Folate Pool Research Articles

Oxidative Stress is Associated with Genetic Polymorphisms in One-Carbon Metabolism in Coronary Artery Disease

In view of growing body of evidence favouring the association of aberrations in one-carbon metabolism and oxidative stress in the aetiology of coronary artery disease (CAD), we investigated the risk associated with polymorphisms regulating the folate uptake and transport such as the glutamate carboxypeptidase II (GCPII) C1561T, reduced folate carrier 1 (RFC1) G80A and cytosolic serine hydroxymethyltransferase (cSHMT) C1420T. We further evaluated the impact of seven putatively functional polymorphisms of this pathway on oxidative stress markers. Genotyping was performed on 288 CAD cases and 266 healthy controls along with the dietary folate assessment. GCPII C1561T polymorphism was found to be an independent risk factor (OR 2.71, 95% CI 1.47-4.98) for CAD, whereas cSHMT C1420T conferred protection (OR 0.51, 95% CI 0.37-0.70). Oxidative stress markers like the plasma levels of malondialdehyde, protein carbonyls and 8-oxo-deoxyguanosine were significantly increased and total glutathione was significantly decreased in CAD cases. Elevated oxidative stress was observed in subjects carrying GCPII 1561T and MTRR 66A-variant alleles and low oxidative stress was observed in the subjects carrying cSHMT 1420T and TYMS 5'-UTR 2R allele. GCPII C1561T, MTHFR C677T and MTRR A66G polymorphisms were observed to influence the homocysteine levels (P<0.05). SHMT and TYMS variants were found to decrease oxidative stress by increasing the folate pool (r=0.38, P=0.003) and also by increasing the antioxidant status (r=0.28, P=0.03). Influence of dietary folate status was not observed. Overall, this study revealed elevated oxidative stress that was associated with the aberrations in one-carbon metabolism which could possibly influence the CAD risk.

The Impact of Folate Status on the Efficacy of Colorectal Cancer Treatment

Over the past three decades, numerous reports have addressed several aspects of drug resistance phenomena. However, little is known regarding the impact that dietary components and nutritional supplements have on the mechanisms of resistance that malignant cells develop to chemotherapeutic agents. The increased fortification of cereals, grains and bread with folic acid (FA) has resulted in a marked rise in folate levels in blood and tissues. Vitamin fortification that includes FA is rather commonly used by cancer patients, but FA is also used to protect against pemetrexed induced side effects in the treatment of non-small cell lung cancer and mesothelioma or that of the antifolate methotrexate in rheumatoid arthritis. Moreover, the reduced folate leucovorin (LV, 5-formyltetrahydrofolate) is also used along with 5-fluorouracil in the treatment of colorectal cancer. Likewise, LV is used to reduce toxicity of methotrexate in the treatment of leukemia. FA can also increase efficacy of unrelated regimens, containing cisplatin. Hence there is growing evidence that dietary supplements as folic acid, can mimic, intensify, or attenuate the effects of unrelated chemotherapeutic agents. The aim of this review is to highlight some new insights in the cellular and molecular mechanisms affected by folate status, leading to chemotherapy resistance, especially towards antifolates in colorectal cancer treatment. This encompasses the effect of folate status on drug export, as well as on the increased expression of mutated target enzymes involved in folate metabolism and on the augmentation of cellular folate pools that impair polyglutamylation of antifolates, ultimately affecting treatment efficacy.

Natural Variation of Folate Content and Composition in Spinach (Spinacia oleracea) Germplasm

