Calcitriol induced a fully active ERα. Cultured breast tumor-derived cells were incubated in the absence (black bars) or presence of calcitriol 1X10-8 M (white bars) for 48 h. Subsequently, cells were coincubated with or without calcitriol plus estradiol (E2, 1x10-8 M), ICI-182,780 (ICI, 1x10-6 M) or vehicle (C) for 24 h. PRL gene expression was determined by qPCR. Results are shown as the mean ± S.D. of PRL/GAPDH mRNA normalized ratio. Data were normalized to 1 for vehicle-treated cells. *P ≤ 0.05 vs. C.