Liver X receptor β and peroxisome proliferator-activated receptor δ regulate cholesterol transport in murine cholangiocytes

PPARβ/δ as a promising molecular drug target for liver diseases: A focused review

Various liver diseases pose great threats to humans. Although the etiologies of these liver diseases are quite diverse, they share similar pathologic phenotypes and molecular mechanisms such as oxidative stress, lipid and glucose metabolism disturbance, hepatic Kupffer cell (KC) proinflammatory polarization and inflammation, insulin resistance, and hepatic stellate cell (HSC) activation and proliferation. Peroxisome proliferator-activated receptor β/δ (PPARβ/δ) is expressed in various types of liver cells with relatively higher expression in KCs and HSCs. Accumulating evidence has revealed the versatile functions of PPARβ/δ such as controlling lipid homeostasis, inhibiting inflammation, regulating glucose metabolism, and restoring insulin sensitivity, suggesting that PPARβ/δ may serve as a potential molecular drug target for various liver diseases. This article aims to provide a concise review of the structure, expression pattern and biological functions of PPARβ/δ in the liver and its roles in various liver diseases, and to discuss potential future research perspectives.

Resistance to Cholesterol Gallstone Disease: Hepatic Cholesterol Metabolism

Cholesterol gallstone disease (CGD) is one of the most common digestive diseases, and it is closely associated with hepatic cholesterol metabolism. Cholesterol gallstones may be caused by abnormal hepatic cholesterol metabolism, such as excessive cholesterol biosynthesis within the liver, interfering with the uptake or export of cholesterol in the liver, and abnormal hepatic cholesterol esterification. In this review, we begin with a brief overview of the clinical diagnosis and treatment of gallstone disease (GSD). Then, we briefly describe the major processes of hepatic cholesterol metabolism and summarize the key molecular expression changes of hepatic cholesterol metabolism in patients with gallstones. We review and analyze the recent advances in elucidating the relationships between these key molecules and CGD, and some targets significantly impacting on CGD via hepatic cholesterol metabolism are also listed. We also provide a significant discussion on the relationship between CGD and nonalcoholic fatty liver disease (NAFLD). Finally, the new discoveries of some therapeutic strategies associated with hepatic cholesterol metabolism to prevent and treat CGD are summarized.

A Phase 3 Trial of Seladelpar in Primary Biliary Cholangitis

BACKGROUND

Effective treatments for patients with primary biliary cholangitis are limited. Seladelpar, a peroxisome proliferator-activated receptor delta agonist, has potential benefits.

METHODS

In this phase 3, 12-month, double-blind, placebo-controlled trial, we randomly assigned (in a 2:1 ratio) patients who had had an inadequate response to or who had a history of unacceptable side effects with ursodeoxycholic acid to receive oral seladelpar at a dose of 10 mg daily or placebo. The primary end point was a biochemical response, which was defined as an alkaline phosphatase level less than 1.67 times the upper limit of the normal range, with a decrease of 15% or more from baseline, and a normal total bilirubin level at month 12. Key secondary end points were normalization of the alkaline phosphatase level at month 12 and a change in the score on the pruritus numerical rating scale (range, 0 [no itch] to 10 [worst itch imaginable]) from baseline to month 6 among patients with a baseline score of at least 4 (indicating moderate-to-severe pruritus).

RESULTS

Of the 193 patients who underwent randomization and treatment, 93.8% received ursodeoxycholic acid as standard-of-care background therapy. A greater percentage of the patients in the seladelpar group than in the placebo group had a biochemical response (61.7% vs. 20.0%; difference, 41.7 percentage points; 95% confidence interval [CI], 27.7 to 53.4, P<0.001). Normalization of the alkaline phosphatase level also occurred in a greater percentage of patients who received seladelpar than of those who received placebo (25.0% vs. 0%; difference, 25.0 percentage points; 95% CI, 18.3 to 33.2, P<0.001). Seladelpar resulted in a greater reduction in the score on the pruritus numerical rating scale than placebo (least-squares mean change from baseline, -3.2 vs. -1.7; least-squares mean difference, -1.5; 95% CI, -2.5 to -0.5, P = 0.005). Adverse events were reported in 86.7% of the patients in the seladelpar group and in 84.6% in the placebo group, and serious adverse events in 7.0% and 6.2%, respectively.

CONCLUSIONS

In this trial involving patients with primary biliary cholangitis, the percentage of patients who had a biochemical response and alkaline phosphatase normalization was significantly greater with seladelpar than with placebo. Seladelpar also significantly reduced pruritus among patients who had moderate-to-severe pruritus at baseline. The incidence and severity of adverse events were similar in the two groups. (Funded by CymaBay Therapeutics; RESPONSE ClinicalTrials.gov number, NCT04620733; EudraCT number, 2020-004348-27.).

The roles of nuclear receptors in cholesterol metabolism and reverse cholesterol transport in nonalcoholic fatty liver disease

As the most prevalent chronic liver disease globally, NAFLD encompasses a pathological process that ranges from simple steatosis to NASH, fibrosis, cirrhosis, and HCC, closely associated with numerous extrahepatic diseases. While the initial etiology was believed to be hepatocyte injury caused by lipid toxicity from accumulated triglycerides, recent studies suggest that an imbalance of cholesterol homeostasis is of greater significance. The role of nuclear receptors in regulating liver cholesterol homeostasis has been demonstrated to be crucial. This review summarizes the roles and regulatory mechanisms of nuclear receptors in the 3 main aspects of cholesterol production, excretion, and storage in the liver, as well as their cross talk in reverse cholesterol transport. It is hoped that this review will offer new insights and theoretical foundations for the study of the pathogenesis and progression of NAFLD and provide new research directions for extrahepatic diseases associated with NAFLD.

