Disrupted liver oxidative metabolism in glycine N-methyltransferase-deficient mice is mitigated by dietary methionine restriction.

OBJECTIVE: One-carbon metabolism is routinely dysregulated in nonalcoholic fatty liver disease. This includes decreased glycine N-methyltransferase (GNMT), a critical regulator of s-adenosylmethionine (SAM). Deletion of GNMT in mice increases SAM and promotes liver steatosis. Lower liver oxidative metabolism, as indicated by a decline in gluconeogenesis, citric acid cycle flux, and oxidative phosphorylation contributes to liver steatosis in GNMT-null mice; however, the extent to which higher SAM mediates this phenotype remains unclear. Here, we determined the SAM-dependent impairment in liver oxidative metabolism by loss of GNMT. METHODS: GNMT knockout (KO) mice were fed a methionine-restricted diet to prevent increased SAM. 2H/13C metabolic flux analysis was performed in conscious, unrestrained mice to quantify liver nutrient fluxes. Metabolomics and high-resolution respirometry were used to quantify liver nutrient pool sizes and mitochondrial oxidative phosphorylation, respectively. Folic acid-supplemented and serine/glycine-deficient diets were used independently to further define the metabolic implications of perturbed one-carbon donor availability. RESULTS: Dietary methionine restriction prevented a 75-fold increase in SAM and a 53% rise in triacylglycerides in livers of KO mice. Dietary methionine restriction increased gluconeogenesis, independent of genotype, and restored cytochrome c oxidase respiratory function in KO mice. Citric acid cycle fluxes remained lower in KO mice irrespective of diet. Restricting dietary methionine abrogated markers of increased lipogenesis and folate cycle dysfunction in KO mice. CONCLUSIONS: The impaired liver oxidative metabolism following loss of GNMT is both dependent and independent of greater SAM availability. Lower in vivo citric acid cycle flux is independent of increased SAM. In contrast, gluconeogenesis and oxidative phosphorylation are negatively regulated by excess SAM. Lipid accumulation in livers of mice lacking GNMT is also linked to higher SAM.

Discovery of an Orally Efficacious MYC Inhibitor for Liver Cancer Using a GNMT-Based High-Throughput Screening System and Structure-Activity Relationship Analysis.

Glycine-N-methyl transferase (GNMT) downregulation results in spontaneous hepatocellular carcinoma (HCC). Overexpression of GNMT inhibits the proliferation of liver cancer cell lines and prevents carcinogen-induced HCC, suggesting that GNMT induction is a potential approach for anti-HCC therapy. Herein, we used Huh7 GNMT promoter-driven screening to identify a GNMT inducer. Compound K78 was identified and validated for its induction of GNMT and inhibition of Huh7 cell growth. Subsequently, we employed structure-activity relationship analysis and found a potent GNMT inducer, K117. K117 inhibited Huh7 cell growth in vitro and xenograft in vivo. Oral administration of a dosage of K117 at 10 mpk (milligrams per kilogram) can inhibit Huh7 xenograft in a manner equivalent to the effect of sorafenib at a dosage of 25 mpk. A mechanistic study revealed that K117 is an MYC inhibitor. Ectopic expression of MYC using CMV promoter blocked K117-mediated MYC inhibition and GNMT induction. Overall, K117 is a potential lead compound for HCC- and MYC-dependent cancers.

Differential expression of NPM, GSTA3, and GNMT in mouse liver following long-term in vivo irradiation by means of uranium tailings.

