The effect of hyperoxic exposure on antioxidant enzyme activities of alveolar type II cells in neonatal and adult rats

Difference in pyruvic acid metabolism between neonatal and adult mouse lungs exposed to hyperoxia

Objective

Neonatal lungs are more tolerant to hyperoxic injury than are adult lungs. This study investigated differences in the response to hyperoxic exposure between neonatal and adult mouse lungs using metabolomics analysis with capillary electrophoresis time-of-flight mass spectrometry (CE- TOFMS).

Methods

Neonatal and adult mice were exposed to 21% or 95% O₂ for four days. Subsequently, lung tissue samples were collected and analyzed by CE-TOFMS. Pyruvate dehydrogenase (PDH) enzyme activity was determined using a microplate assay kit. PDH kinase (Pdk) 1, Pdk2, Pdk3, and Pdk4 mRNA expression levels were determined using quantitative reverse transcription-polymerase chain reaction. Pdk4 protein expression was quantified by Western blotting and Pdk4 protein localization was evaluated by immunohistochemistry.

Results

Levels of 3-phosphoglyceric acid, 2-phosphoglyceric acid, phosphoenolpyruvic acid, and lactic acid were significantly elevated in the lungs of hyperoxia-exposed versus normoxia-exposed adult mice, whereas no significant differences were observed with hyperoxia exposure in neonatal mice. PDH activity was reduced in the lungs of adult mice only. Pdk4 mRNA expression levels after hyperoxic exposure were significantly elevated in adult mice compared with that in neonatal mice. Conversely, gene expression levels of Pdk1, Pdk2, and Pdk3 did not differ after hyperoxic exposure in either neonatal or adult mice. Pdk4 protein levels were also significantly increased in adult mouse lungs exposed to hyperoxia and were localized mainly to the epithelium of terminal bronchiole.

Conclusions

Specific metabolites associated with glycolysis and gluconeogenesis were altered after hyperoxia exposure in the lungs of adult mice, but not in neonates, which was likely a result of reduced PDH activity due to Pdk4 mRNA upregulation under hyperoxia.

Hyperoxia induces paracellular leak and alters claudin expression by neonatal alveolar epithelial cells

BACKGROUND

Premature neonates frequently require oxygen supplementation as a therapeutic intervention that, while necessary, also exposes the lung to significant oxidant stress. We hypothesized that hyperoxia has a deleterious effect on alveolar epithelial barrier function rendering the neonatal lung susceptible to injury and/or bronchopulmonary dysplasia (BPD).

MATERIALS AND METHODS

We examined the effects of exposure to 85% oxygen on neonatal rat alveolar barrier function in vitro and in vivo. Whole lung was measured using wet-to-dry weight ratios and bronchoalveolar lavage protein content and cultured primary neonatal alveolar epithelial cells (AECs) were measured using transepithelial electrical resistance (TEER) and paracellular flux measurements. Expression of claudin-family tight junction proteins, E-cadherin and the Snail transcription factor SNAI1 were measured by Q-PCR, immunoblot and confocal immunofluorescence microscopy.

RESULTS

Cultured neonatal AECs exposed to 85% oxygen showed impaired barrier function. This oxygen-induced increase in paracellular leak was associated with altered claudin expression, where claudin-3 and -18 were downregulated at both the mRNA and protein level. Claudin-4 and -5 mRNA were also decreased, although protein expression of these claudins was largely maintained. Lung alveolarization and barrier function in vivo were impaired in response to hyperoxia. Oxygen exposure also significantly decreased E-cadherin expression and induced expression of the SNAI1 transcription factor in vivo and in vitro.

CONCLUSIONS

These data support a model in which hyperoxia has a direct impact on alveolar tight and adherens junctions to impair barrier function. Strategies to antagonize the effects of high oxygen on alveolar junctions may potentially reverse this deleterious effect.

