Methionine adenosyltransferase 1A knockout mice are predisposed to liver injury and exhibit increased expression of genes involved in proliferation

Mechanisms of non-alcoholic steatohepatitis

In 1980, the term non-alcoholic steatohepatitis was coined to describe a new syndrome occurring in patients who usually were obese (often diabetic) females who had a liver biopsy picture consistent with alcoholic hepatitis, but who denied alcohol use. The causes of this syndrome were unknown, and there was no defined therapy. More than two decades later, this clinical syndrome is only somewhat better understood, and still there is no Food and Drug Administration-approved or even generally accepted drug therapy. Patients with primary non-alcoholic steatohepatitis typically have the insulin resistance syndrome (synonymous with the metabolic syndrome, syndrome X, and so forth), which is characterized by obesity, diabetes, hyperlipidemia, hypertension, and, in some instances, other metabolic abnormalities such as polycystic ovary disease. Secondary non-alcoholic steatohepatitis may be caused by drugs such as tamoxifen, certain industrial toxins, rapid weight loss, and so forth. The cause of non-alcoholic steatohepatitis remains elusive, but most investigators agree that a baseline of steatosis requires a second hit capable of inducing inflammation, fibrosis, or necrosis for non-alcoholic steatohepatitis to develop. Our research group has focused its efforts on the interactions of nutritional abnormalities, cytokines, oxidative stress with lipid peroxidation, and mitochondrial dysfunction in the induction of steatohepatitis, both alcoholic and non-alcoholic in origin. Research findings from other laboratories also support the role of increased cytokine activity, oxidative stress, and mitochondrial dysfunction in the pathogenesis of non-alcoholic steatohepatitis. The objectives of this article are to review the (1) definition and clinical features of non-alcoholic steatohepatitis, (2) potential mechanisms of non-alcoholic steatohepatitis, and (3) potential therapeutic interventions in non-alcoholic steatohepatitis.

Animal models of nonalcoholic fatty liver disease and steatohepatitis

As occurs in people, nonalcoholic fatty liver disease (NAFLD) is associated strongly with obesity, diabetes, and dyslipidemia in experimental animals. There are many animal models that have been used to investigate the pathogenesis of nonalcoholic fatty liver disease. Most of this work has used mice or rats that are fed diets high in fat or carbohydrates, or mice that exhibit a genetic deficiency of a satiety factor such as leptin, 5-adenosylmethionine,or enzyme deficiencies in fatty acid oxidation. The purpose of this article is to update information regarding animal models in the pathogenesis of NAFLD.

The potential role of genes in nonalcoholic fatty liver disease

Although most people with obesity and type 2 diabetes will have steatosis, only a minority will ever develop nonalcoholic steatohepatitis (NASH), fibrosis, and cirrhosis. Family studies suggest that genetic factors are important in disease progression, although dissecting genetic factors playing a role in NASH and fibrosis from those influencing the development established risk factors is difficult. Several approaches can be used to look for genetic factors playing a role in nonalcoholic fatty liver disease (NAFLD). In the future, genome-wide single nucleotide polymorphism (SNP) scanning of cases and controls may become feasible. To date, however,studies have relied on candidate gene, case control, allele association methodology. Recent, and as yet preliminary, studies have reported associations between steatosis severity, NASH, and fibrosis with genes whose products are involved in lipid metabolism,oxidative stress, and endotoxin-cytokine interactions. If confirmed,these associations will enhance understanding of disease pathogenesis,and accordingly, the ability to design effective therapies.

S-adenosylmethionine and its metabolite induce apoptosis in HepG2 cells: Role of protein phosphatase 1 and Bcl-x(S)

S-adenosylmethionine (SAMe) and its metabolite 5'-methylthioadenosine (MTA) are proapoptotic in HepG2 cells. In microarray studies, we found SAMe treatment induced Bcl-x expression. Bcl-x is alternatively spliced to produce two distinct mRNAs and proteins, Bcl-x(L) and Bcl-x(S). Bcl-x(L) is antiapoptotic, while Bcl-x(S) is proapoptotic. In this study we showed that SAMe and MTA selectively induced Bcl-x(S) in a time- and dose-dependent manner in HepG2 cells. There are three transcription start sites in the human Bcl-x gene which yield only Bcl-x(L) in control HepG2 cells. SAMe and MTA treatment did not affect promoter usage, but while one promoter yielded only Bcl-x(L), the other two yielded both Bcl-x(L) and Bcl-x(S), with Bcl-x(S) as the predominant messenger RNA (mRNA) species. Trichostatin A, 3-deaza-adenosine, cycloleucine, and okadaic acid had no effect on Bcl-x(S) induction by SAMe or MTA. Calyculin A and tautomycin, on the other hand, blocked SAMe and MTA-mediated Bcl-x(S) induction and apoptosis in a dose-dependent manner. SAMe and MTA increased protein phosphatase 1 (PP1) catalytic subunit mRNA and protein levels and dephosphorylation of serine-arginine proteins, with the latter blocked by calyculin A. The effects of SAMe and MTA on Bcl-x(S), PP1 expression, and apoptosis were also seen in 293 cells, but not in primary hepatocytes. Induction of Bcl-x(S) by ceramide in HepG2 cells also resulted in apoptosis. In conclusion, we have uncovered a highly novel action of SAMe and MTA, namely the ability to affect the cellular phosphorylation state and alternative splicing of genes, in this case resulting in the induction of Bcl-x(S) leading to apoptosis.

Hyperhomocysteinemia, endoplasmic reticulum stress, and alcoholic liver injury

Deficiencies in vitamins or other factors (B6, B12, folic acid, betaine) and genetic disorders for the metabolism of the non-protein amino acid-homocysteine (Hcy) lead to hyperhomocysteinemia (HHcy). HHcy is an integral component of several disorders including cardiovascular disease, neurodegeneration, diabetes and alcoholic liver disease. HHcy unleashes mediators of inflammation such as NFkappaB, IL-1beta, IL-6, and IL-8, increases production of intracellular superoxide anion causing oxidative stress and reducing intracellular level of nitric oxide (NO), and induces endoplasmic reticulum (ER) stress which can explain many processes of Hcy-promoted cell injury such as apoptosis, fat accumulation, and inflammation. Animal models have played an important role in determining the biological effects of HHcy. ER stress may also be involved in other liver diseases such as alpha (1)-antitrypsin (alpha(1)-AT) deficiency and hepatitis C and/or B virus infection. Future research should evaluate the possible potentiative effects of alcohol and hepatic virus infection on ER stress-induced liver injury, study potentially beneficial effects of lowering Hcy and preventing ER stress in alcoholic humans, and examine polymorphism of Hcy metabolizing enzymes as potential risk-factors for the development of HHcy and liver disease.

