Total intracellular accumulation levels of dietary isothiocyanates determine their activity in elevation of cellular glutathione and induction of Phase 2 detoxification enzymes

Many dietary isothiocyanates (ITCs) have shown cancer chemoprotective activity in animal models. Isothiocyanates rapidly accumulate in cells of various types as glutathione conjugates, and the total intracellular accumulation levels of ITCs (area under time-concentration curve; AUC) were critical for their Phase 2 enzyme inducer activities in murine hepatoma Hepa 1c1c7 cells. Induction of Phase 2 detoxification enzymes is recognized as a major cellular defense against carcinogens and other toxic agents. In order to further define the importance of intracellular AUC of ITCs in stimulating cellular detoxification functions, we have compared the intracellular AUCs and the inducer activities of four common dietary ITCs, allyl-ITC, benzyl-ITC, phenethyl-ITC and sulforaphane [1-isothiocyanato-(4R,S)-(methylsulfinyl)butane], in mouse skin papilloma (PE) cells. When PE cells were incubated with 5 muM of each ITC for 24 h, significant elevations of glutathione content (1.8-4.3-fold), quinone reductase activity (2.1-5.4-fold) and glutathione transferase activity (0.8-1.5-fold) were observed. These elevations were closely correlated with the AUCs of the ITCs. Increasing intracellular AUC of a weaker ITC by multiple dosing also increased its inducer activity. Further studies revealed that the AUC-dependent elevation of the above elements were mediated by the DNA regulatory element EpRE/ARE. In human HepG2 cells, which were stably transfected with a reporter construct under EpRE/ARE control, the intracellular AUC of the four ITCs closely correlated with the levels of reporter gene product (green fluorescent protein). These results showed that cellular accumulation levels of ITCs determine their activity in inducing cellular detoxification capacity and suggested that the intracellular AUC might be a valuable biomarker of the Phase 2 enzyme inducer activity of ITCs.