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

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 enhances Reticulon 2 protein stability and its promotive effects on gastric cancer progression

Our previous study indicated that Reticulon 2 (RTN2) was upregulated and facilitated the progression of gastric cancer. Protein O-linked β-N-acetylglucosaminylation (O-GlcNAcylation) is a general feature during tumorigenesis, and regulates protein activity and stability through post-translational modification on serine/threonine. However, the relationship between RTN2 and O-GlcNAcylation have never been determined. In this study, we explored the influence of O-GlcNAcylation on RTN2 expression and its promotive role in gastric cancer. We found that RTN2 interacted with O-GlcNAc transferase (OGT) and was modified by O-GlcNAc. O-GlcNAcylation enhanced RTN2 protein stability via attenuating its lysosomal degradation in gastric cancer cells. Furthermore, our results demonstrated that RTN2-induced activation of ERK signalling was dependent on O-GlcNAcylation. Consistently, the stimulative effects of RTN2 on cellular proliferation and migration were abrogated by OGT inhibition. Tissue microarray with immumohistochemical staining also confirmed that the expression of RTN2 was positively correlated with the level of total O-GlcNAcylation as well as the phosphorylation level of ERK. Besides, combined RTN2 and O-GlcNAc staining intensity could improve predictive accuracy for gastric cancer patients' survival compared with each alone. Altogether, these findings suggest that O-GlcNAcylation on RTN2 was pivotal for its oncogenic functions in gastric cancer. Targeting RTN2 O-GlcNAcylation might provide new ideas for gastric cancer therapies.

Inhibiting O-GlcNAcylation impacts p38 and Erk1/2 signaling and perturbs cardiomyocyte hypertrophy

The dynamic cycling of O-linked GlcNAc (O-GlcNAc) on and off Ser/Thr residues of intracellular proteins, termed O-GlcNAcylation, is mediated by the conserved enzymes O-GlcNAc transferase (OGT) and O-GlcNAcase. O-GlcNAc cycling is important in homeostatic and stress responses, and its perturbation sensitizes the heart to ischemic and other injuries. Despite considerable progress, many molecular pathways impacted by O-GlcNAcylation in the heart remain unclear. The mitogen-activated protein kinase (MAPK) pathway is a central signaling cascade that coordinates developmental, physiological, and pathological responses in the heart. The developmental or adaptive arm of MAPK signaling is primarily mediated by Erk kinases, while the pathophysiologic arm is mediated by p38 and Jnk kinases. Here, we examine whether O-GlcNAcylation affects MAPK signaling in cardiac myocytes, focusing on Erk1/2 and p38 in basal and hypertrophic conditions induced by phenylephrine. Using metabolic labeling of glycans coupled with alkyne-azide "click" chemistry, we found that Erk1/2 and p38 are O-GlcNAcylated. Supporting the regulation of p38 by O-GlcNAcylation, the OGT inhibitor, OSMI-1, triggers the phosphorylation of p38, an event that involves the NOX2-Ask1-MKK3/6 signaling axis and also the noncanonical activator Tab1. Additionally, OGT inhibition blocks the phenylephrine-induced phosphorylation of Erk1/2. Consistent with perturbed MAPK signaling, OSMI-1-treated cardiomyocytes have a blunted hypertrophic response to phenylephrine, decreased expression of cTnT (key component of the contractile apparatus), and increased expression of maladaptive natriuretic factors Anp and Bnp. Collectively, these studies highlight new roles for O-GlcNAcylation in maintaining a balanced activity of Erk1/2 and p38 MAPKs during hypertrophic growth responses in cardiomyocytes.

