The role of progesterone in endometrial angiogenesis in pregnant and ovariectomised mice

Vascular endothelial growth factor receptor 2 (VEGFR-2) functions to promote uterine decidual angiogenesis during early pregnancy in the mouse

Implantation of an embryo induces rapid proliferation and differentiation of uterine stromal cells, forming a new structure, the decidua. One salient feature of decidua formation is a marked increase in maternal angiogenesis. Vascular endothelial growth factor (VEGF)-dependent pathways are active in the ovary, uterus, and embryo, and inactivation of VEGF function in any of these structures might prevent normal pregnancy development. We hypothesized that decidual angiogenesis is regulated by VEGF acting through specific VEGF receptors (VEGFRs). To test this hypothesis, we developed a murine pregnancy model in which systemic administration of a receptor-blocking antibody would act specifically on uterine angiogenesis and not on ovarian or embryonic angiogenesis. In our model, ovarian function was replaced with exogenous progesterone, and blocking antibodies were administered prior to embryonic expression of VEGFRs. After administration of a single dose of the anti-VEGFR-2 antibody during the peri-implantation period, no embryos were detected on embryonic d 10.5. The pregnancy was disrupted because of a significant reduction in decidual angiogenesis, which under physiological conditions peaks on embryonic d 5.5 and 6.5. Inactivation of VEGFR-3 reduced angiogenesis in the primary decidual zone, whereas administration of VEGFR-1 blocking antibodies had no effect. Pregnancy was not disrupted after administration of anti-VEGFR-3 or anti-VEGFR-1 antibodies. Thus, the VEGF/VEGFR-2 pathway plays a key role in the maintenance of early pregnancy through its regulation of peri-implantation angiogenesis in the uterine decidua. This newly formed decidual vasculature serves as the first exchange apparatus for the developing embryo until the placenta becomes functionally active.

An RNA spiking method demonstrates that 18S rRNA is regulated by progesterone in the mouse uterus

Identifying suitable housekeeping genes for quantitative RT-PCR in the uterus is problematic, as this tissue undergoes significant structural and functional alterations during the oestrous cycle and pregnancy in response to circulating hormones. The suitability of 18S rRNA as a housekeeping gene in mouse uterus was investigated by introducing an 'RNA spike' standard into the reverse transcription reaction. 18S rRNA levels increased by Day 4 of pregnancy and after progesterone administration in ovariectomized mice. We conclude that 18S rRNA is not a suitable housekeeping gene for quantitative RT-PCR analysis in progesterone-responsive tissues, and the RNA spiking method provides a suitable alternative.

Endocrine activity of extraembryonic membranes extends beyond placental amniotes

Background

During development, all amniotes (mammals, reptiles, and birds) form extraembryonic membranes, which regulate gas and water exchange, remove metabolic wastes, provide shock absorption, and transfer maternally derived nutrients. In viviparous (live-bearing) amniotes, both extraembryonic membranes and maternal uterine tissues contribute to the placenta, an endocrine organ that synthesizes, transports, and metabolizes hormones essential for development. Historically, endocrine properties of the placenta have been viewed as an innovation of placental amniotes. However, an endocrine role of extraembryonic membranes has not been investigated in oviparous (egg-laying) amniotes despite similarities in their basic structure, function, and shared evolutionary ancestry. In this study, we ask whether the oviparous chorioallantoic membrane (CAM) of chicken (Gallus gallus) has the capability to synthesize and receive signaling of progesterone, a major placental steroid hormone.

Methodology/Principal Findings

We quantified mRNA expression of key steroidogenic enzymes involved in progesterone synthesis and found that 3β-hydroxysteroid dehydrogenase, which converts pregnenolone to progesterone exhibited a 464 fold increase in the CAM from day 8 to day 18 of embryonic development (F_{5, 68} = 89.282, p<0.0001). To further investigate progesterone synthesis, we performed explant culture and found that the CAM synthesizes progesterone in vitro in the presence of a steroid precursor. Finally, we quantified mRNA expression and performed protein immunolocalization of the progesterone receptor in the CAM.

Conclusions/Significance

Collectively, our data indicate that the chick CAM is steroidogenic and has the capability to both synthesize progesterone and receive progesterone signaling. These findings represent a paradigm shift in evolutionary reproductive biology by suggesting that endocrine activity of extraembryonic membranes is not a novel characteristic of placental amniotes. Rather, we hypothesize that these membranes may share an additional unifying characteristic, steroidogenesis, across amniotes at large.

