Molecular mechanisms of cytochrome P-450 induction by xenobiotics: An expanded role for nuclear hormone receptors

ATP-binding cassette (ABC) drug transporters in the developing blood-brain barrier: role in fetal brain protection

The blood-brain barrier (BBB) provides essential neuroprotection from environmental toxins and xenobiotics, through high expression of drug efflux transporters in endothelial cells of the cerebral capillaries. However, xenobiotic exposure, stress, and inflammatory stimuli have the potential to disrupt BBB permeability in fetal and post-natal life. Understanding the role and ability of the BBB in protecting the developing brain, particularly with respect to drug/toxin transport, is key to promoting long-term brain health. Drug transporters, particularly P-gp and BCRP are expressed in early gestation at the developing BBB and have a crucial role in developmental homeostasis and fetal brain protection. We have highlighted several factors that modulate drug transporters at the developing BBB, including synthetic glucocorticoid (sGC), cytokines, maternal infection, and growth factors. Some factors have the potential to increase expression and function of drug transporters and increase brain protection (e.g., sGC, transforming growth factor [TGF]-β). However, others inhibit drug transporters expression and function at the BBB, increasing brain exposure to xenobiotics (e.g., tumor necrosis factor [TNF], interleukin [IL]-6), negatively impacting brain development. This has implications for pregnant women and neonates, who represent a vulnerable population and may be exposed to drugs and environmental toxins, many of which are P-gp and BCRP substrates. Thus, alterations in regulated transport across the developing BBB may induce long-term changes in brain health and compromise pregnancy outcome. Furthermore, a large portion of neonatal adverse drug reactions are attributed to agents that target or access the nervous system, such as stimulants (e.g., caffeine), anesthetics (e.g., midazolam), analgesics (e.g., morphine) and antiretrovirals (e.g., Zidovudine); thus, understanding brain protection is key for the development of strategies to protect the fetal and neonatal brain.

Regulation of Cytochrome P450 2a5 by Artemisia capillaris and 6,7-Dimethylesculetin in Mouse Hepatocytes

Jaundice is a potentially fatal condition resulting from elevated serum bilirubin levels. For centuries, herbal remedies containing Artemisia capillaris Thunb. including the compound 6,7-dimethylesculetin (DE) have been used in Asia to prevent and treat jaundice in neonates. DE activates an important regulator of bilirubin metabolism, the constitutive androstane receptor (CAR), and increases bilirubin clearance. In addition, murine cytochrome P450 2a5 (Cyp2a5) is known to be involved in the oxidative metabolism of bilirubin. Moreover, treatment of mice with phenobarbital, a known inducer of both CAR and Cyp2a5, increases expression of Cyp2a5 suggesting a potential relationship between CAR and Cyp2a5 expression. The aim of this study is to investigate the influence of Artemisia capillaris and DE on the expression and regulatory control of Cyp2a5 and the potential involvement of CAR. Treatment of mouse hepatocytes in primary culture with DE (50 μM) significant increased Cyp2a5 mRNA and protein levels. In mice, Artemisia capillaris and DE treatment also increased levels of hepatic Cyp2a5 protein. Luciferase reporter assays showed that CAR increases Cyp2a5 gene transcription through a CAR response element in the Cyp2a5 gene promoter. Moreover, DE caused nuclear translocation of CAR in primary mouse hepatocytes and increased Cyp2a5 transcription in the presence of CAR. These results identify a potential CAR-mediated mechanism by which DE regulates Cyp2a5 gene expression and suggests that DE may enhance bilirubin clearance by increasing Cyp2a5 levels. Understanding this process could provide an opportunity for the development of novel therapies for neonatal and other forms of jaundice.

Taxanes in cancer treatment: Activity, chemoresistance and its overcoming

Since 1984, when paclitaxel was approved by the FDA for the treatment of advanced ovarian carcinoma, taxanes have been widely used as microtubule-targeting antitumor agents. However, their historic classification as antimitotics does not describe all their functions. Indeed, taxanes act in a complex manner, altering multiple cellular oncogenic processes including mitosis, angiogenesis, apoptosis, inflammatory response, and ROS production. On the one hand, identification of the diverse effects of taxanes on oncogenic signaling pathways provides opportunities to apply these cytotoxic drugs in a more rational manner. On the other hand, this may facilitate the development of novel treatment modalities to surmount anticancer drug resistance. In the latter respect, chemoresistance remains a major impediment which limits the efficacy of antitumor chemotherapy. Taxanes have shown impact on key molecular mechanisms including disruption of mitotic spindle, mitosis slippage and inhibition of angiogenesis. Furthermore, there is an emerging contribution of cellular processes including autophagy, oxidative stress, epigenetic alterations and microRNAs deregulation to the acquisition of taxane resistance. Hence, these two lines of findings are currently promoting a more rational and efficacious taxane application as well as development of novel molecular strategies to enhance the efficacy of taxane-based cancer treatment while overcoming drug resistance. This review provides a general and comprehensive picture on the use of taxanes in cancer treatment. In particular, we describe the history of application of taxanes in anticancer therapeutics, the synthesis of the different drugs belonging to this class of cytotoxic compounds, their features and the differences between them. We further dissect the molecular mechanisms of action of taxanes and the molecular basis underlying the onset of taxane resistance. We further delineate the possible modalities to overcome chemoresistance to taxanes, such as increasing drug solubility, delivery and pharmacokinetics, overcoming microtubule alterations or mitotic slippage, inhibiting drug efflux pumps or drug metabolism, targeting redox metabolism, immune response, and other cellular functions.

A gene-centered C. elegans protein-DNA interaction network provides a framework for functional predictions

Transcription factors (TFs) play a central role in controlling spatiotemporal gene expression and the response to environmental cues. A comprehensive understanding of gene regulation requires integrating physical protein–DNA interactions (PDIs) with TF regulatory activity, expression patterns, and phenotypic data. Although great progress has been made in mapping PDIs using chromatin immunoprecipitation, these studies have only characterized ~10% of TFs in any metazoan species. The nematode C. elegans has been widely used to study gene regulation due to its compact genome with short regulatory sequences. Here, we delineated the largest gene‐centered metazoan PDI network to date by examining interactions between 90% of C. elegans TFs and 15% of gene promoters. We used this network as a backbone to predict TF binding sites for 77 TFs, two‐thirds of which are novel, as well as integrate gene expression, protein–protein interaction, and phenotypic data to predict regulatory and biological functions for multiple genes and TFs.

A Molecular Aspect in the Regulation of Drug Metabolism: Does PXR-Induced Enzyme Expression Always Lead to Functional Changes in Drug Metabolism?

Pregnane X receptor (PXR, NR112) is a xenobiotic receptor whose primary function is to regulate the expression of drug-metabolizing enzymes (DMEs) and drug transporters. Drug-induced PXR activation and subsequent enzyme and transporter induction has been proposed to be an important mechanism for the drug-drug interactions. In addition to activating PXR, many pharmaceutical chemicals can also function as reversible or irreversible inhibitors of DMEs, which may also impact the pharmacokinetics and pharmacodynamics (PK/PD) of drugs. Therefore, we cannot simply conclude that the PXR-induced alteration in enzyme expression always reflects functional changes. We should consider both PXR activation and DMEs inhibition to improve drug safety in the clinic.

Breast cancer treatment and sulfotransferase

INTRODUCTION

Sustained exposure to excessive estrogen is an established risk factor for breast cancer. Sulfotransferase (SULT)-mediated sulfonation represents an effective approach for estrogen deprivation as estrogen sulfates do not bind and activate estrogen receptors (ERs). The nuclear receptor (NR) superfamily functions as a sensor for xenobiotics as well as endogenous molecules, which can regulate the expression of SULT.