Breeding to increase folate levels in edible parts of plants, termed folate biofortification, is an economical approach to fight against folate deficiency in humans, especially in the developing world. Germplasm with elevated folates are a useful genetic source for both breeding and direct use. Spinach is one of the well-know vegetables that contains a relatively high amount of folate. Currently, little is known about how much folate, and their composition varies in different spinach accessions. The aim of this study was to investigate natural variation in the folate content and composition of spinach genotypes grown under controlled environmental conditions. The folate content and composition in 67 spinach accessions were collected from the United States Department of Agriculture (USDA) and Asian Vegetable Research and Development Center (AVRDC) germplasm collections according to their origin, grown under control conditions to screen for natural diversity. Folates were extracted by a monoenzyme treatment and analyzed by a validated liquid chromatography (LC) method. The total folate content ranged from 54.1 to 173.2 μg/100 g of fresh weight, with 3.2-fold variation, and was accession-dependent. Four spinach accessions (PI 499372, NSL 6095, PI 261787, and TOT7337-B) have been identified as enriched folate content over 150 μg/100 g of fresh weight. The folate forms found were H(4)-folate, 5-CH(3)-H(4)-folate, and 5-HCO-H(4)-folate, and 10-CHO-folic acid also varied among different accessions and was responsible for variation in the total folate content. The major folate vitamer was represented by 5-CH(3)-H(4)-folate, which on average accounted for up to 52% of the total folate pool. The large variation in the total folate content and composition in diverse spinach accessions demonstrates the great genetic potential of diverse genotypes to be exploited by plant breeders.

Dietary Folate Deficiency Blocks Prostate Cancer Progression in the TRAMP Model

Dietary folate is essential in all tissues to maintain several metabolite pools and cellular proliferation. Prostate cells, due to specific metabolic characteristics, have increased folate demand to support proliferation and prevent genetic and epigenetic damage. Although several studies have found that dietary folate interventions can affect colon cancer biology in rodent models, its impact on prostate is unknown. The purpose of this study was to determine whether dietary folate manipulation, possibly being of primary importance for prostate epithelial cell metabolism, could significantly affect prostate cancer progression. Strikingly, mild dietary folate depletion arrested prostate cancer progression in 25 of 26 transgenic adenoma of the mouse prostate (TRAMP) mice, in which tumorigenesis is prostate-specific and characteristically aggressive. The significant effect on prostate cancer growth was characterized by size, grade, proliferation, and apoptosis analyses. Folate supplementation had a mild, nonsignificant, beneficial effect on grade. In addition, characterization of folate pools (correlated with serum), metabolite pools (polyamines and nucleotides), genetic and epigenetic damage, and expression of key biosynthetic enzymes in prostate tissue revealed interesting correlations with tumor progression. These findings indicate that prostate cancer is highly sensitive to folate manipulation and suggest that antifolates, paired with current therapeutic strategies, might significantly improve treatment of prostate cancer, the most commonly diagnosed cancer in American men.

Effect of Genetic Polymorphisms in the Folate Pathway on Methotrexate Therapy in Rheumatic Diseases

Methotrexate (MTX) is the first-line treatment for rheumatoid arthritis and is frequently used in the management of other forms of inflammatory arthritis. It is currently challenging to predict which patients will achieve adequate disease control and which patients will develop adverse effects while taking MTX. As an analog of dihydrofolic acid, MTX enters cells through the reduced folate carrier-1 protein, and is polyglutamated. MTX polyglutamates inhibit key enzymes in the folate pathway to produce an anti-inflammatory effect. It has been suggested that genetic polymorphisms in the folate pathway may influence intracellular folate and MTX polyglutamates pools, and thus MTX response. However, studies to identify genetic predictors have yielded inconclusive results. Nonreplication across studies has been attributed to insufficient statistical power as well as pharmacological and clinical confounders. Prospective studies, standardizing the definitions of response and toxicity, and application of genome-wide approaches may advance the search for genetic predictors of MTX response.

Aberrations in one-carbon metabolism induce oxidative DNA damage in sporadic breast cancer

