Sister-chromatid exchange induction by polycyclic aromatic hydrocarbons in an intact cell system of adult rat-liver epithelial cells

Use of metabolically competent human hepatoma cells for the detection of mutagens and antimutagens

The human hepatoma line (Hep G2) has retained the activities of various phase I and phase II enzymes which play a crucial role in the activation/detoxification of genotoxic procarcinogens and reflect the metabolism of such compounds in vivo better than experimental models with metabolically incompetent cells and exogenous activation mixtures. In the last years, methodologies have been developed which enable the detection of genotoxic effects in Hep G2 cells. Appropriate endpoints are the induction of 6-TGr mutants, of micronuclei and of comets (single cell gel electrophoresis assay). It has been demonstrated that various classes of environmental carcinogens such as nitrosamines, aflatoxins, aromatic and heterocyclic amines and polycyclic aromatic hydrocarbons can be detected in genotoxicity assays with Hep G2 cells. Furthermore, it has been shown that these assays can distinguish between structurally related carcinogens and non-carcinogens, and positive results have been obtained with rodent carcinogens (such as safrole and hexamethylphosphoramide) which give false negative results in conventional in vitro assays with rat liver homogenates. Hep G2 cells have also been used in antimutagenicity studies and can identify mechanisms not detected in conventional in vitro systems such as induction of detoxifying enzymes, inactivation of endogenously formed DNA-reactive metabolites and intracellular inhibition of activating enzymes.

ATSDR evaluation of health effects of chemicals. IV. Polycyclic aromatic hydrocarbons (PAHs): understanding a complex problem

Polycyclic Aromatic Hydrocarbons (PAHs) are a group of chemicals that are formed during the incomplete burning of coal, oil, gas, wood, garbage, or other organic substances, such as tobacco and charbroiled meat. There are more than 100 PAHs. PAHs generally occur as complex mixtures (for example, as part of products such as soot), not as single compounds. PAHs are found throughout the environment in the air, water, and soil. As part of its mandate, the Agency for Toxic Substances and Disease Registry (ATSDR) prepares toxicological profiles on hazardous chemicals, including PAHs (ATSDR, 1995), found at facilities on the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA) National Priorities List (NPL) and which pose the most significant potential threat to human health, as determined by ATSDR and the Environmental Protection Agency (EPA). These profiles include information on health effects of chemicals from different routes and durations of exposure, their potential for exposure, regulations and advisories, and the adequacy of the existing database. Assessing the health effects of PAHs is a major challenge because environmental exposures to these chemicals are usually to complex mixtures of PAHs with other chemicals. The biological consequences of human exposure to mixtures of PAHs depend on the toxicity, carcinogenic and noncarcinogenic, of the individual components of the mixture, the types of interactions among them, and confounding factors that are not thoroughly understood. Also identified are components of exposure and health effects research needed on PAHs that will allow estimation of realistic human health risks posed by exposures to PAHs. The exposure assessment component of research should focus on (1) development of reliable analytical methods for the determination of bioavailable PAHs following ingestion, (2) estimation of bioavailable PAHs from environmental media, particularly the determination of particle-bound PAHs, (3) data on ambient levels of PAHs metabolites in tissues/fluids of control populations, and (4) the need for a critical evaluation of current levels of PAHs found in environmental media including data from hazardous waste sites. The health effects component should focus on obtaining information on (1) the health effects of mixtures of PAHs particularly their noncarcinogenic effects in humans, and (2) their toxicokinetics. This report provides excerpts from the toxicological profile of PAHs (ATSDR, 1995) that contains more detailed information.

Increased sister chromatid exchange (SCE) frequencies in lymphocytes from traffic policemen exposed to automobile exhaust pollution

Sister chromatid exchange (SCE) frequencies were determined in peripheral lymphocytes from traffic policemen drawn from various busy traffic points of Madras metro in India. These policemen were under constant exposure to automobile exhaust pollution during their 8 h work schedule. Analysis of SCE frequencies revealed a significantly greater number of SCE/cell in these policemen as against the matched control population. Sixty percent of these subjects who remained in the service after 5 years showed insignificant but higher values of SCE than in the initial analysis.

