Purification, immunoassay, and tissue distribution of rat C1-tetrahydrofolate synthase

Folate Metabolism in Hepatocellular Carcinoma. What Do We Know So Far?

Cancer cells are characterized by accelerated proliferation and an outstanding adaptation of their metabolic pathways to meet energy demands. The folate cycle, also known as folate metabolism or one-carbon metabolism, through enzymatic interconversions, provides metabolites necessary for nucleotide synthesis, methylation, and reduction power, helping to maintain the high rate of proliferation; therefore, the study of this metabolic pathway is of great importance in the study of cancer. Moreover, multiple enzymes involved in this cycle have been implicated in different types of cancer, corroborating the cell's adaptations under this pathology. During the last decade, nonalcoholic fatty liver disease has emerged as the leading etiology related to the rise in the incidence and deaths of hepatocellular carcinoma. Specifically, cholesterol accumulation has been a determinant promoter of tumor formation, with solid evidence that an enriched-cholesterol diet plays a crucial role in accelerating the development of an aggressive subtype of hepatocellular carcinoma compared to other models. In this review, we will discuss the most recent findings to understand the contribution of folate metabolism to cancer cells and tumor microenvironment while creating a link between the dynamics given by cholesterol and methylenetetrahydrofolate dehydrogenase 1-like, a key enzyme of the cycle located in the mitochondrial compartment.

Rat liver folate metabolism can provide an independent functioning of associated metabolic pathways

Folate metabolism in mammalian cells is essential for multiple vital processes, including purine and pyrimidine synthesis, histidine catabolism, methionine recycling, and utilization of formic acid. It remains unknown, however, whether these processes affect each other via folate metabolism or can function independently based on cellular needs. We addressed this question using a quantitative mathematical model of folate metabolism in rat liver cytoplasm. Variation in the rates of metabolic processes associated with folate metabolism (i.e., purine and pyrimidine synthesis, histidine catabolism, and influxes of formate and methionine) in the model revealed that folate metabolism is organized in a striking manner that enables activation or inhibition of each individual process independently of the metabolic fluxes in others. In mechanistic terms, this independence is based on the high activities of a group of enzymes involved in folate metabolism, which efficiently maintain close-to-equilibrium ratios between substrates and products of enzymatic reactions.

Phenotypes and circadian rhythm in utilization of formate in purine nucleotide biosynthesis de novo in adult humans

AIMS

Folate coenzymes and dependent enzymes introduce one carbon units at positions 2 (C(2)) and 8 (C(8)) of the purine ring during de novo biosynthesis. Formate is one source of one-carbon units. Although much is known about lower organisms, little data exists describing formate utilization for purine biosynthesis in humans.

MAIN METHODS

Mass-spectrometric analysis of urinary uric acid, the final purine catabolite, following 1.0 g oral doses of sodium [(13)C] formate was performed and detected (13)C enrichment at C(2) and C(8) separately.

KEY FINDINGS

Three phenotypes were suggested. One incorporates (13)C 0.72 to 2.0% into C(2) versus only 0 to 0.07% into C(8). Another incorporates only 0 to 0.05% (13)C into C(2) or C(8). A third phenotype incorporates (13)C into C(8) (0.15%) but C(2) incorporation (0.44%) is still greater. In subjects who incorporated (13)C formate into C(2), peak enrichment occurred in voids from 8-12 h (24 h clock) suggesting a circadian rhythm.

SIGNIFICANCE

Evidence that mammalian liver introduces C(8) and that C(2) is introduced in a non-hepatic site would explain our results. Our data are not similar to those in non-mammalian organisms or cells in culture and are not consistent with the hypothesis that formate from folate-dependent metabolism in mitochondria is a major one carbon source for purine biosynthesis. Timing of peak (13)C enrichment at C(2) corresponds to maximal DNA synthesis in human bone marrow. Phenotypes may explain the efficacy (or lack of) of certain anticancer and immunosuppressive drugs.

