Antioxidant enzyme responses to hyperoxia in preterm and term rats after prenatal dexamethasone administration

Oxidative stress in infants born to preeclamptic mothers

BACKGROUND

Oxidative stress may play an important role in the pathophysiology of preeclampsia. An increase in lipid peroxidation products and a decrease in antioxidant activity in preeclamptic women have been reported in many papers. The objective of this study was to evaluate oxidative stress in infants born to preeclamptic mothers.

METHODS

Malondialdehyde (MDA) and glutathione (GSH) levels and glutathione peroxidase (GPx) and superoxide dismutase (SOD) activities were measured in cord plasma of infants born to preeclamptic (n = 18) or normotensive (n = 9) mothers.

RESULTS

Gestational age was similar in both groups. The mean birth weight was significantly lower in the preeclamptic group (P = 0.007). Maternal age, primigravidity, antenatal steroid use, premature rupture of the membranes, clinical chorioamnionitis and adverse neonatal outcomes including sepsis, respiratory distress syndrome and neonatal mortality did not differ between groups. Cesarean delivery was significantly higher in the preeclamptic group. There was no significant difference in cord plasma levels of MDA and GSH, and activity of GPx between the preeclamptic and control groups. SOD was found to be increased in preeclamptic group (P = 0.03).

CONCLUSIONS

We concluded that although cord plasma MDA levels were similar in both the preeclamptic and control groups, increased SOD activity might be an indicator of increased oxidative stress in infants born to preeclamptic mothers.

Modeling and Remodeling of the Lung in Neonatal Chronic Lung Disease

Neonatal chronic lung disease (CLD) is the major long-term pulmonary complication of preterm birth affecting about 20% of infants who need mechanical ventilation. CLD is the result of abnormal repair processes following inflammatory lung injury that lead to remodeling of the lung. Inflammation may be initiated by a variety of stimuli including mechanical ventilation, oxygen toxicity and infection. The resultant neutrophil chemotaxis and degranulation leads to the release of enzymes such as matrix metalloproteinases that can cause proteolysis of the lung extracellular matrix. Abnormal healing with remodeling leads to poorly compliant lungs with reduced capacity for gas exchange. Drugs can influence the normal process of lung modeling or remodeling. Fetal lung development can be influenced by glucocorticosteroids and inflammation. Both can cause abnormal lung modeling with fewer, larger alveoli and accelerated lung maturation, which confers benefits in terms of reduced morbidity and mortality from respiratory distress syndrome but potentially increases the risk of subsequent lung injury. Antioxidants, such as retinol (vitamin A), administered post-natally may reduce the effects of oxidative stress leading to a modest reduction in CLD but they require repeated intramuscular injections. Postnatal glucocorticosteroid therapy can modify the lung inflammatory response and reduce CLD but it can also have detrimental effects on the developing brain and lung, thereby creating a clinical dilemma for neonatologists. Proteinase inhibitors may be a rational therapy but more research is needed before they can be accepted as a treatment for preterm neonates.'Modeling' is defined as planning or forming that follows a set pattern. The term is used to describe the normal process of lung growth and development that culminates in mature branching alveolar air spaces surrounded by a network of capillaries. Normal lung modeling occurs under a variety of genetic and hormonal influences that can be altered, leading to abnormal patterns of growth. 'Remodeling' is defined as altering the structure of or re-making and, in the case of the lung, is used to describe the abnormal patterns of lung growth that occur after lung injury. Modeling and remodeling of the lungs occur to an extent throughout life but never more rapidly than during the fetal and early neonatal periods, and factors that influence this process may lead to development of neonatal CLD. Some of the factors involved in normal and abnormal lung modeling and inflammation and glucocorticosteroid-induced remodeling in the perinatal period, in the context of neonatal CLD, are reviewed with considerations of how various drugs may influence these processes.