Elucidating the primary mechanisms of high-intensity interval training for improved cardiac fitness in obesity

Obesity is a global and rising multifactorial pandemic associated with the emergence of several comorbidities that are risk factors for malignant cardiac remodeling and disease. High-intensity interval training (HIIT) has gained considerable attention due to its favorable outcomes of cardiometabolic health in individuals with overweight or obese. The primary aim of this review is to discuss the fundamental processes through which HIIT improves cardiac impairment in individuals with obesity to develop viable treatments for obesity management. In this review, a multiple database search and collection were conducted from the earliest record to January 2013 for studies included the qualitative component of HIIT intervention in humans and animals with overweight/obesity related to cardiac remodeling and fitness. We attempt to integrate the main mechanisms of HIIT in cardiac remolding improvement in obesity into an overall sequential hypothesis. This work focus on the ameliorative effects of HIIT on obesity-induced cardiac remodeling with respect to potential and pleiotropic mechanisms, including adipose distribution, energy metabolism, inflammatory response, insulin resistance, and related risk profiles in obesity. In conclusion, HIIT has been shown to reduce obesity-induced risks of cardiac remodeling, but the long-term effects of HIIT on obesity-induced cardiac injury and disease are presently unknown. Collective understanding highlights numerous specific research that are needed before the safety and effectiveness of HIIT can be confirmed and widely adopted in patient with obesity.

Seladelpar efficacy and safety at 3 months in patients with primary biliary cholangitis: ENHANCE, a phase 3, randomized, placebo-controlled study

BACKGROUND AND AIMS

ENHANCE was a phase 3 study that evaluated efficacy and safety of seladelpar, a selective peroxisome proliferator-activated receptor-δ (PPAR) agonist, versus placebo in patients with primary biliary cholangitis with inadequate response or intolerance to ursodeoxycholic acid (UDCA).

APPROACH AND RESULTS

Patients were randomized 1:1:1 to oral seladelpar 5 mg (n=89), 10 mg (n=89), placebo (n=87) daily (with UDCA, as appropriate). Primary end point was a composite biochemical response [alkaline phosphatase (ALP) < 1.67×upper limit of normal (ULN), ≥15% ALP decrease from baseline, and total bilirubin ≤ ULN] at month 12. Key secondary end points were ALP normalization at month 12 and change in pruritus numerical rating scale (NRS) at month 6 in patients with baseline score ≥4. Aminotransferases were assessed. ENHANCE was terminated early following an erroneous safety signal in a concurrent, NASH trial. While blinded, primary and secondary efficacy end points were amended to month 3. Significantly more patients receiving seladelpar met the primary end point (seladelpar 5 mg: 57.1%, 10 mg: 78.2%) versus placebo (12.5%) ( p < 0.0001). ALP normalization occurred in 5.4% ( p =0.08) and 27.3% ( p < 0.0001) of patients receiving 5 and 10 mg seladelpar, respectively, versus 0% receiving placebo. Seladelpar 10 mg significantly reduced mean pruritus NRS versus placebo [10 mg: -3.14 ( p =0.02); placebo: -1.55]. Alanine aminotransferase decreased significantly with seladelpar versus placebo [5 mg: 23.4% ( p =0.0008); 10 mg: 16.7% ( p =0.03); placebo: 4%]. There were no serious treatment-related adverse events.

CONCLUSIONS

Patients with primary biliary cholangitis (PBC) with inadequate response or intolerance to UDCA who were treated with seladelpar 10 mg had significant improvements in liver biochemistry and pruritus. Seladelpar appeared safe and well tolerated.

Primary Biliary Cholangitis: Promising Emerging Innovative Therapies and Their Impact on GLOBE Scores

Primary biliary cholangitis (PBC), previously referred to as primary biliary cirrhosis, is an autoimmune disorder leading to the destruction of intra-hepatic bile ducts. If untreated, progressive bile duct damage and cholestasis can lead to ductopenia and result in cirrhosis. Ursodiol, the first drug approved for PBC, has changed the natural history of this disease and improved patient outcomes. Subsequently, several new prediction models incorporating a response to ursodiol were developed. These include the GLOBE score, which was shown to predict long-term outcomes in patients with PBC. In 2016, obeticholic acid (OCA) became the second drug to be approved by the FDA, predominantly based on improvement in alkaline phosphatase (ALP) levels. This trial has subsequently influenced the design of clinical trials. Several drugs are currently being evaluated as therapeutic options for PBC, with improvement in ALP being a main endpoint. In this review, we will discuss the impact of new therapies on GLOBE scores in patients with PBC.

Novel Therapies in Primary Biliary Cholangitis: What Is in the Pipeline?

Primary biliary cholangitis is a chronic autoimmune disease characterized by inflammation and the progressive destruction of small intrahepatic bile ducts. Current first-line treatment includes ursodeoxycholic acid; however, a significant number of patients have an inadequate response to therapy. These patients are at risk of liver failure requiring liver transplantation and experience a poor quality of life due to refractory symptoms. This manuscript aims to shed light on the current and prospective treatment options that may slow disease progression and improve these patients' symptoms.

Seladelpar: an investigational drug for the treatment of early-stage primary biliary cholangitis (PBC)

INTRODUCTION

Generic fibrates are used off-label as add-in therapy for the management of primary biliary cholangitis (PBC) but with unproven long-term liver-related survival benefits. The recently developed fibrate, seladelpar, has shown promising results in clinical trials, but these outcomes have been previously marred by safety concerns.

AREAS COVERED

We summarize existing treatment options in PBC and evaluate current trial data for seladelpar in relation to liver biochemistry, symptomology, and safety.

EXPERT OPINION

Seladelpar leads to marked improvement in liver biochemistry and may improve symptoms. Safety concerns around liver toxicity appear to have been addressed. With likely increasing evidence compared to existing off-label fibrates, seladelpar has the potential as an attractive future second-line agent in PBC.

A phase II, randomized, open-label, 52-week study of seladelpar in patients with primary biliary cholangitis

BACKGROUND & AIMS

We examined the efficacy and safety of seladelpar, a selective peroxisome proliferator-activated receptor-delta agonist, in adults with primary biliary cholangitis (PBC) at risk of disease progression (alkaline phosphatase [ALP] ≥1.67xupper limit of normal [ULN]) who were receiving or intolerant to ursodeoxycholic acid.

METHODS

In this 52-week, phase II, dose-ranging, open-label study, patients were randomized (1:1) to seladelpar 5 mg/day (n = 53) or 10 mg/day (n = 55) or assigned to 2 mg/day (n = 11; United Kingdom sites after interim analysis) for 12 weeks. Doses could then be uptitrated to 10 mg/day. The primary efficacy endpoint was ALP change from baseline to Week 8.

