Proteomic approaches to analyze the dynamic relationships between nucleocytoplasmic protein glycosylation and phosphorylation

Review: Protein O-GlcNAcylation regulates DNA damage response: A novel target for cancer therapy

The DNA damage response (DDR) safeguards the stable genetic information inheritance by orchestrating a complex protein network in response to DNA damage. However, this mechanism can often hamper the effectiveness of radiotherapy and DNA-damaging chemotherapy in destroying tumor cells, causing cancer resistance. Inhibiting DDR can significantly improve tumor cell sensitivity to radiotherapy and DNA-damaging chemotherapy. Thus, DDR can be a potential target for cancer treatment. Post-translational modifications (PTMs) of DDR-associated proteins profoundly affect their activity and function by covalently attaching new functional groups. O-GlcNAcylation (O-linked-N-acetylglucosaminylation) is an emerging PTM associated with adding and removing O-linked N-acetylglucosamine to serine and threonine residues of proteins. It acts as a dual sensor for nutrients and stress in the cell and is sensitive to DNA damage. However, the explanation behind the specific role of O-GlcNAcylation in the DDR remains remains to be elucidated. To illustrate the complex relationship between O-GlcNAcylation and DDR, this review systematically describes the role of O-GlcNAcylation in DNA repair, cell cycle, and chromatin. We also discuss the defects of current strategies for targeting O-GlcNAcylation-regulated DDR in cancer therapy and suggest potential directions to address them.

O-GlcNAcylation and Its Role in Cancer-Associated Inflammation

Cancer cells, as well as surrounding stromal and inflammatory cells, form an inflammatory tumor microenvironment (TME) to promote all stages of carcinogenesis. As an emerging post-translational modification (PTM) of serine and threonine residues of proteins, O-linked-N-Acetylglucosaminylation (O-GlcNAcylation) regulates diverse cancer-relevant processes, such as signal transduction, transcription, cell division, metabolism and cytoskeletal regulation. Recent studies suggest that O-GlcNAcylation regulates the development, maturation and functions of immune cells. However, the role of protein O-GlcNAcylation in cancer-associated inflammation has been less explored. This review summarizes the current understanding of the influence of protein O-GlcNAcylation on cancer-associated inflammation and the mechanisms whereby O-GlcNAc-mediated inflammation regulates tumor progression. This will provide a theoretical basis for further development of anti-cancer therapies.

Demystifying the O-GlcNAc Code: A Systems View

Post-translational modification with O-linked β-N-acetylglucosamine (O-GlcNAc), a process referred to as O-GlcNAcylation, occurs on a vast variety of proteins. Mounting evidence in the past several decades has clearly demonstrated that O-GlcNAcylation is a unique and ubiquitous modification. Reminiscent of a code, protein O-GlcNAcylation functions as a crucial regulator of nearly all cellular processes studied. The primary aim of this review is to summarize the developments in our understanding of myriad protein substrates modified by O-GlcNAcylation from a systems perspective. Specifically, we provide a comprehensive survey of O-GlcNAcylation in multiple species studied, including eukaryotes (e.g., protists, fungi, plants, Caenorhabditis elegans, Drosophila melanogaster, murine, and human), prokaryotes, and some viruses. We evaluate features (e.g., structural properties and sequence motifs) of O-GlcNAc modification on proteins across species. Given that O-GlcNAcylation functions in a species-, tissue-/cell-, protein-, and site-specific manner, we discuss the functional roles of O-GlcNAcylation on human proteins. We focus particularly on several classes of relatively well-characterized human proteins (including transcription factors, protein kinases, protein phosphatases, and E3 ubiquitin-ligases), with representative O-GlcNAc site-specific functions presented. We hope the systems view of the great endeavor in the past 35 years will help demystify the O-GlcNAc code and lead to more fascinating studies in the years to come.

Overexpression of Transcripts Coding for Renin-b but Not for Renin-a Reduce Oxidative Stress and Increase Cardiomyoblast Survival under Starvation Conditions

A stimulated renin-angiotensin system is known to promote oxidative stress, apoptosis, necrosis and fibrosis. Renin transcripts (renin-b; renin-c) encoding a cytosolic renin isoform have been discovered that may in contrast to the commonly known secretory renin (renin-a) exert protective effects Here, we analyzed the effect of renin-a and renin-b overexpression in H9c2 cardiomyoblasts on apoptosis and necrosis as well as on potential mechanisms involved in cell death processes. To mimic ischemic conditions, cells were exposed to glucose starvation, anoxia or combined oxygen–glucose deprivation (OGD) for 24 h. Under OGD, control cells exhibited markedly increased necrotic and apoptotic cell death accompanied by enhanced ROS accumulation, loss of mitochondrial membrane potential and decreased ATP levels. The effects of OGD on necrosis were exaggerated in renin-a cells, but markedly diminished in renin-b cells. However, with respect to apoptosis, the effects of OGD were almost completely abolished in renin-b cells but interestingly also moderately diminished in renin-a cells. Under glucose depletion we found opposing responses between renin-a and renin-b cells; while the rate of necrosis and apoptosis was aggravated in renin-a cells, it was attenuated in renin-b cells. Based on our results, strategies targeting the regulation of cytosolic renin-b as well as the identification of pathways involved in the protective effects of renin-b may be helpful to improve the treatment of ischemia-relevant diseases.

Effect of transient elevation of glucose on contractile properties in non-diabetic rat cardiac muscle

In non-diabetic patients with severe disease, such as acute myocardial infarction or acute heart failure, admission blood glucose level is associated with their short-term and long-term mortality. We examined whether transient elevation of glucose affects contractile properties in non-diabetic hearts. Force, intracellular Ca²⁺ ([Ca²⁺]_i), and sarcomere length were measured in trabeculae from rat hearts. To assess contractile properties, maximum velocity of contraction (Max dF/dt) and minimum velocity of relaxation (Min dF/dt) were calculated. The ratio of phosphorylated troponin I (P-TnI) to troponin I (TnI) was measured. One hour after elevation of glucose from 150 to 400 mg/dL, developed force, Max dF/dt, and Min dF/dt were reduced without changes in [Ca²⁺]_i transients at 2.5 Hz stimulation and 2.0 mM [Ca²⁺]_o, while developed force and [Ca²⁺]_i transients showed no changes at 0.5 Hz stimulation and 0.7 mM [Ca²⁺]_o. In the presence of 1 μM KN-93, a Ca²⁺/calmodulin-dependent protein kinaseII (CaMKII) inhibitor, or 50 μM diazo-5-oxonorleucine, a L-glutamine-D-fructose-6-phosphate amidotransferase inhibitor, the reduction of contractile properties after elevation of glucose was suppressed. Furthermore, 1 h after elevation of glucose to 400 mg/dL at 2.0 mM [Ca²⁺]_o, the ratio of P-TnI to TnI was increased. These results suggest that in non-diabetic hearts under higher Ca²⁺-load, transient elevation of glucose for 1 h reduces contractile properties probably by activating CaMKII through O-GlcNAcylation. Thus, in the patients with severe disease, transient elevation of blood glucose, such as due to stress, may worsen cardiac function and thereby affect their mortality without known diabetes.

