Chronological observation of mouse endometrial carcinogenesis induced by N-methyl-N-nitrosourea and 17 beta-estradiol

Two-sided role of estrogen on endometrial carcinogenesis: stimulator or suppressor?

Endometrial carcinoma (EC) often expresses estrogen receptors (ER), and the growth of EC is stimulated by estrogen. Therefore, EC is considered to be an estrogen-dependent tumor. However, the role of estrogen in endometrial carcinogenesis is somewhat unclear because the majority of EC occurs at peri- or post menopause when serum estrogen levels are generally decreased. In this article, we describe the double-edged role of estrogen in the genesis of EC, especially in terms of mismatch repair functions in vitro and in vivo, i.e. when serum estradiol (E2) levels are relatively low (approximately less than 90 pg/ml), and E2 enhance the carcinogenesis, whereas high E2 levels may suppress the carcinogenesis. This will deepen mechanistic insight into unopposed estrogen.

Effect of letrozole in carcinogen-plus-estrogen-induced endometrial hyperplasia in mice

PURPOSE

To evaluate the effects of letrozole (Ltz) in carcinogen+estrogen-induced endometrial hyperplasia.

METHODS

BALB/c female mice were divided into four groups of 12 animals each receiving an intrauterine dose of N-ethyl-N-nitrosourea (ENU) and weekly subcutaneous injections of estradiol hexaidrobenzoate (EHB), except for group I(control). The groups were divided in I (control), II (ENU+EHB), III (ENU+EHB+MPA) and IV (ENU+EHB+Ltz). Group III also received intramuscular injections of MPA (medroxy progesterone acetate) every four weeks, while group IV received oral doses of Ltz daily. At the end of 16 weeks, the animals were sacrificed, and blood samples were collected for the measurement of serum estradiol and progesterone levels. Uterine histological sections were made to evaluate the presence of endometrial proliferative lesions. Differences between groups were evaluated with student's t test, ANOVA and chi-square test.

RESULTS

Groups ENU+EHB, ENU+EHB+MPA and ENU+EHB+Ltz showed varying degrees of endometrial hyperplasia. The incidence of hyperplasia in groups ENU+EHB and ENU+EHB+Ltz was higher and more severe than in group ENU+EHB+MPA. Control group showed lower levels of serum estradiol than the other groups.

CONCLUSION

There was no evidence that letrozole could act as an antiestrogenic drug in the development of endometrial proliferative lesions.

N-Methyl-N-nitrosourea as a mammary carcinogenic agent

The administration of chemical carcinogens is one of the most commonly used methods to induce tumors in several organs in laboratory animals in order to study oncologic diseases of humans. The carcinogen agent N-methyl-N-nitrosourea (MNU) is the oldest member of the nitroso compounds that has the ability to alkylate DNA. MNU is classified as a complete, potent, and direct alkylating compound. Depending on the animals' species and strain, dose, route, and age at the administration, MNU may induce tumors' development in several organs. The aim of this manuscript was to review MNU as a carcinogenic agent, taking into account that this carcinogen agent has been frequently used in experimental protocols to study the carcinogenesis in several tissues, namely breast, ovary, uterus, prostate, liver, spleen, kidney, stomach, small intestine, colon, hematopoietic system, lung, skin, retina, and urinary bladder. In this paper, we also reviewed the experimental conditions to the chemical induction of tumors in different organs with this carcinogen agent, with a special emphasis in the mammary carcinogenesis.

The role of LEF1 in endometrial gland formation and carcinogenesis

Endometrial carcinoma is the most common gynecologic cancer, yet the mechanisms underlying this disease process are poorly understood. We hypothesized that Lef1 is required for endometrial gland formation within the uterus and is overexpressed in endometrial cancer. Using Lef1 knockout (KO) mice, we compared uterine gland development to wild-type (WT) controls, with respect to both morphology and expression of the Lef1 targets, cyclin D1 and MMP7. We characterized the dynamics of Lef1 protein expression during gland development and the mouse estrus cycle, by immunostaining and Western blot. Finally, we investigated the roles of cyclin D1 and MMP7 in gland and cancer formation in the mouse, and assessed the relevance of Lef1 to human cancer by comparing expression levels in cancerous and normal endometrial tissues. Lef1 upregulation in mouse endometrium correlates with the proliferative stages of the estrus cycle and gland development during the neonatal period. WT mice endometrial glands began to develop by day 5 and were easily identified by day 9, whereas Lef1 KO mice endometrial glands had not developed by day 9 although the endometrial lining was intact. We found that during gland development cyclin D1 is elevated and localized to the gland buds, and that this requires the presence of Lef1. We also noted that Lef1 protein was expressed at higher levels in endometrial cancers within mice and humans when compared to normal endometrium. Our loss-of-function data indicate that Lef1 is required for the formation of endometrial glands in the mouse uterus. Lef1 protein elevation corresponds to gland formation during development, and varies cyclically with the mouse estrus cycle, in parallel with gland regeneration. Finally, Lef1 is overexpressed in human and mouse endometrial tumors, consistent with it playing a role in gland proliferation.

Neoplasms of the reproductive tract: the role of hormone exposure

Cancers of the reproductive system are a major source of morbidity and mortality among women worldwide. Because the uterus, ovaries, and cervix are hormonally responsive tissues, exposure to endogenous or exogenous sex steroids can profoundly affect the carcinogenic process. Animal models developed to date provide valuable but imperfect systems in which to study neoplasms of the reproductive tract. Nonhuman primate models share the unique primate-specific endometrial physiology of humans, but rarely develop neoplasms of the reproductive tract. Therefore a surrogate marker approach is required for the study of hormonally induced cancer risk in primates. Rodents provide practical models in which tumorigenesis can be assayed in a short time and, with appropriate interpretation, can be used for assessment of risk, prevention, and therapeutic strategies. In addition to the spontaneous strain-dependent incidence of female reproductive cancers, the classical chemical and hormonal carcinogenesis models, and the use of xenograft approaches, novel genetically modified animals provide unique insights into relevant molecular mechanisms. Caveats in the use of rodent models include anatomical differences from the human reproductive tract, the greater possibility of different metabolic responses to hormonal agents than humans, strain variations in tumor type and hormonal responsiveness, and unexpected tumor phenotypes in genetically modified animals. Reported nonmammalian models are limited primarily to the study of ovarian carcinogenesis. Recent progress in the understanding of cervical carcinogenesis is encouraging. Unmet needs in this area of research include models of early events in ovarian carcinogenesis and strongly predictive models of endometrial cancer risk. Nonhuman primates remain indispensable for the study of some aspects of reproductive pathophysiology, but the best understanding of carcinogenesis in the reproductive tract requires a broad approach using complementary human, nonhuman primate, and nonprimate studies.