Hypoxia stimulates ecNOS mRNA expression by differentiated human trophoblasts

Oxygen-sensing pathways and the development of mammalian gas exchange

Oxygen-sensing pathways have been extensively explored in the context of homeostatic responses to hypoxic episodes; however, little is known of their involvement in the morphogenesis of respiratory structures (mitochondria, placenta, lung) during development in utero. This review identifies four essential loci where oxygen signalling pathways may cue the development of respiratory structures as: (i). mitochondrial biogenesis coupled with muted oxidative function dependent on the hypoxia-sustained production of NO; (ii). the generation of oxygen gradients which drive trophoblast differentiation and the formation of the chorionic gas exchange interface of the placenta; (iii). the proliferation and epithelial/endothelial differentiation of mesenchyme during the initiation of lung morphogenesis; and (iv). the regulation of epithelial fluid secretion/absorption in the lung. The identification of these oxygen-regulated developmental stages clarifies the close association between oxygen availability, reactive oxygen species and the morphogenesis of gas exchange structures and bears with it the implication that these pathways set the scope for aerobic metabolic performance throughout life.

Global protein synthesis in human trophoblast is resistant to inhibition by hypoxia

Placental growth and function depend on syncytial cell processes which require the continuing synthesis of cellular proteins. The substantial energy demands of protein synthesis are met primarily from oxidative metabolism. Although the responses of individual proteins produced by the syncytiotrophoblast to oxygen deprivation have been investigated previously, there is no information available on global protein synthesis in syncytiotrophoblast under conditions of hypoxia. These studies were designed to test the hypothesis that syncytial protein synthesis is decreased in a dose-dependent manner by hypoxia. Experiments were performed to measure amino acid incorporation into proteins in primary syncytiotrophoblast cells exposed to oxygen concentrations ranging from 0 to 10%. Compared to cells exposed to normoxia (10% O₂), no changes were observed following exposure to 5% or 3% O₂, but after exposure to 1% O₂, protein synthesis after 24 and 48 h decreased by 24% and 23% and with exposure to 0% O₂, by 65% and 50%. As a consequence of these results, we hypothesized that global protein synthesis in conditions of severe hypoxia was being supported by glucose metabolism. Additional experiments were performed therefore to examine the role of glucose in supporting protein synthesis. These demonstrated that at each oxygen concentration there was a significant, decreasing linear trend in protein synthesis as glucose concentration was reduced. Under conditions of near-anoxia and in the absence of glucose, protein synthesis was reduced by >85%. Even under normoxic conditions (defined as 10% O₂) and in the presence of oxidative substrates, reductions in glucose were accompanied by decreases in protein synthesis. These experiments demonstrate that syncytiotrophoblast cells are resistant to reductions in protein synthesis at O₂ concentrations greater than 1%. This could be explained by our finding that a significant fraction of protein synthesis in the syncytiotrophoblast is sustained by glycolytic metabolism. This suggests that with increasing degrees of chronic hypoxia there is a shift from oxidative to glycolytic pathways, allowing a substantial degree of protein synthesis to be maintained.

Maternal and fetal variants in the TGF-beta3 gene and risk of pregnancy-induced hypertension in a predominantly Latino population

OBJECTIVE

We sought to determine whether polymorphisms in the transforming growth factor (TGF)-beta3 gene are associated with risk of pregnancy-induced hypertension (PIH) in case-control mother-baby dyads.

STUDY DESIGN

Patients (n = 136) and control subjects (n = 169) were recruited from our hospital. We genotyped 4 TGF-beta3 polymorphisms and examined association with PIH using logistic regression, adjusting for parity, maternal age, gestational age at delivery, fetal (or maternal) genotypes for the polymorphism in question, and the 3 other polymorphisms within the TGF-beta3 gene.

RESULTS

Only 1 of the TGF-beta3 polymorphisms (rs11466414) was associated with PIH. Mothers who carried a baby with a minor allele were at decreased risk (odds ratio(multi-locus adj), 0.32; 95% confidence interval, 0.14-0.77). Maternal TGF-beta3 variants had no effect on risk of PIH.

CONCLUSION

A fetal TGF-beta3 polymorphism (rs11466414) is associated with PIH in a predominantly Hispanic population.

