The kinase Chk2 and tumor suppressor p53 participate in an ill defined regulatory interaction in mammalian cells. The abundance of Chk2 mRNA and protein has now been shown to be decreased by the induction of p53 in Saos2 cells. Ionizing radiation also triggered the phosphorylation and subsequent down-regulation of Chk2 in human colorectal HCT116 (p53 ؉/؉ ) cancer cells; irradiation of its isogenic mutant HCT116 (p53 ؊/؊ ) cells, which lack functional p53, induced Chk2 phosphorylation but not its down-regulation. In addition, HCT116 (p53 ؉/؉ ) cells constitutively expressing a dominant negative p53 (V143A) failed to suppress Chk2 expression after irradiation. Reporter gene assays in HCT116 (p53 ؉/؉ ) cells revealed that wild-type p53 repressed, whereas a dominant negative p53 mutant increased, the activity of the human Chk2 gene promoter. Mutational analysis showed that a CCAAT box located between nucleotides ؊152 and ؊138 of the promoter was responsible for its negative regulation by p53. Electrophoretic mobility shift assays demonstrated that the transcription factor NF-Y binds to this CCAAT sequence. A dominant negative mutant of NF-YA abolished the effect of p53 on Chk2 promoter activity. These results suggest that p53 negatively regulates Chk2 gene transcription through modulation of NF-Y function and that this regulation may be important for reentry of cells into the cell cycle after DNA damage is repaired.

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