ESTROGEN RESPONSE ELEMENTS CAN MEDIATE AGONIST ACTIVITY OF ANTIESTROGENS IN HUMAN ENDOMETRIAL ISHIKAWA CELLS

Anti-estrogens like hydroxytamoxifen (OHT) have mixed agonist/antagoni st activities, leading to tissue-specific stimulation of cellular prol iferation. Partial agonist activity of OHT can be observed in vitro in endometrial carcinoma cells like Ishikawa. Here, we have compared sev eral anti-estrogens (including extensively characterized OHT and pure anti-estrogens such as ICI164,384 and RU58,668, which are devoid of ut erotrophic activity) for their capacity to stimulate promoters contain ing estrogen response elements (EREs) or AP1-binding sites (12-O-tetra decanoylphorbol-13-acetate response elements, TREs), the two types of DNA motifs known to mediate transcriptional stimulation by estrogen re ceptors. Assays were performed in Ishikawa cells either by transient t ransfection or by using cell lines with stably propagated reporter vec tors. In transient transfection experiments, none of the anti-estrogen s displayed agonist activity on the promoters tested. In contrast, sig nificant transcriptional stimulation was observed with low concentrati ons of OHT and RU39,411 in Ishikawa cells stably propagating reporter constructs containing a minimal ERE3-TATA promoter. In addition, micro molar concentrations of OHT, but not of RU39,411, stimulated stably pr opagated AP1-responsive reporter constructs. No transcriptional stimul ation of ERE- or TRE-containing promoters was observed with the pure a nti-estrogens ICI164,384 and RU58,668. These results indicate that the presence of estrogen response elements in promoters is sufficient to mediate cell-specific agonism of anti-estrogens at the transcriptional level, and that stimulation of AP1 activity may be restricted to a su bset of anti-estrogens possessing agonist activity on EREs. In additio n, our results suggest that transient transfections do not fully recap itulate in vivo conditions required to observe agonist activity of ant i-estrogens.