Current Drug Metabolism - Volume 6, Issue 4, 2005

We are happy to present this hot topic issue of Current Drug Metabolism, on nuclear hormone receptor-mediated regulation of drug metabolizing enzymes and transporters. It has long been appreciated that drug-induced changes in the expression and/or activity of enzymes or transporters can affect the degree of absorption or elimination of drugs, thereby altering the therapeutic or toxicological response to a drug. The molecular mechanisms by which drugs regulate enzymes and transporter expression, have been elusive up till the 1998 cloning and characterization of pregnane X receptor (PXR), pioneered in the laboratories of Ron Evans at the Salk Institute, and Steve Kliewer then at the Glaxo Wellcome. The xenobiotic receptor identity for the constitutive androstane receptor (CAR), an orphan receptor cloned in David Moore's lab in 1994, whose physiological function was then unknown, was subsequently revealed. Since 1998, combinations of molecular biology, mouse genetics, structural biology, and drug metabolism studies have led to the conclusion that PXR and CAR can function as master regulators of the xenobiotic response by regulating both Phase I and Phase II enzymes, as well as members of the drug transporter families. Nuclear receptor-mediated xenobiotic regulation is made up of a complex regulatory network. The complexity is manifested in the observations that multiple receptors are involved in the regulation; each receptor is capable of regulating multiple xenobiotic targets; there is extensive cross talk between receptors; and many xenobiotic receptors exhibit a distinctive, yet overlapping, spectrum of ligands. Finally, the receptor-mediated enzyme and transporter regulation can not only effect drug metabolism, but it can also influence many patho-physiological processes by affecting the homeostasis of endogenous chemicals such as bile acids, bilirubin, and steroid hormones. I would like to personally thank all of the contributing authors, who are experts and are in the forefront of this emerging and exciting research field. Special thanks goes to Dr. Chandra Prakash, Editor-in-Chief of Current Drug Metabolism, who proposed this hot topic issue to me, at the "2004 Gordon Conference on Drug Metabolism".

UDP-glucuronosyltransferases (UGTs) belong to the Phase II drug metabolizing enzymes. UGTs mediate the transfer of glucuronic acid, from UDP glucuronic acid to predominantly hydrophobic xeno- and endobiotic chemicals, thus facilitating their detoxification and excretion. Deficiency in the expression and/or activity of UGTs may lead to genetic and acquired diseases such as Crigler-Najjar syndrome and jaundice. UGT genes are classified into UGT1A and UGT2B subfamily, and each subfamily and each isoform shows tissue-specific distribution pattern. The underlying mechanisms for this tissue specificity are not fully understood. Emerging evidence have demonstrated that nuclear receptors (NR), such as pregnane X receptor (PXR), constitutive androstane receptor (CAR), peroxisome proliferator-activated receptor (PPAR), can regulate UGTs and this NR-mediated regulation may contribute to the tissue-specific expression pattern of UGTs. The regulations are believed to be both receptor- and UGT isoform-specific. In addition, UGTs are also subject to the regulation by aryl hydrocarbon receptor (AhR) and other tissue-specific transcription factors. Based on their capacity to catalyze the glucuronidation of xenobiotics and endobiotics, UGTs play an important role in hormonal homeostasis, energy metabolism, bilirubin clearance, and xenobiotic detoxification. Therefore, elucidating UGT regulation by nuclear receptors has broader significance in understanding UGT's function in various physiological and pathophysiological conditions.

