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 and liver disease

In the early 1930s, Banting and Best, the discoverers of insulin, found that choline could prevent the development of fatty liver disease (steatosis) in pancreatectomized dogs treated with insulin. Later work indicated that in rats and mice, diets deficient in labile methyl groups (choline, methionine, betaine, folate) produced fatty liver and that long-term administration of diets deficient in choline and methionine also caused hepatocellular carcinoma. These experiments not only linked steatosis and diabetes but also provided evidence, for the first time, of the importance of labile methyl group balance to maintain normal liver function. This conclusion is now amply supported by the observation of mice devoid of key enzymes of methionine and folate metabolism and in patients with severe deficiencies in these enzymes. Moreover, treatments with various methionine metabolites in experimental animal models of liver disease show hepatoprotective properties.

Changes in S-adenosylmethionine and GSH homeostasis during endotoxemia in mice

Endotoxemia participates in the pathogenesis of many liver injuries. Lipopolysaccharide (LPS) was shown to inactivate hepatic methionine adenosyltransferase (MAT), the enzyme responsible for S-adenosylmethionine (SAMe) biosynthesis. SAMe treatment was shown to prevent the LPS-induced increase in tumor necrosis factor-alpha, which may be one of its beneficial effects. SAMe is also an important precursor of glutathione (GSH) and GSH was shown to ameliorate LPS-induced hepatotoxicity. The aims of this work were to examine changes in SAMe and GSH homeostasis during endotoxemia and the effect of SAMe. Mice received SAMe or vehicle pretreatment followed by LPS and were killed up to 18 h afterward. Unexpectedly, we found hepatic SAMe level increased 67% following LPS treatment while S-adenosylhomocysteine level fell by 26%, suggesting an increase in SAMe biosynthesis and/or block in transmethylation. The mRNA and protein levels of MAT1A and MAT2A were increased following LPS. However, despite increased MAT1A expression, MAT activity remained inhibited 18 h after LPS. The major methyltransferase that catabolizes hepatic SAMe is glycine N-methyltransferase, whose expression fell by 65% following LPS. Hepatic GSH level fell more than 50% following LPS, coinciding with a comparable fall in the mRNA and protein levels of glutamate-cysteine ligase (GCL) catalytic (GCLC) and modifier subunits (GCLM). SAMe pretreatment prevented the fall in GCLC and attenuated the fall in GCLM expression and GSH level. SAMe pretreatment prevented the LPS-induced increase in plasma alanine transaminases levels but not the LPS-induced increase in hepatic mRNA levels of proinflammatory cytokines. It further enhanced LPS-induced increase in interleukin-10 mRNA level. Taken together, the hepatic response to LPS is to upregulate MAT expression and inhibit SAMe utilization. GSH is markedly depleted largely due to lower expression of GCL. Interestingly, SAMe treatment prevented the fall in GCL and helped to preserve the GSH store and prevent liver injury.

L-methionine toxicity in freshly isolated mouse hepatocytes is gender-dependent and mediated in part by transamination

L-methionine (Met) has been implicated in parenteral nutrition-associated cholestasis in infants and, at high levels, it causes liver toxicity by mechanisms that are not clear. In this study, Met toxicity was characterized in freshly isolated male and female mouse hepatocytes incubated with 5 to 30 mM Met for 0 to 5 h. In male hepatocytes, 20 mM Met was cytotoxic at 4 h as indicated by trypan blue exclusion and lactate dehydrogenase leakage assays. Cytotoxicity was preceded by reduced glutathione (GSH) depletion at 3 h without glutathione disulfide formation. Exposure to 30 mM Met resulted in increased cytotoxicity and GSH depletion. It is interesting to note that female hepatocytes were resistant to Met-induced cytotoxicity at these concentrations and showed increased cellular GSH levels compared with hepatocytes exposed to medium alone. The effects of amino-oxyacetic acid (AOAA), an inhibitor of Met transamination, and 3-deazaadenosine (3-DA), an inhibitor of the Met transmethylation pathway enzyme S-adenosylhomocysteine hydrolase, on Met toxicity in male hepatocytes were then examined. Addition of 0.2 mM AOAA partially blocked Met-induced GSH depletion and cytotoxicity, whereas 0.1 mM 3-DA potentiated Met-induced toxicity. Exposure of male hepatocytes to 0.3 mM 3-methylthiopropionic acid (3-MTP), a known Met transamination metabolite, resulted in cytotoxicity and cellular GSH depletion similar to that observed with 30 mM Met, whereas incubations with D-methionine resulted in no toxicity. Female hepatocytes were less sensitive to 3-MTP toxicity than males, which may partially explain their resistance to Met toxicity. Taken together, these results suggest that Met transamination and not transmethylation plays a major role in Met toxicity in male mouse hepatocytes.

