Impaired vasodilator activity in deoxycorticosterone acetate-salt hypertension is associated with increased protein O-GlcNAcylation

The multifaceted role of intracellular glycosylation in cytoprotection and heart disease

The modification of nuclear, cytoplasmic, and mitochondrial proteins by O-linked β-N-actylglucosamine (O-GlcNAc) is an essential posttranslational modification that is common in metozoans. O-GlcNAc is cycled on and off proteins in response to environmental and physiological stimuli impacting protein function, which, in turn, tunes pathways that include transcription, translation, proteostasis, signal transduction, and metabolism. One class of stimulus that induces rapid and dynamic changes to O-GlcNAc is cellular injury, resulting from environmental stress (for instance, heat shock), hypoxia/reoxygenation injury, ischemia reperfusion injury (heart attack, stroke, trauma hemorrhage), and sepsis. Acute elevation of O-GlcNAc before or after injury reduces apoptosis and necrosis, suggesting that injury-induced changes in O-GlcNAcylation regulate cell fate decisions. However, prolonged elevation or reduction in O-GlcNAc leads to a maladaptive response and is associated with pathologies such as hypertrophy and heart failure. In this review, we discuss the impact of O-GlcNAc in both acute and prolonged models of injury with a focus on the heart and biological mechanisms that underpin cell survival.

Roles of O-GlcNAcylation in Mitochondrial Homeostasis and Cardiovascular Diseases

Protein posttranslational modifications are important factors that mediate the fine regulation of signaling molecules. O-linked β-N-acetylglucosamine-modification (O-GlcNAcylation) is a monosaccharide modification on N-acetylglucosamine linked to the hydroxyl terminus of serine and threonine of proteins. O-GlcNAcylation is responsive to cellular stress as a reversible and posttranslational modification of nuclear, mitochondrial and cytoplasmic proteins. Mitochondrial proteins are the main targets of O-GlcNAcylation and O-GlcNAcylation is a key regulator of mitochondrial homeostasis by directly regulating the mitochondrial proteome or protein activity and function. Disruption of O-GlcNAcylation is closely related to mitochondrial dysfunction. More importantly, the O-GlcNAcylation of cardiac proteins has been proven to be protective or harmful to cardiac function. Mitochondrial homeostasis is crucial for cardiac contractile function and myocardial cell metabolism, and the imbalance of mitochondrial homeostasis plays a crucial role in the pathogenesis of cardiovascular diseases (CVDs). In this review, we will focus on the interactions between protein O-GlcNAcylation and mitochondrial homeostasis and provide insights on the role of mitochondrial protein O-GlcNAcylation in CVDs.

O-GlcNAcylation: cellular physiology and therapeutic target for human diseases

O-linked-β-N-acetylglucosamine (O-GlcNAcylation) is a distinctive posttranslational protein modification involving the coordinated action of O-GlcNAc transferase and O-GlcNAcase, primarily targeting serine or threonine residues in various proteins. This modification impacts protein functionality, influencing stability, protein-protein interactions, and localization. Its interaction with other modifications such as phosphorylation and ubiquitination is becoming increasingly evident. Dysregulation of O-GlcNAcylation is associated with numerous human diseases, including diabetes, nervous system degeneration, and cancers. This review extensively explores the regulatory mechanisms of O-GlcNAcylation, its effects on cellular physiology, and its role in the pathogenesis of diseases. It examines the implications of aberrant O-GlcNAcylation in diabetes and tumorigenesis, highlighting novel insights into its potential role in cardiovascular diseases. The review also discusses the interplay of O-GlcNAcylation with other protein modifications and its impact on cell growth and metabolism. By synthesizing current research, this review elucidates the multifaceted roles of O-GlcNAcylation, providing a comprehensive reference for future studies. It underscores the potential of targeting the O-GlcNAcylation cycle in developing novel therapeutic strategies for various pathologies.

The O-GlcNAc dichotomy: when does adaptation become pathological?

O-Linked attachment of β-N-acetylglucosamine (O-GlcNAc) on serine and threonine residues of nuclear, cytoplasmic, and mitochondrial proteins is a highly dynamic and ubiquitous post-translational modification that impacts the function, activity, subcellular localization, and stability of target proteins. Physiologically, acute O-GlcNAcylation serves primarily to modulate cellular signaling and transcription regulatory pathways in response to nutrients and stress. To date, thousands of proteins have been revealed to be O-GlcNAcylated and this number continues to grow as the technology for the detection of O-GlcNAc improves. The attachment of a single O-GlcNAc is catalyzed by the enzyme O-GlcNAc transferase (OGT), and their removal is catalyzed by O-GlcNAcase (OGA). O-GlcNAcylation is regulated by the metabolism of glucose via the hexosamine biosynthesis pathway, and the metabolic abnormalities associated with pathophysiological conditions are all associated with increased flux through this pathway and elevate O-GlcNAc levels. While chronic O-GlcNAcylation is well associated with cardiovascular dysfunction, only until recently, and with genetically modified animals, has O-GlcNAcylation as a contributing mechanism of cardiovascular disease emerged. This review will address and critically evaluate the current literature on the role of O-GlcNAcylation in vascular physiology, with a view that this pathway can offer novel targets for the treatment and prevention of cardiovascular diseases.

Differential Detection of O-GlcNAcylated proteins in the heart using antibodies

Thousands of mammalian intracellular proteins are dynamically modified by O-linked β-N-acetylglucosamine (O-GlcNAc). Global changes in O-GlcNAcylation have been associated with the development of cardiomyopathy, heart failure, hypertension, and neurodegenerative disease. Levels of O-GlcNAc in cells and tissues can be detected using numerous approaches; however, immunoblotting using GlcNAc-specific antibodies and lectins is commonplace. The goal of this study was to optimize the detection of O-GlcNAc in heart lysates by immunoblotting. Using a combination of tissue fractionation, immunoblotting, and galactosyltransferase labeling, as well as hearts from wild-type and O-GlcNAc transferase transgenic mice, we demonstrate that contractile proteins in the heart are differentially detected by two commercially available antibodies (CTD110.6 and RL2). As CTD110.6 displays poor reactivity toward contractile proteins, and as these proteins represent a major fraction of the heart proteome, a better assessment of cardiac O-GlcNAcylation is obtained in total tissue lysates with RL2. The data presented highlight tissue lysis approaches that should aid the assessment of the cardiac O-GlcNAcylation by immunoblotting.

Physiological and pathological evidence of O-GlcNAcylation regulation during pregnancy related process

O-GlcNAcylation is a dynamic and reversible post-translational modification (PTM) controlled by the enzymes O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA). Changes in its expression lead to a breakdown in cellular homeostasis, which is linked to several pathological processes. Placentation and embryonic development are periods of high cell activity, and imbalances in cell signaling pathways can result in infertility, miscarriage, or pregnancy complications. O-GlcNAcylation is involved in cellular processes such as genome maintenance, epigenetic regulation, protein synthesis/degradation, metabolic pathways, signaling pathways, apoptosis, and stress response. Trophoblastic differentiation/invasion and placental vasculogenesis, as well as zygote viability and embryonic neuronal development, are all dependent on O-GlcNAcylation. This PTM is required for pluripotency, which is a required condition for embryonic development. Further, this pathway is a nutritional sensor and cell stress marker, which is primarily measured by the OGT enzyme and its product, protein O-GlcNAcylation. Yet, this post-translational modification is enrolled in metabolic and cardiovascular adaptations during pregnancy. Finally, evidence of how O-GlcNAc impacts pregnancy during pathological conditions such as hyperglycemia, gestational diabetes, hypertension, and stress disorders are reviewed. Considering this scenario, progress in understanding the role of O- GlcNAcylation in pregnancy is required.

O-GlcNAcylation in Renal (Patho)Physiology

Kidneys maintain internal milieu homeostasis through a well-regulated manipulation of body fluid composition. This task is performed by the correlation between structure and function in the nephron. Kidney diseases are chronic conditions impacting healthcare programs globally, and despite efforts, therapeutic options for its treatment are limited. The development of chronic degenerative diseases is associated with changes in protein O-GlcNAcylation, a post-translation modification involved in the regulation of diverse cell function. O-GlcNAcylation is regulated by the enzymatic balance between O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA) which add and remove GlcNAc residues on target proteins, respectively. Furthermore, the hexosamine biosynthetic pathway provides the substrate for protein O-GlcNAcylation. Beyond its physiological role, several reports indicate the participation of protein O-GlcNAcylation in cardiovascular, neurodegenerative, and metabolic diseases. In this review, we discuss the impact of protein O-GlcNAcylation on physiological renal function, disease conditions, and possible future directions in the field.

