Enzyme induction in the cytochrome P-450 system

Benzimidazole drugs and modulation of biotransformation enzymes

Benzimidazole drugs (e.g., anthelmintics albendazole, fenbendazole, oxfenbendazole, thiabendazole, mebendazole; inhibitors of proton pump omeprazole, lansoprasole, pantoprasole) represent substances used in both human and veterinary medicine; however, from the point of view of induction and inhibition of biotransformation enzymes, research has been carried out mainly due to the initiative of human pharmacologists. The purpose of the present review is to inform about inductive and inhibitive effects of benzimidazole drugs in man, animals and cell cultures. Pharmacological and toxicological consequences of modulation of biotransformation enzymes are discussed and the significance of studies in the field of modulation of biotransformation enzymes in food-producing animals is explained. Since the modulating effect of benzimidazoles strongly varies depending on structure of the individual substances, the particular attention is paid to structure-modulation relationships.

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.

Studies on the metabolism of the thiofurans furfuryl mercaptan and 2-methyl-3-furanthiol in rat liver

The metabolism of two thiofurans, namely furfuryl mercaptan (FM) and 2-methyl-3-furanthiol (MTF), to their corresponding methyl sulphide and methyl sulphoxide derivatives has been studied in male Sprague-Dawley rat hepatocytes and liver microsomes. Rat hepatocytes converted FM to furfuryl methyl sulphoxide (FMSO) and MTF to 2-methyl-3-(methylthio)furan sulphoxide (MMFSO). Liver microsomes catalysed the NADPH-dependent metabolism of furfuryl methyl sulphide (FMS) to FMSO and 2-methyl-3-(methylthio)furan sulphide (MMFS) to MMFSO. FMS and MMFS metabolism to their thiofuran methyl sulphoxide derivatives was induced by the treatment of rats with Aroclor 1254 and inhibited in liver microsomes treated with 1-aminobenzotriazole. The NADPH-dependent metabolism of FM to FMSO and MTF to MMFSO in liver microsomes was observed in the presence of S-adenosylmethionine. In summary, both thiofurans can be metabolised in rat liver to their thiofuran methyl sulphide derivatives which can be subsequently S-oxidised to form thiofuran methyl sulphoxides. FM and MTF appear to be substrates for rat hepatic microsomal thiol methyltransferase and the S-oxidation of FMS and MMFS appears to be primarily catalysed by cytochrome P450 forms.

The effects of albendazole and its metabolites on hepatic cytochromes P450 activities in mouflon and rat

Albendazole (ABZ) is a benzimidazole anthelmintic widely used in veterinary medicine. The effects of ABZ on cytochromes P450 were investigated in primary cultures of mouflon (Ovis musimon) and rat (Rattus norvegicus) hepatocytes. Besides ABZ, its two main metabolites (albendazole-sulphoxide, ABZSO and albendazole-sulphone, ABZSOO) were tested to clarify which compound is responsible for the induction potency of this benzimidazole drug. After 48 h incubation of hepatocytes with benzimidazoles (0.2-25 microM), ethoxyresorufin O-deethylation (EROD) and benzoxyresorufin O-dearylation (BROD) were measured and the P4501A and 3A protein levels were determined by Western blotting. All benzimidazoles provoked a significant increase of EROD and BROD activities in rat hepatocytes. ABZSO and ABZSOO seemed to be responsible for the induction effect of ABZ on P450s in rat. In mouflon, no pharmacologically significant induction of EROD and BROD activities by benzimidazoles tested was observed. From this point of view, anthelmintic therapy of mouflons with ABZ seems to be safe.

Principles of drug therapy for the elderly patient

Physicians will treat larger numbers of elderly patients as the US population ages. Being treated simultaneously for more than 1 condition with multiple prescription drugs is only 1 reason why elderly patients are at greater risk of experiencing adverse drug reactions. The need for physicians to minimize the incidence of these reactions has become incumbent on both physicians and administrators. We review the underlying reasons why the elderly population is at risk of adverse drug reactions and summarize the principles of drug-drug interaction, metabolism, and distribution, which can help elderly patients receive proper pharmacological treatment.

Immunological Findings in a Group of Coke-Oven Workers Exposed to Polycyclic Aromatic Hydrocarbons

Coke-oven workers are exposed to a high concentration of polycyclic aromatic hydrocarbons, which may change the immunologic responses. In this study, we evaluated humoral immunity by measuring serum IgA, IgE, IgG, and tumor necrosis factor-alpha in 251 coke-oven workers and 89 rolling steel workers in Taiwan. Mean values of serum IgE and tumor necrosis factor-alpha levels were 178.8 IU/mL and 8.4 pg/mL in coke-oven workers, respectively, which were significantly higher than in rolling steel workers (102.6 IU/mL and 2.0 pg/mL; P=0.003 and <0.001). In contrast, serum IgA levels were significantly lower in coke-oven workers than in rolling steel workers (mean=264.7 vs 312.0 mg/dL, P<0.001). Our findings suggest that polycyclic aromatic hydrocarbons exposure may alter the immune responses in coke-oven workers.

The effects of mebendazole on P4501A activity in rat hepatocytes and HepG2 cells. Comparison with tiabendazole and omeprazole

Mebendazole is a benzimidazole anthelmintic widely used in veterinary and human therapy. Among benzimidazole derivatives, several drugs with inducing effect on cytochromes P450 can be found. However, the induction capacity of mebendazole on P450s has not been explored yet. In this study, the effects of mebendazole on P4501A activity was tested in primary cultures of rat hepatocytes and in human hepatoma HepG2 cell line. Two known P4501A inducers with benzimidazole structure, tiabendazole and omeprazole, were also included in the experiments with the aim of studying structure-induction relationships. After 24-, 48- and 72-h incubation of rat hepatocytes and HepG2 cells with drugs in various concentrations (0.1-100 microM), enzyme activity associated with P4501A1/2 (EROD, MROD) was measured. In addition, the P4501A1/2 protein levels in both in-vitro systems were determined by Western-blotting. Mebendazole provoked a significant increase in P4501A1/2 protein expression and P4501A activity in both in-vitro systems. Omeprazole caused a significant dose-dependent increase of P4501A activity only in HepG2 cells. Although tiabendazole treatment led to significant increase of P4501A protein level, no effect on P4501A activity was observed in either system. The results demonstrate that mebendazole possesses the ability to significantly induce P4501A. Thus, pharmacological and toxicological consequences of P4501A induction should be taken into account in human therapy. The structure-induction relationships and differences between in-vitro systems used are discussed.

Anguilla anguilla L. liver ethoxyresorufin O-deethylation, glutathione S-transferase, erythrocytic nuclear abnormalities, and endocrine responses to naphthalene and beta-naphthoflavone

The effects of naphthalene (NAP) and beta-naphthoflavone (BNF) on phase I biotransformation and genotoxicity in Anguilla anguilla L. were evaluated. Phase II biotransformation and cortisol levels were also assessed in NAP-treated fish. Two groups of eels were exposed to either a NAP or a BNF concentration range (0.1-2.7 microM) for different exposure periods (2-72 h). An early significant ethoxyresorufin O-deethylation (EROD) activity inhibition was observed, especially for the highest NAP concentrations at 2-6 h exposure and for BNF at 2h exposure. However, a significant EROD activity increase was detected from 16 to 72 h exposure for NAP and from 4 to 72 h exposure for BNF. The cytochrome P450 (P450) content was not dose related. However, with regard to BNF exposure, P450 was the first biomarker to respond. Liver alanine transaminase (ALT) activity was measured as an indicator of hepatic health condition. ALT results demonstrated that the EROD activity decrease, previously described for NAP, was not related to tissue damage. Nevertheless, the highest BNF concentrations were demonstrated to induce liver damage and to impair the EROD activity response. An increased genotoxic response, measured as erythrocytic nuclear abnormalities (ENA), was observed during the first 8h NAP exposure. However, for exposures longer than 8 h, ENA frequency returned to the control levels. This response profile may reflect a considerable DNA repair capacity and/or a metabolic adaptation providing an efficient NAP biotransformation and consequent detoxification. BNF revealed no ENA alterations for all concentrations and exposure lengths. In the NAP experiment a causal relationship between immature erythrocytes (IE) and ENA frequency disappearance was not found. BNF results with regard to IE frequency revealed an ability to alter the balance between erythropoiesis and removal of erythrocytes. Liver glutathione S-transferase activity was significantly induced after 2 and 48 h NAP exposure. A cortisol-impaired response seems to occur from 4 to 24 h NAP exposure, demonstrating an endocrine disruption. However, an adaptation process seems to occur after 48 h, since the plasma cortisol had a tendency to increase. The present findings confirm the usefulness of the adopted biomarkers. The ecological risk associated with aquatic contamination by NAP was also confirmed by the present data.

