Transcriptional activation of cytochrome P450 genes by different classes of chemical inducers

Induction by Phenobarbital of Phase I and II Xenobiotic-Metabolizing Enzymes in Bovine Liver: An Overall Catalytic and Immunochemical Characterization

In cattle, phenobarbital (PB) upregulates target drug-metabolizing enzyme (DME) mRNA levels. However, few data about PB's post-transcriptional effects are actually available. This work provides the first, and an almost complete, characterization of PB-dependent changes in DME catalytic activities in bovine liver using common probe substrates and confirmatory immunoblotting investigations. As expected, PB increased the total cytochrome P450 (CYP) content and the extent of metyrapone binding; moreover, an augmentation of protein amounts and related enzyme activities was observed for known PB targets such as CYP2B, 2C, and 3A, but also CYP2E1. However, contradictory results were obtained for CYP1A, while a decreased catalytic activity was observed for flavin-containing monooxygenases 1 and 3. The barbiturate had no effect on the chosen hydrolytic and conjugative DMEs. For the first time, we also measured the 26S proteasome activity, and the increase observed in PB-treated cattle would suggest this post-translational event might contribute to cattle DME regulation. Overall, this study increased the knowledge of cattle hepatic drug metabolism, and further confirmed the presence of species differences in DME expression and activity between cattle, humans, and rodents. This reinforced the need for an extensive characterization and understanding of comparative molecular mechanisms involved in expression, regulation, and function of DMEs.

In Vitro Assessment of the Interaction Potential of Ocimum basilicum (L.) Extracts on CYP2B6, 3A4, and Rifampicin Metabolism

Ocimum basilicum L. or basilicum is a common culinary herb, used as a traditional medicine for various medical conditions including HIV/AIDS and tuberculosis, in Africa. The objective of this study was to evaluate the effect of methanol, ethanol, aqueous and ethyl acetate extracts of the dried leaves and inflorescence of O. basilicum, on the activity of cytochrome P450 enzymes (CYPs) CYP2B6 and 3A4, as well as esterase-mediated metabolism of rifampicin to 25-O-desacetyl rifampicin (25ODESRIF). Human liver microsomes (HLM) were used to evaluate inhibition and CYP2B6/3A4 mRNA expression HepG2 assays were used to measure induction. Furthermore, the phytoconstituents likely involved in causing the observed effect were analyzed using biochemical tests and LC-MS. The aqueous and methanolic extracts showed reversible and time-dependent inhibition (TDI) of CYP2B6 with TDI-IC₅₀s 33.35 μg/ml (IC₅₀ shift-fold >1.5) and 4.93 μg/ml (IC₅₀ shift-fold >7) respectively, while the methanolic and ethanolic extracts inhibited 25ODESRIF formation (IC₅₀s 31 μg/ml, 8.94 μg/ml). In HepG2 assays, the methanolic and ethanolic extracts moderately induced CYP2B6, 3A4 mRNA with 38%-, 28%-fold shift, and 22%-, 44%-fold shift respectively. LC-MS full scans identified phenols rosmarinic acid [m/z 359 (M-H)^-, approximately 2298 mg/L in aqueous extract] and caftaric acid along with flavones salvigenin [m/z 329 (M+H)⁺, approximately 1855 mg/L in ethanolic extract], eupatorin [m/z 345 (M+H)⁺, 668.772 mg/L in ethanolic extract], rutin [m/z 609 (M-H)^-] and isoquercetin [m/z 463 (M-H)^-] and other compounds—linalool [m/z 153 (M-H)^-], hydroxyjasmonic acid [m/z 225 (M-H)^-], eucommiol [m/z 187 (M-H)^-] and trihydroxy octadecenoic acid [m/z 329 (M-H)^-, 530 mg/L in ethanolic extract]. The putative gastrointestinal tract (GIT) concentration for all extracts was calculated as 2,400 μg/ml and hepatic circulation concentrations were estimated at 805.68 μg/ml for the aqueous extract, and 226.56 μg/ml for methanolic extract. Based on the putative GIT concentration, estimated hepatic circulation concentration [I] and inhibition constant K_i, the predicted percentile of inhibition in vivo was highest for the aqueous extract on CYP2B6 (96.7%). The observations indicated that O. basilicum extracts may have the potential to cause clinically relevant herb-drug interactions (HDI) with CYP2B6 and rifampicin metabolism in vivo, if sufficient hepatic concentrations are reached in humans.

Steroid biosynthesis in adipose tissue

Tissue-specific expression of steroidogenic enzymes allows the modulation of active steroid levels in a local manner. Thus, the measurement of local steroid concentrations, rather than the circulating levels, has been recognized as a more accurate indicator of the steroid action within a specific tissue. Adipose tissue, one of the largest endocrine tissues in the human body, has been established as an important site for steroid storage and metabolism. Locally produced steroids, through the enzymatic conversion from steroid precursors delivered to adipose tissue, have been proven to either functionally regulate adipose tissue metabolism, or quantitatively contribute to the whole body's steroid levels. Most recently, it has been suggested that adipose tissue may contain the steroidogenic machinery necessary for the initiation of steroid biosynthesis de novo from cholesterol. This review summarizes the evidence indicating the presence of the entire steroidogenic apparatus in adipose tissue and discusses the potential roles of local steroid products in modulating adipose tissue activity and other metabolic parameters.

The Use of Long-term Hepatocyte Cultures for Detecting Induction of Drug Metabolising Enzymes: The Current Status

In this report, metabolically competent in vitro systems have been reviewed, in the context of drug metabolising enzyme induction. Based on the experience of the scientists involved, a thorough survey of the literature on metabolically competent long-term culture models was performed. Following this, a prevalidation proposal for the use of the collagen gel sandwich hepatocyte culture system for drug metabolising enzyme induction was designed, focusing on the induction of the cytochrome P450 enzymes as the principal enzymes of interest. The ultimate goal of this prevalidation proposal is to provide industry and academia with a metabolically competent in vitro alternative for long-term studies. In an initial phase, the prevalidation study will be limited to the investigation of induction. However, proposals for other long-term applications of these systems should be forwarded to the European Centre for the Validation of Alternative Methods for consideration. The prevalidation proposal deals with several issues, including: a) species; b) practical prevalidation methodology; c) enzyme inducers; and d) advantages of working with independent expert laboratories. Since it is preferable to include other alternative tests for drug metabolising enzyme induction, when such tests arise, it is recommended that they meet the same level of development as for the collagen gel sandwich long-term hepatocyte system. Those tests which do so should begin the prevalidation and validation process.

Gene and protein expression and cellular localisation of cytochrome P450 enzymes of the 1A, 2A, 2C, 2D and 2E subfamilies in equine intestine and liver

BACKGROUND

Among the cytochrome P450 enzymes (CYP), families 1-3 constitute almost half of total CYPs in mammals and play a central role in metabolism of a wide range of pharmaceuticals. This study investigated gene and protein expression and cellular localisation of CYP1A, CYP2A, CYP2C, CYP2D and CYP2E in equine intestine and liver. Real-time polymerase chain reaction (RT-PCR) was used to analyse gene expression, western blot to examine protein expression and immunohistochemical analyses to investigate cellular localisation.

