Nutrient shortage triggers the hexosamine biosynthetic pathway via the GCN2-ATF4 signalling pathway

Glucose metabolism through the hexosamine biosynthetic pathway drives hepatic de novo lipogenesis via promoting N-linked protein glycosylation

De novo lipogenesis (DNL) converts excess glucose into lipids, whereas the hexosamine biosynthetic pathway (HBP), a glycolytic branch, generates UDP-N-acetylglucosamine for protein glycosylation, including O-GlcNAcylation and N-linked glycosylation. Both pathways are active in hepatocytes and integral to glucose metabolism; however, their functional interplay remains unclear. Here, we investigated the role of HBP in hepatic DNL activation using both in vitro and in vivo models. AML12 hepatocytes were cultured in low- and high-glucose media with or without HBP blockade, both pharmacologically and genetically. For in vivo studies, male C57BL/6J mice were subjected to a fasting-refeeding regimen with or without intraperitoneal administration of azaserine, a competitive inhibitor of glutamine-fructose-6-phosphate transaminase 1 (GFPT1), the rate-limiting enzyme of the HBP. Our results demonstrated that, in AML12 cells, glucose exposure activated both DNL and HBP, leading to triacylglycerol (TAG) accumulation, whereas HBP inhibition ameliorated DNL and TAG accumulation. In mice, refeeding after a 24-h fasting induced hepatic DNL, which was abolished by HBP inhibition, indicating its mechanistic involvement in glucose-driven lipogenesis. Mechanistically, we identified ATF4 as a key regulator of GFPT1 upregulation under high-glucose conditions. As expected, both glucose-treated hepatocytes and livers from fasting-refed mice exhibited increased protein glycosylation. Notably, blocking N-linked glycosylation, but not O-GlcNAcylation, abolished glucose-induced DNL activation, indicating that HBP is essential for glucose-induced DNL pathway activation via promoting N-linked glycosylation, independent of O-GlcNAcylation. In conclusion, our findings establish that an intact HBP is required for glucose-induced hepatic DNL activation, primarily through promoting protein N-linked glycosylation.NEW & NOTEWORTHY High-glucose exposure activates both hepatic HBP and DNL pathways. The glucose metabolism into HBP is essential for the activation of the DNL pathway. ATF4 activation plays a mechanistic role in high glucose-induced HBP activation. HBP drives high glucose-induced hepatic DNL activation via promoting N-linked protein glycosylation.

Uncovering the intricacies of O-GlcNAc modification in cognitive impairment: New insights from regulation to therapeutic targeting

O-linked β-N-acetylglucosamine (O-GlcNAc) represents a post-translational modification that occurs on serine or threonine residues on various proteins. This conserved modification interacts with vital cellular pathways. Although O-GlcNAc is widely distributed throughout the body, it is particularly enriched in the brain, where most proteins are O-GlcNAcylated. Recent studies have established a causal link between O-GlcNAc regulation in the brain and alterations in neurophysiological function. Alterations in O-GlcNAc levels in the brain are associated with the pathogenesis of several neurogenic diseases that can lead to cognitive impairment. Remarkably, manipulation of O-GlcNAc levels demonstrated a protective effect on cognitive function. Although the precise molecular mechanism of O-GlcNAc modification in the nervous system remains elusive, its regulation is fundamental to multiple neural and cognitive functions, fluctuating levels during normal and pathological cognitive processes. In this review, we highlight the significant functional importance of O-GlcNAc modification in pathological cognitive impairments and the potential application of O-GlcNAc as a promising target for the intervention or amelioration of cognitive impairments.

The role of protein O-GlcNAcylation in diabetic cardiomyopathy

It is well established that diabetes markedly increases the risk of multiple types of heart disease including heart failure. However, despite substantial improvements in the treatment of heart failure in recent decades the relative increased risk associated with diabetes remains unchanged. There is increasing appreciation of the importance of the post translational modification by O-linked-N-acetylglucosamine (O-GlcNAc) of serine and threonine residues on proteins in regulating cardiomyocyte function and mediating stress responses. In response to diabetes there is a sustained increase in cardiac O-GlcNAc levels, which has been attributed to many of the adverse effects of diabetes on the heart. Here we provide an overview of potential mechanisms by which increased cardiac O-GlcNAcylation contributes to the adverse effects on the heart and highlight some of the key gaps in our knowledge.

Diabetic neuropathy: understanding the nexus of diabetic neuropathy, gut dysbiosis and cognitive impairment

Objectives

Diabetic neuropathy is a traditional and one of the most prevalent complications of diabetes mellitus. The exact pathophysiology of diabetic neuropathy is not fully understood. However, oxidative stress and inflammation are proven to be one of the major underlying mechanisms of neuropathy which is described in detail. Gut dysbiosis is being studied for various neurological disorders and its impact on diabetic neuropathy is also explained. Diabetic neuropathy also causes loss in an individual's quality of life and one such adverse event is cognitive dysfunction. The interrelation between the neuropathy, cognitive dysfunction and gut is reviewed.

Methods

The exact mechanism is not understood but several hypotheses, cross-sectional studies and systematic reviews suggest a relationship between cognition and neuropathy. The review has collected data from various review and research publications that justifies this inter-relationship.

Results

The multifactorial etiology and pathophysiology of diabetic neuropathy is described with special emphasis on the role of gut dysbiosis. There might exist a correlation between the neuropathy and cognitive impairment caused simultaneously in diabetic patients.

Conclusions

This review summarizes the relationship that might exist between diabetic neuropathy, cognitive dysfunction and the impact of disturbed gut microbiome on its development and progression.

Novel insights into the GCN2 pathway and its targeting. Therapeutic value in cancer and lessons from lung fibrosis development

Defining the mechanisms that allow cells to adapt to environmental stress is critical for understanding the progression of chronic diseases and identifying relevant drug targets. Among these, activation of the pathway controlled by the eIF2-alpha kinase GCN2 is critical for translational and metabolic reprogramming of the cell in response to various metabolic, proteotoxic, and ribosomal stressors. However, its role has frequently been investigated through the lens of a stress pathway signaling via the eIF2α-activating transcription factor 4 (ATF4) downstream axis, while recent advances in the field have revealed that the GCN2 pathway is more complex than previously thought. Indeed, this kinase can be activated through a variety of mechanisms, phosphorylate substrates other than eIF2α, and regulate cell proliferation in a steady state. This review presents recent findings regarding the fundamental mechanisms underlying GCN2 signaling and function, as well as the development of drugs that modulate its activity. Furthermore, by comparing the literature on GCN2's antagonistic roles in two challenging pathologies, cancer and pulmonary diseases, the benefits, and drawbacks of GCN2 targeting, particularly inhibition, are discussed.

O-GlcNAcylation mediates Wnt-stimulated bone formation by rewiring aerobic glycolysis

Wnt signaling is an important target for anabolic therapies in osteoporosis. A sclerostin-neutralizing antibody (Scl-Ab), that blocks the Wnt signaling inhibitor (sclerostin), has been shown to promote bone mass in animal models and clinical studies. However, the cellular mechanisms by which Wnt signaling promotes osteogenesis remain to be further investigated. O-GlcNAcylation, a dynamic post-translational modification of proteins, controls multiple critical biological processes including transcription, translation, and cell fate determination. Here, we report that Wnt3a either induces O-GlcNAcylation rapidly via the Ca2+-PKA-Gfat1 axis, or increases it in a Wnt-β-catenin-dependent manner following prolonged stimulation. Importantly, we find O-GlcNAcylation indispensable for osteoblastogenesis both in vivo and in vitro. Genetic ablation of O-GlcNAcylation in the osteoblast-lineage diminishes bone formation and delays bone fracture healing in response to Wnt stimulation in vivo. Mechanistically, Wnt3a induces O-GlcNAcylation at Serine 174 of PDK1 to stabilize the protein, resulting in increased glycolysis and osteogenesis. These findings highlight O-GlcNAcylation as an important mechanism regulating Wnt-induced glucose metabolism and bone anabolism.

