Dexamethasone responsiveness of a major glucocorticoid-inducible CYP3A gene is mediated by elements unrelated to a glucocorticoid receptor binding motif

Cytochrome P450 3A4, 3A5, and 2C8 expression in breast, prostate, lung, endometrial, and ovarian tumors: relevance for resistance to taxanes

Enzymes of the cytochrome P450 (CYP) subfamily 3A and 2C play a major role in the metabolism of taxane anticancer agents. While their function in hepatic metabolism of taxanes is well established, expression of these enzymes in solid tumors may play a role in the in situ metabolism of drugs as well, potentially affecting the intrinsic taxane susceptibility of these tumors. This article reviews the available literature on intratumoral expression of docetaxel- and paclitaxel-metabolizing enzymes in mammary, prostate, lung, endometrial, and ovarian tumors. Furthermore, the clinical implications of the intratumoral expression of these enzymes are reviewed and the potential of concomitant treatment with protease inhibitors (PIs) as a method to inhibit CYP3A4-mediated metabolism is discussed.

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

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

The genes of all seven CYP3A isoenzymes identified in the equine genome are expressed in the airways of horses

In the present study, we examined the gene expression of cytochrome P450 3A (CYP3A) isoenzymes in the tracheal and bronchial mucosa and in the lung of equines using TaqMan probes. The results show that all seven CYP3A isoforms identified in the equine genome, that is, CYP3A89, CYP3A93, CYP3A94, CYP3A95, CYP3A96, CYP3A97 and CYP3A129, are expressed in the airways of the investigated horses. Though in previous studies, CYP3A129 was found to be absent in equine intestinal mucosa and liver, this CYP3A isoform is expressed in the airways of horses. The gene expression of the CYP3A isoenzymes varied considerably between the individual horses studied. However, in most of the horses CYP3A89, CYP3A93, CYP3A96, CYP3A97 and CYP3A129 were expressed to a high extent, while CYP3A94 and CYP3A95 were expressed to a low extent in the different parts of the airways. The CYP3A isoenzymes present in the airways may play a role in the metabolic degradation of inhaled xenobiotics. In some instances, the metabolism may, however, result in bioactivation of the xenobiotics and subsequent tissue injury.

Brief exposures of human body lice to sublethal amounts of ivermectin over-transcribes detoxification genes involved in tolerance

Transcriptional profiling results, using our non-invasive induction assay {short exposure intervals (2-5 h) to sublethal amounts of insecticides [< lethal concentration 3% (LC(3)) at 24 h] administered by stress-reducing means (contact vs. immersion screen) and with induction assessed in a time frame when tolerance is still present [~lethal concentration 90% (LC(90)) in 2-4 h]}, showed that ivermectin-induced detoxification genes from body lice are identified by quantitative real-time PCR analyses. Of the cytochrome P450 monooxygenase and ATP binding cassette transporter genes induced by ivermectin, CYP6CJ1, CYP9AG1, CYP9AG2 and PhABCC4 were respectively most significantly over-expressed, had high basal expression levels and were most closely related to genes from other organisms that metabolized insecticides, including ivermectin. Injection of double-stranded RNAs (dsRNAs) against either CYP9AG2 or PhABCC4 into non-induced female lice reduced their respective transcript level and resulted in increased sensitivity to ivermectin, indicating that these two genes are involved in the xenobiotic metabolism of ivermectin and in the production of tolerance.

Phthalate induction of CYP3A4 is dependent on glucocorticoid regulation of PXR expression

Cytochrome P450 3A4 (CYP3A4) is responsible for oxidative metabolism of more than 60% of all pharmaceuticals. CYP3A4 is inducible by xenobiotics that activate pregnane X receptor (PXR), and enhanced CYP3A4 activity has been implicated in adverse drug interactions. Recent evidence suggest that the widely used plasticizer, di-2-ethylhexyl phthalate (DEHP), and its primary metabolite mono-2-ethylhexyl phthalate (MEHP) may act as agonists for PXR. Hospital patients are uniquely exposed to high levels of DEHP as well as being administered glucocorticoids. Glucocorticoids positively regulate PXR expression in a glucocorticoid receptor (GR)-mediated mechanism. We suggest that the magnitude of CYP3A4 induction by phthalates is dependent on the expression of PXR and may be significantly higher in the presence of glucocorticoids. DEHP and MEHP induced PXR-mediated transcription of the CYP3A4 promoter in a dose-dependent fashion. Coexposure to phthalates and dexamethasone (Dex) resulted in enhanced CYP3A4 promoter activity; furthermore, this induction was abrogated by both the GR antagonist RU486 and GR small interfering ribonucleic acid. Dex induced PXR protein expression in human hepatocytes and a liver-derived rat cell line. CYP3A4 protein was highly induced by Dex and DEHP coadministration in human hepatocyte cultures. Finally, enhanced 6beta-hydroxytestosterone formation in Dex and phthalate cotreated human hepatocytes confirmed CYP3A4 enzyme induction. Concomitant exposure to glucocorticoids and phthalates resulting in enhanced metabolic activity of CYP3A4 may play a role in altered efficacy of pharmaceutical agents. Understanding the role of glucocorticoid regulation of PXR as a key determinant in the magnitude of CYP3A4 induction by xenobiotics may provide insight into adverse drug effects in a sensitive population.

P-glycoprotein: so many ways to turn it on

Expression of the ABC transporter P-glycoprotein (P-gp or ABCB1) is associated with resistance to chemotherapy in cancer. However, early investigations into the regulation of ABCB1 expression revealed that the process is not a classical induction as observed for certain metabolizing enzymes. The process involves the cellular stress response pathway initiated by either inflicted (e.g., chemotherapy damage) or endogenous (e.g., hypoxia) factors. However, ABCB1 is also expressed in a number of noncancerous tissues. In particular, the protein is found at tissues providing a barrier or secretory function. The localization of ABCB1 in normal tissues will impact significantly on drug pharmacokinetics, in particular the absorption and elimination processes. This review also describes the mechanism underlying ABCB1 expression in noncancerous tissue, a process that does not involve the stress response.

Cell selective glucocorticoid induction of caveolin-1 and caveolae in differentiating pulmonary alveolar epithelial cell cultures

Increased caveolin-1 expression is a marker of the differentiation of lung alveolar epithelial type II cells into a type I phenotype. Here, we show in both a primary differentiating rat alveolar culture, and a human alveolar cell line (A549) that caveolae formation and caveolin-1 expression are dependent upon dexamethasone Dex, and is inhibited by the glucocorticoid receptor (GR) antagonist, mifepristone. Study of a panel of 20 different cell types showed the effect of (Dex) upon caveolin-1 expression to be highly cell selective for lung alveolar epithelial cells. The actions of glucocorticoid upon caveolin-1 appear indirect acting via intermediary genes as evidenced by cycloheximide (CHX) abolition of Dex-induced increases in caveolin-1 mRNA and by recombinant transfection studies using the caveolin-1 promoter cloned upstream of a reporter gene. Treatment with actinomycin D (ACD) revealed that the effects of Dex are also, at least in part, mediated by stabilisation of caveolin-1 mRNA. Collectively, these results indicate that glucocorticoids modulate the expression of caveolin-1 and caveolae biogenesis within alveolar epithelial cells via both transcriptional and translational modifications. The cell-selective effects of glucocorticoid upon caveolin may represent a previously unrecognised mechanism by which glucocorticoids affect lung development.

