Gene/Protein Disease Symptom Drug Enzyme Compound
Pivot Concepts:   Target Concepts:
Query: UMLS:C0008370 (cholestasis)
 9,378 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

Hepatic hydroxylation is an essential step in the metabolism and excretion of bile acids and is necessary to avoid pathologic conditions such as cholestasis and liver damage. In this report, we demonstrate that the human xenobiotic receptor SXR (steroid and xenobiotic receptor) and its rodent homolog PXR (pregnane X receptor) serve as functional bile acid receptors in both cultured cells and animals. In particular, the secondary bile acid derivative lithocholic acid (LCA) is highly hepatotoxic and, as we show here, a metabolic substrate for CYP3A hydroxylation. By using combinations of knockout and transgenic animals, we show that activation of SXR/PXR is necessary and sufficient to both induce CYP3A enzymes and confer resistance to toxicity by LCA, as well as other xenotoxicants such as tribromoethanol and zoxazolamine. Therefore, we establish SXR and PXR as bile acid receptors and a role for the xenobiotic response in the detoxification of bile acids.
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PMID:An essential role for nuclear receptors SXR/PXR in detoxification of cholestatic bile acids. 1124 86

The nuclear pregnane X receptor (PXR; NR1I2) is an integral component of the body's defense mechanism against chemical insult (chemoprotection). PXR is activated by a diverse array of lipophilic chemicals, including xenobiotics and endogenous substances, and regulates the expression of cytochromes P450, conjugating enzymes, and transporters involved in the metabolism and elimination of these potentially harmful chemicals from the body. Among the chemicals that bind and activate PXR is the toxic bile acid lithocholic acid; activation of PXR, in turn, protects against the severe liver damage caused by this bile acid.Thus, PXR serves as a physiological sensor of lithocholic acid and perhaps other bile acids and coordinately regulates genes involved in their detoxification. Interestingly, both the antibiotic rifampicin and the herbal antidepressant St. John's wort activate PXR and have anticholestatic properties, which suggests that more potent, selective PXR agonists may be useful in the treatment of biliary cholestasis or other diseases characterized by the accumulation of bile acids or other toxins in the liver.
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PMID:Regulation of xenobiotic and bile acid metabolism by the nuclear pregnane X receptor. 1189 71

The multidrug resistance-associated protein 2 (MRP2) is an ATP-binding cassette transporter involved in biliary, renal, and intestinal secretion of numerous organic anions, including endogenous compounds such as bilirubin and exogenous compounds such as drugs and toxic chemicals. Its expression can be modulated in various physiopathological situations, notably being markedly decreased during liver cholestasis and upregulated in some cancerous tissues. In addition, MRP2 levels are altered in hepatocytes in response to hormones such as glucocorticoids and to structurally unrelated drugs such as rifampicin, phenobarbital, ritonavir, and cisplatin. The chemical carcinogen 2-acetylaminofluorene and chemopreventive agents such as oltipraz and sulforaphane also markedly increased MRP2 expression in liver parenchymal cells. Interestingly, most of the chemical inducers of MRP2 act on drug-metabolizing enzymes, indicating a coordinated regulation of these detoxifying proteins; cellular mechanisms involved are, at least partly, common and may be related to nuclear hormone receptors such as the pregnane X receptor. Owing to the major role played by MRP2 in elimination of drugs and endogenous compounds, modulation of its expression may lead to adverse effects or to changes in drug pharmacokinetics.
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PMID:The drug efflux pump MRP2: regulation of expression in physiopathological situations and by endogenous and exogenous compounds. 1220 35

