The role of mitochondrial estrogen receptor β in relapse of breast cancer.
e12540 Background: Breast cancer is the most common non-skin cancer in women. Breast cancers are heterogeneous, and treatment by subgroup based on hormone receptor and HER2 made a significant difference in clinical outcomes. Several studies have reported that estrogen receptor beta (ERβ) decreases during tumor development in the breast epithelium. However, the role of ERβ in relapse and metastasis of breast cancer is poorly understood. Methods: In this study, we retrospectively studied 30 case breast carcinomas divided luminal, HER2, and triple negative subtype. Among them, patients relapsed within 5 years are 6 cases. The expression of ERβ gene in breast cancer tissues (30 cases) was estimated using a quantitative PCR, and other marker (ERα, HER2, PR etc) was measured anonymously in formalin-fixed paraffin-embedded tumor sections, by using specific antibodies. Results: A low level of ERβ expression and mitochondrial translocation of ERβ was associated with relapse/metastasis of breast cancer. The ERβ depletion resulted in resistance in response to tamoxifen treatment of MCF-7 breast cancer cells. Conversely, the overexpression of mitochondrial ERβ enhanced the cell death by treatment of tamoxifen in MCF-7 cells. We found that ERβ localizes to the mitochondria via the interaction with Grp75 and improves mitochondrial oxygen consumption rate and ATP production in breast cancer cells. Finally, we showed that ERβ level was a low in the breast cancer stem cells (CD24-CD44+ cells) compared with breast non-cancer stem cells (CD24+CD44- cells), whereas ERα level was a high. The overexpression of mitochondrial ERβ contribute to a decrease of sphere formation showing a tumorigenic ability. Conclusions: The mitochondrial ERβ contribute to suppress survival and stemness of cancer stem cells for relapse/metastasis, promising to the development of novel strategies for the treatment of breast cancer patients.