Alteration of reproductive function but not prenatal sexual development after insertional disruption of the mouse estrogen receptor gene

Estrogen receptor and its ligand, estradiol, have long been thought to be essential for survival, fertility, and female sexual differentiation and development. Consistent with this proposed crucial role, no human estrogen receptor gene mutations are known, unlike the androgen receptor, where many loss of function mutations have been found. We have generated mutant mice lacking responsiveness to estradiol by disrupting the estrogen receptor gene by gene targeting. Both male and female animals survive to adulthood with normal gross external phenotypes. Females are infertile; males have a decreased fertility. Females have hypoplastic uteri and hyperemic ovaries with no detectable corpora lutea. In adult wild-type and heterozygous females, 3-day estradiol treatment at 40 micrograms/kg stimulates a 3- to 4-fold increase in uterine wet weight and alters vaginal cornification, but the uteri and vagina do not respond in the animals with the estrogen receptor gene disruption. Prenatal male and female reproductive tract development can therefore occur in the absence of estradiol receptor-mediated responsiveness.

Generation and reproductive phenotypes of mice lacking estrogen receptor beta

Estrogens influence the differentiation and maintenance of reproductive tissues and affect lipid metabolism and bone remodeling. Two estrogen receptors (ERs) have been identified to date, ERalpha and ERbeta. We previously generated and studied knockout mice lacking estrogen receptor alpha and reported severe reproductive and behavioral phenotypes including complete infertility of both male and female mice and absence of breast tissue development. Here we describe the generation of mice lacking estrogen receptor beta (ERbeta -/-) by insertion of a neomycin resistance gene into exon 3 of the coding gene by using homologous recombination in embryonic stem cells. Mice lacking this receptor develop normally and are indistinguishable grossly and histologically as young adults from their littermates. RNA analysis and immunocytochemistry show that tissues from ERbeta -/- mice lack normal ERbeta RNA and protein. Breeding experiments with young, sexually mature females show that they are fertile and exhibit normal sexual behavior, but have fewer and smaller litters than wild-type mice. Superovulation experiments indicate that this reduction in fertility is the result of reduced ovarian efficiency. The mutant females have normal breast development and lactate normally. Young, sexually mature male mice show no overt abnormalities and reproduce normally. Older mutant males display signs of prostate and bladder hyperplasia. Our results indicate that ERbeta is essential for normal ovulation efficiency but is not essential for female or male sexual differentiation, fertility, or lactation. Future experiments are required to determine the role of ERbeta in bone and cardiovascular homeostasis.

Estrogen receptor null mice: what have we learned and where will they lead us?

All scientific investigations begin with distinct objectives: first is the hypothesis upon which studies are undertaken to disprove, and second is the overall aim of obtaining further information, from which future and more precise hypotheses may be drawn. Studies focusing on the generation and use of gene-targeted animal models also apply these goals and may be loosely categorized into sequential phases that become apparent as the use of the model progresses. Initial studies of knockout models often focus on the plausibility of the model based on prior knowledge and whether the generation of an animal lacking the particular gene will prove lethal or not. Upon the successful generation of a knockout, confirmatory studies are undertaken to corroborate previously established hypotheses of the function of the disrupted gene product. As these studies continue, observations of unpredicted phenotypes or, more likely, the lack of a phenotype that was expected based on models put forth from past investigations are noted. Often the surprising phenotype is due to the loss of a gene product that is downstream from the functions of the disrupted gene, whereas the lack of an expected phenotype may be due to compensatory roles filled by alternate mechanisms. As the descriptive studies of the knockout continue, use of the model is often shifted to the role as a unique research reagent, to be used in studies that 1) were not previously possible in a wild-type model; 2) aimed at finding related proteins or pathways whose existence or functions were previously masked; or 3) the subsequent effects of the gene disruption on related physiological and biochemical systems. The alpha ERKO mice continue to satisfy the confirmatory role of a knockout quite well. As summarized in Table 4, the phenotypes observed in the alpha ERKO due to estrogen insensitivity have definitively illustrated several roles that were previously believed to be dependent on functional ER alpha, including 1) the proliferative and differentiative actions critical to the function of the adult female reproductive tract and mammary gland; 2) as an obligatory component in growth factor signaling in the uterus and mammary gland; 3) as the principal steroid involved in negative regulation of gonadotropin gene transcription and LH levels in the hypothalamic-pituitary axis; 4) as a positive regulator of PR expression in several tissues; 5) in the positive regulation of PRL synthesis and secretion from the pituitary; 6) as a promotional factor in oncogene-induced mammary neoplasia; and 7) as a crucial component in the differentiation and activation of several behaviors in both the female and male. The list of unpredictable phenotypes in the alpha ERKO must begin with the observation that generation of an animal lacking a functional ER alpha gene was successful and produced animals of both sexes that exhibit a life span comparable to wild-type. The successful generation of beta ERKO mice suggests that this receptor is also not essential to survival and was most likely not a compensatory factor in the survival of the alpha ERKO. In support of this is our recent successful generation of double knockout, or alpha beta ERKO mice of both sexes. The precise defects in certain components of male reproduction, including the production of abnormal sperm and the loss of intromission and ejaculatory responses that were observed in the alpha ERKO, were quite surprising. In turn, certain estrogen pathways in the alpha ERKO female appear intact or unaffected, such as the ability of the uterus to successfully exhibit a progesterone-induced decidualization response, and the possible maintenance of an LH surge system in the hypothalamus. [ABSTRACT TRUNCATED]

