Nuclear receptor-mediated repression of human cholesterol 7α-hydroxylase gene transcription by bile acids

The gut microbiota-bile acid axis in cholestatic liver disease

Cholestatic liver diseases (CLD) are characterized by impaired normal bile flow, culminating in excessive accumulation of toxic bile acids. The majority of patients with CLD ultimately progress to liver cirrhosis and hepatic failure, necessitating liver transplantation due to the lack of effective treatment. Recent investigations have underscored the pivotal role of the gut microbiota-bile acid axis in the progression of hepatic fibrosis via various pathways. The obstruction of bile drainage can induce gut microbiota dysbiosis and disrupt the intestinal mucosal barrier, leading to bacteria translocation. The microbial translocation activates the immune response and promotes liver fibrosis progression. The identification of therapeutic targets for modulating the gut microbiota-bile acid axis represents a promising strategy to ameliorate or perhaps reverse liver fibrosis in CLD. This review focuses on the mechanisms in the gut microbiota-bile acids axis in CLD and highlights potential therapeutic targets, aiming to lay a foundation for innovative treatment approaches.

Bile Acids and Farnesoid X Receptor in Renal Pathophysiology

BACKGROUND

Bile acids (BAs) act not only as lipids and lipid-soluble vitamin detergents but also function as signaling molecules, participating in diverse physiological processes. The identification of BA receptors in organs beyond the enterohepatic system, such as the farnesoid X receptor (FXR), has initiated inquiries into their organ-specific functions. Among these organs, the kidney prominently expresses FXR.

SUMMARY

This review provides a comprehensive overview of various BA species identified in kidneys and delves into the roles of renal apical and basolateral BA transporters. Furthermore, we explore changes in BAs and their potential implications for various renal diseases, particularly chronic kidney disease. Lastly, we center our discussion on FXR, a key BA receptor in the kidney and a potential therapeutic target for renal diseases, providing current insights into the protective mechanisms associated with FXR agonist treatments.

KEY MESSAGES

Despite the relatively low concentrations of BAs in the kidney, their presence is noteworthy, with rodents and humans exhibiting distinct renal BA compositions. Renal BA transporters efficiently facilitate either reabsorption into systemic circulation or excretion into the urine. However, adaptive changes in BA transporters are evident during cholestasis. Various renal diseases are accompanied by alterations in BA concentrations and FXR expression. Consequently, the activation of FXR in the kidney could be a promising target for mitigating kidney damage.

Therapeutic implications of impaired nuclear receptor function and dysregulated metabolism in Wilson's disease

Copper is an essential trace element that is required for the activity of many enzymes and cellular processes, including energy homeostasis and neurotransmitter biosynthesis; however, excess copper accumulation results in significant cellular toxicity. The liver is the major organ for maintaining copper homeostasis. Inactivating mutations of the copper-transporting P-type ATPase, ATP7B, result in Wilson's disease, an autosomal recessive disorder that requires life-long medicinal therapy or liver transplantation. Current treatment protocols are limited to either sequestration of copper via chelation or reduction of copper absorption in the gut (zinc therapy). The goal of these strategies is to reduce free copper, redox stress, and cellular toxicity. Several lines of evidence in Wilson's disease animal models and patients have revealed altered hepatic metabolism and impaired hepatic nuclear receptor activity. Nuclear receptors are transcription factors that coordinate hepatic metabolism in normal and diseased livers, and several hepatic nuclear receptors have decreased activity in Wilson's disease and Atp7b-/- models. In this review, we summarize the basic physiology that underlies Wilson's disease pathology, Wilson's disease animal models, and the possibility of targeting nuclear receptor activity in Wilson's disease patients.

Glycocholic acid aggravates liver fibrosis by promoting the up-regulation of connective tissue growth factor in hepatocytes

AIMS

The precise role of bile acid in the progression of liver fibrosis has yet to be elucidated. In this study, common bile duct ligation was used as an in vivo mouse model for the evaluation of bile acids that promote liver connective tissue growth factor expression.

MAIN METHODS

Primary rat and mice hepatocytes, as well as primary rat hepatic stellate and HepaRG cells were evaluated as in vitro models for promoting the expression of connective tissue growth factor by bile acids.

KEY FINDINGS

Compared with taurochenodeoxycholic acid, glycochenodeoxycholic acid, and taurocholic acid, glycocholic acid (GCA) most strongly promoted the secretion of connective tissue growth factor in mouse primary hepatocytes, rat primary hepatocytes and HepaRGs. GCA did not directly promote the activation of hepatic stellate cells. The administration of GCA in mice with ligated bile ducts promotes the progression of liver fibrosis, which may promote the yes-associated protein of hepatocytes into the nucleus, resulting in the hepatocytes secreting more connective tissue growth factor for hepatic stellate cell activation. In conclusion, our data showed that GCA can induce the expression of connective tissue growth factor in hepatocytes by promoting the nuclear translocation of yes-associated protein, thereby activating hepatic stellate cells.

SIGNIFICANCE

Our findings help to elucidate the contribution of GCA to the progression of hepatic fibrosis in cholestatic disease and aid the clinical monitoring of cholestatic liver fibrosis development.

Whey protein hydrolysate alleviated atherosclerosis and hepatic steatosis by regulating lipid metabolism in apoE-/- mice fed a Western diet

Whey protein hydrolysate (WPH) has been proved to possess various biological activities associated with the amelioration of cardiovascular disease (CVD). The objective of this study was to investigate the anti-atherosclerotic and hepatoprotective effects of WPH on apolipoprotein E knockout (apoE^-/-) mice fed with a Western diet for 15 weeks. Results revealed that WPH markedly inhibited the development of atherosclerotic lesions in the aorta and steatosis injury in the liver. The serum lipid and inflammation levels were both reduced after WPH supplemented in apoE^-/- mice. In addition, WPH inhibited the lipid accumulation in the liver, thereby decreasing the hepatic inflammation level and oxidative stress injury. Mechanism investigation revealed that WPH down-regulated the expression of cholesterol biosynthesis genes while up-regulated the expression of cholesterol uptake and excretion genes in the liver. Meanwhile, the de novo lipogenesis was inhibited while the fatty acids β-oxidation was activated in the liver by WPH supplementation. Notably, the n-3 polyunsaturated fatty acid (PUFA)/n-6 PUFA ratio in serum and liver of the WPH-H group were 2.69-fold (p < 0.01) and 3.64-fold (p < 0.01) higher than that of the Model group. Collectively, our results proved WPH possesses potent anti-atherosclerotic and hepatoprotective activities and has the potential to be used as a novel functional ingredient for the management of CVD.

The Medicinal Chemistry and Therapeutic Potential of LRH-1 Modulators

The ligand-activated transcription factor liver receptor homologue 1 (LRH-1, NR5A2) is involved in the regulation of metabolic homeostasis, including cholesterol and glucose balance. Preliminary evidence points to therapeutic potential of LRH-1 modulation in diabetes, hepatic diseases, inflammatory bowel diseases, atherosclerosis, and certain cancers, but because of a lack of suitable ligands, pharmacological control of LRH-1 has been insufficiently studied. Despite the availability of considerable structural knowledge on LRH-1, only a few ligand chemotypes have been developed, and potent, selective, and bioavailable tools to explore LRH-1 modulation in vivo are lacking. In view of the therapeutic potential of LRH-1 in prevalent diseases, improved chemical tools are needed to probe the beneficial and adverse effects of pharmacological LRH-1 modulation in sophisticated preclinical models and to further elucidate the receptor's molecular function.

