Stable expression of rat cytochrome P450IA2 cDNA and hydroxylation of 17 beta-estradiol and 2-aminofluorene in V79 Chinese hamster cells

Predicting Drug Metabolism–dependent Toxicity for Humans with a Genetically Engineered Cell Battery

This paper covers the presentation of an invited lecture - the FRAME Annual Lecture - given in London on 8 November 2006. Investigating the metabolism of chemicals in general, and of drugs and pollutants in particular, is of key importance to understanding pharmacological and toxicological effects. Over more than 15 years, the genes encoding the enzymes involved, have been individually cloned and expressed after gene transfer into V79 Chinese hamster cells, yielding a collection of cell lines - the so called V79 Cell Battery. With this technology, it has become possible to study the relevant enzymes individually, thus avoiding complex in vivo situations. By cloning genes from different species, including humans, species-species comparison became possible, yielding results of immediate predictive value for humans. Since V79 cells had already been approved by the OECD for toxicity studies since the 1980s, the metabolically competent V79 cell lines are of even greater value, as metabolism and toxicity testing are linked in the very same cells in a highly defined fashion. The results obtained so far with the genetically engineered V79 cell lines justify their acceptance as alternatives to animal experimentation in drug development and in the toxicity testing of chemicals, serving the goals of the Three Rs and, in particular, the most important R: Replacement.

N-acetylation of three aromatic amine hair dye precursor molecules eliminates their genotoxic potential

N-acetylation has been described as a detoxification reaction for aromatic amines; however, there is only limited data available showing that this metabolic conversion step changes their genotoxicity potential. To extend this database, three aromatic amines, all widely used as precursors in oxidative hair dye formulations, were chosen for this study: p-phenylenediamine (PPD), 2,5-diaminotoluene (DAT) and 4-amino-2-hydroxytoluene (AHT). Aiming at a deeper mechanistic understanding of the interplay between activation and detoxification for this chemical class, we compared the genotoxicity profiles of the parent compounds with those of their N-acetylated metabolites. While PPD, DAT and AHT all show genotoxic potential in vitro, their N-acetylated metabolites completely lack genotoxic potential as shown in the Salmonella typhimurium reversion assay, micronucleus test with cultured human lymphocytes (AHT), chromosome aberration assay with V79 cells (DAT) and Comet assay performed with V79 cells. For the bifunctional aromatic amines studied (PPD and DAT), monoacetylation was sufficient to completely abolish their genotoxic potential. Detoxification through N-acetylation was further confirmed by comparing PPD, DAT and AHT in the Comet assay using standard V79 cells (N-acetyltransferase (NAT) deficient) and two NAT-proficient cell lines,V79NAT1*4 and HaCaT (human keratinocytes). Here we observed a clear shift of dose-response curves towards decreased genotoxicity of the parent aromatic amines in the NAT-proficient cells. These findings suggest that genotoxic effects will only be found at concentrations where the N-acetylation (detoxifying) capacity of the cells is overwhelmed, indicating that a 'first-pass' effect in skin could be taken into account for risk assessment of these topically applied aromatic amines. The findings also indicate that the use of liver S-9 preparations, which generally underestimate Phase II reactions, contributes to the generation of irrelevant positive results in standard genotoxicity tests for this chemical class.

On the species-specific biotransformation of dibenzo[a,l]pyrene

We were aimed at investigating the activation of the carcinogenic polycyclic aromatic hydrocarbon (PAH) dibenzo[a,l]pyrene (DB[a,l]P) in Chinese hamster V79 cells that express single human, rat or fish cytochrome P450 (CYP) enzymes. DB[a,l]P is detectable in environmental samples and has been characterized as the most potent carcinogenic species among all PAHs as yet tested in rodent bioassays. Metabolite profiles and metabolite-dependent cytotoxic and clastogenic activities were monitored. The total turnover of CYP-mediated transformation of DB[a,l]P was as follows: human CYP1B1>fish CYP1A1 approximately human CYP1A1>>rat CYP1A2>rat CYP1A1. By contrast, enzyme forms that are not classified as being members of family CYP1, such as CYP2A6, 2E1, 2B1, and 3A4, failed to catalyze any detectable conversion of this substrate. All CYP1A1 enzymes tested formed both the K-region trans-8,9- and the trans-11,12-dihydrodiol, whereas human CYP1B1 failed to catalyze K-region activation. In cells expressing human or fish CYP1A1, human CYP1B1, and rat CYP1A2, the (-)-trans-11,12-dihydrodiol was formed enantiospecifically. DB[a,l]P-dependent cytotoxicities (EC(50)) were found in the following order: human CYP1A1 (12 nM)>fish CYP1A1 (30 nM)>human CYP1B1 (45 nM)>>other forms. In addition, an appreciable micronuclei formation was detected in human CYP1A1- and 1B1-expressing cells during exposure to DB[a,l]P. Our study demonstrates that human CYP1A1, 1B1 and fish CYP1A1 are able to transform DB[a,l]P into genotoxic derivatives in appreciable amounts. In contrast, CYP enzymes from rat predominantly target the K-region of DB[a,l]P and thus are serving more a rather protective route of biotransformation. Together our data suggest that humans might be more susceptible to DB[a,l]P-induced carcinogenicity than rats.

