Enzyme induction in the cytochrome P-450 system

The induction of P450-mediated oxidation of all-trans retinoic acid by retinoids in head and neck squamous cell carcinoma cell lines

All-trans retinoic acid (RA) can be catabolized into polar metabolites by cytochrome P450 (P450) in several tissues including the skin. We examined eight different squamous cell carcinoma (SCC) cell lines to determine their capacity to induce P450-mediated oxidation of RA. Among the eight different cell lines, enhanced catabolism was detected in AMC-HN-1, -2, -5, and -6, whereas it was not found in the cell lines of AMC-HN-3, -4, -7, and -8. It was found that the enhanced catabolism brought on by P450 induction was blocked when RA was added to AMC-HN-6 along with actinomycin D or cyclohexamide. Also, this catabolism was inhibited by ketoconazole. P450-mediated oxidation was detectable within 4 hours of RA treatment, and RA catabolism reached its maximum 16 hours after treatment. P450 was induced by 13-cis-RA, 9-cis-RA, and retinal; however, retinol could not induce P450. In conclusion, P450 can be induced by retinoids in head and neck SCC (HNSCC) cells and the ability of retinoids to induce P450 can serve as an important factor in determining the biological effect of retinoids.

3,3'-Diindolylmethane induces CYP1A2 in cultured precision-cut human liver slices

1. The effect of 3,3'-diindolylmethane (DIM), an indole derivative derived from cruciferous vegetables, on cytochrome P450 (CYP) isoforms in the CYP1A and CYP3A subfamilies has been studied in 72-h cultured human liver slices. 2. In cultured human liver slices 50 microM DIM induced 7-ethoxyresorufin O-deethylase and to a lesser extent 7-methoxyresorufin O-demethylase activities. 3. Western immunoblotting of liver slice microsomes was performed with antibodies to rat CYP1A2 and human CYP3A4. Compared with control liver slice microsomes (dimethyl sulphoxide-only treated), DIM induced levels of CYP1A2 but had little effect on levels of CYP3A4. The treatment of human liver slices with 2 microg/ml of the polycholorinated biphenyl mixture Aroclor 1254 also resulted in an induction of levels of CYP1A2, but had no effect on CYP3A4. 4. These results demonstrate that DIM induces CYP1A isoforms in cultured human liver slices. Some variability in the magnitude of induction of enzyme activities by DIM was observed in four human liver samples examined. For 7-ethoxyresorufin O-deethylase, the magnitude of induction by 50 microM DIM ranged from 2.3- to 19.3-fold. 5. These results demonstrate that cultured human liver slices can be used to evaluate the effect of chemicals derived from cruciferous and other vegetables on CYP isoforms.

Antibiotic effects on cytochromes P450 content and mixed-function oxygenase (MFO) activities in the American alligator, Alligator mississippiensis

There are no Food and Drug Administration (FDA)-approved antimicrobial agents for use in cultured American alligators (Alligator mississippiensis) destined for human consumption yet some producers administer antibiotics for prophylaxis. The cytochromes P450-dependent mixed-function oxygenases (MFO) catalyze the oxidation of xenobiotic compounds such as drugs, pesticides and polycyclic aromatic hydrocarbons. Herein, we describe the effects of oxytetracycline, ceftazidime and enrofloxacin on the MFO system of the American alligator, Alligator mississippiensis. Juvenile alligators (4 animals/treatment) were administered these antibiotics intraperitoneally in an effort to induce hepatic microsomal cytochromes P450. Alligators treated with enrofloxacin exhibited emesis and convulsive spasms within 5 min of the initial injection. Total hepatic cytochromes P450 contents were significantly decreased in oxytetracycline-and enrofloxacin-pretreated alligators. In vitro hepatic microsomal benzyloxyresorufin O-dealkylase (BROD) activity was significantly decreased by enrofloxacin pretreatment. Western blots of proteins from antibiotic-pretreated alligator hepatic microsomes incubated with several mammalian and fish cytochromes P450 (CYP) antibodies exhibited little or no induction of CYP1A1, 2B, 2C and 2E1. In vitro incubation with enrofloxacin and oxytetracycline caused a concentration-dependent decrease in alkyl-substituted phenoxazone dealkylase activities catalyzed by phenobarbital- and 3-methylcholanthrene-induced alligator hepatic microsomes.

Time course of enzyme induction in humans: effect of pentobarbital on nortriptyline metabolism

To study the effect of induction we gave six male volunteers 10 mg nortriptyline three times a day for 4 weeks and 0.2 gm pentobarbital on days 8 to 21. Plasma and urinary levels of nortriptyline and metabolites were measured. The rate and extent of induction of the enzyme(s) were estimated by a model with use of nortriptyline concentrations. There was a marked decrease of nortriptyline levels after 2 days of pentobarbital treatment. Total clearance of nortriptyline increased more than twofold (range, 1.6-fold to 4.1-fold). Apparent metabolic clearance by 10-hydroxylation increased markedly. The decrease in nortriptyline levels was more rapid than the increase after pentobarbital cessation, fitting with the theory of the model. The induction of nortriptyline metabolism is probably mainly the result of an increase in a non-CYP 2D6 P450 isozyme, possibly CYP 3A4 or a CYP 2C form. More knowledge of induction characteristics of drugs should lead to better predictions of decreased effects and appearance of adverse effects. The kinetic model used for analysis of our data could then be useful.

Aryl hydrocarbon receptor function in early vertebrates: inducibility of cytochrome P450 1A in agnathan and elasmobranch fish

The mammalian aryl hydrocarbon receptor (AHR) is a ligand-activated transcription factor that controls the expression of cytochrome P450 1A (CYP1A) genes in response to halogenated aromatic hydrocarbons such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). The natural ligand and normal physiologic function of this protein are as yet unknown. One approach to understanding AHR function and significance is to determine the evolutionary history of this receptor and of processes such as CYP1A induction that are controlled by the AHR in mammals. In these studies, AHR function was evaluated in representative cartilaginous fish (little skate, Raja erinacea) and jawless fish (sea lamprey, Petromyzon marinus and Atlantic hagfish, Myxine glutinosa), using CYP1A induction as a model AHR-dependent response. Treatment of skate with beta-naphthoflavone (BNF) caused an 8-fold increase in hepatic ethoxyresorufin O-deethylase (EROD) activity as well as a 37-fold increase in the content of immunodetectable CYP1A protein. Evidence of CYP1A inducibility was also obtained for another cartilaginous fish, the smooth dogfish Mustelus canis. In contrast, hepatic EROD activity was not detected in untreated lamprey nor in lamprey treated with 3,3'4,4'-tetrachlorobiphenyl (TCB), a potent AHR agonist in teleosts. A possible CYP1A homolog was detected in lamprey hepatic microsomes by one of three antibodies to teleost CYP1A, but expression of this protein was not altered by TCB treatment. CYP1A protein and catalytic activity were measurable in hagfish, but neither was induced after treatment with TCB. These results suggest that the AHR-CYP1A signal transduction pathway is highly conserved in gnathostomes, but that there may be fundamental differences in AHR signaling or AHR-CYP1A coupling in agnathan fish. Agnathan fish such as hagfish and lamprey may be interesting model species for examining possible ancestral AHR functions not related to CYP1A regulation.

