Carbohydrate response element binding protein gene expression is positively regulated by thyroid hormone

Understanding the Relationship between Nonalcoholic Fatty Liver Disease and Thyroid Disease

The prevalence of hypothyroidism in patients with nonalcoholic fatty liver disease (NAFLD) is high (22.4%). Thyroid hormones (THs) regulate many metabolic activities in the liver by promoting the export and oxidation of lipids, as well as de novo lipogenesis. They also control hepatic insulin sensitivity and suppress hepatic gluconeogenesis. Because of its importance in lipid and carbohydrate metabolism, the involvement of thyroid dysfunction in the pathogenesis of NAFLD seems plausible. The mechanisms implicated in this relationship include high thyroid-stimulating hormone (TSH) levels, low TH levels, and chronic inflammation. The activity of the TH receptor (THR)-β in response to THs is essential in the pathogenesis of hypothyroidism-induced NAFLD. Therefore, an orally active selective liver THR-β agonist, Resmetirom (MGL-3196), was developed, and has been shown to reduce liver fat content, and as a secondary end point, to improve nonalcoholic steatohepatitis. The treatment of NAFLD with THR-β agonists seems quite promising, and other agonists are currently under development and investigation. This review aims to shine a light on the pathophysiological and epidemiological evidence regarding this relationship and the effect that treatment with THs and selective liver THR-β agonists have on hepatic lipid metabolism.

Thyroid hormone action and liver disease, a complex interplay

Thyroid hormone action is involved in virtually all physiological processes. It is well known that the liver and thyroid are intimately linked, with thyroid hormone playing important roles in de novo lipogenesis, beta-oxidation (fatty acid oxidation), cholesterol metabolism, and carbohydrate metabolism. Clinical and mechanistic research studies have shown that thyroid hormone can be involved in chronic liver diseases, including alcohol-associated or NAFLD and HCC. Thyroid hormone action and synthetic thyroid hormone analogs can exert beneficial actions in terms of lowering lipids, preventing chronic liver disease and as liver anticancer agents. More recently, preclinical and clinical studies have indicated that some analogs of thyroid hormone could also play a role in the treatment of liver disease. These synthetic molecules, thyromimetics, can modulate lipid metabolism, particularly in NAFLD/NASH. In this review, we first summarize the thyroid hormone signaling axis in the context of liver biology, then we describe the changes in thyroid hormone signaling in liver disease and how liver diseases affect the thyroid hormone homeostasis, and finally we discuss the use of thyroid hormone-analog for the treatment of liver disease.

Does Iodine Intake Modify the Effect of Maternal Dysglycemia on Birth Weight in Mild-to-Moderate Iodine-Deficient Populations? A Mother-Newborn Prospective Cohort Study

It is unclear how maternal glycemic status and maternal iodine status influence birth weight among individuals with mild-to-moderate iodine deficiency (ID). We studied the association between birth weight and both maternal glucose levels and iodine intake among pregnant women with mild-to-moderate ID. Glucose values were assessed using a glucose challenge test (GCT) and non-fasting glucose levels that were determined before delivery; individuals' iodine statuses were assessed using an iodine food frequency questionnaire; and serum thyroglobulin (Tg) and urinary iodine concentrations (UIC) were used to assess each group's iodine status. Thyroid antibodies and free thyroxine (FT4) levels were measured. Obstetric and anthropometric data were also collected. Large-for-gestational age (LGA) status was predicted using a Cox proportional hazards model with multiple confounders. Tg > 13 g/L was independently associated with LGA (adjusted hazard ratio = 3.4, 95% CI: 1.4-10.2, p = 0.001). Estimated iodine intake correlated with FT4 among participants who reported consuming iodine-containing supplements (ICS) after adjusting for confounders (β = 0.4, 95% CI: 0.0002-0.0008, p = 0.001). Newborn weight percentiles were inversely correlated with maternal FT4 values (β = -0.2 95% CI:-0.08--56.49, p = 0.049). We conclude that in mild-to-moderate ID regions, insufficient maternal iodine status may increase LGA risk. Iodine status and ICS intake may modify the effect that maternal dysglycemia has on offspring weight.

T3 and glucose increase expression of phosphoenolpyruvate carboxykinase (PCK1) leading to increased β-cell proliferation

OBJECTIVES

Thyroid hormone (T3) and high glucose concentrations are critical components of β-cell maturation and function. In the present study, we asked whether T3 and glucose signaling pathways coordinately regulate transcription of genes important for β-cell function and proliferation.

METHODS

RNA-seq analysis was performed on cadaveric human islets from five different donors in response to low and high glucose concentrations and in the presence or absence of T3. Gene expression was also studies in sorted human β-cells, mouse islets and Ins-1 cells by RT-qPCR. Silencing of the thyroid hormone receptors (THR) was conducted using lentiviruses. Proliferation was assessed by ki67 immunostaining in primary human/mouse islets. Chromatin immunoprecipitation and proximity ligation assay were preformed to validate interactions of ChREBP and THR.

RESULTS

We found glucose-mediated expression of carbohydrate response element binding protein alpha and beta (ChREBPα and ChREBPβ) mRNAs and their target genes are highly dependent on T3 concentrations in rodent and human β-cells. In β-cells, T3 and glucose coordinately regulate the expression of ChREBPβ and PCK1 (phosphoenolpyruvate carboxykinase-1) among other important genes for β-cell maturation. Additionally, we show the thyroid hormone receptor (THR) and ChREBP interact, and their relative response elements are located near to each other on mutually responsive genes. In FACS-sorted adult human β-cells, we found that high concentrations of glucose and T3 induced the expression of PCK1. Next, we show that overexpression of Pck1 together with dimethyl malate (DMM), a substrate precursor, significantly increased β-cell proliferation in human islets. Finally, using a Cre-Lox approach, we demonstrated that ChREBPβ contributes to Pck1-dependent β-cell proliferation in mouse β-cells.

CONCLUSIONS

We conclude that T3 and glucose act together to regulate ChREBPβ, leading to increased expression and activity of Pck1, and ultimately increased β-cell proliferation.

Important Hormones Regulating Lipid Metabolism

There is a wide variety of kinds of lipids, and complex structures which determine the diversity and complexity of their functions. With the basic characteristic of water insolubility, lipid molecules are independent of the genetic information composed by genes to proteins, which determine the particularity of lipids in the human body, with water as the basic environment and genes to proteins as the genetic system. In this review, we have summarized the current landscape on hormone regulation of lipid metabolism. After the well-studied PI3K-AKT pathway, insulin affects fat synthesis by controlling the activity and production of various transcription factors. New mechanisms of thyroid hormone regulation are discussed, receptor α and β may mediate different procedures, the effect of thyroid hormone on mitochondria provides a new insight for hormones regulating lipid metabolism. Physiological concentration of adrenaline induces the expression of extrapituitary prolactin in adipose tissue macrophages, which promotes fat weight loss. Manipulation of hormonal action has the potential to offer a new therapeutic horizon for the global burden of obesity and its associated complications such as morbidity and mortality.

