An essential role for nuclear receptors SXR/PXR in detoxification of cholestatic bile acids

Activation of PXR induces hypercholesterolemia in wild-type and accelerates atherosclerosis in apoE deficient mice

The nuclear hormone receptor pregnane X receptor (PXR; also called SXR) functions as a xenobiotic sensor to coordinately regulate xenobiotic metabolism via transcriptional regulation of xenobiotic-detoxifying enzymes and transporters. Although many clinically relevant PXR ligands have been shown to affect cholesterol levels, the role of PXR in cholesterol homeostasis and atherosclerosis has not been thoroughly investigated. Here, we report that activation of PXR by feeding the PXR agonist pregnenolone 16alpha-carbonitrile (0.02%) for 2 weeks to wild-type (WT) mice significantly increased total cholesterol levels and atherogenic lipoproteins VLDL and LDL levels, but had no effect in PXR knockout (PXR(-/-)) mice. Chronic PXR activation in atherosclerosis prone apolipoprotein E deficient (ApoE(-/-)) mice was found to decrease HDL levels and increase atherosclerotic cross-sectional lesion area at both the aortic root and in the brachiocephalic artery by 54% (P < 0.001) and 116% (P < 0.01), respectively. PXR activation significantly regulated genes in the liver involved in lipoprotein transportation and cholesterol metabolism, including CD36, ApoA-IV, and CYP39A1, in both WT and ApoE(-/-) mice. Furthermore, PXR activation can increase CD36 expression and lipid accumulation in peritoneal macrophages of ApoE(-/-) mice. In summary, PXR activation in WT mice increases levels of the atherogenic lipoproteins VLDL and LDL, whereas in ApoE(-/-) mice, PXR increases atherosclerosis, perhaps by diminishing levels of the antiatherogenic ApoA-IV and increasing lipid accumulation in macrophages.

Nuclear receptors CAR and PXR: Molecular, functional, and biomedical aspects

Nuclear receptors (NRs) are ligand-activated transcription factors sharing a common evolutionary history and having similar sequence features at the protein level. Selective ligand(s) for some NRs is not known, therefore these NRs have been named "orphan receptors". Whenever ligands have been recognized for any of the orphan receptor, it has been categorized and grouped as "adopted" orphan receptor. This group includes the constitutive androstane receptor (CAR) and the pregnane X receptor (PXR). They function as sensors of toxic byproducts derived from endogenous metabolites and of exogenous chemicals, in order to enhance their elimination. This unique function of CAR and PXR sets them apart from the steroid hormone receptors. The broad response profile has established that CAR and PXR are xenobiotic sensors that coordinately regulate xenobiotic clearance in the liver and intestine via induction of genes involved in drug and xenobiotic metabolism. In the past few years, research has revealed new and mostly unsuspected roles for CAR and PXR in modulating hormone, lipid, and energy homeostasis as well as cancer and liver steatosis. The purpose of this review is to highlight the structural and molecular bases of CAR and PXR impact on human health, providing information on mechanisms through which diet, chemical exposure, and environment ultimately impact health and disease.

Function of retinoic acid receptors during embryonic development

Retinoids, the active metabolites of vitamin A, regulate complex gene networks involved in vertebrate morphogenesis, growth, cellular differentiation and homeostasis. Studies performed in vitro, using either acellular systems or transfected cells, have shown that retinoid actions are mediated through heterodimers between the RAR and RXR nuclear receptors. However, in vitro studies indicate what is possible, but not necessarily what is actually occurring in vivo, because they are performed under non-physiological conditions. Therefore, genetic approaches in the animal have been be used to determine the physiological functions of retinoid receptors. Homologous recombination in embryonic stem cells has been used to generate germline null mutations of the RAR- and RXR-coding genes in the mouse. As reviewed here, the generation of such germline mutations, combined with pharmacological approaches to block the RA signalling pathway, has provided genetic evidence that RAR/RXR heterodimers are indeed the functional units transducing the RA signal during prenatal development. However, due to (i) the complexity in “hormonal” signalling through transduction by the multiple RARs and RXRs, (ii) the functional redundancies (possibly artefactually generated by the mutations) within receptor isotypes belonging to a given family, and (iii) in utero or early postnatal lethality of certain germline null mutations, these genetic studies have failed to reveal all the physiological functions of RARs and RXRs, notably in adults. Spatio-temporally-controlled somatic mutations generated in given cell types/tissues and at chosen times during postnatal life, will be required to reveal all the functions of RAR and RXR throughout the lifetime of the mouse.

Bile acids: regulation of synthesis

Bile acids are physiological detergents that generate bile flow and facilitate intestinal absorption and transport of lipids, nutrients, and vitamins. Bile acids also are signaling molecules and inflammatory agents that rapidly activate nuclear receptors and cell signaling pathways that regulate lipid, glucose, and energy metabolism. The enterohepatic circulation of bile acids exerts important physiological functions not only in feedback inhibition of bile acid synthesis but also in control of whole-body lipid homeostasis. In the liver, bile acids activate a nuclear receptor, farnesoid X receptor (FXR), that induces an atypical nuclear receptor small heterodimer partner, which subsequently inhibits nuclear receptors, liver-related homolog-1, and hepatocyte nuclear factor 4alpha and results in inhibiting transcription of the critical regulatory gene in bile acid synthesis, cholesterol 7alpha-hydroxylase (CYP7A1). In the intestine, FXR induces an intestinal hormone, fibroblast growth factor 15 (FGF15; or FGF19 in human), which activates hepatic FGF receptor 4 (FGFR4) signaling to inhibit bile acid synthesis. However, the mechanism by which FXR/FGF19/FGFR4 signaling inhibits CYP7A1 remains unknown. Bile acids are able to induce FGF19 in human hepatocytes, and the FGF19 autocrine pathway may exist in the human livers. Bile acids and bile acid receptors are therapeutic targets for development of drugs for treatment of cholestatic liver diseases, fatty liver diseases, diabetes, obesity, and metabolic syndrome.

