCAM) are related to ST8Sia II activity. In addition, the conformational changes of the PSTD domain due to point mutations in the PBR or PSTD domain verified an intramolecular interaction between the PBR and the PSTD. These findings have been incorporated into Zhou's NCAM polysialylation/cell migration model, which will provide new perspectives on drug research and development related to the tumor-target ST8Sia II.

Biological Functions and Analytical Strategies of Sialic Acids in Tumor

Sialic acids, a subset of nine carbon acidic sugars, often exist as the terminal sugars of glycans on either glycoproteins or glycolipids on the cell surface. Sialic acids play important roles in many physiological and pathological processes via carbohydrate-protein interactions, including cell-cell communication, bacterial and viral infections. In particular, hypersialylation in tumors, as well as their roles in tumor growth and metastasis, have been widely described. Recent studies have indicated that the aberrant sialylation is a vital way for tumor cells to escape immune surveillance and keep malignance. In this article, we outline the present state of knowledge on the metabolic pathway of human sialic acids, the function of hypersialylation in tumors, as well as the recent labeling and analytical techniques for sialic acids. It is expected to offer a brief introduction of sialic acid metabolism and provide advanced analytical strategies in sialic acid studies.

Recent advances in the analysis of polysialic acid from complex biological systems

Polysialic acid (polySia) is a unique, well-characterised carbohydrate polymer highly-expressed on the cell surface of neurons in the early stages of mammalian brain development. Post-embryogenesis, it is also re-expressed in a number of tumours of neuroendocrine origin. It plays important roles in modulating cell-cell, and cell-matrix adhesion and migration, tumour invasion and metastasis. Techniques for structural and quantitative characterisation of polySia from tumours and cancer cells are thus essential in exploring the relationship between polySia expression levels and structural and functional changes associated with cancer progression and metastasis. A variety of techniques have been developed to structurally and quantitatively analyse polySia in clinical tissues and other biological samples. In this review, analytical approaches used for the determination of polySia in biological matrices in the past 20 years are discussed, with a particular focus on chemical approaches, and quantitative analysis.

The Inhibition of Polysialyltranseferase ST8SiaIV Through Heparin Binding to Polysialyltransferase Domain (PSTD)

BACKGROUND

The polysialic acid (polySia) is a unique carbohydrate polymer produced on the surface Of Neuronal Cell Adhesion Molecule (NCAM) in a number of cancer cells, and strongly correlates with the migration and invasion of tumor cells and with aggressive, metastatic disease and poor clinical prognosis in the clinic. Its synthesis is catalyzed by two polysialyltransferases (polySTs), ST8SiaIV (PST) and ST8SiaII (STX). Selective inhibition of polySTs, therefore, presents a therapeutic opportunity to inhibit tumor invasion and metastasis due to NCAM polysialylation. Heparin has been found to be effective in inhibiting the ST8Sia IV activity, but no clear molecular rationale. It has been found that polysialyltransferase domain (PSTD) in polyST plays a significant role in influencing polyST activity, and thus it is critical for NCAM polysialylation based on the previous studies.

OBJECTIVE

To determine whether the three different types of heparin (unfractionated hepain (UFH), low molecular heparin (LMWH) and heparin tetrasaccharide (DP4)) is bound to the PSTD; and if so, what are the critical residues of the PSTD for these binding complexes?

METHODS

Fluorescence quenching analysis, the Circular Dichroism (CD) spectroscopy, and NMR spectroscopy were used to determine and analyze interactions of PSTD-UFH, PSTD-LMWH, and PSTD-DP4.

RESULTS

The fluorescence quenching analysis indicates that the PSTD-UFH binding is the strongest and the PSTD-DP4 binding is the weakest among these three types of the binding; the CD spectra showed that mainly the PSTD-heparin interactions caused a reduction in signal intensity but not marked decrease in α-helix content; the NMR data of the PSTD-DP4 and the PSTDLMWH interactions showed that the different types of heparin shared 12 common binding sites at N247, V251, R252, T253, S257, R265, Y267, W268, L269, V273, I275, and K276, which were mainly distributed in the long α-helix of the PSTD and the short 3-residue loop of the C-terminal PSTD. In addition, three residues K246, K250 and A254 were bound to the LMWH, but not to DP4. This suggests that the PSTD-LMWH binding is stronger than the PSTD-DP4 binding, and the LMWH is a more effective inhibitor than DP4.

