Human Parturition Involves Phosphorylation of Progesterone Receptor-A at Serine-345 in Myometrial Cells

The hormonal control of parturition

Parturition is a complex physiological process that must occur in a reliable manner and at an appropriate gestation stage to ensure a healthy newborn and mother. To this end, hormones that affect the function of the gravid uterus, especially progesterone (P4), 17β-estradiol (E2), oxytocin (OT), and prostaglandins (PGs), play pivotal roles. P4 via the nuclear P4 receptor (PR) promotes uterine quiescence and for most of pregnancy exerts a dominant block to labor. Loss of the P4 block to parturition in association with a gain in prolabor actions of E2 are key transitions in the hormonal cascade leading to parturition. P4 withdrawal can occur through various mechanisms depending on species and physiological context. Parturition in most species involves inflammation within the uterine tissues and especially at the maternal-fetal interface. Local PGs and other inflammatory mediators may initiate parturition by inducing P4 withdrawal. Withdrawal of the P4 block is coordinated with increased E2 actions to enhance uterotonic signals mediated by OT and PGs to promote uterine contractions, cervix softening, and membrane rupture, i.e., labor. This review examines recent advances in research to understand the hormonal control of parturition, with focus on the roles of P4, E2, PGs, OT, inflammatory cytokines, and placental peptide hormones together with evolutionary biology of and implications for clinical management of human parturition.

Decline in corepressor CNOT1 in the pregnant myometrium near term impairs progesterone receptor function and increases contractile gene expression

Progesterone (P4), acting via its nuclear receptor (PR), is critical for pregnancy maintenance by suppressing proinflammatory and contraction-associated protein (CAP)/contractile genes in the myometrium. P4/PR partially exerts these effects by tethering to NF-κB bound to their promot-ers, thereby decreasing NF-κB transcriptional activity. However, the underlying mechanisms whereby P4/PR interaction blocks proinflammatory and CAP gene expression are not fully understood. Herein, we characterized CCR-NOT transcription complex subunit 1 (CNOT1) as a corepressor that also interacts within the same chromatin complex as PR-B. In mouse myome-trium increased expression of CAP genes Oxtr and Cx43 at term coincided with a marked decline in expression and binding of CNOT1 to NF-κB-response elements within the Oxtr and Cx43 promoters. Increased CAP gene expression was accompanied by a pronounced decrease in enrichment of repressive histone marks and increase in enrichment of active histone marks to this genomic region. These changes in histone modification were associated with changes in expression of corresponding histone modifying enzymes. Myometrial tissues from P4-treated 18.5 dpc pregnant mice manifested increased Cnot1 expression at 18.5 dpc, compared to vehicle-treated controls. P4 treatment of PR-expressing hTERT-HM cells enhanced CNOT1 expression and its recruitment to PR bound NF-κB-response elements within the CX43 and OXTR promoters. Furthermore, knockdown of CNOT1 significantly increased expression of contractile genes. These novel findings suggest that decreased expression and DNA-binding of the P4/PR-regulated transcriptional corepressor CNOT1 near term and associated changes in histone modifications at the OXTR and CX43 promoters contribute to the induction of myometrial contractility leading to parturition.

Progesterone inactivation in decidual stromal cells: A mechanism for inflammation-induced parturition

The process of human parturition involves inflammation at the interface where fetal chorion trophoblast cells interact with maternal decidual stromal (DS) cells and maternal immune cells in the decidua (endometrium of pregnancy). This study tested the hypothesis that inflammation at the chorion-decidua interface (CDI) induces labor by negating the capacity for progesterone (P4) to block labor and that this is mediated by inactivation of P4 in DS cells by aldo-keto reductase family 1 member C1 (AKR1C1). In human, Rhesus macaque, and mouse CDI, AKR1C1 expression increased in association with term and preterm labor. In a human DS cell line and in explant cultures of term human fetal membranes containing the CDI, the prolabor inflammatory cytokine, interleukin-1ß (IL-1ß), and media conditioned by LPS-stimulated macrophages increased AKR1C1 expression and coordinately reduced nuclear P4 levels and P4 responsiveness. Loss of P4 responsiveness was overcome by inhibition of AKR1C1 activity, inhibition of AKR1C1 expression, and bypassing AKR1C1 activity with a P4 analog that is not metabolized by AKR1C1. Increased P4 activity in response to AKR1C1 inhibition was prevented by the P4 receptor antagonist RU486. Pharmacologic inhibition of AKR1C1 activity prevented parturition in a mouse model of inflammation-induced preterm parturition. The data suggest that inflammatory stimuli at the CDI drive labor by inducing AKR1C1-mediated P4 inactivation in DS cells and that inhibiting and/or bypassing of AKR1C1-mediated P4 inactivation is a plausible therapeutic strategy to mitigate the risk of inflammation-associated preterm birth.

