Fatty liver and fibrosis in glycine N-methyltransferase knockout mice is prevented by nicotinamide

Abstract

Deletion of glycine N-methyltransferase (GNMT), the main gene involved in liver S-adenosylmethionine (SAM) catabolism, leads to the hepatic accumulation of this molecule and the development of fatty liver and fibrosis in mice. To demonstrate that the excess of hepatic SAM is the main agent contributing to liver disease in GNMT knockout (KO) mice, we treated 1.5-month-old GNMT-KO mice for 6 weeks with nicotinamide (NAM), a substrate of the enzyme NAM N-methyltransferase. NAM administration markedly reduced hepatic SAM content, prevented DNA hypermethylation, and normalized the expression of critical genes involved in fatty acid metabolism, oxidative stress, inflammation, cell proliferation, and apoptosis. More importantly, NAM treatment prevented the development of fatty liver and fibrosis in GNMT-KO mice. Because GNMT expression is down-regulated in patients with cirrhosis, and because some subjects with GNMT mutations have spontaneous liver disease, the clinical implications of the present findings are obvious, at least with respect to these latter individuals. Because NAM has been used for many years to treat a broad spectrum of diseases (including pellagra and diabetes) without significant side effects, it should be considered in subjects with GNMT mutations. Conclusion: The findings of this study indicate that the anomalous accumulation of SAM in GNMT-KO mice can be corrected by NAM treatment leading to the normalization of the expression of many genes involved in fatty acid metabolism, oxidative stress, inflammation, cell proliferation, and apoptosis, as well as reversion of the appearance of the pathologic phenotype. (HEPATOLOGY 2010;52:105-114) Abbreviations: 5mC, 5-methyl-cytosine; a-SMA, a-smooth muscle actin; ADFP, adipose differentiation-related protein; COL1A1, pro-a1-collagen type I; CYP2E1, cytochrome P4502E1; CYP39A1, cytochrome P45039A1; CYP4A10, cytochrome P4504A10; CYP4A14, cytochrome P4504A14; CD36, fatty acid translocase CD36; ERK, extracellular signal-regulated kinase; GAPDH, glyceraldehyde 3-phosphate dehydrogenase; GNMT, glycine N-methyltransferase; GSH, glutathione; HCC, hepatocellular carcinoma; IL6, interleukin-6; iNOS, inducible nitric oxide synthase; JAK, Janus kinase; KO, knockout; MAT, methionine adenosyltransferase; NAM, nicotinamide; NNMT, nicotinamide N-methyltransferase; PARP, poly(ADP-ribose) polymerase; PCR, polymerase chain reaction; PPAR, peroxisome proliferator-activated receptor; RASSF1A, Ras-association domain family/tumor suppressor-1A; SAH, S-adenosylhomocysteine; SAM, Sadenosylmethionine; SEM, standard error of the mean; SIRT1, sirtuin-1; SOCS1, suppressor of cytokine signaling-1; STAT, signal transducer and activator of transcription; TIMP-1, TIMP tissue inhibitor of metalloproteinase-1; TUNEL, terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling; UCP2, uncoupling protein-2; WT, wild-type.From the

Repression of the Nuclear Receptor Small Heterodimer Partner by Steatotic Drugs and in Advanced Nonalcoholic Fatty Liver Disease

“…Moreover, apoptosis increases and genes involved in inflammation are induced. At 9 months, the GNMT 2/2 mouse also spontaneously develops multifocal HCC (Martinez-Chantar et al, 2008;Varela-Rey et al, 2010). SHP was found repressed in the liver of GNMT 2/2 mice, particularly 9-month-old deficient mice that exhibit more manifest liver disease (Fig.…”

“…One major difference among these NAFLD models is that the livers of GNMT 2/2 and MCD diet mice show early symptoms of severe disease with fibrosis and even HCC in GNMT 2/2 (Anstee and Goldin, 2006;Martinez-Chantar et al, 2008;Varela-Rey et al, 2010;Itagaki et al, 2013), whereas MAT1A 2/2 express a less severe phenotype with delayed onset and no fibrosis or HCC (Lu et al, 2001;Martinez-Chantar et al, 2002).…”

“…However, severe fibrosis and HCC do not appear even after prolonged HFD (Takahashi et Imajo et al, 2013). On the contrary, GNMT 2/2 and MCD mice develop steatohepatitis with fibrosis, but without obesity and insulin resistance (Varela-Rey et al, 2010;Takahashi et al, 2012;Imajo et al, 2013). Moreover, the MCD diet induces greater ROS production, mitochondrial DNA damage, and apoptotic cell death than many other diet models of NAFLD (Gao et al, 2004;Anstee and Goldin, 2006).…”

Glycine N-Methyltransferase Deficiency Affects Niemann-Pick Type C2 Protein Stability and Regulates Hepatic Cholesterol Homeostasis

“…Recently, several putative metabolites identified in Gnmt -/-mice serum closely resemble those of NAFLD patients (31). Varela-Rey et al (32) also showed that nicotinamide administration normalized hepatic Sadenosylmethionine (SAM) and fatty acid metabolism, as well as prevented fatty liver development in Gnmt -/-mice. However, the molecular mechanisms of GNMT deficiency-induced hepatic cholesterol accumulation remain unclear.…”

“…Indeed, accumulation of triglyceride and free fatty acids was also observed in Gnmt -/-mice liver (data not shown), indicating that loss of GNMT widely affects lipids metabolism. Because GNMT is abundantly expressed in normal liver cytosol, we cannot rule out other modulations such as GNMT loss-induced high SAM level (32), or other possibilities on GNMT and other lipid metabolism-related protein interactions.…”