p53 interacts with the DNA mismatch repair system to modulate the cytotoxicity and mutagenicity of hydrogen peroxide

This study focused on the question of how the DNA mismatch repair (MMR) sys tem and p53 interact to maintain genomic integrity in the presence of the m utagenic stress induced by hydrogen peroxide (H2O2). The cytotoxic and muta genic effects of H2O2 were compared in four colon carcinoma sublines: HCT11 6, HCT116/E6, HCT116+ch3, and HCT116+ch3/E6, representing MMR-/p53(+), MMR- /p53(-), MMR+/p53(+), and MMR+/p53(-) phenotypes, respectively. Loss of p53 in MMR-proficient cells did not significantly alter cellular sensitivity t o H2O2, but disruption of p53 in MMR-deficient cells resulted in substantia l resistance to H2O2 (IC50 values of 203.8 and 66.2 muM for MMR-/p53(-) and MMR-/p53(+) cells, respectively). The effect of loss of p53 and MMR functi on on sensitivity to the mutagenic effect of H2O2 paralleled the effects on cytotoxic sensitivity. In MMR-deficient cells, loss of p53 resulted in a 3 .5- and 2.2-fold increase in the generation of 6-thiogunaine and ouabain-re sistant clones, respectively. Loss of MMR in combination with loss of p53 s ynergistically increased the frequency of frameshift mutations in the CA re peat tracts of the out-of-frame shuttle vector pZCA29 and further promoted instability of microsatellite sequences under H2O2 stress. Flow cytometric analysis showed that H2O2 treatment produced a G(1) and G(2)/M phase arrest in MMR+/p53(+) cells. Loss of MMR did not alter the ability of H2O2 to act ivate either checkpoint; loss of p53 in either the MMR-proficient or defici ent cells resulted in impairment of the G(1) arrest and a more pronounced G (2)/M arrest. H2O2 caused a greater and more longed increase in p53 protein levels in MMR-proficient than in the MMR-deficient cells. The results demo nstrate that the effect of disabling p53 function is modulated by the profi ciency of the MMR system (and vice versa) and that there is an overlap betw een the functions of p53 and the MMR system with respect to the activation of apoptosis and mutagenesis after an oxidative stress.