Inhibiting Tyrosine Phosphorylation of Protein Kinase C{delta} (PKC{delta}) Protects the Salivary Gland from Radiation Damage [Cell Biology]

February 25th, 2014 by Wie, S. M., Adwan, T. S., DeGregori, J., Anderson, S. M., Reyland, M. E.

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 previously shown that tyrosine phosphorylation at Y64 and Y155 activates PKCδ in response to apoptotic stimuli by facilitating its nuclear import. Here we have identified the tyrosine kinases that mediate activation of PKCδ 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 PKCδ Y155 kinase and c-Src as the Y64 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 PKCδ at both Y64 and Y155. Expression of ″gate-keeper″ mutants of c-Abl or c-Src that are active in the presence of dasatinib restored phosphorylation of PKCδ at Y155 and Y64, respectively. Imatinib, a c-Abl selective inhibitor, also specifically blocked PKCδ Y155 phosphorylation. Dasatinib and imatinib both blocked binding of PKCδ to importin-α and nuclear import, demonstrating that tyrosine kinase inhibitors can inhibit nuclear accumulation of PKCδ. 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 non-tumor tissues in patients undergoing radiotherapy of the head and neck.