1
|
Chai SC, Cherian MT, Wang YM, Chen T. Small-molecule modulators of PXR and CAR. BIOCHIMICA ET BIOPHYSICA ACTA 2016; 1859:1141-1154. [PMID: 26921498 PMCID: PMC4975625 DOI: 10.1016/j.bbagrm.2016.02.013] [Citation(s) in RCA: 49] [Impact Index Per Article: 6.1] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 12/15/2015] [Revised: 02/06/2016] [Accepted: 02/06/2016] [Indexed: 12/27/2022]
Abstract
Two nuclear receptors, the pregnane X receptor (PXR) and the constitutive androstane receptor (CAR), participate in the xenobiotic detoxification system by regulating the expression of drug-metabolizing enzymes and transporters in order to degrade and excrete foreign chemicals or endogenous metabolites. This review aims to expand the perceived relevance of PXR and CAR beyond their established role as master xenosensors to disease-oriented areas, emphasizing their modulation by small molecules. Structural studies of these receptors have provided much-needed insight into the nature of their binding promiscuity and the important elements that lead to ligand binding. Reports of species- and isoform-selective activation highlight the need for further scrutiny when extrapolating from animal data to humans, as animal models are at the forefront of early drug discovery. This article is part of a Special Issue entitled: Xenobiotic nuclear receptors: New Tricks for An Old Dog, edited by Dr. Wen Xie.
Collapse
Affiliation(s)
- Sergio C Chai
- Department of Chemical Biology and Therapeutics, 262 Danny Thomas Place, St. Jude Children's Research Hospital, Memphis, TN 38105, USA
| | - Milu T Cherian
- Department of Chemical Biology and Therapeutics, 262 Danny Thomas Place, St. Jude Children's Research Hospital, Memphis, TN 38105, USA
| | - Yue-Ming Wang
- Department of Chemical Biology and Therapeutics, 262 Danny Thomas Place, St. Jude Children's Research Hospital, Memphis, TN 38105, USA
| | - Taosheng Chen
- Department of Chemical Biology and Therapeutics, 262 Danny Thomas Place, St. Jude Children's Research Hospital, Memphis, TN 38105, USA.
| |
Collapse
|
2
|
Küblbeck J, Zancanella V, Prantner V, Molnár F, Squires EJ, Dacasto M, Honkakoski P, Giantin M. Characterization of ligand-dependent activation of bovine and pig constitutive androstane (CAR) and pregnane X receptors (PXR) with interspecies comparisons. Xenobiotica 2015; 46:200-10. [DOI: 10.3109/00498254.2015.1060374] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/06/2023]
|
3
|
Mathäs M, Burk O, Gödtel-Armbrust U, Herlyn H, Wojnowski L, Windshügel B. Structural and functional similarity of amphibian constitutive androstane receptor with mammalian pregnane X receptor. PLoS One 2014; 9:e96263. [PMID: 24797902 PMCID: PMC4010427 DOI: 10.1371/journal.pone.0096263] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/06/2013] [Accepted: 04/04/2014] [Indexed: 11/19/2022] Open
Abstract
The nuclear receptors and xenosensors constitutive androstane receptor (CAR, NR1I3) and pregnane X receptor (PXR, NR1I2) induce the expression of xenobiotic metabolizing enzymes and transporters, which also affects various endobiotics. While human and mouse CAR feature a high basal activity and low induction upon ligand exposure, we recently identified two constitutive androstane receptors in Xenopus laevis (xlCARα and β) that possess PXR-like characteristics such as low basal activity and activation in response to structurally diverse compounds. Using a set of complementary computational and biochemical approaches we provide evidence for xlCARα being the structural and functional counterpart of mammalian PXR. A three-dimensional model of the xlCARα ligand-binding domain (LBD) reveals a human PXR-like L-shaped ligand binding pocket with a larger volume than the binding pockets in human and murine CAR. The shape and amino acid composition of the ligand-binding pocket of xlCAR suggests PXR-like binding of chemically diverse ligands which was confirmed by biochemical methods. Similarly to PXR, xlCARα possesses a flexible helix 11’. Modest increase in the recruitment of coactivator PGC-1α may contribute to the enhanced basal activity of three gain-of-function xlCARα mutants humanizing key LBD amino acid residues. xlCARα and PXR appear to constitute an example of convergent evolution.
