Carcinogen-induced DNA repair in primary rat liver cell cultures; a possible screen for chemical carcinogens

Evaluation of Pharmaceuticals for DNA Damage in the Chicken Egg Genotoxicity Assay (CEGA)

DNA damage is an established initiating event in the mutagenicity and carcinogenicity of genotoxic chemicals. Accordingly, assessment of this endpoint is critical for chemicals which are being developed for use in humans. To assess the ability of the Chicken Egg Genotoxicity Assay (CEGA) to detect genotoxic pharmaceuticals, a set of 23 compounds with different pharmacological and reported genotoxic effects was tested for the potential to produce nuclear DNA adducts and strand breaks in the embryo-fetal livers using the ³²P-nucleotide postlabeling (NPL) and comet assays, respectively. Due to high toxicity, two aneugens, colchicine and vinblastine, and an autophagy inhibitor, hydroxychloroquine, could not be evaluated. Out of the 20 remaining pharmaceuticals, 10 including estrogen modulators, diethylstilbestrol and tamoxifen, antineoplastics cyclophosphamide, etoposide, and mitomycin C, antifungal griseofulvin, local anesthetics lidocaine and prilocaine, and antihistamines diphenhydramine and doxylamine, yielded clear positive outcomes in at least one of the assays. The antihypertensive vasodilator hydralazine and antineoplastics streptozotocin and teniposide, produced only DNA strand breaks, which were not dose-dependent, and thus, the results with these 3 pharmaceuticals were considered equivocal. No DNA damage was detected for 7 compounds, including the purine antagonist 6-thioguanine, antipyretic analgesics acetaminophen and phenacetin, antibiotic ciprofloxacin, antilipidemic clofibrate, anti-inflammatory ibuprofen, and sedative phenobarbital. However, low solubility of these compounds limited dosages tested in CEGA. Overall, results in CEGA were largely in concordance with the outcomes in other systems in vitro and in vivo, indicating that CEGA provides reliable detection of DNA damaging activity of genotoxic compounds. Further evaluations with a broader set of compounds would support this conclusion.

Genetic toxicity assessment using liver cell models: past, present, and future

Genotoxic compounds may be detoxified to non-genotoxic metabolites while many pro-carcinogens require metabolic activation to exert their genotoxicity in vivo. Standard genotoxicity assays were developed and utilized for risk assessment for over 40 years. Most of these assays are conducted in metabolically incompetent rodent or human cell lines. Deficient in normal metabolism and relying on exogenous metabolic activation systems, the current in vitro genotoxicity assays often have yielded high false positive rates, which trigger unnecessary and costly in vivo studies. Metabolically active cells such as hepatocytes have been recognized as a promising cell model in predicting genotoxicity of carcinogens in vivo. In recent years, significant advances in tissue culture and biological technologies provided new opportunities for using hepatocytes in genetic toxicology. This review encompasses published studies (both in vitro and in vivo) using hepatocytes for genotoxicity assessment. Findings from both standard and newly developed genotoxicity assays are summarized. Various liver cell models used for genotoxicity assessment are described, including the potential application of advanced liver cell models such as 3D spheroids, organoids, and engineered hepatocytes. An integrated strategy, that includes the use of human-based cells with enhanced biological relevance and throughput, and applying the quantitative analysis of data, may provide an approach for future genotoxicity risk assessment.

Antioxidant, antimutagenic, and anticarcinogenic effects of Papaver rhoeas L. extract on Saccharomyces cerevisiae

The aim of this work was to analyze the antioxidant and antimutagenic/anticarcinogenic capacity of Papaver rhoeas L. water extract against standard mutagen/carcinogen methyl methanesulfonate (MMS) and radiomimetic zeocin (Zeo) on a test system Saccharomyces cerevisiae. The following assays were used: 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical scavenging activity, quantitative determination of superoxide anion (antireactive oxygen species [antiROS test]), DNA topology assay, D7ts1 test--for antimutagenic--and Ty1 transposition test--for anticarcinogenic effects. Strong pro-oxidative capacity of Zeo was shown to correlate with its well-expressed mutagenic and carcinogenic properties. The mutagenic and carcinogenic effects of MMS were also confirmed. Our data concerning the antioxidant activity of P. rhoeas L. extract revealed that concentration corresponding to IC(50) in the DPPH assay possessed the highest antioxidant activity in the antiROS biological assay. It was also observed that a concentration with 50% scavenging activity expressed the most pronounced antimutagenic properties decreasing Zeo-induced gene conversion twofold, reverse mutation fivefold, and total aberrations fourfold. The same concentration possessed well-expressed anticarcinogenic properties measured as reduction of MMS-induced Ty1 transposition rate fivefold and fourfold when Zeo was used as an inductor. Based on the well-expressed antioxidant, antimutagenic, and anticarcinogenic properties obtained in this work, the P. rhoeas L. extract could be recommended for further investigations and possible use as a food additive.

A classification framework and practical guidance for establishing a mode of action for chemical carcinogens

The recently released U.S. Environmental Protection Agency (U.S. EPA) Supplemental Guidance for Assessing Risk from Early Life Exposure to Carcinogens (SGAC) provides guidance to account for potential increased early life susceptibility to carcinogens that are acting via a mutagenic mode of action. While determination of the mode of carcinogenic action is central to the SGAC procedures and other regulatory risk assessments, little guidance is given as to the approaches, criteria, and nature of the evidence required to define a mutagenic mode of action. The purpose of this paper is to provide a framework along with practical guidance for the process of assigning a mode of action. Strengths, weaknesses, reliability, and choice of a test battery are discussed for select bacterial, cell culture, whole animal and human cell assays. Common confounding factors of induced pathology, cytolethality, and regenerative cell proliferation in rodent cancer bioassays are discussed along with approaches to account for these effects in assigning a mode of action and in risk assessments. Specific examples are given to illustrate the complexity in generating a data set sufficient to move from the default regulatory position of assuming a genotoxic mode of action to actually assigning a nongenotoxic mode of action. A two-part framework is proposed for assigning a mode of action. First, a weight of evidence approach is used to assess mutagenic potential based on results of genetic toxicology test systems. Second, a descriptor is assigned to classify the degree to which mutagenic activity likely played a role in the mode of action of tumor formation. This option provides a more realistic way of describing the mode of action instead of being bound by the strict genotoxic vs. nongenotoxic choices.