Up-regulation of the alligator CYP3A77 gene by toxaphene and dexamethasone and its short term effect on plasma testosterone concentrations

In this study we describe an alligator hepatic CYP3A gene, CYP3A77, which is inducible by dexamethasone and toxaphene. CYP3A plays a broad role in biotransforming both exogenous compounds and endogenous hormones such as testosterone and estradiol. Alligators collected from sites in Florida that are contaminated with organochlorine compounds exhibit differences in sex steroid concentrations. Many organochlorine compounds induce CYP3A expression in other vertebrates; hence, CYP3A induction by organochlorine contaminants could increase biotransformation and clearance of sex steroids by CYP3A and provide a plausible mechanism for the lowering of endogenous sex steroid concentrations in alligator plasma. We used real time PCR to examine whether known and suspected CYP3A inducers (dexamethasone, metyrapone, rifampicin, and toxaphene) up-regulate steady state levels of hepatic CYP3A77 transcript to determine if induction patterns in female juvenile alligators are similar to those reported in other vertebrates and whether toxaphene, an organochlorine compound found in high concentrations in Lake Apopka alligators, induces this gene. Estrogen receptor alpha (ERalpha), estrogen receptor beta (ERbeta), androgen receptor (AR), glucocorticoid receptor (GR), progesterone receptor (PR), and steroid-xenobiotic receptor (SXR) transcripts were also measured to determine whether any of these nuclear receptors are also regulated by these compounds in alligators. Dexamethasone (4.2-fold) and toxaphene (3.5-fold) significantly induced CYP3A77 gene transcript, whereas rifampicin (2.8-fold) and metyrapone (2.1-fold) up-regulated ERbeta after 24h. None of the compounds significantly up-regulated AR, ERalpha, GR, PR, or SXR over this time period. Plasma testosterone (T) did not change significantly after 24h in alligators from any of the treatment groups. Dexamethasone treated animals exhibited a strong relationship between the 24h plasma T concentrations and CYP3A77 (R(2)=0.9, positive) and SXR (R(2)=0.77, negative) transcripts, which suggests that the expression of these genes is related to plasma T in alligators. In light of our findings, we hypothesized that higher steady state CYP3A77 (and possibly SXR) gene expression would be observed in alligators collected from Lake Apopka, a polluted lake containing organochlorine compounds known to induce CYP3A isoforms in other taxa. Therefore, we measured basal levels of CYP3A77 and SXR gene transcripts in wild juvenile alligators collected from Orange Lake (reference lake), Lake Woodruff (reference lake), and Lake Apopka (contaminated lake). We found that no differences existed in CYP3A77 or SXR gene expression among animals from the lakes sampled suggesting that exposure to organochlorine compounds at concentrations present in Lake Apopka does not lead to variation in the expression of these genes, although capture stress could be interfering with these results since the glucocorticoid dexamethasone induces CYP3A77 transcript in alligators.

Molecular and functional comparison of 1,25-dihydroxyvitamin D(3) and the novel vitamin D receptor ligand, lithocholic acid, in activating transcription of cytochrome P450 3A4

The vitamin D receptor (VDR) binds to and mediates the effects of the 1,25-dihydroxyvitamin D(3) (1,25(OH)(2)D(3)) hormone to alter gene transcription. A newly recognized VDR ligand is the carcinogenic bile acid, lithocholic acid (LCA). We demonstrate that, in HT-29 colon cancer cells, both LCA and 1,25(OH)(2)D(3) induce expression of cytochrome P450 3A4 (CYP3A4), an enzyme involved in cellular detoxification. We also show that LCA-VDR stimulates transcription of gene reporter constructs containing DR3 and ER6 vitamin D responsive elements (VDREs) from the human CYP3A4 gene. Utilizing gel mobility shift, pulldown, and mammalian two-hybrid assays, we observe that: (i) 1,25(OH)(2)D(3) enhances retinoid X receptor (RXR) heterodimerization with VDR more effectively than LCA, (ii) the 1,25(OH)(2)D(3)-liganded VDR-RXR heterodimer recruits full-length SRC-1 coactivator, whereas this interaction is minimal with LCA unless LXXLL-containing fragments of SRC-1 are employed, and (iii) both 1,25(OH)(2)D(3) and LCA enhance the binding of VDR to DRIP205/mediator, but unlike 1,25(OH)(2)D(3)-VDR, LCA-VDR does not interact detectably with NCoA-62 or TRIP1/SUG1, suggesting a different pattern of LCA-VDR comodulator association. Finally, residues in the human VDR (hVDR) ligand binding domain (LBD) were altered to create mutants unresponsive to 1,25(OH)(2)D(3)- and/or LCA-stimulated transactivation, identifying S237 and S225/S278 as critical for 1,25(OH)(2)D(3) and LCA action, respectively. Therefore, these two VDR ligands contact distinct residues in the binding pocket, perhaps generating unique receptor conformations that determine the degree of RXR and comodulator binding. We propose that VDR is a bifunctional regulator, with the 1,25(OH)(2)D(3)-liganded conformation facilitating high affinity endocrine actions, and the LCA-liganded configuration mediating local, lower affinity cellular detoxification by upregulation of CYP3A4 in the colon.

Interaction of the glucocorticoid receptor and the chicken ovalbumin upstream promoter-transcription factor II (COUP-TFII): implications for the actions of glucocorticoids on glucose, lipoprotein, and xenobiotic metabolism

Glucocorticoids exert their extremely diverse effects on numerous biologic activities of humans via only one protein module, the glucocorticoid receptor (GR). The GR binds to the glucocorticoid response elements located in the promoter region of target genes and regulates their transcriptional activity. In addition, GR associates with other transcription factors through direct protein-protein interactions and mutually represses or stimulates each other's transcriptional activities. The latter activity of GR may be more important than the former one, granted that mice harboring a mutant GR, which is active in terms of protein-protein interactions but inactive in terms of transactivation via DNA, survive and procreate, in contrast to mice with a deletion of the entire GR gene that die immediately after birth. We recently found that GR physically interacts with the chicken ovalbumin upstream promoter-transcription factor II (COUP-TFII), which plays a critical role in the metabolism of glucose, cholesterol, and xenobiotics, as well as in the development of the central nervous system in fetus. GR stimulates COUP-TFII-induced transactivation by attracting cofactors via its activation function-1, while COUP-TFII represses the GR-governed transcriptional activity by tethering corepressors, such as the silencing mediator for retinoid and thyroid hormone receptors (SMRT) and the nuclear receptor corepressors (NCoRs) via its C-terminal domain. Their mutual interaction may play an important role in gluconeogenesis, lipoprotein metabolism, and enzymatic clearance of clinically important compounds and bioactive chemicals, by regulating their rate-limiting enzymes and molecules, including the phosphoenolpyruvate carboxykinase (PEPCK), the cytochrome P450 CYP3A and CYP7A, and several apolipoproteins. It appears that glucocorticoids exert their intermediary effects partly via physical interaction with COUP-TFII.

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.

