The hexosamine signaling pathway: deciphering the "O-GlcNAc code"

O-GlcNAcylation Reduces Ischemia-Reperfusion-Induced Brain Injury

O-GlcNAcylation is a common posttranslational modification of nucleocytoplasmic proteins with β-N-acetylglucosamine (GlcNAc) and regulates numerous biological processes. By using mouse models of cerebral ischemia induced by permanent and transient middle cerebral artery occlusion (MCAO), we observed an initial elevation (~1.7-fold, 1–4 hours after ischemia) and then decline of O-GlcNAcylation during cerebral ischemia. We found that moderate increase (<3-fold) of brain O-GlcNAcylation by pharmacological means ameliorated cerebral ischemia-reperfusion injury and the consequent motor and neurological deficits. Interference of the transient elevation of O-GlcNAcylation pharmacologically or genetically aggravates the ischemia-induced brain damage, motor deficits and mortality. The alteration of O-GlcNAcylation was also seen in the ischemic areas of postmortem human brains. This study reveals an important regulation of cerebral ischemia-reperfusion injury by O-GlcNAcylation and also provides a possible therapeutic strategy, i.e., by increasing O-GlcNAcylation, to reduce the cerebral damage and improve the clinical outcome of ischemic stroke.

Glycosylation with O-linked β-N-acetylglucosamine induces vascular dysfunction via production of superoxide anion/reactive oxygen species

Overproduction of superoxide anion (•O₂^-) and O-linked β-N-acetylglucosamine (O-GlcNAc) modification in the vascular system are contributors to endothelial dysfunction. This study tested the hypothesis that increased levels of O-GlcNAc-modified proteins contribute to •O₂^- production via activation of NADPH oxidase, resulting in impaired vasodilation. Rat aortic segments and vascular smooth muscle cells (VSMCs) were incubated with vehicle (methanol) or O-(2-acetamido-2-deoxy-d-glucopyranosylidenamino) N-phenylcarbamate (PUGNAc) (100 μM). PUGNAc produced a time-dependent increase in O-GlcNAc levels in VSMC and decreased endothelium-dependent relaxation, which was prevented by apocynin and tiron, suggesting that •O₂^- contributes to endothelial dysfunction under augmented O-GlcNAc levels. Aortic segments incubated with PUGNAc also exhibited increased levels of reactive oxygen species, assessed by dihydroethidium fluorescence, and augmented •O₂^- production, determined by lucigenin-enhanced chemiluminescence. Additionally, PUGNAc treatment increased Nox-1 and Nox-4 protein expression in aortas and VSMCs. Translocation of the p47^phox subunit from the cytosol to the membrane was greater in aortas incubated with PUGNAc. VSMCs displayed increased p22^phox protein expression after PUGNAc incubation, suggesting that NADPH oxidase is activated in conditions where O-GlcNAc protein levels are increased. In conclusion, O-GlcNAc levels reduce endothelium-dependent relaxation by overproduction of •O₂^- via activation of NADPH oxidase. This may represent an additional mechanism by which augmented O-GlcNAc levels impair vascular function.

O-GlcNAc regulation of autophagy and α-synuclein homeostasis; implications for Parkinson's disease

Post-translational modification on protein Ser/Thr residues by O-linked attachment of ß-N-acetyl-glucosamine (O-GlcNAcylation) is a key mechanism integrating redox signaling, metabolism and stress responses. One of the most common neurodegenerative diseases that exhibit aberrant redox signaling, metabolism and stress response is Parkinson’s disease, suggesting a potential role for O-GlcNAcylation in its pathology. To determine whether abnormal O-GlcNAcylation occurs in Parkinson’s disease, we analyzed lysates from the postmortem temporal cortex of Parkinson’s disease patients and compared them to age matched controls and found increased protein O-GlcNAcylation levels. To determine whether increased O-GlcNAcylation affects neuronal function and survival, we exposed rat primary cortical neurons to thiamet G, a highly selective inhibitor of the enzyme which removes the O-GlcNAc modification from target proteins, O-GlcNAcase (OGA). We found that inhibition of OGA by thiamet G at nanomolar concentrations significantly increased protein O-GlcNAcylation, activated MTOR, decreased autophagic flux, and increased α-synuclein accumulation, while sparing proteasomal activities. Inhibition of MTOR by rapamycin decreased basal levels of protein O-GlcNAcylation, decreased AKT activation and partially reversed the effect of thiamet G on α-synuclein monomer accumulation. Taken together we have provided evidence that excessive O-GlcNAcylation is detrimental to neurons by inhibition of autophagy and by increasing α-synuclein accumulation.

WGA-based lectin affinity gel electrophoresis: A novel method for the detection of O-GlcNAc-modified proteins

Post-translational modification with O-linked β-N-acetylglucosamine (O-GlcNAc) occurs selectively on serine and/or threonine residues of cytoplasmic and nuclear proteins, and dynamically regulates their molecular functions. Since conventional strategies to evaluate the O-GlcNAcylation level of a specific protein require time-consuming steps, the development of a rapid and easy method for the detection and quantification of an O-GlcNAcylated protein has been a challenging issue. Here, we describe a novel method in which O-GlcNAcylated and non-O-GlcNAcylated forms of proteins are separated by lectin affinity gel electrophoresis using wheat germ agglutinin (WGA), which primarily binds to N-acetylglucosamine residues. Electrophoresis of cell lysates through a gel containing copolymerized WGA selectively induced retardation of the mobility of O-GlcNAcylated proteins, thereby allowing the simultaneous visualization of both the O-GlcNAcylated and the unmodified forms of proteins. This method is therefore useful for the quantitative detection of O-GlcNAcylated proteins.

The sweet tooth of the circadian clock

The endogenous circadian clock is a key regulator of daily metabolic processes. On the other hand, circadian clocks in a broad range of tissues can be tuned by extrinsic and intrinsic metabolic cues. The bidirectional interaction between circadian clocks and metabolism involves both transcriptional and post-translational mechanisms. Nuclear receptors exemplify the transcriptional programs that couple molecular clocks to metabolism. The post-translational modifications of the core clock machinery are known to play a key role in metabolic entrainment of circadian clocks. O-linked N-acetylglucosamine modification (O-GlcNAcylation) of intracellular proteins is a key mediator of metabolic response to nutrient availability. This review highlights our current understanding of the role of protein O-GlcNAcylation in mediating metabolic input and output of the circadian clock.

Insights into activity and inhibition from the crystal structure of human O-GlcNAcase

O-GlcNAc hydrolase (OGA) catalyzes removal of βα-linked N-acetyl-D-glucosamine from serine and threonine residues. We report crystal structures of Homo sapiens OGA catalytic domain in apo and inhibited states, revealing a flexible dimer that displays three unique conformations and is characterized by subdomain α-helix swapping. These results identify new structural features of the substrate-binding groove adjacent to the catalytic site and open new opportunities for structural, mechanistic and drug discovery activities.

