A TRANSCRIPTIONAL ENHANCER REQUIRED FOR THE DIFFERENTIAL EXPRESSION OF THE HUMAN ESTROGEN-RECEPTOR IN BREAST CANCERS

Breast cancers lacking estrogen receptor (ER) expression have an adver se prognosis and fail to respond to endocrine therapy. We have identif ied a transcriptional enhancer in the human ER gene which is different ially active in ER-positive (ER(+)) and ER-negative (ER(-)) human brea st cancer cell lines. Enhancer function was mapped to a 35-bp element located from -3778 to -3744 upstream of the major human ER mRNA start site, which we have termed ER-EH0 (for estrogen receptor enhancer). Ge l retardation assays with ER(+) and ER(-) cell lines identified multip le DNA-protein complexes which specifically form on this enhancer. One of these complexes could be supershifted by anti-Jun or anti-Fos anti bodies, identifying it as an AP-l-containing complex. Methylation inte rference assays suggest binding of factors to both the AP-1 site and a djacent base pairs, Enhancer activity requires both the AP-1 site and these adjacent sequences, Mutations introduced into ER-EH0 and the rec ently described proximal promoter element ERF-1 in the context of the full-length promoter confirm ER-EH0 as the dominant cia-acting element involved in differential ER expression.