Tissue distribution and quantitative analysis of estrogen receptor-alpha (ERalpha) and estrogen receptor-beta (ERbeta) messenger ribonucleic acid in the wild-type and ERalpha-knockout mouse

Until recently, only a single type of estrogen receptor (ER) was thought to exist and mediate the genomic effects of the hormone 17beta-estradiol in mammalian tissues. However, the cloning of a gene encoding a second type of ER, termed ERbeta, from the mouse, rat, and human has prompted a reevaluation of the estrogen signaling system. Based on in vitro studies, the ERbeta protein binds estradiol with an affinity similar to that of the classical ER (now referred to as ERalpha) and is able to mediate the effects of estradiol in transfected mammalian cell lines. Essential to further investigations of the possible physiological roles of ERbeta, and its possible interactions with ERalpha, are data on the tissue distribution of the two ER types. Herein, we have described the optimization and use of an RNase protection assay able to detect and distinguish messenger RNA (mRNA) transcripts from both the ERalpha and ERbeta genes in the mouse. Because this assay is directly quantitative, a comparison of the levels of expression within various tissues was possible. In addition, the effect of disruption of the ERalpha gene on the expression of the ERbeta gene was also investigated using the ERalpha-knockout (ERKO) mouse. Transcripts encoding ERalpha were detected in all the wild-type tissues assayed from both sexes. In the female reproductive tract, the highest expression of ERbeta mRNA was observed in the ovary and showed great variation among individual animals; detectable levels were observed in the uterus and oviduct, whereas mammary tissue was negative. In the male reproductive tract, significant expression of ERbeta was seen in the prostate and epididymis, whereas the testes were negative. In other tissues of both sexes, the hypothalamus and lung were clearly positive for both ERalpha and ERbeta mRNA. The ERKO mice demonstrated slightly reduced levels of ERbeta mRNA in the ovary, prostate, and epididymis. These data, in combination with the several described phenotypes in both sexes of the ERKO mouse, suggest that the biological functions of the ERbeta protein may be dependent on the presence of ERalpha in certain cell types and tissues. Further characterization of the physiological phenotypes in the ERKO mice may elucidate possible ERbeta specific actions.

Postnatal sex reversal of the ovaries in mice lacking estrogen receptors alpha and beta

Mice lacking estrogen receptors alpha and beta were generated to clarify the roles of each receptor in the physiology of estrogen target tissues. Both sexes of alphabeta estrogen receptor knockout (alphabetaERKO) mutants exhibit normal reproductive tract development but are infertile. Ovaries of adult alphabetaERKO females exhibit follicle transdifferentiation to structures resembling seminiferous tubules of the testis, including Sertoli-like cells and expression of Müllerian inhibiting substance, sulfated glycoprotein-2, and Sox9. Therefore, loss of both receptors leads to an ovarian phenotype that is distinct from that of the individual ERKO mutants, which indicates that both receptors are required for the maintenance of germ and somatic cells in the postnatal ovary.

