Rat pregnane X receptor: molecular cloning, tissue distribution, and xenobiotic regulation

Bile acids and neurological disease

This review will focus on how bile acids are being used in clinical trials to treat neurological diseases due to their central involvement with the gut-liver-brain axis and their physiological and pathophysiological roles in both normal brain function and multiple neurological diseases. The synthesis of primary and secondary bile acids species and how the regulation of the bile acid pool may differ between the gut and brain is discussed. The expression of several bile acid receptors in brain and their currently known functions along with the tools available to manipulate them pharmacologically are examined, together with discussion of the interaction of bile acids with the gut microbiome and their lesser-known effects upon brain glucose and lipid metabolism. How dysregulation of the gut microbiome, aging and sex differences may lead to disruption of bile acid signalling and possible causal roles in a number of neurological disorders are also considered. Finally, we discuss how pharmacological treatments targeting bile acid receptors are currently being tested in an array of clinical trials for several different neurodegenerative diseases.

Pregnane X receptor promotes liver enlargement in mice through the spatial induction of hepatocyte hypertrophy and proliferation

Nuclear receptor pregnane X receptor (PXR) can induce significant liver enlargement through hepatocyte hypertrophy and proliferation. A previous report showed that during the process of PXR-induced liver enlargement, hepatocyte hypertrophy occurs around the central vein (CV) area while hepatocyte proliferation occurs around the portal vein (PV) area. However, the features of this spatial change remain unclear. Therefore, this study aims to explore the features of the spatial changes in hepatocytes in PXR-induced liver enlargement. PXR-induced spatial changes in hepatocyte hypertrophy and proliferation were confirmed in C57BL/6 mice. The liver was perfused with digitonin to destroy the hepatocytes around the CV or PV areas, and then the regional expression of proteins related to hepatocyte hypertrophy and proliferation was further measured. The results showed that the expression of PXR downstream proteins, such as cytochrome P450 (CYP) 3A11, CYP2B10, P-glycoprotein (P-gp) and organ anion transporting polypeptide 2 (OATP2) was upregulated around the CV area, while the expression of proliferation-related proteins such as cyclin B1 (CCNB1), cyclin D1 (CCND1) and serine/threonine NIMA-related kinase 2 (NEK2) was upregulated around the PV area. At the same time, the expression of cyclin-dependent kinase inhibitors such as retinoblastoma-like protein 2 (RBL2), cyclin-dependent kinase inhibitor 1B (CDKN1B) and CDKN1A was downregulated around the PV area. This study demonstrated that the spatial change in PXR-induced hepatocyte hypertrophy and proliferation is associated with the regional expression of PXR downstream targets and proliferation-related proteins and the regional distribution of triglycerides (TGs). These findings provide new insight into the understanding of PXR-induced hepatomegaly.

Saikosaponins and the deglycosylated metabolites exert liver meridian guiding effect through PXR/CYP3A4 inhibition

ETHNOPHARMACOLOGICAL RELEVANCE

Radix Bupleuri (RB), traditionally used to treat inflammatory disorders and infectious diseases, represents one of the most successful and widely used herbal drugs in Asia over the past 2000 years. Being realized the role in regulating metabolism and controlling Yin/Yang, RB is not only chosen specifically for treating liver meridian and the corresponding organs, but also believed to have liver meridian guiding property and help potentiate the therapeutic effects of liver. However, the ingredients in RB with liver meridian guiding property and the underly mechanism have not been comprehensively investigated.

AIM OF STUDY

Considering the important role of CYP3A4 in first-pass metabolism and the liver exposure of drugs, the present study aimed to determine whether saikosaponins (SSs) and the corresponding saikogenins (SGs) have a role in inhibiting the catalytic activity of CYP3A4 in human liver microsomes and HepG2 hepatoma cells and whether they could suppress CYP3A4 expression by PXR-mediated pathways in HepG2 hepatoma cells.

MATERIALS AND METHODS

The effect of SSs and SGs on CYP3A4-mediated midazolam1'-hydroxylation activities in pooled human liver microsomes (HLMs) was first studied. Dose-dependent experiments were performed to obtain the half inhibit concentration (IC₅₀) values. HepG2 cells were used to assay catalytic activity of CYP3A4, reporter function, mRNA levels, and protein expression. The inhibitory effects of SSa and SSd on CYP3A4 activity are negligible, while the corresponding SGs (SGF and SGG) have obvious inhibitory effects on CYP3A4 activity, with IC₅₀ values of 0.45 and 1.30 μM. The similar results were obtained from testing CYP3A4 catalytic activity in HepG2 cells, which correlated well with the suppression of the mRNA and protein levels of CYP3A4. Time-dependent testing of CYP3A4 mRNA and protein levels, as well as co-transfection experiments using the CYP3A4 promoter luciferase plasmid, further confirmed that SSs and SGs could inhibit the expression of CYP3A4 at the transcription level. Furthermore, PXR protein expression decreased in a concentration- and time-dependent manner after cells were exposed to SSs and SGs. PXR overexpression and RNA interference experiments further showed that SSs and SGs down-regulate the catalytic activity and expression of CYP3A4 in HepG2 may be mainly through PXR-dependent manner.

CONCLUSION

SSs and SGs inhibit the catalytic activity and expression of CYP3A4 in a PXR-dependent manner, which may be highly related to the liver meridian guiding property of RB.

Regulation of CAR and PXR Expression in Health and Disease

Pregnane X receptor (PXR, NR1I2) and constitutive androstane receptor (CAR, NR1I3) are members of the nuclear receptor superfamily that mainly act as ligand-activated transcription factors. Their functions have long been associated with the regulation of drug metabolism and disposition, and it is now well established that they are implicated in physiological and pathological conditions. Considerable efforts have been made to understand the regulation of their activity by their cognate ligand; however, additional regulatory mechanisms, among which the regulation of their expression, modulate their pleiotropic effects. This review summarizes the current knowledge on CAR and PXR expression during development and adult life; tissue distribution; spatial, temporal, and metabolic regulations; as well as in pathological situations, including chronic diseases and cancers. The expression of CAR and PXR is modulated by complex regulatory mechanisms that involve the interplay of transcription factors and also post-transcriptional and epigenetic modifications. Moreover, many environmental stimuli affect CAR and PXR expression through mechanisms that have not been elucidated.

Dechlorination and demethylation of ochratoxin A enhance blocking activity of PXR activation, suppress PXR expression and reduce cytotoxicity

The pregnane X receptor (PXR) has been established to induce chemoresistance and metabolic diseases. Ochratoxin A (OTA), a mycotoxin, decreases the expression of PXR protein in human primary hepatocytes. OTA is chlorinated and has a methylated lactone ring. Both structures are associated with OTA toxicity. The study was to test the hypothesis that structural modifications differentially impact PXR blocking activity over cytotoxicity. To test this hypothesis, OTA-M and OTA-Cl/M were synthesized. OTA-M lacked the methyl group of the lactone-ring, whereas OTA-Cl/M had neither the methyl group nor the chlorine atom. The blocking activity of PXR activation was determined in a stable cell line, harboring both PXR (coding sequence) and its luciferase element reporter. OTA-Cl/M showed the highest blocking activity, followed by OTA-M and OTA. OTA-Cl/M was 60 times as potent as the common PXR blocker ketoconazole based on calculated IC₅₀ values. OTA-Cl/M decreased by 90 % the expression of PXR protein and was the least cytotoxic among the tested compounds. Molecular docking identified that OTA and its derivatives interacted with different sets of residues in PXR, providing a molecular basis for selectivity. Excessive activation of PXR has been implicated in chemoresistance and metabolic diseases. Downregulation of PXR protein expression likely delivers an effective mechanism against structurally diverse PXR agonists.