A phosphomimetic mutation at threonine-57 abolishes transactivation activity and alters nuclear localization pattern of human pregnane x receptor

The pregnane X receptor (PXR) plays crucial roles in multiple physiological processes. However, the signaling mechanisms responsible are not well defined; it is most likely that multiple functions of PXR are modulated by its phosphorylation. Therefore, we sought to determine whether mutation at a highly conserved Thr(57) affects human PXR (hPXR) function. Site-directed mutagenesis was performed to generate phosphorylation-deficient (hPXR(T57A)) and phosphomimetic (hPXR(T57D)) mutants. Gene reporter, Western blotting, immunocytochemistry, mammalian two-hybrid, and electrophoretic mobility shift assays were used to study cytochrome P450 3A4 (CYP3A4) promoter activation, protein levels, localization, cofactor interaction, and CYP3A4 promoter binding of the hPXR mutants, respectively. hPXR(T57D), but not hPXR(T57A), lost its transcriptional activity. Neither mutation altered hPXR's protein levels and interaction with steroid receptor coactivator-1. hPXR and hPXR(T57A) exhibited a homogenous nuclear distribution, whereas hPXR(T57D) exhibited a distinctive punctate nuclear localization pattern similar to that of hPXR mutants with impaired function that colocalize with silencing mediator of retinoid and thyroid receptors (SMRT), although silencing of SMRT did not rescue the altered function of hPXR(T57D). However, hPXR(T57D), but not hPXR(T57A), impaired hPXR's ability to bind to the CYP3A4 promoter, consistent with the mutant's transactivation function. Furthermore, the 70-kDa form of ribosomal protein S6 kinase (p70 S6K) phosphorylated hPXR in vitro and inhibited its transcriptional activity, whereas hPXR(T57A) partially resisted the inhibitory effect of p70 S6K. Our studies identify a functionally significant phosphomimetic mutant (hPXR(T57D)) and show p70 S6K phosphorylation and regulation of hPXR transactivation to support the notion that phosphorylation plays important roles in regulating hPXR function.

Human pregnane X receptor activation and CYP3A4/CYP2B6 induction by 2,3-oxidosqualene:lanosterol cyclase inhibition

The effects of [4'-(6-allyl-methyl-amino-hexyloxy)-2'-fluoro-phenyl]-(4-bromophenyl)-methanone fumarate (Ro 48-8071), an inhibitor of 2,3-oxidosqualene:lanosterol cyclase (cyclase), were evaluated on CYP3A4 and CYP2B6 mRNA content in primary cultured human hepatocytes. In seven hepatocyte culture preparations, 24-h treatment with 3, 10, or 30 microM Ro 48-8071 produced median increases in CYP3A4 mRNA content that were 2.2-, 7.1-, and 8.5-fold greater than untreated control, respectively, and produced increases in CYP2B6 mRNA content that were 3.0-, 4.6-, and 3.4-fold greater than control, respectively. Increases in CYP3A4 immunoreactive protein content were also measured in Ro 48-8071-treated hepatocytes. To evaluate the effects of cyclase inhibitor treatments further, a pregnane X receptor (PXR)-responsive transactivation assay in HepG2 cells was used. Ro 48-8071, trans-N-(4-chlorobenzoyl)-N-methyl-(4-dimethylaminomethylphenyl)-cyclohexylamine (BIBX 79), and 3beta-(2-diethylaminoethoxy)androst-5-en-17-one HCl (U18666A) induced luciferase expression from a PXR-responsive reporter with EC(50)s of 0.113, 0.916, and 0.294 microM, respectively. Treatment of the HepG2 system with (E)N-ethyl-N-(6,6-dimethyl-2-hepten-4-ynyl)-3-[(3,3'-bithiophen-5-yl)methoxy]benzenemethanamine (NB-598), an inhibitor of squalene monooxygenase, at concentrations sufficient to achieve cholesterol biosynthesis inhibition significantly inhibited cyclase inhibitor-mediated, but not rifampicin-mediated, reporter induction. Direct treatment of the HepG2 system with 1 to 10 microM squalene 2,3:22,23-dioxide, but not squalene 2,3-oxide, significantly activated PXR-responsive reporter expression. Also, squalene 2,3:22,23-dioxide bound to human PXR in vitro with an IC(50) of 3.35 microM. These data indicate that cyclase inhibitors are capable of producing CYP3A4 and CYP2B6 induction in primary cultured human hepatocytes, and that an endogenous squalene metabolite is a conserved intracrine activator of PXR.

Role of bile acids and bile acid receptors in metabolic regulation

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

Gene expression profiling in the liver and lung of perfluorooctane sulfonate-exposed mouse fetuses: comparison to changes induced by exposure to perfluorooctanoic acid

Perfluorooctane sulfonate (PFOS) and perfluorooctanoic acid (PFOA) are environmental contaminants found in the tissues of humans and wildlife. They are activators of peroxisome proliferator-activated receptor-alpha (PPAR alpha) and exhibit hepatocarcinogenic potential in rats. PFOS and PFOA are also developmental toxicants in rodents and PFOS has been shown to induce pulmonary deficits in rat offspring. Pregnant CD-1 mice were dosed with 0, 5, or 10mg/kg PFOS from gestation days 1-17. Transcript profiling was conducted on the fetal liver and lung. Results were contrasted to data derived from a previous PFOA study. PFOS-dependent changes were primarily related to activation of PPAR alpha. No remarkable differences were found between PFOS and PFOA. Given that PPAR alpha signaling is required for neonatal mortality in PFOA-treated mice but not those exposed to PFOS, the neonatal mortality observed for PFOS may reflect functional deficits related to the physical properties of the chemical rather than to transcript alterations.

Significance and mechanism of CYP7a1 gene regulation during the acute phase of liver regeneration

Cholesterol 7alpha-hydroxylase (CYP7a1) is the rate-limiting enzyme in the classic pathway of bile acid synthesis. Expression of CYP7a1 is regulated by a negative feedback pathway of bile acid signaling. Previous studies have suggested that bile acid signaling is also required for normal liver regeneration, and CYP7a1 expression is strongly repressed after 70% partial hepatectomy (PH). Both the effect of CYP7a1 suppression on liver regrowth and the mechanism by which 70% PH suppresses CYP7a1 expression are unknown. Here we show that liver-specific overexpression of an exogenous CYP7a1 gene impaired liver regeneration after 70% PH, which was accompanied by increased hepatocyte apoptosis and liver injury. CYP7a1 expression was initially suppressed after 70% PH in an farnesoid X receptor/ small heterodimer partner-independent manner; however, both farnesoid X receptor and small heterodimer partner were required to regulate CYP7a1 expression at the later stage of liver regeneration. c-Jun N-terminus kinase and hepatocyte growth factor signaling pathways are activated during the acute phase of liver regeneration. We determined that hepatocyte growth factor and c-Jun N-terminus kinase pathways were involved in the suppressing of the CYP7a1 expression in the acute phase of live regeneration. Taken together, our results provide the significance that CYP7a1 suppression is required for liver protection after 70% PH and there are two distinct phases of CYP7a1 gene regulation during liver regeneration.

The steroid and xenobiotic receptor (SXR), beyond xenobiotic metabolism

The steroid and xenobiotic receptor (SXR) (also known as pregnane X receptor or PXR) is a nuclear hormone receptor activated by a diverse array of endogenous hormones, dietary steroids, pharmaceutical agents, and xenobiotic compounds. SXR has an enlarged, flexible, hydrophobic ligand binding domain (LBD) which is remarkably divergent across mammalian species and SXR exhibits considerable differences in its pharmacology among mammals. The broad response profile of SXR has led to the development of "the steroid and xenobiotic sensor hypothesis". SXR has been established as a xenobiotic sensor that coordinately regulates xenobiotic clearance in the liver and intestine via induction of genes involved in drug and xenobiotic metabolism. In the past few years, research has revealed new and mostly unsuspected roles for SXR in modulating inflammation, bone homeostasis, vitamin D metabolism, lipid homeostasis, energy homeostasis and cancer. The identification of SXR as a xenobiotic sensor has provided an important tool for studying new mechanisms through which diet, chemical exposure, and environment ultimately impact health and disease. The discovery and pharmacological development of new PXR modulators might represent an interesting and innovative therapeutic approach to combat various diseases.