Thyroid hormone signaling promotes hepatic lipogenesis through the transcription factor ChREBP

Thyroid hormone (TH) action is essential for hepatic lipid synthesis and oxidation. Analysis of hepatocyte-specific thyroid receptor β1 (TRβ1) knockout mice confirmed a role for TH in stimulating de novo lipogenesis and fatty acid oxidation through its nuclear receptor. Specifically, TRβ1 and its principal corepressor NCoR1 in hepatocytes repressed de novo lipogenesis, whereas the TH-mediated induction of lipogenic genes depended on the transcription factor ChREBP. Mice with a hepatocyte-specific deficiency in ChREBP lost TH-mediated stimulation of the lipogenic program, which, in turn, impaired the regulation of fatty acid oxidation. TH regulated ChREBP activation and recruitment to DNA, revealing a mechanism by which TH regulates specific signaling pathways. Regulation of the lipogenic pathway by TH through ChREBP was conserved in hepatocytes derived from human induced pluripotent stem cells. These results demonstrate that TH signaling in the liver acts simultaneously to enhance both lipogenesis and fatty acid oxidation.

Fructose and metabolic diseases: too much to be good

ABSTRACT

Excessive consumption of fructose, the sweetest of all naturally occurring carbohydrates, has been linked to worldwide epidemics of metabolic diseases in humans, and it is considered an independent risk factor for cardiovascular diseases. We provide an overview about the features of fructose metabolism, as well as potential mechanisms by which excessive fructose intake is associated with the pathogenesis of metabolic diseases both in humans and rodents. To accomplish this aim, we focus on illuminating the cellular and molecular mechanisms of fructose metabolism as well as its signaling effects on metabolic and cardiovascular homeostasis in health and disease, highlighting the role of carbohydrate-responsive element-binding protein in regulating fructose metabolism.

Interplay between Thyroid Hormones and Stearoyl-CoA Desaturase 1 in the Regulation of Lipid Metabolism in the Heart

Stearoyl-CoA desaturase 1 (SCD1), an enzyme that is involved in the biosynthesis of monounsaturated fatty acids, induces the reprogramming of cardiomyocyte metabolism. Thyroid hormones (THs) activate both lipolysis and lipogenesis. Many genes that are involved in lipid metabolism, including Scd1, are regulated by THs. The present study used SCD1 knockout (SCD1^−/−) mice to test the hypothesis that THs are important factors that mediate the anti-steatotic effect of SCD1 downregulation in the heart. SCD1 deficiency decreased plasma levels of thyroid-stimulating hormone and thyroxine and the expression of genes that regulate intracellular TH levels (i.e., Slc16a2 and Dio1-3) in cardiomyocytes. Both hypothyroidism and SCD1 deficiency affected genomic and non-genomic TH pathways in the heart. SCD1 deficiency is known to protect mice from genetic- or diet-induced obesity and decrease lipid content in the heart. Interestingly, hypothyroidism increased body adiposity and triglyceride and diacylglycerol levels in the heart in SCD1^−/− mice. The accumulation of triglycerides in cardiomyocytes in SCD1^−/− hypothyroid mice was caused by the activation of lipogenesis, which likely exceeded the upregulation of lipolysis and fatty acid oxidation. Lipid accumulation was also observed in the heart in wildtype hypothyroid mice compared with wildtype control mice, but this process was related to a reduction of triglyceride lipolysis and fatty acid oxidation. We also found that simultaneous SCD1 and deiodinase inhibition increased triglyceride content in HL-1 cardiomyocytes, and this process was related to the downregulation of lipolysis. Altogether, the present results suggest that THs are an important part of the mechanism of SCD1 in cardiac lipid utilization and may be involved in the upregulation of energetic metabolism that is associated with SCD1 deficiency.

Thyroid Hormone Signaling and the Liver

Thyroid hormone (TH) plays a critical role in maintaining metabolic homeostasis throughout life. It is well known that the liver and thyroid are intimately linked, with TH playing important roles in de novo lipogenesis, beta-oxidation (fatty acid oxidation), cholesterol metabolism, and carbohydrate metabolism. Indeed, patients with hypothyroidism have abnormal lipid panels with higher levels of low-density lipoprotein levels, triglycerides (triacylglycerol; TAG), and apolipoprotein B levels. Even in euthyroid patients, lower serum-free thyroxine levels are associated with higher total cholesterol levels, LDL, and TAG levels. In addition to abnormal serum lipids, the risk of nonalcoholic fatty liver disease (NAFLD) increases with lower free thyroxine levels. As free thyroxine rises, the risk of NAFLD is reduced. This has led to numerous animal studies and clinical trials investigating TH analogs and TH receptor agonists as potential therapies for NAFLD and hyperlipidemia. Thus, TH plays an important role in maintaining hepatic homeostasis, and this continues to be an important area of study. A review of TH action and TH actions on the liver will be presented here.

Sterol Carrier Protein-2/Sterol Carrier Protein-x/Fatty Acid Binding Protein-1 Ablation Impacts Response of Brain Endocannabinoid to High-Fat Diet

Brain endocannabinoids (EC) such as arachidonoylethanolamine (AEA) and 2-arachidonoylglycerol (2-AG) primarily originate from serum arachidonic acid (ARA), whose level is regulated in part by a cytosolic ARA-binding protein, that is, liver fatty acid binding protein-1 (FABP1), not expressed in the brain. Ablation of the Fabp1 gene (LKO) increases brain AEA and 2-AG by decreasing hepatic uptake of ARA to increase serum ARA, thereby increasing ARA availability for uptake by the brain. The brain also expresses sterol carrier protein-2 (SCP-2), which is also a cytosolic ARA-binding protein. To further resolve the role of SCP-2 independent of FABP1, mice ablated in the Scp-2/Scp-x gene (DKO) were crossed with mice ablated in the Fabp1 gene (LKO) mice to generate triple knock out (TKO) mice. TKO impaired the ability of LKO to increase brain AEA and 2-AG. While a high-fat diet (HFD) alone increased brain AEA, TKO impaired this effect. Overall, these TKO-induced blocks were not attributable to altered expression of brain proteins in ARA uptake, AEA/2-AG synthesis, or AEA/2-AG degrading enzymes. Instead, TKO reduced serum levels of free ARA and/or total ARA and thereby decreased ARA availability for uptake to the brain and downstream synthesis of AEA and 2-AG therein. In summary, Scp-2/Scp-x gene ablation in Fabp1 null (LKO) mice antagonized the impact of LKO and HFD on brain ARA and, subsequently, EC levels. Thus, both FABP1 and SCP-2 participate in regulating the EC system in the brain.