Gabor Than Award Lecture 2006: pre-eclampsia and villous trophoblast turnover: perspectives and possibilities

Placental apoptosis is exaggerated in pre-eclampsia and cytotrophoblast proliferation is enhanced. This imbalance may be a primary pathogenic event, whereby excessive syncytiotrophoblast apoptosis counters cytotrophoblast fusion, promoting the liberation of syncytial material which perturbs the maternal vascular endothelium. We have previously shown that primary trophoblasts and explant cultured villous fragments from pre-eclamptic pregnancies elicit greater levels of terminal differentiation and apoptosis. This review considers current opinions in trophoblast cell turnover in normal pregnancy and pre-eclampsia. In the context of other findings, this review highlights: (i) the disparity in expression of pro-apoptotic transcription factor p53 in the syncytiotrophoblast in pre-eclampsia, (ii) the importance of reactive oxygen species and hypoxia in initiating villous trophoblast apoptosis and (iii) the concept that aberrant intervillous haemodynamics, as opposed to oxygen per se, initiates excessive syncytiotrophoblast shedding. Finally, therapeutic ways of restoring the syncytiotrophoblast in pre-eclampsia and preventing excessive placental apoptosis are considered, including a role for mitotic manipulators and growth factor replacement strategies.

Chronic hypoxia in vivo reduces placental oxidative stress

Decreased placental oxygenation and increased oxidative stress are implicated in the development of preeclampsia. Oxidative stress arises from imbalance between pro-versus anti-oxidants and can lead to biological oxidation and apoptosis. Because pregnant women living at high altitude (3100 m, HA) have lowered arterial PO2 and an increased incidence of preeclampsia, we hypothesized that HA placentas would have decreased anti-oxidant enzyme activity, increased oxidative stress (lipid peroxidation, protein oxidation and nitration) and greater trophoblast apoptosis than low-altitude (LA) placentas. We measured enzymatic activities, lipid and protein oxidation and co-factor concentrations by spectrophotometric techniques and ELISA in 12 LA and 18 HA placentas. Immunohistochemistry (IHC) was used to evaluate nitrated proteins and specific markers of apoptosis (activated caspase 3 and M30). Superoxide dismutase activity was marginally lower (p=0.05), while glutathione peroxidase activity (p<0.05), thioredoxin concentrations (p<0.005) and thioredoxin reductase activity p<0.01 were all reduced in HA placentas. Decreased anti-oxidant activity was not associated with increased oxidative stress: lipid peroxide content and protein carbonyl formation were lower at HA (p<0.01). We found greater nitrotyrosine residues in the syncytiotrophoblast at 3100 m (p<0.05), but apoptosis did not differ between altitudes. Our data suggest that hypoxia does not increase placental oxidative stress in vivo. Nitrative stress may be a consequence of hypoxia but does not appear to contribute to increased apoptosis. Lowered placental concentrations of anti-oxidants may contribute to the susceptibility of women living at HA to the development of preeclampsia, but are unlikely to be etiological.

Hypoxia-induced leptin production in human trophoblasts does not protect from apoptosis

OBJECTIVE

The ob-gene product, leptin, is an important regulator of placental and fetal development during pregnancy. Leptin, being induced by hypoxia in the placenta, is a known pro-apoptotic molecule in adipose tissue but is also known to inhibit apoptosis in other tissues like neuroblastoma cells. Based on these findings, we investigated if leptin has a pro- or anti-apoptotic effect on a trophoblastic cell line (JAr cells) in the presence or absence of oxygen.

METHODS AND RESULTS

Measurement of leptin in the supernatant by using ELISA showed hypoxia-induced leptin production in JAr cells in vitro. This could be confirmed by a leptin-specific RT-PCR. By analyzing leptin and/or hypoxia exposed cells with FACS cytometry we found that JAr cells can cope with hypoxia down to oxygen tensions of 1%. At this level, only a small number of cells underwent apoptosis. Interestingly, leptin added to the culture medium in high concentrations was not able to interfere with the rate of proliferation or apoptosis in these cells independent of the oxygen tension. Finally, an anti-caspase-3 and anti-caspase-9 Western blot was performed. Again, no difference in the expression of caspase-3 and -9 under the conditions tested was seen.

CONCLUSIONS

These results show that leptin, produced by placental cells after hypoxia in vitro, has no influence on the rate of proliferation of these cells. Furthermore, it does not influence apoptotic pathways in the trophoblastic cell line tested under hypoxic and non-hypoxic conditions.