Bile Acid sulfation: a pathway of bile acid elimination and detoxification

Sulfotransferase-2A1 catalyzes the formation of bile acid-sulfates (BA-sulfates). Sulfation of BAs increases their solubility, decreases their intestinal absorption, and enhances their fecal and urinary excretion. BA-sulfates are also less toxic than their unsulfated counterparts. Therefore, sulfation is an important detoxification pathway of BAs. Major species differences in BA sulfation exist. In humans, only a small proportion of BAs in bile and serum are sulfated, whereas more than 70% of BAs in urine are sulfated, indicating their efficient elimination in urine. The formation of BA-sulfates increases during cholestatic diseases. Therefore, sulfation may play an important role in maintaining BA homeostasis under pathologic conditions. Farnesoid X receptor, pregnane X receptor, constitutive androstane receptor, and vitamin D receptor are potential nuclear receptors that may be involved in the regulation of BA sulfation. This review highlights current knowledge about the enzymes and transporters involved in the formation and elimination of BA-sulfates, the effect of sulfation on the pharmacologic and toxicologic properties of BAs, the role of BA sulfation in cholestatic diseases, and the regulation of BA sulfation.

Vitamin K2 suppresses proliferation and motility of hepatocellular carcinoma cells by activating steroid and xenobiotic receptor

Vitamin K2, known as a cofactor for gamma-carboxylase, also serves as a ligand of a nuclear receptor, Steroid and Xenobiotic Receptor (SXR). Several clinical trials revealed that vitamin K2 reduced de novo formation and recurrence of hepatocellular carcinoma (HCC). To examine the role of SXR in HCC as a receptor activated by vitamin K2, the cells stably overexpressing SXR were established using a HCC cell line, HuH7. Overexpression of SXR resulted in reduced proliferation and motility of the cells. Further suppression of proliferation and motility was observed when SXR overexpressing clones were treated with vitamin K2. These results suggest that the activation of SXR could contribute to tumor suppressive effects of vitamin K2 on HCC cells.

Nuclear receptors: mediators and modifiers of inflammation-induced cholestasis

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

Chlordecone, a mixed pregnane X receptor (PXR) and estrogen receptor alpha (ERalpha) agonist, alters cholesterol homeostasis and lipoprotein metabolism in C57BL/6 mice

Chlordecone (CD) is one of many banned organochlorine (OC) insecticides that are widespread persistent organic pollutants. OC insecticides alter lipid homeostasis in rodents at doses that are not neurotoxic or carcinogenic. Pretreatment of mice or rats with CD altered tissue distribution of a subsequent dose of [(14)C]CD or [(14)C]cholesterol (CH). Nuclear receptors regulate expression of genes important in the homeostasis of CH and other lipids. In this study, we report that CD suppresses in vitro reporter systems for human liver X receptors (LXRs) and activates those for human farnesoid X receptor (FXR), pregnane X receptor (PXR) and estrogen receptor alpha (ERalpha) in a concentration-dependent manner (0-50 muM). Consistent with human PXR activation in vitro, three days after a single dose of CD (15 mg/kg) hepatic microsomal CYP3A11 protein increases in C57BL/6 mice. CD decreases hepatic CH ester content without altering total CH concentration. Apolipoprotein A-I (apoA-I) contents of hepatic lipoprotein-rich and microsomal fractions of CD-treated mice are higher than controls. There is a significant reduction in non-high density lipoprotein CH but not apolipoprotein B-48/100 (apoB-48/100) in plasma from CD-treated mice after a 4 h fast. At 14 days after 15 mg CD/kg apoA-I and apoB-100 proteins but not CYP3A11 protein in hepatic microsomes are similar to controls. This work indicates that altered CH homeostasis is a mode of OC insecticide action of relevance after a single dose. This at least partially explains altered CH tissue distribution in CD-pretreated mice.

Impact of the haplotypes of the human pregnane X receptor gene on the basal and St John's wort-induced activity of cytochrome P450 3A4 enzyme

WHAT IS ALREADY KNOWN ABOUT THIS SUBJECT

Human pregnane X receptor (PXR/NR1I2) is a key regulator of cytochrome P450 3A4. To date, there are 198 reported SNPs for the human PXR/NR1I2 gene. Some of these SNPs are found to affect the inducing ability of PXR to CYP3A4.

WHAT THIS STUDY ADDS

This study, for the first time, has investigated the effect of PXR haplotype on basal and St John's wort-induced CYP3A4 activity in humans. H1/H1 of the PXR gene had weaker basal transcriptional activity but greater inducible transcriptional activity to CYP3A4 than H1/H2 and H2/H2.

AIMS

Human pregnane X receptor (PXR/NR1I2) is the master regulator of CYP3A4, which metabolizes >50% of drugs on the market. This study investigated the relationship between the two most frequent haplotypes [H1 (TCAGGGGCCACC) and H2 (CCGAAAACTAAT)] of PXR and basal and St John's wort (SJW)-induced CYP3A4 activity.

METHODS

Ten healthy subjects carrying H1 and H2 haplotypes (three subjects with H1/H1, four with H1/H2 and three with H2/H2) entered this study. The 10 subjects did not carry CYP3A4*4, *5 and *6. All subjects were administrated a 300-mg SJW tablet three times daily for 14 days, and CYP3A4 activity was measured using nifedipine (NIF) as a probe. The plasma concentrations of NIF and dehydronifedipine (DNIF) were determined by a validated liquid chromatography/mass spectrometry/mass spectrometry method.

RESULTS

After administration of SJW, the AUC(0-infinity) of NIF decreased significantly, and the AUC(0-infinity) of DNIF increased significantly (P < 0.05). For H1/H2, the AUC(0-infinity) of NIF decreased by 42.4%, and the AUC(0-infinity) of DNIF increased by 20.2%; for H2/H2, the AUC(0-infinity) of NIF decreased by 47.9%, and the AUC(0-infinity) of DNIF increased by 33.0%; for H1/H1, the AUC(0-infinity) of NIF decreased by 29.0%, yet the AUC(0-infinity) of DNIF increased by 106.7%. The increase of the AUC(0-infinity) of DNIF in H1/H1 was significantly different from the other two haplotype pairs (P < 0.05). Meanwhile, before administration of SJW, the ratio of AUC(0-infinity(DNIF))/AUC(0-infinity(NIF)) was the lowest for H1/H1 (22.1%), compared with H1/H2 (58.7%) and H2/H2 (30.0%).

CONCLUSIONS

H1/H1 of the human PXR gene had weaker basal transcriptional activity but greater inducible transcriptional activity to CYP3A4 than H1/H2 and H2/H2.

