Phosphorylation Modulates the Activity of Glycine N-Methyltransferase, a Folate Binding Protein

S-Adenosylmethionine: From the Discovery of Its Inhibition of Tumorigenesis to Its Use as a Therapeutic Agent

Alterations of methionine cycle in steatohepatitis, cirrhosis, and hepatocellular carcinoma induce MAT1A decrease and MAT2A increase expressions with the consequent decrease of S-adenosyl-L-methionine (SAM). This causes non-alcoholic fatty liver disease (NAFLD). SAM administration antagonizes pathological conditions, including galactosamine, acetaminophen, and ethanol intoxications, characterized by decreased intracellular SAM. Positive therapeutic effects of SAM/vitamin E or SAM/ursodeoxycholic acid in animal models with NAFLD and intrahepatic cholestasis were not confirmed in humans. In in vitro experiments, SAM and betaine potentiate PegIFN-alpha-2a/2b plus ribavirin antiviral effects. SAM plus betaine improves early viral kinetics and increases interferon-stimulated gene expression in patients with viral hepatitis non-responders to pegIFNα/ribavirin. SAM prevents hepatic cirrhosis, induced by CCl4, inhibits experimental tumors growth and is proapoptotic for hepatocellular carcinoma and MCF-7 breast cancer cells. SAM plus Decitabine arrest cancer growth and potentiate doxorubicin effects on breast, head, and neck cancers. Furthermore, SAM enhances the antitumor effect of gemcitabine against pancreatic cancer cells, inhibits growth of human prostate cancer PC-3, colorectal cancer, and osteosarcoma LM-7 and MG-63 cell lines; increases genomic stability of SW480 cells. SAM reduces colorectal cancer progression and inhibits the proliferation of preneoplastic rat liver cells in vivo. The discrepancy between positive results of SAM treatment of experimental tumors and modest effects against human disease may depend on more advanced human disease stage at moment of diagnosis.

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.

Mammalian Sulfur Amino Acid Metabolism: A Nexus Between Redox Regulation, Nutrition, Epigenetics, and Detoxification

SIGNIFICANCE

Transsulfuration allows conversion of methionine into cysteine using homocysteine (Hcy) as an intermediate. This pathway produces S-adenosylmethionine (AdoMet), a key metabolite for cell function, and provides 50% of the cysteine needed for hepatic glutathione synthesis. The route requires the intake of essential nutrients (e.g., methionine and vitamins) and is regulated by their availability. Transsulfuration presents multiple interconnections with epigenetics, adenosine triphosphate (ATP), and glutathione synthesis, polyol and pentose phosphate pathways, and detoxification that rely mostly in the exchange of substrates or products. Major hepatic diseases, rare diseases, and sensorineural disorders, among others that concur with oxidative stress, present impaired transsulfuration. Recent Advances: In contrast to the classical view, a nuclear branch of the pathway, potentiated under oxidative stress, is emerging. Several transsulfuration proteins regulate gene expression, suggesting moonlighting activities. In addition, abnormalities in Hcy metabolism link nutrition and hearing loss.

CRITICAL ISSUES

Knowledge about the crossregulation between pathways is mostly limited to the hepatic availability/removal of substrates and inhibitors. However, advances regarding protein-protein interactions involving oncogenes, identification of several post-translational modifications (PTMs), and putative moonlighting activities expand the potential impact of transsulfuration beyond methylations and Hcy.

FUTURE DIRECTIONS

Increasing the knowledge on transsulfuration outside the liver, understanding the protein-protein interaction networks involving these enzymes, the functional role of their PTMs, or the mechanisms controlling their nucleocytoplasmic shuttling may provide further insights into the pathophysiological implications of this pathway, allowing design of new therapeutic interventions. Antioxid. Redox Signal. 29, 408-452.

Pretreatment Effect of Folic Acid on 13-Cis-RA-Induced Cellular Damage of Developing Midfacial Processes in Cultured Rat Embryos

OBJECTIVE

Excess treatment of 13-cis-RA (Accutane®) on pregnant women induces craniofacial malformation found in infants. However, the effect of folic acid on 13-cis-RA-induced cellular damages of developing midfacial processes is still unknown. The purpose of this study was to investigate the pretreatment effect of folic acid (FA) on 13-cis-RA-induced cellular damage in developing midfacial processes in rat embryos.

MATERIALS AND METHODS

The rat embryos at developing midfacial processes were performed by whole embryo culture in vitro, in the presence of 13-cis-RA (20 µM) with or without pre-treatment of FA (100 µM). The midfacial morphogenesis score, PCNA and TUNEL assay staining were evaluated for morphogenesis, cell proliferation and apoptosis of the midfacial processes, respectively.