Physiological coupling of growth factor and steroid receptor signaling pathways: estrogen receptor knockout mice lack estrogen-like response to epidermal growth factor

Past studies have shown that epidermal growth factor (EGF) is able to mimic the uterotropic effects of estrogen in the rodent. These studies have suggested a "cross-talk" model in which EGF receptor (EGF-R) signaling results in activation of nuclear estrogen receptor (ER) and its target genes in an estrogen-independent manner. Furthermore, in vitro studies have indicated the requirement for ER in this mechanism. To verify the requirement for ER in an in vivo system, EGF effects were studied in the uteri of ER knockout (ERKO) mice, which lack functional ER. The EGF-R levels, autophosphorylation, and c-fos induction were observed at equivalent levels in both genotypes indicating that removal of ER did not disrupt the EGF responses. Induction of DNA synthesis and the progesterone receptor gene in the uterus were measured after EGF treatment of both ERKO and wild-type animals. Wild-type mice showed increases of 4.3-fold in DNA synthesis, as well as an increase in PR mRNA after EGF treatment. However, these responses were absent in ERKO mice, confirming that the estrogen-like effects of EGF in the mouse uterus do indeed require the ER. These data conclusively demonstrate the coupling of EGF and ER signaling pathways in the rodent reproductive tract.

Vascular estrogen receptors and endothelium-derived nitric oxide production in the mouse aorta. Gender difference and effect of estrogen receptor gene disruption

The present study was designed to test the hypothesis that estrogen receptors (ER) in the blood vessel wall play a role in the modulation of the release of endothelium-derived nitric oxide (EDNO). Both basal and stimulated release of EDNO were determined in aortic rings isolated from female and male wild-type and male homozygous estrogen receptor knock-out (ERKO) mice. 125I-17beta-estradiol binding in aortic tissue showed significantly more high affinity cytosolic- nuclear-binding sites in male compared with female wildtype mice. Estrogen receptor transcripts were present in the aorta of male wild-type mice, but they were absent in male ERKO animals. Basal release of EDNO (determined by endothelium-dependent contraction caused by NG-nitro-arginine) was significantly higher in aorta of wild-type male mice compared with wild-type female mice, and significantly lower in the aorta of male ERKO compared with male wild-type mice. Acetylcholine-induced endothelium-dependent relaxation was similar in all groups studied. No difference was observed in the activity of calcium-dependent nitric oxide synthase in homogenates of lungs and brain taken from male wild-type and ERKO mice. These studies show a significant association between the number of estrogen receptors and basal release of EDNO in the aorta of mice, and suggest that decreased vascular estrogen receptor number may represent a novel risk factor for cardiovascular diseases.

Expression of estrogen receptor beta is developmentally regulated in reproductive tissues of male and female mice

By the use of ribonuclease protection assay (RPA) combined with immunohistochemical techniques, the expression of estrogen receptor (ER) alpha and ERbeta was mapped in the developing gonads and reproductive tracts of male and female mice from fetal day 14 to postnatal day 26 (PND 26). This study was designed to determine the pattern of expression of both ER subtypes in specific tissue compartments during development. In ovaries, ERalpha mRNA was detected at all ages examined; ERbeta mRNA was seen as early as PND 1, and its expression increased with age. Immunolocalization showed ERbeta in differentiating granulosa cells of the ovary, whereas ERalpha was predominantly seen in interstitial cells. The remainder of the female reproductive tract showed ERalpha mRNA at all ages examined with little or no significant levels of ERbeta, except on PND 1 when a low level of message appeared. In males, ERalpha and ERbeta mRNA were detected in the fetal testis; however, ERbeta gradually increased until PND 5 and subsequently diminished to undetectable levels by PND 26. Immunolocalization showed ERalpha in the interstitial compartment of the testis, whereas ERbeta was seen predominantly in developing spermatogonia. The remainder of the male reproductive tract showed varying amounts of both receptors by RPA and immunostaining throughout development. These studies provide information useful in studying the role of both ER subtypes in normal differentiation, and they provide indications of differential tissue expression during development.

