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

Positive Regulation of S-Adenosylmethionine on Chondrocytic Differentiation via Stimulation of Polyamine Production and the Gene Expression of Chondrogenic Differentiation Factors

S-adenosylmethionine (SAM) is considered to be a useful therapeutic agent for degenerative cartilage diseases, although its mechanism is not clear. We previously found that polyamines stimulate the expression of differentiated phenotype of chondrocytes. We also found that the cellular communication network factor 2 (CCN2) played a huge role in the proliferation and differentiation of chondrocytes. Therefore, we hypothesized that polyamines and CCN2 could be involved in the chondroprotective action of SAM. In this study, we initially found that exogenous SAM enhanced proteoglycan production but not cell proliferation in human chondrocyte-like cell line-2/8 (HCS-2/8) cells. Moreover, SAM enhanced gene expression of cartilage-specific matrix (aggrecan and type II collagen), Sry-Box transcription factor 9 (SOX9), CCN2, and chondroitin sulfate biosynthetic enzymes. The blockade of the methionine adenosyltransferase 2A (MAT2A) enzyme catalyzing intracellular SAM biosynthesis restrained the effect of SAM on chondrocytes. The polyamine level in chondrocytes was higher in SAM-treated culture than control culture. Additionally, Alcian blue staining and RT-qPCR indicated that the effects of SAM on the production and gene expression of aggrecan were reduced by the inhibition of polyamine synthesis. These results suggest that the stimulation of polyamine synthesis and gene expression of chondrogenic differentiation factors, such as CCN2, account for the mechanism underlying the action of SAM on chondrocytes.

Structure-function conservation between the methyltransferases SETD3 and SETD6

Among the protein lysine methyltransferases family members, it appears that SETD6 is highly similar and closely related to SETD3. The two methyltransferases show high similarity in their structure, which raised the hypothesis that they share cellular functions. Using a proteomic screen, we identified 52 shared interacting-proteins. Gene Ontology (GO) analysis of the shared proteins revealed significant enrichment of proteins involved in transcription. Our RNA-seq data of SETD6 KO and SETD3 KO HeLa cells identified ∼100 up-regulated and down-regulated shared genes. We have also identified a substantial number of genes that changed dramatically in the double KO cells but did not significantly change in the single KO cells. GO analysis of these genes revealed enrichment of apoptotic genes. Accordingly, we show that the double KO cells displayed high apoptotic levels, suggesting that SETD6 and SETD3 inhibit apoptosis. Collectively, our data strongly suggest a functional link between SETD6 and SETD3 in the regulation of apoptosis.

SAM homeostasis is regulated by CFIm-mediated splicing of MAT2A

S-adenosylmethionine (SAM) is the methyl donor for nearly all cellular methylation events. Cells regulate intracellular SAM levels through intron detention of MAT2A, the only SAM synthetase expressed in most cells. The N6-adenosine methyltransferase METTL16 promotes splicing of the MAT2A detained intron by an unknown mechanism. Using an unbiased CRISPR knock-out screen, we identified CFIm25 (NUDT21) as a regulator of MAT2A intron detention and intracellular SAM levels. CFIm25 is a component of the cleavage factor Im (CFIm) complex that regulates poly(A) site selection, but we show it promotes MAT2A splicing independent of poly(A) site selection. CFIm25-mediated MAT2A splicing induction requires the RS domains of its binding partners, CFIm68 and CFIm59 as well as binding sites in the detained intron and 3´ UTR. These studies uncover mechanisms that regulate MAT2A intron detention and reveal a previously undescribed role for CFIm in splicing and SAM metabolism.

Protein Arginine Methyltransferase 5 in T Lymphocyte Biology

Protein arginine methyltransferase 5 (PRMT5) is the major methyltransferase (MT) catalyzing symmetric dimethylation (SDM). PRMT5 regulates developmental, homeostatic and disease processes in vertebrates and invertebrates, and a carcinogenic role has been observed in mammals. Recently, tools generated for PRMT5 loss of function have allowed researchers to demonstrate essential roles for PRMT5 in mouse and human lymphocyte biology. PRMT5 modulates CD4⁺ and CD8⁺ T cell development in the thymus, peripheral homeostasis, and differentiation into CD4⁺ helper T lymphocyte (Th)17 cell phenotypes. Here, we provide a timely review of the milestones leading to our current understanding of PRMT5 in T cell biology, discuss current tools to modify PRMT5 expression/activity, and highlight mechanistic pathways.

