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

Identification of additional body weight QTLs in the Berlin Fat Mouse BFMI861 lines using time series data

The Berlin Fat Mouse Inbred line (BFMI) is a model for obesity and metabolic syndrome. The sublines BFMI861-S1 and BFMI861-S2 differ in weight despite high genetic similarity and a shared obesity-related locus. This study focused on identifying additional body weight quantitative trait loci (QTLs) by analyzing weekly weight measurements in a male population of the advanced intercross line BFMI861-S1 x BFMI861-S2. QTL analysis, utilizing 200 selectively genotyped mice (GigaMUGA) and 197 males genotyped for top SNPs, revealed a genome-wide significant QTL on Chr 15 (68.46 to 81.40 Mb) for body weight between weeks 9 to 20. Notably, this QTL disappeared (weeks 21 to 23) and reappeared (weeks 24 and 25) coinciding with a diet change. Additionally, a significant body weight QTL on Chr 16 (3.89 to 22.79 Mb) was identified from weeks 6 to 25. Candidate genes, including Gpt, Cbx6, Apol6, Apol8, Sun2 (Chr 15) and Trap1, Rrn3, Mapk1 (Chr 16), were prioritized. This study unveiled two additional body weight QTLs, one of which is novel and responsive to diet changes. These findings illuminate genomic regions influencing weight in BFMI and emphasize the utility of time series data in uncovering novel genetic factors.

MLX plays a key role in lipid and glucose metabolism in humans: Evidence from in vitro and in vivo studies

BACKGROUND AND AIM

Enhanced hepatic de novo lipogenesis (DNL) has been proposed as an underlying mechanism for the development of NAFLD and insulin resistance. Max-like protein factor X (MLX) acts as a heterodimer binding partner for glucose sensing transcription factors and inhibition of MLX or downstream targets has been shown to alleviate intrahepatic triglyceride (IHTG) accumulation in mice. However, its effect on insulin sensitivity remains unclear. As human data is lacking, the aim of the present work was to investigate the role of MLX in regulating lipid and glucose metabolism in primary human hepatocytes (PHH) and in healthy participants with and without MLX polymorphisms.

METHODS

PHH were transfected with non-targeting or MLX siRNA to assess the effect of MLX knockdown on lipid and glucose metabolism, insulin signalling and the hepatocellular transcriptome. A targeted association analysis on imputed genotype data for MLX on healthy individuals was undertaken to assess associations between specific MLX SNPs (rs665268, rs632758 and rs1474040), plasma biochemistry, IHTG content, DNL and gluconeogenesis.

RESULTS

MLX knockdown in PHH altered lipid metabolism (decreased DNL (p < 0.05), increased fatty acid oxidation and ketogenesis (p < 0.05), and reduced lipid accumulation (p < 0.001)). Additionally, MLX knockdown increased glycolysis, lactate secretion and glucose production (p < 0.001) and insulin-stimulated pAKT levels (p < 0.01) as assessed by transcriptomic, steady-state and dynamic measurements. Consistent with the in vitro data, individuals with the rs1474040-A and rs632758-C variants had lower fasting plasma insulin (p < 0.05 and p < 0.01, respectively) and TG (p < 0.05 and p < 0.01, respectively). Although there was no difference in IHTG or gluconeogenesis, individuals with rs632758 SNP had notably lower hepatic DNL (p < 0.01).

CONCLUSION

We have demonstrated using human in vitro and in vivo models that MLX inhibition favored lipid catabolism over anabolism and increased glucose production, despite increased glycolysis and phosphorylation of Akt, suggesting a metabolic mechanism that involves futile cycling.

