Effect of estrogen/ERα on hPRLR hPIII promoter activity in ERα-negative (MDA-MB-231) breast cancer cells. (A) hPIII promoter activity was analyzed in ERα-negative breast cancer cells transiently expressing human ERα (ERα+) in the presence or absence of E2 (100 nM; 16 h). ERα−, cells not expressing ERα. WT, wild-type pGL2 with hPIII promoter sequence (−480/−112) containing SP1 (S) and C/EBPβ (C) cis elements. X, mutated element. (B) Diagram of experimental procedure and sequences of mutated SP1 (SX) or C/EBPβ (CX). Data were presented as relative luciferase activities (Rluc) normalized to the activities of cotransfected β-galactosidase (β-gal). Results are the means ± standard errors from three individual experiments in triplicate. *, **, and #, P < 0.05.