Therapeutic Targeting of Epithelial Plasticity Programs: Focus on the Epithelial-Mesenchymal Transition

Mounting data points to epithelial plasticity programs such as the epithelial-mesenchymal transition (EMT) as clinically relevant therapeutic targets for the treatment of malignant tumors. In addition to the widely realized role of EMT in increasing cancer cell invasiveness during cancer metastasis, the EMT has also been implicated in allowing cancer cells to avoid tumor suppressor pathways during early tumorigenesis. In addition, data linking EMT to innate and acquired treatment resistance further points towards the desire to develop pharmacological therapies to target epithelial plasticity in cancer. In this review we organized our discussion on pathways and agents that can be used to target the EMT in cancer into 3 groups: (1) extracellular inducers of EMT, (2) the transcription factors that orchestrate the EMT transcriptome, and (3) the downstream effectors of EMT. We highlight only briefly specific canonical pathways known to be involved in EMT, such as the signal transduction pathways TGFβ, EFGR, and Axl-Gas6. We emphasize in more detail pathways that we believe are emerging novel pathways and therapeutic targets such as epigenetic therapies, glycosylation pathways, and immunotherapy. The heterogeneity of tumors and the dynamic nature of epithelial plasticity in cancer cells make it likely that targeting only 1 EMT-related process will be unsuccessful or only transiently successful. We suggest that with greater understanding of epithelial plasticity regulation, such as with the EMT, a more systematic targeting of multiple EMT regulatory networks will be the best path forward to improve cancer outcomes.

Posttranslational Modifications of Lipid-Activated Nuclear Receptors: Focus on Metabolism

Posttranslational modifications (PTMs) occur to nearly all proteins, are catalyzed by specific enzymes, and are subjected to tight regulation. They have been shown to be a powerful means by which the function of proteins can be modified, resulting in diverse effects. Technological advances such as the increased sensitivity of mass spectrometry-based techniques and availability of mutant animal models have enhanced our understanding of the complexities of their regulation and the effect they have on protein function. However, the role that PTMs have in a pathological context still remains unknown for the most part. PTMs enable the modulation of nuclear receptor function in a rapid and reversible manner in response to varied stimuli, thereby dramatically altering their activity in some cases. This review focuses on acetylation, phosphorylation, SUMOylation, and O-GlcNAcylation, which are the 4 most studied PTMs affecting lipid-regulated nuclear receptor biology, as well as on the implications of such modifications on metabolic pathways under homeostatic and pathological situations. Moreover, we review recent studies on the modulation of PTMs as therapeutic targets for metabolic diseases.

The UPR reduces glucose metabolism via IRE1 signaling

Neurons are highly dependent on glucose. A disturbance in glucose homeostasis therefore poses a severe risk that is counteracted by activation of stress responses to limit damage and restore the energy balance. A major stress response that is activated under conditions of glucose deprivation is the unfolded protein response (UPR) that is aimed to restore proteostasis in the endoplasmic reticulum. The key signaling of the UPR involves the transient activation of a transcriptional program and an overall reduction of protein synthesis. Since the UPR is strategically positioned to sense and integrate metabolic stress signals, it is likely that - apart from its adaptive response to restore proteostasis - it also directly affects metabolic pathways. Here we investigate the direct role of the UPR in glucose homeostasis. O-GlcNAc is a post-translational modification that is highly responsive to glucose fluctuations. We find that UPR activation results in decreased O-GlcNAc modification, in line with reduced glucose metabolism. Our data indicate that UPR activation has no direct impact on the upstream processes in glucose metabolism; glucose transporter expression, glucose uptake and hexokinase activity. In contrast, prolonged UPR activation decreases glycolysis and mitochondrial metabolism. Decreased mitochondrial respiration is not accompanied by apoptosis or a structural change in mitochondria indicating that the reduction in metabolic rate upon UPR activation is a physiological non-apoptotic response. Metabolic decrease is prevented if the IRE1 pathway of the UPR is inhibited. This indicates that activation of IRE1 signaling induces a reduction in glucose metabolism, as part of an adaptive response.

Effect of glucosamine on expression of type II collagen, matrix metalloproteinase and sirtuin genes in a human chondrocyte cell line

Glucosamine (GlcN) has been widely used to treat osteoarthritis (OA) in humans. However, the effects of GlcN on genes related to cartilage metabolism are still unknown. In the present study, to elucidate the chondroprotective action of GlcN on OA, we examined the effects of GlcN (0.1-10 mM) on the expression of the sirtuin (SIRT) genes as well as type II collagen and matrix metalloproteinases (MMPs) using a human chondrocyte cell line SW 1353. SW 1353 cells were incubated in the absence or presence of GlcN. RT-PCR analyses revealed that GlcN markedly increased the mRNA expression of type II collagen (COL2A1). By contrast, the levels of MMP-1 and MMP-9 mRNA were only slightly increased by GlcN. Furthermore, western blot analyses revealed that GlcN significantly increased the protein level of COL2A1. Importantly, GlcN enhanced the mRNA expression and protein level of SIRT1, an upstream-regulating gene of COL2A1. Moreover, a SIRT1 inhibitor suppressed GlcN-induced COL2A1 gene expression. Together these observations suggest that GlcN enhances the mRNA expression and protein level of SIRT1 and its downstream gene COL2A1 in chondrocytes, thereby possibly exhibiting chondroprotective action on OA.

Chemical Glycoproteomics

Chemical tools have accelerated progress in glycoscience, reducing experimental barriers to studying protein glycosylation, the most widespread and complex form of posttranslational modification. For example, chemical glycoproteomics technologies have enabled the identification of specific glycosylation sites and glycan structures that modulate protein function in a number of biological processes. This field is now entering a stage of logarithmic growth, during which chemical innovations combined with mass spectrometry advances could make it possible to fully characterize the human glycoproteome. In this review, we describe the important role that chemical glycoproteomics methods are playing in such efforts. We summarize developments in four key areas: enrichment of glycoproteins and glycopeptides from complex mixtures, emphasizing methods that exploit unique chemical properties of glycans or introduce unnatural functional groups through metabolic labeling and chemoenzymatic tagging; identification of sites of protein glycosylation; targeted glycoproteomics; and functional glycoproteomics, with a focus on probing interactions between glycoproteins and glycan-binding proteins. Our goal with this survey is to provide a foundation on which continued technological advancements can be made to promote further explorations of protein glycosylation.

Lipopolysaccharide (LPS)-stimulated iNOS Induction Is Increased by Glucosamine under Normal Glucose Conditions but Is Inhibited by Glucosamine under High Glucose Conditions in Macrophage Cells

We investigated the regulatory effect of glucosamine (GlcN) for the production of nitric oxide (NO) and expression of inducible NO synthase (iNOS) under various glucose conditions in macrophage cells. At normal glucose concentrations, GlcN dose dependently increased LPS-stimulated production of NO/iNOS. However, GlcN suppressed NO/iNOS production under high glucose culture conditions. Moreover, GlcN suppressed LPS-induced up-regulation of COX-2, IL-6, and TNF-α mRNAs under 25 mm glucose conditions yet did not inhibit up-regulation under 5 mm glucose conditions. Glucose itself dose dependently increased LPS-induced iNOS expression. LPS-induced MAPK and IκB-α phosphorylation did not significantly differ at normal and high glucose conditions. The activity of LPS-induced nuclear factor-κB (NF-κB) and DNA binding of c-Rel to the iNOS promoter were inhibited under high glucose conditions in comparison with no significant changes under normal glucose conditions. In addition, we found that the LPS-induced increase in O-GlcNAcylation as well as DNA binding of c-Rel to the iNOS promoter were further increased by GlcN under normal glucose conditions. However, both O-GlcNAcylation and DNA binding of c-Rel decreased under high glucose conditions. The NF-κB inhibitor, pyrrolidine dithiocarbamate, inhibited LPS-induced iNOS expression under high glucose conditions but it did not influence iNOS induction under normal glucose conditions. In addition, pyrrolidine dithiocarbamate inhibited NF-κB DNA binding and c-Rel O-GlcNAcylation only under high glucose conditions. By blocking transcription with actinomycin D, we found that stability of LPS-induced iNOS mRNA was increased by GlcN under normal glucose conditions. These results suggest that GlcN regulates inflammation by sensing energy states of normal and fuel excess.

