Reactive oxygen species and extracellular signal-regulated kinase 1/2 mitogen-activated protein kinase mediate hyperoxia-induced cell death in lung epithelium

Protective effects of pituitary adenylate cyclase activating polypeptide in endothelial cells against oxidative stress-induced apoptosis

Pituitary adenylate cyclase activating polypeptide (PACAP) is a widely distributed neuropeptide that has various different functions in the nervous system and in non-neural tissues. Little is known about the effects of PACAP in endothelial cells. The aim of the present study was to investigate the effects of PACAP on endothelial cell survival and apoptotic signaling pathways under oxidative stress. Mouse hemangioendothelioma (EOMA) cells were exposed to 0.5mM H(2)O(2) which resulted in a marked reduction of cell viability and a parallel increase of apoptotic cells assessed by MTT test and flow cytometry. Co-incubation with 20nM PACAP1-38 increased cell viability and reduced the percentage of apoptotic cells. Flow cytometry analysis showed that oxidative stress reduced the phosphorylation of the anti-apoptotic ERK and increased the phosphorylation of the pro-apoptotic JNK and p38 MAP kinases. PACAP1-38 treatment ameliorated these changes: levels of phospho-ERK were elevated and those of phospho-JNK and p38 were decreased. All these effects were abolished by simultaneous treatment with the PACAP antagonist PACAP6-38. In summary, our results show that PACAP effectively protects endothelial cells against the apoptosis-inducing effects of oxidative stress.

Hyperoxia in the intensive care unit: why more is not always better

PURPOSE OF REVIEW

Hyperoxic inspired gas is essential for patients with hypoxic respiratory failure; it is also suspected, however, as a contributor to the pathogenesis of acute lung injury. Several recent studies in humans, animals, and cell culture have identified mechanisms by which hyperoxia may exert deleterious effects on critically ill patients. This review identifies relevant new findings regarding hyperoxic lung injury in the context of providing guidance for future clinical studies.

RECENT FINDINGS

Recent studies have clarified the roles of both receptor-mediated and mitochondrial cell death pathways in experimental hyperoxic lung injury. Studies in animals demonstrate that hyperoxia interacts with mechanical stretch to augment ventilator-induced lung injury. Finally, studies in humans implicate hyperoxia in impairment of host defense responses to infections.

SUMMARY

Although hyperoxia has not been conclusively identified as a clinically important cause of lung injury in humans, animal data strongly implicate it. Reports of interaction effects between hyperoxia and both mechanical ventilation and host defense suggest that clinical studies of hyperoxia must take these variables into account. Accumulating data about how hyperoxia initiates cell death provide guidance for development of both biomarkers to identify hyperoxia-induced injury and pharmacological interventions to limit hyperoxia's adverse effects.

Increased hyperoxia-induced mortality and acute lung injury in IL-13 null mice

IL-13 is a critical effector at sites of Th2 inflammation and remodeling. As a result, anti-IL-13-based therapies are being actively developed to treat a variety of diseases and disorders. However, the beneficial effects of endogenous IL-13 in the normal and diseased lung have not been adequately defined. We hypothesized that endogenous IL-13 is an important regulator of oxidant-induced lung injury and inflammation. To test this hypothesis, we compared the effects of 100% O(2) in mice with wild-type and null IL-13 loci. In this study, we demonstrate that hyperoxia significantly augments the expression of the components of the IL-13R, IL-13Ralpha1, and IL-4Ralpha. We also demonstrate that, in the absence of IL-13, hyperoxia-induced tissue inflammation is decreased. In contrast, in the IL-13 null mice, DNA injury, cell death, caspase expression, and activation and mortality are augmented. Interestingly, the levels of the cytoprotective cytokines vascular endothelial cell growth factor, IL-6, and IL-11 were decreased in the bronchoalveolar lavage fluid. These studies demonstrate that the expression of the IL-13R is augmented and that the endogenous IL-13-IL-13R pathway contributes to the induction of inflammation and the inhibition of injury in hyperoxic acute lung injury.

FLIP inhibits endothelial cell apoptosis during hyperoxia by suppressing Bax

High oxygen tension (hyperoxia) causes pulmonary cell death, involving apoptosis, necrosis, or mixed death phenotypes, though the underlying mechanisms remain unclear. In mouse lung endothelial cells (MLEC) hyperoxia activates both extrinsic (Fas-dependent) and intrinsic (mitochondria-dependent) apoptotic pathways. We examined the hypothesis that FLIP, an inhibitor of caspase-8, can protect endothelial cells against the lethal effects of hyperoxia. Hyperoxia caused the time-dependent downregulation of FLIP in MLEC. Overexpression of FLIP attenuated intracellular reactive oxygen species generation during hyperoxia exposure, by downregulating extracellular-regulated kinase-1/2 activation and p47(phox) expression. FLIP prevented hyperoxia-induced trafficking of the death-inducing signal complex from the Golgi apparatus to the plasma membrane. Furthermore, FLIP blocked the activations of caspase-8/Bid, caspases -3/-9, and inhibited the mitochondrial translocation and activation of Bax, resulting in protection against hyperoxia-induced cell death. Under normoxic conditions, FLIP expression increased the phosphorylation of p38 mitogen-activated protein kinase leading to increased phosphorylation of Bax during hyperoxic stress. Furthermore, FLIP expression markedly inhibited protein kinase C activation and expression of distinct protein kinase C isoforms (alpha, eta, and zeta), and stabilized an interaction of PKC with Bax. In conclusion, FLIP exerted novel inhibitory effects on extrinsic and intrinsic apoptotic pathways, which significantly protected endothelial cells from the lethal effects of hyperoxia.

Hyperoxia increases ventilator-induced lung injury via mitogen-activated protein kinases: a prospective, controlled animal experiment

Introduction

Large-tidal volume (V_T) mechanical ventilation and hyperoxia used in patients with acute respiratory distress syndrome can damage pulmonary epithelial cells through lung inflammation and apoptotic cell death. Hyperoxia has been shown to increase ventilator-induced lung injury, but the mechanisms regulating interaction between large V_T and hyperoxia are unclear. We hypothesized that the addition of hyperoxia to large-V_T ventilation would increase neutrophil infiltration by upregulation of the cytokine macrophage inflammatory protein-2 (MIP-2) and would increase apoptosis via the mitogen-activated protein kinase pathways.

Methods

C57BL/6 mice were exposed to high-V_T (30 ml/kg) mechanical ventilation with room air or hyperoxia for one to five hours.

Results

The addition of hyperoxia to high-V_T ventilation augmented lung injury, as demonstrated by increased apoptotic cell death, neutrophil migration into the lung, MIP-2 production, MIP-2 mRNA expression, increased DNA binding activity of activator protein-1, increased microvascular permeability, and c-Jun NH₂-terminal kinase (JNK) and extracellular signal-regulated kinase (ERK) 1/2 activation. Hyperoxia-induced augmentation of high-V_T-induced lung injury was attenuated in JNK-deficient mice and in mice with pharmacologic inhibition of ERK activity by PD98059. However, only JNK-deficient mice, and not mice with ERK activity inhibition by PD98059, were protected from high-V_T-induced lung injury without hyperoxia.

Conclusion

We conclude that hyperoxia increased high-V_T-induced cytokine production, neutrophil influx, and apoptotic cell death through activation of the JNK and ERK1/2 pathways.

Hyperoxia-induced signal transduction pathways in pulmonary epithelial cells

Mechanical ventilation with hyperoxia is necessary to treat critically ill patients. However, prolonged exposure to hyperoxia leads to the generation of excessive reactive oxygen species (ROS), which can cause acute inflammatory lung injury. One of the major effects of hyperoxia is the injury and death of pulmonary epithelium, which is accompanied by increased levels of pulmonary proinflammatory cytokines and excessive leukocyte infiltration. A thorough understanding of the signaling pathways leading to pulmonary epithelial cell injury/death may provide some insights into the pathogenesis of hyperoxia-induced acute inflammatory lung injury. This review focuses on epithelial responses to hyperoxia and some of the major factors regulating pathways to epithelial cell injury/death, and proinflammatory responses on exposure to hyperoxia. We discuss in detail some of the most interesting players, such as NF-kappaB, that can modulate both proinflammatory responses and cell injury/death of lung epithelial cells. A better appreciation for the functions of these factors will no doubt help us to delineate the pathways to hyperoxic cell death and proinflammatory responses.

Lung epithelial cell apoptosis during acute lung injury in infancy

CONTEXT

Apoptosis of lung epithelial cells is implicated in the pathogenesis of acute lung injury. Most research on this subject has focused on adults. Very little is known about a potential interaction of this process with lung development in children.

OBJECTIVE

To summarize the current literature on lung epithelial cell apoptosis and common causes of acute lung injury in infants and young children and to identify new areas of research.

DESIGN

A Medline-based literature search.

