Tamoxifen-induced apoptosis in association with downregulation of CIP2A and p-Akt in estrogen receptor–negative breast cancer cells. (A) Differential cancerous inhibitor of protein phosphatase 2A (CIP2A) expression in five estrogen receptor (ER)–negative breast cancer cell lines (SK-BR3, MDA-MB-453, MDA-MB-468, MDA-MB-231 and HCC-1937). MCF-7 was used as a positive control for ER expression. (B) Dose and time escalation effects of tamoxifen on apoptosis in five ER-negative breast cancer cell lines. Cells were exposed to tamoxifen at the indicated doses (1 μM, 2 μM, 5 μM, 7.5 μM and 10 μM) for 24 and 36 hours. Apoptotic cells were determined by flow cytometry (sub-G1 analysis of propidium iodide–stained cells). Columns, Mean (n = 3); bars, SD. (C) Dose-dependent analysis of CIP2A, p-Akt and cleaved poly(ADP-ribose) polymerase (PARP). Cells were exposed to tamoxifen at the indicated doses for 36 hours. Cell lysates were prepared and assayed for these molecules by Western blotting. Data are representative of three independent experiments. Note that there was no significant tamoxifen-induced apoptosis in HCC-1937 cells; only the PARP preform is presented.