Hyperoxic ventilated premature baboons have increased p53, oxidant DNA damage and decreased VEGF expression

Hyperoxia-Induced miR-195 Causes Bronchopulmonary Dysplasia in Neonatal Mice

Background: Exposure to hyperoxia is an important factor in the development of bronchopulmonary dysplasia (BPD) in preterm newborns. MicroRNAs (miRs) have been implicated in the pathogenesis of BPD and provide a potential therapeutic target. Methods: This study was conducted utilizing a postnatal animal model of experimental hyperoxia-induced murine BPD to investigate the expression and function of miR-195 as well as its molecular signaling targets within developing mouse lung tissue. Results: miR-195 expression levels increased in response to hyperoxia in male and female lungs, with the most significant elevation occurring in 40% O2 (mild) and 60% O2 (moderate) BPD. The inhibition of miR-195 improved pulmonary morphology in the hyperoxia-induced BPD model in male and female mice with females showing more resistance to injury and better recovery of alveolar chord length, septal thickness, and radial alveolar count. Additionally, we reveal miR-195-dependent signaling pathways involved in BPD and identify PH domain leucine-rich repeat protein phosphatase 2 (PHLPP2) as a novel specific target protein of miR-195. Conclusions: Our data demonstrate that high levels of miR-195 in neonatal lungs cause the exacerbation of hyperoxia-induced experimental BPD while its inhibition results in amelioration. This finding suggests a therapeutic potential of miR-195 inhibition in preventing BPD.

Endothelial deletion of p53 generates transitional endothelial cells and improves lung development during neonatal hyperoxia

Bronchopulmonary dysplasia (BPD), a prevalent and chronic lung disease affecting premature newborns, results in vascular rarefaction and alveolar simplification. Although the vasculature has been recognized as a main player in this disease, the recently found capillary heterogeneity and cellular dynamics of endothelial subpopulations in BPD remain unclear. Here, we show Cap2 cells are damaged during neonatal hyperoxic injury, leading to their replacement by Cap1 cells which, in turn, significantly decline. Single-cell RNA-seq identifies the activation of numerous p53 target genes in endothelial cells, including Cdkn1a (p21). While global deletion of p53 results in worsened vasculature, endothelial-specific deletion of p53 reverses the vascular phenotype and improves alveolar simplification during hyperoxia. This recovery is associated with the emergence of a transitional EC state, enriched for oxidative stress response genes and growth factors. These findings implicate the p53 pathway in EC type transition during injury-repair and highlights the endothelial contributions to BPD.

Endothelial to mesenchymal transition in neonatal hyperoxic lung injury: role of sex as a biological variable

Bronchopulmonary dysplasia (BPD) is characterized by an arrest in alveolarization, abnormal vascular development, and variable interstitial fibroproliferation in the premature lung. Endothelial to mesenchymal transition (EndoMT) may be a source of pathological fibrosis in many organ systems. Whether EndoMT contributes to the pathogenesis of BPD is not known. We tested the hypothesis that pulmonary endothelial cells will show increased expression of EndoMT markers upon exposure to hyperoxia and that sex as a biological variable will modulate differences in expression. Wild-type (WT) and Cdh5-PAC CreERT2 (endothelial reporter) neonatal male and female mice (C57BL6) were exposed to hyperoxia (0.95 [Formula: see text]) either during the saccular stage of lung development (95% [Formula: see text]; postnatal day 1-5 [PND1-5]) or through the saccular and early alveolar stages of lung development (75% [Formula: see text]; PND1-14). Expression of EndoMT markers was measured in whole lung and endothelial cell mRNA. Sorted lung endothelial cells (from room air- and hyperoxia-exposed lungs) were subjected to bulk RNA-Seq. We show that exposure of the neonatal lung to hyperoxia leads to upregulation of key markers of EndoMT. Furthermore, using lung sc-RNA-Seq data from neonatal lung we were able to show that all endothelial cell subpopulations including the lung capillary endothelial cells show upregulation of EndoMT-related genes. Markers related to EndoMT are upregulated in the neonatal lung upon exposure to hyperoxia and show sex-specific differences. Mechanisms mediating EndoMT in the injured neonatal lung can modulate the response of the neonatal lung to hyperoxic injury and need further investigation.NEW & NOTEWORTHY We show that neonatal hyperoxia exposure increased EndoMT markers in the lung endothelial cells and this biological process exhibits sex-specific differences.

Oxygen toxicity: cellular mechanisms in normobaric hyperoxia

In clinical settings, oxygen therapy is administered to preterm neonates and to adults with acute and chronic conditions such as COVID-19, pulmonary fibrosis, sepsis, cardiac arrest, carbon monoxide poisoning, and acute heart failure. In non-clinical settings, divers and astronauts may also receive supplemental oxygen. In addition, under current standard cell culture practices, cells are maintained in atmospheric oxygen, which is several times higher than what most cells experience in vivo. In all the above scenarios, the elevated oxygen levels (hyperoxia) can lead to increased production of reactive oxygen species from mitochondria, NADPH oxidases, and other sources. This can cause cell dysfunction or death. Acute hyperoxia injury impairs various cellular functions, manifesting ultimately as physiological deficits. Chronic hyperoxia, particularly in the neonate, can disrupt development, leading to permanent deficiencies. In this review, we discuss the cellular activities and pathways affected by hyperoxia, as well as strategies that have been developed to ameliorate injury.

SIRT1-Related Signaling Pathways and Their Association With Bronchopulmonary Dysplasia

Bronchopulmonary dysplasia (BPD) is a chronic and debilitating disease that can exert serious and overwhelming effects on the physical and mental health of premature infants, predominantly due to intractable short- and long-term complications. Oxidative stress is one of the most predominant causes of BPD. Hyperoxia activates a cascade of hazardous events, including mitochondrial dysfunction, uncontrolled inflammation, reduced autophagy, increased apoptosis, and the induction of fibrosis. These events may involve, to varying degrees, alterations in SIRT1 and its associated targets. In the present review, we describe SIRT1-related signaling pathways and their association with BPD. Our intention is to provide new insights into the molecular mechanisms that regulate BPD and identify potential therapeutic targets for this debilitating condition.

Effects of antioxidant MnTBAP on angiogenesis in newborn mice with hyperoxic lung injury

BACKGROUND

Oxygen toxicity mediated by reactive oxygen species (ROS) plays an essential role in the development of bronchopulmonary dysplasia in premature infants. By reducing oxidative stress, antioxidants protect the immature lung. We studied the effects of MnTBAP, a catalytic antioxidant on angiogenesis and alveolar growth following neonatal hyperoxia.

METHODS

Newborn mouse litters randomized to room air (RA) or >95% O2 for 72 hours from day 4 (D4) to D7 to receive either MnTBAP (10 mg/kg/d) or saline intraperitoneally (every 24 h for three doses). Lungs harvested for angiogenic gene expression, protein expression, and histopathology post-hyperoxia exposure. Radial alveolar count (RAC), mean linear intercept (MLI) and vessel density assessed by histopathology.

RESULTS

Angiogenic gene expression was significantly lower in the hyperoxia group compared to the RA group. The protein expression for VEGF and its receptor, VEGFR1, was significantly lower following treatment with MnTBAP compared to hyperoxia alone. Expression of VEGFR2, Angiopoietin-1 and TIE2, were substantially higher in the RA groups compared to hyperoxia groups with or without MnTBAP. Hyperoxia groups demonstrated alveolar simplification. MnTBAP reduced vessel density and failed to improve alveolar growth following hyperoxia.

