Inhibition of lung epithelial cell proliferation by hyperoxia. Posttranscriptional regulation of proliferation-related genes

Granulocyte Colony-Stimulating Factor is a Determinant of Severe Bronchopulmonary Dysplasia and Coincident Retinopathy

Bronchopulmonary dysplasia (BPD), also called chronic lung disease of immaturity, afflicts approximately one third of all extremely premature infants, causing lifelong lung damage. There is no effective treatment other than supportive care. Retinopathy of prematurity (ROP), which impairs vision irreversibly, is common in BPD, suggesting a related pathogenesis. However, specific mechanisms of BPD and ROP are not known. Herein, a neonatal mouse hyperoxic model of coincident BPD and retinopathy was used to screen for candidate mediators, which revealed that granulocyte colony-stimulating factor (G-CSF), also known as colony-stimulating factor 3, was up-regulated significantly in mouse lung lavage fluid and plasma at postnatal day 14 in response to hyperoxia. Preterm infants with more severe BPD had increased plasma G-CSF. G-CSF-deficient neonatal pups showed significantly reduced alveolar simplification, normalized alveolar and airway resistance, and normalized weight gain compared with wild-type pups after hyperoxic lung injury. This was associated with a marked reduction in the intensity, and activation state, of neutrophilic and monocytic inflammation and its attendant oxidative stress response, and protection of lung endothelial cells. G-CSF deficiency also provided partial protection against ROP. The findings in this study implicate G-CSF as a pathogenic mediator of BPD and ROP, and suggest the therapeutic utility of targeting G-CSF biology to treat these conditions.

Short-term perinatal oxygen exposure may impair lung development in adult mice

BACKGROUND

Hyperoxia at resuscitation increases oxidative stress, and even brief exposure to high oxygen concentrations during stabilization may trigger organ injury with adverse long-term outcomes in premature infants. We studied the long-term effects of short-term perinatal oxygen exposure on cell cycle gene expression and lung growth in adult mice.

METHODS

We randomized mice litters at birth to 21, 40, or 100%O₂ for 30 min and recovered in room air for 4 or 12 weeks. Cell cycle gene expression, protein analysis, and lung morphometry were assessed at 4 and 12 weeks.

RESULTS

The principal component analysis demonstrated a high degree of correlation for cell cycle gene expression among the three oxygen groups. Lung elastin was significantly lower in the 100%O₂ groups at 4 weeks. On lung morphometry, radial alveolar count, alveolar number, and septal count were similar. However, the mean linear intercept (MLI) and septal length significantly correlated among the oxygen groups. The MLI was markedly higher in the 100%O₂ groups at 4 and 12 weeks of age, and the septal length was significantly lower in the 100%O₂ groups at 12 weeks.

CONCLUSION

Short-term exposure to high oxygen concentrations lead to subtle changes in lung development that may affect alveolarization. The changes are related explicitly to secondary crest formation that may result in alteration in lung elastin. Resuscitation with high oxygen concentrations may have a significant impact on lung development and long-term outcomes such as BPD in premature infants.

Extracellular Vesicle: An Emerging Mediator of Intercellular Crosstalk in Lung Inflammation and Injury

Inflammatory lung responses are one of the characterized features in the pathogenesis of many lung diseases, including acute respiratory distress syndrome (ARDS) and chronic obstructive pulmonary disease (COPD). Alveolar macrophages (AMs) and alveolar epithelial cells are the first line of host defense and innate immunity. Due to their central roles in both the initiation and resolution of inflammatory lung responses, AMs constantly communicate with other lung cells, including the alveolar epithelial cells. In the past, emerging evidence suggests that extracellular vesicles play an essential role in cell–cell crosstalk. In this review, we will discuss the recent findings on the intercellular communications between lung epithelial cells and alveolar macrophages, via EV-mediated signal transfer.

Wnt3a Mediates the Inhibitory Effect of Hyperoxia on the Transdifferentiation of AECIIs to AECIs

The aim of this study is to investigate the effect of Wnt3a in the transdifferentiation of type II alveolar epithelial cells (AECIIs) to type I alveolar epithelial cells (AECIs) under hyperoxia condition. In the in vivo study, preterm rats were exposed in hyperoxia for 21 days. In the in vitro study, primary rat AECIIs were subjected to a hyperoxia and normoxia exposure alternatively every 24 hr for 7 days. siRNA-mediated knockout of Wnt3a and exogenous Wnt3a were used to investigate the effect of Wnt3a on transdifferentiation of AECIIs to AECIs. Wnt5a-overexpressed AECIIs were also used to investigate whether Wnt3a could counteract the effect of Wnt5a. The results showed that hyperoxia induced alveolar damage in the lung of preterm born rats, as well as an increased expression of Wnt3a and nuclear accumulation of β-catenin. In addition, Wnt3a/β-catenin signaling was activated in isolated AECIIs after hyperoxia exposure. Wnt3a knockout blocked the inhibition of the transdifferentiation induced by hyperoxia, and Wnt3a addition exacerbated this inhibition. Furthermore, Wnt3a addition blocked the transdifferentiation-promoting effect of Wnt5a in hyperoxia-exposed Wnt5a-overexpressed AECIIs. In conclusion, our results demonstrate that the activated Wnt3a/β-catenin signal may be involved in the hyperoxia-induced inhibition of AECIIs' transdifferentiation to AECIs.

Activation of the nuclear factor-κB pathway during postnatal lung inflammation preserves alveolarization by suppressing macrophage inflammatory protein-2

A significant portion of lung development is completed postnatally during alveolarization, rendering the immature lung vulnerable to inflammatory stimuli that can disrupt lung structure and function. Although the NF-κB pathway has well-recognized pro-inflammatory functions, novel anti-inflammatory and developmental roles for NF-κB have recently been described. Thus, to determine how NF-κB modulates alveolarization during inflammation, we exposed postnatal day 6 mice to vehicle (PBS), systemic lipopolysaccharide (LPS), or the combination of LPS and the global NF-κB pathway inhibitor BAY 11-7082 (LPS + BAY). LPS impaired alveolarization, decreased lung cell proliferation, and reduced epithelial growth factor expression. BAY exaggerated these detrimental effects of LPS, further suppressing proliferation and disrupting pulmonary angiogenesis, an essential component of alveolarization. The more severe pathology induced by LPS + BAY was associated with marked increases in lung and plasma levels of macrophage inflammatory protein-2 (MIP-2). Experiments using primary neonatal pulmonary endothelial cells (PEC) demonstrated that MIP-2 directly impaired neonatal PEC migration in vitro; and neutralization of MIP-2 in vivo preserved lung cell proliferation and pulmonary angiogenesis and prevented the more severe alveolar disruption induced by the combined treatment of LPS + BAY. Taken together, these studies demonstrate a key anti-inflammatory function of the NF-κB pathway in the early alveolar lung that functions to mitigate the detrimental effects of inflammation on pulmonary angiogenesis and alveolarization. Furthermore, these data suggest that neutralization of MIP-2 may represent a novel therapeutic target that could be beneficial in preserving lung growth in premature infants exposed to inflammatory stress.