Canonical Wnt is inhibited by targeting one-carbon metabolism through methotrexate or methionine deprivation

Metabolism, one of the most conserved features across all domains of life, is interwoven with cellular signaling networks and the posttranslational modification state of proteins. Methionine is an essential amino acid in one-carbon metabolism that serves as the substrate for S-adenosylmethionine, the universal methyl donor for cellular methylation. Here we report that the use of methionine-depleted medium inhibits Wnt-induced endolysosomal formation and canonical Wnt/β-catenin signaling. Use of methotrexate, a folate cycle inhibitor used in cancer chemotherapy, prevented PRMT1 and GSK3 vesicular sequestration, decreased Wnt-driven endolysosomal activity, and reduced canonical Wnt β-catenin luciferase signaling. In sum, these results imply a role for one-carbon metabolism in the regulation of Wnt signaling and endolysosomal biology.

The nutrient-sensing metabolite S-adenosylmethionine (SAM) controls one-carbon metabolism by donating methyl groups to biochemical building blocks, DNA, RNA, and protein. Our recent work uncovered a requirement for cytoplasmic arginine methylation during Wnt signaling through the activity of protein arginine methyltransferase 1 (PRMT1), which transfers one-carbon groups from SAM to many protein substrates. Here, we report that treatments that decrease levels of the universal methyl donor SAM were potent inhibitors of Wnt signaling and of Wnt-induced digestion of extracellular proteins in endolysosomes. Thus, arginine methylation provides the canonical Wnt pathway with metabolic sensing properties through SAM. The rapid accumulation of Wnt-induced endolysosomes within 30 minutes was inhibited by the depletion of methionine, an essential amino acid that serves as the direct substrate for SAM production. We also found that methionine is required for GSK3 sequestration into multivesicular bodies through microautophagy, an essential step in Wnt signaling activity. Methionine starvation greatly reduced Wnt-induced endolysosomal degradation of extracellular serum proteins. Similar results were observed by addition of nicotinamide (vitamin B3), which serves as a methyl group sink. Methotrexate, a pillar in the treatment of cancer since 1948, decreases SAM levels. We show here that methotrexate blocked Wnt-induced endocytic lysosomal activity and reduced canonical Wnt signaling. Importantly, the addition of SAM during methionine depletion or methotrexate treatment was sufficient to rescue endolysosomal function and Wnt signaling. Inhibiting the Wnt signaling pathway by decreasing one-carbon metabolism provides a platform for designing interventions in Wnt-driven disease.

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

SIGNIFICANCE

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

CRITICAL ISSUES

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

FUTURE DIRECTIONS

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

The use of SAMe in chemotherapy-induced liver injury

Drug-induced liver injury (DILI) remains the most common cause of acute liver failure in the Western world. Chemotherapy is one of the major class of drugs most frequently associated with idiosyncratic DILI. For this reason, patients who receive chemotherapy require careful assessment of liver function prior to treatment to determine which drugs may not be appropriate and which drug doses should be modified. S-adenosylmethionine (SAMe) is an endogenous agent derived from methionine. Its supplementation is effective in the treatment of liver disease, in particular intrahepatic cholestasis (IHC). The target of this review is to analyze the mechanisms of hepatotoxicity of the principal anticancer agents and the role of SAMe in the prevention of this complication.