The authors investigated the role of dietary micronutrients and eight functional polymorphisms of one-carbon metabolism in modulating oxidative stress in sporadic breast cancer. PCR-restriction fragment length polymorphism (RFLP) and PCR-amplified fragment length polymorphism (AFLP) methods were used for genetic analysis in 222 sporadic breast cancer cases and 235 controls. Standardized food frequency questionnaire was used for dietary micronutrient assessment. 8-oxo-2'-deoxyguanosine (8-oxodG), folate, and estradiol were estimated using commercial ELISA kits. Reverse-phase HPLC coupled with fluorescence detector was used for plasma homocysteine analysis. Total glutathione was estimated using Ellman's method. Reduced folate carrier 1 (RFC1) G80A and methylenetetrahydrofolate reductase (MTHFR) C677T were associated with risks of 1.34 (95% CI 1.01-1.79)- and 1.84 (95% CI 1.14-3.00)-folds, respectively, for sporadic breast cancer while cytosolic serine hydroxymethyl transferase (cSHMT) C1420T was associated with reduced risk (OR 0.71, 95% CI 0.53-0.94). Significant increase in plasma 8-oxo-2'-deoxyguanosine (P < 0.004) and homocysteine (P < 0.0001); and significant decrease in total glutathione (P < 0.01) and dietary folate (P = 0.006) was observed in cases than in controls. Oxidative DNA damage showed direct association with menopause (P = 0.02), RFC1 G80A (P < 0.05) and homocysteine (P < 0.0001); and inverse association with dietary folate (P < 0.0001), plasma folate (P < 0.0001), cSHMT C1420T (P < 0.05) and glutathione (P < 0.001). To conclude, the aberrations in one-carbon metabolism induce oxidative stress in sporadic breast cancer either by affecting the folate pool or by impairing remethylation.

Folate Polyglutamylation is Required for Rice Seed Development

Abstract In plants, polyglutamylated folate forms account for a significant proportion of the total folate pool. Polyglutamylated folate forms are produced by the enzyme folylpolyglutamate synthetase (FPGS). The FPGS enzyme is encoded by two genes in rice, Os03g02030 and Os10g35940. Os03g02030 represents the major expressed form in developing seed. To determine the function of this FPGS gene in rice, a T-DNA knockout line was characterised. Disrupting Os03g02030 gene expression resulted in delayed seed filling. LC-MS/MS-based metabolite profiling revealed that the abundance of mono- and polyglutamylated folate forms was significantly decreased in seeds of the knockout line. RT-qPCR detected an increase in the transcript abundance of folate biosynthesis genes in seed of the knockout plant, whereas the folate deglutamating enzyme γ-glutamyl hydrolase mRNA level was reduced. Our study has uncovered a novel role for folate polyglutamylation during rice seed development and a potential feedback mechanism to maintain folate abundance.

Abstract 1607: Effect of folate and GGH modulation on chemosensitivity of colon and breast cancer cells to 5-fluorouracil and methotrexate

Abstract Background: γ-Glutamyl hydrolase (GGH) removes terminal glutamates of polyglutamylated folates and antifolates, thereby facilitating hydrolysis and efflux of folates and antifolates from the cell. Therefore, GGH plays an important role in regulating intracellular folates and antifolates for optimal folate-dependent one-carbon transfer reactions and antifolate-induced cytotoxic effects, respectively. We have previously reported that GGH modulation significantly affects chemosensitivity of colon and breast cancer cells to 5-fluorouracil (5FU) and methotrexate (MTX) by changing intracellular retention of a folate cofactor (5,10-methylenetetrahydrofolate) necessary for the cytotoxic effects of 5FU and by changing intracellular retention of MTX, respectively. We investigated whether the GGH modulation-induced changes in chemosensitivity might be counterbalanced by GGH modulation-induced changes in polyglutamylation of other intracellular folate cofactors and total intracellular folate pools. Methods: Human HCT116 colon and MDA-MB-435 breast cancer cells were stably transfected with the sense GGH cDNA or GGH-targeted siRNA, respectively, to generate an in vitro model of GGH overexpression and inhibition. In vitro chemosensitivity to 5FU and MTX under 2.3 μM folic acid (FA) and 50 nM and 100 nM of 5-methyltetrahydrofolate (5MTHF) was determined. Results: GGH overexpression was associated with lower total and long-chain polyglutamylated intracellular folate concentrations, thymidylate synthase (TS) catalytic enzyme activity, and dihydrofolate reductase (DHFR) protein expression than controls, while GGH inhibition showed higher total and long-chain polyglutamylated intracellular folate concentrations, TS activity, and DHFR protein expression than controls (P&amp;lt;0.05). In HCT116 cells, GGH overexpression decreased chemosensitivity to 5FU and MTX at 2.3 uM FA while it enhanced chemosensitivity to 5FU and MTX at 50 nM 5MTHF (P&amp;lt;0.05). At all concentrations, GGH inhibition increased chemosensitivity to 5FU while it decreased chemosensitivity to MTX (P&amp;lt;0.05). In MDA-MB-435 cells, GGH overexpression decreased chemosensitivity to 5FU and MTX at all concentration (P&amp;lt;0.05). GGH inhibition increased chemosensitivity to 5FU at 2.3 uM FA and 100 nM 5MTHF whereas it decreased chemosensitivity to MTX at all concentrations (P&amp;lt;0.05). Conclusions: As proof of principle, we provide functional evidence that GGH overexpression and inhibition can modulate chemosensitivity of colon and breast cancer cells to 5FU and MTX. These GGH modulation-induced changes in chemosensitivity are further influenced by different folate forms and concentrations. Our data suggest that both GGH modulation and folate status affect chemosensitivity of colon and breast cancer cells to 5FU and MTX. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 1607.