The combined action of Esrrb and Nr5a2 is essential for murine naïve pluripotency

The maintenance of pluripotency in mouse embryonic stem cells (ESCs) is governed by the action of an interconnected network of transcription factors. Among them, only Oct4 and Sox2 have been shown to be strictly required for the self-renewal of ESCs and pluripotency, particularly in culture conditions in which differentiation cues are chemically inhibited. Here, we report that the conjunct activity of two orphan nuclear receptors, Esrrb and Nr5a2, parallels the importance of that of Oct4 and Sox2 in naïve mouse ESCs. By occupying a large common set of regulatory elements, these two factors control the binding of Oct4, Sox2 and Nanog to DNA. Consequently, in their absence the pluripotency network collapses and the transcriptome is substantially deregulated, leading to the differentiation of ESCs. Altogether, this work identifies orphan nuclear receptors, previously thought to be performing supportive functions, as a set of core regulators of naïve pluripotency.

Summary: Esrrb and Nr5a2, two orphan nuclear receptors, are identified as essential regulators of pluripotency in mouse embryonic stem cells.

Systemic PFOS and PFOA exposure and disturbed lipid homeostasis in humans: what do we know and what not?

Associations between per- and polyfluoroalkyl substances (PFASs) and increased blood lipids have been repeatedly observed in humans, but a causal relation has been debated. Rodent studies show reverse effects, i.e. decreased blood cholesterol and triglycerides, occurring however at PFAS serum levels at least 100-fold higher than those in humans. This paper aims to present the main issues regarding the modulation of lipid homeostasis by the two most common PFASs, PFOS and PFOA, with emphasis on the underlying mechanisms relevant for humans. Overall, the apparent contrast between human and animal data may be an artifact of dose, with different molecular pathways coming into play upon exposure to PFASs at very low versus high levels. Altogether, the interpretation of existing rodent data on PFOS/PFOA-induced lipid perturbations with respect to the human situation is complex. From a mechanistic perspective, research on human liver cells shows that PFOS/PFOA activate the PPARα pathway, whereas studies on the involvement of other nuclear receptors, like PXR, are less conclusive. Other data indicate that suppression of the nuclear receptor HNF4α signaling pathway, as well as perturbations of bile acid metabolism and transport might be important cellular events that require further investigation. Future studies with human-relevant test systems would help to obtain more insight into the mechanistic pathways pertinent for humans. These studies shall be designed with a careful consideration of appropriate dosing and toxicokinetics, so as to enable biologically plausible quantitative extrapolations. Such research will increase the understanding of possible perturbed lipid homeostasis related to PFOS/ PFOA exposure and the potential implications for human health.

Bile acid metabolism and circadian rhythms

Circadian rhythms are biological systems that synchronize cellular circadian oscillators with the organism's daily feeding-fasting or rest-activity cycles in mammals. Circadian rhythms regulate nutrient absorption and utilization at the cellular level and are closely related to obesity and metabolic disorders. Bile acids are important modulators that facilitate nutrient absorption and regulate energy metabolism. Here, we provide an overview of the current connections and future perspectives between the circadian clock and bile acid metabolism as well as related metabolic diseases. Feeding and fasting cycles influence bile acid pool size and composition, and bile acid signaling can respond to acute lipid and glucose utilization and mediate energy balance. Disruption of circadian rhythms such as shift work, irregular diet, and gene mutations can contribute to altered bile acid metabolism and heighten obesity risk. High-fat diets, alcohol, and gene mutations related to bile acid signaling result in desynchronized circadian rhythms. Gut microbiome also plays a role in connecting circadian rhythms with bile acid metabolism. The underlying mechanism of how circadian rhythms interact with bile acid metabolism has not been fully explored. Sustaining bile acid homeostasis based on circadian rhythms may be a potential therapy to alleviate metabolic disturbance.

Up to date on cholesterol 7 alpha-hydroxylase (CYP7A1) in bile acid synthesis

Cholesterol 7 alpha-hydroxylase (CYP7A1, EC1.14) is the first and rate-limiting enzyme in the classic bile acid synthesis pathway. Much progress has been made in understanding the transcriptional regulation of CYP7A1 gene expression and the underlying molecular mechanisms of bile acid feedback regulation of CYP7A1 and bile acid synthesis in the last three decades. Discovery of bile acid-activated receptors and their roles in the regulation of lipid, glucose and energy metabolism have been translated to the development of bile acid-based drug therapies for the treatment of liver-related metabolic diseases such as alcoholic and non-alcoholic fatty liver diseases, liver cirrhosis, diabetes, obesity and hepatocellular carcinoma. This review will provide an update on the advances in our understanding of the molecular biology and mechanistic insights of the regulation of CYP7A1 in bile acid synthesis in the last 40 years.

Intestinal Absorption of Bile Acids in Health and Disease

The intestinal reclamation of bile acids is crucial for the maintenance of their enterohepatic circulation. The majority of bile acids are actively absorbed via specific transport proteins that are highly expressed in the distal ileum. The uptake of bile acids by intestinal epithelial cells modulates the activation of cytosolic and membrane receptors such as the farnesoid X receptor (FXR) and G protein-coupled bile acid receptor 1 (GPBAR1), which has a profound effect on hepatic synthesis of bile acids as well as glucose and lipid metabolism. Extensive research has focused on delineating the processes of bile acid absorption and determining the contribution of dysregulated ileal signaling in the development of intestinal and hepatic disorders. For example, a decrease in the levels of the bile acid-induced ileal hormone FGF15/19 is implicated in bile acid-induced diarrhea (BAD). Conversely, the increase in bile acid absorption with subsequent overload of bile acids could be involved in the pathophysiology of liver and metabolic disorders such as fatty liver diseases and type 2 diabetes mellitus. This review article will attempt to provide a comprehensive overview of the mechanisms involved in the intestinal handling of bile acids, the pathological implications of disrupted intestinal bile acid homeostasis, and the potential therapeutic targets for the treatment of bile acid-related disorders. Published 2020. Compr Physiol 10:21-56, 2020.

Human cytochrome P450 enzymes 5-51 as targets of drugs and natural and environmental compounds: mechanisms, induction, and inhibition - toxic effects and benefits

Cytochrome P450 (P450, CYP) enzymes have long been of interest due to their roles in the metabolism of drugs, pesticides, pro-carcinogens, and other xenobiotic chemicals. They have also been of interest due to their very critical roles in the biosynthesis and metabolism of steroids, vitamins, and certain eicosanoids. This review covers the 22 (of the total of 57) human P450s in Families 5-51 and their substrate selectivity. Furthermore, included is information and references regarding inducibility, inhibition, and (in some cases) stimulation by chemicals. We update and discuss important aspects of each of these 22 P450s and questions that remain open.