The genome-wide transcriptional response to neonatal hyperoxia identifies Ahr as a key regulator

Premature infants requiring supplemental oxygen are at increased risk for developing bronchopulmonary dysplasia (BPD). Rodent models involving neonatal exposure to excessive oxygen concentrations (hyperoxia) have helped to identify mechanisms of BPD-associated pathology. Genome-wide assessments of the effects of hyperoxia in neonatal mouse lungs could identify novel BPD-related genes and pathways. Newborn C57BL/6 mice were exposed to 100% oxygen for 10 days, and whole lung tissue RNA was used for high-throughput, sequencing-based transcriptomic analysis (RNA-Seq). Significance Analysis of Microarrays and Ingenuity Pathway Analysis were used to identify genes and pathways affected. Expression patterns for selected genes were validated by qPCR. Mechanistic relationships between genes were further tested in cultured mouse lung epithelial cells. We identified 300 genes significantly and substantially affected following acute neonatal hyperoxia. Canonical pathways dysregulated in hyperoxia lungs included nuclear factor (erythryoid-derived-2)-like 2-mediated oxidative stress signaling, p53 signaling, eNOS signaling, and aryl hydrocarbon receptor (Ahr) pathways. Cluster analysis identified Ccnd1, Cdkn1a, and Ahr as critical regulatory nodes in the response to hyperoxia, with Ahr serving as the major effector node. A mechanistic role for Ahr was assessed in lung epithelial cells, and we confirmed its ability to regulate the expression of multiple hyperoxia markers, including Cdkn1a, Pdgfrb, and A2m. We conclude that a global assessment of gene regulation in the acute neonatal hyperoxia model of BPD-like pathology has identified Ahr as one driver of gene dysregulation.

Oxidative stress and inflammation modulate Rev-erbα signaling in the neonatal lung and affect circadian rhythmicity

AIMS

The response to oxidative stress and inflammation varies with diurnal rhythms. Nevertheless, it is not known whether circadian genes are regulated by these stimuli. We evaluated whether Rev-erbα, a key circadian gene, was regulated by oxidative stress and/or inflammation in vitro and in a mouse model.

RESULTS

A unique sequence consisting of overlapping AP-1 and nuclear factor kappa B (NFκB) consensus sequences was identified on the mouse Rev-erbα promoter. This sequence mediates Rev-erbα promoter activity and transcription in response to oxidative stress and inflammation. This region serves as an NrF2 platform both to receive oxidative stress signals and to activate Rev-erbα, as well as an NFκB-binding site to repress Rev-erbα with inflammatory stimuli. The amplitude of the rhythmicity of Rev-erbα was altered by pre-exposure to hyperoxia or disruption of NFκB in a cell culture model of circadian simulation. Oxidative stress overcame the inhibitory effect of NFκB binding on Rev-erbα transcription. This was confirmed in neonatal mice exposed to hyperoxia, where hyperoxia-induced lung Rev-erbα transcription was further increased with NFκB disruption. Interestingly, this effect was not observed in similarly exposed adult mice.

INNOVATION

These data provide novel mechanistic insights into how key circadian genes are regulated by oxidative stress and inflammation in the neonatal lung.

CONCLUSION

Rev-erbα transcription and circadian oscillation are susceptible to oxidative stress and inflammation in the neonate. Due to Rev-erbα's role in cellular metabolism, this could contribute to lung cellular function and injury from inflammation and oxidative stress.

Chronic lung disease in the preterm infant. Lessons learned from animal models

Neonatal chronic lung disease, also known as bronchopulmonary dysplasia (BPD), is the most common complication of premature birth, affecting up to 30% of very low birth weight infants. Improved medical care has allowed for the survival of the most premature infants and has significantly changed the pathology of BPD from a disease marked by severe lung injury to the "new" form characterized by alveolar hypoplasia and impaired vascular development. However, increased patient survival has led to a paucity of pathologic specimens available from infants with BPD. This, combined with the lack of a system to model alveolarization in vitro, has resulted in a great need for animal models that mimic key features of the disease. To this end, a number of animal models have been created by exposing the immature lung to injuries induced by hyperoxia, mechanical stretch, and inflammation and most recently by the genetic modification of mice. These animal studies have 1) allowed insight into the mechanisms that determine alveolar growth, 2) delineated factors central to the pathogenesis of neonatal chronic lung disease, and 3) informed the development of new therapies. In this review, we summarize the key findings and limitations of the most common animal models of BPD and discuss how knowledge obtained from these studies has informed clinical care. Future studies should aim to provide a more complete understanding of the pathways that preserve and repair alveolar growth during injury, which might be translated into novel strategies to treat lung diseases in infants and adults.

High frequency percussive ventilation and low FiO(2)

BACKGROUND

High-frequency percussive ventilation (HFPV) is an effective rescue therapy in ventilated patients with acute lung injury. High levels of inspired oxygen (FiO(2)) are toxic to the lungs. The objective of this study was to review a low FiO(2) (0.25)/HFPV protocol as a protective strategy in burn patients receiving mechanical ventilation greater than 10 days.

METHODS

A single-center, retrospective study in burn patients between December 2002 and May 2005 at the LAC+USC Burn Center. Demographic and physiologic data were recorded from time of admission to extubation, 4 weeks, or death.

RESULTS

32 subjects were included in this study, 1 patient failed the protocol. 23 of 32 (72%) patients were men and mean age was 46±15 years. Average TBSA burn was 30±20 with 9 of 32 (28%) having >40% TBSA involved. Average burn index was 76±21. 22 of 32 (69%) had inhalation injury and 23 of 32 (72%) had significant comorbidities. Average ventilator parameters included ventilator days 24±12, FiO(2) 0.28±0.03, PaO(2) 107±15 Torr, PaCO(2) 42±4 Torr, and PaO(2)/FiO(2) ratio 395±69. 16 of 32 (50%) patients developed pneumonia and 9 of 32 (28%) died. No patient developed ARDS, barotrauma, or died from respiratory failure. There was no association between inhalation injury and mortality in this group of patients.

CONCLUSION

A low FiO(2)/HFPV protocol is a safe and effective way to ventilate critically ill burn patients. Reducing the oxidative stress of high inspired oxygen levels may improve outcome.

Effects of ambient oxygen concentration on the proliferation and viability of cultured human corneal epithelial cells

Ambient oxygen (O(2)) affects the metabolism and other functions of corneal epithelial cells. The effects of O(2) concentration on the proliferation and viability of corneal epithelial cells in culture were investigated. Simian virus 40-transformed human corneal epithelial (HCE) cells were maintained at 37 degrees C in a humidified incubator containing 5% CO(2) and 95% air. The cells were subsequently transferred to a multigas incubator and exposed to 5% CO(2) and either 1, 21, or 60% O(2) plus 94, 74, or 35% N(2), respectively. Cell proliferation was evaluated by determination of cell number and measurement of the incorporation of bromodeoxyuridine. Cell lysis was quantified by measurement of the release of lactate dehydrogenase. Apoptosis was evaluated by flow cytometric analysis of cells stained with annexin V and propidium iodide as well as by immunoblot analysis of cleavage of caspase-7. The phosphorylation (activation) of Akt was also detected by immunoblot analysis. Hyperoxia (60% O(2)) inhibited the increase in cell number and the incorporation of bromodeoxyuridine apparent in HCE cells exposed to normoxia (21% O(2)). It also induced the release of lactate dehydrogenase, an increase in the proportion of apoptotic (annexin V(+), propidium iodide(-)) cells, the cleavage of caspase-7, and the phosphorylation of Akt. None of these effects was observed in cells exposed to hypoxia (1% O(2)). The amounts of the cleaved forms of caspase-3, 6, and 9 did not differ among HCE cells cultured under 1, 21, or 60% O(2). These results indicate that hyperoxia inhibited the proliferation of, and induced death by apoptosis in, cultured human corneal epithelial cells. The antiapoptotic protein Akt was also activated in cells exposed to hyperoxia, possibly reflecting a protective response to oxygen toxicity.