RESULTS

CYP1A and CYP2C were the CYPs with the highest gene expression in the intestine and also showed considerable gene expression in the liver. CYP2E and CYP2A showed the highest gene expression in the liver. CYP2E showed moderate intestinal gene expression, whereas that of CYP2A was very low or undetectable. For CYP2D, rather low gene expression levels were found in both intestine and the liver. In the intestine, CYP gene expression levels, except for CYP2E, exhibited patterns resembling those of the proteins, indicating that intestinal protein expression of these CYPs is regulated at the transcriptional level. For CYP2E, the results showed that the intestinal gene expression did not correlate to any visible protein expression, indicating that intestinal protein expression of this CYP is regulated at the post-transcriptional level. Immunostaining of intestine tissue samples showed preferential CYP staining in enterocytes at the tips of intestinal villi in the small intestine. In the liver, all CYPs showed preferential localisation in the centrilobular hepatocytes.

CONCLUSIONS

Overall, different gene expression profiles were displayed by the CYPs examined in equine intestine and liver. The CYPs present in the intestine may act in concert with those in the liver to affect the oral bioavailability and therapeutic efficiency of substrate drugs. In addition, they may play a role in first-pass metabolism of feed constituents and of herbal supplements used in equine practice.

The tobacco-specific nitrosamine 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) induces mitochondrial and nuclear DNA damage in Caenorhabditis elegans

The metabolites of the tobacco-specific nitrosamine 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) form DNA adducts in animal models. While there are many reports of formation of nuclear DNA adducts, one report also detected NNK-induced damage to the mitochondrial genome in rats. Using a different DNA damage detection technology, we tested whether this finding could be repeated in the nematode Caenorhabditis elegans. We treated N2 strain (wild-type) nematodes with NNK in liquid culture, and applied quantitative PCR to analyze NNK-induced nuclear and mitochondrial DNA (mtDNA) damage. Our results confirm that NNK causes both nuclear and mtDNA damage. However, we did not detect a difference in the level of nuclear versus mtDNA damage in C. elegans. To test whether the mtDNA damage was associated with mitochondrial dysfunction, we used a transgenic nematode strain that permits in vivo measurement of ATP levels and found lower levels of ATP in NNK-exposed animals when compared with the unexposed controls. To test whether the lower levels of ATP could be attributed to inhibition of respiratory chain components, we investigated oxygen consumption in whole C. elegans and found reduced oxygen consumption in exposed animals when compared with the unexposed controls. Our data suggest a model in which NNK exposure causes damage to both C. elegans nuclear and mitochondrial genomes, and support the hypothesis that the mitochondrial damage is functionally important in this model. These results also represent a first step in developing this genetically tractable organism as a model for assessing NNK toxicity.

Regional distribution of drug-metabolizing enzyme activities in the liver and small intestine of cynomolgus monkeys

The cynomolgus monkey is an animal species widely used to study drug metabolism because of its evolutionary closeness to humans. However, drug-metabolizing enzyme activities have not been compared in various parts of the liver and small intestine in cynomolgus monkeys. In this study, therefore, drug-metabolizing enzyme activities were analyzed in the liver (the five lobes) and small intestine (six sections from the duodenum to the distal ileum). 7-Ethoxyresorufin O-deethylation, coumarin 7-hydroxylation, paclitaxel 6α-hydroxylation, diclofenac 4'-hydroxylation, tolbutamide methylhydroxylation, S-mephenytoin 4'-hydroxylation, bufuralol 1'-hydroxylation, chlorzoxazone 6-hydroxylation, midazolam 1'-hydroxylation, and testosterone 6β-, 16α-, 16β-, and 2α-hydroxylation were used as the probe reactions for this investigation. In liver, all probe reactions were detected and enzyme activity levels were similar in all lobes, whereas, in the small intestine, all enzyme activities were detected (except for coumarin 7-hydroxylase and testosterone 16α-hydroxylase activity), but from jejunum to ileum there was a decrease in the level of enzyme activity. This includes midazolam 1'-hydroxylation and testosterone 6β-hydroxylation, which are catalyzed by cynomolgus monkey cytochrome P450 (CYP) 3A4/5, orthologs of human CYP3A4/5, which are important drug-metabolizing enzymes. The data presented in this study are expected to facilitate the use of cynomolgus monkeys in drug metabolism studies.

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

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

Regional distribution of cytochrome p450 mRNA expression in the liver and small intestine of cynomolgus monkeys

The cynomolgus monkey is used to study drug metabolism because of its evolutionary closeness to humans. Despite their importance, regional distribution of cytochrome P450 (CYP) enzymes including CYP3As in the liver and small intestine, the major sites of drug metabolism, has not been fully investigated in cynomolgus monkeys. In this study, we measured mRNA expression levels of 14 CYPs in the CYP1, 2, and 3 subfamilies, including orthologs of human CYP3A4 and CYP3A5, in the liver and small intestine of cynomolgus monkeys. Expression levels of each CYP mRNA in various regions of the liver were quantified and comparisons were made between the right lobe, quadrate lobe, left medial lobe, left lateral lobe, and caudate lobe and with four different sections of the right lobe. In the small intestine, the same mRNAs were measured in the duodenum and six different sections from the proximal jejunum to the distal ileum. Expression levels of the CYP mRNAs were not substantially different between liver samples, but varied between the different sections of the small intestine, including CYP3A4. These results suggest that analysis of distinct sections is required for a better understanding of cynomolgus monkey CYPs in the small intestine.

Glucocorticoid-dependent expression of O(6)-methylguanine-DNA methyltransferase gene modulates dacarbazine-induced hepatotoxicity in mice

O(6)-methylguanine-DNA methyltransferase (MGMT) plays a crucial role in the defense against the alkylating agent-induced cytotoxic lesion O(6)-alkylguanine in DNA. Although a significant circadian variation in MGMT activity has been found in the liver of mice, the exact mechanism of the variation remains poorly understood. In this study, we present evidence that glucocorticoids were required for the 24-h oscillation of MGMT expression in mouse liver. The exposure of mouse hepatic cells (Hepa1-6) to dexamethasone (DEX) significantly increased the mRNA levels of MGMT in a dose-dependent manner. The DEX-induced increase in MGMT expression was reversed by concomitant treatment with RU486 [11beta-[p-(dimethylamino) phenyl]-17beta-hydroxy-17-(1-propynyl)estra-4,9-dien-3-one], a glucocorticoid receptor antagonist. The mRNA levels of MGMT and its enzymatic activity in the liver of mice showed significant 24-h oscillations, which were not observed in adrenalectomized mice. A single administration of DEX to adrenalectomized mice significantly increased the mRNA levels of MGMT in the liver. These findings suggest that the 24-h oscillation in the hepatic expression of MGMT is caused by the endogenous rhythm of glucocorticoid secretion. Dacarbazine (DTIC), a potent O(6)-guanine-alkylating agent, causes serious hepatotoxicity accompanied by hepatocellular necrosis and hepatic vein thrombosis. DTIC-induced hepatotoxicity in mice was attenuated by administering the drug at the time of day when MGMT expression was abundant. The present findings suggest that glucocorticoid-regulated oscillation in the hepatic MGMT expression is the underlying cause of dosing time-dependent changes in DTIC-induced hepatotoxicity.