The expression of CYP2B6, CYP2C9 and CYP3A4 genes: a tangle of networks of nuclear and steroid receptors

Numerous chemicals increase the metabolic capability of organisms by their ability to activate genes encoding various xenochemical-metabolizing enzymes, such as cytochromes P450 (CYPs), transferases and transporters. For example, natural and synthetic glucocorticoids (agonists and antagonists) as well as other clinically important drugs induce the hepatic CYP2B, CYP2C and CYP3A subfamilies in man, and these inductions might lead to clinically important drug-drug interactions. Only recently, the key cellular receptors that mediate such inductions have been identified. They include nuclear receptors, such as the constitutive androstane receptor (CAR, NR1I3), the retinoid X receptor (RXR, NR2B1), the pregnane X receptor (PXR, NR1I2), and the vitamin D receptor (VDR, NR1I1) and steroid receptors such as the glucocorticoid receptor (GR, NR3C1). There is a wide promiscuity of these receptors in the induction of CYPs in response to xenobiotics. Indeed, this adaptive system appears now as a tangle of networks, where receptors share partners, ligands, DNA response elements and target genes. Moreover, they influence mutually their relative expression. This review is focused on these different pathways controlling human CYP2B6, CYP2C9 and CYP3A4 gene expression, and the cross-talk between these pathways.

Regulation of P450 genes by liver-enriched transcription factors and nuclear receptors

Cytochrome P450s (P450s) constitute a superfamily of heme-proteins that play an important role in the activation of chemical carcinogens, detoxification of numerous xenobiotics as well as in the oxidative metabolism of endogenous compounds such as steroids, fatty acids, prostaglandins, and leukotrienes. In addition, some P450s have important roles in physiological processes, such as steroidogenesis and the maintenance of bile acid and cholesterol homeostasis. Given their importance, the molecular mechanisms of P450 gene regulation have been intensely studied. Direct interactions between transcription factors, including nuclear receptors, with the promoters of P450 genes represent one of the primary means by which the expression of these genes is controlled. In this review, several liver-enriched transcription factors that play a role in the tissue-specific, developmental, and temporal regulation of P450s are discussed. In addition, the nuclear receptors that play a role in the fine control of cholesterol and bile acid homeostasis, in part, through their modulation of specific P450s, are discussed.

Two CCAAT/enhancer binding protein sites in the cytochrome P4503A1 locus. Potential role in the glucocorticoid response

Induction of CYP3A genes by the ligand-activated pregnane-X-receptor (PXR) involves the interaction of other as yet unidentified liver transcription factors. Here we show that the CYP3A1 promoter contains two active sites controlled by the CCAAT/enhancer-binding protein alpha (C/EBPalpha), previously shown to regulate a number of liver stress response genes. We have identified two functional C/EBP binding sites at the CYP3A1 promoter that confer luciferase activity to C/EBPalpha cotransfected CHO cells. When inserted upstream of a thymidine kinase promoter, oligonucleotides corresponding to these elements (-350/-311 and -628/-608), increase reporter gene expression when cotransfected with a C/EBPalpha expression vector. Point mutations in the most conserved nucleotides in either element prevent binding of C/EBPalpha and abolish transactivation of the CYP3A1 promoter. Moreover, we demonstrate that C/EBPalpha accumulates in the rat liver nuclei in response to dexamethasone, and that under these conditions C/EBPalpha binds to the CYP3A1 promoter elements. Our results suggest a correlation between transcription of C/EBPalpha, nuclear protein function and induction of CYP3A1 by dexamethasone in the liver. They also support the notion that C/EBPalpha participates in the up-regulation of the CYP3A1 gene in response to synthetic glucocorticoids.

Liganded VDR induces CYP3A4 in small intestinal and colon cancer cells via DR3 and ER6 vitamin D responsive elements

The nuclear vitamin D receptor (VDR) mediates the effects of 1,25-dihydroxyvitamin D(3) (1,25D(3)) to alter intestinal gene transcription and promote calcium absorption. Because 1,25D(3) also exerts anti-cancer effects, we examined the efficacy of 1,25D(3) to induce cytochrome P450 (CYP) enzymes. Exposure of human colorectal adenocarcinoma cells (HT-29) to 10(-8)M 1,25D(3) resulted in >/=3-fold induction of CYP3A4 mRNA and protein as assessed by RT-PCR and Western blotting, respectively. Six vitamin D responsive element (VDRE)-like sequences in the promoter region of the CYP3A4 gene were then individually tested for their ability to enhance transcription. A canonical DR3-type element in the distal region of the promoter (-7719-GGGTCAgcaAGTTCA-7733), and a proximal, non-classical everted repeat with a spacer of 6 bp (ER6; -169-TGAACTcaaaggAGGTCA-152) were identified as functional VDREs in this CYP gene. These data suggest that 1,25D(3)-dependent, VDR-mediated induction of CYP3A4 may constitute a chemoprotective mechanism for detoxification of enteric xenobiotics and carcinogens.

The nuclear pregnane X receptor: a key regulator of xenobiotic metabolism

The nuclear pregnane X receptor (PXR; NR1I2) is an important component of the body's adaptive defense mechanism against toxic substances including foreign chemicals (xenobiotics). PXR is activated by a large number of endogenous and exogenous chemicals including steroids, antibiotics, antimycotics, bile acids, and the herbal antidepressant St. John's wort. Elucidation of the three-dimensional structure of the PXR ligand binding domain revealed that it has a large, spherical ligand binding cavity that allows it to interact with a wide range of hydrophobic chemicals. Thus, unlike other nuclear receptors that interact selectively with their physiological ligands, PXR serves as a generalized sensor of hydrophobic toxins. PXR binds as a heterodimer with the 9-cis retinoic acid receptor (NR2B) to DNA response elements in the regulatory regions of cytochrome P450 3A monooxygenase genes and a number of other genes involved in the metabolism and elimination of xenobiotics from the body. Although PXR evolved to protect the body, its activation by a variety of prescription drugs represents the molecular basis for an important class of harmful drug-drug interactions. Thus, assays that detect PXR activity will be useful in developing safer prescription drugs.

Glucocorticoid-mediated induction of CYP3A4 is decreased by disruption of a protein: DNA interaction distinct from the pregnane X receptor response element

CYP3A4 is the most abundant cytochrome P450 (P450) in human liver, comprising approximately 30% of the total liver P450 content. This enzyme has an important role in steroid catabolism and metabolism of foreign compounds, with the majority of pharmaceutical compounds being substrates for CYP3A4. The molecular mechanisms that underlie transcriptional activation of CYP3A4 are complex with many steroid hormone nuclear receptors, including glucocorticoid receptor, pregnane X receptor (PXR), vitamin D receptor, and constitutive androstane receptor, playing roles. Nowhere is this more evident than in the induction of CYP3A4 gene expression by glucocorticoids. CYP3A genes lack a consensus glucocorticoid receptor response element and yet are highly induced by classical glucocorticoids such as hydrocortisone and dexamethasone. Recent evidence has demonstrated that glucocorticoids are ligands for the orphan nuclear receptor PXR, and induction of CYP3A genes by glucocorticoids may occur primarily through PXR interactions. In this paper, we present a mutant that disrupts a hepatocyte-nuclear-factor-3/CCAAT-enhancer binding protein alpha binding site in the CYP3A4 proximal promoter. This mutation disrupts induction of a reporter gene construct by the glucocorticoids dexamethasone and hydrocortisone; yet induction by the potent PXR ligand rifampicin is unaffected. Such data provides strong evidence that glucocorticoids induce CYP3A4 gene expression both through the established PXR-dependent pathway but also through a PXR-independent pathway.

