Systematic expression analysis and antibody screening do not support the existence of naturally occurring progesterone receptor (PR)-C, PR-M, or other truncated PR isoforms

Luteal phase support in assisted reproductive technology

Infertility affects one in six couples, with in vitro fertilization (IVF) offering many the chance of conception. Compared to the solitary oocyte produced during the natural menstrual cycle, the supraphysiological ovarian stimulation needed to produce multiple oocytes during IVF results in a dysfunctional luteal phase that can be insufficient to support implantation and maintain pregnancy. Consequently, hormonal supplementation with luteal phase support, principally exogenous progesterone, is used to optimize pregnancy rates; however, luteal phase support remains largely 'black-box' with insufficient clarity regarding the optimal timing, dosing, route and duration of treatment. Herein, we review the evidence on luteal phase support and highlight remaining uncertainties and future research directions. Specifically, we outline the physiological luteal phase, which is regulated by progesterone from the corpus luteum, and evaluate how it is altered by the supraphysiological ovarian stimulation used during IVF. Additionally, we describe the effects of the hormonal triggers used to mature oocytes on the degree of luteal phase support required. We explain the histological transformation of the endometrium during the luteal phase and evaluate markers of endometrial receptivity that attempt to identify the 'window of implantation'. We also cover progesterone receptor signalling, circulating progesterone levels associated with implantation, and the pharmacokinetics of available progesterone formulations to inform the design of luteal phase support regimens.

Progesterone control of myometrial contractility

During pregnancy, the primary function of the uterus is to be quiescent and not contract, which allows the growing fetus to develop and mature. A uterine muscle layer, myometrium, is composed of smooth muscle cells (SMCs). Before the onset of labor contractions, the uterine SMCs experience a complex biochemical and molecular transformation involving the expression of contraction-associated proteins. Labor is initiated when genes in SMCs are activated in response to a combination of hormonal, inflammatory and mechanical signals. In this review, we provide an overview of molecular mechanisms regulating the process of parturition in humans, focusing on the hormonal control of the myometrium, particularly the steroid hormone progesterone. The primary reason for discussing the regulation of myometrial contractility by progesterone is the importance of the clinical problem of preterm birth. It is thought that the hormonal mechanisms regulating premature uterine contractions represent an untimely triggering of the normal events occurring during term parturition. Yet, our knowledge of the complex and redundant hormonal pathways controlling uterine contractile activity leading to delivery of the neonate remains incomplete. Finally, we introduce recent animal studies using a novel class of drugs, Selective Progesterone Receptor Modulators, targeting progesterone signaling to prevent premature myometrial contractions.

Progesterone receptors in normal breast development and breast cancer

Progesterone receptors (PR) play a pivotal role in many female reproductive tissues such as the uterus, the ovary, and the mammary gland (MG). Moreover, PR play a key role in breast cancer growth and progression. This has led to the development and study of different progestins and antiprogestins, many of which are currently being tested in clinical trials for cancer treatment. Recent reviews have addressed the role of PR in MG development, carcinogenesis, and breast cancer growth. Thus, in this review, in addition to making an overview on PR action in normal and tumor breast, the focus has been put on highlighting the still unresolved topics on hormone treatment involving PR isoforms and breast cancer prognosis.

Transcriptional control of parturition: insights from gene regulation studies in the myometrium

The onset of labour is a culmination of a series of highly coordinated and preparatory physiological events that take place throughout the gestational period. In order to produce the associated contractions needed for foetal delivery, smooth muscle cells in the muscular layer of the uterus (i.e. myometrium) undergo a transition from quiescent to contractile phenotypes. Here, we present the current understanding of the roles transcription factors play in critical labour-associated gene expression changes as part of the molecular mechanistic basis for this transition. Consideration is given to both transcription factors that have been well-studied in a myometrial context, i.e. activator protein 1, progesterone receptors, oestrogen receptors, and nuclear factor kappa B, as well as additional transcription factors whose gestational event-driving contributions have been demonstrated more recently. These transcription factors may form pregnancy- and labour-associated transcriptional regulatory networks in the myometrium to modulate the timing of labour onset. A more thorough understanding of the transcription factor-mediated, labour-promoting regulatory pathways holds promise for the development of new therapeutic treatments that can be used for the prevention of preterm labour in at-risk women.

Progesterone receptor-mediated actions and the treatment of central nervous system disorders: An up-date of the known and the challenge of the unknown

Progesterone has been shown to exert a wide range of remarkable protective actions in experimental models of central nervous system injury or disease. However, the intimate mechanisms involved in each of these beneficial effects are not fully depicted. In this review, we intend to give the readers a thorough revision on what is known about the participation of diverse receptors and signaling pathways in progesterone-mediated neuroprotective, pro-myelinating and anti-inflammatory outcomes, as well as point out to novel regulatory mechanisms that could open new perspectives in steroid-based therapies.

Myometrial progesterone hyper-responsiveness associated with increased risk of human uterine fibroids

BACKGROUND

Uterine Fibroids (UFs) growth is ovarian steroid-dependent. Previous studies have shown that estrogen and progesterone play an important role in UF development. However, the mechanism underlying progesterone induced UF pathogenesis is largely unknown. In this study, we determined the expression of progesterone receptor and compared the expression level of progesterone-regulated genes (PRGs) in human myometrial cells from normal uteri (MyoN) versus uteri with UFs (MyoF) in response to progesterone.

METHODS

Primary human myometrial cells were isolated from premenopausal patients with structurally normal uteri (PrMyoN). Primary human myometrial cells were also isolated from uterus with UFs (PrMyoF). Isolated tissues were excised at least 2 cm from the closest UFs lesion(s). Progesterone receptor (PR) expression was assessed using Western blot (WB). Expression levels of 15 PRGs were measured by qRT-PCR in PrMyoN and PrMyoF cells in the presence or absence of progesterone.

RESULTS

WB analysis revealed higher expression levels of PR in PrMyoF cells as compared to PrMyoN cells. Furthermore, we compared the expression patterns of 15 UF-related PRGs in PrMyoN and PrMyoF primary cells in response to progesterone hormone treatment. Our studies demonstrated that five PRGs including Bcl2, FOXO1A, SCGB2A2, CYP26a1 and MMP11 exhibited significant progesterone-hyper-responsiveness in human PrMyoF cells as compared to PrMyoN cells (P < 0.05). Another seven PRGs, including CIDEC, CANP6, ADHL5, ALDHA1, MT1E, KIK6, HHI showed gain in repression in response to progesterone treatment (P > 0.05). Importantly, these genes play crucial roles in cell proliferation, apoptosis, cell cycle, tissue remodeling and tumorigenesis in the development of UFs.

CONCLUSION

These data support the idea that progesterone acts as contributing mechanism in the origin of UFs. Identification and analysis of these PRGs will help to further understand the role of progesterone in UF development.

