Further delineation of the phenotypic and metabolomic profile of ALDH1L2-related neurodevelopmental disorder

ALDH1L2, a mitochondrial enzyme in folate metabolism, converts 10-formyl-THF (10-formyltetrahydrofolate) to THF (tetrahydrofolate) and CO2. At the cellular level, deficiency of this NADP+-dependent reaction results in marked reduction in NADPH/NADP+ ratio and reduced mitochondrial ATP. Thus far, a single patient with biallelic ALDH1L2 variants and the phenotype of a neurodevelopmental disorder has been reported. Here, we describe another patient with a neurodevelopmental disorder associated with a novel homozygous missense variant in ALDH1L2, Pro133His. The variant caused marked reduction in the ALDH1L2 enzyme activity in skin fibroblasts derived from the patient as probed by 10-FDDF, a stable synthetic analog of 10-formyl-THF. Additional associated abnormalities in these fibroblasts include reduced NADPH/NADP+ ratio and pool of mitochondrial ATP, upregulated autophagy and dramatically altered metabolomic profile. Overall, our study further supports a link between ALDH1L2 deficiency and abnormal neurodevelopment in humans.

Pancreatic islet protection at the expense of secretory function involves serine-linked mitochondrial one-carbon metabolism

Type 2 diabetes is characterized by insulin hypersecretion followed by reduced glucose-stimulated insulin secretion (GSIS). Here we show that acute stimulation of pancreatic islets with the insulin secretagogue dextrorphan (DXO) or glibenclamide enhances GSIS, whereas chronic treatment with high concentrations of these drugs reduce GSIS but protect islets from cell death. Bulk RNA sequencing of islets shows increased expression of genes for serine-linked mitochondrial one-carbon metabolism (OCM) after chronic, but not acute, stimulation. In chronically stimulated islets, more glucose is metabolized to serine than to citrate, and the mitochondrial ATP/ADP ratio decreases, whereas the NADPH/NADP+ ratio increases. Activating transcription factor-4 (Atf4) is required and sufficient to activate serine-linked mitochondrial OCM genes in islets, with gain- and loss-of-function experiments showing that Atf4 reduces GSIS and is required, but not sufficient, for full DXO-mediated islet protection. In sum, we identify a reversible metabolic pathway that provides islet protection at the expense of secretory function.

Exploratory Metabolomics Underscores the Folate Enzyme ALDH1L1 as a Regulator of Glycine and Methylation Reactions

Folate (vitamin B9) is involved in one-carbon transfer reactions and plays a significant role in nucleic acid synthesis and control of cellular proliferation, among other key cellular processes. It is now recognized that the role of folates in different stages of carcinogenesis is complex, and more research is needed to understand how folate reactions become dysregulated in cancers and the metabolic consequences that occur as a result. ALDH1L1 (cytosolic 10-formyltetrahydrofolate dehydrogenase), an enzyme of folate metabolism expressed in many tissues, is ubiquitously downregulated in cancers and is not expressed in cancer cell lines. The RT4 cell line (derived from papillary bladder cancer) which expresses high levels of ALDH1L1 represents an exception, providing an opportunity to explore the metabolic consequences of the loss of this enzyme. We have downregulated this protein in RT4 cells (shRNA driven knockdown or CRISPR driven knockout) and compared metabolomes of ALDH1L1-expressing and -deficient cells to determine if metabolic changes linked to the loss of this enzyme might provide proliferative and/or survival advantages for cancer cells. In this study, cell extracts were analyzed using Ultra High Performance Liquid Chromatography High Resolution Mass Spectrometry (UHPLC-HR-MS). A total of 13,339 signals were identified or annotated using an in-house library and public databases. Supervised and unsupervised multivariate analysis revealed metabolic differences between RT4 cells and ALDH1L1-deficient clones. Glycine (8-fold decrease) and metabolites derived from S-adenosylmethionine utilizing pathways were significantly decreased in the ALDH1L1-deficient clones, compared with RT4 cells. Other changes linked to ALDH1L1 downregulation include decreased levels of amino acids, Krebs cycle intermediates, and ribose-5-phosphate, and increased nicotinic acid. While the ALDH1L1-catalyzed reaction is directly linked to glycine biosynthesis and methyl group flux, its overall effect on cellular metabolism extends beyond immediate metabolic pathways controlled by this enzyme.

Sex-Specific Metabolic Effects of Dietary Folate Withdrawal in Wild-Type and Aldh1l1 Knockout Mice

ALDH1L1 (10-formyltetrahydrofolate dehydrogenase), an enzyme of folate metabolism, is highly expressed in the liver. It regulates the overall flux of folate-bound one-carbon groups by converting 10-formyltetrahydrofolate to tetrahydrofolate and CO2 in a NADP+-dependent reaction. Our previous study revealed that Aldh1l1 knockout (KO) mice have an altered liver metabotype with metabolic symptoms of folate deficiency when fed a standard chow diet containing 2 ppm folic acid. Here we performed untargeted metabolomic analysis of liver and plasma of KO and wild-type (WT) male and female mice fed for 16 weeks either standard or folate-deficient diet. OPLS-DA, a supervised multivariate technique that was applied to 6595 and 10,678 features for the liver and plasma datasets, respectively, indicated that genotype and diet, alone or in combination, gave distinct metabolic profiles in both types of biospecimens. A more detailed analysis of affected metabolic pathways based on most confidently identified metabolites in the liver and plasma (OL1 and OL2a ontology level) indicated that the dietary folate restriction itself does not fully recapitulate the metabolic effect of the KO. Of note, dietary folate withdrawal enhanced the metabolic perturbations linked to the ALDH1L1 loss only for a subset of metabolites. Importantly, both the ALDH1L1 loss and dietary folate deficiency produced sex-specific metabolic effects.

Abstract 2257: Effect ofALDH1L1folate enzyme on hepatocellular carcinoma inachemical carcinogenesismodel

Background: ALDH1L1 (aldehyde dehydrogenase 1 family member L1, or cytosolic 10-formyltetrahydrofolate dehydrogenase) is an abundant enzyme in folate metabolism representing about 1% of total soluble protein in hepatocytes. Expression of this protein is strongly and ubiquitously downregulated in human cancers through the promoter methylation. Several studies indicate that patients with hepatocellular carcinoma who had low expression of ALDH1L1 had poorer prognosis than those with high expression.

Hypothesis: We propose that in the absence of ALDH1L1, liver tumors proliferate faster and grow to a larger size.

Methods: The effect of the Aldh1l1 knockout (Aldh1l1−/−) on hepatocellular carcinoma was investigated using DEN (diethyl nitrosamine) model of chemical carcinogenesis. Fifteen-days-old male Aldh1l1−/- mice and their wild-type littermate controls (Aldh1l1+/+) were injected intraperitoneally with DEN at the dose of 15 mg/kg body weight. Mice were sacrificed 10, 20, 28- and 36-weeks post-injection and livers were examined for for histopathology.

Results: Significant reduction in total body weight was observed in the DEN-injected knockout compared to wild type animals at 28 and 36 weeks. Immunohistochemical analysis reveals that tumor size was significantly bigger in Aldh1l1−/− compared to Aldh1l1+/+ mice and also indicated fibrosis in livers of knockout vs wild-type mice. Vascular involvement, highlighted by reticulin staining, was present in both Aldh1l1−/− and Aldh1l1+/+ mice as early as 20 weeks after the injection. Tumor multiplicity did not differ significantly between the Aldh1l1−/− and Aldh1l1+/+ animals.