RESULTS

The 13-cis-RA-treated embryos at 24h showed atrophy of midfacial processes with significantly decreased morphogenesis score and cell proliferation, and increased apoptotic cell death. In contrast, the embryos pre-treated with FA for 18h, followed by 13-cis-RA treatment for 24h (FA-RA) showed significantly greater morphogenesis score, increased cell proliferation and lower apoptotic cell death compared to those of the 13-cis-RA-treated embryos.

CONCLUSION

The results suggest that FA reduced the teratogenic effects of 13-cis-RA on midfacial process tissue. Future investigations regarding the anti-teratogenic mechanism of FA on the prevention of damages in midface processes induced by 13-cis-RA on pregnant woman are warranted.

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.

Glycine N-methyltransferase and regulation of S-adenosylmethionine levels

Methylation is a major biological process. It has been shown to be important in formation of compounds such as phosphatidylcholine, creatine, and many others and also participates in epigenetic effects through methylation of histones and DNA. The donor of methyl groups for almost all cellular methylation reactions is S-adenosylmethionine. It seems that the level of S-adenosylmethionine must be regulated in response to developmental stages and metabolic changes, and the enzyme glycine N-methyltransferase has been shown to play a major role in such regulation in mammals. This minireview will focus on the latest discoveries in the elucidation of the mechanism of that regulation.

Methyltetrahydrofolate in folate-binding protein glycine N-methyltransferase

In mammals, folate is used as a carrier of one-carbon units (C(1)) in nucleic acids metabolism and biological methylation. Among all forms of folate the most abundant is 5-methyltetrahydrofolate (5-CH(3)-THF), which is of exceptional importance. Its distinctive role among other forms of folate is in its dual function. As a C(1) carrier it is used for synthesis of methionine by remethylation of homocysteine. In addition, 5-CH(3)-THF is bound to and inhibits glycine-N-methyltransferase (GNMT). GNMT is one of the key enzymes in methionine and S-adenosylmethionine (AdoMet) metabolism. It removes excess AdoMet by using it for methylation of glycine. The interaction of 5-CH(3)-THF and GNMT was proposed as an important regulatory mechanism in AdoMet metabolism and biological methylation. The recent discovery of human individuals with mutant GNMT and the study of a mouse model with the GNMT gene knocked out showed that inactivation of that enzyme, indeed, has a significant impact on AdoMet levels in the liver and plasma. The crystal structure of GNMT complexed with 5-CH(3)-THF revealed that there are two folate molecules bound to one tetrameric form of GNMT, which is a basis for establishing of mechanism of inhibition of GNMT. The role of GNMT as a folate-binding protein and how it affects one-carbon folate metabolism is discussed.

S-adenosylmethionine and its products

S-adenosylmethionine is involved in many processes, mainly methylation, polyamine synthesis and radical-based catalysis. It is synthesised through the catalysis of differently regulated enzyme forms. When it is used, the compounds formed are reutilized in different ways: in case of methylation, its end product is homocysteine, which can be remethylated to methionine, give rise to cysteine in the so-called transsulphuration pathway, or be released; in the case of polyamine synthesis, the methylthioadenosine formed is cleaved and gives rise to compounds which can be reutilized; during radical-based catalysis, 5-deoxyadenosine is formed and this, too, is cleaved and reutilized.

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.

Targeting cellular memory to reprogram the epigenome, restore potential, and improve somatic cell nuclear transfer

