Progesterone Receptor or Cytoskeletal Protein?

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.

Role of sex hormones in cervical changes in a cervical excision-related preterm delivery mouse model

Progesterone and oestrogen play important roles during parturition; however, their roles in the uterine cervix during preterm labour and delivery are unknown. We evaluated the serum progesterone and oestrogen levels and changes in their receptors (PR and ER) in the cervix in a cervical excision-associated preterm delivery mouse model. Adult female C57BL/6 mice were divided into four groups: sham, cervical excision (Ex), lipopolysaccharide (LPS) and Ex + LPS. Mating was permitted at 3 weeks post-Ex. On gestation day 16, mice were administered LPS intrauterine (100 µg/100 µL/mouse) or physiological saline (100 µL) via laparotomy. Uterine cervices and blood were sampled immediately postpartum. As a result, epithelial PR and muscular ERα were down- and upregulated, respectively, in the proximal cervix in Ex + LPS group compared to in the sham group. These results indicate that unique sex hormone effects are exerted on the uterine cervix during cervical excision-associated spontaneous preterm labour and delivery.Impact statementWhat is already known on this subject? Preterm and term parturition require the withdrawal of progesterone and the activation of oestrogen in the uterine body and systemic levels. However, we have little understanding of the role of the sex hormones in the uterine cervix.What do the results of this study add? Increased ERα-to-PR expression ratio in the proximal cervix was associated with preterm labour and delivery. ERα expression in the smooth muscle layer of the proximal cervix was higher and PR expression in the proximal cervix epithelium was lower during preterm labour and delivery.What are the implications of these findings for clinical practice and/or further research? This study revealed the differences between the roles of sex hormones and their receptors in epithelial and muscle layers of proximal and distal cervices in preterm labour and delivery.

Classical and membrane-initiated estrogen signaling in an in vitro model of anterior hypothalamic kisspeptin neurons

The neuropeptide kisspeptin is essential for sexual maturation and reproductive function. In particular, kisspeptin-expressing neurons in the anterior rostral periventricular area of the third ventricle are generally recognized as mediators of estrogen positive feedback for the surge release of LH, which stimulates ovulation. Estradiol induces kisspeptin expression in the neurons of the rostral periventricular area of the third ventricle but suppresses kisspeptin expression in neurons of the arcuate nucleus that regulate estrogen-negative feedback. To focus on the intracellular signaling and response to estradiol underlying positive feedback, we used mHypoA51 cells, an immortalized line of kisspeptin neurons derived from adult female mouse hypothalamus. mHypoA51 neurons express estrogen receptor (ER)-α, classical progesterone receptor (PR), and kisspeptin, all key elements of estrogen-positive feedback. As with kisspeptin neurons in vivo, 17β-estradiol (E2) induced kisspeptin and PR in mHypoA51s. The ERα agonist, 1,3,5-Tris(4-hydroxyphenyl)-4-propyl-1H-pyrazole, produced similar increases in expression, indicating that these events were mediated by ERα. However, E2-induced PR up-regulation required an intracellular ER, whereas kisspeptin expression was stimulated through a membrane ER activated by E2 coupled to BSA. These data suggest that anterior hypothalamic kisspeptin neurons integrate both membrane-initiated and classical nuclear estrogen signaling to up-regulate kisspeptin and PR, which are essential for the LH surge.

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.

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.

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.

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.

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.

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

Functional progesterone withdrawal associated with human parturition has been ascribed to various mechanisms modulating the function of the classical progesterone receptors (PRs), B and A, in utero. These include up-regulation of the inhibitory PR-C isoform, described as a 60-kDa protein occurring from translation initiation at codon 595. Our initial attempts to detect PR-C yielded uninterpretable results. To systematically validate antibodies for immunodetection of PR isoforms, we generated expression vectors for PR variants originating from putative start codons AUG-289, -301, -595, -632, and -692 in addition to those for PR-B and PR-A, and for alternative splice variants PR-T, PR-S, and PR-M. All constructs were subjected to in vitro and in vivo translation and immunoblotting with a panel of 13 PR antibodies. Antibodies raised against full-length PR were generally not capable of detecting N-terminally truncated forms, whereas C-terminal antibodies did not or only weakly reacted with PR-B and PR-A but produced prominent nonspecific signals. Thus, immunodetection of N-terminally truncated PR isoforms is prone to artifacts. Proteins of about 64 kDa were expressed from PR-289 and -301, but no corresponding endogenous forms were observed. PR-T, PR-S, and PR-M cDNAs yielded no detectable translation products. No protein was translated from AUG-595 in our PR-C expression vector unless a Kozak sequence was introduced, and the product was not 60 but 38 kDa in size. Thus, the 60-kDa protein called PR-C does not originate from AUG-595 and is not a naturally occurring PR isoform.

