Mlx is the functional heteromeric partner of the carbohydrate response element-binding protein in glucose regulation of lipogenic enzyme genes

MLX phosphorylation stabilizes the ChREBP-MLX heterotetramer on tandem E-boxes to control carbohydrate and lipid metabolism

The heterodimeric ChREBP-MLX transcription factor complex is a key mediator that couples intracellular sugar levels to carbohydrate and lipid metabolism. To promote the expression of target genes, two ChREBP-MLX heterodimers form a heterotetramer to bind a tandem element with two adjacent E-boxes, called Carbohydrate Responsive Element (ChoRE). How the ChREBP-MLX hetero-tetramerization is achieved and regulated, remains poorly understood. Here we show that MLX phosphorylation on an evolutionarily conserved motif is necessary for the heterotetramer formation on the ChoRE and the transcriptional activity of the ChREBP-MLX complex. We identified CK2 and GSK3 as MLX kinases that coordinately phosphorylate MLX. High intracellular glucose-6-phosphate accumulation inhibits MLX phosphorylation and heterotetramer formation on the ChoRE, impairing ChREBP-MLX activity. Physiologically, MLX phosphorylation is necessary in Drosophila to maintain sugar tolerance and lipid homeostasis. Our findings suggest that MLX phosphorylation is a key mechanism for the ChREBP-MLX heterotetramer formation to regulate carbohydrate and lipid metabolism.

The key role of altered tubule cell lipid metabolism in kidney disease development

Kidney epithelial cells have very high energy requirements, which are largely met by fatty acid oxidation. Complex changes in lipid metabolism are observed in patients with kidney disease. Defects in fatty acid oxidation and increased lipid uptake, especially in the context of hyperlipidemia and proteinuria, contribute to this excess lipid build-up and exacerbate kidney disease development. Recent studies have also highlighted the role of increased de novo lipogenesis in kidney fibrosis. The defect in fatty acid oxidation causes energy starvation. Increased lipid uptake, synthesis, and lower fatty acid oxidation can cause toxic lipid build-up, reactive oxygen species generation, and mitochondrial damage. A better understanding of these metabolic processes may open new treatment avenues for kidney diseases by targeting lipid metabolism.

LncRNA HOTAIR accelerates free fatty acid-induced inflammatory response in HepG2 cells by recruiting SRSF1 to stabilize MLXIPL mRNA

LncRNA HOTAIR has been reported to be associated with metabolic diseases of the liver. However, the effect of HOTAIR on non-alcoholic fatty liver disease (NAFLD) inflammation and its potential mechanism have not been reported. Genes and proteins expression were detected by qRT-PCR and Western blot respectively. The level of inflammatory cytokines was assessed by ELISA. HepG2 cell viability was detected by MTT assay. TG level and lipid accumulation were measured by Assay Kit and Oil red O staining, respectively. Direct binding relationship between HOTAIR and Serine/arginine splicing factor 1 (SRSF1), SRSF1 and MLX interacting protein like (MLXIPL) were confirmed by RNA-pull down and RIP assay. HOTAIR was highly expressed in free fatty acids (FFA)-treated HepG2 cells. HOTAIR knockdown alleviated FFA-induced inflammation of HepG2 cells. Then further analysis showed that HOTAIR and SRSF1 had a mutual binding relationship, and HOTAIR maintained MLXIPL mRNA stability via recruiting SRSF1 in HepG2 cells. Moreover, the inhibitory effect of HOTAIR knockdown on FFA-induced inflammation in HepG2 cells was reversed by MLXIPL overexpression. HOTAIR accelerates inflammation of FFA-induced HepG2 cells by recruiting SRSF1 to stabilize MLXIPL mRNA, which will help to find new effective strategies for NAFLD therapy.

Supplementary Information

The online version contains supplementary material available at 10.1007/s10616-023-00614-x.

Role of Carbohydrate response element-binding protein in mediating dexamethasone-induced glucose transporter 5 expression in Caco-2BBE cells and during the developmental phase in mice

Glucose transporter 5 (GLUT5), the main fructose transporter in mammals, is primarily responsible for absorbing dietary fructose in the small intestine. The expression of this intestinal gene significantly increases in response to developmental and dietary cues that reach the glucocorticoid receptor and carbohydrate response element-binding protein (ChREBP), respectively. Our study demonstrates that ChREBP is involved in the dexamethasone (Dex)-induced expression of GLUT5 in Caco-2BBE cells and the small intestine of both wild-type and ChREBP-knockout mice. Dex, a glucocorticoid, demonstrated an increase in GLUT5 mRNA levels in a dose- and time-dependent manner. While the overexpression of ChREBP moderately increased GLUT5 expression, its synergistic increase in the presence of Dex was noteworthy, whereas the suppression of ChREBP significantly reduced Dex-induced GLUT5 expression. Dex did not increase ChREBP protein levels but facilitated its nuclear translocation, thereby increasing the activity of the GLUT5 promoter. In vivo experiments conducted on 14-day-old mice pups treated with Dex for three days revealed that only wild-type mice (not ChREBP-knockout mice) exhibited Dex-mediated Glut5 gene induction, which further supports the role of ChREBP in regulating GLUT5 expression. Collectively, our results provide insights into the molecular mechanisms involved in the regulation of GLUT5 expression in response to developmental and dietary signals mediated by glucocorticoids and ChREBP. General significance: The transcription factor ChREBP is important for Dex-mediated Glut5 gene expression in the small intestine.

Effects of (-)-epicatechin on hepatic triglyceride metabolism

(-)-Epicatechin (EC) consumption is associated with an improvement of hyperlipemia and other metabolic changes linked to obesity and western-style diets. This work investigated the effects of EC on triglyceride (TG) metabolism both in vivo, where mice were supplemented with EC (2 and 20 mg EC per kg body weight), and in vitro, when human HepG2 hepatocytes were incubated in the presence of EC and the main EC metabolites found in human plasma. Increased hepatic TG levels were only observed after 24 weeks supplementation with EC (20 mg per kg body weight), with a preserved liver structure and absence of inflammation or oxidative stress. EC caused increased expression of diacylglycerol acyltransferases (DGAT2), key enzymes in TG synthesis, and the upregulation of PPARα, which promotes free fatty acid (FFA) oxidation. On the other hand, incubation of HepG2 cells in the presence of high concentrations of EC (1-10 μM) did not affect TG deposition nor DGAT2 expression. In summary, in mouse liver, EC upregulated mechanisms that can neutralize the potential toxicity of FFA, i.e. TG synthesis and FFA β-oxidation. Results in mouse liver and HepG2 cells stress the safety of EC in terms of TG metabolism and development of hepatopathies in doses within the limits given by a rational time and dose for human consumption.

