Comparison of the form(s) of cytochrome P-450 induced by ethanol and glutethimide in cultured chick hepatocytes

Repeated treatment with furazolidone induces multiple cytochrome p450-related activities in chicken liver, but not in rat liver

The nitrofuran antimicrobial agent, furazolidone (FZ), is still used in veterinary medicine in some countries in the Middle and Far Eastern countries. The present study aimed to investigate the effects of successive bolus doses of FZ and its metabolite 3-amino-2-oxazolidinone (AOZ) on cytochrome P450 (CYP)-related activities in the livers of rats and chickens. Female Wistar rats and white Leghorn chickens were orally administered FZ once a day for 4 consecutive days. FZ-treated chickens showed an increase in multiple CYP-related activities, however, rats treated with FZ did not show these changes. In chickens, treatment with FZ also induced production of microsomal CYP2C6-like apoprotein. The present study demonstrated that FZ caused a multiple-type induction of CYP-related activities in chickens, but not in rats.

Role of CYP3A and CYP2E1 in alcohol-mediated increases in acetaminophen hepatotoxicity: comparison of wild-type and Cyp2e1(-/-) mice

CYP2E1 is widely accepted as the sole form of cytochrome P450 responsible for alcohol-mediated increases in acetaminophen (APAP) hepatotoxicity. However, we previously found that alcohol [ethanol and isopentanol (EIP)] causes increases in APAP hepatotoxicity in Cyp2e1(-/-) mice, indicating that CYP2E1 is not essential. Here, using wild-type and Cyp2e1(-/-) mice, we investigated the relative roles of CYP2E1 and CYP3A in EIP-mediated increases in APAP hepatotoxicity. We found that EIP-mediated increases in APAP hepatotoxicity occurred at lower APAP doses in wild-type mice (300 mg/kg) than in Cyp2e1(-/-) mice (600 mg/kg). Although this result suggests that CYP2E1 has a role in the different susceptibilities of these mouse lines, our findings that EIP-mediated increases in CYP3A activities were greater in wild-type mice compared with Cyp2e1(-/-) mice raises the possibility that differential increases in CYP3A may also contribute to the greater APAP sensitivity in EIP-pretreated wild-type mice. At the time of APAP administration, which followed an 11 h withdrawal from the alcohols, alcohol-induced levels of CYP3A were sustained in both mouse lines, whereas CYP2E1 was decreased to constitutive levels in wild-type mice. The CYP3A inhibitor triacetyloleandomycin (TAO) decreased APAP hepatotoxicity in EIP-pretreated wild-type and Cyp2e1(-/-) mice. TAO treatment in vivo resulted in inhibition of microsomal CYP3A-catalyzed activity, measured in vitro, with no inhibition of CYP1A2 and CYP2E1 activities. In conclusion, these findings suggest that both CYP3A and CYP2E1 contribute to APAP hepatotoxicity in alcohol-treated mice.

Arsenite decreases CYP3A23 induction in cultured rat hepatocytes by transcriptional and translational mechanisms

Arsenic is a naturally occurring, worldwide contaminant implicated in numerous pathological conditions in humans, including cancer and several forms of liver disease. One of the contributing factors to these disorders may be the alteration of cytochrome P450 (CYP) levels by arsenic. In rat and human hepatocyte cultures, arsenic, in the form of arsenite, decreases the induction of several CYPs. The present study investigated whether arsenite utilizes transcriptional or post-transcriptional mechanisms to decrease CYP3A23 in primary cultures of rat hepatocytes. In these cultures, a 6-h treatment with 5 microM arsenite abolished dexamethasone (DEX)-mediated induction of CYP3A23 protein and activity, but did not inhibit general protein synthesis. However, arsenite treatment only reduced DEX-induced levels of CYP3A23 mRNA by 30%. The effects of arsenite on CYP3A23 transcription were examined using a luciferase reporter construct containing 1.4 kb of the CYP3A23 promoter. Arsenite caused a 30% decrease in DEX-induced luciferase expression of this reporter. Since arsenite abolished induction of CYP3A23 protein, but caused only a small decrease in CYP3A23 mRNA, the effects of arsenite on translation of CYP3A23 mRNA were investigated. Polysomal distribution analysis showed that arsenite decreased translation by decreasing the DEX-mediated increase in CYP3A23 mRNA association with polyribosomes. Arsenite did not decrease intracellular glutathione or increase lipid peroxidation, suggesting that the effect of arsenite on CYP3A23 does not involve oxidative stress. Overall, the results suggest that low-level arsenite decreases both transcription and translation of CYP3A23 in primary rat hepatocyte cultures.

