Metabolism of exogenous S-adenosylmethionine in isolated rat hepatocyte suspensions: methylation of plasma-membrane phospholipids without intracellular uptake

Methionine metabolism in chronic liver diseases: an update on molecular mechanism and therapeutic implication

As one of the bicyclic metabolic pathways of one-carbon metabolism, methionine metabolism is the pivot linking the folate cycle to the transsulfuration pathway. In addition to being a precursor for glutathione synthesis, and the principal methyl donor for nucleic acid, phospholipid, histone, biogenic amine, and protein methylation, methionine metabolites can participate in polyamine synthesis. Methionine metabolism disorder can aggravate the damage in the pathological state of a disease. In the occurrence and development of chronic liver diseases (CLDs), changes in various components involved in methionine metabolism can affect the pathological state through various mechanisms. A methionine-deficient diet is commonly used for building CLD models. The conversion of key enzymes of methionine metabolism methionine adenosyltransferase (MAT) 1 A and MAT2A/MAT2B is closely related to fibrosis and hepatocellular carcinoma. In vivo and in vitro experiments have shown that by intervening related enzymes or downstream metabolites to interfere with methionine metabolism, the liver injuries could be reduced. Recently, methionine supplementation has gradually attracted the attention of many clinical researchers. Most researchers agree that adequate methionine supplementation can help reduce liver damage. Retrospective analysis of recently conducted relevant studies is of profound significance. This paper reviews the latest achievements related to methionine metabolism and CLD, from molecular mechanisms to clinical research, and provides some insights into the future direction of basic and clinical research.

16 Inhibition of mammalian protein methyltransferases by 5'-methylthioadenosine (MTA): A mechanism of action of dietary same?

5'-deoxy-5'-methylthioadenosine (5'-methylthioadenosine, MTA) is a naturally occurring metabolite. As an experimental reagent, it has proved useful in providing investigators a window onto the role of protein methylation reactions in intact cells, although its mode of action is poorly understood in most cases. This chapter reevaluates its utility as a reagent. It appears now that MTA is at best a poor direct inhibitor of methyltransferases and that its effectiveness in intact cells may depend on its ability to inhibit S-adenosyl-l-homocysteine hydrolase. This chapter reviews recent evidence that points to an important role for MTA as an intermediary in the beneficial pharmaceutical action of orally ingested S-adenosyl-l-methionine (AdoMet, SAMe). These new results suggest that oral AdoMet may function not by enhancing the activity of cellular methyltransferases, as has been previously surmised, but by inhibiting their action. Such inhibition, particularly of protein methyltransferases involved in intracellular communication, may attenuate signal transduction pathways otherwise leading to inflammatory damage to tissues.

S-Adenosylmethionine Exerts a Protective Effect against Thioacetamide-induced Injury in Primary Cultures of Rat Hepatocytes

S-adenosylmethionine (SAMe) has been shown to protect hepatocytes from toxic injury, both experimentally-induced in animals and in isolated hepatocytes. The mechanisms by which SAMe protects hepatocytes from injury can result from the pathways of SAMe metabolism. Unfortunately, data documenting the protective effect of SAMe against mitochondrial damage from toxic injury are not widely available. Thioacetamide is frequently-used as a model hepatotoxin, which causes in vivo centrilobular necrosis. Even though thioacetamide-induced liver necrosis in rats was alleviated by SAMe, the mechanisms of this protective effect remain to be verified. The aim of our study was to determine the protective mechanisms of SAMe on thioacetamide-induced hepatocyte injury by using primary hepatocyte cultures. The release of lactate dehydrogenase (LDH) from cells incubated with thioacetamide for 24 hours, was lowered by simultaneous treatment with SAMe, in a dose-dependent manner. The inhibitory effect of SAMe on thioacetamide-induced lipid peroxidation paralleled the effect on cytotoxicity. A decrease in the mitochondrial membrane potential, as determined by Rhodamine 123 accumulation, was also prevented. The attenuation by SAMe of thioacetamide-induced glutathione depletion was determined after subsequent incubation periods of 48 and 72 hours. SAMe protects both cytoplasmic and mitochondrial membranes. This effect was more pronounced during the development of thioacetamide-induced hepatocyte injury that was mediated by lipid peroxidation. Continuation of the SAMe treatment then led to a reduction in glutathione depletion, as a potential consequence of an increase in glutathione production, for which SAMe is a precursor.