The stress sensor GCN2 differentially controls ribosome biogenesis in colon cancer according to the nutritional context

Nutrient availability is a key determinant of tumor cell behavior. While nutrient-rich conditions favor proliferation and tumor growth, scarcity, and particularly glutamine starvation, promotes cell dedifferentiation and chemoresistance. Here, linking ribosome biogenesis plasticity with tumor cell fate, we uncover that the amino acid sensor general control non-derepressible 2 (GCN2; also known as eIF-2-alpha kinase 4) represses the expression of the precursor of ribosomal RNA (rRNA), 47S, under metabolic stress. We show that blockade of GCN2 triggers cell death by an irremediable nucleolar stress and subsequent TP53-mediated apoptosis in patient-derived models of colon adenocarcinoma (COAD). In nutrient-rich conditions, a cell-autonomous GCN2 activity supports cell proliferation by stimulating 47S rRNA transcription, independently of the canonical integrated stress response (ISR) axis. Impairment of GCN2 activity prevents nuclear translocation of methionyl-tRNA synthetase (MetRS), resulting in nucleolar stress, mTORC1 inhibition and, ultimately, autophagy induction. Inhibition of the GCN2-MetRS axis drastically improves the cytotoxicity of RNA polymerase I (RNA pol I) inhibitors, including the first-line chemotherapy oxaliplatin, on patient-derived COAD tumoroids. Our data thus reveal that GCN2 differentially controls ribosome biogenesis according to the nutritional context. Furthermore, pharmacological co-inhibition of the two GCN2 branches and RNA pol I activity may represent a valuable strategy for elimination of proliferative and metabolically stressed COAD cells.

Uncloaking the viral glycocalyx: How do viruses exploit glycoimmune checkpoints?

The surfaces of cells and enveloped viruses alike are coated in carbohydrates that play multifarious roles in infection and immunity. Organisms across all kingdoms of life make use of a diverse set of monosaccharide subunits, glycosidic linkages, and branching patterns to encode information within glycans. Accordingly, sugar-patterning enzymes and glycan binding proteins play integral roles in cell and organismal biology, ranging from glycoprotein quality control within the endoplasmic reticulum to lymphocyte migration, coagulation, inflammation, and tissue homeostasis. Unsurprisingly, genes involved in generating and recognizing oligosaccharide patterns are playgrounds for evolutionary conflicts that abound in cross-species interactions, exemplified by the myriad plant lectins that function as toxins. In vertebrates, glycans bearing acidic nine-carbon sugars called sialic acids are key regulators of immune responses. Various bacterial and fungal pathogens adorn their cells in sialic acids that either mimic their hosts' or are stolen from them. Yet, how viruses commandeer host sugar-patterning enzymes to thwart immune responses remains poorly studied. Here, we review examples of viruses that interact with sialic acid-binding immunoglobulin-like lectins (Siglecs), a family of immune cell receptors that regulate toll-like receptor signaling and govern glycoimmune checkpoints, while highlighting knowledge gaps that merit investigation. Efforts to illuminate how viruses leverage glycan-dependent checkpoints may translate into new clinical treatments that uncloak viral antigens and infected cell surfaces by removing or masking immunosuppressive sialoglycans, or by inhibiting viral gene products that induce their biosynthesis. Such approaches may hold the potential to unleash the immune system to clear long intractable chronic viral infections.

Regulation of protein O-GlcNAcylation by circadian, metabolic, and cellular signals

O-linked β-N-acetylglucosamine (O-GlcNAcylation) is a dynamic post-translational modification that regulates thousands of proteins and almost all cellular processes. Aberrant O-GlcNAcylation has been associated with numerous diseases, including cancer, neurodegenerative diseases, cardiovascular diseases, and type 2 diabetes. O-GlcNAcylation is highly nutrient-sensitive since it is dependent on UDP-GlcNAc, the end product of the hexosamine biosynthetic pathway (HBP). We previously observed daily rhythmicity of protein O-GlcNAcylation in a Drosophila model that is sensitive to the timing of food consumption. We showed that the circadian clock is pivotal in regulating daily O-GlcNAcylation rhythms given its control of the feeding-fasting cycle and hence nutrient availability. Interestingly, we reported that the circadian clock also modulates daily O-GlcNAcylation rhythm by regulating molecular mechanisms beyond the regulation of food consumption time. A large body of work now indicates that O-GlcNAcylation is likely a generalized cellular status effector as it responds to various cellular signals and conditions, such as ER stress, apoptosis, and infection. In this review, we summarize the metabolic regulation of protein O-GlcNAcylation through nutrient availability, HBP enzymes, and O-GlcNAc processing enzymes. We discuss the emerging roles of circadian clocks in regulating daily O-GlcNAcylation rhythm. Finally, we provide an overview of other cellular signals or conditions that impact O-GlcNAcylation. Many of these cellular pathways are themselves regulated by the clock and/or metabolism. Our review highlights the importance of maintaining optimal O-GlcNAc rhythm by restricting eating activity to the active period under physiological conditions and provides insights into potential therapeutic targets of O-GlcNAc homeostasis under pathological conditions.

Nutrient inputs and social metabolic control of T cell fate

Cells in multicellular organisms experience diverse neighbors, signals, and evolving physical environments that drive functional and metabolic demands. To maintain proper development and homeostasis while avoiding inappropriate cell proliferation or death, individual cells interact with their neighbors via "social" cues to share and partition available nutrients. Metabolic signals also contribute to cell fate by providing biochemical links between cell-extrinsic signals and available resources. In addition to metabolic checkpoints that sense nutrients and directly supply molecular intermediates for biosynthetic pathways, many metabolites directly signal or provide the basis for post-translational modifications of target proteins and chromatin. In this review, we survey the landscape of T cell nutrient sensing and metabolic signaling that supports proper immunity while avoiding immunodeficiency or autoimmunity. The integration of cell-extrinsic microenvironmental cues with cell-intrinsic metabolic signaling provides a social metabolic control model to integrate cell signaling, metabolism, and fate.

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

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

Hexosamine biosynthetic pathway and O-GlcNAc cycling of glucose metabolism in brain function and disease

Impaired brain glucose metabolism is considered a hallmark of brain dysfunction and neurodegeneration. Disruption of the hexosamine biosynthetic pathway (HBP) and subsequent O-linked N-acetylglucosamine (O-GlcNAc) cycling has been identified as an emerging link between altered glucose metabolism and defects in the brain. Myriads of cytosolic and nuclear proteins in the nervous system are modified at serine or threonine residues with a single N-acetylglucosamine (O-GlcNAc) molecule by O-GlcNAc transferase (OGT), which can be removed by β-N-acetylglucosaminidase (O-GlcNAcase, OGA). Homeostatic regulation of O-GlcNAc cycling is important for the maintenance of normal brain activity. Although significant evidence linking dysregulated HBP metabolism and aberrant O-GlcNAc cycling to induction or progression of neuronal diseases has been obtained, the issue of whether altered O-GlcNAcylation is causal in brain pathogenesis remains uncertain. Elucidation of the specific functions and regulatory mechanisms of individual O-GlcNAcylated neuronal proteins in both normal and diseased states may facilitate the identification of novel therapeutic targets for various neuronal disorders. The information presented in this review highlights the importance of HBP/O-GlcNAcylation in the neuronal system and summarizes the roles and potential mechanisms of O-GlcNAcylated neuronal proteins in maintaining normal brain function and initiation and progression of neurological diseases.

The Hexosamine Biosynthesis Pathway: Regulation and Function

The hexosamine biosynthesis pathway (HBP) produces uridine diphosphate-N-acetyl glucosamine, UDP-GlcNAc, which is a key metabolite that is used for N- or O-linked glycosylation, a co- or post-translational modification, respectively, that modulates protein activity and expression. The production of hexosamines can occur via de novo or salvage mechanisms that are catalyzed by metabolic enzymes. Nutrients including glutamine, glucose, acetyl-CoA, and UTP are utilized by the HBP. Together with availability of these nutrients, signaling molecules that respond to environmental signals, such as mTOR, AMPK, and stress-regulated transcription factors, modulate the HBP. This review discusses the regulation of GFAT, the key enzyme of the de novo HBP, as well as other metabolic enzymes that catalyze the reactions to produce UDP-GlcNAc. We also examine the contribution of the salvage mechanisms in the HBP and how dietary supplementation of the salvage metabolites glucosamine and N-acetylglucosamine could reprogram metabolism and have therapeutic potential. We elaborate on how UDP-GlcNAc is utilized for N-glycosylation of membrane and secretory proteins and how the HBP is reprogrammed during nutrient fluctuations to maintain proteostasis. We also consider how O-GlcNAcylation is coupled to nutrient availability and how this modification modulates cell signaling. We summarize how deregulation of protein N-glycosylation and O-GlcNAcylation can lead to diseases including cancer, diabetes, immunodeficiencies, and congenital disorders of glycosylation. We review the current pharmacological strategies to inhibit GFAT and other enzymes involved in the HBP or glycosylation and how engineered prodrugs could have better therapeutic efficacy for the treatment of diseases related to HBP deregulation.

