Omeprazole, an inducer of human CYP1A1 and 1A2, is not a ligand for the Ah receptor

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

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

Influence of omeprazole on multidrug resistance protein 3 expression in human liver

Multidrug resistance protein (MRP) 3 transports bile salts and conjugated xenobiotics from cells (hepatocytes and enterocytes) into the blood. Hepatic MRP3 expression is low under normal conditions but is markedly up-regulated during cholestasis. Since little is known about additional factors increasing human hepatic MRP3 expression, we investigated the variability of MRP3 expression in a large collection of human livers and factors contributing to variable MRP3 expression in liver and HepG2 cells. MRP3 was measured in 62 human livers from patients with and without omeprazole treatment and in HepG2 cells with and without omeprazole or beta-naphthoflavone treatment. Livers of patients treated with omeprazole showed 4.8-fold (P < 0.0001) higher MRP3 protein expression compared with the remainder of the population. Accordingly, MRP3 mRNA and protein were induced 2.4- and 1.8-fold, respectively (P < 0.01 and P < 0.05), in HepG2 cells treated with omeprazole. Finally, MRP3 was induced in HepG2 cells by beta-naphthoflavone. In summary, treatment with omeprazole and beta-naphthoflavone is a determinant of variable human hepatic MRP3 expression.

Identifying toxic mechanisms using DNA microarrays: evidence that an experimental inhibitor of cell adhesion molecule expression signals through the aryl hydrocarbon nuclear receptor

Microarray analysis of gene expression has become a powerful approach for exploring the biological effects of drugs and other chemicals. In toxicology research, gene expression profiling may help identify hazards by comparing results for an experimental compound with a database, and establish mechanistic hypotheses through examination of discrete gene changes. Here we examine the hepatic effects of a thienopyridine inhibitor of NF-kappa B-mediated expression of cellular adhesion proteins. In a 3-day toxicity study in Sprague-Dawley rats, A-277249 induced hypertrophy of the liver and elevated serum levels of alanine aminotransferase (ALT), aspartate aminotransferase (AST) and alkaline phosphatase (ALP). To investigate mechanism, microarray analysis was done on RNA from livers of A-277249-treated rats. Gene expression profiles from A-277249 were compared with a database of profiles from fifteen known hepatotoxins. Agglomerative hierarchical cluster analysis showed A-277249 to have a profile most similar to the aromatic hydrocarbons Aroclor 1254 and 3-methylcholanthrene (3MC), two known activators of the aryl hydrocarbon nuclear receptor (AhR). Several genes regulated by the AhR, including cytochrome P450 1A1, were upregulated by A-277249. In addition, several genes involved in apoptosis and cell cycle were differentially expressed consistent with cell turnover, hypertrophy and hyperplasia observed by histology. Results from this study indicate that A-277249 hepatic toxicity is mediated by the AhR either directly or through effects on NF-kappa B, and demonstrate the utility of microarray analysis for the rapid identification of toxic hazards for new chemical entities.

Ligand binding and activation of the Ah receptor

The Ah receptor (AhR) is a ligand-dependent transcription factor that can be activated by structurally diverse synthetic and naturally-occurring chemicals. Although a significant amount of information is available with respect to the planar aromatic hydrocarbon AhR ligands, the actual spectrum of chemicals that can bind to and activate the AhR is only now being elucidated. In addition, the lack of information regarding the actual three-dimensional structure of the AhR ligand binding domain (LBD) has hindered detailed analysis of the molecular mechanisms by which these ligands bind to and active AhR signal transduction. In this review we describe the current state of knowledge with respect to naturally occurring AhR ligands and present and discuss the first theoretical model of the AhR LBD based on crystal structures of homologous PAS family members.

CYP1A1 induction by pyridine and its metabolites in HepG2 cells

Pyridine and its metabolites have been shown in previous studies to induce cytochrome P4501A1 (CYP1A1) expression in vivo in the rat and in vitro in cultured human lung explants. In this study, we assessed the role of the metabolites in CYP1A1 induction by the parent compound. This was accomplished by comparing pyridine, 2-hydroxypyridine, 3-hydroxypyridine, pyridine N-oxide, and N-methylpyridinium in terms of the induction of CYP1A1 mRNA, CYP1A1 catalytic activity, and a xenobiotic response element-directed chloramphenicol acetyltransferase reporter gene, using HepG2 cells as the experimental system. We also assessed the effect of expression of the pyridine-metabolizing enzyme cytochrome P4502E1 on CYP1A1 induction by the parent pyridine. Only 2-hydroxypyridine significantly induced the CYP1A1 mRNA expression and CYP1A1-preferential activity ethoxyresorufin O-deethylase in wild-type HepG2 cells. Similarly, only 2-hydroxypyridine induced the expression of a xenobiotic response element-directed reporter gene in transfected HepG2 cells. Pyridine elevated CYP1A1 mRNA abundance 4.6-fold in HepG2 cells transfected with a human CYP2E1 expression vector relative to the abundance of the transcript in empty vector-transfected (control) HepG2 cells; the elevation was inhibited by the CYP2E1 inhibitor dimethyl sulfoxide. The results indicate that CYP1A1 induction by pyridine is mediated largely by metabolites, the formation of which may be catalyzed by CYP2E1.

Characterization of human CYP1A1/1A2 induction by DNA microarray and alpha-naphthoflavone

DNA microarrays and real time PCR were used to analyze the mechanism of gene induction by CYP1A1 inducers, beta-naphthoflavone, and omeprazole, in the human hepatocellular carcinoma HepG2 cells. Reproducible and significant inductions were observed in a limited number of genes including CYP1A1 and CYP1A2. Genes induced by omeprazole included several protein tyrosine kinase targets. This result confirmed that omeprazole could modulate gene expressions through protein tyrosine kinase-mediated pathway. Induction ratios were considerably different from CYP1A1 and CYP1A2 (>10-fold) to other induced genes (<5-fold). alpha-Naphthoflavone, which is known as an antagonist to 2,3,7,8-tetrachlorodibenzo-p-dioxin, inhibited the inductions of heme oxygenase 1, glutamate-cysteine ligase (modifier unit), and thioredoxin reductase by beta-naphthoflavone but not those of CYP1A1 and CYP1A2. It unexpectedly enhanced the beta-naphthoflavone-mediated CYP1A1 and CYP1A2 induction. These results suggest that the CYP1A1 and CYP1A2 genes, which share their 5(') enhancer regions, are regulated differently from the other genes.

A high-throughput cell-based reporter gene system for measurement of CYP1A1 induction

INTRODUCTION

Enzyme induction is undesirable in new drug discovery process, with consequences spanning from auto-induction to toxicity. Cytochrome P450 (CYP) 1A1 has long been known to be one of the metabolic enzymes involved in activating many procarcinogens, the first step toward tumor formation during chemical carcinogenesis. Induction of CYP1A1 during drug treatment may predispose the patients to some risk of chemical carcinogenesis.

METHODS

Based on the signal-transduction mechanism of CYP1A1 induction, a high-throughput reporter-gene system was established by stable transformation of H4IIE cells to incorporate the luciferase gene under control of CYP1A1 promoter. This stable cell line was validated with known CYP1A1 inducers, such as 3-methylcholanthrene (3-MC), beta-naphthoflavone (beta-NF), alpha-naphthoflavone (alpha-NF) and 3-indocarbinol. Thirty in-house new chemical entities (NCEs) were then screened with this reporter-gene system, and also administered to rats to evaluate in vivo CYP1A1 induction.

RESULTS

CYP1A1 reporter gene system can be used to identify strong inducers, such as 3-MC, beta-NF and alpha-NF, and weak inducers, such as 3-indocarbinol. In vitro induction of 30 in-house compounds in reporter gene system did not correlate with in vivo induction in rat liver microsome measured by ethoxyresorufin-O-dealkylation (EROD) activity, but had a reasonable correlation with Western blot signals.