4β-Hydroxycholesterol Signals From the Liver to Regulate Peripheral Cholesterol Transporters

Activation of pregnane X receptor (PXR) elevates circulating 4β-hydroxycholesterol (4βHC), an agonist of liver X receptor (LXR). PXR may also regulate 25-hydroxycholesterol and 27-hydroxycholesterol. Our aim was to elucidate the roles of PXR and oxysterols in the regulation of cholesterol transporters. We measured oxysterols in serum of volunteers dosed with PXR agonist rifampicin 600 mg/day versus placebo for a week and analyzed the expression of cholesterol transporters in mononuclear cells. The effect of 4βHC on the transport of cholesterol and the expression of cholesterol transporters was studied in human primary monocyte-derived macrophages and foam cells in vitro. The expression of cholesterol transporters was measured also in rat tissues after dosing with a PXR agonist. The levels of 4βHC were elevated, while 25-hydroxycholesterol and 27-hydroxycholesterol remained unchanged in volunteers dosed with rifampicin. The expression of ATP binding cassette transporter A1 (ABCA1) was induced in human mononuclear cells in vivo. The influx of cholesterol was repressed by 4βHC, as was the expression of influx transporter lectin-like oxidized LDL receptor-1 in vitro. The cholesterol efflux and the expression of efflux transporters ABCA1 and ABCG1 were induced. The expression of inducible degrader of the LDL receptor was induced. In rats, PXR agonist increased circulating 4βHC and expression of LXR targets in peripheral tissues, especially ABCA1 and ABCG1 in heart. In conclusion, PXR activation-elevated 4βHC is a signaling molecule that represses cholesterol influx and induces efflux. The PXR-4βHC-LXR pathway could link the hepatic xenobiotic exposure and the regulation of cholesterol transport in peripheral tissues.

Influence of St. John's Wort on Intravenous Fentanyl Pharmacokinetics, Pharmacodynamics, and Clinical Effects: A Randomized Clinical Trial

BACKGROUND

Patients often use complementary and alternative herbal medicines, hence, potential exists for adverse herb-drug interactions. Fentanyl is metabolized by hepatic CYP3A4 and considered transported by blood-brain barrier P-glycoprotein. Both disposition processes could be upregulated by the herbal St. John's wort. This investigation evaluated effects of St. John's wort on fixed-dose and apparent steady-state IV fentanyl pharmacokinetics, pharmacodynamics, and clinical effects.

METHODS

Healthy volunteers received a fentanyl fixed-dose infusion and an individually tailored target controlled infusion on separate days, before and after 30-day St. John's wort (300 mg thrice daily; n = 8) or placebo control (n = 8) in a randomized parallel-group design. Fentanyl plasma concentrations, pupil diameter, analgesic response to experimental pain (cold pressor), subjective side effects, and cognitive effects were measured. Plasma fentanyl concentrations and changes in pupil diameter were subjected to pharmacokinetic-pharmacodynamic modeling.

RESULTS

St. John's wort did not alter fentanyl pharmacokinetics. Clearance (l/min) before and after St. John's wort (1.13 ± 0.29 and 1.24 ± 0.26, respectively) or placebo (0.96 ± 0.28 and 1.12 ± 0.27, respectively) were not different. St. John's wort also did not affect fentanyl pharmacodynamics as measured by pupil constriction after fixed-dose and tailored fentanyl infusions. EC50 (ng/ml) was 1.1 ± 0.7 and 1.4 ± 0.9 before and after St. John's wort versus 1.2 ± 0.8 and 1.4 ± 1.7 before and after placebo. Effect site equilibration time, T½,ke0 (min), was 12.8 ± 5.3 and 11.3 ± 6.4 before and after St. John's wort versus 11.4 ± 6.4 and 11.1 ± 5.6 before and after placebo. St. John's wort had no influence on analgesia, cognitive performance, or somatic cognitive-affective effects of fentanyl.

CONCLUSIONS

St. John's wort did not alter fentanyl pharmacokinetics, pharmacodynamics or clinical effects, suggesting no effect on hepatic clearance or blood-brain barrier efflux. Patients taking St. John's wort will likely not respond differently to IV fentanyl for anesthesia or analgesia.

Bile Acid-Activated Receptors: A Review on FXR and Other Nuclear Receptors

Nuclear receptors (NRs) are ligand-dependent transcription factors that are involved in various biological processes including metabolism, reproduction, and development. Upon activation by their ligands, NRs bind to their specific DNA elements, exerting their biological functions by regulating their target gene expression. Bile acids are detergent-like molecules that are synthesized in the liver. They not only function as a facilitator for the digestion of lipids and fat-soluble vitamins but also serve as signaling molecules for several nuclear receptors to regulate diverse biological processes including lipid, glucose, and energy metabolism, detoxification and drug metabolism, liver regeneration, and cancer. The nuclear receptors including farnesoid X receptor (FXR), pregnane X receptor (PXR), constitutive androstane receptor (CAR), vitamin D receptor (VDR), and small heterodimer partner (SHP) constitute an integral part of the bile acid signaling. This chapter reviews the role of the NRs in bile acid homeostasis, highlighting the regulatory functions of the NRs in lipid and glucose metabolism in addition to bile acid metabolism.

RNA interference screen identifies NAA10 as a regulator of PXR transcription

The pregnane X receptor (PXR) is a principal xenobiotic receptor crucial in the detection, detoxification, and clearance of toxic substances from the body. PXR plays a vital role in the metabolism and disposition of drugs, and elevated PXR levels contribute to cancer drug resistance. Therefore, to modulate PXR activity and mitigate drug resistance, it is imperative to fully understand its regulation. To this end, we screened a transcription factor siRNA library in pancreatic cancer cells that express high levels of PXR. Through a comprehensive deconvolution process, we identified N-alpha-acetyltransferase 10 (NAA10) as a factor in the transcriptional machinery regulating PXR transcription. Because no one single factor has 100% operational control of PXR transcriptional regulation, our results together with other previous findings suggest that the transcriptional regulation of PXR is complex and that multiple factors contribute to the process including NAA10.

Epigenetic impact of endocrine disrupting chemicals on lipid homeostasis and atherosclerosis: a pregnane X receptor-centric view

Despite the major advances in developing diagnostic techniques and effective treatments, atherosclerotic cardiovascular disease (CVD) is still the leading cause of mortality and morbidity worldwide. While considerable progress has been achieved to identify gene variations and environmental factors that contribute to CVD, much less is known about the role of "gene-environment interactions" in predisposing individuals to CVD. Our chemical environment has significantly changed in the last few decades, and there are more than 100,000 synthetic chemicals in the market. Recent large-scale human population studies have associated exposure to certain chemicals including many endocrine disrupting chemicals (EDCs) with increased CVD risk, and animal studies have also confirmed that some EDCs can cause aberrant lipid homeostasis and increase atherosclerosis. However, the underlying mechanisms of how exposure to those EDCs influences CVD risk remain elusive. Numerous EDCs can activate the nuclear receptor pregnane X receptor (PXR) that functions as a xenobiotic sensor to regulate host xenobiotic metabolism. Recent studies have demonstrated the novel functions of PXR in lipid homeostasis and atherosclerosis. In addition to directly regulating transcription, PXR has been implicated in the epigenetic regulation of gene transcription. Exposure to many EDCs can also induce epigenetic modifications, but little is known about how the changes relate to the onset or progression of CVD. In this review, we will discuss recent research on PXR and EDCs in the context of CVD and propose that PXR may play a previously unrealized role in EDC-mediated epigenetic modifications that affect lipid homeostasis and atherosclerosis.

