Academic literature on the topic 'Placental hypoxia/reoxygenation'

It is now over half a century since Arthur Hertig first reported vascular pathology in the uterine arteries supplying the placenta in women suffering from preeclampsia. His pioneering histological studies have been validated and extended by many others, leading to the general concept that placental perfusion is compromised in these patients. More recent Doppler ultrasound studies have confirmed reduced intervillous blood flow in vivo, and so gradually a consensus has emerged that the placental lesions associated with preeclampsia arise from a state of chronic hypoxia. Whilst hypoxia may undoubtedly play a significant role in the generation of placental pathology, there is considerable evidence that another feature of the intervillous circulation, namely the constancy of the blood flow, may be a more important factor. In this review we propose that hypoxia-reoxygenation, secondary to intermittent perfusion of the intervillous space, is a more physiological approach to take to understanding the pathophysiology of both normal pregnancies, and those complicated by preeclampsia. We further propose that chronic reduction in placental perfusion alone may lead to fetal growth restriction, and that if the two phenomena are superimposed then preeclampsia with growth restriction will result.

An increase in fetoplacental vascular resistance caused by hypoxia is considered one of the key factors of placental hypoperfusion and fetal undernutrition leading to intrauterine growth restriction (IUGR), one of the serious problems in current neonatology. However, although acute hypoxia has been shown to cause fetoplacental vasoconstriction, the effects of more sustained hypoxic exposure are unknown. This study was designed to test the hypothesis that chronic hypoxia elicits elevations in fetoplacental resistance, that this effect is not completely reversible by acute reoxygenation, and that it is accompanied by increased acute vasoconstrictor reactivity of the fetoplacental vasculature. We measured fetoplacental vascular resistance as well as acute vasoconstrictor reactivity in isolated perfused placentae from rats exposed to hypoxia (10% O2) during the last week of a 3-wk pregnancy. We found that chronic hypoxia shifted the relationship between perfusion pressure and flow rate toward higher pressure values (by ∼20%). This increased vascular resistance was refractory to a high dose of sodium nitroprusside, implying the involvement of other factors than increased vascular tone. Chronic hypoxia also increased vasoconstrictor responses to angiotensin II (by ∼75%) and to acute hypoxic challenges (by >150%). We conclude that chronic prenatal hypoxia causes a sustained elevation of fetoplacental vascular resistance and vasoconstrictor reactivity that are likely to produce placental hypoperfusion and fetal undernutrition in vivo.

Background: We have demonstrated that protein aggregation plays a pivotal role in the pathophysiology of preeclampsia and identified several aggregated proteins in the circulation of preeclampsia patients, the most prominent of which is the serum protein TTR (transthyretin). However, the mechanisms that underlie protein aggregation remain poorly addressed. Methods: We examined TTR aggregates in hypoxia/reoxygenation-exposed primary human trophoblasts (PHTs) and the preeclampsia placenta using complementary approaches, including a novel protein aggregate detection assay. Mechanistic analysis was performed in hypoxia/reoxygenation-exposed PHTs and Ttr transgenic mice overexpressing transgene-encoded wild-type human TTR or Ttr −/− mice. High-resolution ultrasound analysis was used to measure placental blood flow in pregnant mice. Results: TTR aggregation was inducible in PHTs and the TCL-1 trophoblast cell line by endoplasmic reticulum stress inducers or autophagy-lysosomal disruptors. PHTs exposed to hypoxia/reoxygenation showed increased intracellular BiP (binding immunoglobulin protein), phosphorylated IRE1α (inositol-requiring enzyme-1α), PDI (protein disulfide isomerase), and Ero-1, all markers of the unfolded protein response, and the apoptosis mediator caspase-3. Blockade of IRE1α inhibited hypoxia/reoxygenation-induced upregulation of Ero-1 in PHTs. Excessive unfolded protein response activation was observed in the early-onset preeclampsia placenta. Importantly, pregnant human TTR mice displayed aggregated TTR in the junctional zone of the placenta and severe preeclampsia-like features. High-resolution ultrasound analysis revealed low blood flow in uterine and umbilical arteries in human TTR mice compared with control mice. However, Ttr −/− mice did not show any pregnancy-associated abnormalities. Conclusions: These observations in the preeclampsia placenta, cultured trophoblasts, and Ttr transgenic mice indicate that TTR aggregation is an important causal contributor to preeclampsia pathophysiology.

