Cellular iron status influences folate metabolism

Iron deficiency and chemical iron chelators are known to modify folate metabolism but the fundamental biochemical mechanisms underlying the modifications are not known. The studies presented in this dissertation describe two mechanisms whereby cellular iron status influences folate metabolism, using human breast cancer cells and neuroblastoma as model systems. Iron chelators, which induce cellular iron deficiency, impair transcription of the folate-dependent enzyme cytoplasmic serine hydroxymethyltransferase (cSHMT) in a tissue-specific manner. Breast cancer cells treated with the iron chelators, mimosine and deferroxamine, are growth arrested and have a 95% reduction in cSHMT protein levels and promoter activity. In contrast, iron chelators have no effect on cell proliferation, cSHMT levels or folate metabolism in human neuroblastoma. These data demonstrate a new and novel regulatory mechanism that accounts for the effects of iron chelators on cell proliferation and folate metabolism, independent of nucleotide precursor concentrations. These initial studies were extended by investigating the effects of the native intracellular iron chelator, heavy chain ferritin (HCF), on folate metabolism. Surprisingly, cells overexpressing HCF have increased cSHMT protein levels due to increased translation of the cSHMT message, and no effects on cSHMT transcription were observed. These data also demonstrated that increased cSHMT expression enhances de novo thymidine biosynthesis. These studies indicate that both chemical iron chelators and HCF influence cSHMT expression and folate metabolism, but by distinct mechanisms. Collectively, this research furthers our understanding of folate metabolism; and in particular this research demonstrates how iron regulates folate metabolism and how cSHMT enzymatic activity is critical to managing the flux of one-carbon units in folate metabolism.