title: Alterations in the methylation status and expression of the raf oncogene in phenobarbital-induced and spontaneous B6C3F1 mouse liver tumors.
abstract: The liver tumor-prone B6C3F1 mouse (C57Bl/6 female x C3H/He male), in conjunction with the more susceptible C3H/He paternal strain and the resistant C57BL/6 maternal strain, is an excellent model for studying the mechanisms involved in carcinogenesis. The study reported here indicated that the B6C3F1 mouse inherited a maternal raf allele containing a methylated site not present in the paternal allele. Seven days after partial hepatectomy or after administration of a promoting dose of phenobarbital (PB) for 14 d; raf in B6C3F1 mouse liver was hypomethylated. The additional methylated site in the allele inherited from C57BL/6 was not maintained. The methylation status of raf in the liver of the C57BL/6 mouse was not affected by PB treatment. This indicates that the B6C3F1 mouse is less capable of maintaining methylation of raf than the C57BL/6 strain is. In both PB-induced and spontaneous B6C3F1 liver tumors, raf was hypomethylated in a nonrandom fashion. The level of raf mRNA increased in seven of 10 PB-induced tumors but in only one of five spontaneous tumors, whereas the level of Ha-ras mRNA increased in nine of 10 PB-induced tumors and in four of five spontaneous tumors. The results of our investigation (a) support the hypothesis that hypomethylation of DNA is a nongenotoxic mechanism involved in tumorigenesis, (b) support the notion that PB promotes liver tumors that develop along a pathway different from that leading to spontaneous tumors, and (c) indicate that differences in DNA methylation between C57BL/6 and B6C3F1 mice could, in part, account for the unusually high tendency of the latter strain to develop liver tumors.
authors.affiliation: Department of Pharmacology and Toxicology, Michigan State University, East Lansing 48824.
authors.forename: J S
authors.initials: JS
authors.lastname: Ray
authors.forename: M L
authors.initials: ML
authors.lastname: Harbison
authors.forename: R M
authors.initials: RM
authors.lastname: McClain
authors.forename: J I
authors.initials: JI
authors.lastname: Goodman
mesh_terms: D000818:Animals; D002455:Cell Division; D005260:Female; D015972:Gene Expression Regulation, Neoplastic; D011905:Genes, ras; D008113:Liver Neoplasms / Q000235:genetics*; D008297:Male; D008745:Methylation; D051379:Mice; D008809:Mice, Inbred C3H; D008810:Mice, Inbred C57BL; D009857:Oncogenes; D010634:Phenobarbital; D012150:Polymorphism, Restriction Fragment Length; D011518:Proto-Oncogene Proteins / Q000235:genetics*; D019908:Proto-Oncogene Proteins c-raf; D012333:RNA, Messenger / Q000235:genetics
chemical_list: D011518:Proto-Oncogene Proteins; D012333:RNA, Messenger; D019908:Proto-Oncogene Proteins c-raf; D010634:Phenobarbital