DISCUSSION

This reporter-gene system may be useful in eliminating compounds that can cause CYP1A1 induction at an early stage of drug discovery.

Black tea polyphenols, theaflavins, prevent cellular DNA damage by inhibiting oxidative stress and suppressing cytochrome P450 1A1 in cell cultures

Tea polyphenols have been demonstrated as chemopreventive agents in a number of experimental models. However, less is known about the mechanism of chemoprevention by black tea compared with that of green tea. Some beneficial properties of theaflavins, the black tea polyphenols, were investigated in the present study. Theaflavins showed inhibitory effects on H(2)O(2)- and tert-butyl hydroperoxide (tBuOOH)-induced cytotoxicity (evaluated by tetrazolium bromide reduction), cellular oxidative stress (detected by oxidation of 2', 7'-dichlorofluorescin), and DNA damage (measured by amount of 8-OHdG and comet assay) in rat normal liver epithelium cell RL-34 cell lines. In addition, theaflavins also exhibited suppression of cytochrome P450 1A1 induced by omeprazole in the human hepatoma HepG2 cell line. Furthermore, when HepG2 cells were pretreated with omeprazole to induce CYP1A1, then exposed to benzo[a]pyrene (B[a]P), DNA damage was observed using the comet assay. However, theaflavins could inhibit this DNA damage. These results indicated that theaflavins could prevent cellular DNA damage by inhibiting oxidative stress and suppressing cytochrome P450 1A1 in cell cultures.

Effects of prochloraz and nonylphenol diethoxylate on hepatic biotransformation enzymes in trout: a comparative in vitro/in vivo-assessment using cultured hepatocytes

The suitability of cultured rainbow trout hepatocytes as a model system for the assessment of xenobiotic effects on hepatic biotransformation enzymes in fish was examined. Two model water pollutants, the imidazole fungicide prochloraz and the alkylphenolic compound nonylphenol diethoxylate (NP2EO), were investigated in a comparative in vitro/in vivo approach. Biotransformation enzymes were measured in cultured rainbow trout hepatocytes following exposure to xenobiotics in vitro, or in the liver of juvenile rainbow trout (Oncorhynchus mykiss) exposed in vivo. The patterns of biochemical responses to the model pollutants were generally similar between in vitro and in vivo investigations. Levels of cytochrome P4501A (CYP1A) protein and the catalytic activity of the CYP1A-dependent enzyme 7-ethoxyresorufin-O-deethylase (EROD) were induced in vitro after 24 h of exposure to 1.0 microM prochloraz. In vitro, higher prochloraz concentrations induced only the levels of CYP1A above control levels, but not EROD activity. In vivo exposure of juvenile trout to 0.27 microM prochloraz resulted in an induction of CYP1A and EROD after 7 and 14 days, while 0.027 microM prochloraz had no effects. In vitro, the 6beta- and 16beta-hydroxylation of testosterone was significantly decreased by 1.0-3.0 microM prochloraz, while in vivo these variables were significantly inhibited after exposure to 0.27 microM prochloraz for 7 and 14 days. NP2EO did not affect EROD activity in vitro. In vivo, EROD activity and CYP1A remained unchanged following 7 days of exposure to 0.32 or 1.30 microM NP2EO. NP2EO (15-50 microM) inhibited the 16beta-hydroxylation and glucuronidation of testosterone in vitro. In vivo, 7 days of exposure to 0.32 or 1.30 microM NP2EO resulted in a significant inhibition of the 6beta- and 16beta-hydroxylation of testosterone. The good qualitative correspondence between in vitro and in vivo results indicates that studies using trout hepatocytes allow the identification of biochemical targets of xenobiotic effects in fish liver. However, more research is needed before quantitative predictions, e.g. of effective concentrations, can be made from in vitro investigations.

Assessment of P450 induction in the marmoset monkey using targeted anti-peptide antibodies

The identity and expression of hepatic P450 enzymes in marmosets was investigated using a panel of anti-peptide antibodies originally targeted against human P450 enzymes. In immunoblotting, of 12 antibodies examined, 10 bound specifically to bands in marmoset liver microsomal fraction corresponding to P450 enzymes. It is proposed that these represent marmoset CYP1A1, CYP1A2, CYP2A, CYP2B, CYP2C forms (CYP2C-1 and CYP2C-2), CYP2D19, CYP3A21 and another CYP3A form (CYP3A-m). The antibodies, together with an anti-marmoset CYP2E1 antibody, were used to investigate the expression of 10 P450 enzymes in marmosets treated with P450-inducing chemicals. Treatment with phenobarbitone caused CYP2B, CYP2C-2 and CYP3A21 levels to increase, rifampicin caused increases in CYP2B and CYP2C-1 and a decrease in CYP3A21 levels, whereas dioxin caused CYP1A1 and CYP1A2 levels to increase and CYP2E1 levels to decrease. Clofibric acid did not induce any P450. P450 enzyme activities were assessed using 8 different substrates and increases were found after treatment with phenobarbitone, rifampicin, and dioxin. However, due to species differences in substrate selectivity, it proved difficult to ascribe these changes to individual P450 enzymes. Thus, the use of anti-peptide antibodies provides a more informative way of assessing the levels of specific P450 enzymes than enzyme activity measurements.

Is CYP1A1 induction always related to AHR signaling pathway?

Humans are daily subjected to ever increasing amounts of exogenous compounds. Some of them are capable of inducing cytochrome P450s, a process that allows the cell to adapt to changes in its chemical environment. One of the most widely CYP studied is CYP1A1 because it metabolises a large number of xenobiotics to cytotoxic and/or mutagenic derivatives. To date, results from the literature indicate that induction of CYP1A1 does not only involve the classical activation cascade of the Ah receptor, e.g. binding of the ligand to the AhR, heterodimerisation with Arnt protein, constitution of a complex with XRE responsive element and subsequent gene activation. Indeed, some xenobiotics do activate CYP1A1 gene expression in spite of their inability to compete with TCDD for binding to the AhR. Other signaling pathways must therefore also be considered. Firstly, the CYP1A1 inducer compounds could be very weak AhR ligands or may be metabolized into a form which is in turn capable of binding to the Ah receptor. A second hypothesis would be that these molecules could act through other signaling cascades. At this time, two of them seem to be implicated. One concerns the RARs signal transduction pathway, as already described for retinoic acid. The second may involve tyrosine kinase activation, but the precise relationship between this activation and CYPA1 induction remains yet to be established. For the moment there is still a black box which needs to be investigated.

Regulation of CYP1A1 transcription via the metabolism of the tryptophan-derived 6-formylindolo[3,2-b]carbazole

A functional cytochrome P4501A1 (CYP1A1) enzyme has been suggested to metabolize endogenous substrates and to autoregulate its own transcription in mouse hepatoma cells. In the present study, the regulation of CYP1A1 gene transcription by 6-formylindolo[3,2-b]carbazole (FICZ), a suggested endogenous ligand for the aryl hydrocarbon receptor (AhR), has been studied in mouse Hepa-1 cell lines. The tryptophan photoproduct, FICZ, has previously been characterized to possess very high AhR binding affinity and to transiently induce CYP1A1 gene expression in cultured cells at picomolar concentrations. The results from this study show that a transient induction of CYP1A1 mRNA at a low concentration of FICZ was only seen in wild-type cells. In c37 cells, deficient in CYP1A1, FICZ caused a sustained induction. Interestingly, we found that a higher amount of tryptophan in culture medium increased the constitutive level of CYP1A1 mRNA expression in the c37 cells but not in the wild-type cells. This suggests that a tryptophan-derived AhR ligand in the medium regulates the basal CYP1A1 expression. In metabolism studies performed with S9 prepared from c37 cells no metabolites were formed from FICZ and no loss of FICZ was observed, while with wild-type cells FICZ was rapidly metabolized. HPLC analysis revealed that at least three metabolites were formed in an NADPH-dependent manner from FICZ when incubated with rat liver S9. The CYP1A1 inhibitor ellipticine totally blocked the metabolism of FICZ. Ellipticine also enhanced both basal and FICZ-induced CYP1A1 mRNA expression. Taken together, these results indicate that tryptophan is a precursor of the endogenous ligand and that the suggested tryptophan-derived ligand FICZ is a substrate for the CYP1A1 enzyme and is involved in autoregulation of CYP1A1 transcription.

