Mitogen-activated protein kinase pathway mediates hyperoxia-induced apoptosis in cultured macrophage cells

Glyoxal-derived advanced glycation end-products, Nε-carboxymethyl-lysine, and glyoxal-derived lysine dimer induce apoptosis-related gene expression in hepatocytes

BACKGROUND

Advanced glycation end-products (AGEs) are proteins or lipids that have been glycated nonenzymatically by reducing sugars and their derivatives such as methylglyoxal. AGEs are known to cause inflammation, oxidative stress, and diseases in the human body. The toxic effects of AGEs and their structures on the origin of the protein being modified have not been well studied.

METHODS AND RESULTS

Five different types of AGEs: AGE1 (glucose-derived), AGE2 (glyceraldehyde-derived), AGE3 (glycolaldehyde-derived), AGE4 (methylglyoxal-derived), and AGE5 (glyoxal-derived); were used to examine the effect of AGEs on HepG2 cells. AGE2 through 5 increase the production of reactive oxygen species (ROS) in liver cells, an initiating factor for apoptosis. At the mRNA and protein levels, AGE5 treatment showed the greatest increase in expression of apoptosis-related factors such as Bax, p53, and Caspase 3. Quantitative analysis revealed that N^ε-carboxymethyl-lysine (CML) and glyoxal-lysine dimer (GOLD) were the important types of AGE5. The ROS generation and the expression of apoptotic factors both increased when cells were treated with CML and GOLD.

CONCLUSION

These findings suggest that AGE5 treatment activates the apoptosis-related gene expression in hapatocytes, with CML and GOLD as potential major AGE compounds.

Modulation of Apoptosis by Plant Polysaccharides for Exerting Anti-Cancer Effects: A Review

Cancer has become a significant public health problem with high disease burden and mortality. At present, radiotherapy and chemotherapy are the main means of treating cancer, but they have shown serious safety problems. The severity of this problem has caused further attention and research on effective and safe cancer treatment methods. Polysaccharides are natural products with anti-cancer activity that are widely present in a lot of plants, and many studies have found that inducing apoptosis of cancer cells is one of their important mechanisms. Therefore, this article reviews the various ways in which plant polysaccharides promote apoptosis of cancer cells. The major apoptotic pathways involved include the mitochondrial pathway, the death receptor pathway, and their upstream signal transduction such as MAPK pathway, PI3K/AKT pathway, and NF-κB pathway. Moreover, the paper has also been focused on the absorption and toxicity of plant polysaccharides with reference to extant literature, making the research more scientific and comprehensive. It is hoped that this review could provide some directions for the future development of plant polysaccharides as anticancer drugs in pharmacological experiments and clinical researches.

Prolonged Hyperoxygenation Treatment Improves Vein Graft Patency and Decreases Macrophage Content in Atherosclerotic Lesions in ApoE3*Leiden Mice

Unstable atherosclerotic plaques frequently show plaque angiogenesis which increases the chance of rupture and thrombus formation leading to infarctions. Hypoxia plays a role in angiogenesis and inflammation, two processes involved in the pathogenesis of atherosclerosis. We aim to study the effect of resolution of hypoxia using carbogen gas (95% O₂, 5% CO₂) on the remodeling of vein graft accelerated atherosclerotic lesions in ApoE3*Leiden mice which harbor plaque angiogenesis. Single treatment resulted in a drastic decrease of intraplaque hypoxia, without affecting plaque composition. Daily treatment for three weeks resulted in 34.5% increase in vein graft patency and increased lumen size. However, after three weeks intraplaque hypoxia was comparable to the controls, as were the number of neovessels and the degree of intraplaque hemorrhage. To our surprise we found that three weeks of treatment triggered ROS accumulation and subsequent Hif1a induction, paralleled with a reduction in the macrophage content, pointing to an increase in lesion stability. Similar to what we observed in vivo, in vitro induction of ROS in bone marrow derived macrophages lead to increased Hif1a expression and extensive DNA damage and apoptosis. Our study demonstrates that carbogen treatment did improve vein graft patency and plaque stability and reduced intraplaque macrophage accumulation via ROS mediated DNA damage and apoptosis but failed to have long term effects on hypoxia and intraplaque angiogenesis.

