Dynamic interplay between O-glycosylation and O-phosphorylation of nucleocytoplasmic proteins: alternative glycosylation/phosphorylation of THR-58, a known mutational hot spot of c-Myc in lymphomas, is regulated by mitogens

Rapid flow-based synthesis of post-translationally modified peptides and proteins: a case study on MYC's transactivation domain

Protein-protein interactions of c-Myc (MYC) are often regulated by post-translational modifications (PTMs), such as phosphorylation, and crosstalk thereof. Studying these interactions requires proteins with unique PTM patterns, which are challenging to obtain by recombinant methods. Standard peptide synthesis and native chemical ligation can produce such modified proteins, but are time-consuming and therefore typically limited to the study of individual PTMs. Herein, we report the development of flow-based methods for the rapid synthesis of phosphorylated MYC sequences (up to 84 AA), and demonstrate the versatility of this approach for the incorporation of other PTMs (N ε-methylation, sulfation, acetylation, glycosylation) and combinations thereof. Peptides containing up to seven PTMs and phosphorylation at up to five sites were successfully prepared and isolated in high yield and purity. We further produced ten PTM-decorated analogues of the MYC Transactivation Domain (TAD) to screen for binding to the tumor suppressor protein, Bin1, using heteronuclear NMR and native mass spectrometry. We determined the effects of phosphorylation and glycosylation on the strength of the MYC:Bin1 interaction, and reveal an influence of MYC sequence length on binding. Our platform for the rapid synthesis of MYC sequences up to 84 AA with distinct PTM patterns thus enables the systematic study of PTM function at a molecular level, and offers a convenient way for expedited screening of constructs.

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.

O-GlcNAcylation: cellular physiology and therapeutic target for human diseases

O-linked-β-N-acetylglucosamine (O-GlcNAcylation) is a distinctive posttranslational protein modification involving the coordinated action of O-GlcNAc transferase and O-GlcNAcase, primarily targeting serine or threonine residues in various proteins. This modification impacts protein functionality, influencing stability, protein-protein interactions, and localization. Its interaction with other modifications such as phosphorylation and ubiquitination is becoming increasingly evident. Dysregulation of O-GlcNAcylation is associated with numerous human diseases, including diabetes, nervous system degeneration, and cancers. This review extensively explores the regulatory mechanisms of O-GlcNAcylation, its effects on cellular physiology, and its role in the pathogenesis of diseases. It examines the implications of aberrant O-GlcNAcylation in diabetes and tumorigenesis, highlighting novel insights into its potential role in cardiovascular diseases. The review also discusses the interplay of O-GlcNAcylation with other protein modifications and its impact on cell growth and metabolism. By synthesizing current research, this review elucidates the multifaceted roles of O-GlcNAcylation, providing a comprehensive reference for future studies. It underscores the potential of targeting the O-GlcNAcylation cycle in developing novel therapeutic strategies for various pathologies.

Best practices in assessing cardiac protein O-GlcNAcylation by immunoblot

The modification of serine and threonine amino acids of proteins by O-linked N-acetylglucosamine (O-GlcNAc) regulates the activity, stability, function, and subcellular localization of proteins. Dysregulation of O-GlcNAc homeostasis is well established as a hallmark of various cardiac diseases, including cardiac hypertrophy, heart failure, complications associated with diabetes, and responses to acute injuries such as oxidative stress and ischemia-reperfusion. Given the limited availability of site-specific O-GlcNAc antibodies, studies of changes in O-GlcNAcylation in the heart frequently use pan-O-GlcNAc antibodies for semiquantitative evaluation of overall O-GlcNAc levels. However, there is a high degree of variability in many published cardiac O-GlcNAc blots. For example, many blots often have regions that lack O-GlcNAc positive staining of proteins either below 50 or above 100 kDa. In some O-GlcNAc blots, only a few protein bands are detected, while in others, intense bands around 75 kDa dominate the gel due to nonspecific IgM band staining, making it difficult to visualize less intense bands. Therefore, the goal of this study was to develop a modifiable protocol that optimizes O-GlcNAc positive banding of proteins in cardiac tissue extracts. We showed that O-GlcNAc blots using CTD110.6 antibody of proteins ranging from <30 to ∼450 kDa could be obtained while also limiting nonspecific staining. We also show that some myofilament proteins are recognized by the CTD110.6 antibody. Therefore, by protocol optimization using the widely available CTD110.6 antibody, we found that it is possible to obtain pan-O-GlcNAc blots of cardiac tissue, which minimizes common limitations associated with this technique.NEW & NOTEWORTHY The post-translational modification of proteins by O-linked N-acetylglucosamine (O-GlcNAc) is recognized as mediating cardiac pathophysiology. However, there is considerable variability in the quality of O-GlcNAc immunoblots used to evaluate changes in cardiac O-GlcNAc levels. Here we show that with relatively minor changes to a commonly used protocol it is possible to minimize the intensity of nonspecific bands while also reproducibly generating O-GlcNAc immunoblots covering a range of molecular weights from <30 to ∼450 kDa.

Reciprocal regulation of TWIST1 and OGT determines the decitabine efficacy in MDS/AML

Chemoresistance poses a significant impediment to effective treatment strategies for myelodysplastic syndromes (MDS) and acute myeloid leukemia (AML). Our previous study unveiled that oncogene TWIST1 interacted with DNA methyltransferase 3a (DNMT3a) to regulate the decitabine (DAC) resistance in MDS/AML. However, the underlying mechanism of TWIST1 dysregulation in DAC resistance remained enigmatic. Here, we found that O-GlcNAc modification was upregulated in CD34+ cells from MDS/AML patients who do not respond to DAC treatment. Functional study revealed that O-GlcNAcylation could stabilize TWIST1 by impeding its interaction with ubiquitin E3 ligase CBLC. In addition, as one typical transcription factor, TWIST1 could bind to the promoter of O-GlcNAc transferase (OGT) gene and activate its transcription. Collectively, we highlighted the crucial role of the O-GlcNAcylated TWIST1 in the chemoresistance capacity of MDS/AML clonal cells, which may pave the way for the development of a new therapeutic strategy targeting O-GlcNAcylated proteins and reducing the ratio of MDS/AML relapse. Video Abstract.

O-GlcNAc Dynamics: The Sweet Side of Protein Trafficking Regulation in Mammalian Cells

The transport of proteins between the different cellular compartments and the cell surface is governed by the secretory pathway. Alternatively, unconventional secretion pathways have been described in mammalian cells, especially through multivesicular bodies and exosomes. These highly sophisticated biological processes rely on a wide variety of signaling and regulatory proteins that act sequentially and in a well-orchestrated manner to ensure the proper delivery of cargoes to their final destination. By modifying numerous proteins involved in the regulation of vesicular trafficking, post-translational modifications (PTMs) participate in the tight regulation of cargo transport in response to extracellular stimuli such as nutrient availability and stress. Among the PTMs, O-GlcNAcylation is the reversible addition of a single N-acetylglucosamine monosaccharide (GlcNAc) on serine or threonine residues of cytosolic, nuclear, and mitochondrial proteins. O-GlcNAc cycling is mediated by a single couple of enzymes: the O-GlcNAc transferase (OGT) which catalyzes the addition of O-GlcNAc onto proteins, and the O-GlcNAcase (OGA) which hydrolyses it. Here, we review the current knowledge on the emerging role of O-GlcNAc modification in the regulation of protein trafficking in mammalian cells, in classical and unconventional secretory pathways.