O-GlcNAc transferase inhibitors: current tools and future challenges

The O-linked N-acetylglucosamine (O-GlcNAc) post-translational modification (O-GlcNAcylation) is the dynamic and reversible attachment of N-acetylglucosamine to serine and threonine residues of nucleocytoplasmic target proteins. It is abundant in metazoa, involving hundreds of proteins linked to a plethora of biological functions with implications in human diseases. The process is catalysed by two enzymes: O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA) that add and remove sugar moieties respectively. OGT knockout is embryonic lethal in a range of animal models, hampering the study of the biological role of O-GlcNAc and the dissection of catalytic compared with non-catalytic roles of OGT. Therefore, selective and potent chemical tools are necessary to inhibit OGT activity in the context of biological systems. The present review focuses on the available OGT inhibitors and summarizes advantages, limitations and future challenges.

Reprogramming glucose metabolism in cancer: can it be exploited for cancer therapy?

In recent years there has been a growing interest among cancer biologists in cancer metabolism. This Review summarizes past and recent advances in our understanding of the reprogramming of glucose metabolism in cancer cells, which is mediated by oncogenic drivers and by the undifferentiated character of cancer cells. The reprogrammed glucose metabolism in cancer cells is required to fulfil anabolic demands. This Review discusses the possibility of exploiting the reprogrammed glucose metabolism for therapeutic approaches that selectively target cancer cells.

Functional analysis of recombinant human and Yarrowia lipolytica O-GlcNAc transferases expressed in Saccharomyces cerevisiae

O-linked β-N-acetylglucosamine (O-GlcNAc) glycosylation is an important post-translational modification in many cellular processes. It is mediated by O-GlcNAc transferases (OGTs), which catalyze the addition of O-GlcNAc to serine or threonine residues of the target proteins. In this study, we expressed a putative Yarrowia lipolytica OGT (YlOGT), the only homolog identified in the subphylum Saccharomycotina through bioinformatics analysis, and the human OGT (hOGT) as recombinant proteins in Saccharomyces cerevisiae, and performed their functional characterization. Immunoblotting assays using antibody against O-GlcNAc revealed that recombinant hOGT (rhOGT), but not the recombinant YlOGT (rYlOGT), undergoes auto-O-GlcNAcylation in the heterologous host S. cerevisiae. Moreover, the rhOGT expressed in S. cerevisiae showed a catalytic activity during in vitro assays using casein kinase II substrates, whereas no such activity was obtained in rYlOGT. However, the chimeric human-Y. lipolytica OGT, carrying the human tetratricopeptide repeat (TPR) domain along with the Y. lipolytica catalytic domain (CTD), mediated the transfer of O-GlcNAc moiety during the in vitro assays. Although the overexpression of full-length OGTs inhibited the growth of S. cerevisiae, no such inhibition was obtained upon overexpression of only the CTD fragment, indicating the role of TPR domain in growth inhibition. This is the first report on the functional analysis of the fungal OGT, indicating that the Y. lipolytica OGT retains its catalytic activity, although the physiological role and substrates of YlOGT remain to be elucidated.

Glucosamine exposure reduces proteoglycan synthesis in primary human endothelial cells in vitro

PURPOSE

Glucosamine (GlcN) supplements are promoted for medical reasons, for example, for patients with arthritis and other joint-related diseases. Oral intake of GlcN is followed by uptake in the intestine, transport in the circulation and thereafter delivery to chondrocytes. Here, it is postulated to have an effect on synthesis and turnover of extracellular matrix constituents expressed by these cells. Following uptake in the intestine, serum levels are transiently increased, and the endothelium is exposed to increased levels of GlcN. We investigated the possible effects of GlcN on synthesis of proteoglycans (PGs), an important matrix component, in primary human endothelial cells.

METHODS

Primary human endothelial cells were cultured in vitro in medium with 5 mM glucose and 0-10 mM GlcN. PGs were recovered and analysed by western blotting, or by SDS-PAGE, gel chromatography or ion-exchange chromatography of (35)S-PGs after (35)S-sulphate labelling of the cells.

RESULTS

The synthesis and secretion of (35)S-PGs from cultured endothelial cells were reduced in a dose- and time-dependent manner after exposure to GlcN. PGs are substituted with sulphated glycosaminoglycan (GAG) chains, vital for PG function. The reduction in (35)S-PGs was not related to an effect on GAG chain length, number or sulphation, but rather to the total expression of PGs.

CONCLUSION

Exposure of endothelial cells to GlcN leads to a general decrease in (35)S-PG synthesis. These results suggest that exposure to high levels of GlcN can lead to decreased matrix synthesis, contrary to what has been claimed by supporters of such supplements.

Glyco-seek: Ultrasensitive Detection of Protein-Specific Glycosylation by Proximity Ligation Polymerase Chain Reaction

We report a non-destructive biochemical technique, termed "Glyco-seek", for analysis of O-GlcNAcylated proteins. Glyco-seek combines chemoenzymatic labeling, proximity ligation, and quantitative polymerase chain reaction to detect O-GlcNAcylated proteins with ultrahigh sensitivity. Our glycan-specific assay can be paired with traditional proximity ligation assays to simultaneously determine the change in total protein levels. We show that Glyco-seek detects attomoles of glycoproteins of interest from cell lysates, with sensitivity several orders of magnitude higher than that of current techniques. We used the method to directly assay the O-GlcNAcylation status of a low-abundance transcription factor from cell lysates without need for isolation or enrichment.

O-GlcNAcylation in Cancer Biology: Linking Metabolism and Signaling

The hexosamine biosynthetic pathway (HBP) is highly dependent on multiple metabolic nutrients including glucose, glutamine, and acetyl-CoA. Increased flux through HBP leads to elevated post-translational addition of β-D-N-acetylglucosamine sugars to nuclear and cytoplasmic proteins. Increased total O-GlcNAcylation is emerging as a general characteristic of cancer cells, and recent studies suggest that O-GlcNAcylation is a central communicator of nutritional status to control key signaling and metabolic pathways that regulate multiple cancer cell phenotypes. This review summarizes our current understanding of changes of O-GlcNAc cycling enzymes in cancer, the role of O-GlcNAcylation in tumorigenesis, and the current challenges in targeting this pathway therapeutically.

O-GlcNAcylation of protein kinase A catalytic subunits enhances its activity: a mechanism linked to learning and memory deficits in Alzheimer's disease

Alzheimer's disease (AD) is characterized clinically by memory loss and cognitive decline. Protein kinase A (PKA)‐CREB signaling plays a critical role in learning and memory. It is known that glucose uptake and O‐GlcNAcylation are reduced in AD brain. In this study, we found that PKA catalytic subunits (PKAcs) were posttranslationally modified by O‐linked N‐acetylglucosamine (O‐GlcNAc). O‐GlcNAcylation regulated the subcellular location of PKAcα and PKAcβ and enhanced their kinase activity. Upregulation of O‐GlcNAcylation in metabolically active rat brain slices by O‐(2‐acetamido‐2‐deoxy‐d‐glucopyranosylidenamino) N‐phenylcarbamate (PUGNAc), an inhibitor of N‐acetylglucosaminidase, increased the phosphorylation of tau at the PKA site, Ser214, but not at the non‐PKA site, Thr205. In contrast, in rat and mouse brains, downregulation of O‐GlcNAcylation caused decreases in the phosphorylation of CREB at Ser133 and of tau at Ser214, but not at Thr205. Reduction in O‐GlcNAcylation through intracerebroventricular injection of 6‐diazo‐5‐oxo‐l‐norleucine (DON), the inhibitor of glutamine fructose‐6‐phosphate amidotransferase, suppressed PKA‐CREB signaling and impaired learning and memory in mice. These results indicate that in addition to cAMP and phosphorylation, O‐GlcNAcylation is a novel mechanism that regulates PKA‐CREB signaling. Downregulation of O‐GlcNAcylation suppresses PKA‐CREB signaling and consequently causes learning and memory deficits in AD.

