The nuclear receptor PXR is a lithocholic acid sensor that protects against liver toxicity

Developmental Regulation of Drug-Processing Genes in Livers of Germ-Free Mice

Very little is known about the effect of gut microbiota on the ontogeny of drug-processing genes (DPGs) in liver. In this study, livers were harvested from conventional (CV) and germ-free (GF) male and female mice from 1 to 90 days of age. RNA-Seq in livers of 90-day-old male mice showed that xenobiotic metabolism was the most downregulated pathway within the mRNA transcriptome in absence of intestinal bacteria. In male livers, the mRNAs of 67 critical DPGs partitioned into 4 developmental patterns (real-time-quantitative polymerase chain reaction): Pattern-1 gradually increased to adult levels in livers of CV mice and were downregulated in livers of GF mice, as exemplified by the major drug-metabolizing enzymes cytochrome 3a (Cyp3a) family, which are prototypical pregnane X receptor (PXR)-target genes. Genes in Pattern-2 include Cyp1a2 (aryl hydrocarbon receptor-target gene), Cyp2c family, and Cyp2e1, which were all upregulated mainly at 90 days of age; as well as the peroxisome proliferator-activated receptor α (PPARα)-target genes Cyp4a family and Aldh3a2, which were upregulated not only in 90-days adult age, but also between neonatal and adolescent ages (from 1 to 30 days of age). Genes in Pattern-3 were enriched predominantly in livers of 15-day-old mice, among which the sterol-efflux transporter dimers Abcg5/Abcg8 were downregulated in GF mice. Genes in Pattern-4 were neonatal-enriched, among which the transporter Octn1 mRNA tended to be lower in GF mice at younger ages but higher in adult GF mice as compared with age-matched CV mice. Protein assays confirmed the downregulation of the PXR-target gene Cyp3a protein (Western-blot and liquid chromatography tandem mass spectroscopy), and decreased Cyp3a enzyme activities in male GF livers. Increased microsomal-Cyp4a proteins and nuclear-PPARα were also observed in male GF livers. Interestingly, in contrast to male livers, the mRNAs of Cyp2c or Cyp4a were not readily upregulated in female GF livers approaching adult age, suggesting the maturation of female-specific hormones interferes with the interactions between intestinal microbiota and DPG ontogeny. In conclusion, intestinal microbiota markedly impacts the ontogeny of many hepatic DPGs in a gender-specific manner.

Bile Acid Metabolism and Signaling in Cholestasis, Inflammation, and Cancer

Bile acids are synthesized from cholesterol in the liver. Some cytochrome P450 (CYP) enzymes play key roles in bile acid synthesis. Bile acids are physiological detergent molecules, so are highly cytotoxic. They undergo enterohepatic circulation and play important roles in generating bile flow and facilitating biliary secretion of endogenous metabolites and xenobiotics and intestinal absorption of dietary fats and lipid-soluble vitamins. Bile acid synthesis, transport, and pool size are therefore tightly regulated under physiological conditions. In cholestasis, impaired bile flow leads to accumulation of bile acids in the liver, causing hepatocyte and biliary injury and inflammation. Chronic cholestasis is associated with fibrosis, cirrhosis, and eventually liver failure. Chronic cholestasis also increases the risk of developing hepatocellular or cholangiocellular carcinomas. Extensive research in the last two decades has shown that bile acids act as signaling molecules that regulate various cellular processes. The bile acid-activated nuclear receptors are ligand-activated transcriptional factors that play critical roles in the regulation of bile acid, drug, and xenobiotic metabolism. In cholestasis, these bile acid-activated receptors regulate a network of genes involved in bile acid synthesis, conjugation, transport, and metabolism to alleviate bile acid-induced inflammation and injury. Additionally, bile acids are known to regulate cell growth and proliferation, and altered bile acid levels in diseased conditions have been implicated in liver injury/regeneration and tumorigenesis. We will cover the mechanisms that regulate bile acid homeostasis and detoxification during cholestasis, and the roles of bile acids in the initiation and regulation of hepatic inflammation, regeneration, and carcinogenesis.

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.

Synthesis and biological activity of novel deoxycholic acid derivatives

We report the synthesis and biological activity of new semi-synthetic derivatives of naturally occurring deoxycholic acid (DCA) bearing 2-cyano-3-oxo-1-ene, 3-oxo-1(2)-ene or 3-oxo-4(5)-ene moieties in ring A and 12-oxo or 12-oxo-9(11)-ene moieties in ring C. Bioassays using murine macrophage-like cells and tumour cells show that the presence of the 9(11)-double bond associated with the increased polarity of ring A or with isoxazole ring joined to ring A, improves the ability of the compounds to inhibit cancer cell growth.

Nuclear bile acid signaling through the farnesoid X receptor

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

Tanshinone IIA exerts protective effects in a LCA-induced cholestatic liver model associated with participation of pregnane X receptor

Tanshinone IIA (Tan IIA) is one of the main natural active ingredients purified from Salvia miltiorrhiza radix, which has long been used in clinical practice in China to treat diseases including liver fibrosis, Alzheimer׳s disease, and cardiovascular diseases. Tan IIA has hepatoprotective properties, and is an efficacious PXR agonist. Our study was designed to observe the function and mechanism of the hepatoprotective properties of Tan IIA. HepG2 cells were used to investigate the vitrol effects of Tan IIA on PXR and CYP3A4. Gut-formed LCA is hepatotoxic, and has been implicated in the pathogenesis of cholestatic diseases. To further investigate the hepatoprotective mechanisms of Tan IIA against LCA-induced cholestasis in vivo, we choose the normal mice and siRNA-treated mice. The in vitro study demonstrated that the effect of Tan IIA on CYP3A4 was mediated by transactivation of PXR in a dose- and time-dependent manner. The in vivo experiments using PXR siRNA revealed that Tan IIA could protect against LCA-induced hepatotoxicity and cholestasis in a dose-dependent manner. These effects were partially caused by the upregulation of PXR, as well as Cyp3a11, Cyp3a13, and Mdr1, which are the enzymes responsible for LCA metabolism. This is the first report showing that the hepatoprotective effects of Tan IIA are partly mediated by PXR.

Transgenic mice and metabolomics for study of hepatic xenobiotic metabolism and toxicity

INTRODUCTION

The study of xenobiotic metabolism and toxicity has been greatly aided by the use of genetically modified mouse models and metabolomics.

AREAS COVERED

Gene knockout mice can be used to determine the enzymes responsible for the metabolism of xenobiotics in vivo and to examine the mechanisms of xenobiotic-induced toxicity. Humanized mouse models are especially important because there exist marked species differences in the xenobiotic-metabolizing enzymes and the nuclear receptors that regulate these enzymes. Humanized mice expressing CYPs and nuclear receptors including the pregnane X receptor, the major regulator of xenobiotic metabolism and transport were produced. With genetically modified mouse models, metabolomics can determine the metabolic map of many xenobiotics with a level of sensitivity that allows the discovery of even minor metabolites. This technology can be used for determining the mechanism of xenobiotic toxicity and to find early biomarkers for toxicity.