Collapse
Affiliation(s)
- Marianne Mathäs
- Department of Pharmacology, University Medical Center, Mainz, Germany
| | - Christian Nußhag
- Department of Pharmacology, University Medical Center, Mainz, Germany
| | - Oliver Burk
- Dr. Margarete Fischer-Bosch-Institute of Clinical Pharmacology, Stuttgart, Germany
| | | | - Holger Herlyn
- Institute of Anthropology, Johannes Gutenberg-University, Mainz, Germany
| | - Leszek Wojnowski
- Department of Pharmacology, University Medical Center, Mainz, Germany
| | - Björn Windshügel
- Centre for Bioinformatics, University of Hamburg, Hamburg, Germany
- European ScreeningPort GmbH, Hamburg, Germany
- * E-mail:
| |
Collapse
|
4
|
Wallace BD, Redinbo MR. Xenobiotic-sensing nuclear receptors involved in drug metabolism: a structural perspective. Drug Metab Rev 2012; 45:79-100. [PMID: 23210723 DOI: 10.3109/03602532.2012.740049] [Citation(s) in RCA: 44] [Impact Index Per Article: 3.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/30/2022]
Abstract
Xenobiotic compounds undergo a critical range of biotransformations performed by the phase I, II, and III drug-metabolizing enzymes. The oxidation, conjugation, and transportation of potentially harmful xenobiotic and endobiotic compounds achieved by these catalytic systems are significantly regulated, at the gene expression level, by members of the nuclear receptor (NR) family of ligand-modulated transcription factors. Activation of NRs by a variety of endo- and exogenous chemicals are elemental to induction and repression of drug-metabolism pathways. The master xenobiotic sensing NRs, the promiscuous pregnane X receptor and less-promiscuous constitutive androstane receptor are crucial to initial ligand recognition, jump-starting the metabolic process. Other receptors, including farnesoid X receptor, vitamin D receptor, hepatocyte nuclear factor 4 alpha, peroxisome proliferator activated receptor, glucocorticoid receptor, liver X receptor, and RAR-related orphan receptor, are not directly linked to promiscuous xenobiotic binding, but clearly play important roles in the modulation of metabolic gene expression. Crystallographic studies of the ligand-binding domains of nine NRs involved in drug metabolism provide key insights into ligand-based and constitutive activity, coregulator recruitment, and gene regulation. Structures of other, noncanonical transcription factors also shed light on secondary, but important, pathways of control. Pharmacological targeting of some of these nuclear and atypical receptors has been instituted as a means to treat metabolic and developmental disorders and provides a future avenue to be explored for other members of the xenobiotic-sensing NRs.
Collapse
Affiliation(s)
- Bret D Wallace
- Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599, USA
| | | |
Collapse
|
5
|
Wang YM, Ong SS, Chai SC, Chen T. Role of CAR and PXR in xenobiotic sensing and metabolism. Expert Opin Drug Metab Toxicol 2012; 8:803-17. [PMID: 22554043 DOI: 10.1517/17425255.2012.685237] [Citation(s) in RCA: 169] [Impact Index Per Article: 14.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/05/2022]
Abstract
INTRODUCTION The xenobiotic detoxification system, which protects the human body from external chemicals, comprises drug-metabolizing enzymes and transporters whose expressions are regulated by pregnane X receptor (PXR) and the constitutive androstane receptor (CAR). The progress made in a large number of recent studies calls for a timely review to summarize and highlight these key discoveries. AREAS COVERED This review summarizes recent advances in elucidating the roles of PXR and CAR in the xenobiotic detoxification system. It also highlights the progress in understanding the regulation of PXR and CAR activity at the post-translational levels, as well as the structural basis for the regulation of these two xenobiotic sensors. EXPERT OPINION Future efforts are needed to discover novel agonists and antagonists with species and isoform selectivity, to systematically understand the regulation of PXR and CAR at multiple levels (transcriptional, post-transcriptional and post-translational levels) in response to xenobiotics exposure, and to solve the structures of the full-length receptors, which will be enabled by improved protein expression and purification techniques and approaches. In addition, more efforts will be needed to validate PXR and CAR as disease-related therapeutic targets and thus expand their roles as master xenobiotic sensors.
