Modification of p53 with O-linked N-acetylglucosamine regulates p53 activity and stability

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.

Relationship Between O-GlcNAcase Expression and Prognosis of Patients With Osteosarcoma

Several studies have demonstrated a role of O-GlcNAcylation (O-GlcNAc) in tumorigenesis of various carcinomas by modification of tumor-associated proteins. However, its implication in the pathogenesis of osteosarcoma remains unclear. This study aimed to investigate the levels of O-GlcNAc and the expressions of O-linked N-acetylglucosamine transferase (OGT) and O-GlcNAcase (OGA) in human osteosarcoma tissues, by using immunohistochemistry; and to find correlations between the levels or expressions and several clinicopathologic parameters. There were 109 first diagnosed osteosarcoma patients, including Enneking stage IIB (n=70) and III (n=39). Correlations between the immunoreactive score (IRS) and clinicopathologic parameters, overall survival, and metastasis-free survival were evaluated. A positive correlation was found between the IRS of OGA and the percentage of postchemotherapeutic tumor necrosis (r=0.308; P=0.017). Univariate analysis revealed significantly lower OGA IRS in metastatic patients (P=0.020) and poor chemotherapeutic-responder patients (P=0.001). By multivariate analysis, presence of tumor metastasis (P=0.002) and lower OGA IRS (P=0.004) was significantly associated with shorter overall survival. Subgroup analysis in stage IIB osteosarcoma (n=70) demonstrated that male sex (P=0.019), presence of tumor recurrence (P=0.026), poor chemotherapeutic responder (P=0.022), and lower OGA IRS (P=0.019) were significantly correlated with short metastasis-free survival. But, lower OGA IRS was the only independent predictor for short metastasis-free survival (P=0.006). Our findings suggested that O-GlcNAc pathway, especially OGA, may involve in pathogenesis and aggressiveness of osteosarcoma. Low level of OGA expression may be used as a poor prognostic indicator.

O-GlcNAcylated p53 in the liver modulates hepatic glucose production

p53 regulates several signaling pathways to maintain the metabolic homeostasis of cells and modulates the cellular response to stress. Deficiency or excess of nutrients causes cellular metabolic stress, and we hypothesized that p53 could be linked to glucose maintenance. We show here that upon starvation hepatic p53 is stabilized by O-GlcNAcylation and plays an essential role in the physiological regulation of glucose homeostasis. More specifically, p53 binds to PCK1 promoter and regulates its transcriptional activation, thereby controlling hepatic glucose production. Mice lacking p53 in the liver show a reduced gluconeogenic response during calorie restriction. Glucagon, adrenaline and glucocorticoids augment protein levels of p53, and administration of these hormones to p53 deficient human hepatocytes and to liver-specific p53 deficient mice fails to increase glucose levels. Moreover, insulin decreases p53 levels, and over-expression of p53 impairs insulin sensitivity. Finally, protein levels of p53, as well as genes responsible of O-GlcNAcylation are elevated in the liver of type 2 diabetic patients and positively correlate with glucose and HOMA-IR. Overall these results indicate that the O-GlcNAcylation of p53 plays an unsuspected key role regulating in vivo glucose homeostasis.

Structure-based prediction of post-translational modification cross-talk within proteins using complementary residue- and residue pair-based features

Post-translational modification (PTM)-based regulation can be mediated not only by the modification of a single residue but also by the interplay of different modifications. Accurate prediction of PTM cross-talk is a highly challenging issue and is in its infant stage. Especially, less attention has been paid to the structural preferences (except intrinsic disorder and spatial proximity) of cross-talk pairs and the characteristics of individual residues involved in cross-talk, which may restrict the improvement of the prediction accuracy. Here we report a structure-based algorithm called PCTpred to improve the PTM cross-talk prediction. The comprehensive residue- and residue pair-based features were designed for paired PTM sites at the sequence and structural levels. Through feature selection, we reserved 23 newly introduced descriptors and 3 traditional descriptors to develop a sequence-based predictor PCTseq and a structure-based predictor PCTstr, both of which were integrated to construct our final prediction model. According to pair- and protein-based evaluations, PCTpred yielded area under the curve values of approximately 0.9 and 0.8, respectively. Even when removing the distance preference of samples or using the input of modeled structures, our prediction performance was maintained or moderately reduced. PCTpred displayed stable and reliable improvements over the state-of-the-art methods based on various evaluations. The source code and data set are freely available at https://github.com/Liulab-HZAU/PCTpred or http://liulab.hzau.edu.cn/PCTpred/.

O-GlcNAcylation: Implications in normal and malignant hematopoiesis

Posttranslational protein modification through addition of the O-linked β-N-acetyl-D-glucosamine (O-GlcNAc) moiety to serine or threonine residues, termed O-GlcNAcylation, is a highly dynamic process conserved throughout eukaryotes. O-GlcNAcylation is reversibly catalyzed by a single pair of enzymes, O-GlcNAc transferase and O-GlcNAcase, and it acts as a fundamental regulator for a wide variety of biological processes including gene expression, cell cycle regulation, metabolism, stress response, cellular signaling, epigenetics, and proteostasis. O-GlcNAcylation is regulated by various intracellular or extracellular cues such as metabolic status, nutrient availability, and stress. Studies over decades have unveiled the profound biological significance of this unique protein modification in normal physiology and pathologic processes of diverse cell types or tissues. In hematopoiesis, recent studies have indicated the essential and pleiotropic roles of O-GlcNAcylation in differentiation, proliferation, and function of hematopoietic cells including T cells, B cells, myeloid progenitors, and hematopoietic stem and progenitor cells. Moreover, aberrant O-GlcNAcylation is implicated in the development of hematologic malignancies with dysregulated epigenetics, metabolism, and gene transcription. Thus, it is now recognized that O-GlcNAcylation is one of the key regulators of normal and malignant hematopoiesis.

Can Glycosylation Mask the Detection of MHC Expressing p53 Peptides by T Cell Receptors?

Proteins of the major histocompatibility complex (MHC) class I, or human leukocyte antigen (HLA) in humans interact with endogenous peptides and present them to T cell receptors (TCR), which in turn tune the immune system to recognize and discriminate between self and foreign (non-self) peptides. Of especial importance are peptides derived from tumor-associated antigens. T cells recognizing these peptides are found in cancer patients, but not in cancer-free individuals. What stimulates this recognition, which is vital for the success of checkpoint based therapy? A peptide derived from the protein p53 (residues 161–169 or p161) was reported to show this behavior. T cells recognizing this unmodified peptide could be further stimulated in vitro to create effective cancer killing CTLs (cytotoxic T lymphocytes). We hypothesize that the underlying difference may arise from post-translational glycosylation of p161 in normal individuals, likely masking it against recognition by TCR. Defects in glycosylation in cancer cells may allow the presentation of the native peptide. We investigate the structural consequences of such peptide glycosylation by investigating the associated structural dynamics.