HIF and ER stress are involved in TGFβ1-mediated wound closure of alveolar epithelial cells

Purpose: Alveolar epithelium dysfunction is associated with a very large spectrum of disease and an abnormal repair capacity of the airway epithelium has been proposed to explain the pathogenesis of Idiopathic Pulmonary Fibrosis (IPF). Following epithelium insult, the damaged cells will activate pathways implicated in the repair process, including proliferation and acquisition of migratory capacities to cover the denuded basement membrane. Induction of Endoplasmic Reticulum stress may be implicated in this process. Interestingly, ER stress excessive activation has been proposed as a central event associated with aberrant repair process and cellular dysfunction observed in IPF. Methods: We study by wound healing assay the molecular targets associated with Alveolar Epithelial Cells (AEC) repair. Results: We demonstrate that the wound recovery of AEC is associated with TGF-β1 signaling and increased transcriptional activity of ER stress and HIF-dependent genes. We further demonstrated that inhibition of TGF-β1 signaling, CHOP expression or HIF-1 expression, limits AECs wound closure. Conclusion: the use of pharmacological drugs targeting the ER/HIF-1 axis could be an attractive approach to limit AEC dysregulation in pathological condition, and confirmed a critical role of theses factor in response to alveolar injury.

HIF: a master regulator of nutrient availability and metabolic cross-talk in the tumor microenvironment

A role for hypoxia-inducible factors (HIFs) in hypoxia-dependent regulation of tumor cell metabolism has been thoroughly investigated and covered in reviews. However, there is limited information available regarding HIF-dependent regulation of nutrient fates in tumor and stromal cells. Tumor and stromal cells may generate nutrients necessary for function (metabolic symbiosis) or deplete nutrients resulting in possible competition between tumor cells and immune cells, a result of altered nutrient fates. HIF and nutrients in the tumor microenvironment (TME) affect stromal and immune cell metabolism in addition to intrinsic tumor cell metabolism. HIF-dependent metabolic regulation will inevitably result in the accumulation or depletion of essential metabolites in the TME. In response, various cell types in the TME will respond to these hypoxia-dependent alterations by activating HIF-dependent transcription to alter nutrient import, export, and utilization. In recent years, the concept of metabolic competition has been proposed for critical substrates, including glucose, lactate, glutamine, arginine, and tryptophan. In this review, we discuss how HIF-mediated mechanisms control nutrient sensing and availability in the TME, the competition for nutrients, and the metabolic cross-talk between tumor and stromal cells.

Genetic Ablation of the Nutrient Sensor Ogt in Endocrine Progenitors Is Dispensable for β-Cell Development but Essential for Maintenance of β-Cell Mass

Previously we utilized a murine model to demonstrate that Ogt deletion in pancreatic progenitors (OgtKOPanc) causes pancreatic hypoplasia, partly mediated by a reduction in the Pdx1-expressing pancreatic progenitor pool. Here, we continue to explore the role of Ogt in pancreas development by deletion of Ogt in the endocrine progenitors (OgtKOEndo). At birth OgtKOEndo, were normoglycemic and had comparable pancreas weight and α-cell, and β-cell mass to littermate controls. At postnatal day 23, OgtKOEndo displayed wide ranging but generally elevated blood glucose levels, with histological analyses showing aberrant islet architecture with α-cells invading the islet core. By postnatal day 60, these mice were overtly diabetic and showed significant loss of both α-cell and β-cell mass. Together, these results highlight the indispensable role of Ogt in maintenance of β-cell mass and glucose homeostasis.

Excessive gluconeogenesis causes the hepatic insulin resistance paradox and its sequelae

Background

Hepatic insulin signaling suppresses gluconeogenesis but promotes de novo lipid synthesis. Paradoxically, hepatic insulin resistance (HIR) enhances both gluconeogenesis and de novo lipid synthesis. Elucidation of the etiology of this paradox, which participates in the pathogenesis of non-alcoholic fatty liver disease (NAFLD), cardiovascular disease, the metabolic syndrome and hepatocellular carcinoma, has not been fully achieved.

Scope of review

This article briefly outlines the previously proposed hypotheses on the etiology of the HIR paradox. It then discusses literature consistent with an alternative hypothesis that excessive gluconeogenesis, the direct effect of HIR, is responsible for the aberrant lipogenesis. The mechanisms involved therein are explained, involving de novo synthesis of fructose and uric acid, promotion of glutamine anaplerosis, and induction of glucagon resistance. Thus, gluconeogenesis via lipogenesis promotes hepatic steatosis, a component of NAFLD, and dyslipidemia. Gluconeogenesis-centred mechanisms for the progression of NAFLD from simple steatosis to non-alcoholic steatohepatitis (NASH) and fibrosis are suggested. That NAFLD often precedes and predicts type 2 diabetes is explained by the ability of lipogenesis to cushion against blood glucose dysregulation in the earlier stages of NAFLD.

Major conclusions

HIR-induced excessive gluconeogenesis is a major cause of the HIR paradox and its sequelae. Such involvement of gluconeogenesis in lipid synthesis rationalizes the fact that several types of antidiabetic drugs ameliorate NAFLD. Thus, dietary, lifestyle and pharmacological targeting of HIR and hepatic gluconeogenesis may be a most viable approach for the prevention and management of the HIR-associated network of diseases.

Integration of O-GlcNAc into Stress Response Pathways

The modification of nuclear, mitochondrial, and cytosolic proteins by O-linked βN-acetylglucosamine (O-GlcNAc) has emerged as a dynamic and essential post-translational modification of mammalian proteins. O-GlcNAc is cycled on and off over 5000 proteins in response to diverse stimuli impacting protein function and, in turn, epigenetics and transcription, translation and proteostasis, metabolism, cell structure, and signal transduction. Environmental and physiological injury lead to complex changes in O-GlcNAcylation that impact cell and tissue survival in models of heat shock, osmotic stress, oxidative stress, and hypoxia/reoxygenation injury, as well as ischemic reperfusion injury. Numerous mechanisms that appear to underpin O-GlcNAc-mediated survival include changes in chaperone levels, impacts on the unfolded protein response and integrated stress response, improvements in mitochondrial function, and reduced protein aggregation. Here, we discuss the points at which O-GlcNAc is integrated into the cellular stress response, focusing on the roles it plays in the cardiovascular system and in neurodegeneration.

Glycan quality control in and out of the endoplasmic reticulum of mammalian cells

The endoplasmic reticulum (ER) is equipped with multiple quality control systems (QCS) that are necessary for shaping the glycoproteome of eukaryotic cells. These systems facilitate the productive folding of glycoproteins, eliminate defective products, and function as effectors to evoke cellular signaling in response to various cellular stresses. These ER functions largely depend on glycans, which contain sugar-based codes that, when needed, function to recruit carbohydrate-binding proteins that determine the fate of glycoproteins. To ensure their functionality, the biosynthesis of such glycans is therefore strictly monitored by a system that selectively degrades structurally defective glycans before adding them to proteins. This system, which is referred to as the glycan QCS, serves as a mechanism to reduce the risk of abnormal glycosylation under conditions where glycan biosynthesis is genetically or metabolically stalled. On the other hand, glycan QCS increases the risk of global hypoglycosylation by limiting glycan availability, which can lead to protein misfolding and the activation of unfolded protein response to maintaining cell viability or to initiate cell death programs. This review summarizes the current state of our knowledge of the mechanisms underlying glycan QCS in mammals and its physiological and pathological roles in embryogenesis, tumor progression, and congenital disorders associated with abnormal glycosylation.

