Downregulation of BAG‑1 in T47D cells promotes resistance to tamoxifen via activation of the PI3K/Akt/mTOR signaling pathway.

Tamoxifen, a selective estrogen receptor (ER) modulator, is the most widely used endocrine therapy for patients with ER‑positive breast cancer. However, ~30% of tamoxifen‑treated breast cancers do not initially respond to tamoxifen, and neither do they eventually develop tamoxifen resistance. Bcl‑2‑associated athanogene 1 (BAG‑1) is a multifunctional protein that interacts with a wide range of molecules to protect cells from apoptosis otherwise induced by cytotoxic drugs, growth factor withdrawal, radiation and stress. The aim of the present study was to investigate the function of BAG‑1 in tamoxifen resistance. Immunohistochemistry techniques were used to determine BAG‑1 expression in 119 stage I‑III primary breast cancer tissues and it was identified that BAG‑1 was significantly overexpressed in ER‑positive breast cancer (P=0.001). Knockdown of BAG‑1 by short interfering RNA was revealed to downregulate ER, and upregulate phospho (p)‑protein kinase B (Akt) and p‑mammalian target of rapamycin (mTOR) levels. Furthermore, significantly decreased tamoxifen‑induced apoptosis (41.70±1.93 vs. 55.03±2.39%; P=0.012) was observed in T47D cells following the silencing of BAG‑1. In contrast, overexpression of BAG‑1 long enhanced apoptosis (65.10±2.35 vs. 55.03±2.39%; P=0.039) in T47D cells treated with tamoxifen. Combination treatment of tamoxifen and an mTOR inhibitor restored the inhibitory effects of tamoxifen in T47D cells exhibiting low BAG‑1 expression levels (66.87±2.27 vs. 57.07±2.46%; P=0.037). In conclusion, there results of the present study indicated that suppression of BAG‑1 expression may activate the phosphoinositide 3‑kinase/Akt/mTOR pathway and protect ER‑positive breast cancer cells from tamoxifen‑induced inhibition of proliferation. ER‑positive breast cancer cells exhibiting low BAG‑1 expression appeared to be more sensitive to treatment with the mTOR inhibitor rapamycin. Furthermore, the results indicated that combination treatment targeting ER with tamoxifen and targeting mTOR with rapamycin may significantly potentiate the inhibitory effect in BAG‑1‑silenced cells.