Nonalcoholic Fatty Liver Disease and Hypercholesterolemia: Roles of Thyroid Hormones, Metabolites, and Agonists

Background: Thyroid hormones (THs) exert a strong influence on mammalian lipid metabolism at the systemic and hepatic levels by virtue of their roles in regulating circulating lipoprotein, triglyceride (TAG), and cholesterol levels, as well as hepatic TAG storage and metabolism. These effects are mediated by intricate sensing and feedback systems that function at the physiological, metabolic, molecular, and transcriptional levels in the liver. Dysfunction in the pathways involved in lipid metabolism disrupts hepatic lipid homeostasis and contributes to the pathogenesis of metabolic diseases, such as nonalcoholic fatty liver disease (NAFLD) and hypercholesterolemia. There has been strong interest in understanding and employing THs, TH metabolites, and TH mimetics as lipid-modifying drugs. Summary: THs regulate many processes involved in hepatic TAG and cholesterol metabolism to decrease serum cholesterol and intrahepatic lipid content. TH receptor β analogs designed to have less side effects than the natural hormone are currently being tested in phase II clinical studies for NAFLD and hypercholesterolemia. The TH metabolites, 3,5-diiodo-l-thyronine (T2) and T1AM (3-iodothyronamine), have different beneficial effects on lipid metabolism compared with triiodothyronine (T3), although their clinical application is still under investigation. Also, prodrugs and glucagon/T3 conjugates have been developed that direct TH to the liver. Conclusions: TH-based therapies show clinical promise for the treatment of NAFLD and hypercholesterolemia. Strategies for limiting side effects of TH are being developed and may enable TH metabolites and analogs to have specific effects in the liver for treatments of these conditions. These liver-specific effects and potential suppression of the hypothalamic/pituitary/thyroid axis raise the issue of monitoring liver-specific markers of TH action to assess clinical efficacy and dosing of these compounds.

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.

Novel Transcriptional Mechanisms for Regulating Metabolism by Thyroid Hormone

The thyroid hormone plays a key role in energy and nutrient metabolisms in many tissues and regulates the transcription of key genes in metabolic pathways. It has long been believed that thyroid hormones (THs) exerted their effects primarily by binding to nuclear TH receptors (THRs) that are associated with conserved thyroid hormone response elements (TREs) located on the promoters of target genes. However, recent transcriptome and ChIP-Seq studies have challenged this conventional view as discordance was observed between TH-responsive genes and THR binding to DNA. While THR association with other transcription factors bound to DNA, TH activation of THRs to mediate effects that do not involve DNA-binding, or TH binding to proteins other than THRs have been invoked as potential mechanisms to explain this discrepancy, it appears that additional novel mechanisms may enable TH to regulate the mRNA expression. These include activation of transcription factors by SIRT1 via metabolic actions by TH, the post-translational modification of THR, the THR co-regulation of transcription with other nuclear receptors and transcription factors, and the microRNA (miR) control of RNA transcript expression to encode proteins involved in the cellular metabolism. Together, these novel mechanisms enlarge and diversify the panoply of metabolic genes that can be regulated by TH.

Direct effects of thyroid hormones on hepatic lipid metabolism

It has been known for a long time that thyroid hormones have prominent effects on hepatic fatty acid and cholesterol synthesis and metabolism. Indeed, hypothyroidism has been associated with increased serum levels of triglycerides and cholesterol as well as non-alcoholic fatty liver disease (NAFLD). Advances in areas such as cell imaging, autophagy and metabolomics have generated a more detailed and comprehensive picture of thyroid-hormone-mediated regulation of hepatic lipid metabolism at the molecular level. In this Review, we describe and summarize the key features of direct thyroid hormone regulation of lipogenesis, fatty acid β-oxidation, cholesterol synthesis and the reverse cholesterol transport pathway in normal and altered thyroid hormone states. Thyroid hormone mediates these effects at the transcriptional and post-translational levels and via autophagy. Given these potentially beneficial effects on lipid metabolism, it is possible that thyroid hormone analogues and/or mimetics might be useful for the treatment of metabolic diseases involving the liver, such as hypercholesterolaemia and NAFLD.

Effects of thyroid hormones on thermogenesis and energy partitioning

Thyroid hormones (TH) are of central importance for thermogenesis, energy homeostasis and metabolism. Here, we will discuss these aspects by focussing on the physiological aspects of TH-dependent regulation in response to cold exposure and fasting, which will be compared to alterations in primary hyperthyroidism and hypothyroidism. In particular, we will summarise current knowledge on regional thyroid hormone status in the central nervous system (CNS) and in peripheral cells. In contrast to hyperthyroidism and hypothyroidism, where parallel changes are observed, local alterations in the CNS differ to peripheral compartments when induced by cold exposure or fasting. Cold exposure is associated with low hypothalamic TH concentrations but increased TH levels in the periphery. Fasting results in a reversed TH pattern. Primary hypothyroidism and hyperthyroidism disrupt these fine-tuned adaptive mechanisms and both, the hypothalamus and the periphery, will have the same TH status. These important mechanisms need to be considered when discussing thyroid hormone replacement and other therapeutical interventions to modulate TH status.

A Renewed Focus on the Association Between Thyroid Hormones and Lipid Metabolism

Thyroid dysfunction, manifesting as either overt or subclinical hypothyroidism, negatively affects lipid metabolism: this leads to hypercholesterolemia which progressively increases the risk for cardiovascular disease and, potentially, mortality. Hypercholesterolemia in hypothyroidism is mainly due to a reduction in low-density lipoprotein (LDL) receptor activity, this accompanied by concomitant diminishing control by triiodothyronine (T3) of sterol regulatory element-binding protein 2 (SREBP-2), which modulates cholesterol biosynthesis by regulating rate-limit degrading enzyme 3-hydroxy-3-methylglutaryl-coenzyme A reductase (HMG-CoA) activity. Recently, 3,5-diiodothyronine (T2), a natural thyroid hormone derivative, was found to repress the transcription factor carbohydrate-response element-binding protein (ChREBP) and also to be involved in lipid catabolism and lipogenesis, though via a different pathway than that of T3. While thyroid hormone could therapeutically reverse the dyslipidemic profile commonly occurring in hypothyroidism, it should be borne in mind that the potency of the effects may be age-and sex-dependent. Thyroid hormone administration possibly also sustains and enhances the efficacy of hypolipidemic drugs, such as statins, ezetimibe and proprotein convertase subtilisin/kexin type 9 (PCSK9), in patients with dyslipidemia and hypothyroidism.

T3 and Glucose Coordinately Stimulate ChREBP-Mediated Ucp1 Expression in Brown Adipocytes From Male Mice

Increasing brown adipose tissue (BAT) activity is regarded as a potential treatment of obese, hyperglycemic patients with metabolic syndrome. Triiodothyronine (T3) is known to stimulate BAT activity by increasing mitochondrial uncoupling protein 1 (Ucp1) gene transcription, leading to increased thermogenesis and decreased body weight. Here we report our studies on the effects of T3 and glucose in two mouse models and in mouse immortalized brown preadipocytes in culture. We identified carbohydrate response element binding protein (ChREBP) as a T3 target gene in BAT by RNA sequencing and studied its effects in brown adipocytes. We found that ChREBP was upregulated by T3 in BAT in both hyperglycemic mouse models. In brown preadipocytes, T3 and glucose synergistically and dose dependently upregulated Ucp1 messenger RNA 1000-fold compared with low glucose concentrations. Additionally, we observed increased ChREBP and Ucp1 protein 11.7- and 19.9-fold, respectively, along with concomitant induction of a hypermetabolic state. Moreover, downregulation of ChREBP inhibited T3 and glucose upregulation of Ucp1 100-fold, whereas overexpression of ChREBP upregulated Ucp1 5.2-fold. We conclude that T3 and glucose signaling pathways coordinately regulate the metabolic state of BAT and suggest that ChREBP is a target for therapeutic regulation of BAT activity.