S-adenosylmethionine and methylthioadenosine inhibit cancer metastasis by targeting microRNA 34a/b-methionine adenosyltransferase 2A/2B axis

MicroRNA-34a (miR-34a) is down-regulated in colorectal cancers (CRC) and required for interleukin-6 (IL-6)-induced CRC metastasis. Mice lacking miR-34a developed more invasive cancer in a colitis-associated cancer model. In the same model, S-adenosylmethionine (SAMe) and methylthioadenosine (MTA) inhibited IL-6/STAT3 and lowered tumor burden. SAMe and MTA reduce the expression of methionine adenosyltransferase 2A (MAT2A) and there are consensus binding sites for miR-34a/b in the MAT2A 3’UTR. Here we examined whether SAMe/MTA influence miR-34a/b expression and cancer metastasis. We found SAMe and MTA raised miR-34a/b expression in CRC cell lines, inhibited migration and invasion in vitro and liver metastasis in vivo. Like CRC, MAT2A and MAT2B expression is induced in human pancreas and prostate cancers. Treatment with SAMe, MTA, miR-34a or miR-34b inhibited MAT2A expression mainly at the protein level. MAT2B protein level also fell because MAT2A and MAT2B enhance each other’s protein stability. Overexpressing miR-34a or miR-34b inhibited while MAT2A or MAT2B enhanced CRC migration and invasion. Co-expressing either miR-34a/b had minimal to no effect on MAT2A/MAT2B’s ability to increase migration, invasion and growth. Taken together, MAT2A and MAT2B are important targets of miR-34a/b and SAMe and MTA target this axis, suppressing MAT2A/MAT2B while raising miR-34a/b expression, inhibiting cancer metastasis.

A chemical probe toolbox for dissecting the cancer epigenome

Cancer cell hallmarks are underpinned by transcriptional programmes operating in the context of a dynamic and complicit epigenomic environment. Somatic alterations of chromatin modifiers are among the most prevalent cancer perturbations. There is a pressing need for targeted chemical probes to dissect these complex, interconnected gene regulatory circuits. Validated chemical probes empower mechanistic research while providing the pharmacological proof of concept that is required to translate drug-like derivatives into therapy for cancer patients. In this Review, we describe chemical probe development for epigenomic effector proteins that are linked to cancer pathogenesis. By annotating these reagents, we aim to share our perspectives on an informative 'epigenomic toolbox' of broad utility to the research community.

Octreotide prevents liver failure through upregulating 5'-methylthioadenosine in extended hepatectomized rats

BACKGROUND & AIMS

Insufficient liver regeneration and hepatocyte injury caused by excessive portal perfusion are considered to be responsible for post-hepatectomy liver failure (PLF) or small-for-size syndrome in living-donor liver transplantation. Somatostatin can decrease portal vein pressure (PVP) but simultaneously inhibits liver regeneration. This interesting paradox motivated us to investigate the outcome of PLF in response to somatostatin treatment.

METHODS

Rats receiving extended partial hepatectomy (90% PH) were treated with octreotide, a somatostatin analogue, or placebo. Animal survival, serum parameters and hepatic histology were evaluated. Metabolomic analysis was performed to investigate the effect of octreotide on hepatocyte metabolism.

RESULTS

Despite significantly inhibiting early regeneration, octreotide application noticeably improved the hepatic histology, liver function and survival after PH but did not decrease the PVP level. Metabolomic analysis exhibited that octreotide profoundly and exclusively altered the levels of five metabolites that participate in or closely associate with the methionine cycle, a biochemical reaction that uniquely produces S-adenosylmethionine (SAMe), an active methyl residual donor for methyltransferase reactions. Among these metabolites, 5'-methylthioadenosine (MTA), a derivate of SAMe, increased three-fold and was found independently improve the hepatic histology and reduce inflammatory cytokines in hepatectomized rats.

CONCLUSIONS

Octreotide exclusively regulates the methionine cycle reaction and augments the MTA level in hepatocytes. MTA prominently protects hepatocytes against shear stress injury and reduces the secondary inflammation, thereby protecting rats from PLF.