S-adenosyl-L-methionine decreases the elevated hepatotoxicity induced by Fas agonistic antibody plus acute ethanol pretreatment in mice

The current study was designed to investigate the effect and potential mechanism of exogenous administration of S-adenosyl-l-methionine (SAM) on the enhanced hepatotoxicity induced by the Fas agonistic Jo2 antibody plus acute ethanol pretreatment in C57BL/6 mice. Acute ethanol plus Jo2 treatment produces liver toxicity under conditions in which ethanol alone or Jo2 alone do not. SAM significantly attenuated this elevated hepatotoxicity in mice as manifested by a decrease of serum aminotransferases and morphological amelioration. Levels of SAM and activity of methionine adenosyltransferase were lowered by the ethanol plus Jo2 treatment but restored by administration of SAM. The ethanol plus Jo2 treatment increased activity and content of CYP2E1, iNOS content and TNF-alpha levels; these increases were blunted by SAM. SAM also protected against the elevated oxidative and nitrosative stress found after ethanol plus Jo2, likely due to the decreases in CYP2E1, iNOS and TNF-alpha. Calcium-induced swelling of mitochondria was enhanced by the ethanol plus Jo2 treatment and this was prevented by SAM. JNK and P38 MAPK were activated by the ethanol plus Jo2 treatment; JNK activation was partially prevented by SAM. It is suggested that SAM protects against the ethanol plus Jo2 toxicity by restoring hepatic SAM levels, preventing the increase in iNOS, CYP2E1 and TNF-alpha and there by lowering the elevated oxidative/nitrosative stress and activation of the JNK signal pathway, ultimately preventing mitochondrial damage.

Metabolic regulatory properties of S-adenosylmethionine and S-adenosylhomocysteine

In mammalian liver, two intersecting pathways, remethylation and transsulfuration, compete for homocysteine that has been formed from methionine. Remethylation of homocysteine, employing either methyltetrahydrofolate or betaine as the methyl donor, forms a methionine cycle that functions to conserve methionine. In contrast, the transsulfuration sequence -- cystathionine synthase and cystathionase -- serves to irreversibly catabolize the homocysteine while synthesizing cysteine. The rate of homocysteine formation and its distribution between these two pathways are the sites for metabolic regulation and coordination. The mechanisms for regulation include both the tissue content and the kinetic properties of the component enzymes as well as the concentrations of their substrates and other metabolic effectors. Adenosylmethionine and adenosylhomocysteine are important regulatory metabolites and may use one or more mechanisms to affect the enzymes. Adenosylmethionine is a positive effector of its own synthesis, cystathionine synthase and glycine methyltransferase but impairs both homocysteine methylases. Thus, the concentration of adenosylmethionine may be self-regulatory in mammalian liver. By means of other enzymatic mechanisms, the hepatic concentration of adenosylhomocysteine, an index of homocysteine accumulation, is also self-regulated. These considerations pertain primarily to liver, which has the unique capacity to synthesize more adenosylmethionine in the presence of excess methionine. However, there are organ-specific patterns of methionine metabolism and its regulation. All tissues possess the methionine cycle with methyltetrahydrofolate as the methyl donor but only liver, kidney, pancreas, intestine and brain also contain the transsulfuration pathway. The limitation of adenosylmethionine concentrations may make adenosylhomocysteine a more significant metabolic regulator in extrahepatic tissues. However, estimates of regulatory changes based on determinations of the plasma concentrations of the two metabolites are of limited value and must be used with caution. In addition, the recent description of "cystathionine (CBS) domains" in proteins not involved with methionine metabolism raises the possibility that abnormal concentrations of the adenosyl metabolites may impact on other metabolic pathways.

