Tamoxifen induces apoptosis through cancerous inhibitor of protein phosphatase 2A–dependent phospho-Akt inactivation in estrogen receptor–negative human breast cancer cells

Chun-Yu Liu, Man-Hsin Hung, Duen-Shian Wang, Pei-Yi Chu, Jung-Chen Su, Tsung-Han Teng, Chun-Teng Huang, Ting-Ting Chao, Cheng-Yi Wang, Chung-Wai Shiau, Ling-Ming Tseng, Kuen-Feng Chen

IntroductionTamoxifen, a selective estrogen receptor (ER) modulator, may affect cancer cell survival through mechanisms other than ER antagonism. Here, we tested the efficacy of tamoxifen in a panel of ER-negative breast cancer cell lines and examined the drug mechanism.MethodsIn total, five ER-negative breast cancer cell lines HCC-1937, MDA-MB-231, MDA-MB-468, MDA-MB-453 and SK-BR-3 were used for in vitro studies. Cell apoptosis was examined by flow-cytometry and western blot. Signal transduction pathways in cells were assessed by western blot. In vivo efficacy of tamoxifen was tested in xenograft nude mice.ResultsTamoxifen induced significant apoptosis in MDA-MB-231, MDA-MB-468, MDA-MB-453, SK-BR-3, but not in HCC-1937 cells. Tamoxifen-induced apoptosis was associated with inhibition of cancerous inhibitor of protein phosphatase 2A (CIP2A) and p-Akt in a dose-dependent manner. Ectopic expression of CIP2A or Akt protected MDA-MB-231 cells from tamoxifen-induced apoptosis. In addition, tamoxifen increased protein phosphatase 2A (PP2A) activity and tamoxifen-induced apoptosis was attenuated by PP2A antagonist okadaic acid in the sensitive cell lines but not in resistant HCC-1937 cells. Moreover, silencing CIP2A by siRNA sensitized HCC-1937 cells to tamoxifen-induced apoptosis. Furthermore, tamoxifen regulated CIP2A protein expression by downregulating CIP2A mRNA. Importantly, tamoxifen inhibited the in vivo growth of MDA-MB-468 xenograft tumors in association with CIP2A downregulation, whereas tamoxifen had no significant effect on CIP2A expression and anti-tumor growth in HCC-1937 tumors.ConclusionsInhibition of CIP2A determines the effects of tamoxifen-induced apoptosis in ER-negative breast cancer cells. Our data suggest a novel ¿off-target¿ mechanism of tamoxifen and suggest that CIP2A/PP2A/p-Akt signaling as a feasible anti-cancer pathway.