The potential existence of ER
The potential existence of ER stress was first examined in EVTs in placental bed biopsy specimens obtained from pregnancies complicated by early-onset preeclampsia. It was then tested whether ER stress can modulate MMP-2 activity before elucidating the role of proinflammatory cytokines in the induction of ER stress and suppression of MMP-2 activity in extravillous-like trophoblastic cells. Finally, the molecular mechanisms by which the UPR pathways may regulate MMP-2 activity were explored.
Materials and Methods
Discussion ER stress has been demonstrated in both the placenta and decidua in cases of idiopathic FGR and early-onset preeclampsia.38, 43 In the placenta, the stress is likely a consequence of hypoxia-reperfusion injury triggered by insufficient remodeling of the spiral arteries. However, the trigger for the decidual stress is unknown, and its effect on pregnancy outcome has not been explored. In this study, proinflammatory cytokines were identified as a potential source of decidual ER stress, and the inhibitory role ER stress exerts on trophoblast invasion through modulation of MMP-2 activity was demonstrated. Furthermore, our results elucidated that the PERK arm of UPR signaling in ER can directly regulate MMP-2 at both the translational and transcriptional levels by PERK-EIF2A and ATF4, respectively. Overall, these results provide new insights into the molecular pathological mechanisms that may underpin cases of reduced trophoblast invasion, and hence spiral artery remodeling, in pregnancy complications. Several pregestational pathological conditions result in aberrant increases of proinflammatory cytokines within the uterine cavity and are associated with an increased risk of preeclampsia. For example, infection with Chlamydia trachomatis increases levels of IFN-γ in cervical secretions, the secretion of IL-1β and TNF-α in dendritic cells, and the risk of preeclampsia. Elevated levels of uterine proinflammatory cytokines likely induce ER stress in the invading EVTs, in a similar fashion to the way that injection of IL-1β and IL-6 induces ER stress in pancreatic Islet cells. Compromise of MMP activity will inhibit invasion into the deeper regions of the Tempol and myometrium, and the clinical outcome will be dependent on the severity of the subsequent deficit in arterial remodeling. Milder cases will result in preeclampsia and/or FGR, whereas severe cases will end in miscarriage. The severity of ER stress determines trophoblast cell fate; at low levels it reduces cell proliferation, whereas at high levels it induces apoptosis. The literature regarding regulation of trophoblast invasion by proinflammatory cytokines is contentious. Although many studies have revealed their inhibitory role, there is also evidence that the same cytokines may promote trophoblast migration and invasion.22, 23, 48, 49 In normal pregnancy, IL-1β (1 to 10 ng/mL) up-regulates the proteases MMPs and urokinase-type plasminogen activator systems to promote trophoblast motility,48, 49 whereas decidual natural killer cell–derived IFN-γ is necessary for spiral artery remodeling and placental formation.22, 23 On the other hand, inhibition of trophoblast invasion by IFN-γ associated with reduced secretion of MMP-2 has been reported. In addition, TNF-α inhibits trophoblast migration and integration into maternal endothelial cellular networks, which also involves the inhibition of MMP-2. In a rat model, TNF-α is causally linked to deficient trophoblast invasion and spiral artery remodeling, leading to features of preeclampsia and FGR. These results reveal the complexity of the regulation of trophoblast invasion by proinflammatory cytokines. Although the mechanisms behind these opposite roles are unknown, the concentration of the cytokines, their spatial and temporal profiles, their sources or origins (immune cells or endometrial cells), and their interactions with other cytokines may explain the differences.24, 50 Changes in proinflammatory cytokine profiles may also alter the interactions between decidual natural killer cells and trophoblast cells, thereby modulating the invasion process. The local milieu is therefore likely to be critical, but mimicking the precise conditions within the decidua in vivo is difficult in reductionist experimental situations. For example, TNF-α and IFN-γ, but not IL-1β, inhibited MMP-2 activity when administered individually, whereas a mixture of all three cytokines produced a synergistic effect (Figure 4). Crucially, similar effects were also observed in their capacity to activate phosphorylation of EIF2A, the PERK arm of UPR pathway in ER.