Lupus autoimmunity altered by cellular methylation metabolism

Modifications of both DNA and protein by methylation are key factors in normal T and B cell immune responses as well as in the development of autoimmune disease. For example, the failure to maintain the methylation status of CpG dinucleotides in DNA triggers T cell autoreactivity. Methylated proteins are known targets of autoimmunity, including the symmetrical dimethylarginine residues of SmD1 and SmD3 in SLE. Herein, we demonstrate that altering the metabolism of S-adenosylmethionine (SAM), the major methyl donor for transmethylation reactions, can suppress T cell immunity. A by-product of SAM metabolism, 5'-deoxy-5'-methylthioadenosine (MTA), and an indirect inhibitor of methyltransferases, inhibits T cell responses including T cell activation markers, Th1/Th2 cytokines and TCR-related signaling events. Moreover, treatment of the lupus-prone MRL/lpr mouse with MTA markedly ameliorates splenomegaly, lymphadenopathy, autoantibody titers as well as IgG deposition and cellular infiltration in the kidney. Incubation of cells with SAM, which increases intracellular MTA levels, inhibits both TCR-mediated T cell proliferation and BCR (anti-IgM)-triggered B cell proliferation in a dose-dependent manner. These studies define the central role of MTA and SAM in immune responses and provide a simple approach to altering lymphocyte transmethylation and T cell mediated autoimmune syndromes.

S-adenosylmethionine in liver health, injury, and cancer

S-adenosylmethionine (AdoMet, also known as SAM and SAMe) is the principal biological methyl donor synthesized in all mammalian cells but most abundantly in the liver. Biosynthesis of AdoMet requires the enzyme methionine adenosyltransferase (MAT). In mammals, two genes, MAT1A that is largely expressed by normal liver and MAT2A that is expressed by all extrahepatic tissues, encode MAT. Patients with chronic liver disease have reduced MAT activity and AdoMet levels. Mice lacking Mat1a have reduced hepatic AdoMet levels and develop oxidative stress, steatohepatitis, and hepatocellular carcinoma (HCC). In these mice, several signaling pathways are abnormal that can contribute to HCC formation. However, injury and HCC also occur if hepatic AdoMet level is excessive chronically. This can result from inactive mutation of the enzyme glycine N-methyltransferase (GNMT). Children with GNMT mutation have elevated liver transaminases, and Gnmt knockout mice develop liver injury, fibrosis, and HCC. Thus a normal hepatic AdoMet level is necessary to maintain liver health and prevent injury and HCC. AdoMet is effective in cholestasis of pregnancy, and its role in other human liver diseases remains to be better defined. In experimental models, it is effective as a chemopreventive agent in HCC and perhaps other forms of cancer as well.

Effects of S-adenosylmethionine and methylthioadenosine on inflammation-induced colon cancer in mice

Chronic inflammation is an underlying risk factor for colon cancer. Tumor necrosis factor alpha (TNF-α) plays a critical role in the development of inflammation-induced colon cancer in a mouse model. S-adenosylmethionine (SAMe) and its metabolite methylthioadenosine (MTA) can inhibit lipopolysaccharide-induced TNF-α expression in macrophages. The aim of this work was to examine whether SAMe and MTA are effective in preventing inflammation-induced colon cancer and if so identify signaling pathways affected. Balb/c mice were treated with azoxymethane (AOM) and dextran sulfate sodium to induce colon cancer. Two days after AOM treatment, mice were divided into three groups: vehicle control, SAMe or MTA. Tumor load, histology, immunohistochemistry, gene and protein expression were determined. SAMe and MTA treatment reduced tumor load by ∼40%. Both treatments raised SAMe and MTA levels but MTA also raised S-adenosylhomocysteine levels. MTA treatment prevented the induction of many genes known to play pathogenetic roles in this model except for TNF-α and inducible nitric oxide synthase (iNOS). SAMe also had no effect on TNF-α or iNOS and was less inhibitory than MTA on the other genes. In vivo, both treatments induced apoptosis but inhibited proliferation, β-catenin, nuclear factor kappa B activation and interleukin (IL) 6 signaling. Effect of SAMe and MTA on IL-6 signaling was examined using Colo 205 colon cancer cells. In these cells, SAMe and MTA inhibited IL-6-induced IL-10 expression. MTA also inhibited IL-10 transcription and signal transducer and activator of transcription 3 activation. In conclusion, SAMe and MTA reduced inflammation-induced colon cancer and inhibited several pathways important in colon carcinogenesis.

