Paracrine-rescued lobulogenesis in chimeric outgrowths comprising progesterone-receptor-null mammary epithelium and redirected wild-type testicular cells

We have previously shown that non-mammary and tumorigenic cells can respond to the signals of the mammary niche and alter their cell fate to that of mammary epithelial progenitor cells. Here we tested the hypothesis that paracrine signals from mammary epithelial cells expressing progesterone receptor (PR) are dispensable for redirection of testicular cells, and that re-directed wild-type testicular-derived mammary cells can rescue lobulogenesis of PR-null mammary epithelium by paracrine signaling during pregnancy. We injected PR-null epithelial cells mixed with testicular cells from wild-type adult male mice into cleared fat-pads of recipient mice. The testicular cells were redirected in vivo to mammary epithelial cell fate during regeneration of the mammary epithelium, and persisted in second-generation outgrowths. In the process, the redirected testicular cells rescued the developmentally deficient PR-null cells, signaling them through the paracrine factor RANKL to produce alveolar secretory structures during pregnancy. This is the first demonstration that paracrine signaling required for alveolar development is not required for cellular reprogramming in the mammary gland, and that reprogrammed testicular cells can provide paracrine signals to the surrounding mammary epithelium.

Progesterone receptor and Stat5 signaling cross talk through RANKL in mammary epithelial cells

Progesterone (P4) stimulates proliferation of the mammary epithelium by a mechanism that involves paracrine signaling mediated from progesterone receptor (PR)-positive to neighboring PR-negative cells. Here we used a primary mouse mammary epithelial cell (MEC) culture system to define the molecular mechanism by which P4 regulates the expression of target gene effectors of proliferation including the paracrine factor receptor and activator of nuclear factor κB ligand (RANKL). MECs from adult virgin mice grown and embedded in three-dimensional basement-membrane medium resemble mammary ducts in vivo structurally and with respect to other properties including a heterogeneous pattern of PR expression, P4 induction of RANKL and other target genes in a PR-dependent manner, and a proliferative response to progestin. RANKL was demonstrated to have multiple functional P4-responsive enhancers that bind PR in a hormone-dependent manner as detected by chromatin immunoprecipitation assay. P4 also stimulated recruitment of signal transducer and activator of transcription (Stat)5a to RANKL enhancers through an apparent tethering with PR. Analysis of primary MECs from Stat5a knockout mice revealed that P4 induction of RANKL and a broad range of other PR target genes required Stat5a, as did P4-stimulated cell proliferation. In the absence of Stat5a, PR binding was lost at selective RANKL enhancers but was retained with others, suggesting that Stat5a acts to facilitate PR DNA binding at selective sites and to function as a coactivator with DNA-bound PR at others. These results show that RANKL is a direct PR target gene and that Stat5a has a novel role as a cofactor in PR-mediated transcriptional signaling in the mammary gland.

Acceleration of the glycolytic flux by steroid receptor coactivator-2 is essential for endometrial decidualization

Early embryo miscarriage is linked to inadequate endometrial decidualization, a cellular transformation process that enables deep blastocyst invasion into the maternal compartment. Although much of the cellular events that underpin endometrial stromal cell (ESC) decidualization are well recognized, the individual gene(s) and molecular pathways that drive the initiation and progression of this process remain elusive. Using a genetic mouse model and a primary human ESC culture model, we demonstrate that steroid receptor coactivator-2 (SRC-2) is indispensable for rapid steroid hormone-dependent proliferation of ESCs, a critical cell-division step which precedes ESC terminal differentiation into decidual cells. We reveal that SRC-2 is required for increasing the glycolytic flux in human ESCs, which enables rapid proliferation to occur during the early stages of the decidualization program. Specifically, SRC-2 increases the glycolytic flux through induction of 6-phosphofructo-2-kinase/fructose-2, 6-bisphosphatase 3 (PFKFB3), a major rate-limiting glycolytic enzyme. Similarly, acute treatment of mice with a small molecule inhibitor of PFKFB3 significantly suppressed the ability of these animals to exhibit an endometrial decidual response. Together, these data strongly support a conserved mechanism of action by which SRC-2 accelerates the glycolytic flux through PFKFB3 induction to provide the necessary bioenergy and biomass to meet the demands of a high proliferation rate observed in ESCs prior to their differentiation into decidual cells. Because deregulation of endometrial SRC-2 expression has been associated with common gynecological disorders of reproductive-age women, this signaling pathway, involving SRC-2 and PFKFB3, promises to offer new clinical approaches in the diagnosis and/or treatment of a non-receptive uterus in patients presenting idiopathic infertility, recurrent early pregnancy loss, or increased time to pregnancy.

