Role of protein O-linked N-acetyl-glucosamine in mediating cell function and survival in the cardiovascular system

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.

Effective glycaemic control critically determines insulin cardioprotection against ischaemia/reperfusion injury in anaesthetized dogs

AIMS

Experimental evidence has shown significant cardioprotective effects of insulin, whereas clinical trials produced mixed results without valid explanations. This study was designed to examine the effect of hyperglycaemia on insulin cardioprotective action in a preclinical large animal model of myocardial ischaemia/reperfusion (MI/R).

METHODS AND RESULTS

Anaesthetized dogs were subjected to MI/R (30 min/4 h) and randomized to normal plasma insulin/euglycaemia (NI/NG), normal-insulin/hyperglycaemia (NI/HG), high-insulin/euglycaemia (HI/NG), and high-insulin/hyperglycaemia (HI/HG) achieved by controlled glucose/insulin infusion. Endogenous insulin production was abolished by peripancreatic vessel ligation. Compared with the control animals (NI/NG), hyperglycaemia (NI/HG) significantly aggravated MI/R injury. Insulin elevation at clamped euglycaemia (HI/NG) protected against MI/R injury as evidenced by reduced infarct size, decreased necrosis and apoptosis, and alleviated inflammatory and oxidative stress (leucocyte infiltration, myeloperoxidase, and malondialdehyde levels). However, these cardioprotective effects of insulin were markedly blunted in hyperglycaemic animals (HI/HG). In vitro mechanistic study in neonatal rat cardiomyocytes revealed that insulin-stimulated tyrosine phosphorylation of insulin receptor substrate-1 (IRS-1) and Akt was significantly attenuated by high glucose, accompanied by markedly increased IRS-1 O-GlcNAc glycosylation following hypoxia/reoxygenation. Inhibition of hexosamine biosynthesis with 6-diazo-5-oxonorleucine abrogated high glucose-induced O-GlcNAc modification and inactivation of IRS-1/Akt as well as cell injury.

CONCLUSIONS

Our results, derived from a canine model of MI/R, demonstrate that hyperglycaemia blunts insulin protection against MI/R injury via hyperglycaemia-induced glycosylation and subsequent inactivation of insulin-signalling proteins. Our findings suggest that prevention of hyperglycaemia is critical for achieving maximal insulin cardioprotection for the ischaemic/reperfused hearts.

The detrimental effects of acute hyperglycemia on myocardial glucose uptake

AIMS

Although acute hyperglycemic (AHG) episodes are linked to lower glucose uptake, underlying mechanisms remain unclear. We hypothesized that AHG triggers reactive oxygen species (ROS) production and increases non-oxidative glucose pathway (NOGP) activation, i.e. stimulation of advanced glycation end products (AGE), polyol pathway (PP), hexosamine biosynthetic pathway (HBP), PKC; thereby decreasing cardiac glucose uptake.

MAIN METHODS

H9c2 cardiomyoblasts were exposed to 25 mM glucose for 24h vs. 5.5mM controls ± modulating agents during the last hour of glucose exposure: a) antioxidant #1 for mitochondrial ROS (250 μM 4-OHCA), b) antioxidant #2 for NADPH oxidase-generated ROS (100 μM DPI), c) NOGP inhibitors - 100 μM aminoguanidine (AGE), 5 μM chelerythrine (PKC); 40 μM DON (HBP); and 10 μM zopolrestat (PP). ROS levels (mitochondrial, intracellular) and glucose uptake were evaluated by flow cytometry.

KEY FINDINGS

AHG elevated ROS, activated NOGPs and blunted glucose uptake. Transketolase activity (pentose phosphate pathway [PPP] marker) did not change. Respective 4-OHCA and DPI treatment blunted ROS production, diminished NOGP activation and normalized glucose uptake. NOGP inhibitory studies identified PKCβII as a key downstream player in lowering insulin-mediated glucose uptake. When we employed an agent (benfotiamine) known to shunt flux away from NOGPs (into PPP), it decreased ROS generation and NOGP activation, and restored glucose uptake under AHG conditions.

SIGNIFICANCE

This study demonstrates that AHG elicits maladaptive events that function in tandem to reduce glucose uptake, and that antioxidant treatment and/or attenuation of NOGP activation (PKC, polyol pathway) may limit the onset of insulin resistance.

Differential hexosamine biosynthetic pathway gene expression with type 2 diabetes

The hexosamine biosynthetic pathway (HBP) culminates in the attachment of O-linked β-N-acetylglucosamine (O-GlcNAc) onto serine/threonine residues of target proteins. The HBP is regulated by several modulators, i.e. O-linked β-N-acetylglucosaminyl transferase (OGT) and β-N-acetylglucosaminidase (OGA) catalyze the addition and removal of O-GlcNAc moieties, respectively; while flux is controlled by the rate-limiting enzyme glutamine:fructose-6-phosphate amidotransferase (GFPT), transcribed by two genes, GFPT1 and GFPT2. Since increased HBP flux is glucose-responsive and linked to insulin resistance/type 2 diabetes onset, we hypothesized that diabetic individuals exhibit differential expression of HBP regulatory genes. Volunteers (n = 60; n = 20 Mixed Ancestry, n = 40 Caucasian) were recruited from Stellenbosch and Paarl (Western Cape, South Africa) and classified as control, pre- or diabetic according to fasting plasma glucose and HbA1c levels, respectively. RNA was purified from leukocytes isolated from collected blood samples and OGT, OGA, GFPT1 and GFPT2 expressions determined by quantitative real-time PCR. The data reveal lower OGA expression in diabetic individuals (P < 0.01), while pre- and diabetic subjects displayed attenuated OGT expression vs. controls (P < 0.01 and P < 0.001, respectively). Moreover, GFPT2 expression decreased in pre- and diabetic Caucasians vs. controls (P < 0.05 and P < 0.01, respectively). We also found ethnic differences, i.e. Mixed Ancestry individuals exhibited a 2.4-fold increase in GFPT2 expression vs. Caucasians, despite diagnosis (P < 0.01). Gene expression of HBP regulators differs between diabetic and non-diabetic individuals, together with distinct ethnic-specific gene profiles. Thus differential HBP gene regulation may offer diagnostic utility and provide candidate susceptibility genes for different ethnic groupings.

Protein O-GlcNAcylation in diabetes and diabetic complications

The post-translational modification of serine and threonine residues of proteins by O-linked β-N-acetylglucosamine (O-GlcNAc) is highly ubiquitous, dynamic and inducible. Protein O-GlcNAcylation serves as a key regulator of critical biological processes including transcription, translation, proteasomal degradation, signal transduction and apoptosis. Increased O-GlcNAcylation is directly linked to insulin resistance and to hyperglycemia-induced glucose toxicity, two hallmarks of diabetes and diabetic complications. In this review, we briefly summarize what is known about protein O-GlcNAcylation and nutrient metabolism, as well as discuss the commonly used tools to probe changes of O-GlcNAcylation in cultured cells and in animal models. We then focus on some key proteins modified by O-GlcNAc, which play crucial roles in the etiology and progression of diabetes and diabetic complications. Proteomic approaches are also highlighted to provide a system view of protein O-GlcNAcylation. Finally, we discuss how aberrant O-GlcNAcylation on certain proteins may be exploited to develop methods for the early diagnosis of pre-diabetes and/or diabetes.