Protective effect of S-adenosylmethionine on cellular and mitochondrial membranes of rat hepatocytes against tert-butylhydroperoxide-induced injury in primary culture

Accumulating evidence that administration of S-adenosylmethionine (SAMe) protects hepatocytes against oxidative stress-mediated injury led us to evaluate the effect of SAMe on hepatocyte injury induced in culture by oxidant substance tert-butylhydroperoxide (1.5 mM tBHP) with regard to prevent mitochondrial injury. The pretreatment of hepatocyte culture with SAMe in doses of 0.25, 0.5, 1, 2.5, 5, 10, 25 and 50 mg/l for 30 min prevented the release of LDH from cells incubated for 30 min with tBHP in a dose dependent manner. The inhibitory effect of SAMe on lipid peroxidation paralleled the effect on cell viability. SAMe also moderated the decrease of the mitochondrial membrane potential induced by tBHP. Our results indicate that the inhibition of lipid peroxidation by SAMe can contribute to the prevention of disruption of both cellular and mitochondrial membranes. While the protective effect of SAMe against tBHP-induced GSH depletion was not confirmed, probably the most potent effect of SAMe on membranes by phospholipid methylation should be verified.

Genetic and Physiological Interactions in the Amoeba-Bacteria Symbiosis1

Amoebae of the xD strain of Amoeba proteus that arose from the D strain by spontaneous infection of Legionella-like X-bacteria are now dependent on their symbionts for survival. Each xD amoeba contains about 42,000 symbionts within symbiosomes, and established xD amoebae die if their symbionts are removed. Thus, harmful infective bacteria changed into necessary cell components. As a result of harboring X-bacteria. xD amoebae exhibit various physiological and genetic characteristics that are different from those of symbiont-free D amoebae. One of the recent findings is that bacterial symbionts control the expression of a host's house-keeping gene. Thus, the expression of the normal amoeba sams gene (sams1) encoding one form of S-adenosylmethionine synthetase is switched to that of sams2 by endosymbiotic X-bacteria. Possible mechanisms for the switching of sams genes brought about by endosymbionts and its significance are discussed.

S-adenosylmethionine (SAMe) protects against acute alcohol induced hepatotoxicity in mice

Although S-Adenosylmethionine (SAMe) has beneficial effects in many hepatic disorders, the effects of SAMe on acute alcohol-induced liver injury are unknown. In the present study, we investigated effects of SAMe on liver injury in mice induced by acute alcohol administration. Male C57BL/6 mice received ethanol (5 g/kg BW) by gavage every 12 hrs for a total of 3 doses. SAMe (5 mg/kg BW) was administrated i.p. once a day for three days before ethanol administration. Subsequent serum ALT level, hepatic lipid peroxidation, enzymatic activity of CYP2E1 and hepatic mitochondrial glutathione levels were measured colorimetrically. Intracellular SAMe concentration was measured by high-performance liquid chromatography (HPLC). Histopathological changes were assessed by H&E staining. Our results showed that acute ethanol administration caused prominent microvesicular steatosis with mild necrosis and an elevation of serum ALT activity. SAMe treatment significantly attenuated the liver injury. In association with the hepatocyte injury, acute alcohol administration induced significant decreases in both hepatic SAMe and mitochondrial GSH levels along with enhanced lipid peroxidation. SAMe treatment attenuated hepatic SAMe and mitochondrial GSH depletion and lipid peroxidation following acute alcohol exposure. These results demonstrate that SAMe protects against the liver injury and attenuates the mitochondrial GSH depletion caused by acute alcohol administration. SAMe may prove to be an effective therapeutic agent in many toxin-induced liver injuries including those induced by alcohol.

The inhibitory role of methylation on the binding characteristics of dopamine receptors and transporter

Excess methylation has been suggested to play a role in the pathogenesis of Parkinson's disease (PD), since the administration of S-adenosylmethionine (SAM), a biological methyl donor, induces PD-like changes in rodents. It was proposed that SAM-induced PD-like changes might be associated with its ability to react with the dopaminergic system. In the present study the effects of SAM on dopamine receptors and transporters were investigated using rats and cloned dopamine receptor proteins. Autoradiographic examination of SAM indicated its tendency to be localized and accumulated in rat striatal region after the intracerebroventricular injection into rat brain. Moreover, results showed that SAM significantly decreased dopamine D1 and D2 receptor binding activities by decreasing the Bmax and increasing the Kd values. At concentrations of 0.1, 0.25 and 0.5 mM, SAM was able to reduce the Bmax from the control value of 848.1 for dopamine D1-specific ligand [3H] SCH 23390 to 760.1, 702.6 and 443.0 fmol/mg protein, respectively. At the same concentrations, SAM was able to increase the Kd values from 0.91 for the control to 1.06, 3.84 and 7.01 nM of [3H] SCH 23390, respectively. The effects of SAM on dopamine D2 binding were similar to those of dopamine D1 binding. SAM also decreased dopamine transporter activity. The interaction of SAM with dopamine receptor proteins produced methanol from methyl-ester formation and hydrolysis. We propose that the SAM effect might be related to its ability to react with dopamine receptor proteins through methyl-ester formation and methanol production following the hydrolysis of the carboxyl-methylated receptor proteins.

