The role of decidual cells in uterine hemostasis, menstruation, inflammation, adverse pregnancy outcomes and abnormal uterine bleeding

Abstract

Background: Human pregnancy requires robust hemostasis to prevent hemorrhage during extravillous trophoblast (EVT) invasion of the decidualized endometrium, modification of spiral arteries and post-partum processes. However, decidual hemorrhage (abruption) can occur throughout pregnancy from poorly transformed spiral arteries, causing fetal death or spontaneous preterm birth (PTB), or it can promote the aberrant placentation observed in intrauterine growth restriction (IUGR) and pre-eclampsia; all leading causes of perinatal or maternal morbidity and mortality. In non-fertile cycles, the decidua undergoes controlled menstrual bleeding. Abnormal uterine bleeding (AUB) accompanying progestin-only, long-acting, reversible contraception (pLARC) accounts for most discontinuations of these safe and highly effective agents, thereby contributing to unwanted pregnancies and abortion. The aim of this study was to investigate the role of decidual cells in uterine hemostasis, menstruation, inflammation, adverse pregnancy outcomes and abnormal uterine bleeding.

Methods: We conducted a critical review of the literature arising from PubMed searches up to December 2015, regarding in situ and in vitro expression and regulation of several specific proteins involved in uterine hemostasis in decidua and cycling endometrium. In addition, we discussed clinical and molecular mechanisms associated with pLARC-induced AUB and pregnancy complications with abruptions, chorioamnionitis or pre-eclampsia.

Results: Progestin-induced decidualization of estradiol-primed human endometrial stromal cells (HESCs) increases in vivo and in vitro expression of tissue factor (TF) and type-1 plasminogen activator inhibitor (PAI-1) while inhibiting plasminogen activators (PAs), matrix metalloproteinases (MMPs), and the vasoconstrictor, endothelin-1 (ET-1). These changes in decidual cell-derived regulators of hemostasis, fibrinolysis, extracellular matrix (ECM) turnover, and vascular tone prevent hemorrhage during EVT invasion and vascular remodeling. In non-fertile cycles, progesterone withdrawal reduces TF and PAI-1 while increasing PA, MMPs and ET-1, causing menstrual-associated bleeding, fibrinolysis, ECM degradation and ischemia. First trimester decidual hemorrhage elicits later adverse outcomes including pregnancy loss, pre-eclampsia, abruption, IUGR and PTB. Decidual hemorrhage generates excess thrombin that binds to decidual cell-expressed protease-activated receptors (PARs) to induce chemokines promoting shallow placentation; such bleeding later in pregnancy generates thrombin to down-regulate decidual cell progesterone receptors and up-regulate cytokines and MMPs linked to PTB. Endometria of pLARC users display ischemia-induced excess vasculogenesis and progestin inhibition of spiral artery vascular smooth muscle cell proliferation and migration leading to dilated fragile vessels prone to bleeding. Moreover, aberrant TF-derived thrombin signaling also contributes to the pathogenesis of endometriosis via induction of angiogenesis, inflammation and cell survival.

Conclusion: Perivascular decidualized HESCs promote endometrial hemostasis during placentation yet facilitate menstruation through progestational regulation of hemostatic, proteolytic, and vasoactive proteins. Pathological endometrial hemorrhage elicits excess local thrombin generation, which contributes to pLARC associated AUB, endometriosis and adverse pregnancy outcomes through several biochemical mechanisms.

Keywords: contraception; decidual cells; pre-eclampsia; preterm birth; thrombin; tissue factor; uterine bleeding.

Figure 1

Schematic demonstration of the coagulation cascade. The tissue factor (TF)–factor (F)VIIa complex of the extrinsic pathway initiates blood coagulation by activating factor (F) X directly or indirectly by activating FIX of the intrinsic pathway. Thrombin plays a pivotal role in this process by activating several proteases and cofactors. Specifically, thrombin cleaves fibrinogen to soluble fibrin monomers. These are cross-linked by FXIIa and thrombin activated platelets (not shown) to form the fibrin clot. Fibrin is degraded into the fibrin degradation products (FDP) by plasmin formed by tissue type plasminogen activator (tPA). The action of tPA is regulated by its fast inactivator, plasminogen activator inhibitor type (PAI)-1.