CONCLUSION

The findings in the present study demonstrate that PSTD domain is a potential target of heparin and may provide new insights into the molecular rationale of heparin-inhibiting NCAM polysialylation.

Molecular retargeting of antibodies converts immune defense against oncolytic viruses into cancer immunotherapy

Virus-neutralizing antibodies are a severe obstacle in oncolytic virotherapy. Here, we present a strategy to convert this unfavorable immune response into an anticancer immunotherapy via molecular retargeting. Application of a bifunctional adapter harboring a tumor-specific ligand and the adenovirus hexon domain DE1 for engaging antiadenoviral antibodies, attenuates tumor growth and prolongs survival in adenovirus-immunized mice. The therapeutic benefit achieved by tumor retargeting of antiviral antibodies is largely due to NK cell-mediated triggering of tumor-directed CD8 T-cells. We further demonstrate that antibody-retargeting (Ab-retargeting) is a feasible method to sensitize tumors to PD-1 immune checkpoint blockade. In therapeutic settings, Ab-retargeting greatly improves the outcome of intratumor application of an oncolytic adenovirus and facilitates long-term survival in treated animals when combined with PD-1 checkpoint inhibition. Tumor-directed retargeting of preexisting or virotherapy-induced antiviral antibodies therefore represents a promising strategy to fully exploit the immunotherapeutic potential of oncolytic virotherapy and checkpoint inhibition.

Antibody-Mediated Endocytosis of Polysialic Acid Enables Intracellular Delivery and Cytotoxicity of a Glycan-Directed Antibody-Drug Conjugate

The specific targeting of differentially expressed glycans in malignant cells has emerged as an attractive anticancer strategy. One such target is the oncodevelopmental antigen polysialic acid (polySia), a polymer of α2,8-linked sialic acid residues that is largely absent during postnatal development but is re-expressed during progression of several malignant human tumors, including small-cell and non-small cell lung carcinomas, glioma, neuroblastoma, and pancreatic carcinoma. In these cancers, expression of polySia correlates with tumor progression and poor prognosis and appears to modulate cancer cell adhesion, invasiveness, and metastasis. To evaluate the potential of PolySia as a target for anticancer therapy, we developed a chimeric human polySia-specific mAb that retained low nanomolar (nmol/L) target affinity and exhibited exquisite selectivity for polySia structures. The engineered chimeric mAb recognized several polySia-positive tumor cell lines in vitro and induced rapid endocytosis of polySia antigens. To determine whether this internalization could be exploited for delivery of conjugated cytotoxic drugs, we generated an antibody-drug conjugate (ADC) by covalently linking the chimeric human mAb to the tubulin-binding maytansinoid DM1 using a bioorthogonal chemical reaction scheme. The resulting polySia-directed ADC demonstrated potent target-dependent cytotoxicity against polySia-positive tumor cells in vitro. Collectively, these results establish polySia as a valid cell-surface, cancer-specific target for glycan-directed ADC and contribute to a growing body of evidence that the tumor glycocalyx is a promising target for synthetic immunotherapies. SIGNIFICANCE: These findings describe a glycan-specific antibody-drug conjugate that establishes polySia as a viable cell surface target within the tumor glycocalyx.

Tumor Targeting via Sialic Acid: [68Ga]DOTA-en-pba as a New Tool for Molecular Imaging of Cancer with PET

PURPOSE

The aim of this study was to demonstrate the potential of Ga-68-labeled macrocycle (DOTA-en-pba) conjugated with phenylboronic vector for tumor recognition by positron emission tomography (PET), based on targeting of the overexpressed sialic acid (Sia).