Suppression of carboxylesterases by imatinib mediated by the down-regulation of pregnane X receptor

BACKGROUND AND PURPOSE

Imatinib mesylate (IM) is a first-line treatment for chronic myeloid leukaemia (CML) as a specific inhibitor of BCR-ABL tyrosine kinase. As IM is widely used in CML, in combination with other drugs, the effects of IM on drug-metabolizing enzymes (DMEs) are crucial to the design of rational drug administration. Carboxylesterases (CESs) are enzymes catalysing the hydrolytic biotransformation of several clinically useful drugs. Although IM is known to inhibit cytochromes P450 (CYPs), its effects on DMEs, and CESs in particular, are still largely undefined.

EXPERIMENTAL APPROACH

Hepatoma cell lines (HepG2 and Huh7) and primary mouse hepatocytes were used. mRNA and protein expression were evaluated by quantitative RT-PCR and Western blot analysis. Reporter luciferase activity was determined by transient co-transfection experiment. Pregnane X receptor (PXR) expression was regulated by overexpression and RNA interference. The activity of CESs was determined by enzymic and toxicological assays. Mice were treated with a range of doses of IM to analyse expression of CESs in mouse liver.

KEY RESULTS

The expression and activity of CESs were markedly repressed by IM, along with the down-regulation of PXR and inhibited expression and activity of CYP3A4 and P-gp.

CONCLUSIONS AND IMPLICATIONS

Down-regulation of PXR mediates IM-induced suppression of CESs. IM may inhibit expression of other genes targeted by PXR, thus inducing a wide range of potential drug-drug interactions during treatment of CML. The data deserve further elucidation including clinical trials.

Inhibition of pregnane X receptor pathway contributes to the cell growth inhibition and apoptosis of anticancer agents in ovarian cancer cells

Epithelial ovarian cancer remains the most devastating gynecologic cancer with drug resistance and rapid recurrence. Pregnane X receptor (PXR) is a nuclear receptor that affects drug metabolism/efflux and drug-drug interaction through control of multiple drug resistance 1 (MDR1), which implies a major role in multidrug resistance, and other genes. We examined whether the inhibition of PXR-mediated pathway using siRNA interference and an antagonist for PXR could influence the paclitaxel and cisplatin cytotoxicity in ovarian cancer cells. PXR agonists, phthalate and pregnenolone had significant positive effects on cytochrome P450 (CYP) 3A4 expression and PXR-mediated transcription through the CYP3A4 promoter, whereas MDR1 expression and PXR-mediated transcription though the MDR1 promoter were significantly increased in the presence of paclitaxel or cisplatin. Downregulation of PXR suppressed the augmented MDR1 expression and PXR-mediated transcription by PXR ligands, and significantly enhanced cell growth inhibition and apoptosis in the presence of paclitaxel or cisplatin. Additionally, ketoconazole, a PXR antagonist, suppressed the augmented MDR1 expression and PXR-mediated transactivation by paclitaxel and cisplatin, and enhanced cell growth inhibition and apoptosis in their presence. In conclusion, inhibition of PXR-mediated pathways could be a novel means of augmenting sensitivity, or overcoming resistance to anticancer agents for ovarian cancer.

MicroRNA-30c-1-3p is a silencer of the pregnane X receptor by targeting the 3'-untranslated region and alters the expression of its target gene cytochrome P450 3A4

The pregnane X receptor (PXR) is a master regulator of genes involved in drug elimination. Recently, activation of PXR has also been linked to the development of many disease conditions such as metabolic disorders and malignancies. MicroRNAs (miRs) emerge as important molecular species involved in these conditions. This study was undertaken to test a large number of miRs for their ability to regulate PXR expression. As many as 58 miRs were tested and miR-30c-1-3p was identified to suppress PXR expression. The suppression was achieved by targeting the 3'-untranslated region, 438 nucleotides from the stop codon. The suppression was detected in multiple cell lines from different organ origins. In addition, miR-30c-1-3p altered basal and induced expression of cytochrome P450 3A4 (CYP3A4), a prototypical target gene of PXR. The alteration varied depending on the time and amounts of miR-30c-1-3p. CYP3A4 is responsible for the metabolism of more than 50% medicines. The interconnection between miR-30c-1-3p and PXR signifies a role of miRs in drug-drug interactions and chemosensitivity. This article is part of a Special Issue entitled: Xenobiotic nuclear receptors: New Tricks for An Old Dog, edited by Dr. Wen Xie.

The pregnane X receptor in tuberculosis therapeutics

INTRODUCTION

Among the infectious diseases, tuberculosis (TB) remains the second most common cause of death after HIV. TB treatment requires the combination of multiple drugs including the rifamycin class. However, rifamycins are activators of human pregnane X receptor (PXR), a transcription factor that regulates drug metabolism, drug resistance, energy metabolism and immune response. Rifamycin-mediated PXR activation may affect the outcome of TB therapy.

AREAS COVERED

This review describes the role of PXR in modulating metabolism, efficacy, toxicity and resistance to anti-TB drugs; as well as polymorphisms of PXR that potentially affect TB susceptibility.

EXPERT OPINION

The wide range of PXR functions that mediate drug metabolism and toxicity in TB therapy are often underappreciated and thus understudied. Further studies are needed to determine the overall impact of PXR activation on the outcome of TB therapy.

Role of pregnane X receptor and aryl hydrocarbon receptor in transcriptional regulation of pxr, CYP2, and CYP3 genes in developing zebrafish

Ligand-activated receptors regulate numerous genes, and mediate effects of a broad set of endogenous and exogenous chemicals in vertebrates. Understanding the roles of these transcription factors in zebrafish (Danio rerio) is important to the use of this non-mammalian model in toxicological, pharmacological, and carcinogenesis research. Response to a potential agonist for the pregnane X receptor (Pxr) [pregnenolone (PN)] was examined in developing zebrafish, to assess involvement of Pxr in regulation of selected genes, including genes in cytochrome P450 subfamilies CYP2 and CYP3. We also examined interaction of Pxr and the aryl hydrocarbon receptor (Ahr) signaling pathways. Pregnenolone caused a dose-dependent increase in mRNA levels of pxr, ahr2, CYP1A, CYP2AA1, CYP2AA12, CYP3A65, and CYP3C1, most of which peaked at 3 µM PN. The well-known Ahr agonist 3,3',4,4',5-pentachlorobiphenyl (PCB126) also upregulated expression of pxr, ahr2, CYP1A, CYP2AA12, CYP3A65, and CYP3C1 in a dose-dependent manner. Inhibition of pxr translation by morpholino antisense oligonucleotides (MO) suppressed PN-induced expression of pxr, ahr2, CYP3A65, and CYP3C1 genes. Levels of CYP2AA1 and CYP2AA12 mRNA were increased in the control-MO group exposed to PN; this was prevented by knocking down Pxr. Similarly, Ahr2-MO treatment blocked PCB126-induced mRNA expression of pxr, CYP1A, CYP2AA12, CYP3A65, and CYP3C1. The present study shows self-regulation of pxr by PN in developing zebrafish. Selected zebrafish CYP1, CYP2 (including several CYP2AAs) and CYP3 genes appear to be under the regulation of both Pxr and Ahr2.

Gambogic acid suppresses cytochrome P450 3A4 by downregulating pregnane X receptor and up-regulating DEC1 in human hepatoma HepG2 cells

Gambogic acid suppresses cytochrome P450 3A4 by downregulating pregnane X receptor and up-regulating DEC1 in human hepatoma HepG2 cells.