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.

The role of progesterone receptor isoforms in the myometrium

Myometrial contraction is stringently controlled throughout pregnancy and parturition. Progesterone signaling, effecting through the progesterone receptor (PR), is pivotal in modulating uterine activity. Evidence has shown that two major PR isoforms, PR-A and PR-B, have distinct activities on gene regulation, and the ratio between these isoforms determines the contractility of the myometrium at different gestational stages. Herein, we focus on the regulation of PR activity in the myometrium, especially the differential actions of the two PR isoforms, which maintain uterine quiescence during pregnancy and regulate the switch to a contractile state at the onset of labor. To demonstrate the PR regulatory network and its mechanisms of actions on myometrial activity, we summarized the findings into three parts: Regulation of PR Expression and Isoform Levels, Progesterone Receptor Interacting Factors, and Biological Processes Regulated by Myometrial Progesterone Receptor Isoforms. Recent genomic and epigenomic data, from human specimens and mouse models, are recruited to support the existing knowledge and offer new insights and future directions in myometrial biology.

Progesterone withdrawal and parturition

The steroid hormone progesterone (P4), acting via the nuclear P4 receptors (PRs) in uterine cells, is essential for the establishment and maintenance of pregnancy. P4/PR signaling maintains pregnancy by promoting uterine quiescence and blocking the contractions of labor. Withdrawal of the P4/PR block to labor induces parturition. The success of pregnancy requires the timely birth of a mature neonate to a healthy mother, and to this end, the mechanism by which the P4/PR block is withdrawn, and how that process is physiologically controlled is critical. This review examines current understanding of cell and molecular biology of P4/PR withdrawal in the control of parturition.

Inflammatory Amplification: A Central Tenet of Uterine Transition for Labor

In preparation for delivery, the uterus transitions from actively maintaining quiescence during pregnancy to an active parturient state. This transition occurs as a result of the accumulation of pro-inflammatory signals which are amplified by positive feedback interactions involving paracrine and autocrine signaling at the level of each intrauterine cell and tissue. The amplification events occur in parallel until they reach a certain threshold, ‘tipping the scale’ and contributing to processes of uterine activation and functional progesterone withdrawal. The described signaling interactions all occur upstream from the presentation of clinical labor symptoms. In this review, we will: 1) describe the different physiological processes involved in uterine transition for each intrauterine tissue; 2) compare and contrast the current models of labor initiation; 3) introduce innovative models for measuring paracrine inflammatory interactions; and 4) discuss the therapeutic value in identifying and targeting key players in this crucial event for preterm birth.

Next generation strategies for preventing preterm birth

Preterm birth (PTB) is defined as delivery before 37 weeks of gestation. Globally, 15 million infants are born prematurely, putting these children at an increased risk of mortality and lifelong health challenges. Currently in the U.S., there is only one FDA approved therapy for the prevention of preterm birth. Makena is an intramuscular progestin injection given to women who have experienced a premature delivery in the past. Recently, however, Makena failed a confirmatory trial, resulting the Center for Drug Evaluation and Research's (CDER) recommendation for the FDA to withdrawal Makena's approval. This recommendation would leave clinicians with no therapeutic options for preventing PTB. Here, we outline recent interdisciplinary efforts involving physicians, pharmacologists, biologists, chemists, and engineers to understand risk factors associated with PTB, to define mechanisms that contribute to PTB, and to develop next generation therapies for preventing PTB. These advances have the potential to better identify women at risk for PTB, prevent the onset of premature labor, and, ultimately, save infant lives.

TBX2, a Novel Regulator of Labour

Background and Objectives: Therapeutic interventions targeting molecular factors involved in the transition from uterine quiescence to overt labour are not substantially reducing the rate of spontaneous preterm labour. The identification of novel rational therapeutic targets are essential to prevent the most common cause of neonatal mortality. Based on our previous work showing that Tbx2 (T-Box transcription factor 2) is a putative upstream regulator preceding progesterone withdrawal in mouse myometrium, we now investigate the role of TBX2 in human myometrium. Materials and Methods: RNA microarray analysis of (A) preterm human myometrium samples and (B) myometrial cells overexpressing TBX2 in vitro, combined with subsequent analysis of the two publicly available datasets of (C) Chan et al. and (D) Sharp et al. The effect of TBX2 overexpression on cytokines/chemokines secreted to the myometrium cell culture medium were determined by Luminex assay. Results: Analysis shows that overexpression of TBX2 in myometrial cells results in downregulation of TNFα- and interferon signalling. This downregulation is consistent with the decreased expression of cytokines and chemokines of which a subset has been previously associated with the inflammatory pathways relevant for human labour. In contrast, CXCL5 (C-X-C motif chemokine ligand 5), CCL21 and IL-6 (Interleukin 6), previously reported in relation to parturition, do not seem to be under TBX2 control. The combined bioinformatical analysis of the four mRNA datasets identifies a subset of upstream regulators common to both preterm and term labour under control of TBX2. Surprisingly, TBX2 mRNA levels are increased in preterm contractile myometrium. Conclusions: We identified a subset of upstream regulators common to both preterm and term labour that are activated in labour and repressed by TBX2. The increased TBX2 mRNA expression in myometrium collected during a preterm caesarean section while in spontaneous preterm labour compared to tissue harvested during iatrogenic preterm delivery does not fit the bioinformatical model. We can only explain this by speculating that the in vivo activity of TBX2 in human myometrium depends not only on the TBX2 expression levels but also on levels of the accessory proteins necessary for TBX2 activity.