PROCEDURES

The imaging reporter DOTA-en-pba was synthesized and labeled with Ga-68 at high efficiency. Cell binding assay on Mel-C and B16-F10 melanoma cells was used to evaluate melanin production and Sia overexpression to determine the best model for demonstrating the capability of [⁶⁸Ga]DOTA-en-pba to recognize tumors. The in vivo PET imaging was done with B16-F10 tumor-bearing SCID mice injected with [⁶⁸Ga]DOTA-en-pba intravenously. Tumor, blood, and urine metabolites were assessed to evaluate the presence of a targeting agent.

RESULTS

The affinity of [⁶⁸Ga]DOTA-en-pba to Sia was demonstrated on B16-F10 melanoma cells, after the production of melanin as well as Sia overexpression was proved to be up to four times higher in this cell line compared to that in Mel-C cells. Biodistribution studies in B16-F10 tumor-bearing SCID mice showed blood clearance at the time points studied, while uptake in the tumor peaked at 60 min post-injection (6.36 ± 2.41 % ID/g). The acquired PET images were in accordance with the ex vivo biodistribution results. Metabolite assessment on tumor, blood, and urine samples showed that [⁶⁸Ga]DOTA-en-pba remains unmetabolized up to at least 60 min post-injection.

CONCLUSIONS

Our work is the first attempt for in vivo imaging of cancer by targeting overexpression of sialic acid on cancer cells with a radiotracer in PET.

Synergistic activation of the NEU4 promoter by p73 and AP2 in colon cancer cells

More than 50% of colon cancers bear mutations in p53, one of the most important tumor suppressors, and its family members p63 or p73 are expected to contribute to inhibiting the progression of colon cancers. The AP2 family also acts as a tumor suppressor. Here we found that p73 and AP2 are able to activate NEU4, a neuraminidase gene, which removes the terminal sialic acid residues from cancer-associated glycans. Under serum starvation, NEU4 was up-regulated and one of the NEU4 target glycans, sialyl Lewis X, was decreased, whereas p73 and AP2 were up-regulated. Sialyl Lewis X levels were not, however, decreased under starvation conditions in p73- or AP2-knockdown cells. p53 and AP2 underwent protein-protein interactions, exerting synergistic effects to activate p21, and interaction of p53 with AP2 was lost in cells expressing the L350P mutation of p53. The homologous residues in p63 and p73 are L423 and L377, respectively. The synergistic effect of p53/p63 with AP2 to activate genes was lost with the L350P/L423P mutation in p53/p63, but p73 bearing the L377P mutation was able to interact with AP2 and exerted its normal synergistic effects. We propose that p73 and AP2 synergistically activate the NEU4 promoter in colon cancer cells.

Polysialic acid is released by human umbilical vein endothelial cells (HUVEC) in vitro

Background

Sialic acids represent common terminal residues on numerous mammalian glycoconjugates, thereby influencing e.g. lumen formation in developing blood vessels. Interestingly, besides monosialylated also polysialylated glycoconjugates are produced by endothelial cells. Polysialic acid (polySia) is formed in several organs during embryonal and postnatal development influencing, for instance, cell migration processes. Furthermore, the function of cytokines like basic fibroblast growth factor (bFGF) is modulated by polySia.

Results

In this study, we demonstrated that human umbilical vein endothelial cells (HUVEC) also secrete polysialylated glycoconjugates. Furthermore, an interaction between polySia and vascular endothelial growth factor (VEGF) was observed. VEGF modulates like bFGF the migration of HUVEC. Since both growth factors interact with polySia, we examined, if polySia modulates the migration of HUVEC. To this end scratch assays were performed showing that the migration of HUVEC is stimulated, when polySia was degraded.

Conclusions

Since polySia can interact with bFGF as well as VEGF and the degradation of polySia resulted in an increased cell migration capacity in the applied scratch assay, we propose that polySia may trap these growth factors influencing their biological activity. Thus, polySia might also contribute to the fine regulation of physiological processes in endothelial cells.

