Effects of Continuous Estradiol Administration by Polydimethylsiloxane and Paraffin Implants on Serum Hormone Levels and Uterine Responses

Estrogen receptors alpha and beta in rat placenta: detection by RT-PCR, real time PCR and Western blotting

BACKGROUND

High levels of estrogens during pregnancy not only retard placental and fetal growth but can lead to reproductive tract abnormalities in male progeny. Estrogens act through estrogen receptors (ER) to modulate the transcription of target genes. These ER exist in two isoforms, ER alpha and ER beta and recently several variants of these isoforms have been identified.

METHODS

The expressions of ER isoforms and variants have been studied in rat placenta at 16, 19 and 21 days gestation (dg). Gene expression was assessed using RT-PCR and real time PCR while protein expression was studied using Western blotting followed by immunodetection. Placental homogenates were probed with: a monoclonal antibody raised against the steroid binding domain of the ER alpha (ER alpha -S), a monoclonal antibody raised against the hinge region of ER alpha (ER alpha -H) and a polyclonal antibody raised against the amino terminus of ER beta.

RESULTS

ER alpha and ER beta mRNA and protein were detected from as early as 16 dg. Two PCR products were detected for ER alpha, one for the wild type ER alpha, and a smaller variant. Real time PCR results suggested the presence of a single product for ER beta. The antibodies used for detection of ER alpha protein both identified a single 67 kDa isoform; however a second 54 kDa band, which may be an ER alpha variant, was identified when using the ER alpha -H antibody. The abundance of both ER alpha bands decreased significantly between 16 and 19 dg. As for ER beta, four bands (76, 59, 54 and 41 kDa) were detected. The abundance of the 59 and 54 kDa bands decreased significantly between 16 and 19 dg.

CONCLUSION

This study shows that both ER protein isoforms and their variants are present in rat placenta. The decrease in their expression near parturition suggests that the placenta may be relatively unresponsive to estrogens at this stage.

Dose-response characteristics of uterine responses in rats exposed to estrogen agonists

Assays for uterine response have played major roles in developing an understanding of estrogen-mediated processes and for identifying compounds with hormonal activity. Data from assays measuring increases in uterine wet weight in rats were evaluated in terms of their dose-response characteristics. Analysis using a Hill equation found inconsistent estimates for the ED50 (concentration giving half-maximal response) and n (steepness of response) among the assays. This variability reflects disparate assay protocols and limitations of the dose-response data collected in the experiments. Although uterine wet weight is easily measured, it arises from several physiological processes (e.g., water retention, cell proliferation). This contributes to the assay variability with different protocols. The potential use of the Hill equation for dose-response analysis to estimate a benchmark dose was also considered using these data sets as surrogates for receptor-mediated toxicological effects. Strengths and weaknesses were identified, but overall the Hill equation should likely become a favored option for determining a benchmark dose, particularly when a data set demonstrates a maximal response. For screening purposes, empirical analysis using the Hill equation provides adequate information for classifying and prioritizing compounds. To develop an understanding of how incremental exposures to compounds with estrogen agonist activities would affect intact adult females, quantitative analyses are required that account for the pharmacokinetics of estradiol and subsequent interactions of the receptor complexes in regulating the responses.

Prenatal dexamethasone or stress but not ACTH or corticosterone alter sexual behavior in male rats

Prenatal maternal stress in rats and mice can demasculinize and feminize the sexual behavior of adult male offspring. Causal mechanisms are unknown, but one attractive hypothesis is that stress activation of maternal adrenal glucocorticoid secretion is the responsible agent. To test this hypothesis, pregnant rats were exposed to a variety of substances which enhance glucocorticoid actions. These included ACTH (20 IU of a gel preparation, SC once daily), corticosterone (CORT; 7 mg/kg SC in oil, three times daily), or dexamethasone (DEX; 0.1 mg/kg, SC once daily). Controls included noninjected dams and a positive stress control group (restraint under bright lights three times daily). All treatments reduced maternal weight gain, DEX most potently. No treatment altered litter size, stillbirths, or sex ratio, but DEX reduced weight at birth, an effect still seen at postnatal day 85. DEX, CORT, and stress reduced male adrenal weight at birth, while DEX and CORT altered sexual differentiation as measured by anogenital distance. Stress impaired adult male sexual performance but not the lordosis quotient following exposure of animals to stud males. DEX affected both measures. No other treatment had any significant effect on sexual behavior. No treatment altered plasma LH levels, either basal or in response to an estrogen challenge in adult gonadectomized males. In adulthood there was no treatment effect on stress reactivity, measured behaviorally or by plasma glucocorticoids. Correlational analysis revealed that weight gain during pregnancy was the single best predictor of subsequent sexual performance. It is concluded that prenatal dexamethasone exposure demasculinizes and feminizes male offspring.(ABSTRACT TRUNCATED AT 250 WORDS)

