Folate metabolism in the human colon: cellular responses identified through in vitro studies

Low folate intake influences the risk of not only neural tube defects and vascular diseases, but also colorectal tumours. Folate forms have different roles in the one-carbon metabolism. This includes DNA synthesis and methylation reactions. Folate is present as 5'-methyltetrahydrofolate (MTHF) in the diet and plasma, while in supplements the synthetic form is present, known as folic acid (pteryol glutamic acid, PGA). In animal studies, PGA seemed to have dual modulatory effects on colorectal carcinogenesis depending on the timing and dose of intervention. In this thesis the effect of different folate forms and dosages in the colon were studied in vitro using a nutrigenomics approach. The DNA microarray technique was applied to study the expression pattern of thousands of genes, and assess known and identify new processes affected by folate. Gene expression analysis revealed that high PGA concentrationsincreased energy metabolism and lowered iron metabolism related genes in the human colorectal cell line, HT29. Further investigation showed that intracellular iron concentration was decreased in three human colorectal cell lines. This effect on iron metabolism was studied in an existing human randomized, placebo-controlled trial. After six months of folate supplementation the serum ferritin, but not the colonic iron, was decreased. We also studied the difference between PGA and MTHF exposure in HT29 cells, using a physiological concentration range of both folates. A dose dependent effect was found for both folates. The normal serum values of PGA increased genes involved in the folate metabolism, compared to identical MTHF concentration. Supplemental concentrations of MTHF affected protein synthesis, endoplasmatic reticulum/Golgi and cancer related genes, compared to identical PGA concentrations. More specifically, high MTHF increased differentiation, adhesion and cell viability; all known cancer related processes. This may suggest a more beneficial effect of MTHF as compared to PGA. However we need to further investigate the effect of PGA and MTHF on iron metabolism, cancer related processes or energy metabolism in humans. Hopefully this will lead to more insight in the effect of folate supplementation with regard to colorectal carcinogenesis.