Multifactorial Regulation of Myometrial Contractility During Pregnancy and Parturition

The steroid hormones progesterone (P₄) and estradiol-17β (E₂), produced by the placenta in humans and the ovaries in rodents, serve crucial roles in the maintenance of pregnancy, and the initiation of parturition. Because of their critical importance for species survival, the mechanisms whereby P₄ and its nuclear receptor (PR) maintain myometrial quiescence during pregnancy, and for the decline in P₄/PR and increase in E₂/estrogen receptor (ER) function leading to parturition, are multifaceted, cooperative, and redundant. These actions of P₄/PR include: (1) PR interaction with proinflammatory transcription factors, nuclear factor κB (NF-κB), and activating protein 1 (AP-1) bound to promoters of proinflammatory and contractile/contraction-associated protein (CAP) genes and recruitment of corepressors to inhibit NF-κB and AP-1 activation of gene expression; (2) upregulation of inhibitors of proinflammatory transcription factor activation (IκBα, MKP-1); (3) induction of transcriptional repressors of CAP genes (e.g., ZEB1). In rodents and most other mammals, circulating maternal P₄ levels remain elevated throughout most of pregnancy and decline precipitously near term. By contrast, in humans, circulating P₄ levels and myometrial PR levels remain elevated throughout pregnancy and into labor. However, even in rodents, wherein P₄ levels decline near term, P₄ levels remain higher than the K_d for PR binding. Thus, parturition is initiated in all species by a series of molecular events that antagonize the P₄/PR maintenance of uterine quiescence. These events include: direct interaction of inflammatory transcription factors (e.g., NF-κB, AP-1) with PR; increased expression of P₄ metabolizing enzymes; increased expression of truncated/inhibitory PR isoforms; altered expression of PR coactivators and corepressors. This article will review various mechanisms whereby P₄ acting through PR isoforms maintains myometrial quiescence during pregnancy as well as those that underlie the decline in PR function leading to labor. The roles of P₄- and E₂-regulated miRNAs in the regulation and integration of these mechanisms will also be considered.

N-terminal truncations in sex steroid receptors and rapid steroid actions

Sex steroid receptors act as ligand activated nuclear transcription factors throughout the body, including the brain. However, post-translational modification of these receptors can direct them to extranuclear sites, including the plasma membrane, where they are able to initiate rapid signaling. Because of the conserved domain structure of these receptors, alternative exon splicing can result in proteins with altered nuclear and extranuclear actions. Although much attention has focused on internal and C-terminal splice variants, both estrogen and androgen receptors undergo N-terminal truncations, as well. These truncated proteins not only influence the transcriptional activity of the full-length receptors, but also associate with caveolin and initiate signaling at the plasma membrane. Such actions may have important physiological consequences in neuronal, endothelial, and cancer signaling and cell survival.

Role of Sex Hormones on Brain Mitochondrial Function, with Special Reference to Aging and Neurodegenerative Diseases

The mitochondria have a fundamental role in both cellular energy supply and oxidative stress regulation and are target of the effects of sex steroids, particularly the neuroprotective ones. Aging is associated with a decline in the levels of different steroid hormones, and this decrease may underline some neural dysfunctions. Besides, modifications in mitochondrial functions associated with aging processes are also well documented. In this review, we will discuss studies that describe the modifications of brain mitochondrial function and of steroid levels associated with physiological aging and with neurodegenerative diseases. A special emphasis will be placed on describing and discussing our recent findings concerning the concomitant study of mitochondrial function (oxidative phosphorylation, oxidative stress) and brain steroid levels in both young (3-month-old) and aged (20-month-old) male and female mice.

Isoform specificity of progesterone receptor antibodies

Progesterone receptors (PR) are prognostic and predictive biomarkers in hormone‐dependent cancers. Two main PR isoforms have been described, PRB and PRA, that differ only in that PRB has 164 extra N‐terminal amino acids. It has been reported that several antibodies empirically exclusively recognize PRA in formalin‐fixed paraffin‐embedded (FFPE) tissues. To confirm these findings, we used human breast cancer xenograft models, T47D‐YA and ‐YB cells expressing PRA or PRB, respectively, MDA‐MB‐231 cells modified to synthesize PRB, and MDA‐MB‐231/iPRAB cells which can bi‐inducibly express either PRA or PRB. Cells were injected into immunocompromised mice to generate tumours exclusively expressing PRA or PRB. PR isoform expression was verified using immunoblots. FFPE samples from the same tumours were studied by immunohistochemistry using H‐190, clone 636, clone 16, and Ab‐6 anti‐PR antibodies, the latter exclusively recognizing PRB. Except for Ab‐6, all antibodies displayed a similar staining pattern. Our results indicate that clones 16, 636, and the H‐190 antibody recognize both PR isoforms. They point to the need for more stringency in evaluating the true specificity of purported PRA‐specific antibodies as the PRA/PRB ratio may have prognostic and predictive value in breast cancer.

Dynamics of progesterone and estrogen receptor alpha in the ventromedial hypothalamus

Cyclic fluctuations of estradiol and progesterone in females influence neuronal activity in the ventrolateral division of the ventromedial hypothalamic nucleus (VMNvl), through the activation of progesterone receptors (PRs) and estrogen receptors (ERs). The expression of ER and PR in the VMNvl is influenced by their cognate ligands and is a central upstream trigger in the pathway of VMNvl-dependent modulation of endocrine responses. By studying the role played by estradiol and progesterone in PR and ERa expression in the VMNvl along the estrous cycle and how the two receptors interact in the same neuron, we aim to evaluate the synergistic action of both ovarian hormones in the regulation of VMNvl activity. In animals at all phases of the estrous cycle, the number of VMN neurons expressing PR or ERa was estimated by stereological methods, and the percentage, and rostro-caudal distribution, of neurons simultaneously expressing both receptors was determined. The highest number of PR-immunoreactive neurons was seen at proestrus, and of ERa-immunoreactive neurons was seen at proestrus and metestrus. The ERa/PR co-localization is increased at caudal levels. Approximately half the neurons expressing PR co-express ERa, a proportion that stays constant along the estrous cycle. The percentage of ERa neurons co-expressing PR changes from 60% at proestrus to 40% at metestrus. Fluctuations in circulating ovarian hormone levels promote coordinated changes in PR and ERa expression and co-localization. This may be an important mechanism in the regulation of input relayed by the VMNvl, allowing a precise modulation of endocrine responses.

Selective progesterone receptor modulators (SPRMs): progesterone receptor action, mode of action on the endometrium and treatment options in gynecological therapies

Introduction: The progesterone receptor plays an essential role in uterine physiology and reproduction. Selective progesterone receptor modulators (SPRMs) have emerged as a valuable treatment option for hormone dependent conditions like uterine fibroids, which have a major impact on women’s quality of life. SPRMs offer potential for longer term medical treatment and thereby patients may avoid surgical intervention.

Areas covered: The authors have reviewed the functional role of the progesterone receptor and its isoforms and their molecular mechanisms of action via genomic and non-genomic pathways. The current knowledge of the interaction of the PR and different SPRMs tested in clinical trials has been reviewed. The authors focused on pharmacological effects of selected SPRMs on the endometrium, their anti-proliferative action, and their suppression of bleeding. Potential underlying molecular mechanisms and the specific histological changes in the endometrium induced by SPRMs (PAEC; Progesterone receptor modulator Associated Endometrial Changes) have been discussed. The clinical potential of this compound class including its impact on quality of life has been covered.

Expert Opinion: Clinical studies indicate SPRMs hold promise for treatment of benign gynecological complaints (fibroids, heavy menstrual bleeding; HMB). There however remains a knowledge gap concerning mechanism of action.

Topographies and isoforms of the progesterone receptor in female human, rat and mouse bladder

Steroid hormones such as progesterone are known to influence bladder function. Progesterone effects are mediated by the progesterone receptor (PR) but no detailed studies of PR in bladder exist. We have investigated the presence, topography and subtypes of PR in mouse, rat and human bladder. Fresh tissue samples were obtained from cystectomies in female humans, rats and mice (n = 7 per group). Tissue samples were processed for immunohistochemistry (IHC), immunofluorescence (IF) and western blot (WB) and, for each species, a panel of specific PR antibody clones was used. Interpretation of IHC/IF was carried out by light/fluorescent microscopy and of WB via standard WB software. IHC/IF in female human bladder showed PR on the interstitial cells in the lamina propria and between detrusor smooth muscle cells, whereas in female rat and mouse bladder, PR was only found on the urothelium. WB in human bladder showed a 78-kD and a 60-kDa band, respectively, corresponding to a modified PR isoform A and PR isoform C. WB in rat and mice bladder showed a 60 kDa band and a 37 kDa band, respectively corresponding with PR isoform C and an unknown isoform. This is the first detailed investigation of the precise location and presence of several isoforms of PR in bladder, together with a comparison of these data between human, rat and mouse. Our study has revealed complex PR families in bladders from the various species studied and demonstrates obvious inter-species differences in PR topography and isoforms.

