Azoxymethane-Induced Colon Carcinogenesis in Mice Occurs Independently of De Novo Thymidylate Synthesis Capacity1–3

Abstract

Folate metabolism affects DNA synthesis, methylation, mutation rates, genomic stability, and gene expression, which are altered in colon cancer. Serine hydroxymethyltransferase 1 (SHMT1) regulates thymidylate (dTMP) biosynthesis and uracil accumulation in DNA, and as such affects genome stability. Previously, we showed that decreased SHMT1 expression inShmt1 knockout mice (Shmt1−/+) or its impaired nuclear localization, as occurs in mice over-expressing anShmt1 transgene (Shmt1tg+), results in elevated uracil incorporation into DNA, which could affect colon cancer risk. We used these 2 models to determine the effect of altered SHMT1 expression and localization, and its interaction with folate insufficiency, on azoxymethane (AOM)-induced colon cancer in mice.Shmt1−/+ andShmt1tg+ mice were weaned to a control or folate-and-choline-deficient (FCD) diet and fed the diet for 28 or 32 wk, respectively. At 6 wk of age, mice were injected weekly for 6 wk with 10 mg/kg AOM (w/v in saline). Colon uracil concentrations in nuclear DNA were elevated 2–7 fold inShmt1−/+ andShmt1tg+ mice. However, colon tumor incidence and numbers were not dependent on SHMT1 expression inShmt1−/+ orShmt1−/− mice. The FCD diet reduced tumor load independent ofShmt1 genotype. In contrast,Shmt1tg+ mice exhibited a 30% reduction in tumor incidence, a 50% reduction in tumor number, and a 60% reduction in tumor load compared with wild-type mice independent of dietary folate intake. Our data indicate that uracil accumulation in DNA does not predict tumor number in AOM-mediated carcinogenesis. Furthermore, enrichment of SHMT1 in the cytoplasm, as observed inShmt1tg+ mice, protects against AOM-mediated carcinogenesis independent of its role in nuclear de novo dTMP biosynthesis.