Inactivation of DNA-dependent protein kinase by protein kinase Cdelta: implications for apoptosis.

Protein kinase Cdelta (PKCdelta) is proteolytically cleaved and activated at the onset of apoptosis induced by DNA-damaging agents, tumor necrosis factor, and anti-Fas antibody. A role for PKCdelta in apoptosis is supported by the finding that overexpression of the catalytic fragment of PKCdelta (PKCdelta CF) in cells is associated with the appearance of certain characteristics of apoptosis. However, the functional relationship between PKCdelta cleavage and induction of apoptosis is unknown. The present studies demonstrate that PKCdelta associates constitutively with the DNA-dependent protein kinase catalytic subunit (DNA-PKcs). The results show that PKCdelta CF phosphorylates DNA-PKcs in vitro. Interaction of DNA-PKcs with PKCdelta CF inhibits the function of DNA-PKcs to form complexes with DNA and to phosphorylate its downstream target, p53. The results also demonstrate that cells deficient in DNA-PK are resistant to apoptosis induced by overexpressing PKCdelta CF. These findings support the hypothesis that functional interactions between PKCdelta and DNA-PK contribute to DNA damage-induced apoptosis.