Targeting epigenetics in diabetic cardiomyopathy: Therapeutic potential of flavonoids

The pathophysiological mechanisms of diabetic cardiomyopathy have been extensively studied, but there is still a lack of effective prevention and treatment methods. The ability of flavonoids to protect the heart from diabetic cardiomyopathy has been extensively described. In recent years, epigenetics has received increasing attention from scholars in exploring the etiology and treatment of diabetes and its complications. DNA methylation, histone modifications and non-coding RNAs play key functions in the development, maintenance and progression of diabetic cardiomyopathy. Hence, prevention or reversal of the epigenetic alterations that have occurred during the development of diabetic cardiomyopathy may alleviate the personal and social burden of the disease. Flavonoids can be used as natural epigenetic modulators in alternative therapies for diabetic cardiomyopathy. In this review, we discuss the epigenetic effects of different flavonoid subtypes in diabetic cardiomyopathy and summarize the evidence from preclinical and clinical studies that already exist. However, limited research is available on the potential beneficial effects of flavonoids on the epigenetics of diabetic cardiomyopathy. In the future, clinical trials in which different flavonoids exert their antidiabetic and cardioprotective effects through various epigenetic mechanisms should be further explored.

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.

Therapeutic potential and recent advances on targeting mitochondrial dynamics in cardiac hypertrophy: A concise review

Pathological cardiac hypertrophy begins as an adaptive response to increased workload; however, sustained hemodynamic stress will lead it to maladaptation and eventually cardiac failure. Mitochondria, being the powerhouse of the cells, can regulate cardiac hypertrophy in both adaptive and maladaptive phases; they are dynamic organelles that can adjust their number, size, and shape through a process called mitochondrial dynamics. Recently, several studies indicate that promoting mitochondrial fusion along with preventing mitochondrial fission could improve cardiac function during cardiac hypertrophy and avert its progression toward heart failure. However, some studies also indicate that either hyperfusion or hypo-fission could induce apoptosis and cardiac dysfunction. In this review, we summarize the recent knowledge regarding the effects of mitochondrial dynamics on the development and progression of cardiac hypertrophy with particular emphasis on the regulatory role of mitochondrial dynamics proteins through the genetic, epigenetic, and post-translational mechanisms, followed by discussing the novel therapeutic strategies targeting mitochondrial dynamic pathways.

O-GlcNAcylation is a key regulator of multiple cellular metabolic pathways

O-GlcNAcylation modifies proteins in serine or threonine residues in the nucleus, cytoplasm, and mitochondria. It regulates a variety of cellular biological processes and abnormal O-GlcNAcylation is associated with diabetes, cancer, cardiovascular disease, and neurodegenerative diseases. Recent evidence has suggested that O-GlcNAcylation acts as a nutrient sensor and signal integrator to regulate metabolic signaling, and that dysregulation of its metabolism may be an important indicator of pathogenesis in disease. Here, we review the literature focusing on O-GlcNAcylation regulation in major metabolic processes, such as glucose metabolism, mitochondrial oxidation, lipid metabolism, and amino acid metabolism. We discuss its role in physiological processes, such as cellular nutrient sensing and homeostasis maintenance. O-GlcNAcylation acts as a key regulator in multiple metabolic processes and pathways. Our review will provide a better understanding of how O-GlcNAcylation coordinates metabolism and integrates molecular networks.

Excessive O-GlcNAcylation Causes Heart Failure and Sudden Death

BACKGROUND

Heart failure is a leading cause of death worldwide and is associated with the rising prevalence of obesity, hypertension, and diabetes. O-GlcNAcylation (the attachment of O-linked β-N-acetylglucosamine [O-GlcNAc] moieties to cytoplasmic, nuclear, and mitochondrial proteins) is a posttranslational modification of intracellular proteins and serves as a metabolic rheostat for cellular stress. Total levels of O-GlcNAcylation are determined by nutrient and metabolic flux, in addition to the net activity of 2 enzymes: O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA). Failing myocardium is marked by increased O-GlcNAcylation, but whether excessive O-GlcNAcylation contributes to cardiomyopathy and heart failure is unknown.

