title: Synergism between etoposide and 17-AAG in leukemia cells: critical roles for Hsp90, FLT3, topoisomerase II, Chk1, and Rad51.
abstract: OBJECTIVE
  DNA-damaging agents, such as etoposide, while clinically useful in leukemia therapy, are limited by DNA repair pathways that are not well understood. 17-(Allylamino)-17-demethoxygeldanamycin (17-AAG), an inhibitor of the molecular chaperone heat shock protein 90 (Hsp90), inhibits growth and induces apoptosis in FLT3(+) leukemia cells. In this study, we evaluated the effects of etoposide and 17-AAG in leukemia cells and the roles of Hsp90, FMS-like tyrosine kinase 3 (FLT3), checkpoint kinase 1 (Chk1), Rad51, and topoisomerase II in this inhibition.
  
  
  METHODS
  The single and combined effects of 17-AAG and etoposide and the mechanism of these effects were evaluated. FLT3 and the DNA repair-related proteins, Chk1 and Rad51, were studied in small interfering RNA (siRNA)-induced cell growth inhibition experiments in human leukemia cells with wild-type or mutated FLT3.
  
  
  RESULTS
  We found that etoposide and the Hsp90/FLT3 inhibitor 17-AAG, had synergistic inhibitory effects on FLT3(+) MLL-fusion gene leukemia cells. Cells with an internal tandem duplication (ITD) FLT3 (Molm13 and MV4;11) were more sensitive to etoposide/17-AAG than leukemias with wild-type FLT3 (HPB-Null and RS4;11). A critical role for FLT3 was shown in experiments with FLT3 ligand and siRNA targeted to FLT3. An important role for topoisomerase II and the DNA repair-related proteins, Chk1 and Rad51, in the synergistic effects was suggested from the results.
  
  
  CONCLUSIONS
  The repair of potentially lethal DNA damage by etoposide in leukemia cells is dependent on intact and functioning FLT3 especially leukemias with ITD-FLT3. These data suggest a rational therapeutic strategy for FLT3(+) leukemias that combines etoposide or other DNA-damaging agents with Hsp90/FLT3 inhibitors such as 17-AAG.
authors.affiliation: The Cancer Center, University of Minnesota MMC 806, 420 Delaware St. SE, Minneapolis, Minnesota, USA.
authors.forename: Qing
authors.initials: Q
authors.lastname: Yao
authors.forename: Brenda
authors.initials: B
authors.lastname: Weigel
authors.forename: John
authors.initials: J
authors.lastname: Kersey
mesh_terms: D000970:Antineoplastic Agents / Q000494:pharmacology*; D016227:Benzoquinones / Q000494:pharmacology*; D015153:Blotting, Western; D045744:Cell Line, Tumor; D000071877:Checkpoint Kinase 1; D045643:DNA Repair Enzymes / Q000187:drug effects*; D004250:DNA Topoisomerases, Type II / Q000187:drug effects / Q000378:metabolism; D004357:Drug Synergism; D005047:Etoposide / Q000494:pharmacology*; D018841:HSP90 Heat-Shock Proteins / Q000187:drug effects / Q000378:metabolism; D006801:Humans; D047029:Lactams, Macrocyclic / Q000494:pharmacology*; D007938:Leukemia / Q000188:drug therapy*; D011494:Protein Kinases / Q000187:drug effects / Q000378:metabolism; D034741:RNA, Small Interfering; D051135:Rad51 Recombinase / Q000187:drug effects / Q000378:metabolism; D051941:fms-Like Tyrosine Kinase 3 / Q000187:drug effects / Q000378:metabolism
chemical_list: D000970:Antineoplastic Agents; D016227:Benzoquinones; D018841:HSP90 Heat-Shock Proteins; D047029:Lactams, Macrocyclic; D034741:RNA, Small Interfering; C112765:tanespimycin; D005047:Etoposide; D011494:Protein Kinases; C497970:FLT3 protein, human; D051941:fms-Like Tyrosine Kinase 3; C000605867:CHEK1 protein, human; D000071877:Checkpoint Kinase 1; D051135:Rad51 Recombinase; D004250:DNA Topoisomerases, Type II; D045643:DNA Repair Enzymes