The role and potential mechanism of O-Glycosylation in gastrointestinal tumors

Glycosylation is a critical post-translational modification (PTM) that affects the function of proteins and regulates cell signaling, thereby regulating various biological processes. Protein oxygen-N-acetylglucosamine (O-GlcNAc) glycosylation modifications are glycochemical modifications that occur within cells in the signal transduction and are frequently found in the cytoplasm and nucleus. Due to the rapid and reversible addition and removal, O-GlcNAc modifications are able to reversibly compete with certain phosphorylation modifications, immediately regulate the activity of proteins, and participate in kinds of cellular metabolic and signal transduction pathways, playing a pivotal role in the regulation of tumors, diabetes, and other diseases. This article provided a brief overview of O-GlcNAc glycosylation modification, introduced its role in altering the progression and immune response regulation of gastrointestinal tumors, and discussed its potential use as a marker of tumor neogenesis.

Nutrient-sensitive protein O-GlcNAcylation shapes daily biological rhythms

O-linked-N-acetylglucosaminylation (O-GlcNAcylation) is a nutrient-sensitive protein modification that alters the structure and function of a wide range of proteins involved in diverse cellular processes. Similar to phosphorylation, another protein modification that targets serine and threonine residues, O-GlcNAcylation occupancy on cellular proteins exhibits daily rhythmicity and has been shown to play critical roles in regulating daily rhythms in biology by modifying circadian clock proteins and downstream effectors. We recently reported that daily rhythm in global O-GlcNAcylation observed in Drosophila tissues is regulated via the integration of circadian and metabolic signals. Significantly, mistimed feeding, which disrupts coordination of these signals, is sufficient to dampen daily O-GlcNAcylation rhythm and is predicted to negatively impact animal biological rhythms and health span. In this review, we provide an overview of published and potential mechanisms by which metabolic and circadian signals regulate hexosamine biosynthetic pathway metabolites and enzymes, as well as O-GlcNAc processing enzymes to shape daily O-GlcNAcylation rhythms. We also discuss the significance of functional interactions between O-GlcNAcylation and other post-translational modifications in regulating biological rhythms. Finally, we highlight organ/tissue-specific cellular processes and molecular pathways that could be modulated by rhythmic O-GlcNAcylation to regulate time-of-day-specific biology.

A nexus of lipid and O-Glcnac metabolism in physiology and disease

Although traditionally considered a glucose metabolism-associated modification, the O-linked β-N-Acetylglucosamine (O-GlcNAc) regulatory system interacts extensively with lipids and is required to maintain lipid homeostasis. The enzymes of O-GlcNAc cycling have molecular properties consistent with those expected of broad-spectrum environmental sensors. By direct protein-protein interactions and catalytic modification, O-GlcNAc cycling enzymes may provide both acute and long-term adaptation to stress and other environmental stimuli such as nutrient availability. Depending on the cell type, hyperlipidemia potentiates or depresses O-GlcNAc levels, sometimes biphasically, through a diversity of unique mechanisms that target UDP-GlcNAc synthesis and the availability, activity and substrate selectivity of the glycosylation enzymes, O-GlcNAc Transferase (OGT) and O-GlcNAcase (OGA). At the same time, OGT activity in multiple tissues has been implicated in the homeostatic regulation of systemic lipid uptake, storage and release. Hyperlipidemic patterns of O-GlcNAcylation in these cells are consistent with both transient physiological adaptation and feedback uninhibited obesogenic and metabolic dysregulation. In this review, we summarize the numerous interconnections between lipid and O-GlcNAc metabolism. These links provide insights into how the O-GlcNAc regulatory system may contribute to lipid-associated diseases including obesity and metabolic syndrome.

Differential effects of glucose and N-acetylglucosamine on genome instability

Aberrant sugar metabolism is linked to an increased risk of pancreatic cancer. Previously, we found that high glucose induces genome instability and de novo oncogenic KRAS mutation preferentially in pancreatic cells through dysregulation of O-GlcNAcylation. Increasing O-GlcNAcylation by extrinsically supplying N-acetyl-D-glucosamine (GlcNAc) causes genome instability in all kinds of cell types regardless of pancreatic origin. Since many people consume excessive amount of sugar (glucose, fructose, and sucrose) in daily life, whether high sugar consumption directly causes genome instability in animals remains to be elucidated. In this communication, we show that excess sugar in the daily drink increases DNA damage and protein O-GlcNAcylation preferentially in pancreatic tissue but not in other kinds of tissue of mice. The effect of high sugar on the pancreatic tissue may be attributed to the intrinsic ratio of GFAT and PFK activity, a limiting factor that dictates UDP-GlcNAc levels. On the other hand, GlcNAc universally induces DNA damage in all six organs examined. Either inhibiting O-GlcNAcylation or supplementing dNTP pool diminishes the induced DNA damage in these organs, indicating that the mechanism of action is similar to that of high glucose treatment in pancreatic cells. Taken together, these results suggest the potential hazards of high sugar drinks and high glucosamine intake to genomic instability and possibly cancer initiation.

The hexosamine pathway and coat complex II promote malignant adaptation to nutrient scarcity

We present adaptive mechanisms of resistance of lung adenocarcinoma to their harsh microenvironment, which typically contains a lower glucose concentration compared with normal tissue.

The glucose-requiring hexosamine biosynthetic pathway (HBP), which produces UDP-N-acetylglucosamine for glycosylation reactions, promotes lung adenocarcinoma (LUAD) progression. However, lung tumor cells often reside in low-nutrient microenvironments, and whether the HBP is involved in the adaptation of LUAD to nutrient stress is unknown. Here, we show that the HBP and the coat complex II (COPII) play a key role in cell survival during glucose shortage. HBP up-regulation withstood low glucose-induced production of proteins bearing truncated N-glycans, in the endoplasmic reticulum. This function for the HBP, alongside COPII up-regulation, rescued cell surface expression of a subset of glycoproteins. Those included the epidermal growth factor receptor (EGFR), allowing an EGFR-dependent cell survival under low glucose in anchorage-independent growth. Accordingly, high expression of the HBP rate-limiting enzyme GFAT1 was associated with wild-type EGFR activation in LUAD patient samples. Notably, HBP and COPII up-regulation distinguished LUAD from the lung squamous-cell carcinoma subtype, thus uncovering adaptive mechanisms of LUAD to their harsh microenvironment.

Comorbidity-associated glutamine deficiency is a predisposition to severe COVID-19

SARS-CoV-2 vaccinations have greatly reduced COVID-19 cases, but we must continue to develop our understanding of the nature of the disease and its effects on human immunity. Previously, we suggested that a dysregulated STAT3 pathway following SARS-Co-2 infection ultimately leads to PAI-1 activation and cascades of pathologies. The major COVID-19-associated metabolic risks (old age, hypertension, cardiovascular diseases, diabetes, and obesity) share high PAI-1 levels and could predispose certain groups to severe COVID-19 complications. In this review article, we describe the common metabolic profile that is shared between all of these high-risk groups and COVID-19. This profile not only involves high levels of PAI-1 and STAT3 as previously described, but also includes low levels of glutamine and NAD+, coupled with overproduction of hyaluronan (HA). SARS-CoV-2 infection exacerbates this metabolic imbalance and predisposes these patients to the severe pathophysiologies of COVID-19, including the involvement of NETs (neutrophil extracellular traps) and HA overproduction in the lung. While hyperinflammation due to proinflammatory cytokine overproduction has been frequently documented, it is recently recognized that the immune response is markedly suppressed in some cases by the expansion and activity of MDSCs (myeloid-derived suppressor cells) and FoxP3+ Tregs (regulatory T cells). The metabolomics profiles of severe COVID-19 patients and patients with advanced cancer are similar, and in high-risk patients, SARS-CoV-2 infection leads to aberrant STAT3 activation, which promotes a cancer-like metabolism. We propose that glutamine deficiency and overproduced HA is the central metabolic characteristic of COVID-19 and its high-risk groups. We suggest the usage of glutamine supplementation and the repurposing of cancer drugs to prevent the development of severe COVID-19 pneumonia.