Transcription factors regulated by cAMP in smooth muscle of the myometrium at human parturition

Cyclic adenosine monophosphate (cAMP) contributes to maintenance of a quiescent (relaxed) state in the myometrium (i.e. uterine smooth muscle) during pregnancy, which most commonly has been attributed to activation of protein kinase A (PKA). PKA-mediated phosphorylation of cytosolic contractile apparatus components in myometrial smooth muscle cells (mSMCs) are known to promote relaxation. Additionally, PKA also regulates nuclear transcription factor (TF) activity to control expression of genes important to the labour process; these are mostly involved in actin-myosin interactions, cell-to-cell connectivity and inflammation, all of which influence mSMC transition from a quiescent to a contractile (pro-labour) phenotype. This review focuses on the evidence that cAMP modulates the activity of TFs linked to pro-labour gene expression, predominantly cAMP response element (CRE) binding TFs, nuclear factor κB (NF-κB), activator protein 1 (AP-1) family and progesterone receptors (PRs). This review also considers the more recently described exchange protein directly activated by cAMP (EPAC) that may oppose the pro-quiescent effects of PKA, as well as explores findings from other cell types that have the potential to be of novel relevance to cAMP action on TF function in the myometrium.

Progesterone receptor isoform B regulates the Oxtr-Plcl2-Trpc3 pathway to suppress uterine contractility

Uterine contractile dysfunction leads to pregnancy complications such as preterm birth and labor dystocia. In humans, it is hypothesized that progesterone receptor isoform PGR-B promotes a relaxed state of the myometrium, and PGR-A facilitates uterine contraction. This hypothesis was tested in vivo using transgenic mouse models that overexpress PGR-A or PGR-B in smooth muscle cells. Elevated PGR-B abundance results in a marked increase in gestational length compared to control mice (21.1 versus 19.1 d respectively, P < 0.05). In both ex vivo and in vivo experiments, PGR-B overexpression leads to prolonged labor, a significant decrease in uterine contractility, and a high incidence of labor dystocia. Conversely, PGR-A overexpression leads to an increase in uterine contractility without a change in gestational length. Uterine RNA sequencing at midpregnancy identified 1,174 isoform-specific downstream targets and 424 genes that are commonly regulated by both PGR isoforms. Gene signature analyses further reveal PGR-B for muscle relaxation and PGR-A being proinflammatory. Elevated PGR-B abundance reduces Oxtr and Trpc3 and increases Plcl2 expression, which manifests a genetic profile of compromised oxytocin signaling. Functionally, both endogenous PLCL2 and its paralog PLCL1 can attenuate uterine muscle cell contraction in a CRISPRa-based assay system. These findings provide in vivo support that PGR isoform levels determine distinct transcriptomic landscapes and pathways in myometrial function and labor, which may help further the understanding of abnormal uterine function in the clinical setting.

Developing a theoretical evolutionary framework to solve the mystery of parturition initiation

Eutherian mammals have characteristic lengths of gestation that are key for reproductive success, but relatively little is known about the processes that determine the timing of parturition, the process of birth, and how they are coordinated with fetal developmental programs. This issue remains one of biology's great unsolved mysteries and has significant clinical relevance because preterm birth is the leading cause of infant and under 5 year old child mortality worldwide. Here, we consider the evolutionary influences and potential signaling mechanisms that maintain or end pregnancy in eutherian mammals and use this knowledge to formulate general theoretical evolutionary models. These models can be tested through evolutionary species comparisons, studies of experimental manipulation of gestation period and birth timing, and human clinical studies. Understanding how gestation time and parturition are determined will shed light on this fundamental biological process and improve human health through the development of therapies to prevent preterm birth.

