Hyperoxic inhibition of newborn rat lung development: protection by deferoxamine

Lung Development and Aging

The onset of chronic obstructive pulmonary disease (COPD) can arise either from failure to attain the normal spirometric plateau or from an accelerated decline in lung function. Despite reports from numerous big cohorts, no single adult life factor, including smoking, accounts for this accelerated decline. By contrast, five childhood risk factors (maternal and paternal asthma, maternal smoking, childhood asthma and respiratory infections) are strongly associated with an accelerated rate of lung function decline and COPD. Among adverse effects on lung development are transgenerational (grandmaternal smoking), antenatal (exposure to tobacco and pollution), and early childhood (exposure to tobacco and pollution including pesticides) factors. Antenatal adverse events can operate by causing structural changes in the developing lung, causing low birth weight and prematurity and altered immunological responses. Also important are mode of delivery, early microbiological exposures, and multiple early atopic sensitizations. Early bronchial hyperresponsiveness, before any evidence of airway inflammation, is associated with adverse respiratory outcomes. Overlapping cohort studies established that spirometry tracks from the preschool years to late middle age, and those with COPD in the sixth decade already had the worst spirometry at age 10 years. Alveolar development is now believed to continue throughout somatic growth and is adversely impacted by early tobacco smoke exposure. Genetic factors are also important, with genes important in lung development and early wheezing also being implicated in COPD. The inescapable conclusion is that the roots of COPD are in early life, and COPD is a disease of childhood adverse factors interacting with genetic factors.

Deferoxamine Improves Alveolar and Pulmonary Vascular Development by Upregulating Hypoxia-inducible Factor-1α in a Rat Model of Bronchopulmonary Dysplasia

Fetal lung development normally occurs in a hypoxic environment. Hypoxia-inducible factor (HIF)-1α is robustly induced under hypoxia and transactivates many genes that are essential for fetal development. Most preterm infants are prematurely exposed to hyperoxia, which can halt hypoxia-driven lung maturation. We were to investigate whether the HIF-1α inducer, deferoxamine (DFX) can improve alveolarization in a rat model of bronchopulmonary dysplasia (BPD). A rat model of BPD was produced by intra-amniotic lipopolysaccharide (LPS) administration and postnatal hyperoxia (85% for 7 days), and DFX (150 mg/kg/d) or vehicle was administered to rat pups intraperitoneally for 14 days. On day 14, the rat pups were sacrificed and their lungs were removed and examined. A parallel in vitro study was performed with a human small airway epithelial cell line to test whether DFX induces the expression of HIF-1α and its target genes. Alveolarization and pulmonary vascular development were impaired in rats with BPD. However, DFX significantly ameliorated these effects. Immunohistochemical analysis showed that HIF-1α was significantly upregulated in the lungs of BPD rats treated with DFX. DFX was also found to induce HIF-1α in human small airway epithelial cells and to promote the expression of HIF-1α target genes. Our data suggest that DFX induces and activates HIF-1α, thereby improving alveolarization and vascular distribution in the lungs of rats with BPD.

5-Lipoxygenase-activating protein (FLAP) inhibitor MK-0591 prevents aberrant alveolarization in newborn mice exposed to 85% oxygen in a dose- and time-dependent manner

Bronchopulmonary dysplasia is characterized by prolonged oxygen dependency due to compromised gas-exchange capability. This is attributable mainly to inadequate and aberrant alveolarization resulting from insults like hyperoxia. Leukotrienes are associated with hyperoxia-induced inhibition of alveolarization. We hypothesized that a 5-lipoxygenase-activating protein (FLAP) inhibitor given while newborn mice were exposed to 85% oxygen would prevent aberrant alveolarization in a dose- and time-dependent manner. Newborn mice were exposed to either room air or hyperoxia for 14 days. Pups were treated with either vehicle or MK-0591 10, 20, or 40 mg/kg subcutaneously daily for days 1-4, 5-9, or 10-14. On day 14, the lungs were inflated, fixed, and stained for histopathological and morphometric analyses. Hyperoxia groups treated with MK-0591 20 or 40 mg/kg during days P1-P4 or P10-P14 showed alveolarization that resembled that of room air controls while untreated hyperoxia groups showed definite evidence of aberrant alveolarization but no inflammation. In a hyperoxia-exposed newborn mice model, a FLAP inhibitor given during critical window periods may prevent aberration of alveolarization in a dose- and time-dependent manner.