RESULTS AND CONCLUSIONS

Few studies have focused on lung epithelial cell apoptosis during common causes of acute lung injury in children. Nevertheless, the limited literature suggests that this may be an important mechanism during respiratory distress syndrome of infants and viral respiratory tract infection. Apoptosis is an essential process during lung development and maturation. Insufficient attention has been paid to potential consequences of this for the short- and long-term outcomes of acute lung injury.

The NOX family of ROS-generating NADPH oxidases: physiology and pathophysiology

For a long time, superoxide generation by an NADPH oxidase was considered as an oddity only found in professional phagocytes. Over the last years, six homologs of the cytochrome subunit of the phagocyte NADPH oxidase were found: NOX1, NOX3, NOX4, NOX5, DUOX1, and DUOX2. Together with the phagocyte NADPH oxidase itself (NOX2/gp91(phox)), the homologs are now referred to as the NOX family of NADPH oxidases. These enzymes share the capacity to transport electrons across the plasma membrane and to generate superoxide and other downstream reactive oxygen species (ROS). Activation mechanisms and tissue distribution of the different members of the family are markedly different. The physiological functions of NOX family enzymes include host defense, posttranlational processing of proteins, cellular signaling, regulation of gene expression, and cell differentiation. NOX enzymes also contribute to a wide range of pathological processes. NOX deficiency may lead to immunosuppresion, lack of otoconogenesis, or hypothyroidism. Increased NOX activity also contributes to a large number or pathologies, in particular cardiovascular diseases and neurodegeneration. This review summarizes the current state of knowledge of the functions of NOX enzymes in physiology and pathology.

VEGF-induced heme oxygenase-1 confers cytoprotection from lethal hyperoxia in vivo

Prolonged exposure to hyperoxia results in hyperoxic acute lung injury (HALI). Vascular endothelial growth factor (VEGF) has been shown to have cytoprotective effects and prolong survival in an in vivo model of HALI. Heme oxygenase-1 (HO-1) has protective effects in hyperoxia; therefore, we hypothesized that induction of HO-1 would be an important contributor to VEGF-induced cytoprotection. Using inducible, lung-specific VEGF overexpressing transgenic mice, we demonstrated that VEGF is a potent inducer of HO-1 mRNA and protein in mouse lung. To evaluate the effect of inhibition of HO-1 on injury, VEGF transgenic mice were treated with HO-1 short interfering RNA (HO-1 siRNA) and exposed to hyperoxia. Total lung lavage protein concentration, TUNEL staining, lipid peroxidation, and wet-to-dry ratio were all increased, consistent with increased injury. These findings were corroborated by survival studies in which inhibition of HO-1 function using tin-protoporphryin or silencing of HO-1 with lentiviral HO-1 short hairpin RNA (ShRNA) significantly decreased survival in hyperoxia. We conclude from these data that VEGF-induced HO-1 is an important contributor to cytoprotection in this in vivo model of acute lung injury and that alterations in VEGF function in the lung are likely to be important determinants of the outcome of acute lung injury.

Carbon monoxide protects against hyperoxia-induced endothelial cell apoptosis by inhibiting reactive oxygen species formation

Hyperoxia causes cell injury and death associated with reactive oxygen species formation and inflammatory responses. Recent studies show that hyperoxia-induced cell death involves apoptosis, necrosis, or mixed phenotypes depending on cell type, although the underlying mechanisms remain unclear. Using murine lung endothelial cells, we found that hyperoxia caused cell death by apoptosis involving both extrinsic (Fas-dependent) and intrinsic (mitochondria-dependent) pathways. Hyperoxia-dependent activation of the extrinsic apoptosis pathway and formation of the death-inducing signaling complex required NADPH oxidase-dependent reactive oxygen species production, because this process was attenuated by chemical inhibition, as well as by genetic deletion of the p47(phox) subunit, of the oxidase. Overexpression of heme oxygenase-1 prevented hyperoxia-induced cell death and cytochrome c release. Likewise, carbon monoxide, at low concentrations, markedly inhibited hyperoxia-induced endothelial cell death by inhibiting cytochrome c release and caspase-9/3 activation. Carbon monoxide, by attenuating hyperoxia-induced reactive oxygen species production, inhibited extrinsic apoptosis signaling initiated by death-inducing signal complex trafficking from the Golgi apparatus to the plasma membrane and downstream activation of caspase-8. We also found that carbon monoxide inhibited the hyperoxia-induced activation of Bcl-2-related proteins involved in both intrinsic and extrinsic apoptotic signaling. Carbon monoxide inhibited the activation of Bid and the expression and mitochondrial translocation of Bax, whereas promoted Bcl-X(L)/Bax interaction and increased Bad phosphorylation. We also show that carbon monoxide promoted an interaction of heme oxygenase-1 with Bax. These results define novel mechanisms underlying the antiapoptotic effects of carbon monoxide during hyperoxic stress.

The cellular basis for diverse responses to oxygen

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

Toll-like receptor 4 deficiency causes pulmonary emphysema

TLRs have been studied extensively in the context of pathogen challenges, yet their role in the unchallenged lung is unknown. Given their direct interface with the external environment, TLRs in the lungs are prime candidates to respond to air constituents, namely particulates and oxygen. The mechanism whereby the lung maintains structural integrity in the face of constant ambient exposures is essential to our understanding of lung disease. Emphysema is characterized by gradual loss of lung elasticity and irreversible airspace enlargement, usually in the later decades of life and after years of insult, most commonly cigarette smoke. Here we show Tlr4(-/-) mice exhibited emphysema as they aged. Adoptive transfer experiments revealed that TLR4 expression in lung structural cells was required for maintaining normal lung architecture. TLR4 deficiency led to the upregulation of what we believe to be a novel NADPH oxidase (Nox), Nox3, in lungs and endothelial cells, resulting in increased oxidant generation and elastolytic activity. Treatment of Tlr4(-/- )mice or endothelial cells with chemical NADPH inhibitors or Nox3 siRNA reversed the observed phenotype. Our data identify a role for TLR4 in maintaining constitutive lung integrity by modulating oxidant generation and provide insights into the development of emphysema.

Endothelial STAT3 is essential for the protective effects of HO‐1 in oxidant‐induced lung injury

Administering high levels of inspired oxygen, or hyperoxia, is commonly used as a life-sustaining measure in critically ill patients. Unfortunately, the oxidant stress generated by prolonged hyperoxia can lead to respiratory failure, multiorgan failure, and death. Although the endothelial cell is known to be a target for hyperoxia-induced injury, its precise role is unclear. Heme oxygenase-1 (HO-1) and "signal transducer and activator of transcription 3" (STAT3) have been found to confer protection against endothelial cell injury. We sought to elucidate the specific roles of HO-1 and STAT3 in hyperoxic lung and endothelial cell injury. Mice or murine lung endothelial cells (MLEC) administered HO-1 siRNA exhibited marked injury and death compared with nonspecific siRNA. Overexpression of either HO-1 or STAT3 confers protection. However, HO-1 and its reaction product carbon monoxide (CO) lose their protective effects in the presence of STAT3 siRNA in MLEC or in endothelial-specific, STAT3-deficient mice. STAT3 overexpression is able to partially rescue HO-1-deficient MLEC from hyperoxia-induced cell death. Our results demonstrate 1) the importance of the endothelium in lethal hyperoxic injury, 2) HO-1 and CO require endothelial STAT3 for their protective effects, and 3) STAT3 confers endothelial cell protection via both HO-1-dependent and independent mechanisms.

Activation of MAP kinases by hexavalent chromium, manganese and nickel in human lung epithelial cells

Epidemiological studies indicate that workers who perform welding operations are at increased risk for bronchitis, siderosis, occupational asthma and lung cancer due to fume exposure. Welding fumes are a complex chemical mixture, and the metal composition is hypothesized to be an etiological factor in respiratory disease due to this exposure. In the present study, human lung epithelial cells in vitro responded to hexavalent chromium, manganese and nickel over a concentration range of 0.2-200 microM with a significant increase in intracellular phosphoprotein (a measure of stress response pathway activation). The mitogen-activated protein kinases ERK1/2, SAPK/JNK and p38 were activated via phosphorylation following 1-h exposures. Hexavalent chromium up-regulated p-38 phosphorylation 23-fold and SAPK/JNK phosphorylation 17-fold, with a comparatively modest 4-fold increase in ERK1/2 phosphorylation. Manganese caused a two- to four-fold increase in SAPK/JNK and ERK 1/2 phosphorylation, with no observed effects on p38 kinase. Nickel caused increased (two-fold) phosphorylation of ERK 1/2 only, and was not cytotoxic over the tested concentration range. The observed effects of welding fume metals on cellular signaling in lung epithelium demonstrate a potentially significant interplay between stress-response signaling (p38 and SAPK/JNK) and anti-apototic signaling (ERK 1/2) that is dependant on the specific metal or combination of metals involved.

