No abstract

Since the toxicological effects of dioxins are mainly mediated by the aryl hydrocarbon receptor (AhR), an in vitro assessment system for AhR activity was used in this study to search for flavonoids that attenuated dioxin toxicity through the intestinal epithelial monolayer. When AhR transformation in Hepa-1c1c7 cells was examined by southwestern ELISA, nine flavonoids among 34 kinds of flavonoids inhibited the transformation by more than one-half. When each flavonoid with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) was added to dioxin-responsive HepG2 cells, seven flavonoids significantly restrained the TCDD-induced transcriptional activity of the CYP1A1 promoter. Furthermore, those seven flavonoids that had permeated the Caco-2 cell monolayers demonstrated an inhibitory effect on both the AhR transformation and on the transcriptional activity of the CYP1A1 promoter. The expression level of the CYP1A1 mRNA and protein induced by TCDD was suppressed by flavone, galangin, and tangeretin. It is proposed from these results that some flavonoids have the ability to suppress dioxin-induced AhR activity after permeating the human intestinal epithelial cell monolayer.

It has been reported that 90% of the amount of dioxin in the whole body is absorbed orally with food. However, a concise and simple system to assess dioxin absorption in the small intestine has not yet been established. The present study reports a new in vitro assessment system for this purpose. A stable dioxin-responsive cell line was established by introducing a plasmid that incorporates a xenobiotic-responsive element upstream of the luciferase gene into human hepatic HepG2 genomic DNA. Dioxin was added to the apical side of differentiated human intestinal epithelial Caco-2 cell monolayers that had been cultured on a semipermeable membrane, and the basal medium was recovered after an appropriate incubation time. To the recovered medium was added dioxin-responsive HepG2, and a luciferase assay was performed. The established stable cell line clearly showed dose-and time-dependent response to dioxin. When a food factor such as chlorophyll, which has been reported to increase dioxin excretion in in vivo studies, was added with dioxin, a significant decrease in dioxin permeability to the Caco-2 monolayer was observed. This assessment system would be useful to search for those food factors that could prevent dioxin absorption in the small intestine.

A system to evaluate the intestinal permeability for 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) was constructed with human intestinal Caco-2 cell monolayers. TCDD was added to the apical side of the cell monolayers and the basal TCDD concentration was estimated by using a HepG2 cell having a luciferase vector with a xenobiotic-responsive element in the promoter region. Addition of such food substances as chlorophyll to the apical side of the Caco-2 cell monolayers markedly decreased the apical-to-basal transport of TCDD. This system would be useful for use in searching for the materials having an inhibitory activity to intestinal TCDD absorption.