Does hypercapnia ameliorate hyperoxia-induced lung injury in neonatal rats?

Therapeutic hypercapnia (TH), an intentional inhalation of CO(2), has been shown to improve pulmonary function in certain models of lung injury. We tested the null hypothesis that TH does not improve hyperoxic lung injury in neonatal rats. The prospective, randomized study was set at Research laboratory in Children's Hospital. Forty-five newborn rats were randomly assigned to three groups (n = 15/group), and exposed to 96 h of normoxia (FiO(2) = 0.21), hyperoxia (FiO(2) > 0.98), and TH (FiO(2) = 0.95, FiCO(2) = 0.05). Lung histology, wet-weight to dry-weight ratio, and concentrations of pro- and anti-inflammatory cytokines (IL-1beta, IL-6, TNF-alpha, and IL-10) were used to evaluate pulmonary damage. Using a scale of 0-4, the total scores for lungs hypercellularity, inflammation, and hemorrhage was significantly increased from a median value of 1.5 in normoxia to 2.5 in hyperoxia (P < 0.05) and 3.0 with TH (P < 0.001, nonparametric ANOVA). The interstitial space relative to the alveolar space, as a measure of hypercellularity, was increased by 18% during hyperoxia and by 44% with TH compared with normoxia. TH significantly increased the size of the interstitial space by 22% compared with hyperoxia (P < 0.001). The lung wet-weight to dry-weight ratio was increased by 10% in both hyperoxic groups (P < 0.001). Both hyperoxic groups showed significant reductions in the concentration of IL-1beta compared with normoxia (P < 0.001), whereas the ratio of IL-1beta to IL-10 was significantly decreased, indicating an anti-inflammatory trend. TH does not prevent histological manifestations of hyperoxic lung injury in spontaneously breathing neonatal rats and may worsen the outcome.

Bone marrow-derived angiogenic cells restore lung alveolar and vascular structure after neonatal hyperoxia in infant mice

Neonatal hyperoxia impairs vascular and alveolar growth in mice and decreases endothelial progenitor cells. To determine the role of bone marrow-derived cells in restoration of neonatal lung structure after injury, we studied a novel bone marrow myeloid progenitor cell population from Tie2-green fluorescent protein (GFP) transgenic mice (bone marrow-derived angiogenic cells; BMDAC). We hypothesized that treatment with BMDAC would restore normal lung structure in infant mice during recovery from neonatal hyperoxia. Neonatal mice (1-day-old) were exposed to 80% oxygen for 10 days. BMDACs (1 x 10(5)), embryonic endothelial progenitor cells, mouse embryonic fibroblasts (control), or saline were then injected into the pulmonary circulation. At 21 days of age, saline-treated mice had enlarged alveoli, reduced septation, and a reduction in vascular density. In contrast, mice treated with BMDAC had complete restoration of lung structure that was indistinguishable from room air controls. BMDAC comprised 12% of distal lung cells localized to pulmonary vessels or alveolar type II (AT2) cells and persist (8.8%) for 8 wk postinjection. Coculture of AT2 cells or lung endothelial cells (luEC) with BMDAC augmented AT2 and luEC cell growth in vitro. We conclude that treatment with BMDAC after neonatal hyperoxia restores lung structure in this model of bronchopulmonary dysplasia.

Hyperoxia reduces bone marrow, circulating, and lung endothelial progenitor cells in the developing lung: implications for the pathogenesis of bronchopulmonary dysplasia

