Chemopreventive effects of dietary folate on intestinal polyps in Apc+/-Msh2-/- mice

Epidemiological and animal studies (reviewed in Y. I. Kim, J, Nutr, Biochem istry, 10: 66-88, 1999; J, B, Mason and T, Levesque, Oncology, 10: 1727-174 3, 1996) suggest that dietary folate intake is inversely related to the ris k of colorectal cancer, However, the optimal timing of folate intervention and mechanisms by which folate modulates colorectal carcinogenesis have not been clearly established. A recently developed murine model of intestinal tumorigenesis, which carries a heterozygous mutation in the Apc gene and a null mutation in the Msh2 gene (Apc +/-Msh2 -/-), was used to determine the effect of dietary folate on intestinal tumorigenesis, Apc +/- Msh2 -/- mic e were randomized to receive either 0 or 8 mg of folate/kg diet starting at either 3 or 6 weeks of age. The 3- and 6-week diet starts represent interv ention before and after the establishment of neoplastic foci, respectively. At 11 weeks of age, mice a ere killed, and the small intestines and colons were analyzed for adenomas and aberrant crypt foci (ACF), Serum folate con centrations were determined by a standard microbiological assay, Genomic DN A methylation was assessed by in vitro [H-3]methyl incorporation into hepat ic DNA and by a methyl-sensitive restriction digestion method. Microsatelli te instability was determined in matched normal and polyp DNA from the smal l intestine and colon at 5 loci. Serum folate concentrations accurately ref lected dietary folate levels (P < 0.005), Folate supplementation, started b efore the establishment of neoplastic foci, significantly decreased the num ber of small intestinal adenomas (by 2.7-fold; P = 0.004) and colonic ACF ( by 2.8-fold; P = 0.028) and colonic adenomas (by 2.8-fold; P = 0.1) compare d with a moderate degree of folate deficiency. In contrast, a moderately fo late-deficient diet, started after the establishment of neoplastic foci, si gnificantly reduced the number of small intestinal adenomas (by 4.2-fold; P = 0.001) but had no effect on colonic ACF and adenomas compared with folat e supplementation. Genomic DNA methylation and microsatellite instability d o not seem to play a major role in folate-modulated intestinal and colonic tumorigenesis in this model. In conclusion, in this murine model, dietary f olate supplementation significantly protects against small intestinal and c olorectal tumarigenesis if it is provided before the establishment of neopl astic foci However, if it is provided after the establishment of neoplastic foci, dietary folate seems to have an opposite effect. These data suggest that the timing of folate intervention is critical in providing an effectiv e and safe chemopreventive effect on intestinal tumorigenesis. Notwithstand ing the limitations associated with this model, our data suggest that the o ptimal timing of folate intervention must be established before folate supp lementation can be used as a safe chemopreventive agent against colorectal cancer.