title: Tamoxifen induces apoptosis through cancerous inhibitor of protein phosphatase 2A-dependent phospho-Akt inactivation in estrogen receptor-negative human breast cancer cells.
abstract: BACKGROUND
  Tamoxifen, 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.
  
  
  METHODS
  In 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.
  
  
  RESULTS
  Tamoxifen 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.
  
  
  CONCLUSIONS
  Inhibition 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.
authors.forename: Chun-Yu
authors.initials: CY
authors.lastname: Liu
authors.forename: Man-Hsin
authors.initials: MH
authors.lastname: Hung
authors.forename: Duen-Shian
authors.initials: DS
authors.lastname: Wang
authors.forename: Pei-Yi
authors.initials: PY
authors.lastname: Chu
authors.forename: Jung-Chen
authors.initials: JC
authors.lastname: Su
authors.forename: Tsung-Han
authors.initials: TH
authors.lastname: Teng
authors.forename: Chun-Teng
authors.initials: CT
authors.lastname: Huang
authors.forename: Ting-Ting
authors.initials: TT
authors.lastname: Chao
authors.forename: Cheng-Yi
authors.initials: CY
authors.lastname: Wang
authors.forename: Chung-Wai
authors.initials: CW
authors.lastname: Shiau
authors.forename: Ling-Ming
authors.initials: LM
authors.lastname: Tseng
authors.forename: Kuen-Feng
authors.initials: KF
authors.lastname: Chen
mesh_terms: D000368:Aged; D000818:Animals; D018931:Antineoplastic Agents, Hormonal / Q000494:pharmacology*; D017209:Apoptosis / Q000187:drug effects*; D001324:Autoantigens / Q000235:genetics / Q000378:metabolism*; D001943:Breast Neoplasms / Q000188:drug therapy* / Q000378:metabolism / Q000401:mortality / Q000473:pathology; D045744:Cell Line, Tumor; D015536:Down-Regulation; D005260:Female; D006801:Humans; D047908:Intracellular Signaling Peptides and Proteins; D053208:Kaplan-Meier Estimate; D008297:Male; D008565:Membrane Proteins / Q000235:genetics / Q000378:metabolism*; D008819:Mice, Nude; D008875:Middle Aged; D051057:Proto-Oncogene Proteins c-akt / Q000378:metabolism*; D011960:Receptors, Estrogen / Q000378:metabolism; D013629:Tamoxifen / Q000494:pharmacology*; D014158:Transcription, Genetic; D047368:Tumor Burden / Q000187:drug effects; D023041:Xenograft Model Antitumor Assays
chemical_list: D018931:Antineoplastic Agents, Hormonal; D001324:Autoantigens; C500266:CIP2A protein, human; D047908:Intracellular Signaling Peptides and Proteins; D008565:Membrane Proteins; D011960:Receptors, Estrogen; D013629:Tamoxifen; D051057:Proto-Oncogene Proteins c-akt