Progesterone receptor signaling in the initiation of pregnancy and preservation of a healthy uterus

Infertility and reproductive-associated disease are global problems in the world today affecting millions of women. A successful pregnancy requires a healthy uterus ready to receive and support an implanting embryo. As an endocrine organ, the uterus is dependent on the secretions of the ovarian hormones estrogen and progesterone which signal via their cognate receptors, the estrogen and progesterone receptors. The progesterone receptor not only functions using classical nuclear receptor signaling, but also participates in non-genomic signaling at the cellular membrane. The complexity of progesterone signaling is further enhanced by the existence of multiple isoforms and post-translational regulation via kinases and transcription coregulators. This dynamic means of regulation of the progesterone receptor is evidenced in its necessary role in a successful pregnancy. Within early pregnancy, the progesterone receptor elicits activation of its target genes in a spatiotemporal manner in order to allow for successful embryo attachment and uterine decidualization. Additionally, appropriate progesterone signaling is important for the prevention of uterine disease such as endometrial cancer, endometriosis, and leiomyoma. The utilization of progesterone receptor modulators in the treatment of these devastating uterine diseases is promising. This review presents a general overview of progesterone receptor structure, function, and regulation and highlights its important role in the establishment of pregnancy and as a therapeutic target in uterine disease.

Cyclic decidualization of the human endometrium in reproductive health and failure

Decidualization denotes the transformation of endometrial stromal fibroblasts into specialized secretory decidual cells that provide a nutritive and immunoprivileged matrix essential for embryo implantation and placental development. In contrast to most mammals, decidualization of the human endometrium does not require embryo implantation. Instead, this process is driven by the postovulatory rise in progesterone levels and increasing local cAMP production. In response to falling progesterone levels, spontaneous decidualization causes menstrual shedding and cyclic regeneration of the endometrium. A growing body of evidence indicates that the shift from embryonic to maternal control of the decidual process represents a pivotal evolutionary adaptation to the challenge posed by invasive and chromosomally diverse human embryos. This concept is predicated on the ability of decidualizing stromal cells to respond to individual embryos in a manner that either promotes implantation and further development or facilitates early rejection. Furthermore, menstruation and cyclic regeneration involves stem cell recruitment and renders the endometrium intrinsically capable of adapting its decidual response to maximize reproductive success. Here we review the endocrine, paracrine, and autocrine cues that tightly govern this differentiation process. In response to activation of various signaling pathways and genome-wide chromatin remodeling, evolutionarily conserved transcriptional factors gain access to the decidua-specific regulatory circuitry. Once initiated, the decidual process is poised to transit through distinct phenotypic phases that underpin endometrial receptivity, embryo selection, and, ultimately, resolution of pregnancy. We discuss how disorders that subvert the programming, initiation, or progression of decidualization compromise reproductive health and predispose for pregnancy failure.

Role of nuclear progesterone receptor isoforms in uterine pathophysiology

BACKGROUND

Progesterone is a key hormonal regulator of the female reproductive system. It plays a major role to prepare the uterus for implantation and in the establishment and maintenance of pregnancy. Actions of progesterone on the uterine tissues (endometrium, myometrium and cervix) are mediated by the combined effects of two progesterone receptor (PR) isoforms, designated PR-A and PR-B. Both receptors function primarily as ligand-activated transcription factors. Progesterone action on the uterine tissues is qualitatively and quantitatively determined by the relative levels and transcriptional activities of PR-A and PR-B. The transcriptional activity of the PR isoforms is affected by specific transcriptional coregulators and by PR post-translational modifications that affect gene promoter targeting. In this context, appropriate temporal and cell-specific expression and function of PR-A and PR-B are critical for normal uterine function.

METHODS

Relevant studies describing the role of PRs in uterine physiology and pathology (endometriosis, uterine leiomyoma, endometrial cancer, cervical cancer and recurrent pregnancy loss) were comprehensively searched using PubMed, Cochrane Library, Web of Science, and Google Scholar and critically reviewed.

RESULTS

Progesterone, acting through PR-A and PR-B, regulates the development and function of the endometrium and induces changes in cells essential for implantation and the establishment and maintenance of pregnancy. During pregnancy, progesterone via the PRs promotes myometrial relaxation and cervical closure. Withdrawal of PR-mediated progesterone signaling triggers menstruation and parturition. PR-mediated progesterone signaling is anti-mitogenic in endometrial epithelial cells, and as such, mitigates the tropic effects of estrogen on eutopic normal endometrium, and on ectopic implants in endometriosis. Similarly, ligand-activated PRs function as tumor suppressors in endometrial cancer cells through inhibition of key cellular signaling pathways required for growth. In contrast, progesterone via PR activation appears to increase leiomyoma growth. The exact role of PRs in cervical cancer is unclear. PRs regulate implantation and therefore aberrant PR function may be implicated in recurrent pregnancy loss (RPL). PRs likely regulate key immunogenic factors involved in RPL. However, the exact role of PRs in the pathophysiology of RPL and the use of progesterone for therapeutic benefit remains uncertain.

CONCLUSIONS

PRs are key mediators of progesterone action in uterine tissues and are essential for normal uterine function. Aberrant PR function (due to abnormal expression and/or function) is a major cause of uterine pathophysiology. Further investigation of the underlying mechanisms of PR isoform action in the uterus is required, as this knowledge will afford the opportunity to create progestin/PR-based therapeutics to treat various uterine pathologies.

Expression of a mitochondrial progesterone receptor in human spermatozoa correlates with a progestin-dependent increase in mitochondrial membrane potential

The hyperactivation of human spermatozoa necessary for fertilization requires a substantial increase in cellular energy production. The factors responsible for increasing cellular energy remain poorly defined. This article proposes a role for a novel mitochondrial progesterone receptor (PR-M) in modulation of mitochondrial activity. Basic science studies demonstrate a 38 kDa protein with western blot analysis, consistent with PR-M; whereas imaging studies with confocal and immunoelectron microscopy demonstrate a PR on the mitochondria. Treatment with a PR-specific progestin shows increased mitochondrial membrane potential, not related to induction of an acrosome reaction. The increase in mitochondrial membrane potential was inhibited by a specific PR antagonist, but not affected by an inhibitor to the progesterone-dependent Catsper voltage-activated channel. In conclusion, these studies suggest expression of a novel mitochondrial PR in human spermatozoa with a progestin-dependent increase in mitochondrial activity. This mechanism may serve to enhance cellular energy production as the spermatozoa traverse the female genital tract being exposed to increasing concentrations of progesterone.

Expression of a mitochondrial progesterone receptor (PR-M) in leiomyomata and association with increased mitochondrial membrane potential

CONTEXT

Clinical evidence supports a role for progestins in the growth of leiomyomata (fibroids). The mechanism(s) for this is thought to involve gene regulation via the nuclear progesterone receptors. Recently a mitochondrial progesterone receptor (PR-M) has been identified with evidence of a progesterone/progestin-dependent increase in cellular respiration. This observation raises a possible new mechanism whereby progesterone/progestin may affect the growth of fibroids.

OBJECTIVE

The goals of this research were to determine differential expression of PR-M in normal myometrium compared with the edge of a fibroid within the same uterus, to demonstrate a progestin-dependent increase in mitochondria membrane potential using an immortalized human myometrial cell line and to examine mitochondrial membrane potential in transfected cells expressing the complete coding sequence of PR-M.