EXPERT OPINION

Metabolomics and genetically modified mouse models can be used for the study of xenobiotic metabolism and toxicity by: i) comparison of the metabolomics profiles between wild-type and genetically modified mice, and searching for genotype-dependent endogenous metabolites; ii) searching for and elucidating metabolites derived from xenobiotics; and iii) discovery of specific alterations of endogenous compounds induced by xenobiotics-induced toxicity.

New paradigms in the treatment of hepatic cholestasis: from UDCA to FXR, PXR and beyond

Cholestasis is an impairment of bile formation/flow at the level of the hepatocyte and/or cholangiocyte. The first, and for the moment, most established medical treatment is the natural bile acid (BA) ursodeoxycholic acid (UDCA). This secretagogue improves, e.g. in intrahepatic cholestasis of pregnancy or early stage primary biliary cirrhosis, impaired hepatocellular and cholangiocellular bile formation mainly by complex post-transcriptional mechanisms. The limited efficacy of UDCA in various cholestatic conditions urges for development of novel therapeutic approaches. These include nuclear and membrane receptor agonists and BA derivatives. The nuclear receptors farnesoid X receptor (FXR), retinoid X receptor (RXR), peroxisome proliferator-activated receptor α (PPARα), and pregnane X receptor (PXR) are transcriptional modifiers of bile formation and at present are under investigation as promising targets for therapeutic interventions in cholestatic disorders. The membrane receptors fibroblast growth factor receptor 4 (FGFR4) and apical sodium BA transporter (ASBT) deserve attention as additional therapeutic targets, as does the potential therapeutic agent norUDCA, a 23-C homologue of UDCA. Here, we provide an overview on established and future promising therapeutic agents and their potential molecular mechanisms and sites of action in cholestatic diseases.

Characterization and treatment of persistent hepatocellular secretory failure

BACKGROUND & AIMS

Hepatocellular secretory failure induced by drugs, toxins or transient biliary obstruction may sometimes persist for months after removal of the initiating factor and may then be fatal without liver transplantation. We characterized patients with severe persistent hepatocellular secretory failure (PHSF) and treated them with the pregnane X receptor (PXR) agonist, rifampicin. We also studied the effect of rifampicin on PXR-dependent expression of genes involved in biotransformation and secretion in vitro.

METHODS

Thirteen patients (age 18-81 years, 6 male) with hepatocellular secretory failure that persisted after removal of the inducing factor (drugs/toxin: 9) or biliary obstruction (4) were identified over 6 years. Six of these patients were screened for ATP8B1 or ABCB11 mutations. All were treated with rifampicin (300 mg daily) for 1-10 weeks. Expression of genes involved in biotransformation and secretion was determined by rtPCR in human hepatocytes and intestinal cells incubated with rifampicin (10 μmol/L).

RESULTS

Serum bilirubin of patients with PHSF ranged from 264 to 755 μmol/L. Normal γGT was found in 10/13 patients of whom 3/6 tested positive for ATP8B1/ABCB11 mutations. Serum bilirubin declined to <33 μmol/L after 1-10 weeks of rifampicin treatment. In vitro, rifampicin PXR-dependently upregulated biotransformation phase 1 (CYP3A4), phase 2 (UGT1A1) and phase 3 (MRP2) enzymes/carriers as well as the basolateral bile salt exporter OSTβ.

CONCLUSION

Persistent hepatocellular secretory failure may develop in carriers of transporter gene mutations. In severe cases, rifampicin may represent an effective therapeutic option of PHSF. PXR-dependent induction of CYP3A4, UGT1A1, MRP2 and OSTβ could contribute to the anticholestatic effect of rifampicin in PHSF.

Pregnane X receptor modulates the inflammatory response in primary cultures of hepatocytes

Bacterial sepsis is characterized by a rapid increase in the expression of inflammatory mediators to initiate the acute phase response in liver. Inflammatory mediator release is counterbalanced by the coordinated expression of anti-inflammatory molecules such as interleukin 1 receptor antagonist (IL1-Ra) through time. This study determined whether activation of pregnane X receptor (PXR, NR1I2) alters the lipopolysaccharide (LPS)-inducible gene expression program in primary cultures of hepatocytes (PCHs). Preactivation of PXR for 24 hours in PCHs isolated from wild-type mice suppressed the subsequent LPS-inducible expression of the key inflammatory mediators interleukin 1β (IL-1β), interleukin 6 (IL-6), and tumor necrosis factor α (TNFα) but not in PCHs isolated from Pxr-null (PXR-knockout [KO]) mice. Basal expression of key inflammatory cytokines was elevated in PCHs from PXR-KO mice. Stimulation of PCHs from PXR-KO mice with LPS alone produced enhanced levels of IL-1β when compared with wild-type mice. Experiments performed using PCHs from both humanized-PXR transgenic mice as well as human donors indicate that prolonged activation of PXR produces an increased secretion of IL1-Ra from cells through time. Our data reveal a working model that describes a pivotal role for PXR in both inhibiting as well as in resolving the inflammatory response in hepatocytes. Understanding the molecular details of how PXR is converted from a positive regulator of drug-metabolizing enzymes into a transcriptional suppressor of inflammation in liver will provide new pharmacologic strategies for modulating inflammatory-related diseases in the liver and intestine.

Environmental contaminants activate human and polar bear (Ursus maritimus) pregnane X receptors (PXR, NR1I2) differently

BACKGROUND

Many persistent organic pollutants (POPs) accumulate readily in polar bears because of their position as apex predators in Arctic food webs. The pregnane X receptor (PXR, formally NR1I2, here proposed to be named promiscuous xenobiotic receptor) is a xenobiotic sensor that is directly involved in metabolizing pathways of a wide range of environmental contaminants.

OBJECTIVES

In the present study, we comparably assess the ability of 51 selected pharmaceuticals, pesticides and emerging contaminants to activate PXRs from polar bears and humans using an in vitro luciferase reporter gene assay.

RESULTS

We found that polar bear PXR is activated by a wide range of our test compounds (68%) but has a slightly more narrow ligand specificity than human PXR that was activated by 86% of the 51 test compounds. The majority of the agonists identified (70%) produces a stronger induction of the reporter gene via human PXR than via polar bear PXR, however with some notable and environmentally relevant exceptions.

CONCLUSIONS

Due to the observed differences in activation of polar bear and human PXRs, exposure of each species to environmental agents is likely to induce biotransformation differently in the two species. Bioinformatics analyses and structural modeling studies suggest that amino acids that are not part of the ligand-binding domain and do not interact with the ligand can modulate receptor activation.

