Hepatic transmethylation reactions in micropigs with alcoholic liver disease

Abstract

Alcoholic liver disease is associated with abnormal hepatic methionine metabolism, including increased levels of homocysteine and S-adenosylhomocysteine (SAH) and reduced levels of Sadenosylmethionine (SAM) and glutathione (GSH). The concept that abnormal methionine metabolism is involved in the pathogenesis of alcoholic liver disease was strengthened by our previous findings in a micropig model where combining dietary folate deficiency with chronic ethanol feeding produced maximal changes in these metabolites together with early onset of microscopic steatohepatitis and an eightfold increase in plasma aspartate aminotransferase. The goal of the present study was to determine potential mechanisms for abnormal levels of these methionine metabolites by analyzing the transcripts and activities of transmethylation enzymes in the livers of the same micropigs. Ethanol feeding or folate deficiency, separately or in combination, decreased transcript levels of methylenetetrahydrofolate reductase (MTHFR), methionine adenosyltransferase (MAT1A), glycine-N-methyltransferase (GNMT) and S-adenosylhomocysteine hydrolase (SAHH). Ethanol feeding alone reduced the activities of methionine synthase (MS) and MATIII and increased the activity of GNMT. Each diet, separately or in combination, decreased the activities of MTHFR and SAHH. In conclusion, the observed abnormal levels of methionine metabolites in this animal model of accelerated alcoholic liver injury can be ascribed to specific effects of ethanol with or without folate deficiency on the expressions and activities of hepatic enzymes that regulate transmethylation reactions. These novel effects on transmethylation reactions may be implicated in the pathogenesis of alcoholic liver disease. ( A bnormal hepatic methionine metabolism can result from chronic exposure to ethanol or from folate deficiency. Hyperhomocysteinemia has been described in chronic alcoholics, 1,2 and decreased liver S-adenosylmethionine (SAM) levels were found in experimental animals and cultured hepatocytes exposed to ethanol. [3][4][5][6] Folate deficiency also results in abnormal methionine metabolism, because 5-methyltetrahydrofolate (5-MTHF) is substrate for the methionine synthase (MS) reaction that converts homocysteine to methionine, the substrate for SAM (Fig. 1). Furthermore, folate deficiency is frequently associated with alcoholic liver disease, in part, because chronic exposure to ethanol reduces the intestinal transport of folic acid and increases folate excretion in the urine. [7][8][9] Previous data support the concept that changes in methionine metabolism are involved in the development of alcoholic liver disease. Studies in rats and micropigs have demonstrated MS activity is reduced during chronic ethanol exposure. 10 -13 Rats that were fed ethanol by gastric tube demonstrated decreased methionine adenosyltransferase MAT1-III activity with decreased SAM and SAM/ S-adenosylhomocysteine (SAH) ratio in association with increased DNA strand breaks. 5 A MAT1A knockout mouse model developed he...

S-Adenosylhomocysteine Hydrolase Overexpression in HEK-293 Cells: Effect on Intracellular Adenosine Levels, Cell Viability, and DNA Methylation

“…Since, during the creation of stable cell lines, the vector has to integrate into the genome, the extent of AdoHcyase overexpression and thus, activity strongly depends on the number of integrated vector fragments (encoding AdoHcyase). This observation is consistent with previous results showing that AdoHcyase activity positively correlates with AdoHcyase mRNA levels [3,21,28]. Moreover, the isolation of several clones with different AdoHcyase activities is a valuable tool to study if AdoHcy metabolism is altered in an enzyme-activity dependent manner.…”

B-Vitamin dependent methionine metabolism and alcoholic liver disease

“…A clinical study comparing gene expressions in liver biopsies from ALD patients with those in control normal liver biopsies found decreased transcripts of MS , MAT1A and C β S which could account for observed reduced levels of hepatic SAM and GSH [15] . A subsequent study of livers from ethanol-fed micropigs that developed ALD with a folate deficient diet found decreased transcript expressions of MTHFR , MS , MAT1A , and SAHH [16] , which accounted for reduced hepatic levels of SAM and the SAM/ SAH ratio with increased levels of homocysteine and SAH in this animal model [7] . Whereas two studies found excellent correlations of liver SAM and GSH levels in experimental ALD [7,17] , others recently linked ethanol induced oxidative liver injury in a rat model to reduced SAM and GSH levels that were corrected by inhibition of the ethanol oxidizing enzyme cytochrome P4502E1 (CYP2E1), which also induces reactive oxygen species (ROS) [18] .…”

“…Studies of hepatocytes from ethanol-fed mice associated elevated SAH levels with increased sensitization of injury pathways to tumor necrosis factor (TNF α ) [54] . The inclusion of dietary folate deficiency with ethanol feeding of micropigs accelerated the onset of the pathology of ALD while increasing plasma homocysteine levels and decreasing liver SAM, the SAM to SAH ratio and GSH levels [7] , as well as reducing the expressions of relevant transmethylation enzymes [16] . Subsequent studies of livers from the same ethanol and control diet fed micropigs demonstrated augmenting effects of the folate deficient diets on the expressions of ethanol-induced enzymes involved in ER stress pathways of lipogenesis and apoptosis [55] .…”

TNFα-dependent hepatic steatosis and liver degeneration caused by mutation of zebrafishs-adenosylhomocysteine hydrolase

“…SAM depletion and/or a reduced SAM:SAH ratio are important consequences of altered methionine metabolism, as evidenced by the ability of SAM or its metabolic precursor, betaine, to delay disease progression in animal liver disease models and in patients with alcoholic liver disease (Lu et al, 2001;Villanueva and Halsted, 2004). SAM depletion in chronic liver conditions arises from nutritional deficiencies coupled with reduced activity of the methionine metabolism enzyme methionine synthase (MAT1A) and reduced BHMT gene expression (Kharbanda, 2007;Lu et al, 2002).…”