Collapse
Affiliation(s)
- Yue-Ming Wang
- St. Jude Children's Research Hospital, Department of Chemical Biology and Therapeutics, 262 Danny Thomas Place, Memphis, TN 38105, USA
| | | | | | | |
Collapse
|
6
|
Windshügel B, Poso A. Constitutive activity and ligand-dependent activation of the nuclear receptor CAR-insights from molecular dynamics simulations. J Mol Recognit 2012; 24:875-82. [PMID: 21812062 DOI: 10.1002/jmr.1132] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022]
Abstract
The constitutive androstane receptor (CAR) possesses, unlike most other nuclear receptors, a pronounced basal activity in vitro whose structural basis is still not fully understood. Using comparative molecular dynamics simulations of CAR X-ray crystal structures, we evaluated the molecular basis for constitutive activity and ligand-dependent receptor activation. Our results suggest that a combination of van der Waals interactions and hydrogen bonds is required to maintain the activation helix in the active conformation also in absence of a ligand. Furthermore, we identified conformational rearrangements within the ligand-binding pocket upon agonist binding and an influence of CAR inducers pregnanedione and CITCO on the helical conformation of the activation helix. Based on the results a model for ligand-dependent CAR activation is suggested.
Collapse
Affiliation(s)
- Björn Windshügel
- Faculty of Health Sciences/School of Pharmacy, University of Eastern Finland, P.O. Box 1627, Kuopio 70211, Finland.
| | | |
Collapse
|
7
|
Jyrkkärinne J, Küblbeck J, Pulkkinen J, Honkakoski P, Laatikainen R, Poso A, Laitinen T. Molecular Dynamics Simulations for Human CAR Inverse Agonists. J Chem Inf Model 2012; 52:457-64. [DOI: 10.1021/ci200432k] [Citation(s) in RCA: 13] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Affiliation(s)
- Johanna Jyrkkärinne
- University of Eastern Finland, Faculty of Health Sciences,
School of Pharmacy, P.O. Box 1627, FI-70211 Kuopio, Finland
| | - Jenni Küblbeck
- University of Eastern Finland, Faculty of Health Sciences,
School of Pharmacy, P.O. Box 1627, FI-70211 Kuopio, Finland
| | - Juha Pulkkinen
- University of Eastern Finland, Faculty of Health Sciences,
School of Pharmacy, P.O. Box 1627, FI-70211 Kuopio, Finland
| | - Paavo Honkakoski
- University of Eastern Finland, Faculty of Health Sciences,
School of Pharmacy, P.O. Box 1627, FI-70211 Kuopio, Finland
| | - Reino Laatikainen
- University of Eastern Finland, Faculty of Health Sciences,
School of Pharmacy, P.O. Box 1627, FI-70211 Kuopio, Finland
| | - Antti Poso
- University of Eastern Finland, Faculty of Health Sciences,
School of Pharmacy, P.O. Box 1627, FI-70211 Kuopio, Finland
| | - Tuomo Laitinen
- University of Eastern Finland, Faculty of Health Sciences,
School of Pharmacy, P.O. Box 1627, FI-70211 Kuopio, Finland
| |
Collapse
|
8
|
Küblbeck J, Laitinen T, Jyrkkärinne J, Rousu T, Tolonen A, Abel T, Kortelainen T, Uusitalo J, Korjamo T, Honkakoski P, Molnár F. Use of comprehensive screening methods to detect selective human CAR activators. Biochem Pharmacol 2011; 82:1994-2007. [DOI: 10.1016/j.bcp.2011.08.027] [Citation(s) in RCA: 32] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/21/2011] [Revised: 08/30/2011] [Accepted: 08/31/2011] [Indexed: 01/20/2023]
|
9
|
Jyrkkärinne J, Windshügel B, Rönkkö T, Tervo AJ, Küblbeck J, Lahtela-Kakkonen M, Sippl W, Poso A, Honkakoski P. Insights into ligand-elicited activation of human constitutive androstane receptor based on novel agonists and three-dimensional quantitative structure-activity relationship. J Med Chem 2009; 51:7181-92. [PMID: 18983136 DOI: 10.1021/jm800731b] [Citation(s) in RCA: 29] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/25/2023]
Abstract