S-Adenosylmethionine in cell growth, apoptosis and liver cancer

S-Adenosylmethionine (SAMe), the principal biological methyl donor, is synthesized from methionine and ATP in a reaction catalyzed by methionine adenosyltransferase (MAT). In mammals, two genes (MAT1A and MAT2A), encode for two homologous MAT catalytic subunits, while a third gene MAT2beta, encodes for the beta-subunit that regulates MAT2A-encoded isoenzyme. Normal liver expresses MAT1A, whereas extrahepatic tissues express MAT2A. MAT2A and MAT2 beta are induced in human hepatocellular carcinoma (HCC), which facilitate cancer cell growth. Patients with cirrhosis of various etiologies, including alcohol, have decreased hepatic MAT activity and SAMe biosynthesis. Consequences of hepatic SAMe deficiency as illustrated by the Mat1a knock-out mouse model include increased susceptibility to steatosis and oxidative liver injury, spontaneous development of steatohepatitis and HCC. Predisposition to HCC can be partly explained by the effect of SAMe on growth. Thus, SAMe inhibits the mitogenic effect of growth factors such as hepatocyte growth factor and, following partial hepatectomy, a fall in SAMe level is required for the liver to regenerate. During liver regeneration, the fall in hepatic SAMe is transient. If the fall were to persist, it would favor a proliferative phenotype and, ultimately, development of HCC. Not only does SAMe control liver growth, it also regulates apoptosis. Interestingly, SAMe is anti-apoptotic in normal hepatocytes but pro-apoptotic in liver cancer cells. In liver cancer cells but not in normal human hepatocytes, SAMe can selectively induce Bcl-x(S), an alternatively spliced isoform of Bcl-x(L) that promotes apoptosis. This should make SAMe an attractive agent for both chemoprevention and treatment of HCC.

Role of methionine adenosyltransferase 2A and S-adenosylmethionine in mitogen-induced growth of human colon cancer cells

BACKGROUND & AIMS

Two genes (MAT1A and MAT2A) encode for methionine adenosyltransferase, an essential enzyme responsible for S-adenosylmethionine (SAMe) biosynthesis. MAT1A is expressed in liver, whereas MAT2A is widely distributed. In liver, increased MAT2A expression is associated with growth, while SAMe inhibits MAT2A expression and growth. The role of MAT2A in colon cancer in unknown. The aims of this study were to examine whether MAT2A expression and SAMe and its metabolite methylthioadenosine (MTA) can modulate growth of colon cancer cells.

METHODS

Studies were conducted using resected colon cancer specimens, polyps from Min mice, and human colon cancer cell lines RKO and HT-29. MAT2A expression was measured by real-time polymerase chain reaction and cell growth by the 3-(4,5-dimethylthiazolyl-2-yl)-2,5-diphenyltetrazolium bromide assay.

RESULTS

In 12 of 13 patients and all 9 polyps from Min mice, the MAT2A messenger RNA levels were 200%-340% of levels in adjacent normal tissues, respectively. Epidermal growth factor, insulin-like growth factor 1, and leptin increased growth and up-regulated MAT2A expression and MAT2A promoter activity in RKO and HT-29 cells. SAMe and MTA lowered the baseline expression of MAT2A and blocked the growth factor-mediated increase in MAT2A expression and growth in colon cancer cell lines. Importantly, the mitogenic effect of these growth factors was inhibited if MAT2A induction was prevented by RNA interference. SAMe and MTA supplementation in drinking water increased intestinal SAMe levels and lowered MAT2A expression.

CONCLUSIONS

Similar to the liver, up-regulation of MAT2A also provides a growth advantage and SAMe and MTA can block mitogenic signaling in colon cancer cells.

Depletion of S-adenosyl-l-methionine with cycloleucine potentiates cytochrome P450 2E1 toxicity in primary rat hepatocytes

S-Adenosyl-l-methionine (SAM) is the principal biological methyl donor. Methionine adenosyltransferase (MAT) catalyzes the only reaction that generates SAM. Hepatocytes were treated with cycloleucine, an inhibitor of MAT, to evaluate whether hepatocytes enriched in cytochrome P450 2E1 (CYP2E1) were more sensitive to a decline in SAM. Cycloleucine decreased SAM and glutathione (GSH) levels and induced cytotoxicity in hepatocytes from pyrazole-treated rats (with an increased content of CYP2E1) to a greater extent as compared to hepatocytes from saline-treated rats. Apoptosis caused by cycloleucine in pyrazole hepatocytes appeared earlier and was more pronounced than control hepatocytes and could be prevented by incubation with SAM, glutathione reduced ethyl ester and antioxidants. The cytotoxicity was prevented by treating rats with chlormethiazole, a specific inhibitor of CYP2E1. Cycloleucine induced greater production of reactive oxygen species (ROS) in pyrazole hepatocytes than in control hepatocytes, and treatment with SAM, Trolox, and chlormethiazole lowered ROS formation. In conclusion, lowering of hepatic SAM levels produced greater toxicity and apoptosis in hepatocytes enriched in CYP2E1. This is due to elevated ROS production by CYP2E1 coupled to lower levels of hepatoprotective SAM and GSH. We speculate that such interactions e.g. induction of CYP2E1, decline in SAM and GSH may contribute to alcohol liver toxicity.

Silencing MAT2A gene by RNA interference inhibited cell growth and induced apoptosis in human hepatoma cells

AIMS

A switch in gene expression from MAT1A to MAT2A was found in liver cancer, suggesting that MAT2A plays an important role in facilitating cancer growth. MAT2A is an interesting target for antineoplastic therapy. The molecular mechanisms of silencing MAT2A by RNA interference inhibited cell growth and induced apoptosis in hepatoma cells was studied.

