Insulin and glucosamine infusions increase O-linked N-acetyl-glucosamine in skeletal muscle proteins in vivo

Smoking cessation only partially reverses cardiac metabolic and structural remodeling in mice

AIMS

Active cigarette smoking is a major risk factor for chronic obstructive pulmonary disease that remains elevated after cessation. Skeletal muscle dysfunction has been well documented after smoking, but little is known about cardiac adaptations to cigarette smoking. The underlying cellular and molecular cardiac adaptations, independent of confounding lifestyle factors, and time course of reversibility by smoking cessation remain unclear. We hypothesized that smoking negatively affects cardiac metabolism and induces local inflammation in mice, which do not readily reverse upon 2-week smoking cessation.

METHODS

Mice were exposed to air or cigarette smoke for 14 weeks with or without 1- or 2-week smoke cessation. We measured cardiac mitochondrial respiration by high-resolution respirometry, cardiac mitochondrial density, abundance of mitochondrial supercomplexes by electrophoresis, and capillarization, fibrosis, and macrophage infiltration by immunohistology, and performed cardiac metabolome and lipidome analysis by mass spectrometry.

RESULTS

Mitochondrial protein, supercomplex content, and respiration (all p < 0.03) were lower after smoking, which were largely reversed within 2-week smoking cessation. Metabolome and lipidome analyses revealed alterations in mitochondrial metabolism, a shift from fatty acid to glucose metabolism, which did not revert to control upon smoking cessation. Capillary density was not different after smoking but increased after smoking cessation (p = 0.02). Macrophage infiltration and fibrosis (p < 0.04) were higher after smoking but did not revert to control upon smoking cessation.

CONCLUSIONS

While cigarette-impaired smoking-induced cardiac mitochondrial function was reversed by smoking cessation, the remaining fibrosis and macrophage infiltration may contribute to the increased risk of cardiovascular events after smoking cessation.

Changes in the amount of nucleotide sugars in aged mouse tissues

Aging affects tissue glycan profiles, which may alter cellular functions and increase the risk of age-related diseases. Glycans are biosynthesized by glycosyltransferases using the corresponding nucleotide sugar, and the availability of nucleotide sugars affects glycosylation efficiency. However, the effects of aging on nucleotide sugar profiles and contents are yet to be elucidated. Therefore, this study aimed to investigate the effects of aging on nucleotide sugars using a new LC-MS/MS method. Specifically, the new method was used to determine the nucleotide sugar contents of various tissues (brain, liver, heart, skeletal muscle, kidney, lung, and colon) of male C57BL/6NCr mice (7- or 26-month-old). Characteristic age-associated nucleotide sugar changes were observed in each tissue sample. Particularly, there was a significant decrease in UDP-glucuronic acid content in the kidney of aged mice and a decrease in the contents of several nucleotide sugars, including UDP-N-acetylgalactosamine, in the brain of aged mice. Additionally, there were variations in nucleotide sugar profiles among the tissues examined regardless of the age. The kidneys had the highest concentration of UDP-glucuronic acid among the seven tissues. In contrast, the skeletal muscle had the lowest concentration of total nucleotide sugars among the tissues; however, CMP-N-acetylneuraminic acid and CDP-ribitol were relatively enriched. Conclusively, these findings may contribute to the understanding of the roles of glycans in tissue aging.

Enzymatic assay for UDP-GlcNAc and its application in the parallel assessment of substrate availability and protein O-GlcNAcylation

O-linked N-acetylglucosaminylation (O-GlcNAcylation) is a ubiquitous and dynamic non-canonical glycosylation of intracellular proteins. Several branches of metabolism converge at the hexosamine biosynthetic pathway (HBP) to produce the substrate for protein O-GlcNAcylation, the uridine diphosphate N-acetylglucosamine (UDP-GlcNAc). Availability of UDP-GlcNAc is considered a key regulator of O-GlcNAcylation. Yet UDP-GlcNAc concentrations are rarely reported in studies exploring the HBP and O-GlcNAcylation, most likely because the methods to measure it are restricted to specialized chromatographic procedures. Here, we introduce an enzymatic method to quantify cellular and tissue UDP-GlcNAc. The method is based on O-GlcNAcylation of a substrate peptide by O-linked N-acetylglucosamine transferase (OGT) and subsequent immunodetection of the modification. The assay can be performed in dot-blot or microplate format. We apply it to quantify UDP-GlcNAc concentrations in several mouse tissues and cell lines. Furthermore, we show how changes in UDP-GlcNAc levels correlate with O-GlcNAcylation and the expression of OGT and O-GlcNAcase (OGA).

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.

Protein O-GlcNAcylation in Metabolic Modulation of Skeletal Muscle: A Bright but Long Way to Go

O-GlcNAcylation is an atypical, dynamic and reversible O-glycosylation that is critical and abundant in metazoan. O-GlcNAcylation coordinates and receives various signaling inputs such as nutrients and stresses, thus spatiotemporally regulating the activity, stability, localization and interaction of target proteins to participate in cellular physiological functions. Our review discusses in depth the involvement of O-GlcNAcylation in the precise regulation of skeletal muscle metabolism, such as glucose homeostasis, insulin sensitivity, tricarboxylic acid cycle and mitochondrial biogenesis. The complex interaction and precise modulation of O-GlcNAcylation in these nutritional pathways of skeletal muscle also provide emerging mechanical information on how nutrients affect health, exercise and disease. Meanwhile, we explored the potential role of O-GlcNAcylation in skeletal muscle pathology and focused on its benefits in maintaining proteostasis under atrophy. In general, these understandings of O-GlcNAcylation are conducive to providing new insights into skeletal muscle (patho) physiology.

The Beginner’s Guide to O-GlcNAc: From Nutrient Sensitive Pathway Regulation to Its Impact on the Immune System

The addition of N-acetyl glucosamine (GlcNAc) on the hydroxy group of serine/threonine residues is known as O-GlcNAcylation (OGN). The dynamic cycling of this monosaccharide on and off substrates occurs via O-linked β-N-acetylglucosamine transferase (OGT) and O-linked β-N-acetylglucosaminase (OGA) respectively. These enzymes are found ubiquitously in eukaryotes and genetic knock outs of the ogt gene has been found to be lethal in embryonic mice. The substrate scope of these enzymes is vast, over 15,000 proteins across 43 species have been identified with O-GlcNAc. OGN has been known to play a key role in several cellular processes such as: transcription, translation, cell signaling, nutrient sensing, immune cell development and various steps of the cell cycle. However, its dysregulation is present in various diseases: cancer, neurodegenerative diseases, diabetes. O-GlcNAc is heavily involved in cross talk with other post-translational modifications (PTM), such as phosphorylation, acetylation, and ubiquitination, by regulating each other’s cycling enzymes or directly competing addition on the same substrate. This crosstalk between PTMs can affect gene expression, protein localization, and protein stability; therefore, regulating a multitude of cell signaling pathways. In this review the roles of OGN will be discussed. The effect O-GlcNAc exerts over protein-protein interactions, the various forms of crosstalk with other PTMs, and its role as a nutrient sensor will be highlighted. A summary of how these O-GlcNAc driven processes effect the immune system will also be included.

