DNA-DEPENDENT PROTEIN-KINASE IS NOT REQUIRED FOR ACCUMULATION OF P53 OR CELL-CYCLE ARREST AFTER DNA-DAMAGE

In response to DNA damage, cells transduce a signal that fends to accu mulation and activation of p53 protein, transcriptional induction of s everal genes, including p21, gadd45, and gadd153, and cell cycle arres t. One hypothesis is that the signal is mediated by DNA-dependent prot ein kinase (DNA-PK), which consists of a catalytic subunit CDNA PKcs) and a regulatory subunit (Ku), DNA-PK has several characteristics that support this hypothesis: Ku binds to DNA damaged hy nicks or double-s trand breaks, DNA-PKcs is activated when Ku binds to DNA, DNA-PK will phosphorylate p53 and other cell cycle regulatory proteins in vitro, a nd DNA-PKcs shares homology with ATM, which is mutated in ataxia telan giectasia and involved in signaling the p53 response to ionizing radia tion, The hypothesis was tested by analyzing early passage fibroblasts from severe combined immunodeficient mice, which are deficient in DNA -PK. After exposure to ionizing radiation, UV radiation, or methyl met hane-sulfonate. severe combined immunodeficient and wild-type cells we re indistinguishable in their response. The accumulation of p53, induc tion of p21, gadd45, and gadd153, and arrest of the cell cycle in G(1) anti G(2) occurred normally. Therefore, DNA-PK is not required for th e p53 response or cell cycle arrest after DNA damage.