METHODS

We investigated the effects of MAT2A on S-adenosyl-methionine (SAM) production, cell growth and apoptotic cell death in hepatoma cell lines (Bel-7402, HepG2, and Hep3B) using an RNA interference approach.

RESULTS

The treatment of three hepatoma cell lines with small interfering RNA (siRNA) targeting to the MAT2A gene resulted in reducing the MAT II activity, facilitating SAM production, increasing SAM : SAH ratio, inhibiting cell growth and inducing cell apoptosis in hepatoma cells. In addition, silencing MAT2A gene resulted in the stimulation of MAT1A mRNA production, which was blocked by 3-deazaadenosine and l-ethionine, but not d-ethionine, suggesting that such effect was specific and mediated by upregulation of SAM level and SAM : S-adenosylethionine (SAH) ratio.

CONCLUSION

Silencing MAT2A by sequence-specific small interfering RNA caused a switch of MAT gene expression from MAT2A to MAT1A, which led the content of SAM to change to a higher steady-state level that resulted in the inhibition of cell growth and the induction of apoptotic cell death in human hepatoma cells. These results also suggested that MAT2A may hold potential as a new target for liver cancer gene therapy.

Sphingolipid signalling and liver diseases

Sphingolipids (SLs) comprise a class of lipids with important structural functions and increasing relevance in cellular signalling. In particular, ceramide has attracted considerable attention owing to its role as a second messenger modulating several cell functions such as proliferation, gene expression, differentiation, cell cycle arrest and cell death. Increasing evidence documents the role of SLs in stress and death ligand-induced hepatocellular death, which contributes to the progression of several liver diseases including steatohepatitis, ischaemia-reperfusion liver injury or hepatocarcinogenesis. Furthermore, recent data indicate that the accumulation of SLs in specific cell subcompartments, characteristic of many sphingolipidoses, contributes to the hepatic dysfunctions that accompany these inherited diseases. Hence, the regulation of the cell biology and metabolism of SLs may open up a novel therapeutic avenue in the treatment of liver diseases.

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.

Amino acids and the regulation of methyl balance in humans

PURPOSE OF REVIEW

To outline recent advances in our understanding of the metabolic basis for the maintenance of cellular S-adenosylmethionine levels and, thus, for facilitating the many crucial methylation reactions in the body. Amino acids are intimately involved in these processes.

RECENT FINDINGS

The application of stable-isotope methodology has permitted accurate estimation of the total transmethylation flux in humans. Chemical balance studies have identified the quantitatively major transmethylation reactions. New evidence points to a key role for deranged S-adenosylmethionine metabolism in the pathogenesis of liver disease. Mutations in key enzymes point to the importance of methyl metabolism in closure of the neural tube, synthesis of creatine and metabolic clearance of methionine. Dietary interventions designed to affect S-adenosylmethionine availability to pregnant mice have been shown to modulate the epigenetic DNA methylation of specific genes.

SUMMARY

These findings are of relevance to the pathogenesis of neural tube defects as well as the interaction between a genetic polymorphism and nutritional status. They also address the issue of methyl group availability and epigenetic regulation. Finally, they are also relevant to the etiology of cirrhosis and steatohepatitis.

Risk factors and mechanisms of hepatocarcinogenesis with special emphasis on alcohol and oxidative stress

Hepatocellular cancer is the fifth most frequent cancer in men and the eighth in women worldwide. Established risk factors are chronic hepatitis B and C infection, chronic heavy alcohol consumption, obesity and type 2 diabetes, tobacco use, use of oral contraceptives, and aflatoxin-contaminated food. Almost 90% of all hepatocellular carcinomas develop in cirrhotic livers. In Western countries, attributable risks are highest for cirrhosis due to chronic alcohol abuse and viral hepatitis B and C infection. Among those with alcoholic cirrhosis, the annual incidence of hepatocellular cancer is 1-2%. An important mechanism implicated in alcohol-related hepatocarcinogenesis is oxidative stress from alcohol metabolism, inflammation, and increased iron storage. Ethanol-induced cytochrome P-450 2E1 produces various reactive oxygen species, leading to the formation of lipid peroxides such as 4-hydroxy-nonenal. Furthermore, alcohol impairs the antioxidant defense system, resulting in mitochondrial damage and apoptosis. Chronic alcohol exposure elicits hepatocyte hyperregeneration due to the activation of survival factors and interference with retinoid metabolism. Direct DNA damage results from acetaldehyde, which can bind to DNA, inhibit DNA repair systems, and lead to the formation of carcinogenic exocyclic DNA etheno adducts. Finally, chronic alcohol abuse interferes with methyl group transfer and may thereby alter gene expression.

