Characterization of glycine-N-methyltransferase-gene expression in human hepatocellular carcinoma

A contribution of metabolic engineering to addressing medical problems: Metabolic flux analysis

Metabolic engineering has served as a systematic discipline for industrial biotechnology as it has offered systematic tools and methods for strain development and bioprocess optimization. Because these metabolic engineering tools and methods are concerned with the biological network of a cell with emphasis on metabolic network, they have also been applied to a range of medical problems where better understanding of metabolism has also been perceived to be important. Metabolic flux analysis (MFA) is a unique systematic approach initially developed in the metabolic engineering community, and has proved its usefulness and potential when addressing a range of medical problems. In this regard, this review discusses the contribution of MFA to addressing medical problems. For this, we i) provide overview of the milestones of MFA, ii) define two main branches of MFA, namely constraint-based reconstruction and analysis (COBRA) and isotope-based MFA (iMFA), and iii) present successful examples of their medical applications, including characterizing the metabolism of diseased cells and pathogens, and identifying effective drug targets. Finally, synergistic interactions between metabolic engineering and biomedical sciences are discussed with respect to MFA.

Folic Acid Ameliorates Renal Injury in Experimental Obstructive Nephropathy: Role of Glycine N-Methyltransferase

Folic acid exerts both anti-inflammatory and antifibrotic effects. Glycine N-methyltransferase (GNMT), the major folic acid-binding protein in the liver, is a crucial enzyme that regulates the cellular methylation process by maintaining S-adenosylmethionine levels. However, as yet neither the therapeutic effects of folic acid in renal fibrosis nor whether GNMT is involved in these folic acid-associated mechanisms has been investigated. First, the expression of GNMT was examined in human kidneys with or without obstructive nephropathy. Later, wild-type and GNMT knockout (GNMT^-/-) mice were subjected to unilateral ureteral obstruction (UUO) and then treated with either folic acid or vehicle for 14 days. Renal tubular injury, inflammation, fibrosis, and autophagy were evaluated by histological analysis and Western blotting. We observed increased expression of GNMT in humans with obstructive nephropathy. Furthermore, UUO significantly increased the expression of GNMT in mice; in addition, it caused renal injury as well as the development of both hydronephrosis and tubular injury. These were all alleviated by folic acid treatment. In contrast, GNMT^-/- mice exhibited exacerbated UUO-induced renal injury, but the protective effect of folic acid was not observed in GNMT^-/- mice. We propose a novel role for folic acid in the treatment of renal fibrosis, which indicates that GNMT may be a therapeutic target.

Serine and glycine metabolism-related gene expression signature stratifies immune profiles of brain gliomas, and predicts prognosis and responses to immunotherapy

Glioma is one of the most lethal cancers and causes more than 200,000 deaths every year. Immunotherapy was an inspiring therapy for multiple cancers but failed in glioma treatment. The importance of serine and glycine and their metabolism has been well-recognized in the physiology of immune cells and microenvironment in multiple cancers. However, their correlation with prognosis, immune cells, and immune microenvironment of glioma remains unclear. In this study, we investigated the relationships between the expression pattern of serine and glycine metabolism-related genes (SGMGs) and clinicopathological features, prognosis, and tumor microenvironment in glioma based on comprehensive analyses of multiple public datasets and our cohort. According to the expression of SGMGs, we conducted the consensus clustering analysis to stratify all patients into four clusters with remarkably distinctive clinicopathological features, prognosis, immune cell infiltration, and immune microenvironment. Subsequently, a serine and glycine metabolism-related genes signature (SGMRS) was constructed based on five critical SGMGs in glioma to stratify patients into SGMRS high- and low-risk groups and tested for its prognostic value. Higher SGMRS expressed genes associated with the synthesis of serine and glycine at higher levels and manifested poorer prognosis. Besides, we confirmed that SGMRS was an independent prognostic factor and constructed nomograms with satisfactory prognosis prediction performance based on SGMRS and other factors. Analyzing the relationship between SGMRS and immune landscape, we found that higher SGMRS correlated with 'hotter' immunological phenotype and more immune cell infiltration. Furthermore, the expression levels of multiple immunotherapy-related targets, including PD-1, PD-L1, and B7-H3, were positively correlated with SGMRS, which was validated by the better predicted response to immune checkpoint inhibitors. In conclusion, our study explored the relationships between the expression pattern of SGMGs and tumor features and created novel models to predict the prognosis of glioma patients. The correlation of SGMRS with immune cells and microenvironment in gliomas suggested an essential role of serine and glycine metabolism in reforming immune cells and microenvironment. Finally, the results of our study endorsed the potential application of SGMRS to guide the selection of immunotherapy for gliomas.

Multi-Channel Metabolomics Analysis Identifies Novel Metabolite Biomarkers for the Early Detection of Fatty Liver Disease in Dairy Cows

Fatty liver disease, a type of metabolic disorder, frequently occurs in dairy cows during the parturition period, causing a high culling rate and, therefore, considerable economic losses in the dairy industry owing to the lack of effective diagnostic methods. Here, metabolite biomarkers were identified and validated for the diagnosis of metabolic disorders. A total of 58 participant cows, including severe fatty liver disease and normal control groups, in the discovery set (liver biopsy tested, n = 18), test set (suspected, n = 20) and verification set (liver biopsy tested, n = 20), were strictly recruited and a sample collected for their feces, urine, and serum. Non-targeted GC-MS-based metabolomics methods were used to characterize the metabolite profiles and to screen in the discovery set. Eventually, ten novel biomarkers involved in bile acid, amino acid, and fatty acid were identified and validated in the test set. Each of them had a higher diagnostic ability than the traditional serum biochemical indicators, with an average area under the receiver operating characteristic curve of 0.830 ± 0.0439 (n = 10) versus 0.377 ± 0.182 (n = 9). Especially, combined biomarker panels via different metabolic pipelines had much better diagnostic sensitivity and specificity than every single biomarker, suggesting their powerful utilization potentiality for the early detection of fatty liver disease. Intriguingly, the serum biomarkers were confirmed perfectly in the verification set. Moreover, common biological pathways were found to be underlying the pathogenesis of fatty liver syndrome in cattle via different metabolic pipelines. These newly-discovered and non-invasive metabolic biomarkers are meaningful in reducing the high culling rate of cows and, therefore, benefit the sustainable development of the dairy industry.

Reversal of High-Fat Diet-Induced Non-Alcoholic Fatty Liver Disease by Metformin Combined with PGG, an Inducer of Glycine N-Methyltransferase

Nonalcoholic fatty liver disease (NAFLD) is a major cause of liver-related morbidities and mortality, and no effective drug treatment currently exists. We aimed to develop a novel treatment strategy to induce the expression of glycine N-methyltransferase (GNMT), which is an important enzyme regulating S-adenosylmethionine metabolism whose expression is downregulated in patients with NAFLD. Because 1,2,3,4,6-pentagalloyl glucose (PGG) is a GNMT inducer, and metformin was shown to upregulate liver mitochondrial GNMT protein expression, the effect of PGG and metformin was evaluated. Biochemical analysis, histopathological examination, immunohistochemical staining, reverse transcription-quantitative PCR (RT-qPCR), Western blotting (WB), proteomic analysis and Seahorse XF Cell Mito Stress Test were performed. The high-fat diet (HFD)-induced NAFLD mice were treated with PGG and metformin. Combination of PGG and metformin nearly completely reversed weight gain, elevation of serum aminotransferases, and hepatic steatosis and steatohepatitis. In addition, the downregulated GNMT expression in liver tissues of HFD-induced NAFLD mice was restored. The GNMT expression was further confirmed by RT-qPCR and WB analysis using both in vitro and in vivo systems. In addition, PGG treatment was shown to increase oxygen consumption rate (OCR) maximum capacity in a dose-dependent manner, and was capable of rescuing the suppression of mitochondrial OCR induced by metformin. Proteomic analysis identified increased expression of glutathione S-transferase mu 4 (GSTM4), heat shock protein 72 (HSP72), pyruvate carboxylase (PYC) and 40S ribosomal protein S28 (RS28) in the metformin plus PGG treatment group. Our findings show that GNMT expression plays an important role in the pathogenesis of NAFLD, and combination of an inducer of GNMT and metformin can be of therapeutic potential for patients with NAFLD.

Nuclear localization dictates hepatocarcinogenesis suppression by glycine N-methyltransferase

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GNMT gene expression contributes to determine hepatocellular carcinoma (HCC) prognosis.

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GNMT expression is genetically determined.