The autoradiographic test for unscheduled DNA synthesis: a sensitive assay for the detection of DNA repair in the HepG2 cell line

We assessed the DNA-repair capacity of HepG2 cells, which were derived from a human hepatoma, by the unscheduled DNA synthesis assay, using the autoradiography protocol (UDS-AR). We evaluated DNA repair following exposure to direct mutagens (4-nitroquinoline-N-oxide (4-NQO), methyl methanesulfonate (MMS)), to mutagens requiring metabolic activation (benzo[a]pyrene (B[a]P), 2-acetylaminofluorene (2-AAF), N-dimethylnitrosoamine (NDMA)) or to structurally related non-mutagens such as pyrene and 4-acetylaminofluorene (4-AAF). All positive compounds tested induced UDS in HepG2 cells. With 4-NQO and MMS, a concentration-dependent increase in net nuclear grains per cell was observed, with 73 and 90% of cells, respectively, in repair at the highest concentration. B[a]P, 2-AAF and NDMA displayed similar dose-dependent UDS responses, but the percentage of cells in repair was lower (about 45%) than that for 4-NQO and MMS. We assessed the genotoxicity of the compounds tested by determining IC(5NNG): the concentration required to induce 5NNG. The compounds studied were ranked in order of IC(5NNG) as follows: 4-NQO = B[a]P > 2-AAF > MMS > NDMA. The UDS assay discriminated between mutagens and non-mutagens, as pyrene and 4-AAF failed to induce DNA repair. The present study demonstrates that UDS can be used as an endpoint for the detection of DNA damage in HepG2 cells.

Genetic toxicology of remifentanil, an opiate analgesic

Compounds that interact with opioid receptors are commonly used as analgesics. Opioid agonists vary in their potency and pharmacokinetic properties as well as in their affinity for distinct opioid receptors. The fentanyl opiate analogues are an important group of analgesics that interact with the mu opioid receptor. Remifentanil (GI87084) is a particularly interesting member of this group of opioids because its action is especially short in duration. This report examines the genetic toxicology of remifentanil. Remifentanil was not genotoxic in an Ames test, an in vitro chromosome aberration assay in Chinese hamster ovary cells, an in vivo micronucleus assay in rat erythrocytes, or an in vivo/in vitro unscheduled DNA synthesis assay in rat hepatocytes. In the in vitro L5178Y tk(+/-) mouse lymphoma assay, remifentanil produced a genotoxic response at dose levels >or=308 microg/mL only in the presence of rat liver S9 metabolic activation; primarily tiny and small mutant colonies were produced. This pattern of activity in a battery of genetic toxicology assays is not unique to remifentanil, but has also been observed for other pharmaceuticals, including the opioid fentanyl. A weight-of-evidence analysis, taking into consideration genotoxic mechanisms, in vivo results, and the conditions of clinical use, suggests remifentanil does not pose a genotoxic risk to patients.

Dimethylhydrazine: a reliable positive control for the short sampling time in the UDS assay in vivo

In the first international guideline addressing the unscheduled DNA synthesis (UDS) assay in vivo (OECD guideline no. 486, adopted July 1997) only the genotoxic liver carcinogen N-nitrosodimethylamine (NDMA) is proposed as positive control for the short sampling time. Since NDMA is extremely volatile, alternative positive controls should be identified to facilitate handling and reduce exposure risk during routine testing. At Bayer AG and at RCC-CCR GmbH, the genotoxic but non-volatile dimethylhydrazine (DMH; as dihydrochloride) was used instead as positive control in livers of Wistar rats and to a limited extent of NRMI mice after 2-4h exposure. As shown by the data presented in this paper DMH induced a positive result in a total of 21 UDS in vivo studies over a period of 7 years. A negative result was never seen for DMH. Due to these results DMH was proven to be a suitable and reliable positive control in the UDS assay in vivo. Consequently, DMH should be considered as positive control for the short sampling time in the next issue of OECD guideline no. 486.

Carcinogen induced unscheduled DNA synthesis in mouse hepatocytes

Mouse primary liver cell cultures were examined for evidence of unscheduled DNA synthesis (UDS) following treatment with the carcinogens; dimethylnitrosamine (DMNA), diethylnitrosamine (DENA), 2-acetylaminofluorene (2-AAF), N-methyl-N'-nitro-N-nitrosoguanidine (MNNG), benzo(a)pyrene (BP), dimethylbenzanthracene (DMBA), 1,1,-bis(p-chlorophenyl)-2,2,2-trichloroethane (DDT), safrole, diethylstilbestrol (DES), aflatoxin B1 (AFB1), and dieldrin and the noncarcinogens; dimethylformamide (DMF), fluorene, and pyrene. Mouse hepatocyte cultures were simultaneously treated with three concentrations of each compound and 3H-thymidine. After 24 hrs, cells were fixed and processed for autoradiography. 3H-thymidine incorporation in both experimental and control cell nuclei, as evidenced by autoradiographic grains, was quantitated microscopically. DMNA, DENA, 2-AAF, MNNG, BP, AFB1 and DMBA significantly increased UDS over untreated cells at all concentrations studied. DDT, DMF, fluorene, pyrene, safrole, DES, and dieldrin were negative for UDS in all concentrations examined. DMNA, 2-AAF and MNNG were also studied for UDS induction in 2 hr old, 1 day old and 4 day old cultures. A progressive decrease in UDS with increased time after plating was found in DMNA and 2-AAF treated cultures. After 4 days DMNA and 2-AAF induced UDS only at the highest concentrations examined (10(-3) M and 10(-4) M respectively). MNNG induced UDS at all time periods and concentrations sampled. An attempt to enhance the sensitivity of the UDS assay by inducing the mixed function oxidative enzyme activity in the hepatocytes with phenobarbital administered in vivo resulted in no statistically significant increase in UDS with DMNA, 2-AAF, MNNG, DDT, and dieldrin when compared with cells from non-induced animals.

Assays for DNA damage

This unit describes several assays for detecting several kinds of DNA damage (strand breaks, internal crosslinking, DNA/protein crosslinks) and repair activity following exposure to genotoxic agents. The methods include single-cell electrophoresis (comet assay), filter eluting, K-SDS precipitation, and measurement of unscheduled DNA synthesis.

Development and utilization of primary hepatocyte culture systems to evaluate metabolism, DNA binding, and DNA repair of xenobiotics

The use of isolated hepatocytes as an approach to evaluate hepatotoxic and hepatocarcinogenic compounds and investigate mechanisms by which chemicals induce liver lesions is well established. This review discusses techniques developed in the author's laboratory describing (1) isolation and primary culture of rodent hepatocytes detailing methods which are optimal for obtaining large numbers of viable cells, (2) DNA damage induced by physical and chemical agents in rodent hepatocytes measured as unscheduled DNA synthesis, and (3) metabolic activation of model hepatocarcinogens, their binding to DNA, and identification of individual adducts thought to be responsible for induction of DNA repair.