A novel response to dioxin. Induction of ecto-ATPase gene expression

We used differential display to discover a new gene that the environmental contaminant 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) regulates in mouse hepatoma cells. Its predicted amino acid sequence suggests that the gene encodes an ecto-ATPase that contains multiple glycosylation sites, conserved cysteine residues, and apyrase conserved regions. cDNA expression experiments in mouse hepatoma cells confirm that the new gene encodes an ecto-ATPase. Wild-type mouse hepatoma cells contain both constitutive and TCDD-inducible ecto-ATPase activity. Induction of ecto-ATPase gene expression by TCDD is direct and occurs at the transcriptional level. Studies in mutant hepatoma cells indicate that induction requires both the aromatic hydrocarbon receptor (AhR) and the AhR nuclear translocator (Arnt). Furthermore, induction requires AhR's transactivation domain, but not that of Arnt. Our findings reveal new aspects of dioxin's biological effects and TCDD-dependent gene regulation.

Induction of CYP1A1, but not CYP1A2, in adrenals of 3, 3'-methylcholanthrene-treated guinea pigs

To test the inducibility of CYP1A homologs in guinea pig adrenal, the effects of 3,3'-methylcholanthrene, an archetypal inducer of CYP1A, were compared in guinea pig adrenal and liver. Western blot analysis showed that levels of both CYP1A1 (53 kDa) and CYP1A2 (56 kDa) increasedin liver microsomes of 3,3'-methylcholanthrene-treated guinea pigs. In adrenals, an immunoreactive protein comigrating with liver CYP1A1 was detected only after 3,3'-methylcholanthrene treatment. Protein comigrating with CYP1A2 was never detected in adrenal microsomes. A third inducible immunoreactive protein (57 kDa) was seen in liver, but not adrenal, after 3, 3'-methylcholanthrene treatment. Another immunoreactive protein (52 kDa), present constitutively in liver and adrenal microsomes, was not induced in either tissue by 3,3'-methylcholanthrene. The precise identities of the inducible 57-kDa and the noninducible 52-kDa proteins remain to be determined. However, the identity of the 53-kDa protein in the adrenal as CYP1A1 was confirmed by RT-PCR, Northern blot, and sequence analysis. Similar analyses demonstrated that, despite the fact that the 56-kDa protein was not detectable in adrenal microsomes, CYP1A2 mRNA was present in adrenals of control animals. Strikingly, CYP1A2 mRNA decreased in adrenal, but increased in liver, following 3,3'-methylcholanthrene treatment, underscoring differences in the regulation of CYP1A expression in the two tissues. Levels of ethoxyresorufin and methyoxyresorufin metabolism correlated with levels of CYP1A1 and CYP1A2 protein, respectively.

Hypoxia and CYP1A1 induction-dependent regulation of proteins involved in glucose utilization in Caco-2 cells

Although induction of cytochrome P-450 1A1 (CYP1A1) in the Caco-2 clone TC7 alters glucose utilization and modifies the expression of sucrase-isomaltase (SI) and hexose transporters, nothing is known of the events that control these effects. In this study, we analyzed the effects of beta-naphthoflavone (beta-NF) and hypoxia on these parameters and expression of key enzymes of glucose metabolism. Both beta-NF and hypoxia induce similar changes: 1) induction of CYP1A1 mRNA; 2) increased glucose consumption and lactic acid production and lower glycogen content; 3) downregulation of SI and upregulation of GLUT1 mRNAs; 4) downregulation of fructose-1,6-bisphosphatase and pyruvate kinase mRNAs and upregulation of phosphoenolpyruvate carboxykinase, pyruvate dehydrogenase, lactate dehydrogenase, and phosphofructokinase mRNAs; and 5) upregulation of c-fos and c-jun mRNAs. Although addition of inhibitors of CYP1A1 catalytic activity to beta-NF-treated cells totally inhibits the enzyme activity, it does not modify CYP1A1 mRNA response and associated effects, thus excluding a direct role for the enzyme per se. These results point to a possible physiological implication of the signal-transduction pathway responsible for CYP1A1 induction.

In vitro interactions between fluoxetine or fluvoxamine and methadone or buprenorphine

Methadone and buprenorphine, widely used in the treatment of opioid abuse, are metabolized by cytochrome P450 3A4, while fluoxetine and fluvoxamine, both selective serotonin reuptake inhibitors, are known to be P450 2D6 and 3A4 inhibitors in vitro. This study deals with the in vitro interactions between methadone or buprenorphine and fluoxetine or fluvoxamine. Fluoxetine inhibited methadone N-demethylation (Ki = 55 microM), but conversely did not inhibit buprenorphine dealkylation. Norfluoxetine inhibited the metabolism of both methadone and buprenorphine metabolisms (Ki 13 and 100 microM, respectively). Fluvoxamine inhibited methadone N-demethylation with a Ki of 7 microM and buprenorphine dealkylation, uncompetitively, with a Ki of 260 microM. Finally, these results suggest that care should be taken when selective serotonin reuptake inhibitors are administered in the treatment of drug craving. This is particularly true in the case of fluvoxamine which is more potent than fluoxetine in inhibiting methadone and buprenorphine metabolism.

Quantitation of three-month intraindividual variability and influence of sex and menstrual cycle phase on CYP1A2, N-acetyltransferase-2, and xanthine oxidase activity determined with caffeine phenotyping

OBJECTIVE

To evaluate intraindividual variability and the effects of sex and menstrual cycle phase on the activity of cytochrome P450 1A2 (CYP1A2), N-acetyltransferase 2 (NAT2), and xanthine oxidase.

METHODS

Ten white men were given 2 mg/kg caffeine orally every 14 days for 3 months. The same dosage of caffeine was given to 10 premenopausal white women during the midfollicular and midluteal phases of three complete menstrual cycles. Phenotype was determined with urinary caffeine metabolite ratios.

RESULTS

For CYP1A2, mean metabolic ratio (+/- SD) was 5.97 +/- 2.78 during the midfollicular phase and 5.32 +/- 1.99 during the midluteal phase (p = 0.2). For extensive and poor metabolizer of NAT2. Mean midfollicular phase metabolite ratios were 0.71 +/- 0.060 and 0.37 +/- 0.030, and mean midluteal phase metabolite ratios were 0.69 +/- 0.076 and 0.39 +/- 0.053 (p = 0.9). For xanthine oxidase, mean midfollicular phase metabolite ratio was 0.63 +/- 0.06 and mean midluteal phase metabolite ratio was 0.63 +/- 0.05 (p = 0.3). Among the men, mean CYP1A2, NAT2 rapid and slow acetylator, and xanthine oxidase indices were 9.42 +/- 10.18, 0.66 +/- 0.021, 0.31 +/- 0.056, and 0.64 +/- 0.03. There were no differences in metabolite ratios between men and women for CYP1A2, NAT2 extensive metabolizers, or xanthine oxidase. A statistically significant sex difference was found for poor metabolizers of NAT2 (p < 0.05). Median coefficients of variation for CYP1A2, NAT2 extensive and poor metabolizers, and xanthine oxidase ratios were 16.8% (range, 4.5% to 49.3%), 2.9% (range, 2.2% to 4.7%), 13.4% (range, 7.5% to 27.2%), and 4.5% (range, 2.3% to 13.0%).

CONCLUSION

Stratification by menstrual cycle phase or sex need not be performed for pharmacokinetic or clinical investigations of substrates for CYP1A2, NAT2, or xanthine oxidase in which the subject are adults.