Urinary bile acids as biomarkers for liver diseases II. Signature profiles in patients

Hepatobiliary diseases result in the accumulation of bile acids (BAs) in the liver, systemic blood, and other tissues leading to an unfavorable prognosis. The BA profile was characterized by the calculation of indices that describe the composition, sulfation, and amidation of total and individual BAs. Comparison of the urinary BA profiles between healthy subjects and patients with hepatobiliary diseases demonstrated significantly higher absolute concentrations of individual and total BAs in patients. The percentage sulfation of some individual BAs were different between the two groups. The percentage amidation of overall and most individual BAs was higher in patients than controls. The percentage of primary BAs (CDCA and CA) was higher in patients, whereas the percentage of secondary BAs (DCA and LCA) was lower in patients. BA indices belonging to percentage amidation and percentage composition were better associated with the severity of the liver disease as determined by the model for end-stage liver disease (MELD) score and disease compensation status compared with the absolute concentrations of individual and total BAs. In addition, BA indices corresponding to percentage amidation and percentage composition of certain BAs demonstrated the highest area under the receiver operating characteristic (ROC) curve suggesting their utility as diagnostic biomarkers in clinic. Furthermore, significant increase in the risk of having liver diseases was associated with changes in BA indices.

Bile acid signaling through farnesoid X and TGR5 receptors in hepatobiliary and intestinal diseases

BACKGROUND

The well-known functions of bile acids (BAs) are the emulsification and absorption of lipophilic xenobiotics. However, the emerging evidences in the past decade showed that BAs act as signaling molecules that not only autoregulate their own metabolism and enterohepatic recirculation, but also as important regulators of integrative metabolism by activating nuclear and membrane-bound G protein-coupled receptors. The present review was to get insight into the role of maintenance of BA homeostasis and BA signaling pathways in development and management of hepatobiliary and intestinal diseases.

DATA SOURCES

Detailed and comprehensive search of PubMed and Scopus databases was carried out for original and review articles.

RESULTS

Disturbances in BA homeostasis contribute to the development of several hepatobiliary and intestinal disorders, such as non-alcoholic fatty liver disease, liver cirrhosis, cholesterol gallstone disease, intestinal diseases and both hepatocellular and colorectal carcinoma.

CONCLUSION

Further efforts made in order to advance the understanding of sophisticated BA signaling network may be promising in developing novel therapeutic strategies related not only to hepatobiliary and gastrointestinal but also systemic diseases.

Bile acid signaling and liver regeneration

The liver is able to regenerate itself in response to partial hepatectomy or liver injury. This is accomplished by a complex network of different cell types and signals both inside and outside the liver. Bile acids (BAs) are recently identified as liver-specific metabolic signals and promote liver regeneration by activating their receptors: Farnesoid X Receptor (FXR) and G-protein-coupled BA receptor 1 (GPBAR1, or TGR5). FXR is a member of the nuclear hormone receptor superfamily of ligand-activated transcription factors. FXR promotes liver regeneration after 70% partial hepatectomy (PHx) or liver injury. Moreover, activation of FXR is able to alleviate age-related liver regeneration defects. Both liver- and intestine-FXR are activated by BAs after liver resection or injury and promote liver regeneration through distinct mechanism. TGR5 is a membrane-bound BA receptor and it is also activated during liver regeneration. TGR5 regulates BA hydrophobicity and stimulates BA excretion in urine during liver regeneration. BA signaling thus represents a novel metabolic pathway during liver regeneration. This article is part of a Special Issue entitled: Nuclear receptors in animal development.

Individual bile acids have differential effects on bile acid signaling in mice

Bile acids (BAs) are known to regulate BA synthesis and transport by the farnesoid X receptor in the liver (FXR-SHP) and intestine (FXR-Fgf15). However, the relative importance of individual BAs in regulating these processes is not known. Therefore, mice were fed various doses of five individual BAs, including cholic acid (CA), chenodeoxycholic acid (CDCA), deoxoycholic acid (DCA), lithocholic acid (LCA), and ursodeoxycholic acid (UDCA) in their diets at various concentrations for one week to increase the concentration of one BA in the enterohepatic circulation. The mRNA of BA synthesis and transporting genes in liver and ileum were quantified. In the liver, the mRNA of SHP, which is the prototypical target gene of FXR, increased in mice fed all concentrations of BAs. In the ileum, the mRNA of the intestinal FXR target gene Fgf15 was increased at lower doses and to a higher extent by CA and DCA than by CDCA and LCA. Cyp7a1, the rate-limiting enzyme in BA synthesis, was decreased more by CA and DCA than CDCA and LCA. Cyp8b1, the enzyme that 12-hydroxylates BAs and is thus responsible for the synthesis of CA, was decreased much more by CA and DCA than CDCA and LCA. Surprisingly, neither a decrease in the conjugated BA uptake transporter (Ntcp) nor increase in BA efflux transporter (Bsep) was observed by FXR activation, but an increase in the cholesterol efflux transporter (Abcg5/Abcg8) was observed with FXR activation. Thus in conclusion, CA and DCA are more potent FXR activators than CDCA and LCA when fed to mice, and thus they are more effective in decreasing the expression of the rate limiting gene in BA synthesis Cyp7a1 and the 12-hydroxylation of BAs Cyp8b1, and are also more effective in increasing the expression of Abcg5/Abcg8, which is responsible for biliary cholesterol excretion. However, feeding BAs do not alter the mRNA or protein levels of Ntcp or Bsep, suggesting that the uptake or efflux of BAs is not regulated by FXR at physiological and pharmacological concentrations of BAs.

Targeting xenobiotic receptors PXR and CAR in human diseases

Nuclear receptors such as the pregnane X receptor (PXR) and constitutive androstane receptor (CAR) are xenobiotic receptors regulating not only drug metabolism and disposition but also various human diseases such as cancer, diabetes, inflammatory disease, metabolic disease and liver diseases, suggesting that PXR and CAR are promising targets for drug discovery. Consequently, there is an urgent need to discover and develop small molecules that target these PXR- and/or CAR-mediated human-disease-related pathways for relevant therapeutic applications. This review proposes approaches to target PXR and CAR, either individually or simultaneously, in the context of various human diseases, taking into consideration the structural differences between PXR and CAR.

Quantitative profiling of bile acids in blood, adipose tissue, intestine, and gall bladder samples using ultra high performance liquid chromatography-tandem mass spectrometry

An ultra high performance liquid chromatography tandem mass spectrometry method (UHPLC-MS/MS) was developed for the determination of 33 target and 28 unknown bile acids (BAs) in biological samples. Sixty-one BAs could be measured in 20 min using only a small amount of sample and with a simple sample preparation. The method proved to be very sensitive (limit of detection 5-350 pg/mL, lower limit of quantitation 0.1-2.6 ng/mL), linear (R(2) > 0.99) and reproducible (typically CV <15 % in biological matrixes). The method was used to analyze human adipose tissue, plasma, and serum (from same subjects) and mouse serum, gall bladder, small intestine, and colon samples (from same animals). Cholic acid, ursodeoxycholic acid, and chenodeoxycholic acid, deoxycholic acid, and their conjugates (mainly glycine, but also taurine conjugates) were the main metabolites in human samples, and cholic acid, glycine cholic acid, and several taurine conjugates in mouse samples. Using the method, 28 unknown BAs could also be detected. UHPLC-MS/MS spectra, accurate mass, and tissue distribution suggested that nine of the unknown bile acids were taurine conjugates, 13 were glycine conjugates, and six were intact BAs, respectively. To our knowledge, this was the first time BAs were detected in adipose tissue. Results showed that 17 targeted BAs were found at ng/g level in human adipose tissue. Our findings give a novel insight of the endogenous role of BAs in adipose tissue and their role as biomarkers (e.g., in metabolic diseases).

