Detection of aneuploidy-inducing carcinogens in the Syrian hamster embryo (SHE) cell transformation assay

Cell transformation assays: are we barking up the wrong tree?

There has been a current resurgence of interest in the use of cell transformation for predicting carcinogenicity, which is based mainly on rodent carcinogenicity data. In view of this renewed interest, this paper critically reviews the published literature concerning the ability of the available assays to detect IARC Group 1 agents (known human carcinogens) and Group 2A agents (probable human carcinogens). The predictivity of the available assays for human and rodent non-genotoxic carcinogens (NGCs), in comparison with standard and supplementary in vitro and in vivo genotoxicity tests, is also discussed. The principal finding is that a surprising number of human carcinogens have not been tested for cell transformation across the three main assays (SHE, Balb/c 3T3 and C3H10T1/2), confounding comparative assessment of these methods for detecting human carcinogens. This issue is not being addressed in the ongoing validation studies for the first two of these assays, despite the lack of any serious logistical issues associated with the use of most of these chemicals. In addition, there seem to be no plans for using exogenous bio-transformation systems for the metabolic activation of pro-carcinogens, as recommended in an ECVAM workshop held in 1999. To address these important issues, it is strongly recommended that consideration be given to the inclusion of more human carcinogens and an exogenous source of xenobiotic metabolism, such as an S9 fraction, in ongoing and future validation studies. While cell transformation systems detect a high level of NGCs, it is considered premature to rely only on this endpoint for screening for such chemicals, as recently suggested. This is particularly important, in view of the fact that there is still doubt as to the relevance of morphological transformation to tumorigenesis in vivo, and the wide diversity of potential mechanisms by which NGCs are known to act. Recent progress with regard to increasing the objectivity of scoring the transformed phenotype, and prospects for developing human cell-based transformation assays, are reviewed.

In vitro genotoxic assessment of xenobiotic diacylglycerols in an in vitro micronucleus assay

Xenobiotic diacylglycerols (DG) may induce pathological disorders by causing abnormal chromosomal segregation, which could be aneuploid. In this study, seven xenobiotic-diacylglycerols (four of drug origin and three of pesticide origin) were evaluated for their ability to induce aneuploidy in mammalian cultures using in vitro cytokinesis blocked micronucleus (CBMN) assay coupled with kinetochore labeling and interphase fluorescent in situ hybridization. Out of seven xeno-DGs, two (ibuprofen-DG and fenbufen-DG) induced statistically significant (P < 0.001) and dose-dependent increase in micronucleus induction, but this apparent micronucleus induction was very weak in case of fenbufen-DG. These MN were produced predominantly by aneugenic and clastogenic mechanisms, respectively, confirmed by immunofluorescent labeling of kinetochores. Fluorescent in situ hybridization analysis revealed that ibuprofen-DG induced significantly higher nondisjunction for chromosomes 10, 17, and 18. Other xenobiotic diacylglycerols (indomethacin-DG, salicylic acid-DG, 4-(2-methyl-4-chlorophenoxy) butanoic acid-DG (MCPB-DG), 2-(2-methyl-4-chlorophenoxy) propanoic acid-DG (MCPP-DG) and 2-(4-dichlorophenoxy)-butanoic acid-DG (2,4 DB-DG) did not induce micronuclei, but the concentrations tested did not reach levels that caused the marked growth suppression typically required for testing for regulatory testing purposes. However, the levels of growth suppression achieved were similar to that seen with ibuprofen-DG, which was positive. This study shows that xeno-DGs, which have been neglected in the past for their possible link to any pathological disorders, need serious assessment of their mutagenic potential.

Genotoxic evaluation of the antimalarial drug, fansidar, in cultured human lymphocytes