CONCLUSIONS

MnTBAP, a catalytic antioxidant, does not offer protection from hyperoxia-induced alveolar impairment. The lack of angiogenic upregulation by MnTBAP may contribute to alveolar simplification in newborn mice.

Preterm Lung Exhibits Distinct Spatiotemporal Proteome Expression at Initiation of Lung Injury

The development of regional lung injury in the preterm lung is not well understood. This study aimed to characterize time-dependent and regionally specific injury patterns associated with early ventilation of the preterm lung using a mass spectrometry-based proteomic approach. Preterm lambs delivered at 124-127 days gestation received 15 or 90 minutes of mechanical ventilation (positive end-expiratory pressure = 8 cm H₂O, Vt = 6-8 ml/kg) and were compared with unventilated control lambs. At study completion, lung tissue was taken from standardized nondependent and dependent regions, and assessed for lung injury via histology, quantitative PCR, and proteomic analysis using Orbitrap-mass spectrometry. Ingenuity pathway analysis software was used to identify temporal and region-specific enrichments in pathways and functions. Apoptotic cell numbers were ninefold higher in nondependent lung at 15 and 90 minutes compared with controls, whereas proliferative cells were increased fourfold in the dependent lung at 90 minutes. The relative gene expression of lung injury markers was increased at 90 minutes in nondependent lung and unchanged in gravity-dependent lung. Within the proteome, the number of differentially expressed proteins was fourfold higher in the nondependent lung than the dependent lung. The number of differential proteins increased over time in both lung regions. A total of 95% of enriched canonical pathways and 94% of enriched cellular and molecular functions were identified only in nondependent lung tissue from the 90-minute ventilation group. In conclusion, complex injury pathways are initiated within the preterm lung after 15 minutes of ventilation and amplified by continuing ventilation. Injury development is region specific, with greater alterations within the proteome of nondependent lung.

Caffeine is associated with improved alveolarization and angiogenesis in male mice following hyperoxia induced lung injury

Background

Caffeine therapy for apnea of prematurity reduces the incidence of bronchopulmonary dysplasia (BPD) in premature neonates. Several mechanisms, including improvement in pulmonary mechanics underly beneficial effects of caffeine in BPD. As vascular development promotes alveologenesis, we hypothesized that caffeine might enhance angiogenesis in the lung, promoting lung growth, thereby attenuating BPD.

Methods

C57Bl/6 mice litters were randomized within 12 h of birth to room air (RA) or 95%O₂ to receive caffeine (20 mg/kg/day) or placebo for 4 days and recovered in RA for 12wks. The lung mRNA and protein expression for hypoxia-inducible factors (HIF) and angiogenic genes performed on day 5. Lung morphometry and vascular remodeling assessed on inflation fixed lungs at 12wks.

Results

Caffeine and hyperoxia in itself upregulate HIF-2α and vascular endothelial growth factor gene expression. Protein expression of HIF-2α and VEGFR1 were higher in hyperoxia/caffeine and angiopoietin-1 lower in hyperoxia. An increase in radial alveolar count, secondary septal count, and septal length with a decrease in mean linear intercept indicate an amelioration of hyperoxic lung injury by caffeine. An increase in vessel surface area and a significant reduction in smooth muscle thickness of the pulmonary arterioles may suggest a beneficial effect of caffeine on vascular remodeling in hyperoxia, especially in male mice.

Conclusions

Postnatal caffeine by modulating angiogenic gene expression early in lung development may restore the pulmonary microvasculature and alveolarization in adult lung.

Effects of preterm birth and ventilation on glomerular capillary growth in the neonatal lamb kidney

OBJECTIVES

Preterm birth is linked to the development of hypertension later in life. This may relate to impaired glomerular capillary growth following preterm birth. The aim of this study was to determine the effects of preterm birth, and/or ventilation, on glomerular capillary growth in the neonatal lamb kidney.

METHODS

Four experimental groups were analysed: preterm lambs delivered at 130 days gestation (term = 147 days) and mechanically ventilated for 3 days (preterm ventilated: n = 9), 133 days gestational controls (gestational control: n = 5), term controls, unassisted breathing for 3 days (term control: n = 8), and term lambs ventilated for 3 days (term ventilated: n = 5). In perfusion-fixed kidneys, total nephron number, average total capillary length, and surface area per renal corpuscle were stereologically assessed, and total renal filtration surface area (TRFSA) was calculated.

RESULTS

In comparison with term controls, preterm lambs had significantly reduced glomerular capillary length, surface area, and TRFSA, indicative of a low renal functional capacity. Term-ventilated lambs exhibited significantly reduced glomerular capillary length and surface area compared with term controls, indicating that ventilation impairs glomerular capillary growth independently of preterm birth.

CONCLUSION

Impaired glomerular capillary growth and subsequent reduced TRFSA following preterm birth may mediate the increased predisposition to hypertension later in life.

Early injury of the neonatal lung contributes to premature lung aging: a hypothesis

Chronic lung disease of the newborn, also known as bronchopulmonary dysplasia (BPD), is the most common chronic lung disease in early infancy and results in an increased risk for long-lasting pulmonary impairment in the adult. BPD develops upon injury of the immature lung by oxygen toxicity, mechanical ventilation, and infections which trigger sustained inflammatory immune responses and extensive remodeling of the extracellular matrix together with dysregulated growth factor signaling. Histopathologically, BPD is characterized by impaired alveolarization, disrupted vascular development, and saccular wall fibrosis. Here, we explore the hypothesis that development of BPD involves disturbance of conserved pathways of molecular aging that may contribute to premature aging of the lung and an increased susceptibility to chronic lung diseases in adulthood.

Hyperoxic Exposure of Immature Mice Increases the Inflammatory Response to Subsequent Rhinovirus Infection: Association with Danger Signals

Infants with a history of prematurity and bronchopulmonary dysplasia have a high risk of asthma and viral-induced exacerbations later in life. We hypothesized that hyperoxic exposure, a predisposing factor to bronchopulmonary dysplasia, modulates the innate immune response, producing an exaggerated proinflammatory reaction to viral infection. Two- to 3-d-old C57BL/6J mice were exposed to air or 75% oxygen for 14 d. Mice were infected intranasally with rhinovirus (RV) immediately after O2 exposure. Lung mRNA and protein expression, histology, dendritic cells (DCs), and airway responsiveness were assessed 1-12 d postinfection. Tracheal aspirates from premature human infants were collected for mRNA detection. Hyperoxia increased lung IL-12 expression, which persisted up to 12 d postexposure. Hyperoxia-exposed RV-infected mice showed further increases in IL-12 and increased expression of IFN-γ, TNF-α, CCL2, CCL3, and CCL4, as well as increased airway inflammation and responsiveness. In RV-infected, air-exposed mice, the response was not significant. Induced IL-12 expression in hyperoxia-exposed, RV-infected mice was associated with increased IL-12-producing CD103(+) lung DCs. Hyperoxia also increased expression of Clec9a, a CD103(+) DC-specific damaged cell-recognition molecule. Hyperoxia increased levels of ATP metabolites and expression of adenosine receptor A1, further evidence of cell damage and related signaling. In human preterm infants, tracheal aspirate Clec9a expression positively correlated with the level of prematurity. Hyperoxic exposure increases the activation of CD103(+), Clec9a(+) DCs, leading to increased inflammation and airway hyperresponsiveness upon RV infection. In premature infants, danger signal-induced DC activation may promote proinflammatory airway responses, thereby increasing respiratory morbidity.

