Effects of estrogens and metabolites on endometrial carcinogenesis in young adult mice initiated with N-ethyl-N′-nitro-N-nitrosoguanidine

The present study assessed effects of estrogens and their steroid metabolites on the endometrial carcinogenesis in young adult mice initiated with N-ethyl-N'-nitro-N-nitrosoguanidine (ENNG). A total of 272 female CD-1 (ICR) mice were used and equally divided into 17 groups. Mice were implanted cholesterol pellets to the back subcutis at 9 weeks of age. Pellets contained nothing (control) or one of the experimental agents, three different estrogens and their 13 different steroid metabolites, at a concentration of 0.5% (w/w). At 10 weeks of age, mice were given a single intra-uterine administration of ENNG at a dose of 25 mg/kg body weight. When reaching the 30 weeks of age (20 weeks after the ENNG treatment), mice were sacrificed to assess the development of endometrial proliferative lesions. While endometrial proliferative lesions, including hyperplasias and adenocarcinomas, were observed in all groups, the incidences of hyperplasias in the groups treated with 2-hydroxyestriol, 2-methoxyestradiol, 2-methoxyestriol and 16-epiestriol were significantly higher than that in the control group. On the other hand, adenocarcinomas were significantly developed in the groups treated with estrone, estradiol, estriol, 16beta-hydroxyestrone, 16alpha-hydroxyestrone and 17-epiestriol. These results indicate that, on the endometrial carcinogenesis in mice initiated with ENNG, estrogens and their metabolites belonging to the 16alpha-hydroxylation pathway and the upstream of the 16beta-hydroxylation pathway exert both promoting and progressing effects, whereas, the estrogen metabolites belonging to the 2- and 4-hydroxylation pathways (catechol estrogens) and the downstream of the 16beta-hydroxylation pathway exert only promoting or no effects. It is thus suggested that a metabolic profile of estrogens may be crucial for the endometrial carcinogenesis and that the rate of the 16alpha-hydroxylation may be associated with the increased carcinogenic risks of estrogens on the endometrium.

Time-dependent promotion activity of 17beta-estradiol on uterine carcinogenesis in mice initiated with N-ethyl-N-nitrosourea

The time-dependent promotion activity of 17beta-estradiol (E2) by initiation with N-ethyl-N-nitrosourea (ENU) on induction of mouse uterine endometrial proliferative lesions was examined. Illumination-induced persistent estrous female CD-1 mice were divided into five groups at 9 weeks of age. At 10 weeks of age, mice in all groups (n=25) were given a single intra-uterine administration of ENU (50 mg/kg), dissolved in polyethylene glycol. Animals in Groups 2 to 5 were then implanted s.c. with an E2 pellet at 9, 11, 14 and 17 weeks of age. The implants were left in place for 8 weeks and then taken out. At the termination of the experiment (week 15 after the ENU-treatment), all surviving mice were killed and the development of uterine proliferative lesions were assessed. All groups demonstrated endometrial hyperplasias and adenocarcinomas and the incidences of the latter in ENU plus E2 treated animals (Groups 2 to 5; 36, 48, 35 and 36%, respectively) were significantly higher compared to 8% for Group 1, without any variation with the age at E2 treatment. However, the incidences of adenocarcinomas plus severe hyperplasias increased from Groups 1 to 5 (28, 40, 56; P<0.05, 61; P<0.05 and 80%; P<0.01, respectively), indicating that promotion effects of E2 on induction of uterine proliferative lesions in the uterine endometrium become more pronounced with the interval after ENU initiation.