The Hexosamine Biosynthesis Pathway: Regulation and Function

The hexosamine biosynthesis pathway (HBP) produces uridine diphosphate-N-acetyl glucosamine, UDP-GlcNAc, which is a key metabolite that is used for N- or O-linked glycosylation, a co- or post-translational modification, respectively, that modulates protein activity and expression. The production of hexosamines can occur via de novo or salvage mechanisms that are catalyzed by metabolic enzymes. Nutrients including glutamine, glucose, acetyl-CoA, and UTP are utilized by the HBP. Together with availability of these nutrients, signaling molecules that respond to environmental signals, such as mTOR, AMPK, and stress-regulated transcription factors, modulate the HBP. This review discusses the regulation of GFAT, the key enzyme of the de novo HBP, as well as other metabolic enzymes that catalyze the reactions to produce UDP-GlcNAc. We also examine the contribution of the salvage mechanisms in the HBP and how dietary supplementation of the salvage metabolites glucosamine and N-acetylglucosamine could reprogram metabolism and have therapeutic potential. We elaborate on how UDP-GlcNAc is utilized for N-glycosylation of membrane and secretory proteins and how the HBP is reprogrammed during nutrient fluctuations to maintain proteostasis. We also consider how O-GlcNAcylation is coupled to nutrient availability and how this modification modulates cell signaling. We summarize how deregulation of protein N-glycosylation and O-GlcNAcylation can lead to diseases including cancer, diabetes, immunodeficiencies, and congenital disorders of glycosylation. We review the current pharmacological strategies to inhibit GFAT and other enzymes involved in the HBP or glycosylation and how engineered prodrugs could have better therapeutic efficacy for the treatment of diseases related to HBP deregulation.

The role of O-GlcNAcylation in development

O-GlcNAcylation is a dynamic post-translational modification performed by two opposing enzymes: O-GlcNAc transferase and O-GlcNAcase. O-GlcNAcylation is generally believed to act as a metabolic integrator in numerous signalling pathways. The stoichiometry of this modification is tightly controlled throughout all stages of development, with both hypo/hyper O-GlcNAcylation resulting in broad defects. In this Primer, we discuss the role of O-GlcNAcylation in developmental processes from stem cell maintenance and differentiation to cell and tissue morphogenesis.

Inhibiting O-GlcNAcylation impacts p38 and Erk1/2 signaling and perturbs cardiomyocyte hypertrophy

The dynamic cycling of O-linked GlcNAc (O-GlcNAc) on and off Ser/Thr residues of intracellular proteins, termed O-GlcNAcylation, is mediated by the conserved enzymes O-GlcNAc transferase (OGT) and O-GlcNAcase. O-GlcNAc cycling is important in homeostatic and stress responses, and its perturbation sensitizes the heart to ischemic and other injuries. Despite considerable progress, many molecular pathways impacted by O-GlcNAcylation in the heart remain unclear. The mitogen-activated protein kinase (MAPK) pathway is a central signaling cascade that coordinates developmental, physiological, and pathological responses in the heart. The developmental or adaptive arm of MAPK signaling is primarily mediated by Erk kinases, while the pathophysiologic arm is mediated by p38 and Jnk kinases. Here, we examine whether O-GlcNAcylation affects MAPK signaling in cardiac myocytes, focusing on Erk1/2 and p38 in basal and hypertrophic conditions induced by phenylephrine. Using metabolic labeling of glycans coupled with alkyne-azide "click" chemistry, we found that Erk1/2 and p38 are O-GlcNAcylated. Supporting the regulation of p38 by O-GlcNAcylation, the OGT inhibitor, OSMI-1, triggers the phosphorylation of p38, an event that involves the NOX2-Ask1-MKK3/6 signaling axis and also the noncanonical activator Tab1. Additionally, OGT inhibition blocks the phenylephrine-induced phosphorylation of Erk1/2. Consistent with perturbed MAPK signaling, OSMI-1-treated cardiomyocytes have a blunted hypertrophic response to phenylephrine, decreased expression of cTnT (key component of the contractile apparatus), and increased expression of maladaptive natriuretic factors Anp and Bnp. Collectively, these studies highlight new roles for O-GlcNAcylation in maintaining a balanced activity of Erk1/2 and p38 MAPKs during hypertrophic growth responses in cardiomyocytes.

OGT Binding Peptide-Tagged Strategy Increases Protein O-GlcNAcylation Level in E. coli

O-GlcNAcylation is a single glycosylation of GlcNAc mediated by OGT, which regulates the function of substrate proteins and is closely related to many diseases. However, a large number of O-GlcNAc-modified target proteins are costly, inefficient, and complicated to prepare. In this study, an OGT binding peptide (OBP)-tagged strategy for improving the proportion of O-GlcNAc modification was established successfully in E. coli. OBP (P1, P2, or P3) was fused with target protein Tau as tagged Tau. Tau or tagged Tau was co-constructed with OGT into a vector expressed in E. coli. Compared with Tau, the O-GlcNAc level of P1Tau and TauP1 increased 4~6-fold. Moreover, the P1Tau and TauP1 increased the O-GlcNAc-modified homogeneity. The high O-GlcNAcylation on P1Tau resulted in a significantly slower aggregation rate than Tau in vitro. This strategy was also used successfully to increase the O-GlcNAc level of c-Myc and H2B. These results indicated that the OBP-tagged strategy was a successful approach to improve the O-GlcNAcylation of a target protein for further functional research.

The Four Homeostasis Knights: In Balance upon Post-Translational Modifications

A cancer outcome is a multifactorial event that comes from both exogenous injuries and an endogenous predisposing background. The healthy state is guaranteed by the fine-tuning of genes controlling cell proliferation, differentiation, and development, whose alteration induces cellular behavioral changes finally leading to cancer. The function of proteins in cells and tissues is controlled at both the transcriptional and translational level, and the mechanism allowing them to carry out their functions is not only a matter of level. A major challenge to the cell is to guarantee that proteins are made, folded, assembled and delivered to function properly, like and even more than other proteins when referring to oncogenes and onco-suppressors products. Over genetic, epigenetic, transcriptional, and translational control, protein synthesis depends on additional steps of regulation. Post-translational modifications are reversible and dynamic processes that allow the cell to rapidly modulate protein amounts and function. Among them, ubiquitination and ubiquitin-like modifications modulate the stability and control the activity of most of the proteins that manage cell cycle, immune responses, apoptosis, and senescence. The crosstalk between ubiquitination and ubiquitin-like modifications and post-translational modifications is a keystone to quickly update the activation state of many proteins responsible for the orchestration of cell metabolism. In this light, the correct activity of post-translational machinery is essential to prevent the development of cancer. Here we summarize the main post-translational modifications engaged in controlling the activity of the principal oncogenes and tumor suppressors genes involved in the development of most human cancers.

Targeting O-GlcNAcylation to overcome resistance to anti-cancer therapies

In cancer cells, metabolic reprogramming is associated with an alteration of the O-GlcNAcylation homeostasis. This post-translational modification (PTM) that attaches O-GlcNAc moiety to intracellular proteins is dynamically and finely regulated by the O-GlcNAc Transferase (OGT) and the O-GlcNAcase (OGA). It is now established that O-GlcNAcylation participates in many features of cancer cells including a high rate of cell growth, invasion, and metastasis but little is known about its impact on the response to therapies. The purpose of this review is to highlight the role of O-GlcNAc protein modification in cancer resistance to therapies. We summarize the current knowledge about the crosstalk between O-GlcNAcylation and molecular mechanisms underlying tumor sensitivity/resistance to targeted therapies, chemotherapies, immunotherapy, and radiotherapy. We also discuss potential benefits and strategies of targeting O-GlcNAcylation to overcome cancer resistance.