Xenobiotic-metabolizing cytochromes p450 in human white adipose tissue: expression and induction

Lipophilic pollutants can accumulate in human white adipose tissue (WAT), and the consequences of this accumulation are still poorly understood. Cytochromes P450 (P450s) have recently been found in rat WAT and shown to be inducible through mechanisms similar to those in the liver. The aim of our study was to describe the cytochrome P450 pattern and their induction mechanisms in human WAT. Explants of subcutaneous and visceral WAT and primary culture of subcutaneous adipocytes were used as WAT models, and liver biopsies and primary culture of hepatocytes were used as liver models to characterize P450 expression in both tissues. The WAT and liver models were then treated with typical P450 inducers (rifampicin, phenobarbital, and 2,3,7,8-tetrachlorodibenzo-p-dioxin) and lipophilic pollutants (lindane, prochloraz, and chlorpyrifos), and the effects on P450 expression were studied. P450 expression was considerably lower in WAT than in the liver, except for CYP1B1 and CYP2U1, which were the most highly expressed adipose P450s in all individuals. 2,3,7,8-Tetrachlorodibenzo-p-dioxin and prochloraz induced CYP1A1 and CYP1B1 expression in both tissues. The aryl hydrocarbon receptor was also present in WAT. In contrast, neither phenobarbital nor rifampicin treatment induced CYP2 or CYP3 mRNA in WAT, and constitutive androstane receptor and pregnane X receptor were almost undetectable. These results suggest that the mechanisms by which P450s of family 1 are regulated in the liver are also functional in human WAT, but those regulating CYP2 and CYP3 expression are not.

Reverse transcriptase-PCR quantification of mRNA levels from cytochrome (CYP)1, CYP2 and CYP3 families in 22 different human tissues

OBJECTIVE

The aim of this work was to study simultaneously the expression profile of the 23 CYP mRNAs of CYP1, CTP2 and CYP3 families in 22 different human tissues namely adrenal gland, bladder, bone marrow, colon, fetal liver, heart, kidney, liver, lung, mammary gland, ovary, placenta, prostate, salivary gland, skeletal muscle, small intestine, spleen, testis, thymus, thyroid, trachea and uterus.

METHODS

Analysis of the mRNA levels of each of these CYP isoforms was performed on total RNA from pooled specimens of human organs using reverse transcriptase-PCR-based CYP mRNA assays previously validated for their sensitivity and their specificity.

RESULTS

Our results confirmed previously reported data in the literature concerning isoforms expression in the most currently studied tissues. Moreover, they provided a great deal of new information, mainly about the expression of mRNA of little-known CYP isoforms. Among the 23 CYP isoforms studied, 12 were mainly hepatic (CYP1A2, 2A6, 2A7, 2A13, 2C8, 2C9, 2C18, 2C19, 2D6, 2E1, 3A4 and 3A43). Two CYP mRNAs were predominantly expressed in several extrahepatic tissues: CYP1B1 mRNA was the predominant CYP in seven extrahepatic tissues (bone marrow, kidney, mammary gland, prostate, spleen, thyroid and uterus) and CYP2J2 in four extrahepatic tissues (heart, placenta, salivary gland and skeletal muscle). Finally, some CYPs were nearly exclusively expressed in only one extrahepatic tissue. CYP2R1 was found in testis, CYP2U1 in the thymus and CYP2F1 in the respiratory tract (lung and trachea).

CONCLUSION

This description will broaden the understanding of the physiological functions of these CYPs.

Induction of cytochrome P450 1a1 by the food flavoring agent, maltol

Maltol is used extensively as a flavor-enhancing agent, food preservative, antioxidant, and also in cosmetic and pharmaceutical formulations. However, a number of studies have shown that maltol may induce carcinogenicity and toxicity but the mechanisms involved remain unknown. Therefore, we examined the ability of maltol to induce the cytochrome P450 1a1 (Cyp1a1), an enzyme known to play an important role in the chemical activation of xenobiotics to carcinogenic derivatives. Our results showed that treatment of Hepa 1c1c7 cells with maltol significantly induced Cyp1a1 at mRNA, protein, and activity levels in a concentration-dependent manner. The RNA synthesis inhibitor, actinomycin D, completely blocked the Cyp1a1 mRNA induction by maltol, indicating a requirement of de novo RNA synthesis through transcriptional activation. In addition, maltol induced aryl hydrocarbon receptor (AhR)-dependent luciferase reporter gene expression in stably transfected H1L1.1c2 cells, suggesting an AhR-dependent mechanism. This is the first demonstration that the food flavoring agent, maltol, can directly induce Cyp1a1 gene expression in an AhR-dependent manner and represents a novel mechanism by which maltol promotes carcinogenicity and toxicity.

The chemical defensome: environmental sensing and response genes in the Strongylocentrotus purpuratus genome

Metazoan genomes contain large numbers of genes that participate in responses to environmental stressors. We surveyed the sea urchin Strongylocentrotus purpuratus genome for homologs of gene families thought to protect against chemical stressors; these genes collectively comprise the 'chemical defensome.' Chemical defense genes include cytochromes P450 and other oxidases, various conjugating enzymes, ATP-dependent efflux transporters, oxidative detoxification proteins, and transcription factors that regulate these genes. Together such genes account for more than 400 genes in the sea urchin genome. The transcription factors include homologs of the aryl hydrocarbon receptor, hypoxia-inducible factor, nuclear factor erythroid-derived 2, heat shock factor, and nuclear hormone receptors, which regulate stress-response genes in vertebrates. Some defense gene families, including the ABCC, the UGT, and the CYP families, have undergone expansion in the urchin relative to other deuterostome genomes, whereas the stress sensor gene families do not show such expansion. More than half of the defense genes are expressed during embryonic or larval life stages, indicating their importance during development. This genome-wide survey of chemical defense genes in the sea urchin reveals evolutionary conservation of this network combined with lineage-specific diversification that together suggest the importance of these chemical stress sensing and response mechanisms in early deuterostomes. These results should facilitate future studies on the evolution of chemical defense gene networks and the role of these networks in protecting embryos from chemical stress during development.

Preventive Effects of Quercetin against Benzo[a]pyrene-Induced DNA Damages and Pulmonary Precancerous Pathologic Changes in Mice

The aim of this study was to investigate the preventive effects of quercetin against benzo[a]pyrene-induced blood lymphocyte DNA damages and pulmonary precancerous pathologic changes in mice, and to reveal the potential mechanism behind these effects. In this study, mice in quercetin-treated groups were given quercetin for 90 days. After one week of treatment, mice in the quercetin-treated groups and the positive control group received a single intraperitoneal dose of benzo[a]pyrene (100 mg/kg body weight). The results of single cell gel electrophoresis assay showed that the average lengths of the comet cell tail and DNA damage in the peripheral blood lymphocytes of mice induced by benzo[a]pyrene decreased significantly as a result of quercetin treatment dose-dependently. Light microscopic examination showed that the degrees of pulmonary precancerous pathologic changes in the quercetin-treated groups decreased significantly compared with those in the positive control group. Meanwhile, the cytochrome P4501A1-linked 7-ethoxyresorufin O-dealkylase activities in lung microsomes of mice decreased as the dose of quercetin increased. The results of this in vivo study revealed that quercetin had a significant preventive effect on benzo[a]pyrene-induced DNA damage, and had a potential chemopreventive effect on the carcinogenesis of lung cancer induced by benzo[a]pyrene. The mechanism of these effects of quercetin could be related to the inhibition of cytochrome P4501A1 activity.

Roles of rifampicin in drug-drug interactions: underlying molecular mechanisms involving the nuclear pregnane X receptor

Rifampicin, an important drug in the treatment of tuberculosis, is used extensively despite its broad effects on drug-drug interactions, creating serious problems. The clinical importance of such interactions includes autoinduction leading to suboptimal or failed treatment. The concomitantly administered effects of rifampicin on other drugs can result in their altered metabolism or transportation that are metabolised by cytochromes P450 or transported by p-glycoprotein in the gastrointestinal tract and liver. This review paper summarises recent findings with emphases on the molecular mechanisms used to explain these broad drug-drug interactions. In general, rifampicin can act on a pattern: rifampicin activates the nuclear pregnane X receptor that in turn affects cytochromes P450, glucuronosyltransferases and p-glycoprotein activities. This pattern of action may explain many of the rifampicin inducing drug-drug interactions. However, effects through other mechanisms have also been reported and these make any explanation of such drug-drug interactions more complex.