Effect of pyrazole and dexamethasone administration on cytochrome P450 2E1 and 3A isoforms in rat liver and kidney: lack of specificity of p-nitrophenol as a substrate of P450 2E1

The induction effects of pyrazole and dexamethasone (known to be specific to P450 2E1 and 3A enzymes, respectively), given alone or simultaneously, were studied in rat liver and kidney microsomes. Pyrazole treatment induced the catalytic activity and the amount of P450 2E1 enzyme in both organs. Immunoreactive P450 2E1 and 4-nitrophenol 2-hydroxylation increased 8- and 13-fold, respectively (versus control), in the kidney, but only 2.4- and 2.7-fold (versus control) in the liver after pyrazole treatment. As assessed by nifedipine oxidation activity, dexamethasone treatment increased the P450 3A catalytic activity approximately 4-fold (versus control) in the liver, but not in the kidney, suggesting that P450 3A was not inducible in the kidney. Pyrazole decreased P450 3A activity in the liver but did not modify it in the kidney. A combination of both chemicals induced both enzymes, but to a lesser extent than treatment with each single chemical compound. Furthermore, the 2-hydroxylation of p-nitrophenol, considered one of the most specific substrates for monitoring the level of P450 2E1, was mediated also by P450 3A, at least in dexamethasone-treated rats. Finally, this experimental work demonstrated that P450 3A induction is organ-specific, and it also demonstrated the lack of specificity of p-nitrophenol as a P450 2E1 substrate.

Induction of multidrug resistance gene expression in rat liver cells in response to acute treatment by the DNA-damaging agent methyl methanesulfonate

Expression of multidrug resistance (mdr) genes encoding the P-glycoprotein (P-gp) drug efflux pump was analysed in cultured rat liver epithelial cells acutely treated by the DNA-damaging agent methyl methanesulfonate (MMS). Exposure to this alkylating agent used at 30 microg/ml for 12 or 24 h was shown to enhance mdr mRNA levels in rat liver cells without alteration of cell viability. Induction of mdr transcripts occurred through increased expression of the mdr1b gene as indicated by reverse transcriptase-polymerase chain reaction analysis using rat mdr gene-specific primers and was not associated with up-regulation of cytochrome P-450 1A1, thereby suggesting that this detoxifying enzyme and P-gp were not coordinately regulated by MMS. In addition, the DNA-damaging agent was found to enhance in a dose-dependent manner cellular efflux of the P-gp substrate rhodamine 123, which was inhibited by the P-gp inhibitor verapamil, thus providing evidence that exposure to MMS led to increased P-gp-related drug transport in rat liver cells. The up-regulation of functional P-gp expression occurring in MMS-treated liver cells may be interpreted as a part of the cellular response to DNA damage.

The CYP2B2 phenobarbital response unit contains an accessory factor element and a putative glucocorticoid response element essential for conferring maximal phenobarbital responsiveness

Hepatic cytochrome P450s play a critical role in the metabolism of hydrophobic xenobiotics. One of the major unsolved problems in xenobiotic metabolism is the molecular mechanism whereby phenobarbital induces hepatic enzymes, particularly CYP2B1 and CYP2B2 in rat liver. By using primary rat hepatocytes for transfection analyses, we previously identified in the CYP2B2 5'-flank a 163-base pair Sau3AI fragment that confers phenobarbital inducibility on a cat reporter gene and that has the properties of a transcriptional enhancer. Transfection experiments with sub-regions of the Sau3AI fragment now indicate that a central core together with an upstream or downstream accessory element within the fragment can confer phenobarbital responsiveness. One such accessory element, AF1, was identified and localized. DNase I footprinting analysis revealed the presence of a footprint overlapping this AF1 element. It also identified three other major protected regions, two of which are putative recognition sites for known transcription factors. Site-directed mutagenesis indicated that a putative glucocorticoid response element as well as a nuclear factor 1 site and an associated nuclear receptor hexamer half-site are essential for conferring maximal phenobarbital inducibility. Taken together, the results indicate that phenobarbital induction of CYP2B2 requires interactions among multiple regulatory proteins and cis-acting elements constituting a phenobarbital response unit.

Activation by diverse xenochemicals of the 51-base pair phenobarbital-responsive enhancer module in the CYP2B10 gene

By extending previous studies of the phenobarbital (PB)-responsive 132-base pair (bp) enhancer sequence in the CYP2B10 gene, we have delimited a 51-bp enhancer element that is fully inducible by PB in mouse primary hepatocytes. Sixteen structurally unrelated phenobarbital-type inducers activated the 51-bp enhancer element in transient transfection assays. The results thus indicate that most PB-type inducers, if not all inducers, increase the transcription of the CYP2B10 gene by activating this 51-bp element, now designated PB-responsive enhancer module or PBREM.

Mutagen content and metabolic activation of promutagens by molluscs as biomarkers of marine pollution

Organisms combat pollutants by inducing biotransformation pathways, which can be used for biomonitoring. Several parameters--biomarkers--change in stressed organisms or populations at different organisation levels. Molecular or cellular biomarkers are early-warning indicators of pollution. Xenobiotics are first biotransformed by phase I enzymes and then conjugated with endogenous metabolites by phase II enzymes. Many organic xenobiotics are initially biotransformed by cytochrome P4501A1; in mammals, it is induced by pollutants via Ah receptor, although in marine invertebrates, its inducibility is much more equivocal. Metallothioneins are small Cys-rich proteins which bind transition metals; they detoxicate pollutant metals and are clearly induced in metal-exposed marine invertebrates. Some pollutants are genotoxins or can be converted into them. Determination of mutagens in bivalve molluscs following extraction with solvents and assay of mutagenicity with bacterial tests is a useful biomarker for marine pollution. While some pollutants are directly mutagenic, others are only mutagenic after they are activated to mutagenic derivatives by monooxygenases or conjugative enzymes. Many of these catalysts are induced by xenobiotics; thus, increased activation of known promutagens can be used as biomarker of environmental pollution. Bioactivation of promutagens requires the simultaneous action of different pathways, thus, reproducing more closely the in vivo situation than the specific assay of individual biotransforming enzymes. Study of molluscs with different pollution levels indicates that polluted animals have higher capacity to activate 2-aminoanthracene and contain more apolar mutagens than those from reference areas.

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.

Quantification of multiple human cytochrome P450 mRNA molecules using competitive reverse transcriptase-PCR

We developed a quantitative competitive reverse transcriptase-polymerase chain reaction (QC RT-PCR) assay to measure mRNA levels of seven human cytochrome P450 (P450, CYP) genes and microsomal epoxide hydrolase (EH) simultaneously. This assay employs an exogenous recombinant RNA (rcRNA) molecule as an internal standard that shares PCR primer and hybridization probe sequences with CYP1A1, CYP1A2, CYP2A6/7, CYP2D6, CYP2E1, CYP2F1, CYP3A4/5/7, and EH mRNA. Because each rcRNA molecule contains several primer sequences, an entire battery of genes that exhibit differential responsiveness to various classes of xenobiotics may be measured simultaneously from one population of cDNA molecules. In this study, we demonstrated the precision and power of the assay using small amounts of human liver total RNA. We also report for the first time quantitative profiles of P450 and EH mRNA abundance in eight human livers. Cytochrome P450 2E1 mRNA maintained the highest abundance (average 6.67 x 10(7) molecules/microg of total RNA) and least variation (13 fold) in all livers examined. Cytochrome P450 1A2, CYP2A6/7, CYP2D6, CYP3A4/5, and EH mRNAs were approximately one order of magnitude less abundant than CYP2E1 transcripts, with CYP2D6 levels exhibiting the greatest variation (220 fold) between individuals. This QC RT-PCR assay should prove valuable for measuring basal and induced mRNAs in different cell types in vitro, as well as in biomonitoring applications where individuals are exposed or hypersusceptible to certain xenobiotic-initiated toxicities.