Poly(ADP-ribose) polymerase-1 (PARP-1) controls lung cell proliferation and repair after hyperoxia-induced lung damage

Oxygen-based therapies expose lung to elevated levels of ROS and induce lung cell damage and inflammation. Injured cells are replaced through increased proliferation and differentiation of epithelial cells and fibroblasts. Failure to modulate these processes leads to excessive cell proliferation, collagen deposition, fibrosis, and chronic lung disease. Poly(ADP-ribose) polymerase-1 (PARP-1) is activated in response to DNA damage and participates in DNA repair, genomic integrity, and cell death. In this study, we evaluated the role of PARP-1 in lung repair during recovery after acute hyperoxia exposure. We exposed PARP-1 -/- and wild-type mice for 64 h to 100% hyperoxia and let them recover in air for 5-21 days. PARP-1-deficient mice exhibited significantly higher lung cell hyperplasia and proliferation than PARP-1 +/+ animals after 5 and 10 days of recovery. This was accompanied by an increased inflammatory response in PARP-1 -/- compared with wild-type animals, characterized by neutrophil infiltration and increased IL-6 levels in bronchoalveolar lavages. These lesions were reversible, since the extent of the hyperplastic regions was reduced after 21 days of recovery and did not result in fibrosis. In vitro, lung primary fibroblasts derived from PARP-1 -/- mice showed a higher proliferative response than PARP-1 +/+ cells during air recovery after hyperoxia-induced growth arrest. Altogether, these results reveal an essential role of PARP-1 in the control of cell repair and tissue remodeling after hyperoxia-induced lung injury.

Heme oxygenase-1/carbon monoxide: from basic science to therapeutic applications

The heme oxygenases, which consist of constitutive and inducible isozymes (HO-1, HO-2), catalyze the rate-limiting step in the metabolic conversion of heme to the bile pigments (i.e., biliverdin and bilirubin) and thus constitute a major intracellular source of iron and carbon monoxide (CO). In recent years, endogenously produced CO has been shown to possess intriguing signaling properties affecting numerous critical cellular functions including but not limited to inflammation, cellular proliferation, and apoptotic cell death. The era of gaseous molecules in biomedical research and human diseases initiated with the discovery that the endothelial cell-derived relaxing factor was identical to the gaseous molecule nitric oxide (NO). The discovery that endogenously produced gaseous molecules such as NO and now CO can impart potent physiological and biological effector functions truly represented a paradigm shift and unraveled new avenues of intense investigations. This review covers the molecular and biochemical characterization of HOs, with a discussion on the mechanisms of signal transduction and gene regulation that mediate the induction of HO-1 by environmental stress. Furthermore, the current understanding of the functional significance of HO shall be discussed from the perspective of each of the metabolic by-products, with a special emphasis on CO. Finally, this presentation aspires to lay a foundation for potential future clinical applications of these systems.

Activation of a novel isoform of methionine adenosyl transferase 2A and increased S-adenosylmethionine turnover in lung epithelial cells exposed to hyperoxia

S-Adenosylmethionine (SAM, AdoMet) is the most important methyl donor used for synthesis of nucleic acids, phospholipids, creatine, and polyamines and for methylation of many bioactive molecules. The metabolic response of the lung to oxidative stress of hyperoxia requires increased RNA and protein synthesis for energy metabolism, growth arrest, and antioxidant defense. We studied the production of SAM and other aspects of methionine metabolism in lung epithelial cells exposed to hyperoxia. Human lung epithelial-like (A549) and primary small airway epithelial (SAE) cells were exposed to normoxia (21% O(2)) or hyperoxia (95% O(2)). Cell methionine and S-adenosylmethionine content increased in response to hyperoxia in SAE and A549 cells. Because methionine adenosyl transferase (MAT) is the rate-limiting enzyme of the pathway, we examined the expression of a lung epithelial isoform of MAT 2A in hyperoxia. Western blots revealed a novel MAT 2A isoform expressed in both cell types, with a lower molecular mass than that described in Jurkat cells. Cloning and sequencing of the MAT 2A cDNA revealed one silent nucleotide substitution compared to that expressed in Jurkat. The lower mass of MAT 2A in both lung epithelial cells indicated that the absence of the major posttranslational modification of MAT 2A found in Jurkat. MAT 2A protein progressively increased during hyperoxic exposure in both transformed and primary lung epithelium. Increased flux of (13)C-labeled methionine to S-adenosylhomocysteine (SAH) in A549 demonstrated that SAM's methyl group was utilized, and increased formation of cystathionine indicated that at least part of SAM generated was directed toward cysteine/GSH in the transsulfuration pathway. These results indicate activation of MAT 2A and the transmethylation pathway in the metabolic response to hyperoxia in lung epithelium.

Altered expression of cyclins and cdks in premature infant baboon model of bronchopulmonary dysplasia

Bronchopulmonary dysplasia (BPD) is a chronic lung disease of premature infants, which results in substantial morbidity. The pathophysiology of BPD includes oxidant injury, baro/volutrauma, and disordered lung repair. As lung development, differentiation, and repair require cell division, we hypothesized dysregulation of the cell cycle in oxygen exposure of premature infants that may contribute to the evolution of BPD. In this investigation, we studied the expression of cyclins and cyclin-dependent kinases (cdks) that regulate transition from G1 and G2 phases of the cell cycle. We report here that expression of cyclin D1, cyclin E, and cyclin A is modulated in premature baboons in respiratory distress. In addition, the expression of cdk1 or cdk4 was also modulated in these premature animals. The phosphorylation of retinoblastoma protein was progressively decreased in 125-day animals and in 140-day animals exposed to 6 or 14 days of PRN oxygen. These results indicate that due to altered cyclin and cdk expression, the repair of injured epithelium may proceed in a disordered manner that is characteristic of BPD. Thus, altered cell cycle regulation may be an important factor in the evolution of BPD.

Pathways of cell signaling in hyperoxia

Administration of high concentrations of oxygen (hyperoxia) is a mainstay of supportive treatment for patients suffering from severe respiratory failure. However, hyperoxia, by generating excess systemic reactive oxygen species (ROS), can exacerbate organ failure by causing cellular injury. Therefore, a better understanding of the signal transduction pathways in hyperoxia may provide the basis for effective therapeutic interventions. The major biological effects of hyperoxia include cell death, induction of stress responses, inflammation, and modulation of cell growth. Major signaling pathways that appear to be involved include the mitogen-activated protein kinases (MAPKs), AP-1, and NF-kappa B, which converge, ultimately, to the expression of a range of stress response genes, cytokines, and growth factors.

