Reduction of spontaneous mutagenesis in mismatch repair-deficient and proficient cells by dietary antioxidants

Abstract

Cells lacking mismatch repair (MMR) exhibit elevated levels of spontaneous mutagenesis. Evidence exists that MMR is involved in repair of some DNA lesions besides mismatches. If some oxidative DNA lesions are substrates for MMR, then the excess mutagenesis in MMR − cells might be blocked by dietary antioxidants. Effects of the dietary antioxidants ascorbate, α-tocopherol, (−)-epigallocatechin gallate (EGCG) and lycopene on spontaneous mutagenesis were studied using mismatch repair-deficient (hMLH1 −) human colon carcinoma HCT116 cells and HCT116/ch3 cells, in which normal human chromosome 3 has been added to restore mismatch repair. HCT116 cells have a 22-fold higher spontaneous mutation rate compared with HCT116/ch3 cells. HCT116 cells cultured in 1% fetal bovine serum (FBS) have twice the spontaneous mutation rate of those cultured in 10% FBS, most likely due to reduction in serum antioxidants in the low serum medium. As expected, α-tocopherol (50 μM) and ascorbate (284 μM) reduced spontaneous mutagenesis in HCT116 cells growing in 1% serum more dramatically than in cells cultured in 10% serum. The strongest antimutagenic compound was lycopene (5 μM), which reduced spontaneous mutagenesis equally (about 70%) in HCT116 cells growing in 10 and 1% FBS and in HCT116/ch3 cells. Since lycopene was equally antimutagenic in cells growing in low and high serum, it may have another antimutagenic mechanism in addition to its antioxidant effect. Surprisingly, EGCG (10 μM) was toxic to cells growing in low serum. It also reduced spontaneous mutagenesis equally (nearly 40%) in HCT116 and HCT116/ch3 cells. The large proportion of spontaneous mutagenesis that can be blocked by antioxidants in mismatch repair-deficient cells support the hypothesis that a major cause of their excess mutagenesis is endogenous oxidants. Blocking spontaneous mutagenesis, perhaps with a cocktail of antioxidants, should reduce the risk of cancer in people with a genetic defect in mismatch repair as well as other individuals.