Exposure to xenobiotic compounds: looking for new biomarkers

A variety of antropogenic compounds that have an estrogenic effect, and are known to be present in the environment, shows a significant potential for interference with the health and reproduction of both wildlife and humans. In this review, the effect of estrogenic and antiestrogenic chemicals with widely divergent potencies-nonylphenol (NP), which acts by binding with the estradiol response element, and beta-naphthoflavone (beta-NF), a dioxin-like compound that exerts its toxic action through the aryl hydrocarbon receptor-was compared with that induced by 17beta-estradiol (E(2)) in a marine teleost, the Gobius niger, under controlled laboratory experiments. The capacity of these compounds to affect the levels of estrogen-regulated proteins such as cathepsin D (CAT D)-in humans, a protein associated with the development of breast cancer, and, in oviparous vertebrates, with reproductive success-was assessed. The results of this study showed that both the estradiol and the higher dose of NP induce CAT D gene expression and its associated activity. On the contrary, beta-NF treatments inhibited CAT D gene expression and, at lengthier exposure (96 h), its enzymatic activity. Based on these results, we suggest CAT D as a novel bioindicator of the presence of endocrine-disrupting substances in the environment. The other biomarker assessed in this study is the Heath Shock Protein 70 (HSP70); this protein protects cells against harmful conditions by binding and refolding damaged proteins. Interestingly, HSP70 was found to be affected by all the toxicant compounds employed in the study. The HSP70 gene expression was significantly increased by both NP concentrations and the exposure time of beta-NF, with the E(2) being the most potent inducer. These data indicate that HSP70 may provide a useful early warning biomarker for studies on the presence of exogenous pollutants in the environment.

Studies on the induction of rat hepatic CYP1A, CYP2B, CYP3A and CYP4A subfamily form mRNAs in vivo and in vitro using precision-cut rat liver slices

1. Real-time quantitative reverse transcription-polymerase chain reaction methodology (TaqMan(R)) was used to examine the induction of some selected rat hepatic cyto-chrome P450 (CYP) forms in vivo and in vitro using cultured precision-cut liver slices. 2. TaqMan primers and probe sets were developed for rat CYP1A1, CYP1A2, CYP2B1, CYP2B1/2, CYP3A1, CYP3A2 and CYP4A1 mRNAs. 3. To characterize the responsiveness of the rat CYP mRNA TaqMan primers and probe sets, rats were treated in vivo with a single intraperitoneal dose of 500 mg kg(-1) Aroclor 1254 (ARO) and with four daily oral doses of either 50 mg kg(-1) day(-1) dexamethasone (DEX) or 75 mg kg(-1) day(-1) methylclofenapate (MCP). Treatment with ARO produced 22 600-, 5480-, 648-, 52-, 47- and 9-fold increases in levels of CYP1A1, CYP2B1, CYP2B1/2, CYP1A2, CYP3A1 and CYP3A2 mRNA, respectively. DEX treatment produced 97-, 24-, 8- and 4-fold increases, respectively, in CYP3A1, CYP2B1, CYP2B1/2 and CYP3A2 mRNA levels, and MCP produced 339-, 126- and 25-fold increases, respectively, in CYP4A1, CYP2B1 and CYP2B1/2 mRNA levels. All three CYP inducers also increased microsomal CYP content and produced corresponding increases in CYP1A, CYP2B, CYP3A and CYP4A form marker enzyme activities. 4. Rat liver slices were cultured for 6 and 24 h in medium containing 0.1 micro M insulin and 0.1 micro M DEX, and also for 24 h in medium containing only 0.1 micro M insulin (DEX-free medium). Liver slices were cultured in control medium or in medium containing either 10 micro M beta-naphthoflavone (BNF), 10 micro g ml(-1) ARO, 500 micro M sodium phenobarbitone (NaPB), 20 micro M pregnenolone-16alpha -carbonitrile (PCN), 50 micro M Wy-14,643 (WY) or 50 micro M MCP. 5. With the exception of the effect of BNF on CYP1A1 mRNA levels, the induction of all the CYP mRNAs studied was greater after 24- than after 6-h treatment. Generally, the magnitude of induction of CYP mRNA levels was greater after 24 h in liver slices cultured in DEX-free than in DEX-supplemented medium. 6. Treatment of liver slices with BNF and ARO for 24 h in DEX-free medium produced 21- and 35-fold increases, respectively, and 38- and 37-fold increases, respectively, in CYP1A1 and CYP1A2 mRNA levels. NaPB, PCN, WY and MCP did not increase either CYP1A1 or CYP1A2 mRNA levels. 7. After 24 h, levels of CYP2B1/2 mRNA were increased 18-, 20-, 9-, 16- and 13-fold by treatment with ARO, NaPB, PCN, WY and MCP, respectively. PCN also produced 56- and 4-fold increases, respectively, in CYP3A1 and CYP3A2 mRNA levels. 8. Treatment with WY and MCP for 24 h produced 437- and 186-fold increases, respectively, in levels of CYP4A1 mRNA. None of the other CYP inducers studied had any effect on CYP4A1 mRNA levels. 9. The results demonstrate the utility of cultured precision-cut liver slices as an in vitro model system to evaluate the effects of xenobiotics on rat CYP1A, CYP2B, CYP3A and CYP4A form mRNA levels.

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

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

Transcriptional analysis in vivo of the hepatic genes, Cyp2b9 and Cyp2b10, by intravenous administration of plasmid DNA in mice

Phenobarbital (PB) responsiveness of CYP2B genes has been shown to be mediated by a PB responsive unit (PBRU). The core of the PBRU contains two nuclear receptor sites, NR-1 and NR-2, and a nuclear factor-1 (NF 1) binding site, which are required for PB responsiveness, but the importance of sequences flanking the core is not clear. We have used intravenous administration of plasmid DNA in the tail veins of mice to transfect hepatocytes in vivo and analyze sequence requirements for PB induction. In this assay PB treatment increased transactivation by the Cyp2b10 PBRU about 100-fold, which is similar to the increase in the expression of the endogenous gene while the Cyp2b9 PBRU was unresponsive. Analysis of chimeras of the two PBRUs and deletion mutants of the Cyp2b10 PBRU indicated that the core region containing the NR-1, NR-2 and NF-1 core sites is not sufficient for PB responsiveness. Additional sequence at the 3' side of the core sequence, which included a previously defined accessory factor-1 (AF-1) site, partially restored responsiveness. This region contained a binding site for NF-1 only in Cyp2b10 and not in Cyp2b9, but the intact site was not required for PB responsiveness. Purified constitutive androstane receptor (CAR)/retinoid X receptor (RXR) bound to the core NR-1 and NR-2 sites and to a third NR-3 site to the 5' side of the core in Cyp2b10. No binding of CAR/RXR to the Cyp2b9 PBRU was observed. These results indicate that changes in the NR sites which eliminate CAR/RXR binding are sufficient for the non-responsiveness to PB of Cyp2b9, but changes in sequences flanking the core independently eliminate PB responsiveness. The results demonstrate the advantages of transfection of mouse hepatocytes in vivo by tail vein injection of DNA as a method for transcriptional analysis of genes in vivo and show that sequences flanking the core region of the PBRU are required for PB induction in vivo.

Transcriptional analysis of the orphan nuclear receptor constitutive androstane receptor (NR1I3) gene promoter: identification of a distal glucocorticoid response element

The constitutive androstane receptor (CAR, NR1I3) transcriptionally activates cytochrome P450 2B6, 2C9, and 3A4 when activated by xenobiotics, such as phenobarbital. Information on the human CAR promoter was obtained by searching the NCBI human genome database. A contig (NT026945) corresponding to a fragment of chromosome 1q21 was found to contain the complete CAR gene. These data were confirmed using chromosomal in situ hybridization. Both primer extension and 5'-rapid amplification of the cDNA end PCR analysis were carried out to determine the transcriptional start site of human CAR, which was found to be 32 nucleotides downstream of a potential TATA box (CATAAAA). In addition, we found that the 5'-untranslated region of CAR mRNA is 110 nucleotides shorter than previously reported. Using genomic PCR, we amplified and cloned approximately 4.9 kb (-4711/+144) of the CAR gene promoter. The activity of this promoter was measured by transient transfection. Deletion analysis suggested the presence of a glucocorticoid responsive element in its distal region (-4477/-4410). From cotransfection experiments, mutagenesis, and gel shift assays, we identified a glucocorticoid response element at -4447/-4432 that was recognized and transactivated by the human glucocorticoid receptor. Finally, using the chromatin immunoprecipitation assay, we demonstrated that the glucocorticoid receptor binds to the distal region of CAR promoter in cultured hepatocytes only in the presence of dexamethasone. Identification of this functional element provides a rational mechanistic basis for CAR induction by glucocorticoids. CAR appears to be a primary glucocorticoid receptor-response gene.

