Activation of the silent progesterone receptor gene by ectopic expression of estrogen receptors in a rat fibroblast cell line

Abstract

We describe the construction and characterization of a novel estrogen (E2)-responsive cell line, Rat1+ER, which ectopically expresses estrogen receptor (ER). Human ER cDNA was introduced by retrovirus-mediated gene transfer into the Rat1 cell line, which does not express functional ER endogenously. Rat1+ER cells express functional ER based on radioreceptor assays, immunoblotting, and transient transfection experiments using E2-responsive reporter plasmids. The effects of this ectopic ER expression were studied on three endogenous E2-responsive genes, prolactin (PRL), progesterone receptor (PR), and epidermal growth factor receptor (EGFR). PRL, usually expressed in the lactotrophs of the pituitary, is not expressed at all in Rat1+ER cells, with or without E2 addition, and appears to require other factors for expression. In contrast, although PR is not expressed in Rat1 cells, it is induced in Rat1+ER cells upon the addition of E2. This induction appears to occur at the transcriptional level and is insensitive to cycloheximide treatment. This is one of the few examples where the expression of one gene activates an otherwise silent gene. Another contrasting observation is that, although EGFR is basally expressed in Rat1+ER cells, the addition of E2 has no effect. Our studies paint a complicated picture of E2 regulation of endogenous genes: the activation of the PR gene may only require the presence of E2 and ER, whereas EGFR and PRL genes require factors in addition to ER for basal as well as E2-regulated expression.

Immunolocalization of estrogen receptor protein in the mouse blastocyst during normal and delayed implantation.

“…This process of expressing ER in all cells initially and restricting receptor expression in a majority of cells later may be necessary if ER is to execute its function at both early and later times. It has been observed that stable transfection, rather than transient transfection, of ER into ER-negative cells is needed for ER to induce certain estrogen-responsive endogenous genes (26). Similar phenomena have also been reported for the progesterone receptor (27).…”

Differentiation of murine embryonic stem cells induces progesterone receptor gene expression

“…The mouse and human PR promoters share some similarity in promoter sequences (especially in the PRA promoter), but are not identical. Others have shown that stable overexpression of ERα in some cell lines followed by estrogen treatment is sufficient to activate endogenous PR expression [63,64]. These data implicate estrogen and ER as important regulators of PR gene expression.…”

Potentiation of Human Estrogen Receptor α Transcriptional Activation through Phosphorylation of Serines 104 and 106 by the Cyclin A-CDK2 Complex

“…The estrogen receptor ␣ (ER), 1 a transcription factor that controls the expression of a number of genes involved in cellular differentiation and proliferation in a wide variety of tissues (1)(2)(3)(4), is regulated by ligand binding and phosphorylation. The receptor is structurally similar to other members of the nuclear receptor superfamily in that separate receptor activities such as DNA and ligand binding are localized to distinct regions of the protein (5).…”