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

Expression of steroid and xenobiotic receptor in uterine carcinosarcoma, leiomyosarcoma and endometrial stromal sarcoma

We analyzed the expression of the steroid and xenobiotic receptor (SXR) in human uterine sarcomas and evaluated its clinical significance. Forty-seven cases with archival specimens were examined for SXR expression using immunohistochemistry. All cases were scored using a semi-quantitative histological scoring (HSCORE) method. Specimens with a HSCORE >40 were regarded as SXR-positive. Various clinicopathological variables, including the expression status of estrogen receptor (ER)-α, progesterone receptor (PR) and Ki67 (MIB-1) were examined. The mean SXR HSCOREs of carcinosarcoma (CS) and leiomyosarcoma (LMS) were 9.13 and 23.6, respectively, and SXR-positive rates were 3 out of 24 (12.5%) and 4 out of 17 (23.5%), respectively. SXR was not detected in endometrial stromal sarcoma (ESS). In CS cases, significant differences were detected between the expression of SXR and age and disease stages. There was no significant correlation between SXR-positive status and either disease-free survival or overall survival. Our results support an association between SXR and malignant behavior. Our results show that overexpression of SXR may represent a useful marker to identify patients with advanced-stage CS. In addition, our results showed that SXR may aid in the diagnosis of uterine sarcomas.

TGF-β-SMAD3 signaling mediates hepatic bile acid and phospholipid metabolism following lithocholic acid-induced liver injury

Transforming growth factor-β (TGFβ) is activated as a result of liver injury, such as cholestasis. However, its influence on endogenous metabolism is not known. This study demonstrated that TGFβ regulates hepatic phospholipid and bile acid homeostasis through MAD homolog 3 (SMAD3) activation as revealed by lithocholic acid-induced experimental intrahepatic cholestasis. Lithocholic acid (LCA) induced expression of TGFB1 and the receptors TGFBR1 and TGFBR2 in the liver. In addition, immunohistochemistry revealed higher TGFβ expression around the portal vein after LCA exposure and diminished SMAD3 phosphorylation in hepatocytes from Smad3-null mice. Serum metabolomics indicated increased bile acids and decreased lysophosphatidylcholine (LPC) after LCA exposure. Interestingly, in Smad3-null mice, the metabolic alteration was attenuated. LCA-induced lysophosphatidylcholine acyltransferase 4 (LPCAT4) and organic solute transporter β (OSTβ) expression were markedly decreased in Smad3-null mice, whereas TGFβ induced LPCAT4 and OSTβ expression in primary mouse hepatocytes. In addition, introduction of SMAD3 enhanced the TGFβ-induced LPCAT4 and OSTβ expression in the human hepatocellular carcinoma cell line HepG2. In conclusion, considering that Smad3-null mice showed attenuated serum ALP activity, a diagnostic indicator of cholangiocyte injury, these results strongly support the view that TGFβ-SMAD3 signaling mediates an alteration in phospholipid and bile acid metabolism following hepatic inflammation with the biliary injury.

Xenobiotic receptor humanized mice and their utility

The nuclear receptors pregnane X receptor, constitutive androstane receptor, and peroxisome proliferator-activated receptor alpha have important endogenous functions and are also involved in the induction of drug-metabolizing enzymes and transporters in response to exogenous xenobiotics. Though not belonging to the same protein family, the Per-Sim-ARNT domain receptor aryl hydrocarbon receptor functionally overlaps with the three nuclear receptors in many aspects and is therefore included in this review. Significant species differences in ligand affinity and biological responses as a result of activation of these receptors have been described. Several xenobiotic receptor humanized mice have been created to overcome these species differences and to provide in vivo models that are more predictive for human responses. This review provides an overview of the different xenobiotic receptor humanized mouse models described to date and will summarize how these models can be applied in basic research and improve drug discovery and development. Some of the key applications in the evaluation of drug induction, drug-drug interactions, nongenotoxic carcinogenicity, other toxicity, or efficacy studies are described. We also discuss relevant considerations in the interpretation of such data and potential future directions for the use of xenobiotic receptor humanized mice.

Sulforaphane is not an effective antagonist of the human pregnane X-receptor in vivo

Sulforaphane (SFN), is an effective in vitro antagonist of ligand activation of the human pregnane and xenobiotic receptor (PXR). PXR mediated CYP3A4 up-regulation is implicated in adverse drug-drug interactions making identification of small molecule antagonists a desirable therapeutic goal. SFN is not an antagonist to mouse or rat PXR in vitro; thus, normal rodent species are not suitable as in vivo models for human response. To evaluate whether SFN can effectively antagonize ligand activation of human PXR in vivo, a three-armed, randomized, crossover trial was conducted with 24 healthy adults. The potent PXR ligand - rifampicin (300mg/d) was given alone for 7days in arm 1, or in daily combination with 450μmol SFN (Broccoli Sprout extract) in arm 2; SFN was given alone in arm 3. Midazolam as an in vivo phenotype marker of CYP3A was administered before and after each treatment arm. Rifampicin alone decreased midazolam AUC by 70%, indicative of the expected increase in CYP3A4 activity. Co-treatment with SFN did not reduce CYP3A4 induction. Treatment with SFN alone also did not affect CYP3A4 activity in the cohort as a whole, although in the subset with the highest basal CYP3A4 activity there was a statistically significant increase in midazolam AUC (i.e., decrease in CYP3A4 activity). A parallel study in humanized PXR mice yielded similar results. The parallel effects of SFN between humanized PXR mice and human subjects demonstrate the predictive value of humanized mouse models in situations where species differences in ligand-receptor interactions preclude the use of a native mouse model for studying human ligand-receptor pharmacology.

Patented TGR5 modulators: a review (2006 - present)

INTRODUCTION

The G protein-coupled receptor TGR5 is a key player of the bile acid signaling network, and its activation has been proved to increase the glycemic control, to enhance energy expenditure and to exert anti-inflammatory actions. Accordingly, TGR5 ligands have emerged in drug discovery and preclinical appraisals as promising agents for the treatment of liver diseases, metabolic syndrome and related disorders.

AREAS COVERED

Recent advances in the field of TGR5 modulators are reviewed, with a particular attention on patent applications and peer-reviewed publications in the past 6 years.

EXPERT OPINION

Activation of TGR5 showed to protect mice from diabesity and insulin resistance, to improve liver functions, as well as to attenuate the development of atherosclerosis. However, although the efficacy of TGR5 agonists in mice is encouraging, further studies are needed to determine their potential in humans and to evaluate carefully the balance between the therapeutic benefits and adverse effects associated with the target. The development of new TGR5 selective ligands to support studies in animal models will surely facilitate the understanding of the complexity of TGR5 signaling network.

Novel insight into mechanisms of cholestatic liver injury

Cholestasis results in a buildup of bile acids in serum and in hepatocytes. Early studies into the mechanisms of cholestatic liver injury strongly implicated bile acid-induced apoptosis as the major cause of hepatocellular injury. Recent work has focused both on the role of bile acids in cell signaling as well as the role of sterile inflammation in the pathophysiology. Advances in modern analytical methodology have allowed for more accurate measuring of bile acid concentrations in serum, liver, and bile to very low levels of detection. Interestingly, toxic bile acid levels are seemingly far lower than previously hypothesized. The initial hypothesis has been based largely upon the exposure of μmol/L concentrations of toxic bile acids and bile salts to primary hepatocytes in cell culture, the possibility that in vivo bile acid concentrations may be far lower than the observed in vitro toxicity has far reaching implications in the mechanism of injury. This review will focus on both how different bile acids and different bile acid concentrations can affect hepatocytes during cholestasis, and additionally provide insight into how these data support recent hypotheses that cholestatic liver injury may not occur through direct bile acid-induced apoptosis, but may involve largely inflammatory cell-mediated liver cell necrosis.

