Glucocorticoid receptor mediates the gluconeogenic activity of the farnesoid X receptor in the fasting condition

Physical exercise plays a role in rebalancing the bile acids of enterohepatic axis in non-alcoholic fatty liver disease

Non-alcoholic fatty liver disease (NAFLD) is considered as one of the most common diseases of lipid metabolism disorders, which is closely related to bile acids disorders and gut microbiota disorders. Bile acids are synthesized from cholesterol in the liver, and processed by gut microbiota in intestinal tract, and participate in metabolic regulation through the enterohepatic circulation. Bile acids not only promote the consumption and absorption of intestinal fat but also play an important role in biological metabolic signaling network, affecting fat metabolism and glucose metabolism. Studies have demonstrated that exercise plays an important role in regulating the composition and function of bile acid pool in enterohepatic axis, which maintains the homeostasis of the enterohepatic circulation and the health of the host gut microbiota. Exercise has been recommended by several health guidelines as the first-line intervention for patients with NAFLD. Can exercise alter bile acids through the microbiota in the enterohepatic axis? If so, regulating bile acids through exercise may be a promising treatment strategy for NAFLD. However, the specific mechanisms underlying this potential connection are largely unknown. Therefore, in this review, we tried to review the relationship among NAFLD, physical exercise, bile acids, and gut microbiota through the existing data and literature, highlighting the role of physical exercise in rebalancing bile acid and microbial dysbiosis.

Hepatic farnesoid X receptor is necessary to facilitate ductular reaction and expression of heme biosynthetic genes

BACKGROUND

Bile, which contains bile acids, the natural ligands for farnesoid x receptor (FXR), moves from the liver to the intestine through bile ducts. Ductular reaction often occurs during biliary obstruction. A subset of patients with erythropoietic protoporphyria, an inherited genetic mutation in heme biosynthetic enzyme ferrochelatase, accumulate porphyrin-containing bile plugs, leading to cholestasis. Here, we examined the link between FXR, bile plug formation, and how heme biosynthesis relates to this connection.

METHODS

We treated female and male wild-type and global and tissue-specific Fxr knockout mice with a diet containing 3,5-diethoxycarbonyl-1,4-dihydrocollidine, an inhibitor of ferrochelatase, and examined the expression of heme biosynthetic genes. We mined FXR mouse ChIP-Seq data, performed biochemical and histological analysis, and tested HepG2 and primary human hepatocytes after treatment with obeticholic acid, an FXR agonist.

RESULTS

We observed that hepatic but not intestinal Fxr loss resulted in reduced bile plugs and ductular reaction in the liver. Then, we examined if FXR plays a regulatory role in heme biosynthesis and found significantly lower porphyrin accumulation in 3,5-diethoxycarbonyl-1, 4-dihydrocollidine-fed Fxr knockout mice. Gene expression and FXR mouse ChIP-Seq atlas analysis revealed that FXR orchestrates the expression of multiple heme biosynthetic enzymes. Finally, human HepG2 cells and primary human hepatocytes treated with obeticholic acid, showed increased expression of several heme biosynthetic genes.

CONCLUSIONS

Overall, our data show that hepatic Fxr is necessary to maintain ductular reaction and accumulation of bile plugs. FXR can direct the expression of multiple heme biosynthetic genes. Thus, modulating FXR activity in EPP patients may help alleviate its associated liver disease.

Circulating Bile Acids as Biomarkers for Disease Diagnosis and Prevention

CONTEXT

Bile acids (BAs) are pivotal signaling molecules that regulate energy metabolism and inflammation. Recent epidemiological studies have reported specific alterations in circulating BA profiles in certain disease states, including obesity, type 2 diabetes mellitus (T2DM), nonalcoholic fatty liver disease (NAFLD), and Alzheimer disease (AD). In the past decade, breakthroughs have been made regarding the translation of BA profiling into clinical use for disease prediction. In this review, we summarize and synthesize recent data on variation in circulating BA profiles in patients with various diseases to evaluate the value of these biomarkers in human plasma for early diagnosis.

EVIDENCE ACQUISITION

This review is based on a collection of primary and review literature gathered from a PubMed search for BAs, obesity, T2DM, insulin resistance (IR), NAFLD, hepatocellular carcinoma (HCC), cholangiocarcinoma (CCA), colon cancer, and AD, among other keywords.

EVIDENCE SYNTHESIS

Individuals with obesity, T2DM, HCC, CCA, or AD showed specific alterations in circulating BA profiles. These alterations may have existed long before the initial diagnosis of these diseases. The intricate relationship between obesity, IR, and NAFLD complicates the establishment of clear and independent associations between BA profiles and nonalcoholic steatohepatitis. Alterations in the levels of total BAs and several BA species were seen across the entire spectrum of NAFLD, demonstrating significant increases with the worsening of histological features.

CONCLUSIONS

Aberrant circulating BA profiles are an early event in the onset and progression of obesity, T2DM, HCC, and AD. The pleiotropic effects of BAs explain these broad connections. Circulating BA profiles could provide a basis for the development of biomarkers for the diagnosis and prevention of a wide range of diseases.

Recent advances on FXR-targeting therapeutics

The bile acid receptor FXR has emerged as a bona fide drug target for chronic cholestatic and metabolic liver diseases, ahead of all non-alcoholic fatty liver disease (NAFLD). FXR is highly expressed in the liver and intestine and activation at both sites differentially contributes to its desired metabolic effects. Unrestricted FXR activation, however, also comes along with undesired effects such as a pro-atherogenic lipid profile, pruritus and hepatocellular toxicity under certain conditions. Several pre-clinical studies have confirmed the potency of FXR activation for cholestatic and metabolic liver diseases, but overall it remains still open whether selective activation of intestinal FXR is advantageous over pan-FXR activation and whether restricted or modulated FXR activation can limit some of the side effects. Even more, FXR antagonist also bear the potential as intestinal-selective drugs in NAFLD models. In this review we will discuss the molecular prerequisites for FXR activation, pan-FXR activation and intestinal FXR in/activation from a therapeutic point of view, different steroidal and non-steroidal FXR agonists, ways to restrict FXR activation and finally what we have learned from pre-clinical models and clinical trials with different FXR therapeutics.

Physiological Role of Bile Acids Modified by the Gut Microbiome

Bile acids (BAs) are produced from cholesterol in the liver and are termed primary BAs. Primary BAs are conjugated with glycine and taurine in the liver and then released into the intestine via the gallbladder. After the deconjugation of glycine or taurine by the gut microbiome, primary BAs are converted into secondary BAs by the gut microbiome through modifications such as dehydroxylation, oxidation, and epimerization. Most BAs in the intestine are reabsorbed and transported to the liver, where both primary and secondary BAs are conjugated with glycine or taurine and rereleased into the intestine. Thus, unconjugated primary Bas, as well as conjugated and unconjugated secondary BAs, have been modified by the gut microbiome. Some of the BAs reabsorbed from the intestine spill into the systemic circulation, where they bind to a variety of nuclear and cell-surface receptors in tissues, whereas some of the BAs are not reabsorbed and bind to receptors in the terminal ileum. BAs play crucial roles in the physiological regulation of various tissues. Furthermore, various factors, such as diet, age, and antibiotics influence BA composition. Here, we review recent findings regarding the physiological roles of BAs modified by the gut microbiome in the metabolic, immune, and nervous systems.