Failure of an embryo to correctly implant into the endometrium is a common cause of pregnancy failure or early embryo miscarriage. Although advances in our understanding of oocyte and embryo development have significantly increased pregnancy success rates, these rates remain unacceptably low due in part to an endometrium that is unreceptive to embryo implantation. Using experimental mouse genetics and a primary human cell culture model, we show here that the development of a receptive endometrium requires steroid receptor coactivator-2, a factor which modulates the response of an endometrial cell to the pregnancy hormone, progesterone. Specifically, we show that SRC-2 increases progesterone-dependent glycolysis in the endometrial cell to provide energy and biomolecules for the next round of cell division. For an endometrium to be receptive to embryo implantation, specific endometrial cells (termed stromal cells) need to divide and numerically increase just prior to development of the receptive state. Therefore, SRC-2 is critical for the metabolic reprogramming of the endometrium to a receptive state, which provides the pretext for considering this factor and its metabolic targets in the design of future clinical approaches to diagnose and therapeutically treat those women at a high risk for early pregnancy loss.

Integrated chromatin immunoprecipitation sequencing and microarray analysis identifies FOXA2 target genes in the glands of the mouse uterus

Uterine glands and their secretions are indispensable for endometrial function and fertility; however, the mechanisms regulating their development and function are not well understood. Forkhead transcription factor box A2 (FOXA2) is uniquely expressed in the glandular epithelial (GE) cells of the uterus, and conditional deletion of Foxa2 after birth impedes uterine gland development. An integrative approach was used here to define the FOXA2 cistrome in the murine uterus. Genome-wide mapping of FOXA2 binding sites was combined with transcriptomic analyses of isolated GE and Foxa2-deleted uteri. ChIP-Seq analyses found the number of FOXA2 target genes was substantially greater in the adult (8893) than neonatal uterus (1101). In the neonatal uterus, FOXA2-bound and GE-expressed genes (469) were enriched for developmentally related processes, including cell cycle, cell junction, focal adhesion, and WNT signaling. In the adult uterus, FOXA2-bound and GE-expressed genes (3730) were enriched for functional processes, including metabolic pathways, focal adhesion, bacterial invasion of epithelial cells, and WNT signaling. Analysis of the uterine FOXA2 cistrome provides novel insights into mechanisms governing endometrial gland development and function, which are important to understand fundamental aspects of uterine differentiation, regeneration and disease.

Progesterone signaling inhibits cervical carcinogenesis in mice

Human papillomavirus is the main cause of cervical cancer, yet other nonviral cofactors are also required for the disease. The uterine cervix is a hormone-responsive tissue, and female hormones have been implicated in cervical carcinogenesis. A transgenic mouse model expressing human papillomavirus oncogenes E6 and/or E7 has proven useful to study a mechanism of hormone actions in the context of this common malignancy. Estrogen and estrogen receptor α are required for the development of cervical cancer in this mouse model. Estrogen receptor α is known to up-regulate expression of the progesterone receptor, which, on activation by its ligands, either promotes or inhibits carcinogenesis, depending on the tissue context. Here, we report that progesterone receptor inhibits cervical and vaginal epithelial cell proliferation in a ligand-dependent manner. We also report that synthetic progestin medroxyprogesterone acetate promotes regression of cancers and precancerous lesions in the female lower reproductive tracts (ie, cervix and vagina) in the human papillomavirus transgenic mouse model. Our results provide the first experimental evidence that supports the hypothesis that progesterone signaling is inhibitory for cervical carcinogenesis in vivo.

