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J Biol Chem, Vol. 274, Issue 19, 13111-13117, May 7, 1999

The Ability of p53 to Activate Downstream Genes p21^WAF1/cip1 and MDM2, and Cell Cycle Arrest following DNA Damage Is Delayed and Attenuated in scid Cells Deficient in the DNA-dependent Protein Kinase

From the Laboratory of Molecular and Cellular Radiation Biology, Department of Radiation Oncology, Massachusetts General Hospital, Harvard Medical School, Boston, Massachusetts 02114

scid mouse embryonic fibroblasts are deficient in DNA-dependent protein kinase activity due to a mutation in the C-terminal domain of the catalytic subunit (DNA-PKcs). When exposed to ionizing radiation, the increase in levels of p53 was the same as in normal mouse embryonic fibroblasts. However, the rise in p21^WAF1/cip1 and mdm2 was found to be delayed and attenuated, which correlated in time with delayed onset of G₁/S arrest by flow cytometric analysis. The p53-dependent G₁ checkpoint was not eliminated: inactivation of p53 by the E6 protein in scid cells resulted in the complete loss of detectable G₁/S arrest after DNA damage. Immunofluorescence analysis of normal cells revealed p53 to be localized predominantly within the cytoplasm prior to irradiation and then translocate to the nucleus after irradiation. In contrast, scid cells show abnormal accumulation of p53 in the nucleus independent of irradiation, which was confirmed by immunoblot analysis of nuclear lysates. Taken together, these data suggest that loss of DNA-PK activity appears to attenuate the kinetics of p53 to activate downstream genes, implying that DNA-PK plays a role in post-translational modification of p53, without affecting the increase in levels of p53 in response to DNA damage.

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