Roles of estrogen and progesterone in endometrial hemodynamics and vascular endothelial growth factor production

BACKGROUND

The endometrium becomes receptive to the embryo after sequential actions of estrogen and progesterone. The purpose of this study was to examine the effects of estrogen and progesterone on endometrial hemodynamics and on secretion of vascular endothelial growth factor (VEGF) from endometrial epithelial cells (EEC).

METHODS

Six early postmenopausal women taking sequential estrogen and progestin [days 1-11: estradiol valerate (estrogen) 2 mg daily; days 12-21: estradiol valerate 2 mg plus norethisterone acetate (progestin) 1 mg daily] were recruited. Three-dimensional power Doppler angiography (3D-PDA) was performed before hormone treatment (phase 0), on days 10-11 of hormone treatment (phase E), and on days 18-20 of hormone treatment (phase E + P). Ishikawa EEC were treated with or without 17-beta-estradiol and progesterone for 24 hours, followed by determination of VEGF concentrations in the supernatants.

RESULTS

The endometrial volume was significantly increased in phase E and phase E + P as compared with that in phase 0. The vascularization index, flow index, and vascularization flow index in the subendometrial region, as measured by 3D-PDA, were significantly higher in phase E + P than in phase 0, but there were no significant differences in these indices between phase 0 and phase E. While treatment of EEC with 17-beta-estradiol had little enhancing effect on VEGF production, progesterone alone or in combination with 17-beta-estradiol significantly increased VEGF secretion from EEC.

CONCLUSION

Our data suggested that progesterone could stimulate VEGF secretion from EEC and subsequently increase subendometrial vascularity and blood flow.

Promotion of angiogenesis by human endometrial lymphocytes

The human endometrium is a unique tissue that undergoes dramatic monthly remodeling during the menstrual cycle in preparation for an implanting conceptus. This remodeling involves sequential proliferation and differentiation of endometrial stromal and epithelial cells, coupled with extensive angiogenesis and infiltration of a specific specialized immune cell subset. Increasing evidence points to an essential role for these maternal leukocytes in stimulating the endometrial angiogenesis, and we propose that they also play a key role in the decidual vascular transformation. Aberrant endometrial angiogenesis, decidualisation and vascular transformation is thought to underlie many pathologies of pregnancy, from infertility to the development of preeclampsia and Intra Uterine Growth Restriction. In this chapter we review the cellular processes associated with each stage of endometrial and decidual transformation, detailing the role of the immune cell populations and the angiogenic and chemotactic factors secreted by them.

Endometrial angiogenesis, vascular maturation, and lymphangiogenesis

Angiogenesis, arteriogenesis or vessel maturation, and lymphangiogenesis comprise a continuum of vascular development, with overlap and interaction between the mechanisms by which they are controlled. These processes are of clinical interest because they play roles in endometrial repair, placental development, and in gynecological disorders including endometrial cancer, endometriosis and abnormal uterine bleeding. Using mouse models we have shown that estrogen can be either proangiogenic or antiangiogenic in endometrium. Progesterone alone is proangiogenic, although this can be moderated by pretreatment with estrogen. Arteriogenesis also increases in response to progesterone, and this effect is not inhibited by estrogen. Lymphatics account for 13% of all vessels in the human functionalis compared to 57% in the basalis. Many of the basalis lymphatic vessels are closely associated with spiral arterioles and this intimate connection may provide a mechanism for paracrine communication between the functionalis and the arteries supplying the endometrium.

Evolution of non-cytotoxic uterine natural killer cells

The immune tolerance and de novo vascularization are two highly intriguing processes at the maternal-fetal interface that appear to be central to normal pregnancy outcome. Immune tolerance occurs despite the local presence of an active maternal immune system including macrophages, dendritic cells and specialized CD56(bright)CD16(-) uterine natural killer (uNK) cells (65-70%). Recent observations indicate that the phenotypic and functional repertoire of uNK cells is distinct from peripheral blood NK and endometrial NK cells, challenging the understanding of their temporal occurrence and function. Origin and specialized programming of uNK cells continue to be debated. uNK cells, replete with an armamentarium to kill the foreign, tolerate the conceptus and facilitate pregnancy. Why do these uNK cells remain non-cytotoxic? Are these NK cells 'multitasking' in nature harboring beneficial and detrimental roles in pregnancy? Are there distinct subpopulations of NK cells that may populate the decidua? We propose that the endometrium/decidua functions as an 'inducible tertiary lymphoid tissue' that supports the recruitment and expansion of CD56(bright)CD16(-) NK cells and induces transcriptional up-regulation of angiogenic machinery in response to exposure to local hormonal factors, cytokine milieu and perhaps hypoxia. The angiogenic features of uNK cells could further result in a 'multitasking' phenotype that still remains to be characterized. This article discusses the factors and pathways that bridge the angiogenic and non-cytotoxic response machineries at the maternal-fetal interface.

