Cracking the O-GlcNAc code in metabolism

Liver X receptor regulates hepatic nuclear O-GlcNAc signaling and carbohydrate responsive element-binding protein activity

Liver X receptor (LXR)α and LXRβ play key roles in hepatic de novo lipogenesis through their regulation of lipogenic genes, including sterol regulatory element-binding protein (SREBP)-1c and carbohydrate responsive element-binding protein (ChREBP). LXRs activate lipogenic gene transcription in response to feeding, which is believed to be mediated by insulin. We have previously shown that LXRs are targets for glucose-hexosamine-derived O-linked β-N-acetylglucosamine (O-GlcNAc) modification enhancing their ability to regulate SREBP-1c promoter activity in vitro. To elucidate insulin-independent effects of feeding on LXR-mediated lipogenic gene expression in vivo, we subjected control and streptozotocin-treated LXRα/β(+/+) and LXRα/β(-/-) mice to a fasting-refeeding regime. We show that under hyperglycemic and hypoinsulinemic conditions, LXRs maintain their ability to upregulate the expression of glycolytic and lipogenic enzymes, including glucokinase (GK), SREBP-1c, ChREBPα, and the newly identified shorter isoform ChREBPβ. Furthermore, glucose-dependent increases in LXR/retinoid X receptor-regulated luciferase activity driven by the ChREBPα promoter was mediated, at least in part, by O-GlcNAc transferase (OGT) signaling in Huh7 cells. Moreover, we show that LXR and OGT interact and colocalize in the nucleus and that loss of LXRs profoundly reduced nuclear O-GlcNAc signaling and ChREBPα promoter binding activity in vivo. In summary, our study provides evidence that LXRs act as nutrient and glucose metabolic sensors upstream of ChREBP by modulating GK expression, nuclear O-GlcNAc signaling, and ChREBP expression and activity.

Chromatin Dynamics of Circadian Transcription

The molecular circadian clock orchestrates the daily cyclical expression of thousands of genes. Disruption of this transcriptional program leads to a variety of pathologies, including insomnia, depression and metabolic disorders. Circadian rhythms in gene expression rely on specific chromatin transitions which are ultimately coordinated by the molecular clock. As a consequence, a highly plastic and dynamic circadian epigenome can be delineated across different tissues and cell types. Intriguingly, genome topology appears to coordinate cyclic transcription at circadian interactomes, in which circadian genes are in physical contact within the cell nucleus in a time-specific manner. Moreover, the clock machinery shows functional interplays with key metabolic regulators, thereby connecting the circadian epigenome to cellular metabolism. Unraveling the molecular aspects of such interplays is likely to reveal new therapeutic strategies towards the treatment of metabolic disorders.

Diabetes Induces Aberrant DNA Methylation in the Proximal Tubules of the Kidney

Epigenetic mechanisms may underlie the progression of diabetic kidney disease. Because the kidney is a heterogeneous organ with different cell types, we investigated DNA methylation status of the kidney in a cell type-specific manner. We first identified genes specifically demethylated in the normal proximal tubules obtained from control db/m mice, and next delineated the candidate disease-modifying genes bearing aberrant DNA methylation induced by diabetes using db/db mice. Genes involved in glucose metabolism, including Sglt2, Pck1, and G6pc, were selectively hypomethylated in the proximal tubules in control mice. Hnf4a, a transcription factor regulating transporters for reabsorption, was also selectively demethylated. In diabetic mice, aberrant hypomethylation of Agt, Abcc4, Cyp4a10, Glut5, and Met and hypermethylation of Kif20b, Cldn18, and Slco1a1 were observed. Time-dependent demethylation of Agt, a marker of diabetic kidney disease, was accompanied by histone modification changes. Furthermore, inhibition of DNA methyltransferase or histone deacetylase increased Agt mRNA in cultured human proximal tubular cells. Aberrant DNA methylation and concomitant changes in histone modifications and mRNA expression in the diabetic kidney were resistant to antidiabetic treatment with pioglitazone. These results suggest that an epigenetic switch involving aberrant DNA methylation causes persistent mRNA expression of select genes that may lead to phenotype changes of the proximal tubules in diabetic kidney disease.