FGF9 Alleviates the Fatty Liver Phenotype by Regulating Hepatic Lipid Metabolism

Although the fatty liver has been linked to numerous impairments of energy homeostasis, the molecular mechanism responsible for fatty liver development remains largely unknown. In the present study, we show that fibroblast growth factors 9 (FGF9) expression is increased in the liver of diet-induced obese (DIO), db/db, and ob/ob mice relative to their respective controls. The long-term knockdown of hepatic FGF9 expression mediated by adeno-associated virus expressing FGF9-specific short hairpin RNA (AAV-shFGF9) aggravated the fatty liver phenotype of DIO mice. Consistently, downregulation of FGF9 expression mediated by adenovirus expressing FGF9-specific shRNA (Ad-shFGF9) in the primary hepatocyte promoted the cellular lipid accumulation, suggesting that FGF9 exerts its effects in an autocrine manner. In contrast, adenoviruses expressing FGF9 (Ad-FGF9) mediated FGF9 overexpression in the liver of DIO mice alleviated hepatic steatosis and improved the insulin sensitivity and glucose intolerance. Moreover, the liver-specific FGF9 transgenic mice phenocopied the Ad-FGF9-infected mice. Mechanistically, FGF9 inhibited the expression of genes involved in lipogenesis and increased the expression of genes involved in fatty acid oxidation, thereby reducing cellular lipid accumulation. Thus, targeting FGF9 might be exploited to treat nonalcoholic fatty liver disease (NAFLD) and metabolic syndrome.

Protective effect of quercetin against the metabolic dysfunction of glucose and lipids and its associated learning and memory impairments in NAFLD rats

BACKGROUND

Quercetin (QUE) is a flavonol reported with anti-inflammatory and antioxidant activities, and previous results from the group of this study have demonstrated its neuroprotective effect against lipopolysaccharide-induced neuropsychiatric injuries. However, little is known about its potential effect on neuropsychiatric injuries induced or accompanied by metabolic dysfunction of glucose and lipids.

METHODS

A nonalcoholic fatty liver disease (NAFLD) rat model was induced via a high-fat diet (HFD), and glucolipid parameters and liver function were measured. Behavioral performance was observed via the open field test (OFT) and the Morris water maze (MWM). The plasma levels of triggering receptor expressed on myeloid cells-1 (TREM1) and TREM2 were measured via enzyme-linked immunosorbent assay (ELISA). The protein expression levels of Synapsin-1 (Syn-1), Synaptatogmin-1 (Syt-1), TREM1 and TREM2 in the hippocampus were detected using western blotting. Morphological changes in the liver and hippocampus were detected by HE and Oil red or silver staining.

RESULTS

Compared with the control rats, HFD-induced NAFLD model rats presented significant metabolic dysfunction, hepatocyte steatosis, and impaired learning and memory ability, as indicated by the increased plasma concentrations of total cholesterol (TC) and triglyceride (TG), the impaired glucose tolerance, the accumulated fat droplets and balloon-like changes in the liver, and the increased escaping latency but decreased duration in the target quadrant in the Morris water maze. All these changes were reversed in QUE-treated rats. Moreover, apart from improving the morphological injuries in the hippocampus, treatment with QUE could increase the decreased plasma concentration and hippocampal protein expression of TREM1 in NAFLD rats and increase the decreased expression of Syn-1 and Syt-1 in the hippocampus.

CONCLUSIONS

These results suggested the therapeutic potential of QUE against NAFLD-associated impairment of learning and memory, and the mechanism might involve regulating the metabolic dysfunction of glucose and lipids and balancing the protein expression of synaptic plasticity markers and TREM1/2 in the hippocampus.

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.

Edematous severe acute malnutrition is characterized by hypomethylation of DNA

Edematous severe acute childhood malnutrition (edematous SAM or ESAM), which includes kwashiorkor, presents with more overt multi-organ dysfunction than non-edematous SAM (NESAM). Reduced concentrations and methyl-flux of methionine in 1-carbon metabolism have been reported in acute, but not recovered, ESAM, suggesting downstream DNA methylation changes could be relevant to differences in SAM pathogenesis. Here, we assess genome-wide DNA methylation in buccal cells of 309 SAM children using the 450 K microarray. Relative to NESAM, ESAM is characterized by multiple significantly hypomethylated loci, which is not observed among SAM-recovered adults. Gene expression and methylation show both positive and negative correlation, suggesting a complex transcriptional response to SAM. Hypomethylated loci link to disorders of nutrition and metabolism, including fatty liver and diabetes, and appear to be influenced by genetic variation. Our epigenetic findings provide a potential molecular link to reported aberrant 1-carbon metabolism in ESAM and support consideration of methyl-group supplementation in ESAM.