Chronic alcoholism-mediated metabolic disorders in albino rat testes

There is good evidence for impairment of spermatogenesis and reductions in sperm counts and testosterone levels in chronic alcoholics. The mechanisms for these effects have not yet been studied in detail. The consequences of chronic alcohol consumption on the structure and/or metabolism of testis cell macromolecules require to be intensively investigated. The present work reports the effects of chronic alcoholism on contents of free amino acids, levels of cytochrome P450 3A2 (CYP3A2) mRNA expression and DNA fragmentation, as well as on contents of different cholesterol fractions and protein thiol groups in rat testes. Wistar albino male rats were divided into two groups: I - control (intact animals), II - chronic alcoholism (15% ethanol self-administration during 150 days). Following 150 days of alcohol consumption, testicular free amino acid content was found to be significantly changed as compared with control. The most profound changes were registered for contents of lysine (-53%) and methionine (+133%). The intensity of DNA fragmentation in alcohol-treated rat testes was considerably increased, on the contrary CYP3A2 mRNA expression in testis cells was inhibited, testicular contents of total and etherified cholesterol increased by 25% and 45% respectively, and protein SH-groups decreased by 13%. Multidirectional changes of the activities of testicular dehydrogenases were detected. We thus obtained complex assessment of chronic alcoholism effects in male gonads, affecting especially amino acid, protein, ATP and NADPH metabolism. Our results demonstrated profound changes in testes on the level of proteome and genome. We suggest that the revealed metabolic disorders can have negative implication on cellular regulation of spermatogenesis under long-term ethanol exposure.

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

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

Pregnane xenobiotic receptors and their effect on drug elimination from the organism

Nuclear receptors are intracellular proteins which, having been activated by their more or less specific ligands, regulate (usually increase) the transcription of target genes. They thus participate in a regulation of a number of physiologic functions. Some of them - especially pregnane xenobiotic receptors - serve primarily as protection of the organism from the xenobiotic intoxication. This is because many xenobiotics activate their function which consists in increasing the gene expression of enzymes involved in the metabolism of xenobiotics and detoxication drug transporters. Clarification of these mechanisms enabled the understanding of the substance of many drug-drug interactions observed in the clinical practice. Polymorphism of the nuclear receptors appears to be one of the causes of the interindividual variability in response to drug administration.

Bile acid receptors as targets for drug development

The intracellular nuclear receptor farnesoid X receptor and the transmembrane G protein-coupled receptor TGR5 respond to bile acids by activating transcriptional networks and/or signalling cascades. These cascades affect the expression of a great number of target genes relevant for bile acid, cholesterol, lipid and carbohydrate metabolism, as well as genes involved in inflammation, fibrosis and carcinogenesis. Pregnane X receptor, vitamin D receptor and constitutive androstane receptor are additional nuclear receptors that respond to bile acids, albeit to a more restricted set of species of bile acids. Recognition of dedicated bile acid receptors prompted the development of semi-synthetic bile acid analogues and nonsteroidal compounds that target these receptors. These agents hold promise to become a new class of drugs for the treatment of chronic liver disease, hepatocellular cancer and extrahepatic inflammatory and metabolic diseases. This Review discusses the relevant bile acid receptors, the new drugs that target bile acid signalling and their possible applications.

Role of pregnane X receptor in obesity and glucose homeostasis in male mice

Clinical obesity is a complex metabolic disorder affecting one in three adults. Recent reports suggest that pregnane X receptor (PXR), a xenobiotic nuclear receptor important for defense against toxic agents and for eliminating drugs and other xenobiotics, may be involved in obesity. Noting differences in ligand specificities between human and mouse PXRs, the role of PXR in high fat diet (HFD)-induced obesity was examined using male PXR-humanized (hPXR) transgenic and PXR-knock-out (PXR-KO) mice in comparison to wild-type (WT) mice. After 16 weeks on either a control diet or HFD, WT mice showed greater weight gain, whereas PXR-KO mice gained less weight due to their resistance to HFD-induced decreases in adipose tissue peroxisome proliferator-activated receptor α and induction of hepatic carnitine palmitoyltransferase 1, suggesting increased energy metabolism. Interestingly, control-fed PXR-KO mice exhibited hepatomegaly, hyperinsulinemia, and hyperleptinemia but hypoadiponectinemia and lower adiponectin receptor R2 mRNA levels relative to WT mice. Evaluation of these biologic indicators in hPXR mice fed a control diet or HFD revealed further differences between the mouse and human receptors. Importantly, although HFD-fed hPXR mice were resistant to HFD-induced obesity, both PXR-KO and hPXR mice exhibited impaired induction of glucokinase involved in glucose utilization and displayed elevated fasting glucose levels and severely impaired glucose tolerance. Moreover, the basal hepatic levels of the gluconeogenic enzyme phosphoenolpyruvate carboxykinase 1 were increased in hPXR mice compared with WT mice. Altogether, although the mouse PXR promotes HFD-induced obesity, the hPXR mouse carries a genetic predisposition for type 2 diabetes and thus provides a model for exploring the role of human PXR in the metabolic syndrome.

Functional characterization of a full length pregnane X receptor, expression in vivo, and identification of PXR alleles, in zebrafish (Danio rerio)

The pregnane X receptor (PXR) (nuclear receptor NR1I2) is a ligand activated transcription factor, mediating responses to diverse xenobiotic and endogenous chemicals. The properties of PXR in fish are not fully understood. Here we report on cloning and characterization of full-length PXR of zebrafish, Danio rerio, and pxr expression in vivo. Initial efforts gave a cDNA encoding a 430 amino acid protein identified as zebrafish pxr by phylogenetic and synteny analysis. The sequence of the cloned Pxr DNA binding domain (DBD) was highly conserved, with 74% identity to human PXR-DBD, while the ligand-binding domain (LBD) of the cloned sequence was only 44% identical to human PXR-LBD. Sequence variation among clones in the initial effort prompted sequencing of multiple clones from a single fish. There were two prominent variants, one sequence with S183, Y218 and H383 and the other with I183, C218 and N383, which we designate as alleles pxr*1 (nr1i2*1) and pxr*2 (nr1i2*2), respectively. In COS-7 cells co-transfected with a PXR-responsive reporter gene, the full-length Pxr*1 (the more common variant) was activated by known PXR agonists clotrimazole and pregnenolone 16α-carbonitrile but to a lesser extent than the full-length human PXR. Activation of full-length Pxr*1 was only 10% of that with the Pxr*1 LBD. Quantitative real time PCR analysis showed prominent expression of pxr in liver and eye, as well as brain and intestine of adult zebrafish. The pxr was expressed in heart and kidney at levels similar to that in intestine. The expression of pxr in liver was weakly induced by ligands for mammalian PXR or constitutive androstane receptor (NR1I3). The results establish a foundation for PXR studies in this vertebrate model. PXR allelic variation and the differences between the full-length PXR and the LBD in reporter assays have implications for assessing the action of PXR ligands in zebrafish.

Sulfotransferase genes: regulation by nuclear receptors in response to xeno/endo-biotics

Pregnane X receptor (PXR) and constitutive active/androstane receptor (CAR), members of the nuclear receptor superfamily, are two major xeno-sensing transcription factors. They can be activated by a broad range of lipophilic xenobiotics including therapeutics drugs. In addition to xenobiotics, endogenous compounds such as steroid hormones and bile acids can also activate PXR and/or CAR. These nuclear receptors regulate genes that encode enzymes and transporters that metabolize and excrete both xenobiotics and endobiotics. Sulfotransferases (SULTs) are a group of these enzymes and sulfate xenobiotics for detoxification. In general, inactivation by sulfation constitutes the mechanism to maintain homeostasis of endobiotics. Thus, deciphering the molecular mechanism by which PXR and CAR regulate SULT genes is critical for understanding the roles of SULTs in the alterations of physiological and pathophysiological processes caused by drug treatment or environmental exposures.

