Induction of cytochrome P450IA1 in mouse hepatoma cells by several chemicals. Phenobarbital and TCDD induce the same form of cytochrome P450

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

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

Induction of Cytochrome P450 IA1 in Mouse Hepatoma Cells as a Short-term Bioassay

The inducibility of aryl hydrocarbon hydroxylase (AHH) in mouse hepatoma Hepa-1 cell culture has been used as a quantitative bioassay for 2,3,7,8-tetrachlorodibenzo- p-dioxin (TCDD). The detection limit was 0.5pM TCDD. The basal AHH activity was about 2U/mg protein, and the maximally induced activity was about 100U/mg protein. ED50 for TCDD was 7pM. Similar data were obtained using 7-ethoxyresorufin as a substrate (EROD activity). Extraction of several consumer paper products with acetone gave an uncharacterised mixture of AHH inducers. The estimated TCDD (equivalents) in these extracts varied between 0 and 42pg/g of paper.

Antiepileptic Drugs Affect Neuronal Androgen Signaling via a Cytochrome P450-Dependent Pathway

Recent data imply an important role for brain cytochrome P450 (P450) in endocrine signaling. In epileptic patients, treatment with P450 inducers led to reproductive disorders; in mouse hippocampus, phenytoin treatment caused concomitant up-regulation of CYP3A11 and androgen receptor (AR). In the present study, we established specific in vitro models to examine whether CYP3A isoforms cause enhanced AR expression and activation. Murine Hepa1c1c7 cells and neuronal-type rat PC-12 cells were used to investigate P450 regulation and its effects on AR after phenytoin and phenobarbital administration. In both cell lines, treatment with antiepileptic drugs (AEDs) led to concomitant up-regulation of CYP3A (CYP3A11 in Hepa1c1c7 and CYP3A2 in PC-12) and AR mRNA and protein. Inhibition of CYP3A expression and activity by the CYP3A inhibitor ketoconazole or by CYP3A11-specific short interfering RNA molecules reduced AR expression to basal levels. The initial up-regulation of AR signal transduction, measured by an androgen-responsive element chloramphenicol-acetyltransferase reporter gene assay, was completely reversed after specific inhibition of CYP3A11. Withdrawal of the CYP3A11 substrate testosterone prevented AR activation, whereas AR mRNA expression remained up-regulated. In addition, recombinant CYP3A11 was expressed heterologously in PC-12 cells, thereby eliminating any direct drug influence on the AR. Again, the initial up-regulation of AR mRNA and activity was reduced to basal levels after silencing of CYP3A11. In conclusion, we show here that CYP3A2 and CYP3A11 are crucial mediators of AR expression and signaling after AED application. These findings point to an important and novel function of P450 in regulation of steroid hormones and their receptors in endocrine tissues such as liver and brain.

Hepatocyte cell lines: their use, scope and limitations in drug metabolism studies

Gaining knowledge on the metabolism of a drug, the enzymes involved and its inhibition or induction potential is a necessary step in pharmaceutical development of new compounds. Primary human hepatocytes are considered a cellular model of reference, as they express the majority of drug-metabolising enzymes, respond to enzyme inducers and are capable of generating in vitro a metabolic profile similar to what is found in vivo. However, hepatocytes show phenotypic instability and have a restricted accessibility. Different alternatives have been explored in the past recent years to overcome the limitations of primary hepatocytes. These include immortalisation of adult or fetal human hepatic cells by means of transforming tumour virus genes, oncogenes, conditionally immortalised hepatocytes, and cell fusion. New strategies are currently being used to upregulate the expression of drug-metabolising enzymes in cell lines or to derive hepatocytes from progenitor cells. This paper reviews the features of liver-derived cell lines, their suitability for drug metabolism studies as well as the state-of-the-art of the strategies pursued in order to generate metabolically competent hepatic cell lines.

