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 microM), the treated MDA-MB468 cells have mainly been affected at a higher dose (20 microM) 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 microM 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.