Role of Posttranslational Modifications of Proteins in Cardiovascular Disease

Cardiovascular disease (CVD) has become a leading cause of mortality and morbidity globally, making it an urgent concern. Although some studies have been performed on CVD, its molecular mechanism remains largely unknown for all types of CVD. However, recent in vivo and in vitro studies have successfully identified the important roles of posttranslational modifications (PTMs) in various diseases, including CVD. Protein modification, also known as PTMs, refers to the chemical modification of specific amino acid residues after protein biosynthesis, which is a key process that can influence the activity or expression level of proteins. Studies on PTMs have contributed directly to improving the therapeutic strategies for CVD. In this review, we examined recent progress on PTMs and highlighted their importance in both physiological and pathological conditions of the cardiovascular system. Overall, the findings of this review contribute to the understanding of PTMs and their potential roles in the treatment of CVD.

O-Linked β-N-Acetylglucosamine Modification: Linking Hypertension and the Immune System

The O-linked β-N-acetylglucosamine modification (O-GlcNAcylation) of proteins dynamically regulates protein function, localization, stability, and interactions. This post-translational modification is intimately linked to cardiovascular disease, including hypertension. An increasing number of studies suggest that components of innate and adaptive immunity, active players in the pathophysiology of hypertension, are targets for O-GlcNAcylation. In this review, we highlight the potential roles of O-GlcNAcylation in the immune system and discuss how those immune targets of O-GlcNAcylation may contribute to arterial hypertension.

The dual role of the hexosamine biosynthetic pathway in cardiac physiology and pathophysiology

The heart is a highly metabolic organ with extensive energy demands and hence relies on numerous fuel substrates including fatty acids and glucose. However, oxidative stress is a natural by-product of metabolism that, in excess, can contribute towards DNA damage and poly-ADP-ribose polymerase activation. This activation inhibits key glycolytic enzymes, subsequently shunting glycolytic intermediates into non-oxidative glucose pathways such as the hexosamine biosynthetic pathway (HBP). In this review we provide evidence supporting the dual role of the HBP, i.e. playing a unique role in cardiac physiology and pathophysiology where acute upregulation confers cardioprotection while chronic activation contributes to the onset and progression of cardio-metabolic diseases such as diabetes, hypertrophy, ischemic heart disease, and heart failure. Thus although the HBP has emerged as a novel therapeutic target for such conditions, proposed interventions need to be applied in a context- and pathology-specific manner to avoid any potential drawbacks of relatively low cardiac HBP activity.

Protein O-GlcNAcylation in the heart

O-GlcNAcylation is a ubiquitous post-translational modification that is extremely labile and plays a significant role in physiology, including the heart. Sustained activation of cardiac O-GlcNAcylation is frequently associated with alterations in cellular metabolism, leading to detrimental effects on cardiovascular function. This is particularly true during conditions such as diabetes, hypertension, cardiac remodelling, heart failure and arrhythmogenesis. Paradoxically, transient elevation of cardiac protein O-GlcNAcylation can also exert beneficial effects in the heart. There is compelling evidence to suggest that a complex interaction between O-GlcNAcylation and phosphorylation also exists in the heart. Beyond direct functional consequences on cardiomyocytes, O-GlcNAcylation also acts indirectly by altering the function of transcription factors that affect downstream signalling. This review focuses on the potential cardioprotective role of protein O-GlcNAcylation during ischaemia-reperfusion injury, the deleterious consequences of chronically elevated O-GlcNAc levels, the interplay between O-GlcNAcylation and phosphorylation in the cardiomyocytes and the effects of O-GlcNAcylation on other major non-myocyte cell types in the heart.

O‑GlcNAcylation contributes to intermittent hypoxia‑associated vascular dysfunction via modulation of MAPKs but not CaMKII pathways

Intermittent hypoxia (IH) leads to vascular dysfunction, and O-linked-β-N-acetylglucosamine (O-GlcNAc)ylation may regulate vascular reactivity through the modulation of intracellular signaling. The present study hypothesized that O-GlcNAc modifications contributed to the vascular effects of acute IH (AIH) and chronic IH (CIH) through the MAPK and Ca²⁺/calmodulin-dependent kinase II (CaMKII) pathways. Rat aortic and mesenteric segments were incubated with DMSO, O-GlcNAcase (OGA) or O-GlcNAc transferase (OGT) inhibitor under either normoxic or AIH conditions for 3 h, and arterial function was then assessed. Meanwhile, arteries isolated from control and CIH rats were exposed to 3 h of incubation under normoxic conditions using DMSO, OGA or OGT as an inhibitor, before assessing arterial reactivity. CIH was found to increase the expression of vascular O-GlcNAc protein and OGT, phosphorylate p38 MAPK and ERK1/2, and decrease OGA levels, but it had no effects on phosphorylated CaMKII levels. OGA inhibition increased global O-GlcNAcylation and the phosphorylation of p38 MAPK, ERK1/2 and CaMKII, whereas OGT blockade had the opposite effects. OGA inhibition preserved acetylcholine-induced relaxation in AIH arteries, whereas OGT blockade attenuated the relaxation responses of arteries under normoxic conditions or undergoing AIH treatments. However, the impairment of acetylcholine dilation in CIH mesenteric arteries was improved. CIH artery contraction was increased following angiotensin II (Ang II) exposure. Blockade of p38 MAPK and ERK1/2, but not CaMKII, attenuated Ang II-induced contractile responses in CIH arteries isolated from the non-OGT inhibitor-treated groups. OGT inhibition significantly blocked contractile responses to Ang II and abolished the inhibitory effects of MAPK inhibitors. These findings indicated that O-GlcNAcylation regulates IH-induced vascular dysfunction, at least partly by modulating MAPK, but not CaMKII, signaling pathways.

Emerging roles of protein O-GlcNAcylation in cardiovascular diseases: Insights and novel therapeutic targets

Cardiovascular diseases (CVDs) are the leading cause of death globally. While the major focus of pharmacological and non-pharmacological interventions has been on targeting disease pathophysiology and limiting predisposing factors, our understanding of the cellular and molecular mechanisms underlying the pathogenesis of CVDs remains incomplete. One mechanism that has recently emerged is protein O-GlcNAcylation. This is a dynamic, site-specific reversible post-translational modification of serine and threonine residues on target proteins and is controlled by two enzymes: O-linked β-N-acetylglucosamine transferase (OGT) and O-linked β-N-acetylglucosaminidase (OGA). Protein O-GlcNAcylation alters the cellular functions of these target proteins which play vital roles in pathways that modulate vascular homeostasis and cardiac function. Through this review, we aim to give insights on the role of protein O-GlcNAcylation in cardiovascular diseases and identify potential therapeutic targets in this pathway for development of more effective medicines to improve patient outcomes.

Pregnancy decreases O-GlcNAc-modified proteins in systemic arteries of normotensive and spontaneously hypertensive rats

AIM

We determined the role played by O-linked N-acetylglucosamine (O-GlcNAc) of proteins in systemic arteries during late pregnancy in normotensive and hypertensive rats.

MAIN METHODS

O-GlcNAc levels and O-GlcNAc modification of endothelial nitric oxide synthase (eNOS) were determined in aorta (conductance vessel) and mesenteric arteries (resistance vessels) of non-pregnant (NP) and pregnant (P) Wistar rats and spontaneously hypertensive rats (SHR). Vascular O-GlcNAc-modified proteins, O-GlcNAcase (OGA) and O-GlcNAc transferase (OGT) expression, and OGA activity were analyzed. Concentration-response to phenylephrine (PE) curves were constructed for arteries with and without endothelium. Arteries were treated with vehicle or PugNAc (OGA inhibitor, 100 μmol/L) in the presence of L-NAME (NOS inhibitor, 100 μmol/L).

