EFFECTS OF TAMOXIFEN ON ENDOMETRIAL CARCINOGENESIS IN MICE

Two experiments were conducted to determine the effect of tamoxifen (T AM) in mouse endometrium in comparison with that of 17 beta-estradiol (E-2), In a medium-term assay, TAM as well as E-2 treatment semi-dose- dependently increased the levels of fos/jun mRNA and their oncoprotein s (Fos/Jun), The long-term effect of TAM on mouse endometrial carcinog enesis was also examined in the following model. A total of 150 female ICR mice, 12-13 weeks of age, were used. Of these, 125 mice received an injection of N-methyl-N-nitrosourea (MNU) solution (1 mg/100 g body weight) into their left uterine tube and saline into the right. One w eek later, they were divided into four groups: groups 1 (35 mice) and 2 (30 mice) were given 25 ppm and 5 ppm E-2-containing diet, respectiv ely, while group 3 (30 mice) was fed 5 ppm TAM-containing diet. Group 5 (30 mice) was fed basal diet alone. The remaining 25 mice (group 4) received 5 ppm TAM-containing diet alone. At the termination of the ex periment (30 weeks), endometrial carcinomas were confirmed to be prese nt in the groups exposed to MNU. TAM increased the incidence of preneo plastic lesions of the endometrium, while E(2 )enhanced the occurrence of the carcinoma. No carcinomas were found in the group given TAM alo ne, In the ovaries, corpora lutea were lacking in most of the mice exp osed to TAM, suggesting that the animals were not cycling. Such findin gs indicate that TAM has an enhancing effect on endometrial carcinogen esis in mice, probably via a mechanism involving overexpression of Fos /Jun proteins.