Protein Glycosylation in Diabetes

Diabetes mellitus is a group of metabolic disorders characterized by the presence of hyperglycaemia. Due to its high prevalence and substantial heterogeneity, many studies have been investigating markers that could identify predisposition for the disease development, differentiate between the various subtypes, establish early diagnosis, predict complications or represent novel therapeutic targets. N-glycans, complex oligosaccharide molecules covalently linked to proteins, emerged as potential markers and functional effectors of various diabetes subtypes, appearing to have the capacity to meet these requirements. For instance, it has been shown that N-glycome changes in patients with type 2 diabetes and that N-glycans can even identify individuals with an increased risk for its development. Moreover, genome-wide association studies identified glycosyltransferase genes as candidate causal genes for both type 1 and type 2 diabetes. N-glycans have also been suggested to have a major role in preventing the impairment of glucose-stimulated insulin secretion by modulating cell surface expression of glucose transporters. In this chapter we aimed to describe four major diabetes subtypes: type 1, type 2, gestational and monogenic diabetes, giving an overview of suggested role for N-glycosylation in their development, diagnosis and management.

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.

Altered N-glycosylation profiles as potential biomarkers and drug targets in diabetes

N-glycosylation is a ubiquitous protein modification, and N-glycosylation profiles are emerging as both biomarkers and functional effectors in various types of diabetes. Genome-wide association studies identified glycosyltransferase genes as candidate causal genes for type 1 and type 2 diabetes. Studies focused on N-glycosylation changes in type 2 diabetes demonstrated that patients can be distinguished from healthy controls based on N-glycome composition. In addition, individuals at an increased risk of future disease development could be identified based on N-glycome profiles. Moreover, accumulating evidence indicates that N-glycans have a major role in preventing the impairment of glucose-stimulated insulin secretion by maintaining the glucose transporter in proper orientation, indicating that interindividual variation in protein N-glycosylation might be a novel risk factor contributing to diabetes development. Defective N-glycosylation of T cells has been implicated in type 1 diabetes pathogenesis. Furthermore, studies of N-glycan alterations have successfully been used to identify individuals with rare types of diabetes (such as the HNF1A-MODY), and also to evaluate functional significance of novel diabetes-associated mutations. In conclusion, both N-glycans and glycosyltransferases emerge as potential therapeutic targets in diabetes.

Effect of Chronic Hyperglycemia on Glucose Metabolism in Subjects With Normal Glucose Tolerance

Chronic hyperglycemia causes insulin resistance, but the inheritability of glucotoxicity and the underlying mechanisms are unclear. We examined the effect of 3 days of hyperglycemia on glucose disposal, enzyme activities, insulin signaling, and protein O-GlcNAcylation in skeletal muscle of individuals without (FH^-) or with (FH⁺) family history of type 2 diabetes. Twenty-five subjects with normal glucose tolerance received a [3-³H]glucose euglycemic insulin clamp, indirect calorimetry, and vastus-lateralis biopsies before and after 3 days of saline (n = 5) or glucose (n = 10 FH^- and 10 FH⁺) infusion to raise plasma glucose by ∼45 mg/dL. At baseline, FH⁺ had lower insulin-stimulated glucose oxidation and total glucose disposal (TGD) but similar nonoxidative glucose disposal and basal endogenous glucose production (bEGP) compared with FH^- After 3 days of glucose infusion, bEGP and glucose oxidation were markedly increased, whereas nonoxidative glucose disposal and TGD were lower versus baseline, with no differences between FH^- and FH⁺ subjects. Hyperglycemia doubled skeletal muscle glycogen content and impaired activation of glycogen synthase (GS), pyruvate dehydrogenase, and Akt, but protein O-GlcNAcylation was unchanged. Insulin resistance develops to a similar extent in FH^- and FH⁺ subjects after chronic hyperglycemia, without increased protein O-GlcNAcylation. Decreased nonoxidative glucose disposal due to impaired GS activation appears to be the primary deficit in skeletal muscle glucotoxicity.

Increased O-Linked N-Acetylglucosamine Modification of NF-ΚB and Augmented Cytokine Production in the Placentas from Hyperglycemic Rats

Inflammation as a result of NF-κB activation may result from the classical (canonical) pathway, with disconnection of the IκB inhibitor and subsequent nuclear translocation or, alternatively, by post-translational modifications of modulatory proteins or NF-κB subunits (non-canonical pathway). We hypothesized that hyperglycemia-induced increased glycosylation with O-linked N-acetylglucosamine (O-GlcNAc) of NF-κB in placental tissue leads to augmented production of pro-inflammatory cytokines, culminating in placental dysfunction and fetal restriction growth. Single injections of streptozotocin (40 mg/kg) or vehicle were used to induce hyperglycemia or normoglycemia, respectively, in female Wistar rats. After 3 days, rats were mated and pregnancy confirmed. Placental tissue was collected at 21 days of pregnancy. Placental expression of p65 subunit was similar between groups. However, nuclear translocation of p65 subunit, showing greater activation of NF-κB, was increased in the hyperglycemic group. Reduced expression of IκB and increased expression of phosphorylated IκB^Ser32 were observed in the placenta from hyperglycemic rats, demonstrating increased classical NF-κB activation. Augmented modification of O-GlcNAc-modified proteins was found in the placenta from hyperglycemic rats and p65 subunit was a key O-GlcNAc target, as demonstrated by immunoprecipitation. Tumor necrosis factor-alpha (TNF-α) and interleukin-6 (IL-6) expressions were increased in the placenta from hyperglycemic rats. Furthermore, placental weight was increased, whereas fetal weight was decreased under hyperglycemic conditions. TNF-α and IL-6 demonstrated positive correlations with placental weight and negative correlations with fetal weight and placental efficiency. Therefore, under hyperglycemic conditions, a modulatory role of O-GlcNAc in NF-κB activity was demonstrated in the placenta, contributing to fetal and placental dysfunction due to inflammatory cytokine exacerbation.

O-linked N-acetylglucosamine transferase promotes cervical cancer tumorigenesis through human papillomaviruses E6 and E7 oncogenes

O-linked N-acetylglucosamine (O-GlcNAc) transferase (OGT) increases O-GlcNAc modification (O-GlcNAcylation), and transcriptional co-regulator host cell factor 1 (HCF-1) is one of OGT targets. High-risk Human Papillomaviruses (HPVs) encode E6 and E7 oncoproteins, which promote cervical cancer. Here, we tested whether O-GlcNAc modification of HCF-1 affects HPV E6 and E7 expressions and tumorigenesis of cervical cancer. We found that depleting OGT with OGT-specific shRNA significantly decreased levels of E6 and E7 oncoproteins, and cervical cancer tumorigenesis, while OGT overexpression greatly increased levels of E6 and E7 oncoproteins. Notably, OGT overexpression caused dose-dependent increases in the transcriptional activity of E6 and E7, and this activity was decreased when HCF-1 was depleted with HCF-1-specific siRNA. Moreover, OGT depletion reduced proliferation, invasion, and metastasis in cervical cancer cells. Further, high glucose enhanced the interaction between OGT and HCF-1, paralleling increased levels of E6 and E7 in cervical cancer cells. Most importantly, we found that reducing OGT in HeLa cells caused decreased tumor growth in vivo. These findings identify OGT as a novel cellular factor involved in E6 and E7 expressions and cervical cancer tumorigenesis, suggesting that targeting OGT in cervical cancer may have potential therapeutic benefit.

