(32)P‐Postlabeling Analysis of DNA Adducts in Rats during Estrogen‐induced Hepatocarcinogenesis and Effect of Tamoxifen on DNA Adduct Level

DNA adduct formation in the liver, pancreas, kidneys and uterus in ethynylestradiol (EE)‐induced carcinogenesis and the effect of tamoxifen (TAM) on DNA adduct formation were evaluated in female Wistar JCL rats using the (32)P‐postlabeling method. Hyperplastic nodules were noted in the liver of all rats 4 months after the first oral administration of 0.075 mg of EE, and hepatocellular carcinoma was detected in 8.1% of rats treated with EE for 12 months. DNA adducts increased in the liver for 4 months, reaching a level of 7.3 adducts/10(7) nucleotides and decreasing thereafter. Formation of DNA adducts was also noted in the pancreas and kidney, but the adduct levels were lower than those in the liver. TAM inhibited estrogen receptors (ER) in liver tissues and completely suppressed the development of hyperplastic nodules or hepatocellular carcinoma but did not affect DNA adduct formation in the liver. In this model, therefore, EE is considered to cause mutations of hepatocytes due to DNA adduct formation without mediation by ER and to induce initiated cells to develop into hepatocellular carcinoma in the presence of ER‐mediated hormonal activities.