title: Nuclear EGFR suppresses ribonuclease activity of polynucleotide phosphorylase through DNAPK-mediated phosphorylation at serine 776.
abstract: Nuclear existence of epidermal growth factor receptor (EGFR) has been documented for more than two decades. Resistance of cancer to radiotherapy is frequently correlated with elevated EGFR expression, activity, and nuclear translocation. However, the role of nuclear EGFR (nEGFR) in radioresistance of cancers remains elusive. In the current study, we identified a novel nEGFR-associated protein, polynucleotide phosphorylase (PNPase), which possesses 3' to 5' exoribonuclease activity toward c-MYC mRNA. Knockdown of PNPase increased radioresistance. Inactivation or knock-down of EGFR enhanced PNPase-mediated c-MYC mRNA degradation in breast cancer cells, and also increased its radiosensitivity. Interestingly, the association of nEGFR with PNPase and DNA-dependent protein kinase (DNAPK) increased significantly in breast cancer cells after exposure to ionizing radiation (IR). We also demonstrated that DNAPK phosphorylates PNPase at Ser-776, which is critical for its ribonuclease activity. The phospho-mimetic S776D mutant of PNPase impaired its ribonuclease activity whereas the nonphosphorylatable S776A mutant effectively degraded c-MYC mRNA. Here, we uncovered a novel role of nEGFR in radioresistance, and that is, upon ionizing radiation, nEGFR inactivates the ribonuclease activity of PNPase toward c-MYC mRNA through DNAPK-mediated Ser-776 phosphorylation, leading to increase of c-MYC mRNA, which contributes to radioresistance of cancer cells.
authors: Graduate Institute of Cancer Biology and Center for Molecular Medicine, China Medical University, Taichung 404, Taiwan. ylyu@mail.cmu.edu.tw
authors: Yung-Luen
authors: YL
authors: Yu
authors: Ruey-Hwang
authors: RH
authors: Chou
authors: Chia-Han
authors: CH
authors: Wu
authors: Ying-Nai
authors: YN
authors: Wang
authors: Wei-Jung
authors: WJ
authors: Chang
authors: Yen-Ju
authors: YJ
authors: Tseng
authors: Wei-Chao
authors: WC
authors: Chang
authors: Chien-Chen
authors: CC
authors: Lai
authors: Hong-Jen
authors: HJ
authors: Lee
authors: Longfei
authors: L
authors: Huo
authors: Chung-Hsuan
authors: CH
authors: Chen
authors: Mien-Chie
authors: MC
authors: Hung
mesh_terms: D019943:Amino Acid Substitution; D045744:Cell Line, Tumor; D051747:DNA-Activated Protein Kinase / Q000235:genetics / Q000378:metabolism*; D066246:ErbB Receptors / Q000235:genetics / Q000378:metabolism*; D005095:Exoribonucleases / Q000235:genetics / Q000378:metabolism*; D005720:Gamma Rays; D006801:Humans; D020125:Mutation, Missense; D009687:Nuclear Proteins / Q000235:genetics / Q000378:metabolism*; D010766:Phosphorylation / Q000235:genetics / Q000528:radiation effects; D016271:Proto-Oncogene Proteins c-myc / Q000235:genetics / Q000378:metabolism*; D020871:RNA Stability / Q000235:genetics / Q000528:radiation effects*; D012333:RNA, Messenger / Q000235:genetics / Q000378:metabolism*; D011836:Radiation Tolerance / Q000235:genetics / Q000528:radiation effects; D024101:Mitochondrial Proteins
chemical_list: D051747:DNA-Activated Protein Kinase; D066246:ErbB Receptors; D005095:Exoribonucleases; D009687:Nuclear Proteins; D016271:Proto-Oncogene Proteins c-myc; D012333:RNA, Messenger; C489427:MYC protein, human; C512478:EGFR protein, human; C497263:PRKDC protein, human; C476417:PNPT1 protein, human; D024101:Mitochondrial Proteins