Relevance of glycans in the interaction between T lymphocyte and the antigen presenting cell

The immunological synapse promotes receptors and ligands interaction in the contact interface between the T lymphocyte and the antigen presenting cell; glycosylation of the proteins involved in this biological process favors regulation of molecular interactions and development of the T lymphocyte effector response. Glycans in the immunological synapse influence cellular and molecular processes such as folding, expression, and structural stability of proteins, they also mediate ligand-receptor interaction and propagation of the intracellular signaling or inhibition of uncontrolled cellular activation that could lead to the development of autoimmunity, among others. It has been suggested that altered glycosylation of proteins that participate in the immunological synapse affects the signaling processes and cell proliferation, as well as exacerbation of the effector mechanisms of T cells that trigger systemic damage and autoimmunity. Understanding the role of glycans in the immune response has allowed for advances in the development of immunotherapies in different fields through the controlled and specific activation of the immune response. This review describes the structural and biological aspects of glycans associated with some molecules present in the immunological synapse, providing information that allows understanding the function of glycosylation in the interaction between the T lymphocyte and the antigen-presenting cell, as well as its impact on signaling and development regulation of T lymphocytes effector response.

Transient Elevation of Glucose Increases Arrhythmia Susceptibility in Non-Diabetic Rat Trabeculae With Non-Uniform Contraction

BACKGROUND

In non-diabetic patients with acute coronary syndrome, stress hyperglycemia occasionally occurs and is related to their mortality. Whether transient elevation of glucose affects arrhythmia susceptibility in non-diabetic hearts with non-uniform contraction was examined.Methods and Results:Force, intracellular Ca²⁺([Ca²⁺]_i), and membrane potential were measured in trabeculae from rat hearts. Non-uniform contraction was produced by a jet of paralyzing solution. Ca²⁺waves and arrhythmias were induced by electrical stimulation (2.0 mmol/L [Ca²⁺]_o). The activity of Ca²⁺/calmodulin-dependent protein kinaseII (CaMKII) was measured. An elevation of glucose from 150 to 400 mg/dL increased the velocity of Ca²⁺waves and the number of spontaneous action potentials triggered by electrical stimulation. Besides, the elevation of glucose increased the CaMKII activity. In the presence of 1 μmol/L KN-93, the elevation of glucose did not increase the velocity of Ca²⁺waves and the number of triggered action potentials. In addition, in the presence of 1 μmol/L autocamtide-2 related inhibitory peptide or 50 μmol/L diazo-5-oxonorleucine, the elevation of glucose did not increase the number of triggered action potentials. Furthermore, the elevation of glucose by adding L-glucose did not increase their number.

CONCLUSIONS

In non-diabetic hearts with non-uniform contraction, transient elevation of glucose increases the velocity of Ca²⁺waves by activating CaMKII,probably through glycosylation with O-linked β-N-acetylglucosamine, thereby increasing arrhythmia susceptibility.

Deciphering the Functions of O-GlcNAc Glycosylation in the Brain: The Role of Site-Specific Quantitative O-GlcNAcomics

The dynamic posttranslational modification O-linked β- N-acetylglucosamine glycosylation (O-GlcNAcylation) is present on thousands of intracellular proteins in the brain. Like phosphorylation, O-GlcNAcylation is inducible and plays important functional roles in both physiology and disease. Recent advances in mass spectrometry (MS) and bioconjugation methods are now enabling the mapping of O-GlcNAcylation events to individual sites in proteins. However, our understanding of which glycosylation events are necessary for regulating protein function and controlling specific processes, phenotypes, or diseases remains in its infancy. Given the sheer number of O-GlcNAc sites, methods for identifying promising sites and prioritizing them for time- and resource-intensive functional studies are greatly needed. Revealing sites that are dynamically altered by different stimuli or disease states will likely go a long way in this regard. Here, we describe advanced methods for identifying O-GlcNAc sites on individual proteins and across the proteome and for determining their stoichiometry in vivo. We also highlight emerging technologies for quantitative, site-specific MS-based O-GlcNAc proteomics (O-GlcNAcomics), which allow proteome-wide tracking of O-GlcNAcylation dynamics at individual sites. These cutting-edge technologies are beginning to bridge the gap between the high-throughput cataloguing of O-GlcNAcylated proteins and the relatively low-throughput study of individual proteins. By uncovering the O-GlcNAcylation events that change in specific physiological and disease contexts, these new approaches are providing key insights into the regulatory functions of O-GlcNAc in the brain, including their roles in neuroprotection, neuronal signaling, learning and memory, and neurodegenerative diseases.

Identification of O-GlcNAcylated proteins in Plasmodium falciparum

BACKGROUND

Post-translational modifications (PTMs) constitute a huge group of chemical modifications increasing the complexity of the proteomes of living beings. PTMs have been discussed as potential anti-malarial drug targets due to their involvement in many cell processes. O-GlcNAcylation is a widespread PTM found in different organisms including Plasmodium falciparum. The aim of this study was to identify O-GlcNAcylated proteins of P. falciparum, to learn more about the modification process and to understand its eventual functions in the Apicomplexans.

METHODS

The P. falciparum strain 3D7 was amplified in erythrocytes and purified. The proteome was checked for O-GlcNAcylation using different methods. The level of UDP-GlcNAc, the donor of the sugar moiety for O-GlcNAcylation processes, was measured using high-pH anion exchange chromatography. O-GlcNAcylated proteins were enriched and purified utilizing either click chemistry labelling or adsorption on succinyl-wheat germ agglutinin beads. Proteins were then identified by mass-spectrometry (nano-LC MS/MS).

RESULTS

While low when compared to MRC5 control cells, P. falciparum disposes of its own pool of UDP-GlcNAc. By using proteomics methods, 13 O-GlcNAcylated proteins were unambiguously identified (11 by click-chemistry and 6 by sWGA-beads enrichment; 4 being identified by the 2 approaches) in late trophozoites. These proteins are all part of pathways, functions and structures important for the parasite survival. By probing clicked-proteins with specific antibodies, Hsp70 and α-tubulin were identified as P. falciparum O-GlcNAc-bearing proteins.

CONCLUSIONS

This study is the first report on the identity of P. falciparum O-GlcNAcylated proteins. While the parasite O-GlcNAcome seems close to those of other species, the structural differences exhibited by the proteomes provides a glimpse of innovative therapeutic paths to fight malaria. Blocking biosynthesis of UDP-GlcNAc in the parasites is another promising option to reduce Plasmodium life cycle.

Nutrient-driven O-GlcNAc in proteostasis and neurodegeneration

Proteostasis is essential in the mammalian brain where post-mitotic cells must function for decades to maintain synaptic contacts and memory. The brain is dependent on glucose and other metabolites for proper function and is spared from metabolic deficits even during starvation. In this review, we outline how the nutrient-sensitive nucleocytoplasmic post-translational modification O-linked N-acetylglucosamine (O-GlcNAc) regulates protein homeostasis. The O-GlcNAc modification is highly abundant in the mammalian brain and has been linked to proteopathies, including neurodegenerative diseases such as Alzheimer's, Parkinson's, and Huntington's. C. elegans, Drosophila, and mouse models harboring O-GlcNAc transferase- and O-GlcNAcase-knockout alleles have helped define the role O-GlcNAc plays in development as well as age-associated neurodegenerative disease. These enzymes add and remove the single monosaccharide from protein serine and threonine residues, respectively. Blocking O-GlcNAc cycling is detrimental to mammalian brain development and interferes with neurogenesis, neural migration, and proteostasis. Findings in C. elegans and Drosophila model systems indicate that the dynamic turnover of O-GlcNAc is critical for maintaining levels of key transcriptional regulators responsible for neurodevelopment cell fate decisions. In addition, pathways of autophagy and proteasomal degradation depend on a transcriptional network that is also reliant on O-GlcNAc cycling. Like the quality control system in the endoplasmic reticulum which uses a 'mannose timer' to monitor protein folding, we propose that cytoplasmic proteostasis relies on an 'O-GlcNAc timer' to help regulate the lifetime and fate of nuclear and cytoplasmic proteins. O-GlcNAc-dependent developmental alterations impact metabolism and growth of the developing mouse embryo and persist into adulthood. Brain-selective knockout mouse models will be an important tool for understanding the role of O-GlcNAc in the physiology of the brain and its susceptibility to neurodegenerative injury.