A highly effective and adjustable dual plasmid system for O-GlcNAcylated recombinant protein production in E. coli

O-GlcNAcylation is a ubiquitous, dynamic and reversible post-translational protein modification in metazoans, and it is catalysed and removed by O-GlcNAc transferase (OGT) and O-GlcNAcase, respectively. Prokaryotes lack endogenous OGT activity. It has been reported that coexpression of mammalian OGT with its target substrates in Escherichia coli produce O-GlcNAcylated recombinant proteins, but the plasmids used were not compatible, and the expression of both OGT and its target protein were induced by the same inducer. Here, we describe a compatible dual plasmid system for coexpression of OGT and its target substrate for O-GlcNAcylated protein production in E. coli. The approach was validated using the CKII and p53 protein as control. This compatible dual plasmid system contains an arabinose-inducible OGT expression vector with a pUC origin and an isopropyl β-d-thiogalactopyranoside-inducible OGT target substrate expression vector bearing a p15A origin. The dual plasmid system produces recombinant proteins with varying O-GlcNAcylation levels by altering the inducer concentration. More importantly, the O-GlcNAcylation efficiency was much higher than the previously reported system. Altogether, we established an adjustable compatible dual plasmid system that can effectively yield O-GlcNAcylated proteins in E. coli.

In vivo and in vitro evidence that chronic activation of the hexosamine biosynthetic pathway interferes with leptin-dependent STAT3 phosphorylation

We previously reported that a 2-day peripheral infusion of glucosamine caused leptin resistance in rats, suggesting a role for the hexosamine biosynthetic pathway (HBP) in the development of leptin resistance. Here we tested leptin responsiveness in mice in which HBP activity was stimulated by offering 30% sucrose solution in addition to chow and water or by infusing glucosamine. Mice were leptin resistant after 33 days of access to sucrose. Resistance was associated with increased activity of the HBP and with phosphorylation of transcription factor signal transducer and activator of transcription-3 Tyr705 [pSTAT3(Y705)] but inhibition of suppressor of cytokine signaling 3 in the liver and hypothalamus. Intravenous infusion of glucosamine for 3 h stimulated pSTAT3(Y705) but prevented leptin-induced phosphorylation of STAT3(S727). In an in vitro system, glucose, glucosamine, and leptin each dose dependently increased O-linked β-N-acetylglucosamine (O-GlcNAc) protein and pSTAT3(Y705) in HepG2 cells. To test the effect of glucose on leptin responsiveness cells were incubated in 5.5 mM (LG) or 20 mM (HG) glucose for 18 h and were treated with 0 or 50 ng/ml leptin for 15 min. HG alone and LG + leptin produced similar increases in O-GlcNAc protein, glutamine fructose-6-phosphate amidotransferase (GFAT), and pSTAT3(Y705) compared with LG media. Leptin did not stimulate these proteins in HG cells, suggesting leptin resistance. Leptin-induced pSTAT3(S727) was prevented by HG media. Inhibition of GFAT with azaserine prevented LG + leptin and HG stimulation of pSTAT3. These data demonstrate development of leptin resistance in sucrose-drinking mice and provide new evidence of leptin-induced stimulation of the HBP.

Neuronal Control of Adaptive Thermogenesis

The obesity epidemic continues rising as a global health challenge, despite the increasing public awareness and the use of lifestyle and medical interventions. The biomedical community is urged to develop new treatments to obesity. Excess energy is stored as fat in white adipose tissue (WAT), dysfunction of which lies at the core of obesity and associated metabolic disorders. By contrast, brown adipose tissue (BAT) burns fat and dissipates chemical energy as heat. The development and activation of "brown-like" adipocytes, also known as beige cells, result in WAT browning and thermogenesis. The recent discovery of brown and beige adipocytes in adult humans has sparked the exploration of the development, regulation, and function of these thermogenic adipocytes. The central nervous system drives the sympathetic nerve activity in BAT and WAT to control heat production and energy homeostasis. This review provides an overview of the integration of thermal, hormonal, and nutritional information on hypothalamic circuits in thermoregulation.