Mechanism of arsenite-mediated decreases in CYP3A23 in rat hepatocytes

In primary cultures of rat hepatocytes, exposure to arsenite causes a major decrease in dexamethasone (DEX)-mediated induction of CYP3A23 hemoprotein, with a minor decrease in CYP3A23 mRNA. Here we show that addition of heme did not prevent the arsenite-mediated decreases in CYP3A23 protein, and arsenite did not decrease intracellular glutathione levels, indicating that heme and glutathione were not limiting for formation of holoCYP3A23. We also investigated whether arsenite decreases CYP3A23 protein by increasing CYP3A23 degradation by the calpain pathway. The calpain inhibitor, calpeptin, caused greater than a 90% inhibition of calpain-mediated proteolysis, but had no effect on DEX-mediated induction of CYP3A23 protein following 24h treatments. However, calpeptin enhanced the effect of arsenite to decrease induction of CYP3A23 protein. In addition, in short-term studies, calpeptin appeared to be a suicidal inhibitor of CYP3A-catalyzed enzyme activity. Our findings suggest that CYP3A23 protein is not degraded by calpain-mediated proteolysis, even in the presence of arsenite.

Comparative expression of liver cytochrome P450-dependent monooxygenases in the horse and in other agricultural and laboratory species

The apoprotein expression and the catalytic activities of cytochrome P450s involved in the biotransformation of xenobiotics were investigated in horse liver microsomes and compared with those of food producing (cattle, pigs, broiler chicks, and rabbits) and laboratory species (rats). Western blot analysis revealed the presence of proteins immunorelated to rat CYP 1A, CYP 2B, CYP 2E, and CYP 3A subfamilies in hepatic microsomes from horses and from any other examined species. With the exception of the N-demethylation of N-nitrosodimethylamine in broiler chicks, all the recorded interspecies differences were quantitative in nature. Equine preparations proved the most active in the biotransformation of the CYP 1A substrates ethoxy- and methoxyresorufin and the least active in the metabolism of aminopyrine and ethoxycoumarin. On a comparative basis, large differences were observed in the rate of the in vitro metabolism of model substrates between "minor" (rabbits, horses) and "major" food producing species. Taken in due consideration the limitations of the in vitro approach, results from this study reinforce the conclusion that studies on drug efficacy and residue depletion should be performed in each target species.

Mutant yields and mutational spectra of the heterocyclic amines MeIQ and PhIP at the S1 locus of human-hamster AL cells with activation by chick embryo liver (CELC) co-cultures