Analysis of the human kidney transcriptome and plasma proteome identifies markers of proximal tubule maladaptation to injury

Acute kidney injury (AKI) is a major risk factor for long-term adverse outcomes, including chronic kidney disease. In mouse models of AKI, maladaptive repair of the injured proximal tubule (PT) prevents complete tissue recovery. However, evidence for PT maladaptation and its etiological relationship with complications of AKI is lacking in humans. We performed single-nucleus RNA sequencing of 120,985 nuclei in kidneys from 17 participants with AKI and seven healthy controls from the Kidney Precision Medicine Project. Maladaptive PT cells, which exhibited transcriptomic features of dedifferentiation and enrichment in pro-inflammatory and profibrotic pathways, were present in participants with AKI of diverse etiologies. To develop plasma markers of PT maladaptation, we analyzed the plasma proteome in two independent cohorts of patients undergoing cardiac surgery and a cohort of marathon runners, linked it to the transcriptomic signatures associated with maladaptive PT, and identified nine proteins whose genes were specifically up- or down-regulated by maladaptive PT. After cardiac surgery, both cohorts of patients had increased transforming growth factor-β2 (TGFB2), collagen type XXIII-α1 (COL23A1), and X-linked neuroligin 4 (NLGN4X) and had decreased plasminogen (PLG), ectonucleotide pyrophosphatase/phosphodiesterase 6 (ENPP6), and protein C (PROC). Similar changes were observed in marathon runners with exercise-associated kidney injury. Postoperative changes in these markers were associated with AKI progression in adults after cardiac surgery and post-AKI kidney atrophy in mouse models of ischemia-reperfusion injury and toxic injury. Our results demonstrate the feasibility of a multiomics approach to discovering noninvasive markers and associating PT maladaptation with adverse clinical outcomes.

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.

Nonalcoholic Fatty Liver Disease and Omega-3 Fatty Acids: Mechanisms and Clinical Use

Nonalcoholic fatty liver disease (NAFLD) is the most common chronic fatty liver disease worldwide, particularly in obese and type 2 diabetic individuals. Currently, there are no therapies for NAFLD that have been approved by the US Food and Drug Administration. Herein, we examine the rationale for using ω3 polyunsaturated fatty acids (PUFAs) in NAFLD therapy. This focus is based on the finding that NAFLD severity is associated with a reduction of hepatic C20-22 ω3 PUFAs. Because C20-22 ω3 PUFAs are pleiotropic regulators of cell function, loss of C20-22 ω3 PUFAs has the potential to significantly impact hepatic function. We describe NAFLD prevalence and pathophysiology as well as current NAFLD therapies. We also present evidence from clinical and preclinical studies that evaluated the capacity of C20-22 ω3 PUFAs to treat NAFLD. Given the clinical and preclinical evidence, dietary C20-22 ω3 PUFA supplementation has the potential to decrease human NAFLD severity by reducing hepatosteatosis and liver injury.

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.

Damaging missense variants in IGF1R implicate a role for IGF-1 resistance in the etiology of type 2 diabetes

Type 2 diabetes (T2D) is a heritable metabolic disorder. While population studies have identified hundreds of common genetic variants associated with T2D, the role of rare (frequency < 0.1%) protein-coding variation is less clear. We performed exome sequence analysis in 418,436 (n = 32,374 T2D cases) individuals in the UK Biobank. We identified previously reported genes (GCK, GIGYF1, HNF1A) in addition to missense variants in ZEB2 (n = 31 carriers; odds ratio [OR] = 5.5 [95% confidence interval = 2.5-12.0]; p = 6.4 × 10^-7), MLXIPL (n = 245; OR = 2.3 [1.6-3.2]; p = 3.2 × 10^-7), and IGF1R (n = 394; OR = 2.4 [1.8-3.2]; p = 1.3 × 10^-10). Carriers of damaging missense variants within IGF1R were also shorter (-2.2 cm [-1.8 to -2.7]; p = 1.2 × 10^-19) and had higher circulating insulin-like growth factor-1 (IGF-1) protein levels (2.3 nmol/L [1.7-2.9]; p = 2.8 × 10^-14), indicating relative IGF-1 resistance. A likely causal role of IGF-1 resistance was supported by Mendelian randomization analyses using common variants. These results increase understanding of the genetic architecture of T2D and highlight the growth hormone/IGF-1 axis as a potential therapeutic target.

Regulation of Normal and Neoplastic Proliferation and Metabolism by the Extended Myc Network

The Myc Network, comprising a small assemblage of bHLH-ZIP transcription factors, regulates many hundreds to thousands of genes involved in proliferation, energy metabolism, translation and other activities. A structurally and functionally related set of factors known as the Mlx Network also supervises some of these same functions via the regulation of a more limited but overlapping transcriptional repertoire. Target gene co-regulation by these two Networks is the result of their sharing of three members that suppress target gene expression as well as by the ability of both Network's members to cross-bind one another's consensus DNA sites. The two Networks also differ in that the Mlx Network's control over transcription is positively regulated by several glycolytic pathway intermediates and other metabolites. These distinctive properties, functions and tissue expression patterns potentially allow for sensitive control of gene regulation in ways that are differentially responsive to environmental and metabolic cues while allowing for them to be both rapid and of limited duration. This review explores how such control might occur. It further discusses how the actual functional dependencies of the Myc and Mlx Networks rely upon cellular context and how they may differ between normal and neoplastic cells. Finally, consideration is given to how future studies may permit a more refined understanding of the functional interrelationships between the two Networks.

