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.
Subject(s)
DNA-Activated Protein Kinase/metabolism , ErbB Receptors/metabolism , Exoribonucleases/metabolism , Gamma Rays , Nuclear Proteins/metabolism , Proto-Oncogene Proteins c-myc/metabolism , RNA Stability/radiation effects , RNA, Messenger/metabolism , Amino Acid Substitution , Cell Line, Tumor , DNA-Activated Protein Kinase/genetics , ErbB Receptors/genetics , Exoribonucleases/genetics , Humans , Mutation, Missense , Nuclear Proteins/genetics , Phosphorylation/genetics , Phosphorylation/radiation effects , Proto-Oncogene Proteins c-myc/genetics , RNA Stability/genetics , RNA, Messenger/genetics , Radiation Tolerance/genetics , Radiation Tolerance/radiation effectsABSTRACT
This study employs a theoretical modeling and an experimental measurement for investigating the dispersion behavior of guided waves propagating in a bi-layer system consisting of a piezoelectric plate and a dielectric fluid layer. The theoretical model is based on a recursive asymptotic stiffness matrix method (RASM) with the fluid layer modeled as an equivalent elastic body. A laser ultrasound technique is used to measure the dispersion relations of the bi-layer system. Behaviors of mode couplings between guided modes propagating in the piezoelectric plate and those in fluid layer are characterized in the modeling and measurements. Dispersion behaviors of guided modes propagating in the bi-layer system are discussed for varying fluid thicknesses. For all of the investigated cases, the theoretical modeled dispersion spectra agree well with the measurements.