Successful cloning by somatic cell nuclear transfer (SCNT) is thought to require reprogramming of a somatic nucleus to a state of restored totipotentiality [Dean, W., Santos, F., Reik, W., 2003. Epigenetic programming in early mammalian development and following somatic cell nuclear transfer. Semin. Cell. Dev. Biol. 14, 93-100; Jouneau, A., Renard, J.P., 2003. Reprogramming in nuclear transfer. Curr. Opin. Genet. Dev. 13, 486-491; ]. Though SCNT-induced reprogramming is reminiscent of the reprogramming that occurs after fertilization, reprogramming a differentiated nucleus to an embryonic state is delayed and incomplete in comparison (for review, see ). This is likely due to the existence of an epigenetic-based cellular memory, or program, that serves to regulate global patterns of gene expression, and is the basis of a genome defense mechanism that silences viruses and transposons. The mechanisms of this memory include CpG methylation and modification of histones. Recent evidence by Feng et al. [Feng, Y.-Q., Desprat, R., Fu, H., Olivier, E., Lin, C.M., Lobell, A., Gowda, S.N., Aladjem, M.I., Bouhasira, E.E., 2006. DNA methylation supports intrinsic epigenetic memory in mammalian cells. PLOS Genet. 2, 0461-0470], using a transgenic experimental system, indicates that these marks may be acquired in more than one order and thus, silent heterochromatic structure can be initiated by either methylation of CpG dinucleotides or by histone modifications. In this system, however, CpG methylation appears to differ from histone modifications because it bestows a persistent epigenetic, or cellular, memory. In other words, CpG methylation can independently confer cellular memory, whereas histone modifications appear to be limited in this capacity. Therefore, in the context of genomic reprogramming induced by SCNT, efficient demethylation is likely a key (if not the only) rate-limiting step to improving the efficiency and outcomes of SCNT cloning. This review discusses the possibility of targeting cellular memory, and in particular inducing demethylation of a somatic nucleus prior to nuclear transfer, to enable reprogramming events typically carried out by oocyte factors and thereby improve developmental competence of SCNT-reconstructed embryos. Several recent published reviews of SCNT, cellular reprogramming and genomic demethylation served as valuable sources for the authors and are recommended as supplemental reading. These include the following: Bird, A., 2002. DNA methylation patterns and epigenetic memory. Gen. Dev. 16, 6-21; Grafi, G., 2004. How cells dedifferentiate: a lesson from plants. Dev. Biol. 268, 1-6; Latham, K.E., 2005. Early and delayed aspects of nuclear reprogramming during cloning. Biol. Cell 97, 119-132; Lyko, F., Brown, R., 2005. DNA methyltransferase inhibitors and the development of epigenetic cancer therapies. J.Natl. Cancer Inst. 97, 1498-1506; Morgan, H.D., Santos, F., Green, K., Dean, W., Reik, W., 2005. Epigenetic reprogramming in mammals. Hum. Mol. Gen. 14, R47-R58; Szyf, M., 2005. DNA methylation and demethylation as targets for anticancer therapy. Biochemistry 70, 533-549; Buszczak, M., Spradling, A.C., 2006. Searching chromatin for stem cell identity. Cell 125, 233-236; Gurdon, J.B., 2006. From nuclear transfer to nuclear reprogramming: the reversal of cell differentiation. Annu. Rev. Cell. Dev. Biol. 22, 1-22; Yoo, C.B., Jones, P.A., 2006. Epigenetic therapy of cancer: past, present and future. Nat. Rev. 5, 37-50.

Disruption of methyl group metabolism by ethanol

Recent studies have focused on establishing a link between the pathogenesis of ethanol and the disruption of metabolic pathways in the liver. Ethanol alters hepatic methionine metabolism, leading to perturbation of S-adenosylmethionine-dependent transmethylation. Therefore, the supply of metabolically related nutrients such as folate may play a role in the hepatotoxic effects of ethanol.

Glycine N-methyltransferases: a comparison of the crystal structures and kinetic properties of recombinant human, mouse and rat enzymes

Glycine N-methyltransferases (GNMTs) from three mammalian sources were compared with respect to their crystal structures and kinetic parameters. The crystal structure for the rat enzyme was published previously. Human and mouse GNMT were expressed in Escherichia coli in order to determine their crystal structures. Mouse GNMT was crystallized in two crystal forms, a monoclinic form and a tetragonal form. Comparison of the three structures reveals subtle differences, which may relate to the different kinetic properties of the enzymes. The flexible character of several loops surrounding the active site, along with an analysis of the active site boundaries, indicates that the observed conformations of human and mouse GNMTs are more open than that of the rat enzyme. There is an increase in kcat when going from rat to mouse to human, suggesting a correlation with the increased flexibility of some structural elements of the respective enzymes.

Triiodothyronine treatment attenuates the induction of hepatic glycine N-methyltransferase by retinoic acid and elevates plasma homocysteine concentrations in rats

Recent studies indicated that hormonal imbalances have a role in modulating the metabolism of methyl groups and homocysteine, interrelated pathways that when disrupted, are associated with a number of pathologies. Retinoic acid (RA) was shown to induce hepatic glycine N-methyltransferase (GNMT), a key regulatory protein in methyl group metabolism, and to reduce circulating homocysteine levels. Because thyroid status influences the hepatic folate-dependent one-carbon pool and retinoids can alter thyroid hormone levels, the aim of this study was to examine the interaction between retinoids and thyroid function. For hypothyroid studies, rats were administered 0.5 g/L propylthiouracil in the drinking water for 15 d, and RA [30 micromol/(kg . d)] for the final 5 d. For hyperthyroid studies, rats were treated with RA [30 micromol/(kg . d)] for 8 d and triiodothyronine [T(3); 50 microg/(100 g . d)] the last 4 d. T(3) treatment prevented the RA-mediated increase in GNMT activity. However, GNMT abundance remained elevated, indicating that GNMT regulation by T(3) in RA-treated rats may be, at least in part, at the post-translational level. In addition, T(3) treatment elevated plasma levels of homocysteine 177%, an elevation that was prevented by RA. T(3)-mediated hyperhomocysteinemia may be due to a 70% decrease in hepatic betaine-homocysteine S-methyltransferase, the enzyme that catalyzes folate-independent remethylation of homocysteine, whereas the RA-mediated stimulation of hepatic homocysteine remethylation by folate-dependent methionine synthase may contribute to lowering plasma homocysteine levels. These findings indicate that thyroid hormones, alone and in conjunction with RA, play an important role in the regulation of methyl group and homocysteine metabolism.