The transcription factor networks that regulate basal and xenobiotic-modulated expression of the hepatic sulfotransferases affect the dynamics of xenobiotic detoxication, carcinogen bioactivation and metabolic homeostasis. Emerging evidence suggests that liver-enriched transcription factors, the aryl hydrocarbon (Ah) receptor and members of the nuclear receptor transcription factor superfamily all play integrated roles in the control of sulfotransferase gene transcription. Unlike the well known up-regulation of CYP1A1, expression of rat hepatic aryl (SULT1A1) and hydroxysteroid (SULT2A) sulfotransferase is suppressed in response to treatment with the prototypic Ah receptor ligand, 2, 3, 7, 8-tetrachlorodibenzo-p-dioxin. Glucocorticoid-inducible rat hepatic SULT1A1 gene transcription occurs through a glucocorticoid receptor (GR)-mediated mechanism, while human hepatic SULT1A1 does not display GRinducible expression. By comparison, liver-enriched transcription factors, such as CCAAT/enhancer binding protein, are essential for the maintenance of basal and GR-inducible rat hepatic SULT2A expression. The transcriptional control of rodent and human hepatic SULT2A expression is subject to trans-activation by the environmental sensor, pregnane X receptor (PXR). IR0 (inverted repeat with zero intervening bases) motifs located in the 5'-flanking regions of rodent SULT2A genes are required for transcriptional activation by PXR and other nuclear receptors, including constitutive androstane receptor, farnesoid X receptor and vitamin D receptor. Peroxisome proliferator activated receptor α (PPARα) mediates the induction of human, but not rat, hepatic SULT2A gene transcription, thus implicating a role for fatty acids as endogenous regulators of hepatic sulfonation in humans. This review focuses on the xenobiotic sensors and transcription factor systems that regulate sulfotransferase gene expression.

Chemicals that increase expression of phase-I and -II biotransformation enzymes in liver, as well as enhance hepatic uptake and biliary excretion are often referred to as microsomal enzyme inducers (MEIs). Early studies suggested that drug metabolism might be coordinately regulated along with drug efflux from hepatocytes as a means for the liver to rid itself of foreign chemicals. Since then, the identification and characterization of nuclear receptors (NRs) has aided in understanding of how various MEIs enhance xeniobiotic uptake, biotransformation, and excretion. In addition, the NRs by which several classes of MEIs induce phase-I and -II drug metabolizing enzymes have been elucidated (i.e. AHR, CAR, PXR, PPARα, Nrf2). Several transporter families which mediate uptake of chemicals into liver and excretion of chemicals from liver into blood and/or bile have been cloned and identified. In general, the organic anion transporting polypeptide family (Oatps) along with Organic cation transporter 1 (Oct1) and Organic anion transporter 2 mediate uptake of a large number of xenobiotics from blood into liver. Conversely, Multidrug resistance proteins (Mdrs), Multidrug resistanceassociated proteins (Mrps), and Breast cancer resistance protein (Bcrp) mediate efflux of xenobiotics from liver into bile or blood. Recent studies have demonstrated that MEIs increase expression of various Oatps, Mrps, and Mdrs in liver, and some occur via activation of nuclear receptors.

The detoxification and elimination of potentially toxic foreign and endogenous compounds depends on the concerted action of xenobiotic metabolizing enzymes. Nuclear hormone receptors (NHRs) have emerged as key regulators of the expression of these enzymes and his review focuses on the xenosenor CAR (Constitutive Androstane Receptor, NR1I3). CAR is highly expressed in the liver and the small intestine, two key tissues expressing xenobiotic metabolizing enzymes, and mediates the induction of their expression by the widely used antiepileptic drug, phenobarbital (PB) and the potent synthetic inducer 1, 4-bis-(2-(3, 5, -dichloropyridyloxy)) benzene (TCPOBOP). TCPOBOP is an agonist ligand for CAR. PB induces its nuclear translocation, which results in increased expression of CAR target genes since, unlike the classical, ligand-dependent nuclear receptors, CAR is an apparently constitutive transactivator. This constitutive activity is inhibited by the inverse agonist ligands androstanol and androstenol. The CAR mediated induction of the expression of xenobiotic metabolizing enzymes is generally protective, but can be deleterious if toxic metabolites are produced. CAR also has a protective role in the stress response elicited by hyperbilirubinemia, as well as lithocholic acid induced cholestasis. In addition, recent studies show that CAR activation disrupts thyroid hormone homeostasis. Finally, CAR activation promotes hepatocyte proliferation and blocks apoptosis, and is essential for the tumorigenesis induced by its activators PB and TCPOBOP. The role of CAR in endobiotic and xenobiotics metabolism has clinical implications in disease prevention, drug-drug interactions, and the development of better drug treatments.