Ubiquitous Importance of Protein Glycosylation

More than half of all proteins are glycosylated. The attached glycans provide proteins with important structural and functional properties and glycan parts of glycoproteins have essential roles in many key biological processes. This chapter describes the effect of glycosylation on the structure and function of proteins, with emphasis on regulation of protein half-life and modulation of protein function by alternative glycosylation. In addition, this chapter highlights the importance of glycan-lectin interactions, the ability of glycans to block phosphorylation of proteins, and the importance of glycans in disease.

The status of diabetic embryopathy

Diabetic embryopathy is a theoretical enigma and a clinical challenge. Both type 1 and type 2 diabetic pregnancy carry a significant risk for fetal maldevelopment, and the precise reasons for the diabetes-induced teratogenicity are not clearly identified. The experimental work in this field has revealed a partial, however complex, answer to the teratological question, and we will review some of the latest suggestions.

Analysis of transition state mimicry by tight binding aminothiazoline inhibitors provides insight into catalysis by human O-GlcNAcase

2′-Aminothiazoline inhibitors of human OGA are tight binding transition state mimics for which binding depends on inhibitor pK_a.

The modification of nucleocytoplasmic proteins with O-linked N-acetylglucosamine (O-GlcNAc) plays diverse roles in multicellular organisms. Inhibitors of O-GlcNAc hydrolase (OGA), the enzyme that removes O-GlcNAc from proteins, lead to increased O-GlcNAc levels in cells and are seeing widespread adoption in the field as a research tool used in cells and in vivo. Here we synthesize and study a series of tight binding carbohydrate-based inhibitors of human OGA (hOGA). The most potent of these 2′-aminothiazolines binds with a sub-nanomolar K_i value to hOGA (510 ± 50 pM) and the most selective has greater than 1 800 000-fold selectivity for hOGA over mechanistically related human lysosomal β-hexosaminidase. Structural data of inhibitors in complex with an hOGA homologue reveals the basis for variation in binding among these compounds. Using linear free energy analyses, we show binding of these 2′-aminothiazoline inhibitors depends on the pK_a of the aminothiazoline ring system, revealing the protonation state of the inhibitor is a key driver of binding. Using series of inhibitors and synthetic substrates, we show that 2′-aminothiazoline inhibitors are transition state analogues of hOGA that bind to the enzyme up to 1-million fold more tightly than the substrate. These collective data support an oxazoline, rather than a protonated oxazolinium ion, intermediate being formed along the reaction pathway. Inhibitors from this series will prove generally useful tools for the study of O-GlcNAc. The new insights gained here, into the catalytic mechanism of hOGA and the fundamental drivers of potency and selectivity of OGA inhibitors, should enable tuning of hOGA inhibitors with desirable properties.

One-step synthesis of glycoprotein mimics in vitro: improvement of protein activity, stability and application in CPP hydrolysis

Glycoprotein mimics produced in vitro by one-step conjugation of glycopolymer and pyrophosphatase have improved bioactivity and stability for potential biomedical applications.

PUGNAc induces protein ubiquitination in C2C12 myotube cells

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

Insulin resistance as a key link for the increased risk of cognitive impairment in the metabolic syndrome

Metabolic syndrome (MetS) is a cluster of cardiovascular risk factors that includes obesity, diabetes, and dyslipidemia. Accumulating evidence implies that MetS contributes to the development and progression of Alzheimer's disease (AD); however, the factors connecting this association have not been determined. Insulin resistance (IR) is at the core of MetS and likely represent the key link between MetS and AD. In the central nervous system, insulin plays key roles in learning and memory, and AD patients exhibit impaired insulin signaling that is similar to that observed in MetS. As we face an alarming increase in obesity and T2D in all age groups, understanding the relationship between MetS and AD is vital for the identification of potential therapeutic targets. Recently, several diabetes therapies that enhance insulin signaling are being tested for a potential therapeutic benefit in AD and dementia. In this review, we will discuss MetS as a risk factor for AD, focusing on IR and the recent progress and future directions of insulin-based therapies.

Role of O-GlcNAcylation in nutritional sensing, insulin resistance and in mediating the benefits of exercise

The purpose of this review is to highlight the role of O-linked β-N-acetylglucosamine (O-GlcNAc) protein modification in metabolic disease states and to summarize current knowledge of how exercise affects this important post-translational signalling pathway. O-GlcNAc modification is an intracellular tool capable of integrating energy supply with demand. The accumulation of excess energy associated with obesity and insulin resistance is mediated, in part, by the hexosamine biosynthetic pathway (HBP), which results in the O-GlcNAcylation of a myriad of proteins, thereby affecting their respective function, stability, and localization. Insulin resistance is related to the excessive O-GlcNAcylation of key metabolic proteins causing a chronic blunting of insulin signalling pathways and precipitating the accompanying pathologies, such as heart and kidney disease. Lifestyle modifications such as diet and exercise also modify the pathway. Exercise is a front-line and cost-effective therapeutic approach for insulin resistance, and recent work shows that the intervention can alter O-GlcNAc gene expression, signalling, and protein modification. However, there is currently no consensus on the effect of frequency, intensity, type, and duration of exercise on O-GlcNAc modification, the HBP, and its related enzymes. On one end of the spectrum, mild, prolonged swim training reduces O-GlcNAcylation, while on the other end, higher intensity treadmill running increases cardiac protein O-GlcNAc modification. Clearly, a balance between acute and chronic stress of exercise is needed to reap the benefits of the intervention on O-GlcNAc signalling.

Identification of nitric oxide as an endogenous inhibitor of 26S proteasomes in vascular endothelial cells

The 26S proteasome plays a fundamental role in almost all eukaryotic cells, including vascular endothelial cells. However, it remains largely unknown how proteasome functionality is regulated in the vasculature. Endothelial nitric oxide (NO) synthase (eNOS)-derived NO is known to be essential to maintain endothelial homeostasis. The aim of the present study was to establish the connection between endothelial NO and 26S proteasome functionality in vascular endothelial cells. The 26S proteasome reporter protein levels, 26S proteasome activity, and the O-GlcNAcylation of Rpt2, a key subunit of the proteasome regulatory complex, were assayed in 26S proteasome reporter cells, human umbilical vein endothelial cells (HUVEC), and mouse aortic tissues isolated from 26S proteasome reporter and eNOS knockout mice. Like the other selective NO donors, NO derived from activated eNOS (by pharmacological and genetic approach) increased O-GlcNAc modification of Rpt2, reduced proteasome chymotrypsin-like activity, and caused 26S proteasome reporter protein accumulation. Conversely, inactivation of eNOS reversed all the effects. SiRNA knockdown of O-GlcNAc transferase (OGT), the key enzyme that catalyzes protein O-GlcNAcylation, abolished NO-induced effects. Consistently, adenoviral overexpression of O-GlcNAcase (OGA), the enzyme catalyzing the removal of the O-GlcNAc group, mimicked the effects of OGT knockdown. Finally, compared to eNOS wild type aortic tissues, 26S proteasome reporter mice lacking eNOS exhibited elevated 26S proteasome functionality in parallel with decreased Rpt2 O-GlcNAcylation, without changing the levels of Rpt2 protein. In conclusion, the eNOS-derived NO functions as a physiological suppressor of the 26S proteasome in vascular endothelial cells.

