N-acetyl-D-mannosamine analogues as potential inhibitors of sialic acid biosynthesis

In Vivo Applications of Bioorthogonal Reactions: Chemistry and Targeting Mechanisms

Bioorthogonal chemistry involves selective biocompatible reactions between functional groups that are not normally present in biology. It has been used to probe biomolecules in living systems, and has advanced biomedical strategies such as diagnostics and therapeutics. In this review, the challenges and opportunities encountered when translating in vitro bioorthogonal approaches to in vivo settings are presented, with a focus on methods to deliver the bioorthogonal reaction components. These methods include metabolic bioengineering, active targeting, passive targeting, and simultaneously used strategies. The suitability of bioorthogonal ligation reactions and bond cleavage reactions for in vivo applications is critically appraised, and practical considerations such as the optimum scheduling regimen in pretargeting approaches are discussed. Finally, we present our own perspectives for this area and identify what, in our view, are the key challenges that must be overcome to maximise the impact of these approaches.

Advances in molecular mechanisms of drugs affecting abnormal glycosylation and metastasis of breast cancer

Breast cancer remains the leading cause of cancer-related death among women worldwide, and its incidence is also increasing. High recurrence rate and metastasis rate are the key causes of poor prognosis and death. It is suggested that abnormal glycosylation plays an important role in the growth, invasion, metastasis and resistance to therapy of breast cancer cells. Meanwhile, it can be used as the biomarkers for the early detection and prognosis of breast cancer and the potential attractive targets for drug treatment. However, only a few attentions have been paid to the molecular mechanism of abnormal glycosylation in the epithelial-mesenchymal transition (EMT) of breast cancer cells and the related intervention of drugs. This manuscript thus investigated the relationship between abnormal glycosylation, the EMT, and breast cancer metastasis. Then, the process of abnormal glycosylation, the classification and their molecular regulatory mechanisms of breast cancer were analyzed in detail. Last, potential drugs are introduced in different categories, which are expected to reverse or intervene the abnormal glycosylation of breast cancer. This review is conducive to an in-depth understanding of the metastasis and drug resistance of breast cancer cells, which will provide new ideas for the clinical regulation of glycosylation and related drug treatments in breast cancer.

Cell Line-, Protein-, and Sialoglycosite-Specific Control of Flux-Based Sialylation in Human Breast Cells: Implications for Cancer Progression

Sialylation, a post-translational modification that impacts the structure, activity, and longevity of glycoproteins has been thought to be controlled primarily by the expression of sialyltransferases (STs). In this report we explore the complementary impact of metabolic flux on sialylation using a glycoengineering approach. Specifically, we treated three human breast cell lines (MCF10A, T-47D, and MDA-MB-231) with 1,3,4-O-Bu₃ManNAc, a "high flux" metabolic precursor for the sialic acid biosynthetic pathway. We then analyzed N-glycan sialylation using solid phase extraction of glycopeptides (SPEG) mass spectrometry-based proteomics under conditions that selectively captured sialic acid-containing glycopeptides, referred to as "sialoglycosites." Gene ontology (GO) analysis showed that flux-based changes to sialylation were broadly distributed across classes of proteins in 1,3,4-O-Bu₃ManNAc-treated cells. Only three categories of proteins, however, were "highly responsive" to flux (defined as two or more sialylation changes of 10-fold or greater). Two of these categories were cell signaling and cell adhesion, which reflect well-known roles of sialic acid in oncogenesis. A third category-protein folding chaperones-was unexpected because little precedent exists for the role of glycosylation in the activity of these proteins. The highly flux-responsive proteins were all linked to cancer but sometimes as tumor suppressors, other times as proto-oncogenes, or sometimes both depending on sialylation status. A notable aspect of our analysis of metabolically glycoengineered breast cells was decreased sialylation of a subset of glycosites, which was unexpected because of the increased intracellular levels of sialometabolite "building blocks" in the 1,3,4-O-Bu₃ManNAc-treated cells. Sites of decreased sialylation were minor in the MCF10A (<25% of all glycosites) and T-47D (<15%) cells but dominated in the MDA-MB-231 line (~60%) suggesting that excess sialic acid could be detrimental in advanced cancer and cancer cells can evolve mechanisms to guard against hypersialylation. In summary, flux-driven changes to sialylation offer an intriguing and novel mechanism to switch between context-dependent pro- or anti-cancer activities of the several oncoproteins identified in this study. These findings illustrate how metabolic glycoengineering can uncover novel roles of sialic acid in oncogenesis.