Conclusion:. Aldh1l1 knockout promoted aggressiveness of liver tumors without affecting the tumor initiation.

Acknowledgement: These studies were supported by the National Institutes of Health grants DK54388 and CA095030 (S.A.K.).

Citation Format: Jaspreet Sharma, Natalia I. Krupenko, Peter Pediaditakis, Kristi Helke, Kyle C. Strickland, Sergey A. Krupenko. Effect ofALDH1L1folate enzyme on hepatocellular carcinoma inachemical carcinogenesismodel [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 2257.

Knockout of Putative Tumor Suppressor Aldh1l1 in Mice Reprograms Metabolism to Accelerate Growth of Tumors in a Diethylnitrosamine (DEN) Model of Liver Carcinogenesis

Simple Summary

Cancers often loose the enzyme of folate metabolism ALDH1L1. We proposed that such loss is advantageous for the malignant tumor growth and tested this hypothesis in mice proficient or deficient (gene knockout) in ALDH1L1 expression. Liver cancer in both groups was induced by injection of chemical carcinogen diethylnitrosamine. While the number of tumors observed in ALDH1L1 proficient and deficient mice was similar, tumors grew faster and to a larger size in the knockout mice. We conclude that the ALDH1L1 loss promotes liver tumor growth without affecting tumor initiation or multiplicity. Accelerated growth of tumors lacking the enzyme was linked to several metabolic pathways, which are beneficial for rapid proliferation.

Abstract

Cytosolic 10-formyltetrahydrofolate dehydrogenase (ALDH1L1) is commonly downregulated in human cancers through promoter methylation. We proposed that ALDH1L1 loss promotes malignant tumor growth. Here, we investigated the effect of the Aldh1l1 mouse knockout (Aldh1l1^−/−) on hepatocellular carcinoma using a chemical carcinogenesis model. Fifteen-day-old male Aldh1l1 knockout mice and their wild-type littermate controls (Aldh1l1^+/+) were injected intraperitoneally with 20 μg/g body weight of DEN (diethylnitrosamine). Mice were sacrificed 10, 20, 28, and 36 weeks post-DEN injection, and livers were examined for tumor multiplicity and size. We observed that while tumor multiplicity did not differ between Aldh1l1^−/− and Aldh1l1^+/+ animals, larger tumors grew in Aldh1l1^−/− compared to Aldh1l1^+/+ mice at 28 and 36 weeks. Profound differences between Aldh1l1^−/− and Aldh1l1^+/+ mice in the expression of inflammation-related genes were seen at 10 and 20 weeks. Of note, large tumors from wild-type mice showed a strong decrease of ALDH1L1 protein at 36 weeks. Metabolomic analysis of liver tissues at 20 weeks showed stronger differences in Aldh1l1^+/+ versus Aldh1l1^−/− metabotypes than at 10 weeks, which underscores metabolic pathways that respond to DEN in an ALDH1L1-dependent manner. Our study indicates that Aldh1l1 knockout promoted liver tumor growth without affecting tumor initiation or multiplicity.

Aldh1l2 knockout mouse metabolomics links the loss of the mitochondrial folate enzyme to deregulation of a lipid metabolism observed in rare human disorder

Background

Mitochondrial folate enzyme ALDH1L2 (aldehyde dehydrogenase 1 family member L2) converts 10-formyltetrahydrofolate to tetrahydrofolate and CO₂ simultaneously producing NADPH. We have recently reported that the lack of the enzyme due to compound heterozygous mutations was associated with neuro-ichthyotic syndrome in a male patient. Here, we address the role of ALDH1L2 in cellular metabolism and highlight the mechanism by which the enzyme regulates lipid oxidation.

Methods

We generated Aldh1l2 knockout (KO) mouse model, characterized its phenotype, tissue histology, and levels of reduced folate pools and applied untargeted metabolomics to determine metabolic changes in the liver, pancreas, and plasma caused by the enzyme loss. We have also used NanoString Mouse Inflammation V2 Code Set to analyze inflammatory gene expression and evaluate the role of ALDH1L2 in the regulation of inflammatory pathways.

Results

Both male and female Aldh1l2 KO mice were viable and did not show an apparent phenotype. However, H&E and Oil Red O staining revealed the accumulation of lipid vesicles localized between the central veins and portal triads in the liver of Aldh1l2^-/- male mice indicating abnormal lipid metabolism. The metabolomic analysis showed vastly changed metabotypes in the liver and plasma in these mice suggesting channeling of fatty acids away from β-oxidation. Specifically, drastically increased plasma acylcarnitine and acylglycine conjugates were indicative of impaired β-oxidation in the liver. Our metabolomics data further showed that mechanistically, the regulation of lipid metabolism by ALDH1L2 is linked to coenzyme A biosynthesis through the following steps. ALDH1L2 enables sufficient NADPH production in mitochondria to maintain high levels of glutathione, which in turn is required to support high levels of cysteine, the coenzyme A precursor. As the final outcome, the deregulation of lipid metabolism due to ALDH1L2 loss led to decreased ATP levels in mitochondria.

Conclusions

The ALDH1L2 function is important for CoA-dependent pathways including β-oxidation, TCA cycle, and bile acid biosynthesis. The role of ALDH1L2 in the lipid metabolism explains why the loss of this enzyme is associated with neuro-cutaneous diseases. On a broader scale, our study links folate metabolism to the regulation of lipid homeostasis and the energy balance in the cell.

Supplementary Information

The online version contains supplementary material available at 10.1186/s40246-020-00291-3.

Sphingolipids and the link between alcohol and cancer

Epidemiological evidence underscores alcohol consumption as a strong risk factor for multiple cancer types, with liver cancer being most commonly associated with alcohol intake. While mechanisms linking alcohol consumption to malignant tumor development are not fully understood, the likely players in ethanol-induced carcinogenesis are genotoxic stress caused by formation of acetaldehyde, increased oxidative stress, and altered nutrient metabolism, including the impairment of methyl transfer reactions. Alterations of sphingolipid metabolism and associated signaling pathways are another potential link between ethanol and cancer development. In particular, ceramides are involved in the regulation of cellular proliferation, differentiation, senescence, and apoptosis and are known to function as important regulators of malignant transformation as well as tumor progression. However, to date, the cross-talk between ceramides and alcohol in cancer disease is largely an open question and only limited data are available on this subject. Most studies linking ceramide to cancer considered liver steatosis as the underlying mechanism, which is not surprising taking into consideration that ceramide pathways are an integral part of the overall lipid metabolism. This review summarizes the latest studies pointing to ceramide as an important mediator of cancer-promoting effects of chronic alcohol consumption and underscores the necessity of understanding the role of sphingolipids and lipid signaling in response to alcohol in order to prevent and/or successfully manage diseases caused by alcohol.

