title: Perturbation of DNA repair pathways by proteasome inhibitors corresponds to enhanced chemosensitivity of cells to DNA damage-inducing agents.
abstract: OBJECTIVE
  Breast cancer treatment often employs DNA double-strand breaks (DSBs), such as that induced by irradiation or anticancer agents. Ubiquitination is required at the site of DNA damage and plays a crucial role in the DSB repair pathway. We investigated the effect of proteasome inhibitors on the pathway after exposure to chemotherapeutic agents and examined its correlation with cytotoxicity.
  
  
  METHODS
  Cells were exposed for 1 h to DNA damage-inducing chemotherapeutic agents. After DNA damage, nuclear foci formation of conjugated ubiquitin (Ub-foci) and cell viability were examined in the absence or presence of proteasome inhibitors MG132 and epoxomicin.
  
  
  RESULTS
  Proteasome inhibitors trapped conjugated ubiquitin in the cytosol and blocked irinotecan (CPT-11)- and epirubicin-induced Ub-foci formation in MCF10A cells and HeLa cells, but not in MCF7 cells. MG132 sensitized MCF10A cells to CPT-11 and epirubicin treatment, demonstrating a synergistic effect. This synergistic effect is likely due to the failure to repair DNA, because a significant rise in unrepaired DNA damage was observed in the cells treated with MG132. On the other hand, no synergy was observed in MCF7 cells or when MG132 was combined with docetaxel.
  
  
  CONCLUSIONS
  The synergistic effect of proteasome inhibitors in combination with DNA damage-inducing agents warrants further investigating into its effectiveness in the treatment of breast cancer.
authors.affiliation: Division of Breast and Endocrine Surgery, St Marianna University School of Medicine, Kawasaki, Japan.
authors.forename: Takashi
authors.initials: T
authors.lastname: Takeshita
authors.forename: Wenwen
authors.initials: W
authors.lastname: Wu
authors.forename: Ayaka
authors.initials: A
authors.lastname: Koike
authors.forename: Mamoru
authors.initials: M
authors.lastname: Fukuda
authors.forename: Tomohiko
authors.initials: T
authors.lastname: Ohta
mesh_terms: D000903:Antibiotics, Antineoplastic / Q000494:pharmacology; D000970:Antineoplastic Agents / Q000494:pharmacology*; D002166:Camptothecin / Q000031:analogs & derivatives / Q000494:pharmacology; D045744:Cell Line, Tumor; D002467:Cell Nucleus / Q000187:drug effects; D002470:Cell Survival / Q000187:drug effects; D020552:Comet Assay; D004249:DNA Damage; D004260:DNA Repair / Q000187:drug effects*; D015251:Epirubicin / Q000494:pharmacology; D005260:Female; D006367:HeLa Cells; D006801:Humans; D007150:Immunohistochemistry; D000077146:Irinotecan; D007976:Leupeptins / Q000494:pharmacology; D009842:Oligopeptides / Q000494:pharmacology; D061988:Proteasome Inhibitors; D025801:Ubiquitin / Q000378:metabolism / Q000494:pharmacology
chemical_list: D000903:Antibiotics, Antineoplastic; D000970:Antineoplastic Agents; D007976:Leupeptins; D009842:Oligopeptides; D061988:Proteasome Inhibitors; D025801:Ubiquitin; D015251:Epirubicin; D000077146:Irinotecan; C072553:benzyloxycarbonylleucyl-leucyl-leucine aldehyde; D002166:Camptothecin; C078846:epoxomicin