Use of [125I]4'-iodoflavone as a tool to characterize ligand-dependent differences in Ah receptor behavior

We have synthesized [(125)I]4'-iodoflavone to study Ah receptor (AhR)-ligand interactions by a class of AhR ligands distinct from the prototypic ligand 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). This radioligand allows the comparison of AhR-ligand interactions using a ligand that differs in AhR affinity, and yet has the same radiospecific activity as [(125)I]2-iodo-7,8-dibromodibenzo-p-dioxin. Specific binding of [(125)I]4'-iodoflavone with the AhR was detected as a single radioactive peak ( approximately 9.7 S) following density sucrose gradient analysis. Cytosolic extracts from both Hepa 1 and HeLa cells were used as the source of mouse and human AhR, respectively. A approximately 6.7 S form of radioligand-bound Ah receptor was detected in the high salt nuclear extracts of both cell lines. In HeLa cells approximately twofold more [(125)I]4'-iodoflavone-AhR 6 S complex, compared with [(125)I]2-iodo-7,8-dibromodibenzo-p-dioxin, was recovered in nuclear extracts. A comparison of the ability of 4'-iodoflavone and TCDD to cause time-dependent translocation of AhR-yellow fluorescent protein revealed that 4'-iodoflavone was more efficient at enhancing nuclear accumulation of the receptor. These results suggest that [(125)I]4'-iodoflavone is a particularly useful and easily synthesized ligand for studying the AhR.

Phenobarbital induces cytochrome P4501A2 hnRNA, mRNA and protein in the liver of C57BL/6J wild type and aryl hydrocarbon receptor knock-out mice

The aryl hydrocarbon receptor mediates the transcriptional response to a variety of hydrocarbons of members of the aryl hydrocarbon gene battery. Phenobarbital does not bind the aryl hydrocarbon receptor with high affinity but induces, in liver cells, expression of cytochrome P4501A. Using both wild type and aryl hydrocarbon receptor knock out C57BL/6J mice, we demonstrate that phenobarbital induced hnRNA, mRNA and protein for the cytochrome P-4501A2 gene in the presence or absence of the aryl hydrocarbon receptor. Using the DNA binding site for the aryl hydrocarbon receptor as a probe, gel retardation analyses showed that phenobarbital treatment induced protein binding, regardless of the presence of the aryl hydrocarbon receptor.

Effect of phenobarbital on hepatic CYP1A1 and CYP1A2 in the Ahr-null mouse

Studies have suggested that phenobarbital (PB) induces members of the CYP1A subfamily by both transcriptional and post-transcriptional mechanisms. Using the Ahr -/- mice, we examined the induction of CYP1A1 and CYP1A2 by PB. CYP1A2 mRNA and protein were induced by PB in the null mice, suggesting that CYP1A2 is regulated by PB by a mechanism independent of the aryl hydrocarbon receptor (AHR). In contrast, the regulation of CYP1A1 is highly dependent on the AHR.

Studies on α-Hexachlorocyclohexane Cytotoxicity, Genotoxicity and Cytochrome P450 Induction in Primary Hepatocytes and Hepatoma Cell Lines from Rodents and Humans

α-Hexachlorocyclohexane (α-HCH) was examined for cytotoxicity, genotoxicity and cytochrome P450 induction in primary cultures of mouse, rat and human hepatocytes and in three hepatoma cell lines (Hepa 1c1c7, FaO and Hep G2, from mouse, rat and man, respectively). The cell lines were much more sensitive to the cytotoxicity of the classical inducers phenobarbital and 3-methylcholanthrene than that of α-HCH, whereas no cytotoxicity was observed in primary hepatocytes. Exposure for 24 hours to 0.32mM α-HCH produced a modest, but statistically significant, frequency of DNA breaks, as measured by the alkaline elution assay, in the mouse Hepa 1c1c7 cell line, and the human Hep G2 cell line, but not in the rat FaO cell line. In the Hep G2 cell line, the amount of DNA fragmentation was found to increase with the length of exposure. Compared with the results of previous observations on primary cultures, with regard to species specificity, only the human cell line gave a concordant positive response. Monooxygenase activity induction in primary hepatocytes, despite rather high initial levels of 7-ethoxycoumarin-O-deethylase activity, was low with the classical inducers phenobarbital and 3-methylcholanthrene. α-HCH caused no induction of monooxygenase. The rat FaO and human Hep G2 cell lines were sensitive to α-HCH, but only after long exposure. The results of this study support the hypothesis that α-HCH might act as a weak genotoxic agent in humans, but they also suggest caution in the extrapolation to the in vivo situation of the observations made in established cell lines.