Role of thyroid hormone in hepatic gene regulation, chromatin remodeling, and autophagy

Thyroid hormone (TH) actions on development and metabolism have been studied ever since the discovery of thyroxine almost a century ago. Initial studies focused on the physiological and biochemical actions of TH. Later, the cloning of the thyroid hormone receptor (THR) isoforms and the development of techniques enabled the study of TH regulation of complex cellular processes (such as gene transcription). Recently we found that TH activates secondary transcription factors such as FOXO1, to amplify gene transcription; and also is a potent inducer of autophagy that was critical for fatty acid β-oxidation in the liver. This review summarizes the recent advancements in our understanding of TH regulation of gene expression of metabolic genes (via co-regulators/transcription factors and epigenetic control) and autophagy in the liver. Our deeper understanding of TH action recently has led to the development of tissue- and THR isoform-specific TH mimetics that may be useful for the treatment of metabolic disorders.

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.

Both 3,5-Diiodo-L-Thyronine and 3,5,3'-Triiodo-L-Thyronine Prevent Short-term Hepatic Lipid Accumulation via Distinct Mechanisms in Rats Being Fed a High-Fat Diet

3,3′,5-triiodo-L-thyronine (T3) improves hepatic lipid accumulation by increasing lipid catabolism but it also increases lipogenesis, which at first glance appears contradictory. Recent studies have shown that 3,5-diiodothyronine (T2), a natural thyroid hormone derivative, also has the capacity to stimulate hepatic lipid catabolism, however, little is known about its possible effects on lipogenic gene expression. Because genes classically involved in hepatic lipogenesis such as SPOT14, acetyl-CoA-carboxylase (ACC), and fatty acid synthase (FAS) contain thyroid hormone response elements (TREs), we studied their transcriptional regulation, focusing on TRE-mediated effects of T3 compared to T2 in rats receiving high-fat diet (HFD) for 1 week. HFD rats showed a marked lipid accumulation in the liver, which was significantly reduced upon simultaneous administration of either T3 or T2 with the diet. When administered to HFD rats, T2, in contrast with T3, markedly downregulated the expression of the above-mentioned genes. T2 downregulated expression of the transcription factors carbohydrate-response element-binding protein (ChREBP) and sterol regulatory element binding protein-1c (SREBP-1c) involved in activation of transcription of these genes, which explains the suppressed expression of their target genes involved in lipogenesis. T3, however, did not repress expression of the TRE-containing ChREBP gene but repressed SREBP-1c expression. Despite suppression of SREBP-1c expression by T3 (which can be explained by the presence of nTRE in its promoter), the target genes were not suppressed, but normalized to HFD reference levels or even upregulated (ACC), partly due to the presence of TREs on the promoters of these genes and partly to the lack of suppression of ChREBP. Thus, T2 and T3 probably act by different molecular mechanisms to achieve inhibition of hepatic lipid accumulation.

Action of Thyroid Hormones, T3 and T2, on Hepatic Fatty Acids: Differences in Metabolic Effects and Molecular Mechanisms

The thyroid hormones (THs) 3,3′,5,5′-tetraiodo-l-thyronine (T4) and 3,5,3′-triiodo-l-thyronine (T3) influence many metabolic pathways. The major physiological function of THs is to sustain basal energy expenditure, by acting primarily on carbohydrate and lipid catabolism. Beyond the mobilization and degradation of lipids, at the hepatic level THs stimulate the de novo fatty acid synthesis (de novo lipogenesis, DNL), through both the modulation of gene expression and the rapid activation of cell signalling pathways. 3,5-Diiodo-l-thyronine (T2), previously considered only a T3 catabolite, has been shown to mimic some of T3 effects on lipid catabolism. However, T2 action is more rapid than that of T3, and seems to be independent of protein synthesis. An inhibitory effect on DNL has been documented for T2. Here, we give an overview of the mechanisms of THs action on liver fatty acid metabolism, focusing on the different effects exerted by T2 and T3 on the regulation of the DNL. The inhibitory action on DNL exerted by T2 makes this compound a potential and attractive drug for the treatment of some metabolic diseases and cancer.

Regulation of hepatic lipogenesis by the zinc finger protein Zbtb20

Hepatic de novo lipogenesis (DNL) converts carbohydrates into triglycerides and is known to influence systemic lipid homoeostasis. Here, we demonstrate that the zinc finger protein Zbtb20 is required for DNL. Mice lacking Zbtb20 in the liver exhibit hypolipidemia and reduced levels of liver triglycerides, along with impaired hepatic lipogenesis. The expression of genes involved in glycolysis and DNL, including that of two ChREBP isoforms, is decreased in livers of knockout mice. Zbtb20 binds to and enhances the activity of the ChREBP-α promoter, suggesting that altered metabolic gene expression is mainly driven by ChREBP. In addition, ChREBP-β overexpression largely restores hepatic expression of genes involved in glucose and lipid metabolism, and increases plasma and liver triglyceride levels in knockout mice. Finally, we show that Zbtb20 ablation protects from diet-induced liver steatosis and improves hepatic insulin resistance. We suggest ZBTB20 is an essential regulator of hepatic lipogenesis and may be a therapeutic target for the treatment of fatty liver disease.

De novo lipogenesis is tightly controlled by hormonal and nutritional signals and plays an important role in energy homoeostasis. Here, Liu et al. show that zinc finger protein ZBTB20 regulates the expression of key glycolytic and lipogenic genes by modulating ChREBP expression and transcriptional activity.

Identification of HNF-4α as a key transcription factor to promote ChREBP expression in response to glucose

Transcription factor carbohydrate responsive element binding protein (ChREBP) promotes glycolysis and lipogenesis in metabolic tissues and cancer cells. ChREBP-α and ChREBP-β, two isoforms of ChREBP transcribed from different promoters, are both transcriptionally induced by glucose. However, the mechanism by which glucose increases ChREBP mRNA levels remains unclear. Here we report that hepatocyte nuclear factor 4 alpha (HNF-4α) is a key transcription factor for glucose-induced ChREBP-α and ChREBP-β expression. Ectopic HNF-4α expression increased ChREBP transcription while knockdown of HNF-4α greatly reduced ChREBP mRNA levels in liver cancer cells and mouse primary hepatocytes. HNF-4α not only directly bound to an E-box-containing region in intron 12 of the ChREBP gene, but also promoted ChREBP-β transcription by directly binding to two DR1 sites and one E-box-containing site of the ChREBP-β promoter. Moreover, HNF-4α interacted with ChREBP-α and synergistically promoted ChREBP-β transcription. Functionally, HNF-4α suppression reduced glucose-dependent ChREBP induction. Increased nuclear abundance of HNF-4α and its binding to cis-elements of ChREBP gene in response to glucose contributed to glucose-responsive ChREBP transcription. Taken together, our results not only revealed the novel mechanism by which HNF-4α promoted ChREBP transcription in response to glucose, but also demonstrated that ChREBP-α and HNF-4α synergistically increased ChREBP-β transcription.