Recent studies have elucidated the mechanism and regulation of hepatic transport of bile acids and organic anions. Bile acids are taken up into hepatocytes by basolateral transporters, Na+-dependently by Na+/taurocholate cotransporting polypeptide (NTCP) and Na+-independently by organic anion transporting polypeptide (OATP) families. Organic anions are taken up into hepatocytes by OATP families. These compounds are then transported in hepatocytes bound to cytosolic binders, and subjected to transport by ATP binding cassette (ABC) transporters at the canalicular membrane. Amidated bile acids are excreted into bile by bile salt export pump (BSEP), and organic anions and bile acid sulfates and glucuronides are excreted by multidrug resistance protein 2 (MRP2). Hepatic transporters are downregulated under cholestasis in rats and humans, except for MRP3, a basolateral ABC transporter, which is upregulated and may have a role in removing bile acids and organic anions from hepatocytes to the blood under cholestatic conditions. Nuclear receptors, which bind bile acids, have been shown to regulate the expression of hepatic transporters. Farnesoid X receptor (FXR), which downregulates CYP7A1, the rate-limiting enzyme of bile acid biosynthesis, upregulates BSEP and downregulates NTCP. MRP2 is upregulated by both FXR and pregnane X receptor (PXR), which upregulates CYP3A.
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PMID:Hepatobiliary transport of bile acids and organic anions. 1248 66

The nuclear receptors, farnesoid X receptor (FXR) and pregnane X receptor (PXR), are important in maintaining bile acid homeostasis. Deletion of both FXR and PXR in vivo by cross-breeding B6;129-Fxrtm1Gonz (FXR-null) and B6;129-Pxrtm1Glaxo-Wellcome (PXR-null) mice revealed a more severe disruption of bile acid, cholesterol, and lipid homeostasis in B6;129-Fxrtm1Gonz Pxrtm1Glaxo-Wellcome (FXR-PXR double null or FPXR-null) mice fed a 1% cholic acid (CA) diet. Hepatic expression of the constitutive androstane receptor (CAR) and its target genes was induced in FXR- and FPXR-null mice fed the CA diet. To test whether up-regulation of CAR represents a means of protection against bile acid toxicity to compensate for the loss of FXR and PXR, animals were pretreated with CAR activators, phenobarbital or 1,4-bis[2-(3,5-dichlorpyridyloxy)]benzene (TCPOBOP), followed by the CA diet. A role for CAR in protection against bile acid toxicity was confirmed by a marked reduction of serum bile acid and bilirubin concentrations, with an elevation of the expression of the hepatic genes involved in bile acid and/or bilirubin metabolism and excretion (CYP2B, CYP3A, MRP2, MRP3, UGT1A, and glutathione S-transferase alpha), following pretreatment with phenobarbital or TCPOBOP. In summary, the current study demonstrates a critical and combined role of FXR and PXR in maintaining not only bile acid but also cholesterol and lipid homeostasis in vivo. Furthermore, FXR, PXR, and CAR protect against hepatic bile acid toxicity in a complementary manner, suggesting that they serve as redundant but distinct layers of defense to prevent overt hepatic damage by bile acids during cholestasis.
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PMID:Complementary roles of farnesoid X receptor, pregnane X receptor, and constitutive androstane receptor in protection against bile acid toxicity. 1292 73

The acute-phase response (APR) induces alterations in lipid metabolism, and our data suggest that this is associated with suppression of type II nuclear hormone receptors that are key regulators of fatty acid, cholesterol, and bile acid metabolism. Recently, the farnesoid X receptor (FXR), constitutive androstane receptor (CAR), and pregnane X receptor (PXR) were found to regulate DHEA sulfotransferase (Sult2A1), which plays an important role in DHEA sulfation and detoxification of bile acids. Because FXR, PXR, and CAR are suppressed during the APR, we hypothesized that Sult2A1 is downregulated during the APR. To induce the APR, mice were treated with LPS, which will then trigger the release of various cytokines, and the mRNA levels of Sult2A1 and the sulfate donor 3'-phosphoadenosine 5'-phosphosulfate synthase 2 (PAPSS2), as well as the enzyme activity of Sult2A1, were determined in the liver. We found that mRNA levels of Sult2A1 decrease in a time- and dose-dependent manner during the LPS-induced APR. Similar changes were observed in the mRNA levels of PAPSS2, the major synthase of PAPS in the liver. Moreover, hepatic Sult2A1 activity and serum levels of DHEA-sulfate (DHEA-S) were significantly decreased in LPS-treated animals. These results suggest that decreased levels or activities of FXR, PXR, and CAR during the APR could contribute to decreases in Sult2A1, resulting in decreased sulfation of DHEA and lower circulating level of DHEA-S. Finally, we found that both TNF and IL-1 caused a significant decrease in the mRNA level of Sult2A1 in Hep3B human hepatoma cells, suggesting that the proinflammatory cytokines TNF and IL-1 mediate the inhibitory effect of LPS on Sult2A1 mRNA level. Our study provides a possible mechanism by which infection and inflammation are associated with altered steroid metabolism and cholestasis.
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PMID:Suppression of DHEA sulfotransferase (Sult2A1) during the acute-phase response. 1519 32

