Relationship between exposure to TPA and appearance of transformed cells in MNNG-initiated transformation of BALB/c 3T3 cells

The cell transformation assay to assess potential carcinogenic properties of nanoparticles

Nanoparticles (NPs) are present in many daily life products with particular physical-chemical properties (size, density, porosity, geometry …) giving very interesting technological properties. Their use is continuously growing and NPs represent a new challenge in terms of risk assessment, consumers being multi-exposed. Toxic effects have already been identified such as oxidative stress, genotoxicity, inflammatory effects, and immune reactions, some of which are leading to carcinogenesis. Cancer is a complex phenomenon implying multiple modes of action and key events, and prevention strategies in cancer include a proper assessment of the properties of NPs. Therefore, introduction of new agents like NPs into the market creates fresh regulatory challenges for an adequate safety evaluation and requires new tools. The Cell Transformation Assay (CTA) is an in vitro test able of highlighting key events of characteristic phases in the cancer process, initiation and promotion. This review presents the development of this test and its use with NPs. The article underlines also the critical issues to address for assessing NPs carcinogenic properties and approaches for improving its relevance.

Post-transcriptional regulation of Rad51c by miR-222 contributes cellular transformation

DNA repair inhibition has been described as an essential event leading to the initiation of carcinogenesis. In a previous study, we observed that the exposure to metal mixture induces changes in the miR-nome of the cells that was correlated with the sub-expression of mRNA involved in processes and diseases associated with metal exposure. From this analysis, one of the miRNAs that shows changes in its expression is miR-222, which is overexpressed in various cancers associated with exposure to metals. In silico studies showed that a possible target for the microRNA-222 could be Rad 51c, a gene involved in the double-stranded DNA repair. We could appreciate that up-regulation of miR-222 reduces the expression both gene and as a protein expression of Rad51c by RT-PCR and immunoblot, respectively. A luciferase assay was performed to validate Rad51c as miR-222 target. Neutral comet assay was performed in order to evaluate DNA double-strand breaks under experimental conditions. Here, we demonstrate that miR-222 up-regulation, directly regulates Rad51c expression negatively, and impairs homologous recombination of double-strand break DNA repair during the initiation stage of cell transformation. This inhibition triggers morphological transformation in a two-stage Balb/c 3T3 cell assay, suggesting that this small RNA acts as an initiator of the carcinogenesis process.

An Interlaboratory Validation Study of the Improved Transformation Assay Employing Balb/c 3T3 Cells: Results of a Collaborative Study on the Two-stage Cell Transformation Assay by the Non-genotoxic Carcinogen Study Group

The Non-genotoxic Carcinogen Study Group of the Environmental Mutagen Society of Japan organised the first step of an interlaboratory validation study on an improved cell transformation assay employing Balb/c 3T3 A31-1-1 cells. Nineteen laboratories participated in this study. The modified transformation assay was evaluated for its responsiveness, its inter-laboratory reproducibility and its transferability. In this study, a mixture of Dulbecco's modified Eagle's medium and nutrient mixture F12, supplemented with insulin–transferrin– ethanolamine–sodium selenite and 2% fetal bovine serum (FBS) was used during the period of expression of transformed foci, intead of the usual minimum essential medium with 10% FBS. 20-Methylcholanthrene (MCA) and 12 -O-tetradecanoylphorbol-13-acetate (TPA) were selected as a prototype initiator and a tumour promoter, respectively. Two series of experiments were conducted. In the first series, the transformation activity of MCA was examined at various concentrations. In the absence of the promoting treatment with TPA, exposure to MCA only weakly induced transformed foci. In the presence of 0.1μg/ml TPA, all laboratories observed significant dose-dependent increases in the number of transformed foci with increasing MCA concentrations. In the second series of experiments, various concentrations of TPA were tested. In the absence of initiating treatment with MCA, exposure to TPA weakly induced transformed foci in about half of the laboratories. In the presence of 0.2μg/ml MCA, all the laboratories observed significant dose-dependent increases in the number of transformed foci with increasing TPA concentrations. The results from this study support the usefulness of this modified two-stage transformation assay with Balb/c 3T3 cells.