Gene switching in Amoeba proteus caused by endosymbiotic bacteria

The expression of genes for S-adenosylmethionine synthetase (SAMS), which catalyzes the synthesis of S-adenosylmethionine (AdoMet), a major methyl donor in cells, was studied in symbiont-free (D) and symbiont-bearing (xD) amoeba strains to determine the effect of bacterial endosymbionts. The symbionts suppressed the expression of the gene in host xD amoebae, but amoebae still exhibited about half the enzyme activity found in symbiont-free D amoebae. The study was aimed at elucidating mechanisms of the suppression of the amoeba's gene and determining the alternative source for the gene product. Unexpectedly, we found a second sams (sams2) gene in amoebae, which encoded 390 amino acids. Results of experiments measuring SAMS activities and amounts of AdoMet in D and xD amoebae showed that the half SAMS activity found in xD amoebae came from the amoeba's SAMS2 and not from their endosymbionts. The expression of amoeba sams genes was switched from sams1 to sams2 as a result of infection with X-bacteria, raising the possibility that the switch in the expression of sams genes by bacteria plays a role in the development of symbiosis and the host-pathogen interactions. This is the first report showing such a switch in the expression of host sams genes by infecting bacteria.

Okadaic acid-induced, naringin-sensitive phosphorylation of glycine N-methyltransferase in isolated rat hepatocytes

Glycine N-methyltransferase (GNMT) is an abundant cytosolic enzyme that catalyses the methylation of glycine into sarcosine, coupled with conversion of the methyl donor, S -adenosylmethionine (AdoMet), into S -adenosylhomocysteine (AdoHcy). GNMT is believed to play a role in monitoring the AdoMet/AdoHcy ratio, and hence the cellular methylation capacity, but regulation of the enzyme itself is not well understood. In the present study, treatment of isolated rat hepatocytes with the protein phosphatase inhibitor okadaic acid, was found to induce an overphosphorylation of GNMT, as shown by proteomic analysis. The analysis comprised two-dimensional gel electrophoretic separation of (32)P-labelled phosphoproteins and identification of individual protein spots by matrix-assisted laser-desorption ionization-time-of-flight mass spectrometry. The identity of GNMT was verified by N-terminal Edman sequencing of tryptic peptides. Chromatographic separation of proteolytic peptides and (32)P-labelled amino acids suggested that GNMT was phosphorylated within a limited region, and only at serine residues. GNMT phosphorylation could be suppressed by naringin, an okadaic acid-antagonistic flavonoid. To assess the possible functional role of GNMT phosphorylation, the effect of okadaic acid on hepatocytic AdoMet and AdoHcy levels was examined, using HPLC separation for metabolite analysis. Surprisingly, okadaic acid was found to have no effect on the basal levels of AdoMet or AdoHcy. An accelerated AdoMet-AdoHcy flux, induced by the addition of methionine (1 mM), was likewise unaffected by okadaic acid. 5-Aminoimidazole-4-carboxamide riboside, an activator of the hepatocytic AMP-activated protein kinase, similarly induced GNMT phosphorylation without affecting AdoMet and AdoHcy levels. Activation of cAMP-dependent protein kinase by dibutyryl-cAMP, reported to cause GNMT phosphorylation under cell-free conditions, also had little effect on hepatocytic AdoMet and AdoHcy levels. Phosphorylation of GNMT would thus seem to play no role in regulation of the intracellular AdoMet/AdoHcy ratio, but could be involved in other GNMT functions, such as the binding of folates or aromatic hydrocarbons.