Genotoxic activity of environmentally important polycyclic aromatic hydrocarbons and their nitro derivatives in the wing spot test of Drosophila melanogaster

The genotoxicity of three polycyclic aromatic hydrocarbons (PAHs) and of three of their nitro derivatives was evaluated in the wing Somatic Mutation And Recombination Test (SMART) in Drosophila melanogaster. Two crosses were used, i.e. the standard cross (ST) and the improved high bioactivation cross (HB) which is characterised by an increased sensitivity to the genotoxic effects of promutagens and procarcinogens. Larvae trans-heterozygous for the two recessive wing cell markers multiple wing hairs (mwh) and flare (flr3) were fed with the test compounds for 48 h. The wings of the surviving flies were analysed for the occurrence of single and twin spots. Naphthalene, 1-nitronaphthalene and 1,5-dinitronaphthalene proved to be more genotoxic in the HB cross than in the ST cross. Anthracene showed a clear genotoxic activity only in the HB cross whereas it was negative in the ST cross. 9-Nitroanthracene gave inconsistent results in both crosses. Phenanthrene was negative in the ST cross, but weakly positive in the HB cross. These results demonstrate that the genotoxic activity of these PAHs and their nitro derivatives can be detected with the somatic cells of the wing imaginal discs of larvae with high bioactivation capacity.

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.

Use of an established human hepatoma cell line with endogenous bioactivation for gene mutation studies

Genetic toxicology assays that rely on S9 microsomal mixes are subject to artifacts related to the generation of mutagenic metabolites by acidic pHs, variation in individual isolations of microsomes and the failure of subcellular fractions to faithfully produce metabolites generated in intact cells. We have developed a gene mutation assay utilizing the human hepatoma cell line HepG2, which has been shown to metabolize a broad spectrum of promutagens. Optimal conditions for assaying the induction of 6-thioguanine-resistant mutants in this cell line include: 1) growth of colonies for three weeks on lethally irradiated feeder layers of 10(6) thioguanine-resistant HepG2 cells (average plating efficiency = 60-80%); 2) a thioguanine concentration in selection dishes of 10(-4) M with a maximum seeding density of 2.5 x 10(5) cells per 100 mm culture dish; and 3) a minimum expression time of 6 days. In addition to ultraviolet light C (254 nm), a cytochrome P450 (cyclophosphamide)-dependent and a cytochrome P448 (aflatoxin B1)-dependent promutagen were shown to induce cytotoxicity and mutations in this test system. The present studies, therefore, suggest that the HepG2 cell line may be useful for a variety of assays in genetic toxicology.

The lack of genotoxicity of sodium fluoride in a battery of cellular tests

In a comprehensive assessment of genotoxicity, sodium fluoride was evaluated in a battery of cellular tests providing different genetic end points and biotransformation capabilities. The tests included the following: rat hepatocyte primary culture/DNA repair assay, Salmonella typhimurium histidine locus reversion assay, adult rat liver epithelial cell/hypoxanthine guanine phosphoribosyl transferase mutation assay, and sister chromatid exchange in two target cell types, human peripheral blood lymphocytes and Chinese hamster ovary cells. Negative findings were made in all assays, indicating that sodium fluoride is not genotoxic in these assays.

Application of a cellular test battery in the decision point approach to carcinogen identification

The assessment of the potential carcinogenicity of a chemical requires a systematic approach taking into account various types of data. Important information on the DNA reactivity and other genetic effects of chemicals can be obtained from a battery of cellular tests. A battery is described which includes DNA repair in hepatocytes, mutagenesis in Salmonella typhimurium, mutagenesis, chromosome alterations, and transformation in mammalian cells. The interpretation of findings in this battery for the identification of potential carcinogenicity of chemicals is discussed.

Induction of cytochrome(s) P450-dependent drug metabolism in cultured MH1C1 hepatoma cells

A cell line derived from a Morris hepatoma, MH1C1, was examined for its in vitro expression of monooxygenases. These cells were found to contain different forms of cytochrome P450, as shown by the response to inducers, namely phenobarbital (PB), 3-methylcholanthrene (MC) and metyrapone (MP). MH1C1 cell monolayers exposed to PB or MC showed an increase in the concentration of two spectrally distinct forms of cytochrome P450. The PB and MC treatments elicited enzyme activities towards the substrates aminopyrine and benzo(a)pyrene, respectively. The cell treatment with metyrapone led to a simultaneous stimulation of aminopyrine demethylase and benzo(a)pyrene hydroxylase activities, so underlining the peculiar features of this inducer.

Incorporation in liposomes as a method for the application of genotoxins of low water solubility in the SCE assay

Benzo[alpha]pyrene BaP), freely dissolved or incorporated in liposomes prepared from egg yolk lecithin, was checked for SCE induction in Chinese hamster V79-E and rat liver RL-19 cells in vitro. SCE induction in V79-E was observed only when freely dissolved BaP was added together with S9 mix. RL-19 cells were per se highly sensitive to SCE induction by BaP either freely dissolved or incorporated in liposomes. It is suggested that the incorporation of genotoxins in liposomes is a practicable method for the application, in mammalian genotoxicity assays, of agents which are barely soluble or completely insoluble in water, provided no exogenous metabolizing system is required.

