Involvement of CYP 2E1 enzyme in ovotoxicity caused by 4-vinylcyclohexene and its metabolites

Ovarian antral follicles metabolize imidacloprid in vitro

Neonicotinoid insecticides are synthetic nicotine derivatives that have high affinity for invertebrate nicotine receptors and low affinity for mammalian nicotine receptors. However, imidacloprid (IMI), the most commonly used neonicotinoid, can be bioactivated by the liver in mammals to desnitro-imidacloprid, an intermediate metabolite that effectively binds and activates mammalian receptors. However, it is not known if other tissues such as the ovaries can metabolize IMI. Thus, the present study tested the hypothesis that ovarian antral follicles metabolize and bioactivate IMI. Antral follicles were dissected from the ovaries of CD-1 mice and cultured in media containing dimethyl sulfoxide or IMI (0.2-200 µg/ml) for 48 and 96 h. Media were subjected to liquid chromatography-mass spectrometry for detection of phase I IMI metabolites. Follicles from the cultures were used for gene expression analysis of metabolic enzymes associated with IMI metabolism. All IMI metabolites were detected at 48 and 96 h. Oxidized IMI intermediates were detected in media from cultured follicles, but not environmental controls. Reduced IMI intermediates were detected in media from cultured follicles and the environmental controls. At 48 h, IMI did not affect expression of any metabolic enzymes compared with control. At 96 h, IMI induced Cyp2e1 and Cyp4f18 compared with control. These data indicate that mouse ovarian follicles metabolize IMI and that IMI induces ovarian Cyp expression over time.

Hepatic and ovarian effects of perfluorooctanoic acid exposure differ in lean and obese adult female mice

Obesity and overweight cause poor oocyte quality, miscarriage, infertility, polycystic ovarian syndrome, and offspring birth defects and affects 40% and 20% of US women and girls, respectively. Perfluorooctanoic acid (PFOA), a per- and poly-fluoroalkyl substance (PFAS), is environmentally persistent and has negative female reproductive effects including endocrine disruption, oxidative stress, altered menstrual cyclicity, and decreased fertility in humans and animal models. PFAS exposure is associated with non-alcoholic fatty liver disease which affects ∼24-26% of the US population. This study investigated the hypothesis that PFOA exposure impacts hepatic and ovarian chemical biotransformation and alters the serum metabolome. At 7 weeks of age, female lean, wild type (KK.Cg-a/a) or obese (KK.Cg-Ay/J) mice received saline (C) or PFOA (2.5 mg/Kg) per os for 15 d. Hepatic weight was increased by PFOA exposure in both lean and obese mice (P < 0.05) and obesity also increased liver weight (P < 0.05) compared to lean mice. The serum metabolome was also altered (P < 0.05) by PFOA exposure and differed between lean and obese mice. Exposure to PFOA altered (P < 0.05) the abundance of ovarian proteins with roles in xenobiotic biotransformation (lean - 6; obese - 17), metabolism of fatty acids (lean - 3; obese - 9), cholesterol (lean - 8; obese - 11), amino acids (lean - 18; obese - 19), glucose (lean - 7; obese - 10), apoptosis (lean - 18; obese - 13), and oxidative stress (lean - 3; obese - 2). Use of qRT-PCR determined that exposure to PFOA increased (P < 0.05) hepatic Ces1 and Chst1 in lean but Ephx1 and Gstm3 in obese mice. Also, obesity basally increased (P < 0.05) Nat2, Gpi and Hsd17b2 mRNA levels. These data identify molecular changes resultant from PFOA exposure that may cause liver injury and ovotoxicity in females. In addition, differences in toxicity induced by PFOA exposure occurs in lean and obese mice.

Scrambled eggs-Negative impacts of heat stress and chemical exposures on ovarian function in swine

Exposure to environmental toxicants and hyperthermia can hamper reproduction in female mammals including swine. Phenotypic manifestations include poor quality oocytes, endocrine disruption, infertility, lengthened time to conceive, pregnancy loss, and embryonic defects. The ovary has the capacity for toxicant biotransformation, regulated in part by the phosphatidylinositol-3 kinase signaling pathway. The impacts of exposure to mycotoxins and pesticides on swine reproduction and the potential for an emerging chemical class of concern, the per- and polyfluoroalkylated substances, to hamper porcine reproduction are reviewed. The negative impairments of heat stress (HS) on swine reproductive outcomes are also described and the cumulative effect of environmental exposures, such as HS, when present in conjunction with a toxicant is considered.