Hyperoxia increases AP-1 DNA binding in rat brain

Oxidative stress appears to contribute to neurodegenerative outcomes after ischemia, hypoxia, and hyperoxia. The AP-1 transcription factor is made up of a family of regulatory proteins that can be activated by oxidative stress. In the present study, we examined AP-1 DNA binding activity in terms of specific participating AP-1 proteins in rat brain after hyperoxia. Male Sprague-Dawley rats were exposed to 100% oxygen under isobaric conditions over time. The AP-1 DNA binding activity present in the rat hippocampus and basal forebrain was characterized by electrophoretic mobility shift analysis (EMSA) and the participating AP-1 proteins identified by immunodepletion/supershift and Western blotting analyses. The Fos and Jun proteins were localized by immunohistochemistry to hippocampus. There were significant increases in AP-1 DNA binding in both hippocampus and basal forebrain after hyperoxia. There was also a significant increase in c-Jun protein levels and the proportion of c-Jun present in AP-1 DNA binding complexes in hippocampal nuclei after hyperoxia. These results suggest that AP-1 activation via c-Jun binding to DNA is an important component of brain responses to oxidative stress.

Hyperoxia increases AP-1 DNA binding in rat brain

Oxidative stress appears to contribute to neurodegenerative outcomes after ischemia, hypoxia, and hyperoxia. The AP-1 transcription factor is made up of a family of regulatory proteins that can be activated by oxidative stress. In the present study, we examined AP-1 DNA binding activity in terms of specific participating AP-1 proteins in rat brain after hyperoxia. Male Sprague-Dawley rats were exposed to 100% oxygen under isobaric conditions over time. The AP-1 DNA binding activity present in the rat hippocampus and basal forebrain was characterized by electrophoretic mobility shift analysis (EMSA) and the participating AP-1 proteins identified by immunodepletion/supershift and Western blotting analyses. The Fos and Jun proteins were localized by immunohistochemistry to hippocampus. There were significant increases in AP-1 DNA binding in both hippocampus and basal forebrain after hyperoxia. There was also a significant increase in c-Jun protein levels and the proportion of c-Jun present in AP-1 DNA binding complexes in hippocampal nuclei after hyperoxia. These results suggest that AP-1 activation via c-Jun binding to DNA is an important component of brain responses to oxidative stress.

Attenuated transcriptional responses to oxidative stress in the aged rat brain

The aged nervous system displays impaired cognitive functions, and these impairments are exacerbated in several neurodegenerative diseases. A role for oxidative stress has been suggested for several of these age-associated dysfunctions. In addition, recovery from more acute traumatic insults that also generate oxidative stress is impaired in the aged. Here we examine the response of aged rat hippocampi to normobaric hyperoxia treatments and demonstrate an attenuation in the DNA binding activity of the AP-1 and nuclear factor-kappa B transcription factors, which are important components of stress response signal transduction pathways and can determine shifts in cellular commitments to necrosis, apoptosis, or functional recovery in the central nervous system.

Endotoxin protection from oxygen toxicity: effect on pulmonary neutrophils and L-selectin

The mechanisms by which sublethal doses of endotoxin protect against hyperoxic lung injury are not completely understood. We hypothesized that endotoxin treatment would result in a decreased inflammatory response to hyperoxia and that this would be accompanied by activation of neutrophils (as evidenced by loss of L-selectin) in the peripheral circulation. Adult rats were injected with endotoxin 0.5 mg/kg prior to and 24 hr after onset of exposure to > or = 98% O2. After 56 hr of hyperoxia, pulmonary neutrophils were lower in the O2/endotoxin group compared to O2 controls as measured by myeloperoxidase in lung homogenates and neutrophil counts in bronchoalveolar lavage fluid. Circulating neutrophils were also significantly lower in the O2/endotoxin group compared to O2 controls at 56 hr. Expression of the neutrophil adhesion molecule, L-selectin, was lower at 4 and 24 hr in the endotoxin-treated rats compared to O2 controls. There were no differences at 48 hr. Expression of CD18 rose significantly in the O2/endotoxin group after 4 hr, but thereafter did not differ from O2 controls. In summary, endotoxin protection from O2 toxicity was associated with reduced neutrophils in the lung and a loss of L-selectin from peripheral blood neutrophils.