Protective role of retinoic acid from antiproliferative action of TNF-alpha on lung epithelial cells

Tumor necrosis factor (TNF)-alpha is a key molecule in lung inflammation. We have established the insulin-like growth factor binding protein 2 (IGFBP-2) as a marker associated with the growth arrest of lung alveolar epithelial cells (AEC). Here, we studied the effects of TNF-alpha on AEC proliferation and the putative protective role of retinoic acid (RA). We documented an antiproliferative action of TNF-alpha that was reversible only at 24 h and then became irreversible with induction of apoptosis. TNF-alpha treatment was associated with a dramatic induction of IGFBP-2. To discover the mechanism of action of IGFBP-2, we further tested the mitogenic potential of IGF-I to counteract TNF-alpha inhibition. Addition of IGF-I to the TNF-alpha containing medium did not stimulate proliferation, whereas des(1-3)IGF-I, an analog of IGF-I that bears low affinity for IGFBPs, was able to restore cell growth. Interestingly, we observed that RA abrogated TNF-alpha-induced growth arrest and that this effect was associated with a dramatic decrease in IGFBP-2 expression. These results suggest a protective role of RA from TNF-alpha antiproliferative action, through mechanisms involving modulation of IGFBP-2 production.

Retinoic acid protects against hyperoxia-mediated cell-cycle arrest of lung alveolar epithelial cells by preserving late G1 cyclin activities

The epithelium of the lung alveolus is a major target for oxidant injury, and its proper repair after injury is dependent on the proliferative response of the alveolar epithelial type 2 cells. Recently, we have provided evidence that retinoic acid (RA) stimulates proliferation of type 2 cells. In the present study, we examined the effects of RA on the proliferative response of alveolar type 2 cells exposed to elevated oxygen (O(2)). We showed that pretreatment by RA was able to prevent the growth arrest and cell loss of O(2)-exposed cells. To gain insights into the mechanisms involved, we studied the effects of RA on the cyclin-dependent kinase (CDK) system. The activity of cyclin E-CDK2 complex was found to be decreased in O(2)-exposed cells. Interestingly, this decrease was no longer observed when cells were pretreated with RA. Analysis of p21(CIP1), an inhibitor of CDK, revealed an increased expression in O(2)-exposed cells that was no longer observed in cells treated with RA. These effects were associated with a reduced association of p21(CIP1) with cyclin E-CDK2 complexes in the presence of RA. In addition, studies of Smad activity strongly suggest that the mechanisms through which RA preserves late G(1) cyclin-CDK complex activity may involve interference with the transforming growth factor-beta signaling pathway.

Extracellular glutathione inhibits oxygen-induced permeability changes in alveolar epithelial monolayers

Exposure to high fractional inspired oxygen for 24 h increases permeability of the alveolar epithelium, contributing to the clinical manifestations of oxygen toxicity. Utilizing a model of the alveolar epithelium in which isolated rat type II cells form polarized monolayers on polycarbonate filters [transepithelial resistance (R(t)) > 1 k Omega x cm(2) by day 4], we evaluated the ability of reduced glutathione (GSH) to ameliorate these changes. On day 4, apical fluid was replaced with culture medium containing 1) no additives, 2) GSH (500 microM), or 3) GSH (500 microM) + glutathione reductase (0.5 U/ml) + nicotinamide adenine dinucleotide phosphate (250 microM). Monolayers were exposed (for 24 h) to room air (control) or 95% O(2), each containing 5% CO(2). After 24 h of hyperoxia, R(t) for condition 1 decreased by 45% compared with control (P < 0.001). In conditions 2 and 3, R(t) did not decrease significantly (P = not significant). Hyperoxia-induced decreases in active ion transport were observed for conditions 1 and 2 (P < 0.05), but not for condition 3 (P = not significant). These findings indicate that extracellular GSH may protect the alveolar epithelium against hyperoxia-induced injury. Addition of glutathione reductase and nicotinamide adenine dinucleotide phosphate may further augment these protective effects of GSH.

DNA damage and cell cycle checkpoints in hyperoxic lung injury: braking to facilitate repair

The beneficial use of supplemental oxygen therapies to increase arterial blood oxygen levels and reduce tissue hypoxia is offset by the knowledge that it injures and kills cells, resulting in increased morbidity and mortality. Although many studies have focused on understanding how hyperoxia kills cells, recent findings reveal that it also inhibits proliferation through activation of cell cycle checkpoints rather than through overt cytotoxicity. Cell cycle checkpoints are thought to be protective because they allow additional time for injured cells to repair damaged DNA and other essential molecules. During recovery in room air, the lung undergoes a burst of proliferation to replace injured and dead cells. Failure to terminate this proliferation has been associated with fibrosis. These observations suggest that growth-suppressive signals, which inhibit proliferation of injured cells and terminate proliferation when tissue repair has been completed, may play an important role in the pulmonary response to hyperoxia. Because DNA replication is coupled with DNA repair, activation of cell cycle checkpoints during hyperoxia may be a mechanism by which cells protect themselves from oxidant genotoxic stress. This review examines the effect of hyperoxia on DNA integrity, pulmonary cell proliferation, and cell cycle checkpoints activated by DNA damage.

The role of p21(CIP1/WAF1) in growth of epithelial cells exposed to hyperoxia

Previous studies have shown that hyperoxia inhibits proliferation and increases the expression of the tumor suppressor p53 and its downstream target, the cyclin-dependent kinase inhibitor p21(CIP1/WAF1), which inhibits proliferation in the G1 phase of the cell cycle. To determine whether growth arrest was mediated through activation of the p21-dependent G1 checkpoint, the kinetics of cell cycle movement during exposure to 95% O2 were assessed in the Mv1Lu and A549 pulmonary adenocarcinoma cell lines. Cell counts, 5-bromo-2'-deoxyuridine incorporation, and cell cycle analyses revealed that growth arrest of both cell lines occurred in S phase, with A549 cells also showing evidence of a G1 arrest. Hyperoxia increased p21 in A549 but not in Mv1Lu cells, consistent with the activation of the p21-dependent G1 checkpoint. The ability of p21 to exert the G1 arrest was confirmed by showing that hyperoxia inhibited proliferation of HCT 116 colon carcinoma cells predominantly in G1, whereas an isogenic line lacking p21 arrested in S phase. The cell cycle arrest in S phase appears to be a p21-independent process caused by a gradual reduction in the rate of DNA strand elongation. Our data reveal that hyperoxia inhibits proliferation in G1 and S phase and demonstrate that p53 and p21 retain their ability to affect G1 checkpoint control during exposure to elevated O2 levels.

Overexpression of manganese superoxide dismutase protects lung epithelial cells against oxidant injury

To determine whether overexpression of antioxidant enzymes in lung epithelial cells prevents damage from oxidant injury, stable cell lines were generated with complementary DNAs encoding manganese superoxide dismutase (MnSOD) and/or catalase (CAT). Cell lines overexpressing MnSOD, CAT, or MnSOD + CAT were assessed for tolerance to hyperoxia or paraquat. After exposure to 95% O(2) for 10 d, 44 to 57% of cells overexpressing both MnSOD and CAT and 37 to 47% of cells overexpressing MnSOD alone were viable compared with 7 to 12% of empty vector or parental cells (P < 0.05). To assess if viable cells were capable of cell division after hyperoxic exposures (up to 5 d), a clonogenicity assay was performed. The clonogenic potential of cells overexpressing MnSOD + CAT and MnSOD alone were significantly better than those expressing CAT alone or empty vector controls. In addition, 54 to 72% of cells overexpressing both MnSOD and CAT survived in 1 mM paraquat compared with 58 to 73% with MnSOD alone and 27% with control cells. Overexpression of CAT alone did not improve survival in hyperoxia or paraquat. The combination of MnSOD + CAT did not provide additional protection from paraquat. Data demonstrate that overexpression of MnSOD protects cells from oxidant injury and CAT offers additional protection from hyperoxic injury when co-expressed with MnSOD.

