Peroxisome Proliferator-induced Transformation of Syrian Hamster Embryo Cells: Influence of Experimental Procedures

Recommended protocol for the Syrian hamster embryo (SHE) cell transformation assay

The Syrian hamster embryo (SHE) cell transformation assay (CTA) is a short-term in vitro assay recommended as an alternative method for testing the carcinogenic potential of chemicals. SHE cells are "normal" cells since they are diploid, genetically stable, non-tumourigenic, and have metabolic capabilities for the activation of some classes of carcinogens. The CTA, first developed in the 1960s by Berwald and Sachs (1963,1964) [3,4], is based on the change of the phenotypic feature of cell colonies expressing the first steps of the conversion of normal to neoplastic-like cells with oncogenic properties. Pienta et al. (1977) [22] developed a protocol using cryopreserved cells to enhance practicality of the assay and limit sources of variability. Several variants of the assay are currently in use, which mainly differ by the pH at which the assay is performed. We present here the common version of the SHE pH 6.7 CTA and SHE pH 7.0 CTA protocols used in the ECVAM (European Centre for the Validation of Alternative Methods) prevalidation study on CTA reported in this issue. It is recommended that this protocol, in combination with the photo catalogues presented in this issue, should be used in the future and serve as a basis for the development of the OECD test guideline.

Transcriptomic effects of di-(2-ethylhexyl)-phthalate in Syrian hamster embryo cells: an important role of early cytoskeleton disturbances in carcinogenesis?

Background

Di-(2-ethylhexyl)-phthalate (DEHP) is a commonly used plasticizer in polyvinylchloride (PVC) formulations and a potentially non-genotoxic carcinogen. The aim of this study was to identify genes whose level of expression is altered by DEHP by using a global wide-genome approach in Syrian hamster embryo (SHE) cells, a model similar to human cells regarding their responses to this type of carcinogen. With mRNA Differential Display (DD), we analysed the transcriptional regulation of SHE cells exposed to 0, 12.5, 25 and 50 μM of DEHP for 24 hrs, conditions which induced neoplastic transformation of these cells. A real-time quantitative polymerase chain reaction (qPCR) was used to confirm differential expression of genes identified by DD.

Results

Gene expression profiling showed 178 differentially-expressed fragments corresponding to 122 genes after tblastx comparisons, 79 up-regulated and 43 down-regulated. The genes of interest were involved in many biological pathways, including signal transduction, regulation of the cytoskeleton, xenobiotic metabolism, apoptosis, lipidogenesis, protein conformation, transport and cell cycle. We then focused particularly on genes involved in the regulation of the cytoskeleton, one of the processes occurring during carcinogenesis and in the early steps of neoplastic transformation. Twenty one cytoskeleton-related genes were studied by qPCR. The down-regulated genes were involved in focal adhesion or cell junction. The up-regulated genes were involved in the regulation of the actin cytoskeleton and this would suggest a role of cellular plasticity in the mechanism of chemical carcinogenesis. The gene expression changes identified in the present study were PPAR-independent.

Conclusion

This study identified a set of genes whose expression is altered by DEHP exposure in mammalian embryo cells. This is the first study that elucidates the genomic changes of DEHP involved in the organization of the cytoskeleton. The latter genes may be candidates as biomarkers predictive of early events in the multistep carcinogenic process.

Perfluorooctanoic acid (PFOA) acts as a tumor promoter on Syrian hamster embryo (SHE) cells

Perfluorooctane sulfonate (PFOS) (C(8)F(17)SO(3)) and perfluorooctanoic acid (PFOA) (C(8)HF(15)O(2)) are synthetic chemicals widely used in industrial applications for their hydrophobic and oleophobic properties. They are persistent, bioaccumulative, and toxic to mammalian species. Their widespread distribution on earth and contamination of human serum raised concerns about long-term side effects. They are suspected to be carcinogenic through a nongenotoxic mode of action, a mechanism supported by recent findings that PFOS induced cell transformation but no genotoxicity in Syrian hamster embryo (SHE) cells. In the present study, we evaluated carcinogenic potential of PFOA using the cell transformation assay on SHE cells. The chemical was applied alone or in combination with a nontransformant concentration of benzo[a]pyrene (BaP, 0.4 μM) in order to detect PFOA ability to act as tumor initiator or tumor promoter. The results showed that PFOA tested alone in the range 3.7 × 10(-5) to 300 μM did not induce SHE cell transformation frequency in a 7-day treatment. On the other side, the combination BaP/PFOA induced cell transformation at all PFOA concentrations tested, which revealed synergistic effects. No genotoxicity of PFOA on SHE cells was detected using the comet assay after 5 and 24 h of exposure. No significant increase in DNA breakage was found in BaP-initiated cells exposed to PFOA in a 7-day treatment. The whole results showed that PFOA acts as a tumor promoter and a nongenotoxic carcinogen. Cell transformation in initiated cells was observed at concentrations equivalent to the ones found in human serum of nonoccupationally and occupationally exposed populations. An involvement of PFOA in increased incidence of cancer recorded in occupationally exposed population cannot be ruled out.

