Long Noncoding Rna Cancer Susceptibility Candidate 9 Promotes Doxorubicin-Resistant Breast Cancer By Binding To Enhancer Of Zeste Homolog 2

The present study aimed to investigate the effect of the long noncoding RNA cancer susceptibility candidate 9 (CASC9) on doxorubicin (DOX)-resistant breast cancer and to reveal the potential underlying mechanisms. The expression of CASC9 in breast cancer tissues and cell lines, in addition to drug-resistant breast cancer cells (MCF-7/DOX), was detected by reverse transcription-quantitative polymerase chain reaction. Subsequently, MCF-7/DOX cells were transfected with the silencing vector pS-CASC9, containing enhancer of zeste homolog 2 (EZH2), multidrug resistance protein 1 (MDR1) or control small interfering (si)RNAs. The viability, apoptosis, migration and invasion of the transfected cells were assessed via an MTT assay, flow cytometry and a Transwell assay, respectively. The expression levels of apoptosis-associated proteins (apoptosis regulator Bcl-2, apoptosis regulator BAX, caspase-3 and caspase-9) were determined by western blotting. An RNA pull-down assay was performed to identify CASC9-binding candidates. In addition, the expression levels of the MDR1 gene and its encoded protein, P-glycoprotein, were detected. CASC9 expression was upregulated in breast cancer tissues and cell lines, and drug-resistant breast cancer cells. CASC9 knockdown significantly inhibited the growth and metastasis of drug-resistant breast cancer cells, and decreased the half-maximal inhibitory concentration DOX in MCF-7/DOX cells. The RNA pull-down assay revealed that CASC9 engaged EZH2; EZH2 siRNA significantly inhibited the cell growth, metastasis and chemoresistance of MCF-7/DOX cells. Additionally, EZH2 may regulate the MDR1 gene. The present study demonstrated the oncogenic role of CASC9 in drug-resistant breast cancer by binding to EZH2 and regulating the MDR1 gene. Modulation of CASC9 expression may be a promising target in the therapy of breast cancer and drug-resistant breast cancer.