Mutagenic activation of arylamines by subcellular fractions of Chamaelea gallina clams exposed to environmental pollutants

Biochemical measurements in the sentinel clam Chamaelea gallina have been used as biomarkers of marine pollution. In this study, S9, cytosolic fractions (CF), and microsomal fractions (MF) prepared from unexposed clams and clams exposed to model pollutants were used to activate 2-aminoanthracene (2-AA) and 2-acetylaminofluorene (AAF) to mutagens in Salmonella typhimurium strain BA149, which overexpresses O-acetyltransferase. Arylamine activation was similar with subcellular fractions from unexposed and Aroclor 1254-exposed clams, but decreased with fractions from As(III)- and Cu(II)-exposed clams. Bioactivation of arylamines by CF was higher than by MF, and higher with NADH than with NADPH as the reducing agent. alpha-Naphthoflavone inhibited AAF activation by CF and MF, but increased 2-AA activation nearly twofold. In contrast to the results with arylamine activation, benzo[a]pyrene hydroxylase (BPH) activity increased twofold in fractions from Aroclor 1254- and Cu(II)-exposed clams. Activation of 2-AA was also evaluated using S9 fractions from clams sampled at littoral sites with different pollutant levels. Compared to a reference site, lower 2-AA bioactivation and higher BPH activity were found in clams containing high levels of copper and organic contaminants, although the differences were not statistically significant. While these findings agree with the results of the model Cu(II) exposure, the effects of other pollutants cannot be ruled out. The results of the study demonstrate that arylamine activation by clams is not preferentially catalyzed by microsomal monooxygenases but by unknown cytosolic system(s), and that bioactivation of 2-AA and AAF appears to occur by different pathways.

Drug-activated nuclear receptors CAR and PXR

The metabolism and elimination of drugs is mainly mediated by cytochrome P450 (CYP) enzymes, aided by conjugative enzymes and transport proteins. An integral aspect of this elimination process is the induction of drug metabolism through activation of gene expression of metabolic and transport proteins. There is compelling evidence that induction is regulated by drug-activated nuclear receptors constitutive androstane receptor (CAR) and pregnane X receptor (PXR). This review outlines the basic properties of CAR and PXR, their ligands and target genes, and the mechanisms of the induction process. The implications of nuclear receptor-mediated induction for drug research are also discussed.

Trophic transfer of persistent organochlorine contaminants (OCs) within an Arctic marine food web from the southern Beaufort-Chukchi Seas

Stable isotope values (13C, 15N) and concentrations of persistent organochlorine contaminants (OCs) were determined to evaluate the near-shore marine trophic status of biota and biomagnification of OCs from the southern Beaufort-Chukchi Seas (1999-2000) near Barrow, AK. The biota examined included zooplankton (Calanus spp.), fish species such as arctic cod (Boreogadus saida), arctic char (Salvelinus alpinus), pink salmon (Oncorhynchus gorbuscha), and fourhorn sculpin (Myoxocephalus quadricornis), along with marine mammals, including bowhead whales (Balaena mysticetus), beluga whales (Delphinapterus leucas), ringed seals (Phoca hispida) and bearded seals (Erignathus barbatus). The isotopically derived trophic position of biota from the Beaufort-Chukchi Seas marine food web, avian fauna excluded, is similar to other coastal food webs in the Arctic. Concentrations of OCs in marine mammals were significantly greater than in fish and corresponded with determined trophic level. In general, OCs with the greatest food web magnification factors (FWMFs) were those either formed due to biotransformation (e.g. p,p'-DDE, oxychlordane) or considered recalcitrant (e.g. -HCH, 2,4,5-Cl substituted PCBs) in most biota, whereas concentrations of OCs that are considered to be readily eliminated (e.g. -HCH) did not correlate with trophic level. Differences in physical-chemical properties of OCs, feeding strategy and possible biotransformation were reflected in the variable biomagnification between fish and marine mammals. The FWMFs in the Beaufort-Chukchi Seas region were consistent with reported values in the Canadian Arctic and temperate food webs, but were statistically different than FWMFs from the Barents and White Seas, indicating that the spatial variability of OC contamination in top-level marine Arctic predators is attributed to differences in regional sources of contamination rather than trophic position.

Effect of 10-aza-substitution on benzo[a]pyrene mutagenicity in vivo and in vitro

Benzo[a]pyrene (BaP), an environmental carcinogen, shows genotoxicity after metabolic transformation into the bay-region diol epoxide, BaP-7,8-diol 9,10-epoxide. 10-Azabenzo[a]pyrene (10-azaBaP), in which a ring nitrogen is located in the bay-region, is also a carcinogen and shows mutagenicity in the Ames test in the presence of the rat liver microsomal enzymes. In order to evaluate the effect of aza-substitution on in vivo genotoxicity, BaP and 10-azaBaP were assayed for their in vivo mutagenicity using the lacZ-transgenic mouse (MutaMouse). BaP was potently mutagenic in all of the organs examined (liver, lung, kidney, spleen, forestomach, stomach, colon, and bone marrow), as described in our previous report, whereas, 10-azaBaP was slightly mutagenic only in the liver and colon. The in vitro mutagenicities of BaP and 10-azaBaP were evaluated by the Ames test using liver homogenates prepared from several sources, i.e. CYP1A-inducer-treated rats, CYP1A-inducer-treated and non-treated mice, and humans. BaP showed greater mutagenicities than 10-azaBaP in the presence of a liver homogenate prepared from CYP1A-inducer-treated rodents. However, 10-azaBaP showed mutagenicities similar to or more potent than BaP in the presence of a liver homogenate or S9 from non-treated mice and humans. These results indicate that 10-aza-substitution markedly modifies the nature of mutagenicity of benzo[a]pyrene in both in vivo and in vitro mutagenesis assays.

Ontogeny of hepatic and renal systemic clearance pathways in infants: part I

Dramatic developmental changes in the physiological and biochemical processes that govern drug pharmacokinetics and pharmacodynamics occur during the first year of life. These changes may have significant consequences for the way infants respond to and deal with drugs. The ontogenesis of systemic clearance mechanisms is probably the most critical determinant of a pharmacological response in the developing infant. In recent years, advances in molecular techniques and an increased availability of fetal and infant tissues have afforded enhanced insight into the ontogeny of clearance mechanisms. Information from these studies is reviewed to highlight the dynamic and complex nature of developmental changes in clearance mechanisms in infants during the first year of life. Hepatic and renal elimination mechanisms constitute the two principal clearance pathways of the developing infant. Drug metabolising enzyme activity is primarily responsible for the hepatic clearance of many drugs. In general, when compared with adult activity levels normalised to amount of hepatic microsomal protein, hepatic cytochrome P450-mediated metabolism and the phase II reactions of glucuronidation, glutathione conjugation and acetylation are deficient in the neonate, but sulfate conjugation is an efficient pathway at birth. Parturition triggers the dramatic development of drug metabolising enzymes, and each enzyme demonstrates an independent rate and pattern of maturation. Marked interindividual variability is associated with their developmental expression, making the ontogenesis of hepatic metabolism a highly variable process. By the first year of life, most enzymes have matured to adult activity levels. When compared with adult values, renal clearance mechanisms are compromised at birth. Dramatic increases in renal function occur in the ensuing postpartum period, and by 6 months of age glomerular filtration rate normalised to bodyweight has approached adult values. Maturation of renal tubular functions exhibits a more protracted time course of development, resulting in a glomerulotubular imbalance. This imbalance exists until adult renal tubule function values are approached by 1 year of age. The ontogeny of hepatic biliary and renal tubular transport processes and their impact on the elimination of drugs remain largely unknown. The summary of the current understanding of the ontogeny of individual pathways of hepatic and renal elimination presented in this review should serve as a basis for the continued accruement of age-specific information concerning the ontogeny of clearance mechanisms in infants. Such information can only help to improve the pharmacotherapeutic management of paediatric patients.

