Bile acids regulate gluconeogenic gene expression via small heterodimer partner-mediated repression of hepatocyte nuclear factor 4 and Foxo1

Hepatic FXR: key regulator of whole-body energy metabolism

The farnesoid X receptor (FXR) is a nuclear receptor whose activation leads to alterations in pathways involved in energy metabolism. For example, it serves as a bile acid receptor in tissues such as the liver, and as an energy metabolism regulator in liver, muscle and adipose tissue. However, the effects of FXR activation are not exclusive to the tissue where it is present, because receptor crosstalk affects tissues throughout the body. It has been demonstrated that FXR regulates the metabolism of not just bile acids, but also of fats and hydrocarbon metabolites. FXR is currently under study as a therapeutic target for the treatment of diseases of excess, such as diabetes. Here we review the effects of FXR activation in the response of an organism to excess energy.

Characterization of novel peroxisome proliferator-activated receptor γ coactivator-1α (PGC-1α) isoform in human liver

Peroxisome proliferator-activated receptor γ coactivator-1α (PGC-1α) is a transcriptional coactivator that contributes to the regulation of numerous transcriptional programs including the hepatic response to fasting. Mechanisms at transcriptional and post-transcriptional levels allow PGC-1α to support distinct biological pathways. Here we describe a novel human liver-specific PGC-1α transcript that results from alternative promoter usage and is induced by FOXO1 as well as glucocorticoids and cAMP-response element-binding protein signaling but is not present in other mammals. Hepatic tissue levels of novel and wild-type transcripts were similar but were only moderately associated (p < 0.003). Novel mRNA levels were associated with a polymorphism located in its promoter region, whereas wild-type transcript levels were not. Furthermore, hepatic PCK1 mRNA levels exhibited stronger associations with the novel than with the wild-type transcript levels. Except for a deletion of 127 amino acids at the N terminus, the protein, termed L-PGC-1α, is identical to PGC-1α. L-PGC-1α was localized in the nucleus and showed coactivation properties that overlap with those of PGC-1α. Collectively, our data support a role of L-PGC-1α in gluconeogenesis, but functional differences predicted from the altered structure suggest that L-PGC-1α may have arisen to adapt PGC-1α to more complex metabolic pathways in humans.

Bile Acid signaling in liver metabolism and diseases

Obesity, diabetes, and metabolic syndromes are increasingly recognized as health concerns worldwide. Overnutrition and insulin resistance are the major causes of diabetic hyperglycemia and hyperlipidemia in humans. Studies in the past decade provide evidence that bile acids are not just biological detergents facilitating gut nutrient absorption, but also important metabolic regulators of glucose and lipid homeostasis. Pharmacological alteration of bile acid metabolism or bile acid signaling pathways such as using bile acid receptor agonists or bile acid binding resins may be a promising therapeutic strategy for the treatment of obesity and diabetes. On the other hand, bile acid signaling is complex, and the molecular mechanisms mediating the bile acid effects are still not completely understood. This paper will summarize recent advances in our understanding of bile acid signaling in regulation of glucose and lipid metabolism, and the potentials of developing novel therapeutic strategies that target bile acid metabolism for the treatment of metabolic disorders.

Inhibitory effects of bile acids and synthetic farnesoid X receptor agonists on rotavirus replication

Rotaviruses (group A rotaviruses) are the most important cause of severe gastroenteritis in infants and children worldwide. Currently, an antiviral drug is not available and information on therapeutic targets for antiviral development is limited for rotavirus infection. Previously, it was shown that lipid homeostasis is important in rotavirus replication. Since farnesoid X receptor (FXR) and its natural ligands bile acids (such as chenodeoxycholic acid [CDCA]) play major roles in cholesterol and lipid homeostasis, we examined the effects of bile acids and synthetic FXR agonists on rotavirus replication in association with cellular lipid levels. In a mouse model of rotavirus infection, effects of oral administration of CDCA on fecal rotavirus shedding were investigated. The results demonstrate the following. First, the intracellular contents of triglycerides were significantly increased by rotavirus infection. Second, CDCA, deoxycholic acid (DCA), and other synthetic FXR agonists, such as GW4064, significantly reduced rotavirus replication in cell culture in a dose-dependent manner. The reduction of virus replication correlated positively with activation of the FXR pathway and reduction of cellular triglyceride contents (r(2) = 0.95). Third, oral administration of CDCA significantly reduced fecal virus shedding in mice (P < 0.05). We conclude that bile acids and FXR agonists play important roles in the suppression of rotavirus replication. The inhibition mechanism is proposed to be the downregulation of lipid synthesis induced by rotavirus infection.

DNA damage induces down-regulation of PEPCK and G6P gene expression through degradation of PGC-1alpha

Hepatic gluconeogenesis plays a crucial role in glucose homeostasis. Although it is well established that various cellular processes are modulated by DNA damage, whether the DNA damage signaling pathway regulates gluconeogenesis has not yet been studied. In this study, we found that mRNA levels of phosphoenolpyruvate carboxykinase (PEPCK) and glucose-6-phosphatase (G6P), key enzymes for gluconeogenesis, were dramatically decreased upon IR- and UV-irradiation. PEPCK and G6P promoter activities were also suppressed by IR- and UV-irradiation, suggesting that PEPCK and G6P gene transcription are down-regulated upon DNA damage. We also found that the protein level of PGC-1α, which is a critical transcription factor for PEPCK gene expression, is decreased upon UV-irradiation. The decreased PGC-1α protein level was abolished by MG132, a potent proteasome inhibitor, suggesting that PGC-1α is degraded through the ubiquitin-proteasome pathway upon UV-irradiation. These results reveal a novel link between glucose metabolism and the DNA damage signaling pathway and suggest a possible role for PEPCK and G6P in the DNA damage response.

FXR an emerging therapeutic target for the treatment of atherosclerosis

Atherosclerosis is the leading cause of illness and death. Therapeutic strategies aimed at reducing cholesterol plasma levels have shown efficacy in either reducing progression of atherosclerotic plaques and atherosclerosis-related mortality. The farnesoid-X-receptor (FXR) is a member of metabolic nuclear receptors (NRs) superfamily activated by bile acids. In entero-hepatic tissues, FXR functions as a bile acid sensor regulating bile acid synthesis, detoxification and excretion. In the liver FXR induces the expression of an atypical NR, the small heterodimer partner, which subsequently inhibits the activity of hepatocyte nuclear factor 4α repressing the transcription of cholesterol 7a-hydroxylase, the critical regulatory gene in bile acid synthesis. In the intestine FXR induces the release of fibroblast growth factor 15 (FGF15) (or FGF19 in human), which activates hepatic FGF receptor 4 (FGFR4) signalling to inhibit bile acid synthesis. In rodents, FXR activation decreases bile acid synthesis and lipogenesis and increases lipoprotein clearance, and regulates glucose homeostasis by reducing liver gluconeogenesis. FXR exerts counter-regulatory effects on macrophages, vascular smooth muscle cells and endothelial cells. FXR deficiency in mice results in a pro-atherogenetic lipoproteins profile and insulin resistance but FXR^−/– mice fail to develop any detectable plaques on high-fat diet. Synthetic FXR agonists protect against development of aortic plaques formation in murine models characterized by pro-atherogenetic lipoprotein profile and accelerated atherosclerosis, but reduce HDL levels. Because human and mouse lipoprotein metabolism is modulated by different regulatory pathways the potential drawbacks of FXR ligands on HDL and bile acid synthesis need to addressed in relevant clinical settings.