Hyperoxia activates ATM independent from mitochondrial ROS and dysfunction

High levels of oxygen (hyperoxia) are often used to treat individuals with respiratory distress, yet prolonged hyperoxia causes mitochondrial dysfunction and excessive reactive oxygen species (ROS) that can damage molecules such as DNA. Ataxia telangiectasia mutated (ATM) kinase is activated by nuclear DNA double strand breaks and delays hyperoxia-induced cell death through downstream targets p53 and p21. Evidence for its role in regulating mitochondrial function is emerging, yet it has not been determined if mitochondrial dysfunction or ROS activates ATM. Because ATM maintains mitochondrial homeostasis, we hypothesized that hyperoxia induces both mitochondrial dysfunction and ROS that activate ATM. In A549 lung epithelial cells, hyperoxia decreased mitochondrial respiratory reserve capacity at 12h and basal respiration by 48 h. ROS were significantly increased at 24h, yet mitochondrial DNA double strand breaks were not detected. ATM was not required for activating p53 when mitochondrial respiration was inhibited by chronic exposure to antimycin A. Also, ATM was not further activated by mitochondrial ROS, which were enhanced by depleting manganese superoxide dismutase (SOD2). In contrast, ATM dampened the accumulation of mitochondrial ROS during exposure to hyperoxia. Our findings suggest that hyperoxia-induced mitochondrial dysfunction and ROS do not activate ATM. ATM more likely carries out its canonical response to nuclear DNA damage and may function to attenuate mitochondrial ROS that contribute to oxygen toxicity.

Role of GDF15 (growth and differentiation factor 15) in pulmonary oxygen toxicity

GDF15 (growth and differentiation factor 15) is a secreted cytokine, a direct target of p53 and plays a role in cell proliferation and apoptosis. It is induced by oxidative stress and has anti-apoptotic effects. The role of GDF15 in hyperoxic lung injury is unknown. We tested the hypothesis that GDF15 will be induced in vitro, in a model of pulmonary oxygen toxicity, and will play a critical role in decreasing cell death and oxidative stress. BEAS-2B (human bronchial epithelial cells) and human pulmonary vascular endothelial cells (HPMEC) were exposed to hyperoxia, and expression of GDF15 and effect of GDF15 disruption on cell viability and oxidative stress was determined. Furthermore, we studied the effect of p53 knockdown on GDF15 expression. In vitro, both BEAS-2B and HPMEC cells showed a significant increase in GDF15 expression upon exposure to hyperoxia. After GDF15 knockdown, there was a significant decrease in cell viability and increase in oxidative stress compared to control cells transfected with siRNA with a scrambled sequence. Knockdown of p53 significantly decreased the induction of GDF15 by hyperoxia. In conclusion, we show that GDF15 has a pro-survival and anti-oxidant role in hyperoxia and that p53 plays a key role in its induction.

Respiratory consequences of prematurity: evolution of a diagnosis and development of a comprehensive approach

Bronchopulmonary dysplasia (BPD) is the most common respiratory consequence of premature birth and contributes to significant short- and long-term morbidity, mortality and resource utilization. Initially defined as a radiographic, clinical and histopathological entity, the chronic lung disease known as BPD has evolved as obstetrical and neonatal care have improved the survival of lower gestational age infants. Now, definitions based on the need for supplementary oxygen at 28 days and/or 36 weeks provide a useful reference point in the neonatal intensive-care unit (NICU), but are no longer based on histopathological findings, and are neither designed to predict longer term respiratory consequences nor to study the evolution of a multifactorial disease. The aims of this review are to critically examine the evolution of the diagnosis of BPD and the challenges inherent to current classifications. We found that the increasing use of respiratory support strategies that administer ambient air without supplementary oxygen confounds oxygen-based definitions of BPD. Furthermore, lack of reproducible, genetic, biochemical and physiological biomarkers limits the ability to identify an impending BPD for early intervention, quantify disease severity for standardized classification and approaches and reliably predict the long-term outcomes. More comprehensive, multidisciplinary approaches to overcome these challenges involve longitudinal observation of extremely preterm infants, not only those with BPD, using genetic, environmental, physiological and clinical data as well as large databases of patient samples. The Prematurity and Respiratory Outcomes Program (PROP) will provide such a framework to address these challenges through high-resolution characterization of both NICU and post-NICU discharge outcomes.

Adapted approach to profile genes while reconciling Vegf-a mRNA expression in the developing and injured lung

During lung development and injury, messenger RNA (mRNA) transcript levels of genes fluctuate over both space and time. Quantitative PCR (qPCR) is a highly sensitive, widely used technique to measure the mRNA levels. The sensitivity of this technique can be disadvantageous and errors amplified when each qPCR assay is not validated. In contrast to other organs, lungs have high RNase activity, resulting in less than optimal RNA integrity. We implemented a strategy to address these limitations in developing and injured lungs. Parameters were established and a filter designed that optimized amplicon length and included or excluded samples based on RNA integrity. This approach was illustrated and validated by measuring mRNA levels including Vegf-a in newborn mouse lungs that were injured by 85% oxygen (hyperoxia) for 12 days and compared with control (normoxia). We demonstrate that, in contrast to contradictory Vegf-a expression when normalized to the least suitable housekeeping genes, application of this filter and normalization to most suitable three housekeeping genes, Hprt, Eef2, and Rpl13a, gave reproducible Vegf-a expression, thus corroborating the sample filter. Accordingly, both short amplicon length and proper normalization to ranked, evaluated genes minimized erroneous fluctuation and qPCR amplification issues associated with nonideal RNA integrity in injured and developing lungs. Furthermore, our work uncovers how RNA integrity, purity, amplicon length, and discovery of stable candidate reference genes enhance precision of qPCR results and utilizes the advantages of qPCR in developmental studies.

Association between oxidative DNA damage and the expression of 8-oxoguanine DNA glycosylase 1 in lung epithelial cells of neonatal rats exposed to hyperoxia

Previous studies have demonstrated that oxidative stress‑induced lung injury is involved in the occurrence and developmental process of bronchopulmonary dysplasia (BPD). The present study assessed whether oxidative DNA damage occurs in the early stages of hyperoxia‑induced BPD in neonatal rats and evaluated the expression and localization of the DNA repair gene, 8‑oxoguanine DNA glycosylase 1 (OGG1), upon exposure to hyperoxia. Neonatal rats and primary cultured neonatal rat alveolar epithelial type II (AECII) cells were exposed to hyperoxia (90% O2) or normoxia (21% O2) and the expression levels of 8‑hydroxy‑2'‑deoxyguanosine (8‑OHdG) in the lung tissues and AECII cells were determined using a competitive enzyme‑linked immunosorbent assay. DNA strand breaks in the AECII cells were detected using a comet assay. The expression and localization of the OGG1 protein in the lung tissues and AECII cells were determined by immunofluorescence confocal microscopy and western blotting. The mRNA expression levels of OGG1 in the lung tissues and AECII cells were determined by reverse transcription polymerase chain reaction. The expression of 8‑OHdG was elevated in the hyperoxia‑exposed neonatal rat lung tissue and the AECII cells compared with the normoxic controls. The occurrence of DNA strand breaks in the AECII cells increased with increasing duration of hyperoxia exposure. The protein expression of OGG1 was significantly increased in the hyperoxia‑exposed lung tissues and AECII cells, with OGG1 preferentially localized to the cytoplasm. No concomitant increase in the mRNA expression of OGG1 was detected. These results revealed that oxidative DNA damage occurred in lung epithelial cells during early‑stage BPD, as confirmed by in vitro and in vivo hyperoxia exposure experiments, and the increased expression of OGG1 was associated with this process.

