Synergistic activation of the Nrf2-signaling pathway by glyceollins under oxidative stress induced by glutathione depletion.

Oxidative stress state such as depletion of the intracellular glutathione (GSH) is associated with the development of cancer. Some dietary phytochemicals have been shown to possess a cancer preventive effect, although the understanding of the involved mechanisms is still limited. Recent study has shown that glyceollins, phytoalexins derived from soybean by biotic elicitor, might have a cancer preventive effect through induction of detoxifying/antioxidant enzymes. The objective of this study was to investigate the effects of glyceollins on the Nrf2 signaling pathway under excessive oxidative stress induced by GSH depletion. In mouse hepatoma cells (Hepa1c1c7) subjected to the buthionine sulfoximine (BSO), an inhibitor of γ-glutamylcysteine synthetase (γGCS), the intracellular GSH content was significantly lowered. On the other hand, incubation with glyceollins in the presence of BSO increased the level of GSH, expression of γGCS, and nuclear translocation of NF-E2-related factor-2 (Nrf2), compared to the cells treated with BSO only. Nrf2-antioxidant responsive element (ARE)-reporter activity assay in HepG2-C8 showed that BSO increased the ARE-reporter activity in a dose-dependent manner, compared to vehicle-treated cells, whereas cotreatment with glyceollins caused further increase in reporter luciferase activity relative to BSO alone. Taken together, glyceollins synergistically activated the Nrf2 signaling pathway and subsequently the expression of phase 2/antioxidant enzymes in the presence of BSO, suggesting that BSO-induced oxidative stress and that glyceollins regulate the expression of phase 2/antioxidant enzymes through different mechanisms from each other.