Mouse promyelocytic leukemia zinc finger protein (PLZF) regulates hepatic lipid and glucose homeostasis dependent on SIRT1

Previous studies have demonstrated that promyelocytic leukemia zinc finger protein (PLZF) promotes the expression of gluconeogenic genes and hepatic glucose output, which leads to hyperglycemia. However, the role played by PLZF in regulating lipid metabolism is not known. In this study, we aimed to examine the function of PLZF in regulating hepatic lipid and glucose homeostasis and the underlying mechanisms. The expression of PLZF was determined in different mouse models with regard to non-alcoholic fatty liver disease (NAFLD). In the next step, adenoviruses that express PLZF (Ad-PLZF) or PLZF-specific shRNA (Ad-shPLZF) were utilized to alter PLZF expression in mouse livers and in primary hepatocytes. For the phenotype of the fatty liver, histologic and biochemical analyses of hepatic triglyceride (TG), serum TG and cholesterol levels were carried out. The underlying molecular mechanism for the regulation of lipid metabolism by PLZF was further explored using luciferase reporter gene assay and ChIP analysis. The results demonstrated that PLZF expression was upregulated in livers derived from ob/ob, db/db and diet-induced obesity (DIO) mice. Liver PLZF-overexpressing C57BL/6J mice showed fatty liver phenotype, liver inflammation, impaired glucose tolerance and insulin sensitivity. On the other hand, hepatic PLZF knockdown in db/db and DIO mice alleviated hepatic steatosis. Of note, we found that PLZF activates SREBP-1c gene transcription through binding directly to the promoter fragment of this gene, which would induce a repressor-to-activator conversion depending on its interaction with SIRT1 in the role played by PLZF in the transcription process through deacetylation. Thus, PLZF is identified as an essential regulator of hepatic lipid and glucose metabolism, where the modulation of its liver expression could open up a therapeutic path for treating NAFLD.

The conserved Mediator subunit cyclin C (CCNC) is required for brown adipocyte development and lipid accumulation

OBJECTIVE

Cyclin C (CCNC) is the most conserved subunit of the Mediator complex, which is an important transcription cofactor. Recently, we have found that CCNC facilitates brown adipogenesis in vitro by activating C/EBPα-dependent transcription. However, the role of CCNC in brown adipose tissue (BAT) in vivo remains unclear.

METHODS

We generated conditional knock-out mice by crossing Ccncflox/flox mice with Myf5Cre, Ucp1Cre or AdipoqCre transgenic mice to investigate the role of CCNC in BAT development and function. We applied glucose and insulin tolerance test, cold exposure and indirect calorimetry to capture the physiological phenotypes and used immunostaining, immunoblotting, qRT-PCR, RNA-seq and cell culture to elucidate the underlying mechanisms.

RESULTS

Here, we show that deletion of CCNC in Myf5+ progenitor cells caused BAT paucity, despite the fact that there was significant neonatal lethality. Mechanistically different from in vitro, CCNC deficiency impaired the proliferation of embryonic brown fat progenitor cells without affecting brown adipogenesis or cell death. Interestingly, CCNC deficiency robustly reduced age-dependent lipid accumulation in differentiated brown adipocytes in all three mouse models. Mechanistically, CCNC in brown adipocytes is required for lipogenic gene expression through the activation of the C/EBPα/GLUT4/ChREBP axis. Consistent with the importance of de novo lipogenesis under carbohydrate-rich diets, high-fat diet (HFD) feeding abolished CCNC deficiency -caused defects of lipid accumulation in BAT. Although insulin sensitivity and response to acute cold exposure were not affected, CCNC deficiency in Ucp1+ cells enhanced the browning of white adipose tissue (beiging) upon prolonged cold exposure.

CONCLUSIONS

Together, these data indicate an important role of CCNC-Mediator in the regulation of BAT development and lipid accumulation in brown adipocytes.

Maladaptive positive feedback production of ChREBPβ underlies glucotoxic β-cell failure

Preservation and expansion of β-cell mass is a therapeutic goal for diabetes. Here we show that the hyperactive isoform of carbohydrate response-element binding protein (ChREBPβ) is a nuclear effector of hyperglycemic stress occurring in β-cells in response to prolonged glucose exposure, high-fat diet, and diabetes. We show that transient positive feedback induction of ChREBPβ is necessary for adaptive β-cell expansion in response to metabolic challenges. Conversely, chronic excessive β-cell-specific overexpression of ChREBPβ results in loss of β-cell identity, apoptosis, loss of β-cell mass, and diabetes. Furthermore, β-cell "glucolipotoxicity" can be prevented by deletion of ChREBPβ. Moreover, ChREBPβ-mediated cell death is mitigated by overexpression of the alternate CHREBP gene product, ChREBPα, or by activation of the antioxidant Nrf2 pathway in rodent and human β-cells. We conclude that ChREBPβ, whether adaptive or maladaptive, is an important determinant of β-cell fate and a potential target for the preservation of β-cell mass in diabetes.

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.

Inhibition of ChREBP ubiquitination via the ROS/Akt-dependent downregulation of Smurf2 contributes to lysophosphatidic acid-induced fibrosis in renal mesangial cells

Background

Mesangial cell fibrosis, a typical symptom of diabetic nephropathy (DN), is a major contributor to glomerulosclerosis. We previously reported that the pharmacological blockade of lysophosphatidic acid (LPA) signaling improves DN. Although LPA signaling is implicated in diabetic renal fibrosis, the underlying molecular mechanisms remain unclear. Here, the role of carbohydrate-responsive element-binding protein (ChREBP) in LPA-induced renal fibrosis and the underlying mechanisms were investigated.

Methods

Eight-week-old wild-type and db/db mice were intraperitoneally injected with the vehicle or an LPAR1/3 antagonist, ki16425 (10 mg/kg), for 8 weeks on a daily basis, following which the mice were sacrificed and renal protein expression was analyzed. SV40 MES13 cells were treated with LPA in the presence or absence of ki16425, and the expression of ChREBP and fibrotic factors, including fibronectin, TGF-β, and IL-1β, was examined. The role of ChREBP in the LPA-induced fibrotic response was investigated by ChREBP overexpression or knockdown. The involvement of Smad ubiquitination regulatory factor-2 (Smurf2), an E3 ligase, in LPA-induced expression of ChREBP and fibrotic factors was investigated by Smurf2 overexpression or knockdown. To identify signaling molecules regulating Smurf2 expression by LPA, pharmacological inhibitors such as A6370 (Akt1/2 kinase inhibitor) and Ly 294002 (PI3K inhibitor) were used.