Abstract 73: Mitochondrial 10-formyltetrahydrofolate dehydrogenase: A novel enzyme in folate metabolism

Abstract Cytosolic 10-formyltetrahydrofolate dehydrogenase (FDH, Aldh1L1) is an important regulator of intracellular folate pools, which displays antiproliferative effects in cancer cells. We have identified a gene at the chromosome locus 12q24.11 in the human genome, the product of which has 87% sequence similarity with cytosolic FDH. This protein has an extra amino-terminal sequence of 22 amino acid residues enriched in arginines, which is predicted to be a mitochondrial translocation signal. The mitochondrial targeting function of the leader has been confirmed in Cos7 cells: green fluorescent protein (GFP)-tagged at the amino-terminus with the leader, localizes to mitochondria. Transfection of Cos7 or A549 cell lines with a construct, in which GFP has been introduced between the leader sequence and the rest of the putative mitochondrial FDH (mtFDH), has also shown mitochondrial localization, suggesting that the identified gene encodes a mitochondrial enzyme. To evaluate the abundance of mtFDH, we have measured its mRNA levels in a wide array of human tissues by real-time PCR, and compared them to the levels of mRNA that encode cytosolic FDH. While cytosolic FDH mRNA is highest in liver, kidney and pancreas, mtFDH mRNA is most highly expressed in pancreas, heart and brain, but not in liver or kidney. In contrast to the cytosolic enzyme, which is non detectable in human cancer cell lines, the presence of mtFDH mRNA was demonstrated in A549 and PC3 cells by conventional and real-time PCR. The presence of the endogenous enzyme in mitochondria of A549 cells has been further confirmed using specific polyclonal antibody generated against purified recombinant mtFDH. We have also shown that recombinant mtFDH, similar to the cytosolic enzyme, catalyzes NADP+-dependent oxidation of the 10-formyltetrahydrofolate to tetrahydrofolate and CO2. Thus, the enzyme is a likely source of CO2 production in mitochondria and we propose that it plays an essential role in distribution of one-carbon groups between cytosolic and mitochondrial compartments of the cell. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 73.

CoFactor: Folate Requirement for Optimization of 5-Fluouracil Activity in Anticancer Chemotherapy

Intracellular reduced folate exists as a “pool” of more than 6 interconvertable forms. One of these forms, 5,10 methylenetetrahydrofolic acid (CH2THF), is the key one-carbon donor and reduced folate substrate for thymidylate synthase (TS). This pathway has been an important target for chemotherapy as it provides one of the necessary nucleotide substrates for DNA synthesis. The fluoropyrimidine 5-fluorouracil (5-FU) exerts its main cytotoxic activity through TS inhibition. Leucovorin (5-formyltetrahydrofolate; LV) has been used to increase the intracellular reduced folate pools and enhance TS inhibition. However, it must be metabolized within the cell through multiple intracellular enzymatic steps to form CH2THF. CoFactor (USAN fotrexorin calcium, (dl)-5,10,-methylenepteroyl-monoglutamate calcium salt) is a reduced folate that potentiates 5-FU cytotoxicity. According to early clinical trials, when 5-FU is modulated by CoFactor instead of LV, there is greater anti-tumor activity and less toxicity. This review presents the emerging role of CoFactor in colorectal and nongastrointestinal malignancies.