PXR: a xenobiotic receptor of diverse function implicated in pharmacogenetics

The pregnane X receptor (PXR; NR1I2), a member of the nuclear receptor superfamily, regulates the expression of drug-metabolic enzymes and transporters involved in the responses of mammals to their chemical environment. The same enzyme and transporter systems are also involved in the homeostasis of numerous endogenous chemicals. The regulatory function of PXR is implicated in normal physiology and diseases, such as drug-drug interactions, hepatic steatosis, vitamin D homeostasis, bile acids homeostasis, steroid hormones homeostasis and inflammatory bowel diseases. As such, any genetic variations of this receptor could potentially have widespread effects on the disposition of xenobiotics and endobiotics. Knowledge concerning the genetic polymorphisms of PXR may help to understand the variations in human drug response and ensure safe drug use. The correlation of PXR genetic polymorphisms with several disease conditions also suggests that this receptor may represent a valid therapeutic for hepato-intestinal disorders such as inflammatory bowel disease and primary sclerosing cholangitis.

Role of vitamin D receptor in the lithocholic acid-mediated CYP3A induction in vitro and in vivo

Lipophilic bile acids are suggested to be involved in the endogenous expression of CYP3A4 in human and experimental animals as ligands of nuclear receptors. To verify the nuclear receptor specificity, the bile acid-mediated induction of CYP3A4 has been studied in vitro and in vivo in the present study. Lithocholic acid (LCA) strongly enhanced the activities of the CYP3A4 reporter gene, which contained multiple nuclear receptor binding elements, in both HepG2 and LS174T cells. The introduction of small interfering RNA for human vitamin D receptor (VDR), but not for human pregnane X receptor, reduced the LCA-induced activation of the reporter gene in these cells, suggesting the major role of VDR in the LCA induction of CYP3A4. Consistently, oral administration of LCA (100 mg/kg/day for 3 days) increased Cyp3a protein levels in the intestine but not in the liver, where a negligible level of VDR mRNA is detected. The selective role of VDR was tested in mice with the adenoviral overexpression of the receptor. Oral administration of LCA had no clear influence on the CYP3A4 reporter activity in the liver of control mice. In mice with the adenovirally expressed VDR, LCA treatment (100 or 400 mg/kg/day for 3 days) resulted in the enhanced reporter activities and increased levels of Cyp3a proteins in the liver. These results indicate the selective involvement of VDR, but not pregnane X receptor, in the LCA-mediated induction of both human and mouse CYP3As in vivo.

Nuclear receptors of the enteric tract: guarding the frontier

In addition to its classical role in mineral homeostasis, the vitamin D receptor has been implicated in diverse physiologic and pathophysiologic processes including immunoregulation and cancer. Interestingly, the vitamin D receptor has been evolutionarily and functionally linked to a select group of nuclear receptors based on a common organism-wide tissue expression profile. These members of the nuclear receptor superfamily, which include the bile acid receptor, xenobiotic receptors, and several orphan nuclear receptors, comprise a transcriptional regulatory network that functions in nutrient uptake, xenobiotic metabolism, and mucosal protection. The major homeostatic functions of the enteric nuclear receptor network are the topic of this review.

The emerging role of nuclear receptor RORalpha and its crosstalk with LXR in xeno- and endobiotic gene regulation

Retinoid-related orphan receptors (RORs), including the alpha, beta and gamma isoforms (NR1F1-3), are orphan nuclear receptors that have been implicated in tissue development, immune responses, and circadian rhythm. Although RORalpha and RORgamma have been shown to be expressed in the liver, the hepatic function of these two RORs remains unknown. We have recently shown that loss of RORalpha and/or RORgamma can positively or negatively influence the expression of multiple Phase I and Phase II drug metabolizing enzymes and transporters in the liver. Among ROR responsive genes, we identified oxysterol 7alpha-hydroxylase (Cyp7b1), which plays a critical role in the homeostasis of cholesterol, as a RORalpha target gene. We showed that RORalpha is both necessary and sufficient for Cyp7b1 activation. Studies of mice deficient of RORalpha or liver X receptors (LXRs) revealed an interesting and potentially important functional crosstalk between RORalpha and LXR. The respective activation of LXR target genes and ROR target genes in RORalpha null mice and LXR null mice led to our hypothesis that these two receptors are mutually suppressive in vivo. LXRs have been shown to regulate a battery of metabolic genes. We conclude that RORs participate in the xeno- and endobiotic regulatory network by regulating gene expression directly or through crosstalk with LXR, which may have broad implications in metabolic homeostasis.

Targeting bile-acid signalling for metabolic diseases

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

Novel polymorphic human UDP-glucuronosyltransferase 2A3: cloning, functional characterization of enzyme variants, comparative tissue expression, and gene induction

UDP-glucuronosyltransferases (UGTs) are critical to the detoxification of numerous drugs, environmental pollutants, and endogenous molecules. However, as yet not all of the human UGTs have been cloned and characterized. cDNA clones from the UGT2A3 gene (located on chromosome 4q13) were isolated using pooled human liver RNA. Approximately 10% of clones contained a c.1489A>G nucleotide substitution, yielding proteins with a residue 497 alanine (UGT2A3.2) instead of a threonine (UGT2A3.1). The allele frequency of this polymorphism (rs13128286) was 0.13 in a European-American population as determined by direct DNA sequencing. Of 81 structurally diverse glucuronidation substrates tested, UGT2A3 expressed by a baculovirus system selectively glucuronidated bile acids, particularly hyodeoxycholic acid at the 6-hydroxy position. Apparent K(m) values of UGT2A3.1 and UGT2A3.2 for hyodeoxycholic acid 6-glucuronidation were 69 +/- 7 and 44 +/- 12 microM, respectively. Of 29 different extrahepatic tissues evaluated by real-time polymerase chain reaction, UGT2A3 mRNA was most highly expressed in small intestine (160% of liver), colon (78% of liver), and adipose tissue (91% of liver). An in silico scan of the proximal UGT2A3 promoter/5'-regulatory region identified transcription factor consensus elements consistent with tissue-selective expression in liver (HNF1) and intestine (CXD2), as well as induction by rifampicin (pregnane X receptor). In LS180 human intestinal cells, rifampicin increased UGT2A3 mRNA by more than 4.5-fold compared with vehicle, whereas levels were not significantly affected by the arylhydrocarbon receptor ligand beta-naphthoflavone. This is the first report establishing UGT2A3 as a functional enzyme, and it represents significant progress toward the goal of having a complete set of recombinant human UGTs for comparative functional analyses.