Enzyme induction and inhibition by new antiepileptic drugs: a review of human studies

The aim of this paper is to review a number of new antiepileptic agents (i.e. felbamate, gabapentin, lamotrigine, levetiracetam, oxcarbazepine, tiagabine, topiramate, vigabatrin and zonisamide) for their inducing and/or inhibitory properties in humans, mainly considering the interactions where they are involved as the cause rather than the object of such interactions. Two aspects have been particularly taken into account: the changes or absence of changes in plasma/serum concentrations of concomitant drugs and the direct or indirect evidence of induction, inhibition or lack of effect on the six major human hepatic CYP isozymes (CYP1A2, CYP2C9, CYP2C19, CYP2D6, CYP2E1 and CYP3A4), as well as on other CYP isozymes or enzyme systems. Felbamate clearly affects the pharmacokinetics of a number of drugs, generally increasing but also decreasing their concentrations. It induces enzymes such as CYP3A4 and inhibits enzymes such as CYP2C19 and those of the beta-oxidation pathway. Topiramate is not devoid of potential interaction properties: it decreases the plasma concentrations of ethinylestradiol, induces CYP3A4 and inhibits CYP2C19. For oxcarbazepine, no inhibitory, only inductive effects have been observed thus far. Felbamate. topiramate and oxcarbazepine may induce the metabolism of steroidal oral contraceptives. In this respect, tiagabine has been studied at a rather low dose. Pharmacodynamic or pharmacokinetic interaction seems to exist between lamotrigine and carbamazepine. Lamotrigine appears to be a weak inducer of UGTs, whereas induction of CYP3A4 seems improbable as the compound does not change the concentrations of oral contraceptives or the urinary excretion of 6beta-hydroxycortisol. Zonisamide has very peculiar pharmacokinetics and an extensive metabolism. Additional information on its enzyme inducing or inhibiting properties would be necessary, as data so far collected on its effect on the pharmacokinetics of other drugs are conflicting. Gabapentin, vigabatrin and in particular levetiracetam appear to be devoid of significant enzyme inducing or inhibiting properties.

Quantitative analysis of constitutive and inducible CYPs mRNA expression in the HepG2 cell line using reverse transcription-competitive PCR

Drug interactions which affect drug metabolism are of clinical importance. It is, however, difficult to estimate drug interactions in human from results obtained in animal experiments. In our previous study, we demonstrated that a combination of the HepG2 cell line and semiquantitative reverse transcription-PCR (RT-PCR) could be used to evaluate the degree of CYP3A mRNA induction by various drugs. Using an RT-competitive PCR (RT-cPCR) with beta-actin as the standard in this study, the constitutive and rifampicin (RFP)-induced expression of CYP3A4, CYP2C9, CYP2E1, and CYP1A2 mRNA in the HepG2 cells could be quantitatively and reproducibly determined. 120 h-treatment of HepG2 cells with 50 micromol/l RFP induced maximally 8.4- and 6.0-fold the expression of CYP3A4 and CYP2C9 mRNA, respectively, in comparison with untreated cells. On the other hand, mRNA level in CYP2E1 and CYP1A2 was not significantly changed by 50 micromol/l RFP after 24 to 120 h. To our knowledge, we report for the first time quantitative profiles of CYPs mRNA in HepG2 cells. This study demonstrates the efficiency of a combination of HepG2 cells and RT-cPCR in the evaluation of CYPs mRNA-induction by drugs.

Induction of CYP1A1 by serum independent of AhR pathway

CYP1A1 is implicated in the bioactivation of procarcinogens such as polycyclic aromatic hydrocarbons. To date, no physiological compounds have been described as inducers of this gene. In this study, we have examined the role of serum in the regulation of CYP1A1 gene expression. After treatment of CaCo-2 cells with fetal bovine serum, CYP1A1 mRNA level increased to the same extent as that observed after 3-methylcholanthrene induction. The same effect was obtained after treatment with adult bovine or human serum. Evaluation of hnRNA level performed on CaCo-2 cells indicates that CYP1A1 induction by serum acts at least in part through transcriptional activation. Promoter region containing the XRE (1.56 kb) was tested in the CAT assay. No stimulation of this reporter gene was detected after serum treatment. These results demonstrate for the first time that physiological compound(s) contained in serum induces CYP1A1 gene expression by transcriptional activation independent of the AhR pathway.

Molecular characteristics of carbaryl, a CYP1A1 gene inducer

Carbaryl belongs to a series of compounds that activate the CYP1A1 gene. This study demonstrates the inability of carbaryl to compete with 2,3,7,8-tetrachlorodibenzo-p-dioxin for binding to the rat aryl hydrocarbon (dioxin) receptor. Structural and physicochemical properties of this insecticide, in relation to the requirements for binding to the aryl hydrocarbon receptor, are described. The crystal structure was determined experimentally using X-ray diffraction. A conformational search using molecular mechanics was performed by means of a Monte-Carlo-type method and a stochastic dynamics simulation. Lipophilicity calculations, log P, and molecular lipophilicity potential are also presented. Common and discriminating properties of carbaryl and aryl hydrocarbon receptor ligands are discussed.

Inhibitory effects of vitamin A and vitamin K on rat cytochrome P4501A1-dependent monooxygenase activity

The inhibitory effects of vitamins A and K toward P4501A1-dependent 7-ethoxycoumarin O-deethylation were examined in the reconstituted system containing the microsomal fraction prepared from the recombinant Saccharomyces cerevisiae cells producing rat P4501A1 and yeast NADPH-P450 reductase. On vitamins A, all-trans-retinol, all-trans-retinal, all-trans-retinoic acid and retinol-palmitate showed competitive inhibition with K(i) values of 0.068, 0.079, 2.6 and 2.0 microM, respectively. Judging from the K(i) values, the inhibitory effects of those vitamins A appear to have physiological significance on the basis of their contents in liver, lung and kidney. On vitamins K, vitamin K(1) showed competitive inhibition with K(i) value of 24 microM, while vitamin K(2) showed noncompetitive inhibition with K(i) value of 60 microM. Judging from these K(i) values together with the contents of these vitamins K in liver, the inhibitory effects of the vitamins K are not as significant as those of vitamins A. These results suggest that the ingestion of enough amounts of vitamins A from foods might lead to the inhibition of the activity of P4501A1 which is known to be induced by smoking, drugs such as omeprazole and lansoprazole, and environmental pollutants like dioxins.

Signal transduction-mediated CYP1A1 induction by omeprazole in human HepG2 cells

Benzimidazole compounds, such as omeprazole and thiabendazole, are a different type of CYP1A1-inducer from Ah receptor-ligands, such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and 3-methylcholanthrene. In HepG2 cells, the commonly used tyrosine kinase-inhibitors, herbimycin-A and a series of tyrphostins, inhibited the induction of CYP1A1 produced by treatment with TCDD. Genistein, another type of tyrosine kinase inhibitor, inhibited the induction of CYP 1A1 whether it was produced by omeprazole or TCDD; however, this inhibition was caused by a dual effect of genistein, that is an anti-tyrosine kinase and an anti-topoisomerase I effect. An antagonist of Ah receptor, 3'-methoxy-4'-aminoflavone (1 microM), did not inhibit the induction of CYP1A1 produced in HepG2 cells by omeprazole or alpha-naphthoflavone (50 microM), but this antagonist did inhibit that produced by TCDD. Thus, omeprazole appears to induce CYP1A1 by initiating a protein tyrosine kinase-mediated signal transduction pathway, a different pathway from that initiated by TCDD.

