Methionine adenosyltransferase 1A ( MAT1A ) and glycine N -methyltransferase ( GNMT ) are the primary genes involved in hepatic S-adenosylmethionine (SAMe) synthesis and degradation, respectively. Mat1a ablation in mice induces a decrease in hepatic SAMe, activation of lipogenesis, inhibition of triglyceride (TG) release, and steatosis. Gnmt -deficient mice, despite showing a large increase in hepatic SAMe, also develop steatosis. We hypothesized that as an adaptive response to hepatic SAMe accumulation, phosphatidylcholine (PC) synthesis by way of the phosphatidylethanolamine (PE) N -methyltransferase (PEMT) pathway is stimulated in Gnmt−/− mice. We also propose that the excess PC thus generated is catabolized, leading to TG synthesis and steatosis by way of diglyceride (DG) generation. We observed that Gnmt−/− mice present with normal hepatic lipogenesis and increased TG release. We also observed that the flux from PE to PC is stimulated in the liver of Gnmt−/− mice and that this results in a reduction in PE content and a marked increase in DG and TG. Conversely, reduction of hepatic SAMe following the administration of a methionine-deficient diet reverted the flux from PE to PC of Gnmt−/− mice to that of wildtype animals and normalized DG and TG content preventing the development of steatosis. Gnmt−/− mice with an additional deletion of perilipin2, the predominant lipid droplet protein, maintain high SAMe levels, with a concurrent increased flux from PE to PC, but do not develop liver steatosis. Conclusion : These findings indicate that excess SAMe reroutes PE towards PC and TG synthesis and lipid sequestration. (Hepatology 2013;58:1296–1305)

Very low-density lipoprotein (VLDL) secretion provides a mechanism to export triglycerides (TG) from the liver to peripheral tissues, maintaining lipid homeostasis. In nonalcoholic fatty liver disease (NAFLD), VLDL secretion disturbances are unclear. Methionine adenosyltransferase (MAT) is responsible for S -adenosylmethionine (SAMe) synthesis and MAT I and III are the products of the MAT1A gene. Deficient MAT I and III activities and SAMe content in the liver have been associated with NAFLD, but whether MAT1A is required for normal VLDL assembly remains unknown. We investigated the role of MAT1A on VLDL assembly in two metabolic contexts: in 3-month-old MAT1A -knockout mice (3-KO), with no signs of liver injury, and in 8-month-old MAT1A -knockout mice (8-KO), harboring nonalcoholic steatohepatitis. In 3-KO mouse liver, there is a potent effect of MAT1A deletion on lipid handling, decreasing mobilization of TG stores, TG secretion in VLDL and phosphatidylcholine synthesis via phosphatidylethanolamine N -methyltransferase. MAT1A deletion also increased VLDL– apolipoprotein B secretion, leading to small, lipid-poor VLDL particles. Administration of SAMe to 3-KO mice for 7 days recovered crucial altered processes in VLDL assembly and features of the secreted lipoproteins. The unfolded protein response was activated in 8-KO mouse liver, in which TG accumulated and the phosphatidylcholine-to-phosphatidylethanolamine ratio was reduced in the endoplasmic reticulum, whereas secretion of TG and apolipoprotein B in VLDL was increased and the VLDL physical characteristics resembled that in 3-KO mice. MAT1A deletion also altered plasma lipid homeostasis, with an increase in lipid transport in low-density lipoprotein subclasses and decrease in high-density lipoprotein subclasses. Conclusion: MAT1A is required for normal VLDL assembly and plasma lipid homeostasis in mice. Impaired VLDL synthesis, mainly due to SAMe deficiency, contributes to NAFLD development in MAT1A -KO mice. (Hepatology 2011

Hyperhomocysteinemia (HHcy) causes increased oxidative stress and is an independent risk factor for cardiovascular disease. Oxidative stress is now believed to be a major contributory factor in the development of non alcoholic fatty liver disease, the most common liver disorder worldwide. In this study, the changes which occur in homocysteine (Hcy) metabolism in high fat-diet induced non alcoholic fatty liver disease (NAFLD) in rats were investigated. After feeding rats a standard low fat diet (control) or a high fat diet (57% metabolisable energy as fat) for 18 weeks, the concentration of homocysteine in the plasma was significantly raised while that of cysteine was lowered in the high fat as compared to the control diet fed animals. The hepatic activities of cystathionine β-synthase (CBS) and cystathionine γ-lyase (CGS), the enzymes responsible for the breakdown of homocysteine to cysteine via the transsulphuration pathway in the liver, were also significantly reduced in the high fat-fed group. These results indicate that high fat diet-induced NAFLD in rats is associated with increased plasma Hcy levels caused by down-regulation of hepatic CBS and CGL activity. Thus, HHcy occurs at an early stage in high fat diet-induced NAFLD and is likely to contribute to the increased risk of cardiovascular disease associated with the condition.