Pretreatment of rats with the inducing agents phenobarbitone and 3-methylcholanthrene ameliorates the toxicity of chromium (VI) in hepatocytes

To exert cytotoxicity chromium VI (Cr(VI)) has to be reduced inside cells. This is achieved through both enzymatic and non-enzymatic mechanisms. Enzymatic mechanisms include DT-diaphorase, cytochrome P450, and NADPH cytochrome c reductase, and non-enzymatic mechanisms involve reduced glutathione (GSH) and ascorbic acid. The extent of cytotoxicity of Cr(VI) may thus be influenced by the availability of non-enzymatic reductants, and by the activities of the reductase enzymes. In the present paper we have investigated the effect of pretreatment with the inducing agents, phenobarbitone (PB) and 3-methylcholanthrene (3-MC), on the response of rat hepatocytes to Cr(VI). Pretreatment with PB increased the activity of NADPH cytochrome c reductase, and 3-MC increased DT-diaphorase activity in hepatocytes. Both inducers increased cytochrome P450 content, while neither influenced intracellular GSH content or the activity of glutathione reductase. Pretreatment with either PB or 3-MC resulted in amelioration of Cr(VI) toxicity both in terms of hepatocyte viability, and to a greater extent, in terms of Cr(VI) induced GSH loss. We propose that the inducing agents increase the amount of enzymatic reduction of Cr(VI) relative to non-enzymatic reduction. Thus, less GSH is used in the reduction of Cr(VI), and intracellular GSH does not fall as rapidly as in cells from control animals therefore cell integrity is better maintained. Exposure to environmental inducing agents in vivo may also alter the response of human tissues to Cr(VI).

Naphthalene and beta-naphthoflavone effects on Anguilla anguilla L. hepatic metabolism and erythrocytic nuclear abnormalities

The effects of a polycyclic aromatic hydrocarbon (PAH) such as naphthalene (NAP)--an environmental contaminant--and beta-naphthoflavone (BNF)--a model substance (PAH-like compound)--were investigated in European eel (Anguilla anguilla L.) over 3-, 6-, and 9-day exposure (0.1-2.7 microM). Both xenobiotics revealed to be strong biotransformation (phase I) inducers. After 3-day exposure, liver ethoxyresorufin O-deethylase (EROD) activity was significantly increased by all NAP and BNF tested concentrations. At 6 and 9 days, liver EROD activity was significantly induced mainly by the highest NAP and BNF concentrations. Liver cytochrome P450 content was significantly induced after 3-day exposure to 0.9 and 2.7 microM BNF and 9-day exposure to 0.1, 0.3 and 0.9 microM NAP. Liver alanine transaminase (ALT) activity was measured as an indicator of hepatic health condition, revealing a significant decrease after 6-day exposure to 0.9 microM BNF. Genotoxicity measured as erythrocytic nuclear abnormalities (ENA) was detected in all BNF treated fish on day 6, whereas on day 9, ENA frequencies returned to control levels, significantly decreasing at 0.9 microM BNF exposure. Immature erythrocytes (IE) frequency demonstrated a decreasing tendency along the BNF experiment and concomitantly with the above ENA response. The present experimental results elect EROD activity in A. anguilla as a useful short- to medium-term biomarker of exposure to both PAH and PAH-like compounds. However, some problems can emerge in the presence of high xenobiotic concentrations. Concerning genotoxicity, it is hypothesized that ENA response depends on different factors such as the exhaustion of the detoxification process, the balance erythropoiesis/erythrocytic catabolism and the DNA repairing capacity.

Use of a quantitative real-time reverse transcription-polymerase chain reaction method to study the induction of CYP1A, CYP2B and CYP4A forms in precision-cut rat liver slices

1. The aim was to employ real-time quantitative reverse transcription-polymerase chain reaction (RT-PCR) technology (TaqMan to examine the induction of some selected cytochrome P450 (CYP) forms in precision-cut rat liver slices. 2. Taqman primers and probe sets were developed for rat CYP1A1, CYP1A2, CYP2B1 and CYP4A1 forms. 3. Rat liver slices were cultured in control medium or medium containing either 10 micro g ml(-1) Aroclor 1254 (ARO), 500 micro M sodium phenobarbitone (NaPB) or 50 micro M Wy-14643 (WY) for 3, 6 and 24 h. 4. Compared with control liver slices, treatment with ARO for 3 and 6 h produced 24- and 184-fold increases, respectively, in CYP1A1 mRNA levels, and after 24h produced an 85-fold increase in CYP1A2 mRNA levels. Levels of CYP1A1 and CYP1A2 mRNA were not markedly affected by NaPB and WY. 5. Treatment with ARO and PB for 24 h produced 10.6- and 23.8-fold increases, respectively, in CYP2B1 mRNA. Levels of CYP2B1 mRNA were not markedly affected by WY. 6. Treatment with WY, but not ARO and NaPB, for 24h produced a 20.4-fold increase in levels of CYP4A1 mRNA. 7. These results demonstrate that cultured liver slices may be used to evaluate the effect of xenobiotics on CYP form mRNA levels.

Cytochrome P450 (CYP1A) induction and DNA adducts in a rat hepatoma cell line (Fao), exposed to environmentally relevant concentrations of organic compounds, singly and in combinations

Cytochrome P4501A (CYP1A) induction and DNA adduct formation were evaluated in the rat hepatoma cell line Fao, as biomarkers of exposure to organic compounds. Cells were exposed to environmentally relevant concentrations of benzo[a]pyrene (B[a]P) or 3,3',4,4'-tetrachlorobiphenyl (TCB), and to combinations of B[a]P and TCB. Both B[a]P and TCB induced CYP1A proteins in a concentration-dependent relationship, up to concentrations of 10 and 1 μM, respectively, detected by Western blotting. DNA adducts, analyzed by (32)P-postlabeling, were found at the highest concentrations of B[a]P (1 and 10 μM). No adducts were found in cells exposed to 0.1 μM TCB alone. The cotreatment of TCB and B[a]P indicated an increase in DNA adduct formation, compared with B[a]P, but no further induction of CYP1A protein compared with TCB alone. This study suggests that Western blotting and (32)P-postlabeling might be suitable methods for detecting CYP1A protein induction and DNA adducts, respectively, after exposure to environmentally relevant concentrations of organic compounds.

Comparison of one-dimensional and two-dimensional gel electrophoresis as a separation tool for proteomic analysis of rat liver microsomes: cytochromes P450 and other membrane proteins

Gel electrophoresis in combination with peptide mass fingerprinting is the method of choice for proteomic profiling of various in vitro and in vivo biological systems. In the investigation reported here we analyzed the protein composition of hepatic microsomes from untreated and phenobarbital treated rats, using one-dimensional (1-DE) and two-dimensional (2-DE) gel electrophoresis, followed by tryptic peptide mapping. To better characterize capabilities of 2-DE 1-DE with regard to microsomal membrane proteins, "ghosts" of microsomal vesicles enriched in membrane proteins were obtained and analyzed. Both 1-DE and 2-DE showed that phenobarbital induces not only cytochromes P450 2B1and 2B2 but such stress related endoplasmic reticulum proteins as protein disulfide isomerase A(3) and A(6) and 78 kDa glucose regulated protein. The analytical performance of 1-DE with regard to endoplasmic reticulum membrane proteins is incomparably greater than that of 2-DE. Twenty-two out of a total of thirty-four known to date microsomal rat membrane proteins were identified by 1-DE in combination with matrix-assisted laser desorption/ionization-mass spectrometry of in-gel digests. At the same time using various types of 2-DE, we were able to identify only three rat microsomal membrane proteins. The data presented in this manuscript clearly demonstrate that 1-DE in combination with peptide mass fingerprinting can be successfully used for cataloging proteins of the endoplasmic reticulum, and that the proteomic analysis of the subcellular organelles containing a considerable number of highly hydrophobic membrane proteins should be performed by combined application of 1-D and 2-D electrophoresis.

PXR, CAR and drug metabolism

Mechanisms that protect the body from a diverse array of harmful chemicals are also involved in drug metabolism, and can cause adverse drug-drug interactions. Two closely related orphan nuclear hormone receptors--the pregnane X receptor (PXR) and the constitutive androstane receptor (CAR)--have recently emerged as transcriptional regulators of cytochrome P450 expression that couple xenobiotic exposure to oxidative metabolism. In this review, we provide an examination of the roles of PXR and CAR as xenobiotic sensors, and discuss the application of this knowledge to toxicological screening in drug discovery.

