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S-adenosylmethionine (SAMe) therapy in liver disease: A review of current evidence and clinical utility

  • Quentin M. Anstee
    Correspondence
    Corresponding author. Address: Institute of Cellular Medicine, The Medical School, Newcastle University, 3rd Floor, William Leech Building, Framlington Place, Newcastle-upon-Tyne NE2 4HH, UK. Tel.: +44 (0) 191 222 7012; fax: +44 (0) 191 222 0723.
    Affiliations
    Liver Research Group, Institute of Cellular Medicine, The Medical School, Newcastle University, Framlington Place, Newcastle-Upon-Tyne NE2 4HH, UK
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  • Christopher P. Day
    Affiliations
    Liver Research Group, Institute of Cellular Medicine, The Medical School, Newcastle University, Framlington Place, Newcastle-Upon-Tyne NE2 4HH, UK
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Open AccessPublished:June 01, 2012DOI:https://doi.org/10.1016/j.jhep.2012.04.041

      Summary

      S-adenosyl-l-methionine (SAMe; AdoMet) is an important, metabolically pleiotropic molecule that participates in multiple cellular reactions as the precursor for the synthesis of glutathione and principle methyl donor required for methylation of nucleic acids, phospholipids, histones, biogenic amines, and proteins. SAMe synthesis is depressed in chronic liver disease and so there has been considerable interest in the utility of SAMe to ameliorate disease severity. Despite encouraging pre-clinical data confirming that SAMe depletion can exacerbate liver injury and supporting a hepatoprotective role for SAMe therapy, to date no large, high-quality randomised clinical trials have been performed that establish clinical utility in specific disease states. Here, we offer an in-depth review of the published scientific literature relating to the physiological and pathophysiological roles of SAMe and its therapeutic use in liver disease, critically assessing implications for clinical practice and offering recommendations for further research.

      Keywords

      Introduction

      The need for therapies to ameliorate liver injury

      Liver disease is considered acute or chronic according to the duration of the injurious process. The process of hepatic fibrogenesis is triggered by tissue damage and continues until the lesion is healed. If the damage persists or is recurrent, the repair process will persist and fibrogenesis will progress towards cirrhosis, liver failure or hepatocellular carcinoma [
      • Malhi H.
      • Gores G.J.
      Cellular and molecular mechanisms of liver injury.
      ,
      • Anstee Q.M.
      • Wright M.
      • Goldin R.
      • Thursz M.R.
      Parenchymal extinction: coagulation and hepatic fibrogenesis.
      ]. Hepatocyte death, through a varying combination of oncotic necrosis and apoptosis, is a characteristic feature of most liver diseases including alcoholic liver disease (ALD), non-alcoholic fatty liver disease (NAFLD), cholestasis, viral hepatitis, ischemia/reperfusion, liver preservation at transplantation and drug/toxin-induced injury [
      • Malhi H.
      • Gores G.J.
      • Lemasters J.J.
      Apoptosis and necrosis in the liver: a tale of two deaths?.
      ]. Correction of the underlying aetiology before the development of cirrhosis and liver failure is the primary goal in managing liver disease. However, where this is not possible, treatment to ameliorate hepatocellular injury or control fibrogenesis offers an attractive therapeutic strategy that may prevent disease progression and, given that fibrosis has a reversible component, allow regression [
      • Benyon R.C.
      • Iredale J.P.
      Is liver fibrosis reversible?.
      ,
      • Iredale J.P.
      • Benyon R.C.
      • Pickering J.
      • McCullen M.
      • Northrop M.
      • Pawley S.
      • et al.
      Mechanisms of spontaneous resolution of rat liver fibrosis. Hepatic stellate cell apoptosis and reduced hepatic expression of metalloproteinase inhibitors.
      ]. At present, there are no accepted anti-fibrotic agents available outside clinical trials and, beyond the use of N-acetylcysteine (NAc) in the treatment of acute acetaminophen (paracetamol) toxicity, there are no widely adopted agents that limit hepatocellular injury in routine clinical use.
      Irrespective of aetiology, progression of liver disease is influenced by the interaction of host genetic factors, the pathogen, and other coincidental environmental influences [
      • Anstee Q.M.
      • Daly A.K.
      • Day C.P.
      Genetics of alcoholic and nonalcoholic fatty liver disease.
      ]. Nutritional status is one such factor [
      • Plauth M.
      • Cabre E.
      • Riggio O.
      • Assis-Camilo M.
      • Pirlich M.
      • Kondrup J.
      • et al.
      ESPEN guidelines on enteral nutrition: liver disease.
      ]. However, it has also become apparent that beyond dietary availability of specific nutrients and essential amino acids, an individual’s metabolic capacity for processing them into active metabolites and the factors that influence this can profoundly affect physiology in health and disease [
      • Lieber C.S.
      S-adenosyl-l-methionine: its role in the treatment of liver disorders.
      ]. The essential amino acid methionine and its biologically active metabolite S-adenosyl-l-methionine (SAMe; AdoMet) are a case in point: there is evidence that SAMe depletion occurs during chronic liver disease [
      • Martinez-Chantar M.L.
      • Garcia-Trevijano E.R.
      • Latasa M.U.
      • Perez-Mato I.
      • Sanchez del Pino M.M.
      • Corrales F.J.
      • et al.
      Importance of a deficiency in S-adenosyl-l-methionine synthesis in the pathogenesis of liver injury.
      ,
      • Mato J.M.
      • Lu S.C.
      Role of S-adenosyl-l-methionine in liver health and injury.
      ] and SAMe has been proposed as treatment for certain disease states [
      • Lieber C.S.
      S-adenosyl-l-methionine: its role in the treatment of liver disorders.
      ,
      • Purohit V.
      • Abdelmalek M.F.
      • Barve S.
      • Benevenga N.J.
      • Halsted C.H.
      • Kaplowitz N.
      • et al.
      Role of S-adenosylmethionine, folate, and betaine in the treatment of alcoholic liver disease: summary of a symposium.
      ]. Due to encouraging data from early studies and the lack of other effective agents, SAMe has been widely adopted in Eastern Europe, Russia, China, Southern Asia, and South America as a therapy for chronic liver disease and intra-hepatic cholestasis. It is therefore timely to discuss the role of SAMe in the pathogenesis of liver disease and critically review the current evidence of clinical utility for SAMe supplementation.

      S-adenosyl-l-methionine (SAMe)

      Hepatic SAMe metabolism

      SAMe is synthesised from dietary l-methionine and ATP by the enzyme methionine adenosyltransferase (MAT; EC 2.5.1.6) in a complex two-step reaction [
      • Mato J.M.
      • Corrales F.J.
      • Lu S.C.
      • Avila M.A.
      S-adenosylmethionine: a control switch that regulates liver function.
      ,
      • Catoni G.L.
      S-adenosylmethionine; a new intermediate formed enzymatically from l-methionine and adenosinetriphosphate.
      ]. The complete tripolyphosphate (PPPi) moiety is cleaved from ATP at the C-5′ atom and the adenosyl moiety is transferred to methionine to form SAMe; PPPi is then hydrolysed to orthophosphate and pyrophosphate (PPi + Pi) at a distinct sub-site within the MAT catalytic domain; and finally SAMe, orthophosphate and pyrophosphate are released (Fig. 1) [
      • Martinez-Chantar M.L.
      • Garcia-Trevijano E.R.
      • Latasa M.U.
      • Perez-Mato I.
      • Sanchez del Pino M.M.
      • Corrales F.J.
      • et al.
      Importance of a deficiency in S-adenosyl-l-methionine synthesis in the pathogenesis of liver injury.
      ]. In mammals, there are three separate forms of the MAT enzyme [
      • Mudd S.H.
      Activation of methionine for transmethylation. V. The mechanism of action of the methionine-activating enzyme.
      ]. The gene MAT1A is predominantly expressed in the adult liver and encodes a 395 amino acid α1 catalytic subunit that is combined into either a homotetramer (MATI) or a homodimer (MATIII) [
      • Kotb M.
      • Mudd S.H.
      • Mato J.M.
      • Geller A.M.
      • Kredich N.M.
      • Chou J.Y.
      • et al.
      Consensus nomenclature for the mammalian methionine adenosyltransferase genes and gene products.
      ]. In contrast, MAT2A is ubiquitously expressed in all mammalian tissues studied including foetal liver (and to a lesser extent in adult liver), erythrocytes, lymphocytes, brain and kidney [
      • Kotb M.
      • Mudd S.H.
      • Mato J.M.
      • Geller A.M.
      • Kredich N.M.
      • Chou J.Y.
      • et al.
      Consensus nomenclature for the mammalian methionine adenosyltransferase genes and gene products.
      ,
      • Kotb M.
      • Geller A.M.
      Methionine adenosyltransferase: structure and function.
      ,
      • Gil B.
      • Casado M.
      • Pajares M.A.
      • Bosca L.
      • Mato J.M.
      • Martin-Sanz P.
      • et al.
      Differential expression pattern of S-adenosylmethionine synthetase isoenzymes during rat liver development.
      ]. It encodes a 396 amino acid α2 catalytic subunit that combines with a non-catalytic 334 amino acid regulatory β subunit encoded by MAT2B to form the MATII isoform of the enzyme [
      • Mato J.M.
      • Corrales F.J.
      • Lu S.C.
      • Avila M.A.
      S-adenosylmethionine: a control switch that regulates liver function.
      ,
      • Kotb M.
      • Mudd S.H.
      • Mato J.M.
      • Geller A.M.
      • Kredich N.M.
      • Chou J.Y.
      • et al.
      Consensus nomenclature for the mammalian methionine adenosyltransferase genes and gene products.
      ,
      • Kotb M.
      • Geller A.M.
      Methionine adenosyltransferase: structure and function.
      ]. MAT is a highly conserved enzyme throughout evolution with a 59% sequence homology between Escherichia coli and humans [
      • Martinez-Chantar M.L.
      • Garcia-Trevijano E.R.
      • Latasa M.U.
      • Perez-Mato I.
      • Sanchez del Pino M.M.
      • Corrales F.J.
      • et al.
      Importance of a deficiency in S-adenosyl-l-methionine synthesis in the pathogenesis of liver injury.
      ]. Both the α1 and α2 subunits share approximately 84% amino acid sequence homology [
      • Kotb M.
      • Mudd S.H.
      • Mato J.M.
      • Geller A.M.
      • Kredich N.M.
      • Chou J.Y.
      • et al.
      Consensus nomenclature for the mammalian methionine adenosyltransferase genes and gene products.
      ] however, the MATII α2/β dimer has lower substrate affinity (km) than MATI/III and its activity is negatively regulated by SAMe as intracellular concentration increases whilst MATI/III is not. These differences in regulatory and kinetic properties limit MATII activity, which is thought to contribute little to hepatic methionine metabolism in healthy adults under normal physiological conditions whilst the MAT1A coded isoforms (MATI/III) maintain high levels of SAMe synthesis (approximately 6–8 g/day) [
      • Mato J.M.
      • Corrales F.J.
      • Lu S.C.
      • Avila M.A.
      S-adenosylmethionine: a control switch that regulates liver function.
      ].
      Figure thumbnail gr1
      Fig. 1Metabolic pathways in methionine/SAMe metabolism. SAMe is synthesised from dietary l-methionine and ATP by the enzyme methionine adenosyltransferase (MAT; •1). In standard conditions, the majority of SAMe generated is used in transmethylation reactions. Glycine-N-methyl transferase (GNMT; •2) is the most abundant methyltransferase in the liver. Irrespective of the specific enzyme mediating the reaction, a common product is S-adenosylhomocysteine (SAH). SAH is cleared by conversion into homocysteine and adenosine in a reversible reaction catalysed by SAH hydrolase (•3). Homocysteine is in turn metabolised through either the remethylation pathways or the transsulfuration pathways. In the former, homocysteine is remethylated by methionine synthase in a process coupled to the folate cycle (MS; •4) or betaine methyltransferase (BHMT; •5) to re-form methionine. Alternatively, the conversion of homocysteine to crystathionine by crystathionine β-synthase (CBS; •6) begins the transsulfuration pathway leading to cysteine and ultimately glutathione (GSH) (•7, •8, •9). Folic acid and the co-factors vitamin B6 and B12 are required for functioning of MS (•4), CBS (•6) and BHMT (•5), respectively. Modified from [
      • Mato J.M.
      • Lu S.C.
      Role of S-adenosyl-l-methionine in liver health and injury.
      ,
      • Purohit V.
      • Abdelmalek M.F.
      • Barve S.
      • Benevenga N.J.
      • Halsted C.H.
      • Kaplowitz N.
      • et al.
      Role of S-adenosylmethionine, folate, and betaine in the treatment of alcoholic liver disease: summary of a symposium.
      ].

