Egr-1 is activated by 17β-estradiol in MCF-7 cells by mitogen-activated protein kinase-dependent phosphorylation of ELK-1

Early growth response‐1 (Egr‐1) is an immediate‐early gene induced by E2 in the rodent uterus and breast cancer cells. E2 induces Egr‐1 mRNA and protein levels in MCF‐7 human breast cancer cells and reporter gene activity in cells transfected with pEgr‐1A, a construct containing the −600 to +12 region of the Egr‐1 promoter linked to the firefly luciferase gene. Deletion analysis of the Egr‐1 promoter identified a minimal E2‐responsive region of the promoter that contained serum response element (SRE)3 (−376 to −350) which bound Elk‐1 and serum response factor (SRF) in gel mobility shift assays. Hormone‐responsiveness of Egr‐1 in MCF‐7 cells was specifically inhibited by PD98059, a mitogen‐activated protein kinase kinase inhibitor, but not by LY294002, an inhibitor of phosphatidylinositol‐3‐kinase (PI3‐K). These results contrasted with hormone‐dependent activation of the SRE in the c‐fos promoter, which was inhibited by both PD98059 and LY294002. Differences in activation of the SREs in Egr‐1 and c‐fos were related to promoter sequence, which defines the affinities of Elk‐1 and SRF to their respective binding sites. Thus, Egr‐1, like c‐fos, is activated through non‐genomic (extranuclear) pathways of estrogen action in breast cancer cells. © 2004 Wiley‐Liss, Inc.