Analysis of activated protooncogenes in B6C3F1 mouse liver tumors induced by ciprofibrate, a potent peroxisome proliferator

Liver tumors from B6C3F1 mice induced by the potent peroxisomeproliferator ciprofibrate, a hypolipidemic drug, were evaluatedfor the presence of transforming genes by the nude mouse tumorigenicityassay. As reported earlier, the tumors were not activated bya point mutation in codon 61 of H-ras. Two of the eight tumorsexamined contained a mutation in codon 13 or an H-ras gene mutatedin codon 117. Screening of another 23 ciprofibrate-induced livertumors by oligonucleotide hybridization analysis and directDNA sequencing resulted in the identification of three tumorDNA samples with point mutations in codon 117 of the H-ras gene.In addition, another tumor sample contained a K-ras gene witha mutation in codon 61. Mutations in these codons have beenseen only rarely in chemically induced liver tumors from thismouse strain. Of 15 spontaneous B6C3F1 liver tumors screenedin the same manner, one exhibited a K-ras gene activated bya mutation in codon 13 and a second contained an H-ras geneactivated by a mutation in codon 117. These ras gene mutationshave not been reported previously from spontaneous liver tumors.The frequency and spectrum of ras oncogene mutations characterizedin ciprofibrate-induced liver tumors differ significantly fromthe frequency and pattern identified in spontaneously occurringliver tumors. The results of this study with a limited numberof samples suggest that ras protooncogene activation or activationof other protooncogenes that can be detected by the nude mousetumorigenicity assay are not frequent events in the mechanismof carcinogenicity of the peroxisome proliferator ciprofibrate.However, the lower frequency and distinct pattern of H-ras mutationsobserved in these tumors disprove the assumption of promotionof spontaneous hepatocarcinogenesis by ciprofibrate.