Hyperoxia disrupts vascular and alveolar growth of the developing lung and contributes to the development of bronchopulmonary dysplasia (BPD). Endothelial progenitor cells (EPC) have been implicated in repair of the vasculature, but their role in lung vascular development is unknown. Since disruption of vascular growth impairs lung structure, we hypothesized that neonatal hyperoxia impairs EPC mobilization and homing to the lung, contributing to abnormalities in lung structure. Neonatal mice (1-day-old) were exposed to 80% O(2) at Denver's altitude (= 65% at sea level) or room air for 10 days. Adult mice were also exposed for comparison. Blood, lung, and bone marrow were harvested after hyperoxia. Hyperoxia decreased pulmonary vascular density by 72% in neonatal but not adult mice. In contrast to the adult, hyperoxia simplified distal lung structure neonatal mice. Moderate hyperoxia reduced EPCs (CD45-/Sca-1+/CD133+/VEGFR-2+) in the blood (55%; P < 0.03), bone marrow (48%; P < 0.01), and lungs (66%; P < 0.01) of neonatal mice. EPCs increased in bone marrow (2.5-fold; P < 0.01) and lungs (2-fold; P < 0.03) of hyperoxia-exposed adult mice. VEGF, nitric oxide (NO), and erythropoietin (Epo) contribute to mobilization and homing of EPCs. Lung VEGF, VEGF receptor-2, endothelial NO synthase, and Epo receptor expression were reduced by hyperoxia in neonatal but not adult mice. We conclude that moderate hyperoxia decreases vessel density, impairs lung structure, and reduces EPCs in the circulation, bone marrow, and lung of neonatal mice but increases EPCs in adults. This developmental difference may contribute to the increased susceptibility of the developing lung to hyperoxia and may contribute to impaired lung vascular and alveolar growth in BPD.

Stimulation of HIF-1alpha, HIF-2alpha, and VEGF by prolyl 4-hydroxylase inhibition in human lung endothelial and epithelial cells

Diminished alveolar and vascular development is characteristic of bronchopulmonary dysplasia (BPD) affecting many preterm newborns. Hypoxia promotes angiogenic responses in developing lung via, for example, vascular endothelial growth factor (VEGF). To determine if prolyl 4-hydroxylase (PHD) inhibition could augment hypoxia-inducible factors (HIFs) and expression of angiogenic proteins essential for lung development, HIF-1alpha and -2alpha proteins were assessed in human developing and adult lung microvascular endothelial cells and alveolar epithelial-like cells treated with either the HIF-PHD-selective inhibitor PHI-1 or the nonselective PHD inhibitors dimethyloxaloylglycine (DMOG) and deferoxamine (DFO). PHI-1 stimulated HIF-1alpha and -2alpha equally or more effectively than did DMOG or DFO, enhanced VEGF release, and elevated glucose consumption, whereas it was considerably less cytotoxic than DMOG or DFO. Moreover, VEGF receptor Flt-1 levels increased, whereas KDR/Flk-1 decreased. PHI-1 treatment also increased PHD-2, but not PHD-1 or -3, protein. These results provide proof of principle that HIF stimulation and modulation of HIF-regulated angiogenic proteins through PHI-1 treatment are feasible, effective, and nontoxic in human lung cells, suggesting the use of PHI-1 to enhance angiogenesis and lung growth in evolving BPD.

Pulmonary antioxidant defenses in the preterm newborn with respiratory distress and bronchopulmonary dysplasia in evolution: implications for antioxidant therapy

Preterm neonates with respiratory distress are exposed not only to the relative hyperoxia ex utero, but also to life-saving mechanical ventilation with high inspired oxygen (O2) concentrations, which is considered a major risk factor for the development of bronchopulmonary dysplasia, also referred to as chronic lung disease of infancy. O2 toxicity is mediated through reactive oxygen species (ROS). ROS are constantly generated as byproducts of normal cellular metabolism, but their production is increased in various pathological states, and also upon exposure to exogenous oxidants, such as hyperoxia. Antioxidants, either enzymatic or nonenzymatic, protect the lung against the deleterious effects of ROS. Expression of various pulmonary antioxidants is developmentally regulated in many species so that the expression is increased toward term gestation, as if in anticipation of birth into an O2-rich extrauterine environment. Therefore, the lungs of prematurely born infants may be ill-adapted for protection against ROS. While premature birth interrupts normal lung development, the clinical condition necessitating the administration of high inhaled O2 concentrations may lead to permanent impairment of alveolar development. An understanding of the processes involved in lung growth, especially in alveolarization and vascularization, as well as in repair of injured lung tissue, may facilitate development of strategies to enhance these processes.

