Reversal of the estrogen receptor-negative phenotype in breast cancer and restoration of antiestrogen response

Purpose: In breast cancer, the presence of estrogen receptor alpha (ER) denotes a better prognosis and response to antiestrogen therapy. Lack of ER alpha correlates with overexpression of epidermal growth factor receptor or c-erbB-2. We have shown that hyperactivation of mitogen-activated protein kinase (MAPK) directly represses ER alpha expression in a reversible manner. In this study, we determine if inhibition of MAPK in established ER alpha(-) breast cancer cell lines and tumors results in reexpression of ER alpha, and further, if reexpression of ER alpha in these ER alpha(-) tumors and cell lines could restore antiestrogen responses. Experimental Design: Established ER alpha(-) breast cancer cell lines, ER alpha(-) breast tumors, and tumor cell cultures obtained from ER alpha(-) tumors were used in this study. Inhibition of hyperactive MAPK was accomplished via the MAPK/ERK kinase 1/2 inhibitor U0126 or via upstream inhibition with Iressa or Herceptin. Western blotting or reverse transcription-PCR for ER alpha was used to assess the reexpression of ER alpha in cells treated with U0126. Growth assays with WST-1 were done to assess restoration of antiestrogen sensitivity in these cells. Results: Inhibition of MAPK activity in ERa breast cancer cell lines results in reexpression of ER alpha; upstream inhibition via targeting epidermal growth factor receptor or c-erbB-2 is equally effective. Importantly, this reexpressed ER alpha can now mediate an antiestrogen response in a subset of these ER alpha(-) breast cancer cell lines. Treatment of ER alpha(-) tumor specimens with MAPK inhibitors results in restoration of ER alpha mRNA, and similarly in epithelial cultures from ER alpha(-) tumors, MAPK inhibition restores both ER alpha protein and antiestrogen response. Conclusions: These data show both the possibility of restoring ERa expression and antiestrogen responses in ER alpha(-) breast cancer and suggest that there exist ER alpha(-) breast cancer patients who would benefit from a combined MAPK inhibition/hormonal therapy.