Local Oxygen Tension Research Articles

Oxygen-Dependent Gene Expression in Development and Cancer: Lessons Learned from the Wilms’ Tumor Gene, WT1

Adequate tissue oxygenation is a prerequisite for normal development of the embryo. Most fetal organs are exquisitely susceptible to hypoxia which occurs when the delivery of oxygen is exceeded by the actual demand. Developmental abnormalities due to insufficient supply with oxygen can result from the impaired expression of genes with essential functions during embryogenesis. As such, the Wilms’ tumor gene, WT1, is among the fetal genes that are regulated by the local oxygen tension. WT1 was originally discovered as a tumor suppressor gene owing to loss-of-function mutations in a subset of pediatric renal neoplasias, known as nephroblastomas or Wilms’ tumors. Wilms’ tumors can arise when pluripotent progenitor cells in the embryonic kidney continue to proliferate rather than differentiating to glomeruli and tubules. WT1 encodes a zinc finger protein, of which multiple isoforms exist due to alternative mRNA splicing in addition to translational and post-translational modifications. While some WT1 isoforms function as transcription factors, other WT1 proteins are presumably involved in post-transcriptional mRNA processing. However, the role of WT1 reaches far beyond that of a tumor suppressor as homozygous disruption of Wt1 in mice caused embryonic lethality with a failure of normal development of the kidneys, gonads, heart, and other tissues. WT1 mutations in humans are associated with malformation of the genitourinary system. A common paradigm of WT1 expressing cells is their capacity to switch between a mesenchymal and epithelial state. Thus, WT1 likely acts as a master switch that enables cells to undergo reciprocal epithelial-to-mesenchymal transition. Impairment of renal precursor cells to differentiate along the epithelial lineage due to WT1 mutations may favor malignant tumor growth. This article shall provide a concise review of the function of WT1 in development and disease with special consideration of its regulation by molecular oxygen.

In vivo T Cell Activation in Lymphoid Tissues is Inhibited in the Oxygen-Poor Microenvironment

Activation of immune cells is under control of immunological and physiological regulatory mechanisms to ensure adequate destruction of pathogens with the minimum collateral damage to “innocent” bystander cells. The concept of physiological negative regulation of immune response has been advocated based on the finding of the critical immunoregulatory role of extracellular adenosine. Local tissue oxygen tension was proposed to function as one of such physiological regulatory mechanisms of immune responses. In the current study, we utilized in vivo marker of local tissue hypoxia pimonidazole hydrochloride (Hypoxyprobe-1) in the flowcytometric analysis of oxygen levels to which lymphocytes are exposed in vivo. The level of exposure to hypoxia in vivo was low in B cells and the levels increased in the following order: T cells < NKT cells < NK cells. The thymus was more hypoxic than the spleen and lymph nodes, suggesting the variation in the degree of oxygenation among lymphoid organs and cell types in normal mice. Based on in vitro studies, tissue hypoxia has been assumed to be suppressive to T cell activation in vivo, but there was no direct evidence demonstrating that T cells exposed to hypoxic environment in vivo are less activated. We tested whether the state of activation of T cells in vivo changes due to their exposure to hypoxic tissue microenvironments. The parallel analysis of more hypoxic and less hypoxic T cells in the same mouse revealed that the degree of T cell activation was significantly stronger in better-oxygenated T cells. These observations suggest that the extent of T cell activation in vivo is dependent on their localization and is decreased in environment with low oxygen tension.