Genotoxicity tests with 6-acetyl-1,1,2,4,4,7-hexamethyltetraline and 1,3,4,6,7,8-hexahydro-4,6,6,7,8,8-hexamethylcyclopenta-gamma-2-benzopyr an

6-Acetyl-1,1,2,4,4,7-hexamethyltetraline (AHTN) and 1,3,4,6,7,8-hexahydro-4,6,6,7,8,8-hexamethylcyclopenta-gamma-2-ben zopyran (HHCB), synthetic fragrance ingredients, were evaluated for potential genotoxicity in a battery of short-term tests. Salmonella typhimurium/Escherichia coli plate incorporation and liquid preincubation assays were conducted on AHTN using tester strains TA97, TA98, TA100, TA102, TA1535, TA1537 and WP2 uvrA +/- S9 activation at doses from 8 to 5000 micrograms/plate. The plate incorporation mutagenicity assay was conducted on HHCB using tester strains TA98, TA100, TA1535, TA1537, TA1538 and WP2 uvrA +/- S9 activation at doses from 10 to 5000 micrograms/plate. An in vitro cytogenetics assay in Chinese hamster ovary (CHO) cells was conducted with AHTN and HHCB at three concentrations each with +/- S9 activation. In the non-activated study, the exposure/harvest periods were 4/20-, 20/20- and 44/44-h. In the S9 activated study, the exposure/harvest periods were 4/20- and 4/44-h. In vitro unscheduled DNA synthesis (UDS) assays were conducted in primary rat hepatocytes at concentrations between 0.15 and 50 micrograms/ml for AHTN and HHCB. In vivo mouse micronucleus assays were conducted with high doses of 1600 mg AHTN/kg and of 1500 mg HHCB/kg in corn oil. No positive responses were observed in any of the tests with HHCB. With AHTN, no positive responses were observed except for cells with structural aberrations in the in vitro cytogenetics assay in CHO cells with S9 activation at the treatment/harvest time of 4/20 h. In initial studies with AHTN, the high dose of 7.8 micrograms/ml showed 0.5% aberrant cells, with the mitotic index at 41% relative to vehicle control and cell growth inhibition in the range of 25-50%. Thus the genotoxicity findings with AHTN were limited to this one positive response; all other genotoxicity tests with AHTN were considered as negative. In particular, the negative finding in the in vivo assay supports AHTN as not likely to be mutagenic in mammalian systems. These considerations, along with other negative published data, lead to the conclusion that both AHTN and HHCB do not have significant potential to act as genotoxic carcinogens.

Application of the in vitro rat hepatocyte micronucleus assay in genetic toxicology testing

The investigation of micronuclei in mitogenic stimulated hepatocytes in vitro is a quite new area of research. Nevertheless, a relatively large database comprising more than 40 tested compounds of various classes has been generated up to now. This paper reviews the available data for the in vitro rat hepatocyte micronucleus assay, showing a sensitivity of this assay in identifying mutagens and genotoxic liver carcinogens of about 85%. Additionally, all of the tested non-carcinogens gave negative results. The use of primary hepatocytes instead of permanently dividing mammalian cell lines for the investigation of micronucleus induction has several advantages. (1) The broad spectrum of metabolizing enzymes expressed in primary hepatocytes ensures an adequate activation of most xenobiotics. (2) No transfer of activated metabolites via the culture medium is necessary in this system, since the metabolizing cells are the target cells themselves. (3) Whilst in experiments with permanently dividing cells the use of S9-mix restricts the treatment period with the test compounds to 2-6 h in the hepatocyte micronucleus assay continuous treatment of up to 48 h is possible. Investigations with the pyrrolizidine alkaloids retrorsine, monocrotaline and isatidine, strong mutagens and liver carcinogens, clearly showed that at least for isatidine a prolonged exposure period is essential to detect its mutagenic potential. This compound gave positive results in rat hepatocytes but not in V79-cells/S9-mix cultures. (4) The results obtained with the hepatocyte micronucleus assay are in good agreement with the genotoxic profiles of most of the compounds tested. Only three polycyclic aromatic hydrocarbons led to 'false-negative' results, since they strongly inhibited hepatocyte proliferation and thereby prevented micronucleus formation. (5) Hepatocytes are target cells of special interest when compounds are investigated which act specifically in the liver. Especially for hepatocarcinogens classified as non-genotoxins in standard genotoxicity tests or for chemicals showing DNA-repair induction in hepatocytes but no mutagenicity in standard tests, the hepatocyte micronucleus assay can contribute to clarify the situation. (6) The rat hepatocyte micronucleus assay can be performed easily and without great efforts in parallel to the in vitro hepatocyte DNA repair test (UDS-test), using the same hepatocyte batches. (7) Similar to the two versions of the UDS-test, the hepatocyte micronucleus assay can be performed following an in vivo-in vitro protocol. In order to further validate the hepatocyte micronucleus assay, as a next step controlled interlaboratory studies should be initiated.

Genotoxicity and subchronic toxicity studies with heated olestra

Olestra is a class of sucrose-fatty acid polyesters intended for use as a non-caloric replacement of edible oil. Genotoxicity and subchronic toxicity studies were conducted to determine whether olestra could form genotoxic or toxic breakdown products during simulated commercial use. Heated olestra was prepared for these studies by batch-frying potato slices in olestra at 177-185 degrees C for 25-32 hr over 5-7 days. Genotoxicity of this previously heated olestra was assessed in four standard in vitro assays: (1) Salmonella mutagenesis (Ames test); (2) forward mutagenesis of mouse lymphoma cells at the thymidine kinase locus; (3) unscheduled DNA synthesis in rat hepatocytes; and (4) clastogenicity in cultured Chinese hamster ovary cells. These tests were conducted with previously heated olestra at concentrations up to at least 5 mg/ml both in the absence of exogenous bioactivation and, for assays (1), (2) and (4) with added liver microsomal (S-9) activation. The Ames and mouse lymphoma assays were performed with olestra (10 mg/ml and 23 mg/litre, respectively) either alone or emulsified with the non-toxic, non-ionic surfactant Pluronics F68, both in the presence and absence of metabolic activation. To test for clastogenicity in vivo, rats were administered previously heated olestra by gavage at 5 g/kg per day for up to 5 days and bone marrow cells were examined for chromosomal aberrations. Heated olestra lacked genotoxic activity detectable by the aforementioned assays. Heated olestra was fed to Fischer 344 rats at up to 10% of the diet (w/w) for 91 days. Evaluation of survival, food consumption, feed efficiency, physical condition, body weight, organ weight, haematological and clinical chemistry parameters, and histomorphology revealed no adverse effects attributable to ingestion of heated olestra at exposure levels in excess of those anticipated for human consumption. It is concluded that olestra used as a deep-frying medium conveys no genotoxic or toxic hazard at anticipated levels of human consumption.

An evaluation of genotoxicity tests with Musk ketone

Musk ketone, a synthetic musk fragrance ingredient that has been found in river water, fish and breast milk, was evaluated for potential genotoxicity in a battery of short-term tests. The mouse lymphoma assay was conducted at musk ketone concentrations ranging from 700 to 4000 micrograms/ml and 2.0 to 35 micrograms/ml in the absence and presence of rat liver S-9, respectively. No increased mutant frequencies were noted. An in vitro cytogenetics assay in Chinese hamster ovary cells was conducted at musk ketone concentrations ranging from 4.3 to 34 micrograms/ml and 1.25 to 10 micrograms/ml in the absence and presence of rat liver S-9, respectively. On the basis of the non-reproducibility of a statistically significant increase at a single concentration and no increases in other test systems, musk ketone was concluded to be negative for chromosome aberrations. An in vitro unscheduled DNA synthesis (UDS) assay was conducted in primary rat hepatocytes at musk ketone concentrations between 0.5 and 5.0 micrograms and 50 micrograms/ml. No increases in net nuclear grain counts were noted. Musk ketone did not show genotoxic potential based on the negative results in the mouse lymphoma, in vitro cytogenetics and in vitro UDS assays.