DESIGN

Protein levels of PR-M, PR-B, PR-A, mitochondrial porin, and glyceraldehyde-3-phosphate dehydrogenase were determined in the myometrium and adjacent edge of a fibroid in 10 subjects undergoing hysterectomy for benign indications. Mitochondrial membrane potential was determined by fluorescent emission of 5,5',6,6'-tetrachloro-1,1',3,3'-tetraethylbenzimidazolecarbocyanide iodine in hTERT-HM cells treated with R5020 and in transfected hTERT-HM cells determined by the fluorescent emission of tetramethylrhodamine methyl ester.

RESULTS

Higher levels of PR-M and mitochondrial porin were found in the fibroid edge compared with adjacent myometrium. Progestin increased mitochondrial membrane potential in hTERT-HM cells, which was not affected by a translation inhibitor. This effect was exaggerated in hTERT-HM cells expressing PR-M after transient transfection.

CONCLUSION

These studies suggest a mechanism whereby progesterone/progestin may affect the growth of fibroids by altering mitochondrial activity.

Endometrial progesterone resistance and PCOS

Polycystic ovary syndrome (PCOS) is a state of altered steroid hormone production and activity. Chronic estrogen exposure or lack of progesterone due to ovarian dysfunction can result in endometrial hyperplasia and carcinoma. A key contributor to our understanding of progesterone as a critical regulator for normal uterine function has been the elucidation of progesterone receptor (PR) expression, regulation, and signaling pathways. Several human studies indicate that PR-mediated signaling pathways in the nucleus are associated with progesterone resistance in women with PCOS. The aim of this review is to provide an overview of endometrial progesterone resistance in women with PCOS; to present the PR structure, its different isoforms, and their expression in the endometrium; to illustrate the possible regulation of PR and PR-mediated signaling in progesterone resistance in women with PCOS; and to discuss current clinical treatments for atypical endometrial hyperplasia and endometrial carcinoma in women with PCOS and accompanying progesterone resistance.

Regulation of progesterone signaling during pregnancy: implications for the use of progestins for the prevention of preterm birth

Preterm birth is a major cause of neonatal morbidity and mortality. Progesterone plays a critical role in suppressing the inflammatory signals that would induce parturition prior to term. Progesterone signaling is regulated in a variety of ways during pregnancy. Endocrine production of high levels of progesterone by the placenta ensures the availability of high levels of progesterone throughout pregnancy. Paracrine regulation of progesterone metabolism in target tissues, particularly the myometrium and cervix, also determines the amount of progesterone ligand available. Progesterone metabolism can also lead to the formation of metabolites that contribute to its effects. In particular, 5β-dihydroprogesterone formation by aldo-keto reductase 1D1 appears to play an important role in maintaining uterine quiescence. Progesterone signaling can also be regulated at the receptor level through changes in the relative expression of the nuclear progesterone receptor isoforms, reduced expression of membrane receptors, and changes in the expression levels of coactivators and/or corepressors, including nuclear factor κB. Progesterone and 17α-hydroxyprogesterone caproate (17OH-PC) have recently been shown to reduce preterm births in women with previous preterm birth or shortened cervix. It is important to realize that these two progestins are likely to act in significantly different ways, which will likely influence their efficacy. The structural differences and resistance to metabolism exhibited by 17OH-PC means that it will be unable to activate some of the pathways that progesterone activates, but that it also will not be subject to paracrine inactivation. The fact that progesterone therapy works for maintaining pregnancy in some women, indicates that for those women insufficient levels of progesterone ligand in target tissues is a determining factor in early parturition, despite high levels of circulating progesterone. This article is part of a Special Issue entitled 'Pregnancy and Steroids'.

Oestrogen and progesterone action on endometrium: a translational approach to understanding endometrial receptivity

Embryo attachment and implantation is critical to successful reproduction of all eutherian mammals, including humans; a better understanding of these processes could lead to improved infertility treatments and novel contraceptive methods. Experience with assisted reproduction, especially oocyte donation cycles, has established that despite the diverse set of hormones produced by the ovary in a cycle-dependent fashion, the sequential actions of only two of them, oestrogen and progesterone, are sufficient to prepare a highly receptive endometrium in humans. Further investigation on the endometrial actions of these two hormones is currently providing significant insight into the implantation process in women, strongly suggesting that an abnormal response to progesterone underlies infertility in some patients.

Protective actions of progesterone in the cardiovascular system: potential role of membrane progesterone receptors (mPRs) in mediating rapid effects

The protective functions of progesterone in the cardiovascular system have received little attention even though evidence has accumulated that progesterone lowers blood pressure, inhibits coronary hyperactivity and has powerful vasodilatory and natriuretic effects. One possible reason why potential beneficial actions of progesterone on cardiovascular functions have not been extensively studied is that divergent effects to those of progesterone have been observed in many clinical trials with synthetic progestins such as medroxyprogesterone acetate which are associated with increased risk of coronary disease. Evidence that progesterone exerts protective effects on cardiovascular functions is briefly reviewed. The finding that progesterone administration decreases blood vessel vasoconstriction in several animal models within a few minutes suggests that rapid, nongenomic progesterone mechanisms are of physiological importance in regulating vascular tone. Rapid activation of second messenger pathways by progesterone has been observed in vascular endothelial and smooth muscle cells, resulting in alterations in endothelial nitric oxide synthase (eNOS) activity and calcium influx, respectively. Both nuclear progesterone receptors (PRs) and novel membrane progesterone receptors (mPRs) are candidates for the intermediaries in these rapid, cell-surface initiated progesterone actions in endothelial and smooth muscle vascular cells. PRs have been detected in both cell types. New data are presented showing mPRα, mPRβ and mPRγ are also present in human endothelial and smooth muscle vascular cells. Preliminary evidence suggests mPRs mediate rapid progestin signaling in these endothelial cells, resulting in down-regulation of cAMP production and increased nitric oxide synthesis. The role of mPRs in progesterone regulation of cardiovascular functions warrants further investigation.

A truncated progesterone receptor (PR-M) localizes to the mitochondrion and controls cellular respiration

The cDNA for a novel truncated progesterone receptor (PR-M) was previously cloned from human adipose and aortic cDNA libraries. The predicted protein sequence contains 16 unique N-terminal amino acids, encoded by a sequence in the distal third intron of the progesterone receptor PR gene, followed by the same amino acid sequence encoded by exons 4 through 8 of the nuclear PR. Thus, PR-M lacks the N terminus A/B domains and the C domain for DNA binding, whereas containing the hinge and hormone-binding domains. In this report, we have localized PR-M to mitochondria using immunofluorescent localization of a PR-M-green fluorescent protein (GFP) fusion protein and in Western blot analyses of purified human heart mitochondrial protein. Removal of the putative N-terminal mitochondrial localization signal obviated association of PR-M with mitochondria, whereas addition of the mitochondrial localization signal to green fluorescent protein resulted in mitochondrial localization. Immunoelectron microscopy and Western blot analysis after mitochondrial fractionation identified PR-M in the outer mitochondrial membrane. Antibody specificity was shown by mass spectrometry identification of a PR peptide in a mitochondrial membrane protein isolation. Cell models of overexpression and gene silencing of PR-M demonstrated a progestin-induced increase in mitochondrial membrane potential and an increase in oxygen consumption consistent with an increase in cellular respiration. This is the first example of a truncated steroid receptor, lacking a DNA-binding domain that localizes to the mitochondrion and initiates direct non-nuclear progesterone action. We hypothesize that progesterone may directly affect cellular energy production to meet the increased metabolic demands of pregnancy.