Ectopic expression of MHC class II genes (RT1.B(I) beta/alpha) in rat hepatocytes in vivo and in culture can be elicited by treatment with the pregnane X receptor agonists pregnenolone 16 alpha-carbonitrile and dexamethasone

The synthetic steroid, pregnenolone-16-alpha-carbonitrile (PCN), has served for decades as a probe for a postulated series of hepatic defenses activated under situations of environmental "stress". PCN, an antiglucocorticoid, and also such glucocorticoids as dexamethasone (Dex) appear to stimulate hepatic metabolism and elimination of xenobiotics by binding to the nuclear pregnane X receptor (PXR) which then interacts with a distinct DNA response element associated with induction of cytochrome P450 3A genes. To explore the full domain of genes controlled by PCN/PXR, we used differential display to detect rat liver mRNA species selectively induced by PCN or by Dex. Sequence analysis identified one of many PCN induced cDNA fragments as RT1.B(I)beta, a member of the major histocompatability class II (MHC) gene family usually found only in antigen presenting cells. Northern blot analysis of RNA from rat liver or from cultured hepatocytes confirmed that amounts of RT1.B(I)beta mRNA and also of its companion gene, RT1.B(I)alpha mRNA, became readily detectable within 3-6 hours following treatment with PCN or Dex, whereas no induction was observed in spleen RNA. Induction by PCN of RT1.B(I)beta immunoreactive protein was localized to the hepatocytes as judged by immunofluorescence. We conclude that ectopic expression of MHC II genes, an unprecedented effect of steroids or drugs, is rapidly evoked by PCN acting on the liver, directly. The concept of a set of genes coordinately controlled to maintain homeostasis in parenchymal tissues during toxic stress must now be extended to include the immune system.

Regulation of cyp3a gene transcription by the pregnane x receptor

The pregnane X receptor (PXR) is a promiscuous nuclear receptor that has evolved to protect the body from toxic chemicals. PXR is activated by a structurally diverse collection of xenobiotics, including several widely used prescription drugs. Various lipophilic compounds produced by the body, such as bile acids and steroids, also activate PXR. PXR stimulates the transcription of cytochrome P450 3A monooxygenases and other genes involved in the detoxification and elimination of these potentially harmful chemicals. Assays that detect PXR activation have important implications for the design of future drugs in two respects. On the one hand, PXR activation assays can be used to determine whether candidate drugs are likely to induce CYP3A gene expression and interact with other medicines. On the other hand, PXR agonists may prove useful in the treatment of diseases in which toxic metabolites accumulate, such as cholestatic liver disease.

Induction of rat organic anion transporting polypeptide 2 by pregnenolone-16alpha-carbonitrile is via interaction with pregnane X receptor

The rat organic anion transporting polypeptide 2 (oatp2; Slc21a5) is a liver transporter that mediates the uptake of a variety of structurally diverse compounds, and has a high affinity for cardiac glycosides. Treatment of rats with pregnenolone-16alpha-carbonitrile (PCN), a ligand for the rodent pregnane X receptor (PXR), significantly enhances the rat oatp2 gene expression. To understand the molecular mechanism of oatp2 induction by PCN, rat oatp2 gene was cloned. The rat oatp2 gene consists of 16 exons; alternative splicing of the second noncoding exon gives rise to the two published rat oatp2 cDNAs. Approximately 8700 base pairs (bp) of the 5'-flanking region of the rat oatp2 gene were linked to the luciferase reporter gene and used in transient transfection assays in H4IIE cells. Treatment of PCN induced the expression of the reporter gene in a dose-dependent manner. Four potential PXR response elements (PXREs) were identified in the 5'-flanking region of the rat oatp2 gene. One element (DR3-1) is located approximately -5000 bp with three more (DR3-2, DR3-3, and DR3-4) clustered at about -8000 bp. Results from electrophoretic mobility shift assays showed that the PXR-retinoid X receptor alpha heterodimer binds to the DR3-2 with the highest affinity, to the DR3-4 and DR3-1 with a lower affinity, and weakly or not at all to the DR3-3. Furthermore, a series of partial deletions of the 5'-flanking region illustrated that both the proximal and distal clusters of PXREs are required for maximal induction of rat oatp2 by PCN. In conclusion, these data elucidate the molecular mechanism by which PCN treatment induces rat oatp2 gene expression. In addition, this study identifies rat oatp2 as a direct PXR-targeted gene and further supports the hypothesis that activation of PXR affects a network of genes that is involved in either metabolism or transport of drugs, steroids, and bile acids.

Receptor-dependent transcriptional activation of cytochrome P4503A genes: induction mechanisms, species differences and interindividual variation in man

1. The importance of CYP3A enzymes in drug metabolism and toxicology has yielded a wealth of information on the structure, function and regulation of this subfamily and recent research emphasis has been placed on the human forms, namely CYP3A4, CYP3A5, CYP3A7 and CYP3A43. 2. The current review will focus on the receptor-dependency of CYP3A regulation and includes consideration of the regulatory roles of the glucocorticoid (GR), pregnane X (PXR) and constitutive androstane (CAR) receptors. 3. Emphasis has been placed on the topics of expression and substrate specificity, assessment of induction, species differences in induction, CYP3A promoter sequences and regulation of gene expression, structural and functional aspects of receptor-mediated, CYP3A gene activation, receptor variants and interindividual variation in human CYP3A expression, the latter encompassing environmental, physiological and genetic aspects. 4. An outline of future research needs will be discussed in the context of receptor-mediated molecular mechanisms of CYP3A gene regulation and the impact on interindividual variations in CYP3A expression. 5. Taken collectively, this review highlights the importance of understanding the molecular mechanisms of CYP3A induction as a means of rationalizing human responses to many clinically used drugs, in addition to providing a mechanistically coherent platform to understand and predict interindividual variations in response and drug-drug interactions.

Hormonal factors in the morbidities associated with extreme prematurity and the potential benefits of hormonal supplement

Cortisol and thyroid hormones are known to modulate the maturation of various fetal organ systems, enzymes, and biochemical pathways. The cortisol furnished by the structural and biochemical immature fetal adrenal gland renders the extremely premature infants relatively cortisol deficient in comparison with the term newborns. The premature infants also have elevated fetal androgens, the production of which persists until approximately 42 weeks of postconceptional age. The androgens produced by the fetal adrenal cortex and the müllerian inhibiting substance produced by the fetal testis have antiglucocorticoid and inhibitory effects on human fetal lung growth and maturation in vitro. Hypothalamic-pituitary-thyroid axis and thyroid function are also immature in extreme prematurity. In addition, there is reduced tissue thyroid hormone responsiveness. Superimposed on this is the reduced thyroid function seen in non-thyroidal illness in which elevated cytokine levels have been implicated. Repeated courses of antenatal steroids and high-dose postnatal dexamethasone appear to be deleterious to lung and brain development. This may be through inhibition of cell replication and catabolism as well as decreased thyroid-stimulating hormone secretion and reduced peripheral conversion of T(4) to T(3). Furthermore, dexamethasone has been found to enhance neurosteroid production in the immature brain, potentially altering brain development. Considered together, the relative cortisol deficiency/androgen excess and reduced thyroid function as well as prolonged high-dose postnatal dexamethasone therapy in these infants may be important factors in their high degree of morbidity. We propose to restrict antenatal steroids to a single course and hypothesize that the overall outcome of low-gestation infants would be improved with (1) hydrocortisone (i.v./p.o.) supplement at a fixed dose of 0.5 mg/kg birth weight every 12 h in infants <30 weeks of gestation from birth till 32 weeks of postconceptional age and (2) T(3) (i.v./p.o.) supplement at a fixed dose of 0.4 microg/kg birth weight every 12 h in those <27 weeks of gestation from birth till 32 weeks of postconceptional age.