The Glycobiology of Pulmonary Arterial Hypertension

Pulmonary arterial hypertension (PAH) is a progressive pulmonary vascular disease of complex etiology. Cases of PAH that do not receive therapy after diagnosis have a low survival rate. Multiple reports have shown that idiopathic PAH, or IPAH, is associated with metabolic dysregulation including altered bioavailability of nitric oxide (NO) and dysregulated glucose metabolism. Multiple processes such as increased proliferation of pulmonary vascular cells, angiogenesis, apoptotic resistance, and vasoconstriction may be regulated by the metabolic changes demonstrated in PAH. Recent reports have underscored similarities between metabolic abnormalities in cancer and IPAH. In particular, increased glucose uptake and altered glucose utilization have been documented and have been linked to the aforementioned processes. We were the first to report a link between altered glucose metabolism and changes in glycosylation. Subsequent reports have highlighted similar findings, including a potential role for altered metabolism and aberrant glycosylation in IPAH pathogenesis. This review will detail research findings that demonstrate metabolic dysregulation in PAH with an emphasis on glycobiology. Furthermore, this report will illustrate the similarities in the pathobiology of PAH and cancer and highlight the novel findings that researchers have explored in the field.

O-GlcNAcylation of MORC2 at threonine 556 by OGT couples TGF-β signaling to breast cancer progression

MORC family CW-type zinc finger 2 (MORC2) is a newly identified chromatin-remodeling enzyme involved in DNA damage response and gene transcription, and its dysregulation has been linked with Charcot-Marie-Tooth disease, neurodevelopmental disorder, and cancer. Despite its functional importance, how MORC2 is regulated remains enigmatic. Here, we report that MORC2 is O-GlcNAcylated by O-GlcNAc transferase (OGT) at threonine 556. Mutation of this site or pharmacological inhibition of OGT impairs MORC2-mediated breast cancer cell migration and invasion in vitro and lung colonization in vivo. Moreover, transforming growth factor-β1 (TGF-β1) induces MORC2 O-GlcNAcylation through enhancing the stability of glutamine-fructose-6-phosphate aminotransferase (GFAT), the rate-limiting enzyme for producing the sugar donor for OGT. O-GlcNAcylated MORC2 is required for transcriptional activation of TGF-β1 target genes connective tissue growth factor (CTGF) and snail family transcriptional repressor 1 (SNAIL). In support of these observations, knockdown of GFAT, SNAIL or CTGF compromises TGF-β1-induced, MORC2 O-GlcNAcylation-mediated breast cancer cell migration and invasion. Clinically, high expression of OGT, MORC2, SNAIL, and CTGF in breast tumors is associated with poor patient prognosis. Collectively, these findings uncover a previously unrecognized mechanistic role for MORC2 O-GlcNAcylation in breast cancer progression and provide evidence for targeting MORC2-dependent breast cancer through blocking its O-GlcNAcylation.

The Beginner’s Guide to O-GlcNAc: From Nutrient Sensitive Pathway Regulation to Its Impact on the Immune System

The addition of N-acetyl glucosamine (GlcNAc) on the hydroxy group of serine/threonine residues is known as O-GlcNAcylation (OGN). The dynamic cycling of this monosaccharide on and off substrates occurs via O-linked β-N-acetylglucosamine transferase (OGT) and O-linked β-N-acetylglucosaminase (OGA) respectively. These enzymes are found ubiquitously in eukaryotes and genetic knock outs of the ogt gene has been found to be lethal in embryonic mice. The substrate scope of these enzymes is vast, over 15,000 proteins across 43 species have been identified with O-GlcNAc. OGN has been known to play a key role in several cellular processes such as: transcription, translation, cell signaling, nutrient sensing, immune cell development and various steps of the cell cycle. However, its dysregulation is present in various diseases: cancer, neurodegenerative diseases, diabetes. O-GlcNAc is heavily involved in cross talk with other post-translational modifications (PTM), such as phosphorylation, acetylation, and ubiquitination, by regulating each other’s cycling enzymes or directly competing addition on the same substrate. This crosstalk between PTMs can affect gene expression, protein localization, and protein stability; therefore, regulating a multitude of cell signaling pathways. In this review the roles of OGN will be discussed. The effect O-GlcNAc exerts over protein-protein interactions, the various forms of crosstalk with other PTMs, and its role as a nutrient sensor will be highlighted. A summary of how these O-GlcNAc driven processes effect the immune system will also be included.

Inhibiting Hyper-O-GlcNAcylation of c-Myc accelerate diabetic wound healing by alleviating keratinocyte dysfunction

Background

Diabetic foot ulcers characterized by delayed healing are one of the main complications of diabetes. Epidermal keratinocyte dysfunction has been found to play a pivotal role in the poor healing ability of diabetic wounds. In this study, we aimed to explore the relationship between c-Myc and its O-linked N-acetylglucosamine (O-GlcNAc) glycosylation (O-GlcNAcylation) modification and keratinocyte dysfunction in diabetic wounds.

Methods

Clinical wound samples were collected and a full-thickness skin defect wound model of diabetic rats was established. Re-epithelialization of wounds was observed by H&E staining and expressions of proliferating cell nuclear antigen, transglutaminase 1, loricrin, c-Myc and O-GlcNAc were measured by immunohistochemistry. The functional changes of proliferation, migration and differentiation of human immortalized epidermal cells (HaCaT) cells after overexpression or knockdown of c-Myc were observed. O-GlcNAcylation of c-Myc was confirmed using immunoprecipitation and proximity ligation assay. Stability of the c-Myc protein was measured using cycloheximide. Wound healing was observed after topical application of compounds that inhibited c-Myc or O-GlcNAc on diabetic wounds.

Results

Keratinocytes at the diabetic wound margin were characterized by active proliferation and division, slow migration and poor differentiation. Similar phenomena were observed in HaCaT cells cultured in 30 mM glucose and keratinocytes at the wound margin of the diabetic rats. The expression of c-Myc was increased in keratinocytes at the wound margin of diabetic rats, patients, and in HaCaT cells cultured with 30 mM glucose. Increased expression of c-Myc promoted the proliferation while inhibiting the migration and differentiation of the HaCaT cells, and inhibition of c-Myc promoted diabetic wound healing. Increased O-GlcNAcylation of c-Myc with 30 mM glucose stabilized the c-Myc proteins. Inhibition of O-GlcNAc ameliorated keratinocyte dysfunction and promoted diabetic wound healing.

Conclusions

Increased expression of c-Myc promoted abnormal proliferation and inhibited migration and differentiation of keratinocytes at the diabetic wound margin. Increased O-GlcNAcylation of c-Myc with 30 mM glucose stabilized the c-Myc proteins. Inhibition of c-Myc or O-GlcNAc alleviated delayed diabetic wound healing. These findings make c-Myc and O-GlcNAc potential therapeutic targets for diabetic wounds.

Advances in chemical probing of protein O-GlcNAc glycosylation: structural role and molecular mechanisms

The addition of O-linked-β-D-N-acetylglucosamine (O-GlcNAc) onto serine and threonine residues of nuclear and cytoplasmic proteins is an abundant, unique post-translational modification governing important biological processes. O-GlcNAc dysregulation underlies several metabolic disorders leading to human diseases, including cancer, neurodegeneration and diabetes. This review provides an extensive summary of the recent progress in probing O-GlcNAcylation using mainly chemical methods, with a special focus on discussing mechanistic insights and the structural role of O-GlcNAc at the molecular level. We highlight key aspects of the O-GlcNAc enzymes, including development of OGT and OGA small-molecule inhibitors, and describe a variety of chemoenzymatic and chemical biology approaches for the study of O-GlcNAcylation. Special emphasis is placed on the power of chemistry in the form of synthetic glycopeptide and glycoprotein tools for investigating the site-specific functional consequences of the modification. Finally, we discuss in detail the conformational effects of O-GlcNAc glycosylation on protein structure and stability, relevant O-GlcNAc-mediated protein interactions and its molecular recognition features by biological receptors. Future research in this field will provide novel, more effective chemical strategies and probes for the molecular interrogation of O-GlcNAcylation, elucidating new mechanisms and functional roles of O-GlcNAc with potential therapeutic applications in human health.