Adaptive hepatic and intestinal alterations in mice after deletion of NADPH-cytochrome P450 Oxidoreductase (Cpr) in hepatocytes

Cytochrome P450 enzymes (P450) play an important role in first-pass metabolism in both the intestine and liver. NADPH-cytochrome P450 oxidoreductase (Cpr) is an essential electron transfer protein required for microsomal P450 activity. Mice with conditional knockout of Cpr in hepatocytes develop normally and survive even with complete loss of liver microsomal P450 activity. Our current studies were performed to determine whether alternative drug-metabolizing pathways increase in an attempt to maintain whole-body homeostasis. In addition to the liver, Cpr is mainly expressed in tissues such as lung, kidney, and gastrointestinal tract. In livers of H-Cpr-null mice, there is a marked increase in mRNA expression of phase I enzymes (Aldh1a1, 1a7, 3a2; Ces1b2, 2a6, and 2a12), antioxidant enzymes (Ho-1, Nqo1, and epoxide hydrolase), phase II enzymes (Ugt1a9; Gsta1/2, m3, m4, m6, t1, and t3; and Sult1a1 and 1d1), and drug transporters (Oatp1a4, Oct3, Mate1, Mdr1a, and Mrp3 and 4). In addition, glucuronide-conjugated bilirubin concentrations are doubled in serum of H-Cpr-null mice. Both constitutive androstane receptor (CAR) and nuclear factor erythroid 2-related factor 2 (Nrf2) protein in nuclei are higher in the livers of H-Cpr-null mice, indicating that CAR and Nrf2 are activated. In the small intestine of H-Cpr-null mice, mRNA expression of Cyp3a11 and Mdr1a, two genes critical for intestinal first-pass metabolism, are markedly up-regulated. In addition, nutrient (Pept1) and cholesterol (Npc1l1) transporters are induced in the small intestine of H-Cpr-null mice. In conclusion, in H-Cpr-null mice, adaptive regulation of alternative detoxification genes in liver and small intestine appear to partially compensate for the loss of microsomal P450 function in liver.

GPBA: a GPCR for bile acids and an emerging therapeutic target for disorders of digestion and sensation

Bile acids (BAs) are digestive secretions that are necessary for the emulsification and absorption of dietary fats. Given the episodic nature of BA secretion and intestinal re-absorption, the circulating and tissue levels of BAs, like those of the gut hormones, fluctuate in fasting and fed states, and BA levels and forms are markedly affected by disease. BAs exert widespread hormonal-like effects by activating receptors in the nucleus and at the plasma membrane. The nuclear steroid receptors mediate the genomic actions of BAs on BA, glucose and lipid homeostasis. GPBA (TGR5) is a G-protein coupled plasma membrane receptor for BAs that mediates many of the rapid, non-genomic actions of BAs. GPBA has been implicated in the control of glucose homeostasis, inflammation and liver functions. Recent observations have revealed an unexpected role for GPBA in the nervous system. GPBA is expressed by enteric neurons and enterochromaffin cells that control peristalsis, and GPBA mediates the prokinetic actions of BAs in the colon that have been known for millennia. GPBA is also present on primary spinal afferent and spinal neurons that are necessary for sensory transduction. BA-induced activation of GPBA in the sensory nervous system promotes scratching behaviours and analgesia, which may contribute to the pruritus and painless jaundice that are observed in some patients with chronic cholestatic disease, where circulating BA concentrations are markedly increased. Thus, GPBA has emerged as an intriguing target for diverse metabolic, inflammatory, digestive and sensory disorders, where agonists and antagonists may be of value.

The role of CYP3A4 in the biotransformation of bile acids and therapeutic implication for cholestasis

CYP3A4 is a major cytochrome P450. It catalyses a broad range of substrates including xenobiotics such as clinically used drugs and endogenous compounds bile acids. Its function to detoxify bile acids could be used for treating cholestasis, which is a condition characterised by accumulation of bile acids. Although bile acids have important physiological functions, they are very toxic when their concentrations are excessively high. The accumulated bile acids in cholestasis can cause liver and other tissue injuries. Thus, control of the concentrations of bile acids is critical for treatment of cholestasis. CYP3A4 is responsively upregulated in cholestasis mediated by the nuclear receptors farnesol X receptor (FXR) and pregnane X receptor (PXR) as a defence mechanism. However, the regulation of CYP3A4 is complicated by estrogen, which is increased in cholestasis and down regulates CYP3A4 expression. The activity of CYP3A4 is also inhibited by accumulated bile acids due to their property of detergent effect. In some cholestasis cases, genetic polymorphisms of the CYP3A4 and PXR genes may interfere with the adaptive response. Further stimulation of CYP3A4 activity in cholestasis could be an effective approach for treatment of the disease. In this review, we summarise recent progress about the roles of CYP3A4 in the metabolism of bile acids, its regulation and possible implication in the treatment of cholestasis.

Combining ligand- and structure-based approaches for the discovery of new inhibitors of the EPHA2-ephrin-A1 interaction

The EPH receptor A2 (EPHA2) represents an attractive anticancer target. With the aim to identify novel EPHA2 receptor antagonists, a virtual screening campaign, combining shape-similarity and docking calculations, was conducted on a set of commercially available compounds. A combined score, taking into account both ligand- and structure-based results, was then used to identify the most promising candidates. Two compounds, selected among the best-ranked ones, were identified as EPHA2 receptor antagonists with micromolar affinity.

Differential regulation of EGFR-MAPK signaling by deoxycholic acid (DCA) and ursodeoxycholic acid (UDCA) in colon cancer

A high-fat diet coincides with increased levels of bile acids. This increase in bile acids, particularly deoxycholic acid (DCA), has been strongly associated with the development of colon cancer. Conversely, ursodeoxycholic acid (UDCA) may have chemopreventive properties. Although structurally similar, DCA and UDCA present different biological and pathological effects in colon cancer progression. The differential regulation of cancer by these two bile acids is not yet fully understood. However, one possible explanation for their diverging effects is their ability to differentially regulate signaling pathways involved in the multistep progression of colon cancer, such as the epidermal growth factor receptor (EGFR)-mitogen-activated protein kinase (MAPK) pathway. This review will examine the biological effects of DCA and UDCA on colon cancer development, as well as the diverging effects of these bile acids on the oncogenic signaling pathways that play a role in colon cancer development, with a particular emphasis on bile acid regulation of the EGFR-MAPK pathway.