Kinetics of arsenic methylation by freshly isolated B6C3F1 mouse hepatocytes

The toxic and carcinogenic effects of arsenic may be mediated by both inorganic and methylated arsenic species. The methylation of arsenic(III) is thought to take place via sequential oxidative methylation and reduction steps to form monomethylarsenic (MMA) and dimethylarsenic (DMA) species, but recent evidence indicates that glutathione complexes of arsenic(III) can be methylated without oxidation. The kinetics of arsenic methylation were determined in freshly isolated hepatocytes from male B6C3F1 mice. Hepatocytes (>90% viability) were isolated by collagenase perfusion and suspended in Williams' Medium E with various concentrations of arsenic(III) (sodium m-arsenite). Aliquots of the lysed cell suspension were analyzed for arsenic species by hydride generation-atomic absorption spectrometry. The formation of MMA(III) from sodium arsenite (1 microM) was linear with respect to time for >90 min. DMA(III) formation did not become significant until 60 min. MMA(V) and DMA(V) were not consistently observed in the incubations. These results suggest that the glutathione complex mechanism of methylation plays an important role in arsenic biotransformation in mouse hepatocytes. Metabolism of arsenic(V) was not observed in mouse hepatocytes, consistent with inhibition of arsenic(V) active cellular uptake by phosphate in the medium. The formation of MMA(III) increased with increasing arsenic(III) concentrations up to approximately 2 microM and declined thereafter. The concentration dependence is consistent with a saturable methylation reaction accompanied by uncompetitive substrate inhibition of the reaction by arsenic(III). Kinetic analysis of the data suggested an apparent K(M) of approximately 3.6 microM arsenic(III), an apparent V(max) of approximately 38.9 microg MMA(III) formed/L/h/million cells, and an apparent K(I) of approximately 1.3 microM arsenic(III). The results of this study can be used in the physiologically based pharmacokinetic model for arsenic disposition in mice to predict the concentration of MMA(III) in liver and other tissues.

Molecular Profiling of Hepatocellular Carcinoma in Mice with a Chronic Deficiency of HepaticS-Adenosylmethionine: Relevance in Human Liver Diseases

S-adenosylmethionine arises as a central molecule in the preservation of liver homeostasis as a chronic hepatic deficiency results in spontaneous development of steatohepatitis and hepatocellular carcinoma. In the present work, we have attempted a comprehensive analysis of proteins associated with hepatocarcinogenesis in MAT1A knock out mice using a combination of two-dimensional electrophoresis and mass spectrometry, to then apply the resulting information to identify hallmarks of human HCC. Our results suggest the existence of individual-specific factors that might condition the development of preneoplastic lesions. Proteomic analysis allowed the identification of 151 differential proteins in MAT1A-/- mice tumors. Among all differential proteins, 27 changed in at least 50% of the analyzed tumors, and some of these alterations were already detected months before the development of HCC in the KO liver. The expression level of genes coding for 13 of these proteins was markedly decreased in human HCC. Interestingly, seven of these genes were also found to be down-regulated in a pretumoral condition such as cirrhosis, while depletion of only one marker was assessed in less severe liver disorders.

Opposite action of S-adenosyl methionine and its metabolites on CYP2E1-mediated toxicity in pyrazole-induced rat hepatocytes and HepG2 E47 cells

S-adenosyl-L-methionine (SAMe) is protective against a variety of hepatotoxins, including ethanol. The ability of SAMe to protect against cytochrome P-450 2E1 (CYP2E1)-dependent toxicity was studied in hepatocytes from pyrazole-treated rats and HepG2 E47 cells, both of which actively express CYP2E1. Toxicity was initiated by the addition of arachidonic acid (AA) or by depletion of glutathione after treatment with L-buthionine sulfoximine (BSO). In pyrazole hepatocytes, SAMe (0.25-1 mM) protected against AA but not BSO toxicity. SAMe elevated GSH levels, thus preventing the decline in GSH caused by AA, and SAMe prevented AA-induced lipid peroxidation. SAMe analogs such as methionine or S-adenosyl homocysteine, which elevate GSH, also protected against AA toxicity. 5'-Methylthioadenosine (MTA), which cannot produce GSH, did not protect. The toxicity of BSO was not prevented by SAMe and the analogs because GSH cannot be synthesized. In contrast, in E47 cells, SAMe and MTA but not methionine or S-adenosyl homocysteine potentiated AA and BSO toxicity. Antioxidants such as trolox or N-acetyl cysteine prevented this synergistic toxicity of SAMe plus AA or SAMe plus BSO, respectively. In pyrazole hepatocytes, SAMe prevented the decline in mitochondrial membrane potential produced by AA, whereas in E47 cells, SAMe potentiated the decline in mitochondrial membrane potential. In E47 cells, but not pyrazole hepatocytes, the combination of SAMe plus BSO lowered levels of the antioxidant transcription factor Nrf2. Because SAMe can be metabolized enzymatically or spontaneously to MTA, MTA may play a role in the potentiation of AA and BSO toxicity by SAMe, but the exact mechanisms require further investigation. In conclusion, contrasting effects of SAMe on CYP2E1 toxicity were observed in pyrazole hepatocytes and E47 cells. In hepatocytes, SAMe protects against CYP2E1 toxicity by a mechanism involving maintaining or elevating GSH levels.