The role of peroxisome proliferator-activated receptors in colorectal cancer

Colorectal cancer is one of the most common cancers in the world. Dietary fat intake is a major risk factor for colorectal cancer. Some nuclear hormone receptors play an important role in regulating nutrient metabolism and energy homeostasis. Among these receptors, special attention has been focused on the role of peroxisome proliferator-activated receptors (PPARs) in colorectal cancer, because PPARs are involved in regulation of lipid and carbohydrate metabolism. PPARs are ligand-activated intracellular transcription factors. The PPAR subfamily consists of three subtypes encoded by distinct genes named PPARα, PPARβ/δ, and PPARγ. PPARγ is the most extensively studied subtype of PPARs. Even though many investigators have studied the expression and clinical implications of PPARs in colorectal cancer, there are still many controversies about the role of PPARs in colorectal cancer. In this paper, the recent progresses in understanding the role of PPARs in colorectal cancer are summarized.

Targeting the pregnane X receptor in liver injury

INTRODUCTION

The nuclear receptor pregnane X receptor (PXR) is a well-characterized hepatic xenobiotic sensor whose activation by chemically diverse compounds results in the induction of drug clearance pathways that rid the body of potentially toxic substances, thus conferring protection from foreign chemicals and endobiotics.

AREAS COVERED

PXR activities are implicated in drug-drug interactions and endocrine disruption. Recent evidence supports a hepatoprotective role for PXR in chronic liver injury, inhibiting liver inflammation through suppression of the NF-κB pathway. However, PXR-mediated induction of CYP3A enhances APAP-induced acute liver injury by generating toxic metabolites. While these observations implicate PXR as a therapeutic target for liver injury, they also caution against PXR activation by pharmaceutical drugs.

EXPERT OPINION

While evidence of PXR involvement in acute and chronic liver injuries identifies it as a possible therapeutic target, it raises additional concerns for all drug candidates. The in vitro and in vivo tests for human PXR activation should be incorporated into the FDA regulations for therapeutic drug approval to identify potential liver toxicities. In addition, PXR pharmacogenetic studies will facilitate the prediction of patient-specific drug reactivities and associated liver disorders.

Ligand dependent hepatic gene expression profiles of nuclear receptors CAR and PXR

Constitutive androstane receptor (CAR) and pregnane X receptor (PXR) are key regulators of drug-metabolizing enzymes and transporters. These two receptors are closely associated with each other and also have overlapping functions. This study investigated the overall hepatic gene expression profiles and the regulatory roles of these nuclear receptors using CAR/PXR single and double knockout mice. Basal and ligand-stimulated gene expression profiles were obtained in each mouse using cDNA microarrays and a reverse transcriptase-polymerase chain reaction. Enzymes such as Cyp2b10, Cyp3a11, Cdc20 and Cdk1 displayed both CAR- and PXR-dependent induction. Inversely, enzymes such as Cyp4a10, Fos and Mme displayed both CAR- and PXR-dependent repression. Enzymes such as Cyp1a1, Cyp1a2 and c-Myc represented the group of genes only induced by CAR. Enzymes such as Aacs represented the group of genes induced only by the PXR. CAR and PXR are closely associated and have diverse roles, both as positive and negative regulators of hepatic genes including xenobiotic metabolism, apoptosis, cholesterol biosynthesis, lipid metabolism, and cytokine signaling pathways.

Alleviation of gut inflammation by Cdx2/Pxr pathway in a mouse model of chemical colitis

Pregnane X Receptor (PXR), a master regulator of drug metabolism and inflammation, is abundantly expressed in the gastrointestinal tract. Baicalein and its O-glucuronide baicalin are potent anti-inflammatory and anti-cancer herbal flavonoids that undergo a complex cycle of interconversion in the liver and gut. We sought to investigate the role these flavonoids play in inhibiting gut inflammation by an axis involving PXR and other potential factors. The consequences of PXR regulation and activation by the herbal flavonoids, baicalein and baicalin were evaluated in vitro in human colon carcinoma cells and in vivo using wild-type, Pxr-null, and humanized (hPXR) PXR mice. Baicalein, but not its glucuronidated metabolite baicalin, activates PXR in a Cdx2-dependent manner in vitro, in human colon carcinoma LS174T cells, and in the murine colon in vivo. While both flavonoids abrogate dextran sodium sulfate (DSS)-mediated colon inflammation in vivo, oral delivery of a potent bacterial β-glucuronidase inhibitor eliminates baicalin’s effect on gastrointestinal inflammation by preventing the microbial conversion of baicalin to baicalien. Finally, reduction of gastrointestinal inflammation requires the binding of Cdx2 to a specific proximal site on the PXR promoter. Pharmacological targeting of intestinal PXR using natural metabolically labile ligands could serve as effective and potent therapeutics for gut inflammation that avert systemic drug interactions.

Daphnia HR96 is a promiscuous xenobiotic and endobiotic nuclear receptor

Daphnia pulex is the first crustacean to have its genome sequenced. The genome project provides new insight and data into how an aquatic crustacean may respond to environmental stressors, including toxicants. We cloned Daphnia pulex HR96 (DappuHR96), a nuclear receptor orthologous to the CAR/PXR/VDR group of nuclear receptors. In Drosophila melanogaster, (hormone receptor 96) HR96 responds to phenobarbital exposure and has been hypothesized as a toxicant receptor. Therefore, we set up a transactivation assay to test whether DappuHR96 is a promiscuous receptor activated by xenobiotics and endobiotics similar to the constitutive androstane receptor (CAR) and the pregnane X-receptor (PXR). Transactivation assays performed with a GAL4-HR96 chimera demonstrate that HR96 is a promiscuous toxicant receptor activated by a diverse set of chemicals such as pesticides, hormones, and fatty acids. Several environmental toxicants activate HR96 including estradiol, pyriproxyfen, chlorpyrifos, atrazine, and methane arsonate. We also observed repression of HR96 activity by chemicals such as triclosan, androstanol, and fluoxetine. Nearly 50% of the chemicals tested activated or inhibited HR96. Interestingly, unsaturated fatty acids were common activators or inhibitors of HR96 activity, indicating a link between diet and toxicant response. The omega-6 and omega-9 unsaturated fatty acids linoleic and oleic acid activated HR96, but the omega-3 unsaturated fatty acids alpha-linolenic acid and docosahexaenoic acid inhibited HR96, suggesting that these two distinct sets of lipids perform opposing roles in Daphnia physiology. This also provides a putative mechanism by which the ratio of dietary unsaturated fats may affect the ability of an organism to respond to a toxic insult. In summary, HR96 is a promiscuous nuclear receptor activated by numerous endo- and xenobiotics.