Sialic acid glycoengineering using N-acetylmannosamine and sialic acid analogs

Sialic acids cap the glycans of cell surface glycoproteins and glycolipids. They are involved in a multitude of biological processes and aberrant sialic acid expression is associated with several pathologies. Sialic acids modulate the characteristics and functions of glycoproteins and regulate cell-cell as well as cell-extracellular matrix interactions. Pathogens such as influenza virus use sialic acids to infect host cells and cancer cells exploit sialic acids to escape from the host's immune system. The introduction of unnatural sialic acids with different functionalities into surface glycans enables the study of the broad biological functions of these sugars and presents a therapeutic option to intervene with pathological processes involving sialic acids. Multiple chemically modified sialic acid analogs can be directly utilized by cells for sialoglycan synthesis. Alternatively, analogs of the natural sialic acid precursor sugar N-Acetylmannosamine (ManNAc) can be introduced into the sialic acid biosynthesis pathway resulting in the intracellular conversion into the corresponding sialic acid analog. Both, ManNAc and sialic acid analogs, have been employed successfully for a large variety of glycoengineering applications such as glycan imaging, targeting toxins to tumor cells, inhibiting pathogen binding, or altering immune cell activity. However, there are significant differences between ManNAc and sialic acid analogs with respect to their chemical modification potential and cellular metabolism that should be considered in sialic acid glycoengineering experiments.

Exploiting metabolic glycoengineering to advance healthcare

Metabolic glycoengineering (MGE) is a technique for manipulating cellular metabolism to modulate glycosylation. MGE is used to increase the levels of natural glycans and, more importantly, to install non-natural monosaccharides into glycoconjugates. In this Review, we summarize the chemistry underlying MGE that has been developed over the past three decades and highlight several recent advances that have set the stage for clinical translation. In anticipation of near-term application to human healthcare, we describe emerging efforts to deploy MGE in diverse applications, ranging from the glycoengineering of biotherapeutic proteins and the diagnosis and treatment of complex diseases such as cancer to the development of new immunotherapies.

Survival of membrane-damaged Escherichia coli in a cytosol-mimicking solution

Selective permeability of cell membrane is critically important for cell survival. The damage caused to cell membrane by pore-forming antimicrobial peptides may result in the loss of selective permeability and leakage of intracellular molecules, eventually leading to cell death. Here, we examined whether the membrane-damaged Escherichia coli cells survive in a cytosol-mimicking solution (CMS), which compensates for the lethal leakage of intracellular molecules. We prepared a CMS comprising 34 low molecular weight compounds from the cytosol and found that the cells were able to grow in CMS even in the presence of a pore-forming peptide, melittin. We confirmed that the melittin-treated cells lost selective membrane permeability by staining with membrane-impermeable dyes, propidium iodide and SYTOX green. Some stained cells maintained the colony formation ability in CMS. These results provide an evidence that E. coli cells can at least partially survive in the CMS even after the temporary impairment of membrane selective permeability. This study demonstrates a technique that allows temporal loss of the selective permeability of the cell membrane while maintaining the viability of cells that may be useful for the introduction of membrane-impermeable molecules into E. coli cells.

Integration of genetic and metabolic features related to sialic acid metabolism distinguishes human breast cell subtypes

In this report we use 'high-flux' tributanoyl-modified N-acetylmannosamine (ManNAc) analogs with natural N-acetyl as well as non-natural azido- and alkyne N-acyl groups (specifically, 1,3,4-O-Bu3ManNAc, 1,3,4-O-Bu3ManNAz, and 1,3,4-O-Bu3ManNAl respectively) to probe intracellular sialic acid metabolism in the near-normal MCF10A human breast cell line in comparison with earlier stage T-47D and more advanced stage MDA-MB-231 breast cancer lines. An integrated view of sialic acid metabolism was gained by measuring intracellular sialic acid production in tandem with transcriptional profiling of genes linked to sialic acid metabolism. The transcriptional profiling showed several differences between the three lines in the absence of ManNAc analog supplementation that helps explain the different sialoglycan profiles naturally associated with cancer. Only minor changes in mRNA transcript levels occurred upon exposure to the compounds confirming that metabolic flux alone can be a key determinant of sialoglycoconjugate display in breast cancer cells; this result complements the well-established role of genetic control (e.g., the transcription of STs) of sialylation abnormalities ubiquitously associated with cancer. A notable result was that the different cell lines produced significantly different levels of sialic acid upon exogenous ManNAc supplementation, indicating that feedback inhibition of UDP-GlcNAc 2-epimerase/ManNAc kinase (GNE)-generally regarded as the 'gatekeeper' enzyme for titering flux into sialic acid biosynthesis-is not the only regulatory mechanism that limits production of this sugar. A notable aspect of our metabolic glycoengineering approach is its ability to discriminate cell subtype based on intracellular metabolism by illuminating otherwise hidden cell type-specific features. We believe that this strategy combined with multi-dimensional analysis of sialic acid metabolism will ultimately provide novel insights into breast cancer subtypes and provide a foundation for new methods of diagnosis.