METHODS

We developed 2 new transgenic mouse models with myocardial overexpression of OGT and OGA to control O-GlcNAcylation independent of pathologic stress.

RESULTS

We found that OGT transgenic hearts showed increased O-GlcNAcylation and developed severe dilated cardiomyopathy, ventricular arrhythmias, and premature death. In contrast, OGA transgenic hearts had lower O-GlcNAcylation but identical cardiac function to wild-type littermate controls. OGA transgenic hearts were resistant to pathologic stress induced by pressure overload with attenuated myocardial O-GlcNAcylation levels after stress and decreased pathologic hypertrophy compared with wild-type controls. Interbreeding OGT with OGA transgenic mice rescued cardiomyopathy and premature death, despite persistent elevation of myocardial OGT. Transcriptomic and functional studies revealed disrupted mitochondrial energetics with impairment of complex I activity in hearts from OGT transgenic mice. Complex I activity was rescued by OGA transgenic interbreeding, suggesting an important role for mitochondrial complex I in O-GlcNAc-mediated cardiac pathology.

CONCLUSIONS

Our data provide evidence that excessive O-GlcNAcylation causes cardiomyopathy, at least in part, attributable to defective energetics. Enhanced OGA activity is well tolerated and attenuation of O-GlcNAcylation is beneficial against pressure overload-induced pathologic remodeling and heart failure. These findings suggest that attenuation of excessive O-GlcNAcylation may represent a novel therapeutic approach for cardiomyopathy.

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.

Heart Failure in Diabetes Mellitus: An Updated Review

Diabetes mellitus (DM) and heart failure (HF) are comorbid conditions associated with significant morbidity and mortality worldwide. Despite the availability of novel and effective therapeutic options and intensive glycaemic control strategies, mortality and hospitalisation rates continue to remain high and the incidence of HF persists. In this review, we described the impact of currently available glucose-lowering therapies in DM with a focus on HF clinical outcomes. Non-conventional modes of management and alternative pathophysiological mechanisms with the potential for therapeutic targeting are also discussed.

Upregulation of miRNA-23a-3p rescues high glucose-induced cell apoptosis and proliferation inhibition in cardiomyocytes

Maternal hyperglycemia potentially inhibits the development of the fetal heart by suppressing cardiomyocyte proliferation and promoting apoptosis. Different studies have indicated that miRNAs are key regulators of cardiomyocyte proliferation, differentiation, and apoptosis and play a protective role in a variety of cardiovascular diseases. However, the biological function of miRNA-23a in hyperglycemia-related cardiomyocyte injury is not fully understood. The present study investigated the effect of miRNA-23a-3p on cell proliferation and apoptosis in a myocardial injury model induced by high glucose. H9c2 cardiomyocytes were exposed to high glucose to establish an in vitro myocardial injury model and then transfected with miRNA-23a-3p mimics. After miRNA-23a-3p transfection, lens-free microscopy was used to dynamically monitor cell numbers and confluence and calculate the cell cycle duration. CCK-8 and EdU incorporation assays were performed to detect cell proliferation. Flow cytometry was used to measured cell apoptosis. Upregulation of miRNA-23a-3p significantly alleviated high glucose-induced cell apoptosis and cell proliferation inhibition (p < 0.01 and p < 0.0001, respectively). The cell cycle of the miRNA-23a-3p mimics group was significantly shorter than that of the negative control group (p < 0.01). The expression of cell cycle–activating and apoptosis inhibition-associated factors Ccna2, Ccne1, and Bcl-2 was downregulated by high glucose and upregulated by miRNA-23a-3p overexpression in high glucose-injured H9c2 cells. miRNA-23a-3p mimics transfection before high glucose treatment had a significantly greater benefit than transfection after high glucose treatment (p < 0.0001), and the rescue effect of miRNA-23a-3p increased as the concentration increased. This study suggests that miRNA-23a-3p exerted a dose- and time-dependent protective effect on high glucose-induced H9c2 cardiomyocyte injury.