Dietary modification of metabolic pathways via nuclear hormone receptors

Nuclear hormone receptors (NHRs), as ligand-dependent transcription factors, have emerged as important mediators in the control of whole body metabolism. Because of the promiscuous nature of several members of this superfamily that have been found to bind ligand with lower affinity than the classical steroid NHRs, they consequently display a broader ligand selectivity. This promiscuous nature has facilitated various bioactive dietary components being able to act as agonist ligands for certain members of the NHR superfamily. By binding to these NHRs, bioactive dietary components are able to mediate changes in various metabolic pathways, including, glucose, cholesterol and triglyceride homeostasis among others. This review will provide a general overview of the nuclear hormone receptors that have been shown to be activated by dietary components. The physiological consequences of such receptor activation by these dietary components will then be discussed in more detail.

FXR and PXR: potential therapeutic targets in cholestasis

Cholestatic liver disorders encompass hepatobiliary diseases of diverse etiologies characterized by the accumulation of bile acids, bilirubin and cholesterol as the result of impaired secretion of bile. Members of the nuclear receptor (NR) family of ligand-modulated transcription factors are implicated in the adaptive response to cholestasis. NRs coordinately regulate bile acid and phospholipid transporter genes required for hepatobiliary transport, as well as the phases I and II metabolizing enzymes involved in processing of their substrates. In this review we will focus on FXR and PXR, two members of the NR family whose activities are regulated by bile acids. In addition, we also discuss the potential of pharmacological modulators of these receptors as novel therapies for cholestatic disorders.

Coordinated regulation of hepatic phase I and II drug-metabolizing genes and transporters using AhR-, CAR-, PXR-, PPARα-, and Nrf2-null mice

The transcription factors aryl hydrocarbon receptor (AhR), constitutive androstane receptor (CAR), pregnane X receptor (PXR), peroxisome proliferator-activated receptor α (PPARα), and nuclear factor erythroid 2-related factor 2 (Nrf2) regulate genes encoding drug-metabolizing enzymes and transporters in livers of mice after chemical activation. However, the specificity of their transcriptional regulation has not been determined systematically in vivo. The purpose of this study was to identify genes encoding drug-metabolizing enzymes and transporters altered by chemical activators in a transcription factor-dependent manner using wild-type and transcription factor-null mice. Chemical activators were administered intraperitoneally to mice once daily for 4 days. Livers were collected 24 h after the final dose, and total RNA was isolated for mRNA quantification of cytochromes P450, NAD(P)H quinone oxidoreductase 1 (Nqo1), aldehyde dehydrogenases (Aldhs), glutathione transferases (Gsts), sulfotransferases (Sults), UDP-glucuronosyltransferases (Ugts), organic anion-transporting polypeptides (Oatps), and multidrug resistance-associated proteins (Mrps). Pharmacological activation of each transcription factor leads to mRNA induction of drug metabolic and transport genes in livers of male and female wild-type mice, but no change in null mice: AhR (Cyp1a2, Nqo1, Aldh7a1, Ugt1a1, Ugt1a6, Ugt1a9, Ugt2b35, Sult5a1, Gstm3, and Mrp4), CAR (Cyp2b10, Aldh1a1, Aldh1a7, Ugt1a1, Ugt2b34, Sult1e1, Sult3a1, Sult5a1, Papps2, Gstt1, Gsta1, Gsta4, Gstm1-4, and Mrp2-4), PXR (Cyp3a11, Ugt1a1, Ugt1a5, Ugt1a9, Gsta1, Gstm1-m3, Oatp1a4, and Mrp3), PPARα (Cyp4a14, Aldh1a1, mGst3, Gstm4, and Mrp4), and Nrf2 (Nqo1, Aldh1a1, Gsta1, Gsta4, Gstm1-m4, mGst3, and Mrp3-4). Taken together, these data reveal transcription factor specificity and overlap in regulating hepatic drug disposition genes by chemical activators. Coordinated regulation of phase I, phase II, and transport genes by activators of transcription factors can have implications in development of pharmaceuticals as well as risk assessment of environmental contaminants.

Intestinal synthesis and secretion of bile salts as an adaptation to developmental biliary atresia in the sea lamprey

Bile salt synthesis is a specialized liver function in vertebrates. Bile salts play diverse roles in digestion and signaling, and their homeostasis is maintained by controlling input (biosynthesis) and intestinal conservation. Patients with biliary atresia (i.e., obliteration of the biliary tree) suffer liver fibrosis and cirrhosis. In contrast, sea lamprey thrives despite developmental biliary atresia. We discovered that the sea lamprey adapts to biliary atresia through a unique mechanism of de novo synthesis and secretion of bile salts in intestine after developmental biliary atresia, in addition to known mechanisms, such as the reduction of bile salt synthesis in liver. During and after developmental biliary atresia, expression of cyp7a1 in intestine increased by more than 100-fold (P < 0.001), whereas in liver it decreased by the same magnitude (P < 0.001). Concurrently, bile salt pools changed in similar patterns and magnitudes in these two organs and the composition shifted from C24 bile alcohol sulfates to taurine-conjugated C24 bile acids. In addition, both in vivo and ex vivo experiments showed that aductular sea lamprey secreted taurocholic acid into its intestinal lumen. Our results indicate that the sea lamprey, a jawless vertebrate, may be in an evolutionarily transitional state where bile salt synthesis occurs in both liver and intestine. Understanding the molecular basis of these mechanisms may shed light on the evolution of bile salt synthesis and possible therapy for infant biliary atresia.

The interplay between retinoic acid receptor-related orphan receptors and human diseases

The retinoic acid receptor-related orphan receptors (RORs) are an important subfamily of transcriptional regulators of the nuclear receptors superfamily. Their discovery over a decade ago by gene cloning strategy have revealed three major isoforms of these orphan receptors in animals. Generation and analyses of isoform-specific ROR null mice have provided revealed-vital roles for the RORs in animals. The RORs undoubtedly participate in a host of biological functions such a metabolism, immunity, development and differentiation, angiogenesis, circadian clock, xenobiotic/drug metabolism and other tissue physiologies for optimal animal survival. Moreover, intense work in the last one decade also revealed a host of human diseases being modulated by the RORs. A number of diseases, such as cancer, autoimmune diseases, inflammation, osteoporosis, metabolic syndrome etc., strongly support the involvement of RORs in their onset and progression. By involving in such diseases, the RORs are indeed a critical factor for optimal cell function and are being intensely investigated as novel targets for drug interventions in the treatment of various diseases. This review focuses on the current knowledge and status about RORs in a number of human disease conditions.

Transcription factor-mediated regulation of carboxylesterase enzymes in livers of mice

The induction of drug-metabolizing enzymes by chemicals is one of the major reasons for drug-drug interactions. In the present study, the regulation of mRNA expression of one arylacetamide deacetylase (Aadac) and 11 carboxylesterases (Cess) by 15 microsomal enzyme inducers (MEIs) was examined in livers of male C57BL/6 mice. The data demonstrated that Aadac mRNA expression was suppressed by three aryl hydrocarbon receptor (AhR) ligands, two constitutive androstane receptor (CAR) activators, two pregnane X receptor (PXR) ligands, and one nuclear factor erythroid 2-related factor 2 (Nrf2) activator. Ces1 subfamily mRNA expression was not altered by most of the MEIs, whereas Ces2 subfamily mRNA was readily induced by the activators of CAR, PXR, and Nrf2 but not by peroxisome proliferator-activated receptor α activators. Studies using null mice demonstrated that 1) AhR was required for the 2,3,7,8-tetrachlorodibenzo-p-dioxin-mediated suppression of Aadac and Ces3a; 2) CAR was involved in the 1,4-bis[2-(3,5-dichloropyridyloxy)]benzene-mediated induction of Aadac, Ces2c, Ces2a, and Ces3a; 3) PXR was required for the pregnenolone-16α-carbonitrile-mediated induction of Aadac, Ces2c, and Ces2a; 4) Nrf2 was required for the oltipraz-mediated induction of Ces1g and Ces2c; and 5) PXR was not required for the DEX-mediated suppression of Cess in livers of mice. In conclusion, the present study systematically investigated the regulation of Cess by MEIs in livers of mice and demonstrated that MEIs modulated mRNA expression of mouse hepatic Cess through the activation of AhR, CAR, PXR, and/or Nrf2 transcriptional pathways.