Functional O-GlcNAc modifications: implications in molecular regulation and pathophysiology

O-linked β-N-acetylglucosamine (O-GlcNAc) is a regulatory post-translational modification of intracellular proteins. The dynamic and inducible cycling of the modification is governed by O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA) in response to UDP-GlcNAc levels in the hexosamine biosynthetic pathway (HBP). Due to its reliance on glucose flux and substrate availability, a major focus in the field has been on how O-GlcNAc contributes to metabolic disease. For years this post-translational modification has been known to modify thousands of proteins implicated in various disorders, but direct functional connections have until recently remained elusive. New research is beginning to reveal the specific mechanisms through which O-GlcNAc influences cell dynamics and disease pathology including clear examples of O-GlcNAc modification at a specific site on a given protein altering its biological functions. The following review intends to focus primarily on studies in the last half decade linking O-GlcNAc modification of proteins with chromatin-directed gene regulation, developmental processes, and several metabolically related disorders including Alzheimer's, heart disease and cancer. These studies illustrate the emerging importance of this post-translational modification in biological processes and multiple pathophysiologies.

Glycosylation of the nuclear pore

The O-linked β-N-acetylglucosamine (O-GlcNAc) posttranslational modification was first discovered 30 years ago and is highly concentrated in the nuclear pore. In the years since the discovery of this single sugar modification, substantial progress has been made in understanding the biochemistry of O-GlcNAc and its regulation. Nonetheless, O-GlcNAc modification of proteins continues to be overlooked, due in large part to the lack of reliable methods available for its detection. Recently, a new crop of immunological and chemical detection reagents has changed the research landscape. Using these tools, approximately 1000 O-GlcNAc-modified proteins have been identified. While other forms of glycosylation are typically associated with extracellular proteins, O-GlcNAc is abundant on nuclear and cytoplasmic proteins. In particular, phenylalanine-glycine nucleoporins are heavily O-GlcNAc-modified. Recent experiments are beginning to provide insight into the functional implications of O-GlcNAc modification on certain proteins, but its role in the nuclear pore has remained enigmatic. However, tantalizing new results suggest that O-GlcNAc may play roles in regulating nucleocytoplasmic transport.

Glucosamine for osteoarthritis: biological effects, clinical efficacy, and safety on glucose metabolism

Osteoarthritis is a chronic degenerative disorder that currently represents one of the main causes of disability within the elderly population and an important presenting complaint overall. The pathophysiologic basis of osteoarthritis entails a complex group of interactions among biochemical and mechanical factors that have been better characterized in light of a recent spike in research on the subject. This has led to an ongoing search for ideal therapeutic management schemes for these patients, where glucosamine is one of the most frequently used alternatives worldwide due to their chondroprotective properties and their long-term effects. Its use in the treatment of osteoarthritis is well established; yet despite being considered effective by many research groups, controversy surrounds their true effectiveness. This situation stems from several methodological aspects which hinder appropriate data analysis and comparison in this context, particularly regarding objectives and target variables. Similar difficulties surround the assessment of the potential ability of glucosamine formulations to alter glucose metabolism. Nevertheless, evidence supporting diabetogenesis by glucosamine remains scarce in humans, and to date, this association should be considered only a theoretical possibility.

Diabetic cardiomyopathy: mechanisms and new treatment strategies targeting antioxidant signaling pathways

Cardiovascular disease is the primary cause of morbidity and mortality among the diabetic population. Both experimental and clinical evidence suggest that diabetic subjects are predisposed to a distinct cardiomyopathy, independent of concomitant macro- and microvascular disorders. 'Diabetic cardiomyopathy' is characterized by early impairments in diastolic function, accompanied by the development of cardiomyocyte hypertrophy, myocardial fibrosis and cardiomyocyte apoptosis. The pathophysiology underlying diabetes-induced cardiac damage is complex and multifactorial, with elevated oxidative stress as a key contributor. We now review the current evidence of molecular disturbances present in the diabetic heart, and their role in the development of diabetes-induced impairments in myocardial function and structure. Our focus incorporates both the contribution of increased reactive oxygen species production and reduced antioxidant defenses to diabetic cardiomyopathy, together with modulation of protein signaling pathways and the emerging role of protein O-GlcNAcylation and miRNA dysregulation in the progression of diabetic heart disease. Lastly, we discuss both conventional and novel therapeutic approaches for the treatment of left ventricular dysfunction in diabetic patients, from inhibition of the renin-angiotensin-aldosterone-system, through recent evidence favoring supplementation of endogenous antioxidants for the treatment of diabetic cardiomyopathy. Novel therapeutic strategies, such as gene therapy targeting the phosphoinositide 3-kinase PI3K(p110α) signaling pathway, and miRNA dysregulation, are also reviewed. Targeting redox stress and protective protein signaling pathways may represent a future strategy for combating the ever-increasing incidence of heart failure in the diabetic population.

Protein O-GlcNAcylation is a novel cytoprotective signal in cardiac stem cells

Clinical trials demonstrate the regenerative potential of cardiac stem cell (CSC) therapy in the postinfarcted heart. Despite these encouraging preliminary clinical findings, the basic biology of these cells remains largely unexplored. The principal requirement for cell transplantation is to effectively prime them for survival within the unfavorable environment of the infarcted myocardium. In the adult mammalian heart, the β-O-linkage of N-acetylglucosamine (i.e., O-GlcNAc) to proteins is a unique post-translational modification that confers cardioprotection from various otherwise lethal stressors. It is not known whether this signaling system exists in CSCs. In this study, we demonstrate that protein O-GlcNAcylation is an inducible stress response in adult murine Sca-1(+) /lin(-) CSCs and exerts an essential prosurvival role. Posthypoxic CSCs responded by time-dependently increasing protein O-GlcNAcylation upon reoxygenation. We used pharmacological interventions for loss- and gain-of-function, that is, enzymatic inhibition of O-GlcNAc transferase (OGT) (adds the O-GlcNAc modification to proteins) by TT04, or inhibition of OGA (removes O-GlcNAc) by thiamet-G (ThG). Reduction in the O-GlcNAc signal (via TT04, or OGT gene deletion using Cre-mediated recombination) significantly sensitized CSCs to posthypoxic injury, whereas augmenting O-GlcNAc levels (via ThG) enhanced cell survival. Diminished O-GlcNAc levels render CSCs more susceptible to the onset of posthypoxic apoptotic processes via elevated poly(ADP-ribose) polymerase cleavage due to enhanced caspase-3/7 activation, whereas promoting O-GlcNAcylation can serve as a pre-emptive antiapoptotic signal regulating the survival of CSCs. Thus, we report the primary demonstration of protein O-GlcNAcylation as an important prosurvival signal in CSCs, which could enhance CSC survival prior to in vivo autologous transfer.

Dynamic O-GlcNAcylation and its roles in the cellular stress response and homeostasis

O-linked N-acetyl-β-D-glucosamine (O-GlcNAc) is a ubiquitous and dynamic post-translational modification known to modify over 3,000 nuclear, cytoplasmic, and mitochondrial eukaryotic proteins. Addition of O-GlcNAc to proteins is catalyzed by the O-GlcNAc transferase and is removed by a neutral-N-acetyl-β-glucosaminidase (O-GlcNAcase). O-GlcNAc is thought to regulate proteins in a manner analogous to protein phosphorylation, and the cycling of this carbohydrate modification regulates many cellular functions such as the cellular stress response. Diverse forms of cellular stress and tissue injury result in enhanced O-GlcNAc modification, or O-GlcNAcylation, of numerous intracellular proteins. Stress-induced O-GlcNAcylation appears to promote cell/tissue survival by regulating a multitude of biological processes including: the phosphoinositide 3-kinase/Akt pathway, heat shock protein expression, calcium homeostasis, levels of reactive oxygen species, ER stress, protein stability, mitochondrial dynamics, and inflammation. Here, we will discuss the regulation of these processes by O-GlcNAc and the impact of such regulation on survival in models of ischemia reperfusion injury and trauma hemorrhage. We will also discuss the misregulation of O-GlcNAc in diseases commonly associated with the stress response, namely Alzheimer's and Parkinson's diseases. Finally, we will highlight recent advancements in the tools and technologies used to study the O-GlcNAc modification.