Bile acid metabolism and the pathogenesis of type 2 diabetes

Type 2 diabetes (T2D) is a growing health problem worldwide, but the currently available strategies for therapy and prevention are insufficient. Recent observations indicate that bile acid homeostasis is altered in T2D. Bile acids are metabolic regulators that act as signaling molecules through receptor-dependent and -independent pathways. The most prominent signaling molecules mediating bile acid signaling are the nuclear receptor farnesoid X receptor (FXR) and the membrane receptor TGR5. Both are implicated in the regulation of lipid, glucose, and energy metabolism. Dysregulation of these pathways might contribute to the development of T2D and associated complications. Interestingly, data from studies with bile acids or bile acid sequestrants indicate that the manipulation of bile acid homeostasis might be an attractive approach for T2D therapy. In this review, we summarize the mechanisms of bile acid-mediated metabolic control that might be relevant in the pathogenesis of T2D.

Ezetimibe decreases SREBP-1c expression in liver and reverses hepatic insulin resistance in mice fed a high-fat diet

Ezetimibe inhibits intestinal cholesterol absorption, thereby reducing serum cholesterol. Recent studies suggest that ezetimibe affects liver steatosis and insulin resistance. We investigated the impact of ezetimibe on insulin sensitivity and glucose metabolism in C57BL/6 mice. We analyzed 4 mouse groups fed the following diets: normal chow (4% fat) for 12 weeks, normal chow for 10 weeks followed by normal chow plus ezetimibe for 2 weeks, high-fat chow (32% fat) for 12 weeks, and high-fat chow for 10 weeks followed by high-fat chow plus ezetimibe for 2 weeks. In the normal chow + ezetimibe group, ezetimibe had no impact on body weight, fat mass, lipid metabolism, liver steatosis, glucose tolerance, or insulin sensitivity. In the high-fat chow + ezetimibe group, ezetimibe had no impact on body weight or fat mass but significantly decreased serum low-density lipoprotein cholesterol, triglyceride, and glutamate pyruvate transaminase levels; liver weight; hepatic triglyceride content; and hepatic cholesterol content and increased the hepatic total bile acid content. In association with increases in IRS-2 and Akt phosphorylation, ezetimibe ameliorated hepatic insulin resistance in the high-fat chow + ezetimibe group, but had no effect on insulin sensitivity in primary cultured hepatocytes. A DNA microarray and Taqman polymerase chain reaction revealed that ezetimibe up-regulated hepatic SREBP2 and SHP expression and down-regulated hepatic SREBP-1c expression. SHP silencing mainly in the liver worsened insulin resistance, and ezetimibe protected against insulin resistance induced by down-regulation of SHP. Ezetimibe down-regulated SREBP-1c in the liver and reversed hepatic insulin resistance in mice fed a high-fat diet.

Lipid-sensing nuclear receptors in the pathophysiology and treatment of the metabolic syndrome

Metabolic syndrome (MS) is a cluster of different diseases, namely central obesity, hypertension, hyperglycemia, and dyslipidemia, together with a pro-thrombotic and pro-inflammatory state. These metabolic abnormalities are often associated with an increased risk for cardiovascular disease (CVD) and cancer. Dietary and lifestyle modifications are currently believed more effective than pharmacological therapies in the management of MS patients. Nevertheless, the relatively low grade of compliance of patients to these recommendations, as well as the failure of current therapies, highlights the need for the discovery of new pharmacological and nutraceutic approaches. A deeper knowledge of the patho-physiological events that initiate and support the MS is mandatory. Lipid-sensing nuclear receptors (NRs) are the master transcriptional regulators of lipid and carbohydrate metabolism and inflammatory responses, thus standing as suitable targets. This review focuses on the physiological relevance of the NRs (peroxisome proliferator-activated receptors, liver X receptors, and farnesoid X receptor) in the control of whole-body homeostasis, with a special emphasis on lipid and glucose metabolism, and on the relationships between metabolic unbalances, systemic inflammation, and the onset of CVD. Future perspectives and possible clinical applications are also presented.

Feedback regulation of hepatic gluconeogenesis through modulation of SHP/Nr0b2 gene expression by Sirt1 and FoxO1

Protein deacetylase Sirt1 has been implicated in the regulation of hepatic gluconeogenesis; however, the mechanisms are not fully understood. To further elucidate how Sirt1 regulates gluconeogenesis, we took a loss-of-function approach by deleting the coding DNA sequence for the catalytic domain of the Sirt1 gene in the liver of a wild-type mouse (LKO(Sirt)¹) or a genetic diabetic mouse in which hepatic insulin receptor substrates 1 and 2 are deleted (DKO(Irs½)). Whereas LKO(Sirt)¹ mice exhibited normal levels of fasting and fed blood glucose, inactivation of Sirt1 in DKO(Irs½) mice (TKO(Irs½:Sirt)¹) reduced blood glucose levels and moderately improved systemic glucose tolerance. Pyruvate tolerance was also significantly improved in TKO(Irs½:Sirt)¹ mice, suggesting that Sirt1 promotes hepatic gluconeogenesis in this diabetic mouse model. To understand why inactivation of hepatic Sirt1 does not alter blood glucose levels in the wild-type background, we searched for a potential cause and found that expression of small heterodimer partner (SHP, encoded by the Nr0b2 gene), an orphan nuclear receptor, which has been shown to suppress the activity of forkhead transcription factor FoxO1, was decreased in the liver of LKO(Sirt)¹ mice. Furthermore, our luciferase reporter assays and chromatin immunoprecipitation analysis revealed that the Nr0b2 gene is a target of FoxO1, which is also regulated by Sirt1. After the gene is upregulated, Nr0b2 can feed back and repress FoxO1- and Sirt1-activated G6pc and Pdk4 gene expression. Thus, our results suggest that Sirt1 can both positively and negatively regulate hepatic gluconeogenesis through FoxO1 and Nr0b2 and keep this physiological process in control.

The relationship between postprandial bile acid concentration, GLP-1, PYY and ghrelin

BACKGROUND

Gut hormones peptide YY (PYY) and glucagon-like peptide-1 (GLP-1) play an integral role in appetite control and energy homeostasis. Entero-endocrine L-cells can be stimulated by nutrients and or bile acids to co-secrete PYY and GLP-1. The aim of this study was to determine the response of bile acids, PYY, GLP-1 and ghrelin after a test meal.

DESIGN

Twelve subjects with a BMI of 22·8 (0·52) kg/m² [mean (SEM)] received a 400 kcal test meal after which blood samples were taken every 30 min from 0 to 180 min. PYY, GLP-1 and ghrelin were measured by radioimmunoassays. Fractionated bile acids were measured by HPLC-MSMS.

RESULTS

PYY positively correlated with glycochenodeoxycholic acid (GCDCA) (rs = 0·23, P = 0·03) and taurochenodeoxycholic acid (TCDCA) (rs = 0·26, P = 0·02). GLP-1 positively correlated with GCDCA (rs = 0·22, P = 0·047) and glycodeoxycholic acid (GDCA) (rs = 0·3, P = 0·005). Ghrelin negatively correlated with GDCA (rs = -0·45, P≤ 0·0001), TCDCA (rs = -0·23, P = 0·034) and taurodeoxycholic acid (TDCA) (rs = -0·44, P≤ 0·0001).