Perinatal corticosteroids and the developing lung

A significant number of infants are born prematurely each year, many of whom will develop respiratory disease and require ventilation. A substantial number of these infants will die and many of the survivors will subsequently develop chronic inflammatory lung disease. Administration of corticosteroids to women prior to a premature delivery is associated with a significant reduction in mortality and in the incidence of respiratory distress syndrome and intracranial haemorrhage in their infants once born. Postnatal administration of corticosteroids to the infant who develops chronic lung disease has been widely practised for many years. Recent meta-analyses have suggested that benefit may be limited. Treatment is also associated with a range of different side-effects but it has been assumed that the cost-benefit ratio favoured treatment. Recent evidence of permanent and highly significant long-term adverse effects has questioned the validity of this judgement.

Na(+)-K(+)-ATPase gene regulation by glucocorticoids in a fetal lung epithelial cell line

The Na(+) pump, Na(+)-K(+)-ATPase, along with the Na(+) channel is essential for the removal of alveolar solute and fluid perinatally. Because Na(+)-pump mRNA and activity increase before birth and maternal glucocorticoids (GCs) influence Na(+)-K(+)-ATPase mRNA expression in fetal rat lung, we hypothesized that GCs increased Na(+)-K(+)-ATPase gene expression in a fetal lung epithelial cell line. After 24 h of exposure, dexamethasone increased the steady-state levels of Na(+)-K(+)-ATPase alpha(1) and beta(1) mRNA in a fetal rat lung epithelial cell line in a dose-dependent fashion (10(-7) to 10(-5) M). The maximal increase in mRNA levels was 3. 8-fold for alpha(1) and 2.8-fold for beta(1). The increase in mRNA was detected as early as 6 h for the beta(1)-subunit and 18 h for the alpha(1)-subunit, and both peaked at 24 h. This gene upregulation was not due to increased mRNA stability based on mRNA half-life determination after actinomycin D inhibition. Transfection experiments with alpha(1) and beta(1) promoter-reporter constructs demonstrated 3.2 +/- 0.5- and 2.6 +/- 0.4-fold increases, respectively, in promoter activity, consistent with transcriptional activation of the promoter-reporter construct. These findings, increased promoter activity with no change in stability, indicate that GCs increased Na(+)-K(+)-ATPase transcription in a fetal lung epithelial cell line.

Induction of apoptosis in the CNS during development by the combination of hyperoxia and inhibition of glutathione synthesis

Apoptosis in the central nervous system (in contrast to necrosis) is an endogenous cell suicide mechanism triggered in response to biological factors and genotoxic stimuli often resulting from oxidative stress. Excessive neural apoptosis may result in longterm brain dysfunction. A significant proportion of prematurely born infants are exposed to high oxygen and nutritional regimens deficient in antioxidant precursors. Such infants frequently display cognitive deficits when studied in later childhood. Studies in cell culture have characterized a close relationship between oxidative stress, glutathione availability and cell death. Here, we assessed this relationship in rat brain, as a model approximation of the situation that occurs in human infants. Two day old rats were exposed to an atmosphere of 95% oxygen and treated with buthionine sulfoximine (BSO), a glutathione synthesis inhibitor. Control groups consisted of rat-pups kept in air, air plus BSO, or oxygen alone. At the end of 5 days of treatment, brains were harvested, dissected and nerve growth factor protein (NGF), glutathione, and extent of apoptosis were measured. Hyperoxia induced a decrease in NGF protein while BSO induced a decrease in glutathione concentrations. Animals treated with both hyperoxia and BSO had a dramatic increase in the extent of brain apoptosis detected. We conclude from these studies that the brains of animals exposed to both oxidative stress and limited antioxidant protection are liable to pro-apoptotic changes. Increased cell death via apoptosis reflecting changes in neurotrophin and glutathione homeostasis may represent the mechanism responsible for the induction of the longterm cognitive deficits observed in some preterm infants.

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.