Expression of the PXR gene in various types of cancer and drug resistance

Pregnane X receptor (PXR) is a member of the nuclear receptor superfamily of ligand-regulated transcription factors. PXR is a key xenobiotic receptor that regulates the expression of genes implicated in drug metabolism, detoxification and clearance, including drug metabolizing enzymes and transporters, suggesting that it is significant in the drug resistance of cancer cells. PXR is expressed in a wide range of tissues in the human body. Studies have demonstrated that PXR is expressed in a variety of tumor types, correlating not only with drug resistance but also with the cell proliferation, apoptosis and prognosis of cancer. The purpose of the present review is to provide a comprehensive review of PXR and its potential roles in multidrug resistance and the biological characteristics of PXR-positive tumors.

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.

Constitutive expression and phenobarbital modulation of drug metabolizing enzymes and related nuclear receptors in cattle liver and extra-hepatic tissues

In humans and rodents, phenobarbital (PB) induces hepatic and extra-hepatic drug metabolizing enzymes (DMEs) through the activation of specific nuclear receptors (NRs). In contrast, few data about PB transcriptional effects in veterinary species are available. The constitutive expression and modulation of PB-responsive NR and DME genes, following an oral PB challenge, were investigated in cattle liver and extra-hepatic tissues (duodenum, kidney, lung, testis, adrenal and muscle). Likewise to humans and rodents, target genes were expressed to a lower extent compared to the liver with few exceptions. Phenobarbital significantly affected hepatic CYP2B22, 2C31, 2C87, 3A and UDP-glucuronosyltransferase 1A1-like, glutathione S-transferase A1-like and sulfotransferase 1A1-like (SULT1A1-like) mRNAs and apoprotein amounts; in extra-hepatic tissues, only duodenum showed a significant down-regulation of SULT1A1-like gene and apoprotein. Nuclear receptor mRNAs were never affected by PB. Presented data are the first evidence about the constitutive expression of foremost DME and NR genes in cattle extra-hepatic tissues, and the data obtained following a PB challenge are suggestive of species-differences in drug metabolism; altogether, these information are of value for the extrapolation of pharmacotoxicological data among species, the characterization of drug-drug interactions as well as the animal and consumer's risk caused by harmful residues formation.

Effects of pharmaceuticals on the expression of genes involved in detoxification in a carp primary hepatocyte model

Fish in many surface freshwaters are exposed to a range of pharmaceuticals via wastewater treatment works effluent discharges. In mammals the pregnane X receptor (PXR) plays a key role in the regulation of a suite of genes involved in drug biotransformation, but information on the role of this response pathway in fish is limited. Here we investigated the effects of exposure of carp (Cyprinus carpio) primary hepatocytes to the human PXR agonist rifampicin (RIF) on expression of target genes involved in phase I (cyp2k, cyp3a) and phase II (gstα, gstπ) drug metabolism and drug transporters mdr1 and mrp2. RIF induced expression of all target genes measured and the PXR antagonist ketoconazole (KET) inhibited responses of cyp2k and cyp3a. Exposure of the primary carp hepatocytes to the pharmaceuticals ibuprofen (IBU), clotrimazole (CTZ), clofibric acid (CFA) and propranolol (PRP), found responses to IBU and CFA, but not CTZ or PRP. This is in contrast with mammals, where CTZ is a potent PXR-agonist. Collectively our data indicate potential PXR involvement in regulating selected genes involved in drug metabolism in fish, but suggest some divergence in the regulation pathways with those in mammals. The carp primary hepatocyte model serves as a useful system for screening for responses in these target genes involved in drug metabolism.

The role of nuclear receptors in the kidney in obesity and metabolic syndrome

Nuclear receptors are ligand-activated transcriptional regulators of several key aspects of renal physiology and pathophysiology. As such, nuclear receptors control a large variety of metabolic processes, including kidney lipid metabolism, drug clearance, inflammation, fibrosis, cell differentiation, and oxidative stress. Derangement of nuclear receptor regulation, that is, mainly due to obesity may induce metabolic syndrome, may contribute to the pathogenesis and progression of chronic renal disease and may result in end-stage renal disease. This places nuclear receptors at the forefront of novel therapeutic approaches for a broad range of kidney disorders and diseases, including glomerulosclerosis, tubulointerstitial disease, renal lipotoxicity, kidney fibrosis, and hypertension. This review focuses on the importance of the transcription factors peroxisome proliferator-activated receptor alpha, peroxisome proliferator-activated receptor beta, peroxisome proliferator-activated receptor gamma, liver X receptors, farnesoid X receptor, and the pregnane X receptor/steroid and xenobiotic receptor (PXR) on the physiology and pathophysiology of renal diseases associated with obesity and metabolic syndrome.

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.

Age-related inducibility of carboxylesterases by the antiepileptic agent phenobarbital and implications in drug metabolism and lipid accumulation

Carboxylesterases (CES) constitute a class of hydrolytic enzymes that play critical roles in drug metabolism and lipid mobilization. Previous studies with a large number of human liver samples have suggested that the inducibility of carboxylesterases is inversely related with age. To directly test this possibility, neonatal (10 days of age) and adult mice were treated with the antiepileptic agent phenobarbital. The expression and hydrolytic activity were determined on six major carboxylesterases including ces1d, the ortholog of human CES1. Without exception, all carboxylesterases tested were induced to a greater extent in neonatal than adult mice. The induction was detected at mRNA, protein and catalytic levels. Ces1d was greatly induced and found to rapidly hydrolyze the antiplatelet agent clopidogrel and support the accumulation of neutral lipids. Phenobarbital represents a large number of therapeutic agents that induce drug metabolizing enzymes and transporters in a species-conserved manner. The higher inducibility of carboxylesterases in the developmental age likely represents a general phenomenon cross species including human. Consequently, individuals in the developmental age may experience greater drug-drug interactions. The greater induction of ces1d also provides a molecular explanation to the clinical observation that children on antiepileptic drugs increase plasma lipids.

Species difference in the regulation of cytochrome P450 2S1: lack of induction in rats by the aryl hydrocarbon receptor agonist PCB126

CYP2S1 is an evolutionarily conserved, mainly extra-hepatic member of the CYP2 family and proposed to be regulated by the aryl hydrocarbon receptor (AhR). The present study explores AhR's regulation of CYP2S1 in male Sprague Dawley rats using PCB126 (3,3',4,4',5-pentachlorobiphenyl), the most potent AhR agonist among the PCBs. Additionally, CYP2S1 expression was examined after treatments with the classic CYP-inducers β-naphthoflavone (β-NF, AhR activator), phenobarbital (PB, CAR activator) and dexamethasone (Dex, PXR activator). CYP2S1 and CYP1A1/2, CYP1B1, CYP2B and CYP3A mRNAs were measured in liver, lung, spleen, stomach, kidney, and thymus at different time points. Constitutive CYP2S1 was expressed at comparable levels to other CYPs with the highest expression levels in stomach, kidney and lung. CYP2S1 mRNA was only non-significantly elevated by β-NF in liver tissues. PCB126 did not increase CYP2S1 mRNA in any organ and at any time point examined despite a significant induction of CYP1 genes. PCB126 reduced CYP2S1 mRNA by 40% (not significant) from the 7th post-exposure day in thymus. PB and Dex had no effect on CYP2S1 mRNA levels. These observations show that in this model CYP2S1 is not, or only weakly, regulated by AhR and not induced by CAR or PXR activators.