Deleterious mutations in ALDH1L2 suggest a novel cause for neuro-ichthyotic syndrome

Neuro-ichthyotic syndromes are a group of rare genetic diseases mainly associated with perturbations in lipid metabolism, intracellular vesicle trafficking, or glycoprotein synthesis. Here, we report a patient with a neuro-ichthyotic syndrome associated with deleterious mutations in the ALDH1L2 (aldehyde dehydrogenase 1 family member L2) gene encoding for mitochondrial 10-formyltetrahydrofolate dehydrogenase. Using fibroblast culture established from the ALDH1L2-deficient patient, we demonstrated that the enzyme loss impaired mitochondrial function affecting both mitochondrial morphology and the pool of metabolites relevant to β-oxidation of fatty acids. Cells lacking the enzyme had distorted mitochondria, accumulated acylcarnitine derivatives and Krebs cycle intermediates, and had lower ATP and increased ADP/AMP indicative of a low energy index. Re-expression of functional ALDH1L2 enzyme in deficient cells restored the mitochondrial morphology and the metabolic profile of fibroblasts from healthy individuals. Our study underscores the role of ALDH1L2 in the maintenance of mitochondrial integrity and energy balance of the cell, and suggests the loss of the enzyme as the cause of neuro-cutaneous disease.

CHIP E3 ligase mediates proteasomal degradation of the proliferation regulatory protein ALDH1L1 during the transition of NIH3T3 fibroblasts from G0/G1 to S-phase

ALDH1L1 is a folate-metabolizing enzyme abundant in liver and several other tissues. In human cancers and cell lines derived from malignant tumors, the ALDH1L1 gene is commonly silenced through the promoter methylation. It was suggested that ALDH1L1 limits proliferation capacity of the cell and thus functions as putative tumor suppressor. In contrast to cancer cells, mouse cell lines NIH3T3 and AML12 do express the ALDH1L1 protein. In the present study, we show that the levels of ALDH1L1 in these cell lines fluctuate throughout the cell cycle. During S-phase, ALDH1L1 is markedly down regulated at the protein level. As the cell cultures become confluent and cells experience increased contact inhibition, ALDH1L1 accumulates in the cells. In agreement with this finding, NIH3T3 cells arrested in G₁/S-phase by a thymidine block completely lose the ALDH1L1 protein. Treatment with the proteasome inhibitor MG-132 prevents such loss in proliferating NIH3T3 cells, suggesting the proteasomal degradation of the ALDH1L1 protein. The co-localization of ALDH1L1 with proteasomes, demonstrated by confocal microscopy, supports this mechanism. We further show that ALDH1L1 interacts with the chaperone-dependent E3 ligase CHIP, which plays a key role in the ALDH1L1 ubiquitination and degradation. In NIH3T3 cells, silencing of CHIP by siRNA halts, while transient expression of CHIP promotes, the ALDH1L1 loss. The downregulation of ALDH1L1 is associated with the accumulation of the ALDH1L1 substrate 10-formyltetrahydrofolate, which is required for de novo purine biosynthesis, a key pathway activated in S-phase. Overall, our data indicate that CHIP-mediated proteasomal degradation of ALDH1L1 facilitates cellular proliferation.

Ceramide Signaling and p53 Pathways

Ceramides, important players in signal transduction, interact with multiple cellular pathways, including p53 pathways. However, the relationship between ceramide and p53 is very complex, and mechanisms underlying their coregulation are diverse and not fully characterized. The role of p53, an important cellular regulator and a transcription factor, is linked to its tumor suppressor function. Ceramides are involved in the regulation of fundamental processes in cancer cells including cell death, proliferation, autophagy, and drug resistance. This regulation, however, can be pro-death or pro-survival depending on cancer type, the balance between ceramide species, the rate of their synthesis and utilization, and the availability of a specific array of downstream targets. This chapter highlights the central role of ceramide in sphingolipid metabolism, its role in cancer, specific effectors in ceramide pathways controlled by p53, and coregulation of ceramide and p53 signaling. We discuss the recent studies, which underscore the function of p53 in the regulation of ceramide pathways and the reciprocal regulation of p53 by ceramide. This complex relationship is based on several molecular mechanisms including the p53-dependent transcriptional regulation of enzymes in sphingolipid pathways, the activation of mutant p53 through ceramide-mediated alternative splicing, as well as modulation of the p53 function through direct and indirect effects on p53 coregulators and downstream targets. Further insight into the connections between ceramide and p53 will allow simultaneous targeting of the two pathways with a potential to yield more efficient anticancer therapeutics.

ALDH1L1 and ALDH1L2 Folate Regulatory Enzymes in Cancer

Epidemiological studies implicate excess ethanol ingestion as a risk factor for several cancers and support the concept of a synergistic effect of chronic alcohol consumption and folate deficiency on carcinogenesis. Alcohol consumption affects folate-related genes and enzymes including two major folate-metabolizing enzymes, ALDH1L1 and ALDH1L2. ALDH1L1 (cytosolic 10-formyltetrahydrofolate dehydrogenase) is a regulatory enzyme in folate metabolism that controls the overall flux of one-carbon groups in folate-dependent biosynthetic pathways. It is strongly and ubiquitously down-regulated in malignant tumors via promoter methylation, and recent studies underscored this enzyme as a candidate tumor suppressor and potential marker of aggressive cancers. A related enzyme, ALDH1L2, is the mitochondrial homolog of ALDH1L1 encoded by a separate gene. In contrast to its cytosolic counterpart, ALDH1L2 is expressed in malignant tumors and cancer cell lines and was implicated in metastasis regulation. This review discusses the link between folate and cancer, modifying effects of alcohol consumption on folate-associated carcinogenesis, and putative roles of ALDH1L1 and ALDH1L2 in this process.

CerS6 Is a Novel Transcriptional Target of p53 Protein Activated by Non-genotoxic Stress

Our previous study suggested that ceramide synthase 6 (CerS6), an enzyme in sphingolipid biosynthesis, is regulated by p53: CerS6 was elevated in several cell lines in response to transient expression of p53 or in response to folate stress, which is known to activate p53. It was not clear, however, whether CerS6 gene is a direct transcriptional target of p53 or whether this was an indirect effect through additional regulatory factors. In the present study, we have shown that the CerS6 promoter is activated by p53 in luciferase assays, whereas transcriptionally inactive R175H p53 mutant failed to induce the luciferase expression from this promoter. In vitro immunoprecipitation assays and gel shift analyses have further demonstrated that purified p53 binds within the CerS6 promoter sequence spanning 91 bp upstream and 60 bp downstream of the transcription start site. The Promo 3.0.2 online tool for the prediction of transcription factor binding sites indicated the presence of numerous putative non-canonical p53 binding motifs in the CerS6 promoter. Luciferase assays and gel shift analysis have identified a single motif upstream of the transcription start as a key p53 response element. Treatment of cells with Nutlin-3 or low concentrations of actinomycin D resulted in a strong elevation of CerS6 mRNA and protein, thus demonstrating that CerS6 is a component of the non-genotoxic p53-dependent cellular stress response. This study has shown that by direct transcriptional activation of CerS6, p53 can regulate specific ceramide biosynthesis, which contributes to the pro-apoptotic cellular response.