Lead facilitates foci formation in a Balb/c-3T3 two-step cell transformation model: role of Ape1 function

Several possible mechanisms have been examined to gain an understanding on the carcinogenic properties of lead, which include among others, mitogenesis, alteration of gene expression, oxidative damage, and inhibition of DNA repair. The aim of the present study was to explore if low concentrations of lead, relevant for human exposure, interfere with Ape1 function, a base excision repair enzyme, and its role in cell transformation in Balb/c-3T3. Lead acetate 5 and 30 μM induced APE1 mRNA and upregulation of protein expression. This increase in mRNA expression is consistent throughout the chronic exposure. Additionally, we also found an impaired function of Ape1 through molecular beacon-based assay. To evaluate the impact of lead on foci formation, a Balb/c-3T3 two-step transformation model was used. Balb/c-3T3 cells were pretreated 1 week with low concentrations of lead before induction of transformation with n-methyl-n-nitrosoguanidine (MNNG) (0.5 μg/mL) and 12-O-tetradecanoylphorbol-13-acetate (TPA) (0.1 μg/mL) (a classical two-step protocol). Morphological cell transformation increased in response to lead pretreatment that was paralleled with an increase in Ape1 mRNA and protein overexpression and an impairment of Ape1 activity and correlating with foci number. In addition, we found that lead pretreatment and MNNG (transformation initiator) increased DNA damage, determined by comet assay. Our data suggest that low lead concentrations (5, 30 μM) could play a facilitating role in cellular transformation, probably through the impaired function of housekeeping genes such as Ape1, leading to DNA damage accumulation and chromosomal instability, one of the most important hallmarks of cancer induced by chronic exposures.

Transformation assay in Bhas 42 cells: a model using initiated cells to study mechanisms of carcinogenesis and predict carcinogenic potential of chemicals

Transformation assays using cultured cells have been applied to the study of carcinogenesis. Although various cell systems exist, few cell types such as BALB/c 3T3 subclones and Syrian hamster embryo cells have been used to study chemically induced two-stage carcinogenesis. Bhas 42 cells were established as a clone by the transfection with the v-Ha-ras gene into mouse BALB/c 3T3 A31-1-1 cells and their subsequent selection based on their sensitivity to 12-O-tetradecanoylphorbol-13-acetate. Using Bhas 42 cells, transformed foci were induced by the treatment with nongenotoxic carcinogens, most of which act as tumor promoters. Therefore, Bhas 42 cells were considered to be a model of initiated cells. Subsequently, not only nongenotoxic carcinogens but also genotoxic carcinogens, most of which act as tumor initiators, were found to induce transformed foci by the modification of the protocol. Furthermore, transformation of Bhas 42 cells was induced by the transfection with genes of oncogenic potential. We interpret this high sensitivity of Bhas 42 cells to various types of carcinogenic stimuli to be related to the multistage model of carcinogenesis, as the transfection of v-Ha-ras gene further advances the parental BALB/c 3T3 A31-1-1 cells toward higher transforming potential. Thus, we propose that Bhas 42 cells are a novel and sensitive cell line for the analysis of carcinogenesis and can be used for the detection of not only carcinogenic substances but also gene alterations related to oncogenesis. This review will address characteristics of Bhas 42 cells, the transformation assay protocol, validation studies, and the various chemicals tested in this assay.

Genotoxic potential and in vitro tumour-promoting potential of 2-dodecylcyclobutanone and 2-tetradecylcyclobutanone, two radiolytic products of fatty acids

