Abstract
Selenium (Se) is an essential trace element that has a role in cancer chemoprevention. DNA methylation is an important factor in carcinogenesis and one-carbon metabolism may influence this by providing methyl groups for DNA methylation. In previous studies Se was shown to be inversely correlated with homocysteine (Hcy), a product of one-carbon metabolism, in humans but it is unclear whether Se has a direct effect on DNA methylation. The relationship between Se and one-carbon metabolism was therefore investigated on two levels: (i) an investigation into the effect of Se supplementation on nutritional factors involved in one-carbon metabolism and (ii) an integrative epigenomic analysis using prostate cell lines and tissues to determine the effect of Se supplementation on DNA methylation. The interaction between Se, tHcy and B vitamins was investigated in a double-blind, placebo-controlled trial where 501 healthy UK elderly volunteers received 100, 200, or 300 mug Se/day as high-Se-yeast, or placebo-yeast for 6 months. Plasma Se, tHcy, folate, vitamin B-12, pyridoxal-5'-phosphate and its catabolite, 4-pyridoxic acid, were measured. At baseline, Se was inversely correlated with tHcy in males (p<0. 05). Before supplementation, tHcy concentration was significantly lower in the highest compared to the lowest Se tertile in males (p<0.05), and in females when folate concentrations were also in the top tertile (p<0.05). After 6 months of Se supplementation, only Se concentration had changed significantly. A genome-wide experimental design combining demethylation with 5-aza-2'-deoxycitidine (5-AZA) and subsequent gene re-expression, with methylome mapping via methylated DNA immunoprecipitation (MeDIP), investigated the global effect of Se supplementation on prostate cell lines RWPE-1, DU145, LnCAP and PC-3 and tissues. Se supplementation increased expression of hypermethylated genes and, in combination with 5-AZA, reduced global methylation levels in the PC-3 prostate cancer cell line (P<0.05). Se and 5-AZA individually activated anti-carcinogenic pathways while Se also inhibited cellular proliferation pathways. Genes that were Se-responsive after demethylation were involved in DNA repair, cell motility, the JAK/STAT pathway and stress-mediated translational regulation. MeDIP identified genes involved in vesicle-mediated transport, in all prostate cancer cell lines. Novel hypermethylated genes included GPx2, an anti-inflammatory and anti-carcinogenic selenoprotein. GPx3, known to be hypermethylated in prostate cancer, was shown to be re-expressed with 5-AZA and had a further increase in gene expression with Se supplementation. Clusters of hypermethylated genes were identified on almost all chromosomes. In conclusion, supplementation with Se does not affect tHcy concentration in the UK elderly population whereas in vitro Se supplementation of prostate cancer cells causes an increase in gene expression of hypermethylated genes. Hypermethylated Se-responsive genes are involved in anti-carcinogenic pathways and can be targeted for cancer therapy by demethylation and Se supplementation.