      Regulation of MAT expression and SAMe synthesis

      Synthesis of SAMe needs significant metabolic energy given the requirement to hydrolyse the three high-energy phosphodiester bonds of ATP for each SAMe molecule synthesised. To control these demands, MATI/III activity is regulated both at transcription level and through post-translational regulation. Sequencing the MAT1A promoter region has identified consensus binding sites for several transcription factors including C/EBP (CAAT enhancer binding protein), NF-1 (nuclear factor 1), and HNF (hepatocyte-enriched nuclear factor) [
      • Mato J.M.
      • Corrales F.J.
      • Lu S.C.
      • Avila M.A.
      S-adenosylmethionine: a control switch that regulates liver function.
      ]. However, studies have demonstrated that, although MAT1A expression is limited to the liver, promoter activity is present in cells from other tissues and so this is unlikely to be the mechanism through which tissue specific expression is mediated. Beyond promoter activity, gene transcription may be controlled by DNA methylation that can directly impede binding of transcription proteins and can additionally influence histone deacetylation and therefore chromatin structure. Hypermethylation of two MAT1A promoter CpG sites has been demonstrated in foetal liver, hepatocellular carcinoma (HCC) and extra-hepatic tissues, whilst these sites were found to be demethylated in normal adult liver where the gene is expressed [
      • Torres L.
      • Avila M.A.
      • Carretero M.V.
      • Latasa M.U.
      • Caballeria J.
      • Lopez-Rodas G.
      • et al.
      Liver-specific methionine adenosyltransferase MAT1A gene expression is associated with a specific pattern of promoter methylation and histone acetylation: implications for MAT1A silencing during transformation.
      ]. Further, the degree of histone acetylation, necessary to maintain chromatin in a decondensed and active state for transcription, is modified so that the degree of H4 histone acetylation associated with the MAT1A promoter is approximately 15-fold greater in the liver than in tissues where MAT1A is not expressed such as the kidney [
      • Torres L.
      • Avila M.A.
      • Carretero M.V.
      • Latasa M.U.
      • Caballeria J.
      • Lopez-Rodas G.
      • et al.
      Liver-specific methionine adenosyltransferase MAT1A gene expression is associated with a specific pattern of promoter methylation and histone acetylation: implications for MAT1A silencing during transformation.
      ]. In human hepatoma cell lines (e.g. HepG2), MAT1A is hypermethylated and so not expressed, while exposure of these cells to the demethylating agent 5′-aza-2-deoxycytidine or to the histone deacetylase inhibitor tricostatin A promotes gene expression [
      • Martinez-Chantar M.L.
      • Garcia-Trevijano E.R.
      • Latasa M.U.
      • Perez-Mato I.
      • Sanchez del Pino M.M.
      • Corrales F.J.
      • et al.
      Importance of a deficiency in S-adenosyl-l-methionine synthesis in the pathogenesis of liver injury.
      ,
      • Torres L.
      • Avila M.A.
      • Carretero M.V.
      • Latasa M.U.
      • Caballeria J.
      • Lopez-Rodas G.
      • et al.
      Liver-specific methionine adenosyltransferase MAT1A gene expression is associated with a specific pattern of promoter methylation and histone acetylation: implications for MAT1A silencing during transformation.
      ]. Together, these findings indicate that tissue-specific MATI/III expression is primarily regulated through DNA methylation and histone deacetylation and that this may be affected by disease states (Fig. 2).
      Figure thumbnail gr2
      Fig. 2Regulation of MAT1A expression and MATI/MATIII activity in health and disease. Tissue-specific MATI/III expression is primarily regulated through DNA methylation and histone deacetylation and this may be affected by disease states. Greater CpG methylation is observed in extra-hepatic tissues and in cirrhosis and HCC where MAT1A expression is suppressed and less MATI/III activity observed. Modified from
      [
      • Martinez-Chantar M.L.
      • Garcia-Trevijano E.R.
      • Latasa M.U.
      • Perez-Mato I.
      • Sanchez del Pino M.M.
      • Corrales F.J.
      • et al.
      Importance of a deficiency in S-adenosyl-l-methionine synthesis in the pathogenesis of liver injury.
      ]
      .
      Beyond limiting MATI/III expression to the adult liver, where there is a high capacity to generate ATP to fuel SAMe synthesis, there is an additional need to rapidly ‘fine tune’ SAMe synthesis according to variations in metabolic demand to prevent hepatocytes becoming ATP depleted at times of metabolic stress [
      • Mato J.M.
      • Corrales F.J.
      • Lu S.C.
      • Avila M.A.
      S-adenosylmethionine: a control switch that regulates liver function.
      ]. Reactive oxygen species (ROS) and nitric oxide (NO) can inactivate MATI/III by oxidation or S-nitrosylation of the cysteine residue at position 121 in the α1 subunit, respectively, inducing a conformational change in the protein that blocks the catalytic site. This change can be equally rapidly reversed by exposure to physiological concentrations of the anti-oxidant glutathione, reactivating MATI/III [
      • Avila M.A.
      • Mingorance J.
      • Martinez-Chantar M.L.
      • Casado M.
      • Martin-Sanz P.
      • Bosca L.
      • et al.
      Regulation of rat liver S-adenosylmethionine synthetase during septic shock: role of nitric oxide.
      ,
      • Sanchez-Gongora E.
      • Ruiz F.
      • Mingorance J.
      • An W.
      • Corrales F.J.
      • Mato J.M.
      Interaction of liver methionine adenosyltransferase with hydroxyl radical.
      ,
      • Ruiz F.
      • Corrales F.J.
      • Miqueo C.
      • Mato J.M.
      Nitric oxide inactivates rat hepatic methionine adenosyltransferase in vivo by S-nitrosylation.
      ]. Importantly, MATII does not possess a cysteine residue at the equivalent position and so its activity (which is regulated by SAMe concentration) is not influenced by oxidative stress. Indeed, inhibition of MATI/III under conditions of oxidative stress causes SAMe levels to fall, disinhibiting MATII.

      Physiological role of SAMe

      SAMe is an important, metabolically pleiotropic molecule that participates in multiple cellular reactions and influences numerous cellular functions. Biochemically, it participates in three types of reaction: transmethylation, transsulfuration and aminopropylation [
      • Lu S.C.
      S-adenosylmethionine.
      ,
      • Lieber C.S.
      • Packer L.
      S-adenosylmethionine: molecular, biological, and clinical aspects – an introduction.
      ]. A comprehensive discussion of these metabolic pathways falls outside the scope of this review however, the key reactions will be briefly summarised.
      SAMe is the principle methyl donor required for methylation of nucleic acids, phospholipids, histones, biogenic amines, and proteins [
      • Lieber C.S.
      • Packer L.
      S-adenosylmethionine: molecular, biological, and clinical aspects – an introduction.
      ]. In standard conditions, the majority of SAMe generated is used in transmethylation reactions (Fig. 1). Glycine-N-methyl transferase (GNMT; EC 2.1.1.20) is the most abundant methyltransferase in the liver and is also present in the exocrine pancreas and prostate. Irrespective of the specific enzyme mediating the reaction, a common product is S-adenosylhomocysteine (SAH). Clearance of SAH by conversion into homocysteine and adenosine in a reversible reaction catalysed by SAH hydrolase (EC 3.3.1.1) is essential as many SAMe-dependent methylation reactions are strongly inhibited by SAH accumulation.
      Homocysteine lies at the intersection of the remethylation pathway and the transsulfuration pathway through which homocysteine may be processed to form the primary endogenous cellular antioxidant, glutathione (Fig. 1) [
      • Mato J.M.
      • Lu S.C.
      Role of S-adenosyl-l-methionine in liver health and injury.
      ]. In the former, homocysteine is remethylated by methionine synthase (MS; EC 2.1.1.13) in a process coupled to the folate cycle or betaine methyltransferase (BHMT; EC 2.1.1.5) to re-form methionine; although it should be noted that the role of BHMT is of much less significance in primates than in rodents [
      • Lieber C.S.
      • Packer L.
      S-adenosylmethionine: molecular, biological, and clinical aspects – an introduction.
      ]. Alternatively, the conversion of homocysteine to crystathionine by crystathionine β-synthase (CBS; EC 4.2.1.22) begins the transsulfuration pathway through which the methionine derived sulphur atom in SAMe is processed stepwise into cysteine and ultimately glutathione (Fig. 1). These pathways are autoregulated by hepatic SAMe concentration which acts as a potent inhibitor of MS and BHMT activity and an activator of CBS so that excess SAMe is preferentially partitioned into glutathione [
      • Ou X.
      • Yang H.
      • Ramani K.
      • Ara A.I.
      • Chen H.
      • Mato J.M.
      • et al.
      Inhibition of human betaine-homocysteine methyltransferase expression by S-adenosylmethionine and methylthioadenosine.
      ]. Folic acid and the co-factors vitamin B6 and B12 are required for functioning of MS, CBS, and BHMT, respectively, and so their availability will limit enzymatic activity and thus homocysteine levels and hepatic methionine handling (Fig. 1) [
      • Mato J.M.
      • Lu S.C.
      Role of S-adenosyl-l-methionine in liver health and injury.
      ]. SAMe is also the precursor for the synthesis of polyamines that are needed to preserve cell viability and proliferation. Here, SAMe is decarboxylated by SAMe decarboxylase (EC 4.1.1.50) and the aminopropyl group used to from polyamines including the biologically active metabolite 5’-methylthioadenosine (MTA).