Cooking meat and fish at high temperature creates heterocyclic amines (HA) including 2-amino-3,4-dimethylimidazo[4,5-f]quinoline (MeIQ) and 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP). Several HA are mutagens in the Ames' S9/Salmonella assay. While PhIP is a substantial Ames' test mutagen, it is 1000-fold less active than the extraordinarily potent MeIQ. In contrast, MeIQ is significantly less mutagenic than PhIP in several mammalian cell assays, especially in repair-deficient Chinese hamster ovary (CHO) cells. HA are suspect human carcinogens on the basis of (i) epidemiological evidence, (ii) induction of tumors in rodents and monkeys, (iii) DNA adduct formation and (iv) mutagenic capacity. In this study, MeIQ and PhIP were significant mutagens at the S1 locus of co-cultivated human/hamster hybrid AL cells following metabolic activation by beta-napthoflavone (betaNF)-induced chick embryonic liver cultures (CELC). MeIQ was more mutagenic than PhIP in the CELC+AL cell assay. The mutant response curves increase with dose and then plateau (PhIP), or decrease (MeIQ). The inflections in these response curves coincide with dose-dependent decreases in cytochrome CYP1A1 activity. Molecular analysis of S1- mutants indicates that a substantial fraction, >65%, of the mutations induced by PhIP are deletions of 4.2 to 133 (Mbp); half are larger than 21 Mbp. Mutations induced by MeIQ were smaller, most (56%) being less than 5.7 Mbp. When appropriate metabolic activation is combined with a target locus, which can detect both small and large chromosomal mutations, both MeIQ and PhIP are significant mutagens and clastogens in repair proficient mammalian cells.

Mechanism of the synergistic induction of CYP2H by isopentanol plus ethanol: comparison to glutethimide and relation to induction of 5-aminolevulinate synthase

We had previously found that combined treatment with isopentanol and ethanol synergistically induced CYP2H protein and activity in cultured chick nepatoytes. Here we investigated the mechanism of induction of CYP2H by the alcohols and whether they caused a coordinate induction of 5-aminolevulinate synthase (ALAS) mRNA. Treatment with isopentanol alone or in combination with ethanol resulted in coordinate increases in CYP2H1 and ALAS mRNAs. With isopentanol alone, the amounts of CYP2H1 and ALAS mRNAs at 4 to 6 h were similar to those observed after treatment with the alcohol combination, but declined by 11 h. Readdition of isopentanol at 11 h again increased the expression of both mRNAs, indicating that the decreases at 11 h were due to limiting amounts of inducer. Similar results were observed in cells exposed to low concentrations of glutethimide. In the combined alcohol treatment, increases in CYP2H1 and ALAS mRNAs were sustained from 4 h to 11 h after addition of the alcohols, but decreased to control levels by 24 h. Using pulse labeling to measure de novo synthesis of CYP2H1/2 protein, we found that the increases in CYP2H1/2 protein reflected the increases in CYP2H1 mRNA. The half-life of CYP2H1/2 protein, measured from pulse-chase experiments, was approximately twofold greater than the half-life of CYP2H1 mRNA. Our results indicate that the alcohols and glutethimide coordinately increase ALAS and CYP2H1 mRNA, and that increases in CYP2H1/2 protein arise from increases in its mRNA.

Effects of mifepristone (RU-486) on heme metabolism and cytochromes P-450 in cultured chick embryo liver cells, possible implications for acute porphyria

Mifepristone (RU-486), a potent progesterone receptor antagonist and inducer of cytochromes P-450, is currently in use in Europe, particularly as a post-coital oral contraceptive. Soon it will be available in the United States, as well. Since progesterone has been implicated in the pathogenesis of acute attacks of porphyria, the use of RU-486 or related compounds might be considered in porphyric patients. However, as with other cytochrome P-450 inducers, RU-486 may have the ability to precipitate or exacerbate attacks of acute porphyria. The acute porphyrias in relapse are associated with an increase in activity of delta-aminolevulinic acid synthase, the first and normally rate-controlling enzyme in heme biosynthesis. We have used primary cultures of chick embryo liver cells to test the ability of RU-486 to induce delta-aminolevulinic acid synthase activity and mRNA, cytochromes P-450, porphyrin accumulation, and heme oxygenase. We found that RU-486, at concentrations observed in human plasma after a single oral dose, induced the mRNA and activity of delta-aminolevulinic acid synthase, both by itself and in the presence of deferoxamine, a potent iron chelator that inhibits ferrochelatase. RU-486 and deferoxamine together also produced significant accumulations of protoporphyrin. These results indicate that RU-486 may pose a risk in patients with known acute porphyria and should be used with caution. RU-486 increased the concentration of total cytochrome P-450, and the activity of erythromycin demethylase, an activity specifically catalyzed by cytochrome P-450 3A. Unlike several other porphyrogens (e.g. hydantoins, barbiturates), RU-486 does not produce accumulation of uroporphyrin or induction of heme oxygenase in chick embryo liver cells.