VEGF-A regulated by progesterone governs uterine angiogenesis and vascular remodelling during pregnancy

The features and regulation of uterine angiogenesis and vascular remodelling during pregnancy are poorly defined. Here we show that dynamic and variable decidual angiogenesis (sprouting, intussusception and networking), and active vigorous vascular remodelling such as enlargement and elongation of ‘vascular sinus folding’ (VSF) and mural cell drop-out occur distinctly in a spatiotemporal manner in the rapidly growing mouse uterus during early pregnancy — just after implantation but before placentation. Decidual angiogenesis is mainly regulated through VEGF-A secreted from the progesterone receptor (PR)-expressing decidual stromal cells which are largely distributed in the anti-mesometrial region (AMR). In comparison, P₄-PR-regulated VEGF-A-VEGFR2 signalling, ligand-independent VEGFR3 signalling and uterine natural killer (uNK) cells positively and coordinately regulate enlargement and elongation of VSF. During the postpartum period, Tie2 signalling could be involved in vascular maturation at the endometrium in a ligand-independent manner, with marked reduction of VEGF-A, VEGFR2 and PR expressions. Overall, we show that two key vascular growth factor receptors — VEGFR2 and Tie2 — strikingly but differentially regulate decidual angiogenesis and vascular remodelling in rapidly growing and regressing uteri in an organotypic manner.

Functional role of progestin and the progesterone receptor in the suppression of spermatogenesis in rodents

Synthetic progestins such as levonorgestrel (LNG) are used in combination with testosterone (T) in male contraceptive clinical trials to suppress gonadotropins secretion, but whether progestins have additional direct effects on the testis are not known. This study aimed to examine the effect of a potent progestin, (LNG), alone or in combination with testosterone (T) on spermatogenesis in adult rats, and to evaluate the functional role of the progesterone receptors (PRs) in the testis. In comparison with a low dose of LNG treatment in adult rats for 4 weeks, T and T + LNG treatment decreased testicular sperm count to 64.1 and 40.2% of control levels respectively. LNG induced germ cell apoptosis at stages I-IV and XII-XIV; T increased apoptosis at stages VII-VIII; LNG + T treatment induced greater germ cell apoptosis at a wider range of seminiferous epithelial stages. RT-PCR and Western Blots showed that PR was present in testes and up-regulated during suppression of spermatogenesis induced by testicular hormonal deprivation. PR knockout (PRKO) mice had larger testes, greater sperm production, increased numbers of Sertoli and Leydig cells. Suppression of gonadotropin and intratesticular T by GnRH-antagonist treatment induced PR promoter driven LacZ expression in Leydig cells of PRKO mice. This suggests that GnRH-antagonist treatment while inducing germ cell apoptosis also up-regulates PR. We conclude that (i) LNG + T induced greater suppression of spermatogenesis through increase in germ cell apoptosis involving a wider range of seminiferous epithelial stages than either treatment alone, (ii) up-regulation of PR was associated with inhibition of spermatogenesis, (iii) PR knockout mice showed increased sperm production suggesting that testicular PR activated events play a physiological and pharmacological inhibitory role in the testis. These data support the hypothesis that in addition to its known suppressive effects on gonadotropins, progestins may have direct inhibitory actions on the testis.