Sialic Acids in Nonenveloped Virus Infections

Sialic acid-based glycoconjugates cover the surfaces of many different cell types, defining key properties of the cell surface such as overall charge or likely interaction partners. Because of this prominence, sialic acids play prominent roles in mediating attachment and entry to viruses belonging to many different families. In this review, we first describe how interactions between viruses and sialic acid-based glycan structures can be identified and characterized using a range of techniques. We then highlight interactions between sialic acids and virus capsid proteins in four different viruses, and discuss what these interactions have taught us about sialic acid engagement and opportunities to interfere with binding.

Structural Basis for Binding of Fluorescent CMP-Neu5Ac Mimetics to Enzymes of the Human ST8Sia Family

Polysialyltransferases synthesize polysialic acid on cell surface-expressed glycoconjugates, which is crucial for developing processes and signaling pathways in eukaryotes. Recent advances in cancer research have rendered polysialyltransferases important drug targets because polysialic acid contributes to cancer cell progression, metastasis, and treatment of resistant tumors. To aid the development of high-throughput screening assays for polysialyltransferase inhibitors, we demonstrate that a previously developed class of fluorescent CMP-sialic acid mimetics for sialyltransferases has nanomolar affinities for oligo- and polysialyltransferases and can be used for the rapid screening of new polysialyltransferase inhibitors. We demonstrate that these CMP-Neu5Ac mimetics inhibit polysialylation in vitro and perform cell culture experiments, where we observe reduced polysialylation of NCAM. Furthermore, we describe the structural basis of CMP-Neu5Ac mimetics binding to the human oligosialyltransferase ST8SiaIII and extrapolate why their affinity is high for human polysialyltransferases. Our results show that this novel class of compounds is a promising tool for the development of potent and selective drugs against polysialyltransferase activity.

Sialylation pattern in lung epithelial cell line and Siglecs expression in monocytic THP-1 cells as cellular indicators of cigarette smoke - induced pathology in vitro

PURPOSE

Cellular response to cigarette smoke (CS) involves activation of recognition receptors resulting in changes in immune status, oxidative stress and cell turnover. We investigated the effects of CS on sialic acid-binding immunoglobulin type lectins (Siglecs) expression and their sialylated ligands in human immune and non-immune cells.

METHODS

Human monocytes (THP-1) and epithelial cells (A549) were cultured in CS-conditioned medium (CSM). Expression of Siglec-8 and Siglec-5/Siglec-14 was analysed in THP-1 cells using flow cytometry. The effects of CS on immune activity was evaluated flow cytometrically in these cells by assessment of phagocytosis and intracellular expression IL-1β and IL-10. Detection and differentiation of sialic acids was analyzed by dot blot, western blot and flow cytometry using plant lectins and antibodies.

RESULTS

Exposure to CS significantly increased expression of Siglec-8 and Siglec-5/Siglec-14 in THP-1 cells. These changes were accompanied by enhanced intracellular level of IL-1β and IL-10 but reduced phagocytic activity. In THP-1 and A549 cells, the level of α2,3-sialic acids, but not α2,6-sialic acid, was significantly increased when compared to naïve cells. The level of α2,8-sialic acids increased significantly in A549 cells, but not in THP-1 cells, after exposure to CS.

CONCLUSION

These results show that cellular response to CS involves changes in expression of Siglec receptors and sialylated ligands functionally associated with immunity.

Polysialic acid is a cellular receptor for human adenovirus 52

We present here that adenovirus type 52 (HAdV-52) attaches to target cells through a mechanism not previously observed in other human pathogenic viruses. The interaction involves unusual, transient, electrostatic interactions between the short fiber capsid protein and polysialic acid (polySia)-containing receptors on target cells. Knowledge about the binding interactions between polySia and its natural ligands is relatively limited, and our results therefore provide additional insight not only into adenovirus biology but also into the structural basis of polySia function. Since polySia can be found in high expression levels in brain and lung cancers where its presence is associated with poor prognosis, we suggest that this polySia-binding adenovirus could be useful for design of vectors for gene therapy of these cancers.

