Effects of (R,S)/(S,R)-4,5-bis(2-chloro-4-hydroxyphenyl)-2-imidazolines and (R,S)/(S,R)-2,3-bis(2-chloro-4-hydroxyphenyl)piperazines on estrogen receptor alpha level and transcriptional activity in MCF-7 cells

4,5-Diaryl-2-imidazolines (Ims) and 2,3-diarylpiperazines (Pips) belong to the type II class of estrogens. These compounds enhance ERα-mediated transcription of ERE-driven reporter genes in MCF-7 cells but do not compete with [3H]estradiol (E2) for receptor binding, because of distinct anchoring modes. The present study examined whether the estrogenic action of Ims and Pips is associated with a down regulation of ERα, as reported for conventional agonists. Im and Pip derivatives displaying a large spectrum of activities in three distinct ERE-dependent transactivation systems were selected for that purpose. ERα immunostaining as well as Western blotting analysis revealed that both classes of compounds down regulated ERα with an efficiency closely related to their transactivation potency. MG-132 abrogated this down regulation, pointing to a proteasomal degradation process. Ims and Pips with strong transactivation potency also altered [3H]E2 binding parameters, leading to a progressive decrease of cellular estrogen binding capacity. This property occurred largely before ERα down regulation and persisted even in presence of MG-132, indicating that it did not result from ERα breakdown but rather from a conformational change of the receptor. The additional finding that the most active agonist tested in this study enhanced the capacity of a purified ERα recombinant to recruit LxxLL co-activators, while its inactive counterpart failed to do so confirmed this hypothesis. Altogether, our data indicate that the association of Ims and Pips with ERα elicits similar responses to conventional agonists, even if they interact with distinct residues of the binding pocket.