Valosin-Containing Protein Stabilizes Mutant p53 to Promote Pancreatic Cancer Growth

Approximately 80% of human pancreatic ductal adenocarcinomas (PDAC) harbor TP53 mutations, among which, R273H is the most frequent. Although p53-R273H is known to possess gain-of-function properties, how it is regulated in PDAC has not been extensively explored. Here we identify valosin-containing protein (VCP) as a regulator of p53-R273H by conducting immunoprecipitation-tandem mass spectrometry analysis. VCP bound p53-R273H at its DNA binding domain. Ectopic or endogenous VCP stabilized p53-R273H by binding to MDM2 and disrupting its association with mutant p53. Inhibition of VCP either by genetic depletion or the pharmacologic inhibitor CB-5083 increased ubiquitination and degradation of p53-R273H, leading to cell death. Consistently, ablation of VCP markedly retarded growth of cultured PDAC cells and xenograft PDAC tumors. Together, these results unveil VCP as a novel partner of p53-R273H in promoting PDAC growth and as a potential target for developing anti-PDAC therapy. SIGNIFICANCE: These findings identify valosin-containing protein (VCP) as a novel regulator of p53-R273H stability and suggest VCP as a potential target for development of pancreatic cancer therapy.

The glyco-redox interplay: Principles and consequences on the role of reactive oxygen species during protein glycosylation

Reactive oxygen species (ROS) carry out prime physiological roles as intracellular signaling agents, yet pathologically high concentrations of ROS cause irreversible damage to biomolecules, alter cellular programs and contribute to various diseases. While decades of intensive research have identified redox-related patterns and signaling pathways, very few addressed how the glycosylation machinery senses and responds to oxidative stress. A common trait among ROS and glycans residing on glycoconjugates is that they are both highly dynamic, as they are quickly fine-tuned in response to stressors such as inflammation, cancer and infectious diseases. On this account, the delicate balance of the redox potential, which is tightly regulated by dozens of enzymes including NOXs, and the mitochondrial electron transport chain as well as the fluidity of glycan biosynthesis resulting from the cooperation of glycosyltransferases, glycosidases, and nucleotide sugar transporters, is paramount to cell survival. Here, we review the broad spectrum of the interplay between redox changes and glycosylation with respect to their principle consequences on human physiology.

Targeting Pyrimidine Metabolism in the Era of Precision Cancer Medicine

Metabolic rewiring is considered as a primary feature of cancer. Malignant cells reprogram metabolism pathway in response to various intrinsic and extrinsic drawback to fuel cell survival and growth. Among the complex metabolic pathways, pyrimidine biosynthesis is conserved in all living organism and is necessary to maintain cellular fundamental function (i.e. DNA and RNA biosynthesis). A wealth of evidence has demonstrated that dysfunction of pyrimidine metabolism is closely related to cancer progression and numerous drugs targeting pyrimidine metabolism have been approved for multiple types of cancer. However, the non-negligible side effects and limited efficacy warrants a better strategy for negating pyrimidine metabolism in cancer. In recent years, increased studies have evidenced the interplay of oncogenic signaling and pyrimidine synthesis in tumorigenesis. Here, we review the recent conceptual advances on pyrimidine metabolism, especially dihydroorotate dehydrogenase (DHODH), in the framework of precision oncology medicine and prospect how this would guide the development of new drug precisely targeting the pyrimidine metabolism in cancer.

New Quinolinone O-GlcNAc Transferase Inhibitors Based on Fragment Growth

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

Stomatin‑like protein 2 induces metastasis by regulating the expression of a rate‑limiting enzyme of the hexosamine biosynthetic pathway in pancreatic cancer

Stomatin-like protein 2 (SLP-2) is associated with poor prognosis in several types of cancer, including pancreatic cancer (PC); however, the molecular mechanism of its involvement remains elusive. The present study aimed to elucidate the role of this protein in the development of PC. Human PC cell lines AsPC-1 and PANC-1 were transfected by a vector expressing SLP-2 shRNA. Analyses of cell proliferation, migration, invasion, chemosensitivity, and glucose uptake were conducted, while a mouse xenograft model was used to evaluate the functional role of SLP-2 in PC. Immunohistochemical analysis was retrospectively performed on human tissue samples to compare expression between the primary site (n=279) and the liver metastatic site (n=22). Furthermore, microarray analysis was conducted to identify the genes correlated with SLP-2. In vitro analysis demonstrated that cells in which SLP-2 was suppressed exhibited reduced cell motility and glucose uptake, while in vivo analysis revealed a marked decrease in the number of liver metastases. Immunohistochemistry revealed that SLP-2 was increased in liver metastatic sites. Microarray analysis indicated that this protein regulated the expression of glutamine-fructose-6-phosphate transaminase 2 (GFPT2), a rate-limiting enzyme of the hexosamine biosynthesis pathway. SLP-2 contributed to the malignant character of PC by inducing liver metastasis. Cell motility and glucose uptake may be induced via the hexosamine biosynthesis pathway through the expression of GFPT2. The present study revealed a new mechanism of liver metastasis and indicated that SLP-2 and its downstream pathway could provide novel therapeutic targets for PC.

pVHL promotes lysosomal degradation of YAP in lung adenocarcinoma

Yes-associated protein (YAP) is a vital transcriptional co-activator that activates cell proliferation and evasion of apoptosis for the promotion of tumorigenesis. The von Hippel-Lindau tumor suppressor protein (pVHL), as a critical component of E3 ubiquitin ligase, targets various substrates to regulate tumor progression. However, the precise molecular mechanisms of pVHL during tumorigenesis remain largely unclear. Herein, we found that there was a significant negative correlation between pVHL and YAP at protein level in the TCGA-LUAD dataset and our cohort. Over-expression of pVHL decreased YAP protein expression and reduced its transcriptional activity. Further study indicated that pVHL did not affect YAP mRNA level but decreased YAP protein stability in a lysosome-dependent manner. In addition, the pVHL-mediated degradation of YAP inhibited cellular proliferation, migration, and enhanced chemosensitivity to cisplatin in lung adenocarcinoma cells. Interestingly, the pVHL-mediated YAP degradation was blocked by elevated O-GlcNAcylation. Collectively, our findings demonstrate that pVHL modulates the lysosomal degradation of YAP, and may provide more clues to better understanding the tumor suppressive effects of pVHL.

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.

Signalling transduction of O-GlcNAcylation and PI3K/AKT/mTOR-axis in prostate cancer

Hexosamine biosynthetic (HBP) and PI3K/AKT/mTOR pathways are found to predominate the proliferation and survival of prostate cancer cells. Both these pathways have their own specific intermediates to propagate the secondary signals in down-stream cascades and besides having their own structured network, also have shared interconnecting branches. These interconnections are either competitive or co-operative in nature depending on the microenvironmental conditions. Specifically, in prostate cancer HBP and mTOR pathways increases the expression and protein level of androgen receptor in order to support cancer cell proliferation, advancement and metastasis. Pharmacological inhibition of a single pathway is therefore insufficient to stop disease progression as the cancer cells manage to alter the signalling channel. This is one of the primary reasons for the therapeutic failure in prostate cancer and emergence of chemoresistance. Inhibition of these multiple pathways at their common junctures might prove to be of benefit in men suffering from an advanced disease state. Hence, a thorough understanding of these cellular intersecting points and their significance with respect to signal transduction mechanisms might assist in the rational designing of combinations for effective management of prostate cancer.