DNA adduct formation in primary mouse embryo cells induced by 7H-dibenzo[c,g]carbazole and its organ-specific carcinogenic derivatives

The nuclease P1 modification of the 32P-postlabeling technique was used to study the biological activity of 7H-dibenzo[c,g]carbazole (DBC) and some of its derivatives, including N-methyldibenzo[c,g]carbazole (N-MeDBC), 5,9-dimethyldibenzo[c,g]carbazole (5,9-diMeDBC), 5,9,N-trimethyldibenzo[c,g]carbazole (5,9,N-triMeDBC), 6-methoxydibenzo[c,g]carbazole (6-McODBC), N-acetyldibenzo[c,g]carbazole (N-AcDBC), N-hydroxymethyldibenzo[c,g]carbazole (N-HMeDBC) in primary mouse embryo cells. A very good correlation was found between carcinogenic specificity in vivo of these N-heterocyclic aromatic hydrocarbons and their DNA-adduction in vitro. Primary mouse embryo cells were able to metabolize and detect tissue-specific sarcomagens N-MeDBC and 6-MeODBC as well as derivatives with both sarcomagenic and hepatocarcinogenic activity, DBC, N-AcDBC, and N-HMeDBC. The strong specific hepatocarcinogen 5,9-diMeDBC in vivo, did not induce any DNA-adducts in the embryo cells, which suggests that the enzymatic composition of the target tissue probably is the determining factor in the organ specificity of this derivative. 5,9,N-triMeDBC, derivative without any carcinogenic activity in vivo, did not induce any DNA-adducts in primary mouse embryo cells. Pretreatment of cells with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) apparently stimulated DNA-adduct formation in the cells exposed to DBC, 6-MeODBC, and N-MeDBC. No or a very slight effect of TCDD on DNA-adduct formation was found in cells exposed to N-HMeDBC and N-AcDBC. Preliminary results have shown that TCDD slightly induced cytochrome P4501A1-linked ethoxyresorufin O-deethylase (EROD) activity in primary mouse embryo cells. These data suggest the role of cytochrome P4501A1 in the metabolism of DBC derivatives with sarcomagenic activity.

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

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

Comparison of expression of aldehyde dehydrogenase 3 and CYP1A1 in dominant and recessive aryl hydrocarbon hydroxylase-deficient mutant mouse hepatoma cells

The mouse hepatoma cell line Hepa-1 is inducible by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) for both CYP1A1 (aryl hydrocarbon hydroxylase, AHH) and class 3 aldehyde dehydrogenase (ALDH3) enzymes. To test the hypothesis of a common regulatory mechanism, several AHH deficient mutants of Hepa-1 were studied for their ALDH3 activities and specific mRNA levels before and after TCDD treatment. The recessive (with respect to the wild-type Hepa-1) mutants have defects in Cypla-1 structural gene (mutant c1) or in the Ah (aryl hydrocarbon) receptor (mutants c2 and c6 with decreased levels of Ah receptor; mutant c4 defective in the DNA binding of the Ah receptor). The results with these mutants suggested that Ah receptor nuclear translocator protein, ARNT, is needed for ALDH3 expression. Two dominant mutants, one of which is characterized by preventing the binding of the Ah receptor complex to DNA, were also studied. Surprisingly, these mutants possessed elevated levels of ALDH3 mRNA and enzyme activities which were also inducible by TCDD. The binding of Ah receptor-ligand complex to DNA was thus not needed for the expression of ALDH3. A dominant repressor for Cypla-1 gene transcription did not prevent the derepression or induction of ALDH3. The results thus suggest that Aldh-3 gene is regulated by a mechanism independent of the Ah receptor.