RESULTS

Mean baseline ALP was 300, 345, and 295 U/L in the 2 mg, 5 mg, and 10 mg cohorts, respectively. Twenty-one percent of patients had cirrhosis, 71% had pruritus. At Week 8, mean ± standard error ALP reductions from baseline were 26 ± 2.8%, 33 ± 2.6%, and 41 ± 1.8% in the 2 mg (n = 11), 5 mg (n = 49), and 10 mg (n = 52) cohorts (all p ≤0.005), respectively. Responses were maintained or improved at Week 52, after dose escalation in 91% and 80% of the 2 mg and 5 mg cohorts, respectively. At Week 52, composite response (ALP <1.67xULN, ≥15% ALP decrease, and normal total bilirubin) rates were 64%, 53%, and 67%, and ALP normalization rates were 9%, 13%, and 33% in the 2 mg, 5 mg, and 10 mg cohorts, respectively. Pruritus visual analog scale score was decreased in the 5 mg and 10 mg cohorts. There were no treatment-related serious adverse events, and 4 patients discontinued due to adverse events.

CONCLUSIONS

Seladelpar demonstrated robust, dose-dependent, clinically significant, and durable improvements in biochemical markers of cholestasis and inflammation in patients with PBC at risk of disease progression. Seladelpar appeared safe and well tolerated and was not associated with any increase in pruritus.

CLINICALTRIALS

GOV NUMBER

NCT02955602 CLINICALTRIALSREGISTER.

EU NUMBER

2016-002996-91 LAY SUMMARY: Current treatment options for patients living with primary biliary cholangitis (PBC) are not optimal due to inadequate effectiveness or undesirable side effects. Patients with PBC who took seladelpar, a new treatment being developed for PBC, at increasing doses (2, 5, or 10 mg/day) for 1 year had clinically significant, dose-dependent improvements in key liver tests. Treatment appeared safe and was not associated with any worsening in patient self-reported itch scores.

Primary Biliary Cholangitis and Primary Sclerosing Cholangitis: Current Knowledge of Pathogenesis and Therapeutics

Cholangiopathies encompass various biliary diseases affecting the biliary epithelium, resulting in cholestasis, inflammation, fibrosis, and ultimately liver cirrhosis. Primary biliary cholangitis (PBC) and primary sclerosing cholangitis (PSC) are the most important progressive cholangiopathies in adults. Much research has broadened the scope of disease biology to genetic risk, epigenetic changes, dysregulated mucosal immunity, altered biliary epithelial cell function, and dysbiosis, all of which interact and arise in the context of ill-defined environmental triggers. An in-depth understanding of the molecular pathogenesis of these cholestatic diseases will help clinicians better prevent and treat diseases. In this review, we focus on the main underlying mechanisms of disease initiation and progression, and novel targeted therapeutics beyond currently approved treatments.

Liver X receptor regulates bile volume and the expression of aquaporins and cystic fibrosis transmembrane conductance regulator in the gallbladder

The gallbladder is considered an important organ in maintaining digestive and metabolic homeostasis. Given that therapeutic options for gallbladder diseases are often limited to cholecystectomy, understanding gallbladder pathophysiology is essential in developing novel therapeutic strategies. Since liver X receptor β (LXRβ), an oxysterol-activated transcription factor, is strongly expressed in gallbladder cholangiocytes, the aim was to investigate LXRβ physiological function in the gallbladder. Thus, we studied the gallbladders of WT and LXRβ-/- male mice using immunohistochemistry, electron microscopy, qRT-PCR, bile duct cannulation, bile and blood biochemistry, and duodenal pH measurements. LXRβ-/- mice presented a large gallbladder bile volume with high duodenal mRNA levels of the vasoactive intestinal polypeptide (VIP), a strong mediator of gallbladder relaxation. LXRβ-/- gallbladders showed low mRNA and protein expression of Aquaporin-1, Aquaporin-8, and cystic fibrosis transmembrane conductance regulator (CFTR). A cystic fibrosis-resembling phenotype was evident in the liver showing high serum cholestatic markers and the presence of reactive cholangiocytes. For LXRβ being a transcription factor, we identified eight putative binding sites of LXR on the promoter and enhancer of the Cftr gene, suggesting Cftr as a novel LXRβ regulated gene. In conclusion, LXRβ was recognized as a regulator of gallbladder bile volume through multiple mechanisms involving CFTR and aquaporins.NEW & NOTEWORTHY This report reveals a novel and specific role of the nuclear receptor liver X receptor β (LXRβ) in controlling biliary tree pathophysiology. LXRβ-/- mice have high gallbladder bile volume and are affected by a cholangiopathy that resembles cystic fibrosis. We found LXRβ to regulate the expression of both aquaporins water channels and the cystic fibrosis transmembrane conductance regulator. This opens a new field in biliary tree pathophysiology, enlightening a possible transcription factor controlling CFTR expression.

The pathogenesis, models and therapeutic advances of primary biliary cholangitis

Primary biliary cholangitis (PBC) is an autoimmune disease characterized by the destruction of intrahepatic small bile ducts and the presence of antimitochondrial antibody (AMA), eventually progresses to liver fibrosis and cirrhosis. Genetic predisposition and environmental factors are involved in the occurrence of PBC, and the epitopes exposure and the imbalance of autoimmune tolerance are the last straw. The apoptosis of biliary epithelial cell (BEC) leads to the release of autoantigen epitopes, which activate the immune system, and the disorder of innate and adaptive immunity eventually leads to the start of disease. Animal models have unique advantages in investigating the pathogenesis and drug exploitation of PBC. Multiple models have been reported, and spontaneous model and induced model have been widely used in relevant research of PBC in recent years. Currently, the only drugs licensed for PBC are ursodesoxycholic acid (UDCA) and obeticholic acid (OCA). In the last few years, as the learned more about the pathogenesis of PBC, more and more targets have been discovered, and multiple targeted drugs are being in developed. In this review, the pathogenesis, murine models and treatment strategies of PBC were summarized, and the current research status was discussed to provide insights for the further study of PBC.

Multiple therapeutic targets in rare cholestatic liver diseases: Time to redefine treatment strategies

Primary biliary cholangitis and primary sclerosing cholangitis are rare diseases affecting the bile ducts and the liver. The limited knowledge of their pathogenesis leads to limited therapeutic options. Nevertheless, the landscape of novel therapies for these cholangiopathies is now rapidly changing, providing new treatment opportunities for patients and clinicians involved in their care. The aim of this review is to summarize the evidence of novel molecules under investigation for primary biliary cholangitis and primary sclerosing cholangitis and to discuss how they can potentially change current treatment paradigms.

