Audrey M. Cummings

Methoxychlor mimics the action of 17β-estradiol on induction of uterine epidermal growth factor receptors in immature female rats☆

Epidermal growth factor (EGF) and its receptor (EGF-R) have been implicated as mediators for estrogen induced cellular growth. This study examines whether the action of the estrogenic pesticide methoxychlor (MXC) parallels the action of 17 beta-estradiol (E2) on uterine EGF-R. Administration of 20 micrograms E2/sexually immature female rat increased 125I-EGF binding to membranes extracted from whole uteri 175% over endogenous levels, while 500 mg MXC/kg led to a 156% increase. E2 in both 20 and 40 micrograms/rat doses and 500 mg MXC/kg led to maximal stimulation over endogenous levels, 12-h posttreatment. Rats were treated with E2, MXC, or vehicle plus 100 micrograms actinomycin-D (ACT-D) or 100 micrograms cycloheximide (CYCLO) per rat to determine if mRNA transcription and translation are involved in the increased EGF-R binding following estrogenic treatment. Only ACT-D inhibited the estrogenic stimulation of EGF-R binding, resulting in a 44% decrease when given concurrently with E2 or MXC, suggesting transcription is required. Additionally, ACT-D decreased endogenous receptor levels by 55%. No other differences were detected. When EGF-R binding data were analyzed by the method of Scatchard, both E2 and MXC, at maximal dosages, elevated uterine EGF-R binding sites by over 200% after 12 h as measured by maximal binding (Bmax) with no significant difference in dissociation constant (Kd) values. These results demonstrate that both E2 and MXC can stimulate the number of EGF-R binding sites without significantly altering the receptor binding affinity (Kd). Further, this stimulation is time dependent and is affected by dose.

Methoxychlor as a model for environmental estrogens

Estrogens can have a variety of physiological effects, especially on the reproductive system. Chemicals with estrogenic activity that are present in the environment may thus be considered potentially hazardous to development and/or reproduction. Methoxychlor is one such chemical, a chlorinated hydrocarbon pesticide with proestrogenic activity. Metabolism of the chemical either in vivo or using liver microsomes produces 2,2-bis(p-hydroxyphenyl)- 1,1,1-trichloroethane (HPTE), the active estrogenic form, and the delineation of this mechanism is reviewed herein. When administered in vivo, methoxychlor has adverse effects on fertility, early pregnancy, and in utero development in females as well as adverse effects on adult males such as altered social behavior following prenatal exposure to methoxychlor. Effects of methoxychlor on the female have been studied extensively, whereas reports on the chemicals effects on males are less common. From the studies reviewed here, the reproductive toxicity of methoxychlor is evident, but the significance of this toxicity with respect to human health remains to be determined.

Induction of endometriosis in mice : a new model sensitive to estrogen

Endometriosis consists of the growth of endometrial tissue outside the uterus. A rat model of endometriosis is available to evaluate the potential for environmental chemicals to promote the disease but may be relatively insensitive for the evaluation of the hazard of certain compounds. Our objective, which was to develop a mouse model for endometriosis, was based on (a) the promotion of endometriosis in primates by 2, 3, 7, 8-tetrachlorodibenzo-p-dioxin (TCDD), (b) the apparent relationship between endometriosis and immunodeficiency, and (c) evidence that humoral immunity is suppressed in mice but not rats following TCDD exposure. In the mouse model, slices of uterus were sutured to intestinal mesenteric vessels. By 3 weeks after surgery, these sites were cyst-like structures. The growth of the sites was hormone dependent. In intact mice, sites measured 3.60 +/- 0.22 mm; vehicle and estrone (0.5 microgram/day) treatments produced site diameters of 0.95 +/- 0.128 and 5.28 +/- 0.355 mm, respectively. This new mouse model provides a sensitive and useful technique for future studies of the potential for specific xenobiotics to promote the development of endometriosis.

Effects of estrogen, progesterone, and methoxychlor on surgically induced endometriosis in rats.

