title: Evasion of a single-step, chemotherapy-induced senescence in breast cancer cells: implications for treatment response.
abstract: OBJECTIVE
  The purpose of this study is to define the mechanistic basis for recovery of proliferative capacity in breast tumor cells after chemotherapy. Here, we test the hypothesis that evasion of senescence confers resistance to chemotherapeutic drugs and ionizing radiation.
  
  
  METHODS
  MCF-7 cells were treated with a single, clinically relevant dose (0.75-1.0 micromol/L) of Adriamycin. Two weeks following induction of senescence, clonal outgrowths were expanded and characterized in terms of senescence-associated beta-galactosidase activity, gene expression profiles (Affymetrix U95 probe sets, Affymetrix, Santa Clara, CA) with confirmatory Western analyses, and telomerase activity following a second drug treatment. Levels of intracellular Adriamycin, as well as cross-resistance to other therapeutic agents, were also determined to define the resistance phenotype.
  
  
  RESULTS
  A senescence-resistant (SR) clone (clone 2) was identified that was largely refractory to both Adriamycin-induced and gamma-irradiation-induced senescence. Clone 2 continued to proliferate and maintain high levels of telomerase activity following a second drug treatment, when treated parental cells expressed very low levels of telomerase and many positive cell cycle regulators. SR clone 2 also expressed substantially more cdc-2 than parental cells and undetectable levels of MDR1, showed an intact p53 checkpoint and only a modestly lower level of intracellular drug accumulation, while exhibiting cross-resistance to other topoisomerase inhibitors.
  
  
  CONCLUSIONS
  SR clone 2 is intrinsically resistant to DNA damage-induced senescence perhaps through an ability to prevent down-regulation of cdc-2. Telomerase is a marker of proliferative recovery for breast cancer cells after chemotherapy exposure. Evasion or escape from a single-step, drug-induced senescence may represent a unique and previously unrecognized drug-resistance phenotype.
authors.affiliation: Department of Pathology, Medical College of Virginia at Virginia Commonwealth University, Richmond, Virginia 23298-230, USA.
authors.forename: Lynne W
authors.initials: LW
authors.lastname: Elmore
authors.forename: Xu
authors.initials: X
authors.lastname: Di
authors.forename: Catherine
authors.initials: C
authors.lastname: Dumur
authors.forename: Shawn E
authors.initials: SE
authors.lastname: Holt
authors.forename: David A
authors.initials: DA
authors.lastname: Gewirtz
mesh_terms: D000903:Antibiotics, Antineoplastic / Q000494:pharmacology*; D015153:Blotting, Western; D001943:Breast Neoplasms / Q000188:drug therapy / Q000473:pathology / Q000532:radiotherapy; D002166:Camptothecin / Q000494:pharmacology; D018797:Cell Cycle Proteins / Q000235:genetics; D045744:Cell Line, Tumor; D049109:Cell Proliferation / Q000187:drug effects / Q000528:radiation effects; D016922:Cellular Senescence / Q000187:drug effects* / Q000528:radiation effects; D002999:Clone Cells / Q000187:drug effects / Q000378:metabolism / Q000528:radiation effects; D050759:Cyclin-Dependent Kinase Inhibitor p21; D004317:Doxorubicin / Q000494:pharmacology*; D019008:Drug Resistance, Neoplasm; D005720:Gamma Rays; D020869:Gene Expression Profiling; D015972:Gene Expression Regulation, Neoplastic / Q000187:drug effects / Q000528:radiation effects; D016147:Genes, Tumor Suppressor; D006801:Humans; D017874:Immediate-Early Proteins / Q000235:genetics; D009687:Nuclear Proteins / Q000235:genetics; D020411:Oligonucleotide Array Sequence Analysis / Q000379:methods; D017239:Paclitaxel / Q000494:pharmacology; D019253:Proto-Oncogene Proteins c-bcl-2 / Q000235:genetics; D019098:Telomerase / Q000378:metabolism; D013713:Teniposide / Q000494:pharmacology; D016159:Tumor Suppressor Protein p53 / Q000235:genetics; D025521:Tumor Suppressor Proteins; D051028:bcl-2-Associated X Protein; D000097642:GADD45 Proteins
chemical_list: D000903:Antibiotics, Antineoplastic; D002166:Camptothecin; D018797:Cell Cycle Proteins; D050759:Cyclin-Dependent Kinase Inhibitor p21; D004317:Doxorubicin; D017874:Immediate-Early Proteins; D009687:Nuclear Proteins; D017239:Paclitaxel; D019253:Proto-Oncogene Proteins c-bcl-2; D019098:Telomerase; D013713:Teniposide; D016159:Tumor Suppressor Protein p53; D025521:Tumor Suppressor Proteins; D051028:bcl-2-Associated X Protein; C493956:CDKN1A protein, human; C439070:GADD45A protein, human; D000097642:GADD45 Proteins; C070114:BTG2 protein, human