Phenobarbitone-mediated translocation of the cytosolic proteins interacting with the 5'-proximal region of rat liver CYP2B1/B2 gene into the nucleus

The positive element (PE) (-69 to -98 bp) within the 5'-proximal region of the CYP2B1/B2 gene (+1 to -179 bp) of rat liver is essential for phenobarbitone (PB) response and gives a single major complex with the rat liver cytosol in gel shift analysis. This complex corresponds to complex I (top) of the three complexes given by the nuclear extracts. PB treatment of rats leads to a decrease in complex I formation with the cytosol and PE and an increase in the same with the nuclear extract in gel shift analysis. Both the changes are counteracted by simultaneous okadaic acid administration. The nuclear protein giving rise to complex I has been isolated and has an M(r) of 26 kDa. The cytosolic counterpart consists of two species, 26 and 28 kDa, as revealed by Southwestern blot analysis using labeled PE. It is concluded that PB treatment leads to the translocation accompanied by processing of the cytosolic protein species into the nucleus that requires protein dephosphorylation. It is suggested that PB may exert a global regulation on the transcription of many genes by modulating the phosphorylation status of different protein factors involved in transcriptional regulation.

Cytochrome P450 3A expression in the human fetal liver: evidence that CYP3A5 is expressed in only a limited number of fetal livers

CYP3A is the major cytochrome P450 subfamily constitutively expressed in the human liver. CYP3A4 is the predominant hepatic P450 form in adults and it is expressed at high but very variable levels among individuals. The fetal liver contains mainly CYP3A7, while the presence of the other CYP3A enzymes in fetal liver has remained controversial. In this study, the relative levels of CYP3A4, CYP3A5 and CYP3A7 expression were determined in a panel of 9-11 fetal livers with a similar gestation age (9-12 weeks) and compared to adult livers. CYP3A7 was found to be the major CYP3A form in all the fetal liver samples. The abundance of CYP3A7 varied more at the mRNA (77-fold variation) than at the protein level (4.8-fold variation). CYP3A5 mRNA was also detected in all of the fetal liver samples, but the average level was 700-fold lower than that of CYP3A7. CYP3A5 protein was detected by immunoblot analysis in only 1 fetal liver out of the 9 investigated, the level of expression being moderately high in this sample. CYP3A4 mRNA was detected in only a subset of the fetal liver samples and its level was the lowest of the CYP3A forms. This is the first study to demonstrate the polymorphic expression of CYP3A5 and the variability of CYP3A7 expression in fetal liver and suggests that significant interindividual differences in the metabolism of xenobiotics may already exist at the prenatal stage. These differences may contribute to individual pharmacological and/or toxicological responses in the fetus.

Evaluation of 7-benzyloxy-4-trifluoromethylcoumarin, some other 7-hydroxy-4-trifluoromethylcoumarin derivatives and 7-benzyloxyquinoline as fluorescent substrates for rat hepatic cytochrome P450 enzymes

1. The aim of this study was to evaluate a number of derivatives of 7-hydroxy-4-trifluoromethylcoumarin (HFC) and 7-benzyloxyquinoline (7BQ) as novel fluorescent substrates for monitoring rat hepatic cytochrome P450 (CYP) enzyme specificity in a 96- well plate format. The HFC derivatives examined comprised 7-benzyloxy-4-trifluoromethylcoumarin (BFC), 2,5-bis(trifluoromethyl)-7-benzyloxy-4-trifluoromethylcoumarin (BFBFC), 3,5-bis(trifluoromethyl)-7-benzyloxy-4-trifluoromethylcoumarin (BTBFC), 2-(trifluoromethyl)-7-benzyloxy-4-trifluoromethylcoumarin (2TFBFC), 3-(trifluoromethyl)-7-benzyloxy-4-trifluoromethylcoumarin (3TFBFC) and 3-(trifluoromethoxy)-7-benzyloxy-4-trifluoromethylcoumarin (3TFMeOBFC). 2. The CYP specificity of the fluorescent probe substrates was examined using characterized liver microsomes from male Sprague-Dawley rats treated with beta naphthoflavone (BNF), sodium phenobarbitone (NaPB), isoniazid, pregnenolone-16alpha-carbonitrile (PCN), dexamethasone (DEX) and methylclofenapate to induce CYP1A, CYP2B, CYP2E, CYP3A, CYP3A and CYP4A forms, respectively. Studies were also performed with microsomes from baculovirus-infected insect cells containing rat cDNA-expressed CYP1A1, CYP1A2, CYP2B1, CYP3A1 and CYP3A2. 3. BFC metabolism was most markedly induced by BNF and NaPB, whereas BFBFC metabolism was most markedly induced by PCN and DEX and BTBFC was not metabolized by rat liver microsomes. BFC was a high-affinity substrate for cDNA-expressed CYP1A1 and CYP2B1, whereas BFBFC exhibited a high affinity for CYP3A1 and CYP3A2. 4. The metabolism of 2TFBFC and 3TFBFC was induced by NaPB, PCN and DEX. 3TFBFC was a relatively specific substrate for cDNA-expressed CYP2B1, whereas 2TFBFC could be metabolized by CYP2B1, CYP3A1 and CYP3A2. 5. 3TFMeOBFC metabolism was markedly induced by BNF treatment and 3TFMeOBFC was extensively metabolized by cDNA-expressed CYP1A1. 6. The metabolism of 7BQ to 7-hydroxyquinoline was induced by treatment with PCN and DEX. 7BQ was a substrate for cDNA-expressed CYP3A2 and to a lesser extent for CYP3A1. 7. In summary, some of the HFC derivatives studied and 7BQ are useful fluorescent probe substrates for rat CYP enzymes. BFC appears to be a probe for CYP1A and CYP2B, 2TFBFC for CYP2B and CYP3A and 3TFBFC for CYP2B. While 3TFMeOBFC appears to be a relatively specific probe for CYP1A1, both BFBFC and 7BQ are good probes for the induction of CYP3A.

Smoking in patients receiving psychotropic medications: a pharmacokinetic perspective

Many psychiatric patients smoke, and are believed to be heavier smokers than those without psychiatric disorders. Cigarette smoking is one of the environmental factors that contributes to interindividual variations in response to an administered drug. Polycyclic aromatic hydrocarbons (PAHs) present in cigarette smoke induce hepatic aryl hydrocarbon hydroxylases, thereby increasing metabolic clearance of drugs that are substrates for these enzymes. PAHs have been shown to induce 3 hepatic cytochrome P450 (CYP) isozymes, primarily CYP1A1, 1A2 and 2E1. Drug therapy can also be affected pharmacodynamically by nicotine. The most common effect of smoking on drug disposition in humans is an increase in biotransformation rate, consistent with induction of drug-metabolising enzymes. Induction of hepatic enzymes has been shown to increase the metabolism and to decrease the plasma concentrations of imipramine, clomipramine, fluvoxamine and trazodone. The effect of smoking on the plasma concentrations of amitriptyline and nortriptyline is variable. Amfebutamone (bupropion) does not appear to be affected by cigarette smoking. Smoking is associated with increased clearance of tiotixene, fluphenazine, haloperidol and olanzapine. Plasma concentrations of chlorpromazine and clozapine are reduced by cigarette smoking. Clinically, reduced drowsiness in smokers receiving chlorpromazine, and benzodiazepines, compared with nonsmokers has been reported. Increased clearance of the benzodiazepines alprazolam, lorazepam, oxazepam, diazepam and demethyl-diazepam is found in cigarette smokers, whereas chlordiazepoxide does not appear to be affected by smoking. Carbamazepine appears to be minimally affected by cigarette smoke, perhaps because hepatic enzymes are already stimulated by its own autoinductive properties. Cigarette smoking can affect the pharmacokinetic and pharmacodynamic properties of many psychotropic drugs. Clinicians should consider smoking as an important factor in the disposition of these drugs.