Revealing the role of peroxisome proliferator-activated receptor β/δ in nonalcoholic fatty liver disease

Nonalcoholic fatty liver disease (NAFLD), a form of chronic liver disease that occurs in individuals with no significant alcohol abuse, has become an increasing concern for global health. NAFLD is defined as the presence of lipid deposits in hepatocytes and it ranges from hepatic steatosis (fatty liver) to steatohepatitis. Emerging data from both preclinical studies and clinical trials suggest that the peroxisome proliferator-activated receptor (PPAR)β/δ plays an important role in the control of carbohydrate and lipid metabolism in liver, and its activation might hinder the progression of NAFLD. Here, we review the latest information on the effects of PPARβ/δ on NAFLD, including its capacity to reduce lipogenesis, to alleviate inflammation and endoplasmic reticulum stress, to ameliorate insulin resistance, and to attenuate liver injury. Because of these effects, activation of hepatic PPARβ/δ through synthetic or natural ligands provides a promising therapeutic option for the management of NAFLD.

Investigational drugs in early phase development for primary biliary cholangitis

Introduction: With a large percentage of patients having an incomplete response or intolerance to current FDA approved medications, new therapies for the treatment of primary biliary cholangitis are in great demand. Areas covered: In this review, we assess currently available drugs as well as promising new therapies for the treatment of primary biliary cholangitis. A literature search was performed with the following search terms: 'PBC treatment,' 'PBC therapeutics,' 'PBC clinical trials,' and included original articles, meta-analyses, and systematic reviews from 1 January 1981, to 1 January 2020. ClinicalTrials.gov was accessed for data from ongoing trials. Expert opinion: Targeted drug therapies offer an alternative for patients who are unable to meet their therapeutic goals with either of the two currently approved treatment options. Specifically, new drugs targeting bile-acid regulation, immune-modulation, and fibrogenic pathways are currently in development with multiple agents showing encouraging early results with the ultimate goal of developing therapies that will achieve high rates of biochemical remission, will be well tolerated, and improve symptoms and quality of life in patients with primary biliary cholangitis. Based on a review of the current literature, PPAR agonists appear to be promising agents, along with FGF19 analogs and FXR agonists.

New Therapeutic Targets in Autoimmune Cholangiopathies

Primary biliary cholangitis (PBC) and primary sclerosing cholangitis (PSC) are autoimmune cholangiopathies characterized by limited treatment options. A more accurate understanding of the several pathways involved in these diseases has fostered the development of novel and promising targeted drugs. For PBC, the characterization of the role of farnesoid X receptor (FXR) and perixosome-proliferator activated receptor (PPAR) has paved the way to several clinical trials including different molecules with choleretic and antinflammatory action. Conversely, different pathogenetic models have been proposed in PSC such as the “leaky gut” hypothesis, a dysbiotic microbiota or a defect in mechanisms protecting against bile acid toxicity. Along these theories, new treatment approaches have been developed, ranging from drugs interfering with trafficking of lymphocytes from the gut to the liver, fecal microbiota transplantation or new biliary acids with possible immunomodulatory potential. Finally, for both diseases, antifibrotic agents are under investigation. In this review, we will illustrate current understanding of molecular mechanisms in PBC and PSC, focusing on actionable biological pathways for which novel treatments are being developed.

High-intensity interval training has a greater effect on reverse cholesterol transport elements compared with moderate-intensity continuous training in obese male rats

OBJECTIVES

The present study compares the effect of high-intensity interval training (HIIT; 18 min) and moderate-intensity continuous training (MIT; 1 h) on reverse cholesterol transport (RCT) elements in obese subjects.

METHODS

Thirty adult male rats were induced high-fat diet (HFD) for 12 weeks. After four weeks, the rats were randomly divided into three groups while simultaneously continuing the HFD for the remaining eight weeks. Group specificities were HFD-control, HFD-MIT and HFD-HIIT. The rats were sacrificed 48 h after the last training session and the samples were collected. Analysis of variance and Pearson's correlation test were used for the statistical analyses (significance level: p ≤ 0.05).

RESULTS

The results showed that both HIIT and MIT improved heart ABCA1, ABCG1, ABCG4, ABCG5, ABCG8, LXR-α and PPARγ gene expression as well as plasma Apo A1, LCAT, lipids and lipoproteins (p ≤ 0.05). Moreover, higher cardiac ABCA1, ABCG1, ABCG4, ABCG5, ABCG8 and PPARγ expression and plasma high-density lipoprotein cholesterol (p ≤ 0.05) concentrations were found in the HFD-HIIT group compared with the HFD-MIT group.

CONCLUSION

HIIT may have more cardioprotective effects than MIT against atherosclerosis, along with saving time, as supported by the changes observed in the main factors involved in the RCT process.

The Effect of Polyphenols on Hypercholesterolemia through Inhibiting the Transport and Expression of Niemann-Pick C1-Like 1

The Niemann-Pick C1-like 1 (NPC1L1) protein is a cholesterol transporter that is expressed in the small intestine. This report describes the discovery of NPC1L1, its transport properties, and the inhibitory effects of polyphenols on NPC1L1. NPC1L1 was identified in 2004 while searching for ezetimibe molecular targets. Excessive synthesis of cholesterol results in hyperlipidemia, which increases the amount of bile cholesterol excreted into the duodenum. The inhibition of NPC1L1 decreases blood cholesterol because food and bile cholesterol are also absorbed from NPC1L1 in the intestine. Some polyphenols, particularly luteolin, have been reported as NPC1L1-mediated anti-dyslipidemia constituents. Luteolin affects NPC1L1 through two mechanisms. Luteolin directly inhibits NPC1L1 by binding to it, which occurs in a short timeframe similar to that for ezetimibe. The other mechanism is the inhibition of NPC1L1 expression. Luteolin reduced the binding of Sterol-regulatory element-binding protein 2 (SREBP2) in the promoter region of the NPC1L1 gene and decreased mRNA levels of SREBP2 and hepatocyte nuclear factor 4α. These data suggest that luteolin decreases the expression of NPC1L1 through regulation of transcription factors. This review also explores the effect of other polyphenols on NPC1L1 and hypercholesterolemia.