Role of methionine adenosyltransferase genes in hepatocarcinogenesis

Hepatocellular carcinoma (HCC) is the most common primary malignant tumor of the liver. Detection of HCC can be difficult, as most of the patients who develop this tumor have no symptoms other than those related to their longstanding liver disease. There is an urgent need to understand the molecular mechanisms that are responsible for the development of this disease so that appropriate therapies can be designed. Methionine adenosyltransferase (MAT) is an essential enzyme required for the biosynthesis of S-adenosylmethionine (AdoMet), an important methyl donor in the cell. Alterations in the expression of MAT genes and a decline in AdoMet biosynthesis are known to be associated with liver injury, cirrhosis and HCC. This review focuses on the role of MAT genes in HCC development and the scope for therapeutic strategies using these genes.

Forced expression of methionine adenosyltransferase 1A in human hepatoma cells suppresses in vivo tumorigenicity in mice

Methionine adenosyltransferase (MAT) catalyzes the synthesis of S-adenosylmethionine, the principal methyl donor, and is encoded by MAT1A and MAT2A in mammals. Normal liver expresses MAT1A, which is silenced in hepatocellular carcinoma. We have shown that hepatoma cells overexpressing MAT1A grew slower, but whether this is also true in vivo remains unknown. To investigate the effect of overexpressing MAT1A on in vivo tumorigenesis, we generated stable transfectants of Huh7 cells overexpressing either MAT1A or empty vector. Real-time PCR and Western blotting were used to measure expression, and BALB/c nude mice were injected subcutaneously with untransfected or Huh7 cells transfected with empty or MAT1A expression vector to establish tumors. Tumor properties such as proliferation, angiogenesis, and apoptosis were compared, and microarray analysis was performed. Huh7 cells overexpressing MAT1A had higher S-adenosylmethionine levels but lower bromodeoxyuridine incorporation than control cells. Tumor growth rates and weights were lower in MAT1A transfected tumors. In addition, microvessel density and CD31 and Ki-67 staining were lower in MAT1A transfected tumors than control tumors, whereas the apoptosis index was higher in MAT1A-transfected tumors. Forced expression of MAT1A induced genes related to apoptosis and tumor suppression and lowered expression of cell growth and angiogenesis proteins. Our data demonstrate in vivo overexpression of MAT1A in liver cancer cells can suppress tumor growth. They also suggest inducing MAT1A expression might be a strategy to treat hepatocellular carcinoma.

Effects of a novel arginine methyltransferase inhibitor on T-helper cell cytokine production

The protein arginine methyltransferase (PRMT) family of enzymes catalyzes the transfer of methyl groups from S-adenosylmethionine to the guanidino nitrogen atom of peptidylarginine to form monomethylarginine or dimethylarginine. We created several less polar analogs of the specific PRMT inhibitor arginine methylation inhibitor-1, and one such compound was found to have improved PRMT inhibitory activity over the parent molecule. The newly identified PRMT inhibitor modulated T-helper-cell function and thus may serve as a lead for further inhibitors useful for the treatment of immune-mediated disease.

Changes in the expression of methionine adenosyltransferase genes and S-adenosylmethionine homeostasis during hepatic stellate cell activation

Hepatic stellate cell (HSC) activation is an essential event during liver fibrogenesis. Methionine adenosyltransferase (MAT) catalyzes biosynthesis of S-adenosylmethionine (SAMe), the principle methyl donor. SAMe metabolism generates two methylation inhibitors, methylthioadenosine (MTA) and S-adenosylhomocysteine (SAH). Liver cell proliferation is associated with induction of two nonliver-specific MATs: MAT2A, which encodes the catalytic subunit alpha2, and MAT2beta, which encodes a regulatory subunit beta that modulates the activity of the MAT2A-encoded isoenzyme MATII. We reported that MAT2A and MAT2beta genes are required for liver cancer cell growth that is induced by the profibrogenic factor leptin. Also, MAT2beta regulates leptin signaling. The strong association of MAT genes with proliferation and leptin signaling in liver cells led us to examine the role of these genes during HSC activation. MAT2A and MAT2beta are induced in culture-activated primary rat HSCs and HSCs from 10-day bile duct ligated (BDL) rat livers. HSC activation led to a decline in intracellular SAMe and MTA levels, a drop in the SAMe/SAH ratio, and global DNA hypomethylation. The decrease in SAMe levels was associated with lower MATII activity during activation. MAT2A silencing in primary HSCs and MAT2A or MAT2beta silencing in the human stellate cell line LX-2 resulted in decreased collagen and alpha-smooth muscle actin (alpha-SMA) expression and cell growth and increased apoptosis. MAT2A knockdown decreased intracellular SAMe levels in LX-2 cells. Activation of extracellular signal-regulated kinase and phosphatidylinositol-3-kinase signaling in LX-2 cells required the expression of MAT2beta but not that of MAT2A.

