The biochemical basis for the species difference in hepatic microsomal 4-vinylcyclohexene epoxidation between female mice and rats

Aged and induced-premature ovarian failure mouse models affect diestrus profile and ovarian features

Sex hormones exert a wide influence on several systems of the human body, especially in women, who undergo intense changes in the trans and postmenopausal periods. Different experimental models are used to mimic these conditions; however, the impact on hormonal profile may be different. This study aimed to analyze and compare vaginal cytology of different post-estropausal mice models, along with their microscopical ovarian features. Forty-six C57BL/6J female mice with the ages of 4, 6 and 18 months at the beginning of the experiment, weighing about 25-28 grams, constituted five groups: NC-(negative control) animals with no treatment, OVX-SHAM-sham ovariectomized, OVX-ovariectomized, VCD-medicated with 160 mg/kg/day of 4-vinylcyclohexene diepoxide via IP for 20 consecutive days, and Aged-senescent mice under physiological estropause. Euthanasia was performed at different periods for the removal of the ovaries, and after diestrus was confirmed by vaginal cytology for 10 consecutive days. For daily vaginal cytology, morphological and histomorphometric microscopic analyzes were performed. Aged mice presented significant increased neutrophils when compared to VCD group, as well as increased cornified epithelial cells when compared to OVX mice, and also increased nucleated epithelial cells when compared to VCD and OVX. NC and OVX-SHAM ovaries presented innumerous follicles at different stages of development, while VCD showed marked follicular atresia, depleted of primordial or developing follicles and a predominance of interstitial cells. The ovaries of aged mice were predominantly constituted by corpus luteum degenerated into corpus albicans, with rare antral follicles. All analyzed models led to different permanent diestrus profiles caused by each model, as indicated by ovarian features. This should be carefully considered when choosing a post-estropausal experimental model, in order to better correlate this challenging phase of female's life with physiological/pathological conditions.

Development of an animal model for ovotoxicity using 4-vinylcyclohexene: a case study

BACKGROUND

The occupational chemical 4-vinylcyclohexene (VCH) has been shown to cause destruction of small pre-antral follicles in ovaries of mice. Further, its monoepoxide metabolites, 1,2-VCH epoxide, 7,8-VCH epoxide, and the diepoxide, VCD, have been shown to cause pre-antral follicle loss in rats as well as mice. Chemicals that destroy small pre-antral follicles are of concern to women because exposure can result in premature ovarian failure (early menopause).

METHODS

Studies working with these chemicals over the past decade have determined a number of aspects of the mechanism(s) of small pre-antral destruction, and a variety of questions have been answered.

RESULTS

Specifically, it has been determined that the diepoxide (VCD) is the bioactive form and it directly targets the ovary in mice and rats. Mice are more susceptible to VCH than rats because they are capable of its metabolic bioactivation. Follicle destruction by VCD is selective for primordial and primary follicles. Mechanistic studies in rats have determined that VCD causes ovotoxicity by accelerating the natural process of atresia (apoptosis) and this requires repeated exposures. Pro-apoptotic signaling events in the Bcl-2 and mitogen activated protein kinase families have been shown to be selectively activated in fractions of small pre-antral follicles (targets for VCD). Finally, a whole ovarian culture system using neonatal mouse and rat ovaries has been developed to expand the potential for more in depth investigations into ovotoxicity caused by VCD.

CONCLUSIONS

This article provides an overview of the questions asked and the approaches taken in studying VCH and VCD to support these conclusions.

Long-term effects of ovarian follicular depletion in rats by 4-vinylcyclohexene diepoxide

4-Vinylcyclohexene diepoxide (VCD) destroys preantral ovarian follicles in rats. Female 28-day Fisher 344 (F344) rats were dosed (30 days) with VCD (80 mg/kg per day, i.p.) or vehicle, and animals were evaluated for reproductive function at subsequent time points for up to 360 days. At each time point animals were killed, and ovaries and plasma collected. VCD reduced (P<0.05) the number of preantral follicles by day 30 relative to control. There were no ultrastructural differences in morphology between VCD-treated and control ovaries. Circulating FSH levels in VCD-treated animals were greater (days 120, 240, and 360, P<0.05) than in controls. Cyclicity was disrupted in the VCD-treated group by day 360. These results show that VCD-induced follicular destruction in rats is associated with a sequence of events (loss of preantral follicles, increased plasma FSH, and cyclic disruption) preceding premature ovarian senescence that is similar to events that occur during the onset of menopause in women.