Cancer and Tumour Suppressor p53 Encounters at the Juncture of Sex Disparity

There are many differences in cancer manifestation between men and women. New understanding of the origin of these point to fundamental distinctions in the genetic code and its demise. Tumour suppressor protein p53 is the chief operating officer of cancer defence and critically acts to safeguard against sustained DNA damaged. P53 cannot be ignored in cancer sex disparity. In this review we discuss the greater prevalence and associated death rates for non-reproductive cancers in males. The major tumour suppressor protein p53, encoded in the TP53 gene is our chosen context. It is fitting to ask why somatic TP53 mutation incidence is estimated to be disproportionately higher among males in the population for these types of cancers compared with females? We scrutinised the literature for evidence of predisposing genetic and epigenetic alterations that may explain this sex bias. Our second approach was to explore whether redox activity, either externally imposed or inherent to males and females, may define distinct risks that could contribute to the clear cancer sex disparities.

Overexpression of p53 due to excess protein O-GlcNAcylation is associated with coronary microvascular disease in type 2 diabetes

AIMS

We previously reported that increased protein O-GlcNAcylation in diabetic mice led to vascular rarefaction in the heart. In this study, we aimed to investigate whether and how coronary endothelial cell (EC) apoptosis is enhanced by protein O-GlcNAcylation and thus induces coronary microvascular disease (CMD) and subsequent cardiac dysfunction in diabetes. We hypothesize that excessive protein O-GlcNAcylation increases p53 that leads to CMD and reduced cardiac contractility.

METHODS AND RESULTS

We conducted in vivo functional experiments in control mice, TALLYHO/Jng (TH) mice, a polygenic type 2 diabetic (T2D) model, and EC-specific O-GlcNAcase (OGA, an enzyme that catalyzes the removal of O-GlcNAc from proteins)-overexpressing TH mice, as well as in vitro experiments in isolated ECs from these mice. TH mice exhibited a significant increase in coronary EC apoptosis and reduction of coronary flow velocity reserve (CFVR), an assessment of coronary microvascular function, in comparison to wild-type mice. The decreased CFVR, due at least partially to EC apoptosis, was associated with decreased cardiac contractility in TH mice. Western blot experiments showed that p53 protein level was significantly higher in coronary ECs from TH mice and T2D patients than in control ECs. High glucose treatment also increased p53 protein level in control ECs. Furthermore, overexpression of OGA decreased protein O-GlcNAcylation and down-regulated p53 in coronary ECs, and conferred a protective effect on cardiac function in TH mice. Inhibition of p53 with pifithrin-α attenuated coronary EC apoptosis and restored CFVR and cardiac contractility in TH mice.

CONCLUSIONS

The data from this study indicate that inhibition of p53 or down-regulation of p53 by OGA overexpression attenuates coronary EC apoptosis and improves CFVR and cardiac function in diabetes. Lowering coronary endothelial p53 levels via OGA overexpression could be a potential therapeutic approach for CMD in diabetes.

The miR-1185-2-3p-GOLPH3L pathway promotes glucose metabolism in breast cancer by stabilizing p53-induced SERPINE1

Background

Phosphatidylinositol-4-phosphate-binding protein GOLPH3L is overexpressed in human ductal carcinoma of the breast, and its expression levels correlate with the prognosis of breast cancer patients. However, the roles of GOLPH3L in breast tumorigenesis remain unclear.

Methods

We assessed the expression and biological function of GOLPH3L in breast cancer by combining bioinformatic prediction, metabolomics analysis and RNA-seq to determine the GOLPH3L-related pathways involved in tumorigenesis. Dual-luciferase reporter assay and coimmunoprecipitation (Co-IP) were used to explore the expression regulation mechanism of GOLPH3L.

Results

We demonstrated that knockdown of GOLPH3L in human breast cancer cells significantly suppressed their proliferation, survival, and migration and suppressed tumor growth in vivo, while overexpression of GOLPH3L promoted aggressive tumorigenic activities. We found that miRNA-1185-2-3p, the expression of which is decreased in human breast cancers and is inversely correlated with the prognosis of breast cancer patients, is directly involved in suppressing the expression of GOLPH3L. Metabolomics microarray analysis and transcriptome sequencing analysis revealed that GOLPH3L promotes central carbon metabolism in breast cancer by stabilizing the p53 suppressor SERPINE1.

Conclusions

In summary, we discovered a miRNA-GOLPH3L-SERPINE1 pathway that plays important roles in the metabolism of breast cancer and provides new therapeutic targets for human breast cancer.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13046-020-01767-9.

Tau Post-translational Modifications: Dynamic Transformers of Tau Function, Degradation, and Aggregation

Post-translational modifications (PTMs) on tau have long been recognized as affecting protein function and contributing to neurodegeneration. The explosion of information on potential and observed PTMs on tau provides an opportunity to better understand these modifications in the context of tau homeostasis, which becomes perturbed with aging and disease. Prevailing views regard tau as a protein that undergoes abnormal phosphorylation prior to its accumulation into the toxic aggregates implicated in Alzheimer's disease (AD) and other tauopathies. However, the phosphorylation of tau may, in fact, represent part of the normal but interrupted function and catabolism of the protein. In addition to phosphorylation, tau undergoes another forms of post-translational modification including (but not limited to), acetylation, ubiquitination, glycation, glycosylation, SUMOylation, methylation, oxidation, and nitration. A holistic appreciation of how these PTMs regulate tau during health and are potentially hijacked in disease remains elusive. Recent studies have reinforced the idea that PTMs play a critical role in tau localization, protein-protein interactions, maintenance of levels, and modifying aggregate structure. These studies also provide tantalizing clues into the possibility that neurons actively choose how tau is post-translationally modified, in potentially competitive and combinatorial ways, to achieve broad, cellular programs commensurate with the distinctive environmental conditions found during development, aging, stress, and disease. Here, we review tau PTMs and describe what is currently known about their functional impacts. In addition, we classify these PTMs from the perspectives of protein localization, electrostatics, and stability, which all contribute to normal tau function and homeostasis. Finally, we assess the potential impact of tau PTMs on tau solubility and aggregation. Tau occupies an undoubtedly important position in the biology of neurodegenerative diseases. This review aims to provide an integrated perspective of how post-translational modifications actively, purposefully, and dynamically remodel tau function, clearance, and aggregation. In doing so, we hope to enable a more comprehensive understanding of tau PTMs that will positively impact future studies.

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.

Proteomic profiling and genome-wide mapping of O-GlcNAc chromatin-associated proteins reveal an O-GlcNAc-regulated genotoxic stress response

O-GlcNAc modification plays critical roles in regulating the stress response program and cellular homeostasis. However, systematic and multi-omics studies on the O-GlcNAc regulated mechanism have been limited. Here, comprehensive data are obtained by a chemical reporter-based method to survey O-GlcNAc function in human breast cancer cells stimulated with the genotoxic agent adriamycin. We identify 875 genotoxic stress-induced O-GlcNAc chromatin-associated proteins (OCPs), including 88 O-GlcNAc chromatin-associated transcription factors and cofactors (OCTFs), subsequently map their genomic loci, and construct a comprehensive transcriptional reprogramming network. Notably, genotoxicity-induced O-GlcNAc enhances the genome-wide interactions of OCPs with chromatin. The dynamic binding switch of hundreds of OCPs from enhancers to promoters is identified as a crucial feature in the specific transcriptional activation of genes involved in the adaptation of cancer cells to genotoxic stress. The OCTF nuclear respiratory factor 1 (NRF1) is found to be a key response regulator in O-GlcNAc-modulated cellular homeostasis. These results provide a valuable clue suggesting that OCPs act as stress sensors by regulating the expression of various genes to protect cancer cells from genotoxic stress.