CONCLUSION

PYY and GLP-1 responses correlated with chenodeoxycholic acid (CDCA) counterparts, whereas ghrelin negatively correlated with deoxycholic acid (DCA) counterparts. Specific bile acids may thus differentially affect entero-endocrine cells.

Glucose-lowering effect of colestimide is associated with baseline HbA1c in type 2 diabetic patients with hypercholesterolemia

We previously reported that colestimide, an anion exchange resin, improved glycemic control in patients with type 2 diabetes. However, the factors associated with the decrease of HbA1c remain unclear. In present study, we retrospectively compared glycemic control between groups receiving colestimide (n=71), atorvastatin (n=99), pravastatin (n=85), and pitavastatin (n=95) until 3 months after the start of treatment. In the colestimide group, fasting plasma glucose decreased significantly from 169 ± 59 to 138 ± 29 mg/dL after 3 months (P<0.01), and glycated hemoglobin (HbA1c) declined from 8.1 ± 1.0% to 7.4 ± 0.8% (an 8% reduction, P<0.01). Fasting plasma glucose and HbA1c did not change in the pravastatin and pitavastatin groups. On the other hand, both parameters increased significantly in the atorvastatin group. Multivariate analysis revealed that baseline HbA1c was the main determinant of the decrease of HbA1c in the colestimide group while age, sex, BMI, and baseline lipid levels were not correlated with the effect of colestimide treatment. The decrease of HbA1c showed a positive correlation with baseline HbA1c (r=0.60, P<0.0001), and patients with a larger change of HbA1c (>8.4%) displayed a better response to colestimide. In conclusion, since patients with type 2 diabetes often have hyperlipidemia as well, colestimide therapy may have a clinically useful dual action in such patients. Baseline HbA1c has the most important independent influence on the glucose-lowering effect of colestimide.

Deciphering the nuclear bile acid receptor FXR paradigm

Originally called retinoid X receptor interacting protein 14 (RIP14), the farnesoid X receptor (FXR) was renamed after the ability of its rat form to bind supra-physiological concentrations of farnesol. In 1999 FXR was de-orphanized since primary bile acids were identified as natural ligands. Strongly expressed in the liver and intestine, FXR has been shown to be the master transcriptional regulator of several entero-hepatic metabolic pathways with relevance to the pathophysiology of conditions such as cholestasis, fatty liver disease, cholesterol gallstone disease, intestinal inflammation and tumors. Furthermore, given the importance of FXR in the gut-liver axis feedbacks regulating lipid and glucose homeostasis, FXR modulation appears to have great input in diseases such as metabolic syndrome and diabetes. Exciting results from several cellular and animal models have provided the impetus to develop synthetic FXR ligands as novel pharmacological agents. Fourteen years from its discovery, FXR has gone from bench to bedside; a novel nuclear receptor ligand is going into clinical use.

Role of nuclear receptor SHP in metabolism and cancer

Small heterodimer partner (SHP, NR0B2) is a unique member of the nuclear receptor (NR) superfamily that contains the dimerization and ligand-binding domain found in other family members, but lacks the conserved DNA-binding domain. The ability of SHP to bind directly to multiple NRs is crucial for its physiological function as a transcriptional inhibitor of gene expression. A wide variety of interacting partners for SHP have been identified, indicating the potential for SHP to regulate an array of genes in different biological pathways. In this review, we summarize studies concerning the structure and target genes of SHP and discuss recent progress in understanding the function of SHP in bile acid, cholesterol, triglyceride, glucose, and drug metabolism. In addition, we review the regulatory role of SHP in microRNA (miRNA) regulation, liver fibrosis and cancer progression. The fact that SHP controls a complex set of genes in multiple metabolic pathways suggests the intriguing possibility of developing new therapeutics for metabolic diseases, including fatty liver, dyslipidemia and obesity, by regulating SHP with small molecules. To achieve this goal, more progress regarding SHP ligands and protein structure will be required. Besides its metabolic regulatory function, studies by us and other groups provide strong evidence that SHP plays a critical role in the development of cancer, particularly liver and breast cancer. An increased understanding of the fundamental mechanisms by which SHP regulates the development of cancers will be critical in applying knowledge of SHP in diagnostic, therapeutic or preventive strategies for specific cancers. This article is part of a Special Issue entitled: Translating nuclear receptors from health to disease.

FoxO1 represses LXRα-mediated transcriptional activity of SREBP-1c promoter in HepG2 cells

Recent studies have demonstrated that FoxO1 modulates the expression of SREBP-1c, but the exact mechanism remains unknown. Our results demonstrate that FoxO1 suppresses the SREBP-1c promoter transcriptional activity in HepG2 cells. This repression was independent of FoxO1 binding to the SREBP-1c promoter, but LXR responsive elements (LXREs) were crucial to this phenomenon. Moreover, FoxO1 also strongly inhibited the LXRα-mediated elevated transcription by SREBP-1c promoter. Electrophoretic mobility shift assay and chromatin immuno-precipitation further suggested the ability of FoxO1 to inhibit LXRα binding with the LXRE in the SREBP-1c promoter. FoxO1-mediated suppression of SREBP-1c promoter activity could be partially alleviated by insulin.

The grapefruit: an old wine in a new glass? Metabolic and cardiovascular perspectives

Grapefruit is a popular, tasty and nutritive fruit enjoyed globally. Biomedical evidence in the last 10 years has, however, shown that consumption of grapefruit or its juice is associated with drug interactions, which, in some cases, have been fatal. Grapefruit-induced drug interactions are unique in that the cytochrome P450 enzyme CYP3A4, which metabolises over 60% of commonly prescribed drugs as well as other drug transporter proteins such as P-glycoprotein and organic cation transporter proteins, which are all expressed in the intestines, are involved. However, the extent to which grapefruit-drug interactions impact on clinical settings has not been fully determined, probably because many cases are not reported. It has recently emerged that grapefruit, by virtue of its rich flavonoid content, is beneficial in the management of degenerative diseases such as diabetes and cardiovascular disorders. This potentially explosive subject is reviewed here.

Discovery of novel forkhead box O1 inhibitors for treating type 2 diabetes: improvement of fasting glycemia in diabetic db/db mice

Excessive hepatic glucose production through the gluconeogenesis pathway is partially responsible for the elevated glucose levels observed in patients with type 2 diabetes mellitus (T2DM). The forkhead transcription factor forkhead box O1 (Foxo1) plays a crucial role in mediating the effect of insulin on hepatic gluconeogenesis. Here, using a db/db mouse model, we demonstrate the effectiveness of Foxo1 inhibitor, an orally active small-molecule compound, as a therapeutic drug for treating T2DM. Using mass spectrometric affinity screening, we discovered a series of compounds that bind to Foxo1, identifying among them the compound, 5-amino-7-(cyclohexylamino)-1-ethyl-6-fluoro-4-oxo-1,4-dihydroquinoline-3-carboxylic acid (AS1842856), which potently inhibits human Foxo1 transactivation and reduces glucose production through the inhibition of glucose-6 phosphatase and phosphoenolpyruvate carboxykinase mRNA levels in a rat hepatic cell line. Oral administration of AS1842856 to diabetic db/db mice led to a drastic decrease in fasting plasma glucose level via the inhibition of hepatic gluconeogenic genes, whereas administration to normal mice had no effect on the fasting plasma glucose level. Treatment with AS1842856 also suppressed an increase in plasma glucose level caused by pyruvate injection in both normal and db/db mice. Taken together, these findings indicate that the Foxo1 inhibitor represents a new class of drugs for use in treating T2DM.