N-(4-hydroxyphenyl)retinamide-induced apoptosis triggered by reactive oxygen species is mediated by activation of MAPKs in head and neck squamous carcinoma cells

N-(4-hydroxyphenyl)retinamide (4HPR), a synthetic retinoid effective in cancer chemoprevention and therapy, is thought to act via apoptosis induction resulting from increased reactive oxygen species (ROS) generation. As ROS can activate MAP kinases and protein kinase C (PKC), we examined the role of such enzymes in 4HPR-induced apoptosis in HNSCC UMSCC22B cells. 4HPR increased ROS level within 1 h and induced activation of caspase 3 and PARP cleavage within 24 h. Activation of MKK3/6 and MKK4, JNK, p38 and ERK was detected between 6 and 12 h, increased up to 24 h and preceded apoptosis. 4HPR-induced activation of these kinases was abrogated by the antioxidants BHA and vitamin C. SP600125, a JNK inhibitor, suppressed 4HPR-induced c-Jun phosphorylation, cytochrome c release from mitochondria and apoptosis. Suppression of JNK1 and JNK2 using siRNA decreased, whereas overexpression of wild type-JNK1 enhanced 4HPR-induced apoptosis. PD169316, a p38, inhibitor suppressed phosphorylation of Hsp27 and apoptosis. PD98059, an MEK1/2 inhibitor, also suppressed ERK1/2 activation and apoptosis induced by 4HPR. Likewise, PKC inhibitor GF109203X suppressed ERK and p38 phosphorylation and PARP cleavage. These data indicate that 4HPR-induced apoptosis is triggered by ROS increase, leading to the activation of the mitogen-activated protein serine/threonine kinases JNK, p38, PKC and ERK, and subsequent apoptosis.

Activation of ERK1/2 pathway mediates oxidant-induced decreases in mitochondrial function in renal cells

Previously, we showed that oxidant exposure in renal proximal tubular cells (RPTC) induces mitochondrial dysfunction mediated by PKC-epsilon. This study examined the role of ERK1/2 in mitochondrial dysfunction induced by oxidant injury and whether PKC-epsilon mediates its effects on mitochondrial function through the Raf-MEK1/2-ERK1/2 pathway. Sublethal injury produced by tert-butylhydroperoxide (TBHP) resulted in three- to fivefold increase in phosphorylation of ERK1/2 and p38 but not JNK. This was followed by decreases in basal and uncoupled respirations (41%), state 3 respiration and ATP production coupled to complex I (46%), and complex I activity (42%). Oxidant exposure decreased aconitase activity 30% but not pyruvate, alpha-ketoglutarate, and malate dehydrogenase activities. Inhibition of ERK1/2 restored basal and state 3 respirations, DeltaPsi(m), ATP production, and complex I activity but not aconitase activity. In contrast, activation of ERK1/2 by expression of constitutively active MEK1 suppressed basal, uncoupled, and state 3 respirations in noninjured RPTC to the levels observed in TBHP-injured RPTC. MEK1/2 inhibition did not change Akt or p38 phosphorylation, demonstrating that the protective effect of MEK1/2 inhibitor was not due to activation of Akt or inhibition of p38 pathway. Inhibition of PKC-epsilon did not block TBHP-induced ERK1/2 phosphorylation in whole RPTC or in mitochondria. We conclude that 1) oxidant-induced activation of ERK1/2 but not p38 or JNK reduces mitochondrial respiration and ATP production by decreasing complex I activity and substrate oxidation through complex I, 2) citric acid cycle dehydrogenases are not under control of the ERK1/2 pathway in oxidant-injured RPTC, 3) the protective effects of ERK1/2 inhibition are not due to activation of Akt, and 4) ERK1/2 and PKC-epsilon mediate oxidant-induced mitochondrial dysfunction through independent pathways.

Copper-induced stimulation of extracellular signal-regulated kinase in trout hepatocytes: the role of reactive oxygen species, Ca2+, and cell energetics and the impact of extracellular signal-regulated kinase signaling on apoptosis and necrosis

The present study investigated if copper (Cu) exposure of trout hepatocytes, which stimulates formation of reactive oxygen species (ROS) and increases intracellular free Ca(2+) (Ca(2+)i), leads to an activation of extracellular signal-regulated kinase (ERK), the mechanisms underlying this activation, and the role of ERK signaling in cell death. Cu stimulated a time- and dose-dependent increase of phosphorylated extracellular signal-regulated kinase (pERK), and preventing the associated Ca(2+) influx or radical formation diminished or inhibited ERK activation, respectively. Furthermore, Cu enhanced caspase 3/7 activity and necrosis, and both effects were inhibited by treatments diminishing radical production and by chelating extracellular Ca(2+). In addition, ERK activity, and to a lesser extent caspase activity, was reduced by inhibiting mitochondrial ATP production, suggesting ATP dependence of the process. Inhibition of the ERK activator MEK, as well as of p38, significantly reduced caspase activation and necrosis, whereas c-Jun N-terminal kinase (JNK) inhibition diminished only caspase activity. Likewise, inhibition of MEK and p38, but not of JNK, prevented Cu-induced ROS production. In summary, we found that stimulation of ERK by Cu exposure of trout hepatocytes is dependent on radical formation and ATP, whereas Ca(2+) only modulates ERK activity. At the same time, activated ERK, as well as p38, contributes to enhanced ROS formation, whereas JNK did not. All three mitogen-activated protein kinases appear to promote apoptotic cell death upon Cu exposure, and ERK and p38 also stimulate necrosis.

Inducible Activation of TLR4 Confers Resistance to Hyperoxia-Induced Pulmonary Apoptosis

TLRs are essential mediators of host defense against infection via recognition of unique microbial structures. Recent observations indicate that TLR4, the principal receptor for bacterial LPS, may also be activated by noninfectious stimuli including host-derived molecules and environmental oxidant stress. In mice, susceptibility to ozone-induced lung permeability has been linked to the wild-type allele of TLR4, whereas deficiency of TLR4 predisposes to lethal lung injury in hyperoxia. To precisely characterize the role of lung epithelial TLR4 expression in the host response to oxidant stress, we have created an inducible transgenic mouse model that targets the human TLR4 signaling domain to the airways. Exposure of induced transgenic mice to hyperoxia revealed a significant reduction in pulmonary apoptosis compared with controls. This phenotype was associated with sustained up-regulation of antiapoptotic molecules such as heme oxygenase-1 and Bcl-2, yet only transient activation of the transcription factor NF-kappaB. Specific in vivo knockdown of pulmonary heme oxygenase-1 or Bcl-2 expression by intranasal administration of short interfering RNA blocked the effect of TLR4 signaling on hyperoxia-induced lung apoptosis. These results define a novel role for lung epithelial TLR4 as a modulator of cellular apoptosis in response to oxidant stress.

Human 8-oxoguanine DNA glycosylase increases resistance to hyperoxic cytotoxicity in lung epithelial cells and involvement with altered MAPK activity

It is unknown whether base excision DNA repair (BER) proteins interact with mitogen-activated protein kinases (MAPK) under oxidation. Here, we explored roles of BER proteins in signaling transduction involving MAPK during hyperoxia. We demonstrated that ERK1/2 phosphorylation in A549 cells was increased in 95% O₂. p38 activity in A549 cells was also increased by exposure to 95% O₂. To evaluate regulatory roles of MAPK, we have transduced A549 cells and primary alveolar epithelial type II cells (AECII) to overexpress 8-oxoguanine DNA glycosylase (hOgg1). Overexpression of hOgg1 reduced hyperoxic toxicity in A549 and AECII cells. Furthermore, protection by BER against hyperoxia appeared to involve an upregulation of ERK1/2 and downregulation of p38. These observations demonstrate, for the first time, that reduction of hyperoxic toxicity by BER proteins may be involved with MAPK activity, thereby impacting cell survival. Furthermore, our studies suggest that modulation of MAPK may be used in combination with BER proteins to counteract hyperoxic toxicity.

Molecular characterization of muscle atrophy and proteolysis associated with spawning in rainbow trout

Severe muscle deterioration is a physiological response to the energetic demands of fish spawning. This response represents a suitable model to study mechanisms of muscle degradation in fish where typical tetrapod methods, such as muscle unloading, are not applicable. Enzyme activities and mRNA accumulations of genes in major proteolytic pathways, including cathepsins, calpains and the multi-catalytic proteasome, were measured in white muscles of rainbow trout during spawning and post-spawning seasons of gravid fish for comparisons to sterile fish. Fertile fish at spawning had less muscle tissue and less muscle protein compared to sterile fish and post-spawning fertile fish. Muscle deterioration of the fertile fish during spawning was associated with greater mRNA accumulation and elevated activity of cathepsin-L. Concurrently, muscle of spawning fish showed increased mRNA accumulations of cathepsin-D, the calpain regulatory subunit and the proteasome catalytic subunit alpha without corresponding increases in enzyme activities. In addition, elevated activity and increased mRNA accumulation of caspase-9, but not caspase-3, were observed in fertile fish during spawning. This study indicates that cathepsins mediate protein catabolism during spawning in rainbow trout and the catabolic process may involve activation of the apoptosis mediator, caspase-9, but not the apoptosis executioner, caspase-3.