Developmental differences in hyperoxia-induced oxidative stress and cellular responses in the murine lung

Exposure of newborn mice to high inspired oxygen elicits a distinct phenotype of compromised alveolar and vascular development, although lethality during long-term exposure is lower in newborns compared to adults. As the effects of hyperoxia are mediated by excessive reactive oxygen species (ROS) generation, we hypothesized that newborn mice may exhibit enhanced expression of antioxidant defenses or attenuated ROS generation compared with adults. We measured subcellular oxidant responses to acute hyperoxia in lung slices and alveolar epithelial cells at varying time points during postnatal murine lung development. Oxidant stress was assessed using RoGFP, a ratiometric protein thiol redox sensor, targeted to the cytosol or the mitochondrial matrix. In contrast to newborn resistance to oxygen-induced mortality, cells of lung slices from younger mice demonstrated exaggerated mitochondrial matrix oxidant stress compared to adults, whereas oxidant stress responses in the cytosol were absent. Cell death in lung slices from newborn mice exposed to 48h of hyperoxia was also greater than for adults. Consistent with these findings, expression of antioxidant enzymes in newborn lungs was lower than in adults, and induction of antioxidant levels and activity during 24h of in vivo exposure was absent. However, expression of the reactive oxygen species-generating enzyme NADPH oxidase 1 was increased with hyperoxic exposure in the young but not the adult lung. Collectively, these results suggest that the greater lethality in adult animals may be more likely attributed to processes such as inflammation than to differences in antioxidant defenses. Therapies for neonatal and adult oxidative lung injury should therefore consider and address developmental differences in oxidative stress responses.

The impact of hyperoxia on the neonatal and adult developing dendritic cell

Oxygen is essential therapy for neonates with acute respiratory failure, including those with infections. However, high oxygen levels may be counterproductive for overcoming infections because hyperoxia may kill cells, including dendritic cells that are essential to the emergence of the pulmonary immune system and pivotal in mounting immune responses to infections. We studied the impact of hyperoxia on developing dendritic cells from neonatal cord blood and adult blood monocytes, comparing viability, development of maturation, and endocytic function. Our data suggest that cord blood-derived dendritic cells may be more resistant to hyperoxic-induced cell death than adult blood-derived cells. Moreover, the surviving cells in either group are those that maintain an immature phenotype. This may impair their ability to perform optimal immune function.

Global effects of xanthine oxidase stress on alveolar type II cells

OBJECTIVE: To delineate biochemical details of graded xanthine oxidase stress toward cultured alveolar type II cells, particularly oxidant-mediated damage of type II cell nucleic acid, protein, and lipid, as an in vitro model of distant ischemia-reperfusion lung injury. DESIGN: In vitro injury model using native rat and immortalized mouse alveolar type II cells and exogenous xanthine oxidase. SETTING: Research laboratory. Measurement: Cultured type II cells were subjected to xanthine oxidase-derived reactive oxygen stress at variable concentrations and incubation times. Reduction of type II cell double-stranded DNA, inhibition of de novo phosphatidyl choline synthesis, enhancement of lipid peroxidation, and suppression of mitochondrial redox capacity were analyzed in relation to high-intensity (xanthine oxidase, 25 munits/mL) oxidant stress. Alterations in type II cell cellular glutathione-related antioxidant repertoire were assessed at both high-intensity and low-intensity (xanthine oxidase, 1 munits/mL) oxidant stress. MAIN RESULTS: High-intensity xanthine oxidase stress significantly increased type II cell DNA strand breakage, inhibited de novo phosphatidyl choline synthesis, diminished mitochondrial integrity, and enhanced lipid peroxidation in the absence of overt cytolysis. This injury was modulated with addition of exogenous glutathione peroxidase, or catalase/superoxide dismutase, but not glutathione or N-acetylcysteine. Although aspects of the glutathione antioxidant repertoire were similarly diminished with high-intensity xanthine oxidase stress, low-dose (long duration) xanthine oxidase stress augmented the activities of type II cell glutathione peroxidase and gamma-glutamyl transferase (the rate-limiting enzyme in glutathione synthesis). CONCLUSION: High-intensity xanthine oxidase stress (in vitro model of in vivo ischemia-reperfusion) may overwhelm type II cell antioxidant defenses and mediate oxidant injury to nucleic acid, protein, and lipid in the absence of cell lysis. Immortalized murine type II cells seem to appropriately model xanthine oxidase-mediated nucleic acid and protein injury of native rat type II cells. Exogenous glutathione peroxidase reduces oxidant injury in this in vitro model. Depending on magnitude (and possibly duration) of the xanthine oxidase stress, type II cell glutathione antioxidant elements may be diminished or enhanced.