Sulfur containing amino acids and human disease

Sulfur containing amino acids contribute substantially to the maintenance and integrity of cellular systems by influencing cellular redox state and cellular capacity to detoxify toxic compounds, free radicals and reactive oxygen species. Methionine and cysteine are the two primary sulfur-containing amino acids in mammals. Methionine is an essential amino acid, obtained by dietary intake while cysteine is non-essential and a metabolite of methionine metabolism. Each of these amino acids contributes significantly to the cellular pool of organic sulfur and generally to sulfur homeostasis as well as playing a significant role in regulation of one carbon metabolism. Genetic defects in the enzymes regulating sulfur pools produce a variety of human pathologies, including homo- and cystinuria, homo- and cysteinemia, and neural tube defects. In addition, thiol imbalance has been associated with multiple disorders, including vascular disease, Alzheimer's, HIV and cancer. Possible treatments to restore the thiol balance are also discussed.

Modulation of methyl group metabolism by streptozotocin-induced diabetes and all-trans-retinoic acid

The hepatic enzyme glycine N-methyltransferase (GNMT) plays a major role in the control of methyl group and homocysteine metabolism. Because disruption of these vital pathways is associated with numerous pathologies, understanding GNMT control is important for evaluating methyl group regulation. Recently, gluconeogenic conditions have been shown to modulate homocysteine metabolism and treatment with glucocorticoids and/or all-trans-retinoic acid (RA)-induced active GNMT protein, thereby leading to methyl group loss. This study was conducted to determine the effect of diabetes, alone and in combination with RA, on GNMT regulation. Diabetes and RA increased GNMT activity 87 and 148%, respectively. Moreover, the induction of GNMT activity by diabetes and RA was reflected in its abundance. Cell culture studies demonstrated that pretreatment with insulin prevented GNMT induction by both RA and dexamethasone. There was a significant decline in homocysteine concentrations in diabetic rats, owing in part to a 38% increase in the abundance of the transsulfuration enzyme cystathionine beta-synthase; treatment of diabetic rats with RA prevented cystathionine beta-synthase induction. A diabetic state also increased the activity of the folate-independent homocysteine remethylation enzyme betaine-homocysteine S-methyltransferase, whereas the activity of the folate-dependent enzyme methionine synthase was diminished 52%. In contrast, RA treatment attenuated the streptozotocin-mediated increase in betaine-homocysteine S-methyltransferase, whereas methionine synthase activity remained diminished. These results indicate that both a diabetic condition and RA treatment have marked effects on the metabolism of methyl groups and homocysteine, a finding that may have significant implications for diabetics and their potential sensitivity to retinoids.

Retinoic acid and glucocorticoid treatment induce hepatic glycine N-methyltransferase and lower plasma homocysteine concentrations in rats and rat hepatoma cells

Perturbation of folate and methyl group metabolism is associated with a number of pathological conditions, including cardiovascular disease and neoplastic development. Glycine N-methyltransferase (GNMT) is a key protein that functions to regulate the supply and utilization of methyl groups for S-adenosylmethionine (SAM)-dependent transmethylation reactions. Factors or conditions that have the ability to regulate GNMT and the generation of homocysteine, a product of transmethylation, have important implications in the potential perturbation of methyl group metabolism. We showed that retinoid compounds induce active hepatic GNMT, resulting in compromised transmethylation processes. Because retinoids can stimulate gluconeogenesis, a condition known to alter methyl group and homocysteine metabolism, the current study was undertaken to determine the relationship between all-trans-retinoic acid (RA) and gluconeogenic hormones on these metabolic pathways. Intact adrenal function was not required for RA to induce and activate hepatic GNMT; however, treatment of rats with dexamethasone (DEX) was as effective as RA in inducing GNMT in rat liver. The marked increase in plasma total homocysteine levels observed in adrenalectomized rats was reduced to normal levels by treatment with either RA or DEX, indicating that the transsulfuration and/or remethylation pathways may be enhanced. Moreover, coadministration of RA and DEX had an additive effect on GNMT induction. Similar findings were also observed in a rat hepatoma cell culture model using H4IIE cells. Taken together, these results demonstrate that both RA and DEX independently induce GNMT, thereby having substantial implications for the potential interaction of retinoid administration with diabetes.