The role of glycans in the development and progression of prostate cancer

Prostate cancer is a unique and heterogeneous disease. Currently, a major unmet clinical need exists to develop biomarkers that enable indolent disease to be distinguished from aggressive disease. The prostate is an abundant secretor of glycoproteins of all types, and alterations in glycans are, therefore, attractive as potential biomarkers and therapeutic targets. Despite progress over the past decade in profiling the genome and proteome, the prostate cancer glycoproteome remains relatively understudied. A wide range of alterations in the glycoproteins on prostate cancer cells can occur, including increased sialylation and fucosylation, increased O-β-N-acetylglucosamine (GlcNAc) conjugation, the emergence of cryptic and high-mannose N-glycans and alterations to proteoglycans. Glycosylation can alter protein function and has a key role in many important biological processes in cancer including cell adhesion, migration, interactions with the cell matrix, immune surveillance, cell signalling and cellular metabolism; altered glycosylation in prostate cancer might modify some, or all of these processes. In the past three years, powerful tools such as glycosylation-specific antibodies and glycosylation gene signatures have been developed, which enable detailed analyses of changes in glycosylation. Thus, emerging data on these often overlooked modifications have the potential to improve risk stratification and therapeutic strategies in patients with prostate cancer.

Hijacking the Hexosamine Biosynthetic Pathway to Promote EMT-Mediated Neoplastic Phenotypes

The epithelial-mesenchymal transition (EMT) is a highly conserved program necessary for orchestrating distant cell migration during embryonic development. Multiple studies in cancer have demonstrated a critical role for EMT during the initial stages of tumorigenesis and later during tumor invasion. Transcription factors (TFs) such as SNAIL, TWIST, and ZEB are master EMT regulators that are aberrantly overexpressed in many malignancies. Recent evidence correlates EMT-related transcriptomic alterations with metabolic reprograming in cancer. Metabolic alterations may allow cancer to adapt to environmental stressors, supporting the irregular macromolecular demand of rapid proliferation. One potential metabolic pathway of increasing importance is the hexosamine biosynthesis pathway (HBP). The HBP utilizes glycolytic intermediates to generate the metabolite UDP-GlcNAc. This and other charged nucleotide sugars serve as the basis for biosynthesis of glycoproteins and other glycoconjugates. Recent reports in the field of glycobiology have cultivated great curiosity within the cancer research community. However, specific mechanistic relationships between the HBP and fundamental pathways of cancer, such as EMT, have yet to be elucidated. Altered protein glycosylation downstream of the HBP is well positioned to mediate many cellular changes associated with EMT including cell-cell adhesion, responsiveness to growth factors, immune system evasion, and signal transduction programs. Here, we outline some of the basics of the HBP and putative roles the HBP may have in driving EMT-related cancer processes. With novel appreciation of the HBP's connection to EMT, we hope to illuminate the potential for new therapeutic targets of cancer.

Two-way communication between the metabolic and cell cycle machineries: the molecular basis

The relationship between cellular metabolism and the cell cycle machinery is by no means unidirectional. The ability of a cell to enter the cell cycle critically depends on the availability of metabolites. Conversely, the cell cycle machinery commits to regulating metabolic networks in order to support cell survival and proliferation. In this review, we will give an account of how the cell cycle machinery and metabolism are interconnected. Acquiring information on how communication takes place among metabolic signaling networks and the cell cycle controllers is crucial to increase our understanding of the deregulation thereof in disease, including cancer.

Downregulation of O-linked N-acetylglucosamine transferase by RNA interference decreases MMP9 expression in human esophageal cancer cells

O-linked N-acetylglucosamine transferase (OGT) catalyzes O-linked glycosylation (O-GlcNAcylation). O-GlcNAcylation is a post-translational carbohydrate modification of diverse nuclear and cytosolic proteins by the addition of O-linked β-N-acetylglucosamine. It was recently demonstrated that OGT and the level of O-GlcNAcylation are upregulated in esophageal cancer; however, the physiological consequences of this upregulation remain unknown. The current study reports that OGT knockdown by short hairpin RNA (shRNA) did not affect cell viability; however, cell migration in esophageal cancer Eca-109 cells was significantly reduced. OGT-specific shRNA vectors efficiently decreased the protein and mRNA levels of OGT and the RL2 level (a marker of O-GlcNAcylation levels) in Eca-109 esophageal cancer cells. In addition, colony formation and cell proliferation assays demonstrated that OGT-specific shRNA decreased the proliferation of Eca-109 cells; however, there was no significant statistical difference between OGT-specific shRNA and control shRNA. Notably, transwell assays demonstrated that the migratory ability of Eca-109 cells was significantly suppressed following knockdown of the OGT gene. Correspondingly, western blot analyses demonstrated that OGT knockdown significantly downregulated the expression of matrix metalloproteinase 9 (MMP9) in Eca-109 cells. These results suggest that OGT may promote the migration, invasion and metastasis of esophageal cancer cells by enhancing the stability or expression of MMP9.

Nucleocytoplasmic human O-GlcNAc transferase is sufficient for O-GlcNAcylation of mitochondrial proteins

O-linked N-acetylglucosamine modification (O-GlcNAcylation) is a nutrient-dependent protein post-translational modification (PTM), dynamically and reversibly driven by two enzymes: O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA) that catalyse the addition and the removal of the O-GlcNAc moieties to/from serine and threonine residues of target proteins respectively. Increasing evidence suggests involvement of O-GlcNAcylation in many biological processes, including transcription, signalling, neuronal development and mitochondrial function. The presence of a mitochondrial O-GlcNAc proteome and a mitochondrial OGT (mOGT) isoform has been reported. We explored the presence of mOGT in human cell lines and mouse tissues. Surprisingly, analysis of genomic sequences indicates that this isoform cannot be expressed in most of the species analysed, except some primates. In addition, we were not able to detect endogenous mOGT in a range of human cell lines. Knockdown experiments and Western blot analysis of all the predicted OGT isoforms suggested the expression of only a single OGT isoform. In agreement with this, we demonstrate that overexpression of the nucleocytoplasmic OGT (ncOGT) isoform leads to increased O-GlcNAcylation of mitochondrial proteins, suggesting that ncOGT is necessary and sufficient for the generation of the O-GlcNAc mitochondrial proteome.