Human protection against non-genotoxic carcinogens in the US without the Delaney Clause

Cancers of many types are major chronic diseases with a high fatality rate and a high cost to society. In the USA, the Delaney Clause was implemented in 1958 because the public believed that many cancers stem from food additives and food contaminants. In the intervening years, research has provided key information about the mechanisms of carcinogenesis and demonstrated that there are two major classes of carcinogens, genotoxic and non-genotoxic. Two case reports are presented, of sodium saccharin and ethylenebisdithiocarbamates that were banned based on the Delaney Clause in an unjustified manner, based on the underlying mechanisms not relevant for non-genotoxic carcinogens. Also, the causes of major cancers have been discovered. Most cancers are associated with lifestyle, specifically tobacco and excessive alcohol use, inappropriate nutritional traditions, and lack of exercise. These lifestyle components involve now known genotoxic carcinogens and importantly, non-genotoxic carcinogens. The effect of non-genotoxic carcinogens is highly dose dependent and also reversible upon lowering the dose below a threshold. Thus, it is quite possible to lower human cancer risk, and also the risk of related chronic diseases such as coronary heart disease, hypertension and stroke, adult on-set diabetes, by proper lifestyle adjustments. Clearly, the Delaney Clause plays no role in disease prevention.

An evaluation of musk xylene in a battery of genotoxicity tests

Musk xylene (CAS no. 81-15-2), a synthetic musk fragrance ingredient, was evaluated in a battery of short-term genotoxicity tests that included the mouse lymphoma assay, an in vitro cytogenetics assay in Chinese hamster ovary (CHO) cells, the in vitro unscheduled DNA synthesis (UDS) assay in primary rat hepatocytes and an in vivo UDS assay. Musk xylene gave uniformly negative results in these genotoxicity tests. These observation, combined with previously reported negative Ames tests, suggest a non-genotoxic mechanism for the induction of mouse liver tumours by musk xylene.

Evaluation studies on the in vitro rat hepatocyte micronucleus assay

Based on a previous study with 8 chemicals (Müller et al., 1993) the applicability of the in vitro rat hepatocyte micronucleus assay was evaluated by testing a further 21 compounds of different chemical classes. The obtained results are in good agreement with the known genotoxic profiles of about 90% of the in total tested compounds. Several known mutagens and carcinogens, i.e., alkylating agents, aromatic amines, nitrosamines, nitro compounds, cross-linking agents, and pyrrolizidine alkaloids gave clear positive results in this assay, whereas all of the tested non-carcinogens were negative. The hepatocyte micronucleus assay was shown to distinguish between carcinogenic/non-carcinogenic isomers, such as 2- and 4-acetylaminofluorene (AAF) and 2- and 1-nitropropane (NP). Furthermore, the non-genotoxic nature of several hepatocarcinogens, i.e., the peroxisome proliferating agents fenofibrate, nafenopin, Wy-14,643, diethyl(hexyl)phthalate (DEHP), and the sedative phenobarbital, could be confirmed in this assay. The hepatocarcinogen coumarin exerted mitogenic but no mutagenic properties in the rat hepatocyte micronucleus assay. This compound may act as a liver tumor promoter. Benzo[a]pyrene (B[a]P) and 7,12-dimethylbenzanthacene (DMBA), both belonging to the group of known carcinogenic and mutagenic polycyclic aromatic hydrocarbons, failed to induce micronucleus formation in rat hepatocytes. The high susceptibility of in vitro proliferating hepatocytes to mitotic inhibition, exerted by the strong cytotoxic actions of these compounds, seems to be responsible for these negative results. A strongly reduced mitotic activity can prevent the formation of micronuclei, even when clastogenic effects may have occurred. In the present stage, the in vitro rat hepatocyte micronucleus assay cannot be recommended for screening genotoxicity testing. It should rather be used for special purposes, e.g., when liver-specific mutagenic effects are expected.

Human bioassays to assess environmental genotoxicity: development of a DNA repair assay in HepG2 cells

A direct assessment of the effects of environmental chemicals on human health has been hampered by the lack of suitable experimental systems. We have recently employed a human liver cell line (HepG2) to assess the biological effects of pollutants at both cellular and DNA levels. A Neutral Red dye uptake assay was used to assess potential cytotoxic effects of xenobiotics. DNA damage was quantified using an unscheduled DNA synthesis assay that measures repair that is induced following exposure to genotoxic compounds. HepG2 cells responded to the known mutagens, 4-nitroquinoline N-oxide and methylmethane sulfonate, both in the Neutral Red assay for cytotoxicity and two DNA repair assays for genotoxicity (monitored autoradiographically or by liquid scintillation counting). The HepG2 DNA repair and cytotoxicity assays also responded to an extract (containing polycyclic aromatic hydrocarbons) of sediment obtained from a polluted site in the Great Lakes. Results indicate that this system can be deployed further to assess potential cyto- and genotoxicity of pollutants. The development of human cell culture assays is a critical step towards a full assessment of the risk that such pollutants pose to human health.

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.

Genotoxicity studies in semiconductor industry. 1. In vitro mutagenicity and genotoxicity studies of waste samples resulting from plasma etching

Solid waste samples taken from the etching reactor, the turbo pump, and the waste air system of a plasma etching technology line in semiconductor production were studied as to their genotoxic properties in a bacterial repair test, in the Ames/Salmonella microsome assay, in the SOS chromotest, in primary mouse hepatocytes, and in Chinese hamster V79 cell cultures. All three waste samples were found to be active by inducing of unscheduled DNA-synthesis in mouse hepatocytes in vitro. In the bacterial rec-type repair test with Proteus mirabilis, waste samples taken from the turbo pump and the vacuum pipe system were not genotoxic. The waste sample taken from the chlorine-mediated plasma reactor was clearly positive in the bacterial repair assay and in the SOS chromotest wit Escherichia coli. Mutagenic activity was demonstrated for all samples in the presence and absence of S9 mix made from mouse liver homogenate. Again, highest mutagenic activity was recorded for the waste sample taken from the plasma reactor, while samples collected from the turbo pump and from the waste air system before dilution and liberation of the air were less mutagenic. For all samples chromosomal damage in V79 cells was not detected, indicating absence of clastogenic activity in vitro. Altogether, these results indicate generation of genotoxic and mutagenic products as a consequence of chlorine-mediated plasma etching in the microelectronics industry and the presence of genotoxins even in places distant from the plasma reactor. Occupational exposure can be expected both from the precipitated wastes and from chemicals reaching the environment with the air stream.