Studies on transcriptional regulation of Cyp3a16 gene in mouse livers by application of direct DNA injection method

CYP3A16 is expressed in the mouse liver specifically during a fetal and puberty life. Functional regions responsible for the transcriptional regulation of the Cyp3a16 gene were identified by an in vivo direct DNA injection into mouse livers followed by a luciferase assay. The results of the deletion analysis of a 5'-flanking sequence suggested the existence of an adult-specific repressor(s) interacting with the Cyp3a16 gene. In addition, a positive regulatory element was assumed to be present in a region from -146 to -56 of the Cyp3a16 gene. A hepatocyte nuclear factor 4 (HNF-4)-binding consensus sequence was found in this region. In fact, HNF-4alpha bound to this sequence as evidenced by a gel mobility shift assay. The role of the HNF-4-binding sequence was further examined by an introduction of mutations in this sequence. The introduction of the mutations resulted in a reduced activity of a luciferase in the assay. These results obtained by an application of the direct DNA injection method suggest that the HNF-4alpha activates the transcription of the Cyp3a16 gene in vivo.

A novel glucocorticoid regulatory unit mediates the hormone responsiveness of the beta1-adrenergic receptor gene

The effects of glucocorticoids on expression of the beta1-adrenergic receptor (beta1AR) gene have been varied. To study the mechanism underling hormonal regulation of the beta1AR, transient transfection of progressively deleted ovine beta1AR promoter fragments was used to identify a 43-bp region (-1274 to -1232 from the translation start site) that contains a novel glucocorticoid regulatory unit (GRU) and confers glucocorticoid responsiveness. Using DNase I footprinting and electrophoretic mobility shift assays (EMSA), we demonstrated the GRU was composed of a palindrome, 5'-TAATTA-3', which is a core binding motif for the homeodomain proteins, an E-box (5'-CACGTG-3'), binding site for the Myc/Max family proteins, and an overlapping glucocorticoid response element (GRE) half-site (5'-TGTTCT-3'). EMSA demonstrated that the GRE half-site is critical for GRU-protein interactions, which also require binding of proteins to the E-box and the homeodomain region. Co-transfection of a plasmid expressing a c-myc antisense construct significantly reduced glucocorticoid responsiveness of the ovine beta1AR promoter. Furthermore, expression of proteins binding to the GRU was shown to be developmentally regulated, being high in embryonic, reduced in newborn and not detectable in adult heart. We conclude that the ovine beta1AR promoter contains a novel, functional GRU and that glucocorticoid receptor (GR) and the Myc/Max family proteins are involved in the cell-specific nuclear factor binding and transactivation via this element. The results suggest an alternative pathway through which glucocorticoids may exert their effects on genes lacking a full consensus GRE.

Phenobarbital response elements of cytochrome P450 genes and nuclear receptors

Phenobarbital (PB) response elements are composed of various nuclear receptor (NR)-binding sites. A 51-bp distal element PB-responsive enhancer module (PBREM) conserved in the PB-inducible CYP2B genes contains two NR-binding direct repeat (DR)-4 motifs. Responding to PB exposure in liver, the NR constitutive active receptor (CAR) translocates to the nucleus, forms a dimer with the retinoid X receptor (RXR), and activates PBREM via binding to DR-4 motifs. For CYP3A genes, a common NR site [DR-3 or everted repeat (ER)-6] is present in proximal promoter regions. In addition, the distal element called the xenobiotic responsive module (XREM) is found in human CYP3A4 genes, which contain both DR-3 and ER-6 motifs. Pregnane X receptor (PXR) could bind to all of these sites and, upon PB induction, a PXR:RXR heterodimer could transactivate XREM. These response elements and NRs are functionally versatile, and capable of responding to distinct but overlapping groups of xenochemicals.

Regulation of cytochrome P450 (CYP) genes by nuclear receptors

Members of the nuclear-receptor superfamily mediate crucial physiological functions by regulating the synthesis of their target genes. Nuclear receptors are usually activated by ligand binding. Cytochrome P450 (CYP) isoforms often catalyse both formation and degradation of these ligands. CYPs also metabolize many exogenous compounds, some of which may act as activators of nuclear receptors and disruptors of endocrine and cellular homoeostasis. This review summarizes recent findings that indicate that major classes of CYP genes are selectively regulated by certain ligand-activated nuclear receptors, thus creating tightly controlled networks.

Use of the nuclear receptor PXR to predict drug interactions

We recently cloned the human, rabbit, rat, and mouse orthologs of a novel member of the steroid/retinoid/thyroid hormone receptor family, which we have named the Pregnane X Receptor (PXRs). The discovery and characterization of PXR has led to an increased understanding of the molecular basis of many drug-drug interactions as well as a better understanding of xenobiotic metabolism in general. The key insights into PXR action was the finding that this nuclear receptor is linked to regulation of the cytochrome P450 3A monooxygenase (CYP3A) genes. Several lines of evidence indicate that PXR mediates the induction of CYP3A gene transcription. First, PXR is selectively expressed in the liver and intestine, the same tissues in which CYP3A gene expression is induced. Second, PXR binds as a heterodimer with the retinoid X receptor (RXR) to xenobiotic response elements that have been identified in CYP3A gene promoters. Third, PXR is activated by the remarkable array of compounds that are known to induce CYP3A gene transcription. And finally, PXRs from different species are differentially activated by certain compounds such as rifampicin and pregnenolone 16alpha-carbonitrile (PCN) in a manner that correlates with species-specific induction of CYP3A gene expression. We are now employing high throughput PXR activation and binding assays to identify drug candidates that induce CYP3A gene expression so that these compounds can be removed from the drug development process.

A conserved nuclear receptor consensus sequence (DR-4) mediates transcriptional activation of the chicken CYP2H1 gene by phenobarbital in a hepatoma cell line

Phenobarbital-responsive DNA elements were identified in the 5'-flanking region of the chicken CYP2H1 gene by in reporter gene assays in a chicken hepatoma cell line (leghorn male hepatoma (LMH)). A 264-base pair (bp) enhancer sequence (phenobarbital-responsive unit (PBRU)) responded to phenobarbital and a variety of phenobarbital-type inducers. Analysis of putative transcription factor binding sites within the 264-bp element revealed a nuclear receptor half-site repeat (DR-4) neighboring a putative nuclear factor-1 site. This motif resembles phenobarbital response elements in the flanking regions of three phenobarbital-inducible genes, rat CYP2B2, mouse Cyp2b10, and human CYP2B6. Activation of the 264-bp element was eliminated after site-directed mutagenesis of the DR-4 hexamer half-sites. Evidence for evolutionary conservation of this recognition site was indicated by activation in LMH cells of a mouse Cyp2b10 phenobarbital-responsive enhancer by the same spectrum of inducers that activate the CYP2H1 264-bp PBRU. Inhibition of this activation by okadaic acid may explain the reported inhibitory effects on induction of CYP2B1/2 and Cyp2b10 by this phosphatase inhibitor. We show that this inhibition occurs directly on the 264-bp PBRU, whereas the proximal promoter of CYP2H1 is induced by okadaic acid in reporter gene assays. These experiments exploit the unique phenobarbital inducibility of the hepatoma-derived cell line LMH.