AMPK-dependent repression of hepatic gluconeogenesis via disruption of CREB.CRTC2 complex by orphan nuclear receptor small heterodimer partner

Orphan nuclear receptor small heterodimer partner (SHP) plays a key role in transcriptional repression of gluconeogenic enzyme gene expression. Here, we show that SHP inhibited protein kinase A-mediated transcriptional activity of cAMP-response element-binding protein (CREB), a major regulator of glucose metabolism, to modulate hepatic gluconeogenic gene expression. Deletion analysis of phosphoenolpyruvate carboxykinase (PEPCK) promoter demonstrated that SHP inhibited forskolin-mediated induction of PEPCK gene transcription via inhibition of CREB transcriptional activity. In vivo imaging demonstrated that SHP inhibited CREB-regulated transcription coactivator 2 (CRTC2)-mediated cAMP-response element-driven promoter activity. Furthermore, overexpression of SHP using adenovirus SHP decreased CRTC2-dependent elevations in blood glucose levels and PEPCK or glucose-6-phosphatase (G6Pase) expression in mice. SHP and CREB physically interacted and were co-localized in vivo. Importantly, SHP inhibited both wild type CRTC2 and S171A (constitutively active form of CRTC2) coactivator activity and disrupted CRTC2 recruitment on the PEPCK gene promoter. In addition, metformin or overexpression of a constitutively active form of AMPK (Ad-CA-AMPK) inhibited S171A-mediated PEPCK and G6Pase gene expression, and hepatic glucose production and knockdown of SHP partially relieved the metformin- and Ad-CA-AMPK-mediated repression of hepatic gluconeogenic enzyme gene expression in primary rat hepatocytes. In conclusion, our results suggest that a delayed effect of metformin-mediated induction of SHP gene expression inhibits CREB-dependent hepatic gluconeogenesis.

Transgenic expression of cholesterol 7alpha-hydroxylase in the liver prevents high-fat diet-induced obesity and insulin resistance in mice

Cholesterol 7alpha-hydroxylase (CYP7A1) is the rate-limiting enzyme in the bile acid biosynthetic pathway that converts cholesterol into bile acids in the liver. Recent studies have shown that bile acids may play an important role in maintaining lipid, glucose, and energy homeostasis. However, the role of CYP7A1 in the development of obesity and diabetes is currently unclear. In this study, we demonstrated that transgenic mice overexpressing Cyp7a1 in the liver [i.e., Cyp7a1 transgenic (Cyp7a1-tg) mice] were resistant to high-fat diet (HFD)-induced obesity, fatty liver, and insulin resistance. Cyp7a1-tg mice showed increased hepatic cholesterol catabolism and an increased bile acid pool. Cyp7a1-tg mice had increased secretion of hepatic very low density lipoprotein but maintained plasma triglyceride homeostasis. Gene expression analysis showed that the hepatic messenger RNA expression levels of several critical lipogenic and gluconeogenic genes were significantly decreased in HFD-fed Cyp7a1-tg mice. HFD-fed Cyp7a1-tg mice had increased whole body energy expenditure and induction of fatty acid oxidation genes in the brown adipose tissue.

CONCLUSION

This study shows that Cyp7a1 plays a critical role in maintaining whole body lipid, glucose, and energy homeostasis. The induction of CYP7A1 expression with the expansion of the hydrophobic bile acid pool may be a potential therapeutic strategy for treating metabolic disorders such as fatty liver diseases, obesity, and diabetes in humans.

Bile acids regulate hepatic gluconeogenic genes and farnesoid X receptor via G(alpha)i-protein-coupled receptors and the AKT pathway

Bile acids are important regulatory molecules that can activate specific nuclear receptors and cell signaling pathways in the liver and gastrointestinal tract. In the current study, the chronic bile fistula (CBF) rat model and primary rat hepatocytes (PRH) were used to study the regulation of gluconeogenic genes phosphoenolpyruvate carboxykinase (PEPCK) and glucose-6-phosphatase (G-6-Pase) and the gene encoding short heterodimeric partner (SHP) by taurocholate (TCA). The intestinal infusion of TCA into the CBF rat rapidly (1h) activated the AKT (approximately 9-fold) and ERK1/2 (3- to 5-fold) signaling pathways, downregulated (approximately 50%, 30 min) the mRNA levels of PEPCK and G-6-Pase, and induced (14-fold in 3 h) SHP mRNA. TCA rapidly ( approximately 50%, 1-2 h) downregulated PEPCK and G-6-Pase mRNA levels in PRH. The downregulation of these genes by TCA was blocked by pretreatment of PRH with pertussis toxin (PTX). In PRH, TCA plus insulin showed a significantly stronger inhibition of glucose secretion/synthesis from lactate and pyruvate than either alone. The induction of SHP mRNA in PRH was strongly blocked by inhibition of PI3 kinase or PKCzeta by specific chemical inhibitors or knockdown of PKCzeta by siRNA encoded by a recombinant lentivirus. Activation of the insulin signaling pathway appears to be linked to the upregulation of farnesoid X receptor functional activity and SHP induction.

Targeting orphan nuclear receptor SHP in the treatment of metabolic diseases

IMPORTANCE OF THE FIELD

The orphan nuclear receptor small heterodimer partner (SHP; NR0B2) is an atypical nuclear receptor that contains a ligand-binding domain, but lacks the conserved DNA-binding domain. Since its discovery, SHP has been identified as a key transcriptional regulatory factor of genes involved in diverse metabolic pathways.

AREAS COVERED IN THIS REVIEW

We performed a Medline/Pubmed search to find published studies on the role of SHP in the regulation of metabolism in the liver, pancreatic islets, blood vessel, and kidney and on the feasibility of using SHP as a therapeutic target in metabolic disease.

WHAT THE READER WILL GAIN

In this review, we discuss the function of SHP as regulator of cholesterol, lipid and glucose metabolism, and the role of SHP in metabolic and fibrotic diseases.

TAKE HOME MESSAGE

The reviewed studies suggested that SHP could be a major target for therapeutic intervention in metabolic and fibrotic diseases, including metabolic syndrome and its complications. However, for SHP-targeted therapy, there are some outstanding issues, including the small number of known inducers of SHP and the lack of sufficient data in humans.

A comparative study of genome-wide transcriptional profiles of primary hepatocytes in collagen sandwich and monolayer cultures

Two commonly used culture systems in hepatic tissue engineering are the collagen sandwich (CS) and monolayers of cells. In this study, genome-wide gene expression profiles of primary hepatocytes were measured over an 8-day period for each cell culture system using Affymetrix GeneChips and compared via gene set enrichment analysis to elicit biologically meaningful information at the level of gene sets. Our results demonstrate that gene expression in hepatocytes in CS cultures steadily and comprehensively diverges from that in monolayer cultures. Gene sets up-regulated in CS cultures include several associated with liver metabolic and synthesis functions, such as metabolism of lipids, amino acids, carbohydrates, and alcohol, and synthesis of bile acids. Monooxygenases such as Cytochrome-P450 enzymes do not show any change between the culture systems after 1 day, but exhibit significant up-regulation in CS cultures after 3 days in comparison to hepatocyte monolayers. These data provide insights into the up- and down-regulation of several liver-critical gene sets and their subsequent effects on liver-specific functions. These results provide a baseline for further explorations into the systems biology of engineered liver mimics.