Alternative splicing of the nuclear progestin receptor in a perciform teleost generates novel mechanisms of dominant-negative transcriptional regulation

In mammals, downstream function of the nuclear progestin receptor (PGR) can be differentially regulated in each target tissue by altering the expression levels of PGR mRNA variants. Such PGR isoforms have also been identified in birds and reptiles, but not in non-amniote vertebrates. Based upon extensive phylogenetic, syntenic and functional analyses, here we show that higher orders of Teleostei retain a single pgr gene, and that four different pgr transcript variants of the extant gene are expressed in the ovary of an evolutionary advanced perciform teleost, the gilthead seabream (Sparus aurata). Three of the isoforms (pgr_tv2, pgr_tv3 and pgr_tv4) arise from alternative pre-mRNA splicing resulting in different N-terminally truncated receptors, whereas one isoform (pgr_tv1) is a deletion variant. Seabream wild-type Pgr shows the highest transactivational response to native euteleostean progestins, 17α,20β-dihydroxy-4-pregnen-3-one and 17α,20β,21-trihydroxy-4-pregnen-3-one, whereas the Pgr_tv3 and Pgr_tv4 isoforms independently regulate novel nuclear and cytosolic mechanisms of dominant-negative repression of Pgr-mediated transcription. In the seabream ovary, the wild-type Pgr protein is localized in oogonia, in the nuclei of primary (previtellogenic) oocytes, as well as in follicular (granulosa) cells and the oocyte cytoplasm of early and late vitellogenic ovarian follicles. Expression of wild-type pgr, pgr_tv3 and pgr_tv4 was the highest in seabream primary ovaries, while expression of both inhibitory receptor isoforms, but not of pgr, decreased during vitellogenesis. Stimulation of primary ovarian explants in vitro with recombinant piscine follicle-stimulating hormone and estrogen differentially regulated the temporal expression of pgr, pgr_tv3 and pgr_tv4. These findings suggest that, as in mammals, ovarian progestin responsiveness in the seabream, particularly during early oogenesis, may be regulated through alternative splicing of the nuclear pgr mRNA. Thus, the dominant-negative mechanism of PGR transcriptional regulation likely evolved prior to the separation of Actinopterygii (ray-finned fishes) from Sarcopterygii (lobe-finned fishes).

Progesterone receptor (PR) variants exist in breast cancer cells characterised as PR negative

Progesterone receptor (PR) expression is measured in breast cancer by immunohistochemistry using N-terminally targeted antibodies and serves as a biomarker for endocrine therapeutic decisions. Extensive PR alternative splicing has been reported which may generate truncated PR variant proteins which are not detected by current breast cancer screening or may alter the function of proteins detected in screening. However, the existence of such truncated PR variants remains controversial. We have characterised PR protein expression in breast cancer cell lines using commercial PR antibodies targeting different epitopes. Truncated PR proteins are detected in reportedly PR negative MDA-MB-231 cells using a C-terminally targeted antibody. Antibody specificity was confirmed by immunoblotting following siRNA knockdown of PR expression. We have further demonstrated that alternatively spliced PR mRNA is present in MDA-MB-231 cells and in reportedly PR-negative breast tumour tissue which could encode the truncated PR proteins detected by the C-terminal antibody. The potential function of PR variant proteins present in MDA-MB-231 cells was also assessed, indicating the ability of these PR variants to bind progesterone, interact with a nuclear PR co-factor and bind DNA. These findings suggest that alternative splicing may generate functional truncated PR variant proteins which are not detected by breast cancer screening using N-terminally targeted antibodies leading to misclassification as PR negative.

Primate-specific melanoma antigen-A11 regulates isoform-specific human progesterone receptor-B transactivation

Progesterone acting through the progesterone receptor (PR) and its coregulators prepares the human endometrium for receptivity to embryo implantation and maintains pregnancy. The menstrual cycle-dependent expression of melanoma antigen-A11 (MAGE-11) in the mid-secretory human endometrium suggested a novel function in human PR signaling. Here we show that MAGE-11 is an isoform-specific coregulator responsible for the greater transcriptional activity of human PR-B relative to PR-A. PR was recruited to progesterone response regions of progesterone-regulated FK506-binding protein 5 (FKBP5) immunophilin and small Ras family G protein cell growth inhibitor RASD1 genes. Expression of MAGE-11 lentivirus shRNA in human endometrial Ishikawa cells expressing PR-B showed that MAGE-11 is required for isoform-specific PR-B up-regulation of FKBP5. In contrast, MAGE-11 was not required for progesterone up-regulation of RASD1 in endometrial cells expressing the PR-A/B heterodimer. Target gene specificity of PR-B depended on the synergistic actions of MAGE-11 and p300 mediated by the unique PR-B NH(2)-terminal (110)LLXXVLXXLL(119) motif that interacts with the MAGE-11 F-box region in a phosphorylation- and ubiquitinylation-dependent manner. A progesterone-dependent mechanism is proposed in which MAGE-11 and p300 increase PR-B up-regulation of the FKBP5 gene. MAGE-11 down-regulates PR-B, similar to the effects of progesterone, and interacts with FKBP5 to stabilize a complex with PR-B. We conclude that the coregulator function of MAGE-11 extends to isoform-specific regulation of PR-B during the cyclic development of the human endometrium.

Effect of prolonged in vivo administration of progesterone in pregnancy on myometrial gene expression, peripheral blood leukocyte activation, and circulating steroid hormone levels

OBJECTIVE

We aimed to investigate the effects of progesterone on gene expression and function of both myometrium and circulating leukocytes.

METHODS

We recruited women participating in a randomized clinical trial of progesterone to prevent preterm delivery. These participants had a twin pregnancy and were managed in 1 of 2 tertiary referral centers. Participants were treated with progesterone (90 mg vaginally) or placebo from 24 to 34 weeks of pregnancy. The outcome measures were myometrial and leukocyte gene expression and expression of cell surface markers in circulating leukocytes, all quantified ex vivo.

RESULTS

Prolonged in vivo administration of progesterone inhibited myometrial expression of connexins 26 and 43, endothelial nitric acid synthase (eNOS), and the prostaglandin receptor EP2 ex vivo. Administration of progesterone also increased numbers of circulating neutrophils while decreasing lymphocyte proportions and decreasing neutrophil CD11b expression.

CONCLUSION

The observed effects of prolonged in vivo administration of progesterone will minimize the ability of the uterus to contract as a synctium and the ability of peripheral blood leukocytes to migrate into the myometrium during parturition. We suggest that these are putative mechanisms by which progesterone might prevent preterm birth in women at high risk.

Mechanisms of endometrial progesterone resistance

Throughout the reproductive years, the rise and fall in ovarian hormones elicit in the endometrium waves of cell proliferation, differentiation, recruitment of inflammatory cells, apoptosis, tissue breakdown and regeneration. The activated progesterone receptor, a member of the superfamily of ligand-dependent transcription factors, is the master regulator of this intense tissue remodelling process in the uterus. Its activity is tightly regulated by interaction with cell-specific transcription factors and coregulators as well as by specific posttranslational modifications that respond dynamically to a variety of environmental and inflammatory signals. Endometriosis, a chronic inflammatory disorder, disrupts coordinated progesterone responses throughout the reproductive tract, including in the endometrium. This phenomenon is increasingly referred to as 'progesterone resistance'. Emerging evidence suggests that progesterone resistance in endometriosis is not just a consequence of perturbed progesterone signal transduction caused by chronic inflammation but associated with epigenetic chromatin changes that determine the intrinsic responsiveness of endometrial cells to differentiation cues.