Retinoid X receptors (RXRs) form heterodimers with pregnane X receptor (PXR) and constitutive androstane receptor (CAR), two prototypical xenobiotic receptors, and mediate metabolism of xenobiotics (foreign compounds) and endobiotics (endogenous compounds). Establishment of gene knockout and transgenic mouse models of RXRs, PXR, and CAR greatly enhanced the study of the biology of nuclear receptors, leading to considerable research progress in understanding the molecular mechanism underlying the nuclear receptor-mediated pathways in xenobiotic and endobiotic metabolism. These animal models are widely used in screening nuclear receptor ligands, identifying nuclear receptor target genes, and defining physiological and pharmacological pathways mediated by these xenobiotic nuclear receptors. In addition, "humanized" PXR and CAR mouse models, which avoid species specificity, provide valuable tools for investigating human xenobiotic response. Moreover, generations of multiple gene knockout mouse models further allow us to identify unique and redundant pathways mediated by each xenosensor. In this article, we review the progress made by using animal models of RXRs, PXR, and CAR in understanding the biological functions of these nuclear receptors in physiology, pharmacology, and pathology.

The pregnane X receptor (PXR) is a member of the nuclear receptor family of ligand-regulated transcription factors. Like many former orphan nuclear receptors, it contains both DNA and ligand binding domains and binds to response elements in the regulatory regions of target genes as a heterodimer with RXRα. Unlike the vast majority of nuclear receptors, however, PXR responds to a wide variety of chemically distinct xenobiotics and endobiotics, regulating the expression of genes central to both drug and bile acid metabolism. We review the structural basis of PXR's promiscuity in ligand binding, its recruitment of transcriptional coregulators, its potential formation of higher-order nuclear receptor complexes, and its control of target gene expression. Structural flexibility appears to be central to the receptor's ability to conform to ligands that differ both in size and shape. We also discuss the clinical implications of PXR's role in the drugdrug interactions, cancer, and cholestatic liver disease.

The defense mechanisms responsible for protecting the body from endogenous toxins are also involved in the metabolism of drugs and are composed of phase I and phase II drug metabolizing enzymes, as well as drug transporters. Numerous drugs and chemicals have been shown to modulate the expression of the genes involved in these three drugdetoxifying processes. Induction of these genes contributes to both auto-induction of drug clearance and to drug-drug interactions in combination therapies. The orphan nuclear receptors PXR (pregnane X receptor) and CAR (Constitutive androstane receptor) are xenosensors that mediate drug-induced changes by increasing transcription of genes that are involved in drug clearance and disposition. Co-administration of drugs, one of which is a nuclear receptor agonist or antagonist, can either lead to altered clearance of the second drug and severe toxicity, or a loss of therapeutic efficacy or an imbalance in physiological substrate concentrations, providing a novel molecular mechanism for drug-drug interactions. Thus, genetic variability in these nuclear receptors will contribute to human variation in the magnitude of clinically significant drug-drug interactions. This review describes common PXR and CAR genetic variants that have been identified to date in the human population and the functional consequence of these variant alleles. In addition, alternatively spliced variants of PXR and CAR that may also contribute to individual variability as well as tissue specific expression of these receptors are also described. Identification of PXR and CAR genetic variants and alternatively spliced mRNAs may ultimately allow predictions of interindividual differences in target gene induction and drug-drug interactions.

It is becoming increasingly evident that constitutive, induced, and regulated expression of genes important to the drug disposition process such as drug transporters, phase I and II metabolic enzymes are largely under the transcriptional control of certain nuclear receptor (NR) family members. In the past decade, important new insights regarding the role and relevance of ligand-activated nuclear receptors such as such as the pregnane X receptor (PXR) and constitutive androstane receptor (CAR) in terms of their activation by endogenous biochemicals, natural products, as well as synthetic compounds have led to a much better understanding of the xenobiotic-mediated induction process and the clinical relevance of such NRs to drug therapy in general. However, in addition to CAR and PXR, many orphan and adopted orphan NRs have recently been identified as key regulators of drug disposition genes. Indeed, nuclear receptors including farnesoid X receptor, peroxisome proliferator-activated receptor, and hepatocyte nuclear factors (1α, 3 and 4α) exhibit overlapping ligand specificities and regulate multiple gene targets, resulting in tissue- and organ-specific expression of drug disposition genes. In this review, the biology, pathophysiology, and the potential clinical relevance of such NRs to drug disposition and response are discussed.