      SAMe metabolism in liver disease

      Recognition that methionine metabolism is impaired in patients with chronic liver disease extends back more than 60 years to studies demonstrating reduced methionine clearance after liver injury [
      • Kinsell L.W.
      • Harper H.A.
      • Barton H.C.
      • Hutchin M.E.
      • Hess J.R.
      Studies in methionine and sulfur metabolism. I. The fate of intravenously administered methionine, in normal individuals and in patients with liver damage.
      ]. Independent of aetiology, patients with hepatic cirrhosis have been shown to have reduced MAT1A expression, lower MATI/III activity, with no compensatory increase in MAT2A expression and therefore hepatocellular accumulation of methionine and significantly reduced levels of SAMe [
      • Duce A.M.
      • Ortiz P.
      • Cabrero C.
      • Mato J.M.
      S-adenosyl-l-methionine synthetase and phospholipid methyltransferase are inhibited in human cirrhosis.
      ,
      • Avila M.A.
      • Berasain C.
      • Torres L.
      • Martin-Duce A.
      • Corrales F.J.
      • Yang H.
      • et al.
      Reduced mRNA abundance of the main enzymes involved in methionine metabolism in human liver cirrhosis and hepatocellular carcinoma.
      ]. These changes are due to hypermethylation of the MAT1A promoter limiting gene expression in cirrhotic patients, however, the mechanism through which this occurs is not known (Fig. 2) [
      • Avila M.A.
      • Berasain C.
      • Torres L.
      • Martin-Duce A.
      • Corrales F.J.
      • Yang H.
      • et al.
      Reduced mRNA abundance of the main enzymes involved in methionine metabolism in human liver cirrhosis and hepatocellular carcinoma.
      ]. Furthermore, conditions that promote oxidative stress such as alcohol consumption, viral hepatitis, septic shock, and toxin exposure; or increase nitric oxide (NO) synthesis such as hypoxia and inflammatory cytokines (e.g. TNFα, IL-6) will inactivate residual MATI/III [
      • Mato J.M.
      • Corrales F.J.
      • Lu S.C.
      • Avila M.A.
      S-adenosylmethionine: a control switch that regulates liver function.
      ]. Given that SAMe is a biochemical intermediary in glutathione synthesis, this in turn will further downgrade hepatocellular defences against oxidative stress and worsen liver injury.
      The effects of chronic SAMe depletion may be studied in the Mat1a null (MATO) mice [
      • Lu S.C.
      • Alvarez L.
      • Huang Z.Z.
      • Chen L.
      • An W.
      • Corrales F.J.
      • et al.
      Methionine adenosyltransferase 1A knockout mice are predisposed to liver injury and exhibit increased expression of genes involved in proliferation.
      ]. In these mice, hepatic hyperplasia is evident by 3 months of age; young animals are more sensitive to choline-deficient diet induced steatosis; and, consistent with the effects of methionine/choline-deficient dietary models [
      • Anstee Q.M.
      • Goldin R.D.
      Mouse models in non-alcoholic fatty liver disease and steatohepatitis research.
      ], the mice develop spontaneous steatohepatitis by age 8 months [
      • Lu S.C.
      • Alvarez L.
      • Huang Z.Z.
      • Chen L.
      • An W.
      • Corrales F.J.
      • et al.
      Methionine adenosyltransferase 1A knockout mice are predisposed to liver injury and exhibit increased expression of genes involved in proliferation.
      ]. Gene expression profiling reveals that, even when the liver appears histologically normal, numerous growth, dedifferentiation and acute phase response genes are upregulated including proliferating cell nuclear antigen, α-fetoprotein, and Mat2a. MATO mice are also significantly more sensitive to the effects of carbon tetrachloride exposure than wild type animals, exhibiting higher levels of ALT, AST, and greater histological liver injury [
      • Mato J.M.
      • Corrales F.J.
      • Lu S.C.
      • Avila M.A.
      S-adenosylmethionine: a control switch that regulates liver function.
      ]. Few human examples of spontaneous MAT1A deficiency have been reported, however, it is notable that neurological rather than hepatic dysfunction is the primary phenotype described (OMIM #610550).
      Beyond its role as a metabolic intermediate, there is evidence that the balance between MATI/III and MATII activation and therefore intracellular SAMe concentration can modulate cell proliferation in the liver. SAMe levels are high in quiescent hepatocytes but are much lower in proliferating cells [
      • Cai J.
      • Mao Z.
      • Hwang J.J.
      • Lu S.C.
      Differential expression of methionine adenosyltransferase genes influences the rate of growth of human hepatocellular carcinoma cells.
      ]. Studies in HepG2 and HuH7 cells demonstrate that when MAT2A is expressed, low SAMe levels promote more rapid cell growth whilst increased SAMe synthesis due to MAT1A expression or SAMe treatment is associated with slower growth [
      • Cai J.
      • Mao Z.
      • Hwang J.J.
      • Lu S.C.
      Differential expression of methionine adenosyltransferase genes influences the rate of growth of human hepatocellular carcinoma cells.
      ]. These findings are also supported by reports that the potent mitogen, hepatocyte growth factor (HGF), induces acetylation of histones associated with the MAT2A promoter, an effect that can be overcome by administration of exogenous SAMe [
      • Latasa M.U.
      • Boukaba A.
      • Garcia-Trevijano E.R.
      • Torres L.
      • Rodriguez J.L.
      • Caballeria J.
      • et al.
      Hepatocyte growth factor induces MAT2A expression and histone acetylation in rat hepatocytes: role in liver regeneration.
      ,
      • Garcia-Trevijano E.R.
      • Latasa M.U.
      • Carretero M.V.
      • Berasain C.
      • Mato J.M.
      • Avila M.A.
      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.
      ]. In vivo, following partial hepatectomy in the rat, SAMe levels are markedly reduced, coinciding with the start of DNA synthesis and regeneration. Once again, hepatocellular regeneration can be inhibited by SAMe administration [
      • Huang Z.Z.
      • Mao Z.
      • Cai J.
      • Lu S.C.
      Changes in methionine adenosyltransferase during liver regeneration in the rat.
      ]. Underlining the clinical significance of the effect of SAMe on hepatocellular proliferation, SAMe depletion in Mat1a knockout mice is associated with spontaneous development of HCC [
      • Martinez-Chantar M.L.
      • Corrales F.J.
      • Martinez-Cruz L.A.
      • Garcia-Trevijano E.R.
      • Huang Z.Z.
      • Chen L.
      • et al.
      Spontaneous oxidative stress and liver tumors in mice lacking methionine adenosyltransferase 1A.
      ,
      • Chen L.
      • Zeng Y.
      • Yang H.
      • Lee T.D.
      • French S.W.
      • Corrales F.J.
      • et al.
      Impaired liver regeneration in mice lacking methionine adenosyltransferase 1A.
      ] whilst development of HCC in carcinogen-exposed rats can be blocked by SAMe treatment [
      • Pascale R.M.
      • Marras V.
      • Simile M.M.
      • Daino L.
      • Pinna G.
      • Bennati S.
      • et al.
      Chemoprevention of rat liver carcinogenesis by S-adenosyl-l-methionine: a long-term study.
      ,
      • Pascale R.M.
      • Simile M.M.
      • De Miglio M.R.
      • Nufris A.
      • Daino L.
      • Seddaiu M.A.
      • et al.
      Chemoprevention by S-adenosyl-l-methionine of rat liver carcinogenesis initiated by 1,2-dimethylhydrazine and promoted by orotic acid.
      ]. Thus, SAMe depletion promotes increased cellular proliferation and growth. As a short lived response to an acute insult these changes may be beneficial: SAMe synthesis is reset to a new lower steady state preserving ATP, hepatocyte proliferation and growth are prioritised and the original liver mass restored. However, if injury becomes chronic, SAMe depletion may be deleterious as it favours malignant transformation and so SAMe supplementation, at least to physiological levels, may be beneficial.

      Role of SAMe in modifying response to injury

      Given our current understanding of methionine metabolism and the physiological role of SAMe, it has been proposed that supplementation could both ameliorate liver injury and reduce the development of HCC in chronic liver disease. These effects cannot be achieved by giving methionine, even a 7-fold increase in methionine consumption was unable to significantly increase hepatic SAMe and could exacerbate the potentially harmful accumulation of methionine that is observed in cirrhotic patients [
      • Finkelstein J.D.
      • Martin J.J.
      Methionine metabolism in mammals. Adaptation to methionine excess.
      ,
      • Hardwick D.F.
      • Applegarth D.A.
      • Cockcroft D.M.
      • Ross P.M.
      • Cder R.J.
      Pathogenesis of methionine-induced toxicity.
      ]. Administration of SAMe would, however, bypass MAT and theoretically avoid these problems. Before considering the evidence for specific disease states, we will examine the role of SAMe in hepatic pathophysiology and how this may be manipulated through therapeutic supplementation.