Inhibition of liver methionine adenosyltransferase gene expression by 3-methylcolanthrene: protective effect of S-adenosylmethionine

Methionine adenosyltransferase (MAT) is an essential enzyme that catalyzes the synthesis of S-adenosylmethionine (AdoMet), the most important biological methyl donor. Liver MAT I/III is the product of the MAT1A gene. Hepatic MAT I/III activity and MAT1A expression are compromised under pathological conditions such as alcoholic liver disease and hepatic cirrhosis, and this gene is silenced upon neoplastic transformation of the liver. In the present work, we evaluated whether MAT1A expression could be targeted by the polycyclic arylhydrocarbon (PAH) 3-methylcholanthrene (3-MC) in rat liver and cultured hepatocytes. MAT1A mRNA levels were reduced by 50% following in vivo administration of 3-MC to adult male rats (100 mg/kg, p.o., 4 days' treatment). This effect was reproduced in a time- and dose-dependent fashion in cultured rat hepatocytes, and was accompanied by the induction of cytochrome P450 1A1 gene expression. This action of 3-MC was mimicked by other PAHs such as benzo[a]pyrene and benzo[e]pyrene, but not by the model arylhydrocarbon receptor (AhR) activator 2,3,7,8-tetrachlorodibenzo-p-dioxin. 3-MC inhibited transcription driven by a MAT1A promoter-reporter construct transfected into rat hepatocytes, but MAT1A mRNA stability was not affected. We recently showed that liver MAT1A expression is induced by AdoMet in cultured hepatocytes. Here, we observed that exogenously added AdoMet prevented the negative effects of 3-MC on MAT1A expression. Taken together, our data demonstrate that liver MAT1A gene expression is targeted by PAHs, independently of AhR activation. The effect of AdoMet may be part of the protective action of this molecule in liver damage.

S-adenosylmethionine regulates MAT1A and MAT2A gene expression in cultured rat hepatocytes: a new role for S-adenosylmethionine in the maintenance of the differentiated status of the liver

Methionine metabolism starts with the formation of S-adenosylmethionine (AdoMet), the most important biological methyl donor. This reaction is catalyzed by methionine adenosyltransferase (MAT). MAT is the product of two different genes: MAT1A, which is expressed only in the adult liver, and MAT2A, which is widely distributed, expressed in the fetal liver, and replaces MAT1A in hepatocarcinoma. In the liver, preservation of high expression of MAT1A and low expression of MAT2A is critical for the maintenance of a functional and differentiated organ. Here we describe that in cultured rat hepatocytes MAT1A expression progressively decreased, as described for other liver-specific genes, and MAT2A expression was induced. We find that this switch in gene expression was prevented by adding AdoMet to the culture medium. We also show that in cultured hepatocytes with decreased MAT1A expression AdoMet addition markedly increased MAT1A transcription in a dose-dependent fashion. This effect of AdoMet was mimicked by methionine, and blocked by 3-deazaadenosine and L-ethionine, but not D-ethionine, indicating that the effect was specific and mediated probably by a methylation reaction. These findings identify AdoMet as a key molecule that differentially regulates MAT1A and MAT2A expression and helps to maintain the differentiated status of the hepatocyte.

S-adenosylmethionine and Pneumocystis carinii

We previously reported that S-adenosylmethionine (AdoMet), a key molecule in methylation reactions and polyamine biosynthesis, enhances axenic culture of the AIDS-associated opportunistic fungal pathogen Pneumocystis carinii. Here we report that AdoMet is absolutely required for continuous growth. Two transporters are present, one high affinity, K(m) = 4.5 microm, and one low affinity, K(m) = 333 microm. The physiologically relevant high affinity transporter has a pH optimum of 7.5 and no related natural compounds compete for uptake. Transport is 98% inhibited at 4 degrees C, 24% inhibited by 20 mm sodium azide, and 95% inhibited by the combination of 20 mm sodium azide and 1 mm salicylhydroxamic acid; thus transport is active and dependent on both a cytochrome chain and an alternative oxidase. In vitro, AdoMet is used at a rate of 1. 40 x 10(7) molecules cell(-1) min(-1). AdoMet synthetase activity was not detected by a sensitive radiolabel incorporation assay capable of detecting 0.1% of the activity in rat liver. In addition, the AdoMet plasma concentration of rats is inversely correlated with the number of P. carinii in the lungs. These findings demonstrate that P. carinii is an AdoMet auxotroph. The uptake and metabolism of this compound are rational chemotherapeutic targets.

