Glucose-dependent transcriptional regulation by an evolutionarily conserved glucose-sensing module

An acetylated Lysine Residue of Its Low-glucose Inhibitory Domain Controls Activity and Protein Interactions of ChREBP

Carbohydrate response element-binding protein (ChREBP) is a transcription factor activated by glucose metabolites that orchestrates the expression of genes involved in glycolysis, de novo lipogenesis, and ATP homeostasis. Inadequate ChREBP activity impairs the cellular adaptations to glucose exposure and in humans associates with dyslipidemia, fatty liver disease, and type 2 diabetes. ChREBP activity is regulated by cytosolic-nuclear translocation involving its low-glucose inhibitory domain (LID). Whether this domain is targeted by post-translational lysine acetylation is unknown. Here we report a novel LID acetylation site that controls activity and protein interactions of ChREBP. Mutation of this residue increased glucose-induced activity and target gene expression of ChREBP. Mechanistically, mutant ChREBP protein showed more nuclear localization and enhanced genomic binding to a target promoter. Interactions with proteins that exhibit differential binding upon glucose exposure were attenuated by the mutation, demonstrating the importance of the LID in the formation of the protein interactome. Particularly interactions with 14-3-3 proteins, factors that regulate cytosolic/nuclear trafficking of ChREBP, were reduced, whereas interactions with proteins of the nucleosome remodeling deacetylase complex (NuRD) were increased. These molecular insights may shape new therapeutic strategies to target ChREBP activity and counteract metabolic diseases.

Indian lychee honey ameliorates hepatic glucose uptake by regulating the ChREBP/Glut4 axis under insulin-resistant conditions

Many traditional treatments include honey owing to its magnificent health beneficiary effects. Recent studies have demonstrated the potent anti-diabetic activity of honey. However, its actual mechanism of action remains elusive. Moreover, being rich in sugar (75%-80%), its role in maintaining glucose homeostasis remains questionable. Although the polyphenol content of honey aids its hypoglycaemic activity, the small quantity of bioactive compounds in honey (0.5%-1.0%) may not be solely responsible for this. In the current study, an attempt was made to understand the role of Indian lychee honey (LyH) in regulating blood glucose levels under diabetic conditions. This study investigated whether LyH, although rich in sugars, can be used as an alternative to regulate glucose and lipid homeostasis under insulin-resistant conditions by regulating the ChREBP/Glut4 signalling pathway. This study was first performed in vitro in palmitic acid-induced insulin-resistant HepG2 cells. Various assays, such as FACS, GCMS, qRT-PCR, immunoblot and ChIP-qPCR, were performed to establish the anti-hyperglycaemic role of LyH in vitro. The in vitro results were subsequently confirmed in vivo using a high-fat diet-induced diabetic C57BL/6 mice model. The in vivo study was supported by several experiments, such as examining blood parameters, histopathology, double-immunohistochemistry and ELISA. Finally, the finding was validated by comparing it with a couple of GEO datasets from the NCBI database. This study found that LyH is an excellent choice for regulating blood sugar levels under diabetic conditions without significant harmful side effects. Moreover, LyH showed excellent hepatic glucose uptake activity in an insulin-independent manner. This activity is mainly governed by sugars as its main ingredient. LyH treatment also regulates hepatic lipid homeostasis by maintaining a balance between saturated and unsaturated fatty acids in insulin-resistant HepG2 cells. Further, sugar, when supplemented individually, caused severe inflammation, which was validated through histopathology, ELISA and IHC. Collectively, the findings of this study indicate that Indian LyH provides a better food matrix (the right proportion of sugars and different bioactive compounds), which significantly improves hyperglycemia and inflammation under diabetic conditions by regulating the hepatic ChREBP/Glut4 axis.

Glucose-Sensing Carbohydrate Response Element-Binding Protein in the Pathogenesis of Diabetic Retinopathy

Glucose-sensing ChREBP and MondoA are transcriptional factors involved in the lipogenic, inflammatory, and insulin signaling pathways implicated in metabolic disorders; however, limited ocular studies have been conducted on these proteins. We aimed to investigate the potential role of ChREBP in the pathogenesis of diabetic retinopathy (DR). We used diabetic human and mouse retinal cryosections analyzed by immunohistochemistry. qRT-PCR was performed to quantify gene expression. To explore the role of ChREBP in rods, we generated caChREBPRP mice with constitutively active (ca) ChREBP. These mice underwent retinal functional testing, which was followed by proteomic analysis using LC-MS. Furthermore, ARPE-19 cells were infected with lentiviral particles expressing human ChREBP (ARPE-19ChREBP) and subjected to global proteomics. Our results demonstrate that both proteins were expressed across the retina, although with distinct distribution patterns: MondoA was more prominently expressed in cones, while ChREBP was broadly expressed throughout the retina. Elevated expression of both proteins was observed in DR. This may have contributed to rod photoreceptor degeneration, as we observed diminished scotopic ERG amplitudes in caChREBPRP mice at P35. The retinal proteomic landscape revealed a decline in the KEGG pathways associated with phototransduction, amino acid metabolism, and cell adhesion. Furthermore, rod-specific caChREBP induced TXNIP expression. Consistent with altered retinal proteomics, ARPE-19ChREBP cells exhibit a metabolic shift toward increased glyoxylate signaling, sugar metabolism, and lysosomal activation. Our study demonstrates that ChREBP overexpression causes significant metabolic reprogramming triggering retinal functional loss in mice.

A genome-first approach to variants in MLXIPL and their association with hepatic steatosis and plasma lipids

BACKGROUND

Common variants of the max-like protein X (MLX)-interacting protein-like (MLXIPL) gene, encoding the transcription factor carbohydrate-responsive element-binding protein, have been shown to be associated with plasma triglyceride levels. However, the role of these variants in steatotic liver disease (SLD) is unclear.

METHODS

We used a genome-first approach to analyze a variety of metabolic phenotypes and clinical outcomes associated with a common missense variant in MLXIPL, Gln241His, in 2 large biobanks: the UK Biobank and the Penn Medicine Biobank.

RESULTS

Carriers of MLXIPL Gln241His were associated with significantly lower serum levels of triglycerides, apolipoprotein-B, gamma-glutamyl transferase, and alkaline phosphatase. Additionally, MLXIPL Gln241His carriers were associated with significantly higher serum levels of HDL cholesterol and alanine aminotransferase. Carriers homozygous for MLXIPL Gln241His showed a higher risk of SLD in 2 unrelated cohorts. Carriers of MLXIPL Gln241His were especially more likely to be diagnosed with SLD if they were female, obese, and/or also carried the PNPLA3 I148M variant. Furthermore, the heterozygous carriage of MLXIPL Gln241His was associated with significantly higher all-cause, liver-related, and cardiovascular mortality rates. Nuclear magnetic resonance metabolomics data indicated that carriage of MLXIPL Gln241His was significantly associated with lower serum levels of VLDL and increased serum levels of HDL cholesterol.

CONCLUSIONS

Analyses of the MLXIPL Gln241His polymorphism showed a significant association with a higher risk of SLD diagnosis and elevated serum alanine aminotransferase as well as significantly lower serum triglycerides and apolipoprotein-B levels. MLXIPL might, therefore, be a potential pharmacological target for the treatment of SLD and hyperlipidemia, notably for patients at risk. More mechanistic studies are needed to better understand the role of MLXIPL Gln241His on lipid metabolism and steatosis development.

