Bisphenol-A reduces DNA methylation after metabolic activation

Bisphenol-A (BPA) is an important environmental contaminant with adverse health effects suspected to be mediated through epigenetic mechanisms. We had reported that the FLO1-dependent flocculation of transgenic yeast expressing human DNA methyltransferase (DNMT yeast) is a useful tool in epigenotoxicology studies. In this report, we have investigated the effects of BPA in the presence of metabolic activation (S-9 mix) on the transcription level of the FLO1 gene in the DNMT yeast. In the presence of metabolic activation, BPA inhibited the intensity of green fluorescence reporter protein (GFP) driven by the FLO1 promoter. A metabolite of BPA, 4-methyl-2,4-bis(p-hydroxyphenyl) pent-1-ene (MBP), also exhibited similar inhibitory effect. Furthermore, BPA in the presence of S-9 mix had only a weak while MBP had no inhibitory effects on the expression of modified GFP reporter gene under the control of FLO1 promoter with reduced CpG motifs. Aforementioned behavior was confirmed by the inhibition of flocculation as well as FLO1 gene mRNA expression. In addition, the global DNA methylation level in the human HEK293 cells was also reduced by MBP. These results indicate that BPA metabolites have inhibitory effect on DNA methylation. Our approach offers a novel in vitro method for screening for chemicals that can alter the epigenome by a mechanism dependent on their metabolic activation.

[New trend in genotoxicity research taking into account genome instability]

Since mutagenicity which can induce permanent transmissible changes in the structure of the genetic material is one of the major causes of cancer, research for genotoxicity including mutagenicity has focused on cancer hazard identification. Thus, it has been assumed that there was no threshold in mutagenesis. On the other hand, tumor development induced by not only non-genotoxic carcinogen but also genotoxic carcinogens will likely show a practical threshold. Therefore, statistical evaluation can provide value of the benchmark dose lower confidence limit (BMDL) calculated by approaches for the determination of genetic toxicity point of departure (PoD). In addition, disruption of epigenetic regulation which affect transcription through alteration of chromatin structure is considered to be important in future genotoxicity research. Taking into account benchmark dose or epigenetics will help improve assessment of genotoxicity, which offer promising insight into understanding genomic instability. Overall, this review presents current trends for future assessments of genotoxicity.

Epigenetic effect of the mycotoxin Fumonisin B1 on DNA methylation

Mycotoxin Fumonisin B1 (FB1) is a secondary metabolite that is produced by certain Fusarium species. Although numerous studies demonstrate toxic and carcinogenic effects of FB1, the underlying mechanisms have not been fully elucidated. In this study, we evaluated the epigenetic effects of FB1 for the first time using FLO assays, which detect epigenetic changes that affect the flocculation gene (FLO1) promoter activity in budding yeast. FLO assays showed increased reporter activities of the FLO1 promoter in the presence of 10- and 20-µM FB1. FB1 (20 µM) treatments also promoted flocculation. In subsequent in vitro methylation assays of a bacterial DNA methyltransferase (DNMT), FB1 treatments increased DNMT activities. Moreover, global DNA methylation was significantly increased in HEK293 cells treated with 100-µM FB1. Taken together, these results suggest that FB1 exposure leads to unique epigenetic alterations due to increased DNMT activities and demonstrate that FB1 may be an important risk factor for epigenetic dysfunction-associated human diseases including cancer.