Acidic sphingomyelinase downregulates the liver-specific methionine adenosyltransferase 1A, contributing to tumor necrosis factor-induced lethal hepatitis

S-adenosyl-L-methionine (SAM) is synthesized by methionine adenosyltransferases (MATs). Ablation of the liver-specific MAT1A gene results in liver neoplasia and sensitivity to oxidant injury. Here we show that acidic sphingomyelinase (ASMase) mediates the downregulation of MAT1A by TNF-alpha. The levels of MAT1A mRNA as well as MAT I/III protein decreased in cultured rat hepatocytes by in situ generation of ceramide from exogenous human placenta ASMase. Hepatocytes lacking the ASMase gene (ASMase-/-) were insensitive to TNF-alpha but were responsive to exogenous ASMase-induced downregulation of MAT1A. In an in vivo model of lethal hepatitis by TNF-alpha, depletion of SAM preceded activation of caspases 8 and 3, massive liver damage, and death of the mice. In contrast, minimal hepatic SAM depletion, caspase activation, and liver damage were seen in ASMase-/- mice. Moreover, therapeutic treatment with SAM abrogated caspase activation and liver injury, thus rescuing ASMase+/+ mice from TNF-alpha-induced lethality. Thus, we have demonstrated a new role for ASMase in TNF-alpha-induced liver failure through downregulation of MAT1A, and maintenance of SAM may be useful in the treatment of acute and chronic liver diseases.

Hepatic transmethylation reactions in micropigs with alcoholic liver disease

Alcoholic liver disease is associated with abnormal hepatic methionine metabolism, including increased levels of homocysteine and S-adenosylhomocysteine (SAH) and reduced levels of S-adenosylmethionine (SAM) and glutathione (GSH). The concept that abnormal methionine metabolism is involved in the pathogenesis of alcoholic liver disease was strengthened by our previous findings in a micropig model where combining dietary folate deficiency with chronic ethanol feeding produced maximal changes in these metabolites together with early onset of microscopic steatohepatitis and an eightfold increase in plasma aspartate aminotransferase. The goal of the present study was to determine potential mechanisms for abnormal levels of these methionine metabolites by analyzing the transcripts and activities of transmethylation enzymes in the livers of the same micropigs. Ethanol feeding or folate deficiency, separately or in combination, decreased transcript levels of methylenetetrahydrofolate reductase (MTHFR), methionine adenosyltransferase (MAT1A), glycine-N-methyltransferase (GNMT) and S-adenosylhomocysteine hydrolase (SAHH). Ethanol feeding alone reduced the activities of methionine synthase (MS) and MATIII and increased the activity of GNMT. Each diet, separately or in combination, decreased the activities of MTHFR and SAHH. In conclusion, the observed abnormal levels of methionine metabolites in this animal model of accelerated alcoholic liver injury can be ascribed to specific effects of ethanol with or without folate deficiency on the expressions and activities of hepatic enzymes that regulate transmethylation reactions. These novel effects on transmethylation reactions may be implicated in the pathogenesis of alcoholic liver disease.

Microarray analysis of mouse brain gene expression following acute ethanol treatment

Alterations in gene expression are thought to help mediate certain effects of alcohol in the brain. We have analyzed the expression of approximately 24,000 genes using oligonucleotide microarrays to examine the brain expression profiles in two strains of inbred mice, C57BL/6J and DBA/2J, following exposure to an acute dose of ethanol. Our screen identified 61 genes responding to the ethanol treatment beyond a 1.5-fold threshold, with 46 genes altered in both mouse strains and 15 altered in only one strain. Approximately 25% of the genes were selected for confirmation by reverse transcriptase polymerase chain reaction with an 87% success rate. The genes identified have roles in cell signaling, gene regulation, and homeostasis/stress response. Although some of the genes were previously known to be ethanol responsive, we have for the most part identified novel genes involved in the acute murine brain response to ethanol. Such genes have the potential to represent candidate genes in the search to elucidate the molecular pathways mediating ethanol's effects in the brain.

An integrated reverse functional genomic and metabolic approach to understanding orotic acid-induced fatty liver

In functional genomics, DNA microarrays for gene expression profiling are increasingly being used to provide insights into biological function or pathology. To better understand the significance of the multiple transcriptional changes across a time period, the temporal changes in phenotype must be described. Orotic acid-induced fatty liver disease was investigated at the transcriptional and metabolic levels using microarrays and metabolic profiling in two strains of rats. High-resolution 1H-NMR spectroscopic analysis of liver tissue indicated that Kyoto rats compared with Wistar rats are predisposed to the insult. Metabolite analysis and gene expression profiling following orotic acid treatment identified perturbed metabolic pathways, including those involved in fatty acid, triglyceride, and phospholipid synthesis, beta-oxidation, altered nucleotide, methyl donor, and carbohydrate metabolism, and stress responses. Multivariate analysis and statistical bootstrapping were used to investigate co-responses with transcripts involved in metabolism and stress responses. This reverse functional genomic strategy highlighted the relationship between changes in the transcription of stearoyl-CoA desaturase 1 and those of other lipid-related transcripts with changes in NMR-derived lipid profiles. The results suggest that the integration of 1H-NMR and gene expression data sets represents a robust method for identifying a focused line of research in a complex system.

Starvation response in mouse liver shows strong correlation with life-span-prolonging processes

We have monitored global changes in gene expression in mouse liver in response to fasting and sugar-fed conditions using high-density microarrays. From ∼20,000 different genes, the significantly regulated ones were grouped into specific signaling and metabolic pathways. Striking changes in lipid signaling cascade, insulin and dehydroepiandrosterone (DHEA) hormonal pathways, urea cycle and S-adenosylmethionine-based methyl transfer systems, and cell apoptosis regulators were observed. Since these pathways have been implicated to play a role in the aging process, and since we observe significant overlap of genes regulated upon starvation with those regulated upon caloric restriction, our analysis suggests that starvation may elicit a stress response that is also elicited during caloric restriction. Therefore, many of the signaling and metabolic components regulated during fasting may be the same as those which mediate caloric restriction-dependent life-span extension.

Impaired liver regeneration in mice lacking methionine adenosyltransferase 1A

Methionine adenosyltransferase (MAT) is an essential enzyme because it catalyzes the formation of S-adenosylmethionine (SAMe), the principal biological methyl donor. Of the two genes that encode MAT, MAT1A is mainly expressed in adult liver and MAT2A is expressed in all extrahepatic tissues. Mice lacking MAT1A have reduced hepatic SAMe content and spontaneously develop hepatocellular carcinoma. The current study examined the influence of chronic hepatic SAMe deficiency on liver regeneration. Despite having higher baseline hepatic staining for proliferating cell nuclear antigen, MAT1A knockout mice had impaired liver regeneration after partial hepatectomy (PH) as determined by bromodeoxyuridine incorporation. This can be explained by an inability to up-regulate cyclin D1 after PH in the knockout mice. Upstream signaling pathways involved in cyclin D1 activation include nuclear factor kappaB (NFkappaB), the c-Jun-N-terminal kinase (JNK), extracellular signal-regulated kinases (ERKs), and signal transducer and activator of transcription-3 (STAT-3). At baseline, JNK and ERK are more activated in the knockouts whereas NFkappaB and STAT-3 are similar to wild-type mice. Following PH, early activation of these pathways occurred, but although they remained increased in wild-type mice, c-jun and ERK phosphorylation fell progressively in the knockouts. Hepatic SAMe levels fell progressively following PH in wild-type mice but remained unchanged in the knockouts. In culture, MAT1A knockout hepatocytes have higher baseline DNA synthesis but failed to respond to the mitogenic effect of hepatocyte growth factor. Taken together, our findings define a critical role for SAMe in ERK signaling and cyclin D1 regulation during regeneration and suggest chronic hepatic SAMe depletion results in loss of responsiveness to mitogenic signals.