Oxidative stress response results in increased p21WAF1/CIP1 degradation in cystic fibrosis lung epithelial cells

Lung epithelium in cystic fibrosis (CF) patients is characterized by structural damage and altered repair due to oxidative stress. To gain insight into the oxidative stress-related damage in CF, we studied the effects of hyperoxia in CF and normal lung epithelial cell lines. In response to a 95% O2 exposure, both cell lines exhibited increased reactive oxygen species. Unexpectedly, the cyclin-dependent kinase inhibitor p21WAF1/CIP1 protein was undetectable in CF cells under hyperoxia, contrasting with increased levels of p21WAF1/CIP1 in normal cells. In both cell lines, exposure to hyperoxia led to S-phase arrest. Apoptotic features including nuclear condensation, DNA laddering, Annexin V incorporation, and elevated caspase-3 activity were not readily observed in CF cells in contrast to normal cells. Interestingly, treatment of hyperoxia-exposed CF cells with two proteasome inhibitors, MG132 and lactacystin, restored p21WAF1/CIP1 protein and was associated with an increase of caspase-3 activity. Moreover, transfection of p21WAF1/CIP1 protein in CF cells led to increased caspase-3 activity and was associated with increased apoptotic cell death, specifically under hyperoxia. Taken together, our data suggest that modulating p21WAF1/CIP1 degradation may have the therapeutic potential of reducing lung epithelial damage related to oxidative stress in CF patients.

Cutting edge: TLR4 deficiency confers susceptibility to lethal oxidant lung injury

TLRs have been studied extensively in pathogen-mediated host responses. We use a murine model of lethal oxidant-mediated injury to demonstrate for the first time that mammalian TLR4 is required for survival and lung integrity. Administering high levels of inspired oxygen, or hyperoxia, is commonly used as a life-sustaining measure in critically ill patients. However, prolonged exposures can lead to respiratory failure and death. TLR4-deficient mice exhibited increased mortality and lung injury during hyperoxia. The enhanced susceptibility of TLR4-deficient mice to hyperoxia was associated with an inability to up-regulate Bcl-2 and phospho-Akt. Restoration of Bcl-2 and phospho-Akt levels by the exogenous transfer of the antioxidant gene heme oxygenase-1 markedly attenuated hyperoxia-induced injury, apoptosis, and mortality in TLR4-deficient mice. Taken together, our results suggest a protective role of TLR4 in oxidant-mediated injury, providing novel mechanistic links among innate immunity, oxidant stress, and apoptosis.

Wood smoke extract induces oxidative stress-mediated caspase-independent apoptosis in human lung endothelial cells: role of AIF and EndoG

Although a link between toxic smoke and oxidant lung vascular injury has been indicated, the cellular mechanisms of smoke-induced injury to lung endothelial cells are unknown. We investigated oxidative stress and apoptosis induced by wood smoke extract (SE) in human pulmonary artery endothelial cells (HPAECs) and delineated their relationship. We found that SE increased intracellular reactive oxygen species (ROS), depleted intracellular glutathione, and upregulated Cu/Zn superoxide dismutase and heme oxygenase-1 (2 antioxidant enzymes), but it failed to alter the expression of catalase and glutathione peroxidase. In addition, SE promoted apoptosis as indicated by the external exposure of membrane phosphatidylserine, the loss of mitochondrial membrane potential, an increase in the level of Bax (a proapoptotic protein), and enhanced DNA fragmentation. This apoptosis was associated with mitochondrial-to-nuclear translocation of apoptosis-inducing factor (AIF) and endonuclease G (EndoG) (2 apoptogenic proteins) but was independent of caspase cascade activation. Whereas N-acetylcysteine (an antioxidant) effectively reversed the SE-induced increase in ROS and depletion of glutathione, it also suppressed SE-induced nuclear translocation of either AIF or EndoG and prevented the enhanced DNA fragmentation that would have resulted from this. We conclude that 1) although SE upregulates Cu/Zn superoxide dismutase and heme oxygenase-1, it nevertheless increases intracellular oxidative stress in HPAECs, and 2) SE promotes oxidative stress-mediated caspase-independent HPAEC apoptosis that involves mitochondrial-to-nuclear translocation of AIF and EndoG. Thus modulations of the expression of antioxidant enzymes and the caspase-independent apoptotic pathway are possible target choices for potential therapeutic regimes to treat smoke-induced lung injury.

Genipin-induced apoptosis in hepatoma cells is mediated by reactive oxygen species/c-Jun NH2-terminal kinase-dependent activation of mitochondrial pathway

Genipin, the aglycone of geniposide, exhibits anti-inflammatory and anti-angiogenic activities. Here we demonstrate that genipin induces apoptotic cell death in FaO rat hepatoma cells and human hepatocarcinoma Hep3B cells, detected by morphological cellular changes, caspase activation and release of cytochrome c. During genipin-induced apoptosis, reactive oxygen species (ROS) level was elevated, and N-acetyl-l-cysteine (NAC) and glutathione (GSH) suppressed activation of caspase-3, -7 and -9. Stress-activated protein kinase/c-Jun NH2-terminal kinase 1/2(SAPK/JNK1/2) but neither MEK1/2 nor p38 MAPK was activated in genipin-treated hepatoma cells. SP600125, an SAPK/JNK1/2 inhibitor, markedly suppressed apoptotic cell death in the genipin-treated cells. The FaO cells stably transfected with a dominant-negative c-Jun, TAM67, was less susceptible to apoptotic cell death triggered by genipin. Diphenyleneiodonium (DPI), an inhibitor of NADPH oxidase, inhibited ROS generation, apoptotic cell death, caspase-3 activation and JNK activation. Consistently, the stable expression of Nox1-C, a C-terminal region of Nox1 unable to generate ROS, blocked the formation of TUNEL-positive apoptotic cells, and activation of caspase-3 and JNK in FaO cells treated with genipin. Our observations imply that genipin signaling to apoptosis of hepatoma cells is mediated via NADPH oxidase-dependent generation of ROS, which leads to downstream of JNK.

Catalase inhibition by amino triazole induces oxidative stress in goldfish brain

The effects of in vivo inhibition of catalase by 3-amino 1,2,4-triazole (AMT) on the levels of damage products resulting from reactive oxygen species attack on proteins and lipids as well as on the activities of five antioxidant and associated enzymes were studied in the brain of goldfish, Carassius auratus. Intraperitoneal injection of AMT at a concentration of 0.1 mg/g wet weight caused a gradual decrease in brain catalase activity over 72 h, whereas higher AMT concentrations (0.5 or 1.0 mg/g) reduced catalase activity by about two-thirds within 5-10 h. AMT effects on antioxidant enzyme activities and oxidative stress markers were studied in detail using fish treated with 0.5 mg/g AMT for 24 or 168 h. The levels of thiobarbituric acid-reactive substances (a lipid damage product) increased 6.5-fold by 24 h after AMT injection but fell again after 168 h. The content of carbonylproteins (CP) also rose within 24 h (by approximately 2-fold) and remained 1.5-fold higher compared with respective sham-injected fish after 168 h. CP levels correlated inversely with catalase activity (R(2) = 0.83) suggesting that catalase may protect proteins in vivo against oxidative modification. The activities of both glutathione peroxidase and glutathione-S-transferase increased by approximately 50% and 80%, respectively, in brain of AMT-treated fish and this might represent a compensatory response to lowered catalase activity. Possible functions of catalase in the maintenance of prooxidant/antioxidant balance in goldfish brain are discussed.

NADPH oxidase-derived reactive oxygen species-mediated activation of ERK1/2 is required for apoptosis of human neutrophils induced by Entamoeba histolytica

The extracellular tissue penetrating protozoan parasite Entamoeba histolytica has been known to induce host cell apoptosis. However, the intracellular signaling mechanism used by the parasite to trigger apoptosis is poorly understood. In this study, we investigated the roles of reactive oxygen species (ROS), and of MAPKs in the Entamoeba-induced apoptosis of human neutrophils. The neutrophils incubated with live trophozoites of E. histolytica revealed a marked increase of receptor shedding of CD16 as well as phosphatidylserine (PS) externalization on the cell surface. The Entamoeba-induced apoptosis was effectively blocked by pretreatment of cells with diphenyleneiodonium chloride (DPI), a flavoprotein inhibitor of NADPH oxidase. A large amount of intracellular ROS was detected after exposure to viable trophozoites, and the treatment with DPI strongly inhibited the Entamoeba-induced ROS generation. However, a mitochondrial inhibitor rotenone did not attenuate the Entamoeba-induced ROS generation and apoptosis. Although E. histolytica strongly induced activation of ERK1/2 and p38 MAPK in neutrophils, the activation of ERK1/2 was closely associated with ROS-mediated apoptosis. Pretreatment of neutrophils with MEK1 inhibitor PD98059, but not p38 MAPK inhibitor SB202190, prevented Entamoeba-induced apoptosis. Moreover, DPI almost completely inhibited Entamoeba-induced phosphorylation of ERK1/2, but not phosphorylation of p38 MAPK. These results strongly suggest that NADPH oxidase-derived ROS-mediated activation of ERK1/2 is required for the Entamoeba-induced neutrophil apoptosis.