Ovarian toxicity of 4-vinylcyclohexene diepoxide: a mechanistic model

Female mammals are born with a finite number of ovarian primordial follicles that cannot be regenerated; thus, chemicals that destroy oocytes contained in these follicles can produce premature ovarian failure (early menopuase in women). Exposure of women to known ovotoxicants, such as contaminants in cigarette smoke, is associated with early menopause. Thus, the potential risks posed by ovotoxic chemicals is of concern. Our studies have focused on the environmental chemical 4-vinylcyclohexene (VCH), which is produced during the manufacture of rubber tires, flame retardants, insecticides, plasticizers, and antioxidants. Dosing of female rats and mice with the ovotoxic diepoxide metabolite of VCH, 4-vinylcyclohexene diepoxide (VCD), for 30 days destroyed the majority of ovarian primordial follicles. Using VCD in rats as a generalized model for ovotoxicity, we determined that 1) repeated daily dosing is required, 2) cell death is via apoptosis, and 3) altered expression of specific genes is involved. An integrated approach at the morphologic, biochemical, and molecular level was used to support these conclusions. Studies in isolated rat small preantral follicles (targeted for VCD-induced ovotoxicity) focused on the role of cell death genes, mitochondrion-associated events, and VCD metabolism. We also evaluated how this information relates to human risk for early menopause. These animal research results provide a better understanding of the potential risk of human exposure to environmental ovarian toxicants and greater insight as to the impact of these toxicants on reproductive health in women.

In vitro metabolism of 4-vinylcyclohexene in rat and mouse liver, lung, and ovary

4-Vinylcyclohexene (4-VCH), the dimer of 1,3-butadiene, is an ovarian toxicant in mice due to the formation of a diepoxide metabolite, but the tissue-specific site of formation of the metabolites is unknown. Microsomal preparations from liver, lung, and ovaries obtained from female Crl:CD BR rats and female B6C3F1 mice were tested for their ability to metabolize the following reactions: 4-VCH to 4-VCH-1,2-epoxide and 4-VCH-7,8-epoxide; 4-VCH-1,2-epoxide to 4-VCH diepoxide and 4-VCH-1,2-diol; 4-VCH-7,8-epoxide to 4-VCH diepoxide and 4-VCH-7,8-diol; and hydrolysis of 4-VCH diepoxide. Microsomes were incubated with the test chemical and the reaction products were analyzed by gas chromatography. Rat liver and lung microsomes and mouse liver and lung microsomes metabolized 4-VCH to 4-VCH-1,2-epoxide at detectable rates. Mouse liver had a Vmax for the reaction that was 56-fold higher than that for rat liver (11.1 and 0.20 nmol/min/mg protein, respectively). The Vmax for mouse lung was 2-fold higher than that for rat lung. 4-VCH-1,2-epoxide formation was not detected in ovarian microsomes from rats or mice. Metabolism of 4-VCH to 4-VCH-7,8-epoxide was detected in microsomes from rat liver and mouse liver and lung, at rates very low compared to those for metabolism to the 1,2-epoxide. Rat and mouse liver had very similar K(m) and Vmax values for metabolism of 4-VCH-1,2-epoxide to 4-VCH diepoxide. The Vmax for rat liver was 3.69 and for mouse liver was 5.35 nmol/min/mg protein. Rat and mouse ovaries did not have detectable capacity to metabolize 4-VCH-1,2-epoxide to the diepoxide. Rat and mouse liver and lung have very similar K(m) and Vmax values for metabolism of 4-VCH-7,8-epoxide to the diepoxide, while ovaries did not have detectable rates for this reaction. Hydrolysis of 4-VCH-1,2-epoxide to 4-VCH-1,2-diol was at similar rates in rat and mouse liver microsomes. Hydrolysis of 4-VCH-7,8-epoxide to 4-VCH-7,8-diol was detected only in rat liver microsomes. Hydrolysis of 4-VCH diepoxide was detected in rat and mouse liver and lung, and in rat ovary microsomes. The Vmax for rat liver was 9-fold greater than that for mouse liver (5.51 and 0.63 nmol/min/mg protein, respectively), and lung and ovary tissues were not as active as rat liver. The balance of activation versus detoxication reactions in rats and mice suggests that the mouse may be more susceptible to 4-VCH toxicity because of generation of high levels of epoxide metabolites. In general, the mouse is more efficient at metabolism of 4-VCH to epoxides than is the rat. In contrast, the rat may be more efficient at hydrolysis of epoxides. Thus, the rat would tend to produce a lower concentration of epoxide metabolites than the mouse, at equal doses of 4-VCH.

Toxicological considerations in evaluating indoor air quality and human health: impact of new carpet emissions

This review article considers evidence regarding the toxicological impact of new carpet emissions on indoor air quality and human health. It compares emissions data from several studies and describes the dominant compounds found in those emissions. The toxicity of each these compounds is assessed for animal and human data, with a focus on inhalation exposure. Data for acute and chronic exposures are presented, and synergistic effects are considered. Differences and similarities between health responses caused by toxicity and/or by immunological reactions are discussed. Possible neurogenic pathways and associations between these and immune changes are considered as they might relate to inflammatory-based human reactions. Additionally, factors affecting human odor responses are described. The roles that a variety of psychological factors may also play in the etiology of potentially related phenomena, such as the sick building syndrome, pathogenic illness, and multiple chemical sensitivity, are considered. Gaps in the literature are identified within the article and suggestions for future research are offered. In particular, it is noted that few, if any, prior studies have evaluated both neurogenic and immune-mediated inflammation status within the same study. Based on the present information available, it is concluded that under normal environmental circumstances, VOC emissions from new carpets are sufficiently low such that they should not adversely affect indoor air quality or pose significant health risk to people.