Pituitary component of the aromatic hydrocarbon-mediated expression of CYP2B and CYP2C11

1. The aim was to determine if the ethylbenzene (EB)-mediated expression of CYP2B and CYP2C11 involved a hormonally controlled component. 2. The hypophysectomized (HX) and intact rats were treated with EB for 1 or 2 days, and the effects on specific CYP levels measured. 3. Differences were observed in the inducibility of CYP2B by EB in the HX rat when compared with intact controls. Whereas significant elevations of CYP2B-dependent activities and protein levels were observed after both 1 and 2 days of EB injection in intact controls, CYP2B levels were significantly elevated in the HX rat only after 2 days of hydrocarbon treatment. 4. Both CYP2C11-dependent activities and protein levels were decreased after EB administration to the intact rat. In contrast, CYP2C11 levels were unaffected by EB in the HX rat at any of the time points indicated. 5. CYP2C11 protein levels were unaffected by treatment with EB for 24 h in cultured hepatocytes, also supporting the hypothesis that hormones are involved in CYP2C11 expression. 6. This study indicates that pituitary input influences the EB-mediated changes in both CYP2B and CYP2C11. CYP2C11 is affected by EB administration in a manner similar to other xenobiotics such as phenobarbital. On the other hand, the smaller induction of CYP2B1/2 in response to EB differs from that observed with phenobarbital where HX augmented the response of the inducer.

Modulation of snake hepatic cytochrome P450 by 3-methylcholanthrene and phenobarbital

Induction mode of the hepatic drug-metabolizing enzymes was studied in Corn snake (Elaphe guttata emoryi). Treatment of snakes with 3-methylcholanthrene or phenobarbital produced no effects on liver weight and total content of cytochromes P450 and b5. Treatment with 3-methylcholanthrene significantly induced the activities of arylhydrocarbon hydroxylase, 7-ethoxyresorufin O-deethylase and 7-pentoxyresorufin O-dealkylase, whereas those of ethoxycoumarin O-deethylase, benzphetamine N-demethylase, erythromycin N-demethylase and testosterone hydroxylases were not affected. 3-Methylcholanthrene-induced activities of 7-ethoxyresorufin O-deethylase and 7-pentoxyresorufin O-dealkylase were inhibited by 20 microM alpha-naphthoflavone by 98% and 73%, respectively. Phenobarbital-treatment caused a significant induction of the activities of erythromycin N-demethylase and testosterone 6 beta-hydroxylase, but did not affect those of the other phase I enzymes and the other testosterone hydroxylases. The activities of UDP-glucuronyltransferase and glutathione S-transferase were not affected by either 3-methylcholanthrene or phenobarbital administration. Immunoblotting showed that 3-methylcholanthrene-treatment induced a protein band related to hamster CYP1A2, and decreased the intensity of the two bands detected with anti-rat CYP2B1. Phenobarbital-treatment did not affect the intensity of CYP2B-related proteins. The results suggest that snake liver has multiple forms of cytochrome P450, notably those inducible by 3-methylcholanthrene.

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.

Mutagenic activation of aromatic amines by molluscs as a biomarker of marine pollution

Mutagenic activation of arylamines by mollusc S9 fractions was evaluated as a biomarker for marine pollution. Two bivalve species were used as bioindicators, the common mussel (Mytilus edulis) and the striped venus (Chameleo gallina). A strain of Salmonella typhimurium overproducing O-acetyltransferase was used as indicator of mutagenicity. Mussels from an area of the North Atlantic Spanish zone that was exposed to an accidental crude oil spill were compared to bivalves from a reference area. C. gallina samples were from low polluted and highly polluted areas of the South Atlantic Spanish littoral. The promutagen 2-aminoanthracene (2-AA) was activated to mutagenic derivative(s) by S9 fractions from both C. gallina and M. edulis. Animals from contaminated sites showed higher arylamine activation capabilities than reference animals. This was further correlated with the mutagenic activities of corresponding cyclopentone-dichloromethane animal extracts. 2-AA activation by mollusc S9 was potentiated by alpha-naphthoflavone (ANF), known to inhibit PAH-inducible CYP1A cytochromes from vertebrates, but inhibited by methimazole (MZ), a substrate of the flavin monooxygenase (FMO) system. 2-AA-activating enzymes were mainly cytosolic; this localization clearly suggests that such activity could be attributed to soluble enzymes, different from the CYP1A or FMO systems. In conclusion, mutagenic activation of arylamines by mollusc S9, using as indicator a strain of Salmonella typhimurium that overproduces O-acetyltransferase, could be a reliable biomarker for marine pollution.

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

1. We review here the molecular mechanisms underlying the xenobiotic induction of genes encoding cytochrome P450 (CYP) enzymes in the liver and other tissues. We will focus on four major families of CYP genes. 2. Members of the CYP1 gene family are induced by polycyclic aromatic hydrocarbons and this process is mediated by the basic helix-loop-helix proteins: the Ah receptor and its heterodimeric partner Arnt. Considerable progress has been made in elucidating the molecular details of this induction process. 3. CYP4 genes are activated by peroxisomal proliferators, a group of structurally diverse chemicals that also induce peroxisome proliferation. The transcriptional response is dependent on the peroxisome proliferator-activated receptor and its partner RXR, both members of the nuclear receptor superfamily; their role in the induction process has been well characterized at the molecular level. 4. In contrast, the mechanism of gene induction of CYP2 genes by phenobarbital and other structurally diverse inducers is not well understood and a specific phenobarbital-responsive receptor has not been identified. 5. Induction of the CYP3 gene family by the glucocorticoid dexamethasone appears to involve the glucocorticoid receptor, but this receptor is not apparently required for induction by metapyrone and a complete molecular understanding of the induction processes is lacking at present.

Xenobiotic-metabolizing cytochrome P450 enzymes in the human feto-placental unit: role in intrauterine toxicity

Practically all lipid-soluble xenobiotics enter the conceptus through placental transfer. Many xenobiotics, including a number of clinically used drugs, are known to cause unwanted effects in the embryo or fetus, including in utero death, initiation of birth defects, and production of functional abnormalities. It is well established that numerous xenobiotics are not necessarily toxic as such, but are enzymatically transformed in the body to reactive and toxic intermediates. The cytochrome P450 (CYP) enzymes are known to catalyze oxidative metabolism of a vast number of compounds, including many proteratogens, procarcinogens, and promutagens. About 20 xenobiotic-metabolizing CYP forms are known to exist in humans. Most of these forms are most abundant in the liver, but examples of exclusively extrahepatic CYP forms also exist. Unlike rodents, the liver of the human fetus and even embryo possesses relatively well-developed metabolism of xenobiotics. There is experimental evidence for the presence of CYP1A1, CYP1B1, CYP2C8, CYP2D6, CYP2E1, CYP3A4, CYP3A5, and CYP3A7 in the fetal liver after the embryonic phase (after 8 to 9 weeks of gestation). Significant xenobiotic metabolism occurs also during organogenesis (before 8 weeks of gestation). Also, some fetal extrahepatic tissues, most notably the adrenal, contain substantial levels of CYP enzymes. The full-term human placenta is devoid of many CYP activities present in liver. Placental CYP1A1 is highly inducible by maternal cigarette smoking. Other forms present in full-term placenta include CYP4B1 and CYP19 (steroid aromatase), which also contribute to the oxidation of some xenobiotics. At earlier stages of pregnancy, the placenta may express a wider array of CYP genes, including CYP2C, CYP2D6, and CYP3A7. Due to the small size of the fetus and low abundance of CYPs in placenta, the contribution of feto-placental metabolism to overall gestational pharmacokinetics of drugs is probably minor. In contrast, several toxic outcomes have been ascribed to altered metabolic patterns in the feto-placental unit, including a putative association between reduced placental oxidative capacity and birth defects. Examples of human teratogens that are substrates for CYP enzymes include thalidomide, phenytoin, ethanol, and several hormonal agents. Recent studies have improved our understanding of the expression and regulation of individual CYP genes in the fetus and placenta, and the stage is set for applying this knowledge with more precision to the role of xenobiotic metabolism in abnormal intrauterine development in humans.