Figure 2

Role of tissue factor generated thrombin in the pathogenesis of endometriosis. Expression of tissue factor (TF) is limited to endometrial stromal cells in normal endometrium, whereas in endometriosis, in addition to endometrial stromal cells TF is also expressed by epithelial and endothelial cells. TF expressed by stromal and epithelial cells generates excess thrombin by cleaving prothrombin present in retrograde menstrual cell debris and blood. Additional prothrombin cleavage occurs by TF expressed by endometriotic endothelial cells. The resulting excess thrombin binds to Protease-Activated Receptors (PARs) in endometriotic stromal cells to induce thrombin-mediated signaling cascades that initiate secretion of several cytokines and growth factors, specifically IL-8 and VEGF. Both cytokines trigger endometriotic angiogenesis, and IL-8 also recruits leukocytes, dominated by neutrophils. During retrograde menstruation, hypoxia also promotes vascularization of endometriotic implants. TF: tissue factor/factor VII/VIIa.

Figure 3

Cellular and molecular mechanisms leading to preterm birth. In the presence of decidual hemorrhage, thrombin generated from decidual cell-expressed TF activates NF-κB and p38 MAPK signaling pathways. Both pathways enhance production of several cytokines (IL-8, IL-11, CSF2, etc.), which recruit neutrophils that elaborate various matrix metalloproteinases (MMPs) as well as other proteinases. Thrombin directly enhances MMP-1 and -3 production in decidual cells. Collectively, these proteinases promote fetal membrane rupture and cervical change. Thrombin also suppresses decidual cell progesterone receptor (PR) levels by increasing ERK1/2 phosphorylation. The resulting functional progesterone withdrawal in the decidua further induces MMP expression and decidual inflammation. In the case of chorioamnionitis, pro-inflammatory mediators (IL-1β or TNF-α) and/or bacterial products (e.g. endotoxins) activate NF-κB, ERK1/2 and p38 MAPK mediated signaling pathways in decidual cells. NF-κB and p38 MAPK activation enhances production of cytokines (IL-8, IL6, IL11, etc.) and expression of MMP-1, -3, and -9 as well as cyclooxygenase (COX2), which in turn induce prostaglandins (PG) production in decidual cells. Moreover, the activated ERK1/2 MAPK signaling cascade lowers PR isoform levels causing functional progesterone withdrawal. Taken together, functional progesterone withdrawal, MMPs and increased PG production trigger uterine contractions, fetal membrane rupture and cervical ripening leading to chorioamnionitis-associated preterm birth (PTB).

Figure 4

Cellular mechanisms of AUB in progestin-only, long-acting, reversible contraception (pLARC). Administration of pLARCs induces human endometrial stromal cell (HESC) decidualization and reduces endometrial blood flow causing local hypoxia. In decidualized HESCs, this hypoxia inhibits expression of angiopoietin (Ang)-1 while increasing expression of vascular endothelial growth factor (VEGF), interleukin (IL)-8 and matrix metalloproteinases (MMPs). In human endometrial epithelial cells (HEECs), hypoxia and ROS increase expression of Ang-2, a potent angiogenic agent. Hypoxia and reperfusion generate reactive oxygen species (ROS) that directly damages blood vessels, resulting in enhanced vascular permeability. The latter delivers circulating factor VII to HESC membrane bound tissue factor (TF), which activates factor Xa to generate thrombin. The resulting excess thrombin exacerbates hypoxia/ROS effects on endometrial angiogenesis and inflammation. pLARCs-induced hypoxia/ROS enhances HESC secreted NPTX1 which trigger HEEC apoptosis by inhibiting AKT signaling, causing mitochondrial dysfunction, enhancing cytochrome c release, and activating a caspase 3 mediated cascade. Moreover, pLARCs directly inhibit vascular smooth muscle cells (VSMC) proliferation and migration by blocking CCL2-mediated STAT1 signaling. The combined impact of these molecular mechanisms promotes aberrant angiogenesis and endothelial apoptosis, and reduces numbers of VSMCs to generate thin-walled hyperdilated fragile vessels prone to leakage and bleeding.