O-GlcNAcylation links oncogenic signals and cancer epigenetics

Prevalent dysregulation of epigenetic modifications plays a pivotal role in cancer. Targeting epigenetic abnormality is a new strategy for cancer therapy. Understanding how conventional oncogenic factors cause epigenetic abnormality is of great basic and translational value. O-GlcNAcylation is a protein modification which affects physiology and pathophysiology. In mammals, O-GlcNAcylation is catalyzed by one single enzyme OGT and removed by one single enzyme OGA. O-GlcNAcylation is affected by the availability of the donor, UDP-GlcNAc, generated by the serial enzymatic reactions in the hexoamine biogenesis pathway (HBP). O-GlcNAcylation regulates a wide spectrum of substrates including many proteins involved in epigenetic modification. Like epigenetic modifications, abnormality of O-GlcNAcylation is also common in cancer. Studies have revealed substantial impact on HBP enzymes and OGT/OGA by oncogenic signals. In this review, we will first summarize how oncogenic signals regulate HBP enzymes, OGT and OGA in cancer. We will then integrate this knowledge with the up to date understanding how O-GlcNAcylation regulates epigenetic machinery. With this, we propose a signal axis from oncogenic signals through O-GlcNAcylation dysregulation to epigenetic abnormality in cancer. Further elucidation of this axis will not only advance our understanding of cancer biology but also provide new revenues towards cancer therapy.

Laminar Flow on Endothelial Cells Suppresses eNOS O-GlcNAcylation to Promote eNOS Activity

[Figure: see text].

Nutrients in the fate of pluripotent stem cells

Pluripotent stem cells model certain features of early mammalian development ex vivo. Medium-supplied nutrients can influence self-renewal, lineage specification, and earliest differentiation of pluripotent stem cells. However, which specific nutrients support these distinct outcomes, and their mechanisms of action, remain under active investigation. Here, we evaluate the available data on nutrients and their metabolic conversion that influence pluripotent stem cell fates. We also discuss key questions open for investigation in this rapidly expanding area of increasing fundamental and practical importance.

MEF2D-NR4A1-FAM134B2-mediated reticulophagy contributes to amino acid homeostasis

We recently identified FAM134B2, which is an N-terminal truncated reticulophagy receptor highly induced by starvation such as fasting of mice and treatment of mammalian cells with a starvation medium that does not contain amino acids, glucose and growth factors. However, which starvation signal mediates the induction of FAM134B2 is still obscure. In this study, we found that amino acid deficiency (AAD) could mimic the starvation condition to induce FAM134B2 expression. Unexpectedly, EIF2AK4/GCN2-mediated integrated signal response (ISR) and MTOR (mechanistic target of rapamycin kinase) signals, which constitute two major signaling pathways that respond to AAD, did not contribute to AAD-induced FAM134B2 induction. mRNA-seq and siRNA screenings identified two ISR-independent transcription factors, MEF2D (myocyte enhancer factor 2D) and NR4A1 (nuclear receptor subfamily 4 group A member 1), involved in AAD-induced FAM134B2 expression. AAD induces MEF2D, resulting in the induction of NR4A1, which in turn induces FAM134B2-mediated reticulophagy to maintain intracellular amino acid levels. In conclusion, the MEF2D-NR4A1-FAM134B2 cascade is a critical signal in amino acid homeostasis.

The Warburg effect is necessary to promote glycosylation in the blastema during zebrafish tail regeneration

Throughout their lifetime, fish maintain a high capacity for regenerating complex tissues after injury. We utilized a larval tail regeneration assay in the zebrafish Danio rerio, which serves as an ideal model of appendage regeneration due to its easy manipulation, relatively simple mixture of cell types, and superior imaging properties. Regeneration of the embryonic zebrafish tail requires development of a blastema, a mass of dedifferentiated cells capable of replacing lost tissue, a crucial step in all known examples of appendage regeneration. Using this model, we show that tail amputation triggers an obligate metabolic shift to promote glucose metabolism during early regeneration similar to the Warburg effect observed in tumor forming cells. Inhibition of glucose metabolism did not affect the overall health of the embryo but completely blocked the tail from regenerating after amputation due to the failure to form a functional blastema. We performed a time series of single-cell RNA sequencing on regenerating tails with and without inhibition of glucose metabolism. We demonstrated that metabolic reprogramming is required for sustained TGF-β signaling and blocking glucose metabolism largely mimicked inhibition of TGF-β receptors, both resulting in an aberrant blastema. Finally, we showed using genetic ablation of three possible metabolic pathways for glucose, that metabolic reprogramming is required to provide glucose specifically to the hexosamine biosynthetic pathway while neither glycolysis nor the pentose phosphate pathway were necessary for regeneration.

FHL2 anchors mitochondria to actin and adapts mitochondrial dynamics to glucose supply

Mitochondrial movement and distribution are fundamental to their function. Here we report a mechanism that regulates mitochondrial movement by anchoring mitochondria to the F-actin cytoskeleton. This mechanism is activated by an increase in glucose influx and the consequent O-GlcNAcylation of TRAK (Milton), a component of the mitochondrial motor-adaptor complex. The protein four and a half LIM domains protein 2 (FHL2) serves as the anchor. FHL2 associates with O-GlcNAcylated TRAK and is both necessary and sufficient to drive the accumulation of F-actin around mitochondria and to arrest mitochondrial movement by anchoring to F-actin. Disruption of F-actin restores mitochondrial movement that had been arrested by either TRAK O-GlcNAcylation or forced direction of FHL2 to mitochondria. This pathway for mitochondrial immobilization is present in both neurons and non-neuronal cells and can thereby adapt mitochondrial dynamics to changes in glucose availability.

O-GlcNAcylation and O-GlcNAc Cycling Regulate Gene Transcription: Emerging Roles in Cancer

Simple Summary

O-linked β-N-acetylglucosamine (O-GlcNAc) is a post-translational modification (PTM) linking nutrient flux through the hexosamine biosynthetic pathway (HBP) to gene transcription. Mounting experimental and clinical data implicates aberrant O-GlcNAcylation in the development and progression of cancer. Herein, we discuss how alteration of O-GlcNAc-regulated transcriptional mechanisms leads to atypical gene expression in cancer. We discuss the challenges associated with studying O-GlcNAc function and present several new approaches for studies of O-GlcNAc-regulated transcription.

Abstract

O-linked β-N-acetylglucosamine (O-GlcNAc) is a single sugar post-translational modification (PTM) of intracellular proteins linking nutrient flux through the Hexosamine Biosynthetic Pathway (HBP) to the control of cis-regulatory elements in the genome. Aberrant O-GlcNAcylation is associated with the development, progression, and alterations in gene expression in cancer. O-GlcNAc cycling is defined as the addition and subsequent removal of the modification by O-GlcNAc Transferase (OGT) and O-GlcNAcase (OGA) provides a novel method for cells to regulate various aspects of gene expression, including RNA polymerase function, epigenetic dynamics, and transcription factor activity. We will focus on the complex relationship between phosphorylation and O-GlcNAcylation in the regulation of the RNA Polymerase II (RNAP II) pre-initiation complex and the regulation of the carboxyl-terminal domain of RNAP II via the synchronous actions of OGT, OGA, and kinases. Additionally, we discuss how O-GlcNAcylation of TATA-box binding protein (TBP) alters cellular metabolism. Next, in a non-exhaustive manner, we will discuss the current literature on how O-GlcNAcylation drives gene transcription in cancer through changes in transcription factor or chromatin remodeling complex functions. We conclude with a discussion of the challenges associated with studying O-GlcNAcylation and present several new approaches for studying O-GlcNAc regulated transcription that will advance our understanding of the role of O-GlcNAc in cancer.

Endogenous glutamate determines ferroptosis sensitivity via ADCY10-dependent YAP suppression in lung adenocarcinoma

Rationale: Ferroptosis, a newly identified form of regulated cell death, can be induced following the inhibition of cystine-glutamate antiporter system X_C^- because of the impaired uptake of cystine. However, the outcome following the accumulation of endogenous glutamate in lung adenocarcinoma (LUAD) has not yet been determined. Yes-associated protein (YAP) is sustained by the hexosamine biosynthesis pathway (HBP)-dependent O-linked beta-N-acetylglucosaminylation (O-GlcNAcylation), and glutamine-fructose-6-phosphate transaminase (GFPT1), the rate-limiting enzyme of the HBP, can be phosphorylated and inhibited by adenylyl cyclase (ADCY)-mediated activation of protein kinase A (PKA). However, whether accumulated endogenous glutamate determines ferroptosis sensitivity by influencing the ADCY/PKA/HBP/YAP axis in LUAD cells is not understood.