The DNA-damaging and tumour-promoting effects of two 2-alkylcyclobutanones (2-ACBs), which are found in irradiated fat-containing foods, were investigated by use of the comet assay and in an azoxymethane (AOM)-induced colon-carcinogenesis study in rats, respectively. We conducted genotoxicity tests of 2-dodecylcyclobutanone (2-dDCB) and 2-tetradecylcyclobutanone (2-tDCB) according to the test guidelines for chemicals or drugs. In addition, a cell-transformation assay with Bhas 42 cells was performed to investigate their promoting potential in vitro. The Salmonella typhimurium mutagenicity assay (Ames test), conducted with five tester strains, revealed that neither 2-dDCB nor 2-tDCB possessed mutagenic activity. Moreover, both in the in vitro chromosomal aberration test on CHL/IU cells and the in vivo bone-marrow micronucleus test where mice were given 2-dDCB and 2-tDCB (orally, up to 2000 mg/kg bw/day), we did not detect any clastogenic effects. Furthermore, DNA strand-breaks were not detected in the in vitro comet assay with CHL/IU cells, and DNA adducts derived from 2-dDCB and 2-tDCB were not detected in the colon tissues of the mice used for the micronucleus tests, in rats from a repeated dose 90-day oral toxicity test (0.03% 2-tDCB in the diet), or in rats from the AOM-induced carcinogenesis study (0.025% 2-tDCB in the diet). An in vitro tumour-promotion assay with Bhas 42 cells revealed that the number of transformed foci increased significantly following treatment of cells in the stationary phase with 2-dDCB or 2-tDCB for 10 days. Our results indicate that neither 2-dDCB nor 2-tDCB were genotoxic chemicals. However, they exhibited promoting activity, at least in vitro, when Bhas 42 cells were continuously exposed to these chemicals at toxic doses.

Role of oxidative stress in transformation induced by metal mixture

Metals are ubiquitous pollutants present as mixtures. In particular, mixture of arsenic-cadmium-lead is among the leading toxic agents detected in the environment. These metals have carcinogenic and cell-transforming potential. In this study, we used a two step cell transformation model, to determine the role of oxidative stress in transformation induced by a mixture of arsenic-cadmium-lead. Oxidative damage and antioxidant response were determined. Metal mixture treatment induces the increase of damage markers and the antioxidant response. Loss of cell viability and increased transforming potential were observed during the promotion phase. This finding correlated significantly with generation of reactive oxygen species. Cotreatment with N-acetyl-cysteine induces effect on the transforming capacity; while a diminution was found in initiation, in promotion phase a total block of the transforming capacity was observed. Our results suggest that oxidative stress generated by metal mixture plays an important role only in promotion phase promoting transforming capacity.

Application of the Improved BALB/c 3T3 Cell Transformation Assay to the Examination of the Initiating and Promoting Activities of Chemicals: The Second Inter-laboratory Collaborative Study by the Non-genotoxic Carcinogen Study Group of Japan

The Non-genotoxic Carcinogen Study Group in the Environmental Mutagen Society of Japan organised the second step of the inter-laboratory collaborative study on one-stage and two-stage cell transformation assays employing BALB/c 3T3 cells, with the objective of confirming whether the respective laboratories could independently produce results relevant to initiation or promotion. The method was modified to use a medium consisting of DMEM/F12 supplemented with 2% fetal bovine serum and a mixture of insulin, transferrin, ethanolamine and sodium selenite, at the stationary phase of cell growth. Seventeen laboratories collaborated in this study, and each chemical was tested by three to five laboratories. Comparison between the one-stage and two-stage assays revealed that the latter method would be beneficial in the screening of chemicals. In the test for initiating activity with the two-stage assay (post-treated with 0.1μg/ml 12- O-tetradecanoylphorbol-13-acetate), the relevant test laboratories all obtained positive results for benzo[ a]pyrene and methylmethane sulphonate, and negative results for phenanthrene. Of those laboratories assigned phenacetin for the initiation phase, two returned positive results and two returned negative results, where the latter laboratories tested up to one dose lower than the maximum dose used by the former laboratories. In the exploration of promoting activity with the two-stage assay (pretreated with 0.2μg/ml 3-methylcholanthrene), the relevant test laboratories obtained positive results for mezerein, sodium orthovanadate and TGF-β1, and negative results for anthralin, phenacetin and phorbol. Two results returned for phorbol 12,13-didecanoate were positive, but one result was negative — again, the maximum dose to achieve the latter result was lower than that which produced the former results. These results suggest that this modified assay method is relevant, reproducible and transferable, provided that dosing issues, such as the determination of the maximum dose, are adequately considered. The application of this two-stage assay for screening the initiating and promoting potential of chemicals is recommended for consideration by other research groups and regulatory authorities.