Effects of beta-naphthoflavone on the cytochrome P450 system, and phase II enzymes in gilthead seabream (Sparus aurata)

The effect of beta-naphthoflavone (beta-NF) on several catalytic activities of cytochrome P450 (CYP) and phase II enzymes putatively controlled by [Ah]-receptor activation in the liver, heart and kidney of gilthead seabream, was investigated. In the liver, beta-NF treatment [intraperitoneal injection (i.p.) 50 mg/kg] resulted in an increase of CYP content, immunoreactive CYP 1A and methoxyresorufin-O-demethylase (MEROD), pentoxyresorufin O-depentylase (PROD) and ethoxyresorufin-O-deethylase (EROD) activities. However, beta-NF had no effect on any of the hepatic phase II enzymes examined (benzaldehyde dehydrogenase, propionaldehyde dehydrogenase, glutathione S-transferase, UDP-glucuronyl-transferase, DT-diaphorase). Single i.p. injection of 10 mg/kg beta-NF showed a maximal induction of CYP 1A-like protein and EROD activity after 3-7 days. CYP 1A and EROD returned to control levels 18-days post-treatment. beta-NF injection also caused a rapid increase of a single band size of mRNA recognized by a CYP 1A1 cDNA fragment from sea bass (Dicentrarchus labrax). Expression of mRNA preceded the increase of EROD activity and declined rapidly by 96 h. Dose-response experiments demonstrated that EROD was significantly enhanced in liver by a single injection of 0.3 mg/kg beta-NF and was the most sensitive measurement for CYP 1A-like induction. beta-NF treatments also increased the expression of CYP 1A-like protein, mRNA and EROD, but not MEROD and PROD activities in heart and kidney.

Persistent, low-dose 2,3,7,8-tetrachlorodibenzo-p-dioxin exposure: effect on aryl hydrocarbon receptor expression in a dioxin-resistance model

Most toxic effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) are mediated by the aryl hydrocarbon receptor (AHR). A single, acute dose of TCDD can alter its own receptor levels thus complicating evaluation of dose-response relationships for AHR-mediated events. Since environmental exposure to dioxins is typically of a repeated low-dose nature, we examined the effect of such exposure on AHR expression. Three rat strains differing greatly in their sensitivity to acute TCDD lethality, Long-Evans (Turku AB) (L-E) (LD50 approximately 10 microg/kg); Sprague Dawley (SD) (LD50 approximately 50 microg/kg); and Han/Wistar (Kuopio) (H/W) (LD50 > 9600 microg/kg), were administered TCDD intragastrically, biweekly for 22 weeks producing doses equivalent to 0, 10, 30, and 100 ng/kg/day. Changes in hepatic AHR levels were quantitated at the protein level by radioligand binding and immunoblotting and at the mRNA level by RT-PCR. Cytosolic AHR protein was elevated at 10 or 30 ng/kg/day TCDD in SD and L-E rats; AHR mRNA was also elevated at these doses, suggesting a pretranslational mechanism. There was no apparent relationship between TCDD-induced AHR regulation and strain sensitivity to TCDD. Overall, "subchronic" TCDD did not greatly perturb AHR expression. The maintenance of relatively constant receptor levels in the face of persistent agonist stimulation is in contrast to the sustained depletion of AHR by TCDD observed in cell culture and to the fluctuations in AHR observed hours to days following acute TCDD exposure in vivo. Changes in AHR levels may affect dose-response relationships; the effect of TCDD on its own receptor at environmentally relevant dosing schemes is therefore important to risk assessment.

Effect of 4-vinylcyclohexene on micronucleus formation in the bone marrow of rats and mice

This study was conducted to evaluate the potential of 4-vinylcyclohexene (VCH) to induce micronuclei in the bone marrow of mice and rats. Male and female Crl:CD BR (Sprague-Dawley) rats and B6C3F1/CrBR mice were exposed to VCH 6 hr/day for 2 days or for 13 weeks. In the 2-day study, mice were exposed by inhalation to 0, 250, 500, or 1000 ppm, and rats were exposed to 0, 500, 1000, or 2000 ppm. In the 13-week study, mice were exposed to 0, 50, 250, or 1000 ppm, and rats were exposed to 0, 250, 1000, or 1500 ppm. In each study, a separate group of mice was exposed to 1000 ppm 1,3-butadiene (BD) so that a comparison could be made between the two compounds. Likewise, cyclophosphamide was also included for rats as a positive control. Bone marrow was collected from VCH-exposed animals approximately 24 h and 48 h after the final exposure. There were no statistically significant increases in micronucleatedpolychromatic erythrocytes (MN-PCEs) among VCH-treated mice and rats at any dose level or sampling interval at either 2-days or 13-weeks. Also, no statistically significant differences in the polychromatic erythrocytes (PCE) to normochromatic erythrocytes (NCE) ratios were observed in any of the VCH-treated mice and rats compared to air-exposed animals. As expected, both the butadiene-treated mice and the cyclophosphamide-treated rats showed significantly more MN-PCEs than the control animals.

Implications and consequences of enzyme induction on preclinical and clinical drug development

1. Enzyme induction has traditionally been studied during drug development to assess the potential of drug entities to interact with concomitant medications and alter their pharmacological effects, and clearly it is an unwanted phenomenon. However, another hurdle caused by induction occurs during preclinical development via the attainment of safety data, obtained by dosing high quantities of compound to species used in toxicology assessment. This review considers the techniques that can now be utilized in drug discovery, their relevance, the pharmacokinetic aspects of this phenomenon, and it discusses the consequences and implications of induction during preclinical and clinical development. 2. It is becoming increasingly routine to employ hepatocyte cultures and novel techniques such as quantitative real-time reverse transcriptase PCR to identify enzyme inducers in vitro. The major challenge is to utilize these in vitro data to predict the consequences of induction in vivo. From an understanding of pharmacokinetic principles and low clinical doses relative to preclinical studies, there is limited potential for induction by a development candidate to significantly alter the pharmacological efficacy of a co-administered drug. 3. The most comprehensive approach when considering induction involves integrating quantitative in vitro data, information on the pharmacokinetic behaviour of the compound and the PK/PD) relationship in order to predict its consequences. The generation of this holistic strategy would enable more detailed and informed decision-making about both the suitability of molecules for development and the development strategy itself.

Cloning, sequencing, heterologous expression, and characterization of murine cytochrome P450 3a25*(Cyp3a25), a testosterone 6beta-hydroxylase

A full-length cDNA clone encoding a novel form of the cytochrome P450 3A subfamily (Cyp3a-25) has been isolated from a mouse liver cDNA library. The sequence contained 2010 base pairs and encoded a protein with 503 amino acids. The amino acid sequence shared greater identities with rat CYP3A18 (90%) and golden hamster CYP3A10 (81%) sequences than with known mouse sequences (Cyp3a-11, Cyp3a-13, Cyp3a-16, and Cyp3a-41 [68--70%]). CYP3A25 was expressed in the Escherichia coli PCWori(+) expression vector following slight modifications of the N- and C-terminals of the cDNA. The purified CYP3A25 was recognized on an immunoblot by CYP3A1 antibody and has a molecular weight of 50 kD. CYP3A25 was catalytically active in the 6 beta-hydroxylation of testosterone and the N-demethylation of benzphetamine and erythromycin. It was demonstrated by RT-PCR that the CYP3A25 mRNA is present in both fetal and adult tissues, including liver, lung, intestines, kidney, and brain. Northern blotting demonstrated that expression is greatest in the liver and small intestine.

Use of cDNA microarrays to analyze dioxin-induced changes in human liver gene expression

One mechanism by which cells adapt to environmental changes is by altering gene expression. Here, we have used cDNA microarrays to identify genes whose expression is altered by exposure to the environmental contaminant 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). The goal of our study was to enhance our understanding of toxicity mediated through the pathway by which TCDD stimulates gene expression. To model this toxicity response, we exposed human hepatoma (HepG2) cells to TCDD (10 nM for 18 h) and analyzed mRNA by two-color fluorescent hybridization to cDNA sequences immobilized on glass microscope slides (2.5 x 7.5 cm) covering a surface area of 2.25 cm(2). We analyzed approximately one-third of the genes expressed in HepG2 cells and found that TCDD up- or down-regulates 112 genes two-fold or more. Most changes are relatively subtle (two- to four-fold). We verified the regulation of protooncogene cot, XMP, and human enhancer of filamentation-1 (HEF1), genes involved in cellular proliferation, as well as metallothionein, plasminogen activator inhibitor (PAI1), and HM74, genes involved in cellular signaling and regeneration. To characterize the response in more detail, we performed time-course, dose-dependence studies, and cycloheximide experiments. We observed direct and indirect responses to TCDD implying that adaptation to TCDD (and other related environmental stimuli) is substantially more complex than we previously realized.