Gene expression of drug metabolizing enzymes in adult and aged mouse liver: A modulation by immobilization stress

The role of stress in the regulation of enzymatic systems involved in the biotransformation of xenobiotics, as well as endogenous substrates in the liver was investigated using single immobilization stress as a model. Adult (3 months of age) and aged (26 months) C3H/a male mice were used. Cytochrome P450 1A1 and 1A2 (CYP1A1 and CYP1A2), glutathione S-transferase M1 (GSTM1), aryl hydrocarbon receptor (AHR), aryl hydrocarbon receptor nuclear translocator (ARNT) and catechol-O-methyltransferase (COMT) mRNA levels in the mouse liver were measured by a semi-quantitative reverse transcription-polymerase chain reaction (RT-PCR) method. Excluding CYP1A1, experiments revealed significant differences in the expression of these genes between adult- and aged-control animals. The influence of stress on the expression of genes studied was shown to be higher in adult mice than in aged ones. Our results clearly demonstrate the lack of response or even the attenuation of gene expression in aged animals that may play an important role in age-related pathologies and diseases.

St. John's Wort (Hypericum perforatum) induces overexpression of multidrug resistance protein 2 (MRP2) in rats: a 30-day ingestion study

St. John's Wort (Hypericum perforatum, SJW) has been used as a herbal medicine for the treatment of depression in oral doses of 900-1050 mg/day in humans. However, the ingestion of SJW was reported to cause interactions with drugs. In the present study, we examined the effects of SJW treatment on the induction of drug transporters and enzymes in rats. An immunoblot analysis was performed to quantify the expression of the transporters and enzymes. SJW was given at a dose of 400 mg/kg/day, since it was reported that 400 mg/kg/day is antidepressant effective dose in rats. When SJW was administered for 10 days, the amounts of multidrug resistance protein 2 (MRP2), glutathione S-transferase-P (GST-P) and cytochrome P450 1A2 (CYP1A2) in the liver were increased to 304%, 252% and 357% of controls, respectively, although the amounts of P-glycoprotein and multidrug resistance protein 1 were not changed. Under the same conditions, an increase of MRP2 in the kidney was not observed. The increase in the levels of each protein was maximal at 10 days after SJW treatment and lasted for at least 30 consecutive days. These results suggest that SJW induces hepatic MRP2, GST-P and CYP1A2 overexpressions, and thus, it could affect drug metabolism, conjugation and disposition.

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.

Different alterations of cytochrome P450 3A4 isoform and its gene expression in livers of patients with chronic liver diseases

AIM: To determine whether parenchymal cells or hepatic cytochrome P450 protein was changed in chronic liver diseases, and to compare the difference of CYP3A4 enzyme and its gene expression between patients with hepatic cirrhosis and obstructive jaundice, and to investigate the pharmacologic significance behind this difference.

METHODS: Liver samples were obtained from patients undergoing hepatic surgery with hepatic cirrhosis (n = 6) and obstructive jaundice (n = 6) and hepatic angeioma (controls, n = 6). CYP3A4 activity and protein were determined by Nash and western bloting using specific polychonal antibody, respectively. Total hepatic RNA was extracted and CYP3A4cDNA probe was prepared according the method of random primer marking, and difference of cyp3a4 expression was compared among those patients by Northern blotting.

RESULTS: Compared to control group, the CYP3A4 activity and protein in liver tissue among patients with cirrhosis were evidently reduced. (P < 0.01) Northern blot showed the same change in its mRNA levels. In contrast, the isoenzyme and its gene expression were not changed among patients with obstructive jaundice.

CONCLUSION: Hepatic levels of P450s and its CYP3A4 isoform activity were selectively changed in different chronic liver diseases. CYP3A4 isoenzyme and its activity declined among patients with hepatic cirrhosis as expression of cyp3a4 gene was significantly reduced. Liver's ability to eliminate many clinical therateutic drug substrates would decline consequently, These findings may have practical implications for the use of drugs in patients with cirrhosis and emphasize the need to understand the metabolic fate of therapeutic compounds. Elucidation of the reasons for these different changes in hepatic CYP3A4 may provide insight into more fundamental aspects and mechanisms of imparied liver function.

Time-dependent transcriptional induction of CYP1A1, CYP1A2 and CYP1B1 mRNAs by H+/K+ -ATPase inhibitors and other xenobiotics

1. Xenobiotic-mediated regulation of mRNA expression of all members of the human cytochrome P450 (CYP) 1 family has been measured by RT-PCR in the hepatoma cell line, HepG2. Besides the positive control beta -naphthoflavone, the H(+)/K(+)-ATPase inhibitors omeprazole, lansoprazole, pantoprazole and rabeprazole and the anti-malaria drug primaquine were included in this study. 2. beta-Naphthoflavone, primaquine, omeprazole and lansoprazole increased mRNA levels of CYP1A1, CYP1A2 and CYP1B1. Induction by rabeprazole was significant only for CYP1A1 and CYP1A2, whereas none of the CYP1 mRNAs was induced by pantoprazole. This result was confirmed in primary human hepatocytes. 3. Transcriptional regulation was proved by inhibition of induction with actinomycin D. 4. Increase of CYP1 mRNA was significant after 1 h and maximal after 4 h. CYP1B1, but not CYP1A1 or CYP1A2, was dramatically down-regulated between 4 and 24 h. This decrease was prevented by treatment of cells with actinomycin D after induction, indicating an active transcription-dependent mechanism of CYP1B1 mRNA degradation. 5. In conclusion, xenobiotics inducing CYP1A1 mRNA expression have been shown also to induce CYP1A2 and CYP1B1, differing only with regard to level and time course of induction.

Evidence that the coactivator CBP/p300 is important for phenobarbital-induced but not basal expression of the CYP2H1 gene

We have previously identified an upstream 556-bp enhancer domain for the chicken CYP2H1 gene that responds to phenobarbital and binds several transcription factors, including the orphan chicken xenobiotic receptor (CXR). By contrast, the promoter lacks a CXR site and is not inducible by phenobarbital. Although it has been established that CXR can interact with the coactivator SRC-1, there are no reports as to whether other coactivators may be important for phenobarbital-mediated inducibility. Our studies using the adenovirus E1A wild-type protein, which inhibits the coactivators cAMP response element binding protein (CBP) and CBP associated factor (p/CAF), provide evidence for the involvement of one or both of these coactivators at the enhancer but not at the promoter of the CYP2H1 gene. The observations that mutant E1A proteins did not affect the enhancer activity and that inhibition by wild-type E1A was reversed by CBP and p/CAF confirmed the involvement of these coactivators in the induction process. We propose that the intrinsic histone acetyl transferase activity of one or both of these coactivators participates in chromatin remodeling thereby stimulating drug induction of the promoter. This proposal was supported by experiments with the histone deacetylase inhibitor, trichostatin A, which resulted in the superinduction of the drug response but had little effect on basal expression of the CYP2H1 gene. The work provides evidence for the first time for the involvement of the coactivators CBP and p/CAF in the phenobarbital-mediated induction of the CYP2H1 gene.