Decreased bile-acid synthesis in livers of hepatocyte-conditional NADPH-cytochrome P450 reductase-null mice results in increased bile acids in serum

NADPH-cytochrome P450 reductase (Cpr) is essential for the function of microsomal cytochrome P450 monooxygenases (P450), including those P450s involved in bile acid (BA) synthesis. Mice with hepatocyte-specific deletion of NADPH-cytochrome P450 reductase (H-Cpr-null) have been engineered to understand the in vivo function of hepatic P450s in the metabolism of xenobiotics and endogenous compounds. However, the impact of hepatic Cpr on BA homeostasis is not clear. The present study revealed that H-Cpr-null mice had a 60% decrease in total BA concentration in liver, whereas the total BA concentration in serum was almost doubled. The decreased level of cholic acid (CA) in both serum and livers of H-Cpr-null mice is likely due to diminished enzyme activity of Cyp8b1 that is essential for CA biosynthesis. Feedback mechanisms responsible for the reduced liver BA concentrations and/or increased serum BA concentrations in H-Cpr-null mice included the following: 1) enhanced alternative BA synthesis pathway, as evidenced by the fact that classic BA synthesis is diminished but chenodeoxycholic acid still increases in both serum and livers of H-Cpr-null mice; 2) inhibition of farnesoid X receptor activation, which increased the mRNA of Cyp7a1 and 8b1; 3) induction of intestinal BA transporters to facilitate BA absorption from the intestine to the circulation; 4) induction of hepatic multidrug resistance-associated protein transporters to increase BA efflux from the liver to blood; and 5) increased generation of secondary BAs. In summary, the present study reveals an important contribution of the alternative BA synthesis pathway and BA transporters in regulating BA concentrations in H-Cpr-null mice.

Bile acid signaling in metabolic disease and drug therapy

Bile acids are the end products of cholesterol catabolism. Hepatic bile acid synthesis accounts for a major fraction of daily cholesterol turnover in humans. Biliary secretion of bile acids generates bile flow and facilitates hepatobiliary secretion of lipids, lipophilic metabolites, and xenobiotics. In the intestine, bile acids are essential for the absorption, transport, and metabolism of dietary fats and lipid-soluble vitamins. Extensive research in the last 2 decades has unveiled new functions of bile acids as signaling molecules and metabolic integrators. The bile acid-activated nuclear receptors farnesoid X receptor, pregnane X receptor, constitutive androstane receptor, vitamin D receptor, and G protein-coupled bile acid receptor play critical roles in the regulation of lipid, glucose, and energy metabolism, inflammation, and drug metabolism and detoxification. Bile acid synthesis exhibits a strong diurnal rhythm, which is entrained by fasting and refeeding as well as nutrient status and plays an important role for maintaining metabolic homeostasis. Recent research revealed an interaction of liver bile acids and gut microbiota in the regulation of liver metabolism. Circadian disturbance and altered gut microbiota contribute to the pathogenesis of liver diseases, inflammatory bowel diseases, nonalcoholic fatty liver disease, diabetes, and obesity. Bile acids and their derivatives are potential therapeutic agents for treating metabolic diseases of the liver.

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.

Bile acid sequestrants in type 2 diabetes: potential effects on GLP1 secretion

Bile acid sequestrants have been used for decades for the treatment of hypercholesterolaemia. Sequestering of bile acids in the intestinal lumen interrupts enterohepatic recirculation of bile acids, which initiate feedback mechanisms on the conversion of cholesterol into bile acids in the liver, thereby lowering cholesterol concentrations in the circulation. In the early 1990s, it was observed that bile acid sequestrants improved glycaemic control in patients with type 2 diabetes. Subsequently, several studies confirmed the finding and recently - despite elusive mechanisms of action - bile acid sequestrants have been approved in the USA for the treatment of type 2 diabetes. Nowadays, bile acids are no longer labelled as simple detergents necessary for lipid digestion and absorption, but are increasingly recognised as metabolic regulators. They are potent hormones, work as signalling molecules on nuclear receptors and G protein-coupled receptors and trigger a myriad of signalling pathways in many target organs. The most described and well-known receptors activated by bile acids are the farnesoid X receptor (nuclear receptor) and the G protein-coupled cell membrane receptor TGR5. Besides controlling bile acid metabolism, these receptors are implicated in lipid, glucose and energy metabolism. Interestingly, activation of TGR5 on enteroendocrine L cells has been suggested to affect secretion of incretin hormones, particularly glucagon-like peptide 1 (GLP1 (GCG)). This review discusses the role of bile acid sequestrants in the treatment of type 2 diabetes, the possible mechanism of action and the role of bile acid-induced secretion of GLP1 via activation of TGR5.

Biology of PXR: role in drug-hormone interactions

Hormonal homeostasis is essential for a variety of physiological and pathological processes. Elimination and detoxification of xenobiotics, such as drugs introduced into the human body, could disrupt the balance of hormones due to the induction of drug metabolizing enzymes (DMEs) and transporters. Pregnane X receptor (PXR, NR1I2) functions as a master xenobiotic receptor involved in drug metabolism and drug-drug interactions by its coordinated transcriptional regulation of phase I and phase II DMEs and transporters. Recently, increasing evidences indicate that PXR can also mediate the endocrine disruptor function and thus impact the integrity of the endocrine system. This review focuses primarily on the recent advances in our understanding of the function of PXR in glucocorticoid, mineralocorticoid, androgen and estrogen homeostasis. The elucidation of PXR-mediated drug-hormone interactions might have important therapeutic implications in dealing with hormone-dependent diseases and safety assessment of drugs.

Clinical application of transcriptional activators of bile salt transporters

Hepatobiliary bile salt (BS) transporters are critical determinants of BS homeostasis controlling intracellular concentrations of BSs and their enterohepatic circulation. Genetic or acquired dysfunction of specific transport systems causes intrahepatic and systemic retention of potentially cytotoxic BSs, which, in high concentrations, may disturb integrity of cell membranes and subcellular organelles resulting in cell death, inflammation and fibrosis. Transcriptional regulation of canalicular BS efflux through bile salt export pump (BSEP), basolateral elimination through organic solute transporters alpha and beta (OSTα/OSTβ) as well as inhibition of hepatocellular BS uptake through basolateral Na(+)-taurocholate cotransporting polypeptide (NTCP) represent critical steps in protection from hepatocellular BS overload and can be targeted therapeutically. In this article, we review the potential clinical implications of the major BS transporters BSEP, OSTα/OSTβ and NTCP in the pathogenesis of hereditary and acquired cholestatic syndromes, provide an overview on transcriptional control of these transporters by the key regulatory nuclear receptors and discuss the potential therapeutic role of novel transcriptional activators of BS transporters in cholestasis.