AREAS COVERED

In this review, we summarize the mechanisms of SULT regulation by NRs and inactivation of estrogen by SULT. Furthermore, we discuss the potential of clinical therapy targeting SULT in breast cancer treatment. Gaps in current knowledge that require further study are also highlighted.

EXPERT OPINION

The prevention of estrogen binding to ER by antiestrogen and inhibition of estrogen synthesis by aromatase or sulfatase inhibitor have been used in clinical therapy for breast cancer. Although the induction of SULT has been proven effective to estrogen inactivation, reports on this method applied to breast cancer treatment are rare. Targeted activation of SULT may open up a new means of treating hormone-dependent breast cancer.

The role of CYP3A4 in the biotransformation of bile acids and therapeutic implication for cholestasis

CYP3A4 is a major cytochrome P450. It catalyses a broad range of substrates including xenobiotics such as clinically used drugs and endogenous compounds bile acids. Its function to detoxify bile acids could be used for treating cholestasis, which is a condition characterised by accumulation of bile acids. Although bile acids have important physiological functions, they are very toxic when their concentrations are excessively high. The accumulated bile acids in cholestasis can cause liver and other tissue injuries. Thus, control of the concentrations of bile acids is critical for treatment of cholestasis. CYP3A4 is responsively upregulated in cholestasis mediated by the nuclear receptors farnesol X receptor (FXR) and pregnane X receptor (PXR) as a defence mechanism. However, the regulation of CYP3A4 is complicated by estrogen, which is increased in cholestasis and down regulates CYP3A4 expression. The activity of CYP3A4 is also inhibited by accumulated bile acids due to their property of detergent effect. In some cholestasis cases, genetic polymorphisms of the CYP3A4 and PXR genes may interfere with the adaptive response. Further stimulation of CYP3A4 activity in cholestasis could be an effective approach for treatment of the disease. In this review, we summarise recent progress about the roles of CYP3A4 in the metabolism of bile acids, its regulation and possible implication in the treatment of cholestasis.

Cytochrome P450 2J2: distribution, function, regulation, genetic polymorphisms and clinical significance

Cytochrome P450 2J2 (CYP2J2) is an enzyme mainly found in human extrahepatic tissues, with predominant expression in the cardiovascular systems and lower levels in the intestine, kidney, lung, pancreas, brain, liver, etc. During the past 15 years, CYP2J2 has attracted much attention for its epoxygenase activity in arachidonic acid (AA) metabolism. It converts AA to four epoxyeicosatrienoic acids (EETs) that have various biological effects, especially in the cardiovascular systems. In recent publications, CYP2J2 is shown highly expressed in various human tumor cells, and its EET metabolites are demonstrated to implicate in the pathologic development of human cancers. CYP2J2 is also a human CYP that involved in phase I xenobiotics metabolism. Antihistamine drugs and many other compounds were identified as the substrates of CYP2J2, and studies have demonstrated that these substrates have a broad structural diversity. CYP2J2 is found not readily induced by known P450 inducers; however, its expression could be regulated in some pathological conditions, might through the activator protein-1(AP-1), the AP-1-like element and microRNA let-7b. Several genetic mutations in the CYP2J2 gene have been identified in humans, and some of them have been shown to have potential associations with some diseases. With the increasing awareness of its roles in cancer disease and drug metabolism, studies about CYP2J2 are still going on, and various inhibitors of CYP2J2 have been determined. Further studies are needed to delineate the roles of CYP2J2 in disease pathology, drug development and clinical practice.

Transcriptional regulation of chicken cytochrome P450 2D49 basal expression by CCAAT/enhancer-binding protein α and hepatocyte nuclear factor 4α

Chicken cytochrome P450 (CYP)2D49 is structurally and functionally related to human CYP2D6, which is an important drug-metabolizing enzyme. To date, little is known about the transcriptional regulation of this cytochrome. Through deletion analysis of the CYP2D49 promoter, we identified two putative degenerate CCAAT/enhancer-binding protein (C/EBP)-binding sites and an imperfect DR1 element (the site contains direct repeats of the hexamer AGGTCA separated by a one-nucleotide spacer motif) within regions -296/-274, -274/-226, and -226/-183, respectively, which may play critical roles in the transcriptional activation of the CYP2D49 gene. Electrophoretic mobility shift assays and chromatin immunoprecipitation assays showed that the putative C/EBP boxes and DR1 element in the CYP2D49 promoter are functional motifs that bind to C/EBPα and hepatocyte nuclear factor 4α (HNF4α), respectively. Furthermore, we studied the functional importance and relationships of these transcription factor-binding sites by examining the effects of mutation and deletion of these regions on promoter activity. These studies revealed that the two C/EBP-binding sites show a compensatory relationship and work cooperatively with the DR1 element to modulate the transcription of CYP2D49. The results of overexpressing C/EBPα and HNF4α in culture cells further confirmed that both C/EBPα and HNF4α contribute significantly to sustaining a high level of CYP2D49 transcription. In conclusion, the data indicate that the constitutive hepatic expression of CYP2D49 is governed by both C/EBPα and HNF4α. Further studies will be required to fully characterize the molecular mechanisms that modulate CYP2D49 expression.

Modulation of xenobiotic receptors by steroids

Nuclear receptors (NRs) are ligand-activated transcription factors that regulate the expression of their target genes. NRs play important roles in many human diseases, including metabolic diseases and cancer, and are therefore a key class of therapeutic targets. Steroids play important roles in regulating nuclear receptors; in addition to being ligands of steroid receptors, steroids (and their metabolites) also regulate other NRs, such as the pregnane X receptor and constitutive androstane receptor (termed xenobiotic receptors), which participate in steroid metabolism. Xenobiotic receptors have promiscuous ligand-binding properties, and their structurally diverse ligands include steroids and their metabolites. Therefore, steroids, their metabolism and metabolites, xenobiotic receptors, steroid receptors, and the respective signaling pathways they regulate have functional interactions. This review discusses these functional interactions and their implications for activities mediated by steroid receptors and xenobiotic receptors, focusing on steroids that modulate pathways involving the pregnane X receptor and constitutive androstane receptor. The emphasis of the review is on structure-function studies of xenobiotic receptors bound to steroid ligands.

Induced CYP3A4 expression in confluent Huh7 hepatoma cells as a result of decreased cell proliferation and subsequent pregnane X receptor activation

We have previously shown that confluent growth of the human hepatoma cell line Huh7 substantially induces the CYP3A4 mRNA, protein, and activity levels. Here, the mechanisms behind were investigated, and a transcriptome analysis revealed significant up-regulation of liver-specific functions, whereas pathways related to proliferation and cell cycle were down-regulated in the confluent cells. Reporter analysis revealed that the CYP3A4 gene was transcriptionally activated during confluence in a process involving pregnane X receptor (PXR). PXR expression was increased, and PXR protein accumulated in the nuclei during confluent growth. The PXR ligand rifampicin further increased the expression of CYP3A4, and siRNA-mediated knock-down of PXR in confluent cells resulted in decreased CYP3A4 expression. Cyclin-dependent kinase 2 (CDK2), a known modulator of the cell cycle and a negative regulator of PXR, was more highly expressed in proliferating control cells. Trypsinization of the confluent cells and replating them subconfluent resulted in a decrease in CYP3A4 and PXR expression back to levels observed in subconfluent control cells, whereas the CDK2 levels increased. Knock-down of CDK2 in proliferating control cells increased the CYP3A4 and PXR protein levels. Moreover, the CDK inhibitor roscovitine stimulated the expression of CYP3A4. A phosphorylation-deficient mutation (S350A) in the PXR protein significantly induced the CYP3A4 transcription. In conclusion, the data strongly suggest that the increased CYP3A4 expression in confluent Huh7 cells is caused by the endogenous induction of PXR as a result of cell-cell contact inhibited proliferation and subsequent decreased CDK2 activities, indicating an endogenous, non-ligand-dependent regulation of PXR and CYP3A4, possibly of physiologic and pharmacological significance.

