title: CIP2A is a target of bortezomib in human triple negative breast cancer cells.
abstract: BACKGROUND
  Triple negative breast cancer (TNBC) is very aggressive and currently has no specific therapeutic targets, such as hormone receptors or human epidermal growth factor receptor type 2 (HER2); therefore, prognosis is poor. Bortezomib, a proteasome inhibitor, may exert efficacy in TNBC through its multiple cellular effects. Here, we tested the efficacy of bortezomib and examined the drug mechanism in breast cancer cells.
  
  
  METHODS
  Five breast cancer cell lines: TNBC HCC-1937, MDA-MB-231, and MDA-MB-468; HER2-overexpressing MDA-MB-453; and estrogen receptor positive MCF-7 were used for in vitro studies. Apoptosis was examined by both flow cytometry and Western Blot. Signal transduction pathways in cells were assessed by Western Blot. Gene silencing was done by small interfering RNA (siRNA). In vivo efficacy of bortezomib was tested in nude mice with breast cancer xenografts. Immunohistochemical study was performed on tumor tissues from patients with TNBC.
  
  
  RESULTS
  Bortezomib induced significant apoptosis, which was independent of its proteasome inhibition, in the three TNBC cell lines, but not in MDA-MB-453 or MCF-7 cells. Furthermore, cancerous inhibitor of protein phosphatase 2A (CIP2A), a cellular inhibitor of protein phosphatase 2A (PP2A), mediated the apoptotic effect of bortezomib. We showed that bortezomib inhibited CIP2A in association with p-Akt downregulation in a dose- and time-dependent manner in all sensitive TNBC cells, whereas no alterations in CIP2A expression and p-Akt were noted in bortezomib-resistant cells. Overexpression of CIP2A upregulated p-Akt and protected MDA-MB-231 and MDA-MB-468 cells from bortezomib-induced apoptosis, whereas silencing CIP2A by siRNA overcame the resistance to bortezomib-induced apoptosis in MCF-7 cells. In addition, bortezomib downregulated CIP2A mRNA but did not affect the degradation of CIP2A protein. Furthermore, bortezomib exerted in vivo antitumor activity in HCC-1937 xenografted tumors, but not in MCF-7 tumors. Bortezomib downregulated CIP2A expression in the HCC-1937 tumors but not in the MCF-7 tumors. Importantly, CIP2A expression is readily detectable in tumor samples from TNBC patients.
  
  
  CONCLUSIONS
  CIP2A is a major determinant mediating bortezomib-induced apoptosis in TNBC cells. CIP2A may thus be a potential therapeutic target in TNBC.
authors.affiliation: Department of Surgery, Taipei Veterans General Hospital, No. 201 Sec. 2 Shih-Pai Road, Taipei 112, Taiwan.
authors.forename: Ling-Ming
authors.initials: LM
authors.lastname: Tseng
authors.forename: Chun-Yu
authors.initials: CY
authors.lastname: Liu
authors.forename: Kung-Chi
authors.initials: KC
authors.lastname: Chang
authors.forename: Pei-Yi
authors.initials: PY
authors.lastname: Chu
authors.forename: Chung-Wai
authors.initials: CW
authors.lastname: Shiau
authors.forename: Kuen-Feng
authors.initials: KF
authors.lastname: Chen
mesh_terms: D000818:Animals; D000970:Antineoplastic Agents / Q000494:pharmacology*; D017209:Apoptosis / Q000187:drug effects; D001324:Autoantigens / Q000235:genetics / Q000378:metabolism*; D001897:Boronic Acids / Q000494:pharmacology*; D000069286:Bortezomib; D001943:Breast Neoplasms / Q000188:drug therapy* / Q000378:metabolism / Q000473:pathology; D045744:Cell Line, Tumor; D004305:Dose-Response Relationship, Drug; D019008:Drug Resistance, Neoplasm; D005260:Female; D006801:Humans; D047908:Intracellular Signaling Peptides and Proteins; D061986:MCF-7 Cells; D008565:Membrane Proteins / Q000235:genetics / Q000378:metabolism*; D051379:Mice; D008819:Mice, Nude; D061988:Proteasome Inhibitors / Q000494:pharmacology; D051057:Proto-Oncogene Proteins c-akt / Q000378:metabolism; D011719:Pyrazines / Q000494:pharmacology*; D011960:Receptors, Estrogen / Q000378:metabolism; D023041:Xenograft Model Antitumor Assays
chemical_list: D000970:Antineoplastic Agents; D001324:Autoantigens; D001897:Boronic Acids; C500266:CIP2A protein, human; D047908:Intracellular Signaling Peptides and Proteins; D008565:Membrane Proteins; D061988:Proteasome Inhibitors; D011719:Pyrazines; D011960:Receptors, Estrogen; D000069286:Bortezomib; D051057:Proto-Oncogene Proteins c-akt