Triple-negative breast cancer (TNBC) accounts for 15–20% of all breast cancer. TNBC does not express the estrogen receptor, progesterone receptor, or human epidermal growth factor receptor 2. Cytotoxic chemotherapy and surgery are the current therapeutic strategies for TNBC patients, but the chemoresistance of TNBC limits the efficiency of this strategy and shortens the lifespan of patients. The exploration of targeted therapy is ongoing in TNBC research. The aim of the present study was to identify the mechanism underlying acquired resistance in TNBC through the exploration of the relationship between the expression of USP7 and of ABCB1. We found that ubiquitin specific protease 7 (USP7) is a potential therapeutic target for overcoming the chemoresistance of TNBC. USP7 overexpression increased the chemoresistance of TNBC, while the knockdown of USP7 effectively increased the chemosensitivity of chemoresistant TNBC. A USP7 inhibitor effectively induced apoptosis and suppressed metastasis in chemoresistant TNBC. We further clarified that USP7 is a specific deubiquitinating enzyme for ABCB1 that plays an essential role in drug resistance. USP7 directly interacted with ABCB1 and regulated its stability. We concluded that USP7 promotes the chemoresistance of TNBC by stabilizing the ABCB1 protein.

• Nafamostat mesylate (NM) causes apoptosis and suppresses metastasis of endocrine-resistant ER-positive breast cancer (ERPBC). • Epigenetic downregulation of CDK4/CDK6 by NM in endocrine-resistant ERPBC via disruption of binding of H3K27Ac on promoter region. • Combination of nafamostat mesylate and CDK4/6 inhibitor synergistically overcomes endocrine resistance of breast cancer. • Nafamostat mesylate would be a well-efficient drug for endocrine-resistant ERPBC.

Estrogen receptor-positive breast cancer (ERPBC) accounts for approximately 70% of breast cancers in women worldwide. The therapeutic strategy process for ERPBC is well-established and significantly reduces the mortality rate. The discovery of new therapeutic targets remains essential for ERPBC patients with metastasis or endocrine resistance. This study indicated that USP7 is highly expressed in ERBPC and promotes tumor progression and metastasis. Inhibition of USP7 activity repressed proliferation, induced apoptosis, suppressed migration and invasive activities, and reversed the epithelial-mesenchymal transition of ERPBC. Mass spectrometry analysis indicated that USP7 regulates CDK1 expression, which is highly expressed and correlates with a poor overall survival rate in ERPBC. USP7 directly interacts with CDK1 and regulates its stability. The combined inhibition of USP7 and CDK1 by GNE-6776 and Ro-3306 synergistically represses the malignant process and metastasis of ERPBC. These findings proved that targeting USP7 and CDK1 is a potential strategy for overcoming endocrine resistance in patients with advanced ERPBC.