Loss of CDH1 and Pten accelerates cellular invasiveness and angiogenesis in the mouse uterus

E-cadherin (CDH1) is a cell adhesion molecule that coordinates key morphogenetic processes regulating cell growth, cell proliferation, and apoptosis. Loss of CDH1 is a trademark of the cellular event epithelial to mesenchymal transition, which increases the metastatic potential of malignant cells. PTEN is a tumor-suppressor gene commonly mutated in many human cancers, including endometrial cancer. In the mouse uterus, ablation of Pten induces epithelial hyperplasia, leading to endometrial carcinomas. However, loss of Pten alone does not affect longevity until around 5 mo. Similarly, conditional ablation of Cdh1 alone does not predispose mice to cancer. In this study, we characterized the impact of dual Cdh1 and Pten ablation (Cdh1(d/d) Pten(d/d)) in the mouse uterus. We observed that Cdh1(d/d) Pten(d/d) mice died at Postnatal Days 15-19 with massive blood loss. Their uteri were abnormally structured with curly horns, disorganized epithelial structure, and increased cell proliferation. Co-immunostaining of KRT8 and ACTA2 showed invasion of epithelial cells into the myometrium. Further, the uteri of Cdh1(d/d) Pten(d/d) mice had prevalent vascularization in both the endometrium and myometrium. We also observed reduced expression of estrogen and progesterone receptors, loss of cell adherens, and tight junction molecules (CTNNB1 and claudin), as well as activation of AKT in the uteri of Cdh1(d/d) Pten(d/d) mice. However, complex hyperplasia was not found in the uteri of Cdh1(d/d) Pten(d/d) mice. Collectively, these findings suggest that ablation of Pten with Cdh1 in the uterus accelerates cellular invasiveness and angiogenesis and causes early death.

Uterine-specific loss of Tsc2 leads to myometrial tumors in both the uterus and lungs

Lymphangioleiomyomatosis (LAM) is a rare disease characterized by proliferation of abnormal smooth-muscle cells in the lungs, leading to functional loss and sometimes lung transplantation. Although the origin of LAM cells is unknown, several features of LAM provide clues. First, LAM cells contain inactivating mutations in genes encoding Tsc1 or Tsc2, proteins that limit mTORC1 activity. Second, LAM tumors recur after lung transplantation, suggesting a metastatic pathogenesis. Third, LAM is found almost exclusively in women. Finally, LAM shares features with uterine leiomyomas, benign tumors of myometrial cells. From these observations, we proposed that LAM cells might originate from uterine leiomyomas containing Tsc mutations. To test our hypothesis, and to develop mouse models for leiomyoma and LAM, we targeted Tsc2 deletion primarily in uterine cells. In fact, nearly 100% of uteri from uterine-specific Tsc2 knockout mice developed myometrial proliferation and uterine leiomyomas by 12 and 24 weeks, respectively. Myometrial proliferation and mTORC1/S6 activity were abrogated by the mTORC1 inhibitor rapamycin or by elimination of sex steroid production through ovariectomy or aromatase inhibition. In ovariectomized Tsc2 null mice, mTORC1/S6 activity and myometrial growth were restored by estrogen but not progesterone. Thus, even without Tsc2, estrogen appears to be required for myometrial mTORC1/S6 signaling and proliferation. Finally, we found Tsc2 null myometrial tumors in lungs of older Tsc2 uterine-specific knockout females, suggesting that lung LAM-like myometrial lesions may indeed originate from the uterus. This mouse model may improve our understanding of LAM and leiomyomas and might lead to novel therapeutic strategies for both diseases.

