Dysregulation Of O-GlcNAcylation Research Articles

Biological Functions and Potential Therapeutic Significance of O-GlcNAcylation in Hepatic Cellular Stress and Liver Diseases

O-linked-β-D-N-acetylglucosamine (O-GlcNAc) glycosylation (O-GlcNAcylation), which is dynamically regulated by O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA), is a post-translational modification involved in multiple cellular processes. O-GlcNAcylation of proteins can regulate their biological functions via crosstalk with other post-translational modifications, such as phosphorylation, ubiquitination, acetylation, and methylation. Liver diseases are a major cause of death worldwide; yet, key pathological features of the disease, such as inflammation, fibrosis, steatosis, and tumorigenesis, are not fully understood. The dysregulation of O-GlcNAcylation has been shown to be involved in some severe hepatic cellular stress, viral hepatitis, liver fibrosis, nonalcoholic fatty acid liver disease (NAFLD), malignant progression, and drug resistance of hepatocellular carcinoma (HCC) through multiple molecular signaling pathways. Here, we summarize the emerging link between O-GlcNAcylation and hepatic pathological processes and provide information about the development of therapeutic strategies for liver diseases.

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.

Cryo-EM structure of human O-GlcNAcylation enzyme pair OGT-OGA complex

O-GlcNAcylation is a conserved post-translational modification that attaches N-acetyl glucosamine (GlcNAc) to myriad cellular proteins. In response to nutritional and hormonal signals, O-GlcNAcylation regulates diverse cellular processes by modulating the stability, structure, and function of target proteins. Dysregulation of O-GlcNAcylation has been implicated in the pathogenesis of cancer, diabetes, and neurodegeneration. A single pair of enzymes, the O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA), catalyzes the addition and removal of O-GlcNAc on over 3,000 proteins in the human proteome. However, how OGT selects its native substrates and maintains the homeostatic control of O-GlcNAcylation of so many substrates against OGA is not fully understood. Here, we present the cryo-electron microscopy (cryo-EM) structures of human OGT and the OGT-OGA complex. Our studies reveal that OGT forms a functionally important scissor-shaped dimer. Within the OGT-OGA complex structure, a long flexible OGA segment occupies the extended substrate-binding groove of OGT and positions a serine for O-GlcNAcylation, thus preventing OGT from modifying other substrates. Conversely, OGT disrupts the functional dimerization of OGA and occludes its active site, resulting in the blocking of access by other substrates. This mutual inhibition between OGT and OGA may limit the futile O-GlcNAcylation cycles and help to maintain O-GlcNAc homeostasis.

Eogt-catalyzed <i>O</i>-GlcNAcylation

O-GlcNAcylation is critical in human physiology. The dysregulation of O-GlcNAcylation on various intracellular proteins impacts cardiovascular diseases, the immune system, type 2 diabetes, neurodegeneration diseases, and cancers. In the past decade, extracellular O-GlcNAcylation was discovered from the conserved epidermal growth factor-like (EGF) domain in Drosophila and mammals. Extracellular O-GlcNAcylation is catalyzed by EGF domain O-GlcNAc transferase (EOGT) in the endoplasmic reticulum, and, unlike intracellular O-GlcNAcylation, it is further elongated with galactose and sialic acid in the Golgi apparatus. In this review, we retrospect the studies addressing the catalysis of EOGT, the conserved consensus sequence, the impacts of abnormal extracellular O-GlcNAcylation, and the diseases related to EOGT mutation.

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.

O-GlcNAcylation and phosphorylation of β-actin Ser199 in diabetic nephropathy.

The function of actin is regulated by various posttranslational modifications. We have previously shown that in the kidneys of nonobese type 2 diabetes model Goto-Kakizaki rats, increased O-GlcNAcylation of β-actin protein is observed. It has also been reported that both O-GlcNAcylation and phosphorylation occur on Ser199 of β-actin. However, their roles are not known. To elucidate their roles in diabetic nephropathy, we examined the rat kidney for changes in O-GlcNAcylation of Ser199 (gS199)-actin and in the phosphorylation of Ser199 (pS199)-actin. Both gS199- and pS199-actin molecules had an apparent molecular weight of 40 kDa and were localized as nonfilamentous actin in both the cytoplasm and nucleus. Compared with the normal kidney, the immunostaining intensity of gS199-actin increased in podocytes of the glomeruli and in proximal tubules of the diabetic kidney, whereas that of pS199-actin did not change in podocytes but decreased in proximal tubules. We confirmed that the same results could be observed in the glomeruli of the human diabetic kidney. In podocytes of glomeruli cultured in the presence of the O-GlcNAcase inhibitor Thiamet G, increased O-GlcNAcylation was accompanied by a concomitant decrease in the amount of filamentous actin and in morphological changes. Our present results demonstrate that dysregulation of O-GlcNAcylation and phosphorylation of Ser199 occurred in diabetes, which may contribute partially to the causes of the morphological changes in the glomeruli and tubules. gS199- and pS199-actin will thus be useful for the pathological evaluation of diabetic nephropathy.