Deiodinase knockdown during early zebrafish development affects growth, development, energy metabolism, motility and phototransduction

Thyroid hormone (TH) balance is essential for vertebrate development. Deiodinase type 1 (D1) and type 2 (D2) increase and deiodinase type 3 (D3) decreases local intracellular levels of T3, the most important active TH. The role of deiodinase-mediated TH effects in early vertebrate development is only partially understood. Therefore, we investigated the role of deiodinases during early development of zebrafish until 96 hours post fertilization at the level of the transcriptome (microarray), biochemistry, morphology and physiology using morpholino (MO) knockdown. Knockdown of D1+D2 (D1D2MO) and knockdown of D3 (D3MO) both resulted in transcriptional regulation of energy metabolism and (muscle) development in abdomen and tail, together with reduced growth, impaired swim bladder inflation, reduced protein content and reduced motility. The reduced growth and impaired swim bladder inflation in D1D2MO could be due to lower levels of T3 which is known to drive growth and development. The pronounced upregulation of a large number of transcripts coding for key proteins in ATP-producing pathways in D1D2MO could reflect a compensatory response to a decreased metabolic rate, also typically linked to hypothyroidism. Compared to D1D2MO, the effects were more pronounced or more frequent in D3MO, in which hyperthyroidism is expected. More specifically, increased heart rate, delayed hatching and increased carbohydrate content were observed only in D3MO. An increase of the metabolic rate, a decrease of the metabolic efficiency and a stimulation of gluconeogenesis using amino acids as substrates may have been involved in the observed reduced protein content, growth and motility in D3MO larvae. Furthermore, expression of transcripts involved in purine metabolism coupled to vision was decreased in both knockdown conditions, suggesting that both may impair vision. This study provides new insights, not only into the role of deiodinases, but also into the importance of a correct TH balance during vertebrate embryonic development.

Thyroid hormone regulation of hepatic lipid and carbohydrate metabolism

Thyroid hormone (TH) has important roles in regulating hepatic lipid, cholesterol, and glucose metabolism. Recent findings suggest that clinical conditions such as non-alcoholic fatty liver disease and type 2 diabetes mellitus, which are associated with dysregulated hepatic metabolism, may involve altered intracellular TH action. In addition, TH has key roles in lipophagy in lipid metabolism, mitochondrial quality control, and the regulation of metabolic genes. In this review, we discuss recent findings regarding the functions of TH in hepatic metabolism, the relationship between TH and metabolic disorders, and the potential therapeutic use of thyromimetics to treat metabolic dysfunction in the liver.

Liver X receptor regulation of thyrotropin-releasing hormone transcription in mouse hypothalamus is dependent on thyroid status

Reversing the escalating rate of obesity requires increased knowledge of the molecular mechanisms controlling energy balance. Liver X receptors (LXRs) and thyroid hormone receptors (TRs) are key physiological regulators of energetic metabolism. Analysing interactions between these receptors in the periphery has led to a better understanding of the mechanisms involved in metabolic diseases. However, no data is available on such interactions in the brain. We tested the hypothesis that hypothalamic LXR/TR interactions could co-regulate signalling pathways involved in the central regulation of metabolism. Using in vivo gene transfer we show that LXR activation by its synthetic agonist GW3965 represses the transcriptional activity of two key metabolic genes, Thyrotropin-releasing hormone (Trh) and Melanocortin receptor type 4 (Mc4r) in the hypothalamus of euthyroid mice. Interestingly, this repression did not occur in hypothyroid mice but was restored in the case of Trh by thyroid hormone (TH) treatment, highlighting the role of the triiodothyronine (T₃) and TRs in this dialogue. Using shLXR to knock-down LXRs in vivo in euthyroid newborn mice, not only abrogated Trh repression but actually increased Trh transcription, revealing a potential inhibitory effect of LXR on the Hypothalamic-Pituitary-Thyroid axis. In vivo chromatin immunoprecipitation (ChIP) revealed LXR to be present on the Trh promoter region in the presence of T₃ and that Retinoid X Receptor (RXR), a heterodimerization partner for both TR and LXR, was never recruited simultaneously with LXR. Interactions between the TR and LXR pathways were confirmed by qPCR experiments. T₃ treatment of newborn mice induced hypothalamic expression of certain key LXR target genes implicated in metabolism and inflammation. Taken together the results indicate that the crosstalk between LXR and TR signalling in the hypothalamus centres on metabolic and inflammatory pathways.

Transcription factor networks regulating hepatic fatty acid metabolism

Tight regulation of lipid levels is critical for cellular and organismal homeostasis, not only in terms of energy utilization and storage, but also to prevent potential toxicity. The liver utilizes a set of hepatic transcription factors to regulate the expression of genes implicated in all aspects of lipid metabolism including catabolism, transport, and synthesis. In this article, we will review the main transcriptional mechanisms regulating the expression of genes involved in hepatic lipid metabolism. The principal regulatory pathways are composed of simple modules of transcription factor crosstalks, which correspond to building blocks of more complex regulatory networks. These transcriptional networks contribute to the regulation of proper lipid homeostasis in parallel to posttranslational mechanisms and end product-mediated modulation of lipid metabolizing enzymes. This article is part of a Special Issue entitled Linking transcription to physiology in lipodomics.

Role of ChREBP and its target gene PNPLA3 in pathogenesis of nonalcoholic fatty liver disease

Nonalcoholic fatty liver disease (NAFLD) is a clinical and pathological syndrome caused by excessive triglyceride accumulation in liver cells. Its hepatic histological changes are similar to those of alcoholic liver disease (ALD), but the patients do not have a history of heavy alcohol drinking. NAFLD includes nonalcoholic simple fatty liver (NASFL), nonalcoholic steatohepatitis (NASH) and nonalcoholic fatty hepatitis related to liver fibrosis and cirrhosis. At present, the detailed pathogenesis of NAFLD is unclear. Many studies have reported the relationship between ChREBP and its target gene PNPLA3 and NAFLD occurrence. ChREBP and PNPLA3 are associated with liver fat content and inflammation. This article reviews the role of ChREBP and its target gene PNPLA3 in the pathogenesis of NAFLD, in order to provide a theoretical basis for further research of the occurrence and treatment of NAFLD.

Identification of thyroid hormone receptor binding sites in developing mouse cerebellum

Background

Thyroid hormones play an essential role in early vertebrate development as well as other key processes. One of its modes of action is to bind to the thyroid hormone receptor (TR) which, in turn, binds to thyroid response elements (TREs) in promoter regions of target genes. The sequence motif for TREs remains largely undefined as does the precise chromosomal location of the TR binding sites. A chromatin immunoprecipitation on microarray (ChIP-chip) experiment was conducted using mouse cerebellum post natal day (PND) 4 and PND15 for the thyroid hormone receptor (TR) beta 1 to map its binding sites on over 5000 gene promoter regions. We have performed a detailed computational analysis of these data.

