Inhibitory effect of ochratoxin A on DNMT-mediated flocculation of yeast.

The mycotoxin ochratoxin A (OTA) is considered to be a human carcinogen. However, the mode of its carcinogenetic action has not been elucidated. Recently, it has become evident that epigenetic changes influence the risk of developing cancer. Since it has been revealed that the yeast flocculation displayed by the strains transformed with human DNA methyltransferases (DNMT) can be regulated by epigenetic mechanisms, we examined the effect of OTA on the transcription level of FLO1, which mediates the flocculation phenotype. OTA but not a non-carcinogenetic mycotoxin deoxynivalenol (DON) inhibited the intensity of GFP fluorescence under the transcriptional regulation of FLO1 promoter in a dose-dependent manner. At the same time, OTA had no effect on the reporter activity under the control of modified FLO1 promoter with reduced CpG motifs. In addition, it was confirmed that the flocculation and FLO1 mRNA of DNMT gene-transformed yeast (DNMT yeast) were decreased by OTA. In vitro methylation assay using a bacterial DNMT revealed an inhibitory effect of OTA on the DNMT activity, and OTA treatment reduced the frequency of abnormally shaped nuclei which were often observed in DNMT yeast. These results suggest that the carcinogenicity of OTA may involve inhibition of DNMT-mediated epigenetic regulation.