Regulation of cytochrome P450 (CYP) genes by nuclear receptors

Members of the nuclear-receptor superfamily mediate crucial physiological functions by regulating the synthesis of their target genes. Nuclear receptors are usually activated by ligand binding. Cytochrome P450 (CYP) isoforms often catalyse both formation and degradation of these ligands. CYPs also metabolize many exogenous compounds, some of which may act as activators of nuclear receptors and disruptors of endocrine and cellular homoeostasis. This review summarizes recent findings that indicate that major classes of CYP genes are selectively regulated by certain ligand-activated nuclear receptors, thus creating tightly controlled networks.

Methodologies to study the induction of rat hepatic and intestinal cytochrome P450 3A at the mRNA, protein, and catalytic activity level

Studies were conducted to characterize assays for the isolation and quantitation of rat cytochrome P450 (CYP) 3A isoforms from hepatic and intestinal tissues. Isolated intestinal microsomes were analyzed for their alkaline phosphatase activity and CYP 3A immunoreactivity. The involvement of CYP 3A in the in vitro hydroxylation of midazolam (MDZ) was also evaluated using isoform specific chemical and antibody inhibitors. The effect of glycerol (a common constituent of the microsomal reconstitution buffer) concentration on in vitro MDZ hydroxylation was also investigated. Additionally, to verify that the intestinal preparation was adequate for use in studies investigating the induction of CYP3A at the MRNA, protein, and catalytic activity within a single animal, a separate induction study was carried out with the CYP 3A inducer dexamethasone (DEX). A reverse transcription-polymerase chain reaction (RT-PCR) assay and a quantitative Western blotting method were used to reliably detect differences in CYP 3A mRNA and immunoreactivity between DEX- and vehicle (VH)-treated tissues. The in vitro hydroxylation of MDZ evaluated CYP 3A catalytic activity and identified increases in CYP 3A activity caused by DEX in comparison to VH. Collectively, these described techniques provide an experimental model to study xenobiotic induction of rat hepatic and intestinal CYP 3A from the molecular to the catalytic level in individual rats without the need for pooling of tissue.

Differential glucocorticoid responses of CYP3A23 and CYP3A2 are mediated by selective binding of orphan nuclear receptors

CYP3A2 and CYP3A23 are two cytochrome P450 genes in rat that are differentially regulated in both their constitutive activities and their responsiveness to glucocorticoids, the prototypic CYP3A inducers. CYP3A2 displays 20-25% of the response to glucocorticoids as CYP3A23 despite extensive sequence homology in their 5'-regulatory regions. Promoter deletion analyses revealed that the CYP3A2 -57 to -168 region, homologous to the CYP3A23 dexamethasone-responsive region, mediated its low level activation. When this region was analyzed by DNase I footprinting, three binding sites were shown to correspond to the functional elements described for CYP3A23: DexRE-1, DexRE-2, and Site A (J. M. Huss and C. B. Kasper (1998) J. Biol. Chem. 273: 16155-16162). The CYP3A2 DexRE-2 and Site A elements bear two mismatches each from the CYP3A23 elements but displayed similar binding patterns in footprinting and gel-shift analyses as their CYP3A23 counterparts. The region containing 3A2DexRE-1 has six mismatches and displayed unique footprinting and gel-shift patterns compared to 3A23DexRE-1. Functional assays revealed that four mismatches within the DexRE-1 and DexRE-2 elements accounted for the differential inducibility of the two isoforms. We propose that the reduced responsiveness of CYP3A2 is the result of preferential binding of COUP-TF at the CYP3A2 DexRE-1 site. In contrast, CYP3A23 DexRE-1 associates with an accessory factor(s) that acts in concert with downstream sites to mediate the strong glucocorticoid induction response observed for CYP3A23. Site A mismatches did not influence induction magnitude but were responsible for basal activity differences. Higher CYP3A23 basal activity appears to be due to an E-box in 3A23SiteA that interacts with USF1, a ubiquitous bHLH/leucine zipper transcription factor. This site is disrupted in the corresponding 3A2SiteA. Hence, 4 nucleotide mismatches within two elements account for the difference in glucocorticoid induction, and a single mismatch is responsible for the fivefold difference in the basal activities of CYP3A2 and CYP3A23.

P450 gene induction by structurally diverse xenochemicals: central role of nuclear receptors CAR, PXR, and PPAR

The biochemistry of foreign compound metabolism and the roles played by individual cytochrome P450 (CYP) enzymes in drug metabolism and in the toxification and detoxification of xenochemicals prevalent in the environment are important areas of molecular pharmacology and toxicology that have been widely studied over the past decade. Important advances in our understanding of the mechanisms through which foreign chemicals impact on these P450-dependent metabolic processes have been made during the past 2 years with several key discoveries relating to the mechanisms through which xenochemicals induce the expression of hepatic P450 enzymes. Roles for three "orphan" nuclear receptor superfamily members, designated CAR, PXR, and PPAR, in respectively mediating the induction of hepatic P450s belonging to families CYP2, CYP3, and CYP4 in response to the prototypical inducers phenobarbital (CAR), pregnenolone 16alpha-carbonitrile and rifampicin (PXR), and clofibric acid (PPAR) have now been established. Two other nuclear receptors, designated LXR and FXR, which are respectively activated by oxysterols and bile acids, also play a role in liver P450 expression, in this case regulation of P450 cholesterol 7alpha-hydroxylase, a key enzyme of bile acid biosynthesis. All five P450-regulatory nuclear receptors belong to the same nuclear receptor gene family (family NR1), share a common heterodimerization partner, retinoid X-receptor (RXR), and are subject to cross-talk interactions with other nuclear receptors and with a broad range of other intracellular signaling pathways, including those activated by certain cytokines and growth factors. Endogenous ligands of each of those nuclear receptors have been identified and physiological receptor functions are emerging, leading to the proposal that these receptors may primarily serve to modulate hepatic P450 activity in response to endogenous dietary or hormonal stimuli. Accordingly, P450 induction by xenobiotics may in some cases lead to a perturbation of endogenous regulatory circuits with associated pathophysiological consequences.

Effects of glucocorticoids on the trabecular meshwork: towards a better understanding of glaucoma

Glucocorticoid effects on the human trabecular meshwork can be used as a model system in which to study glaucomatous damage to the trabecular meshwork. One of the most important risk factors for glaucoma is an elevated intraocular pressure. The administration of glucocorticoids also can cause elevated intraocular pressure in some individuals. In addition, there is suggestive evidence linking glucocorticoids with the development of glaucoma. Glucocorticoids cause multiple effects on the human trabecular meshwork including changes in extracellular matrix metabolism, organisation of the cytoskeleton, and changes in gene expression and cell function. New discoveries on the molecular mechanisms of glucocorticoid receptor action provide new opportunities to study the possible role of this receptor in the development of glaucoma. For example, alternate spliced forms of the glucocorticoid receptor, glucocorticoid receptor response element half-sites, numerous modulatory factors, and direct effects of nuclear transcription factors have been recently described. Other recent information has shown that the new glaucoma gene (GLC1A/myocilin) is induced in the human trabecular meshwork by glucocorticoids. Although the exact function of myocilin is currently unknown, it offers the opportunity to dissect the molecular pathways regulating aqueous humor outflow. Future challenges include determining (1) which glucocorticoid effects in the human trabecular meshwork are responsible for elevated intraocular pressure; and (2) the significance of these findings to the development of glaucoma.

