Protein kinase Cδ overexpression enhances radiation sensitivity via extracellular regulated protein kinase 1/2 activation, abolishing the radiation-induced G₂-M arrest

Protein kinase C (PKC) has been widely implicated in regulation of cell growth/cell cycle progression and apoptosis. However, the role of PKCδ in radiosensitivity and cell cycle regulation remains unclear. Overexpression of PKCδ increased Ca²⁺independent PKC activity without altering other PKC isoforms (PKCα, -β1, -ε, and -ζ), and extracellular regulated protein kinase (ERK) 1/2 activity was also increased in PKCδ-specific manner. A clonogenic survival assay showed that PKCδ-overexpressed cells had more radiosensitivity and pronounced induction of apoptosis than control cells. Flow cytometric analysis revealed that PKCδ made the cells escape from radiation-induced G₂-M arrest. Moreover, p53 and p21^waf induction by radiation were higher in PKCδoverexpressed cells than control cells, and PKCδmediated apoptosis was reduced, when radiation induced ERK1/2 activity was inhibited by PD98059. Furthermore, PKCδ antisense and rottlerin, PKC inhibitor-abrogated PKCδ-mediated radiosensitivity and reduced ERK1/2 activity to the control vector level. These results demonstrated that PKCδ overexpression enhanced radiation-induced apoptosis and radiosensitivity via ERK1/2 activation, thereby abolishing the radiation-induced G₂-M arrest and finally apoptosis.