Inhibition of O-GlcNAc transferase (OGT) by peptidic hybrids

Discovery of two non-UDP-mimic inhibitors of O-GlcNAc transferase by screening a DNA-encoded library

Finding potent inhibitors of O-GlcNAc transferase (OGT) has proven to be a challenge, especially because the diversity of published inhibitors is low. The large majority of available OGT inhibitors are uridine-based or uridine-like compounds that mimic the main interactions of glycosyl donor UDP-GlcNAc with the enzyme. Until recently, screening of DNA-encoded libraries for discovering hits against protein targets was dedicated to a few laboratories around the world, but has become accessible to wider public with the recent launch of the DELopen platform. Here we report the results and follow-up of a DNA-encoded library screening by using the DELopen platform. This led to the discovery of two new hits with structural features not resembling UDP. Small focused libraries bearing those two scaffolds were made, leading to low micromolar inhibition of OGT and elucidation of their structure-activity relationship.

Targeting O-GlcNAcylation in cancer therapeutic resistance: The sugar Saga continues

O-linked-N-acetylglucosaminylation (O-GlcNAcylation), a dynamic post-translational modification (PTM), holds profound implications in controlling various cellular processes such as cell signaling, metabolism, and epigenetic regulation that influence cancer progression and therapeutic resistance. From the therapeutic perspective, O-GlcNAc modulates drug efflux, targeting and metabolism. By integrating signals from glucose, lipid, amino acid, and nucleotide metabolic pathways, O-GlcNAc acts as a nutrient sensor and transmits signals to exerts its function on genome stability, epithelial-mesenchymal transition (EMT), cell stemness, cell apoptosis, autophagy, cell cycle. O-GlcNAc also attends to tumor microenvironment (TME) and the immune response. At present, several strategies aiming at targeting O-GlcNAcylation are under mostly preclinical evaluation, where the newly developed O-GlcNAcylation inhibitors markedly enhance therapeutic efficacy. Here we systematically outline the mechanisms through which O-GlcNAcylation influences therapy resistance and deliberate on the prospects and challenges associated with targeting O-GlcNAcylation in future cancer treatments.

Design of OSMI-4 Analogs Using Scaffold Hopping: Investigating the Importance of the Uridine Mimic in the Binding of OGT Inhibitors

β-N-Acetylglucosamine transferase (OGT) inhibition is considered an important topic in medicinal chemistry. The involvement of O-GlcNAcylation in several important biological pathways is pointing to OGT as a potential therapeutic target. The field of OGT inhibitors drastically changed after the discovery of the 7-quinolone-4-carboxamide scaffold and its optimization to the first nanomolar OGT inhibitor: OSMI-4. While OSMI-4 is still the most potent inhibitor reported to date, its physicochemical properties are limiting its use as a potential drug candidate as well as a biological tool. In this study, we have introduced a simple modification (elongation) of the peptide part of OSMI-4 that limits the unwanted cyclisation during OSMI-4 synthesis while retaining OGT inhibitory potency. Secondly, we have kept this modified peptide unchanged while incorporating new sulfonamide UDP mimics to try to improve binding of newly designed OGT inhibitors in the UDP-binding site. With the use of computational methods, a small library of OSMI-4 derivatives was designed, prepared and evaluated that provided information about the OGT binding pocket and its specificity toward quinolone-4-carboxamides.

An overview of tools to decipher O-GlcNAcylation from historical approaches to new insights

O-GlcNAcylation is a post-translational modification which affects approximately 5000 human proteins. Its involvement has been shown in many if not all biological processes. Variations in O-GlcNAcylation levels can be associated with the development of diseases. Deciphering the role of O-GlcNAcylation is an important issue to (i) understand its involvement in pathophysiological development and (ii) develop new therapeutic strategies to modulate O-GlcNAc levels. Over the past 30 years, despite the development of several approaches, knowledge of its role and regulation have remained limited. This review proposes an overview of the currently available tools to study O-GlcNAcylation and identify O-GlcNAcylated proteins. Briefly, we discuss pharmacological modulators, methods to study O-GlcNAcylation levels and approaches for O-GlcNAcylomic profiling. This review aims to contribute to a better understanding of the methods used to study O-GlcNAcylation and to promote efforts in the development of new strategies to explore this promising modification.

