Differences between oestrogen receptor activation by oestrogen and antioestrogen

Triphenylethylene antioestrogens such as tamoxifen, nafoxidine and Ci 628 specifically inhibit oestrogen action at the target cell level, probably by interacting with the oestrogen receptor (ER) and competitively displacing oestrogens from their binding sites. It is not clear, however, why these ligands are less biologically active than oestrogens when the bind to the ER, as no reliable difference has been found either in the binding affinity of these two series of ligands to the ER or in their ability to translocate the ER to the nucleus. In fact, these antioestrogens are transformed in vivo into hydroxylated metabolites which display a better antioestrogenic activity than the injected compound and at least the same high affinity as oestradiol for the ER. With the aim of finding an in vitro criterion to predict the agonistic or antagonistic properties of ER ligands, we have stabilized the ER in its 'native' or non-activated form by the use of molybdate and have compared the binding of oestradiol (E2) and of 4-hydroxytamoxifen (OHT), an active metabolite of tamoxifen, to the molybdate-treated and to the activated ER. We report here that molybdate prevented the DNA binding and the 4S to 5S transformation of the ER bound to both ligands, and that it increased the dissociation rate of oestrogens but not that of antioestrogens. Moreover, in the absence of molybdate, receptor activation by heating decreased the dissociation rate of E2 but not that of OHT. We conclude that a difference exists between the ER activation triggered by oestrogens and antioestrogens and propose that antioestrogens are acting as allosteric ligands of the ER.