Resuming Sensitivity of Tamoxifen-Resistant Breast Cancer Cells to Tamoxifen by Tetrandrine

BACKGROUND: Tamoxifen is one of the medicines for adjuvant endocrine therapy of hormone-dependent breast cancer. However, development of resistance to tamoxifen occurs inevitably during treatment. This study aimed to determine whether sensitivity of tamoxifen-resistant breast cancer cells (TAM-R) could be reinstated by tetrandrine (Tet). METHODS: All experiments were conducted in TAM-R cells derived from the MCF-7 breast cancer cell line by long-term tamoxifen exposure. Cell growth, apoptosis, and autophagy were end-points that evaluated the effect of Tet (0.9 μg/ml, 1.8 μg/ml, and 3.75 μg/ml) alone or in combination with TAM (1 μM). Cell apoptosis was determined by an ELISA assay and autophagy was determined by fluorescent staining using the Enzo autophagy detection kit. Immunoblotting was used to evaluate markers for apoptosis, autophagy, and related signal pathway molecules. RESULTS: Growth of TAM-R cells was significantly inhibited by Tet. Combination of Tet with tamoxifen induced a greater inhibition on cell growth than tamoxifen alone, which was predominantly due to enhancement of pro-apoptotic effect of TAM by Tet. Autophagy was significantly inhibited in TAM-R cells treated with Tet plus TAM as shown by increased autophagosomes and the levels of LC3-II and p62. At 0.9 μg/ml, Tet increased the levels of both apoptosis and autophagy markers. Among them increase in p53 levels was more dramatic. CONCLUSIONS: Tet as a monotherapy inhibits TAM-R cells. Tet potentiates the pro-apoptotic effect of TAM via inhibition of autophagy.