KEY FINDINGS

The content of vascular O-GlcNAc-modified proteins was lower, OGT and OGA expression did not change, and OGA activity was higher in arteries of P-Wistar rats and P-SHR compared to arteries of NP-groups. Reactivity to PE increased in arteries of P-Wistar rats treated with PugNAc compared to vehicle. O-GlcNAcylation of eNOS decreased in P-SHR compared to NP-SHR. PugNAc partially inhibited the effects of endothelium removal and L-NAME on reactivity to PE in arteries of P-Wistar rats. However, PugNAc did not alter reactivity to PE in arteries of P-SHR. Our data showed that pregnancy decreased the content of vascular O-GlcNAc-modified proteins.

SIGNIFICANCE

Increased OGA activity and decreased O-GlcNAc modification of eNOS boosts eNOS activity in arteries of P-Wistar rats. In P-SHR, altered OGA activity may lower the content of O-GlcNAc-modified proteins, but decreased OGT activity seems a potential mechanism to reduce glycosylation.

O-GlcNAcylation Mediates Glucose-Induced Alterations in Endothelial Cell Phenotype in Human Diabetes Mellitus

Background Posttranslational protein modification with O-linked N-acetylglucosamine (O-GlcNAc) is linked to high glucose levels in type 2 diabetes mellitus (T2DM) and may alter cellular function. We sought to elucidate the involvement of O-GlcNAc modification in endothelial dysfunction in patients with T2DM. Methods and Results Freshly isolated endothelial cells obtained by J-wire biopsy from a forearm vein of patients with T2DM (n=18) was compared with controls (n=10). Endothelial O-GlcNAc levels were 1.8-ford higher in T2DM patients than in nondiabetic controls (P=0.003). Higher endothelial O-GlcNAc levels correlated with serum fasting blood glucose level (r=0.433, P=0.024) and hemoglobin A1c (r=0.418, P=0.042). In endothelial cells from patients with T2DM, normal glucose conditions (24 hours at 5 mmol/L) lowered O-GlcNAc levels and restored insulin-mediated activation of endothelial nitric oxide synthase, whereas high glucose conditions (30 mmol/L) maintained both O-GlcNAc levels and impaired insulin action. Treatment of endothelial cells with Thiamet G, an O-GlcNAcase inhibitor, increased O-GlcNAc levels and blunted the improvement of insulin-mediated endothelial nitric oxide synthase phosphorylation by glucose normalization. Conclusions Taken together, our findings indicate a role for O-GlcNAc modification in the dynamic, glucose-induced impairment of endothelial nitric oxide synthase activation in endothelial cells from patients with T2DM. O-GlcNAc protein modification may be a treatment target for vascular dysfunction in T2DM.

O-GlcNAc impairs endothelial function in uterine arteries from virgin but not pregnant rats: The role of GSK3β

Increased O-Linked β-N-acetylglucosamine (O-GlcNAc) is observed in several pathologies, and unbalanced O-GlcNAcylation levels favor endothelial dysfunction. Whether augmented O-GlcNAc impacts the uterine artery (UA) function and how it affects the UA during pregnancy remains to be elucidated. We hypothesized that glucosamine treatment increases O-GlcNAc, leading to uterine artery dysfunction and this effect is prevented by pregnancy. Pregnant (P) and non-pregnant (NP) Wistar rats were treated with glucosamine (300 mg/kg; i.p.) for 21 days. Concentration response-curves (CRC) to acetylcholine (in the presence or absence of L-NAME) and sodium nitroprusside were performed in UAs. In NP rats, glucosamine treatment increased O-GlcNAc expression in UAs accompanied by decreased endothelium-dependent relaxation, which was abolished by L-NAME. Endothelial nitric oxide synthase (eNOS) activity and total Akt expression were decreased by glucosamine-treatment in NP rats. Further, NP rats treated with glucosamine displayed increased glycogen synthase kinase 3 beta (GSK3β) activation and O-GlcNAc-transferase (OGT) expression in the UA. P rats treated with glucosamine displayed decreased O-GlcNAc in UAs and it was accompanied by improved relaxation to acetylcholine, whereas eNOS and GSK3β activity and total Akt and OGT expression were unchanged. Sodium nitroprusside-induced relaxation was not changed in all groups, indicating that glucosamine treatment led to endothelial dysfunction in NP rats. The underlying mechanism is, at least in part, dependent on Akt/GSK3β/OGT modulation. We speculate that during pregnancy, hormonal alterations play a protective role in preventing O-GlcNAcylation-induced endothelial dysfunction in the UAs.

O-GlcNAc Transferase Regulates Angiogenesis in Idiopathic Pulmonary Arterial Hypertension

Idiopathic pulmonary arterial hypertension (IPAH) is considered a vasculopathy characterized by elevated pulmonary vascular resistance due to vasoconstriction and/or lung remodeling such as plexiform lesions, the hallmark of the PAH, as well as cell proliferation and vascular and angiogenic dysfunction. The serine/threonine hydroxyl-linked N-Acetylglucosamine (O-GlcNAc) transferase (OGT) has been shown to drive pulmonary arterial smooth muscle cell (PASMC) proliferation in IPAH. OGT is a cellular nutrient sensor that is essential in maintaining proper cell function through the regulation of cell signaling, proliferation, and metabolism. The aim of this study was to determine the role of OGT and O-GlcNAc in vascular and angiogenic dysfunction in IPAH. Primary isolated human control and IPAH patient PASMCs and pulmonary arterial endothelial cells (PAECs) were grown in the presence or absence of OGT inhibitors and subjected to biochemical assessments in monolayer cultures and tube formation assays, in vitro vascular sprouting 3D spheroid co-culture models, and de novo vascularization models in NODSCID mice. We showed that knockdown of OGT resulted in reduced vascular endothelial growth factor (VEGF) expression in IPAH primary isolated vascular cells. In addition, specificity protein 1 (SP1), a known stimulator of VEGF expression, was shown to have higher O-GlcNAc levels in IPAH compared to control at physiological (5 mM) and high (25 mM) glucose concentrations, and knockdown resulted in decreased VEGF protein levels. Furthermore, human IPAH PAECs demonstrated a significantly higher degree of capillary tube-like structures and increased length compared to control PAECs. Addition of an OGT inhibitor, OSMI-1, significantly reduced the number of tube-like structures and tube length similar to control levels. Assessment of vascular sprouting from an in vitro 3D spheroid co-culture model using IPAH and control PAEC/PASMCs and an in vivo vascularization model using control and PAEC-embedded collagen implants demonstrated higher vascularization in IPAH compared to control. Blocking OGT activity in these experiments, however, altered the vascular sprouting and de novo vascularization in IPAH similar to control levels when compared to controls. Our findings in this report are the first to describe a role for the OGT/O-GlcNAc axis in modulating VEGF expression and vascularization in IPAH. These findings provide greater insight into the potential role that altered glucose uptake and metabolism may have on the angiogenic process and the development of plexiform lesions. Therefore, we believe that the OGT/O-GlcNAc axis may be a potential therapeutic target for treating the angiogenic dysregulation that is present in IPAH.

Metabolic Stress and Cardiovascular Disease in Diabetes Mellitus: The Role of Protein O-GlcNAc Modification

Mammalian cells metabolize glucose primarily for energy production, biomass synthesis, and posttranslational glycosylation; and maintaining glucose metabolic homeostasis is essential for normal physiology of cells. Impaired glucose homeostasis leads to hyperglycemia, a hallmark of diabetes mellitus. Chronically increased glucose in diabetes mellitus promotes pathological changes accompanied by impaired cellular function and tissue damage, which facilitates the development of cardiovascular complications, the major cause of morbidity and mortality of patients with diabetes mellitus. Emerging roles of glucose metabolism via the hexosamine biosynthesis pathway (HBP) and increased protein modification via O-linked β-N-acetylglucosamine (O-GlcNAcylation) have been demonstrated in diabetes mellitus and implicated in the development of diabetic cardiovascular complications. This review will discuss the biological outcomes of the glucose metabolism via the hexosamine biogenesis pathway and protein O-GlcNAcylation in regulating cellular homeostasis, and highlight the regulations and contributions of elevated O-GlcNAcylation to the pathogenesis of diabetic cardiovascular disease.

Cholinergic drugs ameliorate endothelial dysfunction by decreasing O-GlcNAcylation via M3 AChR-AMPK-ER stress signaling

AIMS

Obesity is associated with increased cardiovascular morbidity and mortality. It is accompanied by augmented O-linked β-N-acetylglucosamine (O-GlcNAc) modification of proteins via increasing hexosamine biosynthetic pathway (HBP) flux. However, the changes and regulation of the O-GlcNAc levels induced by obesity are unclear.