Identification of a 16-Nucleotide Sequence That Mediates Post-transcriptional Regulation of Rat CYP2E1 by Insulin

Insulin directly down-regulates the gene expression of the rat CYP2E1 by altering its mRNA stability (De Waziers, I., Garlatti, M., Bouguet, J., Beaune, P. H., and Barouki, R. (1995) Mol. Pharmacol. 47, 474-479). Because the regulation of CYP mRNA stability was poorly understood, the molecular mechanisms involved in this regulation in the rat hepatoma H4IIEC3 cell line were studied. By using RNase T1 protection methods, the formation of a major CYP2E1 RNA-protein complex was observed. By competition experiments, the binding site of this complex was located on a 16-nucleotide sequence in the 5'-proximal region of the CYP2E1-coding sequence. Insulin did not modify the binding pattern of proteins to this sequence. and transfections of expression vectors or antisense oligonucleotides were undertaken to demonstrate the actual functionality of the 16-mer sequence. The insertion of this sequence in a luciferase gene was sufficient to render the chimeric mRNA sensitive to insulin. Furthermore, transfection of H4IIEC3 cells with antisense oligonucleotide complementary to this sequence blocked the insulin effect on the CYP2E1 mRNA expression, i.e. its rapid degradation. All these results demonstrate that this 16-nucleotide sequence is implicated in the CYP2E1 post-transcriptional regulation by insulin.

A systematic gene expression screen of Caenorhabditis elegans cytochrome P450 genes reveals CYP35 as strongly xenobiotic inducible

The soil nematode Caenorhabditis elegans is one of the simplest animals having the status of a laboratory model. Its genome contains 80 cytochrome P450 genes (CYP). In order to study CYP gene expression in C. elegans mixed stages and synchronized hermaphrodites were exposed to 18 known xenobiotic cytochrome P450 inducers. Messenger RNA expression was detected by DNA arrays and semiquantitative RT-PCR. Using subfamily-specific primers, a pooled set of exon-rich CYP fragments could be amplified. In this way it was possible to systematically check the influence of different inducers on CYP expression at the same time. The well-known CYP1A inducers beta-naphthoflavone, PCB52, and lansoprazol were the most active and in particular they strongly induced almost all CYP35 isoforms. A few number of further CYP forms were found to be inducible by other xenobiotics like phenobarbital, atrazine, and clofibrate. In addition, a transgenic C. elegans line expressing GFP under control of the CYP35A2 promoter showed a strong induction of the fusion by beta-naphthoflavone in the intestine.

A duplicated HNF-3 binding site in the CYP2H2 promoter underlies the weak phenobarbital induction response

We are investigating induction of chicken cytochrome P450 genes by the sedative phenobarbital in chick embryo hepatocytes. The steady-state level of induced mRNA for the gene CYP2H1 is about 10-fold higher than that of a second gene, CYP2H2. Here, we show that a difference in drug-responsive enhancer activity does not underlie the differential response of these genes to phenobarbital since upstream enhancer regions are identical in these genes. The first 198 bp of CYP2H2 promoter sequence is identical to the CYP2H1 gene promoter, except that the functional HNF-3 binding site in the CYP2H1 promoter is replaced with a duplicated HNF-3 sequence in the CYP2H2 promoter. Transient expression analysis established that the promoter activity of the CYP2H2 gene was about ninefold lower than the CYP2H1 gene. Mutagenesis of either of the partially overlapping HNF-3 sites in the CYP2H2 gene substantially induced drug induction. Gel-shift analysis established that each of these HNF-3 sites bound HNF-3, most likely HNF-3beta. In-vitro footprint analysis demonstrated that all the identified sites in the CYP2H2 promoter bound protein except the duplicated HNF-3 region. However, protein binding was observed by in-vitro footprint analysis if either of the HNF-3 sites was mutated in the CYP2H2 promoter. Hence, duplication of the HNF-3 site in the CYP2H2 promoter does not allow binding of HNF-3 in the promoter context and may be predominantly, if not exclusively, responsible for the poor response of the CYP2H2 gene to phenobarbital.

Phosphorylation of native and heme-modified CYP3A4 by protein kinase C: a mass spectrometric characterization of the phosphorylated peptides

As an initial approach toward the characterization of the phosphorylation of cumene hydroperoxide (CuOOH)-inactivated cytochrome P450 (CYP3A4, the major human liver drug-metabolizing enzyme) and its role in the degradation of the inactivated protein, we have identified one of the major participating cytosolic kinase(s) as rat liver cytosolic protein kinase C (PKC) with the use of specific and general kinase inhibitors. Accordingly, we employed a model phosphorylation system consisting of purified PKC, gamma-S-[(32)P]ATP, and either native or CuOOH-inactivated purified recombinant His(6)-tagged CYP3A4. Lysylendoprotease (Lys)-C digestion of the phosphorylated CuOOH-inactivated CYP3A4(His)(6) followed by HPLC-peptide mapping and mass spectrometric (LC/MS/MS) analyses led to the isolation and the unambiguous identification of two PKC-phosphorylated CYP3A4 peptides: E(258)SRLEDT(p)QK(266) and F(414)LPERFS(p)K(421). Similar analyses of the PKC-phosphorylated native enzyme predominantly yielded E(258)SRLEDT(p)QK(266) as the phosphorylated peptide. Studies are currently in progress to determine whether phosphorylation of any or both of these peptides is required for the Ub-dependent 26S proteasomal degradation of CuOOH-inactivated CYP3A4.

The effect of dexamethasone on P450 activities in regenerating rat liver

The aim of our study was to detect four P450s (CYP1A, CYP2B, CYP2E1, CYP3A) on the basis of selective enzyme activities and protein amount, and to investigate the effect of dexamethasone treatment during liver regeneration. Partial hepatectomy of rats resulted in the loss of CYP1A, CYP2B, CYP2E1, and CYP3A activities. The reduction of enzyme activities and the loss of enzyme protein of CYP2B1/2, CYP2E1, and CYP3A1/2 were the most pronounced. In the case of CYP1A1, only slight decrease was observed. Dexamethasone treatment seems to counteract this loss mainly in the first 12 h.

The antiglucocorticoid RU486 inhibits phenobarbital induction of the chicken CYP2H1 gene in primary hepatocytes

The cytochrome P450 gene CYP2H1 is highly induced by phenobarbital in chick embryo hepatocytes. Recent studies have established that the orphan nuclear receptor CAR plays a critical role in the induction mechanism. Here, we show that a high concentration of the potent glucocorticoid and progesterone receptor antagonist RU486 almost completely blocks phenobarbital-induced accumulation of CYP2H1 mRNA in hepatocytes yet has no effect on basal expression. In marked contrast, CYP2H1 mRNA induced by the phenobarbital-type inducers glutethimide and 2-allylisopropylacetamide is not affected by RU486. RU486 inhibition is not mediated through the glucocorticoid or progesterone receptors. Transient transfection studies showed that RU486 does not repress through activation of the orphan receptor PXR and subsequent competition with CAR for binding to the upstream drug-responsive 556-base-pair enhancer. Additionally, none of the known functional transcription factor binding sites found in the enhancer region was a target of RU486 inhibition. Using an artificial construct containing multiple CAR binding sites, we also established that RU486 has no direct effect on the activity of exogenously expressed CAR. There is no evidence that phenobarbital binds to CAR; we propose that RU486 inhibits phenobarbital induction, either by interfering with a phenobarbital-dependent mechanism responsible for nuclear import of CAR or with the metabolism of phenobarbital to the true inducer. Whether a novel nuclear receptor that binds RU486 at high concentrations plays a role in the inhibitory action of RU486 is an interesting possibility.