Circulating Bile Acid Profiles: A Need for Further Examination

CONTEXT

Bile acids (BAs) are increasingly recognized as metabolic and chronobiologic integrators that synchronize the systemic metabolic response to nutrient availability. Alterations in the concentration and/or composition of circulating BAs are associated with a number of metabolic disorders, such as obesity, type 2 diabetes mellitus (T2DM), insulin resistance (IR), and metabolic associated fatty liver disease (MAFLD). This review summarizes recent evidence that links abnormal circulating BA profiles to multiple metabolic disorders, and discusses the possible mechanisms underlying the connections to determine the role of BA profiling as a novel biomarker for these abnormalities.

EVIDENCE ACQUISITION

The review is based on a collection of primary and review literature gathered from a PubMed search of BAs, T2DM, IR, and MAFLD, among other keywords.

EVIDENCE SYNTHESIS

Obese and IR subjects appear to have elevated fasting circulating BAs but lower postprandial increase when compared with controls. The possible underlying mechanisms are disruption in the synchronization between the feeding/fasting cycle and the properties of BA-regulated metabolic pathways. Whether BA alterations are associated per se with MAFLD remains inconclusive. However, increased fasting circulating BAs level was associated with higher risk of advanced fibrosis stage. Thus, for patients with MAFLD, dynamically monitoring the circulating BA profiles may be a promising tool for the stratification of MAFLD.

CONCLUSIONS

Alterations in the concentration, composition, and rhythm of circulating BAs are associated with adverse events in systemic metabolism. Subsequent investigations regarding these aspects of circulating BA kinetics may help predict future metabolic disorders and guide therapeutic interventions.

Synthesis of 12β-Methyl-18-nor-bile Acids

Decoupling the roles of the farnesoid X nuclear receptor and Takeda G-protein-coupled bile acid receptor 5 is essential for the development of novel bile acid therapeutics targeting metabolic and neurodegenerative diseases. Herein, we describe the synthesis of 12β-methyl-18-nor-bile acids which may serve as probes in the search for new bile acid analogues with clinical applicability. A Nametkin-type rearrangement was applied to protected cholic acid derivatives, giving rise to tetra-substituted Δ13,14- and Δ13,17-unsaturated 12β-methyl-18-nor-bile acid intermediates (24a and 25a). Subsequent catalytic hydrogenation and deprotection yielded 12β-methyl-18-nor-chenodeoxycholic acid (27a) and its 17-epi-epimer (28a) as the two major reaction products. Optimization of the synthetic sequence enabled a chromatography-free route to prepare these bile acids at a multi-gram scale. In addition, the first cis-C-D ring-junctured bile acid and a new 14(13 → 12)-abeo-bile acid are described. Furthermore, deuteration experiments were performed to provide mechanistic insights into the formation of the formal anti-hydrogenation product 12β-methyl-18-nor-chenodeoxycholic acid (27a).

FXR in liver physiology: Multiple faces to regulate liver metabolism

The liver is the central metabolic hub which coordinates nutritional inputs and metabolic outputs. Food intake releases bile acids which can be sensed by the bile acid receptor FXR in the liver and the intestine. Hepatic and intestinal FXR coordinately regulate postprandial nutrient disposal in a network of interacting metabolic nuclear receptors. In this review we summarize and update the "classical roles" of FXR as a central integrator of the feeding state response, which orchestrates the metabolic processing of carbohydrates, lipids, proteins and bile acids. We also discuss more recent and less well studied FXR effects on amino acid, protein metabolism, autophagic turnover and inflammation. In addition, we summarize the recent understanding of how FXR signaling is affected by posttranslational modifications and by different FXR isoforms. These modifications and variations in FXR signaling might be considered when FXR is targeted pharmaceutically in clinical applications.

Transcriptomic and Quantitative Proteomic Profiling Reveals Signaling Pathways Critical for Pancreatic Islet Maturation

Pancreatic β-cell dysfunction and reduced insulin secretion play a key role in the pathogenesis of diabetes. Fetal and neonatal islets are functionally immature and have blunted glucose responsiveness and decreased insulin secretion in response to stimuli and are far more proliferative. However, the mechanisms underlying functional immaturity are not well understood. Pancreatic islets are composed of a mixture of different cell types, and the microenvironment of islets and interactions between these cell types are critical for β-cell development and maturation. RNA sequencing and quantitative proteomic data from intact islets isolated from fetal (embryonic day 19) and 2-week-old Sprague-Dawley rats were integrated to compare their gene and protein expression profiles. Ingenuity Pathway Analysis (IPA) was also applied to elucidate pathways and upstream regulators modulating functional maturation of islets. By integrating transcriptome and proteomic data, 917 differentially expressed genes/proteins were identified with a false discovery rate of less than 0.05. A total of 411 and 506 of them were upregulated and downregulated in the 2-week-old islets, respectively. IPA revealed novel critical pathways associated with functional maturation of islets, such as AMPK (adenosine monophosphate-activated protein kinase) and aryl hydrocarbon receptor signaling, as well as the importance of lipid homeostasis/signaling and neuronal function. Furthermore, we also identified many proteins enriched either in fetal or 2-week-old islets related to extracellular matrix and cell communication, suggesting that these pathways play critical roles in islet maturation. Our present study identified novel pathways for mature islet function in addition to confirming previously reported mechanisms, and provided new mechanistic insights for future research on diabetes prevention and treatment.

Characterization of stress response involved in chicken myopathy

Myopathies (Woody Breast (WB) and White Striping (WS)) of broiler chickens have been correlated with fast growth. Recent studies reported that localized hypoxia and metabolic impairment may involve in these myopathies of birds. In order to better understand the stress response mechanisms affecting myopathies of broilers, the aim of this study was to examine effects of WB and both WB/WS on stress hormone corticosterone (CORT) levels and expressional changes of stress response genes including glucocorticoid (GC) receptor (GR), 11β-Hydroxysteroid dehydrogenase type 1 (11β-HSD1), DNA methylation regulators (DNMTs), and arginine vasotocin receptor 1a and 1b (V1aR, V1bR). Results of radioimmunoassay showed that CORT levels of WB and WB/WS birds were significantly higher compared to Con (p < 0.05), however, the combination of WB/WS was not significantly higher than WB birds, implying that the effects of WB and WS on CORT are not synergistic. Hepatic GR expression of both WB and WB/WS birds were significantly higher compared to Con (p < 0.05). However, GR expression levels in breast muscle of both WB and WB/WS birds were decreased compared to Con (p < 0.05). Hepatic 11β-HSD1 expression was increased only in WB/WS birds compared to Con birds with no significant difference between Con and WB birds. 11β-HSD1 expression was decreased and increased in WB and WB/WS birds compared to Con, respectively, in breast muscle (p < 0.05). DNMT1 expression was significantly decreased in both muscle and liver of WB birds, and in muscle of WB/WS birds, but not in liver of WB/WS birds, indicating differential effects of WS on the epigenetical stress response of muscle and liver compared to WB. V1aR expression was significantly increased in muscle of WB birds, and in liver of WB/WS birds compared to Con birds (p < 0.05). V1bR was not changed in muscle and liver of WB birds compared to Con birds. Taken together, results suggest that GC-induced myopathies occur in fast-growing broiler chickens and circulating CORT level might be a significant biochemical marker of myopathies (WB and WS) of birds. In addition, chronic stress responses in breast muscle and tissue-specific epigenetic changes of stress response genes by DNMTs may play a critical role in the occurrence of myopathies.