Glycerol-3-phosphate acyltransferase-1 upregulation by O-GlcNAcylation of Sp1 protects against hypoxia-induced mouse embryonic stem cell apoptosis via mTOR activation

Oxygen signaling is critical for stem cell regulation, and oxidative stress-induced stem cell apoptosis decreases the efficiency of stem cell therapy. Hypoxia activates O-linked β-N-acetyl glucosaminylation (O-GlcNAcylation) of stem cells, which contributes to regulation of cellular metabolism, as well as cell fate. Our study investigated the role of O-GlcNAcylation via glucosamine in the protection of hypoxia-induced apoptosis of mouse embryonic stem cells (mESCs). Hypoxia increased mESCs apoptosis in a time-dependent manner. Moreover, hypoxia also slightly increased the O-GlcNAc level. Glucosamine treatment further enhanced the O-GlcNAc level and prevented hypoxia-induced mESC apoptosis, which was suppressed by O-GlcNAc transferase inhibitors. In addition, hypoxia regulated several lipid metabolic enzymes, whereas glucosamine increased expression of glycerol-3-phosphate acyltransferase-1 (GPAT1), a lipid metabolic enzyme producing lysophosphatidic acid (LPA). In addition, glucosamine-increased O-GlcNAcylation of Sp1, which subsequently leads to Sp1 nuclear translocation and GPAT1 expression. Silencing of GPAT1 by gpat1 siRNA transfection reduced glucosamine-mediated anti-apoptosis in mESCs and reduced mammalian target of rapamycin (mTOR) phosphorylation. Indeed, LPA prevented mESCs from undergoing hypoxia-induced apoptosis and increased phosphorylation of mTOR and its substrates (S6K1 and 4EBP1). Moreover, mTOR inactivation by rapamycin (mTOR inhibitor) increased pro-apoptotic proteins expressions and mESC apoptosis. Furthermore, transplantation of non-targeting siRNA and glucosamine-treated mESCs increased cell survival and inhibited flap necrosis in mouse skin flap model. Conversely, silencing of GPAT1 expression reversed those glucosamine effects. In conclusion, enhancing O-GlcNAcylation of Sp1 by glucosamine stimulates GPAT1 expression, which leads to inhibition of hypoxia-induced mESC apoptosis via mTOR activation.

Drosophila O-GlcNAcase Deletion Globally Perturbs Chromatin O-GlcNAcylation

Gene expression during Drosophila development is subject to regulation by the Polycomb (Pc), Trithorax (Trx), and Compass chromatin modifier complexes. O-GlcNAc transferase (OGT/SXC) is essential for Pc repression suggesting that the O-GlcNAcylation of proteins plays a key role in regulating development. OGT transfers O-GlcNAc onto serine and threonine residues in intrinsically disordered domains of key transcriptional regulators; O-GlcNAcase (OGA) removes the modification. To pinpoint genomic regions that are regulated by O-GlcNAc levels, we performed ChIP-chip and microarray analysis after OGT or OGA RNAi knockdown in S2 cells. After OGA RNAi, we observed a genome-wide increase in the intensity of most O-GlcNAc-occupied regions including genes linked to cell cycle, ubiquitin, and steroid response. In contrast, O-GlcNAc levels were strikingly insensitive to OGA RNAi at sites of polycomb repression such as the Hox and NK homeobox gene clusters. Microarray analysis suggested that altered O-GlcNAc cycling perturbed the expression of genes associated with morphogenesis and cell cycle regulation. We then produced a viable null allele of oga (oga(del.1)) in Drosophila allowing visualization of altered O-GlcNAc cycling on polytene chromosomes. We found that trithorax (TRX), absent small or homeotic discs 1 (ASH1), and Compass member SET1 histone methyltransferases were O-GlcNAc-modified in oga(del.1) mutants. The oga(del.1) mutants displayed altered expression of a distinct set of cell cycle-related genes. Our results show that the loss of OGA in Drosophila globally impacts the epigenetic machinery allowing O-GlcNAc accumulation on RNA polymerase II and numerous chromatin factors including TRX, ASH1, and SET1.

O-GlcNAcylation: a bridge between glucose and cell differentiation

Glucose is the major energy supply and a critical metabolite for most cells and is especially important when cell is differentiating. High or low concentrations of glucose enhances or inhibits the osteogenic, chondrogenic and adipogenic differentiation of cell via the insulin, transforming growth factor‐β and peroxisome proliferator‐activated receptor γ pathways, among others. New evidence implicates the hexosamine biosynthetic pathway as a mediator of crosstalk between glucose flux, cellular signalling and epigenetic regulation of cell differentiation. Extracellular glucose flux alters intracellular O‐GlcNAcylation levels through the hexosamine biosynthetic pathway. Signalling molecules that are important for cell differentiation, including protein kinase C, extracellular signal‐regulated kinase, Runx2, CCAAT/enhancer‐binding proteins, are modified by O‐GlcNAcylation. Thus, O‐GlcNAcylation markedly alters cell fate during differentiation via the post‐transcriptional modification of proteins. Furthermore, O‐GlcNAcylation and phosphorylation show complex interactions during cell differentiation: they can either non‐competitively occupy different sites on a substrate or competitively occupy a single site or proximal sites. Therefore, the influence of glucose on cell differentiation via O‐GlcNAcylation offers a potential target for controlling tissue homoeostasis and regeneration in ageing and disease. Here, we review recent progress establishing an emerging relationship among glucose concentration, O‐GlcNAcylation levels and cell differentiation.

Transcriptome responses in alfalfa associated with tolerance to intensive animal grazing

Tolerance of alfalfa (Medicago sativa L.) to animal grazing varies widely within the species. However, the molecular mechanisms influencing the grazing tolerant phenotype remain uncharacterized. The objective of this study was to identify genes and pathways that control grazing response in alfalfa. We analyzed whole-plant de novo transcriptomes from grazing tolerant and intolerant populations of M. sativa ssp. falcata subjected to grazing by sheep. Among the Gene Ontology terms which were identified as grazing responsive in the tolerant plants and differentially enriched between the tolerant and intolerant populations (both grazed), most were associated with the ribosome and translation-related activities, cell wall processes, and response to oxygen levels. Twenty-one grazing responsive pathways were identified that also exhibited differential expression between the tolerant and intolerant populations. These pathways were associated with secondary metabolite production, primary carbohydrate metabolic pathways, shikimate derivative dependent pathways, ribosomal subunit composition, hormone signaling, wound response, cell wall formation, and anti-oxidant defense. Sequence polymorphisms were detected among several differentially expressed homologous transcripts between the tolerant and intolerant populations. These differentially responsive genes and pathways constitute potential response mechanisms for grazing tolerance in alfalfa. They also provide potential targets for molecular breeding efforts to develop grazing-tolerant cultivars of alfalfa.

Mixed Lineage Leukemia 5 (MLL5) Protein Stability Is Cooperatively Regulated by O-GlcNac Transferase (OGT) and Ubiquitin Specific Protease 7 (USP7)

Mixed lineage leukemia 5 (MLL5) protein is a trithorax family histone 3 lysine 4 (H3K4) methyltransferase that regulates diverse biological processes, including cell cycle progression, hematopoiesis and cancer. The mechanisms by which MLL5 protein stability is regulated have remained unclear to date. Here, we showed that MLL5 protein stability is cooperatively regulated by O-GlcNAc transferase (OGT) and ubiquitin-specific protease 7 (USP7). Depletion of OGT in cells led to a decrease in the MLL5 protein level through ubiquitin/proteasome-dependent proteolytic degradation, whereas ectopic expression of OGT protein suppressed MLL5 ubiquitylation. We further identified deubiquitinase USP7 as a novel MLL5-associated protein using mass spectrometry. USP7 stabilized the MLL5 protein through direct binding and deubiquitylation. Loss of USP7 induced degradation of MLL5 protein. Conversely, overexpression of USP7, but not a catalytically inactive USP7 mutant, led to decreased ubiquitylation and increased MLL5 stability. Co-immunoprecipitation and co-immunostaining assays revealed that MLL5, OGT and USP7 interact with each other to form a stable ternary complex that is predominantly located in the nucleus. In addition, upregulation of MLL5 expression was correlated with increased expression of OGT and USP7 in human primary cervical adenocarcinomas. Our results collectively reveal a novel molecular mechanism underlying regulation of MLL5 protein stability and provide new insights into the functional interplay among O-GlcNAc transferase, deubiquitinase and histone methyltransferase.

