Novel roles of microRNA-200b in reversing the chemoimmunoresistance by inhibiting BRCA1 nuclear export in triple negative breast cancer

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 chemo-resistance 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 down-regulated 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 roles 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