Protein O-GlcNAcylation is involved in regulating gene expression and maintaining cellular homeostasis. Here, the authors develop a chemical reporter-based strategy for the proteomic profiling and genome-wide mapping of genotoxic stress-induced O-GlcNAcylated chromatin-associated proteins.

O-GlcNAcylation of MEK2 promotes the proliferation and migration of breast cancer cells

Mitogen-activated protein kinase kinases are an important part of evolutionary conserved signaling modules that are involved in a variety of cellular processes in response to environmental stimuli. Among them, mitogen-activated protein kinase kinase 2 (MEK2) is the most crucial upstream signaling pathway of ERK1/2 cascade as a therapeutic target for overcoming Ras-driven cancers. However, the mechanisms of MEK2 regulation during tumor progression remain not fully elucidated. Herein, we identified that MEK2 was post-translationally regulated by O-GlcNAcylation. We found that MEK2 associated with OGT and was modified by O-GlcNAc. Mass spectrometry analysis further verified that O-GlcNAcylation of MEK2 occurred at Thr13, which was in the docking domain for specifically identifying its target proteins. While total O-GlcNAcylation stimulated the protein stability and phosphorylation of MEK2, Thr13 O-GlcNAcylation of MEK2 specifically enhanced its Thr394 phosphorylation as well as downstream ERK1/2 activation. Genetic ablation of MEK2 O-GlcNAcylation at Thr13 abrogated its ability to promote the proliferation and migration of breast cancer cells. Together, our data demonstrate that O-GlcNAcylation of MEK2 might be a key regulatory mechanism during tumorigenesis and is a potential therapeutic target for tumor treatment.

O-GlcNAc stabilizes SMAD4 by inhibiting GSK-3β-mediated proteasomal degradation

O-linked β-N-acetylglucosamine (O-GlcNAc) is a post-translational modification which occurs on the hydroxyl group of serine or threonine residues of nucleocytoplasmic proteins. It has been reported that the presence of this single sugar motif regulates various biological events by altering the fate of target proteins, such as their function, localization, and degradation. This study identified SMAD4 as a novel O-GlcNAc-modified protein. SMAD4 is a component of the SMAD transcriptional complex, a major regulator of the signaling pathway for the transforming growth factor-β (TGF-β). TGF-β is a powerful promoter of cancer EMT and metastasis. This study showed that the amount of SMAD4 proteins changes according to cellular O-GlcNAc levels in human lung cancer cells. This observation was made based on the prolonged half-life of SMAD4 proteins. The mechanism behind this interaction was that O-GlcNAc impeded interactions between SMAD4 and GSK-3β which promote proteasomal degradation of SMAD4. In addition, O-GlcNAc modification on SMAD4 Thr63 was responsible for stabilization. As a result, defects in O-GlcNAcylation on SMAD4 Thr63 attenuated the reporter activity of luciferase, the TGF-β-responsive SMAD binding element (SBE). This study’s findings imply that cellular O-GlcNAc may regulate the TGF-β/SMAD signaling pathway by stabilizing SMAD4.

Roles of ten-eleven translocation family proteins and their O-linked β-N-acetylglucosaminylated forms in cancer development

Members of the ten-eleven translocation (TET) protein family of which three mammalian TET proteins have been discovered so far, catalyze the sequential oxidation of 5-methylcytosine to 5-hydroxymethylcytosine, 5-formylcytosine, and 5-carboxylcytosine which serve an important role in embryonic development and tumor progression. O-GlcNAcylation (O-linked β-N-acetylglucosaminylation) is a reversible post-translational modification known to serve important roles in tumorigenesis and metastasis especially in hematopoietic malignancies such as myelodysplastic syndromes, chronic myelomonocytic leukemia and acute myeloid leukemia. O-GlcNAcylation activity requires only two enzymes: O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA). OGT catalyzes attachment of GlcNAc sugar to serine, threonine and cytosine residues in proteins, while OGA hydrolyzes O-GlcNAc attached to proteins. Numerous recent studies have demonstrated that TETs can be O-GlcNAcylated by OGT, with consequent alteration of TET activity and stability. The present review focuses on the cellular, biological and biochemical functions of TET and its O-GlcNAcylated form and proposes a model of the role of TET/OGT complex in regulation of target proteins during cancer development. In addition, the present review provides directions for future research in this area.

Cancer Metabolism: Phenotype, Signaling and Therapeutic Targets

Aberrant metabolism is a major hallmark of cancer. Abnormal cancer metabolism, such as aerobic glycolysis and increased anabolic pathways, has important roles in tumorigenesis, metastasis, drug resistance, and cancer stem cells. Well-known oncogenic signaling pathways, such as phosphoinositide 3-kinase (PI3K)/AKT, Myc, and Hippo pathway, mediate metabolic gene expression and increase metabolic enzyme activities. Vice versa, deregulated metabolic pathways contribute to defects in cellular signal transduction pathways, which in turn provide energy, building blocks, and redox potentials for unrestrained cancer cell proliferation. Studies and clinical trials are being performed that focus on the inhibition of metabolic enzymes by small molecules or dietary interventions (e.g., fasting, calorie restriction, and intermittent fasting). Similar to genetic heterogeneity, the metabolic phenotypes of cancers are highly heterogeneous. This heterogeneity results from diverse cues in the tumor microenvironment and genetic mutations. Hence, overcoming metabolic plasticity is an important goal of modern cancer therapeutics. This review highlights recent findings on the metabolic phenotypes of cancer and elucidates the interactions between signal transduction pathways and metabolic pathways. We also provide novel rationales for designing the next-generation cancer metabolism drugs.

Mutual regulation between OGT and XIAP to control colon cancer cell growth and invasion

O-GlcNAc transferase (OGT) is an enzyme that catalyzes the O-GlcNAc modification of nucleocytoplasmic proteins and is highly expressed in many types of cancer. However, the mechanism regulating its expression in cancer cells is not well understood. This study shows that OGT is a substrate of the E3 ubiquitin ligase X-linked inhibitor of apoptosis (XIAP) which plays an important role in cancer pathogenesis. Although LSD2 histone demethylase has already been reported as an E3 ubiquitin ligase in lung cancer cells, we identified XIAP as the main E3 ubiquitin ligase in colon cancer cells. Interestingly, OGT catalyzes the O-GlcNAc modification of XIAP at serine 406 and this modification is required for the E3 ubiquitin ligase activity of XIAP toward specifically OGT. Moreover, O-GlcNAcylation of XIAP suppresses colon cancer cell growth and invasion by promoting the proteasomal degradation of OGT. Therefore, our findings regarding the reciprocal regulation of OGT and XIAP provide a novel molecular mechanism for controlling cancer growth and invasion regulated by OGT and O-GlcNAc modification.