Cooperative effects of sequential PGF2α and IL-1β on IL-6 and COX-2 expression in human myometrial cells†

The change from the state of pregnancy to the state of parturition, which we call uterine transitioning, requires the actions of inflammatory mediators and results in an activated uterus capable of performing the physiology of labor. Interleukin (IL)-1β and prostaglandin (PG)F2α are two key mediators implicated in preparing the uterus for labor by regulating the expression of uterine activation proteins (UAPs) and proinflammatory cytokines and chemokines. To investigate this process, primary human myometrial smooth muscle cells (HMSMC) isolated from the lower segment of women undergoing elective cesarean sections at term (not in labor) were used to test the inflammatory cytokine and UAP outputs induced by PGF2α and IL-1β alone or in sequential combinations. PGF2α and IL-1β regulate mRNA abundance of the PGF2α receptor FP, the IL-1 receptor system, interleukin 6, and other UAPs (OXTR, COX2), driving positive feedback interactions to further amplify their own proinflammatory effects. Sequential stimulation of HMSMC by PGF2α and IL-1β in either order results in amplified upregulation of IL-6 and COX-2 mRNA and protein, compared to their effects individually. These profound increases were unique to myometrium and not observed with stimulation of human fetal membrane explants. These results suggest that PGF2α and IL-1β act cooperatively upstream in the birth cascade to maximize amplification of IL-6 and COX-2, to build inflammatory load and thereby promote uterine transition. Targeting PGF2α or IL-1β, their actions, or intermediates (e.g. IL-6) would be an effective therapeutic intervention for preterm birth prevention or delay.

Interplay of transcriptional signaling by progesterone, cyclic AMP, and inflammation in myometrial cells: implications for the control of human parturition

Parturition involves cellular signaling changes driven by the complex interplay between progesterone (P4), inflammation, and the cyclic adenosine monophosphate (cAMP) pathway. To characterize this interplay, we performed comprehensive transcriptomic studies utilizing eight treatment combinations on myometrial cell lines and tissue samples from pregnant women. We performed genome-wide RNA-sequencing on the hTERT-HM${}^{A/B}$ cell line treated with all combinations of P4, forskolin (FSK) (induces cAMP), and interleukin-1$\beta$ (IL-1$\beta$). We then performed gene set enrichment and regulatory network analyses to identify pathways commonly, differentially, or synergistically regulated by these treatments. Finally, we used tissue similarity index (TSI) to characterize the correspondence between cell lines and tissue phenotypes. We observed that in addition to their individual anti-inflammatory effects, P4 and cAMP synergistically blocked specific inflammatory pathways/regulators including STAT3/6, CEBPA/B, and OCT1/7, but not NF$\kappa$B. TSI analysis indicated that FSK + P4- and IL-1$\beta$-treated cells exhibit transcriptional signatures highly similar to non-laboring and laboring term myometrium, respectively. Our results identify potential therapeutic targets to prevent preterm birth and show that the hTERT-HM${}^{A/B}$ cell line provides an accurate transcriptional model for term myometrial tissue.

Progesterone receptor ligands for the treatment of endometriosis: the mechanisms behind therapeutic success and failure

BACKGROUND

Despite intense research, it remains intriguing why hormonal therapies in general and progestins in particular sometimes fail in endometriosis.

OBJECTIVE AND RATIONALE

We review here the action mechanisms of progesterone receptor ligands in endometriosis, identify critical differences between the effects of progestins on normal endometrium and endometriosis and envisage pathways to escape drug resistance and improve the therapeutic response of endometriotic lesions to such treatments.

SEARCH METHODS

We performed a systematic Pubmed search covering articles published since 1958 about the use of progestins, estro-progestins and selective progesterone receptor modulators, to treat endometriosis and its related symptoms. Two reviewers screened the titles and abstracts to select articles for full-text assessment.

OUTCOMES

Progesterone receptor signalling leads to down-regulation of estrogen receptors and restrains local estradiol production through interference with aromatase and 17 beta-hydroxysteroid dehydrogenase type 1. Progestins inhibit cell proliferation, inflammation, neovascularisation and neurogenesis in endometriosis. However, progesterone receptor expression is reduced and disrupted in endometriotic lesions, with predominance of the less active isoform (PRA) over the full-length, active isoform (PRB), due to epigenetic abnormalities affecting the PGR gene transcription. Oxidative stress is another mechanism involved in progesterone resistance in endometriosis. Among the molecular targets of progesterone in the normal endometrium that resist progestin action in endometriotic cells are the nuclear transcription factor FOXO1, matrix metalloproteinases, the transmembrane gap junction protein connexin 43 and paracrine regulators of estradiol metabolism. Compared to other phenotypes, deep endometriosis appears to be more resistant to size regression upon medical treatments. Individual genetic characteristics can affect the bioavailability and pharmacodynamics of hormonal drugs used to treat endometriosis and, hence, explain part of the variability in the therapeutic response.