Involvement of O-GlcNAcylation in the Skeletal Muscle Physiology and Physiopathology: Focus on Muscle Metabolism

Skeletal muscle represents around 40% of whole body mass. The principal function of skeletal muscle is the conversion of chemical energy toward mechanic energy to ensure the development of force, provide movement and locomotion, and maintain posture. This crucial energy dependence is maintained by the faculty of the skeletal muscle for being a central place as a "reservoir" of amino acids and carbohydrates in the whole body. A fundamental post-translational modification, named O-GlcNAcylation, depends, inter alia, on these nutrients; it consists to the transfer or the removal of a unique monosaccharide (N-acetyl-D-glucosamine) to a serine or threonine hydroxyl group of nucleocytoplasmic and mitochondrial proteins in a dynamic process by the O-GlcNAc Transferase (OGT) and the O-GlcNAcase (OGA), respectively. O-GlcNAcylation has been shown to be strongly involved in crucial intracellular mechanisms through the modulation of signaling pathways, gene expression, or cytoskeletal functions in various organs and tissues, such as the brain, liver, kidney or pancreas, and linked to the etiology of associated diseases. In recent years, several studies were also focused on the role of O-GlcNAcylation in the physiology and the physiopathology of skeletal muscle. These studies were mostly interested in O-GlcNAcylation during muscle exercise or muscle-wasting conditions. Major findings pointed out a different "O-GlcNAc signature" depending on muscle type metabolism at resting, wasting and exercise conditions, as well as depending on acute or long-term exhausting exercise protocol. First insights showed some differential OGT/OGA expression and/or activity associated with some differential stress cellular responses through Reactive Oxygen Species and/or Heat-Shock Proteins. Robust data displayed that these O-GlcNAc changes could lead to (i) a differential modulation of the carbohydrates metabolism, since the majority of enzymes are known to be O-GlcNAcylated, and to (ii) a differential modulation of the protein synthesis/degradation balance since O-GlcNAcylation regulates some key signaling pathways such as Akt/GSK3β, Akt/mTOR, Myogenin/Atrogin-1, Myogenin/Mef2D, Mrf4 and PGC-1α in the skeletal muscle. Finally, such involvement of O-GlcNAcylation in some metabolic processes of the skeletal muscle might be linked to some associated diseases such as type 2 diabetes or neuromuscular diseases showing a critical increase of the global O-GlcNAcylation level.

Effects of hypoglycemia on myocardial susceptibility to ischemia-reperfusion injury and preconditioning in hearts from rats with and without type 2 diabetes

BACKGROUND

Hypoglycemia is associated with increased mortality rate in patients with diabetes. The underlying mechanisms may involve reduced myocardial tolerance to ischemia and reperfusion (IR) or reduced capacity for ischemic preconditioning (IPC). As IPC is associated with increased myocardial glucose uptake (MGU) during reperfusion, cardioprotection is linked to glucose metabolism possibly by O-linked β-N-acetylglucosamine (O-GlcNAc). We aimed to investigate the impact of hypoglycemia in hearts from animals with diabetes on myocardial IR tolerance, on the efficacy of IPC and whether modulations of MGU and O-GlcNAc levels are involved in the underlying mechanisms.

METHODS

In a Langendorff model using diabetic ZDF (fa/fa) and non-diabetic (fa/+) rats (n = 6-7 in each group) infarct size (IS) was evaluated after 40 min of global ischemia and 120 min reperfusion during hypoglycemia [(glucose) = 3 mmol/l] and normoglycemia [(glucose) = 11 mmol/l]. Myocardial glucose uptake and O-GlcNAc levels were evaluated during reperfusion. IPC was induced by 2 × 5 min of global ischemia prior to index ischemia.

RESULTS

IS increased in hearts from animals with (p < 0.01) and without (p < 0.01) diabetes during hypoglycemia compared to normoglycemia. IPC reduced IS during normoglycemia in both animals with (p < 0.01) and without (p < 0.01) diabetes. During hypoglycemia, however, IPC only reduced IS in hearts from animals with diabetes (p < 0.05). IPC increased MGU during reperfusion and O-GlcNAc levels in animals with diabetes during hypo- (MGU: p < 0.05, O-GlcNAc: p < 0.05) and normoglycemia (MGU: p < 0.01, O-GlcNAc: p < 0.05) and in animals without diabetes only during normoglycemia (MGU: p < 0.05, O-GlcNAc: p < 0.01).

CONCLUSIONS

Hypoglycemia increases myocardial susceptibility to IR injury in hearts from animals with and without diabetes. In contrast to hearts from animals without diabetes, the hearts from animals with diabetes are amenable to cardioprotection during hypoglycemia. In parallel with IPC induced cardioprotection, MGU and O-GlcNAc levels increase suggesting that increased MGU and O-GlcNAc levels are involved in the mechanisms of IPC.

Camelus dromedarius glucose transporter 4: in silico analysis, cloning, expression, purification and characterisation in E. coli

Camels have exceptional carbohydrate metabolism as their plasma glucose level is high and have low whole body insulin sensitivity, similar to that observed in type 2 diabetes patients. We aimed at studing an important component of insulin signalling pathway, the GLUT4, in camel. Camelus dromedarius GLUT4 (CdGLUT4) CDS is 1530 nucleotide in length that encodes for a 55KDa protein. CdGLUT4 has 23 amino acid substitutions and 3N-glycosylation sites, compared to 2 in Human GLUT4. 3 D structures of CdGLUT4 and HsGLUT4 generated by homology modelling revealed conservation of characteristic signature motifs. CdGLUT4 was cloned and expressed optimally in C43(DE3)pLysS strain and maximum detergent solubility was observed in n-Dodecyl-β-d-maltopyranoside. These preliminary data provide information on residual differences between CdGLUT4 and HsGLUT4 that may be responsible for camel's unique glucose metabolism. These differences are postulated to assist in designing and development of efficacious GLUT4 that might help in management of diabetic patients.

Exercise training increases protein O-GlcNAcylation in rat skeletal muscle

Protein O-GlcNAcylation has emerged as an important intracellular signaling system with both physiological and pathophysiological functions, but the role of protein O-GlcNAcylation in skeletal muscle remains elusive. In this study, we tested the hypothesis that protein O-GlcNAcylation is a dynamic signaling system in skeletal muscle in exercise and disease. Immunoblotting showed different protein O-GlcNAcylation pattern in the prototypical slow twitch soleus muscle compared to fast twitch EDL from rats, with greater O-GlcNAcylation level in soleus associated with higher expression of the modulating enzymes O-GlcNAc transferase (OGT), O-GlcNAcase (OGA), and glutamine fructose-6-phosphate amidotransferase isoforms 1 and 2 (GFAT1, GFAT2). Six weeks of exercise training by treadmill running, but not an acute exercise bout, increased protein O-GlcNAcylation in rat soleus and EDL There was a striking increase in O-GlcNAcylation of cytoplasmic proteins ~50 kDa in size that judged from mass spectrometry analysis could represent O-GlcNAcylation of one or more key metabolic enzymes. This suggests that cytoplasmic O-GlcNAc signaling is part of the training response. In contrast to exercise training, postinfarction heart failure (HF) in rats and humans did not affect skeletal muscle O-GlcNAcylation level, indicating that aberrant O-GlcNAcylation cannot explain the skeletal muscle dysfunction in HF Human skeletal muscle displayed extensive protein O-GlcNAcylation that by large mirrored the fiber-type-related O-GlcNAcylation pattern in rats, suggesting O-GlcNAcylation as an important signaling system also in human skeletal muscle.