All-trans-retinoic acid rapidly induces glycine N-methyltransferase in a dose-dependent manner and reduces circulating methionine and homocysteine levels in rats

Glycine N-methyltransferase (GNMT) regulates the methyl group supply for S-adenosylmethionine-dependent transmethylation reactions. Retinoids have been shown to perturb methyl group metabolism by increasing the abundance and activity of GNMT, thereby leading to the loss of methyl groups. Previous studies used pharmacologic doses (30 micro mol/kg body weight) of various retinoids administered daily for a total of 10 d. Here, we examined the dose- and time-dependent relationships between all-trans-retinoic acid (ATRA) administration and induction of GNMT, as well as determining additional indices of methyl group and folate metabolism. For the dose-response study, rats were administered 0, 1, 5, 10, 15 or 30 micro mol ATRA/kg body weight for 10 d. For the time-course study, rats were given 30 micromol ATRA/kg body weight for 0, 1, 2, 4, or 8 d. A significant increase (105%) in GNMT activity was observed with doses as low as 5 micromol/kg body weight, whereas maximal induction (231%) of GNMT activity was achieved at 30 micromol/kg body weight. Induction of hepatic GNMT by ATRA was rapid, exhibiting a 31% increase after a single dose (1 d) and achieving maximal induction (95%) after 4 d. Plasma methionine and homocysteine concentrations were decreased 42 and 53%, respectively, in ATRA-treated rats compared with controls. In support of this finding, the hepatic activity of methionine synthase, the folate-dependent enzyme required for homocysteine remethylation, was elevated 40% in ATRA-treated rats. This work demonstrates that ATRA administration exerts a rapid effect on hepatic methyl group, folate and homocysteine metabolism at doses that are within the therapeutic range used by humans.

Cobalamin deficiency with and without neurologic abnormalities: differences in homocysteine and methionine metabolism

The unknown biochemical basis for neurologic dysfunction in cobalamin deficiency and the frequent divergence between neurologic and hematologic manifestations led us to study homocysteine metabolism in 22 patients with pernicious anemia. Serum levels of total homocysteine (tHcy), methionine, S-adenosylmethionine (AdoMet), cysteine, cysteinylglycine (cys-gly), and glutathione (GSH) were measured. Only levels of tHcy and cysteine were increased and only GSH was decreased in cobalamin deficiency as a whole, compared with 17 control subjects. AdoMet correlated only with methionine levels (P =.015) and cysteine only with cys-gly (P =.007) in healthy subjects, but in cobalamin-deficient patients AdoMet correlated instead with cysteine, cys-gly, and folate levels only (P =.008, P =.03, and P =.03, respectively). Significant differences appeared in clinically subgrouped cobalamin-deficient patients. The 11 patients with neurologic defects had higher mean levels of folate (27.9 versus 15.4 nM), AdoMet (117.2 versus 78.6 nM), cysteine (462 versus 325 microM), and cys-gly (85.0 versus 54.7 microM) than the 11 neurologically unaffected patients. Cobalamin therapy restored all metabolic changes to normal. The results indicate that changes in several metabolic pathways differ in patients with and without neurologic dysfunction. Cysteine levels were the most significant predictors of neurologic dysfunction, but it is unclear if they are direct or indirect indicators of neurotoxicity. The higher AdoMet levels in neurologically affected patients may result from inhibition of glycine N-methyltransferase by those patients' higher folate levels. The origin of the folate differences is unclear and possibly varied. Low AdoMet and GSH levels were independent predictors of anemia.

Hepatic glycine N-methyltransferase is up-regulated by excess dietary methionine in rats

Glycine N-methyltransferase (GNMT) regulates S-adenosylmethionine (SAM) levels and the ratio of SAM:S-adenosylhomocysteine (SAH). In liver, methionine availability, both from the diet and via the folate-dependent one-carbon pool, modulates GNMT activity to maintain an optimal SAM:SAH ratio. The regulation of GNMT activity is accomplished via posttranslational and allosteric mechanisms. We more closely examined GNMT regulation in various tissues as a function of excess dietary methyl groups. Sprague Dawley rats were fed either a control diet (10% casein plus 0.3% L-methionine) or the control diet supplemented with graded levels (0.5-2%) of L-methionine. Pair-fed control groups of rats were included due to the toxicity associated with high methionine consumption. As expected, the hepatic activity of GNMT was significantly elevated in a dose-dependent fashion after 10 d of feeding the diets containing excess methionine. Moreover, the abundance of hepatic GNMT protein was similarly increased. The kidney had a significant increase in GNMT as a function of dietary methionine, but to a much lesser extent than in the liver. For pancreatic tissue, neither the activity of GNMT nor the abundance of the protein was responsive to excess dietary methionine. These data suggest that additional mechanisms contribute to regulation of GNMT such that synthesis of the protein is greater than its degradation. In addition, methionine-induced regulation of GNMT is dose dependent and appears to be tissue specific, the latter suggesting that the role it plays in the kidney and pancreas may in part differ from its hepatic function.