Diabetes and vascular disease: pathophysiology, clinical consequences, and medical therapy: part I

Hyperglycemia and insulin resistance are key players in the development of atherosclerosis and its complications. A large body of evidence suggest that metabolic abnormalities cause overproduction of reactive oxygen species (ROS). In turn, ROS, via endothelial dysfunction and inflammation, play a major role in precipitating diabetic vascular disease. A better understanding of ROS-generating pathways may provide the basis to develop novel therapeutic strategies against vascular complications in this setting. Part I of this review will focus on the most current advances in the pathophysiological mechanisms of vascular disease: (i) emerging role of endothelium in obesity-induced insulin resistance; (ii) hyperglycemia-dependent microRNAs deregulation and impairment of vascular repair capacities; (iii) alterations of coagulation, platelet reactivity, and microparticle release; (iv) epigenetic-driven transcription of ROS-generating and proinflammatory genes. Taken together these novel insights point to the development of mechanism-based therapeutic strategies as a promising option to prevent cardiovascular complications in diabetes.

O-GlcNAcylation of αB-crystallin regulates its stress-induced translocation and cytoprotection

Under normal conditions, the ubiquitously expressed αB-crystallin functions as a chaperone. αB-crystallin has been implicated in a variety of pathologies, consistent with a build-up of protein aggregates, such as neuromuscular disorders, myofibrillar myopathies, and cardiomyopathies. αB-crystallins' cardioprotection is partially attributed to its translocation and binding to cytoskeletal elements in response to stress. The triggers for this translocation are not clearly understood. In the heart, αB-crystallin undergoes at least three significant post-translational modifications: phosphorylation at ser-45 and 59 and O-GlcNAcylation (O-linked attachment of the monosaccharide β-N-acetyl-glucosamine) at thr-170. Whether phosphorylation status drives translocation remains controversial. Therefore, we evaluated the role of αB-crystallins' O-GlcNAcylation in its stress-induced translocation and cytoprotection in cardiomyocytes under stress. Immunoblotting and precipitation experiments with anti-O-GlcNAc antibody (CTD110.6) and glycoprotein staining (Pro-Q Emerald) both demonstrate robust stress-induced O-GlcNAcylation of αB-crystallin. A non-O-GlcNAcylatable αB-crystallin mutant (αB-T170A) showed diminished translocation in response to heat shock and robust phosphorylation at both ser-45 and ser-59. Cell survival assays show a loss of overexpression-associated cytoprotection with the non-glycosylatable mutant to multiple stresses. While ectopic expression of wild-type αB-crystallin strongly stabilized ZsProSensor, a fusion protein rapidly degraded by the proteasome, the non-O-GlcNAcylatable version did not. Therefore, we believe the O-GlcNAcylation of αB-crystallin is a dynamic and important regulator of both its localization and function.

Metabolic effects of glutamine on the heart: anaplerosis versus the hexosamine biosynthetic pathway

Glutamine, the most abundant amino acid in plasma, has attracted considerable interest for its cardioprotective properties. The primary effect of glutamine in the heart is commonly believed to be mediated via its anaplerotic metabolism to citric acid cycle (CAC) intermediates; however, there is little direct evidence to support this concept. Another potential candidate is the hexosamine biosynthetic pathway (HBP), which has recently been shown to modulate cardiomyocyte function and metabolism. Therefore, the goal of this study was to evaluate the contribution of anaplerosis and the HBP to the acute metabolic effects of glutamine in the heart. Normoxic ex vivo working rat hearts were perfused with (13)C-labeled substrates to assess relevant metabolic fluxes either with a physiological mixture of carbohydrates and a fatty acid (control) or under conditions of restricted pyruvate anaplerosis. Addition of a physiological concentration of glutamine (0.5mM) had no effect on contractile function of hearts perfused under the control condition, but improved that of hearts perfused under restricted pyruvate anaplerosis. Changes in CAC intermediate concentrations as well as (13)C-enrichment from [U-(13)C]glutamine did not support a major role of glutamine anaplerosis under any conditions. Under the control condition, however, glutamine significantly increased the contribution of exogenous oleate to β-oxidation, 1.6-fold, and triglyceride formation, 2.8-fold. Glutamine had no effect on malonyl-CoA or AMP kinase activity levels; however, it resulted in a higher plasma membrane level of the fatty acid transporter CD36. These metabolic effects of glutamine were reversed by azaserine, which inhibits glucose entry into the HPB. Our results reveal a metabolic role of physiological concentration of glutamine in the healthy working heart beyond anaplerosis. This role appears to involve the HBP and regulation of fatty acid entry and metabolism via CD36. This article is part of a Special Issue entitled "Focus on Cardiac Metabolism".

Zinc-induced cardiomyocyte relaxation in a rat model of hyperglycemia is independent of myosin isoform

It has been reported previously that diabetic cardiomyopathy can be inhibited or reverted with chronic zinc supplementation. In the current study, we hypothesized that total cardiac calcium and zinc content is altered in early onset diabetes mellitus characterized in part as hyperglycemia (HG) and that exposure of zinc ion (Zn²⁺) to isolated cardiomyocytes would enhance contraction-relaxation function in HG more so than in nonHG controls. To better control for differential cardiac myosin isoform expression as occurs in rodents after β-islet cell necrosis, hypothyroidism was induced in 16 rats resulting in 100% β-myosin heavy chain expression in the heart. β-Islet cell necrosis was induced in half of the rats by streptozocin administration. After 6 wks of HG, both HG and nonHG controls rats demonstrated similar myofilament performance measured as thin filament calcium sensitivity, native thin filament velocity in the myosin motility assay and contractile velocity and power. Extracellular Zn²⁺ reduced cardiomyocyte contractile function in both groups, but enhanced relaxation function significantly in the HG group compared to controls. Most notably, a reduction in diastolic sarcomere length with increasing pacing frequencies, i.e., incomplete relaxation, was more pronounced in the HG compared to controls, but was normalized with extracellular Zn²⁺ application. This is a novel finding implicating that the detrimental effect of HG on cardiomyocyte Ca²⁺ regulation can be amelioration by Zn²⁺. Among the many post-translational modifications examined, only phosphorylation of ryanodine receptor (RyR) at S-2808 was significantly higher in HG compared to nonHG. We did not find in our hypothyroid rats any differentiating effects of HG on myofibrillar protein phosphorylation, lysine acetylation, O-linked N-acetylglucosamine and advanced glycated end-products, which are often implicated as complicating factors in cardiac performance due to HG. Our results suggest that the relaxing effects of Zn²⁺ on cardiomyocyte function are more pronounced in the HG state due an insulin-dependent effect of enhancing removal of cytosolic Ca²⁺ via SERCA2a or NCX or by reducing Ca²⁺ influx via L-type channel or Ca²⁺ leak through the RyR. Investigations into the effects of Zn²⁺ on these mechanisms are now underway.

Cardiac myofilaments: from proteome to pathophysiology

This review addresses the functional consequences of altered post-translational modifications of cardiac myofilament proteins in cardiac diseases such as heart failure and ischemia. The modifications of thick and thin filament proteins as well as titin are addressed. Understanding the functional consequences of altered protein modifications is an essential step in the development of targeted therapies for common cardiac diseases.

