title: Role of drug transporters and drug accumulation in the temporal acquisition of drug resistance.
abstract: BACKGROUND
  Anthracyclines and taxanes are commonly used in the treatment of breast cancer. However, tumor resistance to these drugs often develops, possibly due to overexpression of drug transporters. It remains unclear whether drug resistance in vitro occurs at clinically relevant doses of chemotherapy drugs and whether both the onset and magnitude of drug resistance can be temporally and causally correlated with the enhanced expression and activity of specific drug transporters. To address these issues, MCF-7 cells were selected for survival in increasing concentrations of doxorubicin (MCF-7DOX-2), epirubicin (MCF-7EPI), paclitaxel (MCF-7TAX-2), or docetaxel (MCF-7TXT). During selection cells were assessed for drug sensitivity, drug uptake, and the expression of various drug transporters.
  
  
  RESULTS
  In all cases, resistance was only achieved when selection reached a specific threshold dose, which was well within the clinical range. A reduction in drug uptake was temporally correlated with the acquisition of drug resistance for all cell lines, but further increases in drug resistance at doses above threshold were unrelated to changes in cellular drug uptake. Elevated expression of one or more drug transporters was seen at or above the threshold dose, but the identity, number, and temporal pattern of drug transporter induction varied with the drug used as selection agent. The pan drug transporter inhibitor cyclosporin A was able to partially or completely restore drug accumulation in the drug-resistant cell lines, but had only partial to no effect on drug sensitivity. The inability of cyclosporin A to restore drug sensitivity suggests the presence of additional mechanisms of drug resistance.
  
  
  CONCLUSIONS
  This study indicates that drug resistance is achieved in breast tumour cells only upon exposure to concentrations of drug at or above a specific selection dose. While changes in drug accumulation and the expression of drug transporters does occur at the threshold dose, the magnitude of resistance cannot be attributed solely to changes in drug accumulation or the activity of drug transporters. The identities of these additional drug-transporter-independent mechanisms are discussed, including their likely clinical relevance.
authors.affiliation: Regional Cancer Program, Sudbury Regional Hospital, and Department of Chemistry and Biochemistry, Laurentian University, Sudbury, ON Canada. shembruff@hrsrh.on.ca
authors.forename: Stacey L
authors.initials: SL
authors.lastname: Hembruff
authors.forename: Monique L
authors.initials: ML
authors.lastname: Laberge
authors.forename: David J
authors.initials: DJ
authors.lastname: Villeneuve
authors.forename: Baoqing
authors.initials: B
authors.lastname: Guo
authors.forename: Zachary
authors.initials: Z
authors.lastname: Veitch
authors.forename: Melanie
authors.initials: M
authors.lastname: Cecchetto
authors.forename: Amadeo M
authors.initials: AM
authors.lastname: Parissenti
mesh_terms: D018435:ATP Binding Cassette Transporter, Subfamily B; D020168:ATP Binding Cassette Transporter, Subfamily B, Member 1 / Q000378:metabolism; D000070997:ATP Binding Cassette Transporter, Subfamily G, Member 2; D018528:ATP-Binding Cassette Transporters / Q000378:metabolism*; D000704:Analysis of Variance; D000970:Antineoplastic Agents / Q000378:metabolism / Q000494:pharmacology*; D045744:Cell Line, Tumor; D016572:Cyclosporine / Q000494:pharmacology; D000077143:Docetaxel; D004317:Doxorubicin / Q000378:metabolism / Q000494:pharmacology; D019008:Drug Resistance, Neoplasm; D015251:Epirubicin / Q000378:metabolism / Q000494:pharmacology; D006801:Humans; D000089762:Multidrug Resistance-Associated Protein 2; D027425:ATP-Binding Cassette, Sub-Family C Proteins / Q000378:metabolism; D009363:Neoplasm Proteins / Q000378:metabolism; D020411:Oligonucleotide Array Sequence Analysis; D017239:Paclitaxel / Q000378:metabolism / Q000494:pharmacology; D018709:Statistics, Nonparametric; D043823:Taxoids / Q000378:metabolism / Q000494:pharmacology; D020394:Vault Ribonucleoprotein Particles / Q000378:metabolism; D000099187:Major Vault Protein
chemical_list: D018435:ATP Binding Cassette Transporter, Subfamily B; D020168:ATP Binding Cassette Transporter, Subfamily B, Member 1; D000070997:ATP Binding Cassette Transporter, Subfamily G, Member 2; D018528:ATP-Binding Cassette Transporters; D000970:Antineoplastic Agents; D016572:Cyclosporine; D000077143:Docetaxel; D004317:Doxorubicin; D015251:Epirubicin; D000089762:Multidrug Resistance-Associated Protein 2; D027425:ATP-Binding Cassette, Sub-Family C Proteins; D009363:Neoplasm Proteins; D017239:Paclitaxel; D043823:Taxoids; D020394:Vault Ribonucleoprotein Particles; D000099187:Major Vault Protein; C513055:ABCB1 protein, human; C073492:ABCC4 protein, human; C116614:ABCG2 protein, human; C421109:multidrug resistance-associated protein 1