Abnormal hepatic methionine and glutathione metabolism in patients with alcoholic hepatitis

BACKGROUND

Abnormal methionine metabolism occurs in animals fed ethanol and in end-stage cirrhotic patients. Expected consequences of these abnormalities include reduced hepatic S-adenosylmethionine and glutathione (GSH) levels, impaired transmethylation, and reduced homocysteine catabolism, resulting in the often-observed hyperhomocystinemia in cirrhotic patients. These parameters have not been examined simultaneously in patients with less advanced alcoholic liver disease.

METHODS

Six patients hospitalized for alcoholic hepatitis were studied. Plasma was analyzed for homocysteine, methionine, and GSH levels. Liver biopsies diagnosed acute alcoholic hepatitis and underlying fibrosis. Liver specimens were processed for messenger RNA (mRNA) levels and various metabolites and were compared with those of six normal controls.

RESULTS

Three patients had cirrhosis, and three had only portal fibrosis. Plasma levels of homocysteine and methionine were increased in two of the three patients with cirrhosis but not in the patients with fibrosis. All patients had markedly lower plasma GSH levels (mean +/- SD: 0.27 +/- 0.19 microM, which is at least 10-fold lower than the normal range). Hepatic S-adenosylmethionine levels were reduced by 50%, whereas methionine, GSH, and cysteine levels were reduced by 70-80%. The mRNA levels of most enzymes involved in methionine metabolism and GSH synthesis were decreased, whereas albumin expression was unchanged. Despite the well known induction of cytochrome P450 2E1 in chronic alcoholics, its mRNA levels were nearly 70% lower in these patients.

CONCLUSIONS

In alcoholic hepatitis, abnormal hepatic gene expression in methionine and GSH metabolism occurs and often contributes to decreased hepatic methionine, S-adenosylmethionine, cysteine, and GSH levels. It may be important to replenish these thiols in patients hospitalized with alcoholic hepatitis.

Epigenetic events and protection from colon cancer in New Zealand

The incidence of colon cancer is high in many developed nations, especially New Zealand. Molecular understanding of the nature of colon cancer shows a disease whose well-characterized morphological progression is paralleled at the cellular level by increased numbers of gene or chromosome mutations, loss of heterozygosity, changed methylation patterns, and genomic instability. In the present study, we consider whether an imbalance of factors that affect DNA methylation patterns might explain at least part of the high colon cancer incidence in New Zealand. Folate is the major micronutrient whose intake impacts methylation, particularly through interaction with choline and methionine. Folate is generally somewhat deficient in the New Zealand diet, with the voluntary addition of folate to white flour not producing desired levels. Selenium affects methylation status in several ways and is recognized as being low in New Zealand soils and, therefore, diet. Zinc is also low in the diets of some New Zealand population groups, which can lead to hypomethylation. Several of the components of fruits and vegetables affect methylation patterns, and the average New Zealand intake, at two to three servings per day, is considerably below recommended amounts. Low dietary fiber, high tobacco use, and increasing rates of obesity are also likely New Zealand risk factors that may impact on methylation status. Dietary supplementation is not as common in New Zealand as in countries such as the United States, but may provide a way to raise the levels of nutrients and phytochemicals affecting methylation status, thereby enhancing colon cancer protection.

Microarrays, antiobesity and the liver

In this review, the microarray technology and especially oligonucleotide arrays are exemplified with a practical example taken from the perilipin-/- mice and using the dChip software, available for non-lucrative purposes. It was found that the liver of perilipin-/- mice was healthy and normal, even under high-fat diet when compared with the results published for the scd1-/- mice, which under high-fat diets had a darker liver, suggestive of hepatic steatosis. Scd1 is required for the biosynthesis of monounsaturated fatty acids and plays a key role in the hepatic synthesis of triglycerides and of very-low-density lipoproteins. Both models of obesity resistance share many similar phenotypic antiobesity features, however, the perilipin-/- mice had a significant downregulation of stearoyl CoA desaturases scd1 and scd2 in its white adipose tissue, but a normal level of both genes inside the liver, even under high-fat diet. Here, different microarray methodologies are discussed, and also some of the most recent discoveries and perspectives regarding the use of microarrays, with an emphasis on obesity gene expression, and a personal remark on my findings of increased expression for hemoglobin transcripts and other hemo related genes (hemo-like), and for leukocyte like (leuko-like) genes inside the white adipose tissue of the perilipin-/- mice. In conclusion, microarrays have much to offer in comparative studies such as those in antiobesity, and also they are methodologies adequate for new astounding molecular discoveries [free full text of this article Online].

Lipid peroxidation, stellate cell activation and hepatic fibrogenesis in a rat model of chronic steatohepatitis

BACKGROUND/AIMS

We explored the involvement of cell types, cytokines and lipid peroxidation in a rat dietary model of fibrosing steatohepatitis.

METHODS

Male rats were fed a high fat diet deficient in methionine and choline (MCD) for up to 17 weeks. Whole liver, hepatocytes and non-parenchymal cells were analysed for reduced glutathione (GSH) levels, products of lipid peroxidation (thiobarbituric acid reactive substances, TBARS), liver injury, and fibrosis.

RESULTS

MCD diet-fed rats developed hepatic steatosis at week 2 and focal necroinflammatory change by week 5, while pericellular fibrosis evolved and progressed between weeks 12 and 17. Collagen alpha(1)(1) gene expression was upregulated by week 5 and increased fivefold by week 17. Stellate cells were the unique source of collagen gene expression. TIMP-1 and -2 were increased at week 12. Livers of MCD diet-fed rats exhibited lowered levels of GSH and elevated TBARS. Hepatocytes were the source of lipid peroxidation, and mRNA levels for TGFbeta1 were increased only in this cell type.

CONCLUSIONS

The MCD model of 'fibrosing steatohepatitis' replicates the histologic features of human steatohepatitis, and the sequence of steatosis, inflammatory cell injury and fibrogenesis. The temporal sequence is consistent with a concept for involvement of oxidative injury in inflammatory recruitment and pathogenesis of hepatic fibrogenesis.