Elevated expression of soluble vascular endothelial growth factor receptor-1 (sFlt-1) in preeclampsia plays a major role in the pathogenesis of this serious disorder of human pregnancy. Although reduced placental oxygenation is thought to be involved in the pathogenesis of preeclampsia, it is unclear how oxygen regulates placental sFlt-1 expression. The aims herein were to investigate sFlt-1 expression in in vivo and in vitro physiological and pathological models of human placental hypoxia and to understand the role of hypoxia inducible factor-1 (HIF-1) in regulating the expression of this molecule. sFlt-1 expression in placental villi was significantly increased under physiological low oxygen conditions in early first-trimester and in high-altitude placentae, as well as in pathological low oxygen conditions, such as preeclampsia. In high-altitude and in preeclamptic tissue, sFlt-1 localized within villi to perivascular regions, the syncytiotrophoblast layer, and syncytial knots. In first-trimester villous explants, low oxygen, but not hypoxia-reoxygenation (HR), increased sFlt-1 expression. Moreover, exposure of villous explants to dimethyloxalyl-glycin, a pharmacological inhibitor of prolyl-hydroxylases, which mimics hypoxia by increasing HIF-1α stability, increased sFlt-1 expression. Conversely, HIF-1α knockdown using antisense oligonucleotides, decreased sFlt-1 expression. In conclusion, placental sFlt-1 expression is increased by both physiologically and pathologically low levels of oxygen. This oxygen-induced effect is mediated via the transcription factor HIF-1. Low oxygen levels, as opposed to intermittent oxygen tension (HR) changes, play an important role in regulating sFlt-1 expression in the developing human placenta and hence may contribute to the development of preeclampsia.

Preeclampsia (PE) is a pregnancy-specific disorder that affects 3 to 5% of pregnancies worldwide and is one of the leading causes of maternal and fetal morbidity and mortality. Nevertheless, how these events occur remains unclear. We hypothesized that the induction of hypoxic conditions in vitro in primary human trophoblast cells would mimic several characteristics of PE found in vivo. We applied and characterized a model of primary cytotrophoblasts isolated from healthy pregnancies that were placed under different oxygen concentrations: ambient O2 (5% pCO2, 21%pO2, 24 h, termed “normoxia”), low O2 concentration (5% pCO2, 1.5% pO2, 24 h, termed “hypoxia”), or “hypoxia/reoxygenation” (H/R: 6 h intervals of normoxia and hypoxia for 24 h). Various established preeclamptic markers were assessed in this cell model and compared to placental tissues obtained from PE pregnancies. Seventeen PE markers were analyzed by qPCR, and the protein secretion of soluble fms-like tyrosine kinase 1 (sFlT-1) and the placenta growth factor (PlGF) was determined by ELISA. Thirteen of seventeen genes associated with angiogenesis, the renin–angiotensin system, oxidative stress, endoplasmic reticulum stress, and the inflammasome complex were susceptible to H/R and hypoxia, mimicking the expression pattern of PE tissue. In cell culture supernatants, the secretion of sFlT-1 was increased in hypoxia, while PlGF release was significantly reduced in H/R and hypoxia. In the supernatants of our cell models, the sFlT-1/PlGF ratio in hypoxia and H/R was higher than 38, which is a strong indicator for PE in clinical practice. These results suggest that our cellular models reflect important pathological processes occurring in PE and are therefore suitable as PE in vitro models.

We have previously described that placental activation of autophagy is a central feature of normal pregnancy, whereas autophagy is impaired in preeclampsia (PE). Here, we show that hypoxia–reoxygenation (H/R) treatment dysregulates key molecules that maintain autophagy–lysosomal flux in primary human trophoblasts (PHTs). Ultrastructural analysis using transmission electron microscopy reveals a significant reduction in autophagosomes and autolysosomes in H/R-exposed PHTs. H/R-induced accumulation of protein aggregates follows a similar pattern that occurs in PHTs treated with a lysosomal disruptor, chloroquine. Importantly, the placenta from early-onset PE deliveries exhibits the same features as seen in H/R-treated PHTs. Taken together, our results indicate that H/R disrupts autophagic machinery in PHTs and that impaired autophagy in the placenta from early-onset PE deliveries mimics the events in H/R-treated PHTs. Notably, assessment of key regulators at each stage of autophagic processes, especially lysosomal integrity, and verification of autophagic ultrastructure are essential for an accurate evaluation of autophagy activity in human trophoblasts and placental tissue from PE deliveries.