Aldehyde dehydrogenase homologous folate enzymes: Evolutionary switch between cytoplasmic and mitochondrial localization

Cytosolic and mitochondrial 10-formyltetrahydrofolate dehydrogenases are products of separate genes in vertebrates but only one such gene is present in invertebrates. There is a significant degree of sequence similarity between the two enzymes due to an apparent origin of the gene for the mitochondrial enzyme (ALDH1L2) from the duplication of the gene for the cytosolic enzyme (ALDH1L1). The primordial ALDH1L gene originated from a natural fusion of three unrelated genes, one of which was an aldehyde dehydrogenase. Such structural organization defined the catalytic mechanism of these enzymes, which is similar to that of aldehyde dehydrogenases. Here we report the analysis of ALDH1L1 and ALDH1L2 genes from different species and their phylogeny and evolution. We also performed sequence and structure comparison of ALDH1L enzymes possessing aldehyde dehydrogenase catalysis to those lacking this feature in an attempt to explain mechanistic differences between cytoplasmic ALDH1L1 and mitochondrial ALDH1L2 enzymes and to better understand their functional roles.

Rho GTPases RhoA and Rac1 mediate effects of dietary folate on metastatic potential of A549 cancer cells through the control of cofilin phosphorylation

Folate, an important nutrient in the human diet, has been implicated in cancer, but its role in metastasis is not established. We have shown previously that the withdrawal of medium folate leads to the inhibition of migration and invasion of A549 lung carcinoma cells. Here we have demonstrated that medium folate regulates the function of Rho GTPases by enabling their carboxyl methylation and translocation to plasma membrane. Conversely, the lack of folate leads to the retention of these proteins in endoplasmic reticulum. Folate also promoted the switch from inactive (GDP-bound) to active (GTP-bound) GTPases, resulting in the activation of downstream kinases p21-activated kinase and LIM kinase and phosphorylation of the actin-depolymerizing factor cofilin. We have further demonstrated that in A549 cells two GTPases, RhoA and Rac1, but not Cdc42, are immediate sensors of folate status: the siRNA silencing of RhoA or Rac1 blocked effects of folate on cofilin phosphorylation and cellular migration and invasion. The finding that folate modulates metastatic potential of cancer cells was confirmed in an animal model of lung cancer using tail vein injection of A549 cells in SCID mice. A folate-rich diet enhanced lung colonization and distant metastasis to lymph nodes and decreased overall survival (35 versus 63 days for mice on a folate-restricted diet). High folate also promoted epithelial-mesenchymal transition in cancer cells and experimental mouse tumors. Our study provides experimental evidence for a mechanism of metastasis promotion by dietary folate and highlights the interaction between nutrients and metastasis-related signaling.

JNK1/2 regulate Bid by direct phosphorylation at Thr59 in response to ALDH1L1

BH3 interacting-domain death agonist (Bid) is a BH3-only pro-apoptotic member of the Bcl-2 family of proteins. Its function in apoptosis is associated with the proteolytic cleavage to the truncated form tBid, mainly by caspase-8. tBid translocates to mitochondria and assists Bax and Bak in induction of apoptosis. c-Jun N-terminal kinase (JNK)-dependent alternative processing of Bid to jBid was also reported. We have previously shown that the folate stress enzyme 10-formyltetrahydrofolate dehydrogenase (ALDH1L1) activates JNK1 and JNK2 in cancer cells as a pro-apoptotic response. Here we report that in PC-3 prostate cancer cells, JNK1/2 phosphorylate Bid at Thr59 within the caspase cleavage site in response to ALDH1L1. In vitro, all three JNK isoforms, JNK 1–3, phosphorylated Thr59 of Bid with JNK1 being the least active. Thr59 phosphorylation protected Bid from cleavage by caspase-8, resulting in strong accumulation of the full-length protein and its translocation to mitochondria. Interestingly, although we did not observe jBid in response to ALDH1L1 in PC-3 cells, transient expression of Bid mutants lacking the caspase-8 cleavage site resulted in strong accumulation of jBid. Of note, a T59D mutant mimicking constitutive phosphorylation revealed more profound cleavage of Bid to jBid. JNK-driven Bid accumulation had a pro-apoptotic effect in our study: small interfering RNA silencing of either JNK1/2 or Bid prevented Bid phosphorylation and accumulation, and rescued ALDH1L1-expressing cells. As full-length Bid is a weaker apoptogen than tBid, we propose that the phosphorylation of Bid by JNKs, followed by the accumulation of the full-length protein, delays attainment of apoptosis, and allows the cell to evaluate the stress and make a decision regarding the response strategy. This mechanism perhaps can be modified by the alternative cleavage of phospho-T59 Bid to jBid at some conditions.

Molecular mechanisms underlying the potentially adverse effects of folate

The importance of proper consumption of dietary folate for human health has been highlighted by an extensive number of publications over several decades. Fortification of grain products with folic acid was initiated with the specific intent to prevent neural tube defects, and the scope of this endeavor is unique in that its target population (women of the periconceptional period) is many times smaller than the population it affects (everyone who ingests fortified grain products). Folate fortification has been wildly successful in terms of its goal; since its inception, the incidence of neural tube defects has markedly decreased. In the wake of this public health triumph, it is important to catalog both the serendipitous benefits and potential side effects of folic acid supplementation. The vitamin is generally regarded as a harmless nutrient based on studies evaluating the safe upper limits of folate intake. In recent years, however, a concern has been raised with respect to a potential downside to folate supplementation; namely, its proposed ability to enhance proliferation of malignant tumors. The current review summarizes the available literature on the effects of folate supplementation and the molecular mechanisms by which high doses of folate may have negative consequences on human health, especially with regard to cancer.

A novel tumor suppressor function of glycine N-methyltransferase is independent of its catalytic activity but requires nuclear localization

Glycine N-methyltransferase (GNMT), an abundant cytosolic enzyme, catalyzes the transfer of a methyl group from S-adenosylmethionine (SAM) to glycine generating S-adenosylhomocysteine and sarcosine (N-methylglycine). This reaction is regulated by 5-methyltetrahydrofolate, which inhibits the enzyme catalysis. In the present study, we observed that GNMT is strongly down regulated in human cancers and is undetectable in cancer cell lines while the transient expression of the protein in cancer cells induces apoptosis and results in the activation of ERK1/2 as an early pro-survival response. The antiproliferative effect of GNMT can be partially reversed by treatment with the pan-caspase inhibitor zVAD-fmk but not by supplementation with high folate or SAM. GNMT exerts the suppressor effect primarily in cells originated from malignant tumors: transformed cell line of non-cancer origin, HEK293, was insensitive to GNMT. Of note, high levels of GNMT, detected in regenerating liver and in NIH3T3 mouse fibroblasts, do not produce cytotoxic effects. Importantly, GNMT, a predominantly cytoplasmic protein, was translocated into nuclei upon transfection of cancer cells. The presence of GNMT in the nuclei was also observed in normal human tissues by immunohistochemical staining. We further demonstrated that the induction of apoptosis is associated with the GNMT nuclear localization but is independent of its catalytic activity or folate binding. GNMT targeted to nuclei, through the fusion with nuclear localization signal, still exerts strong antiproliferative effects while its restriction to cytoplasm, through the fusion with nuclear export signal, prevents these effects (in each case the protein was excluded from cytosol or nuclei, respectively). Overall, our study indicates that GNMT has a secondary function, as a regulator of cellular proliferation, which is independent of its catalytic role.