Induction of human methionine adenosyltransferase 2A expression by tumor necrosis factor alpha. Role of NF-kappa B and AP-1

Two genes (MAT1A and MAT2A) encode for methionine adenosyltransferase (MAT), an essential cellular enzyme responsible for S-adenosylmethionine biosynthesis. MAT1A is expressed mostly in the liver, whereas MAT2A is widely distributed. We showed a switch from MAT1A to MAT2A expression in human hepatocellular carcinoma (HCC), which facilitates cancer cell growth. Using DNase I footprinting analysis, we previously identified a region in the MAT2A promoter protected from DNase I digestion in HCC. This region contains NF-kappa B and AP-1 elements, and the present study examined whether they regulate MAT2A promoter activity. We found nuclear binding of NF-kappa B and AP-1 to the MAT2A promoter increased in HCC. Tumor necrosis factor alpha (TNFalpha), which activates both NF-kappa B and AP-1, increased MAT2A expression in a dose- and time-dependent manner, binding of both NF-kappa B and AP-1 to the MAT2A promoter and MAT2A promoter activity, with the latter effect blocked by site-directed mutagenesis of the NF-kappa B and AP-1 binding sites. Blocking NF-kappa B with I kappa B super-repressor or AP-1 with dominant-negative c-Jun led to decreased basal MAT2A expression and prevented the TNF alpha-induced increase in MAT2A expression. Although blocking NF-kappa B had no influence on the ability of TNF alpha to increase AP-1 nuclear binding, blocking AP-1 with dominant-negative c-Jun prevented the TNF alpha-mediated increase in NF-kappa B binding. In conclusion, both NF-kappa B and AP-1 are required for basal MAT2A expression in HepG2 cells and mediate the increase in MAT2A expression in response to TNF alpha treatment. Increased trans-activation of these two sites also contributes to MAT2A up-regulation in HCC.

S-adenosylmethionine (AdoMet) modulates endotoxin stimulated interleukin-10 production in monocytes

IL-10 is produced by a large variety of cells including monocytes, macrophages, B and T lymphocytes, as well as natural killer cells and is an important suppressor for both immunoproliferative and inflammatory responses. IL-10 exerts antifibrotic effects in the liver, and decreased monocyte synthesis of IL-10 is well documented in alcoholic cirrhosis. Intracellular deficiency of S-adenosylmethionine (AdoMet) is a hallmark of toxin-induced liver injury. Although the administration of exogenous AdoMet attenuates this injury, the mechanisms of its actions are not fully established. This study was performed to investigate the effect of exogenous AdoMet on IL-10 production in LPS-stimulated RAW 264.7 cells, a murine macrophage cell line. Our results demonstrated that exogenous AdoMet administration enhanced both protein production and gene expression of IL-10 in RAW 264.7 cells. Ethionine, an inhibitor for methionine adenosyltransferases, inhibited LPS-stimulated IL-10 both at the protein and mRNA levels. Exogenous AdoMet increased the intracellular cAMP concentration as early as 3 h and continued for 24 h after AdoMet treatment; however, the inhibitors for both adenylyl cyclase and PKA did not significantly affect IL-10 production. On the basis of these results, we conclude that AdoMet administration may exert its anti-inflammatory and hepatoprotective effects, at least in part, by enhancing LPS-stimulated IL-10 production.

L-methionine availability regulates expression of the methionine adenosyltransferase 2A gene in human hepatocarcinoma cells: role of S-adenosylmethionine

In mammals, methionine adenosyltransferase (MAT), the enzyme responsible for S-adenosylmethionine (AdoMet) synthesis, is encoded by two genes, MAT1A and MAT2A. In liver, MAT1A expression is associated with high AdoMet levels and a differentiated phenotype, whereas MAT2A expression is associated with lower AdoMet levels and a dedifferentiated phenotype. In the current study, we examined regulation of MAT2A gene expression by l-methionine availability using HepG2 cells. In l-methionine-deficient cells, MAT2A gene expression is rapidly induced, and methionine adenosyltransferase activity is increased. Restoration of l-methionine rapidly down-regulates MAT2A mRNA levels; for this effect, l-methionine needs to be converted into AdoMet. This novel action of AdoMet is not mediated through a methyl transfer reaction. MAT2A gene expression was also regulated by 5'-methylthioadenosine, but this was dependent on 5'-methylthioadenosine conversion to methionine through the salvage pathway. The transcription rate of the MAT2A gene remained unchanged during l-methionine starvation; however, its mRNA half-life was significantly increased (from 100 min to more than 3 h). The effect of l-methionine withdrawal on MAT2A mRNA stabilization requires both gene transcription and protein synthesis. We conclude that MAT2A gene expression is modulated as an adaptive response of the cell to l-methionine availability through its conversion to AdoMet.

Nonalcoholic steatohepatitis: summary of an AASLD Single Topic Conference

Fatty liver disease that develops in the absence of alcohol abuse is recognized increasingly as a major health burden. This report summarizes the presentations and discussions at a Single Topic Conference held September 20-22, 2002, and sponsored by the American Association for the Study of Liver Diseases. The conference focused on fatty liver disorders. Estimates based on imaging and autopsy studies suggest that about 20% to 30% of adults in the United States and other Western countries have excess fat accumulation in the liver. About 10% of these individuals, or fully 2% to 3% of adults, are estimated to meet current diagnostic criteria for nonalcoholic steatohepatitis (NASH). Sustained liver injury leads to progressive fibrosis and cirrhosis in a fraction, possibly up to one third, of those with NASH, and NASH may be a cause of cryptogenic cirrhosis. NASH is now a significant health issue for obese children as well, leading to cirrhosis in some. The diagnostic criteria for NASH continue to evolve and rely on the histologic findings of steatosis, hepatocellular injury (ballooning, Mallory bodies), and the pattern of fibrosis. Generally recognized indications for biopsy include establishing the diagnosis and staging of the injury, but strict guidelines do not exist. Liver enzymes are insensitive and cannot be used reliably to confirm the diagnosis or stage the extent of fibrosis. Older age, obesity, and diabetes are predictive of fibrosis. The pathogenesis of NASH is multifactorial. Insulin resistance may be an important factor in the accumulation of hepatocellular fat, whereas excess intracellular fatty acids, oxidant stress, adenosine triphosphate (ATP) depletion, and mitochondrial dysfunction may be important causes of hepatocellular injury in the steatotic liver. Efforts are underway to refine the role of insulin resistance in NASH and determine whether improving insulin sensitivity pharmacologically is an effective treatment. An altered lifestyle may be a more effective means of improving insulin sensitivity. The research agenda for the future includes establishing the role of insulin resistance and abnormal lipoprotein metabolism in NASH, determining the pathogenesis of cellular injury, defining predisposing genetic abnormalities, identifying better noninvasive predictors of disease, and defining effective therapy.

Methionine adenosyltransferase II beta subunit gene expression provides a proliferative advantage in human hepatoma

BACKGROUND & AIMS

Of the 2 genes (MAT1A, MAT2A) encoding methionine adenosyltransferase, the enzyme that synthesizes S-adenosylmethionine, MAT1A, is expressed in liver, whereas MAT2A is expressed in extrahepatic tissues. In liver, MAT2A expression associates with growth, dedifferentiation, and cancer. Here, we identified the beta subunit as a regulator of proliferation in human hepatoma cell lines. The beta subunit has been cloned and shown to lower the K(m) of methionine adenosyltransferase II alpha2 (the MAT2A product) for methionine and to render the enzyme more susceptible to S-adenosylmethionine inhibition.