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.

Increased vascular O-GlcNAcylation augments reactivity to constrictor stimuli - VASOACTIVE PEPTIDE SYMPOSIUM

BACKGROUND

O-Linked N-acetylglucosaminylation (O-GlcNAcylation) plays a role in many aspects of protein function. Whereas elevated O-GlcNAc levels contribute to diabetes related end-organ damage, O-GlcNAcylation is also physiologically important. Because proteins that play a role in vascular tone regulation can be O-GlcNAcylated, we hypothesized that O-GlcNAcylation increases vascular reactivity to constrictor stimuli.

METHODS AND RESULTS

Aortas from male Sprague-Dawley rats and C57BL/6 mice were incubated for 24 h with vehicle or PugNAc (O-GlcNAcase inhibitor, 100muM). PugNAc incubation significantly increased O-GlcNAc-proteins, as determined by Western blot. PugNAc also increased vascular contractions to phenylephrine and serotonin, an effect not observed in the presence of L-NAME or in endothelium-denuded vessels. Acetylcholine-induced relaxation, but not that to sodium nitroprusside was decreased by PugNAc treatment, an effect accompanied by decreased levels of phosphorylated eNOS(Ser-1177) and Akt(Ser-473).

CONCLUSION

Augmented O-GlcNAcylation increases vascular reactivity to constrictor stimuli, possibly due to its effects on eNOS expression and activity, reinforcing the concept that O-GlcNAcylation modulates vascular reactivity and may play a role in pathological conditions associated with abnormal vascular function.

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."

O-GlcNAcylation, novel post-translational modification linking myocardial metabolism and cardiomyocyte circadian clock

The cardiomyocyte circadian clock directly regulates multiple myocardial functions in a time-of-day-dependent manner, including gene expression, metabolism, contractility, and ischemic tolerance. These same biological processes are also directly influenced by modification of proteins by monosaccharides of O-linked β-N-acetylglucosamine (O-GlcNAc). Because the circadian clock and protein O-GlcNAcylation have common regulatory roles in the heart, we hypothesized that a relationship exists between the two. We report that total cardiac protein O-GlcNAc levels exhibit a diurnal variation in mouse hearts, peaking during the active/awake phase. Genetic ablation of the circadian clock specifically in cardiomyocytes in vivo abolishes diurnal variations in cardiac O-GlcNAc levels. These time-of-day-dependent variations appear to be mediated by clock-dependent regulation of O-GlcNAc transferase and O-GlcNAcase protein levels, glucose metabolism/uptake, and glutamine synthesis in an NAD-independent manner. We also identify the clock component Bmal1 as an O-GlcNAc-modified protein. Increasing protein O-GlcNAcylation (through pharmacological inhibition of O-GlcNAcase) results in diminished Per2 protein levels, time-of-day-dependent induction of bmal1 gene expression, and phase advances in the suprachiasmatic nucleus clock. Collectively, these data suggest that the cardiomyocyte circadian clock increases protein O-GlcNAcylation in the heart during the active/awake phase through coordinated regulation of the hexosamine biosynthetic pathway and that protein O-GlcNAcylation in turn influences the timing of the circadian clock.

Activation of hexosamine pathway impairs nitric oxide (NO)-dependent arteriolar dilations by increased protein O-GlcNAcylation

We hypothesized that under high glucose conditions, activation of the hexosamine pathway leads to impaired nitric oxide (NO)-dependent arteriolar dilation. Skeletal muscle arterioles (diameter: ~160μm) isolated from male Wistar rats were exposed to normal glucose (NG, 5.5mmol/L) or high glucose concentrations (HG, 30mmol/L, for 2h) and agonist-induced diameter changes were measured with videomicroscopy. Western blots were performed to identify the vascular levels of protein O-linked-N-acetyl-glucosamine (O-GlcNAc) and phosphorylated endothelial NO synthase (eNOS). In arterioles exposed to HG, dilations to histamine were abolished compared to those exposed to NG (max: -6±6% and 69±9%, respectively), while acetylcholine-induced responses were not affected. Inhibition of NO synthesis with N(G)-nitro-l-arginine methyl ester (L-NAME) reduced histamine-induced dilations in NG arterioles, but it had no effect on microvessels exposed to HG. Dilations to the NO donor, sodium nitroprusside and constrictions to norepinephrine and serotonin were similar in the two groups. In the presence of the inhibitor of hexosamine pathway, azaserine, histamine-induced dilations were significantly augmented in arterioles exposed to HG (max: 67±2%). Moreover, exposure of vessels to glucosamine (5mmol/L, for 2h) resulted in reduced histamine-induced arteriolar dilations (max: 26±3%). The level of protein O-GlcNAcylation was increased, whereas the P-eNOS (Ser-1177) was decreased in HG exposed vessels. These findings indicate that a high concentration of glucose may lead to glucosamine formation, which impairs histamine-induced, NO-mediated arteriolar dilations. We propose that interfering with the hexosamine pathway may prevent microvascular complications in diabetes.

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.

Cardiac O-GlcNAc signaling is increased in hypertrophy and heart failure

Reversible protein O-GlcNAc modification has emerged as an essential intracellular signaling system in several tissues, including cardiovascular pathophysiology related to diabetes and acute ischemic stress. We tested the hypothesis that cardiac O-GlcNAc signaling is altered in chronic cardiac hypertrophy and failure of different etiologies. Global protein O-GlcNAcylation and the main enzymes regulating O-GlcNAc, O-GlcNAc transferase (OGT), O-GlcNAcase (OGA), and glutamine-fructose-6-phosphate amidotransferase (GFAT) were measured by immunoblot and/or real-time RT-PCR analyses of left ventricular tissue from aortic stenosis (AS) patients and rat models of hypertension, myocardial infarction (MI), and aortic banding (AB), with and without failure. We show here that global O-GlcNAcylation was increased by 65% in AS patients, by 47% in hypertensive rats, by 81 and 58% post-AB, and 37 and 60% post-MI in hypertrophic and failing hearts, respectively (P < 0.05). Noticeably, protein O-GlcNAcylation patterns varied in hypertrophic vs. failing hearts, and the most extensive O-GlcNAcylation was observed on proteins of 20-100 kDa in size. OGT, OGA, and GFAT2 protein and/or mRNA levels were increased by pressure overload, while neither was regulated by myocardial infarction. Pharmacological inhibition of OGA decreased cardiac contractility in post-MI failing hearts, demonstrating a possible role of O-GlcNAcylation in development of chronic cardiac dysfunction. Our data support the novel concept that O-GlcNAc signaling is altered in various etiologies of cardiac hypertrophy and failure, including human aortic stenosis. This not only provides an exciting basis for discovery of new mechanisms underlying pathological cardiac remodeling but also implies protein O-GlcNAcylation as a possible new therapeutic target in heart failure.

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.

Effects of miglitol in platelet-derived microparticle, adiponectin, and selectin level in patients with type 2 diabetes mellitus

Background:

Platelet-derived microparticles (PDMP), selectins, and adiponectin play an important role in the development of atherosclerosis in diabetes. Miglitol has been shown to have a beneficial effect on postprandial hyperglycemia in diabetic patients. However, its influence on platelet activation markers (PDMP and soluble CD40 ligand [sCD40L]), selectins, and adiponectin in these patients is poorly understood.

Aim:

We investigated the effect of miglitol on circulating levels of PDMP, sCD40L, selectins, and adiponectin in patients with type 2 diabetes.

