Regulation of radiation-induced protein kinase Cdelta activation in radiation-induced apoptosis differs between radiosensitive and radioresistant mouse thymic lymphoma cell lines

Protein kinase Cd(PKCd) has an important role in radiation-induced apoptosis. The expression and function of PKCd in radiation-induced apoptosis were assessed in a radiation-sensitive mouse thymic lymphoma cell line, 3SBH5, and its radioresistant variant, XR223. Rottlerin, a PKCd-specific inhibitor, completely abolished radiation-induced apoptosis in 3SBH5. Radiation-induced PKCd activation correlated with the degradation of PKCd, indicating that PKCd activation through degradation is involved in radiation-induced apoptosis in radiosensitive 3SBH5. In radioresistant XR223, radiation-induced PKCd activation was lower than that in radiosensitive 3SBH5. Cytosol PKCd levels in 3SBH5 decreased markedly after irradiation, while those in XR223 did not. There was no apparent change after irradiation in the membrane fractions of either cell type. In addition, basal cytosol PKCd levels in XR223 were higher than those in 3SBH5. These results suggest that the radioresistance in XR223 to radiation-induced apoptosis is due to a difference in the regulation of radiation-induced PKCd activation compared to that of 3SBH5. On the other hand, Atm-/- mouse thymic lymphoma cells were more radioresistant to radiation-induced apoptosis than wild-type mouse thymic lymphoma cells. Irradiated wild-type cells, but not Atm-/- cells, had decreased PKCd levels, indicating that the Atm protein is involved in radiation-induced apoptosis through the induction of PKCd degradation. The decreased Atm protein levels induced by treatment with Atm small interference RNA had no effect on radiation-induced apoptosis in 3SBH5 cells. These results suggest that the regulation of radiation-induced PKCd activation, which is distinct from the Atm-mediated cascade, determines radiation sensitivity in radiosensitive 3SBH5 cells