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/

The effects of different liver tumor-promoting treatments (i.e., a cho line-devoid, methionine-deficient (CMD) diet, phenobarbital (PB), or b oth) on Ha-ras and raf methylation status and expression were determin ed in mouse strains with different susceptibilities to liver tumor for mation: the relatively sensitive B6C3F1 and the relatively resistant C 57BL/6. Additionally, B6C3F1 mouse liver tumors, spontaneous or PB ind uced, were assessed for alterations in global DNA methylation status a nd expression of Ha-ras and raf. The CMD diet led to hypomethylation o f Ha-ras and raf after 12 wk of administration in B6C3F1 and C57BL/6 m ice. At this early phase of tumor promotion, the frequency of increase d 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 ev ident. Carcinomas were significantly more hypomethylated than adenomas . The level of raf mRNA was not changed in spontaneous or PB-induced B 6C3F1 mouse liver tumors. Increased expression of Ha-ras was evident i n some spontaneous B6C3F1 liver tumors and in most of the PB-induced l iver tumors. These experiments support the concept that altered DNA me thylation plays a key role in tumorigenesis and indicate that the high propensity of the B6C3F1 mice to liver tumorigenesis may be due, in p art, to a decreased ability to maintain normal methylation status. (C) 1997 Wiley-Liss, Inc.