Abstract P3-05-11: The evolution of the estrogen receptor (ER) complex conformation governs estrogen-induced apoptosis in antihormone-resistant breast cancer cells

Abstract

Abstract Over the past decade new insights have been gained into the acquired resistance to tamoxifen and the Aromatase Inhibitors (AIs) with the discovery of the new biology of estrogen-induced apoptosis. However, it has also been learned that estrogens can be classified into planar class I and angular class II estrogens. Using model systems of long-term estrogen-deprived breast cancer cells in vitro (MCF-7:5C), it was previously shown that class I estrogens cause immediate apoptosis over a 3-4 day period. Paradoxically, class II estrogens actually block apoptosis caused by planar class I estrogens. To gain insight into this paradox we have successfully crystallized an angular class II triphenylethylene (TPE) estrogen bound to the ligand binding domain (LBD) of the ER and derived a new conformation for the TPE:ER complex (code 3Q97 in the PDB). Surprisingly, Helix 12 seals the LBD with the class II estrogen, but not the same conformation is observed with the planar class I estrogen 17β-estradiol (E2). There would seem to be no reason why the 3Q97 complex would not cause immediate apoptosis. To address this issue we have used Western blot analysis for protein and qRT-PCR for mRNA levels for the ER. ER parameters were monitored for up to 72 hours and results compared and contrasted between E2, the Class II estrogens, 4-hydroxytamoxifen (4OHT) and endoxifen (Endox). ER protein and mRNA levels with 4OHT or Endox accumulated and remained high throughout the study period. In contrast, the planar estrogen E2 produced a rapid decline in the protein and mRNA levels for the ER complex. The angular class II estrogens initially produced an accumulation of the ER protein complex, which decreased with time. Using a chromatin immunoprecipitation (ChIP) technique we demonstrated that the class II estrogens recruit only half of the ER to the estrogen-responsive genes promoters (TFF1 And BREB1) and less than half co-activator binding compared to E2. The TPEs were only partial agonists compared to planar estrogen. These results explain why the Class II estrogens induce delayed apoptosis. We conclude that, for the first time, we have observed the binding of a ligand to a receptor that changes conformation against time and evolves from an antagonist to an agonist conformation to trigger apoptosis. These observations have current relevance to the decryption of the protective effects of estrogen alone therapy against breast cancer incidence in the Women's Health Initiative (WHI) trial. This work was supported by the Susan G. Komen for the Cure Foundation award SAC100009. Citation Format: Maximov PY, Sengupta S, Fernandes DJ, Fan P, Curpan RF, Rajan SS, Greene GL, Jordan VC. The evolution of the estrogen receptor (ER) complex conformation governs estrogen-induced apoptosis in antihormone-resistant breast cancer cells. [abstract]. In: Proceedings of the Thirty-Eighth Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2015 Dec 8-12; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2016;76(4 Suppl):Abstract nr P3-05-11.