Alizarin, an Agonist of AHR Receptor, Enhances CYP1A1 Enzyme Activity and Induces Transcriptional Changes in Hepatoma Cells

The phytopigment alizarin was previously characterized as an anti-tumor drug owing to its antioxidant or antigenotoxic activities. However, the safety of alizarin is currently still under dispute. In this study, we explored the activity of alizarin in the AHR-CYP1A1 pathway and analyzed the transcriptional changes affected by alizarin using human hepatoma cell line HepG2-based assays. The results showed that alizarin decreased HepG2 cell viability in a dose-dependent manner, with IC50 values between 160.4 and 216.8 μM. Furthermore, alizarin significantly upregulated the expression of CYP1A1 and increased the ethoxyresorufin-O-deethylase activity. Alizarin also exhibited agonistic activity toward the AHR receptor in the XRE-mediated luciferase reporter gene assay, which was further confirmed via the molecular docking assay. In addition, the transcriptional analysis indicated that alizarin may act as a potential carcinogen through significantly enriching several items related to cancer in both DO and KEGG analysis. In brief, our findings indicated that alizarin shows agonistic activities to the AHR receptor through activating the AHR-CYP1A1 signaling pathway in HepG2 cells, which may lead to the risks for cancer developing.

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.

Functional role of DNA methylation at the FLO1 promoter in budding yeast

We have previously reported that the transformation of the budding yeast with plasmids encoding the human DNA methyltransferases DNMT1 and DNMT3B cDNAs induces the mRNA of flocculin gene FLO1 and the flocculation phenotype. In the present study, we evaluated the effect of DNMT inhibitor in the transformed yeasts using a FLO1 promoter-based green fluorescent protein (GFP) reporter gene assay. The DNMT inhibitor, 5-aza-2΄-deoxycytidine (5AZ), decreased GFP fluorescence driven by FLO1 promoter in DNMT-genes transformed yeast (DNMT yeast). Surprisingly, the GFP activity driven by cytosine-phosphate-guanine (CpG) motif-reduced FLO1 promoter decreased both in DNMTs gene-transformed and control strains. Yeast cells transformed with expression vector encoding a maintenance enzyme DNMT1 cDNA showed a flocculation phenotype that was associated with an enhanced mRNA level of FLO1. Bisulfite sequencing revealed methylated CpG sites at the FLO1 promoter in a control strain not expressing any DNMT transgenes, and no detectable methylation at the sites was observed in cells treated with 5AZ. These results suggest that the FLO1 promoter is endogenously de novo methylated leading to the activation of FLO1 gene transcription. Furthermore, the methylation level at the FLO1 promoter is responsible for the significant differences in FLO1 promoter-driven expression of GFP in DNMT yeast.