Proteomic methods for biomarker discovery in a rat model of alcohol steatosis

Understanding changes in the expression of specific proteins and/or alterations in their posttranslational modifications is crucial to elucidating the molecular mechanisms underlying disease states such as alcoholic liver disease. Protein separation and analysis techniques such as two-dimensional electrophoresis and mass spectrometry can be used for identifying biomarker proteins that are altered during progression of alcoholic liver disease. In this chapter, we outline methods for resolving liver tissue proteins from a rodent model of alcoholic liver disease using two-dimensional electrophoresis and identifying differentially expressed proteins using mass spectrometry. In addition, since oxidative stress strongly correlates with alcoholic liver disease, we also describe methods for identifying oxidatively modified proteins from liver tissue. We specifically focus on identifying proteins that are carbonylated as protein carbonylation is a permanent modification and considered deleterious to cells. The combination of two-dimensional electrophoresis for protein resolution, mass spectrometry for protein identification, and affinity-based methods for enriching and identifying carbonylated proteins is a powerful methodology for protein biomarker identification.

Toward in silico structure-based ADMET prediction in drug discovery

Quantitative structure-activity relationship (QSAR) methods and related approaches have been used to investigate the molecular features that influence the absorption, distribution, metabolism, excretion and toxicity (ADMET) of drugs. As the three-dimensional structures of several major ADMET proteins become available, structure-based (docking-scoring) computations can be carried out to complement or to go beyond QSAR studies. Applying docking-scoring methods to ADMET proteins is a challenging process because they usually have a large and flexible binding cavity; however, promising results relating to metabolizing enzymes have been reported. After reviewing current trends in the field we applied structure-based methods in the context of receptor flexibility in a case study involving the phase II metabolizing sulfotransferases. Overall, the explored concepts and results suggested that structure-based ADMET profiling will probably join the mainstream during the coming years.

Use of transgenic mice in UDP-glucuronosyltransferase (UGT) studies

Transgenic mouse models are useful to understand the function and regulation of drug-metabolizing enzymes in vivo. This article is intended to describe the general strategies and to discuss specific examples on how to use transgenic, gene knockout, and humanized mice to study the function as well as genetic and pharmacological regulation of UDP-glucuronosyltransferases (UGTs). The physiological and pharmacological implications of transcription factor-mediated UGT regulation will also be discussed. The UGT-regulating transcription factors to be discussed in this article include nuclear hormone receptors (NRs), aryl hydrocarbon receptor (AhR), and nuclear factor erythroid 2-related factor 2 (Nrf2).

The anti-tumorigenic properties of peroxisomal proliferator-activated receptor alpha are arachidonic acid epoxygenase-mediated

Prevalence and mortality make cancer a health challenge in need of effective and better tolerated therapeutic approaches, with tumor angiogenesis identified as a promising target for drug development. The epoxygenase products, the epoxyeicosatrienoic acids, are pro-angiogenic, and down-regulation of their biosynthesis by peroxisomal proliferator-activated receptor alpha (PPARalpha) ligands reduces tumor angiogenesis and growth. Endothelial cells lacking a Cyp2c44 epoxygenase, a PPARalpha target, show reduced proliferative and tubulogenic activities that are reversed by the enzyme's metabolites. In a mouse xenograft model of tumorigenesis, disruption of the host Cyp2c44 gene causes marked reductions in tumor volume, mass, and vascularization. The relevance of these studies to human cancer is indicated by the demonstration that: (a) activation of human PPARalpha down-regulates endothelial cell CYP2C9 epoxygenase expression and blunts proliferation and tubulogenesis, (b) in a PPARalpha-humanized mouse model, activation of the receptor inhibits tumor angiogenesis and growth, and (c) the CYP2C9 epoxygenase is expressed in the vasculature of human tumors. The identification of anti-angiogenic/anti-tumorigenic properties of PPARalpha points to a role for the receptor and its epoxygenase regulatory target in the pathophysiology of cancer, and for its ligands as candidates for the development of a new generation of safer and better tolerated anti-cancer drugs.

Technical pentabromodiphenyl ether and hexabromocyclododecane as activators of the pregnane-X-receptor (PXR)

Technical pentabrominated diphenyl ether (pentaBDE mix) is a mixture of polybrominated diphenyl ethers (PBDEs) which has been widely used as a flame retardant. Since its ban in several countries it has been replaced by other brominated flame retardants such as hexabromocyclododecane (HBCD). Both certain PBDE congeners and HBCD are present in environmental and human samples reflecting their persistent and bioaccumulative properties. PentaBDE mix and HBCD have recently been found to induce cytochrome P450 (CYP) 3 enzymes in rat liver. In this study we tested both technical pentaBDE mix and HBCD for their potency to induce CYP3A enzymes in rat hepatocytes in primary culture, and in rat H4IIE and human HepG2 hepatoma cells. In rat hepatocytes, HBCD was a more effective CYP3A1 inducer than pentaBDE mix, being less effective, however, than the prototype inducer dexamethasone. In human HepG2 cells, both compounds and the prototype inducer rifampicin were about equally effective. In contrast, in HepG2 cells, HBCD failed to induce luciferin-PFBE dealkylase, a common catalytic activity of a number of CYP3A enzymes, possibly reflecting enzyme inhibition. A significant induction of catalytic activity was observed in rat hepatocytes with both compounds. Analysis of a XREM-driven reporter gene activity in transfected cells confirmed that both compounds act as agonists of the human and rat pregnane-X-receptor, which was detectable in all cell types used.

Induction of hepatic glutathione S-transferases in male mice by prototypes of various classes of microsomal enzyme inducers

The underlying need for glutathione S-transferase (Gst) induction is thought to be an adaptive response to chemical stress within the cell. Classical microsomal enzyme inducers (MEIs) increase the expression of biotransformation enzymes (phase I and II) and transporters through transcription factors, such as the aryl hydrocarbon receptor (AhR), constitutive androstane receptor (CAR), pregnane X receptor (PXR), peroxisome proliferator-activated receptor (PPAR) alpha, and nuclear factor erythroid-derived 2-related factor 2 (Nrf2). The effects of MEIs on the induction of hepatic Gsts in mice have not been comprehensively characterized. The purpose of this study was to determine the effects of 15 MEIs on the mRNA expression of 19 mouse Gsts. Male C57BL/6 mice were treated with three different activators each for AhR, CAR, PXR, PPARalpha, and Nrf2. In general, the Gsts are readily induced. All five transcription factors appear to play a role in Gst induction. The Nrf2 activators induced most Gsts (10), followed by the CAR, PXR, and PPARalpha activators (6-7), whereas the AhR ligands induced the least (1). Clofibrate, a PPARalpha agonist, induced most of the Gsts; however, all three PPARalpha agonists decreased Gstp1/2 mRNA. None of the 15 inducers was able to increase or only minimally increased eight of the Gsts (Gsta3, Gstk1, Gstm6, Gsto1, Gstp1/2, Gstt3, Gstz1, and MGst1). Thus, the protection afforded by a ligand for one of these transcription factors will depend on the activator, as well as which Gst that detoxifies the chemicals of interest.