Validation of 4-nitrophenol as an in vitro substrate probe for human liver CYP2E1 using cDNA expression and microsomal kinetic techniques

The involvement of human cytochrome P450 (CYP) 2E1 in the hydroxylation of 4-nitrophenol (4NP) to 4-nitrocatechol (4NC) has been investigated using cDNA expression and liver microsomal kinetic and inhibitor techniques. 4NP hydroxylation by human liver microsomes and cDNA-expressed human CYP2E1 exhibited Michaelis-Menten kinetics; the respective apparent Km values were 30 +/- 7 and 21 microM. Mutual competitive inhibition was observed for 4NP and chlorzoxazone (CZ) (an alternative human CYP2E1 substrate) in liver microsomes, with close similarities between the calculated apparent Km and Ki values for each individual compound. 4NP and CZ hydroxylase activities in microsomes from 18 liver donors varied to a similar extent (3.3- and 3.0-fold, respectively) and 4NP hydroxylase activity correlated significantly (rs > or = 0.75, P < 0.005) with both CZ hydroxylation and immunoreactive CYP2E1 content. The prototypic CYP2E1 inhibitor, diethyldithiocarbamate, was a potent inhibitor of 4NC formation and decreased 4NP hydroxylation by cDNA-expressed CYP2E1 and human liver microsomes in parallel. Probes for other human CYP isoforms namely (alpha-naphthoflavone, coumarin, sulphaphenazole, quinidine, troleandomycin and mephenytoin) caused < 15% inhibition of liver microsomal 4NP hydroxylation. These data confirm that, as in animal species, 4NP hydroxylation is catalysed largely by CYP2E1 in human liver and 4NP may therefore be used as an in vitro substrate probe for the human enzyme.

Biochemical characterization of coumarin 7-hydroxylase activity in chick embryo liver microsomes

Coumarin occurs naturally in the diet and can induce and inhibit cytochrome P450 enzymes. Hepatic coumarin 7-hydroxylase activity is the major pathway for coumarin metabolism in humans but not in rats, most strains of mice, or other laboratory animals. Coumarin 7-hydroxylase activity and the effects of chemical inhibitors and inducers on this activity were studied in 19-day-old chick embryo liver microsomes. Activity was between 35 and 75 nmol/mg protein/hr which is approximately 2-fold higher than reported for human liver microsomes. The pH optimum was 7.8 and the Km determined by both an ether extraction and a high performance liquid chromatography method was 7.3 +/- 0.9 (+/- SD) microM. Substrate inhibition was evident at coumarin concentrations above 250 microM (activities at 1000 and 4000 microM coumarin were 84 and 40% of Vmax, respectively). The Ki values (+/- SD) for inhibitors of microsomal coumarin 7-hydroxylase activity in vitro were: alpha-naphthoflavone, 46.9 +/- 19.8 nM; metyrapone, 0.8 +/- 0.9 microM; aniline, 12.3 +/- 8.2 microM; cimetidine, 70.9 +/- 27.9 microM; N-nitrosodimethylamine, 0.7 +/- 0.9 mM; and dimethyl sulfoxide, 7.9 +/- 1.9 mM. Treatment of chick embryos with pyrazole (40 mumol) increased coumarin 7-hydroxylase by 50% at 24 hr, but this activity was unaffected by treatment of embryos with 3-methylcholanthrene (2 mumol) or glutethimide (8 mumol). Thus, hepatic coumarin 7-hydroxylase activity in 19-day-old chick embryos is higher than in most laboratory animals and has similar biochemical properties as the enzyme in humans and mice. The chick embryo liver may be a useful system for studies on the biochemical effects of coumarin and the regulation of cytochrome P450-dependent coumarin 7-hydroxylase.