Orphan nuclear receptor small heterodimer partner inhibits angiotensin II- stimulated PAI-1 expression in vascular smooth muscle cells

Angiotensin II is a major effector molecule in the development of cardiovascular disease. In vascular smooth muscle cells (VSMCs), angiotensin II promotes cellular proliferation and extracellular matrix accumulation through the upregulation of plasminogen activator inhibitor-1 (PAI-1) expression. Previously, we demonstrated that small heterodimer partner (SHP) represses PAI-1 expression in the liver through the inhibition of TGF-beta signaling pathways. Here, we investigated whether SHP inhibited angiotensin II-stimulated PAI-1 expression in VSMCs. Adenovirus-mediated overexpression of SHP (Ad- SHP) in VSMCs inhibited angiotensin II- and TGF-beta-stimulated PAI-1 expression. Ad-SHP also inhibited angiotensin II-, TGF-beta- and Smad3-stimulated PAI-1 promoter activity, and angiotensin II-stimulated AP-1 activity. The level of PAI-1 expression was significantly higher in VSMCs of SHP(-/-) mice than wild type mice. Moreover, loss of SHP increased PAI-1 mRNA expression after angiotensin II treatment. These results suggest that SHP inhibits PAI-1 expression in VSMCs through the suppression of TGF-beta/Smad3 and AP-1 activity. Thus, agents that target the induction of SHP expression in VSMCs might help prevent the development and progression of atherosclerosis.

Emerging molecular targets for the treatment of nonalcoholic fatty liver disease

Nonalcoholic fatty liver disease (NAFLD) is characterized by hepatic fat accumulation in the absence of significant ethanol consumption, viral infection, or other specific causes of liver disease. Currently the most common chronic liver disease, affecting 30% of the Western world, NAFLD may progress to cirrhosis and end-stage liver disease and may increase the risk of developing diabetes and cardiovascular disease. Although its pathogenesis is unclear, NAFLD is tightly associated with insulin resistance and the metabolic syndrome. No established treatment exists, and current research is targeting new molecular mechanisms that underlie NAFLD and associated cardiometabolic disorders. This review discusses some of these emerging molecular mechanisms and their therapeutic implications for the treatment of NAFLD: microRNAs, incretin analogs/antagonists, liver-specific thyromimetics, AMP-activated protein kinase activators, and nuclear receptors farnesoid X receptor and pregane X receptor.

The nuclear receptor FXR is expressed in pancreatic beta-cells and protects human islets from lipotoxicity

Farnesoid X receptor (FXR) is highly expressed in liver and intestine where it controls bile acid (BA), lipid and glucose homeostasis. Here we show that FXR is expressed and functional, as assessed by target gene expression analysis, in human islets and beta-cell lines. FXR is predominantly cytosolic-localized in the islets of lean mice, but nuclear in obese mice. Compared to FXR+/+ mice, FXR-/- mice display a normal architecture and beta-cell mass but the expression of certain islet-specific genes is altered. Moreover, glucose-stimulated insulin secretion (GSIS) is impaired in the islets of FXR-/- mice. Finally, FXR activation protects human islets from lipotoxicity and ameliorates their secretory index.

Colesevelam lowers glucose and lipid levels in type 2 diabetes: the clinical evidence

Simultaneous control of blood glucose and other risk factors such as hypertension and dyslipidaemia is essential for reducing the risk of complications associated with type 2 diabetes mellitus (T2DM). As relatively few patients with T2DM have their risk factors managed to within the limits recommended by the American Diabetes Association, American College of Endocrinology or National Cholesterol Education Program Adult Treatment Panel III guidelines, treatment that can simultaneously control more than one risk factor is of therapeutic benefit. Clinical studies have shown that bile acid sequestrants have glucose-lowering effects in addition to their low-density lipoprotein cholesterol-lowering effects in patients with T2DM. The bile acid sequestrant colesevelam hydrochloride is approved as an adjunct to antidiabetes therapy for improving glycaemic control in adults with T2DM. This review examines data from three phase III clinical trials that evaluated the glucose- and lipid-lowering effects of colesevelam when added to the existing antidiabetes treatment regimen of patients with T2DM.

Colesevelam improves insulin resistance in a diet-induced obesity (F-DIO) rat model by increasing the release of GLP-1

Bile acid sequestrants have been shown to lower glucose levels in patients with type 2 diabetes. To investigate how colesevelam (CL) HCl improves hyperglycemia, studies were conducted in diet-induced obesity (F-DIO) rats, which develop insulin resistance when fed a high-energy (high fat/high sucrose) diet (HE). The rats were fed HE; HE + 2% CL; HE + 0.02% SC-435 (SC), an apical sodium-dependent bile acid transporter inhibitor; and regular chow (controls). After 4 wk of treatment, both in the HE group and the SC + HE group, plasma glucose and insulin levels remained elevated compared with baseline values throughout an oral glucose tolerance test (OGTT). In contrast, in the CL + HE group, plasma glucose levels returned to baseline by the end of the test, and insulin peaked in 15-30 min and then returned to baseline. CL induced release of glucagon-like peptide-1 (GLP-1) because the area under the curve of plasma total GLP-1 in the CL + HE group was significantly greater than in the HE group during the OGTT. Bile acid concentrations in the portal blood did not decrease in the HE group but declined significantly both in the CL + HE and SC + HE groups with reduced farnesoid X receptor activation compared with controls. We concluded that CL reduces plasma glucose levels by improving insulin resistance in this rat model. It is unlikely that the improvement is attributable to decreased bile acid flux to the liver but is likely secondary to induced GLP-1 secretion, which improves insulin release.

Bile acid-activated receptors in the treatment of dyslipidemia and related disorders

Dyslipidemia is a metabolic disorder that constitutes a major risk factor for cardiovascular diseases and stroke and is often associated with diabetes mellitus and atherosclerosis. In recent years a number of ligand-activated receptors have been found to exert a role in integrating essential steps of lipid and glucose metabolism. Bile acid-activated receptors are a defined subset of nuclear and G-protein coupled receptors mainly expressed in entero-hepatic tissues for which bile acids function as signaling molecules. Primary bile acids (chenodeoxycholic acid and cholic acid) are physiological ligands/activators of farnesoid-X-receptor (FXR), pregnane-X-receptor (PXR) and constitutive androstane receptor (CAR), while litocholic acid is a ligand for the Vitamin D receptor (VDR) and the G-protein coupled receptor TGR5. Despite FXR demonstrates a high selectivity for bile acids, PXR and CAR are relatively promiscuous receptors integrating lipid homeostasis with xenobiotic metabolism. FXR, PXR, CAR and TGR exert synergistic activities in regulating lipid and glucose homeostasis and energy expenditure and liver and peripheral insulin sensitivity. Ligands for these receptors hold promise in the treatment of dyslipidemic conditions as revealed by results of a number of preclinical models but carry a defined risk for potential side effects.

Pharmacotherapy of hyperglycemia

Type 2 diabetes mellitus (T2DM) is a chronic, progressive disorder that affects more than 230 million people worldwide and is expected to affect 366 million by 2030. Both the prevalence of T2DM and the cost of its long term complications has driven the focus and emphasis on treatments aimed at reducing hyperglycemia and controlling hypertension and dyslipidemia. In the last 5 years new glucose lowering drugs acting on novel pathways have been developed, licensed and launched. These drugs include the glucagon-like peptide (GLP-1) agonists, exenatide, and dipeptidyl peptidase (DPP-IV) inhibitors such as sitagliptin and saxagliptin. This review describes current approaches to T2DM treatment, focusing on newer agents which tend to be associated with less hypoglycemia and possible weight loss, and addresses the potential roles of novel oral pharmacologic agents in the late-stages of development that might provide new options for the management of this disease.