Distinct patterns of insulin-like growth factor binding protein (IGFBP)-2 and IGFBP-3 expression in oxidant exposed lung epithelial cells

Oxygen (O(2)) species are involved in a large variety of pulmonary diseases. Among the various cell types that compose the lung, the epithelial cells of the alveolar structure appear to be a major target for oxidant injury. Despite their importance in the repair processes, the mechanisms which regulate the replication of the stem cells of the alveolar epithelium, the type 2 cells, remain poorly understood. Based on the results of several studies which have documented the involvement of the insulin-like growth factor (IGF) system in lung epithelial cell replication, and which have also suggested a role for IGF binding proteins (IGFBPs) in the control of cell proliferation, the aim of the present work was to determine whether IGFBPs could be involved in the modulation of growth of human lung epithelial cells exposed to oxidants. Experiments were performed using a human lung adenocarcinoma cell line (A549) which was exposed for various durations to hyperoxia (95% O(2)). We observed a rapid and reversible growth arrest of the cells after only 24 h of O(2) exposure. When oxidant injury was prolonged, growth arrest was followed by induction of apoptosis with activation of the Fas pathway. These effects were associated with an increased expression of IGFBP-2 and IGFBP-3. In addition, study of localization of these proteins revealed distinct patterns of distribution. IGFBP-3 was mainly present in the extracellular compartment. In comparison, the fraction of IGFBP-2 secreted was less abundant whereas the IGFBP-2 fraction in the intracellular compartment appeared stronger. In addition, analysis of the subcellular localization provided data indicating the presence of IGFBP-2 in the nucleus. Taken together these data support a role for IGFBP-2 and IGFBP-3 in the processes of growth arrest and apoptosis in lung epithelial cells upon oxidant exposure. They also suggest that distinct mechanisms may link IGFBP-2 and IGFBP-3 to the key regulators of the cell cycle.

Signal transduction pathways in hyperoxia-induced lung cell death

Acute lung injury is an unfortunate consequence of oxygen therapy. Increasing evidence suggests that pulmonary dysfunction resulting from acute oxygen toxicity is at least in part due to the injury and death of lung cells. Studies using morphological and biochemical analyses revealed that hyperoxia-induced pulmonary cell death is multimodal, involving not only necrosis, but also apoptosis. A correlative relationship between the severity of hyperoxic acute lung injury and increased apoptosis has been supported by numerous studies in a variety of animal models, although future experiments are necessary to determine whether it is an actual causal relationship. Altered expression of several apoptotic regulatory proteins, such as p53 and Bcl-2, and DNA damage-induced proteins is associated with hyperoxic cell death and lung injury. Stress-responsive proteins, such as heme oxygenase (HO)-1, have been shown to protect animals against hyperoxic cell injury and death. Redox-sensitive transcription factors and mitogen-activated protein kinase signal transduction pathways may play important roles in regulating the expression of stress-responsive and apoptotic regulatory genes. A better understanding of signal transduction pathways leading to hyperoxic cell death may provide new approaches to the treatment of hyperoxia-induced lung injury.

Attenuation of hyperoxia-induced growth inhibition in H441 cells by gene transfer of mitochondrially targeted glutathione reductase

Reactive oxygen species (ROS) are implicated as agents of cellular damage in pulmonary oxygen toxicity. Glutathione (GSH) and GSH-dependent antioxidant enzymes protect against damage by ROS, and recycling of glutathione disulfide (GSSG) to GSH by glutathione reductase (GR) is essential for the optimum functioning of this system. Exposure to hyperoxia inhibits lung development in newborn animals and humans, and attenuates cell growth in proliferating cell cultures. Considerable evidence supports a role for ROS as growth-altering molecules. Previously, we have observed that gene transfer of GR to mitochondria in H441 cells, using a vector containing a mitochondrial leader sequence (LGR), protected these cells against t-BuOOH-induced cytotoxicity. The present studies tested the hypothesis that gene transfer of LGR would attenuate the cytostatic effects of hyperoxia exposure in H441 cells. H441 cells (0.9 x 10(6) cells/plate) transfected with adenovirus containing LGR or the complementary DNA (cDNA) for manganese superoxide dismutase in reverse orientation (DOS) as a control construct, and untransfected cells (CON) were maintained in 21% oxygen (normoxia) or 95% oxygen (hyperoxia) for 48 h, and cell growth was assessed by cell counts and by reduction of the tetrazolium dye 3-(4,5 dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT) to formazan. Cells maintained in normoxia achieved normal growth (CON, 1.98; DOS, 1.91; LGR, 2.0 x 10(6) cells/plate). Hyperoxia inhibited cell growth and the reduction of MTT; however, cells transfected with LGR had greater mitochondrial GR activities (CON, 16+/-2; DOS, 19+/-3; LGR, 322+/-18 mU/mg of protein), sustained more normal growth patterns (CON, 1.25+/-0.12; DOS, 1.24 +/-0.21, LGR, 1.8+/-0.25 x 10(6) cells/plate), and had less inhibition of MTT reduction (CON, 29; DOS, 27; LGR, 16% inhibition, P<0.01) after exposure to hyperoxia for 48 h than was observed in cells transfected with DOS or in control cells not infected with virus. In addition, resistant cells had higher mitochondrial GSH levels and maintained mitochondrial GSH/GSSG ratios in hyperoxia, suggesting that maintaining mitochondrial GSH homeostasis determined critical aspects of cell division in these studies. The mechanisms for sustaining cell growth during hyperoxia in H441 cells with enhanced mitochondrial GR activities are unknown, but similar effects in infants exposed to supplemental oxygen could be highly beneficial.

Hyperoxia inhibits proliferation of Mv1Lu epithelial cells independent of TGF-beta signaling

High concentrations of O(2) inhibit epithelial cell proliferation that resumes on recovery in room air. To determine whether growth arrest is mediated by transforming growth factor-beta (TGF-beta), changes in cell proliferation during exposure to hyperoxia were assessed in the mink lung epithelial cell line Mv1Lu and the clonal variant R1B, which is deficient for the type I TGF-beta receptor. Mv1Lu cells treated with TGF-beta accumulated in the G(1) phase of the cell cycle as determined by propidium iodide staining, whereas proliferation of R1B cells was unaffected by TGF-beta. In contrast, hyperoxia inhibited proliferation of both cell lines within 24 h of exposure through an accumulation in the S phase. Mv1Lu cells treated with TGF-beta and exposed to hyperoxia accumulated in the G(1) phase, suggesting that TGF-beta can inhibit the S phase accumulation observed with hyperoxia alone. Cyclin A was detected in cultures exposed to room air or growth arrested by hyperoxia while decreasing in cells growth arrested in the G(1) phase by TGF-beta. Finally, hyperoxia failed to activate a TGF-beta-dependent transcriptional reporter in both Mv1Lu and R1B cells. These findings reveal that simple growth arrest by hyperoxia involves a defect in S phase progression that is independent of TGF-beta signaling.