Transcriptional control of intestinal cholesterol absorption, adipose energy expenditure and lipid handling by Sortilin

The sorting receptor Sortilin functions in the regulation of glucose and lipid metabolism. Dysfunctional lipid uptake, storage, and metabolism contribute to several major human diseases including atherosclerosis and obesity. Sortilin associates with cardiovascular disease; however, the role of Sortilin in adipose tissue and lipid metabolism remains unclear. Here we show that in the low-density lipoprotein receptor-deficient (Ldlr^−/−) atherosclerosis model, Sortilin deficiency (Sort1^−/−) in female mice suppresses Niemann-Pick type C1-Like 1 (Npc1l1) mRNA levels, reduces body and white adipose tissue weight, and improves brown adipose tissue function partially via transcriptional downregulation of Krüppel-like factor 4 and Liver X receptor. Female Ldlr^−/−Sort1^−/− mice on a high-fat/cholesterol diet had elevated plasma Fibroblast growth factor 21 and Adiponectin, an adipokine that when reduced is associated with obesity and cardiovascular disease-related factors. Additionally, Sort1 deficiency suppressed cholesterol absorption in both female mice ex vivo intestinal tissue and human colon Caco-2 cells in a similar manner to treatment with the NPC1L1 inhibitor ezetimibe. Together our findings support a novel role of Sortilin in energy regulation and lipid homeostasis in female mice, which may be a potential therapeutic target for obesity and cardiovascular disease.

Phosphorylation of P27 by AKT is required for inhibition of cell cycle progression in cholangiocarcinoma

BACKGROUND AND OBJECTIVE

P27 is a putative tumor suppressor when located in the nucleus and AKT is an inhibitor of P27 which promotes growth of cholangiocarcinoma. We hypothesized that AKT-dependent phosphorylation at the P27 nuclear localization sequence T157 leads to nuclear export of P27, and thus loss of its tumor suppressive function. This study investigated whether loss of cell cycle regulation in cholangiocarcinoma due to subcellular localization of P27.

METHODS

Human cholangiocarcinoma cells were transfected with AKT. P27 was tagged with yellow fluorescence protein. Cell cycle progression was determined by flow cytometry. Migration and invasion of was measured by transwell assay.

RESULTS

Overexpression of wildtype P27 or P27-T157A in Mz-ChA-1 cells resulted in G1 arrest; expression of myr-AKT caused translocation of P27-YFP and endogenous P27 from the nucleus to the cytoplasm, leading to inhibition of P27-dependent G1 arrest; the AKT inhibitor and expression of dnAKT increased P27-YFP accumulation in the nucleus and promoted G1 arrest. In contrast, cells expressing YFP-P27-T157A or P27-YFP accumulated only in the nucleus. Co-expression of myr-AKT failed to induce P27-YFP translocation to the cytoplasm or inhibit G1 arrest. Overexpression of P27-T157A significantly increased migration and invasion.

CONCLUSIONS

Cholangiocarcinoma growth is associated with nuclear export of P27 that is due to AKT-mediated phosphorylation of P27 at T157.

Environmental Xenoestrogens Super-Activate a Variant Murine ER Beta in Cholangiocytes

High systemic levels of oestrogens are cholestatic and primary biliary cholangitis (PBC)-which is characterized by hepatic ductular inflammation-is thought to be triggered by exposure to xenobiotics such as those around landfill sites. Xenoestrogens may be a component of this chemical trigger. We therefore hypothesized that xenoestrogens are present at higher levels in the proximity of landfill sites. To test this hypothesis, soil samples were collected, extracts prepared and biological oestrogenic activity examined using cell-based reporter gene assays. Extracts from several sample sites around a landfill site contained a chemical(s) which activated the human ERα in a dose-dependent manner. Extracts from 3 separate control sampling sites were absent of any detectable activity. The mouse ERα and 2 variant mouse ERβ cDNAs were cloned and extracts from sample sites around a landfill site also activated these receptors. One variant murine ERβ was constitutively active when expressed in cholangiocytes, was readily inactivated by ICI182780 and activated in a dose-responsive, ICI182780-inhibitable manner by oestrogen. However, when this receptor was activated by extracts from landfill site soils, ICI182780 failed to antagonize activation. ERβ was readily detectable in murine cholangiocytes and exposing mice acutely to a pooled ER activating soil extracts also gave rise to a mild cholestatic injury. These data indicate that the environment around landfill sites may contain higher levels of xenoestrogens; that these chemicals have "super-activating" characteristics with a variant ERβ and therefore these chemicals could be a component of a xenobiotic insult that triggers PBC.

Bile acid receptors in the biliary tree: TGR5 in physiology and disease

Bile salts represent signalling molecules with a variety of endocrine functions. Bile salt effects are mediated by different receptor molecules, comprising ligand-activated nuclear transcription factors as well as G protein-coupled membrane-bound receptors. The farnesoid X receptor (FXR) and the plasma membrane-bound G protein-coupled receptor TGR5 (Gpbar-1) are prototypic bile salt receptors of both classes and are highly expressed in the liver including the biliary tree as well as in the intestine. In liver, TGR5 is localized in different non-parenchymal cells such as sinusoidal endothelial cells, Kupffer cells, hepatic stellate cells and small and large cholangiocytes. Through TGR5 bile salts can mediate choleretic, cell-protective as well as proliferative effects in cholangiocytes. A disturbance of these signalling mechanisms can contribute to the development of biliary diseases. In line with the important role of TGR5 for bile salt signalling, TGR5 knockout mice are more susceptible to cholestatic liver damage. Furthermore, in absence of TGR5 cholangiocyte proliferation in response to cholestasis is attenuated and intrahepatic and extrahepatic bile ducts show increased cell damage, underscoring the role of the receptor for biliary physiology. Decreased TGR5 expression may also contribute to the development or progression of cholangiopathies like primary biliary cholangitis (PBC) and primary sclerosing cholangitis (PSC) since reduced TGR5-dependent cell-protective mechanisms such as bicarbonate secretion renders cholangiocytes more vulnerable towards bile salt toxicity. Nevertheless, TGR5 overexpression or constant stimulation of the receptor can promote cholangiocyte proliferation leading to cyst growth in polycystic liver disease or even progression of cholangiocarcinoma. Not only the stimulation of TGR5-mediated pathways by suitable TGR5 agonists but also the inhibition of TGR5 signalling by the use of antagonists represent potential therapeutic approaches for different types of biliary diseases. This article is part of a Special Issue entitled: Cholangiocytes in Health and Disease edited by Jesus Banales, Marco Marzioni, Nicholas LaRusso and Peter Jansen.