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

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

Induction of metabolism and transport in human intestine: validation of precision-cut slices as a tool to study induction of drug metabolism in human intestine in vitro

Induction of drug enzyme activity in the intestine can strongly determine plasma levels of drugs. It is therefore important to predict drug-drug interactions in human intestine in vitro. We evaluated the applicability of human intestinal precision-cut slices for induction studies in vitro. Morphological examination and intracellular ATP levels indicated tissue integrity up to 24 h of incubation, whereas in proximal jejunum slices, the metabolic rate toward most substrates remained at 40 to 50% of initial values. In colon slices, the cytochrome P450 conversions were below the detection limit, but conjugation rates remained relatively constant during incubation. The inducibility of drug-metabolizing enzymes and P-glycoprotein was evaluated using prototypical inducers for five induction pathways. beta-Naphthoflavone (aryl hydrocarbon receptor ligand) induced CYP1A1 (132-fold in colon and 362-fold in proximal jejunum) and UDP glucuronosyltransferase (UGT) 1A6 mRNA (9.8-fold in colon and 3.2-fold in proximal jejunum). In proximal jejunum, rifampicin (RIF) [pregnane X receptor (PXR) ligand] induced CYP3A4 (5.2-fold), CYP2B6 (2-fold), UGT1A6 (2.2-fold), and multidrug resistance-1 (MDR1)/ABCB1 mRNA (2.7-fold), whereas 6beta-hydroxytestosterone formation (CYP3A4) increased 2-fold. In colon, RIF induced UGT1A6 32-fold and MDR1 2.2-fold. Dexamethasone (glucocorticoid receptor and PXR ligand) induced CYP3A4 mRNA (3.5-fold) and activity (5-fold) in proximal jejunum. Phenobarbital (constitutive androstane receptor activator) induced CYP3A4 (4.1-fold, only in jejunum), CYP2B6 (4.9-fold in colon and 2.3-fold in proximal jejunum), and MDR1/ABCB1 mRNA and CYP3A4 activity (2-fold only proximal jejunum). Quercetin (nuclear factor-E2-related factor 2 activator) induced UGT1A6 mRNA (6.7-fold in colon and 2.2-fold in proximal jejunum). In conclusion, this study shows that human intestinal precision-cut slices are useful to study induction of drug-metabolizing enzymes and transporters in the human intestine.

Roles of the cytochrome P450 arachidonic acid monooxygenases in the control of systemic blood pressure and experimental hypertension

Studies of the cytochrome P450 arachidonic acid (AA) monooxygenase, now established as a major pathway for the bioactivation of this physiological important fatty acid, have uncovered new and important roles for this enzyme system in the regulation of kidney function, including renal hemodynamics and tubular ion transport. Associations between genetically controlled alterations in blood pressure and the activity and/or transcriptional regulation of the kidney Cyp2c AA epoxygenases and Cyp4a omega-hydroxylases revealed a role for these enzymes in the pathophysiology of hypertension, a leading cause of cardiovascular, cerebral, and renal morbidity and mortality. Furthermore, analysis of associations between genetic variants of human CYP4A11 and hypertension suggest a potential role for this gene as a determinant of polygenic blood pressure control in humans. These results are providing conceptually novel approaches for studies of the molecular basis of human hypertension that could lead to new strategies for the early diagnosis and clinical management of this devastating disease.

Guide to drug porphyrogenicity prediction and drug prescription in the acute porphyrias

AIMS

This paper addresses two common problems in the care of carriers of acute porphyria: the choice of safe drugs for pharmacotherapy and the strategy to apply when potentially unsafe drugs cannot be avoided.

METHODS AND RESULTS

A technique is presented for prediction of risk that a certain drug may activate the disease in a gene carrier for acute porphyria. It is based on a model explaining the clinical manifestations as a result of the acute overloading of a deficient enzyme within the hepatic heme biosynthetic chain. The capacity of the drug for induction of the rate-limiting enzyme in heme biosynthesis, e.g. housekeeping 5-aminolevulinate synthase (ALAS1), is assessed by critical appraisal of reports of the outcomes of clinical use of the drug, and by theoretical criteria. The assessment occurs within the frame of a flow-scheme employing variables of increasing specificity, i.e. endocrine properties of the drug, structure and metabolism pointing to affinity to cytochrome P450, hepatic load in therapeutic use, recognized affinity to major CYP species, capacity for CYP-induction or irreversible inhibition, and capacity to activate or modulate the transduction mechanisms of nuclear receptors affecting ALAS1-gene transcription. It is proposed that in the absence of a safer alternative, an urgently needed drug not should be withheld on the grounds of potential porphyrogenicity. After risk-benefit analysis it should be prescribed, but individualized preventive measures adapted to patient vulnerability may be needed.

CONCLUSIONS

About 1000 therapeutic drugs categorized with regard to porphyrogenicity by the technique proposed are presented on the internet (http://www.drugs-porphyria.org).

Cytochrome P450 gene regulation and physiological functions mediated by the aryl hydrocarbon receptor

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor that functions as an intracellular mediator in the xenobiotic signaling pathway. Although a number of studies have examined AhR-mediated CYP1A1 induction in detail, recent studies of AhR-null mice have revealed that AhR plays important regulatory roles in the normal homeostasis and development of animals. In this short review, we summarize the present state of knowledge about the molecular mechanisms of AhR-mediated CYP1 induction, and we also focus on recent advances in the study of the physiological functions of AhR.

Peroxisomal proliferator-activated receptor-alpha-dependent inhibition of endothelial cell proliferation and tumorigenesis

The peroxisomal proliferator-activated nuclear receptor-alpha (PPARalpha), the target for most hypolipidemic drugs in current clinical use, regulates the transcription of genes involved in lipid metabolism and transport, and energy homeostasis. More recently, PPARalpha and its ligands have been implicated in inflammatory responses and the regulation of cell proliferation. PPARalpha also regulates the expression of Cyp4a fatty acid omega-hydroxylases and Cyp2c arachidonic acid epoxygenase genes. To study the role of the PPARalpha receptor and of its Cyp2c epoxygenase gene target in tumorigenesis, we treated mice injected with tumor cells with Wy-14,643, a PPARalpha-selective ligand. Compared with untreated controls, Wy-14643-treated animals showed marked reductions in tumor growth and vascularization, which were accompanied by decreases in the plasma levels of pro-angiogenic epoxygenase metabolites (EETs), hepatic EET biosynthesis, and Cyp2c epoxygenase expression. All these Wy-14643-induced responses were absent in PPARalpha(-/-) mice and are thus PPARalpha-mediated. Primary cultures of mouse lung endothelial cells treated with Wy-14643 showed reductions in cell proliferation and in the formation of capillary-like structures. These effects were absent in cells obtained from PPRAalpha(-/-) mice and reversed by the addition of EETs. These results identify important anti-angiogenic and anti-tumorigenic roles for PPARalpha, characterize the contribution of its Cyp2c epoxygenases gene target to these responses, and suggest potential anti-cancer roles for this nuclear receptor and its ligands.