The role of post-translational modifications in cardiac hypertrophy

Pathological cardiac hypertrophy involves excessive protein synthesis, increased cardiac myocyte size and ultimately the development of heart failure. Thus, pathological cardiac hypertrophy is a major risk factor for many cardiovascular diseases and death in humans. Extensive research in the last decade has revealed that post‐translational modifications (PTMs), including phosphorylation, ubiquitination, SUMOylation, O‐GlcNAcylation, methylation and acetylation, play important roles in pathological cardiac hypertrophy pathways. These PTMs potently mediate myocardial hypertrophy responses via the interaction, stability, degradation, cellular translocation and activation of receptors, adaptors and signal transduction events. These changes occur in response to pathological hypertrophy stimuli. In this review, we summarize the roles of PTMs in regulating the development of pathological cardiac hypertrophy. Furthermore, PTMs are discussed as potential targets for treating or preventing cardiac hypertrophy.

O-GlcNAc transferase associates with the MCM2-7 complex and its silencing destabilizes MCM-MCM interactions

O-GlcNAcylation of proteins is governed by O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA). The homeostasis of O-GlcNAc cycling is regulated during cell cycle progression and is essential for proper cellular division. We previously reported the O-GlcNAcylation of the minichromosome maintenance proteins MCM2, MCM3, MCM6 and MCM7. These proteins belong to the MCM2-7 complex which is crucial for the initiation of DNA replication through its DNA helicase activity. Here we show that the six subunits of MCM2-7 are O-GlcNAcylated and that O-GlcNAcylation of MCM proteins mainly occurs in the chromatin-bound fraction of synchronized human cells. Moreover, we identify stable interaction between OGT and several MCM subunits. We also show that down-regulation of OGT decreases the chromatin binding of MCM2, MCM6 and MCM7 without affecting their steady-state level. Finally, OGT silencing or OGA inhibition destabilizes MCM2/6 and MCM4/7 interactions in the chromatin-enriched fraction. In conclusion, OGT is a new partner of the MCM2-7 complex and O-GlcNAcylation homeostasis might regulate MCM2-7 complex by regulating the chromatin loading of MCM6 and MCM7 and stabilizing MCM/MCM interactions.

Elevated O-GlcNAcylation promotes gastric cancer cells proliferation by modulating cell cycle related proteins and ERK 1/2 signaling

O-GlcNAc transferase (OGT) is the only enzyme in mammals that catalyzes the attachment of β-D-N-acetylglucosamine (GlcNAc) to serine or threonine residues of target proteins. Hyper-O-GlcNAcylation is becoming increasingly realized as a general feature of cancer and contributes to rapid proliferation of cancer cells. In this study, we demonstrated that O-GlcNAc and OGT levels were increased in all six gastric cancer (GC) cell lines as compared with immortal gastric epithelial cells. Downregulation of the O-GlcNAcylation level by silencing OGT inhibited cell viability and growth rate via the cdk-2, cyclin D1 and ERK 1/2 pathways. In vivo xenograft assays also demonstrated that the hyper-O-GlcNAc level markedly promoted the proliferation of tumors. Moreover, compared with noncancerous tissues, the O-GlcNAcylation level was increased in cancerous tissues. GC patients with higher levels of O-GlcNAcylation exhibited large tumor sizes (≥5 cm), deep tumor invasion (T3 and T4), high AJCC stages (stage III and IV), more lymph node metastases and lower overall survival. Notably, the phosphorylation level of ERK 1/2 was increased progressively with the increase of O-GlcNAcylation in both SGC 7901 and AGS cells. Consistently, human GC tissue arrays also revealed that ERK 1/2 signaling was positively correlated to O-GlcNAcylation (r = 0.348; P = 0.015). Taken together, here we reported that hyper-O-GlcNAcylation significantly promotes GC cells proliferation by modulating cell cycle related proteins and ERK 1/2 signaling, suggesting that inhibition of OGT may be a potential novel therapeutic target of GC.

