DNA damage-induced cell-cycle phase regulation of p53 and p21waf1 in normal and ATM-defective cells.

The ATM-dependent accumulation of p53 and induction of p21waf1 are key events for G1 cell-cycle checkpoint arrest following DNA damage. In ATM-null AT cells, even though the p53 and p21waf1 responses are kinetically delayed and quantitatively reduced, the G1 checkpoint is virtually disrupted, suggesting that these proteins arrive too late in G1 to enforce the arrest. As the precise mechanism remains unclear, we examined the response to DNA double-strand breaks generated by gamma-radiation (IR), to determine if ATM deficiency affects the cell-cycle phase regulation of these molecules. We find that, after irradiation, whereas normal LCL-N cells markedly increase their levels of p53 in all phases of the cell cycle, AT cells fail to show any p53 increase in the G1 phase. In addition, whereas in LCL-N p21waf1 is induced in G1 and G2-M, in AT cells this induction is partly seen in G2-M, but not in G1, indicating a different cell-cycle phase regulation of p53 and p21waf1 as a result of ATM deficiency. The levels and catalytic activity of the p53-targeting kinases ATR and DNA-PK in LCL-N and AT cells are very similar throughout the cell cycle, both before and after IR, thus excluding a phase-specific activity for these kinases. Collectively, our findings demonstrate that, in ATM-deficient cells, the p53-dependent p21waf1 response to DNA damage is not only quantitatively reduced, but also specifically suppressed in the G1 phase, thus providing a mechanistic explanation for the severe disruption of the G1 checkpoint in AT cells.