Induction of cytochrome P450 2B1 by pyrethroids in primary rat hepatocyte cultures11Abbreviations: CYP, cytochrome P450; EGF, epidermal growth factor; GAPDH, glyceraldehyde-3-phosphate dehydrogenase; MC, methylcholanthrene; PB, phenobarbital; PBO, piperonyl butoxide; PBREM, phenobarbital-responsive enhancer module; and PROD, pentoxyresorufin-O-depentylase

Numerous xenobiotics are capable of inducing their own metabolism and by enzyme induction can also lead to enhanced biotransformation of other xenobiotics. In this project, we examined the influence of pyrethroids (permethrin, cypermethrin, and fenvalerate) on the expression and activity of the phenobarbital (PB)-inducible cytochrome P450 2B1 isoform (CYP2B1) in primary rat hepatocyte cultures. Incubation of hepatocyte cultures with pyrethroids resulted in a marked CYP2B1 induction. Among the tested pyrethroids, permethrin elicited the most pronounced induction of CYP2B1 mRNA, which exceeded maximal induction achieved by PB at concentrations approximately 10-fold higher. Furthermore, permethrin induced CYP3A1 mRNA expression, while the expression of the CYP1A1 isoform, which in vivo is not responsive to PB treatment, was not significantly affected by pyrethroids. Permethrin-dependent enhancement of CYP2B1 and CYP3A1 mRNA expression was repressed by the hepatotrophic cytokine epidermal growth factor, which is known to also inhibit PB-dependent induction of CYP2B1. Several metabolites of permethrin formed by hepatocytes (3-(2',2'-dichlorovinyl)-2,2-dimethylcyclopropanecarboxylic acid, 3-phenoxybenzyl alcohol, and 3-phenoxybenzoic acid) were ineffective in inducing CYP2B1 mRNA. Furthermore, permethrin stimulated the expression of the luciferase reporter gene under control of the CYP2B1 promoter (comprising the PB-responsive enhancer module) in transiently transfected primary hepatocyte cultures. Thus, permethrin-stimulated gene expression occurred on the transcriptional level. Taken together, these results indicate that the pyrethroid permethrin is a PB-like inducer. Due to its superior potency in induction, permethrin appears as a useful substance for mechanistic studies to elucidate the mechanism of enzyme induction by phenobarbital.

Effect of a complex environmental mixture from coal tar containing polycyclic aromatic hydrocarbons (PAH) on the tumor initiation, PAH-DNA binding and metabolic activation of carcinogenic PAH in mouse epidermis

Human exposure to polycyclic aromatic hydrocarbons (PAH) occurs through complex mixtures such as coal tar. The effect of complex PAH mixtures on the activation of carcinogenic PAH to DNA-binding derivatives and carcinogenesis were investigated in mice treated topically with NIST (National Institute of Standards and Technology) Standard Reference Material 1597 (SRM), a complex mixture of PAH extracted from coal tar, and either additional benzo[a]pyrene (B[a]P) or dibenzo[a,l]pyrene (DB[a,l]P). In an initiation-promotion study using 12-O-tetradecanoylphorbol-13-acetate as the promoter for 25 weeks, the SRM and B[a]P co-treated mice had a similar incidence of papillomas per mouse compared with the group exposed to B[a]P alone as the initiator. PAH-DNA adduct analysis of epidermal DNA by 33P-post-labeling and reversed-phase high-performance liquid chromatography found the SRM co-treatment led to a significant decrease in the total level of DNA adducts and B[a]P-DNA adducts to less than that observed in mice treated with B[a]P alone at 6, 12 and 72 h exposure. After 24 and 48 h exposure, there was no significant difference in the levels of adducts between these groups. In the DB[a,l]P initiation-promotion study, the co-treated group had significantly fewer papillomas per mouse than mice treated with DB[a,l]P alone as initiator. Averaging over the times of exposure gave strong evidence that mice co-treated with SRM and DB[a,l]P had a significantly lower level of PAH-DNA adducts than mice treated with DB[a,l]P alone. Western immunoblots showed that both cytochrome P450 (CYP) 1A1 and 1B1 were induced by the SRM. These results are consistent with the hypothesis that two major factors determining the carcinogenic activity of PAH within a complex mixture are (i) the persistence of certain PAH-DNA adducts as well as total adduct levels, and (ii) the ability of the components present in the mixture to inhibit the activation of carcinogenic PAH by the induced CYP enzymes.

Phenobarbital response elements of cytochrome P450 genes and nuclear receptors

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

What will be the role of pharmacogenetics in evaluating drug safety and minimising adverse effects?

In the US, adverse drug reactions (ADRs) rank between the fourth to sixth leading cause of death, ahead of pneumonia and diabetes mellitus. An important reason for the high incidence of serious and fatal ADRs is that the existing drug development paradigms do not generate adequate information on the mechanistic sources of marked variability in pharmacokinetics and pharmacodynamics of new therapeutic candidates, precluding treatments from being tailored for individual patients. Pharmacogenetics is the study of the hereditary basis of person-to-person variations in drug response. The focus of pharmacogenetic investigations has traditionally been unusual and extreme drug responses resulting from a single gene effect. The Human Genome Project and recent advancements in molecular genetics now present an unprecedented opportunity to study all genes in the human genome, including genes for drug metabolism, drug targets and postreceptor second messenger machinery, in relation to variability in drug safety and efficacy. In addition to sequence variations in the genome, high throughput and genome-wide transcript profiling for differentially regulated mRNA species before and during drug treatment will serve as important tools to uncover novel mechanisms of drug action. Pharmacogenetic-guided drug discovery and development represent a departure from the conventional approach which markets drugs for broad patient populations, rather than smaller groups of patients in whom drugs may work more optimally. Pharmacogenetics provides a rational framework to minimise the uncertainty in outcome of drug therapy and clinical trials and thereby should significantly reduce the risk of drug toxicity.

Effects of fumonisin B1 present in Fusarium moniliforme culture material on drug metabolising enzyme activities in ducks

The effects of fumonisin B1 (0, 5, 15 and 45 mg/kg/day), obtained from culture material of Fusarium moniliforme, on drug metabolising enzyme activities were investigated in four groups of five growing ducks by daily oral administration over 12 days. No lethality or sign of toxicosis occurred. The liver and kidney weights were increased, whereas microsomal and cytosolic tissue fractions were unaffected. Although the total microsomal P450 content was unaffected, benzphetamine, ethylmorphine, erythromycin N-demethylases and ethoxyresorufin O-deethylase activities were together increased (respectively by 114, 242, 57 and 27% with 5 mg/kg/day and 1024, 969, 200 and 147% with 45 mg/kg/day). By contrast, aminopyrine and nitrosodimethylamine N-demethylases, methoxyresorufin and pentoxyresorufin O-dealkylases, and UDP-glucuronyltransferase activities were only increased by using 45 mg/kg/day, whereas glutathione S-transferases activities remained unaffected.

Phenobarbital inducibility and differences in protein expression of an animal model

Aldehyde dehydrogenases (ALDHs) are a group of enzymes which catalyze the conversion of aldehydes to the corresponding carboxylic acids in a NAD(P)(+)-dependent reaction. In mammals, different ALDHs are constitutively expressed in liver, stomach, eye and skin. In addition, inducible ALDH-isoenzymes are detectable in many tissues; apart from other physico- and immuno-chemical differences, two cytosolic ALDHs (ALDH1A3 and ALDH3A1) are known to be activated in rat liver, by different types of inducers of drug metabolism. Phenobarbital-type inducers increase the ALDH1A3, while polycyclic hydrocarbons (such as BaP and TCDD) increase the expression of the two members of ALDH3A subfamily (3A1 and 3A2). In this study, we used two Wistar rat substrains which have been well-characterized for different inducibility of ALDH1A3 enzyme activity after treatment with phenobarbital. Animals that respond (RR) or do not respond (rr) to treatment have been inbred for almost 25 years, offering a useful experimental model. Apart from the level of ALDH1A3 induced enzyme expression after phenobarbital treatment, no other differences between the two substrains have been noticed, as far as drug metabolizing enzyme activities (like the pentoxy- and ethoxy-O-dealkylation rate) are concerned. According to the present results, the ALDH1A3 expression is still the only difference between the two substrains. Immunoblotting experiments with polyclonal antibodies raised against CYP2B1 or/and CYP1A1/1A2 showed no differences between the two substrains. Additionally, data concerning time- and dose-response induction of ALDH1A3 after phenobarbital and griseofulvin treatment are presented. It is concluded that these two Wistar rat substrains represent a unique animal model for studying what seems to be the only difference between these substrains - the genetic basis of the phenobarbital induction.