Pharmacological, Physiochemical, and Drug-Relevant Biological Properties of Short Chain Fatty Acid Hexosamine Analogues Used in Metabolic Glycoengineering

In this study, we catalog structure activity relationships (SAR) of several short chain fatty acid (SCFA)-modified hexosamine analogues used in metabolic glycoengineering (MGE) by comparing in silico and experimental measurements of physiochemical properties important in drug design. We then describe the impact of these compounds on selected biological parameters that influence the pharmacological properties and safety of drug candidates by monitoring P-glycoprotein (Pgp) efflux, inhibition of cytochrome P450 3A4 (CYP3A4), hERG channel inhibition, and cardiomyocyte cytotoxicity. These parameters are influenced by length of the SCFAs (e.g., acetate vs n-butyrate), which are added to MGE analogues to increase the efficiency of cellular uptake, the regioisomeric arrangement of the SCFAs on the core sugar, the structure of the core sugar itself, and by the type of N-acyl modification (e.g., N-acetyl vs N-azido). By cataloging the influence of these SAR on pharmacological properties of MGE analogues, this study outlines design considerations for tuning the pharmacological, physiochemical, and the toxicological parameters of this emerging class of small molecule drug candidates.

Reactivation of hyperglycemia-induced hypocretin (HCRT) gene silencing by N-acetyl-d-mannosamine in the orexin neurons derived from human iPS cells

Orexin neurons regulate critical brain activities for controlling sleep, eating, emotions, and metabolism, and impaired orexin neuron function results in several neurologic disorders. Therefore, restoring normal orexin function and understanding the mechanisms of loss or impairment of orexin neurons represent important goals. As a step toward that end, we generated human orexin neurons from induced pluripotent stem cells (hiPSCs) by treatment with N-acetyl-d-mannosamine (ManNAc) and its derivatives. The generation of orexin neurons was associated with DNA hypomethylation, histone H3/H4 hyperacetylation, and hypo-O-GlcNAcylation on the HCRT gene locus, and, thereby, the treatment of inhibitors of SIRT1 and OGT were effective at inducing orexin neurons from hiPSCs. The prolonged exposure of orexin neurons to high glucose in culture caused irreversible silencing of the HCRT gene, which was characterized by H3/H4 hypoacetylation and hyper-O-GlcNAcylation. The DNA hypomethylation status, once established in orexin neurogenesis, was maintained in the HCRT-silenced orexin neurons, indicating that histone modifications, but not DNA methylation, were responsible for the HCRT silencing. Thus, the epigenetic status of the HCRT gene is unique to the hyperglycemia-induced silencing. Intriguingly, treatment of ManNAc and its derivatives reactivated HCRT gene expression, while inhibitors SIRT1 and the OGT did not. The present study revealed that the HCRT gene was silenced by the hyperglycemia condition, and ManNAc and its derivatives were useful for restoring the orexin neurons.

Metabolic Remodeling of Cell-Surface Sialic Acids: Principles, Applications, and Recent Advances

Cell-surface sialic acids are essential in mediating a variety of physiological and pathological processes. Sialic acid chemistry and biology remain challenging to investigate, demanding new tools for probing sialylation in living systems. The metabolic glycan labeling (MGL) strategy has emerged as an invaluable chemical biology tool that enables metabolic installation of useful functionalities into cell-surface sialoglycans by "hijacking" the sialic acid biosynthetic pathway. Here we review the principles of MGL and its applications in study and manipulation of sialic acid function, with an emphasis on recent advances.