Development of a minimal growth medium for Lactobacillus plantarum

A medium with minimal requirements for the growth of Lactobacillus plantarum WCFS was developed. The composition of the minimal medium was compared to a genome-scale metabolic model of L. plantarum. By repetitive single omission experiments, two minimal media were developed: PMM5 (true minimal medium) and PMM7 [a pseudominimal medium, supporting proper biomass formation of 350 mg l(-1) dry weight (DW)]. The specific growth rate of L. plantarum on PMM7 was found to be 50% and 63% lower when compared to growth on established growth media (chemically defined medium and MRS, respectively). Using a genome-scale metabolic model of L. plantarum, it was predicted that PMM5 and PMM7 would not support the growth of L. plantarum. This is because the biosynthesis of para-aminobenzoic acid (pABA) was predicted to be essential for growth. The discrepancy in simulated growth and experimental growth on PMM7 was further investigated for pABA; a molecule which plays an important role in folate production. The growth performance and folate production were determined on PMM7 in the presence and absence of pABA. It was found that a 12,000-fold reduction in folate pools exerted no influence on formation of biomass or growth rate of L. plantarum cultures when grown in the absence of pABA. Largely reduced folate production pools do not have an effect on the growth of L. plantarum, showing that L. plantarum makes folate in a large excess. These experiments illustrate the importance of combining genome-scale metabolic models with growth experiments on minimal media.

249 Biology of mesothelioma

Members of the ATP-binding cassette (ABC) transporters including P-glycoprotein (Pgp/ABCB1), multidrug resistance proteins (MRPs/ABCC) as well as breast cancer resistance protein (BCRP/ABCG2) function as ATP-dependent drug efflux transporters, which form a unique defense network against multiple chemotherapeutic drugs and cellular toxins. Among antitumor agents is the important group of folic acid antimetabolites known as antifolates. Antifolates such as methotrexate (MTX), pemetrexed and raltitrexed exert their cytotoxic activity via potent inhibition of folate-dependent enzymes essential for purine and pyrimidine nucleotide biosynthesis and thereby block DNA replication. Overexpression of MRPs and BCRP confers resistance upon malignant cells to various hydrophilic and lipophilic antifolates. Apart from their central role in mediating resistance to antifolates and other anticancer drugs, MRPs and BCRP have been recently shown to transport naturally occurring reduced folates. This was inferred from various complementary systems as follows: (a) Cell-free systems including ATP-dependent uptake of radiolabeled folate/MTX into purified inside-out membrane vesicles from stable transfectants and/or cells overexpressing these transporters, (b) Decreased accumulation of radiolabeled folate/MTX in cultured tumor cells overexpressing these transporters, as well as (c) In vivo rodent models such as Eisi hyperbillirubinemic rats (EHBR) that hereditarily lack MRP2 in their canalicular membrane and thereby display a bile that is highly deficient in various reduced folate cofactors and MTX, when compared with wild type Sprague–Dawley (SD) rats. In all cases, these folate/antifolate transporters functioned as high capacity, low affinity ATP-driven exporters. While the mechanism of cellular retention of (anti)folates is mediated via (anti)folylpolyglutamylation, certain efflux transporters including MRP5 (ABCC5) and BCRP were shown to transport both mono-, di- as well as triglutamate derivatives of MTX and folic acid. Furthermore, overexpression of MRPs and BCRP has been shown to result in decreased cellular folate pools, whereas loss of ABC transporter expression brought about a significant expansion in the intracellular reduced folate pool. The latter finding has important implications to antifolate-based chemotherapy as an augmented cellular folate pool results in a significant level of resistance to certain antifolates. Hence, the aims of the present review are: (a) To summarize and discuss the cumulative evidence supporting a functional role for various multidrug resistance efflux transporters of the ABC superfamily which mediate resistance to hydrophilic and lipophilic antifolates, (b) To describe and evaluate the recent data suggesting a role for these efflux transporters in regulation of cellular folate homeostasis under folate replete and deplete conditions. Furthermore, novel developments and future perspectives regarding the identification of novel antifolate target proteins and mechanisms of action, as well as rationally designed emerging drug combinations containing antifolates along with receptor tyrosine kinase inhibitors are being discussed.