Upregulation of decorin by FXR in vascular smooth muscle cells

Decorin is a member of the family of small leucine-rich proteoglycans that are present in blood vessels and synthesized by vascular smooth muscle cells (VSMCs). Decorin plays complex roles in both normal vascular physiology and the pathogenesis of various types of vascular disorders. However, the mechanisms of regulation of decorin expression in vasculature are not clearly understood. Particularly little information is available about a role of nuclear receptors in the regulation of decorin expression. In the present study, we report that activation of vascular FXR by a specific ligand resulted in upregulation of decorin at the levels of both mRNA and protein. FXR appears to induce decorin expression at a transcriptional level because (1) upregulation of decorin mRNA expression was abolished by the treatment of a transcription inhibitor, actinomycin D; and (2) decorin promoter activity was significantly increased by activation of FXR. Functional analysis of human decorin promoter identified an imperfect inverted repeat DNA motif, IR8 (-2313TGGTCAtagtgtcaTGACCT-2294), as a likely FXR-responsive element that is involved in decorin regulation.

Hepatocyte-specific ablation of Foxa2 alters bile acid homeostasis and results in endoplasmic reticulum stress

Production of bile by the liver is crucial for the absorption of lipophilic nutrients. Dysregulation of bile acid homeostasis can lead to cholestatic liver disease and endoplasmic reticulum (ER) stress. We show by global location analysis ('ChIP-on-chip') and cell type-specific gene ablation that the winged helix transcription factor Foxa2 is required for normal bile acid homeostasis. As suggested by the location analysis, deletion of Foxa2 in hepatocytes in mice using the Cre-lox system leads to decreased transcription of genes encoding bile acid transporters on both the basolateral and canalicular membranes, resulting in intrahepatic cholestasis. Foxa2-deficient mice are strikingly sensitive to a diet containing cholic acid, which results in toxic accumulation of hepatic bile salts, ER stress and liver injury. In addition, we show that expression of FOXA2 is markedly decreased in liver samples from individuals with different cholestatic syndromes, suggesting that reduced FOXA2 abundance could exacerbate the injury.

[[Inflammation and drug metabolism: NF-kappB and the CAR and PXR xeno-receptors]

Decreased drug metabolism, hyperbilirubinemia and intrahepatic cholestasis are frequently observed during inflammation. Additionally, it has long been appreciated that exposure to drug metabolism-inducing xenobiotics can impair immune function. The nuclear receptor CAR (constitutive androstane receptor or NR1I3) and PXR (pregnane X receptor, NR1I2) control phase I (cytochrome P450 2B and 3A), phase II (GSTA, UGT1A1), and transporter (MDR1, SLC21A6, MRP2) genes involved in drugs metabolism, bile acids and bilirubin clearance in response to xenobiotics. It is well known that inflammation, through the activation of NF-kappaB pathway, leads to a decrease of CAR, PXR and RXRalpha expression and the expression of their target genes. In addition, a new study reveals the mutual repression between PXR and NF-kappaB signaling pathways, providing a molecular mechanism linking xenobiotic metabolism and inflammation.

Inhibition and induction of human cytochrome P450 enzymes: current status

Variability of drug metabolism, especially that of the most important phase I enzymes or cytochrome P450 (CYP) enzymes, is an important complicating factor in many areas of pharmacology and toxicology, in drug development, preclinical toxicity studies, clinical trials, drug therapy, environmental exposures and risk assessment. These frequently enormous consequences in mind, predictive and pre-emptying measures have been a top priority in both pharmacology and toxicology. This means the development of predictive in vitro approaches. The sound prediction is always based on the firm background of basic research on the phenomena of inhibition and induction and their underlying mechanisms; consequently the description of these aspects is the purpose of this review. We cover both inhibition and induction of CYP enzymes, always keeping in mind the basic mechanisms on which to build predictive and preventive in vitro approaches. Just because validation is an essential part of any in vitro-in vivo extrapolation scenario, we cover also necessary in vivo research and findings in order to provide a proper view to justify in vitro approaches and observations.

Crosstalk between cholesterol homeostasis and drug metabolism

Napjainkban a cardiovascularis megbetegedések vezető halálozási oknak számítanak világszerte. A szív- és érrendszeri megbetegedések kialakulásában jelentős szerepet játszik a magas szérumkoleszterin-szint, illetve az atherosclerosis. A vér koleszterinszintjének csökkentésével kedvezően befolyásolható a káros folyamatok kialakulása, és a már kialakult betegségekben is javulás érhető el. Az általánosan alkalmazott sztatinalapú gyógyszeres terápia ade novokoleszterin-bioszintézist gátolja a májban. Más hatóanyagok (például ezetimib) a koleszterin táplálékból történő felszívódását gátolják. A leghatékonyabb megoldást ezek kombinált alkalmazása jelentheti. A gyógyszeres terápia során azonban figyelembe kell venni, hogy számos vegyület (gyógyszer) képes specifikusan megváltoztatni – a koleszterinhomeosztázis fenntartásában szerepet játszó enzimek mellett – a gyógyszer-metabolizáló enzimek indukciójával a citokróm P450 enzimek mennyiségét is (például sztatinok), ami a terápia módosítását teszi szükségessé. A koleszterin-anyagcsere és a gyógyszer-metabolizmus szabályozásában ugyanis több kapcsolódási pont is található. A kapcsolat az úgynevezett nukleáris receptorokon keresztül valósul meg, ezért a koleszterinhomeosztázis és a gyógyszer-metabolizmus közti összefüggés megértése és ismereteink bővítése elengedhetetlen egy megfelelő terápiás stratégia kidolgozásához, illetve új gyógyszerek fejlesztéséhez.

Cytosol-nucleus traffic and colocalization with FXR of conjugated bile acids in rat hepatocytes

Bile acids (BAs) are natural ligands of nuclear receptors, in particular farnesoid X receptor (FXR). Whether, in addition to protein-mediated cytosolic-nuclear BA translocation, other mechanisms are involved in the access of BAs to nuclear FXR was investigated. When rat hepatocytes were incubated with radiolabeled taurocholic acid, taurodeoxycholic acid, taurochenodeoxycholic acid, and tauroursodeoxycholic acid, their nuclear accumulation was proportional to their intracellular levels. With the use of flow cytometry analysis, the accumulation by nuclei isolated from rat liver cells was found to differ for several fluorescent compounds of similar molecular weight and different charge, including fluorescein-tagged BAs [cholylglycyl amidofluorescein (CGamF), ursodeoxycholylglycyl amidofluorescein, or chenodeoxycholylglycyl amidofluorescein]. When we varied nuclear volume by incubation with different sucrose concentrations, a similar relationship between nuclear volume and content of FITC and 4-kDa FITC-dextran was found. In contrast, this relationship was markedly lower for CGamF. Confocal microscopy studies revealed that fluorescein-tagged BAs, but also FITC or 10-kDa FITC-dextran were found in the nuclear envelope and concentrated in regions where DNA was less densely packed. In contrast to the cytosolic subcellular localization of peroxisome proliferator-activated receptor-alpha, FXR and nucleolin (a marker of transcriptional active chromatin) were also localized by immunoreactivity in these intranuclear regions. In conclusion, although intranuclear levels of small organic molecules including conjugated BAs depend on their concentrations in the extranuclear space, the existence of certain molecular selectivity (not strictly dependent on molecular weight or charge) suggests that, in addition to simple diffusional exchange, other mechanisms may be also involved in determining their overall nuclear content in regions where these compounds coincide and may interact with nuclear receptors such as FXR.