Hochachka's "Hypoxia Defense Strategies" and the development of the pathway for oxygen

Hochachka's "Hypoxia Defense Strategies" identify oxygen signalling, metabolic arrest, channel arrest and coordinated suppression of ATP turnover rates as key factors that determine the ability of organisms to survive exposure to chronic hypoxia. In this review, I assess the developmental role played by these phenomena in the morphogenesis of the gas exchange tissues that define the pathway for oxygen transport to cytochrome c oxidase. Key areas of regulation lie in: (I) the suppression of fetal mitochondrial oxidative function in hand with mitochondrial biogenesis (metabolic arrest), (II) the role of hypoxia-driven oxygen signalling pathways in directing the scope of non-differentiated stem cell proliferation in placenta and lung development and (III) the regulation of epithelial fluid secretion/absorption in the lung through the oxygen-dependent modulation of Na+ conductance pathways. The identification of developmental roles for Hochachka's "Hypoxia Defense Strategies" in directing the morphogenesis of gas exchange structures bears with it the implication that these strategies are fundamental to establishing the scope for aerobic metabolic performance throughout life.

Glutathione peroxidase levels throughout normal pregnancy and in pre-eclampsia

OBJECTIVE

Evidence suggests that hemoglobin, in addition to its function as a carrier of oxygen, also serves to transport nitric oxide, as S-nitroso cysteine, from the lungs to the peripheral circulation, where it can be released. Glutathione peroxidase, besides being an important antioxidant, is known to catalyze the release of nitric oxide from smaller carrier molecules, and may play a role in the distribution of nitric oxide throughout the body. In light of these findings, we sought to determine whether glutathione peroxidase levels differed throughout gestation, and specifically between pre-eclamptic and normal women.

METHODS

A nested case-control study of women receiving routine prenatal care was conducted. Pre-eclampsia was defined by a blood pressure of at least 140 mmHg systolic and/or 90 mmHg diastolic as well as proteinuria > 300 mg/24 h or > 2+ by dipstick, both occurring on two occasions at least 6 h apart. Blood was collected in heparinized tubes and was then centrifuged in a clinical centrifuge for 10 min. Plasma was frozen promptly at -80 degrees C for later enzyme-linked immunosorbent assay (ELISA), with which plasma glutathione peroxidase was determined.

RESULTS

The maternal demographics of the pre-eclamptic and non-pre-eclamptic study groups did not significantly vary with respect to mean maternal age, gravidity, parity and gestational age at the time of delivery. The median maternal ages were 33 and 34 years, and the median gestational ages at the time of birth were 37.5 and 38.1 weeks, respectively. In evaluating the glutathione peroxidase levels of all patients across the three trimesters, we found that there was essentially no difference in mean levels (83.7, 81.0 and 89.5 ng/ml, respectively). There was no difference between the pre-eclamptic and non-pre-eclamptic patients, again stratified by trimester. A linear regression analysis indicated that the plasma glutathione peroxidase concentration did not correlate with gestational age or the presence of pre-eclampsia.

CONCLUSIONS

Plasma glutathione peroxidase expression is similar across all trimesters. There is no change in the glutathione peroxidase levels in pre-eclamptic patients.

Selective overexpression of the hypoxia-inducible transcription factor, HIF-2alpha, in placentas from women with preeclampsia

Transcription factors orchestrate the development of extraembryonic tissues. Because placental hypoxia likely plays an important role in both normal and abnormal placentation, we have been investigating the hypoxia-inducible transcription factors (HIFs) in the human placenta. In this report, we focus on the placentas from women with preeclampsia. Because the placenta is a large, heterogeneous organ, we employed a systematic and unbiased approach to placental sampling, and our results are based on the analyses of eight biopsy sites per placenta. We observed no significant differences in HIF-1alpha or -2alpha mRNA expression between normal term and preeclamptic placentas. Nor was HIF protein expression significantly different, with the notable exception of HIF-2alpha, which, on average, was increased by 1.7-fold in the preeclamptic placentas (P: < 0.03 vs. normal term placentas). Considering all 48 paired placental biopsy sites (eight sites each for six normal term and six preeclamptic placentas), HIF-2alpha protein levels in the preeclamptic placentas exceeded those in the normal term placentas in 39, or 81%, of the paired sites (P: < 0.0013). The HIF-2alpha immunoreactivity was mainly located in the nuclei of the syncytiotrophoblast and fetoplacental vascular endothelium in the preeclamptic villous placenta. To control for the earlier gestational age of the preeclamptic placentas, an additional group of placentas from preterm deliveries without preeclampsia were also evaluated. The HIF protein expression was comparable in these preterm specimens and the normal term placentas. We conclude that protein expression of HIF-2alpha, but not of HIF-1alpha or -1beta, is selectively increased in the preeclamptic placenta. The molecular mechanism(s) of this abnormality as well as the genes affected downstream are currently under investigation. To our knowledge, this is the first report of abnormal HIF-2alpha expression in human disease other than cancer.