METHODS

Methionine adenosyltransferase II alpha2 and beta subunit expression was analyzed in human and rat liver and hepatoma cell lines and their interaction studied in HuH7 cells. beta Subunit expression was up- and down-regulated in human hepatoma cell lines and the effect on DNA synthesis determined.

RESULTS

We found that beta subunit is expressed in rat extrahepatic tissues but not in normal liver. In human liver, beta subunit expression associates with cirrhosis and hepatoma. beta Subunit is expressed in most (HepG2, PLC, and Hep3B) but not all (HuH7) hepatoma cell lines. Transfection of beta subunit reduced S-adenosylmethionine content and stimulated DNA synthesis in HuH7 cells, whereas down-regulation of beta subunit expression diminished DNA synthesis in HepG2. The interaction between methionine adenosyltransferase II alpha2 and beta subunit was demonstrated in HuH7 cells.

CONCLUSIONS

Our findings indicate that beta subunit associates with cirrhosis and cancer providing a proliferative advantage in hepatoma cells through its interaction with methionine adenosyltransferase II alpha2 and down-regulation of S-adenosylmethionine levels.

Functional proteomics of nonalcoholic steatohepatitis: mitochondrial proteins as targets of S-adenosylmethionine

Recent work shows that S-adenosylmethionine (AdoMet) helps maintain normal liver function as chronic hepatic deficiency results in spontaneous development of steatohepatitis and hepatocellular carcinoma. The mechanisms by which these nontraditional functions of AdoMet occur are unknown. Here, we use knockout mice deficient in hepatic AdoMet synthesis (MAT1A(-/-)) to study the proteome of the liver during the development of steatohepatitis. One hundred and seventeen protein spots, differentially expressed during the development of steatohepatitis, were selected and identified by peptide mass fingerprinting. Among them, 12 proteins were found to be affected from birth, when MAT1A(-/-) expression is switched on in WT mouse liver, to the rise of histological lesions, which occurs at approximately 8 months. Of the 12 proteins, 4 [prohibitin 1 (PHB1), cytochrome c oxidase I and II, and ATPase beta-subunit] have known roles in mitochondrial function. We show that the alteration in expression of PHB1 correlates with a loss of mitochondrial function. Experiments in isolated rat hepatocytes indicate that AdoMet regulates PHB1 content, thus suggesting ways by which steatohepatitis may be induced. Importantly, we found the expression of these mitochondrial proteins was abnormal in obob mice and obese patients who are at risk for nonalcoholic steatohepatitis.

Role of S-adenosylmethionine in two experimental models of pancreatitis

Severe necrotizing pancreatitis occurs in young female mice fed a choline-deficient and ethionine-supplemented (CDE) diet. Although the mechanism of the pancreatitis is unknown, one consequence of this diet is depletion of hepatic S-adenosylmethionine (SAM). SAM formation is catalyzed by methionine adenosyltransferases (MATs), which are encoded by liver-specific (MAT1A) and non-liver-specific (MAT2A) genes. In this work, we examined changes in pancreatic SAM homeostasis in mice receiving the CDE diet and the effect of SAM treatment. We found that both MAT forms are expressed in normal pancreas and pancreatic acini. After 48 h of the CDE diet, SAM levels decreased 50% and MAT1A-encoded protein disappeared via post-translational mechanisms, whereas MAT2A-encoded protein increased via pretranslational mechanisms. CDE-fed mice exhibited extensive necrosis, edema, and acute pancreatic inflammatory infiltration, which were prevented by SAM treatment. However, old female mice consuming the CDE diet that do not develop pancreatitis showed a similar fall in pancreatic SAM level. SAM was also protective in cerulein-induced pancreatitis in the rat, but the protection was limited. Although the pancreatic SAM level fell by more than 80% in the MAT1A knockout mice, no pancreatitis developed. This study thus provides several novel findings. First, the so-called liver-specific MAT1A is highly expressed in the normal pancreas and pancreatic acini. Second, the CDE diet causes dramatic changes in the expression of MAT isozymes by different mechanisms. Third, in contrast to the situation in the liver, where absence of MAT1A and decreased hepatic SAM level can lead to spontaneous tissue injury, in the pancreas the roles of SAM and MAT1A appear more complex and remain to be defined.

Genetic effects of methylation diets

DNA methylation at cytosines in CpG dinucleotides can lead to changes in gene expression and function without altering the primary sequence of the DNA. Methylation can be affected by dietary levels of methyl-donor components, such as folic acid. This may be an important mechanism for environmentally induced changes in gene expression. Recent literature supports a role for DNA-methylation changes in a number of adult-onset disorders and during development. These changes may be significant for better understanding certain birth defects (e.g., neural tube defects) and the long-term consequences of early environmental influences on gene expression (metabolic programming). Optimal "methylation diets" should be investigated as part of the prevention and treatment of all these conditions, as well as in disorders such as Rett syndrome, whose primary defects may lie in DNA methylation-dependent gene regulation.

Acute valproate administration impairs methionine metabolism in rats

Valproate (VPA) is a drug widely used to treat epilepsy, but it has serious adverse effects including hepatotoxicity, teratogenicity and antifolate activity. The mechanism underlying VPA toxicity is unclear although an interaction with folate and other metabolites involved in methionine metabolism has been suggested. The present study was undertaken to evaluate potential changes in the metabolic function of the methionine cycle after acute exposure to a single dose of valproate. Female Wistar rats (n = 30) were treated with 400 mg/kg of VPA. Different groups of six rats were killed at 1 (t1), 3 (t3), 6 (t6), 9 (t9), and 24 (t24) hours after the injection. One group of rats was untreated (n = 6) and was considered the control group. The most pronounced effects of VPA administration were observed 1 h after drug injection. VPA induced a 56% reduction in methionine adenosyltransferase activity and a 54% reduction in plasma vitamin B-6. Increases in the hepatic concentration of S-adenosylhomocysteine and oxidized glutathione, and a reduction in the S-adenosylmethionine/S-adenosylhomocysteine transmethylation ratio also occurred at 1 h. All of these alterations, however, were normalized within 24 h, parallel with a decrease in serum VPA concentration. The acute effects of VPA suggest that the alterations in the methionine cycle could be the common mechanism underlying the hepatotoxic, teratogenic and antifolate effects of the drug.