Methods:

Miglitol (150 mg/day) was administered for 4 months. Levels of PDMP, sCD40L, soluble P-selectin (sP-selectin), soluble E-selectin (sE-selectin), soluble L-selectin (sL-selectin), and adiponectin were measured by enzyme-linked immunosorbent assay at baseline, and after 1 and 4 months of treatment.

Results:

The levels of PDMP, sCD40L, sP-selectin, sE-selectin, and sL-selectin were higher in diabetic patients than in hypertensive patients, while there were no significant differences between hypertensive and hyperlipidemic patients. Before miglitol treatment, the adiponectin level of diabetic patients was lower than that of hypertensive patients. Miglitol therapy significantly decreased the plasma PDMP and sCD40L levels relative to baseline. Miglitol also caused a significant decrease of sP-selectin, sE-selectin, and sL-selectin. On the other hand, miglitol therapy led to a significant increase in adiponectin after 4 months of administration compared with baseline. Furthermore, the reduction of platelet activation markers and selectins during miglitol therapy was significantly greater in the responder (adiponectin-improved) group than the nonresponder group of diabetic patients.

Conclusion:

Miglitol has an adiponectin-dependent anti-atherothrombotic effect that may be beneficial for primary prevention of atherothrombosis in patients with type 2 diabetes.

Acute regulation of cardiac metabolism by the hexosamine biosynthesis pathway and protein O-GlcNAcylation

Objective

The hexosamine biosynthesis pathway (HBP) flux and protein O-linked N-acetyl-glucosamine (O-GlcNAc) levels have been implicated in mediating the adverse effects of diabetes in the cardiovascular system. Activation of these pathways with glucosamine has been shown to mimic some of the diabetes-induced functional and structural changes in the heart; however, the effect on cardiac metabolism is not known. Therefore, the primary goal of this study was to determine the effects of glucosamine on cardiac substrate utilization.

Methods

Isolated rat hearts were perfused with glucosamine (0–10 mM) to increase HBP flux under normoxic conditions. Metabolic fluxes were determined by ¹³C-NMR isotopomer analysis; UDP-GlcNAc a precursor of O-GlcNAc synthesis was assessed by HPLC and immunoblot analysis was used to determine O-GlcNAc levels, phospho- and total levels of AMPK and ACC, and membrane levels of FAT/CD36.

Results

Glucosamine caused a dose dependent increase in both UDP-GlcNAc and O-GlcNAc levels, which was associated with a significant increase in palmitate oxidation with a concomitant decrease in lactate and pyruvate oxidation. There was no effect of glucosamine on AMPK or ACC phosphorylation; however, membrane levels of the fatty acid transport protein FAT/CD36 were increased and preliminary studies suggest that FAT/CD36 is a potential target for O-GlcNAcylation.

Conclusion/Interpretation

These data demonstrate that acute modulation of HBP and protein O-GlcNAcylation in the heart stimulates fatty acid oxidation, possibly by increasing plasma membrane levels of FAT/CD36, raising the intriguing possibility that the HBP and O-GlcNAc turnover represent a novel, glucose dependent mechanism for regulating cardiac metabolism.

The dynamic stress-induced "O-GlcNAc-ome" highlights functions for O-GlcNAc in regulating DNA damage/repair and other cellular pathways

The modification of nuclear, mitochondrial, and cytoplasmic proteins by O-linked β-N-acetylglucosamine (O-GlcNAc) is a dynamic and essential post-translational modification of metazoans. Numerous forms of cellular injury lead to elevated levels of O-GlcNAc in both in vivo and in vitro models, and elevation of O-GlcNAc levels before, or immediately after, the induction of cellular injury is protective in models of heat stress, oxidative stress, endoplasmic reticulum (ER) stress, hypoxia, ischemia reperfusion injury, and trauma hemorrhage. Together, these data suggest that O-GlcNAc is a regulator of the cellular stress response. However, the molecular mechanism(s) by which O-GlcNAc regulates protein function leading to enhanced cell survival have not been identified. In order to determine how O-GlcNAc modulates stress tolerance in these models we have used stable isotope labeling with amino acids in cell culture to determine the identity of proteins that undergo O-GlcNAcylation in response to heat shock. Numerous proteins with diverse functions were identified, including NF-90, RuvB-like 1 (Tip49α), RuvB-like 2 (Tip49β), and several COPII vesicle transport proteins. Many of these proteins bind double-stranded DNA-dependent protein kinase (PK), or double-stranded DNA breaks, suggesting a role for O-GlcNAc in regulating DNA damage signaling or repair. Supporting this hypothesis, we have shown that DNA-PK is O-GlcNAc modified in response to numerous forms of cellular stress.

Augmented O-GlcNAc signaling attenuates oxidative stress and calcium overload in cardiomyocytes

O-linked β-N-acetylglucosamine (O-GlcNAc) is an inducible, dynamically cycling and reversible post-translational modification of Ser/Thr residues of nucleocytoplasmic and mitochondrial proteins. We recently discovered that O-GlcNAcylation confers cytoprotection in the heart via attenuating the formation of mitochondrial permeability transition pore (mPTP) and the subsequent loss of mitochondrial membrane potential. Because Ca(2+) overload and reactive oxygen species (ROS) generation are prominent features of post-ischemic injury and favor mPTP formation, we ascertained whether O-GlcNAcylation mitigates mPTP formation via its effects on Ca(2+) overload and ROS generation. Subjecting neonatal rat cardiac myocytes (NRCMs, n ≥ 6 per group) to hypoxia, or mice (n ≥ 4 per group) to myocardial ischemia reduced O-GlcNAcylation, which later increased during reoxygenation/reperfusion. NRCMs (n ≥ 4 per group) infected with an adenovirus carrying nothing (control), adenoviral O-GlcNAc transferase (adds O-GlcNAc to proteins, AdOGT), adenoviral O-GlcNAcase (removes O-GlcNAc to proteins, AdOGA), vehicle or PUGNAc (blocks OGA; increases O-GlcNAc levels) were subjected to hypoxia-reoxygenation or H(2)O(2), and changes in Ca(2+) levels (via Fluo-4AM and Rhod-2AM), ROS (via DCF) and mPTP formation (via calcein-MitoTracker Red colocalization) were assessed using time-lapse fluorescence microscopy. Both OGT and OGA overexpression did not significantly (P > 0.05) alter baseline Ca(2+) or ROS levels. However, AdOGT significantly (P < 0.05) attenuated both hypoxia and oxidative stress-induced Ca(2+) overload and ROS generation. Additionally, OGA inhibition mitigated both H(2)O(2)-induced Ca(2+) overload and ROS generation. Although AdOGA exacerbated both hypoxia and H(2)O(2)-induced ROS generation, it had no effect on H(2)O(2)-induced Ca(2+) overload. We conclude that inhibition of Ca(2+) overload and ROS generation (inducers of mPTP) might be one mechanism through which O-GlcNAcylation reduces ischemia/hypoxia-mediated mPTP formation.

Glucose-insulin-potassium reduces the incidence of low cardiac output episodes after aortic valve replacement for aortic stenosis in patients with left ventricular hypertrophy: results from the Hypertrophy, Insulin, Glucose, and Electrolytes (HINGE) trial

BACKGROUND

Patients undergoing aortic valve replacement for critical aortic stenosis often have significant left ventricular hypertrophy. Left ventricular hypertrophy has been identified as an independent predictor of poor outcome after aortic valve replacement as a result of a combination of maladaptive myocardial changes and inadequate myocardial protection at the time of surgery. Glucose-insulin-potassium (GIK) is a potentially useful adjunct to myocardial protection. This study was designed to evaluate the effects of GIK infusion in patients undergoing aortic valve replacement surgery.