Uranium tailings (UT) are formed as a byproduct of uranium mining and are of potential risk to living organisms. In the present study, we sought to identify potential biomarkers associated with chronic exposure to low dose rate γ radiation originating from UT. We exposed C57BL/6J mice to 30, 100, or 250 µGy/h of gamma radiation originating from UT samples. Nine animals were included in each treatment group. We observed that the liver central vein was significantly enlarged in mice exposed to dose rates of 100 and 250 µGy/h, when compared with nonirradiated controls. Using proteomic techniques, we identified 18 proteins that were differentially expressed (by a factor of at least 2.5-fold) in exposed animals, when compared with controls. We chose glycine N-methyltransferase (GNMT), glutathione S-transferase A3 (GSTA3), and nucleophosmin (NPM) for further investigations. Our data showed that GNMT (at 100 and 250 µGy/h) and NPM (at 250 µGy/h) were up-regulated, and GSTA3 was down-regulated in all of the irradiated groups, indicating that their expression is modulated by chronic gamma radiation exposure. GNMT, GSTA3, and NPM may therefore prove useful as biomarkers of gamma radiation exposure associated with UT. The mechanisms underlying those changes need to be further studied.

Sarcosine Up-Regulates Expression of Genes Involved in Cell Cycle Progression of Metastatic Models of Prostate Cancer.

The effects of sarcosine on the processes driving prostate cancer (PCa) development remain still unclear. Herein, we show that a supplementation of metastatic PCa cells (androgen independent PC-3 and androgen dependent LNCaP) with sarcosine stimulates cells proliferation in vitro. Similar stimulatory effects were observed also in PCa murine xenografts, in which sarcosine treatment induced a tumor growth and significantly reduced weight of treated mice (p < 0.05). Determination of sarcosine metabolism-related amino acids and enzymes within tumor mass revealed significantly increased glycine, serine and sarcosine concentrations after treatment accompanied with the increased amount of sarcosine dehydrogenase. In both tumor types, dimethylglycine and glycine-N-methyltransferase were affected slightly, only. To identify the effects of sarcosine treatment on the expression of genes involved in any aspect of cancer development, we further investigated expression profiles of excised tumors using cDNA electrochemical microarray followed by validation using the semi-quantitative PCR. We found 25 differentially expressed genes in PC-3, 32 in LNCaP tumors and 18 overlapping genes. Bioinformatical processing revealed strong sarcosine-related induction of genes involved particularly in a cell cycle progression. Our exploratory study demonstrates that sarcosine stimulates PCa metastatic cells irrespectively of androgen dependence. Overall, the obtained data provides valuable information towards understanding the role of sarcosine in PCa progression and adds another piece of puzzle into a picture of sarcosine oncometabolic potential.

Identification of 1,2,3,4,6-Penta-O-galloyl-ß-d-glucopyranoside as a Glycine N-Methyltransferase Enhancer by High-Throughput Screening of Natural Products Inhibits Hepatocellular Carcinoma.

Glycine N-methyltransferase (GNMT) expression is vastly downregulated in hepatocellular carcinomas (HCC). High rates of GNMT knockout mice developed HCC, while overexpression of GNMT prevented aflatoxin-induced carcinogenicity and inhibited liver cancer cell proliferation. Therefore, in this study, we aimed for the identification of a GNMT inducer for HCC therapy. We established a GNMT promoter-driven luciferase reporter assay as a drug screening platform. Screening of 324 pure compounds and 480 crude extracts from Chinese medicinal herbs resulted in the identification of Paeonia lactiflora Pall (PL) extract and the active component 1,2,3,4,6-penta-O-galloyl-ß-d-glucopyranoside (PGG) as a GNMT inducer. Purified PL extract and PGG induced GNMT mRNA and protein expression in Huh7 human hepatoma cells and in xenograft tumors. PGG and PL extract had potent anti-HCC effects both in vitro and in vivo. Furthermore, PGG treatment induced apoptosis in Huh7 cells. Moreover, PGG treatment sensitized Huh7 cells to sorafenib treatment. Therefore, these results indicated that identifying a GNMT enhancer using the GNMT promoter-based assay might be a useful approach to find drugs for HCC. These data also suggested that PGG has therapeutic potential for the treatment of HCC.

Relation of exposure to amino acids involved in sarcosine metabolic pathway on behavior of non-tumor and malignant prostatic cell lines.