DNA-PKcs Ser2056 auto-phosphorylation is affected by an O-GlcNAcylation/phosphorylation interplay

BACKGROUND

DNA dependent Protein Kinase (DNA-PK) is an heterotrimeric complex regulating the Non Homologous End Joining (NHEJ) double strand break (DSB) repair pathway. The activity of its catalytic subunit (DNA-PKcs) is regulated by multiple phosphorylations, like the Ser2056 one that impacts DSB end processing and telomeres integrity. O-GlcNAcylation is a post translational modification (PTM) closely related to phosphorylation and its implication in the modulation of DNA-PKcs activity during the DNA Damage Response (DDR) is unknown.

METHODS

Using IP techniques, and HeLa cell line, we evaluated the effect of pharmacological or siOGT mediated O-GlcNAc level modulation on DNA-PKcs O-GlcNAcylation. We used the RPA32 phosphorylation as a DNA-PKcs activity reporter substrate to evaluate the effect of O-GlcNAc modulators.

RESULTS

We show here that human DNA-PKcs is an O-GlcNAc modified protein and that this new PTM is responsive to the cell O-GlcNAcylation level modulation. Our findings reveal that DNA-PKcs hypo O-GlcNAcylation affects its kinase activity and that the bleomycin-induced Ser2056 phosphorylation, is modulated by DNA-PKcs O-GlcNAcylation.

CONCLUSIONS

DNA-PKcs Ser2056 phosphorylation is antagonistically linked to DNA-PKcs O-GlcNAcylation level modulation.

GENERAL SIGNIFICANCE

Given the essential role of DNA-PKcs Ser2056 phosphorylation in the DDR, this study brings data about the role of cell O-GlcNAc level on genome integrity maintenance.

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.

Understanding p53 functions through p53 antibodies

TP53 is the most frequently mutated gene across all cancer types. Our understanding of its functions has evolved since its discovery four decades ago. Initially thought to be an oncogene, it was later realized to be a critical tumour suppressor. A significant amount of our knowledge about p53 functions have come from the use of antibodies against its various forms. The early anti-p53 antibodies contributed to the recognition of p53 accumulation as a common feature of cancer cells and to our understanding of p53 DNA-binding and transcription activities. They led to the concept that conformational changes can facilitate p53’s activity as a growth inhibitory protein. The ensuing p53 conformational-specific antibodies further underlined p53’s conformational flexibility, collectively forming the basis for current efforts to generate therapeutic molecules capable of altering the conformation of mutant p53. A subsequent barrage of antibodies against post-translational modifications on p53 has clarified p53’s roles further, especially with respect to the mechanistic details and context-dependence of its activity. More recently, the generation of p53 mutation-specific antibodies have highlighted the possibility to go beyond the general framework of our comprehension of mutant p53—and promises to provide insights into the specific properties of individual p53 mutants. This review summarizes our current knowledge of p53 functions derived through the major classes of anti-p53 antibodies, which could be a paradigm for understanding other molecular events in health and disease.

O-GlcNAcylation on LATS2 disrupts the Hippo pathway by inhibiting its activity

The Hippo pathway plays a crucial role in maintaining tissue homeostasis. Generally, activated Hippo pathway effectors, YAP/TAZ, induce the transcription of their negative regulators, NF2 and LATS2, and this negative feedback loop maintains homeostasis of the Hippo pathway. However, YAP and TAZ are consistently hyperactivated in various cancer cells, enhancing tumor growth. Our study found that LATS2, a direct-inhibiting kinase of YAP/TAZ and a core component of the negative feedback loop in the Hippo pathway, is modified with O-GlcNAc. LATS2 O-GlcNAcylation inhibited its activity by interrupting the interaction with the MOB1 adaptor protein, thereby activating YAP and TAZ to promote cell proliferation. Thus, our study suggests that LATS2 O-GlcNAcylation is deeply involved in Hippo pathway dysregulation in cancer cells.

The Hippo pathway controls organ size and tissue homeostasis by regulating cell proliferation and apoptosis. The LATS-mediated negative feedback loop prevents excessive activation of the effectors YAP/TAZ, maintaining homeostasis of the Hippo pathway. YAP and TAZ are hyperactivated in various cancer cells which lead to tumor growth. Aberrantly increased O-GlcNAcylation has recently emerged as a cause of hyperactivation of YAP in cancer cells. However, the mechanism, which induces hyperactivation of TAZ and blocks LATS-mediated negative feedback, remains to be elucidated in cancer cells. This study found that in breast cancer cells, abnormally increased O-GlcNAcylation hyperactivates YAP/TAZ and inhibits LATS2, a direct negative regulator of YAP/TAZ. LATS2 is one of the newly identified O-GlcNAcylated components in the MST-LATS kinase cascade. Here, we found that O-GlcNAcylation at LATS2 Thr436 interrupted its interaction with the MOB1 adaptor protein, which connects MST to LATS2, leading to activation of YAP/TAZ by suppressing LATS2 kinase activity. LATS2 is a core component in the LATS-mediated negative feedback loop. Thus, this study suggests that LATS2 O-GlcNAcylation is deeply involved in tumor growth by playing a critical role in dysregulation of the Hippo pathway in cancer cells.

O-GlcNAcylation of myosin phosphatase targeting subunit 1 (MYPT1) dictates timely disjunction of centrosomes

The role of O-linked N-acetylglucosamine (O-GlcNAc) modification in the cell cycle has been enigmatic. Previously, both O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA) disruptions have been shown to derail the mitotic centrosome numbers, suggesting that mitotic O-GlcNAc oscillation needs to be in concert with mitotic progression to account for centrosome integrity. Here, using both chemical approaches and biological assays with HeLa cells, we attempted to address the underlying molecular mechanism and observed that incubation of the cells with the OGA inhibitor Thiamet-G strikingly elevates centrosomal distances, suggestive of premature centrosome disjunction. These aberrations could be overcome by inhibiting Polo-like kinase 1 (PLK1), a mitotic master kinase. PLK1 inactivation is modulated by the myosin phosphatase targeting subunit 1 (MYPT1)-protein phosphatase 1cβ (PP1cβ) complex. Interestingly, MYPT1 has been shown to be abundantly O-GlcNAcylated, and the modified residues have been detected in a recent O-GlcNAc-profiling screen utilizing chemoenzymatic labeling and bioorthogonal conjugation. We demonstrate here that MYPT1 is O-GlcNAcylated at Thr-577, Ser-585, Ser-589, and Ser-601, which antagonizes CDK1-dependent phosphorylation at Ser-473 and attenuates the association between MYPT1 and PLK1, thereby promoting PLK1 activity. We conclude that under high O-GlcNAc levels, PLK1 is untimely activated, conducive to inopportune centrosome separation and disruption of the cell cycle. We propose that too much O-GlcNAc is equally deleterious as too little O-GlcNAc, and a fine balance between the OGT/OGA duo is indispensable for successful mitotic divisions.

O-linked N-acetylgalactosamine modification is present on the tumor suppressor p53

BACKGROUND

Mucin-type O-glycosylation (referred to as O-GalNAc glycosylation) is the most abundant O-glycosylation on membrane and secretory proteins. Recently several evidences suggest that nuclear or cytoplasmic proteins might also have O-GalNAc glycosylation. However, what nucleocytoplasmic proteins are O-GalNAc glycosylated and what the biological function of this modification in cells are still poorly understood. Previously, we reported the tumor suppressor p53 could be O-GalNAc glycosylated in vitro. To investigate the existence and function of O-GalNAc glycosylation on nucleocytoplasmic proteins in cell, p53 as a representative nucleocytoplasmic protein was studied.