Bile Microinfarcts in Cholestasis Are Initiated by Rupture of the Apical Hepatocyte Membrane and Cause Shunting of Bile to Sinusoidal Blood

Bile duct ligation (BDL) is an experimental procedure that mimics obstructive cholestatic disease. One of the early consequences of BDL in rodents is the appearance of so-called bile infarcts that correspond to Charcot-Gombault necrosis in human cholestasis. The mechanisms causing bile infarcts and their pathophysiological relevance are unclear. Therefore, intravital two photon-based imaging of BDL mice was performed with fluorescent bile salts (BS) and non-BS organic anion analogues. Key findings were followed up by matrix-assisted laser desorption ionization imaging, clinical chemistry, immunostaining, and gene expression analyses. In the acute phase, 1-3 days after BDL, BS concentrations in bile increased and single-cell bile microinfarcts occurred in dispersed hepatocytes throughout the liver caused by the rupture of the apical hepatocyte membrane. This rupture occurred after loss of mitochondrial membrane potential, followed by entry of bile, cell death, and a "domino effect" of further death events of neighboring hepatocytes. Bile infarcts provided a trans-epithelial shunt between bile canaliculi and sinusoids by which bile constituents leaked into blood. In the chronic phase, ≥21 days after BDL, uptake of BS tracers at the sinusoidal hepatocyte membrane was reduced. This contributes to elevated concentrations of BS in blood and decreased concentrations in the biliary tract. Conclusion: Bile microinfarcts occur in the acute phase after BDL in a limited number of dispersed hepatocytes followed by larger infarcts involving neighboring hepatocytes, and they allow leakage of bile from the BS-overloaded biliary tract into blood, thereby protecting the liver from BS toxicity; in the chronic phase after BDL, reduced sinusoidal BS uptake is a dominant protective factor, and the kidney contributes to the elimination of BS until cholemic nephropathy sets in.

Bile acids and intestinal microbiota in autoimmune cholestatic liver diseases

Autoimmune cholestatic liver diseases, including primary biliary cholangitis (PBC) and primary sclerosing cholangitis (PSC), are manifested as an impairment of normal bile flow and excessive accumulation of potentially toxic bile acids. Endogenous bile acids are involved in the pathogenesis and progression of cholestasis. Consequently, chronic cholestasis affects the expression of bile acid transporters and nuclear receptors, and results in liver injury. Several lines of evidence suggest that intestinal microbiota plays an important role in the etiopathogenesis of cholestatic liver diseases by regulating metabolism and immune responses. However, progression of the disease may also affect the composition of gut microbiota, which in turn exacerbates the progression of cholestasis. In addition, the interaction between intestinal microbiota and bile acids is not unidirectional. Bile acids can shape the gut microbiota community, and in turn, intestinal microbes are able to alter bile acid pool. In general, gut microbiota actively communicates with bile acids, and together play an important role in the pathogenesis of PBC and PSC. Targeting the link between bile acids and intestinal microbiota offers exciting new perspectives for the treatment of those cholestatic liver diseases. This review highlights current understanding of the interactions between bile acids and intestinal microbiota and their roles in autoimmune cholestatic liver diseases. Further, we postulate a bile acids-intestinal microbiota-cholestasis triangle in the pathogenesis of autoimmune cholestatic liver diseases and potential therapeutic strategies by targeting this triangle.

The ascending pathophysiology of cholestatic liver disease

In this review we develop the argument that cholestatic liver diseases, particularly primary biliary cholangitis and primary sclerosing cholangitis (PSC), evolve over time with anatomically an ascending course of the disease process. The first and early lesions are in "downstream" bile ducts. This eventually leads to cholestasis, and this causes bile salt (BS)-mediated toxic injury of the "upstream" liver parenchyma. BS are toxic in high concentration. These concentrations are present in the canalicular network, bile ducts, and gallbladder. Leakage of bile from this network and ducts could be an important driver of toxicity. The liver has a great capacity to adapt to cholestasis, and this may contribute to a variable symptom-poor interval that is often observed. Current trials with drugs that target BS toxicity are effective in only about 50%-60% of primary biliary cholangitis patients, with no effective therapy in PSC. This motivated us to develop and propose a new view on the pathophysiology of primary biliary cholangitis and PSC in the hope that these new drugs can be used more effectively. These views may lead to better stratification of these diseases and to recommendations on a more "tailored" use of the new therapeutic agents that are currently tested in clinical trials. Apical sodium-dependent BS transporter inhibitors that reduce intestinal BS absorption lower the BS load and are best used in cholestatic patients. The effectiveness of BS synthesis-suppressing drugs, such as farnesoid X receptor agonists, is greatest when optimal adaptation is not yet established. By the time cytochrome P450 7A1 expression is reduced these drugs may be less effective. Anti-inflammatory agents are probably most effective in early disease, while drugs that antagonize BS toxicity, such as ursodeoxycholic acid and nor-ursodeoxycholic acid, may be effective at all disease stages. Endoscopic stenting in PSC should be reserved for situations of intercurrent cholestasis and cholangitis, not for cholestasis in end-stage disease. These are arguments to consider a step-wise pathophysiology for these diseases, with therapy adjusted to disease stage. An obstacle in such an approach is that disease stage-defining biomarkers are still lacking. This review is meant to serve as a call to prioritize the development of biomarkers that help to obtain a better stratification of these diseases. (Hepatology 2017;65:722-738).

Sirtuin 1 activation alleviates cholestatic liver injury in a cholic acid-fed mouse model of cholestasis

Sirtuin1 (Sirt1; mammalian homolog of Saccharomyces cerevisiae enzyme Sir2) is a transcriptional and transactivational regulator of murine farnesoid X receptor (Fxr), which is the primary bile acid (BA) sensor, and critical regulator of BA metabolism in physiological and pathophysiological conditions. Previous studies have suggested compromised Sirt1 expression in rodent models of cholestatic liver injury. We hypothesized that Sirt1 could be potentially targeted to alleviate cholestatic liver injury. In cultured primary human hepatocytes, SIRT1 messenger RNA was down-regulated after GCA treatment, potentially through induction of microRNA (miR)-34a, whereas tauroursodeoxycholic acid induced SIRT1 expression without affecting miR-34a expression. Sirt1 expression was also significantly down-regulated in three mouse models of liver injury (bile duct ligation, 1% cholic acid [CA] fed, and the Mdr2^-/- mouse). Mice fed CA diet also demonstrated hepatic FXR hyperacetylation and induction of the Janus kinase/p53 pathway. Mice fed a CA diet and concurrently administered the Sirt1 activator, SRT1720 (50 mg/kg/day, orally), demonstrated 40% and 45% decrease in plasma alanine aminotransferase and BA levels, respectively. SRT1720 increased hepatic BA hydrophilicity by increasing tri- and tetrahydroxylated and decreasing the dihydroxylated BA fraction. SRT1720 administration also inhibited hepatic BA synthesis, potentially through ileal fibroblast growth factor 15- and Fxr-mediated inhibition of cytochrome p450 (Cyp) 7a1 and Cyp27a1, along with increased hepatic BA hydroxylation in association with Cyp2b10 induction. SRT1720 administration significantly induced renal multidrug resistance-associated protein 2 and 4, peroxisome proliferator-activated receptor gamma coactivator 1-α, and constitutive androstance receptor expression along with ∼2-fold increase in urinary BA concentrations.

CONCLUSION

SRT1720 administration alleviates cholestatic liver injury in mice by increasing hydrophilicity of hepatic BA composition and decreasing plasma BA concentration through increased BA excretion into urine. Thus, use of small-molecule activators of Sirt1 presents a novel therapeutic target for cholestatic liver injury. (Hepatology 2016;64:2151-2164).