Comutagenesis-III. In vitro metabolism of 2-amino-3-methylpyridine: the effect of various potential inhibitors, activators and inducers

1. The effects of various potential inhibitors, activators and inducers on the metabolism of the comutagen 2-amino-3-methylpyridine (2A3MP) by rabbit hepatic microsomes and S9 supernatants have been studied. 2. The 1-N-oxidation of 2A3MP to 2-amino-3-methylpyridine-1-N-oxide (2A3M-PNO) was inhibited by 2,4-dichloro-6-phenylphenoxyethylamine (DPEA), 2-diethylaminoethyl-2,2-diphenylvalerate (SKF 525-A) and n-octylamine. 3. The C-oxidation products of 2A3MP, i.e. 2-amino-3-hydroxymethylpyridine (2A3HMP) and 2-amino-3-methyl-5-hydroxypyridine (2A3M5HP), were also inhibited by these compounds. 4. Pretreatment of animals with phenobarbitone (PB) resulted in an increase in the production of 2A3MPNO and 2A3HMP, whereas beta-naphthoflavone (BNF) pretreatment had a greater effect on the formation of 2A3M5HP. 5. Pretreatment with pyridine or pyrazine also had an appreciable effect on the formation of 2A3HMP. 6. It is suggested that different cytochrome P450 isozymes are responsible for the metabolic profile of 2A3MP. CYP2B was involved in the N-oxidation; 2E and/or 2B in the formation of 2A3HMP, and 3A and/or 1A in the formation of 2A3M5HP.

Induction of CYP3A isoforms in cultured precision-cut human liver slices

1. The effect of rifampicin on cytochrome P450 isoforms in the CYP1A and CYP3A subfamilies has been studied in 72-h cultured precision-cut human liver slices. 2. In cultured human liver slices 50 microM rifampicin induced testosterone 6 beta-hydroxylase activity, but had no effect on 7-ethoxyresorufin O-deethylase and 7-methoxyresorufin O-demethylase activities. 3. Western immunoblotting of liver slice microsomes was performed with antibodies to rat CYP1A2 and human CYP3A4. Compared with control (dimethyl sulphoxide only treated) liver slice microsomes, rifampicin increased levels of CYP3A4 but had no effect on CYP1A2. 4. These results demonstrate that rifampicin induces CYP3A isoforms, but not CYP1A2, in cultured human liver slices. Some variability in the magnitude of induction by rifampicin was observed in the six human liver samples examined. 5. These results demonstrate that cultured human liver slices may be used to evaluate the effects of xenobiotics on CYP3A isoforms.

Metabolic capacity of nasal tissue interspecies comparisons of xenobiotic-metabolizing enzymes

High levels of xenobiotic-metabolizing enzymes occur in the nasal mucosa of all species studied. In certain species, including rats and rabbits, unique enzymes are present in the nasal mucosa. The function of these enzymes is not well understood, but it is thought that they play a role in protecting the lungs from toxicity of inhalants. The observation that several nasal xenobiotic-metabolizing enzymes accept odorants as substrates may indicate that these enzymes also play a role in the olfactory process. Xenobiotic-metabolizing enzymes were found in the nasal cavity around 15 years ago. Since that time, much has been learned about the nature of the enzymes and the substrates they accept. In the present review, this information is summarized with special attention to species differences in xenobiotic-metabolizing enzymes of the nasal cavity. Such differences may be important in interpreting the results of toxicity assays in animals because rodents are apparently more susceptible to nasal toxicity after exposure to inhalants than are humans.

Experimental models for evaluating enzyme induction potential of new drug candidates in animals and humans and a strategy for their use

Experimental models that have application for evaluating enzyme induction potential have been described in order of increasing complexity. The main focus was on models that have had wide application thus far. However, many new models are currently being developed that may have future applications in evaluating enzyme induction potential. A strategy to evaluate the enzyme induction potential of drug candidates was outlined. This scheme uses a combination of new and established techniques to evaluate data in a stepwise manner that is appropriate to the drug's current stage of development.

A microassay for the detection of low levels of cytochrome P450 O-deethylation activities with alkoxyresorufin substrates

A microfluorometric method for the detection of low levels of cytochrome P450 was developed to increase the sensitivity of the assay, since a low level of CYP450 associated enzymatic activities was expected in human placenta tissues and small samples of placenta (approximately 10 g) could be easily collected, stored and processed. The dual fluorescence assay of Kennedy et al. [1], which was developed to simultaneously quantitate microsomal proteins and ethoxyresorufin-O-deethylase (EROD) activity was adapted for 96 wells microtiter plates. Placental microsomes samples were analyzed. For samples obtained from non-smoking mothers from the general southern Quebec population, results ranged from less than 1-3.3 pmol/mg protein.min. Samples collected from smoking mothers showed activity levels ranging from 30-69 pmol/mg protein.min. These results showed the suitability of the microassay for measuring low level of CYP450 activity in tissues such as placenta.

Ontogenic expression of detoxication enzymes in an Australian marsupial, the brushtail possum (Trichosurus vulpecula)

Marsupials and eutherians display vastly different reproductive strategies. Marsupials are characterised by the production of altricial neonates with little functional capacity. An investigation of the ontogenic expression of phase I (mixed function oxidase) and phase II (glutathione transferase) enzyme systems in the marsupial, the brushtail possum was undertaken. Enzyme expression in the youngest age group studied (60 days old) was between 5% and 10% of the adult level. A gradual increase in expression was then observed until a significant 3-fold increase to adult levels of expression of cytochrome P450, cytochrome b5 and glutathione transferase content and ECOD and AE activity was observed in brushtail possum young between the ages of 150 +/- 15 and 180 +/- 15 days. The expression of EROD activity reached adult levels by the age of 150 +/- days, while the expression of NADPH-cytochrome c reductase activity was delayed and adult levels had not yet been achieved by the oldest group studied (> 200 days). The ontogenic expression of detoxication enzymes was significantly delayed in the marsupial in comparison to eutherians. Adult levels were achieved during the weaning period, suggesting that dietary xenobiotics act as a regulatory mechanism in the developmental expression of these enzymes in the brushtail possum.

Molecular cloning of cDNA for guinea pig CYP1A2 comparison with guinea pig CYP1A1

Guinea pig CYP1A2 cDNA was isolated by RT-PCR from liver tissue of 3,3'-methylcholanthrene-treated guinea pigs. It shares considerable sequence identity with guinea pig CYP1A1 (nt 77%, aa 65%), but differs in levels of constitutive expression, function, and inducibility. Western blot analysis of protein expressed by full-length cDNA in COS-1 cells identified CYP1A2 (56 kDa) and CYP1A1 (53 kDa) proteins in corun liver microsomes. CYP1A2 transfectants metabolized methoxyresorufin and ethoxyresorufin, while CYP1A1 transfectants metabolized only ethoxyresorufin. Constitutive expression of CYP1A2 mRNA (2.0 kb) and protein was much lower than that of CYP1A1 mRNA (2.6 kb) and protein, but the fold induction of CYP1A2 by 3,3'-methylcholanthrene was greater than that of CYP1A1. Changes in splicing of CYP1A2 pre-mRNA occur upon treatment with 3,3'-methylcholanthrene.

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.

