Abstract 3924: Alteration of the shape of the ligand estrogen receptor complex controls estrogen induced apoptosis in breast cancer

Abstract

Abstract Aromatase inhibitors (AIs) are the standard of care for long term adjuvant treatment of estrogen receptor (ER) positive breast cancer in postmenopausal patients. However, acquired drug resistance eventually develops and evolves to expose a vulnerability in breast cancer: estrogen induced apoptosis. The discovery of the new biology of estrogen induced apoptosis has translated to treatment of patients in clinical trials [Jordan V, Ford L. Cancer Prev Res 2011;4:633-7]. We recently reported the sequence of events involved in triggering of estrogen induced apoptosis [Ariazi et al. Proc Natl Acad Sci USA. 2011]. Our goal here is to define the mechanics of ligand ER action to trigger apoptosis. 4-Hydroxytamoxifen (4OHT) reverses estradiol (E2) induced apoptosis in long term estrogen deprived MCF-7 cells. As an investigative tool, we synthesized a range of estrogenic triphenylethylenes (TPEs), which are structurally similar to 4OHT. We discovered that although the TPEs are all classified as estrogens for breast cancer growth they, like 4OHT, inhibit E2 induced apoptosis. We previously identified that the surface amino acid D351 within the ligand binding domain is critical for the estrogenic actions of 4OHT. Unlike raloxifene, whose antiestrogen side chain shields and neutralizes D351, the side chain in 4OHT is too short. In order to elucidate the biological activity of the TPE:ER, we devised an assay using induction of the mRNA for the transforming growth factor (TGF-β) gene in situ[Jordan et al. Cancer Res. 2001;61:6619-6623]. In MDA-MB231 cells stably transfected with wild type ER (MC2 cells), TPEs and tamoxifen metabolites, 4OHT and endoxifen induce TGF- ≤ mRNA expression in a similar fashion to 17α estradiol (E2). Cells stably expressing D351G ER (JM6 cells) i.e. without the charged D351, ERα retain responsiveness to planar estrogens; however the nonplanar estrogens and 4OHT, lose their ability to induce TGF-β mRNA and become completely antiestrogenic. This response therefore is characteristic of an antiestrogenic conformation of the ER complex where charged D351 would interact with activating function 1(AF-1) to enhance estrogen action for growth. The TPEs prefer to dock with the ER in a conformation related to 4OHT: the antiestrogenic conformation. The fact that the TPEs also stimulate growth of breast cancer demonstrates the extreme sensitivity of this estrogenic endpoint. Thus the observation that planar and nonplanar compounds have different abilities to change the shape of the ER complex may have important implication for the modulation of the E2 induced apoptosis. We conclude that AF-1 is required for E2 induced growth but AF1 and AF2 together are required to deliver coactivators and cause E2 induced apoptosis. Acknowledgements This work (VCJ) was supported by the Susan G Komen for the Cure Foundation under Award number SAC100009. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 3924. doi:1538-7445.AM2012-3924