Transcriptional corepressor SMILE recruits SIRT1 to inhibit nuclear receptor estrogen receptor-related receptor gamma transactivation

SMILE (small heterodimer partner interacting leucine zipper protein) has been identified as a corepressor of the glucocorticoid receptor, constitutive androstane receptor, and hepatocyte nuclear factor 4alpha. Here we show that SMILE also represses estrogen receptor-related receptor gamma (ERRgamma) transactivation. Knockdown of SMILE gene expression increases ERRgamma activity. SMILE directly interacts with ERRgamma in vitro and in vivo. Domain mapping analysis showed that SMILE binds to the AF2 domain of ERRgamma. SMILE represses ERRgamma transactivation partially through competition with coactivators PGC-1alpha, PGC-1beta, and GRIP1. Interestingly, the repression of SMILE on ERRgamma is released by SIRT1 inhibitors, a catalytically inactive SIRT1 mutant, and SIRT1 small interfering RNA but not by histone protein deacetylase inhibitor. In vivo glutathione S-transferase pulldown and coimmunoprecipitation assays validated that SMILE physically interacts with SIRT1. Furthermore, the ERRgamma inverse agonist GSK5182 enhances the interaction of SMILE with ERRgamma and SMILE-mediated repression. Knockdown of SMILE or SIRT1 blocks the repressive effect of GSK5182. Moreover, chromatin immunoprecipitation assays revealed that GSK5182 augments the association of SMILE and SIRT1 on the promoter of the ERRgamma target PDK4. GSK5182 and adenoviral overexpression of SMILE cooperate to repress ERRgamma-induced PDK4 gene expression, and this repression is released by overexpression of a catalytically defective SIRT1 mutant. Finally, we demonstrated that ERRgamma regulates SMILE gene expression, which in turn inhibits ERRgamma. Overall, these findings implicate SMILE as a novel corepressor of ERRgamma and recruitment of SIRT1 as a novel repressive mechanism for SMILE and ERRgamma inverse agonist.

DAX-1 acts as a novel corepressor of orphan nuclear receptor HNF4alpha and negatively regulates gluconeogenic enzyme gene expression

DAX-1 (dosage-sensitive sex reversal adrenal hypoplasia congenital critical region on X chromosome, gene 1) is an atypical member of the nuclear receptor family and acts as a corepressor of a number of nuclear receptors. HNF4alpha (hepatocyte nuclear factor 4alpha) is a liver-enriched transcription factor that controls the expression of a variety of genes involved in cholesterol, fatty acid, and glucose metabolism. Here we show that DAX-1 inhibits transcriptional activity of HNF4alpha and modulates hepatic gluconeogenic gene expression. Hepatic DAX-1 expression is increased by insulin and SIK1 (salt-inducible kinase 1), whereas it is decreased in high fat diet-fed and diabetic mice. Coimmunoprecipitation assay from mouse liver samples depicts that endogenous DAX-1 interacts with HNF4alpha in vivo. In vivo chromatin immunoprecipitation assay affirms that the recruitment of DAX-1 on the phosphoenolpyruvate carboxykinase (PEPCK) gene promoter is inversely correlated with the recruitment of PGC-1alpha and HNF4alpha under fasting and refeeding, showing that DAX-1 could compete with the coactivator PGC-1alpha for binding to HNF4alpha. Adenovirus-mediated expression of DAX-1 decreased both HNF4alpha- and forskolin-mediated gluconeogenic gene expressions. In addition, knockdown of DAX-1 partially reverses the insulin-mediated inhibition of gluconeogenic gene expression in primary hepatocytes. Finally, DAX-1 inhibits PEPCK and glucose-6-phosphatase gene expression and significantly lowers fasting blood glucose level in high fat diet-fed mice, suggesting that DAX-1 can modulate hepatic gluconeogenesis in vivo. Overall, this study demonstrates that DAX-1 acts as a corepressor of HNF4alpha to negatively regulate hepatic gluconeogenic gene expression in liver.

Bile acids as regulatory molecules

In the past, bile acids were considered to be just detergent molecules derived from cholesterol in the liver. They were known to be important for the solubilization of cholesterol in the gallbladder and for stimulating the absorption of cholesterol, fat-soluble vitamins, and lipids from the intestines. However, during the last two decades, it has been discovered that bile acids are regulatory molecules. Bile acids have been discovered to activate specific nuclear receptors (farnesoid X receptor, preganane X receptor, and vitamin D receptor), G protein coupled receptor TGR5 (TGR5), and cell signaling pathways (c-jun N-terminal kinase 1/2, AKT, and ERK 1/2) in cells in the liver and gastrointestinal tract. Activation of nuclear receptors and cell signaling pathways alter the expression of numerous genes encoding enzyme/proteins involved in the regulation of bile acid, glucose, fatty acid, lipoprotein synthesis, metabolism, transport, and energy metabolism. They also play a role in the regulation of serum triglyceride levels in humans and rodents. Bile acids appear to function as nutrient signaling molecules primarily during the feed/fast cycle as there is a flux of these molecules returning from the intestines to the liver following a meal. In this review, we will summarize the current knowledge of how bile acids regulate hepatic lipid and glucose metabolism through the activation of specific nuclear receptors and cell signaling pathways.

Colesevelam hydrochloride for the treatment of type 2 diabetes mellitus

BACKGROUND

Colesevelam hydrochloride is a bile acid sequestrant approved in January 2008 by the US Food and Drug Administration (FDA) for the treatment of adult patients with type 2 diabetes mellitus (DM) in combination with a sulfonylurea, metformin, and/or insulin therapy.

OBJECTIVE

The purpose of this article was to review the pharmacology, pharmacokinetics, efficacy, adverse effects and tolerability, drug-drug interactions, contraindications/precautions, dosage and administration, pharmacoeconomics, and the overall role of colesevelam in the management of adult patients with type 2 DM.

METHODS

A literature search using MEDLINE (1966-October 27, 2008), PubMed (1950-October 27, 2008), Science Direct (1994-October 27, 2008), Web of Science (1980-October 27, 2008), American Diabetes Association Scientific Abstracts (2004-2008), and International Pharmaceutical Abstracts (1970-October 27, 2008) was performed using the term colesevelam. English-language, original research and review articles were examined, and citations from these articles were assessed. Manufacturer prescribing information and the FDA review of the new drug application for colesevelam were also examined.

RESULTS

Colesevelam is a hydrophilic, water-insoluble polymer, with negligible absorption and systemic distribution, that is excreted primarily in the feces. Through a mechanism still under investigation, colesevelam effectively lowers glycosylated hemoglobin (HbA(1c)) when used in combination with a sulfonylurea, metformin, and/or insulin therapy. Three completed, published Phase III clinical trials investigating colesevelam for the treatment of type 2 DM were evaluated for information, data, and conclusions. At dosing of 1.875 g BID or 3.75 g once daily in combination with one of the aforementioned agents versus placebo, reductions in HbA(1c) in all 3 Phase III clinical trials of colesevelam ranged from 0.5% to 0.7% (P < 0.02). In clinical trials, colesevelam was well tolerated, with hypoglycemia occurring in approximately 3% of studied patients.