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Nuclear GNMT binds to CYP1A1, PREX2, PARP1, and NFKB gene promoters and strongly inhibits their expression.

Background

GNMT (glycine N-methyltransferase) is a tumor suppressor gene, but the mechanisms mediating its suppressive activity are not entirely known.

Methods

We investigated the oncosuppressive mechanisms of GNMT in human hepatocellular carcinoma (HCC). GNMT mRNA and protein levels were evaluated by quantitative RT-PCR and immunoblotting. GNMT effect in HCC cell lines was modulated through GNMT cDNA induced overexpression or anti-GNMT siRNA transfection.

Results

GNMT was expressed at low level in human HCCs with a better prognosis (HCCB) while it was almost absent in fast-growing tumors (HCCP). In HCCB, the nuclear localization of the GNMT protein was much more pronounced than in HCCP. In Huh7 and HepG2 cell lines, GNMT forced expression inhibited the proliferation and promoted apoptosis. At the molecular level, GNMT overexpression inhibited the expression of CYP1A (Cytochrome p450, aromatic compound-inducible), PREX2 (Phosphatidylinositol-3,4,5-trisphosphate-dependent Rac exchange factor 2), PARP1 [Poly (ADP-ribose) polymerase 1], and NFKB (nuclear factor-kB) genes. By chromatin immunoprecipitation, we found GNMT binding to the promoters of CYP1A1, PREX2, PARP1, and NFKB genes resulting in their strong inhibition. These genes are implicated in hepatocarcinogenesis, and are involved in the GNMT oncosuppressive action.

Conclusion

Overall, the present data indicate that GNMT exerts a multifaceted suppressive action by interacting with various cancer-related genes and inhibiting their expression.

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.

Sex Differences in the Incidence of Obesity-Related Gastrointestinal Cancer

Cancer is the second leading cause of death worldwide, with 9.6 million people estimated to have died of cancer in 2018. Excess body fat deposition is a risk factor for many types of cancer. Men and women exhibit differences in body fat distribution and energy homeostasis regulation. This systematic review aimed to understand why sex disparities in obesity are associated with sex differences in the incidence of gastrointestinal cancers. Cancers of the esophagus, liver, and colon are representative gastrointestinal cancers, and obesity is a convincing risk factor for their development. Numerous epidemiological studies have found sex differences in the incidence of esophageal, liver, and colorectal cancers. We suggest that these sexual disparities are partly explained by the availability of estrogens and other genetic factors regulating inflammation, cell growth, and apoptosis. Sex differences in gut microbiota composition may contribute to differences in the incidence and phenotype of colorectal cancer. To establish successful practices in personalized nutrition and medicine, one should be aware of the sex differences in the pathophysiology and associated mechanisms of cancer development.

Proteome characteristics of liver tissue from patients with parenteral nutrition-associated liver disease

Background

Parenteral nutrition (PN)-associated liver disease (PNALD) is a common and life-threatening complication in patients receiving PN. However, its definitive etiology is not yet clear. Therefore, performed proteomic analyses of human liver tissue to explore the same.

Methods

Liver tissue was derived and compared across selected patients with (n = 3) /without (n = 4) PNALD via isobaric Tag for Relative and Absolute Quantitation (iTRAQ)-based quantitative proteomics. Bioinformatics analysis was performed using Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) databases to explore the mechanisms of PNALD based on differentially expressed proteins (DEPs). The essential proteins that were differentially expressed between the two groups were explored and verified by western blotting.

Results

A total of 112 proteins were found to be differentially expressed, of which 73 were downregulated, and 39 were upregulated in the PNALD group. Bioinformatics analysis showed DEPs to be associated with mitochondrial oxidative phosphorylation (mainly involved in mitochondrial respiratory chain complex I assembly), hepatic glycolipid metabolism (involved primarily in glycogen formation and gluconeogenesis), and oxidative stress (mainly involved in antioxidant change).

Conclusion

Overall, our results indicated that mitochondrial energy metabolism impairment, hepatic glycolipid metabolism disorder, and excessive oxidative stress injury might explain the comprehensive mechanism underlying PNALD. Moreover, we have provided multiple potential targets for further exploring the PNALD mechanism.

Chemical Constituents with GNMT-Promoter-Enhancing and NRF2-Reduction Activities from Taiwan Agarwood Excoecaria formosana

Hepatocellular carcinoma (HCC) is considered to be a silent killer, and was the fourth leading global cause of cancer deaths in 2018. For now, sorafenib is the only approved drug for advanced HCC treatment. The introduction of additional chemopreventive agents and/or adjuvant therapies may be helpful for the treatment of HCC. After screening 3000 methanolic extracts from the Formosan plant extract bank, Excoecaria formosana showed glycine N-methyltransferase (GNMT)-promoter-enhancing and nuclear factor erythroid 2-related factor 2 (NRF2)-suppressing activities. Further, the investigation of the whole plant of E. formosana led to the isolation of a new steroid, 7α-hydroperoxysitosterol-3-O-β-d-(6-O-palmitoyl)glucopyranoside (1); two new coumarinolignans, excoecoumarin A (2) and excoecoumarin B (3); a new diterpene, excoeterpenol A (4); and 40 known compounds (5-44). Among them, Compounds 38 and 40-44 at a 100 μM concentration showed a 2.97 ± 0.27-, 3.17 ± 1.03-, 2.73 ± 0.23-, 2.63 ± 0.14-, 6.57 ± 0.13-, and 2.62 ± 0.05-fold increase in GNMT promoter activity, respectively. In addition, Compounds 40 and 43 could reduce NRF2 activity, a transcription factor associated with drug resistance, in Huh7 cells with relative activity of 33.1 ± 0.2% and 45.2 ± 2.5%. These results provided the basis for the utilization of Taiwan agarwood for the development of anti-HCC agents.

miR-873-5p targets mitochondrial GNMT-Complex II interface contributing to non-alcoholic fatty liver disease

Objective

Non-alcoholic fatty liver disease (NAFLD) is a complex pathology in which several dysfunctions, including alterations in metabolic pathways, mitochondrial functionality and unbalanced lipid import/export, lead to lipid accumulation and progression to inflammation and fibrosis. The enzyme glycine N-methyltransferase (GNMT), the most important enzyme implicated in S-adenosylmethionine catabolism in the liver, is downregulated during NAFLD progression. We have studied the mechanism involved in GNMT downregulation by its repressor microRNA miR-873-5p and the metabolic pathways affected in NAFLD as well as the benefit of recovery GNMT expression.

Methods

miR-873-5p and GNMT expression were evaluated in liver biopsies of NAFLD/NASH patients. Different in vitro and in vivo NAFLD murine models were used to assess miR-873-5p/GNMT involvement in fatty liver progression through targeting of the miR-873-5p as NAFLD therapy.

Results

We describe a new function of GNMT as an essential regulator of Complex II activity in the electron transport chain in the mitochondria. In NAFLD, GNMT expression is controlled by miR-873-5p in the hepatocytes, leading to disruptions in mitochondrial functionality in a preclinical murine non-alcoholic steatohepatitis (NASH) model. Upregulation of miR-873-5p is shown in the liver of NAFLD/NASH patients, correlating with hepatic GNMT depletion. Importantly, NASH therapies based on anti-miR-873-5p resolve lipid accumulation, inflammation and fibrosis by enhancing fatty acid β-oxidation in the mitochondria. Therefore, miR-873-5p inhibitor emerges as a potential tool for NASH treatment.

Conclusion

GNMT participates in the regulation of metabolic pathways and mitochondrial functionality through the regulation of Complex II activity in the electron transport chain. In NAFLD, GNMT is repressed by miR-873-5p and its targeting arises as a valuable therapeutic option for treatment.

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The microRNA miR-873-5p is upregulated in human and murine NAFLD/NASH livers.

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miR-873-5p upregulation downregulates GNMT in the liver.

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miR-873-5p inhibition reduces liver steatosis, inflammation and fibrosis in in vivo NAFLD mouse models.

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GNMT is a hepatic metabolic hub with mitochondria activity through the regulation of Complex II of the ETC.

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Mitochondrial GNMT deficiency compromises ETC functionality and metabolism.