Methods: Cell viability, cell death and the generation of lipid reactive oxygen species (ROS) and malondialdehyde (MDA) were measured to evaluate the responses to the induction of ferroptosis following the inhibition of system X_C^-. Tandem mass tags (TMTs) were employed to explore potential factors critical for the ferroptosis sensitivity of LUAD cells. Immunoblotting (IB) and quantitative RT-PCR (qPCR) were used to analyze protein and mRNA expression. Co-immunoprecipitation (co-IP) assays were performed to identify protein-protein interactions and posttranslational modifications. Metabolite levels were measured using the appropriate kits. Transcriptional regulation was evaluated using a luciferase reporter assay, chromatin immunoprecipitation (ChIP), and electrophoretic mobility shift assay (EMSA). Drug administration and limiting dilution cell transplantation were performed with cell-derived xenograft (CDX) and patient-derived xenograft (PDX) mouse models. The associations among clinical outcome, drug efficacy and ADCY10 expression were determined based on data from patients who underwent curative surgery and evaluated with patient-derived primary LUAD cells and tissues.

Results: The accumulation of endogenous glutamate following system X_C^- inhibition has been shown to determine ferroptosis sensitivity by suppressing YAP in LUAD cells. YAP O-GlcNAcylation and expression cannot be sustained in LUAD cells upon impairment of GFPT1. Thus, Hippo pathway-like phosphorylation and ubiquitination of YAP are enhanced. ADCY10 acts as a key downstream target and diversifies the effects of glutamate on the PKA-dependent suppression of GFPT1. We also discovered that the protumorigenic and proferroptotic effects of ADCY10 are mediated separately. Advanced-stage LUADs with high ADCY10 expression are sensitive to ferroptosis. Moreover, LUAD cells with acquired therapy resistance are also prone to higher ADCY10 expression and are more likely to respond to ferroptosis. Finally, a varying degree of secondary labile iron increase is caused by the failure to sustain YAP-stimulated transcriptional compensation for ferritin at later stages further explains why ferroptosis sensitivity varies among LUAD cells.

Conclusions: Endogenous glutamate is critical for ferroptosis sensitivity following the inhibition of system X_C^- in LUAD cells, and ferroptosis-based treatment is a good choice for LUAD patients with later-stage and/or therapy-resistant tumors.

Suppression of the HBP Function Increases Pancreatic Cancer Cell Sensitivity to a Pan-RAS Inhibitor

Pancreatic ductal adenocarcinoma (PDAC) is a leading cause of cancer-related death and the search for a resolutive therapy is still a challenge. Since KRAS is commonly mutated in PDAC and is one of the main drivers of PDAC progression, its inhibition should be a key strategy for treatment, especially considering the recent development of specific KRAS inhibitors. Nevertheless, the effects of KRAS inhibition can be increased through the co-inhibition of other nodes important for cancer development. One of them could be the hexosamine biosynthetic pathway (HBP), whose enhancement is considered fundamental for PDAC. Here, we demonstrate that PDAC cells expressing oncogenic KRAS, owing to an increase in the HBP flux, become strongly reliant on HBP for both proliferation and survival. In particular, upon treatment with two different compounds, 2-deoxyglucose and FR054, inhibiting both HBP and protein N-glycosylation, these cells undergo apoptosis significantly more than PDAC cells expressing wild-type KRAS. Importantly, we also show that the combined treatment between FR054 and the pan-RAS inhibitor BI-2852 has an additive negative effect on cell proliferation and survival by means of the suppression of both Akt activity and cyclin D1 expression. Thus, co-inhibition of HBP and oncogenic RAS may represent a novel therapy for PDAC patients.

Role of O-Linked N-Acetylglucosamine Protein Modification in Cellular (Patho)Physiology

In the mid-1980s, the identification of serine and threonine residues on nuclear and cytoplasmic proteins modified by a N-acetylglucosamine moiety (O-GlcNAc) via an O-linkage overturned the widely held assumption that glycosylation only occurred in the endoplasmic reticulum, Golgi apparatus, and secretory pathways. In contrast to traditional glycosylation, the O-GlcNAc modification does not lead to complex, branched glycan structures and is rapidly cycled on and off proteins by O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA), respectively. Since its discovery, O-GlcNAcylation has been shown to contribute to numerous cellular functions, including signaling, protein localization and stability, transcription, chromatin remodeling, mitochondrial function, and cell survival. Dysregulation in O-GlcNAc cycling has been implicated in the progression of a wide range of diseases, such as diabetes, diabetic complications, cancer, cardiovascular, and neurodegenerative diseases. This review will outline our current understanding of the processes involved in regulating O-GlcNAc turnover, the role of O-GlcNAcylation in regulating cellular physiology, and how dysregulation in O-GlcNAc cycling contributes to pathophysiological processes.

Regulation of cardiac O-GlcNAcylation: More than just nutrient availability

The post-translational modification of serine and threonine residues of nuclear, cytosolic, and mitochondrial proteins by O-linked β-N-acetyl glucosamine (O-GlcNAc) has long been seen as an important regulatory mechanism in the cardiovascular system. O-GlcNAcylation of cardiac proteins has been shown to contribute to the regulation of transcription, metabolism, mitochondrial function, protein quality control and turnover, autophagy, and calcium handling. In the heart, acute increases in O-GlcNAc have been associated with cardioprotection, such as those observed during ischemia/reperfusion. Conversely, chronic increases in O-GlcNAc, often associated with diabetes and nutrient excess, have been shown to contribute to cardiac dysfunction. Traditionally, many studies have linked changes in O-GlcNAc with nutrient availability and as such O-GlcNAcylation is often seen as a nutrient driven process. However, emerging evidence suggests that O-GlcNAcylation may also be regulated by non-nutrient dependent mechanisms, such as transcriptional and post-translational regulation. Therefore, the goals of this review are to provide an overview of the impact of O-GlcNAcylation in the cardiovascular system, how this is regulated and to discuss the emergence of regulatory mechanisms other than nutrient availability.

Ribosome binding protein GCN1 regulates the cell cycle and cell proliferation and is essential for the embryonic development of mice

Amino acids exert many biological functions, serving as allosteric regulators and neurotransmitters, as constituents in proteins and as nutrients. GCN2-mediated phosphorylation of eukaryotic initiation factor 2 alpha (elF2α) restores homeostasis in response to amino acid starvation (AAS) through the inhibition of the general translation and upregulation of amino acid biosynthetic enzymes and transporters by activating the translation of Gcn4 and ATF4 in yeast and mammals, respectively. GCN1 is a GCN2-binding protein that possesses an RWD binding domain (RWDBD) in its C-terminus. In yeast, Gcn1 is essential for Gcn2 activation by AAS; however, the roles of GCN1 in mammals need to be established. Here, we revealed a novel role of GCN1 that does not depend on AAS by generating two Gcn1 mutant mouse lines: Gcn1-knockout mice (Gcn1 KO mice (Gcn1^-/-)) and RWDBD-deleted mutant mice (Gcn1^ΔRWDBD mice). Both mutant mice showed growth retardation, which was not observed in the Gcn2 KO mice, such that the Gcn1 KO mice died at the intermediate stage of embryonic development because of severe growth retardation, while the Gcn1^ΔRWDBD embryos showed mild growth retardation and died soon after birth, most likely due to respiratory failure. Extension of pregnancy by 24 h through the administration of progesterone to the pregnant mothers rescued the expression of differentiation markers in the lungs and prevented lethality of the Gcn1^ΔRWDBD pups, indicating that perinatal lethality of the Gcn1^ΔRWDBD embryos was due to simple growth retardation. Similar to the yeast Gcn2/Gcn1 system, AAS- or UV irradiation-induced elF2α phosphorylation was diminished in the Gcn1^ΔRWDBD mouse embryonic fibroblasts (MEFs), suggesting that GCN1 RWDBD is responsible for GCN2 activity. In addition, we found reduced cell proliferation and G2/M arrest accompanying a decrease in Cdk1 and Cyclin B1 in the Gcn1^ΔRWDBD MEFs. Our results demonstrated, for the first time, that GCN1 is essential for both GCN2-dependent stress response and GCN2-independent cell cycle regulation.