Activation and induction of glycine N-methyltransferase by retinoids are tissue- and gender-specific

Glycine N-methyltransferase (GNMT) is a key protein in the liver that functions to regulate S-adenosylmethionine (SAM) and the SAM/S-adenosylhomocysteine ratio. Significant GNMT expression is also present in the kidney and pancreas. Inappropriate regulation of GNMT may have negative consequences on methyl group and folate metabolism. We have demonstrated that retinoid compounds significantly elevated hepatic GNMT activity and abundance (approximately 2-fold) in male rats. However, pancreatic GNMT activity and abundance were not altered by retinoid treatment. Likewise, retinoid administration was without effect on renal GNMT activity. Hepatic GNMT activity was also elevated in female rats treated with all-trans-retinoic acid, but to a lesser extent compared to males. Collectively, these results indicate that the modulation of methyl group metabolism by retinoids is tissue- and gender-specific, and may compromise the availability of methyl groups for SAM-dependent transmethylation reactions. In support of this, SAM-dependent synthesis of creatinine was significantly reduced 21% following all-trans-retinoic acid treatment.

Vitamin A and its derivatives induce hepatic glycine N-methyltransferase and hypomethylation of DNA in rats

Regulation of S-adenosylmethionine (SAM) and the SAM/S-adenosylhomocysteine (SAH) ratio by the key cytosolic enzyme glycine N-methyltransferase (GNMT) is essential in optimizing methyl group supply and subsequent functioning of methyltransferase enzymes. Therefore, inappropriate activation of GNMT may lead to the loss of methyl groups vital for many SAM-dependent transmethylation reactions. Previously, we demonstrated that the retinoid derivatives 13-cis- (CRA) and all-trans-retinoic acid (ATRA) mediated both the activity of GNMT and its abundance. The present study was conducted to determine whether vitamin A had a similar ability to up-regulate GNMT and to assess the biological importance of GNMT modulation by examining both the transmethylation and transsulfuration pathways after retinoid treatment. Rats were fed a control (10% casein + 0.3% L-methionine) diet and orally given retinyl palmitate (RP), CRA, ATRA or vehicle daily for 10 d. RP, CRA and ATRA elevated hepatic GNMT activity 32, 74 and 124%, respectively, compared with the control group. Moreover, the retinoid-mediated changes in GNMT activity were reflected in GNMT abundance (38, 89 and 107% increases for RP-, CRA-, and ATRA-treated rats, respectively). In addition, hepatic DNA, a substrate for SAM-dependent transmethylation, was hypomethylated (approximately 100%) after ATRA treatment compared with the control group. In contrast, the transsulfuration product glutathione was unaffected by retinoid treatment. These results provide evidence of the following: 1) vitamin A, like its retinoic acid derivatives, can induce enzymatically active GNMT; and 2) inappropriate induction of GNMT can lead to a biologically important loss of methyl groups and the subsequent impairment of essential transmethylation processes.

Hyperglucagonemia in rats results in decreased plasma homocysteine and increased flux through the transsulfuration pathway in liver

An elevated plasma level of homocysteine is a risk factor for the development of cardiovascular disease. The purpose of this study was to investigate the effect of glucagon on homocysteine metabolism in the rat. Male Sprague-Dawley rats were treated with 4 mg/kg/day (3 injections per day) glucagon for 2 days while control rats received vehicle injections. Glucagon treatment resulted in a 30% decrease in total plasma homocysteine and increased hepatic activities of glycine N-methyltransferase, cystathionine beta-synthase, and cystathionine gamma-lyase. Enzyme activities of the remethylation pathway were unaffected. The 90% elevation in activity of cystathionine beta-synthase was accompanied by a 2-fold increase in its mRNA level. Hepatocytes prepared from glucagon-injected rats exported less homocysteine, when incubated with methionine, than did hepatocytes of saline-treated rats. Flux through cystathionine beta-synthase was increased 5-fold in hepatocytes isolated from glucagon-treated rats as determined by production of (14)CO(2) and alpha-[1-(14)C]ketobutyrate from l-[1-(14)C]methionine. Methionine transport was elevated 2-fold in hepatocytes isolated from glucagon-treated rats resulting in increased hepatic methionine levels. Hepatic concentrations of S-adenosylmethionine and S-adenosylhomocysteine, allosteric activators of cystathionine beta-synthase, were also increased following glucagon treatment. These results indicate that glucagon can regulate plasma homocysteine through its effects on the hepatic transsulfuration pathway.

