title: Triapine disrupts CtIP-mediated homologous recombination repair and sensitizes ovarian cancer cells to PARP and topoisomerase inhibitors.
abstract: PARP inhibitors exploit synthetic lethality to target epithelial ovarian cancer (EOC) with hereditary BRCA mutations and defects in homologous recombination repair (HRR). However, such an approach is limited to a small subset of EOC patients and compromised by restored HRR due to secondary mutations in BRCA genes. Here, it was demonstrated that triapine, a small-molecule inhibitor of ribonucleotide reductase, enhances the sensitivity of BRCA wild-type EOC cells to the PARP inhibitor olaparib and the topoisomerase II inhibitor etoposide. Triapine abolishes olaparib-induced BRCA1 and Rad51 foci, and disrupts the BRCA1 interaction with the Mre11-Rad50-Nbs1 (MRN) complex in BRCA1 wild-type EOC cells. It has been shown that phosphorylation of CtIP (RBBP8) is required for the interaction with BRCA1 and with MRN to promote DNA double-strand break (DSB) resection during S and G(2) phases of the cell cycle. Mechanistic studies within reveal that triapine inhibits cyclin-dependent kinase (CDK) activity and blocks olaparib-induced CtIP phosphorylation through Chk1 activation. Furthermore, triapine abrogates etoposide-induced CtIP phosphorylation and DSB resection as evidenced by marked attenuation of RPA32 phosphorylation. Concurrently, triapine obliterates etoposide-induced BRCA1 foci and sensitizes BRCA1 wild-type EOC cells to etoposide. Using a GFP-based HRR assay, it was determined that triapine suppresses HRR activity induced by an I-SceI-generated DSB. These results suggest that triapine augments the sensitivity of BRCA wild-type EOC cells to drug-induced DSBs by disrupting CtIP-mediated HRR.
  
  
  CONCLUSIONS
  These findings provide a strong rationale for combining triapine with PARP or topoisomerase inhibitors to target HRR-proficient EOC cells.
authors.affiliation: Department of Pharmacology, Yale Cancer Center,Yale University School of Medicine, New Haven, Connecticut.
authors.forename: Z Ping
authors.initials: ZP
authors.lastname: Lin
authors.affiliation: Department of Obstetrics and Gynecology, Division of Gynecologic Oncology, Yale Cancer Center,Yale University School of Medicine, New Haven, Connecticut.
authors.forename: Elena S
authors.initials: ES
authors.lastname: Ratner
authors.affiliation: Department of Obstetrics and Gynecology, Division of Gynecologic Oncology, Yale Cancer Center,Yale University School of Medicine, New Haven, Connecticut.
authors.forename: Margaret E
authors.initials: ME
authors.lastname: Whicker
authors.affiliation: Department of Internal Medicine, Section of Nephrology, Yale Cancer Center,Yale University School of Medicine, New Haven, Connecticut.
authors.forename: Yashang
authors.initials: Y
authors.lastname: Lee
authors.affiliation: Department of Pharmacology, Yale Cancer Center,Yale University School of Medicine, New Haven, Connecticut.
authors.forename: Alan C
authors.initials: AC
authors.lastname: Sartorelli
mesh_terms: D000971:Antineoplastic Combined Chemotherapy Protocols / Q000494:pharmacology*; D000077216:Carcinoma, Ovarian Epithelial; D002352:Carrier Proteins / Q000235:genetics / Q000378:metabolism*; D045744:Cell Line, Tumor; D004357:Drug Synergism; D005260:Female; D006801:Humans; D009375:Neoplasms, Glandular and Epithelial / Q000188:drug therapy* / Q000201:enzymology / Q000235:genetics; D010051:Ovarian Neoplasms / Q000188:drug therapy* / Q000201:enzymology / Q000235:genetics; D010793:Phthalazines / Q000008:administration & dosage / Q000494:pharmacology; D010879:Piperazines / Q000008:administration & dosage / Q000494:pharmacology; D000067856:Poly(ADP-ribose) Polymerase Inhibitors; D011065:Poly(ADP-ribose) Polymerases / Q000378:metabolism; D011725:Pyridines / Q000008:administration & dosage / Q000494:pharmacology*; D011995:Recombination, Genetic; D059767:Recombinational DNA Repair / Q000187:drug effects*; D013882:Thiosemicarbazones / Q000008:administration & dosage / Q000494:pharmacology*; D059003:Topoisomerase Inhibitors / Q000008:administration & dosage / Q000494:pharmacology*; D014162:Transfection
chemical_list: D002352:Carrier Proteins; D010793:Phthalazines; D010879:Piperazines; D000067856:Poly(ADP-ribose) Polymerase Inhibitors; D011725:Pyridines; D013882:Thiosemicarbazones; D059003:Topoisomerase Inhibitors; C078157:3-aminopyridine-2-carboxaldehyde thiosemicarbazone; D011065:Poly(ADP-ribose) Polymerases; C531550:olaparib