Induction of chromosomal aberrations by phenolic compounds: possible role of reactive oxygen species

Phenolic molecules are widely present in the environment and some of them are well known carcinogens. Some phenolic molecules are also genotoxic but the mechanisms involved in this process are not fully understood. We have studied the induction of chromosomal aberrations by phenol, catechol and pyrogallol in V79 cells at different pH values (6.0, 7.4 and 8.0). At the same pH values, the production of hydroxyl radicals was assessed by measuring the degradation of deoxyribose. Apart from phenol, which only induces a non-significant increase in chromosomal aberration in this experimental system, catechol and pyrogallol showed clear clastogenic effect in a pH-dependent way. Experiments carried out at pH 7.4 in the presence of S9 Mix, SOD, catalase and catalase + SOD suggest that the formation of reactive oxygen species is not the main mechanism involved in the genotoxicity of catechol. However, concerning pyrogallol, our results suggest that its genotoxicity is almost exclusively mediated by reactive oxygen species. Taken together, these results suggest that, in spite of the structural similarity between the different molecules studied, the mechanisms of genotoxicity of these molecules could be considerably different. The existence of several mechanisms of genotoxicity, partially shared by this class of compounds, could explain the synergistic effects observed between these compounds in several genotoxicity test systems. Accurate knowledge of their mechanisms of genotoxicity could improve considerably the assessment of their relevance to human health, since these compounds, once absorbed, are subject to a wide range of pH values in vivo.

Comparison of three different in vitro mutation assays used for the investigation of cytochrome P450-mediated mutagenicity of nitro-polycyclic aromatic hydrocarbons

Three different in vitro mutation assays were used to investigate the involvement of cytochrome P450 enzymes in the activation of the nitro-polycyclic aromatic hydrocarbons (nitroPAHs) 1-nitropyrene and 2-nitrofluorene and their reduced metabolites amino-polycyclic aromatic hydrocarbons (aminoPAHs) 1-aminopyrene and 2-aminofluorene. Mutagenicity was investigated at the HPRT locus in Chinese hamster V79 cells with (V79-NH) or without (V79-MZ) endogenous acetyltransferase activity, stably expressing human cytochrome P450 cDNAs; in NIH/3T3 control or stably expressing human CYP1A2 cells, in combination with a shuttle vector containing a reporter gene; and in Salmonella typhimurium TA98, by inhibition of cytochrome P450 enzymes in rat liver S9 mix. Both the HPRT assay and the Ames test did not show any involvement of CYP3A in the activation of 1-nitropyrene to a mutagenic metabolite. In addition, a clear involvement of CYP1A2 in the activation of the nitroPAH 1-nitropyrene was demonstrated in both mutation assays using eukaryotic cells. However, no activation of 1-nitropyrene was seen in the eukaryotic cell lines when expressing only CYP1A2 (V79-MZ1A2) or acetyltransferase (V79-NH, 3T3-LNCX). The reduced metabolite of 1-nitropyrene, 1-aminopyrene, was also shown to be activated to a mutagenic metabolite by CYP1A2, using 3T3-1A2 cells in combination with a shuttle vector, and the Amestest in combination with the specific CYP1A2 inhibitor furafylline. No clear involvement of cytochrome P450 could be demonstrated for activation of 2-nitrofluorene to a mutagenic metabolite, whereas a role for CYP1A2 in the bioactivation of 2-aminofluorene is suggested. In the present study, we have demonstrated the complementary value of the three in vitro mutation assays in the examination of promutagen activation pathways.

Genetically engineered cells stably expressing cytochrome P450 and their application to mutagen assays