In vitro and in vivo (32)P-postlabeling analysis of 4-vinyl-1-cyclohexene (butadiene dimer) diepoxide-DNA adducts

4-Vinyl-1-cyclohexene diepoxide (VCD), and industrial chemical, and its parent compound, 4-vinyl-1-cyclohexene (VCH), are potential health hazards, a they destroy oocytes in follicles in rodents. VCD is also a skin carcinogen at the site of application in both female and male mice and rats and after gavage, induces ovarian tumors in mice and forestomach tumors in rats. A (32)P-postlabeling assay was developed for the detection and measurement of VCD-DNA adducts. VCD, a direct-acting carcinogen, was reacted with DNA in vitro, as well as through mouse skin painting for 3 days with different doses of VCD. (32)P-Labeled adducts were separated by polyethyleneimine (PEI)-cellulose TLC and detected by screen-enhanced autoradiography. Comparable adduct profiles were obtained in vitro and in vivo. At higher doses (36-225 micro mol/mouse), adduct levels in vivo showed a linear dose response, while there was no difference between 14 and 36 micro mol/mouse. The limit of detection was estimated to be 1-3 adducts in 10(8) DNA nucleotides. The results show that VCD exposure gives rise to (presumably pre-mutagenic) DNA adducts in vivo and that (32)P-postlabeling can be applied to biomonitoring of VCD exposure.

Follicular mechanisms associated with 4-vinylcyclohexene diepoxide-induced ovotoxicity in rats

The mechanism of 4-vinylcyclohexene diepoxide (VCD)-induced oocyte destruction in small preantral follicles of rats and mice has not been elucidated. This study examined the effects of daily dosing of female rats with VCD on protein synthesis and follicle viability. An investigation of granulosa cells as a target for VCD was also made. Small preantral follicles (25 to 100 microns) isolated from untreated immature rats (day 28) as well as from rats injected daily for 10 d with VCD (0.57 mmol/kg, IP) or vehicle control (sesame oil) were incubated for 3, 6, or 10 h in vitro with or without VCD. Viability (trypan blue dye exclusion) or protein synthesis (3H-leucine incorporation) in follicles was measured. Large preantral follicles (100 to 250 microns), isolated oocytes or granulosa cells from small preantral follicles, hepatocytes, and adrenal cells served as controls. Viability was not compromised in small follicles isolated from untreated or VCD-injected rats. However, following in vitro incubation of small preantral follicles with VCD, there was a significant decrease in viability by 6 h. This loss in viability was observed in granulosa cells and was even greater in follicles from dosed as compared with undosed animals. The various cell types were incubated in vitro with or without VCD for 3 h and the rate of protein synthesis was measured by 3H-leucine incorporation during the last hour of incubation. Incubation of small preantral follicles from untreated animals with VCD for 3 h produced significant inhibition in the rate of protein synthesis. This effect was reversed and significantly stimulated after 6 and 10 h of incubation with VCD. Follicles from animals that had been dosed daily with VCD for 10 d demonstrated similar inhibition of protein synthesis following 3 h in vitro incubation with VCD; however, unlike those from undosed rats, follicles from dosed rats did not recover from this inhibition after 6 or 10 h of in vitro incubation with VCD. In vitro incubation with VCD stimulated the rate of protein synthesis in large preantral follicles; however, no effect on the rate of protein synthesis was observed in isolated oocytes and granulosa cells, hepatocytes, or adrenal cells. These observations suggest that VCD affects follicular viability via an effect on granulosa cells and that daily dosing of rats with VCD makes small preantral follicles more susceptible to ovotoxicity by VCD.

Destruction of preantral follicles in adult rats by 4-vinyl-1-cyclohexene diepoxide

4-vinyl-1-cyclohexene diepoxide (VCD) is known to destroy oocytes in ovaries of immature rats. Since ovaries functionally differ between immature and adult animals, we examined the effect of VCD on oocytes in adult rats. Adult (58 days) and immature (28 days) rats were injected daily (30 days) with vehicle or VCD. Each group contained 10 rats. During this time, cyclicity was determined daily by vaginal cytology. Animals were terminated on day 31 and tissues were collected. Oocytes were counted; livers, spleens, and uteri were weighed. VCD reduced the number of regular estrous cycles/30 days in adults, but not immature rats (n = 20, P < 0.05). VCD reduced the number of oocytes in adult and immature rats (n = 20, P < 0.05). Liver, spleen, or ovarian weights were not affected by VCD in either group. VCD reduced uterine weight in adult (n = 20, P < 0.05) but not in immature rats. These results demonstrate that VCD decreases uterine weight in adult rats and as with immature rats, selectively destroys oocytes in ovaries of adults.