Extracellular Toxoplasma gondii tachyzoites metabolize and incorporate unnatural sugars into cellular proteins

Toxoplasma gondii is an obligate intracellular parasite that infects all nucleated cell types in diverse warm-blooded organisms. Many of the surface antigens and effector molecules secreted by the parasite during invasion and intracellular growth are modified by glycans. Glycosylated proteins in the nucleus and cytoplasm have also been reported. Despite their prevalence, the complete inventory and biological significance of glycosylated proteins in Toxoplasma remain unknown. In this study, we aimed to globally profile parasite glycoproteins using a bioorthogonal chemical reporter strategy. This strategy involves the metabolic incorporation of unnatural functional groups (i.e., "chemical reporters") into Toxoplasma glycans, followed by covalent labeling with visual probes or affinity tags. The two-step approach enables the visualization and identification of newly biosynthesized glycoconjugates in the parasite. Using a buffer that mimics intracellular conditions, extracellular Toxoplasma tachyzoites were found to metabolize and incorporate unnatural sugars (equipped with bioorthogonal functional groups) into diverse proteins. Covalent chemistries were used to visualize and retrieve these labeled structures. Subsequent mass spectrometry analysis revealed 89 unique proteins. This survey identified novel proteins as well as previously characterized proteins from lectin affinity analyses.

Dysregulated metabolism contributes to oncogenesis

Cancer is a disease characterized by unrestrained cellular proliferation. In order to sustain growth, cancer cells undergo a complex metabolic rearrangement characterized by changes in metabolic pathways involved in energy production and biosynthetic processes. The relevance of the metabolic transformation of cancer cells has been recently included in the updated version of the review "Hallmarks of Cancer", where dysregulation of cellular metabolism was included as an emerging hallmark. While several lines of evidence suggest that metabolic rewiring is orchestrated by the concerted action of oncogenes and tumor suppressor genes, in some circumstances altered metabolism can play a primary role in oncogenesis. Recently, mutations of cytosolic and mitochondrial enzymes involved in key metabolic pathways have been associated with hereditary and sporadic forms of cancer. Together, these results demonstrate that aberrant metabolism, once seen just as an epiphenomenon of oncogenic reprogramming, plays a key role in oncogenesis with the power to control both genetic and epigenetic events in cells. In this review, we discuss the relationship between metabolism and cancer, as part of a larger effort to identify a broad-spectrum of therapeutic approaches. We focus on major alterations in nutrient metabolism and the emerging link between metabolism and epigenetics. Finally, we discuss potential strategies to manipulate metabolism in cancer and tradeoffs that should be considered. More research on the suite of metabolic alterations in cancer holds the potential to discover novel approaches to treat it.

Evidence for a Functional O-Linked N-Acetylglucosamine (O-GlcNAc) System in the Thermophilic Bacterium Thermobaculum terrenum

Post-translational modification of proteins is a ubiquitous mechanism of signal transduction in all kingdoms of life. One such modification is addition of O-linked N-acetylglucosamine to serine or threonine residues, known as O-GlcNAcylation. This unusual type of glycosylation is thought to be restricted to nucleocytoplasmic proteins of eukaryotes and is mediated by a pair of O-GlcNAc-transferase and O-GlcNAc hydrolase enzymes operating on a large number of substrate proteins. Protein O-GlcNAcylation is responsive to glucose and flux through the hexosamine biosynthetic pathway. Thus, a close relationship is thought to exist between the level of O-GlcNAc proteins within and the general metabolic state of the cell. Although isolated apparent orthologues of these enzymes are present in bacterial genomes, their biological functions remain largely unexplored. It is possible that understanding the function of these proteins will allow development of reductionist models to uncover the principles of O-GlcNAc signaling. Here, we identify orthologues of both O-GlcNAc cycling enzymes in the genome of the thermophilic eubacterium Thermobaculum terrenum. The O-GlcNAcase and O-GlcNAc-transferase are co-expressed and, like their mammalian orthologues, localize to the cytoplasm. The O-GlcNAcase orthologue possesses activity against O-GlcNAc proteins and model substrates. We describe crystal structures of both enzymes, including an O-GlcNAcase·peptide complex, showing conservation of active sites with the human orthologues. Although in vitro activity of the O-GlcNAc-transferase could not be detected, treatment of T. terrenum with an O-GlcNAc-transferase inhibitor led to inhibition of growth. T. terrenum may be the first example of a bacterium possessing a functional O-GlcNAc system.

Large-scale production of pharmaceutical proteins in plant cell culture-the Protalix experience

Protalix Biotherapeutics develops recombinant human proteins and produces them in plant cell culture. Taliglucerase alfa has been the first biotherapeutic expressed in plant cells to be approved by regulatory authorities around the world. Other therapeutic proteins are being developed and are currently at various stages of the pipeline. This review summarizes the major milestones reached by Protalix Biotherapeutics to enable the development of these biotherapeutics, including platform establishment, cell line selection, manufacturing process and good manufacturing practice principles to consider for the process. Examples of the various products currently being developed are also presented.

The omniscient placenta: Metabolic and epigenetic regulation of fetal programming

Fetal development could be considered a sensitive period wherein exogenous insults and changes to the maternal milieu can have long-term impacts on developmental programming. The placenta provides the fetus with protection and necessary nutrients for growth, and responds to maternal cues and changes in nutrient signaling through multiple epigenetic mechanisms. The X-linked enzyme O-linked-N-acetylglucosamine transferase (OGT) acts as a nutrient sensor that modifies numerous proteins to alter various cellular signals, including major epigenetic processes. This review describes epigenetic alterations in the placenta in response to insults during pregnancy, the potential links of OGT as a nutrient sensor to placental epigenetics, and the implications of placental epigenetics in long-term neurodevelopmental programming. We describe the role of placental OGT in the sex-specific programming of hypothalamic-pituitary-adrenal (HPA) axis programming deficits by early prenatal stress as an example of how placental signaling can have long-term effects on neurodevelopment.

O-GlcNAcylation regulates ischemia-induced neuronal apoptosis through AKT signaling

Apoptosis plays an important role in neural development and neurological disorders. In this study, we found that O-GlcNAcylation, a unique protein posttranslational modification with O-linked β-N-acetylglucosamine (GlcNAc), promoted apoptosis through attenuating phosphorylation/activation of AKT and Bad. By using co-immunoprecipitation and mutagenesis techniques, we identified O-GlcNAc modification at both Thr308 and Ser473 of AKT. O-GlcNAcylation-induced apoptosis was attenuated by over-expression of AKT. We also found a dynamic elevation of protein O-GlcNAcylation during the first four hours of cerebral ischemia, followed by continuous decline after middle cerebral artery occlusion (MCAO) in the mouse brain. The elevation of O-GlcNAcylation coincided with activation of cell apoptosis. Finally, we found a negative correlation between AKT phosphorylation and O-GlcNAcylation in ischemic brain tissue. These results indicate that cerebral ischemia induces a rapid increase of O-GlcNAcylation that promotes apoptosis through down-regulation of AKT activity. These findings provide a novel mechanism through which O-GlcNAcylation regulates ischemia-induced neuronal apoptosis through AKT signaling.