Coordinate regulation of heterogeneous nuclear ribonucleoprotein dynamics by steroid hormones in the human fallopian tube and endometrium in vivo and in vitro

Heterogeneous nuclear ribonucleoproteins (hnRNPs), which are chromatin-associated RNA-binding proteins, participate in mRNA stability, transport, intracellular localization, and translation by acting as transacting factors. Several studies have shown that steroid hormones can regulate hnRNP expression. However, to date, the regulation of hnRNPs and their interactions with steroid hormone signaling in fallopian tubes and endometrium are not fully elucidated. In the present study, we determined whether hnRNP expression is regulated during the menstrual cycle and correlates with estrogen receptor (ER) and progesterone receptor (PR) levels in human fallopian tubes in vivo. Because of the limited availability of human tubal tissues for the research, we also explored the mechanisms of hnRNP regulation in human endometrium in vitro. Fallopian tissue was obtained from patients in the early, late, and postovulatory phases and the midsecretory phase and endometrial tissue from premenopausal and postmenopausal women undergoing hysterectomy. We measured expression of hnRNPs and assessed their intracellular localization and interactions with ERs and PRs. We also determined the effects of human chorionic gonadotropin, 17β-estradiol (E(2)), and progesterone (P(4)) on hnRNP expression. In fallopian tubes, mRNA and protein levels of hnRNP A1, AB, D, G, H, and U changed dynamically during ovulation and in the midsecretory phase. In coimmunolocation and coimmunoprecipitation experiments, hnRNPs interacted with each other and with ERs and PRs in fallopian tubes. After treatment with E(2) and/or P(4) to activate ERs and PRs, hnRNP A1, AB, D, G, and U proteins displayed overlapping but distinct patterns of regulation in the endometrium in vitro. Our findings expand the physiological repertoire of hnRNPs in human fallopian tubes and endometrium and suggest that steroid hormones regulate different hnRNPs directly by interacting with ERs and/or PRs or indirectly by binding other hnRNPs. Both actions may contribute to regulation of gene transcription.

Evaluation of human first trimester decidual and telomerase-transformed endometrial stromal cells as model systems of in vitro decidualization

BACKGROUND

Decidualization, the differentiation process of maternal uterine stromal cells into secretory decidual cells, is a prerequisite for successful implantation and progression of pregnancy. For in vitro differentiation mostly primary human endometrial stromal cells (HESC) isolated from uterine samples after hysterectomy for benign gynaecological diseases are utilised. However, a continuous supply of endometrial tissue is often lacking. Hence, we analysed whether cultivated human decidual stromal cells (HDSC) prepared from first trimester pregnancy terminations may represent an alternative model system for in vitro decidualization. Moreover, based on the expression of critical marker genes these cells were compared to a previously established endometrial stromal cell line during in vitro differentiation.

METHODS

HDSC isolated from decidual tissue attached to first trimester placentae, and telomerase-transformed human endometrial stromal cells (THESC) were characterised by immunofluorescence and differentiated in vitro using either cyclic adenosine monophosphate (cAMP) and/or estrogen (E2)/progesterone (P4). Proliferation was measured by analyzing cumulative cell numbers. Expression of mRNAs encoding progesterone receptor (PR), prolactin (PRL), insulin-like growth factor binding protein-1 (IGFBP1), and Dickkopf-1 (DKK1) was evaluated using quantitative PCR after 3, 6, 9 and 12 days of in vitro differentiation. PRL and IGFBP-1 protein expression was investigated by enzyme-linked immunosorbent assay (ELISA) and Western blotting, respectively. Furthermore, forkhead box O1A (FOXO1A), a critical transcription factor in decidualization, was analysed by immunofluorescence and Western blotting at two different time points of differentiation.

RESULTS

Treatment with cAMP provoked morphological changes and growth arrest of THESC and HDSC, the latter showing loss of cells after 6 days of treatment. E2P4 stimulation did neither affect cell morphology nor proliferation of THESC and HDSC. Upon cAMP stimulation PR mRNA was suppressed in HDSC but not in THESC, whereas E2P4 did not alter transcript levels in both cell types. Protein expression of PR-A and PR-B was detectable in HDSC and diminished under cAMP, whereas THESC failed to produce the nuclear receptors. Supplementation of cAMP induced mRNA and protein expression of PRL and IGFBP-1 in both cell types at day 3, 6, 9, and 12 of treatment. In HDSC stimulation with E2P4 increased PRL and IGFBP-1 mRNA and protein production, whereas hormone treatment did not induce the two factors in THESC. E2P4 increased DKK1 mRNA at all time points in HDSC and cAMP provoked induction at day 9 and 12 of differentiation. In contrast, cAMP suppressed DKK1 mRNA in THESC, whereas E2P4 was ineffective. In both cell types combined treatments with cAMP and E2P4 provoked higher expression levels of PRL and IGFBP1 mRNA and protein as compared to cAMP stimulation alone. FOXO1A protein and its nuclear abundance were increased by cAMP in both cell types. However, reduction of its nuclear localisation upon E2P4 treatment could only be observed in HDSC.

CONCLUSION

Both HDSC and THESC may represent suitable model systems for cAMP-dependent in vitro decidualization. Since cAMP decreases cell viability of HDSC after 6 days of incubation, this substance should be preferentially used in short-term experiments. Progesterone treatment of THESC might not be applicable since these cells lack progesterone response and PR protein. In contrast, stimulation of PR-expressing HDSC with E2P4 or cAMP/E2P4 may represent an appropriate protocol for human in vitro decidualization inducing and maintaining expression of critical marker genes in a time-dependent manner.

Progesterone signals through membrane progesterone receptors (mPRs) in MDA-MB-468 and mPR-transfected MDA-MB-231 breast cancer cells which lack full-length and N-terminally truncated isoforms of the nuclear progesterone receptor

The functional characteristics of membrane progesterone receptors (mPRs) have been investigated using recombinant mPR proteins over-expressed in MDA-MB-231 breast cancer cells. Although these cells do not express the full-length progesterone receptor (PR), it is not known whether they express N-terminally truncated PR isoforms which could possibly account for some progesterone receptor functions attributed to mPRs. In the present study, the presence of N-terminally truncated PR isoforms was investigated in untransfected and mPR-transfected MDA-MB-231 cells, and in MDA-MB-468 breast cancer cells. PCR products were detected in PR-positive T47D Yb breast cancer cells using two sets of C-terminus PR primers, but not in untransfected and mPR-transfected MDA-MB-231 cells, nor in MDA-MB-468 cells. Western blot analysis using a C-terminal PR antibody, 2C11F1, showed the same distribution pattern for PR in these cell lines. Another C-terminal PR antibody, C-19, detected immunoreactive bands in all the cell lines, but also recognized α-actinin, indicating that the antibody is not specific for PR. High affinity progesterone receptor binding was identified on plasma membranes of MDA-MB-468 cells which was significantly decreased after treatment with siRNAs for mPRα and mPRβ. Plasma membranes of MDA-MB-468 cells showed very low binding affinity for the PR agonist, R5020, ≤1% that of progesterone, which is characteristic of mPRs. Progesterone treatment caused G protein activation and decreased production of cAMP in MDA-MB-468 cells, which is also characteristic of mPRs. The results indicate that the progestin receptor functions in these cell lines are mediated through mPRs and do not involve any N-terminally truncated PR isoforms.

Stromal cell-specific apoptotic and antiestrogenic mechanisms may explain uterine defects in humans after clomiphene citrate therapy

OBJECTIVE

The purpose of this study was to investigate clomiphene citrate (CC)-induced modulation of uterine cell function in vivo.

STUDY DESIGN

Prepubertal female Sprague-Dawley rats were treated intraperitoneally with CC for 6 or 24 hours or with a combination of CC and/or 17-beta-estradiol (E2) for 4 days.

RESULTS

Chronic CC treatment induced apoptosis in a fraction of uterine stromal cells by activating the caspase-3-mediated apoptotic pathway. The damage was prevented by successive E2 treatment; however, pretreatment or concomitant treatment with E2 did not protect against CC-induced uterine apoptosis. CC decreased the protein expression of estrogen receptor alpha and increased its phosphorylation but did not affect estrogen receptor beta expression or phosphorylation. Furthermore, changes in Hoxa11, p27, and progesterone receptor protein levels and localization were associated with CC treatment.