Retinoid compounds activate and induce hepatic glycine N-methyltransferase in rats

Glycine N-methyltransferase (GNMT) functions to regulate S-adenosylmethionine (SAM) levels and the ratio of SAM/S-adenosylhomocysteine (SAH). SAM is a universal methyl group donor and the up-regulation of GNMT may lead to wastage of methyl groups required for transmethylation reactions. Previously, we demonstrated that dietary treatment of rats with 13-cis-retinoic acid (CRA) decreased the hepatic concentration of SAM and the SAH ratio. Here, we examined the ability of CRA, as well as all-trans-retinoic acid (ATRA), to regulate hepatic GNMT as a potential basis for our earlier observations. Rats were fed either a control (10% casein + 0.3% L-methionine) diet or a control diet supplemented with L-methionine (10 g/kg diet). Rats from each group were orally given ATRA, CRA (both at 30 micromol/kg body), or vehicle daily for 7 d. For control rats, administration of both CRA and ATRA elevated the hepatic GNMT activity 49% and 34%, respectively, compared with the control group. Similar results were exhibited by rats fed the methionine-supplemented diet. Moreover, the retinoid-induced elevations in enzyme activity were reflected in the abundance of GNMT protein. To our knowledge, this is the first report of a nutritional compound that induces GNMT activity at the transcriptional and/or translational level.

Folic acid

Folic acid is an essential nutrient from the B complex group of vitamins. Folate, as a cofactor, is involved in numerous intracellular reactions, and this is reflected in the various derivatives that have been isolated from biological sources. Folic acid is involved in single carbon transfer reactions and serves as a source of single carbon units in different oxidative states. The processes involved in the absorption, transport, and intracellular metabolism of this cofactor are complex. Much of folate is bound tightly to enzymes, indicating that there is not excess of this cofactor and that its cellular availability is protected as well as being strictly regulated. In animals, the liver controls the supply of folate through first pass metabolism, biliary secretion, enterohepatic recirculation, as well as through senescent erythrocyte recycling. Deficiencies of folate can occur for many reasons, including reduced intake, increased metabolism, and/or increased requirements as well as through genetic defects. The effects of folate deficiency include hyperhomocysteinemia, megaloblastic anemia, and mood disorders. Folate deficiency has also been implicated in disorders associated with neural tube defects. Supplementation of grain products such as cereals has been undertaken in several countries as a cost-effective means of reducing the prevelance of neural tube defects. Recently, common polymorphisms have been discovered in several genes associated with folate pathways that may play a role in diseases associated with folate deficiency, particularly mild folate deficiency.

Plasma homocysteine is decreased in the hypothyroid rat

Recent clinical studies have indicated that plasma homocysteine was significantly increased in hypothyroid patients. Since hyperhomocysteinemia is an independent risk factor for cardiovascular disease we investigated homocysteine metabolism in hypothyroid rats. Hypothyroidism was induced in one study by addition of propylthiouracil (PTU) to the drinking water for 2 weeks. In a second study, thyroidectomized and sham-operated rats were used with thyroid hormone replacement via mini-osmotic pumps. Unlike the human hypothyroid patients, both groups of hypothyroid rats exhibited decreased total plasma homocysteine (30% in PTU rats, 50% in thyroidectomized rats) versus their respective controls. Thyroid replacement normalised homocysteine levels in the thyroidectomized rat. Increased activities of the hepatic trans-sulfuration enzymes were found in both models of hypothyroidism. These results provide a possible explanation for the decreased plasma homocysteine concentrations. The hypothyroid rat cannot be used as a model to study homocysteine metabolism in hypothyroid patients.Key words: homocysteine, cystathionine beta-synthase, cystathionine gamma-lyase, thyroid hormone.

The existence of the 4S polycyclic aromatic hydrocarbon-protein binding in 14-day-old chick embryo liver

Cytochrome P-450IA1, the isozyme most closely associated with aryl hydrocarbon hydroxylase (AHH), is regulated by two high-affinity binding proteins, the 4S polycyclic aromatic hydrocarbon (PAH)-binding protein which primarily binds PAHs and the 8S Ah (dioxin) receptor which binds 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and like congeners. The present study was conducted to determine whether the 4S protein existed in 14-day-old chick embryo liver when AHH activity is maximal to determine if they are linked as is the 8S Ah receptor and to confirm the existence of the dioxin receptor by investigating their ligand binding characteristics in the presence and absence of sodium molybdate, an agent that stabilizes steroid hormone receptors and partially stabilizes the dioxin receptor. Competitive ligand binding studies were performed with liver cytosol from livers of male 14-day-old chick embryos using [3H]-benzo[a]pyrene (B[a]P) or [3H]-TCDD in the presence and absence of a 200-fold excess of B[a]P, benzo[e]pyrene (B[e]P), 3-methylcholanthrene (3-MC), and tetrachlorodibenzofuran (TCDBF). Specific PAH-binding activity was assayed using sucrose gradient analysis. In the absence of molybdate, the 4S PAH-binding protein had high affinity for B[a]P, B[e]P, 3-MC, but very low affinity for TCDBF; the Ah receptor exhibited high affinity for TCDBF. In the presence of sodium molybdate, the Ah receptor was stabilized while the 4S PAH-binding protein was relatively unaffected. These results affirm the existence of two distinct PAH-binding proteins in 14-day-old chick embryo liver cytosol and suggest a linkage of the 4S protein to AHH.