Discovery of a New Drug-Like Series of OGT Inhibitors by Virtual Screening

O-GlcNAcylation is an essential post-translational modification installed by the enzyme O-β-N-acetyl-d-glucosaminyl transferase (OGT). Modulating this enzyme would be extremely valuable to better understand its role in the development of serious human pathologies, such as diabetes and cancer. However, the limited availability of potent and selective inhibitors hinders the validation of this potential therapeutic target. To explore new chemotypes that target the active site of OGT, we performed virtual screening of a large library of commercially available compounds with drug-like properties. We purchased samples of the most promising virtual hits and used enzyme assays to identify authentic leads. Structure-activity relationships of the best identified OGT inhibitor were explored by generating a small library of derivatives. Our best hit displays a novel uridine mimetic scaffold and inhibited the recombinant enzyme with an IC50 value of 7 µM. The current hit represents an excellent starting point for designing and developing a new set of OGT inhibitors that may prove useful for exploring the biology of OGT.

O-GlcNAcylated peptides and proteins for structural and functional studies

O-GlcNAcylation is an enzymatic post-translational modification occurring in hundreds of protein substrates. This modification occurs through the addition of the monosaccharide N-acetylglucosamine to serine and threonine residues on intracellular proteins in the cytosol, nucleus, and mitochondria. As a highly dynamic form of modification, changes in O-GlcNAc levels coincide with alterations in metabolic state, the presence of stressors, and cellular health. At the protein level, the consequences of the sugar modification can vary, thus necessitating biochemical investigations on protein-specific and site-specific effects. To this end, enzymatic and chemical methods to 'encode' the modification have been developed and the utilization of these synthetic glycopeptides and glycoproteins has since been instrumental in the discovery of the mechanisms by which O-GlcNAcylation can affect a diverse array of biological processes.

New Quinolinone O-GlcNAc Transferase Inhibitors Based on Fragment Growth

O-GlcNAcylation is an important post-translational and metabolic process in cells that must be carefully regulated. O-GlcNAc transferase (OGT) is ubiquitously present in cells and is the only enzyme that catalyzes the transfer of O-GlcNAc to proteins. OGT is a promising target in various pathologies such as cancer, immune system diseases, or nervous impairment. In our previous work we identified the 2-oxo-1,2-dihydroquinoline-4-carboxamide derivatives as promising compounds by a fragment-based drug design approach. Herein, we report the extension of this first series with several new fragments. As the most potent fragment, we identified 3b with an IC₅₀ value of 116.0 μM. If compared with the most potent inhibitor of the first series, F20 (IC₅₀ = 117.6 μM), we can conclude that the new fragments did not improve OGT inhibition remarkably. Therefore, F20 was used as the basis for the design of a series of compounds with the elongation toward the O-GlcNAc binding pocket as the free carboxylate allows easy conjugation. Compound 6b with an IC₅₀ value of 144.5 μM showed the most potent OGT inhibition among the elongated compounds, but it loses inhibition potency when compared to the UDP mimetic F20. We therefore assume that the binding of the compounds in the O-GlcNAc binding pocket is likely not crucial for OGT inhibition. Furthermore, evaluation of the compounds with two different assays revealed that some inhibitors most likely interfere with the commercially available UDP-Glo™ glycosyltransferase assay, leading to false positive results. This observation calls for caution, when evaluating UDP mimetic as OGT inhibitors with the UDP-Glo™ glycosyltransferase assay, as misinterpretations can occur.