A potential mechanism of breakthrough bleeding associated with progestin: involvement in alteration of endometrial endothelial cells

OBJECTIVE

To explore the potential mechanism of breakthrough bleeding associated with progestin with in vitro methods.

METHODS

The isolation and culture of human endometrial endothelial cells (HEECs) was performed with the method established in our laboratory. The content and activity of urokinase-type plasminogen activator (uPA) and the content of plasminogen activator inhibitor-1 (PAI-1) in cell supernatants after incubated with different concentrations of progesterone (0-5 micromol/L) and 17beta-estradiol (0, 0.1, or 1 nmol/L) were measured by method of ELISA. Apoptosis rate of HEECs was measured by flow cytometry. Viable cell count was measured by MTT.

RESULTS

The increased level of progesterone (0.5-5 micromol/L) combined with 17beta-estradiol elevated content and activity of uPA while the production of PAI-1 remained unchanged. The apoptosis of HEECs was inhibited along with the increment of total viable cell counts at higher concentrations of progesterone with 17beta-estradiol.

CONCLUSION

The inhibition of apoptosis and increased content and activity of uPA may contribute to the occurrence of irregular bleeding associated with progestin use to some extent

Gonadotropin-releasing hormone agonist pretreatment did not decrease postoperative adhesion formation after abdominal myomectomy in a randomized control trial

OBJECTIVE

To determine if 3 months of preoperative gonadotropin-releasing hormone agonist (GnRH-a) treatment decreases postoperative uterine adhesions after open abdominal surgery for the removal of uterine fibroids.

DESIGN

Prospective, randomized, clinical study.

SETTING

A tertiary care medical center.

PATIENT(S)

Women of reproductive age with symptomatic uterine fibroids not amenable to hysteroscopic removal.

INTERVENTION(S)

Twenty patients underwent an initial abdominal myomectomy followed by a second-look laparoscopy for evaluating uterine adhesions after random allocation to groups receiving either GnRH analog or placebo for 3 months before the initial surgery.

MAIN OUTCOME MEASURE(S)

Adhesion formation between treatment groups and by incision number and aggregate length.

RESULT(S)

Presurgical GnRH-a treatment did not decrease adhesion formation compared with placebo. For every additional centimeter of incision length, the total adhesion area over the uterine serosal surface increased by 0.55 cm(2). The number of myomas removed and the number of incisions were positively correlated with total adhesion area.

CONCLUSION(S)

Preoperative treatment with GnRH-a for 3 months before open abdominal myomectomy did not decrease postoperative uterine adhesions. Following the standards of good surgical technique, adhesions are minimized with fewer and smaller incisions.

Progesterone, but not estrogen, stimulates vessel maturation in the mouse endometrium

The human endometrium undergoes regular periods of growth and regression, including concomitant changes in the vasculature, and is one of the few adult tissues where significant angiogenesis and vascular maturation occurs on a routine, physiological basis. The aim of this study was to investigate the effects of estrogen and progesterone on endometrial vascular maturation in mice. Endometrial tissues were collected from early pregnant mice (d 1-4) and ovariectomized mice given a single 17beta-estradiol (100 ng) injection 24 h before dissection (short-term estrogen regime) or three consecutive daily injections of progesterone (1 mg) with/without estrogen priming (progesterone regime). Experiments were then repeated with the inclusion of mice treated concurrently with progesterone and either RU486 or a vascular endothelial growth factor-A antiserum. Proliferating vascular mural cells (PVMC) were observed on d 3-4 of pregnancy, corresponding with an increase in circulating progesterone. A significant increase in PVMC and alpha-smooth muscle actin (labels mural cells) coverage of vessel profiles were observed in mice treated with progesterone in comparison to controls; no significant change was noted in mice treated with estrogen or with vascular endothelial growth factor antiserum. RU486 treatment did not inhibit the progesterone-induced increases in PVMC and mural cell coverage, although progesterone-induced changes in endothelial and epithelial cell proliferation were inhibited. These results show that progesterone, but not estrogen, stimulates vessel maturation in the mouse endometrium. The work illustrates the relevancy of the mouse model for understanding endometrial vascular remodeling during the menstrual cycle and in response to the clinically important progesterone receptor antagonist RU486.