Depleted folate pool and dysfunctional mitochondria associated with defective mitochondrial folate proteins sensitize Chinese ovary cell mutants to tert-butylhydroperoxide-induced oxidative stress and apoptosis

The functional role of mitochondrial (mt) folate-associated proteins in mammalian cells is not clearly understood. We investigated the respiratory function and apoptosis phenotype of Chinese hamster ovary (CHO) mutant cells with defective mt serine hydroxymethyltransferase (SHMT) activities (glyA) or with defective mt folate transporter (glyB) in the absence/presence of oxidant challenge. The mechanisms underlying their aberrant phenotypes were explored. Compared with CHOK1 wild-type cells, both mutants carried dysfunctional mitochondria with reduced respiratory complex IV activity and depolarized mt membrane potential ( P<.05). Elevated superoxide levels and accumulated mtDNA large deletions were observed in glyB in association with a depleted compartmental folate pool ( P<.05). tert-Butylhydroperoxide (tBH) treatment at 50 μM for 72 h significantly depleted mt and cytosolic folate levels, impaired antioxidant defenses, and aggravated mt oxidative dysfunction in both mutants ( P<.05), more severely in glyB. Only tBH-treated glyB cells displayed an elevated ratio of mt Bax/Bcl-2, activation of procaspases 9 and 3, and apoptosis promotion. The apoptotic phenotype of tBH-treated glyB could be partially corrected by folate supplementation (10–1000 μM), which enriched compartmental folate levels, restored antioxidant defenses, eliminated mt oxidative injuries, and normalized mt membrane function. Our data identify previously unrecognized roles of mt folate-associated proteins in the protection of mitochondria against oxidative insults. Defective mt folate transporter sensitized glyB cells to elevated oxidative stress and tBH-induced apoptosis, partly mediated by depleted compartmental folate and mt dysfunction. Defective mt SHMT sensitized glyA to respiratory dysfunction and tBH-induced oxidative injury without apoptosis promotion.

Tissue Levels of Reduced Folates in Patients with Colorectal Carcinoma After Infusion of Folinic Acid at Various Dose Levels

In patients with colorectal cancer (CRC), modulation of 5-fluorouracil (5-FU) by folinic acid (FA) improves response rate and overall survival compared with 5-FU alone. However, the optimal dose of FA is still debated. We investigated reduced folate pools in various tissues from patients with CRC without and after prior administration of FA. A total of 186 specimens (normal colorectal mucosa, primary colorectal tumor, normal liver, and liver metastases) from 86 consecutive patients with CRC were obtained and investigated for levels of reduced folates. Before surgery, patients did (n = 52) or did not (n = 34) receive FA as 15-minute i.v. infusion. FA-dose levels chosen were 20, 200, or 500 mg/m2. Tissue lysates were analyzed for reduced folate levels by means of the tritium release assay. In normal mucosa, combined pools of tetrahydrofolate and 5,10-methylenetetra-hydrofolate were significantly elevated at all FA dose levels compared with untreated controls. In primary tumor, only 200 and 500 mg/m2 FA resulted in a significant increase of reduced folates with highest values measured after 500 mg/m2 FA. In specimens from normal liver, folate levels did not increase after administration of FA. By contrast, in specimens from liver metastases, reduced folate levels were low without FA pretreatment compared with levels from normal liver samples. Infusion of 500 mg/m2 FA caused a significant increase of reduced folate levels in liver metastases. From a pharmacologic point of view, high-dose FA should be recommended for optimal modulation of 5-FU in patients with mCRC.

A Genome-Wide and Metabolic Analysis Determined the Adaptive Response of Arabidopsis Cells to Folate Depletion Induced by Methotrexate