Understanding enantioselective processes: a laboratory rat model for alpha-hexachlorocyclohexane accumulation

Since cyclodextrin gas chromatography columns became popular for chiral separations, many researchers have noticed high enantiomeric ratios [ER: (+)-enantiomer/(-)-enantiomer] for alpha-HCH in the brains of wildlife. This investigation used the laboratory rat as a model for these phenomena. Rats were either pretreated with phenobarbital (PB) or left untreated and then dosed with alpha-HCH. Animals were sacrificed after 1 or 24 h. The ER averaged 0.95 +/- 0.01 in blood, 1.29 +/- 0.02 in fat, and 0.77 +/- 0.004 in liver. ERs in brain ranged from 2.8 +/- 0.5 to 13.5 +/- 0.4. Both the tissue concentration distribution and the ERs agree well with those previously reported in wildlife. To determine whether high brain ERs were due to enantioselective metabolism or transport through the blood-brain barrier, alpha-HCH exposed brain and liver tissue slices were compared. Concentrations in the brain slices did not decrease with PB pretreatment but did decrease in the liver slices. Enantiomeric ratios in the brain slices averaged 1.11 +/- 0.02 and were 0.76 +/- 0.03 in liver slices for the PB pretreated rats. These data indicate that the enantioselective metabolism of alpha-HCH by the brain is not the mechanism responsible for high ERs in this tissue.

The Human Cytochrome P450 1A1 mRNA Is Rapidly Degraded In HepG2 Cells

The cytochromes P450 are a superfamily of enzymes that can carry out a wide range of oxidative reactions. While the transcriptional control of the cytochrome P450 genes has been relatively well-studied, posttranscriptional regulatory mechanisms that contribute to the regulation of P450s are much less well understood. We followed the decay of CYP1A1, CYP1A2, and CYP1B1 mRNAs after induction by the AH receptor ligand 2,3,7,8,-tetrachlorodibenzo-p-dioxin. CYP1A2 and CYP1B1 mRNAs were long-lived in this cell line (to > 24 h). In contrast, the CYP1A1 mRNA decays remarkably quickly. To determine if this rapid decay was unique to CYP1A1, we assessed the decay of selected human P450 and liver-specific mRNAs in HepG2 cells as a comparison. We analyzed albumin, phosphofructokinase, and GAPDH mRNAs and found that they were long-lived, with half-lives >24 h. We show that CYP2E1 mRNA can be detected in HepG2 cells by RT-PCR and that this mRNA also has a basal half-life of >24 h. Thus the CYP1A1 mRNA with its half-life of 2.4 h was one of the shortest-lived mRNA studied and is the most unstable of the cytochrome P450 mRNAs we have tested. The rapid decay of CYP1A1 mRNA is associated with a rapid loss in poly(A) tail length, suggesting that deadenylation is the first step in the decay pathway. The short half-life appears to be conserved across species, which suggests that this characteristic of the CYP1A1 mRNA is important for its function.

Subcellular localization of the aryl hydrocarbon receptor is modulated by the immunophilin homolog hepatitis B virus X-associated protein 2

The hepatitis B virus X-associated protein 2 (XAP2) is an immunophilin homolog and core component of the aryl hydrocarbon receptor (AhR). Immunophilins are components of many steroid receptor complexes, serving a largely unknown function. Transiently expressed AhR.YFP (yellow fluorescent protein) localized to the nuclei of COS-1 and NIH-3T3 cells. Co-expression of AhR.YFP with XAP2 restored cytoplasmic localization, which was reversed by 2,3,7, 8-tetrachlorodibenzo-p-dioxin treatment (TCDD). The effect of XAP2 on AhR localization was specific involving a nuclear localization signal-mediated pathway. Examination of the ratio of AhR to XAP2 in the AhR complex revealed that approximately 25% of transiently expressed AhR was associated with XAP2, in contrast with approximately 100% when the AhR and XAP2 were co-expressed. Strikingly, TCDD did not influence these ratios, suggesting that ligand binding initiates nuclear translocation prior to complex dissociation. Analysis of endogenous AhR in Hepa-1 cells revealed that approximately 40% of the AhR complex was associated with XAP2, predicting observed AhR localization to cytoplasm and nuclei. This study reveals a novel functional role for the immunophilin-like component of a soluble receptor complex and provides new insight into the mechanism of AhR-mediated signal transduction, demonstrating the existence of two structurally distinct and possibly functionally unique forms of the AhR.

Does selenium reduce the risk of threatened preterm delivery associated with placental cytochrome P450-1A1 activity?

Selenium (Se), an essential trace element in human nutrition, is thought to have an important role in the prevention of oxygen damage by organic hydroperoxides generated by oxidative metabolism. Epidemiological studies have shown an association between placental cytochrome P450-1A1 (CYP1) activity and threatened preterm delivery (TPD), and other experimental studies have shown alterations in fetal development with CYP1 activity or toxicity. The present study examined the possible protective effect of selenium on the potential toxicity of maternal exposure to polycyclic aromatic hydrocarbons (PAHs) on the normal course of pregnancy. Placental CYP1 activity was used as a risk factor resulting from maternal exposure to PAHs. TPD occurrence was used as a general indicator of troubles in the normal course of pregnancy. A group of TPD patients and a group of controls were selected from 178 pregnant women attending obstetrical care in a maternity hospital. Selenium concentrations in maternal plasma were lower in the TPD group: 63.7 ng/ml (CI 95% confidence bounds = 43.6-82.2) vs 69.2 ng/ml (CI 95% confidence bounds = 49. 3-96.3) (t test, P<0.01). When placental CYP1 was induced, an association between TPD and selenium was found, with an increase of 10 ng/ml for the latter. An adjusted odds ratio of 0.55 (CI 95% confidence bounds = 0.34-0.88; chi(2), P<0.01) was estimated. When placental CYP1 was not activated, the odds ratio was estimated at 0.99 (CI 95% confidence bounds=0.95-1.03; NS). This epidemiologic finding suggests that antioxidant Se status may be a protective factor against the potential toxic effect of PAHs on the normal course of pregnancy. The downward trend that we observed supports the hypothesis that the one-electron pathway metabolism of PAHs may explain a large fraction of TPD and some preterm deliveries.

Phase I trial of carmustine plus O6-benzylguanine for patients with recurrent or progressive malignant glioma

PURPOSE

The major mechanism of resistance to alkylnitrosourea therapy involves the DNA repair protein O(6)-alkylguanine-DNA alkyltransferase (AGT), which removes chloroethylation or methylation damage from the O(6) position of guanine. O(6)-benzylguanine (O(6)-BG) is an AGT substrate that inhibits AGT by suicide inactivation. We conducted a phase I trial of carmustine (BCNU) plus O(6)-BG to define the toxicity and maximum-tolerated dose (MTD) of BCNU in conjunction with the preadministration of O(6)-BG with recurrent or progressive malignant glioma.

PATIENTS AND METHODS

Patients were treated with O(6)-BG at a dose of 100 mg/m(2) followed 1 hour later by BCNU. Cohorts of three to six patients were treated with escalating doses of BCNU, and patients were observed for at least 6 weeks before being considered assessable for toxicity. Plasma samples were collected and analyzed for O(6)-BG, 8-oxo-O(6)-BG, and 8-oxoguanine concentration.