Long-term effects of postnatal exposure to diethylstilbestrol on uterine estrogen receptor and growth

Diethylstilbestrol (DES) treatment of female rats on postnatal days (PND) 1-5 reduces uterine growth, estrogen receptor (ER) level and gland number by PND 25, while daily DES treatment on PND 1-25 increases uterine growth 4-fold, further reduces ER level and completely suppresses gland formation. We now report the persistence of these effects in adults. By PND 60, uterine weight was 70% of controls in rats injected with DES on PND 1-5 but only 10% of controls in rats injected PND 1-10 or longer. In fact, uterine weights were the same on PND 10 and 60. Uterine gland numbers were reduced to 30% of controls in all DES-treated rats regardless of exposure length; however, luminal and glandular epithelial cell heights were reduced to less than 50 and 70%, respectively, of controls when DES was given on PND 1-25 but not when given on PND 1-5. Ovariectomy 7 days prior to sacrifice on PND 60 reduced uterine weight in controls by 67% and in rats injected with DES on PND 1-5 by 53%, but had no effect in rats injected with DES on PND 1-10. DES exposure at either PND 1-5 or 1-10 lowered ER levels by 35-50% at both 60 and 90 days. Treatment with a high dose of estradiol (E2) 1 week before sacrifice significantly down-regulated ER to the same concentration in all treatment groups at PND 60 and 90. Following E2 treatment, all groups also showed increased uterine weight at PND 60 and 90. These data show there is a short period of development (PND 5-10) in which further DES exposure indirectly inhibits uterine growth.

Detection of estrogen receptor (ER) in the rat brain using rat anti-ER monoclonal IgG with the unlabeled antibody method

Application of Sternberger's unlabeled antibody enzyme method for detection of the estrogen receptor (ER) using a rat primary antibody with rat tissues has been discouraged, presumably because nonspecific staining of endogenous IgG was expected with the required anti-rat IgG bridging antibody. Because the blood-brain barrier greatly reduces immunoglobulin infiltration into the brain, we hypothesized that rat brain tissue could be specifically immunostained using rat IgG primary antibodies. A rat monoclonal anti-ER antibody (H222) specifically stained ERs in the brains of ovariectomized but not in ovariectomized estrogen-treated rats. In contrast, the uterus, a well-perfused target organ stained intensely in a nonspecific fashion. Dense populations of estrogen receptors were observed in the medial preoptic area, the bed nucleus of the stria terminalis, and the arcuate and ventromedial nuclei of the hypothalamus. A monoclonal rat IgG directed against alpha-tubulin labeled primarily cortical dendrites quite distinct from the neuronal nuclei that are the primary antigenic sites for the estrogen receptor antibody. These results confirm that the sensitive unlabeled antibody method can be applied to rat brain tissues, even when the primary antibody is rat IgG and that labeling of endogenous IgG may be used as a simple method to evaluate the integrity of the blood brain barrier.

Short-term effects of physiological and pharmacological doses of estradiol on estrogen receptor and uterine growth

Estradiol (E2) regulation of estrogen receptor (ER) concentrations has been shown to be both time- and dose-dependent. E2 concentrations of 0.5 mg/ml or greater contained in Silastic capsules suppressed uterine ER concentrations after one day's exposure. In this study, we looked at the effects of physiological (1.0 and 10.0 micrograms subcutaneous injections) and pharmacological (5.0 mg/ml implants) doses of E2 on ER concentrations at times less than 24 hours. The implanted rats had maximum E2 plasma levels of approximately 2000 pg/ml for at least six hours which fell to around 800 pg/ml by 12 hours where they remained up to 24 hours. The physiological doses resulted in plasma levels at one hour of 2000 pg/ml (10 micrograms dose) and 250 pg/ml (1 microgram dose) both of which fell to less than 60 pg/ml by six hours. All treatments caused maximal ER suppression by six hours; however, the implants caused a greater reduction in ER levels than either of the physiological doses. The reduction of ER levels was due primarily to a decrease in the "cytosolic" receptor. Despite the decrease in ER, all doses caused a significant and equivalent increase in uterine weight at six hours, however, only the implanted animals maintained the maximal uterine weight gain through 24 hours. This maintenance of uterine weight appears to be correlated with a small but significant increase in the nuclear ER level over this same time period. Thus, while E2 can cause a short-term suppression of its receptor concentration with no effect on short-term uterine weight gain, uterine growth is positively correlated with the level of nuclear ER.