WIDER IMPLICATIONS

Medical treatment of endometriosis needs urgent innovation, which should start by deeper understanding of the disease core features and diverse phenotypes and idiosyncrasies, while moving from pure hormonal treatments to drug combinations or novel molecules capable of restoring the various homeostatic mechanisms disrupted by endometriotic lesions.

Maternal and fetal intrauterine tissue crosstalk promotes proinflammatory amplification and uterine transition†

Birth is a complex biological event requiring genetic, cellular, and physiological changes to the uterus, resulting in a uterus activated for completing the physiological processes of labor. We define the change from the state of pregnancy to the state of parturition as uterine transitioning, which requires the actions of inflammatory mediators and localized paracrine interactions between intrauterine tissues. Few studies have examined the in vitro interactions between fetal and maternal gestational tissues within this proinflammatory environment. Thus, we designed a co-culture model to address this gap, incorporating primary term human myometrium smooth muscle cells (HMSMCs) with human fetal membrane (hFM) explants to study interactions between the tissues. We hypothesized that crosstalk between tissues at term promotes proinflammatory expression and uterine transitioning for parturition. Outputs of 40 cytokines and chemokines encompassing a variety of proinflammatory roles were measured; all but one increased significantly with co-culture. Eighteen of the 39 cytokines increased to a higher abundance than the sum of the effect of each tissue cultured separately. In addition, COX2 and IL6 but not FP and OXTR mRNA abundance significantly increased in both HMSMCs and hFM in response to co-culture. These data suggest that synergistic proinflammatory upregulation within intrauterine tissues is involved with uterine transitioning.

The impact of progesterone and RU-486 on classic pro-labour proteins & contractility in human myometrial tissues during 24-hour exposure to tension & interleukin-1β

Increased expression of pro-labour genes that encode cyclooxygenase-2 (COX-2), oxytocin receptor (OTR) and connexin-43 (Cx43) at parturition is often attributed to P4 functional withdrawal, based on findings from animal models and human primary myometrial cells. However, the cause of reduced myometrial P4 responsiveness that promotes contractions at labour is not fully determined. Uterine stretch occurs with advancing gestation but most in vitro experimental models do not take this into consideration. We aimed to examine whether tissue-level myometrial stretch influences the ability of P4 to regulate pro-labour protein abundance by using myometrial biopsies from term gestation pregnant women to assess the impact of 24 h exposure to combinations of (i) stretch-mediated tension, (ii) P4 (100 nM) and (iii) an anti-progestin, RU-486 (1 μM). Firstly, we observed baseline COX-2 and Cx43 protein levels increased, whereas P4 content along with calponin-1 and progesterone receptor (PR) protein abundance decreased, in vehicle-treated tissues. P4 supplementation subtly reduced COX-2 levels in un-stretched tissues. Spontaneous and oxytocin-augmented contractility were unchanged by tissue culture exposure to P4 and/or RU-486. Interleukin-1β (IL-1β; 1 ng/ml) enhanced COX-2 protein and PGE₂ content in un-stretched tissues. Overall, tissue stretch may, in part, regulate P4-sensitive pro-labour protein levels, but this is likely to be reliant on interaction with other in utero factors that were absent in our tissue cultures. More complex culture conditions should be evaluated in future to aid further development of a physiologically relevant model to improve our understanding of in utero myometrial P4 responsiveness.

Progestin therapy to prevent preterm birth: History and effectiveness of current strategies and development of novel approaches

In the 1930s the "progestin" hormone produced by the corpus luteum was isolated and found to be a Δ4-keto-steroid. It was aptly named progesterone (P4) and in the following 30 years the capacity of P4 and derivatives to prevent preterm birth (PTB) was examined. Outcomes of multiple small studies suggested that progestin prophylaxis beginning at mid-gestation decreases the risk for PTB. Subsequent larger trials found that prophylaxis with weekly intramuscular injections of 17α-hydroxyprogesterone caproate (17HPC) beginning at mid-gestation decreased PTB risk in women with a history of PTB. Other trials found that daily vaginal P4 prophylaxis, also beginning at mid-gestation decreased PTB risk in women with a short cervix. Currently, prophylaxis with 17HPC (in women with a history of PTB) or vaginal P4 (in women with a short cervix) are used to prevent PTB. Recent advances in understanding the molecular biology of P4 signaling in uterine cells is revealing novel progestin-based targets for PTB prevention. One possibility is to use selective P4 receptor (PR) modulators (SPRMs) to boost PR anti-inflammatory activity that blocks labor, while simultaneously preventing PR phosphorylation that causes loss of P4/PR anti-inflammatory activity. This may be achieved by SPRMs that induce a specific PR conformation that prevents site-specific serine phosphorylation that inhibits anti-inflammatory activity. Further advances in understanding how P4 promotes uterine quiescence and how its labor blocking actions are withdrawn to trigger parturition will reveal novel therapeutic targets to more effectively prevent PTB.

