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.
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