Oxygen induces S-phase growth arrest and increases p53 and p21(WAF1/CIP1) expression in human bronchial smooth-muscle cells

Hyperoxia increases free radical production, leading to DNA damage. Recent studies indicate that oxygen augments the expression of p53 and p21(WAF1/CIP1), and increases apoptotic labeling of airway epithelial cells. Similar changes in regulatory gene products have not been reported in other pulmonary cells, nor have these changes been investigated in conjunction with alterations in cell-cycle distribution. The present study was conducted to determine whether oxygen alters the expression of p53 and p21(WAF1/CIP1) in human bronchial smooth-muscle cells (BSMC). BSMC placed in room air (RA), 40% O(2), or 95% O(2) were examined for 3 d to determine cell number, thymidine incorporation, cell-cycle distribution, and lactate dehydrogenase release. Apoptosis was assessed through the terminal deoxynucleotidyl transferase-deoxyuridine triphosphate end-nick labeling (TUNEL) technique, and p53 and p21(WAF1/CIP1) protein levels were determined through enzyme-linked immunosorbent assay. Exposure of BSMC to 95% O(2) decreased proliferation and DNA synthesis within 24 h, and was accompanied by an increase in S-phase cells (72 h; RA: 12.9 +/- 4.6%, versus 95% O(2): 34.6 +/- 7.0%; P < 0.01). By comparison, exposure to 40% O(2) resulted in decreased proliferation at 48 h without significant alterations in cell-cycle distribution. Both p53 and p21(WAF1/CIP1) levels were increased by 95% O(2), with maximal differences noted at 24 and 48 h, respectively. All atmospheres showed < 8% cell death and few TUNEL-positive cells. Our results indicate that oxygen-mediated alterations in BSMC proliferation are time- and concentration-dependent. Furthermore, high oxygen levels induce S-phase arrest and increased expression of p53 and p21(WAF1/CIP1). Activation of these genes may prevent replication without inducing apoptosis to allow for the repair of oxidative damage.

Extracellular superoxide dismutase in the airways of transgenic mice reduces inflammation and attenuates lung toxicity following hyperoxia

Extracellular superoxide dismutase (EC-SOD, or SOD3) is the major extracellular antioxidant enzyme in the lung. To study the biologic role of EC-SOD in hyperoxic-induced pulmonary disease, we created transgenic (Tg) mice that specifically target overexpression of human EC-SOD (hEC-SOD) to alveolar type II and nonciliated bronchial epithelial cells. Mice heterozygous for the hEC-SOD transgene showed threefold higher EC-SOD levels in the lung compared with wild-type (Wt) littermate controls. A significant amount of hEC-SOD was present in the epithelial lining fluid layer. Both Tg and Wt mice were exposed to normobaric hyperoxia (>99% oxygen) for 48, 72, and 84 hours. Mice overexpressing hEC-SOD in the airways attenuated the hyperoxic lung injury response, showed decreased morphologic evidence of lung damage, had reduced numbers of recruited inflammatory cells, and had a reduced lung wet/dry ratio. To evaluate whether reduced numbers of neutrophil infiltration were directly responsible for the tolerance to oxygen toxicity observed in the Tg mice, we made Wt and Tg mice neutropenic using anti-neutrophil antibodies and subsequently exposed them to 72 hours of hyperoxia. Both Wt and Tg neutrophil-depleted (ND) mice have less severe lung injury compared with non-ND animals, thus providing direct evidence that neutrophils recruited to the lung during hyperoxia play a distinct role in the resultant acute lung injury. We conclude that oxidative and inflammatory processes in the extracellular lung compartment contribute to hyperoxic-induced lung damage and that overexpression of hEC-SOD mediates a protective response to hyperoxia, at least in part, by attenuating the neutrophil inflammatory response.

Role for NF-kappa B in mediating the effects of hyperoxia on IGF-binding protein 2 promoter activity in lung alveolar epithelial cells

The surface of the pulmonary alveolus is a major target for oxidant injury, and its proper repair following injury is dependent on the proliferative response of the stem cells of the alveolar epithelium, the type 2 cells. In previous studies on the mechanisms controlling this response, we have documented involvement of several components of the IGF system, and mainly of the IGF binding protein-2 (IGFBP-2). We have provided evidence that this binding protein was associated with inhibition of DNA synthesis of type 2 cells exposed to oxidants and that its expression was regulated mostly at the level of transcription. In the present study, we focused on the factors involved in this regulation. From examination of the IGFBP-2 gene promoter sequence which revealed the presence of four potential binding sites for transcription factors of the NF-kappa B/Rel family, we hypothesized that NF-kappa B might be involved in the transcriptional activation of IGFBP-2 in oxidant-exposed cells. Data reported herein demonstrated that NF-kappa B activated IGFBP-2 promoter in transient transfection assays, and that exposure of cells to hyperoxia was associated with accumulation of the active form of NF-kappa B. Using gel shift analysis, we documented in O2-treated cells an increased binding to the four NF-kappa B binding sites. We also showed that accumulation of NF-kappa B was associated with a decrease in the inhibitory molecule I kappa B-alpha. Based on the current knowledge on NF-kappa B regulation, it is likely that in a number of situations associated with injury of lung alveolar epithelial cells signaling events involving accumulation of NF-kappa B converge to activate IGFBP-2 and to block entry into S phase.

Accumulation of p21(Cip1/WAF1) during hyperoxic lung injury in mice

Hyperoxic lung injury results in decreased cell proliferation, DNA damage, and cell death. Because the cyclin-dependent kinase inhibitor p21(Cip1/WAF1) (p21) inhibits cell proliferation in G1/S, enhances DNA repair, and regulates apoptosis in some cells, we hypothesized that the expression of p21 would increase in lungs of C57Bl/6J male mice exposed to and recovered from > 95% oxygen. A low level of p21 messenger RNA (mRNA) expression was detected by Northern blot analysis of room air-exposed lungs. Exposure to hyperoxia resulted in a modest increase in p21 mRNA expression by 24 h, followed by a marked induction by 48 to 72 h. In situ hybridization revealed that p21 mRNA abundance increased in bronchiolar epithelium and in resident alveolar cells, but not in smooth-muscle cells or large airway epithelium. Hyperoxia increased the expression of p21 protein by 24 h and continued to increase at 48 and 72 h. Immunohistochemical staining showed that p21 protein accumulated in the bronchiolar epithelium and in alveolar regions that had increased p21 mRNA expression. In contrast, the expression of the cyclin-dependent kinase inhibitor p27(Kip1) was not altered by hyperoxia. To determine whether p21 expression was altered during the repair process, mice were exposed to hyperoxia for 64 h and allowed to recover for up to 4 d in room air. The abundance of p21 mRNA and protein decreased by 1 to 2 d of recovery and returned to room air-exposed levels by 3 to 4 d of recovery. These findings support the concept that bronchiolar epithelial and alveolar cells damaged by hyperoxia express molecules such as p21, which may participate in regulating cell proliferation, DNA repair, and cell death.