Structural and mechanistic aspects of transcriptional induction of cytochrome P450 1A1 by benzimidazole derivatives in rat hepatoma H4IIE cells

The effect of several structurally different benzimidazole compounds on CYP1A1 expression at the transcriptional, mRNA and protein levels was investigated in the rat hepatoma H4IIE cell line. Omeprazole, thiabendazole, carbendazim, 2-mercaptobenzimidazole and 2-mercapto-5-methoxybenzimidazole caused a dose-dependent increase in CYP1A1 protein levels that reached maximum effect at 250 microm, as measured by Western blot. In addition, hydroxyomeprazole, 2-aminobenzimidazole and 2-mercapto-5-nitro-benzimidazole caused a notable increase in CYP1A1 protein expression, whereas 5-O-desmethylomeprazole, 2-hydroxybenzimidazole, 2-benzimidazole propionic acid and 5-benzimidazole carboxylic acid were ineffective. Thus, benzimidazole substituted with a thiol or an amino group in the 2-position were active inducers. Northern blot analysis confirmed an extensive increase of CYP1A1 mRNA induced by omeprazole and 2-mercapto-5-methoxybenzimidazole which was 32% and 49% of maximal induction by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) respectively, whereas thiabendazole and carbendazim showed approximately 15% increase as compared to TCDD. Transient transfection of H4IIE cells, with a XRE-pGL3 reporter gene construct revealed a 2.3-4.3-fold induction by carbendazim, thiabendazole, and 2-mercapto-5-methoxybenzimidazole as compared to a 3.3- and 23-fold induction by omeprazole and TCDD, respectively. Thus, these data indicate that the benzimidazoles utilize the aryl hydrocarbon receptor-arnt-XRE-mediated signal-transduction pathway for induction of the CYP1A1 gene.

Cytochrome 1A1 induction by primaquine in human hepatocytes and HepG2 cells: absence of binding to the aryl hydrocarbon receptor

Malaria remains the most prevalent infectious disease of tropical and subtropical areas of the world. It represents a crucial problem in public health care, affecting 750 million people annually, of whom at least two million die. Various antimalarials currently used were studied for their capability to induce expression of the cytochrome P450 1A1 (CYP1A1) gene, an enzyme that plays an important role in the activation of xenobiotics to genotoxic derivatives. Studies on human hepatocytes and HepG2 cell lines showed that primaquine was capable of dose dependently increasing both the ethoxyresorufin-O-deethylase activity and CYP1A1 mRNAs, suggesting a transcriptional activation of this gene. Moreover, alpha-naphthoflavone, a partial aryl hydrocarbon receptor (AhR) antagonist, and 8-methoxypsoralen, which interferes with the binding of activated AhR to the xenobiotic responsive element, were shown to suppress CYP1A1 induction when added to the cultures. However, neither primaquine nor its metabolites were able to displace [3H]2,3,7,8-tetrachlorodibenzo-p-dioxin from AhR in competitive binding studies using 9S-enriched fractions of human cytosol. These data, together with the induction of CYP1A1 promoter-directed chloramphenicol acetyl transferase gene expression, suggest that CYP1A1 induction involves the participation of the AhR but not a direct primaquine-receptor interaction. This supports the notion that an alternative ligand-independent mechanism has to be considered. Given the pharmaco-toxicological significance of CYP1A1 induction, these findings may have important implications in the treatment of malaria with primaquine and new analogs.

Induction of cytochrome P-450 1A1 by omeprazole in human HepG2 cells is protein tyrosine kinase-dependent and is not inhibited by alpha-naphthoflavone

Benzimidazole compounds, such as omeprazole and thiabendazole, are a different type of CYP1A1 inducer from Ah receptor-ligands, such as TCDD (2,3,7,8-tetrachlorodibenzo-p-dioxin) and 3-methylcholanthrene. In HepG2 cells, the commonly used tyrosine kinase inhibitors, herbimycin-A and a series of tyrphostins, inhibited the induction of CYP1A1 produced by treatment with TCDD. Genistein, another type of tyrosine kinase inhibitor, inhibited the induction of CYP1A1 whether it was produced by omeprazole or TCDD; however, this inhibition was caused by a dual effect of genistein, that is an anti-tyrosine kinase and an anti-topoisomerase I effect. An antagonist of Ah receptor, alpha-naphthoflavone (0.1-10 microM), and 3'-methoxy-4'-aminoflavone (1 microM), did not inhibit the induction of CYP1A1 produced in HepG2 cells by omeprazole, but both of them did inhibit that produced by TCDD. In one of a number of human lung tumor cell lines, S6T, the inducibility of CYP1A1 was high by TCDD, whereas the inducibility by omeprazole was low. Thus, omeprazole appears to induce CYP1A1 by initiating a protein tyrosine kinase-mediated signal transduction pathway, a different pathway from that inhibited by TCDD.

Effects of chemical inducers on human microsomal epoxide hydrolase in primary hepatocyte cultures

Human microsomal epoxide hydrolase (mEH; EC 3.3.2.3) is an important biotransformation enzyme and potential risk determinant for pathologies such as cancer and teratogenesis. Currently, the effects of chemical exposures on human mEH gene expression are largely unknown, but they may constitute a unique modifier of disease susceptibility. To examine this issue, we exposed cultures of primary human hepatocytes isolated from seven donors to prototypic chemical inducers [such as phenobarbital (PB), polyaromatic hydrocarbons, dexamethasone, butylated hydroxyanisole, and ciprofibrate]. Basal levels of mEH RNA and protein were detected readily in untreated cells. Chemical treatment of cultured hepatocytes resulted in variable mEH RNA and protein expression, but, in general, only modest modulatory effects were detected following these exposures. The maximum increase in mEH RNA expression observed was approximately 3.5-fold following Arochlor 1254 exposure. Immunochemical levels of mEH protein were quantified for all treatment groups in three cultures and demonstrated less overall variation and, in general, a lack of concordance with corresponding mEH RNA levels. Cytochrome P450 (CYP) 1A2 and 3A mRNA levels were measured before and following exposure to beta-naphthaflavone and PB, respectively, to permit independent evaluation of hepatocyte inducer responsiveness. Substantial increases in RNA expression levels for both the CYP1A2 and CYP3A genes demonstrated that the hepatocyte cultures were robust and highly responsive to inducer treatment. These results indicate that the mEH gene in human hepatocytes is only modestly responsive to chemical exposures.

2-Methoxy-4-nitroaniline and its isomers induce cytochrome P4501A (CYP1A) enzymes with different selectivities in the rat liver

We reported previously that 2-methoxy-4-nitroaniline (2-MeO-4-NA) is a selective inducer of cytochrome P4501A2 (CYP1A2) in the rat liver, and its molecular size is the smallest among known CYP1A2-selective inducers. In the present study, a structure-activity relationship on the CYP1A2-selective induction has been investigated using isomeric nitroanisidines and their related chemicals. Western blot analyses revealed that the chemicals removed a substituent (amino, methoxyl or nitro group) from a 2-MeO-4-NA molecule had no capacity for inducing CYP1A enzymes in rat livers. On the other hand, isomeric nitroanisidines such as 2-MeO-4-NA, 2-MeO-5-NA and 4-MeO-2-NA induced both CYP1A2 and CYP1A1 enzymes with different selectivities. As judged from the induced levels of CYP1A proteins, 2-MeO-4-NA (CYP1A2/CYP1A1 ratio; 9.5) and 4-MeO-2-NA (0.3) were the most selective inducers of CYP1A2 and CYP1A1, respectively, among the isomeric nitroanisidines (0.44 mmol/kg) used. The induced level of CYP1A2 protein was in the order 2-MeO-4-NA > 2-MeO-5-NA > 4-MeO-2-NA, although no significant difference was observed on their CYP1A2 mRNA level. On the contrary, increases in the levels of CYP1A1 mRNA and protein were in the order 4-MeO-2-NA > 2-MeO-5-NA > 2-MeO-4-NA. The present findings indicate that all three substituents (amino, methoxyl and nitro groups) are necessary components of nitroanisidines for induction of CYP1A enzymes, and also show that regio-isomeric positions of these substituents determine the selectivity in the induction of CYP1A enzymes.