Establishment of a Stable Cell Line Expressing Green Fluorescence Protein-fused Hypoxia Inducible Factor-1α for Assessment of Carcinogenicity of Chemical Toxicants

Hypoxia inducible factor 1α (HIF-1α) is a potential marker of carcicnogenesis since it is overexpresssed in many human cancers such as brain, breast, and uterus, and its role has implicated in tumor cell growth and metastasis. In this study, we established a stable cell line that express green fluorescence protein (GFP)-fused hypoxia inducible factor 1α (HIF-1α) and evaluated the potential use of this cell line for assessment of carcinogenicity of chemical toxicants. Western blot analysis as well as fluorescence measurements showed that protein-level of GFP-HIF-1α was significantly enhanced in a dose-dependent manner upon treatment of hypoxia mimicking agents such as dexferrioxamine and CoCl₂. Well-Known tumor promoters such as mitomycin and methyl methane-sulfonate. significantly induced the fluorescence intensity of GFP-HIF-1α, whereas the known negative controls such as o-anthranilic acid and benzethonium chloride, did not. These results indicate that HIF-1α could be a biological parameter for detection of tumor initiators/promoters and suggest that the GFP-HIF-1α cell line is a useful system for screening of carcinogenic toxicants.

Altered gene expression and miRNA expression associated with cancerous IEC-6 cell transformed by MNNG

Background

Tumorigenesis is thought to be the consequence of gene mutation and disordered gene expression. However, the detailed molecular mechanism underlying the development and progress of colon cancer have not been elucidate completely. This study aimed to find out the genes associated with cancer biological pathways involved in transformation and tumorigenesis.

Methods

Normal intestinal cell line 6 (IEC-6) cells were transformed to cancer cells by treatment with cancerogenic agent of N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) and Phorbol 12-myristate 13 acetate (PMA). Then we investigated the altered gene expression of transformed IEC-6 cells by the microarray containing 113 genes associated with cancer pathway. Also the altered miRNAs of transformed IEC-6 cells were analyzed by array hybridization (miRCURY Array v9.2, Exiqon). The levels of acetylated histone H3 in transformed IEC-6 cells was evaluated by western blot.

Results

Cell proliferation was significantly increased as IEC-6 cells were transformed and tumor xenografts could be detected in animals as transformed IEC-6 cells were inoculated subcutaneously in nude mice. Result of microarray showed nine genes were increased and two decreased, as well as 13 miRNA were increased and 97 decreased. Verification by real-time PCR implies that the data obtained from microarray analysis were reliable. Western blot showed the levels of acetylated histone H3 were increased dramatically after MNNG/PMA treatment.

Conclusion

Our results showed many important biological pathways and miRNAs were involved in transformation and tumorigenesis of IEC-6 cells, which suggested the transformation of normal cells was involved with large mount of genetic and epigenetic variation.

Technical Modification of the Balb/c 3T3 Cell Transformation Assay: The use of Serum-reduced Medium to Optimise the Practicability of the Protocol

The two-stage Balb/c 3T3 model of cell transformation can mimic the two-stage carcinogenicity bioassay, and has been recognised as a screening method for detecting potential tumour initiators and promoters. A technical modification to the original protocol (which involved the use of M10F medium, consisting of MEM plus 10% fetal bovine serum [FBS]) has been previously proposed, in order to increase its efficacy, namely: the introduction of enriched, serum-reduced medium (DF2F medium, comprising DMEM/F12 plus 2% FBS and other supplements). The aim of this study was to further modify the protocol, so as to attain higher practicability for the assay. The protocol was further optimised by: a) reducing the number of plates required, through the use of larger plates; b) reducing the cost of the assay by retaining the reduced serum concentration and by using 2μg/ml insulin, rather than the more-complex insulin–transferrin–ethanolamine–sodium selenite (ITES) supplement (i.e. DF2F2I medium); and c) extending the culture period from 24–25 days to 31–32 days, resulting in clearer foci (the number of medium changes did not increase, as less-frequent medium changes were performed during the extended culture period). Growth curve construction revealed that variations in the saturation densities of the parental Balb/c 3T3 cell line and its three transformed clones were highest when M10F medium was replaced with DF2F2I medium just before cells reached confluence. We applied this newly-optimised protocol to the assessment of: a) the tumour initiating activity of 3-methylcholanthrene (MCA), N-methyl- N’-nitro- N-nitrosoguanidine, mitomycin C, methylmethane sulphonate, CdCl2 and phenacetin, combining a post-treatment of 100ng/ml 12- O-tetradecanoylphorbol-13-acetate at the promotion stage; and b) the tumour promoting activity of insulin, lithocholic acid, CdCl2 and phenobarbital, with pre-treatment of 0.2μg/ml MCA at the initiation stage. In the present study, only phenobarbital was negative when tested by using the modified protocol.