Phosphorylated and O-GlcNAc Modified IRS-1 (Ser1101) and -2 (Ser1149) Contribute to Human Diabetes Type II

BACKGROUND

The prevalence of the chronic metabolic disorder Type 2 diabetes mellitus (T2DM) is increasing steadily, and has even turned into an epidemic in some countries. T2DM results from defective responses to insulin and obesity is a major factor behind insulin resistance in T2DM. Insulin receptor substrate (IRS) proteins are adaptor proteins in the insulin receptor signalling pathway. The insulin signalling is controlled through tyrosine phosphorylation of IRS-1 and IRS-2, and dysregulation of IRS proteins signalling may lead to glucose intolerance and eventually insulin resistance.

OBJECTIVE

In this work, we suggest that both glycosylation (O-GlcNAc modification) and phosphorylation of IRS-1 and -2 are involved in the pathogenesis of T2DM.

METHODS

Phosphorylation and O-GlcNAc modifications (Ser1101 in IRS-1 and Ser1149 in IRS-2) proteins were determined experimentally by sandwich ELISA with specific antibodies and with bioinformatics tools.

RESULTS

When IRS-1 (on Ser1101) and IRS-2 (Ser1149) become glycosylated following an increase in UDP-GlcNAc pools, it may contribute to insulin resistance. Whereas when the same (IRS-1 on Ser1101 and IRS-2 on Ser1149) are phosphorylated, the insulin signalling is inhibited.

DISCUSSION

In this work OGlcNAc-modified proteins were specifically detected using O-Glc- NAc-specific antibodies, suggesting that elevated levels of O-GlcNAc-modified proteins are found, independently of their possible involvement in Advanced Glycation End products (AGEs).

CONCLUSION

This study suggests a mechanism, which is controlled by posttranslational modifications, and may contribute to the pathogenesis of type II diabetes.

Detection and Analysis of Proteins Modified by O-Linked N-Acetylglucosamine

O-GlcNAc is a common post-translational modification of nuclear, mitochondrial, and cytoplasmic proteins that regulates normal physiology and the cell stress response. Dysregulation of O-GlcNAc cycling is implicated in the etiology of type II diabetes, heart failure, hypertension, and Alzheimer's disease, as well as cardioprotection. These protocols cover simple and comprehensive techniques for detecting proteins modified by O-GlcNAc and studying the enzymes that add or remove O-GlcNAc. © 2021 The Authors. Current Protocols published by Wiley Periodicals LLC. Basic Protocol 1: Increasing the stoichiometry of O-GlcNAc on proteins before analysis Basic Protocol 2: Detection of proteins modified by O-GlcNAc using antibodies Basic Protocol 3: Detection of proteins modified by O-GlcNAc using the lectin sWGA Support Protocol 1: Control for O-linked glycosylation Basic Protocol 4: Detection and enrichment of proteins using WGA-agarose Support Protocol 2: Digestion of proteins with hexosaminidase Alternate Protocol: Detection of proteins modified by O-GlcNAc using galactosyltransferase Support Protocol 3: Autogalactosylation of galactosyltransferase Support Protocol 4: Assay of galactosyltransferase activity Basic Protocol 5: Characterization of labeled glycans by β-elimination and chromatography Basic Protocol 6: Detection of O-GlcNAc in 96-well plates Basic Protocol 7: Assay for OGT activity Support Protocol 5: Desalting of O-GlcNAc transferase Basic Protocol 8: Assay for O-GlcNAcase activity.

Advances in Strategies and Tools Available for Interrogation of Protein O-GlcNAcylation

The attachment of a single O-linked β-N-acetylglucosamine (O-GlcNAc) to serine and threonine residues of numerous proteins in the nucleus, cytoplasm, and mitochondria is a reversible post-translational modification (PTM) and plays an important role as a regulator of various cellular processes in both healthy and disease states. Advances in strategies and tools that allow for the detection of dynamic O-GlcNAcylation on cellular proteins have helped to enhance our initial and ongoing understanding of its dynamic effects on cellular stimuli and given insights into its link to the pathogenesis of several chronic diseases. Furthermore, chemical genetic strategies and related tools have been successfully applied to a myriad of biological systems with a new level of spatiotemporal and molecular precision. These strategies have started to be used in studying and controlling O-GlcNAcylation both in vivo and in vitro. In this minireview, overviews of recent advances in molecular tools being applied to the detection and identification of O-GlcNAcylation on cellular proteins as well as on individual proteins are provided. In addition, chemical genetic strategies that have already been applied or are potentially usable in O-GlcNAc functional are also discussed.

High-Fat Diet Leads to Reduced Protein O-GlcNAcylation and Mitochondrial Defects Promoting the Development of Alzheimer's Disease Signatures

The disturbance of protein O-GlcNAcylation is emerging as a possible link between altered brain metabolism and the progression of neurodegeneration. As observed in brains with Alzheimer's disease (AD), flaws of the cerebral glucose uptake translate into reduced protein O-GlcNAcylation, which promote the formation of pathological hallmarks. A high-fat diet (HFD) is known to foster metabolic dysregulation and insulin resistance in the brain and such effects have been associated with the reduction of cognitive performances. Remarkably, a significant role in HFD-related cognitive decline might be played by aberrant protein O-GlcNAcylation by triggering the development of AD signature and mitochondrial impairment. Our data support the impairment of total protein O-GlcNAcylation profile both in the brain of mice subjected to a 6-week high-fat-diet (HFD) and in our in vitro transposition on SH-SY5Y cells. The reduction of protein O-GlcNAcylation was associated with the development of insulin resistance, induced by overfeeding (i.e., defective insulin signaling and reduced mitochondrial activity), which promoted the dysregulation of the hexosamine biosynthetic pathway (HBP) flux, through the AMPK-driven reduction of GFAT1 activation. Further, we observed that a HFD induced the selective impairment of O-GlcNAcylated-tau and of O-GlcNAcylated-Complex I subunit NDUFB8, thus resulting in tau toxicity and reduced respiratory chain functionality respectively, highlighting the involvement of this posttranslational modification in the neurodegenerative process.

Nutrient regulation of the flow of genetic information by O-GlcNAcylation

O-linked-β-N-acetylglucosamine (O-GlcNAc) is a post-translational modification (PTM) that is actively added to and removed from thousands of intracellular proteins. As a PTM, O-GlcNAcylation tunes the functions of a protein in various ways, such as enzymatic activity, transcriptional activity, subcellular localization, intermolecular interactions, and degradation. Its regulatory roles often interplay with the phosphorylation of the same protein. Governed by 'the Central Dogma', the flow of genetic information is central to all cellular activities. Many proteins regulating this flow are O-GlcNAc modified, and their functions are tuned by the cycling sugar. Herein, we review the regulatory roles of O-GlcNAcylation on the epigenome, in DNA replication and repair, in transcription and in RNA processing, in protein translation and in protein turnover.

Chemical Synthesis and Biological Applications of O-GlcNAcylated Peptides and Proteins

All human cells use O-GlcNAc protein modifications (O-linked N-acetylglucosamine) to rapidly adapt to changing nutrient and stress conditions through signaling, epigenetic, and proteostasis mechanisms. A key challenge for biologists in defining precise roles for specific O-GlcNAc sites is synthetic access to homogenous isoforms of O-GlcNAc proteins, a result of the non-genetically templated, transient, and heterogeneous nature of O-GlcNAc modifications. Toward a solution, this review details the state of the art of two strategies for O-GlcNAc protein modification: advances in "bottom-up" O-GlcNAc peptide synthesis and direct "top-down" installation of O-GlcNAc on full proteins. We also describe key applications of synthetic O-GlcNAc peptide and protein tools as therapeutics, biophysical structure-function studies, biomarkers, and as disease mechanistic probes to advance translational O-GlcNAc biology.

