BCAR4 facilitates trastuzumab resistance and EMT in breast cancer via sponging miR‐665 and interacting with YAP1

Abstract Breast cancer antiestrogen resistance 4 (BCAR4) has been suggested that can modulate cell behavior, resulting in tumorigenesis and chemoresistance. However, the underlying mechanisms of BCAR4 in trastuzumab resistance (TR) is still elusive. Here, we explored the function and the underlying mechanism of BCAR4 involving in TR. We found that BCAR4 is significantly upregulated in trastuzumab‐resistant BC cells. Knockdown of BCAR4 could sensitize the BC cells to trastuzumab and suppress epithelial–mesenchymal transition (EMT). Mechanically, BCAR4 promotes yes‐associated protein 1 (YAP1) expression by competitively sponging miR‐665, to activated TGF‐β signaling. Reciprocally, YAP1 could occupy the BCAR4 promoter to enhance its transcription, suggesting that there exists a positive feedback regulation between YAP1 and BCAR4. Targeting the BCAR4/miR‐665/YAP1 axis may provide a novel insight of therapeutic approaches for TR in BC.