Roles of O-GlcNAc in chronic diseases of aging

O-GlcNAcylation, a dynamic nutrient and stress sensitive post-translational modification, occurs on myriad proteins in the cell nucleus, cytoplasm and mitochondria. O-GlcNAcylation serves as a nutrient sensor to regulate signaling, transcription, translation, cell division, metabolism, and stress sensitivity in all cells. Aberrant protein O-GlcNAcylation plays a critical role both in the development, as well as in the progression of a variety of age related diseases. O-GlcNAcylation underlies the etiology of diabetes, and changes in specific protein O-GlcNAc levels and sites are responsible for insulin expression and sensitivity and glucose toxicity. Abnormal O-GlcNAcylation contributes directly to diabetes related dysfunction of the heart, kidney and eyes and affects progression of cardiomyopathy, nephropathy and retinopathy. O-GlcNAcylation is a critical modification in the brain and plays a role in both plaque and tangle formation, thus making its study important in neurodegenerative disorders. O-GlcNAcylation also affects cellular growth and metabolism during the development and metastasis of cancer. Finally, alterations in O-GlcNAcylation of transcription factors in macrophages and lymphocytes affect inflammation and cytokine production. Thus, O-GlcNAcylation plays key roles in many of the major diseases associated with aging. Elucidation of its specific functions in both normal and diseased tissues is likely to uncover totally novel avenues for therapeutic intervention.

Glucosamine enhances body weight gain and reduces insulin response in mice fed chow diet but mitigates obesity, insulin resistance and impaired glucose tolerance in mice high-fat diet

OBJECTIVE

This study investigated the potential of glucosamine (GlcN) to affect body weight gain and insulin sensitivity in mice normal and at risk for developing diabetes.

METHODS

Male C57BL/6J mice were fed either chow diet (CD) or a high fat diet (HFD) and the half of mice from CD and HFD provided with a solution of 10% (w/v) GlcN. Total cholesterol and nonesterified free fatty acid levels were determined. Glucose tolerance test and insulin tolerance test were performed. HepG2 human hepatoma cells or differentiated 3T3-L1 adipocytes were stimulated with insulin under normal (5 mM) or high glucose (25 mM) conditions. Effect of GlcN on 2-deoxyglucose (2-DG) uptake was determined. JNK and Akt phosphorylation and nucleocytoplasmic protein O-GlcNAcylation were assayed by Western blotting.

RESULTS

GlcN administration stimulated body weight gain (6.58±0.82 g vs. 11.1±0.42 g), increased white adipose tissue fat mass (percentage of bodyweight, 3.7±0.32 g vs. 5.61±0.34 g), and impaired the insulin response in livers of mice fed CD. However, GlcN treatment in mice fed HFD led to reduction of body weight gain (18.02±0.66 g vs. 16.22±0.96 g) and liver weight (2.27±0.1 vs. 1.85±0.12 g). Furthermore, obesity-induced insulin resistance and impaired Akt insulin signaling in the liver were alleviated by GlcN administration. GlcN inhibited the insulin response under low (5 mM) glucose conditions, whereas it restored the insulin response for Akt phosphorylation under high (25 mM) glucose conditions in HepG2 and 3T3-L1 cells. Uptake of 2-DG increased upon GlcN treatment under 5 mM glucose compared to control, whereas insulin-stimulated 2-DG uptake decreased under 5 mM and increased under 25 mM glucose in differentiated 3T3-L1 cells.

CONCLUSION

Our results show that GlcN increased body weight gain and reduced the insulin response for glucose maintenance when fed to normal CD mice, whereas it alleviated body weight gain and insulin resistance in HFD mice. Therefore, the current data support the integrative function of the HBP reflecting the nutrient status of lipids or glucose and further implicate the importance of the pathway in insulin signaling for the regulation of metabolism.

O-GlcNAcylation, contractile protein modifications and calcium affinity in skeletal muscle

O-GlcNAcylation, a generally undermined atypical protein glycosylation process, is involved in a dynamic and highly regulated interplay with phosphorylation. Akin to phosphorylation, O-GlcNAcylation is also involved in the physiopathology of several acquired diseases, such as muscle insulin resistance or muscle atrophy. Recent data underline that the interplay between phosphorylation and O-GlcNAcylation acts as a modulator of skeletal muscle contractile activity. In particular, the O-GlcNAcylation level of the phosphoprotein myosin light chain 2 seems to be crucial in the modulation of the calcium activation properties, and should be responsible for changes in calcium properties observed in functional atrophy. Moreover, since several key structural proteins are O-GlcNAc-modified, and because of the localization of the enzymes involved in the O-GlcNAcylation/de-O-GlcNAcylation process to the nodal Z disk, a role of O-GlcNAcylation in the modulation of the sarcomeric structure should be considered.

Protein O-GlcNAcylation in diabetes and diabetic complications

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

Borate-aided anion exchange high-performance liquid chromatography of uridine diphosphate-sugars in brain, heart, adipose and liver tissues

In this paper we describe a method optimized for the purification of uridine diphosphate (UDP)-sugars from liver, adipose tissue, brain, and heart, with highly reproducible up to 85% recoveries. Rapid tissue homogenization in cold ethanol, lipid removal by butanol extraction, and purification with a graphitized carbon column resulted in isolation of picomolar quantities of the UDP-sugars from 10 to 30mg of tissue. The UDP-sugars were baseline separated from each other, and from all major nucleotides using a CarboPac PA1 anion exchange column eluted with a gradient of acetate and borate buffers. The extraction and purification protocol produced samples with few unidentified peaks. UDP-N-acetylglucosamine was a dominant UDP-sugar in all the rat tissues studied. However, brain and adipose tissue showed high UDP-glucose levels, equal to that of UDP-N-acetylglucosamine. The UDP-N-acetylglucosamine showed 2.3-2.7 times higher levels than UDP-N-acetylgalactosamine in all tissues, and about the same ratio was found between UDP-glucose and UDP-galactose in adipose tissue and brain (2.6 and 2.8, respectively). Interestingly, the UDP-glucose/UDP-galactose ratio was markedly lower in liver (1.1) and heart (1.7). The UDP-N-acetylglucosamine/UDP-glucuronic acid ratio was also constant, between 9.7 and 7.7, except in liver with the ratio as low as 1.8. The distinct UDP-glucose/galactose ratio, and the abundance of UDP-glucuronic acid may reflect the specific role of liver in glycogen synthesis, and metabolism of hormones and xenobiotics, respectively, using these UDP-sugars as substrates.

O-GlcNAc cycling: a link between metabolism and chronic disease

To maintain homeostasis under variable nutrient conditions, cells rapidly and robustly respond to fluctuations through adaptable signaling networks. Evidence suggests that the O-linked N-acetylglucosamine (O-GlcNAc) posttranslational modification of serine and threonine residues functions as a critical regulator of intracellular signaling cascades in response to nutrient changes. O-GlcNAc is a highly regulated, reversible modification poised to integrate metabolic signals and acts to influence many cellular processes, including cellular signaling, protein stability, and transcription. This review describes the role O-GlcNAc plays in governing both integrated cellular processes and the activity of individual proteins in response to nutrient levels. Moreover, we discuss the ways in which cellular changes in O-GlcNAc status may be linked to chronic diseases such as type 2 diabetes, neurodegeneration, and cancers, providing a unique window through which to identify and treat disease conditions.

