Coactivation of ERα/Sp1 in ZR-75 (A, B), MCF-7 (C) and MDA-MB-231 (D) cells. Cells were transfected with pSp13 and different amounts of ERα expression plasmid (A, C and D) or pSp13 alone (B), treated with DMSO or 10 nM E2, and luciferase activity was determined as described in the Materials and Methods. Significant (p < 0.05) induction or activation of Sp1 by E2 (*) and coactivation by DRIP150 (**) are indicated. Transfection experiments illustrated in this and all other figures are results of at least three replicate experiments for each treatment group and are expressed as means ± SE. All coactivation experiments have been confirmed in two or more experiments.