Can Excess Glucocorticoid, Predispose to Cardiovascular and Metabolic Disease in Middle Age?

For many years, both human and animal studies correlated changes in behaviour of the young offspring with the degree of maternal stress or glucocorticoid exposure of the foetus/neonate. In the past ten years there has been overwhelming epidemiological evidence to suggest that growth retardation in utero is a very important risk factor for the development of cardiovascular and metabolic disease in adult life. More recently, it has been shown that one important, even key, determinant is the exposure of the foetus to excess glucocorticoid. Even a brief period (48 h) of dexamethasone exposure very early in pregnancy was able to programme permanently hypertensive adult sheep. Understanding how such programming works, and the underlying physiological changes that occur, provides one of the most exciting challenges in contemporary endocrinology and developmental biology.

The repressed nuclear receptor CAR responds to phenobarbital in activating the human CYP2B6 gene

The endogenous CYP2B6 gene becomes phenobarbital (PB) inducible in androstenol-treated HepG2 cells either transiently or stably transfected with a nuclear receptor CAR expression vector. The PB induction mediated by CAR is regulated by a conserved 51-base pair element called PB-responsive enhancer module (PBREM) that has now been located between -1733 and -1683 bp in the gene's 5'-flanking region. An in vitro translated CAR acting as a retinoid X receptor alpha heterodimer binds directly to the two nuclear receptor sites NR1 and NR2 within PBREM. In a stably transfected HepG2 cell line, both PBREM and NR1 are activated by PB and PB-type compounds such as chlorinated pesticides, polychlorinated biphenyls and chlorpromazine. In addition to PBREM, CAR also transactivates the steroid/rifampicin-response element of the human CYP3A4 gene in HepG2 cells. Thus, activation of the repressed nuclear receptor CAR appears to be a versatile mediator that regulates PB induction of the CYP2B and other genes.

A reporter gene assay to assess the molecular mechanisms of xenobiotic-dependent induction of the human CYP3A4 gene in vitro

1. A plasmid containing 1 kb of the CYP3A4 regulatory (promoter) region coupled to a reporter gene for secretary placental alkaline phosphatase (SPAP) was transfected into HepG2 cells. Transfected cells were dosed with several known inducers of CYP3A4 and the levels of SPAP were measured. The effect of co-transfecting a plasmid encoding the human glucocorticoid receptor on reporter gene activity was also examined. 2. Dexamethasone induced CYP3A4-dependent reporter gene expression in a concentration-dependent manner and induction was approximately doubled in the presence of the glucocorticoid receptor. Dexamethasone-dependent induction was blocked by RU-486 (a glucocorticoid receptor antagonist), in the presence of the co-transfected glucocorticoid receptor. 3. Induction of CYP3A4-dependent reporter gene expression and enhancement of the induction by the glucocorticoid receptor was also observed with pregnenolone-16alpha-carbonitrile (PCN), rifampicin, phenytoin, carbamazepine, phenylbutazone and phenobarbitone, all known in vivo inducers of CYP3A4 in man. 4. Metyrapone and sulfinpyrazone induced CYP3A4-dependent reporter gene expression, but induction was not enhanced by the glucocorticoid receptor. 5. Clotrimazole, erythromycin and triacetyloleandomycin (TAO) did not induce CYP3A4-dependent reporter gene expression, consistent with the observation that these inducers act through post-transcriptional mechanisms. 6. These results highlight differences in the molecular mechanisms of induction of CYP3A4 by the xenobiotics studied and indicate that the glucocorticoid receptor is involved in the induction of the CYP3A4 gene by some, but not all, CYP3A4 inducers. 7. We propose that the approach described here provides a useful in vitro approach for the identification of transcriptional regulators of the CYP3A4 gene.

Sp1 and egr-1 have opposing effects on the regulation of the rat Pgp2/mdr1b gene

The promoter of the rat pgp2/mdr1b gene has a GC-rich region (pgp2GC) that is highly conserved in mdr genes and contains an consensus Sp1 site. Sp1's role in transactivation of the pgp2/mdr1b promoter was tested in Drosophila Schneider cells. The pgp2/mdr1b promoter was strongly activated by co-transfected wild type Sp1 but not mutant Sp1 and mutation of the Sp1 site abrogated Sp1-dependent transactivation. In gel shift assays, the same mutations abolished Sp1-DNA complex formation. Moreover, basal activity of the pgp2/mdr1b Sp1 mutant promoter was dramatically lower. Enforced ectopic overexpression of Sp1 in H35 rat hepatoma cells revealed that cell lines overexpressing Sp1 had increased endogenous pgp2/mdr1b mRNA, demonstrating that Sp1 activates the endogenous pgp2/mdr1b gene. Pgp2GC oligonucleotide also bound Egr-1 in gel shift assays and Egr-1 competitively displaced bound Sp1. In transient transfections of H35 cells (and human LS180 and HepG2 cells) Egr-1 potently and specifically suppressed pgp2/mdr1b promoter activity and mutations in the Egr-1 site decreased Egr-1 binding and correlated with pgp2/mdr1b up-regulation. Ectopic overexpression of Egr-1 in H35 cells decreased Pgp expression and selectively increased vinblastine sensitivity. In conclusion, Sp1 positively regulates while Egr-1 negatively regulates the rat pgp2/mdr1b gene. Moreover, competitive interactions between Sp1 and Egr-1 in all likelihood determine the constitutive expression of the pgp2/mdr1b gene in H35 cells.

The human orphan nuclear receptor PXR is activated by compounds that regulate CYP3A4 gene expression and cause drug interactions

The cytochrome P-450 monooxygenase 3A4 (CYP3A4) is responsible for the oxidative metabolism of a wide variety of xenobiotics including an estimated 60% of all clinically used drugs. Although expression of the CYP3A4 gene is known to be induced in response to a variety of compounds, the mechanism underlying this induction, which represents a basis for drug interactions in patients, has remained unclear. We report the identification of a human (h) orphan nuclear receptor, termed the pregnane X receptor (PXR), that binds to a response element in the CYP3A4 promoter and is activated by a range of drugs known to induce CYP3A4 expression. Comparison of hPXR with the recently cloned mouse PXR reveals marked differences in their activation by certain drugs, which may account in part for the species-specific effects of compounds on CYP3A gene expression. These findings provide a molecular explanation for the ability of disparate chemicals to induce CYP3A4 levels and, furthermore, provide a basis for developing in vitro assays to aid in predicting whether drugs will interact in humans.

Nuclear receptor involvement in the regulation of rat cytochrome P450 3A23 expression

Many genes of the cytochrome P450 3A (CYP3A) subfamily, including several human and rat isoforms, are inducible by glucocorticoids. In the rat CYP3A23 gene, a 110-base pair segment of the proximal 5'-flanking region mediates dexamethasone activation. Three binding sites (DexRE-1, DexRE-2, and Site A), identified by DNase I footprinting analysis, were characterized for their relative contribution to both basal activity and dexamethasone inducibility. Site-directed mutagenesis of DexRE-1 (-144 to -169) and DexRE-2 (-118 to -136) demonstrated that each contained a core imperfect AGGTCA direct repeat, which comprised a consensus nuclear receptor binding site, and was essential for dexamethasone responsiveness but was not required for basal activity. Competition gel shift and supershift analyses revealed that both sites can bind the orphan nuclear receptor chicken ovalbumin upstream promoter-transcription factor. Site A (-85 to -110) was shown to be important for both basal activity and dexamethasone responsiveness. Point mutants displayed a reduced (2-3-fold) induction response, compared with 15-fold for wild-type, which was accompanied by a 40-60% drop in basal activity. Site A was shown to bind the liver-enriched nuclear receptor hepatocyte nuclear factor 4. Our studies demonstrate that the mechanism mediating glucocorticoid-inducible transcriptional activity of CYP3A23 involves multiple binding sites for members of the nuclear receptor superfamily.