Coumarin-induced changes in delta-aminolaevulinic acid synthase and cytochrome P-450 in chick embryo liver

Coumarin occurs naturally in the diet and inhibits several cytochrome P-450 enzymes in laboratory animals. The effect of coumarin was examined on haem biosynthesis and cytochrome P-450 activities in the 18-day-old chick embryo liver in ovo. At 40 and 50 mumol/embryo coumarin increased delta-aminolaevulinic acid synthase, porphyrins, cytochrome P-450, benzphetamine N-demethylase and benzo[a]pyrene hydroxylase. At 10 mumol/embryo coumarin decreased aniline 4-hydroxylase, and at both 10 and 50 mumol/embryo it decreased 7-ethoxyresorufin O-deethylase, coumarin 7-hydroxylase and nitrosodimethylamine N-demethylase. 7-Hydroxycoumarin and 5, 7-methoxycoumarin at 40 mumol/embryo had none of these effects. Coumarin (5-500 microM) added to liver microsomes inhibited aniline hydroxylase by 45%, but not nitrosodimethylamine N-demethylase, and inhibited 7-ethoxyresorufin O-deethylase in microsomes from 3-methylcholanthrene-treated embryos by 15 and 100% at coumarin concentrations of 250 and 500 microM, respectively. Coumarin 7-hydroxylase activity in chick embryo liver was comparable with that reported for human liver and greater than in the rat. The data indicate that coumarin can both increase and decrease cytochrome P-450 activities in chick embryo liver and can induce haem biosynthesis. Because the chick embryo liver hydroxylates coumarin at position 7 in a manner similar to humans, it may be a more suitable model than the rat for studying some of the metabolic effects of coumarin.

Ethanol increases cytochromes P450IIE, IIB1/2, and IIIA in cultured rat hepatocytes

In intact rats, ethanol treatment has been associated with increases in hepatic levels of both P450IIB1/2 and P450IIE. When rat hepatocytes were cultured on an extracellular tumor matrix (Matrigel), exposure to ethanol from 48 to 96 h in culture resulted in increases in cytochromes P450IIE, IIB1/2, and IIIA. Cytochrome P450IIE was detected immunologically and enzymatically, using two activities associated with cytochrome P450IIE, p-nitrophenol hydroxylation, and acetaminophen activation to a metabolite that binds to glutathione. The content of cytochrome P450IIE in freshly isolated cells decreased when the cells were placed in culture. Exposure of the cultured hepatocytes to ethanol from 48 to 96 h after inoculation resulted in an increase in cytochrome P450IIE compared to untreated cultured cells. In addition, in culture, the amount of enzymatically active protein after ethanol treatment was equal to that in hepatocytes freshly isolated from intact animals. Ethanol treatment resulted in increases in cytochrome P450IIB1/2 compared to untreated cells, as shown immunologically and by increased benzyloxyresorufin dealkylase activity. However, phenobarbital induced cytochrome P450IIB1/2 to higher levels, compared to ethanol. Ethanol and phenobarbital treatments both increased P450IIIA, as determined immunologically and by the amount of propoxycoumarin depropylase activity that is inhibited by triacetyloleandomycin. However, the amount of P450IIIA increased after ethanol treatment was less than that increased after treatment with dexamethasone in these cells. The ethanol-mediated increases in all four forms of cytochrome P450 in culture suggest that these increases in the intact animal result from direct effects of ethanol on the liver.