A novel mouse model for genetic variation in 10-formyltetrahydrofolate synthetase exhibits disturbed purine synthesis with impacts on pregnancy and embryonic development

Genetic variants in one-carbon folate metabolism have been identified as risk factors for disease because they may impair the production or use of one-carbon folates required for nucleotide synthesis and methylation. p.R653Q (1958G>A) is a single-nucleotide polymorphism (SNP) in the 10-formyltetrahydrofolate (formylTHF) synthetase domain of the trifunctional enzyme MTHFD1; this domain produces the formylTHF which is required for the de novo synthesis of purines. Approximately 20% of Caucasians are homozygous for the Q allele. MTHFD1 p.R653Q has been proposed as a risk factor for neural tube defects (NTDs), congenital heart defects (CHDs) and pregnancy losses. We have generated a novel mouse model in which the MTHFD1 synthetase activity is inactivated without affecting protein expression or the other activities of this enzyme. Complete loss of synthetase activity (Mthfd1S(-/-)) is incompatible with life; embryos die shortly after 10.5 days gestation, and are developmentally delayed or abnormal. The proportion of 10-formylTHF in the plasma and liver of Mthfd1S(+/-) mice is reduced (P < 0.05), and de novo purine synthesis is impaired in Mthfd1S(+/-) mouse embryonic fibroblasts (MEFs, P < 0.005). Female Mthfd1S(+/-) mice had decreased neutrophil counts (P < 0.05) during pregnancy and increased incidence of developmental defects in embryos (P = 0.052). These findings suggest that synthetase deficiency may lead to pregnancy complications through decreased purine synthesis and reduced cellular proliferation. Additional investigation of the impact of synthetase polymorphisms on human pregnancy is warranted.

Folate stress induces apoptosis via p53-dependent de novo ceramide synthesis and up-regulation of ceramide synthase 6

We have investigated the role of ceramide in the cellular adaptation to folate stress induced by Aldh1l1, the enzyme involved in the regulation of folate metabolism. Our previous studies demonstrated that Aldh1l1, similar to folate deficiency, evokes metabolic stress and causes apoptosis in cancer cells. Here we report that the expression of Aldh1l1 in A549 or HCT116 cells results in the elevation of C16-ceramide and a transient up-regulation of ceramide synthase 6 (CerS6) mRNA and protein. Pretreatment with ceramide synthesis inhibitors myriocin and fumonisin B1 or siRNA silencing of CerS6 prevented C16-ceramide accumulation and rescued cells supporting the role of CerS6/C16-ceramide as effectors of Aldh1l1-induced apoptosis. The CerS6 activation by Aldh1l1 and increased ceramide generation were p53-dependent; this effect was ablated in p53-null cells. Furthermore, the expression of wild type p53 but not transcriptionally inactive R175H p53 mutant strongly elevated CerS6. Also, this dominant negative mutant prevented accumulation of CerS6 in response to Aldh1l1, indicating that CerS6 is a transcriptional target of p53. In support of this mechanism, bioinformatics analysis revealed the p53 binding site 3 kb downstream of the CerS6 transcription start. Interestingly, ceramide elevation in response to Aldh1l1 was inhibited by silencing of PUMA, a proapoptotic downstream effector of p53 whereas the transient expression of CerS6 elevated PUMA in a p53-dependent manner indicating reciprocal relationships between ceramide and p53/PUMA pathways. Importantly, folate withdrawal also induced CerS6/C16-ceramide elevation accompanied by p53 accumulation. Overall, these novel findings link folate and de novo ceramide pathways in cellular stress response.

Activation of p21-Dependent G1/G2 Arrest in the Absence of DNA Damage as an Antiapoptotic Response to Metabolic Stress

The folate enzyme, FDH (10-formyltetrahydrofolate dehydrogenase, ALDH1L1), a metabolic regulator of proliferation, activates p53-dependent G1 arrest and apoptosis in A549 cells. In the present study, we have demonstrated that FDH-induced apoptosis is abrogated upon siRNA knockdown of the p53 downstream target PUMA. Conversely, siRNA knockdown of p21 eliminated FDH-dependent G1 arrest and resulted in an early apoptosis onset. The acceleration of FDH-dependent apoptosis was even more profound in another cell line, HCT116, in which the p21 gene was silenced through homologous recombination (p21(-/-) cells). In contrast to A549 cells, FDH caused G2 instead of G1 arrest in HCT116 p21(+/+) cells; such an arrest was not seen in p21-deficient (HCT116 p21(-/-)) cells. In agreement with the cell cycle regulatory function of p21, its strong accumulation in nuclei was seen upon FDH expression. Interestingly, our study did not reveal DNA damage upon FDH elevation in either cell line, as judged by comet assay and the evaluation of histone H2AX phosphorylation. In both A549 and HCT116 cell lines, FDH induced a strong decrease in the intracellular ATP pool (2-fold and 30-fold, respectively), an indication of a decrease in de novo purine biosynthesis as we previously reported. The underlying mechanism for the drop in ATP was the strong decrease in intracellular 10-formyltetrahydrofolate, a substrate in two reactions of the de novo purine pathway. Overall, we have demonstrated that p21 can activate G1 or G2 arrest in the absence of DNA damage as a response to metabolite deprivation. In the case of FDH-related metabolic alterations, this response delays apoptosis but is not sufficient to prevent cell death.

Epigenetic Silencing of ALDH1L1, a Metabolic Regulator of Cellular Proliferation, in Cancers

FDH (10-formyltetrahydrofolate dehydrogenase, the product of the ALDH1L1 gene), a major folate-metabolizing enzyme in the cytosol, is involved in the regulation of cellular proliferation. We have previously demonstrated that FDH is strongly and ubiquitously down-regulated in malignant human tumors and cancer cell lines. Here, we report that promoter methylation is a major mechanism controlling FDH levels in human cancers. A computational analysis has identified an extensive CpG island in the ALDH1L1 promoter region. It contains 96 CpG pairs and covers the region between -525 and +918 bp of the ALDH1L1 gene including the promoter, the entire exon 1, and a part of intron 1 immediately downstream of the exon. Bisulfite sequencing analysis revealed extensive methylation of the island (76%-95% of CpGs) in cancer cell lines. In agreement with these findings, treatment of FDH-deficient A549 cells with the methyltransferase inhibitor 5-aza-2'-deoxycytidine restored FDH expression. Analysis of the samples from patients with lung adenocarcinomas demonstrated methylation of the ALDH1L1 CpG island in tumor samples and a total lack of methylation in respective normal tissues. The same phenomenon was observed in liver tissues: the CpG island was methylation free in DNA extracted from normal hepatocytes but was extensively methylated in a hepatocellular carcinoma. Levels of ALDH1L1 mRNA and protein correlated with the methylation status of the island, with tumor samples demonstrating down-regulation of expression or even complete silencing of the gene. Our studies have also revealed that exon 1 significantly increases transcriptional activity of ALDH1L1 promoter in a luciferase reporter assay. Interestingly, the exon is extensively methylated in samples with a strongly down-regulated or silenced ALDH1L1 gene.