Genetically engineered cells transiently and stably expressing cytochrome P450 (P450), a key enzyme for biotransformation of a wide variety of compounds, have provided new tools for investigation of P450 functions such as P450-mediated metabolic activation of chemicals. This review will focus on the development of mammalian cell lines stably expressing P450s and application to toxicology testings. Stable expression systems have an advantage over transient ones in that a series of the process from metabolic activation of test compounds to the appearance of toxicological consequences occurs entirely in the same intact cells. Indeed, many cell lines stably expressing a single form of mammalian P450 have been established so far and applied to cytotoxic or genotoxic assays, the endpoints of which contained mutations at hprt and other gene loci, chromosomal aberrations, sister chromatid exchanges, micronuclei, morphological transformation, and 32P-postlabeling. Analyses of metabolites of toxic substances have also been carried out, using the intact cells or microsomal fractions prepared from the cells. The stable expression systems clearly indicate the form of P450 enzyme capable of activating a certain chemical. More recently, coexpression of P450 together with other components of microsomal electron transfer systems such as NADPH-cytochrome P450 reductase has been successfully performed to increase the metabolic capacity of the heterologously expressed P450. In addition, to reconstruct the entire metabolic activation system for certain heterocyclic amines, cell lines which simultaneously express a form of human P450 and a phase II enzyme, N-acetyltransferase, were established. These cells were highly sensitive to some carcinogenic heterocyclic amines. In genetic toxicology, such a coexpression system for two or more enzymes will provide useful materials which mimic in vivo activation systems.

Regio- and stereoselectivity in the metabolism of benzo[c]phenanthrene mediated by genetically engineered V79 Chinese hamster cells expressing rat and human cytochromes P450

Regio- and stereoselective metabolism mediated by cytochrome P450 (CYP) and metabolite-dependent cytotoxicity of benzo[c]phenanthrene (B[c]Ph) and its trans-3,4-dihydrodiol, the metabolic precursor of the carcinogenic fjord-region B[c]Ph-3,4-dihydrodiol 1,2-epoxides (B[c]PhDE), were investigated with V79 Chinese hamster cells genetically engineered for three rat and six human CYP isoforms. The order of the capabilities of the CYP isoforms to metabolize B[c]Ph was as follows: h1A1>r1A1>r1A2>h1B1>h1A2>r2B1>>h2E1>h2A6>h3A4. Regardless of the species, all individual CYP isoforms preferentially catalyzed the oxidation of B[c]Ph at the 5,6-position (K-region) except human CYP1A1 and human CYP1A2, which oxidized both the 5,6- and the 3,4-position with similar efficiency. While human CYP1A1, rat CYP1A1 and rat CYP1A2 formed almost exclusively the (-)-B[c]Ph-3R,4R-dihydrodiol, human CYP1A2 produced both the (-)-3R,4R- and the (+)-3S,4S-dihydrodiol enantiomers in a ratio of 2:1. Stereoselective activation of B[c]Ph, the (±)-B[c]Ph-3,4-dihydrodiol and its (-)-3R,4R-enantiomer to the fjord-region (-)-anti-B[c]PhDE occurred upon incubation with rat CYP1A1 and rat CYP1A2 as indicated by the formation of two stereoisomeric tetraols, the hydrolysis products of the labile anti-B[c]PhDE. The formation of tetraols in the culture medium was accompanied by a concentration-dependent increase in cytotoxicity indicating that this effect was mediated by the fjord-region (-)-anti-B[c]PhDE formed as reactive intermediate. All human and rat CYP-expressing V79 cell lines investigated did not show any significant capacity to metabolize the (+)-3S,4S-dihydrodiol. The present study indicates that the human CYP isoforms 1A1 and 1B1 have complementary catalytic properties to activate B[c]Ph to its fjord-region B[c]PhDE, whereas other human isoforms play a minor role. Activation of B[c]Ph by human CYP1A1 and 1B1 is less efficient than by rat CYP1A1 or rat CYP1A2, but proceeds with similar stereoselectivity via the (-)-3R,4R-dihydrodiol to the strong carcinogen (-)-anti-B[c]PhDE with (R,S,S,R)-configuration.

Sulfotransferase-mediated activation of mutagens studied using heterologous expression systems

Sulfation is a common final step in the biotransformation of xenobiotics and is traditionally associated with inactivation. However, the sulfate group is electron-withdrawing and may be cleaved off heterolytically in some molecules leading to electrophilic cations which may form adducts with DNA and other important cellular structures. Since endogenous sulfotransferases do not appear to be expressed in indicator cells of standard mutagenicity tests, rat and human sulfotransferases have been stably expressed in his- Salmonella typhimurium strain TA1538 and Chinese hamster V79 cells. Using these recombinant indicator cells, sulfotransferase-dependent genotoxic activities were detected with N-hydroxy-2-acetylaminofluorene, 2-acetylaminofluorene (in the presence of co-expressed rat cytochrome P450 1A2), hycanthone, 1'-hydroxysafrole, alpha-hydroxytamoxifen and various benzylic alcohols derived from polycyclic aromatic hydrocarbons. In several cases, it was critical that the reactive sulfuric acid conjugates were formed directly within the indicator cells, owing to the inefficient penetration of cell membranes. In other cases, spontaneous benzylic substitution reactions with medium components, such as halogenide ions or amino acids, led to secondary, membrane-penetrating reactive species. Different sulfotransferases, including related forms from rat and human, substantially differed in their substrate specificity towards the investigated promutagens. It is known that some sulfotransferases are expressed with high tissue and cell type specificities. This site-dependent expression together with the limitations in the distribution of reactive sulfuric acid conjugates may explain organotropic effects of compounds activated by this metabolic pathway.