Cervix Stromal Cells and the Progesterone Receptor A Isoform Mediate Effects of Progesterone for Prepartum Remodeling

The prepartum transition from a soft to ripening cervix is an inflammatory process that occurs well before birth when systemic progesterone is near peak concentration. This 2-part study first determined that stromal fibroblasts but not macrophages in the cervix have progesterone receptors (PRs). Neither the number of PR cells in cervix sections nor the relative abundance or ratio of nuclear PR isoforms (PR-A/PR-B) were diminished in mice between day 15 of pregnancy and term. Second in mice lacking PR-B ( Pgr^tm20mc), the number of cells that expressed the PR-A isoform were maintained during this period of prepartum cervix remodeling. Thus, progesterone effects to sustain pregnancy, as well as soften and ripen the cervix, are mediated by a stable stromal cell population that expresses PR-A and, through interactions with resident macrophages, are likely to mediate inflammatory ripening processes in preparation for birth.

Progesterone and cAMP synergize to inhibit responsiveness of myometrial cells to pro-inflammatory/pro-labor stimuli

Progesterone (P4) acting through the P4 receptor (PR) isoforms, PR-A and PR-B, promotes uterine quiescence for most of pregnancy, in part, by inhibiting the response of myometrial cells to pro-labor inflammatory stimuli. This anti-inflammatory effect is inhibited by phosphorylation of PR-A at serine-344 and -345 (pSer^344/345-PRA). Activation of the cyclic adenosine monophosphate (cAMP) signaling pathway also promotes uterine quiescence and myometrial relaxation. This study examined the cross-talk between P4/PR and cAMP signaling to exert anti-inflammatory actions and control pSer^344/345-PRA generation in myometrial cells. In the hTERT-HM^A/B immortalized human myometrial cell line P4 inhibited responsiveness to interleukin (IL)-1β and forskolin (increases cAMP) and 8-Br-cAMP increased this effect in a concentration-dependent and synergistic manner that was mediated by activation of protein kinase A (PKA). Forskolin also inhibited the generation of pSer^344/345-PRA and expression of key contraction-associated genes. Generation of pSer^344/345-PRA was catalyzed by stress-activated protein kinase/c-Jun NH₂-terminal kinase (SAPK/JNK). Forskolin inhibited pSer^344/345-PRA generation, in part, by increasing the expression of dual specificity protein phosphatase 1 (DUSP1), a phosphatase that inactivates mitogen-activated protein kinases (MAPKs) including SAPK/JNK. P4/PR and forskolin increased DUSP1 expression. The data suggest that P4/PR promotes uterine quiescence via cross-talk and synergy with cAMP/PKA signaling in myometrial cells that involves DUSP1-mediated inhibition of SAPK/JNK activation.

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.

Progesterone Receptor Regulation of Uterine Adaptation for Pregnancy

Progesterone acts through the progesterone receptor to direct physiological adaption of the uterus in preparation and completion of pregnancy. Genome-wide transcriptome and cistrome analyses have uncovered new members and novel modifiers of the progesterone signaling pathway. Genetically engineered mice allow functional assessment of newly identified genes in vivo and provide insights on the impact of progesterone receptor-dependent molecular mechanisms on pregnancy at the organ system level. Progesterone receptor isoforms collectively mediate progesterone signaling via their distinct and common downstream target genes, which makes the stoichiometry of isoforms relevant in modifying the progesterone activity. This review discusses recent advances on the discovery of the progesterone receptor network, with special focus on the endometrium at early pregnancy and myometrium during parturition.

Vaginal progesterone for preventing preterm birth and adverse perinatal outcomes in singleton gestations with a short cervix: a meta-analysis of individual patient data

BACKGROUND

The efficacy of vaginal progesterone for preventing preterm birth and adverse perinatal outcomes in singleton gestations with a short cervix has been questioned after publication of the OPPTIMUM study.

OBJECTIVE

To determine whether vaginal progesterone prevents preterm birth and improves perinatal outcomes in asymptomatic women with a singleton gestation and a midtrimester sonographic short cervix.

