INVOLVEMENT OF ESTROGEN RECEPTOR ALPHA IN PROGESTERONE RECEPTOR DRIVEN MAMMARY TUMOR GROWTH

SEBASTIAN GIULIANELLI; ROCIO SOLDATI; CLAUDIA LANARI

San Diego, California. USA

Conferencia; AACR. Advances in Breast Cancer Research: Genetics, Biology, and Clinical Applications; 2007

The expression of estrogen receptor alpha (ERa) in breast cancer identifies a group of tumors which may respond to an endocrine therapy. Most of these tumors are also progesterone receptor (PR) positive and this has been used as a marker of estrogen action. It has also been postulated that cytosolic PR isoform B (PRB) interacts with ERa and activates c-Src inducing rapid non-genomic effects. We have developed an experimental model of breast cancer in BALB/c female mice in which, ductal mammary carcinomas are maintained by serial transplantations in syngeneic mice. Hormone-dependent (HD) tumors need the exogenous administration of medroxyprogesterone acetate (MPA) to grow in vivo while the hormone-independent (HI) variants are able to grow in untreated female mice without hormone supply. Both tumor types express high levels of ERa and PR and, when grown in plastic, they have the same hormone requirements. 17b-estradiol (E2) or antiprogestin treatment may induce complete tumor regression and, paradoxically, the pure antiestrogen ICI 182780 (ICI) had the same effect in primary cultures of HD tumors, suggesting that ERá are as necessary as PR to maintain tumor growth. This might be because ERa induces PR expression or because an interaction between ERá and PR is required to induce cell proliferation. The main goal of this study was to evaluate the role of ERá in tumor growth in this model using the C4-HD and C4-HI tumors. We studied a) the activation state of ERa in HD and HI tumors, b) the effect of ICI and siRNA ERá on cell proliferation and c) nuclear interactions between ERa and PR. By immunohistochemistry, Western blot and Immunofluorescence (IF), we detected high levels of pSer118 and pSer167-ERa in both C4-HI and C4-HD tumors growing with MPA. In vitro, in primary cultures of C4-HI cells, high levels of pERá were detected in quiescent cells suggesting that ERa activation is not enough to induce cell proliferation. The blockage of ERá either by incubation with ICI or by treatment with siRNA ERá did not affect significantly PR expression. However, the proliferation induced by MPA was significantly reduced (HD: ICI 0.1ìM p<0.001; HI: ICI 0.5ìM p<0,001; ERa-siRNA: HD 33.5±2.1 % inhibition p<0.002; HI 35.15±11.5 % inhibition p<0.05). In addition, in co-immunoprecipitation assays from nuclear lysates of HD tumors we found a physical interaction between ERa and the two PR isoforms (PRA and PRB). Those experiments were confirmed by IF using confocal microscopy in both, tumors and primary cultures. As a whole, our results indicate that ERá is a key receptor involved in tumor growth. However, ERa activation is not enough to induce tumor growth. The nuclear physical association between ERa and PR would appear as a novel mechanism in the proliferation mediated by progesterone receptor in this model.