Modifying effects of ethinylestradiol but not methoxychlor on N-ethyl-N-nitrosourea-induced uterine carcinogenesis in heterozygous p53-deficient CBA mice

It is unknown whether endocrine-disrupting chemicals (EDCs) with estrogenic activities have any modifying effects on uterine carcinogenesis. In our pr evious study, we established a uterine-carcinogenesis model that is useful for detecting tumor-modifying effects of EDCs by the administration of N-et hyl-N-nitrosourea (ENU) to female heterozygous p53-deficient CBA mice [p53 (+/-) mice]. To investigate the effects of ethinylestradiol (EE) and methox ychlor (MXC) on development of ENU-induced uterine tumors, female p53 (+/-) mice and their wild-type littermates [p53 (+/+) mice] received an intraper itoneal injection of 120 mg/kg body weight (bw) of ENU, followed, in Group 1, by no further treatment; in Group 2, by a diet containing 1 ppm EE; in G roup 3, by a diet containing 5 ppm EE for 4 weeks and 2.5 ppm EE thereafter ; and in Group 4, by a diet containing 2000 ppm MXC for 26 weeks. Uterine p roliferative lesions that were induced were composed of both endometrial-st romal and epithelial-cell types. Endometrial stromal sarcomas were induced in p53 (+/-) mice of Groups 1 to 4, and the incidence (87%) in Group 3 was significantly increased compared to Group 1 (47%). Atypical hyperplasias (c lear-cell type) of the endometrial gland in p53 (+/-) mice were seen at inc idences of 0, 14, 60, and 0% in Groups 1, 2, 3, and 4, respectively, while their incidence in p53 (+/+) mice was 0, 7, 53, and 0%, respectively, with a significant difference between Groups 1 and 3 in both cases. One p53 (+/- ) mouse in Group 3 also had an adenocarcinoma consisting of clear cells, an d the PCNA labeling indices of the clear-cell atypical hyperplasias, and th is endometrial adenocarcinoma, were higher than those of glandular hyperpla sias. The present study suggests that 2.5 ppm EE, but not MXC, exerts tumor -promoting effects on stromal and epithelial proliferative lesions of the u teri in p53 (+/-) mice initiated with ENU.