Nuclear factor-E2-related factor 2 is a major determinant of bile acid homeostasis in the liver and intestine

The transcription factor nuclear factor-E2-related factor 2 (Nrf2) is a key regulator for induction of hepatic detoxification and antioxidant mechanisms, as well as for certain hepatobiliary transporters. To examine the role of Nrf2 in bile acid homeostasis and cholestasis, we assessed the determinants of bile secretion and bile acid synthesis and transport before and after bile duct ligation (BDL) in Nrf2(-/-) mice. Our findings indicate reduced rates of biliary bile acid and GSH excretion, higher levels of intrahepatic bile acids, and decreased expression of regulators of bile acid synthesis, Cyp7a1 and Cyp8b1, in Nrf2(-/-) compared with wild-type control mice. The mRNA expression of the bile acid transporters bile salt export pump (Bsep) and organic solute transporter (Ostα) were increased in the face of impaired expression of the multidrug resistance-associated proteins Mrp3 and Mrp4. Deletion of Nrf2 also decreased ileal apical sodium-dependent bile acid transporter (Asbt) expression, leading to reduced bile acid reabsorption and increased loss of bile acid in feces. Finally, when cholestasis is induced by BDL, liver injury was not different from that in wild-type BDL mice. These Nrf2(-/-) mice also had increased pregnane X receptor (Pxr) and Cyp3a11 mRNA expression in association with enhanced hepatic bile acid hydroxylation. In conclusion, this study finds that Nrf2 plays a major role in the regulation of bile acid homeostasis in the liver and intestine. Deletion of Nrf2 results in a cholestatic phenotype but does not augment liver injury following BDL.

Nuclear receptors in the multidrug resistance through the regulation of drug-metabolizing enzymes and drug transporters

Chemotherapy is one of the three most common treatment modalities for cancer. However, its efficacy is limited by multidrug resistant cancer cells. Drug metabolizing enzymes (DMEs) and efflux transporters promote the metabolism, elimination, and detoxification of chemotherapeutic agents. Consequently, elevated levels of DMEs and efflux transporters reduce the therapeutic effectiveness of chemotherapeutics and, often, lead to treatment failure. Nuclear receptors, especially pregnane X receptor (PXR, NR1I2) and constitutive androstane activated receptor (CAR, NR1I3), are increasingly recognized for their role in xenobiotic metabolism and clearance as well as their role in the development of multidrug resistance (MDR) during chemotherapy. Promiscuous xenobiotic receptors, including PXR and CAR, govern the inducible expressions of a broad spectrum of target genes that encode phase I DMEs, phase II DMEs, and efflux transporters. Recent studies conducted by a number of groups, including ours, have revealed that PXR and CAR play pivotal roles in the development of MDR in various human carcinomas, including prostate, colon, ovarian, and esophageal squamous cell carcinomas. Accordingly, PXR/CAR expression levels and/or activation statuses may predict prognosis and identify the risk of drug resistance in patients subjected to chemotherapy. Further, PXR/CAR antagonists, when used in combination with existing chemotherapeutics that activate PXR/CAR, are feasible and promising options that could be utilized to overcome or, at least, attenuate MDR in cancer cells.

Opposing regulation of cytochrome P450 expression by CAR and PXR in hypothyroid mice

Clinical hypothyroidism affects various metabolic processes including drug metabolism. CYP2B and CYP3A are important cytochrome P450 drug metabolizing enzymes that are regulated by the xenobiotic receptors constitutive androstane receptor (CAR, NR1I3) and pregnane X receptor (PXR, NR1I2). We evaluated the regulation of the hepatic expression of CYPs by CAR and PXR in the hypothyroid state induced by a low-iodine diet containing 0.15% propylthiouracil. Expression of Cyp3a11 was suppressed in hypothyroid C57BL/6 wild type (WT) mice and a further decrement was observed in hypothyroid CAR-/- mice, but not in hypothyroid PXR-/- mice. In contrast, expression of Cyp2b10 was induced in both WT and PXR-/- hypothyroid mice, and this induction was abolished in CAR-/- mice and in and CAR-/- PXR-/- double knockouts. CAR mRNA expression was increased by hypothyroidism, while PXR expression remained unchanged. Carbamazepine (CBZ) is a commonly used antiepileptic that is metabolized by CYP3A isoforms. After CBZ treatment of normal chow fed mice, serum CBZ levels were highest in CAR-/- mice and lowest in WT and PXR-/- mice. Hypothyroid WT or PXR-/- mice survived chronic CBZ treatment, but all hypothyroid CAR-/- and CAR-/- PXR-/- mice died, with CAR-/-PXR-/- mice surviving longer than CAR-/- mice (12.3±3.3 days vs. 6.3±2.1 days, p=0.04). All these findings suggest that hypothyroid status affects xenobiotic metabolism, with opposing responses of CAR and PXR and their CYP targets that can cancel each other out, decreasing serious metabolic derangement in response to a xenobiotic challenge.

An insulin-like growth factor 1 receptor inhibitor induces CYP3A4 expression through a pregnane X receptor-independent, noncanonical constitutive androstane receptor-related mechanism

Inhibition of insulin-like growth factor-1 receptor (IGF-1R) signaling represents an attractive therapeutic strategy for cancer treatment. A first-generation IGF-1R inhibitor (R)-4-(3-(3-chlorophenyl)-3-hydroxypropyl)-3-(4-methyl-6-morpholino-1H-benzo[d]imidazol-2-yl)pyridin-2(1H)-one (BMS-536924), however, was associated with potent CYP3A4 induction mediated by pregnane X receptor (PXR; NR1I2) transactivation. Structural activity-based modification led to the synthesis of 4-(1-(2-(4-((2-(4-chloro-1H-pyrazol-1-yl)ethyl)amino)-2-oxo-1,2-dihydropyridin-3-yl)-4-methyl-1H-benzo[d]imidazol-6-yl)piperidin-4-yl) piperazine-1-carboxylate (BMS-665351) with no PXR activity while maintaining its ability to inhibit IGF-1R. However, BMS-665351 significantly induces CYP3A4 expression in human primary hepatocytes (HPHs). Here, we report a novel nonclassical constitutive androstane receptor (CAR; NR1I3)-related pathway of BMS-665351-mediated CYP3A4 induction. BMS-665351 treatment resulted in the significant induction of CYP3A4 in HPHs and HepG2 cells, but failed to activate either PXR or CAR in cell-based reporter assays. Moreover, BMS-665351 at concentrations that induce CYP3A4 expression was unable to translocate human CAR from the cytoplasm to the nucleus of HPHs, which represents the initial step of CAR activation. Nevertheless, quantitative polymerase chain reaction analysis demonstrated that BMS-665351 significantly enhanced the expression of CYP3A4 in CAR- but not PXR-transfected HepG2 and Huh7 cells. It is noteworthy that BMS-665351 selectively induced the expression of CAR but not PXR in all tested hepatic cell systems. Synergistic induction of CYP3A4 was observed in HPHs cotreated with BMS-665351 and prototypical activators of CAR but not PXR. In summary, our results indicate that BMS-665351-mediated induction of CYP3A4 is CAR-dependent, but BMS-665351 itself is not a typical activator of either CAR or PXR, rather it functions as a selective inducer of CAR expression and increases CYP3A4 through a noncanonical CAR-related mechanism.