Does probiotic Kefir reduce dyslipidemia, hematological disorders and oxidative stress induced by zearalenone toxicity in wistar rats?

Zearalenone (ZEA) is a toxic metabolite of the genus Fusarium, which causes hepatotoxicity and induces oxidative stress. Kefir is an important probiotic dairy-product showing important in vitro antioxidant potential. In this study, the effect of Kefir supplementation to mitigate ZEA toxicity in rats was investigated. Animals were divided into four groups of five rats each, which received sterile milk (200 μL/day) during the first week. Then, they were switched to Kefir (200 μL/day), ZEA (40 mg/kg b. w./day) and Kefir + ZEA for the second week. Hematological and biochemical parameters, as well as liver histological analysis were determined. Kefir administration prevented the changes occurred in the count of all blood cells, and improved the antioxidant enzymes in the liver, such as catalase, glutathione peroxidase and superoxide dismutase activities that increased by 6, 4.5 and 1.3 folds, respectively, compared to ZEA group. Interestingly, the concurrent regimen Kefir + ZEA removed ZEA residues in the serum and liver. Furthermore, the Kefir + ZEA group showed a reduction in the levels of bilirubin, alanine aminotransferase, aspartate aminotransferase, alkaline phosphatase and hepatic malonaldehyde by ∼82, 54, 66, 50 and 36%, respectively, compared to the ZEA group. The histopathological analysis showed a normal liver histological architecture in Kefir + ZEA group, while degenerative changes were observed in ZEA group. These results suggest that Kefir as probiotic consortium may have a hepatoprotective effect against ZEA poisoning.

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Zearalenone caused oxidative stress and liver damage in rats.

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Zearalenone induced disruption of hematological and biochemical parameters.

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Kefir improved the antioxidant defense systems in rats subjected to Zearalenone.

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Kefir prevented Zearalenone-induced hepatotoxicity in rats.

Assessing the toxicant effect of spontaneously volatilized 4-vinylcyclohexane exposure in nymphs of the lobster cockroach nauphoeta cinerea

Vinylcyclohexene (VCH) is an environmental contaminant well known for its ovotoxicant effects in several organisms. However, the mechanisms underlying the toxicity of VCH as well as its harmful effects toward other organs are until unclear. In this work, we assess some endpoint signals of toxicity induced by volatilized VCH exposure using nymphs of the lobster cockroach Nauphoeta cinerea. Nymphs were exposed to VCH via inhalation for 70 days. The levels of volatilized VCH were quantified by headspace gas chromatography and the concentration varied between 3.41 and 7.03 nmol/μl. VCH inhalation caused a reduction of 35% in the survival rate of the exposed animals. Nymphs exposed to volatilized VCH for 35 and 70 days had a reduction in the body weight gain of 1.8- and 2.6-fold, respectively with a reduction in dissected head, fat body, and maturing reproductive organs. The exposure did not change water consumption, excepting on the 20th day (with a 3-fold change) and decreased the food intake significantly. Regarding biochemical markers, we found that the activity of GST from the dissected organs was increased by volatilized VCH after both 35 and 70 days of exposure. The fat body presented the most prominent GST activity especially after 35 days of exposure with 1.6-fold higher than the control group. Exposure also caused an increase in RS levels in the fat body of 1.35-fold and 1.47-fold after 35 and 70 days, respectively and did not affect the activity of the AChE from the head. Our findings support the harmful impact of volatilized VCH inhalation, highlighting the cockroach N.cinerea as a valuable insect model to investigate environmental toxicants.

