O-GlcNAcylation increases ChREBP protein content and transcriptional activity in the liver

New insights into phytochemicals via protein glycosylation focused on aging and diabetes

BACKGROUND

Protein glycosylation as a common post-translational modification that has significant impacts on protein folding, enzymatic activity, and interfering with receptor functioning. In recent years, with the rapid development of glycopeptide enrichment and analysis technology and the deepening of glycosylation research, glycosylation has gradually become a sign of disease occurrence and development. Multiple investigations suggest that protein glycosylation affect the advances of diabetes and aging.

PURPOSE AND METHODS

This review was focused on the action mechanisms of glycosylated proteins production, permanent abnormalities in extracellular matrix component function, inflammatory and reactive oxygen species production, as well as the glycosylated characterizations of diabetes and aging. Further, advances in glycosylation analysis and detection methods are presented for the first time, highlighting for needed future developments. All literatures were gathered from PubMed and Google Scholar.

RESULTS

Herein, we review how protein glycosylation impacts the progression of diabetes and aging. Specifically, we focus on various types of glycosylation, including N-linked glycosylation, O-linked glycosylation, C-glycosylation, S-glycosylation, and glycophosphatidylinositol (GPI) anchors. N-linked glycosylation and O-linked glycosylation are commonly observed glycosylation forms, wherein O-GlcNAcylation plays a significant role in diabetes, while N-glycan could serve as biomarkers for identifying inflammation and aging.

CONCLUSIONS

Protein glycosylation produces a vastly larger number of core glycan structures through utilizing at least 173 glycosyltransferases and repeated common scaffolds. Single protein may contain multiple glycosylation sites, and the structure and occupancy of glycan at each site may be different, resulting in the macro heterogeneity of protein glycosylation. This review will contribute to how protein glycosylation impacts the life progress of cells and its association with diseases.

Functional genomic profiling of O-GlcNAc reveals its context-specific interplay with RNA polymerase II

BACKGROUND

How reversible glycosylation of DNA-bound proteins acts on transcription remains scarcely understood. O-linked β-N-acetylglucosamine (O-GlcNAc) is the only known form of glycosylation modifying nuclear proteins, including RNA polymerase II (RNA Pol II) and many transcription factors. Yet, the regulatory function of the O-GlcNAc modification in mammalian chromatin remains unclear.

RESULTS

Here, we combine genome-wide profiling of O-GlcNAc-modified proteins with perturbations of intracellular glycosylation, RNA Pol II-degron, and super-resolution microscopy. Genomic profiling of O-GlcNAc-modified proteins shows a non-random distribution across the genome, with high densities in heterochromatin regions as well as on actively transcribed gene promoters. Large-scale intersection of the O-GlcNAc signal at promoters with public ChIP-seq datasets identifies a high overlap with RNA Pol II and specific cofactors. Knockdown of O-GlcNAc Transferase (Ogt) shows that most direct target genes are downregulated, supporting a global positive role of O-GlcNAc on the transcription of cellular genes. Rapid degradation of RNA Pol II results in the decrease of the O-GlcNAc levels at promoters encoding transcription factors and DNA modifying enzymes. RNA Pol II depletion also unexpectedly causes an increase of O-GlcNAc levels at a set of promoters encoding for the transcription machinery.

CONCLUSIONS

This study provides a deconvoluted genomic profiling of O-GlcNAc-modified proteins in murine and human cells. Perturbations of O-GlcNAc or RNA Pol II uncover a context-specific reciprocal functional interplay between the transcription machinery and the O-GlcNAc modification.

Opportunities for Therapeutic Modulation of O-GlcNAc

O-Linked β-N-acetylglucosamine (O-GlcNAc) is an essential, dynamic monosaccharide post-translational modification (PTM) found on serine and threonine residues of thousands of nucleocytoplasmic proteins. The installation and removal of O-GlcNAc is controlled by a single pair of enzymes, O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA), respectively. Since its discovery four decades ago, O-GlcNAc has been found on diverse classes of proteins, playing important functional roles in many cellular processes. Dysregulation of O-GlcNAc homeostasis has been implicated in the pathogenesis of disease, including neurodegeneration, X-linked intellectual disability (XLID), cancer, diabetes, and immunological disorders. These foundational studies of O-GlcNAc in disease biology have motivated efforts to target O-GlcNAc therapeutically, with multiple clinical candidates under evaluation. In this review, we describe the characterization and biochemistry of OGT and OGA, cellular O-GlcNAc regulation, development of OGT and OGA inhibitors, O-GlcNAc in pathophysiology, clinical progress of O-GlcNAc modulators, and emerging opportunities for targeting O-GlcNAc. This comprehensive resource should motivate further study into O-GlcNAc function and inspire strategies for therapeutic modulation of O-GlcNAc.

O-GlcNAc signaling: Implications for stress-induced adaptive response pathway in the tumor microenvironment

The tumor microenvironment (TME) consists of tumor cells, non-tumor cells, extracellular matrix, and signaling molecules, which can contribute to tumor initiation, progression, and therapy resistance. In response to starvation, hypoxia, and drug treatments, tumor cells undergo a variety of deleterious endogenous stresses, such as hypoxia, DNA damage, and oxidative stress. In this context, to survive the difficult situation, tumor cells evolve multiple conserved adaptive responses, including metabolic reprogramming, DNA damage checkpoints, homologous recombination, up-regulated antioxidant pathways, and activated unfolded protein responses. In the last decades, the protein O-GlcNAcylation has emerged as a crucial causative link between glucose metabolism and tumor progression. Here, we discuss the relevant pathways that regulate the above responses. These pathways are adaptive adjustments induced by endogenous stresses in cells. In addition, we systematically discuss the role of O-GlcNAcylation-regulated stress-induced adaptive response pathways (SARPs) in TME remodeling, tumor progression, and treatment resistance. We also emphasize targeting O-GlcNAcylation through compounds that modulate OGT or OGA activity to inhibit tumor progression. It seems that targeting O-GlcNAcylated proteins to intervene in TME may be a novel approach to improve tumor prognosis.

Biological Functions and Potential Therapeutic Significance of O-GlcNAcylation in Hepatic Cellular Stress and Liver Diseases

O-linked-β-D-N-acetylglucosamine (O-GlcNAc) glycosylation (O-GlcNAcylation), which is dynamically regulated by O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA), is a post-translational modification involved in multiple cellular processes. O-GlcNAcylation of proteins can regulate their biological functions via crosstalk with other post-translational modifications, such as phosphorylation, ubiquitination, acetylation, and methylation. Liver diseases are a major cause of death worldwide; yet, key pathological features of the disease, such as inflammation, fibrosis, steatosis, and tumorigenesis, are not fully understood. The dysregulation of O-GlcNAcylation has been shown to be involved in some severe hepatic cellular stress, viral hepatitis, liver fibrosis, nonalcoholic fatty acid liver disease (NAFLD), malignant progression, and drug resistance of hepatocellular carcinoma (HCC) through multiple molecular signaling pathways. Here, we summarize the emerging link between O-GlcNAcylation and hepatic pathological processes and provide information about the development of therapeutic strategies for liver diseases.