Seladelpar (MBX-8025), a selective PPAR-δ agonist, in patients with primary biliary cholangitis with an inadequate response to ursodeoxycholic acid: a double-blind, randomised, placebo-controlled, phase 2, proof-of-concept study

BACKGROUND

Many patients with primary biliary cholangitis have an inadequate response to first-line therapy with ursodeoxycholic acid. Seladelpar is a potent, selective agonist for the peroxisome proliferator-activated receptor-delta (PPAR-δ), which is implicated in bile acid homoeostasis. This first-in-class study evaluated the anti-cholestatic effects and safety of seladelpar in patients with an inadequate response to ursodeoxycholic acid.

METHODS

The study was a 12-week, double-blind, placebo-controlled, phase 2 trial of patients with alkaline phosphatase of at least 1·67 times the upper limit of normal (ULN) despite treatment with ursodeoxycholic acid. Patients, recruited at 29 sites in North America and Europe, were randomly assigned to placebo, seladelpar 50 mg/day, or seladelpar 200 mg/day while ursodeoxycholic acid was continued. Randomisation was done centrally (1:1:1) by a computerised system using an interactive voice-web response system with a block size of three. Randomisation was stratified by region (North America and Europe). The primary outcome was the percentage change from baseline in alkaline phosphatase over 12 weeks, analysed in the modified intention-to-treat (ITT) population (any randomised patient who received at least one dose of medication and had at least one post-baseline alkaline phosphatase evaluation). This study is registered with ClinicalTrials.gov (NCT02609048) and the EU Clinical Trials Registry (EudraCT2015-002698-39).

FINDINGS

Between Nov 4, 2015, and May 26, 2016, 70 patients were screened at 29 sites in North America and Europe. During recruitment, three patients treated with seladelpar developed fully reversible, asymptomatic grade 3 alanine aminotransferase increases (one on 50 mg, two on 200 mg), ranging from just over five to 20 times the ULN; as a result, the study was terminated after 41 patients were randomly assigned. The modified ITT population consisted of 12 patients in the placebo group, 13 in the seladelpar 50 mg group, and 10 in the seladelpar 200 mg group. Mean changes from baseline in alkaline phosphatase were -2% (SD 16) in the placebo group, -53% (14) in the seladelpar 50 mg group, and -63% (8) in the seladelpar 200 mg group. Changes in both seladelpar groups versus placebo were significant (p<0·0001 for both groups vs placebo), with no significant difference between the two seladelpar groups (p=0·1729). All five patients who received seladelpar for 12 weeks had normal alkaline phosphatase values at the end of treatment, based on a central laboratory ULN for alkaline phosphatase of 116 U/L. The most frequently reported adverse events were pruritus (16%; one patient on placebo, four on seladelpar 50 mg, and one on seladelpar 200 mg), nausea (13%; one patient on placebo, three on seladelpar 50 mg, and one on seladelpar 200 mg), diarrhoea (10%; two patients on placebo, one on seladelpar 50 mg, and one on seladelpar 200 mg), dyspepsia (8%; two patients on seladelpar 50 mg and one on seladelpar 200 mg), muscle spasms (8%; three patients on seladelpar 200 mg), myalgia (8%; one patient on placebo and two on seladelpar 200 mg), and dizziness (8%; one patient on placebo and two on seladelpar 50 mg).

INTERPRETATION

Seladelpar normalised alkaline phosphatase levels in patients who completed 12 weeks of treatment. However, treatment was associated with grade 3 increases in aminotransferases and the study was stopped early. The effects of seladelpar should be explored at lower doses.

FUNDING

CymaBay Therapeutics.

Hepatic circadian clock oscillators and nuclear receptors integrate microbiome-derived signals

The liver is a key organ of metabolic homeostasis with functions that oscillate in response to food intake. Although liver and gut microbiome crosstalk has been reported, microbiome-mediated effects on peripheral circadian clocks and their output genes are less well known. Here, we report that germ-free (GF) mice display altered daily oscillation of clock gene expression with a concomitant change in the expression of clock output regulators. Mice exposed to microbes typically exhibit characterized activities of nuclear receptors, some of which (PPARα, LXRβ) regulate specific liver gene expression networks, but these activities are profoundly changed in GF mice. These alterations in microbiome-sensitive gene expression patterns are associated with daily alterations in lipid, glucose, and xenobiotic metabolism, protein turnover, and redox balance, as revealed by hepatic metabolome analyses. Moreover, at the systemic level, daily changes in the abundance of biomarkers such as HDL cholesterol, free fatty acids, FGF21, bilirubin, and lactate depend on the microbiome. Altogether, our results indicate that the microbiome is required for integration of liver clock oscillations that tune output activators and their effectors, thereby regulating metabolic gene expression for optimal liver function.

Stimulation of nuclear receptor peroxisome proliferator-activated receptor-γ limits NF-κB-dependent inflammation in mouse cystic fibrosis biliary epithelium

Cystic fibrosis-associated liver disease is a chronic cholangiopathy that negatively affects the quality of life of cystic fibrosis patients. In addition to reducing biliary chloride and bicarbonate secretion, up-regulation of toll-like receptor 4/nuclear factor kappa light-chain-enhancer of activated B cells (NF-κB)-dependent immune mechanisms plays a major role in the pathogenesis of cystic fibrosis-associated liver disease and may represent a therapeutic target. Nuclear receptors are transcription factors that regulate several intracellular functions. Some nuclear receptors, including peroxisome proliferator-activated receptor-γ (PPAR-γ), may counterregulate inflammation in a tissue-specific manner. In this study, we explored the anti-inflammatory effect of PPAR-γ stimulation in vivo in cystic fibrosis transmembrane conductance regulator (Cftr) knockout mice exposed to dextran sodium sulfate and in vitro in primary cholangiocytes isolated from wild-type and from Cftr-knockout mice exposed to lipopolysaccharide. We found that in CFTR-defective biliary epithelium expression of PPAR-γ is increased but that this does not result in increased receptor activity because the availability of bioactive ligands is reduced. Exogenous administration of synthetic agonists of PPAR-γ (pioglitazone and rosiglitazone) up-regulates PPAR-γ-dependent genes, while inhibiting the activation of NF-κB and the secretion of proinflammatory cytokines (lipopolysaccharide-induced CXC chemokine, monocyte chemotactic protein-1, macrophage inflammatory protein-2, granulocyte colony-stimulating factor, keratinocyte chemoattractant) in response to lipopolysaccharide. PPAR-γ agonists modulate NF-κB-dependent inflammation by up-regulating nuclear factor of kappa light polypeptide gene enhancer in B cells inhibitor alpha, a negative regulator of NF-κB. Stimulation of PPAR-γ in vivo (rosiglitazone) significantly attenuates biliary damage and inflammation in Cftr-knockout mice exposed to a dextran sodium sulfate-induced portal endotoxemia.