Gene Expression and DNA Methylation Alterations in the Glycine N-Methyltransferase Gene in Diet-Induced Nonalcoholic Fatty Liver Disease-Associated Carcinogenesis

Nonalcoholic fatty liver disease (NAFLD) is becoming a major etiological risk factor for hepatocellular carcinoma (HCC) in the United States and other Western countries. In this study, we investigated the role of gene-specific promoter cytosine DNA methylation and gene expression alterations in the development of NAFLD-associated HCC in mice using (1) a diet-induced animal model of NAFLD, (2) a Stelic Animal Model of nonalcoholic steatohepatitis-derived HCC, and (3) a choline- and folate-deficient (CFD) diet (CFD model). We found that the development of NAFLD and its progression to HCC was characterized by down-regulation of glycine N-methyltransferase (Gnmt) and this was mediated by progressive Gnmt promoter cytosine DNA hypermethylation. Using a panel of genetically diverse inbred mice, we observed that Gnmt down-regulation was an early event in the pathogenesis of NAFLD and correlated with the extent of the NAFLD-like liver injury. Reduced GNMT expression was also found in human HCC tissue and liver cancer cell lines. In in vitro experiments, we demonstrated that one of the consequences of GNMT inhibition was an increase in genome methylation facilitated by an elevated level of S-adenosyl-L-methionine. Overall, our findings suggest that reduced Gnmt expression caused by promoter hypermethylation is one of the key molecular events in the development of NAFLD-derived HCC and that assessing Gnmt methylation level may be useful for disease stratification.

Tumor suppressor gene glycine N-methyltransferase and its potential in liver disorders and hepatocellular carcinoma

Glycine N-methyltransferase is a protein with many functions. In addition to catalyzing the production of sarcosine in the one carbon metabolism pathway, it plays a role in the detoxification of environmental carcinogens such as benzo[a]pyrene, aflatoxin B1, and aristocholic acid. There is also increasing evidence suggesting a role of GNMT deficiency in liver carcinogenesis. In this review, we discuss the role of GNMT in the detoxification of xenobiotics and the mechanism of GNMT suppression during liver tumorigenesis. The protective role of GNMT in the liver allows GNMT to not only serve as a marker of liver disease, but also potentially be applied in the treatment of liver disorders and hepatocellular carcinoma. We describe the potential use of GNMT in gene therapy and we introduce the development of a GNMT promoter reporter assay that can be used to screen medicinal drugs and herbal libraries for natural compounds with anti-cancer properties.

Dysregulated transmethylation leading to hepatocellular carcinoma compromises redox homeostasis and glucose formation

Objective

The loss of liver glycine N-methyltransferase (GNMT) promotes liver steatosis and the transition to hepatocellular carcinoma (HCC). Previous work showed endogenous glucose production is reduced in GNMT-null mice with gluconeogenic precursors being used in alternative biosynthetic pathways that utilize methyl donors and are linked to tumorigenesis. This metabolic programming occurs before the appearance of HCC in GNMT-null mice. The metabolic physiology that sustains liver tumor formation in GNMT-null mice is unknown. The studies presented here tested the hypothesis that nutrient flux pivots from glucose production to pathways that incorporate and metabolize methyl groups in GNMT-null mice with HCC.

Methods

²H/¹³C metabolic flux analysis was performed in conscious, unrestrained mice lacking GNMT to quantify glucose formation and associated nutrient fluxes. Molecular analyses of livers from mice lacking GNMT including metabolomic, immunoblotting, and immunochemistry were completed to fully interpret the nutrient fluxes.

Results

GNMT knockout (KO) mice showed lower blood glucose that was accompanied by a reduction in liver glycogenolysis and gluconeogenesis. NAD⁺ was lower and the NAD(P)H-to-NAD(P)⁺ ratio was higher in livers of KO mice. Indices of NAD⁺ synthesis and catabolism, pentose phosphate pathway flux, and glutathione synthesis were dysregulated in KO mice.

Conclusion

Glucose precursor flux away from glucose formation towards pathways that regulate redox status increase in the liver. Moreover, synthesis and scavenging of NAD⁺ are both impaired resulting in reduced concentrations. This metabolic program blunts an increase in methyl donor availability, however, biosynthetic pathways underlying HCC are activated.

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Loss of glycine N-methyltransferase results in hepatocellular carcinoma.

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Metabolic reprogramming ensues to attenuate the increased S-adenosylmethionine.

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The metabolic changes include dysregulated liver NAD⁺ homeostasis and redox state.

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Liver glucose formation is reduced and precursors directed to biosynthetic pathways.

Loss of ALDH1L1 folate enzyme confers a selective metabolic advantage for tumor progression

ALDH1L1 (cytosolic 10-formyltetrahydrofolate dehydrogenase) is the enzyme in folate metabolism commonly downregulated in human cancers. One of the mechanisms of the enzyme downregulation is methylation of the promoter of the ALDH1L1 gene. Recent studies underscored ALDH1L1 as a candidate tumor suppressor and potential marker of aggressive cancers. In agreement with the ALDH1L1 loss in cancer, its re-expression leads to inhibition of proliferation and to apoptosis, but also affects migration and invasion of cancer cells through a specific folate-dependent mechanism involved in invasive phenotype. A growing body of literature evaluated the prognostic value of ALDH1L1 expression for cancer disease, the regulatory role of the enzyme in cellular proliferation, and associated metabolic and signaling cellular responses. Overall, there is a strong indication that the ALDH1L1 silencing provides metabolic advantage for tumor progression at a later stage when unlimited proliferation and enhanced motility become critical processes for the tumor expansion. Whether the ALDH1L1 loss is involved in tumor initiation is still an open question.

Induction of GNMT by 1,2,3,4,6-penta-O-galloyl-beta-D-glucopyranoside through proteasome-independent MYC downregulation in hepatocellular carcinoma

Glycine-N-methyl transferase (GNMT) a tumor suppressor for hepatocellular carcinoma (HCC) plays a crucial role in liver homeostasis. Its expression is downregulated in almost all the tumor tissues of HCC while the mechanism of this downregulation is not yet fully understood. Recently, we identified 1,2,3,4,6-penta-O-galloyl-beta-D-glucopyranoside (PGG) as a GNMT promoter enhancer compound in HCC. In this study, we aimed to delineate the mechanism by which PGG enhances GNMT expression and to investigate its effect on GNMT suppression in HCC. Microarray and pathway enrichment analysis revealed that MYC was a major target of PGG. PGG suppressed MYC mRNA and protein expression in Huh7 and Hep G2 cells in a dose- and time-dependent fashion. Furthermore, MYC expression was also reduced in xenograft tumors in PGG treated mice. Moreover, shRNA-mediated knocked-down or pharmacological inhibition of MYC resulted in a significant induction of GNMT promoter activity and endogenous GNMT mRNA expression in Huh7 cells. In contrast, overexpression of MYC significantly inhibited GNMT promoter activity and endogenous GNMT protein expression. In addition, antibodies against MYC effectively precipitated the human GNMT promoter in a chromatin immunoprecipitation assay. Lastly, GNMT expression was negatively correlated with MYC expression in human HCC samples. Interestingly, PGG not only inhibited MYC gene expression but also promoted MYC protein degradation through proteasome-independent pathways. This work reveals a novel anticancer mechanism of PGG via downregulation of MYC expression and establishes a therapeutic rationale for treatment of MYC overexpressing cancers using PGG. Our data also provide a novel mechanistic understanding of GNMT regulation through MYC in the pathogenesis of HCC.

ZNF300 stimulates fatty acid oxidation and alleviates hepatosteatosis through regulating PPARα

ZNF300 plays an important role in the regulation of HBV-related hepatocellular carcinoma. However, little is known about the role of ZNF300 in lipid metabolism and NAFLD. In the present study, we observed that ZNF300 expression was markedly decreased in free fatty acid (FFA)-induced fatty liver. Overexpressed ZNF300 alleviated hepatic lipid accumulation, whereas knockdown of ZNF300 enhanced the FFA-induced lipid accumulation. Investigations of the underlying mechanisms revealed that ZNF300 directly binds to and regulates the PPARα expression, thus promoting fatty acid oxidation. Furthermore, bisulfite pyrosequencing PCR (BSP) analysis identified the hypermethylation status of ZNF300 gene in FFA-treated hepatocytes. Importantly, the suppression of ZNF300 could be blocked by DNA methyltransferase inhibitor (5-azadC) or DNMT3a-siRNA. These results suggested that ZNF300 plays an important role in hepatic lipid metabolism via PPARα promoting fatty acid oxidation and this effect might be blocked by DNMT3a-mediated methylation of ZNF300. Therefore, in addition to ZNF300 expression levels, the methylation status of this gene also has a potential as a prognostic biomarker.

