O-GlcNAcase Fragment Discovery with Fluorescence Polarimetry

O-GlcNAcylation promotes malignancy and cisplatin resistance of lung cancer by stabilising NRF2

BACKGROUND

The transcription factor NRF2 plays a significant role in regulating genes that protect cells from oxidative damage. O-GlcNAc modification, a type of posttranslational modification, is crucial for cellular response to stress. Although the involvement of both NRF2 and O-GlcNAc in maintaining cellular redox balance and promoting cancer malignancy has been demonstrated, the potential mechanisms remain elusive.

METHODS

The immunoblotting, luciferase reporter, ROS assay, co-immunoprecipitation, and immunofluorescence was used to detect the effects of global cellular O-GlcNAcylation on NRF2. Mass spectrometry was utilised to map the O-GlcNAcylation sites on NRF2, which was validated by site-specific mutagenesis and O-GlcNAc enzymatic labelling. Human lung cancer samples were employed to verify the association between O-GlcNAc and NRF2. Subsequently, the impact of NRF2 O-GlcNAcylation in lung cancer malignancy and cisplatin resistance were evaluated in vitro and in vivo.

RESULTS

NRF2 is O-GlcNAcylated at Ser103 residue, which hinders its binding to KEAP1 and thus enhances its stability, nuclear localisation, and transcription activity. Oxidative stress and cisplatin can elevate the phosphorylation of OGT at Thr444 through the activation of AMPK kinase, leading to enhanced binding of OGT to NRF2 and subsequent elevation of NRF2 O-GlcNAcylation. Both in cellular and xenograft mouse models, O-GlcNAcylation of NRF2 at Ser103 promotes the malignancy of lung cancer. In human lung cancer tissue samples, there was a significant increase in global O-GlcNAcylation, and elevated levels of NRF2 and its O-GlcNAcylation compared to paired adjacent normal tissues. Chemotherapy promotes NRF2 O-GlcNAcylation, which in turn decreases cellular ROS levels and drives lung cancer cell survival.

CONCLUSION

Our findings indicate that OGT O-GlcNAcylates NRF2 at Ser103, and this modification plays a role in cellular antioxidant, lung cancer malignancy, and cisplatin resistance.

Isoindoline-based fluorogenic probes bearing a self-immolative linker for the sensitive and selective detection of O-GlcNAcase activity

O-GlcNAcase (OGA) is implicated in several important biological and disease-relevant processes. Here, we synthesized fluorogenic probes for OGA by grafting GlcNAc directly or using a self-immolative linker to the hydroxyl position of 4-hydroxylisoindoline (BHID), a typical excited-state intramolecular proton transfer (ESIPT) probe. The probe was used for a fluorogenic assay to determine the half maximal inhibitory concentration of a known OGA inhibitor and differentiate between OGA and hexosaminidase when GlcNAc is replaced by GlcNPr, where a propionyl group is used instead of an acetyl group.

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.

Bicyclic Picomolar OGA Inhibitors Enable Chemoproteomic Mapping of Its Endogenous Post-translational Modifications

Owing to its roles in human health and disease, the modification of nuclear, cytoplasmic, and mitochondrial proteins with O-linked N-acetylglucosamine residues (O-GlcNAc) has emerged as a topic of great interest. Despite the presence of O-GlcNAc on hundreds of proteins within cells, only two enzymes regulate this modification. One of these enzymes is O-GlcNAcase (OGA), a dimeric glycoside hydrolase that has a deep active site cleft in which diverse substrates are accommodated. Chemical tools to control OGA are emerging as essential resources for helping to decode the biochemical and cellular functions of the O-GlcNAc pathway. Here we describe rationally designed bicyclic thiazolidine inhibitors that exhibit superb selectivity and picomolar inhibition of human OGA. Structures of these inhibitors in complex with human OGA reveal the basis for their exceptional potency and show that they extend out of the enzyme active site cleft. Leveraging this structure, we create a high affinity chemoproteomic probe that enables simple one-step purification of endogenous OGA from brain and targeted proteomic mapping of its post-translational modifications. These data uncover a range of new modifications, including some that are less-known, such as O-ubiquitination and N-formylation. We expect that these inhibitors and chemoproteomics probes will prove useful as fundamental tools to decipher the mechanisms by which OGA is regulated and directed to its diverse cellular substrates. Moreover, the inhibitors and structures described here lay out a blueprint that will enable the creation of chemical probes and tools to interrogate OGA and other carbohydrate active enzymes.

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.

Molecular Interrogation to Crack the Case of O-GlcNAc

The O-linked β-N-acetylglucosamine (O-GlcNAc) modification, termed O-GlcNAcylation, is an essential and dynamic post-translational modification in cells. O-GlcNAc transferase (OGT) installs this modification on serine and threonine residues, whereas O-GlcNAcase (OGA) hydrolyzes it. O-GlcNAc modifications are found on thousands of intracellular proteins involved in diverse biological processes. Dysregulation of O-GlcNAcylation and O-GlcNAc cycling enzymes has been detected in many diseases, including cancer, diabetes, cardiovascular and neurodegenerative diseases. Here, recent advances in the development of molecular tools to investigate OGT and OGA functions and substrate recognition are discussed. New chemical approaches to study O-GlcNAc dynamics and its potential roles in the immune system are also highlighted. It is hoped that this minireview will encourage more research in these areas to advance the understanding of O-GlcNAc in biology and diseases.

Genetic recoding to dissect the roles of site-specific protein O-GlcNAcylation

Modification of specific Ser and Thr residues of nucleocytoplasmic proteins with O-GlcNAc, catalyzed by O-GlcNAc transferase (OGT), is an abundant posttranslational event essential for proper animal development and is dysregulated in various diseases. Due to the rapid concurrent removal by the single O-GlcNAcase (OGA), precise functional dissection of site-specific O-GlcNAc modification in vivo is currently not possible without affecting the entire O-GlcNAc proteome. Exploiting the fortuitous promiscuity of OGT, we show that S-GlcNAc is a hydrolytically stable and accurate structural mimic of O-GlcNAc that can be encoded in mammalian systems with CRISPR-Cas9 in an otherwise unperturbed O-GlcNAcome. Using this approach, we target an elusive Ser 405 O-GlcNAc site on OGA, showing that this site-specific modification affects OGA stability.

Assembly of Divalent Ligands and Their Effect on Divalent Binding to Pseudomonas aeruginosa Lectin LecA

Divalent ligands were prepared as inhibitors for the adhesion protein of the problematic Pseudomonas aeruginosa pathogen. Bridging two binding sites enables simultaneous binding of two galactose moieties, which strongly enhances binding. An alternating motif of glucose and triazole and aryl groups was shown to have the right mix of rigidity, solubility, and ease of synthesis. Spacers were varied with respect to the core unit as well as the aglycon portions in an attempt to optimize dynamics and enhance interactions with the protein. Affinities of the divalent ligands were measured by ITC, and K_d’s as low as 12 nM were determined, notably for a compounds with either a rigid (phenyl) or flexible (butyl) unit at the core. Introducing a phenyl aglycon moiety next to the galactoside ligands on both termini did indeed lead to a higher enthalpy of binding, which was more than compensated by entropic costs. The results are discussed in terms of thermodynamics and theoretical calculations of the expected and observed multivalency effects.