Methoxychlor Inhibits Growth and Induces Atresia of Antral Follicles through an Oxidative Stress Pathway

Abstract

The mammalian ovary contains antral follicles, which are responsible for the synthesis and secretion of hormones that regulate estrous cyclicity and fertility. The organochlorine pesticide methoxychlor (MXC) causes atresia (follicle death via apoptosis) of antral follicles, but little is known about the mechanisms by which MXC does so. Oxidative stress is known to cause apoptosis in nonreproductive and reproductive tissues. Thus, we tested the hypothesis that MXC inhibits growth and induces atresia of antral follicles through an oxidative stress pathway. To test this hypothesis, antral follicles isolated from 39-day-old CD-1 mice were cultured with vehicle control (dimethylsulfoxide [DMSO]), MXC (1-100 microg/ml), or MXC + the antioxidant N-acetyl cysteine (NAC) (0.1-10 mM). During culture, growth was monitored daily. At the end of culture, follicles were processed for quantitative real-time polymerase chain reaction of Cu/Zn superoxide dismutase (SOD1), glutathione peroxidase (GPX), and catalase (CAT) mRNA expression or for histological evaluation of atresia. The results indicate that exposure to MXC (1-100 microg/ml) inhibited growth of follicles compared to DMSO controls and that NAC (1-10 mM) blocked the ability of MXC to inhibit growth. MXC induced follicular atresia, whereas NAC (1-10 mM) blocked the ability of MXC to induce atresia. In addition, MXC reduced the expression of SOD1, GPX, and CAT, whereas NAC reduced the effects of MXC on their expression. Collectively, these data indicate MXC causes slow growth and increased atresia by inducing oxidative stress.

Mechanisms of Ovarian Atresia Induced by Xenobiotic Exposures

“…MXC-treated mouse ovaries had decreased Sod1 mRNA and increased Gpx and Cat mRNA expression after 48 hours of MXC treatment, however, at the time of MXCinduced antral follicle atresia (96 hours), there was a decrease in SOD1, GPA and CAT (Gupta et al, 2006b). At that time, MXC treatment also increased H 2 O 2 levels .…”

“…MXC has been shown to increase oxidative stress-induced mitochondrial damage (Gupta et al, 2006a;2006b). MXC-treated mouse ovaries had decreased Sod1 mRNA and increased Gpx and Cat mRNA expression after 48 hours of MXC treatment, however, at the time of MXCinduced antral follicle atresia (96 hours), there was a decrease in SOD1, GPA and CAT (Gupta et al, 2006b).…”

“…At that time, MXC treatment also increased H 2 O 2 levels . N-acetyl cysteine (NAC), an antioxidant, prevented the detrimental effects of MXC on antral follicle growth and atresia and the MXC-induced changes in SOD1, GPX and CAT (Gupta et al, 2006b). Further, treatment with Vitamin E prevented MXC-induced OSE damage (Symonds et al, 2008).…”

Ovarian Follicular Atresia

“…methoxychlor, a pesticide) on reproductive function reveal impaired follicular development, including an increase in the rate of atresia of antral follicles. Methoxychlor (MXC) reduces mRNA expression of antioxidant enzymes (SOD1, GPx, and CAT) in mouse antral follicles, and treatment with an exogenous antioxidant (N-acetyl cysteine) significantly decreases the rate of atresia induced by MXC (Gupta et al, 2006). Although the current evidence does not resolve the precise pathway(s) by which MXC induces follicle atresia, it may do so by either inducing changes in Bcl-2 followed by OS or, conversely, by leading to oxidative stress that results in Bcl-2 changes.…”

“…Moreover, atresia is a developmental process occurring along a temporal continuum; it is not a sudden process, and the timing of when follicles are experimentally analyzed during atresia might impact the determination of oxidative status. Such dynamic relationships are observed following ovarian exposure to MXC: increases followed by decreases in antioxidant enzyme expression occur prior to the time follicular atresia is evident (Gupta et al, 2006). In an initial attempt for protection, the follicle may first respond to an insult or its atretic fate by upregulating antioxidants.…”