Animal models of bronchopulmonary dysplasia. The preterm and term rabbit models

Bronchopulmonary dysplasia (BPD) is an important lung developmental pathophysiology that affects many premature infants each year. Newborn animal models employing both premature and term animals have been used over the years to study various components of BPD. This review describes some of the neonatal rabbit studies that have contributed to the understanding of BPD, including those using term newborn hyperoxia exposure models, premature hyperoxia models, and a term newborn hyperoxia model with recovery in moderate hyperoxia, all designed to emulate aspects of BPD in human infants. Some investigators perturbed these models to include exposure to neonatal infection/inflammation or postnatal malnutrition. The similarities to lung injury in human premature infants include an acute inflammatory response with the production of cytokines, chemokines, and growth factors that have been implicated in human disease, abnormal pulmonary function, disordered lung architecture, and alveolar simplification, development of fibrosis, and abnormal vascular growth factor expression. Neonatal rabbit models have the drawback of limited access to reagents as well as the lack of readily available transgenic models but, unlike smaller rodent models, are able to be manipulated easily and are significantly less expensive than larger animal models.

Oxygen differentially affects the hox proteins Hoxb5 and Hoxa5 altering airway branching and lung vascular formation

Hoxb5 and Hoxa5 transcription factor proteins uniquely impact lung morphogenesis at the developmental time point when extremely preterm infants are born. The effect of O2 exposure (0.4 FiO2) used in preterm infant care on these Hox proteins is unknown. We used ex vivo fetal mouse lung organ cultures to explore the effects of 0.4 FiO2 on lung airway and vascular formation in the context of Hoxb5 and Hoxa5 expression and regulation. Compared to room air, 48 h (h) 0.4 FiO2 adversely attenuated airway and microvasculature formation while reducing lung growth and epithelial cell volume, and increasing mesenchymal volume. 0.4 FiO2 decreased pro-angiogenic Hoxb5 and VEGFR2 while not altering protein levels of angiostatic Hoxa5. Lungs returned to RA after 24 h 0.4FiO2 had partial structural recovery but remained smaller and less developed. Mesenchymal cell apoptosis increased and proliferation decreased with time in O2 while epithelial cell proliferation significantly increased. Hoxb5 overexpression led to prominent peri-airway VEGFR2 expression and promoted lung vascular and airway patterning. Hoxa5 overexpression had the opposite effects. We conclude that 0.4 FiO2 exposure causes a profound loss of airway and lung microvascular development that occurs partially via reduction in pro-angiogenic Hoxb5 while angiostatic Hoxa5 expression is maintained.

Differential concentration-specific effects of caffeine on cell viability, oxidative stress, and cell cycle in pulmonary oxygen toxicity in vitro

Caffeine is used to prevent bronchopulmonary dysplasia (BPD) in premature neonates. Hyperoxia contributes to the development of BPD, inhibits cell proliferation and decreases cell survival. The mechanisms responsible for the protective effect of caffeine in pulmonary oxygen toxicity remain largely unknown. A549 and MLE 12 pulmonary epithelial cells were exposed to hyperoxia or maintained in room air, in the presence of different concentrations (0, 0.05, 0.1 and 1mM) of caffeine. Caffeine had a differential concentration-specific effect on cell cycle progression, oxidative stress and viability, with 1mM concentration being deleterious and 0.05 mM being protective. Reactive oxygen species (ROS) generation during hyperoxia was modulated by caffeine in a similar concentration-specific manner. Caffeine at 1mM, but not at the 0.05 mM concentration decreased the G2 arrest in these cells. Taken together this study shows the novel funding that caffeine has a concentration-specific effect on cell cycle regulation, ROS generation, and cell survival in hyperoxic conditions.

The role of hyperoxia in the pathogenesis of experimental BPD

Supplemental oxygen is often used as a life-saving therapy in the treatment of preterm infants. However, its protracted use can lead to the development of bronchopulmonary dysplasia (BPD), and more recently, has been associated with adversely affecting the general health of children and adolescents who were born preterm. Efforts to understand how exposure to excess oxygen can disrupt lung development have historically focused on the interplay between oxidative stress and antioxidant defense mechanisms. However, there has been a growing appreciation for how changes in gene-environment interactions occurring during critically important periods of organ development can profoundly affect human health and disease later in life. Here, we review the concept that oxygen is an environmental stressor that may play an important role at birth to control normal lung development via its interactions with genes and cells. Understanding how changes in the oxygen environment have the potential to alter the developmental programing of the lung, such that it now proceeds along a different developmental trajectory, could lead to novel therapies in the prevention and treatment of respiratory diseases, such as BPD.

Neonatal hyperoxia: effects on nephrogenesis and long-term glomerular structure

Preterm neonates are born while nephrogenesis is ongoing and are commonly exposed to factors in the extrauterine environment that may impair renal development. Supplemental oxygen therapy exposes the preterm infant to a hyperoxic environment that may induce oxidative stress. Our aim was to determine the immediate and long-term effects of exposure to hyperoxia, during the period of postnatal nephrogenesis, on renal development. Newborn mice (C57BL/6J) were kept in a normoxic (room air, 21% oxygen) or a controlled hyperoxic (65% oxygen) environment from birth to postnatal day 7 (P7d). From P7d, animals were maintained in room air until early adulthood at postnatal day 56 (P56d) or middle age (10 mo; P10mo). Pups were assessed for glomerular maturity and renal corpuscle cross-sectional area at P7d (control n = 14; hyperoxic n = 14). Nephron number and renal corpuscle size were determined stereologically at P56d (control n = 14; hyperoxic n = 14) and P10mo (control n = 10; hyperoxic n = 10). At P7d, there was no effect of hyperoxia on glomerular size or maturity. In early adulthood (P56d), body weights, relative kidney weights and volumes, and nephron number were not different between groups, but the renal corpuscles were significantly enlarged. This was no longer evident at P10mo, with relative kidney weights and volumes, nephron number, and renal corpuscle size not different between groups. Furthermore, hyperoxia exposure did not significantly accelerate glomerulosclerosis in middle age. Hence, our findings show no overt long-term deleterious effects of early life hyperoxia on glomerular structure.

Zfp148 deficiency causes lung maturation defects and lethality in newborn mice that are rescued by deletion of p53 or antioxidant treatment

The transcription factor Zfp148 (Zbp-89, BFCOL, BERF1, htβ) interacts physically with the tumor suppressor p53 and is implicated in cell cycle control, but the physiological role of Zfp148 remains unknown. Here we show that Zfp148 deficiency leads to respiratory distress and lethality in newborn mice. Zfp148 deficiency prevented structural maturation of the prenatal lung without affecting type II cell differentiation or surfactant production. BrdU analyses revealed that Zfp148 deficiency caused proliferation arrest of pulmonary cells at E18.5–19.5. Similarly, Zfp148-deficient fibroblasts exhibited proliferative arrest that was dependent on p53, raising the possibility that cell stress is part of the underlying mechanism. Indeed, Zfp148 deficiency lowered the threshold for activation of p53 under oxidative conditions. Moreover, both in vivo and cellular phenotypes were rescued on Trp53^+/− or Trp53^−/− backgrounds and by antioxidant treatment. Thus, Zfp148 prevents respiratory distress and lethality in newborn mice by attenuating oxidative stress–dependent p53-activity during the saccular stage of lung development. Our results establish Zfp148 as a novel player in mammalian lung maturation and demonstrate that Zfp148 is critical for cell cycle progression in vivo.