Quercetin inhibition of SREBPs and ChREBP expression results in reduced cholesterol and fatty acid synthesis in C6 glioma cells

Quercetin (Que), a widely distributed flavonoid in the human diet, exerts neuroprotective action because of its property to antagonize oxidative stress. Here, we investigated the effects of Que on lipid synthesis in C6 glioma cells. A rapid Que-induced inhibition of cholesterol and, to a lesser extent, of fatty acid synthesis from [1-¹⁴C]acetate was observed. The maximum decrease was detected at the level of palmitate, the end product of de novo fatty acid synthesis. The effect of Que on the enzyme activities of acetyl-CoA carboxylase 1 (ACC1) and fatty acid synthase (FAS), the two enzymes of this pathway, was investigated directly in situ in permeabilized C6 cells. An inhibitory effect on ACC1 was observed after 4 h of 25 μM Que treatment, while FAS activity was not affected. A reduction of polar lipid biosynthesis was also detected. A remarkable decrease of 3-hydroxy-3-methylglutaryl-CoA reductase (HMGCR) activity, regulatory enzyme of cholesterol synthesis, was evidenced. Expression studies demonstrated that Que acts at transcriptional level, by reducing the mRNA abundance and protein amount of ACC1 and HMGCR. Deepening the molecular mechanism, we found that Que decreased the expression of SREBP-1 and SREBP-2, transcriptional factors representing the main regulators of de novo fatty acid and cholesterol synthesis, respectively. Que also reduced the nuclear content of ChREBP, a glucose-induced transcription factor involved in the regulation of lipogenic genes. Our results represent the first evidence that a direct and rapid downregulatory effect of Que on cholesterol and de novo fatty acid synthesis is elicited in C6 cells.

3,5-diiodo-L-thyronine increases de novo lipogenesis in liver from hypothyroid rats by SREBP-1 and ChREBP-mediated transcriptional mechanisms

Hepatic de novo lipogenesis (DNL), the process by which carbohydrates are converted into lipids, is strictly controlled by nutritional and hormonal status. 3,5-Diiodo-L-thyronine (T2), a product of the 3,5,3'-triiodo-L-thyronine (T3) peripheral metabolism, has been shown to mimic some T3 effects on lipid metabolism by a short-term mechanism independent of protein synthesis. Here, we report that T2, administered for 1 week to hypothyroid rats, increases total fatty acid synthesis from acetate in isolated hepatocytes. Studies carried out on liver subcellular fractions demonstrated that T2 not only increases the activity and the expression of acetyl-CoA carboxylase and fatty acid synthase but also of other proteins linked to DNL such as the mitochondrial citrate carrier and the cytosolic ATP citrate lyase. Parallelly, T2 stimulates the activities of enzymes supplying cytosolic NADPH needed for the reductive steps of DNL. With respect to both euthyroid and hypothyroid rats, T2 administration decreases the hepatic mRNA level of SREBP-1, a transcription factor which represents a master regulator of DNL. However, when compared to hypothyroid rats T2 significantly increases, without bringing to the euthyroid value, the content of both mature (nSREBP-1), and precursor (pSREBP-1) forms of the SREBP-1 protein as well as their ratio. Moreover, T2 administration strongly augmented the nuclear content of ChREBP, another crucial transcription factor involved in the regulation of lipogenic genes. Based on these results, we can conclude that in the liver of hypothyroid rats the transcriptional activation by T2 of DNL genes could depend, at least in part, on SREBP-1- and ChREBP-dependent mechanisms. © 2019 IUBMB Life, 2019.