Evidence for a tissue-specific induction of cutaneous CYP2E1 by dexamethasone

We studied in mouse the effect of topical application of dexamethasone or salicylic acid, on CYP2E1 and CYP3A expression (proteins and/or mRNA) in liver and skin. Dexamethasone was also administered by intraperitoneal injection. Topical application or intraperitoneal injection of dexamethasone increased cutaneous CYP2E1 (8 and 4-fold respectively) whereas the hepatic level of this isoform showed a slight decrease and hepatic CYP3A expression was increased (3-fold). Cutaneous CYP2E1 was increased (3-fold) after topical treatment by salicylic acid. This compound had no effect on hepatic CYP3A and CYP2E1 expression. Cutaneous CYP3A (protein and mRNA) was not detectable in all groups (control or treated animals). Dexamethasone and salicylic acid increased cutaneous CYP2E1 mRNA level (2.5 and 1.4-fold respectively). In conclusion, dexamethasone and salicylic acid induced cutaneous CYP2E1 protein and mRNA level. Cutaneous CYP2E1 induction by dexamethasone is a tissue-specific process.

Regulation of hepatic CYP1A isozymes by piperonyl butoxide and acenaphthylene in the mouse

The regulation of CYP1A1 and CYP1A2 isozymes by piperonyl butoxide (PBO) and acenaphthylene (ACN) was studied in the liver of male C57BL/6 and DBA/2 mice. These two cytochrome P450 genes are known to be regulated by the aromatic hydrocarbon-responsive receptor (AHR); however, it has been suggested that CYP1A2 is also induced by an AHR-independent mechanism. In this study, PBO induced hepatic CYP1A1 considerably more in C57BL/6 (Ahrb-I) than in DBA/2 (Ahrd) mice. In addition, the superinduction of CYP1A1 in wildtype hepa1c1c7 cells, which is AHR-dependent, resulted from PBO and cycloheximide treatment of the cells. In other studies in this laboratory using AHR knock-out (AHR-/-) mice, a hybrid of 129/SV and C57BL/6 strains, no induction of CYP1A1 occurred with PBO or ACN. [D.-Y. Ryu, P.E. Levi, P. Fernandez-Salguero, F.J. Gonzalez, E. Hodgson, Mol. Pharmacol., 50 (1996) 443-446.] ACN, however, did not induce CYP1A1 under the experimental conditions used. These results suggest that PBO, but not ACN, induces CYP1A1 through a weak activation of AHR. On the other hand, hepatic CYP1A2 mRNA and hnRNA were induced by PBO in both C57BL/6 and DBA/2 strains, but were not induced by ACN, a strong inducer of CYP1A2 in the B6C3F1 strain. However, both PBO and ACN induced CYP1A2 in AHR-/- mice. It is assumed, therefore, that the transcriptional induction of CYP1A2 by PBO and ACN is AHR-independent. In addition, the induction of CYP1A2 by ACN depends upon the strain of mice. Immunohistochemical studies for CYP1A1/CYP1A2 apoproteins showed that PBO induced CYP1A1/CYP1A2 around the central veins as did 3-methylcholanthrene (3-MC). The induction of CYP1A1/CYP1A2 by ACN, however, was not observed, consistent with the northern blot results.

Up-regulation of P-glycoprotein expression in rat liver cells by acute doxorubicin treatment

Expression of P-glycoprotein, a plasma-membrane glycoprotein involved in multidrug resistance and encoded by mdr genes, was investigated in cultured rat liver cells acutely exposed to doxorubicin. This anticancer drug was shown to increase mdr mRNA levels in a dose-dependent manner in both rat liver epithelial (RLE) cells and primary rat hepatocytes. This induction of mdr transcripts was detected as early as a 4-h exposure to doxorubicin used at 0.5 microg/ml. It occurred through increased expression of the mdr1 gene as assessed by northern blot analysis using rat mdr-gene-specific probes. In addition, RLE cells exposed to doxorubicin displayed an overexpression of a 140-kDa P-glycoprotein as demonstrated by western blotting. Moreover, doxorubicin-treated RLE cells displayed enhanced cellular efflux of the P-glycoprotein substrate rhodamine 123 that was inhibited by the P-glycoprotein blocker verapamil, thus providing evidence that doxorubicin-induced P-glycoprotein was functional in liver cells. Doxorubicin-mediated mdr mRNA induction was found to be fully inhibited by actinomycin D, thus indicating its dependence on RNA synthesis; it was demonstrated to be not associated with alteration of protein synthesis, suggesting it differed from the known mdr mRNA overexpression occurring in response to cycloheximide. In contrast to P-glycoprotein, other liver detoxification pathways such as cytochromes P-450 1A were not induced by doxorubicin treatment. These data indicate that doxorubicin can act as a potent acute inducer of functional P-glycoprotein in rat liver cells and therefore may modulate both chemosensitivity of hepatic cells and P-glycoprotein-mediated biliary secretion of xenobiotics.

Pharmacokinetic study of the interaction between rifampin and delavirdine mesylate

OBJECTIVE

To study the effect of rifampin (INN, rifampicin), a potent inducer of cytochrome P450, on the steady-state pharmacokinetics of delavirdine.

METHODS

Twelve patients who were positive for human immunodeficiency virus, with CD4 counts ranging from 110 to 483/mm3, were randomized to two groups and studied in parallel. Both the control group (n = 5) and the rifampin group (n = 7) received 400 mg delavirdine mesylate every 8 hours for 30 days; subjects in the rifampin group took a 600 mg once-daily dose of rifampin on days 16 through 30. Harvested plasma from serial blood samples collected after dosing on days 15, 16, and 30 was assayed for delavirdine and its N-desalkyl metabolite concentrations with a reversed-phase HPLC method. Blood samples obtained on days 16 and 30 were also assayed for rifampin by HPLC.

RESULTS

Delavirdine mesylate alone and in combination with rifampin was well tolerated. On day 30, statistically significant differences between groups were observed for all delavirdine pharmacokinetic parameters (p < 0.049). In the rifampin group, delavirdine oral clearance increased by about 27-fold (p = 0.022), resulting in virtually negligible (< 0.09 mumol/L) steady-state through drug concentrations in all patients after 2 weeks of concurrent dosing of delavirdine mesylate and rifampin. The ratio of metabolite formation to elimination clearance for desalkyldelavirdine was significantly higher (3.9 +/- 1.2 versus 0.23 +/- 0.10) and delavirdine elimination half-life was significantly shorter (1.7 +/- 1.4 versus 4.3 +/- 1.3 hours) when delavirdine mesylate was taken with rifampin. Rifampin pharmacokinetic parameters on days 16 and 30 were similar to those previously reported for normal volunteers.

CONCLUSIONS

The findings of this study indicate that rifampin induces the metabolism of delavirdine. Therefore therapy with rifampin is contraindicated in patients receiving delavirdine mesylate.