Bile acids and signal transduction: role in glucose homeostasis

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

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

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

Evolution of pharmacologic specificity in the pregnane X receptor

Background

The pregnane X receptor (PXR) shows the highest degree of cross-species sequence diversity of any of the vertebrate nuclear hormone receptors. In this study, we determined the pharmacophores for activation of human, mouse, rat, rabbit, chicken, and zebrafish PXRs, using a common set of sixteen ligands. In addition, we compared in detail the selectivity of human and zebrafish PXRs for steroidal compounds and xenobiotics. The ligand activation properties of the Western clawed frog (Xenopus tropicalis) PXR and that of a putative vitamin D receptor (VDR)/PXR cloned in this study from the chordate invertebrate sea squirt (Ciona intestinalis) were also investigated.

Results

Using a common set of ligands, human, mouse, and rat PXRs share structurally similar pharmacophores consisting of hydrophobic features and widely spaced excluded volumes indicative of large binding pockets. Zebrafish PXR has the most sterically constrained pharmacophore of the PXRs analyzed, suggesting a smaller ligand-binding pocket than the other PXRs. Chicken PXR possesses a symmetrical pharmacophore with four hydrophobes, a hydrogen bond acceptor, as well as excluded volumes. Comparison of human and zebrafish PXRs for a wide range of possible activators revealed that zebrafish PXR is activated by a subset of human PXR agonists. The Ciona VDR/PXR showed low sequence identity to vertebrate VDRs and PXRs in the ligand-binding domain and was preferentially activated by planar xenobiotics including 6-formylindolo-[3,2-b]carbazole. Lastly, the Western clawed frog (Xenopus tropicalis) PXR was insensitive to vitamins and steroidal compounds and was activated only by benzoates.

Conclusion

In contrast to other nuclear hormone receptors, PXRs show significant differences in ligand specificity across species. By pharmacophore analysis, certain PXRs share similar features such as human, mouse, and rat PXRs, suggesting overlap of function and perhaps common evolutionary forces. The Western clawed frog PXR, like that described for African clawed frog PXRs, has diverged considerably in ligand selectivity from fish, bird, and mammalian PXRs.

Molecular bases of the fetal liver-placenta-maternal liver excretory pathway for cholephilic compounds

Potentially toxic endogenous compounds, such as bile acids (BAs) and biliary pigments, as well as many xenobiotics, such as drugs and food components, are biotransformed and eliminated by the hepatobiliary system with the collaboration of the kidney. However, the situation is very different during pregnancy because the fetal liver produces biliary compounds despite the fact that this organ, owing to its immaturity, is not able to eliminate them into bile. Moreover, the excretory ability of the fetal kidneys is also very limited. Thus, during the intra-uterine life, the major route to eliminate fetal BAs and biliary pigments is their transfer to the mother across the placenta. The maternal liver and, to a lesser extent, the maternal kidney, are then in charge of their biotransformation and elimination into faeces and urine respectively. This review describes current knowledge of the machinery responsible for the detoxification and excretion of cholephilic compounds through the pathway formed by the fetal liver-placenta-maternal liver trio.

Pregnane X receptor is essential for normal progression of liver regeneration

Pregnane X receptor (PXR) mediates xenobiotic and endobiotic metabolism as well as hepatocyte proliferation. To determine the role of PXR in liver regeneration, 2/3 partial hepatectomy (PH) was performed on wild-type and PXR-null mice. Our results showed that hepatic steatosis was markedly suppressed in mice lacking PXR 36 hours after PH, concomitant with reduction of hepatocyte proliferation at the same time point. Gene expression analysis revealed the role of PXR in regulating the transcription of genes involved in lipid uptake, transport, biosynthesis, oxidation, and storage during liver regeneration. When PXR was absent, the second wave of hepatocyte proliferation was severely suppressed, which was accompanied by the inactivation of STAT3. Lack of PXR inhibited the second phase of liver growth, leading to 17% less liver mass at the anticipated end point of liver regeneration (day 10).

CONCLUSION

PXR is required for normal progression of liver regeneration by modulating lipid homeostasis and regulating hepatocyte proliferation.

The tangle of nuclear receptors that controls xenobiotic metabolism and transport: crosstalk and consequences

The expression of many genes involved in xenobiotic/drug metabolism and transport is regulated by at least three nuclear receptors or xenosensors: aryl hydrocarbon receptor (AhR), constitutive androstane receptor (CAR), and pregnane X receptor (PXR). These receptors establish crosstalk with other nuclear receptors or transcription factors controlling signaling pathways that regulate the homeostasis of bile acids, lipids, glucose, inflammation, vitamins, hormones, and others. These crosstalks are expected to modify profoundly our vision of xenobiotic/drug disposition and toxicity. They provide molecular mechanisms to explain how physiopathological stimuli affect xenobiotic/drug disposition, and how xenobiotics/drugs may affect physiological functions and generate toxic responses. In addition, the possibility that xenosensors may control other signaling pathways opens the way to new pharmacological opportunities.

Pharmacogenetics of the constitutive androstane receptor

The constitutive androstane receptor (CAR), a member of the NR1I group of nuclear hormone receptors, has been implicated in regulating the expression of genes that are critical in xenobiotic and endobiotic metabolism, uptake and elimination as well as genes involved in various other physiological processes. Hence, functional variation in CAR associated with its expression and/or activity can influence the transcriptional activation of its target genes and could contribute to the observed variation in drug response and toxicity. Moreover, coadministration of agents that are also CAR activators contributes to clinically relevant drug-drug interactions in patients receiving certain combination therapies. This review will discuss the functional significance of known genetic variants in CAR and the most common alternatively spliced isoforms of CAR. We will also discuss the influence of gender and ethnicity on CAR and its target genes. Although genetic polymorphisms in CAR may have an indirect effect on drug disposition, understanding the association of genetic polymorphisms in CAR with the expression of its target genes might help us to better understand the molecular mechanisms underlying the interindividual variation in drug disposition in addition to drug-drug interactions.

Post-transcriptional regulation of human pregnane X receptor by micro-RNA affects the expression of cytochrome P450 3A4

Pregnane X receptor (PXR) is a major transcription factor regulating the inducible expression of a variety of transporters and drug-metabolizing enzymes, including CYP3A4 (cytochrome P450 3A4). We first found that the PXR mRNA level was not correlated with the PXR protein level in a panel of 25 human livers, indicating the involvement of post-transcriptional regulation. Notably, a potential miR-148a recognition element was identified in the 3'-untranslated region of human PXR mRNA. We investigated whether PXR might be regulated by miR-148a. A reporter assay revealed that miR-148a could recognize the miR-148a recognition element of PXR mRNA. The PXR protein level was decreased by the overexpression of miR-148a, whereas it was increased by inhibition of miR-148a. The miR-148a-dependent decrease of PXR protein attenuated the induction CYP3A4 mRNA. Furthermore, the translational efficiency of PXR (PXR protein/PXR mRNA ratio) was inversely correlated with the expression levels of miR-148a in a panel of 25 human livers, supporting the miR-148a-dependent regulation of PXR in human livers. Eventually, the PXR protein level was significantly correlated with the CYP3A4 mRNA and protein levels. In conclusion, we found that miR-148a post-transcriptionally regulated human PXR, resulting in the modulation of the inducible and/or constitutive levels of CYP3A4 in human liver. This study will provide new insight into the unsolved mechanism of the large interindividual variability of CYP3A4 expression.