O-GlcNAc and the epigenetic regulation of gene expression

O-GlcNAcylation is an abundant nutrient-driven modification linked to cellular signaling and regulation of gene expression. Utilizing precursors derived from metabolic flux, O-GlcNAc functions as a homeostatic regulator. The enzymes of O-GlcNAc cycling, OGT and O-GlcNAcase, act in mitochondria, the cytoplasm, and the nucleus in association with epigenetic "writers" and "erasers" of the histone code. Both O-GlcNAc and O-phosphate modify repeats within the RNA polymerase II C-terminal domain (CTD). By communicating with the histone and CTD codes, O-GlcNAc cycling provides a link between cellular metabolic status and the epigenetic machinery. Thus, O-GlcNAcylation is poised to influence trans-generational epigenetic inheritance.

O-GlcNAcase: promiscuous hexosaminidase or key regulator of O-GlcNAc signaling?

O-GlcNAc signaling is regulated by an opposing pair of enzymes: O-GlcNAc transferase installs and O-GlcNAcase (OGA) removes the modification from proteins. The dynamics and regulation of this process are only beginning to be understood as the physiological functions of both enzymes are being probed using genetic and pharmacological approaches. This minireview charts the discovery and functional and structural analysis of OGA and summarizes the insights gained from recent studies using OGA inhibition, gene knock-out, and overexpression. We identify several areas of “known unknowns” that would benefit from future research, such as the enigmatic C-terminal domain of OGA.

Glucose regulates mitochondrial motility via Milton modification by O-GlcNAc transferase

Cells allocate substantial resources toward monitoring levels of nutrients that can be used for ATP generation by mitochondria. Among the many specialized cell types, neurons are particularly dependent on mitochondria due to their complex morphology and regional energy needs. Here, we report a molecular mechanism by which nutrient availability in the form of extracellular glucose and the enzyme O-GlcNAc Transferase (OGT), whose activity depends on glucose availability, regulates mitochondrial motility in neurons. Activation of OGT diminishes mitochondrial motility. We establish the mitochondrial motor-adaptor protein Milton as a required substrate for OGT to arrest mitochondrial motility by mapping and mutating the key O-GlcNAcylated serine residues. We find that the GlcNAcylation state of Milton is altered by extracellular glucose and that OGT alters mitochondrial motility in vivo. Our findings suggest that, by dynamically regulating Milton GlcNAcylation, OGT tailors mitochondrial dynamics in neurons based on nutrient availability.

The potential role of O-GlcNAc modification in cancer epigenetics

There is no doubt that cancer is not only a genetic disease but that it can also occur due to epigenetic abnormalities. Diet and environmental factors can alter the scope of epigenetic regulation. The results of recent studies suggest that O-GlcNAcylation, which involves the addition of N-acetylglucosamine on the serine or threonine residues of proteins, may play a key role in the regulation of the epigenome in response to the metabolic status of the cell. Two enzymes are responsible for cyclic O-GlcNAcylation: O-GlcNAc transferase (OGT), which catalyzes the addition of the GlcNAc moiety to target proteins; and O-GlcNAcase (OGA), which removes the sugar moiety from proteins. Aberrant expression of O-GlcNAc cycling enzymes, especially OGT, has been found in all studied human cancers. OGT can link the cellular metabolic state and the epigenetic status of cancer cells by interacting with and modifying many epigenetic factors, such as HCF-1, TET, mSin3A, HDAC, and BAP1. A growing body of evidence from animal model systems also suggests an important role for OGT in polycomb-dependent repression of genes activity. Moreover, O-GlcNAcylation may be a part of the histone code: O-GlcNAc residues are found on all core histones.