Development of a V79 cell line expressing human cytochrome P450 2D6 and its application as a metabolic screening tool

Expression of human cytochrome P450 (CYP) in heterologous cells is a means of specifically studying the role of these enzymes in drug metabolism. The complete cDNA encoding CYP2D6-VAL(374) was inserted into an expression vector containing the strong mycloproliferative sarcoma virus promotor in combination with the enhancer of the cytomegalovirus and stably expressed in V79 Chinese hamster cells. The presence of genomically integrated CYP2D6 cDNA was confirmed by polymerase chain reaction analysis. The protein expression was shown by Western blotting. Functional expression could be demonstrated by O-demethylation of dextromethorphan to dextrorphan in live cells. The enzymatic activity of 154 ± 16 pmol min(-1) mg(-1) protein was comparable with dextromethorphan-O-demethylation activities of human liver. The metabolism of two dopaminergic ergoline derivatives was investigated in whole recombinant V19 cells. Both lisuride and terguride were monodeethylated; in case of lisuride a correlation to the in vivo situation was demonstrated comparing poor and extensive metabolizers.

Noninterference of cytochrome P4501A2 in the cytotoxicity of tacrine using genetically engineered V79 Chinese hamster cells for stable expression of the human or rat isoform and two human hepatocyte cell lines

Tacrine (THA) is the only drug currently approved for the treatment of Alzheimer's disease. A common side effect of this drug in humans is major hepatotoxicity. THA-induced toxicity may be related to a metabolic pathway implicating cytochrome P450 1A2 (CYP1A2). The purpose of this study was to clarify the role of the metabolic conversion of THA by CYP1A2 in the cytotoxicity of THA. The cytotoxicity of THA was evaluated in two human hepatocyte cell lines, HepG2 and Chang liver, and on the V79 Chinese hamster cell line, which does not express cytochrome P450 activity, and its variants, genetically engineered for expression of human or rat CYP1A2. Cells expressing human CYP1A2 metabolized THA to form its 1-OH derivative (Vmax = 9.36 +/- 0.57 pmol min(-1) mg(-1) total protein), whereas no metabolism was observed with the nonexpressing parental cells. In all cell lines, THA induced a marked decrease in cell viability and a strong inhibition of RNA and protein synthesis. However, these cytotoxic effects did not differ in parental V79 cells and variant cells expressing human or rat CYP1A2. The IC50 were tenfold higher for cell viability than for RNA and protein inhibition after 3 hr of incubation but were similar after 24 hr (P < 0.0001), indicating that this early inhibition was not a transient effect and could lead to cell death. These results strongly suggest that THA-induced cytotoxicity is not mediated by CYP1A2.

Development and validation of alternative metabolic systems for mutagenicity testing in short-term assays

We present here the results obtained within the framework of an EU funded project aimed to develop and validate alternative metabolic activating systems to be used in short-term mutagenicity assays, in order to reduce the use of laboratory animals for toxicology testing. The activating systems studied were established cell lines (Hep G2, CHEL), genetically engineered V79 cell lines expressing specific rat cytochromes P450, erythrocyte-derived systems, CYP-mimetic chemical systems and plant homogenates. The metabolically competent cell lines were used as indicator cells for genotoxic effects as well as for the preparation of external activating systems using other indicator cells. The following endpoints were used: micronuclei, chromosomal aberrations and sister chromatid exchanges, mutations at the hprt locus, gene mutations in bacteria (Ames test), unscheduled DNA synthesis and DNA breaks detected in the comet assay. All metabolic systems employed activated some promutagens. With some of them, promutagens belonging to many different classes of chemicals were activated to genotoxicants, including carcinogens negative in liver S9-mediated assays. In other cases, the use of the new activating systems allowed the detection of mutagens at much lower substrate concentrations than in liver S9-mediated assays. Therefore, the alternative metabolizing systems, which do not require the use of laboratory animals, have a substantial potential in in vitro toxicology, in the basic genotoxicity testing as well as in the elucidation of activation mechanisms. However, since the data basis is much smaller for the new systems than for the activating systems produced from subcellular liver preparations, the overlapping use of both systems is recommended for the present and near future. For example, liver S9 preparations may be used with some indicator systems (e.g., bacterial mutagenicity), and metabolically competent mammalian cell lines may be used with other indicator systems (e.g., a cytogenetic endpoint) in a battery of basic tests.