BACKGROUND: Sarcosine (N-methylglycine) was previously delineated as a substantial oncometabolite of prostate cancer (PCa) and its metabolism seems to be significantly involved in PCa development and behavior. METHODS: We focused on investigation whether the exposure of prostate cells (PNT1A, 22Rv1, and PC-3) to sarcosine-related amino acids (glycine, dimethylglycine, and sarcosine) affects their aggressiveness (cell mobility and division rates, using real-time cell based assay). The effect of supplementation on expression of glycine-N-methyltransferase (GNMT) mRNA was examined using qRT-PCR. Finally, post-treatment amino acids patterns were determined with consequent statistical processing using the Ward's method, factorial ANOVA and principal component analysis (P < 0.05). RESULTS: The highest migration induced sarcosine and glycine in metastatic PC-3 cells (a decrease in relative free area about 53% and 73%). The highest cell division was achieved after treatment of 22Rv1 and PC-3 cells with sarcosine (time required for division decreased by 65% or 45%, when compared to untreated cells). qRT-PCR revealed also significant effects on expression of GNMT. Finally, amino acid profiling shown specific amino acid patterns for each cell line. In both, treated and untreated PC-3 cells significantly higher levels of serine, glutamic acid, and aspartate, linked with prostate cancer progression were found. CONCLUSIONS: Sarcosine-related amino acids can exceptionally affect the behavior of benign and malignant prostate cells.

Betaine supplementation prevents fatty liver induced by a high-fat diet: effects on one-carbon metabolism.

The purpose of this study was to examine the effects of betaine supplementation on the regulation of one-carbon metabolism and liver lipid accumulation induced by a high-fat diet in rats. Rats were fed one of three different liquid diets: control diet, high-fat diet and high-fat diet supplemented with betaine. The control and high-fat liquid diets contained, respectively, 35 and 71 % of energy derived from fat. Betaine supplementation involved the addition of 1 % (g/L) to the diet. After three weeks on the high-fat diet the rats had increased total liver fat concentration, liver triglycerides, liver TBARS and plasma TNF-α. The high-fat diet decreased the hepatic S-adenosylmethionine concentration and the S-adenosylmethionine/S-adenosylhomocysteine ratio compared to the control as well as altering the expression of genes involved in one-carbon metabolism. Betaine supplementation substantially increased the hepatic S-adenosylmethionine concentration (~fourfold) and prevented fatty liver and hepatic injury induced by the high-fat diet. It was accompanied by the normalization of the gene expression of BHMT, GNMT and MGAT, which code for key enzymes of one-carbon metabolism related to liver fat accumulation. In conclusion, the regulation of the expression of MGAT by betaine supplementation provides an additional and novel mechanism by which betaine supplementation regulates lipid metabolism and prevents accumulation of fat in the liver.

Proteomic investigation of signatures for geniposide-induced hepatotoxicity.

Evaluating the safety of traditional medicinal herbs and their major active constituents is critical for their widespread usage. Geniposide, a major active constituent with a defined structure from the traditional medicinal herb Gardenia jasminoides ELLIS fruit, exhibits remarkable anti-inflammatory, antiapoptotic, and antifibrotic properties and has been used in a variety of medical fields, mainly for the treatment of liver diseases. However, geniposide-induced hepatotoxicity and methods for the early detection of hepatotoxicity have yet to be reported. In this study, geniposide-induced hepatotoxicity was investigated. In addition, candidate biomarkers for the earlier detection of geniposide-induced hepatotoxicity were identified using a label-free quantitative proteomics approach on a geniposide overdose-induced liver injury in a rat model. Using an accurate intensity-based, absolute quantification (iBAQ)-based, one-step discovery and verification approach, a candidate biomarker panel was easily obtained from individual samples in response to different conditions. To determine the biomarkers' early detection abilities, five candidate biomarkers were selected and tested using enzyme-linked immunosorbent assays (ELISAs). Two biomarkers, glycine N-methyltransferase (GNMT) and glycogen phosphorylase (PYGL), were found to indicate hepatic injuries significantly earlier than the current gold standard liver biomarker. This study provides a first insight into geniposide-induced hepatotoxicity in a rat model and describes a method for the earlier detection of this hepatotoxicity, facilitating the efficient monitoring of drug-induced hepatotoxicity.

