title: Inhibition of Homologous Recombination and Promotion of Mutagenic Repair of DNA Double-Strand Breaks Underpins Arabinoside-Nucleoside Analogue Radiosensitization.
abstract: In concurrent chemoradiotherapy, drugs are used to sensitize tumors to ionizing radiation. Although a spectrum of indications for simultaneous treatment with drugs and radiation has been defined, the molecular mechanisms underpinning tumor radiosensitization remain incompletely characterized for several such combinations. Here, we investigate the mechanisms of radiosensitization by the arabinoside nucleoside analogue 9-β-D-arabinofuranosyladenine (araA) placing particular emphasis on the repair of DNA double-strand breaks (DSB), and compare the results to those obtained with fludarabine (F-araA) and cytarabine (araC). Postirradiation treatment with araA strongly sensitizes cells to ionizing radiation, but leaves unchanged DSB repair by NHEJ in logarithmically growing cells, in sorted G1 or G2 phase populations, as well as in cells in the plateau phase of growth. Notably, araA strongly inhibits DSB repair by homologous recombination (HRR), as assessed by scoring ionizing radiation-induced RAD51 foci, and in functional assays using integrated reporter constructs. Cells compromised in HRR by RNAi-mediated transient knockdown of RAD51 show markedly reduced radiosensitization after treatment with araA. Remarkably, mutagenic DSB repair compensates for HRR inhibition in araA-treated cells. Compared with araA, F-araA and araC are only modestly radiosensitizing under the conditions examined. We propose that the radiosensitizing potential of nucleoside analogues is linked to their ability to inhibit HRR and concomitantly promote the error-prone processing of DSBs. Our observations pave the way to treatment strategies harnessing the selective inhibitory potential of nucleoside analogues and the development of novel compounds specifically utilizing HRR inhibition as a means of tumor cell radiosensitization.
authors.affiliation: Institute of Medical Radiation Biology, University of Duisburg-Essen Medical School, Essen, Germany.
authors.forename: Simon
authors.initials: S
authors.lastname: Magin
authors.affiliation: Institute of Medical Radiation Biology, University of Duisburg-Essen Medical School, Essen, Germany.
authors.forename: Maria
authors.initials: M
authors.lastname: Papaioannou
authors.affiliation: Institute of Medical Radiation Biology, University of Duisburg-Essen Medical School, Essen, Germany.
authors.forename: Janapriya
authors.initials: J
authors.lastname: Saha
authors.affiliation: Institute of Medical Radiation Biology, University of Duisburg-Essen Medical School, Essen, Germany.
authors.forename: Christian
authors.initials: C
authors.lastname: Staudt
authors.affiliation: Institute of Medical Radiation Biology, University of Duisburg-Essen Medical School, Essen, Germany. Georg.Iliakis@uk-essen.de.
authors.forename: George
authors.initials: G
authors.lastname: Iliakis
mesh_terms: D015153:Blotting, Western; D045744:Cell Line, Tumor; D049109:Cell Proliferation / Q000187:drug effects / Q000528:radiation effects; D003561:Cytarabine / Q000494:pharmacology*; D053903:DNA Breaks, Double-Stranded / Q000187:drug effects* / Q000528:radiation effects; D004260:DNA Repair / Q000187:drug effects* / Q000528:radiation effects; D004305:Dose-Response Relationship, Drug; D004307:Dose-Response Relationship, Radiation; D049452:Green Fluorescent Proteins / Q000235:genetics / Q000378:metabolism; D059765:Homologous Recombination / Q000187:drug effects* / Q000528:radiation effects; D006801:Humans; D018613:Microscopy, Confocal; D034622:RNA Interference; D051135:Rad51 Recombinase / Q000235:genetics / Q000378:metabolism; D011838:Radiation-Sensitizing Agents / Q000494:pharmacology; D014740:Vidarabine / Q000031:analogs & derivatives* / Q000494:pharmacology*; D014965:X-Rays
chemical_list: D011838:Radiation-Sensitizing Agents; D003561:Cytarabine; D049452:Green Fluorescent Proteins; D051135:Rad51 Recombinase; D014740:Vidarabine; C024352:fludarabine