Imaging tumor hypoxia by magnetic resonance methods

Tumor hypoxia results from the negative balance between the oxygen demands of the tissue and the capacity of the neovasculature to deliver sufficient oxygen. The resulting oxygen deficit has important consequences with regard to the aggressiveness and malignancy of tumors, as well as their resistance to therapy, endowing the imaging of hypoxia with vital repercussions in tumor prognosis and therapy design. The molecular and cellular events underlying hypoxia are mediated mainly through hypoxia-inducible factor, a transcription factor with pleiotropic effects over a variety of cellular processes, including oncologic transformation, invasion and metastasis. However, few methodologies have been able to monitor noninvasively the oxygen tensions in vivo. MRI and MRS are often used for this purpose. Most MRI approaches are based on the effects of the local oxygen tension on: (i) the relaxation times of (19)F or (1)H indicators, such as perfluorocarbons or their (1)H analogs; (ii) the hemodynamics and magnetic susceptibility effects of oxy- and deoxyhemoglobin; and (iii) the effects of paramagnetic oxygen on the relaxation times of tissue water. (19)F MRS approaches monitor tumor hypoxia through the selective accumulation of reduced nitroimidazole derivatives in hypoxic zones, whereas electron spin resonance methods determine the oxygen level through its influence on the linewidths of appropriate paramagnetic probes in vivo. Finally, Overhauser-enhanced MRI combines the sensitivity of EPR methodology with the resolution of MRI, providing a window into the future use of hyperpolarized oxygen probes.

Towards a quantitative understanding of oxygen tension and cell density evolution in fibrin hydrogels

The in vitro culture of hydrogel-based constructs above a critical size is accompanied by problems of unequal cell distribution when diffusion is the primary mode of oxygen transfer. In this study, an experimentally-informed mathematical model was developed to relate cell proliferation and death inside fibrin hydrogels to the local oxygen tension in a quantitative manner. The predictive capacity of the resulting model was tested by comparing its outcomes to the density, distribution and viability of human periosteum derived cells (hPDCs) that were cultured inside fibrin hydrogels in vitro. The model was able to reproduce important experimental findings, such as the formation of a multilayered cell sheet at the hydrogel periphery and the occurrence of a cell density gradient throughout the hydrogel. In addition, the model demonstrated that cell culture in fibrin hydrogels can lead to complete anoxia in the centre of the hydrogel for realistic values of oxygen diffusion and consumption. A sensitivity analysis also identified these two parameters, together with the proliferation parameters of the encapsulated cells, as the governing parameters for the occurrence of anoxia. In conclusion, this study indicates that mathematical models can help to better understand oxygen transport limitations and its influence on cell behaviour during the in vitro culture of cell-seeded hydrogels.

Oxygen Tension Modulates Neurite Outgrowth in PC12 Cells Through A Mechanism Involving HIF and VEGF

Cell-based approaches are a promising therapeutic strategy for treating injuries to the nervous system, but the optimal means to promote neurite extension and direct cellular behavior are unclear. Previous studies have examined the behavior of neural-like cells in ambient air (21% oxygen tension), yet these conditions are not representative of the physiological oxygen microenvironment of neural tissues. We hypothesized that neuronal differentiation of a model neural cell line (PC12) could be controlled by modulating local oxygen tension. Compared to ambient conditions, PC12 cells cultured in reduced oxygen exhibited significant increases in neurite extension and total neurite length, with 4% oxygen yielding the highest levels of both indicators. We confirmed neurite extension was mediated through oxygen-responsive mechanisms using small molecules that promote or inhibit HIF-1α stabilization. The hypoxic target gene Vegf was implicated as a neurotrophic factor, as neurite formation at 21% oxygen was mimicked with exogenous VEGF, and a VEGF-neutralizing antibody attenuated neurite formation under reduced oxygen conditions. These findings demonstrate that behavior of neural-like cells is driven by the oxygen microenvironment via VEGF function, and suggest promising approaches for future applications in neural repair.