Multiple-dose pharmacokinetics of ritonavir in human immunodeficiency virus-infected subjects

The multiple-dose pharmacokinetics of ritonavir were investigated in four groups of human immunodeficiency virus-positive male subjects (with 16 subjects per group) under nonfasting conditions; a 3:1 ritonavir:placebo ratio was used. Ritonavir was given at 200 (group I), 300 (group II), 400 (group III), or 500 (group IV) mg every 12 h for 2 weeks. The multiple-dose pharmacokinetics of ritonavir were moderately dose dependent, with the clearance for group IV (6.8 +/- 2.7 liters/h) being an average of 32% lower than that for group I (10.0 +/- 3.2 liters/h). First-pass metabolism should be minimal for ritonavir. The functional half-life, estimated from peak and trough concentrations, were similar among the dosage groups, averaging 3.1 and 5.7 h after the morning and evening doses, respectively. The area under the concentration-time curve at 24 h (AUC24) and apparent terminal-phase elimination rate constant remained relatively time invariant, but predose concentrations decreased 30 to 70% over time. Concentration-dependent autoinduction is the most likely mechanism for the time-dependent pharmacokinetics. The Km and initial maximum rate of metabolism (Vmax) values estimated from population pharmacokinetic modeling (nonlinear mixed-effects models) were 3.43 microg/ml and 46.9 mg/h, respectively. The group IV Vmax increased to 68 mg/h after 2 weeks. The maximum concentration of ritonavir in serum (Cmax) and AUC after the evening doses were an average of 30 to 40% lower than the values after the morning doses, while the concentration at 12 h was an average of 32% lower than the predose concentration, probably due to protracted absorption. Less than 2% of the dose was eliminated unchanged in the urine. Triglyceride levels increased from the levels at the baseline, and the levels were correlated with baseline triglyceride levels and AUC, Cmax, or predose concentrations.

In vitro metabolism of 4-vinylcyclohexene in rat and mouse liver, lung, and ovary

4-Vinylcyclohexene (4-VCH), the dimer of 1,3-butadiene, is an ovarian toxicant in mice due to the formation of a diepoxide metabolite, but the tissue-specific site of formation of the metabolites is unknown. Microsomal preparations from liver, lung, and ovaries obtained from female Crl:CD BR rats and female B6C3F1 mice were tested for their ability to metabolize the following reactions: 4-VCH to 4-VCH-1,2-epoxide and 4-VCH-7,8-epoxide; 4-VCH-1,2-epoxide to 4-VCH diepoxide and 4-VCH-1,2-diol; 4-VCH-7,8-epoxide to 4-VCH diepoxide and 4-VCH-7,8-diol; and hydrolysis of 4-VCH diepoxide. Microsomes were incubated with the test chemical and the reaction products were analyzed by gas chromatography. Rat liver and lung microsomes and mouse liver and lung microsomes metabolized 4-VCH to 4-VCH-1,2-epoxide at detectable rates. Mouse liver had a Vmax for the reaction that was 56-fold higher than that for rat liver (11.1 and 0.20 nmol/min/mg protein, respectively). The Vmax for mouse lung was 2-fold higher than that for rat lung. 4-VCH-1,2-epoxide formation was not detected in ovarian microsomes from rats or mice. Metabolism of 4-VCH to 4-VCH-7,8-epoxide was detected in microsomes from rat liver and mouse liver and lung, at rates very low compared to those for metabolism to the 1,2-epoxide. Rat and mouse liver had very similar K(m) and Vmax values for metabolism of 4-VCH-1,2-epoxide to 4-VCH diepoxide. The Vmax for rat liver was 3.69 and for mouse liver was 5.35 nmol/min/mg protein. Rat and mouse ovaries did not have detectable capacity to metabolize 4-VCH-1,2-epoxide to the diepoxide. Rat and mouse liver and lung have very similar K(m) and Vmax values for metabolism of 4-VCH-7,8-epoxide to the diepoxide, while ovaries did not have detectable rates for this reaction. Hydrolysis of 4-VCH-1,2-epoxide to 4-VCH-1,2-diol was at similar rates in rat and mouse liver microsomes. Hydrolysis of 4-VCH-7,8-epoxide to 4-VCH-7,8-diol was detected only in rat liver microsomes. Hydrolysis of 4-VCH diepoxide was detected in rat and mouse liver and lung, and in rat ovary microsomes. The Vmax for rat liver was 9-fold greater than that for mouse liver (5.51 and 0.63 nmol/min/mg protein, respectively), and lung and ovary tissues were not as active as rat liver. The balance of activation versus detoxication reactions in rats and mice suggests that the mouse may be more susceptible to 4-VCH toxicity because of generation of high levels of epoxide metabolites. In general, the mouse is more efficient at metabolism of 4-VCH to epoxides than is the rat. In contrast, the rat may be more efficient at hydrolysis of epoxides. Thus, the rat would tend to produce a lower concentration of epoxide metabolites than the mouse, at equal doses of 4-VCH.

Murine Cyp1a-1 induction in mouse hepatoma Hepa-1C1C7 cells by myristicin

Mouse hepatoma Hepa-1c1c7 (Hepa-1) cells were treated with myristicin to assess the role of myristicin in the process of Cyp1a-1 induction. Treatment of Hepa-1 cells with myristicin increased Cyp1a-1 transcription in a dose-dependent manner as shown by analysis of 7-ethoxyresorufin O-deethylase activity, Cyp1a-1 protein level, and Cyp1a-1 mRNA. Myristicin, however, did not competitively displace [3H]2,3,7,8-tetrachlorodibenzo-p-dioxin from the Hepa-1 cytosolic aryl hydrocarbon (Ah) receptor in a competitive Ah receptor binding analysis using sucrose density gradient sedimentation and did not affect formation of DNA-protein complexes between the Ah receptor and its DRE target in a gel mobility shift assay using oligonucleotides corresponding to DRE 3 of the Cyp1a-1. These results suggest that the induction of Cyp1a-1 gene expression by myristicin in Hepa-1 cells might occur through an Ah receptor-independent pathway.

Fluvoxamine and fluoxetine do not interact in the same way with the metabolism of the enantiomers of methadone

Six and seven addicts treated with racemic methadone (MTD) were comedicated with fluvoxamine (FLV) and fluoxetine (FLX), respectively. The plasma concentrations of both (R)- (the active enantiomer) and (S)-MTD were increased by FLV, whereas only (R)-MTD concentrations were increased by the addition of FLX. This suggests that cytochrome P450IID6 (CYP2D6), an enzyme that is strongly inhibited by FLX, preferentially metabolizes (R)-MTD, whereas CYP1A2, which is strongly inhibited by FLV, metabolizes both enantiomers. The choice of a selective serotonin reuptake inhibitor in depressive addicted patients treated with MTD and the possible use of FLX or FLV to potentiate the effects of MTD in some cases of therapeutic failure are discussed.

Microsomal and peroxisomal fatty acid oxidation in bile duct ligated rats: a comparative study between liver and kidney

1. Microsomal cytochrome P-450 and peroxisomal fatty acid oxidation was studied in the kidney of rats 7 days after bile duct ligation (BDL) and a comparative study between kidney and liver was done. 2. Only in the liver did cholestasis decrease the cytocrome P-450 content and the peroxisomal fatty acid beta-oxidation, the catalase activity, and the microsomal metabolism of lauric acid and aminopyrine. 3. In contrast, cholestasis did not influence these activities in the kidney. The microsomal and peroxisomal activities studied responded in a coordinate way to cholestasis. 4. These results could suggest the possibility of a cause-and-effect relationship between microsomal cytochrome P-450 and peroxisomal activity.