Elevated expression of surfactant proteins in newborn rats during adaptation to hyperoxia

The mechanisms whereby lung adaptation to hyperoxia occurs in the newborn period are incompletely understood. Pulmonary surfactant has been implicated in lung protection against hyperoxic injury, and elevated expression of certain surfactant proteins occurs in lungs of adult rats during adaptation to sublethal oxygen (85% O(2)). Here we report that newborn rats, which can adapt to even higher levels of hyperoxia (100% O(2)) than do adult rats, manifest changes in the lung surfactant proteins (SP), especially SP-A and SP-D. In newborn rats exposed to hyperoxia on Days 3 through 10 of life, lung messenger RNAs (mRNAs) for SP-A and SP-B gradually and progressively increased, relative to levels in age-matched, air-exposed newborns, over this 8-d period. By contrast, SP-C and SP-D mRNAs were maximally increased relative to values in simultaneously air-exposed control rats after 4 d of exposure. Lung mRNA for CC-10, a protein specific for Clara cells, was greater in hyperoxia-exposed rats than in air-exposed control rats on Day 4 of exposure, but not on other days. Lung mRNA for thyroid transcription factor (TTF)-1 was marginally increased on Days 1, 2, 4, and 6, and significantly increased on Day 8. Both SP-A and SP-D proteins were increased in lung lavage samples taken from hyperoxia-exposed newborns, relative to those taken from air-exposed controls, with the greatest increases occurring on Days 6 and 8 of exposure. However, the patterns of increase of the proteins were not identical to those of the respective mRNAs. In situ hybridization studies demonstrated increases in SP-D, and to a lesser extent in SP-A, in peripheral lung tissues from oxygen-exposed newborns. Taken together, these data indicate that specific surfactant proteins are upregulated at both the pretranslational and post-translational levels in distal lung epithelium during adaptation to hyperoxia in the newborn rat.

Hypoxia induces hexokinase II gene expression in human lung cell line A549

During adaptation to hypoxic and hyperoxic conditions, the genes involved in glucose metabolism are upregulated. To probe involvement of the transcription factor hypoxia-induced factor-1 (HIF-1) in hexokinase (HK) II expression in human pulmonary cells, A549 cells and small-airway epithelial cells (SAECs) were exposed to stimuli such as hypoxia, deferoxamine (DFO), and metal ions. The largest increase in HK-II (20-fold for mRNA and 2.5-fold for enzymatic activity) was observed in A549 cells when exposed to DFO. All stimuli selectively increased the 5.5-kb rather than 4-kb transcript in A549 cells. Cycloheximide and actinomycin D inhibited these responses. In addition, cells were transfected with luciferase reporter constructs driven by the full-length HK-II 5'-regulatory region (4.0 kb) or various deletions of that region. A549 cells transfected with the 4.0-kb construct and exposed to hypoxia or DFO increased their luciferase activity 7- and 10-fold, respectively, indicating that HK-II induction is, at least in part, due to increased gene transcription. Sixty percent of the inducible activity of the 4.0-kb construct was shown to reside within the proximal 0.5 kb. Additionally, cotransfection with a stable HIF-1 mutant and the 4.0-kb promoter construct resulted in increased luciferase activity under normoxic conditions. These results strongly suggest that HK-II is selectively regulated in pulmonary cells by a HIF-1-dependent mechanism.

Effect of the 21-aminosteroid U74389G on oxygen-induced free radical production, lipid peroxidation, and inhibition of lung growth in neonatal rats

Bronchopulmonary dysplasia is a chronic pneumopathy of preterm infants, with significant associated mortality and morbidity, for which there is no effective preventive therapy. Pulmonary O2 toxicity is thought to be a major contributor to the development of bronchopulmonary dysplasia, and antioxidant interventions hold significant promise for therapy. The relative importance of specific reactive oxygen species in the development of O2-mediated lung injury is unknown. In this study, we tested the effect of a synthetic 21-aminosteroid, U74389G, on 95% O2-induced free radical production, lipid peroxidation, and inhibition of postnatal lung growth in a neonatal rat model. Lipid peroxidation products, as measured by total 8-isoprostane and aldehydes, and hydroxyl radical formation, assessed using salicylate metabolites, in rat lungs and serum were significantly increased after exposure to 95% O2. These changes could be completely or partially attenuated by U74389G. However, U74389G did not improve the survival rate or lung wet-to-dry weight ratio. Expression of proliferating cell nuclear antigen, a marker for DNA synthesis, was examined by immunohistochemistry. Four- or 7-d-old control rat lungs had active DNA synthesis, which was inhibited by exposure to 95% O2. U74389G had a protective effect against 95% O2-mediated inhibition of DNA synthesis. Air-exposed animals treated with U74389G had a modest reduction in lung DNA synthesis, consistent with a role for hydroxyl radicals or lipid hydroperoxides as second messengers in the normal regulation of lung growth.