A peptide panel investigation reveals the acceptor specificity of O-GlcNAc transferase

O-linked β-N-acetylglucosaminylation (O-GlcNAcylation) is widely distributed on nucleocytoplasmic proteins and participates in various physiological processes. But O-GlcNAc status on numerous proteins remains unknown. To better understand this modification, computational analysis combined with experimental study was performed in this work. Structural analysis of many O-GlcNAcylation sites indicated that the modification occurred predominantly in a random coil region. Frequency analysis on many O-GlcNAcylated peptides revealed a signature sequence, PPVS/TSATT, around the modification site (underlined, position 0). Based on the sequence, a peptide panel was designed to investigate key positions affecting O-GlcNAcylation of peptides and their amino acid preference. It was indicated that 3 positions (-2, -1, and +2) had an important role for this modification, where the presence of uncharged amino acids with small side chains could confer high reactivity. The amino acid preference at key positions was further investigated on bovine crystalline α via site-directed mutagenesis. The preferred amino acids were Pro > Ala > Gly at position -2, Ala > Thr > Val > Lys > Pro at position -1, and Ala > Gly > Arg > Glu at position +2. Altogether, these findings suggested that a substrate (peptide or protein) with Pro, Ala at position -2, and/or Val, Ala, Thr, Ser at position -1, and/or Ala, Ser, Pro, Thr, Gly at position +2 would have more chances for O-GlcNAcylation. To test the rule, 2 O-GlcNAcylation sites on sOGT (S52 and T449) were predicted and confirmed by Western blot. The present work systematically investigated the sequence signature for O-GlcNAcylation. The result will contribute to predicting the O-GlcNAc status of a protein and further functional studies.

O-GlcNAcylation involvement in high glucose-induced cardiac hypertrophy via ERK1/2 and cyclin D2

Continuous hyperglycemia is considered to be the most significant pathogenesis of diabetic cardiomyopathy, which manifests as cardiac hypertrophy and subsequent heart failure. O-GlcNAcylation has attracted attention as a post-translational protein modification in the past decade. The role of O-GlcNAcylation in high glucose-induced cardiomyocyte hypertrophy remains unclear. We studied the effect of O-GlcNAcylation on neonatal rat cardiomyocytes that were exposed to high glucose and myocardium in diabetic rats induced by streptozocin. High glucose (30 mM) incubation induced a greater than twofold increase in cell size and increased hypertrophy marker gene expression accompanied by elevated O-GlcNAcylation protein levels. High glucose increased ERK1/2 but not p38 MAPK or JNK activity, and cyclin D2 expression was also increased. PUGNAc, an inhibitor of β-N-acetylglucosaminidase, enhanced O-GlcNAcylation and imitated the effects of high glucose. OGT siRNA and ERK1/2 inhibition with PD98059 treatment blunted the hypertrophic response and cyclin D2 upregulation. OGT inhibition also prevented ERK1/2 activation. We also observed concentric hypertrophy and similar changes of O-GlcNAcylation level, ERK1/2 activation and cyclin D2 expression in myocardium of diabetic rats induced by streptozocin. In conclusion, O-GlcNAcylation plays a role in high glucose-induced cardiac hypertrophy via ERK1/2 and cyclin D2.

Immediate effects of a single exercise bout on protein O-GlcNAcylation and chromatin regulation of cardiac hypertrophy

Cardiac hypertrophy induced by pathological stimuli is regulated by a complex formed by the repressor element 1-silencing transcription factor (REST) and its corepressor mSin3A. We previously reported that hypertrophic signaling is blunted by O-linked attachment of β-N-acetylglucosamine (O-GlcNAc) to proteins. Regular exercise induces a physiological hypertrophic phenotype in the heart that is associated with decreased O-GlcNAc levels, but a link between O-GlcNAc, the REST complex, and initiation of exercise-induced cardiac hypertrophy is not known. Therefore, mice underwent a single 15- or 30-min bout of moderate- to high-intensity treadmill running, and hearts were harvested immediately and compared with sedentary controls. Cytosolic O-GlcNAc was lower (P < 0.05) following 15 min exercise with no differences in nuclear levels (P > 0.05). There were no differences in cytosolic or nuclear O-GlcNAc levels in hearts after 30 min exercise (P > 0.05). Cellular compartment levels of O-GlcNAc transferase (OGT, the enzyme that removes the O-GlcNAc moiety from proteins), REST, mSin3A, and histone deacetylases (HDACs) 1, 2, 3, 4, and 5 were not changed with exercise. Immunoprecipitation revealed O-GlcNAcylation of OGT and HDACs 1, 2, 4, and 5 that was not changed with acute exercise; however, exercised hearts did exhibit lower interactions between OGT and REST (P < 0.05) but not between OGT and mSin3A. These data suggest that hypertrophic signaling in the heart may be initiated by as little as 15 min of exercise via intracellular changes in protein O-GlcNAcylation distribution and reduced interactions between OGT and the REST chromatin repressor.

Effect of posttranslational modifications on enzyme function and assembly

The detailed examination of enzyme molecules by mass spectrometry and other techniques continues to identify hundreds of distinct PTMs. Recently, global analyses of enzymes using methods of contemporary proteomics revealed widespread distribution of PTMs on many key enzymes distributed in all cellular compartments. Critically, patterns of multiple enzymatic and nonenzymatic PTMs within a single enzyme are now functionally evaluated providing a holistic picture of a macromolecule interacting with low molecular mass compounds, some of them being substrates, enzyme regulators, or activated precursors for enzymatic and nonenzymatic PTMs. Multiple PTMs within a single enzyme molecule and their mutual interplays are critical for the regulation of catalytic activity. Full understanding of this regulation will require detailed structural investigation of enzymes, their structural analogs, and their complexes. Further, proteomics is now integrated with molecular genetics, transcriptomics, and other areas leading to systems biology strategies. These allow the functional interrogation of complex enzymatic networks in their natural environment. In the future, one might envisage the use of robust high throughput analytical techniques that will be able to detect multiple PTMs on a global scale of individual proteomes from a number of carefully selected cells and cellular compartments. This article is part of a Special Issue entitled: Posttranslational Protein modifications in biology and Medicine.

Chemical approaches to study O-GlcNAcylation

The enzymatic addition of a single β-D-N-acetylglucosamine sugar molecule on serine and/or threonine residues of protein chains is referred to as O-GlcNAcylation. This novel form of post-translational modification, first reported in 1984, is extremely abundant on nuclear and cytoplasmic proteins and has site specific cycling dynamics comparable to that of protein-phosphorylation. A nutrient and stress sensor, O-GlcNAc abnormalities underlie insulin resistance and glucose toxicity in diabetes, neurodegenerative disorders and dysregulation of tumor suppressors and oncogenic proteins in cancer. Recent advances have helped understand the biochemical mechanisms of GlcNAc addition and removal and have opened the door to developing key inhibitors towards this type of protein modification. Advanced methods in detecting and measuring O-GlcNAcylation have assisted in delineating its biological roles in a variety of cellular processes and diseased states. Availability of facile glycomic techniques are allowing for the exponential growth in the study of protein O-GlcNAcylation and are helping to elucidate key biological roles of this novel PTM.