Comparative induction of CYP3A and CYP2B in rat liver by 3-benzoylpyridine and metyrapone

3-Benzoylpyridine (3BP) is a major metabolite of HGG-12, and oxime that has been synthesized as a potential antidote to the toxic effects of soman and other anticholinesterases. Structural similarities exist between 3BP, the cytochrome P450 (CYP)-inducer metyrapone (MET) and other 3-substituted pyridines that interact with CYPs. The present study evaluated the regulatory effects of 3BP on CYP expression in rat liver. Both 3BP and MET (100 mg/kg) increased total hepatic microsomal holo-CYP content significantly 24 h after administration to male rats. Pronounced increases in activities mediated by CYP2B (androstenedione 16 beta-hydroxylation and 7-pentylresorufin O-depentylation) were produced by 3BP and MET, which correlated with respective 9- and 14-fold increases in CYP2B immunoreactive protein. In addition, both agents slightly increased rates of microsomal CYP3A-dependent steroid 6 beta-hydroxylation, troleandomycin metabolite complex formation and total CYP3A immunoreactive protein. Induction of the dexamethasone-inducible CYP3A23 mRNA to 4.5- and 2.5-fold of control was detected in liver of MET- and 3BP-induced rats; CYP3A2 mRNA levels were unchanged. Analogous in vitro studies revealed that MET was a preferential inhibitor of CYP3A-mediated steroid 6 beta-hydroxylation activity, but 3BP was inactive against constitutive steroid hydroxylase CYPs. These findings indicate that the structurally related 3BP and MET elicit similar induction effects on CYPs 2B and 3A23 in rat liver after in vivo administration, but differential inhibitory effects of the chemicals on CYP activity in vitro. Recent reports have implicated a microsomal binding site in the induction of CYP3A1/3A23 in rat liver. In light of the present findings, substituted pyridines like 3BP may be useful tools in structure-activity studies to evaluate the physicochemical requirements for binding to this protein.

Endocrine factors modulate the phenobarbital-mediated induction of cytochromes P450 and phase II enzymes in a similar strain-dependent manner

Phenobarbital (PB)-mediated induction of five forms of cytochrome P450 (CYP2B1, CYP2B2, CYP3A1, CYP2A1, and CYP2C6) and epoxide hydrolase is highly suppressed, at the transcriptional level, in Wistar Furth (WF) relative to Fischer 344 (F344) female rats. Either hypophysectomy or thyroid hormone depletion by methimazole largely reverses the suppression in WF animals. Here we show that this strain-dependent polymorphism and unusual endocrine regulation extend to PB induction of phase II enzymes UGT2B1 uridine diphosphate-glucuronosyl transferase (UDPGT), PB-inducible aldehyde dehydrogenase (ALDH), and glutathione transferases Ya1 and Ya2 (GSTYa1 and GSTYa2). UDPGT, ALDH, GSTYa1, and GSTYa2 had mRNA levels induced by PB in a similar strain-dependent manner (F344 > WF). The extent to which mRNA induction was favored in female F344 relative to female WF was gene dependent (UDPGT 5 x; ALDH 15 x; GSTYa1 2 x; GSTYa2 3-5 x). Again, thyroid suppression by methimazole treatment selectively enhanced mRNA induced levels in female WF animals to remove much of the strain difference. Since thyroid hormone action is linked to fatty acid (FA) homeostasis, we tested the possibility that FAs participated in this endocrine polymorphism by using three isocaloric diets: low fat (LFD), polyunsaturated fatty acid (PUFAD), or saturated fatty acid (SFAD). The LFD suppressed PB-induction of CYP mRNA and protein in WF but not F344 rats. This had no parallel in phase II mRNA induction, possibly indicating that FA and thyroid hormone effects are uncoupled. We conclude that the PB-response mechanism for induction of multiple P450 and phase II genes share a pathway that has as a common feature the linkage between chemical stimulation and thyroid hormone suppression that is seen in female WF relative to female F344 rats but not in male rats.

Characterization of DNA-binding proteins required for glucocorticoid induction of CYP3A23

Cytochrome P450 (CYP) 3A23 is transcriptionally regulated in rat liver by such glucocorticoids as dexamethasone (DEX) and by such antiglucocorticoids as pregnenolone 16 alpha-carbonitrile (PCN). Based on studies of CYP3A23 gene fragments expressed in primary cultures of adult rat hepatocytes and tested for DNA-protein interactions, we have proposed that the mechanism of CYP3A23 induction by these steroid hormones involves the glucocorticoid receptor or a protein induced by glucocorticoids indirectly interacting with proteins constitutively bound to an enhancer element consisting of a direct repeat of 7-bp separated by two nucleotides in the 5'-flanking region of the CYP3A23 gene (L. Quattrochi et al., J. Biol. Chem. 270, 28,917, 1995). In the present study, we prepared and transiently expressed in cultured rat hepatocytes 20-bp double-stranded (ds)-oligonucleotides containing this direct repeat or various mutations of this direct repeat inserted into a chloramphenicol acetyltransferase (CAT) reporter plasmid. We found that both repeats were necessary for induction of CAT by either DEX or PCN. Analysis of proteins bound to CYP3A23 enhancer through the use of uv cross-linking revealed two rat liver nuclear proteins with molecular masses of approximately 130 and 100 kDa, as well as several proteins of molecular masses between 45 and 60 kDa, that specifically bind to the 20-bp ds-oligonucleotide CYP3A23 enhancer. Methylation interference assays determined that all guanine residues within the direct repeats of this oligonucleotide are important for protein binding. Mutations of these guanine residues abolished binding of nuclear proteins and eliminated DEX or PCN inducibility of CAT. These data suggest that constitutively bound proteins, interacting with the CYP3A23 enhancer possibly as a heterodimeric complex, play a role in the glucocorticoid inducibility of CYP3A23.

Sexually dimorphic expression of rat CYP3A9 and CYP3A18 genes is regulated by growth hormone

The cDNAs for two CYP3A genes were isolated from the livers of rats using an RT-PCR approach with CYP3A subfamily-specific primers. Sequence analysis revealed these cDNAs to be identical to CYP3A9, which had previously been isolated from rat brain and nasal epithelium and the recently described CYP3A18. The hepatic expression of both genes was sexually dimorphic. Thus CYP3A18 mRNA levels were 25-fold higher in male livers compared to females, while CYP3A9 showed a reverse pattern with 6-fold higher expression in the liver of females. Exposure of male rats to the female pattern of growth hormone secretion led to an increase in hepatic CYP3A9 mRNA expression and suppressed expression of CYP3A18. These findings indicate that the CYP3A subfamily in rats has both male- and female-specific isoforms which are regulated by growth hormone in a manner similar to some other sexually dimorphic cytochrome P450s.