CDK6 inhibits de novo lipogenesis in white adipose tissues but not in the liver

Increased de novo lipogenesis (DNL) in white adipose tissue is associated with insulin sensitivity. Under both Normal-Chow-Diet and High-Fat-Diet, mice expressing a kinase inactive Cyclin-dependent kinase 6 (Cdk6) allele (K43M) display an increase in DNL in visceral white adipose tissues (VAT) as compared to wild type mice (WT), accompanied by markedly increased lipogenic transcriptional factor Carbohydrate-responsive element-binding proteins (CHREBP) and lipogenic enzymes in VAT but not in the liver. Treatment of WT mice under HFD with a CDK6 inhibitor recapitulates the phenotypes observed in K43M mice. Mechanistically, CDK6 phosphorylates AMP-activated protein kinase, leading to phosphorylation and inactivation of acetyl-CoA carboxylase, a key enzyme in DNL. CDK6 also phosphorylates CHREBP thus preventing its entry into the nucleus. Ablation of runt related transcription factor 1 in K43M mature adipocytes reverses most of the phenotypes observed in K43M mice. These results demonstrate a role of CDK6 in DNL and a strategy to alleviate metabolic syndromes.

mTORC1 controls murine postprandial hepatic glycogen synthesis via Ppp1r3b

In response to a meal, insulin drives hepatic glycogen synthesis to help regulate systemic glucose homeostasis. The mechanistic target of rapamycin complex 1 (mTORC1) is a well-established insulin target and contributes to the postprandial control of liver lipid metabolism, autophagy, and protein synthesis. However, its role in hepatic glucose metabolism is less understood. Here, we used metabolomics, isotope tracing, and mouse genetics to define a role for liver mTORC1 signaling in the control of postprandial glycolytic intermediates and glycogen deposition. We show that mTORC1 is required for glycogen synthase activity and glycogenesis. Mechanistically, hepatic mTORC1 activity promotes the feeding-dependent induction of Ppp1r3b, a gene encoding a phosphatase important for glycogen synthase activity whose polymorphisms are linked to human diabetes. Reexpression of Ppp1r3b in livers lacking mTORC1 signaling enhances glycogen synthase activity and restores postprandial glycogen content. mTORC1-dependent transcriptional control of Ppp1r3b is facilitated by FOXO1, a well characterized transcriptional regulator involved in the hepatic response to nutrient intake. Collectively, we identify a role for mTORC1 signaling in the transcriptional regulation of Ppp1r3b and the subsequent induction of postprandial hepatic glycogen synthesis.

A high throughput screen for pharmacological inhibitors of the carbohydrate response element

A central regulator of metabolism, transcription factor carbohydrate response element binding protein (ChREBP) senses and responds to dietary glucose levels by stimulating the transcription of glycolytic and lipogenic enzymes. Genetic depletion of ChREBP rescues β-cell dysfunction arising from high glucose levels, suggesting that inhibiting ChREBP might represent an attractive therapeutic approach to manage diabetes and other metabolic diseases. However, the molecular mechanisms governing ChREBP activation are poorly understood and chemical tools to probe the cellular activity of ChREBP are lacking. Here, we report a high-throughput pharmacological screen in INS-1E β-cells that identified novel inhibitors of ChREBP-driven transcription at carbohydrate response element sites, including three putative covalent inhibitors and two likely non-covalent chemical scaffolds. This work affords a pharmacological toolkit to help uncover the signaling logic controlling ChREBP activation and may ultimately reveal potential therapeutic approaches for treating metabolic disease.

Insulin Regulation of Hepatic Lipid Homeostasis

The incidence of obesity, insulin resistance, and type II diabetes (T2DM) continues to rise worldwide. The liver is a central insulin-responsive metabolic organ that governs whole-body metabolic homeostasis. Therefore, defining the mechanisms underlying insulin action in the liver is essential to our understanding of the pathogenesis of insulin resistance. During periods of fasting, the liver catabolizes fatty acids and stored glycogen to meet the metabolic demands of the body. In postprandial conditions, insulin signals to the liver to store excess nutrients into triglycerides, cholesterol, and glycogen. In insulin-resistant states, such as T2DM, hepatic insulin signaling continues to promote lipid synthesis but fails to suppress glucose production, leading to hypertriglyceridemia and hyperglycemia. Insulin resistance is associated with the development of metabolic disorders such as cardiovascular and kidney disease, atherosclerosis, stroke, and cancer. Of note, nonalcoholic fatty liver disease (NAFLD), a spectrum of diseases encompassing fatty liver, inflammation, fibrosis, and cirrhosis, is linked to abnormalities in insulin-mediated lipid metabolism. Therefore, understanding the role of insulin signaling under normal and pathologic states may provide insights into preventative and therapeutic opportunities for the treatment of metabolic diseases. Here, we provide a review of the field of hepatic insulin signaling and lipid regulation, including providing historical context, detailed molecular mechanisms, and address gaps in our understanding of hepatic lipid regulation and the derangements under insulin-resistant conditions. © 2023 American Physiological Society. Compr Physiol 13:4785-4809, 2023.

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.

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.

MEK1-dependent MondoA phosphorylation regulates glucose uptake in response to ketone bodies in colorectal cancer cells

The Mondo family transcription factor MondoA plays a pivotal role in sensing metabolites, such as glucose, glutamine, and lactic acid, to regulate glucose metabolism and cell proliferation. Ketone bodies are important signals for reducing glucose uptake. However, it is unclear whether MondoA functions in ketone body-regulated glucose transport. Here we reported that ketone bodies promoted MondoA nuclear translocation and binding to the promoter of its target gene TXNIP. Ketone bodies reduced glucose uptake, increased apoptosis and decreased proliferation of colorectal cancer cells, which was impeded by MondoA knockdown. Moreover, we identified MEK1 as a novel component of the MondoA protein complex using a proteomic approach. Mechanistically, MEK1 interacted with MondoA and enhanced tyrosine 222, but not serine or threonine, phosphorylation of MondoA, inhibiting MondoA nuclear translocation and transcriptional activity. Ketone bodies decreased MEK1-dependent MondoA phosphorylation by blocking MondoA and MEK1 interaction, leading to MondoA nuclear translocation, TXNIP transcription, and inhibition of glucose uptake. Therefore, our study not only demonstrated that ketone bodies reduce glucose uptake, promote apoptosis, and inhibit cell proliferation in colorectal cancer cells by regulating MondoA phosphorylation but also identified MEK1-dependent phosphorylation as a new mechanism to manipulate MondoA activity.