Cytochrome P450-mediated activation of phenanthrene in genetically engineered V79 Chinese hamster cells

V79 Chinese hamster cells genetically engineered for rat cytochromes P450 1A1, 1A2, 2B1 and human cytochromes P450 1A1, 1A2, 2A6, 2E1, and 3A4 are being applied in metabolism studies on polycyclic aromatic hydrocarbons. This study presents the results on phenanthrene as the prototypic polycyclic aromatic hydrocarbon possessing a bay region. Phenanthrene is of less importance regarding cytotoxicity and carcinogenicity as compared to e.g. benzo[a]pyrene or 7,12-dimethylbenz[a]anthracene. However, phenanthrene is more readily converted to metabolites which are exreted in higher amounts than those from any other polycyclic aromatic hydrocarbon. Therefore, its metabolites are of diagnostic value in epidemiological and occupational exposure studies. For this reason, it is worthwhile to understand the metabolism of phenanthrene in detail, e.g. allocating metabolites and cytochromes P450s. In accordance to previous observations cytochromes P450 1A1 and 1A2 were the most active forms towards phenanthrene. However, metabolite profiles differed between rat and human homologues of cytochromes P450, in particular for cytochrome P450 1A2. The predominant metabolite formed by rat cytochrome P450 1A2 was the K region trans-9,10-dihydrodiol, whereas human cytochrome P450 1A2 produced similar amounts of the trans-1,2-, trans-3,4- and trans-9,10-dihydrodiol. High amounts of trans-1,2-dihydrodiol, the metabolic precursor of the bay-region dihydrodiol epoxide, were also formed by human cytochrome P450 1A1 compared to its rat homologue. Unexpectedly, human cytochrome P450 2E1 showed a remarkable catalytic activity to metabolize phenanthrene to its trans-9,10-dihydrodiol. Utilizing recombinant CYPs in live V79 cells appears to be a valuable too yielding results important for the evaluation of exposure data and risk assessment for humans.

Genetically engineered mammalian cells and applications

In general, cells genetically engineered for stable and defined expression of xenobiotic-metabolizing enzymes are useful tools whenever a metabolism-related problem in toxicology and pharmacology is to be solved. It is the genetic and phenotypic nature of a given cell that determines its applicability. Mammalian cells have useful characteristics not given in bacterial, yeast or insect cells, which also may express xenobiotic-metabolizing enzymes. It is the problem to be solved and the question to be answered which determine the optimal choice for the best-suited expression system. There may even be subtle differences between mammalian cells of different species and organ origin, which might play a role in choosing a mammalian expression system. Thus, the level and specificity of the xenobiotic-metabolizing enzyme, the experimental testing conditions, and the biological endpoints present in a chosen cell are the most important criteria to be observed in the application of the mammalian expression systems.

Stable expression of human cytochrome P450 3A4 in V79 cells and its application for metabolic profiling of ergot derivatives

Expression of human cytochrome (CYP) in heterologous cells is a means of specifically studying the role of these enzymes in drug metabolism. The complete cDNA encoding CYP3A4 (PCN1) was inserted into an expression vector containing the strong myeloproliferative sarcoma virus promoter in combination with the enhancer of the cytomegalovirus and stably expressed in V79 Chinese hamster cells. The presence of genomically integrated CYP3A4 cDNA cell clones was confirmed by polymerase chain reaction analysis. Transcription was detected by reverse transcribed polymerase chain reaction analysis. Functional expression could be demonstrated by conversion of testosterone to the specific 6beta-hydroxylated product. In recombinant V79 cells expressing CYP3A4 about 6% of the substrate was converted to 6beta-hydroxytestosterone. The metabolism of two dopaminergic ergot derivatives was investigated in live recombinant V79 cells. Both lisuride and terguride were monodeethylated.

Genotoxicity assessment of aromatic amines and amides in genetically engineered V79 cells

A genetically engineered V79 cell line expressing rat CYP1A2 and another cell line expressing rat CYP1A2 as well as endogenous acetyltransferase activity, as well as CYP-deficient parental V79 cell lines, were used to assess the genotoxicity of the aromatic amines and amides 2-aminoanthracene, 2-aminofluorene, 2-acetylaminofluorene, 4-acetylaminofluorene and 2-amino-3-methylimidazo[4,5-f]quinoline, with chromosomal aberrations and sister chromatid exchanges as the end-points. None of the test compounds showed a clear effect on the frequency of chromosomal aberrations in any cell line used. Sister chromatid exchanges, however, were induced by 2-aminoanthracene, 2-aminofluorene and 2-acetylaminofluorene in the CYP1A2-proficient cells, but not in the CYP1A2-deficient cells. The presence of acetyltransferase activity enhanced the effect of 2-aminoanthracene, 2-aminofluorene and 2-acetylaminofluorene. 4-Acetylaminofluorene and 2-amino-3-methylimidazo[4,5-f]quinoline did not induce sister chromatid exchanges in the investigated cell lines. The use of cell lines with defined metabolic capabilities seems to be a valuable tool to study specific metabolic pathways important in the activation of procarcinogens.

