Combination of siRNA-directed gene silencing with epigallocatechin-3-gallate (EGCG) reverses drug resistance in human breast cancer cells

Elevated expression of NF-E2-related factor 2 (Nrf2), a nuclear transcription factor, is a frequent genetic abnormality seen in this malignancy and is an important contributor to chemoresistance in cancer therapy. In the present study, we investigated if Nrf2 was associated with drug resistance in tamoxifen-resistant MCF-7 (MCF-7/TAM) cells, and whether EGCG, major flavonoid isolated from green tea, could reverse drug resistance in MCF-7/TAM cells. Our results showed that the endogenous expression of Nrf2 as well as its target proteins heme oxygenase-1, NADP (H):quinone oxidoreductase in MCF-7/TAM cells was higher than that in MCF-7 cells. Epicatechin gallate (EGCG) significantly sensitizes MCF-7/TAM cells to tamoxifen and dramatically reduced Nrf2 expression at both the messenger RNA and protein, leading to a reduction of Nrf2-downstream genes. In addition, using siRNA technique, we found that the intracellular Nrf2 protein level was significantly decreased in MCF-7/TAM cells and tamoxifen resistance was partially reversed by Nrf2 siRNA. Combination of siRNA-directed gene silencing with EGCG downregulated the Nrf2-dependent response and partly reversed tamoxifen resistance in MCF-7/TAM cells in a synergic manner. These results suggested that combining the chemotherapeutic effect of EGCG with siRNA-mediated Nrf2 knock-down results in the feasibility of using Nrf2 inhibitors to increase efficacy of chemotherapeutic drugs.