Similar adaptative mechanism but divergent demographic history of four sympatric desert rodents in Eurasian inland

Phenotypes associated with metabolism and water retention are thought to be key to the adaptation of desert species. However, knowledge on the genetic changes and selective regimes on the similar and divergent ways to desert adaptation in sympatric and phylogenetically close desert organisms remains limited. Here, we generate a chromosome level genome assembly for Northern three-toed jerboa (Dipus sagitta) and three other high-quality genome assemblies for Siberian jerboa (Orientallactaga sibirica), Midday jird (Meriones meridianus), and Desert hamster (Phodopus roborovskii). Genomic analyses unveil that desert adaptation of the four species mainly result from similar metabolic pathways, such as arachidonic acid metabolism, thermogenesis, oxidative phosphorylation, insulin related pathway, DNA repair and protein synthesis and degradation. However, the specific evolved genes in the same adaptative molecular pathway often differ in the four species. We also reveal similar niche selection but different demographic histories and sensitivity to climate changes, which may be related to the diversified genomic adaptative features. In addition, our study suggests that nocturnal rodents have evolved some specific adaptative mechanism to desert environments compared to large desert animals. Our genomic resources will provide an important foundation for further research on desert genetic adaptations.

Association between the rs3812316 Single Nucleotide Variant of the MLXIPL Gene and Alpha-Linolenic Acid Intake with Triglycerides in Mexican Mestizo Women

The carbohydrate response element binding protein (ChREBP) is a key transcription factor to understand the gene−diet−nutrient relationship that leads to metabolic diseases. We aimed to analyze the association between the rs17145750 and rs3812316 SNVs (single nucleotide variants) of the MLXIPL gene with dietary, anthropometric, and biochemical variables in Mexican Mestizo subjects. This is a cross-sectional study of 587 individuals. Genotyping was performed by allelic discrimination. In addition, liver and adipose tissue biopsies were obtained from a subgroup of 24 subjects to analyze the expression of the MLXIPL gene. An in silico test of the protein stability and allelic imbalance showed that rs17145750 and rs3812316 showed a high rate of joint heritability in a highly conserved area. The G allele of rs3812316 was associated with lower triglyceride levels (OR = −0.070 ± 0.027, p < 0.011, 95% CI = −0.124 to −0.016), the production of an unstable protein (ΔΔG −0.83 kcal/mol), and probably lower tissue mRNA levels. In addition, we found independent factors that also influence triglyceride levels, such as insulin resistance, HDL-c, and dietary protein intake in women. Our data showed that the association of rs3812316 on triglycerides was only observed in patients with an inadequate alpha-linolenic acid intake (1.97 ± 0.03 vs. 2.11 ± 0.01 log mg/dL, p < 0.001).

Transcriptional regulation of Acsl1 by CHREBP and NF-kappa B in macrophages during hyperglycemia and inflammation

Acyl-CoA synthetase 1 (ACSL1) is an enzyme that converts fatty acids to acyl-CoA-derivatives for lipid catabolism and lipid synthesis in general and can provide substrates for the production of mediators of inflammation in monocytes and macrophages. Acsl1 expression is increased by hyperglycemia and inflammatory stimuli in monocytes and macrophages, and promotes the pro-atherosclerotic effects of diabetes in mice. Yet, surprisingly little is known about the mechanisms underlying Acsl1 transcriptional regulation. Here we demonstrate that the glucose-sensing transcription factor, Carbohydrate Response Element Binding Protein (CHREBP), is a regulator of the expression of Acsl1 mRNA by high glucose in mouse bone marrow-derived macrophages (BMDMs). In addition, we show that inflammatory stimulation of BMDMs with lipopolysaccharide (LPS) increases Acsl1 mRNA via the transcription factor, NF-kappa B. LPS treatment also increases ACSL1 protein abundance and localization to membranes where it can exert its activity. Using an Acsl1 reporter gene containing the promoter and an upstream regulatory region, which has multiple conserved CHREBP and NF-kappa B (p65/RELA) binding sites, we found increased Acsl1 promoter activity upon CHREBP and p65/RELA expression. We also show that CHREBP and p65/RELA occupy the Acsl1 promoter in BMDMs. In primary human monocytes cultured in high glucose versus normal glucose, ACSL1 mRNA expression was elevated by high glucose and further enhanced by LPS treatment. Our findings demonstrate that CHREBP and NF-kappa B control Acsl1 expression under hyperglycemic and inflammatory conditions.

CHREBP suppresses gastric cancer progression via the cyclin D1-Rb-E2F1 pathway

Accumulating evidence has demonstrated that carbohydrate response element binding protein (CHREBP) has a crucial function in tumor pathology. In this study, we found CHREBP downregulation in gastric cancer (GC) tissues, and CHREBP was determined to be an independent diagnostic marker of GC. The downregulation of CHREBP promoted cell proliferation and inhibited apoptosis. Moreover, the level of cyclin D1 was significantly correlated with CHREBP expression in GC and paracancerous normal samples. In addition, CHREBP transcriptionally inhibited cyclin D1 expression in GC cells. Tumor suppressor activity of CHREBP could be affected by the upregulation of cyclin D1. In summary, CHREBP was found to be an independent diagnostic marker of GC and to influence GC growth and apoptosis via targeting the cyclin D1-Rb-E2F1 pathway.

The polyol pathway is an evolutionarily conserved system for sensing glucose uptake

Cells must adjust the expression levels of metabolic enzymes in response to fluctuating nutrient supply. For glucose, such metabolic remodeling is highly dependent on a master transcription factor ChREBP/MondoA. However, it remains elusive how glucose fluctuations are sensed by ChREBP/MondoA despite the stability of major glycolytic pathways. Here, we show that in both flies and mice, ChREBP/MondoA activation in response to glucose ingestion involves an evolutionarily conserved glucose-metabolizing pathway: the polyol pathway. The polyol pathway converts glucose to fructose via sorbitol. It has been believed that this pathway is almost silent, and its activation in hyperglycemic conditions has deleterious effects on human health. We show that the polyol pathway regulates the glucose-responsive nuclear translocation of Mondo, a Drosophila homologue of ChREBP/MondoA, which directs gene expression for organismal growth and metabolism. Likewise, inhibition of the polyol pathway in mice impairs ChREBP’s nuclear localization and reduces glucose tolerance. We propose that the polyol pathway is an evolutionarily conserved sensing system for glucose uptake that allows metabolic remodeling.

The polyol pathway, which converts glucose to fructose via sorbitol, was thought to be largely silent, and only the negative effects of its activation were known. This study reveals that the polyol pathway is involved in glucose-responsive activation of Mondo/ChREBP-mediated metabolic remodeling in both mice and flies.

Normal and Neoplastic Growth Suppression by the Extended Myc Network

Among the first discovered and most prominent cellular oncogenes is MYC, which encodes a bHLH-ZIP transcription factor (Myc) that both activates and suppresses numerous genes involved in proliferation, energy production, metabolism and translation. Myc belongs to a small group of bHLH-ZIP transcriptional regulators (the Myc Network) that includes its obligate heterodimerization partner Max and six "Mxd proteins" (Mxd1-4, Mnt and Mga), each of which heterodimerizes with Max and largely opposes Myc's functions. More recently, a second group of bHLH-ZIP proteins (the Mlx Network) has emerged that bears many parallels with the Myc Network. It is comprised of the Myc-like factors ChREBP and MondoA, which, in association with the Max-like member Mlx, regulate smaller and more functionally restricted repertoires of target genes, some of which are shared with Myc. Opposing ChREBP and MondoA are heterodimers comprised of Mlx and Mxd1, Mxd4 and Mnt, which also structurally and operationally link the two Networks. We discuss here the functions of these "Extended Myc Network" members, with particular emphasis on their roles in suppressing normal and neoplastic growth. These roles are complex due to the temporal- and tissue-restricted expression of Extended Myc Network proteins in normal cells, their regulation of both common and unique target genes and, in some cases, their functional redundancy.

