Effects of methoxychlor and its metabolites on mouse antral follicles

In the female reproductive system, the ovary is the primary functional organ responsible for oocyte production, follicle maturation and synthesis of sex steroid hormones including estrogens. Throughout the lifetime of a female, normal levels of estrogen are critical for the proper development and functioning of the ovaries. An alteration in the levels of estrogen can result in impaired development of the ovaries, which then adversely affects their response to endogenous hormones or exogenous chemicals. Currently, humans are exposed to numerous chemicals including organochlorine pesticides that are introduced into the environment on a daily basis. Some of these chemicals exert their toxicity by binding to estrogen receptors in inappropriate manners or by inhibiting the ability of endogenous estrogen to bind to its receptors. Methoxychlor (MXC; 1,1,1-Trichloro-2,2-bis(4-methoxyphenyl)ethane) is an estrogenic organochlorine pesticide that has been shown to cause adverse reproductive outcomes in mammalian males and females. Cytochrome P450 enzymes metabolize MXC to mono-hydroxy MXC (MOH; 1,1,1-trichloro-2-(4-hydroxyphenyl)-2-(4-methoxyphenyl)ethane) and bis-hydroxy MXC (HPTE; 1,1,1-trichloro-2,2-bis(4-hydroxyphenyl)ethane). MXC as well as its metabolites can interact with ESRs and are known to cause ovarian toxicity. The goal of my doctoral dissertation is to determine whether endocrine disruption in the normal estrogenic pathway leads to altered sensitivity of the ovaries to environmental chemicals. Specifically, I have used a transgenic mouse model in which estrogen receptor alpha (ESR1) is overexpressed (OE) in several reproductive and non-reproductive tissues. I hypothesized that the ovaries of ESR1 OE mice will be more sensitive to growth inhibition and atresia induced by MXC and its metabolites. in vitro as well as in vivo. Results indicate that ovarian antral follicles of ESR1 OE are more sensitive to growth inhibition and atresia induced by MXC and its metabolites compared to controls. Moreover, MXC and its metabolites cause differential gene expression of nuclear receptors and pro-apoptotic markers in ESR1 OE mouse ovaries compared to control ovaries. Furthermore, ESR1 OE mice have lower levels of Cyp3a4 (an enzyme involved in the metabolizing pathway of MXC) compared to controls, suggesting that MXC may not be clearing out of ESR1 OE animals compared to controls. Collectively, these results suggest that altered gene expression of Cyp3a4, nuclear receptors that bind to MXC and pro-apoptotic markers may play a vital role in causing ESR1 OE mouse antral follicles to be more sensitive to growth inhibition and atresia caused by MXC and its metabolites compare to controls