RESULTS

Twenty-three patients were treated (22 with glioblastoma multiforme and one with anaplastic astrocytoma). Four dose levels of BCNU (13.5, 27, 40, and 55 mg/m(2)) were evaluated, with the highest dose level being complicated by grade 3 or 4 thrombocytopenia and neutropenia. O(6)-BG rapidly disappeared from plasma (elimination half-life = 0. 54 +/- 0.14 hours) and was converted to a longer-lived metabolite, 8-oxo-O(6)-BG (elimination half-life = 5.6 +/- 2.7 hours) and further to 8-oxoguanine. There was no detectable O(6)-BG 5 hours after the start of the O(6)-BG infusion; however, 8-oxo-O(6)-BG and 8-oxoguanine concentrations were detected 25 hours after O(6)-BG infusion. The mean area under the concentration-time curve (AUC) of 8-oxo-O(6)-BG was 17.5 times greater than the mean AUC for O(6)-BG.

CONCLUSION

These results indicate that the MTD of BCNU when given in combination with O(6)-BG at a dose of 100 mg/m(2) is 40 mg/m(2) administered at 6-week intervals. This study provides the foundation for a phase II trial of O(6)-BG plus BCNU in nitrosourea-resistant malignant glioma.

CXR, a chicken xenobiotic-sensing orphan nuclear receptor, is related to both mammalian pregnane X receptor (PXR) and constitutive androstane receptor (CAR)

Nuclear receptors constitute a large family of ligand-modulated transcription factors that mediate cellular responses to small lipophilic molecules, including steroids, retinoids, fatty acids, and exogenous ligands. Orphan nuclear receptors with no known endogenous ligands have been discovered to regulate drug-mediated induction of cytochromes P450 (CYP), the major drug-metabolizing enzymes. Here, we report the cloning of an orphan nuclear receptor from chicken, termed chicken xenobiotic receptor (CXR), that is closely related to two mammalian xenobiotic-activated receptors, the pregnane X receptor (PXR) and the constitutive androstane receptor (CAR). Expression of CXR is restricted to tissues where drug induction of CYPs predominantly occurs, namely liver, kidney, small intestine, and colon. Furthermore, CXR binds to a previously identified phenobarbital-responsive enhancer unit (PBRU) in the 5'-flanking region of the chicken CYP2H1 gene. A variety of drugs, steroids, and chemicals activate CXR in CV-1 monkey cell transactivation assays. The same agents induce PBRU-dependent reporter gene expression and CYP2H1 transcription in a chicken hepatoma cell line. These results provide convincing evidence for a major role of CXR in the regulation of CYP2H1 and add a member to the family of xenobiotic-activated orphan nuclear receptors.

Effect of phenobarbital on intralobular expression of CYP2B1/2 in livers of rats: difference in the expression between single and repetitive administrations

Phenobarbital (PB) was shown to induce the major PB-inducible cytochrome P450 (CYP) isoforms, CYP2B1/2, in perivenular hepatocytes by a single injection, and in midzonal and periportal hepatocytes in addition to perivenular hepatocytes by injections of the same dosage once a day for 3 days in rat livers. The present study was undertaken to determine whether the spread of enzyme induction to midzonal and periportal hepatocytes is caused by the increase in total dose of the drug by repetitive injections or by the repetitive injections of the drug themselves. Male adult rats were administered PB by a single injection (80 mg/kg) or repetitive injections (20 mg/kg once a day for 4 days; a total dose of 80 mg/kg), and the molar content of CYP2B1/2 was measured by quantitative immunohistochemistry in the cytoplasm of perivenular, midzonal, and periportal hepatocytes. In addition, the molar content of total CYP in the cytoplasm was measured by microphotometry, and the expression of CYP2B2 mRNA was examined by in situ hybridization. When animals received the single injection, the isoforms and CYP2B2 mRNA increased markedly in perivenular hepatocytes, increased somewhat in midzonal hepatocytes, and remained unchanged in periportal hepatocytes. If animals received the repetitive injections, however, although the isoforms and the mRNA increased markedly in perivenular hepatocytes, they also increased markedly in midzonal hepatocytes and somewhat in periportal hepatocytes. These findings demonstrated that the enlargement of the sublobular area in which induction of the isoforms occurred was caused by the repetitive injections of PB themselves.

Cell biology of cytochrome P-450 in the liver

Cytochromes P-450 (P-450) are members of a multigene superfamily of hemoproteins consisting the microsomal monooxygenase system with NADPH P-450 reductase (reductase) and/or reducing equivalents. Expression of many P-450 isoforms in hepatocytes is shown to be regulated at the level of transcription through interaction between cis-acting elements in the genes and DNA-binding (transacting) factors. Some isoforms of the CYP1A, 2B, 2E, and 3A subfamilies are regulated at the posttranscriptional level. For the topology of P-450 and reductase molecules in ER membrane of hepatocytes, models from stopped flow analysis and electron spin resonance are proposed. The densities of total P-450 and reductase molecules are revealed to be high enough to support the cluster model, suggesting that about ten P-450 molecules form an aggregate and surround one reductase molecule, and therefore the two enzymes form large micelles. ER proliferation after PB administration, which had been correlated with increase in P-450 level, is shown to be probably independent of the increase in P-450 level. There are considerable discrepancies among results reported on sublobular expression of various P-450 isoforms. Causes of the discrepancies are likely to be differences in experimental conditions of histochemical detection carried out and/or in species, strain, and/or sex.

Angiotensin II modulates ion transport in rat proximal tubules through CYP metabolites

To assess the effect of angiotensin II on ion transport in rat isolated proximal tubules and establish the arachidonic acid cytochrome P450 metabolites' role mediating angiotensin II effect and to analyze whether corticosteroids play a role modulating this effect, we studied the effect of low (10 and 100 pM) and high (0.1-1 microM) angiotensin II concentrations on proximal tubule ion transport, measured as (86)Rb uptake. Low angiotensin II produced a stimulation on the (86)Rb uptake (195.79 +/- 35, 377.9 +/- 81, and 300 +/- 49 pg (86)Rb/microg protein/2 min, for control and 10 and 100 pM angiotensin II, respectively). High angiotensin II concentration inhibited ion transport (0.1 microM, 57.9 +/- 5 and 1 microM, 47.3 +/- 4 pg (86)Rb/microg protein/2 min), this effect was prevented by 17-ODYA and by losartan, while indomethacin had no effect. Dexamethasone treatment increased angiotensin II-induced (86)Rb uptake inhibition and arachidonic acid metabolism (19-, 20-HETE and 12-HETE), while adrenalectomy partly prevented angiotensin II-induced inhibition and decreased cytochrome P450-dependent arachidonic acid metabolism. In conclusion, high doses of angiotensin II produce inhibition of ion transport in rat isolated proximal tubules; this effect is mediated by AT(1) receptors, involves cytochrome P450-dependent arachidonic acid metabolites, and is upregulated by corticosteroids.

The effect of smoking on postoperative nausea and vomiting

In an attempt to quantify the postoperative effects of smoking, 327 consecutive patients undergoing arthroscopic day case knee surgery were given a standard anaesthetic consisting of an intravenous induction with propofol and fentanyl followed by inhalational maintenance using isoflurane in an oxygen and nitrous oxide mixture. Pre-operatively, patients were asked inter alia to give details of social smoking habits. Postoperatively, patients were given standard analgesic and anti-emetic drugs. Prior to discharge patients were asked to give details of postoperative nausea and vomiting together with details of the severity of postoperative pain. There were 85 smokers and 242 nonsmokers. Of the 327 patients, a total of 42 (13%) complained of postoperative nausea and vomiting. Of the smokers, only 6% complained of postoperative nausea and vomiting in contrast to 15% of the nonsmokers (p < 0.05). It is postulated that enzyme induction is the most likely reason for this anti-emetic effect. Possible ways in which this clinically beneficial mechanism can be utilised to improve outcome after anaesthesia are discussed.