Molecular Physiology of Bile Acid Signaling in Health, Disease, and Aging

Over the past two decades, bile acids (BAs) have become established as important signaling molecules that enable fine-tuned inter-tissue communication from the liver, their site of production, over the intestine, where they are modified by the gut microbiota, to virtually any organ, where they exert their pleiotropic physiological effects. The chemical variety of BAs, to a large extent determined by the gut microbiome, also allows for a complex fine-tuning of adaptive responses in our body. This review provides an overview of the mechanisms by which BA receptors coordinate several aspects of physiology and highlights new therapeutic strategies for diseases underlying pathological BA signaling.

Metabolic dysregulation in the Atp7b-/- Wilson's disease mouse model

Inactivating mutations in the copper transporter Atp7b result in Wilson's disease. The Atp7b-/- mouse develops hallmarks of Wilson's disease. The activity of several nuclear receptors decreased in Atp7b-/- mice, and nuclear receptors are critical for maintaining metabolic homeostasis. Therefore, we anticipated that Atp7b-/- mice would exhibit altered progression of diet-induced obesity, fatty liver, and insulin resistance. Following 10 wk on a chow or Western-type diet (40% kcal fat), parameters of glucose and lipid homeostasis were measured. Hepatic metabolites were measured by liquid chromatography-mass spectrometry and correlated with transcriptomic data. Atp7b-/- mice fed a chow diet presented with blunted body-weight gain over time, had lower fat mass, and were more glucose tolerant than wild type (WT) littermate controls. On the Western diet, Atp7b-/- mice exhibited reduced body weight, adiposity, and hepatic steatosis compared with WT controls. Atp7b-/- mice fed either diet were more insulin sensitive than WT controls; however, fasted Atp7b-/- mice exhibited hypoglycemia after administration of insulin due to an impaired glucose counterregulatory response, as evidenced by reduced hepatic glucose production. Coupling gene expression with metabolomic analyses, we observed striking changes in hepatic metabolic profiles in Atp7b-/- mice, including increases in glycolytic intermediates and components of the tricarboxylic acid cycle. In addition, the active phosphorylated form of AMP kinase was significantly increased in Atp7b-/- mice relative to WT controls. Alterations in hepatic metabolic profiles and nuclear receptor signaling were associated with improved glucose tolerance and insulin sensitivity as well as with impaired fasting glucose production in Atp7b-/- mice.

AKR1D1 is a novel regulator of metabolic phenotype in human hepatocytes and is dysregulated in non-alcoholic fatty liver disease

OBJECTIVE

Non-alcoholic fatty liver disease (NAFLD) is the hepatic manifestation of metabolic syndrome. Steroid hormones and bile acids are potent regulators of hepatic carbohydrate and lipid metabolism. Steroid 5β-reductase (AKR1D1) is highly expressed in human liver where it inactivates steroid hormones and catalyzes a fundamental step in bile acid synthesis.

METHODS

Human liver biopsies were obtained from 34 obese patients and AKR1D1 mRNA expression levels were measured using qPCR. Genetic manipulation of AKR1D1 was performed in human HepG2 and Huh7 liver cell lines. Metabolic assessments were made using transcriptome analysis, western blotting, mass spectrometry, clinical biochemistry, and enzyme immunoassays.

RESULTS

In human liver biopsies, AKR1D1 expression decreased with advancing steatosis, fibrosis and inflammation. Expression was decreased in patients with type 2 diabetes. In human liver cell lines, AKR1D1 knockdown decreased primary bile acid biosynthesis and steroid hormone clearance. RNA-sequencing identified disruption of key metabolic pathways, including insulin action and fatty acid metabolism. AKR1D1 knockdown increased hepatocyte triglyceride accumulation, insulin sensitivity, and glycogen synthesis, through increased de novo lipogenesis and decreased β-oxidation, fueling hepatocyte inflammation. Pharmacological manipulation of bile acid receptor activation prevented the induction of lipogenic and carbohydrate genes, suggesting that the observed metabolic phenotype is driven through bile acid rather than steroid hormone availability.

CONCLUSIONS

Genetic manipulation of AKR1D1 regulates the metabolic phenotype of human hepatoma cell lines, driving steatosis and inflammation. Taken together, the observation that AKR1D1 mRNA is down-regulated with advancing NAFLD suggests that it may have a crucial role in the pathogenesis and progression of the disease.

Pharmacologic Modulation of Bile Acid-FXR-FGF15/FGF19 Pathway for the Treatment of Nonalcoholic Steatohepatitis

Nonalcoholic steatohepatitis (NASH) is within the spectrum of nonalcoholic fatty liver disease (NAFLD) and can progress to fibrosis, cirrhosis, and even hepatocellular carcinoma (HCC). The prevalence of NASH is rising and has become a large burden to the medical system worldwide. Unfortunately, despite its high prevalence and severe health consequences, there is currently no therapeutic agent approved to treat NASH. Therefore, the development of efficacious therapies is of utmost urgency and importance. Many molecular targets are currently under investigation for their ability to halt NASH progression. One of the most promising and well-studied targets is the bile acid (BA)-activated nuclear receptor, farnesoid X receptor (FXR). In this chapter, the characteristics, etiology, and prevalence of NASH will be discussed. A brief introduction to FXR regulation of BA homeostasis will be described. However, for more details regarding FXR in BA homeostasis, please refer to previous chapters. In this chapter, the mechanisms by which tissue and cell type-specific FXR regulates NASH development will be discussed in detail. Several FXR agonists have reached later phase clinical trials for treatment of NASH. The progress of these compounds and summary of released data will be provided. Lastly, this chapter will address safety liabilities specific to the development of FXR agonists.

Acute fructose intake suppresses fasting-induced hepatic gluconeogenesis through the AKT-FoxO1 pathway

Excessive intake of fructose increases lipogenesis in the liver, leading to hepatic lipid accumulation and development of fatty liver disease. Metabolic alterations in the liver due to fructose intake have been reported in many studies, but the effect of fructose administration on hepatic gluconeogenesis is not fully understood. The aim of this study was to evaluate the acute effects of fructose administration on fasting-induced hepatic gluconeogenesis. C57BL/6J mice were administered fructose solution after 14 h of fasting and plasma insulin, glucose, free fatty acids, and ketone bodies were analysed. We also measured phosphorylated AKT and forkhead box O (FoxO) 1 protein levels and gene expression related to gluconeogenesis in the liver. Furthermore, we measured glucose production from pyruvate after fructose administration. Glucose-administered mice were used as controls. Fructose administration enhanced phosphorylation of AKT in the liver, without increase of blood insulin levels. Blood free fatty acids and ketone bodies concentrations were as high as those in the fasting group after fructose administration, suggesting that insulin-induced inhibition of lipolysis did not occur in mice administered with fructose. Fructose also enhanced phosphorylation of FoxO1 and suppressed gluconeogenic gene expression, glucose-6-phosphatase activity, and glucose production from pyruvate. The present study suggests that acute fructose administration suppresses fasting-induced hepatic gluconeogenesis in an insulin-independent manner.

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Fructose administration does not increase blood glucose and insulin levels.

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Fructose administration suppressed fasting-induced hepatic gluconeogenic gene expression and G6Pase activity.

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Fructose accelerates FoxO1 phosphorylation through the AKT-FoxO1 pathway.

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We propose that fructose intake suppresses fasting-induced hepatic gluconeogenesis in an insulin-independent manner.