Control of folate homeostasis is essential to sustain the demand for one-carbon (C1) units that are necessary for major biological functions, including nucleotide synthesis and methylation reactions. In this study, we analyzed the genome-wide and metabolic adaptive response of Arabidopsis (Arabidopsis thaliana) cells to folate depletion induced by the antifolate methotrexate. Drug treatment induced a response typical to xenobiotic stress and important changes in folate content and composition. This resulted in a reduction of cell division and primary energy metabolism that was likely associated with perturbation of nucleotide homeostasis. Through a modification of serine metabolism, folate depletion also induced O-acetylserine accumulation and mimicked sulfur deficiency response. The major adaptive response to folate limitation concerned the composition of the folate pool rather than the intracellular level of cofactors. Thus, no significant change in the expression of genes involved in cofactor synthesis, degradation, or trafficking was observed. However, changes in the distribution of C1 derivative pools and increased expression levels for transcripts coding enzymes manipulating C1 moieties in plastids suggested a reorientation of C1 units toward the synthesis of purine and thymidylate. Also, no genomic or metabolic adaptation was built up to counterbalance the major impairment of the methyl index, which controls the efficiency of methylation reactions in the cell. Together, these data suggested that the metabolic priority of Arabidopsis cells in response to folate limitation was to shuttle the available folate derivatives to the synthesis of nucleotides at the expense of methylation reactions.

Role of Global Regulators and Nucleotide Metabolism in Antibiotic Tolerance in Escherichia coli

Bacterial populations produce a small number of persister cells that exhibit multidrug tolerance. Persister cells are largely responsible for the antibiotic recalcitrance of biofilm infections. The mechanism of persister cell formation largely remains unknown due to the challenges in identifying persister genes. We screened an ordered comprehensive library of 3,985 Escherichia coli knockout strains to identify mutants with altered antibiotic tolerance. Stationary-state cultures in 96-well plates were exposed to ofloxacin at a concentration which allows only tolerant persister cells to survive. The persister cell level of each culture was determined. A total of 150 mutants with decreased persistence were identified in the initial screen, and subsequent validation confirmed that neither the growth rate nor the ofloxacin MIC was affected for 10 of them. The genes affected in these strains were dnaJ and dnaK (chaperones), apaH (diadenosine tetraphosphatase), surA (peptidyl-prolyl cis-trans isomerase), fis and hns (global regulators), hnr (response regulator of RpoS), dksA (transcriptional regulator of rRNA transcription), ygfA (5-formyl-tetrahydrofolate cyclo-ligase), and yigB (flavin mononucleotide [FMN] phosphatase). The prominent presence of global regulators among these strains pointed to the likely redundancy of persister cell formation mechanisms: the elimination of a regulator controlling several redundant persister genes would be expected to produce a phenotype. This observation is consistent with previous findings for a possible role of redundant genes such as toxin/antitoxin modules in persister cell formation. ygfA and yigB were of special interest. The mammalian homolog of YgfA (methenyltetrahydrofolate synthetase) catalyzes the conversion of 5-formyl-tetrahydrofolate (THF) into the rapidly degraded 5,10-methenyl-THF, depleting the folate pool. The YigB protein is a phosphatase of FMN which would deplete the pool of this cofactor. Stochastic overexpression of these genes could lead to dormancy and, hence, tolerance by depleting the folate and FMN pools, respectively. Consistent with this scenario, the overexpression of both genes produced increased tolerance to ofloxacin.

Methionine synthase reductase undergoes phosphate‐sensitive loss of flavin mononucleotide upon dilution in vitro

Hyperhomocyst(e)inemia is a metabolic derangement that is linked to the distribution of folate pools, which provide one‐carbon units for biosynthesis of purines and thymidylate and for remethylation of homocysteine to form methionine. In humans, methionine synthase deficiency results in the accumulation of methyltetrahydrofolate at the expense of folate derivatives required for purine and thymidylate biosynthesis. Methionine synthase reductase (MSR) is absolutely required for methionine synthase activity in vivo. MSR is a dual flavoprotein, containing one mol of FAD and one mol of FMN per mol of enzyme. While free FMN is highly fluorescent, enzyme‐bound FMN fluorescence is quenched. Flavin mononucleotide (FMN) binding in MSR is weak compared to other dual flavin‐containing enzymes (Kd = ~85 μM; 0 mM Pi), especially in the presence of phosphate (Kd = ~450 μM; 50 mM Pi) at constant ionic strength. Release of FMN from MSR is associated with loss of enzyme activity, as determined by cytochrome c reductase assay, while FMN‐reconstituted enzyme regains all activity. We found that the presence of 50 mM phosphate increased the overall loss of reductase activity upon dilution by ~50% and decreased the rate of rebinding of FMN ~5‐fold compared to phosphate free buffer. These findings point to a mechanism by which MSR may be inactivated under conditions that limit the availability of FMN in vivo in a phosphate sensitive manner.

