Comparison of effect of tumor promoter treatments on DNA methylation status and gene expression in B6C3F1 and C57BL/6 mouse liver and in B6C3F1 mouse liver tumors

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Abstract
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. Mol. Carcinog. 18:97–106, 1997.