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

In Vivo Genotoxicity and Toxicity Assessment of Sterigmatocystin Individually and in Mixture with Aflatoxin B1

Mycotoxins are natural food and feed contaminants produced by several molds. The primary mode of exposure in humans and animals is through mixtures. Aflatoxin B1 (AFB1) and sterigmatocystin (STER) are structurally related mycotoxins that share the same biosynthetic route. Few in vivo genotoxicity assays have been performed with STER. In the present genotoxicity study, Wistar rats were dosed orally with STER (20 mg/kg b.w.), AFB1 (0.25 mg/kg b.w.) or a mixture of both in an integrated micronucleus (bone marrow) and comet study (liver and kidney). STER was dosed at the highest feasible dose in corn oil. No increase in the percentage of micronuclei in bone marrow was observed at any condition. Slight DNA damage was detected in the livers of animals treated with AFB1 or the mixture (DNA strand breaks and Fpg (Formamidopyrimidine DNA glycosylase)-sensitive sites, respectively). Plasma, liver, and kidney samples were analyzed with LC-MS/MS demonstrating exposure to both mycotoxins. General toxicity parameters (organs absolute weight, biochemistry, and histopathology) were not altered either individually or in the mixture. The overall absence of individual genotoxicity did not allow us to set any type of interaction in the mixture. However, a possible toxicokinetic interaction was observed.

Following the adverse outcome pathway from micronucleus to cancer using H2B-eGFP transgenic healthy stem cells

In vitro assessment of genotoxicity as an early warning tool for carcinogenicity mainly relies on recording cytogenetic damages (micronuclei, nucleoplasmic bridges) in tumour-derived mammalian cell lines like V79 or CHO. The forecasting power of the corresponding standardised test is based on epidemiological evidence between micronuclei frequencies and cancer incidence. As an alternative to destructive staining of nuclear structures a fish stem cell line transgenic for a fusion protein of histone 2B (H2B) and enhanced green fluorescent protein (eGFP) was established. The cells are derived from koi carp brain (KCB) and distinguish from mammalian culturable cells by non-tumour-driven self-renewal. This technology enables the analysis of genotoxic- and malign downstream effects in situ in a combined approach. In proof-of concept-experiments, we used known carcinogens (4-Nitroquinoline 1-oxide, colchicine, diethylstilbestrol, ethyl methanesulfonate) and observed a significant increase in micronuclei (MNi) frequencies in a dose-dependent manner. The concentration ranges for MNi induction were comparable to human/mammalian cells (i.e. VH-16, CHL and HepG2). Cannabidiol caused the same specific cytogenetic damage pattern as observed in human cells, in particular nucleoplasmic bridges. Metabolic activation of aflatoxin B1 and cyclophosphamide could be demonstrated by pre-incubation of the test compounds using either conventional rat derived S9 mix as well as an in vitro generated biotechnological alternative product ewoS9R. The presented high throughput live H2B-eGFP imaging technology using non-transformed stem cells opens new perspectives in the field of in vitro toxicology. The technology offers experimental access to investigate the effects of carcinogens on cell cycle control, gene expression pattern and motility in the course of malign transformation. The new technology enables the definition of Adverse Outcome Pathways leading to malign cell transformation and contributes to the replacement of animal testing. Summary: Complementation of genotoxicity testing by addressing initiating events leading to malign transformation is suggested. A vertebrate cell model showing "healthy" stemness is recommended, in contrast to malign transformed cells used in toxicology/oncocology.