The mitochondrial O-linked N-acetylglucosamine transferase (mOGT) in the diabetic patient could be the initial trigger to develop Alzheimer disease

Diabetes mellitus (DM) is considered a risk factor for the development of Alzheimer disease (AD); however, how DM favors evolution of AD is still insufficiently understood. Hyperglycemia in DM is associated to an increase in mitochondrial reactive oxygen species (ROS) generation, as well as damage of hippocampal cells, reflected by changes in morphological and mitochondrial functionality. Similar mitochondrial damage has been observed when amyloid beta (Aβ) accumulates in the brain of AD patients. In DM, the excess of glucose in the brain induces higher activity of the hexosamine biosynthesis pathway (HBP), it synthesizes UDP-N-acetylglucosamine (UDP-GlcNAc), which is used by O-linked N-acetylglucosamine transferase (OGT) to catalyze O-GlcNAcylation of numerous proteins. Although O-GlcNAcylation plays an important role in maintaining structure and cellular functionality, chronic activity of this pathway has been associated with insulin resistance and hyperglycemia-induced glucose toxicity. Three different forms of OGT are known: nucleocytoplasmic (ncOGT), short (sOGT), and mitochondrial (mOGT). Previous reports showed that overexpression of ncOGT is not toxic to the cell; in contrast, overexpression of mOGT is associated with cellular apoptosis. In this work, we suggest that hyperglycemia in the diabetic patient could induce greater expression and activity of mOGT, modifying the structure and functionality of mitochondria in hippocampal cells, accelerating neuronal damage, and favoring the start of AD. In consequence, mOGT activity could be a key point for AD development in patients with DM.

Transcriptional coregulators: fine-tuning metabolism

Metabolic homeostasis requires that cellular energy levels are adapted to environmental cues. This adaptation is largely regulated at the transcriptional level, through the interaction between transcription factors, coregulators, and the basal transcriptional machinery. Coregulators, which function as both metabolic sensors and transcriptional effectors, are ideally positioned to synchronize metabolic pathways to environmental stimuli. The balance between inhibitory actions of corepressors and stimulatory effects of coactivators enables the fine-tuning of metabolic processes. This tight regulation opens therapeutic opportunities to manage metabolic dysfunction by directing the activity of cofactors toward specific transcription factors, pathways, or cells/tissues, thereby restoring whole-body metabolic homeostasis.

Dynamic O-linked N-acetylglucosamine modification of proteins affects stress responses and survival of mesothelial cells exposed to peritoneal dialysis fluids

The ability of cells to respond and survive stressful conditions is determined, in part, by the attachment of O-linked N-acetylglucosamine (O-GlcNAc) to proteins (O-GlcNAcylation), a post-translational modification dependent on glucose and glutamine. This study investigates the role of dynamic O-GlcNAcylation of mesothelial cell proteins in cell survival during exposure to glucose-based peritoneal dialysis fluid (PDF). Immortalized human mesothelial cells and primary mesothelial cells, cultured from human omentum or clinical effluent of PD patients, were assessed for O-GlcNAcylation under normal conditions or after exposure to PDF. The dynamic status of O-GlcNAcylation and effects on cellular survival were investigated by chemical modulation with 6-diazo-5-oxo-L-norleucine (DON) to decrease or O-(2-acetamido-2-deoxy-D-glucopyranosylidene)amino N-phenyl carbamate (PUGNAc) to increase O-GlcNAc levels. Viability was decreased by reducing O-GlcNAc levels by DON, which also led to suppressed expression of the cytoprotective heat shock protein 72. In contrast, increasing O-GlcNAc levels by PUGNAc or alanyl-glutamine led to significantly improved cell survival paralleled by higher heat shock protein 72 levels during PDF treatment. Addition of alanyl-glutamine increased O-GlcNAcylation and partly counteracted its inhibition by DON, also leading to improved cell survival. Immunofluorescent analysis of clinical samples showed that the O-GlcNAc signal primarily originates from mesothelial cells. In conclusion, this study identified O-GlcNAcylation in mesothelial cells as a potentially important molecular mechanism after exposure to PDF. Modulating O-GlcNAc levels by clinically feasible interventions might evolve as a novel therapeutic target for the preservation of peritoneal membrane integrity in PD.