ChREBPβ is dispensable for the control of glucose homeostasis and energy balance

Impaired glucose metabolism is observed in obesity and type 2 diabetes. Glucose controls gene expression through the transcription factor ChREBP in liver and adipose tissues. Mlxipl encodes 2 isoforms: ChREBPα, the full-length form (translocation into the nucleus is under the control of glucose), and ChREBPβ, a constitutively nuclear shorter form. ChREBPβ gene expression in white adipose tissue is strongly associated with insulin sensitivity. Here, we investigated the consequences of ChREBPβ deficiency on insulin action and energy balance. ChREBPβ-deficient male and female C57BL6/J and FVB/N mice were produced using CRISPR/Cas9-mediated gene editing. Unlike global ChREBP deficiency, lack of ChREBPβ showed modest effects on gene expression in adipose tissues and the liver, with variations chiefly observed in brown adipose tissue. In mice fed chow and 2 types of high-fat diets, lack of ChREBPβ had moderate effects on body composition and insulin sensitivity. At thermoneutrality, ChREBPβ deficiency did not prevent the whitening of brown adipose tissue previously reported in total ChREBP-KO mice. These findings revealed that ChREBPβ is dispensable for metabolic adaptations to nutritional and thermic challenges.

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.

Signal Transduction and Molecular Regulation in Fatty Liver Disease

Significance: Fatty liver disease is a major liver disorder in the modern societies. Comprehensive understanding of the pathophysiology and molecular mechanisms is essential for the prevention and treatment of the disease. Recent Advances: Remarkable progress has been made in the recent years in basic and translational research in the field of fatty liver disease. Multiple signaling pathways have been implicated in the development of fatty liver disease, including AMP-activated protein kinase, mechanistic target of rapamycin kinase, endoplasmic reticulum stress, oxidative stress, inflammation, transforming growth factor β, and yes1-associated transcriptional regulator/transcriptional coactivator with PDZ-binding motif (YAP/TAZ). In addition, critical molecular regulations at the transcriptional and epigenetic levels have been linked to the pathogenesis of fatty liver disease. Critical Issues: Some critical issues remain to be solved so that research findings can be translated into clinical applications. Robust and reliable biomarkers are needed for diagnosis of different stages of the fatty liver disease. Effective and safe molecular targets remain to be identified and validated. Prevention strategies require solid scientific evidence and population-wide feasibility. Future Directions: As more data are generated with time, integrative approaches are needed to comprehensively understand the disease pathophysiology and mechanisms at multiple levels from population, organismal system, organ/tissue, to cell. The interactions between genes and environmental factors require deeper investigation for the purposes of prevention and personalized treatment of fatty liver disease. Antioxid. Redox Signal. 35, 689-717.

What fuels the fly: Energy metabolism in Drosophila and its application to the study of obesity and diabetes

The organs and metabolic pathways involved in energy metabolism, and the process of ATP production from nutrients, are comparable between humans and Drosophila melanogaster This level of conservation, together with the power of Drosophila genetics, makes the fly a very useful model system to study energy homeostasis. Here, we discuss the major organs involved in energy metabolism in Drosophila and how they metabolize different dietary nutrients to generate adenosine triphosphate. Energy metabolism in these organs is controlled by cell-intrinsic, paracrine, and endocrine signals that are similar between Drosophila and mammals. We describe how these signaling pathways are regulated by several physiological and environmental cues to accommodate tissue-, age-, and environment-specific differences in energy demand. Last, we discuss several genetic and diet-induced fly models of obesity and diabetes that can be leveraged to better understand the molecular basis of these metabolic diseases and thereby promote the development of novel therapies.

MondoA Is Required for Normal Myogenesis and Regulation of the Skeletal Muscle Glycogen Content in Mice

Background

Skeletal muscle is the largest tissue in the human body, and it plays a major role in exerting force and maintaining metabolism homeostasis. The role of muscle transcription factors in the regulation of metabolism is not fully understood. MondoA is a glucose-sensing transcription factor that is highly expressed in skeletal muscle. Previous studies suggest that MondoA can influence systemic metabolism homeostasis. However, the function of MondoA in the skeletal muscle remains unclear.

Methods

We generated muscle-specific MondoA knockout (MAKO) mice and analyzed the skeletal muscle morphology and glycogen content. Along with skeletal muscle from MAKO mice, C2C12 myocytes transfected with small interfering RNA against MondoA were also used to investigate the role and potential mechanism of MondoA in the development and glycogen metabolism of skeletal muscle.

Results

MAKO caused muscle fiber atrophy, reduced the proportion of type II fibers compared to type I fibers, and increased the muscle glycogen level. MondoA knockdown inhibited myoblast proliferation, migration, and differentiation by inhibiting the phosphatase and tensin homolog (PTEN)/phosphoinositide 3-kinase (PI3K)/Akt pathway. Further mechanistic experiments revealed that the increased muscle glycogen in MAKO mice was caused by thioredoxin-interacting protein (TXNIP) downregulation, which led to upregulation of glucose transporter 4 (GLUT4), potentially increasing glucose uptake.

Conclusion

MondoA appears to mediate mouse myofiber development, and MondoA decreases the muscle glycogen level. The findings indicate the potential function of MondoA in skeletal muscle, linking the glucose-related transcription factor to myogenesis and skeletal myofiber glycogen metabolism.

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".

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.

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.

Role for carbohydrate response element-binding protein (ChREBP) in high glucose-mediated repression of long noncoding RNA Tug1

Long noncoding RNAs (lncRNAs) have been shown to play key roles in a variety of biological activities of the cell. However, less is known about how lncRNAs respond to environmental cues and what transcriptional mechanisms regulate their expression. Studies from our laboratory have shown that the lncRNA Tug1 (taurine upregulated gene 1) is crucial for the progression of diabetic kidney disease, a major microvascular complication of diabetes. Using a combination of proximity labeling with the engineered soybean ascorbate peroxidase (APEX2), ChIP-qPCR, biotin-labeled oligonucleotide pulldown, and classical promoter luciferase assays in kidney podocytes, we extend our initial observations in the current study and now provide a detailed analysis on a how high-glucose milieu downregulates Tug1 expression in podocytes. Our results revealed an essential role for the transcription factor carbohydrate response element binding protein (ChREBP) in controlling Tug1 transcription in the podocytes in response to increased glucose levels. Along with ChREBP, other coregulators, including MAX dimerization protein (MLX), MAX dimerization protein 1 (MXD1), and histone deacetylase 1 (HDAC1), were enriched at the Tug1 promoter under high-glucose conditions. These observations provide the first characterization of the mouse Tug1 promoter's response to the high-glucose milieu. Our findings illustrate a molecular mechanism by which ChREBP can coordinate glucose homeostasis with the expression of the lncRNA Tug1 and further our understanding of dynamic transcriptional regulation of lncRNAs in a disease state.