Effects of transgene expression of superoxide dismutase and glutathione peroxidase on pulmonary epithelial cell growth in hyperoxia

Prolonged exposure to supraphysiological oxygen concentrations results in the generation of reactive oxygen species, which can cause significant lung injury in critically ill patients. Supplementation with human recombinant antioxidant enzymes (AOE) may mitigate hyperoxic lung injury, but it is unclear which combination and concentration will optimally protect pulmonary epithelial cells. First, stable cell lines were generated in alveolar epithelial cells (MLE12) overexpressing one or more of the following AOE: Mn superoxide dismutase (MnSOD), CuZnSOD, or glutathione peroxidase 1. Next, A549 cells were transduced with 50–300 particles/cell of recombinant adenovirus containing either LacZ or each of the three AOE (alone or in combination). Cells were then exposed to 95% O2 for up to 3 days, with cell number and viability determined daily. Overexpression of either MnSOD (primarily mitochondrial) or CuZnSOD (primarily cytosolic) reversed the growth inhibitory effects of hyperoxia within the first 48 h of exposure, resulting in a significant increase in viable cells ( P < 0.05), with 1.5- to 3-fold increases in activity providing optimal protection. Protection from mitochondrial oxidation was confirmed by assessing aconitase activity, which was significantly improved in cells overexpressing MnSOD ( P < 0.05). Data indicate that optimal protection from hyperoxic injury occurs in cells coexpressing MnSOD and glutathione peroxidase 1, with prevention of mitochondrial oxidation being a critical factor. This has important implications for clinical trials in preterm infants receiving SOD supplementation to prevent acute and chronic lung injury.

(-)Epigallocatechin gallate and quercetin enhance survival signaling in response to oxidant-induced human endothelial apoptosis

We reported recently that (-)epigallocatechin gallate and quercetin inhibited H2O2-induced apoptosis through modulation of the expression of apoptosis-related Bcl-2 and Bax in endothelial cells. This study attempted to identify possible regulatory sites and mechanisms of antiapoptotic flavonoids, focusing on ROS-mediated signaling in HUVEC. The effects of apigenin on the signaling pathway downstream were compared. Submillimolar H2O2 caused >30% cell killing with intracellular oxidant generation. H2O2-induced oxidant generation markedly decreased total intracellular glutathione (GSH) levels. Micromolar (-)epigallocatechin gallate and quercetin partially eliminated the dichlorodihydrofluorescein (DCF) and phospho-p53 staining, suggesting that these flavonoids inhibited the accumulation of intracellular oxidants and nuclear transactivation of p53 in H2O2-exposed cells. In contrast, cells treated with apigenin remained DCF and phospho-p53 staining positive in response to H2O2. (-)Epigallocatechin gallate significantly raised the total GSH level that had been depleted by H2O2. Caspase-3 activity was enhanced by H2O2, and this increase was inhibited by (-)epigallocatechin gallate and quercetin. Additionally, the upregulation of caspase-3 activation was reversed by these flavonoids at > or =10 micromol/L; these inhibitory effects were dose dependent. Western blot data revealed that H2O2 upregulated phosphorylation of c-Jun N-terminal kinase (JNK) and p38 mitogen-activated protein kinase (MAPK), which was rapidly reversed by quercetin within 30 min; H2O2 activation of c-Jun was downregulated. (-)Epigallocatechin gallate inhibited H2O2-induced phosphorylation of JNK and p38 MAPK after 60 min. These results reveal that quercetin blocks JNK- and p38 MAPK-related signaling triggered by the oxidant and may regulate expression of apoptotic downstream genes, preventing apoptosis and promoting cell survival. (-)Epigallocatechin gallate may function as an antiapoptotic agent through other antiapoptotic pathways.

Loss of Gadd45a does not modify the pulmonary response to oxidative stress

It is well established that exposure to high levels of oxygen (hyperoxia) injures and kills microvascular endothelial and alveolar type I epithelial cells. In contrast, significant death of airway and type II epithelial cells is not observed at mortality, suggesting that these cell types may express genes that protect against oxidative stress and damage. During a search for genes induced by hyperoxia, we previously reported that airway and alveolar type II epithelial cells uniquely express the growth arrest and DNA damage ( Gadd) 45a gene. Because Gadd45a has been implicated in protection against genotoxic stress, adult Gadd45a (+/+) and Gadd45a (−/−) mice were exposed to hyperoxia to investigate whether it protected epithelial cells against oxidative stress. During hyperoxia, Gadd45a deficiency did not affect loss of airway epithelial expression of Clara cell secretory protein or type II epithelial cell expression of pro-surfactant protein C. Likewise, Gadd45a deficiency did not alter recruitment of inflammatory cells, edema, or overall mortality. Consistent with Gadd45a not affecting the oxidative stress response, p21Cip1/WAF1and heme oxygenase-1 were comparably induced in Gadd45a (+/+) and Gadd45a (−/−) mice. Additionally, Gadd45a deficiency did not affect oxidative DNA damage or apoptosis as assessed by oxidized guanine and terminal deoxyneucleotidyl transferase-mediated dUTP nick-end labeling staining. Overexpression of Gadd45a in human lung adenocarcinoma cells did not affect viability or survival during exposure, whereas it was protective against UV-radiation. We conclude that increased tolerance of airway and type II epithelial cells to hyperoxia is not attributed solely to expression of Gadd45a.

Diesel exhaust activates redox-sensitive transcription factors and kinases in human airways

Diesel exhaust (DE) is a major component of airborne particulate matter. In previous studies we have described the acute inflammatory response of the human airway to inhaled DE. This was characterized by neutrophil, mast cell, and lymphocyte infiltration into the bronchial mucosa with enhanced epithelial expression of IL-8, Gro-alpha, and IL-13. In the present study, we investigated whether redox-sensitive transcription factors were activated as a consequence of DE exposure, consistent with oxidative stress triggering airway inflammation. In archived biopsies from 15 healthy subjects exposed to DE [particulates with a mass median diameter of <10 mum, 300 microg/m3] and air, immunohistochemical staining was used to quantify the expression of the transcription factors NF-kappaB (p65) and AP-1 (c-jun and c-fos), as well their upstream MAPKs, p38 and JNK, in the bronchial epithelium. In addition, phosphorylation of tyrosine residues was examined. DE induced a significant increase in the nuclear translocation of NF-kappaB (P = 0.02), AP-1 (P = 0.02), phosphorylated JNK (P = 0.04), and phosphorylated p38 (P = 0.01), as well as an increase in total (cytoplasmic + nuclear) immunostaining of phosphorylated p38 (P = 0.03). A significant increase in nuclear phosphorylated tyrosine was also observed (P < 0.05). These observations demonstrate that DE activates redox-sensitive transcription factors in vivo consistent with oxidative stress triggering the increased synthesis of proinflammatory cytokines.

Sulindac sulfide-induced apoptosis in sinonasal cancer cells

CONCLUSIONS

These results demonstrate that sulindac sulfide can induce cell death in maxillary cancer cells, and that sulindac sulfide-induced apoptosis is related to the extracellular signal-regulated kinase/p38 MAPK-caspase 3 signaling pathway.

OBJECTIVE

Head and neck cancer is the sixth commonest cancer in the human body. Squamous cell carcinoma accounts for most sinonasal cancers. However, little is known regarding the biochemical mechanism(s) of cell death in sinonasal cancers. Recently, human epidemiological and clinical intervention studies have indicated that sulindac, a non-steroidal anti-inflammatory drug, exhibits chemopreventive activity in colorectal cancer. In this study, we aimed to investigate whether sulindac sulfide can induce apoptosis in sinonasal cancer cells and what type of molecular mechanisms induces the death of sinonasal cancer cells.

MATERIAL AND METHODS

Sinonasal cancer cells (Asan Medical Center Head and Neck Cancer 5) were treated with various concentrations of sulindac sulfide. The degree of cell death was determined by means of a fluorescence-activated cell scan and the signal transduction pathway for cell death was examined.