Effect of pregnane X receptor (PXR) prototype agonists on chemoprotective and drug metabolizing enzymes in mice

The effects of known PXR inducers; spironolactone [SPL, (i.p.)], pregnenolone-16 alpha-carbonitrile [PCN, (i.g.)] and dexamethasone (i.p.) on hepatic drug metabolizing enzymes in the male CF1 mouse were examined 24 h after 3 daily doses (50, 100, or 200 mg/kg) in corn oil vehicle. All three compounds produced dose-dependent elevations in cytochrome P450 [CYP3A], glutathione S-transferase [GST] and NAD(P)H quinone oxidoreductase [NQO] activities. Only elevations in CYP3A produced after dexamethasone were statistically significant. An elevation in microsomal epoxide hydrolase [mEH, Ephx1] activity was seen after almost all treatments but was erratic with dose. UDP-glucuronosyltransferase and thioredoxin reductase activities were not increased by any agent. Dexamethasone elevated Cyp1a1/2 mRNA at the low dose but reduced the mRNA transcript and activity of the enzyme at the mid and high doses. The mRNA responses of Ephx1 and Nqo1 showed a close parallel to each other with no increases after dexamethasone or SPL treatment, and elevations at the mid dose of PCN. With the exception of dexamethasone at the high dose, elevations in Gst mRNAs were seen with all doses of the three agents. When a large number of hepatic enzymes are examined, the responses to dexamethasone, SPL and PCN are far from identical, and any dose dependency is agent specific. Induction of enzymes seems more complicated to be controlled by one orphan receptor. This study not only filled the void about the murine PXR-induction profile but also will help in the course of drug development research with respect to extrapolation to human risk assessment.

Rapid, nongenomic stimulation of multidrug resistance protein 2 (Mrp2) activity by glucocorticoids in renal proximal tubule

In renal proximal tubule, multidrug resistance protein 2 (Mrp2) actively transports many organic anions into urine, including drugs and metabolic wastes. Upon exposure to nephrotoxicants or during endotoxemia, both Mrp2 activity and expression are up-regulated. This may result from induced de novo synthesis of Mrp2 or post-transcriptional events involving specific signaling pathways. Here, we investigated glucocorticoid signaling to Mrp2 in killifish renal proximal tubules, a model system in which transport activity can be measured using a fluorescent substrate and confocal imaging. Exposure of tubules to dexamethasone rapidly increased Mrp2-mediated fluorescein methotrexate transport. Other glucocorticoid receptor (GR) ligands, cortisol and triamcinolone acetonide, also stimulated Mrp2-mediated transport. The GR antagonist, mifepristone 17β-hydroxy-11β-[4-dimethylamino phenyl]-17α-[1-propynyl]estra-4,9-dien-3-one (RU486), abolished stimulation by all three ligands, whereas the mineralocorticoid receptor antagonist, spironolactone, was ineffective. Consistent with action through a nongenomic mechanism, dexamethasone stimulation of Mrp2-mediated transport was insensitive to cycloheximide and actinomycin D, and immunohistochemistry revealed no alterations in Mrp2 expression at the luminal membrane. (9S-(9α,10β,12α))-2,3,9,10,11,12-hexahydro-10-hydroxy-10-(methoxycarbonyl)-9-methyl-9,12-epoxy-1H-diindolo[1,2,3-fg:3',2',1'-kl]pyrrolo[3,4-i][1,6]benzodiazocin-1-one (K252a), an inhibitor of the tyrosine receptor kinase subfamily, reduced the dexamethasone effect, as did the specific hepatocyte growth factor receptor (c-Met) tyrosine kinase inhibitor, (2R)-1-[[5-[(Z)-[5-[[(2,6-dichlorophenyl)methyl]sulfonyl]-1,2-dihydro-2-oxo-3H-indol-3-ylidene]methyl]-2,4-dimethyl-1H-pyrrol-3-yl]carbonyl]-2-(1-pyrrolidinylmethyl)pyrrolidine (PHA-665752). Hepatocyte growth factor (HGF), an endogenous ligand for c-Met, stimulated Mrp2-mediated transport. This effect was reversed by PHA-665752 but not by RU486. Inhibition of mitogen-activated protein kinase/extracellular signal-regulated kinase kinase (MEK 1/2) also abolished the effects of dexamethasone and HGF. Our results disclose a novel mechanism by which glucocorticoids acting through GR, c-Met, and MEK1/2 cause rapid, nongenomic stimulation of Mrp2-mediated transport in renal proximal tubules. This up-regulation may be nephroprotective, enhancing efflux of metabolic wastes and toxicants during cell and tissue stress.

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.

Byakangelicin induces cytochrome P450 3A4 expression via transactivation of pregnane X receptors in human hepatocytes

BACKGROUND AND PURPOSE

Byakangelicin is found in extracts of the root of Angelica dahurica, used in Korea and China as a traditional medicine to treat colds, headache and toothache. As byakangelicin can inhibit the effects of sex hormones, it may increase the catabolism of endogenous hormones. Therefore, this study investigated the effects of byakangelicin on the cytochrome P450 isoform cytochrome (CY) P3A4 in human hepatocytes.

EXPERIMENTAL APPROACH

Cultures of human hepatocytes and a hepatoma cell line (Huh7 cells) were used. mRNA and protein levels were measured by quantitative reverse transcription-polymerase chain reaction and Western blot. Plasmid constructs and mutants were prepared by cloning and site-directed mutagenesis. Reporter (luciferase) activity was determined by transient co-transfection experiments.

KEY RESULTS

In human primary hepatocytes, byakangelicin markedly induced the expression of CYP3A4 both at the mRNA level (approximately fivefold) and the protein level (approximately threefold) but did not affect expression of human pregnane X receptor (hPXR). In reporter assays, byakangelicin activated CYP3A4 promoter in a concentration-dependent manner (EC₅₀ = 5 µM), and this activation was enhanced by co-transfection with hPXR. Further reporter assays demonstrated that the eNR4 binding element in the CYP3A4 promoter was required for the transcriptional activation of CYP3A4 by byakangelicin.

CONCLUSIONS AND IMPLICATIONS

Byakangelicin induced expression and activity of CYP3A4 in human hepatocytes. This induction was achieved by the transactivation of PXR and not by increased expression of PXR. Therefore, byakangelicin is likely to increase the expression of all PXR target genes (such as MDR1) and induce a wide range of drug-drug interactions.

The suitability of rat hepatoma cell line H4IIE for evaluating the potentials of compounds to induce CYP3A23 expression

To investigate the suitability of H4IIE cells for detecting cytochrome P450 (CYP) induction in vitro, we compared CYP induction by typical CYP inducers in H4IIE cells and rat primary hepatocytes by examining gene expression and enzyme activity, and by immunocytochemistry. The cells were preincubated with 0.1 μM of dexamethasone (DEX) for 24 h, followed by 48 h of exposure to 10 μM of beta-naphthoflavone (bNF), 100 μM of phenobarbital (PB) and 10 μM of DEX. Cyp1a1, Cyp2b1/2 and Cyp3a23/3a1 (Cyp3a23) expressions in H4IIE cells were up-regulated 280-, 1.5- and 65-fold relative to those in vehicle-treated cells, respectively. The fold inductions of those expressions in rat primary hepatocytes were 80-, 33- and 152-fold, respectively. Comprehensive gene expression analysis using DNA microarrays showed that Cyp3a23, Gsta2, Ugt2b12, Udpgt and Sult2a1 expressions were up-regulated in H4IIE cells exposed to 10 μM of DEX. CYP3A activity was not increased, but some H4IIE cells exposed to DEX were stained strongly with anti-CYP3A antibody. We cloned these cells and obtained cloned H4IIE (cH4IIE) cells with expression level of Cyp3a23 higher than those of vehicle-treated cells. It was confirmed that preincubation with 0.1 μM of DEX increased pregnane X receptor (Pxr) expression level and enhanced the Cyp3a23 induction effects of test compounds significantly. Retrospective examination of in vitro CYP induction assay using cH4IIE cells resulted in 80% correlation with the data from in vivo rat toxicity studies. These results suggested that cH4IIE cells are suitable for evaluating the potentials of a compound to induce CYP3A23 expression.