CONCLUSION

MAT2A and MAT2beta genes are induced during HSC activation and are essential for this process. The SAMe level falls, resulting in global DNA hypomethylation.

S-Adenosylmethionine and methylthioadenosine inhibit cellular FLICE inhibitory protein expression and induce apoptosis in colon cancer cells

S-Adenosylmethionine (SAMe) and its metabolite 5'-methylthioadenosine (MTA) inhibit mitogen-induced proliferative response in liver and colon cancer cells. SAMe and MTA are also proapoptotic in liver cancer cells by selectively inducing Bcl-x(S) expression. The aims of this work were to assess whether these agents are proapoptotic in colon cancer cells, and if so, to elucidate the molecular mechanisms. We found that both SAMe and MTA are proapoptotic in HT-29 and RKO cells in a dose- and time-dependent manner. Gene microarray uncovered down-regulation of cellular FLICE inhibitory protein (cFLIP). SAMe and MTA treatment led to a decrease in the mRNA and protein levels of both the long and short cFLIP isoforms. This required de novo RNA synthesis and was associated with activation of procaspase-8, Bid cleavage, and release of cytochrome c from the mitochondria. Inhibiting caspase 8 activity or overexpression of cFLIP protected against apoptosis, whereas supplementing with polyamines did not. SAMe and MTA treatment sensitized RKO cells to tumor necrosis factor alpha-related apoptosis-inducing ligand-induced apoptosis. Although SAMe and MTA are proapoptotic in colon cancer cells, they have no toxic effects in NCM460 cells, a normal colon epithelial cell line. In contrast to liver cancer cells, SAMe and MTA had no effect on Bcl-x(S) expression in colon cancer cells. In conclusion, SAMe and MTA are proapoptotic in colon cancer cells but not normal colon epithelial cells. One molecular mechanism identified is the inhibition of cFLIP expression. SAMe and MTA may be attractive agents in the chemoprevention and treatment of colon cancer.

Methionine metabolism and liver disease

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

S-adenosylmethionine inhibits lipopolysaccharide-induced gene expression via modulation of histone methylation

We previously showed that S-adenosylmethionine (SAMe) and its metabolite methylthioadenosine (MTA) blocked lipopolysaccharide (LPS)-induced tumor necrosis factor alpha (TNFalpha) expression in RAW (murine macrophage cell line) and Kupffer cells at the transcriptional level without affecting nuclear factor kappa B nuclear binding. However, the exact molecular mechanism or mechanisms of the inhibitory effect were unclear. While SAMe is a methyl donor, MTA is an inhibitor of methylation. SAMe can convert to MTA spontaneously, so the effect of exogenous SAMe may be mediated by MTA. The aim of our current work is to examine whether the mechanism of SAMe and MTA's inhibitory effect on proinflammatory mediators might involve modulation of histone methylation. In RAW cells, we found that LPS induced TNFalpha expression by both transcriptional and posttranscriptional mechanisms. SAMe and MTA treatment inhibited the LPS-induced increase in gene transcription. Using the chromatin immunoprecipitation assay, we found that LPS increased the binding of trimethylated histone 3 lysine 4 (H3K4) to the TNFalpha promoter, and this was completely blocked by either SAMe or MTA pretreatment. Similar effects were observed with LPS-mediated induction of inducible nitric oxide synthase (iNOS). LPS increased the binding of histone methyltransferases Set1 and myeloid/lymphoid leukemia to these promoters, which was unaffected by SAMe or MTA. The effects of MTA in RAW cells were confirmed in vivo in LPS-treated mice. Exogenous SAMe is unstable and converts spontaneously to MTA, which is stable and cell-permeant. Treatment with SAMe doubled intracellular MTA and S-adenosylhomocysteine (SAH) levels. SAH also inhibited H3K4 binding to TNFalpha and iNOS promoters.