LXRα Regulates ChREBPα Transactivity in a Target Gene-Specific Manner through an Agonist-Modulated LBD-LID Interaction

The cholesterol-sensing nuclear receptor liver X receptor (LXR) and the glucose-sensing transcription factor carbohydrate responsive element-binding protein (ChREBP) are central players in regulating glucose and lipid metabolism in the liver. More knowledge of their mechanistic interplay is needed to understand their role in pathological conditions like fatty liver disease and insulin resistance. In the current study, LXR and ChREBP co-occupancy was examined by analyzing ChIP-seq datasets from mice livers. LXR and ChREBP interaction was determined by Co-immunoprecipitation (CoIP) and their transactivity was assessed by real-time quantitative polymerase chain reaction (qPCR) of target genes and gene reporter assays. Chromatin binding capacity was determined by ChIP-qPCR assays. Our data show that LXRα and ChREBPα interact physically and show a high co-occupancy at regulatory regions in the mouse genome. LXRα co-activates ChREBPα and regulates ChREBP-specific target genes in vitro and in vivo. This co-activation is dependent on functional recognition elements for ChREBP but not for LXR, indicating that ChREBPα recruits LXRα to chromatin in trans. The two factors interact via their key activation domains; the low glucose inhibitory domain (LID) of ChREBPα and the ligand-binding domain (LBD) of LXRα. While unliganded LXRα co-activates ChREBPα, ligand-bound LXRα surprisingly represses ChREBPα activity on ChREBP-specific target genes. Mechanistically, this is due to a destabilized LXRα:ChREBPα interaction, leading to reduced ChREBP-binding to chromatin and restricted activation of glycolytic and lipogenic target genes. This ligand-driven molecular switch highlights an unappreciated role of LXRα in responding to nutritional cues that was overlooked due to LXR lipogenesis-promoting function.

Glucose-6-Phosphate Upregulates Txnip Expression by Interacting With MondoA

The major metabolic fates of glucose in cells are glycolysis and the pentose phosphate pathway, and they share the first step: converting glucose to glucose-6-phosphate (G6P). Here, we show that G6P can be sensed by the transcription factor MondoA/Mlx to modulate Txnip expression. Endogenous knockdown and EMSA (gel migration assay) analyses both confirmed that G6P is the metabolic intermediate that activates the heterocomplex MondoA/Mlx to elicit the expression of Txnip. Additionally, the three-dimensional structure of MondoA is modeled, and the binding mode of G6P to MondoA is also predicted by in silico molecular docking and binding free energy calculation. Finally, free energy decomposition and mutational analyses suggest that certain residues in MondoA, GKL139-141 in particular, mediate its binding with G6P to activate MondoA, which signals the upregulation of the expression of Txnip.

S-Equol ameliorates insulin secretion failure through Chrebp/Txnip signaling via modulating PKA/PP2A activities

Background

S-Equol, produced from daidzein by gut microbiota, has been suggested as an potential anti-diabetic agent, but the underlying mechanisms remain unclear. Recent evidences demonstrated that carbohydrate response element-binding protein (Chrebp)/Thioredoxin-interacting protein (Txnip) signaling played central roles on diabetes progression, particularly in relation to the function maintenance and apoptosis of pancreatic β-cell. Here, we investigated the effects of S-Equol on β-cell function and Chrebp/Txnip signaling.

Methods

Zucker diabetic fatty rats were treated with racemic Equol (120 mg/kg.BW.d) for 6 weeks. The glucose and lipid metabolism were monitored during the supplementation, and the Chrebp and Txnip expression were measured by using Western blotting. INS-1 cells were incubated with high glucose (26.2 mM) with or without S-Equol (0.1 μM, 1 μM, 10 μM) for 48 h. Glucose-stimulated insulin secretion (GSIS) was evaluated by radioimmunoassay, and the apoptosis of INS-1 cells was analyzed using Annexin V-FITC/PI and TUNEL assay. The dual luciferase reporter assay, chromatin immunoprecipitation assay and Western-blotting followed by Chrebp small interfering RNAs were utilized to clarify the mechanism of transcriptional regulation of S-Equol on Chrebp/Txnip signaling and the activities of protein kinase A (PKA) and protein phophatase (PP2A) were also detected.

Results

In vivo, Equol supplementation delayed the onset of the hyperglycemia and hyperlipemia, ameliorated insulin secretion failure, enhanced GSIS in isolated islets, and significantly reduced Chrebp and Txnip expression in islets. In vitro, S-Equol treatment enhanced GSIS of high glucose cultured INS-1 cell, and reduced apoptosis of INS-1 cells were also observed. Moreover, S-Equol dramatically suppressed Txnip transcription, as evident by the reduction of Txnip protein and mRNA levels and decrease in the Txnip promoter-driven luciferase activity. Meanwhile, S-Equol significantly inhibited Chrebp/Mlx expression and decreased occupancy of Chrebp on the Txnip promoter, and combined with siChrebp, we confirmed that S-Equol improvement of insulin secretion was partially through the Chrebp/Txnip pathway. Furthermore, S-Equol significantly decrease nuclear translocation of Chrebp, which was related with the decrease activity of protein kinase A (PKA) and the increase activity of protein phophatase (PP2A).

Conclusions

S-Equol could ameliorate insulin secretion failure, which was dependent on the suppression of Chrebp/Txnip signaling via modulating PKA/PP2A activities.

The Protective Role of the Carbohydrate Response Element Binding Protein in the Liver: The Metabolite Perspective

The Carbohydrate response element binding protein, ChREBP encoded by the MLXIPL gene, is a transcription factor that is expressed at high levels in the liver and has a prominent function during consumption of high-carbohydrate diets. ChREBP is activated by raised cellular levels of phosphate ester intermediates of glycolysis, gluconeogenesis and the pentose phosphate pathway. Its target genes include a wide range of enzymes and regulatory proteins, including G6pc, Gckr, Pklr, Prkaa1,2, and enzymes of lipogenesis. ChREBP activation cumulatively promotes increased disposal of phosphate ester intermediates to glucose, via glucose 6-phosphatase or to pyruvate via glycolysis with further metabolism by lipogenesis. Dietary fructose is metabolized in both the intestine and the liver and is more lipogenic than glucose. It also induces greater elevation in phosphate ester intermediates than glucose, and at high concentrations causes transient depletion of inorganic phosphate, compromised ATP homeostasis and degradation of adenine nucleotides to uric acid. ChREBP deficiency predisposes to fructose intolerance and compromised cellular phosphate ester and ATP homeostasis and thereby markedly aggravates the changes in metabolite levels caused by dietary fructose. The recent evidence that high fructose intake causes more severe hepatocyte damage in ChREBP-deficient models confirms the crucial protective role for ChREBP in maintaining intracellular phosphate homeostasis. The improved ATP homeostasis in hepatocytes isolated from mice after chronic activation of ChREBP with a glucokinase activator supports the role of ChREBP in the control of intracellular homeostasis. It is hypothesized that drugs that activate ChREBP confer a protective role in the liver particularly in compromised metabolic states.