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.

Suppression of Glutamine:fructose-6-phosphate amidotransferase-1 inhibits adipogenesis in 3T3-L1 adipocytes

O-linked N-acetylglucosamine (O-GlcNAc) protein modification has been implicated in the regulation of signaling pathways, cell function, and gene expression. Glutamine:fructose-6-phosphate amidotransferase-1 (GFAT-1) is the rate-limiting enzyme in the hexosamine biosynthetic pathway (HBP), which generates the sugar nucleotide UDP-GlcNAc, where this nucleotide acts as the donor for O-GlcNAc modification. In this study, we determined whether GFAT-1 regulates adipogenesis in adipocytes. 3T3-L1 preadipocytes were differentiated using medium containing high glucose, insulin, dexamethasone, and isobutylmethylxanthine. Cells were harvested 4, 8, and 12 h and 1, 2, 3, 4, 6, and 8 days after the initiation of differentiation. Global level of O-GlcNAc modification increased 4 h after induction and persisted for 8 days of observation. GFAT-1 mRNA and protein expression was also upregulated beginning 4 h after induction. Pharmacological inhibition of GFAT-1 or GFAT-1 siRNA treatment blocked the increase in O-GlcNAcylation and the formation of lipid droplets in adipocytes. GFAT-1 may regulate the expression of C/EBPβ, PPARγ, SREBP-1, fatty acid synthase, S3-12, perilipin, or adipophilin during adipogenesis. Our results suggest that GFAT-1 plays a critical role in modulating adipogenesis via the regulation of protein O-GlcNAcylation in adipocytes.

O-linked-N-acetylglucosamine cycling and insulin signaling are required for the glucose stress response in Caenorhabditis elegans

In a variety of organisms, including worms, flies, and mammals, glucose homeostasis is maintained by insulin-like signaling in a robust network of opposing and complementary signaling pathways. The hexosamine signaling pathway, terminating in O-linked-N-acetylglucosamine (O-GlcNAc) cycling, is a key sensor of nutrient status and has been genetically linked to the regulation of insulin signaling in Caenorhabditis elegans. Here we demonstrate that O-GlcNAc cycling and insulin signaling are both essential components of the C. elegans response to glucose stress. A number of insulin-dependent processes were found to be sensitive to glucose stress, including fertility, reproductive timing, and dauer formation, yet each of these differed in their threshold of sensitivity to glucose excess. Our findings suggest that O-GlcNAc cycling and insulin signaling are both required for a robust and adaptable response to glucose stress, but these two pathways show complex and interdependent roles in the maintenance of glucose-insulin homeostasis.

O-GlcNAcylation determines the solubility, filament organization, and stability of keratins 8 and 18

Keratins 8 and 18 (K8/18) are intermediate filament proteins expressed specifically in simple epithelial tissues. Dynamic equilibrium of these phosphoglycoproteins in the soluble and filament pool is an important determinant of their cellular functions, and it is known to be regulated by site-specific phosphorylation. However, little is known about the role of dynamic O-GlcNAcylation on this keratin pair. Here, by comparing immortalized (Chang) and transformed hepatocyte (HepG2) cell lines, we have demonstrated that O-GlcNAcylation of K8/18 exhibits a positive correlation with their solubility (Nonidet P-40 extractability). Heat stress, which increases K8/18 solubility, resulted in a simultaneous increase in O-GlcNAc on these proteins. Conversely, increasing O-GlcNAc levels were associated with a concurrent increase in their solubility. This was also associated with a notable decrease in total cellular levels of K8/18. Unaltered levels of transcripts and the reduced half-life of K8 and K18 indicated their decreased stability on increasing O-GlcNAcylation. On the contrary, the K18 glycosylation mutant (K18 S29A/S30A/S48A) was notably more stable than the wild type K18 in Chang cells. The K18-O-GlcNAc mutant accumulated as aggregates upon stable expression, which possibly altered endogenous filament architecture. These results strongly indicate the involvement of O-GlcNAc on K8/18 in regulating their solubility and stability, which may have a bearing on the functions of these keratins.

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

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

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

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

Increasing O-GlcNAc levels: An overview of small-molecule inhibitors of O-GlcNAcase

The O-GlcNAc modification is found on many nucleocytoplasmic proteins. The dynamic nature of O-GlcNAc, which in some ways is reminiscent of phosphorylation, has enabled investigators to modulate the stoichiometry of O-GlcNAc on proteins in order to study its function. Although several genetic and pharmacological methods for manipulating O-GlcNAc levels have been described, one of the most direct approaches of increasing global O-GlcNAc levels is by using small-molecule inhibitors of O-GlcNAcase (OGA). As the interest in increasing O-GlcNAc levels has grown, so too has the number of OGA inhibitors. This review provides an overview of the available methods of increasing O-GlcNAc levels, with a special emphasis on inhibition of OGA by small molecules. Known inhibitors of OGA are discussed with particular attention on those most suitable for cell-based biological studies. Several examples in which OGA inhibitors have been used to study the functional role of the O-GlcNAc modification in biological systems are discussed, highlighting the pros and cons of different inhibitors.

Mapping of O-linked beta-N-acetylglucosamine modification sites in key contractile proteins of rat skeletal muscle

O-linked beta-N-acetylglucosamine (O-GlcNAc) is a widespread modification of serine/threonine residues of nucleocytoplasmic proteins. Recently, several key contractile proteins in rat skeletal muscle (i.e., myosin heavy and light chains and actin) were identified as O-GlcNAc modified. Moreover, it was demonstrated that O-GlcNAc moieties involved in contractile protein interactions could modulate Ca(2+) activation parameters of contraction. In order to better understand how O-GlcNAc can modulate the contractile activity of muscle fibers, we decided to identify the sites of O-GlcNAc modification in purified contractile protein homogenates. Using an MS-based method that relies on mild beta-elimination followed by Michael addition of DTT (BEMAD), we determined the localization of one O-GlcNAc site in the subdomain four of actin and four O-GlcNAc sites in the light meromyosin region of myosin heavy chains (MHC). According to previous reports concerning the role of these regions, our data suggest that O-GlcNAc sites might modulate the actin-tropomyosin interaction, and be involved in MHC polymerization or interactions between MHC and other contractile proteins. Thus, the results suggest that this PTM might be involved in protein-protein interactions but could also modulate the contractile properties of skeletal muscle.

Increased O-GlcNAc causes disrupted lens fiber cell differentiation and cataracts

Diminished proteolytic functionality in the lens may cause cataracts. We have reported that O-GlcNAc is an endogenous inhibitor of the proteasome. We hypothesize that in the lens there is a cause-and-effect relationship between proteasome inhibition by O-GlcNAc, and cataract formation. To demonstrate this, we established novel transgenic mouse models to over-express a dominant-negative form of O-GlcNAcase, GK-NCOAT, in the lens. Expression of GK-NCOAT suppresses removal of O-GlcNAc from proteins, resulting in increased levels of O-GlcNAc in the lenses of our transgenic mice, along with decreased proteasome function. We observed that transgenic mice developed markedly larger cataracts than controls and lens fiber cell denucleation was inhibited. Our study suggests that increased O-GlcNAc in the lens could lead to cataract formation and attenuation of lens fiber cell denucleation by inhibition of proteasome function. These findings may explain why cataract formation is a common complication of diabetes since O-GlcNAc is derived from glucose.

