title: Inhibition of protein phosphatase 2A radiosensitizes pancreatic cancers by modulating CDC25C/CDK1 and homologous recombination repair.
abstract: OBJECTIVE
  To identify targets whose inhibition may enhance the efficacy of chemoradiation in pancreatic cancer and thus improve survival, we conducted an siRNA library screen in pancreatic cancer cells. We investigated PPP2R1A, a scaffolding subunit of protein phosphatase 2A (PP2A) as a lead radiosensitizing target.
  
  
  METHODS
  We determined the effect of PP2A inhibition by genetic (PPP2R1A siRNA) and pharmacologic (LB100, a small molecule entering phase I clinical trials) approaches on radiosensitization of Panc-1 and MiaPaCa-2 pancreatic cancer cells both in vitro and in vivo.
  
  
  RESULTS
  PPP2R1A depletion by siRNA radiosensitized Panc-1 and MiaPaCa-2 cells, with radiation enhancement ratios of 1.4 (P < 0.05). Likewise, LB100 produced similar radiosensitization in pancreatic cancer cells, but minimal radiosensitization in normal small intestinal cells. Mechanistically, PPP2R1A siRNA or LB100 caused aberrant CDK1 activation, likely resulting from accumulation of the active forms of PLK1 (pPLK1 T210) and CDC25C (pCDC25C T130). Furthermore, LB100 inhibited radiation-induced Rad51 focus formation and homologous recombination repair (HRR), ultimately leading to persistent radiation-induced DNA damage, as reflected by γ-H2AX expression. Finally, we identified CDC25C as a key PP2A substrate involved in LB100-mediated radiosensitization as depletion of CDC25C partially reversed LB100-mediated radiosensitization. In a mouse xenograft model of human pancreatic cancer, LB100 produced significant radiosensitization with minimal weight loss.
  
  
  CONCLUSIONS
  Collectively, our data show that PP2A inhibition radiosensitizes pancreatic cancer both in vitro and in vivo via activation of CDC25C/CDK1 and inhibition of HRR, and provide proof-of-concept evidence that PP2A is a promising target for the improvement of local therapy in pancreatic cancer.
authors.affiliation: Department of Radiation Oncology, University of Michigan Medical School, Ann Arbor, Michigan 48109-5637, USA.
authors.forename: Dongping
authors.initials: D
authors.lastname: Wei
authors.forename: Leslie A
authors.initials: LA
authors.lastname: Parsels
authors.forename: David
authors.initials: D
authors.lastname: Karnak
authors.forename: Mary A
authors.initials: MA
authors.lastname: Davis
authors.forename: Joshua D
authors.initials: JD
authors.lastname: Parsels
authors.forename: Amanda C
authors.initials: AC
authors.lastname: Marsh
authors.forename: Lili
authors.initials: L
authors.lastname: Zhao
authors.forename: Jonathan
authors.initials: J
authors.lastname: Maybaum
authors.forename: Theodore S
authors.initials: TS
authors.lastname: Lawrence
authors.forename: Yi
authors.initials: Y
authors.lastname: Sun
authors.forename: Meredith A
authors.initials: MA
authors.lastname: Morgan
mesh_terms: D000818:Animals; D016203:CDC2 Protein Kinase / Q000378:metabolism*; D045744:Cell Line, Tumor; D004249:DNA Damage / Q000187:drug effects / Q000528:radiation effects; D004195:Disease Models, Animal; D004789:Enzyme Activation / Q000187:drug effects; D006801:Humans; D051379:Mice; D010190:Pancreatic Neoplasms / Q000235:genetics* / Q000378:metabolism* / Q000532:radiotherapy; D054648:Protein Phosphatase 2 / Q000037:antagonists & inhibitors* / Q000235:genetics / Q000378:metabolism; D034622:RNA Interference; D011836:Radiation Tolerance / Q000187:drug effects / Q000235:genetics; D011838:Radiation-Sensitizing Agents / Q000494:pharmacology; D059767:Recombinational DNA Repair / Q000187:drug effects*; D023041:Xenograft Model Antitumor Assays; D020687:cdc25 Phosphatases / Q000378:metabolism*
chemical_list: C517191:PPP2R1A protein, human; D011838:Radiation-Sensitizing Agents; D016203:CDC2 Protein Kinase; D054648:Protein Phosphatase 2; D020687:cdc25 Phosphatases