O-GlcNAcylation as a Therapeutic Target for Alzheimer's Disease

Alzheimer's disease (AD) is the most common cause of dementia and the number of elderly patients suffering from AD has been steadily increasing. Despite worldwide efforts to cope with this disease, little progress has been achieved with regard to identification of effective therapeutics. Thus, active research focusing on identification of new therapeutic targets of AD is ongoing. Among the new targets, post-translational modifications which modify the properties of mature proteins have gained attention. O-GlcNAcylation, a type of PTM that attaches O-linked β-N-acetylglucosamine (O-GlcNAc) to a protein, is being sought as a new target to treat AD pathologies. O-GlcNAcylation has been known to modify the two important components of AD pathological hallmarks, amyloid precursor protein, and tau protein. In addition, elevating O-GlcNAcylation levels in AD animal models has been shown to be effective in alleviating AD-associated pathology. Although studies investigating the precise mechanism of reversal of AD pathologies by targeting O-GlcNAcylation are not yet complete, it is clearly important to examine O-GlcNAcylation regulation as a target of AD therapeutics. This review highlights the mechanisms of O-GlcNAcylation and its role as a potential therapeutic target under physiological and pathological AD conditions.

Effects of Moderate Chronic Food Restriction on the Development of Postprandial Dyslipidemia with Ageing

Ageing is a major risk factor for the development of metabolic disorders linked to dyslipidemia, usually accompanied by increased adiposity. The goal of this work was to investigate whether avoiding an excessive increase in adiposity with ageing, via moderate chronic food restriction (FR), ameliorates postprandial dyslipidemia in a rat model of metabolic syndrome associated with ageing. Accordingly, we performed an oral lipid loading test (OLLT) in mature middle-aged (7 months) and middle-old-aged (24 months) Wistar rats fed ad libitum (AL) or under moderate FR for 3 months. Briefly, overnight fasted rats were orally administered a bolus of extra-virgin olive oil (1 mL/Kg of body weight) and blood samples were taken from the tail vein before fat load (t = 0) and 30, 60, 90, 120, 180, and 240 min after fat administration. Changes in serum lipids, glucose, insulin, and glucagon levels were measured at different time-points. Expression of liver and adipose tissue metabolic genes were also determined before (t = 0) and after the fat load (t = 240 min). Postprandial dyslipidemia progressively increased with ageing and this could be associated with hepatic ChREBP activity. Interestingly, moderate chronic FR reduced adiposity and avoided excessive postprandial hypertriglyceridemia in 7- and 24-month-old Wistar rats, strengthening the association between postprandial triglyceride levels and adiposity. The 24-month-old rats needed more insulin to maintain postprandial normoglycemia; nevertheless, hyperglycemia occurred at 240 min after fat administration. FR did not alter the fasted serum glucose levels but it markedly decreased glucagon excursion during the OLLT and the postprandial rise of glycemia in the 24-month-old rats, and FGF21 in the 7-month-old Wistar rats. Hence, our results pointed to an important role of FR in postprandial energy metabolism and insulin resistance in ageing. Lastly, our data support the idea that the vWAT might function as an ectopic site for fat deposition in 7-month-old and in 24-month-old Wistar rats that could increase their browning capacity in response to an acute fat load.

Carbohydrate Sensing Through the Transcription Factor ChREBP

Carbohydrate response element binding protein (ChREBP) is a carbohydrate-signaling transcription factor that in the past years has emerged as a central metabolic regulator. ChREBP expression is mostly abundant in active sites of de novo lipogenesis including liver and white and brown adipose tissues. ChREBP is also expressed in pancreatic islets, small intestine and to a lesser extent in the kidney and the brain. In response to glucose, ChREBP undergoes several post-translational modifications (PTMs) (phosphorylation, acetylation and/or O-GlcNAcylation) that will either modulate its cellular location, stability and/or its transcriptional activity. ChREBPβ is a shorter isoform of ChREBP that was first described in adipose tissue and later found to be expressed in other sites including liver and pancreatic β cells. ChREBPβ lacks an important regulatory inhibitory domain, known as LID (low glucose inhibitory domain), in its N-terminal domain and is therefore reported as a highly active isoform. In this review, we recapitulate a recent progress concerning the mechanisms governing the activity of the ChREBP isoforms, including PTMs, partners/cofactors as well as novel metabolic pathways regulated by ChREBP in key metabolic tissues, by discussing phenotypes associated with tissue-specific deletion of ChREBP in knockout mice.

Recent insights into the role of ChREBP in intestinal fructose absorption and metabolism

Fructose in the form of sucrose and high fructose corn syrup is absorbed by the intestinal transporter and mainly metabolized in the small intestine. However, excess intake of fructose overwhelms the absorptive capacity of the small intestine, leading to fructose malabsorption. Carbohydrate response element-binding protein (ChREBP) is a basic helix-loop-helix leucine zipper transcription factor that plays a key role in glycolytic and lipogenic gene expression in response to carbohydrate consumption. While ChREBP was initially identified as a glucose-responsive factor in the liver, recent evidence suggests that ChREBP is essential for fructoseinduced lipogenesis and gluconeogenesis in the small intestine as well as in the liver. We recently identified that the loss of ChREBP leads to fructose intolerance via insufficient induction of genes involved in fructose transport and metabolism in the intestine. As fructose consumption is increasing and closely associated with metabolic and gastrointestinal diseases, a comprehensive understanding of cellular fructose sensing and metabolism via ChREBP may uncover new therapeutic opportunities. In this mini review, we briefly summarize recent progress in intestinal fructose metabolism, regulation and function of ChREBP by fructose, and delineate the potential mechanisms by which excessive fructose consumption may lead to irritable bowel syndrome.

ChREBP Reciprocally Regulates Liver and Plasma Triacylglycerol Levels in Different Manners

Carbohydrate response element-binding protein (ChREBP) has an important role in the carbohydrate-mediated regulation of hepatic de novo lipogenesis, but the mechanism for how it regulates plasma triacylglycerol (TAG) levels has not been established. This study aimed to clarify the role of ChREBP in regulation of plasma TAG levels. We analyzed the metabolic changes in mice infected with an adenovirus expressing ChREBP Δ196 (Ad-ChREBP). Compared with adenovirus harboring green fluorescent protein infected mice, Ad-ChREBP-infected mice had higher plasma free fatty acid levels and paradoxically lower plasma 3-hydroxybutyrate levels through decreased fatty acid oxidation, rather than ketogenesis. Consistent with their hepatomegaly and increased lipogenic gene expression, the liver TAG contents were much higher. Regarding lipid composition, C16:0 was much lower and C18:1n-9 was much higher, compatible with increased stearoyl CoA desaturase-1 and ELOVL fatty acid elongase 6 expression. Furthermore, Ad-ChREBP-infected mice had decreased plasma TAG and very low density lipoprotein (VLDL)-TAG levels, consistent with decreased Angiopoietin-like protein 3 (Angptl3) and increased fibroblast growth factor (Fgf21) mRNA and protein levels. Finally, Ad-ChREBP infection increased white adipose tissue Ucp1 mRNA levels with increased plasma Fgf21 levels. Because Fgf21 and Angptl3 are known to activate and suppress lipolysis in adipose tissues and oxidative tissues, ChREBP appears to regulate plasma TAG levels by modulating Fgf21 and Angptl3 levels. Thus, ChREBP overexpression led to dissociation of hepatic steatosis from hyperlipidemia.