Regulation of drug-metabolizing enzymes by xenobiotic receptors: PXR and CAR

Drug-metabolizing enzymes (DMEs) and transporters play pivotal roles in the disposition and detoxification of numerous foreign and endogenous chemicals. To accommodate chemical challenges, the expression of many DMEs and transporters is up-regulated by a group of ligand-activated transcription factors namely nuclear receptors (NRs). The importance of NRs in xenobiotic metabolism and clearance is best exemplified by the most promiscuous xenobiotic receptors: pregnane X receptor (PXR, NR1I2) and constitutive androstane/activated receptor (CAR, NR1I3). Together, these two receptors govern the inductive expression of a largely overlapping array of target genes encoding phase I and II DMEs, and drug transporters. Moreover, PXR and CAR also represent two distinctive mechanisms of NR activation, whereby CAR demonstrates both constitutive and ligand-independent activation. In this review, recent advances in our understanding of PXR and CAR as xenosensors are discussed with emphasis placed on the differences rather than similarities of these two xenobiotic receptors in ligand recognition and target gene regulation.

Interactions of pharmaceuticals and other xenobiotics on hepatic pregnane X receptor and cytochrome P450 3A signaling pathway in rainbow trout (Oncorhynchus mykiss)

The pregnane X receptor (PXR) belongs to the nuclear hormone receptor (NR) superfamily and is commonly described as a xenophore or a pharmacophore, as it can be activated by a wide array of xenobiotics, including numerous pharmaceuticals and other environmental pollutants. The PXR regulates expression of e.g. cytochrome P450 3A (CYP3A) and the P-glycoprotein (P-gp) that are involved in excretion of lipophilic xenobiotics and endobiotics. A full length PXR cDNA was isolated from rainbow trout liver and it was expressed in a descending order of magnitude in liver>intestine>kidney>heart. A rainbow trout PXR reporter assay was developed and a suite of pharmaceuticals and other xenobiotics were screened. However, no specific activation of rainbow trout PXR was observed with the substances tested. Interactions of prototypical PXR agonists on PXR signaling in rainbow trout were further investigated in cells of hepatic origin exposed in vitro and in juvenile rainbow trout exposed in vivo. The rainbow trout hepatoma cell line (RTH-149), displayed 600 times lower expression of CYP3A mRNA compared to primary cultures of hepatocytes, and did not respond to treatment with either pregnenolone 16α-carbonitrile (PCN), ketoconazole (KCZ) or rifampicin (RIF), which implies a non-functional PXR in this cell line. Exposure of hepatocytes to PCN and lithocholic acid (LA), resulted in a weak concentration-dependent induction of CYP3A and P-gp mRNA levels, though, exposure to the higher concentration of LA (50 μM) decreased PXR mRNA levels. Exposure to dexamethasone (DEX) resulted in a decrease in PXR mRNA, without affecting CYP3A mRNA levels in hepatocytes in vitro. Injections of rainbow trout in vivo with 1 mg LA/kg fish resulted in a slight (albeit not significant) increase in CYP3A mRNA levels without affecting PXR mRNA levels. Although, injection with 10mg omeprazole (OME)/kg fish had no effect on PXR and CYP3A mRNA levels, a 60% inhibition of CYP3A enzyme activities was evident. An in vitro screening of the chemicals used showed that OME and RIF acted as weak CYP3A inhibitors whereas LA and DEX did not affect the CYP3A activity. In contrast, PCN acted as an activator of the CYP3A enzyme activity in vitro. Taken together, these data show that some prototypical PXR agonists weakly affect PXR activation in rainbow trout. Besides, some of these agonists have a stronger effect on the CYP3A catalyst. This study demonstrates the importance of investigation effects of pharmaceuticals on the PXR signaling pathway in non-target animals such as fish.

The aryl hydrocarbon receptor pathway and the response to 3-methylcholanthrene are altered in the liver of adrenalectomized rats

The aryl hydrocarbon receptor (AHR) is activated by 3-methylcholanthrene (MC), a polycyclic aromatic hydrocarbon, and environmental contaminants, such as 2,3,7,8-tetrachlorodibenzo-p-dioxin. Adrenalectomized (ADX) rats have decreased hepatic AHR protein and lower levels of MC-induced CYP1B1 mRNA. To further characterize the effects of decreased AHR protein and the response to MC in ADX rats, we measured AHR-mediated responses in the liver of sham-operated (SHAM) and ADX rats, 6 and 54 h after MC treatment. CYP1A2 mRNA was suppressed by 46 to 60% 4 days after ADX in vehicle-treated animals. AHR mRNA was induced 4-fold 6 h after MC in SHAM rats, but no induction was observed in ADX rats. The MC-induced 7-ethoxyresorufin O-deethylation (EROD) activity in ADX rats was 35% of the activity in the MC-treated SHAM group at 6 h. At 54 h after treatment, the induction of EROD activity by MC was more pronounced in ADX rats than at 6 h. To assess the overall capacity for hepatic P450-mediated metabolism, we measured NADPH-cytochrome P450 oxidoreductase (POR) activity. POR activity was decreased by 50% after ADX. We have shown that the response to MC in ADX rats is suppressed for some, but not all, AHR-mediated responses and that reduced POR activity after ADX could contribute to a decreased capacity for P450-dependent metabolism. The current study contributes to our understanding of how adrenal-dependent factors modulate the AHR pathway and the response to MC in vivo.

Dexamethasone transcriptionally increases the expression of the pregnane X receptor and synergistically enhances pyrethroid esfenvalerate in the induction of cytochrome P450 3A23

The pregnane X receptor (PXR) is recognized as a key regulator for the induction of a large number of genes in drug metabolism and transport. The transactivation of PXR is enhanced by the glucocorticoid dexamethasone and the enhancement is linked to the induction of PXR in humans and rats. The present study was undertaken to determine the mechanism for the induction and ascertain the synergistic effect on the expression of CYP3A23, a rat PXR target. In primary hepatocytes, significant induction of PXR was detected as early as 2h after the treatment and the maximal induction occurred at 1 microM dexamethasone. Similar induction kinetics was observed in the hepatoma line H4-II-E-C3. The induction was abolished by actinomycin D and dexamethasone efficaciously stimulated the rat PXR promoter. In addition, dexamethasone synergized esfenvalerate (an insecticide and a PXR activator) in inducing CYP3A23 and stimulating the CYP3A23 promoter. The full promoter of CYP3A23 (-1445/+74) was activated in a similar pattern as the changes in PXR mRNA in response to dexamethasone, esfenvalerate and co-treatment. In contrast, different responding patterns were detected on the stimulation of the CYP3A23 proximal promoter. Synergistic stimulation was also observed on the CYP3A4-DP-Luc reporter, the human counterpart of CYP3A23. These findings establish that transactivation is responsible for the induction of rat PXR and the induction presents potential interactions with insecticides in a species-conserved manner. The different responding patterns among CYP3A23 reporters point to an involvement of multiple transcriptional events in the regulation of CYP3A23 expression by dexamethasone, esfenvalerate and both.

