p27Kip1 Is Required to Mediate a G1 Cell Cycle Arrest Downstream of ATM following Genotoxic Stress

The DNA damage response (DDR) is a coordinated signaling network that ensures the maintenance of genome stability under DNA damaging stress. In response to DNA lesions, activation of the DDR leads to the establishment of cell cycle checkpoints that delay cell-cycle progression and allow repair of the defects. The tumor suppressor p27(Kip1) is a cyclinCDK inhibitor that plays an important role in regulating quiescence in a variety of tissues. Several studies have suggested that p27(Kip1) also plays a role in the maintenance of genomic integrity. Here we demonstrate that p27(Kip1) is essential for the establishment of a G1 checkpoint arrest after DNA damage. We also uncovered that ATM phosphorylates p27(Kip1) on a previously uncharacterized residue (Ser-140), which leads to its stabilization after induction of DNA double-strand breaks. Inhibition of this stabilization by replacing endogenous p27(Kip1) with a Ser-140 phospho-mutant (S140A) significantly sensitized cells to IR treatments. Our findings reveal a novel role for p27(Kip1) in the DNA damage response pathway and suggest that part of its tumor suppressing functions relies in its ability to mediate a G1 arrest after the induction of DNA double strand breaks.