The detoxification effect of vitamin C on zearalenone toxicity in piglets

Zearalenone (ZEN), one of the more virulent mycotoxins occurred in various cereals and feed during recent decades and made serious health hazards to plants, animals and humans. Vitamin C (Vc) has been shown to be an effective antidote to zearalenone. In this paper, the effects of diets containing zearalenone on the growth performance, genital organ and immunoglobulin of weaning piglets and the toxicity alleviation of vitamin C were studied. Piglets were weaned at 21 days of age and 32 healthy female hybrid weaning piglets (Duroc × Landrace × Large white) with a mean weight of 12.27 ± 0.30 kg were randomly selected. The thirty-two female weaning piglets were divided into four treatment groups according to body weight: control; basal diet + vitamin C (150 mg/kg); basal diet + 1.0 mg/kg ZEN; basal diet + 1 mg/kg ZEN+vitamin C (150 mg/kg). There were eight replicates in each group. The test period was twenty-eight days. The results demonstrated that dietary zearalenone could significantly increase the length, width and area of vulva (P < 0.05), the genital organ coefficient (P < 0.05), the level of IgA, IgG and IgM (P < 0.05), the level of BUN, CRE, AST and TBIL (P < 0.05), and significantly decrease the level of E2, PROG, LH and FSH (P < 0.05). However, the addition of 150 mg/kg vitamin C to dietary zearalenone prevented deformities in the vulva, decrease in immune response capacity, changes in serum biochemical indicators and disorders in hormones level of the piglets that received the diet containing only zearalenone. In conclusion, feeding ZEN of 1.0 mg/kg can result in a deleterious effect on piglets, which was totally or partly ameliorated by dietary supplementation of vitamin C at concentrations about 150 mg/kg diet. This study systematically investigated the inhibition mechanism of vitamin C on ZEN-induced reproductive toxicity, immunotoxicity and hematological toxicity of piglets, and which provided new ideas for reducing the harm of mycotoxins to the animals through means of nutrition regulation.

Progressive obesity alters ovarian insulin, phosphatidylinositol-3 kinase, and chemical metabolism signaling pathways and potentiates ovotoxicity induced by phosphoramide mustard in mice

Mechanisms underlying obesity-associated reproductive impairment are ill defined. Hyperinsulinemia is a metabolic perturbation often observed in obese subjects. Insulin activates phosphatidylinositol 3-kinase (PI3K) signaling, which regulates ovarian folliculogenesis, steroidogenesis, and xenobiotic metabolism. The impact of progressive obesity on ovarian genes encoding mRNA involved in insulin-mediated PI3K signaling and xenobiotic biotransformation [insulin receptor (Insr), insulin receptor substrate 1 (Irs1), 2 (Irs2), and 3 (Irs3); kit ligand (Kitlg), stem cell growth factor receptor (Kit), protein kinase B (AKT) alpha (Akt1), beta (Akt2), forkhead transcription factor (FOXO) subfamily 1 (Foxo1), and subfamily 3 (Foxo3a), microsomal epoxide hydrolase (Ephx1), cytochrome P450 family 2, subfamily E, polypeptide 1 (Cyp2e1), glutathione S-transferase (GST) class Pi (Gstp1) and class mu 1 (Gstm1)] was determined in normal wild-type nonagouti (a/a; lean) and lethal yellow mice (KK.CG-Ay/J; obese) at 6, 12, 18, or 24 weeks of age. At 6 weeks, ovaries from obese mice had increased (P < 0.05) Insr and Irs3 but decreased (P < 0.05) Kitlg, Foxo1, and Cyp2e1 mRNA levels. Interestingly, at 12 weeks, an increase (P < 0.05) in Kitlg and Kit mRNA, pIRS1Ser302, pAKTThr308, EPHX1, and GSTP1 protein level was observed due to obesity, while Cyp2e1 mRNA and protein were reduced. A phosphoramide mustard (PM) challenge increased (P < 0.05) ovarian EPHX1 protein abundance in lean but not obese females. In addition, lung tissue from PM-exposed animals had increased (P < 0.05) EPHX1 protein with no impact of obesity thereon. Taken together, progressive obesity affected ovarian signaling pathways potentially involved in obesity-associated reproductive disorders.

Danzhi Qing'e (DZQE) activates AMPK pathway and regulates lipid metabolism in a rat model of perimenopausal hyperlipidaemia