Oxygen tension regulates heme oxygenase-1 gene expression in mammalian cell lines

The gene expression of heme oxygenase-1 (HO-1) was studied in mammalian cell lines exposed to hyperoxia. Northern blot analysis demonstrated that hyperoxic exposure increased the HO-1 mRNA levels in various types of cells, including human hepatoma (HepG2) cells. This increase was time- and dose-dependent, and reversible. The HO-1 mRNA levels in HepG2 cells were increased to 2.3- and 4.2-fold of the control by hyperoxic exposure of 6 and 23 h, respectively. Cycloheximide and actinomycin D inhibited the increases in the HO-1 mRNA level produced by hyperoxia, indicating that response to hyperoxia is dependent on de novo protein synthesis and mRNA transcription. Antioxidants, desferrioxamine (DES) and o-phenanthroline (OP) partially inhibited the HO-1 mRNA elevation by hyperoxia. In addition to hyperoxia, sodium arsenite (NaAsO2), cadmium chloride (CdCl(2)) and hydrogen peroxide (H2O2), which are reactive oxygen intermediates (ROI) generators, increased the HO-1 mRNA level by 11-, 22- and 2.5-fold, respectively. OP, an antioxidant and a bivalent metal chelator, blocked the HO-1 mRNA elevation induced either by hyperoxia or by the three ROI generators. In contrast to OP, N-acetylcysteine (NAC), an antioxidant and membrane-permeable reducing reagent, enhanced the HO-1 mRNA elevation induced by hyperoxia, although NAC inhibited the mRNA elevation induced by NaAsO2, CdCl2 and H2O2. These results indicate that oxygen tension regulates HO-1 gene expression and suggest that hyperoxia-specific and redox-sensitive regulators may be involved in hyperoxia-mediated HO-1 gene expression.

The effects of hyperoxic injury and antioxidant vitamins on death and proliferation of human small airway epithelial cells

Previously it was reported that hyperoxia induced death of the human lung adenocarcinoma cell line (A549 cells) by necrosis, not by apoptosis. This study examined proliferation and death of untransformed human small airway epithelial (SAE) cells in normoxia or hyperoxia in comparison with A549 cells. We tested the hypothesis that SAE cells respond differently to hyperoxic injury than do A549 cells. We measured total cell number and viability, thymidine incorporation (SAE cells only), lactate dehydrogenase (LDH) release, and apoptotic changes as markers for cell proliferation and death. Protective effects of antioxidant vitamins also were examined in SAE cells. In normoxia, subconfluent SAE cells had less apoptosis and fewer detached cells, but higher thymidine incorporation than did near-confluent cells. Hyperoxia suppressed thymidine incorporation and augmented apoptosis in both subconfluent and near-confluent SAE cells. Hyperoxia decreased the total cell number only in subconfluence, whereas SAE cell viability declined with hyperoxia in near confluence, but not in subconfluence. For SAE cells, necrosis assessed by LDH release was minimal in all conditions and was not augmented by hyperoxia in SAE cells. In contrast, normoxic A549 cells proliferated more rapidly than did SAE cells with a large number of cells detached during the culture. A549 cells underwent necrotic cell death under confluent or in hyperoxic conditions, but had much less apoptotic cell death. In SAE cells, vitamin E partially prevented the decline of thymidine incorporation with hyperoxia in subconfluence and protected against apoptotic changes with hyperoxia in both subconfluent and near-confluent conditions. Vitamin C prevented apoptosis with hyperoxia only in near-confluent SAE cells. Thus, SAE cells maintained balanced apoptosis and cell proliferation that were altered by cell density and hyperoxia and demonstrated very little necrosis with hyperoxia. Although A549 cells underwent cell death mainly by necrosis, they also were influenced by cell density and hyperoxia. Cell density also determined specific antioxidant vitamin protection in SAE cells.

Functional and pathological effects of prolonged hyperoxia in neonatal mice

Bronchopulmonary dysplasia (BPD) commonly develops in premature infants. An improved understanding of the pathophysiology of BPD requires better models. In this study, neonatal FVB/N mice were exposed to room air or 85% oxygen for 28 days. Neonatal hyperoxia resulted in decreased alveolar septation, increased terminal air space size, and increased lung fibrosis. These changes were evident after 7 days and more pronounced by 28 days. Decreased alveolarization was preceded by decreased proliferation of lung cells. After 3 days of hyperoxia, cell proliferation was decreased compared with room air littermates. Cell proliferation continued to be decreased in the first 2 wk but normalized by 4 wk. Hyperoxia caused an increased number of inflammatory cells in lung tissue and in lung lavage fluid. Analysis of lung tissue RNA by RT-PCR showed that hyperoxia increased expression of the proinflammatory cytokines interleukin-1alpha and macrophage inflammatory protein-1alpha. Prolonged neonatal hyperoxia caused functional changes, decreasing lung volume and pulmonary compliance. We conclude that prolonged exposure of neonatal mice to hyperoxia creates a lesion that is very similar to human BPD and suggests that altered cell proliferation may be important in the pathogenesis of chronic neonatal lung disease.

Retinoic acid-induced proliferation of lung alveolar epithelial cells: relation with the IGF system

Retinoids, including retinol and retinoic acid (RA) derivatives, are important molecules for lung growth and homeostasis. The presence of RA receptors and of RA-binding proteins in the alveolar epithelium led to suggest a role for RA on alveolar epithelial cell replication. In the present study, we examined the effects of RA on proliferation of the stem cells of the alveolar epithelium, the type 2 cells. We showed that treatment of serum-deprived type 2 cells with RA led to a stimulation of cell proliferation, with an increase in cell number in a dose-dependent manner. To gain some insights into the mechanisms involved, we studied the effects of RA on the expression of several components of the insulin-like growth factor (IGF) system that have been shown to be associated with the growth arrest of type 2 cells, mainly the IGF-binding protein-2 (IGFBP-2), IGF-II, and the type 2 IGF receptor. We documented a marked decrease in the expression of these components upon RA treatment. Using conditioned media from RA-treated cells, we provided evidence that the proliferative response of type 2 cells to RA was mediated through production of growth factor(s) distinct from IGF-I. We also showed that RA was able to reduce the decrease in cell number observed when type 2 cells were treated with transforming growth factor (TGF)-beta1. These results together with the known stimulatory effect of TGF-beta1 on IGFBP-2 expression led to suggest that RA may be associated with type 2 cell proliferation through mechanisms interfering with the TGF-beta1 pathway.

Hyperoxia inhibits fetal rat lung fibroblast proliferation and expression of procollagens

The direct effects of hyperoxia on collagen production by fetal lung fibroblasts are unknown and would be important to the understanding of the molecular mechanisms involved in bronchopulmonary dysplasia in premature infants. We studied the effect of hyperoxia on 1) proliferation, 2) mRNA levels for type I and III procollagens, and 3) net collagen production in primary cultures of fetal rat lung fibroblasts. Fibroblasts from 19-day-old rat fetuses (term is 22 days) were obtained. Test plates were incubated in hyperoxia and controls in room air for varying time periods. Cell viability in both conditions was >97% as determined by trypan blue exclusion. Fibroblast proliferation in nonconfluent cultures was found to be significantly reduced with exposure to hyperoxia (P < 0.001). Steady-state levels of type I and III procollagen mRNAs, analyzed on Northern blots hybridized to [32P]cDNA probes, were significantly decreased in hyperoxia (P < 0.01). This effect was noted as early as 4 h of exposure to hyperoxia and persisted for 5 days. There was a significant inverse correlation between duration of exposure to O2 and steady-state levels of mRNA for alpha1(I)-procollagen (r = -0.904) and alpha1(III)-procollagen (r = -0.971). There were no significant changes in steady-state levels of beta-actin mRNA. We also found a significant decrease in collagen synthesis in hyperoxia-exposed fibroblasts (P < 0.05). We conclude that hyperoxia directly effects a reduction in fetal lung fibroblast proliferation and net collagen production at a pretranslational level.