Glutamine and glutamate limit the shortening of action potential duration in anoxia-challenged rabbit hearts

In clinical conditions, amino acid supplementation is applied to improve contractile function, minimize ischemia/reperfusion injury, and facilitate postoperative recovery. It has been shown that glutamine enhances myocardial ATP/APD (action potential duration) and glutathione/oxidized glutathione ratios, and can increase hexosamine biosynthesis pathway flux, which is believed to play a role in cardioprotection. Here, we studied the effect of glutamine and glutamate on electrical activity in Langendorff-perfused rabbit hearts. The hearts were supplied by Tyrode's media with or without 2.5 mmol/L glutamine and 150 μmol/L glutamate, and exposed to two 6-min anoxias with 20-min recovery in between. Change in APD was detected using a monophasic action potential probe. A nonlinear mixed-effects regression technique was used to evaluate the effect of amino acids on APD over the experiment. Typically, the dynamic of APD change encompasses three phases: short transient increase (more prominent in the first episode), slow decrease, and fast increase (starting with the beginning of recovery). The effect of both anoxic challenge and glutamine/glutamate was cumulative, being more pronounced in the second anoxia. The amino acids' protective effect became largest by the end of anoxia – 20.0% (18.9, 95% CI: [2.6 ms, 35.1 ms]), during the first anoxia and 36.6% (27.1, 95% CI: [7.7 ms, 46.6 ms]), during the second. Following the second anoxia, APD difference between control and supplemented hearts progressively increased, attaining 10.8% (13.6, 95% CI: [4.1 ms, 23.1 ms]) at the experiments' end. Our data reveal APD stabilizing and suggest an antiarrhythmic capacity of amino acid supplementation in anoxic/ischemic conditions.

OGT mediated histone H2B S112 GlcNAcylation regulates DNA damage response

O-GlcNAcylation is an important post-translational modification and has been implicated in many fundamental cellular processes. Recent studies showed that O-linked N-acetylglucosamine (GlcNAc) transferase (OGT) mediated O-GlcNAcylation of histone H2B Ser 112 (H2B S112 GlcNAcylation) plays an important role in gene transcription. However, the role of this histone modification in DNA damage response has not been studied yet. In this study, we found that OGT and OGT mediated H2B S112 GlcNAcylation are involved in DNA damage response for maintaining genomic stability and are required for resistance to many DNA-damaging and replication stress-inducing agents. OGT mediated H2B S112 GlcNAcylation increased locally upon the induction of double-strand breaks (DSBs), and depletion of OGT or overexpression of H2B S112A mutant impaired homologous recombination (HR) and nonhomologous end-joining (NHEJ). Mechanistically, H2B S112 GlcNAcylation could bind Nijmegen breakage syndrome 1 (NBS1) and regulate NBS1 foci formation. Taken together, our results demonstrate a new function of histone O-GlcNAcylation in DNA damage response (DDR).

A small molecule that inhibits OGT activity in cells

O-GlcNAc transferase (OGT) is an essential mammalian enzyme that regulates numerous cellular processes through the attachment of O-linked N-acetylglucosamine (O-GlcNAc) residues to nuclear and cytoplasmic proteins. Its targets include kinases, phosphatases, transcription factors, histones, and many other intracellular proteins. The biology of O-GlcNAc modification is still not well understood, and cell-permeable inhibitors of OGT are needed both as research tools and for validating OGT as a therapeutic target. Here, we report a small molecule OGT inhibitor, OSMI-1, developed from a high-throughput screening hit. It is cell-permeable and inhibits protein O-GlcNAcylation in several mammalian cell lines without qualitatively altering cell surface N- or O-linked glycans. The development of this molecule validates high-throughput screening approaches for the discovery of glycosyltransferase inhibitors, and further optimization of this scaffold may lead to yet more potent OGT inhibitors useful for studying OGT in animal models.

The Utilities of Chemical Reactions and Molecular Tools for O-GlcNAc Proteomic Studies

The post-translational modification of nuclear and cytoplasmic proteins with O-linked β-N-acetylglucosamine (O-GlcNAc) is involved in a wide variety of cellular processes and is associated with the pathological progression of chronic diseases. Considering its emerging biological significance, systematic identification, site mapping, and quantification of O-GlcNAc proteins are essential and have led to the development of several approaches for O-GlcNAc protein profiling. This minireview mainly focuses on the various useful chemical reactions and molecular tools with detailed reaction mechanisms widely adopted for O-GlcNAc protein/peptide enrichment and its quantification for comprehensive O-GlcNAc protein profiling.

High glucose induces mitochondrial dysfunction independently of protein O-GlcNAcylation

Diabetes is characterized by hyperglycaemia and perturbations in intermediary metabolism. In particular, diabetes can augment flux through accessory pathways of glucose metabolism, such as the hexosamine biosynthetic pathway (HBP), which produces the sugar donor for the β-O-linked-N-acetylglucosamine (O-GlcNAc) post-translational modification of proteins. Diabetes also promotes mitochondrial dysfunction. Nevertheless, the relationships among diabetes, hyperglycaemia, mitochondrial dysfunction and O-GlcNAc modifications remain unclear. In the present study, we tested whether high-glucose-induced increases in O-GlcNAc modifications directly regulate mitochondrial function in isolated cardiomyocytes. Augmentation of O-GlcNAcylation with high glucose (33 mM) was associated with diminished basal and maximal cardiomyocyte respiration, a decreased mitochondrial reserve capacity and lower Complex II-dependent respiration (P<0.05); however, pharmacological or genetic modulation of O-GlcNAc modifications under normal or high glucose conditions showed few significant effects on mitochondrial respiration, suggesting that O-GlcNAc does not play a major role in regulating cardiomyocyte mitochondrial function. Furthermore, an osmotic control recapitulated high-glucose-induced changes to mitochondrial metabolism (P<0.05) without increasing O-GlcNAcylation. Thus, increased O-GlcNAcylation is neither sufficient nor necessary for high-glucose-induced suppression of mitochondrial metabolism in isolated cardiomyocytes.

Nucleotide substitutions revealing specific functions of Polycomb group genes

POLYCOMB group (PCG) proteins belong to the family of epigenetic regulators of genes playing important roles in differentiation and development. Mutants of PcG genes were isolated first in the fruit fly, Drosophila melanogaster, resulting in spectacular segmental transformations due to the ectopic expression of homeotic genes. Homologs of Drosophila PcG genes were also identified in plants and in vertebrates and subsequent experiments revealed the general role of PCG proteins in the maintenance of the repressed state of chromatin through cell divisions. The past decades of gene targeting experiments have allowed us to make significant strides towards understanding how the network of PCG proteins influences multiple aspects of cellular fate determination during development. Being involved in the transmission of specific expression profiles of different cell lineages, PCG proteins were found to control wide spectra of unrelated epigenetic processes in vertebrates, such as stem cell plasticity and renewal, genomic imprinting and inactivation of X-chromosome. PCG proteins also affect regulation of metabolic genes being important for switching programs between pluripotency and differentiation. Insight into the precise roles of PCG proteins in normal physiological processes has emerged from studies employing cell culture-based systems and genetically modified animals. Here we summarize the findings obtained from PcG mutant fruit flies and mice generated to date with a focus on PRC1 and PRC2 members altered by nucleotide substitutions resulting in specific alleles. We also include a compilation of lessons learned from these models about the in vivo functions of this complex protein family. With multiple knockout lines, sophisticated approaches to study the consequences of peculiar missense point mutations, and insights from complementary gain-of-function systems in hand, we are now in a unique position to significantly advance our understanding of the molecular basis of in vivo functions of PcG proteins.