Structure, function and physiological role of glycine N-methyltransferase

Glycine N-methyltransferase (EC 2.1.1.20) catalyzes the transfer of the methyl group of S-adenosylmethionine (AdoMet) to glycine to form S-adenosylhomocysteine and sarcosine. Unlike most AdoMet-dependent methyltransferases, glycine N-methyltransferase is a tetramer of identical subunits. Crystallography of recombinant rat glycine N-methyltransferase indicates that four nearly spherical subunits are arranged to form a flat, square tetramer with a large hole in the centre. The enzyme occurs abundantly in the livers of rat, rabbit and mouse. Glycine N-methyltransferases from rat, rabbit, human and pig livers are shown to have similar amino acid sequences and, with the enzymes from rat and rabbit livers, it is demonstrated that the N-terminal valine is acetylated. Glycine N-methyltransferases from livers exhibit sigmoidal rate behaviour with respect to AdoMet and hyperbolic behaviour with respect to glycine at all pH tested. However, recombinant rat glycine N-methyltransferase which lacks the N-terminal acetyl group shows no cooperativity toward AdoMet at neutral pH, suggesting that elimination of the positive charge at the N-terminus is required for cooperative behaviour. Glycine N-methyltransferase binds 5-methyltetrahydropteroylpentaglutamate tightly, resulting in inhibition of the catalytic activity. The nature of these unique functional features is discussed in the light of the three-dimensional structure of the enzyme. The tissue and subcellular localization of the enzyme and its possible role in methionine metabolism are reviewed.

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

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

ATP depletion affects the phosphorylation state, ligand binding, and nuclear transport of the 4 S polycyclic aromatic hydrocarbon-binding protein in rat hepatoma cells

In the rat, cytochrome P4501A1 gene expression is thought to be regulated by several trans-acting factors including the 4 S polycyclic aromatic hydrocarbon (PAH)-binding protein. Phosphorylation and dephosphorylation have been suggested to influence the function of many cytosolic receptors and transcription factors. The ATP level within H4IIE rat hepatoma cells could be depleted by treatment with sodium azide or 2,4-dinitrophenol; restoration of the original ATP levels occurred with addition of glucose to the cell culture. ATP depletion reduced the phosphate content of the 4 S protein by approximately 25-30%, which lowered the binding of benzo[a]pyrene (B[a]P) to the 4 S protein by >60%. This effect could not be reversed by the addition of ATP to the binding reaction mixtures. Alkaline phosphatase treatment of the purified 4 S protein in a cell-free system also reduced the B[a]P binding to the protein. Cells treated with a protein phosphatase inhibitor, okadaic acid, and a protein kinase inhibitor, staurosporin, affected the B[a]P binding of the 4 S protein positively and negatively, respectively. These data suggested that phosphorylation is involved in the interaction of the 4 S protein with the PAH. The nuclear translocation of the predominantly cytosolic binding protein has been investigated after ligand binding. Western blots with the immunopurified 4 S PAH-binding protein from cytosolic and nuclear lysates showed significant differences in the distribution of the 4 S receptor between cytosolic and nuclear compartments in control and ATP-depleted cells. Ligand binding stimulated the movement of the receptor into the nucleus, which was completely blocked by reducing the intracellular ATP concentration. These findings provide new information on the role of ATP and phosphorylation on the interaction of B[a]P with 4 S PAH-binding protein and its nuclear translocation.

Mammalian small molecule methyltransferases: their structural and functional features

Structural and functional features of mammalian S-adenosyl-methionine-dependent small molecule methyltransferases are reviewed. The methyltransferases have similar protomer molecular weights in the range of 25,000-35,000. Two common sequence motifs are found in all enzymes of known sequence. Whereas the kinetic mechanisms may be different, the methyltransferases in the free form bind S-adenosylmethionine. Most, if not all, of mammalian small molecule methyltransferases appear to have vicinal thiols in a catalytically important area of the enzyme.

How is rat liver S-adenosylmethionine synthetase regulated?

The in vivo regulation of S-adenosylmethionine synthetase, a key enzyme in methionine metabolism, is so far unknown. The enzyme activity has been shown to be modulated by glutathione and the oxidation state of its sulfhydryl groups. Analysis of the protein sequence has revealed the presence of putative phosphorylation sites. A mixed regulatory mechanism combining phosphorylation and the oxido/reduction of sulfhydryl groups is proposed. The role of glutathione in this mechanism is also discussed.