Human adenovirus 52 (HAdV-52) is one of only three known HAdVs equipped with both a long and a short fiber protein. While the long fiber binds to the coxsackie and adenovirus receptor, the function of the short fiber in the virus life cycle is poorly understood. Here, we show, by glycan microarray analysis and cellular studies, that the short fiber knob (SFK) of HAdV-52 recognizes long chains of α-2,8-linked polysialic acid (polySia), a large posttranslational modification of selected carrier proteins, and that HAdV-52 can use polySia as a receptor on target cells. X-ray crystallography, NMR, molecular dynamics simulation, and structure-guided mutagenesis of the SFK reveal that the nonreducing, terminal sialic acid of polySia engages the protein with direct contacts, and that specificity for polySia is achieved through subtle, transient electrostatic interactions with additional sialic acid residues. In this study, we present a previously unrecognized role for polySia as a cellular receptor for a human viral pathogen. Our detailed analysis of the determinants of specificity for this interaction has general implications for protein–carbohydrate interactions, particularly concerning highly charged glycan structures, and provides interesting dimensions on the biology and evolution of members of Human mastadenovirus G.

Targeting polysialic acid-abundant cancers using oncolytic adenoviruses with fibers fused to active bacteriophage borne endosialidase

Genetic replacement of adenoviral fiber knobs by ligands that enable tumor specific targeting of oncolytic adenoviruses is challenging because the fiber knob contributes to virus assembly. Here, we present a novel concept by describing stable recombinant adenoviruses with tumor specific infection mode. The fiber knob was replaced by endosialidaseNF (endoNF), the tailspike protein of bacteriophage K1F. EndoNF recognizes polysialic acid, an oncofetal antigen characteristic for high malignant tumors of neuroendocrine origin. An intramolecular chaperone contained in endoNF warrants folding and compensates for the knob function in virus assembly. Obtained recombinant viruses demonstrated polysialic acid dependent infection modes, strong oncolytic capacity with polysialic acid positive cells in culture and a high potential to inhibit tumor growth in a therapeutic mouse model of subcutaneous neuroblastoma. With a single genetic manipulation we achieved ablation of the fiber knob, introduction of a tumor specific ligand, and folding control over the chimeric fiber construct.

Autopolysialylation of polysialyltransferases is required for polysialylation and polysialic acid chain elongation on select glycoprotein substrates

Polysialic acid (polySia) is a large glycan polymer that is added to some glycoproteins by two polysialyltransferases (polySTs), ST8Sia-II and ST8Sia-IV. As polySia modulates cell adhesion and signaling, immune cell function, and tumor metastasis, it is of interest to determine how the polySTs recognize their select substrates. We have recently identified residues within the ST8Sia-IV polybasic region (PBR) that are required for neural cell adhesion molecule (NCAM) recognition and subsequent polysialylation. Here, we compared the PBR sequence requirements for NCAM, neuropilin-2 (NRP-2), and synaptic cell adhesion molecule 1 (SynCAM 1) for polysialylation by their respective polySTs. We found that the polySTs use unique but overlapping sets of PBR residues for substrate recognition, that the NCAM-recognizing PBR sites in ST8Sia-II and ST8Sia-IV include homologous residues, but that the ST8Sia-II site is larger, and that fewer PBR residues are involved in NRP-2 and SynCAM 1 recognition than in NCAM recognition. Noting that the two sites for ST8Sia-IV autopolysialylation flank the PBR, we evaluated the role of PBR residues in autopolysialylation and found that the requirements for polyST autopolysialylation and substrate polysialylation overlap. These data together with the evaluation of the polyST autopolysialylation mechanism enabled us to further identify PBR residues potentially playing dual roles in substrate recognition and in polySia chain polymerization. Finally, we found that ST8Sia-IV autopolysialylation is required for NRP-2 polysialylation and that ST8Sia-II autopolysialylation promotes the polymerization of longer polySia chains on SynCAM 1, suggesting a critical role for polyST autopolysialylation in substrate selection and polySia chain elongation.