TH-D-304A-06: Comparison of Cu-ATSM and FMISO Uptake to Variations in Oxygen Tension

Purpose: Unique uptake and retention mechanisms of PET hypoxia tracers make comparison between them challenging. Two common hypoxia surrogates, [ 61 Cu ]Cu‐ATSM and [ 18 F ]FMISO, were compared by modeling their uptake as a function of local oxygen tension and comparing these with clinical measurements. Materials and Methods: An electrochemical formalism describing bioreductive retention mechanisms under equilibrium conditions was adopted to relate time‐averaged tracer concentration to tissue partial oxygen tension (pO2). Chemical equilibrium constants describing product concentration to reactant concentration ratios were determined from free energy changes and reduction‐oxidation potentials of the most probable reactions. Calculated pO2 distributions from imaged Cu‐ATSM tracer activity concentrations of head and neck squamous cell carcinoma (HNSCC) patients were compared to microelectrode pO2 measurements in 69 patients with HNSCC. Additionally, simulated Cu‐ATSM and FMISO uptake distributions were derived from the mean HNSCC population pO2 distribution. Results: Cu‐ATSM and FMISO uptake differ in sensitivity to changes in oxygen tension, as Cu‐ATSM finely samples low pO2 over broad uptake ranges while FMISO uniformly samples pO2 above low pO2 distribution peaks. Cu‐ATSM uptake is higher than FMISO uptake under severe hypoxia (pO2 10 mmHg) FMISO is retained more than Cu‐ATSM. Based on population averages, clinical hypoxia thresholds (5 mmHg and 2.5 mmHg) correspond to higher Cu‐ATSM SUV (3.1 and 3.7) and lower FMISO SUV (1.2 and 2.4). Conclusions: Differing retention mechanisms of Cu‐ATSM and FMISO make direct comparison inaccurate without relating their uptake to common metrics. Results indicate that Cu‐ATSM is more selective for low pO2 than FMISO and may explain the relatively low correlation between uptakes of these tracers in certain small animal models. When using various hypoxia tracers as a basis for dose painting, uptake and resulting dose distributions should not be interpreted interchangeably but rather as yielding complementary information.

Mechanisms of oxygen sensing: a key to therapy of pulmonary hypertension and patent ductus arteriosus.

Specialized tissues that sense acute changes in the local oxygen tension include type 1 cells of the carotid body, neuroepithelial bodies in the lungs, and smooth muscle cells of the resistance pulmonary arteries and the ductus arteriosus (DA). Hypoxia inhibits outward potassium current in carotid body type 1 cells, leading to depolarization and calcium entry through L-type calcium channels. Increased intracellular calcium concentration ([Ca+ +]i) leads to exocytosis of neurotransmitters, thus stimulating the carotid sinus nerve and respiration. The same K+ channel inhibition occurs with hypoxia in pulmonary artery smooth muscle cells (PASMCs), causing contraction and providing part of the mechanism of hypoxic pulmonary vasoconstriction (HPV). In the SMCs of the DA, the mechanism works in reverse. It is the shift from hypoxia to normoxia that inhibits K+ channels and causes normoxic ductal contraction. In both PA and DA, the contraction is augmented by release of Ca+ + from the sarcoplasmic reticulum, entry of Ca+ + through store-operated channels (SOC) and by Ca+ + sensitization. The same three 'executive' mechanisms are partly responsible for idiopathic pulmonary arterial hypertension (IPAH). While vasoconstrictor mediators constrict both PA and DA and vasodilators dilate both vessels, only redox changes mimic oxygen by having directly opposite effects on the K+ channels, membrane potential, [Ca(++)]i and tone in the PA and DA. There are several different hypotheses as to how redox might alter tone, which remain to be resolved. However, understanding the mechanism will facilitate drug development for pulmonary hypertension and patent DA.