Roles of xenobiotic receptors in vascular pathophysiology

The pregnane X receptor (PXR) and constitutive androstane receptor (CAR), 2 closely related and liver-enriched members of the nuclear receptor superfamily, and aryl hydrocarbon receptor (AhR), a nonnuclear receptor transcription factor (TF), are major receptors/TFs regulating the expression of genes for the clearance and detoxification of xenobiotics. They are hence defined as "xenobiotic receptors". Recent studies have demonstrated that PXR, CAR and AhR also regulate the expression of key proteins involved in endobiotic responses such as the metabolic homeostasis of lipids, glucose, and bile acid, and inflammatory processes. It is suggested that the functions of PXR, CAR and AhR may be closely implicated in the pathogeneses of metabolic vascular diseases, such as hyperlipidemia, atherogenesis, and hypertension. Therefore, manipulation of the activities of these receptors may provide novel strategies for the treatment of vascular diseases. Here, we review the pathophysiological roles of PXR, CAR and AhR in the vascular system.

Secondary bile acids: an underrecognized cause of colon cancer

Bile acids were first proposed as carcinogens in 1939. Since then, accumulated evidence has linked exposure of cells of the gastrointestinal tract to repeated high physiologic levels of bile acids as an important risk factor for gastrointestinal cancers. High exposure to bile acids may occur in a number of settings, but most importantly, is prevalent among individuals who have a high dietary fat intake. A rapid effect on cells of high bile acid exposure is the generation of reactive oxygen species and reactive nitrogen species, disruption of the cell membrane and mitochondria, induction of DNA damage, mutation and apoptosis, and development of reduced apoptosis capability upon chronic exposure. Here, we review the substantial evidence of the mechanism of secondary bile acids and their role in colon cancer.

Expression of CYP3A in chronic ethanol-fed mice is mediated by endogenous pregnane X receptor ligands formed by enhanced cholesterol metabolism

Pregnane X receptor (PXR) is a nuclear receptor that plays a key regulatory role in xenobiotic metabolism in a ligand-dependent manner. Recently, ethanol was reported to be either an inducer or inhibitor of Cytochrome P450 (CYP) 3A expression. According to our recent microarray data, chronic ethanol upregulates the expression of the genes associated with oxidative phase I drug metabolism, phase II conjugation reaction and phase III xenobiotic transport, most of which are known to be regulated by PXR. In this study, we investigated the effects of chronic ethanol on the expression and activity of CYP3A11 in mice and the role of PXR. Ethanol was administrated to male ICR mice by feeding a standard Lieber-DeCarli diet containing 36 % ethanol for 4 weeks. Ethanol significantly increased hepatic mRNA expression of Pxr and Cyp3a11. Treatment of mice with ethanol increased nuclear translocation of PXR. Consistent with the increase in nuclear PXR, ethanol significantly increased the binding of PXR to the Cyp3a11 promoter. Hepatic cholesterol level and bile acid synthesis are increased by ethanol treatment. The level of some cholesterol metabolites, such as 5β-cholestane-3α,7α,12α-triol, 7α-hydroxy-4-cholestene-3-one and lithocholic acid, that have been identified as potent PXR agonists are increased in the livers of ethanol-treated mice. In summary, chronic ethanol upregulates the expression of Pxr and Cyp3a11 mRNAs and proteins in mice by PXR activation mediated by enhanced cholesterol metabolism and bile acid synthesis. Our data provide some critical information needed to understand the molecular mechanisms of ethanol-induced CYP3A expression.

Analysis of flavonoids regulating the expression of UGT1A1 via xenobiotic receptors in intestinal epithelial cells

UDP-glucuronosyltransferase (UGT) 1A1 is one of the major metabolic enzymes for the detoxification of harmful xenobiotics in intestines, and its expression is regulated by transcription factors like the aryl hydrocarbon receptor (AhR) and the pregnane X receptor (PXR). A screening assay using real-time PCR showed that baicalein and 3-hydroxyflavone induced human UGT1A1 mRNA expression in LS180 cells. Experimental results confirmed that these flavonoids increased UGT1A protein expression as well as its enzymatic activity. The results indicated that baicalein and 3-hydroxyflavone increased the transcriptional activity of UGT1A1 via AhR and PXR, respectively. Observation via immunofluorescence microscopy suggested that baicalein and 3-hydroxyflavone further induced nuclear translocation of AhR and PXR, respectively. In addition, direct interaction between baicalein and AhR or 3-hydroxyflavone and PXR were observed using the quartz crystal microbalance method. These results elucidate the molecular mechanism of flavonoid-induced UGT1A1 gene expression via xenobiotic receptors in the intestines.

5β-Reduced steroids and human Δ(4)-3-ketosteroid 5β-reductase (AKR1D1)

5β-Reduced steroids are non-planar steroids that have a 90° bend in their structure to create an A/B cis-ring junction. This novel property is required for bile-acids to act as emulsifiers, but in addition 5β-reduced steroids have remarkable physiology and may act as potent tocolytic agents, endogenous cardiac glycosides, neurosteroids, and can act as ligands for orphan and membrane bound receptors. In humans there is only a single 5β-reductase gene AKR1D1, which encodes Δ(4)-3-ketosteroid-5β-reductase (AKR1D1). This enzyme is a member of the aldo-keto reductase superfamily, but possesses an altered catalytic tetrad, in which Glu120 replaces the conserved His residue. This predominant liver enzyme generates all 5β-dihydrosteroids in the C19-C27 steroid series. Mutations exist in the AKR1D1 gene, which result in loss of protein stability and are causative in bile-acid deficiency.

Low dose of oleanolic acid protects against lithocholic acid-induced cholestasis in mice: potential involvement of nuclear factor-E2-related factor 2-mediated upregulation of multidrug resistance-associated proteins

Oleanolic acid (OA) is a natural triterpenoid and has been demonstrated to protect against varieties of hepatotoxicants. Recently, however, OA at high doses was reported to produce apparent cholestasis in mice. In this study, we characterized the protective effect of OA at low doses against lithocholic acid (LCA)-induced cholestasis in mice and explored further mechanisms. OA cotreatment (5, 10, and 20 mg/kg, i.p.) significantly improved mouse survival rate, attenuated liver necrosis, and decreased serum alanine aminotransferase, aspartate aminotransferase, and alkaline phosphatase; more importantly, serum total bile acids and bilirubin, as well as hepatic total bile acids were also remarkably reduced. Gene and protein expression analysis showed that hepatic expression of multidrug resistance-associated protein 2 (Mrp2), Mrp3, and Mrp4 was significantly increased by OA cotreatment, whereas other bile acid metabolism- and transport-related genes, including Na+/taurocholate cotransporter, organic anion transporter 1b2, bile salt export pump, multidrug resistance protein 3, Cyp3a11, Cyp2b10, Sulfotransferase 2a1 (Sult2a1), and UDP-glucuronosyltransferase 1a1 (Ugt1a1), were only slightly changed. OA also caused increased nuclear factor-E2-related factor (Nrf2) mRNA expression and nuclear protein accumulation, whereas nuclear receptors farnesoid X receptor (FXR), pregnane X receptor (PXR), and constitutive androstane receptor were not significantly influenced by OA. Luciferase (Luc) assays performed in HepG2 cells illustrated that OA was a strong Nrf2 agonist with moderate PXR and weak FXR agonism. Finally, in mouse primary cultured hepatocytes, OA dose- and time-dependently induced expression of Mrp2, Mrp3, and Mrp4; however, this upregulation was abrogated when Nrf2 was silenced. In conclusion, OA produces a protective effect against LCA-induced hepatotoxicity and cholestasis, possibly due to Nrf2-mediated upregulation of Mrp2, Mrp3, and Mrp4.

