Generation of a stable antioxidant response element-driven reporter gene cell line and its use to show redox-dependent activation of Nrf2 by cancer chemotherapeutic agents

The NF-E2 p45-related factor 2 (Nrf2) regulates cytoprotective genes that contain an antioxidant response element (ARE) in their promoters. To investigate whether anticancer drugs can induce ARE-driven gene expression, we have developed a stable human mammary MCF7-derived reporter cell line called AREc32, which contains a luciferase gene construct controlled by eight copies of the cis-element. In these cells, luciferase activity was increased up to 50-fold following treatment with 50 mumol/L tert-butylhydroquinone (t-BHQ). Basal and inducible luciferase activities in AREc32 cells were increased by forced overexpression of Nrf2 and reduced by knockdown of endogenous Nrf2 expression with RNA interference. Depletion of cellular reduced glutathione (GSH) by treatment of AREc32 cells with l-buthionine-S,R-sulfoximine (BSO) did not influence basal levels of luciferase activity, but pretreatment with BSO augmented induction of luciferase activity by t-BHQ. Induction of reporter activity by t-BHQ in AREc32 cells was suppressed markedly by the antioxidants N-acetylcysteine and GSH but only modestly by vitamins C or E, suggesting that ARE-luciferase expression is induced primarily by thiol-active electrophiles rather than free radicals. The anticancer drugs cisplatin, etoposide, mitoxantrone, chlorambucil, melphalan, and carmustine [1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU)] weakly induced luciferase activity in AREc32 cells. Moreover, treatment of AREc32 cells with BSO immediately before exposure to anticancer drugs enhanced induction of ARE-driven luciferase activity by cisplatin, BCNU, chlorambucil, and melphalan and also induced endogenous AKR1C (AKR1C refers to AKR1C1 and AKR1C2), a target gene of Nrf2. Our findings show that Nrf2 can be activated by certain anticancer agents, and this will influence the effectiveness of chemotherapy.