Targeting O-GlcNAcylation in cancer therapeutic resistance: The sugar Saga continues

O-linked-N-acetylglucosaminylation (O-GlcNAcylation), a dynamic post-translational modification (PTM), holds profound implications in controlling various cellular processes such as cell signaling, metabolism, and epigenetic regulation that influence cancer progression and therapeutic resistance. From the therapeutic perspective, O-GlcNAc modulates drug efflux, targeting and metabolism. By integrating signals from glucose, lipid, amino acid, and nucleotide metabolic pathways, O-GlcNAc acts as a nutrient sensor and transmits signals to exerts its function on genome stability, epithelial-mesenchymal transition (EMT), cell stemness, cell apoptosis, autophagy, cell cycle. O-GlcNAc also attends to tumor microenvironment (TME) and the immune response. At present, several strategies aiming at targeting O-GlcNAcylation are under mostly preclinical evaluation, where the newly developed O-GlcNAcylation inhibitors markedly enhance therapeutic efficacy. Here we systematically outline the mechanisms through which O-GlcNAcylation influences therapy resistance and deliberate on the prospects and challenges associated with targeting O-GlcNAcylation in future cancer treatments.

Whole-body deletion of Endospanin 1 protects from obesity-associated deleterious metabolic alterations

The importance of the proper localization of most receptors at the cell surface is often underestimated, although this feature is essential for optimal receptor response. Endospanin 1 (Endo1) (also known as OBRGRP or LEPROT) is a protein generated from the same gene as the human leptin receptor and regulates the trafficking of proteins to the surface, including the leptin receptor. The systemic role of Endo1 on whole-body metabolism has not been studied so far. Here, we report that general Endo1-KO mice fed a high-fat diet develop metabolically healthy obesity with lipid repartitioning in organs and preferential accumulation of fat in adipose tissue, limited systematic inflammation, and better controlled glucose homeostasis. Mechanistically, Endo1 interacts with the lipid translocase CD36, thus regulating its surface abundance and lipid uptake in adipocytes. In humans, the level of Endo1 transcripts is increased in the adipose tissue of patients with obesity, but low levels rather correlate with a profile of metabolically healthy obesity. We suggest here that Endo1, most likely by controlling CD36 cell surface abundance and lipid uptake in adipocytes, dissociates obesity from diabetes and that its absence participates in metabolically healthy obesity.

O-GlcNAc transferase acts as a critical nutritional node for the control of liver homeostasis

Background & Aims

O-GlcNAcylation is a reversible post-translational modification controlled by the activity of two enzymes, O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA). In the liver, O-GlcNAcylation has emerged as an important regulatory mechanism underlying normal liver physiology and metabolic disease.

Methods

To address whether OGT acts as a critical hepatic nutritional node, mice with a constitutive hepatocyte-specific deletion of OGT (OGTLKO) were generated and challenged with different carbohydrate- and lipid-containing diets.

Results

Analyses of 4-week-old OGTLKO mice revealed significant oxidative and endoplasmic reticulum stress, and DNA damage, together with inflammation and fibrosis, in the liver. Susceptibility to oxidative and endoplasmic reticulum stress-induced apoptosis was also elevated in OGTLKO hepatocytes. Although OGT expression was partially recovered in the liver of 8-week-old OGTLKO mice, hepatic injury and fibrosis were not rescued but rather worsened with time. Interestingly, weaning of OGTLKO mice on a ketogenic diet (low carbohydrate, high fat) fully prevented the hepatic alterations induced by OGT deletion, indicating that reduced carbohydrate intake protects an OGT-deficient liver.

Conclusions

These findings pinpoint OGT as a key mediator of hepatocyte homeostasis and survival upon carbohydrate intake and validate OGTLKO mice as a valuable model for assessing therapeutical approaches of advanced liver fibrosis.

Impact and Implications

Our study shows that hepatocyte-specific deletion of O-GlcNAc transferase (OGT) leads to severe liver injury, reinforcing the importance of O-GlcNAcylation and OGT for hepatocyte homeostasis and survival. Our study also validates the Ogt liver-deficient mouse as a valuable model for the study of advanced liver fibrosis. Importantly, as the severe hepatic fibrosis of Ogt liver-deficient mice could be fully prevented upon feeding on a ketogenic diet (i.e. very-low-carbohydrate, high-fat diet) this work underlines the potential interest of nutritional intervention as antifibrogenic strategies.

Glucosamine increases macrophage lipid accumulation by regulating the mammalian target of rapamycin signaling pathway

Elevated blood glucose is associated with an increased risk of atherosclerosis. Data from the current study showed that glucosamine (GlcN), a normal glucose metabolite of the hexosamine biosynthetic pathway (HBP), promoted lipid accumulation in RAW264.7 macrophage cells. Oleic acid- and lipopolysaccharide (LPS)-induced lipid accumulation was further enhanced by GlcN in RAW264.7 cells, although there was no a significant change in the rate of fatty acid uptake. GlcN increased acetyl CoA carboxylase (ACC), fatty acid synthase (FAS), scavenger receptor class A, liver X receptor, and sterol regulatory elementbinding protein-1c (SREBP-1c) mRNA expression, and; conversely, suppressed ATP-binding cassette transporter A1 (ABCA-1) and ABCG-1 expression. Additionally, GlcN promoted O-GlcNAcylation of nuclear SREBP-1 but did not affect its DNA binding activity. GlcN stimulated phosphorylation of mammalian target of rapamycin (mTOR) and S6 kinase. Rapamycin, a mTOR-specific inhibitor, suppressed GlcN-induced lipid accumulation in RAW264.7 cells. The GlcN-mediated increase in ACC and FAS mRNA was suppressed, while the decrease in ABCA-1 and ABCG-1 by GlcN was not significantly altered by rapamycin. Together, our results highlight the importance of the mTOR signaling pathway in GlcN-induced macrophage lipid accumulation and further support a potential link between mTOR and HBP signaling in lipogenesis. [BMB Reports 2024; 57(2): 92-97].

Protein O-GlcNAcylation: The sweet hub in liver metabolic flexibility from a (patho)physiological perspective

O-GlcNAcylation is a dynamic, reversible and atypical O-glycosylation that regulates various cellular physiological processes via conformation, stabilisation, localisation, chaperone interaction or activity of target proteins. The O-GlcNAcylation cycle is precisely controlled by collaboration between O-GlcNAc transferase and O-GlcNAcase. Uridine-diphosphate-N-acetylglucosamine, the sole donor of O-GlcNAcylation produced by the hexosamine biosynthesis pathway, is controlled by the input of glucose, glutamine, acetyl coenzyme A and uridine triphosphate, making it a sensor of the fluctuation of molecules, making O-GlcNAcylation a pivotal nutrient sensor for the metabolism of carbohydrates, amino acids, lipids and nucleotides. O-GlcNAcylation, particularly prevalent in liver, is the core hub for controlling systemic glucose homeostasis due to its nutritional sensitivity and precise spatiotemporal regulation of insulin signal transduction. The pathology of various liver diseases has highlighted hepatic metabolic disorder and dysfunction, and abnormal O-GlcNAcylation also plays a specific pathological role in these processes. Therefore, this review describes the unique features of O-GlcNAcylation and its dynamic homeostasis maintenance. Additionally, it explains the underlying nutritional sensitivity of O-GlcNAcylation and discusses its mechanism of spatiotemporal modulation of insulin signal transduction and liver metabolic homeostasis during the fasting and feeding cycle. This review emphasises the pathophysiological implications of O-GlcNAcylation in nonalcoholic fatty liver disease, nonalcoholic steatohepatitis and hepatic fibrosis, and focuses on the adverse effects of hyper O-GlcNAcylation on liver cancer progression and metabolic reprogramming.