Cytochrome P450 isoenzyme activities in cultured rat and mouse liver slices

1. The objective of this study was to determine the basal and inducible activities of several cytochrome P450 (CYP) isozymes and monitor the acinar and hepatocyte morphology in precision cut, cultured rat and mouse liver slices. 2. The slices were cultured up to 96 h in Chee's essential medium supplemented with insulin, transferrin, selenium, DMSO, dexamethasone and epidermal growth factor. A dynamic roller system was used to incubate the slices at 37 degrees C in an atmosphere of 95% O2:5% CO2. 3. Histopathology of the liver slices revealed maintenance of normal hepatic lobular architecture with time in culture. 4. CYP isozyme activities were measured at various times of culture. In rat liver slices, at 72 h, CYP1A1/1A2 activity was induced 4-fold by beta NF and 37-fold by dioxin (TCDD) whereas in mouse liver slices, 1A1/1A2 activity was not inducible by beta NF but was induced 19-fold by TCDD. At 72 h, CYP2A5 (coumarin-7-hydroxylase) activity was not detected in rat liver slices but in mouse liver slices, 2A5 was induced 2-fold by beta NF, 11-fold by phenobarbital (PB) and 3-fold by TCDD. 5. Hydroxylation of testosterone at specific positions was used as an indication of the activities of various P450 isoforms. Testosterone was added to the cultures at 0 and 72 h and the metabolites were measured at 24 and 96 h respectively by hplc analysis. Depending upon the species, the treatment and the time in culture, CYP1A, 2A, 3A, 2B and 2C activities were detectable. 3A activity was highly induced by PB in both rat and mouse liver slices. These results demonstrate that this culture system can be used to assess and compare xenobiotic metabolism in liver slices from rodent species.

The expression of insecticide resistance-related cytochrome P450 forms is regulated by molting hormone in Drosophila melanogaster

The expression and enzymatic activities of insecticide resistance-related cytochrome P450B are increased by the treatment with 20-hydroxyecdysone (20HE) in D. melanogaster Oregon R flies. We have explored the role of this hormone in the maintenance of P450B basal expression. Arrest of ecdysone synthesis led to a decrease in CYP6A2 mRNA level, as well as in P450B expression and activities. This effect occurred both in insecticide susceptible (ecd1) and resistant (IRED) strains carrying the temperature-sensitive ecd mutation. The role of the 20HE in the regulation of cytochrome P450-mediated insecticide resistance has been proposed.

Ethylbenzene modulates the expression of different cytochrome P-450 isozymes by discrete multistep processes

Ethylbenzene (EB) treatment to male Holtzman rats was shown to alter the expression of cytochrome P-450s 1A1, 2B, 2C11, 2E1, and 3A, with several isozymes exhibiting complex multiphasic induction patterns when treated for 1 and 3 days with the alkylbenzene. Male rats were treated with daily i.p. injections of EB for either one or three days, and the effects on P-450 dependent activities, P-450 immunoreactive protein levels and their corresponding mRNA levels were measured. Although levels of P-450 2B, 2C11, 2E1, and 3A were all modulated by EB treatment, each exhibited different temporal characteristics. P-450 2B1/2B2 were induced after a single EB exposure and continued to be elevated after EB treatment for 3 days. However, P-450 2B1 and 2B2 mRNA levels were elevated about 50-fold after a single injection, and returned to control values after continued EB administration. P-450 2C11 expression was decreased to about 45% of controls after either single or repeated EB exposure with corresponding changes being observed in the levels of 2C11 mRNA. P-450 2E1 was induced by EB according to a complex multistep induction pattern. Both P-450 2E1 protein and RNA levels were increased 2-4-fold after a single EB treatment but returned to control values after continued administration. P-450 3A-dependent testosterone 2beta-hydroxylation and P-450 3A immunoreactive protein levels were both increased about 3-fold after a single EB treatment, whereas levels were only elevated 2-fold after EB treatment for 3 days. In contrast, P-450 3A2 mRNA was unaffected by a single EB injection but was increased 3.5-fold with repeated administration. Changes in P-450 3A1/2 were similar to those observed with P-450 3A2, whereas changes in P-450 3A1/23 and 3A23 mRNAs were not detectable. These data indicate that while EB can influence the expression of several P-450 isozymes, the hydrocarbon appears to alter P-450 expression by acting at different regulatory steps.

Binding of 2,3,7,8-tetrachlorodibenzo-p-dioxin to AH receptor in placentas from normal versus abnormal pregnancy outcomes

Binding of [3H]2,3,7,8-tetrachlorodibenzo-p-dioxin to AH receptor was characterized in cytosol from human placentas in which the pregnancy outcome was normal compared with pregnancies in which there was some adverse outcome (premature birth; intrauterine growth retardation; structural abnormality). No significant difference was detected between normal and adverse outcomes in the concentration of AH receptor sites (Bmax) nor in the affinity with which [3H]TCDD bound to the receptor (Kd). Aryl hydrocarbon hydroxylase activity, a CYP1A1 enzyme regulated by the AH receptor, was elevated in placental microsomes from smokers; this elevation was associated with intrauterine growth retardation.

Localization of multiple forms of inducible cytochromes P450 in rat liver mitochondria: immunological characteristics and patterns of xenobiotic substrate metabolism

Hepatic mitochondria contain inducible cytochromes P450 that cross-react with antibodies to P4501A1/2 and 2B1/2. In the present study, we present evidence for the occurrence of additional P450 forms in rat liver mitochondria that cross-react with antibodies to microsomal P4503A1/2 and 2E1. Protease protection and also immunoelectron microscopy studies were carried out to support the mitochondrial location of the immunoreactive P450s. The solubility of immunoreactive proteins in 0.1 M Na2CO3 suggests that the mitochondrial P450 forms tested are not membrane-integral proteins. The mitochondrial-associated P450 forms are capable of metabolizing resorufin derivatives, erythromycin, and p-nitrophenol in an adrenodoxin- and adrenodoxin reductase-supported system. Treatment of rats with phenobarbital (PB) resulted in the induction of mitochondrial pentoxyresorufin O-deethylase (PROD), benzoxyresorufin O-deethylase (BROD), and erythromycin N-demethylase (ERND) activities by 17-, 23-, and 2-fold, respectively. These activities were inhibited by 33 to 64% by antibodies to P4502B1/2 and P4503A1/2. The induction of the above monooxygenase activities correlated with the levels of mitochondrial proteins cross-reacting with antibodies to P4502B1/2 and P4503A1/2 in PB-treated livers. Similarly, administration of beta-naphthoflavone (BNF) resulted in a marked elevation of O-deethylation of ethoxy-, benzoxy-, and methoxyresorufins and a 2-fold increase in ERND activity. Immunoblot and immunoinhibition experiments using P4501A1/2, P4502B1/2, P4503A1/2, and P4502E1 antibodies revealed the presence of P450 forms closely related to the microsomal inducible forms. Results of immunoinhibition studies, using antibodies to adrenodoxin and reconstitution of enzyme activity with purified P450 forms, suggested a role for the mitochondrial P450 in the metabolism of xenobiotic substrates. The purified mitochondrial P450s also exhibited overlapping substrate specificities for resorufin derivatives and erythromycin.

Interaction of methadone with substrates of human hepatic cytochrome P450 3A4

Methadone, a synthetic drug, is one of the most widely used drugs for opiate dependency treatment. This drug has been demonstrated to be extensively metabolized by cytochrome P450 3A4 in human liver microsomes. Thus, the aim of this in vitro study was to determine if methadone is an inhibitor of other P450s characterized by their specific catalytic activities. Enzymatic activities specific to P450 2E1, P450 1A, P450 2B and P450 2C were not inhibited by methadone. Conversely, nifedipine oxidation, mediated by cytochrome P450 3A4, was potently inhibited by methadone by a mixed-type inhibition mechanism with a Ki of 100 microM. Fluvoxamine, a new antidepressant, was shown to be a potent mixed-type inhibitor of methadone N-demethylation with a Ki of 7 microM. Finally, methadone appears to be a mixed-type inhibitor and not a suicide inhibitor of cytochrome P450 3A family. Accordingly, caution should be advised in the clinical use of methadone when other drugs are administered that are able to induce or inhibit P450 3A4, such as rifampicin or nifedipine, diazepam and fluvoxamine.