Estrogen and progesterone metabolism in the cervix during pregnancy and parturition

CONTEXT

Experimental and clinical studies in a variety of nonprimate species demonstrate that progesterone withdrawal leads to changes in gene expression that initiate parturition at term. Mice deficient in 5alpha-reductase type I fail to undergo cervical ripening at term despite the timely onset of luteolysis and progesterone withdrawal in blood.

OBJECTIVE

Our objective was to test the hypothesis that estrogen and progesterone metabolism is regulated in cervical tissues during pregnancy, even in species in which parturition is not characterized by progesterone withdrawal in blood.

DESIGN

Estradiol and progesterone metabolism was quantified in intact cervical tissues from nonpregnant and pregnant women at term before or after labor.

SETTING

The study was conducted at a university hospital.

PATIENTS

Tissues were obtained from five nonpregnant and 21 pregnant women (nine before labor and 12 in labor).

MAIN OUTCOME MEASURES

Enzyme activity measurements, Northern blot analysis, quantitative real-time RT-PCR, and immunohistochemistry were used to quantify steroid hormone metabolizing enzymes in cervical and myometrial tissues.

RESULTS

During pregnancy, 17beta-hydroxysteroid dehydrogenase type 2 was induced in glandular epithelial cells to catalyze the conversion of estradiol to estrone and stroma-derived 20alpha-hydroxyprogesterone to progesterone. During parturition, 17beta-hydroxysteroid dehydrogenase type 2 was down-regulated in endocervical cells, thereby creating a microenvironment favorable for cervical ripening.

CONCLUSIONS

Together, the data indicate that cervical ripening during parturition involves localized regulation of estrogen and progesterone metabolism through a complex relationship between cervical epithelium and stroma, and that steroid hormone metabolism in cervical tissues from pregnant women is unique from that in mice.

Delayed parturition and altered myometrial progesterone receptor isoform A expression in mice null for Krüppel-like factor 9

Preterm and delayed labor conditions are devastating health problems with currently unknown etiologies. We previously showed that the transcription factor Krüppel-like factor 9 (KLF9) influences the expression and/or transcriptional activity of receptors for estrogen and progesterone in endometrial cells in vivo and in vitro. Given that estrogen and progesterone differentially regulate uterine myometrial contractility during gestation, we hypothesized that lack of KLF9 could compromise myometrial function, leading to defects in parturition. To test this, we used mice null for Klf9 to evaluate gestation length, response to the progesterone receptor (PGR) antagonist RU486, expression levels of steroid receptor proteins, nuclear receptor coactivator and contractility-associated genes, and nuclear factor-kappaB (NF-kappaB) DNA binding activity in myometrium near term. Klf9 knockout (KO) mice exhibited delayed parturition by 1-2 days relative to wild-type (WT) counterparts, in the absence of fetal genotype contribution and differences in serum estrogen and progesterone levels. Knockout mice near term were refractory to the abortive action of RU486, and they displayed aberrant myometrial expression patterns of nuclear PGR-A and NF-kappaB p65/RELA relative to WT mice. Myometrial expression levels of nuclear estrogen receptor-alpha did not differ, whereas those for Oxtr and Crebbp mRNAs were lower, in KO versus WT mice. Results indicate that KLF9 contributes to the regulation of PGR-associated components in the myometrium necessary for timely onset of parturition in mice. The present study highlights the potential utility of Klf9 null mice to investigate the pathophysiology of parturition defects involving PGR signaling.