Counts J L, McClain R M, Goodman J I
Department of Pharmacology and Toxicology, Michigan State University, East Lansing 48824, USA.
Mol Carcinog. 1997 Feb;18(2):97-106. doi: 10.1002/(sici)1098-2744(199702)18:2<97::aid-mc5>3.0.co;2-n.
The effects of different liver tumor-promoting treatments (i.e., a choline-devoid, methionine-deficient (CMD) diet, phenobarbital (PB), or both) on Ha-ras and raf methylation status and expression were determined in mouse strains with different susceptibilities to liver tumor formation: the relatively sensitive B6C3F1 and the relatively resistant C57BL/6. Additionally, B6C3F1 mouse liver tumors, spontaneous or PB induced, were assessed for alterations in global DNA methylation status and expression of Ha-ras and raf. The CMD diet led to hypomethylation of Ha-ras and raf after 12 wk of administration in B6C3F1 and C57BL/6 mice. At this early phase of tumor promotion, the frequency of increased expression of both Ha-ras and raf mRNAs was higher in the B6C3F1 but not the C57BL/6 mice. This is a mechanism that may, in part, underlie the heightened sensitivity of the B6C3F1 mouse to liver tumorigenesis. Subpopulations of B6C3F1 mouse liver tumors displayed altered global methylation status, with both hypomethylation and hypermethylation evident. Carcinomas were significantly more hypomethylated than adenomas. The level of raf mRNA was not changed in spontaneous or PB-induced B6C3F1 mouse liver tumors. Increased expression of Ha-ras was evident in some spontaneous B6C3F1 liver tumors and in most of the PB-induced liver tumors. These experiments support the concept that altered DNA methylation plays a key role in tumorigenesis and indicate that the high propensity of the B6C3F1 mice to liver tumorigenesis may be due, in part, to a decreased ability to maintain normal methylation status.
在对肝肿瘤形成具有不同易感性的小鼠品系中，即相对敏感的B6C3F1和相对抗性的C57BL/6，确定了不同的肝肿瘤促进处理（即缺乏胆碱、蛋氨酸的（CMD）饮食、苯巴比妥（PB）或两者）对Ha-ras和raf甲基化状态及表达的影响。此外，对B6C3F1小鼠的自发或PB诱导的肝肿瘤进行了全基因组DNA甲基化状态改变以及Ha-ras和raf表达的评估。在B6C3F1和C57BL/6小鼠中，给予CMD饮食12周后导致Ha-ras和raf的低甲基化。在肿瘤促进的这个早期阶段，B6C3F1小鼠中Ha-ras和raf mRNA表达增加的频率较高，但C57BL/6小鼠中并非如此。这是一种机制，可能部分解释了B6C3F1小鼠对肝肿瘤发生较高的敏感性。B6C3F1小鼠肝肿瘤亚群显示出全基因组甲基化状态改变，低甲基化和高甲基化均很明显。癌组织的低甲基化程度明显高于腺瘤。在自发或PB诱导的B6C3F1小鼠肝肿瘤中，raf mRNA水平没有变化。在一些自发的B6C3F1肝肿瘤和大多数PB诱导的肝肿瘤中，Ha-ras表达增加明显。这些实验支持了DNA甲基化改变在肿瘤发生中起关键作用的概念，并表明B6C3F1小鼠对肝肿瘤发生的高易感性可能部分归因于维持正常甲基化状态的能力下降。