The polysialic acid mimetics idarubicin and irinotecan stimulate neuronal survival and neurite outgrowth and signal via protein kinase C

Polysialic acid (PSA) is a large, negatively charged, linear homopolymer of alpha2-8-linked sialic acid residues. It is generated by two polysialyltransferases and attached to N- and/or O-linked glycans, and its main carrier is the neural cell adhesion molecule (NCAM). PSA controls the development and regeneration of the nervous system by enhancing cell migration, axon pathfinding, synaptic targeting, synaptic plasticity, by regulating the differentiation of progenitor cells and by modulating cell-cell and cell-matrix adhesions. In the adult, PSA plays a role in the immune system, and PSA mimetics promote functional recovery after nervous system injury. In search for novel small molecule mimetics of PSA that are applicable for therapy, we identified idarubicin, an antineoplastic anthracycline, and irinotecan, an antineoplastic agent of the topoisomerase I inhibitor class, as PSA mimetics using a competition enzyme-linked immunosorbent assay. Idarubicin and irinotecan compete with the PSA-mimicking peptide and colominic acid, the bacterial analog of PSA, for binding to the PSA-specific monoclonal antibody 735. Idarubicin and irinotecan stimulate neurite outgrowth and survival of cultured cerebellar neurons after oxidative stress via protein kinase C and Erk1/2 in a similar manner as colominic acid, whereas Fyn, casein kinase II and the phosphatase and tensin homolog are only involved in idarubicin and irinotecan-stimulated neurite outgrowth. These novel results show that the structure and function of PSA can be mimicked by the small organic compounds irinotecan and idarubicin which trigger the same signaling cascades as PSA, thus introducing the possibility of retargeting these drugs to treat nervous system injuries.

X-ray crystallographic structure of a bacterial polysialyltransferase provides insight into the biosynthesis of capsular polysialic acid

Polysialic acid (polySia) is a homopolymeric saccharide that is associated with some neuroinvasive pathogens and is found on selective cell types in their eukaryotic host. The presence of a polySia capsule on these bacterial pathogens helps with resistance to phagocytosis, cationic microbial peptides and bactericidal antibody production. The biosynthesis of bacterial polySia is catalysed by a single polysialyltransferase (PST) transferring sialic acid from a nucleotide-activated donor to a lipid-linked acceptor oligosaccharide. Here we present the X-ray structure of the bacterial PST from Mannheimia haemolytica serotype A2, thereby defining the architecture of this class of enzymes representing the GT38 family. The structure reveals a prominent electropositive groove between the two Rossmann-like domains forming the GT-B fold that is suitable for binding of polySia chain products. Complex structures of PST with a sugar donor analogue and an acceptor mimetic combined with kinetic studies of PST active site mutants provide insight into the principles of substrate binding and catalysis. Our results are the basis for a molecular understanding of polySia biosynthesis in bacteria and might assist the production of polysialylated therapeutic reagents and the development of novel antibiotics.

Exploring and Exploiting Acceptor Preferences of the Human Polysialyltransferases as a Basis for an Inhibitor Screen

α2,8-Linked polysialic acid (polySia) is an oncofoetal antigen with high abundance during embryonic development. It reappears in malignant tumours of neuroendocrine origin. Two polysialyltransferases (polySTs) ST8SiaII and IV are responsible for polySia biosynthesis. During development, both enzymes are essential to control polySia expression. However, in tumours ST8SiaII is the prevalent enzyme. Consequently, ST8SiaII is an attractive target for novel cancer therapeutics. A major challenge is the high structural and functional conservation of ST8SiaII and -IV. An assay system that enables differential testing of ST8SiaII and -IV would be of high value to search for specific inhibitors. Here we exploited the different modes of acceptor recognition and elongation for this purpose. With DMB-DP3 and DMB-DP12 (fluorescently labelled sialic acid oligomers with a degree of polymerisation of 3 and 12, respectively) we identified stark differences between the two enzymes. The new acceptors enabled the simple comparative testing of the polyST initial transfer rate for a series of CMP-activated and N-substituted sialic acid derivatives. Of these derivatives, the non-transferable CMP-Neu5Cyclo was found to be a new, competitive ST8SiaII inhibitor.