MiR-873-5p acts as an epigenetic regulator in early stages of liver fibrosis and cirrhosis

Glycine N-methyltransferase (GNMT) is the most abundant methyltransferase in the liver and a master regulator of the transmethylation flux. GNMT downregulation leads to loss of liver function progressing to fibrosis, cirrhosis, and hepatocellular carcinoma. Moreover, GNMT deficiency aggravates cholestasis-induced fibrogenesis. To date, little is known about the mechanisms underlying downregulation of GNMT levels in hepatic fibrosis and cirrhosis. On this basis, microRNAs are epigenetic regulatory elements that play important roles in liver pathology. In this work, we aim to study the regulation of GNMT by microRNAs during liver fibrosis and cirrhosis. Luciferase assay on the 3ʹUTR-Gnmt was used to confirm in silico analysis showing that GNMT is potentially targeted by the microRNA miR-873-5p. Correlation between GNMT and miR-873-5p in human cholestasis and cirrhosis together with miR-873-5p inhibition in vivo in different mouse models of liver cholestasis and fibrosis [bile duct ligation and Mdr2 (Abcb4)^-/- mouse] were then assessed. The analysis of liver tissue from cirrhotic and cholestatic patients, as well as from the animal models, showed that miR-873-5p inversely correlated with the expression of GNMT. Importantly, high circulating miR-873-5p was also detected in cholestastic and cirrhotic patients. Preclinical studies with anti-miR-873-5p treatment in bile duct ligation and Mdr2^-/- mice recovered GNMT levels in association with ameliorated inflammation and fibrosis mainly by counteracting hepatocyte apoptosis and cholangiocyte proliferation. In conclusion, miR-873-5p emerges as a novel marker for liver fibrosis, cholestasis, and cirrhosis and therapeutic approaches based on anti-miR-873-5p may be effective treatments for liver fibrosis and cholestatic liver disease.

AAV serotype 8-mediated liver specific GNMT expression delays progression of hepatocellular carcinoma and prevents carbon tetrachloride-induced liver damage

Glycine N-methyltransferase (GNMT) is abundantly expressed in normal livers and plays a protective role against tumor formation. GNMT depletion leads to progression of hepatocellular carcinoma (HCC). In this study, we investigated the activity of ectopic GNMT delivered using recombinant adeno-associated virus (AAV) gene therapy in mouse models of liver cirrhosis and HCC. Injection of AAV serotype 8 (AAV8) vector carrying the GNMT gene (AAV8-GNMT) in Gnmt^−/− mice increased GNMT expression and downregulated pro-inflammatory responses, resulting in reduced liver damage and incidence of liver tumors. Moreover, AAV8-GNMT resulted in the amelioration of carbon tetrachloride (CCl₄)-induced liver fibrosis in BALB/c mice. We showed that AAV8-GNMT protected hepatocytes from CCl₄-induced liver damage.  AAV8-GNMT significantly attenuated the levels of pro-fibrotic markers and increased efficiency of hepatocyte proliferation. These results suggest that correction of hepatic GNMT by gene therapy of AAV8-mediated gene enhancement may provide a potential strategy for preventing and delaying development of liver diseases.

MicroRNA-224 down-regulates Glycine N-methyltransferase gene expression in Hepatocellular Carcinoma

Glycine N-methyltransferase (GNMT) is a tumor suppressor for HCC. It is down-regulated in HCC, but the mechanism is not fully understood. MicroRNA-224 (miR-224) acts as an onco-miR in HCC. This study is the first to investigate miR-224 targeting the coding region of GNMT transcript. The GNMT-MT plasmid containing a miR-224 binding site silent mutation of the GNMT coding sequence can escape the suppression of miR-224 in HEK293T cells. Expression of both exogenous and endogenous GNMT was suppressed by miR-224, while miR-224 inhibitor enhanced GNMT expression. miR-224 counteracts the effects of GNMT on the reduction of cell proliferation and tumor growth. The levels of miR-224 and GNMT mRNA showed a significant inverse relationship in tumor specimens from HCC patients. Utilizing CCl4-treated hepatoma cells and mice as a cell damage of inflammatory or liver injury model, we observed that the decreased expression levels of GNMT were accompanied with the elevated expression levels of miR-224 in hepatoma cells and mouse liver. Finally, hepatic AAV-mediated GNMT also reduced CCl4-induced miR-224 expression and liver fibrosis. These results indicated that AAV-mediated GNMT has potential liver protection activity. miR-224 can target the GNMT mRNA coding sequence and plays an important role in GNMT suppression during liver tumorigenesis.

Alterations of methionine metabolism in hepatocarcinogenesis: the emergent role of glycine N-methyltransferase in liver injury

The methionine and folate cycles play a fundamental role in cell physiology and their alteration is involved in liver injury and hepatocarcinogenesis. Glycine N-methyltransferase is implicated in methyl group supply, DNA methylation, and nucleotide biosynthesis. It regulates the cellular S-adenosylmethionine/S-adenosylhomocysteine ratio and S-adenosylmethionine-dependent methyl transfer reactions. Glycine N-methyltransferase is absent in fast-growing hepatocellular carcinomas and present at a low level in slower growing HCC ones. The mechanism of tumor suppression by glycine N-methyltransferase is not completely known. Glycine N-methyltransferase inhibits hepatocellular carcinoma growth through interaction with Dep domain-containing mechanistic target of rapamycin (mTor)-interacting protein, a binding protein overexpressed in hepatocellular carcinoma. The interaction of the phosphatase and tensin homolog inhibitor, phosphatidylinositol 3,4,5-trisphosphate-dependent rac exchanger, with glycine N-methyltransferase enhances proteasomal degradation of this exchanger by the E3 ubiquitin ligase HectH. Glycine N-methyltransferase also regulates genes related to detoxification and antioxidation pathways. It supports pyrimidine and purine syntheses and minimizes uracil incorporation into DNA as consequence of folate depletion. However, recent evidence indicates that glycine N-methyltransferase targeted into nucleus still exerts strong anti-proliferative effects independent of its catalytic activity, while its restriction to cytoplasm prevents these effects. Our current knowledge suggest that glycine N-methyltransferase plays a fundamental, even if not yet completely known, role in cellular physiology and highlights the need to further investigate this role in normal and cancer cells.

Glycine N-methyltransferase deletion in mice diverts carbon flux from gluconeogenesis to pathways that utilize excess methionine cycle intermediates

Glycine N-methyltransferase (GNMT) is the most abundant liver methyltransferase regulating the availability of the biological methyl donor, S-adenosylmethionine (SAM). Moreover, GNMT has been identified to be down-regulated in hepatocellular carcinoma (HCC). Despite its role in regulating SAM levels and association of its down-regulation with liver tumorigenesis, the impact of reduced GNMT on metabolic reprogramming before the manifestation of HCC has not been investigated in detail. Herein, we used ²H/¹³C metabolic flux analysis in conscious, unrestrained mice to test the hypothesis that the absence of GNMT causes metabolic reprogramming. GNMT-null (KO) mice displayed a reduction in blood glucose that was associated with a decline in both hepatic glycogenolysis and gluconeogenesis. The reduced gluconeogenesis was due to a decrease in liver gluconeogenic precursors, citric acid cycle fluxes, and anaplerosis and cataplerosis. A concurrent elevation in both hepatic SAM and metabolites of SAM utilization pathways was observed in the KO mice. Specifically, the increase in metabolites of SAM utilization pathways indicated that hepatic polyamine synthesis and catabolism, transsulfuration, and de novo lipogenesis pathways were increased in the KO mice. Of note, these pathways utilize substrates that could otherwise be used for gluconeogenesis. Also, this metabolic reprogramming occurs before the well-documented appearance of HCC in GNMT-null mice. Together, these results indicate that GNMT deletion promotes a metabolic shift whereby nutrients are channeled away from glucose formation toward pathways that utilize the elevated SAM.