Estrogen receptor-alpha knockout mice exhibit resistance to the developmental effects of neonatal diethylstilbestrol exposure on the female reproductive tract

Data indicate that estrogen-dependent and -independent pathways are involved in the teratogenic/carcinogenic syndrome that follows developmental exposure to 17beta-estradiol or diethylstilbestrol (DES), a synthetic estrogen. However, the exact role and extent to which each pathway contributes to the resulting pathology remain unknown. We employed the alphaERKO mouse, which lacks estrogen receptor-alpha (ERalpha), to discern the role of ERalpha and estrogen signaling in mediating the effects of neonatal DES exposure. The alphaERKO provides the potential to expose DES actions mediated by the second known ER, ERbeta, and those that are ER-independent. Wild-type and alphaERKO females were treated with vehicle or DES (2 microg/pup/day for Days 1-5) and terminated after 5 days and 2, 4, 8, 12, and 20 months for biochemical and histomorphological analyses. Assays for uterine expression of the genes Hoxa10, Hoxa11, and Wnt7a shortly after treatment indicated significant decreases in DES-treated wild-type but no effect in the alphaERKO. In contrast, the DES effect on uterine expression of Wnt4 and Wnt5a was preserved in both genotypes, suggesting a developmental role for ERbeta. Adult alphaERKO mice exhibited complete resistance to the chronic effects of neonatal DES exposure exhibited in treated wild-type animals, including atrophy, decreased weight, smooth muscle disorganization, and epithelial squamous metaplasia in the uterus; proliferative lesions of the oviduct; and persistent vaginal cornification. Therefore, the lack of DES effects on gene expression and tissue differentiation in the alphaERKO provides unequivocal evidence of an obligatory role for ERalpha in mediating the detrimental actions of neonatal DES exposure in the murine reproductive tract.

Analysis of transcription and estrogen insensitivity in the female mouse after targeted disruption of the estrogen receptor gene

We employed homologous recombination in mouse embryonic stem cells to disrupt the estrogen receptor (ER) gene. Subsequently generated mice that are homozygous for the gene disruption, termed ERKO, possess no demonstrable wild-type ER by Western blot analysis. However, the presence of residual high affinity binding, as detected by [3H]estradiol binding assays and sucrose gradients in uterine extracts from ERKO females prompted further investigation of transcription and translation products from the disrupted ER gene. Analysis of ERKO uterine messenger RNA (mRNA) by reverse transcriptase-polymerase chain reaction demonstrated that although no full-length wild-type ER mRNA was present, two smaller transcripts, labeled E1 and E2, were identified and partially sequenced. Both ERKO transcripts are splicing variants that result in the disrupting NEO sequence being partially or completely removed from the mRNA. In the ERKO-E2 variant, this results in a frame shift and the creation of at least two stop codons downstream. In the ERKO-E1 variant, the ER reading frame is preserved and encodes for a smaller mutant ER that could be the source of the residual estradiol binding. When this mutant form is overexpressed and characterized in vitro, it results in a smaller protein of the predicted size that possesses significantly reduced estrogen-dependent transcriptional activity compared with that of the wild-type ER. Despite residual amounts of an impaired ER variant, estrogen insensitivity in the female ERKOs was confirmed by the failure of estrogen treatment to induce known uterine markers of estrogen action, such as increased DNA synthesis, and transcription of the progesterone receptor, lactoferrin, and glucose-6-phosphate dehydrogenase genes. Furthermore, serum levels of estradiol in the ERKO female are more than 10-fold higher than those in the wild type, consistent with a syndrome of hormone insensitivity.

Targeted disruption of the estrogen receptor-alpha gene in female mice: characterization of ovarian responses and phenotype in the adult

