DNA-PK_CS binding to p53 on the p21^WAF1/CIP1 promoter blocks transcription resulting in cell death

A key determinant of p53-mediated cell fate following various DNA damage modalities is p21^WAF1/CIP1 expression, with elevated p21 expression triggering cell cycle arrest and repressed p21 expression promoting apoptosis. We show that under pro-death DNA damage conditions, the DNA-dependent protein kinase (DNA-PK_CS) is recruited to the p21 promoter where it forms a protein complex with p53. The DNA-PK_CS-associated p53 displays post-translational modifications that are distinct from those under pro-arrest conditions, ablating p21 transcription and inducing cell death. Inhibition of DNA-PK activity prevents DNA-PK_CS binding to p53 on the p21 promoter, restores p21 transcription and significantly reduces cell death. These data demonstrate that DNA-PK_CS negatively regulates p21 expression by directly interacting with the p21 transcription machinery via p53, driving the cell towards apoptosis.