O-GlcNAc transferase activates stem-like cell potential in hepatocarcinoma through O-GlcNAcylation of eukaryotic initiation factor 4E.

O‐GlcNAcylation catalysed by O‐GlcNAc transferase (OGT) is a reversible post‐translational modification. O‐GlcNAcylation participates in transcription, epigenetic regulation, and intracellular signalling. Dysregulation of O‐GlcNAcylation in response to high glucose or OGT expression has been implicated in metabolic diseases and cancer. However, the underlying mechanisms by which OGT regulates hepatoma development remain largely unknown. Here, we employed the lentiviral shRNA‐based system to knockdown OGT to analyse the contribution of OGT in hepatoma cell proliferation and stem‐like cell potential. The sphere‐forming assay and western blot analysis of stem‐related gene expression were used to evaluate stem‐like cell potential of hepatoma cell. We found that the level of total O‐GlcNAcylation or OGT protein was increased in hepatocellular carcinoma. OGT activated stem‐like cell potential in hepatoma through eukaryotic initiation factor 4E (eIF4E) which bound to stem‐related gene Sox2 5'‐untranslated region. O‐GlcNAcylation of eIF4E at threonine 168 and threonine 177 protected it from degradation through proteasome pathway. Expression of eIF4E in hepatoma was determined by immunostaining in 232 HCC patients, and Kaplan‐Meier survival analysis was used to determine the correlation of eIF4E expression with prognosis. High glucose promoted stem‐like cell potential of hepatoma cell through OGT‐eIF4E axis. Collectively, our findings indicate that OGT promotes the stem‐like cell potential of hepatoma cell through O‐GlcNAcylation of eIF4E. These results provide a mechanism of HCC development and a cue between the pathogenesis of HCC and high glucose condition.

Regulation of Mitophagy By O-Linked N-Acetylglucosamine Transferase Is Essential for Hematopoietic Stem Cell Maintenance

Hematopoietic stem cells (HSCs) reside in hypoxic niche in the bone marrow (BM), where they are maintained in quiescent state. To adapt to this hypoxic microenvironment, HSCs generate ATP mainly from glycolysis, while suppressing oxidative phosphorylation in mitochondria. Through this metabolic regulation, HSCs protect themselves from oxidative stress caused by reactive oxygen species (ROS). ROS levels are controlled by various mechanisms including autophagy, which is one of the critical mechanisms regulating HSC function mainly through maintaining mitochondrial homeostasis. Glycosylation of serine or threonine residues with O-linked N-acetylglucosamine (O-GlcNAcylation) by O-linked N-acetylglucosamine transferase (OGT) is crucial for regulating various protein functions, and dysregulation of O-GlcNAcylation is critically linked to metabolic or degenerative diseases (i.e. diabetes, Alzheimer disease), tumorigenesis and aging. In hematopoietic system, OGT is essential for differentiation and proliferation of T and B cells. However, a role for OGT in the function of hematopoietic stem and progenitor cells (HSPCs) remains elusive. To elucidate a role for OGT in HSPCs, we analyzed the effect of Ogt loss on HSPCs using Ogt-conditional knockout mice. Conditional disruption of Ogt in adult mice led to pancytopenia and significantly reduced numbers of HSPCs in the BM. In addition, Ogt-deficient HSCs exhibited loss of quiescence, increased apoptosis and impaired long-term repopulation as well as self-renewal capacities by serial competitive repopulation assays. Interestingly, ROS levels were significantly increased in Ogt-deficient HSPCs, and the treatment of mice with N-acetyl cysteine (NAC), a scavenger for ROS, at least partially rescued the phenotype of Ogt-deficient hematopoietic cells in in vivo settings. MitoSOX analysis showed that excessive ROS was generated mainly from mitochondria in Ogt-deficient HSCs. In addition, mitochondrial mass was significantly increased in Ogt-deficient HSCs. Assessment of mitochondrial quality by its membrane potential and extracellular flux analysis revealed that mitochondria of Ogt-deficient HSPCs had significantly lower membrane potential and lower spare respiratory capacity. Morphologically, mitochondria in Ogt-deficient HSPCs were enlarged and swollen with disorganized cristae. These findings suggest that inactivation of Ogt leads to loss of HSPCs and impaired HSC homeostasis, which is due, at least in part, to elevated ROS levels with accumulation of defective mitochondria. Since mitophagy (mitochondria-specific autophagy) plays an important role in quality control of mitochondria, we examined gene expressions of mitophagy regulators by RNA sequencing. This revealed that disruption of Ogt led to down-regulation of key mitophagy inducers, Pink1 and Bnip3. These findings together with the accumulation of defective mitochondria indicated that inactivation of Ogt impairs mitophagy probably due to reduced expression of Pink1 and Bnip3. OGT is known to regulate gene expressions by modifying trimethylation of lysine 4 on the histone H3 (H3K4me3) through proteolytic activation of HCF-1, which is a critical component of SET1/COMPASS complex. Chromatin immunoprecipitation (ChIP) assay showed significantly reduced levels of H3K4me3 at the transcriptional start sites of Pink1 and Bnip3 in Ogt-deficient HSPCs. Next we asked whether defective HSPC homeostasis caused by Ogt disruption could be rescued by restoring mitophagy in Ogt-deficient HSPCs through overexpressing Pink1, a critical initiator of mitophagy. As expected, Pink1 overexpression efficiently restored mitophagy in Ogt-deficient HSCs as shown by normalization of mitochondrial mass. Interestingly, a number and apoptosis of Ogt-deficient HSCs were restored to levels similar to those of wild-type HSCs. These data strongly indicate that Pink1 functions as a key downstream effector of OGT in HSC maintenance. In summary, our results revealed that OGT plays an essential role in HSC maintenance by assuring mitochondrial quality through mitophagy. DisclosuresNo relevant conflicts of interest to declare.