Anti-angiogenic effects of imatinib target smooth muscle cells but not endothelial cells

Knowing that progesterone up-regulates PDGF-A, which plays a relevant role in angiogenesis, and that imatinib mesylate targets PDGF receptor tyrosine kinase activity, the aim of the present study was to examine the effects of imatinib on Human Aortic Smooth Muscle Cells (HAoSMC) and Human Umbilical Vein Endothelial Cells (HUVEC) after incubation with progesterone. Expression of phosphorylated (activated) PDGFR-alpha was detected in HAoSMC, but in a very low extent in HUVEC. In agreement with the lack of active PDGFR-alpha, imatinib was unable to prevent HUVEC growth, survival or migration ability. In contrast, HAoSMC viability and proliferation were effectively inhibited by imatinib, as evaluated by MTT and BrdU incorporation assay, respectively. Corroborating these findings, a significant increase in the percentage of apoptotic cells was also observed after treatment with imatinib. Cell migration assays also showed a reduction in the migratory ability after incubation with imatinib. Altogether, these facts reveal that imatinib is able to affect HAoSMC survival, growth and migration. Furthermore, incubation with recombinant PDGF as well as, with progesterone seems to sustain PDGFR-alpha activity, prompting these cells to the inhibitory action of imatinib. These findings were restricted to smooth muscle cells, leading to the assumption that imatinib is probably preventing vessel stabilization, a crucial event for neovascular maturation. Our findings indicate that imatinib might be a good therapeutic agent against atherosclerosis and other vascular-associated disorders that carry in common smooth muscle cells abnormal growth.

Placental growth, angiogenic gene expression, and vascular development in undernourished adolescent sheep

Limiting maternal nutrient intake during ovine adolescent pregnancy progressively depleted maternal body reserves, impaired fetal nutrient supply, and slowed fetal soft tissue growth. The present study examined placental growth, angiogenic gene expression, and vascular development in this undernourished adolescent model at Days 90 and 130 of gestation. Singleton pregnancies were established, and ewes were offered an optimal control (C; n = 14) or low (L [0.7 x C]; n = 21) dietary intake. Seven ewes receiving L intakes were switched to C intakes on Day 90 of gestation (L-C). Fetal body weight (P < 0.01) and glucose concentrations (P < 0.03) were reduced in L versus C pregnancies by Day 130, whereas L-C group values were intermediate. Placental cellular proliferation, gross morphology, and mass were independent of maternal nutrition at both Day 90 and 130. In contrast, capillary area density in the maternal caruncular portion of the placentome was reduced by 20% (P < 0.001) at both stages of gestation in L compared with C groups. Caruncular capillary area density was equivalent in the L and L-C groups at Day 130. Placental mRNA expression of five key angiogenic ligands or receptors increased (P < 0.001) between Days 90 and 130 of gestation. VEGFA mRNA expression was higher (P < 0.04) in L compared with C and L-C pregnancies at Day 130, but otherwise gene expression of the remaining angiogenic factors and receptors analyzed was unaffected by maternal intake. Undernourishing the pregnant adolescent dam restricts fetal growth independently of changes in placental mass. Alterations in maternal placental vascular development may, however, play a role in mediating the previously reported reduction in maternal and hence fetal nutrient supply.

Mast cell degranulation in rat uterine cervix during pregnancy correlates with expression of vascular endothelial growth factor mRNA and angiogenesis

Vascular growth of the uterine cervix during pregnancy is associated with mast cell (MC) degranulation. To better understand the mechanism underlying this process, uterine cervices of intact pregnant rats were dissected and endothelial cell proliferation was measured by a bromodeoxyuridine incorporation technique. Total vascular endothelial growth factor (VEGF) mRNA expression and the relative abundance of VEGF splice variants (120, 164, and 188) were determined by RT-PCR. VEGF protein expression was evaluated by immunohistochemistry. To investigate the role of MCs on cervical angiogenesis, a second set of pregnant animals were treated with an MC stabilizer (disodium cromoglycate) to inhibit MC degranulation. Furthermore, 17β-estradiol (E2) serum levels were established by RIA. In intact pregnant rats, VEGF mRNA expression was positively correlated with endothelial cell proliferation and circulating E2levels. All selected splice variants ofVEGFgene were detected and their relative abundance did not show any change throughout pregnancy. Animals treated with disodium cromoglycate showed a decrease in endothelial cell proliferation and in VEGF mRNA expression compared with controls. Relative abundance of VEGF mRNA splice variants and E2serum levels showed no differences between these experimental groups. These results show a time-dependent correlation between VEGF mRNA expression and E2serum levels in the uterine cervix of intact pregnant rats, while MC stabilizer-treated animals reduced the VEGF expression without modifying E2serum levels. We suggest that cervical angiogenesis during pregnancy could be regulated by a mechanism which involves endogenous E2and chemical mediators stored in MC granules via a VEGF-dependent pathway.