Mitochondria, oxygen sensing, and the regulation of HIF-2α. Focus on “Induction of HIF-2α is dependent on mitochondrial O2 consumption in an O2-sensitive adrenomedullary chromaffin cell line”

oxygen gradients in tissues and cells are critical signals in a range of physiological processes including development, wound healing, and adaptation to hypoxia ([12][1]). Eukaryotic cells have evolved mechanisms for sensing changes in local oxygen tension and initiation of a response designed to

Long‐term effects of low‐dose NOS inhibition in mouse renal medulla

Chronic NO deficiency causes endothelial dysfunction of the renal medullary vasculature and increases the expression of the Na,K,2Cl‐cotransporter (NKCC2) of the thick ascending limb. We hypothesized that impaired medullary perfusion and increased NKCC2 activity may promote local hypoxia and fibrosis. To test this, adult male 129J mice were treated with the NOS inhibitor, NG‐methyl‐L‐arginine (L‐NMMA; 0.3 or 0.8 mg/ml in drinking water). Blood pressure was measured by tail plethysmography. Density and structure of medullary blood vessels were evaluated using histochemistry and fine structural analysis. Distribution and quantity of CD31, CD34 and phospho‐NKCC2 were studied. Hypoxic areas were visualized by hypoxyprobe (pimonidazole) staining. L‐NMMA treated groups showed focal expansions of connective tissue deposits in or near vascular bundles. Blood pressure and overall structure and density of medullary microvessels were unaffected. Outer medullary phospho‐NKCC2 immunoreactivity was markedly increased in treated animals, whereas hypoxic signs were unrelated to treatment. We conclude that long term application of sub‐pressor doses of NOS inhibitor in mice increases the activity of outer medullary NKCC2 without major effects on local oxygen tension and vascular integrity. Activation of NKCC2 may be related to fibrotic changes in the renal medulla under reduced availability of NO.

Hypoxia and the HIF system in kidney disease

The kidney is sensitive to changes in oxygen delivery. This sensitivity has the merit of facilitating the kidneys in their adjustment of erythropoietin (EPO) production to changes in oxygen supply. The main determinant of EPO synthesis is the transcriptional activity of its gene in kidneys, which is related to local oxygen tensions. Regulation of EPO production is mediated by hypoxia-inducible factor (HIF). When local oxygen tension decreases, accumulated HIF binds to the key sequence of the EPO gene, the hypoxia-responsive element (HRE), and activates transcription of EPO. HIF consists of a constitutive beta-subunit and one of alternative oxygen-regulated HIF alpha-subunits (HIF-1alpha, HIF-2alpha, and HIF-3alpha), and HIF-2alpha is responsible for erythropoietin production. However, the high sensitivity to changes in oxygen tension also makes the kidney prone to hypoxic injury. Severe energy depletion and subsequent activation of a number of critical alterations in metabolism occurs under hypoxic conditions. Hypoxia is also a profibrogenic stimulus. In addition to ischemic acute renal failure, hypoxia can also play a crucial role in the development of nephrotoxic acute kidney injury, radiocontrast nephropathy, and acute glomerulonephritis. Furthermore, accumulating evidence suggests that chronic hypoxia is a final common pathway to end-stage kidney failure in chronic kidney disease. Given that renal hypoxia has pivotal roles on the development and progression of both acute and chronic kidney disease, hypoxia can be a valid therapeutic target for chronic kidney disease. Activation of HIF leads to expression of a variety of adaptive genes in a coordinated manner. Studies utilizing HIF-stimulating agents proved efficacy in various kidney disease models, suggesting that HIF activation is an ideal target of future therapeutic approaches.

Arteriogenesis: basic mechanisms and therapeutic stimulation

Pharmacological attempts to stimulate the growth of collateral arteries (arteriogenesis) are evolving towards a new treatment option for patients with vascular occlusive diseases. This enlargement of small pre-existing anastomoses towards large conductance arteries takes place independent of local oxygen tension and is driven by changes in luminal shear stress and infiltration of circulating cells. With the increasing knowledge regarding the distinct differences between capillary sprouting (angiogenesis) and arteriogenesis, several cytokines and growth factors have been demonstrated to stimulate the growth of arterial blood vessels in preclinical models of vascular disease. However, the translation towards clinical practice remains difficult and first in-man trials show limited success. Intensive research especially regarding new drug delivery platforms and the potentially serious side effects of pro-arteriogenic therapeutics is warranted before stimulation of arteriogenesis could become a significant treatment option for vascular occlusive diseases. This review focuses on the recent advances in the field of collateral artery growth. In addition, possible means to overcome the hurdles that have hampered the clinical implementation of pro-arteriogenic therapies will be discussed.