Inhibition of cytochromes P4501A by nitric oxide

Inflammatory stimulation of the liver leads to the induction of nitric oxide (NO) biosynthesis. Because NO binds to the catalytic heme moiety of cytochromes P450 (CYPs), we investigated whether NO interferes with specific CYP-dependent metabolic pathways. In a first experimental approach V79 Chinese hamster cells genetically engineered for stable expression of rat and human CYP1A1 and -1A2 were used. Incubation with the NO donors sodium nitroprusside and S-nitrosylacetylpenicillamine led to a concentration-dependent inhibition of all four CYP enzymes. CYP1A1 was more sensitive to the inhibitory effect of NO than CYP1A2. In the second part of the study, endogenous NO synthesis was induced in rat hepatocytes by incubation with a mixture of cytokines and endotoxin. Concurrently, as NO production in hepatocytes increased within 24 hr, a decrease in CYP1A1-dependent benzo[a]pyrene turnover was observed to almost undetectable levels. The competitive inhibitor of NO synthesis, NG-monomethyl-L-arginine, was able to significantly restore CYP1A1 activity in the presence of cytokines and endotoxin. Inhibition of hepatocellular CYP activity by NO was predominantly due to a direct effect on the enzymes. However, NO-dependent inhibition of CYP expression at a transcriptional level was also demonstrated. Our results indicate that inhibition of NO biosynthesis in patients suffering from systemic inflammatory response syndromes may help to restore biotransformation capacity of the liver.

V79 Chinese hamster cells genetically engineered for cytochrome P450 and their use in mutagenicity and metabolism studies

V79 Chinese hamster cells are being genetically engineered for stable expression of cytochromes P450 using a SV40 early promoter containing plasmid as the eukaryotic expression vector. V79 cells lack endogenous cytochrome P450 activity. Therefore, genetically engineered V79 cell lines are defined for the cytochrome P450 isoform acquired upon cDNA mediated gene transfer. V79 cells have a longstanding tradition as indicator cells for various biological endpoints, e.g. mutation, chromosomal aberration, cytotoxicity. The genetically engineered V79 cells combine these useful biological endpoints with metabolic competence. In this sense, V79 cell lines genetically engineered for stable expression of cytochromes P450 present newly developed tools for studying and understanding metabolism related problems in toxicology and pharmacology.

A comparative study of the use of primary Chinese hamster liver cultures and genetically engineered immortal V79 Chinese hamster cell lines expressing rat liver CYP1A1, 1A2 and 2B1 cDNAs in micronucleus assays

Liver microsome preparations (S9 mix) have been extensively used for in vitro genotoxicity studies to provide the capacity for the activation of indirect genotoxins. However, the use of S9 preparations with mammalian cell cultures has raised considerable toxicity problems which limit their use to exposure times which are only a small fraction of the cell cycle. In addition, false negative results may be obtained if reactive metabolites are unable to penetrate the cell membrane or have short half-lives. The generation and detection of a promutagen within a single cell would therefore be advantageous. To this end, we have studied the bioactivation of a panel of promutagens (benzo[a]pyrene, cyclophosphamide, 2-aminoanthracene and sterigmatocystin) in low passage Chinese hamster fibroblasts of hepatic origin (LiC2 cells) and in a series of V79 Chinese hamster cell lines genetically engineered to express rat liver cytochrome P450 cDNAs. These include strains XEM2 (expresses CYP1A1), SD1 (CYP2B1) and strains XEMd-MZ and XEMd-NH which express CYP1A2. The end point selected for study was the induction of micronuclei. The protocol incorporated a cytochalasin B-induced cytokinesis block and the enumeration of micronuclei in the resulting binucleate cells which have undergone one nuclear division following the induction of chromosome damage. Micronuclei containing whole chromosomes and chromosome fragments were distinguished by the use of CREST antibody specific for kinetochore protein as a measure for the presence of centromeres. Micronuclei were induced by the test agents in low passage liver fibroblasts and in immortal V79 cultures only in the presence of Aroclor-induced S9 preparations. The data obtained from micronucleus assays of the genetically engineered V79 cell lines demonstrated the utility of each strain for the optimal detection and quantification of the activity of the individual test compounds. Kinetochore antibody demonstrated differences in the kinetics of induction of micronuclei containing chromosome fragments and whole chromosomes with chemicals such as benzo[a]pyrene. As part of this cytogenetic study, we also conducted karyotypic analyses and spindle fidelity assays of the V79 cell lines to investigate the presence of chromosomal instabilities which may arise as a consequence of the genetic engineering procedure. Such studies represent an important quality control step in the validation of the suitability of each cell line prior to their use in genotoxicity studies.