MAIN METHODS

High fat diet (HFD) model was induced obesity in mice with or without the cholinergic drug pyridostigmine (PYR, 3 mg/kg/d) for 22 weeks and in vitro human umbilical vein endothelial cells (HUVECs) was treated with high glucose (HG, 30 mM) with or without acetylcholine (ACh).

KEY FINDINGS

PYR significantly reduced body weight, blood glucose, and O-GlcNAcylation levels and attenuated vascular endothelial cells detachment in HFD-fed mice. HG addition induced endoplasmic reticulum (ER) stress and increased O-GlcNAcylation levels and apoptosis in HUVECs in a time-dependent manner. Additionally, HG decreased levels of phosphorylated AMP-activated protein kinase (AMPK). Interestingly, ACh significantly blocked damage to HUVECs induced by HG. Furthermore, the effects of ACh on HG-induced ER stress, O-GlcNAcylation, and apoptosis were prevented by treating HUVECs with 4-diphenylacetoxy-N-methylpiperidine methiodide (4-DAMP, a selective M3 AChR antagonist) or compound C (Comp C, an AMPK inhibitor). Treatment with 5-aminoimidazole-4-carboxamide ribose (AICAR, an AMPK activator), 4-phenyl butyric acid (4-PBA, an ER stress inhibitor), and 6-diazo-5-oxonorleucine (DON, a GFAT antagonist) reproduced a similar effect with ACh.

SIGNIFICANCE

Activation of cholinergic signaling ameliorated endothelium damage, reduced levels of ER stress, O-GlcNAcylation, and apoptosis in mice and HUVECs under obese conditions, which may function through M3 AChR-AMPK signaling.

O-Glycosylation with O-linked β-N-acetylglucosamine increases vascular contraction: Possible modulatory role on Interleukin-10 signaling pathway

AIMS

The interleukin-10 (IL-10) is an immuno-regulatory cytokine that plays a protective effect in the vasculature. IL-10 binding to its receptor, activating the IL-10/JAK1/STAT3 cascade to exert its effects. Therefore, STAT3 phosphorylation is essential for IL-10 actions. O-Glycosylation with linked β-N-acetylglucosamine (O-GlcNAc) is a post-translational modification able to regulate many proteins by interfering with protein on a phosphorylation level. Our aim was to determine whether O-GlcNAc promotes the inhibition of IL-10-pathway (JAK1/STAT3/IL-10), inactivationg its action in the vasculature.

MAIN METHODS

Mice (C57BL/6) aortic segments were incubated with vehicle or Thiamet G (0.1 mM, for 24 h) to increase global O-GlcNAc levels. Aortas from knockout mice for IL-10 were also used. Vascular reactivity and western blot tests were performed to evaluate protein expression.

KEY FINDINGS

High levels of O-GlcNAc, induced by Thiamet G incubation, increased vascular expression of JAK1, but decreased expression and activity of STAT3. In addition, IL-10 levels were diminished in arteries treated with Thiamet G. Absence of IL-10, as well as augmented O-GlcNAcylation, increased vascular reactivity to constrictor stimuli, an effect that was abolished by ERK 1/2 inhibitor. High levels of O-GlcNAc and the absence of IL-10 also leads to increased vascular expression of ERK1/2.

SIGNIFICANCE

Our data suggest that O-GlcNAc modification seems to (dys)regulate IL-10 signaling pathway and consequently, compromise the protective effect of this cytokine in vasculature. It is possible that there is a promising relationship in pathophysiological conditions where changes in O-GlcNAcylation and IL-10 levels are observed, such as hypertension and diabetes.

O-GlcNAcylation reduces proximal tubule protein reabsorption and promotes proteinuria in spontaneously hypertensive rats

Hypertensive individuals are at greater risk for developing chronic kidney disease (CKD). Reducing proteinuria has been suggested as a possible therapeutic approach to treat CKD. However, the mechanisms underlying the development of proteinuria in hypertensive conditions are incompletely understood. Cardiac and vascular dysfunction is associated with changes in the O-GlcNAcylation pathway in hypertensive models. We hypothesized that O-GlcNAcylation is also involved in renal damage, especially development of proteinuria, associated with hypertension. Using the spontaneously hypertensive rat (SHR) model, we observed higher renal cortex O-GlcNAcylation, glutamine-fructose aminotransferase (GFAT), and O-GlcNAc transferase (OGT) protein expression, which positively correlated with proteinuria. Interestingly, this was observed in hypertensive, but not pre-hypertensive, rats. Pharmacological inhibition of GFAT decreased renal cortex O-GlcNAcylation, proteinuria, and albuminuria in SHR. Using a proximal tubule cell line, we observed that increased O-GlcNAcylation reduced megalin surface expression and albumin endocytosis in vitro, and the effects were correlated in vivo Moreover, megalin is O-GlcNAcylated both in vitro and in vivo In conclusion, our results demonstrate a new mechanism involved in hypertension-associated proteinuria.

O-linked N-acetyl-glucosamine deposition in placental proteins varies according to maternal glycemic levels

AIMS

Hyperglycemia increases glycosylation with O-linked N-acetyl-glucosamine (O-GlcNAc) contributing to placental dysfunction and fetal growth impairment. Our aim was to determine how O-GlcNAc levels are affected by hyperglycemia and the O-GlcNAc distribution in different placental regions.

MAIN METHODS

Female Wistar rats were divided into the following groups: severe hyperglycemia (>300 mg/dL; n = 5); mild hyperglycemia (>140 mg/dL, at least than two time points during oral glucose tolerance test; n = 7) or normoglycemia (<120 mg/dL; n = 6). At 21 days of pregnancy, placental tissue was collected and processed for morphometry and immunohistochemistry analyses, or properly stored at -80 °C for protein quantification by western blot.

KEY FINDINGS

Placental index was increased only in severe hyperglycemic rats. Morphometric analysis showed increased junctional zone and decreased labyrinth region in placentas exclusively from the severe hyperglycemic group. Proteins targeted by O-GlcNAc were detected in all regions, with increased O-GlcNAc levels in the hyperglycemic group compared to control and mild hyperglycemic rats. Proteins in endothelial and trophoblast cells were the main target for O-GlcNAc. Whereas no changes in O-GlcNAc transferase (OGT) expression were detected, O-GlcNAcase (OGA) expression was reduced in placentas from the severe hyperglycemic group and augmented in placentas from the mild hyperglycemic group, compared with their respective control groups.

SIGNIFICANCE

Placental O-GlcNAc overexpression may contribute to placental dysfunction, as indicated by the placental index. Additionally, morphometric alterations, occurring simultaneously with increased O-GlcNAc accumulation in the placental tissue may contribute to placental dysfunction during hyperglycemia.

Brief Overview of a Decade of Genome-Wide Association Studies on Primary Hypertension

Primary hypertension is widely believed to be a complex polygenic disorder with the manifestation influenced by the interactions of genomic and environmental factors making identification of susceptibility genes a major challenge. With major advancement in high-throughput genotyping technology, genome-wide association study (GWAS) has become a powerful tool for researchers studying genetically complex diseases. GWASs work through revealing links between DNA sequence variation and a disease or trait with biomedical importance. The human genome is a very long DNA sequence which consists of billions of nucleotides arranged in a unique way. A single base-pair change in the DNA sequence is known as a single nucleotide polymorphism (SNP). With the help of modern genotyping techniques such as chip-based genotyping arrays, thousands of SNPs can be genotyped easily. Large-scale GWASs, in which more than half a million of common SNPs are genotyped and analyzed for disease association in hundreds of thousands of cases and controls, have been broadly successful in identifying SNPs associated with heart diseases, diabetes, autoimmune diseases, and psychiatric disorders. It is however still debatable whether GWAS is the best approach for hypertension. The following is a brief overview on the outcomes of a decade of GWASs on primary hypertension.