METHODS

Using lectin blotting with GalNAc specific lectins, enzymatic treatments with O-GlcNAcase, core 1 β1, 3-galactosyltransferase and O-glycosidase, and metabolic labeling with un-O-acetylated GalNAz in UDP-Gal/UDP-GalNAc 4-epimerase (GALE) knockout cells, we validated the O-GalNAc glycosylation on p53. Using mass spectrometry analysis and site-directed mutagenesis, we identified the glycosylated sites and studied the functions of O-GalNAc glycosylation on p53.

RESULTS

The p53 was O-GalNAc glycosylated in cells. Ser121 residue was one of the glycosylated sites on p53. The O-GalNAc glycosylation at Ser121 was associated with the stability and activity of p53.

CONCLUSIONS

These results revealed that the O-GalNAc glycosylation was a novel modification on p53.

GENERAL SIGNIFICANCE

Our study provided a pilot evidence that the O-GalNAc glycosylation existed on nucleocytoplasmic protein.

O-GlcNAcylation and its role in the immune system

O-linked-N-acetylglucosaminylation (O-GlcNAcylation) is a type of glycosylation that occurs when a monosaccharide, O-GlcNAc, is added onto serine or threonine residues of nuclear or cytoplasmic proteins by O-GlcNAc transferase (OGT) and which can be reversibly removed by O-GlcNAcase (OGA). O-GlcNAcylation couples the processes of nutrient sensing, metabolism, signal transduction and transcription, and plays important roles in development, normal physiology and physiopathology. Cumulative studies have indicated that O-GlcNAcylation affects the functions of protein substrates in a number of ways, including protein cellular localization, protein stability and protein/protein interaction. Particularly, O-GlcNAcylation has been shown to have intricate crosstalk with phosphorylation as they both modify serine or threonine residues. Aberrant O-GlcNAcylation on various protein substrates has been implicated in many diseases, including neurodegenerative diseases, diabetes and cancers. However, the role of protein O-GlcNAcylation in immune cell lineages has been less explored. This review summarizes the current understanding of the fundamental biochemistry of O-GlcNAcylation, and discusses the molecular mechanisms by which O-GlcNAcylation regulates the development, maturation and functions of immune cells. In brief, O-GlcNAcylation promotes the development, proliferation, and activation of T and B cells. O-GlcNAcylation regulates inflammatory and antiviral responses of macrophages. O-GlcNAcylation promotes the function of activated neutrophils, but inhibits the activity of nature killer cells.

Role of nutrient-driven O-GlcNAc-post-translational modification in pancreatic exocrine and endocrine islet development

Although the developing pancreas is exquisitely sensitive to nutrient supply in utero, it is not entirely clear how nutrient-driven post-translational modification of proteins impacts the pancreas during development. We hypothesized that the nutrient-sensing enzyme O-GlcNAc transferase (Ogt), which catalyzes an O-GlcNAc-modification onto key target proteins, integrates nutrient-signaling networks to regulate cell survival and development. In this study, we investigated the heretofore unknown role of Ogt in exocrine and endocrine islet development. By genetic manipulation in vivo and by using morphometric and molecular analyses, such as immunofluorescence imaging and single cell RNA sequencing, we show the first evidence that Ogt regulates pancreas development. Genetic deletion of Ogt in the pancreatic epithelium (OgtKOPanc) causes pancreatic hypoplasia, in part by increased apoptosis and reduced levels of of Pdx1 protein. Transcriptomic analysis of single cell and bulk RNA sequencing uncovered cell-type heterogeneity and predicted upstream regulator proteins that mediate cell survival, including Pdx1, Ptf1a and p53, which are putative Ogt targets. In conclusion, these findings underscore the requirement of O-GlcNAcylation during pancreas development and show that Ogt is essential for pancreatic progenitor survival, providing a novel mechanistic link between nutrients and pancreas development.

The O-GlcNAc Modification on Kinases

O-Linked N-acetyl glucosamine (O-GlcNAc) is a protein modification found on thousands of nuclear, cytosolic, and mitochondrial proteins. Many O-GlcNAc sites occur in proximity to protein sites that are likewise modified by phosphorylation. While several studies have uncovered crosstalk between these two signaling modifications on individual proteins and pathways, an understanding of the role of O-GlcNAc in regulating kinases, the enzymes that install the phosphate modification, is still emerging. Here we review recent methods to profile the O-GlcNAc modification on a global scale that have revealed more than 100 kinases are modified by O-GlcNAc and highlight existing studies about regulation of these kinases by O-GlcNAc. Continuing efforts to profile the O-GlcNAc proteome and understand the role of O-GlcNAc on kinases will reveal new mechanisms of regulation and potential avenues for manipulation of the signaling mechanisms at the intersection of O-GlcNAc and phosphorylation.

Blockage of O-linked GlcNAcylation induces AMPK-dependent autophagy in bladder cancer cells

Background

High levels of the post-translational modification O-GlcNAcylation (O-GlcNAc) are found in multiple cancers, including bladder cancer. Autophagy, which can be induced by stress from post-translational modifications, plays a critical role in maintaining cellular homeostasis and regulating tumorigenesis. The impact of O-GlcNAcylation on autophagy in bladder cancer remains unclear. Here, we evaluate the change in autophagic activity in response to O-GlcNAcylation and explore the potential mechanisms.

Methods

O-GlcNAcylation levels in bladder cancer cells were altered through pharmacological or genetic manipulations: treating with 6-diazo-5-oxo-norleucine (DON) or thiamet-G (TG) or up- and downregulation of O-GlcNAc transferase (OGT) or O-GlcNAcase (OGA). Autophagy was determined using fluorescence microscopy and western blotting. Co-immunoprecipitation (Co-IP) assays were performed to evaluate whether the autophagy regulator AMP-activated protein kinase (AMPK) was O-GlcNAc modified.

Results

Cellular autophagic flux was strikingly enhanced as a result of O-GlcNAcylation suppression, whereas it decreased at high O-GlcNAcylation levels. Phosphorylation of AMPK increased after the suppression of O-GlcNAcylation. We found that O-GlcNAcylation of AMPK suppressed the activity of this regulator, thereby inhibiting ULK1 activity and autophagy.

Conclusion

We characterized a new function of O-GlcNAcylation in the suppression of autophagy via regulation of AMPK.

Graphical abstract

Blockage of O-linked GlcNAcylation induces AMPK dependent autophagy in bladder cancer cells.