CONCLUSION

The mechanism of SAMe's pharmacologic inhibitory effect on proinflammatory mediators is mainly mediated by MTA and SAH at the level of histone methylation.

Leptin's mitogenic effect in human liver cancer cells requires induction of both methionine adenosyltransferase 2A and 2beta

Leptin is an adiopokine that plays a pivotal role in the progression of liver fibrogenesis and carcinogenesis. Recently, leptin was shown to be mitogenic in human liver cancer cell lines HepG2 and Huh7. Whether leptin can act as a mitogen in normal hepatocytes is unclear. Methionine adenosyltransferase (MAT) is an essential enzyme that catalyzes the formation of S-adenosylmethionine (SAMe), the principal methyl donor and precursor of polyamines. Two genes (MAT1A and MAT2A) encode for the catalytic subunit of MAT, whereas a third gene (MAT2beta) encodes for a regulatory subunit that modulates the activity of MAT2A-encoded isoenzyme. The aims of this study were to examine whether leptin's mitogenic activity involves MAT2A and MAT2beta and whether this can be modulated. We found that leptin is mitogenic in HepG2 cells but not in primary human or mouse hepatocytes. Leptin induced the expression of MAT2A and MAT2beta in HepG2 cells and normal human and mouse hepatocytes, but although it increased SAMe level in HepG2 cells, it had no effect on SAMe level in normal hepatocytes. Leptin-mediated induction of MAT genes and growth in HepG2 cells required activation of extracellular signal-regulated kinase and phosphatidylinositol-3-kinase signaling pathways. Treatment with SAMe or its metabolite methylthioadenosine (MTA) lowered expression of MAT2A and MAT2beta and blocked leptin-induced signaling, including an increase in MAT gene expression and growth. Increased expression of MAT2A and MAT2beta is required for leptin to be mitogenic, although by entirely different mechanisms.

CONCLUSION

Leptin induces MAT2A and MAT2beta expression in HepG2 cells and normal hepatocytes but is mitogenic only in HepG2 cells. Pharmacological doses of SAMe or MTA lower expression of both MAT2A and MAT2beta and interfere with leptin signaling.

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

BACKGROUND & AIMS

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Role of S-adenosyl-L-methionine in liver health and injury

S-adenosylmethionine (SAMe) has rapidly moved from being a methyl donor to a key metabolite that regulates hepatocyte growth, death, and differentiation. Biosynthesis of SAMe occurs in all mammalian cells as the first step in methionine catabolism in a reaction catalyzed by methionine adenosyltransferase (MAT). Decreased hepatic SAMe biosynthesis is a consequence of all forms of chronic liver injury. In an animal model of chronic liver SAMe deficiency, the liver is predisposed to further injury and develops spontaneous steatohepatitis and hepatocellular carcinoma. However, impaired SAMe metabolism, which occurs in patients with mutations of glycine N-methyltransferase (GNMT), can also lead to liver injury. This suggest that hepatic SAMe level needs to be maintained within a certain range, and deficiency or excess can both lead to abnormality. SAMe treatment in experimental animal models of liver injury shows hepatoprotective properties. Meta-analyses also show it is effective in patients with cholestatic liver diseases. Recent data show that exogenous SAMe can regulate hepatocyte growth and death, independent of its role as a methyl donor. This raises the question of its mechanism of action when used pharmacologically. Indeed, many of its actions can be recapitulated by methylthioadenosine (MTA), a by-product of SAMe that is not a methyl donor. A better understanding of why liver injury occurs when SAMe homeostasis is perturbed and mechanisms of action of pharmacologic doses of SAMe are essential in defining which patients will benefit from its use.