Comparison of S9 mix and hepatocytes as external metabolizing systems in mammalian cell cultures: cytogenetic effects of 7,12-dimethylbenzanthracene and aflatoxin B1

The development and prevalidation of an in vitro mutagenicity assay based on MutaMouse primary hepatocytes, Part I: Isolation, structural, genetic, and biochemical characterization

To develop an improved in vitro mammalian cell gene mutation assay, it is imperative to address the known deficiencies associated with existing assays. Primary hepatocytes isolated from the MutaMouse are ideal for an in vitro gene mutation assay due to their metabolic competence, their "normal" karyotype (i.e., neither transformed nor immortalized), and the presence of the MutaMouse transgene for rapid and reliable mutation scoring. The cells were extensively characterized to confirm their utility. Freshly isolated cells were found to have a hepatocyte-like morphology, predominantly consisting of binucleated cells. These cells maintain hepatocyte-specific markers for up to 3 days in culture. Analyses revealed a normal murine hepatocyte karyotype with a modal ploidy number of 4n. Fluorescence in situ hybridization analysis confirmed the presence of the lambda shuttle vector on chromosome 3. The doubling time was determined to be 22.5 ± 3.3 h. Gene expression and enzymatic activity of key Phase I and Phase II metabolic enzymes were maintained for at least 8 and 24 h in culture, respectively. Exposure to β-naphthoflavone led to approximately 900- and 9-fold increases in Cyp1a1 and Cyp1a2 gene expression, respectively, and approximately twofold induction in cytochrome P450 (CYP) 1A1/1A2 activity. Exposure to phenobarbital resulted in an approximately twofold increase in CYP 2B6 enzyme activity. Following this characterization, it is evident that MutaMouse primary hepatocytes have considerable promise for in vitro mutagenicity assessment. The performance of these cells in an in vitro gene mutation assay is assessed in Part II. Environ. Mol. Mutagen. 60:331-347, 2019. © 2018 The Authors. Environmental and Molecular Mutagenesis published by Wiley Periodicals, Inc. on behalf of Environmental Mutagen Society.

Activation of P53 in HepG2 cells as surrogate to detect mutagens and promutagens in vitro

The current genotoxicity tests of the standard in vitro battery, especially those using mammalian cells, are limited by their low specificity and highlight the importance of new in vitro tools. This study aimed to evaluate the suitability of HepG2 cells for assaying mutagens and promutagens. We determined P53 activity as surrogate genotoxicity endpoint in HepG2 cells. Our results revealed a significant P53-induction by actinomycin D, methyl methanesulfonate and etoposide. Prior to the investigation of promutagens we characterized HepG2 cells by analyzing the expression of 45 genes involved in xenobiotic metabolism and measuring the activity of selected Cytochrome-P450 (CYP) enzymes. We determined a limited metabolic capacity prompting us to employ a co-treatment with rat liver S9 as metabolic activation system (MAS) for promutagens. While cyclophosphamide showed an elevation of activated P53 in the presence of S9, 7,12-dimethylbenz[a]anthracene and aflatoxin B(1) responded without the MAS. Inhibition of cellular CYP3A4 or CYP1A/1B suppressed the aflatoxin B(1)- and dimethylbenz[a]anthracene-mediated P53 response, respectively, indicating that HepG2 cells are capable of metabolizing these compounds in a CYP1A/B/3A4-dependent manner. In summary, our results indicate that P53 activation in HepG2 cells combined with a MAS can be used for the identification of new (pro)genotoxicants.

Cytogenetic evaluation of extractable agents from airborne particulate matter generated in the city of Catania (Italy)

In order to document cytogenetic damage associated with air pollution and, possibly, with health effects in the city of Catania, Sicily (Italy), we analyzed the induction of chromosomal aberrations by extractable agents from airborne particulate matter in a Chinese hamster epithelial liver (CHEL) cells. These cells retain their metabolic competence to activate different classes of promutagens/procarcinogens into biologically active metabolites. Airborne particulate matter was obtained from two stationary samplers (stations I and II) in two areas endowed by an elevated car transit in the centre of Catania. The results obtained clearly indicated that airborne particulate matter from both stations I and II proved to be clastogens in CHEL cells but not in Chinese hamster ovary (CHO) cells without metabolic activation, indicating that airborne particulate mixtures need to be metabolically converted before exerting their genotoxic potential. On the basis of these results we can assert that the test system employed to identify the cytogenetic potential of airborne particulate matter is useful and profitable for environmental control, and helpful to plan specific actions aimed at reducing the hazards derived from exposure to polluted air.