O-GlcNAcylation: a novel post-translational mechanism to alter vascular cellular signaling in health and disease: focus on hypertension

O-Linked attachment of beta-N-acetyl-glucosamine (O-GlcNAc) on serine and threonine residues of nuclear and cytoplasmic proteins is a highly dynamic posttranslational modification that plays a key role in signal transduction pathways. Preliminary data show that O-GlcNAcylation may represent a key regulatory mechanism in the vasculature, modulating contractile and relaxant responses. Proteins with an important role in vascular function, such as endothelial nitric oxide synthase, sarcoplasmic reticulum Ca(2+)-ATPase, protein kinase C, mitogen-activated protein kinases, and proteins involved in cytoskeleton regulation and microtubule assembly are targets for O-GlcNAcylation, indicating that this posttranslational modification may play an important role in vascular reactivity. Here, we will focus on a few specific pathways that contribute to vascular function and cardiovascular disease-associated vascular dysfunction, and the implications of their modification by O-GlcNAc. New chemical tools have been developed to detect and study O-GlcNAcylation, including inhibitors of O-GlcNAc enzymes, chemoenzymatic tagging methods, and quantitative proteomics strategies; these will also be briefly addressed. An exciting challenge in the future will be to better understand the cellular dynamics of this posttranslational modification, as well as the signaling pathways and mechanisms by which O-GlcNAc is regulated on specific proteins in the vasculature in health and disease.

Regulation of the proteasome by AMPK in endothelial cells: the role of O-GlcNAc transferase (OGT)

26S proteasome is a macromolecular multi-subunit complex responsible for recognizing, unfolding, and ultimately destroying proteins. It remains poorly understood how 26S proteasome activity is regulated. The present study was to investigate if AMP-activated protein kinase (AMPK) functions as a physiological suppressor of the 26S proteasome in endothelial cells. 26S proteasome assembly, activity, and O-GlcNAcylation of P700 were assayed in cultured human umbilical vein endothelial cells (HUVEC) and mouse aortas isolated from C57BL6 wild type and AMPKα2 knockout mice with or without being exposed to selective AMPK activators or inhibitors. Pharmacological and genetic activation of AMPK effectively suppresses 26S proteasomes in endothelial cells. Conversely, inactivation of AMPK either pharmacologically or genetically increases 26S proteasome activity; furthermore, the inactivation decreases the O-GlcNAcylation of PA700/S10B (the regulatory complex in 26S proteasomes) and increases the assembly of 26S proteasomes. In contrast, AMPK activation increases levels of O-GlcNAcylated PA700/S10B, likely through enhanced association of PA700 with O-GlcNAc transferase (OGT), the enzyme that catalyzes protein O-GlcNAcylation. Finally, aortas from AMPK-KO vs wild type mice exhibit elevated 26S proteasome activity in parallel with decreased PA700/S10B O-GlcNAcylation and PA700/S10B-OGT association. Taken together, we conclude that AMPK functions as a physiological suppressor of 26S proteasomes.

Identification and Quantification of Protein Glycosylation

Glycosylation is one of the most abundant posttranslation modifications of proteins, and accumulating evidence indicate that the vast majority of proteins in eukaryotes are glycosylated. Glycosylation plays a role in protein folding, interaction, stability, and mobility, as well as in signal transduction. Thus, by regulating protein activity, glycosylation is involved in the normal functioning of the cell and in the development of diseases. Indeed, in the past few decades there has been a growing realization of the importance of protein glycosylation, as aberrant glycosylation has been implicated in metabolic, neurodegenerative, and neoplastic diseases. Thus, the identification and quantification of protein-borne oligosaccharides have become increasingly important both in the basic sciences of biochemistry and glycobiology and in the applicative sciences, particularly biomedicine and biotechnology. Here, we review the state-of-the-art methodologies for the identification and quantification of oligosaccharides, specifically N- and O-glycosylated proteins.

Protein O-linked β-N-acetylglucosamine: a novel effector of cardiomyocyte metabolism and function

The post-translational modification of serine and threonine residues of nuclear and cytoplasmic proteins by the O-linked attachment of the monosaccharide β-N-acetyl-glucosamine (O-GlcNAc) is emerging as an important mechanism for the regulation of numerous biological processes critical for normal cell function. Active synthesis of O-GlcNAc is essential for cell viability and acute activation of pathways resulting in increased protein O-GlcNAc levels improves the tolerance of cells to a wide range of stress stimuli. Conversely sustained increases in O-GlcNAc levels have been implicated in numerous chronic disease states, especially as a pathogenic contributor to diabetic complications. There has been increasing interest in the role of O-GlcNAc in the heart and vascular system and acute activation of O-GlcNAc levels have been shown to reduce ischemia/reperfusion injury, attenuate vascular injury responses as well mediate some of the detrimental effects of diabetes and hypertension on cardiac and vascular function. Here we provide an overview of our current understanding of pathways regulating protein O-GlcNAcylation, summarize the different methodologies for identifying and characterizing O-GlcNAcylated proteins and subsequently focus on two emerging areas: 1) the role of O-GlcNAc as a potential regulator of cardiac metabolism and 2) the cross talk between O-GlcNAc and reactive oxygen species. This article is part of a Special Section entitled "Post-translational Modification."

Cardioprotection requires flipping the 'posttranslational modification' switch

Minimizing damage during reperfusion of the heart following an ischemic event is an important part of the recovery process, as is preventing future recurrences; however, restoring blood perfusion to the heart following ischemia can lead to apoptosis, necrosis, and finally, diminished cardiac function. Exercise reduces risk of heart disease and has been shown to improve the recovery of the heart following ischemia and reperfusion. Brief intermittent ischemic events administered prior to or following a myocardial infarction have also been demonstrated to reduce the infarct size and improve cardiac function, thereby providing cardioprotection. Many signaling transduction pathways are known to regulate cardioprotection, including but not limited to calcium regulation, antioxidant scavenging, and kinase activation. Although posttranslational modifications (PTM) such as phosphorylation, O-GlcNAcylation, methylation, and acetylation are essential regulators of these pathways, their contributions are often overlooked in the literature. This review will examine how PTMS are important regulators of cardioprotection and demonstrate why they should be targeted when developing future therapies for the minimization of damage caused by cardiac ischemia and reperfusion.

Cross talk between O-GlcNAcylation and phosphorylation: roles in signaling, transcription, and chronic disease

O-GlcNAcylation is the addition of β-D-N-acetylglucosamine to serine or threonine residues of nuclear and cytoplasmic proteins. O-linked N-acetylglucosamine (O-GlcNAc) was not discovered until the early 1980s and still remains difficult to detect and quantify. Nonetheless, O-GlcNAc is highly abundant and cycles on proteins with a timescale similar to protein phosphorylation. O-GlcNAc occurs in organisms ranging from some bacteria to protozoans and metazoans, including plants and nematodes up the evolutionary tree to man. O-GlcNAcylation is mostly on nuclear proteins, but it occurs in all intracellular compartments, including mitochondria. Recent glycomic analyses have shown that O-GlcNAcylation has surprisingly extensive cross talk with phosphorylation, where it serves as a nutrient/stress sensor to modulate signaling, transcription, and cytoskeletal functions. Abnormal amounts of O-GlcNAcylation underlie the etiology of insulin resistance and glucose toxicity in diabetes, and this type of modification plays a direct role in neurodegenerative disease. Many oncogenic proteins and tumor suppressor proteins are also regulated by O-GlcNAcylation. Current data justify extensive efforts toward a better understanding of this invisible, yet abundant, modification. As tools for the study of O-GlcNAc become more facile and available, exponential growth in this area of research will eventually take place.

