Pre-eclampsia and gestational age differently alter binding of endothelin-1 to placental and trophoblast membrane preparations

Temporal changes of the endothelin system in human cytotrophoblasts during the first trimester of pregnancy

The first trimester of pregnancy is characterized by continuous proliferation, invasion and differentiation of cytotrophoblasts. These processes are precisely controlled both, in space and time by molecules such as endothelin-1 (ET-1). ET-1 is expressed in human first trimester trophoblast and is known to stimulate cytotrophoblast proliferation through endothelin A and B receptor subtypes (ET(A) and ET(B)), and cytotrophoblast invasion through ET(B). However, temporal changes of the ET system during the first trimester of pregnancy have not been previously studied. This study tested the hypothesis that ET-1 release, ET(A) and ET(B) expression are increased towards the end of the first trimester of pregnancy (weeks 10-12 vs. weeks 6-9), resulting in increased cytotrophoblast proliferation and invasion. Tissue samples were obtained from 17 surgical pregnancy interruptions (week 6-9: n=9; week 10-12: n=8). After cytotrophoblast isolation, the invasive and proliferative phenotypes were immune-separated by an alpha(6)-integrin antibody. Both proliferative and invasive cytotrophoblasts were cultured separately on plastic or Matrigel for 24 h. ET-1 release into the culture medium of both cytotrophoblast subtypes was measured by radioimmunoassay. ET(A) and ET(B) mRNA expression was measured by RT-PCR, and the ET-1 effect on cytotrophoblast proliferation and invasion was determined using proliferation and invasion assays, respectively. ET-1 release increased from early to late first trimester of pregnancy in both proliferative (1.8-4.5 fold) and invasive cytotrophoblasts (9.3-28 fold), especially when cultured on Matrigel. This was paralleled by less ET(B) mRNA on invasive cytotrophoblasts independent of the time period in first trimester, whereas ET(A) expression was similar on proliferative an invasive cytotrophoblasts. Proliferation and invasion of cytotrophoblasts under control conditions decreased from early to late first trimester. ET-1 stimulated both processes at both periods with the most pronounced effect (7-fold) on invasion in late first trimester. The ET-1/ET-receptor system changes between weeks 6-9 and 10-12 in pregnancy. Our data suggest an autocrine and endocrine ET-1 effect, which is stronger in late than in early first trimester of pregnancy paralleled by different stimulatory effects on trophoblast invasion and proliferation. In general, this suggests time as an additional effector of the critical processes governing placental development in the first trimester of human pregnancy.

Endothelin-1 Attenuates Apoptosis in Cultured Trophoblasts From Term Human Placentas

The authors test the hypothesis that endothelin-1 (ET-1) modulates apoptosis in human term trophoblasts. Primary cultures of cytotrophoblasts from term human placentas (n = 5) were cultured for 16 hours total or 24 hours prior to harvest at 72 hours in atmospheres of <1%, 8%, and 20% oxygen, in the presence of 10% serum, ET-1 (1-100 pmol/mL), both, or neither. The apoptotic cleavage products of poly-ADP-ribose polymerase and cytokeratin 18 filaments were quantified by Western analysis and immunocytochemistry. The expression of BAD, pBAD-serine 112, p53, and 2 isoforms of MDM2 were quantified by immunoblotting, and endothelin A and B receptors were analyzed by immunocytochemistry. Compared to vehicle control, increasing concentrations of ET-1 reduce by 3- to 6-fold the level of apoptosis in cytotrophoblasts exposed to serum-free conditions at 20% oxygen. Similarly, syncytiotrophoblast cultures grown for 24 hours without serum in 100 pmol/mL ET-1 show a 3-fold lower level of apoptosis compared with vehicle control. ET-1 significantly reduces apoptosis in cultures exposed to 20% oxygen but not in cultures exposed to 8% or 1% oxygen. The effect of ET-1 on apoptosis in 20% oxygen is accompanied by reduced p53 expression and is correlated with enhanced expression of endothelin B receptor, compared to cultures in 8% or 1% oxygen. ET-1 reduces apoptosis in cultured human trophoblasts, and this finding suggests a role for ET-1 in protecting trophoblasts against injury.

Increased risk for the development of preeclampsia in obese pregnancies: weighing in on the mechanisms

Preeclampsia (PE) is a pregnancy-specific disorder typically presenting as new-onset hypertension and proteinuria. While numerous epidemiological studies have demonstrated that obesity increases the risk of PE, the mechanisms have yet to be fully elucidated. Growing evidence from animal and human studies implicate placental ischemia in the etiology of this maternal syndrome. It is thought that placental ischemia is brought about by dysfunctional cytotrophoblast migration and invasion into the uterus and subsequent lack of spiral arteriole widening and placental perfusion. Placental ischemia/hypoxia stimulates the release of soluble placental factors into the maternal circulation where they cause endothelial dysfunction, particularly in the kidney, to elicit the clinical manifestations of PE. The most recognized of these factors are the anti-angiogenic sFlt-1 and pro-inflammatory TNF-α and AT1-AA, which promote endothelial dysfunction by reducing levels of the provasodilator nitric oxide and stimulating production of the potent vasoconstrictor endothelin-1 and reactive oxygen species. We hypothesize that obesity-related metabolic factors increase the risk for developing PE by impacting various stages in the pathogenesis of PE, namely, 1) cytotrophoblast migration and placental ischemia; 2) release of soluble placental factors into the maternal circulation; and 3) maternal endothelial and vascular dysfunction. This review will summarize the current experimental evidence supporting the concept that obesity and metabolic factors like lipids, insulin, glucose, and leptin affect placental function and increase the risk for developing hypertension in pregnancy by reducing placental perfusion; enhancing placental release of soluble factors; and by increasing the sensitivity of the maternal vasculature to placental ischemia-induced soluble factors.

Endothelin A and B receptors change their expression levels during development of human placental villi

Endothelin receptors have recently been found in non-vascular tissues including the human placenta. The present study investigated developmental changes in location and expression levels of endothelin A and B receptors (ETA-R, ETB-R) in human placentae and isolated trophoblast by comparing first and third trimester tissues. In the first trimester all cells and tissues were immunolabelled for ETA-R and ETB-R, whereas in third trimester placentae the syncytiotrophoblast (ETA-R, ETB-R) and macrophages (ETA-R) were unstained. Immunoblotting for both receptors revealed up to three bands at 33-35, 50 and 75 kDa, respectively, which were differentially present in the first and third trimester. Pre-adsorption of antibodies with oligopeptides used for antigen-generation weakened the immunoreactions. ETA-R protein levels decreased (P< 0.05) in total villous tissue and isolated trophoblast, whereas ETB-R was unchanged. ETB-R transcripts (RT-PCR) prevailed in both stages and tissues, but in contrast to the protein levels its preponderance decreased from first trimester to term in villous tissue (P< 0.01), because of a four to five-fold increase in ETA-R and only a two-fold (P< 0.05) increase in ETB-R mRNA levels (P< 0.01). We conclude that ET receptor location, intracellular processing and expression levels in human villous tissue change between the first and third trimester. This may reflect changing functions of ET-1 during placental development.