Overview of the Assays to Probe O-Linked β-N-Acetylglucosamine Transferase Binding and Activity

O-GlcNAcylation is a posttranslational modification that occurs at serine and threonine residues of protein substrates by the addition of O-linked β-d-N-acetylglucosamine (GlcNAc) moiety. Two enzymes are involved in this modification: O-GlcNac transferase (OGT), which attaches the GlcNAc residue to the protein substrate, and O-GlcNAcase (OGA), which removes it. This biological balance is important for many biological processes, such as protein expression, cell apoptosis, and regulation of enzyme activity. The extent of this modification has sparked interest in the medical community to explore OGA and OGT as therapeutic targets, particularly in degenerative diseases. While some OGA inhibitors are already in phase 1 clinical trials for the treatment of Alzheimer's disease, OGT inhibitors still have a long way to go. Due to complex expression and instability, the discovery of potent OGT inhibitors is challenging. Over the years, the field has grappled with this problem, and scientists have developed a number of techniques and assays. In this review, we aim to highlight assays and techniques for OGT inhibitor discovery, evaluate their strength for the field, and give us direction for future bioassay methods.

Novel bisubstrate uridine-peptide analogues bearing a pyrophosphate bioisostere as inhibitors of human O-GlcNAc transferase

Protein O-linked β-D-N-acetylglucosamine (O-GlcNAc) modification (O-GlcNAcylation), an essential post-translational as well as cotranslational modification, is the attachment of β-D-N-acetylglucosamine to serine and threonine residues of nucleocytoplasmic proteins. An aberrant O-GlcNAc profile on certain proteins has been implicated in metabolic diseases such as diabetes and cancer. Inhibitors of O-GlcNAc transferase (OGT) are valuable tools to study the cell biology of protein O-GlcNAc modification. In this study we report novel uridine-peptide conjugate molecules composed of an acceptor peptide covalently linked to a catalytically inactive donor substrate analogue that bears a pyrophosphate bioisostere and explore their inhibitory activities against OGT by a radioactive hOGT assay. Further, we investigate the structural basis of their activities via molecular modelling, explaining their lack of potency towards OGT inhibition.

Monitoring and modulating O-GlcNAcylation: assays and inhibitors of O-GlcNAc processing enzymes

O-linked N-acetylglucosamine (O-GlcNAc) is protein modification that is emerging as a regulator of diverse aspects of cellular physiology. Aberrant O-GlcNAcylation has been linked to several diseases, spurring the creation of methods to detect and perturb the activity of the two enzymes that govern this modification - O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA). Here we summarize assays used for these two enzymes. We also detail the latest structure-guided development of inhibitors of these two enzymes and touch on selected reports that underscore the utility of inhibitors as tools for uncovering the diverse roles of O-GlcNAc in cell function. Finally, we summarize recent reports on the potential therapeutic benefits of antagonizing these enzymes and comment on outstanding challenges within the field.

Bisubstrate Ether-Linked Uridine-Peptide Conjugates as O-GlcNAc Transferase Inhibitors

The O-linked β-N-acetylglucosamine (O-GlcNAc) transferase (OGT) is a master regulator of installing O-GlcNAc onto serine or threonine residues on a multitude of target proteins. Numerous nuclear and cytosolic proteins of varying functional classes, including translational factors, transcription factors, signaling proteins, and kinases are OGT substrates. Aberrant O-GlcNAcylation of proteins is implicated in signaling in metabolic diseases such as diabetes and cancer. Selective and potent OGT inhibitors are valuable tools to study the role of OGT in modulating a wide range of effects on cellular functions. We report linear bisubstrate ether-linked uridine-peptide conjugates as OGT inhibitors with micromolar affinity. In vitro evaluation of the compounds revealed the importance of donor substrate, linker and acceptor substrate in the rational design of bisubstrate analogue inhibitors. Molecular dynamics simulations shed light on the binding of this novel class of inhibitors and rationalized the effect of amino acid truncation of acceptor peptide on OGT inhibition.

Biological evaluation and molecular modeling of peptidomimetic compounds as inhibitors for O-GlcNAc transferase (OGT)

The vital enzyme O-linked β-N-acetylglucosamine transferase (OGT) catalyzes the O-GlcNAcylation of intracellular proteins coupling the metabolic status to cellular signaling and transcription pathways. Aberrant levels of O-GlcNAc and OGT have been linked to metabolic diseases as cancer and diabetes. Here, a new series of peptidomimetic OGT inhibitors was identified highlighting the compound LQMed 330, which presented better IC₅₀ compared to the most potent inhibitors found in the literature. Molecular modeling study of selected inhibitors into the OGT binding site provided insight into the behavior by which these compounds interact with the enzyme. The results obtained in this study provided new perspectives on the design and synthesis of highly specific OGT inhibitors.