A novel transcriptional mechanism of cell type-specific regulation of vascular gene expression by glucose

OBJECTIVE

Vascular diabetic complications are associated with abnormal extracellular matrix and dysfunction of vascular cells, which later result in aberrant angiogenesis and development of atherosclerotic lesions. The tissue and cell specificity of the effects of high glucose are well recognized, but the underlying cell type-specific molecular mechanisms controlled by glucose are still unclear. We sought to identify cell type-specific mechanisms by which high glucose regulates transcription of genes in vascular cells.

METHODS AND RESULTS

Thrombospondin-1 is a potent antiangiogenic protein associated with development of several diabetic complications and regulated by high glucose in multiple cell types. We report that distinct cell type-specific mechanisms regulate thrombospondin-1 gene (THBS1) transcription in endothelial cells (ECs) and vascular smooth muscle cells (VSMCs) in response to high glucose: although a proximal fragment of 280 nucleotides is sufficient to drive transcription in ECs, THBS1 was regulated cooperatively by interaction between proximal (-272 to -275) and distal (-1016 to -1019) promoter elements in VSMCs. Transcription factors activated by high glucose in VSMCs were cell type-specific. The formation of a single complex interacting with both distal and proximal glucose-responsive elements of THBS1 promoter in VSMCs was confirmed using gel-shift assays, binding sequence decoy oligomers, and specific mutant promoter fragments.

CONCLUSIONS

Transcriptional response of vascular cells to high glucose is cell type-specific and involves activation of distinct transcription factors, providing a basis for tissue-specific changes of vasculature in diabetics.

Hypoxia upregulates the expression of the O-linked N-acetylglucosamine containing epitope H in human ependymal cells

Epitope H contains an O-linked N-acetylglucosamine (O-GlcNAc) residue in a specific conformation and/or environment recognized by mouse IgM monoclonal antibody H (mabH). Epitope H is present in several types of cells and in several polypeptides outside the CNS. Previous results have shown that in the adult human brains, epitope H is confined mostly to a minority of fibrous astrocytes, and it is greatly upregulated in the reactive astrocytes. Post-translational modification with O-GlcNAc occurs on many proteins involved in several cell processes, such as cell cycle progression, apoptosis, proteasome degradation pathways, and modulation of cellular function in response to nutrition and stress. Hypoxia is one of the major causes of cellular stress. Therefore, in this study, we used the mAbH and the indirect immunoperoxidase method to investigate the expression of epitope H in ependymal cells in brains of persons who died with signs of hypoxic encephalopathy. The results of the present study showed that practically all ependymal cells showed cytoplasmic staining for epitope H in supranuclear cytoplasm in the brain of two premature neonates and in ten infants who died with signs of hypoxic encephalopathy. However, the overwhelming majority of ependymal cells of the nine human embryos taken from legal abortions, ranging from 26 days until 13 weeks of gestational age, and of the ten infants' brains without any sign of hypoxic encephalopathy remained negative. Only occasionally did the ependymal cells show weak cytoplasmic staining in some foci. In addition, the reactive astrocytes in the hypoxic brains showed strong cytoplasmic staining, confirming previous results.

Regulation of insulin receptor substrate 1 (IRS-1)/AKT kinase-mediated insulin signaling by O-Linked beta-N-acetylglucosamine in 3T3-L1 adipocytes

Increased O-linked beta-N-acetylglucosamine (O-GlcNAc) is associated with insulin resistance in muscle and adipocytes. Upon insulin treatment of insulin-responsive adipocytes, O-GlcNAcylation of several proteins is increased. Key insulin signaling proteins, including IRS-1, IRS-2, and PDK1, are substrates for OGT, suggesting potential O-GlcNAc control points within the pathway. To elucidate the roles of O-GlcNAc in dampening insulin signaling (Vosseller, K., Wells, L., Lane, M. D., and Hart, G. W. (2002) Proc. Natl. Acad. Sci. U. S. A. 99, 5313-5318), we focused on the pathway upstream of AKT. Increasing O-GlcNAc in 3T3-L1 adipocytes decreases phosphoinositide 3-kinase (PI3K) interactions with both IRS-1 and IRS-2. Elevated O-GlcNAc also reduces phosphorylation of the PI3K p85 binding motifs (YXXM) of IRS-1 and results in a concomitant reduction in tyrosine phosphorylation of Y(608)XXM in IRS-1, one of the two main PI3K p85 binding motifs. Additionally, insulin signaling stimulates the interaction of OGT with PDK1. We conclude that one of the steps at which O-GlcNAc contributes to insulin resistance is by inhibiting phosphorylation at the Y(608)XXM PI3K p85 binding motif in IRS-1 and possibly at PDK1 as well.

Regulation of AMPA receptor trafficking by O-glycosylation

The present study investigated the role of O-linked beta-N-acetylglucosamine (O-GlcNAc) glycosylation (O-GlcNAcylation) in AMPA receptor trafficking. Alloxan, an inhibitor of O-GlcNAc transferase, potentiated responses of AMPA receptors composed of the GluR1 subunit expressed in Xenopus oocytes. No potentiating effect of alloxan was obtained with mutant GluR1 (S831A) receptor lacking CaMKII phosphorylation site. Alloxan facilitated basal synaptic transmission to approximately 120% of basal levels and enhanced Schaffer collateral-CA1 long-term potentiation (LTP) in rat hippocampal slices, especially in the late phase of the LTP. Alloxan stimulated translocation of the GluR1 and GluR2 subunit from the cytosol towards the plasma membrane in rat hippocampal slices with the LTP, although it had no effect on subcellular distribution of the NR1 subunit. Taken together, the results of the present study show that alloxan regulates AMPA receptor trafficking by inhibiting O-GlcNAcylation, to modulate hippocampal synaptic transmission and synaptic plasticity.

The role of protein O-linked beta-N-acetylglucosamine in mediating cardiac stress responses

The modification of serine and threonine residues of nuclear and cytoplasmic proteins by O-linked beta-N-acetylglucosamine (O-GlcNAc) has emerged as a highly dynamic post-translational modification that plays a critical role in regulating numerous biological processes. Much of our understanding of the mechanisms underlying the role of O-GlcNAc on cellular function has been in the context of its adverse effects in mediating a range of chronic disease processes, including diabetes, cancer and neurodegenerative diseases. However, at the cellular level it has been shown that O-GlcNAc levels are increased in response to stress; augmentation of this response improved cell survival while attenuation decreased cell viability. Thus, it has become apparent that strategies that augment O-GlcNAc levels are pro-survival, whereas those that reduce O-GlcNAc levels decrease cell survival. There is a long history demonstrating the effectiveness of acute glucose-insulin-potassium (GIK) treatment and to a lesser extent glutamine in protecting against a range of stresses, including myocardial ischemia. A common feature of these approaches for metabolic cardioprotection is that they both have the potential to stimulate O-GlcNAc synthesis. Consequently, here we examine the links between metabolic cardioprotection with the ischemic cardioprotection associated with acute increases in O-GlcNAc levels. Some of the protective mechanisms associated with activation of O-GlcNAcylation appear to be transcriptionally mediated; however, there is also strong evidence to suggest that transcriptionally independent mechanisms also play a critical role. In this context we discuss the potential link between O-GlcNAcylation and cardiomyocyte calcium homeostasis including the role of non-voltage gated, capacitative calcium entry as a potential mechanism contributing to this protection.