Results

The renal expression of ChREBP increased in diabetic db/db mice, and was reduced following treatment with the ki16425. Treatment with LPA induced the expression of ChREBP and fibrotic factors, including fibronectin, TGF-β, and IL-1β, in SV40 MES13 cells, which were positively correlated. The LPA-induced expression of fibrotic factors increased or decreased following ChREBP overexpression and knockdown, respectively. The production of reactive oxygen species (ROS) mediated the LPA-induced expression of ChREBP and fibrotic factors, and LPA decreased Smurf2 expression via Traf4-mediated ubiquitination. The LPA-induced expression of ubiquitinated-ChREBP increased or decreased following Smurf2 overexpression and knockdown, respectively. Additionally, Smurf2 knockdown significantly increased the expression of ChREBP and fibrotic factors. The pharmacological inhibition of Akt signaling suppressed the LPA-induced alterations in the expression of ChREBP and Smurf2.

Conclusion

Collectively, the results demonstrated that the ROS/Akt-dependent downregulation of Smurf2 and the subsequent increase in ChREBP expression might be one of the mechanisms by which LPA induces mesangial cell fibrosis in DN.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12929-022-00814-1.

Intestine-to-neuronal signaling alters risk-taking behaviors in food-deprived Caenorhabditis elegans

Animals integrate changes in external and internal environments to generate behavior. While neural circuits detecting external cues have been mapped, less is known about how internal states like hunger are integrated into behavioral outputs. Here, we use the nematode C. elegans to examine how changes in internal nutritional status affect chemosensory behaviors. We show that acute food deprivation leads to a reversible decline in repellent, but not attractant, sensitivity. This behavioral change requires two conserved transcription factors MML-1 (MondoA) and HLH-30 (TFEB), both of which translocate from the intestinal nuclei to the cytoplasm during food deprivation. Next, we identify the insulin-like peptide INS-31 as a candidate ligand relaying food-status signals from the intestine to other tissues. Further, we show that neurons likely use the DAF-2 insulin receptor and AGE-1/PI-3 Kinase, but not DAF-16/FOXO to integrate these intestine-released peptides. Altogether, our study shows how internal food status signals are integrated by transcription factors and intestine-neuron signaling to generate flexible behaviors via the gut-brain axis.

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.

ChREBP deficiency alleviates apoptosis by inhibiting TXNIP/oxidative stress in diabetic nephropathy

AIMS

In the present study, we investigated the effect of carbohydrate responsive element binding protein (ChREBP) on the TXNIP/oxidative stress and apoptosis in diabetic nephropathy.

METHODS

ChREBP^-/- mice (8-week old) were produced using the CRISPR/Cas9 gene editing approach. Diabetes was induced in C57BL/6 mice with streptozotocin. HK-2 cells was transfected with plasmid containing either ChREBP shRNA or TXNIP siRNA.

RESULTS

Renal expression of ChREBP and thioredoxin-interacting protein (TXNIP) was increased in patients with type 2 diabetes mellitus (T2DM) and diabetic mice. ChREBP deficiency improved renal function, apoptosis as well as endoplasmic reticulum (ER) stress in diabetic mice. In addition, ChREBP deficiency prevented expression levels of TXNIP and NADPH oxidase 4 (Nox4), 8-hydroxydeoxyguanosine (8-OHdG) and heme oxygenase-1 (HO-1) in diabetic kidneys. The increased urinary 8-OHdG level induced by diabetes was also attenuated in ChREBP deficiency mice. Similarly, HG was shown to induce ChREBP expression and nuclear translocation in HK-2 cells. HG-induced apoptosis was inhibited by transfection of ChREBP shRNA plasmid. Moreover, we found that knockdown of ChREBP suppressed HG-induced TXNIP and Nox4 expression, reactive oxygen species (ROS) generation and ER stress in HK-2 cells. Furthermore, TXNIP knockdown effectively abrogated HG-induced apoptosis in HK-2 cells.

CONCLUSIONS

These results suggest that ChREBP deficiency prevents diabetes-induced apoptosis via inhibiting oxidative stress and ER stress, highlighting ChREBP as a potential therapy target for diabetic nephropathy.

Dietary interventions as regulators of stem cell behavior in homeostasis and disease

Stem cells maintain tissues by balancing self-renewal with differentiation. A stem cell's local microenvironment, or niche, informs stem cell behavior and receives inputs at multiple levels. Increasingly, it is becoming clear that the overall metabolic status of an organism or metabolites themselves can function as integral members of the niche to alter stem cell fate. Macroscopic dietary interventions such as caloric restriction, the ketogenic diet, and a high-fat diet systemically alter an organism's metabolic state in different ways. Intriguingly, however, they all converge on a propensity to enhance self-renewal. Here, we highlight our current knowledge on how dietary changes feed into stem cell behavior across a wide variety of tissues and illuminate possible explanations for why diverse interventions can result in similar stem cell phenotypes. In so doing, we hope to inspire new avenues of inquiry into the importance of metabolism in stem cell homeostasis and disease.

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.

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.

Carbohydrate response element-binding protein regulates lipid metabolism via mTOR complex1 in diabetic nephropathy

Lipid deposition caused by the disorder of renal lipid metabolism is involved in diabetic nephropathy (DN). Carbohydrate response element-binding protein (ChREBP) is a key transcription factor in high glucose-induced cellular fat synthesis. At present, the regulation and mechanism of ChREBP on fat metabolism in diabetic kidneys are still unclear. In this study, we showed that lack of ChREBP significantly improved renal injury, inhibited oxidative stress, lipid deposition, fatty acid synthase (FASN), acetyl-CoA carboxylase (ACC) and thioredoxin-interacting protein (TXNIP) expression, as well as the activity of mammalian target of rapamycin complex 1 (mTORC1) in diabetic kidneys. Meanwhile, ChREBP deficiency upregulated the expression of peroxisome proliferator-activated receptor-α (PPARα), carnitine palmitoyltransferaser 1A (CPT1A) and acyl-coenzyme A oxidase 1 (ACOX1) in diabetic kidneys. In vitro, knockdown of ChREBP attenuated lipid deposition, mTORC1 activation, and expression of FASN and ACC, increased PPARα, CPT1A, and ACOX1 expression in HK-2 cells and podocytes under high glucose (HG) conditions. Moreover, HG-induced lipid deposition, increased expression of FASN and ACC and decreased expression of PPARα, CPT1A, and ACOX1 were reversed by rapamycin, a specific inhibitor of mTORC1, in HK-2 cells. These results indicate that ChREBP deficiency alleviates diabetes-associated renal lipid accumulation by inhibiting mTORC1 activity and suggest that reduction of ChREBP is a potential therapeutic strategy to treat DN.