Targeted disruption of the mouse estrogen receptor-alpha gene (estrogen receptor-alpha knockout; ERKO) results in a highly novel ovarian phenotype in the adult. The ERKO mouse model was used to characterize ER alpha-dependent processes in the ovary. Visualization of the ovaries of 10-, 20-, and 50-day-old wild-type (WT) and ERKO mice showed that the ERKO phenotype developed between 20 and 50 days of age. Developmental progression through the primordial, primary, and antral follicle stages appeared normal, but functional maturation of preovulatory follicles was arrested resulting in atresia or in anovulatory follicles, which in many cases formed large, hemorrhagic cysts. Corpora lutea were absent, which also indicates that the normal biochemical and mechanical processes that accomplish ovulation were compromised. Northern and ribonuclease protection analyses indicated that ERKO ovary FSH receptor (FSHR) messenger RNA (mRNA) expression was approximately 4-fold greater than in WT controls. Ovarian LH receptor (LHR) mRNA expression was also higher in the ERKO animals. Cellular localization studies by in situ hybridization analysis of ERKO ovaries showed a high level of LHR mRNA expression in the granulosa and thecal layers of virtually all the antral follicles. Ribonuclease protection analyses showed that ovarian progesterone receptor and androgen receptor mRNA expression were similar in the two groups. These results indicated that ER alpha action was not a prerequisite for LHR mRNA expression by thecal or granulosa cells or for ovarian expression of progesterone receptor mRNA. Ovarian estrogen receptor beta (ER beta) was detected immunohistochemically, was sharply compartmentalized to the granulosa cells, and was expressed approximately equally in the ERKO animals and the WT controls. In contrast, ER alpha staining was present in the thecal cells but not the granulosa cells of the WT animals. The summary findings indicate that in the adult the major cause of the ERKO phenotype is high circulating LH interacting with functional LHR of the theca and granulosa cells. These features result in a failure of the normal maturational events leading to successful ovulation and luteinization and presumably involve both hypothalamic-pituitary and intraovarian mechanisms dependent upon ER alpha action. The presence of ER beta in the granulosa cells did not rescue the phenotype of the ovary.

Effects of castration and chronic steroid treatments on hypothalamic gonadotropin-releasing hormone content and pituitary gonadotropins in male wild-type and estrogen receptor-alpha knockout mice

Testicular androgens are integral components of the hormonal feedback loops that regulate circulating levels of LHbeta and FSH. The sites of feedback include hypothalamic areas regulating GnRH neurons and pituitary gonadotropes. To better define the roles of androgen receptor (AR), estrogen receptor-alpha (ERalpha), and estrogen receptor-beta (ERbeta) in mediating feedback effects of sex steroids on reproductive neuroendocrine function, we have determined the effects of castration and steroid replacement therapy on hypothalamic GnRH content, pituitary LHbeta and FSHbeta messenger RNA (mRNA) levels, and serum gonadotropins in male wild-type (WT) and estrogen receptor-alpha knockout (ERKO) mice. Hypothalami from intact WT and ERKO males contained similar amounts of GnRH, whereas castration significantly reduced GnRH contents in both genotypes. Replacement therapy with estradiol (E2), testosterone (T), or dihydrotestosterone (DHT) restored hypothalamic GnRH content in castrated (CAST) WT mice; only the androgens were effective in CAST ERKOs. Analyses of pituitary function revealed that LHbeta mRNA and serum LHbeta levels in intact ERKOs were 2-fold higher than those in intact WT males. Castration increased levels of LHbeta mRNA (1.5- to 2-fold) and serum LHbeta (4- to 5-fold) in both genotypes. Both E2 and T treatments significantly suppressed LHbeta mRNA and serum LH levels in CAST WT males. However, E2 was completely ineffective, and T was only partially effective in suppressing these two indexes in the CAST ERKO males. DHT treatments stimulated a 50% increase in LHbeta mRNA and serum LH levels in WT males, whereas serum LH was significantly suppressed in DHT-treated ERKO males. Although the pituitaries from intact ERKO males contained similar amounts of FSHbeta mRNA, serum FSH levels were 20% higher than those in the intact WT males. Castration increased FSHbeta mRNA levels only in WT males, but significantly increased serum FSH levels in both genotypes. Both E2 and T treatments significantly suppressed serum FSH in CAST WT males, whereas only E2 suppressed FSHbeta mRNA. DHT treatments of CAST WT mice stimulated a small increase in serum FSH, but failed to alter FSHbeta mRNA levels. None of the steroid treatments exerted any significant effect on FSHbeta mRNA or serum FSH levels in CAST ERKOs. These data suggest that hypothalamic GnRH contents can be maintained solely through AR signaling pathways. However, normal regulation of gonadotrope function requires aromatization of T and activation of ERalpha signaling pathways in the gonadotrope. In addition, serum FSH levels in male ERKOs appear to be regulated largely by nonsteroidal testicular factors such as inhibin. Finally, these data suggest that hypothalamic ERbeta may not be involved in mediating the negative feedback effects of T on serum LH and FSH in male mice.