Exercise training decreases gene expression of endo- and xeno-sensors in rat small intestine

The purpose of the study was to test the hypothesis that gene expression of members of the nuclear receptor (NR) superfamily known to act as endo- and xeno-sensors is reduced in the ileum of exercise-trained (Tr) rats. Healthy female rats were either treadmill-trained for 8 weeks, 5 times/week, or remained sedentary (Sed). Training resulted in a significant (p < 0.05) decrease in plasma free fatty acid (0.18 ± 0.01 to 0.15 ± 0.01 mmol/L) and glycerol (24.8 ± 0.8 to 18.7 ± 0.8 mg/L) concentrations. Gene expressions of NRs farnesoid X receptor (FXR; p < 0.05), liver X receptor (LXR; p < 0.05), pregnane X receptor (PXR; p < 0.01), and retinoid X receptor (RXR; p < 0.06) were reduced in the ileum of Tr compared with Sed animals. Tr was also associated with a reduction (p < 0.05) in gene expression of FXR downstream heterodimeric organite solute transporters α (OSTα) and β (OSTβ) involved in the transport of bile acids, LXR downstream genes heterodimeric ATP-binding cassette transporters (ABCG5/G8) involved in transport of absorbed cholesterol back to the lumen, and Niemann-Pick C1-like 1 (NPC1L1) involved in cholesterol absorption. These data indicate that exercise training lowers the expression of molecules involved in the defense system of the ileum against endobiotic and xenobiotic insults under normal conditions, thus, suggesting that regular exercise contributes to the intestinal maintenance of cholesterol and bile acid homeostasis.

The role of canalicular ABC transporters in cholestasis

Cholestasis, a hallmark feature of hepatobiliary disease, is characterized by the retention of biliary constituents. Some of these constituents, such as bile acids, inflict damage to hepatocytes and bile duct cells. This damage may lead to inflammation, fibrosis, cirrhosis, and eventually carcinogenesis, sequelae that aggravate the underlying disease and deteriorate clinical outcome. Canalicular ATP-binding cassette (ABC) transporters, which mediate the excretion of individual bile constituents, play a key role in bile formation and cholestasis. The study of these transporters and their regulatory nuclear receptors has revolutionized our understanding of cholestatic disease. This knowledge has served as a template to develop novel treatment strategies, some of which are currently already undergoing phase III clinical trials. In this review we aim to provide an overview of the structure, function, and regulation of canalicular ABC transporters. In addition, we will focus on the role of these transporters in the pathogenesis and treatment of cholestatic bile duct and liver diseases.

Tissue-specific regulation of pregnane X receptor in cancer development and therapy

As a ligand-dependent transcription factor of the nuclear hormone receptor superfamily, the pregnane X receptor (PXR) has a multitude of functions including regulating xenobiotic and cholesterol metabolism, energy homeostasis, gut mucosal defense, and cancer development. Whereas the detoxification functions of PXR have been widely studied and well established, the role of PXR in cancer has become controversial. With more than 60% of non-prescription and prescription drugs being metabolized by cytochrome P450 enzyme 3A4 (CYP3A4), a transcriptional target of PXR, insights into the regulation of PXR during systemic administration of novel treatment modalities will lead to a better understanding of PXR function in the context of human disease. Previous studies have suggested that PXR activation decreases drug sensitivity and augments chemoresistance in certain colon cancers mainly through the upregulation of CYP3A4 and multidrug resistance protein-1 (MDR1). Later studies suggest that downregulation of PXR expression may be oncogenic in hormone-dependent breast and endometrial cancers by reducing estrogen metabolism via CYP3A4; thus, higher estradiol concentrations contribute to carcinogenesis. These results suggest a differential role of PXR in tumor growth regulation dependent on tissue type and tumor microenvironment. Here, we will summarize the various mechanisms utilized by PXR to induce its diverse effects on cancerous tissues. Moreover, current approaches will be explored to evaluate the exploitation of PXR-mediated pathways as a novel mechanistic approach to cancer therapy.

Drug-induced perturbations of the bile acid pool, cholestasis, and hepatotoxicity: mechanistic considerations beyond the direct inhibition of the bile salt export pump

The bile salt export pump (BSEP) is located on the canalicular plasma membrane of hepatocytes and plays an important role in the biliary clearance of bile acids (BAs). Therefore, any drug or new chemical entity that inhibits BSEP has the potential to cause cholestasis and possibly liver injury. In reality, however, one must consider the complexity of the BA pool, BA enterohepatic recirculation (EHR), extrahepatic (renal) BA clearance, and the interplay of multiple participant transporters and enzymes (e.g., sulfotransferase 2A1, multidrug resistance-associated protein 2, 3, and 4). Moreover, BAs undergo extensive enzyme-catalyzed amidation and are subjected to metabolism by enterobacteria during EHR. Expression of the various enzymes and transporters described above is governed by nuclear hormone receptors (NHRs) that mount an adaptive response when intracellular levels of BAs are increased. The intracellular trafficking of transporters, and their ability to mediate the vectorial transport of BAs, is governed by specific kinases also. Finally, bile flow, micelle formation, canalicular membrane integrity, and BA clearance can be influenced by the inhibition of multidrug resistant protein 3- or ATPase-aminophospholipid transporter-mediated phospholipid flux. Consequently, when screening compounds in a discovery setting or conducting mechanistic studies to address clinical findings, one has to consider the direct (inhibitory) effect of the parent drug and metabolites on multiple BA transporters, as well as inhibition of BA sulfation and amidation and NHR function. Vectorial BA transport, in addition to BA EHR and homoeostasis, could also be impacted by drug-dependent modulation of kinases and enterobacteria.