The recently identified ferroptotic cell death is characterized by excessive accumulation of hydroperoxy-arachidonoyl (C20:4)- or adrenoyl (C22:4)- phosphatidylethanolamine (Hp-PE). The selenium-dependent glutathione peroxidase 4 (GPX4) inhibits ferroptosis, converting unstable ferroptotic lipid hydroperoxides to nontoxic lipid alcohols in a tissue-specific manner. While placental oxidative stress and lipotoxicity are hallmarks of placental dysfunction, the possible role of ferroptosis in placental dysfunction is largely unknown. We found that spontaneous preterm birth is associated with ferroptosis and that inhibition of GPX4 causes ferroptotic injury in primary human trophoblasts and during mouse pregnancy. Importantly, we uncovered a role for the phospholipase PLA2G6 (PNPLA9, iPLA2beta), known to metabolize Hp-PE to lyso-PE and oxidized fatty acid, in mitigating ferroptosis induced by GPX4 inhibition in vitro or by hypoxia/reoxygenation injury in vivo. Together, we identified ferroptosis signaling in the human and mouse placenta, established a role for PLA2G6 in attenuating trophoblastic ferroptosis, and provided mechanistic insights into the ill-defined placental lipotoxicity that may inspire PLA2G6-targeted therapeutic strategies.

Preeclampsia (PE) is a dangerous complication of pregnancy, especially when it presents at <34 wk of gestation (PE < 34 wk). It is a major cause of maternal and fetal morbidity and mortality and also increases the risk of cardiometabolic diseases in later life for both mother and offspring. Placental oxidative stress induced by defective placentation sits at the epicenter of the pathophysiology. The placenta is susceptible to activation of the unfolded protein response (UPR), and we hypothesized this may affect mitochondrial function. We first examined mitochondrial respiration before investigating evidence of mitochondrial UPR (UPRmt) in placentas of PE < 34 wk patients. Reduced placental oxidative phosphorylation (OXPHOS) capacity measured in situ was observed despite no change in protein or mRNA levels of electron transport chain complexes. These results were fully recapitulated by subjecting trophoblast cells to repetitive hypoxia–reoxygenation and were associated with activation of a noncanonical UPRmt pathway; the quality-control protease CLPP, central to UPRmt signal transduction, was reduced, while the cochaperone, TID1, was increased. Transcriptional factor ATF5, which regulates expression of key UPRmt genes including HSP60 and GRP75, showed no nuclear translocation. Induction of the UPRmt with methacycline reduced OXPHOS capacity, while silencing CLPP was sufficient to reduce OXPHOS capacity, membrane potential, and promoted mitochondrial fission. CLPP was negatively regulated by the PERK-eIF2α arm of the endoplasmic reticulum UPR pathway, independent of ATF4. Similar changes in the UPRmt pathway were observed in placentas from PE < 34 wk patients. Our results identify UPRmt as a therapeutic target for restoration of placental function in early-onset preeclampsia.