Rhynchophylline Ameliorates Endothelial Dysfunction via Src-PI3K/Akt-eNOS Cascade in the Cultured Intrarenal Arteries of Spontaneous Hypertensive Rats

Objectives: To examine the protective effect of Rhynchophylline (Rhy) on vascular endothelial function in spontaneous hypertensive rats (SHRs) and the underlying mechanism. Methods: Intrarenal arteries of SHRs and Wistar rats were suspended in myograph for force measurement. Expression and phosphorylation of endothelial nitric oxide (NO) synthase (eNOS), Akt, and Src kinase (Src) were examined by Western blotting. NO production was assayed by ELISA. Results: Rhy time- and concentration-dependently improved endothelium-dependent relaxation in the renal arteries from SHRs, but had no effect on endothelium-independent relaxation in SHR renal arteries. Wortmannin (an inhibitor of phosphatidylinositol 3-kinase) or PP2 (an inhibitor of Src) inhibited the improvement of relaxation in response to acetylcholine by 12 h-incubation with 300 μM Rhy. Western blot analysis revealed that Rhy elevated phosphorylations of eNOS, Akt, and Src in SHR renal arteries. Moreover, wortmannin reversed the increased phosphorylations of Akt and eNOS induced by Rhy, but did not affect the phosphorylation of Src. Furthermore, the enhanced phosphorylations of eNOS, Akt, and Src were blunted by PP2. Importantly, Rhy increased NO production and this effect was blocked by inhibition of Src or PI3K/Akt. Conclusion: The present study provides evidences for the first time that Rhy ameliorates endothelial dysfunction in SHRs through the activation of Src-PI3K/Akt-eNOS signaling pathway.

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

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

O-GlcNAcylation and cardiovascular disease

The post-translational modification of serine and threonine residues of proteins found in numerous subcellular locations by O-linked N-acetylglucosamine (O-GlcNAc) is emerging as a key mediator of many cardiovascular pathophysiological processes. Early studies implicated increased protein O-GlcNAcylation as contributing to the cardiovascular complications associated with diabetes, whereas subsequent studies demonstrated that acute increases in O-GlcNAc levels were protective against ischemia/reperfusion injury. There is now a growing understanding that O-GlcNAc modification of proteins influences numerous cellular functions, including transcription, protein turnover, calcium handling, and bioenergetics. As a result, a more nuanced view of the role of protein O-GlcNAcylation in the cardiovascular system is emerging along with the recognition that it is required for normal cellular function and homeostasis. Consequently, the impact of changes in O-GlcNAc cycling due to stress or disease on the heart is complex and highly dependent on the specific context of these events. The goal of this review is to provide an overview of some of the more recent advances in our understanding of the role O-GlcNAcylation plays in mediating cardiovascular function and disease.

Antihypertensive effects of peroxisome proliferator-activated receptor-β/δ activation

Peroxisome proliferator-activated receptors (PPARs) are members of the nuclear hormone receptor superfamily of ligand-activated transcription factors, which is composed of three members encoded by distinct genes: PPARα, PPARβ/δ, and PPARγ. The biological actions of PPARα and PPARγ and their potential as a cardiovascular therapeutic target have been extensively reviewed, whereas the biological actions of PPARβ/δ and its effectiveness as a therapeutic target in the treatment of hypertension remain less investigated. Preclinical studies suggest that pharmacological PPARβ/δ activation induces antihypertensive effects in direct [spontaneously hypertensive rat (SHR), ANG II, and DOCA-salt] and indirect (dyslipemic and gestational) models of hypertension, associated with end-organ damage protection. This review summarizes mechanistic insights into the antihypertensive effects of PPARβ/δ activators, including molecular and functional mechanisms. Pharmacological PPARβ/δ activation induces genomic actions including the increase of regulators of G protein-coupled signaling (RGS), acute nongenomic vasodilator effects, as well as the ability to improve the endothelial dysfunction, reduce vascular inflammation, vasoconstrictor responses, and sympathetic outflow from central nervous system. Evidence from clinical trials is also examined. These preclinical and clinical outcomes of PPARβ/δ ligands may provide a basis for the development of therapies in combating hypertension.

High-fat diet increases O-GlcNAc levels in cerebral arteries: a link to vascular dysfunction associated with hyperlipidaemia/obesity?

Obesity and high fat intake induce alterations in vascular function and structure. Aberrant O-GlcNAcylation (O-GlcNAc) of vascular proteins has been implicated in vascular dysfunction associated with cardiovascular and metabolic diseases. In the present study, we tested the hypothesis that high-fat diet (HFD)-mediated increases in O-GlcNAc-modified proteins contribute to cerebrovascular dysfunction. O-GlcNAc-protein content was increased in arteries from male Wistar rats treated with a HFD (45% fat) for 12 weeks compared with arteries from rats on control diet (CD). HFD augmented body weight [(g) 550±10 compared with 502±10 CD], increased plasma triacylglycerols [(mg/dl) 160±20 compared with 95±15 CD] and increased contractile responses of basilar arteries to serotonin [5-hydroxytryptamine (5-HT)] [(pD2) 7.0±0.1 compared with 6.7±0.09 CD] and the thromboxane analogue 9,11-dideoxy-9α,11α-methanoepoxy prostaglandin F2α (U-46619) [(pD2) 7.2±0.1 compared with 6.8±0.09 CD]. Of importance, increased levels of O-GlcNAc [induced by 24 h-incubation of vessels with a potent inhibitor of O-GlcNAcase (OGA), O-(2-acetamido-2-deoxy-D-glucopyranosylidene)amino-N-phenylcarbamate (PugNAc)] increased basilar artery contractions in response to U-46619 [(pD2) 7.4±0.07 compared with 6.8±0.08 CD] and 5-HT [(pD2) 7.5±0.06 compared with 7.1±0.1 CD]. Vessels from rats on the HFD for 12 weeks and vessels treated with PugNAc displayed increased phosphorylation of p38 (Thr(180/182)) and extracellular signal-regulated kinase 1/2 (Erk1/2) (Ser(180/221)). Increased 5HT-induced contractions in arteries from rats on the HFD or in arteries incubated with PugNAc were abrogated by mitogen-activated protein kinase (MAPK) inhibitors. Our data show that HFD augments cerebrovascular O-GlcNAc and this modification contributes to increased contractile responses and to the activation of the MAPK pathway in the rat basilar artery.

O-GlcNAcase overexpression reverses coronary endothelial cell dysfunction in type 1 diabetic mice

Cardiovascular disease is the primary cause of morbidity and mortality in diabetes, and endothelial dysfunction is commonly seen in these patients. Increased O-linked N-acetylglucosamine (O-GlcNAc) protein modification is one of the central pathogenic features of diabetes. Modification of proteins by O-GlcNAc (O-GlcNAcylation) is regulated by two key enzymes: β-N-acetylglucosaminidase [O-GlcNAcase (OGA)], which catalyzes the reduction of protein O-GlcNAcylation, and O-GlcNAc transferase (OGT), which induces O-GlcNAcylation. However, it is not known whether reducing O-GlcNAcylation can improve endothelial dysfunction in diabetes. To examine the effect of endothelium-specific OGA overexpression on protein O-GlcNAcylation and coronary endothelial function in diabetic mice, we generated tetracycline-inducible, endothelium-specific OGA transgenic mice, and induced OGA by doxycycline administration in streptozotocin-induced type 1 diabetic mice. OGA protein expression was significantly decreased in mouse coronary endothelial cells (MCECs) isolated from diabetic mice compared with control MCECs, whereas OGT protein level was markedly increased. The level of protein O-GlcNAcylation was increased in diabetic compared with control mice, and OGA overexpression significantly decreased the level of protein O-GlcNAcylation in MCECs from diabetic mice. Capillary density in the left ventricle and endothelium-dependent relaxation in coronary arteries were significantly decreased in diabetes, while OGA overexpression increased capillary density to the control level and restored endothelium-dependent relaxation without changing endothelium-independent relaxation. We found that connexin 40 could be the potential target of O-GlcNAcylation that regulates the endothelial functions in diabetes. These data suggest that OGA overexpression in endothelial cells improves endothelial function and may have a beneficial effect on coronary vascular complications in diabetes.

