Avila M A, Carretero M V, Rodriguez E N, Mato J M
Instituto de Investigaciones Biomédicas, Consejo Superior de Investigaciones Científicas, Madrid, Spain.
Gastroenterology. 1998 Feb;114(2):364-71. doi: 10.1016/s0016-5085(98)70489-5.
BACKGROUND & AIMS: Oxygen supply to the hepatic parenchyma is compromised by long- or short-term ethanol consumption and pathological conditions such as cirrhosis. Impairment in the production of S-adenosyl-L-methionine, the major methylating agent, occurs during hypoxia. In this study, the molecular mechanisms implicated in the regulation of S-adenosyl-L-methionine synthesis by oxygen levels were investigated.
Rat hepatocytes were isolated and cultured under normoxic (21% O2) or hypoxic (3% O2) conditions for different periods. Methionine adenosyltransferase activity, messenger RNA levels, and nuclear transcription were evaluated.
Methionine adenosyltransferase was inactivated in hepatocytes kept under low oxygen levels. Hypoxia induced the expression of nitric oxide (NO) synthase, and the inactivation of methionine adenosyltransferase was prevented by the NO synthase inhibitor N(G)-monomethyl-L-arginine methyl ester. Methionine adenosyltransferase messenger RNA levels were down-regulated by hypoxia, through a mechanism that might involve a hemoprotein. Hypoxia dramatically reduced methionine adenosyltransferase gene transcription, and messenger stability was also decreased, although to a lesser extent.
We have established the molecular basis for the regulation of methionine adenosyltransferase activity and gene expression by hypoxia. NO-mediated inactivation and transcriptional arrest seem to be the two major pathways by which oxygen levels control hepatic methionine adenosyltransferase, the enzyme necessary for methylation reactions and for the synthesis of polyamines and glutathione.
长期或短期饮酒以及诸如肝硬化等病理状况会损害肝实质的氧气供应。在缺氧状态下，主要甲基化剂S-腺苷-L-蛋氨酸的生成会受到损害。在本研究中，我们调查了氧气水平调控S-腺苷-L-蛋氨酸合成的分子机制。
分离大鼠肝细胞，并在常氧（21% O₂）或低氧（3% O₂）条件下培养不同时间段。评估蛋氨酸腺苷转移酶活性、信使核糖核酸水平及核转录情况。
处于低氧水平的肝细胞中蛋氨酸腺苷转移酶失活。缺氧诱导一氧化氮（NO）合酶表达，且NO合酶抑制剂N（G）-单甲基-L-精氨酸甲酯可防止蛋氨酸腺苷转移酶失活。缺氧通过一种可能涉及血红蛋白的机制下调蛋氨酸腺苷转移酶信使核糖核酸水平。缺氧显著降低蛋氨酸腺苷转移酶基因转录，信使稳定性也降低，尽管程度较小。
我们已确立缺氧调控蛋氨酸腺苷转移酶活性及基因表达的分子基础。NO介导的失活和转录停滞似乎是氧气水平控制肝脏蛋氨酸腺苷转移酶的两条主要途径，该酶是甲基化反应以及多胺和谷胱甘肽合成所必需的。