Nutrition and colonic health: the critical role of the microbiota

To highlight mechanisms whereby diet affects colonic function and disease patterns. Topical nutrients are preferentially used by the gut mucosa to maintain structure and function. With the colon, topical nutrients are generated by the colonic microbiota to maintain mucosal health. Most importantly, short chain fatty acids control proliferation and differentiation, thereby reducing colon cancer risk. In patients with massive loss of small intestine, short chain fatty acid production supports survival by releasing up to 1000 kcal energy/day. Human studies show that the microbiota synthesizes a large pool of utilizable folate which may support survival in impoverished populations. Unfortunately, the microbiota may also elaborate toxic products from food residues such as genotoxic hydrogen sulfide by sulfur-reducing bacteria in response to a high-meat diet. The employment of culture-free techniques based on 16S regions of DNA has revealed that our colons harbor over 800 bacterial species and 7000 different strains. Evidence suggests that the diet directly influences the diversity of the microbiota, providing the link between diet, colonic disease, and colon cancer. The microbiota, however, can determine the efficiency of food absorption and risk of obesity. Our investigations have focused on a small number of bacterial species: characterization of microbiota and its metabolism can be expected to provide the key to colonic health and disease.

The Many Flavors of Hyperhomocyst(e)inemia: Insights from Transgenic and Inhibitor-Based Mouse Models of Disrupted One-Carbon Metabolism

Mouse models that perturb homocysteine metabolism, including genetic mouse models that result in deficiencies of methylenetetrahydrofolate reductase, methionine synthase, methionine synthase reductase, and cystathionine beta-synthase, and a pharmaceutically induced mouse model with a transient deficiency in betainehomocysteine methyl transferase, have now been characterized and can be compared. Although each of these enzyme deficiencies is associated with moderate to severe hyperhomocyst(e)inemia, the broader metabolic profiles are profoundly different. In particular, the various models differ in the degree to which tissue ratios of S-adenosylmethionine to S-adenosylhomocysteine are reduced in the face of elevated plasma homocyst(e)ine, and in the distribution of the tissue folate pools. These different metabolic profiles illustrate the potential complexities of hyperhomocyst(e)inemia in humans and suggest that comparison of the disease phenotypes of the various mouse models may be extremely useful in dissecting the underlying risk factors associated with human hyperhomocyst(e)inemia.

Effects of folate and folylpolyglutamyl synthase modulation on chemosensitivity of breast cancer cells

Folylpolyglutamyl synthase (FPGS) converts intracellular folates and antifolates to polyglutamates. Polyglutamylated folates and antifolates are retained in cells longer and are better substrates than their monoglutamate counterparts for enzymes involved in one-carbon transfer. FPGS modulation affects the chemosensitivity of cancer cells to antifolates, such as methotrexate, and 5-fluorouracil (5FU) by altering polyglutamylation of antifolates and specific target intracellular folate cofactors. However, this effect may be counterbalanced by FPGS modulation-induced changes in polyglutamylation of other intracellular folate cofactors and total intracellular folate pools. We generated an in vitro model of FPGS overexpression and inhibition in breast cancer cells by stably transfecting human MDA-MB-435 breast cancer cells with the sense FPGS cDNA or FPGS-targeted small interfering RNA, respectively, and investigated the effects of FPGS modulation on chemosensitivity to 5FU and methotrexate. FPGS modulation-induced changes in polyglutamylation of both antifolates and folate cofactors and in intracellular folate pools affected chemosensitivity of breast cancer cells to pemetrexed and trimetrexate whose cytotoxic effects do or do not depend on polyglutamylation, respectively, in a predictable manner. However, the effects of FPGS modulation on the chemosensitivity of breast cancer cells to 5FU and methotrexate seem to be highly complex and depend not only on polyglutamylation of a specific target intracellular folate cofactor or methotrexate, respectively, but also on total intracellular folate pools and polyglutamylation of other intracellular folate cofactors. Whether or not FPGS modulation may be an important clinical determinant of chemosensitivity of breast cancer cells to 5FU and methotrexate-based chemotherapy needs further exploration.