Exploring the role of sex steroids through studies of receptor deficient mice

Decades of study have described a number roles fulfilled by the steroid hormones and their respective receptors in sexual differentiation and development, reproductive function and behavior, and more recently in the function and maintenance of non-reproductive organ systems, such as skeletal muscle, bone and coronary tissues. The biological effects of the steroid hormones are believed to be mediated in part by specific receptor proteins that demonstrated great specificity for their respective steroid ligands. Much of the experimental research of the functions of the sex steroid receptors has depended upon in vitro systems as well as in vivo methods that require surgical castration or the pharmacological administration of hormone antagonists. However, recently developed techniques that allow for manipulation of the mouse genome have been utilized to generate transgenic animals that lack functional estrogen or progesterone receptors. These transgenic animals, combined with the naturally existing Tfm mice which lack functional androgen receptor, now provide in vivo models for further study of the various actions of the sex steroids and their receptors. This review attempts to describe and compare the various phenotypes that result in each of these lines of mice, with emphasis on the development and function of the reproductive systems as well as reproductive behavior.

Prevention of the polycystic ovarian phenotype and characterization of ovulatory capacity in the estrogen receptor-alpha knockout mouse

Ovarian-derived estradiol plays a critical endocrine role in the regulation of gonadotropin synthesis and secretion from the hypothalamic-pituitary axis. In turn, several para/autocrine effects of estrogen within the ovary are known, including increased ovarian weight, stimulation of granulosa cell growth, augmentation of FSH action, and attenuation of apoptosis. The estrogen receptor-alpha (ERalpha) is present in all three components of the hypothalamic-pituitary-ovarian axis of the mouse. In contrast, estrogen receptor-beta (ERbeta) is easily detectable in ovarian granulosa cells but is low to absent in the pituitary of the adult mouse. This distinct expression pattern for the two ERs suggests the presence of separate roles for each in the regulation of ovarian function. Herein, we definitively show that a lack of ERalpha in the hypothalamic-pituitary axis of the ERalpha-knockout (alphaERKO) mouse results in chronic elevation of serum LH and is the primary cause of the ovarian phenotype of polycystic follicles and anovulation. Prolonged treatment with a GnRH antagonist reduced serum LH levels and prevented the alphaERKO cystic ovarian phenotype. To investigate a direct role for ERalpha within the ovary, immature alphaERKO females were stimulated to ovulate with exogenous gonadotropins. Ovulatory capacity in the immature alphaERKO female was reduced compared with age-matched wild-type (14.5+/-2.9 vs. 40.6+/-2.6 oocytes/animal, respectively); however, oocytes collected from the alphaERKO were able to undergo successful in vitro fertilization. A similar discrepancy in oocyte yield was observed after superovulation of peripubertal (42 days) wild-type and alphaERKO females. In addition, ovaries from immature superovulated alphaERKO females possessed several ovulatory but unruptured follicles. Investigations of the possible reasons for the reduced number of ovulations in the alphaERKO included ribonuclease protection assays to assess the mRNA levels of several markers of follicular maturation and ovulation, including ERbeta, LH-receptor, cyclin-D2, P450-side chain cleavage enzyme, prostaglandin synthase-2, and progesterone receptor. No marked differences in the expression pattern for these mRNAs during the superovulation regimen were observed in the immature alphaERKO ovary compared with that of the wild-type. Serum progesterone levels just before ovulation were slightly lower in the alphaERKO compared with wild-type. These studies indicate that treatment of alphaERKO females with a GnRH antagonist decreased the serum LH levels to within the wild-type range and concurrently prevented development of the characteristic ovarian phenotype of cystic and hemorrhagic follicles. Furthermore, a lack of functional ERalpha within the ovary had no effect on the regulation of several genes required for follicular maturation and ovulation. However, the reduced numbers of ovulations following the administration of exogenous gonadotropins in the alphaERKO suggests an intraovarian role for ERalpha in follicular development and ovulation.