Regulation of Carbohydrate Energy Metabolism in Drosophila melanogaster

Carbohydrate metabolism is essential for cellular energy balance as well as for the biosynthesis of new cellular building blocks. As animal nutrient intake displays temporal fluctuations and each cell type within the animal possesses specific metabolic needs, elaborate regulatory systems are needed to coordinate carbohydrate metabolism in time and space. Carbohydrate metabolism is regulated locally through gene regulatory networks and signaling pathways, which receive inputs from nutrient sensors as well as other pathways, such as developmental signals. Superimposed on cell-intrinsic control, hormonal signaling mediates intertissue information to maintain organismal homeostasis. Misregulation of carbohydrate metabolism is causative for many human diseases, such as diabetes and cancer. Recent work in Drosophila melanogaster has uncovered new regulators of carbohydrate metabolism and introduced novel physiological roles for previously known pathways. Moreover, genetically tractable Drosophila models to study carbohydrate metabolism-related human diseases have provided new insight into the mechanisms of pathogenesis. Due to the high degree of conservation of relevant regulatory pathways, as well as vast possibilities for the analysis of gene-nutrient interactions and tissue-specific gene function, Drosophila is emerging as an important model system for research on carbohydrate metabolism.

A Specific ChREBP and PPARα Cross-Talk Is Required for the Glucose-Mediated FGF21 Response

While the physiological benefits of the fibroblast growth factor 21 (FGF21) hepatokine are documented in response to fasting, little information is available on Fgf21 regulation in a glucose-overload context. We report that peroxisome-proliferator-activated receptor α (PPARα), a nuclear receptor of the fasting response, is required with the carbohydrate-sensitive transcription factor carbohydrate-responsive element-binding protein (ChREBP) to balance FGF21 glucose response. Microarray analysis indicated that only a few hepatic genes respond to fasting and glucose similarly to Fgf21. Glucose-challenged Chrebp^−/− mice exhibit a marked reduction in FGF21 production, a decrease that was rescued by re-expression of an active ChREBP isoform in the liver of Chrebp^−/− mice. Unexpectedly, carbohydrate challenge of hepatic Pparα knockout mice also demonstrated a PPARα-dependent glucose response for Fgf21 that was associated with an increased sucrose preference. This blunted response was due to decreased Fgf21 promoter accessibility and diminished ChREBP binding onto Fgf21 carbohydrate-responsive element (ChoRE) in hepatocytes lacking PPARα. Our study reports that PPARα is required for the ChREBP-induced glucose response of FGF21.

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Fgf21 is a unique hepatic gene inducible by both catabolic and anabolic signals

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The ChREBP-mediated induction of Fgf21 in hepatocytes requires PPARα

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Loss of PPARα impairs Fgf21 promoter accessibility at the ChoRE

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PPARα is required for the control of sucrose preference in vivo

Association Between a Melatonin Receptor 1B Genetic Polymorphism and Its Protein Expression in Gestational Diabetes Mellitus

AIMS

This study was conducted to investigate the relationship between a genetic polymorphism and the expression of melatonin receptor 1B (MTNR1B) in the placenta of Han Chinese women with gestational diabetes mellitus (GDM).

METHODS

In this study, 215 patients with GDM and 243 healthy controls were genotyped using direct sequencing for the MTNR1B single-nucleotide polymorphism rs10830963. The expression of MTNR1B in placenta was detected by immunohistochemistry and Western blotting. The association of rs10830963 with the expression of MTNR1B, plasma glucose, and insulin levels as well as blood lipid levels was investigated.

RESULTS

The genotype and allele frequencies of rs10830963 were significantly different between women with GDM and controls (P < .05). Fasting blood glucose, fasting insulin, and homeostasis model assessment for insulin resistance in women with GDM with the GG and GC genotypes were significantly higher than those with the CC genotype (P < .05). The expression level of MTNR1B in placenta was significantly higher in the GDM group than in the control group (P < .05). The expression of MTNR1B was significantly higher in all participants with the GG and GC genotypes (1.31 [0.74]) than in pregnant women with the CC genotype (0.92 [0.52], P < .05).

CONCLUSIONS

The genetic polymorphism rs10830963 in MTNR1B and its protein expression levels in placenta are associated with an increased risk of developing GDM. Furthermore, rs10830963 may tag a molecular mechanism leading to insulin resistance in Han Chinese women with GDM.

Sugar sensing by ChREBP/Mondo-Mlx-new insight into downstream regulatory networks and integration of nutrient-derived signals

Animals regulate their physiology with respect to nutrient status, which requires nutrient sensing pathways. Simple carbohydrates, sugars, are sensed by the basic-helix-loop-helix leucine zipper transcription factors ChREBP/Mondo, together with their heterodimerization partner Mlx, which are well-established activators of sugar-induced lipogenesis. Loss of ChREBP/Mondo-Mlx in mouse and Drosophila leads to sugar intolerance, that is, inability to survive on sugar containing diet. Recent evidence has revealed that ChREBP/Mondo-Mlx responds to sugar and fatty acid-derived metabolites through several mechanisms and cross-connects with other nutrient sensing pathways. ChREBP/Mondo-Mlx controls several downstream transcription factors and hormones, which mediate not only readjustment of metabolic pathways, but also control feeding behavior, intestinal digestion, and circadian rhythm.

Pak1 mediates the stimulatory effect of insulin and curcumin on hepatic ChREBP expression

Insulin can stimulate hepatic expression of carbohydrate-responsive element-binding protein (ChREBP). As recent studies revealed potential metabolic beneficial effects of ChREBP, we asked whether its expression can also be regulated by the dietary polyphenol curcumin. We also aimed to determine mechanisms underlying ChREBP stimulation by insulin and curcumin. The effect of insulin on ChREBP expression was assessed in mouse hepatocytes, while the effect of curcumin was assessed in mouse hepatocytes and with curcumin gavage in mice. Chemical inhibitors for insulin signaling molecules were utilized to identify involved signaling molecules, and the involvement of p21-activated protein kinase 1 (Pak1) was determined with its chemical inhibitor and Pak1-/- hepatocytes. We found that both insulin and curcumin-stimulated ChREBP expression in Akt-independent but MEK/ERK-dependent manner, involving the inactivation of the transcriptional repressor Oct-1. Aged Pak1-/- mice showed reduced body fat volume. Pak1 inhibition or its genetic deletion attenuated the stimulatory effect of insulin or curcumin on ChREBP expression. Our study hence suggests the existence of a novel signaling cascade Pak1/MEK/ERK/Oct-1 for both insulin and curcumin in exerting their glucose-lowering effect via promoting hepatic ChREBP production, supports the recognition of beneficial functions of ChREBP, and brings us a new overview on dietary polyphenols.

The regulation and role of carbohydrate response element-binding protein in metabolic homeostasis and disease

The transcription factor carbohydrate response element-binding protein (ChREBP) is a member of the basic helix-loop-helix leucine zipper transcription factor family. Under high-glucose conditions, it has a role in regulating the expression of key genes involved in various pathways, including glycolysis, gluconeogenesis and lipogenesis. It does this by forming a tetrameric complex made up of two ChREBP/Mlx heterodimers, which enables it to bind to the carbohydrate response element (ChoRE) in the promoter region of its target genes to regulate transcription. Because ChREBP plays a key role in glucose signalling and metabolism, and aberrations in glucose homeostasis are often present in metabolic diseases, this transcription factor presents itself as an enticing target with respect to further understanding metabolic disease mechanisms and potentially uncovering new therapeutic targets.