CONCLUSION

These studies unravel a novel function of PPAR-γ in controlling biliary epithelium inflammation and suggest that impaired activation of PPAR-γ contributes to the chronic inflammatory state of CFTR-defective cholangiocytes.

Nuclear receptors in acute and chronic cholestasis

BACKGROUND

Nuclear receptors (NRs) form a family of 48 members. NRs control hepatic processes such as bile acid homeostasis, lipid metabolism and mechanisms involved in fibrosis and inflammation. Due to their central role in the regulation of hepatoprotective mechanisms, NRs are promising therapeutic targets in cholestatic disorders.

KEY MESSAGES

NRs can be classified into five different physiological clusters. NRs from the 'bile acids and xenobiotic metabolism' and from the 'lipid metabolism and energy homeostasis' clusters are strongly expressed in the liver. Furthermore, NRs from these clusters, such as farnesoid X receptor α (FXRα), pregnane X receptor (PXR) and peroxisome proliferator-activated receptors (PPARs), have been associated with the pathogenesis and the progression of cholestasis. The latter observation is also true for vitamin D receptor (VDR), which is barely detectable in the whole liver, but has been linked to cholestatic diseases. Involvement of VDR in cholestasis is ascribed to a strong expression in nonparenchymal liver cells, such as biliary epithelial cells, Kupffer cells and hepatic stellate cells. Likewise, NRs from other physiological clusters with low hepatic expression, such as estrogen receptor α (ERα) or reverse-Erb α/β (REV-ERB α/β), may also control pathophysiological processes in cholestasis.

CONCLUSIONS

In this review, we will describe the impact of individual NRs on cholestasis. We will then discuss the potential role of these transcription factors as therapeutic targets.

Recruited metastasis suppressor NM23-H2 attenuates expression and activity of peroxisome proliferator-activated receptor δ (PPARδ) in human cholangiocarcinoma

BACKGROUND

Peroxisome proliferator-activated receptor δ (PPARδ) is a versatile regulator of distinct biological processes and overexpression of PPARδ in cancer may be partially related to its suppression of its own co-regulators.

AIMS

To determine whether recruited suppressor proteins bind to and regulate PPARδ expression, activity and PPARδ-dependent cholangiocarcinoma proliferation.

METHODS

Yeast two-hybrid assays were done using murine PPARδ as bait. PPARδ mRNA expression was determined by qPCR. Protein expression was measured by western blot. Immunohistochemistry and fluorescence microscopy were used to determine PPARδ expression and co-localization with NDP Kinase alpha (NM23-H2). Cell proliferation assays were performed to determine cell numbers.

RESULTS

Yeast two-hybrid screening identified NM23-H2 as a PPARδ binding protein and their interaction was confirmed. Overexpressed PPARδ or treatment with the agonist GW501516 resulted in increased cell proliferation. NM23-H2 siRNA activated PPARδ luciferase promoter activity, upregulated PPARδ RNA and protein expression and increased GW501516-stimulated CCA growth. Overexpression of NM23-H2 inhibited PPARδ luciferase promoter activity, downregulated PPARδ expression and AKT phosphorylation and reduced GW501516-stimulated CCA growth.

CONCLUSIONS

We report the novel association of NM23-H2 with PPARδ and the negative regulation of PPARδ expression by NM23-H2 binding to the C-terminal region of PPARδ. These findings provide evidence that the metastasis suppressor NM23-H2 is involved in the regulation of PPARδ-mediated proliferation.

Cell-type-resolved quantitative proteomics of murine liver

Mass spectrometry (MS)-based proteomics provides a powerful approach to globally investigate the biological function of individual cell types in mammalian organs. Here, we applied this technology to the in-depth analysis of purified hepatic cell types from mouse. We quantified 11,520 proteins, making this the most comprehensive proteomic resource of any organ to date. Global protein copy number determination demonstrated that a large proportion of the hepatocyte proteome is dedicated to fatty acid and xenobiotic metabolism. We identified as-yet-unknown components of the TGF-β signaling pathway and extracellular matrix in hepatic stellate cells, uncovering their regulative role in liver physiology. Moreover, our high-resolution proteomic data set enabled us to compare the distinct functional roles of hepatic cell types in cholesterol flux, cellular trafficking, and growth factor receptor signaling. This study provides a comprehensive resource for liver biology and biomedicine.

The mechanism of enterohepatic circulation in the formation of gallstone disease

Bile acids entering into enterohepatic circulating are primary acids synthesized from cholesterol in hepatocyte. They are secreted actively across canalicular membrane and carried in bile to gallbladder, where they are concentrated during digestion. About 95 % BAs are actively taken up from the lumen of terminal ileum efficiently, leaving only approximately 5 % (or approximately 0.5 g/d) in colon, and a fraction of bile acids are passively reabsorbed after a series of modifications in the human large intestine including deconjugation and oxidation of hydroxy groups. Bile salts hydrolysis and hydroxy group dehydrogenation reactions are performed by a broad spectrum of intestinal anaerobic bacteria. Next, hepatocyte reabsorbs bile acids from sinusoidal blood, which are carried to liver through portal vein via a series of transporters. Bile acids (BAs) transporters are critical for maintenance of the enterohepatic BAs circulation, where BAs exert their multiple physiological functions including stimulation of bile flow, intestinal absorption of lipophilic nutrients, solubilization, and excretion of cholesterol. Tight regulation of BA transporters via nuclear receptors (NRs) is necessary to maintain proper BA homeostasis. In conclusion, disturbances of enterohepatic circulation may account for pathogenesis of gallstones diseases, including BAs transporters and their regulatory NRs and the metabolism of intestinal bacterias, etc.