Inhibition of DNMT1 methyltransferase activity via glucose-regulated O-GlcNAcylation alters the epigenome

The DNA methyltransferase activity of DNMT1 is vital for genomic maintenance of DNA methylation. We report here that DNMT1 function is regulated by O-GlcNAcylation, a protein modification that is sensitive to glucose levels, and that elevated O-GlcNAcylation of DNMT1 from high glucose environment leads to alterations to the epigenome. Using mass spectrometry and complementary alanine mutation experiments, we identified S878 as the major residue that is O-GlcNAcylated on human DNMT1. Functional studies in human and mouse cells further revealed that O-GlcNAcylation of DNMT1-S878 results in an inhibition of methyltransferase activity, resulting in a general loss of DNA methylation that preferentially occurs at partially methylated domains (PMDs). This loss of methylation corresponds with an increase in DNA damage and apoptosis. These results establish O-GlcNAcylation of DNMT1 as a mechanism through which the epigenome is regulated by glucose metabolism and implicates a role for glycosylation of DNMT1 in metabolic diseases characterized by hyperglycemia.

Targeting protein modifications in metabolic diseases: molecular mechanisms and targeted therapies

The ever-increasing prevalence of noncommunicable diseases (NCDs) represents a major public health burden worldwide. The most common form of NCD is metabolic diseases, which affect people of all ages and usually manifest their pathobiology through life-threatening cardiovascular complications. A comprehensive understanding of the pathobiology of metabolic diseases will generate novel targets for improved therapies across the common metabolic spectrum. Protein posttranslational modification (PTM) is an important term that refers to biochemical modification of specific amino acid residues in target proteins, which immensely increases the functional diversity of the proteome. The range of PTMs includes phosphorylation, acetylation, methylation, ubiquitination, SUMOylation, neddylation, glycosylation, palmitoylation, myristoylation, prenylation, cholesterylation, glutathionylation, S-nitrosylation, sulfhydration, citrullination, ADP ribosylation, and several novel PTMs. Here, we offer a comprehensive review of PTMs and their roles in common metabolic diseases and pathological consequences, including diabetes, obesity, fatty liver diseases, hyperlipidemia, and atherosclerosis. Building upon this framework, we afford a through description of proteins and pathways involved in metabolic diseases by focusing on PTM-based protein modifications, showcase the pharmaceutical intervention of PTMs in preclinical studies and clinical trials, and offer future perspectives. Fundamental research defining the mechanisms whereby PTMs of proteins regulate metabolic diseases will open new avenues for therapeutic intervention.

The Function of MondoA and ChREBP Nutrient-Sensing Factors in Metabolic Disease

Obesity is a major global public health concern associated with an increased risk of many health problems, including type 2 diabetes, heart disease, stroke, and some types of cancer. Obesity is also a critical factor in the development of insulin resistance and type 2 diabetes. Insulin resistance is associated with metabolic inflexibility, which interferes with the body's ability to switch from free fatty acids to carbohydrate substrates, as well as with the ectopic accumulation of triglycerides in non-adipose tissue, such as that of skeletal muscle, the liver, heart, and pancreas. Recent studies have demonstrated that MondoA (MLX-interacting protein or MLXIP) and the carbohydrate response element-binding protein (ChREBP, also known as MLXIPL and MondoB) play crucial roles in the regulation of nutrient metabolism and energy homeostasis in the body. This review summarizes recent advances in elucidating the function of MondoA and ChREBP in insulin resistance and related pathological conditions. This review provides an overview of the mechanisms by which MondoA and ChREBP transcription factors regulate glucose and lipid metabolism in metabolically active organs. Understanding the underlying mechanism of MondoA and ChREBP in insulin resistance and obesity can foster the development of new therapeutic strategies for treating metabolic diseases.

Physiological and pathological roles of lipogenesis

Lipids are essential metabolites, which function as energy sources, structural components and signalling mediators. Most cells are able to convert carbohydrates into fatty acids, which are often converted into neutral lipids for storage in the form of lipid droplets. Accumulating evidence suggests that lipogenesis plays a crucial role not only in metabolic tissues for systemic energy homoeostasis but also in immune and nervous systems for their proliferation, differentiation and even pathophysiological roles. Thus, excessive or insufficient lipogenesis is closely associated with aberrations in lipid homoeostasis, potentially leading to pathological consequences, such as dyslipidaemia, diabetes, fatty liver, autoimmune diseases, neurodegenerative diseases and cancers. For systemic energy homoeostasis, multiple enzymes involved in lipogenesis are tightly controlled by transcriptional and post-translational modifications. In this Review, we discuss recent findings regarding the regulatory mechanisms, physiological roles and pathological importance of lipogenesis in multiple tissues such as adipose tissue and the liver, as well as the immune and nervous systems. Furthermore, we briefly introduce the therapeutic implications of lipogenesis modulation.

Immunometabolism at the crossroads of obesity and cancer-a Keystone Symposia report

Immunometabolism considers the relationship between metabolism and immunity. Typically, researchers focus on either the metabolic pathways within immune cells that affect their function or the impact of immune cells on systemic metabolism. A more holistic approach that considers both these viewpoints is needed. On September 5-8, 2022, experts in the field of immunometabolism met for the Keystone symposium "Immunometabolism at the Crossroads of Obesity and Cancer" to present recent research across the field of immunometabolism, with the setting of obesity and cancer as an ideal example of the complex interplay between metabolism, immunity, and cancer. Speakers highlighted new insights on the metabolic links between tumor cells and immune cells, with a focus on leveraging unique metabolic vulnerabilities of different cell types in the tumor microenvironment as therapeutic targets and demonstrated the effects of diet, the microbiome, and obesity on immune system function and cancer pathogenesis and therapy. Finally, speakers presented new technologies to interrogate the immune system and uncover novel metabolic pathways important for immunity.

Tcf7l2 in hepatocytes regulates de novo lipogenesis in diet-induced non-alcoholic fatty liver disease in mice

AIMS/HYPOTHESIS

Non-alcoholic fatty liver disease (NAFLD) associated with type 2 diabetes may more easily progress towards severe forms of non-alcoholic steatohepatitis (NASH) and cirrhosis. Although the Wnt effector transcription factor 7-like 2 (TCF7L2) is closely associated with type 2 diabetes risk, the role of TCF7L2 in NAFLD development remains unclear. Here, we investigated how changes in TCF7L2 expression in the liver affects hepatic lipid metabolism based on the major risk factors of NAFLD development.

METHODS

Tcf7l2 was selectively ablated in the liver of C57BL/6N mice by inducing the albumin (Alb) promoter to recombine Tcf7l2 alleles floxed at exon 5 (liver-specific Tcf7l2-knockout [KO] mice: Alb-Cre;Tcf7l2f/f). Alb-Cre;Tcf7l2f/f and their wild-type (Tcf7l2f/f) littermates were fed a high-fat diet (HFD) or a high-carbohydrate diet (HCD) for 22 weeks to reproduce NAFLD/NASH. Mice were refed a standard chow diet or an HCD to stimulate de novo lipogenesis (DNL) or fed an HFD to provide exogenous fatty acids. We analysed glucose and insulin sensitivity, metabolic respiration, mRNA expression profiles, hepatic triglyceride (TG), hepatic DNL, selected hepatic metabolites, selected plasma metabolites and liver histology.