The stress response at the translational level is an energetically cost-saving mechanism because translation consumes a considerable amount of energy. Upon exposure to stresses such as that from amino acid starvation (AAS), the translational initiation factor eIF2α is phosphorylated, which represses general translation to save energy. At the same time, eIF2α phosphorylation increases the selective translation of cytoprotective proteins, such as ATF4, that transcriptionally activate the stress response, promoting cell survival. Among four eIF2α kinases, GCN2 responds to AAS and phosphorylates eIF2α. In yeast, Gcn1 is required for Gcn2 activation by AAS, but the roles of GCN1 in mammals remain to be established. Here, we show that GCN1 is involved in GCN2-mediated eIF2α phosphorylation after AAS and UV radiation by generating Gcn1 mutant mice. Interestingly, GCN1 not only regulates the eIF2α-mediated stress response but also the cell cycle and cell proliferation in a GCN2-independent manner. Taking these findings together, we propose that GCN1 integrates cellular information and coordinates the cellular stress response to enhance viability.

Starvation and antimetabolic therapy promote cytokine release and recruitment of immune cells

Cellular starvation is typically a consequence of tissue injury that disrupts the local blood supply but can also occur where cell populations outgrow the local vasculature, as observed in solid tumors. Cells react to nutrient deprivation by adapting their metabolism, or, if starvation is prolonged, it can result in cell death. Cell starvation also triggers adaptive responses, like angiogenesis, that promote tissue reorganization and repair, but other adaptive responses and their mediators are still poorly characterized. To explore this issue, we analyzed secretomes from glucose-deprived cells, which revealed up-regulation of multiple cytokines and chemokines, including IL-6 and IL-8, in response to starvation stress. Starvation-induced cytokines were cell type-dependent, and they were also released from primary epithelial cells. Most cytokines were up-regulated in a manner dependent on NF-κB and the transcription factor of the integrated stress response ATF4, which bound directly to the IL-8 promoter. Furthermore, glutamine deprivation, as well as the antimetabolic drugs 2-deoxyglucose and metformin, also promoted the release of IL-6 and IL-8. Finally, some of the factors released from starved cells induced chemotaxis of B cells, macrophages, and neutrophils, suggesting that nutrient deprivation in the tumor environment can serve as an initiator of tumor inflammation.

Role of O-GlcNAcylation and endoplasmic reticulum stress on obesity and insulin resistance

Background

Obesity is a global public health problem. Obesity closely associated with various metabolic diseases such as; insulin resistance, hypertension, dyslipidemia and cardiovascular diseases. Endoplasmic reticulum (ER) stress is a critical factor for insulin resistance. O-linked N-acetyl-glucosamine (O-GlcNAc); is the post-translational modification which is has a vital role in biological processes; including cell signaling, in response to nutrients, stress and other extracellular stimuli.

Materials and methods

In this study, we aimed to investigate the role of O-GlcNAc modification in the context of obesity and obesity-associated insulin resistance in adipose tissue. For this purpose, first, the visceral and epididymal adipose tissues of obese and insulin resistant C57BL/6 Lepob/Lepob and wild-type mice were used to determine the O-GlcNAc modification pattern by western blot. Secondly, the external stimulation of O-GlcNAc modification in wild-type mice achieved by intraperitoneal 5 mg/kg/day glucosamine injection every 24 h for 5 days. The effect of increased O-GlcNAc modification on insulin resistance and ER stress investigated in adipose tissues of glucosamine challenged wild-type mice through regulation of the insulin signaling pathway and unfolded protein response (UPR) elements by western blot. In addition to that, the O-GlcNAc status of the insulin receptor substrate-1 (IRS1) investigated in epididymal and visceral adipose tissues of ob/ob, wild-type and glucosamine challenged mice by immunoprecipitation.

Results

We found that reduced O-GlcNAc levels in visceral and epididymal adipose tissues of obese and insulin-resistant ob/ob mice, although interestingly we observed that increased O-GlcNAc modification in glucosamine challenged wild-type mice resulted in insulin resistance and ER stress. Furthermore, we demonstrated that the IRS1 was modified with O-GlcNAc in visceral and epididymal adipose tissues in both ob/ob mice and glucosamine-injected mice, and was compatible with the serine phosphorylation of this modification.

Conclusion

Our results suggest that O-GlcNAcylation of proteins is a crucial factor for intracellular trafficking regulates insulin receptor signaling and UPR depending on the cellular state of insulin resistance.

Fueling the fire: emerging role of the hexosamine biosynthetic pathway in cancer

Altered metabolism and deregulated cellular energetics are now considered a hallmark of all cancers. Glucose, glutamine, fatty acids, and amino acids are the primary drivers of tumor growth and act as substrates for the hexosamine biosynthetic pathway (HBP). The HBP culminates in the production of an amino sugar uridine diphosphate N-acetylglucosamine (UDP-GlcNAc) that, along with other charged nucleotide sugars, serves as the basis for biosynthesis of glycoproteins and other glycoconjugates. These nutrient-driven post-translational modifications are highly altered in cancer and regulate protein functions in various cancer-associated processes. In this review, we discuss recent progress in understanding the mechanistic relationship between the HBP and cancer.

Impact and Relevance of the Unfolded Protein Response in HNSCC

Head and neck squamous cell carcinomas (HNSCC) encompass a heterogeneous group of solid tumors that arise from the upper aerodigestive tract. The tumor cells face multiple challenges including an acute demand of protein synthesis often driven by oncogene activation, limited nutrient and oxygen supply and exposure to chemo/radiotherapy, which forces them to develop adaptive mechanisms such as the Unfolded Protein Response (UPR). It is now well documented that the UPR, a homeostatic mechanism, is induced at different stages of cancer progression in response to intrinsic (oncogenic activation) or extrinsic (microenvironment) perturbations. This review will discuss the role of the UPR in HNSCC as well as in the key processes that characterize the physiology of HNSCC. The role of the UPR in the clinical context of HNSCC will also be addressed.

Quantitative Proteomic Analysis Reveals Unfolded-Protein Response Involved in Severe Fever with Thrombocytopenia Syndrome Virus Infection

Severe fever with thrombocytopenia syndrome (SFTS) is an emerging, highly pathogenic, infectious disease caused by infection with a newly discovered tick-borne phlebovirus, SFTS virus (SFTSV). Limited information on the molecular mechanism of SFTSV infection and pathogenesis impedes the development of effective vaccines and drugs for SFTS prevention and treatment. In this study, an isobaric tag for relative and absolute quantification (iTRAQ)-based quantitative proteomic analysis of SFTSV-infected HEK 293 cells was performed to explore dynamic host cellular protein responses toward SFTSV infection. A total of 433 of 5,606 host proteins involved in different biological processes were differentially regulated by SFTSV infection. The proteomic results highlighted a potential role of endoplasmic reticular stress-triggered unfolded-protein response (UPR) in SFTSV infection. Further functional studies confirmed that all three major branches of the UPR, including the PKR-like endoplasmic reticulum kinase (PERK), the activating transcription factor-6 (ATF6), and the inositol-requiring protein-1 (IRE1)/X-box-binding protein 1 (XBP1) pathways, were activated by SFTSV. However, only the former two pathways play a crucial role in SFTSV infection. Furthermore, expression of SFTSV glycoprotein (GP) alone was sufficient to stimulate the UPR, whereas suppression of PERK and ATF6 notably decreased GP expression. Interestingly, two other newly discovered phleboviruses, Heartland virus and Guertu virus, also stimulated the UPR, suggesting a common mechanism shared by these genetically related phleboviruses. This study provides a global view to our knowledge on how host cells respond to SFTSV infection and highlights that host cell UPR plays an important role in phlebovirus infection.IMPORTANCE Severe fever with thrombocytopenia syndrome virus (SFTSV) is an emerging tick-borne bunyavirus that causes severe fever with thrombocytopenia syndrome in humans, with a mortality rate reaching up to 30% in some outbreaks. There are currently no U.S. Food and Drug Administration-approved vaccines or specific antivirals available against SFTSV. To comprehensively understand the molecular interactions occurring between SFTSV and the host cell, we exploit quantitative proteomic approach to investigate the dynamic host cellular responses to SFTSV infection. The results highlight multiple biological processes being regulated by SFTSV infection. Among these, we focused on exploration of the mechanism of how SFTSV infection stimulates the host cell's unfolded-protein response (UPR) and identified the UPR as a common feature shared by SFTSV-related new emerging phleboviruses. This study, for the first time to our knowledge, provides a global map for host cellular responses to SFTSV infection and highlighted potential host targets for further research.