Spontaneous oxidative stress and liver tumors in mice lacking methionine adenosyltransferase 1A

In mammals, methionine metabolism occurs mainly in the liver via methionine adenosyltransferase-catalyzed conversion to S-adenosylmethionine. Of the two genes that encode methionine adenosyltransferase(MAT1Aand MAT2A), MAT1A is mainly expressed in adult liver whereas MAT2A is expressed in all extrahepatic tissues. Mice lacking MAT1A have reduced hepatic S-adenosylmethionine content and hyperplasia and spontaneously develop nonalcoholic steatohepatitis. In this study, we examined whether chronic hepatic S-adenosylmethionine deficiency generates oxidative stress and predisposes to injury and malignant transformation. Differential gene expression in MAT1A knockout mice was analyzed following the criteria of the Gene Ontology Consortium. Susceptibility of MAT1A knockout mice to CCl4-induced hepatotoxicity and malignant transformation was determined in 3- and 18-month-old mice, respectively. Analysis of gene expression profiles revealed an abnormal expression of genes involved in the metabolism of lipids and carbohydrates in MAT1A knockout mice, a situation that is reminiscent of that found in diabetes, obesity, and other conditions associated with nonalcoholic steatohepatitis. This aberrant expression of metabolic genes in the knockout mice was associated with hyperglycemia, increased hepatic CYP2E1 and UCP2 expression and triglyceride levels, and reduced hepatic glutathione content. The knockout animals have increased lipid peroxidation and enhanced sensitivity to CCl4-induced liver damage, which was largely due to increased CYP2E1 expression because diallyl sulfide, an inhibitor of CYP2E1, prevented CCl4-induced liver injury. Hepatocellular carcinoma developed in more than half of the knockout mice by 18 months of age. Taken together, our findings define a critical role for S-adenosylmethionine in maintaining normal hepatic function and tumorigenesis of the liver.

Dietary protein modifies hepatic gene expression associated with oxidative stress responsiveness in growing pigs

Understanding the basis for differences in nutrient requirements and for nutrient effects on health and performance requires an appreciation of the links between nutrition and gene expression. We developed and applied molecular probes to characterize diet-associated postabsorptive hepatic gene expression in growing pigs chronically fed protein-restricted diets based on either casein (CAS) or soy protein isolate (SPI). Eighty-eight expressed sequence tags (ESTs) were identified on the basis of diet-related changes in expression, by using an mRNA differential display method. Expression profiling based on transcription analysis by real-time reverse transcriptase-polymerase chain reaction showed that the SPI diet significantly changed the pattern of gene expression as compared with the CAS diet and allowed identification of coregulated genes. The expression of six genes involved in the metabolism of stress response (glutathione S-transferase, peptide methionine sulfoxide reductase, apolipoprotein A-I, organic anion transport polypeptide 2, calnexin, heat shock transcription factor 1) exhibited significant changes in the transcription level and indicated an increased oxidative stress response in pigs fed the SPI diet. Hierarchical clustering of gene expression data of all 33 ESTs analyzed across 14 pigs fed the two different diets resulted in clustering of genes related to the oxidative stress response with genes related to the regulation of gene expression and neuronal signaling.

Regulation of mammalian liver methionine adenosyltransferase

S-adenosylmethionine (SAM) is an essential metabolite in all cells. SAM is the most important biological methyl group donor and is a precursor in the synthesis of polyamines. Methionine adenosyltransferase (MAT; EC 2.5.1.6) catalyzes the only known SAM biosynthetic reaction from methionine and ATP. In mammalian tissues, three different forms of MAT (MAT I, MAT III and MAT II) have been identified that are the product of two different genes (MAT1A and MAT2A). Although MAT2A is expressed in all mammalian tissues, the expression of MAT1A is primarily restricted to adult liver. In mammals, up to 85% of all methylation reactions and as much as 48% of methionine metabolism occurs in the liver, which indicates the important role of this organ in the regulation of blood methionine. Recent evidence indicates that not only is SAM the main biological methyl group donor and an intermediate metabolite in methionine catabolism, but it is also an intracellular control switch that regulates essential hepatic functions such as liver regeneration and differentiation as well as the sensitivity of this organ to injury. Therefore, knowledge of factors that regulate the activity of MAT I/III, the specific liver enzyme, is essential to understand how cellular SAM levels are controlled.

Diet, methyl donors and DNA methylation: interactions between dietary folate, methionine and choline

DNA methylation influences the expression of some genes and depends upon the availability of methyl groups from S-adenosylmethionine (SAM). Dietary methyl groups derive from foods that contain methionine, one-carbon units and choline (or the choline metabolite betaine). Humans ingest approximately 50 mmol of methyl groups per day; 60% of them are derived from choline. Transmethylation metabolic pathways closely interconnect choline, methionine, methyltetrahydrofolate (methyl-THF) and vitamins B-6 and B-12. The pathways intersect at the formation of methionine from homocysteine. Perturbing the metabolism of one of these pathways results in compensatory changes in the others. For example, methionine can be formed from homocysteine using methyl groups from methyl-THF, or using methyl groups from betaine that are derived from choline. Similarly, methyl-THF can be formed from one-carbon units derived from serine or from the methyl groups of choline via dimethylglycine, and choline can be synthesized de novo using methyl groups derived from methionine (via SAM). When animals and humans are deprived of choline, they use more methyl-THF to remethylate homocysteine in the liver and increase dietary folate requirements. Conversely, when they are deprived of folate, they use more methyl groups from choline, increasing the dietary requirement for choline. The availability of transgenic and knockout mice has made possible additional studies that demonstrate the interrelationship of these methyl sources. In summary, as we consider dietary requirements and possible effects on DNA methylation, it is important to realize that methionine, methyl-THF and choline can be fungible sources of methyl groups, and the design of our studies should reflect this.

Folate deficiency disturbs hepatic methionine metabolism and promotes liver injury in the ethanol-fed micropig

Alcoholic liver disease is associated with abnormal hepatic methionine metabolism and folate deficiency. Because folate is integral to the methionine cycle, its deficiency could promote alcoholic liver disease by enhancing ethanol-induced perturbations of hepatic methionine metabolism and DNA damage. We grouped 24 juvenile micropigs to receive folate-sufficient (FS) or folate-depleted (FD) diets or the same diets containing 40% of energy as ethanol (FSE and FDE) for 14 wk, and the significance of differences among the groups was determined by ANOVA. Plasma homocysteine levels were increased in all experimental groups from 6 wk onward and were greatest in FDE. Ethanol feeding reduced liver methionine synthase activity, S-adenosylmethionine (SAM), and glutathione, and elevated plasma malondialdehyde (MDA) and alanine transaminase. Folate deficiency decreased liver folate levels and increased global DNA hypomethylation. Ethanol feeding and folate deficiency acted together to decrease the liver SAM/S-adenosylhomocysteine (SAH) ratio and to increase liver SAH, DNA strand breaks, urinary 8-oxo-2'-deoxyguanosine [oxo(8)dG]/mg of creatinine, plasma homocysteine, and aspartate transaminase by more than 8-fold. Liver SAM correlated positively with glutathione, which correlated negatively with plasma MDA and urinary oxo(8)dG. Liver SAM/SAH correlated negatively with DNA strand breaks, which correlated with urinary oxo(8)dG. Livers from ethanol-fed animals showed increased centrilobular CYP2E1 and protein adducts with acetaldehyde and MDA. Steatohepatitis occurred in five of six pigs in FDE but not in the other groups. In summary, folate deficiency enhances perturbations in hepatic methionine metabolism and DNA damage while promoting alcoholic liver injury.