Cell density and antioxidant vitamins determine the effects of hyperoxia on proliferation and death of MDCK epithelial cells

Epithelial cells are prone to oxidant injury, which could change epithelial cell homeostasis and lead to degenerative diseases. We examined the effects of hyperoxia on death and proliferation off Madin-Darby canine kidney (MDCK) epithelial cells and antioxidant vitamin protection. Subconfluent and near-confluent MDCK cells were cultured under normoxia or hyperoxia for two days. We measured cell number and viability, mitochondria enzymatic activity, thymidine incorporation, necrosis [lactate dehydrogenase (LDH) release], and apoptosis (DNA fragmentation and morphological changes). When the cells were subconfluent, hyperoxia decreased the number of adherent cells, mitochondrial enzymatic activity, and thymidine incorporation, but neither LDH release nor apoptotic changes increased compared with normoxic controls. In normoxia, near-confluent cells had lower nonadherent cell numbers, mitochondrial enzymatic activity, and thymidine incorporation than subconfluent cells; hyperoxia further decreased the latter two parameters and increased apoptotic changes and LDH release in near-confluent cells. Vitamin E protected mitochondrial enzymatic activity, apoptotic changes, and LDH release against hyperoxic injury but did not affect changes in thymidine incorporation with hyperoxia. Vitamin C partially protected the mitochondrial enzymatic activity and thymidine incorporation in subconfluence, but not in near confluence. These results indicate that cell density is a major determinant of the effects of hyperoxic injury and the profile of antioxidant vitamin protection.

Altered regulation of G1 cyclins in oxidant-induced growth arrest of lung alveolar epithelial cells. Accumulation of inactive cyclin E-DCK2 complexes

The alveolar surface of the lung is a major target for oxidant injury, and its repair following injury is dependent on the ability of its stem cells, the type 2 cells, to initiate proliferation. From previous studies it is likely that events located before the entry into the S phase of the cell cycle and involving several components of the insulin-like growth factor system as well as of transforming growth factor-beta (TGF-beta) play a key role in growth regulation of oxidant-exposed type 2 epithelial cells. To gain further insights into these mechanisms, we explored the effects of O2 exposure on G1 cyclins and their cyclin-dependent kinases (CDKs). We documented an increased expression of these genes in O2-treated type 2 cells. However, despite this induction, a dramatic decrease in cyclin E-CDK2 activity, but not in cyclin D-CDK4 activity, was found. The concomitant induction of CDK inhibitory proteins (CKIs), mainly p21(CIP1), suggests that accumulation of inactive cyclin E-CDK2 activity is due to CKI binding. We also provided evidence that the mechanisms regulating this process involved TGF-beta as anti-TGF-beta antibody treatment was able to reduce the oxidant-induced inhibition of cyclin E-CDK2 activity. Taken together, these results suggest that oxidants may block entry into S phase by acting on a subset of late G1 events whose alterations are sufficient to impair the activation of cyclin E-CDK2 complexes.

Cellular oxygen toxicity. Oxidant injury without apoptosis

All forms of aerobic life are faced with the threat of oxidation from molecular oxygen (O2) and have evolved antioxidant defenses to cope with this potential problem. However, cellular antioxidants can become overwhelmed by oxidative insults, including supraphysiologic concentrations of O2 (hyperoxia). Oxidative cell injury involves the modification of cellular macromolecules by reactive oxygen intermediates (ROI), often leading to cell death. O2 therapy, which is a widely used component of life-saving intensive care, can cause lung injury. It is generally thought that hyperoxia injures cells by virtue of the accumulation of toxic levels of ROI, including H2O2 and the superoxide anion (O2-), which are not adequately scavenged by endogenous antioxidant defenses. These oxidants are cytotoxic and have been shown to kill cells via apoptosis, or programmed cell death. If hyperoxia-induced cell death is a result of increased ROI, then O2 toxicity should kill cells via apoptosis. We studied cultured epithelial cells in 95% O2 and assayed apoptosis using a DNA-binding fluorescent dye, in situ end-labeling of DNA, and electron microscopy. Using all approaches we found that hyperoxia kills cells via necrosis, not apoptosis. In contrast, lethal concentrations of either H2O2 or O2- cause apoptosis. Paradoxically, apoptosis is a prominent event in the lungs of animals injured by breathing 100% O2. These data indicate that O2 toxicity is somewhat distinct from other forms of oxidative injury and suggest that apoptosis in vivo is not a direct effect of O2.

Cell-density-dependent modulation of the rat insulin-like-growth-factor-binding protein 2 and its gene

The steady-state level of the rat insulin-like-growth-factor-binding protein 2 (IGFBP-2) and insulin-like-growth-factor-II (IGF-II) mRNA increased approximately 20-fold when BRL-3A cells were cultured at increasingly higher cell densities. This increase could not be accounted for by paracrine or autocrine factors, or by the addition of insulin, IGF-I, transforming growth factor beta (TGF-beta), cAMP or IGFBP-2 to the culture medium. A reporter gene assay carrying the promoter domain of the IGFBP-2 gene indicated that the promoter-dependent IGFBP-2 transcription is tenfold higher in high-density cells. The increase in the IGFBP-2 message was accompanied by an increase in the level of protein in the medium. When confluent BRL-3A cells were reseeded at low cell density, the IGFBP-2 mRNA disappeared at a rate significantly faster than in normal conditions. A protein synthesis inhibitor, cycloheximide, was able to prevent the decay of the message observed after the switch from high to low densities. In summary, these findings suggest a regulatory link between cell density and IGFBP-2.

Oxygen modulates production of bFGF and TGF-beta by retinal cells in vitro

Vasoproliferative retinopathies result from retinal capillary non-perfusion and consequent inner retinal hypoxia. However, it is not known whether oxygen mediates vasoproliferation directly (at the nuclear level) or indirectly by regulating the production of growth factors. We have investigated the effect of oxygen on the production of basic fibroblast growth factor and transforming-growth-factor-beta by a variety of retinal cell types in culture. Confluent cultures were maintained for 48 hr under varying oxygen tensions ranging from 135 to 18 mmHg. A reduction in basic fibroblast growth factor levels was observed in the cell lysates and extracellular matrix from retinal microvascular endothelial cell, retinal microvascular pericyte and retinal pigment epithelial cell cultures when the oxygen tension of the medium was reduced from 135 to 18 mmHg. Levels of basic fibroblast growth factor in conditioned media from microvascular endothelial and retinal pigment epithelial cell cultures also decreased when the oxygen tension of the medium was reduced from 135 to 18 mmHg. Total transforming-growth-factor-beta (and specifically isoforms 1 and 2) in the conditioned media from all three cell types was similarly modulated by oxygen i.e. it decreased as the oxygen tension of the medium was reduced from 135 to 18 mmHg. In contrast, the steady state messenger RNA levels for both basic fibroblast growth factor and transforming-growth-factor-beta 1 genes in RPE cells increased significantly when the oxygen tension of the medium was reduced from 135 to 18 mmHg. These results support the putative role of oxygen in influencing the balance of growth factors during the development of preretinal new vessels.