Effects of TK promotor and hepatocyte nuclear factor-4 in CAR-mediated transcriptional activity of phenobarbital responsive unit of CYP2B gene in monkey kidney epithelial-derived cell line COS-7

Previous studies reported that constitutive androstane receptor (CAR) does not transactivate phenobarbital responsive unit (PBRU)2C1luciferase reporter gene in COS cells in which endogenous CYP2B1 gene is not induced with PB. In order to understand molecular mechanism(s) whereby PBRU is transactivated, this article determined if the use of strong thymidine kinase (TK) promotor rather than the minimal CYP2C1 promotor, and hepatocyte nuclear factor-4 (HNF-4) can affect CAR-mediated transactivation of PBRU in the monkey kidney epithelial-derived COS-7 cells. To examine CAR-mediated transactivation, cultured COS-7 cells were transfected with CAR expression plasmid, pEGFP-mCAR1, and confirmed for high level of the protein expression. In COS-7 cells, TK promotor induced CAR-mediated PBRU transactivation in a dose-dependent manner. Whereas expression of HNF-4 slightly promoted PBRU transactivation with low amount of CAR transfected, it repressed PBRU transactivation in a dose-dependent manner with high amount of CAR. Consistent with the previous reports in Hep G2 cells, CAR transactivated PBRU2C1luciferase in a dose-dependent manner and this CAR-mediated transactivation required functional NR-1 and NF-1 sites. However, HNF-4 did not affect CAR-mediated PBRU transactivation in Hep G2 cells. These results suggest that proximal promotor and a trans-acting factor, HNF-4, can affect CAR-mediated transactivation of PBRU in COS-7 cells.

Induction of phase I and II drug metabolism in rat small intestine and colon in vitro

The aim of this study was to evaluate drug metabolism in rat small intestinal and colon precision-cut slices during 24 h of incubation and the applicability of these slices for enzyme induction studies. Various parameters were evaluated: intracellular levels of ATP (general viability marker), alkaline phosphatase activity (specific epithelial marker), villin expression (specific epithelial marker), and metabolic rates of 7-ethoxycoumarin (CYP1A), testosterone (CYP3A and CYP2B), and 7-hydroxycoumarin (glucuronide and sulfate conjugation) conversions. ATP and villin remained constant up to, respectively, 5 and 8 h in small intestine and up to 24 h in colon. The metabolic rate remained constant in small intestinal slices up to 8 h and decreased afterward to 24 to 92%, depending on the substrate studied. The inducibility of metabolism in small intestinal and colon slices was tested with several inducers at various concentrations and incubation times. The following inducers were used: 3-methylcholanthrene, beta-naphthoflavone, indirubin, and tert-butylhydroquinone (aryl hydrocarbon receptor ligands), dexamethasone (glucocorticoid receptor/pregnane X receptor ligand) and phenobarbital (constitutive androstane receptor ligand). After incubation with inducers, metabolic rates were evaluated with 7-ethoxycoumarin and testosterone (phase I) and 7-hydroxycoumarin (phase II) as substrate. All inducers elevated the metabolic rates consistent with the available published in vivo induction data. Induction of enzyme activity was already detectable after 5 h (small intestine) and after 8 h (colon) for 3-methylcholanthrene and beta-naphthoflavone and was clearly detectable for all tested inducers after 24 h (up to 20-fold compared with noninduced controls). In conclusion, small intestinal and colon precision-cut slices are useful for metabolism and enzyme induction studies.

CYP4 isoform specificity in the omega-hydroxylation of phytanic acid, a potential route to elimination of the causative agent of Refsum's disease

The saturated C20 isoprenoid phytanic acid is physiologically derived from phytol released in the degradation of chlorophyll. The presence of a C-3 methyl group in this substrate blocks normal beta-oxidation, so phytanic acid degradation primarily occurs by initial peroxisomal alpha-oxidation to shift the register of the methyl group. However, individuals with Refsum's disease are genetically deficient in the required phytanoyl-CoA alpha-hydroxylase and suffer from neurological pathologies caused by the accumulation of phytanic acid. Recent work has shown that phytanic acid can also be catabolized by a pathway initiated by omega-hydroxylation of the hydrocarbon chain, followed by oxidation of the alcohol to the acid and conventional beta-oxidation. However, the enzymes responsible for the omega-hydroxylation of phytanic acid have not been identified. In this study, we have determined the activities of all of the rat and human CYP4A enzymes and two of the rat CYP4F enzymes, with respect to the omega-hydroxylation of phytanic acid. Furthermore, we have shown that the ability to omega-hydroxylate phytanic acid is elevated in microsomes from rats pretreated with clofibrate. The results support a possible role for CYP4 enzyme elevation in the elimination of phytanic acid in Refsum's disease patients.

Salt-sensitive hypertension is associated with dysfunctional Cyp4a10 gene and kidney epithelial sodium channel

Functional and biochemical data have suggested a role for the cytochrome P450 arachidonate monooxygenases in the pathophysiology of hypertension, a leading cause of cardiovascular, cerebral, and renal morbidity and mortality. We show here that disruption of the murine cytochrome P450, family 4, subfamily a, polypeptide 10 (Cyp4a10) gene causes a type of hypertension that is, like most human hypertension, dietary salt sensitive. Cyp4a10-/- mice fed low-salt diets were normotensive but became hypertensive when fed normal or high-salt diets. Hypertensive Cyp4a10-/- mice had a dysfunctional kidney epithelial sodium channel and became normotensive when administered amiloride, a selective inhibitor of this sodium channel. These studies (a) establish a physiological role for the arachidonate monooxygenases in renal sodium reabsorption and blood pressure regulation, (b) demonstrate that a dysfunctional Cyp4a10 gene causes alterations in the gating activity of the kidney epithelial sodium channel, and (c) identify a conceptually novel approach for studies of the molecular basis of human hypertension. It is expected that these results could lead to new strategies for the early diagnosis and clinical management of this devastating disease.

Molecular mechanisms of AhR functions in the regulation of cytochrome P450 genes

AhR, a ligand-activated transcription factor, mediates xenobiotic signaling to enhance the expression of target genes, including drug-metabolizing cytochrome P450s. The recent development of several new techniques, including chromatin immunoprecipitation and RNA interference, has expanded and deepened our knowledge of AhR function in the xenobiotic signal transduction. In this review, we briefly summarize our current understanding of the activation and inactivation of AhR activities and discuss the future directions of AhR research.

CYP35: xenobiotically induced gene expression in the nematode Caenorhabditis elegans

Although over 80 cytochrome P450 (CYP) encoding genes have been identified in the genome of the nematode Caenorhabditis elegans very little is known about their involvement in biotransformation. This paper demonstrates a concentration-dependent relationship of C. elegans CYP35A1, A2, A5, and C1 gene expression in response to four organic xenobiotics, namely atrazine, PCB52, fluoranthene, and lansoprazole. The toxicity of these xenobiotics was determined using a reproduction assay. CYP-specific messenger RNA expression was analyzed by semi-quantitative RT-PCR resulting in a strongly increasing, concentration-dependent induction well below the EC50 for reproduction. For PCB52, approximately 0.5% of the EC50 induces a 2-fold increase of CYP35 gene expression. Using a double mutant and multiple RNAi of CYP35A/C it was possible to diminish the reproduction decline caused by PCB52 and fluoranthene.