Mitochondrial DNA damage mediates hyperoxic dysmorphogenesis in rat fetal lung explants

BACKGROUND

Numerous studies in cultured cells indicate that damage to mitochondrial DNA (mtDNA) dictates cellular responses to oxidant stress, yet the consequences of mtDNA damage have not been studied directly in the preterm lung.

OBJECTIVE

We sought to determine whether hyperoxia-induced fetal lung dysmorphogenesis is linked to mtDNA damage and establish mtDNA repair as a potential therapeutic approach for treating lung dysplasia in the preterm neonate.

METHODS

Hyperoxia-induced mtDNA damage was assessed by quantitative alkaline gel electrophoresis in normoxic (3% O2) and hyperoxic (21% O2) fetal rat lung explants. A fusion protein construct targeting the DNA repair enzyme endonuclease III (Endo III) to the mitochondria was used to augment mtDNA repair. Fetal lung branching and surfactant protein C (SFPTC) were assessed in these tissues.

RESULTS

Hyperoxia induced mtDNA damage in lung explants and was accompanied by impaired branching morphogenesis and decreased SFPTC mRNA expression. Treatment of lung explants with Endo III fusion protein prevented hyperoxia-induced mtDNA damage and restored normal branching morphogenesis and SFPTC mRNA expression.

CONCLUSION

These findings support the concept that mtDNA governs cellular responses to oxidant stress in the fetal lung and suggest that modulation of mtDNA repair is a potential pharmacologic strategy in the prevention of hyperoxic lung injury.

Lung injury in preterm neonates: the role and therapeutic potential of stem cells

Continuous improvements in perinatal care have allowed the survival of ever more premature infants, making the task of protecting the extremely immature lung from injury increasingly challenging. Premature infants at risk of developing chronic lung disease or bronchopulmonary dysplasia (BPD) are now born at the late canalicular stage of lung development, just when the airways become juxtaposed to the lung vasculature and when gas-exchange becomes possible. Readily available strategies, including improved antenatal management (education, regionalization, steroids, and antibiotics), together with exogenous surfactant and exclusive/early noninvasive ventilatory support, will likely decrease the incidence/severity of BPD over the next few years. Nonetheless, because of the extreme immaturity of the developing lung, the extent to which disruption of lung growth after prematurity and neonatal management lead to an earlier or more aggravated decline in respiratory function in later life is a matter of concern. Consequently, much more needs to be learned about the mechanisms of lung development, injury, and repair. Recent insight into stem cell biology has sparked interest for stem cells to repair damaged organs. This review summarizes the exciting potential of stem cell-based therapies for lung diseases in general and BPD in particular.

Genome-wide transcriptional profiling reveals connective tissue mast cell accumulation in bronchopulmonary dysplasia

RATIONALE

Bronchopulmonary dysplasia (BPD) is a major complication of premature birth. Risk factors for BPD are complex and include prenatal infection and O(2) toxicity. BPD pathology is equally complex and characterized by inflammation and dysmorphic airspaces and vasculature. Due to the limited availability of clinical samples, an understanding of the molecular pathogenesis of this disease and its causal mechanisms and associated biomarkers is limited.

OBJECTIVES

Apply genome-wide expression profiling to define pathways affected in BPD lungs.

METHODS

Lung tissue was obtained at autopsy from 11 BPD cases and 17 age-matched control subjects without BPD. RNA isolated from these tissue samples was interrogated using microarrays. Standard gene selection and pathway analysis methods were applied to the data set. Abnormal expression patterns were validated by quantitative reverse transcriptase-polymerase chain reaction and immunohistochemistry.

MEASUREMENTS AND MAIN RESULTS

We identified 159 genes differentially expressed in BPD tissues. Pathway analysis indicated previously appreciated (e.g., DNA damage regulation of cell cycle) as well as novel (e.g., B-cell development) biological functions were affected. Three of the five most highly induced genes were mast cell (MC)-specific markers. We confirmed an increased accumulation of connective tissue MC(TC) (chymase expressing) mast cells in BPD tissues. Increased expression of MC(TC) markers was also demonstrated in an animal model of BPD-like pathology.

CONCLUSIONS

We present a unique genome-wide expression data set from human BPD lung tissue. Our data provide information on gene expression patterns associated with BPD and facilitated the discovery that MC(TC) accumulation is a prominent feature of this disease. These observations have significant clinical and mechanistic implications.

Maternal and fetal origins of lung disease in adulthood

This review focuses on genetic and environmental influences that result in long term alterations in lung structure and function. Environmental factors operating during fetal and early postnatal life can have persistent effects on lung development and so influence lung function and respiratory health throughout life. Common factors affecting the quality of the intrauterine environment that can alter lung development include fetal nutrient and oxygen availability leading to intrauterine growth restriction, fetal intrathoracic space, intrauterine infection or inflammation, maternal tobacco smoking and other drug exposures. Similarly, factors that operate during early postnatal life, such as mechanical ventilation and high FiO(2) in the case of preterm birth, undernutrition, exposure to tobacco smoke and respiratory infections, can all lead to persistent alterations in lung structure and function. Greater awareness of the many prenatal and early postnatal factors that can alter lung development will help to improve lung development and hence respiratory health throughout life.

VEGF levels in humans and animal models with RDS and BPD: temporal relationships

Respiratory distress syndrome (RDS) and bronchopulmonary dysplasia (BPD) contribute significantly to neonatal morbidity and mortality. Pulmonary function depends on the interaction between alveolar microvasculature and airspace development. While it has been shown in various animal models that vascular endothelial growth factor (VEGF) and its receptors increase in normal animal lung development, its pathophysiological role in neonatal respiratory failure is not yet entirely clear. Current animal and human studies exhibit controversial results. Though animal models are invaluable tools in the study of human lung disease, inherent differences in physiology mandate clarification of the timing of these studies to ensure that they appropriately correlate with the human stages of lung development. The purpose of this review article is to highlight the importance of considering the temporal relationship of VEGF and lung development in human neonates and developmentally-appropriate animal models with RDS and BPD.

Neonatal hyperoxia causes pulmonary vascular disease and shortens life span in aging mice

Bronchopulmonary dysplasia is a chronic lung disease observed in premature infants requiring oxygen supplementation and ventilation. Although the use of exogenous surfactant and protective ventilation strategies has improved survival, the long-term pulmonary consequences of neonatal hyperoxia are unknown. Here, we investigate whether neonatal hyperoxia alters pulmonary function in aging mice. By 67 weeks of age, mice exposed to 100% oxygen between postnatal days 1 to 4 showed significantly a shortened life span (56.6% survival, n = 53) compared to siblings exposed to room air as neonates (100% survival, n = 47). Survivors had increased lung compliance and decreased elastance. There was also right ventricular hypertrophy and pathological evidence for pulmonary hypertension, defined by reduction of the distal microvasculature and the presence of numerous dilated arterioles expressing von Willebrand factor and α-smooth muscle actin. Consistent with recent literature implicating bone morphogenetic protein (BMP) signaling in pulmonary vascular disease, BMP receptors and downstream phospho-Smad1/5/8 were reduced in lungs of aging mice exposed to neonatal oxygen. BMP signaling alterations were not observed in 8-week-old mice. These data suggest that loss of BMP signaling in aged mice exposed to neonatal oxygen is associated with a shortened life span, pulmonary vascular disease, and associated cardiac failure. People exposed to hyperoxia as neonates may be at increased risk for pulmonary hypertension.