Induction of micronuclei by 7H-dibenzo[c,g]carbazole and its tissue specific derivatives in Chinese hamster V79MZh1A1 cells

The clastogenicity/aneugenicity of N-heterocyclic polycyclic aromatic pollutant 7H-dibenzo[c,g]carbazole (DBC) and its two synthetic derivatives N-methyl DBC (MeDBC) and 5,9-dimethyl DBC (diMeDBC) was evaluated in the genetically engineered Chinese hamster V79 cell line V79MZh1A1 with stable expression of human cytochrome P4501A1 and in the parental V79MZ cell line without any cytochrome P450 activity. While none of the three carbazoles changed significantly the level of micronuclei in the parental V79MZ cells, a variable, but statistically significant rise of micronucleus frequencies was assessed in V79MZh1A1 cells. DBC induced dose-dependent increase in the number of micronuclei at harvest times of 24 and 48h and MeDBC at sampling time of 48h in V79MZh1A1 cells in comparison to untreated cells, however, no significant time-dependent increase in micronucleus frequencies was found. The use of the antikinetochore immunostaining revealed that DBC and MeDBC induced approximately equal levels of both kinetochore positive (C+) and kinetochore negative (C-) micronuclei. DiMeDBC, a strict hepatocarcinogen, did not manifest any effect on micronucleus induction in V79MZh1A1 cells. These studies suggest that genetically engineered Chinese hamster V79 cell lines expressing individual CYP cDNAs are a useful in vitro model for evaluation the role of particular cytochromes P450 in biotransformation of DBC and its tissue and organ specific derivatives.

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.

Formation of micronuclei in incubated hen's eggs as a measure of genotoxicity

The formation of micronuclei (MN) is a widely used and accepted endpoint of genotoxicity testing. The micronucleus assay provides a simple and rapid indirect measure of the induction of structural or numerical chromosome aberrations. In this work we describe hen's eggs, incubated for 11 days, as ex vivo assay system for the detection of micronucleus formation in young erythrocytes (Hen's Egg Test for Micronucleus Induction, HET-MN). At this stage of development the chick embryo presents a high metabolic competency which allows an adequate activation of several types of promutagens, as previously reported by several authors. As all stages of maturing erythrocytes are present in the bloodstream of the chick embryo, we could conveniently use samples of peripheral blood for scoring micronuclei as well as for determining the ratio between mature and immature erythrocytes as a measure of an undisturbed erythropoiesis. The obtained blood smears were stained by a modified May-Gruenwald-Giemsa procedure and scored microscopically. The examinations were facilitated by using a semiautomatic image analysis system. We could demonstrate a strong increase of the micronucleus frequency after the administration of the promutagens diethylnitrosamine (DENA), 7,12-dimethyl-benz[a]anthracene (DMBA), cyclophosphamide (CP), ifosphamide (IF), mitomycin C (MMC), and the direct-acting mutagen methanesulfonic acid methyl ester (MMS) compared to the concomitant negative controls. CP was used to demonstrate a dose-response relation and the effect of using two different routes of application (air cell and albumen). Nuclear aberrations, other than MN, were demonstrated after application of high doses of CP or IF. Expanded exposure times revealed a similar effect. The HET-MN, as an ex vivo assay, is a simple, inexpensive, and rapid assay system for genotoxicity testing, positioned between pure in vitro and in vivo assays, strictly in line with animal protection regulations and ethical aspects.

A critical evaluation of centromeric labeling to distinguish micronuclei induced by chromosomal loss and breakage in vitro

The in vitro micronucleus assay in conjunction with CREST-staining and fluorescence in situ hybridization (FISH) with centromere-specific DNA probes is being increasingly utilized for the detection of clastogenic and aneuploidy-inducing agents. Although potentially powerful techniques, both methods have unique characteristics that can influence sample processing and the interpretation of results. In this article, the use of the CREST and the FISH modifications of the in vitro micronucleus assay have been used to characterize the origin of the micronuclei induced by cyclophosphamide, 4,4'-methylene-bis(2-chloroaniline), 4-nitroquinoline N-oxide and ionizing radiation in metabolically competent MCL-5 cells or a derived cell line lacking metabolic activation. Using these results and our previous experiences with these techniques, a detailed comparison including the strengths and limitations of each technique as well as potential problems in performing each assay and in analyzing the data is discussed. In spite of their limitations, our results to date indicate that CREST-staining as well as FISH with centromere-specific DNA probes can be used to accurately distinguish micronuclei formed from chromosome loss from those originating from chromosome breakage and that these techniques can be valuable complements to the in vitro micronucleus assay.