New insights into structure and function of mitochondria and their role in aging and disease

This review covers some novel findings on mitochondrial biochemistry and discusses diseases due to mitochondrial DNA mutations as a model of the changes occurring during physiological aging. The random collision model of organization of the mitochondrial respiratory chain has been recently challenged on the basis of findings of supramolecular organization of respiratory chain complexes. The source of superoxide in Complex I is discussed on the basis of laboratory experiments using a series of specific inhibitors and is presumably iron sulfur center N2. Maternally inherited diseases due to mutations of structural genes in mitochondrial DNA are surveyed as a model of alterations mimicking those occurring during normal aging. The molecular defects in senescence are surveyed on the basis of the "Mitochondrial Theory of Aging", establishing mitochondrial DNA somatic mutations, caused by accumulation of oxygen radical damage, to be at the basis of cellular senescence. Mitochondrial production of reactive oxygen species increases with aging and mitochondrial DNA mutations and deletions accumulate and may be responsible for oxidative phosphorylation defects. Evidence is presented favoring the mitochondrial theory, with primary mitochondrial alterations, although the problem is made more complex by changes in the cross-talk between nuclear and mitochondrial DNA.

Dysregulated cytokine metabolism, altered hepatic methionine metabolism and proteasome dysfunction in alcoholic liver disease

Alcoholic liver disease (ALD) remains an important complication and cause of morbidity and mortality from alcohol abuse. Major developments in our understanding of the mechanisms of ALD over the past decade are now being translated into new forms of therapy for this disease process which currently has no FDA approved treatment. Cytokines are low molecular weight mediators of cellular communication, and the pro-inflammatory cytokine tumor necrosis factor (TNF) has been shown to play a pivotal role in the development of experimental ALD. Similarly, TNF levels are elevated in the serum of alcoholic hepatitis patients. Abnormal methionine metabolism is well documented in patients with ALD, with patients having elevated serum methionine levels, but low S-adenosylmethionine levels in the liver. On the other hand, S-adenosylhomocysteine and homocysteine levels are elevated in ALD. Recent studies have documented potential interactions between homocysteine and S-adenosylhomocysteine with TNF in the development of ALD. Altered proteasome function also is now well documented in ALD, and decreased proteasome function can cause hepatocyte apoptosis. Recently it has been shown that decreased proteasome function can also act synergistically to enhance TNF hepatotoxicity. Hepatocytes dying of proteasome dysfunction release pro-inflammatory cytokines such as Interleukin-8 to cause sustained inflammation. This article reviews the interactions of cytokines, altered methionine metabolism, and proteasome dysfunction in the development of ALD.

Effects of betaine supplementation on hepatic metabolism of sulfur-containing amino acids in mice

BACKGROUND/AIMS

We previously reported that acute betaine treatment induced significant changes in the hepatic glutathione and cysteine levels in mice and rats. The present study was aimed to determine the effects of dietary betaine on the metabolism of sulfur-containing amino acids.

METHODS/RESULTS

Male mice were supplemented with betaine (1%) in drinking water for up to 3 weeks. Changes in hepatic levels of major sulfur amino acid metabolites and products were stabilized after 2 weeks of betaine supplementation. Betaine intake increased methionine, S-adenosylmethionine, and S-adenosylhomocysteine levels significantly, but homocysteine and cystathionine were reduced. Methionine adenosyltransferase activity was elevated to three-fold of control. Cysteine catabolism to taurine was inhibited as evidenced by a decrease in cysteine dioxygenase activity and taurine levels in liver and plasma. Despite the significant changes in the transsulfuration reactions, neither hepatic cysteine nor glutathione was altered. Betaine supplementation decreased the hepatotoxicity induced by chloroform (0.5 ml/kg, ip) significantly.

CONCLUSIONS

Betaine supplementation enhances recycling of homocysteine for the generation of methionine and S-adenosylmethionine while reducing its utilization for the synthesis of cystathionine and cysteine. However, the hepatic levels of cysteine or glutathione are not affected, most probably due to the depression of taurine generation from cysteine.