      Oxidative stress and cellular damage

      A common feature in the pathophysiology of most inflammatory processes is the involvement of reactive oxygen species (ROS) such as superoxide, peroxynitrite, hydrogen peroxide, and the hydroxyl radical [
      • Jaeschke H.
      Reactive oxygen and mechanisms of inflammatory liver injury: present concepts.
      ]. Due to their high metabolic activity, ROS are generated continuously in hepatocytes and also, following liver injury, by cells of the innate immune system (e.g. Kupffer cells, infiltrating monocytes/macrophages, and neutrophils). These highly reactive chemical species react with cell membranes causing lipid peroxidation, induce DNA damage, activate c-jun-N-terminal kinase (JNK) and caspase signalling to promote apoptosis and cause oncotic necrosis [
      • Jaeschke H.
      Reactive oxygen and mechanisms of inflammatory liver injury: present concepts.
      ]. To counter this, intracellular ROS generation is largely confined to specific organelles including mitochondria and peroxisomes; several anti-oxidant enzyme systems have developed including superoxide dismutase (SOD), glutathione peroxidise, and catalase that detoxify ROS; and cells contain endogenous anti-oxidants which scavenge free radicals to mitigate against cellular damage, the most abundant of which is glutathione [
      • Copple B.L.
      • Jaeschke H.
      • Klaassen C.D.
      Oxidative stress and the pathogenesis of cholestasis.
      ]. As discussed above, SAMe is a precursor for the synthesis of cysteine and thus glutathione (Fig. 1) [
      • Mato J.M.
      • Corrales F.J.
      • Lu S.C.
      • Avila M.A.
      S-adenosylmethionine: a control switch that regulates liver function.
      ]. SAMe has been shown to effectively increase intracellular glutathione concentration in murine models [
      • Song Z.
      • Zhou Z.
      • Chen T.
      • Hill D.
      • Kang J.
      • Barve S.
      • et al.
      S-adenosylmethionine (SAMe) protects against acute alcohol induced hepatotoxicity in mice.
      ,
      • Garcia-Ruiz C.
      • Morales A.
      • Colell A.
      • Ballesta A.
      • Rodes J.
      • Kaplowitz N.
      • et al.
      Feeding S-adenosyl-l-methionine attenuates both ethanol-induced depletion of mitochondrial glutathione and mitochondrial dysfunction in periportal and perivenous rat hepatocytes.
      ,
      • Colell A.
      • Garcia-Ruiz C.
      • Miranda M.
      • Ardite E.
      • Mari M.
      • Morales A.
      • et al.
      Selective glutathione depletion of mitochondria by ethanol sensitizes hepatocytes to tumor necrosis factor.
      ,
      • Colell A.
      • García-Ruiz C.
      • Morales A.
      • Ballesta A.
      • Ookhtens M.
      • Rodés J.
      • et al.
      Transport of reduced glutathione in hepatic mitochondria and mitoplasts from ethanol-treated rats: effect of membrane physical properties and S-adenosyl-l-methionine.
      ] and in patients with liver disease [
      • Vendemiale G.
      • Altomare E.
      • Trizio T.
      • Le Grazie C.
      • Di Padova C.
      • Salerno M.T.
      • et al.
      Effects of oral S-adenosyl-l-methionine on hepatic glutathione in patients with liver disease.
      ]. Importantly, SAMe can replenish hepatic mitochondrial glutathione and normalise fluidity of the inner mitochondrial membrane, which is critical for maintenance of function [
      • Garcia-Ruiz C.
      • Morales A.
      • Colell A.
      • Ballesta A.
      • Rodes J.
      • Kaplowitz N.
      • et al.
      Feeding S-adenosyl-l-methionine attenuates both ethanol-induced depletion of mitochondrial glutathione and mitochondrial dysfunction in periportal and perivenous rat hepatocytes.
      ,
      • Colell A.
      • García-Ruiz C.
      • Morales A.
      • Ballesta A.
      • Ookhtens M.
      • Rodés J.
      • et al.
      Transport of reduced glutathione in hepatic mitochondria and mitoplasts from ethanol-treated rats: effect of membrane physical properties and S-adenosyl-l-methionine.
      ]. It is tempting to speculate that these effects could underlie a reported amelioration of biochemical markers of chemotherapy-related liver injury described in three retrospective, observational studies from Italy, however, hepatic glutathione levels were not measured and so it would require further, prospective assessment to confirm these findings and determine mechanism of action [
      • Santini D.
      • Vincenzi B.
      • Massacesi C.
      • Picardi A.
      • Gentilucci U.V.
      • Esposito V.
      • et al.
      S-adenosylmethionine (AdoMet) supplementation for treatment of chemotherapy-induced liver injury.
      ,
      • Vincenzi B.
      • Daniele S.
      • Frezza A.M.
      • Berti P.
      • Vespasiani U.
      • Picardi A.
      • et al.
      The role of S-adenosylmethionine in preventing oxaliplatin-induced liver toxicity: a retrospective analysis in metastatic colorectal cancer patients treated with bevacizumab plus oxaliplatin-based regimen.
      ,
      • Vincenzi B.
      • Santini D.
      • Frezza A.M.
      • Berti P.
      • Vespasiani U.
      • Picardi A.
      • et al.
      The role of S-adenosyl methionine in preventing FOLFOX-induced liver toxicity: a retrospective analysis in patients affected by resected colorectal cancer treated with adjuvant FOLFOX regimen.
      ].
      There is evidence that, beyond its effect on hepatocellular ROS, SAMe may have additional beneficial effects modulating the balance between pro- and anti-inflammatory cytokines in liver injury [
      • Purohit V.
      • Abdelmalek M.F.
      • Barve S.
      • Benevenga N.J.
      • Halsted C.H.
      • Kaplowitz N.
      • et al.
      Role of S-adenosylmethionine, folate, and betaine in the treatment of alcoholic liver disease: summary of a symposium.
      ]. Experimental evidence of the ability of SAMe to attenuate pro-inflammatory TNFα-mediated liver injury, likely through downregulation of transcription factor NFκB, comes from a number of studies [
      • Purohit V.
      • Abdelmalek M.F.
      • Barve S.
      • Benevenga N.J.
      • Halsted C.H.
      • Kaplowitz N.
      • et al.
      Role of S-adenosylmethionine, folate, and betaine in the treatment of alcoholic liver disease: summary of a symposium.
      ,
      • Veal N.
      • Hsieh C.L.
      • Xiong S.
      • Mato J.M.
      • Lu S.
      • Tsukamoto H.
      Inhibition of lipopolysaccharide-stimulated TNF-alpha promoter activity by S-adenosylmethionine and 5’-methylthioadenosine.
      ]. Elevated glutathione levels may also help reduce TNFα-induced necrosis [
      • Colell A.
      • Garcia-Ruiz C.
      • Miranda M.
      • Ardite E.
      • Mari M.
      • Morales A.
      • et al.
      Selective glutathione depletion of mitochondria by ethanol sensitizes hepatocytes to tumor necrosis factor.
      ]. In rats fed a methionine–choline deficient diet to induce steatohepatitis, SAMe was shown to reduce the induction of TNFα expression caused by exposure to bacterial lipopolysaccharide (LPS) [
      • Chawla R.K.
      • Watson W.H.
      • Eastin C.E.
      • Lee E.Y.
      • Schmidt J.
      • McClain C.J.
      S-adenosylmethionine deficiency and TNF-alpha in lipopolysaccharide-induced hepatic injury.
      ]. Both SAMe and its metabolite from the polyamine pathway, MTA, have also been shown to reduce TNFα production in vivo and in vitro following LPS stimulation of the murine monocyte cell line RAW264.7 [
      • Song Z.
      • Uriarte S.
      • Sahoo R.
      • Chen T.
      • Barve S.
      • Hill D.
      • et al.
      S-adenosylmethionine (SAMe) modulates interleukin-10 and interleukin-6, but not TNF, production via the adenosine (A2) receptor.
      ,
      • Hevia H.
      • Varela-Rey M.
      • Corrales F.J.
      • Berasain C.
      • Martinez-Chantar M.L.
      • Latasa M.U.
      • et al.
      5′-Methylthioadenosine modulates the inflammatory response to endotoxin in mice and in rat hepatocytes.
      ], and to induce anti-inflammatory IL-10 production in the same models [
      • Song Z.
      • Uriarte S.
      • Sahoo R.
      • Chen T.
      • Barve S.
      • Hill D.
      • et al.
      S-adenosylmethionine (SAMe) modulates interleukin-10 and interleukin-6, but not TNF, production via the adenosine (A2) receptor.
      ,
      • Hevia H.
      • Varela-Rey M.
      • Corrales F.J.
      • Berasain C.
      • Martinez-Chantar M.L.
      • Latasa M.U.
      • et al.
      5′-Methylthioadenosine modulates the inflammatory response to endotoxin in mice and in rat hepatocytes.
      ,
      • Song Z.
      • Barve S.
      • Chen T.
      • Nelson W.
      • Uriarte S.
      • Hill D.
      • et al.
      S-adenosylmethionine (AdoMet) modulates endotoxin stimulated interleukin-10 production in monocytes.
      ].

      Apoptosis

      The interactions between SAMe and apoptotic cell death pathways are complex. SAMe has been shown to inhibit bile acid-induced apoptosis in vitro [
      • Benz C.
      • Angermuller S.
      • Kloters-Plachky P.
      • Sauer P.
      • Stremmel W.
      • Stiehl A.
      Effect of S-adenosylmethionine versus tauroursodeoxycholic acid on bile acid-induced apoptosis and cytolysis in rat hepatocytes.
      ,
      • Webster C.R.
      • Boria P.
      • Usechak P.
      • Anwer M.S.
      S-adenosylmethionine and cAMP confer differential cytoprotection against bile acid-induced apoptosis in canine renal tubular cells and primary rat hepatocytes.
      ] and both SAMe and MTA can protect normal cultured rat hepatocytes against okadaic acid-induced apoptosis in a dose dependent manner, an effect thought to be mediated through reduced mitochondrial cytochrome-c release, caspase-3 activation, and poly(ADP-ribose) polymerase cleavage [
      • Purohit V.
      • Abdelmalek M.F.
      • Barve S.
      • Benevenga N.J.
      • Halsted C.H.
      • Kaplowitz N.
      • et al.
      Role of S-adenosylmethionine, folate, and betaine in the treatment of alcoholic liver disease: summary of a symposium.
      ,
      • Ishii H.
      • Adachi M.
      • Fernandez-Checa J.C.
      • Cederbaum A.I.
      • Deaciuc I.V.
      • Nanji A.A.
      Role of apoptosis in alcoholic liver injury.
      ]. However, SAMe has the opposite effect on cancer cells [
      • Ansorena E.
      • Garcia-Trevijano E.R.
      • Martinez-Chantar M.L.
      • Huang Z.Z.
      • Chen L.
      • Mato J.M.
      • et al.
      S-adenosylmethionine and methylthioadenosine are antiapoptotic in cultured rat hepatocytes but proapoptotic in human hepatoma cells.
      ]. For example, SAMe induces apoptosis in HepG2 and HuH7 cell lines via the mitochondrial death pathway [
      • Ansorena E.
      • Garcia-Trevijano E.R.
      • Martinez-Chantar M.L.
      • Huang Z.Z.
      • Chen L.
      • Mato J.M.
      • et al.
      S-adenosylmethionine and methylthioadenosine are antiapoptotic in cultured rat hepatocytes but proapoptotic in human hepatoma cells.
      ]. The reasons for the differential effect remain incompletely understood but two mechanisms have been proposed [
      • Mato J.M.
      • Lu S.C.
      Role of S-adenosyl-l-methionine in liver health and injury.
      ]. Firstly, SAMe and MTA alter cellular phosphorylation state and alternative splicing of genes in cancer cells resulting in Bcl-xs induction and apoptosis; and secondly, SAMe and MTA have been shown to directly inhibit BHMT activity in cancer cells but not normal hepatocytes, perturbing homocysteine metabolism, and promoting endoplasmic reticulum stress and therefore apoptosis [
      • Mato J.M.
      • Lu S.C.
      Role of S-adenosyl-l-methionine in liver health and injury.
      ]. These effects are consistent with the observed in vivo chemopreventive effects of SAMe in murine models of HCC where more apoptotic bodies were seen in tumours of SAMe-treated animals than in untreated controls [
      • Pascale R.M.
      • Marras V.
      • Simile M.M.
      • Daino L.
      • Pinna G.
      • Bennati S.
      • et al.
      Chemoprevention of rat liver carcinogenesis by S-adenosyl-l-methionine: a long-term study.
      ,
      • Pascale R.M.
      • Simile M.M.
      • De Miglio M.R.
      • Nufris A.
      • Daino L.
      • Seddaiu M.A.
      • et al.
      Chemoprevention by S-adenosyl-l-methionine of rat liver carcinogenesis initiated by 1,2-dimethylhydrazine and promoted by orotic acid.
      ].

      Transmethylation reactions and intra-hepatic cholestasis

      Cholestasis occurs in a number of disease states and results in an accumulation of potentially toxic bile salts in the liver and blood which in turn leads to oxidative stress, hepatocellular injury, bile duct proliferation, and ultimately hepatic fibrosis [
      • Copple B.L.
      • Jaeschke H.
      • Klaassen C.D.
      Oxidative stress and the pathogenesis of cholestasis.
      ,
      • Golovanova E.V.
      • Petrakov A.V.
      • Noskova K.K.
      Intrahepatic cholestasis in chronic liver diseases.
      ]. Methylation status is recognised to be of fundamental importance to cell membrane function with phospholipid methylation influencing membrane fluidity and transport of metabolites and transmission of signals across membranes [
      • Hirata F.
      • Axelrod J.
      Phospholipid methylation and biological signal transmission.
      ,
      • Hirata F.
      • Toyoshima S.
      • Axelrod J.
      • Waxdal M.J.
      Phospholipid methylation: a biochemical signal modulating lymphocyte mitogenesis.
      ,
      • Hirata F.
      • Viveros O.H.
      • Diliberto Jr., E.J.
      • Axelrod J.
      Identification and properties of two methyltransferases in conversion of phosphatidylethanolamine to phosphatidylcholine.
      ]. As a major methyl donor, availability of SAMe potentially has profound effects on these processes. Therefore, it has been suggested that SAMe depletion contributes to the development of intra-hepatic cholestasis by interfering with bile salt export pump (BSEP) activity [
      • Lieber C.S.
      S-adenosyl-l-methionine: its role in the treatment of liver disorders.
      ]. Furthermore, hepatocellular injury in cholestasis is frequently associated with glutathione depletion and so SAMe may help correct this [
      • Copple B.L.
      • Jaeschke H.
      • Klaassen C.D.
      Oxidative stress and the pathogenesis of cholestasis.
      ].

      SAMe therapy for chronic liver disease

      Intra-hepatic cholestasis

      The efficacy of SAMe therapy has been examined in a range of chronic liver conditions, although historically the greatest interest has been in the area of intra-hepatic cholestasis (IHC). IHC is a syndrome that develops from impaired bile flow at the sub-lobular level. This may occur due to: (1) hepatocellular damage, including viral hepatitis, alcoholic hepatitis or prolonged TPN use; (2) canalicular membrane changes, often seen in drug-induced liver injury (e.g. oral contraceptives, antibiotics, etc.); (3) genetic defects in bile transporters; (4) obstruction of the canaliculi or ductules; and (5) ductopenia [
      ]. Clinically, IHC is characterised by the presence of pruritus or jaundice with elevated serum total bilirubin, alkaline phosphatise, and gamma-glutamyltransferase levels [
      ].