Suppression of efflux transporters in the intestines of endotoxin-treated rats

Infection and inflammation suppress the expression and activity of several drug transporters in the liver. In the intestine, P-glycoprotein (PGP/mdr1) and the multidrug resistance-associated protein 2 (MRP2) are important barriers to the absorption of many clinically important drugs. The protein expression and activity of these transporters were examined during inflammation induced by lipopolysaccharide (LPS). The transport of rhodamine123 (Rho123) and 5-carboxyfluorescein (5-CF) was determined in isolated ileal segments from endotoxin-treated or control rats in the presence or absence of inhibitors. The reverse transcription-polymerase chain reaction was used to measure mRNA levels. Compared with the controls, the mRNA levels of mdr1a and mrp2 were significantly decreased by approximately 50% in the ilea of the LPS-treated rats. Corresponding reductions in the basolateral-apical efflux of Rho123 and 5-CF were observed, resulting in significant increases in the apical-basolateral absorption of these compounds. Neither the permeability of fluorescein isothiocyanate labeled dextran 4000 (FD-4), a paracellular marker, nor membrane resistance was altered. These results indicate that endotoxin-induced inflammation reduces the intestinal expression and activity of PGP and MRP2 in rats, which eliciting corresponding changes in the intestinal transport of their substrates. Hence, infection and inflammatory diseases may induce variability in drug bioavailability through alterations in the intestinal expression and activity of drug transporters.

Bile acid sequestrants: more than simple resins

PURPOSE OF REVIEW

Bile acid sequestrants (BAS) have been used for more than 50 years in the treatment of hypercholesterolemia. The last decade, bile acids are emerging as integrated regulators of metabolism via induction of various signal transduction pathways. Consequently, BAS treatment may exert unexpected side-effects. We discuss a selection of recently published studies that evaluated BAS in several metabolic diseases.

RECENT FINDINGS

Recently, an increasing body of evidence has shown that BAS in addition to ameliorating hypercholesterolemia are also effective in improving glycemic control in patients with type 2 diabetes, although the mechanism is not completely understood. Furthermore, some reports suggested using these compounds to modulate energy expenditure. Many of these effects have been related to the local effects of BAS in the intestine by directly binding bile acids in the intestine or indirectly by interfering with signaling processes.

SUMMARY

A substantial effort is being made by researchers to fully define the mechanism by which BAS improve glycemic control in type 2 diabetic patients. A new challenge will be to confirm in clinical trials the recent discoveries coming from animal experiments suggesting a role for bile acids in energy metabolism.

Reduced expression of UGT1A1 in intestines of humanized UGT1 mice via inactivation of NF-κB leads to hyperbilirubinemia

BACKGROUND & AIMS

Bilirubin is a natural and potent antioxidant that accumulates in the blood of newborn children and leads to physiological jaundice. Breastfed infants have higher serum levels of bilirubin than formula-fed infants and are at risk for bilirubin-induced neurological dysfunction (BIND). Clearance of bilirubin requires the expression of uridine diphosphate glucuronosyltransferase (UGT) 1A1; we investigated its role in the association between breast feeding with jaundice in mice.

METHODS

We studied mice in which the original Ugt1 locus was disrupted and replaced with the human UGT1 locus (hUGT1 mice); these mice spontaneously develop neonatal hyperbilirubinemia and BIND. We fed human breast milk or formula to neonatal hUGT1 mice and examined activation of the intestinal xenobiotic receptors pregnane X receptor and constitutive androstane receptor. We also examined inflammatory signaling pathways in mice with disruptions in IκB-kinase-α and IκB kinase-β in the intestinal epithelium.

RESULTS

hUGT1 mice that were fed breast milk developed severe hyperbilirubinemia because of suppression of UGT1A1 in the gastrointestinal tract. Formula-fed hUGT1 mice had lower serum levels of bilirubin, which resulted from induction of UGT1A1 in the gastrointestinal tract. hUGT1/Pxr-null mice did not develop severe hyperbilirubinemia, whereas hUGT1/Car-null mice were susceptible to BIND when they were fed breast milk. Breast milk appeared to suppress intestinal IκB kinase α and β, resulting in inactivation of nuclear factor-κB and loss of expression of UGT1A1, leading to hyperbilirubinemia.

CONCLUSIONS

Breast milk reduces expression of intestinal UGT1A1, which leads to hyperbilirubinemia and BIND; suppression of this gene appears to involve inactivation of nuclear factor-κB. Hyperbilirubinemia can be reduced by activation of pregnane X receptor, constitutive androstane receptor, or nuclear factor-κB.

Cytochrome P450s in the synthesis of cholesterol and bile acids--from mouse models to human diseases

The present review describes the transgenic mouse models that have been designed to evaluate the functions of the cytochrome P450s involved in cholesterol and bile acid synthesis, as well as their link with disease. The knockout of cholesterogenic Cyp51 is embrionally lethal, with symptoms of Antley-Bixler syndrome occurring in mice, whereas the evidence for this association is conflicting in humans. Disruption of Cyp7a1 from classic bile acid synthesis in mice leads to either increased postnatal death or a milder phenotype with elevated serum cholesterol. The latter is similar to the case in humans, where CYP7A1 mutations associate with high plasma low-density lipoprotein and hepatic cholesterol content, as well as deficient bile acid excretion. Disruption of Cyp8b1 from an alternative bile acid pathway results in the absence of cholic acid and a reduced absorption of dietary lipids; however, the human CYP8B1 polymorphism fails to explain differences in bile acid composition. Unexpectedly, apparently normal Cyp27a1(-/-) mice still synthesize bile acids that originate from the compensatory pathway. In humans, CYP27A1 mutations cause cerebrotendinous xanthomatosis, suggesting that only mice can compensate for the loss of alternative bile acid synthesis. In line with this, Cyp7b1 knockouts are also apparently normal, whereas human CYP7B1 mutations lead to a congenital bile acid synthesis defect in children or spastic paraplegia in adults. Mouse knockouts of the brain-specific Cyp46a1 have reduced brain cholesterol excretion, whereas, in humans, CYP46A1 polymorphisms associate with cognitive impairment. At present, cytochrome P450 family 39 is poorly characterized. Despite important physiological differences between humans and mice, mouse models prove to be an invaluable tool for understanding the multifactorial facets of cholesterol and bile acid-related disorders.

Pregnane X receptor regulates drug metabolism and transport in the vasculature and protects from oxidative stress

AIMS

Circulating endogenous, dietary, and foreign chemicals can contribute to vascular dysfunction. The mechanism by which the vasculature protects itself from these chemicals is unknown. This study investigates whether the pregnane X receptor (PXR), the major transcriptional regulator of hepatic drug metabolism and transport that responds to such xenobiotics, mediates vascular protection by co-ordinating a defence gene programme in the vasculature.