Previous studies show that feedback inhibition of bile acid production by bile acids is mediated by multiple mechanisms, including activation of pregnane X receptor (PXR). Consistent with these studies, the antibiotic rifampicin, a ligand for human PXR, reduces hepatic bile acid levels in cholestasis patients. To delineate the mechanisms underlying PXR-mediated suppression of bile acid biosynthesis, we examined the functional cross-talk between human PXR and HNF-4, a key hepatic activator of genes involved in bile acid biosynthesis including the cholesterol 7-alpha hydroxylase (CYP7A1) and sterol 12-alpha hydroxylase (CYP8B1) genes. Treatment with rifampicin resulted in repression of endogenous human CYP7A1 expression in HepG2 cells that was reversed by PXR small interfering RNA. The coactivator PGC-1 enhanced transcriptional activity of HNF-4, and this enhancement was suppressed by rifampicin-activated PXR. Endogenous PGC-1 from mouse liver extracts bound to PXR, and recombinant PGC-1 directly interacted with both PXR and HNF-4 in vitro. Rifampicin-dependent interaction of PXR with PGC-1 was shown in cells by coimmunoprecipitation, and intranuclear localization studies using confocal microscopy provided further evidence for this interaction. In chromatin immunoprecipitation studies, rifampicin treatment did not inhibit HNF-4 binding to the native promoters of CYP7A1 and CYP8B1 but resulted in dissociation of PGC-1 and concomitant gene repression. Most interestingly, these rifampicin effects were also observed in the phosphoenolpyruvate carboxykinase gene that contains a functional HNF-4-binding site and is central to hepatic gluconeogenesis. Our study suggests that ligand-activated PXR interferes with HNF-4 signaling by targeting the common coactivator PGC-1, which underlies physiologically relevant inhibitory cross-talk between drug metabolism and cholesterol/glucose metabolism.
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PMID:Ligand-activated pregnane X receptor interferes with HNF-4 signaling by targeting a common coactivator PGC-1alpha. Functional implications in hepatic cholesterol and glucose metabolism. 1532 3

Bile acids, steroids, and drugs activate steroid and xenobiotic receptor pregnane X receptor (PXR; NR1I2), which induces human cytochrome P4503A4 (CYP3A4) in drug metabolism and cholesterol 7 alpha-hydroxylase (CYP7A1) in bile acid synthesis in the liver. Rifampicin, a human PXR agonist, inhibits bile acid synthesis and has been used to treat cholestatic diseases. The objective of this study is to elucidate the mechanism by which PXR inhibits CYP7A1 gene transcription. The mRNA expression levels of CYP7A1 and several nuclear receptors known to regulate the CYP7A1 gene were assayed in human primary hepatocytes by quantitative real-time PCR (Q-PCR). Rifampicin reduced CYP7A1 and small heterodimer partner (SHP; NR02B) mRNA expression suggesting that SHP was not involved in PXR inhibition of CYP7A1. Rifampicin inhibited CYP7A1 reporter activity and a PXR binding site was localized to the bile acid response element-I. Mammalian two-hybrid assays revealed that PXR interacted with hepatic nuclear factor 4 alpha (HNF4 alpha, NR2A1) and rifampicin was required. Coimmunoprecipitation assay confirmed PXR interaction with HNF4 alpha. PXR also interacted with peroxisome proliferator-activated receptor gamma coactivator (PGC-1 alpha), which interacted with HNF4 alpha and induced CYP7A1 gene transcription. Rifampicin enhanced PXR interaction with HNF4 alpha and reduced PGC-1 alpha interaction with HNF4 alpha. Chromatin immunoprecipitation assay showed that PXR, HNF4 alpha, and PGC-1 alpha bound to CYP7A1 chromatin, and rifampicin dissociated PGC-1 alpha from chromatin. These results suggest that activation of PXR by rifampicin promotes PXR interaction with HNF4 alpha and blocks PGC-1 alpha activation with HNF4 alpha and results in inhibition of CYP7A1 gene transcription. Rifampicin inhibition of bile acid synthesis may be a protective mechanism against drug and bile acid-induced cholestasis.
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PMID:Mechanism of rifampicin and pregnane X receptor inhibition of human cholesterol 7 alpha-hydroxylase gene transcription. 1533 48