O-GlcNAc signaling in cancer metabolism and epigenetics

The covalent attachment of β-D-N-acetylglucosamine monosaccharides (O-GlcNAc) to serine/threonine residues of nuclear and cytoplasmic proteins is a major regulatory mechanism in cell physiology. Aberrant O-GlcNAc modification of signaling proteins, metabolic enzymes, and transcriptional and epigenetic regulators has been implicated in cancer. Relentless growth of cancer cells requires metabolic reprogramming that is intertwined with changes in the epigenetic landscape. This review highlights the emerging role of protein O-GlcNAcylation at the interface of cancer metabolism and epigenetics.

Chromosome imbalance as a driver of sex disparity in disease

It has long been recognized that men and women exhibit different risks for diverse disorders ranging from metabolic to autoimmune diseases. However, the underlying causes of these disparities remain obscure. Analysis of patients with chromosomal abnormalities, including Turner syndrome (45X) and Klinefelter syndrome (47XXY), has highlighted the importance of X-linked gene dosage as a contributing factor for disease susceptibility. Escape from X-inactivation and X-linked imprinting can result in transcriptional differences between normal men and women as well as in patients with sex chromosome abnormalities. Animal models support a role for X-linked gene dosage in disease with O-linked N-acetylglucosamine transferase (OGT) emerging as a prime candidate for a pleiotropic effector. OGT encodes a highly regulated nutrient-sensing epigenetic modifier with established links to immunity, metabolism and development.

Hepatic glucose sensing and integrative pathways in the liver

The hepatic glucose-sensing system is a functional network of enzymes and transcription factors that is critical for the maintenance of energy homeostasis and systemic glycemia. Here we review the recent literature on its components and metabolic actions. Glucokinase (GCK) is generally considered as the initial postprandial glucose-sensing component, which acts as the gatekeeper for hepatic glucose metabolism and provides metabolites that activate the transcription factor carbohydrate response element binding protein (ChREBP). Recently, liver receptor homolog 1 (LRH-1) has emerged as an upstream regulator of the central GCK-ChREBP axis, with a critical role in the integration of hepatic intermediary metabolism in response to glucose. Evidence is also accumulating that O-linked β-N-acetylglucosaminylation (O-GlcNAcylation) and acetylation can act as glucose-sensitive modifications that may contribute to hepatic glucose sensing by targeting regulatory proteins and the epigenome. Further elucidation of the components and functional roles of the hepatic glucose-sensing system may contribute to the future treatment of liver diseases associated with deregulated glucose sensors.

A guide into glycosciences: How chemistry, biochemistry and biology cooperate to crack the sugar code

BACKGROUND

The most demanding challenge in research on molecular aspects within the flow of biological information is posed by the complex carbohydrates (glycan part of cellular glycoconjugates). How the 'message' encoded in carbohydrate 'letters' is 'read' and 'translated' can only be unraveled by interdisciplinary efforts.

SCOPE OF REVIEW

This review provides a didactic step-by-step survey of the concept of the sugar code and the way strategic combination of experimental approaches characterizes structure-function relationships, with resources for teaching.

MAJOR CONCLUSIONS

The unsurpassed coding capacity of glycans is an ideal platform for generating a broad range of molecular 'messages'. Structural and functional analyses of complex carbohydrates have been made possible by advances in chemical synthesis, rendering production of oligosaccharides, glycoclusters and neoglycoconjugates possible. This availability facilitates to test the glycans as ligands for natural sugar receptors (lectins). Their interaction is a means to turn sugar-encoded information into cellular effects. Glycan/lectin structures and their spatial modes of presentation underlie the exquisite specificity of the endogenous lectins in counterreceptor selection, that is, to home in on certain cellular glycoproteins or glycolipids.

GENERAL SIGNIFICANCE

Understanding how sugar-encoded 'messages' are 'read' and 'translated' by lectins provides insights into fundamental mechanisms of life, with potential for medical applications.