Maintenance of GLUT4 expression in smooth muscle prevents hypertension-induced changes in vascular reactivity

Previous studies have shown that expression of GLUT4 is decreased in arterial smooth muscle of hypertensive rats and mice and that total body overexpression of GLUT4 in mice prevents enhanced arterial reactivity in hypertension. To demonstrate that the effect of GLUT4 overexpression on vascular responses is dependent on vascular smooth muscle GLUT4 rather than on some systemic effect we developed and tested smooth-muscle-specific GLUT4 transgenic mice (SMG4). When made hypertensive with angiotensin II, both wild-type and SMG4 mice exhibited similarly increased systolic blood pressure. Responsiveness to phenylephrine, serotonin, and prostaglandin F2α was significantly increased in endothelium-intact aortic rings from hypertensive wild-type mice but not in aortae of SMG4 mice. Inhibition of Rho-kinase equally reduced serotonin-stimulated contractility in aortae of hypertensive wild-type and SMG4-mice. In addition, acetylcholine-stimulated relaxation was significantly decreased in aortic rings of hypertensive wild-type mice, but not in rings of SMG4 mice. Inhibition of either prostacylin receptors or cyclooxygenase-2 reduced relaxation in rings of hypertensive SMG4 mice. Inhibition of cyclooxygenase-2 had no effect on relaxation in rings of hypertensive wild-type mice. Cyclooxygenase-2 protein expression was decreased in hypertensive wild-type aortae but not in hypertensive SMG4 aortae compared to nonhypertensive controls. Our results demonstrate that smooth muscle expression of GLUT4 exerts a major effect on smooth muscle contractile responses and endothelium-dependent vasorelaxation and that normal expression of GLUT4 in vascular smooth muscle is required for appropriate smooth muscle and endothelial responses.

MicroRNA-539 is up-regulated in failing heart, and suppresses O-GlcNAcase expression

Derangements in metabolism and related signaling pathways characterize the failing heart. One such signal, O-linked β-N-acetylglucosamine (O-GlcNAc), is an essential post-translational modification regulated by two enzymes, O-GlcNAc transferase and O-GlcNAcase (OGA), which modulate the function of many nuclear and cytoplasmic proteins. We recently reported reduced OGA expression in the failing heart, which is consistent with the pro-adaptive role of increased O-GlcNAcylation during heart failure; however, molecular mechanisms regulating these enzymes during heart failure remain unknown. Using miRNA microarray analysis, we observed acute and chronic changes in expression of several miRNAs. Here, we focused on miR-539 because it was predicted to target OGA mRNA. Indeed, co-transfection of the OGA-3'UTR containing reporter plasmid and miR-539 overexpression plasmid significantly reduced reporter activity. Overexpression of miR-539 in neonatal rat cardiomyocytes significantly suppressed OGA expression and consequently increased O-GlcNAcylation; conversely, the miR-539 inhibitor rescued OGA protein expression and restored O-GlcNAcylation. In conclusion, this work identifies the first target of miR-539 in the heart and the first miRNA that regulates OGA. Manipulation of miR-539 may represent a novel therapeutic target in the treatment of heart failure and other metabolic diseases.

Vascular O-GlcNAcylation augments reactivity to constrictor stimuli by prolonging phosphorylated levels of the myosin light chain

O-GlcNAcylation is a modification that alters the function of numerous proteins. We hypothesized that augmented O-GlcNAcylation levels enhance myosin light chain kinase (MLCK) and reduce myosin light chain phosphatase (MLCP) activity, leading to increased vascular contractile responsiveness. The vascular responses were measured by isometric force displacement. Thoracic aorta and vascular smooth muscle cells (VSMCs) from rats were incubated with vehicle or with PugNAc, which increases O-GlcNAcylation. In addition, we determined whether proteins that play an important role in the regulation of MLCK and MLCP activity are directly affected by O-GlcNAcylation. PugNAc enhanced phenylephrine (PE) responses in rat aortas (maximal effect, 14.2 ± 2 vs 7.9 ± 1 mN for vehicle, n=7). Treatment with an MLCP inhibitor (calyculin A) augmented vascular responses to PE (13.4 ± 2 mN) and abolished the differences in PE-response between the groups. The effect of PugNAc was not observed when vessels were preincubated with ML-9, an MLCK inhibitor (7.3 ± 2 vs 7.5 ± 2 mN for vehicle, n=5). Furthermore, our data showed that differences in the PE-induced contractile response between the groups were abolished by the activator of AMP-activated protein kinase (AICAR; 6.1 ± 2 vs 7.4 ± 2 mN for vehicle, n=5). PugNAc increased phosphorylation of myosin phosphatase target subunit 1 (MYPT-1) and protein kinase C-potentiated inhibitor protein of 17 kDa (CPI-17), which are involved in RhoA/Rho-kinase-mediated inhibition of myosin phosphatase activity. PugNAc incubation produced a time-dependent increase in vascular phosphorylation of myosin light chain and decreased phosphorylation levels of AMP-activated protein kinase, which decreased the affinity of MLCK for Ca(2+)/calmodulin. Our data suggest that proteins that play an important role in the regulation of MLCK and MLCP activity are directly affected by O-GlcNAcylation, favoring vascular contraction.

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.

O-GlcNAc and the cardiovascular system

The cardiovascular system is capable of robust changes in response to physiologic and pathologic stimuli through intricate signaling mechanisms. The area of metabolism has witnessed a veritable renaissance in the cardiovascular system. In particular, the post-translational β-O-linkage of N-acetylglucosamine (O-GlcNAc) to cellular proteins represents one such signaling pathway that has been implicated in the pathophysiology of cardiovascular disease. This highly dynamic protein modification may induce functional changes in proteins and regulate key cellular processes including translation, transcription, and cell death. In addition, its potential interplay with phosphorylation provides an additional layer of complexity to post-translational regulation. The hexosamine biosynthetic pathway generally requires glucose to form the nucleotide sugar, UDP-GlcNAc. Accordingly, O-GlcNAcylation may be altered in response to nutrient availability and cellular stress. Recent literature supports O-GlcNAcylation as an autoprotective response in models of acute stress (hypoxia, ischemia, oxidative stress). Models of sustained stress, such as pressure overload hypertrophy, and infarct-induced heart failure, may also require protein O-GlcNAcylation as a partial compensatory mechanism. Yet, in models of Type II diabetes, O-GlcNAcylation has been implicated in the subsequent development of vascular, and even cardiac, dysfunction. This review will address this apparent paradox and discuss the potential mechanisms of O-GlcNAc-mediated cardioprotection and cardiovascular dysfunction. This discussion will also address potential targets for pharmacologic interventions and the unique considerations related to such targets.

O-GlcNAcylation in cellular functions and human diseases

O-GlcNAcylation is dynamic and a ubiquitous post-translational modification. O-GlcNAcylated proteins influence fundamental functions of proteins such as protein-protein interactions, altering protein stability, and changing protein activity. Thus, aberrant regulation of O-GlcNAcylation contributes to the etiology of chronic diseases of aging, including cancer, cardiovascular disease, metabolic disorders, and Alzheimer's disease. Diverse cellular signaling systems are involved in pathogenesis of these diseases. O-GlcNAcylated proteins occur in many different tissues and cellular compartments and affect specific cell signaling. This review focuses on the O-GlcNAcylation in basic cellular functions and human diseases.

Chitosan oligosaccharides block LPS-induced O-GlcNAcylation of NF-κB and endothelial inflammatory response

It is known that chitosan oligosaccharides (COS) suppress LPS-induced vascular endothelial inflammatory response by mechanism involving NF-κB blockade. It remains unknown how COS inhibit NF-κB. We provided evidence both in cultured endothelial cells and mouse model supporting a new mechanism. Regardless of the endothelial cell types, the LPS-induced NF-κB-dependent inflammatory gene expression was suppressed by COS, which was associated with reduced NF-κB nucleus translocation. LPS enhanced O-GlcNAc modification of NF-κB/p65 and activated NF-κB pathway, which could be prevented either by siRNA knockdown of O-GlcNAc transferase (OGT) or pretreatment with COS. Inhibition of either mitogen-activated protein kinase or superoxide generation abolishes LPS-induced NF-κB O-GlcNAcylation. Consistently, aortic tissues from LPS-treated mice presented enhanced NF-κB/p65 O-GlcNAcylation in association with upregulated gene expression of inflammatory cytokines in vascular tissues; however, pre-administration of COS prevented these responses. In conclusion, COS decreased OGT-dependent O-GlcNAcylation of NF-κB and thereby attenuated LPS-induced vascular endothelial inflammatory response.