Molecular modelling and quantitative structure-activity relationship studies on the interaction of omeprazole with cytochrome P450 isozymes

Molecular modelling of the anti-ulcerative agent, omeprazole, with the putative active sites of cytochromes P4503A4 and P4502C19, enzymes which are the major catalysts of omeprazole metabolism in man, are reported. Interactive docking of omeprazole in both CYP3A4 and CYP2C19 gives rise to binding orientations which are consistent with both the known sites of metabolism reported for these isoforms and with evidence from site-directed mutagenesis experiments on CYP2C19, a P450 associated with genetic polymorphism in human drug metabolism. The potential P450 enzymic interactions, inhibition and induction of omeprazole are discussed in the light of molecular modelling and QSAR (quantitative structure-activity relationship) studies on related compounds.

The 4S benzo(a)pyrene-binding protein is not a transcriptional activator of Cyp1a1 gene in Ah receptor-deficient (AHR -/-) transgenic mice

In an effort to better understand the role of the 4S benzo(a)pyrene-binding protein in the induction of CYP1A1 by PAHs, we used a genetically engineered mouse line deficient in Ah receptor (AHR -/-). First, we demonstrated through binding experiments analyzed by sucrose gradient sedimentation and gel permeation chromatography that AHR -/- mice have no detectable AHR protein. In contrast, this AHR-deficient line expressed a 4S protein which efficiently binds BP as it does in hepatic cytosol from C57BL/6 mice. In vivo BP exposure in AHR-deficient mice proved the inability to sustain any CYP1A1 mRNA or CYP1A1 protein induction. These findings demonstrate the requirement of an active AHR to sustain the transactivation pathway leading to CYP1A1 induction. Surprisingly, the 4S BP-binding protein, which was previously characterized as the glycine N-methyltransferase, was completely devoid of such an enzymatic activity after purification by Sephacryl gel permeation chromatography. Moreover, sedimentation and chromatographic experiments, under nondenaturing conditions, do not support the assumption of 4S protein as a subunit of a multimeric protein (GNMT) displaying a molecular mass of 150 kDa.

Signal transduction-mediated activation of the aryl hydrocarbon receptor in rat hepatoma H4IIE cells

We have investigated mechanisms of omeprazole (OME)-mediated induction of CYP1A1 and CYP3A, using the rat hepatoma H4IIE cell line, in comparison with mechanisms exerted by traditional aryl hydrocarbon receptor (AhR) ligands such as benso(a)pyrene (B(a)P) and 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). OME did not bind specifically to AhR, and it could not activate the AhR complex in rat cytosol to a xenobiotic-responsive element (XRE)-binding form in vitro. Genistein, a tyrosine kinase inhibitor, and daidzein, an inhibitor of casein kinase II, efficiently inhibited OME-mediated but not B(a)P- or TCDD-mediated induction of CYP1A1, as monitored at the transcriptional, mRNA, and protein levels as well as by analysis of activation of XRE-luciferase reporter constructs transfected into H4IIE cells. The protease inhibitor Nalpha-p-tosyl-L-lysine chloromethyl ketone (TLCK) and lavendustin A also had similar OME-specific effects. In addition, insulin pretreatment caused an almost complete inhibition of OME-dependent CYP1A1 induction but only partially affected TCDD and B(a)P-mediated induction of CYP1A1. Staurosporine, an inhibitor of protein kinase C, impaired the induction by both B(a)P and OME. OME caused an approximately 2-fold increase in the level of CYP3A expression, but all inhibitors used were ineffective in preventing this induction. Gel shift analysis with radiolabeled XRE and specific peptide antibodies toward AhR and aryl hydrocarbon receptor nuclear translocator protein (Arnt) revealed an OME-mediated translocation of the AhR.Arnt complex into the nuclei. Genistein inhibited the specific nuclear XRE binding caused by OME, but it potentiated the formation of the TCDD-induced XRE.AhR complex. Although daidzein was able to effectively inhibit the OME-stimulated CYP1A1 gene transcription, it did not influence the OME-dependent AhR.XRE complex formation. The data are consistent with a mechanism for OME-mediated induction of CYP1A1 that involves activation of the AhR complex via intracellular signal transduction systems and that is distinct from induction mediated by AhR ligands.

Preclinical evaluation of drug-drug interaction potential: present status of the application of primary human hepatocytes in the evaluation of cytochrome P450 induction

Cytochrome P450 (CYP) inhibition and induction are the key mechanisms in drug-drug interactions. Aside from clinical studies, primary human hepatocytes may represent the most appropriate experimental system for the evaluation of CYP induction in humans. A consensus of an international panel on the present status and future research directions in the application of primary human hepatocytes in the evaluation of CYP-induction is presented here. The following observations are concluded to be generally true: (1) Human hepatocytes isolated from both biopsy samples and transplantable livers are suitable for induction studies. (2) Hormonally-defined media can be used for the evaluation of CYP induction. (3) Isozyme-selective induction of CYP1A and 3A by known inducers are observed. (4) Reproducibility of induction could be improved by using hepatocytes plated as confluent cultures. (5) Induction could be observed for hepatocytes treated at 1-3 days after culturing. (6) Treatment duration of 2 days in general leads to near maximal induction. (7) In general, there is a good qualitative correlation between human hepatocyte results in vitro and clinical observations in vivo. (8) When the same inducers were evaluated in independent laboratories, similar data were generally observed. We conclude that primary human hepatocytes represent an appropriate model for mechanistic evaluation of CYP induction and as a screening tool for CYP induction potential of xenobiotics. A set of data acceptance criteria are proposed: (1) Positive response should be observed with concurrent positive control chemicals; (2) reproducible observation should be observed with multiple human donors; (3) for negative response, the doses used should not be cytotoxic; and (4) replicate treatment and/or multiple dose treatment should be performed to allow statistical analysis. Future studies should include the further development of on: (1) The inducibility of CYP isozymes other than CYP1A and 3A, and phase II enzymes; (2) further development of culturing condition to allow optimal gene expression; (3) evaluation of the involvement of nonparenchymal cells on CYP induction of parenchymal cells; (4) the and validation of quantitative approaches to extrapolate in vitro data to in vivo data; (5) evaluation of possible individual variations and potential genetic polymorphism in inducibility; (6) further definition of species differences in CYP induction; (7) development of a 'normal' human hepatocyte cell line for CYP induction studies; (8) improvement of cryopreservation procedure of human hepatocytes; (9) definition of the molecular mechanisms of CYP induction; and (10) evaluation of the induction of phase II metabolic pathways.