RESULTS

Human nasal cavity cancer cells treated with sulindac sulfide underwent cell death, and the induction of apoptosis occurred in a dose-dependent manner. Moreover, sulindac sulfide-induced apoptosis was abolished by treatment with the caspase inhibitor Z-VAD-fmk and the mitogen-activated protein kinase (MAPK) inhibitors PD98059 and SB203580.

Neuroprotection by fructose-1,6-bisphosphate involves ROS alterations via p38 MAPK/ERK

Fructose-1,6-bisphosphate (FBP) is a glucose metabolism intermediate that shows a neuroprotective action in animal models of ischemia and other injuries. The intracellular mechanism of FBP on neuroprotection has not been previously defined. Here, we examined whether FBP has a neuroprotective effect against excitotoxicity, and whether it affects the production of reactive oxygen species (ROS), which are involved in the MAPK pathway in cortical neurons. FBP prevented neuronal death in a dose-dependent manner following 24 h of treatment with the excitotoxin, NMDA. After 8 h of NMDA treatment, we observed FBP-induced inhibition of the production of intracellular ROS, and at the earlier time FBP suppressed NMDA-induced p-p38 and p-ERK expression. In addition, MAPK inhibitors reduced NMDA-induced excitotoxicity and also ROS production. Taken together, our results suggest that the neuroprotective effects of FBP could be explained by down-regulation of free radical production through the p38MAPK/ERK pathway.

Characterization of thiol-specific antioxidant 1 (TSA1) ofCandida albicans

We previously identified several proteins that are differentially expressed in pathogenic hyphae by comparing protein profiles of yeast and hyphae of Candida albicans. One of these, thiol-specific antioxidant 1 (TSA1), attracted our attention because it may play some roles in surviving an unfavourable oxidative environment created by host cells. Two alleles of the C. albicans TSA1 (CaTSA1) gene are located in opposite orientation on the same chromosome. Using PCR-directed disruption cassettes and URA-Blaster, a series of deletion mutants that lack one to four copies were constructed to examine the functions of CaTSA1. Northern and Western analyses showed that both the transcript and protein products of CaTSA1 decreased proportionally to the disrupted copy number and were completely absent in the null mutant, indicating that all four TSA1 copies are equally functional at the transcriptional level. Intracellular H2O2 increased by an order of magnitude in deletion mutants lacking three to four copies, suggesting that CaTsa1p is not a redundant H2O2 scavenger. CaTsa1p was indispensable for yeast-to-hyphal transition when C. albicans was cultured under oxidative stress. The level of its oxidized form increased approximately five-fold in hyphal cells, whereas that of the reduced form increased two-fold compared to yeast cells. The ratio of oxidized to reduced form was increased three-fold in hyphal cells. This overall increase was found to be controlled at the post-transcriptional level. Interestingly, CaTsa1p is translocated to the nucleus of hyphal cells. These findings may be of biological significance for differentiation and pathogenicity.

Extracellular Signal–Regulated Kinase Activation Delays Hyperoxia-Induced Epithelial Cell Death in Conditions of Akt Downregulation

Hyperoxia (fraction of inspired oxygen = 95%) induces death of lung epithelial cells. The duration of cell survival in the setting of hyperoxia depends on hyperoxia-induced activation of intracellular survival pathways. Two survival pathways with known effects on lung epithelial cells are the propidium iodide 3-kinase/Akt and extracellular signal-regulated kinase (ERK)/mitogen-activated protein (MAP) kinase pathways. We investigated the effect of hyperoxia on activity of both the Akt and ERK pathways in the A549 lung epithelial cell line. Hyperoxia-exposed cells show progressive loss of Akt activation and total Akt protein. Hyperoxia decreases Akt mRNA, consistent with the loss of total Akt. In addition, hyperoxia induces ERK activation. Inhibition of ERK with the MAP kinase kinase 1/2 inhibitor, U0126, shortens the survival time of cells in hyperoxia, suggesting that increased ERK activity partially compensates for the hyperoxia-induced Akt downregulation. Our findings show, for the first time, that hyperoxia has divergent effects on two survival pathways (Akt and ERK), and that ERK activity compensates for the loss of the Akt survival effects, delaying the death of hyperoxia-exposed lung epithelial cells.

Inhibition of c-Jun NH2-terminal kinase or extracellular signal-regulated kinase improves lung injury

Background

Although in vitro studies have determined that the activation of mitogen-activated protein (MAP) kinases is crucial to the activation of transcription factors and regulation of the production of proinflammatory mediators, the roles of c-Jun NH₂-terminal kinase (JNK) and extracellular signal-regulated kinase (ERK) in acute lung injury have not been elucidated.

Methods

Saline or lipopolysaccharide (LPS, 6 mg/kg of body weight) was administered intratracheally with a 1-hour pretreatment with SP600125 (a JNK inhibitor; 30 mg/kg, IO), or PD98059 (an MEK/ERK inhibitor; 30 mg/kg, IO). Rats were sacrificed 4 hours after LPS treatment.

Results

SP600125 or PD98059 inhibited LPS-induced phosphorylation of JNK and ERK, total protein and LDH activity in BAL fluid, and neutrophil influx into the lungs. In addition, these MAP kinase inhibitors substantially reduced LPS-induced production of inflammatory mediators, such as CINC, MMP-9, and nitric oxide. Inhibition of JNK correlated with suppression of NF-κB activation through downregulation of phosphorylation and degradation of IκB-α, while ERK inhibition only slightly influenced the NF-κB pathway.

Conclusion

JNK and ERK play pivotal roles in LPS-induced acute lung injury. Therefore, inhibition of JNK or ERK activity has potential as an effective therapeutic strategy in interventions of inflammatory cascade-associated lung injury.

NF-kappaB protects lung epithelium against hyperoxia-induced nonapoptotic cell death-oncosis

Prolonged exposure to hyperoxia induces pulmonary epithelial cell death and acute lung injury. Although both apoptotic and nonapoptotic morphologies are observed in hyperoxic animal lungs, nonapoptotic cell death had only been recorded in transformed lung epithelium cultured in hyperoxia. To test whether the nonapoptotic characteristics in hyperoxic animal lungs are direct effects of hyperoxia, the mode of cell death was determined both morphologically and biochemically in human primary lung epithelium exposed to 95% O(2). In contrast to characteristics observed in apoptotic cells, hyperoxia induced swelling of nuclei and an increase in cell size, with no evidence for any augmentation in the levels of either caspase-3 activity or annexin V incorporation. These data suggest that hyperoxia can directly induce nonapoptotic cell death in primary lung epithelium. Although hyperoxia-induced nonapoptotic cell death was associated with NF-kappaB activation, it is unknown whether NF-kappaB activation plays any causal role in nonapoptotic cell death. This study shows that inhibition of NF-kappaB activation can accelerate hyperoxia-induced epithelial cell death in both primary and transformed lung epithelium. Corresponding to the reduced cell survival in hyperoxia, the levels of MnSOD were also low in NF-kappaB-deficient cells. These results demonstrate that NF-kappaB protects lung epithelial cells from hyperoxia-induced nonapoptotic cell death.

Involvement of NADPH oxidase-mediated generation of reactive oxygen species in the apototic cell death by capsaicin in HepG2 human hepatoma cells

Although capsaicin (8-methyl-N-vanillyl-6-nonenamide), a pungent ingredient in a variety of red peppers of the genus Capsicum, has been shown to induce apoptotic cell death in many cancer cells, the exact mechanism of this action of capsaicin is not completely understood. In this study, we investigated the possible mediation of the NADPH oxidase-modulated production of reactive oxygen species (ROS) in the apoptotic mechanism of capsaicin in HepG2 human hepatoblastoma cells. Capsaicin induced apoptotic cell death in a time- and dose-dependent manner. Capsaicin at the concentration of inducing apoptosis also markedly increased the level of ROS. The capsaicin-induced generation of ROS and apoptosis was significantly suppressed by treatment with antioxidants, DPPD and tocopherol. In addition, inhibitors of NADPH oxidase, diphenylene iodonium, apocynin and neopterine, profoundly blocked the capsaicin-induced ROS generation and apoptosis. The expression of Rac1N17, a dominant negative mutant of Rac1, also significantly inhibited the capsaicin-induced apoptosis. Activation of nuclear factor-kappaB, a transcription factor essentially involved in ROS-induced apoptosis, was also observed by treatment with capsaicin. Collectively, these results suggest that the NADPH oxidase-mediated generation of ROS may be essentially involved in the mechanism of capsaicin-induced apoptosis in HepG2 cells. These results further suggest that capsaicin may be a valuable agent for the therapeutic intervention of human hepatomas.

