Inhibiting Tyrosine Phosphorylation of Protein Kinase Cδ( PKCδ) Protects the Salivary Gland from Radiation Damage

Background: Nuclear import of protein kinase C is required for DNA-damage-induced apoptosis. Results: c-Src and c-Abl phosphorylate PKC to regulate nuclear import. Tyrosine kinase inhibitors block nuclear translocation of PKC and suppress apoptosis. Conclusion: Tyrosine kinase inhibitors can regulate the pro-apoptotic function of protein kinase C. Significance: Tyrosine kinase inhibitors may improve the quality of life in cancer patients receiving radiation therapy. Radiation therapy for head and neck cancer can result in extensive damage to normal adjacent tissues such as the salivary gland and oral mucosa. We have shown previously that tyrosine phosphorylation at Tyr-64 and Tyr-155 activates PKC in response to apoptotic stimuli by facilitating its nuclear import. Here we have identified the tyrosine kinases that mediate activation of PKC in apoptotic cells and have explored the use of tyrosine kinase inhibitors for suppression of irradiation-induced apoptosis. We identify the damage-inducible kinase, c-Abl, as the PKC Tyr-155 kinase and c-Src as the Tyr-64 kinase. Depletion of c-Abl or c-Src with shRNA decreased irradiation- and etoposide-induced apoptosis, suggesting that inhibitors of these kinases may be useful therapeutically. Pretreatment with dasatinib, a broad spectrum tyrosine kinase inhibitor, blocked phosphorylation of PKC at both Tyr-64 and Tyr-155. Expression of gate-keeper mutants of c-Abl or c-Src that are active in the presence of dasatinib restored phosphorylation of PKC at Tyr-155 and Tyr-64, respectively. Imatinib, a c-Abl-selective inhibitor, also specifically blocked PKC Tyr-155 phosphorylation. Dasatinib and imatinib both blocked binding of PKC to importin- and nuclear import, demonstrating that tyrosine kinase inhibitors can inhibit nuclear accumulation of PKC. Likewise, pretreatment with dasatinib also suppressed etoposide and radiation induced apoptosis in vitro. In vivo, pre-treatment of mice with dasatinib blocked radiation-induced apoptosis in the salivary gland by >60%. These data suggest that tyrosine kinase inhibitors may be useful prophylactically for protection of nontumor tissues in patients undergoing radiotherapy of the head and neck.