Genotoxic effects of selected peroxisome proliferators

Peroxisome proliferators, a class of structurally dissimilar chemicals including hypolipidemic drugs and industrial plasticizers, have been shown to be associated with hepatocarcinogenesis although an initiating effect could not yet be demonstrated in the cell systems utilized. For this reason the genotoxic potential of the peroxisome proliferators nafenopin, ciprofibrate and di(2-ethylhexyl)adipate (DEHA) was determined in primary cultures of adult rat hepatocytes. To further test if these compounds are genotoxic per se or the genotoxic effect is due to peroxisome proliferation, the cultures were exposed for 3 and 51 h. Treatment for 3 h with the hypolipidemic drugs nafenopin and ciprofibrate induced statistically significant increases of SCE at concentrations > or = 30 and 100 microM respectively. At higher concentrations statistically significant increases of chromosomal aberrations (nafenopin: 100 microM; ciprofibrate: > or = 100 microM) and micronuclei (ciprofibrate: > or = 250 microM) were also found. The presence of peroxisome proliferators in the media until harvesting (51 h) did not significantly alter the dose response of SCE, micronuclei and chromosomal aberration induction by ciprofibrate, while long-term exposure to nafenopin resulted in statistically significant increases of chromosomal aberrations and micronuclei at concentrations > or = 30 microM. The differences were statistically significant at 30 and 100 microM for micronuclei, and at 30 microM for chromosomal aberrations. Neither short- nor long-term exposure to DEHA produced a significant genotoxic effect up to 200 microM. The peroxisome proliferators tested were not cytotoxic at any concentration, as determined by mitotic index. These results clearly demonstrate that the peroxisome proliferators nafenopin and ciprofibrate can cause genotoxic effects in primary cultures of adult rat hepatocytes. The comparison of short- and long-term exposure does not suggest a strong correlation between the induction of peroxisome proliferation and genotoxicity, since long-term exposure did not significantly alter the dose response and--except for nafenopin--the extent of the genotoxic effects.

Effects of selected secondary metabolites of Fusarium moniliforme on unscheduled synthesis of DNA by rat primary hepatocytes

The Fusarium moniliforme mycotoxins--fusarin C, fumonisin B1, moniliformin and bikaverin--were evaluated for genotoxicity by their ability to induce unscheduled DNA synthesis (UDS) in primary rat hepatocytes. Isolated hepatocytes were exposed to several concentrations of moniliformin (5.0-500 microM), bikaverin (1.0-500 microM), fumonisin B1 (0.5-250 microM), or fusarin C (1.0-100 microM). Aflatoxin B1, a known inducer of UDS, was included as a positive control. UDS was determined by autoradiography of cells after their exposure to [3H]thymidine. The highest doses of fusarin C and bikaverin caused cell death, but no cytotoxicity was observed in cells exposed to moniliformin or fumonisin B1. Fumonisin B1, moniliformin and bikaverin were not genotoxic in the UDS assay. The results of the UDS assay with fusarin C were inconclusive since a marginal effect on UDS was obtained.

Differential cytotoxicity and mycotoxin content among isolates of Fusarium moniliforme

Ten different isolates of the common corn fungus, Fusarium moniliforme, were cultured on corn, and the production by the isolates of two important mycotoxins, fusarin C and fumonisin B1, was compared. Additionally, both aqueous and organic extracts of the cultures were tested for cytotoxicity to rat primary hepatocytes by measuring the effects of three dose levels on the ability of the cells to take up valine and to cause the release of the cytoplasmic enzyme, lactate dehydrogenase. The fungal isolates differed drastically in their ability to produce the two mycotoxins and in their cytotoxicity. However the toxic effects could not be accounted for by the content of the two toxins measured. Therefore it appears that there are other toxins, both organic and aqueous soluble compounds, that are toxic to liver cells.

The effect of deuterium labeling on the genotoxicity of N-nitrosodimethylamine, epichlorohydrin and dimethyl sulfate

Deuterated and non-deuterated N-nitrosodimethylamine, epichlorohydrin and dimethyl sulfate were evaluated for the ability to induce DNA single-strand breaks in rat hepatocytes as measured by alkaline elution. Non-deuterated nitrosodimethylamine induced twice the amount of DNA-strand breaks as the deuterated form. No evidence of a deuterium isotope effect was seen for the direct-acting alkylating agents epichlorohydrin and dimethyl sulfate.

The application of a wedge perfusion technique to the in vivo-in vitro rat hepatocyte DNA-repair assay

The in vivo-in vitro rat hepatocyte DNA-repair assay is regarded as labour-intensive and time-consuming to perform. This has tended to impose limitations on its use as a routine procedure for assessing the potential genotoxicity of chemicals. We have developed a simple wedge-perfusion technique which enables hepatocytes to be isolated from several different rats simultaneously. Hepatocyte yield and metabolic capacity are comparable to those isolated by conventional whole-liver perfusion. Hepatocyte viability was generally superior to that obtained when performing multiple in situ perfusions for the rat hepatocyte UDS assay. The median lobe is routinely used but no difference was observed in the UDS response to the positive control genotoxic agents, methyl methanesulphonate (MMS, CAS No. 66-27-3) and 2-acetylaminofluorene (AAF, CAS No. 53-96-3), in hepatocytes isolated from the median or either lateral lobe. The use of Williams medium E or Leibovitz L15 culture medium did not influence the response. This perfusion technique greatly reduces the time, equipment and personnel required and therefore the cost for hepatocyte isolation. It also facilitates the inclusion of concurrent control groups at each time point of assay.

Evaluation of four methods for scoring cytoplasmic grains in the in vitro unscheduled DNA synthesis (UDS) assay

The in vitro unscheduled DNA synthesis (UDS) assay measures DNA repair (incorporation of [3H]thymidine) following in vitro treatment of rat primary hepatocytes. The autoradiographic method was used to detect UDS by counting developed silver grains in the photographic emulsion overlaying nuclei and cytoplasmic areas of the hepatocytes. In this communication we report results using 4 scoring methods: (1) the 2 most heavily labeled cytoplasmic areas adjacent to the nucleus (our standard method), (2) the cytoplasmic area left of the nucleus, (3) the cytoplasmic areas left and right of the nucleus, and (4) 2 cytoplasmic areas whose positions were selected at random. Rat primary hepatocyte cultures treated with a medium control, a solvent control (dimethyl sulfoxide) and 5 known genotoxic chemicals (2-acetylaminofluorene, dimethylnitrosamine, diethylnitrosamine, methyl methanesulfonate and ethyl methanesulfonate) were scored using these 4 methods. The average or maximum cytoplasmic grain count was subtracted from the nuclear grain count to yield net grains/nucleus (NG). In general, NG counts for Methods 2, 3 and 4 were similar, although shifted about 3-10 grains higher than Method 1 for controls and most treated groups. Methods 2, 3 and 4 showed more experiment-to-experiment variability in sensitivity for detecting statistically significant increases in treated groups than did our standard method. Thus, the alternative methods afforded no consistent improvements in sensitivity or reduction of variability for this assay. Subtraction of the average or the highest cytoplasmic count had virtually no effect on the sensitivity of the assay, but simply requires an appropriate adjustment of the criteria for a positive response.