Glycine N-methyltransferase inhibits aristolochic acid nephropathy by increasing CYP3A44 and decreasing NQO1 expression in female mouse hepatocytes

Plants containing aristolochic acids (AA) are nephrotoxins. Glycine N-methyltransferase (GNMT) acts to bind environmental toxins such as benzo(a)pyrene and aflatoxin B1, translocate into nucleus, and alter hepatic metabolism. This study aims to determine the role of GNMT in AA-induced nephropathy. We established an AA nephropathy mouse model and found that AA type I (AAI)-induced nephropathy at a lower concentration in male than in female mice, implying sex differences in AAI resistance. Microarray analysis and AAI-treated mouse models showed that GNMT moderately reduced AAI-induced nephropathy by lowering the upregulated level of NQO1 in male, but significantly improved the nephropathy additionally by increasing Cyp3A44/3A41 in female. The protective effects of GNMT were absent in female GNMT knockout mice, in which re-expression of hepatic GNMT significantly decreased AAI-induced nephropathy. Mechanism-wise, AAI enhanced GNMT nuclear translocation, resulting in GNMT interaction with the promoter region of the genes encoding Nrf2 and CAR/PXR, the transcription factors for NQO1 and CYP3A44/3A41, respectively. Unlike the preference for Nrf2/NQO1 transcriptions at lower levels of GNMT, overexpression of GNMT preferred CAR/PXR/CYP3A44/3A41 transcriptions and alleviated kidney injury upon AAI treatment. In summary, hepatic GNMT protected mice from AAI nephropathy by enhancing CAR/PXR/CYP3A44/3A41 transcriptions and reducing Nrf2/NQO1 transcriptions.

Characterization of a new murine cell line of sarcomatoid hepatocellular carcinoma and its application for biomarker/therapy development

Sarcomatoid hepatocellular carcinoma (SHC) is a rare type of HCC with significantly poorer survival than ordinary HCC. Little is known about the mechanism associated with SHC and its biomarkers and therapy. Here, we established a mouse liver cancer cell line and designated as Ymac-1. A sarcomatous appearance was observed in the allograft tumor arose from Ymac-1. Liver-secreted plasma proteins were found in Ymac-1 cultured supernatant by proteomics analysis. The positive staining of CK7, CK8, Vimentin and the suppressed expression of AFP indicated that Ymac-1 is a SHC cell line. Compared to its original tumor, an elevated level of EMT markers, N-cadherin and Vimentin, was found in Ymac-1. Ymac-1 displayed a higher migration rate and side population percentage than a mouse ordinary HCC cell line-Hepa1-6. Microarray analysis was performed to identify potential biomarkers/therapeutic targets for SHC. G6pd, a vital enzyme in pentose phosphate pathway, is highly expressed in Ymac-1. Depletion of G6pd in Ymac-1 reduced CD133 expression and sphere formation. Positive correlations between G6PD and CD133 were observed in human specimen. Higher expression of both G6PD and CD133 in tumor were associated with poor survival. In summary Ymac-1 can be a useful SHC cell model for novel biomarker and therapy development.

Characterization of the GNMT-HectH9-PREX2 tripartite relationship in the pathogenesis of hepatocellular carcinoma

The pathogenesis of hepatocellular carcinoma (HCC) involves many molecular pathways. Glycine N-methyltransferase (GNMT) is downregulated in almost all HCC and its gene knockout mice developed HCC with high penetrance. We identified PREX2, a novel PTEN inhibitor, as a GNMT-interacting protein. Such interaction enhanced degradation of PREX2 through an E3 ligase HectH9-mediated proteasomal ubiquitination pathway. Depletion of GNMT or HectH9 resulted in AKT activation in a PREX2 dependent manner and enhanced cell proliferation. An elevated PREX2 protein expression accompanied by activation of AKT was observed in the liver of Gnmt knockout mice. PREX2 protein expression was upregulated in 54.9% of human HCC samples, while its mRNA level was comparable in tumor and tumor-adjacent tissue, suggesting a post-translational alteration of PREX2 expression. Higher level of PREX2 in the tumor tissues was associated with poorer survival. These results reveal a novel mechanism in which GNMT participates in AKT signaling and HCC tumorigenesis by promoting HectH9-mediated PREX2 degradation.

Consensus recommendations for the diagnosis, treatment and follow-up of inherited methylation disorders

Inherited methylation disorders are a group of rarely reported, probably largely underdiagnosed disorders affecting transmethylation processes in the metabolic pathway between methionine and homocysteine. These are methionine adenosyltransferase I/III, glycine N-methyltransferase, S-adenosylhomocysteine hydrolase and adenosine kinase deficiencies. This paper provides the first consensus recommendations for the diagnosis and management of methylation disorders. Following search of the literature and evaluation according to the SIGN-methodology of all reported patients with methylation defects, graded recommendations are provided in a structured way comprising diagnosis (clinical presentation, biochemical abnormalities, differential diagnosis, newborn screening, prenatal diagnosis), therapy and follow-up. Methylation disorders predominantly affect the liver, central nervous system and muscles, but clinical presentation can vary considerably between and within disorders. Although isolated hypermethioninemia is the biochemical hallmark of this group of disorders, it is not always present, especially in early infancy. Plasma S-adenosylmethionine and S-adenosylhomocysteine are key metabolites for the biochemical clarification of isolated hypermethioninemia. Mild hyperhomocysteinemia can be present in all methylation disorders. Methylation disorders do not qualify as primary targets of newborn screening. A low-methionine diet can be beneficial in patients with methionine adenosyltransferase I/III deficiency if plasma methionine concentrations exceed 800 μmol/L. There is some evidence that this diet may also be beneficial in patients with S-adenosylhomocysteine hydrolase and adenosine kinase deficiencies. S-adenosylmethionine supplementation may be useful in patients with methionine adenosyltransferase I/III deficiency. Recommendations given in this article are based on general principles and in practice should be adjusted individually according to patient's age, severity of the disease, clinical and laboratory findings.

Glycine N-methyltransferase deficiency in female mice impairs insulin signaling and promotes gluconeogenesis by modulating the PI3K/Akt pathway in the liver

Background

Glycine N-methyltransferase (GNMT) is abundantly expressed in the normal liver but is down-regulated in liver cancer tissues. GNMT knockout (Gnmt−/−) mice can spontaneously develop chronic hepatitis, fatty liver, and liver cancer. We previously demonstrated that hepatic GNMT is decreased in high-fat-diet-induced type 2 diabetes mellitus, but its contribution to metabolic syndrome is unclear. Here we show that GNMT modulates key aspects of metabolic syndrome in mice.

Methods

Eleven-week-old Gnmt−/− and wild-type (WT) mice with a C57BL/6 genetic background were used in this study. The metabolic defects of GNMT deficiency were measured by glucose and insulin tolerance tests, lipid homeostasis, gluconeogenesis, and insulin signaling.

Results

Gnmt−/− mice, especially females, exhibited glucose intolerance and insulin resistance. However, their body fat and lean mass, food and water intakes, and energy expenditure did not differ from those of WT mice. In addition, glucose-stimulated insulin secretion and insulin-stimulated glucagon secretion were normal in the serum and pancreatic islets of Gnmt−/− mice. Importantly, we found that GNMT deficiency increased lipogenesis and triglycerides in the liver. The elevated triglycerides disrupted the ability of insulin to induce Akt and S6 ribosomal protein phosphorylation, and then triggered insulin resistance and gluconeogenesis in female Gnmt−/− mice.

Conclusions

Our data indicate that hepatic GNMT regulates lipid and glucose homeostasis, and provide insight into the development of insulin resistance through modulating the PI3K/Akt pathway.

Electronic supplementary material

The online version of this article (doi:10.1186/s12929-016-0278-8) contains supplementary material, which is available to authorized users.

Glycine N-Methyltransferase Deficiency: A Member of Dysmethylating Liver Disorders?

Glycine N-methyltransferase deficiency is an inherited disorder of methionine metabolism, reported so far in only four patients and characterised by permanent hypermethioninemia. This disorder has been considered as probably benign because moderate hepatomegaly in two patients was the only obvious symptom and mild to moderate elevation of aminotransferases the only laboratory abnormality. Our experience with the current novel patient points out that this disease, due to very high hypermethioninemia, is not harmless and that there may be diagnostic pitfalls in interpretation of biochemical hallmarks of the disease. Since the first description of glycine N-methyltransferase deficiency, other disorders of this metabolic pathway affecting the liver have been reported pointing to dysmethylation as the common pathogenetic mechanism. Therefore, we suggest the whole group to be named dysmethylating liver diseases.

Identification of 1,2,3,4,6-Penta-O-galloyl-β-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-β-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.