CONCLUSIONS

When used in combination with a sulfonylurea, metformin, and/or insulin therapy, colesevelam has been reported to significantly reduce HbA(1c) in adult patients with type 2 DM. Colesevelam's role in the management of type 2 DM remains undefined, however; further investigation into its mechanism of action and long-term efficacy and safety should be performed.

Bile acids and insulin resistance: implications for treating nonalcoholic fatty liver disease

Nonalcoholic fatty liver disease is characterized by an accumulation of excess triglycerides in hepatocytes, and insulin resistance is now considered the fundamental operative mechanism throughout the prevalence and progression of the disease. Besides their role in dietary lipid absorption and cholesterol homeostasis, evidence has accumulated that bile acids are also signaling molecules that play two important roles in glucose and lipid metabolism: in the nuclear hormone receptors as farnesoid X receptors (FXR), as well as ligands for G-protein-coupled receptors TGR5. The activated FXR-SHP pathway regulates the enterohepatic recycling and biosynthesis of bile acids and underlies the down-regulation of hepatic fatty acid and triglyceride biosynthesis and very low density lipoprotein production mediated by sterol-regulatory element-binding protein-1c. The bile acid-TGR5-cAMP-D2 signaling pathway in human skeletal muscle in the fasting-feeding cycle increases energy expenditure and prevents obesity. Therefore, a molecular basis has been provided for a link between bile acids, lipid metabolism and glucose homeostasis, which can open novel pharmacological approaches against insulin resistance and nonalcoholic fatty liver disease.

Interrelationship between liver X receptor alpha, sterol regulatory element-binding protein-1c, peroxisome proliferator-activated receptor gamma, and small heterodimer partner in the transcriptional regulation of glucokinase gene expression in liver

Liver glucokinase (LGK) plays an essential role in controlling blood glucose levels and maintaining cellular metabolic functions. Expression of LGK is induced mainly regulated by insulin through sterol regulatory element-binding protein-1c (SREBP-1c) as a mediator. Since LGK expression is known to be decreased in the liver of liver X receptor (LXR) knockout mice, we have investigated whether LGK might be directly activated by LXRalpha. Furthermore, we have studied interrelationship between transcription factors that control gene expression of LGK. In the current studies, we demonstrated that LXRalpha increased LGK expression in primary hepatocytes and that there is a functional LXR response element in the LGK gene promoter as shown by electrophoretic mobility shift and chromatin precipitation assay. In addition, our studies demonstrate that LXRalpha and insulin activation of the LGK gene promoter occurs through a multifaceted indirect mechanism. LXRalpha increases SREBP-1c expression and then insulin stimulates the processing of the membrane-bound precursor SREBP-1c protein, and it activates LGK expression through SREBP sites in its promoter. LXRalpha also activates the LGK promoter by increasing the transcriptional activity and induction of peroxisome proliferator-activated receptor (PPAR)-gamma, which also stimulates LGK expression through a peroxisome proliferator-responsive element. This activation is tempered through a negative mechanism, where a small heterodimer partner (SHP) decreases LGK gene expression by inhibiting the transcriptional activity of LXRalpha and PPARgamma by directly interacting with their common heterodimer partner RXRalpha. From these data, we propose a mechanism for LXRalpha in controlling the gene expression of LGK that involves activation through SREBP-1c and PPARgamma and inhibition through SHP.

Hematopoietically expressed homeobox is a target gene of farnesoid X receptor in chenodeoxycholic acid-induced liver hypertrophy

Farnesoid X receptor (FXR/Fxr) is a bile acid-regulated nuclear receptor that promotes hepatic bile acid metabolism, detoxification, and liver regeneration. However, the adaptive pathways under conditions of bile acid stress are not fully elucidated. We found that wild-type but not Fxr knockout mice on diets enriched with chenodeoxycholic acid (CDCA) increase their liver/body weight ratios by 50% due to hepatocellular hypertrophy. Microarray analysis identified Hex (Hematopoietically expressed homeobox), a central transcription factor in vertebrate embryogenesis and liver development, as a novel CDCA- and Fxr-regulated gene. HEX/Hex was also regulated by FXR/Fxr and CDCA in primary mouse hepatocytes and human HepG2 cells. Comparative genomic analysis identified a conserved inverted repeat-1-like DNA sequence within a 300 base pair enhancer element of intron-1 in the human and mouse HEX/Hex gene. A combination of chromatin immunoprecipitation, electromobility shift assay, and transcriptional reporter assays demonstrated that FXR/Fxr binds to this element and mediates HEX/Hex transcriptional activation.

CONCLUSION

HEX/Hex is a novel bile acid-induced FXR/Fxr target gene during adaptation of hepatocytes to chronic bile acid exposure.

Role of bile acids and bile acid receptors in metabolic regulation

The incidence of the metabolic syndrome has taken epidemic proportions in the past decades, contributing to an increased risk of cardiovascular disease and diabetes. The metabolic syndrome can be defined as a cluster of cardiovascular disease risk factors including visceral obesity, insulin resistance, dyslipidemia, increased blood pressure, and hypercoagulability. The farnesoid X receptor (FXR) belongs to the superfamily of ligand-activated nuclear receptor transcription factors. FXR is activated by bile acids, and FXR-deficient (FXR(-/-)) mice display elevated serum levels of triglycerides and high-density lipoprotein cholesterol, demonstrating a critical role of FXR in lipid metabolism. In an opposite manner, activation of FXR by bile acids (BAs) or nonsteroidal synthetic FXR agonists lowers plasma triglycerides by a mechanism that may involve the repression of hepatic SREBP-1c expression and/or the modulation of glucose-induced lipogenic genes. A cross-talk between BA and glucose metabolism was recently identified, implicating both FXR-dependent and FXR-independent pathways. The first indication for a potential role of FXR in diabetes came from the observation that hepatic FXR expression is reduced in animal models of diabetes. While FXR(-/-) mice display both impaired glucose tolerance and decreased insulin sensitivity, activation of FXR improves hyperglycemia and dyslipidemia in vivo in diabetic mice. Finally, a recent report also indicates that BA may regulate energy expenditure in a FXR-independent manner in mice, via activation of the G protein-coupled receptor TGR5. Taken together, these findings suggest that modulation of FXR activity and BA metabolism may open new attractive pharmacological approaches for the treatment of the metabolic syndrome and type 2 diabetes.

FGF15/FGFR4 integrates growth factor signaling with hepatic bile acid metabolism and insulin action

The current studies show FGF15 signaling decreases hepatic forkhead transcription factor 1 (FoxO1) activity through phosphatidylinositol (PI) 3-kinase-dependent phosphorylation. The bile acid receptor FXR (farnesoid X receptor) activates expression of fibroblast growth factor (FGF) 15 in the intestine, which acts through hepatic FGFR4 to suppress cholesterol-7alpha hydroxylase (CYP7A1) and limit bile acid production. Because FoxO1 activity and CYP7A1 gene expression are both increased by fasting, we hypothesized CYP7A1 might be a FoxO1 target gene. Consistent with recently reported results, we show CYP7A1 is a direct target of FoxO1. Additionally, we show that the PI 3-kinase pathway is key for both the induction of CYP7A1 by fasting and the suppression by FGF15. FGFR4 is the major hepatic FGF receptor isoform and is responsible for the hepatic effects of FGF15. We also show that expression of FGFR4 in liver was decreased by fasting, increased by insulin, and reduced by streptozotocin-induced diabetes, implicating FGFR4 as a primary target of insulin regulation. Because insulin and FGF both target the PI 3-kinase pathway, these observations suggest FoxO1 is a key node in the convergence of FGF and insulin signaling pathways and functions as a key integrator for the regulation of glucose and bile acid metabolism.

