Mechanisms of Protection by the Betaine-Homocysteine Methyltransferase/Betaine System in HepG2 Cells and Primary Mouse Hepatocytes

Betaine-homocysteine methyltransferase (BHMT) is a cytosolic zinc metalloenzyme that is highly expressed in the liver and kidneys.1–6 BMHT catalyzes methyl transfer from betaine, a product of choline oxidation, to homocysteine, yielding methionine and N,N-dimethylglycine. Homocysteine remethylation is also catalyzed by a cobalamin-dependent enzyme, methionine synthase (MS), with 5-methylfolate as a cosubstrate supplied by 5,10-methylenetetrahydrofolate reductase. Both BHMT and MS have a low Michaelis-Menten constant (Km) for homocysteine. Elevated S-adenosylmethionine (SAM), resulting from a methionine excess, inhibits BHMT and the formation of 5-methyltetrahydrofolate catalyzed by 5,10-methylenetetrahydrofolate reductase.7 Hence, homocysteine remethylation is predominant at low levels of homocysteine and methionine. At high levels, SAM stimulates 2 high-Km enzymes, methionine adenosyltransferase-III and cystathionine β-synthase (CBS). The latter converts homocysteine toward the transsulfuration pathway for the production of cysteine.8,9 Thus, BHMT is a component of the methionine cycle, and when folate-dependent methionine synthesis is impaired by either genetic or environmental factors (for example, a chronic alcohol treatment), the BHMT/betaine system plays a critical role in homocysteine homeostasis.10 Impaired BHMT results in elevated homocysteine levels and could contribute to the risk for vascular, hepatic, and neurological diseases.4,10–13 Betaine supplementation ameliorates the biochemical abnormalities and the clinical course in homocystinuria due to a deficiency of CBS or to several remethylation defects.14 We and others have previously observed that betaine supplementation protects against alcohol-induced fatty liver and endoplasmic reticulum (ER) stress and, at the same time, prevents alcohol-induced hyperhomocysteinemia.15–20 Other potential mechanisms of protection by betaine may contribute to the amelioration of an alcoholic fatty liver/injury, including increasing the SAM to S-adenosylhomocysteine (SAH) ratio and phosphatidylethanolamine methyltransferase activity and acting as a molecular chaperone. In addition, the expression of apolipoprotein B (ApoB) is increased in McArdle RH-7777 expressing BHMTand in rat livers following the in vivo induction of BHMT,21–24 which could increase triglyceride mobilization/secretion to minimize fatty liver. In the liver, BHMT is responsible for 50% of the homocysteine remethylation.3–5,25 A severe reduction of BHMT messenger RNA(mRNA) was found in 90% of patients with hepatitis C virus–induced cirrhosis and in about 50% of patients with chronic alcohol– induced cirrhosis.26 In order to investigate the direct role of the BHMT/betaine system in liver steatosis and injury, we generated BHMT transgenic cell models and silenced BHMT expression in primary mouse hepatocytes. We compared the response of the transgenic models and wild type to homo-cysteine challenge versus other inducers of ER stress and found that BHMT/betaine protected specifically against homocysteine-induced ER stress and cell death in the hepatocytes and decreased hepatocellular lipids, which correlated with decreased sterol regulatory element binding protein 1 (SREBP-1) induction, increased ApoB expression, and restored SAM/SAH.