RESULTS

Alb-Cre;Tcf7l2f/f essentially exhibited increased lipogenic genes, but there were no changes in hepatic lipid content in mice fed a normal chow diet. However, following 22 weeks of diet-induced NAFLD/NASH conditions, liver steatosis was exacerbated owing to preferential metabolism of carbohydrate over fat. Indeed, hepatic Tcf7l2 deficiency enhanced liver lipid content in a manner that was dependent on the duration and amount of exposure to carbohydrates, owing to cell-autonomous increases in hepatic DNL. Mechanistically, TCF7L2 regulated the transcriptional activity of Mlxipl (also known as ChREBP) by modulating O-GlcNAcylation and protein content of carbohydrate response element binding protein (ChREBP), and targeted Srebf1 (also called SREBP1) via miRNA (miR)-33-5p in hepatocytes. Eventually, restoring TCF7L2 expression at the physiological level in the liver of Alb-Cre;Tcf7l2f/f mice alleviated liver steatosis without altering body composition under both acute and chronic HCD conditions.

CONCLUSIONS/INTERPRETATION

In mice, loss of hepatic Tcf7l2 contributes to liver steatosis by inducing preferential metabolism of carbohydrates via DNL activation. Therefore, TCF7L2 could be a promising regulator of the NAFLD associated with high-carbohydrate diets and diabetes since TCF7L2 deficiency may lead to development of NAFLD by promoting utilisation of excess glucose pools through activating DNL.

DATA AVAILABILITY

RNA-sequencing data have been deposited into the NCBI GEO under the accession number GSE162449 ( www.ncbi.nlm.nih.gov/geo/query/acc.cgi?acc=GSE162449 ).

The role of ChREBP in carbohydrate sensing and NAFLD development

Excessive sugar consumption and defective glucose sensing by hepatocytes contribute to the development of metabolic diseases including type 2 diabetes mellitus (T2DM) and nonalcoholic fatty liver disease (NAFLD). Hepatic metabolism of carbohydrates into lipids is largely dependent on the carbohydrate-responsive element binding protein (ChREBP), a transcription factor that senses intracellular carbohydrates and activates many different target genes, through the activation of de novo lipogenesis (DNL). This process is crucial for the storage of energy as triglycerides in hepatocytes. Furthermore, ChREBP and its downstream targets represent promising targets for the development of therapies for the treatment of NAFLD and T2DM. Although lipogenic inhibitors (for example, inhibitors of fatty acid synthase, acetyl-CoA carboxylase or ATP citrate lyase) are currently under investigation, targeting lipogenesis remains a topic of discussion for NAFLD treatment. In this Review, we discuss mechanisms that regulate ChREBP activity in a tissue-specific manner and their respective roles in controlling DNL and beyond. We also provide in-depth discussion of the roles of ChREBP in the onset and progression of NAFLD and consider emerging targets for NAFLD therapeutics.

Hepatocyte Kctd17 Inhibition Ameliorates Glucose Intolerance and Hepatic Steatosis Caused by Obesity-induced Chrebp Stabilization

BACKGROUND & AIMS

Obesity predisposes to type 2 diabetes (T2D) and nonalcoholic fatty liver disease (NAFLD), but underlying mechanisms are incompletely understood. Potassium channel tetramerization domain-containing protein 17 (Kctd17) levels are increased in livers from obese mice and humans. In this study, we investigated the mechanism of increased Kctd17 and whether it is causal to obesity-induced metabolic complications.

METHODS

We transduced Rosa26-LSL-Cas9 knockin mice with AAV8-TBG-Cre (Control), AAV8-U6-Kctd17 sgRNA-TBG-Cre (L-Kctd17), AAV8-U6-Oga sgRNA-TBG-Cre (L-Oga), or AAV8-U6-Kctd17/Oga sgRNA-TBG-Cre (DKO). We fed mice a high-fat diet (HFD) and assessed for hepatic glucose and lipid homeostasis. We generated Kctd17, O-GlcNAcase (Oga), or Kctd17/Oga-knockout hepatoma cells by CRISPR-Cas9, and Kctd17-directed antisense oligonucleotide to test therapeutic potential in vivo. We analyzed transcriptomic data from patients with NAFLD.

RESULTS

Hepatocyte Kctd17 expression was increased in HFD-fed mice due to increased Srebp1c activity. HFD-fed L-Kctd17 or Kctd17 antisense oligonucleotide-treated mice show improved glucose tolerance and hepatic steatosis, whereas forced Kctd17 expression caused glucose intolerance and hepatic steatosis even in lean mice. Kctd17 induced Oga degradation, resulting in increasing carbohydrate response element-binding protein (Chrebp) protein, so concomitant Oga knockout negated metabolic benefits of hepatocyte Kctd17 deletion. In patients with NAFLD, KCTD17 messenger RNA was positively correlated with expression of Chrebp target and other lipogenic genes.

CONCLUSIONS

Srebp1c-induced hepatocyte Kctd17 expression in obesity disrupted glucose and lipid metabolism by stabilizing Chrebp, and may represent a novel therapeutic target for obesity-induced T2D and NAFLD.

Emerging Role of Protein O-GlcNAcylation in Liver Metabolism: Implications for Diabetes and NAFLD

O-linked b-N-acetyl-glucosaminylation (O-GlcNAcylation) is one of the most common post-translational modifications of proteins, and is established by modifying the serine or threonine residues of nuclear, cytoplasmic, and mitochondrial proteins. O-GlcNAc signaling is considered a critical nutrient sensor, and affects numerous proteins involved in cellular metabolic processes. O-GlcNAcylation modulates protein functions in different patterns, including protein stabilization, enzymatic activity, transcriptional activity, and protein interactions. Disrupted O-GlcNAcylation is associated with an abnormal metabolic state, and may result in metabolic disorders. As the liver is the center of nutrient metabolism, this review provides a brief description of the features of the O-GlcNAc signaling pathway, and summarizes the regulatory functions and underlying molecular mechanisms of O-GlcNAcylation in liver metabolism. Finally, this review highlights the role of O-GlcNAcylation in liver-associated diseases, such as diabetes and nonalcoholic fatty liver disease (NAFLD). We hope this review not only benefits the understanding of O-GlcNAc biology, but also provides new insights for treatments against liver-associated metabolic disorders.

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.