Compartmentalization of Mammalian folate-mediated one-carbon metabolism

The recognition that mitochondria participate in folate-mediated one-carbon metabolism grew out of pioneering work beginning in the 1950s from the laboratories of D.M. Greenberg, C.G. Mackenzie, and G. Kikuchi. These studies revealed mitochondria as the site of oxidation of one-carbon donors such as serine, glycine, sarcosine, and dimethylglycine. Subsequent work from these laboratories and others demonstrated the participation of folate coenzymes and folate-dependent enzymes in these mitochondrial processes. Biochemical and molecular genetic approaches in the 1980s and 1990s identified many of the enzymes involved and revealed an interdependence of cytoplasmic and mitochondrial one-carbon metabolism. These studies led to the development of a model of eukaryotic one-carbon metabolism that comprises parallel cytosolic and mitochondrial pathways, connected by one-carbon donors such as serine, glycine, and formate. Sequencing of the human and other mammalian genomes has facilitated identification of the enzymes that participate in this intercompartmental one-carbon metabolism, and animal models are beginning to clarify the roles of the cytoplasmic and mitochondrial isozymes of these enzymes. Identifying the mitochondrial transporters for the one-carbon donors and elucidating how flux through these pathways is controlled are two areas ripe for exploration.

Mthfd1 is an essential gene in mice and alters biomarkers of impaired one-carbon metabolism

Cytoplasmic folate-mediated one carbon (1C) metabolism functions to carry and activate single carbons for the de novo synthesis of purines, thymidylate, and for the remethylation of homocysteine to methionine. C1 tetrahydrofolate (THF) synthase, encoded by Mthfd1, is an entry point of 1Cs into folate metabolism through its formyl-THF synthetase (FTHFS) activity that catalyzes the ATP-dependent conversion of formate and THF to 10-formyl-THF. Disruption of FTHFS activity by the insertion of a gene trap vector into the Mthfd1 gene results in embryonic lethality in mice. Mthfd1gt/+ mice demonstrated lower hepatic adenosylmethionine levels, which is consistent with formate serving as a source of 1Cs for cellular methylation reactions. Surprisingly, Mthfd1gt/+ mice exhibited decreased levels of uracil in nuclear DNA, indicating enhanced de novo thymidylate synthesis, and suggesting that serine hydroxymethyltransferase and FTHFS compete for a limiting pool of unsubstituted THF. This study demonstrates the essentiality of the Mthfd1 gene and indicates that formate-derived 1Cs are utilized for de novo purine synthesis and the remethylation of homocysteine in liver. Further, the depletion of cytoplasmic FTHFS activity enhances thymidylate synthesis, affirming the competition between thymidylate synthesis and homocysteine remethylation for THF cofactors.

Human mitochondrial C1-tetrahydrofolate synthase: submitochondrial localization of the full-length enzyme and characterization of a short isoform

Mammalian mitochondrial C(1)-tetrahydrofolate (THF) synthase (MTHFDIL gene product) is a monofunctional 10-formyl-THF synthetase, lacking the 5,10-methylene-THF dehydrogenase and 5,10-methenyl-THF cyclohydrolase activities typically found in the trifunctional cytoplasmic proteins. Here, we report the submitochondrial localization of epitope-tagged human mitochondrial C(1)-THF synthase expressed in Chinese hamster ovary cells. Mitochondrial fractionation experiments show that human mitochondrial C(1)-THF synthase behaves as a peripheral membrane protein, tightly associated with the matrix side of the mitochondrial inner membrane. Inner mitochondrial membrane association was also observed for the endogenous mitochondrial C(1)-THF synthase in adult rat spleen. We also purified and characterized the recombinant protein product (short isoform) of the alternatively spliced short transcript of the mitochondrial isozyme. Methylene-THF dehydrogenase assays confirmed that the short isoform is not enzymatically active. The purified short isoform was used in the production of polyclonal antibodies specific for the mitochondrial isozyme. These antibodies detected endogenous full-length mitochondrial C(1)-THF synthase in mitochondria from adult rat spleen and human placenta, confirming the expression of the mitochondrial isozyme in adult mammalian tissues.

The folate metabolic enzyme ALDH1L1 is restricted to the midline of the early CNS, suggesting a role in human neural tube defects