Inhibition of methotrexate-induced chromosomal damage by folinic acid in V79 cells

Methotrexate (MTX), an anticancer compound, is widely used in the treatment of leukemia. It induces cytogenetic damage as well as cytostatic effects on a variety of cell systems. Folinic acid (Leucovorin) is generally administered along with MTX as a rescue agent to decrease MTX-induced toxicity. However, information regarding the inhibitory effect of folinic acid against cytogenetic damage caused by MTX is limited. This study was conducted to assess the cytogenetic effect of MTX and its inhibition by folinic acid (FA) using the micronucleus and chromosomal aberration assays concurrently. Exponentially growing V79 cells were treated with MTX at five different concentrations (5-100 micrograms ml-1) with S9 microsomal fraction for 6 h and post-treated with two concentrations of FA (5 or 50 micrograms) for 40 h. Results indicate that MTX alone induced a concentration-related increase in % micronucleated binucleated cells (MNBN) and % aberrant cells (Abs). There was a decrease in nuclear division index (NDI) with increase in MTX concentration. Similarly, the mitotic index (MI) also decreased in all concentrations of MTX tested. The addition of FA at 50 micrograms ml-1 significantly reduced % MNBN (40-68%) and % Abs (36-77%). Inhibition was also seen at 5 micrograms FA (12 to 54% for MNBN and 20 to 61% for Abs). These results indicate that FA is capable of reducing the cytogenetic damage induced by MTX and appears to be an anticlastogenic agent.

An assessment of the in vitro hepatocyte micronucleus assay

The in vitro hepatocyte micronucleus assay was tested for its practicability and its usefulness in detecting mutagens. The assay protocol developed by Alati et al. (1989) was shown to give reproducible levels of proliferating hepatocytes and the formation of micronuclei could be readily assessed by fluorescence microscopy. Epidermal growth factor and insulin were used as mitogens, yielding mitotic indices of 2.4 +/- 0.74% after 72 h of culture. The high number of 8.0 +/- 3.33% micronucleated hepatocytes in control cultures at that time, typically for in vitro stimulated hepatocytes, is probably due to disordered mitoses frequently leading to chromosome loss. The direct acting mutagen N-methyl-N'-nitro-N-nitrosoguanidine and the clastogens cyclophosphamide and retrorsine, which require metabolic activation, induced dose dependent increases in the frequencies of micronucleated hepatocytes. The carcinogen 2-AAF also yielded significantly enhanced rates of micronuclei. The non-mutagen KCl as well as the peroxisome proliferator clofibrate, which is considered to be a non-genotoxic hepatocarcinogen, yielded consistently negative results. Problems occurred when chemicals exerting strong cytotoxic effects were tested in this assay. The mutagen and hepatocarcinogen aflatoxin B1 did not enhance the number of micronucleated hepatocytes. Rather a reduction of micronuclei and of mitoses was observed at AFB1 concentrations considered positive in other genotoxicity assays. Hepatocyte proliferation seems to be highly susceptible to the cytotoxic action of chemicals. A decrease in the proliferating activity of hepatocytes can obviously prevent the detection of mutagenic effects. Further studies on the in vitro hepatocyte micronucleus assay are necessary to clarify its role in mutagenicity testing.

Sister-chromatid exchange: second report of the Gene-Tox Program

This paper reviews the ability of a number of chemicals to induce sister-chromatid exchanges (SCEs). The SCE data for animal cells in vivo and in vitro, and human cells in vitro are presented in 6 tables according to their relative effectiveness. A seventh table summarizes what is known about the effects of specific chemicals on SCEs for humans exposed in vivo. The data support the concept that SCEs provide a useful indication of exposure, although the mechanism and biological significance of SCE formation still remain to be elucidated.

Evaluation of genotoxicity of tert.-butylhydroquinone in an hepatocyte-mediated assay with V79 Chinese hamster lung cells and in strain D7 of Saccharomyces cerevisiae