PO2-dependent Differential Regulation of Multidrug Resistance 1 Gene Expression by the c-Jun NH2-terminal Kinase Pathway

Hypoxia-induced multidrug resistance 1 (MDR1) gene expression is known to be mediated by c-Jun NH(2)-terminal kinase (JNK) activation. However, the molecular mechanisms underlying this action of JNK remain elusive. On the contrary, there has been increasing evidence for a negative correlation of JNK activity with MDR1 expression under normoxic conditions. Here, we present evidence that the JNK pathway represses MDR1 expression in normoxia and activates MDR1 expression in hypoxia. Our data show that JNK pathway-induced MDR1 repression in normoxia is mediated by increased c-Jun binding to activator protein 1 site, located in the MDR1 promoter, and requires the activity of histone deacetylase 5. In contrast, JNK pathway-induced MDR1 activation in hypoxia is independent of the activator protein 1 site. Rather, this action is dependent on increased hypoxia-inducible factor 1 (HIF1) binding to the hypoxia response element in the MDR1 promoter, which is promoted by the interaction of HIF1alpha with c-Jun in the nucleus and requires the activity of the p300/CBP (CREB-binding protein) coactivator.

Angiogenesis and vasculogenesis: Inducing the growth of new blood vessels and wound healing by stimulation of bone marrow–derived progenitor cell mobilization and homing

During embryonic development, the vasculature is among the first organs to form and is in charge of maintaining metabolic homeostasis by supplying oxygen and nutrients and removing waste products. As one would expect, blood vessels are critical not only for organ growth in the embryo but also for repair of wounded tissue in the adult. An imbalance in angiogenesis (a time-honored term that globally refers to the growth of new blood vessels) contributes to the pathogenesis of numerous malignant, inflammatory, ischemic, infectious, immune, and wound-healing disorders. This review focuses on the central role of the growth of new blood vessels in ischemic and diabetic wound healing and defines the most current nomenclature that describes the neovascularization process in wounds. There are now two well-defined, distinct, yet interrelated processes for the formation of postnatal new blood vessels, angiogenesis, and vasculogenesis. Reviewed are recent new data on vasculogenesis that promise to advance the field of wound healing.

Composition of Bone Marrow‐Derived Progenitor Cells in the Cellular Infiltrate of Infarcted Hearts: Role of Local Oxygen Tension

Our objective was to determine the origin and phenotypes of cells in the early mononuclear infiltrate of infarcted hearts. Methods. In mice transplanted with bone marrow from LacZ-expressing donors, we performed (a) permanent coronary ligation (n=5), or (b) ligation for 30 min, followed by release (n=5). These mice, and (c) sham-operated controls, were sacrificed after 7 days. The cellular composition in the hearts was assessed by immunocytochemistry, after staining for beta-galactosidase expression. Using a novel in vivo EPR oximetry method, we also measured the time course of oxygenation of the infarcted hearts. Results. In the infarcted areas, we found a mosaic distribution of the following markers: LacZ (bone marrow origin); F4/80 (macrophages); smooth muscle alpha actin; troponin I (in mononuclear cells located mainly in the border zone), and ABC-G2 (‘side population’ progenitors). At the core of infarcted territory we noted a paucity of organized microvessels, indicating a state of chronic ischemia. This was confirmed by the dramatic decrease in the local, EPR-detectable oxygen tension. Re-vascularized hearts displayed only minimal, focally distributed damage. Conclusion. At one week post-infarction, we detected bone marrow derived progenitor cells with phenotypes topographically correlated to the pattern of oxygenation. This finding advocates for the potential benefit of post-infarct re-vascularization therapy.