Detection of initiating as well as promoting activity of chemicals by a novel cell transformation assay using v-Ha-ras-transfected BALB/c 3T3 cells (Bhas 42 cells)

Cell transformation assay using BALB/c 3T3 cells, C3H10T1/2 cells and others, can simulate the two-stage carcinogenesis utilized for formation of transformed foci. A sensitive cell transformation assay for tumor initiators as well as promoters has been developed using a v-Ha-ras-transfected BALB/c 3T3 cell line, Bhas 42; these cells are regarded as initiated in the two-stage paradigm of carcinogenesis. To distinguish between initiation and promotion, the initiation assay involves a 2-day treatment of low-density cells, obtained one day after plating, with a test chemical, and the promotion assay involves treatment of near-confluent cells with a test chemical for a period of 12 days (Day 3-14). When Bhas 42 cells were treated with tumor initiators, N-methyl-N'-nitro-N-nitrosoguanidine and 3-methylcholanthrene, transformed foci were induced in the initiation assay but not in the promotion assay. In contrast, tumor promoters, 12-O-tetradecanoylphorbol-13-acetate, lithocholic acid and okadaic acid, gave negative responses in the initiation assay but positive responses in the promotion assay. The results were reproducible with various treatment protocols. Sixteen polycyclic aromatic hydrocarbons were examined using both assays. Benzo[a]pyrene and 7,12-dimethylbenz[a]anthracene induced focus formation only in the initiation assay. Increase of focus formation was observed in the promotion assay with benzo[e]pyrene, benzo[ghi]perylene, 1-nitropyrene and pyrene. Benz[a]anthracene, benz[b]anthracene, chrysene and perylene showed positive responses in both initiation and promotion assays. Results of initiation and promotion assays of acenaphthylene, anthracene, coronene, 9,10-diphenylanthracene, naphthalene and phenanthrene were negative or equivocal. The present Bhas assays for the detection of either/both initiating and promoting activities of chemicals are sensitive and of high performance compared with other cell transformation assays.

Cell transformation assay using Balb/c 3T3 cells or Bhas 42 cells for the efficient detection of tumour promoters

In this paper, we report three interlaboratory validation studies with special interest in detecting tumour promoters. The former two validation studies were on the improved two-stage in vitro transformation assay using Balb/c 3T3 cells. The third one was on the Bhas 42 cell assay. From the results, we could confirm that, on the whole, the improved two-stage transformation assay was sensitive, reproducible and reliable, while the Bhas 42 cell assay was a very sensitive and reproducible method for detecting tumour-promoting potential. However, there was some variation in the sensitivity among the laboratories. In order to cope with this sensitivity variation, further refinement is necessary, which will increase the applicability of the methods.

Lipopolysaccharide enhancement of 12-o-tetradecanoylphorbol 13-acetate-mediated transformation in rat glioma C6, accompanied by induction of inducible nitric oxide synthase