Roles of CNC Transcription Factors NRF1 and NRF2 in Cancer

Simple Summary

Although NRF1 (nuclear factor erythroid 2-like 1; NFE2L1) and NRF2 (nuclear factor erythroid 2-like 2; NFE2L2) belong to the CNC (cap‘n’collar) transcription factor family and share DNA recognition elements, their functions in vivo are substantially different. In cancer cells, while NRF2 confers therapeutic resistance via increasing antioxidant capacity and modulating glucose and glutamine metabolism, NRF1 confers therapeutic resistance via triggering proteasome bounce back response. Proteasome inhibition activates NRF1, and NRF1, in turn, activates the proteasome by inducing the transcriptional activation of proteasome subunit genes. One of the oncometabolites, UDP-GlcNAc (uridine diphosphate N-acetylglucosamine), has been found to be a key to the NRF1-mediated proteasome bounce back response. In this review, we introduce the roles of NRF1 in the cancer malignancy in comparison with NRF2.

Abstract

Cancer cells exhibit unique metabolic features and take advantage of them to enhance their survival and proliferation. While the activation of NRF2 (nuclear factor erythroid 2-like 2; NFE2L2), a CNC (cap‘n’collar) family transcription factor, is effective for the prevention and alleviation of various diseases, NRF2 contributes to cancer malignancy by promoting aggressive tumorigenesis and conferring therapeutic resistance. NRF2-mediated metabolic reprogramming and increased antioxidant capacity underlie the malignant behaviors of NRF2-activated cancer cells. Another member of the CNC family, NRF1, plays a key role in the therapeutic resistance of cancers. Since NRF1 maintains proteasome activity by inducing proteasome subunit genes in response to proteasome inhibitors, NRF1 protects cancer cells from proteotoxicity induced by anticancer proteasome inhibitors. An important metabolite that activates NRF1 is UDP-GlcNAc (uridine diphosphate N-acetylglucosamine), which is abundantly generated in many cancer cells from glucose and glutamine via the hexosamine pathway. Thus, the metabolic signatures of cancer cells are closely related to the oncogenic and tumor-promoting functions of CNC family members. In this review, we provide a brief overview of NRF2-mediated cancer malignancy and elaborate on NRF1-mediated drug resistance affected by an oncometabolite UDP-GlcNAc.

When Glycosylation Meets Blood Cells: A Glance of the Aberrant Glycosylation in Hematological Malignancies

Among neoplasia-associated epigenetic alterations, changes in cellular glycosylation have recently received attention as a key component of hematological malignancy progression. Alterations in glycosylation appear to not only directly impact cell growth and survival, but also alter the adhesion of tumor cells and their interactions with the microenvironment, facilitating cancer-induced immunomodulation and eventual metastasis. Changes in glycosylation arise from altered expression of glycosyltransferases, enzymes that catalyze the transfer of saccharide moieties to a wide range of acceptor substrates, such as proteins, lipids, and other saccharides in the endoplasmic reticulum (ER) and Golgi apparatus. Novel glycan structures in hematological malignancies represent new targets for the diagnosis and treatment of blood diseases. This review summarizes studies of the aberrant expression of glycans commonly found in hematological malignancies and their potential mechanisms and defines the specific roles of glycans as drivers or passengers in the development of hematological malignancies.

ELT-2 promotes O-GlcNAc transferase OGT-1 expression to modulate Caenorhabditis elegans lifespan

O-GlcNAc transferase (OGT) is the enzyme catalyzing protein O-GlcNAcylation by addition of a single O-linked-β-N-acetylglucosamine molecule (O-GlcNAc) to nuclear and cytoplasmic targets, and it uses uridine diphosphate-N-acetylglucosamine (UDP-GlcNAc) as a donor. As UDP-GlcNAc is the final product of the nutrient-sensing hexosamine signaling pathway, overexpression or knockout of ogt in mammals or invertebrate models influences cellular nutrient-response signals and increases susceptibility to chronic diseases of aging. Evidence shows that OGT expression levels decrease in tissues of older mice and rats. However, how OGT expression is modulated in the aging process remains poorly understood. In Caenorhabditis elegans, the exclusive mammalian OGT ortholog OGT-1 is crucial for lifespan control. Here, we observe that worm OGT-1 expression gradually reduces during aging. By combining prediction via the "MATCH" algorithm and luciferase reporter assays, GATA factor ELT-2, the homolog of human GATA4, is identified as a transcriptional factor driving OGT-1 expression. Chromatin immunoprecipitation-quantitative polymerase chain reaction and electrophoretic mobility shift assays show ELT-2 directly binds to and activates the ogt-1 promoter. Knockdown of elt-2 decreases the global O-GlcNAc modification level and reduces the lifespan of wild-type worms. The reduction in lifespan caused by elt-2 RNA interference is abrogated by the loss of ogt-1. These results imply that GATA factors are able to activate OGT expression, which could be beneficial for longevity and the development of therapeutic treatment for aging-related diseases.

Tools for functional dissection of site-specific O-GlcNAcylation

Protein O-GlcNAcylation is an abundant post-translational modification of intracellular proteins with the monosaccharide N-acetylglucosamine covalently tethered to serines and threonines. Modification of proteins with O-GlcNAc is required for metazoan embryo development and maintains cellular homeostasis through effects on transcription, signalling and stress response. While disruption of O-GlcNAc homeostasis can have detrimental impact on cell physiology and cause various diseases, little is known about the functions of individual O-GlcNAc sites. Most of the sites are modified sub-stoichiometrically which is a major challenge to the dissection of O-GlcNAc function. Here, we discuss the application, advantages and limitations of the currently available tools and technologies utilised to dissect the function of O-GlcNAc on individual proteins and sites in vitro and in vivo. Additionally, we provide a perspective on future developments required to decipher the protein- and site-specific roles of this essential sugar modification.

Role of O-Linked N-Acetylglucosamine Protein Modification in Cellular (Patho)Physiology

In the mid-1980s, the identification of serine and threonine residues on nuclear and cytoplasmic proteins modified by a N-acetylglucosamine moiety (O-GlcNAc) via an O-linkage overturned the widely held assumption that glycosylation only occurred in the endoplasmic reticulum, Golgi apparatus, and secretory pathways. In contrast to traditional glycosylation, the O-GlcNAc modification does not lead to complex, branched glycan structures and is rapidly cycled on and off proteins by O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA), respectively. Since its discovery, O-GlcNAcylation has been shown to contribute to numerous cellular functions, including signaling, protein localization and stability, transcription, chromatin remodeling, mitochondrial function, and cell survival. Dysregulation in O-GlcNAc cycling has been implicated in the progression of a wide range of diseases, such as diabetes, diabetic complications, cancer, cardiovascular, and neurodegenerative diseases. This review will outline our current understanding of the processes involved in regulating O-GlcNAc turnover, the role of O-GlcNAcylation in regulating cellular physiology, and how dysregulation in O-GlcNAc cycling contributes to pathophysiological processes.