STUDY DESIGN

We searched MEDLINE, EMBASE, LILACS, and CINAHL (from their inception to September 2017); Cochrane databases; bibliographies; and conference proceedings for randomized controlled trials comparing vaginal progesterone vs placebo/no treatment in women with a singleton gestation and a midtrimester sonographic cervical length ≤25 mm. This was a systematic review and meta-analysis of individual patient data. The primary outcome was preterm birth <33 weeks of gestation. Secondary outcomes included adverse perinatal outcomes and neurodevelopmental and health outcomes at 2 years of age. Individual patient data were analyzed using a 2-stage approach. Pooled relative risks with 95% confidence intervals were calculated. Quality of evidence was assessed using the GRADE methodology.

RESULTS

Data were available from 974 women (498 allocated to vaginal progesterone, 476 allocated to placebo) with a cervical length ≤25 mm participating in 5 high-quality trials. Vaginal progesterone was associated with a significant reduction in the risk of preterm birth <33 weeks of gestation (relative risk, 0.62; 95% confidence interval, 0.47-0.81; P = .0006; high-quality evidence). Moreover, vaginal progesterone significantly decreased the risk of preterm birth <36, <35, <34, <32, <30, and <28 weeks of gestation; spontaneous preterm birth <33 and <34 weeks of gestation; respiratory distress syndrome; composite neonatal morbidity and mortality; birthweight <1500 and <2500 g; and admission to the neonatal intensive care unit (relative risks from 0.47-0.82; high-quality evidence for all). There were 7 (1.4%) neonatal deaths in the vaginal progesterone group and 15 (3.2%) in the placebo group (relative risk, 0.44; 95% confidence interval, 0.18-1.07; P = .07; low-quality evidence). Maternal adverse events, congenital anomalies, and adverse neurodevelopmental and health outcomes at 2 years of age did not differ between groups.

CONCLUSION

Vaginal progesterone decreases the risk of preterm birth and improves perinatal outcomes in singleton gestations with a midtrimester sonographic short cervix, without any demonstrable deleterious effects on childhood neurodevelopment.

Vaginal progesterone for preventing preterm birth and adverse perinatal outcomes in singleton gestations with a short cervix: a meta-analysis of individual patient data

BACKGROUND

The efficacy of vaginal progesterone for preventing preterm birth and adverse perinatal outcomes in singleton gestations with a short cervix has been questioned after publication of the OPPTIMUM study.

OBJECTIVE

To determine whether vaginal progesterone prevents preterm birth and improves perinatal outcomes in asymptomatic women with a singleton gestation and a midtrimester sonographic short cervix.

STUDY DESIGN

We searched MEDLINE, EMBASE, LILACS, and CINAHL (from their inception to September 2017); Cochrane databases; bibliographies; and conference proceedings for randomized controlled trials comparing vaginal progesterone vs placebo/no treatment in women with a singleton gestation and a midtrimester sonographic cervical length ≤25 mm. This was a systematic review and meta-analysis of individual patient data. The primary outcome was preterm birth <33 weeks of gestation. Secondary outcomes included adverse perinatal outcomes and neurodevelopmental and health outcomes at 2 years of age. Individual patient data were analyzed using a 2-stage approach. Pooled relative risks with 95% confidence intervals were calculated. Quality of evidence was assessed using the GRADE methodology.

RESULTS

Data were available from 974 women (498 allocated to vaginal progesterone, 476 allocated to placebo) with a cervical length ≤25 mm participating in 5 high-quality trials. Vaginal progesterone was associated with a significant reduction in the risk of preterm birth <33 weeks of gestation (relative risk, 0.62; 95% confidence interval, 0.47-0.81; P = .0006; high-quality evidence). Moreover, vaginal progesterone significantly decreased the risk of preterm birth <36, <35, <34, <32, <30, and <28 weeks of gestation; spontaneous preterm birth <33 and <34 weeks of gestation; respiratory distress syndrome; composite neonatal morbidity and mortality; birthweight <1500 and <2500 g; and admission to the neonatal intensive care unit (relative risks from 0.47-0.82; high-quality evidence for all). There were 7 (1.4%) neonatal deaths in the vaginal progesterone group and 15 (3.2%) in the placebo group (relative risk, 0.44; 95% confidence interval, 0.18-1.07; P = .07; low-quality evidence). Maternal adverse events, congenital anomalies, and adverse neurodevelopmental and health outcomes at 2 years of age did not differ between groups.

CONCLUSION

Vaginal progesterone decreases the risk of preterm birth and improves perinatal outcomes in singleton gestations with a midtrimester sonographic short cervix, without any demonstrable deleterious effects on childhood neurodevelopment.