Rapid decrease of cytochrome P-450IIE1 in primary hepatocyte culture and its maintenance by added 4-methylpyrazole

Studies were conducted to evaluate the possible induction or the maintenance of cytochrome P-450IIE1 in primary hepatocyte cultures by the inducing agent 4-methylpyrazole. Hepatocytes were isolated from control (noninduced) rats and from rats treated in vivo with either pyrazole or 4-methylpyrazole to induce P-450IIE1. The content of P-450IIE1 was determined by Western blots with antipyrazole P-450 IgG, and catalytic activity was assessed by assays of dimethylnitrosamine demethylase activity. The treatment with 4-methylpyrazole in vivo increased the content of P-450IIE1 and dimethylnitrosamine demethylase activity sevenfold and fourfold, respectively. In cultures prepared from noninduced hepatocytes, P-450IIE1 levels fell to values of 76%, 65%, 31% and 1% of freshly isolated hepatocytes after 1, 3, 6 and 9 days in culture. A similar decrease in dimethylnitrosamine demethylase was observed during this time. In cultures prepared from induced hepatocytes, the decline in P-450IIE1 was more rapid as levels fell to 77%, 31%, 3% and 3% of initial values after 1, 3, 6 and 9 days in culture. Again, the fall in dimethylnitrosamine demethylase activity paralleled the decline in content of P-450IIE1 and was more rapid with the induced hepatocytes. With cultures prepared from noninduced or induced hepatocytes, the addition of 4-methylpyrazole in vitro did not increase the content of P-450IIE1 or the activity of dimethylnitrosamine demethylase over the initial values. However, 4-methylpyrazole appeared to stabilize the P-450IIE1 and to decrease its rate of decline in culture. In noninduced cultures, the percent remaining content of P-450IIE1 after 6 days was 31% in the absence of and 52% in the presence of 5 mol/L 4-methylpyrazole. In cultures from 4-methylpyrazole-induced hepatocytes, the percent remaining P-450IIE1 after 3 days was 31% in the absence of inducer and 59% with 4-methylpyrazole added in vitro. Similarly 4-methylpyrazole helped to prevent the rapid decline of dimethylnitrosamine demethylase activity in induced and noninduced cultures. Viability of the induced and noninduced cultures in the absence or presence of added 4-methylpyrazole was similar. Levels of mRNA for P-450IIE1 were similar for livers from control rats and from rats treated in vivo with 4-methylpyrazole. The mRNA levels rapidly declined in induced and noninduced cultures, and this decline, unlike the fall in P-450IIE1 or dimethylnitrosamine demethylase activity, could not be prevented by the addition of 4-methylpyrazole in vitro to the cultures.(ABSTRACT TRUNCATED AT 400 WORDS)

Isolation of four forms of acetone-induced cytochrome P-450 in chicken liver by h.p.l.c. and their enzymic characterization

The purpose of this study was to purify and characterize the forms of cytochrome P-450 induced in chicken liver by acetone or ethanol. Using high performance liquid ion-exchange chromatography, we were able to isolate at least four different forms of cytochrome P-450 which were induced by acetone in chicken liver. All four forms of cytochrome P-450 proved to be distinct proteins, as indicated by their N-terminal amino acid sequences and their reconstituted catalytic activities. Two of these forms, also induced by glutethimide in chicken embryo liver, appeared to be cytochromes P450IIH1 and P450IIH2. Both of these cytochromes P-450 have identical catalytic activities towards benzphetamine demethylation. However, they differ in their abilities to hydroxylate p-nitrophenol and to convert acetaminophen into a metabolite that forms a covalent adduct with glutathione at the 3-position. Another form of cytochrome P-450 induced by acetone is highly active in the hydroxylation of p-nitrophenol and in the conversion of acetaminophen to a reactive metabolite, similar to reactions catalysed by mammalian cytochrome P450IIE. Yet the N-terminal amino acid sequence of this form has only 30-33% similarity with cytochrome P450IIE purified from rat, rabbit and human livers. A fourth form of cytochrome P-450 was identified whose N-terminal amino acid sequence and enzymic activities do not correspond to any mammalian cytochromes P-450 reported to be induced by acetone or ethanol.