Role of methionine adenosyltransferase and S-adenosylmethionine in alcohol-associated liver cancer

Two genes (MAT1A and MAT2A) encode for the essential enzyme methionine adenosyltransferase (MAT), which catalyzes the biosynthesis of S-adenosylmethionine (SAMe), the principal methyl donor and, in the liver, a precursor of glutathione. MAT1A is expressed mostly in the liver, whereas MAT2A is widely distributed. MAT2A is induced in the liver during periods of rapid growth and dedifferentiation. In human hepatocellular carcinoma (HCC) MAT1A is replaced by MAT2A. This is important pathogenetically because MAT2A expression is associated with lower SAMe levels and faster growth, whereas exogenous SAMe treatment inhibits growth. Rats fed ethanol intragastrically for 9 weeks also exhibit a relative switch in hepatic MAT expression, decreased SAMe levels, hypomethylation of c-myc, increased c-myc expression, and increased DNA strand break accumulation. Patients with alcoholic liver disease have decreased hepatic MAT activity owing to both decreased MAT1A expression and inactivation of the MAT1A-encoded isoenzymes, culminating in decreased SAMe biosynthesis. Consequences of chronic hepatic SAMe depletion have been examined in the MAT1A knockout mouse model. In this model, the liver is more susceptible to injury. In addition, spontaneous steatohepatitis develops by 8 months, and HCC develops by 18 months. Accumulating evidence shows that, in addition to being a methyl donor, SAMe controls hepatocyte growth response and death response. Whereas transient SAMe depletion is necessary for the liver to regenerate, chronic hepatic SAMe depletion may lead to malignant transformation. It is interesting that SAMe is antiapoptotic in normal hepatocytes, but proapoptotic in liver cancer cells. This should make SAMe an attractive agent for both chemoprevention and treatment of HCC.

Methylthioadenosine phosphorylase gene expression is impaired in human liver cirrhosis and hepatocarcinoma

Methylthioadenosine phosphorylase (MTAP) is a key enzyme in the methionine and adenine salvage pathways. In mammals, the liver plays a central role in methionine metabolism, and this essential function is lost in the progression from liver cirrhosis to hepatocarcinoma. Deficient MTAP gene expression has been recognized in many transformed cell lines and tissues. In the present work, we have studied the expression of MTAP in human and experimental liver cirrhosis and hepatocarcinoma. We observe that MTAP gene expression is significantly reduced in human hepatocarcinoma tissues and cell lines. Interestingly, MTAP gene expression was also impaired in the liver of CCl4-cirrhotic rats and cirrhotic patients. We provide evidence indicating that epigenetic mechanisms, involving DNA methylation and histone deacetylation, may play a role in the silencing of MTAP gene expression in hepatocarcinoma. Given the recently proposed tumor suppressor activity of MTAP, our observations can be relevant to the elucidation of the molecular mechanisms of multistep hepatocarcinogenesis.

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.

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.

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.

5-Aza-2′-deoxycytidine and paclitaxel inhibit inducible nitric oxide synthase activation in fibrosarcoma cells

Given the important role of gaseous free radical nitric oxide (NO) in tumor cell biology, we investigated the ability of the anti-cancer drugs 5-Aza-2'-deoxycytidine (ADC) and paclitaxel to modulate NO production in mouse L929 fibrosarcoma cells. Both drugs reduced IFN-gamma-stimulated NO release in cultures of L929 and primary fibroblasts, but not in mouse peritoneal macrophages. The inhibitory effect was due to the reduced expression of inducible NO synthase (iNOS), the enzyme responsible for cytokine-induced intracellular NO synthesis, as both agents markedly suppressed the interferon-gamma (IFN-gamma)-triggered increase in iNOS concentration in L929 cells. In addition, ADC and paclitaxel prevented the IFN-gamma-triggered activation of p44/p42 mitogen-activated protein (MAP) kinase in L929 fibroblasts, suggesting a possible mechanism for the observed inhibition of iNOS expression. These results might have important implications for the therapeutic effect of ADC and paclitaxel, since their inhibitory action on NO release partly neutralized the NO-dependent toxicity of IFN-gamma on L929 fibrosarcoma cells.