Targeting the plasma membrane of neoplastic cells through alkylation: a novel approach to cancer chemotherapy

BACKGROUND

Although DNA-directed alkylating agents and related compounds have been a mainstay in chemotherapeutic protocols due to their ability to readily interfere with the rapid mitotic progression of malignant cells, their clinical utility is limited by DNA repair mechanisms and immunosuppression. However, the same destructive nature of alkylation can be reciprocated at the cell surface using novel plasma membrane alkylating agents.

RESULTS

Plasma membrane alkylating agents have elicited long term survival in mammalian models challenged with carcinomas, sarcomas, and leukemias. Further, a specialized group of plasma membrane alkylating agents known as tetra-O-acetate haloacetamido carbohydrate analogs (Tet-OAHCs) potentiates a substantial leukocyte influx at the administration and primary tumor site, indicative of a potent immune response. The effects of plasma membrane alkylating agents may be further potentiated through the use of another novel class of chemotherapeutic agents, known as dihydroxyacetone phosphate (DHAP) inhibitors, since many cancer types are known to rely on the DHAP pathway for lipid synthesis.

CONCLUSION

Despite these compelling data, preliminary clinical trials for plasma membrane-directed agents have yet to be considered. Therefore, this review is intended for academics and clinicians to postulate a novel approach of chemotherapy; altering critical malignant cell signaling at the plasma membrane surface through alkylation, thereby inducing irreversible changes to functions needed for cell survival.

Metabolic glycoengineering bacteria for therapeutic, recombinant protein, and metabolite production applications

Metabolic glycoengineering is a specialization of metabolic engineering that focuses on using small molecule metabolites to manipulate biosynthetic pathways responsible for oligosaccharide and glycoconjugate production. As outlined in this article, this technique has blossomed in mammalian systems over the past three decades but has made only modest progress in prokaryotes. Nevertheless, a sufficient foundation now exists to support several important applications of metabolic glycoengineering in bacteria based on methods to preferentially direct metabolic intermediates into pathways involved in lipopolysaccharide, peptidoglycan, teichoic acid, or capsule polysaccharide production. An overview of current applications and future prospects for this technology are provided in this report.

Metabolic oligosaccharide engineering: implications for selectin-mediated adhesion and leukocyte extravasation

Metabolic oligosaccharide engineering is an emerging technology wherein non-natural monosaccharide analogs are exogenously supplied to living cells and are biosynthetically incorporated into cell surface glycans. A recently reported application of this methodology employs fluorinated analogs of ManNAc, GlcNAc, and GalNAc to modulate selectin-mediated adhesion associated with leukocyte extravasation and cancer cell metastasis. This monograph outlines possible mechanisms underlying the altered adhesion observed in analog-treated cells; these range from the most straightforward explanation (e.g., structural changes to the selectin ligands ablate interaction with their receptors) to the alternative mechanism where the analogs inhibit or otherwise perturb ligand production to more indirect mechanisms (e.g., changes to the biophysical properties of the selectin binding partner, the nanoenviroment of the binding partners, or the entire cell surface).

Metabolic glycoengineering: sialic acid and beyond

This report provides a perspective on metabolic glycoengineering methodology developed over the past two decades that allows natural sialic acids to be replaced with chemical variants in living cells and animals. Examples are given demonstrating how this technology provides the glycoscientist with chemical tools that are beginning to reproduce Mother Nature's control over complex biological systems - such as the human brain - through subtle modifications in sialic acid chemistry. Several metabolic substrates (e.g., ManNAc, Neu5Ac, and CMP-Neu5Ac analogs) can be used to feed flux into the sialic acid biosynthetic pathway resulting in numerous - and sometime quite unexpected - biological repercussions upon nonnatural sialoside display in cellular glycans. Once on the cell surface, ketone-, azide-, thiol-, or alkyne-modified glycans can be transformed with numerous ligands via bioorthogonal chemoselective ligation reactions, greatly increasing the versatility and potential application of this technology. Recently, sialic acid glycoengineering methodology has been extended to other pathways with analog incorporation now possible in surface-displayed GalNAc and fucose residues as well as nucleocytoplasmic O-GlcNAc-modified proteins. Finally, recent efforts to increase the "druggability" of sugar analogs used in metabolic glycoengineering, which have resulted in unanticipated "scaffold-dependent" activities, are summarized.