What is the central question of this study? Does Danzhi Qing'e (DZQE) regulate lipid metabolism and improve ovarian function in a rat model of perimenopausal hyperlipidaemia, and could this effect be mediated through the AMPK pathway? What is the main finding and its importance? We revealed that DZQE is a pharmacotherapy that could activate the AMPK pathway to improve ovarian function and lipid metabolism during perimenopause complicated with hyperlipidaemia syndrome in an animal model. Thus, this study provides a novel therapeutic option for treating perimenopausal syndrome and highlights the therapeutic potential of DZQE in perimenopausal rats. Menopause is an important event in a woman's life. During perimenopause, accompanied by development of osteoporosis and dyslipidaemia, ovarian function gradually declines. Dyslipidaemia is a risk factor for cardiovascular disorders, cerebrovascular disease and breast cancer in postmenopausal women. All of these contribute to impairment of liver function, particularly fatty liver disease, because liver dysfunction is associated with ovarian dysfunction and hyperlipidaemia. The aim of this study was to define a therapeutic approach to improve ovarian function and attenuate lipid accumulation in order to prevent perimenopause-induced ovarian dysfunction and hyperlipidaemia. Four-week-old female Wistar rats were injected i.p. with 4-vinylcyclohexene diepoxide (4-VCD) and fed with a high-fat diet (HFD) to serve as a model of perimenopause complicated with hyperlipidaemia. The 4-VCD induces perimenopause, while the HFD causes hyperlipidaemia. Five days after administration of 4-VCD, the 4-VCD + HFD-treated rats were assessed daily for oestrous cycle stage by vaginal cytology. Rats were then assigned into groups, in which 2.5, 5.0 or 10.0 g kg^-1 Danzhi Qing'e (DZQE) or estradiol valerate was administered intragastrically for 8 weeks. Expression levels of follicle-stimulating hormone (FSH), luteinizing hormone (LH), oestrogen and testosterone measured by enzyme-linked immunosorbent assay served as biomarkers for perimenopause and ovarian dysfunction. The expression levels of AMPK and acetyl-CoA carboxylase in the liver were determined with Western blotting, and aspartate aminotransferase and alanine aminotransferase were analysed using an automated biochemical analyser to examine liver function. The DZQE improved ovarian function by upregulating oestrogen and testosterone concentrations in serum and downregulating FSH and LH serum concentrations. Moreover, DZQE reduced serum concentrations of triglyceride, total cholesterol and low-density lipoprotein in a dose-dependent manner to regulate lipid levels during perimenopause. Furthermore, DZQE increased AMPK at both the transcriptional and translational levels and decreased the expression of SREBP-1c gene as well as its downstream target gene, fatty acid synthase. Danzhi Qing'e improved dyslipidaemia during menopause and also had an effect on liver function. Danzhi Qing'e is an effective Chinese herbal compound, which improves ovarian function and lipid metabolism in perimenopause complicated with hyperlipidaemia at least in part through the AMPK pathway.

Hepatic and renal toxicological evaluations of an industrial ovotoxic chemical, 4-vinylcyclohexene diepoxide, in both sexes of Wistar rats

4-Vinylcyclohexene diepoxide (VCD) is an industrial occupational health hazard chemical because it induces ovotoxicity in rodents. The current study investigated the impacts of VCD on selected hepatic and renal markers of oxidative stress and inflammation in both sexes of Wistar rats. Thus, male and female rats were randomly distributed into four groups of ten rats per group, and dosed orally with VCD for 28days. The control male and female groups of rats received corn oil only, while each of the three remaining groups of both sexes of rats received VCD (100, 250 and 500mg/kg BW) respectively. Thereafter, biomarkers of hepatic and renal oxidative damage, inflammation and immunohistochemical expressions of iNOS, COX-2, caspase-9 and caspase-3 were evaluated. The results revealed that VCD increased markers of liver and kidney functions, oxidative damage and inflammation, and disrupted the antioxidant homeostasis of the rats (p<0.05). Lastly, VCD enhanced the immunohistochemical expressions of iNOS, COX-2, caspase-9 and caspase-3 in the liver of the rats. Thus, our data imply that VCD induced toxicity in the liver and kidney of rats via the combined impacts of oxidative damage and inflammation.

Conditions and possible mechanisms of VCD-induced ovarian failure

Perimenopause is an important period in women's lives, in which they experience a series of physiological changes. Current animal models of perimenopause fail to adequately replicate this particular stage in female life, while current in vitro models are too simplistic and cannot account for systemic effects. Neither the naturally-ageing animal model, nor the ovariectomised animal model, mimic the natural transitional process that is the menopause. In vivo and in vitro studies have confirmed that the occupational chemical, 4-vinylcyclohexene diepoxide (VCD), can cause selective destruction of the ovarian primordial and primary follicles of rats and mice by accelerating the apoptotic process, which successfully mimics the perimenopausal state in women. However, it is the in vivo VCD-induced rodent perimenopausal models that are currently the most widely used in research, rather than any of the available in vitro models. Studies on the mechanisms involved have found that VCD induces ovotoxicity via interference with the c-kit/kit ligand and apoptotic signalling pathways, among others. Overall, the VCD-induced perimenopausal animal models have provided some insight into female perimenopause, but they are far from ideal models of the human situation.