Strain differences in in vitro rat hepatocyte unscheduled DNA synthesis (UDS): effect of UV is independent of strain while increased sensitivity is apparent using Fischer-344 instead of Sprague-Dawley rats

The unscheduled DNA synthesis (UDS) assay measures DNA repair following in vitro treatment of rat primary hepatocytes. This report compares the UDS response of primary hepatocytes from 2 widely used rat strains, the Fischer-344 (F344) and Sprague-Dawley (SD) strains. Ultraviolet (UV) light and 5 known genotoxic chemicals were evaluated in each strain in parallel experiments. The chemicals tested were 2-acetylaminofluorene (2-AAF), 4-aminobiphenyl (4-AB), benzidine, dimethylnitrosamine (DMN) and N-propyl-N'-nitro-N-nitrosoguanidine (PNNG). Four of these compounds (2-AAF, 4-AB, benzidine and DMN) require metabolic activation. Benzidine and PNNG were both negative using SD rat hepatocytes, but were weakly positive using F344 rat hepatocytes. In the first of 2 experiments, 4-AB was inconclusive in SD hepatocytes, but strongly positive in F344 cells. In the second experiment, 4-AB was positive in hepatocytes from both strains. 2-AAF was more strongly positive in F344 cells than in SD cells. DMN and UV light induced positive dose responses with little or no differences between strains. It is concluded that hepatocytes from F344 rats may be more sensitive, qualitatively and quantitatively, than hepatocytes from SD rats as indicators of UDS. This difference is not due to intrinsic differences in DNA repair mechanisms but is probably due to differences in drug-metabolizing enzymes between these strains. Thus, for routine screening, F344 rats are preferable for measurement of the in vitro UDS-inducing potential of compounds.

Genotoxicity evaluation of lithium hypochlorite

Lithium hypochlorite (LiOCl), the pool and spa sanitizer/algicide, was evaluated for genotoxicity in a battery of studies designed to evaluate potential mutagenicity, DNA damage and chromosome aberrations. LiOCl was not mutagenic in the Ames test when tested in Salmonella typhimurium strains TA98, TA100, TA1535, TA1537, TA1538 or in the hypoxanthine-guanine phosphoribosyl transferase (HGPRT) mutation assay in Chinese hamster ovary (CHO) cells without metabolic activation. LiOCl did not induce DNA damage in the unscheduled DNA synthesis assay using rat primary hepatocytes. Effects on metaphase chromosomes were evaluated in vitro in CHO cells at 12 and 18 h exposure without S9 and at 12 and 22 h following a 2 h exposure with S9. LiOCl induced a statistically significant increase in chromosome aberrations at the high dose only at both harvest times without S9 and at the late harvest time with S9. There were significant increases in chromosome aberrations at the low dose, low-mid and high doses, but not at the high mid-dose at the early harvest time with S9. However, LiOCl did not increase chromosome aberrations when tested orally in rats at maximally tolerated doses. Bone marrow cells, collected 6, 24 and 48 h after a single oral dose of LiOCl to rats (100, 500, 1000 mg/kg in males; 50, 250, 500 mg/kg in females) showed no increase in the incidence of aberrations. In general, the weight of the evidence indicates that LiOCl is not genotoxic.

Induction of DNA-repair synthesis in primary rat hepatocytes by epoxides

The genotoxicity of 10 epoxides was investigated in the UDS test with primary rat hepatocytes. The sensitivity of the assay was demonstrated using 2-acetylaminofluorence. The epoxides 1,2-epoxyoctane, 1,2-epoxydecane, epoxycyclooctane, epoxycyclododecane, (+)-limoneoxide, alpha-pinaneoxide, transstilbeneoxide, and cis-2,3-epoxysuccinic acid, which are known to be non-mutagenic in the Ames test, as well as the bacterial mutagen, 1,2-epoxyphenoxypropane did not induce UDS in primary hepatocytes of the rat. However, a positive UDS response obtained with glycidyltrimethylammonium chloride showed that metabolic inactivation of the oxirane ring in hepatocytes is influenced by further structural substituents.

DNA repair induced by various mutagens in rat hepatocyte primary cultures measured in the presence of hydroxyurea, guanazole or aphidicolin

Guanazole and aphidicolin were chosen as candidates in the search for a selective, non-genotoxic inhibitor of DNA replication which could be used instead of hydroxyurea to measure DNA repair synthesis in rat hepatocyte primary cultures by liquid scintillation counting. The genotoxicity of these 3 chemicals was studied using the Salmonella/liver homogenate assay and the autoradiographic UDS test in hepatocytes. Hydroxyurea was positive in both of these assays. Guanazole and aphidicolin did not induce DNA repair in hepatocytes. Aphidicolin was not mutagenic for Salmonella typhimurium, whereas guanazole increased the revertant numbers of strain TA102 slightly. The incorporation of [3H]thymidine was measured by liquid scintillation to determine DNA repair induced by 2-acetylaminofluorene (2-AAF), aflatoxin B1, benzo[a]pyrene, cyclophosphamide, H2O2, 6-hydroxydopamine, N-methyl-N'-nitro-N-nitrosoguanidine (MNNG), methylnitrosourea (MNU), 4-nitroquinoline-N-oxide and UV irradiation in the presence of either 10 mM hydroxyurea, 15 mM guanazole or 0.015 mM aphidicolin. Aphidicolin had an inhibitory effect on DNA repair. Except for the 3 chemicals mentioned below, the sensitivity of the DNA repair measurement was the same, no matter whether hydroxyurea or guanazole was used to inhibit replicative DNA synthesis. In the presence of hydroxyurea, DNA repair synthesis was found at lower concentrations in the case of aflatoxin B1, due to differences in the solvent control values, and in the case of H2O2, possibly due to a synergistic effect between hydroxyurea and H2O2. Guanazole allowed the detection of DNA repair induced by MNNG at lower concentrations, probably because of an antagonistic effect between hydroxyurea and MNNG. Based on these results, it was concluded that guanazole, but not aphidicolin, could be used instead of hydroxyurea to measure DNA repair synthesis by liquid scintillation in rat hepatocyte primary cultures. Although guanazole does not completely fulfill the criteria for an ideal DNA replication inhibitor, it has the advantage of being less genotoxic than hydroxyurea, and also appears to have a smaller potential to falsify the results by interacting with the test compounds.

Structure-activity relationships in the rat hepatocyte DNA-repair test for 300 chemicals

312 chemicals/mixtures were tested for genotoxicity in the rat hepatocyte/DNA-repair test. A variety of structure-activity relationships was evident. Of the 309 pure chemicals, 142 were positive. Of these, 43 were judged by IARC to have sufficient or limited evidence of carcinogenicity and none of the remainder was a proven noncarcinogen. Among the 167 negative chemicals, 44 were carcinogens. Some of these are known to be genotoxic in other systems, but based on several lines of evidence, many are considered to be epigenetic carcinogens that lack the ability to react with DNA and rather lead to neoplasia by nongenotoxic mechanisms.

