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Androgens inhibit the proliferation of breast cancer cells and tumor growth. While this inhibitory effect of androgens appears to be dependent on the intracellular mediator of androgens the androgen receptor (AR), the precise mechanism is not known. In this study we demonstrate that the AR can interact with the estrogen receptor alpha (ERα) to inhibit estrogen signaling in breast cancer cells. This interaction occurs in the absence of both AR and ERα ligands, but is enhanced by both ligands. Full-length wild type AR (wtAR) and a truncated, constitutively active AR (AR-T707) inhibited the activity of ectopically expressed ERα in MDA-MB-231 breast cancer cells (ERα- and AR-negative), with inhibition at a 4:1 molar ratio being approximately 70% and 85%, respectively. In addition, expression of AR-T707 inhibited the activity of the endogenous ERα in T-47D breast cancer cells (ERα- and AR-positive). To investigate the mechanism of inhibition of ERα signaling by AR, we deleted or mutated the activation functions (AF-1 and AF-5) within the amino-terminal transactivation domain of the AR, or the DNA binding domain (DBD). Deletion of the DBD and mutation of a key amino acid required for DNA binding (ie C617Y), abrogated the inhibitory effect of AR on ERα activity. Additionally, we demonstrated that the DBD of AR alone was sufficient to inhibit ERα activity. Consistent with a requirement for the DBD of AR for optimal inhibition of ERα activity, mobility shift assays demonstrated binding of wtAR to a consensus estrogen response element (ERE). Our findings suggest that the inhibitory effect of AR on ERα activity could result from either AR homodimers competing for ERα binding to EREs, or formation of a non-functional heterodimer between AR and ERα to either prevent binding to EREs or activation of estrogen responsive target genes.

98th AACR Annual Meeting-- Apr 14-18, 2007; Los Angeles, CA