Ah receptor-dependent CYP1A induction by two carotenoids, canthaxanthin and beta-apo-8'-carotenal, with no affinity for the TCDD binding site

The assays of several phase I and phase II xenobiotic-metabolizing enzyme activities, as well as CYP1A immunoblot analysis, were performed in liver microsomes and cytosol of male C57BL/6 mice (Ah receptor-responsive), of male DBA/2 mice (Ah receptor-low responsive) and of female Ah receptor gene knockout mice that were fed diets containing 300 mg/kg of a nonprovitamin A carotenoid, canthaxanthin, or a provitamin A carotenoid, beta-apo-8'-carotenal for 14 days, or which were injected i.p. with 3-methylcholanthrene. Previous studies have shown that some carotenoids, such as canthaxanthin and beta-apo-8'-carotenal, are strong inducers of liver CYP1A1 and 1A2 when given to rats. In this work, only canthaxanthin induced both CYP1A1 and 1A2 in C57BL/6 mice, whereas beta-apo-8'-carotenal induced only CYP1A2 in this strain. Neither of the two carotenoids modified CYP1A1/2 protein contents or enzyme activities in Ah receptor-low responsive DBA/2 or in Ah receptor gene knockout mice. Cytosol prepared from C57BL/6 mice liver tissue was incubated with [3H] 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in the presence of canthaxanthin or beta-apo-8'-carotenal and analyzed by sucrose density gradient sedimentation: neither of the carotenoids, even when present in large excess, competed with TCDD for the TCDD binding site of the cytosolic Ah receptor of C57BL/6 mice. In brief, the carotenoids canthaxanthin or beta-apo-8'-carotenal induced Cyp1a genes in mice through an Ah receptor-dependent pathway, but did not bind to the Ah receptor.

Regulation of dioxin receptor function by omeprazole

The intracellular dioxin (aryl hydrocarbon) receptor mediates signal transduction by dioxin (2,3,7,8-tetrachlorodibenzo-p-dioxin) and related environmental pollutants and functions as a ligand-activated transcription factor. In this study we have examined the effects on dioxin receptor function of a potentially novel ligand, omeprazole, which is widely clinically used as a gastric anti-ulcer drug. In primary human hepatocytes omeprazole potently induced cytochrome P4501A1 mRNA expression, whereas this effect was not detected in mouse primary hepatocytes. In human hepatoma cells omeprazole was found to induce transcription of reporter genes via the xenobiotic response element that is recognized by the ligand-activated dioxin receptor. In contrast, the human dioxin receptor was not activated by omeprazole upon expression in a receptor-deficient mouse hepatoma cell line. In a reconstituted yeast (Saccharomyces cerevisiae) model system, however, both the mouse and human dioxin receptors were potently activated by omeprazole. Although omeprazole failed to displace dioxin in in vitro ligand binding assays, a residue within the ligand binding domain that is critical for dioxin binding in vitro was also critical for omeprazole responsiveness in vivo. Consistent with this observation, both omeprazole and dioxin responsiveness of the dioxin receptor was inhibited in mutant yeast cells expressing low levels of the molecular chaperone hsp90 that is critical for ligand binding activity. The sulfoxide group that is essential for formation of a planar conversion product of omeprazole was found to be critical for dioxin receptor activation. Taken together, these data suggest that omeprazole represents a precursor for a novel class of dioxin receptor agonists that are bona fide dioxin receptor ligands but generated in a strictly species-specific manner.

Effects of benzimidazole derivatives on cytochrome P450 1A1 expression in a human hepatoma cell line

1. Induction of endogenous cytochrome P4501A1 (CYP1A1) by benzimidazole derivatives has been investigated in the human hepatoma cell line HepG2. 2. By Northern and Western blot analysis, omeprazole has been shown to be a more potent inducer of CYP1A1 than both lansoprazole and E3810, whereas pantoprazole did not induce CYP1A1. Similar results were obtained for the CYP1A1 enzyme-specific deethylation of 7-ethoxyresorufin. 3. The induction of CYP1A1 in the permanent cell line HepG2 corresponds to results observed in human hepatocytes in primary culture. 4. The results provide experimental evidence that HepG2 cells can be used as an appropriate tool to examine inducing effects of drugs on the expression of CYP1A1.

Induction of liver and kidney CYP1A1/1A2 by caffeine in rat

Caffeine metabolism by hepatic microsomal P450 enzymes is well documented in experimental animals and humans. However, its induction effect on P450 enzymes has not been thoroughly studied. In a preliminary experiment, the time-dependent incubation of 1 mM caffeine with rat hepatocyte culture resulted in an increase of its own metabolic rate. The dose-dependent expression of rat hepatic and renal cytochromes (CYP) 1A1/1A2 was then investigated after per os administration of caffeine. P450 expression was monitored by using specific enzymatic activities and Northern blot analysis. Caffeine caused a dose-dependent elevation of hepatic CYP1A1/1A2 activities in microsomal preparations, which ranged from 1.7- to 6-fold for ethoxyresorufin O-deethylase and 3- to 8.9-fold for methoxy-resorufin O-demethylase according to the dose regimen of 50 and 150 mg caffeine/kg/day for 3 days, respectively. Northern blot analysis demonstrated that caffeine treatment increased liver CYP1A1 and CYP1A2 mRNA levels over the dose regimen of 50-150 mg caffeine/kg/day for 3 days, respectively. The result of this study demonstrates that caffeine increases its own metabolism in a dose-dependent manner and induces CYP1A1/1A2 expression through either transcriptional activation or mRNA stabilization.

Phenobarbital induction of CYP1A1 gene expression in a primary culture of rainbow trout hepatocytes

In mammals, phenobarbital (PB) is an in vivo inducer of the cytochrome P4502B (CYP2B) family, whereas in teleosts PB induction of cytochrome P450 is unclear. We show that teleost cytochrome P4502K1 (CYP2K1) protein levels and 7-pentoxyresorufin-O-deethylase activity were not induced by exposure of primary cultures of rainbow trout hepatocytes to PB. Instead, cytochrome P4501A1 (CYP1A1) gene expression was strongly induced by PB, based upon observations of marked increases in CYP1A1 mRNA, CYP1A1 protein, and 7-ethoxyresorufin-O-deethylase activity. In accordance with these data we provide a temporal study employing antibodies for the aromatic hydrocarbon (Ah) receptor that showed an increase in Ah receptor in nuclear extracts prepared from cells exposed to PB. Employment of the electrophoretic mobility shift assay (EMSA) showed PB to cause activation or "transformation" of the Ah receptor in nuclear extracts. Studies employing actinomycin D and cycloheximide indicated that PB induction of CYP1A1 was regulated at both the transcriptional and post-transcriptional levels. Nuclear run-off experiments confirm that PB causes an increase in CYP1A1 transcription. Inhibition of protein synthesis led to the superinduction of CYP1A1 mRNA, suggesting the regulation of teleost CYP1A1 may involve a labile repressor protein. These findings suggest that PB induction of the CYP1A1 gene involves the Ah receptor and is via transcriptional activation.

Induction of CYP1A1 gene by benzimidazole derivatives during Caco-2 cell differentiation. Evidence for an aryl-hydrocarbon receptor-mediated mechanism

The Caco-2 cell line, derived from a human colon adenocarcinoma, is unique in its property of spontaneously differentiating into a mature enterocyte cell type during its growth in culture. In this work, we compared the response of the CYP1A1 gene with the benzimidazole derivatives omeprazole and lansoprazole, and with the classical inducer beta-naphthoflavone in the Caco-2 cells at various culture stages. In addition, we characterized the Caco-2 aryl-hydrocarbon receptor. The protein-synthesis inhibitor cycloheximide led to a derepression of the CYP1A1 gene transcription, and to a superinduction when combined with either beta-naphthoflavone or benzimidazoles. Taking advantage of the spontaneous differentiation of Caco-2 cells in long-term cultures, we observed a difference in behavior between the classical inducer beta-naphthoflavone and the atypical inducer omeprazole. In the poorly differentiated cells, both compounds elicited comparable dose/response and rate of induction of CYP1A1 gene expression. In the fully differentiated cells, in contrast, the induction by omeprazole was only transient, whereas the response to beta-naphthoflavone was long lasting. The Caco-2 aryl-hydrocarbon receptor exhibited binding characteristics similar to those determined for human liver and other tissues. The induction of CYP1A1 transcription by benzimidazole derivatives in Caco-2 cells occurred with no direct binding of benzimidazole derivatives to the aryl-hydrocarbon receptor, as in human hepatocytes. However, transient transfection experiments clearly showed that the xenobiotic-responsive element enhancer, with which the activated aryl-hydrocarbon receptor interacts, could drive the induction of a heterologous promoter in the presence of benzimidazoles. Finally the presence of the activated aryl-hydrocarbon receptor in the nuclei of the Caco-2 cells exposed to these molecules was clearly demonstrated by gel-retardation experiments. These results question about the mechanism of ligand-independent activation of the aryl-hydrocarbon receptor and intracellular signaling, initiated by benzimidazole derivatives.

