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.

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.

Labor (term and preterm) is associated with changes in the placental mRNA expression of corticotrophin-releasing factor

Because maternal plasma corticotrophin-releasing factor (CRF) levels increase during the last weeks of pregnancy and at parturition, the present study evaluated whether placental mRNA expression of CRF and CRF-binding protein (CRF-BP) are modified in preterm delivery. A group of 30 women with singleton pregnancies were enrolled in the study. A placental tissue specimen was collected from pregnant women (1) at term after cesarean delivery (39.3 +/- 0.1 gestational weeks; n = 10), (2) at term after spontaneous vaginal delivery (40.1 +/- 0.2 gestational weeks; n = 10), or (3) at preterm delivery (32.4 +/- 0.4 gestational weeks; n = 10). Changes of placental mRNA expression were evaluated by real-time quantitative reverse transcriptase polymerase chain reaction analysis. Placental CRF mRNA expression at term (P < .001) and preterm delivery (P < .001) was significantly higher than in cesarean delivery and highest in preterm placentas. With respect to CRF-BP, no significant difference in placental mRNA expression was observed among samples collected after term or preterm delivery and cesarean delivery. The present study showed for the first time that both term and preterm labor are associated with increased expression of placental CRF but not CRF-BP mRNA.

The preterm parturition syndrome

The implicit paradigm that has governed the study and clinical management of preterm labour is that term and preterm parturition are the same processes, except for the gestational age at which they occur. Indeed, both share a common pathway composed of uterine contractility, cervical dilatation and activation of the membranes/decidua. This review explores the concept that while term labour results from physiological activation of the components of the common pathway, preterm labour arises from pathological signalling and activation of one or more components of the common pathway of parturition. The term "great obstetrical syndromes" has been coined to reframe the concept of obstetrical disease. Such syndromes are characterised by: (1) multiple aetiology; (2) long preclinical stage; (3) frequent fetal involvement; (4) clinical manifestations that are often adaptive in nature; and (5) gene-environment interactions that may predispose to the syndromes. This article reviews the evidence indicating that the pathological processes implicated in the preterm parturition syndrome include: (1) intrauterine infection/inflammation; (2) uterine ischaemia; (3) uterine overdistension; (4) abnormal allograft reaction; (5) allergy; (6) cervical insufficiency; and (7) hormonal disorders (progesterone related and corticotrophin-releasing factor related). The implications of this conceptual framework for the prevention, diagnosis, and treatment of preterm labour are discussed.

Reduced maternal plasma urocortin concentrations and impaired uterine artery blood flow at human mid pregnancy

OBJECTIVE

Urocortin is a placental neuropeptide belonging to the family of corticotropin-releasing factors (CRFs), playing a role in the uteroplacental blood flow regulation through the binding to specific CRF receptors. Since CRF receptors are expressed in the uterine vascular bed of pregnant rats, and because urocortin has a relaxant effect on uterine vasculature, we evaluated mid-gestation plasma urocortin levels in women with impaired blood flow through uterine arteries.

METHODS

Maternal plasma urocortin was assayed by specific radioimmunoassay and uterine artery resistance index (RI) by Doppler evaluation at 22-24 weeks' gestation in 57 healthy pregnant women, of which 29 showed a monolateral or bilateral uterine artery notch. Statistical analysis was performed by one-way analysis of variance (ANOVA), followed by post-hoc Tukey test for multiple comparison and Pearson correlation coefficient test.

RESULTS

The mean uterine artery RI was significantly (P <.001) higher in women with a notch than healthy controls. Mean +/- SEM maternal plasma urocortin levels were significantly (P <.001) lower in women with unilateral (52.03 +/- 3.25 pg/mL) or bilateral (47.01 +/- 4.16 pg/mL) uterine artery notch than in healthy control pregnant women (84.01 +/- 3.5 pg/mL). While no difference was found in urocortin levels between patients with unilateral or bilateral uterine artery notch, urocortin concentrations inversely correlated with the mean RI (Pearson r = -0.7318; 95% confidence interval -0.8334 to -0.5822; P <.0001).

CONCLUSIONS

The present findings suggest that reduced levels of circulating urocortin are associated with increased uterine artery resistances and support the hypothesis that urocortin may regulate uterine artery tone at mid gestation.

Mifepristone: effect on plasma corticotropin-releasing hormone, adrenocorticotropic hormone, and cortisol in term pregnancy

OBJECTIVE

To compare the effects of in vivo mifepristone with placebo on plasma corticotropin-releasing hormone (CRH), adrenocorticotropic hormone (ACTH), and cortisol levels concentrations in term human pregnancies.

STUDY DESIGN

In all, 24 women participating in a randomized controlled trial of mifepristone for preinduction cervical ripening were enrolled in this ancillary study. Participants with uncomplicated singleton pregnancies beyond 41 weeks gestation and undilated, uneffaced cervices were randomized to either placebo or mifepristone 200 mg orally and observed for 24 hours prior to receiving either intravaginal misoprostol and/or intravenous oxytocin. Blood samples were obtained before medication administration, 3 and 6 hours later, and then every 6 hours until delivery. Plasma hormone levels were measured by radioimmunoassay.

RESULTS

Basal levels of CRH, ACTH, and cortisol were similar in the placebo (n=13) and mifepristone groups (n=11). Compared to placebo treatment, exposure to mifepristone resulted in significant elevation of plasma cortisol within 18 hours. Plasma CRH and ACTH were unaffected. Progression of labor was associated with significant increases in cortisol in both groups, while CRH and ACTH levels were not altered. Compared to basal levels within each group, plasma cortisol at delivery was significantly elevated within both the mifepristone (156.8+/-17.7 vs 332.6+/-48.5 ng/ml, p=0.008) and the placebo (166.6+/-34.3 vs 342.4+/-46.4 ng/ml, p=0.003) groups. However, plasma CRH, ACTH, and cortisol levels at delivery did not differ between the groups.

CONCLUSION

Mifepristone exposure and induced labor were associated with significant increases in plasma cortisol without alterations of systemic CRH or ACTH levels.