Ovotoxicants 4-vinylcyclohexene 1,2-monoepoxide and 4-vinylcyclohexene diepoxide disrupt redox status and modify different electrophile sensitive target enzymes and genes in Drosophila melanogaster

The compounds 4-vinylcyclohexene 1,2-monoepoxide (VCM) and 4-Vinylcyclohexene diepoxide (VCD) are the two downstream metabolites of 4-vinylcyclohexene (VCH), an ovotoxic agent in mammals. In addition, VCM and VCD may be found as by-products of VCH oxidation in the environment. Recently, we reported the involvement of oxidative stress in the toxicity of VCH in Drosophila melanogaster. However, it was not possible to determine the individual contributions of VCM and VCD in VCH toxicity. Hence, we investigated the toxicity of VCM and VCD (10–1000 µM) in flies after 5 days of exposure via the diet. Our results indicated impairments in climbing behaviour and disruptions in antioxidant balance and redox status evidenced by an increase in DCFH oxidation, decreases in total thiol content and glutathione-S-transferase (GST) activity in the flies exposed to VCM and VCD (p<0.05). These effects were accompanied by disruptions in the transcription of the genes encoding the proteins superoxide dismutase (SOD1), kelch-like erythroid-derived cap-n-collar (CNC) homology (ECH)-associated protein 1 (Keap-1), mitogen activated protein kinase 2 (MAPK-2), catalase, Cyp18a1, JAFRAC 1 (thioredoxin peroxidase 1) and thioredoxin reductase 1 (TrxR-1) (p<0.05). VCM and VCD inhibited acetylcholinesterase (AChE) and delta aminolevulinic acid dehydratase (δ-ALA D) activities in the flies (p<0.05). Indeed, here, we demonstrated that different target enzymes and genes were modified by the electrophiles VCM and VCD in the flies. Thus, D. melanogaster has provided further lessons on the toxicity of VCM and VCD which suggest that the reported toxicity of VCH may be mediated by its transformation to VCM and VCD.

Involvement of oxidative stress in 4-vinylcyclohexene-induced toxicity in Drosophila melanogaster

4-Vinylcyclohexene (VCH) is a dimer of 1,3-butadiene produced as a by-product of pesticides, plastic, rubber, flame retardants, and tire production. Although, several studies have reported the ovotoxicity of VCH, information on a possible involvement of oxidative stress in the toxicity of this occupational chemical is scarce. Hence, this study was carried out to investigate further possible mechanisms of toxicity of VCH with a specific emphasis on oxidative stress using a Drosophila melanogaster model. D. melanogaster (both genders) of 1 to 3 days old were exposed to different concentrations of VCH (10 µM-1 mM) in the diet for 5 days. Subsequently, the survival and negative geotaxis assays and the quantification of reactive oxygen species (ROS) generation were determined. In addition, we evaluated RT-PCR expressions of selected oxidative stress and antioxidant mRNA genes (HSP27, 70, and 83, SOD, Nrf-2, MAPK2, and catalase). Furthermore, catalase, glutathione-S-transferase (GST), delta aminolevulinic acid dehydratase (δ-ALA-D), and acetylcholinesterase (AChE) activities were determined. VCH exposure impaired negative geotaxic behavior and induced the mRNA of SOD, Nrf-2, and MAPK2 genes expressions. There were increases in catalase and ROS production, as well as inhibitions of GST, δ-ALA-D, and AChE activities (P<0.05). Our results suggest that the VCH mechanism of toxicity is associated with oxidative damage, as evidenced by the alteration in the oxidative stress-antioxidant balance, and possible neurotoxic consequences due to decreased AChE activity, and impairments in negative geotaxic behavior. Thus, we conclude that D. melanogaster is a useful model for investigating the toxicity of VCH exposure, and here, we have provided further insights on the mechanism of VCH-induced toxicity.