Folate supplementation prevents up to 70% of human neural tube defects (NTDs), although the precise cellular and metabolic sites of action remain undefined. One possibility is that folate modulates the function of metabolic enzymes expressed in cellular populations involved in neural tube closure. Here we show that the folate metabolic enzyme ALDH1L1 is cell-specifically expressed in PAX3-negative radial glia at the midline of the neural tube during early murine embryogenesis. Midline restriction is not a general property of this branch of folate metabolism, as MTHFD1 displays broad and apparently ubiquitous expression throughout the neural tube. Consistent with previous work showing antiproliferative effects in vitro, ALDH1L1 upregulation during central nervous system (CNS) development correlates with reduced proliferation and most midline ALDH1L1(+) cells are quiescent. These data provide the first evidence for localized differences in folate metabolism within the early neural tube and suggest that folate might modulate proliferation via effects on midline Aldh1l1(+) cells. To begin addressing its role in neurulation, we analyzed a microdeletion mouse strain lacking Aldh1l1 and observed neither increased failure of neural tube closure nor detectable proliferation defects. Although these results indicate that loss-of-function Aldh1l1 mutations do not impair these processes in mice, the specific midline expression of ALDH1L1 and its ability to dominantly suppress proliferation in a folate responsive manner may suggest that mutations contributing to disease are gain-of-function, rather than loss-of-function. Moreover, a role for loss-of-function mutations in human NTDs remains possible, as Mthfr null mice do not develop NTDs even though MTHFR mutations increase human NTD risk.

Enzymatic characterization of human mitochondrial C1-tetrahydrofolate synthase

A human mitochondrial isozyme of C1-tetrahydrofolate (THF) synthase was previously identified by its similarity to the human cytoplasmic C1-THF synthase. All C1-THF synthases characterized to date, from yeast to human, are trifunctional, containing the activities of 5,10-methylene-THF dehydrogenase, 5,10-methenyl-THF cyclohydrolase, and 10-formyl-THF synthetase. Here we report on the enzymatic characterization of the recombinant human mitochondrial isozyme. Enzyme assays of purified human mitochondrial C1-THF synthase protein revealed only the presence of 10-formyl-THF synthetase activity. Gel filtration and crosslinking studies indicated that human mitochondrial C1-THF synthase exists as a homodimer in solution. Steady-state kinetic characterization of the 10-formyl-THF synthetase activity was performed using (6R,S)-H4-PteGlu1, (6R,S)-H4-PteGlu3, and (6R,S)-H4-PteGlu5 substrates. The (6R,S)-H4-PteGlun Km dropped from greater than 500 microM for the monoglutamate to 15 microM and 3.6 microM for the tri- and pentaglutamates, respectively. The Km values for formate and ATP also are lowered when THF polyglutamates are used. The formate Km dropped 79-fold and the ATP Km dropped more than 5-fold when (6R,S)-H4-PteGlu5 was used as the substrate in place of (6R,S)-H4-PteGlu1.

Characterization of the rat cytoplasmic C1-tetrahydrofolate synthase gene and analysis of its expression in liver regeneration and fetal development

The eukaryotic trifunctional enzyme, C(1)-tetrahydrofolate (THF) synthase, interconverts folic acid derivatives between various oxidation states and is critical for normal cellular function, growth, and differentiation. Using a rat C(1)-THF synthase cDNA and synthetic oligonucleotides, the rat C(1)-THF synthase gene was isolated and characterized. The gene consists of 28 exons and spans 67.5 kbp. Primer extension, RNase protection, and rapid amplification of cDNA ends (RACE) experiments indicate the presence of multiple transcription start points (tsp) within a 250-bp window located between 50 and 300 bp upstream from the start codon. The 5' flanking region is devoid of a TATA consensus sequence motif, but putative regulatory elements, including NF-kappabeta, HNF-4alpha1, RARalpha1, C/EBP, and PPAR are present in the promoter region. The 5' flanking region also contains two sets of tetranucleotide repeats and two short interspersed nuclear elements (SINES). The initial 2500 bp of 5' flanking sequences of the rat and mouse cytoplasmic C(1)-THF synthase genes share 70% identity. However, comparison with the human gene from the Human Genome Data Bank revealed no significant homology in the 5' flanking region. The gene structure characterization led to the identification of a pseudogene that is 94% identical to the C(1)-THF synthase gene and probably diverged 10-12 million years ago. In addition, the gene expression patterns of C(1)-THF synthase were investigated during liver regeneration and liver and kidney organogenesis, two highly regulated events. In both processes, C(1)-THF synthase expression correlated with increased nucleotide metabolism. This pattern suggests that the gene is regulated in response to changes in the demand for folate-dependent one-carbon units.