O-GlcNAc profiling: from proteins to proteomes

O-linked β-D-N-acetylglucosamine (O-GlcNAc) modification (O-GlcNAcylation) onto serine and threonine residues of proteins is an important post-translational modification (PTM), which is involved in many crucial biological processes including transcription, translation, proteasomal degradation, and signal transduction. Aberrant protein O-GlcNAcylation is directly linked to the pathological progression of chronic diseases including diabetes, cancer, and neurodegenerative disorders. Identification, site mapping, and quantification of O-GlcNAc proteins are a prerequisite to decipher their functions. In this review, we mainly focus on technological developments regarding O-GlcNAc protein profiling. Specifically, on one hand, we show how these techniques are being used for the comprehensive characterization of certain targeted proteins in which biologists are most interested. On the other hand, we present several newly developed approaches for O-GlcNAcomic profiling as well as how they provide us with a systems perspective to crosstalk amongst different PTMs and complicated biological events. Promising technical trends are also highlighted to evoke more efforts by diverse laboratories, which would further expand our understanding of the physiological and pathological roles of protein O-GlcNAcylation in chronic diseases.

Fatty acid metabolism, energy expenditure and insulin resistance in muscle

Fatty acids (FAs) are essential elements of all cells and have significant roles as energy substrates, components of cellular structure and signalling molecules. The storage of excess energy intake as fat in adipose tissue is an evolutionary advantage aimed at protecting against starvation, but in much of today's world, humans are faced with an unlimited availability of food, and the excessive accumulation of fat is now a major risk for human health, especially the development of type 2 diabetes (T2D). Since the first recognition of the association between fat accumulation, reduced insulin action and increased risk of T2D, several mechanisms have been proposed to link excess FA availability to reduced insulin action, with some of them being competing or contradictory. This review summarises the evidence for these mechanisms in the context of excess dietary FAs generating insulin resistance in muscle, the major tissue involved in insulin-stimulated disposal of blood glucose. It also outlines potential problems with models and measurements that may hinder as well as help improve our understanding of the links between FAs and insulin action.

Recognition of diazirine-modified O-GlcNAc by human O-GlcNAcase

The mammalian O-GlcNAc hydrolase (OGA) removes O-GlcNAc from serine and threonine residues on intracellular glycoproteins. OGA activity is sensitive to N-acyl substitutions to O-GlcNAc, with alkyl diazirine-modified O-GlcNAc (O-GlcNDAz) being completely resistant to removal by OGA. Using homology modeling, we identified OGA residues proximal to the N-acyl position of O-GlcNAc substrate. Mutation of one of these residues, C215, results in mutant enzymes that are able to hydrolytically remove O-GlcNDAz from a model compound. Further, the C215A mutant is capable of removing O-GlcNDAz from a peptide substrate. These results can be used to improve metabolism of O-GlcNAc analogs in cells. In addition, the enzyme specificity studies reported here provide new insight into the active site of OGA, an important drug target.

O-GlcNAc: A Bittersweet Switch in Liver

The liver is a vital organ responsible for maintaining nutrient homeostasis. After a meal, insulin stimulates glycogen and lipid synthesis in the liver; in the fasted state, glucagon induces gluconeogenesis and ketogenesis, which produce glucose and ketone bodies for other tissues to use as energy sources. These metabolic changes involve spatiotemporally co-ordinated signaling cascades. O-linked β-N-acetylglucosamine (O-GlcNAc) modification has been recognized as a nutrient sensor and regulatory molecular switch. This review highlights mechanistic insights into spatiotemporal regulation of liver metabolism by O-GlcNAc modification and discusses its pathophysiological implications in insulin resistance, non-alcoholic fatty liver disease, and fibrosis.

Synthesis of o-linked glycoconjugates in the nervous system

Glycoproteins carrying O-linked N-acetylgalactosamine, N-acetylgluco-samine, mannose, fucose, glucose, and xylose are found in the nervous system. Lipids can be glycosylated as well. Membrane lipid, ceramide, is modified by the addition of either glucose or galactose to form glycosphingolipid, galactosylceramide, or glucosylceramide. Recent analyses have identified glucosylated lipids of cholesterol and phosphatidic acid. These O-linked carbohydrate residues are found primarily on the outer surface of the plasma membrane or in the extracellular space. Their expression is cell or tissue specific and developmentally regulated. Due to their structural diversity, they play important roles in a variety of biological processes such as membrane transport and cell-cell interactions.