Kinetics of methionine transport and metabolism by Trypanosoma brucei brucei and Trypanosoma brucei rhodesiense

Methionine is an essential amino acid for both prokaryotic and eukaryotic organisms; however, little is known concerning its utilization in African trypanosomes, protozoa of the Trypanosoma brucei group. This study explored the Michaelis-Menten kinetic constants for transport and pool formation as well as metabolic utilization of methionine by two divergent strains of African trypanosomes, Trypanosoma brucei brucei (a veterinary pathogen), highly sensitive to trypanocidal agents, and Trypanosoma brucei rhodesiense (a human pathogenic isolate), highly refractory to trypanocidal arsenicals. The Michaelis-Menten constants derived by Hanes-Woolf analysis for transport of methionine for T. b. brucei and T. b. rhodesiense, respectively, were as follows: K(M) values, 1. 15 and 1.75 mM; V(max) values, 3.97 x 10(-5) and 4.86 x 10(-5) mol/L/min. Very similar values were obtained by Lineweaver-Burk analysis (K(M), 0.25 and 1.0 mM; V(max), 1 x 10(-5) and 2.0 x 10(-5) mol/L/min, T. b. brucei and T. b. rhodesiense, respectively). Cooperativity analyses by Hill (log-log) plot gave Hill coefficients (n) of 6 and 2 for T. b. brucei and T. b. rhodesiense, respectively. Cytosolic accumulation of methionine after 10-min incubation with 25 mM exogenous methionine was 1.8-fold greater in T. b. rhodesiense than T. b. brucei (2.1 vs 1.1 mM, respectively). In African trypanosomes as in their mammalian host, S-adenosylmethionine (AdoMet) is the major product of methionine metabolism. Accumulation of AdoMet was measured by HPLC analysis of cytosolic extracts incubated in the presence of increasing cytosolic methionine. In trypanosomes incubated for 10 min with saturating methionine, both organisms accumulated similar amounts of AdoMet (approximately 23 microM), but the level of trans-sulfuration products (cystathionine and cysteine) in T. b. rhodesiense was double that of T. b. brucei. Methionine incorporation during protein synthesis in T. b. brucei was 2.5 times that of T. b. rhodesiense. These results further confirm our belief that the major pathways of methionine utilization, for polyamine synthesis, protein transmethylation and the trans-sulfuration pathway, are excellent targets for chemotherapeutic intervention against African trypanosomes.

S-adenosylmethionine attenuates the lipopolysaccharide-induced expression of the gene for tumour necrosis factor alpha

Intracellular deficiency of S-adenosylmethionine (AdoMet) and elevated serum concentrations of tumour necrosis factor alpha (TNF) are hallmarks of toxin-induced liver injury. In these models, the administration of either exogenous AdoMet or antibody/soluble receptor for TNF attenuates the injury. We have demonstrated previously that the administration of exogenous AdoMet to AdoMet-deficient rats attenuated lipopolysaccharide (LPS)-induced liver injury and serum TNF concentrations. Here we report that AdoMet lowered the amount of TNF secreted by LPS-stimulated murine macrophage cells (RAW 264.7) in a dose-dependent manner. The inhibition of TNF release was correlated with changes in the steady-state TNF mRNA concentrations. Changes in TNF mRNA were not due to its altered stability and might have been due to an attenuation of the transcription rate of the TNF gene. The inhibition of TNF release in RAW cells was not mediated by GSH because treatment with AdoMet did not increase intracellular GSH. In addition, N-acetylcysteine, whereas it did increase GSH concentration, had no effect on LPS-stimulated TNF release in these cells. Exogenous AdoMet also attenuated LPS-induced serum TNF levels in normal rats sensitized with lead. Thus AdoMet administration might exert its hepatoprotective effects at least in part by its inhibitory effect on expression of the gene for TNF.

Novel evidence for an ecto-phospholipid methyltransferase in isolated rat hepatocytes

Phospholipids of isolated rat hepatocytes were labelled by preincubation with either 2 microM -methyl-14C-S-adenosylmethionine (AdoMet) or 2 microM [methyl-14C]methionine. Subsequent addition of phospholipase C to the suspension removed 95% of the radioactivity from phospholipids methylated by [methyl-14C]AdoMet within a few minutes, but was without effect on phospholipids methylated by [methyl-14C]methionine radioactivity from the latter could, nevertheless, be removed by phospholipase C after permeabilization of the cells with digitonin. The results clearly show that the methyl group of exogenous AdoMet, contrary to that of methionine, is transferred on to phospholipids located on the external face of the plasma membrane. Accordingly, pretreatment of isolated hepatocytes with trypsin prevented the methylation of phospholipids from exogenous AdoMet by 60-80%, whereas it was almost without effect when exogenous methionine was the methyl donor. Our data corroborate previous work [Bontemps and Van den Berghe (1997) Biochem. J. 327, 383-389], which indicated that AdoMet methylates hepatocyte phospholipids without penetrating the cells.