title: A Small Molecule Screen Exposes mTOR Signaling Pathway Involvement in Radiation-Induced Apoptosis.
abstract: Individuals are at risk of exposure to acute ionizing radiation (IR) from a nuclear accident or terrorism, but we lack effective therapies to mitigate the lethal IR effects. In the current study, we exploited an optimized, cell-based, high throughput screening assay to interrogate a small molecule library comprising 3437 known pharmacologically active compounds for mitigation against IR-induced apoptosis. Thirty-three library compounds significantly reduced apoptosis when administered 1 h after 4 Gy IR. Two- or three-dimensional computational structural analyses of the compounds indicated only one or two chemical clusters with most of the compounds being unique structures. The mechanistic target of rapamycin complex 1 (mTORC1) inhibitor, rapamycin, was the most potent compound, and it mitigated apoptosis by 50% at 200 ± 50 pM. Other mTOR inhibitors, namely everolimus, AZD8055, and torin 1, also suppressed apoptosis, providing additional pharmacological evidence for mTOR pathway involvement in regulating cell death after IR. Everolimus and torin 1 treatment after IR decreased the S phase population and enforced both G1 and G2 phase arrest. This prorogation of cell cycle progression was accompanied by decreased IR-induced DNA damage measured by γH2AX phosphorylation at Ser139. RNA interference-mediated knockdown of the respective mTORC1 and mTORC2 subunits, Raptor or Rictor, also mitigated IR-induced apoptosis. Collectively, this study suggests a central role for the mTOR signaling in the cytotoxic response to IR and offers a useful platform to probe for additional agents.
authors.forename: Elizabeth R
authors.initials: ER
authors.lastname: Sharlow
authors.forename: Stephanie
authors.initials: S
authors.lastname: Leimgruber
authors.forename: Ana
authors.initials: A
authors.lastname: Lira
authors.forename: Michael J
authors.initials: MJ
authors.lastname: McConnell
authors.forename: Andrés
authors.initials: A
authors.lastname: Norambuena
authors.forename: George S
authors.initials: GS
authors.lastname: Bloom
authors.affiliation: Department of Radiation Oncology, University of Pittsburgh , Pittsburgh, Pennsylvania, United States.
authors.forename: Michael W
authors.initials: MW
authors.lastname: Epperly
authors.affiliation: Department of Radiation Oncology, University of Pittsburgh , Pittsburgh, Pennsylvania, United States.
authors.forename: Joel S
authors.initials: JS
authors.lastname: Greenberger
authors.forename: John S
authors.initials: JS
authors.lastname: Lazo
mesh_terms: D017209:Apoptosis / Q000187:drug effects* / Q000528:radiation effects*; D002453:Cell Cycle / Q000187:drug effects / Q000528:radiation effects; D002460:Cell Line; D006801:Humans; D011837:Radiation-Protective Agents / Q000737:chemistry / Q000494:pharmacology*; D015398:Signal Transduction / Q000187:drug effects* / Q000528:radiation effects; D020123:Sirolimus / Q000737:chemistry / Q000494:pharmacology; D054852:Small Molecule Libraries / Q000737:chemistry / Q000494:pharmacology*; D058570:TOR Serine-Threonine Kinases / Q000037:antagonists & inhibitors / Q000378:metabolism*
chemical_list: D011837:Radiation-Protective Agents; D054852:Small Molecule Libraries; D058570:TOR Serine-Threonine Kinases; D020123:Sirolimus