Accelerated onset of uterine tumors in transgenic mice with aberrant expression of the estrogen receptor after neonatal exposure to diethylstilbestrol

The role of estrogen and the estrogen receptor (ER) in the induction and promotion of tumors was investigated by using transgenic MT-mER mice, which overexpress the ER. It was hypothesized that because of this abnormal expression of the ER, the reproductive-tract tissues of the MT-mER mice may be more susceptible to tumors after neonatal exposure to the potent synthetic estrogen diethylstilbestrol (DES). Normally non-estrogen responsive tissues that may have expressed ER as a result of the transgene were also studied for DES-induced tumors. Wild-type and MT-mER littermates were treated with 2 micrograms/pup/d DES 1-5 d after birth and then killed at 4, 8, 12, and 18 mo of age. The DES-treated MT-mER mice demonstrated a significantly higher incidence of uterine adenocarcinoma at 8 mo (73%) than the DES-treated wild-type mice (46%). The tumors of the MT-mER mice were often more aggressive than those in the wild-type animals. These tumors were also preceeded at 4 mo by a significantly higher incidence of the preneoplastic lesion atypical hyperplasia in the MT-mER mice (26% compared with 0% in the wild-type mice). Other DES-induced abnormalities were observed at equal rates in the wild-type and MT-mER mice. Although no tumors were observed in untreated wild-type females, a single untreated MT-mER female had uterine adenocarcinoma at 18 mo. These data indicate that the level of ER present in a tissue may also be a determining factor in development of estrogen-responsive tumors.

Contrasting phenotypes in reproductive tissues of female estrogen receptor null mice

The estrogen receptor (ER) exists in two known forms, ERalpha and ERbeta, and acts as a ligand-inducible transcription factor to fulfill critical roles in reproductive physiology. Although in vitro studies suggest the ERs may play redundant roles, a dissimilar tissue distribution indicates otherwise. Therefore, to gain insight into the role of each ER form, individual lines of mice lacking each respective receptor, as well as mice lacking both ER forms, were generated. alphaERKO and betaERKO female mice possess a normally developed reproductive tract and maintain expression of the opposite ER. The alphaERKO female is infertile and exhibits a hypoplastic uterus that is refractory to estrogens. The ovaries of the alphaERKO female are consistently polycystic and lack indications of spontaneous ovulation. In contrast, the betaERKO female exhibits a hormonally responsive uterus and grossly normal ovaries, but is subfertile in terms of the frequency and size of litters. Immature females of both ERKO lines successfully ovulate viable ova when superovulated with exogenous gonadotropins, yet the average yield of ooctyes is reduced. Mice lacking both known ER forms (alphabetaERKO) are infertile, possess the expected reproductive tract structures, but exhibit a remarkably distinct ovarian phenotype characterized by postnatal loss of oocytes and redifferentiation of the remaining somatic cells to Sertoli-like cells. This "sex-reversal" in the alphabetaERKO ovary is accompanied by the ectopic expression of testis-specific genes, for example, Sox9 and sulfatedglycoprotein-2.

17beta-estradiol and ICI-182780 regulate the hair follicle cycle in mice through an estrogen receptor-alpha pathway

Estradiol (E(2)) applied topically twice weekly to mouse skin at doses as low as 1 nmol inhibited hair growth by blocking the transition of the hair follicle from the resting phase (telogen) to the growth phase (anagen). In contrast, application of </=10 nmol of other steroids produced limited inhibition. Topical treatment with the estrogen receptor (ER) antagonist ICI-182780 reversed the effects of E(2), and when applied alone, ICI-182780 caused a telogen-to-anagen transition. Both E(2) and ICI-182780 were highly effective at their site of application but not at distant sites, indicating the direct rather than secondary systemic nature of their effects. Western analysis detected a 65-kDa ER-alpha immunoreactive dermal protein, and Northern analysis revealed the presence of a 6.7-kb ER-alpha mRNA. A ribonuclease protection assay confirmed the presence of ER-alpha transcripts but failed to detect ER-beta transcripts. These findings implicate a skin-specific ER-alpha pathway in the regulation of the hair follicle cycle.