The Influence of an Obesogenic Diet on Oxysterol Metabolism in C57BL/6J Mice

Our current understanding of oxysterol metabolism during different disease states such as obesity and dyslipidemia is limited. Therefore, the aim of this study was to determine the effect of diet-induced obesity on the tissue distribution of various oxysterols and the mRNA expression of key enzymes involved in oxysterol metabolism. To induce obesity, male C57BL/6J mice were fed a high fat-cholesterol diet for 24 weeks. Following diet-induced obesity, plasma levels of 4β-hydroxycholesterol, 5,6α-epoxycholesterol, 5,6β-epoxycholesterol, 7α-hydroxycholesterol, 7β-hydroxycholesterol, and 27-hydroxycholesterol were significantly (P < 0.05) increased. In the liver and adipose tissue of the obese mice, 4β-hydroxycholesterol was significantly (P < 0.05) increased, whereas 27-hydroxycholesterol was increased only in the adipose tissue. No significant changes in either hepatic or adipose tissue mRNA expression were observed for oxysterol synthesizing enzymes 4β-hydroxylase, 27-hydroxylase, or 7α-hydroxylase. Hepatic mRNA expression of SULT2B1b, a key enzyme involved in oxysterol detoxification, was significantly (P < 0.05) elevated in the obese mice. Interestingly, the appearance of the large HDL₁ lipoprotein was observed with increased oxysterol synthesis during obesity. In diet-induced obese mice, dietary intake and endogenous enzymatic synthesis of oxysterols could not account for the increased oxysterol levels, suggesting that nonenzymatic cholesterol oxidation pathways may be responsible for the changes in oxysterol metabolism.

Targeting nuclear receptors with marine natural products

Nuclear receptors (NRs) are important pharmaceutical targets because they are key regulators of many metabolic and inflammatory diseases, including diabetes, dyslipidemia, cirrhosis, and fibrosis. As ligands play a pivotal role in modulating nuclear receptor activity, the discovery of novel ligands for nuclear receptors represents an interesting and promising therapeutic approach. The search for novel NR agonists and antagonists with enhanced selectivities prompted the exploration of the extraordinary chemical diversity associated with natural products. Recent studies involving nuclear receptors have disclosed a number of natural products as nuclear receptor ligands, serving to re-emphasize the translational possibilities of natural products in drug discovery. In this review, the natural ligands of nuclear receptors will be described with an emphasis on their mechanisms of action and their therapeutic potentials, as well as on strategies to determine potential marine natural products as nuclear receptor modulators.

Total gastrectomy may result in reduced drug effectiveness due to an increase in the expression of the drug-metabolizing enzyme Cytochrome P450, in the liver

In patients with gastrectomy, it is possible that drug effectiveness is reduced compared to healthy subjects due to the increased of the drug-metabolizing enzyme, Cytochrome P450 (CYP). The purpose of this study is to verify this possibility. Gastrectomy model mice were prepared to evaluate the expression level of various CYPs in the liver from 2 to 24 weeks post-operation. No significant differences were observed in the protein expression levels of CYP3A, CYP1A, CYP2C, and CYP2D between the sham operation group and the gastrectomy group up to 4 weeks after the gastrectomy. On the other hand, significant increases in the protein expression levels of any CYPs were observed in the gastrectomy group compared to the sham operation group from 12 weeks after the gastrectomy onward. These increases in expression levels were maintained until 24 weeks after the gastrectomy. The examination of metabolic activity in the liver in the gastrectomy group using triazolam revealed that the metabolic activity at 12 weeks after the gastrectomy was significantly increased in the gastrectomy group. The administration of the anticancer drug imatinib, which is a substrate of CYP3A, to mice at 12weeks after gastrectomy resulted in an increase in the metabolic rate, suggesting a possible decrease in drug effectiveness. It has been revealed that drug effectiveness may be reduced after gastrectomy because the expression levels of various CYPs in the liver were increased over a prolonged period. The results of this study can serve as valuable fundamental knowledge for drug therapy in patients with gastrectomy.

Hepatic bile acid metabolism and expression of cytochrome P450 and related enzymes are altered in Bsep (-/-) mice

The bile salt export pump (BSEP/Bsep; gene symbol ABCB11/Abcb11) translocates bile salts across the hepatocyte canalicular membrane into bile in humans and mice. In humans, mutations in the ABCB11 gene cause a severe childhood liver disease known as progressive familial intrahepatic cholestasis type 2. Targeted inactivation of mouse Bsep produces milder persistent cholestasis due to detoxification of bile acids through hydroxylation and alternative transport pathways. The purpose of the present study was to determine whether functional expression of hepatic cytochrome P450 (CYP) and microsomal epoxide hydrolase (mEH) is altered by Bsep inactivation in mice and whether bile acids regulate CYP and mEH expression in Bsep (-/-) mice. CYP expression was determined by measuring protein levels of Cyp2b, Cyp2c and Cyp3a enzymes and CYP-mediated activities including lithocholic acid hydroxylation, testosterone hydroxylation and alkoxyresorufin O-dealkylation in hepatic microsomes prepared from female and male Bsep (-/-) mice fed a normal or cholic acid (CA)-enriched diet. The results indicated that hepatic lithocholic acid hydroxylation was catalyzed by Cyp3a/Cyp3a11 enzymes in Bsep (-/-) mice and that 3-ketocholanoic acid and murideoxycholic acid were major metabolites. CA feeding of Bsep (-/-) mice increased hepatic Cyp3a11 protein levels and Cyp3a11-mediated testosterone 2β-, 6β-, and 15β-hydroxylation activities, increased Cyp2b10 protein levels and Cyp2b10-mediated benzyloxyresorufin O-debenzylation activity, and elevated Cyp2c29 and mEH protein levels. We propose that bile acids upregulate expression of hepatic Cyp3a11, Cyp2b10, Cyp2c29 and mEH in Bsep (-/-) mice and that Cyp3a11 and multidrug resistance-1 P-glycoproteins (Mdr1a/1b) are vital components of two distinct pathways utilized by mouse hepatocytes to expel bile acids.

Impact of new-generation lipid emulsions on cellular mechanisms of parenteral nutrition-associated liver disease

Parenteral nutrition (PN) is a life-saving nutritional support for a large population of hospitalized infants, and lipids make a substantial contribution to their energy and essential fatty acid (FA) needs. A challenge in the care of these infants is that their metabolic needs require prolonged PN support that increases the risk of PN-associated liver disease (PNALD). In recent years, the emergence of new parenteral lipid emulsions containing different source lipids and FA profiles has created nutritional alternatives to the first-generation, soybean oil-based lipid emulsion Intralipid. The limited U.S. introduction of the new-generation fish-oil emulsion Omegaven has generated promising results in infants with PNALD and spawned a renewed interest in how PN and lipid emulsions, in particular, contribute to this disease. Studies suggest that the lipid load and constituents, such as specific FAs, ratio of n-3 (ω-3) to n-6 (ω-6) long-chain polyunsaturated FAs, phytosterols, and vitamin E content, may be involved. There is an existing literature describing the molecular mechanisms whereby these specific nutrients affect hepatic metabolism and function via lipid and bile acid sensing nuclear receptors, such as peroxisome proliferator-activated receptor α, liver X receptor, and farnesoid X receptor, yet virtually no information as to how they interact and modulate liver function in the context of PN in pediatric patients or animal models. This article will review the recent development of parenteral lipid emulsions and their influence on PNALD and highlight some of the emerging molecular mechanisms that may explain the effects on liver function and disease.