Biochemical studies of apoptosis induced by tamoxifen in estrogen receptor positive and negative breast cancer cell lines

Objectives Tamoxifen has been reported to show an efficacy in the treatment of breast cancer. Apoptosis could be a major mechanism of its antitumor effect. Therefore, this study has been designed to investigate the biochemical mechanisms of tamoxifen-induced apoptosis in both ER+ MCF-7 and ER− MDA-MB468 breast cancer cell lines. Methods Trypan blue dye exclusion test, Annexin V-Fluorescein/PI flow cytometry, MTT assay and Hoechst 33258 staining were used to detect cytotoxicity and apoptosis. The activation of caspase-3 was assayed by colorimetric assay kit. Bcl-2 and Bax proteins were estimated by western immunoblotting method. Results Tamoxifen induced apoptosis in both cell lines (chi-square test, p< 0.05). Unlike the MCF-7 cells, which responded to the low concentration (1 μM), the treated MDA-MB468 cells have mainly been affected at a higher dose (20 μM) at which a significant increase was also obtained in the caspase-3 activity (chi-square test, p< 0.05). Interestingly, tamoxifen at doses higher than 2.5 μM increased cell proliferation in the MCF-7 cells. The levels of Bcl-2 and Bax remained unchanged. Conclusion Since tamoxifen has induced apoptosis in both cell lines by different mechanisms, it might be concluded that there exists ER+ and ER− pathways for the induction of apoptosis.