AMPK activation counteracts cardiac hypertrophy by reducing O-GlcNAcylation

AMP-activated protein kinase (AMPK) has been shown to inhibit cardiac hypertrophy. Here, we show that submaximal AMPK activation blocks cardiomyocyte hypertrophy without affecting downstream targets previously suggested to be involved, such as p70 ribosomal S6 protein kinase, calcineurin/nuclear factor of activated T cells (NFAT) and extracellular signal-regulated kinases. Instead, cardiomyocyte hypertrophy is accompanied by increased protein O-GlcNAcylation, which is reversed by AMPK activation. Decreasing O-GlcNAcylation by inhibitors of the glutamine:fructose-6-phosphate aminotransferase (GFAT), blocks cardiomyocyte hypertrophy, mimicking AMPK activation. Conversely, O-GlcNAcylation-inducing agents counteract the anti-hypertrophic effect of AMPK. In vivo, AMPK activation prevents myocardial hypertrophy and the concomitant rise of O-GlcNAcylation in wild-type but not in AMPKα2-deficient mice. Treatment of wild-type mice with O-GlcNAcylation-inducing agents reverses AMPK action. Finally, we demonstrate that AMPK inhibits O-GlcNAcylation by mainly controlling GFAT phosphorylation, thereby reducing O-GlcNAcylation of proteins such as troponin T. We conclude that AMPK activation prevents cardiac hypertrophy predominantly by inhibiting O-GlcNAcylation.

Chronic O-GlcNAcylation and Diabetic Cardiomyopathy: The Bitterness of Glucose

Type 2 diabetes (T2D) is a major risk factor for heart failure. Diabetic cardiomyopathy (DC) is characterized by diastolic dysfunction and left ventricular hypertrophy. Epidemiological data suggest that hyperglycaemia contributes to the development of DC. Several cellular pathways have been implicated in the deleterious effects of high glucose concentrations in the heart: oxidative stress, accumulation of advanced glycation end products (AGE), and chronic hexosamine biosynthetic pathway (HBP) activation. In the present review, we focus on the effect of chronic activation of the HBP on diabetic heart function. The HBP supplies N-acetylglucosamine moiety (O-GlcNAc) that is O-linked by O-GlcNAc transferase (OGT) to proteins on serine or threonine residues. This post-translational protein modification modulates the activity of the targeted proteins. In the heart, acute activation of the HBP in response to ischaemia-reperfusion injury appears to be protective. Conversely, chronic activation of the HBP in the diabetic heart affects Ca²⁺ handling, contractile properties, and mitochondrial function and promotes stress signaling, such as left ventricular hypertrophy and endoplasmic reticulum stress. Many studies have shown that O-GlcNAc impairs the function of key protein targets involved in these pathways, such as phospholamban, calmodulin kinase II, troponin I, and FOXO1. The data show that excessive O-GlcNAcylation is a major trigger of the glucotoxic events that affect heart function under chronic hyperglycaemia. Supporting this finding, pharmacological or genetic inhibition of the HBP in the diabetic heart improves heart function. In addition, the SGLT2 inhibitor dapagliflozin, a glucose lowering agent, has recently been shown to lower cardiac HBP in a lipodystophic T2D mice model and to concomitantly improve the diastolic dysfunction of these mice. Therefore, targeting cardiac-excessive O-GlcNAcylation or specific target proteins represents a potential therapeutic option to treat glucotoxicity in the diabetic heart.