O-GlcNAcylation and Metabolic Reprograming in Cancer

Although cancer metabolism has received considerable attention over the past decade, our knowledge on its specifics is still fragmentary. Altered cellular metabolism is one of the most important hallmarks of cancer. Cancer cells exhibit aberrant glucose metabolism characterized by aerobic glycolysis, a phenomenon known as Warburg effect. Accelerated glucose uptake and glycolysis are main characteristics of cancer cells that allow them for intensive growth and proliferation. Accumulating evidence suggests that O-GlcNAc transferase (OGT), an enzyme responsible for modification of proteins with N-acetylglucosamine, may act as a nutrient sensor that links hexosamine biosynthesis pathway to oncogenic signaling and regulation of factors involved in glucose and lipid metabolism. Recent studies suggest that metabolic reprograming in cancer is connected to changes at the epigenetic level. O-GlcNAcylation seems to play an important role in the regulation of the epigenome in response to cellular metabolic status. Through histone modifications and assembly of gene transcription complexes, OGT can impact on expression of genes important for cellular metabolism. This paper reviews recent findings related to O-GlcNAc-dependent regulation of signaling pathways, transcription factors, enzymes, and epigenetic changes involved in metabolic reprograming of cancer.

Regulation of transcription factor activity by interconnected post-translational modifications

Transcription factors comprise just over 7% of the human proteome and serve as gatekeepers of cellular function, integrating external signal information into gene expression programs that reconfigure cellular physiology at the most basic levels. Surface-initiated cell signaling pathways converge on transcription factors, decorating these proteins with an array of post-translational modifications (PTMs) that are often interdependent, being linked in time, space, and combinatorial function. These PTMs orchestrate every activity of a transcription factor over its entire lifespan--from subcellular localization to protein-protein interactions, sequence-specific DNA binding, transcriptional regulatory activity, and protein stability--and play key roles in the epigenetic regulation of gene expression. The multitude of PTMs of transcription factors also offers numerous potential points of intervention for development of therapeutic agents to treat a wide spectrum of diseases. We review PTMs most commonly targeting transcription factors, focusing on recent reports of sequential and linked PTMs of individual factors.

O-GlcNAcylation under hypoxic conditions and its effects on the blood-retinal barrier in diabetic retinopathy

An increase in O-linked N-acetylglucosamine (O-GlcNAc) protein modifications has been observerd in db/db mouse retinas. O-GlcNAc-modified proteins in the db/db mouse retina have been shown to be localized in the ganglion cell layer, the inner nuclear layer, the retina pigment epithelium (RPE) layer and the inner plexiform layer, in which hypoxia-inducible factor 1α (HIF1α) has also been shown to be localized. In the current study, we examined whether hypoxia increases O-GlcNAcylation in retinal vascular cells under high glucose conditions and whether HIF1α activation is consistent with the response to and activation of O-GlcNAcylation in retinal lesions in diabetic retinopathy. In addition, the effects of O-GlcNAcylation on the blood-retinal barrier were verified in vitro by the inhibition of O-GlcNAcylation. A time-dependent increase in the O-GlcNAcylation in bovine retinal vascular endothelial cells (BRVECs) was observed following incubation of the cells with high glucose medium (glucose 4.5 g/l) under hypoxic (1-3% O2) conditions. Hypoxia-induced BRVEC O-GlcNAcylation was not observed when the BRVECs were transfected with siRNA targeting O-GlcNAc transferase (OGT) or treated with alloxan (an OGT inhibitor) prior to exposure to high glucose. The increase in BRVEC O-GlcNAcylation induced by high glucose, as well as by thiamet G [an O-GlcNAcase (OGA) inhibitor] led to a reduction in occludin expression levels in vitro, which was prevented by treatment with OGT siRNA and alloxan. In conclusion, the current study demonstrates the relationship between O-GlcNAc glycosylation and hypoxia during diabetic retinopathy and that hyperglycemia induced O2 consumption activates HIF1α and O-GlcNAc modification protein in the same retinal layer. The reduced protein BRVEC O-GlcNAcylation levels exert protective effects on the blood-retinal barrier and thus represent a potential therapeutic target for the treatment of diabetic retinopathy.