Impact of obesity on ovotoxicity induced by 7,12-dimethylbenz[a]anthracene in mice

Insulin, elevated during obesity, regulates xenobiotic biotransformation enzymes, potentially through phosphatidylinositol 3-kinase (PI3K) signaling, in extraovarian tissues. PI3K regulates oocyte viability, follicular activation, and ovarian chemical biotransformation. 7,12-Dimethylbenz[a]anthracene (DMBA), a carcinogen and ovotoxicant, destroys all stages of follicles, leading to premature ovarian failure. Obesity has been reported to promote DMBA-induced tumors, but it remains unknown whether obesity affects ovarian xenobiotic metabolism. Therefore, we investigated ovarian expression of xenobiotic metabolism genes-microsomal epoxide hydrolase (Ephx1), glutathione S-transferase (GST) class Pi (Gstp1) and class mu 1 (Gstm1), and PI3K-signaling members (protein kinase B [AKT] alpha [Akt1], beta [Akt2], and the forkhead transcription factor subfamily 3 [Foxo3])-in lean and obese female mice after DMBA exposure (1 mg/kg; intraperitoneal injection for 14 days). Relative to lean, obese mice had decreased (P < 0.05) healthy primordial and primary follicle numbers but increased (P < 0.05) secondary and preovulatory follicles numbers. Obesity increased (P < 0.05) Akt1, Akt2, Gstm1, and Ephx1 mRNA and pAKT(Ser473/Thr308), GSTM1, GSTP1, and EPHX1 protein levels. DMBA decreased (P < 0.05) ovarian weight in lean and obese mice, however, obese DMBA-treated females had a greater reduction (P < 0.05) in ovarian weight. In both lean and obese mice, DMBA decreased (P < 0.05) all stages of healthy follicle numbers, increased Gstp1 and Ephx1 mRNA as well as GSTM1, GSTP1, and EPHX1 protein levels, and decreased Akt1 and Akt2 mRNA as well as pAKT(Ser473) or pAKT(Thr308), FOXO3, and pFOXO3(Ser253) protein expression. There was an additive effect between obesity and DMBA exposure for increased Gstm1 and Ephx1 mRNA as well as GSTM1 and EPHX1 protein expression.

Glutathione S-transferase class μ regulation of apoptosis signal-regulating kinase 1 protein during VCD-induced ovotoxicity in neonatal rat ovaries

4-Vinylcyclohexene diepoxide (VCD) destroys ovarian primordial and small primary follicles via apoptosis. In mice, VCD exposure induces ovarian mRNA expression of glutathione S-transferase (GST) family members, including isoform mu (Gstm). Extra-ovarian GSTM negatively regulates pro-apoptotic apoptosis signal-regulating kinase 1 (ASK1) through protein complex formation, which dissociates during stress, thereby initiating ASK1-induced apoptosis. The present study investigated the ovarian response of Gstm mRNA and protein to VCD. Induction of Ask1 mRNA at VCD-induced follicle loss onset was determined. Ovarian GSTM:ASK1 protein complex formation was investigated and VCD exposure effects thereon evaluated. Phosphatidylinositol-3 kinase (PI3K) regulation of GSTM protein was also studied. Postnatal day (PND) 4 rat ovaries were cultured in control media ± 1) VCD (30 μM) for 2-8 days; 2) VCD (30 μM) for 2 days, followed by incubation in control media for 4 days (acute VCD exposure); or 3) LY294002 (20 μM) for 6 days. VCD exposure did not alter Gstm mRNA expression, however, GSTM protein increased (P<0.05) after 6 days of both the acute and chronic treatments. Ask1 mRNA increased (0.33-fold; P<0.05) relative to control after 6 days of VCD exposure. Ovarian GSTM:ASK1 protein complex formation was confirmed and, relative to control, the amount of GSTM bound to ASK1 increased 33% (P<0.05) by chronic but with no effect of acute VCD exposure. PI3K inhibition increased (P<0.05) GSTM protein by 40% and 71% on d4 and d6, respectively. These findings support involvement of GSTM in the ovarian response to VCD exposure, through regulation of pro-apoptotic ASK1.

4-vinylcyclohexene diepoxide: a model chemical for ovotoxicity

The occupational chemical 4-vinylcyclohexene diepoxide (VCD) has been shown to cause selective destruction of ovarian small pre-antral (primordial and primary) follicles in rats and mice by accelerating the natural, apoptotic process of atresia. Chemicals that destroy primordial follicles are of concern to women because exposure can result in premature ovarian failure (early menopause). Initial studies using in vivo exposure of rats determined that VCD specifically targets primordial and primary (small pre-antral) follicles and that repeated dosing is required. Through a method of isolation of ovarian small follicles, biochemical and molecular studies determined that intracellular pro-apoptotic pathways are activated following VCD dosing in rats. Subsequently an in vitro system using cultured whole neonatal rat ovaries was developed to provide more mechanistic information. That approach was used to demonstrate that the cell survival c-kit/kit ligand signaling pathway is the direct target for VCD-induced ovotoxicity. Specifically, VCD directly interacts with the oocyte-associated c-kit receptor to inhibit its autophosphorylation, and thereby impair oocyte viability. The cellular and molecular approach developed to determine these findings is described in this article.