Recombinant human superoxide dismutase enhances the effect of inhaled nitric oxide in persistent pulmonary hypertension

We investigated the pulmonary vascular effects of superoxide dismutase (SOD) alone and in combination with inhaled nitric oxide (iNO) in newborn lambs with persistent pulmonary hypertension (PPHN) following prenatal ligation of the ductus arteriosus. In in vitro experiments, pretreatment with SOD significantly enhanced vascular relaxation in response to the NO donor S-nitrosyl-acetylpenicillamine (SNAP) in fifth-generation pulmonary arteries isolated from lambs with PPHN. In vivo treatment of fully instrumented newborn lambs with a single intratracheal dose of recombinant human CuZn SOD (rhSOD; 5 mg/kg) produced selective dilation of the pulmonary circulation. Further studies, of the combination of rhSOD and iNO, showed enhancement of the pulmonary vascular effects of iNO after brief periods of inhalation of 5 ppm and 80 ppm NO. We conclude that rhSOD reduces pulmonary vascular resistance and facilitates the action of iNO in a lamb model of PPHN. This suggests that rhSOD may prove to be an effective adjunctive treatment for newborns with PPHN.

Cellular response of antioxidant metalloproteins in Cu/Zn SOD transgenic mice exposed to hyperoxia

Ceruloplasmin, metallothionein, and ferritin are metal-binding proteins with potential antioxidant activity. Despite evidence that they are upregulated in pulmonary tissue after oxidative stress, little is known regarding their influence on trace metal homeostasis. In this study, we have used copper- and zinc-containing superoxide dismutase (Cu/Zn SOD) transgenic-overexpressing and gene knockout mice and hyperoxia to investigate the effects of chronic and acute oxidative stress on the expression of these metalloproteins and to identify their influence on copper, zinc, and iron homeostasis. We found that the oxidative stress-mediated induction of ceruloplasmin and metallothionein in the lung had no effect on tissue levels of copper, iron, or zinc. However, Cu/Zn SOD expression had a marked influence on hepatic copper and iron as well as circulating copper homeostasis. These results suggest that ceruloplasmin and metallothionein may function as antioxidants independent of their role in trace metal homeostasis and that Cu/Zn SOD functions in copper homeostasis via mechanisms distinct from its superoxide scavenging properties.

Ventilatory responses to ozone are reduced in immature rats

During ozone (O(3)) exposure, adult rats decrease their minute ventilation (VE). To determine whether such changes are also observed in immature animals, Sprague-Dawley rats, aged 2, 4, 6, 8, or 12 wk, were exposed to O(3) (2 ppm) in nose-only-exposure plethysmographs. Baseline VE normalized for body weight decreased with age from 2.1 +/- 0.1 ml. min(-1). g(-1) in 2-wk-old rats to 0. 72 +/- 0.03 ml. min(-1). g(-1) in 12-wk-old rats, consistent with the higher metabolic rates of younger animals. In adult (8- and 12-wk-old) rats, O(3) caused 40-50% decreases in VE that occurred primarily as the result of a decrease in tidal volume. In 6-wk-old rats, O(3)-induced changes in VE were significantly less, and in 2- and 4-wk-old rats, no significant changes in VE were observed during O(3) exposure. The increased baseline VE and the smaller decrements in VE induced by O(3) in the immature rats imply that their delivered dose of O(3) is much higher than in adult rats. To determine whether these differences in O(3) dose influence the extent of injury, we measured bronchoalveolar lavage protein concentrations. The magnitude of the changes in bronchoalveolar lavage induced by O(3) was significantly greater in 2- than in 8-wk-old rats (267 +/- 47 vs. 165 +/- 22%, respectively, P < 0.05). O(3) exposure also caused a significant increase in PGE(2) in 2-wk-old but not in adult rats. The results indicate that the ventilatory response to O(3) is absent in 2-wk-old rats and that lack of this response, in conjunction with a greater specific ventilation, leads to greater lung injury.