CYP3A4, the most abundant cytochrome P450 in human liver, is responsible for the metabolism of numerous xenobiotics and endobiotics. CYP3A4 expression is highly variable and is induced by numerous compounds of exogenous and endogenous origin, including elevated concentrations of secondary bile acids via the pregnane X receptor (PXR). We show that physiological concentrations of the primary bile acid chenodeoxycholic acid regulate the expression of CYP3A4 via the bile acid receptor FXR. Experiments performed in vitro in different cell culture systems, gel-mobility shift assays and experiments performed in vivo in transgenic mice lacking FXR or PXR and treated with the synthetic FXR agonist GW4064 were undertaken to study the implication of FXR in the regulation of CYP3A. Our data provide evidence for the presence of two functional FXR recognition sites located in a 345-bp element within the 5'-flanking region of CYP3A4. Mutational analysis of these sites and experiments in transgenic mice lacking FXR or PXR support the relevance of FXR activation for CYP3A regulation. Thus, whereas elevated concentrations of precursors of bile acids and secondary bile acids induce CYP3A via PXR, primary bile acids can modulate the expression of CYP3A via FXR. These findings may explain elevated CYP3A expression in cholestasis and part of the variability of drug responsiveness and toxicity between individuals.
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PMID:Regulation of CYP3A4 by the bile acid receptor FXR: evidence for functional binding sites in the CYP3A4 gene. 1545 28

Cholestasis is associated with accumulation of bile acids and lipids, and liver injury. The constitutive androstane receptor (CAR) and pregnane X receptor (PXR) are xenobiotic nuclear receptors that coordinate protective hepatic responses to potentially toxic stimuli, including bile acids. We investigated the role of these receptors in the regulation of bile acid and lipid metabolism in a bile duct ligation (BDL) model of cholestasis applied to receptor knockout mice. Hepatic damage from bile acid accumulation was increased in both CAR knockout (CARKO) and PXR knockout mice, but bile acid concentrations were lower in CARKO mice. High-density lipoprotein (HDL) cholesterol was elevated in CARKO mice, and serum total cholesterol increased less in CARKO or PXR knockout mice than WT mice after BDL. Gene expression analysis of the BDL knockout animals demonstrated that, in response to cholestasis, PXR and CAR both repressed and induced the specific hepatic membrane transporters Oatp-c (organic anion transporting polypeptide C) and Oatp2 (Na+-dependent organic anion transporter 2), respectively. Induction of the xenobiotic transporter multidrug resistance protein 1 in cholestasis was independent of either PXR or CAR, in contrast to the known pattern of induction of multidrug resistance protein 1 by xenobiotics. These results demonstrate that CAR and PXR influence cholesterol metabolism and bile acid synthesis, as well as multiple detoxification pathways, and suggest their potential role as therapeutic targets for the treatment of cholestasis and lipid disorders.
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PMID:Nuclear receptors constitutive androstane receptor and pregnane X receptor ameliorate cholestatic liver injury. 1568 63


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