Regulation of eNOS enzyme activity by posttranslational modification

The regulation of endothelial NO synthase (eNOS) employs multiple different cellular control mechanisms impinging on level and activity of the enzyme. This review aims at summarizing the current knowledge on the posttranslational modifications of eNOS, including acylation, nitrosylation, phosphorylation, acetylation, glycosylation and glutathionylation. Sites, mediators and impact on enzyme localization and activity of the single modifications will be discussed. Moreover, interdependence, cooperativity and competition between the different posttranslational modifications will be elaborated with special emphasis on the susceptibility of eNOS to metabolic cues.

Visualization of O-GlcNAc glycosylation stoichiometry and dynamics using resolvable poly(ethylene glycol) mass tags

O-linked N-acetylglucosamine (O-GlcNAc) glycosylation is a dynamic protein posttranslational modification with roles in processes such as transcription, cell cycle regulation, and metabolism. Detailed mechanistic studies of O-GlcNAc have been hindered by a lack of methods for measuring O-GlcNAc stoichiometries and the interplay of glycosylation with other posttranslational modifications. We recently developed a method for labeling O-GlcNAc-modified proteins with resolvable poly(ethylene glycol) mass tags. This mass-tagging approach enables the direct measurement of glycosylation stoichiometries and the visualization of distinct O-GlcNAc-modified subpopulations. Here, we describe procedures for labeling O-GlcNAc glycoproteins in cell lysates with mass tags.

O-GlcNAc transferase enables AgRP neurons to suppress browning of white fat

Induction of beige cells causes the browning of white fat and improves energy metabolism. However, the central mechanism that controls adipose tissue browning and its physiological relevance are largely unknown. Here, we demonstrate that fasting and chemical-genetic activation of orexigenic AgRP neurons in the hypothalamus suppress the browning of white fat. O-linked β-N-acetylglucosamine (O-GlcNAc) modification of cytoplasmic and nuclear proteins regulates fundamental cellular processes. The levels of O-GlcNAc transferase (OGT) and O-GlcNAc modification are enriched in AgRP neurons and are elevated by fasting. Genetic ablation of OGT in AgRP neurons inhibits neuronal excitability through the voltage-dependent potassium channel, promotes white adipose tissue browning, and protects mice against diet-induced obesity and insulin resistance. These data reveal adipose tissue browning as a highly dynamic physiological process under central control, in which O-GlcNAc signaling in AgRP neurons is essential for suppressing thermogenesis to conserve energy in response to fasting.

Chemical tools to explore nutrient-driven O-GlcNAc cycling

Posttranslational modifications (PTM) including glycosylation, phosphorylation, acetylation, methylation and ubiquitination dynamically alter the proteome. The evolutionarily conserved enzymes O-linked N-acetylglucosamine (O-GlcNAc) transferase (OGT) and O-GlcNAcase are responsible for the addition and removal, respectively, of the nutrient-sensitive PTM of protein serine and threonine residues with O-GlcNAc. Indeed, the O-GlcNAc modification acts at every step in the "central dogma" of molecular biology and alters signaling pathways leading to amplified or blunted biological responses. The cellular roles of OGT and the dynamic PTM O-GlcNAc have been clarified with recently developed chemical tools including high-throughput assays, structural and mechanistic studies and potent enzyme inhibitors. These evolving chemical tools complement genetic and biochemical approaches for exposing the underlying biological information conferred by O-GlcNAc cycling.

Glycosylation alterations in lung and brain cancer

Alterations in glycosylation are common in cancer and are thought to contribute to disease. Lung cancer and primary malignant brain cancer, most commonly glioblastoma, are genetically heterogeneous diseases with extremely poor prognoses. In this review, we summarize the data demonstrating that glycosylation is altered in lung and brain cancer. We then use specific examples to highlight the diverse roles of glycosylation in these two deadly diseases and illustrate shared mechanisms of oncogenesis. In addition to alterations in glycoconjugate biosynthesis, we also discuss mechanisms of postsynthetic glycan modification in cancer. We suggest that alterations in glycosylation in lung and brain cancer provide novel tumor biomarkers and therapeutic targets.

O-linked β-N-acetylglucosamine transferase directs cell proliferation in idiopathic pulmonary arterial hypertension

BACKGROUND

Idiopathic pulmonary arterial hypertension (IPAH) is a cardiopulmonary disease characterized by cellular proliferation and vascular remodeling. A more recently recognized characteristic of the disease is the dysregulation of glucose metabolism. The primary link between altered glucose metabolism and cell proliferation in IPAH has not been elucidated. We aimed to determine the relationship between glucose metabolism and smooth muscle cell proliferation in IPAH.

METHODS AND RESULTS

Human IPAH and control patient lung tissues and pulmonary artery smooth muscle cells (PASMCs) were used to analyze a specific pathway of glucose metabolism, the hexosamine biosynthetic pathway. We measured the levels of O-linked β-N-acetylglucosamine modification, O-linked β-N-acetylglucosamine transferase (OGT), and O-linked β-N-acetylglucosamine hydrolase in control and IPAH cells and tissues. Our data suggest that the activation of the hexosamine biosynthetic pathway directly increased OGT levels and activity, triggering changes in glycosylation and PASMC proliferation. Partial knockdown of OGT in IPAH PASMCs resulted in reduced global O-linked β-N-acetylglucosamine modification levels and abrogated PASMC proliferation. The increased proliferation observed in IPAH PASMCs was directly impacted by proteolytic activation of the cell cycle regulator, host cell factor-1.

CONCLUSIONS

Our data demonstrate that hexosamine biosynthetic pathway flux is increased in IPAH and drives OGT-facilitated PASMC proliferation through specific proteolysis and direct activation of host cell factor-1. These findings establish a novel regulatory role for OGT in IPAH, shed a new light on our understanding of the disease pathobiology, and provide opportunities to design novel therapeutic strategies for IPAH.

Conditional knock-out reveals a requirement for O-linked N-Acetylglucosaminase (O-GlcNAcase) in metabolic homeostasis

O-GlcNAc cycling is maintained by the reciprocal activities of the O-GlcNAc transferase and the O-GlcNAcase (OGA) enzymes. O-GlcNAc transferase is responsible for O-GlcNAc addition to serine and threonine (Ser/Thr) residues and OGA for its removal. Although the Oga gene (MGEA5) is a documented human diabetes susceptibility locus, its role in maintaining insulin-glucose homeostasis is unclear. Here, we report a conditional disruption of the Oga gene in the mouse. The resulting homozygous Oga null (KO) animals lack OGA enzymatic activity and exhibit elevated levels of the O-GlcNAc modification. The Oga KO animals showed nearly complete perinatal lethality associated with low circulating glucose and low liver glycogen stores. Defective insulin-responsive GSK3β phosphorylation was observed in both heterozygous (HET) and KO Oga animals. Although Oga HET animals were viable, they exhibited alterations in both transcription and metabolism. Transcriptome analysis using mouse embryonic fibroblasts revealed deregulation in the transcripts of both HET and KO animals specifically in genes associated with metabolism and growth. Additionally, metabolic profiling showed increased fat accumulation in HET and KO animals compared with WT, which was increased by a high fat diet. Reduced insulin sensitivity, glucose tolerance, and hyperleptinemia were also observed in HET and KO female mice. Notably, the respiratory exchange ratio of the HET animals was higher than that observed in WT animals, indicating the preferential utilization of glucose as an energy source. These results suggest that the loss of mouse OGA leads to defects in metabolic homeostasis culminating in obesity and insulin resistance.