Intracellular Hydrolysis of Small-Molecule O-Linked N-Acetylglucosamine Transferase Inhibitors Differs among Cells and Is Not Required for Its Inhibition

O-GlcNAcylation is an essential post-translational modification that occurs on nuclear and cytoplasmic proteins, regulating their function in response to cellular stress and altered nutrient availability. O-GlcNAc transferase (OGT) is the enzyme that catalyzes this reaction and represents a potential therapeutic target, whose biological role is still not fully understood. To support this research field, a series of cell-permeable, low-nanomolar OGT inhibitors were recently reported. In this study, we resynthesized the most potent OGT inhibitor of the library, OSMI-4, and we used it to investigate OGT inhibition in different human cell lines. The compound features an ethyl ester moiety that is supposed to be cleaved by carboxylesterases to generate its active metabolite. Our LC-HRMS analysis of the cell lysates shows that this is not always the case and that, even in the cell lines where hydrolysis does not occur, OGT activity is inhibited.

Nutrient Sensor mTOR and OGT: Orchestrators of Organelle Homeostasis in Pancreatic β-Cells

The purpose of this review is to integrate the role of nutrient-sensing pathways into β-cell organelle dysfunction prompted by nutrient excess during type 2 diabetes (T2D). T2D encompasses chronic hyperglycemia, hyperlipidemia, and inflammation, which each contribute to β-cell failure. These factors can disrupt the function of critical β-cell organelles, namely, the ER, mitochondria, lysosomes, and autophagosomes. Dysfunctional organelles cause defects in insulin synthesis and secretion and activate apoptotic pathways if homeostasis is not restored. In this review, we will focus on mTORC1 and OGT, two major anabolic nutrient sensors with important roles in β-cell physiology. Though acute stimulation of these sensors frequently improves β-cell function and promotes adaptation to cell stress, chronic and sustained activity disturbs organelle homeostasis. mTORC1 and OGT regulate organelle function by influencing the expression and activities of key proteins, enzymes, and transcription factors, as well as by modulating autophagy to influence clearance of defective organelles. In addition, mTORC1 and OGT activity influence islet inflammation during T2D, which can further disrupt organelle and β-cell function. Therapies for T2D that fine-tune the activity of these nutrient sensors have yet to be developed, but the important role of mTORC1 and OGT in organelle homeostasis makes them promising targets to improve β-cell function and survival.

O-GlcNAc Modification Protects against Protein Misfolding and Aggregation in Neurodegenerative Disease

Post-translational modifications (PTMs) of proteins are becoming the focus of intense research due to their implications in a broad spectrum of neurodegenerative diseases. Various PTMs have been identified to alter the toxic profiles of proteins which play critical roles in disease etiology. In Alzheimer's disease (AD), dysregulated phosphorylation is reported to promote pathogenic processing of the microtubule-associated tau protein. Among the PTMs, the enzymatic addition of N-acetyl-d-glucosamine (GlcNAc) residues to Ser/Thr residues is reported to deliver protective effects against the pathogenic processing of both amyloid precursor protein (APP) and tau. Modification of tau with as few as one single O-GlcNAc residue inhibits its toxic self-assembly. This modification also has the same effect on the assembly of the Parkinson's disease (PD) associated α-synuclein (ASyn) protein. In fact, O-GlcNAcylation ( O-linked GlcNAc modification) affects the processing of numerous proteins implicated in AD, PD, amyotrophic lateral sclerosis (ALS), and Huntington's disease (HD) in a similar manner. As such, manipulation of a protein's O-GlcNAcylation status has been proposed to offer therapeutic routes toward addressing multiple neurodegenerative pathologies. Here we review the various effects that O-GlcNAc modification, and its modulated expression, have on pathogenically significant proteins involved in neurodegenerative disease.