      Pre-clinical and clinical therapeutic evidence in non-pregnancy related cholestasis

      The effects of SAMe treatment in vivo in rat models of surgical cholestasis (bile duct ligation) have shown some benefit. Rats treated with SAMe and then subjected to bile duct ligation for 7 days were found to exhibit less oxidative stress as measured by thiobarbituric acid reactive substances (TBARS) and to have a reduced ratio of oxidised to total glutathione [
      • Gonzalez-Correa J.A.
      • De La Cruz J.P.
      • Martin-Aurioles E.
      • Lopez-Egea M.A.
      • Ortiz P.
      • Sanchez de la Cuesta F.
      Effects of S-adenosyl-l-methionine on hepatic and renal oxidative stress in an experimental model of acute biliary obstruction in rats.
      ]. Perhaps unsurprisingly, given the nature of the model, both this and a similar study failed to detect any change in biochemical cholestasis (ALT, ALP, and bilirubin), area of hepatocellular necrosis or fibrosis [
      • Copple B.L.
      • Jaeschke H.
      • Klaassen C.D.
      Oxidative stress and the pathogenesis of cholestasis.
      ,
      • Gonzalez-Correa J.A.
      • De La Cruz J.P.
      • Martin-Aurioles E.
      • Lopez-Egea M.A.
      • Ortiz P.
      • Sanchez de la Cuesta F.
      Effects of S-adenosyl-l-methionine on hepatic and renal oxidative stress in an experimental model of acute biliary obstruction in rats.
      ,
      • Yang H.
      • Ramani K.
      • Xia M.
      • Ko K.S.
      • Li T.W.
      • Oh P.
      • et al.
      Dysregulation of glutathione synthesis during cholestasis in mice. molecular mechanisms and therapeutic implications.
      ].
      The two largest studies that have examined the utility of SAMe therapy in this setting have both been conducted in patients with features of IHC due to a mixture of different aetiologies [
      • Fiorelli G.
      S-adenosylmethionine in the treatment of intrahepatic cholestasis of chronic liver disease: a field trial.
      ,
      • Frezza M.
      • Surrenti C.
      • Manzillo G.
      • Fiaccadori F.
      • Bortolini M.
      • Di Padova C.
      Oral S-adenosylmethionine in the symptomatic treatment of intrahepatic cholestasis. A double-blind, placebo-controlled study.
      ]. One of the earliest was a multi-centre, double-blind placebo-controlled trial conducted in 220 IHC patients, many of indeterminate aetiology and of differing disease stage (68% cirrhosis, 26% chronic viral hepatitis, 6% PBC) [
      • Frezza M.
      • Surrenti C.
      • Manzillo G.
      • Fiaccadori F.
      • Bortolini M.
      • Di Padova C.
      Oral S-adenosylmethionine in the symptomatic treatment of intrahepatic cholestasis. A double-blind, placebo-controlled study.
      ]. This demonstrated a significant reduction in clinical biochemical indices of cholestasis and improvement in symptoms of fatigue and pruritus after oral SAMe treatment (1600 mg/day). This study also showed that significantly more SAMe-treated patients reported a >50% increase in general well-being (SAMe 84% vs. placebo 29%, p <0.01) [
      • Frezza M.
      • Surrenti C.
      • Manzillo G.
      • Fiaccadori F.
      • Bortolini M.
      • Di Padova C.
      Oral S-adenosylmethionine in the symptomatic treatment of intrahepatic cholestasis. A double-blind, placebo-controlled study.
      ]. These findings were supported by a subsequent Italian study. Here, 640 IHC patients were allocated to one of two different parenteral dosing schedules (500 mg/day im or 800 mg/day iv) for 15 days in a non-randomised, non-placebo controlled, observational study [
      • Fiorelli G.
      S-adenosylmethionine in the treatment of intrahepatic cholestasis of chronic liver disease: a field trial.
      ]. The majority of patients recruited had chronic viral hepatitis with or without concomitant excess alcohol consumption and approximately 60% were cirrhotic at enrolment. Little additional information as to whether patients were inpatients or outpatients at the time of the study was provided. Over two-thirds of participants reported substantial improvements in subjective symptoms of pruritus and fatigue using a visual analogue scale and reductions in serum markers of cholestasis were also observed [
      • Fiorelli G.
      S-adenosylmethionine in the treatment of intrahepatic cholestasis of chronic liver disease: a field trial.
      ]. Neither parenteral regimen was found to be superior to the other. Although these studies provide supportive evidence for the use of SAMe in chronic liver disease, the study cohorts were poorly defined and methodology was not to current standards for clinical trials. This is particularly so for the latter study, which was not randomised and had no control arm, making it difficult to confidently draw conclusions that influence practice. Importantly, neither study addressed duration of effect or whether this translated into any tangible prognostic benefit.
      There have been several smaller studies that have also examined symptom severity (fatigue and/or pruritus) as an end point [
      • Frezza M.
      • Surrenti C.
      • Manzillo G.
      • Fiaccadori F.
      • Bortolini M.
      • Di Padova C.
      Oral S-adenosylmethionine in the symptomatic treatment of intrahepatic cholestasis. A double-blind, placebo-controlled study.
      ,
      • Manzillo G.
      • Piccinino F.
      • Surrenti C.
      • Frezza M.
      • Giudici G.A.
      • Le Grazie C.
      Multicentre double-blind placebo-controlled study of intravenous and oral s-adenosyl-l-methionine (SAMe) in cholestatic patients with liver disease.
      ,
      • Qin B.
      • Guo S.
      • Zhao Y.
      • Zou S.
      • Zhang Q.
      • Wang Z.
      • et al.
      A trial of ademetionine in the treatment of intrahepatic biliary stasis viral hepatitis.
      ]. The value of SAMe as a treatment for pruritus was addressed by the Agency for Healthcare Research and Quality in 2002 (http://archive.ahrq.gov/clinic/tp/sametp.htm) [
      • Hardy M.L.
      • Coulter I.
      • Morton S.C.
      • Favreau J.
      • Venuturupalli S.
      • Chiappelli F.
      • et al.
      S-adenosyl-l-methionine for treatment of depression, osteoarthritis, and liver disease.
      ]. In a meta-analysis of four smaller, heterogeneous studies addressing IHC of mixed aetiology unrelated to pregnancy (Table 1), the authors concluded that SAMe therapy was superior to placebo, significantly reducing pruritus (relative risk 0.45, 95% CI 0.37–0.55) and serum bilirubin levels (pooled estimate, p = 0.02) however, there was insufficient evidence to compare efficacy with other agents such as ursodeoxycholic acid [
      • Hardy M.L.
      • Coulter I.
      • Morton S.C.
      • Favreau J.
      • Venuturupalli S.
      • Chiappelli F.
      • et al.
      S-adenosyl-l-methionine for treatment of depression, osteoarthritis, and liver disease.
      ].
      Table 1Selected clinical trials of SAMe in non-pregnancy related IHC.
      Included in systematic review
      • Hardy M.L.
      • Coulter I.
      • Morton S.C.
      • Favreau J.
      • Venuturupalli S.
      • Chiappelli F.
      • et al.
      S-adenosyl-l-methionine for treatment of depression, osteoarthritis, and liver disease.
      .
      RCT, randomised controlled trial.

      Pre-clinical and clinical therapeutic evidence in cholestasis of pregnancy

      Cholestasis of pregnancy is associated with a high risk of adverse perinatal outcomes, including preterm birth (19–60%), meconium passage (27%), foetal distress (22–41%), and foetal death (0.4–4.1%) [
      • Pusl T.
      • Beuers U.
      Intrahepatic cholestasis of pregnancy.
      ]. Risk increases with fasting serum bile acid levels >40 μmol/L [
      • Pusl T.
      • Beuers U.
      Intrahepatic cholestasis of pregnancy.
      ]. The use of SAMe to relieve symptoms and lower bile salt levels in this condition has been studied by several groups. The largest study randomised 78 patients with cholestasis of pregnancy at <36 weeks gestation to SAMe, ursodeoxycholic acid (UDCA) or combination therapy until delivery. Both therapies were equally effective at improving pruritus, however, women receiving UDCA had a significantly greater improvement in the concentration of serum bile acids (p = 0.001) and clinical biochemistry than those receiving SAMe [
      • Roncaglia N.
      • Locatelli A.
      • Arreghini A.
      • Assi F.
      • Cameroni I.
      • Pezzullo J.C.
      • et al.
      A randomised controlled trial of ursodeoxycholic acid and S-adenosyl-l-methionine in the treatment of gestational cholestasis.
      ]. A systematic review by members of the Cochrane Collaboration (performed in 2001 and updated 2010) identified five randomised control trials that met their inclusion criteria (these and others are listed in Table 2), however, analysis was hampered by their heterogeneity and several methodological problems were identified in the studies [
      • Burrows R.F.
      • Clavisi O.
      • Burrows E.
      Interventions for treating cholestasis in pregnancy.
      ]. The drug was generally well tolerated in all studies with no serious adverse events reported, however, the authors concluded that there was insufficient evidence to support the use of SAMe or UDCA in cholestasis of pregnancy [
      • Burrows R.F.
      • Clavisi O.
      • Burrows E.
      Interventions for treating cholestasis in pregnancy.
      ].
      Table 2Selected clinical trials of SAMe in cholestasis of
      • Nicastri P.L.
      • Diaferia A.
      • Tartagni M.
      • Loizzi P.
      • Fanelli M.
      A randomised placebo-controlled trial of ursodeoxycholic acid and S-adenosylmethionine in the treatment of intrahepatic cholestasis of pregnancy.
      ,
      • Ribalta J.
      • Reyes H.
      • Gonzalez M.C.
      • Iglesias J.
      • Arrese M.
      • Poniachik J.
      • et al.
      S-adenosyl-l-methionine in the treatment of patients with intrahepatic cholestasis of pregnancy: a randomized, double-blind, placebo-controlled study with negative results.
      ,
      • Frezza M.
      • Centini G.
      • Cammareri G.
      • Le Grazie C.
      • Di Padova C.
      S-adenosylmethionine for the treatment of intrahepatic cholestasis of pregnancy. Results of a controlled clinical trial.
      ,
      • Frezza M.
      • Pozzato G.
      • Chiesa L.
      • Stramentinoli G.
      • di Padova C.
      Reversal of intrahepatic cholestasis of pregnancy in women after high dose S-adenosyl-l-methionine administration.
      ,
      • Floreani A.
      • Paternoster D.
      • Melis A.
      • Grella P.V.
      S-adenosylmethionine versus ursodeoxycholic acid in the treatment of intrahepatic cholestasis of pregnancy: preliminary results of a controlled trial.
      ,
      • Lafuenti G.
      • Plotti G.
      • Nicolanti G.
      • Gaglione R.
      • Tibollo F.G.
      • Mancuso S.
      Evaluation of the obstetrical risk in pregnant women with intrahepatic cholestasis treated with S-adenosyl-l-methionine.
      ,
      • Catalino F.
      • Scarponi S.
      • Cesa F.
      • Loiacono G.
      • Bortolini M.
      Efficacy and safety of intravenous S-adenosyl-l-methionine therapy in the management of intrahepatic cholestasis of pregnancy.
      .
      Included in Cochrane systematic review
      • Burrows R.F.
      • Clavisi O.
      • Burrows E.
      Interventions for treating cholestasis in pregnancy.
      .
      RCT, randomised controlled trial; n.a., not available.