Chemiluminescence because of the production of reactive oxygen species in the lungs of newborn piglets during resuscitation periods after asphyxiation load

Reactive oxygen species are regarded as a possible cause of many diseases. However, there are few reports offering in vivo and in situ proof of the direct involvement of reactive oxygen species in the pathogenesis of disease. In the present study, the luciferin derivative 2-methyl-6-[4-methoxyphenyl]-3,7-dihydroimidazo [1,2-alpha] pyrazin-3-one (MCLA) was used to investigate the amount of reactive oxygen species produced during resuscitation after asphyxiation load in newborn piglets. The animals were first asphyxiated by stopping respiration for 4 min, and then resuscitated using 100% oxygen. When physiologic saline solution was administered, lung surface chemiluminescence had a mean value of 2, whereas with MCLA, a maximum luminescence of 580 was seen, demonstrating the possibility of measuring reactive oxygen species in vivo and in situ using MCLA. In a group in which resuscitation after acute asphyxiation was performed with 21% oxygen, the relative maximum lung surface chemiluminescence was 59.5+/-39, whereas that for a group in which resuscitation was performed using 100% oxygen had a significantly higher value of 186.1+/-72.5. Consequently, ventilation and especially resuscitation by 100% oxygen may represent a potential danger.

Tracheal ligation and mechanical ventilation do not improve the antioxidant enzyme status in the lamb model of congenital diaphragmatic hernia

BACKGROUND/PURPOSE

The antioxidant enzyme (AOE) system is the primary intracellular defense system of the lung against oxygen toxicity. The authors recently demonstrated depressed levels of catalase, glutathione peroxidase, and superoxide dismutase in congenital diaphragmatic hernia (CDH) lambs compared with controls. The aim of this study was to determine whether tracheal ligation (TL) or mechanical ventilation (recently shown to stimulate growth and surfactant metabolism, respectively) could induce an elevation in AOE activity.

METHODS

Four nonventilated lambs with surgically created CDH and TL and five CDH lambs ventilated for 4 hours were studied. Lung tissue was analyzed for AOE and the results compared with untreated CDH lambs.

RESULTS

Both ventilation and TL failed to elevate AOE activity above that of untreated CDH lambs.

CONCLUSIONS

The data provide further evidence that TL does not improve lung metabolism or maturation. Mechanical ventilation, which often involves high oxygen delivery, is a necessary and often beneficial therapeutic modality. In the CDH neonate compromised not only by low baseline levels of the AOE, but also by an inability to induce enzyme synthesis in response to hyperoxia, mechanical ventilation may, by causing lung injury, be contributing to the high morbidity and mortality rate associated with CDH.

Effect of oxygen on lung superoxide dismutase activities in premature baboons with bronchopulmonary dysplasia

We investigated the effects of gestational age and oxygen exposure on superoxide dismutase (SOD) activities in distal fetal lung tissue in primate models of bronchopulmonary dysplasia. During the final third of fetal life, lung coppper-zinc SOD (Cu,ZnSOD) specific activity decreased, whereas lung manganese SOD (MnSOD) specific activity tended to increase. In the premature newborn (140 days, 78% of term gestation), lung total SOD and Cu,ZnSOD specific activities decreased after 6-10 days of ventilation with as needed [pro re nada (PRN)] or 100% oxygen compared with fetal control animals. Neither Cu,ZnSOD mRNA nor protein expression changed after either oxygen exposure at this gestation (140 days) relative to fetal control animals. At this age (6-10 days), lung MnSOD specific activity did not change in oxygen-exposed relative to fetal control animals, even though lung expression of MnSOD mRNA and protein increased after PRN or 100% oxygen exposure. In the very premature 125-day newborn (69% of term), lung Cu,ZnSOD specific activity and protein decreased, whereas Cu,ZnSOD mRNA increased, after 6-10 days of ventilation with PRN oxygen compared with fetal control animals. In fetal lung explants, hyperoxia also decreased expression of SOD activity acutely (16-h exposure, 125 and 140 days gestation). To conclude, expression of SOD activity in the premature primate lung did not increase in response to elevated oxygen tension, apparently due to effects occurring subsequent to the expression of these mRNAs.

