Estradiol Modulates Cytotoxic Responses to Topoisomerase II Targeting Agents in Estrogen Receptor Positive Breast Cancer Cells
Endocrine therapy is the primary treatment for estrogen receptor-positive (ER+) breast cancer. Paradoxically high dose estradiol can induce ER+ breast cancer cell death in an ER dependent manner. Although a well-established phenomenon, the mechanism of estradiol (E2)-induced breast cancer cell death has remained elusive. More recent evidence has revealed that E2 drives estrogen receptor alpha (ERa)-dependent transcription through TOP2 mediated DNA double-strand breaks. This raises two important questions. Firstly, is E2-dependent breast cancer cell death dependent on TOP2 mediated DNA double strand cleavage, and, secondly, does E2 signaling alter sensitivity of ER+ breast cancer cells to TOP2 poisons. Two ER+ breast cancer cell lines T47D and MCF-7 cells were treated with E2, doxorubicin (TOP2 poison) or T638 (catalytic TOP2 inhibitor), alone, and with each drug combined with E2 for 72 h. Viability was measured and normalized to untreated controls and IC50 values were calculated from dose-response curve. E2 enhanced doxorubicin potency in T47D and MCF7 ER+ breast cancer cell lines. E2 was toxic to T47D and MCF7 cells in a concentration dependent manner. The TOP2 catalytic inhibitor, T638, which blocks TOP2-mediated DNA cleavage reduced viability of both cell lines when used alone but decreased the cytotoxic effects of E2 about 7.6-fold, consistent with a role for TOP2-mediated DNA cleavage in the cytotoxic effects of E2. Thus, E2 sensitizes ER+ breast cancer cells to the TOP2 poison, whereas T638, the catalytic inhibitor, diminishes the cytotoxic response to E2. These opposing effects are consistent with E2 acting through TOP2-dependent DNA breaks.