Identification of liver CYP51 as a gene responsive to circulating cholesterol in a hamster model

Hypercholesterolaemia is a risk factor for CVD, which is a leading cause of death in industrialised societies. The biosynthetic pathways for cholesterol metabolism are well understood; however, the regulation of circulating cholesterol by diet is still not fully elucidated. The present study aimed to gain more comprehensive understanding of the relationship between circulating cholesterol levels and molecular effects in target tissues using the hamster model. Male golden Syrian hamsters were fed with chow or diets containing 36 % energy from fat with or without 1 % cholesteyramine (CA) as a modulator of circulating cholesterol levels for 35 d. It was revealed that the expression of lanosterol 14α-demethylase (CYP51) instead of 3-hydroxy-3-methyl-glutaryl (HMG)-CoA reductase mRNA expression was responsive to circulating cholesterol in hamsters fed hypercholesterolaemic diets. The high-fat diet increased circulating cholesterol and down-regulated CYP51, but not HMG-CoA reductase. The CA diet decreased cholesterol and increased CYP51 expression, but HMG-CoA reductase expression was not affected. The high-fat diet and CA diet altered the expression level of cholesterol, bile acids and lipid metabolism-associated genes (LDL receptor, cholesterol 7α-hydroxylase (CYP7A1), liver X receptor (LXR) α, and ATP-binding cassette subfamily G member 5/8 (ABCG5/8)) in the liver, which were significantly correlated with circulating cholesterol levels. Correlation analysis also showed that circulating cholesterol levels were regulated by LXR/retinoid X receptor and PPAR pathways in the liver. Using the hamster model, the present study provided additional molecular insights into the influence of circulating cholesterol on hepatic cholesterol metabolism pathways during hypercholesterolaemia.

Elevated copper impairs hepatic nuclear receptor function in Wilson's disease

Wilson's disease (WD) is an autosomal recessive disorder that results in accumulation of copper in the liver as a consequence of mutations in the gene encoding the copper-transporting P-type ATPase (ATP7B). WD is a chronic liver disorder, and individuals with the disease present with a variety of complications, including steatosis, cholestasis, cirrhosis, and liver failure. Similar to patients with WD, Atp7b⁻/⁻ mice have markedly elevated levels of hepatic copper and liver pathology. Previous studies have demonstrated that replacement of zinc in the DNA-binding domain of the estrogen receptor (ER) with copper disrupts specific binding to DNA response elements. Here, we found decreased binding of the nuclear receptors FXR, RXR, HNF4α, and LRH-1 to promoter response elements and decreased mRNA expression of nuclear receptor target genes in Atp7b⁻/⁻ mice, as well as in adult and pediatric WD patients. Excessive hepatic copper has been described in progressive familial cholestasis (PFIC), and we found that similar to individuals with WD, patients with PFIC2 or PFIC3 who have clinically elevated hepatic copper levels exhibit impaired nuclear receptor activity. Together, these data demonstrate that copper-mediated nuclear receptor dysfunction disrupts liver function in WD and potentially in other disorders associated with increased hepatic copper levels.

Negative feedback loop of cholesterol regulation is impaired in the livers of patients with Alagille syndrome

AIM

To characterize cholesterol regulation in the liver of patients with Alagille syndrome (AGS).

METHODS

Serum total cholesterol (TC) and total bile acid (TBA) levels were measured in 23 AGS patients. The expressions of genes involved in cholesterol regulation, including low-density lipoprotein receptor (LDLR), scavenger receptor class B type I (SR-BI), 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMGCR), cholesterol 7α-hydroxylase (CYP7A1), ATP-binding cassette transporter (ABC) A1, and ABCG1/5/8, were measured in liver tissues from five of these patients. Expression of regulators for these genes, including farnesoid X receptor/small heterodimer partner (SHP), liver X receptor α (LXRα) and mature Sterol regulatory element-binding protein 2 (SREBP2) was measured. The expression of mature SREBP2 protein was also examined.

RESULTS

Serum TC and TBA levels were correlated in the AGS patients. Liver cholesterol was also increased compared with controls, and correlated with bile acid contents. LDLR, SR-BI, HMGCR, and ABCGs mRNA expression were upregulated, while CYP7A1 mRNA expression was downregulated in AGS livers. SHP and LXRα mRNA expression was also increased, but maturation of SREBP2 was not suppressed in the patients.

CONCLUSIONS

The major upregulators of liver cholesterol might be increased in AGS patients, indicating an impaired negative feedback mechanism and accelerated liver cholesterol accumulation.

Lepidozenolide from the liverwort Lepidozia fauriana acts as a farnesoid X receptor agonist

Lepidozenolide is a sesquiterpenoid isolated from the liverwort Lepidozia fauriana and its possible bioactivity is unclear. The farnesoid X receptor (FXR) is a member of nuclear receptor superfamily that has been widely targeted for developing treatments for chronic liver disease and hyperglycemia. In this study, whether lepidozenolide may act as a FXR agonist was determined. Indeed, in mammalian one-hybrid and transient transfection reporter assays, lepidozenolide transactivated FXR to modulate promoter action including GAL4, CYP7A1, and PLTP promoters in a dose-dependent manner, while it exhibited slightly less agonistic activity than chenodeoxycholic acid, an endogenous FXR agonist. Through the molecular modeling docking studies lepidozenolide was shown to bind to FXR ligand binding pocket fairly well. All these results indicate that lepidozenolide acts as a FXR agonist.

Bile acid flux through portal but not peripheral veins inhibits CYP7A1 expression without involvement of ileal FGF19 in rabbits

It was proposed that CYP7A1 expression is suppressed through the gut-hepatic signaling pathway fibroblast growth factor (FGF) 15/19-fibroblast growth factor receptor 4, which is initiated by activation of farnesoid X receptor in the intestine rather than in the liver. The present study tested whether portal bile acid flux alone without ileal FGF19 could downregulate CYP7A1 expression in rabbits. A rabbit model was developed by infusing glycodeoxycholic acid (GDCA) through the splenic vein to bypass ileal FGF19. Study was conducted in four groups of rabbits: control; bile fistula + bovine serum albumin solution perfusion (BF); BF + GDCA (by portal perfusion); and BF + GDCA-f (by femoral perfusion). Compared with only BF, BF + GDCA (6 h portal perfusion) suppressed CYP7A1 mRNA, whereas BF + GDCA-f (via femoral vein) with the same perfusion rate of GDCA did not show inhibitory effects. Meanwhile, there was a decrease in ileal FGF19 expression and portal FGF19 protein levels, but an equivalent increase in biliary bile acid outputs in both GDCA perfusion groups. This study demonstrated that portal bile acid flux alone downregulated CYP7A1 expression with diminished FGF19 expression and protein levels, whereas the same bile acid flux reaching the liver through the hepatic artery via femoral vein had no inhibitory effect on CYP7A1. We propose that bile acid flux through the portal venous system may be a kind of "intestinal factor" that suppresses CYP7A1 expression.

