Estrogen receptor beta activates the human retinoic acid receptor alpha-1 promoter in response to tamoxifen and other estrogen receptor antagonists, but not in response to estrogen

Human estrogen receptor-alpha (hER alpha) or -beta (hER beta) transfected i nto Hep G2 or COS1 cells each responded to estrogen to increase transcripti on from an estrogen-responsive element (ERE)-driven reporter vector with si milar fold induction through a classical mechanism involving direct recepto r binding to DNA. ER antagonists inhibited this estrogen induction through both hER alpha and hER beta, although raloxifene was more potent through ER alpha than ER beta, and tamoxifen was more potent via ER beta than ER alph a. We have shown previously that estrogen stimulated the human retinoic aci d receptor-alpha-1 (hRAR alpha-1) promoter through nonclassical EREs by a m echanism that was ER alpha dependent, but that did not involve direct recep tor binding to DNA. We show here that in contrast to hER alpha, hER beta di d not induce reporter activity driven by the hRAR alpha-1 promoter in the p resence of estrogen. While hER beta did not confer estrogen responsiveness on this promoter, it did elicit transcriptional activation in the presence of 6-hydroxytamoxifen (4-OH-Tam). Additionally, this 4-OH-Tam agonist activ ity via ER beta was completely blocked by estrogen. Like ER alpha, transcri ptional activation of this promoter by ER beta was not mediated by direct r eceptor binding to DNA. While hER alpha was shown to act through two estrog en-responsive sequences within the promoter, hER beta acted only at the 3'- region, through two Sp1 sites, in response to 4-OH-Tam. Other ER antagonist s including raloxifene, ICI-164,384 and ICI-182,780 also acted as agonists through ER beta via the hRAR alpha-1 promoter. Through the use of mutant an d chimeric receptors, it was shown that the 4-OH-Tam activity via ER beta f rom the hRAR alpha-1 promoter in Hep G2 cells required the amino-terminal r egion of ER beta, a region that was not necessary for estrogen-induced ER b eta activity from an ERE in Hep G2 cells. Additionally, the progesterone re ceptor (PR) antagonist RU486 acted as a weak (IC50 >1 mu M) antagonist via hER alpha and as a fairly potent (IC50 similar to 200 nM) antagonist via hE R beta from an ERE-driven reporter in cells that do not express PR. Althoug h RU486 bound only weakly to ER alpha or ER beta in vitro, it did bind to E R beta in whole-cell binding assays, and therefore, it is likely metabolize d to an ER beta-interacting compound in the cell. Interestingly, RU486 acte d as an agonist through ER beta to stimulate the hRAR alpha-1 promoter in R ep G2 cells. These findings may have ramifications in breast cancer treatme nt regimens utilizing tamoxifen or other ER antagonists and may explain som e of the known estrogenic or antiestrogenic biological actions of RU486.