METHODS AND RESULTS

PXR was detected in primary human and rat aortic endothelial and smooth muscle cells (SMC) and blood vessels including the human and rat aorta. Metabolic PXR target genes cytochrome P450 3A, 2B, 2C, and glutathione S-transferase mRNA and activity were induced by PXR ligands in rodent and human vascular cells and absent in the aortas from PXR-null mice stimulated in vivo or in rat aortic SMC expressing dominant-negative PXR. Activation of aortic PXR by classical agonists had several protective effects: increased xenobiotic metabolism demonstrated by bioactivation of the pro-drug clopidogrel, which reduced adenosine diphosphate-induced platelet aggregation; increased expression of multidrug resistance protein 1, mediating chemical efflux from the vasculature; and protection from reactive oxygen species-mediated cell death.

CONCLUSION

PXR co-ordinately up-regulates drug metabolism, transport, and antioxidant genes to protect the vasculature from endogenous and exogenous insults, thus representing a novel gatekeeper for vascular defence.

Pregnane X receptor- and CYP3A4-humanized mouse models and their applications

Pregnane X receptor (PXR) is a pivotal nuclear receptor modulating xenobiotic metabolism primarily through its regulation of CYP3A4, the most important enzyme involved in drug metabolism in humans. Due to the marked species differences in ligand recognition by PXR, PXR-humanized (hPXR) mice, and mice expressing human PXR and CYP3A4 (Tg3A4/hPXR) were established. hPXR and Tg3A4/hPXR mice are valuable models for investigating the role of PXR in xenobiotic metabolism and toxicity, in lipid, bile acid and steroid hormone homeostasis, and in the control of inflammation.

Sterol regulation of metabolism, homeostasis, and development

Sterol metabolites are critical signaling molecules that regulate metabolism, development, and homeostasis. Oxysterols, bile acids (BAs), and steroids work primarily through cognate sterol-responsive nuclear hormone receptors to control these processes through feed-forward and feedback mechanisms. These signaling pathways are conserved from simple invertebrates to mammals. Indeed, results from various model organisms have yielded fundamental insights into cholesterol and BA homeostasis, lipid and glucose metabolism, protective mechanisms, tissue differentiation, development, reproduction, and even aging. Here, we review how sterols act through evolutionarily ancient mechanisms to control these processes.

Expression, localization and polymorphisms of the nuclear receptor PXR in Barrett's esophagus and esophageal adenocarcinoma

BACKGROUND

The continuous exposure of esophageal epithelium to refluxate may induce ectopic expression of bile-responsive genes and contribute to the development of Barrett's esophagus (BE) and esophageal adenocarcinoma. In normal physiology of the gut and liver, the nuclear receptor Pregnane × Receptor (PXR) is an important factor in the detoxification of xenobiotics and bile acid homeostasis. This study aimed to investigate the expression and genetic variation of PXR in reflux esophagitis (RE), Barrett's esophagus (BE) and esophageal adenocarcinoma.

METHODS

PXR mRNA levels and protein expression were determined in biopsies from patients with adenocarcinoma, BE, or RE, and healthy controls. Esophageal cell lines were stimulated with lithocholic acid and rifampicin. PXR polymorphisms 25385C/T, 7635A/G, and 8055C/T were genotyped in 249 BE patients, 233 RE patients, and 201 controls matched for age and gender.

RESULTS

PXR mRNA levels were significantly higher in adenocarcinoma tissue and columnar Barrett's epithelium, compared to squamous epithelium of these BE patients (P<0.001), and RE patients (P=0.003). Immunohistochemical staining of PXR showed predominantly cytoplasmic expression in BE tissue, whereas nuclear expression was found in adenocarcinoma tissue. In cell lines, stimulation with lithocholic acid did not increase PXR mRNA levels, but did induce nuclear translocation of PXR protein. Genotyping of the PXR 7635A/G polymorphism revealed that the G allele was significantly more prevalent in BE than in RE or controls (P=0.037).

CONCLUSIONS

PXR expresses in BE and adenocarcinoma tissue, and showed nuclear localization in adenocarcinoma tissue. Upon stimulation with lithocholic acid, PXR translocates to the nuclei of OE19 adenocarcinoma cells. Together with the observed association of a PXR polymorphism and BE, this data implies that PXR may have a function in prediction and treatment of esophageal disease.

Influence of gender, feeding regimen, and exposure duration on gene expression associated with xenobiotic metabolism in fathead minnows (Pimephales promelas)

Little is known about how gender and diet affect the metabolism of environmental contaminants in fish. This study was undertaken to compare these differences in hepatic mRNA expression of two genes associated with xenobiotic metabolism: the pregnane x-receptor (PXR) and cytochrome P450 3A4 (CYP3A4). PXR is an ideal receptor to study xenobiotic metabolism as it binds to a vast array of anthropogenic compounds and induces transcription of genes involved in metabolizing xenobiotic compounds, such as CYP3A4. In this study FHM were separated by gender and feeding regimen (fed versus unfed), and injected with control (corn oil), clotrimazole, or pregnene-16α-olone, and expression monitored after 2- and 5-days of treatment. Our research indicates that there are gender- and diet-specific differences in the expression of PXR and CYP3A4, the most striking of which was a significant increase in the expression of PXR and CYP3A4 in unfed females as compared to all other groups. Understanding the relationship among xenobiotic metabolism, gender and diet would be of particular interest in the study of contaminated habitats, where fish may have to adapt to reductions in food quantity or quality while being exposed to toxic compounds.

Regulation of drug-metabolizing cytochrome P450 enzymes by glucocorticoids

The regulation of drug-metabolizing cytochrome P450 enzymes (CYP) is a complex process involving multiple mechanisms. Among them, transcriptional regulation through ligand-activated nuclear receptors is the crucial mechanism involved in hormone-controlled and xenobiotic-induced expression of drug-metabolizing CYPs. In this article, we focus, in detail, on the role of the glucocorticoid receptor (GR) in the transcriptional regulation of human drug-metabolizing CYP enzymes and the mechanisms of the regulation. There are at least three distinct transcriptional mechanisms by which GR controls the expression of CYPs: 1) direct binding of GR to a specific gene-promoter sequence called the glucocorticoid responsive element (GRE); 2) indirect binding of GR in the form of a multiprotein complex to gene promoters without a direct contact between GR and promoter DNA; and 3) up- or downregulation of other CYP transcriptional regulators or nuclear receptors (i.e., transcriptional regulatory cross-talk). However, due to the general effect of glucocorticoids on numerous cellular pathways and functions, the net transcriptional effect of glucocorticoids on drug-metabolizing enzymes is usually a combination of several mechanisms. Since synthetic glucocorticoids are widely prescribed in human pharmacotherapy for the treatment of many diseases, comprehensive understanding of the transcriptional regulation of drug-metabolizing CYPs via GR with respect to glucocorticoid therapy or glucocorticoid hormonal status will aid in the development of efficient individualized pharmacotherapy without drug-drug interactions.

