The p38 Mitogen-Activated Protein Kinase Pathway Links the DNA Mismatch Repair System to the G₂ Checkpoint and to Resistance to Chemotherapeutic DNA-Methylating Agents

Although human cells exposed to DNA-methylating agents undergo mismatch repair (MMR)-dependent G₂ arrest, the basis for the linkage between MMR and the G₂ checkpoint is unclear. We noted that mitogen-activated protein kinase p38α was activated in MMR-proficient human glioma cells exposed to the chemo-therapeutic methylating agent temozolomide (TMZ) but not in paired cells made MMR deficient by expression of a short inhibitory RNA (siRNA) targeted to the MMR protein Mlh1. Furthermore, activation of p38α in MMR-proficient cells was associated with nuclear inactivation of the cell cycle regulator Cdc25C phosphatase and its downstream target Cdc2 and with activation of the G₂ checkpoint, actions which were suppressed by the p38α/β inhibitors SB203580 and SB202590 or by expression of a p38α siRNA. Finally, pharmacologic or genetic inhibition of p38α increased the sensitivity of MMR-proficient cells to the cytotoxic actions of TMZ by increasing the percentage of cells that underwent mitotic catastrophe as a consequence of G₂ checkpoint bypass. These results suggest that p38α links DNA MMR to the G₂ checkpoint and to resistance to chemo-therapeutic DNA-methylating agents. The p38 pathway may therefore represent a new target for the development of agents to sensitize tumor cells to chemotherapeutic methylating agents.