Hepatic microsomal metabolism of indole to indoxyl, a precursor of indoxyl sulfate

The aim of our study was to determine which microsomal cytochrome P450 isozyme(s) were responsible for the microsomal oxidation of indole to indoxyl, an important intermediate in the information of the uremic toxin indoxyl sulfate. Indole was incubated together with an NADPH-generating system and rat liver microsomes. Formation of indigo, an auto-oxidation product of indoxyl, was used to determine the indole-3-hydroxylation activity. Apparent Km and Vmax values of 0.85 mM and 1152 pmol min(-1) mg(-1) were calculated for the formation of indoxyl from indole using rat liver microsomes. The effects of various potential inducers and inhibitors on the metabolism of indole to indoxyl by rat liver microsomes were studied to elucidate the enzymes responsible for metabolism. Studies with general and isozyme-specific P450 inhibitors demostrated that P450 enzymes and not FMO are responsible for the formation of indoxyl. In the induction studies, rate of indoxyl formation in the microsomes from untreated vs induced rats correlated nearly exactly with the CYP2E1 activity (4-nitrophenol 2-hydroxylation). These results suggests that CYP2E1 is the major isoform for the microsomal oxidation of indole to indoxyl.

Characterization of xenobiotic-metabolizing cytochrome P450 (CYP) forms in ringed and grey seals from the Baltic Sea and reference sites

Earlier studies have shown that members of the cytochrome P4501 (CYP1) enzyme family are constitutively expressed, and are elevated in the livers of ringed seals (Phoca hispida) and grey seals (Halichoerus grypus) living in the heavily polluted Baltic Sea. In this study, we compared the expression profiles of several additional CYP enzymes in the liver and extrahepatic tissues of Baltic ringed and grey seals with the corresponding CYP expression in seals from relatively unpolluted waters. We used marker enzyme activity levels, diagnostic inhibitors and immunoblot analysis to assess members of the CYP2A, CYP2B, CYP2C, CYP2D, CYP2E and CYP3A sub-families. Coumarin 7-hydroxylation (COH), a marker of CYP2A activity, was high in the liver and the lungs of all the studied seal populations. The presence of a putative CYP2A form in these seals was further supported by the strong inhibition of COH activity by a chemical inhibitor and by an anti-CYP2A5 antibody. However, antibodies to human and rodent CYP2B, CYP2C and CYP2E forms did not recognize any proteins in these seal species. Dextromethorphan O-demethylation (marker for CYP2D activity) and chlorzoxazone 6-hydroxylation (marker for CYP2E activity) were measurable in the livers of all the seals we studied. Both activities were elevated in the Baltic seal populations, showed a strong positive correlation with CYP1A activity and were at least partly inhibited by a typical CYP1A inhibitor, alpha-naphthoflavone. Further studies are needed to determine the presence and characteristics of CYP2D and CYP2E enzymes in ringed and grey seals. Testosterone 6beta-hydroxylation, a CYP3A marker, showed a relatively high level of activity in the livers of both seal species and was potently inhibited by ketoconazole, a CYP3A-selective inhibitor. The putative CYP3A activity showed an opposing geographical trend to that of CYP2D and CYP2E, since it was elevated in the control area. CYP3A protein levels, revealed by immunoblotting, showed a positive correlation with testosterone 6beta-hydroxylation. We conclude tentatively that CYP2A- and CYP3A-like enzymes are expressed in ringed and grey seals, but that CYP2B- and CYP2C-like ones are not. Further information on the individual contaminant profile is needed before any conclusions can be drawn on a possible connection between the varying CYP expressions and the contaminant load.

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.

Quantitative structure-activity relationships in a series of endogenous and synthetic steroids exhibiting induction of CYP3A activity and hepatomegaly associated with increased DNA synthesis

The results of a quantitative structure-activity relationship (QSAR) study on a total of 14 steroids exhibiting induction of a CYP3A-associated activity and increase in liver weight/DNA synthesis is reported. It is found that different, but related, structural descriptors correlate with increase in ethylmorphine N-demethylase activity (r=0.92) and with the increase in liver weight (r=0.78) and DNA synthesis (r=0.78). Although there is a strong correlation between increase in liver weight and DNA content (r=0.999), neither of these correlated with ethylmorphine N-demethylase activity. These findings are discussed in the light of CYP3A induction, substrate specificity and inhibition; a proposed model of human CYP3A4 based on sequence homology with CYP102, a bacterial P450 of known crystal structure, demonstrates the possible mode of interaction between substrates and inhibitors within the putative active site.

An uncommon phenotype of poor inducibility of CYP1A1 in human lung is not ascribable to polymorphisms in the AHR, ARNT, or CYP1A1 genes

Cigarette smoking can induce CYP1A1 in the lung. Induction requires the aryl hydrocarbon receptor (AHR) and aryl hydrocarbon receptor nuclear translocator (ARNT) proteins. Lung samples from seven of 75 Finnish patients who smoked until the time of surgery exhibited absent or low levels of CYP1A1 protein, mRNA and enzymatic activity, suggesting that these individuals might be genetically non or poorly inducible for CYP1A1. All seven lung samples expressed normal levels of AHR mRNA and ARNT mRNA, indicating that they did not carry inactivating polymorphisms in the 5' upstream regulatory regions of these genes. Sequencing of cDNAs encompassing the complete coding regions of AHR and ARNT identified a previously known codon 554 polymorphism in AHR, which was present in the homozygous state in one individual. This polymorphism, which leads to an amino acid substitution, has previously been reported either to have no effect or to enhance CYP1A1 induction. Previously unreported silent single nucleotide polymorphisms were identified in codon 44 of AHR and codon 189 of ARNT. 1500 bp of genomic sequence from the 5' upstream regulatory sequence of the CYP1A1 gene was also sequenced in the non-inducible individuals. A nucleotide substitution polymorphism at position -459 was detected in the heterozygous state in two individuals. This polymorphic site does not reside in any known regulatory sequence. The complete CYP1A1 coding sequence and intron/exon boundaries were then sequenced. None of the non or poorly inducible individuals exhibited any polymorphisms, either homozygous or heterozygous compared to representative inducible individuals or the previously published CYP1A1 sequence. Thus, no polymorphisms in the AHR, ARNT or CYP1A1 genes were identified that could be responsible for the non/low inducibility phenotype observed.

Pharmacological evaluation of the role of cytochrome P450 in intracellular calcium signalling in rat pancreatic acinar cells

We have investigated whether the cytochrome P450 system is involved in Ca(2+) signalling in rat pancreatic acinar cells. Intracellular free [Ca(2+)] ([Ca(2+)](i)) was measured in collagenase-isolated cells using fura-2 microspectrofluorimetry and imaging. The imidazole P450 inhibitor ketoconazole (5 - 50 microM) inhibited [Ca(2+)](i) oscillations induced by cholecystokinin octapeptide (CCK). However, ketoconazole also raised baseline [Ca(2+)](i) when applied in the absence of CCK. These effects were mimicked by 5 - 50 microM SKF96365, an imidazole widely used as an inhibitor of Ca(2+) entry. The non-imidazole P450 inhibitor proadifen (SKF525A) inhibited CCK-induced [Ca(2+)](i) oscillations at a concentration of 10 - 50 microM. Proadifen alone caused intracellular Ca(2+) release at 25 or 50 microM, but not at 10 microM. Octadecynoic acid and 1-aminobenzotriazole, structurally-unrelated non-imidazole P450 inhibitors, did not alter baseline [Ca(2+)](i) or CCK-evoked oscillations. We compared cumulative CCK dose-response relationship in control cells and in cells where P450 had been induced by prior injection of animals with beta-naphthoflavone. Only minor differences were apparent, with induced cells showing some decrease in responsiveness at moderate and higher concentration of CCK (30 pM - 3 nM). Direct assessment of depletion-activated Ca(2+) entry showed no clear differences between control and induced cells. In conclusion, we could find no compelling evidence for a role of P450 in controlling Ca(2+) signalling generally, or Ca(2+) entry in particular, in pancreatic acinar cells. Induction of P450 is therefore probably toxic to acinar cells via a Ca(2+)-independent mechanism.

Induction of drug metabolism by nuclear receptor CAR: molecular mechanisms and implications for drug research

Recent findings indicate that induction of drug metabolism is regulated by activation of specific members of the nuclear receptor gene family. This minireview deals with the mechanisms by which phenobarbital and phenobarbital-type chemicals induce cytochrome P450 and other genes, and summarises the knowledge on the role of the constitutively active receptor CAR in the induction process. The potential implications of CAR-mediated induction for drug research and possible uses of CAR are also discussed.