N-acetylcysteine protects from glutathione depletion in rats exposed to hyperoxia

BACKGROUND

N-acetylcysteine (NAC) may protect against oxidative injury by providing cysteine for glutathione (GSH) biosynthesis or by direct reactions with electrophiles. We have recently shown that hyperoxic exposure of rats prior to liver perfusion is associated with significant decreases in hepatic GSH and significant changes in biliary amino acid concentrations. We hypothesized that NAC administration during hyperoxic exposure would prevent depletion of hepatic GSH by providing cysteine for GSH biosynthesis.

METHODS

NAC was administered during two conditions known to induce GSH depletion: hyperoxic exposure and biochemical inhibition of GSH synthesis using buthionine sulfoximine (BSO). After 48 hours, GSH concentrations in bile, liver and perfusate and biliary amino acid concentrations were determined using isolated perfused liver preparations.

RESULTS

Administration of NAC to rats maintained in normoxic or hyperoxic conditions, prior to liver perfusion, resulted in dose-dependent increases in GSH concentrations in bile, liver and perfusate, increases in bile flow rates and changes in biliary amino acid concentrations. When BSO was given concurrently with NAC in normal or hyperoxic conditions, these effects were not observed, and oxidant stress was evident.

CONCLUSIONS

NAC prevents oxidant stress during hyperoxic exposure, most likely by supplying cysteine as a precursor for GSH synthesis.

Influence of post-hypoxia reoxygenation conditions on energy metabolism and superoxide production in cultured neurons from the rat forebrain

Brain reperfusion and/or reoxygenation may be of particular importance in the etiology of neuronal damage after hypoxic-ischemic insult in neonates, especially with reference to the generation of free radicals. To investigate this issue, the influence of either standard reoxygenation or transient hyperoxia was studied on the consequences of severe hypoxia in a model of cultured neurons isolated from the fetal rat brain. Culture dishes were exposed for 6 h to hypoxia (95% N2/5% CO2). They were then placed under normoxia (95% air/5% CO2) or hyperoxia (95% O2/5% CO2) for 3 h, and finally returned to normoxia. Control cultures were kept under normoxic conditions. Cell morphology, protein concentrations, lactate dehydrogenase leakage, energy metabolism, as reflected by specific transport and incorporation of 2-D-[3H]deoxyglucose, as well as superoxide radical formation were analyzed as a function of time. Po2 values in the cell incubating medium were decreased by 78% by hypoxia and increased by 221% by hyperoxia. No morphologic alteration could be noticed before 72 h posthypoxia, when cell degeneration became apparent, with a concomitant reduction in protein contents. Hypoxia-reoxygenation induced a transient cellular hypermetabolism, as shown by a 36% increase in 2-D-[3H]deoxyglucose uptake 24 h after hypoxia, and then a 23% decrease below control values at 72 h. It also led to a sharp increase in the formation of superoxide radicals (+108%). Transient hyperoxia during reoxygenation did not exacerbate these events, and thus would not enhance their deterimental effects on cell integrity.

In vivo ESR studies of antioxidant activity on free radical reaction in living mice under oxidative stress

In vivo antioxidant activity seems to be quite complicate due to multiple interaction with biomaterials and differs from results by in vitro experiments. In vivo estimation of antioxidant activity is performed by measuring TBA reactive substances in blood or hydrocarbon gases in breath, but these systems do not measure free radical reaction but the final products of oxidative reaction. In the present study, we applied in vivo ESR to evaluate antioxidant activity by monitoring the redox reaction of nitroxide radical and clearly found that the nitroxide is very susceptible to oxidative stress in vivo and quite useful to evaluate antioxidant activity non-invasively.