Glycosidic linkage, N-acetyl side-chain, and other structural properties of methyl 2-acetamido-2-deoxy-β-D-glucopyranosyl-(1→4)-β-D-mannopyranoside monohydrate and related compounds

The crystal structure of methyl 2-acetamido-2-deoxy-β-D-glycopyranosyl-(1→4)-β-D-mannopyranoside monohydrate, C₁₅H₂₇NO₁₁·H₂O, was determined and its structural properties compared to those in a set of mono- and disaccharides bearing N-acetyl side-chains in βGlcNAc aldohexopyranosyl rings. Valence bond angles and torsion angles in these side chains are relatively uniform, but C-N (amide) and C-O (carbonyl) bond lengths depend on the state of hydrogen bonding to the carbonyl O atom and N-H hydrogen. Relative to N-acetyl side chains devoid of hydrogen bonding, those in which the carbonyl O atom serves as a hydrogen-bond acceptor display elongated C-O and shortened C-N bonds. This behavior is reproduced by density functional theory (DFT) calculations, indicating that the relative contributions of amide resonance forms to experimental C-N and C-O bond lengths depend on the solvation state, leading to expectations that activation barriers to amide cis-trans isomerization will depend on the polarity of the environment. DFT calculations also revealed useful predictive information on the dependencies of inter-residue hydrogen bonding and some bond angles in or proximal to β-(1→4) O-glycosidic linkages on linkage torsion angles φ and ψ. Hypersurfaces correlating φ and ψ with the linkage C-O-C bond angle and total energy are sufficiently similar to render the former a proxy of the latter.

O-GlcNAcylation of light chain serine 12 mediates rituximab production doubled by thiamet G

O-Glycosylation occurs in recombinant proteins produced by CHO cells, but this phenomenon has not been studied extensively. Here, we report that rituximab is an O-linked N-acetyl-glucosaminylated (O-GlcNAcylated) protein and the production of rituximab is increased by thiamet G, an inhibitor of O-GlcNAcase. The production of rituximab doubled with OGA inhibition and decreased with O-GlcNAc transferase inhibition. O-GlcNAc-specific antibody and metabolic labelling with azidO-GlcNAc confirmed the increased O-GlcNAcylation with thiamet G. Protein mass analysis revealed that serine 7, 12, and 14 of the rituximab light chain were O-GlcNAcylated. S12A mutation of the light chain decreased rituximab stability and failed to increase the production with thiamet G without any significant changes of mRNA level. Cytotoxicity and thermal stability assays confirmed that there were no differences in the biological and physical properties of rituximab produced by thiamet G treatment. Therefore, thiamet G treatment improves the production of rituximab without significantly altering its function.

The nutrient sensor OGT regulates Hipk stability and tumorigenic-like activities in Drosophila

Environmental cues such as nutrients alter cellular behaviors by acting on a wide array of molecular sensors inside cells. Of emerging interest is the link observed between effects of dietary sugars on cancer proliferation. Here, we identify the requirements of hexosamine biosynthetic pathway (HBP) and O-GlcNAc transferase (OGT) for Drosophila homeodomain-interacting protein kinase (Hipk)-induced growth abnormalities in response to a high sugar diet. On a normal diet, OGT is both necessary and sufficient for inducing Hipk-mediated tumor-like growth. We further show that OGT maintains Hipk protein stability by blocking its proteasomal degradation and that Hipk is O-GlcNAcylated by OGT. In mammalian cells, human HIPK2 proteins accumulate posttranscriptionally upon OGT overexpression. Mass spectrometry analyses reveal that HIPK2 is at least O-GlcNAc modified at S852, T1009, and S1147 residues. Mutations of these residues reduce HIPK2 O-GlcNAcylation and stability. Together, our data demonstrate a conserved role of OGT in positively regulating the protein stability of HIPKs (fly Hipk and human HIPK2), which likely permits the nutritional responsiveness of HIPKs.

Mitochondria-targeted tetrahedral DNA nanostructures for doxorubicin delivery and enhancement of apoptosis

d-(KLAKLAK)₂-and Cy5-modified tetrahedral DNA nanostructures for imaging-guided mitochondria-targeted drug delivery

O-GlcNAcylation as a Therapeutic Target for Alzheimer's Disease

Alzheimer's disease (AD) is the most common cause of dementia and the number of elderly patients suffering from AD has been steadily increasing. Despite worldwide efforts to cope with this disease, little progress has been achieved with regard to identification of effective therapeutics. Thus, active research focusing on identification of new therapeutic targets of AD is ongoing. Among the new targets, post-translational modifications which modify the properties of mature proteins have gained attention. O-GlcNAcylation, a type of PTM that attaches O-linked β-N-acetylglucosamine (O-GlcNAc) to a protein, is being sought as a new target to treat AD pathologies. O-GlcNAcylation has been known to modify the two important components of AD pathological hallmarks, amyloid precursor protein, and tau protein. In addition, elevating O-GlcNAcylation levels in AD animal models has been shown to be effective in alleviating AD-associated pathology. Although studies investigating the precise mechanism of reversal of AD pathologies by targeting O-GlcNAcylation are not yet complete, it is clearly important to examine O-GlcNAcylation regulation as a target of AD therapeutics. This review highlights the mechanisms of O-GlcNAcylation and its role as a potential therapeutic target under physiological and pathological AD conditions.

O-GlcNAc-Modification of NSL3 at Thr755 Site Maintains the Holoenzyme Activity of MOF/NSL Histone Acetyltransfease Complex

Both OGT1 (O-linked β-N-acetylglucosamine (O-GlcNAc) transferase isoform 1) and NSL3 (nonspecific lethal protein 3) are crucial components of the MOF (males absent on the first)/NSL histone acetyltransferase complex. We previously described how global histone H4 acetylation levels were modulated by OGT1/O-GlcNAcylation-mediated NSL3 stability. However, the specific modification site of NSL3 and its molecular mechanism of protein stability remain unknown. Here, we present evidence from biochemical experiments arguing that O-GlcNAcylation of NSL3 at Thr755 is tightly associated with holoenzyme activity of the MOF/NSL complex. Using in vitro O-GlcNAc-transferase assays combined with mass spectrometry, we suppose that the residue Thr755 on NSL3 C-terminus is the major site O-GlcNAc-modified by OGT1. Importantly, O-GlcNAcylation of this site is involved in the regulation of the ubiquitin-degradation of NSL3, because this site mutation (T755A) promotes the ubiquitin-mediated degradation of NSL3. Further in-depth research found that ubiquitin conjugating enzyme E2 S (UBE2S) accelerated the degradation of NSL3 via direct binding to it. Interestingly, OGT1 and UBE2S competitively bind to NSL3, suggesting the coordination of OGT1-UBE2S in regulating NSL3 stability. Furthermore, O-GlcNAcylation of NSL3 Thr755 site regulates the histone H4 acetylation levels at lysine 5, 8, and 16, suggesting that the O-GlcNAcylation of NSL3 at Thr755 is required for maintaining the integrity and holoenzyme activity of the MOF/NSL complex. In colony formation assays, we found that the integrity of the complex impacts the proliferation of the lung carcinoma type II epithelium-like A549 cells. Taken together, our results provide new insight into the elucidation of the molecular mechanism of the MOF/NSL complex.