Impact of environmental exposures on ovarian function and role of xenobiotic metabolism during ovotoxicity

The mammalian ovary is a heterogeneous organ and contains oocyte-containing follicles at varying stages of development. The most immature follicular stage, the primordial follicle, comprises the ovarian reserve and is a finite number, defined at the time of birth. Depletion of all follicles within the ovary leads to reproductive senescence, known as menopause. A number of chemical classes can destroy follicles, thus hastening entry into the menopausal state. The ovarian response to chemical exposure can determine the extent of ovotoxicity that occurs. Enzymes capable of bioactivating as well as detoxifying xenobiotics are expressed in the ovary and their impact on ovotoxicity has been partially characterized for trichloroethylene, 7,12-dimethylbenz[a]anthracene, and 4-vinylcyclohexene. This review will discuss those studies, as well as illustrate where knowledge gaps remain for chemicals that have also been established as ovotoxicants.

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

4-vinylcyclohexene diepoxide (VCD) is a metabolite of 4-vinylcyclohexene (VCH) which has the potential to be formed in the ovary through CYP2E1 activity. VCD specifically destroys primordial and small primary follicles in the rodent ovary. Mouse ovaries exposed to VCD demonstrate increased mRNA and protein expression of microsomal epoxide hydrolase (mEH), and an inactive tetrol metabolite (4-(1,2-dihydroxy)ethyl-1,2-dihydroxycyclohexane) can be formed in mouse ovarian follicles, potentially through detoxification action of mEH. In contrast, mEH can bioactivate another ovotoxic chemical, 7,12-dimethylbenz[a]anthracene (DMBA) to a more toxic compound, DMBA-3,4-diol-1,2-epoxide. Thus, the present study evaluated a functional role for mEH during detoxification of VCD. Additionally, because inhibition of the phosphatidyinositol-3 kinase (PI3K) signaling pathway in a previous study protected primordial follicles from VCD-induced destruction, but accelerated DMBA-induced ovotoxicity, a role for PI3K in ovarian mEH regulation was evaluated. Using a post-natal day (PND) 4 Fischer 344 rat whole ovary culture system inhibition of mEH using cyclohexene oxide during VCD exposure resulted in a greater (P<0.05) loss of primordial and small primary follicles relative to VCD-treated ovaries. Also, relative to controls, meh mRNA was increased (P<0.05) on day 4 of VCD (30 μM) exposure, followed by increased (P<0.05) mEH protein after 6 days. Furthermore, inhibition of PI3K signaling increased mEH mRNA and protein expression. Thus, these results support a functional role for mEH in the rat ovary, and demonstrate the involvement of PI3K signaling in regulation of ovarian xenobiotic metabolism by mEH.

Ovarian metabolism of xenobiotics

At birth, the mammalian ovary contains a finite number of primordial follicles, which once depleted, cannot be replaced. Xenobiotic exposures can destroy primordial follicles resulting in premature ovarian failure and, consequently, early entry into menopause. A number of chemical classes can induce premature ovarian failure, including environmental, chemotherapeutic and industrial exposures. While our knowledge on the mechanistic events that occur in the ovary with chemical exposures is increasing, our understanding of the ovary's capacity to metabolize such compounds is less established. This review will focus on three chemicals for which information on ovarian metabolism is known: trichloroethylene, 7,12-dimethylbenz[a]anthracene and 4-vinylcyclohexene. The current state of understanding of ovarian bioactivation and detoxification processes for each will be described.

