Enhancing Effects of Estrogens on Endometrial Carcinogenesis Initiated by N‐Methyl‐N‐nitrosourea in ICR Mice

The present study was undertaken to examine the effects of estrogens, such as estrone (E(1)), 17β‐estradiol (E(2)) and estriol (E(3)), on endometrial Carcinogenesis initiated by N‐methyl‐N‐nitrosourea (MNU) in mice. A total of 120 female ICR mice received MNU solution (1 mg/100 g body wt.) and normal saline at 10 weeks of age into their left and right uterine corpora, respectively. One week later, they were divided into four groups and treated as follows: Group 1 (30 mice) was given 25 ppm E(1)‐containing diet; Group 2 (30 mice) was fed 5 ppm E(2)‐containing diet; Group 3 (30 mice) was given 25 ppm E(3)‐containing diet; and Group 4 (30 mice) was fed the basal diet alone. At the termination of the experiment (Week 30), all surviving animals were autopsied and histopathological examinations revealed that endometrial adenocarcinomas had developed in all groups. The incidence of adenocarcinomas in the MNU‐treated uterine corpus in Group 1 (25 ppm E(1)‐feeding, 9/23, 39%) was significantly higher than that in Group 4 (basal diet, 3/26, 12%, P<0.05). Also, the incidences of adenocarcinomas in the MNU‐treated uterine corpus in Groups 2 (5 ppm E(2)‐feeding, 8/24, 33%) and 3 (25 ppm E(3)‐feeding, 7/26, 28%) were higher than in Group 4, but the difference was not statistically significant. Feeding of diet containing E(1), E(2) and E(3) increased the incidences of the preneoplastic endometrial lesions (atypical, adenomatous or cystic glandular hyperplasia). In the uterine cervix, small numbers of squmous cell carcinomas, dysplasias or hyperplasias were occasionally found in all groups. These results indicate enhancing effects of the above three types of estrogens on the endometrial carcinogenesis induced by MNU in ICR mice.