Neutrophil elastase induces IL-8 synthesis by lung epithelial cells via the mitogen-activated protein kinase pathway

The sequestration of neutrophils in the lung and the release of proinflammatory mediators, including neutrophil elastase, are responsible for sepsis-induced microvascular permeability and alveolar epithelial cell damage. To assess the underlying mechanism, human neutrophil elastase (0.01-0.5 microg/ml) was added to cultured A549 epithelial cells in the presence or absence of inhibitors. IL-8 was analyzed by ELISA or by RT-PCR to measure the IL-8 synthesis capacity. Mitogen-activated protein kinase (MAPK) activity was detected by Western blot analysis. Neutrophil elastase dose-dependently increased IL-8 release from cultured A549 epithelial cells. Pretreatment with a specific elastase inhibitor, elastase inhibitor II (at 0.5, 5, and 50 microg/ml), dose-dependently inhibited neutrophil elastase-induced IL-8 release. The activities of MAPK, p38, and extracellular signal-regulated kinase (ERK) were upregulated by neutrophil elastase. Nuclear transcriptional factor-kappa B (NF-kappaB) and activator protein 1 (AP-1) were also activated. These responses were significantly inhibited by elastase inhibitor II. A specific inhibitor of p38 MAPK (SB203580) and an NF-kappaB inhibitor (pyrrolidine dithiocarbamate), but not an ERK inhibitor (PD 98059), significantly inhibited neutrophil elastase-induced IL-8 release and mRNA expression. The specific tyrosine kinase inhibitor, genistein, and the protein kinase C (PKC) inhibitor, Ro 31-8220, also inhibited IL-8 release and mRNA expression as well as p38 and NF-kappaB activation. There was no significant effect by the protein kinase A inhibitor, H-89, on neutrophil elastase-induced IL-8 synthesis or p38 MAPK activation. Our results indicate that neutrophil elastase activates p38 MAPK which upregulates NF-kappaB and AP-1 activities, thus inducing IL-8 mRNA expression and protein synthesis. Tyrosine kinase and PKC are implicated in neutrophil elastase activation of the MAPK pathway.

Lack of integrin alpha1beta1 leads to severe glomerulosclerosis after glomerular injury

Severity of fibrosis after injury is determined by the nature of the injury and host genetic susceptibility. Metabolism of collagen, the major component of fibrotic lesions, is, in part, regulated by integrins. Using a model of glomerular injury by adriamycin, which induces reactive oxygen species (ROS) production, we demonstrated that integrin alpha1-null mice develop more severe glomerulosclerosis than wild-type mice. Moreover, primary alpha1-null mesangial cells produce more ROS both at baseline and after adriamycin treatment. Increased ROS synthesis leads to decreased cell proliferation and increased glomerular collagen IV accumulation that is reversed by antioxidants both in vivo and in vitro. Thus, we have identified integrin alpha1beta1 as a modulator of glomerulosclerosis. In addition, we showed a novel pathway where integrin alpha1beta1 modulates ROS production, which in turn controls collagen turnover and ultimately fibrosis. Because integrin alpha1beta1 is expressed in many cell types this may represent a generalized mechanism of controlling matrix accumulation, which has implications for numerous diseases characterized by fibrosis.

Hyperoxia causes oxygen free radical-mediated membrane injury and alters myocardial function and hemodynamics in the newborn

Newborn children can be exposed to high oxygen levels (hyperoxia) for hours to days during their medical and/or surgical management, and they also can have poor myocardial function and hemodynamics. Whether hyperoxia alone can compromise myocardial function and hemodynamics in the newborn and whether this is associated with oxygen free radical release that overwhelms naturally occurring antioxidant enzymes leading to myocardial membrane injury was the focus of this study. Yorkshire piglets were anesthetized with pentobarbital sodium (65 mg/kg), intubated, and ventilated to normoxia. Once normal blood gases were confirmed, animals were randomly allocated to either 5 h of normoxia [arterial Po(2) (Pa(O(2))) = 83 +/- 5 mmHg, n = 4] or hyperoxia (Pa(O(2)) = 422 +/- 33 mmHg, n = 6), and myocardial functional and hemodynamic assessments were made hourly. Left ventricular (LV) biopsies were taken for measurements of antioxidant enzyme activities [superoxide dismutase (SOD), glutathione peroxidase (GPx), and catalase (CAT)] and malondialdehyde (MDA) and 4-hydroxynonenal (4-HNE) as an indicator of oxygen free radical-mediated membrane injury. Hyperoxic piglets suffered significant reductions in contractility (P < 0.05), systolic blood pressure (P < 0.03), and mean arterial blood pressure (P < 0.05). Significant increases were seen in heart rate (P < 0.05), whereas a significant 11% (P < 0.05) and 61% (P < 0.001) reduction was seen in LV SOD and GPx activities, respectively, after 5 h of hyperoxia. Finally, MDA and 4-HNE levels were significantly elevated by 45% and 38% (P < 0.001 and P = 0.02), respectively, in piglets exposed to hyperoxia. Thus, in the newborn, hyperoxia triggers oxygen free radical-mediated membrane injury together with an inability of the newborn heart to upregulate its antioxidant enzyme defenses while impairing myocardial function and hemodynamics.

Reactive oxygen species (ROS) control the expression of Bcl-2 family proteins by regulating their phosphorylation and ubiquitination

We examined the influence of ROS on the phosphorylation and complex formation of Bcl-2 family proteins in Mn-superoxide dismutase (SOD) antisense-transfected squamous cell carcinoma cells, OSC-4 cells. The increase of intracellular ROS level induced by cis-diamminedichloroplatinum (CDDP) and gamma-ray treatment was greater in antisense-transfected cells than in control vector-transfected cells, and apoptosis was more extensively induced in the former. Antisense-transfected cells expressed high levels of Bax and Bak, but low levels of Bcl-2 and Bcl-XL when treated with CDDP, peplomycin, 5-fluorouracil or gamma-rays. After treatment with these agents, the phosphorylation of protein kinase A, Bcl-2 (Thr56) and Bad (Ser155) was increased, especially in antioxidant (N-acetylcysteine and pyrrolidine dithiocarbamate)-pretreated control cells, but the phosphorylation levels were very low in the antisense-transfected cells. Bcl-2 ubiquitination was increased, but ubiquitination of Bad and Bax was decreased in the antisense-transfected cells, although their ubiquitination was increased by the antioxidants. These results reveal that ROS induce apoptosis by regulating the phosphorylation and ubiquitination of Bcl-2 family proteins, resulting in increased proapoptotic protein levels and decreased antiapoptotic protein expression.

NADPH oxidase and ERK signaling regulates hyperoxia-induced Nrf2-ARE transcriptional response in pulmonary epithelial cells

Oxidative stress plays a major role in hyperoxia-induced acute lung injury. We have shown previously that mice lacking the Nrf2 are more susceptible to hyperoxia than are wild-type mice. Nrf2 activates antioxidant response element (ARE)-mediated gene expression involved in cellular protection against toxic insults. The present study was designed to investigate the mechanisms that control the activation of Nrf2 by hyperoxia using a non-malignant murine alveolar epithelial cell line, C10. No significant alteration in the levels of Nrf2 mRNA and protein was found following exposure to hyperoxia. In contrast, hyperoxia caused the translocation of Nrf2 from the cytoplasm to the nucleus within 30-60 min of exposure. Consistent with these observations, gel shift and reporter analyses demonstrated a correlation between the hyperoxia-enhanced ARE DNA-binding activity of Nrf2 and an up-regulation of ARE-driven transcription. Inhibition of NADPH oxidase with diphenyleneiodonium (DPI) blocked both Nrf2 translocation and ARE-mediated transcription. Inhibition of the MEK/ERK pathway caused a similar effect. Consistent with this finding, hyperoxia stimulated ERK-1 and ERK-2 phosphorylation, whereas DPI or N-acetyl-l-cysteine blocked such activation. Hyperoxia stimulated the phosphorylation of endogenous Nrf2, but not in the presence of U0126, suggesting a critical role for ERK signaling in the activation of Nrf2. Consistent with this notion, hyperoxia did not stimulate the phosphorylation of Nrf2 in fibroblasts lacking the ERK-1. Collectively, our findings suggest that hyperoxia-induced, ARE-driven, Nrf2-dependent transcription is controlled by NADPH oxidase and ERK-1 signaling.

