Hepatic Methionine Homeostasis Is Conserved in C57BL/6N Mice on High-Fat Diet Despite Major Changes in Hepatic One-Carbon Metabolism

Obesity is an underlying risk factor in the development of cardiovascular disease, dyslipidemia and non-alcoholic fatty liver disease (NAFLD). Increased hepatic lipid accumulation is a hallmark in the progression of NAFLD and impairments in liver phosphatidylcholine (PC) metabolism may be central to the pathogenesis. Hepatic PC biosynthesis, which is linked to the one-carbon (C1) metabolism by phosphatidylethanolamine N-methyltransferase, is known to be important for hepatic lipid export by VLDL particles. Here, we assessed the influence of a high-fat (HF) diet and NAFLD status in mice on hepatic methyl-group expenditure and C1-metabolism by analyzing changes in gene expression, protein levels, metabolite concentrations, and nuclear epigenetic processes. In livers from HF diet induced obese mice a significant downregulation of cystathionine β-synthase (CBS) and an increased betaine-homocysteine methyltransferase (BHMT) expression were observed. Experiments in vitro, using hepatoma cells stimulated with peroxisome proliferator activated receptor alpha (PPARα) agonist WY14,643, revealed a significantly reduced Cbs mRNA expression. Moreover, metabolite measurements identified decreased hepatic cystathionine and L-α-amino-n-butyrate concentrations as part of the transsulfuration pathway and reduced hepatic betaine concentrations, but no metabolite changes in the methionine cycle in HF diet fed mice compared to controls. Furthermore, we detected diminished hepatic gene expression of de novo DNA methyltransferase 3b but no effects on hepatic global genomic DNA methylation or hepatic DNA methylation in the Cbs promoter region upon HF diet. Our data suggest that HF diet induces a PPARα-mediated downregulation of key enzymes in the hepatic transsulfuration pathway and upregulates BHMT expression in mice to accommodate to enhanced dietary fat processing while preserving the essential amino acid methionine.

肥胖是心血管疾病、血脂异常以及非酒精性脂肪性肝病（non-alcoholic fatty liver disease, NAFLD）发生发展的潜在危险因素。肝脏脂质堆积增加是非酒精性脂肪性肝病进展的标志性特征，而肝脏磷脂酰胆碱（phosphatidylcholine, PC）代谢异常可能是其发病机制的核心环节。肝脏磷脂酰胆碱的生物合成通过磷脂酰乙醇胺N-甲基转移酶（phosphatidylethanolamine N-methyltransferase）与一碳代谢（one-carbon, C1）相联系，已知其对极低密度脂蛋白（very low density lipoprotein, VLDL）颗粒介导的肝脏脂质输出至关重要。
本研究通过分析基因表达、蛋白质水平、代谢物浓度以及细胞核表观遗传过程的变化，评估了小鼠的高脂（high-fat, HF）饮食与非酒精性脂肪性肝病状态对肝脏甲基基团消耗及一碳代谢的影响。在高脂饮食诱导的肥胖小鼠肝脏中，我们观察到胱硫醚β合酶（cystathionine β-synthase, CBS）的表达显著下调，而甜菜碱-同型半胱氨酸甲基转移酶（betaine-homocysteine methyltransferase, BHMT）的表达则上调。
在体外实验中，使用过氧化物酶体增殖物激活受体α（peroxisome proliferator activated receptor alpha, PPARα）激动剂WY14,643刺激肝癌细胞，结果显示Cbs的mRNA表达水平显著降低。此外，代谢物检测结果显示，与对照组相比，高脂饮食喂养小鼠的肝脏中作为转硫途径（transsulfuration pathway）组成部分的胱硫醚与L-α-氨基正丁酸浓度降低，甜菜碱浓度也有所下降，但甲硫氨酸循环（methionine cycle）中的代谢物未出现明显变化。
此外，我们还发现高脂饮食处理后，小鼠肝脏中从头合成DNA甲基转移酶3b（de novo DNA methyltransferase 3b）的基因表达水平降低，但并未观察到肝脏全基因组DNA甲基化或胱硫醚β合酶启动子区域（promoter region）的DNA甲基化水平发生变化。
本研究数据表明，高脂饮食可通过过氧化物酶体增殖物激活受体α介导肝脏转硫途径关键酶的下调，并上调小鼠体内甜菜碱-同型半胱氨酸甲基转移酶的表达，以适应膳食脂肪加工能力的增强，同时保留必需氨基酸甲硫氨酸。