Targeting glycosylation pathways and the cell cycle: sugar-dependent activity of butyrate-carbohydrate cancer prodrugs

Short-chain fatty acid (SCFA)-carbohydrate hybrid molecules that target both histone deacetylation and glycosylation pathways to achieve sugar-dependent activity against cancer cells are described in this article. Specifically, n-butyrate esters of N-acetyl-D-mannosamine (But4ManNAc, 1) induced apoptosis, whereas corresponding N-acetyl-D-glucosamine (But4GlcNAc, 2), D-mannose (But5Man, 3), or glycerol (tributryin, 4) derivatives only provided transient cell cycle arrest. Western blots, reporter gene assays, and cell cycle analysis established that n-butyrate, when delivered to cells via any carbohydrate scaffold, functioned as a histone deacetylase inhibitor (HDACi), upregulated p21WAF1/Cip1 expression, and inhibited proliferation. However, only 1, a compound that primed sialic acid biosynthesis and modulated the expression of a different set of genes compared to 3, ultimately killed the cells. These results demonstrate that the biological activity of butyrate can be tuned by sugars to improve its anticancer properties.

Metabolic oligosaccharide engineering: perspectives, applications, and future directions

Many adhesion and signaling molecules critical for development, as well as surface markers implicated in diseases ranging from cancer to influenza, contain oligosaccharides that modify their functions. Inside a cell, complex glycosylation pathways assemble these oligosaccharides and attach them to proteins and lipids as they traffic to the cell surface. Until recently, practical technologies to manipulate glycosylation have lagged unlike the molecular biologic and genetic methods available to intervene in nucleic acid and protein biochemistry; now, metabolic oligosaccharide engineering shows promise for manipulating glycosylation. In this methodology, exogenously-supplied non-natural sugars intercept biosynthetic pathways and exploit the remarkable ability of many of the enzymes involved in glycosylation to process metabolites with slightly altered chemical structures. To date, non-natural forms of sialic acid, GalNAc, GlcNAc, and fucose have been incorporated into glycoconjugates that appear on the cell surface; in addition O-GlcNAc protein modification involved in intracellular signaling has been tagged with modified forms of this sugar. Reactive functional groups, including ketones, azides, and thiols, have been incorporated into glycoconjugates and thereby provide chemical 'tags' that can be used for diverse purposes ranging from drug delivery to new modes of carbohydrate-based cell adhesion that can be used to control stem cell destiny. Finally, strategies for further engineering non-natural sugars to improve their pharmacological properties and provide complementary biological activities, such as addition of short chain fatty acids, are discussed in this article.

Versatile intermediates in the selective modification of the amino function of 2-amino-2-deoxy-D-mannopyranose and the 3-position of 2-acetamido-2-deoxy-D-mannose: potential membrane modifiers in neoplastic control

A general method has been developed to selectively modify the amino group of 2-amino-2-deoxy-D-mannopyranose (D-mannosamine), a precursor of the terminal membrane sugar, sialic acid. 1,3,4,6-Tetra-O-acetyl-2-amino-2-deoxy-alpha-D-mannopyranose oxalate was prepared via two routes that allowed introduction of various acyl groups onto the amino function. These compounds were evaluated for their antineoplastic properties. The most significant preclinical therapeutic finding was the antileukemic activity found in mice for tetra-O-acetyl-2-epi-streptozotocin (the acetylated alpha-mannosamine epimer of streptozotocin). Administration of 50 mg/kg/day x 5 to leukemia L1210-bearing DBA/2Ha mice resulted in 5/5 35-day survivors. Neutralization of 1,3,4,6-tetra-O-acetyl-2-amino-2-deoxy-alpha-D-mannopyranose oxalate under aqueous conditions led to 2-acetamido-1,4,6-tri-O-acetyl-2-deoxy-alpha-D-mannopyranose, the oxidation of which gave 2-acetamido-4,6-di-O-acetyl-1,5-anhydro-2-deoxy-D-erythro-hex-1-en-3- ulose. This agent demonstrated an IC50(2) of 25 microM with a murine L1210 cell culture. Administration of 100 mg/kg/day x 5 resulted in 42% ILS3 in DBA/2 mice with ip L1210 leukemia. Several other nonacetylated derivatives were also prepared by direct N-acylation, producing, for example, fluorescently tagged N-dansylmannosamine.