Oxidative stress of the placenta is considered a key intermediary step in the pathogenesis of preeclampsia, but the cause for the stress remains unknown. Ischaemia-reperfusion injury, as a result of intermittent placental perfusion secondary to deficient trophoblast invasion of the endometrial arteries, is a possible mechanism. This thesis therefore tests whether hypoxia-reoxygenation (H/R) in vitro can induce placental oxidative stress, and cause increased apoptosis and production of tumour necrosis factor-α as seen in the preeclamptic placenta. The first aim was to examine the oxidative status of human placental tissues during periods of hypoxia and reoxygenation in vitro. Rapid generation of reactive oxygen species (ROS) was detected using a fluorescent marker when hypoxic villous samples were reoxygenated. The expression of oxidative stress markers including nitrotyrosine residues, 4-hydroxy-2-nonenal adducts, and inducible heat shock protein 72 was greatly increased in villous samples subjected to H/R compared to the controls maintained under constant hypoxia. Furthermore, preloading villous samples with ROS scavengers such as desferrioxamine and α-phenyl-N-tert-butylnitrone significantly reduced the levels of oxidative stress in H/R. Having demonstrated that in vitro H/R is capable of inducing oxidative stress in a reproducible and manipulable manner, investigations were next carried out to study the effects of resultant oxidative stress on apoptosis within the trophoblast. Compared to hypoxic and normoxic controls, there was a significant increase in the release of cytochrome c from mitochondria, activation of caspase , and cleavage of poly (ADP-ribose) polymerase in villous samples subjected to H/R. These events were associated with an increased number of syncytiotrophoblastic nuclei displaying apoptotic changes and increased lactate dehydrogenase release into the medium. The causal relationship between the generation of ROS and these apoptotic changes was revealed by the fact that pre-administration of desferrioxamine attenuated the insult.

Affecting 6-8% of all pregnancies, preeclampsia is the leading cause of maternal morbidity in the western world and is charactensed by hypertension, proteinuria, edema and platelet aggregation. Despite its prevalence and severity, no comprehensive theory or single factor has been suggested to explain the pathophysiology of this multi system disorder of pregnancy, with the only therapies being bed rest, pharmacological symptom management and if necessary early delivery. Oxidative stress plays an important role in the pathophysiology of preeclampsia, resulting from defective trophoblast invasion, reductions in placental perfusion and placental hypoxia/reoxygenation. The inability of endogenous antioxidant systems up regulated in normal pregnancy, to control increased levels of oxidative stress, is suggested as a possible factor in the feed forward generation of reactive oxygen species and placental oxidative stress. That in turn may stimulate increased syncytiotrophoblast apoptosis, endothelial cell activation and the maternal hyper immune response characteristic of preeclampsia. Analysis of the research literature revealed that previous evaluations of placental oxidation and antioxidant enzyme activity in preeclampsia were by no means comprehensive, and exhibited significant inter-study variations. It was the aim of this thesis to clarify the placental oxidative state and the endogenous antioxidant activity of glutathione peroxidase, thioredoxin reductase, thioredoxin and superoxide dismutase in human placentae in an attempt to determine if variations in antioxidant function were due to changes in gene expression or protein oxidation. The findings reported in this thesis indicate the presence of increased levels of oxidative stress in the preeclamptic placenta, associated with significant reductions in antioxidant enzyme capacity. Quantitative real-time PCR analysis of placental samples revealed that deceases in antioxidant capacity in the placenta are more likely to be related to the significant oxidative burden within the tissue rather than reductions in gene expression. A number of animal models exist to investigate components of preeclampsia pathophysiology, however the ability of these models to mimic the oxidative and antioxidant features of preeclampsia remains unclear. The exposure of pregnant rats to N(G)-nitro-L-arginine methyl ester is a widely used model of endothelial cell dysfunction during preeclampsia. It was the aim of this thesis to determine the biochemical characteristics of this model in an attempt to assess its effectiveness in mimicking oxidative changes in the preeclamptic placenta. Although this model is capable of producing a syndiome in rats similar to the disorder in terms of physiology, this is not manifest in terms of placental biochemistry. The importance of selenium in the synthesis of selenobased antioxidants such as glutathione peroxidase and thioredoxin reductase is well documented. Increasing demand for selenium by the developing fetus may be linked to reductions in selenium status during pregnancy. Considering preeclampsia is associated with significant reductions in selenium status it may be hypothesised that reductions in antioxidant function may be linked to selenium inadequacy. The modulation of dietary selenium in pregnant rats was used to determine the importance of selenium during pregnancy and its effect on antioxidant function and placental oxidative stress. The results of this analysis revealed that selenium deficiency causes a pregnancy specific condition similar to preeclampsia. This condition was found to be associated with increased placental oxidative stress and significant reductions in the systemic activity of selenobased antioxidants that could be modified through selenium supplementation. In summary, data obtained in this thesis indicate that placental oxidative stress and reduced antioxidant enzyme activity play a significant role in the pathogenesis of preeclampsia. These studies support the hypothesis that antioxidant sufficiency is crucial in the maintenance of oxidative balance and that antioxidant dysfunction may result in damage to the placenta and the progression of the disease. These novel data further our understanding of the pathophysiology of preeclampsia and provide new insight into the pathogenesis of clinical complications exhibited in this condition, suggesting antioxidant therapy as a possible means for improving the health outcomes of both mother and baby.