Human mitochondrial C1-tetrahydrofolate synthase: gene structure, tissue distribution of the mRNA, and immunolocalization in Chinese hamster ovary calls

C1-tetrahydrofolate (THF) synthase is a trifunctional enzyme found in eukaryotes that contains the activities 10-formyl-THF synthetase, 5,10-methenyl-THF cyclohydrolase, and 5,10-methylene-THF dehydrogenase. The cytoplasmic isozyme of C1-THF synthase is well characterized in a number of mammals, including humans; but a mitochondrial isozyme has been previously identified only in the yeast Saccharomyces. Here, we report the identification and characterization of the human gene encoding a functional mitochondrial C1-THF synthase. The gene spans 236 kilobase pairs on chromosome 6 and consists of 28 exons plus one alternative exon. The gene encodes a protein of 978 amino acids, including an N-terminal mitochondrial targeting sequence. The mitochondrial isozyme is 61% identical to the human cytoplasmic isozyme. Expression of the gene was detected in most human tissues, but transcripts were highest in placenta, thymus, and brain. Two mRNAs were detected, a 3.6-kb transcript and a 1.1-kb transcript, and both transcripts were observed in varying ratios in each tissue. The shorter transcript results from an alternative splicing event, where exon 7 is spliced to exon 8a instead of exon 8. Exon 8a is derived from an exonized Alu sequence, sharing no homology with exon 8 of the long transcript, and encodes just 15 amino acids followed by a stop codon and a polyadenylation signal. This short transcript potentially encodes a bifunctional enzyme lacking 10-formyl-THF synthetase activity. Both transcripts initiate at the same 5'-site, 107 nucleotides up-stream of the ATG start codon. The full-length (2934 bp) cDNA fused to a C-terminal V5 epitope tag was expressed in Chinese hamster ovary cells. Immunoblots of subfractionated cells revealed a 107-kDa protein only in the mitochondrial fractions of these cells, confirming the mitochondrial localization of the protein. Yeast cells expressing the full-length human cDNA exhibited elevated 10-formyl-THF synthetase activity, confirming its identification as the human mitochondrial C1-THF synthase.

Inflammatory bowel disease is associated with changes of enterocytic junctions

Changes of the intestinal mucosal barrier are considered to play a role in the pathogenesis of inflammatory bowel disease (IBD). Our experiments were designed to identify dysregulation of epithelial junctional molecules in the IBD intestinum and to address whether altered expression of these molecules is a primary event in IBD or a phenomenon secondary to the inflammatory process. Noninflamed and inactively and actively inflamed mucosal tissues from patients with ulcerative colitis or Crohn's disease as well as tissues from control subjects were analyzed for the expression of junctional molecules by different methods. Marked downregulation of junctional proteins and their respective mRNAs was observed in actively inflamed IBD tissues. In IBD tissues with inactive inflammation, only a few junctional molecules such as E-cadherin and alpha-catenin were affected, whereas expression of desmosomal or tight junction-associated proteins appeared almost unchanged. In noninflamed IBD tissues, junctional protein expression was not different from that seen in normal control subjects. In IBD, downregulation of junctional molecule expression is apparently associated with the inflammatory process and does not likely represent a primary phenomenon.

Transcriptional regulation of murine NADP(+)-dependent methylenetetrahydrofolate dehydrogenase-cyclohydrolase-synthetase

The cytosolic NADP(+)-dependent methylenetetrahydrofolate dehydrogenase-methenyltetrahydrofolate cyclohydrolase-formyltetrahydrofolate synthetase is ubiquitously expressed in all mouse tissues and cell lines examined. Northern analyses of the RNA indicated that there is an extensive variation in the levels of mRNA in different tissues. However, the gene is refractory to induction by serum, phorbol esters or growth factors in cultured fibroblasts. The mRNA of the NADP(+)-dependent trifunctional enzyme is stabilized post-transcriptionally by insulin-like growth factor-1.

Molecular genetic analysis of Saccharomyces cerevisiae C1-tetrahydrofolate synthase mutants reveals a noncatalytic function of the ADE3 gene product and an additional folate-dependent enzyme