METHODS AND RESULTS

Patients undergoing aortic valve replacement for aortic stenosis with evidence of left ventricular hypertrophy were randomly assigned to GIK or placebo. The trial was double-blind and conducted at a single center. The primary outcome was the incidence of low cardiac output syndrome. Left ventricular biopsies were analyzed to assess changes in 5' adenosine monophosphate-activated protein kinase (AMPK), Akt phosphorylation, and protein O-linked β-N-acetylglucosamination (O-GlcNAcylation). Over a 4-year period, 217 patients were randomized (107 control, 110 GIK). GIK treatment was associated with a significant reduction in the incidence of low cardiac output state (odds ratio, 0.22; 95% confidence interval, 0.10 to 0.47; P=0.0001) and a significant reduction in inotrope use 6 to 12 hours postoperatively (odds ratio, 0.30; 95% confidence interval, 0.15 to 0.60; P=0.0007). These changes were associated with a substantial increase in AMPK and Akt phosphorylation and a significant increase in the O-GlcNAcylation of selected protein bands.

CONCLUSIONS

Perioperative treatment with GIK was associated with a significant reduction in the incidence of low cardiac output state and the need for inotropic support. This benefit was associated with increased signaling protein phosphorylation and O-GlcNAcylation. Multicenter studies and late follow-up will determine whether routine use of GIK improves patient prognosis.

O-GlcNAcylation: a novel pathway contributing to the effects of endothelin in the vasculature

Glycosylation with O-linked β-N-acetylglucosamine (O-GlcNAc) or O-GlcNAcylation on serine and threonine residues of nuclear and cytoplasmic proteins is a posttranslational modification that alters the function of numerous proteins important in vascular function, including kinases, phosphatases, transcription factors, and cytoskeletal proteins. O-GlcNAcylation is an innovative way to think about vascular signaling events both in physiological conditions and in disease states. This posttranslational modification interferes with vascular processes, mainly vascular reactivity, in conditions where endothelin-1 (ET-1) levels are augmented (e.g. salt-sensitive hypertension, ischemia/reperfusion, and stroke). ET-1 plays a crucial role in the vascular function of most organ systems, both in physiological and pathophysiological conditions. Recognition of ET-1 by the ET(A) and ET(B) receptors activates intracellular signaling pathways and cascades that result in rapid and long-term alterations in vascular activity and function. Components of these ET-1-activated signaling pathways (e.g., mitogen-activated protein kinases, protein kinase C, RhoA/Rho kinase) are also targets for O-GlcNAcylation. Recent experimental evidence suggests that ET-1 directly activates O-GlcNAcylation, and this posttranslational modification mediates important vascular effects of the peptide. This review focuses on ET-1-activated signaling pathways that can be modified by O-GlcNAcylation. A brief description of the O-GlcNAcylation biology is presented, and its role on vascular function is addressed. ET-1-induced O-GlcNAcylation and its implications for vascular function are then discussed. Finally, the interplay between O-GlcNAcylation and O-phosphorylation is addressed.

O-GlcNAcylation contributes to the vascular effects of ET-1 via activation of the RhoA/Rho-kinase pathway

AIMS

Glycosylation with β-N-acetylglucosamine (O-GlcNAcylation) is one of the most complex post-translational modifications. The cycling of O-GlcNAc is controlled by two enzymes: UDP-NAc transferase (OGT) and O-GlcNAcase (OGA). We recently reported that endothelin-1 (ET-1) augments vascular levels of O-GlcNAcylated proteins. Here we tested the hypothesis that O-GlcNAcylation contributes to the vascular effects of ET-1 via activation of the RhoA/Rho-kinase pathway.

METHODS AND RESULTS

Incubation of vascular smooth muscle cells (VSMCs) with ET-1 (0.1 μM) produces a time-dependent increase in O-GlcNAc levels. ET-1-induced O-GlcNAcylation is not observed when VSMCs are previously transfected with OGT siRNA, treated with ST045849 (OGT inhibitor) or atrasentan (ET(A) antagonist). ET-1 as well as PugNAc (OGA inhibitor) augmented contractions to phenylephrine in endothelium-denuded rat aortas, an effect that was abolished by the Rho kinase inhibitor Y-27632. Incubation of VSMCs with ET-1 increased expression of the phosphorylated forms of myosin phosphatase target subunit 1 (MYPT-1), protein kinase C-potentiated protein phosphatase 1 inhibitor protein (protein kinase C-potentiated phosphatase inhibitor-17), and myosin light chain (MLC) and RhoA expression and activity, and this effect was abolished by both OGT siRNA transfection or OGT inhibition and atrasentan. ET-1 also augmented expression of PDZ-Rho GEF (guanine nucleotide exchange factor) and p115-Rho GEF in VSMCs and this was prevented by OGT siRNA, ST045849, and atrasentan.

CONCLUSION

We suggest that ET-1 augments O-GlcNAcylation and this modification contributes to increased vascular contractile responses via activation of the RhoA/Rho-kinase pathway.

The role of uncoupling protein 2 in the apoptosis induced by free fatty acid in rat cardiomyocytes

The apoptotic loss of cardiomyocytes contributes to numerous cardiovascular disorders. Evidence suggests that free fatty acids induce cellular apoptosis, and recent studies have shown that free fatty acids dramatically elevate mRNA levels of uncoupling protein 2 (UCP2) in some cell lines. In this study, we investigated the possibility that free fatty acids induce the expression of UCP2 through the peroxisome proliferator-activated receptor pathway, thereby increasing cell apoptosis in adult rat cardiomyocytes. Primary cultured adult rat cardiomyocytes exposed to free fatty acids exhibited a dose-dependent increase in apoptosis. Quantitative real-time reverse transcription-polymerase chain reaction and Western blotting showed significant increases in the level of UCP2 expression at 6, 12, and at 24 hours after treatment of adult rat cardiomyocytes with free fatty acids. Expression of UCP2 was suppressed with RNA interference, and knockdown of UCP2 attenuated free fatty acid-induced apoptosis in the cardiomyocytes. In summary, free fatty acids induced UCP2 expression through peroxisome proliferator-activated receptor alpha in adult rat cardiomyocytes.

O-linked beta-N-acetylglucosamine (O-GlcNAc) regulates stress-induced heat shock protein expression in a GSK-3beta-dependent manner

To investigate the mechanisms by which O-linked β-N-acetylglucosamine modification of nucleocytoplasmic proteins (O-GlcNAc) confers stress tolerance to multiple forms of cellular injury, we explored the role(s) of O-GlcNAc in the regulation of heat shock protein (HSP) expression. Using a cell line in which deletion of the O-GlcNAc transferase (OGT; the enzyme that adds O-GlcNAc) can be induced by 4-hydroxytamoxifen, we screened the expression of 84 HSPs using quantitative reverse transcriptase PCR. In OGT null cells the stress-induced expression of 18 molecular chaperones, including HSP72, were reduced. GSK-3β promotes apoptosis through numerous pathways, including phosphorylation of heat shock factor 1 (HSF1) at Ser(303) (Ser(P)(303) HSF1), which inactivates HSF1 and inhibits HSP expression. In OGT null cells we observed increased Ser(P)(303) HSF1; conversely, in cells in which O-GlcNAc levels had been elevated, reduced Ser(P)(303) HSF1 was detected. These data, combined with those showing that inhibition of GSK-3β in OGT null cells recovers HSP72 expression, suggests that O-GlcNAc regulates the activity of GSK-3β. In OGT null cells, stress-induced inactivation of GSK-3β by phosphorylation at Ser(9) was ablated providing a molecular basis for these findings. Together, these data suggest that stress-induced GlcNAcylation increases HSP expression through inhibition of GSK-3β.