The nuclear bile acid receptor FXR is a PKA- and FOXA2-sensitive activator of fasting hepatic gluconeogenesis

BACKGROUND & AIMS

Embedded into a complex signaling network that coordinates glucose uptake, usage and production, the nuclear bile acid receptor FXR is expressed in several glucose-processing organs including the liver. Hepatic gluconeogenesis is controlled through allosteric regulation of gluconeogenic enzymes and by glucagon/cAMP-dependent transcriptional regulatory pathways. We aimed to elucidate the role of FXR in the regulation of fasting hepatic gluconeogenesis.

METHODS

The role of FXR in hepatic gluconeogenesis was assessed in vivo and in mouse primary hepatocytes. Gene expression patterns in response to glucagon and FXR agonists were characterized by quantitative reverse transcription PCR and microarray analysis. FXR phosphorylation by protein kinase A was determined by mass spectrometry. The interaction of FOXA2 with FXR was identified by cistromic approaches and in vitro protein-protein interaction assays. The functional impact of the crosstalk between FXR, the PKA and FOXA2 signaling pathways was assessed by site-directed mutagenesis, transactivation assays and restoration of FXR expression in FXR-deficient hepatocytes in which gene expression and glucose production were assessed.

RESULTS

FXR positively regulates hepatic glucose production through two regulatory arms, the first one involving protein kinase A-mediated phosphorylation of FXR, which allowed for the synergistic activation of gluconeogenic genes by glucagon, agonist-activated FXR and CREB. The second arm involves the inhibition of FXR's ability to induce the anti-gluconeogenic nuclear receptor SHP by the glucagon-activated FOXA2 transcription factor, which physically interacts with FXR. Additionally, knockdown of Foxa2 did not alter glucagon-induced and FXR agonist enhanced expression of gluconeogenic genes, suggesting that the PKA and FOXA2 pathways regulate distinct subsets of FXR responsive genes.

CONCLUSIONS

Thus, hepatic glucose production is regulated during physiological fasting by FXR, which integrates the glucagon/cAMP signal and the FOXA2 signal, by being post-translationally modified, and by engaging in protein-protein interactions, respectively.

LAY SUMMARY

Activation of the nuclear bile acid receptor FXR regulates gene expression networks, controlling lipid, cholesterol and glucose metabolism, which are mostly effective after eating. Whether FXR exerts critical functions during fasting is unknown. The results of this study show that FXR transcriptional activity is regulated by the glucagon/protein kinase A and the FOXA2 signaling pathways, which act on FXR through phosphorylation and protein-protein interactions, respectively, to increase hepatic glucose synthesis.

HS218 as an FXR antagonist suppresses gluconeogenesis by inhibiting FXR binding to PGC-1α promoter

INTRODUCTION

Farnesoid X receptor (FXR) as a member of nuclear receptor is tightly associated with glucose metabolism. Accumulated evidence has addressed the potential of FXR antagonist in the treatment of type 2 diabetes mellitus (T2DM), although the related mechanisms remain unclear. Here, we determined a specific FXR antagonist HS218 (N-benzyl-N-(3-(tert-butyl)-4-hydroxyphenyl)-2,4-dichlorobenzamide), which exhibited high activities in suppressing gluconeogenesis and ameliorating glucose homeostasis in db/db and HFD/STZ-induced T2DM mice. We would like to investigate the mechanisms underlying FXR antagonism in the regulation of gluconeogenesis by using HS218 as a probe.

METHODS

HS218 was evaluated by glucose output assay. Binding affinity of HS218 to the ligand binding domain of FXR (FXR-LBD) was detected by Surface Plasmon Resonance (SPR) technology-based Biacore and fluorescence quenching assays. Mammalian one-hybrid and transactivation assays were carried out to detect the antagonistic effect of HS218 on FXR. Real-time PCR assay was performed to measure the expressions of FXR-target and gluconeogenic genes. Anti-diabetic efficiencies of HS218 were determined in db/db and HFD/STZ-induced T2DM mice. Assays by promoter 5'-deletion analysis and Chromatin immunoprecipitation (ChIP) were performed to detect the binding of FXR to peroxisome proliferator-activated receptor γ coactivator-1α (PGC-1α) promoter. Western blot assay was used to determine the protein level in either cells or the liver tissues of mice.

RESULTS

We determined that HS218 as a new FXR specific antagonist could FXR-dependently suppress gluconeogenesis in mouse primary hepatocytes, and effectively improve glucose homeostasis in db/db and HFD/STZ-induced T2DM mice. HS218 decreased gluconeogenesis by inhibiting the FXR-induced increase in the promoter activity of the key gluconeogenic gene PGC-1α, leading to the repression of PGC-1α and its target gene peroxisome proliferator-activated receptor α (PPARα).

CONCLUSIONS

To our knowledge, our work might be the first report on the mechanism underlying FXR antagonist in the regulation of gluconeogenesis, and all results have also highlighted the potential of HS218 in the treatment of T2DM.

Farnesoid X receptor: A "homeostat" for hepatic nutrient metabolism

The Farnesoid X receptor (FXR) is a nuclear receptor activated by bile acids (BAs). BAs are amphipathic molecules that serve as fat solubilizers in the intestine under postprandial conditions. In the post-absorptive state, BAs bind FXR in the hepatocytes, which in turn provides feedback signals on BA synthesis and transport and regulates lipid, glucose and amino acid metabolism. Therefore, FXR acts as a homeostat of all three classes of nutrients, fats, sugars and proteins. Here we re-analyze the function of FXR in the perspective of nutritional metabolism, and discuss the role of FXR in liver energy homeostasis in postprandial, post-absorptive and fasting/starvation states. FXR, by regulating nutritional metabolism, represses autophagy in conditions of nutrient abundance, and controls the metabolic needs of proliferative cells. In addition, FXR regulates inflammation via direct effects and via its impact on nutrient metabolism. These functions indicate that FXR is an attractive therapeutic target for liver diseases.

Role of HDAC9-FoxO1 Axis in the Transcriptional Program Associated with Hepatic Gluconeogenesis

Histone deacetylase 9 (HDAC9) regulates hepatic gluconeogenesis by deacetylating Forkhead box O 1 (FoxO1). HDAC9 upregulation is involved in hepatitis C virus (HCV)-associated exaggerated gluconeogenesis. Herein, we found in addition to FoxO1, HDAC9 also regulates other gluconeogenic transcription factors, including peroxisomeproliferator-activated receptor-γ coactivator-1α (PGC-1α), cyclic AMP-responsive element-binding protein (CREB), and glucocorticoid receptor (GR). Unlike FoxO1, which is regulated by post-translational modification responses to HDAC9, HDAC9 regulates PGC-1α, CREB and GR by altering gene expression. Similar to PGC-1α, CREB and GR were found to be novel regulatory targets of FoxO1 by examination of the FoxO1 binding site in their promoter. PGC-1α, CREB and GR were upregulated in response to HDAC9 via FoxO1 deacetylation. These findings indicate that HDAC9-FoxO1 signalling contributes to gluconeogenesis by modulating the expression of gluconeogenic transcription factors. In particular, metabolic profiling demonstrated a clear shift towards gluconeogenesis metabolism, and HDAC9-FoxO1 signalling can be strongly induced to upregulate gluconeogenic transcription factors following HCV infection. The positive correlation between HDAC9 and gluconeogenic transcription factor expression levels in the livers of both HCV-infected patients and normal individuals further emphasizes the clinical relevance of these results. Thus, HDAC9-FoxO1 signalling axis is involved in regulating gluconeogenic transcription factors, gluconeogenesis, and HCV-induced type 2 diabetes.