S-Adenosylmethionine revisited: its essential role in the regulation of liver function

Dietary methionine is mainly metabolized in the liver where it is converted into S-adenosylmethionine (AdoMet), the main biologic methyl donor. This reaction is catalyzed by methionine adenosyltransferase I/III (MAT I/III), the product of MAT1A gene, which is exclusively expressed in this organ. It was first observed that serum methionine levels were elevated in experimental models of liver damage and in liver cirrhosis in human beings. Results of further studies showed that this pathological alteration was due to reduced MAT1A gene expression and MAT I/III enzyme inactivation associated with liver injury. Synthesis of AdoMet is essential to all cells in the organism, but it is in the liver where most of the methylation reactions take place. The central role played by AdoMet in cellular function, together with the observation that AdoMet administration reduces liver damage caused by different agents and improves survival of alcohol-dependent patients with cirrhosis, led us to propose that alterations in methionine metabolism could play a role in the onset of liver disease and not just be a consequence of it. In the present work, we review the recent findings that support this hypothesis and highlight the mechanisms behind the hepatoprotective role of AdoMet.

Folate deficiency, methionine metabolism, and alcoholic liver disease

Methionine metabolism is regulated by folate, and both folate deficiency and abnormal hepatic methionine metabolism are recognized features of alcoholic liver disease (ALD). Previously, histological features of ALD were induced in castrated male micropigs fed diets containing ethanol at 40% of kilocalories for 12 months, whereas in male micropigs fed the same diets for 12 months abnormal methionine metabolism and hepatocellular apoptosis developed. Folate deficiency may promote the development of ALD by accentuating abnormal methionine metabolism. Intact male micropigs received eucaloric diets that were folate sufficient, folate deficient, or each containing 40% of kilocalories as ethanol for 14 weeks. Folate deficiency alone reduced hepatic folates by one half, and ethanol feeding alone reduced methionine synthase, S-adenosylmethionine (SAM), and glutathione (GSH) levels and elevated plasma malondialdehyde (MDA) levels. The combined regimen elevated plasma homocysteine, hepatic S-adenosylhomocysteine (SAH), urinary 8-hydroxy-2-deoxyguanosine (oxy(8)dG), an index of DNA oxidation, and serum aspartate aminotransferase (AST) levels. Terminal hepatic histopathologic characteristics included typical features of steatonecrosis and focal inflammation in pigs fed the combined diet, with no changes in the other groups. Hepatic SAM levels correlated with those of GSH, whereas urinary oxy(8)dG and plasma MDA levels correlated with the SAM:SAH ratio and to hepatic GSH. The results demonstrate the linkage of abnormal methionine metabolism to products of DNA and lipid oxidation and to liver injury. The finding of steatonecrosis and focal inflammation only in the combined diet group supports the suggestion that folate deficiency promotes and folate sufficiency protects against the early onset of methionine cycle-mediated ALD.

Role of abnormal methionine metabolism in alcoholic liver injury

Methionine catabolism occurs mostly in the liver through the formation of S-adenosylmethionine (SAM) in a reaction catalyzed by methionine adenosyltransferase (MAT). S-adenosylmethionine is the principal biologic methyl donor, a precursor for polyamines, and in liver, it is also a precursor for reduced glutathione (GSH). Liver-specific and non-liver-specific MAT are products of two different genes, MAT1A and MAT2A, respectively. Mature liver expresses MAT1A, whereas MAT2A is expressed in extrahepatic tissues and induced during liver growth and de-differentiation. The type of MAT expressed by the cell affects the steady-state SAM level, DNA methylation, and growth rate. This has been demonstrated further by using the MAT1A knockout mouse model in which hepatic SAM and GSH levels decrease, the liver becomes larger and more susceptible to injury, and steatohepatitis develops spontaneously. Altered methionine metabolism in alcoholic liver disease results in decreased transmethylation and transsulfuration, changes that may play important pathogenic roles. Major changes include a relative switch in MAT expression; decreased hepatic SAM, GSH, and DNA methylation levels; decreased homocysteine metabolism; and hyperhomocysteinemia. Consequences of hepatic DNA hypomethylation include increased expression of c-myc and DNA strand break accumulation. One possible consequence of hyperhomocysteinemia is increased fibrogenesis. Abnormal methionine metabolism may also occur in Kupffer cells, which express both forms of MAT. If SAM levels also decrease in these cells, this may contribute to the induction of tumor necrosis factor (TNF) expression and release. In summary, altered hepatic methionine metabolism can have serious consequences that affect not only hepatocytes, but also hepatic stellate and Kupffer cells. These changes can lead to impaired antioxidant defense, altered gene expression, promotion of fibrogenesis, and even hepatocarcinogenesis.

S-Adenosylmethionine, cytokines, and alcoholic liver disease

Hepatic deficiency of S-adenosylmethionine (AdoMet) is a critical acquired metabolic abnormality in alcoholic liver disease (ALD) and in many experimental models of hepatotoxicity. Subnormal AdoMet, elevated serum tumor necrosis factor (TNF), and endotoxemia (LPS) are hallmarks of ALD and experimental liver injury. AdoMet deficiency is attributed to its subnormal synthesis, but mechanisms for increased TNF are not known. AdoMet deficiency may affect the critical balance of proinflammatory (e.g., TNF) and antiinflammatory [e.g., interleukin (IL)-10] cytokines. Rats maintained on a choline-deficient diet with limited amounts of methionine (MCD diet) developed AdoMet deficiency. When challenged with LPS, rats fed MCD diet had significantly increased serum TNF levels and worse liver injury compared with findings for controls. Exogenous AdoMet attenuated liver injury and serum TNF levels. Results of in vitro studies with the use of RAW 264.7 cells demonstrated that exogenous AdoMet supplementation lowered LPS-induced TNF formation in a dose-dependent manner, and AdoMet deficiency enhanced TNF secretion and TNF gene expression. AdoMet also dose-dependently decreased LPS-stimulated TNF production from monocytes obtained from patients with alcoholic hepatitis. Finally, AdoMet supplementation stimulated production of the antiinflammatory cytokine IL-10. Interleukin-10 plays a critical role in the modulation of TNF production, and IL-10 may inhibit hepatic fibrosis. This article will review (1) the role of AdoMet in ALD/liver injury, (2) the role of TNF/proinflammatory cytokines in ALD, (3) potential roles of AdoMet in TNF/proinflammatory cytokine regulation in ALD, and (4) conclusions and future directions.

