Roles of Cyclin A, Myc, Jun and Ppm1l in tumourigenic transformation of NIH3T3 cell

Predicting Carcinogenic Mechanisms of Non-Genotoxic Carcinogens via Combined Analysis of Global DNA Methylation and In Vitro Cell Transformation

An in vitro cell transformation assay (CTA) is useful for the detection of non-genotoxic carcinogens (NGTXCs); however, it does not provide information on their modes of action. In this study, to pursue a mechanism-based approach in the risk assessment of NGTXCs, we aimed to develop an integrated strategy comprising an in vitro Bhas 42 CTA and global DNA methylation analysis. For this purpose, 10 NGTXCs, which were also predicted to be negative through Derek/Sarah structure-activity relationship analysis, were first tested for transforming activity in Bhas 42 cells. Methylation profiles using reduced representation bisulfite sequencing were generated for seven NGTXCs that were positive in CTAs. In general, the differentially methylated regions (DMRs) within promoter regions showed slightly more bias toward hypermethylation than the DMRs across the whole genome. We also identified 13 genes associated with overlapping DMRs within the promoter regions in four NGTXCs, of which seven were hypermethylated and six were hypomethylated. Using ingenuity pathway analysis, the genes with DMRs at the CpG sites were found to be enriched in cancer-related categories, including "cell-to-cell signaling and interaction" as well as "cell death and survival". Moreover, the networks related to "cell death and survival", which were considered to be associated with carcinogenesis, were identified in six NGTXCs. These results suggest that epigenetic changes supporting cell transformation processes occur during non-genotoxic carcinogenesis. Taken together, our combined system can become an attractive component for an integrated approach for the testing and assessment of NGTXCs.

Genome-wide DNA methylation changes in transformed foci induced by nongenotoxic carcinogens

In vitro cell transformation assays (CTA) have been proposed as a method to identify possible nongenotoxic carcinogens. However, the current protocols do not provide information on the mechanism of action of the test articles. In this study, we combined an in vitro Bhas 42 CTA and sequencing-based DNA methylation profiling analysis to elucidate the carcinogenic mechanism associated with nongenotoxic carcinogens. Three nongenotoxic carcinogens were evaluated: cadmium chloride, methyl carbamate, and lithocholic acid. Methylation profiles were generated for the two nongenotoxic carcinogens (cadmium chloride and lithocholic acid) that were positive in Bhas 42 CTA. Methyl carbamate did not exhibit any promoter activity. Approximately 9.8% of all differentially methylated regions (DMRs) identified in cadmium chloride-induced transformed foci overlapped with DMRs in lithocholic acid-induced transformed foci. Interestingly, overlapping DMRs showed more hypermethylation than individual DMRs. In addition, the DMRs in CpG island elements common to both nongenotoxic carcinogens showed considerably more bias toward hypermethylated DMRs than those unique to either cadmium chloride or lithocholic acid. Pathway enrichment analysis revealed that genes harboring hypermethylated DMRs were significantly enriched in Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways including pathways in cancer, basal cell carcinoma, and Wnt signaling. The genes harboring hypomethylated DMRs were significantly related to mRNA surveillance pathway, RNA transport, and autophagy. Taken together, our preliminary results on genome-wide methylation analysis of cell clones from nongenotoxic carcinogen-induced foci could be exploited for CTAs improvement, but further research will be required to standardize and assess the specificity and sensitivity of this combined approach. Environ. Mol. Mutagen. 2019. © 2019 Wiley Periodicals, Inc.