BAR502, a dual FXR and GPBAR1 agonist, promotes browning of white adipose tissue and reverses liver steatosis and fibrosis

Non-alcoholic steatohepatitis (NASH) is a highly prevalent chronic liver disease. Here, we have investigated whether BAR502, a non-bile acid, steroidal dual ligand for FXR and GPBAR1, reverses steato-hepatitis in mice fed a high fat diet (HFD) and fructose. After 9 week, mice on HFD gained ≈30% of b.w (P < 0.01 versus naïve) and were insulin resistant. These overweighting and insulin resistant mice were randomized to receive HFD or HFD in combination with BAR502. After 18 weeks, HFD mice developed NASH like features with severe steato-hepatitis and fibrosis, increased hepatic content of triacylglycerol and cholesterol and expression of SREPB1c, FAS, ApoC2, PPARα and γ, α-SMA, α1 collagen and MCP1 mRNAs. Treatment with BAR502 caused a ≈10% reduction of b.w., increased insulin sensitivity and circulating levels of HDL, while reduced steatosis, inflammatory and fibrosis scores and liver expression of SREPB1c, FAS, PPARγ, CD36 and CYP7A1 mRNA. BAR502 increased the expression of SHP and ABCG5 in the liver and SHP, FGF15 and GLP1 in intestine. BAR502 promoted the browning of epWAT and reduced liver fibrosis induced by CCl₄. In summary, BAR502, a dual FXR and GPBAR1 agonist, protects against liver damage caused by HFD by promoting the browning of adipose tissue.

Glucocorticoid Signaling: An Update from a Genomic Perspective

Glucocorticoid hormones (GC) regulate essential physiological functions including energy homeostasis, embryonic and postembryonic development, and the stress response. From the biomedical perspective, GC have garnered a tremendous amount of attention as highly potent anti-inflammatory and immunosuppressive medications indispensable in the clinic. GC signal through the GC receptor (GR), a ligand-dependent transcription factor whose structure, DNA binding, and the molecular partners that it employs to regulate transcription have been under intense investigation for decades. In particular, next-generation sequencing-based approaches have revolutionized the field by introducing a unified platform for a simultaneous genome-wide analysis of cellular activities at the level of RNA production, binding of transcription factors to DNA and RNA, and chromatin landscape and topology. Here we describe fundamental concepts of GC/GR function as established through traditional molecular and in vivo approaches and focus on the novel insights of GC biology that have emerged over the last 10 years from the rapidly expanding arsenal of system-wide genomic methodologies.

Decoding the vasoregulatory activities of bile acid-activated receptors in systemic and portal circulation: role of gaseous mediators

Bile acids are end products of cholesterol metabolism generated in the liver and released in the intestine. Primary and secondary bile acids are the result of the symbiotic relation between the host and intestinal microbiota. In addition to their role in nutrient absorption, bile acids are increasingly recognized as regulatory signals that exert their function beyond the intestine by activating a network of membrane and nuclear receptors. The best characterized of these bile acid-activated receptors, GPBAR1 (also known as TGR5) and the farnesosid-X-receptor (FXR), have also been detected in the vascular system and their activation mediates the vasodilatory effects of bile acids in the systemic and splanchnic circulation. GPBAR1, is a G protein-coupled receptor, that is preferentially activated by lithocholic acid (LCA) a secondary bile acid. GPBAR1 is expressed in endothelial cells and liver sinusoidal cells (LSECs) and responds to LCA by regulating the expression of both endothelial nitric oxide synthase (eNOS) and cystathionine-γ-lyase (CSE), an enzyme involved in generation of hydrogen sulfide (H₂S). Activation of CSE by GPBAR1 ligands in LSECs is due to genomic and nongenomic effects, involves protein phosphorylation, and leads to release of H₂S. Despite that species-specific effects have been described, vasodilation caused by GPBAR1 ligands in the liver microcirculation and aortic rings is abrogated by inhibition of CSE but not by eNOS inhibitor. Vasodilation caused by GPBAR1 (and FXR) ligands also involves large conductance calcium-activated potassium channels likely acting downstream to H₂S. The identification of GPBAR1 as a vasodilatory receptor is of relevance in the treatment of complex disorders including metabolic syndrome-associated diseases, liver steatohepatitis, and portal hypertension.

The Interactome of the Glucocorticoid Receptor and Its Influence on the Actions of Glucocorticoids in Combatting Inflammatory and Infectious Diseases

Glucocorticoids (GCs) have been widely used for decades as a first-line treatment for inflammatory and autoimmune diseases. However, their use is often hampered by the onset of adverse effects or resistance. GCs mediate their effects via binding to glucocorticoid receptor (GR), a transcription factor belonging to the family of nuclear receptors. An important aspect of GR's actions, including its anti-inflammatory capacity, involves its interactions with various proteins, such as transcription factors, cofactors, and modifying enzymes, which codetermine receptor functionality. In this review, we provide a state-of-the-art overview of the protein-protein interactions (PPIs) of GR that positively or negatively affect its anti-inflammatory properties, along with mechanistic insights, if known. Emphasis is placed on the interactions that affect its anti-inflammatory effects in the presence of inflammatory and microbial diseases.

Farnesoid X receptor activation promotes cell proliferation via PDK4-controlled metabolic reprogramming

Farnesoid X receptor (FXR) plays a pivotal role in the regulation of various metabolic pathways as well as liver regeneration. However, the casual link between cell proliferative effects during liver regeneration and metabolic regulation of FXR was elusive. In this study, we found that FXR activation significantly promotes HepG2 cell proliferation accompanied with metabolic switch towards the excessive accumulation of aerobic glycolytic intermediates including lactic acid, pyruvate and the subsequently increased biosynthesis of glycine. This FXR-induced metabolic switch was found dependent on an up-regulation of pyruvate dehydrogenate kinase 4 (PDK4), a FXR target gene. FXR agonists were found to promote liver regeneration in the murine model of APAP induced liver injury, which was associated with a metabolic switch favoring the accumulation of glycolytic intermediates as precursors for generation of biomass. However, FXR activation has little effect on the glycolytic metabolism in healthy primary hepatocytes in vitro and the liver of healthy mice in vivo. Therefore, we conclude that FXR may promote the proliferation of tumor cells and the hepatocytes in the process of liver regeneration by activating the PDK4-mediated metabolic reprogramming to generate glycolytic intermediates essential for rapid biomass generation, establishing a mechanistic link between cell proliferation and metabolic switch.

Metabolic Enzymes Enjoying New Partnerships as RNA-Binding Proteins

In the past century, few areas of biology advanced as much as our understanding of the pathways of intermediary metabolism. Initially considered unimportant in terms of gene regulation, crucial cellular fate changes, cell differentiation, or malignant transformation are now known to involve 'metabolic remodeling' with profound changes in the expression of many metabolic enzyme genes. This review focuses on the recent identification of RNA-binding activity of numerous metabolic enzymes. We discuss possible roles of this unexpected second activity in feedback gene regulation ('moonlighting') and/or in the control of enzymatic function. We also consider how metabolism-driven post-translational modifications could regulate enzyme-RNA interactions. Thus, RNA emerges as a new partner of metabolic enzymes with far-reaching possible consequences to be unraveled in the future.