Regulation of CYP3A4 by the bile acid receptor FXR

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

GABAA/central benzodiazepine receptor and peripheral benzodiazepine receptor ligands as inducers of phenobarbital-inducible CYP2B and CYP3A

A sequence critical for phenobarbital (PB) induction, the PB response unit (PBRU), situated upstream of the rat CYP2B1 and CYP2B2 genes, includes two nuclear receptor binding sites, NR1 and NR2. When NR1 and NR2 are mutated PB responsiveness is abolished. While no nuclear receptor for which PB is an agonist ligand has yet been identified, PB is a ligand of GABA(A) receptors and it can displace [(3)H] 1-(2-chlorophenyl)-N-methyl-N-(1-methylpropyl)-3-isoquinolinecarboxamide (PK 11195) from its binding site on the peripheral benzodiazepine receptor (PBR). We assessed CYP2B levels in primary rat hepatocytes following treatment with 10 ligands of either or both of these receptors. All compounds tested were found to be CYP2B1/CYP2B2 inducers and most were CYP3A inducers. Five had not previously been described as CYP2B1/CYP2B2 inducers: bicuculline, flunitrazepam, 4'-chlorodiazepam (Ro5-4864), N,N-dihexyl-2-(4-fluorophenyl)indole-3-acetamide (FGIN 1-27) and 7-(dimethylcarbamoyloxy)-6-phenylpyrrolo-[2,1-d][1,5]benzothiazepine (DCPPBT). Reporter gene analysis demonstrated that CYP2B induction by these agents and other PBR or GABA(A) receptor ligands is mediated through the PBRU and the NR1/NR2 sites, suggesting a molecular mechanism similar to that for PB induction. The potencies for PBRU-dependent induction by 11 ligands of PBR or the GABA(A) receptor was evaluated. FGIN-127, DCPPBT and PK 11195 exhibited EC(50) values for PBRU-dependent transcription activation about three orders of magnitude higher than the reported affinities of the PBR for these agents, arguing against the involvement of the PBR in PB induction. However the EC(50) values found for the agents tested encourage further investigation on the possible involvement of the GABA(A) receptor in PB induction.

Effect of pioglitazone on endotoxin-induced decreases in hepatic drug-metabolizing enzyme activity and expression of CYP3A2 and CYP2C11

It has been reported that peroxisome proliferator-activated receptor-gamma (PPAR-gamma) ligands ameliorate the expression of inducible nitric oxide synthase (iNOS) by endotoxin. In the present study, we investigated the effect of pioglitazone, a potent PPAR-gamma ligand, on the endotoxin-induced reduction of hepatic drug-metabolizing enzyme activity and on the down-regulation of the expression of hepatic cytochrome P450 (CYP) 3A2 and CYP2C11 proteins in rats. Endotoxin (1 mg/kg) significantly decreased hepatic drug-metabolizing enzyme activity in vivo, as represented by the systemic clearance of antipyrine and protein levels of CYP3A2 and CYP2C11 24 h after intraperitoneal injection. Pretreatment with pioglitazone (10 mg/kg, 4 times at 10-min intervals) significantly protected the endotoxin-induced decreases in the systemic clearance of antipyrine and protein levels of CYP3A2, but not CYP2C11, with no biochemical and histopathological changes in the liver. Pioglitazone alone had no effect on the systemic clearance of antipyrine and protein levels of CYP3A2 or CYP2C11. Pioglitazone significantly protected endotoxin-induced overexpression of iNOS in the liver, but not the overproduction of nitric oxide (NO) in plasma. It is unlikely that the protective effect of pioglitazone against endotoxin-induced decreases in the hepatic drug-metabolizing enzyme activity and protein levels of CYP3A2 in the liver is due to the inhibition of the overproduction of NO.

Induction of Drug Metabolism: The Role of Nuclear Receptors

Induction of drug metabolism was described more than 40 years ago. Progress in understanding the molecular mechanism of induction of drug-metabolizing enzymes was made recently when the important roles of the pregnane X receptor (PXR) and the constitutive androstane receptor (CAR), two members of the nuclear receptor superfamily of transcription factors, were discovered to act as sensors for lipophilic xenobiotics, including drugs. CAR and PXR bind as heterodimeric complexes with the retinoid X receptor to response elements in the regulatory regions of the induced genes. PXR is directly activated by xenobiotic ligands, whereas CAR is involved in a more complex and less well understood mechanism of signal transduction triggered by drugs. Most recently, analysis of these xenobiotic-sensing nuclear receptors and their nonmammalian precursors such as the chicken xenobiotic receptor suggests an important role of PXR and CAR also in endogenous pathways, such as cholesterol and bile acid biosynthesis and metabolism. In this review, recent findings regarding xenosensors and their target genes are summarized and are put into an evolutionary perspective in regard to how a living organism has derived a system that is able to deal with potentially toxic compounds it has not encountered before.

Genomic characterization and regulation of CYP3a13: role of xenobiotics and nuclear receptors

We report that CYP3a13 gene, located on mouse chromosome 5, spans 27.5 Kb and contains 13 exons. The transcription start site is 35 bp upstream of the coding region and results in a 109 bp 5' untranslated region. CYP3a13 promoter shows putative binding sites for retinoid X receptor, pregnane X receptor, and estrogen receptor. CYP3a13 shows a broad tissue distribution with predominant expression in liver. Although CYP3a13 shares 92% nucleotide identity with the female-specific rat CYP3A9, its expression does not exhibit sexual dimorphism. Ligand activation of peroxisomal proliferator-activated receptor-gamma and retinoid X receptor inhibit expression of CYP3a13 at the transcription level in a tissue-specific manner. Another novel finding is hepatic induction of CYP3a13 by dexamethasone occurring only in pregnane X receptor null mice. We also report that pregnane X receptor is essential to maintain robust in vivo basal levels of CYP3a13 in contrast to CYP3a11. CYP3a13 protein expressed in vitro can metabolize clinically active drugs ethylmorphine and erythromycin, as well as benzphetamine. We conclude that CYP3a13 is regulated differentially by various nuclear receptors. In humans this may lead to altered drug metabolism, as many of the newly synthesized ligands/drugs targeted toward these nuclear receptors could influence CYP3A gene expression.

Coactivator binding promotes the specific interaction between ligand and the pregnane X receptor

The pregnane X receptor (PXR) detects the presence of a wide variety of endogenous and xenobiotic compounds, and is a master regulator of the expression of genes central to drug metabolism and excretion. We present the 2.0A crystal structure of the human PXR ligand-binding domain (LBD) in complex with the cholesterol-lowering compound SR12813 and a 25 amino acid residue fragment of the human steroid receptor coactivator-1 (SRC-1) containing one LXXLL motif. PXR crystallizes as a homodimer in the asymmetric unit in this structure and possesses a novel alpha2 helix adjacent to its ligand-binding cavity. The SRC-1 peptide forms two distinct helices and binds adjacent to the ligand-dependent transactivation AF-2 helix on the surface of PXR. In contrast with previous PXR structures, in which SR12813 bound in multiple orientations, the small SR12813 agonist in this structure binds in a single, unique orientation within the receptor's ligand-binding pocket and contacts the AF-2 helix. Thermal denaturation studies reveal that the SR12813 ligand and SRC-1 coactivator peptide each stabilize the LBD of PXR, and that together they exert an additive effect on the stability of the receptor. These results indicate that the binding of coactivator to the surface of PXR limits the ability of this promiscuous receptor to "breathe" and helps to trap a single, active conformation of SR12813. They further reveal that specificity is required for PXR activation.

A binuclear zinc transcription factor binds the host isoflavonoid-responsive element in a fungal cytochrome p450 gene responsible for detoxification

The PDA1 gene of the filamentous fungus Nectria haematococca MPVI (anamorph: Fusarium solani) encodes pisatin demethylase, a cytochrome P450. Pisatin is a fungistatic isoflavonoid produced by garden pea (Pisum sativum), a host for this fungus. Pisatin demethylase detoxifies pisatin and functions as a virulence factor for this fungus. Pisatin induces PDA1 expression both in cultured mycelia as well as during pathogenesis on pea. The regulatory element within PDA1 that provides pisatin-responsive expression was identified using a combination of in vivo functional analysis and in vitro binding analysis. The 40 bp pisatin-responsive element is located 635 bp upstream of the PDA1 transcription start site. This element was sufficient to provide strong pisatin-induced expression to a minimal promoter in vivo and was required for pisatin regulation of the PDA1 promoter. A gene encoding a DNA-binding protein specific to this 40 bp element was isolated from a N. haematococca cDNA library using the yeast one-hybrid screen. The cloned gene possesses sequence motifs found in the binuclear zinc (Cys 6-Zn 2) family of transcription factors unique to fungi. The results suggest that it is a regulator of this fungal cytochrome P450 gene and may provide pisatin-responsive regulation.