The human constitutive androstane receptor (CAR, NR1I3) is an important regulator of xenobiotic metabolism and other physiological processes. So far, only few CAR agonists are known and no explicit mechanism has been proposed for their action. Thus, we aimed to generate a 3D QSAR model that could explain the molecular determinants of CAR agonist action. To obtain a sufficient number of agonists that cover a wide range of activity, we applied a virtual screening approach using both structure- and ligand-based methods. We identified 27 novel human CAR agonists on which a 3D QSAR model was generated. The model, complemented by coregulator recruitment and mutagenesis results, suggests a potential activation mechanism for human CAR and may serve to predict potential activation of CAR for compounds emerging from drug development projects or for chemicals undergoing toxicological risk assessment.
Collapse
Affiliation(s)
- Johanna Jyrkkärinne
- Departments of Pharmaceutics and Pharmaceutical Chemistry, University of Kuopio, P.O. Box 1627, 70211 Kuopio, Finland
| | | | | | | | | | | | | | | | | |
Collapse
|
10
|
Repo S, Jyrkkärinne J, Pulkkinen JT, Laatikainen R, Honkakoski P, Johnson MS. Ligand specificity of constitutive androstane receptor as probed by induced-fit docking and mutagenesis. J Med Chem 2009; 51:7119-31. [PMID: 18973326 DOI: 10.1021/jm800337r] [Citation(s) in RCA: 13] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/11/2022]
Abstract
Constitutive androstane receptor (CAR, NR1I3) belongs to the nuclear receptor family of transcription factors and acts as a chemical sensor of drugs and endogenous compounds. The ligand-binding preferences of CAR are diverse, and more importantly, there are significant species differences in ligand specificity. Here, we show that while certain residues are critical for the basal activity of mouse CAR (mCAR) and/or affect the binding of all tested ligands, mutation of some ligand-binding pocket (LBP) residues (e.g., F171 and Y336) paradoxically decreased the activity of a specific ligand while increasing that of others. Comparisons to previously reported human CAR (hCAR) residues indicated that the function of key CAR residues (e.g., N175, L253) is dramatically different between species. The docking results provide some mechanistic rationale for the ability of 17alpha-ethinyl-3,17beta-estradiol (EE2) to both activate mCAR and repress hCAR.
Collapse
Affiliation(s)
- Susanna Repo
- Structural Bioinformatics Laboratory, Department of Biochemistry and Pharmacy, Abo Akademi University, Tykistökatu 6, FI-20520 Turku, Finland
| | | | | | | | | | | |
Collapse
|
11
|
Küblbeck J, Jyrkkärinne J, Poso A, Turpeinen M, Sippl W, Honkakoski P, Windshügel B. Discovery of substituted sulfonamides and thiazolidin-4-one derivatives as agonists of human constitutive androstane receptor. Biochem Pharmacol 2008; 76:1288-97. [PMID: 18786510 DOI: 10.1016/j.bcp.2008.08.014] [Citation(s) in RCA: 24] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/24/2008] [Revised: 08/11/2008] [Accepted: 08/13/2008] [Indexed: 02/04/2023]
Abstract
The constitutive androstane receptor (CAR; NR1I3) is a nuclear receptor responsible for the recognition of potentially toxic endo- and exogenous compounds whose elimination from the body is accelerated by the CAR-mediated inducible expression of metabolizing enzymes and transporters. Despite the importance of CAR, few human agonists are known so far. Following a sequential virtual screening procedure using a 3D pharmacophore and molecular docking approach, we identified 17 novel agonists that could activate human CAR in vitro and enhance its association with the nuclear receptor co-activator SRC1. Selected agonists also increased the expression of the human CAR target CYP2B6 mRNA in primary hepatocytes. Composed of substituted sulfonamides and thiazolidin-4-one derivatives, these agonists represent two novel chemotypes capable of human CAR activation, thus broadening the agonist spectrum of CAR.
