Regulation of the human endogenous retroviral Syncytin‐1 and cell–cell fusion by the nuclear hormone receptors PPARγ/RXRα in placentogenesis

Abstract

Cytotrophoblast (CT) cell fusion into a syncytiotrophoblast is obligatory for placentation and mediated by the human endogenous retrovirus (HERV)-W envelope gene Syncytin-1. Abnormal placentation is associated with preeclampsia (PE), HELLP and intrauterine growth restriction (IUGR). In placentogenesis, the MAP-kinase p38α regulates PPARγ/RXRα signaling and target genes, like leptin, resistin, ABCG2, and hCG. The aim of this study was to analyze PPARγ/RXRα signaling and target gene regulation using primary CT cultures, the trophoblastic cell line BeWo and placental tissues from patients with normal and abnormal placentation. CT from four different human control placentae and BeWo cells demonstrated that Syncytin-1, other signaling members and CT cell fusions were regulated with PPARγ/RXRα activators troglitazone and 9-cis retinoic acid, via protein kinase A and p38α inhibition. Significant discordant regulations between CTs and BeWo were found. Two PPARγ/RXRα-response-elements from upstream regulatory elements and the 5'LTR of HERV-W were confirmed with DNA-protein binding assays using nuclear extracts and recombinant PPARγ/RXRα proteins. These promoter elements were validated with luciferase assays in the presence of PPARγ/RXRα modulators. Furthermore, troglitazone or 9-cis retinoic acid treatment of siRNA-PPARγ and siRNA-RXRα transfected BeWo cells proved the requirement of these proteins for Syncytin-1 regulation. Thirty primary abnormal placentae from PE, HELLP and IUGR patients compared to 10 controls showed significant deregulation of leptin RNA and protein, p38α, phospho-p38α, PPARγ, ABCG2, INSL4 and Syncytin-1. Our study characterized PPARγ/RXRα signaling in human CT and cell fusions identifying Syncytin-1 as a new target gene. Based on these results, a disturbed PPARγ/RXRα pathway could contribute to pathological human pregnancies.

Corticotropin-releasing hormone stimulates expression of leptin, 11beta-HSD2 and syncytin-1 in primary human trophoblasts

“…Stimulation with CRH (1.0 and 2.0 μg/ml) did not influence the amount of β-hCG in the supernatant at 48 and 72 h (Figure 1), indicating that CRH does not alter maturation of trophoblastic cells in vitro. Additionally we measured syncytin-1 (Syn1) expression, as previously described [33,35]. Syn1 is essential for mediating trophoblast cell fusion events [37].…”

“…Moreover, 11β-HSD2 expression was dose-dependently induced by 2.0 μg/ml CRH after 48 h. Formation of a functional syncytiotrophoblast occurred after 24 h in the CRH-stimulated and the control group to the same degree, as determined by increasing β-hCG secretion without a concomitant increase of cell lysis reflected by constant LDH levels in the supernatant throughout the experiment [31]. Syncytin-1 expression, a key regulator gene of trophoblast syncytialisation [33,35,37], was dose-dependently induced after 48 h of CRH stimulation only.…”

“…After DNase treatment, 1.0 mg RNA was transcribed into cDNA using M-MLV-RT (Promega, Madison, WI, USA) and Oligo dT-primer (MWG-Biotech AG, Ebersberg, Germany). DNase treatment and cDNA synthesis were carried out as previously [33,35]. …”

Activation of Peroxisome Proliferator Activator Receptor Delta in Mouse Impacts Lipid Composition and Placental Development at Early Stage of Gestation1

“…Among the PPAR family, the role of PPARG in regulating syncytin-1 mRNA expression as well as trophoblast differentiation and invasion is well established [58,59]. However, Ppard effects on cytotrophoblast differentiation in placentas are unknown.…”

Transglutaminase 2-dependent Deamidation of Glyceraldehyde-3-phosphate Dehydrogenase Promotes Trophoblastic Cell Fusion

“…In addition to the cAMP/PKA pathway, two mitogen-activated protein kinase (MAPK) family members, ERK1/2 and p38, are suggested to mediate trophoblast cell fusion and differentiation downstream from epidermal growth factor receptor activation. Induction of these MAPKs activates the PPAR␥/RXR␣ signal directly regulating syncytin-1 for cell fusion (10). Although syncytin is a key factor mediating cell fusion of cytotrophoblasts, many other proteins and signaling pathways, including those involved in cytoskeletal remodeling and degradation of adhesion proteins, also participate in trophoblast fusion, and the whole picture of the syncytialization process is not yet completely understood.…”