Effects of canthaxanthin, astaxanthin, lycopene and lutein on liver xenobiotic-metabolizing enzymes in the rat

1. The catalytic activities of several phase I and II xenobiotic-metabolizing enzymes and the immunochemical detection of P4501A and 2B have been investigated in liver microsomes and cytosol of male rats fed for 15 days with diets containing canthaxanthin, astaxanthin, lycopene or lutein (as lutein esters) (300 mg/kg diet) and in rats fed increasing levels (10, 30, 100 and 300 ppm) of canthaxanthin or astaxanthin in the diet. 2. Canthaxanthin increased the liver content of P450, the activities of NADH- and NADPH-cytochrome c reductase, and produced a substantial increase of some P450-dependent activities, especially ethoxyresorufin O-deethylase (EROD) (x 139) and methoxyresorufin O-demethylase (MROD) (x 26). Canthaxanthin also increased pentoxy-(PROD) and benzoxyresorufin O-dealkylases (BROD), but did not affect. NADPH-cytochrome c reductase and erythromycin N-demethylase (ERDM) activities and decreased nitrosodimethylamine N-demethylase (NDMAD) activity. Phase II p-nitrophenol UDP-glucuronosyl transferase (4NP-UGT) and quinone reductase (QR) activities were also increased by canthaxanthin treatment. These enhancing effects on EROD, MROD and 4NP-UGT were clearly detectable at a dose as low as 10 ppm of canthaxanthin in the diet; the induction of QR was only observed in rats fed > or = 100 ppm. Astaxanthin induced the same pattern of enzymes activities as canthaxanthin, but to a lesser extent: its effects on phase I enzymes and 4NP-UGT were observed in rats fed > or = 100 ppm, and QR was not increased. Western blots of microsomal proteins clearly showed the induction of P4501A1 and 1A2 by canthaxanthin and astaxanthin. By contrast, lutein had no effect on the phase I and II xenobiotic-metabolizing enzymes activities measured. Lycopene only decreased NDMAD activity. 3. The two 4-oxocarotenoids canthaxanthin and astaxanthin are substantial inducers of liver P4501A1 and 1A2 in the rat, and coinduce 4NP-UGT and QR, just like polycyclic aromatic hydrocarbon, beta-naphtoflavone or dioxin (TCDD). However, these latter classical P4501A inducers also induce aldehyde dehydrogenase class 3 (ALDH3); this enzyme is not increased, or only marginally, by canthaxanthin and astaxanthin. These two oxocarotenoids form a new class of inducers of P4501A, are structurally very different from the classical inducers quoted above, which are ligands of the AH receptor.

Review article: omeprazole and the cytochrome P450 system

Enzymes of the cytochrome P450 superfamily play a key role in xenobiotic metabolism. Their properties and significance are discussed with particular reference to interactions with the H+,K(+)-ATPase blocker, omeprazole. Such interactions include both inhibitory (subfamily 2C) and inducing effects (subfamily 1A). Delayed metabolic elimination of diazepam, warfarin, carbamazepin and phenytoin is probably due to omeprazole competition for the concerned isoform of subfamily 2C; however, these effects are modest to negligible in magnitude and, for phenytoin, not consistently reproducible. Also, induction of subfamily 1A is only minor as assessed from the resultant changes in N-3-demethylation of caffeine, a reaction specific to this subfamily. Concerns about a possible activation of procarcinogens that might arise from subfamily 1A induction appear ill-founded given the fact that cruciferous vegetables such as broccoli and Brussels sprouts are potent inducers, but rather seem to lower the incidence of certain types of cancer. Likewise, the idea that the toxicity of acetaminophen might increase upon subfamily 1A induction appears far-fetched, mainly because much stronger inducers of subfamily 1A (cigarette smoke and charcoaled beef) are unable to alter acetaminophen metabolism.

CYP1 induction, binding to the hepatic aromatic hydrocarbon receptor and mutagenicity of a series of 11-alkoxy cyclopenta[a]phenanthren-17-ones: a structure activity relationship

A series of four 11-alkoxy cyclopenta[a]phenanthren-17-ones, ranging from the methoxy to the butoxy derivative, has been synthesised in order to investigate the effect of the size of the 11-substituent on the mutagenicity and ability of these compounds to induce hepatic CYP1 activity in rats. The latter was monitored by using as diagnostic probes methoxy and ethoxy-resorufin, and immunologically in Western blots employing anti-CYP1A1 antibodies. All four members of the series induced both CYP1A1 and CYP1A2 activities and apoprotein levels, but the methoxy- and ethoxy-CPP-17-ones were clearly the most potent. Of the four isomers, only 11-methoxy-CPP-17-one displaced 3H-TCDD from the cytosolic Ah receptor. Similarly only 11-methoxy-CPP-17-one elicited a positive mutagenic response in the Ames test in the presence of an Aroclor 1254-induced activation system. The relevance of these findings to the carcinogenicity of these compounds in the mouse skin painting model is discussed.

Pretranslational induction of cytochrome P4501A enzymes by beta-naphthoflavone and 3-methylcholanthrene occurs in different liver zones

Most of the cytochrome P450 (CYP) genes are expressed in an uneven, zonated pattern in the liver. Factors regulating this regionally restricted expression are not well known. In this study we have analysed cell lysates obtained from opposite zones of rat liver by infusing digitonin to the perfused liver to study the zonation of CYP1A1 and CYP1A2 induction. 3-Methylcholanthrene induced CYP1A1 protein in perivenous cells, while a low dose of beta-naphthoflavone caused periportal induction. Analysis of CYP1A1 mRNA from cell lysates by reverse transcriptase-coupled polymerase chain reaction (RT-PCR) and in situ hybridization experiments both demonstrated that this inducer-specific differently localized effect occurred at the pretranslational level. A corresponding difference in the regional pattern of CYP1A2 induction was seen: induction by beta-naphthoflavone reversed the constitutive perivenous pattern into a periportal CYP1A2 mRNA pattern while induction after 3-methylcholanthrene treatment was more panacinar. Attempts to identify the regiospecific factors involved were made by comparing the in vitro induction of CYP1A1 by beta-naphthoflavone and 3-methylcholanthrene in hepatocytes isolated from the periportal and perivenous region. However, after isolation, induction seemed to be independent of the source of the cells. Our results demonstrate the existence in the liver of regionally acting factors that mediate the induction of CYP1A1 and 1A2 in a local and inducer-specific fashion. These factors could be Ah receptor associated binding proteins operating in vivo, but no longer in isolated cells.