Fansidar (pyrimethamine-sulfadoxine) has been used extensively worldwide for the treatment of chloroquine resistant Plasmodium falciparum malaria, toxoplasmosis and Pneumocystis carinii pneumonia in patients with the acquired immunodeficiency syndrome. Because of the wide usage of pyrimethamine-sulfadoxine in developing countries and the lake of information from open literature and reports from manufacturers about the genotoxicity of such antimalarial drug, the present work was suggested. The possible genetic toxicity of fansidar has been evaluated in human peripheral blood lymphocyte cultures. The frequencies of sister-chromatid exchanges (SCE) and micronuclei (MN) were scored as genetic endpoints. Both tests covering a wide range of induced genetic damage as primary DNA damage, clastogenicity and aneugenicity. Cultures were set up by using blood samples from two healthy donors and the treatment was done using different fansidar concentrations ranging from 1:20 to 10:200 microg/ml. From our results, it appears that this drug is able to induce moderate genotoxic effects, as revealed by the increases found in SCE and MN frequencies in cultures from the two donors at the two highest concentrations tested (5:100 and 10:200 microg/ml). In addition, cyotoxic/cytostatic effects of fansidar were revealed by a decrease in the proliferative rate index (PRI) and in the cytokinesis block proliferation index (CBPI). Our findings suggest that the use of this drug should be restricted to situations where other antimalarial drugs cannot be used. The drug should never be given to pregnant women.

Methods of in vitro toxicology

In vitro methods are common and widely used for screening and ranking chemicals, and have also been taken into account sporadically for risk assessment purposes in the case of food additives. However, the range of food-associated compounds amenable to in vitro toxicology is considered much broader, comprising not only natural ingredients, including those from food preparation, but also compounds formed endogenously after exposure, permissible/authorised chemicals including additives, residues, supplements, chemicals from processing and packaging and contaminants. A major promise of in vitro systems is to obtain mechanism-derived information that is considered pivotal for adequate risk assessment. This paper critically reviews the entire process of risk assessment by in vitro toxicology, encompassing ongoing and future developments, with major emphasis on cytotoxicity, cellular responses, toxicokinetics, modelling, metabolism, cancer-related endpoints, developmental toxicity, prediction of allergenicity, and finally, development and application of biomarkers. It describes in depth the use of in vitro methods in strategies for characterising and predicting hazards to the human. Major weaknesses and strengths of these assay systems are addressed, together with some key issues concerning major research priorities to improve hazard identification and characterisation of food-associated chemicals.

Perspectives on cadmium toxicity research

Since there are a plethora of studies on cadmium toxicity and poisoning in laboratory animals and humans, we have limited this review to studies that are relevant to human health issues by focusing on carcinogenicity, genotoxicity, circulatory disease, nephrotoxicity and life expectancy. Cadmium exposure has been established to induce cancer in various tissues of laboratory animals. Contrary to early findings of the lack of genotoxicity by cadmium, recent findings of mammalian cell culture studies have revealed genotoxic effects. Furthermore, cadmium exposure at relatively low doses induces circulatory diseases in laboratory animals. Despite such results of various cadmium toxicities in animal studies, data from human studies are lacking and insufficient to support the cause-effect relationship. Although cadmium is currently considered to be a human carcinogen by the International Agency for Research and Cancer, it is inappropriate to conclude that sufficient evidence on the carcinogenicity of cadmium in humans exists. It is also thought that epidemiological studies so far reported do not support the occurrence of cadmium-induced circulatory disease in humans. Since there are inconsistent reports on the relationship of cadmium exposure with the life expectancy of people living in cadmium-polluted areas, further studies are needed for clarification. It is also necessary to examine apparent discrepancies in result between humans and experimental animals. It has been established that long-term exposure to cadmium causes renal dysfunction in both humans and experimental animals, and whether there are any differences in the inducibility of metallothionein in the kidney warrants further study.

In vitro prediction of carcinogenicity using a bovine papillomavirus DNA--carrying C3H/10T 1/2 cell line (T1). II: Results from the testing of 100 chemicals

A new in vitro test for identifying carcinogens is evaluated against a testing database of 100 chemicals including the following groups: steroids, antineoplastics, PCBs, dioxins, alkyl halides, aromatic amines, nitrogen heterocycles, polyaromatic hydrocarbons, mustards, and benzodioxoles. The assay uses focus formation in a stable, BPV-1-DNA-carrying C3H/10T 1/2 mouse embryo fibroblast cell line (T1), which does not require transfection, infection with virus, or isolation of primary cells from animals. For this group of chemicals, the T1 assay correctly predicted the rodent carcinogenicity or noncarcinogenicity of 77% of the chemicals for which carcinogenicity is reported. Based on published data the bacterial mutagenicity assay would have correctly predicted carcinogenicity or noncarcinogenicity of 53% of the chemicals. The Syrian hamster embryo test would have correctly predicted carcinogenicity or noncarcinogenicity of 61% of the chemicals. We also demonstrate dose--response relationships for two of the chemicals. We report the responses of T1 cells to the group of chemicals used in the International Life Sciences Institute's program for screening of alternative methods of predicting carcinogenicity.

