ACTIVATION OF THE SILENT PROGESTERONE-RECEPTOR GENE BY ECTOPIC EXPRESSION OF ESTROGEN-RECEPTORS IN A RAT FIBROBLAST CELL-LINE

We describe the construction and characterization of a novel estrogen (E2)-responsive cell line, Rat1+ER, which ectopically expresses estrog en receptor (ER). Human ER cDNA was introduced by retrovirus-mediated gene transfer into the Rat1 cell line, which does not express function al ER endogenously. Rat1+ER cells express functional ER based on radio receptor assays, immunoblotting, and transient transfection experiment s using E2-responsive reporter plasmids. The effects of this ectopic E R expression were studied on three endogenous E2-responsive genes, pro lactin (PRL), progesterone receptor (PR), and epidermal growth factor receptor (EGFR). PRL, usually expressed in the lactotrophs of the pitu itary, is not expressed at all in Rat1+ER cells, with or without E2 ad dition, and appears to require other factors for expression. In contra st, although PR is not expressed in Rat1 cells, it is induced in Rat1ER cells upon the addition of E2. This induction appears to occur at t he transcriptional level and is insensitive to cycloheximide treatment . This is one of the few examples where the expression of one gene act ivates an otherwise silent gene. Another contrasting observation is th at, although EGFR is basally expressed in Rat1+ER cells, the addition of E2 has no effect. Our studies paint a complicated picture of E2 reg ulation of endogenous genes: the activation of the PR gene may only re quire the presence of E2 and ER, whereas EGFR and PRL genes require fa ctors in addition to ER for basal as well as E2-regulated expression.