Liver receptor homolog-1 is essential for pregnancy

Successful pregnancy requires coordination of an array of signals and factors from multiple tissues. One such element, the liver receptor homolog-1 (Lrh-1, NR5A2), is an orphan nuclear receptor that regulates metabolism and hormone synthesis¹. It is strongly expressed in granulosa cells of ovarian follicles and in the corpus luteum of rodents² and humans. Germline ablation of the Lrh-1 gene in mice is embryo-lethal at gastrulation³. Depletion of Lrh-1 in the ovarian follicle demonstrates that it regulates genes required for both steroid synthesis and ovulation⁴. To study the effects of Lrh-1 on mouse gestation, we disrupted its expression in the corpus luteum, resulting in luteal insufficiency. Hormone replacement permitted embryo implantation but was followed by gestational failure with impaired endometrial decidualization, compromised placental formation, fetal growth retardation, and fetal death. Lrh-1 is expressed in the mouse and human endometrium. In a human model of primary culture of endometrial stromal cells, depletion of Lrh-1 by siRNA abrogated decidualization. These findings demonstrate that Lrh-1 is necessary for maintenance of the corpus luteum, for promotion of decidualization and for placental formation. It therefore plays multiple, indispensible roles in establishing and sustaining pregnancy.

Critical tumor suppressor function mediated by epithelial Mig-6 in endometrial cancer

Endometrial cancer is preceded by endometrial hyperplasia, unopposed estrogen exposure, and genetic alterations, but the precise causes of endometrial cancer remain uncertain. Mig-6, mainly known as a negative regulator of the EGF receptor, is an important mediator of progesterone signaling in the uterus, where it mediates tumor suppression by modulating endometrial stromal-epithelial communications. In this study, we investigated the function of Mig-6 in the uterine epithelium using a tissue-specific gene knockout strategy, in which floxed Mig-6 (Mig-6(f/f)) mice were crossed to Wnt7a-Cre mice (Wnt7a(cre+)Mig-6(f/f)). Wnt7a(cre+)Mig-6(f/f) mice developed endometrial hyperplasia and estrogen-dependent endometrial cancer, exhibiting increased proliferation in epithelial cells as well as apoptosis in subepithelial stromal cells. We documented increased expression of NOTCH1 and BIRC3 in epithelial cells of Wnt7a(cre+)Mig-6(f/f) mice and decreased expression of the progesterone receptor (PR) in stromal cells. Progesterone therapy controls endometrial growth and prevents endometrial cancer, but the effectiveness of progesterone as a treatment for women with endometrial cancer is less clear. We noted that the hyperplasic phenotype of Wnt7a(cre+)Mig-6(f/f) mice was prevented by progesterone treatment, whereas this treatment had no effect in PR(cre/+)Mig-6(f/f) mice where Mig-6 was deleted in both the epithelial and stromal compartments of the uterus. In contrast, activation of progesterone signaling in the stroma regulated proliferation and apoptosis in the epithelium via suppression of ERα signaling. In summary, our results establish that epithelial Mig-6 functions as a critical tumor suppressor that mediates the ability of progesterone to prevent the development of endometrial cancer.

BMPR2 is required for postimplantation uterine function and pregnancy maintenance

Abnormalities in cell-cell communication and growth factor signaling pathways can lead to defects in maternal-fetal interactions during pregnancy, including immunologic rejection of the fetal/placental unit. In this study, we discovered that bone morphogenetic protein receptor type 2 (BMPR2) is essential for postimplantation physiology and fertility. Despite normal implantation and early placental/fetal development, deletion of Bmpr2 in the uterine deciduae of mice triggered midgestation abnormalities in decidualization that resulted in abnormal vascular development, trophoblast defects, and a deficiency of uterine natural killer cells. Absence of BMPR2 signaling in the uterine decidua consequently suppressed IL-15, VEGF, angiopoietin, and corin signaling. Disruption of these pathways collectively lead to placental abruption, fetal demise, and female sterility, thereby placing BMPR2 at a central point in the regulation of several physiologic signaling pathways and events at the maternal-fetal interface. Since trophoblast invasion and uterine vascular modification are implicated in normal placentation and fetal growth in humans, our findings suggest that abnormalities in uterine BMPR2-mediated signaling pathways can have catastrophic consequences in women for the maintenance of pregnancy.