Photosensitizing properties of quinine and synthetic antimalarials

Quinine, an alkaloid that occurs in the bark of trees of the genus Cinchona, has been used for the treatment of malaria in humans for over 150 years. In 1888 it was reported that quinine was more toxic to plant tissues and frog eggs in the light than in the dark; thus it is probably one of the first pure compounds shown to be a photosensitizer for biological systems. During this century, because of the toxic side effects of quinine and the appearance of quinine-resistant malarial strains, a search was begun to identify synthetic antimalarial compounds with improved properties. A number have been identified and are now in widespread use; but like quinine, most of these are also photosensitizers. Because of the very large numbers of patients receiving antimalarials, many studies have been made of the photophysical, photochemical and photosensitizing properties of quinine and several of the most commonly used synthetic antimalarials (chloroquine, primaquine, quinacrine and mefloquine). The results of these studies are summarized in this review. Most antimalarials photosensitize in part by the generation of singlet oxygen, although free radical pathways may also be involved. The carcinogenic and photocarcinogenic properties of antimalarials and related compounds are briefly surveyed.

Induction of micronuclei in Syrian hamster embryo cells: comparison to results in the SHE cell transformation assay for National Toxicology Program test chemicals

Sixteen chemicals currently being tested in National Toxicology Program (NTP) carcinogenicity studies were evaluated in the Syrian hamster embryo (SHE) cell in vitro micronucleus assay. Results from these studies were compared to the results from the SHE cell transformation assay for the same chemicals The overall concordance between induction of micronuclei and transformation of SHE cells was 56%, which is far lower that the 93% concordance between these two tests reported previously by Fritzenschaf et al. (1993; Mutation Res. 319, 47-53). The difference between our results appears to be due to differences in the types of chemicals in the two studies. Overall, there is good agreement between the SHE cell micronucleus and transformation assays for mutagenic chemicals, but, as our study highlights, the SHE cell transformation assay has the added utility of detecting nonmutagenic carcinogens. The utility of a multi-endpoint assessment in SHE cells for carcinogen screening is discussed.

Aneuploidy and consistent structural chromosome changes associated with transformation of Syrian hamster embryo cells

To gain a better understanding of the role of specific numerical and structural chromosome changes in the multistage process of transformation of Syrian hamster embryo (SHE) cells, we analyzed seven benzo(a)pyrene (BP)-induced immortal SHE cell lines, and one spontaneously immortalized cell line. In addition, we analyzed chromosome changes in early passage tumor-derived cell lines induced by injection of four immortalized cell lines into neonate hamsters. Of particular interest was the observation of a deletion in the short arm of chromosome 2 in four of the seven BP-immortalized cell lines. Other types of alterations in chromosome 2 were observed in two other cell lines. Loss of one copy of chromosome 16 was also observed in more than 90 to 100% of the cells in three of seven BP-immortalized cell lines. In contrast, the only chromosome alteration seen in the spontaneously immortalized cell line was a deletion in the short arm of chromosome 20. Genetic instability, as indicated by increased numerical or structural chromosome changes, was observed in all tumor-derived cell lines compared to the immortal cell line from which they originated. These results, along with previous reports in the literature, suggest that alterations in specific chromosomes, like chromosome 2, may be involved in transformation of SHE cells.

Application of in vitro cell transformation assays to predict the carcinogenic potential of chemicals

Genotoxicity test batteries have become a standard fool for identifying chemicals that may have potential carcinogenic risk to humans. It is now apparent, however, that the use of genotoxicity batteries for assessing carcinogenic potential has limitations including an overall low specificity and a limited ability to detect carcinogens acting via 'nongenotoxic' mechanisms. In vitro cell transformation models, because they measure a chemical's ability to induce preneoplastic or neoplastic endpoints regardless of mechanism, may fulfil the current need for an in vitro biologically relevant model with increased predictiveness for determining carcinogenic potential. This review will focus on data demonstrating the similarities of chemically induced cell transformation in vitro to carcinogenesis in vivo. Furthermore, a growing database demonstrating a high overall correlation between cell transformation results with those of the rodent bioassay will also be discussed. Finally, the inclusion of cell transformation approaches for assessing the carcinogenic potential of chemicals relative to currently used genotoxicity batteries will be presented.