Mutational analysis of Ctnnb1 and Apc in tumors from rats given 1,2-dimethylhydrazine or 2-amino-3-methylimidazo[4,5-f]quinoline: mutational 'hotspots' and the relative expression of beta-catenin and c-jun.
There is growing interest in beta-catenin and its role in various human cancers. We recently reported that 2-amino-3-methylimidazo[4,5-f]quinoline (IQ)- and 1,2-dimethylhydrazine (DMH)-induced colon tumors in the rat contain mutations in Ctnnb1, the gene for beta-catenin, but the mutation spectrum was influenced by postinitiation exposure to chlorophyllin (CHL) and indole-3-carbinol (I3C) [Blum et al., Carcinogenesis 2001;22:315-320]. The present paper describes a follow-up study in which all of the target organs for IQ- and DMH-induced tumorigenesis were screened; Ctnnb1 mutations were found in 44 of 119 DMH-induced colon tumors, six of 13 IQ-induced colon tumors, 28 of 81 DMH-induced small intestine tumors, none of five IQ-induced small intestine tumors, four of 106 IQ-induced liver tumors, none of 14 DMH-induced Zymbal's gland tumors, none of 24 IQ-induced Zymbal's gland tumors, and none of 29 IQ-induced skin tumors. In tumors from rats given carcinogen alone, or carcinogen plus CHL or I3C, Ctnnb1 mutations frequently substituted amino acids adjacent to Ser33, a critical Ser/Thr residue in the glycogen synthase kinase-3beta regulatory domain of beta-catenin. However, substitution of critical Ser/Thr residues themselves was detected in only three of 24 (12.5%) of the tumors from rats given carcinogen alone, compared with 23 of 58 (40%) of the tumors from rats given carcinogen and treated postinitiation with I3C or CHL (P < 0.02). More than 50 of the colon tumors with wild-type beta-catenin were examined further for their Apc status; the overall frequency of Apc mutations was <10%, and these genetic changes occurred exclusively in the 'Mutation Cluster Region' of Apc. A subset of colon tumors also was examined for expression of beta-catenin and c-jun; these proteins were overexpressed in all tumors containing Ctnnb1 mutations, but the expression was highest in tumors with Ctnnb1 mutations affecting Thr41 and Ser45 residues in the glycogen synthase kinase-3beta region of beta-catenin. Thus, Ctnnb1 mutations occurred more frequently than Apc mutations in colon and small intestine tumors of the rat, and certain mutations upregulated beta-catenin/T-cell factor target genes more effectively than others, perhaps influencing the response to phytochemicals administered postinitiation.
Blum
Carmen A
CA
Linus Pauling Institute, Department of Environmental and Molecular Toxicology, Oregon State University, Corvallis, OR 97331-6512, USA.
Tanaka
Tomoko
T
Zhong
Xiaoying
X
Li
Qingjie
Q
Dashwood
Wan-Mohaiza
WM
Pereira
Clifford
C
Xu
Meirong
M
Dashwood
Roderick H
RH
D019813:1,2-Dimethylhydrazine / Q000633:toxicity*; D000818:Animals; D001483:Base Sequence; D002273:Carcinogens / Q000633:toxicity; D003062:Codon / Q000235:genetics; D003110:Colonic Neoplasms / Q000139:chemically induced*; D016384:Consensus Sequence; D003598:Cytoskeletal Proteins / Q000235:genetics*; D004252:DNA Mutational Analysis / Q000379:methods*; D017491:Genes, APC; D016758:Genes, jun / Q000235:genetics*; D007414:Intestinal Neoplasms / Q000139:chemically induced*; D008113:Liver Neoplasms / Q000139:chemically induced*; D008297:Male; D020641:Polymorphism, Single Nucleotide; D011804:Quinolines / Q000633:toxicity*; D051381:Rats; D011916:Rats, Inbred F344; D012878:Skin Neoplasms / Q000139:chemically induced; D015534:Trans-Activators / Q000235:genetics*; D051176:beta Catenin
D002273:Carcinogens; D003062:Codon; C495268:Ctnnb1 protein, rat; D003598:Cytoskeletal Proteins; D011804:Quinolines; D015534:Trans-Activators; D051176:beta Catenin; C029216:2-amino-3-methylimidazo(4,5-f)quinoline; D019813:1,2-Dimethylhydrazine