Osmotic regulation of hepatic choline metabolism

Betaine critically contributes to the control of hepatocellular hydration and provides protection of the liver from different kinds of stress. This study investigates to what extent hepatocellular hydration changes affect the expression levels of enzymes involved in the metabolism of betaine and related organic osmolytes by using qRT-PCR gene expression studies in rat hepatoma cells as well as metabolic and gene expression profiling in 5,10 - methylene tetrahydrofolate reductase (MTHFR) deficient primary hepatocytes. The results demonstrate a coordinated regulation of betaine degradation and synthesis under anisoosmotic conditions. Expression of betaine degrading enzymes is downregulated by hyperosmolarity and strongly induced by hypoosmolarity. In contrast, synthesis of glycerophosphocholine from phosphoethanolamine and conversion of choline to betaine are both induced by hyperosmolarity but decreased under hypoosmotic conditions. In addition we evaluated the flux of choline and its derivates in liver and plasma of methylene tetrahydrofolate reductase knockout (Mthfr-/-) mice by tandem mass spectrometry. Analyses of system-wide alterations of osmolyte metabolism with microarray studies revealed expression changes similar to those after hypoosmotic exposure in this betaine depletion model. In conclusion, regulation of betaine synthesis and degradation and concomitant changes in intracellular osmolyte concentrations contribute to long-term adaptation to anisoosmotic exposure of the liver. Expression of 280 genes were analyzed in wild type and mthr-/- mice (n=7) with spotted oligonucleotides.甜菜碱（Betaine）在调控肝细胞水合稳态中发挥关键作用，可保护肝脏免受多种应激损伤。本研究采用大鼠肝癌细胞的实时定量PCR（qRT-PCR）基因表达分析，以及5,10-亚甲基四氢叶酸还原酶（5,10-methylene tetrahydrofolate reductase, MTHFR）缺陷原代肝细胞的代谢与基因表达谱分析，探究肝细胞水合状态改变对甜菜碱及相关有机渗透调节剂（organic osmolytes）代谢相关酶表达水平的影响程度。 研究结果显示，在异渗（anisoosmotic）条件下，甜菜碱的降解与合成存在协同调控机制：高渗性（hyperosmolarity）会下调甜菜碱降解酶的表达，而低渗性（hypoosmolarity）则可显著诱导其表达。与之相反，由磷酸乙醇胺（phosphoethanolamine）合成甘油磷酸胆碱（glycerophosphocholine），以及胆碱（choline）转化为甜菜碱的过程，均会被高渗性诱导，却在低渗性条件下受到抑制。 此外，本研究通过串联质谱（tandem mass spectrometry）分析了MTHFR敲除（Mthfr-/-）小鼠肝脏与血浆中胆碱及其衍生物的代谢通量。通过基因芯片（microarray）对渗透调节剂代谢的全系统改变进行分析后发现，在该甜菜碱耗竭模型中，基因表达变化与低渗暴露后的情况高度相似。 综上，甜菜碱合成与降解的调控，以及细胞内渗透调节剂浓度的伴随性变化，有助于肝脏长期适应异渗环境暴露。本研究采用点样寡核苷酸芯片（spotted oligonucleotides），对野生型及Mthfr-/-小鼠（n=7）的280个基因的表达水平进行了分析。