Endometriosis is a disease of women where endometrial tissue is found growing at ectopic sites. While evidence suggesting a role for the ovarian hormones in endometriosis exists, no complete studies of the roles of estrogen and progesterone have heretofore been performed. Also, if estrogen has a role in the growth and/or maintenance of endometriosis, it is likely that the proestrogenic pesticide, methoxychlor (MXC), might also have such an effect. Sixty rats underwent surgery on Day 0 to induce endometriosis. On Day 21, all rats were ovariectomized. During surgery, the diameters of all endometriotic implants (which were fully developed) were measured. Starting on Day 21, groups of rats were treated daily, for 3 weeks, with (a) vehicle (b) estrone, 1 micrograms/rat, E;(c) progesterone, 2 mg/rat, P; (d) E + P, 1 micrograms + 2 mg; (e) MXC, 250 mg/kg; or (f) MXC + P, 250 mg/kg + 2 mg/rat. On Day 42, all rats were killed, and the diameters of all endometriotic sites were measured. While no differences in diameter were found across groups prior to ovariectomy, ovariectomy plus treatment altered the growth of endometriosis tissue. Progesterone and vehicle treatments produced results that were identical: regression of endometriotic sites. Both estrogen and MXC treatments maintained endometriotic site size at a level greater than that in the vehicle-treated group. The combination of progesterone with either estrone or MXC did not alter the effect of either chemical. We conclude that while estrogen promotes the growth of endometriosis, progesterone either produces regression or fails to maintain the sites. MXC, at a relatively high dose, supports the development of endometriosis. Concurrent progesterone treatment does not modulate the effects of estrone or MXC. These results suggest that exposure of women to high doses of MXC may exacerbate the development of endometriosis or contribute to its recurrence.

Effects of methyl benzimidazolecarbamate during early pregnancy in the rat

Methyl 2-benzimidizolecarbamate (MBC), an agricultural fungicide, and its parent compound benomyl have adverse reproductive effects on male rats and exhibit embryotoxicity, including teratogenicity, when administered to rats during mid to late pregnancy. This study was designed to assess potential maternal effects of MBC during early pregnancy, to distinguish maternal from embryotoxic effects of the chemical, and to differentiate between early pregnancy failure and late embryonic loss. MBC was administered to rats by gavage at 0, 25, 50, 100, 200, 400, and 1000 mg/kg/day during Days 1 through 8 of pregnancy (Day 0 = sperm positive). A range of maternal and embryonic parameters was assessed following euthanasia on Day 9, including the number of implantation sites, body weight gain, uterine weight, implantation site size, and serum ovarian and pituitary hormones. In a separate experiment, pseudopregnant rats were administered 0 or 400 mg/kg/day MBC during Days 1-8, received bilateral uterine decidual induction on Day 4, and were killed on Day 9 at which time the decidual cell response was evaluated as a measure of uterine competency. When dosages of up to 400 mg/kg/day of MBC were administered during early pregnancy, the chemical had no significant effect on any measured parameter but a trend toward increased resorptions was evident. The 1000 mg/kg/day dosage of MBC produced reductions in body weight gain, implantation site weight, and serum LH and an increase in serum estradiol. When administered during pseudopregnancy, 400 mg/kg/day MBC partially reduced uterine decidual growth but affected no other parameter.(ABSTRACT TRUNCATED AT 250 WORDS)

Assessment of estrogenicity by using the delayed implanting rat model and examples

Endocrine disrupting chemicals have recently drawn increased interest. The delayed implanting rat model is a method that can identify and quantify the estrogenic activity of a chemical. In rats hypophysectomized after breeding, the administration of progesterone delays embryo implantation, and exposure to one dose of an estrogenic substance initiates implantation. Although methoxychlor was ineffective at dosages below 400 mg/kg when given by injection, the administration of the chemical by gavage resulted in an increase in the percent of fertilized rats exhibiting implantation sites. These results were statistically significant at dosages of 50, 100, 200, and 300 mg methoxychlor/kg. When bisphenol A was administered, by subcutaneous injection, dosages of 50, 100, and 200 mg/kg induced implantation. Only the 400 mg/kg dose of 4-tert-octylphenol was effective. Doses of beta-sitosterol up to 30 mg/kg failed to initiate implantation. These data confirm previous evidence of the availability of this model for evaluating estrogenic activity and provide estimates of the estrogenic potencies of several environmentally important chemicals.