O-GlycNacylation Remission Retards the Progression of Non-Alcoholic Fatty Liver Disease

Non-alcoholic fatty liver disease (NAFLD) is a metabolic disease spectrum associated with insulin resistance (IR), from non-alcoholic fatty liver (NAFL) to non-alcoholic steatohepatitis (NASH), cirrhosis, and hepatocellular carcinoma (HCC). O-GlcNAcylation is a posttranslational modification, regulated by O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA). Abnormal O-GlcNAcylation plays a key role in IR, fat deposition, inflammatory injury, fibrosis, and tumorigenesis. However, the specific mechanisms and clinical treatments of O-GlcNAcylation and NAFLD are yet to be elucidated. The modification contributes to understanding the pathogenesis and development of NAFLD, thus clarifying the protective effect of O-GlcNAcylation inhibition on liver injury. In this review, the crucial role of O-GlcNAcylation in NAFLD (from NAFL to HCC) is discussed, and the effect of therapeutics on O-GlcNAcylation and its potential mechanisms on NAFLD have been highlighted. These inferences present novel insights into the pathogenesis and treatments of NAFLD.

Posttranslational modifications in diabetes: Mechanisms and functions

As one of the most widespread chronic diseases, diabetes and its accompanying complications affect approximately one tenth of individuals worldwide and represent a growing cause of morbidity and mortality. Accumulating evidence has proven that the process of diabetes is complex and interactive, involving various cellular responses and signaling cascades by posttranslational modifications (PTMs). Therefore, understanding the mechanisms and functions of PTMs in regulatory networks has fundamental importance for understanding the prediction, onset, diagnosis, progression, and treatment of diabetes. In this review, we offer a holistic summary and illustration of the crosstalk between PTMs and diabetes, including both types 1 and 2. Meanwhile, we discuss the potential use of PTMs in diabetes treatment and provide a prospective direction for deeply understanding the metabolic diseases.

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

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

Protein O-GlcNAcylation and the regulation of energy homeostasis: lessons from knock-out mouse models

O-GlcNAcylation corresponds to the addition of N-Acetylglucosamine (GlcNAc) on serine or threonine residues of cytosolic, nuclear and mitochondrial proteins. This reversible modification is catalysed by a unique couple of enzymes, O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA). OGT uses UDP-GlcNAc produced in the hexosamine biosynthesis pathway, to modify proteins. UDP-GlcNAc is at the cross-roads of several cellular metabolisms, including glucose, amino acids and fatty acids. Therefore, OGT is considered as a metabolic sensor that post-translationally modifies proteins according to nutrient availability. O-GlcNAcylation can modulate protein–protein interactions and regulate protein enzymatic activities, stability or subcellular localization. In addition, it can compete with phosphorylation on the same serine or threonine residues, or regulate positively or negatively the phosphorylation of adjacent residues. As such, O-GlcNAcylation is a major actor in the regulation of cell signaling and has been implicated in numerous physiological and pathological processes. A large body of evidence have indicated that increased O-GlcNAcylation participates in the deleterious effects of glucose (glucotoxicity) in metabolic diseases. However, recent studies using mice models with OGT or OGA knock-out in different tissues have shown that O-GlcNAcylation protects against various cellular stresses, and indicate that both increase and decrease in O-GlcNAcylation have deleterious effects on the regulation of energy homeostasis.

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.

The physiological and pathophysiological roles of carbohydrate response element binding protein in the kidney

Glucose is not only the energy fuel for most cells, but also the signaling molecule which affects gene expression via carbohydrate response element binding protein (ChREBP), a Mondo family transcription factor. In response to high glucose conditions, ChREBP regulates glycolytic and lipogenic genes by binding to carbohydrate response elements (ChoRE) in the regulatory region of its target genes, thus elucidating the role of ChREBP for converting excessively ingested carbohydrates to fatty acids as an energy storage in lipogenic tissues such as the liver and adipose tissue. While the pathophysiological roles of ChREBP for fatty liver and obesity in these tissues are well known, much of the physiological and pathophysiological roles of ChREBP in other tissues such as the kidney remains unclear despite its high levels of expression in them. This review will thus highlight the roles of ChREBP in the kidney and briefly introduce the latest research results that have been reported so far.

Demystifying the O-GlcNAc Code: A Systems View

Post-translational modification with O-linked β-N-acetylglucosamine (O-GlcNAc), a process referred to as O-GlcNAcylation, occurs on a vast variety of proteins. Mounting evidence in the past several decades has clearly demonstrated that O-GlcNAcylation is a unique and ubiquitous modification. Reminiscent of a code, protein O-GlcNAcylation functions as a crucial regulator of nearly all cellular processes studied. The primary aim of this review is to summarize the developments in our understanding of myriad protein substrates modified by O-GlcNAcylation from a systems perspective. Specifically, we provide a comprehensive survey of O-GlcNAcylation in multiple species studied, including eukaryotes (e.g., protists, fungi, plants, Caenorhabditis elegans, Drosophila melanogaster, murine, and human), prokaryotes, and some viruses. We evaluate features (e.g., structural properties and sequence motifs) of O-GlcNAc modification on proteins across species. Given that O-GlcNAcylation functions in a species-, tissue-/cell-, protein-, and site-specific manner, we discuss the functional roles of O-GlcNAcylation on human proteins. We focus particularly on several classes of relatively well-characterized human proteins (including transcription factors, protein kinases, protein phosphatases, and E3 ubiquitin-ligases), with representative O-GlcNAc site-specific functions presented. We hope the systems view of the great endeavor in the past 35 years will help demystify the O-GlcNAc code and lead to more fascinating studies in the years to come.

O-GlcNAcylation promotes fatty acid synthase activity under nutritional stress as a pro-survival mechanism in cancer cells

Protein O-GlcNAcylation is a specific form of protein glycosylation that targets a wide range of proteins with important functions. O-GlcNAcylation is known to be deregulated in cancer and has been linked to multiple aspects of cancer pathology. Despite its ubiquity and importance, the current understanding of the role of O-GlcNAcylation in the stress response remains limited. In this study, we performed a quantitative chemical proteomics-based open study of the O-GlcNAcome in HeLa cells, and identified 163 differentially-glycosylated proteins under starvation, involving multiple metabolic pathways. Among them, fatty acid metabolism was found to be targeted and subsequent analysis confirmed that fatty acid synthase (FASN) is O-GlcNAcylated. O-GlcNAcylation led to enhanced de novo fatty acid synthesis activity, and fatty acids contributed to the cytoprotective effects of O-GlcNAcylation under starvation. Moreover, dual inhibition of O-GlcNAcylation and FASN displayed a strong synergistic effect in vitro in inducing cell death in cancer cells. Together, the results from this study provide novel insights into the role of O-GlcNAcylation in the nutritional stress response and suggest the potential of combining inhibition of O-GlcNAcylation and fatty acid synthesis in cancer therapy. This article is protected by copyright. All rights reserved.

Insulin Resistance: From Mechanisms to Therapeutic Strategies

Insulin resistance is the pivotal pathogenic component of many metabolic diseases, including type 2 diabetes mellitus, and is defined as a state of reduced responsiveness of insulin-targeting tissues to physiological levels of insulin. Although the underlying mechanism of insulin resistance is not fully understood, several credible theories have been proposed. In this review, we summarize the functions of insulin in glucose metabolism in typical metabolic tissues and describe the mechanisms proposed to underlie insulin resistance, that is, ectopic lipid accumulation in liver and skeletal muscle, endoplasmic reticulum stress, and inflammation. In addition, we suggest potential therapeutic strategies for addressing insulin resistance.

