Abstract 498: Dual functions of miR-200b in triple-negative breast cancer metastasis and chemoimmuno-resistance
Authors: Tan Xiaohui , Ren Shuchang , Lee Woojin , Wu Xiaoling , Rezaei Katayoon , Man Yan-gao , Latham Patricia , Siegel1 Robert S. , Brem Rachel F. , Li Zhongwu , Chang Xiaofeng , Fu Sidney W. 
Published: August 2018

Abstract
Triple-negative breast cancer (TNBC) is a subtype of breast cancer that is negative for estrogen and progesterone receptors (ER/PR) and human epidermal growth factor receptor 2 (HER2). It is typically associated with high rate of metastasis and limited targeted treatment options. Chemotherapy is the standard treatment for metastatic TNBC. However, the development of chemoresistance limits its clinical application. Elevated expression of immune-related genes in TNBC suggests that immunotherapy strategies may provide new therapeutic options for TNBC. Programmed death 1 (PD-1) and programmed death-ligand 1 (PD-L1) immune checkpoint inhibitors have been approved by the FDA for TNBC treatment. However, tumor immune evasion is considered an important obstacle. BRCA1 is a nuclear-cytoplasmic shuttling protein that plays a key role in preventing the development of a malignant phenotype. BRCA1 dysregulation and nuclear export are an important mechanism in cancer development and chemoresistance, especially in TNBC. Blocking BRCA1 nuclear export could be used as a strategy to prevent resistance. BRCA1 nuclear export has been reported to be mediated by several proteins such as BRCA1-binding protein 2 (BRAP2) and chromosomal maintenance 1 (CRM1, also known as exportin 1, XPO1). CRM1-mediated events have been implicated in breast cancer and involved in chemoimmunotherapy. microRNA-200b (miR-200b) is a cell-autonomous suppressor of EMT (epithelial-mesenchymal transition) and involved in tumor metastasis. In our present work, we discovered that miR-200b overexpression resulted in significant BRCA1 nuclear retention accompanied by downregulated expression of CRM1 and STAT1 (signal transducer and activator of transcription 1). Bioinformatics analysis indicated that miR-200b directly targets STAT1, which was confirmed by luciferase assay. We demonstrated that STAT1 is a transcription factor (TF) of CRM1, by both Transfac analysis and chromatin immunoprecipitation (ChIP)-qPCR assay. In patient tissue samples, we found that miR-200b expression was relatively lower in TNBC compared to non-TNBCs. Furthermore, we demonstrated that miR-200b-mediated BRCA1 nuclear retention is associated with significant PD-L1 downregulation, and sensitizes the TNBC cells to chemotherapeutic agents. In addition, high level PD-L1 expression is associated with not only chemoimmunoresistance but also tumor metastasis. These data provide strong evidence that miR-200b-mediated regulation of BRCA1 nucleus retention is through transcriptional regulation of CRM1 by STAT1, and miR-200b regulates PD-L1 expression in TNBC. In conclusion, this novel dual role of miR-200b may serve as a strategy in metastatic TNBC therapy by repressing STAT1-mediated CRM1 transcription regulation, and reversing the chemoimmunoresistance via PD-L1 inhibition.
Citation Format: Xiaohui Tan, Shuchang Ren, Woojin Lee, Xiaoling Wu, Katayoon Rezaei, Yan-gao Man, Patricia Latham, Robert S. Siegel1, Rachel F. Brem, Zhongwu Li, Xiaofeng Chang, Sidney W. Fu. Dual functions of miR-200b in triple-negative breast cancer metastasis and chemoimmuno-resistance [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 498.