ERα induction restored the response to antiestrogens in ER-negative breast cancer cells. A) Cultured breast tumor-derived cells, B) SUM-229PE and C) MCF-7 were incubated in the absence (black bars) or presence of calcitriol 1X10-8 M (white bars) for 48 h. Afterwards, cells were coincubated without (black bars) or with calcitriol (white bars) plus estradiol (E2, 1X10-8 M), tamoxifen (Tx, 1X10-6 M), ICI-182,780 (ICI, 1X10-6 M), ethanol (C), or combination of antagonists with E2 for 6 days. Cell growth assays by the XTT colorimetric method were performed. Bars represent the mean ± S.D. Data were normalized to 100% using the activity of vehicle-treated cells. Results are representative from two independent experiments performed in sextuplicates.* P ≤ 0.05 vs. control for each group (black bars vs black control or white bars vs white control).