ER dimerization: a key factor for the weak estrogenic activity of an ER alpha modulator unable to compete with estradiol in binding assays

摘要

Estrothiazine (ESTZ) is a weak estrogen sharing structural similarities with coumestrol. ESTZ failed to compete with [H-3]17 beta-estradiol ([H-3]17 beta-E-2) for binding to the estrogen receptor alpha (ER alpha), questioning its ability to interact with the receptor. However, detection by atomic force spectroscopy (AFS) of an ESTZ-induced ER alpha dimerization has eliminated any remaining doubts. The effect of the compound on the proliferation of ER alpha-positive and negative breast cancer cells confirmed the requirement of the receptor. The efficiency of ESTZ in MCF-7 cells was weak without any potency to modify the proliferation profile of estradiol and coumestrol. Growth enhancement was associated with a proteasomal degradation of ER without substantial recruitment of LxxLL coactivators. This may be related to an unusual delay between the acquisition by the receptor of an ERE-binding capacity and the subsequent estrogen-dependent transcription. A complementary ability to enhance TPA-induced AP-1 transcription was observed, even at concentrations insufficient to activate the ER, suggesting a partly independent mechanism. ESTZ also rapidly and transiently activated ERK1/2 likely through membrane estrogenic pathways provoking a reorganization of the actin network. Finally, the systematic absence of biological responses with an ESTZ derivative unable to induce ER dimerization stresses the importance of this step in the action of the compound, as reported for conventional estrogens. In view of the existence of many other ER modulators (endocrine disruptors such as, for example, pesticides, environmental contaminants or phytoestrogens) with extremely weak or similar apparent lack of binding ability, our work may appear as pilot investigation for assessing their mechanism of action.