Targeting O-GlcNAcylation to develop novel therapeutics

O-linked β-D-N-acetylglucosamine (O-GlcNAc) is an abundant post-translational modification (PTM) that modifies the serine or threonine residues of thousands of proteins in the nucleus, cytoplasm and mitochondria. Being a major "nutrient sensor" in cells, the O-GlcNAc pathway is sensitive to cellular metabolic states. Extensive crosstalk is observed between O-GlcNAcylation and protein phosphorylation. O-GlcNAc regulates protein functions at multiple levels, including enzymatic activity, transcriptional activity, subcellular localization, intermolecular interactions and degradation. Abnormal O-GlcNAcylation is associated with many human diseases including cancer, diabetes and neurodegenerative diseases. Though research on O-GlcNAc is still in its infantry, accumulating evidence suggest O-GlcNAcylation to be a promising therapeutic target. In this review, we briefly discuss the basic features of this PTM, the O-GlcNAc signaling pathway, its regulatory functions on different proteins, and its involvement in human diseases. We hope this review will provide insights to researchers who study human disease, as well as researchers who are interested in the fundamental roles of O-GlcNAcylation in all cells.

Glycogen synthase kinase (GSK)-3 and the double-strand RNA-dependent kinase, PKR: When two kinases for the common good turn bad

Glycogen synthase kinase (GSK)-3α/β and the double-stranded RNA-dependent kinase PKR are two sentinel kinases that carry-out multiple similar yet distinct functions in both the cytosol and the nucleus. While these kinases belong to separate signal transduction cascades, they demonstrate an uncanny propensity to regulate many of the same proteins either through direct phosphorylation or by altering transcription/translation, including: c-MYC, NF-κB, p53 and TAU, as well as each another. A significant number of studies centered on the GSK3 kinases have led to the identification of the GSK3 interactome and a number of substrates, which link GSK3 activity to metabolic control, translation, RNA splicing, ribosome biogenesis, cellular division, DNA repair and stress/inflammatory signaling. Interestingly, many of these same pathways and processes are controlled by PKR, but unlike the GSK3 kinases, a clear picture of proteins interacting with PKR and a complete listing of its substrates is still missing. In this review, we take a detailed look at what is known about the PKR and GSK3 kinases, how these kinases interact to influence common cellular processes (innate immunity, alternative splicing, translation, glucose metabolism) and how aberrant activation of these kinases leads to diseases such as Alzheimer's disease (AD), diabetes mellitus (DM) and cancer.

•

GSK3α/β and PKR are major regulators of cellular homeostasis and the response to stress/inflammation and infection.

•

GSK3α/β and PKR interact with and/or modify many of the same proteins and affect the expression of similar genes.

•

A balance between AKT and PKR nuclear signaling may be responsible for regulating the activation of nuclear GSK3β.

•

GSK3α/β- and PKR-dependent signaling influence major molecular mechanisms of the cell through similar intermediates.

•

Aberrant activation of GSK3α/β and PKR is highly involved in cancer, metabolic disorders, and neurodegenerative diseases.

Prediction of human tau 3D structure, and interplay between O-β-GlcNAc and phosphorylation modifications in Alzheimer's disease: C. elegans as a suitable model to study these interactions in vivo

Tau protein regulates, maintains and stabilizes microtubule assembly under normal physiological conditions. In certain pathological circumstances, tau is post-translationally modified predominantly via phosphorylation and glycosylation. Hyper-phosphorylation of tau in Alzheimer's disease (AD) resulted in aggregated neurofibrillary tangles (NFTs) formation. Unfortunately, absence of tau 3D structure makes difficult to understand exact mechanism involved in tau pathology. Here by using ab-initio modelling, we predicted a tau 3D structure that not only explains its binding with microtubules but also elucidates NFTs formation. O-linked β-N-acetylglucosaminylation (O-β-GlcNAc) is thought to regulate tau phosphorylation on single or proximal Ser/Thr residues (called as Yin-Yang sites). In this study, we not only validate the previously described three-serine residues (208, 238 and 400) as Yin-Yang sites but also predicted 22 more possible Ser/Thr O-glycosylation sites. Among them seventeen residues were predicted as possible Yin-Yang sites and are proposed to mediate NFT formation in AD. These predicted Yin-Yang sites may act as attractive therapeutic targets for the drug development in AD. Predicted 3D structure of tau₄₄₁ was highly accessible for phosphorylation and hyperphosphorylation, and showed higher surface accessibility for interplay between O-β-GlcNAc and phosphorylation modifications. Kinases and phosphatases involved in tau phosphorylation are conserved in human and other organisms. Homology modelling revealed conserved catalytic domain for both human and C. elegans O-GlcNAc transferase (OGT), suggesting that transgenic C. elegans expressing human tau may be a suitable model system to study these modifications.

Crosstalk of intracellular post-translational modifications in cancer

Post-translational modifications (PTMs) have been reported to play pivotal roles in numerous cellular biochemical and physiological processes. Multiple PTMs can influence the actions of each other positively or negatively, termed as PTM crosstalk or PTM code. During recent years, development of identification strategies for PTMs co-occurrence has revealed abundant information of interplay between PTMs. Increasing evidence demonstrates that deregulation of PTMs crosstalk is involved in the genesis and development of various diseases. Insight into the complexity of PTMs crosstalk will help us better understand etiology and provide novel targets for drug therapy. In the present review, we will discuss the important functional roles of PTMs crosstalk in proteins associated with cancer diseases.

iPTMnet: an integrated resource for protein post-translational modification network discovery

Protein post-translational modifications (PTMs) play a pivotal role in numerous biological processes by modulating regulation of protein function. We have developed iPTMnet (http://proteininformationresource.org/iPTMnet) for PTM knowledge discovery, employing an integrative bioinformatics approach—combining text mining, data mining, and ontological representation to capture rich PTM information, including PTM enzyme-substrate-site relationships, PTM-specific protein-protein interactions (PPIs) and PTM conservation across species. iPTMnet encompasses data from (i) our PTM-focused text mining tools, RLIMS-P and eFIP, which extract phosphorylation information from full-scale mining of PubMed abstracts and full-length articles; (ii) a set of curated databases with experimentally observed PTMs; and iii) Protein Ontology that organizes proteins and PTM proteoforms, enabling their representation, annotation and comparison within and across species. Presently covering eight major PTM types (phosphorylation, ubiquitination, acetylation, methylation, glycosylation, S-nitrosylation, sumoylation and myristoylation), iPTMnet knowledgebase contains more than 654 500 unique PTM sites in over 62 100 proteins, along with more than 1200 PTM enzymes and over 24 300 PTM enzyme-substrate-site relations. The website supports online search, browsing, retrieval and visual analysis for scientific queries. Several examples, including functional interpretation of phosphoproteomic data, demonstrate iPTMnet as a gateway for visual exploration and systematic analysis of PTM networks and conservation, thereby enabling PTM discovery and hypothesis generation.

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.

Essential role of O-GlcNAcylation in stabilization of oncogenic factors

A reversible post-translational protein modification which involves addition of N-acetylglucosamine (GlcNAc) onto hydroxyl groups of serine and/or threonine residues which is known as O-GlcNAcylation, has emerged as a potent competitor of phosphorylation. This glycosyltransfer reaction is catalyzed by the enzyme O-linked β-N-acetylglucosamine transferase (OGT). This enzyme uses uridine diphosphate N-acetylglucosamine (UDP-GlcNAc), the end product of hexosamine biosynthetic pathway, to modify numerous nuclear and cytosolic proteins. O-GlcNAcylation influences cancer cell metabolism in such a way that hyper-O-GlcNAcylation is considered as a prominent trait of many cancers, and is proposed as a major factor enabling cancer cell proliferation and progression. Growing evidence supports a connection between O-GlcNAcylation and major oncogenic factors, including for example, c-MYC, HIF-1α, and NF-κB. A comprehensive study of the roles of O-GlcNAc modification of oncogenic factors is warranted as a thorough understanding may help drive advances in cancer diagnosis and therapy. The focus of this article is to highlight the interplay between oncogenic factors and O-GlcNAcylation along with OGT in cancer cell proliferation and survival. The prospects for OGT inhibitors will also be discussed.