Signal transducer and activator of transcription-3 (Stat3) plays a critical role in implantation via progesterone receptor in uterus

Recent studies have shown that activation of the signal transducer and activator of transcription-3 (Stat3) is required for decidualization, interacting with progesterone receptor (PR) in uterus. Based on previous reports, we hypothesized that crosstalk between STAT3 and PR signaling is required for successful implantation. To identify the interaction between STAT3 and PR isoforms, we performed immunoprecipitation following transient cotransfection and found that STAT3 physically interacted with PR-A, which is known to be important for uterine development and function, but not with PR-B. To further investigate the role of Stat3 in uterine function, Stat3 was conditionally ablated only in the PR-positive cells (PR(cre/+) Stat3(f/f); Stat3(d/d)). Our studies revealed that ovarian function and uterine development of Stat3(d/d) mice were normal. However, Stat3(d/d) female mice were infertile due to defective embryo implantation. Unlike Stat3(f/f) mice, Stat3(d/d) mice exhibited an unclosed uterine lumen. Furthermore, uteri of Stat3(d/d) mice were unable to undergo a well-characterized hormonally induced decidual reaction. The expression of stromal PR was decreased during decidualization and preimplantation period in Stat3(d/d) mice, and PR target genes were significantly down-regulated after progesterone induction. Our results suggest that STAT3 and PR crosstalk is required for successful implantation in the mouse uterus.

Progesterone drives mammary secretory differentiation via RankL-mediated induction of Elf5 in luminal progenitor cells

Progesterone-RankL paracrine signaling has been proposed as a driver of stem cell expansion in the mammary gland, and Elf5 is essential for the differentiation of mammary epithelial progenitor cells. We demonstrate that Elf5 expression is induced by progesterone and that Elf5 and progesterone cooperate to promote alveolar development. The progesterone receptor and Elf5 are expressed in a mutually exclusive pattern, and we identify RankL as the paracrine mediator of the effects of progesterone on Elf5 expression in CD61+ progenitor cells and their consequent differentiation. Blockade of RankL action prevented progesterone-induced side branching and the expansion of Elf5(+) mature luminal cells. These findings describe a mechanism by which steroid hormones can produce the expansion of steroid hormone receptor-negative mammary epithelial cells.

Nuclear receptor coactivator-6 attenuates uterine estrogen sensitivity to permit embryo implantation

Uterine receptivity to embryo implantation is coordinately regulated by 17β-estradiol (E(2)) and progesterone (P(4)). Although increased E(2) sensitivity causes infertility, the mechanisms underlying the modulation of E(2) sensitivity are unknown. We show that nuclear receptor coactivator-6 (NCOA6), a reported coactivator for estrogen receptor α (ERα), actually attenuates E(2) sensitivity to determine uterine receptivity to embryo implantation under normal physiological conditions. Specifically, conditional knockout of Ncoa6 in uterine epithelial and stromal cells does not decrease, but rather markedly increases E(2) sensitivity, which disrupts embryo implantation and inhibits P(4)-regulated genes and decidual response. NCOA6 enhances ERα ubiquitination and accelerates its degradation, while loss of NCOA6 causes ERα accumulation in stromal cells during the preimplantation period. During the same period, NCOA6 deficiency also caused a failure in downregulation of steroid receptor coactivator-3 (SRC-3), a potent ERα coactivator. Therefore, NCOA6 controls E(2) sensitivity and uterine receptivity by regulating multiple E(2)-signaling components.