Prolonged mechanical ventilation induces cell cycle arrest in newborn rat lung

RATIONALE

The molecular mechanism(s) by which mechanical ventilation disrupts alveolar development, a hallmark of bronchopulmonary dysplasia, is unknown.

OBJECTIVE

To determine the effect of 24 h of mechanical ventilation on lung cell cycle regulators, cell proliferation and alveolar formation in newborn rats.

METHODS

Seven-day old rats were ventilated with room air for 8, 12 and 24 h using relatively moderate tidal volumes (8.5 mL.kg⁻¹).

MEASUREMENT AND MAIN RESULTS

Ventilation for 24 h (h) decreased the number of elastin-positive secondary crests and increased the mean linear intercept, indicating arrest of alveolar development. Proliferation (assessed by BrdU incorporation) was halved after 12 h of ventilation and completely arrested after 24 h. Cyclin D1 and E1 mRNA and protein levels were decreased after 8-24 h of ventilation, while that of p27(Kip1) was significantly increased. Mechanical ventilation for 24 h also increased levels of p57(Kip2), decreased that of p16(INK4a), while the levels of p21(Waf/Cip1) and p15(INK4b) were unchanged. Increased p27(Kip1) expression coincided with reduced phosphorylation of p27(Kip1) at Thr¹⁵⁷, Thr¹⁸⁷ and Thr¹⁹⁸ (p<0.05), thereby promoting its nuclear localization. Similar -but more rapid- changes in cell cycle regulators were noted when 7-day rats were ventilated with high tidal volume (40 mL.kg⁻¹) and when fetal lung epithelial cells were subjected to a continuous (17% elongation) cyclic stretch.

CONCLUSION

This is the first demonstration that prolonged (24 h) of mechanical ventilation causes cell cycle arrest in newborn rat lungs; the arrest occurs in G₁ and is caused by increased expression and nuclear localization of Cdk inhibitor proteins (p27(Kip1), p57(Kip2)) from the Kip family.

Highly conserved transcriptional responses to mechanical ventilation of the lung

Cross-species analysis of microarray data has shown improved discriminating power between healthy and diseased states. Computational approaches have proven effective in deciphering the complexity of human disease by identifying upstream regulatory elements and the transcription factors that interact with them. Here we used both methods to identify highly conserved transcriptional responses during mechanical ventilation, an important therapeutic treatment that has injurious side effects. We generated control and ventilated whole lung samples from the premature baboon model of bronchopulmonary dysplasia (BPD), processed them for microarray, and combined them with existing whole lung oligonucleotide microarray data from 85 additional control samples from mouse, rat, and human and 19 additional ventilated samples from mouse and rat. Of the 2,531 orthologs shared by all 114 samples, 60 were modulated by mechanical ventilation [false discovery rate (FDR)-adjusted q value (q(FDR)) = 0.005, ANOVA]. These included transcripts encoding the transcription factors ATF3 and FOS. Because of compelling known roles for these transcription factors, we used computational methods to predict their targets in the premature baboon model of BPD, which included elastin (ELN), gastrin-releasing polypeptide (GRP), and connective tissue growth factor (CTGF). This approach identified highly conserved transcriptional responses to mechanical ventilation and may facilitate identification of therapeutic targets to reduce the side effects of this valuable treatment.

Chronic lung disease in preterm lambs: effect of daily vitamin A treatment on alveolarization

Neonatal chronic lung disease is characterized by failed formation of alveoli and capillaries, and excessive deposition of matrix elastin, which are linked to lengthy mechanical ventilation (MV) with O(2)-rich gas. Vitamin A supplementation has improved respiratory outcome of premature infants, but there is little information about the structural and molecular manifestations in the lung that occur with vitamin A treatment. We hypothesized that vitamin A supplementation during prolonged MV, without confounding by antenatal steroid treatment, would improve alveolar secondary septation, decrease thickness of the mesenchymal tissue cores between distal air space walls, and increase alveolar capillary growth. We further hypothesized that these structural advancements would be associated with modulated expression of tropoelastin and deposition of matrix elastin, phosphorylated Smad2 (pSmad2), cleaved caspase 3, proliferating cell nuclear antigen (PCNA), VEGF, VEGF-R2, and midkine in the parenchyma of the immature lung. Eight preterm lambs (125 days' gestation, term approximately 150 days) were managed by MV for 3 wk: four were treated with daily intramuscular Aquasol A (vitamin A), 5,000 IU/kg, starting at birth; four received vehicle alone. Postmortem lung assays included quantitative RT-PCR and in situ hybridization, immunoblot and immunohistochemistry, and morphometry and stereology. Daily vitamin A supplementation increased alveolar secondary septation, decreased thickness of the mesenchymal tissue cores between the distal air space walls, and increased alveolar capillary growth. Associated molecular changes were less tropoelastin mRNA expression, matrix elastin deposition, pSmad2, and PCNA protein localization in the mesenchymal tissue core of the distal air space walls. On the other hand, mRNA expression and protein abundance of VEGF, VEGF-R2, midkine, and cleaved caspase 3 were increased. We conclude that vitamin A treatment partially improves lung development in chronically ventilated preterm neonates by modulating expression of tropoelastin, deposition of elastin, and expression of vascular growth factors.

The 'new' bronchopulmonary dysplasia: challenges and commentary

Lung development is orchestrated by highly integrated morphogenic programs of interrelated patterns of gene and protein expression. Injury to the developing lung in the canalicular and saccular phases of lung development alters subsequent alveolar and vascular development resulting in simplified alveolar structures, dysmorphic capillary configuration, variable interstitial cellularity and fibroproliferation that are characteristic of the 'new' bronchopulmonary dysplasia (BPD). Fetal and neonatal infection, abnormal stretch of the developing airways and alveoli, altered expression of surfactant proteins (or genetically altered proteins), polymorphisms of genes encoding for vascular endothelial growth factors, and reactive oxygen species result in imparied gas exchange in the developing lung. However, the 'new' BPD represents only one form of neonatal chronic lung disease and the consistent use of both the physiologic definition and severity scale would provide greater accuracy in determining the impact of the disease currently defined by its treatment. Our present labelling of the clinical state of oxygen supplementation and/or ventilatory support at 36 weeks' postmenstrual age and the histopathologic severity of alveolar arrest and vascular 'simplification' may not always be predictive of the degree of altered lung development and thus longer-term pulmonary function evaluations are needed to determine the impact of this disorder in specific infants. The proposed role of novel molecular therapies, and the combined effects of currently established therapies, as well as exogenous surfactant and inhaled nitric oxide or repetitive surfactant dosing, on the severity and incidence of new BPD hold considerable promise for reducing the long-term pulmonary morbidity among infants delivered prematurely.