ChREBP downregulates SNAT2 amino acid transporter expression through interactions with SMRT in response to a high-carbohydrate diet

Carbohydrate responsive element-binding protein (ChREBP) has been identified as a primary transcription factor that maintains energy homeostasis through transcriptional regulation of glycolytic, lipogenic, and gluconeogenic enzymes in response to a high-carbohydrate diet. Amino acids are important substrates for gluconeogenesis, but nevertheless, knowledge is lacking about whether this transcription factor regulates genes involved in the transport or use of these metabolites. Here, we demonstrate that ChREBP represses the expression of the amino acid transporter sodium-coupled neutral amino acid transporter 2 (SNAT2) in response to a high-sucrose diet in rats by binding to a carbohydrate response element (ChoRE) site located -160 bp upstream of the transcriptional start site in the SNAT2 promoter region. Additionally, immunoprecipitation assays revealed that ChREBP and silencing mediator of retinoic acid and thyroid hormone receptor (SMRT) interact with each other, as part of the complex that repress SNAT2 expression. The interaction between these proteins was confirmed by an in vivo chromatin immunoprecipitation assay. These findings suggest that glucogenic amino acid uptake by the liver is controlled by ChREBP through the repression of SNAT2 expression in rats consuming a high-carbohydrate diet.NEW & NOTEWORTHY This study highlights the key role of carbohydrate responsive element-binding protein (ChREBP) in the fine-tuned regulation between glucose and amino acid metabolism in the liver via regulation of the amino acid transporter sodium-coupled neutral amino acid transporter 2 (SNAT2) expression after the consumption of a high-carbohydrate diet. ChREBP binds to a carbohydrate response element (ChoRE) site in the SNAT2 promoter region and recruits silencing mediator of retinoic acid and thyroid hormone receptor (SMRT) corepressor to reduce SNAT2 transcription. This study revealed that ChREBP prevents the uptake of glucogenic amino acids upon the consumption of a high-carbohydrate diet.

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.

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.

Glucose Availability Alters Gene and Protein Expression of Several Newly Classified and Putative Solute Carriers in Mice Cortex Cell Culture and D. melanogaster

Many newly identified solute carriers (SLCs) and putative transporters have the possibility to be intricately involved in glucose metabolism. Here we show that many transporters of this type display a high degree of regulation at both mRNA and protein level following no or low glucose availability in mouse cortex cultures. We show that this is also the case in Drosophila melanogaster subjected to starvation or diets with different sugar content. Interestingly, re-introduction of glucose to media, or refeeding flies, normalized the gene expression of a number of the targets, indicating a fast and highly dynamic control. Our findings demonstrate high conservation of these transporters and how dependent both cell cultures and organisms are on gene and protein regulation during metabolic fluctuations. Several transporter genes were regulated simultaneously maybe to initiate alternative metabolic pathways as a response to low glucose levels, both in the cell cultures and in D. melanogaster. Our results display that newly identified SLCs of Major Facilitator Superfamily type, as well as the putative transporters included in our study, are regulated by glucose availability and could be involved in several cellular aspects dependent of glucose and/or its metabolites. Recently, a correlation between dysregulation of glucose in the central nervous system and numerous diseases such as obesity, type 2 diabetes mellitus as well as neurological disease such as Alzheimer’s and Parkinson’s diseases indicate a complex regulation and fine tuning of glucose levels in the brain. The fact that almost one third of transporters and transporter-related proteins remain orphans with unknown or contradictive substrate profile, location and function, pinpoint the need for further research about them to fully understand their mechanistic role and their impact on cellular metabolism.

Expressions of Carbohydrate Response Element Binding Protein and Glucose Transporters in Liver Cancer and Clinical Significance

Carbohydrate response element binding protein (ChREBP) is a glucose-sensing transcription factor that mediates the induction of glycolytic and lipogenic genes in response to glucose. We investigated the expression patterns of ChREBP and glucose transporters (GLUTs) in human hepatocellular carcinoma (HCC) and their association with HCC progression. ChREBP, GLUT2 and GLUT1 immunohistochemistry were performed on liver tissue array containing normal liver tissue, HCC adjacent tissue and cancer tissue of different HCC stages. The effect of HCC malignancy on protein expression was analyzed with one-way ANOVA. The correlations between protein expressions were analyzed with Pearson Correlation test. We found that ChREBP protein expression tended to be positively correlated to liver malignancy. GLUT2 protein expression was significantly reduced in human HCC as compared to normal liver tissue and its expression in HCC was inversely associated to malignancy (p < 0.001). In contrast, GLUT1 was significantly increased in cancer cells and its expression was positively correlated to malignancy (p < 0.001). Furthermore, GLUT1 expression was positively associated to ChREBP expression (r = 0.481, p < 0.0001, n = 70) but negatively correlated to GLUT2 expression (r = -0.320, p = 0.007, n = 70). Notably, ChREBP-expressing hepatocytes did not express GLUT2 but GLUT1. This is the first report unveiling expressions of ChREBP and GLUT2/GLUT1 and their relations in HCC. The expression patterns are related to malignancy and this information would facilitate evaluation of clinical behavior and treatment of HCC.

Expression and pathophysiological significance of carbohydrate response element binding protein (ChREBP) in the renal tubules of diabetic kidney

Carbohydrate response element binding protein (ChREBP), a glucose responsive transcription factor, mainly regulates expression of genes involved in glucose metabolism and lipogenesis. Recently, ChREBP is speculated to be involved in the onset and progression of diabetic nephropathy (DN). However, there exists no report regarding the localization and function of ChREBP in the kidney. Therefore, we analyzed the localization of Chrebp mRNA expression in the wild type (WT) mice kidney using laser microdissection method, and observed its dominant expression in the proximal tubules. In diabetic mice, mRNA expression of Chrebp target genes in the proximal tubules, including Chrebpβ and thioredoxin-interacting protein (Txnip), significantly increased comparing with that of WT mice. Co-overexpression of ChREBP and its partner Mlx, in the absence of glucose, also increased TXNIP mRNA expression as well as high glucose in human proximal tubular epithelial cell line HK-2. Since TXNIP is well known to be involved in the production of reactive oxygen species (ROS), we next examined the effect of ChREBP/Mlx co-overexpression, in the absence of glucose, on ROS production in HK-2 cells. Interestingly, ChREBP/Mlx co-overexpression also induced ROS production significantly as well as high glucose. Moreover, both high glucose-induced increase of TXNIP mRNA expression and ROS production were abrogated by ChREBP small interfering RNA transfection. Taken together, high glucose-activated ChREBP in the renal proximal tubules induce the expression of TXNIP mRNA, resulting in the production of ROS which may cause renal tubular damage. It is therefore speculated that ChREBP is involved in the onset and progression of DN.

