Expression of ovarian microsomal epoxide hydrolase and glutathione S-transferase during onset of VCD-induced ovotoxicity in B6C3F1 mice

Abstract

Abstract4-vinylcyclohexene diepoxide (VCD) specifically destroys small pre-antral follicles in the rodent ovary. VCD can be detoxified to an inactive tetrol by microsomal epoxide hydrolase (mEH), or by conjugation to glutathione (GSH) by glutathione S-transferase (GST). Formation of VCD-GSH adducts in the mouse ovary 4 h after VCD exposure (0.57mmol/kg/day) has been demonstrated. Because the mouse ovary expresses both mEH and GST, expression of mEH and GST pi and mu during a time-course of VCD-induced ovotoxicity was evaluated in a neonatal mouse ovarian culture system. Ovaries from postnatal day 4 (PND4) B6C3F 1 mice were incubated with VCD (15μM) for 2, 4, 6, 8, 10, 12, or 15 days. Following incubation, ovaries were histologically evaluated, or assessed for mRNA or protein expression. VCD did not cause follicle loss (P>0.05) on days 2, 4, or 6 of culture. At days 8, 10, 12, and 15, VCD reduced (P<0.05) both primordial and primary follicle numbers. Increased (P<0.05) expression of mEH, GST pi and GST mu mRNA was detected after 4 days of VCD exposure. This expression was reduced on days 6 and 8, when follicle loss was underway, but increased (P<0.05) after 10 days of exposure. mEH and GST pi proteins were elevated (P<0.05) following 8 days of VCD-exposure however there was no increase in GST mu protein.These findings suggest that with continuous exposure to VCD, increased expression of detoxification enzymes may participate in retarding the onset of follicle loss, but that this loss cannot ultimately be prevented.

4-vinylcyclohexene diepoxide: a model chemical for ovotoxicity

“…Subsequently, studies addressing the metabolism of VCH and VCD led to the hypothesis that in mice VCH is more readily bioactivated to VCD, and VCD is less readily detoxified, as compared with rats [Hoyer and Sipes 2007]. Further studies have determined that the mouse and rat ovary possess the enzymatic capabilities to bioactivate and detoxify VCH and VCD, respectively [Cannady et al 2002; 2003; Rajapaksa et al 2007; Keating et al 2008a; Keating et al 2008b; Keating et al 2010]. Therefore, the ovary itself may directly contribute to the degree of follicle damage produced by exposure to xenobiotic agents.…”

Ovarian expressed microsomal epoxide hydrolase: Role in detoxification of 4-vinylcyclohexene diepoxide and regulation by phosphatidylinositol-3 kinase signaling

“…mEH mRNA and enzyme activity expression were shown to increase in small pre-antral follicles of mouse ovaries following repeated daily dosing (15d) with VCD (0.57 mmol/kg/day) (Cannady et al ., 2002). Furthermore, mEH mRNA and protein were up-regulated in cultured postnatal day 4 (PND4) B6C3F 1 mouse ovaries in response to VCD exposure (Keating et al ., 2008a), and formation of the inactive tetrol metabolite [4-(1,2-dihydroxy)ethyl-1,2-dihydroxycyclohexane] in ovarian follicles of VCD dosed mice has been demonstrated (Flaws et al ., 1994). Based on these collective data, a functional role for mEH in VCD detoxification is hypothesized but has not yet been established.…”

Mechanisms of Ovarian Atresia Induced by Xenobiotic Exposures

“…How these effects are produced is becoming better understood and detailed for a number of chemical classes discussed herein. While outside of the scope of this chapter, it is important to note that the ovary has the capability to biotransform chemicals to more or less toxic metabolites, and these metabolism processes are highly active in ovarian tissues (Igawa et al, 2009;Keating et al, 2008a;2008b;Rajapaksa et al, 2007a;Rajapaksa et al, 2007b) and contribute to the extent of ovotoxicity observed.…”