      Alcoholic liver disease

      Internationally, alcoholic liver disease (ALD) poses a major burden on healthcare resources. Excess alcohol consumption (typically a threshold of >20 g/day for women and >30 g/day for men is adopted) is associated with progressive liver injury ranging from steatosis, through alcoholic steatohepatitis to cirrhosis. Pathogenesis is driven by a combination of direct hepatotoxic effects of alcohol; ROS generation by cytochrome P450 (CYP2E1); bacterial translocation from the gut promoting LPS-induced Kupffer cell TNFα and pro-inflammatory cytokine release; and innate immune system activation (recently reviewed elsewhere [
      • Gao B.
      • Bataller R.
      Alcoholic liver disease: pathogenesis and new therapeutic targets.
      ]). In addition, patients with ALD frequently exhibit multiple nutritional deficiencies including protein energy malnutrition and thiamine (vitamin B1), vitamin B6, vitamin B12, and folate deficiency [
      • Halsted C.H.
      • Medici V.
      Vitamin-dependent methionine metabolism and alcoholic liver disease.
      ]. The reasons for these deficiencies are multifactorial and include poor diet, intestinal malabsorption, and reduced hepatic uptake [
      • Halsted C.H.
      • Medici V.
      Vitamin-dependent methionine metabolism and alcoholic liver disease.
      ]. Additionally, specific chemical interactions also interfere with normal physiological processes, for example, the ethanol metabolite acetaldehyde displaces the active form of vitamin B6 (pyridoxal phosphatase) from its hepatic binding site [
      • Lumeng L.
      The role of acetaldehyde in mediating the deleterious effect of ethanol on pyridoxal 5’-phosphate metabolism.
      ]. As discussed above, these nutrients are essential for normal methionine metabolism and so deficiency will have important deleterious effects, impairing remethylation of homocysteine by MS and BHMT and its metabolism to form glutathione, thereby degrading defenses against oxidative stress. The consequent increase in hepatocellular homocysteine will alter the catalytic equilibrium of the reversible enzyme SAH hydrolase, altering the SAH:SAMe ratio and inhibiting many SAMe dependent methylation reactions (Fig. 1). Confirming this, liver biopsies from patients with alcoholic hepatitis demonstrate significant SAMe depletion and 50% reduced expression of MAT1A as well as the genes encoding MS and CBS [
      • Lee T.D.
      • Sadda M.R.
      • Mendler M.H.
      • Bottiglieri T.
      • Kanel G.
      • Mato J.M.
      • et al.
      Abnormal hepatic methionine and glutathione metabolism in patients with alcoholic hepatitis.
      ]. Similar results are found in patients with both alcoholic and non-alcoholic cirrhosis [
      • Duce A.M.
      • Ortiz P.
      • Cabrero C.
      • Mato J.M.
      S-adenosyl-l-methionine synthetase and phospholipid methyltransferase are inhibited in human cirrhosis.
      ,
      • Cabrero C.
      • Duce A.M.
      • Ortiz P.
      • Alemany S.
      • Mato J.M.
      Specific loss of the high-molecular-weight form of S-adenosyl-l- methionine synthetase in human liver cirrhosis.
      ] where MAT activity was suppressed, and in a range of animal models of alcoholic liver injury including mice [
      • Song Z.
      • Zhou Z.
      • Chen T.
      • Hill D.
      • Kang J.
      • Barve S.
      • et al.
      S-adenosylmethionine (SAMe) protects against acute alcohol induced hepatotoxicity in mice.
      ], rats [
      • Colell A.
      • Garcia-Ruiz C.
      • Miranda M.
      • Ardite E.
      • Mari M.
      • Morales A.
      • et al.
      Selective glutathione depletion of mitochondria by ethanol sensitizes hepatocytes to tumor necrosis factor.
      ,
      • Barak A.J.
      • Beckenhauer H.C.
      • Tuma D.J.
      • Badakhsh S.
      Effects of prolonged ethanol feeding on methionine metabolism in rat liver.
      ], baboons [
      • Lieber C.S.
      • Casini A.
      • DeCarli L.M.
      • Kim C.I.
      • Lowe N.
      • Sasaki R.
      • et al.
      S-adenosyl-l-methionine attenuates alcohol-induced liver injury in the baboon.
      ], and micropigs [
      • Halsted C.H.
      • Villanueva J.A.
      • Devlin A.M.
      • Niemela O.
      • Parkkila S.
      • Garrow T.A.
      • et al.
      Folate deficiency disturbs hepatic methionine metabolism and promotes liver injury in the ethanol-fed micropig.
      ,
      • Halsted C.H.
      • Villanueva J.
      • Chandler C.J.
      • Stabler S.P.
      • Allen R.H.
      • Muskhelishvili L.
      • et al.
      Ethanol feeding of micropigs alters methionine metabolism and increases hepatocellular apoptosis and proliferation.
      ].

      Pre-clinical and clinical therapeutic evidence

      The effects of SAMe supplementation in ALD have been explored in a number of animal models and in several clinical trials. In general, the results of pre-clinical studies have been positive although this may be influenced by publication bias. In isolated perfused rat liver, SAMe attenuated ethanol toxicity by restoring mitochondrial and total liver glutathione [
      • Gigliozzi A.
      • Romeo R.
      • Fraioli F.
      • Cantafora A.
      • Delle Monache M.
      • Cardilli A.
      • et al.
      Effect of S-adenosyl-l-methionine and dilinoleoylphosphatidylcholine on liver lipid composition and ethanol hepatotoxicity in isolated perfused rat liver.
      ]. SAMe supplementation has been shown to reverse SAMe and glutathione depletion in ethanol fed baboons [
      • Lieber C.S.
      • Casini A.
      • DeCarli L.M.
      • Kim C.I.
      • Lowe N.
      • Sasaki R.
      • et al.
      S-adenosyl-l-methionine attenuates alcohol-induced liver injury in the baboon.
      ] and rats [
      • Garcia-Ruiz C.
      • Morales A.
      • Colell A.
      • Ballesta A.
      • Rodes J.
      • Kaplowitz N.
      • et al.
      Feeding S-adenosyl-l-methionine attenuates both ethanol-induced depletion of mitochondrial glutathione and mitochondrial dysfunction in periportal and perivenous rat hepatocytes.
      ]; reduce hepatic fibrogenesis following carbon-tetrachloride exposure in rats [
      • Corrales F.
      • Gimenez A.
      • Alvarez L.
      • Caballeria J.
      • Pajares M.A.
      • Andreu H.
      • et al.
      S-adenosylmethionine treatment prevents carbon tetrachloride-induced S- adenosylmethionine synthetase inactivation and attenuates liver injury.
      ] (although as noted earlier, not following bile duct ligation [
      • Copple B.L.
      • Jaeschke H.
      • Klaassen C.D.
      Oxidative stress and the pathogenesis of cholestasis.
      ,
      • Gonzalez-Correa J.A.
      • De La Cruz J.P.
      • Martin-Aurioles E.
      • Lopez-Egea M.A.
      • Ortiz P.
      • Sanchez de la Cuesta F.
      Effects of S-adenosyl-l-methionine on hepatic and renal oxidative stress in an experimental model of acute biliary obstruction in rats.
      ,
      • Yang H.
      • Ramani K.
      • Xia M.
      • Ko K.S.
      • Li T.W.
      • Oh P.
      • et al.
      Dysregulation of glutathione synthesis during cholestasis in mice. molecular mechanisms and therapeutic implications.
      ]); restore hepatocyte mitochondrial inner-membrane fluidity in ethanol fed rats [
      • Colell A.
      • García-Ruiz C.
      • Morales A.
      • Ballesta A.
      • Ookhtens M.
      • Rodés J.
      • et al.
      Transport of reduced glutathione in hepatic mitochondria and mitoplasts from ethanol-treated rats: effect of membrane physical properties and S-adenosyl-l-methionine.
      ]; normalise mitochondrial glutathione handling in ethanol fed rats [
      • Colell A.
      • García-Ruiz C.
      • Morales A.
      • Ballesta A.
      • Ookhtens M.
      • Rodés J.
      • et al.
      Transport of reduced glutathione in hepatic mitochondria and mitoplasts from ethanol-treated rats: effect of membrane physical properties and S-adenosyl-l-methionine.
      ]; and prevent TNFα-mediated glutathione depletion, ameliorate steatosis, hepatocyte necrosis, and ALT elevations in ethanol fed mice [
      • Song Z.
      • Zhou Z.
      • Chen T.
      • Hill D.
      • Kang J.
      • Barve S.
      • et al.
      S-adenosylmethionine (SAMe) protects against acute alcohol induced hepatotoxicity in mice.
      ].
      Encouraged by these data, several groups have conducted clinical trials examining the efficacy of SAMe therapy across a range of different end points in patients with ALD. Many were small, observational studies with significant design flaws and have only been published in abstract form. Selected trials are summarized in Table 3. The largest of these was a 2-year Spanish multi-center study examining the effect of oral SAMe in 123 patients with cirrhosis due to alcoholic liver disease [
      • Mato J.M.
      • Camara J.
      • Fernandez de Paz J.
      • Caballeria L.
      • Coll S.
      • Caballero A.
      • et al.
      S-adenosylmethionine in alcoholic liver cirrhosis: a randomized, placebo-controlled, double-blind, multicenter clinical trial.
      ]. 62 patients (53 male, 9 female) were randomised to SAMe 1.2 g/day and 61 patients (53 male, 8 female) placebo. Of note, approximately a quarter of the patients also had chronic viral hepatitis [
      • Rambaldi A.
      • Gluud C.
      S-adenosyl-l-methionine for alcoholic liver diseases.
      ]. Outcomes studied were all-cause mortality, liver transplantation, complications of liver disease, and clinical biochemistry. This study found that a combined all-cause mortality/transplantation end point fell from 30% in the placebo arm to 16% in those treated with SAMe, however, the effect did not reach statistical significance (p = 0.077) unless those with more advanced disease (Child score C) were excluded from the analysis, 29% vs. 12% (p = 0.025) [
      • Mato J.M.
      • Camara J.
      • Fernandez de Paz J.
      • Caballeria L.
      • Coll S.
      • Caballero A.
      • et al.
      S-adenosylmethionine in alcoholic liver cirrhosis: a randomized, placebo-controlled, double-blind, multicenter clinical trial.
      ]. No significant difference between arms in the number of patients that experienced severe complications of cirrhosis was observed during the study.
      Table 3Selected clinical trials of SAMe in
      • Altomare E.
      • Vendemiale G.
      • Marchesini G.
      • Legrazie C.
      • Dipadova C.
      Increased bioavailability of sulfurated compounds after S-adenosylmethionine (same) administration to alcoholics.
      ,
      • Chawla R.K.
      • Gaetke L.
      • McClain C.J.
      S-adenosylmethionine (Adomet) and plasma methionine clearance in alcoholic subjects.
      ,
      • Cibin M.
      • Gentile N.
      • Ferri M.
      • Canton G.
      • Legrazie C.
      • Gallimberti L.
      S-adenosylmethionine (same) is effective in reducing ethanol abuse in an outpatient program for alcoholics.
      ,
      • Corrales F.
      • Pajares M.
      • Pliego M.
      • Ortiz P.
      • Moreno J.
      • Puerta J.
      • et al.
      Effect of S adenosylmethionine treatment on methionine intolerance in alcoholic cirrhosis.
      ,
      • Diaz Belmont A.
      • Dominguez Henkel R.
      • Uribe Ancira F.
      Parenteral S-adenosylmethionine compared to placebos in the treatment of alcoholic liver diseases.
      ,
      • Loguercio C.
      • Nardi G.
      • Argenzio F.
      • Aurilio C.
      • Petrone E.
      • Grella A.
      • et al.
      Effect of S-adenosyl-l-methionine administration on red blood cell cysteine and glutathione levels in alcoholic patients with and without liver disease.
      ,
      • Trespi E.
      • Vigoni R.
      • Matti C.
      • Colla C.
      TUDCA, UDCA and ademethionine (Ade) in the treatment of alcohol-induced liver damage.
      .
      Included in Cochrane systematic review
      • Rambaldi A.
      • Gluud C.
      S-adenosyl-l-methionine for alcoholic liver diseases.
      .
      #Mixed chronic liver disease including ALD.
      Published only as abstract.
      RCT, randomised controlled trial; NR, non-randomised.
      An update of the 2001 systematic review by members of the Cochrane Collaboration identified nine randomised control trials that met their inclusion criteria (Table 3) [
      • Rambaldi A.
      • Gluud C.
      S-adenosyl-l-methionine for alcoholic liver diseases.
      ]. These studies, including the trial discussed above, which was the only study considered to have used adequate methodology, recruited a total of 434 patients. No significant effect of SAMe on all cause mortality (relative risk (RR) 0.62, 95% confidence interval (CI) 0.30–1.26); liver-related mortality (RR 0.68, 95% CI 0.31–1.48); combined all-cause mortality or transplantation RR 0.55, 95% CI 0.27–1.09); or liver-related complications (RR 1.35, 95% CI 0.84–2.16) was identified [
      • Rambaldi A.
      • Gluud C.
      S-adenosyl-l-methionine for alcoholic liver diseases.
      ]. The authors concluded that the available evidence was insufficient to either support or refute the benefit of SAMe therapy in ALD and that, to address this, there was a need for a large, well conducted, placebo-controlled, randomised clinical trial that stratified patients not only by alcohol consumption but also by presence of co-existent diseases such as chronic viral hepatitis.
      A recent North American study, published since the Cochrane analysis was completed, randomised 37 abstinent ALD patients to 1.2 g/day SAMe or placebo for 6 months. Eleven patients were excluded from the final analysis due to on-going alcohol consumption but 14 of the remaining patients had undergone paired liver biopsies at the start and end of the study. Fasting serum SAMe levels were found to have increased during the course of the study in the SAMe treatment group. The entire cohort showed an overall improvement of AST, ALT, and bilirubin levels after 24 weeks, most probably due to abstinence from alcohol, but no differences between the treatment groups in any clinical, biochemical or histological parameters (steatosis, inflammation, fibrosis, and Mallory–Denk hyaline bodies) were identified [
      • Medici V.
      • Virata M.C.
      • Peerson J.M.
      • Stabler S.P.
      • French S.W.
      • Gregory 3rd, J.F.
      • et al.
      S-adenosyl-l-methionine treatment for alcoholic liver disease: a double-blinded, randomized, placebo-controlled trial.
      ]. Although this study is relatively small and arguably 6-month follow-up is too little time for substantial histological changes in fibrosis to occur, the methodology used was sound. The results led the authors to conclude that ‘SAMe was no more effective than placebo’ in the treatment of ALD [
      • Halsted C.H.
      • Medici V.
      Vitamin-dependent methionine metabolism and alcoholic liver disease.
      ,
      • Medici V.
      • Virata M.C.
      • Peerson J.M.
      • Stabler S.P.
      • French S.W.
      • Gregory 3rd, J.F.
      • et al.
      S-adenosyl-l-methionine treatment for alcoholic liver disease: a double-blinded, randomized, placebo-controlled trial.
      ]. It is interesting to note that, one previous study has also performed biopsies pre- and post-6 months of SAMe therapy [
      • Vendemiale G.
      • Altomare E.
      • Trizio T.
      • Le Grazie C.
      • Di Padova C.
      • Salerno M.T.
      • et al.
      Effects of oral S-adenosyl-l-methionine on hepatic glutathione in patients with liver disease.
      ]. However, the authors did not present any histological data (despite stating in the methods section that tissue underwent histopathological assessment), only showing data that demonstrated SAMe increased intra-hepatic glutathione and improved clinical biochemistry [
      • Vendemiale G.
      • Altomare E.
      • Trizio T.
      • Le Grazie C.
      • Di Padova C.
      • Salerno M.T.
      • et al.
      Effects of oral S-adenosyl-l-methionine on hepatic glutathione in patients with liver disease.
      ].

