Preferential Estrogen Receptor β Ligands Reduce Bcl-2 Expression in Hormone-Resistant Breast Cancer Cells to Increase Autophagy

Acquired resistance to selective estrogen receptor (ER) modulators (SERM) and downregulators (SERD) is a significant clinical problem in the treatment of estrogen (E2) receptor-positive (ER+) breast cancers. There are two ER subtypes, ER alpha and ER beta, which promote and inhibit breast cancer cell proliferation, respectively. Although ER+ breast cancers typically express a high ratio of ER alpha to ER beta, the acquisition of SERM resistance in vitro and in vivo is associated with increased relative expression of the ER beta. On some gene enhancers, ER beta has been shown to function in opposition to the ER alpha in the presence of E2. Here, we demonstrate that two different ERb agonists, WAY-20070 and a novel A-CD estrogen called L17, produce a marked reduction in G(2)-Mphase correlated with effects on cyclin D1 and cyclin E expression in a SERM/SERD-resistant breast cancer cell line. ER beta agonists recruited both the ER alpha and ER beta to the Bcl-2 E2-response element strongly reducing Bcl-2 mRNA and protein in an ER beta-dependent manner. L17 recruited RIP140 to the Bcl-2 promoter in cells overexpressing ER beta. Exposure to the ER beta ligands also resulted in increased processing of LC3-I to LC3-II, indicative of enhanced autophagic flux. The coaddition of ER beta agonist and the autophagy inhibitor chloroquine resulted in a significant accumulation of sub-G(1) DNA which was completely prevented by the addition of the caspase inhibitor Z-VAD-FMK. We propose that combined therapies with an ER beta agonist and an inhibitor of autophagy may provide the basis for a novel approach to the treatment of SERM/SERD-resistant breast cancers. Mol Cancer Ther; 13(7); 1882-93. (C) 2014 AACR.