Repetitive prenatal glucocorticoid therapy reduces oxidative stress in the lungs of preterm lambs

Repetitive courses of maternal prenatal glucocorticoids are often used in high-risk pregnancies with threatening preterm labor to induce lung maturation, but the effects on the cellular oxidant-antioxidant balance in the fetal lung have not been evaluated. We investigated the effect of repetitive treatment with glucocorticoids, beginning early in gestation, on oxidative stress in the preterm ovine lung. Pregnant ewes were randomized to receive one, two, three, or four doses of 0.5 mg/kg betamethasone or saline placebo at 7-day intervals on 104, 111, 118, and 124 days gestation (n = 11 for each group). All lambs were delivered preterm at 125 days gestation, and lung tissue was assayed for antioxidant enzymes, lipid hydroperoxides, and carbonyl proteins. Lung manganese superoxide dismutase, catalase, and glutathione peroxidase activity increased after 1 dose of betamethasone given at 104 days gestation, whereas copper-zinc superoxide dismutase activity increased after 2 doses given at 104 and 111 days gestation. The activity of all four antioxidant enzymes further increased with additional doses and was maximal after four doses of betamethasone. Lung lipid hydroperoxide levels and carbonyl protein content decreased stepwise after each dose of betamethasone and were lowest after four doses. Repetitive prenatal glucocorticoid therapy increases antioxidant enzyme activity and reduces oxidative stress in the lungs of preterm lambs, and these effects begin early in gestation and persist for 2-3 wk.

Dexamethasone induced alterations in enzymatic and nonenzymatic antioxidant status in heart and kidney of rats

This study was designed to investigate the alterations in thiobarbituric acid reactants (TBA-reactants) and enzymatic and nonenzymatic antioxidant levels induced by dexamethasone (Dex) in heart and kidney and to find out whether these alterations induced by Dex and its hypertensive effect had any role in the maintenance of hypertension in this model. Administration of dexamethasone induced severe loss of body weight, significant increase in heart and kidney weights and also marked electrocardiographic changes. The protein content in heart and kidney increased significantly during Dex administration and returned to near normalcy after withdrawal. Total activity of lactate dehydrogenase showed a significant increase in heart till day 8 of treatment, whereas in serum, it exhibited a significant decrease. The activity of CK in heart showed an increase till day 8 of treatment and approached normalcy thereafter. In serum, CK exhibited a decrease till day 8, remaining insignificant thereafter. CKMB in heart showed an insignificant increase initially, reaching normal levels on Dex withdrawal, whereas in serum, it showed a significant decrease throughout the experimental period. Mean arterial pressure (MAP) and heart rate increased significantly, while a significant elevation in the ST segment was noticed during administration as well as after withdrawal of Dex. The TBA-reactants levels were found to increase in heart and kidney during days 12 and 16 of administration with Dex and even after withdrawal of Dex, the levels were insignificantly elevated. The level of glutathione in heart and kidney increased from day 4 onwards and reached normalcy during the later stages of treatment and after withdrawal of Dex. The total sulfhydryl groups exhibited a significant increase in both heart and kidney throughout the experiment. The antioxidant enzymes such as catalase, superoxide dismutase, glutathione peroxidase and glutathione S-transferase exhibited a significant decrease in heart during Dex administration whereas, in kidney, they exhibited a significant increase during treatment and after withdrawal of Dex. Thus, Dex induced rise in mean arterial pressure, significant alterations in electrocardiographic parameters and also marked alterations in enzymatic and nonenzymatic antioxidant levels and in the TBA-reactants level in heart and kidney.

Prenatal dexamethasone administration to premature rats exposed to prolonged hyperoxia: a new rat model of pulmonary fibrosis (bronchopulmonary dysplasia)

OBJECTIVE

To evaluate the postnatal effects of prenatal dexamethasone treatment of preterm rats and to test the hypothesis that prenatal dexamethasone treatment projects against pulmonary oxygen toxicity in the preterm rats and stimulates lung antioxidant enzyme levels in response to hyperoxia.