Generation of functional cholangiocyte-like cells from human pluripotent stem cells and HepaRG cells

Cholangiocytes are biliary epithelial cells, which, like hepatocytes, originate from hepatoblasts during embryonic development. In this study we investigated the potential of human embryonic stem cells (hESCs) to differentiate into cholangiocytes and we report a new approach, which drives differentiation of hESCs toward the cholangiocytic lineage using feeder-free and defined culture conditions. After differentiation into hepatic progenitors, hESCs were differentiated further into cholangiocytes using growth hormone, epidermal growth factor, interleukin-6, and then sodium taurocholate. These conditions also allowed us to generate cholangiocytes from HepaRG-derived hepatoblasts. hESC- and HepaRG-derived cholangiocyte-like cells expressed markers of cholangiocytes including cytokeratin 7 and osteopontin, and the transcription factors SOX9 and hepatocyte nuclear factor 6. The cells also displayed specific proteins important for cholangiocyte functions including cystic fibrosis transmembrane conductance regulator, secretin receptor, and nuclear receptors. They formed primary cilia and also responded to hormonal stimulation by increase of intracellular Ca(2+) . We demonstrated by integrative genomics that the expression of genes, which signed hESC- or HepaRG-cholangiocytes, separates hepatocytic lineage from cholangiocyte lineage. When grown in a 3D matrix, cholangiocytes developed epithelial/apicobasal polarity and formed functional cysts and biliary ducts. In addition, we showed that cholangiocyte-like cells could also be generated from human induced pluripotent stem cells, demonstrating the efficacy of our approach with stem/progenitor cells of diverse origins.

CONCLUSION

We have developed a robust and efficient method for differentiating pluripotent stem cells into cholangiocyte-like cells, which display structural and functional similarities to bile duct cells in normal liver. These cells will be useful for the in vitro study of the molecular mechanisms of bile duct development and have important potential for therapeutic strategies, including bioengineered liver approaches.

Action mechanisms of Liver X Receptors

The two Liver X Receptors, LXRα and LXRβ, are nuclear receptors belonging to the superfamily of ligand-activated transcription factors. They share more than 78% homology in amino acid sequence, a common profile of oxysterol ligands and the same heterodimerization partner, Retinoid X Receptor. LXRs play crucial roles in several metabolic pathways: lipid metabolism, in particular in preventing cellular cholesterol accumulation; glucose homeostasis; inflammation; central nervous system functions and water transport. As with all nuclear receptors, the transcriptional activity of LXR is the result of an orchestration of numerous cellular factors including ligand bioavailability, presence of corepressors and coactivators and cellular context i.e., what other pathways are activated in the cell at the time the receptor recognizes its ligand. In this mini-review we summarize the factors regulating the transcriptional activity and the mechanisms of action of these two receptors.

The orphan nuclear receptors at their 25-year reunion

The nuclear receptor superfamily includes many receptors, identified based on their similarity to steroid hormone receptors but without a known ligand. The study of how these receptors are diversely regulated to interact with genomic regions to control a plethora of biological processes has provided critical insight into development, physiology, and the molecular pathology of disease. Here we provide a compendium of these so-called orphan receptors and focus on what has been learned about their modes of action, physiological functions, and therapeutic promise.

FXR-induced secretion of FGF15/19 inhibits CYP27 expression in cholangiocytes through p38 kinase pathway

Cholangiocytes, bile duct lining cells, actively adjust the amount of cholesterol and bile acids in bile through expression of enzymes and channels involved in transportation and metabolism of the cholesterol and bile acids. Herein, we report molecular mechanisms regulating bile acid biosynthesis in cholangiocytes. Among the cytochrome p450 (Cyp) enzymes involved in bile acid biosynthesis, sterol 27-hydroxylase (Cyp27) that is the rate-limiting enzyme for the acidic pathway of bile acid biosynthesis expressed in cholangiocytes. Expression of other Cyp enzymes for the basic bile acid biosynthesis was hardly detected. The Cyp27 expression was negatively regulated by a hydrophobic bile acid through farnesoid X receptor (FXR), a nuclear receptor activated by bile acid ligands. Activated FXR exerted the negative effects by inducing an expression of fibroblast growth factor 15/19 (FGF15/19). Similar to its repressive function against cholesterol 7α-hydroxylase (Cyp7a1) expression in hepatocytes, secreted FGF15/19 triggered Cyp27 repression in cholangiocytes through interaction with its cognate receptor fibroblast growth factor receptor 4 (FGFR4). The involvements of FXR and FGFR4 for the bile acid-induced Cyp27 repression were confirmed in vivo using knockout mouse models. Different from the signaling in hepatocytes, wherein the FGF15/19-induced repression signaling is mediated by c-Jun N-terminal kinase (JNK), FGF15/19-induced Cyp27 repression in cholangiocytes was mediated by p38 kinase. Thus, the results collectively suggest that cholangiocytes may be able to actively regulate bile acid biosynthesis in cholangiocytes and even hepatocyte by secreting FGF15/19. We suggest the presence of cholangiocyte-mediated intrahepatic feedback loop in addition to the enterohepatic feedback loop against bile acid biosynthesis in the liver.

Ciliary subcellular localization of TGR5 determines the cholangiocyte functional response to bile acid signaling

TGR5, the G protein-coupled bile acid receptor that transmits bile acid signaling into a cell functional response via the intracellular cAMP signaling pathway, is expressed in human and rodent cholangiocytes. However, detailed information on the localization and function of cholangiocyte TGR5 is limited. We demonstrated that in human (H69 cells) and rat cholangiocytes, TGR5 is localized to multiple, diverse subcellular compartments, with its strongest expression on the apical plasma, ciliary, and nuclear membranes. To evaluate the relationship between ciliary TGR5 and the cholangiocyte functional response to bile acid signaling, we used a model of ciliated and nonciliated H69 cells and demonstrated that TGR5 agonists induce opposite changes in cAMP and ERK levels in cells with and without primary cilia. The cAMP level was increased in nonciliated cholangiocytes but decreased in ciliated cells. In contrast, ERK signaling was induced in ciliated cholangiocytes but suppressed in cells without cilia. TGR5 agonists inhibited proliferation of ciliated cholangiocytes but activated proliferation of nonciliated cells. The observed differential effects of TGR5 agonists were associated with the coupling of TGR5 to Gαi protein in ciliated cells and Gαs protein in nonciliated cholangiocytes. The functional responses of nonciliated and ciliated cholangiocytes to TGR5-mediated bile acid signaling may have important pathophysiological significance in cilia-related liver disorders (i.e., cholangiociliopathies), such as polycystic liver disease. In summary, TGR5 is expressed on diverse cholangiocyte compartments, including a primary cilium, and its ciliary localization determines the cholangiocyte functional response to bile acid signaling.