Adhesion development and the expression of endothelial nitric oxide synthase

OBJECTIVE

This study was conducted to determine whether nitric oxide (NO), a potent vasodilator and inhibitor of thrombus formation, is involved in the formation and maintenance of adhesions.

METHODS

Skin, subcutaneous tissues, peritoneum and adhesions were collected from surgical patients and total RNA was isolated. Quantitative reverse transcription polymerase chain reaction (QRT-PCR) was performed to quantitate endothelial nitric oxide synthase (eNOS) and beta-actin mRNA levels.

RESULTS

eNOS mRNA levels for skin, subcutaneous tissue, peritoneum and adhesions were < or = 3.12 x 10(-4), < or = 3.12 x 10(-4), 6.24 x 10(-4) and 2.5 x 10(-3) attomoles/microl, respectively. Beta-actin mRNA levels for all tissues were between 1.25 x 10(-1) and 6.25 x 10(-2) attomoles/microl.

CONCLUSION

eNOS mRNA can be identified in tissue adhesions, and may therefore play a role in adhesion formation and maintenance.

Expression of angiopoietin-2 by human endometrial endothelial cells: regulation by hypoxia and inflammation

The functional endometrial layer receives the implanting blastocyst, but is sloughed off during menstruation. Angiogenesis regulates growth and repair of cycling human endometrium. While vascular endothelial growth factor initiates angiogenesis, the angiopoietins (Angs) acting via the Tie2 receptor, are key regulators of subsequent angiogenic steps. This study is the first to localize Ang-2 and Tie2 in human endometrium and to study Ang-2 regulation in cultured human endometrial endothelial cells (HEECs). Immunohistochemistry revealed that expression of Ang-2 and Tie2 was absent from the glands, low in stromal cells, and intense in the endothelial cells. In contrast, only weak expression of Ang-1 was detected. The phase of the menstrual cycle did not appear to affect the expression of Ang-2 or Tie2. In vitro studies were carried out utilizing isolated HEECs, the most relevant model for endometrial microvascular biology studies. Both hypoxia and phorbol-myristate-acetate enhanced Ang-2 mRNA levels in HEECs. These results suggest that Ang-2 plays a role in endometrial pathologies complicated by impaired blood flow and inflammation.

Expression, ontogeny, and regulation of hypoxia-inducible transcription factors in the human placenta

Placental hypoxia likely plays an important role in both normal placental development and pathology. Yet, the molecular mechanisms of hypoxia signaling in this organ are virtually unexplored. Therefore, we investigated the expression of the hypoxia inducible transcription factors (HIF) in normal human placentas spanning the first trimester to term. Several key observations emerged: 1) HIF-1 alpha and -2 alpha mRNA were present in placentas of all gestational ages but with greater variability during early pregnancy; 2) overall, HIF-1 alpha mRNA was expressed at a constant level in all placentas, whereas HIF-2 alpha mRNA increased significantly with gestational age; 3) both HIF-1 alpha and -2 alpha protein decreased significantly with gestational age; and 4) HIF-1 alpha and -2 alpha immunoreactivity were overlapping in cellular distribution being expressed by the syncytiotrophoblast, villous cytotrophoblast, and fetoplacental vasculature with both nuclear and cytoplasmic localization. Next, we studied the regulation of these transcription factors by oxygen using placental villous explants in culture from first-trimester and term placentas. The major findings were 1) HIF-1 alpha and -2 alpha protein, but not mRNA, was induced by hypoxia in the placental villous explants; 2) HIF-1 alpha DNA-binding activity was also stimulated by hypoxia; and 3) glucose transporter-1 mRNA (a known target of HIF) was also increased by hypoxia in placental villous explants. We suggest that physiological hypoxia contributes to the increased expression of HIF-1 alpha and -2 alpha protein in early placentas and that regulation of these transcription factors by hypoxia in the human placenta occurs at the level of protein and not mRNA.

Placental oxygen consumption. Part II: in vitro studies--a review

The human placenta in vivo consumes large amounts of oxygen. After delivery the placenta is exposed to anoxia and under in vitro experimental conditions oxygen consumption is only a fraction of in vivo estimates. In spite of a reduced oxygen supply, structural and functional integrity of the tissue is surprisingly well preserved. Special metabolic adjustments in the sense of 'partial metabolic arrest' may be the explanation for a remarkable survival capacity of placental tissue and reduction of protein synthesis seems to be an important component of metabolic slowdown. The potential significance of this special feature of placental metabolism for the in vivo situation is discussed.

