Spl binding sites and an estrogen response element half-site are involved in regulation of the human progesterone receptor a promoter

Progesterone receptor gene expression is induced by estrogen in MCF-7 human breast cancer cells. Although it is generally thought that estrogen responsiveness is mediated through estrogen response elements (EREs), the progesterone receptor gene lacks an identifiable ERE. The progesterone receptor A promoter does, however, contain a half-ERE/Sp1 binding site comprised of an ERE half-site upstream of two Spl binding sites. We have used in vivo deoxyribonuclease I (DNase I) footprinting to demonstrate that the half-ERE/Sp1 binding site is more protected when MCF-7 cells are treated with estrogen than when cells are not exposed to hormone, suggesting that this region is involved in estrogen-regulated gene expression. The ability of the half-ERE/Sp1 binding site to confer estrogen responsiveness to a simple heterologous promoter was confirmed in transient cotransfection assays. In vitro DNase I footprinting and gel mobility shift assays demonstrated that Spl present in MCF-7 nuclear extracts and purified Spl protein bound to the two Spl sites and that the estrogen receptor enhanced Spl binding. In addition to its effects on Spl binding, the estrogen receptor also bound directly to the ERE half-site. Taken together, these findings suggest that the estrogen receptor and Spl play a role in activation of the human progesterone receptor A promoter.