Role of glycine N-methyltransferase in the regulation of T-cell responses in experimental autoimmune encephalomyelitis

Glycine N-methyltransferase (GNMT) is known for its function as a tumor suppressor gene. Since 100% of female Gnmt(-/-) mice developed hepatocellular carcinoma, we hypothesized that Gnmt(-/-) mice may have defective immune surveillance. In this study, we examined the immune modulation of GNMT in T-cell responses using experimental autoimmune encephalomyelitis (EAE). The results showed that EAE severity was reduced significantly in Gnmt(-/-) mice. Pathological examination of the spinal cords revealed that Gnmt(-/-) mice had significantly lower levels of mononuclear cell infiltration and demyelination than the wild-type mice. In addition, quantitative real-time PCR showed that expression levels of proinflammatory cytokines, including interferon (IFN)-γ and interleukin (IL)-17A, were much lower in the spinal cord of Gnmt(-/-) than in that of wild-type mice. Accordingly, myelin oligodendrocyte glycoprotein (MOG)-specific T-cell proliferation and induction of T-helper (Th)1 and Th17 cells were markedly suppressed in MOG(35-55)-induced Gnmt(-/-) mice. Moreover, the number of regulatory T (Treg) cells was increased significantly in these mice. When the T-cell receptor was stimulated, the proliferative capacity and the activation status of mTOR-associated downstream signaling were decreased significantly in Gnmt(-/-) CD4(+) T cells via an IL-2- and CD25-independent manner. Moreover, GNMT deficiency enhanced the differentiation of Treg cells without affecting the differentiation of Th1 and Th17 cells. Furthermore, the severity of EAE in mice adoptive transferred with GNMT-deficient CD4(+) T cells was much milder than in those with wild-type CD4(+) T cells. In summary, our findings suggest that GNMT is involved in the pathogenesis of EAE and plays a crucial role in the regulation of CD4(+) T-cell functions.

Modeling progressive non-alcoholic fatty liver disease in the laboratory mouse

Non-alcoholic fatty liver disease (NAFLD) is the most common liver disease in the world and its prevalence is rising. In the absence of disease progression, fatty liver poses minimal risk of detrimental health outcomes. However, advancement to non-alcoholic steatohepatitis (NASH) confers a markedly increased likelihood of developing severe liver pathologies, including fibrosis, cirrhosis, organ failure, and cancer. Although a substantial percentage of NAFLD patients develop NASH, the genetic and molecular mechanisms driving this progression are poorly understood, making it difficult to predict which patients will ultimately develop advanced liver disease. Deficiencies in mechanistic understanding preclude the identification of beneficial prognostic indicators and the development of effective therapies. Mouse models of progressive NAFLD serve as a complementary approach to the direct analysis of human patients. By providing an easily manipulated experimental system that can be rigorously controlled, they facilitate an improved understanding of disease development and progression. In this review, we discuss genetically- and chemically-induced models of NAFLD that progress to NASH, fibrosis, and liver cancer in the context of the major signaling pathways whose disruption has been implicated as a driving force for their development. Additionally, an overview of nutritional models of progressive NAFLD is provided.

Role of glycine N-methyltransferase in experimental ulcerative colitis

BACKGROUND AND AIM

Inflammatory bowel diseases (IBDs) are chronic inflammatory disorders with unclear etiology and mechanism(s). Glycine N-methyltransferase (GNMT) plays a central role in inflammatory diseases such as hepatitis and atherosclerosis. However, little is known about the impact of GNMT and the involved mechanism in the pathogenesis of IBD. In the current study, we investigated the role of GNMT in the mouse model of dextran sulfate sodium (DSS)-induced colitis.

METHODS

Protein expression was determined by Western blotting or immunohistochemistry. Histopathology was examined by hematoxylin and eosin staining. Levels of pro-inflammatory cytokines were evaluated by ELISA kits.

RESULTS

GNMT was expressed in the epithelium of the colon under normal conditions, and with DSS treatment, its expression was predominant in infiltrated leukocytes of lesions. Mice with genetic deletion of GNMT (GNMT(-/-) ) showed increased susceptibility to DSS induction of colitis, as revealed by the progression of colitis. Additionally, severe colonic inflammation, including increased crypt loss, leukocyte infiltration, and hemorrhage, was greater with DSS treatment in GNMT(-/-) than wild-type mice. Furthermore, the expression of adhesion molecule and inflammatory mediators in the colon was significantly higher with DSS treatment in GNMT(-/-) than wild-type mice. Moreover, loss of GNMT decreased cell apoptosis in colitis lesions with DSS treatment.

CONCLUSIONS

Collectively, our findings suggest that GNMT may be a crucial molecule in the pathogenesis of DSS-induced colitis. This finding may provide new information for a potential therapeutic target in treating IBD.

Expression of DNA methyltransferases is influenced by growth hormone in the long-living Ames dwarf mouse in vivo and in vitro

Methyltransferase expression and DNA methylation are linked to aging and age-related disease. We utilized 3-, 12-, and 24-month-old Ames dwarf and their wild-type siblings to examine the genotype and age-related differences in the expression of methyltransferase enzymes related to DNA methylation in the liver, glycine-N-methyltransferase and DNA methyltransferase (DNMT). We found that DNMT proteins and transcripts are differentially expressed in dwarf mice compared with wild-type siblings that can be attributed to age and/or genotype. However, DNMT1 protein expression is drastically reduced compared with wild-type controls at every age. DNMT3a protein levels coincide with differences observed in DNMT activity. Growth hormone appears to modulate expression of DNMT1 and 3a in dwarf liver tissue and primary hepatocytes. Therefore, growth hormone may contribute to age-related processes, DNA methylation, and, ultimately, longevity.

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 DNA methylation signature associated with the epigenetic repression of glycine N-methyltransferase in human hepatocellular carcinoma

The basic mechanisms underlying promoter DNA hypermethylation in cancer are still largely unknown. It has been proposed that the levels of the methyl donor group in DNA methylation reactions, S-adenosylmethionine (SAMe), might be involved. SAMe levels depend on the glycine-N-methyltransferase (GNMT), a one-carbon group methyltransferase, which catalyzes the conversion of SAMe to S-adenosylhomocysteine in hepatic cells. GNMT has been proposed to display tumor suppressor activity and to be frequently repressed in hepatocellular carcinoma (HCC). In this study, we show that GNMT shows aberrant DNA hypermethylation in some HCC cell lines and primary tumors (20 %). GNMT hypermethylation could contribute to gene repression and its restoration in cell lines displaying hypermethylation-reduced tumor growth in vitro. In agreement, human primary tumors expressing GNMT were of smaller size than tumors showing GNMT hypermethylation. Genome-wide analyses of gene promoter methylation identified 277 genes whose aberrant methylation in HCC was associated with GNMT methylation/expression. The findings in this manuscript indicate that DNA hypermethylation plays an important role in the repression of GNMT in HCC and that loss of GNMT in human HCC could promote the establishment of aberrant DNA methylation patterns at specific gene promoters.

Proteomic differences between hepatocellular carcinoma and nontumorous liver tissue investigated by a combined gel-based and label-free quantitative proteomics study

Proteomics-based clinical studies have been shown to be promising strategies for the discovery of novel biomarkers of a particular disease. Here, we present a study of hepatocellular carcinoma (HCC) that combines complementary two-dimensional difference in gel electrophoresis (2D-DIGE) and liquid chromatography (LC-MS)-based approaches of quantitative proteomics. In our proteomic experiments, we analyzed a set of 14 samples (7 × HCC versus 7 × nontumorous liver tissue) with both techniques. Thereby we identified 573 proteins that were differentially expressed between the experimental groups. Among these, only 51 differentially expressed proteins were identified irrespective of the applied approach. Using Western blotting and immunohistochemical analysis the regulation patterns of six selected proteins from the study overlap (inorganic pyrophosphatase 1 (PPA1), tumor necrosis factor type 1 receptor-associated protein 1 (TRAP1), betaine-homocysteine S-methyltransferase 1 (BHMT)) were successfully verified within the same sample set. In addition, the up-regulations of selected proteins from the complements of both approaches (major vault protein (MVP), gelsolin (GSN), chloride intracellular channel protein 1 (CLIC1)) were also reproducible. Within a second independent verification set (n = 33) the altered protein expression levels of major vault protein and betaine-homocysteine S-methyltransferase were further confirmed by Western blots quantitatively analyzed via densitometry. For the other candidates slight but nonsignificant trends were detectable in this independent cohort. Based on these results we assume that major vault protein and betaine-homocysteine S-methyltransferase have the potential to act as diagnostic HCC biomarker candidates that are worth to be followed in further validation studies.