2,4-Dichlorophenoxyacetic acid: effects on Syrian hamster embryo (SHE) cell transformation, c-Myc expression, DNA damage and apoptosis

2,4-Dichlorophenoxyacetic acid (2,4-D) is a selective, systemic auxin-type herbicide extensively used throughout the world. The present research was aimed at studying effects of low and non-cytotoxic concentrations of 2,4-D on SHE cells in relation with carcinogenicity. Effects were studied on Syrian hamster morphological cell transformation, c-Myc expression - both at the gene and protein level - DNA damage and apoptosis. 2,4-D significantly induced cell transformation at 11.5 microM and 23 microM (i.e. 2.5 microg/mL and 5 microg/mL). An increase in the expression of the transcription factor c-Myc, measured by use of RT-PCR with respect to mRNA level and by Western blotting for protein level was registered at these concentrations, as well as genotoxic effects evaluated with the single-cell gel electrophoresis (Comet) assay. Consequences for apoptosis of 2,4-D treatment were also investigated. The fluorochrome acridine orange was used to study DNA fragmentation as a marker of apoptosis. No effect on apoptosis was found at 2,4-D concentrations that induced cell transformation. This was confirmed by the unchanged expression of Bcl-2 and Bax, two regulator genes of the mitochondrial pathway of apoptosis. Our results demonstrate the transforming and genotoxic effects of low concentrations of 2,4-D in mammalian cells. This information contributes to a better understanding of the mechanism of 2,4-D toxicity in mammalian cells and demonstrates that 2,4-D should be considered as potentially hazardous to humans.

Di-(2-ethylhexyl)phthalate (DEHP) increases Bcl-2/Bax ratio and modifies c-myc expression in Syrian hamster embryo (SHE) cells

The objective of this work was to study the anti-apoptotic properties of the non-genotoxic rodent carcinogen, di(2-ethylhexyl)phthalate (DEHP) in Syrian hamster embryo (SHE) cells. We demonstrated that a 24 h pre-treatment of SHE cells with 50 microM DEHP inhibited apoptosis triggered by growth factors deprivation. The RNA expression levels of the regulator genes involved in the apoptotic pathway, bcl-2, bax and of c-myc were measured using Western blotting and RT-PCR. We showed that a 24 h treatment of SHE cells with 50 microM DEHP increased (P < 0.05) the bcl-2 expression, while c-myc expression was decreased. No effect on bax expression was observed in the range of 10-50 microM. The defective regulation of apoptosis caused by DEHP treatment could contribute to its carcinogenicity.

Application of a two-stage Syrian hamster embryo cell transformation assay to cigarette smoke particulate matter

The induction of transformation in Syrian hamster embryo (SHE) cells is a multifactorial process, in comparison to endpoints induced in in vitro genotoxicity assays such as Ames, mouse lymphoma and cytogenetics [Y. Berwald, L. Sachs, In vitro cell transformation with chemical carcinogens, Nature (London) 200 (1963) 1182-1184]. Furthermore, a number of non-genotoxic carcinogens and promoters such as clofibrate and diethylhexylphthalate, have been positively identified in this assay, while giving false negative results in traditional genotoxicity assays [H. Yamasaki, J. Ashby, M. Bignami, W. Jongen, K. Linnainmaa, R.F. Newbold, G. Nguyen-Ba, S. Parodi, E. Rivedal, D. Schiffmann, J.W.I.M. Simons, P. Vasseur, Nongenotoxic carcinogens: development of detection methods based on mechanisms: a European project, Mutat. Res. 353 (1996) 47-63]. A high concordance between results obtained in this assay when compared with rodent carcinogenesis bioassays has also been noted [R.J. Isfort, G.A. Kerckaert, R.A. LeBoeuf, Comparison of the standard and reduced pH Syrian hamster embryo (SHE) in vitro cell transformation assays to predict the carcinogenic potential of chemicals, Mutat. Res. 356 (1996) 11-63]. Carcinogenesis is known to be a multistage process, with agents potentially acting at each stage. Specifically, mouse skin painting experiments established that tumour induction could be mechanistically divided into two distinct phases, termed initiation and promotion. Initiation, is defined as the stage at which a normal cell is converted to a latent tumour cell, followed by promotion where the latent tumour cell progresses to a tumour [W.F. Friedwald, P. Rous, The initiating and promoting elements in tumour production: analysis of the effects of tar, benzpyrene and methylcholanthrene on rabbit skin, J. Exp. Med. 80 (1944) 101-125]. A protocol for the pH 6.7 SHE transformation assay has been developed which allows separation of cell transformation process into two phases, potentially analogous to initiation and promotion in vivo. This allows chemicals found to be positive in the traditional SHE cell transformation assay to be further classified as initiators or promoters. Following validation with known initiators, benzo(a)pyrene and N-methyl-N'-nitro-N-nitrosoguanidine and promoters, 12-O-tetradecanoyl-phorbol-13-acetate and phenobarbitone, the two-stage model was applied to cigarette smoke particulates which was found to act both at the initiation and promotion stage of cell transformation.

ZnCl2 induces Syrian hamster embryo (SHE) cell transformation

In order to test the hypothesis of a relationship between apoptosis and neoplastic transformation, we studied the transforming potency of zinc, known for its antiapoptotic effects. In this study, zinc chloride (100 microM) was shown to induce morphological transformation (MT) in Syrian hamster embryo (SHE) cells. It was also tested in combination with benzo(a)pyrene (BaP), a positive control for carcinogenicity, or fomesafen, a carcinogenic pesticide with hepatic peroxisomal proliferation properties. A co-exposure of the two carcinogens with 100 microM zinc increased cell transformation in SHE cells. These results were in agreement with the theory of a relationship between the inhibition of apoptosis and induction of cell transformation. The cloning efficiency (CE) of SHE cells seeded at clonal density was raised by zinc, fomesafen and furthermore by the mixture of the two chemicals, which could be explained by the antiapoptotic action of zinc and fomesafen on SHE cells. No change in myc and bax expressions was observed in zinc-treated SHE cells.