Results

By analysing a recent spike-in study, the optimal normalization and peak identification approaches were determined for our dataset. Application of these techniques led to the identification of 211 ChIP-chip peaks enriched for TR binding in cerebellum samples. ChIP-PCR validation of 25 peaks led to the identification of 16 true positive TREs. Following a detailed literature review to identify all known mouse TREs, a position weight matrix (PWM) was created representing the classic TRE sequence motif. Various classes of promoter regions were investigated for the presence of this PWM, including permuted sequences, randomly selected promoter sequences, and genes known to be regulated by TH. We found that while the occurrence of the TRE motif is strongly correlated with gene regulation by TH for some genes, other TH-regulated genes do not exhibit an increased density of TRE half-site motifs. Furthermore, we demonstrate that an increase in the rate of occurrence of the half-site motifs does not always indicate the specific location of the TRE within the promoter region. To account for the fact that TR often operates as a dimer, we introduce a novel dual-threshold PWM scanning approach for identifying TREs with a true positive rate of 0.73 and a false positive rate of 0.2. Application of this approach to ChIP-chip peak regions revealed the presence of 85 putative TREs suitable for further in vitro validation.

Conclusions

This study further elucidates TRβ gene regulation in mouse cerebellum, with 211 promoter regions identified to bind to TR. While we have identified 85 putative TREs within these regions, future work will study other mechanisms of action that may mediate the remaining observed TR-binding activity.

Crosstalk between thyroid hormone receptor and liver X receptor in the regulation of selective Alzheimer's disease indicator-1 gene expression

Selective Alzheimer’s disease (AD) indicator 1 (Seladin-1) has been identified as a gene down-regulated in the degenerated lesions of AD brain. Up-regulation of Seladin-1 reduces the accumulation of β-amyloid and neuronal death. Thyroid hormone (TH) exerts an important effect on the development and maintenance of central nervous systems. In the current study, we demonstrated that Seladin-1 gene and protein expression in the forebrain was increased in thyrotoxic mice compared with that of euthyroid mice. However, unexpectedly, no significant decrease in the gene and protein expression was observed in hypothyroid mice. Interestingly, an agonist of liver X receptor (LXR), TO901317 (TO) administration in vivo increased Seladin-1 gene and protein expression in the mouse forebrain only in a hypothyroid state and in the presence of mutant TR-β, suggesting that LXR-α would compensate for TR-β function to maintain Seladin-1 gene expression in hypothyroidism and resistance to TH. TH activated the mouse Seladin-1 gene promoter (−1936/+21 bp) and site 2 including canonical TH response element (TRE) half-site in the region between −159 and −154 bp is responsible for the positive regulation. RXR-α/TR-β heterodimerization was identified on site 2 by gel-shift assay, and chromatin immunoprecipitation assay revealed the recruitment of TR-β to site 2 and the recruitment was increased upon TH administration. On the other hand, LXR-α utilizes a distinct region from site 2 (−120 to −102 bp) to activate the mouse Seladin-1 gene promoter. Taking these findings together, we concluded that TH up-regulates Seladin-1 gene expression at the transcriptional level and LXR-α maintains the gene expression.

Recent progress on the role of ChREBP in glucose and lipid metabolism

Carbohydrate response element binding protein (ChREBP) is a transcription factor activated by glucose that is highly expressed in liver, pancreatic β-cells, brown and white adipose tissues, and muscle. We reported that hepatic suppression of the Chrebp gene improves hepatic steatosis, glucose intolerance, and obesity in genetically obese mice. Moreover, we have studied the role of ChREBP with special reference to feedforward and feedback looping in liver and pancreatic β-cells. Recently, several groups reported that (1) glucose activates ChREBP-α transactivity and in turn ChREBP-α induces ChREBP-β on both transcriptional and translational levels in adipose tissues, and (2) ChREBP regulates glucose transporter type 4 mRNA levels, which may affect glucose uptake in adipose tissues. Moreover, in adipose tissues of obese patients, Chrebpb mRNA levels were much lower than those in lean subjects, while the levels were much higher in liver of obese patients than those in lean subjects. These findings suggest that Chrebpb mRNA levels are different in various tissues and probably in the stages of diabetes mellitus. Herein, we review recent progress in the study of ChREBP with special references to (1) the mechanisms regulating ChREBP transactivity (posttranslational modifications, intramolecular glucose sensing module, feedforward mechanism, and the feedback loop between ChREBP and its target genes), and (2) the role of ChREBP in liver, pancreatic islets and adipose tissues. Understanding the role of ChREBP in each tissue will provide important insight into the pathogenesis of metabolic syndrome.

Human stearoyl-CoA desaturase 1 (SCD-1) gene expression is negatively regulated by thyroid hormone without direct binding of thyroid hormone receptor to the gene promoter

Stearoyl-CoA desaturase-1 (SCD-1) plays a pivotal role in an increase of triglyceride by an excess of dietary carbohydrate intake. Dietary carbohydrates increase SCD-1 gene expression in liver by sterol response element binding protein (SREBP)-1c-dependent and SREBP-1c -independent pathways. Previous report demonstrated that thyroid hormone (TH) negatively regulates mouse SCD-1 gene promoter before SREBP-1c was revealed. We reported that TH negatively regulates SREBP-1c recently. Therefore, in the current study, we examined whether and how TH regulates human SCD-1 gene expression and evaluated SREBP-1c effect on the negative regulation. Luciferase assays revealed that TH suppresses both mouse and human SCD-1 gene promoter activity. In SREBP-1 knockdown HepG2 cells, TH still suppresses SCD-1 gene promoter activity, and it also exerted the negative regulation under cotransfection of a small amount of SREBP-1c. These data indicated that SREBP-1c does not play the decisive role for the negative regulation by TH. The responsible region for the negative regulation in human SCD-1 gene promoter turned out to be between -124 and -92 bp, referred to as site A. Chromatin immunoprecipitation assays demonstrated that TH receptor-β is recruited to the region upon T(3) administration, although TR-β does not bind directly to site A. In conclusion, TH negatively regulates human SCD-1 gene expression in without direct binding of the TH receptor to the SCD-1 gene promoter.