Lipopolysaccharide (LPS) from Gram-negative bacterial has been identified as an important molecule involved in the inflammatory process through inducing nitric oxide (NO) production. However, the effect of LPS in carcinogenesis is still undefined. In the present study, the biological effect of LPS was examined in 12-o-tetradecanoylphorbol 13-acetate (TPA)-treated rat glioma cells C6. Results of MTT assay showed that LPS and TPA exhibited no significant cytotoxicity in glioma C6 cells. Interestingly, transformation foci were found in LPS/TPA-treated glioma C6 cells, but not in LPS- or TPA-treated cells. The transformation foci induced by LPS/TPA were also observed in the absence of serum. It indicates that induction of transformation foci formation by LPS and TPA is independent on the serum in glioma C6 cells. Induction of iNOS gene expression and NO production was examined in LPS/TPA-treated cells, but not obvious in LPS- or TPA-treated cells. NO donor sodium nitroprusside (SNP) induces transformation in glioma C6 cells in according with elevating NO production. In addition, LPS/TPA induces metalloproteinase 9 (MMP9) activity by gelatin activity assay in gel. Wogonin and quercetin but not rutin, inhibitors of iNOS gene expression and NO production induced by LPS, showed the significant inhibition on LPS/TPA-induced transformation foci formation, accompanied by inhibiting iNOS gene expression, NO production and MMP9 activity. Results of the present study provide scientific evidences to link the inflammatory responses and carcinogenesis, and suggest that NO derived from inflammation may contribute to the progression of carcinogenesis; natural products with anti-inflammatory effects such as wogonin and quercetin possess the ability to block transformation induced by LPS/TPA.

Verification of the BALB/c 3T3 cell transformation assay after improvement by using an ITES-medium

We carried out the first-step verification study on our ITES-medium-improved BALB/c 3T3 cell transformation assay. In order to estimate its potential use as a short-term screening method for putative carcinogens, 31 chemicals were tested in the improved transformation assay. The test chemicals consisted of 18 carcinogens and 13 noncarcinogens. The present improved transformation assay did not use an exogenous metabolizing system. Data analysis was carried out on 34 chemicals, including assay data for three chemicals reported previously by the authors. As a result, the improved transformation assay showed a concordance of 73.5% with a rodent bioassay, a sensitivity for carcinogens of 71.4%, and a specificity for detection of noncarcinogens of 76.9%. The improved transformation assay detected all of the genotoxic carcinogens, and five of 11 nongenotoxic carcinogens as positive. It can be expected that the improved transformation assay will be able to detect not only genotoxic carcinogens with high probability but also approximately 50% of nongenotoxic carcinogens within about 3 weeks. Hence, these preliminary findings suggest that our improved transformation assay will be a reliable and useful short-term test procedure of screening for potential carcinogens, and it encourages us to conduct further experiments on many carcinogens and noncarcinogens.

Use of a cell transformation assay with established cell lines, and a metabolic cooperation assay with V79 cells for the detection of tumour promoters: a review

Extensive studies on the safety evaluation of chemicals have indicated that a considerable number of non-genotoxic chemicals are carcinogenic. Tumour promoters are likely to be among these non-genotoxic carcinogens, and their detection is considered to be an important approach to the prevention of cancer. In this review, the results are summarised for in vitro transformation assays involving established cell lines, and for an assay for inhibition of gap junctional intercellular communication for the detection of tumour promoters, which involves V79 cells. Although the number of chemicals examined is still too small to permit a full evaluation of the correlation between in vitro cell transformation and in vivo carcinogenicity, it is clear that the sensitivity of the focus formation assay is very high. In the case of the metabolic cooperation assay, the sensitivity appears to be rather poor, but the assay can be considered to be useful because of its simple procedure and its considerable database. These in vitro assays for tumour promoters are recommended as useful tools for the detection of non-genotoxic carcinogens.

Establishment and characterization of normal and initiated hamster tracheal epithelial cell system

The development and characterization of normal hamster tracheal epithelial (HTE) cell system and its initiated subline is described in the present study. Normal HTE cells grew in a monolayer, had a stable diploid karyotype, were anchorage dependent and non-tumorigenic. The presence of desmosomal attachments and keratin filaments confirmed the epithelial nature of these cells. An initiated subline DTC8 was isolated after treatment of HTE cells with a suboptimal dose of 9,10-dimethyl-1,2-benz[a]anthracene (DMBA). These DTC8 cells grew in a monolayer, had a higher growth rate and saturation density, were weakly anchorage independent and non-tumorigenic. Treatment of DTC8 cells with 100 ng 12-O-tetradecanoyl phorbol-13-acetate (TPA), resulted in transformation of these cells which then showed anchorage independent growth on semisolid agar and formed tumours in 85% animals. As DTC8 cells showed heterogeneity in chromosome number, they were further cloned by the limiting dilution method using gamma-irradiated hamster embryonic fibroblasts as a feeder layer. The clone H7I, isolated among these clones had all the properties of initiated cells.