Enhancement of Radiation Response in Breast Cancer Stem Cells by Inhibition of Thioredoxin- and Glutathione-Dependent Metabolism

The goal of this study was to determine if depletion of glutathione (GSH) and inhibition of thioredoxin (Trx) reductase (TrxR) activity could enhance radiation responses in human breast cancer stem cells by a mechanism involving thiol-dependent oxidative stress. The following were used to inhibit GSH and Trx metabolism: buthionine sulfoximine (BSO), a GSH synthesis inhibitor; sulfasalazine (SSZ), an inhibitor of x_c^– cysteine/glutamate antiporter; auranofin (Au), a thioredoxin reductase inhibitor; or 2-AAPA, a GSH-reductase inhibitor. Clonogenic survival, Matrigel assays, flow cytometry cancer stem cell assays (CD44⁺CD24^–ESA⁺ or ALDH1) and human tumor xenograft models were used to determine the antitumor activity of drug and radiation combinations. Combined inhibition of GSH and Trx metabolism enhanced cancer cell clonogenic killing and radiation responses in human breast and pancreatic cancer cells via a mechanism that could be inhibited by N-acetylcysteine (NAC). Au, BSO and radiation also significantly decreased breast cancer cell migration and invasion in a thiol-dependent manner that could be inhibited by NAC. In addition, pretreating cells with Au sensitized breast cancer stem cell populations to radiation in vitro as determined by CD44⁺CD24^–ESA⁺ or ALDH1. Combined administration of Au and BSO, given prior to irradiation, significantly increased the survival of mice with human breast cancer xenografts, and decreased the number of ALDH1⁺ cancer stem cells. These results indicate that combined inhibition of GSH- and Trx-dependent thiol metabolism using pharmacologically relevant agents can enhance responses of human breast cancer stem cells to radiation both in vitro and in vivo.