Nitrilotriacetic acid does not induce sister-chromatid exchanges in hamster or human cells

The effect of nitrilotriacetic acid on sister-chromatid exchanges (SCEs) in human peripheral blood lymphocytes and Chinese hamster ovary cells was studied. The chemical did not increase the frequency of SCEs in either cell type. The negative response of these cells supports other evidence that nitrilotriacetic acid is not genotoxic and that its carcinogenicity to the urinary tract may involve an epigenetic mechanism.

SCE induction by indirect mutagens/carcinogens in metabolically active cultured mammalian cell lines

Sister chromatid exchange (SCE) analysis in a total of 20 cultured mammalian cell lines of various types exposed to a variety of indirect mutagens/carcinogens suggested that, among these, several human and rat tumor cell lines are intrinsically capable of metabolizing a wide range of mutagens/carcinogens into genotoxic forms to induce SCEs. These cells may therefore serve as simple in vitro SCE test systems for screening of activation-dependent genotoxins without the use of an exogenous activating system. In addition, a comparison of the relevant metabolizing enzyme activity and inducibility of SCEs by mutagens/carcinogens in such cell systems appears to provide clues about the metabolic pathways related to SCE induction. Based on our own data and those in the literature, practical and theoretical advantages of the use of a direct-activating SCE test system in genetic toxicology are discussed.

Benzo(a)pyrene and aniline increase sister chromatid exchanges in cultured rat liver fibroblasts without addition of activating enzymes

The RL4 rat liver epithelial cell line possesses enzymes capable of bioactivating xenobiotics. This cell line was used without the addition of exogenous activating enzymes to study the genotoxicity of benzo(a)pyrene B(a)P and aniline as measured by the sister chromatid exchange technique. Both increased sister-chromatid exchange (SCE) formation in RL4 cells although B(a)P was more effective than aniline. Pyrene, a structural analogue of B(a)P, demonstrated no genotoxicity. These results indicate that the RL4 cell line containing endogenous bioactivation ability may be useful for genotoxicity studies.

Sister-chromatid exchange induction by indirect mutagens/carcinogens, aryl hydrocarbon hydroxylase activity and benzo[alpha]pyrene metabolism in cultured human hepatoma cells

2 human hepatoma cell lines (C-HC-4 and C-HC-20), in which aryl hydrocarbon hydroxylase activity was induced with benz[alpha]anthracene in vitro to about 140- and 64-fold of the respective basal levels, yielded an increased frequency of sister-chromatid exchanges (SCEs) when exposed to benzo[alpha]pyrene (BP), 7,12-dimethylbenz[alpha]anthracene and 3-methylcholanthrene in vitro. Analysis of the metabolism of BP by these cells by high-pressure liquid chromatography revealed that both cell lines produced various BP metabolites including the proximate form BP-7,8-dihydrodiol which has been reported to be the most potent inducer of SCEs among the metabolites of BP. In addition, aflatoxin B1 and cyclophosphamide also induced SCEs in these cell lines. The above findings suggest that these cells may be capable of metabolizing a range of indirect mutagens/carcinogens into DNA-active forms. These cells may therefore serve as a useful test system in vitro for the detection of genotoxic agents, without the use of an exogenous activating system.

Evaluation of a human hepatoma cell line as a target cell in genetic toxicology

A cell line derived from a human hepatoblastoma, HepG2, was examined for its ability to activate cyclophosphamide (CY) to a genotoxic form. Metabolism of CY to genotoxic product(s) was determined by the induction of sister-chromatid exchanges (SCE). The dose-dependent response pattern in HepG2 was compared to the patterns obtained by three other mammalian cell lines. HepG2 and a rat hepatoma cell line, H4-II-E, show similar dose-dependent increases of induced SCE, whereas non-hepatic-derived fibroblast lines show little or no CY-induced SCE. Microsomal enzyme activities characteristic of cytochromes P450 and P448 and epoxide hydrolase were examined in the two hepatoma cell lines and compared to levels in rat liver microsomal preparations. Although no cultured cell line can be a universal surrogate for in vivo metabolism, we propose that HepG2 may be useful to determine in a qualitative manner whether human cells possess the ability to activate a chemical to a genetically damaging form.

Carcinogen testing: current problems and new approaches

The classic procedures for testing potential carcinogens in animals have basically not changed in the past 50 years. Considerable knowledge of the mechanisms of carcinogenesis has accrued in the last 20 years, particularly concepts on the metabolic activation of chemicals to reactive electrophilic compounds that can interact with nucleophilic including DNA. These developments, in turn, have yielded a framework for integrating into carcinogen testing the determination of genetic effects of chemicals. A systematic decision point approach to carcinogen testing has been developed which entails a sequential decision-making process as specific tests are performed and evaluated prior to initiation of higher order, more complex tests. Compared to conventional bioassays in rodents, this approach provides knowledge based on mechanisms of carcinogenesis, yields a substantial amount of data at minimal cost, and forms a solid base for eventual heath risk assessment.