HB-EGF Signaling Is Required for Glucose-Induced Pancreatic β-Cell Proliferation in Rats

The molecular mechanisms of β-cell compensation to metabolic stress are poorly understood. We previously observed that nutrient-induced β-cell proliferation in rats is dependent on epidermal growth factor receptor (EGFR) signaling. The aim of this study was to determine the role of the EGFR ligand heparin-binding EGF-like growth factor (HB-EGF) in the β-cell proliferative response to glucose, a β-cell mitogen and key regulator of β-cell mass in response to increased insulin demand. We show that exposure of isolated rat and human islets to HB-EGF stimulates β-cell proliferation. In rat islets, inhibition of EGFR or HB-EGF blocks the proliferative response not only to HB-EGF but also to glucose. Furthermore, knockdown of HB-EGF in rat islets blocks β-cell proliferation in response to glucose ex vivo and in vivo in transplanted glucose-infused rats. Mechanistically, we demonstrate that HB-EGF mRNA levels are increased in β-cells in response to glucose in a carbohydrate-response element-binding protein (ChREBP)-dependent manner. In addition, chromatin immunoprecipitation studies identified ChREBP binding sites in proximity to the HB-EGF gene. Finally, inhibition of Src family kinases, known to be involved in HB-EGF processing, abrogated glucose-induced β-cell proliferation. Our findings identify a novel glucose/HB-EGF/EGFR axis implicated in β-cell compensation to increased metabolic demand.

ChREBP-Mediated Regulation of Lipid Metabolism: Involvement of the Gut Microbiota, Liver, and Adipose Tissue

Carbohydrate response element-binding protein (ChREBP) plays an important role in the development of type 2 diabetes, dyslipidemia, and non-alcoholic fatty liver disease, as well as tumorigenesis. ChREBP is highly expressed in lipogenic organs, such as liver, intestine, and adipose tissue, in which it regulates the production of acetyl CoA from glucose by inducing Pklr and Acyl expression. It has recently been demonstrated that ChREBP plays a role in the conversion of gut microbiota-derived acetate to acetyl CoA by activating its target gene, Acss2, in the liver. ChREBP regulates fatty acid synthesis, elongation, and desaturation by inducing Acc1 and Fasn, elongation of long-chain fatty acids family member 6 (encoded by Elovl6), and Scd1 expression, respectively. ChREBP also regulates the formation of very low-density lipoprotein by inducing the expression of Mtp. Furthermore, it plays a crucial role in peripheral lipid metabolism by inducing Fgf21 expression, as well as that of Angptl3 and Angptl8, which are known to reduce peripheral lipoprotein lipase activity. In addition, ChREBP is involved in the production of palmitic-acid-5-hydroxystearic-acid, which increases insulin sensitivity in adipose tissue. Curiously, ChREBP is indirectly involved in fatty acid β-oxidation and subsequent ketogenesis. Thus, ChREBP regulates whole-body lipid metabolism by controlling the transcription of lipogenic enzymes and liver-derived cytokines.

Glucose-Sensing Transcription Factor MondoA/ChREBP as Targets for Type 2 Diabetes: Opportunities and Challenges

The worldwide increase in type 2 diabetes (T2D) is becoming a major health concern, thus searching for novel preventive and therapeutic strategies has become urgent. In last decade, the paralogous transcription factors MondoA and carbohydrate response element-binding protein (ChREBP) have been revealed to be central mediators of glucose sensing in multiple metabolic organs. Under normal nutrient conditions, MondoA/ChREBP plays vital roles in maintaining glucose homeostasis. However, under chronic nutrient overload, the dysregulation of MondoA/ChREBP contributes to metabolic disorders, such as insulin resistance (IR) and T2D. In this review, we aim to provide an overview of recent advances in the understanding of MondoA/ChREBP and its roles in T2D development. Specifically, we will briefly summarize the functional similarities and differences between MondoA and ChREBP. Then, we will update the roles of MondoA/ChREBP in four T2D-associated metabolic organs (i.e., the skeletal muscle, liver, adipose tissue, and pancreas) in physiological and pathological conditions. Finally, we will discuss the opportunities and challenges of MondoA/ChREBP as drug targets for anti-diabetes. By doing so, we highlight the potential use of therapies targeting MondoA/ChREBP to counteract T2D and its complications.

Mono-2-ethylhexyl phthalate induces the expression of genes involved in fatty acid synthesis in HepG2 cells

Mono-2-ethylhexyl phthalate (MEHP) is a major bioactive metabolite in the widely used industrial plasticizer diethylhexyl phthalate (DEHP) that has been found to be toxic to the liver. The aim of this study is to determine whether MEHP exposure can change the expression of fatty acid metabolism-related genes in HepG2 cells, which might be related to non-alcoholic fatty liver disease (NAFLD). The results revealed that exposure to MEHP promoted lipid accumulation in HepG2 cells. The levels of intracellular triglycerides in the hepatocytes increased after exposure to 0.8-100 μM MEHP for 24 h and 48 h. The genetic expressions of SREBP-1c, ChREBP, ACC1, FASN, and SCD significantly increased at 6 h after exposure to MEHP. At 24 h, the expression of the SREBP-1c and ChREBP genes remained increased, while the expression of the FASN and SCD genes decreased. At 48 h, the expression of SREBP-1c, ChREBP, ACC1, FASN, and SCD decreased. Furthermore, the levels of proteins including ACC1, FASN, SCD, and ChREBP (except SREBP-1c) increased at 24 h. These findings suggest that MEHP exposure can promote fatty acid synthesis in hepatocytes by regulating the expression of relevant genes and proteins, contributing to NAFLD.

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.