Up-regulation of O-GlcNAc transferase with glucose deprivation in HepG2 cells is mediated by decreased hexosamine pathway flux

O-Linked N-acetylglucosamine (O-GlcNAc) is a post-translational modification of proteins that functions as a nutrient sensing mechanism. We have previously shown a significant induction of O-GlcNAc modification under conditions of glucose deprivation. Increased O-GlcNAc modification was mediated by increased mRNA for nucleocytoplasmic O-linked N-acetylglucosaminyltransferase (ncOGT). We have investigated the mechanism mediating ncOGT induction with glucose deprivation. The signal does not appear to be general energy depletion because no differences in AMP-dependent kinase protein levels or phosphorylation were observed between glucose-deprived and normal glucose-treated cells. However, treatment of glucose-deprived cells with a small dose (1 mm) of glucosamine blocked the induction of ncOGT mRNA and subsequent increase in O-GlcNAc protein modification, suggesting that decreased hexosamine flux is the signal for ncOGT up-regulation. Consistent with this, treatment of glucose-deprived cells with an inhibitor of O-GlcNAcase (O-(2-acetamido-2-deoxy-D-glucopyranosylidene) amino N-phenyl carbamat) completely prevented the subsequent up-regulation of ncOGT. Glucosamine treatment also resulted in a 40% rescue of the down-regulation of glycogen synthase activity normally seen after glucose deprivation. We conclude that deglycosylation of proteins within the first few hours of glucose deprivation promotes ncOGT induction. These findings suggest a novel negative feedback regulatory loop for OGT and O-GlcNAc regulation.

Cross-talk between GlcNAcylation and phosphorylation: roles in insulin resistance and glucose toxicity

O-linked beta-N-acetylglucosamine (O-GlcNAc) is a dynamic posttranslational modification that, analogous to phosphorylation, cycles on and off serine and/or threonine hydroxyl groups. Cycling of O-GlcNAc is regulated by the concerted actions of O-GlcNAc transferase and O-GlcNAcase. GlcNAcylation is a nutrient/stress-sensitive modification that regulates proteins involved in a wide array of biological processes, including transcription, signaling, and metabolism. GlcNAcylation is involved in the etiology of glucose toxicity and chronic hyperglycemia-induced insulin resistance, a major hallmark of type 2 diabetes. Several reports demonstrate a strong positive correlation between GlcNAcylation and the development of insulin resistance. However, recent studies suggest that inhibiting GlcNAcylation does not prevent hyperglycemia-induced insulin resistance, suggesting that other mechanisms must also be involved. To date, proteomic analyses have identified more than 600 GlcNAcylated proteins in diverse functional classes. However, O-GlcNAc sites have been mapped on only a small percentage (<15%) of these proteins, most of which were isolated from brain or spinal cord tissue and not from other metabolically relevant tissues. Mapping the sites of GlcNAcylation is not only necessary to elucidate the complex cross-talk between GlcNAcylation and phosphorylation but is also key to the design of site-specific mutational studies and necessary for the generation of site-specific antibodies, both of which will help further decipher O-GlcNAc's functional roles. Recent technical advances in O-GlcNAc site-mapping methods should now finally allow for a much-needed increase in site-specific analyses to address the functional significance of O-GlcNAc in insulin resistance and glucose toxicity as well as other major biological processes.

The metabolic syndrome and the heart--a considered opinion

The metabolic syndrome (MS) is a multifactorial, heterogeneous group of risk factors for the development of cardiovascular disease. Here we review the evidence in support of the hypothesis that metabolic dysregulation of the body as a whole leads to contractile dysfunction of the heart due to an imbalance of substrate uptake (increased) and substrate oxidation (decreased). The consequences of this imbalance were already recognized 150 years ago by Virchow when he described "fatty atrophy" of the heart as a "true metamorphosis of the heart muscle cell."

Phosphoinositide signalling links O-GlcNAc transferase to insulin resistance

Glucose flux through the hexosamine biosynthetic pathway leads to the post-translational modification of cytoplasmic and nuclear proteins by O-linked beta-N-acetylglucosamine (O-GlcNAc). This tandem system serves as a nutrient sensor to couple systemic metabolic status to cellular regulation of signal transduction, transcription, and protein degradation. Here we show that O-GlcNAc transferase (OGT) harbours a previously unrecognized type of phosphoinositide-binding domain. After induction with insulin, phosphatidylinositol 3,4,5-trisphosphate recruits OGT from the nucleus to the plasma membrane, where the enzyme catalyses dynamic modification of the insulin signalling pathway by O-GlcNAc. This results in the alteration in phosphorylation of key signalling molecules and the attenuation of insulin signal transduction. Hepatic overexpression of OGT impairs the expression of insulin-responsive genes and causes insulin resistance and dyslipidaemia. These findings identify a molecular mechanism by which nutritional cues regulate insulin signalling through O-GlcNAc, and underscore the contribution of this modification to the aetiology of insulin resistance and type 2 diabetes.

Defining the regulated secreted proteome of rodent adipocytes upon the induction of insulin resistance

Insulin resistance defines the metabolic syndrome and precedes, as well is the hallmark of, type II diabetes. Adipocytes, besides being a major site for energy storage, are endocrine in nature and secrete a variety of proteins, adipocytokines (adipokines), that can modulate insulin sensitivity, inflammation, obesity, hypertension, food intake (anorexigenic and orexigenic), and general energy homeostasis. Recent data demonstrates that increased intracellular glycosylation of proteins via O-GlcNAc can induce insulin resistance and that a rodent model with genetically elevated O-GlcNAc levels in muscle and fat displays hyperleptinemia. The link between O-GlcNAc levels, insulin resistance, and adipocytokine secretion is further explored here. First, with the use of immortalized and primary rodent adipocytes, the secreted proteome of differentiated adipocytes is more fully elucidated by the identification of 97 and 203 secreted proteins, respectively. Mapping of more than 80 N-linked glycosylation sites on adipocytokines from the cell lines further defines this proteome. Importantly, adipocytokines that are modulated when cells are shifted from insulin responsive to insulin resistant conditions are determined. By the use of two protocols for inducing insulin resistance, classical hyperglycemia with chronic insulin exposure and pharmacological elevation of O-GlcNAc levels, several proteins are identified that are regulated in a similar fashion under both conditions including HCNP, Quiescin Q6, Angiotensin, lipoprotein lipase, matrix metalloproteinase 2, and slit homologue 3. Detection of these potential prognostic/diagnostic biomarkers for metabolic syndrome, type II diabetes, and the resulting complications of both diseases further establishes the central role of the O-GlcNAc modification of intracellular proteins in the pathophysiology of these conditions.