Reversal of Endothelial Dysfunction by GPBAR1 Agonism in Portal Hypertension Involves a AKT/FOXOA1 Dependent Regulation of H2S Generation and Endothelin-1

Background

GPBAR1 is a bile acids activated receptor expressed in entero-hepatic tissues. In the liver expression of GPBAR1 is restricted to sinusoidal and Kuppfer cells. In the systemic circulation vasodilation caused by GPBAR1 agonists is abrogated by inhibition of cystathione-γ-liase (CSE), an enzyme essential to the generation of hydrogen sulfide (H₂S), a vasodilatory agent. Portal BAR501 is a semisynthetic bile acid derivative endowed with a potent and selective agonistic activity toward GPBAR1.

Methods

Cirrhosis was induced in mice by carbon tetrachloride (CCL₄) administration for 9 weeks. Liver endothelial dysfunction was induced by feeding wild type and Gpbar1^-/- mice with methionine for 4 weeks. In both models, mice were administered BAR501, 15 mg/kg/day.

Results

By transactivation assay we demonstrate that BAR501 is a selective GPBAR1 agonist devoid of any FXR agonistic activity. In naïve rats, BAR501 effectively reduced hepatic perfusion pressure and counteracted the vasoconstriction activity of norepinephrine. In the CCl₄ model, 9 weeks treatment with BAR501 effectively protected against development of endothelial dysfunction by increasing liver CSE expression and activity and by reducing endothelin (ET)-1 gene expression. In mice feed methionine, treatment with BAR501 attenuated endothelial dysfunction and caused a GPBAR1-dependent regulation of CSE. Using human liver sinusoidal cells, we found that modulation of CSE expression/activity is mediated by both genomic (recruitment of CREB to CRE in the CSE promoter) and non-genomic effects, involving a Akt-dependent phosporylation of CSE and endothelial nitric oxide (NO) synthase (eNOS). BAR501, phosphorylates FOXO1 and inhibits ET-1 transcription in liver sinusoidal cells.

Conclusions

BAR501, a UDCA-like GPBAR1 agonist, rescues from endothelial dysfunction in rodent models of portal hypertension by exerting genomic and non-genomic effects on CSE, eNOS and ET-1 in liver sinusoidal cells.

Regulation of Glucose Homeostasis by Glucocorticoids

Glucocorticoids are steroid hormones that regulate multiple aspects of glucose homeostasis. Glucocorticoids promote gluconeogenesis in liver, whereas in skeletal muscle and white adipose tissue they decrease glucose uptake and utilization by antagonizing insulin response. Therefore, excess glucocorticoid exposure causes hyperglycemia and insulin resistance. Glucocorticoids also regulate glycogen metabolism. In liver, glucocorticoids increase glycogen storage, whereas in skeletal muscle they play a permissive role for catecholamine-induced glycogenolysis and/or inhibit insulin-stimulated glycogen synthesis. Moreover, glucocorticoids modulate the function of pancreatic α and β cells to regulate the secretion of glucagon and insulin, two hormones that play a pivotal role in the regulation of blood glucose levels. Overall, the major glucocorticoid effect on glucose homeostasis is to preserve plasma glucose for brain during stress, as transiently raising blood glucose is important to promote maximal brain function. In this chapter we will discuss the current understanding of the mechanisms underlying different aspects of glucocorticoid-regulated mammalian glucose homeostasis.

Glucocorticoid hormones and energy homeostasis

Glucocorticoids (GC) and their cognate intracellular receptor, the glucocorticoid receptor (GR), have been characterised as critical checkpoints in the endocrine control of energy homeostasis in mammals. Indeed, aberrant GC action has been linked to a variety of severe metabolic diseases, including obesity, insulin resistance and type 2 diabetes. As a steroid-binding member of the nuclear receptor superfamily of transcription factors, the GR translocates into the cell nucleus upon GC binding where it serves as a transcriptional regulator of distinct GC-responsive target genes that are - in many cases - associated with glucose and lipid regulatory pathways and thereby intricately control both physiological and pathophysiological systemic energy homeostasis. Here, we summarize the current knowledge of GC/GR function in energy metabolism and systemic metabolic dysfunction, particularly focusing on glucose and lipid metabolism.

Nuclear bile acid signaling through the farnesoid X receptor

Bile acids (BAs) are amphipathic molecules produced from cholesterol by the liver. Expelled from the gallbladder upon meal ingestion, BAs serve as fat solubilizers in the intestine. BAs are reabsorbed in the ileum and return via the portal vein to the liver where, together with nutrients, they provide signals to coordinate metabolic responses. BAs act on energy and metabolic homeostasis through the activation of membrane and nuclear receptors, among which the nuclear receptor farnesoid X receptor (FXR) is an important regulator of several metabolic pathways. Highly expressed in the liver and the small intestine, FXR contributes to BA effects on metabolism, inflammation and cell cycle control. The pharmacological modulation of its activity has emerged as a potential therapeutic strategy for liver and metabolic diseases. This review highlights recent advances regarding the mechanisms by which the BA sensor FXR contributes to global signaling effects of BAs, and how FXR activity may be regulated by nutrient-sensitive signaling pathways.

Cystathionine γ-lyase, a H2S-generating enzyme, is a GPBAR1-regulated gene and contributes to vasodilation caused by secondary bile acids

GPBAR1 is a bile acid-activated receptor (BAR) for secondary bile acids, lithocholic (LCA) and deoxycholic acid (DCA), expressed in the enterohepatic tissues and in the vasculature by endothelial and smooth muscle cells. Despite that bile acids cause vasodilation, it is unclear why these effects involve GPBAR1, and the vascular phenotype of GPBAR1 deficient mice remains poorly defined. Previous studies have suggested a role for nitric oxide (NO) in regulatory activity exerted by GPBAR1 in liver endothelial cells. Hydrogen sulfide (H2S) is a vasodilatory agent generated in endothelial cells by cystathionine-γ-lyase (CSE). Here we demonstrate that GPBAR1 null mice had increased levels of primary and secondary bile acids and impaired vasoconstriction to phenylephrine. In aortic ring preparations, vasodilation caused by chenodeoxycholic acid (CDCA), a weak GPBAR1 ligand and farnesoid-x-receptor agonist (FXR), was iberiotoxin-dependent and GPBAR1-independent. In contrast, vasodilation caused by LCA was GPBAR1 dependent and abrogated by propargyl-glycine, a CSE inhibitor, and by 5β-cholanic acid, a GPBAR1 antagonist, but not by N(5)-(1-iminoethyl)-l-ornithine (l-NIO), an endothelial NO synthase inhibitor, or iberiotoxin, a large-conductance calcium-activated potassium (BKCa) channels antagonist. In venular and aortic endothelial (HUVEC and HAEC) cells GPBAR1 activation increases CSE expression/activity and H2S production. Two cAMP response element binding protein (CREB) sites (CREs) were identified in the CSE promoter. In addition, TLCA stimulates CSE phosphorylation on serine residues. In conclusion we demonstrate that GPBAR1 mediates the vasodilatory activity of LCA and regulates the expression/activity of CSE. Vasodilation caused by CDCA involves BKCa channels. The GPBAR1/CSE pathway might contribute to endothelial dysfunction and hyperdynamic circulation in liver cirrhosis.

Intrafollicular cortisol levels inversely correlate with cumulus cell lipid content as a possible energy source during oocyte meiotic resumption in women undergoing ovarian stimulation for in vitro fertilization

OBJECTIVE

To determine whether follicular fluid (FF) cortisol levels affect cumulus cell (CC) lipid content during oocyte meiotic resumption, and whether CCs express genes for glucocorticoid action.