Collapse
Affiliation(s)
- Jenni Küblbeck
- Department of Pharmaceutics, University of Kuopio, Yliopistonranta 1C, FI-70210 Kuopio, Finland
| | | | | | | | | | | | | |
Collapse
|
12
|
Pelkonen O, Turpeinen M, Hakkola J, Honkakoski P, Hukkanen J, Raunio H. Inhibition and induction of human cytochrome P450 enzymes: current status. Arch Toxicol 2008; 82:667-715. [PMID: 18618097 DOI: 10.1007/s00204-008-0332-8] [Citation(s) in RCA: 386] [Impact Index Per Article: 24.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/23/2008] [Accepted: 06/16/2008] [Indexed: 02/07/2023]
Abstract
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.
Collapse
Affiliation(s)
- Olavi Pelkonen
- Department of Pharmacology and Toxicology, Institute of Biomedicine, University of Oulu, PO Box 5000 (Aapistie 5 B), 90014 Oulu, Finland.
| | | | | | | | | | | |
Collapse
|
13
|
Rapid clinical induction of hepatic cytochrome P4502B6 activity by ritonavir. Antimicrob Agents Chemother 2008; 52:1663-9. [PMID: 18285471 DOI: 10.1128/aac.01600-07] [Citation(s) in RCA: 67] [Impact Index Per Article: 4.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/19/2023] Open
Abstract
Ritonavir is the most potent and efficacious inhibitor of cytochrome P4503A (CYP3A), and it is used accordingly for the pharmacoenhancement of other antiretrovirals. Paradoxically, ritonavir induces the clinical metabolism and clearance of many drugs. The mechanism by which ritonavir inhibits and induces clinical drug metabolism is unknown. Ritonavir induces CYP2B6 in human hepatocytes. This investigation tested the hypothesis that ritonavir induces human CYP2B6 in vivo. Thirteen healthy human immunodeficiency virus-negative volunteers underwent a three-way sequential crossover protocol, receiving racemic bupropion after nothing (control), 3 days of treatment with ritonavir, and 2.5 weeks of treatment with ritonavir (400 mg twice a day). Stereoselective bupropion hydroxylation was used as an in vivo probe for CYP2B6 activity. Plasma and urine (R)- and (S)-bupropion and (R,R)- and (S,S)-hydroxybupropion concentrations were measured by liquid chromatography-mass spectrometry. Racemic, (R)-, and (S)-bupropion plasma ratios of the area under the concentration-time curve from 0 h to infinity (AUC(0-infinity)) (ritonavir/control) were significantly reduced to 0.84, 0.86, and 0.80, respectively, after 3 days of ritonavir treatment and to 0.67, 0.69, and 0.60 after steady-state ritonavir treatment. Apparent oral clearances for racemic, (R)-, and (S)-bupropion all were significantly increased by 1.2-fold after 3 days of ritonavir treatment and by 1.4-, 1.7-, and 1.5-fold after steady-state ritonavir treatment. The plasma (S,S)-hydroxybupropion/(S)-bupropion AUC(0-72) ratio was significantly increased by ritonavir. Formation clearances of both (R,R)- and (S,S)-hydroxybupropion were increased 1.8-fold after 3 days of ritonavir treatment and 2.1-fold after steady-state ritonavir treatment. These results show that ritonavir induces human CYP2B6 activity. Induction is rapid, occurring after only 3 days of ritonavir, and is sustained for at least 2 weeks. The ritonavir induction of CYP2B6 activity may have significant implications for drug interactions and clarify previously unexplained interactions.