Demystifying O-GlcNAcylation: hints from peptide substrates

O-GlcNAcylation, analogous to phosphorylation, is an essential post-translational modification of proteins at Ser/Thr residues with a single β-N-acetylglucosamine moiety. This dynamic protein modification regulates many fundamental cellular processes and its deregulation has been linked to chronic diseases such as cancer, diabetes and neurodegenerative disorders. Reversible attachment and removal of O-GlcNAc is governed only by O-GlcNAc transferase and O-GlcNAcase, respectively. Peptide substrates, derived from natural O-GlcNAcylation targets, function in the catalytic cores of these two enzymes by maintaining interactions between enzyme and substrate, which makes them ideal models for the study of O-GlcNAcylation and deglycosylation. These peptides provide valuable tools for a deeper understanding of O-GlcNAc processing enzymes. By taking advantage of peptide chemistry, recent progress in the study of activity and regulatory mechanisms of these two enzymes has advanced our understanding of their fundamental specificities as well as their potential as therapeutic targets. Hence, this review summarizes the recent achievements on this modification studied at the peptide level, focusing on enzyme activity, enzyme specificity, direct function, site-specific antibodies and peptide substrate-inspired inhibitors.

Critical observations that shaped our understanding of the function(s) of intracellular glycosylation (O-GlcNAc)

Almost 100 years after the first descriptions of proteins conjugated to carbohydrates (mucins), several studies suggested that glycoproteins were not restricted to the serum, extracellular matrix, cell surface, or endomembrane system. In the 1980s, key data emerged demonstrating that intracellular proteins were modified by monosaccharides of O-linked β-N-acetylglucosamine (O-GlcNAc). Subsequently, this modification was identified on thousands of proteins that regulate cellular processes as diverse as protein aggregation, localization, post-translational modifications, activity, and interactions. In this Review, we will highlight critical discoveries that shaped our understanding of the molecular events underpinning the impact of O-GlcNAc on protein function, the role that O-GlcNAc plays in maintaining cellular homeostasis, and our understanding of the mechanisms that regulate O-GlcNAc-cycling.

Investigation of post-translational modifications in type 2 diabetes

The investigation of post-translational modifications (PTMs) plays an important role for the study of type 2 diabetes. The importance of PTMs has been realized with the advancement of analytical techniques. The challenging detection and analysis of post-translational modifications is eased by different enrichment methods and by high throughput mass spectrometry based proteomics studies. This technology along with different quantitation methods provide accurate knowledge about the changes happening in disease conditions as well as in normal conditions. In this review, we have discussed PTMs such as phosphorylation, N-glycosylation, O-GlcNAcylation, acetylation and advanced glycation end products in type 2 diabetes which have been characterized by high throughput mass spectrometry based proteomics analysis.

Multiple Roles of Glycans in Hematological Malignancies

The three types of blood cells (red blood cells for carrying oxygen, white blood cells for immune protection, and platelets for wound clotting) arise from hematopoietic stem/progenitor cells in the adult bone marrow, and function in physiological regulation and communication with local microenvironments to maintain systemic homeostasis. Hematological malignancies are relatively uncommon malignant disorders derived from the two major blood cell lineages: myeloid (leukemia) and lymphoid (lymphoma). Malignant clones lose their regulatory mechanisms, resulting in production of a large number of dysfunctional cells and destruction of normal hematopoiesis. Glycans are one of the four major types of essential biological macromolecules, along with nucleic acids, proteins, and lipids. Major glycan subgroups are N-glycans, O-glycans, glycosaminoglycans, and glycosphingolipids. Aberrant expression of glycan structures, resulting from dysregulation of glycan-related genes, is associated with cancer development and progression in terms of cell signaling and communication, tumor cell dissociation and invasion, cell-matrix interactions, tumor angiogenesis, immune modulation, and metastasis formation. Aberrant glycan expression occurs in most hematological malignancies, notably acute myeloid leukemia, myeloproliferative neoplasms, and multiple myeloma, etc. Here, we review recent research advances regarding aberrant glycans, their related genes, and their roles in hematological malignancies. Our improved understanding of the mechanisms that underlie aberrant patterns of glycosylation will lead to development of novel, more effective therapeutic approaches targeted to hematological malignancies.

Cross regulation between mTOR signaling and O-GlcNAcylation

The hexosamine biosynthetic pathway (HBP) integrates glucose, amino acids, fatty acids and nucleotides metabolisms for uridine diphosphate N-acetylglucosamine (UDP-GlcNAc) synthesis. UDP-GlcNAc is the nucleotide sugar donor for O-linked β-N-acetylglucosaminylation (O-GlcNAcylation) processes. O-GlcNAc transferase (OGT) is the enzyme which transfers the N-acetylglucosamine (O-GlcNAc) residue onto target proteins. Several studies previously showed that glucose metabolism dysregulations associated with obesity, diabetes or cancer correlated with an increase of OGT expression and global O-GlcNAcylation levels. Moreover, these diseases present an increased activation of the nutrient sensing mammalian target of rapamycin (mTOR) pathway. Other works demonstrate that mTOR regulates protein O-GlcNAcylation in cancer cells through stabilization of OGT. In this context, we studied the cross-talk between these two metabolic sensors in vivo in obese mice predisposed to diabetes and in vitro in normal and colon cancer cells. We report that levels of OGT and O-GlcNAcylation are increased in obese mice colon tissues and colon cancer cells and are associated with a higher activation of mTOR signaling. In parallel, treatments with mTOR regulators modulate OGT and O-GlcNAcylation levels in both normal and colon cancer cells. However, deregulation of O-GlcNAcylation affects mTOR signaling activation only in cancer cells. Thus, a crosstalk exists between O-GlcNAcylation and mTOR signaling in contexts of metabolism dysregulation associated to obesity or cancer.

Drug resistance related to aberrant glycosylation in colorectal cancer

Colorectal cancer (CRC) is the fourth leading cause of cancer-related deaths in the world. Drug resistance of tumour cells remains the main challenge toward curative treatments efficiency. Several epidemiologic studies link emergence and recurrence of this cancer to metabolic disorders. Glycosylation that modifies more than 80% of human proteins is one of the most widepread nutrient-sensitive post-translational modifications. Aberrant glycosylation participates in the development and progression of cancer. Thus, some of these glycan changes like carbohydrate antigen CA 19-9 (sialyl Lewis a, sLea) or those found on carcinoembryonic antigen (CEA) are already used as clinical biomarkers to detect and monitor CRC. The current review highlights emerging evidences accumulated mainly during the last decade that establish the role played by altered glycosylations in CRC drug resistance mechanisms that induce resistance to apoptosis and activation of signaling pathways, alter drug absorption and metabolism, and led to stemness acquisition. Knowledge in this field of investigation could aid to the development of better therapeutic approaches with new predictive biomarkers and targets tied in with adapted diet.

Functional significance of O-GlcNAc modification in regulating neuronal properties

Post-translational modifications (PTMs) covalently modify proteins and diversify protein functions. Along with protein phosphorylation, another common PTM is the addition of O-linked β-N-acetylglucosamine (O-GlcNAc) to serine and/or threonine residues. O-GlcNAc modification is similar to phosphorylation in that it occurs to serine and threonine residues and cycles on and off with a similar time scale. However, a striking difference is that the addition and removal of the O-GlcNAc moiety on all substrates are mediated by the two enzymes regardless of proteins, O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA), respectively. O-GlcNAcylation can interact or potentially compete with phosphorylation on serine and threonine residues, and thus serves as an important molecular mechanism to modulate protein functions and activation. However, it has been challenging to address the role of O-GlcNAc modification in regulating protein functions at the molecular level due to the lack of convenient tools to determine the sites and degrees of O-GlcNAcylation. Studies in this field have only begun to expand significantly thanks to the recent advances in detection and manipulation methods such as quantitative proteomics and highly selective small-molecule inhibitors for OGT and OGA. Interestingly, multiple brain regions, especially hippocampus, express high levels of both OGT and OGA, and a number of neuron-specific proteins have been reported to undergo O-GlcNAcylation. This review aims to discuss the recent updates concerning the impacts of O-GlcNAc modification on neuronal functions at multiple levels ranging from intrinsic neuronal properties to synaptic plasticity and animal behaviors.