Liver X receptor α is involved in the transcriptional regulation of the 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase gene

The activity of 6-phosphofructo-1-kinase is strictly controlled by fructose-2,6-bisphosphate, the level of which is regulated by another enzyme, 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase (PFK2/FBP2). PFK2/FBP2 is a bifunctional enzyme, having kinase and phosphatase activities, and regulates both glycolysis and gluconeogenesis. Here, we examined the hormonal regulation of the PFK2/FBP2 gene in vitro using the reporter assay, the electromobility shift assay (EMSA), and the chromatin immunoprecipitation (ChIP) assay in HuH7 cells and also using the mouse liver in vivo. We found that the transcriptional activity of the PFK2/FBP2 gene was stimulated by insulin and inhibited by cAMP and glucocorticoid. Liver X receptor (LXR) α showed a potent and specific stimulatory effect on PFK2/FBP2 gene transcription. Deletion and mutagenesis analyses identified the LXR response element (LXRE) in the 5'-promoter region of the PFK2/FBP2 gene. Binding of LXRα was confirmed by the EMSA and ChIP assay. Endogenous PFK2/FBP2 mRNA in the mouse liver was increased in the fasting/refeeding state compared with the fasting state. Altogether, PFK2/FBP2 gene transcription is found to be regulated in a way that is more similar to other glycolytic enzyme genes than to gluconeogenic genes. Furthermore, our data strongly suggest that LXRα is one of the key regulators of PFK2/FBP2 gene transcription.

Glucose sensing by ChREBP/MondoA-Mlx transcription factors

The paralogous transcription factors ChREBP and MondoA, together with their common binding partner Mlx, have emerged as key mediators of intracellular glucose sensing. By regulating target genes involved in glycolysis and lipogenesis, they mediate metabolic adaptation to changing glucose levels. As disturbed glucose homeostasis plays a central role in human metabolic diseases and as cancer cells often display altered glucose metabolism, better understanding of cellular glucose sensing will likely uncover new therapeutic opportunities. Here we review the regulation, function and evolutionary conservation of the ChREBP/MondoA-Mlx glucose sensing system and discuss possible directions for future research.

The effect of thyroid disorders on lipid levels and metabolism

Thyroid hormones regulate cholesterol and lipoprotein metabolism, whereas thyroid disorders, including overt and subclinical hypothyroidism, considerably alter lipid profile and promote cardiovascular disease. Good evidence shows that high thyroid-stimulating hormone (TSH) is associated with a nonfavorable lipid profile, although TSH has no cutoff threshold for its association with lipids. Thyromimetics represent a new class of hypolipidemic drugs: their imminent application in patients with severe dyslipidemias, combined or not with statins, will improve the lipid profile, potentially accelerate energy expenditure and, as a consequence, vitally lessen the risk of cardiovascular disease.

Free Triiodothyronine and Cholesterol Levels in Euthyroid Elderly T2DM Patients

Thyroid function regulates lipid metabolism. Despite the fact that T2DM is more prevalent in the elderly, often associates with thyroid dysfunction and increases cardiovascular risk both per se and via high TC and LDL-C levels, the association of the latter with FT(3) and FT(4) levels has not yet been fully investigated in T2DM. While trying to fill this gap in 296 elderly outpatients with T2DM, we found that TC and LDL-C correlated negatively with FT(4) and positively with FT(3). When divided according to treatment by oral hypoglycaemic agents (OHA) and insulin (IT), they reacted differently with respect to investigated associations: in the OHA's TC and LDL-C correlated negatively with FT(4) and showed no association with FT(3), whereas, in the IT's TC and LDL-C correlated positively with FT(3) and negatively with FT(4). When controlled for possible confounding factors, these associations did not change in the IT's but were missing in the OHA's. Recent literature reports upon complex hypothalamic and peripheral interactions between T2DM and thyroid, and suggests T(3) to enhance cholesterol synthesis and to have a role in insulin resistance states. Further investigations are needed to understand the intimate mechanisms of lipid metabolism in T2DM with respect to thyroid function.

Thyroid hormone-regulated gene expression in juvenile mouse liver: identification of thyroid response elements using microarray profiling and in silico analyses

Background

Disruption of thyroid hormone signalling can alter growth, development and energy metabolism. Thyroid hormones exert their effects through interactions with thyroid receptors that directly bind thyroid response elements and can alter transcriptional activity of target genes. The effects of short-term thyroid hormone perturbation on hepatic mRNA transcription in juvenile mice were evaluated, with the goal of identifying genes containing active thyroid response elements. Thyroid hormone disruption was induced from postnatal day 12 to 15 by adding goitrogens to dams' drinking water (hypothyroid). A subgroup of thyroid hormone-disrupted pups received intraperitoneal injections of replacement thyroid hormones four hours prior to sacrifice (replacement). An additional group received only thyroid hormones four hours prior to sacrifice (hyperthyroid). Hepatic mRNA was extracted and hybridized to Agilent mouse microarrays.

Results

Transcriptional profiling enabled the identification of 28 genes that appeared to be under direct thyroid hormone-regulation. The regulatory regions of the genome adjacent to these genes were examined for half-site sequences that resemble known thyroid response elements. A bioinformatics search identified 33 thyroid response elements in the promoter regions of 13 different genes thought to be directly regulated by thyroid hormones. Thyroid response elements found in the promoter regions of Tor1a, 2310003H01Rik, Hect3d and Slc25a45 were further validated by confirming that the thyroid receptor is associated with these sequences in vivo and that it can bind directly to these sequences in vitro. Three different arrangements of thyroid response elements were identified. Some of these thyroid response elements were located far up-stream (> 7 kb) of the transcription start site of the regulated gene.

Conclusions

Transcriptional profiling of thyroid hormone disrupted animals coupled with a novel bioinformatics search revealed new thyroid response elements associated with genes previously unknown to be responsive to thyroid hormone. The work provides insight into thyroid response element sequence motif characteristics.

Thyroid hormone action in metabolic regulation

Thyroid hormone plays pivotal roles in growth, differentiation, development and metabolic homeostasis via thyroid hormone receptors (TRs) by controlling the expression of TR target genes. The transcriptional activity of TRs is modulated by multiple factors including various TR isoforms, diverse thyroid hormone response elements, different heterodimeric partners, coregulators, and the cellular location of TRs. In the present review, we summarize recent advance in understanding the molecular mechanisms of thyroid hormone action obtained from human subject research, thyroid hormone mimetics application, TR isoform-specific knock-in mouse models, and mitochondrion study with highlights in metabolic regulations. Finally, as future perspectives, we share our thoughts about current challenges and possible approaches to promote our knowledge of thyroid hormone action in metabolism.

Cross-regulation of hepatic glucose metabolism via ChREBP and nuclear receptors

There is a worldwide epidemic of obesity and type 2 diabetes, two major public health concerns associated with alterations in both insulin and glucose signaling pathways. Glucose is not only an energy source but also controls the expression of key genes involved in energetic metabolism, through the glucose-signaling transcription factor, Carbohydrate Responsive Element Binding Protein (ChREBP). ChREBP has emerged as a central regulator of de novo fatty acid synthesis (lipogenesis) in response to glucose under both physiological and physiopathological conditions. Glucose activates ChREBP by regulating its entry from the cytosol to the nucleus, thereby promoting its binding to carbohydrate responsive element (ChoRE) in the promoter regions of glycolytic (L-PK) and lipogenic genes (ACC and FAS). We have previously reported that the inhibition of ChREBP in liver of obese ob/ob mice improves the metabolic alterations linked to obesity, fatty liver and insulin-resistance. Therefore, regulating ChREBP activity could be an attractive target for lipid-lowering therapies in obesity and diabetes. However, before this is possible, a better understanding of the mechanism(s) regulating its activity is needed. In this review, we summarize recent findings on the role and regulation of ChREBP and particularly emphasize on the cross-regulations that may exist between key nuclear receptors (LXR, TR, HNF4α) and ChREBP for the control of hepatic glucose metabolism. These novel molecular cross-talks may open the way to new pharmacological opportunities. This article is part of a Special Issue entitled: Translating nuclear receptors from health to disease.