Expression of Wilms' tumor suppressor in the liver with cirrhosis: relation to hepatocyte nuclear factor 4 and hepatocellular function

The Wilms' tumor suppressor WT1 is a transcriptional regulator present in the fetal but not in the mature liver. Its expression and functional role in liver diseases remains unexplored. In this study, we analyzed WT1 expression by reverse-transcription polymerase chain reaction (RT-PCR) and by immunohistochemistry in normal and diseased livers. In addition, we performed in vitro studies in isolated rat hepatocytes to investigate WT1 regulation and function. We detected WT1 messenger RNA (mRNA) in 18% of normal livers, 17% of chronic hepatitis with minimal fibrosis, 49% of chronic hepatitis with bridging fibrosis, and 71% of cirrhotic livers. In cirrhosis, WT1 immunoreactivity was localized to the nucleus of hepatocytes. WT1 mRNA abundance correlated inversely with prothrombin time (P =.04) and directly with serum bilirubin (P =.002) and with the MELD score (P =.001) of disease severity. In rats, WT1 expression was present in fetal hepatocytes and in the cirrhotic liver but not in normal hepatic tissue. In vitro studies showed that isolated primary hepatocytes express WT1 when stimulated with transforming growth factor beta (TGF-beta) or when the cells undergo dedifferentiation in culture. Moreover, we found that WT1 down-regulates hepatocyte nuclear factor 4 (HNF-4), a factor that is essential to maintain liver function and metabolic regulation in the mature organ. Hepatic expression of HNF-4 was impaired in advanced human cirrhosis and negatively correlated with WT1 mRNA levels (P =.001). In conclusion, we show that WT1 is induced by TGF-beta and down-regulates HNF-4 in liver cells. WT1 is reexpressed in the cirrhotic liver in relation to disease progression and may play a role in the development of hepatic insufficiency in cirrhosis.

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.

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.

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.

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.

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.

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.

S-adenosylmethionine and methylthioadenosine are antiapoptotic in cultured rat hepatocytes but proapoptotic in human hepatoma cells

S-adenosylmethionine (AdoMet) is an essential compound in cellular transmethylation reactions and a precursor of polyamine and glutathione synthesis in the liver. In liver injury, the synthesis of AdoMet is impaired and its availability limited. AdoMet administration attenuates experimental liver damage, improves survival of alcoholic patients with cirrhosis, and prevents experimental hepatocarcinogenesis. Apoptosis contributes to different liver injuries, many of which are protected by AdoMet. The mechanism of AdoMet's hepatoprotective and chemopreventive effects are largely unknown. The effect of AdoMet on okadaic acid (OA)-induced apoptosis was evaluated using primary cultures of rat hepatocytes and human hepatoma cell lines. AdoMet protected rat hepatocytes from OA-induced apoptosis dose dependently. It attenuated mitochondrial cytochrome c release, caspase 3 activation, and poly(ADP-ribose) polymerase cleavage. These effects were independent from AdoMet-dependent glutathione synthesis, and mimicked by 5'-methylthioadenosine (MTA), which is derived from AdoMet. Interestingly, AdoMet and MTA did not protect HuH7 cells from OA-induced apoptosis; conversely both compounds behaved as proapoptotic agents. AdoMet's proapoptotic effect was dose dependent and observed also in HepG2 cells. In conclusion, AdoMet exerts opposing effects on apoptosis in normal versus transformed hepatocytes that could be mediated through its conversion to MTA. These effects may participate in the hepatoprotective and chemopreventive properties of this safe and well-tolerated drug.

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.

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

Liver-specific and nonliver-specific methionine adenosyltransferases (MATs) are products of two genes, MAT1A and MAT2A, respectively, that catalyze the formation of S-adenosylmethionine (AdoMet), the principal biological methyl donor. Mature liver expresses MAT1A, whereas MAT2A is expressed in extrahepatic tissues and is induced during liver growth and dedifferentiation. To examine the influence of MAT1A on hepatic growth, we studied the effects of a targeted disruption of the murine MAT1A gene. MAT1A mRNA and protein levels were absent in homozygous knockout mice. At 3 months, plasma methionine level increased 776% in knockouts. Hepatic AdoMet and glutathione levels were reduced by 74 and 40%, respectively, whereas S-adenosylhomocysteine, methylthioadenosine, and global DNA methylation were unchanged. The body weight of 3-month-old knockout mice was unchanged from wild-type littermates, but the liver weight was increased 40%. The Affymetrix genechip system and Northern and Western blot analyses were used to analyze differential expression of genes. The expression of many acute phase-response and inflammatory markers, including orosomucoid, amyloid, metallothionein, Fas antigen, and growth-related genes, including early growth response 1 and proliferating cell nuclear antigen, is increased in the knockout animal. At 3 months, knockout mice are more susceptible to choline-deficient diet-induced fatty liver. At 8 months, knockout mice developed spontaneous macrovesicular steatosis and predominantly periportal mononuclear cell infiltration. Thus, absence of MAT1A resulted in a liver that is more susceptible to injury, expresses markers of an acute phase response, and displays increased proliferation.

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.