title: Modification of topoisomerases in mammospheres derived from breast cancer cell line: clinical implications for combined treatments with tyrosine kinase inhibitors.
abstract: BACKGROUND
  Accumulating evidences suggest that tumors are driven by a small population of cells, termed "cancer stem cells" (CSCs), which may be resistant to current therapeutic approaches. In breast carcinoma, the CSCs have been identified as a CD44+/CD24- cell population. These rare cells are able to grow as non-adherent sphere-like structures, termed "mammospheres", which enables their isolation and expansion in culture. To design efficient strategies for the complete eradication of CSCs, it is important to identify enzymes and proteins that are known as anti-cancer targets, and differ in their properties from those present in the none CSCs. Here we investigated the activity and expression of type I and type II DNA topoisomerases (topo I and topo II) in CSCs and their response to anti-topoisomerase inhibitors.
  
  
  METHODS
  MCF7 breast cancer cells, PC3 prostate cancer cells and 4 T1-Luc-Oct3/4pG mouse mammary carcinoma cells were grown on low-attachment dishes in specific medium and allowed to form spheres. Enrichment of CSC population was verified by immunostaining, flow cytometry or fluorescent microscopy imaging. Nuclear protein extracts were prepared and topoisomerases activity and protein levels were determined. Cell viability was examined by the MTT and Neutral Red assays.
  
  
  RESULTS
  Unlike the adherent MCF7 cell line, topo I activity is decreased and topo II activity is increased in the CSCs. However, the relative levels of the enzyme proteins were similar in both mammospheres and adherent cells. Topo I activity in mammospheres is regulated, at least in part, by PARP-1, as observed by the recovery of topo I activity after treatment with PARP-1 inhibitor 3-Aminobenzamide. Mammosphere-derived cells show reduced sensitivity to topo I inhibitor, camptothecin, and increased sensitivity to topo II inhibitor etoposide. Intact mammospheres show increased resistance to both drugs. A combined treatment of intact mammospheres with either CPT and gefitinib, or etoposide and erlotinib, increased the anti-cancer effect of both drugs.
  
  
  CONCLUSIONS
  The data of this study suggest that the understanding of biological behavior of essential enzymes such as topoisomerases, in CSCs' progression and early stages of tumor development, is important for developing new strategies for cancer treatment as well as new therapies for advanced disease.
authors.forename: Refael
authors.initials: R
authors.lastname: Peleg
authors.forename: Marianna
authors.initials: M
authors.lastname: Romzova
authors.forename: Inga
authors.initials: I
authors.lastname: Kogan-Zviagin
authors.forename: Ron N
authors.initials: RN
authors.lastname: Apte
authors.affiliation: The Shraga Segal Department of Microbiology, Immunology & Genetics, Faculty of Health Sciences, Ben-Gurion University of the Negev, Beer Sheva, Israel. priel@bgu.ac.il.
authors.forename: Esther
authors.initials: E
authors.lastname: Priel
mesh_terms: D000818:Animals; D000954:Antigens, Surface / Q000378:metabolism; D001943:Breast Neoplasms / Q000378:metabolism*; D002166:Camptothecin / Q000494:pharmacology; D045744:Cell Line, Tumor; D002470:Cell Survival / Q000187:drug effects; D026942:DNA Topoisomerases / Q000378:metabolism*; D004264:DNA Topoisomerases, Type I / Q000378:metabolism; D004250:DNA Topoisomerases, Type II / Q000378:metabolism; D004357:Drug Synergism; D004789:Enzyme Activation; D005047:Etoposide / Q000494:pharmacology; D005260:Female; D000077156:Gefitinib; D006801:Humans; D016130:Immunophenotyping; D061986:MCF-7 Cells; D051379:Mice; D014411:Neoplastic Stem Cells / Q000378:metabolism; D011799:Quinazolines / Q000494:pharmacology; D018874:Spheroids, Cellular; D059004:Topoisomerase I Inhibitors / Q000494:pharmacology; D059005:Topoisomerase II Inhibitors / Q000494:pharmacology; D014407:Tumor Cells, Cultured
chemical_list: D000954:Antigens, Surface; D011799:Quinazolines; D059004:Topoisomerase I Inhibitors; D059005:Topoisomerase II Inhibitors; D005047:Etoposide; D026942:DNA Topoisomerases; D004264:DNA Topoisomerases, Type I; D004250:DNA Topoisomerases, Type II; D000077156:Gefitinib; D002166:Camptothecin