Regulation of CYP3A4 expression in human hepatocytes by pharmaceuticals and natural products

Human CYP3A4 metabolizes a majority of clinically important substrates at variable rates. Accounting for these unpredictable rates is the wide variation noted in expression of this enzyme that is due, in part, to xenobiotic exposure. We used primary cultures of human hepatocytes from 17 individuals to assess the inducibility of CYP3A4 mRNA by prototypical inducers, dietary flavonoids, and botanicals. Those agents producing the greatest mRNA accumulation were 10 microM RIF (699 +/- 307% of control levels) 100 microM phenytoin (707 +/- 188% of control), 1 mM phenobarbital (536 +/- 207% of control), and 100 microM omeprazole (404 +/- 8% of control). Various concentrations of RIF were found to exhibit a typical dose-response curve for CYP3A4 mRNA content. A reporter gene assay using the human pregnane X receptor (hPXR) and promoter regions of CYP3A4 transiently transfected into HepG2 cells, exhibited inductive properties by the aforementioned therapeutics that were similar to those observed in hepatocytes. Several flavonoids including quercetin, resveratrol, and curcumin were also examined for their ability to induce CYP3A4 in human hepatocytes. Only quercetin produced accumulation of CYP3A4 mRNA (230 +/- 73% of control). When examined in a reporter gene assay, this flavonoid exhibited negligible increases in luciferase activity suggesting that quercetin induced CYP3A4 by mechanisms that may not involve PXR. We also examined the effects of herbals on CYP3A4 expression in human hepatocytes. Grapeseed extract, ginseng, silymarin, and kava-kava produced 270 +/- 73, 155 +/- 83, 100 +/- 10, and 386 +/- 185% of control CYP3A4 mRNA, respectively. Of these botanicals only kava-kava produced enhanced luciferase activity (11.6 +/- 2.1 fold above DMSO treated cells). Such results indicate that kava-kava required PXR to mediate CYP3A4 induction. Collectively, results demonstrated that several botancials induce CYP3A4, suggesting the potential for drug-herbal interactions.

Effects of prototypical microsomal enzyme inducers on cytochrome P450 expression in cultured human hepatocytes

Cultured human hepatocytes are a valuable in vitro system for evaluating new molecular entities as inducers of cytochrome P450 (P450) enzymes. The present study summarizes data obtained from 62 preparations of cultured human hepatocytes that were treated with vehicles (saline or dimethylsulfoxide, 0.1%), beta-naphthoflavone (33 microM), phenobarbital (100 or 250 microM), isoniazid (100 microM) and/or rifampin (20 or 50 microM), and examined for the expression of P450 enzymes based on microsomal activity toward marker substrates, or in the case of CYP2C8, the level of immunoreactive protein. The results show that CYP1A2 activity was markedly induced by beta-naphthoflavone (on average 13-fold, n = 28 preparations), and weakly induced by phenobarbital (1.9-fold, n = 25) and rifampin (2.3-fold, n = 22); CYP2A6 activity tended to be increased with phenobarbital (n = 7) and rifampin (n = 3) treatments, but the effects were not statistically significant; CYP2B6 was induced by phenobarbital (6.5-fold, n = 13) and rifampin (13-fold, n = 14); CYP2C8 was induced by phenobarbital (4.0-fold, n = 4) and rifampin (5.2-fold, n = 4); CYP2C9 was induced by phenobarbital (1.8-fold, n = 14) and rifampin (3.5-fold, n = 10); CYP2C19 was markedly induced by rifampin (37-fold, n = 10), but relatively modestly by phenobarbital (7-fold, n = 9); CYP2D6 was not significantly induced by phenobarbital (n = 5) or rifampin (n = 5); CYP2E1 was induced by phenobarbital (1.7-fold, n = 5), rifampin (2.2-fold, n = 5), and isoniazid (2.3-fold, n = 5); and, CYP3A4 was induced by phenobarbital (3.3-fold, n = 42) and rifampin (10-fold, n = 61), but not by beta-naphthoflavone. Based on these observations, we generalize that beta-naphthoflavone induces CYP1A2 and isoniazid induces CYP2E1, whereas rifampin and, to a lesser extent phenobarbital, tend to significantly and consistently induce enzymes of the CYP2A, CYP2B, CYP2C, CYP2E, and CYP3A subfamilies but not the 2D subfamily.

Assessment of porcine and human 16-ene-synthase, a third activity of P450c17, in the formation of an androstenol precursor. Role of recombinant cytochrome b5 and P450 reductase

Recently, we have shown that the biosynthesis of androstenol, a potential endogenous ligand for the orphan receptors constitutive androstane receptor and pregnane-X-receptor, requires the presence of enzymes of the steroidogenic pathway, such as 3 beta-hydroxysteroid dehydrogenase, 5 alpha-reductase and 3 alpha-hydroxysteroid dehydrogenase. In this report, we examine at the molecular level whether the enzyme 17 alpha-hydroxylase/17,20-lyase (P450c17), which possesses dual 17 alpha-hydroxylase and 17,20-lyase activities and catalyzes the production of precursors for glucocorticoids and sex steroids, is also able to catalyze the formation of a third class of active steroids, 16-ene steroids (including androstenol). The role of components of the P450 complex is also assessed. We transfected human embryonic kidney (HEK-293) cells with various amounts of vectors expressing P450c17, NADPH-cytochrome P450 reductase, and cytochrome b5. Our results showed that P450c17 possesses a 16-ene-synthase activity able to transform pregnenolone into 5,16-androstadien-3 beta-ol, without the formation of the precursor 17-hydroxypregnenolone. Cytochrome b5 has a much stronger effect on the 16-ene-synthase activity than on the 17 alpha-hydroxylase/17,20-lyase activities. On the other hand, P450reductase has a drastic effect on the latter, but a negligible one on 5,16-androstadien-3 beta-ol synthesis. Our results therefore demonstrate that human P450c17, as other enzymes of the classical steroidogenic pathway, is involved in the biosynthetic pathway leading to the formation of androstenol.

Phenobarbital induction of drug/steroid-metabolizing enzymes and nuclear receptor CAR

Phenobarbital (PB) increases hepatic drug/steroid-metabolic capability by coordinately activating transcription of the genes encoding various metabolizing enzymes. The nuclear receptor CAR was first implicated as a transcription factor that activates the cytochrome P450 Cyp2b10 gene. In response to PB, CAR forms a heterodimer with the retinoid X receptor (RXR), binds to a PB response element (typified by DR-4 motif), and activates transcription of the gene. In the CAR-null mouse, PB does not only induce the Cyp2b10 gene, but also induces genes encoding various metabolizing enzymes. Thus, CAR is a general nuclear receptor that is essential for PB induction of drug/steroid metabolizing enzymes. PB also induces amino levulinate synthase 1 (ALAS-1), the rate-limiting enzyme in heme biosynthesis, to increase heme supply. However, PB induction of the synthase occurs in CAR-null mice, suggesting that CAR does not coordinate the heme synthesis for the induction of drug/steroid metabolism.

