Different effects of methotrexate on DNA mismatch repair proficient and deficient cells

Antifolates exert their antiproliferative activity through the inhibition o f dihydrofolate reductase and, as a consequence, of thymidylate synthesis, thereby inducing nucleotide misincorporation and impairment of DNA synthesi s. We investigated the processes involved in the repair of antifolate-induc ed damage and their relationship with cell death. Since misincorporated bas es may be removed by DNA mismatch repair (MMR), the study was carried out o n the MMR-proficient human cell lines HeLa and HCT116 + chr3, and, in paral lel, on the MMR-deficient cell lines HeLa cell-clone12, defective in the pr otein hPMS2, and HCT116, with an inactive hMLH1. After treatment with metho trexate (MTX), we observed that DNA repair synthesis occurs independently o f the cellular MMR function. Clear signs of apoptosis such as nuclear shrin kage, chromatin condensation and degradation, DNA laddering, and poly (ADP- ribose) polymerase (PARP) proteolysis, were visible in both MMR+ and MMR- c ells. Remarkably, cell viability was lower and the apoptotic process was tr iggered more efficiently in the MMR-competent cells. (C) 2001 Elsevier Scie nce Ltd. All rights reserved.