Human FXR regulates SHP expression through direct binding to an LRH-1 binding site, independent of an IR-1 and LRH-1

Background

Farnesoid X receptor/retinoid X receptor-alpha (FXR/RXRα) is the master transcriptional regulator of bile salt synthesis and transport in liver and intestine. FXR is activated by bile acids, RXRα by the vitamin A–derivative 9-cis retinoic acid (9cRA). Remarkably, 9cRA inhibits binding of FXR/RXRα to its response element, an inverted repeat-1 (IR-1). Still, most FXR/RXRα target genes are maximally expressed in the presence of both ligands, including the small heterodimer partner (SHP). Here, we revisited the FXR/RXRα-mediated regulation of human SHP.

Methods

A 579-bp hSHP promoter element was analyzed to locate FXR/chenodeoxycholic acid (CDCA)- and RXRα/9cRA-responsive elements. hSHP promoter constructs were analyzed in FXR/RXRα-transfected DLD-1, HEK293 and HepG2 cells exposed to CDCA, GW4064 (synthetic FXR ligand) and/or 9cRA. FXR-DNA interactions were analyzed by in vitro pull down assays.

Results

hSHP promoter elements lacking the previously identified IR-1 (−291/−279) largely maintained their activation by FXR/CDCA, but were unresponsive to 9cRA. FXR-mediated activation of the hSHP promoter was primarily dependent on the −122/−69 region. Pull down assays revealed a direct binding of FXR to the −122/−69 sequence, which was abrogated by site-specific mutations in a binding site for the liver receptor homolog-1 (LRH-1) at −78/−70. These mutations strongly impaired the FXR/CDCA-mediated activation, even in the context of a hSHP promoter containing the IR-1. LRH-1 did not increase FXR/RXRα-mediated activation of hSHP promoter activity.

Conclusion

FXR/CDCA-activated expression of SHP is primarily mediated through direct binding to an LRH-1 binding site, which is not modulated by LRH-1 and unresponsive to 9cRA. 9cRA-induced expression of SHP requires the IR-1 that overlaps with a direct repeat-2 (DR-2) and DR-4. This establishes for the first time a co-stimulatory, but independent, action of FXR and RXRα agonists.

Nuclear receptor control of enterohepatic circulation

Enterohepatic circulation is responsible for the capture of bile acids and other steroids produced or metabolized in the liver and secreted to the intestine, for reabsorption back into the circulation and transport back to the liver. Bile acids are secreted from the liver in the form of mixed micelles that also contain phosphatidylcholines and cholesterol that facilitate the uptake of fats and vitamins from the diet due to the surfactant properties of bile acids and lipids. Bile acids are synthesized in the liver from cholesterol by a cascade of enzymes that carry out oxidation and conjugation reactions, and transported to the bile duct and gall bladder where they are stored before being released into the intestine. Bile flow from the gall bladder to the small intestine is triggered by food intake in accordance with its role in lipid and vitamin absorption from the diet. Bile acids are further metabolized by gut bacteria and are transported back to the circulation. Metabolites produced in the liver are termed primary bile acids or primary conjugated bile salts, while the metabolites generated by bacterial are called secondary bile acids. About 95% of bile acids are reabsorbed in the proximal and distal ileum into the hepatic portal vein and then into the liver sinusoids, where they are efficiently transported into the liver with little remaining in circulation. Each bile acid is reabsorbed about 20 times on average before being eliminated. Enterohepatic circulation is under tight regulation by nuclear receptor signaling, notably by the farnesoid X receptor (FXR).

FXR signaling in the enterohepatic system

Enterohepatic circulation serves to capture bile acids and other steroid metabolites produced in the liver and secreted to the intestine, for reabsorption back into the circulation and reuptake to the liver. This process is under tight regulation by nuclear receptor signaling. Bile acids, produced from cholesterol, can alter gene expression in the liver and small intestine via activating the nuclear receptors farnesoid X receptor (FXR; NR1H4), pregnane X receptor (PXR; NR1I2), vitamin D receptor (VDR; NR1I1), G protein coupled receptor TGR5, and other cell signaling pathways (JNK1/2, AKT and ERK1/2). Among these controls, FXR is known to be a major bile acid-responsive ligand-activated transcription factor and a crucial control element for maintaining bile acid homeostasis. FXR has a high affinity for several major endogenous bile acids, notably cholic acid, deoxycholic acid, chenodeoxycholic acid, and lithocholic acid. By responding to excess bile acids, FXR is a bridge between the liver and small intestine to control bile acid levels and regulate bile acid synthesis and enterohepatic flow. FXR is highly expressed in the liver and gut, relative to other tissues, and contributes to the maintenance of cholesterol/bile acid homeostasis by regulating a variety of metabolic enzymes and transporters. FXR activation also affects lipid and glucose metabolism, and can influence drug metabolism.

Specific bile acids inhibit hepatic fatty acid uptake in mice

Bile acids are known to play important roles as detergents in the absorption of hydrophobic nutrients and as signaling molecules in the regulation of metabolism. We tested the novel hypothesis that naturally occurring bile acids interfere with protein-mediated hepatic long chain free fatty acid (LCFA) uptake. To this end, stable cell lines expressing fatty acid transporters as well as primary hepatocytes from mouse and human livers were incubated with primary and secondary bile acids to determine their effects on LCFA uptake rates. We identified ursodeoxycholic acid (UDCA) and deoxycholic acid (DCA) as the two most potent inhibitors of the liver-specific fatty acid transport protein 5 (FATP5). Both UDCA and DCA were able to inhibit LCFA uptake by primary hepatocytes in a FATP5-dependent manner. Subsequently, mice were treated with these secondary bile acids in vivo to assess their ability to inhibit diet-induced hepatic triglyceride accumulation. Administration of DCA in vivo via injection or as part of a high-fat diet significantly inhibited hepatic fatty acid uptake and reduced liver triglycerides by more than 50%.

CONCLUSION

The data demonstrate a novel role for specific bile acids, and the secondary bile acid DCA in particular, in the regulation of hepatic LCFA uptake. The results illuminate a previously unappreciated means by which specific bile acids, such as UDCA and DCA, can impact hepatic triglyceride metabolism and may lead to novel approaches to combat obesity-associated fatty liver disease.

Insulin-dependent suppression of cholesterol 7α-hydroxylase is a possible link between glucose and cholesterol metabolisms

Cholesterol 7α-hydroxylase (CYP7A1) regulates the balance between cholesterol supply and metabolism by catalyzing the rate-limiting step of bile acid biosynthesis. The transcriptional activity of CYP7A1 is tightly controlled by various nuclear receptors. A forkhead transcription factor O1 (FOXO1) plays a critical role in metabolism, and insulin inactivates FOXO1 through Akt-dependent phosphorylation and nuclear exclusion. We investigated the role of insulin- Akt-FOXO1 signaling pathway in CYP7A1 transcriptional regulation since we found putative insulin-response elements, FOXO1 binding sequences, in both rat and human CYP7A1 promoters. However, ectopic expression of FOXO1 increased the rat CYP7A1-, but mildly reduced human CYP7A1-promoter activities in a dose-dependent manner. Similarly to bile acids, insulin treatment increased small heterodimer partner (SHP) mRNA rapidly and transiently, leading to the suppression of CYP7A1 transcription in both human and rodents. Chromatin immunoprecipitation showed that FOXO1 directly bound to rat CYP1A1 promoter in the absence of insulin. FOXO1 binding to the rat promoter was diminished by insulin treatment as well as by expression of SHP. Our results suggest that the stimulation of insulin- signaling pathway of Akt-FOXO1 and SHP expression may regulate cholesterol/bile acid metabolisms in liver, linking carbohydrate and cholesterol metabolic pathways. A prolonged exposure of insulin in hyperinsulinemic insulin resistance or diabetic status represses CYP7A1 transcription and bile acid biosynthesis through SHP induction and FOXO1 inactivation, leading to impairment of the hepatic cholesterol/bile acid metabolisms.

