The Cell Transformation Assay: A Historical Assessment of Current Knowledge of Applications in an Integrated Approach to Testing and Assessment for Non-Genotoxic Carcinogens

The history of the development of the cell transformation assays (CTAs) is described, providing an overview of in vitro cell transformation from its origin to the new transcriptomic-based CTAs. Application of this knowledge is utilized to address how the different types of CTAs, variously addressing initiation and promotion, can be included on a mechanistic basis within the integrated approach to testing and assessment (IATA) for non-genotoxic carcinogens. Building upon assay assessments targeting the key events in the IATA, we identify how the different CTA models can appropriately fit, following preceding steps in the IATA. The preceding steps are the prescreening transcriptomic approaches, and assessment within the earlier key events of inflammation, immune disruption, mitotic signaling and cell injury. The CTA models address the later key events of (sustained) proliferation and change in morphology leading to tumor formation. The complementary key biomarkers with respect to the precursor key events and respective CTAs are mapped, providing a structured mechanistic approach to represent the complexity of the (non-genotoxic) carcinogenesis process, and specifically their capacity to identify non-genotoxic carcinogenic chemicals in a human relevant IATA.

In Vitro-In Vivo Carcinogenicity

The evaluation of the carcinogenic potential of chemicals constitutes an essential step in assessing the risk that the chemicals pose to human health. The "gold standard" method to evaluate the carcinogenic potential of chemicals is the carcinogenicity test in laboratory animals. However, because carcinogenicity studies in vivo are extremely time-consuming, expensive, make use of a high number of animals, and cannot be used to screen a high number of compounds at the same time, various different in vitro cell transformation assays have been developed. In this report, procedures to test the carcinogenicity in vivo and in vitro are described, whereby in the latter case three extensively evaluated test systems (the BALB/c 3T3 cell transformation assay, the Bhas 42 cell transformation assay, and the Syrian hamster embryo assay) are presented. Their performance shows that they are a useful complement to in vitro genotoxicity test batteries, can be used to identify non-genotoxic carcinogens, and as screening assays may significantly limit the number of chemicals to undergo an in vivo carcinogenicity testing, thereby strongly reducing the number of laboratory animals to be used. In the future, the development of human cell line-based transformation assays may contribute to increase further their relevance and the willingness to incorporate them into existing in vitro toxicity test batteries.

Classification of agents using Syrian hamster embryo (SHE) cell transformation assay (CTA) with ATR-FTIR spectroscopy and multivariate analysis

The Syrian hamster embryo (SHE) cell transformation assay (pH 6.7) has a reported sensitivity of 87% and specificity of 83%, and an overall concordance of 85% with in vivo rodent bioassay data. To date, the SHE assay is the only in vitro assay that exhibits multistage carcinogenicity. The assay uses morphological transformation, the first stage towards neoplasm, as an endpoint to predict the carcinogenic potential of a test agent. However, scoring of morphologically transformed SHE cells is subjective. We treated SHE cells grown on low-E reflective slides with 2,6-diaminotoluene, N-nitroso-N-ethylnitroguanidine, N-nitroso-N-methylurea, N-nitroso-N-ethylurea, EDTA, dimethyl sulphoxide (DMSO; vehicle control), methyl methanesulfonate, benzo[e]pyrene, mitomycin C, ethyl methanesulfonate, ampicillin or five different concentrations of benzo[a]pyrene. Macroscopically visible SHE colonies were located on the slides and interrogated using attenuated total reflection Fourier-transform infrared (ATR-FTIR) spectroscopy acquiring five spectra per colony. The acquired IR data were analysed using Fisher's linear discriminant analysis (LDA) followed by principal component analysis (PCA)-LDA cluster vectors to extract major and minor discriminating wavenumbers for each treatment class. Each test agent vs. DMSO and treatment-induced transformed cells vs. corresponding non-transformed were classified by a unique combination of major and minor discriminating wavenumbers. Alterations associated with Amide I, Amide II, lipids and nucleic acids appear to be important in segregation of classes. Our findings suggest that a biophysical approach of ATR-FTIR spectroscopy with multivariate analysis could facilitate a more objective interrogation of SHE cells towards scoring for transformation and ultimately employing the assay for risk assessment of test agents.