Dual protective role for glutathione S-transferase class pi against VCD-induced ovotoxicity in the rat ovary

The occupational chemical 4-vinylcyclohexene diepoxide (VCD) selectively destroys ovarian small pre-antral follicles in rats and mice via apoptosis. Detoxification of VCD can occur through glutathione conjugation, catalyzed by glutathione S-transferase (GST) enzymes. Further, GST class pi (GSTp) can negatively regulate JNK activity through protein:protein interactions in extra-ovarian tissues. Dissociation of this protein complex in the face of chemical exposure releases the inhibition of pro-apoptotic JNK. Increased JNK activity during VCD-induced ovotoxicity has been shown in isolated ovarian small pre-antral follicles following in vivo dosing of rats (80mg/kg/day; 15days, i.p.). The present study investigated the pattern of ovarian GSTp expression during VCD exposure. Additionally, the effect of VCD on an ovarian GSTp:JNK protein complex was investigated. PND4 F344 rat ovaries were incubated in control medium+/-VCD (30muM) for 2-8days. VCD increased ovarian GSTp mRNA (P <0.05) relative to control on d4-d8; whereas GSTp protein was increased (P<0.05) on d6-d8. A GSTp:JNK protein complex was detected by immunoprecipitation and Western blotting in ovarian tissues. Relative to control, the amount of GSTp-bound JNK was increased (P=0.09), while unbound JNK was decreased (P<0.05) on d6 of VCD exposure. The VCD-induced decrease in unbound JNK was preceded by a decrease in phosphorylated c-Jun which occurred on d4. These findings are in support of a possible dual protective role for GSTp in the rat ovary, consisting of metabolism of VCD and inhibition of JNK-initiated apoptosis.

Current methods in investigating the development of the female reproductive system

The female reproductive system is important as the site for development and fertilization of an oocyte, for implantation and development of an embryo, and for growth and delivery of the fetus. It also produces protein and steroid hormones that help maintain a female's health. Although the female phenotype is the default pathway for the development of the urogenital system, many processes can become disrupted during and after development which may originate from developmental problems. Improper development can be the underlying cause of structural malformations, sub- or infertility, hormonal abnormalities, endometriosis, carcinogenesis, or other detrimental outcomes. Our research programs examine the normal physiology and function of the female reproductive system and how it can become damaged due to pathologies or environmental/therapeutic exposures, with a focus on the ovary, ovarian follicles, and ovarian hormones. This chapter will describe detailed protocols of an in vitro organ culture system and methods to analyze changes in follicle formation, follicle development, and ovarian physiology. These methods can also be applied to the study of other aspects of female reproduction.

Effect of CYP2E1 gene deletion in mice on expression of microsomal epoxide hydrolase in response to VCD exposure

Females are born with a finite number of primordial follicles. 4-Vinylcyclohexene diepoxide (VCD) is a metabolite formed by epoxidation of 4-vinylcyclohexene (VCH) via its two monoepoxides 1,2- and 7,8-4-vinylcyclohexene monoepoxide (VCM). VCD specifically destroys small preantral (primordial and small primary) follicles in the rodent ovary. The phase I enzyme, cytochrome P450 isoform 2E1 (CYP2E1) is involved in ovarian metabolism of VCM to VCD. Further, microsomal epoxide hydrolase (mEH) can detoxify VCD to an inactive tetrol (4-(1,2-dihydroxy)ethyl-1,2-dihydroxycyclohexane). This study evaluated the effects of VCD-induced ovotoxicity on mEH in CYP2E1+/+ and -/- mice (129S(1)/SvImJ background strain) using a postnatal day 4 mouse whole ovary culture system. The hypothesis of our study is that there is a relationship between CYP2E1 and mEH gene expression in the mouse ovary. Relative to control, VCD exposure caused follicle loss (p < 0.05) in ovaries from both genotypes; however, after 15 days, this loss was greater (p < 0.05) in CYP2E1+/+ ovaries. In a time course (2-15 days), relative to control, VCD (5 microM) caused an increase (p < 0.05) in mEH mRNA by 0.5-fold (day 10) and 1.84-fold (day 15) in CYP2E1-/- but not +/+ ovaries. 7,12-Dimethylbenz[a]anthracene (DMBA) also destroys ovarian follicles but, unlike VCD, is bioactivated by mEH to an ovotoxic 3,4-diol-1,2-epoxide metabolite. Incubation of ovaries in increasing concentrations of DMBA (0.5-1 microM, 15 days) resulted in greater (p < 0.05) follicle loss in CYP2E1-/-, relative to +/+ ovaries. With greater mEH (CYP2E1-/-), increased follicle loss with DMBA (bioactivation) and decreased follicle loss with VCD (detoxification) support that ovarian expression of CYP2E1 and mEH may be linked.

Expression of ovarian microsomal epoxide hydrolase and glutathione S-transferase during onset of VCD-induced ovotoxicity in B6C3F(1) mice

4-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 (0.57 mmol/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 (15 microM) 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.