Exogenous surfactant prevents hyperoxia-induced lung injury in adult mice

Background

In addition to the risk of developing ventilator-induced lung injury, patients with ARDS are at risk of developing hyperoxic injury due the supra-physiological oxygen supplementation clinically required to reverse hypoxemia. Alterations of endogenous surfactant system participate in the pulmonary dysfunction observed in ARDS. Administration of exogenous surfactant could have protective effects during hyperoxia.

Methods

Male BALB/c mice (8–10 weeks), a strain highly sensitive to hyperoxia, received the exogenous surfactant-containing protein SP-B and SP-C by intranasal instillation 12 h before starting 24 h of exposure to hyperoxia in an inhalation chamber and were compared to mice receiving hyperoxia alone and to controls subjected to normoxia.

Results

Compared to the hyperoxia group, the administration of exogenous surfactant was able to reduce lung inflammation through a reduction in the influx of neutrophils and inflammatory biomarkers such as TNF, IL-17, and HMGB1 expression. The antioxidant activity prevented oxidative damage by reducing lipid peroxidation and protein carbonylation and increasing superoxide dismutase activity when compared to the hyperoxia group.

Conclusion

Our results offer new perspectives on the effects and the mechanism of exogenous surfactant in protecting the airway and lungs, in oxygen-rich lung microenvironment, against oxidative damage and aggravation of acute inflammation induced by hyperoxia.

Time Dependent Pathway Activation of Signalling Cascades in Rat Organs after Short-Term Hyperoxia

Administration of oxygen is one of the most common interventions in medicine. Previous research showed that differential regulated proteins could be linked to hyperoxia-associated signaling cascades in different tissues. However, it still remains unclear which signaling pathways are activated by hyperoxia. The present study analyses hyperoxia-induced protein alterations in lung, brain, and kidney tissue using a proteomic and bioinformatic approach. Pooled data of 36 Wistar rats exposed to hyperoxia were used. To identify possible hyperoxia biomarkers, and to evaluate the relationship between protein alterations in hyperoxia affected organs and blood, proteomics data from brain, lung, and kidney were analyzed. Functional network analyses (IPA^®, PathwaysStudio^®, and GENEmania^®) in combination with hierarchical cluster analysis (Perseus^®) was used to identify relevant pathways and key proteins. Data of 54 2D-gels with more than 2500 significantly regulated spots per gel were collected. Thirty-eight differentially expressed proteins were identified and consecutively analyzed by bioinformatic methods. Most differences between hyperoxia and normoxia (21 proteins up-regulated, 17 proteins down-regulated) were found immediately after hyperoxia (15 protein spots), followed by day 3 (13 spots), and day 7 (10 spots). A highly significant association with inflammation and the inflammatory response was found. Cell proliferation, oxidative stress, apoptosis and cell death as well as cellular functions were revealed to be affected. Three hours of hyperoxia resulted in significant alterations of protein expression in different organs (brain, lung, kidney) up to seven days after exposure. Further studies are required to interpret the relevance of protein alterations in signaling cascades during/after hyperoxia.

Probucol attenuates hyperoxia-induced lung injury in mice

Hyperoxic lung injury is pathologically characterized by alveolar edema, interlobular septal edema, hyaline membrane disease, lung inflammation, and alveolar hemorrhage. Although the precise mechanism by which hyperoxia causes lung injury is not well defined, oxidative stress, epithelial cell death, and proinflammatory cytokines are thought to be involved. Probucol—a commercially available drug for treating hypercholesterolemia—has been suggested to have antioxidant and antiapoptotic effects. This study aimed to assess whether probucol could attenuate hyperoxic lung injury in mice. Mice were exposed to 95% O₂ for 72 h, with or without pre-treatment with 130 μg/kg probucol intratracheally. Probucol treatment significantly decreased both the number of inflammatory cells in the bronchoalveolar lavage fluid and the degree of lung injury in hyperoxia-exposed mice. Probucol treatment reduced the number of cells positive for 8-hydroxyl-2′-deoxyguanosine or terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) and suppressed NF-κB activation, Bax expression, and caspase-9 activation in lung tissues from hyperoxia-exposed mice. These results suggest that probucol can reduce oxidative DNA damage, apoptotic cell death, and inflammation in lung tissues. Intratracheal administration of probucol may be a novel treatment for lung diseases induced by oxidative stress, such as hyperoxic lung injury and acute respiratory distress syndrome.