Nrf2 inhibits LXRα-dependent hepatic lipogenesis by competing with FXR for acetylase binding

AIMS

The nuclear receptor liver X receptor-α (LXRα) stimulates lipogenesis, leading to steatosis. Nuclear factor erythroid-2-related factor-2 (Nrf2) contributes to cellular defense mechanism by upregulating antioxidant genes, and may protect the liver from injury inflicted by fat accumulation. However, whether Nrf2 affects LXRα activity is unknown. This study investigated the inhibitory role of Nrf2 in hepatic LXRα activity and the molecular basis.

RESULTS

A deficiency of Nrf2 enhanced the ability of LXRα agonist to promote hepatic steatosis, as mediated by lipogenic gene induction. In hepatocytes, Nrf2 overexpression repressed gene transactivation by LXR-binding site activation. Consistently, treatment of mice with sulforaphane (an Nrf2 activator) suppressed T0901317-induced lipogenesis, as confirmed by the experiments using hepatocytes. Nrf2 activation promoted deacetylation of farnesoid X receptor (FXR) by competing for p300, leading to FXR-dependent induction of small heterodimer partner (SHP), which was responsible for the repression of LXRα-dependent gene transcription. In human steatotic samples, the transcript levels of LXRα and SREBP-1 inversely correlated with those of Nrf2, FXR, and SHP.

INNOVATION

Our findings offer the mechanism to explain how decrease in Nrf2 activity in hepatic steatosis could contribute to the progression of NAFLD, providing the use of Nrf2 as a molecular biomarker to diagnose NAFLD. As certain antioxidants have the abilities to activate Nrf2, clinicians might utilize the activators of Nrf2 as a new therapeutic approach to prevent and/or treat NAFLD.

CONCLUSION

Nrf2 activation inhibits LXRα activity and LXRα-dependent liver steatosis by competing with FXR for p300, causing FXR activation and FXR-mediated SHP induction. Our findings provide important information on a strategy to prevent and/or treat steatosis.

Transcriptional regulation of hepatic stellate cells

Hepatic stellate cell (HSC) activation is a process of cellular transdifferentiation in which, upon liver injury, the quiescent vitamin A storing perisinusoidal HSC is converted into a wound-healing myofibroblast and acquires potent pro-inflammatory and pro-fibrogenic activities. This remarkable phenotypic transformation is underpinned by changes in the expression of a vast number of genes. In this review we survey current knowledge of the transcription factors that either control HSC activation or which regulate specific fibrogenic functions of the activated HSC such as collagen expression, proliferation and resistance to apoptosis.

Pyrazolidine-3,5-dione derivatives as potent non-steroidal agonists of farnesoid X receptor: virtual screening, synthesis, and biological evaluation

The identification of a novel pyrazolidine-3,5-dione based scaffold hit compound as Farnesoid X receptor (FXR) partial or full agonist has been accomplished by means of virtual screening techniques. A series of pyrazolidine-3,5-dione derivatives (1a-u and 7) was designed, synthesized, and evaluated by a cell-based luciferase transactivation assay for their agonistic activities against FXR. Most of them showed agonistic potencies and 10 of them (1a, 1b, 1d-f, 1j, 1n, 1t, 5b, and 7) exhibited lower EC(50) values than the reference drug CDCA. Molecular modeling studies for the representative compounds 1a, 1d, 1f, 1j, 1n, 1u, 5b, and 7 were also presented. The novel structural scaffold has provided a new direction for finding potent and selective FXR partial and full agonists (referred to as 'selective bile acid receptor modulators', SBARMs).

Bile acids and signal transduction: role in glucose homeostasis

Bile acids are mainly recognized for their role in dietary lipid absorption and cholesterol homeostasis. However, recent progress in bile acid research suggests that bile acids are important signaling molecules that play a role in glucose homeostasis. Among the various supporting evidence, several reports have demonstrated an improvement of the glycemic index of type 2 diabetic patients treated with diverse bile acid binding resins. Herein, we review the diverse interactions of bile acids with various signaling/response pathways, including calcium mobilization and protein kinase activation, membrane receptor-mediated responses, and nuclear receptor responses. Some of the effects of the bile acids are direct through the activation of specific receptors, i.e., TGR5, CAR, VDR, and FXR, while others imply modulation of the hormonal, growth factor and/or neuromediator responses, i.e., glucagon, EGF, and acetylcholine. We also discuss recent evidence implicating the interaction of bile acids with glucose homeostasis mechanisms, with the integration of our understanding of how the signaling mechanisms modulated by bile acid could regulate glucose metabolism.

Molecular basis of endocrine regulation by orphan nuclear receptor Small Heterodimer Partner

Nuclear receptors (NRs) are a unique superfamily of transcription factors (TFs) which are involved in and play a crucial role in almost all aspects of mammalian physiology. Small Heterodimer Partner (SHP; NR0B2), an exceptional member of this superfamily of NRs, have been identified as a key regulatory factor of the transcription of a variety of genes involved in diverse metabolic pathways, and are thereby an important factor in a variety of physiological functions. Since its discovery a decade ago, considerable progress has been made in the elucidation of the underlying mechanism by which SHP regulates various metabolic processes, and the results of previous studies support its importance in the maintenance of metabolic homeostasis. In this review, we have evaluated the current state of understanding of the molecular mechanisms and the resultant physiological interpretations governed by SHP.

Interplay of pregnane X receptor with other nuclear receptors on gene regulation

Human body needs to protect itself from a diverse array of harmful chemicals. These chemicals are also involved in drug metabolism, enzyme induction, and can cause adverse drug-drug interactions. Being a member of nuclear receptors (NRs), pregnane X receptor (PXR) has recently emerged as transcriptional regulators of cytochrome P450 (CYP) and transporters expression so as to against xenobiotics exposure. This review describes some common nuclear receptors, i.e. farnesoid X receptor (FXR), small heterodimer partner (SHP), hepatocyte nuclear factor-4alpha (HNF-4alpha), liver X receptor (LXR), glucocorticoid receptor (GR), constitutive androstane receptor (CAR) that crosstalk with PXR and involvement of coregulators thus control target genes expression.