Evaluation of the xenobiotic biotransformation capability of six rodent hepatoma cell lines in comparison with rat hepatocytes

Phase I and II activities were examined in six rodent hepatoma cell lines and compared with those of cultured rat hepatocytes both in basal conditions and after exposure to 5 microM methylcholanthrene, 2 mM phenobarbital, and 15 microM beta-naphtoflavone. The metabolic profile of testosterone was also studied. The highest aryl hydrocarbon hydroxylase and 7-ethoxycoumarin O-deethylase activities were found in MH1C1 cells. Comparable values for 7-ethoxyresorufin O-deethylase activity, ranging from 21.6 to 42.9 pmol/mg x min, were observed in the hepatocytes and hepatoma cells, except the HTC cells. In contrast, only Fao cells showed 7-pentoxyresorufin O-depentylase activity at levels similar to those of hepatocytes (6.2 +/- 1.0 and 7.4 +/- 1.2 pmol/mg x min, respectively). Rat hepatocytes actively hydroxylated p-nitrophenol, but this activity was not measurable in hepatoma cells. Glutathione transferase activity was maintained in all the hepatoma cell lines at similar levels to those found in hepatocytes (684 +/- 56 nmol/mg x min). The seven hydroxylated metabolites of testosterone produced by cultured hepatocytes were negligible in hepatoma cells. Exposure of cells to inducers revealed that aryl hydrocarbon hydroxylase activity was mainly increased after treatment with 3-methylcholanthrene and beta-naphtoflavone, and the highest values were found in rat hepatocytes followed by MH1C1 and Fao cells. 3-Methylcholanthrene and naphtoflavone treatment also resulted in a marked increase in 7-ethoxyresorufin O-deethylase activity in hepatocytes as well as in H4IIC3, McA-Rh7777, MH1C1, and Fao cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Fulminant hepatic failure associated with status epilepticus in children: three cases and a review of potential mechanisms

Fulminant hepatic failure is a rare complication of status epilepticus. Although many of the anticonvulsants used to treat the seizures are known to have hepatotoxic properties, the exact mechanism leading to massive destruction of the liver following a prolonged seizure remains unclear. Three children are presented who developed fulminant hepatic failure following status epilepticus and subsequently died of multiple organ failure. The literature is reviewed with particular attention to the possible interaction between the anticonvulsants and the metabolic consequences of status epilepticus. We postulate that it is a combination of hypoxia and ischemia that occurs during a prolonged seizure with altered metabolism of free radicals secondary to the anticonvulsant drugs which leads to widespread hepatocyte membrane damage.

Comparison of CYP1A1 induction and genotoxicity in vitro as indicators of potentially harmful effects of environmental samples

Cytochrome P450IA1 (CYP1A1) induction of Hepa-1 mouse and H4IIE rat hepatoma cell lines was compared using selected environmental samples. The results were in agreement for both cell lines: no induction was observed for the fly ash extract from peat combustion, an intermediate induction was found for the fly ash extract from biosludge combustion, and a strong induction was detected for natural peat extract. However, Hepa-1 responded to the samples more sensitively than did H4IIE: the half maximal induction (ED50) values for Hepa-1 were smaller than those for H4IIE. In a bacterial DNA repair assay without metabolic activation and in a mammalian sister chromatid exchange test in the presence of metabolic activation the samples were virtually non-genotoxic. Thus the CYP1A1-inducing potency and genotoxicity of the samples were not correlated. In light of these results, the CYP1A1 induction test might be a useful addition to conventional genotoxicity tests, which may fail to detect potentially harmful compounds/mixtures.

