Dietary phytate lowers K-ras mutational frequency, decreases DNA-adduct and hydroxyl radical formation in azoxymethane-induced colon cancer

OBJECTIVE(S): Dietary phytate is known to protect against azoxymethane (AOM)-induced preneoplastic lesions. The present study was designed to determine whether dietary phytate affects mutation frequency in colon epithelial cells challenged with azoxymethane in vivo, through lowering the formation of O(6)-methyl guanosine (O(6)-MeG) and 8-hydroxy deoxyguanosine (8-OHdG) adducts. MATERIALS AND METHODS: We used Fisher F344 rats induced with AOM for 20 weeks and undertook 1% or 2% phytate supplementation for subsequent 16 weeks to monitor the mutation frequencies of one of the candidate genes, K-ras, along with DNA adduct load. RESULTS: Dietary phytate significantly suppressed aberrant crypt foci formation and effectively inhibited colon tumor formation in a dose-dependent manner. DNA sequencing results demonstrated that 60% of the colon tumors from AOM-treated and control diet fed animals showed GGT to GAT transition and 40% of the tumors showed GGT to GTT transversion at codon 12, along with 18% of the tumors showing GGC to CGC transversion at codon 13. Phytate supplementation at 1 and 2% lowered the frequency of GGT > GAT to 30 and 10%, respectively. Phytate supplementation also nullified the codon 13 mutations. No mutations were observed at codon 61 in any of the experimental groups. CONCLUSION: The lowered frequency of K-ras mutations correlated with decreased formation of hydroxyl radicals, O(5)-meG and 8-OH-dG levels in phytate-supplemented animals with lowered tumor burden.