A new isoform of steroid receptor coactivator-1 is crucial for pathogenic progression of endometriosis

Endometriosis is considered to be an estrogen-dependent inflammatory disease, but its etiology is unclear. Thus far, a mechanistic role for steroid receptor coactivators (SRCs) in the progression of endometriosis has not been elucidated. An SRC-1-null mouse model reveals that the mouse SRC-1 gene has an essential role in endometriosis progression. Notably, a previously unidentified 70-kDa SRC-1 proteolytic isoform is highly elevated both in the endometriotic tissue of mice with surgically induced endometriosis and in endometriotic stromal cells biopsied from patients with endometriosis compared to normal endometrium. Tnf⁻/⁻ and Mmp9⁻/⁻ mice with surgically induced endometriosis showed that activation of tumor necrosis factor a (TNF-α)-induced matrix metallopeptidase 9 (MMP9) activity mediates formation of the 70-kDa SRC-1 C-terminal isoform in endometriotic mouse tissue. In contrast to full-length SRC-1, the endometriotic 70-kDa SRC-1 C-terminal fragment prevents TNF-α-mediated apoptosis in human endometrial epithelial cells and causes the epithelial-mesenchymal transition and the invasion of human endometrial cells that are hallmarks of progressive endometriosis. Collectively, the newly identified TNF-α-MMP9-SRC-1 isoform functional axis promotes pathogenic progression of endometriosis.

Progesterone suppresses the mTOR pathway and promotes generation of induced regulatory T cells with increased stability

While induced FoxP3(+) T cells (iTreg cells) are promising cellular therapeutics to treat inflammatory diseases, a limitation in utilizing iTreg cells prepared in vitro is their low stability in inflammatory conditions. Progesterone (P4) is an immune regulatory nuclear hormone with a potent Treg induction activity. We reasoned that this function of progesterone would be utilized to generate iTreg cells with highly suppressive activity and improved stability in vivo. Here we generated iTreg cells with progesterone in vitro and found that progesterone generates iTreg cells that are highly stable in inflammatory conditions. Moreover, P4-induced iTreg cells highly express latency-associated peptide TGF-β1 and are efficient in regulating inflammation in multiple tissues, whereas control iTreg cells induced with TGF-β1 alone are less stable and ineffective in suppressing inflammation. The function of progesterone in inducing iTreg cells with improved regulatory activity is associated with the function of P4 in suppressing the mTOR pathway. Moreover, the function of progesterone in inducing FoxP3(+) T cells is decreased but not completely abolished on nuclear progesterone receptor-deficient T cells, suggesting that both nuclear and nonnuclear progesterone receptors are involved in mediating the function. We conclude that P4 can be utilized to generate iTreg cells with a high therapeutic potential in treatment of tissue inflammation.

Uterine development and fertility are dependent on gene dosage of the nuclear receptor coregulator REA

Although the effectiveness of nuclear hormone-receptor complexes is known to depend on coregulator partner proteins, relatively little is known about the roles of coregulators in uterine development and early stages of pregnancy and implantation. Because conventional genetic deletion of the coregulator, repressor of estrogen receptor activity (REA), was embryonic lethal, we here study REA conditional knockout mice generated by cre-loxP recombination, in which REA function was abrogated only in progesterone receptor-expressing tissues, to define the roles of REA in postembryonic stages and in a tissue-specific manner. We find that REA has gene dose-dependent activity impacting uterine development and fertility. Conditional homozygous mutant (REA(d/d)) mice developed to adulthood and showed normal ovarian function, but females were infertile with severely compromised uterine development and function characterized by cell cycle arrest, apoptosis, and altered adenogenesis (endometrial gland morphogenesis), resulting in failure of implantation and decidualization. By contrast, mice heterozygous for REA (REA(f/d)) had a very different phenotype, with estradiol treatment resulting in hyperstimulated, large uteri showing increased proliferation of luminal epithelial cells, and enhanced fluid imbibition associated with altered regulation of aquaporins. These REA(f/d) female mice showed a subfertility phenotype with reduced numbers and sizes of litters. These findings highlight that uterine development and regulation of estrogen receptor activities show a bimodal dependence on the gene dosage of REA. Optimal uterine development and functional activities require the normal gene dosage of REA, with partial or complete deletion resulting in hyperresponsiveness or underresponsiveness to hormone and subfertility or infertility, respectively.