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.

The logic of the hepatic methionine metabolic cycle.

This review describes our current understanding of the "traffic lights" that regulate sulfur flow through the methionine bionetwork in liver, which supplies two major homeostatic systems governing cellular methylation and antioxidant potential. Theoretical concepts derived from mathematical modeling of this metabolic nexus provide insights into the properties of this system, some of which seem to be paradoxical at first glance. Cellular needs supported by this network are met by use of parallel metabolic tracks that are differentially controlled by intermediates in the pathway. A major task, i.e. providing cellular methylases with the methylating substrate, S-adenosylmethionine, is met by flux through the methionine adenosyltransferase I isoform. On the other hand, a second important function, i.e., stabilization of the blood methionine concentration in the face of high dietary intake of this amino acid, is achieved by switching to an alternative isoform, methionine adenosyltransferase III, and to glycine N-methyl transferase, which together bypass the first two reactions in the methionine cycle. This regulatory strategy leads to two metabolic modes that differ in metabolite concentrations and metabolic rates almost by an order of magnitude. Switching between these modes occurs in a narrow trigger zone of methionine concentration. Complementary experimental and theoretical analyses of hepatic methionine metabolism have been richly informative and have the potential to illuminate its response to oxidative challenge, to methionine restriction and lifespan extension studies and to diseases resulting from deficiencies at specific loci in this pathway.

Folic acid supplementation affects ROS scavenging enzymes, enhances Vegf-A, and diminishes apoptotic state in yolk sacs of embryos of diabetic rats.

We aimed to investigate the extent to which maternal diabetes with or without folic acid (FA) supplementation affects mRNA levels and protein distribution of ROS scavenging enzymes, vascular endothelial growth factor-A (Vegf-A), folate binding protein-1 (Folbp-1), and apoptosis-associated proteins in the yolk sacs of rat embryos on gestational days 10 and 11. Commencing at conception and throughout pregnancy, half of the streptozotocin-diabetic and half of the control rats received daily FA injections. Maternal diabetes impaired vascular morphology and decreased CuZnSOD and GPX-1 gene expression in yolk sacs. Maternal diabetes also increased the levels of CuZnSOD protein, increased the Bax/Bcl-2 protein ratio and decreased Vegf-A protein distribution. FA treatment normalized vascular morphology, decreased mRNA levels of all three SOD isoforms and increased Vegf-A mRNA levels, rectified CuZnSOD protein distribution and Bax/Bcl-2 ratio. A teratogenic diabetic environment produces a state of vasculopathy, oxidative stress, and mild apoptosis in the yolk sac. FA administration normalizes vascular morphology, diminishes apoptotic rate, and increases Vegf-A gene expression and protein distribution in the yolk sac of diabetic rats.

Dietary selenium affects homocysteine metabolism differently in Fisher-344 rats and CD-1 mice.

In our previous work with rats, plasma and tissue homocysteine concentrations were decreased by selenium deprivation. The purpose of this study was to follow up and expand on that work by determining the effects of selenium status (deficient, adequate, and supranutritional) on several aspects of homocysteine metabolism involving methionine recycling and transsulfuration. A 2nd objective was to determine whether there are differences in how selenium status affects homocysteine metabolism in rats and mice. Male weanling Fischer-344 rats and male weanling CD-1 mice were fed diets containing 0, 0.2, or 2.0 microg selenium (as sodium selenite)/g for 72 d or 60 d, respectively. Plasma homocysteine and cysteine were significantly decreased by feeding rats or mice the selenium-deficient diet compared with feeding adequate or supranutritional selenium. On the other hand, plasma glutathione was increased by selenium deficiency only in rats. Also, the specific activities of liver betaine homocysteine methyltransferase and glycine N-methyltransferase were decreased by selenium deficiency in rats, but were unaffected by selenium status in mice. Real-time RT-PCR was used to determine the expression of the subunits of glutamate-cysteine ligase, which catalyzes the rate-limiting step in glutathione biosynthesis. The expression of Gclc, the catalytic subunit of glutamate-cysteine ligase, was upregulated by selenium deprivation in both rat and mouse liver. Gclm, the modifier subunit of glutamate-cysteine ligase, was downregulated in rats fed 2 microg Se/g compared with rats fed adequate or deficient selenium. Based on these findings, it is evident that selenium deficiency has different outcomes in mice and rats. These variables are all related to methionine/methyl metabolism. Although only one strain of rat was compared with one strain of mouse, this work suggests that differences between species may prove vital in determining which animal model is used in studies of selenium deficiency or in studies that are designed to ascertain chemopreventive mechanisms of selenium.