Dual and opposing roles of ERK in regulating G1 and S-G2/M delays in A549 cells caused by hyperoxia

This study explores the role of ERK activation in regulating G(1) and S-G(2)/M delays during hyperoxia. We demonstrate here that exposing A549 human alveolar type 2 adenocarcinoma cells to hyperoxia (95% O(2)) for 0.5-24 h time-dependently increases phospho-ERK, phospho-p53(Ser15), p53, and p21(CIP1) protein levels. Decreasing phospho-ERK with the pharmacological inhibitors, PD98059 and U0126, markedly suppresses hyperoxia-stimulated phospho-p53(Ser15), p53, and p21(CIP1), and also restores the hyperoxia-reduced kinase activities of cyclin D1/E1-Cdks. Our results suggest that ERK activation during hyperoxia contributes to the p53/p21-mediated G(1) checkpoint. However, inhibition of ERK signaling during hyperoxia further delays S-phase entry and progression. Hyperoxia induces significant expression of cyclin A/B1 and translocation of cyclin A into nuclei while marginally decreasing cyclin A/B1-Cdks kinase activities, which may be related to nuclear association with p21. Interestingly, inhibition of ERK signaling markedly suppresses the elevation of cyclin A/B1 proteins and cyclin A/B1-Cdks kinase activities during hyperoxia. Taken together, the results presented here suggest that hyperoxia-activated ERK acts upstream of p53 and p21 to suppress G(1)-Cdk activities; however, it is also required for induction of cyclin A/B1 and maintenance of cyclin A/B1-Cdk activities that oppose delays in S-phase entry and progression.

Reactive oxygen species produced by NAD(P)H oxidase inhibit apoptosis in pancreatic cancer cells

One reason why pancreatic cancer is so aggressive and unresponsive to treatments is its resistance to apoptosis. We report here that reactive oxygen species (ROS) are a prosurvival, antiapoptotic factor in pancreatic cancer cells. Human pancreatic adenocarcinoma MIA PaCa-2 and PANC-1 cells generated ROS, which was stimulated by growth factors (serum, insulin-like growth factor I, or fibroblast growth factor-2). Growth factors also stimulated membrane NAD(P)H oxidase activity in these cells. Both intracellular ROS and NAD(P)H oxidase activity were inhibited by antioxidants tiron and N-acetylcysteine and the inhibitor of flavoprotein-dependent oxidases, diphenylene iodonium, but not by inhibitors of various other ROS-generating enzymes. Using Rho(0) cells deficient in mitochondrial DNA, we showed that a nonmitochondrial NAD(P)H oxidase is a major source of growth factor-induced ROS in pancreatic cancer cells. Among proteins that have been implicated in NAD(P)H oxidase activity, MIA PaCa-2 and PANC-1 cells do not express the phagocytic gp91(phox) subunit but express several nonphagocytic oxidase (NOX) isoforms. Transfection with Nox4 antisense oligonucleotide inhibited NAD(P)H oxidase activity and ROS production in MIA PaCa-2 and PANC-1 cells. Inhibiting ROS with the antioxidants, Nox4 antisense, or MnSOD overexpression all stimulated apoptosis in pancreatic cancer cells as measured by internucleosomal DNA fragmentation, phosphatidylserine externalization, cytochrome c release, and effector caspase activation. The results show that growth factor-induced ROS produced by NAD(P)H oxidase (probably Nox4) protect pancreatic cancer cells from apoptosis. This mechanism may play an important role in pancreatic cancer resistance to treatment and thus represent a novel therapeutic target.

Disruption of the Ah receptor gene alters the susceptibility of mice to oxygen-mediated regulation of pulmonary and hepatic cytochromes P4501A expression and exacerbates hyperoxic lung injury

Administration of supplemental oxygen is frequently encountered in infants suffering from pulmonary insufficiency and in adults with acute respiratory distress syndrome. However, hyperoxia causes acute lung damage in experimental animals. In the present study, we investigated the roles of the Ah receptor (AHR) in the modulation of cytochrome P4501A (CYP1A) enzymes and in the development of lung injury by hyperoxia. Adult male wild-type [AHR (+/+)] mice and AHR-deficient animals [AHR (-/-)] were maintained in room air or exposed to hyperoxia (>95% oxygen) for 24 to 72 h, and pulmonary and hepatic expression of CYP1A and lung injury were studied. Hyperoxia caused significant increases in pulmonary and hepatic CYP1A1 activities (ethoxyresorufin O-deethylase) and mRNA levels in wild-type (C57BL/6J) AHR (+/+), but not AHR (-/-) mice, suggesting that AHR-dependent mechanisms contributed to CYP1A1 induction. On the other hand, hyperoxia augmented hepatic CYP1A2 expression in both wild-type and AHR (-/-) animals, suggesting that AHR-independent mechanisms contributed to the CYP1A2 regulation by hyperoxia. AHR (-/-) mice exposed to hyperoxia were more susceptible than wild-type mice to lung injury and inflammation, as indicated by significantly higher lung weight/body weight ratios, increased pulmonary edema, and enhanced neutrophil recruitment into the lungs. In conclusion, our results support the hypothesis that the hyperoxia induces CYP1A1, but not CYP1A2, expression in vivo by AHR-dependent mechanisms, a phenomenon that may mechanistically contribute to the beneficial effects of the AHR in hyperoxic lung injury.

Extracellular ATP-mediated signaling for survival in hyperoxia-induced oxidative stress

Respiratory failure is a serious consequence of lung cell injury caused by treatment with high inhaled oxygen concentrations. Human lung microvascular endothelial cells (HLMVEC) are a principal target of hyperoxic injury (hyperoxia). Cell stress can cause release of ATP, and this extracellular nucleotide can activate purinoreceptors and mediate responses essential for survival. In this investigation, exposure of endothelial cells to an oxidative stress, hyperoxia, caused rapid but transient ATP release (20.03 +/- 2.00 nm/10(6) cells in 95% O(2) versus 0.08 +/- 0.01 nm/10(6) cells in 21% O2 at 30 min) into the extracellular milieu without a concomitant change in intracellular ATP. Endogenously produced extracellular ATP-enhanced mTOR-dependent uptake of glucose (3467 +/- 102 cpm/mg protein in 95% oxygen versus 2100 +/- 112 cpm/mg protein in control). Extracellular addition of ATP-activated important cell survival proteins like PI 3-kinase and extracellular-regulated kinase (ERK-1/2). These events were mediated primarily by P2Y receptors, specifically the P2Y2 and/or P2Y6 subclass of receptors. Extracellular ATP was required for the survival of HLMVEC in hyperoxia (55 +/- 10% surviving cells with extracellular ATP scavengers [apyrase + adenosine deaminase] versus 95 +/- 12% surviving cells without ATP scavengers at 4 d of hyperoxia). Incubation with ATP scavengers abolished ATP-dependent ERK phosphorylation stimulated by hyperoxia. Further, ERK activation also was found to be important for cell survival in hyperoxia, as treatment with PD98059 enhanced hyperoxia-mediated cell death. These findings demonstrate that ATP release and subsequent ATP-mediated signaling events are vital for survival of HLMVEC in hyperoxia.

Mitochondrial cytochromecrelease is a key event in hyperoxia-induced lung injury: protection by cyclosporin A

Hyperoxia is known to induce extensive alveolar cell death by still poorly defined mechanisms. In this study, the mitochondria-dependent cell death pathway was explored during hyperoxia-induced lung injury in mice. We observed a progressive release of cytochrome c from the mitochondria into the cytosol of alveolar cells. This release was accompanied by the translocation of the proapoptotic protein Bax from cytosol to mitochondria without detectable activation of caspase-3. As cytochrome c release can be induced by mitochondrial membrane alteration and permeability transition (MPT), mice were treated with cyclosporin A, which specifically inhibits MPT. Cyclosporin A treatment prevented mitochondrial release of cytochrome c during hyperoxia and concomitantly preserved mitochondria from extensive swelling and crista disorganization, as assessed by electron microscopy analysis of alveolar epithelial cells. These morphological and biochemical observations correlated with decreased lung tissue damage, as evaluated by morphological score and lung weight. In conclusion, mitochondrial damage and cytochrome c release are important linked events in hyperoxia-induced lung injury and can be efficiently blocked by cyclosporin A.

In vivo exposure to hyperoxia induces DNA damage in a population of alveolar type II epithelial cells

It is well established that hyperoxia injures and kills alveolar endothelial and type I epithelial cells of the lung. Although type II epithelial cells remain morphologically intact, it remains unclear whether they are also damaged. DNA integrity was investigated in adult mice whose type II cells were identified by their endogenous expression of pro-surfactant protein C or transgenic expression of enhanced green fluorescent protein. In mice exposed to room air, punctate perinuclear 8-oxoguanine staining was detected in approximately 4% of all alveolar cells and in 30% of type II cells. After 48 or 72 h of hyperoxia, 8-oxoguanine was detected in 11% of all alveolar cells and in >60% of type II cells. 8-Oxoguanine colocalized by confocal microscopy with the mitochondrial transmembrane protein cytochrome oxidase subunit 1. Type II cells isolated from hyperoxic lungs exhibited nuclear DNA strand breaks by comet assay even though they were viable and morphologically indistinguishable from cells isolated from lungs exposed to room air. These data reveal that type II cells exposed to in vivo hyperoxia have oxidized and fragmented DNA. Because type II cells are essential for lung remodeling, our findings raise the possibility that they are proficient in DNA repair.