Peroxidase-dependent bioactivation and oxidation of DNA and protein in benzo[a]pyrene-initiated micronucleus formation

Micronucleus formation initiated by benzo[a]pyrene (B[a]P) and related xenobiotics is widely believed to reflect potential carcinogenic initiation, yet neither a dependence upon bioactivation nor the critical enzymes have been demonstrated. Using rat skin fibroblasts, protein oxidation (carbonyl formation) and content of prostaglandin H synthase (PHS) and cytochrome P4501A1 (CYP1A1) protein were determined by Western blot/immunodetection with enhanced chemiluminescence. DNA oxidation as 8-hydroxy-2'-deoxyguanosine formation was quantified using high-performance liquid chromatography with electrochemical detection. Fibroblast CYP1A1 activity assessed as ethoxyresorufin-O-deethylase was not detectable, and even CYP1A1 protein was measurable only after induction with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). However, TCDD additionally induced prostaglandin H synthase (PHS), which also was detectable constitutively. B[a]P 10 microM initiated the oxidation of DNA and protein, and the formation of micronuclei, all of which were enhanced over 2-fold by the dual CYP1A1/PHS inducer TCDD 10 nM, as well as by other PHS inducers, 12-O-tetradecanoylphorbol-13-acetate 1 microM and interleukin-1alpha 0.625 or 1.25 ng/ml, that do not induce CYP1A1 (p < .05). Conversely, B[a]P target oxidation and micronucleus formation were abolished by 1-aminobenzotriazole 1 mM (p < .05), which was a potent inhibitor of both peroxidases and P450. These results provide the first direct evidence that B[a]P-initiated micronucleus formation, like carcinogenic initiation, requires enzymatic bioactivation, and that peroxidase-dependent, reactive oxygen species-mediated oxidation of DNA, and possibly protein, constitutes a molecular mechanism of initiation in uninduced cells. Induction of either CYP1A1 or peroxidases such as PHS substantially enhances this genotoxic initiation, which may reflect cancer risk.

The Capacity of Murine Epithelial Cell Lines with Different Degrees of Malignant Transformation to Metabolise Benzo[a]pyrene

Cultures of three murine cell lines derived from fetal mouse liver (FL, FLT3 and FLT5) were treated with benzo[a]pyrene to evaluate their ability to activate promutagens. DNA adducts were detected in all three cell lines when they were treated with the major active metabolite of benzo[a]pyrene, benzo[a]pyrene diolepoxide. DNA adducts were also produced in FL and FLT3 cells when they were treated with the promutagen, benzo[a]pyrene, while treatment of FLT5 cells resulted in very few DNA adducts. In treated FL and FLT3 cells, the most evident adduct spot detected showed a chromatographic position similar to that of the main adduct formed in cells treated with (±)-r-7,t-8-dihydroxy-t-9,10-oxy-7,8,9,10-tetrahydro-benzo[a]pyrene.

Metabolic activation of benzo[a]pyrene in two fetal mouse hepatocyte lines: induction of DNA adducts and micronuclei

We have studied the metabolic competence of two non-transformed epithelial-like cell lines derived from fetal mouse liver, C 6 and C 2.8, to activate the promutagen benzo[a]pyrene by measuring both the induction of DNA adducts through the nuclease P1-enhanced 32P-postlabeling assay and the formation of micronuclei. The pattern and level of DNA adducts detected in C 6 and C 2.8 cells treated with benzo[a]pyrene were compared with those obtained in human peripheral blood lymphocytes treated with the same compound and with [3H]anti-benzo[a]pyrene diolepoxide. In both the cell lines and in human lymphocytes we observed a consistent induction of distinct DNA adducts. In C 6 and C 2.8 cells, the most evident adduct showed a position similar to that of the main adduct induced by [3H]-anti-benzo[a]pyrene diolepoxide in human lymphocytes. In addition, benzo[a]pyrene caused a significant increase of micronucleated C 6 and C 2.8 cells, whereas the frequency of micronuclei did not increase in CHO cells treated, for comparison, in the same way.

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.