Collapse
|
14
|
Karkola S, Höltje HD, Wähälä K. A three-dimensional model of CYP19 aromatase for structure-based drug design. J Steroid Biochem Mol Biol 2007; 105:63-70. [PMID: 17583493 DOI: 10.1016/j.jsbmb.2006.11.023] [Citation(s) in RCA: 28] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 07/03/2006] [Accepted: 11/15/2006] [Indexed: 10/23/2022]
Abstract
Aromatase (CYP450(arom), CYP19) is an enzyme responsible for converting the aliphatic androgens androstenedione and testosterone to the aromatic estrogens estrone and estradiol, respectively. These endogenous hormones are a key factor in cancer tumor formation and proliferation through a cascade starting from estrogen binding to estrogen receptor. To interfere with the overproduction of estrogens especially in tumor tissue, it is possible to inhibit aromatase activity. This can be achieved using aromatase inhibitors. In order to design novel aromatase inhibitors, it is necessary to have an understanding of the active site of aromatase. As no crystal structure of the enzyme has yet been published, we built a homology model of aromatase using the first crystallized mammalian cytochrome enzyme, rabbit 21-progesterone hydroxylase 2C5, as a template structure. The initial model was validated with exhaustive molecular dynamics simulation with and without the natural substrate androstenedione. The resulting enzyme-substrate complex shows very good stability and only two of the residues are in disallowed regions in a Ramachandran plot.
Collapse
Affiliation(s)
- Sampo Karkola
- Laboratory of Organic Chemistry, Department of Chemistry, Faculty of Science, P.O. Box 55, University of Helsinki, FIN-00014, Finland
| | | | | |
Collapse
|
15
|
Timsit YE, Negishi M. CAR and PXR: the xenobiotic-sensing receptors. Steroids 2007; 72:231-46. [PMID: 17284330 PMCID: PMC1950246 DOI: 10.1016/j.steroids.2006.12.006] [Citation(s) in RCA: 321] [Impact Index Per Article: 18.9] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 10/05/2006] [Revised: 12/07/2006] [Accepted: 12/11/2006] [Indexed: 01/13/2023]
Abstract
The xenobiotic receptors CAR and PXR constitute two important members of the NR1I nuclear receptor family. They function as sensors of toxic byproducts derived from endogenous metabolism 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. In contrast, the steroid receptors, exemplified by the estrogen receptor (ER) and glucocorticoid receptor (GR), are the sensors that tightly monitor and respond to changes in circulating steroid hormone levels to maintain body homeostasis. This divergence of the chemical- and steroid-sensing functions has evolved to ensure the fidelity of the steroid hormone endocrine regulation while allowing development of metabolic elimination pathways for xenobiotics. The development of the xenobiotic receptors CAR and PXR also reflect the increasing complexity of metabolism in higher organisms, which necessitate novel mechanisms for handling and eliminating metabolic by-products and foreign compounds from the body. The purpose of this review is to discuss similarities and differences between the xenobiotic receptors CAR and PXR with the prototypical steroid hormone receptors ER and GR. Interesting differences in structure explain in part the divergence in function and activation mechanisms of CAR/PXR from ER/GR. In addition, the physiological roles of CAR and PXR will be reviewed, with discussion of interactions of CAR and PXR with endocrine signaling pathways.
Collapse
Affiliation(s)
| | - Masahiko Negishi
- *CORRESPONDING AUTHOR ADDRESS: Dr, Masahiko Negishi, Ph.D., Head, Pharmacogenetics Section, Laboratory of Reproductive and Developmental Toxicology, National Institute of Environmental Health Sciences, 111 T.W. Alexander Drive, Research Triangle Park, NC, 27709, Tel: (919) 541-2942, Fax (919) 541-0696,
| |
Collapse
|