Abstract
A comparative study on the possible involvement of several genes in the susceptibility of chemical carcinogenesis was carried out using carcinogen-resistant DRH rat and -sensitive Donryu and F344 rats. Previously, we observed that the induction of glutathione S-transferase placental form (GST-P) in the liver of Donryu rats by 3'-methyl-4-dimethylaminoazobenzene (3'-Me-DAB) was significantly greater than that of DRH rats. In the present study, we tentatively determined base sequences of the enhancer region including GPE-Ⅰ and GPE-Ⅱ (GST-P enhancersⅠ and Ⅱ) of GST-P genes of DRH, Donryu and F344 rats, but we did not observe any nucleotide polymorphism around these regions. Furthermore, the mRNA levels of silencer binding protein (NFA-1) for the GST-P promoter of rat liver were also similar in the DRH and Donryu rats. Since clonal expansion of putative preneoplastic GST-P-positive foci in the DRH rat liver was significantly suppressed during 3'-Me-DAB administration, we examined whether two opposite growth controling factors, TGF-α and TGF-β, may participate in such suppression of growth. It was supposed that mannose 6-phosphate / insulin-like growth factor 2 receptor (M6P / IGF2R), at least in part, activates TGF-β preproprotein. However, we observed that the levels of M6P / IGF2R mRNA in the livers of DRH were not higher than those of Donryu rats after being fed 3'-Me-DAB for 8 weeks. Another important factor in the carcinogenesis is insulin-like growth factorⅡ itself. Although liver tumors induced by 3'-Me-DAB in F344 had high levels of IGF-Ⅱ mRNA, little IGF-Ⅱ gene expression existed in normal adult livers of Donryu, F344 and DRH rats. High levels of IGF-Ⅱ mRNA were detected similarly in the livers of neonates from all these three strains of rats. Finally, we detected a significant increase of AFP (α-fetoprotein) mRNA in the livers of Donryu rats around 6 to 8 weeks from the start of 3'-Me-DAB feeding, which is in parallel with detrimental effects of this carcinogen on these rats. A reduced induction of AFP mRNA was observed in DRH rats under the same conditions. Further study will be needed to explain the lower tumor susceptibility in the DRH rat.