Neonatal oxygen adversely affects lung function in adult mice without altering surfactant composition or activity

Despite its potentially adverse effects on lung development and function, supplemental oxygen is often used to treat premature infants in respiratory distress. To understand how neonatal hyperoxia can permanently disrupt lung development, we previously reported increased lung compliance, greater alveolar simplification, and disrupted epithelial development in adult mice exposed to 100% inspired oxygen fraction between postnatal days 1 and 4. Here, we investigate whether oxygen-induced changes in lung function are attributable to defects in surfactant composition and activity, structural changes in alveolar development, or both. Newborn mice were exposed to room air or 40%, 60%, 80%, or 100% oxygen between postnatal days 1 and 4 and allowed to recover in room air until 8 wk of age. Lung compliance and alveolar size increased, and airway resistance, airway elastance, tissue elastance, and tissue damping decreased, in mice exposed to 60-80% oxygen; changes were even greater in mice exposed to 100% oxygen. These alterations in lung function were not associated with changes in total protein content or surfactant phospholipid composition in bronchoalveolar lavage. Moreover, surface activity and total and hydrophobic protein content were unchanged in large surfactant aggregates centrifuged from bronchoalveolar lavage compared with control. Instead, the number of type II cells progressively declined in 60-100% oxygen, whereas levels of T1alpha, a protein expressed by type I cells, were comparably increased in mice exposed to 40-100% oxygen. Thickened bundles of elastin fibers were also detected in alveolar walls of mice exposed to > or = 60% oxygen. These findings support the hypothesis that changes in lung development, rather than surfactant activity, are the primary causes of oxygen-altered lung function in children who were exposed to oxygen as neonates. Furthermore, the disruptive effects of oxygen on epithelial development and lung mechanics are not equivalently dose dependent.

Manipulation of gene expression by oxygen: a primer from bedside to bench

For nearly 100 y, pediatricians have regularly used oxygen to treat neonatal and childhood diseases. During this time, it has become clear that oxygen is toxic and that overzealous use can lead to significant morbidity. As we have learned more about the appropriate clinical indications for oxygen therapy, studies at the bench have begun to elucidate the molecular mechanisms by which cells respond to hyperoxia. In this review, we discuss transcription factors whose activity is regulated by oxygen, including nuclear factor, erythroid 2-related factor 2 (Nrf2), activator protein 1 (AP-1), p53, nuclear factor kappaB (NF-kappaB), signal transducers and activators of transcription protein (STAT), and ccat/enhancer binding protein (CEBP). Special attention is paid to the mechanisms by which hyperoxia affects these transcription factors in the lung. Finally, we identify downstream targets of these transcription factors, with a focus on heme oxygenase-1. A better understanding of how oxygen affects various signaling pathways could lead to interventions aimed at preventing hyperoxic injury.

Soluble vascular endothelial growth factor receptor 1 in tracheal aspirate fluid of preterm neonates at birth may be predictive of bronchopulmonary dysplasia/chronic lung disease

OBJECTIVE

We tested the hypothesis that soluble vascular endothelial growth factor receptors are involved in the development of bronchopulmonary dysplasia/chronic lung disease.

PATIENTS AND METHODS

Neonates with a birth weight of < or =1500 g and/or < or =30 weeks' gestation, with respiratory failure, requiring O(2) and mechanical ventilation within 24 hours, were eligible. Tracheal aspirate fluid samples were collected from 65 neonates before surfactant and/or assisted mechanical ventilation (baseline), at 3 and 7 days after birth, and weekly thereafter until extubation. Samples were analyzed for total vascular endothelial growth factor, soluble vascular endothelial growth factor receptor 1 and 2 levels and compared in infants with bronchopulmonary dysplasia/chronic lung disease (n = 31) versus those with no bronchopulmonary dysplasia/chronic lung disease (n = 34).

RESULTS

Mean gestational age and birth weight were lower in infants with bronchopulmonary dysplasia/chronic lung disease. At baseline, vascular endothelial growth factor levels in the tracheal aspirate fluid were significantly lower, whereas soluble vascular endothelial growth factor receptor 1 levels were higher in the bronchopulmonary dysplasia/chronic lung disease infants compared with infants with no bronchopulmonary dysplasia/chronic lung disease. Vascular endothelial growth factor levels progressively increased from baseline to 4 weeks in all of the infants developing bronchopulmonary dysplasia/chronic lung disease. Conversely, soluble vascular endothelial growth factor receptor 1 declined in both groups from baseline to 5 weeks of age. Similarly, soluble vascular endothelial growth factor receptor 2 declined from baseline to 5 weeks in the control infants, but there were significant increases at 3 and 4 weeks in infants developing bronchopulmonary dysplasia/chronic lung disease.

CONCLUSIONS

We speculate that low vascular endothelial growth factor levels in tracheal aspirate fluid, concurrent with elevated soluble vascular endothelial growth factor receptor 1 levels on the first day of life, are biological markers for the development of bronchopulmonary dysplasia/chronic lung disease in very low birth weight infants requiring O(2) and assisted mechanical ventilation.

Alterations of the thioredoxin system by hyperoxia: implications for alveolar development

Alterations in vascular endothelial growth factor (VEGF) contribute to alveolar simplification seen in animal models of bronchopulmonary dysplasia, and VEGF expression is redox regulated by thioredoxin (Trx)-1 in other diseases. The present studies tested the hypothesis that exposure to 85% O2 negatively impacts the Trx1 system and VEGF expression in the lungs of newborn mice. There was no effect of fraction of inspired oxygen on lung Trx1 or Trx reductase-1 protein levels; however, lung Trx1 protein was predominantly oxidized in the lungs of newborn mice exposed to 85% O2 by 24 hours of exposure. In room air (RA), lung Trx interacting protein (Txnip) levels decreased developmentally through Day 7 (1.0 +/- 0.06 [Day 1] vs. 0.49 +/- 0.10 [Day 3] vs. 0.29 +/- 0.03 [Day 7]; P < 0.01), whereas VEGF expression increased (1.25 +/- 0.16 [Day 1] vs. 4.35 +/- 1.51 [Day 3] vs. 13.23 +/- 0.37 [Day 7]; P < 0.01). Newborn mice exposed to 85% O2 had no developmental decrease in Txnip protein levels and a delayed increase in VEGF protein levels. Lung Txnip and VEGF protein levels were different than in corresponding RA controls at Day 3, before the detection of lung morphologic abnormalities in our model. Txnip and VEGF protein levels were inversely correlated in both the RA and hyperoxia-exposed groups (n = 18; R = -0.66; P = 0.003). In conclusion, oxidation of Trx1 and sustained Txnip expression in the lungs of newborn mice exposed to 85% oxygen is likely to severely attenuate normal Trx1 function. The inverse correlation of Txnip with VEGF expression suggests that decreased Trx1 function contributes to the observed lung developmental abnormalities.

Pulse oxygen saturation levels and arterial oxygen tension values in newborns receiving oxygen therapy in the neonatal intensive care unit: is 85% to 93% an acceptable range?

OBJECTIVE

Our aim was to define the relationship of PaO(2) and pulse oxygen saturation values during routine clinical practice and to evaluate whether pulse oxygen saturation values between 85% and 93% were associated with PaO(2) levels of <40 mmHg.

METHODS

Prospective comparison of PaO(2) and pulse oxygen saturation values in 7 NICUs at sea level in 2 countries was performed. The PaO(2) measurements were obtained from indwelling arterial catheters; simultaneous pulse oxygen saturation values were recorded if the pulse oxygen saturation values changed <1% before, during, and after the arterial gas sample was obtained.

RESULTS

We evaluated 976 paired PaO(2)/pulse oxygen saturation values in 122 neonates. Of the 976 samples, 176 (18%) from infants breathing room air had a mean pulse oxygen saturation of 93.9 +/- 4.3% and a median of 95.5%. The analysis of 800 samples from infants breathing supplemental oxygen revealed that, when pulse oxygen saturation values were 85% to 93%, the mean PaO(2) was 56 +/- 14.7 mmHg and the median 54 mmHg. At this pulse oxygen saturation level, 86.8% of the samples had PaO(2) values of 40 to 80 mmHg, 8.6% had values of <40 mmHg, and 4.6% had values of >80 mmHg. When the pulse oxygen saturation values were >93%, the mean PaO(2) was 107.3 +/- 59.3 mmHg and the median 91 mmHg. At this pulse oxygen saturation level, 39.5% of the samples had PaO(2) values of 40 to 80 mmHg and 59.5% had values of >80 mmHg.