Correlation between low levels of estrogen receptors and estrogen responsiveness in two rat osteoblast-like cell lines

With the knowledge that estrogen replacement therapy can circumvent postmenopausal osteoporosis and with the discovery of estrogen receptors (ER) in cultures of normal osteoblast-like cells, extensive investigations have been directed toward understanding the role of the ER in normal bone homeostasis. ROS 17/2.8 and UMR-106-01, two established osteoblast-like cell lines derived from rat osteosarcomas, have been shown to have estrogen-regulated biologic responses. Only the ROS 17/2.8 cell line has been reported to contain ER. In this study, high-affinity, saturable binding sites characteristic of the ER were detected in UMR-106-01 cells by binding assays with the high-affinity ligand, [125I]17 beta-estradiol. An initial immunoconcentration step before western blot analysis also allowed detection of the full-length ER protein. In addition, northern blot analysis indicated that the entire ER transcript was expressed and that the half-life of the ER message was increased following cycloheximide treatment. Message levels were also regulated by removal of serum and treatment with estradiol. An estrogen-regulated reporter vector, ERET81CAT, was transfected into the UMR-106-01 cells to determine whether the detected level of ER was transcriptionally functional. Using this assay, estrogen responsiveness was evident; however, the response was inconsistent. Multiple factors, such as serum, estradiol, and cell density, influence the ER levels in these cells and probably cause fluctuations in the abundance of receptors available to induce the CAT response. When the cells are responsive, the ICI 164,384 antagonist could block the estrogen-induced activation of CAT.(ABSTRACT TRUNCATED AT 250 WORDS)

Aberrant reproductive phenotypes evident in transgenic mice expressing the wild-type mouse estrogen receptor

The estrogen receptor (ER) acts as a transcription factor to regulate multiple cellular functions involved in normal physiology, differentiation, and reproduction. To date, there is no known animal model for studying aberrant ER expression. Therefore, we created transgenic mice expressing the wild-type mouse ER under the control of the mouse metallothionein-I (MT) promoter to determine whether overexpression of the ER would disrupt normal reproductive processes. Five male and one female founder mice were produced, and all were fertile. The progeny from these mice were screened for MT-mER expression by the ribonuclease protection assay. Mice in all six lines were found to express the transgene in a variety of tissues, although generally at low levels. The highest level of expression was observed in the female reproductive tract of line E. Females in all six lines demonstrated aberrant reproductive phenotypes involving processes at parturition and, with some of the lines, a tendency toward reduced fertility. Gestational length was prolonged up to 4 days beyond the normal gestation of 19 days, providing evidence of delayed parturition. In addition, prolonged labor (up to 3 days in length to deliver all pups) and labors requiring cesarean sections for maternal survival demonstrated the occurrence of dystocia in the MT-mER females. As maternal age increased, the incidence of stillborn litters, delayed parturition, and dystocia approached 100% in the transgenic dams. Difficulties at parturition were not observed in nontransgenic control females. These phenotypes suggest that the mechanisms regulating parturition may be perturbed by improper expression of the ER. The MT-mER transgenic mice may provide a novel approach for studying the estrogen-regulated signals involved in parturition and fertility as well as a unique animal model for the human reproductive phenotypes of delayed parturition and dystocia.

17beta-estradiol induces apoptosis in the developing rodent prostate independently of ERalpha or ERbeta

Estrogens induce both proliferative and antiproliferative responses in the prostate gland. To date, antiproliferative effects of estrogens are generally considered to be due to systemic antiandrogenic actions. However, estrogen action mediated through estrogen receptor (ER) beta was recently suggested as another mechanism of induction of apoptosis in the prostate. This study aimed to explore the hypothesis that the antiproliferative effects of estrogen are directly mediated through ERbeta using a prostate organ culture system. We previously reported effects of 17beta-estradiol (E2) using rat ventral prostate (VP) tissues, and adapted the system for culturing mouse tissues. In both rat and mouse models, estrogen-induced apoptosis was detected that was spatially and regionally localized to the epithelium of the distal tips. Using organ cultures of alphaER knockout (alphaERKO) and betaERKO prostates, we failed to demonstrate that apoptosis induced by E2 was mediated through either receptor subtype. Activation of ER-selective ligands (ERalpha, propyl pyrazole triol, ERbeta, diaryl-proprionitrile, and 5alpha-androstane-3beta,17beta-diol) in organ culture experiments failed to induce apoptosis, as did the membrane impermeable conjugate E2:BSA, discounting the possibility of nongenomic effects. Consequently, E2 regulation of androgen receptor (AR) expression was examined and, in the presence of nanomolar testosterone levels, E2 caused a specific reduction in AR protein expression in wild-type, alphaERKO, and betaERKO mice, particularly in the distal region where apoptosis was detected. This down-regulation of AR protein provides a possible mechanism for the proapoptotic action of E2 that is independent of ERs or nongenomic effects.