Sp1 Binding Sites and An Estrogen Response Element Half-Site Are Involved in Regulation of the Human Progesterone Receptor A Promoter

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Abstract
Progesterone receptor gene expression is induced by estrogen in MCF-7 human breast cancer cells. Although it is generally thought that estrogen re- sponsiveness is mediated through estrogen re- sponse 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 Sp1 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 hor- mone, suggesting that this region is involved in es- trogen-regulated gene expression. The ability of the half-ERE/Sp1 binding site to confer estrogen respon- siveness to a simple heterologous promoter was confirmed in transient cotransfection assays. In vitro DNase I footprinting and gel mobility shift assays demonstrated that Sp1 present in MCF-7 nuclear ex- tracts and purified Sp1 protein bound to the two Sp1 sites and that the estrogen receptor enhanced Sp1 binding. In addition to its effects on Sp1 binding, the estrogen receptor also bound directly to the ERE half-site. Taken together, these findings suggest that the estrogen receptor and Sp1 play a role in activa- tion of the human progesterone receptor A promoter. (Molecular Endocrinology 14: 972-985, 2000)

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