Oxygen and placental vascular development

Human embryogenesis takes place in a hypoxic environment because the trophoblast shell excludes entry of maternal blood. The first fetal-placental villi develop as trophoblast sprouts. These are invaded by allantoic mesoderm to form secondary villi and are transformed, by vasculogenesis, into tertiary villi. The placental barrier to maternal blood is gradually breached between 8-12 weeks of gestation, due to invasion of placental-bed uteroplacental spiral arteries by the extravillous trophoblast (EVT). Placental oxygen tension thus rises and a phase of branching angiogenesis continues until 24 weeks. Thereafter a gradual shift takes place favoring non-branching angiogenesis. Gas-exchanging terminal villi thus form which are essential for rapid fetal growth and development of a high-flow, low-resistance fetal-placental circulation. Inadequate invasion of the uteroplacental spiral arteries by EVT results in placental ischemia and the development of obstetrical complications--preeclampsia and/or intrauterine growth restriction (IUGR). Placental villi often show evidence of continued branching angiogenesis, as is the case with anemic pregnancy, and pregnancy at high altitude. These structural alterations may reflect continued hypoxia-driven activity of vascular endothelial growth factor (VEGF). By contrast, a minority of severe early-onset IUGR pregnancies exhibit reduced fetal-placental blood flow with elongated maldeveloped villous capillaries. Placenta-like growth factor (PIGF) expression is increased while trophoblast proliferation is reduced, suggesting "hyperoxia" in the placental villous tree. IUGR may thus have two phenotypes--a more common hypoxic and a rarer hyperoxic type. While this concept is gaining acceptance, we have no insight as to the initiating mechanism(s).

Expression of endothelial nitric oxide synthase by extravillous trophoblast cells in the human placenta

Previous reports have documented the expression of endothelial nitric oxide synthase (eNOS) expression by the syncytiotrophoblast layer of the villus in the human placenta. In contrast, the underlying villous cytotrophoblast cells do not express the enzyme. Because extravillous cytotrophoblasts have not been as extensively investigated, our objective was to test whether these cells express eNOS. Using both a mouse monoclonal and a rabbit polyclonal antibody, we demonstrated immunoreactive eNOS in trophoblast cell columns emanating from anchoring villi in second trimester placentae. Cytokeratin positive trophoblast cells lying beneath remnant anchoring villi, lining decidual blood vessels and scattered throughout the basal plate of normal term and pre-eclamptic placentae also expressed immunoreactive eNOS. By Western analysis, the monoclonal and polyclonal antibodies were shown to be absolutely and relatively specific for eNOS, respectively. The finding of immunoreactive eNOS expression by extravillous trophoblast cells was substantiated by in situ hybridization. Using riboprobes generated from a bovine eNOS cDNA, we demonstrated specific hybridization in the endothelium of blood vessels in the umbilical cord, thus validating the in situ hybridization methodology, as well as specific hybridization in the extravillous trophoblast cells of the basal plate in normal term placenta. In conclusion, several different populations of extravillous trophoblast cells in the basal plate of the human placenta express eNOS.

Expression of endothelial (type III) nitric oxide synthase in cytotrophoblastic cell lines: regulation by hypoxia and inflammatory cytokines

Expression of endothelial nitric oxide synthase (eNOS) has been localized to the villous syncytiotrophoblasts suggesting that NO release from these cells could prevent platelet adhesion and aggregation in the intervillous space. Hypoxia- or inflammation-dependent changes in the release of this vasoactive substance may result in thrombus formation and altered vascular resistance which occur in the placental bed of pre-eclamptic patients. To evaluate the influence of low-oxygen tension and inflammation on eNOS production in the trophoblast steady-state eNOS mRNA and protein levels were investigated in cytotrophoblastic BeWo and Jeg-3 cells cultured at 3.5 per cent oxygen and/or in the presence of the pro-inflammatory cytokines IL-1 and TNF-alpha. By RT-PCR and immunocytochemistry we demonstrate that BeWo cells produce eNOS mRNA and protein while eNOS polypeptide was undetectable in JEG-3 cells. In BeWo cells addition of both cytokines decreases eNOS mRNA and protein abundancies within 24 h of incubation while each substance alone had no effect. Compared to controls, the amount of eNOS transcripts was found to be elevated at low-oxygen tension, however, cNOS protein was downregulated after 24 h in the hypoxic environment, as shown by immunocytochemistry and Western blot analysis. Forskolin and methotrexate, which induce biochemical differentiation/ growth arrest in choriocarcinoma cells, stimulate eNOS mRNA and protein synthesis, but cannot overcome the decline of eNOS polypeptide levels during hypoxic incubation. It is speculated that acute hypoxia and inflammation impair eNOS/NO production of the trophoblast in vivo, which might contribute to pathological conditions of gestational diseases.