      Non-alcoholic fatty liver disease

      Non-alcoholic fatty liver disease (NAFLD), represents a spectrum encompassing steatosis, non-alcoholic steatohepatitis (NASH) and cirrhosis in the absence of alcohol abuse and has been recognised as the most common cause of liver dysfunction in the majority of developed countries [
      • Anstee Q.M.
      • Daly A.K.
      • Day C.P.
      Genetics of alcoholic and nonalcoholic fatty liver disease.
      ]. NAFLD is strongly associated with obesity, insulin resistance or type 2 diabetes mellitus and dyslipidaemia and so may be considered the hepatic manifestation of the metabolic syndrome [
      • Sanyal A.J.
      American gastroenterological A. AGA technical review on nonalcoholic fatty liver disease.
      ,
      • Marchesini G.
      • Brizi M.
      • Morselli-Labate A.M.
      • Bianchi G.
      • Bugianesi E.
      • McCullough A.J.
      • et al.
      Association of nonalcoholic fatty liver disease with insulin resistance.
      ,
      • Powell E.E.
      • Cooksley W.G.
      • Hanson R.
      • Searle J.
      • Halliday J.W.
      • Powell L.W.
      The natural history of nonalcoholic steatohepatitis: a follow-up study of forty-two patients for up to 21 years.
      ]. Perhaps unsurprisingly, given the gross histological similarities between ALD and NAFLD, there is now increasing evidence to suggest that both conditions share common pathogenic processes [
      • Nagata K.
      • Suzuki H.
      • Sakaguchi S.
      Common pathogenic mechanism in development progression of liver injury caused by non-alcoholic or alcoholic steatohepatitis.
      ]. The initiating events in NAFLD are founded on the development of obesity and insulin resistance and increased hepatic free fatty acid (FFA) flux. This imbalance between the rate of import/synthesis and the rate of export/catabolism of fatty acids leads to development of steatosis, which represents an adaptive response through which potentially lipotoxic FFAs are partitioned into relatively stable triglyceride stores [
      • Yamaguchi K.
      • Yang L.
      • McCall S.
      • Huang J.
      • Yu X.X.
      • Pandey S.K.
      • et al.
      Inhibiting triglyceride synthesis improves hepatic steatosis but exacerbates liver damage and fibrosis in obese mice with nonalcoholic steatohepatitis.
      ]. When these are overwhelmed, steatohepatitis develops due to the increased oxidative stress produced during β- and ω-fatty acid oxidation; direct lipotoxicity; endotoxin/TLR4 induced activation of the innate immune system causing cytokine release; and endoplasmic reticulum (ER) stress. Consequent cellular damage triggers a mixture of immune mediated hepatocellular injury and both necrotic and apoptotic cell death pathways culminating in hepatic fibrosis [
      • Malhi H.
      • Gores G.J.
      • Lemasters J.J.
      Apoptosis and necrosis in the liver: a tale of two deaths?.
      ,
      • Anstee Q.M.
      • Concas D.
      • Kudo H.
      • Levene A.
      • Pollard J.
      • Charlton P.
      • et al.
      Impact of pan-caspase inhibition in animal models of established steatosis and non-alcoholic steatohepatitis.
      ,
      • Farrell G.C.
      • Larter C.Z.
      • Hou J.Y.
      • Zhang R.H.
      • Yeh M.M.
      • Williams J.
      • et al.
      Apoptosis in experimental NASH is associated with p53 activation and TRAIL receptor expression.
      ,
      • Iredale J.P.
      Models of liver fibrosis: exploring the dynamic nature of inflammation and repair in a solid organ.
      ].

