Regulation of Folate-mediated One-carbon Metabolism by 10-Formyltetrahydrofolate Dehydrogenase

Abstract

10-Formyltetrahydrofolate dehydrogenase (FDH) catalyzes the NADP؉ -dependent conversion of 10-formyltetrahydrofolate to CO 2 and tetrahydrofolate (THF) and is an abundant high affinity folatebinding protein. Although several activities have been ascribed to FDH, its metabolic role in folate-mediated one-carbon metabolism is not well understood. FDH has been proposed to: 1) inhibit purine biosynthesis by depleting 10-formyl-THF pools, 2) maintain cellular folate concentrations by sequestering THF, 3) deplete the supply of folateactivated one-carbon units, and 4) stimulate the generation of THFactivated one-carbon unit synthesis by channeling folate cofactors to other folate-dependent enzymes. The metabolic functions of FDH were investigated in neuroblastoma, which do not contain detectable levels of FDH. Both low and high FDH expression reduced total cellular folate concentrations by 60%, elevated rates of folate catabolism, and depleted cellular 5-methyl-THF and S-adenosylmethionine levels. Low FDH expression increased the formyl-THF/THF ratio nearly 10-fold, whereas THF accounted for nearly 50% of total folate in neuroblastoma with high FDH expression. FDH expression did not affect the enrichment of exogenous formate into methionine, serine, or purines and did not suppress de novo purine nucleotide biosynthesis. We conclude that low FDH expression facilitates the incorporation of one-carbon units into the one-carbon pool, whereas high levels of FDH expression deplete the folate-activated one-carbon pool by catalyzing the conversion of 10-formyl-THF to THF. Furthermore, FDH does not increase cellular folate concentrations by sequestering THF in neuroblastoma nor does it inhibit or regulate de novo purine biosynthesis. FDH expression does deplete cellular 5-methyl-THF and S-adenosylmethionine levels indicating that FDH impairs the folate-dependent homocysteine remethylation cycle. Tetrahydrofolate (THF)2 polyglutamates are cofactors that function as one-carbon donors and acceptors in a set of reactions known as folate-mediated one-carbon metabolism, which occurs both in the cytoplasm and in mitochondria (see Fig. 1) (1). THF cofactors carry onecarbon units at three oxidation states ranging from formate to methanol (2). The biologically active THF derivatives contain a reduced pteridine and a polyglutamate peptide consisting of five to eight glutamate residues linked by ␥-peptide bonds (3). Cytoplasmic folate-mediated onecarbon metabolism is required for the de novo synthesis of purines (supplies the carbon-2 and carbon-8 of the purine ring) and thymidylate (methylation of dUMP to dTMP), and also for remethylation of homocysteine to methionine (2, 3). Methionine can be converted to S-adenosylmethionine (SAM) and serve as a methyl donor for numerous methylation reactions, including the methylation of DNA, RNA, and proteins. Serine is a primary source of folate-activated one-carbon units (4). Cytoplasmic serine hydroxymethyltransferase (cSHMT) catalyzes the THF-dependent aldol cleavage of serine to methylene-THF and...

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“…Although the binding between GNMT and 5-methyl-THF has been well characterized, the significance of maintaining optical folate status in human pathological conditions is less clear, and the specific impact of GNMT expression on hepatic methylfolate-dependent reactions has not been elucidated. In recent years, stable isotope tracers have been widely utilized to elucidate how specific folate enzymes mediate and regulate the fluxes of one-carbon units among folate-dependent reactions (19)(20). We demonstrated that transformed human lymphoblasts with reduced methylene tetrahydrofolate reductase (MTHFR) have advantages in de novo purine synthesis when folate is adequate, but they are more susceptible to S-adenosylmethionine depletion when folate is restricted (21).…”

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“…31 The enzyme functions as a metabolic regulator, which removes carbon groups, in the form of CO 2 , from the reduced folate pool, thus diverting them from biosynthetic pathways. 32,33 This function results from FDH-catalyzed reaction of the conversion of 10-formyltetrahydrofolate (10-fTHF) to tetrahydrofolate (THF) and CO 2 in a NADP + -dependent manner. 33 FDH, being abundant in normal tissues, is commonly down-regulated or silenced in cancers.…”

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“…Although the enzymatic function of FDH, that is the conversion of 10-formyltetrahydrofolate to tetrahydrofolate, was known for decades, the main physiological role of the enzyme is still in question. Proposed FDH functions include regulation of reduced folate pools (Champion et al, 1994), storage of the intracellular folate (Min et al, 1988), regulation of de novo purine biosynthesis (Krupenko and Oleinik, 2002) and regulation of cellular methylation (Anguera et al, 2006).…”