Fructose Consumption, Lipogenesis, and Non-Alcoholic Fatty Liver Disease

Increased fructose consumption has been suggested to contribute to non-alcoholic fatty liver disease (NAFLD), dyslipidemia, and insulin resistance, but a causal role of fructose in these metabolic diseases remains debated. Mechanistically, hepatic fructose metabolism yields precursors that can be used for gluconeogenesis and de novo lipogenesis (DNL). Fructose-derived precursors also act as nutritional regulators of the transcription factors, including ChREBP and SREBP1c, that regulate the expression of hepatic gluconeogenesis and DNL genes. In support of these mechanisms, fructose intake increases hepatic gluconeogenesis and DNL and raises plasma glucose and triglyceride levels in humans. However, epidemiological and fructose-intervention studies have had inconclusive results with respect to liver fat, and there is currently no good human evidence that fructose, when consumed in isocaloric amounts, causes more liver fat accumulation than other energy-dense nutrients. In this review, we aim to provide an overview of the seemingly contradicting literature on fructose and NAFLD. We outline fructose physiology, the mechanisms that link fructose to NAFLD, and the available evidence from human studies. From this framework, we conclude that the cellular mechanisms underlying hepatic fructose metabolism will likely reveal novel targets for the treatment of NAFLD, dyslipidemia, and hepatic insulin resistance. Finally, fructose-containing sugars are a major source of excess calories, suggesting that a reduction of their intake has potential for the prevention of NAFLD and other obesity-related diseases.

Sweet Sixteenth for ChREBP: Established Roles and Future Goals

With the identification of ChREBP in 2001, our interest in understanding the molecular control of carbohydrate sensing has surged. While ChREBP was initially studied as a master regulator of lipogenesis in liver and fat tissue, it is now clear that ChREBP functions as a central metabolic coordinator in a variety of cell types in response to environmental and hormonal signals, with wide implications in health and disease. Celebrating its sweet sixteenth birthday, we review here the current knowledge about the function and regulation of ChREBP throughout usual and less explored tissues, to recapitulate ChREBP's role as a whole-body glucose sensor.

mTOR controls ChREBP transcriptional activity and pancreatic β cell survival under diabetic stress

Through in vivo analyses of mTOR deficiency and in vitro studies of human and mouse pancreatic islets, Chau et al. show that mTOR plays a critical role in β cell survival in diabetes. mTOR associates with and inhibits the transcriptional ChREBP–Mlx complex, suppressing TXNIP expression and β cell death.

Impaired nutrient sensing and dysregulated glucose homeostasis are common in diabetes. However, how nutrient-sensitive signaling components control glucose homeostasis and β cell survival under diabetic stress is not well understood. Here, we show that mice lacking the core nutrient-sensitive signaling component mammalian target of rapamycin (mTOR) in β cells exhibit reduced β cell mass and smaller islets. mTOR deficiency leads to a severe reduction in β cell survival and increased mitochondrial oxidative stress in chemical-induced diabetes. Mechanistically, we find that mTOR associates with the carbohydrate-response element–binding protein (ChREBP)–Max-like protein complex and inhibits its transcriptional activity, leading to decreased expression of thioredoxin-interacting protein (TXNIP), a potent inducer of β cell death and oxidative stress. Consistent with this, the levels of TXNIP and ChREBP were highly elevated in human diabetic islets and mTOR-deficient mouse islets. Thus, our results suggest that a nutrient-sensitive mTOR-regulated transcriptional network could be a novel target to improve β cell survival and glucose homeostasis in diabetes.

MondoA/ChREBP: The usual suspects of transcriptional glucose sensing; Implication in pathophysiology

Identification of the Mondo glucose-responsive transcription factors family, including the MondoA and MondoB/ChREBP paralogs, has shed light on the mechanism whereby glucose affects gene transcription. They have clearly emerged, in recent years, as key mediators of glucose sensing by multiple cell types. MondoA and ChREBP have overlapping yet distinct expression profiles, which underlie their downstream targets and separate roles in regulating genes involved in glucose metabolism. MondoA can restrict glucose uptake and influences energy utilization in skeletal muscle, while ChREBP signals energy storage through de novo lipogenesis in liver and white adipose tissue. Because Mondo proteins mediate metabolic adaptations to changing glucose levels, a better understanding of cellular glucose sensing through Mondo proteins will likely uncover new therapeutic opportunities in the context of the imbalanced glucose homeostasis that accompanies metabolic diseases such as type 2 diabetes and cancer. Here, we provide an overview of structural homologies, transcriptional partners as well as the nutrient and hormonal mechanisms underlying Mondo proteins regulation. We next summarize their relative contribution to energy metabolism changes in physiological states and the evolutionary conservation of these pathways. Finally, we discuss their possible targeting in human pathologies.

The Role of Carbohydrate Response Element Binding Protein in Intestinal and Hepatic Fructose Metabolism

Many articles have discussed the relationship between fructose consumption and the incidence of obesity and related diseases. Fructose is absorbed in the intestine and metabolized in the liver to glucose, lactate, glycogen, and, to a lesser extent, lipids. Unabsorbed fructose causes bacterial fermentation, resulting in irritable bowl syndrome. Therefore, understanding the mechanisms underlying intestinal and hepatic fructose metabolism is important for the treatment of metabolic syndrome and fructose malabsorption. Carbohydrate response element binding protein (ChREBP) is a glucose-activated transcription factor that controls approximately 50% of de novo lipogenesis in the liver. ChREBP target genes are involved in glycolysis (Glut2, liver pyruvate kinase), fructolysis (Glut5, ketohexokinase), and lipogenesis (acetyl CoA carboxylase, fatty acid synthase). ChREBP gene deletion protects against high sucrose diet-induced and leptin-deficient obesity, because Chrebp^−/− mice cannot consume fructose or sucrose. Moreover, ChREBP contributes to some of the physiological effects of fructose on sweet taste preference and glucose production through regulation of ChREBP target genes, such as fibroblast growth factor-21 and glucose-6-phosphatase catalytic subunits. Thus, ChREBP might play roles in fructose metabolism. Restriction of excess fructose intake will be beneficial for preventing not only metabolic syndrome but also irritable bowl syndrome.

Physiological Adaptations to Sugar Intake: New Paradigms from Drosophila melanogaster

Sugars are important energy sources, but high sugar intake poses a metabolic challenge and leads to diseases. Drosophila melanogaster is a generalist fruit breeder that encounters high levels of dietary sugars in its natural habitat. Consequently, Drosophila displays adaptive responses to dietary sugars, including highly conserved and unique metabolic adaptations not described in mammals. Carbohydrate homeostasis is maintained by a network comprising intracellular energy sensors, transcriptional regulators, and hormonal and neuronal mechanisms that together coordinate animal behavior, gut function, and metabolic flux. Here we give an overview of the physiological responses associated with sugar intake and discuss some of the emerging themes and applications of the Drosophila model in understanding sugar sensing and carbohydrate metabolism.