CONCLUSION

We provide novel mechanistic insights into cellular and molecular events by which CC regulates uterine stromal cell function and hence the implantation process and pregnancy outcome.

Minireview: rapid actions of sex steroids in the endothelium

The endothelium is a dynamic interface between the blood vessel and the circulating blood that plays a pivotal role in vascular homeostasis. As such, studies on sex steroid regulation of endothelial function are critical to understanding the role of sex steroids in cardiovascular health and disease. The classical model of steroid action involves liganded steroid receptors binding to specific response elements on target genes to regulate gene transcription. In whole organisms, the time lag between steroid administration and observable effects produced by newly synthesized protein is typically in the order of hours to days. And yet, some effects of steroids, such as vasodilatation, occur within seconds to minutes of steroid administration. Studies in multiple cell types have also shown that steroids can cause the rapid initiation of multiple signaling cascades and second messenger systems, prompting investigations into alternate, transcription independent mechanisms of steroid action. Studies of the endothelium over the past two decades have revealed fundamental mechanisms in rapid sex steroid signaling. In particular, endothelium-dependent vasodilatation by estradiol-induced activation of endothelial nitric oxide synthase has proven to be an uniquely informative model to study sex steroid signaling via classical sex steroid receptors localized to the cell membrane. Despite the complexity of feedback and cross talk between rapid sex steroid signaling and other modes of steroid action, recent studies in this field are facilitating the development of steroidal drugs that selectively target the ability of sex steroids to initiate signaling cascades.

Activation of progestin receptors in female reproductive behavior: Interactions with neurotransmitters

The steroid hormone, progesterone (P), modulates neuroendocrine functions in the central nervous system resulting in alterations in physiology and reproductive behavior in female mammals. A wide body of evidence indicates that these neural effects of P are predominantly mediated via their intracellular progestin receptors (PRs) functioning as "ligand-dependent" transcription factors in the steroid-sensitive neurons regulating genes and genomic networks. In addition to P, intracellular PRs can be activated by neurotransmitters, growth factors and cyclic nucleotides in a ligand-independent manner via crosstalk and convergence of pathways. Furthermore, recent studies indicate that rapid signaling events associated with membrane PRs and/or extra-nuclear, cytoplasmic PRs converge with classical PR activated pathways in neuroendocrine regulation of female reproductive behavior. The molecular mechanisms, by which multiple signaling pathways converge on PRs to modulate PR-dependent female reproductive behavior, are discussed in this review.

Role of progesterone in endometrial cancer

Progesterone is a key hormone in the endometrium that opposes estrogen-driven growth. Insufficient progesterone will result in unopposed estrogen action that could lead to the development of endometrial hyperplasia and adenocarcinoma. Although these endometrial neoplasias can regress in response to progestin treatment, this does not occur in all instances. To understand this resistance to progesterone and to improve on existing hormonal therapies, it is imperative that the molecular mechanisms of progesterone action through its receptor be deciphered in endometrial cancer. This review highlights what is known thus far regarding the efficacy of progestin therapy in the clinic and the role of progesterone in endometrial cancer cell behavior and gene regulation.

Progesterone action in human tissues: regulation by progesterone receptor (PR) isoform expression, nuclear positioning and coregulator expression

Progesterone is a critical regulator of normal female reproductive function, with diverse tissue-specific effects in the human. The effects of progesterone are mediated by its nuclear receptor (PR) that is expressed as two isoforms, PRA and PRB, which are virtually identical except that PRA lacks 164 amino acids that are present at the N-terminus of PRB. Considerable in vitro evidence suggests that the two PRs are functionally distinct and in animals, tissue-specific distribution patterns of PRA and PRB may account for some of the diversity of progesterone effects. In the human, PRA and PRB are equivalently expressed in most target cells, suggesting that alternative mechanisms control the diversity of progesterone actions. PR mediates the effects of progesterone by association with a range of coregulatory proteins and binding to specific target sequences in progesterone-regulated gene promoters. Ligand activation of PR results in redistribution into discrete subnuclear foci that are detectable by immunofluorescence, probably representing aggregates of multiple transcriptionally active PR-coregulator complexes. PR foci are aberrant in cancers, suggesting that the coregulator composition and number of complexes is altered. A large family of coregulators is now described and the range of proteins known to bind PR exceeds the complement required for transcriptional activation, suggesting that in the human, tissue-specific coregulator expression may modulate progesterone response. In this review, we examine the role of nuclear localization of PR, coregulator association and tissue-specific expression in modulating progesterone action in the human.

Expression of progesterone receptor A form and its role in the interaction of progesterone with cortisol on cyclooxygenase-2 expression in amnionic fibroblasts

CONTEXT

Human amnion fibroblasts produce abundant prostaglandins toward the end of gestation, which is believed to be one of the major events leading to parturition. Glucocorticoids have been shown to up-regulate cyclooxygenase-2 (COX-2) expression, the crucial enzyme catalyzing prostaglandin synthesis, in human amnion fibroblasts. Although a major propregnancy hormone, the effect of progesterone and the associated progesterone receptor subtypes in the regulation of both basal and glucocorticoid-induced COX-2 expression in human amnion fibroblasts have not been resolved.

METHODS AND RESULTS

Cultured human amnion fibroblasts prepared from the fetal membranes at term pregnancy without labor mainly expressed the progesterone receptor A form (PRA). Inhibition of endogenous progesterone production with trilostane or knockdown of PRA expression with small interfering RNA significantly enhanced the glucocorticoid receptor (GR)-mediated COX-2 induction by cortisol, whereas overexpression of PRA attenuated the induction by cortisol. Co-immunoprecipitation assay revealed PRA in the GR protein complex. Although exogenous progesterone did not alter COX-2 expression under basal conditions, it attenuated cortisol-induced COX-2 expression at concentrations about 10- to 50-fold higher, which might be achieved by competition with cortisol for GR.

CONCLUSIONS

We demonstrated in this study that endogenous progesterone might counteract the induction of prostaglandin synthesis by cortisol via PRA transdominant repression of GR function, whereas high levels of progesterone might further inhibit the induction by cortisol via competitive binding to GR in human amnion fibroblasts. These inhibitory actions of progesterone and PRA on glucocorticoids and GR may partly explain the inconsistent effects of glucocorticoids on parturition in humans.

Characterization of a novel telomerase-immortalized human endometrial stromal cell line, St-T1b

BACKGROUND

Coordinated differentiation of the endometrial compartments in the second half of the menstrual cycle is a prerequisite for the establishment of pregnancy. Endometrial stromal cells (ESC) decidualize under the influence of ovarian progesterone to accommodate implantation of the blastocyst and support establishment of the placenta. Studies into the mechanisms of decidualization are often hampered by the lack of primary ESC. Here we describe a novel immortalized human ESC line.

METHODS

Primary ESC were immortalized by the transduction of telomerase. The resultant cell line, termed St-T1b, was characterized for its morphological and biochemical properties by immunocytochemistry, RT-PCR and immunoblotting. Its progestational response was tested using progesterone and medroxyprogesterone acetate with and without 8-Br-cAMP, an established inducer of decidualization in vitro.

RESULTS

St-T1b were positive for the fibroblast markers vimentin and CD90 and negative for the epithelial marker cytokeratin-7. They acquired a decidual phenotype indistinguishable from primary ESC in response to cAMP stimulation. The decidual response was characterized by transcriptional activation of marker genes, such as PRL, IGFBP1, and FOXO1, and enhanced protein levels of the tumor suppressor p53 and the metastasis suppressor KAI1 (CD82). Progestins alone had no effect on St-T1b cells, but medroxyprogesterone acetate greatly enhanced the cAMP-stimulated expression of IGFBP-1 after 3 and 7 days. Progesterone, albeit more weakly, also augmented the cAMP-induced IGFBP-1 production but only after 7 days of treatment. The cell line remained stable in continuous culture for more than 150 passages.