Mechanisms of cholestasis

This article gives an overview of the molecular and cellular mechanisms of cholestasis. Topics reviewed include the pathomechanisms of hereditary cholestasis syndromes, such as progressive familial intrahepatic cholestasis, and hepatocellular transporter defects encountered in various acquired cholestatic disorders, such as intrahepatic cholestasis of pregnancy, drug-induced cholestasis, inflammatory cholestasis, primary sclerosing cholangitis, and primary biliary cirrhosis. In addition, current concepts regarding adaptive hepatocellular mechanisms counteracting cholestatic liver damage are discussed.

Biotransformation of lithocholic acid by rat hepatic microsomes: metabolite analysis by liquid chromatography/mass spectrometry

Lithocholic acid is a lipid-soluble hepatotoxic bile acid that accumulates in the liver during cholestasis. A potential detoxification pathway for lithocholic acid involves hydroxylation by hepatic cytochrome P450 (P450) enzymes. The purpose of the present study was to identify the hepatic microsomal metabolites of lithocholic acid by liquid chromatography/mass spectrometry and to determine the P450 enzymes involved. Incubation of lithocholic acid with rat hepatic microsomes and NADPH produced murideoxycholic acid (MDCA), isolithocholic acid (ILCA), and 3-keto-5beta-cholanic acid (3KCA) as major metabolites and 6-ketolithocholic acid and ursodeoxycholic acid as minor metabolites. Experiments with hepatic microsomes prepared from rats pretreated with P450 inducers and with inhibitory antibodies indicated that CYP2C and CYP3A enzymes contribute to microsomal MDCA formation. Results obtained with a panel of recombinant P450 enzymes and CYP2D6 antiserum showed that CYP2D1 can also catalyze MDCA formation. Similar experimental evidence revealed that formation of 3KCA was mediated primarily by CYP3A enzymes. ILCA formation appeared to be catalyzed by a distinct pathway mediated largely by microsomal non-P450 enzymes. Based on the results obtained using lithocholic acid and 3KCA as substrates, a mechanism for the formation of ILCA involving a geminal diol intermediate is outlined. In conclusion, lithocholic acid was extensively metabolized by multiple P450 enzymes with the predominant biotransformation pathway being hydroxylation at the 6beta-position. This study provides an insight into possible routes of detoxification of lithocholic acid.

Lithocholic acid derivatives act as selective vitamin D receptor modulators without inducing hypercalcemia

1alpha,25-Dihydroxyvitamin D(3) [1,25(OH)(2)D(3)], a vitamin D receptor (VDR) ligand, regulates calcium homeostasis and also exhibits noncalcemic actions on immunity and cell differentiation. In addition to disorders of bone and calcium metabolism, VDR ligands are potential therapeutic agents in the treatment of immune disorders, microbial infections, and malignancies. Hypercalcemia, the major adverse effect of vitamin D(3) derivatives, limits their clinical application. The secondary bile acid lithocholic acid (LCA) is an additional physiological ligand for VDR, and its synthetic derivative, LCA acetate, is a potent VDR agonist. In this study, we found that an additional derivative, LCA propionate, is a more selective VDR activator than LCA acetate. LCA acetate and LCA propionate induced the expression of the calcium channel transient receptor potential vanilloid type 6 (TRPV6) as effectively as that of 1alpha,25-dihydroxyvitamin D(3) 24-hydroxylase (CYP24A1), whereas 1,25(OH)(2)D(3) was more effective on TRPV6 than on CYP24A1 in intestinal cells. In vivo experiments showed that LCA acetate and LCA propionate effectively induced tissue VDR activation without causing hypercalcemia. These bile acid derivatives have the ability to function as selective VDR modulators.

The antiapoptotic role of pregnane X receptor in human colon cancer cells

The orphan nuclear receptor pregnane X receptor (PXR) plays an important role in the detoxification of foreign and endogenous chemicals, including bile acids. PXR promotes bile acid elimination by activating bile acid-detoxifying enzymes and transporters. Certain bile acids are known to promote colonic carcinogenesis by inducing colon cancer cell apoptosis. However, whether and how PXR plays a role in colon cancer apoptosis has not been reported. In this study, we showed that activation of PXR by genetic (using a constitutively activated PXR) or pharmacological (using PXR agonist rifampicin) means protected the PXR-overexpressing colon cancer HCT116 cells from deoxycholic acid-induced apoptosis. Interestingly, activation of PXR also protected HCT116 cells from adriamycin-induced cell death, suggesting that the antiapoptotic effect of PXR was not bile acid specific. Moreover, the antiapoptotic effect of PXR in HCT116 cells appeared to be independent of xenobiotic enzyme regulation, because these cells had little basal and inducible expression of bile acid-detoxifying enzymes. Instead, SuperArray analysis showed that PXR-mediated deoxycholic acid resistance was associated with up-regulation of multiple antiapoptotic genes, including BAG3, BIRC2, and MCL-1, and down-regulation of proapoptotic genes, such as BAK1 and TP53/p53. Treatment with rifampicin in colon cancer LS180 cells, a cell line known to express endogenous PXR, also inhibited apoptosis. Activation of PXR in transgenic mice inhibited bile acid-induced colonic epithelial apoptosis and sensitized mice to dimethylhydrazine-induced colonic carcinogenesis, suggesting that the antiapoptotic effect of PXR is conserved in normal colon epithelium. In summary, our results have established the antiapoptotic role of PXR in both human colon cancer cells and normal mouse colon epithelium.

The phytoestrogen coumestrol is a naturally occurring antagonist of the human pregnane X receptor

Antagonizing the action of the human nuclear xenobiotic receptor pregnane X receptor (PXR) may have important clinical implications in preventing drug-drug interactions and improving therapeutic efficacy. We provide evidence that a naturally occurring phytoestrogen, coumestrol, is an antagonist of the nuclear receptor PXR (NR1I2). In transient transfection assays, coumestrol was able to suppress the agonist effects of SR12813 on human PXR activity. PXR activity was assessed and correlated with effects on the metabolism of the anesthetic tribromoethanol and on gene expression in primary human hepatocytes. We found that coumestrol was able to suppress the effects of PXR agonists on the expression of the known PXR target genes, CYP3A4 and CYP2B6, in primary human hepatocytes as well as inhibit metabolism of tribromoethanol in humanized PXR mice. Coumestrol at concentrations above 1.0 microm competed in scintillation proximity assays with a labeled PXR agonist for binding to the ligand-binding cavity. However, mammalian two-hybrid assays and transient transcription data using ligand-binding-cavity mutant forms of PXR show that coumestrol also antagonizes coregulator recruitment. This effect is likely by binding to a surface outside the ligand-binding pocket. Taken together, these data imply that there are antagonist binding site(s) for coumestrol on the surface of PXR. These studies provide the basis for development of novel small molecule inhibitors of PXR with the ultimate goal of clinical applications toward preventing drug-drug interactions.