ULK1 O-GlcNAcylation Is Crucial for Activating VPS34 via ATG14L during Autophagy Initiation

Unc-51-like-kinase 1 (ULK1) is a target of both the mechanistic target of rapamycin (mTOR) and AMP-activated protein kinase (AMPK), whose role is to facilitate the initiation of autophagy in response to starvation. Upon glucose starvation, dissociation of mTOR from ULK1 and phosphorylation by AMPK leads to the activation of ULK1 activity. Here, we provide evidence that ULK1 is the attachment of O-linked N-acetylglucosamine (O-GlcNAcylated) on the threonine 754 site by O-linked N-acetylglucosamine transferase (OGT) upon glucose starvation. ULK1 O-GlcNAcylation occurs after dephosphorylation of adjacent mTOR-dependent phosphorylation on the serine 757 site by protein phosphatase 1 (PP1) and phosphorylation by AMPK. ULK1 O-GlcNAcylation is crucial for binding and phosphorylation of ATG14L, allowing the activation of lipid kinase VPS34 and leading to the production of phosphatidylinositol-(3)-phosphate (PI(3)P), which is required for phagophore formation and initiation of autophagy. Our findings provide insights into the crosstalk between dephosphorylation and O-GlcNAcylation during autophagy and specify a molecular framework for potential therapeutic intervention in autophagy-related diseases.

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.

Glucosamine impedes transforming growth factor β1-mediated corneal fibroblast differentiation by targeting Krüppel-like factor 4

BACKGROUND

Transforming growth factor (TGF) family members play important roles in the regulation of corneal integrity, and the pathogenesis of corneal fibrosis. Currently, there are no effective agents targeting TGF-β signaling to diminish corneal fibrosis. Glucosamine (GlcN), which is widely used in the treatment of osteoarthritis, abrogates the morphologic effects of TGF-β2 on retinal pigmented epithelial cells in a mouse disease model. Here, we sought to determine whether GlcN would exert beneficial effects against TGF-β1-induced corneal fibrosis.

METHODS

In human corneal fibroblasts (HCFs) treated with GlcN, the expression of Krüppel-like factor 4 (KLF4) and its downstream signaling effects were determined in the presence and absence of TGF-β1 using immunoblot analysis. We further explored GlcN inhibition of fibroblast-to-myofibroblast differentiation via KLF4 siRNA. The effect of cycloheximide on KLF4 protein levels with or without GlcN administration was assessed to determine whether GlcN affects the stability of the KLF4 protein.

RESULTS

In HCFs, GlcN induced the expression of KLF4, which regulated the maturation and maintenance of the ocular surface. GlcN partially suppressed the TGF-β1-induced expression of alpha-smooth muscle actin (α-SMA) and reduced the collagen contraction capacity in HCFs, suggesting a decrease in fibroblast-to-myofibroblast differentiation. This effect appeared to be mediated through suppression of Smad2 phosphorylation and ERK-dependent signaling. The levels of KLF4 mRNA were increased by GlcN and decreased by TGF-β1 and the TGF-β1-induced α-SMA mRNA expression was upregulated when the KLF4 gene was silenced. GlcN also appeared to stabilize the KLF4 protein, reducing its turnover in corneal fibroblasts.

CONCLUSION

These findings shed light on a novel mechanism by which GlcN suppresses TGF-β1-induced fibroblast-to-myofibroblast differentiation through the upregulation of KLF4 expression. Current strategies for treating corneal fibrosis were not effective. Elevating KLF4 levels through the use of GlcN might provide an effective alternative to alleviate the development and progression of corneal fibrosis.

Next-generation unnatural monosaccharides reveal that ESRRB O-GlcNAcylation regulates pluripotency of mouse embryonic stem cells

Unnatural monosaccharides such as azidosugars that can be metabolically incorporated into cellular glycans are currently used as a major tool for glycan imaging and glycoproteomic profiling. As a common practice to enhance membrane permeability and cellular uptake, the unnatural sugars are per-O-acetylated, which, however, can induce a long-overlooked side reaction, non-enzymatic S-glycosylation. Herein, we develop 1,3-di-esterified N-azidoacetylgalactosamine (GalNAz) as next-generation chemical reporters for metabolic glycan labeling. Both 1,3-di-O-acetylated GalNAz (1,3-Ac₂GalNAz) and 1,3-di-O-propionylated GalNAz (1,3-Pr₂GalNAz) exhibit high efficiency for labeling protein O-GlcNAcylation with no artificial S-glycosylation. Applying 1,3-Pr₂GalNAz in mouse embryonic stem cells (mESCs), we identify ESRRB, a critical transcription factor for pluripotency, as an O-GlcNAcylated protein. We show that ESRRB O-GlcNAcylation is important for mESC self-renewal and pluripotency. Mechanistically, ESRRB is O-GlcNAcylated by O-GlcNAc transferase at serine 25, which stabilizes ESRRB, promotes its transcription activity and facilitates its interactions with two master pluripotency regulators, OCT4 and NANOG.

Per-O-acetylated unnatural monosaccharides are popular tools for glycan labeling in live cells but can undergo unwanted side reactions with cysteines. Here, the authors develop unnatural sugars in a partially esterified form that are inert towards cysteines, and use them to probe O-GlcNAcylation in mESCs.

p53 modifications: exquisite decorations of the powerful guardian

The last 40 years have witnessed how p53 rose from a viral binding protein to a central factor in both stress responses and tumor suppression. The exquisite regulation of p53 functions is of vital importance for cell fate decisions. Among the multiple layers of mechanisms controlling p53 function, posttranslational modifications (PTMs) represent an efficient and precise way. Major p53 PTMs include phosphorylation, ubiquitination, acetylation, and methylation. Meanwhile, other PTMs like sumoylation, neddylation, O-GlcNAcylation, adenosine diphosphate (ADP)-ribosylation, hydroxylation, and β-hydroxybutyrylation are also shown to play various roles in p53 regulation. By independent action or interaction, PTMs affect p53 stability, conformation, localization, and binding partners. Deregulation of the PTM-related pathway is among the major causes of p53-associated developmental disorders or diseases, especially in cancers. This review focuses on the roles of different p53 modification types and shows how these modifications are orchestrated to produce various outcomes by modulating p53 activities or targeted to treat different diseases caused by p53 dysregulation.

Fatty acid-induced CD36 expression via O-GlcNAcylation drives gastric cancer metastasis

Metastasis is the primary cause of death in patients with advanced cancer. Recently, a high-fat diet was shown to specifically promote the metastatic potential of specific cancer cells in a CD36-dependent manner. However, the molecular basis of the fatty acid (FA)-induced upregulation of CD36 has remained unclear.

Methods: RT-qPCR, FACS analysis, immunoblotting and immunohistochemistry, as well as retrieving TCGA database, were carried out to quantitate CD36 expression in gastric cancer (GC) tissues and cell lines. Transwell assay and xenografts were used to assess cell metastasis abilities in vitro and in vivo after indicated treatment. Luciferase reporter assay was carried out to evaluate the changes in signaling pathways when O-GlcNAcylation level was increased in GC cells and in vitro O-GlcNAcylation assay was utilized for wild and mutant types of CD36 protein to explore the potential O-GlcNAcylation sites.

Results: High CD36 expression is a predictor of poor survival and promotes metastasis of GC cells and the use of neutralizing antibodies to block CD36 inhibits GC metastasis in mice. FA or a HFD promotes the metastatic potential of GC cells by upregulating CD36 via increasing the O-GlcNAcylation level. Increased O-GlcNAcylation levels promote the transcription of CD36 by activating the NF-κB pathway and also increase its FA uptake activity by directly modifying CD36 at S468 and T470.

Conclusion: FA-induced hyper-O-GlcNAcylation promotes the transcription and function of CD36 by activating the NF-κB pathway and directly modifying CD36 at S468 and T470, which drives GC metastasis.