STUDY DESIGN

We administered dexamethasone (0.4 mg/kg, intraperitoneally), or equivolume saline solution to pregnant rats at 48 and 24 hours before premature delivery at gestation day 21. Both groups of prematurely delivered rat pups were randomly assigned to other > 95% O2 or room air immediately after birth and brief resuscitation.

RESULTS

The hyperoxic survival rates from day 1 through day 14 were similar in both dexamethasone-treated and control preterm O2 groups. At 7 days of hyperoxia, the preterm pups demonstrated similar lung antioxidant enzyme activity and sufactant content responses to high O2 in the dexamethasone-treated and control groups. Lung quantitative morphometry changes were similar (equal degree of inhibition of normal alveolar development) in both groups. Unexpectedly, the lungs of the preterm O2 control rats showed evidence of septal fibrosis and the pups that received dexamethasone-O2 showed even greater severity of septal fibrosis and a greater increase (+50%) of lung hydroxyproline compared with the O2 groups control rats.

CONCLUSIONS

In preterm animals, prenatal dexamethasone administration does not show any of the hypothesized protective effects against hyperoxia or protective biochemical lung changes during prolonged O2 exposure. However, prenatal dexamethasone administration with prolonged exposure of the preterm rat to hyperoxia results in a pulmonary pathologic picture quite similar to bronchopulmonary dysplasia.

Induction of antioxidant enzymes by dexamethasone in the adult rat lung

Catalase, glutathione peroxidase and superoxide dismutase enzymes were determined after administering dexamethasone. Catalase increased its activity over six times (0.388 U/mg DNA) the normal rate, while glutathione peroxidase caused 3 times an increase one hour after dexamethasone injection. Superoxide dismutase increased gradually during the 3 hour treatment. The antioxidant enzyme activities decreased to basal values in the presence of protein synthesis (Cycloheximide) and RNA synthesis (Actinomycin D) inhibitors. The current report demonstrates that the increase of antioxidant enzymes is due to an enzymatic induction mechanism, and not due to an activation process.

Maternal dexamethasone treatment enhances the expression of surfactant apoprotein A in the hypoplastic lung of rabbit fetuses induced by oligohydramnios

Previously the authors reported that oligohydramnios induced lung hypoplasia in rabbit fetuses and showed that sustained oligohydramnios, which was induced by amniotic shunting from gestational sacs into the maternal peritoneal cavity between 23 and 30 days' gestation, significantly retards not only lung structural growth but also the functional development of alveolar type II cells in surfactant apoprotein A (SP-A) expression. In the present study, the authors examined, both immunohistochemically and morphometrically, whether the maternal administration of dexamethasone restored SP-A synthesis in fetal hypoplastic lungs. The fetal rabbits were treated through maternal administration of dexamethasone (0.25 mg/kg/d) or saline 48 and 24 hours before delivery, at 30 days' gestation. The ratio of lung weight to body weight was significantly greater for the dexamethasone-treated fetuses compared with the saline-treated fetuses in both the shunted and the nonshunted groups (P < .05). Compared with the lungs of the saline-treated fetuses, those of the dexamethasone-treated fetuses had a statistically significant increase in SP-A expression, namely the number of SP-A-positive type II cells per unit area (P < .001), the ratio of SP-A-positive cells to the total number of cells (P < .01), and the percentage of SP-A-positive area per unit area (P < .05) in the shunted group. An increase in the ratio of SP-A-positive area to lung interstitial was found for the shunted group. However, similar findings were not observed in the nonshunted group. The results suggest that maternal dexamethasone treatment accelerates the functional development of alveolar type II cells in SP-A expression, even in hypoplastic lungs induced by oligohydramnios.