Phylogeny and evolution of aldehyde dehydrogenase-homologous folate enzymes

Folate coenzymes function as one-carbon group carriers in intracellular metabolic pathways. Folate-dependent reactions are compartmentalized within the cell and are catalyzed by two distinct groups of enzymes, cytosolic and mitochondrial. Some folate enzymes are present in both compartments and are likely the products of gene duplications. A well-characterized cytosolic folate enzyme, FDH (10-formyltetrahydro-folate dehydrogenase, ALDH1L1), contains a domain with significant sequence similarity to aldehyde dehydrogenases. This domain enables FDH to catalyze the NADP(+)-dependent conversion of short-chain aldehydes to corresponding acids in vitro. The aldehyde dehydrogenase-like reaction is the final step in the overall FDH mechanism, by which a tetrahydrofolate-bound formyl group is oxidized to CO(2) in an NADP(+)-dependent fashion. We have recently cloned and characterized another folate enzyme containing an ALDH domain, a mitochondrial FDH. Here the biological roles of the two enzymes, a comparison of the respective genes, and some potential evolutionary implications are discussed. The phylogenic analysis suggests that the vertebrate ALDH1L2 gene arose from a duplication event of the ALDH1L1 gene prior to the emergence of osseous fish >500 millions years ago.

ALDH1L1 inhibits cell motility via dephosphorylation of cofilin by PP1 and PP2A

Here we report that ALDH1L1 (FDH, a folate enzyme with tumor suppressor-like properties) inhibits cell motility. The underlying mechanism involves F-actin stabilization, re-distribution of cytoplasmic actin towards strong preponderance of filamentous actin, and formation of actin stress fibers. A549 cells expressing FDH demonstrated a much slower recovery of GFP-actin fluorescence in a FRAP assay, as well as an increase in G-actin polymerization and a decrease in F-actin depolymerization rates in pyren-actin fluorescence assays indicating the inhibition of actin dynamics. These effects were associated with robust dephosphorylation of the actin depolymerizing factor cofilin by PP1 and PP2A serine/threonine protein phosphatases but not the cofilin-specific phosphatases slingshot and chronophin. In fact, the PP1/PP2A inhibitor calyculin prevented cofilin dephosphorylation and restored motility. Inhibition of FDH-induced apoptosis by the JNK inhibitor SP600125 or the pan-caspase inhibitor zVAD-fmk did not restore motility or levels of phospho-cofilin, indicating that the observed effects are independent from FDH function in apoptosis. Interestingly, cofilin siRNA or expression of phosphorylation-deficient S3A cofilin mutant resulted in a decrease of G-actin and the actin stress fiber formation, the effects seen upon FDH expression. In contrast, the expression of S3D mutant, mimicking constitutive phosphorylation, prevented these effects further supporting the cofilin-dependent mechanism. Dephosphorylation of cofilin and inhibition of motility in response to FDH can be also prevented by the increased folate in media. Furthermore, folate depletion itself, in the absence of FDH, resulted in cofilin dephosphorylation and inhibition of motility in several cell lines. Our experiments showed that these effects were folate-specific and not a general response to nutrient starvation. Overall, this study demonstrates the presence of distinct intracellular signaling pathways regulating motility in response to folate status and points toward mechanisms involving folates in promoting a malignant phenotype.

ALDH1L2 is the mitochondrial homolog of 10-formyltetrahydrofolate dehydrogenase

Cytosolic 10-formyltetrahydrofolate dehydrogenase (FDH, ALDH1L1) is an abundant enzyme of folate metabolism. It converts 10-formyltetrahydrofolate to tetrahydrofolate and CO(2) in an NADP(+)-dependent reaction. We have identified a gene at chromosome locus 12q24.11 of the human genome, the product of which has 74% sequence similarity with cytosolic FDH. This protein has an extra N-terminal sequence of 22 amino acid residues, predicted to be a mitochondrial translocation signal. Transfection of COS-7 or A549 cell lines with a construct in which green fluorescent protein was introduced between the leader sequence and the rest of the putative mitochondrial FDH (mtFDH) has demonstrated mitochondrial localization of the fusion protein, suggesting that the identified gene encodes a mitochondrial enzyme. Purified pig liver mtFDH displayed dehydrogenase/hydrolase activities similar to cytosolic FDH. Real-time PCR performed on an array of human tissues has shown that although cytosolic FDH mRNA is highest in liver, kidney, and pancreas, mtFDH mRNA is most highly expressed in pancreas, heart, and brain. In contrast to the cytosolic enzyme, which is not detectable in cancer cells, the presence of mtFDH was demonstrated in several human cancer cell lines by conventional and real-time PCR and by Western blot. Analysis of genomes of different species indicates that the mitochondrial enzyme is a later evolutionary product when compared with the cytosolic enzyme. We propose that this novel mitochondrial enzyme is a likely source of CO(2) production from 10-formyltetrahydrofolate in mitochondria and plays an essential role in the distribution of one-carbon groups between the cytosolic and mitochondrial compartments of the cell.

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

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.

Acyl carrier protein-specific 4'-phosphopantetheinyl transferase activates 10-formyltetrahydrofolate dehydrogenase

4'-Phosphopantetheinyl transferases (PPTs) catalyze the transfer of 4'-phosphopantetheine (4-PP) from coenzyme A to a conserved serine residue of their protein substrates. In humans, the number of pathways utilizing the 4-PP post-translational modification is limited and may only require a single broad specificity PPT for all phosphopantetheinylation reactions. Recently, we have shown that one of the enzymes of folate metabolism, 10-formyltetrahydrofolate dehydrogenase (FDH), requires a 4-PP prosthetic group for catalysis. This moiety acts as a swinging arm to couple the activities of the two catalytic domains of FDH and allows the conversion of 10-formyltetrahydrofolate to tetrahydrofolate and CO2. In the current study, we demonstrate that the broad specificity human PPT converts apo-FDH to holoenzyme and thus activates FDH catalysis. Silencing PPT by small interfering RNA in A549 cells prevents FDH modification, indicating the lack of alternative enzymes capable of accomplishing this transferase reaction. Interestingly, PPT-silenced cells demonstrate significantly reduced proliferation and undergo strong G(1) arrest, suggesting that the enzymatic function of PPT is essential and nonredundant. Our study identifies human PPT as the FDH-modifying enzyme and supports the hypothesis that mammals utilize a single enzyme for all phosphopantetheinylation reactions.

