Acquisition of doxorubicin resistance in ovarian carcinoma cells accompanies activation of the NRF2 pathway.

It has been firmly established that the transcription factor NRF2 is a critical element in the survival of healthy cells in response to oxidative stress because it up-regulates a wide array of antioxidant genes by binding to the antioxidant-response element (ARE). However, adaptive activation of the NRF2 system after an exposure of cancer cells to chemotherapy can be hypothesized, implying the acquisition of chemoresistance by tumors. In this study we have investigated the potential role of NRF2 signaling in the development of acquired resistance to doxorubicin. The human ovarian carcinoma cell line A2780, which is highly sensitive to doxorubicin, showed low levels of ARE binding and ARE-driven luciferase activity, as well as repressed expression of its target genes compared with resistant ovarian carcinoma SKOV3 and OV90 cells. Doxorubicin-resistant A2780DR cells, established after exposure to stepwise increasing concentrations of doxorubicin, displayed a refractoriness to doxorubicin-induced cell death. Acquisition of doxorubicin resistance in A2780 cells was accompanied by an elevation in NRF2 activity and consequent increase in the expression of the catalytic subunit of gamma-glutamylcysteine ligase and total GSH content. A critical role for NRF2 in the acquired chemoresistance of A2780DR cells could be confirmed by the restoration of doxorubicin sensitivity after stable expression of NRF2-specific shRNA in A2780DR cells, whereas inhibition of NRF2 could not further enhance doxorubicin sensitivity in the parental A2780 cells. These results suggest that the level of NRF2 activity might be a determining factor for doxorubicin sensitivity in ovarian carcinoma cell lines and adaptive activation of the NRF2 system can participate in the development of acquired resistance to anthracycline therapy.