Indian hedgehog is a major mediator of progesterone signaling in the mouse uterus

The hedgehog family of morphogens are regulators of cell proliferation, differentiation and cell-cell communication. These morphogens have been shown to have important roles in organogenesis, spermatogenesis, stem cell maintenance and oncogenesis. Indian hedgehog (encoded by Ihh) has been shown to be expressed in the uterine epithelium under the control of the steroid hormone, progesterone. Although in vivo and in vitro studies have shown that progesterone achieves its effects on uterine function through epithelial-stromal cross-talk, molecular mediator(s) for this cellular communication pathway have not been elucidated. Using new experimental approaches that ablate Ihh specifically in Pgr-positive uterine cells of the mouse, we demonstrate that Ihh is an essential mediator of Pgr action in the uterus, and expression of this factor is critical in mediating the communication between the uterine epithelium and stroma required for embryo implantation.

Identification of label-retaining cells in mouse endometrium

Human and mouse endometrium (lining of the uterus) undergo cycles of growth and regression as part of each reproductive cycle. A well-known method to identify somatic stem/progenitor cells and their location in the stem cell niche is the label-retaining cell (LRC) approach. We hypothesized that mouse endometrium contains small populations of both epithelial and stromal somatic stem/progenitor cells that may be detected by the LRC technique. The overall objective of this study was to identify and quantify LRCs in mouse endometrium, to determine their location, and to identify their niche in this highly regenerative tissue. Endometrium was labeled for 3 days with bromodeoxyuridine (BrdU) in postnatal day 3 (P3) mice prior to gland development and prepubertal (P19) mice after glands had formed, followed by chase periods of up to 12 weeks. After an 8-week chase, 3% of epithelial nuclei immunostained with BrdU antibody and were considered epithelial LRCs. These were primarily located in the luminal epithelium. Epithelial LRCs did not express estrogen receptor-alpha (ER-alpha). Stromal LRCs (6%) were found adjacent to luminal epithelium, at the endometrial-myometrial junction, and near blood vessels after a 12-week chase. Stromal LRCs were stem cell antigen-1, CD45(-), and some (16%) expressed ER-alpha, indicating their capacity to respond to estrogen and transmit paracrine signals to epithelial cells for endometrial epithelium regeneration. Both epithelial LRCs and some stromal LRCs, mainly located at the endometrial-myometrial junction, were recruited into the cell cycle after estrogen-stimulated endometrial regeneration, indicating a functional response to proliferative signals. This study has demonstrated for the first time the presence of both epithelial and stromal LRCs in mouse endometrium, suggesting that these stem-like cells may be responsible for endometrial regeneration.

Recent advances in endometrial angiogenesis research

This review summarises recent research into the mechanisms and regulation of endometrial angiogenesis. Understanding of when and by what mechanisms angiogenesis occurs during the menstrual cycle is limited, as is knowledge of how it is regulated. Significant endometrial endothelial cell proliferation occurs at all stages of the menstrual cycle in humans, unlike most animal models where a more precise spatial relationship exists between endothelial cell proliferation and circulating levels of oestrogen and progesterone. Recent stereological data has identified vessel elongation as a major endometrial angiogenic mechanism in the mid-late proliferative phase of the cycle. In contrast, the mechanisms that contribute to post-menstrual repair and secretory phase remodelling have not yet been determined. Both oestrogen and progesterone/progestins appear to have paradoxical actions, with recent studies showing that under different circumstances both can promote as well as inhibit endometrial angiogenesis. The relative contribution of direct versus indirect effects of these hormones on the vasculature may help to explain their pro- or anti-angiogenic activities. Recent work has also identified the hormone relaxin as a player in the regulation of endometrial angiogenesis. While vascular endothelial growth factor (VEGF) is fundamental to endometrial angiogenesis, details of how and when different endometrial cell types produce VEGF, and how production and activity is controlled by oestrogen and progesterone, remains to be elucidated. Evidence is emerging that the different splice variants of VEGF play a major role in regulating endometrial angiogenesis at a local level. Intravascular neutrophils containing VEGF have been identified as having a role in stimulating endometrial angiogenesis, although other currently unidentified mechanisms must also exist. Future studies to clarify how endometrial angiogenesis is regulated in the human, as well as in relevant animal models, will be important for a better understanding of diseases such as breakthrough bleeding, menorrhagia, endometriosis and endometrial cancer.