10-formyltetrahydrofolate dehydrogenase-induced c-Jun-NH2-kinase pathways diverge at the c-Jun-NH2-kinase substrate level in cells with different p53 status

10-Formyltetrahydrofolate dehydrogenase (FDH) suppresses cancer cell proliferation through p53-dependent apoptosis but also induces strong cytotoxicity in p53-deficient prostate cells. In the present study, we have shown that FDH induces apoptosis in PC-3 prostate cells through simultaneous activation of the c-Jun-NH(2)-kinase (JNK) and extracellular signal-regulated kinase (ERK) pathways with JNK phosphorylating c-Jun and ERK1/2 phosphorylating Elk-1. The JNK1/2 inhibitor SP600125 or ERK1/2 inhibitor PD98059 prevented phosphorylation of c-Jun and Elk-1, correspondingly and partially protected PC-3 cells from FDH-induced cytotoxicity. Combination of the two inhibitors produced an additive effect. The contribution from the JNK cascade to FDH-induced apoptosis was significantly stronger than from the ERK pathway. siRNA knockdown of JNK1/2 or "turning off" the downstream target c-Jun by either siRNA or expression of the dominant-negative c-Jun mutant, TAM67, rescued PC-3 cells from FDH-induced apoptosis. The pull-down assays on immobilized c-Jun showed that c-Jun is directly phosphorylated by JNK2 in FDH-expressing cells. Interestingly, the FDH-induced apoptosis in p53-proficient A549 cells also proceeds through activation of JNK1/2, but the down-stream target for JNK2 is p53 instead of c-Jun. Furthermore, in A549 cells, FDH activates caspase 9, whereas in PC-3 cells, it activates caspase 8. Our studies indicate that the JNK pathways are common downstream mechanisms of FDH-induced cytotoxicity in different cell types, whereas the end point target in the cascade is cell type specific. JNK activation in response to FDH was inhibited by high supplementation of reduced folate leucovorin, further indicating a functional connection between folate metabolism and mitogen-activated protein kinase pathways.

10-formyltetrahydrofolate dehydrogenase requires a 4'-phosphopantetheine prosthetic group for catalysis

10-Formyltetrahydrofolate dehydrogenase (FDH) consists of two independent catalytic domains, N- and C-terminal, connected by a 100-amino acid residue linker (intermediate domain). Our previous studies on structural organization and enzymatic properties of rat FDH suggest that the overall enzyme reaction, i.e. NADP(+)-dependent conversion of 10-formyltetrahydrofolate to tetrahydrofolate and CO(2), consists of two steps: (i) hydrolytic cleavage of the formyl group in the N-terminal catalytic domain, followed by (ii) NADP(+)-dependent oxidation of the formyl group to CO(2) in the C-terminal aldehyde dehydrogenase domain. In this mechanism, it was not clear how the formyl group is transferred between the two catalytic domains after the first step. This study demonstrates that the intermediate domain functions similarly to an acyl carrier protein. A 4'-phosphopantetheine swinging arm bound through a phosphoester bond to Ser(354) of the intermediate domain transfers the formyl group between the catalytic domains of FDH. Thus, our study defines the intermediate domain of FDH as a novel carrier protein and provides the previously lacking component of the FDH catalytic mechanism.

Cooperation between JNK1 and JNK2 in activation of p53 apoptotic pathway

FDH (10-formyltetrahydrofolate dehydrogenase) is strongly downregulated in tumors while its elevation suppresses proliferation of cancer cells and induces p53-dependent apoptosis. We have previously shown that FDH induces phosphorylation of p53 at Ser6, which is a required step in the activation of apoptosis. In the present study, we report that FDH-induced p53 phosphorylation is carried out by JNK1 and JNK2 (c-Jun N-terminal kinases) working in concert. We have demonstrated that FDH induces phosphorylation of JNK1 and JNK2, while treatment of FDH-expressing cells with JNK inhibitor SP600125, as well as knockdown of JNK1 or JNK2 by siRNA, prevents phosphorylation of p53 at Ser6 and protects cells from apoptosis. Interestingly, the knockdown of JNK1 abolished phosphorylation of JNK2 in response to FDH, while knockdown of JNK2 did not prevent JNK1 phosphorylation. Pull-down assay with the p53-specific antibody has shown that JNK2, but not JNK1, is physically associated with p53. Our studies revealed a novel mechanism in which phosphorylation of JNK2 is mediated by JNK1 before phosphorylation of p53, and then p53 is directly phosphorylated by JNK2 at Ser6.

Metabolic derangement of methionine and folate metabolism in mice deficient in methionine synthase reductase

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. Complete ablation of methionine synthase activity in mice results in embryonic lethality. Other mouse models for hyperhomocyst(e)inemia have normal or reduced levels of methyltetrahydrofolate and are not embryonic lethal, although they have decreased ratios of AdoMet/AdoHcy and impaired methylation. We have constructed a mouse model with a gene trap insertion in the Mtrr gene specifying methionine synthase reductase, an enzyme essential for the activity of methionine synthase. This model is a hypomorph, with reduced methionine synthase reductase activity, thus avoiding the lethality associated with the absence of methionine synthase activity. Mtrr(gt/gt) mice have increased plasma homocyst(e)ine, decreased plasma methionine, and increased tissue methyltetrahydrofolate. Unexpectedly, Mtrr(gt/gt) mice do not show decreases in the AdoMet/AdoHcy ratio in most tissues. The different metabolite profiles in the various genetic mouse models for hyperhomocyst(e)inemia may be useful in understanding biological effects of elevated homocyst(e)ine.

Crystal structures of the carboxyl terminal domain of rat 10-formyltetrahydrofolate dehydrogenase: implications for the catalytic mechanism of aldehyde dehydrogenases

10-Formyltetrahydrofolate dehydrogenase (FDH) catalyzes an NADP+-dependent dehydrogenase reaction resulting in conversion of 10-formyltetrahydrofolate to tetrahydrofolate and CO2. This reaction is a result of the concerted action of two catalytic domains of FDH, the amino-terminal hydrolase domain and the carboxyl-terminal aldehyde dehydrogenase domain. In addition to participation in the overall FDH mechanism, the C-terminal domain is capable of NADP+-dependent oxidation of short chain aldehydes to their corresponding acids. We have determined the crystal structure of the C-terminal domain of FDH and its complexes with oxidized and reduced forms of NADP. Compared to other members of the ALDH family, FDH demonstrates a new mode of binding of the 2'-phosphate group of NADP via a water-mediated contact with Gln600 that may contribute to the specificity of the enzyme for NADP over NAD. The structures also suggest how Glu673 can act as a general base in both acylation and deacylation steps of the reaction. In the apo structure, the general base Glu673 is positioned optimally for proton abstraction from the sulfur atom of Cys707. Upon binding of NADP+, the side chain of Glu673 is displaced from the active site by the nicotinamide ring and contacts a chain of highly ordered water molecules that may represent a pathway for translocation of the abstracted proton from Glu673 to the solvent. When reduced, the nicotinamide ring of NADP is displaced from the active site, restoring the contact between Cys707 and Glu673 and allowing the latter to activate the hydrolytic water molecule in deacylation.

Methotrexate γ-hydroxamate derivatives as potential dual target antitumor drugs

A series of new aminopteroyl-based hydroxamate derivatives were synthesized and tested in vitro in cell culture models as potential dual target drugs. These compounds were designed to target two families of enzymes, matrix metalloproteinases (MMP) and a folate enzyme, dihydrofolate reductase (DHFR). These enzymes are the components of two unrelated cellular pathways and they are often over-expressed in metastasizing tumors. In addition to the synthesis and full structural characterization of the hybrid molecules, we describe their inhibitory activities against a series of MMPs (MMP-2, MMP-7, MMP-9, MMP-14) and DHFR, as well as their antiproliferative activity in three cancer cell lines. The new hydroxamate derivatives of MTX proved to be effective inhibitors of MMPs and DHFR in the micromolar and nanomolar range, respectively. Furthermore, they showed strong antiproliferative activity against A549 cells (non-small cell lung carcinoma), and PPC-1 and Tsu-Pr1 prostate cancer cell lines. Therefore, based on the present results, these bi-functional drugs may be good candidates to target specific tumors in animal models due to potential combined effects on two pathways crucial for tumor development.