Induction of drug metabolising enzymes: pharmacokinetic and toxicological consequences in humans

Currently, 5 different main mechanisms of induction are distinguished for drug-metabolising enzymes. The ethanol type of induction is mediated by ligand stabilisation of the enzyme, but the others appear to be mediated by intracellular 'receptors'. These are the aryl hydrocarbon (Ah) receptor, the peroxisome proliferator activated receptor (PPAR), the constitutive androstane receptor (CAR, phenobarbital induction) and the pregnane X receptor [PXR, rifampicin (rifampin) induction]. Enzyme induction has the net effect of increasing protein levels. However, many inducers are also inhibitors of the enzymes they induce, and the inductive effects of a single drug may be mediated by more than one mechanism. Therefore, it appears that every inducer has its own pattern of induction; knowledge of the main mechanism is often not sufficient to predict the extent and time course of induction, but may serve to make the clinician aware of potential dangers. The possible pharmacokinetic consequences of enzyme induction depend on the localisation of the enzyme. They include decreased or absent bioavailability for orally administered drugs, increased hepatic clearance or accelerated formation of reactive metabolites, which is usually related to local toxicity. Although some severe drug-drug interactions are caused by enzyme induction, most of the effects of inducers are not detected in the background of nonspecific variation. For any potent inducer, however, its addition to, or withdrawal from, an existing drug regimen may cause pronounced concentration changes and should be done gradually and with appropriate monitoring of therapeutic efficacy and adverse events. The toxicological consequences of enzyme induction in humans are rare, and appear to be mainly limited to hepatoxicity in ethanol-type induction.

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.

Rapid determination of rat hepatocyte mRNA induction potential using oligonucleotide probes for CYP1A1, 1A2, 3A and 4A1

1. A new assay to quantify mRNA levels in small numbers of rat hepatocytes has been developed for cytochrome P450 (CYP) isoforms 1A1, 1A2, 3A and 4A1. The assay uses sets of oligonucleotide probes end-labelled with [35S]-dATP to hybridize to mRNA in control- or drug-treated rat hepatocytes cultured on Cytostar-T 96-well scintillating microplates. 2. The rat hepatocyte induction potential (RHIP) assays for CYP3A, 1A1, 1A2 and 4A1 are sensitive and selective and have an excellent qualitative relationship with CYP induction data ex vivo. The robustness of the CYP3A assay was determined following a run of > 40 plates. The variation of the dexamethasone (DEX) response on each plate, calculated as %coefficient of variation, showed that there was no significant difference between the variability of the response to DEX. 3. Assay specificity for each CYP isoform was achieved by designing probes (four per isoform) antisense to coding regions of each CYP gene sequence. In the CYP3A RHIP assay, pregnenalone 16alpha-carbonitrile (PCN), DEX, clotrimazole (CLOT) and miconazole (MIC) were all good inducers of CYP3A mRNA; beta-napthoflavone (BNF) and methylclofenapate (MCP), however, did not induce CYP3A mRNA, further defining the specificity of this methodology. Specificity was similarly confirmed for the other CYP isoforms. 4. Ind50, the concentration of inducer required to elicit a 50% induction of CYP-specific mRNA, was derived for prototypical CYP inducers: BNF 0.54 and 0.17 microM (CYP1A1 and 1A2 respectively), 3-methylcholanthrene (3MC) 0.11 and 0.04 microM (CYP1A1 and 1A2 respectively), PCN 0.03 microM, DEX 0.17 microM, CLOT 0.48 microM, MIC 3 microM, TAO 3 microM (CYP3A), MCP 1.8 microM, clofibrate (CLOF) 65 microM and ciprofibrate (CIP) 1.9 microM (CYP4A1). Ind50 for BNF and 3MC at CYP1A2 was 3-fold lower than that at CYP1A1 indicating a subfamily difference in inducer potency. 5. Reducing the numbers of animals and the amount of compound required to study CYP induction is an important advantage of the RHIP assays over conventional evaluations in vivo. Typically four rats are dosed for 4 days using oral doses in the range 50-500 mg kg(-1) day(-1). In comparison, the amount of hepatocytes required to carry out all the studies reported herein may be obtained from a single animal (< 2 x 10(8) viable cells) and CYP induction investigated using microg rather than g quantities of drug substance. 6. With appropriately designed oligonucleotide probes, the RHIP technology can assess CYP induction in human hepatocytes, which together with preclinical data can contribute to improving the quality of compounds progressing into the expensive process of drug development.

Transcriptional regulation of 5-aminolevulinate synthase by phenobarbital and cAMP-dependent protein kinase

5-Aminolevulinate synthase (ALA-S) is a mitochondrial matrix enzyme that catalyzes the first and rate-limiting step of the heme biosynthesis. There are two ALA-S isozymes encoded by distinct genes. One gene encodes an isozyme that is expressed exclusively in erythroid cells, and the other gene encodes a housekeeping isozyme that is apparently expressed in all tissues. In this report we examine the mechanisms by which phenobarbital and cAMP regulate housekeeping ALA-S expression. We have determined that cAMP and phenobarbital effects are additive and the combined action is necessary to observe the cAMP effect on ALA-S mRNA in rat hepatocytes. The role of the cAMP-dependent protein kinase (PKA) has been examined. A synergism effect on ALA-S mRNA induction is observed in rat hepatocytes treated with pairs of selective analogs by each PKA cAMP binding sites. A 870-bp fragment of ALA-S 5'-flanking region is able to provide cAMP and phenobarbital stimulation to chloramphenicol O-acetyltranferase fusion vectors in transiently transfected HepG2 cells. ALA-S promoter activity is induced by cotransfection with an expression vector containing the catalytic subunit of PKA. Furthermore, cotransfection with a dominant negative mutant of the PKA regulatory subunit impairs the cAMP analog-mediated increase, but the phenobarbital-mediated induction is not modified. Our data suggest that the transcription factor cAMP-response element binding protein (CREB) is probably involved in PKA induction of ALA-S gene expression. Finally, heme addition greatly decreases the basal and phenobarbital or cAMP analog-mediated induction of ALA-S promoter activity. The present work provides evidence that cAMP, through PKA-mediated CREB phosphorylation, and phenobarbital induce ALA-S expression at the transcriptional level, while heme represses it.

Differential gene expression in drug metabolism and toxicology: practicalities, problems and potential

1. An important feature of the work of many molecular biologists is identifying which genes are switched on and off in a cell under different environmental conditions or subsequent to xenobiotic challenge. Such information has many uses, including the deciphering of molecular pathways and facilitating the development of new experimental and diagnostic procedures. However, the student of gene hunting should be forgiven for perhaps becoming confused by the mountain of information available as there appears to be almost as many methods of discovering differentially expressed genes as there are research groups using the technique. 2. The aim of this review was to clarify the main methods of differential gene expression analysis and the mechanistic principles underlying them. Also included is a discussion on some of the practical aspects of using this technique. Emphasis is placed on the so-called 'open' systems, which require no prior knowledge of the genes contained within the study model. Whilst these will eventually be replaced by 'closed' systems in the study of human, mouse and other commonly studied laboratory animals, they will remain a powerful tool for those examining less fashionable models. 3. The use of suppression-PCR subtractive hybridization is exemplified in the identification of up- and down-regulated genes in rat liver following exposure to phenobarbital, a well-known inducer of the drug metabolizing enzymes. 4. Differential gene display provides a coherent platform for building libraries and microchip arrays of 'gene fingerprints' characteristic of known enzyme inducers and xenobiotic toxicants, which may be interrogated subsequently for the identification and characterization of xenobiotics of unknown biological properties.