Short-term glucosamine infusion increases islet blood flow in anesthetized rats

Impaired glucose tolerance and type 2 diabetes in rodents are associated with increased islet blood flow. If this is important for modulation of the endocrine function is at present unknown. We evaluated if glucosamine infusion, which induces peripheral insulin resistance and glucose intolerance, could be used to acutely increase islet blood flow. We infused anaesthetized Sprague-Dawley rats for 2 h with glucosamine (6 mg/kg body weight), in some cases followed by glucose administration. The former induced a 2-fold increase in serum insulin concentrations while plasma glucose remained unchanged. In vitro an augmented insulin response to hyperglycemia and decreased insulin content in batch type islet incubations with glucosamine for 24 h were seen. After 2 h glucosamine exposure in vitro, insulin release was decreased. In vivo glucosamine infusion increased islet blood flow, without affecting other regional blood flow values. Glucose increased islet blood flow to the same extent in control and glucosamine-infused rats. When exposed to 10 mmol/L glucosamine arterioles of isolated perfused islets showed a 10% dilation of their vascular smooth muscle. Thus, application of this model leads to acute hyperinsulinemia in vivo but a decreased insulin release in vitro, which suggests that effects not located to β cells are responsible for the effects seen in vivo. An increased islet blood flow in previously healthy animals was also seen after glucose administration, which can be used to further dissect the importance of blood flow changes in islet function.

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.

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.

O-GlcNAcylation and oxidation of proteins: is signalling in the cardiovascular system becoming sweeter?

O-GlcNAcylation is an unusual form of protein glycosylation, where a single-sugar [GlcNAc (N-acetylglucosamine)] is added (via β-attachment) to the hydroxyl moiety of serine and threonine residues of nuclear and cytoplasmic proteins. A complex and extensive interplay exists between O-GlcNAcylation and phosphorylation. Many phosphorylation sites are also known glycosylation sites, and this reciprocal occupancy may produce different activities or alter the stability in a target protein. The interplay between these two post-translational modifications is not always reciprocal, as some proteins can be concomitantly phosphorylated and O-GlcNAcylated, and the adjacent phosphorylation or O-GlcNAcylation can regulate the addition of either moiety. Increased cardiovascular production of ROS (reactive oxygen species), termed oxidative stress, has been consistently reported in various chronic diseases and in conditions where O-GlcNAcylation has been implicated as a contributing mechanism for the associated organ injury/protection (for example, diabetes, Alzheimer's disease, arterial hypertension, aging and ischaemia). In the present review, we will briefly comment on general aspects of O-GlcNAcylation and provide an overview of what has been reported for this post-translational modification in the cardiovascular system. We will then specifically address whether signalling molecules involved in redox signalling can be modified by O-GlcNAc (O-linked GlcNAc) and will discuss the critical interplay between O-GlcNAcylation and ROS generation. Experimental evidence indicates that the interactions between O-GlcNAcylation and oxidation of proteins are important not only for cell regulation in physiological conditions, but also under pathological states where the interplay may become dysfunctional and thereby exacerbate cellular injury.

Epicatechin: endothelial function and blood pressure

Epidemiological studies indicate an inverse relationship between flavanol intake and the risk of cardiovascular disease. Potential mechanisms include their effects on endothelial function and hypertension. A number of studies have shown that flavanol-rich cocoa reduces blood pressure and endothelial dysfunction, whereas black tea may have opposite effects. These results highlight the importance of the different effects of the multitude of phytochemical constituents in these foods and the need for studying the individual flavanols. Epicatechin seems to be a major bioactive constituent of cocoa and other flavanol-rich foods and beverages. It has been shown to improve endothelial function in animals and humans. In salt-sensitive animal models of hypertension, epicatechin lowers blood pressure and the associated end-organ damage. Nitric oxide (NO) seems to play a key role in the protection of both hypertension and endothelial dysfunction.

Acute O-GlcNAcylation prevents inflammation-induced vascular dysfunction

Acute increases in cellular protein O-linked N-acetyl-glucosamine (O-GlcNAc) modification (O-GlcNAcylation) have been shown to have protective effects in the heart and vasculature. We hypothesized that d-glucosamine (d-GlcN) and Thiamet-G, two agents that increase protein O-GlcNAcylation via different mechanisms, inhibit TNF-α-induced oxidative stress and vascular dysfunction by suppressing inducible nitric oxide (NO) synthase (iNOS) expression. Rat aortic rings were incubated for 3h at 37°C with d-GlcN or its osmotic control l-glucose (l-Glc) or with Thiamet-G or its vehicle control (H(2)O) followed by the addition of TNF-α or vehicle (H(2)O) for 21 h. After incubation, rings were mounted in a myograph to assess arterial reactivity. Twenty-four hours of incubation of aortic rings with TNF-α resulted in 1) a hypocontractility to 60 mM K(+) solution and phenylephrine, 2) blunted endothelium-dependent relaxation responses to ACh and substance P, and 3) unaltered relaxing response to the Ca(2+) ionophore A-23187 and the NO donor sodium nitroprusside compared with aortic rings cultured in the absence of TNF-α. d-GlcN and Thiamet-G pretreatment suppressed the TNF-α-induced hypocontractility and endothelial dysfunction. Total protein O-GlcNAc levels were significantly higher in aortic segments treated with d-GlcN or Thiamet-G compared with controls. Expression of iNOS protein was increased in TNF-α-treated rings, and this was attenuated by pretreatment with either d-GlcN or Thiamet-G. Dense immunostaining for nitrotyrosylated proteins was detected in the endothelium and media of the aortic wall, suggesting enhanced peroxynitrite production by iNOS. These findings demonstrate that acute increases in protein O-GlcNAcylation prevent TNF-α-induced vascular dysfunction, at least in part, via suppression of iNOS expression.

Intravital microscopy reveals endothelial dysfunction in resistance arterioles in Angiotensin II-induced hypertension

It is known that hypertension is associated with endothelial dysfunction and that Angiotensin II (Ang II) is a key player in the pathogenesis of hypertension. We aimed to elucidate whether endothelial dysfunction is a specific feature of Ang II-mediated hypertension or a common finding of hypertension, independently of underlying etiology. We studied endothelial-dependent vasorelaxation in precapillary resistance arterioles and in various large-caliber conductance arteries in wild-type mice with Ang II-dependent hypertension (2-kidney 1-clip (2K1C) model) or Ang II-independent (volume overload) hypertension (1-kidney 1-clip model (1K1C)). Normotensive sham mice were used as controls. Aortic mechanical properties were also evaluated. Intravital microscopy of precapillary arterioles revealed a significantly impaired endothelium-dependent vasorelaxation in 2K1C mice compared with sham mice, as quantified by the ratio of acetylcholine (ACh)-induced over S-nitroso-N-acetyl-D,L-penicillamine (SNAP)-induced vasorelaxation (2K1C: 0.49±0.12 vs. sham: 0.87±0.11, P=0.018). In contrast, the ACh/SNAP ratio in volume-overload hypertension 1K1C mice was not significantly different from sham mice, indicating no specific endothelial dysfunction (1K1C: 0.77±0.27 vs. sham: 0.87±0.11, P=0.138). Mechanical aortic wall properties and endothelium-dependent vasorelaxation, assessed ex vivo in rings of large-caliber conductance (abdominal and thoracic aorta, carotid and femoral arteries), were not different between 2K1C, 1K1C and sham mice. Endothelial dysfunction is an early feature of Ang II- but not volume-overload-mediated hypertension. This occurs exclusively at the level of precapillary arterioles and not in conduit arteries. Our findings, if confirmed in clinical studies, will provide a better understanding of the pathophysiological mechanisms of hypertension.

Chronic ingestion of a Western diet increases O-linked-β-N-acetylglucosamine (O-GlcNAc) protein modification in the rat heart

AIMS

Protein O-GlcNAcylation is both a nutrient sensing and cellular stress response that mediates signal transduction in the heart. Chronically elevated O-GlcNAc has been associated with the development of cardiac dysfunction at both the cellular and organ levels in obesity, insulin resistance and diabetes. Development of these pathologies is often attributed to diets high in saturated fat and sugar (a "Western" diet; WES) but a role for O-GlcNAc in diet-induced cardiac dysfunction has not been established. The aims of this study were to examine the effect of chronic consumption of WES on cardiac O-GlcNAcylation and investigate associations of O-GlcNAc with cardiac function and markers of cellular stress.

MAIN METHODS

Young male rats received either a control diet (CON; n=9) or WES (n=8) diet for 52 weeks.

