Abstract
This study focused on the question of how the DNA mismatch repair (MMR) system and p53 interact to maintain genomic integrity in the presence of the mutagenic stress induced by hydrogen peroxide (H2O2). The cytotoxic and mutagenic effects of H2O2 were compared in four colon carcinoma sublines: HCT116, 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 to H2O2, but disruption of p53 in MMR-deficient cells resulted in substantial resistance to H2O2 (IC50 values of 203.8 and 66.2 μM for MMR−/p53− and MMR−/p53+ cells, respectively). The effect of loss of p53 and MMR function 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-resistant clones, respectively. Loss of MMR in combination with loss of p53 synergistically increased the frequency of frameshift mutations in the CA repeat 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 Gl and G2/M phase arrest in MMR+/p53+ cells. Loss of MMR did not alter the ability of H2O2 to activate either checkpoint; loss of p53 in either the MMR-proficient or deficient cells resulted in impairment of the Gl arrest and a more pronounced G2/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 demonstrate that the effect of disabling p53 function is modulated by the proficiency of the MMR system (and vice versa) and that there is an overlap between the functions of p53 and the MMR system with respect to the activation of apoptosis and mutagenesis after an oxidative stress.
Footnotes
- Received April 18, 2000.
- Accepted September 1, 2000.
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Send reprint requests to: Stephen B. Howell, MD, Department of Medicine 0058, University of California, San Diego, La Jolla, CA 92093. E-mail: showell{at}ucsd.edu
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This work was supported in part by Grant CA78648 from the National Institutes of Health and conducted in part by the Clayton Foundation for Research—California Division. X.L. and S.B.H. are Clayton Foundation investigators. A preliminary account of this work was presented at the 1999 DNA Repair and Mutagenesis Meeting of the American Society for Microbiology.
- The American Society for Pharmacology and Experimental Therapeutics
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