Farnesoid X receptor represses hepatic human APOA gene expression

High plasma concentrations of lipoprotein(a) [Lp(a), which is encoded by the APOA gene] increase an individual's risk of developing diseases, such as coronary artery diseases, restenosis, and stroke. Unfortunately, increased Lp(a) levels are minimally influenced by dietary changes or drug treatment. Further, the development of Lp(a)-specific medications has been hampered by limited knowledge of Lp(a) metabolism. In this study, we identified patients suffering from biliary obstructions with very low plasma Lp(a) concentrations that rise substantially after surgical intervention. Consistent with this, common bile duct ligation in mice transgenic for human APOA (tg-APOA mice) lowered plasma concentrations and hepatic expression of APOA. To test whether farnesoid X receptor (FXR), which is activated by bile acids, was responsible for the low plasma Lp(a) levels in cholestatic patients and mice, we treated tg-APOA and tg-APOA/Fxr-/- mice with cholic acid. FXR activation markedly reduced plasma concentrations and hepatic expression of human APOA in tg-APOA mice but not in tg-APOA/Fxr-/- mice. Incubation of primary hepatocytes from tg-APOA mice with bile acids dose dependently downregulated APOA expression. Further analysis determined that the direct repeat 1 element between nucleotides -826 and -814 of the APOA promoter functioned as a negative FXR response element. This motif is also bound by hepatocyte nuclear factor 4α (HNF4α), which promotes APOA transcription, and FXR was shown to compete with HNF4α for binding to this motif. These findings may have important implications in the development of Lp(a)-lowering medications.

Role of nuclear receptor SHP in metabolism and cancer

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

Nuclear receptor PXR, transcriptional circuits and metabolic relevance

The pregnane X receptor (PXR, NR1I2) is a ligand activated transcription factor that belongs to the nuclear hormone receptor (NR) superfamily. PXR is highly expressed in the liver and intestine, but low levels of expression have also been found in many other tissues. PXR plays an integral role in xenobiotic and endobiotic metabolism by regulating the expression of drug-metabolizing enzymes and transporters, as well as genes implicated in the metabolism of endobiotics. PXR exerts its transcriptional regulation by binding to its DNA response elements as a heterodimer with the retinoid X receptor (RXR) and recruitment of a host of coactivators. The biological and physiological implications of PXR activation are broad, ranging from drug metabolism and drug-drug interactions to the homeostasis of numerous endobiotics, such as glucose, lipids, steroids, bile acids, bilirubin, retinoic acid, and bone minerals. The purpose of this article is to provide an overview on the transcriptional circuits and metabolic relevance controlled by PXR. This article is part of a Special Issue entitled: Translating Nuclear Receptors from Health to Disease.

Role of nuclear receptors for bile acid metabolism, bile secretion, cholestasis, and gallstone disease

Nuclear receptors (NRs) play a key role in the transcriptional control of critical steps of hepatobiliary transport and phase I/II metabolism of endo- and xenobiotics such as bile acids and drugs. Apart from these metabolic roles, NRs may also play a key role in the control of hepatic inflammation. Hereditary and acquired alterations of NRs contribute to our understanding of the pathogenesis of cholestasis and gallstone disease. Moreover, NRs may represent attractive drug targets for these disorders. This article is part of a Special Issue entitled: Translating nuclear receptors from health to disease.

Nitric oxide mimics transcriptional and post-translational regulation during α-tocopherol cytoprotection against glycochenodeoxycholate-induced cell death in hepatocytes

BACKGROUND & AIMS

Reactive oxygen species (ROS) and nitric oxide (NO) exert a relevant role during bile acid-induced hepatotoxicity. Whether α-Tocopherol regulates oxidative and nitrosative stress, bile acid transporter expression and their NO-dependent post-translational modifications, and cell death were assessed in vitro and in vivo.

METHODS

α-Tocopherol and/or NO donors (DETA-NONOate or CSNO, and V-PYRRO/NO) were administered to glycochenodeoxycholic acid (GCDCA)-treated cultured human hepatocytes or to bile duct obstructed rats. Cell injury, superoxide anion (O⁻₂) production, as well as inducible nitric oxide synthase (NOS-2), cytochrome P4507A1 (CYP7A1), heme oxygenase-1, (HO-1) and bile acid transporter expression were determined. Cysteine S-nitrosylation and tyrosine nitration of Na(+)-taurocholate co-transporting polypeptide (NTCP), as well as taurocholic acid (TC) uptake were also evaluated.

RESULTS

GCDCA-induced cell death was associated with increased (O⁻₂) production, NTCP and HO-1 expression, and with a reduction of CYP7A1 and NOS-2 expression. α-Tocopherol reduced cell death, (O⁻₂) production, CYP7A1, NTCP, and HO-1 expression, as well as increased NOS-2 expression and NO production in GCDCA-treated hepatocytes. α-Tocopherol and NO donors increased NTCP cysteine S-nitrosylation and tyrosine nitration, and reduced TC uptake in hepatocytes. α-Tocopherol and V-PYRRO/NO reduced liver injury and NTCP expression in obstructed rats.

CONCLUSIONS

The regulation of CYP7A1, NTCP, and HO-1 expression may be relevant for the cytoprotective properties of α-Tocopherol and NO against mitochondrial dysfunction, oxidative stress and cell death in GCDCA-treated hepatocytes. The regulation of NO-dependent post-translational modifications of NTCP by α-Tocopherol and NO donors reduces the uptake of toxic bile acids by hepatocytes.

Estrogen modulates transactivations of SXR-mediated liver X receptor response element and CAR-mediated phenobarbital response element in HepG2 cells

The nuclear receptors, steroid and xenobiotic receptor (SXR) and constitutive androstane receptor (CAR) play important functions in mediating lipid and drug metabolism in the liver. The present study demonstrates modulatory actions of estrogen in transactivations of SXR-mediated liver X receptor response element (LXRE) and CAR-mediated phenobarbital response element (PBRU). When human estrogen receptor (hERα) and SXR were exogenously expressed, treatment with either rifampicin or corticosterone promoted significantly the SXR-mediated transactivation of LXRE reporter gene in HepG2. However, combined treatment with estrogen plus either rifampicin or corticosterone resulted in less than 50% of the mean values of the transactivation by rifampicin or corticosterone alone. Thus, it is suggested that estrogen may repress the SXR-mediated transactivation of LXRE via functional cross-talk between ER and SXR. The CAR-mediated transactivation of PBRU was stimulated by hERa in the absence of estrogen. However, the potentiation by CAR agonist, TCPOBOP, was significantly repressed by moxestrol in the presence of ER. Thus, ER may play both stimulatory and inhibitory roles in modulating CAR-mediated transactivation of PBRU depending on the presence of their ligands. In summary, this study demonstrates that estrogen modulates transcriptional activity of SXR and CAR in mediating transactivation of LXRE and PBRU, respectively, of the nuclear receptor target genes through functional cross-talk between ER and the corresponding nuclear receptors.

The bile acid membrane receptor TGR5: a valuable metabolic target

Bile acids (BAs) are amphipathic molecules that facilitate the uptake of lipids, and their levels fluctuate in the intestines as well as in the circulation depending on food intake. Besides their role in dietary lipid absorption, BAs function as signaling molecules that activate specific BA receptors and trigger downstream signaling cascades. The BA receptors and the signaling pathways they control are not only important in the regulation of BA synthesis and their metabolism, but they also regulate glucose homeostasis, lipid metabolism and energy expenditure - processes relevant in the context of the metabolic syndrome. In addition to the function of the nuclear receptor FXRα in regulating local effects of BAs in the organs of the enterohepatic axis, increasing evidence points to a crucial role of the G-protein-coupled receptor TGR5 in mediating systemic actions of BAs. Here we review the current knowledge on BA receptors, with a strong focus on the cell membrane receptor TGR5, which has emerged as a promising target for intervention in metabolic diseases.

