Increased expression of NLRP3 inflammasome in placentas from pregnant women with severe preeclampsia

Abstract

Preeclampsia is a pregnancy disorder characterized by imbalance between pro- and anti-inflammatory cytokines associated with high plasma levels of uric acid and Interleukin-1 beta (IL-1β). The inflammasome is a protein complex that mediates innate immune responses via caspase-1 activation promoting secretion of IL-1β and IL-18 in their active forms, and also release of the high-mobility group box 1 protein (HMGB1). As the placenta seems to play an important role in the pathogenesis of PE, the present study investigated the expression of genes and proteins related to the inflammasome in placentas from pregnant women with severe preeclampsia. Placental tissue was collected from 20 normotensive pregnant women and 20 preeclamptic women, and inflammasome components, NLRP3 (NOD-like receptor family, pyrin domain-containing protein 3), caspase-1, IL-1β and IL-18, as well as tumor necrosis factor-alpha (TNF-α) and HMGB1 were evaluated by immunohistochemistry, enzyme-linked immunosorbent assay (ELISA) and also quantified by reverse transcription-qPCR (RT-qPCR). Compared with normotensive pregnant women, placenta from women with PE showed a significant increase in NLRP3, caspase-1, IL-1β, TNF-α and HMGB1 mRNA. Immunohistochemical staining of NLRP3, caspase-1, IL-1β and TNF-α in placental villi, as well as the levels of caspase-1, IL-1β, TNF-α and HMGB1 in placental homogenate were significantly higher in the preeclamptic group than in the normotensive group. However, mRNA expression of IL-18 and its protein concentrations were lower in placentas from preeclamptic women. The results suggest that placentas from pregnant women with preeclampsia show higher expression of NLRP3 inflammasome, which may be involved in the exaggerated inflammatory state in preeclampsia.

Placental inflammation in pre-eclampsia by Nod-like receptor protein (NLRP)3 inflammasome activation in trophoblasts

“…The cell-specific co-localization of components essential for NLRP3 activity strongly supports a link between complement priming, NLRP3 inflammasome activation and IL-1b production in the syncytiotrophoblast. Our findings are sustained in a recent report showing NLRP3, caspase-1 and IL-1b protein expression in the syncytiotrophoblast layer [32] and by NLRP3 expression in placental lysates and primary trophoblasts [31,33,34,45,46]. Syncytium IL-1b production in first-and third-trimester placentas is in accordance with previous findings [47][48][49].…”

“…Studies of the NLRP3 inflammasome pathway in pre-eclampsia are limited. Recently, Weel et al [32] showed elevated NLRP3 and caspase-1 in placental tissue from pre-eclamptic pregnancies; however, our study is the first to quantify cell-specific NLRP3 protein expression in the syncytiotrophoblast layer. Our identification of elevated placental IL-1b expression in pre-eclampsia is in accordance with previous findings [31,32,50].…”

“…Recently, Weel et al [32] showed elevated NLRP3 and caspase-1 in placental tissue from pre-eclamptic pregnancies; however, our study is the first to quantify cell-specific NLRP3 protein expression in the syncytiotrophoblast layer. Our identification of elevated placental IL-1b expression in pre-eclampsia is in accordance with previous findings [31,32,50]. Maternal serum IL-1b in pre-eclampsia has been suggested to originate from an inflamed placenta [50][51][52], and our findings point to NLRP3 activation at the syncytium as a probable source.…”

“…NLRP3 inflammasome activation requires an already exaggerated inflammatory setting, as a priming signal is needed to achieve sufficient intracellular levels of NLRP3 and pro-IL-1b [15,54]. The choice of priming signal in the placenta was based on reports of elevated C5a and TNF-a in pre-eclampsia [28,32,[35][36][37][38][55][56][57], and supported by our discovery of C5a and TCC in the syncytium of pre-eclamptic placentas. The functional response confirms that C5a and TNF-a may serve as endogenous priming factors potentiating placental NLRP3 inflammasome activation.…”

“…It has been reported that maternal serum levels of cholesterol and uric acid are elevated in pre-eclampsia and FGR [21][22][23][24][25][26][27][28], that cholesterol is accumulated in pre-eclamptic placentas [29] and that uric acid crystals activate the NLRP3 inflammasome in human placentas [30]. Trophoblast NLRP3 activation has been assigned a role in pre-eclampsia development [31][32][33][34]. Raised serum and placental levels of IL-1b and excessive complement activation in women with pre-eclampsia [3,28,31,[35][36][37][38][39] further indicates a role for placental NLRP3 inflammasome involvement in this disease, but a detailed mechanistic outline with defined cellular involvement is missing.…”