Using Fourier transform IR spectroscopy to analyze biological materials

IR spectroscopy is an excellent method for biological analyses. It enables the nonperturbative, label-free extraction of biochemical information and images toward diagnosis and the assessment of cell functionality. Although not strictly microscopy in the conventional sense, it allows the construction of images of tissue or cell architecture by the passing of spectral data through a variety of computational algorithms. Because such images are constructed from fingerprint spectra, the notion is that they can be an objective reflection of the underlying health status of the analyzed sample. One of the major difficulties in the field has been determining a consensus on spectral pre-processing and data analysis. This manuscript brings together as coauthors some of the leaders in this field to allow the standardization of methods and procedures for adapting a multistage approach to a methodology that can be applied to a variety of cell biological questions or used within a clinical setting for disease screening or diagnosis. We describe a protocol for collecting IR spectra and images from biological samples (e.g., fixed cytology and tissue sections, live cells or biofluids) that assesses the instrumental options available, appropriate sample preparation, different sampling modes as well as important advances in spectral data acquisition. After acquisition, data processing consists of a sequence of steps including quality control, spectral pre-processing, feature extraction and classification of the supervised or unsupervised type. A typical experiment can be completed and analyzed within hours. Example results are presented on the use of IR spectra combined with multivariate data processing.

Exploiting biospectroscopy as a novel screening tool for cervical cancer: towards a framework to validate its accuracy in a routine clinical setting

Biospectroscopy is an emerging field that harnesses the platform of physical sciences with computational analysis in order to shed novel insights on biological questions. An area where this approach seems to have potential is in screening or diagnostic clinical settings, where there is an urgent need for new approaches to objectively interrogate large numbers of samples in an objective fashion with acceptable levels of sensitivity and specificity. This review outlines the benefits of biospectroscopy in screening for precancer lesions of the cervix due to its ability to separate different grades of dysplasia. It evaluates the feasibility of introducing this technique into cervical screening programs on the basis of its ability to identify biomarkers of progression within derived spectra (‘biochemical‑cell fingerprints’).

Miniaturization of cytotoxicity tests for concentration range-finding studies prior to conducting the pH 6.7 Syrian hamster embryo cell-transformation assay

The Syrian hamster embryo (SHE) cell-transformation assay (SHE assay) is a promising alternative method to animal testing for the identification of potential carcinogens in vitro. Prior to conducting the SHE assay the appropriate concentration range for each test chemical must be established, with a maximum concentration causing approximately 50% cytotoxicity. Concentration range-finding is done in separate experiments, which are similar to the final SHE assay but with less replicates and more concentrations. Here we present an alternative for the cytotoxicity testing by miniaturization of the test procedure by use of 24-well plates and surpluses from feeder-cell preparations as target cells. In addition, we integrated the photometry-based neutral red (NR) assay. For validation of the assay, incubations with dimethyl sulf-oxide, p-phenylenediamine-2HCl, aniline, o-toluidine-HCl, 2,4-diaminotoluene, and 2-naphthylamine were carried out in the miniaturized approach and compared with the standard procedure in terms of calculating the relative plating efficiencies (RPEs). To directly compare both methods, concentrations that produced 50% cytotoxicity (IC50) were calculated. Excellent associations were observed between the number of colonies and NR uptake. For all test substances a concentration-dependent, concomitant decrease of NR uptake in the miniaturized approach and RPEs in the standard test was observed after a 7-day incubation. The results from both test setups showed a comparable order of magnitude and the IC50 values differed by a factor <2 (1.4-1.9), depending on the substance in question. Overall, the miniaturized approach should be considered an improved alternative for cytotoxicity testing in the SHE assay, as it saves valuable SHE cells and speeds-up the time, to obtain test results more rapidly.

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.