CONCLUSIONS

High PaO(2) occurs very rarely in neonates breathing supplemental oxygen when their pulse oxygen saturation values are 85% to 93%. This pulse oxygen saturation range also is infrequently associated with low PaO(2) values. Pulse oxygen saturation values of >93% are frequently associated with PaO(2) values of >80 mmHg, which may be of risk for some newborns receiving supplemental oxygen.

Expression of angiostatin and its related factors in the plasma of newborn pigs with hypoxia and reoxygenation

Little is known about angiostatin and its related factors in the hypoxia-reoxygenation of neonates. In this study we compared the effect of 21% and 100% reoxygenation on temporal changes in the plasma level of these factors in newborn piglets subjected to hypoxia. Newborn piglets were subjected to 2 h hypoxia followed by 1 h of reoxygenation with either 21% or 100% oxygen and observed for 4 days. On day 4 of recovery in 100% hypoxic-reoxygenated group, there were increases in total angiostatin, plasminogen/plasmin and MMP-2 levels, and decreases in VEGF levels (vs. respective baseline levels, all P <0.001), whereas no significant temporal changes were found in the 21% hypoxic-reoxygenated and sham-operated groups. Angiostatin levels correlated positively with the levels of MMP-2 and HIF-1alpha and negatively with VEGF levels in 100% hypoxic-reoxygenated group (all P <0.05). In comparison to 21% oxygen, neonatal resuscitation with 100% oxygen was found to increase the levels anti-angiogenic factors.

Hyperoxia enhances VEGF release from A549 cells via post-transcriptional processes

Exposure of animals to hyperoxia decreases lung VEGF mRNA expression concomitant with an acute increase in VEGF protein within the epithelial lining fluid (ELF). The VEGF concentration in ELF is in excess of that found in the plasma, leading to the hypothesis that hyperoxia stimulates the release of VEGF protein from stores within the extracellular matrix. To test this hypothesis in a cell culture system, we exposed A549 cells to 95% O(2) (Ox) for 48 h followed by recovery in room air (RA) for 24 h. We found that Ox increased VEGF protein two- to threefold within the medium at 48 h of exposure and during recovery. Heparin clearing revealed the medium to contain a 50/50 mixture of the heparin-binding (VEGF(165)) and heparin-nonbinding (VEGF(121)) proteins and that Ox increased both proteins equally. Transcriptional activation of VEGF seems unlikely to explain the increase in VEGF protein, as expression of full-length and splice variant VEGF mRNA was unchanged by hyperoxia. Analysis of cell-associated VEGF proteins found that Ox increased the expression of VEGF(121) and VEGF(165) proteins. Blocking binding sites with exogenous heparin enhanced VEGF protein in the medium from RA-grown cells, whereas heparinase digestion of bound VEGF revealed a greater reserve of VEGF protein in RA cells. Collectively these findings indicate that hyperoxia enhances the expression of VEGF(121/165) proteins and facilitates the release of VEGF(165) from cell-associated stores. Increases in VEGF in ELF may represent an adaptive response fostering cell survival and type II cell proliferation in O(2)-induced lung injury.

Oxygen as a neonatal health hazard: call for détente in clinical practice

Education in oxygenation and in how oxygen is given to newborns needs to increase. Treatment with oxygen should no longer be considered proverbial and customary, regardless of our 'past experience' or consensus recommendations in clinical guidelines, since oxygen may lead to acute or chronic health effects.

CONCLUSION

Inappropriate oxygen use is a neonatal health hazard associated with aging, DNA damage and cancer, retinopathy of prematurity, injury to the developing brain, infection and others. Neonatal exposure to pure O2, even if brief, or to pulse oximetry >95% when breathing supplemental O2 must be avoided as much as possible.

The cellular basis for diverse responses to oxygen

Mammalian cells have divergent responses to varying oxygen levels. Cells exposed to low oxygen levels (hypoxia) activate the transcription factor hypoxia-inducible factor-1 (HIF-1) as an adaptive response. Cells exposed to hypoxia do not undergo senescence or cell death and do not diminish ATP levels. By contrast, cells exposed to high oxygen levels (hyperoxia) undergo senescence and cell death and decrease their ATP levels, yet do not activate HIF-1. Despite these divergent responses with respect to senescence, cell death, metabolism, and gene expression, the signaling events in both systems are mediated by the generation of mitochondrial-derived reactive oxygen species (ROS). This perspective reviews the role of signaling through mitochondrial ROS in hypoxic and hyperoxic environments.

Inflammation and bronchopulmonary dysplasia: a continuing story

Increasing evidence indicates that bronchopulmonary dysplasia (BPD) results, at least in part, from an imbalance between pro-inflammatory and anti-inflammatory mechanisms, with a persistent imbalance that favours pro-inflammatory mechanisms. The inflammatory response is characterised by an accumulation of neutrophils and macrophages in the airways and pulmonary tissue of preterm infants and, moreover, by an arsenal of pro-inflammatory mediators which affect the alveolar capillary unit and tissue integrity. As well as pro-inflammatory cytokines and toxic oxygen radicals, various lipid mediators as well as potent proteases may be responsible for acute lung injury. During the last decade it has become evident that multiple pre- and postnatal events contribute to the development of BPD in preterm infants. Chorioamnionitis and cytokine exposure in utero, plus sequential lung injury caused by postnatal resuscitation, oxygen toxicity, volu-, barotrauma and infection all lead to a pulmonary inflammatory response which is most probably associated with aberrant wound healing and an inhibition of alveolarisation as well as vascular development in the immature lungs of very preterm infants, causing the 'new BPD'.

SERPINB1 upregulation is associated with in vivo complex formation with neutrophil elastase and cathepsin G in a baboon model of bronchopulmonary dysplasia

Bronchopulmonary dysplasia (BPD) continues to be a major cause of morbidity in premature infants. An imbalance between neutrophil elastase and its inhibitors has been implicated in BPD. Serine protease inhibitor (SERPIN)B1 is an inhibitor of neutrophil proteases, including neutrophil elastase (NE) and cathepsin G (cat G). Recent studies suggest that SERPINB1 could provide protection in the airways by regulating excess protease activity associated with inflammatory lung disorders. In this study, we determined the distribution and ontogeny of SERPINB1 in the baboon lung and characterized the expression of SERPINB1 in baboon models of BPD. SERPINB1 expression was detected in the conducting airway and glandular epithelial cells in addition to neutrophils, macrophages, and mast cells. SERPINB1 mRNA and protein expression increased with advancing gestational age and in the new BPD model. In contrast, SERPINB1 expression levels were decreased in the old BPD model. Furthermore, SERPINB1 was detected as a high-molecular-mass (HMM) complex in lung tissue and bronchoalveolar lavage fluid samples from the BPD group. Analysis of the HMM complex by coimmunoprecipitation showed that these complexes were formed between SERPINB1 and NE or cat G. High-performance liquid chromatography (HPLC) ion trap mass spectrometry verified the presence of SERPINB1 in HMM complexes. Finally, NE activity level was compared between new and old baboon models of BPD and was found to be significantly lower in new BPD. Thus SERPINB1 upregulation in new BPD may be protective by contributing to the regulation of neutrophil proteases NE and cat G.