The multi-functional roles of GNMT in toxicology and cancer

Although glycine N-methyltransferase (GNMT) has been discovered for five decades, its function was not elucidated until recently. In this review, we discuss the multiple roles of GNMT in toxicology and cancer. Besides catalyzing the production of methylglycine (sarcosine) in one carbon metabolism pathway, GNMT was found to be able to bind a number of polycyclic aromatic hydrocarbons and inhibit DNA adducts formation. Moreover, GNMT exerts protective effects against the cytotoxicity and carcinogenicity of benzo(a)pyrene and aflatoxin B(1) in vitro and in vivo. Occupational study showed that workers who had genotypes with higher GNMT promoter activity may have lower content of oxidative damaged DNA products in their urine. In terms of cancer, recent studies using GNMT knockout mouse models demonstrated that GNMT deficiency has high penetrance in inducing the development of steatohepatitis and hepatocellular carcinoma. In terms of the mechanism, besides dysregulation of epigenetic modification, insights have been provided by recent identification of two novel proteins interacting with GNMT-DEPTOR and NPC2. These studies suggest that GNMT not only is involved in mTOR signaling pathway, but also plays an important role in the intracellular trafficking of cholesterol. The implication of these findings to the preventive medicine and translational research will be discussed.

S-adenosylmethionine in liver health, injury, and cancer

S-adenosylmethionine (AdoMet, also known as SAM and SAMe) is the principal biological methyl donor synthesized in all mammalian cells but most abundantly in the liver. Biosynthesis of AdoMet requires the enzyme methionine adenosyltransferase (MAT). In mammals, two genes, MAT1A that is largely expressed by normal liver and MAT2A that is expressed by all extrahepatic tissues, encode MAT. Patients with chronic liver disease have reduced MAT activity and AdoMet levels. Mice lacking Mat1a have reduced hepatic AdoMet levels and develop oxidative stress, steatohepatitis, and hepatocellular carcinoma (HCC). In these mice, several signaling pathways are abnormal that can contribute to HCC formation. However, injury and HCC also occur if hepatic AdoMet level is excessive chronically. This can result from inactive mutation of the enzyme glycine N-methyltransferase (GNMT). Children with GNMT mutation have elevated liver transaminases, and Gnmt knockout mice develop liver injury, fibrosis, and HCC. Thus a normal hepatic AdoMet level is necessary to maintain liver health and prevent injury and HCC. AdoMet is effective in cholestasis of pregnancy, and its role in other human liver diseases remains to be better defined. In experimental models, it is effective as a chemopreventive agent in HCC and perhaps other forms of cancer as well.

Deficiency of glycine N-methyltransferase aggravates atherosclerosis in apolipoprotein E-null mice

The mechanism underlying the dysregulation of cholesterol metabolism and inflammation in atherogenesis is not understood fully. Glycine N-methyltransferase (GNMT) has been implicated in hepatic lipid metabolism and the pathogenesis of liver diseases. However, little is known about the significance of GNMT in atherosclerosis. We showed the predominant expression of GNMT in foamy macrophages of mouse atherosclerotic aortas. Genetic deletion of GNMT exacerbated the hyperlipidemia, inflammation and development of atherosclerosis in apolipoprotein E-deficient mice. In addition, ablation of GNMT in macrophages aggravated oxidized low-density lipoprotein-mediated cholesterol accumulation in macrophage foam cells by downregulating the expression of reverse cholesterol transporters including ATP-binding cassette transporters-A1 and G1 and scavenger receptor BI. Furthermore, tumor necrosis factor-α-induced inflammatory response was promoted in GNMT-null macrophages. Collectively, our data suggest that GNMT is a crucial regulator in cholesterol metabolism and in inflammation, and contributes to the pathogenesis of atherosclerosis. This finding may reveal a potential therapeutic target for atherosclerosis.

Glycine N-methyltransferase deficiency affects Niemann-Pick type C2 protein stability and regulates hepatic cholesterol homeostasis

Nonalcoholic fatty liver disease (NAFLD) is associated with the development of metabolic syndromes and hepatocellular carcinoma (HCC). Cholesterol accumulation is related to NAFLD, whereas its detailed mechanism is not fully understood. Previously, we reported that glycine N-methyltransferase (GNMT) knockout (Gnmt(-/-)) mice develop chronic hepatitis and HCC. In this study, we showed that Gnmt(-/-) mice had hyperlipidemia and steatohepatitis. Single photon emission computed tomography images of mice injected with (131)I-labeled 6β-iodocholesterol demonstrated that Gnmt(-/-) mice had slower hepatic cholesterol uptake and excretion rates than wild-type mice. In addition, genes related to cholesterol uptake (scavenger receptor class B type 1 [SR-B1] and ATP-binding cassette A1 [ABCA1]), intracellular trafficking (Niemann-Pick type C1 protein [NPC1] and Niemann-Pick type C2 protein [NPC2]) and excretion (ATP-binding cassette G1 [ABCG1]) were downregulated in Gnmt(-/-) mice. Yeast two-hybrid screenings and coimmunoprecipitation assays elucidated that the C conserved region (81-105 amino acids) of NPC2 interacts with the carboxyl-terminal fragment (171-295 amino acids) of GNMT. Confocal microscopy demonstrated that when cells were treated with low-density lipoprotein, NPC2 was released from lysosomes and interacts with GNMT in the cytosol. Overexpression of GNMT doubled the half-lives of both NPC2 isoforms and reduced cholesterol accumulation in cells. Furthermore, GNMT was downregulated in the liver tissues from patients suffering with NAFLD as well as from mice fed a high-fat diet, high-cholesterol diet or methionine/choline-deficient diet. In conclusion, our study demonstrated that GNMT regulates the homeostasis of cholesterol metabolism, and hepatic cholesterol accumulation may result from downregulation of GNMT and instability of its interactive protein NPC2. Novel therapeutics for steatohepatitis and HCC may be developed by using this concept.

Designation of enzyme activity of glycine-N-acyltransferase family genes and depression of glycine-N-acyltransferase in human hepatocellular carcinoma

The human glycine-N-acyltransferase (hGLYAT) gene and two related-genes (GLYATL1 and GLYATL2) were isolated. Human GLYAT, GLYATL1, and GLYATL2 cDNAs were isolated and shown to encode polypeptides of 295, 302, and 294 amino acids, respectively. GLYAT catalyzes glycine-N-acyltransfer reaction with benzoyl-CoA acting as a typical aralkyl transferase, while GLYATL1 catalyzed glutamine-N-acyltransfer reaction with phenylacetyl-CoA as an arylacetyl transferase. GLYAT was shown to be expressed specifically in the liver and kidney, and the cellular localization of GLYAT protein was restricted to the mitochondria. Interestingly, labeling using highly affinity purified anti-GLYAT antibody revealed that GLYAT expression was suppressed in all hepatocellular carcinomas, but not in other liver diseases. hGLYAT repression in cancerous cells in the liver was controlled at the transcriptional level. hGLYAT is a good candidate as a novel marker of hepatocellular carcinoma and may be a key molecule in the transition between differentiation and carcinogenesis of liver cells.

The important role of glycine N-methyltransferase in the carcinogenesis and progression of prostate cancer

Glycine N-methyltransferase (GNMT) has a role in the metabolism of methionine as well as in gluconeogenesis. It has recently been reported that the GNMT gene acts as a tumor-susceptible gene. However, little is known about the specific function of GNMT in carcinogenesis and malignant progression. To better our understanding of the function of GNMT in prostate cancer, we used siRNAs to examine the effects of GNMT knockdown on cell proliferation and the cell cycle. In addition, the relation between immunohistochemical GNMT expression and clinicopathologic parameters was investigated in 148 prostate cancer tissues. Here, we show that siRNA-mediated GNMT knockdown results in an inhibition of proliferation, and induces G1 arrest and apoptosis in prostate cancer cell lines. Moreover, high cytoplasmic GNMT expression was also correlated with a higher Gleason score (P<0.001) and higher pT stage (P=0.027). The patients with high GNMT cytoplasmic expression showed significantly lower disease-free survival rates than patients with low expression (P<0.001). High GNMT cytoplasmic expression had a significant impact on patient disease-free survival in multivariate analysis (P=0.005). This is the first investigation to reveal the novel finding that GNMT may have an important role in promoting prostate cancer cell growth via the regulation of apoptosis and contribute to the progression of prostate cancer. The modulation of GNMT expression or function may be a strategy for developing novel therapeutics for prostate cancer. GNMT may represent a novel marker of malignant progression and poor prognosis in prostate cancer.