Crosstalk of thyroid hormone receptor and liver X receptor in lipid metabolism and beyond [Review]

Thyroid hormone receptors (TRs) and liver X receptors (LXRs) are members of the nuclear receptor superfamily. Although LXRs and TRs belong to two distinct receptor subgroups with respect to ligand-binding affinity, the two receptor systems show similarity with respect to molecular mechanism, target genes, and physiological roles. Since both TRs and LXRs play an important role in metabolic regulation, form heterodimers with retinoid X receptors (RXRs), and bind to direct repeat-4 (DR-4) with identical geometry and polarity, crosstalk between these two receptors has been reported, especially on lipid metabolism-related genes. Recently, several types of crosstalk between TRs and LXRs have been identified and crosstalk has also been observed in other physiological systems such as central nervous system rather than lipid metabolism. In this review, recent advances in elucidating the molecular mechanisms of the crosstalk between these two nuclear receptors are discussed, with the aim of finding a perspective on unknown roles of TRs and LXRs.

Thyroid hormone receptor beta (TRbeta) and liver X receptor (LXR) regulate carbohydrate-response element-binding protein (ChREBP) expression in a tissue-selective manner

Thyroid hormone (TR) and liver X (LXR) receptors are transcription factors involved in lipogenesis. Both receptors recognize the same consensus DNA-response element in vitro. It was previously shown that their signaling pathways interact in the control of cholesterol elimination in the liver. In the present study, carbohydrate-response element-binding protein (ChREBP), a major transcription factor controlling the activation of glucose-induced lipogenesis in liver, is characterized as a direct target of thyroid hormones (TH) in liver and white adipose tissue (WAT), the two main lipogenic tissues in mice. Using genetic and molecular approaches, ChREBP is shown to be specifically regulated by TRbeta but not by TRalpha in vivo, even in WAT where both TR isoforms are expressed. However, this isotype specificity is not found in vitro. This TRbeta specific regulation correlates with the loss of TH-induced lipogenesis in TRbeta(-/-) mice. Fasting/refeeding experiments show that TRbeta is not required for the activation of ChREBP expression particularly marked in WAT following refeeding. However, TH can stimulate ChREBP expression in WAT even under fasting conditions, suggesting completely independent pathways. Because ChREBP has been described as an LXR target, the interaction of LXR and TRbeta in ChREBP regulation was assayed both in vitro and in vivo. Each receptor recognizes a different response element on the ChREBP promoter, located only 8 bp apart. There is a cross-talk between LXR and TRbeta signaling on the ChREBP promoter in liver but not in WAT where LXR does not regulate ChREBP expression. The molecular basis for this cross-talk has been determined in in vitro systems.

Lipogenesis in arterial wall and vascular smooth muscle cells of Psammomys obesus: its regulation and abnormalities in diabetes

AIM

Lipogenesis is expressed in vascular smooth muscle cells (VSMCs), and such in situ lipogenesis could be providing the fatty acids for triglyceride synthesis and cholesterol esterification, and contributing to lipid accumulation in the arterial wall. This study investigated both the expression and regulation of lipogenesis in VSMCs to determine if they are modified in Psammomys obesus gerbils fed a high-fat diet as a model of insulin resistance and diabetes.

METHODS

Aortas were collected from diabetic and non-diabetic P. obesus for histological examination, measurement of lipogenic gene expression and VSMC culture.

RESULTS

The aortas of diabetic animals exhibited lipid deposits and foam cells as well as disorganization of elastic fibres. However, lipogenic gene expression was not modified. VSMCs in vitro from the aortas of diabetic animals had, compared with cells from non-diabetic animals, lower mRNA levels of SREBP-1c and ChREBP. An adipogenic medium stimulated moderate FAS and ACC1 expression in cells from both diabetic and non-diabetic animals, but glucose and insulin on their own had no such stimulatory action. Also, triiodothyronine (T3) had a clear stimulatory action, while angiotensin II had a moderate effect, in cells from non-diabetic P. obesus, but not from diabetic animals, whereas LXR agonists stimulated lipogenesis in cells from both animal groups.

CONCLUSION

Lipogenesis is expressed in the arterial walls and VSMCs of P. obesus. However, its expression was not increased in diabetes, and did not respond to either T3 or angiotensin II. Therefore, lipogenesis in situ is unlikely to contribute to the accumulation of lipids in the arterial walls of diabetic P. obesus gerbils.

Thyroid hormone crosstalk with nuclear receptor signaling in metabolic regulation

Thyroid hormone influences diverse metabolic pathways important in lipid and glucose metabolism, lipolysis and regulation of body weight. Recently, it has been recognized that thyroid hormone receptor interacts with transcription factors that predominantly respond to nutrient signals including the peroxisome proliferator-activated receptors, liver X receptor and others. Crosstalk between thyroid hormone signaling and these nutrient responsive factors occurs through a variety of mechanisms: competition for retinoid X receptor heterodimer partners, DNA binding sites and transcriptional cofactors. This review focuses on the mechanisms of interaction of thyroid hormone signaling with other metabolic pathways and the importance of understanding these interactions to develop therapeutic agents for treatment of metabolic disorders, such as dyslipidemias, obesity and diabetes.

Endotoxin, zymosan, and cytokines decrease the expression of the transcription factor, carbohydrate response element binding protein, and its target genes

Carbohydrate response element binding protein (ChREBP) is a recently discovered transcription factor whose levels and activity are increased by glucose leading to the activation of target genes, which include acetyl-CoA carboxylase, fatty acid synthase, and liver-type pyruvate kinase. Here, we demonstrate that lipopolysaccharide (LPS) treatment causes a marked decrease in ChREBP mRNA and protein levels in the liver of mice fed a normal chow diet or in mice fasted for 24 h and then re-fed a high carbohydrate diet. This decrease occurs rapidly and is a sensitive response (half-maximal dose 0.1 μg/mouse). The decrease in ChREBP is accompanied by a decrease in the expression of ChREBP target genes. Zymosan and turpentine treatment also decrease hepatic ChREBP levels and the expression of its target genes. Additionally, tumor necrosis factor alpha (TNF-α) and interleukin-1 beta (IL-1β) decrease liver ChREBP expression both in vivo and in Hep3B cells in culture. Finally, LPS decreased ChREBP expression in muscle and adipose tissue. These studies demonstrate that ChREBP is down-regulated during the acute phase response resulting in alterations in the expression of ChREBP regulated target genes. Thus, ChREBP joins a growing list of transcription factors that are regulated during the acute phase response.