title: Tumor-initiating cells of HER2-positive carcinoma cell lines express the highest oncoprotein levels and are sensitive to trastuzumab.
abstract: OBJECTIVE
  The existence of tumor-initiating cells in breast cancer has profound implications for cancer therapy. In this study, we investigated the sensitivity of tumor-initiating cells isolated from human epidermal growth factor receptor type 2 (HER2)-overexpressing carcinoma cell lines to trastuzumab, a compound used for the targeted therapy of breast cancer.
  
  
  METHODS
  Spheres were analyzed by indirect immunofluorescence for HER2 cell surface expression and by real-time PCR for HER2 mRNA expression in the presence or absence of the Notch1 signaling inhibitor (GSI) or Notch1 small interfering RNA. Xenografts of HER2-overexpressing breast tumor cells were treated with trastuzumab or doxorubicin. The sphere-forming efficiency (SFE) and serial transplantability of tumors were assessed.
  
  
  RESULTS
  In HER2-overexpressing carcinoma cell lines, cells with tumor-initiating cell properties presented increased HER2 levels compared with the bulk cell population without modification in HER2 gene amplification. HER2 levels were controlled by Notch1 signaling, as shown by the reduction of HER2 cell surface expression and lower SFE following gamma-secretase inhibition or Notch1 specific silencing. We also show that trastuzumab was able to effectively target tumor-initiating cells of HER2-positive carcinoma cell lines, as indicated by the significant decrease in SFE and the loss of serial transplantability, following treatment of HER2-overexpressing xenotransplants.
  
  
  CONCLUSIONS
  Here, we provide evidence for the therapeutic efficacy of trastuzumab in debulking and in targeting tumor-initiating cells of HER2-overexpressing tumors. We also propose that Notch signaling regulates HER2 expression, thereby representing a critical survival pathway of tumor-initiating cells.
authors.affiliation: Molecular Biology Unit, Department of Experimental Oncology, IRCCS Foundation: National Cancer Institute, Milan, Italy.
authors.forename: Alessandra
authors.initials: A
authors.lastname: Magnifico
authors.forename: Luisa
authors.initials: L
authors.lastname: Albano
authors.forename: Stefano
authors.initials: S
authors.lastname: Campaner
authors.forename: Domenico
authors.initials: D
authors.lastname: Delia
authors.forename: Fabio
authors.initials: F
authors.lastname: Castiglioni
authors.forename: Patrizia
authors.initials: P
authors.lastname: Gasparini
authors.forename: Gabriella
authors.initials: G
authors.lastname: Sozzi
authors.forename: Enrico
authors.initials: E
authors.lastname: Fontanella
authors.forename: Sylvie
authors.initials: S
authors.lastname: Menard
authors.forename: Elda
authors.initials: E
authors.lastname: Tagliabue
mesh_terms: D000818:Animals; D000911:Antibodies, Monoclonal / Q000494:pharmacology*; D061067:Antibodies, Monoclonal, Humanized; D000970:Antineoplastic Agents / Q000494:pharmacology*; D045744:Cell Line, Tumor; D006801:Humans; D000077341:Lapatinib; D051379:Mice; D011799:Quinazolines / Q000494:pharmacology; D012333:RNA, Messenger / Q000032:analysis; D018719:Erb-b2 Receptor Tyrosine Kinases / Q000032:analysis* / Q000037:antagonists & inhibitors* / Q000235:genetics; D051881:Receptor, Notch1 / Q000502:physiology; D000068878:Trastuzumab; D023041:Xenograft Model Antitumor Assays
chemical_list: D000911:Antibodies, Monoclonal; D061067:Antibodies, Monoclonal, Humanized; D000970:Antineoplastic Agents; D000077341:Lapatinib; D011799:Quinazolines; D012333:RNA, Messenger; D018719:Erb-b2 Receptor Tyrosine Kinases; D051881:Receptor, Notch1; D000068878:Trastuzumab; C497761:NOTCH1 protein, human; C508053:ERBB2 protein, human