Biotransformation of methylxanthines in mammalian cell lines genetically engineered for expression of single cytochrome P450 isoforms. Allocation of metabolic pathways to isoforms and inhibitory effects of quinolones

V79 Chinese hamster cells genetically engineered for stable expression of single forms of rat cytochromes P450IA1, P450IA2, P450IIB1, human P450IA2, and rat liver epithelial cells expressing murine P450IA2 were used to allocate metabolic pathways of methylxanthines to specific isoforms and to test the suitability of such cell lines for investigations on drug interactions occurring at the cytochrome expressed. The cell lines were exposed to caffeine and/or theophylline and concentrations of metabolites formed in the medium were determined by HPLC. Caffeine was metabolized by human, rat and murine P450IA2, resulting in the formation of four primary demethylated and hydroxylated metabolites. However, there were differences in the relative amounts of the metabolites. The human and the mouse P450IA2 isoforms predominantly mediated 3-demethylation of caffeine. The rat cytochrome P450IA2 mediated both 3-demethylation and 1-demethylation of caffeine to a similar extent. The results support the hypothesis that caffeine plasma clearance is a specific in vivo probe for determining human P450IA2 activity. Addition of the quinolone antibiotic agents pipemidic acid or pefloxacin, both known to inhibit caffeine metabolism in vivo and in human liver microsomes, reduced formation rates of all metabolites of caffeine in cells expressing rat and human P450IA2. Theophylline was mainly metabolized via 8-hydroxylation. All cell lines tested were able to carry out this reaction, with highest activities in cell lines expressing rat or human P450IA2, or rat P450IA1.

Stable expression of human cytochrome P450 1A1 cDNA in V79 Chinese hamster cells and metabolic activation of benzo[a]pyrene

A V79 Chinese hamster cell line stably expressing human cytochrome P450 1A1 (CYP1A1) was obtained by chromosomal integration of the human CYP1A1 cDNA under the control of the SV40 early promoter. Chromosomal integration was verified by Southern analysis, and effective transcription of the human CYP1A1 cDNA was demonstrated by Northern analysis. The CYP1A1 cDNA-encoded protein was characterized by Western analysis using anti-rat CYP1A1. Intracellular association of CYP1A1 with the endoplasmic reticulum could be visualized by in situ immunofluorescence. Crude cell lysates of the V79 derived cell line was able to catalyze 7-ethoxyresorufin-O-deethylation (EROD) with an activity of about 50 pmol min-1 mg-1 total protein, and an aryl hydrocarbon hydroxylase activity (AHH) of 25 pmol min-1 mg-1. CYP1A1 dependent cytotoxicity, measured by neutral red uptake, and genotoxicity, determined by the frequency of micronucleus formation, of benzo[a]pyrene (B[a]P) and trans-7,8-dihydroxy-7,8-dihydrobenzo[a]pyrene (B[a]P-7,8-diol) could be demonstrated at substrate concentrations as low as 10 nM. Thus, this cell line presents a sensitive tool for studying CYP1A1 mediated metabolism of polycyclic aromatic hydrocarbons (PAH). B[a]P and the purified (+)- and (-)-enantiomers of B[a]P-7,8-diol were compared for their mutagenicity. The (-)-enantiomer was found to be 3-5-fold more mutagenic than the (+)-enantiomer.

Cytogenetic effects of promutagens in genetically engineered V79 Chinese hamster cells expressing cytochromes P450

V79 Chinese hamster cell lines genetically engineered to express rat CYP2B1, CYP1A1, CYP1A2, and their parental cell lines V79-MZ, without acetyltransferase, and V79-NH, with acetyltransferase, were studied for chromosome aberrations and sister chromatid exchange induced by aflatoxin B1, cyclophosphamide, benzo[a]pyrene, 7,12-dimethylbenz[a]anthracene and dimethylnitrosamine. The parental V79 cell lines did not show clastogenic effects. Significant clastogenic effects were observed after an 18 h exposure to aflatoxin B1 and cyclophosphamide in CYP2B1 expressing cells, to benzo[a]pyrene in CYP1A1 and CYP1A2 expressing cells, to 7,12-dimethylbenz[a]anthracene and dimethylnitrosamine in cells, expressing CYP1A2 with or without acetyltransferase, and to cyclophosphamide in cells expressing both CYP1A2 and acetyltransferase. A significant sister chromatid exchange inducing effect was found after a 24 h exposure in each of the genetically engineered cell lines, except for benzo[a]pyrene and 7,12-dimethylbenz[a]anthracene in CYP2B1 expressing cells, and for benzo[a]pyrene in cells expressing both CYP1A2 and acetyltransferase. Thus, a battery of cell lines genetically engineered for metabolic competence may serve as a tool for investigating chromosomal changes induced by activated xenobiotics.