Elevated O-GlcNAc-dependent signaling through inducible mOGT expression selectively triggers apoptosis

O-linked N-acetylglucosamine transferase (OGT) catalyzes O-GlcNAc addition to numerous cellular proteins including transcription and nuclear pore complexes and plays a key role in cellular signaling. One differentially spliced isoform of OGT is normally targeted to mitochondria (mOGT) but is quite cytotoxic when expressed in cells compared with the ncOGT isoform. To understand the basis of this selective cytotoxicity, we constructed a fully functional ecdysone-inducible GFP–OGT. Elevated GFP–OGT expression induced a dramatic increase in intracellular O-GlcNAcylated proteins. Furthermore, enhanced OGT expression efficiently triggered programmed cell death. Apoptosis was dependent upon the unique N-terminus of mOGT, and its catalytic activity. Induction of mOGT expression triggered programmed cell death in every cell type tested including INS-1, an insulin-secreting cell line. These studies suggest that deregulated activity of the mitochondrially targeted mOGT may play a role in triggering the programmed cell death observed with diseases such as diabetes mellitus and neurodegeneration.

O-GlcNAc Modification: Friend or Foe in Diabetic Cardiovascular Disease

O-Linked β-N-acetyl glucosaminylation (O-GlcNAcylation) is a dynamic post-translational modification that occurs on serine and threonine residues of cytosolic and nuclear proteins in all cell types, including those involved in the cardiovascular system. O-GlcNAcylation is thought to act in a manner analogous to protein phosphorylation. O-GlcNAcylation rapidly cycles on/off proteins in a time scale similar to that for phosphorylation/dephosphorylation of proteins. Several studies indicate that O-GlcNAc might induce nuclear localization of some transcription factors and may affect their DNA binding activities. However, at the cellular level, it has been shown that O-GlcNAc levels increase in response to stress and augmentation of this response suppresses cell survival. Increased levels of O-GlcNAc have been implicated as a pathogenic contributor to glucose toxicity and insulin resistance, which are major hallmarks of type 2 diabetes and diabetes-related cardiovascular complications. Thus, O-GlcNAc and its metabolic functions are not yet well-understood; focusing on the role of O-GlcNAc in the cardiovascular system is a viable target for biomedical investigation. In this review, we summarize our current understanding of the role of O-GlcNAc on the regulation of cell function and survival in the cardiovascular system.

Obesogenic high fat western diet induces oxidative stress and apoptosis in rat heart

Feeding Wistar rats a high calorie "Western" diet (45% fat) for up to 48 weeks induces obesity and cardiac dysfunction, while a high fat diet (60% fat) induces obesity only. Here we investigated the molecular "footprints" of the two forms of diet-induced obesity in the heart. In rats fed Western diet for a long term, cardiac mRNA transcript levels of malic enzyme were decreased (-72%, P < 0.05), suggesting impaired anaplerotic flux of the Krebs cycle (KC) and mitochondrial dysfunction. In addition, there was a marked decrease in the expression of the transcription factor MEF2C (myocyte enhancer factor 2C) and its target gene SERCA2a (sarco-endo-plasmic reticulum Ca(2+)-ATPase). Oxidative stress was reflected in reduced transcript levels of manganese superoxide dismutase, glutathione peroxidase 1, and increased protein levels of mitochondrial transcription factor A, suggesting compensatory mitochondrial biogenesis in the face of increased mitochondrial damage. Oxidant injury was accompanied by increased protein glycosylation, increased transcript levels of glutamine fructose 6-phosphate amidotransferase 2, and decreased protein levels of acetyl Co-A carboxylase. Lastly, apoptosis was evident by TUNEL positivity and elevated mRNA transcript levels and activity of caspase 3. Consistent with these results, protein levels of Bcl2 were markedly reduced. We conclude that inadequate supplementation of KC intermediates due to reduced levels of malic enzyme, downregulation of MEF2C and its target gene SERCA2a, oxidative stress, and programmed cell death are all potential contributors to contractile dysfunction of the heart.

Activation of the hexosamine biosynthesis pathway and protein O-GlcNAcylation modulate hypertrophic and cell signaling pathways in cardiomyocytes from diabetic mice

Patients with diabetes have a much greater risk of developing heart failure than non-diabetic patients, particularly in response to an additional hemodynamic stress such as hypertension or infarction. Previous studies have shown that increased glucose metabolism via the hexosamine biosynthesis pathway (HBP) and associated increase in O-linked-β-N-acetylglucosamine (O-GlcNAc) levels on proteins contributed to the adverse effects of diabetes on the heart. Therefore, in this study we tested the hypothesis that diabetes leads to impaired cardiomyocyte hypertrophic and cell signaling pathways due to increased HBP flux and O-GlcNAc modification on proteins. Cardiomyocytes isolated from type 2 diabetic db/db mice and non-diabetic controls were treated with 1 μM ANG angiotensin II (ANG) and 10 μM phenylephrine (PE) for 24 h. Activation of hypertrophic and cell signaling pathways was determined by assessing protein expression levels of atrial natriuretic peptide (ANP), α-sarcomeric actin, p53, Bax and Bcl-2 and phosphorylation of p38, ERK and Akt. ANG II and PE significantly increased levels of ANP and α-actin and phosphorylation of p38 and ERK in the non-diabetic but not in the diabetic group; phosphorylation of Akt was unchanged irrespective of group or treatment. Constitutive Bcl-2 levels were lower in diabetic hearts, while there was no difference in p53 and Bax. Activation of the HBP and increased protein O-GlcNAcylation in non-diabetic cardiomyocytes exhibited a significantly decreased hypertrophic signaling response to ANG or PE compared to control cells. Inhibition of the HBP partially restored the hypertrophic signaling response of diabetic cardiomyocytes. These results suggest that activation of the HBP and protein O-GlcNAcylation modulates hypertrophic and cell signaling pathways in type 2 diabetes.

O-GlcNAc signaling in the cardiovascular system

Cardiovascular function is regulated at multiple levels. Some of the most important aspects of such regulation involve alterations in an ever-growing list of posttranslational modifications. One such modification orchestrates input from numerous metabolic cues to modify proteins and alter their localization and/or function. Known as the beta-O-linkage of N-acetylglucosamine (ie, O-GlcNAc) to cellular proteins, this unique monosaccharide is involved in a diverse array of physiological and pathological functions. This review introduces readers to the general concepts related to O-GlcNAc, the regulation of this modification, and its role in primary pathophysiology. Much of the existing literature regarding the role of O-GlcNAcylation in disease addresses the protracted elevations in O-GlcNAcylation observed during diabetes. In this review, we focus on the emerging evidence of its involvement in the cardiovascular system. In particular, we highlight evidence of protein O-GlcNAcylation as an autoprotective alarm or stress response. We discuss recent literature supporting the idea that promoting O-GlcNAcylation improves cell survival during acute stress (eg, hypoxia, ischemia, oxidative stress), whereas limiting O-GlcNAcylation exacerbates cell damage in similar models. In addition to addressing the potential mechanisms of O-GlcNAc-mediated cardioprotection, we discuss technical issues related to studying protein O-GlcNAcylation in biological systems. The reader should gain an understanding of what protein O-GlcNAcylation is and that its roles in the acute and chronic disease settings appear distinct.