Lung manganese superoxide dismutase protein expression increases in the baboon model of bronchopulmonary dysplasia and is regulated at a posttranscriptional level

The expression of lung manganese superoxide dismutase (MnSOD) mRNA and protein were examined in a premature baboon model of hyperoxia-induced bronchopulmonary dysplasia (BPD) and BPD superimposed with bacterial infection. When 140-d gestation baboons were delivered by hysterotomy and treated for 16 d with appropriate ventilatory and oxygen support (pro re nada controls), there was an increase in both MnSOD mRNA and protein compared with 140-d or 156-d gestation, nonventilated controls. The concentration of MnSOD protein was also elevated when the prematurely delivered baboons were ventilated with a high fraction of inspired O2 to produce a primate homolog of BPD, but there was a significant decrease in the concentration of MnSOD mRNA in BPD animals compared with pro re nada controls. In the lungs of premature baboons in which Escherichia coli infection was superimposed on hyperoxia-induced BPD, MnSOD mRNA was diminished to approximately the same extent as in BPD alone, but MnSOD protein was significantly increased compared with all other groups. Taken together these data indicate that the premature baboon is capable of mounting an antioxidant response and that increased MnSOD protein expression in BPD and BPD-infected premature baboons is regulated, at least in part, at a posttranscriptional level.

The effects of hyperoxia, mechanical ventilation, and dexamethasone on pulmonary antioxidant enzyme activity in the newborn piglet

It has been previously shown that prophylactic, intravenous dexamethasone (DEX) and intratracheal recombinant human Cu/Zn superoxide dismutase (SOD) ameliorate lung injury in newborn piglets treated with 48 hr of hyperoxia and mechanical ventilation. DEX has many pharmacologic effects, including the possible induction of antioxidant enzyme systems. To investigate whether the effects of DEX are mediated by an increase in endogenous antioxidant enzyme activity, 5 groups of term newborn piglets were studied: Group 1 piglets were ventilated with room air for 48 hr; Group 2 animals were ventilated with 100% O2 for 48 hr; Group 3 animals were ventilated with room air for 48 hr and received DEX (0.7 mg/kg) every 12 h; Group 4 were ventilated with 100% O2 for 48 hr and also received DEX; Group 5 animals were no ventilated and were sacrificed at time 0. At the conclusion of the studies, bronchoalveolar lavage (BAL) was performed and the lungs were removed and homogenized. Lung tissue and BAL were analyzed for SOD, catalase, GPX activities, and total protein concentration. No significant differences in any of these assays were seen in either lung tissue or BAL in the 5 groups. These observations indicate that 48 hr of hyperoxia, mechanical ventilation, or dexamethasone treatment does not induce activity of SOD, catalase, or glutathione peroxidase (GPX) in the lungs of newborn piglets. Thus postnatal DEX appears to minimize neonatal lung injury by mechanisms that are independent of these enzymes.

Effect of dexamethasone and oxygen exposure on neonatal rat lung retinoic acid receptor proteins

Retinol deficiency in animal models results in histopathologic airway changes that appear similar to those found in human premature infants with bronchopulmonary dysplasia (BPD). Dexamethasone (DEX), a steroid now often used in the treatment of BPD, might potentially affect lung vitamin A homeostasis since it alters serum and liver retinoid stores in certain models. Our objective was to determine the effect of DEX on neonatal rat lung retinoid status and the binding of retinoic acid (RA) to cytosolic and nuclear receptor proteins. We examined this effect both in room air and when the animals breathed 95% oxygen (O2). Twenty-four 1-day-old rat pups received either 1 microgram/g DEX subcutaneously, an equal volume of normal saline (NS) subcutaneously at 0 (start experiment time), 24, and 48 hours, or no injection at all, and were sacrificed at 72 hours. Twelve rats in each treatment group were housed in room air and 12 in each group were exposed to > 95% O2 for the 3 day period. Lung and liver were analyzed for retinyl palmitate (RP). Nuclear retinoic acid receptor (RAR) and cellular retinoic acid binding protein (CRABP) were measured by specific binding assays. DEX decreased liver RP by 33-55% and rat pup lung RP by over 60%; it also decreased lung RAR binding (mean dpm/microgram protein +/- SEM) in both room air and oxygen groups: Air (11.2 +/- 1.0) vs. Air/DEX (4.6 +/- 1.3, n = 6; P < 0.01), and O2 (18.2 +/- 0.6) vs. O2/DEX (3.2 +/- 0.6, n = 6; P < 0.001).(ABSTRACT TRUNCATED AT 250 WORDS)