Collateral Artery Growth

See related article, pages 656–662 The primary role of the vascular system is to carry blood and oxygen to tissue. This task is complicated by the vastly differing needs of tissues for oxygen according to their metabolism. Moreover, the metabolic activity in a given organ, and thereby its need of blood flow and oxygen, may dramatically change in time scales varying from seconds (during the onset of muscular exercise for example) to months or years (growth of organs, ischemic processes). Therefore, to ensure good organ function, the vasculature must adapt to these heterogeneous needs. Because blood vessels are highly distributed in space, one marvels at how a vascular bed manages to adapt structurally to the local environment, and how individual segments within the vascular network succeed in supplying the right amount of blood and oxygen to tissues in the most economical way. In fact, such intricate control requires 2 adaptation processes that exist in parallel: short term release of vasoactive agonists or changes in vascular tone provide a quick functional adaptation to accommodate rapid changes in metabolic demand, whereas growth or regression of blood vessels (termed vascular remodeling) represent long term structural adaptation to new, lasting metabolic and blood flow conditions (Figure 1). Both acute and chronic adaptations are complementary and are driven by the same environmental variables, which include local oxygen tension, vascular wall tension, and shear stress. For example, when arteries remodel in response to a new and lasting elevated blood flow, the initial acute vasodilation, mediated by endothelium-derived nitric oxide (NO) in large arteries and possibly involving prostaglandins in the microvasculature, leads the way to expansive medial remodeling that alters the resting diameter of the vessel. Figure 1. Acute and chronic vascular adaptations to changes in local metabolic demands. However, simple vasodilatory or remodeling processes may …

Neutrophil apoptosis in rheumatoid arthritis is regulated by local oxygen tensions within joints

Neutrophils are normally short-lived cells and die by apoptosis, but when recruited into tissues, their apoptosis is delayed, and they survive for much longer time periods. In inflammatory diseases, such as rheumatoid arthritis (RA), this delayed apoptosis may lead to increased tissue damage and a failure of the inflammation to resolve. However, there are conflicting reports in the literature as to whether neutrophil apoptosis is delayed or accelerated in rheumatoid joints. In this report, we show that neutrophils isolated from the synovial fluid (SF) of patients with RA show accelerated rates of apoptosis when incubated ex vivo and that SF, despite containing a variety of antiapoptotic cytokines, is proapoptotic. Paradoxically, levels of the key neutrophil survival protein Mcl-1 are elevated in freshly isolated SF neutrophils compared with matched peripheral blood samples from the same patients, indicating that delayed neutrophil apoptosis has been signaled in vivo as the cells enter the joints. However, when SF was added to neutrophils and incubated under hypoxia (1% O(2)), conditions known to exist in vivo within joints, the SF was antiapoptotic. These data reveal that the rheumatoid synovial joint contains a complex mixture of pro- and antiapoptotic factors and that the low, local oxygen tensions that exist within these joints can exert profound effects on neutrophil survival. These experiments also highlight the importance of performing in vitro experiments under laboratory conditions that closely mimic those that occur in vivo; otherwise, misleading conclusions may be drawn.

Analysis of Cell Growth in Three-Dimensional Scaffolds

The in vitro growth of pre-osteoblasts in multi-layer, three-dimensional scaffolds was determined from experimental measurements and was compared to a mathematical model. Immediately following cell seeding, the initial cell density was uniform throughout the scaffold. After 10 days, the cell density increased from 2.1 x 10(5) cells/cm(3) to 1.3 x 10(7) cells/cm(3) at the fluid-scaffold interface. The increase in cell density was largely confined to the outermost 200 microm from the fluid-scaffold interface. The cell density profile was in good agreement with a mathematical model that simulated the cell growth based on the local oxygen tension. The improved understanding derived from this mathematical model may be useful in the design of three-dimensional scaffolds that can support more uniform growth of cells.

