The role of the Yes-associated protein (YAP) in oncogenesis and progression of breast cancer remains controversial. Meanwhile, development of therapeutic resistance to trastuzumab, a common breast cancer treatment administered after chemotherapy, is a significant challenge in the treatment of HER2-positive breast cancer. We, therefore, analyzed the role of YAP in trastuzumab resistance in HER2-positive-breast carcinoma cells in vitro and evaluated the status of YAP and related proteins in patient-derived breast carcinoma tissues by immunohistochemistry. YAP expression was observed in both BT474-TS (trastuzumab-sensitive) and BT474-TR (trastuzumab-resistant) cells. Treatment with trastuzumab increased expression of nuclear-YAP (N-YAP) in BT474-TS cells, whereas BT474-TR cells showed a decrease in N-YAP expression following trastuzumab treatment. YAP silencing significantly reduced trastuzumab-induced inhibitory effects in BT474-TS cells. YAP-silenced cells also showed decreased apoptosis and significantly lower p73 levels following trastuzumab treatment. Combined protein kinase B (AKT) inhibitor-trastuzumab treatment significantly inhibited BT474-TR cell proliferation, resulting in increased N-YAP and p73 expression, as well as apoptosis. In both paclitaxel, doxorubicin and cyclophosphamide (TAC)-treated, and docetaxel, carboplatin, and trastuzumab (TCbH)-treated groups; the pathological complete response (pCR) ratios were inversely correlated with p-AKT status in biopsy specimens, while YAP and p73 status were positively correlated with the pCR ratio in the biopsy specimens of the TCbH group. Our results show that YAP is involved in trastuzumab resistance in HER2-positive breast carcinoma cells and that YAP and AKT may be developed as prognostic markers of neoadjuvant trastuzumab therapy in patients with HER2-positive breast cancer.
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