Evaluation of a Luciferase-Based Reporter Assay as a Screen for Inhibitors of Estrogen-ERα-Induced Proliferation of Breast Cancer Cells

Estrogens, acting through estrogen receptor alpha (ER alpha), stimulate breast cancer proliferation, making ER alpha an attractive drug target. Since 384-well format screens for inhibitors of proliferation can be challenging for some cells, inhibition of luciferase-based reporters is often used as a surrogate end point. To identify novel small-molecule inhibitors of 17 beta-estradiol (E-2)-ER alpha-stimulated cell proliferation, we established a cell-based screen for inhibitors of E-2-ER alpha induction of an estrogen response element (ERE)(3)-luciferase reporter. Seventy-five "hits" were evaluated in tiered follow-up assays to identify where hits failed to progress and evaluate their effectiveness as inhibitors of E-2-ER alpha-induced proliferation of breast cancer cells. Only 8 of 75 hits from the luciferase screen inhibited estrogen-induced proliferation of ER alpha-positive MCF-7 and T47D cells but not control ER alpha-negative MDA-MB-231 cells. Although 12% of compounds inhibited E-2-ER alpha-stimulated proliferation in only one of the ER alpha-positive cell lines, 40% of compounds were toxic and inhibited growth of all the cell lines, and similar to 37% exhibited little or no ability to inhibit E-2-ER alpha-stimulated cell proliferation. Representative compounds were evaluated in more detail, and a lead ER alpha inhibitor was identified.