Folate status modulates the induction of hepatic glycine N-methyltransferase and homocysteine metabolism in diabetic rats.

A diabetic state induces the activity and abundance of glycine N-methyltransferase (GNMT), a key protein in the regulation of folate, methyl group, and homocysteine metabolism. Because the folate-dependent one-carbon pool is a source of methyl groups and 5-methyltetrahydrofolate allosterically inhibits GNMT, the aim of this study was to determine whether folate status has an impact on the interaction between diabetes and methyl group metabolism. Rats were fed a diet containing deficient (0 ppm), adequate (2 ppm), or supplemental (8 ppm) folate for 30 days, after which diabetes was initiated in one-half of the rats by streptozotocin treatment. The activities of GNMT, phosphatidylethanolamine N-methyltransferase (PEMT), and betaine-homocysteine S-methyltransferase (BHMT) were increased about twofold in diabetic rat liver; folate deficiency resulted in the greatest elevation in GNMT activity. The abundance of GNMT protein and mRNA, as well as BHMT mRNA, was also elevated in diabetic rats. The marked hyperhomocysteinemia in folate-deficient rats was attenuated by streptozotocin, likely due in part to increased BHMT expression. These results indicate that a diabetic state profoundly modulates methyl group, choline, and homocysteine metabolism, and folate status may play a role in the extent of these alterations. Moreover, the upregulation of BHMT and PEMT may indicate an increased choline requirement in the diabetic rat.

Differential effects of dietary selenium (se) and folate on methyl metabolism in liver and colon of rats.

A previous study compared the effects of folate on methyl metabolism in colon and liver of rats fed a selenium-deficient diet (< 3 microg Se/kg) to those of rats fed a diet containing supranutritional Se (2 mg selenite/kg). The purpose of this study was to investigate the effects of folate and adequate Se (0.2 mg/kg) on methyl metabolism in colon and liver. Weanling, Fischer-344 rats (n = 8/diet) were fed diets containing 0 or 0.2 mg selenium (as selenite)/kg and 0 or 2 mg folic acid/kg in a 2 x 2 design. After 70 d, plasma homocysteine was increased (p < 0.0001) by folate deficiency; this increase was markedly attenuated (p < 0.0001) in rats fed the selenium-deficient diet compared to those fed 0.2 mg Se/kg. The activity of hepatic glycine N-methyltransferase (GNMT), an enzyme involved in the regulation of tissue S-adenosylmethionine (SAM) and S-adenosylhomocysteine (SAH), was increased by folate deficiency (p < 0.006) and decreased by selenium deprivation (p < 0.0003). Colon and liver SAH were highest (p < 0.006) in rats fed deficient folate and adequate selenium. Although folate deficiency decreased liver SAM (p < 0.001), it had no effect on colon SAM. Global DNA methylation was decreased (p<0.04) by selenium deficiency in colon but not liver; folate had no effect. Selenium deficiency did not affect DNA methyltransferase (Dnmt) activity in liver but tended to decrease (p < 0.06) the activity of the enzyme in the colon. Dietary folate did not affect liver or colon Dnmt. These results in rats fed adequate selenium are similar to previous results found in rats fed supranutritional selenium. This suggests that selenium deficiency appears to be a more important modifier of methyl metabolism than either adequate or supplemental selenium.