Affecting 6-8% of all pregnancies, preeclampsia is the leading cause of maternal morbidity in the western world and is charactensed by hypertension, proteinuria, edema and platelet aggregation. Despite its prevalence and severity, no comprehensive theory or single factor has been suggested to explain the pathophysiology of this multi system disorder of pregnancy, with the only therapies being bed rest, pharmacological symptom management and if necessary early delivery. Oxidative stress plays an important role in the pathophysiology of preeclampsia, resulting from defective trophoblast invasion, reductions in placental perfusion and placental hypoxia/reoxygenation. The inability of endogenous antioxidant systems up regulated in normal pregnancy, to control increased levels of oxidative stress, is suggested as a possible factor in the feed forward generation of reactive oxygen species and placental oxidative stress. That in turn may stimulate increased syncytiotrophoblast apoptosis, endothelial cell activation and the maternal hyper immune response characteristic of preeclampsia. Analysis of the research literature revealed that previous evaluations of placental oxidation and antioxidant enzyme activity in preeclampsia were by no means comprehensive, and exhibited significant inter-study variations. It was the aim of this thesis to clarify the placental oxidative state and the endogenous antioxidant activity of glutathione peroxidase, thioredoxin reductase, thioredoxin and superoxide dismutase in human placentae in an attempt to determine if variations in antioxidant function were due to changes in gene expression or protein oxidation. The findings reported in this thesis indicate the presence of increased levels of oxidative stress in the preeclamptic placenta, associated with significant reductions in antioxidant enzyme capacity. Quantitative real-time PCR analysis of placental samples revealed that deceases in antioxidant capacity in the placenta are more likely to be related to the significant oxidative burden within the tissue rather than reductions in gene expression. A number of animal models exist to investigate components of preeclampsia pathophysiology, however the ability of these models to mimic the oxidative and antioxidant features of preeclampsia remains unclear. The exposure of pregnant rats to N(G)-nitro-L-arginine methyl ester is a widely used model of endothelial cell dysfunction during preeclampsia. It was the aim of this thesis to determine the biochemical characteristics of this model in an attempt to assess its effectiveness in mimicking oxidative changes in the preeclamptic placenta. Although this model is capable of producing a syndiome in rats similar to the disorder in terms of physiology, this is not manifest in terms of placental biochemistry. The importance of selenium in the synthesis of selenobased antioxidants such as glutathione peroxidase and thioredoxin reductase is well documented. Increasing demand for selenium by the developing fetus may be linked to reductions in selenium status during pregnancy. Considering preeclampsia is associated with significant reductions in selenium status it may be hypothesised that reductions in antioxidant function may be linked to selenium inadequacy. The modulation of dietaty selenium in pregnant rats was used to determine the importance of selenium during pregnancy and its effect on antioxidant function and placental oxidative stress. The results of this analysis revealed that selenium deficiency causes a pregnancy specific condition similar to preeclampsia. This condition was found to be associated with increased placental oxidative stress and significant reductions in the systemic activity of selenobased antioxidants that could be modified through selenium supplementation. In summary, data obtained in this thesis indicate that placental oxidative stress and reduced antioxidant enzyme activity play a significant role in the pathogenesis of preeclampsia. These studies support the hypothesis that antioxidant sufficiency is crucial in the maintenance of oxidative balance and that antioxidant dysfunction may result in damage to the placenta and the progression of the disease. These novel data further our understanding of the pathophysiology of preeclampsia and provide new insight into the pathogenesis of clinical complications exhibited in this condition, suggesting antioxidant therapy as a possible means for improving the health outcomes of both mother and baby.
Thesis (PhD Doctorate)
Doctor of Philosophy (PhD)
School of Medical Science
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