The Role of ERK1/2 in the Development of Diabetic Cardiomyopathy

Diabetes mellitus is a chronic metabolic condition that affects carbohydrate, lipid and protein metabolism and may impair numerous organs and functions of the organism. Cardiac dysfunction afflicts many patients who experience the oxidative stress of the heart. Diabetic cardiomyopathy (DCM) is one of the major complications that accounts for more than half of diabetes-related morbidity and mortality cases. Chronic hyperglycemia and hyperlipidemia from diabetes mellitus cause cardiac oxidative stress, endothelial dysfunction, impaired cellular calcium handling, mitochondrial dysfunction, metabolic disturbances, and remodeling of the extracellular matrix, which ultimately lead to DCM. Although many studies have explored the mechanisms leading to DCM, the pathophysiology of DCM has not yet been fully clarified. In fact, as a potential mechanism, the associations between DCM development and mitogen-activated protein kinase (MAPK) activation have been the subjects of tremendous interest. Nonetheless, much remains to be investigated, such as tissue- and cell-specific processes of selection of MAPK activation between pro-apoptotic vs. pro-survival fate, as well as their relation with the pathogenesis of diabetes and associated complications. In general, it turns out that MAPK signaling pathways, such as extracellular signal-regulated kinase 1/2 (ERK1/2), c-Jun N-terminal protein kinase (JNK) and p38 MAP kinase, are demonstrated to be actively involved in myocardial dysfunction, hypertrophy, fibrosis and heart failure. As one of MAPK family members, the activation of ERK1/2 has also been known to be involved in cardiac hypertrophy and dysfunction. However, many recent studies have demonstrated that ERK1/2 signaling activation also plays a crucial role in FGF21 signaling and exerts a protective environment of glucose and lipid metabolism, therefore preventing abnormal healing and cardiac dysfunction. The duration, extent, and subcellular compartment of ERK1/2 activation are vital to differential biological effects of ERK1/2. Moreover, many intracellular events, including mitochondrial signaling and protein kinases, manipulate signaling upstream and downstream of MAPK, to influence myocardial survival or death. In this review, we will summarize the roles of ERK1/2 pathways in DCM development by the evidence from current studies and will present novel opinions on "differential influence of ERK1/2 action in cardiac dysfunction, and protection against myocardial ischemia-reperfusion injury".

O-GlcNAcylation, enemy or ally during cardiac hypertrophy development?

O-linked attachment of the monosaccharide β-N-acetyl-glucosamine (O-GlcNAcylation) is a post-translational modification occurring on serine and threonine residues, which is evolving as an important mechanism for the regulation of various cellular processes. The present review will, first, provide a general background on the molecular regulation of protein O-GlcNAcylation and will summarize the role of this post-translational modification in various acute cardiac pathologies including ischemia-reperfusion. Then, we will focus on research studies examining protein O-GlcNAcylation in the context of cardiac hypertrophy. A particular emphasis will be laid on the convergent but also divergent actions of O-GlcNAcylation according to the type of hypertrophy investigated, including physiological, pressure overload-induced and diabetes-linked cardiac hypertrophy. In an attempt to distinguish whether O-GlcNAcylation is detrimental or beneficial, this review will present the different O-GlcNAcylated targets involved in hypertrophy development. We will finally argue on potential interest to target O-GlcNAc processes to treat cardiac hypertrophy. This article is part of a Special Issue entitled: The role of post-translational protein modifications on heart and vascular metabolism edited by Jason R.B. Dyck & Jan F.C. Glatz.

