Ji Cheng, Shinohara Masao, Vance Dennis, Than Tin Aung, Ookhtens Murad, Chan Christine, Kaplowitz Neil
Research Center for Liver Disease, Southern California Research Center for Alcoholic Liver and Pancreatic Diseases, Keck School of Medicine, University of Southern California, Los Angeles, California 90033, USA.
Alcohol Clin Exp Res. 2008 Jun;32(6):1049-58. doi: 10.1111/j.1530-0277.2008.00666.x.
Chronic alcohol feeding induces hyperhomocysteinemia (HHcy). Previously, we reported a protective role of betaine-homocysteine methyltransferase (BHMT) in homocysteine-induced injury in cultured hepatocytes. In this study, we investigated the direct role of BHMT in alcohol or homocysteine-induced liver injury.
Betaine-homocysteine methyltransferase transgenic (Tg) mice were generated. Comparisons were made between the Tg and wild type (WT) mice in their response to intragastric alcohol infusion or to oral feeding of a high methionine low folate diet (HMLF).
Expression of the Tg BHMT was increased in organs peripheral to the liver. The alcohol infusion for 4 weeks increased: plasma ALT by 5-fold in WT mice and 2.7-fold in Tg mice; plasma homocysteine by 7-fold in WT mice and 2-fold in Tg mice; liver triglycerides by 4-fold in WT mice and 2.5-fold in Tg mice. The alcohol-induced fatty liver was more severe in WT than in Tg mice based on H&E staining. The HMLF feeding for 4 weeks increased plasma ALT by 2-fold in WT mice and 1-fold in Tg mice; plasma homocysteine by 21-fold in WT mice and 3.3-fold in Tg mice; liver triglycerides by 2.5-fold in WT mice and 1.5-fold in Tg mice. HMLF induced accumulation of macro fat droplets in WT but not Tg mice. Betaine supplementation decreased partially the alcohol or HMLF-induced increase of ALT, homocysteine and liver lipids in WT mice. However, Tg mice were normal when fed both HMLF and betaine. In WT mice, both alcohol and HMLF induced moderate increase of sterol regulatory element binding protein 1 (SREBP1) protein which was partially reduced by betaine supplementation. In Tg mice, alcohol but not HMLF increased SREBP1. Carbohydrate responsive element-binding protein was increased by alcohol in either WT or Tg mice which was not affected by betaine supplementation. Ratio of S-adenosylmethionine (SAM) to S-adenosylhomocysteine (SAH) was reduced by 50% in WT and by 20% in Tg mice fed alcohol. Ratio of phosphatidylcholine (PC) to phosphatidylethanolamine (PE) was reduced in WT but not Tg mice fed alcohol. Changes in PE methyltransferase activities were not detected in response to alcohol or HMLF feeding but were increased by betaine.
The BHMT Tg mice are resistant to alcohol or HMLF-induced HHcy and liver steatosis indicating that peripheral metabolism of homocysteine protected the liver without a direct effect of BHMT in the liver. Multiple mechanisms are involved in protection by betaine including increased SAM/SAH and PC/PE ratios.
长期饮酒会导致高同型半胱氨酸血症（HHcy）。此前，我们报道了甜菜碱-同型半胱氨酸甲基转移酶（BHMT）在同型半胱氨酸诱导的培养肝细胞损伤中的保护作用。在本研究中，我们调查了BHMT在酒精或同型半胱氨酸诱导的肝损伤中的直接作用。
构建了甜菜碱-同型半胱氨酸甲基转移酶转基因（Tg）小鼠。比较了Tg小鼠和野生型（WT）小鼠对灌胃酒精或口服高蛋氨酸低叶酸饮食（HMLF）的反应。
Tg BHMT在肝脏外周器官中的表达增加。连续4周灌胃酒精后，WT小鼠血浆谷丙转氨酶（ALT）升高5倍，Tg小鼠升高2.7倍；WT小鼠血浆同型半胱氨酸升高7倍，Tg小鼠升高2倍；WT小鼠肝脏甘油三酯升高4倍，Tg小鼠升高2.5倍。基于苏木精-伊红染色，WT小鼠酒精性脂肪肝比Tg小鼠更严重。连续4周喂食HMLF后，WT小鼠血浆ALT升高2倍，Tg小鼠升高1倍；WT小鼠血浆同型半胱氨酸升高21倍，Tg小鼠升高3.3倍；WT小鼠肝脏甘油三酯升高2.5倍，Tg小鼠升高1.5倍。HMLF诱导WT小鼠而非Tg小鼠出现大脂肪滴积聚。补充甜菜碱可部分降低WT小鼠酒精或HMLF诱导的ALT、同型半胱氨酸和肝脏脂质增加。然而，同时喂食HMLF和甜菜碱时，Tg小鼠表现正常。在WT小鼠中，酒精和HMLF均诱导固醇调节元件结合蛋白1（SREBP1）蛋白适度增加，补充甜菜碱可使其部分降低。在Tg小鼠中，酒精而非HMLF增加SREBP1。在WT或Tg小鼠中，酒精均增加碳水化合物反应元件结合蛋白，补充甜菜碱对此无影响。喂食酒精的WT小鼠中，S-腺苷甲硫氨酸（SAM）与S-腺苷同型半胱氨酸（SAH）的比值降低50%，Tg小鼠降低20%。喂食酒精的WT小鼠中，磷脂酰胆碱（PC）与磷脂酰乙醇胺（PE）的比值降低，Tg小鼠未降低。未检测到酒精或HMLF喂养后PE甲基转移酶活性的变化，但甜菜碱可使其增加。
BHMT转基因小鼠对酒精或HMLF诱导的HHcy和肝脂肪变性具有抗性，表明同型半胱氨酸的外周代谢可保护肝脏，而BHMT在肝脏中无直接作用。甜菜碱的保护作用涉及多种机制，包括增加SAM/SAH和PC/PE比值。