The in vitro unscheduled DNA synthesis (UDS) assay in rat primary hepatocytes: evaluation of 2-furoic acid and 7 drug candidates

The in vitro unscheduled DNA synthesis assay (UDS) is part of the routine genetic toxicology screening at The Upjohn Company. The purpose of this paper is to report results for 8 compounds which were tested in the in-house genetic toxicology program. These compounds represent diverse chemical structure and most of them entered the screening program because they are biologically active in efficacy screens. All tests were carried out under Good Laboratory Practices Regulations of the U.S. Food and Drug Administration. None of the materials reported here produced an increase in UDS and therefore the UDS results with these compounds do not suggest potential for genotoxicity.

The in vitro unscheduled DNA synthesis (UDS) assay in rat primary hepatocytes: evaluation of 24 drug candidates

The in vitro unscheduled DNA synthesis assay (UDS) is part of the routine genetic toxicology screening at The Upjohn Company. The purpose of this paper is to report results for 24 drug candidates which were tested as coded compounds. These compounds are very diverse in chemical structure and represent classes of compounds selected because of biological activity in a variety of preliminary drug efficacy screens. None of the compounds reported here produced an increase in UDS, and therefore, the UDS results with these materials do not suggest potential for mutagenesis or carcinogenesis.

The in vitro unscheduled DNA synthesis (UDS) assay in rat primary hepatocytes. Validation of improved methods for primary culture including data on the lack of effect of ionizing radiation

The in vitro unscheduled DNA synthesis (UDS) assay was evaluated for inclusion in a battery of assays used at The Upjohn Company for evaluation of lead compounds in the development of new and existing drug entities. This evaluation process encompassed aspects of the isolation of hepatocytes and tests of reference mutagens and genotoxins. The flow rate of perfusion solutions and their temperatures were critical in the isolation of high viability hepatocytes in good yield. The attachment of freshly isolated hepatocytes to coverslips was greatly enhanced by coating the coverslips with type III collagen. Results of testing 12 known genotoxic agents (UV light, cyclophosphamide, 7,12-dimethylbenzanthracene, dimethylnitrosamine, diethylnitrosamine, 2-acetylaminofluorene, benzo[a]pyrene, methyl methanesulfonate, ethyl methanesulfonate, N-propyl-N'-nitro-N-nitrosoguanidine, benzidine and 4-aminobiphenyl) were in agreement with the literature. The use of X-ray did not induce unscheduled DNA synthesis in hepatocytes. This latter finding draws attention to the inability of this assay to detect agents which result in 'short-patch' repair of damage.

Enhancement of bovine papillomavirus-induced cell transformation by tumour promoters

Cultured C3H/10T1/2 cells transfected with the plasmid pdBPV-1 were used as targets, and the frequency of transformed colonies as the endpoint to test the enhancing capacity of four promoters: 12-O-tetradecanoylphorbol-13-acetate (TPA), 4-O-methyl-tetradecanoylphorbol-13-acetate (4-O-methyl-TPA), mezerein and phorbol-12-retinoate-13-acetate (PRA). The frequency of the transfected C3H/10T1/2 cells to form transformed colonies was enhanced in the following order: mezerein greater than PRA greater than TPA greater than 4-O-methyl-TPA. The amount of promoters required to promote a tenfold increase in transformed cells was 0.24, 0.81, 30 and 100 ng/ml mezerein, PRA, TPA and 4-O-methyl-TPA, respectively. A significant promoting effect was obtained by a 3.5-day exposure to mezerein regardless of whether it was added at different time intervals after transfection with BPV-DNA. The examined promoters lacked genotoxic activity, as tested on Chinese hamster ovary cells, using chromatid aberrations and exchanges, frequency of macronuclei, unscheduled DNA synthesis (UDS) and inhibition of UDS as endpoints. The usefulness of BPV-1-induced transformation as a bioassay for detecting chemicals with promoting activities is discussed.

Mutagenicity studies on ketone solvents: methyl ethyl ketone, methyl isobutyl ketone, and isophorone

3 ketone solvents (methyl ethyl ketone (MEK), methyl isobutyl ketone (MiBK), and isophorone) were tested for potential genotoxicity. The assays of MEK and MiBK included the Salmonella/microsome (Ames) assay, L5178Y/TK+/- mouse lymphoma (ML) assay, BALB/3T3 cell transformation (CT) assay, unscheduled DNA synthesis (UDS) assay, and micronucleus (MN) assay. Only the ML, UDS, and MN assays were conducted on samples of isophorone. No genotoxicity was found for MEK or isophorone. The presence of a marginal response only at the highest, cytotoxic concentration tested in the ML assay, the lack of reproducibility in the CT assay, and clearly negative results in the Ames assay, UDS and MN assays, suggest that MiBK is unlikely to be genotoxic in mammalian systems.

International Commission for Protection against Environmental Mutagens and Carcinogens. ICPEMC working paper No. 5. Genotoxicity tests as predictors of carcinogens: an analysis

Differences between the results of numerical validation studies comparing in vitro and in vivo genotoxicity tests with the rodent cancer bioassay are leading to the perception that short-term tests predict carcinogenicity only with uncertainty. Consideration of factors such as the pharmacokinetic distribution of chemicals, the systems available for metabolic activation and detoxification, the ability of the active metabolite to move from the site of production to the target DNA, and the potential for expression of the induced lesions, strongly suggests that the disparate sensitivity of the different test systems is a major reason why numerical validation is not more successful. Furthermore, genotoxicity tests should be expected to detect only a subset of carcinogens, namely genotoxic carcinogens, rather than those carcinogens that appear to act by non-genetic mechanisms. Instead of relying primarily on short-term in vitro genotoxicity tests to predict carcinogenic activity, these tests should be used in a manner that emphasizes the accurate determination of mutagenicity or clastogenicity. It must then be determined whether the mutagenic activity is further expressed as carcinogenicity in the appropriate studies using test animals. The prospects for quantitative extrapolation of in vitro or in vivo genotoxicity test results to carcinogenicity requires a much more precise understanding of the critical molecular events in both processes.

Rat liver foci and in vitro assays to detect initiating and promoting effects of chlorinated ethanes and ethylenes

Nine chlorinated aliphatics (CAs) were examined in a rat liver foci assay for tumor initiating and promoting activities. In this model, young adult male Osborne Mendel rats were first subjected to a partial hepatectomy, the test chemical was then administered at the maximum tolerated dose in the initiation or promotion phase in conjunction with diethylnitrosamine (DEN; 30 mg/kg b.w.) or phenobarbital (PB; 0.05 percent, w/w, in the diet), and gamma glutamyltranspeptidase (GGT) was used as a putative preneoplastic indicator. When administered in the promotion protocol after initiation with DEN, 1,1-dichloroethane, 1,1,2-trichloroethane (1,1,2-TCE), 1,1,2,2-tetrachloroethane (1,1,2,2-TTCE), tetrachloroethylene (TTCY), and hexachloroethane induced significant increases in GGT+-foci above control levels. 1,1,2,2-TTCE, TTCY, and 1,1,2-TCE also induced significant increases in GGT+-foci when administered in the promotion protocol without DEN initiation. Two variants of GGT+-foci were observed: the classical type associated with PB promotion, and the other, which was more diffuse, less intensely stained, resembling foci undergoing redifferentiation and associated with CAs. A number of CAs were also genotoxic in short-term in vitro tests. Taken together, the studies suggest that CAs may be complete carcinogens in vivo with weak initiating activity and stronger promoting activity.