Human and rodent temporal lobe epilepsy is characterized by changes in O-GlcNAc homeostasis that can be reversed to dampen epileptiform activity

Temporal Lobe Epilepsy (TLE) is frequently associated with changes in protein composition and post-translational modifications (PTM) that exacerbate the disorder. O-linked-β-N-acetyl glucosamine (O-GlcNAc) is a PTM occurring at serine/threonine residues that is derived from and closely associated with metabolic substrates. The enzymes O-GlcNActransferase (OGT) and O-GlcNAcase (OGA) mediate the addition and removal, respectively, of the O-GlcNAc modification. The goal of this study was to characterize OGT/OGA and protein O-GlcNAcylation in the epileptic hippocampus and to determine and whether direct manipulation of these proteins and PTM's alter epileptiform activity. We observed reduced global and protein specific O-GlcNAcylation and OGT expression in the kainate rat model of TLE and in human TLE hippocampal tissue. Inhibiting OGA with Thiamet-G elevated protein O-GlcNAcylation, and decreased both seizure duration and epileptic spike events, suggesting that OGA may be a therapeutic target for seizure control. These findings suggest that loss of O-GlcNAc homeostasis in the kainate model and in human TLE can be reversed via targeting of O-GlcNAc related pathways.

ER Stress is Involved in Epithelial-To-Mesenchymal Transition of Alveolar Epithelial Cells Exposed to a Hypoxic Microenvironment

Background: Idiopathic pulmonary fibrosis (IPF) is a chronic, progressive and fatal interstitial lung disease of unknown origin. Alveolar epithelial cells (AECs) play an important role in the fibrotic process as they undergo sustained endoplasmic reticulum (ER) stress, and may acquire a mesenchymal phenotype through epithelial-to-mesenchymal transition (EMT), two phenomena that could be induced by localized alveolar hypoxia. Here we investigated the potential links between hypoxia, ER stress and EMT in AECs. Methods: ER stress and EMT markers were assessed by immunohistochemistry, western blot and qPCR analysis, both in vivo in rat lungs exposed to normoxia or hypoxia (equivalent to 8% O₂) for 48 h, and in vitro in primary rat AECs exposed to normoxia or hypoxia (1.5% O₂) for 2–6 days. Results: Hypoxia induced expression of mesenchymal markers, pro-EMT transcription factors, and the activation of ER stress markers both in vivo in rat lungs, and in vitro in AECs. In vitro, pharmacological inhibition of ER stress by 4-PBA limited hypoxia-induced EMT. Calcium chelation or hypoxia-inducible factor (HIF) inhibition also prevented EMT induction under hypoxic condition. Conclusions: Hypoxia and intracellular calcium are both involved in EMT induction of AECs, mainly through the activation of ER stress and HIF signaling pathways.

EZH2 is overexpressed in transitional preplasmablasts and is involved in human plasma cell differentiation

Plasma cells (PCs) play a major role in the defense of the host organism against pathogens. We have shown that PC generation can be modeled using multi-step culture systems that reproduce the sequential cell differentiation occurring in vivo. Using this unique model, we investigated the role of EZH2 during PC differentiation (PCD) using H3K27me3 and EZH2 ChIP-binding profiles. We then studied the effect of the inhibition of EZH2 enzymatic activity to understand how EZH2 regulates the key functions involved in PCD. EZH2 expression significantly increases in preplasmablasts with H3K27me3 mediated repression of genes involved in B cell and plasma cell identity. EZH2 was also found to be recruited to H3K27me3-free promoters of transcriptionally active genes known to regulate cell proliferation. Inhibition the catalytic activity of EZH2 resulted in B to PC transcriptional changes associated with PC maturation induction, as well as higher immunoglobulin secretion. Altogether, our data suggest that EZH2 is involved in the maintenance of preplasmablast transitory immature proliferative state that supports their amplification.

The role of the unfolded protein response in cancer progression: From oncogenesis to chemoresistance

Tumour cells endure both oncogenic and environmental stresses during cancer progression. Transformed cells must meet increased demands for protein and lipid production needed for rapid proliferation and must adapt to exist in an oxygen- and nutrient-deprived environment. To overcome such challenges, cancer cells exploit intrinsic adaptive mechanisms such as the unfolded protein response (UPR). The UPR is a pro-survival mechanism triggered by accumulation of unfolded or misfolded proteins in the endoplasmic reticulum (ER), a condition referred to as ER stress. IRE1, PERK and ATF6 are three ER anchored transmembrane receptors. Upon induction of ER stress, they signal in a coordinated fashion to re-establish ER homoeostasis, thus aiding cell survival. Over the past decade, evidence has emerged supporting a role for the UPR in the establishment and progression of several cancers, including breast cancer, prostate cancer and glioblastoma multiforme. This review discusses our current knowledge of the UPR during oncogenesis, tumour growth, metastasis and chemoresistance.

mTORC2 modulates the amplitude and duration of GFAT1 Ser-243 phosphorylation to maintain flux through the hexosamine pathway during starvation

The mechanistic target of rapamycin (mTOR) controls metabolic pathways in response to nutrients. Recently, we have shown that mTOR complex 2 (mTORC2) modulates the hexosamine biosynthetic pathway (HBP) by promoting the expression of the key enzyme of the HBP, glutamine:fructose-6-phosphate aminotransferase 1 (GFAT1). Here, we found that GFAT1 Ser-243 phosphorylation is also modulated in an mTORC2-dependent manner. In response to glutamine limitation, active mTORC2 prolongs the duration of Ser-243 phosphorylation, albeit at lower amplitude. Blocking glycolysis using 2-deoxyglucose robustly enhances Ser-243 phosphorylation, correlating with heightened mTORC2 activation, increased AMPK activity, and O-GlcNAcylation. However, when 2-deoxyglucose is combined with glutamine deprivation, GFAT1 Ser-243 phosphorylation and mTORC2 activation remain elevated, whereas AMPK activation and O-GlcNAcylation diminish. Phosphorylation at Ser-243 promotes GFAT1 expression and production of GFAT1-generated metabolites including ample production of the HBP end-product, UDP-GlcNAc, despite nutrient starvation. Hence, we propose that the mTORC2-mediated increase in GFAT1 Ser-243 phosphorylation promotes flux through the HBP to maintain production of UDP-GlcNAc when nutrients are limiting. Our findings provide insights on how the HBP is reprogrammed via mTORC2 in nutrient-addicted cancer cells.

The Nutrient-Sensing Hexosamine Biosynthetic Pathway as the Hub of Cancer Metabolic Rewiring

Alterations in glucose and glutamine utilizing pathways and in fatty acid metabolism are currently considered the most significant and prevalent metabolic changes observed in almost all types of tumors. Glucose, glutamine and fatty acids are the substrates for the hexosamine biosynthetic pathway (HBP). This metabolic pathway generates the “sensing molecule” UDP-N-Acetylglucosamine (UDP-GlcNAc). UDP-GlcNAc is the substrate for the enzymes involved in protein N- and O-glycosylation, two important post-translational modifications (PTMs) identified in several proteins localized in the extracellular space, on the cell membrane and in the cytoplasm, nucleus and mitochondria. Since protein glycosylation controls several key aspects of cell physiology, aberrant protein glycosylation has been associated with different human diseases, including cancer. Here we review recent evidence indicating the tight association between the HBP flux and cell metabolism, with particular emphasis on the post-transcriptional and transcriptional mechanisms regulated by the HBP that may cause the metabolic rewiring observed in cancer. We describe the implications of both protein O- and N-glycosylation in cancer cell metabolism and bioenergetics; focusing our attention on the effect of these PTMs on nutrient transport and on the transcriptional regulation and function of cancer-specific metabolic pathways.