Comparative proteomic profiling reveals a pathogenic role for the O-GlcNAcylated AIMP2-PARP1 complex in aging-related hepatic steatosis in mice

The prevalence of non-alcoholic fatty liver disease (NAFLD) increases with aging. However, the mechanism of aging-related NAFLD remains unclear. Herein, we constructed an aging-related hepatic steatosis model and analyzed the differentially expressed proteins (DEPs) in livers from young and old mice using liquid chromatography-mass spectrometry. Five hundred and eighty-eight aging-related DEPs and novel pathways were identified. Aminoacyl tRNA synthetase complex-interacting multifunctional protein 2 (AIMP2), the most significantly upregulated protein, promoted poly(ADP-ribose) polymerase 1 (PARP1) activation in aging-related hepatic steatosis. Additionally, mice liver-specific O-GlcNAcase knockout promoted AIMP2 and PARP1 expression. O-GlcNAc transferase (OGT) overexpression and O-GlcNAcase inhibition by genetic or pharmaceutical manipulations increased AIMP2 and PARP1 levels in vitro. Mechanistically, O-GlcNAcylation increased AIMP2 protein stability, leading to its aggregation. Our study reveals O-GlcNAcylated AIMP2 as a novel pathogenic regulator of aging-related hepatic steatosis.

Whole-Genome Selective Scans Detect Genes Associated With Important Phenotypic Traits in Sheep (Ovis aries)

Sheep (Ovis aries) is one of the important livestock with diverse phenotypic traits. However, little is known about the molecular mechanism of diverse phenotypic traits in domestic sheep. Using the genome-wide high-density SNP data (600K) in 253 samples from 13 populations, we conducted the tests of selective sweeps (i.e., pairwise F_ST and XP-CLR) associated with several important phenotypic traits (e.g., tail types, horn morphology, prolificacy, coat pigmentation, ear size, milk production, meat production, body size and wool fineness). We identified strong selective signatures in previously reported (e.g., T, RXFP2, BMPR1B, TYRP1, MSRB3, TF, CEBPA, GPR21 and HOXC8) and novel genes associated with the traits, such as CERS6, BTG1, RYR3, SLC6A4, NNAT and OGT for fat deposition in the tails, FOXO4 for fertility, PTCH1 and EMX2 for ear size, and RMI1 and SCD5 for body size. Further gene annotation analysis showed that these genes were identified to be the most probable genes accounting for the diverse phenotypic traits. Our results provide novel insights into the genetic mechanisms underlying the traits and also new genetic markers for genetic improvement in sheep and other livestock.

O-GlcNAcylation inhibits hepatic stellate cell activation

BACKGROUND AND AIM

Protein O-GlcNAcylation is a critical post-translational modification regulating gene expression and fundamental cell functions. O-GlcNAc transferase (OGT) emerged as a key regulator of liver pathophysiology and disease. In this study, we aimed to evaluate the role of OGT in hepatic stellate cells (HSCs) and its consequent role in liver fibrosis.

METHODS

Primary HSCs were isolated from C57/B6 mice. Cell morphology and α-SMA immunofluorescence staining were observed under scanning confocal microscope. Transcriptomic profile was evaluated by RNAseq (Illumina). Promoter activity was examined by luciferase and β--Galactosidase reporter assays. Liver fibrosis mouse models were induced either by intraperitoneal injection of CCl4 at 3 times/week for 4 weeks or by feeding with methionine and choline deficient (MCD) diet for 4 weeks.

RESULTS

OGT protein expression and protein O-GlcNAcylation were significantly decreased in CCl4 - or MCD diet-induced liver fibrosis as compared with normal liver in mice. OGT expression and protein O-GlcNAcylation were also decreased in primary HSCs isolated from liver with CCl4 -induced fibrosis compared with those from normal liver. RNA-seq showed that OGT knockdown in HSCs modulated key signaling pathways involved in HSC activation. Promoter sequence analysis of the differentially expressed genes predicted serum response factor (SRF) as a key transcription factor regulated by OGT. Luciferase reporter assay confirmed that OGT repressed activity of SRF to induce α-SMA transcription. Mutations of specific O-GlcNAcylation sites on SRF increased its transcriptional activity, validating negative regulation of SRF by OGT-mediated O-GlcNAcylation.

CONCLUSIONS

Our results suggest that OGT functions as a negative regulator of HSC activation by promoting SRF O-GlcNAcylation to protect against liver fibrosis.

Pleiotropic actions of IP6K1 mediate hepatic metabolic dysfunction to promote nonalcoholic fatty liver disease and steatohepatitis

OBJECTIVE

Obesity and insulin resistance greatly increase the risk of nonalcoholic fatty liver disease and steatohepatitis (NAFLD/NASH). We have previously discovered that whole-body and adipocyte-specific Ip6k1deletion protects mice from high-fat-diet-induced obesity and insulin resistance due to improved adipocyte thermogenesis and insulin signaling. Here, we aimed to determine the impact of hepatocyte-specific and whole-body Ip6k1 deletion (HKO and Ip6k1-KO or KO) on liver metabolism and NAFLD/NASH.

METHODS

Body weight and composition; energy expenditure; glycemic profiles; and serum and liver metabolic, inflammatory, fibrotic and toxicity parameters were assessed in mice fed Western and high-fructose diet (HFrD) (WD: 40% kcal fat, 1.25% cholesterol, no added choline and HFrD: 60% kcal fructose). Mitochondrial oxidative capacity was evaluated in isolated hepatocytes. RNA-Seq was performed in liver samples. Livers from human NASH patients were analyzed by immunoblotting and mass spectrometry.

RESULTS

HKO mice displayed increased hepatocyte mitochondrial oxidative capacity and improved insulin sensitivity but were not resistant to body weight gain. Improved hepatocyte metabolism partially protected HKO mice from NAFLD/NASH. In contrast, enhanced whole-body metabolism and reduced body fat accumulation significantly protected whole-body Ip6k1-KO mice from NAFLD/NASH. Mitochondrial oxidative pathways were upregulated, whereas gluconeogenic and fibrogenic pathways were downregulated in Ip6k1-KO livers. Furthermore, IP6K1 was upregulated in human NASH livers and interacted with the enzyme O-GlcNAcase that reduces protein O-GlcNAcylation. Protein O-GlcNAcylation was found to be reduced in Ip6k1-KO and HKO mouse livers.

CONCLUSION

Pleiotropic actions of IP6K1 in the liver and other metabolic tissues mediate hepatic metabolic dysfunction and NAFLD/NASH, and thus IP6K1 deletion may be a potential treatment target for this disease.

Hexosaminidase A (HEXA) regulates hepatic sphingolipid and lipoprotein metabolism in mice

Hexosaminidase A (HexA), a heterodimer consisting of HEXA and HEXB, converts the ganglioside sphingolipid GM2 to GM3 by removing a terminal N-acetyl-d-galactosamine. HexA enzyme deficiency in humans leads to GM2 accumulation in cells, particularly in neurons, and is associated with neurodegeneration. While HexA and sphingolipid metabolism have been extensively investigated in the context of neuronal lipid metabolism, little is known about the metabolic impact of HexA and ganglioside degradation in other tissues. Here, we focussed on the role of HexA in the liver, which is a major regulator of systemic lipid metabolism. We find that hepatic Hexa expression is induced by lipid availability and increased in the presence of hepatic steatosis, which is associated with increased hepatic GM3 content. To assess the impact of HEXA on hepatic lipid metabolism, we used an adeno-associated virus to overexpress HEXA in the livers of high-fat diet fed mice. HEXA overexpression was associated with increased hepatic GM3 content and increased expression of enzymes involved in the degradation of glycated sphingolipids, ultimately driving sphingomyelin accumulation in the liver. In addition, HEXA overexpression led to substantial proteome remodeling in cell surface lipid rafts, which was associated with increased VLDL processing and secretion, hypertriglyceridemia and ectopic lipid accumulation in peripheral tissues. This study established an important role of HEXA in modulating hepatic sphingolipid and lipoprotein metabolism.