Iron regulates hyperoxia-dependent human heme oxygenase 1 gene expression in pulmonary endothelial cells

The endothelium of the lung is sensitive to the toxic effects of oxygen, and early evidence of toxicity is characterized by protein leak and extravasation of red blood cells. The overproduction of oxygen free radicals plays a critical role in the pathophysiology of a hyperoxic lung injury. Recently, heme oxygenase 1 (HO-1), the rate-limiting enzyme in the metabolism of heme, has been found to have a protective role in oxidant injury. Our laboratory and others have identified HO-1 as a hyperoxia-inducible protein. In this study, we characterized HO-1 expression and evaluated its regulation in human pulmonary endothelial cells. Hyperoxia results in a relatively small increase in HO-1 expression; however, this induction is potentiated by heme and dramatically potentiated in the presence of free iron. This is probably more reflective of the in vivo situation in which there is extravasation of heme and iron products. We also found that HO-1 expression depended on chelatable iron. The iron chelator desferrioxamine not only inhibited the iron- dependent potentiation of HO-1 in response to hyperoxia but also inhibited both hyperoxia and basal expression. On the basis of inhibitor studies and nuclear run-on assays, we demonstrated that this induction is transcriptionally dependent. We also evaluated 4.5 kb of the human HO-1 promoter region and demonstrated that this region has promoter activity to the stimulus heme; however, there was no evidence of promoter activity to either iron or hyperoxia. This diversity of promoter activity to heme, heavy metals, and hyperoxia is unique to the human HO-1 gene.

Iron and oxidized beta-casein in the lavages of hyperoxic Fischer-344 rats

We reported previously that Fischer-344 (F344) rats were more susceptible to hyperoxic lung injury than were Sprague Dawley (SD) rats. In the present study we exposed adult male F344 and SD rats to >95% oxygen for up to 48 h, and measured lung wet-to-dry weight ratios and lavage protein concentrations as indices of lung injury. In addition, we measured nonheme iron contents in the lung subcellular fractions and in bronchoalveolar lavages (BAL), and we derivatized samples from the subcellular compartments and lavages with 2,4-dinitrophenylhydrazine (DNPH), separated the proteins electrophoretically, and detected DNPH-derivatized proteins by western blotting. After 48 h of hyperoxia, BAL protein and nonheme iron concentrations were higher in F344 rats than in SD rats (2.17+/-0.77 versus 0.17+/-0.17 mg/ml, and 1.61+/-0.45 versus 0.45+/-0.18 nmol/ml, respectively, both P<0.05). In addition, two DNPH-reactive proteins of about 40 and 120 kDa were identified in the lavage fluids of hyperoxic F-344 rats that were not observed similarly in hyperoxic SD rats or in air-breathing rats of either strain. N-terminal amino acid sequences of the two DNPH-reactive proteins 100% identical over 16 residues to rat beta-casein, which is a potent neutrophil chemotaxin, and has been reported to be a product of cytotoxic T-lymphocytes. There were no significant alterations in iron contents in lung subcellular fractions in either strain of rat as a consequence of hyperoxia-exposure, nor were there any significant alterations in DNPH-reactive carbonyls, as determined by western blotting. These data suggest that increased iron concentrations in the airspaces reflect altered iron homeostasis, which may contribute to the greater susceptibility of F344 rats than SD rats to hyperoxic lung injury. The identification of oxidized beta-casein in the BAL of the hyperoxic F344 rats suggests a role for cytotoxic T-lymphocytes in hyperoxic lung inflammation and injury, although the nature of this possible involvement is not known at this time.