Liver cell proliferation requires methionine adenosyltransferase 2A mRNA up-regulation

Regulation of liver cell proliferation is a key event to control organ size during development and liver regeneration. Methionine adenosyltransferase (MAT) 2A is expressed in proliferating liver, whereas MAT1A is the form expressed in adult quiescent hepatocytes. Here we show that, in H35 hepatoma cells, growth factors such as hepatocyte growth factor (HGF) and insulin up-regulated MAT2A expression. HGF actions were time- and dose-response dependent and required transcriptional activity. Mitogen-activated protein (MAP) kinase and phosphatidylinositol 3-phosphate kinase (PI 3-K) pathways were required for both HGF-induced cell proliferation and MAT2A up-regulation. Furthermore, in H35 cells treated with HGF, the inhibition of these pathways was associated with the switch from the expression of fetal liver MAT2A to the adult liver MAT1A isoform. Fetal liver hepatocytes exhibited an identical response pattern. Treatment of H35 hepatoma cells with MAT2A antisense oligonucleotides decreased cell proliferation induced by HGF; this decrease correlated with the decay in MAT2A messenger RNA (mRNA) levels. Finally, growth inhibitors such as transforming growth factor (TGF) beta blocked HGF-induced MAT2A up-regulation while increasing MAT1A mRNA levels in H35 cells. In conclusion, our results show that MAT2A expression not only correlates with liver cell proliferation but is required for this process.

NO sensitizes rat hepatocytes to proliferation by modifying S-adenosylmethionine levels

BACKGROUND & AIMS

Liver regeneration is a fundamental response of this organ to injury. Hepatocyte proliferation is triggered by growth factors, such as hepatocyte growth factor. However, hepatocytes need to be primed to react to mitogenic signals. It is known that nitrous oxide (NO), generated after partial hepatectomy, plays an important role in hepatocyte growth. Nevertheless, the molecular mechanisms behind this priming event are not completely known. S-adenosylmethionine (AdoMet) synthesis by methionine adenosyltransferase is the first step in methionine metabolism, and NO regulates hepatocyte S-adenosylmethionine levels through specific inhibition of this enzyme. We have studied the modulation of hepatocyte growth factor-induced proliferation by NO through the regulation of S-adenosylmethionine levels.

METHODS

Studies were conducted in cultured rat hepatocytes isolated by collagenase perfusion, which triggers NO synthesis.

RESULTS

The mitogenic response to hepatocyte growth factor was blunted when inducible NO synthase was inhibited; this process was overcome by the addition of an NO donor. This effect was dependent on methionine concentration in culture medium and intracellular S-adenosylmethionine levels. Accordingly, we found that S-adenosylmethionine inhibits hepatocyte growth factor-induced cyclin D1 and D2 expression, activator protein 1 induction, and hepatocyte proliferation.

CONCLUSIONS

Together our findings indicate that NO may switch hepatocytes into a hepatocyte growth factor-responsive state through the down-regulation of S-adenosylmethionine levels.

Equilibrium unfolding studies of the rat liver methionine adenosyltransferase III, a dimeric enzyme with intersubunit active sites

The reversible unfolding of rat liver methionine adenosyltransferase dimer by urea under equilibrium conditions has been monitored by fluorescence spectroscopy, CD, size-exclusion chromatography, analytical ultracentrifugation and enzyme activity measurements. The results obtained indicate that unfolding takes place through a three-state mechanism, involving an inactive monomeric intermediate. This intermediate has a 70% native secondary structure, binds less 8-anilinonaphthalene-1-sulphonic acid than the native dimer and has a sedimentation coefficient of 4.24+/-0.15. The variations of free energy in the absence of denaturant [DeltaG(H(2)O)] and its coefficients of urea dependence (m), calculated by the linear extrapolation model, were 36.15+/-2.3 kJ.mol(-1) and 19.87+/-0.71 kJ.mol(-1).M(-1) for the dissociation of the native dimer and 14.77+/-1.63 kJ.mol(-1) and 5.23+/-0.21 kJ.mol(-1).M(-1) for the unfolding of the monomeric intermediate respectively. Thus the global free energy change in the absence of denaturant and the m coefficient were calculated to be 65.69 kJ.mol(-1) and 30.33 kJ.mol(-1).M(-1) respectively. Analysis of the calculated thermodynamical parameters indicate the instability of the dimer in the presence of denaturant, and that the major exposure to the solvent is due to dimer dissociation. Finally, a minimum-folding mechanism for methionine adenosyltransferase III is established.

S-Adenosylmethionine: a control switch that regulates liver function

Genome sequence analysis reveals that all organisms synthesize S-adenosylmethionine (AdoMet) and that a large fraction of all genes is AdoMet-dependent methyltransferases. AdoMet-dependent methylation has been shown to be central to many biological processes. Up to 85% of all methylation reactions and as much as 48% of methionine metabolism occur in the liver, which indicates the crucial importance of this organ in the regulation of blood methionine. Of the two mammalian genes (MAT1A, MAT2A) that encode methionine adenosyltransferase (MAT, the enzyme that makes AdoMet), MAT1A is specifically expressed in adult liver. It now appears that growth factors, cytokines, and hormones regulate liver MAT mRNA levels and enzyme activity and that AdoMet should not be viewed only as an intermediate metabolite in methionine catabolism, but also as an intracellular control switch that regulates essential hepatic functions such as regeneration, differentiation, and the sensitivity of this organ to injury. The aim of this review is to integrate these recent findings linking AdoMet with liver growth, differentiation, and injury into a comprehensive model. With the availability of AdoMet as a nutritional supplement and evidence of its beneficial role in various liver diseases, this review offers insight into its mechanism of action.

S-Adenosylmethionine modulates inducible nitric oxide synthase gene expression in rat liver and isolated hepatocytes

BACKGROUND/AIMS

Hepatocellular availability of S-adenosylmethionine, the principal biological methyl donor, is compromised in situations of liver damage. S-Adenosylmethionine administration alleviates experimental liver injury and increases survival in cirrhotic patients. The mechanisms behind these beneficial effects of S-adenosylmethionine are not completely known. An inflammatory component is common to many of the pathological conditions in which S-adenosylmethionine grants protection to the liver. This notion led us to study the effect of S-adenosylmethionine administration on hepatic nitric oxide synthase-2 induction in response to bacterial lipopolysaccharide and proinflammatory cytokines.

METHODS

The effect of S-adenosylmethionine on nitric oxide synthase-2 expression was assessed in rats challenged with bacterial lipopolysaccharide and in isolated rat hepatocytes treated with proinflammatory cytokines. Interactions between S-adenosylmethionine and cytokines on nuclear factor kappa B activation and nitric oxide synthase-2 promoter transactivation were studied in isolated rat hepatocytes and HepG2 cells, respectively.

RESULTS

S-Adenosylmethionine attenuated the induction of nitric oxide synthase-2 in the liver of lipopolysaccharide-treated rats and in cytokine-treated hepatocytes. S-Adenosylmethionine accelerated the resynthesis of inhibitor kappa B alpha, blunted the activation of nuclear factor kappa B and reduced the transactivation of nitric oxide synthase-2 promoter.

CONCLUSIONS

Our findings indicate that the hepatoprotective actions of S-adenosylmethionine may be mediated in part through the modulation of nitric oxide production.