tert.-Butylhydroquinone (TBHQ) has been reported to be genotoxic in some short-term assays but non-genotoxic in others. We have examined cytotoxicity and genotoxicity of TBHQ, a principal metabolite of the phenolic antioxidant 2(3)-tert.-butyl-4-hydroxyanisole (BHA), in an hepatocyte-mediated assay with V79 Chinese hamster lung cells including both sister-chromatid exchange (SCE) and thioguanine-resistance (TGR) endpoints. The ability of BHA and of TBHQ to elicit a genotoxic response in Saccharomyces cerevisiae strain D7 was also investigated. In V79 cytotoxicity tests, TBHQ without hepatocytes produced a 50% reduction in colony formation at 4.2 micrograms/ml and was lethal to 100% of the cells at concentrations above 5 micrograms/ml. At partially cytotoxic dose levels, (0.17-3.4 micrograms/ml of medium), TBHQ sometimes increased significantly the frequency of SCE. TBHQ also produced sporadic statistically significant increases in the mutation frequency at the HGPRTase (TGR) gene locus when tested alone or with activation by rat or hamster hepatocytes. Mitotic gene conversion and reverse mutation were not induced in strain D7 of Saccharomyces cerevisiae by exposure to BHA or to TBHQ for 4 h at concentrations as high as 200 micrograms/ml for BHA or 500 micrograms/ml for TBHQ, either alone or with activation by rat-liver S9. Incubation of the yeast cells with BHA or TBHQ for 24 h in growth medium without activation also did not induce genotoxic activity. The slight and sporadic response to TBHQ in the V79 test system may indicate weak genotoxicity which is sensitive to slight differences in test conditions. The classification and test strategies adopted for compounds such as TBHQ could have important implications for regulatory decisions and for the validation of short-term tests.

The clastogenic potential in vitro of pyrrolizidine alkaloids employing hepatocyte metabolism

Three pyrrolizidine alkaloids (PAs), monocrotaline, retrorsine and isatidine, were tested for their clastogenic activity under different conditions of metabolic activation in vitro. All three compounds exhibited a weak activity when V79 cells were treated at very high concentrations for 18 h in the absence of a metabolizing system. Short-term (2 h) treatment with rat liver S9 mix led to a strong and concentration-dependent increase in chromosomal aberrations for retrorsine. Isatidine was not mutagenic with S9 mix and monocrotaline was positive at high concentrations only. In contrast, a prolonged treatment (18 h) in vitro under activation conditions in the presence of primary hepatocytes led to clear concentration-dependent positive responses for all three PAs investigated. Particularly the results with isatidine demonstrate that in vitro tests using S9 mix for metabolization can generate misleading results. It is not clear whether the results could be attributed to a better activation of the test compounds by intact hepatocytes in comparison to S9 mix or if the fact that only hepatocytes allow a treatment for the whole culture period under activation conditions was more important. Owing to its strong cytotoxicity the exposure to S9 mix is generally limited to 2-4 h, limiting also the exposure of the target cells to a test chemical as well as its metabolites. The results presented show significant differences in mutagenic potency of PAs due to variations in the activation system. This underlines the usefulness of primary hepatocytes, e.g., for the detection of pre-mutagens. The PAs investigated are present in plants which are used for phytotherapeutic medicinal products. They do not contribute to their efficacy and are, therefore, not to be tolerated in amounts that may impose a risk for the user.

Application of an epithelial liver cell line, metabolically competent, for mutation studies of promutagens

Recently numerous attempts have been made to reduce the use of vertebrate animals in laboratory experiments to evaluate general and acute toxicity, mutagenesis and teratogenesis of new drugs or chemicals. One common approach is to use established, proliferating cell lines that preserve differentiated functions such as the competence to metabolize xenobiotics. To this end a continuous Chinese hamster epithelial liver cell line (CHEL cells) was established, cultured as used for mutagenesis studies. Structurally different promutagens, such as 7,12-dimethylbenz[a]anthracene (7,12-DMBA), benzo[a]pyrene (B(a)P), aflatoxin B1 (AB1) and cyclophosphamide (CP), were used in order to check and validate the test system. anti-Chrysene-1,2-diol 3,4-epoxide (CDE) and mitomycin C (MMC) were taken as representatives of direct mutagens. The genetic change induced by the mutagens was quantified by measuring mutation frequencies at the HGPRT locus. Several parameters, such as mutant expression time for each chemical, cell density for selection of mutants and enzymatic characterization for HGPRT phenotype, were examined to establish the optimal assay conditions. All promutagens analyzed significantly affected either the cloning efficiency and/or the mutant frequency of CHEL cells after 24 h of exposure. In addition, various enzyme activities involved in the metabolism of the promutagens were determined in CHEL cells, under the experimental conditions of chemical exposure used in the mutagenesis assay. The enzyme activities were compared with those found in uninduced Chinese hamster liver.