AMPK-Mediated Regulation of Alpha-Arrestins and Protein Trafficking

The adenosine monophosphate-activated protein kinase (AMPK) plays a central role in the regulation of cellular metabolism. Recent studies reveal a novel role for AMPK in the regulation of glucose and other carbohydrates flux by controlling the endocytosis of transporters. The first step in glucose metabolism is glucose uptake, a process mediated by members of the GLUT/SLC2A (glucose transporters) or HXT (hexose transporters) family of twelve-transmembrane domain glucose transporters in mammals and yeast, respectively. These proteins are conserved from yeast to humans, and multiple transporters—each with distinct kinetic properties—compete for plasma membrane occupancy in order to enhance or limit the rate of glucose uptake. During growth in the presence of alternative carbon sources, glucose transporters are removed and replaced with the appropriate transporter to help support growth in response to this environment. New insights into the regulated protein trafficking of these transporters reveal the requirement for specific α-arrestins, a little-studied class of protein trafficking adaptor. A defining feature of the α-arrestins is that each contains PY-motifs, which can bind to the ubiquitin ligases from the NEDD4/Rsp5 (Neural precursor cell Expressed, Developmentally Down-regulated 4 and Reverses Spt- Phenotype 5, respectively) family. Specific association of α-arrestins with glucose and carbohydrate transporters is thought to bring the ubiquitin ligase in close proximity to its membrane substrate, and thereby allows the membrane cargo to become ubiquitinated. This ubiquitination in turn serves as a mark to stimulate endocytosis. Recent results show that AMPK phosphorylation of the α-arrestins impacts their abundance and/or ability to stimulate carbohydrate transporter endocytosis. Indeed, AMPK or glucose limitation also controls α-arrestin gene expression, adding an additional layer of complexity to this regulation. Here, we review the recent studies that have expanded the role of AMPK in cellular metabolism to include regulation of α-arrestin-mediated trafficking of transporters and show that this mechanism of regulation is conserved over the ~150 million years of evolution that separate yeast from man.

Interaction between hormone-sensitive lipase and ChREBP in fat cells controls insulin sensitivity

Impaired adipose tissue insulin signalling is a critical feature of insulin resistance. Here we identify a pathway linking the lipolytic enzyme hormone-sensitive lipase (HSL) to insulin action via the glucose-responsive transcription factor ChREBP and its target, the fatty acid elongase ELOVL6. Genetic inhibition of HSL in human adipocytes and mouse adipose tissue results in enhanced insulin sensitivity and induction of ELOVL6. ELOVL6 promotes an increase in phospholipid oleic acid, which modifies plasma membrane fluidity and enhances insulin signalling. HSL deficiency-mediated effects are suppressed by gene silencing of ChREBP and ELOVL6. Mechanistically, physical interaction between HSL, independent of lipase activity, and the isoform activated by glucose metabolism ChREBPα impairs ChREBPα translocation into the nucleus and induction of ChREBPβ, the isoform with high transcriptional activity that is strongly associated with whole-body insulin sensitivity. Targeting the HSL-ChREBP interaction may allow therapeutic strategies for the restoration of insulin sensitivity.

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.

MiR-451a attenuates free fatty acids-mediated hepatocyte steatosis by targeting the thyroid hormone responsive spot 14 gene

The thyroid hormone responsive spot 14 (THRSP) gene is a de novo lipogenesis-related gene that plays a significant role in the initiation and development of nonalcoholic fatty liver disease (NAFLD). Several previous studies had shown that endogenous and environmental factors could regulate the expression of THRSP. The role of microRNAs (miRNAs), however, in controlling THRSP expression has not been investigated. In this study, we first constructed the hepatic steatosis cell model by using a mixture of free fatty acids (FFAs; oleate/palmitate, 2:1 ratio) to treat and demonstrate the promotive role of THRSP in lipid accumulation in hepatic cells. By analyzing the photoactivatable ribonucleoside-enhanced crosslinking and immunoprecipitation (PAR-CLIP) database and performing a luciferase reporter assay, we confirmed that microRNA-451a specifically binds to mouse and human THRSP 3'UTR and inhibits its activity. Overexpression of miR-451a efficiently reduced THRSP mRNA and protein expression as well as triglyceride (TG) accumulation in cultured hepatic cells (AML12 and HepG2). Moreover, overexpression of miR-451a significantly decreases TG accumulation in the livers of mice injected with an miR-451a agomir. All these results demonstrated that miR-451a might participate in the FFA-induced hepatic steatosis by regulating the expression of the THRSP gene which represents a new potential target for NAFLD therapy.

Regulation and Metabolic Significance of De Novo Lipogenesis in Adipose Tissues

De novo lipogenesis (DNL) is a complex and highly regulated process in which carbohydrates from circulation are converted into fatty acids that are then used for synthesizing either triglycerides or other lipid molecules. Dysregulation of DNL contributes to human diseases such as obesity, type 2 diabetes, and cardiovascular diseases. Thus, the lipogenic pathway may provide a new therapeutic opportunity for combating various pathological conditions that are associated with dysregulated lipid metabolism. Hepatic DNL has been well documented, but lipogenesis in adipocytes and its contribution to energy homeostasis and insulin sensitivity are less studied. Recent reports have gained significant insights into the signaling pathways that regulate lipogenic transcription factors and the role of DNL in adipose tissues. In this review, we will update the current knowledge of DNL in white and brown adipose tissues with the focus on transcriptional, post-translational, and central regulation of DNL. We will also summarize the recent findings of adipocyte DNL as a source of some signaling molecules that critically regulate energy metabolism.

ChREBP deficiency leads to diarrhea-predominant irritable bowel syndrome

OBJECTIVE

Fructose malabsorption is a common digestive disorder in which absorption of fructose in the small intestine is impaired. An abnormality of the main intestinal fructose transporter proteins has been proposed as a cause for fructose malabsorption. However the underlying molecular mechanism for this remains unclear. In this study, we investigated whether carbohydrate response element-binding protein (ChREBP) plays a role in intestinal fructose absorption through the regulation of genes involved in fructose transport and metabolism and ion transport.

METHODS

Wild type (WT) and Chrebp knockout (KO) mice (6 or 8 weeks old) were fed a control diet (55% starch, 15% maltodextrin 10) or high-fructose diet (HFrD, 60% fructose, 10% starch) for 3-12 days. Body weight and food intake were measured, signs of fructose malabsorption were monitored, and the expression of genes involved in fructose transport/metabolism and ion transport was evaluated. Furthermore, transient transfection and chromatin immunoprecipitation were performed to show the direct interaction between ChREBP and carbohydrate response elements in the promoter of Slc2A5, which encodes the fructose transporter GLUT5.

