Assessing anti-estrogenic effects of AHR ligands in primary human and rat endometrial epithelial cells

Unopposed estrogenic action in the uterus can lead to the development of endometrial cancer in both humans and rats. Aryl hydrocarbon receptor (AHR) activation gives rise to anti-estrogenic actions and may consequently reduce the development of endometrial cancer. In this study, the anti-estrogenic potential of the AHR ligands 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and DELAQ, a metabolite of the pharmaceutical laquinimod, was assessed in in primary human and rat endometrial epithelial cells (EECs) with and without co-exposure to endogenous hormones. In human EECs, estradiol and progesterone did not affect AHR gene expression, but in rat EECs, progesterone decreased Ahre xpression (1.4-fold). In accordance, AHR-mediated induction of Cyp1a1/1b1 expression by DELAQ and TCDD decreased in hormone-treated rat EECs. DELAQ was 22-fold more potent than TCDD in human EECs in inducing CYP1A1/1B1 gene expression, while DELAQ was approximately 16-33-fold less potent than TCDD in rat EECs. In human EECs, 10 nM DELAQ decreased estradiol-induced expression of growth-regulated estrogen receptor binding 1 (GREB1) by 1.8-fold. In rat EECs, both DELAQ and TCDD did not affect the expression of estradiol-induced genes. This study shows that AHR ligand DELAQ, but not TCDD, causes anti-estrogenic effects in primary human EECs. Furthermore, although AHR-mediated CYP1A1/1B1/Cyp1a1/1b1 induction by DELAQ and TCDD was stronger in rat EECs than human EECs, this did not result in apparent anti-estrogenic effects in the rat cells. This study shows that primary human and rat endometrial cells respond differently towards hormones and AHR ligands. This should be considered in human risk assessment based on rodent studies.

Enhancement of Uterine Cancer Development after Oocyte Depletion by Juvenile Exposure to Gamma Radiation in Rats: - A Comparative Study on Sensitivity of Damage to Female Reproductive Organs by Radiation between Juvenile and Adulthood Using a Rat Model

To clarify sensitivities of juvenile exposure to radiation on uterine carcinogenesis, female Donryu rats, a high yield strain of uterine corpus cancer, were exposed to 0.2 and 1.0 Gy of gamma radiation at postnatal day 14. Sequential changes in their reproductive organs and hematology, and the effects on uterine tumor development were compared to those in adult rats exposed to the same doses. Half number of the rats in each group was treated with N-ethyl-N'-nitro-N-nitrosoguanidine (ENNG) after the radiation to accelerate the development of uterine cancer. Severe apoptosis and depletion of oocytes in the primordial/primary follicles were immediately induced after juvenile exposure at 1.0 Gy only. The ovaries in rats exposed to 1.0 Gy at juvenile showed severe atrophy characterized by the loss of all types of follicles and a lack of corpora lutea by 2 months of age, and all rats elicited an early onset of persistent estrus corresponding to the atrophy. At the termination of 9 months of age, juvenile 1.0 Gy exposure with ENNG treatment increased the incidence of endometrial adenocarcinoma and the multiplicities of combined endometrial adenocarcinomas and their precancerous lesions. Enhancement of uterine cancer development was not apparent at the same exposure without ENNG. In comet assays, neither 0.2 nor 1.0 Gy juvenile exposure induced direct DNA damage to uteri though the damage was found in the ovary at 1.0 Gy. The present results indicated that juvenile exposure to gamma radiation indirectly enhanced uterine cancer development in rats through direct damage to oocytes resulting in serious atrophy of the ovary accompanying early onset of persistent estrus. The damage to ovary was more sensitive at juvenile than adults. The result in comet assay suggested that direct DNA damage to the uterus by radiation was excluded.

Predictive modes of action of pesticides in uterine adenocarcinoma development in rats

Endometrial adenocarcinoma in the uterine corpus is a malignant cancer that occurs in menopausal women and aged rodents. Because of the similarities in pathogenesis and morphology of endometrial adenocarcinoma in rodents and humans, prediction of the modes of action (MOA) in uterine carcinogenesis is important for extrapolation of rodent data to humans. Three MOAs have been accepted as major pathways for uterine carcinogenesis in rodents: 1) estrogenic activity, 2) increased serum 17beta-estradiiol (E2) to progesterone (P4) ratio and 3) modulation of estrogen metabolism to produce 4-hydroxyestradiol via P450 induction. Inhibition of estrogen excretion and increased aromatase in situ in the tumor are also a potential pathway. Here, chemicals showing uterine carcinogenicity were chosen from approximately 300 pesticides evaluated in Japan within the past decade, and their mechanisms were predicted using parameters from mechanistic and toxicity studies. Seven pesticides increased uterine tumor formation in rats, and the pathways of 4 pesticides could be predicted based on various mechanistic studies. The MOAs of cyenopyrafen and benthiavalicarb-isopropyl were predicted to be modulation of estrogen metabolism, while those of pyriminobac-methyl and spirodiclofen were predicted to be increased E2 to P4 ratio. The driven pathways of metazosulfuron and isopyrazam could not be predicted using several mechanistic studies. No mechanistic studies have been reported for sedaxane, which has a chemical structure and toxicological profile similar to isopyrazam. Our results indicated that appropriate mechanistic studies are useful for mechanism prediction in risk assessment. From this analysis, a flowchart showing a decision tree for predictive MOAs in uterine carcinogenesis was proposed.