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

Abstract

IntroductionTamoxifen, a selective estrogen receptor (ER) modulator, may affect cancer cell survival through mechanisms other than ER antagonism. In the present study, 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. Cellular apoptosis was examined by flow cytometry and Western blot analysis. Signal transduction pathways in cells were assessed by Western blot analysis. The 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 and SK-BR-3 cells, but not in HCC-1937 cells. Tamoxifen-induced apoptosis was associated with inhibition of cancerous inhibitor of protein phosphatase 2A (CIP2A) and phospho-Akt (p-Akt) in a dose-dependent manner. Ectopic expression of either 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 the PP2A antagonist okadaic acid in the sensitive cell lines, but not in resistant HCC-1937 cells. Moreover, silencing CIP2A by small interfering RNA 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 may be a feasible anti-cancer pathway.Electronic supplementary materialThe online version of this article (doi:10.1186/s13058-014-0431-9) contains supplementary material, which is available to authorized users.

LB100, a small molecule inhibitor of PP2A with potent chemo- and radio-sensitizing potential

“…These include okadaic acid, a cytotoxin produced by certain algae with potent apoptotic effects in human cancer cell lines as a single agent or in combination with standard chemotherapeutic agents. [56][57][58] Another natural compound, cantharidin, a toxin produced by the Mylabris beetle, also exhibits selective PP2A inhibition and tumor suppressive properties in cancer cells. [59][60][61] Inhibition of PP2A with either of these compounds results in abnormal mitotic figures characteristic of mitotic catastrophe, a form of cell death distinct from apoptosis.…”

ERRα Is a Marker of Tamoxifen Response and Survival in Triple-Negative Breast Cancer

“…Nonetheless, stratification of TNBC based on molecular expression signatures revealed that ERβ may serve as a predictive factor of tamoxifen response in ER-negative breast cancer (4, 5). In addition, it was shown that co-targeting Akt may sensitize ER-negative cells to tamoxifen (6, 7). These data underscore the importance of considering the role of oncogenes and members of the steroid hormone receptor family as predictors of tamoxifen sensitivity in TNBC.…”

Targeting ceramide metabolic pathway induces apoptosis in human breast cancer cell lines

“…However, its significance over cell proliferation has also been reported in ER À breast cancer cells indicating its ER independent effect [4]. Eventhough, Tamoxifen has been approved to be the gold standard for Breast cancer therapy the major problem encountered with Tamoxifen is resistance after long term treatment [5,6].…”