Cancer Biology and Signal Transduction Preferential Estrogen Receptor b 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 (ERþ) breast cancers. There are two ER subtypes, ERa and ERb, which promote and inhibit breast cancer cell proliferation, respectively. Although ERþ breast cancers typically express a high ratio of ERa to ERb, the acquisition of SERM resistance in vitro and in vivo is associated with increased relative expression of the ERb. On some gene enhancers, ERb has been shown to function in opposition to the ERa in the presence of E2. Here, we demonstrate that two different ERb agonists,WAY-20070 and a novel "A-CD" estrogen called L17, produce amarked reduction inG2–Mphase correlated with effects on cyclin D1 and cyclin E expression in a SERM/SERD-resistant breast cancer cell line. ERb agonists recruited both the ERa and ERb to the Bcl-2 E2-response element strongly reducing Bcl-2mRNA and protein in an ERb-dependent manner. L17 recruited RIP140 to the Bcl-2 promoter in cells overexpressing ERb. Exposure to the ERb ligands also resulted in increased processing of LC3-I to LC3-II, indicative of enhancedautophagic flux. The coaddition of ERb agonist and the autophagy inhibitor chloroquine resulted in a significant accumulation of sub-G1 DNA which was completely prevented by the addition of the caspase inhibitorZ-VAD-FMK.Wepropose that combined therapieswith anERb agonist andan 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. 2014 AACR.