Metabolism of phenacetin in V79 Chinese hamster cell cultures expressing rat liver cytochrome P4501A2 compared to isolated rat hepatocytes

In a genetically engineered V79 cell line (XEMd-MZ) expressing rat cytochrome P4501A2 the activity of phenacetin-O-deethylase was determined and compared with freshly isolated rat hepatocytes. In the V79 cells the apparent Km was 0.99 microM (N = 4), compared to the high affinity Km (0.23 microM, N = 4) found in freshly isolated rat hepatocytes, where as the Vmax found in the XEMd-MZ cells (14.90 pmol/min/10(6) cells) corresponded to the Vmax for the high affinity site of the hepatocytes (18.2 pmol/min/10(-6) cells). The metabolic capacity of phenacetin was quantitatively comparable to that of freshly isolated hepatocytes on a cell to cell basis.

On the bioactivation and genotoxic action of fluoranthene

Fluoranthene (FA) was studied with respect to possible mechanisms of its high mutagenicity but low carcinogenicity, in comparison with the corresponding properties of benzo[a]pyrene (BaP), and with regard to the synergism of these two compounds shown by van Duuren and Goldschmidt (J Natl Cancer Inst 56, 1976, 1237). FA and BaP activated by S9 from Aroclor 1254 (PCB)-treated rats induce HPRT mutations in CHO cells with about equal effectiveness at the same exposure doses, which also lead to the same frequencies of repairable DNA adducts, enzyme-induced strand breaks being used as an indirect measure of adducts to DNA. FA was also shown to be an efficient inducer of SCE in human peripheral lymphocytes cocultivated with PCB-treated HepG2 cells or with liver cells from PCB-pretreated rats. For the induction of SCE, FA and BaP were shown to act additively. From metabolic studies with liver microsomes from C57Bl/6 mice it is concluded that, whereas BaP induces the metabolism of BaP to the mutagenic epoxide, neither BaP nor FA is able to induce the metabolism of FA. In mutation experiments with V79 cells (XEM2) constitutive for P450 IA1 activity, BaP 7,8-diol but not FA 2,3-diol provokes a high frequency of HPRT mutations. In cells constitutive for P450 IA2 enzymatic activity FA and BaP are but weakly mutagenic and practically nonmutagenic, respectively. Due to the additivity of the genotoxic effects of FA and BaP, induction of an error-prone condition by the latter compound seems to be excluded.(ABSTRACT TRUNCATED AT 250 WORDS)

Applications of stable V79-derived cell lines expressing rat cytochromes P4501A1, 1A2, and 2B1

1. Chinese hamster V79-derived cell lines, stably expressing cytochromes P4501A1, 1A2, and 2B1 activities, were constructed by genetic engineering in continuation of our work to establish a battery of V79 derived cell lines designed to study the metabolism of xenobiotics. 2. Cell lines XEM1 and XEM2, expressing cytochrome P4501A1, were capable of the O-dealkylation of 7-ethoxycoumarin and the hydroxylation of benzo[a]pyrene. 3. Cell lines XEMd.MZ and XEMd.NH, expressing P4501A2, were shown to hydroxylate 17 beta-estradiol and 2-aminofluorene. 4. Cell line SD1, expressing cytochrome P4502B1, was able to hydroxylate testosterone stereo- and regio-specifically at the 16 alpha and 16 beta positions. 5. Cell lines were validated in mutagenicity, cytotoxicity, and metabolism studies employing benzo[a]pyrene, trans-7,8-dihydroxy-7,8-dihydrobenzo[a]pyrene, cyclophosphamide, ifosfamide, and picene. 6. Construction of metabolically-competent V79-derived cell lines be recombinant DNA technology will be a fundamental improvement for the evaluation of the cytotoxic, genotoxic and pharmacological properties of a chemical.

Genetically engineered V79 Chinese hamster cells for stable expression of human cytochrome P450IA2

V79 Chinese hamster cells were genetically engineered for stable expression of human P450IA2. Full length cDNA, encoding human P450IA2, was inserted into an SV40 early promoter containing eukaryotic expression vector and cointroduced with the selection marker neomycin phosphotransferase (conferring resistance to the neomycin derivative G418) into V79 Chinese hamster cells. The recombinant expression vector was introduced into two different V79 sublines, one expressing an endogenous acetyltransferase (V79-NH), the other not (V79-MZ). The presence of human cytochrome CYP1A2 cDNA in the G418 resistant V79 cell clones was confirmed by Southern blotting. The transcription of the cDNA into mRNA was detected by Northern blotting and the translation into an authentic cytochrome P450IA2 protein was shown by Western blotting. The enzymatic activity in these cells was determined by the cytochrome P450IA2-dependent methoxy-, ethoxy-, benzoxy-, and pentoxyresorufin dealkylation activity.