Glucose deprivation stimulates O-GlcNAc modification of proteins through up-regulation of O-linked N-acetylglucosaminyltransferase

O-Linked N-acetylglucosamine (O-GlcNAc) is a post-translational modification of proteins that functions as a nutrient sensing mechanism. Here we report on regulation of O-GlcNAcylation over a broad range of glucose concentrations. We have discovered a significant induction of O-GlcNAc modification of a limited number of proteins under conditions of glucose deprivation. Beginning 12 h after treatment, glucose-deprived human hepatocellular carcinoma (HepG2) cells demonstrate a 7.8-fold increase in total O-GlcNAc modification compared with cells cultured in normal glucose (5 mm; p = 0.008). Some of the targets of glucose deprivation-induced O-GlcNAcylation are distinct from those modified in response to high glucose (20 mm) or glucosamine (10 mm) treatment, suggesting differential targeting with glucose deprivation and glucose excess. O-GlcNAcylation of glycogen synthase is significantly increased with glucose deprivation, and this O-GlcNAc increase contributes to a 60% decrease (p = 0.004) in glycogen synthase activity. Increased O-GlcNAc modification is not mediated by increased UDP-GlcNAc, the rate-limiting substrate for O-GlcNAcylation. Rather, the mRNA for nucleocytoplasmic O-linked N-acetylglucosaminyltransferase (OGT) increases 3.4-fold within 6 h of glucose deprivation (p = 0.006). Within 12 h, OGT protein increases 1.7-fold (p = 0.01) compared with normal glucose-treated cells. In addition, 12-h glucose deprivation leads to a 49% decrease in O-GlcNAcase protein levels (p = 0.03). We conclude that increased O-GlcNAc modification stimulated by glucose deprivation results from increased OGT and decreased O-GlcNAcase levels and that these changes affect cell metabolism, thus inactivating glycogen synthase.

Proposed regulation of gene expression by glucose in rodent heart

Background

During pressure overload-induced hypertrophy, unloading-induced atrophy, and diabetes mellitus, the heart induces ‘fetal’ genes (e.g. myosin heavy chain β; mhcβ).

Hypothesis

We propose that altered glucose homeostasis within the cardiomyocyte acts as a central mechanism for the regulation of gene expression in response to environmental stresses. The evidence is as follows.

Methods and Results

Forced glucose uptake both ex vivo and in vivo results in mhc isoform switching. Restricting dietary glucose prevents mhc isoform switching in hearts of both GLUT1-Tg mice and rats subjected to pressure overload-induced hypertrophy. Thus, glucose availability correlates with mhc isoform switching under all conditions investigated. A potential mechanism by which glucose affects gene expression is through O-linked glycosylation of specific transcription factors. Glutamine:fructose-6-phosphate amidotransferase (GFAT) catalyzes the flux generating step in UDP-N-acetylglucosamine biosynthesis, the rate determining metabolite in protein glycosylation. Ascending aortic constriction increased intracellular levels of UDP-N-acetylglucosamine, and the expression of gfat2, but not gfat1, in the rat heart.

Conclusions

Collectively, the results strongly suggest glucose-regulated gene expression in the heart, and the involvement of glucose metabolites in isoform switching of sarcomeric proteins characteristic for the fetal gene program.

o-GlcNAc transferase is activated by CaMKIV-dependent phosphorylation under potassium chloride-induced depolarization in NG-108-15 cells

Post-translational modification of cellular proteins by beta-o-linked N-acetylglucosamine (o-GlcNAc) moieties plays a significant role in signal transduction by modulating protein stability, protein-protein interactions, transactivation processes, and the enzyme activities of target proteins. Though various classes of proteins are known to be regulated by o-GlcNAc modification (o-GlcNAcylation), the mechanism that regulates o-linked GlcNAc transferase (OGT) activity remains unknown. Here, we report that potassium chloride-induced depolarization provokes the activation of OGT and subsequent o-GlcNAcylation of proteins in neuroblastoma NG-108-15 cells. Moreover, such an induction of protein o-GlcNAcylation was abolished by treating cells with either a voltage-gated calcium channel inhibitor or a calcium/calmodulin-dependent protein kinase (CaMK) inhibitor. In addition, CaMKIV was found to specifically phosphorylate and activate OGT in vivo and in vitro, which implies that CaMKIV is required for depolarization-induced activation of OGT. Furthermore, we found that OGT is involved in depolarization-induced and CaMKIV-dependent activation of activator protein-1 (AP-1) and subsequent tissue inhibitor of metalloproteinase-1 (Timp-1) gene expression. Taken together, our findings suggest that CaMKIV activated OGT, and OGT has an essential role on the process of CaMKIV-dependent AP-1 activation under depolarization in neuronal cells.

O-GlcNAc modification in diabetes and Alzheimer's disease

Similar to phosphorylation, O-GlcNAcylation (or simply GlcNAcylation) is an abundant, dynamic, and inducible post-translational modification. In some cases, GlcNAcylation and phosphorylation occur at the same or adjacent sites, modulating each other. GlcNAcylated proteins are crucial in regulating virtually all cellular processes, including signaling, cell cycle, and transcription, among others. GlcNAcylation affects protein-protein interactions, activity, stability, and expression. Several GlcNAcylated proteins are involved in diabetes and Alzheimer's disease. Hyperglycemia increases GlcNAcylation of proteins within the insulin signaling pathway and contributes to insulin resistance. In addition, hyperinsulinemia and hyperlipidemia are also associated with increased GlcNAcylation, which affect and regulate several insulin signaling proteins, as well as proteins involved on the pathology of diabetes. With respect to Alzheimer's disease, several proteins involved in the etiology of the disease, including tau, neurofilaments, beta-amyloid precursor protein, and synaptosomal proteins are GlcNAcylated in normal brain. The impairment of brain glucose uptake/metabolism is a known metabolic defect in Alzheimer's neurons. Data support the hypothesis that hypoglycemia within the brain may reduce the normal GlcNAcylation of tau, exposing kinase acceptor sites, thus leading to hyperphosphorylation, which induces tangle formation and neuronal death. Alzheimer's disease and type II diabetes represent two metabolic disorders where dysfunctional protein GlcNAcylation/phosphorylation may be important for disease pathology.

Cycling of O-linked beta-N-acetylglucosamine on nucleocytoplasmic proteins

All animals and plants dynamically attach and remove O-linked beta-N-acetylglucosamine (O-GlcNAc) at serine and threonine residues on myriad nuclear and cytoplasmic proteins. O-GlcNAc cycling, which is tightly regulated by the concerted actions of two highly conserved enzymes, serves as a nutrient and stress sensor. On some proteins, O-GlcNAc competes directly with phosphate for serine/threonine residues. Glycosylation with O-GlcNAc modulates signalling, and influences protein expression, degradation and trafficking. Emerging data indicate that O-GlcNAc glycosylation has a role in the aetiology of diabetes and neurodegeneration.