The current view of cancer progression highlights that cancer cells must undergo through a post-translational regulation and metabolic reprogramming to progress in an unfriendly environment. In here, the importance of neddylation modification in liver cancer was investigated. We found that hepatic neddylation was specifically enriched in liver cancer patients with bad prognosis. In addition, the treatment with the neddylation inhibitor MLN4924 in Phb1-KO mice, an animal model of hepatocellular carcinoma showing elevated neddylation, reverted the malignant phenotype. Tumor cell death in vivo translating into liver tumor regression was associated with augmented phosphatidylcholine synthesis by the PEMT pathway, known as a liver-specific tumor suppressor, and restored mitochondrial function and TCA cycle flux. Otherwise, in protumoral hepatocytes, neddylation inhibition resulted in metabolic reprogramming rendering a decrease in oxidative phosphorylation and concomitant tumor cell apoptosis. Moreover, Akt and LKB1, hallmarks of proliferative metabolism, were altered in liver cancer being new targets of neddylation. Importantly, we show that neddylation-induced metabolic reprogramming and apoptosis were dependent on LKB1 and Akt stabilization. Overall, our results implicate neddylation/signaling/metabolism, partly mediated by LKB1 and Akt, in the development of liver cancer, paving the way for novel therapeutic approaches targeting neddylation in hepatocellular carcinoma.

We aimed to characterize the primary abnormalities associated with fat accumulation and vulnerability to hepatocellular injury of obesity-related fatty liver. We performed functional analyses and comparative transcriptomics of isolated primary hepatocytes from livers of obese insulin-resistant Zucker rats (comprising mild to severe hepatic steatosis) and age-matched lean littermates, searching for novel genes linked to chronic hepatic steatosis. Of the tested genome, 1.6% was identified as steatosis linked. Overexpressed genes were mainly dedicated to primary metabolism (100%), signaling, and defense/acute phase (approximately 70%); detoxification, steroid, and sulfur metabolism (approximately 65%) as well as cell growth/proliferation and protein synthesis/transformation (approximately 70%) genes were downregulated. The overexpression of key genes involved in de novo lipogenesis, fatty acid and glycerolipid import and synthesis, as well as acetyl-CoA and cofactor provision was paralleled by enhanced hepatic lipogenesis and production of large triacylglycerol-rich VLDL. Greatest changes in gene expression were seen in those encoding the lipogenic malic enzyme (up to 7-fold increased) and cell-to-cell interacting cadherin 17 (up to 8-fold decreased). Among validated genes, fatty acid synthase, stearoyl-CoA desaturase 1, fatty acid translocase/Cd36, malic enzyme, cholesterol-7 alpha hydroxylase, cadherin 17, and peroxisome proliferator-activated receptor alpha significantly correlated with severity of hepatic steatosis. In conclusion, dysregulated expression of metabolic and survival genes accompany hepatic steatosis in obese insulin-resistant rats and may render steatotic hepatocytes more vulnerable to cell injury in progressive nonalcoholic fatty liver disease.

Several etiologies result in chronic liver diseases including chronic hepatitis C virus infection (HCV). Despite its high incidence and the severe economic and medical consequences, liver disease is still commonly overlooked due to the lack of efficient non-invasive diagnostic methods. While several techniques have been tested for the detection of fibrosis, the available biomarkers still present severe limitations that preclude their use in clinical diagnostics. Liver diseases have also been the subject of metabolomic analysis. Here, we demonstrate the suitability of ¹H NMR spectroscopy for characterizing the metabolism of liver fibrosis induced by HCV. Serum samples from HCV patients without fibrosis or with liver cirrhosis were analyzed by NMR spectroscopy and the results were submitted to multivariate and univariate statistical analysis. PLS-DA test was able to discriminate between advanced fibrotic and non-fibrotic patients and several metabolites were found to be up or downregulated in patients with cirrhosis. The suitability of the most significantly regulated metabolites was validated by ROC analysis. Our study reveals that choline, acetoacetate and low-density lipoproteins are the most informative biomarkers for predicting cirrhosis in HCV patients. Our results demonstrate that statistical analysis of ¹H-NMR spectra is able to distinguish between fibrotic and non-fibrotic patients suffering from HCV, representing a novel diagnostic application for NMR spectroscopy.