O-GlcNAc modification of proteins affects volume regulation in Jurkat cells

An increasing amount of recent research has demonstrated that the hexosamine biosynthesis pathway (HBP) plays a significant role in the modulation of intracellular signaling transduction pathways, and affects cellular processes via modification of protein by O-linked beta-N-acetylglucosamine (O-GlcNAc). Besides the many known and postulated effects of protein O-GlcNAc modifications, there is little available data on the role of O-GlcNAc in cellular volume regulation. Our objective was to test the effect of increased O-GlcNAc levels on hypotonia-induced volume changes in Jurkat cells. We pretreated Jurkat cells for 1 h with glucosamine (GlcN), PUGNAc (O-(2-acetamido-2-deoxy-D-glucopyranosylidene)-amino-N-phenylcarbamate) an inhibitor of O-GlcNAcase, or a high level of glucose to induce elevated levels of O-GlcNAc. We found that the response of Jurkat cells to hypotonic stress was significantly altered. The hypotonia induced cell-swelling was augmented in both GlcN and PUGNAc-treated cells and, to a lesser extent, in high glucose concentration-treated cells. Evaluated by NMR measurements, GlcN and PUGNAc treatment also significantly reduced intracellular water diffusion. Taken together, increased cell swelling and reduced water diffusion caused by elevated O-GlcNAc show notable analogy to the regulatory volume changes seen by magnetic resonance methods in nervous and other tissues in different pathological states. In conclusion, we demonstrate for the first time that protein O-GlcNAc could modulate cell volume regulation.

Synthesis of a pyridyl disulfide end-functionalized glycopolymer for conjugation to biomolecules and patterning on gold surfaces

A pyridyl disulfide end-functionalized polymer with N-acetyl-d-glucosamine pendant side-chains was synthesized by atom transfer radical polymerization (ATRP). The glycopolymer was prepared from a pyridyl disulfide initiator catalyzed by a Cu(I)/Cu(II)/2,2'-bipyridine system in a mixture of methanol and water at 30 degrees C. The final polymer had a number-average molecular weight (M(n)) of 13.0 kDa determined by (1)H NMR spectroscopy and a narrow polydispersity index (1.12) determined by gel permeation chromatography (GPC). The pyridyl disulfide end-group was then utilized to conjugate the glycopolymer to a double-stranded short interfering RNA (siRNA). Characterization of the glycopolymer-siRNA by polyacrylamide gel electrophoresis (PAGE) showed 97% conjugation. The activated disulfide polymer was also patterned on gold via microcontact printing. The pyridyl disulfide allowed for ready immobilization of the glycopolymer into 200 microm sized features on the surface.

Mass spectrometry (LC-MS/MS) site-mapping of N-glycosylated membrane proteins for breast cancer biomarkers

Cancer cell membrane proteins are released into the plasma/serum by exterior protein cleavage, membrane sloughing, cellular secretion or cell lysis, and represent promising candidates for interrogation. Because many known disease biomarkers are both glycoproteins and membrane bound, we chose the hydrazide method to specifically target, enrich, and identify glycosylated proteins from breast cancer cell membrane fractions using the LTQ Orbitrap mass spectrometer. Our initial goal was to select membrane proteins from breast cancer cell lines and then to use the hydrazide method to identify the N-linked proteome as a prelude to evaluation of plasma/serum proteins from cancer patients. A combination of steps facilitated identification of the glycopeptides and also defined the glycosylation sites. In MCF-7, MDA-MB-453 and MDA-MB-468 cell membrane fractions, use of the hydrazide method facilitated an initial enrichment and site mapping of 27 N-linked glycosylation sites in 25 different proteins. However, only three N-linked glycosylated proteins, galectin-3 binding protein, lysosome associated membrane glycoprotein 1, and oxygen regulated protein, were identified in all three breast cancer cell lines. In addition, MCF-7 cells shared an additional 3 proteins with MDA-MB-453. Interestingly, the hydrazide method isolated a number of other N-linked glycoproteins also known to be involved in breast cancer, including epidermal growth factor receptor (EGFR), CD44, and the breast cancer 1, and early onset isoform 1 (BRCA1) biomarker. Analyzing the N-glycoproteins from membranes of breast cancer cell lines highlights the usefulness of the procedure for generating a practical set of potential biomarkers.

Dysregulation of the nutrient/stress sensor O-GlcNAcylation is involved in the etiology of cardiovascular disorders, type-2 diabetes and Alzheimer's disease

O-GlcNAcylation is widespread within the cytosolic and nuclear compartments of cells. This post-translational modification is likely an indicator of good health since its intracellular level correlates with the availability of extracellular glucose. Apart from its status as a nutrient sensor, O-GlcNAcylation may also act as a stress sensor since it exerts its fundamental effects in response to stress. Several studies report that the cell quickly responds to an insult by elevating O-GlcNAcylation levels and by unmasking a newly described Hsp70-GlcNAc binding property. From a more practical point of view, it has been shown that O-GlcNAcylation impairments contribute to the etiology of cardiovascular diseases, type-2 diabetes and Alzheimer's disease (AD), three illnesses common in occidental societies. Many studies have demonstrated that O-GlcNAcylation operates as a powerful cardioprotector and that by raising O-GlcNAcylation levels, the organism more successfully resists trauma-hemorrhage and ischemia/reperfusion injury. Recent data have also shown that insulin resistance and, more broadly, type-2 diabetes can be controlled by O-GlcNAcylation of the insulin pathway and O-GlcNAcylation of the gluconeogenesis transcription factors FoxO1 and CRCT2. Lastly, the finding that AD may correspond to a type-3 diabetes offers new perspectives into the knowledge of the neuropathology and into the search for new therapeutic avenues.

eNOS activation by physical forces: from short-term regulation of contraction to chronic remodeling of cardiovascular tissues

Nitric oxide production in response to flow-dependent shear forces applied on the surface of endothelial cells is a fundamental mechanism of regulation of vascular tone, peripheral resistance, and tissue perfusion. This implicates the concerted action of multiple upstream "mechanosensing" molecules reversibly assembled in signalosomes recruiting endothelial nitric oxide synthase (eNOS) in specific subcellular locales, e.g., plasmalemmal caveolae. Subsequent short- and long-term increases in activity and expression of eNOS translate this mechanical stimulus into enhanced NO production and bioactivity through a complex transcriptional and posttranslational regulation of the enzyme, including by shear-stress responsive transcription factors, oxidant stress-dependent regulation of transcript stability, eNOS regulatory phosphorylations, and protein-protein interactions. Notably, eNOS expressed in cardiac myocytes is amenable to a similar regulation in response to stretching of cardiac muscle cells and in part mediates the length-dependent increase in cardiac contraction force. In addition to short-term regulation of contractile tone, eNOS mediates key aspects of cardiac and vascular remodeling, e.g., by orchestrating the mobilization, recruitment, migration, and differentiation of cardiac and vascular progenitor cells, in part by regulating the stabilization and transcriptional activity of hypoxia inducible factor in normoxia and hypoxia. The continuum of the influence of eNOS in cardiovascular biology explains its growing implication in mechanosensitive aspects of integrated physiology, such as the control of blood pressure variability or the modulation of cardiac remodeling in situations of hemodynamic overload.