DNA cross-linking in mammalian cells by pyrrolizidine alkaloids: structure-activity relationships

Pyrrolizidine alkaloids (PAs) are common constituents of many species of flowering plants which possess carcinogenic as well as anticarcinogenic activity in vivo. Pyrrolizidine alkaloids are genotoxic in various short-term assays. The mechanisms by which these compounds exert these effects is still unclear. In this study, we characterized the ability of eight bifunctional PAs, with differing stereochemistry and functional groups, to cross-link cellular DNA in cultured bovine kidney epithelial cells. PAs representative of three major structural classes, the macrocycles (seneciphylline, riddelline, retrorsine, senecionine, monocrotaline), the open diesters (heliosupine, latifoline), and pyrrolizidine base (retronecine) were cultured for 2 hr with cells and an external metabolizing system. Every PA induced DNA cross-links which consisted primarily of proteinase-sensitive cross-links (DPC), but also to a smaller extent, DNA interstrand cross-links (ISC). None of the PAs induced detectable amounts of DNA single-strand breaks. The PAs which produced DPC and/or ISC (ranked from most potent to least) were: seneciphylline (DPC greater than ISC); riddelline (DPC greater than ISC); retrorsine (DPC greater than ISC); senecionine (DPC greater than ISC); heliosupine (DPC greater than ISC); monocrotaline (ISC = DPC); latifoline (DPC greater than ISC); and retronecine (ISC greater than DPC). Although the PAs induced DNA cross-linking to varying degrees, cell viabilities for all treatment groups were greater than 90% as determined by trypan blue dye exclusion. Since the cross-linking ability of these PAs paralleled their ability to inhibit colony formation, cross-link formation may be involved in the biological activity of these compounds. Two structural determinants of biological activity appear to be the presence of both a macrocyclic necic acid ester and an alpha,beta-unsaturated ester function since the cross-linking ability of seneciphylline, riddelline, retrorsine, and senecionine far exceeded that of monocrotaline, heliosupine, latifoline, and retronecine. In addition, the stereochemical orientation of the ester linkage was found to have no effect on biological activity.

Further investigations on the clastogenicity of paracetamol and acetylsalicylic acid in vitro

Paracetamol (PCM) and acetylsalicylic acid (ASA), both widely used analgesics, were tested for their clastogenicity in V79 cells in vitro. Rat liver S9 mix and primary rat hepatocytes (PRH) were used as external activation systems. ASA was found to be negative with and without activation system in concentrations up to 10(-2) M. In contrast PCM induced concentration-dependent chromosomal aberrations with and without activation system within the range of 3 x 10(-3) and 10(-2) M. The greatest effects were observed following continuous treatment with PRH activation and without external metabolization. Pulse treatments without external metabolization, with S9 mix and PRH were less effective. The clastogenic potency of PCM seems to be partly independent of metabolic activation. Although clastogenic effects in vitro were observed only in very high concentrations pharmacokinetic data and other published mutagenicity data indicate that there might be a risk for human use. Peak plasma levels of more than 10(-4) M have been reported (Forrest et al., 1982) and 2 groups of investigators (Kocisova et al., 1988; Hongslo et al., 1990) found PCM to be weakly clastogenic in human lymphocytes in vivo in the maximum human therapeutic dose range.

Induced hepatocytes as a metabolic activation system for the mouse-lymphoma assay

We have developed methods for the coculture of hepatocytes and mouse lymphoma cells and have shown that this system can be used for evaluating promutagens from several chemical classes (Brock et al., 1987). In the present study we investigated the use of hepatocytes isolated from rats pretreated with a cytochrome P-450 inducer (PB) or a P-448 inducer (BNF). CP-induced mutagenicity was higher in the presence of PB-induced hepatocytes than in control hepatocytes. Control and BNF-induced hepatocytes were evaluated with B(a)P, B(l)A, and BA. A dose-related positive response was observed with B(a)P and B(l)A both in the presence of control or induced hepatocytes; however, somewhat higher mutant frequencies were obtained in the presence of BNF-induced hepatocytes. BA induced a very weak positive response (approx. 2 X b.g.) in the presence of control hepatocytes and was weakly positive in the presence of BNF-induced hepatocytes. Benzene was tested using control and both PB- and BNF-induced hepatocytes. Neither of these approaches were successful in activating benzene to a mutagenic metabolite. These studies indicate that for some chemicals the mutagenic response of mouse lymphoma cells can be increased by inducing hepatocytes prior to isolation and cocultivation, and expands the use of hepatocytes for research evaluating chemicals requiring metabolic activation.

