Methoxychlor-Induced Atresia in the Mouse Involves Bcl-2 Family Members, but Not Gonadotropins or Estradiol1

Abstract

Methoxychlor (MXC) is an organochlorine pesticide that increases the rate of ovarian atresia. To date, little is known about the mechanism by which MXC induces atresia. Because Bcl-2 (an antiapoptotic factor), Bax (a proapoptotic factor), gonadotropins, and estradiol are important regulators of atresia in the ovary, the purpose of this study was first to examine whether MXC-induced atresia occurred through alterations in Bcl-2 or Bax, and second, to examine the effect of MXC on gonadotropins, estradiol, and their receptors. CD-1 mice were dosed with 8-64 mg kg(-1) day(-1) MXC or vehicle (sesame oil). Ovaries were subjected to analysis of antral follicle numbers, Bcl-2, Bax, estrogen receptor, and follicle-stimulating hormone receptor levels. Blood was used to measure gonadotropins and estradiol. In some experiments, mice that overexpressed Bcl-2 or mice that were deficient in Bax were dosed with MXC or vehicle and their ovaries were analyzed for atresia. MXC caused a dose-dependent increase in the percentage of atretic antral follicles compared with controls at the 32 and 64 mg kg(-1) day(-1) doses of MXC. MXC treatment did not result in changes in Bcl-2 levels, but it did result in an increase in Bax levels in antral follicles. MXC treatment did not affect gonadotropin or estradiol levels, nor did it affect the levels of follicle-stimulating hormone or estrogen receptors. Mice that overexpressed Bcl-2 or mice that were deficient in Bax were protected from MXC-induced atresia. These data suggest that MXC induces atresia through direct effects on the Bax and Bcl-2 signaling pathways in the ovary.

Early postnatal methoxychlor exposure inhibits folliculogenesis and stimulates anti-Mullerian hormone production in the rat ovary

“…However, whether MXC affects the hypothalamus and pituitary is controversial. Injection of MXC (8-64 mg/kg per day) in adult mice for 20 days did not change the serum FSH levels or levels of the FSH receptor in the ovary (Borgeest et al 2004). In addition, feeding MXC (24-1200 ppm) to pregnant rats between embryonic day 15 (E15) and P10 does not affect the size of the sexually dimorphic nucleus of the pre-optic area of the hypothalamus in the female offspring (Masutomi et al 2003).…”

Mechanisms of Ovarian Atresia Induced by Xenobiotic Exposures

“…MXC took a longer time to inhibit growth of antral follicles from both Bax deficient and Bcl-2 over expressing mice (96 hours) when compared to the wild type controls (72 hours; Miller et al, 2005). Interestingly, anti-apoptotic Bcl-2 over-expressing mice were shown to have more healthy antral follicles following MXC treatment when compared to MXC-treated wild type controls following 20 days (Borgeest et al, 2004). Fig.…”

“…Mechanistic investigations have indicated that MXC accelerates atresia via increased apoptosis involving the BCL-2 proto-oncogene family (Figure 4) (Borgeest et al, 2002, Miller et al, 2005 in isolated mouse antral follicles. Immunohistochemical analysis demonstrated increased pro-apoptotic BAX protein staining in MXC-treated antral follicles (Borgeest et al, 2004). MXC increased Bax mRNA expression in cultured mouse antral follicles in vitro after 48, 72 and 96 hours, but decreased mRNA expression of Bcl-2 only after 96 hours (Miller et al, 2005).…”

BAX is involved in regulating follicular growth, but is dispensable for follicle atresia in adult mouse ovaries

“…As BAX is a proapoptotic protein that is expressed in the ovary, and has been shown to be involved in regulating immature follicle atresia and the mediation BAX is dispensable for follicle atresia in mice of follicle atresia induced by a variety of toxicants , Springer et al 1996, Hu et al 2001, Matikainen et al 2001, Borgeest et al 2004, Miller et al 2005, we hypothesized that Bax deletion would enhance follicle survival and lead to the presence of increased numbers of healthy follicles. This was not the case, and instead, we observed a defect in folliculogenesis following Bax deletion.…”

“…For example, Tilly et al (1995) found that Bax expression is reduced following in vivo eCG (an FSH analog) treatment in rats, and that follicles cultured in serum-free media in the absence of eCG underwent atresia and exhibited greatly increased Bax expression. Further, 4-vinylcyclohexene diepoxide (VCD) and the pesticide methoxychlor (MXC) induce atresia of small preantral and antral follicles respectively, and do so at least in part, through BAXmediated mechanisms (Springer et al 1996, Hu et al 2001, Borgeest et al 2004, Miller et al 2005.…”