O-GlcNAcylation: a bridge between glucose and cell differentiation

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

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

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

Investigating the Mechanism of Hyperglycemia-Induced Fetal Cardiac Hypertrophy

Hyperglycemia in diabetic mothers enhances the risk of fetal cardiac hypertrophy during gestation. However, the mechanism of high-glucose-induced cardiac hypertrophy is not largely understood. In this study, we first demonstrated that the incidence rate of cardiac hypertrophy dramatically increased in fetuses of diabetic mothers using color ultrasound examination. In addition, human fetal cardiac hypertrophy was successfully mimicked in a streptozotocin (STZ)-induced diabetes mouse model, in which mouse cardiac hypertrophy was diagnosed using type-M ultrasound and a histological assay. PH3 immunofluorescent staining of mouse fetal hearts and in vitro-cultured H9c2 cells indicated that cell proliferation decreased in E18.5, E15.5 and E13.5 mice, and cell apoptosis in H9c2 cells increased in the presence of high glucose in a dose-dependent manner. Next, we found that the individual cardiomyocyte size increased in pre-gestational diabetes mellitus mice and in response to high glucose exposure. Meanwhile, the expression of β-MHC and BMP-10 was up-regulated. Nkx2.5 immunofluorescent staining showed that the expression of Nkx2.5, a crucial cardiac transcription factor, was suppressed in the ventricular septum, left ventricular wall and right ventricular wall of E18.5, E15.5 and E13.5 mouse hearts. However, cardiac hypertrophy did not morphologically occur in E13.5 mouse hearts. In cultured H9c2 cells exposed to high glucose, Nkx2.5 expression decreased, as detected by both immunostaining and western blotting, and the expression of KCNE1 and Cx43 was also restricted. Taken together, alterations in cell size rather than cell proliferation or apoptosis are responsible for hyperglycemia-induced fetal cardiac hypertrophy. The aberrant expression of Nkx2.5 and its regulatory target genes in the presence of high glucose could be a principal component of pathogenesis in the development of fetal cardiac hypertrophy.

Post-translational modifications of the cardiac proteome in diabetes and heart failure

Cardiovascular complications are the leading cause of death in diabetic patients. Decades of research has focused on altered gene expression, altered cellular signaling, and altered metabolism. This work has led to better understanding of disease progression and treatments aimed at reversing or stopping this deadly process. However, one of the pieces needed to complete the puzzle and bridge the gap between altered gene expression and changes in signaling/metabolism is the proteome and its host of modifications. Defining the mechanisms of regulation includes examining protein levels, localization, and activity of the functional component of cellular machinery. Excess or misutilization of nutrients in obesity and diabetes may lead to PTMs contributing to cardiovascular disease progression. PTMs link regulation of metabolic changes in the healthy and diseased heart with regulation of gene expression itself (e.g. epigenetics), protein enzymatic activity (e.g. mitochondrial oxidative capacity), and function (e.g. contractile machinery). Although a number of PTMs are involved in each of these pathways, we will highlight the role of the serine and threonine O‐linked addition of β‐N‐acetyl‐glucosamine or O‐GlcNAcylation. This nexus of nutrient supply, utilization, and storage allows for the modification and translation of mitochondrial function to many other aspects of the cell.

Role of Nodal-PITX2C signaling pathway in glucose-induced cardiomyocyte hypertrophy

Pathological cardiac hypertrophy is a major cause of morbidity and mortality in cardiovascular disease. Recent studies have shown that cardiomyocytes, in response to high glucose (HG) stimuli, undergo hypertrophic growth. While much work still needs to be done to elucidate this important mechanism of hypertrophy, previous works have showed that some pathways or genes play important roles in hypertrophy. In this study, we showed that sublethal concentrations of glucose (25 mmol/L) could induce cardiomyocyte hypertrophy with an increase in the cellular surface area and the upregulation of the atrial natriuretic peptide (ANP) gene, a hypertrophic marker. High glucose (HG) treatments resulted in the upregulation of the Nodal gene, which is under-expressed in cardiomyocytes. We also determined that the knockdown of the Nodal gene resisted HG-induced cardiomyocyte hypertrophy. The overexpression of Nodal was able to induce hypertrophy in cardiomyocytes, which was associated with the upregulation of the PITX2C gene. We also showed that increases in the PITX2C expression, in response to Nodal, were mediated by the Smad4 signaling pathway. This study is highly relevant to the understanding of the effects of the Nodal-PITX2C pathway on HG-induced cardiomyocyte hypertrophy, as well as the related molecular mechanisms.