The proline-rich domain of MML-1 is biologically important but not required for localization to target promoters

The only representative of the MYC superfamily transcription factors in C. elegans, MML-1 (Myc and Mondo-like 1), was shown to promote extended lifespan in a variety of models and to regulate some aspects of C. elegans development. This previous research did not involve molecular characterization of MML-1. Here we use available mml-1 mutant alleles and other reagents to demonstrate that MML-1 is modified by O-GlcNAc, binds to promoters of some genes directly regulated by the DOT-1.1 histone methyltransferase complex, and has a role in promoting neuronal migration. Surprisingly, we found that the deletion allele mml-1(ok849), which was considered a null, produces an internally truncated protein resulting from an in-frame deletion. Localization of this truncated product to MML-1 target promoters was not impaired. The deleted region of MML-1 is proline-rich, and its function is poorly understood in mammalian homologs of MML-1. Based on our work and previously published data we conclude that the internal proline-rich region of MML-1 is dispensable for DNA binding but is biologically important.

The Roles of Carbohydrate Response Element Binding Protein in the Relationship between Carbohydrate Intake and Diseases

Carbohydrates are macronutrients that serve as energy sources. Many studies have shown that carbohydrate intake is nonlinearly associated with mortality. Moreover, high-fructose corn syrup (HFCS) consumption is positively associated with obesity, cardiovascular disease, and type 2 diabetes mellitus (T2DM). Accordingly, products with equal amounts of glucose and fructose have the worst effects on caloric intake, body weight gain, and glucose intolerance, suggesting that carbohydrate amount, kind, and form determine mortality. Understanding the role of carbohydrate response element binding protein (ChREBP) in glucose and lipid metabolism will be beneficial for elucidating the harmful effects of high-fructose corn syrup (HFCS), as this glucose-activated transcription factor regulates glycolytic and lipogenic gene expression. Glucose and fructose coordinately supply the metabolites necessary for ChREBP activation and de novo lipogenesis. Chrebp overexpression causes fatty liver and lower plasma glucose levels, and ChREBP deletion prevents obesity and fatty liver. Intestinal ChREBP regulates fructose absorption and catabolism, and adipose-specific Chrebp-knockout mice show insulin resistance. ChREBP also regulates the appetite for sweets by controlling fibroblast growth factor 21, which promotes energy expenditure. Thus, ChREBP partly mimics the effects of carbohydrate, especially HFCS. The relationship between carbohydrate intake and diseases partly resembles those between ChREBP activity and diseases.

New targets for NAFLD

Non-alcoholic fatty liver disease (NAFLD) is a growing cause of chronic liver disease worldwide. It is characterised by steatosis, liver inflammation, hepatocellular injury and progressive fibrosis. Several preclinical models (dietary and genetic animal models) of NAFLD have deepened our understanding of its aetiology and pathophysiology. Despite the progress made, there are currently no effective treatments for NAFLD. In this review, we will provide an update on the known molecular pathways involved in the pathophysiology of NAFLD and on ongoing studies of new therapeutic targets.

Trefoil factor 2 secreted from damaged hepatocytes activates hepatic stellate cells to induce fibrogenesis

Liver fibrosis is a common characteristic of chronic liver diseases. The activation of hepatic stellate cells (HSCs) plays a key role in fibrogenesis in response to liver injury, yet the mechanism by which damaged hepatocytes modulate the activation of HSCs is poorly understood. Our previous studies have established that liver-specific deletion of O-GlcNAc transferase (OGT)leads to hepatocyte necroptosis and spontaneous fibrosis. Here, we report that OGT-deficient hepatocytes secrete trefoil factor 2 (TFF2) that activates HSCs and contributes to the fibrogenic process. The expression and secretion of TFF2 are induced in OGT-deficient hepatocytes but not in WT hepatocytes. TFF2 activates the platelet-derived growth factor receptor beta signaling pathway that promotes the proliferation and migration of primary HSCs. TFF2 protein expression is elevated in mice with carbon tetrachloride-induced liver injury. These findings identify TFF2 as a novel factor that mediates intercellular signaling between hepatocytes and HSCs and suggest a role of the hepatic OGT–TFF2 axis in the process of fibrogenesis.

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

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

From Food to Genes: Transcriptional Regulation of Metabolism by Lipids and Carbohydrates

Lipids and carbohydrates regulate gene expression by means of molecules that sense these macronutrients and act as transcription factors. The peroxisome proliferator-activated receptor (PPAR), activated by some fatty acids or their derivatives, and the carbohydrate response element binding protein (ChREBP), activated by glucose-derived metabolites, play a key role in metabolic homeostasis, especially in glucose and lipid metabolism. Furthermore, the action of both factors in obesity, diabetes and fatty liver, as well as the pharmacological development in the treatment of these pathologies are indeed of high relevance. In this review we present an overview of the discovery, mechanism of activation and metabolic functions of these nutrient-dependent transcription factors in different tissues contexts, from the nutritional genomics perspective. The possibility of targeting these factors in pharmacological approaches is also discussed. Lipid and carbohydrate-dependent transcription factors are key players in the complex metabolic homeostasis, but these factors also drive an adaptive response to non-physiological situations, such as overeating. Possibly the decisive role of ChREBP and PPAR in metabolic regulation points to them as ideal therapeutic targets, but their pleiotropic functions in different tissues makes it difficult to "hit the mark".

Hepatic transcriptional responses to fasting and feeding

Mammals undergo regular cycles of fasting and feeding that engage dynamic transcriptional responses in metabolic tissues. Here we review advances in our understanding of the gene regulatory networks that contribute to hepatic responses to fasting and feeding. The advent of sequencing and -omics techniques have begun to facilitate a holistic understanding of the transcriptional landscape and its plasticity. We highlight transcription factors, their cofactors, and the pathways that they impact. We also discuss physiological factors that impinge on these responses, including circadian rhythms and sex differences. Finally, we review how dietary modifications modulate hepatic gene expression programs.

The hexosamine biosynthetic pathway and cancer: Current knowledge and future therapeutic strategies

The hexosamine biosynthetic pathway (HBP) is a glucose metabolism pathway that results in the synthesis of a nucleotide sugar UDP-GlcNAc, which is subsequently used for the post-translational modification (O-GlcNAcylation) of intracellular proteins that regulate nutrient sensing and stress response. The HBP is carried out by a series of enzymes, many of which have been extensively implicated in cancer pathophysiology. Increasing evidence suggests that elevated activation of the HBP may act as a cancer biomarker. Inhibition of HBP enzymes could suppress tumor cell growth, modulate the immune response, reduce resistance, and sensitize tumor cells to conventional cancer therapy. Therefore, targeting the HBP may serve as a novel strategy for treating cancer patients. Here, we review the current findings on the significance of HBP enzymes in various cancers and discuss future approaches for exploiting HBP inhibition for cancer treatment.