Regulation of cancer metabolism by O-GlcNAcylation

Cancer cells exhibit increased uptake of glucose and glutamine, and rewire the metabolic flux toward anabolic pathways important for cell growth and proliferation. Understanding how this altered metabolism is regulated has recently emerged as an intense research focus in cancer biology. O-linked β-N-acetylglucosamine (O-GlcNAc) is a reversible posttranslational modification of serine and/or threonine residues of nuclear and cytosolic proteins. O-GlcNAcylation has been identified in numerous proteins that are involved in many important cellular functions, including transcription, translation, signal transduction, and stress responses. More recently, increasing evidence indicates that O-GlcNAcylation plays important roles in regulating cancer metabolic reprogramming by modifying key transcription factors, metabolic enzymes and major oncogenic signaling pathways. Thus, O-GlcNAcylation emerges as a novel regulatory mechanism linking altered metabolism to cancer pathogenesis.

O-GlcNAcylation at promoters, nutrient sensors, and transcriptional regulation

Post-translational modifications play important roles in transcriptional regulation. Among the less understood PTMs is O-GlcNAcylation. Nevertheless, O-GlcNAcylation in the nucleus is found on hundreds of transcription factors and coactivators and is often found in a mutually exclusive ying-yang relationship with phosphorylation. O-GlcNAcylation also links cellular metabolism directly to the proteome, serving as a conduit of metabolic information to the nucleus. This review serves as a brief introduction to O-GlcNAcylation, emphasizing its important thematic roles in transcriptional regulation, and highlights several recent and important additions to the literature that illustrate the connections between O-GlcNAc and transcription.

Regulation of protein degradation by O-GlcNAcylation: crosstalk with ubiquitination

The post-translational modification of intracellular proteins by O-linked N-acetylglucosamine (O-GlcNAc) regulates essential cellular processes such as signal transduction, transcription, translation, and protein degradation. Misfolded, damaged, and unwanted proteins are tagged with a chain of ubiquitin moieties for degradation by the proteasome, which is critical for cellular homeostasis. In this review, we summarize the current knowledge of the interplay between O-GlcNAcylation and ubiquitination in the control of protein degradation. Understanding the mechanisms of action of O-GlcNAcylation in the ubiquitin-proteosome system shall facilitate the development of therapeutics for human diseases such as cancer, metabolic syndrome, and neurodegenerative diseases.

O-GlcNAcylation: A New Cancer Hallmark?

O-linked N-acetylglucosaminylation (O-GlcNAcylation) is a reversible post-translational modification consisting in the addition of a sugar moiety to serine/threonine residues of cytosolic or nuclear proteins. Catalyzed by O-GlcNAc-transferase (OGT) and removed by O-GlcNAcase, this dynamic modification is dependent on environmental glucose concentration. O-GlcNAcylation regulates the activities of a wide panel of proteins involved in almost all aspects of cell biology. As a nutrient sensor, O-GlcNAcylation can relay the effects of excessive nutritional intake, an important cancer risk factor, on protein activities and cellular functions. Indeed, O-GlcNAcylation has been shown to play a significant role in cancer development through different mechanisms. O-GlcNAcylation and OGT levels are increased in different cancers (breast, prostate, colon…) and vary during cell cycle progression. Modulating their expression or activity can alter cancer cell proliferation and/or invasion. Interestingly, major oncogenic factors have been shown to be directly O-GlcNAcylated (p53, MYC, NFκB, β-catenin…). Furthermore, chromatin dynamics is modulated by O-GlcNAc. DNA methylation enzymes of the Tet family, involved epigenetic alterations associated with cancer, were recently found to interact with and target OGT to multi-molecular chromatin-remodeling complexes. Consistently, histones are subjected to O-GlcNAc modifications which regulate their function. Increasing number of evidences point out the central involvement of O-GlcNAcylation in tumorigenesis, justifying the attention received as a potential new approach for cancer treatment. However, comprehension of the underlying mechanism remains at its beginnings. Future challenge will be to address directly the role of O-GlcNAc-modified residues in oncogenic-related proteins to eventually propose novel strategies to alter cancer development and/or progression.