Hyperglycemia-mediated activation of the hexosamine biosynthetic pathway results in myocardial apoptosis

The mechanisms mediating hyperglycemia-mediated myocardial cell death are poorly defined. Since elevated flux through the hexosamine biosynthetic pathway (HBP) is closely linked with the diabetic phenotype, we hypothesized that hyperglycemia-mediated oxidative stress results in greater O-GlcNAcylation (HBP end product) of the proapoptotic peptide BAD, thereby increasing myocardial apoptosis. H9c2 cardiomyoblasts were exposed to high glucose (33 mM) ± HBP modulators ± antioxidant treatment for 5 days vs. matched controls (5.5 mM), and we subsequently evaluated apoptosis by immunoblotting, immunofluorescence staining, and caspase activity measurements. In vitro reactive oxygen species (ROS) levels were quantified by 2′,7′-dichlorodihydrofluorescein diacetate staining (fluorescence microscopy and flow cytometry). We determined total and BAD O-GlcNAcylation, respectively, by immunoblotting and immunofluorescence microscopy. The current study shows that high glucose treatment of cells significantly increased the degree of apoptosis. In parallel, overall O-GlcNAcylation, BAD O-GlcNAcylation, and ROS levels were increased. HBP inhibition and antioxidant treatment attenuated these effects, while increased end product levels exacerbated it. As BAD-Bcl-2 dimer formation enhances apoptosis, we performed immunoprecipitation analysis and colocalization and found increased dimerization in cells exposed to hyperglycemia. Our study identified a novel pathway whereby hyperglycemia results in greater oxidative stress and increased HBP activation and BAD O-GlcNAcylation in H9c2 cardiomyoblasts. Since greater BAD-Bcl-2 dimerization increases myocardial apoptosis, this pathway may play a crucial role in diabetes-related onset of heart diseases.

Diabetic cardiomyopathy: signaling defects and therapeutic approaches

Diabetes mellitus is the world's fastest growing disease with high morbidity and mortality rates, predominantly as a result of heart failure. A significant number of diabetic patients exhibit diabetic cardiomyopathy; that is, left ventricular dysfunction independent of coronary artery disease or hypertension. The pathogenesis of diabetic cardiomyopathy is complex, and is characterized by dysregulated lipid metabolism, insulin resistance, mitochondrial dysfunction and disturbances in adipokine secretion and signaling. These abnormalities lead to impaired calcium homeostasis, ultimately resulting in lusitropic and inotropic defects. This article discusses the impact of these hallmark factors in diabetic cardiomyopathy, and concludes with a survey of available and emerging therapeutic modalities.

Inhibition of phospholamban phosphorylation by O-GlcNAcylation: implications for diabetic cardiomyopathy

Cardiac-type sarco(endo)plasmic reticulum Ca(2)-ATPase (SERCA2a) plays a major role in cardiac muscle contractility. Phospholamban (PLN) regulates the function of SERCA2a via its Ser(16)-phosphorylation. Since it has been proposed that the Ser/Thr residues on cytoplasmic and nuclear proteins are modified by O-linked N-acetylglucosamine (O-GlcNAc), we examined the effect of O-GlcNAcylation on PLN function in rat adult cardiomyocytes. Studies using enzymatic labeling and co-immunoprecipitation of wild type and a series of mutants of PLN showed that PLN was O-GlcNAcylated and Ser(16) of PLN might be the site for O-GlcNAcylation. In cardiomyocytes treated with O-(2-acetamido-2-deoxy-D-glucopyranosylidene)amino-N-phenylcarbamate (PUGNAc), the O-GlcNAcylation was significantly increased compared to non-treated cells. Simultaneously, Ser(16)-phosphorylation of PLN was reduced. In Chinese hamster ovary cells where PLN cDNA and O-GlcNAc transferase siRNA were co-transfected, the Ser(16)-phosphorylation of PLN was significantly increased compared to controls. The same results were observed in heart homogenates from diabetic rats. In a co-immunoprecipitation of PLN with SERCA2a, the physical interaction between the two proteins was increased in PUGNAc-treated cardiomyocytes. Unlike non-treated cells, the activity of SERCA2a and the profiles of calcium transients in PUGNAc-treated cardiomyocytes were not significantly changed even after treatment with catecholamine. These data suggest that PLN is O-GlcNAcylated to induce the inhibition of its phosphorylation, which correlates to the deterioration of cardiac function. This might define a novel mechanism by which PLN regulation of SERCA2a is altered under conditions where O-GlcNAcylation is increased, such as those occurring in diabetes.

The cardiomyocyte circadian clock: emerging roles in health and disease

Circadian misalignment has been implicated in the development of obesity, diabetes mellitus, and cardiovascular disease. Time-of-day-dependent synchronization of organisms with their environment is mediated by circadian clocks. This cell autonomous mechanism has been identified within all cardiovascular-relevant cell types, including cardiomyocytes. Recent molecular- and genetic-based studies suggest that the cardiomyocyte circadian clock influences multiple myocardial processes, including transcription, signaling, growth, metabolism, and contractile function. Following an appreciation of its physiological roles, the cardiomyocyte circadian clock has recently been linked to the pathogenesis of heart disease in response to adverse stresses, such as ischemia/reperfusion, in animal models. The purpose of this review is therefore to highlight recent advances regarding the roles of the cardiomyocyte circadian clock in both myocardial physiology and pathophysiology (ie, health and disease).

Myocardium of type 2 diabetic and obese patients is characterized by alterations in sphingolipid metabolic enzymes but not by accumulation of ceramide

Data from animal experiments strongly suggest that ceramide is an important mediator of lipotoxicity in the heart and that accumulation of ceramide contributes to cardiomyocyte apoptosis associated with type 2 diabetes and obesity. However, it remains unknown whether a similar relationship is present also in the human heart. Therefore, we aimed to examine whether myocardial apoptosis in obese and type 2 diabetic patients is associated with elevated ceramide level. The study included 11 lean and 26 overweight or moderately obese subjects without (n = 11, OWT) or with (n = 15, T2D-OWT) a history of type 2 diabetes. Samples of the right atrial appendage were obtained from patients at the time of coronary bypass surgery. Compared with lean subjects, the extent of DNA fragmentation (a marker of apoptosis) was significantly higher in the myocardium of OWT patients and increased further in T2D-OWT subjects. However, the content of ceramide and sphingoid bases remained stable. Interestingly, the mRNA level of enzymes involved in synthesis and degradation of ceramide including serine palmitoyltransferase, sphingosine kinase 1, neutral sphingomyelinase, and ceramidases was markedly higher in the myocardium of OWT and T2D-OWT patients compared with lean subjects. Our results indicate that in the human heart, or at least in the atrium, ceramide is not a major factor in cardiomyocyte apoptosis associated with obesity and type 2 diabetes.

Protein O-GlcNAcylation: A critical regulator of the cellular response to stress

The post-translational modification of serine and threonine residues of nuclear and cytoplasmic proteins by the O-linked attachment of the monosaccharide ß-N-acetyl-glucosamine (O-GlcNAc) is a highly dynamic and ubiquitous protein 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 chronic disease processes. However, there is increasing evidence that O-GlcNAc levels are increased in response to stress and that acute augmentation of this response is cytoprotective, at least in the short term. Conversely, a reduction in O-GlcNAc levels appears to be associated with decreased cell survival in response to an acute stress. Here we summarize our current understanding of protein O-GlcNAcylation on the cellular response to stress and in mediating cellular protective mechanisms focusing primarily on the cardiovascular system as an example. We consider the potential link between O-GlcNAcylation and cardiomyocyte calcium homeostasis and explore the parallels between O-GlcNAc signaling and redox signaling. We also discuss the apparent paradox between the reported adverse effects of increased O-GlcNAcylation with its recently reported role in mediating cell survival mechanisms.