O-linked N-acetylglucosaminylation is involved in the Ca2+ activation properties of rat skeletal muscle

O-Linked N-acetylglucosaminylation termed O-GlcNAc is a dynamic cytosolic and nuclear glycosylation that is dependent both on glucose flow through the hexosamine biosynthesis pathway and on phosphorylation because of the existence of a balance between phosphorylation and O-GlcNAc. This glycosylation is a ubiquitous post-translational modification, which probably plays an important role in many aspects of protein functions. We have previously reported that, in skeletal muscle, proteins of the glycolytic pathway, energetic metabolism, and contractile proteins were O-GlcNAc-modified and that O-Glc-NAc variations could control the muscle protein homeostasis and be implicated in the regulation of muscular atrophy. In this paper, we report O-N-acetylglucosaminylation of a number of key contractile proteins (i.e. myosin heavy and light chains and actin), which suggests that this glycosylation could be involved in skeletal muscle contraction. Moreover, our results showed that incubation of skeletal muscle skinned fibers in N-acetyl-d-glucosamine, in a concentration solution known to inhibit O-GlcNAc-dependent interactions, induced a decrease in calcium sensitivity and affinity of muscular fibers, whereas the cooperativity of the thin filament proteins was not modified. Thus, our results suggest that O-GlcNAc is involved in contractile protein interactions and could thereby modulate muscle contraction.

Alois Alzheimer revisited: differences in origin of the disease carrying his name

Based on the means of his time, Alois Alzheimer supposed that the disease, later carrying his name, is a disease of older age, and that the pathomorphological structures he described are due to disturbances in brain metabolism. In this contribution, it is discussed which cellular metabolic abnormalities may be representative for age-related sporadic Alzheimer disease (SAD) the predominant form of SAD in contrast to the very rare hereditary early-onset form. In focus are disturbances in glucose/energy metabolism which involve the deficits in acetylcholine, cholesterol and UDP-N-acetylglucosamine beside ATP. Another leading abnormality is the defect in cell membrane composition. The interrelation between abnormal glucose/energy metabolism and membrane defect may be assumed to form the basis for the induction of both the perturbed metabolism of the amyloid precursor protein leading to increased formation of beta-amyloid and hyperphosphorylation of tau-protein destroying cell structures. Alois Alzheimer may have been so prescient to assume most of this 100 years ago.

Inhibition of phospholipase C-beta1-mediated signaling by O-GlcNAc modification

Here we report inhibition of phospholipase C-beta1 (PLC-beta1)-mediated signaling by post-translational glycosylation with beta-N-acetylglucosamine (O-GlcNAc modification). In C2C12 myoblasts, isoform-specific knock-down experiments using siRNA showed that activation of bradykinin (BK) receptor led to stimulation of PLC-beta1 and subsequent intracellular Ca2+ mobilization. In C2C12 myotubes, O-GlcNAc modification of PLC-beta1 was markedly enhanced in response to treatment with glucosamine (GlcNH2), an inhibitor of O-GlcNAase (PUGNAc) and hyperglycemia. This was associated with more than 50% inhibition of intracellular production of IP3 and Ca2+ mobilization in response to BK. Since the abundance of PLC-beta1 remained unchanged, these data suggest that O-GlcNAc modification of PLC-beta1 led to inhibition of its activity. Moreover, glucose uptake stimulated by BK was significantly blunted by treatment with PUGNAc. These data support the notion that O-GlcNAc modification negatively modulates the activity of PLC-beta1.

Erythrocyte glycohydrolases in subjects with trisomy 21: could Down's syndrome be a model of accelerated ageing?

We studied some erythrocyte glycohydrolases, erythrocyte membrane fluidity, plasma hydroperoxides and total antioxidant defences in 23 Down syndrome (DS) individuals in comparison with healthy age-matched and elderly controls. With regard to erythrocyte plasma membrane fluidity, plasma hydroperoxides and total plasma oxidative defences, DS subjects resembled the age-matched controls more than the elderly ones. Membrane glycohydrolases in DS, however, presented a pattern partly similar to age-matched controls and partly to elderly controls. Concerning cytosol glycohydrolases, DS subjects had lower levels of hexosaminidase and N-acetyl-beta-D-glucosaminidase, the latter specific for the hydrolysis of GlcNAc residues O-linked to proteins. In general, erythrocyte membrane and cytosol glycohydrolases decreased during erythrocyte ageing in DS subjects and in all controls. The increased levels of the same enzymes in DS plasma might be attributed to an alteration of their release-uptake mechanisms between the two different compartments, on account of the higher plasma hydroperoxide levels. These findings indicate that erythrocyte ageing in DS differs partially from that of age-matched and elderly controls. In any case, the accelerated ageing seen in DS is no fully comparable to physiological ageing.

Glycomics investigation into insulin action

Defects in glycosylation are becoming increasingly associated with a range of human diseases. In some cases, the disease is caused by the glycosylation defect, whereas in others, the aberrant glycosylation may be a consequence of the disease. The implementation of highly sensitive and rapid mass spectrometric screening strategies for profiling the glycans present in model biological systems is revealing valuable insights into disease phenotypes. In addition, glycan screening is proving useful in the analysis of knock-out mice where it is possible to assess the role of glycosyltransferases and glycosidases and what function they have at the cellular and whole organism level. In this study, we analysed the effect of insulin on the glycosylation of 3T3-L1 cells and the effect of insulin resistance on glycosylation in a mouse model. Transcription profiling of 3T3-L1 cells treated with and without insulin revealed expression changes of several glycogenes. In contrast, mass spectrometric screening analysis of the glycans from these cells revealed very similar profiles suggesting that any changes in glycosylation were most likely on specific proteins rather than a global phenomenon. A fat-fed versus carbohydrate-fed mouse insulin resistant model was analysed to test the consequences of chronic insulin resistance. Muscle and liver N-glycosylation profiles from these mice are reported.

Hexosamines, insulin resistance, and the complications of diabetes: current status

The hexosamine biosynthesis pathway (HBP) is a relatively minor branch of glycolysis. Fructose 6-phosphate is converted to glucosamine 6-phosphate, catalyzed by the first and rate-limiting enzyme glutamine:fructose-6-phosphate amidotransferase (GFAT). The major end product is UDP-N-acetylglucosamine (UDP-GlcNAc). Along with other amino sugars generated by HBP, it provides essential building blocks for glycosyl side chains, of proteins and lipids. UDP-GlcNAc regulates flux through HBP by regulating GFAT activity and is the obligatory substrate of O-GlcNAc transferase. The latter is a cytosolic and nuclear enzyme that catalyzes a reversible, posttranslational protein modification, transferring GlcNAc in O-linkage (O-GlcNAc) to specific serine/threonine residues of proteins. The metabolic effects of increased flux through HBP are thought to be mediated by increasing O-GlcNAcylation. Several investigators proposed that HBP functions as a cellular nutrient sensor and plays a role in the development of insulin resistance and the vascular complications of diabetes. Increased flux through HBP is required and sufficient for some of the metabolic effects of sustained, increased glucose flux, which promotes the complications of diabetes, e.g., diminished expression of sarcoplasmic reticulum Ca(2+)-ATPase in cardiomyocytes and induction of TGF-beta and plasminogen activator inhibitor-1 in vascular smooth muscle cells, mesangial cells, and aortic endothelial cells. The mechanism was consistent with enhanced O-GlcNAcylation of certain transcription factors. The role of HBP in the development of insulin resistance has been controversial. There are numerous papers showing a correlation between increased flux through HBP and insulin resistance; however, the causal relationship has not been established. More recent experiments in mice overexpressing GFAT in muscle and adipose tissue or exclusively in fat cells suggest that the latter develop in vivo insulin resistance via cross talk between fat cells and muscle. Although the relationship between HBP and insulin resistance may be quite complex, it clearly deserves further study in concert with its role in the complications of diabetes.