MDA-MB-231 cells (The Cell Bank of Type Culture Collection of the Chinese Academy of Sciences) were incubated in Dulbecco's modified Eagle's medium or RPMI-1640 medium (Beijing Solarbio Science & Technology Co., Ltd.) with penicillin (100 U/ml), 10% FBS and streptomycin 100 µg/ml, and cultured at 37°C in an atmosphere of 90% relative humidity and 5% CO2. HP-β-CD (BBI Life Sciences, Shanghai, China) was dissolved in phosphate-buffered saline (PBS) then filtered with a syringe-driven filter (Guangzhou Jet Bio-Filtration Co., Ltd.). The cells were treated with HP-β-CD diluted to the desired concentrations with complete medium. To choose the optimal concentration and treatment time of HP-β-CD, a previous study was referred to and cells treated with different concentrations (0, 2.5, 5, 10 mmol/l) for 48 h (22 (link)). Then the expression of EMT markers vimentin and E-cadherin was detected by western blotting. Cells were stimulated with 10 ng/ml TGF-β1 (13 (link)) (cat. no. 10804-HNAC; Sino Biological Inc.) dissolved in PBS for 48 h and the same amount of PBS was added to control group. For inhibition of ER stress, 5 mmol/l 4-PBA (23 (link)) (Shanghai Macklin Biochemical Co., Ltd.) was dissolved in DMSO and then diluted to the desired concentrations with complete medium; DMSO (<0.1%) was then added to the culture medium.