Estrogen receptor alpha transcriptional activity is regulated by distinct conformational states that are the result of ligand binding. Phage display was used to identify peptides that interact specifically with either estradiol- or tamoxifen-activated estrogen receptor alpha. When these peptides were coexpressed with estrogen receptor alpha in cells, they functioned as ligand-specific antagonists, indicating that estradiol-agonist and tamoxifen-partial agonist activities do not occur by the same mechanism. The ability to regulate estrogen receptor alpha transcriptional activity by targeting sites outside of the ligand-binding pocket has implications for the development of estrogen receptor alpha antagonists for the treatment of tamoxifen-refractory breast cancers.

SUMMARY HOXB13 is a member of the homeodomain family of sequence-specific transcription factors and together with the androgen receptor (AR) plays a critical role in the normal development of the prostate gland. We demonstrate here that in prostate cancer cells HOXB13 is a key determinant of the response to androgens. Specifically, it was determined that HOXB13 interacts with the DNA binding domain of AR and inhibits the transcription of genes that contain an androgen response element (ARE). In contrast, the AR:HOXB13 complex confers androgen responsiveness to promoters that contain a specific HOXB13-response element. Further, HOXB13 and AR synergize to enhance the transcription of genes that contain a HOX element juxtaposed to an ARE. The profound effects of HOXB13 knockdown on androgen regulated proliferation, migration and lipogenesis, in prostate cancer cells highlight the importance of the observed changes in gene expression.

We have utilized a genetic selection system in yeast to identify novel estrogen-responsive genes within the human genome and to define the sequences in the BRCA-1 gene responsible for its estrogen responsiveness. This approach led to the identification of a new subclass within the Alu family of DNA repeats which have diverged from known Alu sequences and have acquired the ability to function as estrogen receptor-dependent enhancers. Importantly, these new elements confer receptor-dependent estrogen responsiveness to a heterologous promoter when assayed in mammalian cells. This transcriptional activity can be attenuated by the addition of either of three different classes of estrogen receptor antagonists, indicating that these elements function as classical estrogen receptor-dependent enhancers. Furthermore, this enhancer activity is restricted to a specific subset of DNA repeats because consensus Alu elements of four major subfamilies do not respond to the estrogen receptor. Previously, most Alu sequences have been considered to be functionally inert. However, this work provides strong evidence that a significant subset can confer estrogen responsiveness upon a promoter within which they are located. Clearly, Alu sequences must now be considered as important contributors to the regulation of gene transcription in estrogen receptor-containing cells.The steroid hormone estrogen is a key intracellular modulator of the processes involved in establishment and maintenance of female reproductive function (1). In addition, its actions play an important role in maintaining female cardiovascular tone and regulating bone cell differentiation (2). In pathological states, the mitogenic activity of estrogen facilitates progression of breast cancers (3) and is implicated in the abnormalities of uterine function observed in endometriosis and possibly uterine fibroids (4). These actions of estrogen all appear to be mediated through specific high affinity receptors located within target cell nuclei (1). Molecular cloning has revealed that the estrogen receptor (ER) 1 is a member of a superfamily of receptors which mediate the nuclear actions of the sex steroids, retinoic acid, vitamin D 3 , and thyroid hormones (5). In the absence of hormone, ER resides in a latent form in target cell nuclei associated within a large macromolecular complex comprising heat shock protein 90 (hsp90), hsp70, p59, and other proteins (6). Upon ligand binding, the receptor undergoes a dramatic conformational change (7,8), initiating a cascade of events leading ultimately to the association of an ER dimer with specific estrogen response elements (EREs) within the regulatory regions of target genes (9). The mechanism by which the bound receptor modulates gene transcription is unknown. Because of its diverse biological functions and the implied complexity of its targets, there has been a keen interest in defining the genes which are regulated by estrogen. To date, a relatively small number of genes have been identified in humans which are modulated direc...