Protective effect of exogenous transferrin against hyperoxia: a study on premature rabbits

We hypothesized that an increase in plasma iron binding capacity would decrease the generation of oxygen radicals and of lipid peroxides. To test this hypothesis, we studied whether supplementation of transferrin (TF) in premature rabbits would modify the degree of hyperoxic lung injury. Animals, delivered prematurely at 29 days of gestation (term 31 days), were randomized and given either 0.5 g/kg of albumin (Alb) (n = 116) or 0.5 g/kg of iron-free TF (n = 132) intravenously within 2 hours after birth. Another group was randomized to receive saline (n = 15), or either 0.35 g/kg (n = 12) or 0.70 g/kg of iron-free TF (n = 8). After exposure to a 100% oxygen environment for 2 or 4 days, the animals were killed, and plasma and bronchoalveolar lavage (BAL) fluid was recovered. Infusion of TF caused a dose-dependent increase in the concentration of TF and an increase in the unsaturated iron-binding capacity. Administration of TF at birth increased the gradient of TF between serum and alveolar epithelial lining fluid on day 4, suggesting decreased alveolar-capillary permeability. BAL fluid and plasma from TF-supplemented animals contained less lipid peroxidation products and more inhibitor of lipid peroxidation than BAL fluid or plasma from Alb-treated animals. In TF-treated animals, the recovery of protein in BAL fluid (TF group, 1.26 +/- 0.07 mg; Alb group, 1.78 +/- 0.10 mg; P = 0.02) and the water content of the extravascular lung tissue (TF group, 78.5 +/- 1.4%; Alb group, 83.2 +/- 1.3%; P = 0.05) were lower than in Alb-treated animals. We propose that supplementation of iron-free TF decreases iron-catalyzed redox reactions and may decrease hyperoxic lung injury in the premature.

Recombinant human erythropoietin: possible role as an antioxidant in premature rabbits

Iron is an important catalyst for free oxygen radicals and lipid peroxidation reactions which may play a role in the pathogenesis of several diseases in premature infants. During the early neonatal period, extracellular iron is available in excessive amounts. We hypothesized that administration of erythropoietin (EPO) mobilizes iron from plasma and inhibits iron-catalyzed reactions. To evaluate this hypothesis, recombinant human EPO (rhEPO) was administered s.c. to premature rabbits delivered at 29-d gestation: one group was kept in room air (RA) and the other in a 100% oxygen environment. Within each group, the animals were randomized to receive placebo or rhEPO at 400 or at 800 U/kg on d 0 and 2 of life. On d 3 or 4, plasma iron and iron saturation of transferrin were assessed. Lipid peroxidation was analyzed in plasma and bronchoalveolar lavage fluid (BAL). Nonsedimentable protein (NSP) and phospholipid content were measured in BAL. Erythropoiesis was evaluated in liver and bone marrow. Treatment with rhEPO decreased plasma iron, decreased iron saturation of transferrin, increased reticulocytes, and increased erythropoiesis in liver and bone marrow in both RA and hyperoxia group. Oxygen exposure increased NSP in BAL and decreased the ability of BAL to inhibit lipid peroxidation as measured by malondialdehyde (MDA) generation compared with RA exposure. In O2-exposed animals, EPO treatment increased the ability of both plasma (EPO 800) and BAL (EPO 400 and 800) to inhibit lipid peroxidation and decreased NSP in BAL (EPO 400). In addition, rhEPO treatment decreased alveolar thickening and proteinaceous exudate in the hyperoxia group. We propose that by stimulating erythropoiesis, rhEPO mobilizes non-heme iron and decreases oxidant injury that depends on the availability of transient metal.

Biologically relevant metal ion-dependent hydroxyl radical generation. An update

Transition metal ions, especially iron, appear to be important mediators of oxidative damage in vivo. Iron(II) reacts with H2O2 to give more-reactive radicals. On the basis of ESR spin-trapping data with DMPO, supported by aromatic hydroxylation studies and patterns of DNA base modification, it is concluded that hydroxyl radical (OH.) is likely to be the major damaging species formed in Fenton Systems under biologically-relevant conditions (which include iron concentrations no higher than the micromolar range). Although reactive oxo-iron species (such as ferryl and perferryl) may also be important, direct chemical evidence for their formation and identity in biologically relevant Fenton systems is currently lacking. Studies at alkaline pH values show that iron(IV) and iron(V) species are highly oxidizing under those reaction conditions, with a pattern of reactivity different from that of OH..