Fluctuation of lipoprotein metabolism linked with bile acid-activated liver nuclear receptors in Alagille syndrome

Alagille syndrome (AGS) is a rare hereditary disorder exhibiting fluctuating cholestasis and dyslipidemia. Farnesoid X receptor (FXR) and liver X receptor (LXR) are hepatic nuclear receptors that regulate bile acid and lipoprotein metabolism. To investigate whether cholestasis is related to dyslipidemia and hepatic nuclear receptor expression in AGS patients, we determined the blood levels of total bile acid (TBA) and lipoprotein parameters, and examined hepatic nuclear receptor expression in three AGS children and their three incomplete AGS parents repeatedly over several years. In the AGS children, TBA level showed significant positive correlations with low-density lipoprotein-cholesterol, apolipoprotein E (apoE)-rich high-density lipoprotein-cholesterol (HDL-C), apoA-I, apoE, and cholesteryl ester transfer protein (CETP) concentrations, but negative correlation with apoE-poor HDL-C concentration. Western blot analysis of liver biopsy specimens revealed that FXR and LXR expression increased in parallel with TBA level. CETP- and ATP-binding cassette transporter A1 expression also increased with TBA level, while scavenger receptor class B type-I expression showed the opposite response. However, apoA-I expression was similar to the control level at any TBA level. In the incomplete AGS parents, TBA and lipoprotein parameters showed little fluctuation. In summary, cholestasis is closely related to dyslipidemia and hepatic nuclear receptor expression in AGS patients.

Liver receptor homolog 1 transcriptionally regulates human bile salt export pump expression

The metabolic conversion of cholesterol into bile acids in liver is initiated by the rate-limiting cholesterol 7 alpha-hydroxylase (CYP7A1), whereas the bile salt export pump (BSEP) is responsible for the canalicular secretion of bile acids. Liver receptor homolog 1 (LRH-1) is a key transcriptional factor required for the hepatic expression of CYP7A1. We hypothesized that LRH-1 was also involved in the transcriptional regulation of BSEP. In support of our hypothesis, we found that overexpression of LRH-1 induced, whereas knockdown of LRH-1 decreased, BSEP expression. Consistent with its role in transcriptional regulation, LRH-1 dose-dependently transactivated the BSEP promoter. In addition, such transactivation by LRH-1 was required for maximal induction of BSEP expression through the bile acid/farnesoid X receptor (FXR) activation pathway. Bioinformatic and mutational analysis led to the identification of a functional liver receptor homolog 1-responsive element (LRHRE) in the BSEP promoter. Specific binding of LRH-1 to the LRHRE and recruitment of LRH-1 to the BSEP promoter were demonstrated by electrophoretic mobility shift assay and chromatin immunoprecipitation assay, respectively. In conclusion, LRH-1 transcriptionally activated the BSEP promoter and functioned as a modulator in bile acid/FXR-mediated BSEP regulation. These results suggest that LRH-1 plays a supporting role to FXR in maintaining hepatic bile acid levels by coordinately regulating CYP7A1 and BSEP for bile acid synthesis and elimination, respectively.

P-glycoprotein: so many ways to turn it on

Expression of the ABC transporter P-glycoprotein (P-gp or ABCB1) is associated with resistance to chemotherapy in cancer. However, early investigations into the regulation of ABCB1 expression revealed that the process is not a classical induction as observed for certain metabolizing enzymes. The process involves the cellular stress response pathway initiated by either inflicted (e.g., chemotherapy damage) or endogenous (e.g., hypoxia) factors. However, ABCB1 is also expressed in a number of noncancerous tissues. In particular, the protein is found at tissues providing a barrier or secretory function. The localization of ABCB1 in normal tissues will impact significantly on drug pharmacokinetics, in particular the absorption and elimination processes. This review also describes the mechanism underlying ABCB1 expression in noncancerous tissue, a process that does not involve the stress response.

Hepatocyte growth factor signaling pathway inhibits cholesterol 7alpha-hydroxylase and bile acid synthesis in human hepatocytes

Bile acid synthesis in the liver is regulated by the rate-limiting enzyme cholesterol 7alpha-hydroxylase (CYP7A1). Transcription of the CYP7A1 gene is inhibited by bile acids and cytokines. The rate of bile acid synthesis is reduced immediately after partial hepatectomy and during the early stage of liver regeneration. Hepatocyte growth factor (HGF) released from stellate cells activates a receptor tyrosine kinase c-Met, in hepatocytes and stimulates signaling pathways that regulate cell growth, proliferation, and apoptosis. This study demonstrated that HGF strongly and rapidly repressed CYP7A1 mRNA expression and the rate of bile acid synthesis in primary human hepatocytes. HGF rapidly induced c-Jun and small heterodimer partner mRNA and protein expression and increased phosphorylation of ERK1/2, JNK, and c-Jun. Specific inhibitors of protein kinase C, extracellular signal-regulated kinase 1/2 (ERK1/2), and c-Jun N-terminal kinase (JNK) blocked HGF inhibition of CYP7A1 expression. Knockdown of c-Met by small interfering RNA resulted in a significant increase in CYP7A1 and blocked HGF inhibition of CYP7A1 mRNA expression. Chromatin immunoprecipitation assays showed that HGF induced recruitment of c-Jun and small heterodimer partner (SHP) but reduced recruitment of the coactivators peroxisome proliferators activated receptor rho coactivator 1alpha (PGC-1alpha) and cAMP response element binding protein (CREB)-binding protein (CBP) to chromatin.

CONCLUSION

This study demonstrated that HGF is a novel regulator of CYP7A1 and bile acid synthesis in human hepatocytes and may protect hepatocytes from accumulating toxic bile acids and developing intrahepatic cholestasis during the early stage of liver regeneration.

An overlapping binding site in the CYP7A1 promoter allows activation of FXR to override the stimulation by LXRalpha

The aim of this study was to explore why in rabbits activation of farnesoid X receptor (FXR) is dominant over activated liver X receptor-alpha (LXRalpha) in the regulation of CYP7A1. We cloned the rabbit CYP7A1 promoter and found a fetoprotein transcription factor (FTF) binding element embedded within the LXRalpha binding site (LXRE). Gel shift assays demonstrated that FTF competes with LXRalpha for binding to LXRE. Short heterodimer partner (SHP) enhances the competitive ability of FTF. Studies in HepG2 cells showed that SHP combined with FTF had more powerful effect to offset the stimulation of CYP7A1 by LXRalpha. Gel shift and chromatin immunoprecipitation assays demonstrated that SHP with FTF diminished LXRalpha binding to the CYP7A1 promoter. In vivo studies in rabbits fed cholesterol for 10 days showed that hepatic expression of SHP but not FTF rose and LXRalpha-bound LXRE decreased. We propose that the SHP/FTF heterodimer occupies LXRE via the embedded FTF binding element and blocks LXRalpha from recruiting to LXRE. Therefore, activation of FXR, which upregulates SHP expression, will eliminate the stimulatory effect of LXRalpha on the CYP7A1 promoter because increased levels of SHP combined with FTF diminish the recruitment of LXRalpha to CYP7A1 promoter.

Genomic profiling in nuclear receptor-mediated toxicity

Nuclear receptors (NRs) are attractive drug targets due to their role in regulation of a wide range of physiologic responses. In addition to providing therapeutic value, many pharmaceutical agents along with environmental chemicals are ligands for NRs and can cause adverse health effects that are directly related to activation of NRs. Identifying the molecular events that produce a toxic response may be confounded by the fact that there is a significant overlap in the biological processes that NRs regulate. Microarrays and other methods for gene expression profiling have served as useful, sensitive tools for discerning the mechanisms by which therapeutics and environmental chemicals invoke toxic effects. The capability to probe thousands of genes simultaneously has made genomics a prime technology for identifying drug targets, biomarkers of exposure/toxicity and key players in the mechanisms of disease. The complex intertwining networks regulated by NRs are hard to probe comprehensively without global approaches and genomics has become a key technology that facilitates our understanding of NR-dependent and -independent events. The future of drug discovery, design and optimization, and risk assessment of chemical toxicants that activate NRs will inevitably involve genomic profiling. This review will focus on genomics studies related to PPAR, CAR, PXR, RXR, LXR, FXR, and AHR.