KEY FINDINGS

There was no evidence of cardiac dysfunction, advanced glycation endproduct (AGE) accumulation, pathological cardiac hypertrophy, calcium handling impairment, fibrosis or endoplasmic reticulum stress in WES hearts. However, cardiac O-GlcNAc protein, particularly in the higher molecular weight range, was significantly higher in WES hearts compared to CON (P<0.05). Protein levels of the enzymes that regulate O-GlcNAc attachment were not different between groups; thus, the increased O-GlcNAcylation in WES hearts appears to be due to increased nutrient availability rather than enzymatic regulation of cellular stress.

SIGNIFICANCE

These data suggest that diets high in saturated fat and sugar may contribute to the adverse effects of metabolic syndrome and diabetes by an O-GlcNAc-mediated process and that this may occur in the absence of overt cellular stress.

The roles of O-linked β-N-acetylglucosamine in cardiovascular physiology and disease

More than 1,000 proteins of the nucleus, cytoplasm, and mitochondria are dynamically modified by O-linked β-N-acetylglucosamine (O-GlcNAc), an essential post-translational modification of metazoans. O-GlcNAc, which modifies Ser/Thr residues, is thought to regulate protein function in a manner analogous to protein phosphorylation and, on a subset of proteins, appears to have a reciprocal relationship with phosphorylation. Like phosphorylation, O-GlcNAc levels change dynamically in response to numerous signals including hyperglycemia and cellular injury. Recent data suggests that O-GlcNAc appears to be a key regulator of the cellular stress response, the augmentation of which is protective in models of acute vascular injury, trauma hemorrhage, and ischemia-reperfusion injury. In contrast to these studies, O-GlcNAc has also been implicated in the development of hypertension and type II diabetes, leading to vascular and cardiac dysfunction. Here we summarize the current understanding of the roles of O-GlcNAc in the heart and vasculature.

Epicatechin lowers blood pressure, restores endothelial function, and decreases oxidative stress and endothelin-1 and NADPH oxidase activity in DOCA-salt hypertension

Flavanol-rich diets have been reported to exert beneficial effects in preventing cardiovascular diseases, such as hypertension. We studied the effects of chronic treatment with epicatechin on blood pressure, endothelial function, and oxidative status in deoxycorticosterone acetate (DOCA)-salt-induced hypertension. Rats were treated for 5 weeks with (-)-epicatechin at 2 or 10 mg kg(-1)day(-1). The high dose of epicatechin prevented both the increase in systolic blood pressure and the proteinuria induced by DOCA-salt. Plasma endothelin-1 and malondialdehyde levels and urinary iso-prostaglandin F(2α) excretion were increased in animals of the DOCA-salt group and reduced by the epicatechin 10 mg kg(-1) treatment. Aortic superoxide levels were enhanced in the DOCA-salt group and abolished by both doses of epicatechin. However, only epicatechin at 10 mg kg(-1) reduced the rise in aortic nicotinamide adenine dinucleotide phosphate (NADPH) oxidase activity and p47(phox) and p22(phox) gene overexpression found in DOCA-salt animals. Epicatechin increased the transcription of nuclear factor-E2-related factor-2 (Nrf2) and Nrf2 target genes in aortas from control rats. Epicatechin also improved the impaired endothelium-dependent relaxation response to acetylcholine and increased the phosphorylation of both Akt and eNOS in aortic rings. In conclusion, epicatechin prevents hypertension, proteinuria, and vascular dysfunction. Epicatechin also induced a reduction in ET-1 release, systemic and vascular oxidative stress, and inhibition of NADPH oxidase activity.

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.

The role of uridine adenosine tetraphosphate in the vascular system

The endothelium plays a pivotal role in vascular homeostasis, and endothelial dysfunction is a major feature of cardiovascular diseases, such as arterial hypertension, atherosclerosis, and diabetes. Recently, uridine adenosine tetraphosphate (Up(4)A) has been identified as a novel and potent endothelium-derived contracting factor (EDCF). Up(4)A structurally contains both purine and pyrimidine moieties, which activate purinergic receptors. There is an accumulating body of evidence to show that Up(4)A modulates vascular function by actions on endothelial and smooth muscle cells. In this paper, we discuss the effects of Up(4)A on vascular function and a potential role for Up(4)A in cardiovascular diseases.

Alterations in vasoconstrictor responses to the endothelium-derived contracting factor uridine adenosine tetraphosphate are region specific in DOCA-salt hypertensive rats

Uridine adenosine tetraphosphate (Up(4)A) has been recently identified as a novel and potent endothelium-derived contracting factor and contains both purine and pyrimidine moieties, which activate purinergic P2X and P2Y receptors. The present study was designed to compare contractile responses to Up(4)A and other nucleotides such as ATP (P2X/P2Y agonist), UTP (P2Y(2)/P2Y(4) agonist), UDP (P2Y(6) agonist), and α,β-methylene ATP (P2X(1) agonist) in different vascular regions [thoracic aorta, basilar, small mesenteric, and femoral arteries] from deoxycorticosterone acetate-salt (DOCA-salt) and control rats. In DOCA-salt rats [vs. control uninephrectomized (Uni) rats]: (1) in thoracic aorta, Up(4)A-, ATP-, and UTP-induced contractions were unchanged; (2) in basilar artery, Up(4)A-, ATP-, UTP- and UDP-induced contractions were increased, and expression for P2X(1), but not P2Y(2) or P2Y(6) was decreased; (3) in small mesenteric artery, Up(4)A-induced contraction was decreased and UDP-induced contraction was increased; expression of P2Y(2) and P2X(1) was decreased whereas P2Y(6) expression was increased; (4) in femoral artery, Up(4)A-, UTP-, and UDP-induced contractions were increased, but expression of P2Y(2), P2Y(6) and P2X(1) was unchanged. The α,β-methylene ATP-induced contraction was bell-shaped and the maximal contraction was reached at a lower concentration in basilar and mesenteric arteries from Uni rats, compared to arteries from DOCA-salt rats. These results suggest that Up(4)A-induced contraction is heterogenously affected among various vascular beds in arterial hypertension. P2Y receptor activation may contribute to enhancement of Up(4)A-induced contraction in basilar and femoral arteries. These changes in vascular reactivity to Up(4)A may be adaptive to the vascular alterations produced by hypertension.

Uridine adenosine tetraphosphate-induced contraction is increased in renal but not pulmonary arteries from DOCA-salt hypertensive rats

Uridine adenosine tetraphosphate (Up(4)A) was reported as a novel endothelium-derived contracting factor. Up(4)A contains both purine and pyrimidine moieties, which activate purinergic (P2)X and P2Y receptors. However, alterations in the vasoconstrictor responses to Up(4)A in hypertensive states remain unclear. The present study examined the effects of Up(4)A on contraction of isolated renal arteries (RA) and pulmonary arteries (PA) from DOCA-salt rats using isometric tension recording. RA from DOCA-salt rats exhibited increased contraction to Up(4)A versus arteries from control uninephrectomized rats in the absence and presence of N(G)-nitro-l-arginine (nitric oxide synthase inhibitor). On the other hand, the Up(4)A-induced contraction in PA was similar between the two groups. Up(4)A-induced contraction was inhibited by suramin (nonselective P2 antagonist) but not by diinosine pentaphosphate pentasodium salt hydrate (Ip(5)I; P2X(1) antagonist) in RA from both groups. Furthermore, 2-thiouridine 5'-triphosphate tetrasodium salt (2-ThioUTP; P2Y(2) agonist)-, uridine-5'-(γ-thio)-triphosphate trisodium salt (UTPγS; P2Y(2)/P2Y(4) agonist)-, and 5-iodouridine-5'-O-diphosphate trisodium salt (MRS 2693; P2Y(6) agonist)-induced contractions were all increased in RA from DOCA-salt rats. Protein expression of P2Y(2)-, P2Y(4)-, and P2Y(6) receptors in RA was similar between the two groups. In DOCA-salt RA, the enhanced Up(4)A-induced contraction was reduced by PD98059, an ERK pathway inhibitor, and Up(4)A-stimulated ERK activation was increased. These data are the first to indicate that Up(4)A-induced contraction is enhanced in RA from DOCA-salt rats. Enhanced P2Y receptor signaling and activation of the ERK pathway together represent a likely mechanism mediating the enhanced Up(4)A-induced contraction. Up(4)A might be of relevance in the pathophysiology of vascular tone regulation and renal dysfunction in arterial hypertension.