Abstract 4610: Nuclear interaction between estrogen and progesterone receptors in experimental breast cancer is blocked by antiestrogens but not by RU 486
Authors: Giulianelli Sebastian J. , Lanari Claudia 
Published: February 2011

Abstract
The expression of estrogen receptor alpha (ERα) and progesterone receptor (PR) in breast cancer is used as a prognostic factor and identifies a group of tumors which may respond to an endocrine therapy aimed to block estrogen synthesis or ER activation. There is however, compelling evidence that indicates that PR is also involved in breast cancer growth. We have developed an experimental model of breast cancer in BALB/c female mice in which mammary carcinomas are induced by the administration of medroxyprogesterone acetate (MPA). The hormone-dependent (HD) tumor, C4-HD, grows in vivo only in the presence of MPA, and expresses ERα and both PR isoforms (PRA and PRB). PR are essential for tumor growth in this model, since antisense oligonucleotides of PR and different antiprogestins inhibit their growth. Interestingly, 17β-estradiol (E2) and the pure antiestrogen ICI 182780 (ICI) also induce tumor regression. It is controversial how progesterone (Pg) induces non genomic effects. Although it has been demonstrated that Pg binding to cytosolic PRB activates c-Src through an interaction with ERα, others have reported that this interaction is not necessary for c-Src activation in human T47D breast cancer cells. In our Laboratory, we have shown a nuclear physical association between ERα and PR in C4-HD tumors, and we postulate that this interaction is necessary to induce progestin stimulated cell proliferation. The aim of this study was to further investigate this nuclear interaction in C4-HD and in T47D epithelial cells. We found, by confocal immunofluorescence, that MPA (10nM) treatment induced an early increase in the nuclear co-localization between ERα and PRA in both tumor models (p&lt;0.001). These results were corroborated by co-immunoprecipitation assays showing that both PR isoforms interact with ERα in the cell nuclei. We also found nuclear interaction between both pSer118 ERα and pSer294 PR after 15 minutes of MPA incubation in C4-HD cells (p&lt;0.001), indicating that this complex could be transcriptionally active. E2 (1nM) and ICI (1μM) treatment inhibited this interaction whereas the antiprogestin RU 486 (RU, 10nM), which also inhibits cell proliferation as E2 and ICI, behaved as a PR agonist supporting previous data of our lab that indicates that RU activates MAPK as MPA, and that the inhibitory effects induced by RU are due to genomic events. Our data support the hypothesis that ERα is involved in MPA-induced cell proliferation; we postulate however that the nuclear interaction between ERα and PR participates in the progestin induced stimulatory effects in these breast cancer models. In addition, the results shown herein reinforce our previous findings in which we showed early agonistic effects of RU mediating non genomic events, despite inducing tumor regression. These results highlight the importance of targeting both ERα and PR in breast cancer therapies.
Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 4610.