Bile salts induce resistance to apoptosis through NF-kappaB-mediated XIAP expression

Apoptosis plays a critical role in intestinal mucosal homeostasis. We previously showed that the bile salt taurodeoxycholate has a beneficial effect on the intestinal mucosa through an increase in resistance to apoptosis mediated by nuclear factor (NF)-kappaB. The current study further characterizes the effect of bile salts on intestinal epithelial cell susceptibility to apoptosis and determines if the X-linked inhibitor of apoptosis protein (XIAP) regulates bile salt-induced resistance to apoptosis. Exposure of normal intestinal epithelial cells (IEC-6) to the conjugated bile salts taurodeoxycholate (TDCA) and taurochenodeoxycholate (TCDCA) resulted in an increase in resistance to tumor necrosis factor (TNF)-alpha and cycloheximide (CHX)-induced apoptosis, and NF-kappaB activation. Treatment with TDCA and TCDCA resulted in an increase in XIAP expression. Specific inhibition of NF-kappaB by infection with an adenoviral vector that expresses the IkappaBalpha super-repressor (IkappaBSR) prevented the induction of XIAP expression and the bile salt-mediated resistance to apoptosis. Treatment with the specific XIAP inhibitor Smac also overcame this increase in enterocyte resistance to apoptosis. Bile salts inhibited formation of the active caspase-3 from its precursor procaspase-3. Smac prevented the inhibitory effect of bile salts on caspase-3 activation. These results indicate that bile salts increase intestinal epithelial cell resistance to apoptosis through NF-kappaB-mediated XIAP expression. Bile salt-induced XIAP mediates resistance to TNF-alpha/CHX-induced apoptosis, at least partially, through inhibition of caspase-3 activity. These data support an important beneficial role of bile salts in regulation of mucosal integrity. Decreased enterocyte exposure to luminal bile salts, as occurs during starvation and parenteral nutrition, may have a detrimental effect on mucosal integrity.

Expression profiling of liver receptor homologue 1 (LRH-1) in mouse tissues using tissue microarray

Liver receptor homologue 1 (LRH-1) is a nuclear receptor that plays important roles in lipid homeostasis and embryogenesis. To elucidate systemic physiological functions of LRH-1, we used tissue microarray-based immunohistochemistry to examine the tissue distribution and localization of LRH-1 in adult mouse tissues. LRH-1 immunoreactivity was observed in the nucleus of multiple epithelial lineage cells in the digestive system (including absorptive epithelial cells in the small and large intestines, goblet cells, acinar cells of the exocrine glands, chief cells and mucus neck cells in the stomach, granular and prickle layer cells in the tongue and forestomach, and gall bladder epithelium); respiratory system (alveolar type II cells); and urinary system (transitional epithelium). Nuclear LRH-1 immunoreactivity was also localized in cells involved in fatty acid/glucose metabolism, including hepatocytes, brown adipocytes, and cardiomyocytes, and neurons involved in the regulation of food intake, including the arcuate nucleus in the hypothalamus and paraventricular nucleus of thalamus. Additionally, LRH-1 immunoreactivity was observed in testicular Leydig cells and ovarian follicular cells. These data suggest that LRH-1 functions in multiple organ systems to regulate epithelial cell physiology and differentiation, energy metabolism, and reproduction.

Structure and function of the atypical orphan nuclear receptor small heterodimer partner

The small heterodimer partner (SHP; NROB2) is a member of the nuclear receptor superfamily and is classified as an "orphan" subgroup, as its ligand has not yet been identified. SHP lacks the classical DNA-binding domain found in most nuclear receptors and functions as a transcriptional coregulator by directly interacting with nuclear receptors and other transcription factors. SHP regulates the transcription of a variety of target genes and controls a variety of physiological functions. For the past 10 years, great progress has been made in our understanding of the mechanism of action of SHP and the regulation of SHP gene expression. Many of the results imply that SHP has a variety of roles in the regulation of metabolic homeostasis. In this review, we discuss the current state of understanding of the structure, expression, and function of the orphan nuclear receptor, SHP.

Recognition of LXXLL by Ligand Binding Domain of the Farnesoid X Receptor in Molecular Dynamics Simulation

The Farnesoid X receptor (FXR) has recently become a potential therapeutical target. The recruitment of coactivator protein (specified by LXXLL sequence) is the initial step in transcriptional activation of nuclear receptors (NRs). In this paper, the process of recognition of the LXXLL motif by the ligand binding domain (LBD) of FXR is observed in a 25 ns molecular dynamics simulation. The hydrophobic and hydrogen bonding interactions between the LBD and the coactivator are fully analyzed. This observation provides justification for the 'on deck' model proposed by Nettles and Greene. At last, insight to the protein-polypeptide interactions and protein conformational changes are discussed.

Identification of a lead pharmacophore for the development of potent nuclear receptor modulators as anticancer and X syndrome disease therapeutic agents

A series of tetrahydroisoquinoline-N-phenylamide derivatives were designed, synthesized, and tested for their relative binding affinity and antagonistic activity against androgen receptor (AR). Compound 1b (relative binding affinity, RBA = 6.4) and 1h (RBA = 12.6) showed higher binding affinity than flutamide (RBA = 1), a potent AR antagonist. These two compounds also exerted optimal antagonistic activity against AR in reporter assays. The derivatives were also tested for their activities against another nuclear receptor, farnesoid x receptor (FXR), with most compounds acting as weak antagonists, however, compound 1h behaved as a FXR agonist with activity slightly less than that of chenodeoxycholic acid (CDCA), a natural FXR agonist.

Unveiling hidden features of orphan nuclear receptors: The case of the small heterodimer partner (SHP)

The small heterodimer partner (SHP) is an atypical nuclear receptor lacking the N-terminal ligand-independent activation domain and the DNA binding domain. SHP acts as transcriptional inhibitor of a large set of nuclear receptors, among which ER, AR, CAR, RXR, GR, LXR and ERRgamma. The repression mechanism of SHP involves several actions including competition with coactivators binding on the AF-2 of nuclear receptors and recruitment of transcriptional inhibitors such as EID-1. The investigation of the structure and repression mechanism of SHP is a challenging task for a full understanding of nuclear receptor interaction pathways and functions. So far, mutational analyses in multiple populations identified loss of function mutants of SHP gene involved in mild obesity, increased birth weight and insulin levels. Furthermore, experimental mutagenesis has been exploited to characterize the interactions between SHP and the transcriptional inhibitor EID-1. With the aim of gaining insight into the structural basis of SHP repression mechanism, we modelled SHP and EID-1 structures. Docking experiments were carried out to identify the EID-1 binding surface on SHP structure. The results obtained in this study allow for the first time a unique interpretation of many experimental data available from the published literature. In addition, a fascinating hypothesis raises from the inspection of the proposed SHP structure: the presence of a potential unexpected ligand binding site, supported by recently available experimental data that may represent a breakthrough in the design and development of synthetic modulators of SHP functions.