The effect of inducers of mixed-function oxidases on hepatic microsome-mediated aflatoxin B1 transformation in C3H/10T1/2 cells

The potent hepatotoxin and hepatocarcinogen aflatoxin B1 (AFB1) is metabolized by different forms of cytochrome P450 associated with the hepatic mixed-function oxidase system. C3H/10T1/2(10T1/2) cells, which have limited inherent capacity to metabolize AFB1, were treated with AFB1 in the presence of hepatic microsomes isolated from chemically treated rats to investigate the effects of the induction of specific cytochromes P450 on AFB1-mediated toxicity and transformation. Relative to uninduced microsomes, phenobarbital (PB) treatment induced AFB1-DNA binding (essentially representing the formation of AFB1-8,9-oxide bound to DNA) 3.2-fold, while pretreatment with Aroclor 1254, 3-methylcholanthrene (3-MC), or 5,6-benzoflavone (beta-NF) preferentially induced aflatoxin M1 (AFM1) formation from 2- to 5-fold. 10T1/2 cells were exposed to a multiple treatment regimen with 4 microM AFB1 and hepatic microsomes from uninduced, PB-, Aroclor 1254-, 3-MC-, or beta-NF-treated rats; respective cumulative toxicities of approximately 90, 95, 70, 60, and 40% control (no microsomes) values resulted. An enhanced AFB1 transformation response correlated with the increasing toxicities observed for the different treatments, with uninduced or PB-induced microsomes yielding approximately four foci/dish, while treatments with Aroclor 1254-, 3-MC-, or beta-NF-induced microsomes resulted in only one to two foci/dish. These results demonstrate that AFB1 is a complete carcinogen in the 10T1/2 system when repetitive incubations are used in conjunction with an appropriate hepatic microsomal activation system. These data also correlate the induction of AFB1-4-hydroxylase with a decrease in AFB1-mediated toxicity and transformation of 10T1/2 cells, and support the hypothesis that the Phase I metabolic conversion of AFB1 to AFM1 in the liver represents an effective detoxification pathway per se.

Induction of sister-chromatid exchanges by direct and indirect mutagens in human lymphocytes, co-cultured with intact rat liver cells. Effect of enzyme induction and preservation of the liver cells by freezing in liquid nitrogen

An in vitro assay system using intact rat hepatocytes and human peripheral lymphocytes is described which has been developed with the aim of bringing test conditions closer to in vivo conditions, thereby broadening the available battery of simple in vitro assays. A culture vessel, which contains an inner chamber with a semipermeable bottom, has been designed to allow easy removal of the hepatocytes. Determination of sister-chromatid exchange rate was used as the experimental end point. For validation, a series of chemicals were used which have been tested previously in a large interlaboratory investigation of short-term test methods. Our study supplies complementary information to this investigation in as much as some chemicals could be correctly assigned as positive or negative, in contrast to what was found in the earlier tests. Furthermore, we show that the metabolic capacity of both normal and induced liver cells can be preserved in liquid nitrogen for long periods.

Promutagen activation by freshly isolated and cryopreserved rat hepatocytes

The metabolic activation of promutagens by freshly isolated and cryopreserved rat hepatocytes was compared using the Chinese hamster ovary/hypoxanthine-guanine phosphoribosyltransferase assay (CHO/HGPRT). Cryopreserved rat hepatocytes were equivalent to freshly isolated hepatocytes in their ability to metabolize dimethylbenz(a)anthracene (DMBA) and dimethylnitrosamine (DMN) to active mutagens. Similar dose-response curves were observed using either freshly isolated or cryopreserved hepatocytes as activating systems after treatment with DMBA (0.1-1 micrograms/ml) and DMN (0.075-0.6 mg/ml). Our results suggest that cryopreserved hepatocytes are similar to freshly isolated hepatocytes as an experimental system for studies on promutagen activation.

Hepatocyte-mediated SCE induction by indirect mutagens: importance of hepatocyte density and cell-to-cell contact

The SCE-inducing effects of the indirectly acting mutagens cyclophosphamide (CP), dimethylnitrosamine (DMN) and aflatoxin B1 (AFB1) were analysed in hepatocyte (hpc)/mammalian cell coculture systems with regard to the importance of the hpc density. V79 cells and human lymphocytes served as target cells. For all 3 compounds steadily increasing genetic effects were observed when the hpc density was increased from 3.2 X 10(4) up to 3.2 X 10(6) viable hpc per culture (25-cm2 flask), i.e. the more hpc available for metabolisation, the more genetic effects induced. The frequency distributions of the CP-induced SCE values were clearly different from those obtained with DMN, especially when high hpc densities were used: distribution patterns obtained for the mutagen with stable metabolites (CP) are characterized by the presence of distinct maxima and the absence of cells with SCE control values, whereas distribution patterns for the mutagen with very short-lived metabolites (DMN) can be described by the absence of maxima and the presence of cells with SCE control values. The frequency distributions of the AFB1-induced SCE values were more similar to the CP type than to the DMN type. From these results it is deduced that close contact between metabolising and target cells is necessary for the detection of the genotoxic effect of DMN. For CP and AFB1 a direct contact seems not to be essential, i.e. reactive intermediates may also be transported via the culture medium to the target cells.