Replacement of estrogen by methoxychlor in the artificially-induced decidual cell response in the rat

The pesticide methoxychlor (MXC) exhibits estrogenic activity although it is not a steroid. Therefore its mode of action may differ from that of estrogen. Here we evaluated the ability of MXC to replace estrogen in the ovariectomized, hormone-treated decidual cell response (DCR) model. Following priming with estrone, ovariectomized rats were treated with estrone plus progesterone, progesterone alone, or progesterone plus various dosage levels of MXC. Within a narrow dose range, MXC can replace estrone and, in combination with progesterone, produce a maximal DCR. In the same manner as that seen with progesterone plus estrone, progesterone plus MXC produced no effect different from progesterone alone at low to intermediate dosages and an inhibition of decidual growth at high doses. The data support the hypothesis that MXC exhibits classical mechanisms of estrogenic activity.

Effect of 5-azacytidine administration during very early pregnancy.

The chemotherapeutic agent 5-azacytidine (5AZ) is cytotoxic via nucleic acid hypomethylation. Malformations and embryolethality result when rats or mice are exposed to 5AZ on any of Days 9 through 12 of pregnancy. To investigate the effect of 5AZ exposure during the pre- and early postimplantation period, we administered 0.15, 0.30, 0.60, or 1.2 mg/kg 5AZ/day to rats during Days 1-8 of pregnancy and evaluated outcome on Days 9 or 20. No adverse effects were detected on Day 9; the numbers and weights of implantation sites, the numbers of resorptions, maternal body weight gains, and hormone measures were not different from those of controls. However, when pregnancy outcome was evaluated on Day 20, dose-dependent decreases in offspring survival and fetal weight were observed and the incidences of two malformations, microphthalmia and exencephaly, were increased. In a follow-up study, 5AZ was administered during the preimplantation period (Days 1-3) or during the postimplantation period (Days 4-8) and pregnancy outcome was evaluated on Day 20. When rats were exposed to 5AZ during the preimplantation period, no adverse effects were seen. Postimplantation dosing produced an increase in resorptions and a decrease in fetal survival and fetal weight, with no gross external malformations evident. At the doses used in this study, 5AZ was embryotoxic, with limited embryonic vulnerability prior to Day 4 of pregnancy but serious consequences following postimplantation exposure. This embryotoxicity is not detectable by our measures on Day 9 but is evident on Day 20.

Evaluation of the effects of methanol during early pregnancy in the rat

Recent attention to methanol (MeOH) as a potential alternative fuel prompted an evaluation of the chemicals effects during early pregnancy. Rats were dosed by gavage during Days 1-8 of pregnancy at 0, 1.6, 2.4, or 3.2 g MeOH/kg/day. Groups of animals were killed on Days 9, 11, or 20 of pregnancy, and maternal, embryonic, or fetal parameters were assessed, depending on the stage of pregnancy. The decidual cell response (DCR) technique was also applied to rats treated with MeOH during pseudopregnancy. Reductions in pregnant uterine and implantation site weights seen on Day 9 are the result of MeOH impedance of normal decidualization, as demonstrated by effects on the DCR. An increase in the extravasation of blood at implantation sites seen on Day 9 did not result in an increase in resorptions by Day 20. The 3.2 g/kg/day dose of MeOH produced a reduction in body weight gain by Day 9, which may be considered an indication of non-specific maternal toxicity. No effect on Day 11 or Day 20 embryo-fetal survival, or development was observed.