The bile acid membrane receptor TGR5 as an emerging target in metabolism and inflammation

Bile acids (BAs) are amphipathic molecules that facilitate the uptake of lipids, and their levels fluctuate in the intestine as well as in the blood circulation depending on food intake. Besides their role in dietary lipid absorption, bile acids function as signaling molecules capable to activate specific receptors. These BA receptors are not only important in the regulation of bile acid synthesis and their metabolism, but also regulate glucose homeostasis, lipid metabolism, and energy expenditure. These processes are important in diabetes and other facets of the metabolic syndrome, which represents a considerable increasing health burden. In addition to the function of the nuclear receptor FXRα in regulating local effects in the organs of the enterohepatic axis, increasing evidence points to a crucial role of the G-protein coupled receptor (GPCR) TGR5 in mediating systemic actions of BAs. Here we discuss the current knowledge on BA receptors, with a strong focus on the cell membrane receptor TGR5, which emerges as a valuable target for intervention in metabolic diseases.

A critical analysis of the interplay between cytochrome P450 3A and P-glycoprotein: recent insights from knockout and transgenic mice

CYP3A is one of the most important drug-metabolizing enzymes, determining the first-pass metabolism, oral bioavailability, and elimination of many drugs. It is also an important determinant of variable drug exposure and is involved in many drug-drug interactions. Recent studies with CYP3A knockout and transgenic mice have yielded a number of key insights that are important to consider during drug discovery and development. For instance, studies with tissue-specific CYP3A-transgenic mice have highlighted the importance of intestinal CYP3A-dependent metabolism. They also revealed that intestinal CYP3A plays an important role in the regulation of various drug-handling systems in the liver. Intestinal CYP3A activity can thus have far-reaching pharmacological effects. Besides CYP3A, the active drug efflux transporter P-glycoprotein also has a strong effect on the pharmacokinetics of numerous drugs. CYP3A and P-glycoprotein have an extensive overlap in their substrate spectrum. It has been hypothesized that for many drugs, the combined activity of CYP3A and P-glycoprotein makes for efficient intestinal first-pass metabolism of orally administered drugs as a result of a potentially synergistic collaboration. However, there is only limited in vitro and in vivo evidence for this hypothesis. There has also been some confusion in the field about what synergy actually means in this case. Our recent studies with Cyp3a/P-glycoprotein combination knockout mice have provided further insights into the CYP3A-P-glycoprotein interplay. We here present our view of the status of the synergy hypothesis and an attempt to clarify the existing confusion about synergy. We hope that this will facilitate further critical testing of the hypothesis and improve communication among researchers. Above all, the recent findings and insights into the interplay between CYP3A and P-glycoprotein may have implications for improving oral drug bioavailability and reducing adverse side effects.

PXR prevents cholesterol gallstone disease by regulating biosynthesis and transport of bile salts

BACKGROUND & AIMS

Cholesterol gallstone disease (CGD) results from a biochemical imbalance of lipids and bile salts in the gallbladder bile. We investigated whether the xenobiotic receptor pregnane X receptor (PXR) has a role in pathogenesis of CGD.

METHODS

Wild-type, PXR-null (PXR-/-), and CGD-sensitive C57L mice were placed on a lithogenic diet and then analyzed for CGD at the biochemical, histological, and gene-regulation levels.

RESULTS

Loss of PXR sensitized mice to lithogenic diet-induced CGD, characterized by decreases in biliary concentrations of bile salts and phospholipids and an increases in the cholesterol saturation index and formation of cholesterol crystals. The decreased bile acid pool size in PXR-/- mice that received lithogenic diets was associated with reduced expression of cholesterol 7α-hydroxylase, the rate-limiting enzyme of cholesterol catabolism and bile acid formation. The reduced expression of cholesterol 7α-hydroxylase most likely resulted from activation of farnesoid X receptor and induction of fibroblast growth factor 15 in the intestine. In C57L mice given the PXR agonist, pregnenolone-16α-carbonitrile, or the herbal medicine, St John's wort, cholesterol precipitation was prevented by increases in concentrations of biliary bile salt and a reduced cholesterol saturation index. PXR prevented CGD via its coordinate regulation of the biosynthesis and transport of bile salts in the liver and intestine.

CONCLUSIONS

PXR maintains biliary bile acid homeostasis and may be developed as a therapeutic target for CGD.

Bile acids regulate cardiovascular function

Research over the last decade has uncovered roles for bile acids (BAs) that extend beyond their traditional functions in regulating lipid digestion and cholesterol metabolism. BAs are now recognized as signaling molecules that interact with both plasma membrane and nuclear receptors. Emerging evidence indicates that by interacting with these receptors, BAs regulate their own synthesis, glucose and energy homeostasis, and other important physiological events. Herein, we provide a comprehensive review of the actions of BAs on cardiovascular function. In the heart and the systemic circulation, BAs interact with plasma membrane G-protein-coupled receptors, for example, TGR5 and muscarinic receptors, and nuclear receptors, for example, the farnesoid (FXR) and pregnane (PXR) xenobiotic receptors. BA receptors are expressed in cardiovascular tissue, however, the mechanisms underlying BA-mediated regulation of cardiovascular function remain poorly understood. BAs reduce heart rate by regulating channel conductance and calcium dynamics in sino-atrial and ventricular cardiomyocytes and regulate vascular tone via both endothelium-dependent and -independent mechanisms. End-stage liver disease, obstructive jaundice, and intrahepatic cholestasis of pregnancy are prominent conditions in which elevated serum BAs alter vascular dynamics. This review focuses on BAs as newly recognized signaling molecules that modulate cardiovascular function.

Aflatoxins upregulate CYP3A4 mRNA expression in a process that involves the PXR transcription factor

Pregnane X receptor (PXR) is a member of the nuclear hormone receptor (NHR) superfamily, which regulates xenobiotic and endobiotic metabolism in the liver. This transcription factor is activated by structurally diverse ligands, including drugs and environmental pollutants. PXR regulates the expression of numerous genes that function in biotransformation and the disposition of xenobiotics upon binding to an AG(G/T)TCA DNA motif in target promoter regions. We performed a screen of mycotoxins that pose a known environmental threat to human and animal health for the ability to activate PXR function in a human hepatocyte cell line, HepG2. We found that aflatoxins B1, M1, and G1 activated PXR. This activation was associated with upregulation of CYP3A4 expression and increased occupancy of PXR protein on the CYP3A4 promoter. Using a microarray approach, we also found that aflatoxin B1 upregulated the expression of multiple genes involved in xenobiotic metabolism, including genes known to be regulated in a PXR-dependent fashion. We also observed an effect of aflatoxin B1 on the expression in other functional groups of genes, including the downregulation of genes involved in cholesterologenesis. The results of this study indicate that aflatoxin B1 is able to activate PXR, a known regulator of liver xenobiotic metabolism, in human hepatocytes, and it can upregulate the expression of PXR-dependent genes responsible for aflatoxin B1 biotransformation, including CYP3A4.