Leucovorin-induced resistance against FDH growth suppressor effects occurs through DHFR up-regulation

10-Formyltetrahydrofolate dehydrogenase (FDH) converts 10-formyltetrahydrofolate to tetrahydrofolate (THF). Expression of the enzyme in FDH-deficient cancer cells induces cytotoxicity that can be reversed by supplementation with high concentrations of a reduced folate, 5-formyl-THF (leucovorin). In contrast, non-tumor cells are resistant to FDH. The present study was undertaken to investigate mechanisms that could protect cells against FDH suppressor effects. Using 10 microM leucovorin supplementation of FDH-sensitive A549 cells transfected for FDH expression, we selected clones that have acquired resistance against FDH. Resistant cells expressed high levels of FDH and were capable of growing after withdrawal of leucovorin. These cells, however, have increased doubling time due to prolonged S phase. They also have significantly increased levels of total folate pool and THF/5,10-methylene-THF pool while the level of 10-formyl-THF was two-fold lower than in parental FDH-sensitive cells. We have shown that the FDH-catalyzed reaction proceeds at about a three-fold slower rate at the ratio of 10-formyl-THF/THF corresponding to the resistant cells than at the ratio corresponding to parental sensitive cells, due to product inhibition (KI is 2.35 microM). FDH-resistant cells have strongly up-regulated dihydrofolate reductase (DHFR) that is proposed to be a mechanism for the alteration of folate pools and a key component of the acquired resistance. Elevation of DHFR in A549 cells by transient transfection decreased sensitivity to FDH toxicity and allowed selection of FDH-resistant clones. DHFR-induced repression of FDH catalysis could be an S phase-related metabolic adjustment that provides protection against FDH suppressor effects.

Transport of rat liver glycine N-methyltransferase into rat liver nuclei

Rat liver cytosolic glycine N-methyltransferase (GNMT) catalyzes the S-adenosylmethionine-dependent methylation of glycine to sarcosine. It is comprised of four identical 292-amino acid residue subunits. Recently, evidence has been provided to show that GNMT is identical to the cytosolic receptor for benzo[a]pyrene, which induces cytochrome P450 1A1 gene expression. In the present study we show that chemical modification of purified rat liver GNMT with fluorescein isothiocyanate (FITC) resulted in dissociation of the tetrameric enzyme and was accompanied by loss of enzyme activity. Amino acid sequence analysis of the FITC-labeled peptides obtained by hydrolysis of the modified protein with Staphylococcus aureus V8 protease revealed that lysines 45, 89, 92, 96, 122, and 147 were modified. Lys-122 and Lys-147 were derivatized in tetrameric, dimeric, and monomeric forms of the enzyme. Lysines 45, 89, 92, and 96 were derivatized only in monomeric GNMT, suggesting that modification of these residues resulted in GNMT dissociation. The modified monomeric GNMT was quickly transported into isolated rat liver nuclei. This transport was specific for the GNMT monomer, since neither tetramer nor dimer was able to enter the nuclei. Bovine carbonic anhydrase, similar in size to the GNMT monomer, was labeled with FITC to a similar extent but was not transported into the nuclei. Disruption of the nuclei containing fluorescein-labeled GNMT and subsequent extraction of the nuclear lysate with both high and low salt buffers recovered FITC-GNMT only in the chromatin pellet. Our study supports the suggestion of an additional function for GNMT, probably connected with regulation of cytochrome P450 1A1 gene expression.

Organization of the transcortin-binding domain on placental plasma membranes

Complex formation between transcortin (corticosteroid-binding globulin) and 20 kDa sialoglycoprotein from human syncytiotrophoblast plasma membranes (presumably a transcortin-recognizing subunit of the transcortin membrane receptor) was studied using FPLC and cross-linking with bifunctional reagents. The action of 1,5-difluoro-2,4-dinitrobenzene (DFDNB) on a solution of the purified 20 kDa sialoglycoprotein and transcortin resulted in formation of covalently linked complexes of 95 kDa and 140 kDa consisting of one transcortin molecule and either two or four molecules of the membrane sialoglycoprotein (the molecular mass of transcortin is 55 kDa). Additionally, cross-linking resulted in the appearance of a 43 kDa species which is the cross-linked dimer of the membrane protein. The dimer was also observed during chromatography on a Superose 12 column in the absence of DFDNB treatment. Treatment of intact syncytiotrophoblast membranes with DFDNB resulted in isolation of the transcortin binding protein dimer as the major portion of total pool of the protein. Formation of the transcortin complexes with two and four molecules of the membrane protein was also observed when the membranes were incubated with 125I-labeled transcortin and treated with DFDNB, but formation of the latter complexes predominated. The results obtained suggest that the recognizing and binding domain for transcortin in placental membranes is organized as dimers consisting of non-covalently linked sialoglycoprotein monomers of a 20 kDa each and that transcortin has two sites for interaction with this dimer. Apparently, binding of two dimers results in the formation of the functional form of the transcortin-receptor complex. The possible biological role of such a complex is discussed.

Interaction of sex hormone-binding globulin with plasma membranes from the rat epididymis and other tissues

The binding of human sex hormone-binding globulin (hSHBG) to plasma membranes prepared from the adult rat epididymis and other potential target and non-target tissues was examined. Specific binding sites were detected in the epididymis, testis, prostate, skeletal muscle and liver. The first three organs exhibited a higher (KD approx. 0.1 nM; Bmax approx. 0.05-0.10 pmol/mg membrane protein, Site I) and a lower (KD approx. 5 nM; Bmax approx. 1.0-2.5 pmol/mg membrane protein, Site II) affinity binding site. Only Site I was detected in muscle membranes and only Site II was detected in membranes isolated from liver. Specific binding was not detectable in either spleen or brain. Regional distribution of hSHBG binding sites occurred in the epididymis. Both Site I and Site II were present in the proximal caput and distal cauda. The distal caput and proximal cauda contained only Site II; no specific binding was detected in the corpus. Binding of hSHBG to epididymal membranes was time- and temperature-dependent. The presence of Ca2+ did not affect binding. Non-liganded [125I]-labeled hSHBG can bind to both sites in epididymal membranes. The affinity of hSHBG for Site I increased 2-fold when it was complexed with 5 alpha-dihydrotestosterone, testosterone or estradiol. The hSHBG-androgen complex had little effect on Site II versus steroid-free SHBG. However, the affinity of the hSHBG-estradiol complex for these sites was increased 10-fold. Cortisol, which has a low affinity for hSHBG, did not influence its binding to either the higher or lower affinity membrane sites.

Binding of human sex hormone-binding globulin-androgen complexes to the placental syncytiotrophoblast membrane

We have found that human SHBG complexed with androgens binds specifically to the plasma membrane of human placental syncytiotrophoblast. Apparent equilibrium association constants were 5.3.10(11) M-1 for SHBG-testosterone complex and 1.1.10(11) M-1 for SHBG-5 alpha-dihydrotestosterone. Devoid of steroid, SHBG did not bind to the membrane. This suggests that the specific membrane binding of SHBG-androgen complexes is a step of the mechanism of androgen action on syncytiotrophoblast.