CONCLUSION

St-T1b express the appropriate phenotypic ESC markers and their decidual response closely mimics that of primary cultures. Decidualization is efficiently induced by cAMP analog and enhanced by medroxyprogesterone acetate, and, to a lesser extent, by natural progesterone. St-T1b cells therefore serve as a useful model for primary ESC.

Are animal models relevant to key aspects of human parturition?

Preterm birth remains the most serious complication of pregnancy and is associated with increased rates of infant death or permanent neurodevelopmental disability. Our understanding of the regulation of parturition remains inadequate. The scientific literature, largely derived from rodent animal models, suggests two major mechanisms regulating the timing of parturition: the withdrawal of the steroid hormone progesterone and a proinflammatory response by the immune system. However, available evidence strongly suggests that parturition in the human has significantly different regulators and mediators from those in most of the animal models. Our objectives are to critically review the data and concepts that have arisen from use of animal models for parturition and to rationalize the use of a new model. Many animal models have contributed to advances in our understanding of the regulation of parturition. However, we suggest that those animals dependent on progesterone withdrawal to initiate parturition clearly have a limitation to their translation to the human. In such models, a linear sequence of events (e.g., luteolysis, progesterone withdrawal, uterine activation, parturition) gives rise to the concept of a "trigger" mechanism. Conversely, we propose that human parturition may arise from the concomitant maturation of several systems in parallel. We have termed this novel concept "modular accumulation of physiological systems" (MAPS). We also emphasize the urgency to determine the precise role of the immune system in the process of parturition in situations other than intrauterine infection. Finally, we accentuate the need to develop a nonprimate animal model whose physiology is more relevant to human parturition. We suggest that the guinea pig displays several key physiological characteristics of gestation that more closely resemble human pregnancy than do currently favored animal models. We conclude that the application of novel concepts and new models are required to advance translational research in parturition.

The cytoplasmic 60 kDa progesterone receptor isoform predominates in the human amniochorion and placenta at term

BACKGROUND

The mechanism that initiates human parturition has been proposed to be 'functional progesterone withdrawal' whereby the 116 kDa B-isoform of the progesterone receptor (PR-B) switches in favour of the 94 kDa A-isoform (PR-A) in reproductive tissues. Recently, other PR isoforms, PR-S, PR-C and PR-M generated from the same gene have been identified and partially characterised.

METHODS AND RESULTS

Using immunohistochemical, western blotting and RT-PCR techniques, evidence is provided that indicates the major PR isoform present in human term fetal membranes (amnion and chorion) and syncytiotrophoblast of the placenta is neither of the classical nuclear PR-B or PR-A isoforms but is the N-terminally truncated 60 kDa PR-C isoform. Evidence is also provided that this 60 kDa isoform resides in the cytoplasm of the expressing cell types. Data are also presented to show that PR-B, PR-A and PR-S isoforms are essentially absent from the amnion and chorion, whereas PR isoforms A, B, C and S are all present in the decidua, with PR-A being the major isoform. The syncytiotrophoblast of the placenta contains the cytoplasmic 60 kDa isoform, but not isoforms PR-A, PR-B or PR-S.

CONCLUSION

The major PR isoform in the amnion, chorion and placenta is a 60 kDa protein that could be PR-C, suggesting that the cytoplasmic isoform has a specific role in extra-embryonic tissues and may be involved in the regulation of human parturition.

Minireview: fetal-maternal hormonal signaling in pregnancy and labor

Mechanisms underlying the initiation of parturition remain unclear. Throughout most of pregnancy, uterine quiescence is maintained by elevated progesterone acting through progesterone receptor (PR). Although in most mammals, parturition is associated with a marked decline in maternal progesterone, in humans, circulating progesterone and uterine PR remain elevated throughout pregnancy, suggesting a critical role for functional PR inactivation in the initiation of labor. Both term and preterm labor in humans and rodents are associated with an inflammatory response. In preterm labor, intraamniotic infection likely provides the stimulus for increased amniotic fluid interleukins and migration of inflammatory cells into the uterus and cervix. However, at term, the stimulus for this inflammatory response is unknown. Increasing evidence suggests that the developing fetus may produce physical and hormonal signals that stimulate macrophage migration to the uterus, with release of cytokines and activation of inflammatory transcription factors, such as nuclear factor kappaB (NF-kappaB) and activator protein 1 (AP-1), which also is activated by myometrial stretch. We postulate that the increased inflammatory response and NF-kappaB activation promote uterine contractility via 1) direct activation of contractile genes (e.g. COX-2, oxytocin receptor, and connexin 43) and 2) impairment of the capacity of PR to mediate uterine quiescence. PR function near term may be compromised by direct interaction with NF-kappaB, altered expression of PR coregulators, increased metabolism of progesterone within the cervix and myometrium, and increased expression of inhibitory PR isoforms. Alternatively, we propose that uterine quiescence during pregnancy is regulated, in part, by PR antagonism of the inflammatory response.

Nuclear progesterone receptor expression in the human fetal membranes and decidua at term before and after labor

To explore how progesterone affects human pregnancy, we identified the progesterone target cells within the fetal membranes (amnion, chorion, and decidua) at term by assessing the extent of expression and localization of the nuclear progesterone receptors, progesterone receptor-A and progesterone receptor-B. Fetal membranes (separated into amnion and chorion-decidua) were obtained after term cesarean deliveries performed before (n = 7) and after (n = 7) labor onset. Nuclear progesterone receptor expression was determined by the abundance of nuclear progesterone receptor mRNAs (by quantitative reverse transcriptase-polymerase chain reaction) and proteins (by western blotting). Localization of nPRs was determined by immunohistochemistry. Progesterone receptor-A and progesterone receptor-B mRNA and protein levels were highest in the chorion-decidua and did not change in association with labor. Nuclear progesterone receptor mRNAs and proteins were barely detectable in amnion. Nuclear progesterone receptor immunostaining was detected only in the nucleus of decidual cells. These findings suggest that the decidua, and not the amnion and chorion, is a direct target for nuclear progesterone receptor-mediated progesterone actions during human pregnancy.

Non-genomic progesterone actions in female reproduction

BACKGROUND

The steroid hormone progesterone is indispensable for mammalian procreation by controlling key female reproductive events that range from ovulation to implantation, maintenance of pregnancy and breast development. In addition to activating the progesterone receptors (PRs)-B and -A, members of the superfamily of ligand-dependent transcription factors, progesterone also elicits a variety of rapid signalling events independently of transcriptional or genomic regulation. This review covers our current knowledge on the mechanisms and relevance of non-genomic progesterone signalling in female reproduction.

METHODS

PubMed was searched up to August 2008 for papers on progesterone actions in ovary/breast/endometrium/myometrium/brain, focusing primarily on non-genomic signalling mechanisms.

RESULTS

Convergence and intertwining of rapid non-genomic events and the slower transcriptional actions critically determine the functional response to progesterone in the female reproductive system in a cell-type- and environment-specific manner. Several putative progesterone-binding moieties have been implicated in rapid signalling events, including the 'classical' PR and its variants, progesterone receptor membrane component 1, and the novel family of membrane progestin receptors. Progesterone and its metabolites have also been implicated in the allosteric regulation of several unrelated receptors, such as gamma-aminobutyric acid type A, oxytocin and sigma(1) receptors.

CONCLUSIONS

Identification of the mechanisms and receptors that relay rapid progesterone signalling is an area of research fraught with difficulties and controversy. More in-depth characterization of the putative receptors is required before the non-genomic progesterone pathway in normal and pathological reproductive function can be targeted for pharmacological intervention.