Pathophysiology of preterm labor with intact membranes

Preterm labor with intact membranes is a major cause of spontaneous preterm birth (sPTB). To prevent sPTB a clear understanding is needed of the hormonal interactions that initiate labor. The steroid hormone progesterone acting via its nuclear progesterone receptors (PRs) in uterine cells is essential for the establishment and maintenance of pregnancy and disruption of PR signaling (i.e., functional progesterone/PR withdrawal) is key trigger for labor. The process of parturition is also associated with inflammation within the uterine tissues and it is now generally accepted that inflammatory stimuli from multiple extrinsic and intrinsic sources induce labor. Recent studies suggest inflammatory stimuli induce labor by affecting PR transcriptional activity in uterine cells to cause functional progesterone/PR withdrawal. Advances in understanding the functional interaction of inflammatory load on the pregnancy uterus and progesterone/PR signaling is opening novel areas of research and may lead to rational therapeutic strategies to effectively prevent sPTB.

Control of Progesterone Receptor-A Transrepressive Activity in Myometrial Cells: Implications for the Control of Human Parturition

Uterine quiescence during pregnancy is maintained by progesterone primarily via signaling mediated by the type-B progesterone receptor (PR-B) in myometrial cells. Withdrawal of PR-B-mediated progesterone activity is a principal trigger for labor. One mechanism for PR-B withdrawal is by inhibition of its activity by the type-A PR (PR-A) isoform in myometrial cells. We hypothesized that human parturition involves hormonal interactions that induce the capacity for PR-A to inhibit PR-B in myometrial cells and that pro-inflammatory cytokines are major regulators of this process. We tested this hypothesis in an immortalized human myometrial cell line, hTERT-HM^A/B, in which levels of PR-A and PR-B can be experimentally controlled. We found that the capacity for PR-A to repress PR-B, assessed by activity of a transiently transfected reporter DNA controlled by the progesterone response element, and expression of FK506 binding protein 5 ( FKBP5) an endogenous PR-B responsive gene, was increased by serum supplementation and interleukin-1β. In pregnant uterus, FKBP5 was detected exclusively in myometrial cells and its expression decreased with advancing gestation and in association with the onset of labor at term. These findings suggest that in myometrial cells the repressive activity of PR-A on PR-B increases with advancing gestation and is induced by pro-inflammatory cytokines. This may be a key mechanism linking inflammation with the onset of labor.

The transcriptional repressor GATAD2B mediates progesterone receptor suppression of myometrial contractile gene expression

The mechanisms whereby progesterone (P₄), acting via the progesterone receptor (PR), inhibits proinflammatory/contractile gene expression during pregnancy are incompletely defined. Using immortalized human myometrial (hTERT-HM) cells stably expressing wild-type PR-A or PR-B (PR_WT), we found that P₄ significantly inhibited IL-1β induction of the NF-κB target genes, COX-2 and IL-8 P₄-PR_WT transrepression occurred at the level of transcription initiation and was mediated by decreased recruitment of NF-κB p65 and RNA polymerase II to COX-2 and IL-8 promoters. However, in cells stably expressing a PR-A or PR-B DNA-binding domain mutant (PR_mDBD), P₄-mediated transrepression was significantly reduced, suggesting a critical role of the PR DBD. ChIP analysis of hTERT-HM cells stably expressing PR_WT or PR_mDBD revealed that P₄ treatment caused equivalent recruitment of PR_WT and PR_mDBD to COX-2 and IL-8 promoters, suggesting that PR inhibitory effects were not mediated by its direct DNA binding. Using immunoprecipitation, followed by MS, we identified a transcriptional repressor, GATA zinc finger domain-containing 2B (GATAD2B), that interacted strongly with PR_WT but poorly with PR_mDBD P₄ treatment of PR_WT hTERT-HM cells caused enhanced recruitment of endogenous GATAD2B to COX-2 and IL-8 promoters. Further, siRNA knockdown of endogenous GATAD2B significantly reduced P₄-PR_WT transrepression of COX-2 and IL-8 Notably, GATAD2B expression was significantly decreased in pregnant mouse and human myometrium during labor. Our findings suggest that GATAD2B serves as an important mediator of P₄-PR suppression of proinflammatory and contractile genes during pregnancy. Decreased GATAD2B expression near term may contribute to the decline in PR function, leading to labor.

Progesterone Receptor Signaling in Uterine Myometrial Physiology and Preterm Birth

Myometrium holds the structural integrity for the uterus and generates force for parturition with its primary component, the smooth muscle cells. The progesterone receptor mediates progesterone-dependent signaling and connects to a network of pathways for regulation of contractility and inflammatory responses in myometrium. Dysfunctional progesterone signaling has been linked to pregnancy complications including preterm birth. In the present review, we summarize recent findings on modifiers and effectors of the progesterone receptor signaling. Discussions include novel conceptual discoveries and new development in legacy pathways such as the signal transducers NF-κB, ZEB, microRNA, and the unfolded protein response pathways. We also discuss the impact of progesterone receptor isoform composition and ligand accessibility in modification of the progesterone receptor genomic actions.