Higher susceptibility to aflatoxin B(1)-related hepatocellular carcinoma in glycine N-methyltransferase knockout mice

In both humans and rodents, males are known to be more susceptible than females to hepatocarcinogenesis. We have previously reported that glycine N-methyltransferase (GNMT) interacts with aflatoxin B(1) (AFB(1)) and reduces both AFB(1)-DNA adduct formation and hepatocellular carcinoma (HCC) in mice. We also reported that 50% of the males and 100% of the females in a small group of Gnmt null (Gnmt-/-) mice developed HCC, with first dysplastic hepatocellular nodules detected at mean ages of 17 and 16.5 months, respectively. In our study, we tested our hypothesis that male and female Gnmt-/- mice are susceptible to AFB(1) carcinogenesis, and that the absence of Gnmt expression may accelerate AFB(1)-induced liver tumorigenesis. We inoculated Gnmt-/- and wild-type mice intraperitoneally with AFB(1) at 7 days and 9 weeks of age and periodically examined them using ultrasound. Dysplastic hepatocellular nodules were detected in six of eight males and five of five females at 12.7 and 12 months of ages, respectively. Dysplastic hepatocellular nodules from 5/8 (62.5%) male and 4/5 (80%) female Gnmt-/- mice were diagnosed as having HCC, ∼6 months earlier than AFB(1)-treated wild-type mice. Results from microarray and real-time PCR analyses indicate that five detoxification pathway-related genes were downregulated in AFB(1)-treated Gnmt-/- mice: Cyp1a2, Cyp3a44, Cyp2d22, Gsta4 and Abca8a. In summary, we observed overall higher susceptibility to AFB(1)-related HCC in Gnmt-/- mice, further evidence that GNMT overexpression is an important contributing factor to liver cancer resistance.

GNMT expression increases hepatic folate contents and folate-dependent methionine synthase-mediated homocysteine remethylation

Glycine N-methyltransferase (GNMT) is a major hepatic enzyme that converts S-adenosylmethionine to S-adenosylhomocysteine while generating sarcosine from glycine, hence it can regulate mediating methyl group availability in mammalian cells. GNMT is also a major hepatic folate binding protein that binds to, and, subsequently, may be inhibited by 5-methyltetrafolate. GNMT is commonly diminished in human hepatoma; yet its role in cellular folate metabolism, in tumorigenesis and antifolate therapies, is not understood completely. In the present study, we investigated the impacts of GNMT expression on cell growth, folate status, methylfolate-dependent reactions and antifolate cytotoxicity. GNMT-diminished hepatoma cell lines transfected with GNMT were cultured under folate abundance or restriction. Folate-dependent homocysteine remethylation fluxes were investigated using stable isotopic tracers and gas chromatography/mass spectrometry. Folate status was compared between wild-type (WT), GNMT transgenic (GNMT(tg)) and GNMT knockout (GNMT(ko)) mice. In the cell model, GNMT expression increased folate concentration, induced folate-dependent homocysteine remethylation, and reduced antifolate methotrexate cytotoxicity. In the mouse models, GNMT(tg) had increased hepatic folate significantly, whereas GNMT(ko) had reduced folate. Liver folate levels correlated well with GNMT expressions (r = 0.53, P = 0.002); and methionine synthase expression was reduced significantly in GNMT(ko), demonstrating impaired methylfolate-dependent metabolism by GNMT deletion. In conclusion, we demonstrated novel findings that restoring GNMT assists methylfolate-dependent reactions and ameliorates the consequences of folate depletion. GNMT expression in vivo improves folate retention and bioavailability in the liver. Studies on how GNMT expression impacts the distribution of different folate cofactors and the regulation of specific folate dependent reactions are underway.

Enhanced expression of glycine N-methyltransferase by adenovirus-mediated gene transfer in CNS culture is neuroprotective

Glycine N-methyltransferase (GNMT) is the most abundant hepatic methyltransferase and plays important roles in regulating methyl group metabolism. In the central nervous system, GNMT expression is low and its function has not been revealed. The present study examines the effect of GNMT overexpression by adenovirus-mediated transfer in cortical mixed neuron-glial cultures. Infection of adenovirus encoding green fluorescence protein to cultures demonstrates high preference for non-neuronal cells. Optimal GNMT overexpression in cultures by adenoviral GNMT (Ad-GNMT) infection not only induces protein kinase C phosphorylation, but also increases neuronal/oligodendroglial survival. Furthermore, these Ad-GNMT-infected cultures are significantly resistant to H(2)O(2) toxicity and lipopolysaccharide stimulation. Conditioned media from Ad-GNMT-infected microglia also significantly enhance neuronal survival. Taken together, enhanced GNMT expression in mixed neuronal-glial cultures is neuroprotective, most likely mediated through non-neuronal cells.

Fatty liver and fibrosis in glycine N-methyltransferase knockout mice is prevented by nicotinamide

Deletion of glycine N-methyltransferase (GNMT), the main gene involved in liver S-adenosylmethionine (SAM) catabolism, leads to the hepatic accumulation of this molecule and the development of fatty liver and fibrosis in mice. To demonstrate that the excess of hepatic SAM is the main agent contributing to liver disease in GNMT knockout (KO) mice, we treated 1.5-month-old GNMT-KO mice for 6 weeks with nicotinamide (NAM), a substrate of the enzyme NAM N-methyltransferase. NAM administration markedly reduced hepatic SAM content, prevented DNA hypermethylation, and normalized the expression of critical genes involved in fatty acid metabolism, oxidative stress, inflammation, cell proliferation, and apoptosis. More importantly, NAM treatment prevented the development of fatty liver and fibrosis in GNMT-KO mice. Because GNMT expression is down-regulated in patients with cirrhosis, and because some subjects with GNMT mutations have spontaneous liver disease, the clinical implications of the present findings are obvious, at least with respect to these latter individuals. Because NAM has been used for many years to treat a broad spectrum of diseases (including pellagra and diabetes) without significant side effects, it should be considered in subjects with GNMT mutations.

CONCLUSION

The findings of this study indicate that the anomalous accumulation of SAM in GNMT-KO mice can be corrected by NAM treatment leading to the normalization of the expression of many genes involved in fatty acid metabolism, oxidative stress, inflammation, cell proliferation, and apoptosis, as well as reversion of the appearance of the pathologic phenotype.

Deficiency of glycine N-methyltransferase results in deterioration of cellular defense to stress in mouse liver

PURPOSE

Previously, we reported that glycine N-methyltransferase (GNMT) interacts with benzo[a]pyrene (BaP) and inhibits BaP-DNA adducts formation. In addition, Gnmt knockout (Gnmt(-/-)) mice developed chronic hepatitis and hepatocellular carcinoma (HCC). The aims of this study were to understand the gene expression profile of Gnmt(-/-) mice and to study the interaction between BaP and GNMT deficiency in vivo.

EXPERIMENTAL DESIGN

Gene expression profiles of Gnmt(-/-) mice were analyzed by 2-D PAGE and real-time PCR. Both wild-type and Gnmt(-/-) mice were challenged with BaP and sacrificed at the age of 13 months.

RESULTS

Compared with the wild-type mice, proteins involved in the anti-oxidation/detoxification response, glycolytic energy metabolism and one-carbon metabolism pathways were down-regulated significantly in Gnmt(-/-) mice. Malondialdehyde assay showed that lipid peroxidation was significantly increased in the Gnmt(-/-) mice liver. H(2)O(2) treatment demonstrated that the survival rate of HuH-7 cells overexpressing GNMT was significantly higher than the controls. BaP challenge experiments showed that 71.4% (5/7) of male and all (7/7) female Gnmt(-/-) mice developed HCC, while only 16.7% (1/6) of male and 20% (1/5) of female wild-type mice had HCC.

CONCLUSION AND CLINICAL RELEVANCE

GNMT regulates genes related to detoxification and anti-oxidation pathways. BaP is a liver cancer carcinogen especially during GNMT deficiency.

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.

Inherited disorders in the conversion of methionine to homocysteine

During the last decade much important new information relating to the metabolic pathway from methionine to homocysteine has been gained. Interest has been stimulated by the discovery of two novel disorders, glycine N-methyltransferase deficiency and S-adenosylhomocysteine hydrolase deficiency. Another disorder in this pathway, methionine adenosyltransferase deficiency, has been increasingly detected, thanks to the expansion of newborn screening programmes by tandem mass spectrometry technology. These significant steps allow important insight into the pathogenesis of these three disorders, as well as into the mechanisms of damage to various organs (liver, brain, muscle) and point to the relevance of these disorders for crucial biological processes such as methylation, transsulfuration or carcinogenesis in mammals, the pathogenesis of numerous pathological conditions, in particular those associated with hyperhomocysteinaemia, the action and possible toxicity of some drugs or consequences of nutritional variations. This review summarizes current knowledge of three inherited disorders in this metabolic pathway and draws attention to their much broader significance for human health and understanding of important biological processes.