PUGNAc induces protein ubiquitination in C2C12 myotube cells.

O-linked β-N-acetylglucosaminylation (O-GlcNAcylation) regulates many cellular processes including the cell cycle, cell signaling, and protein trafficking. Dysregulation of O-GlcNAcylation may be involved in the development of insulin resistance and type 2 diabetes. Therefore, it is necessary to identify cellular proteins that are induced by elevated O-GlcNAcylation. Here, using adenosine 5'-triphosphate affinity chromatography, we employed a proteomic approach in order to identify differentially expressed proteins in response to treatment with the O-GlcNAcase inhibitor, O-(2-acetamido-2-deoxy-d-glucopyranosylidene)amino-N-phenylcarbamate (PUGNAc), in mouse C2C12 myotube cells. Among 205 selected genes, we identified 68 nucleotide-binding proteins, 14 proteins that have adenosinetriphosphatase activity, and 10 proteins with ligase activity. Upregulation of proteins, including ubiquitin-activating enzyme E1, proteasome subunit 20S, cullin-associated NEDD8-dissociated protein 1, ezrin, and downregulation of the protein nucleoside diphosphate kinase B, were confirmed by western blot analysis. In particular, we found that the protein ubiquitination level in C2C12 cells was increased by PUGNAc treatment. This is the first report of quantitative proteomic profiles of myotube cells after treatment with PUGNAc, and our results demonstrate the potential to enhance understanding of the relationship between insulin resistance, O-GlcNAc, and PUGNAc in the future.

O-GlcNAc cycling enzymes control vascular development of the placenta by modulating the levels of HIF-1α

IntroductionPlacental vasculogenesis is essential for fetal growth and development, and is affected profoundly by oxygen tension (hypoxia). Hypoxia-inducible factor-1α (HIF-1α), which is stabilized at the protein level in response to hypoxia, is essential for vascular morphogenesis in the placenta. Many studies suggested that responses to hypoxia is influenced by O-GlcNAcylation. O-GlcNAcylation is regulated by O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA) that catalyze the addition and removal of O-GlcNAc respectively. MethodsWe generated OGA deficient mice and evaluated OGA−/− placentas. The analysis of OGA−/− placentas was focused on morphological change and placental vasculogenesis. HIF-1α protein stability or transcriptional activity under dysregulation of O-GlcNAcylation were evaluated by Western blot, RT-qPCR and luciferase reporter gene assays in MEFs or MS1 cell line. ResultsDeletion of OGA results in defective placental vasculogenesis. OGA−/− placentas showed an abnormal placental shape and reduced vasculature in the labyrinth, which caused a developmental delay in the embryos. OGA deletion, which elevates O-GlcNAcylation and downregulates O-GlcNAc transferase (OGT), suppressed HIF-1α stabilization and the transcription of its target genes. In contrast, the overexpression of O-GlcNAc cycling enzymes enhanced the expression and transcriptional activity of HIF-1α. DiscussionThese results suggest that OGA plays a critical role in placental vasculogenesis by modulating HIF-1α stabilization. Control of O-GlcNAcylation is essential for placental development.