RESULTS

Chrebp KO mice fed the control diet maintained a constant body weight, whereas those fed a HFrD showed significant weight loss within 3-5 days. In addition, Chrebp KO mice fed the HFrD exhibited a markedly distended cecum and proximal colon containing both fluid and gas, suggesting incomplete fructose absorption. Fructose-induced increases of genes involved in fructose transport (GLUT5), fructose metabolism (fructokinase, aldolase B, triokinase, and lactate dehydrogenase), and gluconeogenesis (glucose-6-phosphatase and fructose-1,6-bisphosphatase) were observed in the intestine of WT but not of Chrebp KO mice. Moreover the Na⁺/H⁺ exchanger NHE3, which is involved in Na⁺ and water absorption in the intestine, was significantly decreased in HFrD-fed Chrebp KO mice. Consistent with this finding, the high-fructose diet-fed Chrebp KO mice developed severe diarrhea. Results of chromatin immunoprecipitation assays showed a direct interaction of ChREBP with the Glut5 promoter, but not the Nhe3 promoter, in the small intestine. Ectopic co-expression of ChREBP and its heterodimer partner Max-like protein X activated the Glut5 promoter in Caco-2BBE cells.

CONCLUSIONS

ChREBP plays a key role in the dietary fructose transport as well as conversion into lactate and glucose through direct transcriptional control of genes involved in fructose transport, fructolysis, and gluconeogenesis. Moreover, ablation of Chrebp results in a severe diarrhea in mice fed a high-fructose diet, which is associated with the insufficient induction of GLUT5 in the intestine.

Histone modifications in fatty acid synthase modulated by carbohydrate responsive element binding protein are associated with non‑alcoholic fatty liver disease

Non-alcoholic fatty liver disease (NAFLD) is a manifestation of metabolic syndrome in the liver and is closely associated with diabetes; however, its pathogenesis remains to be elucidated. Carbohydrate responsive element binding protein (ChREBP), the hub of glucolipid metabolism, regulates the induction of fatty acid synthase (FASN), the key enzyme of de novo lipogenesis, by directly binding to carbohydrate response element (ChoRE) in its promoter. Investigations of histone modifications on NAFLD remain in their infancy. In the present study, by using ChIP, the association between histone modifications and FASN transcription was investigated and histone modifications in FASN modulated by ChREBP were measured. It was demonstrated that ChREBP induced FASN ChREBP-ChoRE binding to accelerate the expression of FASN, leading to hepatocellular steatosis by facilitating H3 and H4 acetylation, H3K4 trimethylation and the phosphorylation of H3S10, but inhibiting the trimethylation of H3K9 and H4K20 in FASN promoter regions of HepG2 and L02 cells. It was also found that ChREBP-ChoRE binding of FASN relied on histone acetylation and that the transcriptional activity of ChREBP on FASN is required, based on the premise that histone acetylation causes conformational changes in FASN chromatin. This indicated histone acetylation as a crucial mechanism involved in the transcription of FASN modulated by ChREBP. Consequently, the present study provides further insight into the pathophysiology and a novel therapeutic potential of NAFLD based on epigenetic mechanisms.

Multi-tissue transcriptomic study reveals the main role of liver in the chicken adaptive response to a switch in dietary energy source through the transcriptional regulation of lipogenesis

Background

Because the cost of cereals is unstable and represents a large part of production charges for meat-type chicken, there is an urge to formulate alternative diets from more cost-effective feedstuff. We have recently shown that meat-type chicken source is prone to adapt to dietary starch substitution with fat and fiber. The aim of this study was to better understand the molecular mechanisms of this adaptation to changes in dietary energy sources through the fine characterization of transcriptomic changes occurring in three major metabolic tissues – liver, adipose tissue and muscle – as well as in circulating blood cells.

Results

We revealed the fine-tuned regulation of many hepatic genes encoding key enzymes driving glycogenesis and de novo fatty acid synthesis pathways and of some genes participating in oxidation. Among the genes expressed upon consumption of a high-fat, high-fiber diet, we highlighted CPT1A, which encodes a key enzyme in the regulation of fatty acid oxidation. Conversely, the repression of lipogenic genes by the high-fat diet was clearly associated with the down-regulation of SREBF1 transcripts but was not associated with the transcript regulation of MLXIPL and NR1H3, which are both transcription factors. This result suggests a pivotal role for SREBF1 in lipogenesis regulation in response to a decrease in dietary starch and an increase in dietary PUFA. Other prospective regulators of de novo hepatic lipogenesis were suggested, such as PPARD, JUN, TADA2A and KAT2B, the last two genes belonging to the lysine acetyl transferase (KAT) complex family regulating histone and non-histone protein acetylation. Hepatic glycogenic genes were also down-regulated in chickens fed a high-fat, high-fiber diet compared to those in chickens fed a starch-based diet. No significant dietary-associated variations in gene expression profiles was observed in the other studied tissues, suggesting that the liver mainly contributed to the adaptation of birds to changes in energy source and nutrients in their diets, at least at the transcriptional level. Moreover, we showed that PUFA deposition observed in the different tissues may not rely on transcriptional changes.

Conclusion

We showed the major role of the liver, at the gene expression level, in the adaptive response of chicken to dietary starch substitution with fat and fiber.

Electronic supplementary material

The online version of this article (10.1186/s12864-018-4520-5) contains supplementary material, which is available to authorized users.

PNPLA3 expression and its impact on the liver: current perspectives

A single-nucleotide polymorphism occurring in the sequence of the human patatin-like phospholipase domain-containing 3 gene (PNPLA3), known as I148M variant, is one of the best characterized and deeply investigated variants in several clinical scenarios, because of its tight correlation with increased risk for developing hepatic steatosis and more aggressive part of the disease spectrum, such as nonalcoholic steatohepatitis, advanced fibrosis and cirrhosis. Further, the I148M variant is positively associated with alcoholic liver diseases, chronic hepatitis C-related cirrhosis and hepatocellular carcinoma. The native gene encodes for a protein that has not yet a fully defined role in liver lipid metabolism and, according to recent observations, seems to be divergently regulated among distinct liver cells type, such as hepatic stellate cells. Therefore, the aim of this review is to collect the latest data regarding PNPLA3 expression in human liver and to analyze the impact of its genetic variant in human hepatic pathologies. Moreover, a description of the current biochemical and metabolic data pertaining to PNPLA3 function in both animal models and in vitro studies is summarized to allow a better understanding of the relevant pathophysiological role of this enzyme in the progression of hepatic diseases.