Methoxychlor induces atresia of antral follicles in ERα-overexpressing mice

Methoxychlor (MXC) is a pesticide that is known to bind to estrogen receptor alpha (ERα) and to induce atresia of antral ovarian follicles. Although studies have shown that MXC is toxic to the ovary, we hypothesize that perturbation to the estrogen-signaling system (i.e., increase or decrease in estrogen sensitivity) might alter ovarian responsiveness to MXC. Thus, we examined whether ERα overexpression alters the ability of MXC to increase follicle atresia. To do so, we employed a transgenic mouse model in which ERα can be inducibly overexpressed in animal tissues (ERα overexpressors). We dosed female controls and ERα overexpressors with sesame oil (vehicle control) or MXC (32 and 64 mg/kg/day) for 20 days. After dosing, the ovaries were collected for histological evaluation of follicle numbers and follicle atresia, while blood was collected for measurements of hormones. Estrous cycles were determined in all animals to ensure that all were terminated during estrus. Although there were no significant effects of MXC on the numbers of primordial, primary, and preantral follicles in both controls and ERα overexpressors, there was an effect on antral follicles. Specifically, our data indicate that 32 and 64 mg/kg MXC increased the percentage of atretic follicles compared to vehicle in both control and ERα overexpressor groups. Moreover, there was a clear trend toward greater sensitivity to 64 mg/kg MXC in ERα-overexpressing mice compared to control animals. Specifically, at the 64-mg/kg MXC dose, ERα-overexpressing mice had a significantly higher percentage of atretic follicles compared to control animals (controls 21.5 ± 3%, n = 5; ERα overexpressors = 37 ± 23%, n = 9, p ≤ 0.05 vs. controls). After 20 days of dosing, there were no differences in estradiol levels between controls and ERα-overexpressing mice in all treatment groups. Follicle-stimulating hormone (FSH) levels were similar in sesame oil-treated control mice and control mice treated with 32 mg/kg MXC, while control mice treated with 64 mg/kg MXC had significantly lower levels of FSH compared to sesame oil-treated controls (sesame oil = 4.31 ± 0.7, MXC [64 mg/kg/day] = 1.89 ± 0.4, n = 3, p ≤ 0.02 vs. sesame oil). ERα-overexpressing mice treated with sesame oil, 32 or 64 mg/kg MXC, had similar FSH levels. Thus, we observed an increased percentage of atretic antral follicles in ERα-overexpressing mice treated with MXC compared to control mice treated with the same compound, suggesting that the ERα-signaling pathway plays an important role in MXC-induced atresia. The trend toward greater sensitivity to MXC in ERα-overexpressing mice compared to control animals cannot be explained by alterations in estradiol and/or FSH levels.