ERβ shifts from mitochondria to nucleus during estrogen-induced neoplastic transformation of human breast epithelial cells and is involved in estrogen-induced synthesis of mitochondrial respiratory chain proteins

Both estrogen receptors (ER) alpha (ERα) and beta (ERβ) are localized in the nucleus, plasma membrane, and mitochondria, where they mediate the different physiological effects of estrogens. It has been observed that the relative subcellular localization of ERs is altered in several cancer cells. We have demonstrated that MCF-10F cells, the immortal and non-tumorigenic human breast epithelial cells (HBEC) that are ERα-negative and ERβ-positive, are transformed in vitro by 17β-estradiol (E2), generating highly invasive cells that are tumorigenic in severe combined immunodeficient mice. E2-transformed MCF-10F (trMCF) cells exhibit progressive loss of ductulogenesis, invasive (bsMCF) and tumorigenic (caMCF) phenotypes. Immunolocalization of ERβ  by confocal fluorescent microscopy and electron microscopy revealed that ERβ is predominantly localized in mitochondria of MCF-10F and trMCF cells. Silencing ERβ expression with ERβ-specific small interference RNA (siRNA-ERβ) markedly diminishes both nuclear and mitochondrial ERβ in MCF-10F cells. The ERβ shifts from its predominant localization in the mitochondria of MCF-10F and trMCF cells to the nucleus of bsMCF cells, becoming predominantly nuclear in caMCF cells. Furthermore, we demonstrated that the mitochondrial ERβ in MCF-10F cells is involved in E2-induced expression of mitochondrial DNA (mtDNA)-encoded respiratory chain (MRC) proteins. This is the first report of an association of changes in the subcellular localization of ERβ with various stages of E2-induced transformation of HBEC and a functional role of mitochondrial ERβ in mediating E2-induced MRC protein synthesis. Our findings provide a new insight into one of the potential roles of ERβ in human breast cancer.