Isoalantolactone from Inula helenium Caused Nrf2-Mediated Induction of Detoxifying Enzymes

Our previous study demonstrated that methanolic extract of Inula helenium (Elecampane) has the potential to induce detoxifying enzymes such as NAD(P)H:(quinone acceptor) oxidoreductase 1 (EC 1.6.99.2) (NQO1, QR) activity and glutathione S-transferase (GST) and found isoalantolactone and alantolactone as the active components. In this study we investigated the detoxifying enzyme-inducing potential of isoalantolactone, which is present in I. helenium and has a structure similar to that of alantolactone. The compound induced QR in a dose-dependent manner in both Hepa1c1c7 cells and its mutant BPRc1 cells lacking the arylhydrocarbon receptor translocator. Like with most phase 2 enzyme inducers, other phase 2 detoxifying enzymes, including GST, glutathione reductase, gamma-glutamylcysteine synthetase, and heme oxygenase-1, were also induced by isoalantolactone in a dose-dependent manner in the cultured cells. Furthermore, isoalantolactone caused a proportionate increase in luciferase activity depending upon concentration and exposure time in the reporter assay in which HepG2-C8 cells, transfectants carrying antioxidant response element-luciferase gene, were used. The nuclear translocation of nuclear factor-E2-related factor 2 (Nrf2) was stimulated by the compound and attenuated by phosphatidylinositol 3-kinase inhibitors such as LY294002 and wortmannin. In conclusion, isoalantolactone is a candidate for chemoprevention and acts as potent phase 2 enzyme inducer by stimulating the accumulation of Nrf2 in the nucleus.