Truncated Estrogen Receptor Product-1 Stimulates Estrogen Receptor α Transcriptional Activity by Titration of Repressor Proteins

Abstract

The truncated estrogen receptor product-1 (TERP-1, or TERP) is a pituitary-specific isoform of estrogen receptor ␣ (ER␣), and its expression is regulated by estrogen. TERP modulates the transcriptional activity of ER␣ but has no independent effect on transcription of estrogen-response element-containing promoters. At low concentrations, TERP stimulates ER␣ transcriptional activity in transient transfection assays. At TERP concentrations equal to or greater than full-length ER␣, TERP forms dimers with ER␣ and reduces both liganddependent and -independent transcription. A dimerization mutant of TERP, TERP L509R, stimulated ER␣ transcription at all concentrations. We hypothesized that TERP stimulates ER␣ transcriptional activity by titrating suppressors of ER␣ activity. We found that repressor of estrogen receptor activity (REA), originally isolated from human breast cancer cells, is present in mouse pituitary gonadotrope cell lines. Levels of REA vary slightly throughout the rat reproductive cycle, but TERP mRNA and protein vary much more dramatically. In transfection experiments, REA suppressed ER␣ transcriptional activity, and TERP L509R was able to alleviate transcriptional suppression by REA. In glutathione S-transferase pull-down assays, TERP bound to REA more efficiently than did ER␣ at equivalent concentrations, suggesting that REA will preferentially bind to TERP. Our findings suggest that the stimulation of pituitary ER␣ activity by low concentrations of TERP can occur by titration of corepressors such as REA. The ligand-bound estrogen receptor (ER)1 activates transcription and exerts its biological actions in a number of responsive tissues such as uterus, mammary glands, brain, and pituitary (1-3). Like other nuclear receptors, the ER has five conserved structural domains. The N-terminal A/B region contains the ligand-independent activation function-1 (AF-1) and is the most variable in length and in sequence. It is thought to contribute to the ligand-independent activation of the receptor and is involved in promoter-and cell-specific activity (4 -6). The C-terminal ligand-binding domain (LBD) consists of 12 conserved helices. It contains the ligand-dependent activation function-2 (AF-2) and is also important in heat-shock protein binding, nuclear localization, and dimerization of the receptor (7). Both AF-1 and AF-2 domains contribute to the transcription activation of ER. The most conserved domain is the DNAbinding domain (DBD), which has two zinc fingers and recognizes specific DNA sequences, or estrogen-response elements (EREs), on hormone-responsive genes. The hinge region links the DNA-binding and ligand-binding domains and allows rotation of the DBD (7). Activation of the receptor by ligand requires a conformational change in the LBD in which helix 12 moves toward helix 3 and helix 4, forming a hydrophobic pocket and a surface that allows interaction with coactivators and other regulatory proteins (5, 8). There are two subtypes of the estrogen receptor, ER␣ and ER␤, which are encoded by separate ge...

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