Rifampicin Inhibits the LPS-induced Expression of Toll-like Receptor 2 via the Suppression of NF-kappaB DNA-binding Activity in RAW 264.7 Cells

Rifampicin is a macrocyclic antibiotic which is used extensively for treatment against Mycobacterium tuberculosis and other mycobacterial infections. Recently, a number of studies have focused on the immune-regulatory effects of rifampicin. Therefore, we hypothesized that rifampicin may influence the TLR2 expression in LPS-activated RAW 264.7 cells. In this study, we determined that rifampicin suppresses LPS-induced TLR2 mRNA expression. The down-regulation of TLR2 expression coincided with decreased production of TNF-alpha. Since NF-kappaB is a major transcription factor that regulates genes for TLR2 and TNF-alpha, we examined the effect of rifampicin on the LPS-induced NF-kappaB activation. Rifampicin inhibited NF-kappaB DNA-binding activity in LPS-activated RAW 264.7 cells, while it did not affect IKKalpha/beta activity. However, rifampicin slightly inhibited the nuclear translocation of NF-kappaB p65. In addition, rifampicin increased physical interaction between pregnane X receptor, a receptor for rifampicin, and NF-kappaB p65, suggesting pregnane X receptor interferes with NF-kappaB binding to DNA. Taken together, our results demonstrate that rifampicin inhibits LPS-induced TLR2 expression, at least in part, via the suppression of NF-kappaB DNA-binding activity in RAW 264.7 cells. Thus, the present results suggest that the rifampicin-mediated inhibition of TLR2 via the suppression of NF-kappaB DNA-binding activity may be a novel mechanism of the immune-suppressive effects of rifampicin.

Activation of pregnane X receptor (PXR) and constitutive androstane receptor (CAR) by herbal medicines

Pregnane X receptor (PXR) and constitutive androstane receptor (CAR) are transcription factors that control the expression of a broad array of genes involved not only in transcellular transport and biotransformation of many drugs, other xenochemicals, and endogenous substances, such as bile acid, bilirubin, and certain vitamins, but also in various physiological/pathophysiological processes such as lipid metabolism, glucose homeostasis, and inflammation. Ligands of PXR and CAR are chemicals of diverse structures, including naturally occurring compounds present in herbal medicines. The overall aim of this article is to provide an overview of our current understanding of the role of herbal medicines as modulators of PXR and CAR.

Pyrethroid insecticides: isoform-dependent hydrolysis, induction of cytochrome P450 3A4 and evidence on the involvement of the pregnane X receptor

Pyrethroids account for more than one-third of the insecticides currently marketed in the world. In mammals, these insecticides undergo extensive metabolism by carboxylesterases and cytochrome P450s (CYPs). In addition, some pyrethroids are found to induce the expression of CYPs. The aim of this study was to determine whether pyrethroids induce carboxylesterases and CYP3A4, and whether the induction is correlated inversely with their hydrolysis. Human liver microsomes were pooled and tested for the hydrolysis of 11 pyrethroids. All pyrethroids were hydrolyzed by the pooled microsomes, but the hydrolytic rates varied by as many as 14 fold. Some pyrethroids such as bioresmethrin were preferably hydrolyzed by carboxylesterase HCE1, whereas others such as bifenthrin preferably by HCE2. In primary human hepatocytes, all pyrethroids except tetramethrin significantly induced CYP3A4. In contrast, insignificant changes were detected on the expression of carboxylesterases. The induction of CYP3A4 was confirmed in multiple cell lines including HepG2, Hop92 and LS180. Overall, the magnitude of the induction was correlated inversely with the rates of hydrolysis, but positively with the activation of the pregnane X receptor (PXR). Transfection of a carboxylesterase markedly decreased the activation of PXR, and the decrease was in agreement with carboxylesterase-based preference for hydrolysis. In addition, human PXR variants as well as rat PXR differed from human PXR (wild-type) in responding to certain pyrethroids (e.g., lambda-cyhalothrin), suggesting that induction of PXR target genes by these pyrethroids varies depending on polymorphic variants and the PXR species identity.

Interplay between the nuclear receptor pregnane X receptor and the uptake transporter organic anion transporter polypeptide 1A2 selectively enhances estrogen effects in breast cancer

The ligand-activated nuclear receptor pregnane X receptor (PXR) is known to play a role in the regulated expression of drug metabolizing enzymes and transporters. Recent studies suggest a potential clinically relevant role of PXR in breast cancer. However, the relevant pathway or target genes of PXR in breast cancer biology and progression have not yet been fully clarified. In this study, we show that mRNA expression of organic anion transporter polypeptide 1A2 (OATP1A2), a transporter capable of mediating the cellular uptake of estrogen metabolites, is nearly 10-fold greater in breast cancer compared with adjacent healthy breast tissues. Immunohistochemistry revealed exclusive expression of OATP1A2 in breast cancer tissue. Interestingly, treatment of breast cancer cells in vitro with the PXR agonist rifampin induced OATP1A2 expression in a time-dependent and concentration-dependent manner. Consistent with its role as a hormone uptake transporter, induction of OATP1A2 was associated with increased uptake of estrone 3-sulfate. The rifampin response was abrogated after small interfering RNA targeting of PXR. We then identified a PXR response element in the human OATP1A2 promoter, located approximately 5.7 kb upstream of the transcription initiation site. The specificity of PXR-OATP1A2 promoter interaction was confirmed using chromatin immunoprecipitation. Importantly, we used a novel potent and specific antagonist of PXR (A-792611) to show the reversal of the rifampin effect on the cellular uptake of E(1)S. These data provide important new insights into the interplay between a xenobiotic nuclear receptor PXR and OATP1A2 that could contribute to the pathogenesis of breast cancer and may also prove to be heretofore unrecognized targets for breast cancer treatment.

The pregnane X receptor: from bench to bedside

BACKGROUND

The pregnane X receptor (PXR; NR1I2), a member of the nuclear receptor superfamily, regulates the expression of metabolic enzymes and transporters involved in the response of mammals to their chemical environment.

OBJECTIVE

To summarize the functions and clinical implications of PXR.

METHODS

In the current review, the clinical implications of PXR are discussed, and the use of genetically engineered PXR mouse models is highlighted.

RESULTS/CONCLUSION

Recent advances in mouse models, including Pxr-null and PXR-humanized mice, provide in vivo tools for evaluating the physiological functions of PXR and its role in controlling xenobiotic metabolism and transport. By using the PXR knockout and humanized mouse models, PXR was found to influence drug-drug interactions, hepatic steatosis, and the homeostasis of vitamin D, bile acids, and steroid hormones. PXR was also shown to influence inflammatory bowel diseases.

Function and Regulation of Multidrug Resistance Proteins (MRPs) in the Renal Elimination of Organic Anions

The reabsorptive and excretory capacity of the kidney has an important influence on the systemic concentration of drugs. Multidrug resistance proteins (MRP/ABCC) expressed in the kidney play a critical role in the tubular efflux of a wide variety of drugs and toxicants, and, in particular, of their negatively charged phase II metabolites. Nine structurally and functionally related MRP family members have been identified (MRP1-9), which differ from each other by their localization, expression levels, and substrate specificity. During altered physiological circumstances, adaptations in these transporters are required to avoid systemic toxicity as well as renal tubular damage. Key players in these events are hormones, protein kinases, nuclear receptors, and disease conditions, which all may affect transporter protein expression levels. This review discusses current knowledge on the renal characteristics of MRP1-9, with specific focus on their regulation.