Estrogen Receptor Alpha Mediates Progestin-Induced Mammary Tumor Growth by Interacting with Progesterone Receptors at the Cyclin D1/MYC Promoters

Abstract

Synthetic progesterone used in contraception drugs (progestins) can promote breast cancer growth, but the mechanisms involved are unknown. Moreover, it remains unclear whether cytoplasmic interactions between the progesterone receptor (PR) and estrogen receptor alpha (ERá) are required for PR activation. In this study, we used a murine progestin-dependent tumor to investigate the role of ERá in progestin-induced tumor cell proliferation. We found that treatment with the progestin medroxyprogesterone acetate (MPA) induced the expression and activation of ERá, as well as rapid nuclear colocalization of activated ERá with PR. Treatment with the pure antiestrogen fulvestrant to block ERá disrupted the interaction of ERá and PR in vitro and induced the regression of MPA-dependent tumor growth in vivo. ERá blockade also prevented an MPA-induced increase in CYCLIN D1 (CCND1) and MYC expression. Chromatin immunoprecipitation studies showed that MPA triggered binding of ERá and PR to the CCND1 and MYC promoters. Interestingly, blockade or RNAi-mediated silencing of ERá inhibited ERá, but not PR binding to both regulatory sequences, indicating that an interaction between ERá and PR at these sites is necessary for MPA-induced gene expression and cell proliferation. We confirmed that nuclear colocalization of both receptors also occurred in human breast cancer samples. Together, our findings argued that ERá-PR association on target gene promoters is essential for progestin-induced cell proliferation.