Adaptive and maladaptive roles for ChREBP in the liver and pancreatic islets

Excessive sugar consumption is a contributor to the worldwide epidemic of cardiometabolic disease. Understanding mechanisms by which sugar is sensed and regulates metabolic processes may provide new opportunities to prevent and treat these epidemics. Carbohydrate Responsive-Element Binding Protein (ChREBP) is a sugar-sensing transcription factor that mediates genomic responses to changes in carbohydrate abundance in key metabolic tissues. Carbohydrate metabolites activate the canonical form of ChREBP, ChREBP-alpha, which stimulates production of a potent, constitutively active ChREBP isoform called ChREBP-beta. Carbohydrate metabolites and other metabolic signals may also regulate ChREBP activity via posttranslational modifications including phosphorylation, acetylation, and O-GlcNAcylation that can affect ChREBP’s cellular localization, stability, binding to cofactors, and transcriptional activity. In this review, we discuss mechanisms regulating ChREBP activity and highlight phenotypes and controversies in ChREBP gain- and loss-of-function genetic rodent models focused on the liver and pancreatic islets.

New Insight Into Metformin-Induced Cholesterol-Lowering Effect Crosstalk Between Glucose and Cholesterol Homeostasis via ChREBP (Carbohydrate-Responsive Element-Binding Protein)-Mediated PCSK9 (Proprotein Convertase Subtilisin/Kexin Type 9) Regulation

[Figure: see text].

The Role of Mondo Family Transcription Factors in Nutrient-Sensing and Obesity

In the past several decades obesity has become one of the greatest health burdens worldwide. Diet high in fats and fructose is one of the main causes for the prevalence of metabolic disorders including obesity. Promoting brown or beige adipocyte development and activity is regarded as a potential treatment of obesity. Mondo family transcription factors including MondoA and carbohydrate response element binding protein (ChREBP) are critical for nutrient-sensing in multiple metabolic organs including the skeletal muscle, liver, adipose tissue and pancreas. Under normal nutrient conditions, MondoA and ChREBP contribute to maintaining metabolic homeostasis. When nutrient is overloaded, Mondo family transcription factors directly regulate glucose and lipid metabolism in brown and beige adipocytes or modulate the crosstalk between metabolic organs. In this review, we aim to provide an overview of recent advances in the understanding of MondoA and ChREBP in sensing nutrients and regulating obesity or related pathological conditions.

Dual regulation of TxNIP by ChREBP and FoxO1 in liver

TxNIP (Thioredoxin-interacting protein) is considered as a potential drug target for type 2 diabetes. Although TxNIP expression is correlated with hyperglycemia and glucotoxicity in pancreatic β cells, its regulation in liver cells has been less investigated. In the current study, we aim at providing a better understanding of Txnip regulation in hepatocytes in response to physiological stimuli and in the context of hyperglycemia in db/db mice. We focused on regulatory pathways governed by ChREBP (Carbohydrate Responsive Element Binding Protein) and FoxO1 (Forkhead box protein O1), transcription factors that play central roles in mediating the effects of glucose and fasting on gene expression, respectively. Studies using genetically modified mice reveal that hepatic TxNIP is up-regulated by both ChREBP and FoxO1 in liver cells and that its expression strongly correlates with fasting, suggesting a major role for this protein in the physiological adaptation to nutrient restriction.

The human O-GlcNAcome database and meta-analysis

Over the past 35 years, ~1700 articles have characterized protein O-GlcNAcylation. Found in almost all living organisms, this post-translational modification of serine and threonine residues is highly conserved and key to biological processes. With half of the primary research articles using human models, the O-GlcNAcome recently reached a milestone of 5000 human proteins identified. Herein, we provide an extensive inventory of human O-GlcNAcylated proteins, their O-GlcNAc sites, identification methods, and corresponding references ( www.oglcnac.mcw.edu ). In the absence of a comprehensive online resource for O-GlcNAcylated proteins, this list serves as the only database of O-GlcNAcylated proteins. Based on the thorough analysis of the amino acid sequence surrounding 7002 O-GlcNAc sites, we progress toward a more robust semi-consensus sequence for O-GlcNAcylation. Moreover, we offer a comprehensive meta-analysis of human O-GlcNAcylated proteins for protein domains, cellular and tissue distribution, and pathways in health and diseases, reinforcing that O-GlcNAcylation is a master regulator of cell signaling, equal to the widely studied phosphorylation.

NF-κB p65 regulates hepatic lipogenesis by promoting nuclear entry of ChREBP in response to a high carbohydrate diet

Overconsumption of sucrose and other sugars has been associated with nonalcoholic fatty liver disease (NAFLD). Reports suggest hepatic de novo lipogenesis (DNL) as an important contributor to and regulator of carbohydrate-induced hepatic lipid accumulation in NAFLD. The mechanisms responsible for the increase in hepatic DNL due to overconsumption of carbohydrate diet are less than clear; however, literatures suggest high carbohydrate diet to activate the lipogenic transcription factor carbohydrate response element-binding protein (ChREBP), which further transcribes genes involved in DNL. Here, we provide an evidence of an unknown link between nuclear factor kappa-light chain enhancer of activated B cells (NF-κB) activation and increased DNL. Our data indicates high carbohydrate diet to enforce nuclear shuttling of hepatic NF-κB p65 and repress transcript levels of sorcin, a cytosolic interacting partner of ChREBP. Reduced sorcin levels, further prompted ChREBP nuclear translocation, leading to enhanced DNL and intrahepatic lipid accumulation both in vivo and in vitro. We further report that pharmacological inhibition of NF-κB abrogated high carbohydrate diet-mediated sorcin repression and thereby prevented ChREBP nuclear translocation and this, in turn, attenuated hepatic lipid accumulation both in in vitro and in vivo. Additionally, sorcin knockdown blunted the lipid-lowering ability of the NF-κB inhibitor in vitro. Together, these data suggest a heretofore unknown role for NF-κB in regulating ChREBP nuclear localization and activation, in response to high carbohydrate diet, for further explorations in lines of NAFLD therapeutics.

The glucose-sensing transcription factor ChREBP is targeted by proline hydroxylation

Cellular energy demands are met by uptake and metabolism of nutrients like glucose. The principal transcriptional regulator for adapting glycolytic flux and downstream pathways like de novo lipogenesis to glucose availability in many cell types is carbohydrate response element-binding protein (ChREBP). ChREBP is activated by glucose metabolites and post-translational modifications, inducing nuclear accumulation and regulation of target genes. Here we report that ChREBP is modified by proline hydroxylation at several residues. Proline hydroxylation targets both ectopically expressed ChREBP in cells and endogenous ChREBP in mouse liver. Functionally, we found that specific hydroxylated prolines were dispensable for protein stability but required for the adequate activation of ChREBP upon exposure to high glucose. Accordingly, ChREBP target gene expression was rescued by re-expressing WT but not ChREBP that lacks hydroxylated prolines in ChREBP-deleted hepatocytes. Thus, proline hydroxylation of ChREBP is a novel post-translational modification that may allow for therapeutic interference in metabolic diseases.