Electrophilic probes for deciphering substrate recognition by O-GlcNAc transferase

O-linked β-N-acetylglucosamine (O-GlcNAc) transferase (OGT) is an essential human glycosyltransferase that adds O-GlcNAc modifications to numerous proteins. However, little is known about the mechanism with which OGT recognizes various protein substrates. Here we report on GlcNAc electrophilic probes (GEPs) to expedite the characterization of OGT-substrate recognition. Data from mass spectrometry, X-ray crystallization, and biochemical and radiolabeled kinetic assays support the application of GEPs to rapidly report the impacts of OGT mutations on protein substrate or sugar binding and to discover OGT residues crucial for protein recognition. Interestingly, we found that the same residues on the inner surface of the N-terminal domain contribute to OGT interactions with different protein substrates. By tuning reaction conditions, a GEP enables crosslinking of OGT with acceptor substrates in situ, affording a unique method to discover genuine substrates that weakly or transiently interact with OGT. Hence, GEPs provide new strategies to dissect OGT-substrate binding and recognition.

The emerging link between O-GlcNAcylation and neurological disorders

O-linked β-N-acetylglucosaminylation (O-GlcNAcylation) is involved in the regulation of many cellular cascades and neurological diseases such as Alzheimer's disease (AD), Parkinson's disease (PD), and stroke. In the brain, the expression of O-GlcNAcylation is notably heightened, as is that of O-linked N-acetylglucosaminyltransferase (OGT) and β-N-acetylglucosaminidase (OGA), the presence of which is prominent in many regions of neurological importance. Most importantly, O-GlcNAcylation is believed to contribute to the normal functioning of neurons; conversely, its dysregulation participates in the pathogenesis of neurological disorders. In neurodegenerative diseases, O-GlcNAcylation of the brain's key proteins, such as tau and amyloid-β, interacts with their phosphorylation, thereby triggering the formation of neurofibrillary tangles and amyloid plaques. An increase of O-GlcNAcylation by pharmacological intervention prevents neuronal loss. Additionally, O-GlcNAcylation is stress sensitive, and its elevation is cytoprotective. Increased O-GlcNAcylation ameliorated brain damage in victims of both trauma-hemorrhage and stroke. In this review, we summarize the current understanding of O-GlcNAcylation's physiological and pathological roles in the nervous system and provide a foundation for development of a therapeutic strategy for neurological disorders.

Hyper-O-GlcNAcylation activates nuclear factor κ-light-chain-enhancer of activated B cells (NF-κB) signaling through interplay with phosphorylation and acetylation

O-GlcNAcylation is the covalent addition of an O-linked β-N-acetylglucosamine (O-GlcNAc) sugar moiety to hydroxyl groups of serine/threonine residues of cytosolic and nuclear proteins. O-GlcNAcylation, analogous to phosphorylation, plays critical roles in gene expression through direct modification of transcription factors, such as NF-κB. Aberrantly increased NF-κB O-GlcNAcylation has been linked to NF-κB constitutive activation and cancer development. Therefore, it is of a great biological and clinical significance to dissect the molecular mechanisms that tune NF-κB activity. Recently, we and others have shown that O-GlcNAcylation affects the phosphorylation and acetylation of NF-κB subunit p65/RelA. However, the mechanism of how O-GlcNAcylation activates NF-κB signaling through phosphorylation and acetylation is not fully understood. In this study, we mapped O-GlcNAcylation sites of p65 at Thr-305, Ser-319, Ser-337, Thr-352, and Ser-374. O-GlcNAcylation of p65 at Thr-305 and Ser-319 increased CREB-binding protein (CBP)/p300-dependent activating acetylation of p65 at Lys-310, contributing to NF-κB transcriptional activation. Moreover, elevation of O-GlcNAcylation by overexpression of OGT increased the expression of p300, IKKα, and IKKβ and promoted IKK-mediated activating phosphorylation of p65 at Ser-536, contributing to NF-κB activation. In addition, we also identified phosphorylation of p65 at Thr-308, which might impair the O-GlcNAcylation of p65 at Thr-305. These results indicate mechanisms through which both non-pathological and oncogenic O-GlcNAcylation regulate NF-κB signaling through interplay with phosphorylation and acetylation.

O-GlcNAcylation of SKN-1 modulates the lifespan and oxidative stress resistance in Caenorhabditis elegans

In C. elegans, the transcription factor skinhead-1 (SKN-1), the ortholog of human NF-E2-related factor 2 (Nrf-2), plays important roles in oxidative stress defense and aging processes. It has been documented that the activity of SKN-1 is regulated by its phosphorylation modification. However, whether other posttranslational modifications of SKN-1 affect its function remains unclear to date. Here we report, for the first time, that SKN-1 is O-GlcNAcylated at Ser470 and Thr493 by O-GlcNActransferase OGT-1. By generating the double mutations of Ser470/Thr493 in the wild type and skn-1(zu67) worms, respectively, we found that disruption of O-GlcNAc modification on SKN-1 repressed the accumulation of SKN-1 in the intestinal nuclei, and decreased the activities of SKN-1 in modulating lifespan and oxidative stress resistance. Moreover, under oxidative stress, SKN-1 was highly O-GlcNAcylated, resulting in the decrease of GSK-3-mediated phosphorylation at Ser483 adjacent to the O-GlcNAcylated residues (Ser470 and Thr493). These data suggest that O-GlcNAcylation of SKN-1 is crucial for regulating lifespan and oxidative stress resistance via the crosstalk with its phosphorylation in C. elegans. These findings have important implications for studying the functions of O-GlcNAcylation on Nrf-2 in human aging-related diseases.

O-GlcNAcylation promotes migration and invasion in human ovarian cancer cells via the RhoA/ROCK/MLC pathway

O-GlcNAcylation is a dynamic and reversible post-translational modification associated with the regulation of multiple cellular functions. The addition and removal of O-Linked β-N-acetylglucosamine (O-GlcNAc) on target proteins is catalyzed by O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA), respectively. Accumulating evidence suggests that O-GlcNAcylation is associated with the malignancy of several types of human cancer. To investigate the effect of O-GlcNAcylation on ovarian cancer phenotypes, global O-GlcNAc levels were decreased by OGT silencing through RNA interference and increased by inhibiting OGA activity with Thiamet-G. Transwell assay results demonstrated that OGT silencing inhibited the migration and invasion of SKOV3 and 59M ovarian cells in vitro, while Thiamet-G treatment promoted migration and invasion. Furthermore, a pull-down assay and western blot analysis demonstrated that Thiamet-G treatment enhanced RhoA activity and the phosphorylation of the Rho-associated protein kinase (ROCK) substrate, myosin light chain (MLC), while OGT silencing attenuated RhoA activity and MLC phosphorylation. In addition, RhoA silencing via RNA interference and inhibition of ROCK activity with Y-27632 prevented Thiamet-G-induced increases in cell migration and invasion. These data suggest that O-GlcNAcylation augments the motility of ovarian cancer cells via the RhoA/ROCK/MLC signaling pathway. Therefore, O-GlcNAcylation may be a potential target for the diagnosis and treatment of ovarian cancer.