Transplacental estrogen responses in the fetal rat: increased uterine weight and ornithine decarboxylase activity

The synthetic estrogens, diethylstilbestrol (DES) and ethynylestradiol (EE2), are more potent than 17 beta-estradiol (E2) in inducing uterine weight gain in the neonatal rat, due to the binding of E2 to serum alpha-fetoprotein (AFP). However, all three hormones are equipotent in inducing neonatal uterine ornithine decarboxylase (ODC) activity. The present study assessed estrogen potency in fetal rats. Pregnant CD rats were injected sc daily on gestation days (GD) 16-20 with DES, EE2, or E2 in sesame oil. Both DES and EE2, but not E2, significantly increased uterine weight at birth, to more than twice that of controls. In addition, implants which continuously release E2 only slightly increased uterine weight at birth. Alternatively, dams were given a single estrogen injection on GD 20 and were sacrificed at various times after injection. Peak fetal uterine ODC activity occurred at 6-8 hours after maternal injection for all three estrogens. E2 had a relative potency about tenfold less than either DES or EE2 in stimulating fetal ODC activity, in contrast to equal potencies of the three estrogens in the postnatal rat uterus. Similar patterns were found following direct fetal injection with E2 or DES. In summary, these data demonstrate a transplacental induction of fetal uterine ODC activity and uterine weight gain by both DES and EE2. In addition, the lack of correlation between these endpoints in response to E2 suggests that they may be useful as selective indicators of potential toxicity of both natural and synthetic estrogens.

Estrogen- and antiestrogen-induced ornithine decarboxylase activity and uterine growth in the rat

The estrogen antagonists tamoxifen and monohydroxytamoxifen are also classified as partial estrogen agonists. In infantile rats, estradiol induced a single peak of uterine ODC activity at 6h following injection regardless of the extent of induction by various estradiol doses. By contrast, the timing of the ODC activity peak induced by tamoxifen and monohydroxytamoxifen was highly dependent upon the dosing conditions and was delayed to 18 h at lower tamoxifen doses. In immature rats, tamoxifen and monohydroxytamoxifen induced two peaks of uterine ODC activity resembling those induced by estradiol. Both ODC activity peaks were delayed by 9 h, without decreases in peak heights, by a 50-fold tamoxifen dose reduction. In all experiments the initial appearance of antiestrogen- and estradiol-induced ODC activity corresponded to initial uterine wet weight gain regardless of dosing condition. Thus, when dose-related temporal shifts are taken into account, tamoxifen and monohydroxytamoxifen are complete agonists with respect to induction of uterine weight gain and ODC activity.

Identification of uterine nuclear type II estrogen binding sites in estrogen treated rats

Uterine nuclear fractions from estrogen-treated rats contain both the estrogen receptor and a lower affinity estrogen binding site (type II site). In Scatchard plots of estrogen binding, two types of curves are seen. The hook-shaped form is composed of a linear component (the estrogen receptor) and a convex component (the type II site) while the curvilinear form is resolvable into two linear binding species (the estrogen receptor and a secondary site). To clarify the relationship between the two forms, we examined the curvilinear form from immature rats injected for 4 days with estradiol (E2) for type II site properties. Like the hook-shaped type II, this form could be detected in a nuclear exchange assay at both 37 and 4 degrees C, but at neither temperature in the presence of reducing agent. Additionally, the steroid specificity of the curvilinear form was identical to the hook-shaped form. The hook-shaped form was found in both immature and ovariectomized adult rats implanted for 6 days with an E2-releasing Silastic capsule to provide pharmacological E2 levels. When uteri from implanted animals displaying the hook-shaped form were mixed in various ratios with uteri lacking type II sites, the curvilinear form was produced. Animals given an E2 implant for 3 days, followed by a 3 day hormone-free period showed a curvilinear form. In vivo E2 dose-response experiments showed the curvilinear form at low E2 doses and the hook-shaped form at the high dose and in implanted animals. We conclude that curvilinear Scatchard plots result from the presence of authentic type II at lower concentrations than those giving rise to the hook-shaped form.