An increased flux through the glucose 6-phosphate pool in enterocytes delays glucose absorption in Fxr-/- mice

The farnesoid X receptor (FXR) is involved in regulation of bile acid and lipid metabolism. Recently, a role for FXR in control of glucose metabolism became evident. Because FXR is expressed along the length of the small intestine, we evaluated the potential role of FXR in glucose absorption and processing. During intravenous infusion of a trace amount of d-[6,6-(2)H(2)]glucose, a d-[U-(13)C]glucose-enriched oral glucose bolus was given, and glucose kinetics were determined in wild-type and Fxr(-/-) mice. Compared with wild-type mice, Fxr(-/-) mice showed a delayed plasma appearance of orally administered glucose. Multicompartmental kinetic modeling revealed that this delay was caused by an increased flux through the glucose 6-phosphate pool in enterocytes. Thus, our results show involvement of FXR in intestinal glucose absorption, representing a novel physiological function for this nuclear receptor.

SMILE, a new orphan nuclear receptor SHP-interacting protein, regulates SHP-repressed estrogen receptor transactivation

SHP (small heterodimer partner) is a well-known NR (nuclear receptor) co-regulator. In the present study, we have identified a new SHP-interacting protein, termed SMILE (SHP-interacting leucine zipper protein), which was previously designated as ZF (Zhangfei) via a yeast two-hybrid system. We have determined that the SMILE gene generates two isoforms [SMILE-L (long isoform of SMILE) and SMILE-S (short isoform of SMILE)]. Mutational analysis has demonstrated that the SMILE isoforms arise from the alternative usage of initiation codons. We have confirmed the in vivo interaction and co-localization of the SMILE isoforms and SHP. Domain-mapping analysis indicates that the entire N-terminus of SHP and the middle region of SMILE-L are involved in this interaction. Interestingly, the SMILE isoforms counteract the SHP repressive effect on the transactivation of ERs (estrogen receptors) in HEK-293T cells (human embryonic kidney cells expressing the large T-antigen of simian virus 40), but enhance the SHP-repressive effect in MCF-7, T47D and MDA-MB-435 cells. Knockdown of SMILE gene expression using siRNA (small interfering RNA) in MCF-7 cells increases ER-mediated transcriptional activity. Moreover, adenovirus-mediated overexpression of SMILE and SHP down-regulates estrogen-induced mRNA expression of the critical cell-cycle regulator E2F1. Collectively, these results indicate that SMILE isoforms regulate the inhibition of ER transactivation by SHP in a cell-type-specific manner and act as a novel transcriptional co-regulator in ER signalling.

Nuclear receptors: mediators and modifiers of inflammation-induced cholestasis

Inflammation-induced cholestasis (IIC) is a frequently occurring phenomenon. A central role in its pathogenesis is played by nuclear receptors (NRs). These ligand-activated transcription factors not only regulate basal expression of hepatobiliary transport systems, but also mediate adaptive responses to inflammation and possess anti-inflammatory characteristics. The latter two functions may be exploited in the search for new treatments for IIC as well as for cholestasis in general. Current knowledge of the pathogenesis of IIC and the dual role NRs in this process are reviewed. Special interest is given to the use of NRs as potential targets for intervention.

Reducing cardiovascular complications of type 2 diabetes by targeting multiple risk factors

Insulin resistance syndrome is characterized by hyperglycemia, atherogenic dyslipidemia, hypertension, and abdominal obesity. Hyperglycemia is the major risk factor for microvascular complications in type 2 diabetes. However, 70% to 80% of patients with type 2 diabetes will die of macrovascular disease. Atherogenic dyslipidemia-characterized by elevated triglyceride levels, low high-density lipoprotein cholesterol (HDL-c) levels, and a preponderance of small, dense, low-density lipoprotein (LDL) particles-is the major cause of atherosclerosis in individuals with type 2 diabetes. Therefore, treatment of type 2 diabetes must address hyperglycemia to prevent microvascular disease (retinopathy, neuropathy, and nephropathy) and atherogenic dyslipidemia to prevent macrovascular complications. Emerging evidence indicates lipid and glucose homeostasis are interrelated via bile acid-activated nuclear hormone receptor signaling pathways. Agents that act on these pathways could simultaneously address hyperglycemia and dyslipidemia in patients with type 2 diabetes. Recent studies have shown that bile acid sequestrants, including cholestyramine, colestimide, and colesevelam HCl, significantly improve glycemic control and reduce LDL cholesterol levels in patients with type 2 diabetes. This paper will review the effects of bile acid sequestrants on both glucose and lipid metabolism in patients with type 2 diabetes.

Novel pathways for glycaemic control in type 2 diabetes: focus on bile acid modulation

Type 2 diabetes is a common disorder with high risk of macrovascular and microvascular complications. These complications are largely driven by hyperglycaemia, dyslipidaemia and hypertension, for which aggressive treatment is thus warranted. Achieving and maintaining control of all three risk factors is especially difficult, however, and new therapeutic approaches could be useful. Bile acids have a well-established and important role in cholesterol homeostasis. Normally, their levels are maintained primarily by ileal reabsorption and enterohepatic recycling. Bile acid sequestrants bind bile acids in the intestine, reduce this recycling and deplete the bile acid pool, thereby stimulating use of hepatic cholesterol for bile acid synthesis, which leads to accelerated removal of LDL from the plasma and a decrease in LDL-cholesterol levels. Interestingly, recent evidence suggests that bile acid sequestrants can lower glucose levels to a clinically meaningful degree. This review presents this evidence and the possible mechanisms by which these glucose-lowering effects occur and discusses the apparently unique ability of bile acid sequestrants among lipid-lowering agents to significantly improve two cardiovascular risk factors, hyperglycaemia and dyslipidaemia. There is renewed interest in the use of bile acid sequestrants in individuals with type 2 diabetes, most of whom would benefit from additional reductions in both LDL-cholesterol and glycaemia.

Endocrine and paracrine role of bile acids

Bile acids are not only important for the absorption of dietary lipids and fat soluble vitamins but are signalling molecules with diverse endocrine and paracrine functions. Bile acids regulate bile acid, lipid and glucose metabolism and modulate temperature and energy homeostasis. Furthermore, bile acids can not only promote cell proliferation and liver regeneration but can also induce programmed cell death. Bile acid functions are mediated through different pathways which comprise the activation of nuclear hormone receptors, of intracellular kinases and of the plasma membrane-bound, G-protein coupled bile acid receptor TGR5/Gpbar-1.

Targeting bile-acid signalling for metabolic diseases

Bile acids are increasingly being appreciated as complex metabolic integrators and signalling factors and not just as lipid solubilizers and simple regulators of bile-acid homeostasis. It is therefore not surprising that a number of bile-acid-activated signalling pathways have become attractive therapeutic targets for metabolic disorders. Here, we review how the signalling functions of bile acids can be exploited in the development of drugs for obesity, type 2 diabetes, hypertriglyceridaemia and atherosclerosis, as well as other associated chronic diseases such as non-alcoholic steatohepatitis.