A cell-based reporter gene assay for determining induction of CYP3A4 in a high-volume system

Assessing the inducibility of CYP3A4 by various xenobiotics can predict potential drug interactions. In the present investigation, human hepatoma cells were stably integrated with either the CYP3A4 enhancer region and a luciferase reporter gene or the CYP3A4-luciferase construct and the human pregnane X receptor (PXR). Several colonies containing one to three copies of luciferase per cell were identified by Southern blot analysis. Those transformants producing high luciferase activity in response to rifampicin were used to standardize a 96-well plate screening system with minimal inter- and intraplate variability. Standardization also consisted of assessing viability of cells cultured in medium containing various serum concentrations. In cells maintained for 48 h in medium with less than 5% serum, a significant (p < 0.01) decline was observed in viability accompanied by altered induction. A defined serum-free medium also produced less viable cells but did not alter the inductive response. Treatment of transformants with various concentrations of rifampicin produced a dose-response curve with maximal induction at 10 microM (5.6 +/- 0.18- and 2.1 +/- 0.3-fold above dimethyl sulfoxide (DMSO)-treated cells in transformants with and without PXR, respectively). Of additional agents examined for their ability to induce CYP3A4, omeprazole (200 microM) was the most potent inducer (12.8 +/- 1.9- and 2.4 +/- 0.2-fold above DMSO-treated cells in transformants with and without PXR, respectively). Mifepristone and mevastatin produced modest induction (approximately 3-fold) in the cell line containing exogenous PXR, but produced less than 1.2-fold increases in cells lacking PXR. Thus, only potent inducers can be identified in the cell line without PXR. In contrast, cells containing the receptor can be used to rank CYP3A4 induction. Because a high volume of chemicals can be readily and accurately screened for their ability to induce CYP3A4 with this format, such a system could be valuable in the initial stages of preclinical drug development.

The CYP4A isoforms hydroxylate epoxyeicosatrienoic acids to form high affinity peroxisome proliferator-activated receptor ligands

Cytochromes P450 of the CYP2C and CYP4A gene subfamilies metabolize arachidonic acid to 5,6-, 8,9-, 11,12-, and 14,15-epoxyeicosatrienoic acids (EETs) and to 19- and 20-hydroxyeicosatetraenoic acids (HETEs), respectively. Abundant functional studies indicate that EETs and HETEs display powerful and often opposing biological activities as mediators of ion channel activity and regulators of vascular tone and systemic blood pressures. Incubation of 8,9-, 11,12-, and 14,15-EETs with microsomal and purified forms of rat CYP4A isoforms led to rapid NADPH-dependent metabolism to the corresponding 19- and 20-hydroxylated EETs. Comparisons of reaction rates and catalytic efficiency with those of arachidonic and lauric acids showed that EETs are one of the best endogenous substrates so far described for rat CYP4A isoforms. CYP4A1 exhibited a preference for 8,9-EET, whereas CYP4A2, CYP4A3, and CYP4A8 preferred 11,12-EET. In general, the closer the oxido ring is to the carboxylic acid functionality, the higher the rate of EET metabolism and the lower the regiospecificity for the EET omega-carbon. Analysis of cis-parinaric acid displacement from the ligand-binding domain of the human peroxisome proliferator-activated receptor-alpha showed that omega-hydroxylated 14,15-EET bound to this receptor with high affinity (K(i) = 3 +/- 1 nm). Moreover, at 1 microm, the omega-alcohol of 14,15-EET or a 1:4 mixture of the omega-alcohols of 8,9- and 11,12-EETs activated human and mouse peroxisome proliferator-activated receptor-alpha in transient transfection assays, suggesting a role for them as endogenous ligands for these orphan nuclear receptors.

Aryl hydrocarbon receptors: diversity and evolution

Animals have evolved inducible enzymatic defenses to facilitate the biotransformation and elimination of toxic compounds encountered in the environment. The sensory component of this system consists of soluble receptors that regulate the expression of certain isoforms of cytochrome P450, other enzymes, and transporters in response to environmental chemicals. These receptors include several members of the steroid/nuclear receptor superfamily as well as the aryl hydrocarbon receptor (AHR), a member of the bHLH-PAS gene superfamily. In addition to its adaptive functions, the AHR serves poorly understood physiological roles; interference with those roles by dioxins and related chemicals causes toxicity. One approach to understanding the physiological significance of the AHR is to characterize its structure, function, and regulation in diverse species, including mammals, birds, fish, and invertebrates. These animal groups include model species with unique features that can be exploited to broaden our understanding of AHR function. Studies carried out in diverse species also provide phylogenetic information that allows inferences about the evolutionary history of the AHR. This review summarizes the current understanding of AHR diversity among animal species and the evolution of the AHR signaling pathway, as inferred from molecular studies in vertebrate and invertebrate animals. The AHR gene has undergone duplication and diversification in vertebrate animals, resulting in at least three members of an AHR gene family: AHR1, AHR2, and AHR repressor. The inability of invertebrate AHR homologs to bind dioxins and related chemicals, along with other evidence, suggests that the adaptive role of the AHR as a regulator of xenobiotic metabolizing enzymes may have been a vertebrate innovation. The physiological functions of the AHR during development appear to be ancestral to the adaptive functions. Sensitivity to the developmental toxicity of dioxins and related chemicals may have had its origin in the evolution of dioxin-binding capacity of the AHR in the vertebrate lineage.

Drugs mediate the transcriptional activation of the 5-aminolevulinic acid synthase (ALAS1) gene via the chicken xenobiotic-sensing nuclear receptor (CXR)

Heme is an essential component in oxygen transport and metabolism in living systems. In non-erythropoietic cells, 5-aminolevulinate synthase (ALAS1) is the first and rate-limiting enzyme in the heme biosynthesis pathway. ALAS1 expression and heme levels are increased in vivo by drugs and other chemical inducers of cytochrome P450 hemoproteins through mechanisms that are poorly understood. In the present studies, a chicken genomic cosmid library was employed to isolate a major portion of the ALAS1 gene. Two drug-responsive enhancer sequences, 176 and 167 base pairs in length, were identified in the 5'-flanking region of the gene in reporter gene assays in the hepatoma cell line LMH. The relative potency of inducers to activate these enhancers corresponds to induction of ALAS1 mRNA levels in LMH cells. Analysis of putative transcription factor binding sites within the enhancers revealed DR5 and DR4 type recognition sequences for nuclear receptors. Drug activation of the enhancer elements was reduced at least 60% after mutagenesis of individual nuclear receptor binding sites and was virtually eliminated following alteration of both recognition sites within the respective elements. Electrophoretic mobility shift assays and transactivation studies demonstrate direct interactions between the nuclear receptor binding sites and the recently described chicken xenobiotic-sensing receptor, (CXR) implicating drug activation mechanisms for ALAS1 similar to those found in inducible cytochrome(s) P450. This is the first report describing direct transcriptional activation of ALAS1 by drugs via drug-responsive enhancer sequences.

Cholesterol and bile acids regulate xenosensor signaling in drug-mediated induction of cytochromes P450

Cytochromes P450 (CYP) constitute the major enzymatic system for metabolism of xenobiotics. Here we demonstrate that transcriptional activation of CYPs by the drug-sensing nuclear receptors pregnane X receptor, constitutive androstane receptor, and the chicken xenobiotic receptor (CXR) can be modulated by endogenous cholesterol and bile acids. Bile acids induce the chicken drug-activated CYP2H1 via CXR, whereas the hydroxylated metabolites of bile acids and oxysterols inhibit drug induction. The cholesterol-sensing liver X receptor competes with CXR, pregnane X receptor, or constitutive androstane receptor for regulation of drug-responsive enhancers from chicken CYP2H1, human CYP3A4, or human CYP2B6, respectively. Thus, not only cholesterol 7 alpha-hydroxylase (CYP7A1), but also drug-inducible CYPs, are diametrically affected by these receptors. Our findings reveal new insights into the increasingly complex network of nuclear receptors regulating lipid homeostasis and drug metabolism.