      Pre-clinical and clinical therapeutic evidence

      SAMe may influence the pathogenesis of NAFLD both through its role as a precursor for glutathione synthesis and also as a methyl donor in the synthesis of phosphatidylcholine, which is required for VLDL assembly and hepatic triglyceride export. Evidence of a role for methionine metabolism and SAMe in the pathogenesis of NAFLD has largely been based on the study of pre-clinical models. Prolonged consumption of a methionine-choline deficient (MCD) diet is associated with hepatic SAMe depletion and development of a histological fibrosing steatohepatitis in rodents [
      • Anstee Q.M.
      • Goldin R.D.
      Mouse models in non-alcoholic fatty liver disease and steatohepatitis research.
      ,
      • Wortham M.
      • He L.
      • Gyamfi M.
      • Copple B.L.
      • Wan Y.J.
      The transition from fatty liver to NASH associates with SAMe depletion in db/db mice fed a methionine choline-deficient diet.
      ]. Similar changes are observed in MATO mice that, as discussed earlier, lack Mat1a and so are unable to synthesize SAMe [
      • Lu S.C.
      • Alvarez L.
      • Huang Z.Z.
      • Chen L.
      • An W.
      • Corrales F.J.
      • et al.
      Methionine adenosyltransferase 1A knockout mice are predisposed to liver injury and exhibit increased expression of genes involved in proliferation.
      ,
      • Anstee Q.M.
      • Goldin R.D.
      Mouse models in non-alcoholic fatty liver disease and steatohepatitis research.
      ]. Investigation of the mechanisms underlying these observations demonstrates that, even before NAFLD is histologically apparent, there is a potent effect of Mat1a deletion on lipid handling [
      • Cano A.
      • Buque X.
      • Martinez-Una M.
      • Aurrekoetxea I.
      • Menor A.
      • Garcia-Rodriguez J.L.
      • et al.
      Methionine adenosyltransferase 1A gene deletion disrupts hepatic very low-density lipoprotein assembly in mice.
      ]. Three-month old MATO mice exhibited decreased mobilization of TG stores, TG secretion in VLDL, and phosphatidylcholine synthesis via phosphatidylethanolamine N-methyltransferase. In addition, the authors report that administration of SAMe for 7 days was sufficient to rectify the deficits in VLDL assembly and features of the secreted lipoproteins [
      • Cano A.
      • Buque X.
      • Martinez-Una M.
      • Aurrekoetxea I.
      • Menor A.
      • Garcia-Rodriguez J.L.
      • et al.
      Methionine adenosyltransferase 1A gene deletion disrupts hepatic very low-density lipoprotein assembly in mice.
      ]. They conclude that Mat1a is required for normal VLDL assembly and plasma lipid homeostasis in mice and that impaired VLDL synthesis, mainly due to SAMe deficiency, contributes to NAFLD development in MATO mice [
      • Cano A.
      • Buque X.
      • Martinez-Una M.
      • Aurrekoetxea I.
      • Menor A.
      • Garcia-Rodriguez J.L.
      • et al.
      Methionine adenosyltransferase 1A gene deletion disrupts hepatic very low-density lipoprotein assembly in mice.
      ]. A recent human study has attempted to determine the extent to which methionine metabolism and SAMe contribute to the pathogenesis of NASH in man [
      • Kalhan S.C.
      • Edmison J.
      • Marczewski S.
      • Dasarathy S.
      • Gruca L.L.
      • Bennett C.
      • et al.
      Methionine and protein metabolism in non-alcoholic steatohepatitis: evidence for lower rate of transmethylation of methionine.
      ]. Studying a cohort of 15 patients with biopsy proven NASH and 19 healthy controls, the authors determined that the rates of remethylation of homocysteine and transmethylation of methionine (Fig. 1) were significantly reduced in NASH and hypothesised that this may in part be due to inactivation of MATI/III by increased oxidative stress [
      • Kalhan S.C.
      • Edmison J.
      • Marczewski S.
      • Dasarathy S.
      • Gruca L.L.
      • Bennett C.
      • et al.
      Methionine and protein metabolism in non-alcoholic steatohepatitis: evidence for lower rate of transmethylation of methionine.
      ].
      No pharmacological agents are currently licenced specifically for NASH therapy and as yet there have been no clinical studies that address the utility of SAMe in this condition [
      • Anstee Q.M.
      • McPherson S.
      • Day C.P.
      How big a problem is non-alcoholic fatty liver disease?.
      ]. Several studies have, however, reported encouraging results with the use of the anti-oxidant vitamin E [
      • Sanyal A.J.
      • Chalasani N.
      • Kowdley K.V.
      • McCullough A.
      • Diehl A.M.
      • Bass N.M.
      • et al.
      Pioglitazone, vitamin E, or placebo for nonalcoholic steatohepatitis.
      ,
      • Lavine J.E.
      • Schwimmer J.B.
      • Van Natta M.L.
      • Molleston J.P.
      • Murray K.F.
      • Rosenthal P.
      • et al.
      Effect of vitamin E or metformin for treatment of nonalcoholic fatty liver disease in children and adolescents: the TONIC randomized controlled trial.
      ] and so it may be that the anti-oxidant effects of SAMe via augmenting hepatic glutathione synthesis could provide clinical benefit. The methyl donor betaine (trimethylglycine) is required for BMHT-mediated remethylation of homocysteine to methionine and has been shown to reduce hepatic SAH and increase SAMe in animal models (Fig. 1) [
      • Purohit V.
      • Abdelmalek M.F.
      • Barve S.
      • Benevenga N.J.
      • Halsted C.H.
      • Kaplowitz N.
      • et al.
      Role of S-adenosylmethionine, folate, and betaine in the treatment of alcoholic liver disease: summary of a symposium.
      ]. Betaine has been tested as a therapy for NASH in two clinical trials, an initial pilot study that yielded positive results and a subsequent large randomised placebo-control trial, which did not [
      • Abdelmalek M.F.
      • Angulo P.
      • Jorgensen R.A.
      • Sylvestre P.B.
      • Lindor K.D.
      Betaine, a promising new agent for patients with nonalcoholic steatohepatitis: results of a pilot study.
      ,
      • Abdelmalek M.F.
      • Sanderson S.O.
      • Angulo P.
      • Soldevila-Pico C.
      • Liu C.
      • Peter J.
      • et al.
      Betaine for nonalcoholic fatty liver disease: results of a randomized placebo-controlled trial.
      ]. This second study randomised 55 patients with biopsy proven NASH to betaine (20 g/day) or placebo for 1 year. The analysis demonstrated that betaine therapy effectively increased serum methionine and SAMe levels but did not significantly reduce SAH or ameliorate histological steatohepatitis [
      • Abdelmalek M.F.
      • Sanderson S.O.
      • Angulo P.
      • Soldevila-Pico C.
      • Liu C.
      • Peter J.
      • et al.
      Betaine for nonalcoholic fatty liver disease: results of a randomized placebo-controlled trial.
      ]. It would seem timely to formally assess the efficacy of direct SAMe supplementation in NAFLD.

      Viral hepatitis

      Several of the studies so far discussed have included in the cohorts a significant number of patients with viral hepatitis and have indicated that SAMe may be effective in IHC of viral aetiology. An effect supported in a viral hepatitis cohort by an open-label study comparing SAMe and traditional Chinese remedies for the treatment of jaundice [
      • Bao-en W.
      Ademetionine 1,4 butanedisulphonate vs traditional Chinese medicine for the treatment of hepatocellular jaundice complicating chronic viral hepatitis.
      ]. Arguably of greater interest, however, are recent studies demonstrating that SAMe may be an effective adjunctive therapy in the treatment of chronic hepatitis C (HCV).

      Pre-clinical and clinical therapeutic evidence

      Despite recent advances in our understanding of genetic modifiers of host response to Pegylated IFNα/Ribavarin based anti-viral regimens [
      • Tanaka Y.
      • Nishida N.
      • Sugiyama M.
      • Kurosaki M.
      • Matsuura K.
      • Sakamoto N.
      • et al.
      Genome-wide association of IL28B with response to pegylated interferon-alpha and ribavirin therapy for chronic hepatitis C.
      ,
      • Suppiah V.
      • Moldovan M.
      • Ahlenstiel G.
      • Berg T.
      • Weltman M.
      • Abate M.L.
      • et al.
      IL28B is associated with response to chronic hepatitis C interferon-alpha and ribavirin therapy.
      ,
      • Ge D.
      • Fellay J.
      • Thompson A.J.
      • Simon J.S.
      • Shianna K.V.
      • Urban T.J.
      • et al.
      Genetic variation in IL28B predicts hepatitis C treatment-induced viral clearance.
      ], the mechanisms that determine response remain incompletely understood. One mechanism appears to be HCV-induced viral interference with IFNα signal transduction and Jak-STAT signalling due to STAT1 hypomethylation, facilitating binding of its inhibitor, protein inhibitor of activated STAT1 (PIAS1). [
      • Duong F.H.
      • Filipowicz M.
      • Tripodi M.
      • La Monica N.
      • Heim M.H.
      Hepatitis C virus inhibits interferon signaling through up-regulation of protein phosphatase 2A.
      ]. This effect is reversed in vitro by SAMe and betaine which restore STAT1 methylation, improve IFNα signalling and enhance the antiviral effect of IFNα in cell culture [
      • Duong F.H.
      • Christen V.
      • Filipowicz M.
      • Heim M.H.
      S-adenosylmethionine and betaine correct hepatitis C virus induced inhibition of interferon signaling in vitro.
      ]. These effects have also been described in vivo. In an open-label pilot study of combined SAMe (1200 mg/day) and betaine (6 g/day) treatment in HCV previous non-responders, there was no evidence of any direct anti-viral effect of SAMe, however, SAMe treatment was associated with an increase in early virological response but this did not translate into greater sustained virological response [
      • Filipowicz M.
      • Bernsmeier C.
      • Terracciano L.
      • Duong F.H.
      • Heim M.H.
      S-adenosyl-methionine and betaine improve early virological response in chronic hepatitis C patients with previous nonresponse.
      ]. A further clinical study in a group of 24 HCV genotype 1 non-responders found that addition of SAMe was associated with improved viral kinetics in the second-phase slope of viral decline (Control 0.11 ± 0.04 log10 IU/ml/wk vs. SAMe 0.27 ± 0.06; p = 0.009) and higher rates of early and sustained viral clearance [
      • Feld J.J.
      • Modi A.A.
      • El-Diwany R.
      • Rotman Y.
      • Thomas E.
      • Ahlenstiel G.
      • et al.
      S-adenosyl methionine improves early viral responses and interferon-stimulated gene induction in hepatitis C nonresponders.
      ]. This effect was associated with significantly greater interferon-stimulated gene (ISG) expression in peripheral blood mononuclear cells (PBMCs) and, once again, SAMe was shown to increase induction of ISGs and the antiviral effects of interferon by increasing STAT1 methylation in vitro [
      • Feld J.J.
      • Modi A.A.
      • El-Diwany R.
      • Rotman Y.
      • Thomas E.
      • Ahlenstiel G.
      • et al.
      S-adenosyl methionine improves early viral responses and interferon-stimulated gene induction in hepatitis C nonresponders.
      ]. Whilst these effects will need to be replicated in large randomised-controlled trails, the authors conclude that SAMe represents the first interferon sensitising agent with in vivo efficacy and that it may be a useful adjunct to interferon-based therapy [
      • Feld J.J.
      • Modi A.A.
      • El-Diwany R.
      • Rotman Y.
      • Thomas E.
      • Ahlenstiel G.
      • et al.
      S-adenosyl methionine improves early viral responses and interferon-stimulated gene induction in hepatitis C nonresponders.
      ].
      A study examining the carcinogenic effects of hepatitis B (HBV) showed that the hepatocellular cytoplasmic level of HBV X protein (HBx) was strongly correlated with MAT2A expression in HCC samples [
      • Liu Q.
      • Chen J.
      • Liu L.
      • Zhang J.
      • Wang D.
      • Ma L.
      • et al.
      The X protein of hepatitis B virus inhibits apoptosis in hepatoma cells through enhancing the methionine adenosyltransferase 2A gene expression and reducing S-adenosylmethionine production.
      ]. In vitro, HBx was shown to reduce MAT1A gene expression and SAMe production whilst directly activating MAT2A expression in a dose-dependent manner through binding of NF-κB and CREB to the MAT2A gene promoter. In addition, HBx and/or overexpression of MAT2A were able to inhibit apoptosis in HCC cells [
      • Liu Q.
      • Chen J.
      • Liu L.
      • Zhang J.
      • Wang D.
      • Ma L.
      • et al.
      The X protein of hepatitis B virus inhibits apoptosis in hepatoma cells through enhancing the methionine adenosyltransferase 2A gene expression and reducing S-adenosylmethionine production.
      ]. Although this has not been formally examined, these data offer a biological rationale for the use of SAMe to prevent HCC in HBV. Clinical trials to validate this are, however, required.

      Conclusions and future directions

      In conclusion, there is strong pre-clinical evidence that SAMe has important physiological roles in health and that liver disease of various aetiologies may perturb these. Furthermore, it is apparent that hepatocellular SAMe concentration can influence diverse pathophysiological processes including tissue oxidative stress, mitochondrial function, hepatocellular apoptosis, and malignant transformation, not to mention the intriguing data suggesting that chronic viral hepatitis may modulate interferon sensitivity through SAMe. These data suggest that SAMe could offer substantial clinical benefits, however, very few large, high-quality clinical trials have been performed to prove or refute this. The challenge is now to address this evidence deficit. We suggest that, rather than examining abstract features of IHC, future clinical studies should be conducted in defined, well-characterised patient groups and should be focussed on the effects of SAMe on clinically relevant ‘hard end points’, possibly in three key areas where there is a good pre-clinical rationale for efficacy:
      • The role of SAMe, alone or in combination with vitamin B6/B12 and folate supplementation [
        • Halsted C.H.
        • Medici V.
        Vitamin-dependent methionine metabolism and alcoholic liver disease.
        ], in the treatment of inflammation in NASH;
      • The utility of SAMe as an adjunct to pegylated IFNα/Ribavarin based anti-viral therapy in chronic HCV infection. This would be of particular interest in HCV genotype 4 patients which have low sustained viral response rates with standard therapy and are currently under-represented in trials of novel protease inhibitors;
      • The value of SAMe as prophylaxis to reduce the incidence of HCC in chronic liver disease (e.g. HBV).
      It is hoped that, by addressing these, the therapeutic potential of SAMe can be translated from bench to the bedside.

      Conflict of interest

      CPD has received consultancy fees and both authors have received speakers’ fees from Abbott Laboratories. The authors declare that this work was supported by an educational grant from Abbott Laboratories.

      Acknowledgements

      Q.M.A. is the recipient of a Clinical Senior Lectureship Award from the Higher Education Funding Council for England (HEFCE). This work was supported by an educational grant from Abbott Laboratories.

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