DESIGN

Prospective cohort study.

SETTING

Academic medical center.

PATIENT(S)

Thirty-seven nonobese women underwent ovarian stimulation for in vitro fertilization (IVF).

INTERVENTION(S)

At oocyte retrieval, FF was aspirated from the first follicle (>16 mm in size) of each ovary and pooled CCs were collected.

MAIN OUTCOME MEASURE(S)

Follicular fluid cortisol and cortisone analysis was performed with the use of liquid chromatography-tandem mass spectrometry. CCs were stained with lipid fluorescent dye Bodipy FL C16 to determine lipid content with the use of confocal microscopy. Quantitative real-time polymerase chain reaction was used to detect CC gene expression of 11β-hydroxysteroid dehydrogenase (11β-HSD) types 1 and 2, glucocorticoid receptor (NR3C1), lipoprotein lipase (LPL), and hormone-sensitive lipase (HSL).

RESULT(S)

Adjusting for maternal age, FF cortisol levels negatively correlated with CC lipid content and positively correlated with numbers of total and mature oocytes. CCs expressed genes for 11β-HSD type 1 as the predominant 11β-HSD isoform, NR3C1, LPL, and HSL.

CONCLUSION(S)

FF cortisol levels may regulate CC lipolysis during oocyte meiotic resumption and affect oocyte quality during IVF.

Modification on ursodeoxycholic acid (UDCA) scaffold. discovery of bile acid derivatives as selective agonists of cell-surface G-protein coupled bile acid receptor 1 (GP-BAR1)

Bile acids are signaling molecules interacting with the nuclear receptor FXR and the G-protein coupled receptor 1 (GP-BAR1/TGR5). GP-BAR1 is a promising pharmacological target for the treatment of steatohepatitis, type 2 diabetes, and obesity. Endogenous bile acids and currently available semisynthetic bile acids are poorly selective toward GP-BAR1 and FXR. Thus, in the present study we have investigated around the structure of UDCA, a clinically used bile acid devoid of FXR agonist activity, to develop a large family of side chain modified 3α,7β-dihydroxyl cholanoids that selectively activate GP-BAR1. In vivo and in vitro pharmacological evaluation demonstrated that administration of compound 16 selectively increases the expression of pro-glucagon 1, a GP-BAR1 target, in the small intestine, while it had no effect on FXR target genes in the liver. Further, compound 16 results in a significant reshaping of bile acid pool in a rodent model of cholestasis. These data demonstrate that UDCA is a useful scaffold to generate novel and selective steroidal ligands for GP-BAR1.

Minireview: new molecular mediators of glucocorticoid receptor activity in metabolic tissues

The glucocorticoid receptor (GR) was one of the first nuclear hormone receptors cloned and represents one of the most effective drug targets available today for the treatment of severe inflammation. The physiologic consequences of endogenous or exogenous glucocorticoid excess are well established and include hyperglycemia, insulin resistance, fatty liver, obesity, and muscle wasting. However, at the molecular and tissue-specific level, there are still many unknown protein mediators of glucocorticoid response and thus, much remains to be uncovered that will help determine whether activation of the GR can be tailored to improve therapeutic efficacy while minimizing unwanted side effects. This review summarizes recent discoveries of tissue-selective modulators of glucocorticoid signaling that are important in mediating the unwanted side effects of therapeutic glucocorticoid use, emphasizing the downstream molecular effects of GR activation in the liver, adipose tissue, muscle, and pancreas.

Coordinated Actions of FXR and LXR in Metabolism: From Pathogenesis to Pharmacological Targets for Type 2 Diabetes

Type 2 diabetes (T2D) is the most prevalent metabolic disease, and many people are suffering from its complications driven by hyperglycaemia and dyslipidaemia. Nuclear receptors (NRs) are ligand-inducible transcription factors that mediate changes to metabolic pathways within the body. As metabolic regulators, the farnesoid X receptor (FXR) and the liver X receptor (LXR) play key roles in the pathogenesis of T2D, which remains to be clarified in detail. Here we review the recent progress concerning the physiological and pathophysiological roles of FXRs and LXRs in the regulation of bile acid, lipid and glucose metabolism and the implications in T2D, taking into account that these two nuclear receptors are potential pharmaceutical targets for the treatment of T2D and its complications.

Farnesoid X receptor alpha: a molecular link between bile acids and steroid signaling?

Bile acids are cholesterol metabolites that have been extensively studied in recent decades. In addition to having ancestral roles in digestion and fat solubilization, bile acids have recently been described as signaling molecules involved in many physiological functions, such as glucose and energy metabolisms. These signaling pathways involve the activation of the nuclear receptor farnesoid X receptor (FXRα) or of the G protein-coupled receptor TGR5. In this review, we will focus on the emerging role of FXRα, suggesting important functions for the receptor in steroid metabolism. It has been described that FXRα is expressed in the adrenal glands and testes, where it seems to control steroid production. FXRα also participates in steroid catabolism in the liver and interferes with the steroid signaling pathways in target tissues via crosstalk with steroid receptors. In this review, we discuss the potential impacts of bile acid (BA), through its interactions with steroid metabolism, on glucose metabolism, sexual function, and prostate and breast cancers. Although several of the published reports rely on in vitro studies, they highlight the need to understand the interactions that may affect health. This effect is important because BA levels are increased in several pathophysiological conditions related to liver injuries. Additionally, BA receptors are targeted clinically using therapeutics to treat liver diseases, diabetes, and cancers.

Metabolic control through glucocorticoid hormones: an update

In the past decades, glucocorticoid (GC) hormones and their cognate, intracellular receptor, the glucocorticoid receptor (GR), have been well established as critical checkpoints in mammalian energy homeostasis. Whereas many aspects in healthy nutrient metabolism require physiological levels and/or action of GC, aberrant GC/GR signalling has been linked to severe metabolic dysfunction, including obesity, insulin resistance and type 2 diabetes. Consequently, studies of the molecular mechanisms within the GC signalling axis have become a major focus in biomedical research, up-to-date particularly focusing on systemic glucose and lipid handling. However, with the availability of novel high throughput technologies and more sophisticated metabolic phenotyping capabilities, as-yet non-appreciated, metabolic functions of GC have been recently discovered, including regulatory roles of the GC/GR axis in protein and bile acid homeostasis as well as metabolic inter-organ communication. Therefore, this review summarises recent advances in GC/GR biology, and summarises findings relevant for basic and translational metabolic research.

Treatment of mouse liver slices with cholestatic hepatotoxicants results in down-regulation of Fxr and its target genes

BACKGROUND

Unexpected cholestasis substantially contributes to drug failure in clinical trials. Current models used for safety assessment in drug development do not accurately predict cholestasis in humans. Therefore, it is of relevance to develop new screening models that allow identifying drugs with cholestatic properties.

METHODS

We employed mouse precision cut liver slices (PCLS), which were incubated 24 h with two model cholestatic compounds: cyclosporin A (CsA) and chlorpromazine (CPZ). Subsequently, transcriptome analysis using DNA microarrays and q-PCR were performed to identify relevant biological processes and biomarkers. Additionally, histology was carried out and levels of triglycerides (TG) and bile acids (BA) were measured. To verify the ex vivo mouse data, these were compared with publically available human data relevant for cholestasis.