Hepa-1 enzyme induction assay as an in vitro indicator of the CYP1A1-inducing potencies of laboratory rodent diets in vivo

The Hepa-1 enzyme induction assay (assay of the induction of CYP1A1 catalytic activities in the Hepa-1 mouse hepatoma cell line by various compounds or mixtures) was evaluated as an in vitro indicator of the CYP1A1-inducing potencies of laboratory rodent diets in vivo. C57BL/6J mice were fed for three weeks four selected commercially available diets (one semisynthetic and three standard natural ingredient diets) exhibiting different enzyme-inducing effects in the Hepa-1 assay. beta-Naphthoflavone mixed in a semisynthetic diet (33 and 330 mg/kg of diet) was used as a model inducer. CYP1A1-dependent enzyme activities (aryl hydrocarbon hydroxylase and 7-ethoxyresorufin O-deethylase) were measured in the small intestinal mucosa and liver. There was good agreement between the induction of CYP1A1 in vitro and in vivo: the rank order of the enzyme activities elicited by the diets was the same in the mice as in the Hepa-1 cells. The standard diets were less effective inducers than beta-naphthoflavone in the Hepa-1 cells and in the mice, especially in the small intestinal mucosa. The Hepa-1 enzyme induction assay thus seems to be a mechanistically sound, reliable and sensitive in vitro indicator of the CYP1A1-inducing potencies of laboratory rodent diets in vivo.

Directly acting geno- and cytotoxic agents from a wild mushroom Dermocybe sanguinea

The wild mushroom, Dermocybe sanguinea, contains several anthraquinone pigments, of which emodin (1,3,8-trihydroxy-6-methylanthraquinone) is quantitatively the most important. In our preliminary tests, Dermocybe sanguinea extracts were genotoxic without metabolic activation. The ethanol extract of Dermocybe sanguinea was fractionated by flash chromatography, and the emodin contents of the fractions were determined by HPLC. Their genotoxicities were assayed using a bacterial repair assay and sister-chromatid exchange analysis. The cytotoxicity of the fractions was assayed with mouse hepatoma cells using growth inhibition as the endpoint. The results of the biological tests were compared with those obtained with pure emodin. It was concluded that, in addition to emodin, Dermocybe sanguinea contains several other geno- and cytotoxic compounds.

Induction of class 3 aldehyde dehydrogenase in the mouse hepatoma cell line Hepa-1 by various chemicals

The mouse hepatoma cell line Hepa-1 was shown to express an aldehyde dehydrogenase (ALDH) isozyme which was inducible by TCDD and carcinogenic polycyclic aromatic hydrocarbons. The induced activity could be detected with benzaldehyde as substrate and NADP as cofactor (B/NADP ALDH). As compared with rat liver and hepatoma cell lines, the response was moderate (maximally 5-fold). There was an apparent correlation between this specific form of ALDH and aryl hydrocarbon hydroxylase (AHH) in the Hepa-1 wild-type cell line--in terms of inducibility by several chemicals. However, the magnitude of the response was clearly smaller for ALDH than for AHH. Southern blot analysis showed that a homologous gene (class 3 ALDH) was present in the rat and mouse genome. The gene was also expressed in Hepa-1 and there was a good correlation between the increase of class 3 ALDH-specific mRNA and B/NADP ALDH enzyme activity after exposure of the Hepa-1 cells to TCDD. It is concluded that class 3 ALDH is inducible by certain chemicals in the mouse hepatoma cell line, although the respective enzyme is not inducible in mouse liver in vivo.

Cytotoxic and aryl hydrocarbon hydroxylase-inducing effects of laboratory rodent diets. A cell culture study

Extracts of several rodent diets were studied for their cytotoxic and aryl hydrocarbon hydroxylase-inducing properties by an in vitro method. The cell culture system based on a mouse hepatoma cell line (Hepa-1) was shown to be a convenient and sensitive method for screening of diets for these parameters implying the presence of compounds potentially harmful in vivo. Considerable differences among diets and batches were detected. Smallest effects were observed with a semipurified diet and with the unrefined diet which - contrary to other four unrefined diets - contained no fish.