Contrast Echocardiography: A Valid and Sensitive Imaging Method in the Investigation of Transmyocardial Laser-Revascularization in an Acute Ischemia Model

In an acute porcine myocardial ischemia model we examined the effect of transmyocardial laser-revascularization (TMLR) on the regional micro perfusion and oxygen supply. In clinical practice, contrast echocardiography is a reliable tool for the measurement of changes in regional blood flow in the ischemic myocardium. We compared contrast echocardiography with the laser Doppler measurement of micro perfusion and with the quantification of regional tissue oxygen tension using a Clark electrode. 22 pigs were randomised in the interventional group and 12 in the control group. Measurements with all three methods were performed before and after occlusion of the first diagonal branch of the left anterior descending coronary artery and, in the interventional group, after TMLR. We investigated not only the effects in the dependent ischemic myocardium, but also in two other myocardial areas, not involved in the coronary occlusion. There was a significant effect of TMLR on regional micro perfusion and local oxygen tension in the dependent ischemic myocardial area. Contrast echocardiography is a successful experimental tool to measure changes in regional myocardial perfusion which cannot be perceived using clinical imaging methods.

Regulation of erythropoietin production

The glycoprotein hormone erythropoietin (EPO) is an essential growth and survival factor for erythroid progenitor cells, and the rate of red blood cell production is normally determined by the serum EPO concentration. EPO production is inversely related to oxygen availability, so that an effective feedback loop is established, which controls erythropoiesis. Since recombinant EPO became available as an effective therapeutic agent, significant progress has also been made in understanding the basis of this feedback control. The main determinant of EPO synthesis is the transcriptional activity of its gene in liver and kidneys, which is related to local oxygen tensions. This control is achieved by hypoxia-inducible transcription factors (HIF), consisting of a constitutive beta-subunit and one of two alternative oxygen-regulated HIFalpha subunits (HIF-1alpha and HIF-2alpha). In the presence of oxygen (normoxia) the HIFalpha subunits are hydroxylated, which targets them for proteasomal degradation. Under hypoxia, because of the lack of molecular oxygen, HIF cannot be hydroxylated and is thereby stabilized. Although HIF-1alpha was the first transcription factor identified through its ability to bind to an enhancer sequence of the EPO gene, more recent evidence suggests that HIF-2alpha is responsible for the regulation of EPO. Although EPO is a prime example for an oxygen- regulated gene, the role of the HIF system goes far beyond the regulation of EPO, because it operates widely in almost all cells and controls a broad transcriptional response to hypoxia, including genes involved in cell metabolism, angiogenesis and vascular tone. Further evidence suggests that apart from its effect as an erythropoietic hormone EPO acts as a paracrine, tissue-protective protein in the brain and possibly also in other organs.

Trophoblast CD274 (B7-H1) Is Differentially Expressed Across Gestation: Influence of Oxygen Concentration1

Modulation of the maternal immune system by the placenta is a mechanism by which the fetus ensures its own survival in a genetically foreign environment. The immunoinhibitor CD274 (also called B7-H1 or PD-L1) is highly expressed in the placenta, positioned to interact with maternal leukocytes. Further, immunoblot analysis of first- and second-trimester placental lysates showed that CD274 expression is low in the first trimester but dramatically rises around the onset of the second trimester. As this coincides with the expected onset of maternal blood flow to the placenta and a corresponding rise in local oxygen tension, we explored the possibility that oxygen regulates CD274 expression in trophoblast cells by culturing term trophoblast cells under oxygen concentrations similar to those found in vivo. Indeed, CD274 protein levels paralleled the in vivo situation: expression increased with rising oxygen concentrations. Furthermore, downregulation of CD274 mRNA by low oxygen was rapid, occurring within 4-12 h. We conclude that oxygen is a potential mediator of CD274 expression in vivo such that it is induced coincidentally on exposure of fetal tissues to maternal blood. Further, the regulation of this immunomodulator by oxygen may implicate its alteration during and involvement in the pathogenesis of complications of pregnancy such as preeclampsia.