Effect of varying the exposure and 3H-thymidine labeling period upon the outcome of the primary hepatocyte DNA repair assay

The results presented in this report demonstrate that an 18-20 hour exposure/3H-thymidine DNA labeling period is superior to a 4 hour incubation interval for general genotoxicity screening studies in the rat primary hepatocyte DNA repair assay. When DNA damaging agents which give rise to bulky-type DNA base adducts such as 2-acetylaminofluorene, aflatoxin B1 and benzidine were evaluated, little or no difference was observed between the 4 hour or an 18-20-hour exposure/labeling period. Similar results were also noted for the DNA ethylating agent diethylnitrosamine. However, when DNA damaging chemicals which produce a broader spectrum of DNA lesions were studied, differences in the amount of DNA repair as determined by autoradiographic analysis did occur. Methyl methanesulfonate and dimethylnitrosamine induced repairable DNA damage that was detected at lower dose levels with the 18-20 hour exposure/labeling period. Similar results were also observed for the DNA cross-linking agents, mitomycin C and nitrogen mustard. Ethyl methanesulfonate produced only a marginal amount of DNA repair in primary hepatocytes up to a dose level of 10(-3) M during the 4 hour incubation period, whereas a substantial amount of DNA repair was detectable at a dose level of 2.5 X 10(-4) M when the 18-20 hour exposure/labeling period was employed. The DNA alkylating agent 4-nitroquinoline-1-oxide, which creates DNA base adducts that are slowly removed from mammalian cell DNA, induced no detectable DNA repair in hepatocytes up to a toxic dose level of 2 X 10(-5) M with the 4 hour exposure period, whereas a marked DNA repair response was observed at 10(-5) M when the 18-20 hour exposure/labeling period was used.

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.

Griseofulvin

Griseofulvin (GF) is a mycotoxin produced by various species of Penicillium including P. griseofulvum Dierckx, P. janczewski (P. nigricans) and P. patulum. It is active against dermatophytic fungi of different species in the genera Microsporum, Trychophyton and Epidermophyton. Because of its capacity to concentrate in the keratinous layer of the epidermis and its relatively low toxicity in man, it has been extensively used in the therapy of dermatophytoses by oral administration. The biological activity of GF towards fungi is manifested as nuclear and mitotic abnormalities followed by distortions in the hyphal morphology. Mitotic segregation is also induced in fungi by GF treatment. In higher eukaryotes the cytostatic action of GF is essentially due to a mitotic arrest at late metaphase/early anaphase. The cytological effects observable both in vivo and in vitro on different plant and animal cell systems, include C-mitoses, multipolar mitoses and multinuclearity. Prolonged GF treatment in experimental animals provokes biochemical changes consisting mainly of disturbances of porphyrin metabolism, variation in the microsomal cytochrome levels and formation of Mallory bodies. In mice these alterations are followed by the development of multiple hepatomas. Evidence of tumor induction by GF has been obtained in mice and rats, but not in hamsters. GF may also act either as a promoting or a co-carcinogenic agent, depending on the circumstances of its administration. It has been found to increase the frequency of cell transformation induced by polyoma virus, but not to induce cell transformation per se. Induction of sperm abnormalities has been observed in GF-treated mice. The embryotoxic and teratogenic action of GF has been demonstrated in pregnant rats exposed during organogenesis. Genetic effects of GF have been investigated by the following tests: Salmonella/microsome mutagenicity assay, point mutations in mammalian and plant cells, DNA damage and repair, SCE, chromosome aberrations, micronuclei, dominant lethals, aneuploidy in lower and higher eukaryotes. A positive response has been obtained in the assays on numerical chromosome changes in all the systems analyzed; limited or inconclusive evidence has been obtained for SCE and structural chromosome changes. Doubled or highly polyploid sets can be detected in all types of cells during or immediately after GF treatment. A marked increase in chromosome number variation is observed at various times after withdrawal of the drug, with prevailing hyperdiploid and reduced sets in animal cells and plant cells respectively.(ABSTRACT TRUNCATED AT 400 WORDS)

Species and sex differences in genotoxicity of heterocyclic amine pyrolysis and cooking products in the hepatocyte primary culture/DNA repair test using rat, mouse, and hamster hepatocytes

Eleven mutagenic heterocyclic amines, 3-amino-1,4-dimethyl-5H-pyrido[4,3-b]-indole (Trp-P-1), 3-amino-1-methyl-5H-pyrido[4,3]indole (Trp-P-2), 2-amino-6-methyl-dipyrido[1,2-a:3',2'-d]imidazole (Glu-P-1), 2-aminodipyrido[1,2-a:3',2'-d]imidazole (Glu-P-2), 2-amino-9H-pyrido[2,3-b]indole (A alpha C), 2-amino-3-methyl-9H-pyrido[2,3-b]indole (MeA alpha C), 2-amino-3-methylimidazo[4,5-f]quinoline (IQ), 2-amino-3,4-dimethylimidazo[4,5-f]quinoline (MeIQ), 2-amino-3,8-dimethylimidazo [4,5-f]quinoline (MeIQX), 2-amino-3,4,8-trimethylimidazo[4,5-f]quinoxaline (4,8-diMeIQX), and 2-amino-3,7,8-trimethylimidazo[4,5-f]quinoxaline (7,8-diMeIQX), were studied for genotoxicity in the hepatocyte/DNA repair test employing hepatocytes of male rats, male and female mice, and male hamsters. In these four assay systems, all compounds elicited DNA repair in at least three systems, except Trp-P-2, which was uniformly inactive. However, there were several significant differences in the responses of different systems. Rat and hamster hepatocytes responded to nine of the ten genotoxic compounds with the exception of Glu-P-2. Male and female mouse hepatocytes responded to Glu-P-2, whereas female, but not male, mouse hepatocytes responded to MeIQX and 4,8-diMeIQX. These results illustrate species and sex differences in response to these heterocyclic amines and suggest that a number of these compounds are carcinogenic in hamsters, as they have been in rats and mice.

A protocol and guide for the in vivo rat hepatocyte DNA-repair assay

The in vivo rat hepatocyte DNA-repair assay is a valuable tool in assessing the genotoxic activity of chemical agents. An advantage of the system is that it reflects the complex patterns of uptake, distribution, metabolism, detoxification and excretion that actually occur in the whole animal. This article provides a typical procedure and guidelines for conducting the rat in vivo hepatocyte DNA-repair assay.