Cardioprotection of salidroside from ischemia/reperfusion injury by increasing N-acetylglucosamine linkage to cellular proteins

The modification of proteins with O-linked N-acetylglucosamine (O-GlcNAc) is increasingly recognized as an important posttranslational modification that modulates cellular function. Recent studies suggested that augmentation of O-GlcNAc levels increase cell survival following stress. Salidroside, one of the active components of Rhodiola rosea, shows potent anti-hypoxia property. In the present study, we reported the cardioprotection of salidroside from ischemia and reperfusion. Cardiomyocytes were exposed to 4 h of ischemia and 16 h of reperfusion, and then cell viability, apoptosis, glucose uptake, ATP levels and cytosolic Ca(2+) concentration were determined, and O-GlcNAc levels were assessed by Western blotting. Salidroside (80 uM) was added 24 h before ischemia/reperfusion was induced. Treatment with salidroside markedly improved cell viability from 64.7+/-4.5% to 85.8+/-3.1%, decreased lactate dehydrogenase (LDH) release from 38.5+/-2.1% to 21.2+/-1.7%, reduced cell apoptosis from 27.2+/-3.2% to 12.2+/-1.9%, significantly improved cardiomyocytes glucose uptake by 1.7-fold and increased O-GlcNAc levels by 1.6-fold, as well as reducing cytosolic Ca(2+) concentration compared to untreated cells following ischemia/reperfusion. Furthermore, the improved cell survival and the increase in O-GlcNAc with salidroside were attenuated by alloxan, an inhibitor of O-GlcNAc transferase. These results suggested that salidroside significantly enhances glucose uptake and increases protein O-GlcNAc levels and this is associated with decreased cardiomyocytes injury following ischemia/reperfusion.

Therapeutically targeting protein-glycan interactions

Glycosylation is the most common form of post-translational modifications by which oligosaccharide side chains are covalently attached to specific residues of the core protein. Especially O-linked glycan structures like the glycosaminoglycans were found to contribute significantly to many (patho-)biological processes like inflammation, coagulation, cancer and viral infections. Glycans exert their function by interacting with proteins thereby changing the structure of the interacting proteins and consequently modulating their function. Given the complex nature of cell-surface and extracellular matrix glycan structures, this therapeutic site has been neglected for a long time, the only exception being the antithrombin III-glycan interaction which has been successfully targeted by unfractionated and low-molecular weight heparins for many decades. Due to the recent breakthrough in the '-ome' sciences, among them proteomics and glycomics, protein-glycan interactions became more amenable for therapeutic approaches so that novel inhibitors of this interaction are currently in preclinical and clinical studies. An overview of current approaches, their advantages and disadvantages, is given and the promising potential of pharmacologically interfering with protein-glycan interactions is highlighted here.

Protein O-GlcNAcylation: a new signaling paradigm for the cardiovascular system

The posttranslational modification of serine and threonine residues of nuclear and cytoplasmic proteins by the O-linked attachment of the monosaccharide beta-N-acetylglucosamine (O-GlcNAc) is a highly dynamic and ubiquitous protein modification. Protein O-GlcNAcylation is rapidly emerging as a key regulator of critical biological processes including nuclear transport, translation and transcription, signal transduction, cytoskeletal reorganization, proteasomal degradation, and apoptosis. Increased levels of O-GlcNAc have been implicated as a pathogenic contributor to glucose toxicity and insulin resistance, which are both major hallmarks of diabetes mellitus and diabetes-related cardiovascular complications. Conversely, there is a growing body of data demonstrating that the acute activation of O-GlcNAc levels is an endogenous stress response designed to enhance cell survival. Reports on the effect of altered O-GlcNAc levels on the heart and cardiovascular system have been growing rapidly over the past few years and have implicated a role for O-GlcNAc in contributing to the adverse effects of diabetes on cardiovascular function as well as mediating the response to ischemic injury. Here, we summarize our present understanding of protein O-GlcNAcylation and its effect on the regulation of cardiovascular function. We examine the pathways regulating protein O-GlcNAcylation and discuss, in more detail, our understanding of the role of O-GlcNAc in both mediating the adverse effects of diabetes as well as its role in mediating cellular protective mechanisms in the cardiovascular system. In addition, we also explore the parallels between O-GlcNAc signaling and redox signaling, as an alternative paradigm for understanding the role of O-GlcNAcylation in regulating cell function.

New insights into metabolic signaling and cell survival: the role of beta-O-linkage of N-acetylglucosamine

The involvement of glucose in fundamental metabolic pathways represents a core element of biology. Late in the 20th century, a unique glucose-derived signal was discovered, which appeared to be involved in a variety of cellular processes, including mitosis, transcription, insulin signaling, stress responses, and potentially, Alzheimer's disease, and diabetes. By definition, this glucose-fed signaling system was a post-translational modification to proteins. However, unlike classical cotranslational N-glycosylation occurring in the endoplasmic reticulum and Golgi apparatus, this process occurs elsewhere throughout the cell in a highly dynamic fashion, similar to the quintessential post-translational modification, phosphorylation. This more recently described post-translational modification, the beta-O-linkage of N-acetylglucosamine (i.e., O-GlcNAc) to nucleocytoplasmic proteins, represents an under-investigated area of biology. This signaling system operates in all of the tissues examined and seems to have persisted throughout all multicellular eukaryotes. Thus, it comes with little surprise that O-GlcNAc signaling is an integral system and viable target for biomedical investigation. This system may be a boundless source for insight into a variety of diseases and yield numerous opportunities for drug design. This Perspective will address recent insights into O-GlcNAc signaling in the cardiovascular system as a paradigm for its involvement in other biological systems.

Characterization of beta-N-acetylglucosaminidase cleavage by caspase-3 during apoptosis

Beta-O-linked N-acetylglucosamine is a dynamic post-translational modification involved in protein regulation in a manner similar to phosphorylation. Removal of N-acetylglucosamine is regulated by beta-N-acetylglucosaminidase (O-GlcNAcase), which was previously shown to be a substrate of caspase-3 in vitro. Here we show that O-GlcNAcase is cleaved by caspase-3 into two fragments during apoptosis, an N-terminal fragment containing the O-GlcNAcase active site and a C-terminal fragment containing a region with homology to GCN5 histone acetyl-transferases. The caspase-3 cleavage site of O-GlcNAcase, mapped by Edman sequencing, is a noncanonical recognition site that occurs after Asp-413 of the SVVD sequence in human O-GlcNAcase. A point mutation, D413A, abrogates cleavage by caspase-3 both in vitro and in vivo. Finally, we show that O-GlcNAcase activity is not affected by caspase-3 cleavage because the N- and C-terminal O-GlcNAcase fragments remain associated after the cleavage. Furthermore, when co-expressed simultaneously in the same cell, the N-terminal and C-terminal caspase fragments associate to reconstitute O-GlcNAcase enzymatic activity. These studies support the identification of O-GlcNAcase as a caspase-3 substrate with a novel caspase-3 cleavage site and provide insight about O-GlcNAcase regulation during apoptosis.

Regulation of the O-linked beta-N-acetylglucosamine transferase by insulin signaling

O-Linked beta-N-acetylglucosamine (O-GlcNAc) transferase (OGT) catalyzes the addition of O-linked beta-N-acetylglucosamine (O-GlcNAc) onto serine and threonine residues in response to stimuli or stress analogous to phosphorylation by Ser/Thr-kinases. Like protein phosphatases, OGT appears to be targeted to myriad specific substrates by transiently interacting with specific targeting subunits. Here, we show that OGT is activated by insulin signaling. Insulin treatment of 3T3-L1 adipocytes stimulates both tyrosine phosphorylation and catalytic activity of OGT. A subset of OGT co-immunoprecipitates with the insulin receptor. Insulin stimulates purified insulin receptor to phosphorylate OGT in vitro. OGT is a competitive substrate with reduced and carboxyamidomethylated lysozyme (RCAM-lysozyme), a well characterized insulin receptor substrate. Insulin stimulation of 3T3-L1 adipocytes results in a partial translocation of OGT from the nucleus to the cytoplasm. The insulin activation of OGT results in increased O-GlcNAc modification of OGT and other proteins including, signal transducer and activator of transcription 3 (STAT3). We conclude that insulin stimulates the tyrosine phosphorylation and activity of OGT.