Induction in vitro and complete coding region sequence of cytochrome P4501A1 cDNA from cultured whole rat conceptuses during early organogenesis

Exposures of cultured whole rat conceptuses during organogenesis to 3-methylcholanthrene (MC; 0.025-25 microM), 5,6-benzoflavone (BNF; 5-100 microM) or benz[a]anthracene (BA; 5-100 microM) were effected by placement of each of these "MC-type" inducing agents in the culture medium at the time of explantation on day 9.5 of gestation. Conceptuses were then cultured for 48 hr and evaluated on day 11.5 for increased expression of inducible conceptal cytochrome P450 (P450). The three agents each elicited concentration-dependent increases in 7,8-benzoflavone (ANF)-inhibitable ethoxyresorufin O-deethylase (EROD) activities and increased P4501A1 mRNA as detected by primer-specific reverse transcriptase-polymerase chain reaction (RT-PCR) in cell-free preparations of the treated, cultured conceptuses. At effective inducing concentrations, dysmorphogenic or other embryotoxic effects were not detectable. At 20 microM concentrations, the three agents exhibited roughly equal induction that was approximately equivalent in magnitude (6- to 13-fold) to that achieved previously with exposures to MC in utero. Additions to the culture medium of 2.5 to 10 microM concentrations of dexamethasone (DEX) did not alter significantly the magnitude of MC-elicited induction in vitro. Repeated full-length sequencing of an RT-PCR-amplified cDNA revealed a coding region sequence identical to that reported for the P4501A1 sequence from adult rat liver. The results provide a basis for investigations, in the absence of maternal influences, of the regulation of mammalian conceptal P4501A1 in intact tissues during organogenesis, a gestational period critical in terms of the dysmorphogenic and other embryotoxic effects of foreign organic chemicals. The results are also pertinent to studies of embryotoxicity, particularly to the transplacental carcinogenicity, mutagenicity and dysmorphogenicity of P4501A1 substrates.

Assessment of liver metabolic function. Clinical implications

Inter- and intraindividual variability in pharmacokinetics of most drugs is largely determined by variable liver function as described by parameters of hepatic blood flow and metabolic capacity. These parameters may be altered as a result of disease affecting the liver, genetic differences in metabolising enzymes, and various types of drug interactions, including enzyme induction, enzyme inhibition or down-regulation. With the now known large number of drug metabolising enzymes, their differential substrate specificity, and their differential induction or inhibition, each test substance of liver function should be used as a probe for its specific metabolising enzyme. Thus, the concept of model test-substances providing general information about liver function has severe limitations. To test the metabolic activity of several enzymes, either several test substances may be given (cocktail approach) or several metabolites of a single test substance may be analysed (metabolic fingerprint approach). The enzyme-specific analysis of liver function results in a preference for analysis of the metabolites rather than analysis of the clearance of the parent test substance. There are specific methods to quantify the activity of cytochrome P450 enzymes such as CYP1A2, CYP2C9, CYP2C19MEPH, CYP2D6, CYP2E1, and CYP3A, and phase II enzymes, such as glutathione S-transferases, glucuronyl-transferases or N-acetyltransferases, in vivo. Interactions based on competitive or noncompetitive inhibition should be analysed specifically for the cytochrome P450 enzyme involved. At least 5 different types of cytochrome P450 enzyme induction may result in major variability of hepatic function; this may be quantified by biochemical parameters, clearance methods, or highly enzyme-specific methods such as Western blot analysis or molecular biological techniques such as mRNA quantification in blood and tissues. Therapeutic drug monitoring is already implicitly used for quantification of the enzyme activities relevant for a specific drug. Selective impairment of hepatic enzymes due to gene mutations may have an effect on the pharmacokinetics of certain drugs similar to that caused by cirrhosis. Assessment of this heritable source of variability in liver function is possible by in vivo or ex vivo enzymological methods. For genetically polymorphic enzymes and carrier proteins involved in drug disposition, molecular genetic methods using a patient's blood sample may be used for classification of the individual into: (i) the impaired or poor metaboliser (homozygous deficient); (ii) the extensive (homozygous active) metaboliser group; and (iii) the moderately extensive metaboliser (heterozygous) group. For hepatic blood flow determinations, galactose or sorbitol given at relatively low doses may be much better indicators than the indocyanine green.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of beta-carotene and canthaxanthin on liver xenobiotic-metabolizing enzymes in the rat

The activities of several phase I and phase II xenobiotic-metabolizing enzymes have been measured in liver microsomes and cytosol of male rats that had been fed for 15 days with diets containing beta-carotene or canthaxanthin (300 mg/kg diet) or an excess of vitamin A (70,000 IU/kg diet), or to which beta-carotene had been administered by ip injections (7 x 10 mg/kg body weight). Microsomal cytochrome P-450 and the associated NADH- and NADPH-cytochrome c reductases were assayed, as well as several phase I and phase II enzyme activities. Phase I activities were markers of the families 1, 2, 3 and 4 of P-450; phase II activities were microsomal UDP glucuronosyl transferases (UGT) and cytosolic glutathione S-transferase (GST). Canthaxanthin accumulated in liver to a much higher level than did ingested or injected beta-carotene. Canthaxanthin increased the liver content of cytochrome P-450 (control value x 1.7), and the activity of NADH-cytochrome c reductase (x 1.5), and of some P-450-dependent enzymes (ethoxy-, methoxy-, pentoxy- and benzoxyresorufin O-dealkylases; x98, x15, x6.5 and x13, respectively), but not of others (erythromycin N-demethylase, nitrosodimethylamine N-demethylase and laurate omega-hydroxylase). Phase II activities were also increased: UGT1 (x3.4), UGT2 (x1.2) and GST (x1.2). This induction profile, characterized by the very strong increase of the activity associated with P4501A1, and the co-induction of UGT1, closely resemble that of a classical inducer, 3-methylcholanthrene. By contrast, neither beta-carotene (fed or injected), nor an excess of vitamin A induced any significant variation of the enzyme activities measured.

Aberrant CYP1A1 induction: discrepancy of CYP1A1 mRNA and aryl hydrocarbon hydroxylase activity in mutant cells of mouse hepatoma line, Hepa-1

We have isolated new benzo[a]pyrene-resistant clones, cl-21 and cl-32, of the mouse hepatoma line, Hepa-1. CYP1A1-dependent aryl hydrocarbon hydroxylase activity is not inducible by 2,3,7,8-tetrachlorodibenzo-p-dioxin or 3-methylcholanthrene in these two cell lines. However, mRNA of CYP1A1 is inducible in cl-21 and cl-32 cells, as in the wild-type cells, in spite of an undetectable level of cytosolic Ah receptor. The cl-21 cDNA of Cyp1a-1 was found to have a single mutation leading to an amino acid substitution from Leu (118) to Arg (118). However, the CYP1A1 protein band was not detected on Western immunoblots. The cDNA of cl-32 was found to have a single mutation leading to an amino acid change from Arg (359) to Trp (359). The presence of the mature protein in cl-32 was confirmed by Western blot analysis. Somatic cell hybridization experiments demonstrated that the phenotype of cl-21 and cl-32 is recessive and that these clones belong to the same complementation group. These data suggest that there may be a non-Ah receptor-mediated mechanism of CYP1A1 induction.

Characterization of cytochromes P-450 purified from untreated and 14C-2,3,7,8-tetrachlorodibenzo-p-dioxin--treated marmoset monkeys: identification of the major form as a possible orthologue of P-450 1A2

Three purified cytochrome P-450 (P-450) forms obtained from liver microsomes of marmoset monkeys induced with 14C-2,3,7,8-tetrachlorodibenzo-p-dioxin (14C-TCDD) were characterized. Comparison of electrophoretic and spectral properties and reconstituted methoxy-and ethoxyresorufin O-dealkylase (MROD and EROD) activities with those of forms isolated from untreated marmosets indicated that one of these (form F) is likely constitutive. Another form (D) had MROD and EROD activities which were 100 and 15 times those observed for form F. A form having biophysical properties similar to those of form D was also found in untreated animals. A third form (C) had an appreciable capacity to bind its inducing agent and showed a TCDD-to-P-450 molar ratio for detergent-free solutions of 0.66 +/- 0.13 to 1. In immunoblot analyses of these forms with antibodies raised against specific peptide sequences derived from rat P-450 1A1 and 1A2, the only positive reactions seen were those for untreated and inducible forms D with anti-rat 1A2. This provides evidence that the main or sole P-450 1A form in marmoset liver microsomes is 1A2, as in humans, and that this is inducible by TCDD.