In eucaryotes, 10-formyltetrahydrofolate (formyl-THF) synthetase, 5,10-methenyl-THF cyclohydrolase, and NADP(+)-dependent 5,10-methylene-THF dehydrogenase activities are present on a single polypeptide termed C1-THF synthase. This trifunctional enzyme, encoded by the ADE3 gene in the yeast Saccharomyces cerevisiae, is thought to be responsible for the synthesis of the one-carbon donor 10-formyl-THF for de novo purine synthesis. Deletion of the ADE3 gene causes adenine auxotrophy, presumably as a result of the lack of cytoplasmic 10-formyl-THF. In this report, defined point mutations that affected one or more of the catalytic activities of yeast C1-THF synthase were generated in vitro and transferred to the chromosomal ADE3 locus by gene replacement. In contrast to ADE3 deletions, point mutations that inactivated all three activities of C1-THF synthase did not result in an adenine requirement. Heterologous expression of the Clostridium acidiurici gene encoding a monofunctional 10-formyl-THF synthetase in an ade3 deletion strain did not restore growth in the absence of adenine, even though the monofunctional synthetase was catalytically competent in vivo. These results indicate that adequate cytoplasmic 10-formyl-THF can be produced by an enzyme(s) other than C1-THF synthase, but efficient utilization of that 10-formyl-THF for purine synthesis requires a nonenzymatic function of C1-THF synthase. A monofunctional 5,10-methylene-THF dehydrogenase, dependent on NAD+ for catalysis, has been identified and purified from yeast cells (C. K. Barlowe and D. R. Appling, Biochemistry 29:7089-7094, 1990). We propose that the characteristics of strains expressing full-length but catalytically inactive C1-THF synthase could result from the formation of a purine-synthesizing multienzyme complex involving the structurally unchanged C1-THF synthase and that production of the necessary one-carbon units in these strains is accomplished by an NAD+ -dependent 5,10-methylene-THF dehydrogenase.

Molecular genetic analysis of Saccharomyces cerevisiae C1-tetrahydrofolate synthase mutants reveals a noncatalytic function of the ADE3 gene product and an additional folate-dependent enzyme

In eucaryotes, 10-formyltetrahydrofolate (formyl-THF) synthetase, 5,10-methenyl-THF cyclohydrolase, and NADP(+)-dependent 5,10-methylene-THF dehydrogenase activities are present on a single polypeptide termed C1-THF synthase. This trifunctional enzyme, encoded by the ADE3 gene in the yeast Saccharomyces cerevisiae, is thought to be responsible for the synthesis of the one-carbon donor 10-formyl-THF for de novo purine synthesis. Deletion of the ADE3 gene causes adenine auxotrophy, presumably as a result of the lack of cytoplasmic 10-formyl-THF. In this report, defined point mutations that affected one or more of the catalytic activities of yeast C1-THF synthase were generated in vitro and transferred to the chromosomal ADE3 locus by gene replacement. In contrast to ADE3 deletions, point mutations that inactivated all three activities of C1-THF synthase did not result in an adenine requirement. Heterologous expression of the Clostridium acidiurici gene encoding a monofunctional 10-formyl-THF synthetase in an ade3 deletion strain did not restore growth in the absence of adenine, even though the monofunctional synthetase was catalytically competent in vivo. These results indicate that adequate cytoplasmic 10-formyl-THF can be produced by an enzyme(s) other than C1-THF synthase, but efficient utilization of that 10-formyl-THF for purine synthesis requires a nonenzymatic function of C1-THF synthase. A monofunctional 5,10-methylene-THF dehydrogenase, dependent on NAD+ for catalysis, has been identified and purified from yeast cells (C. K. Barlowe and D. R. Appling, Biochemistry 29:7089-7094, 1990). We propose that the characteristics of strains expressing full-length but catalytically inactive C1-THF synthase could result from the formation of a purine-synthesizing multienzyme complex involving the structurally unchanged C1-THF synthase and that production of the necessary one-carbon units in these strains is accomplished by an NAD+ -dependent 5,10-methylene-THF dehydrogenase.

Immunolocalization of C1-tetrahydrofolate synthase in the rat kidney

The distribution of the trifunctional enzyme C1-tetrahydrofolate synthase (C1-THF synthase) was examined in the rat kidney by immunolocalization with anti-C1-THF synthase serum using the peroxidase-antiperoxidase method. C1-THF synthase immunoreactivity was detected in both distal and proximal epithelial cells. Staining of the distal tubule epithelia was more intense and granular whereas staining of the proximal tubule epithelia was diffuse. All cells of the cortical collecting duct showed positive granular staining. In the outer medullary collecting duct, the intercalated cells showed intense granular cytoplasmic staining and the principal cells were either negative or weakly positive. The ascending thick limb of Henle's loop was also positive. Glomeruli and the inner medulla showed no staining for C1-THF synthase.