RESULTS

Whole genome gene expression analysis showed that CsA up-regulated pathways related to NF-κB, ER stress and inflammation. Both CsA and CPZ down-regulated processes related to extracellular matrix (ECM) remodelling, BA homeostasis, Fxr signalling, and energy metabolism. The differential expression of a number of characteristic genes (e.g. Abcg5, Abcg8, Klf15, and Baat) could be confirmed by q-PCR. Histology revealed that CsA but not CPZ induced "ballooning" of hepatocytes. No effects on TG and BA levels were observed after incubation of PCLS with CsA and CPZ. A substantial number of processes altered in CsA- and CPZ-treated mouse PCLS ex vivo was also found to be affected in liver biopsies of cholestatic patients.

CONCLUSION

The present study demonstrated that mouse PCLS can be used as a tool to identify mechanisms of action of cholestatic model compounds. The induction of general stress responses and down-regulated Fxr signalling could play a role in the development of drug induced cholestasis. Importantly, comparative data analysis showed that the ex vivo mouse findings are also relevant for human pathology. Moreover, this work provides a set of genes that are potentially useful to assess drugs for cholestatic properties.

Dissociation of intestinal and hepatic activities of FXR and LXRα supports metabolic effects of terminal ileum interposition in rodents

The farnesoid X receptor (FXR) and the liver x receptors (LXRs) are bile acid-activated receptors that are highly expressed in the enterohepatic tissues. The mechanisms that support the beneficial effects of bariatric surgery are only partially defined. We have investigated the effects of ileal interposition (IT), a surgical relocation of the distal ileum into the proximal jejunum, on FXR and LXRs in rats. Seven months after surgery, blood concentrations of total bile acids, taurocholic acid, an FXR ligand, and taurohyocholic acid, an LXRα ligand, were significantly increased by IT (P < 0.05). In contrast, liver and intestinal concentrations of conjugated and nonconjugated bile acids were decreased (P < 0.05). These changes were associated with a robust induction of FXR and FXR-regulated genes in the intestine, including Fgf15, a negative regulator of bile acid synthesis. IT repressed the liver expression of glucose-6-phosphatase (G6PC) and phosphoenolpyruvate carboxykinase (Pepck), two gluconeogenetic genes, along with the expression of LXRα and its target genes sterol regulatory element-binding protein (Srebp) 1c and fatty acid synthase (Fas) in the liver. Treating IT rats with chenodeoxycholic acid ameliorated insulin signaling in the liver. Whether confirmed in human settings, these results support the association of pharmacological therapies with bariatric surgeries to exploit the selective activation of intestinal nuclear receptors.

Cholesterol and male fertility: what about orphans and adopted?

The link between cholesterol homeostasis and male fertility has been clearly suggested in patients who suffer from hyperlipidemia and metabolic syndrome. This has been confirmed by the generation of several transgenic mouse models or in animals fed with high cholesterol diet. Next to the alteration of the endocrine signaling pathways through steroid receptors (androgen and estrogen receptors); "orphan" and "adopted" nuclear receptors, such as the Liver X Receptors (LXRs), the Proliferating Peroxisomal Activated Receptors (PPARs) or the Liver Receptor Homolog-1 (LRH-1), have been involved in this cross-talk. These transcription factors show distinct expression patterns in the male genital tract, explaining the large panel of phenotypes observed in transgenic male mice and highlighting the importance of lipid homesostasis and the complexity of the molecular pathways involved. Increasing our knowledge of the roles of these nuclear receptors in male germ cell differentiation could help in proposing new approaches to either treat infertile men or define new strategies for contraception.

The bile acid sensor FXR is required for immune-regulatory activities of TLR-9 in intestinal inflammation

BACKGROUND

Toll like receptors (TLRs) sense the intestinal microbiota and regulate the innate immune response. A dysregulation of TLRs function participates into intestinal inflammation. Farnesoid X Receptor (FXR) is a nuclear receptor and bile acid sensor highly expressed in entero-hepatic tissues. FXR regulates lipid metabolism and innate immunity.

METHODOLOGY/PRINCIPAL FINDINGS

In this study we have investigated whether FXR gene expression/function in the intestine is modulated by TLRs. We found that in human monocytes activation of membrane TLRs (i.e. TLR2, 4, 5 and 6) downregulates, while activation of intracellular TLRs (i.e. TLR3, 7, 8 and 9) upregulates the expression of FXR and its target gene SHP, small heterodimer partner. This effect was TLR9-dependent and TNFα independent. Intestinal inflammation induced in mice by TNBS downregulates the intestinal expression of FXR in a TLR9-dependent manner. Protection against TNBS colitis by CpG, a TLR-9 ligand, was lost in FXR(-/-) mice. In contrast, activation of FXR rescued TLR9(-/-) and MyD88(-/-) mice from colitis. A putative IRF7 response element was detected in the FXR promoter and its functional characterization revealed that IRF7 is recruited on the FXR promoter under TLR9 stimulation.

CONCLUSIONS/SIGNIFICANCE

Intestinal expression of FXR is selectively modulated by TLR9. In addition to its role in regulating type-I interferons and innate antiviral immunity, IRF-7 a TLR9-dependent factor, regulates the expression of FXR, linking microbiota-sensing receptors to host's immune and metabolic signaling.

Role of nuclear receptors in lipid dysfunction and obesity-related diseases

This article is a report on a symposium sponsored by the American Society for Pharmacology and Experimental Therapeutics and held at the Experimental Biology 12 meeting in San Diego, CA. The presentations discussed the roles of a number of nuclear receptors in regulating glucose and lipid homeostasis, the pathophysiology of obesity-related disease states, and the promise associated with targeting their activities to treat these diseases. While many of these receptors (in particular, constitutive androstane receptor and pregnane X receptor) and their target enzymes have been thought of as regulators of drug and xenobiotic metabolism, this symposium highlighted the advances made in our understanding of the endogenous functions of these receptors. Similarly, as we gain a better understanding of the mechanisms underlying bile acid signaling pathways in the regulation of body weight and glucose homeostasis, we see the importance of using complementary approaches to elucidate this fascinating network of pathways. The observation that some receptors, like the farnesoid X receptor, can function in a tissue-specific manner via well defined mechanisms has important clinical implications, particularly in the treatment of liver diseases. Finally, the novel findings that agents that selectively activate estrogen receptor β can effectively inhibit weight gain in a high-fat diet model of obesity identifies a new role for this member of the steroid superfamily. Taken together, the significant findings reported during this symposium illustrate the promise associated with targeting a number of nuclear receptors for the development of new therapies to treat obesity and other metabolic disorders.

Plakilactones from the marine sponge Plakinastrella mamillaris. Discovery of a new class of marine ligands of peroxisome proliferator-activated receptor γ

In this paper we report the isolation and the molecular characterization of a new class of PPARγ ligands from the marine environment. Biochemical characterization of a library of 13 oxygenated polyketides isolated from the marine sponge Plakinastrella mamillaris allowed the discovery of gracilioether B and plakilactone C as selective PPARγ ligands in transactivation assays. Both agents covalently bind to the PPARγ ligand binding domain through a Michael addition reaction involving a protein cysteine residue and the α,β-unsaturated ketone in their side chains. Additionally, gracilioether C is a noncovalent agonist for PPARγ, and methyl esters 1 and 2 are noncovalent antagonists. Structural requirements for the interaction of these agents within the PPARγ ligand binding domain were obtained by docking analysis. Gracilioether B and plakilactone C regulate the expression of PPARγ-dependent genes in the liver and inhibit the generation of inflammatory mediators by macrophages.