Regulation of transporters by nuclear hormone receptors: implications during inflammation

Membrane transporters play a critical role in the absorption, distribution, and elimination of both endogenous substrates and xenobiotics. Defects in transporter function can lead to altered drug disposition including toxicity or loss of efficacy. Inflammation is one condition during which variable drug response has been demonstrated, and this can be attributed, at least in part, to changes in the expression of transporter genes. Thus, knowledge of the mechanisms behind transporter regulation can significantly contribute to our ability to predict variations in drug disposition among individuals and during inflammatory disease. The discovery of several xenobiotic-activated nuclear hormone receptors during the past decade including the pregnane X receptor, constitutive androstane receptor, and farnesoid X receptor has contributed greatly toward this endeavor. These receptors regulate the expression of transporters such as P-glycoprotein, MRP2, MRP3, BCRP, and OATP2 (Oatp1a1/OATP1B1), all of which undergo altered expression during an inflammatory response. Nuclear receptors may therefore play an important role in mediating this effect. This review presents what is currently known about the role of nuclear receptors in transporter regulation during inflammation. The use of this knowledge toward understanding interindividual variation in drug response and drug interactions during inflammation as well toward the development of therapeutics to treat transporter-related diseases will also be discussed.

PXR and LXR in hepatic steatosis: a new dog and an old dog with new tricks

PXR was isolated as a "xenobiotic receptor" that regulates drug-metabolizing enzymes and transporters, whereas LXR is known to promote hepatic lipogenesis by activating the lipogenic transcriptional factor sterol regulatory element-binding protein (SREBP). We have recently shown that PXR can mediate a SREBP-independent lipogenic pathway by activating the free fatty acid (FFA) uptake transporter CD36, PPARgamma, and several accessory lipogenic enzymes, such as stearoyl CoA desaturase-1 (SCD-1) and long-chain free fatty acid elongase (FAE). More recently, we found activation of LXR also induced the expression of CD36. Promoter analysis established CD36 as a novel transcriptional target of LXRalpha. Moreover, the steatotic effect of LXR agonists was largely abolished in CD36 null mice, suggesting an essential role for CD36 and FFA uptake in LXR-mediated steatosis. We also showed that PPARgamma, a positive regulator of CD36, is also a transcriptional target of PXR. Thus, PXR can regulate CD36 directly or through its activation of PPARgamma. Interestingly, PXR- and LXR-mediated CD36 activation and PXR-mediated PPARgamma activation are all liver-specific. We conclude that CD36 is a shared target of LXR, PXR, and PPARgamma. The network of CD36 regulation controlled by LXR, PXR, and PPARgamma establishes this FFA transporter as a common target of orphan nuclear receptors in their mediation of hepatic steatosis. It is hoped that the nuclear receptor-mediated CD36 regulation may offer novel targets for the therapeutic management of alcoholic and nonalcoholic steatosis.

Review article: The function and regulation of proteins involved in bile salt biosynthesis and transport

BACKGROUND

Bile salts are produced and secreted by the liver and are required for intestinal absorption of fatty food components and excretion of endobiotics and xenobiotics. They are reabsorbed in the terminal ileum and transported back to the liver via the portal tract. Dedicated bile salt transporters in hepatocytes and enterocytes are responsible for the unidirectional transport of bile salts in the enterohepatic cycle.

AIM

To give an overview of the function and regulations of proteins involved in bile salt synthesis and transport.

METHODS

Data presented are obtained from PubMed-accessible literature combined with our own recent research.

RESULT

Hepatocytes and enterocytes contain unique bile salt importers (sodium-taurocholate cotransporting polypeptide and apical sodium-dependent bile acid transporter, respectively) and exporters (bile salt export pump and organic solute transporter alpha-beta, respectively). Enzymes involved in bile salt biosynthesis reside in different subcellular locations, including the endoplasmic reticulum, mitochondria, cytosol and peroxisomes. Defective expression or function of the transporters or enzymes may lead to cholastasis. The bile salt-activated transcription factor Farnesoid X receptor controls expression of genes involved in bile salt biosynthesis and transport.

CONCLUSIONS

Detailed knowledge is available about the enzymes and transporters involved in bile salt homeostasis and how their defective function is associated with cholestasis. In contrast, the process of intracellular bile salt transport is largely unexplored.

Epigenetic effects of the continuous exposure to peroxisome proliferator WY-14,643 in mouse liver are dependent upon peroxisome proliferator activated receptor alpha

Peroxisome proliferators are potent rodent liver carcinogens that act via a non-genotoxic mechanism. The mode of action of these agents in rodent liver includes increased cell proliferation, decreased apoptosis, secondary oxidative stress and other events; however, it is not well understood how peroxisome proliferators are triggering the plethora of the molecular signals leading to cancer. Epigenetic changes have been implicated in the mechanism of liver carcinogenesis by a number of environmental agents. Short-term treatment with peroxisome proliferators and other non-genotoxic carcinogens leads to global and locus-specific DNA hypomethylation in mouse liver, events that were suggested to correlate with a burst of cell proliferation. In the current study, we investigated the effects of long-term exposure to a model peroxisome proliferator WY-14,643 on DNA and histone methylation. Male SV129mice were fed a control or WY-14,643-containing (1000ppm) diet for one week, five weeks or five months. Treatment with WY-14,643 led to progressive global hypomethylation of liver DNA as determined by an HpaII-based cytosine extension assay with the maximum effect reaching over 200% at five months. Likewise, trimethylation of histone H4 lysine 20 and H3 lysine 9 was significantly decreased at all time points. The majority of cytosine methylation in mammals resides in repetitive DNA sequences. In view of this, we measured the effect of WY-14,643 on the methylation status of major and minor satellites, as well as in IAP, LINE1 and LINE2 elements in liver DNA. Exposure to WY-14,643 resulted in a gradual loss of cytosine methylation in major and minor satellites, IAP, LINE1 and LINE2 elements. The epigenetic changes correlated with the temporal effects of WY-14,643 on cell proliferation rates in liver, but no sustained effect on c-Myc promoter methylation was observed. Finally, WY-14,643 had no effect on DNA and histone methylation status in Pparalpha-null mice at any of the time points considered in this study. These data indicate the importance of epigenetic alterations in the mechanism of action of peroxisome proliferators and the key role of Pparalpha.