Detoxification of Mitochondrial Oxidants and Apoptotic Signaling Are Facilitated by Thioredoxin-2 and Peroxiredoxin-3 during Hyperoxic Injury

Mitochondria play a fundamental role in the regulation of cell death during accumulation of oxidants. High concentrations of atmospheric oxygen (hyperoxia), used clinically to treat tissue hypoxia in premature newborns, is known to elicit oxidative stress and mitochondrial injury to pulmonary epithelial cells. A consequence of oxidative stress in mitochondria is the accumulation of peroxides which are detoxified by the dedicated mitochondrial thioredoxin system. This system is comprised of the oxidoreductase activities of peroxiredoxin-3 (Prx3), thioredoxin-2 (Trx2), and thioredoxin reductase-2 (TrxR2). The goal of this study was to understand the role of the mitochondrial thioredoxin system and mitochondrial injuries during hyperoxic exposure. Flow analysis of the redox-sensitive, mitochondrial-specific fluorophore, MitoSOX, indicated increased levels of mitochondrial oxidant formation in human adenocarcinoma cells cultured in 95% oxygen. Increased expression of Trx2 and TrxR2 in response to hyperoxia were not attributable to changes in mitochondrial mass, suggesting that hyperoxic upregulation of mitochondrial thioredoxins prevents accumulation of oxidized Prx3. Mitochondrial oxidoreductase activities were modulated through pharmacological inhibition of TrxR2 with auranofin and genetically through shRNA knockdown of Trx2 and Prx3. Diminished Trx2 and Prx3 expression was associated with accumulation of mitochondrial superoxide; however, only shRNA knockdown of Trx2 increased susceptibility to hyperoxic cell death and increased phosphorylation of apoptosis signal-regulating kinase-1 (ASK1). In conclusion, the mitochondrial thioredoxin system regulates hyperoxic-mediated death of pulmonary epithelial cells through detoxification of oxidants and regulation of redox-dependent apoptotic signaling.

ERK-mediated phosphorylation of BIS regulates nuclear translocation of HSF1 under oxidative stress

B-cell lymphoma (BCL)-2-interacting cell death suppressor (BIS) has diverse cellular functions depending on its binding partners. However, little is known about the effects of biochemical modification of BIS on its various activities under oxidative stress conditions. In this study, we showed that H₂O₂ reduced BIS mobility on SDS–polyacrylamide gels in a time-dependent manner via the activation of extracellular signaling-regulated kinase (ERK). The combined results of mass spectroscopy and computational prediction identified Thr285 and Ser289 in BIS as candidate residues for phosphorylation by ERK under oxidative stress conditions. Deletion of these sites resulted in a partial reduction in the H₂O₂-induced mobility shift relative to that of the wild-type BIS protein; overexpression of the deletion mutant sensitized A172 cells to H₂O₂-induced cell death without increasing the level of intracellular reactive oxygen species. Expression of the BIS deletion mutant decreased the level of heat shock protein (HSP) 70 mRNA following H₂O₂ treatment, which was accompanied by impaired nuclear translocation of heat shock transcription factor (HSF) 1. Co-immunoprecipitation assays revealed that the binding of wild-type BIS to HSF1 was decreased by oxidative stress, while the binding of the BIS deletion mutant to HSF1 was not affected. These results indicate that ERK-dependent phosphorylation of BIS has a role in the regulation of nuclear translocation of HSF1 likely through modulation of its interaction affinity with HSF1, which affects HSP70 expression and sensitivity to oxidative stress.

Novel Double-Hit Model of Radiation and Hyperoxia-Induced Oxidative Cell Damage Relevant to Space Travel

Spaceflight occasionally requires multiple extravehicular activities (EVA) that potentially subject astronauts to repeated changes in ambient oxygen superimposed on those of space radiation exposure. We thus developed a novel in vitro model system to test lung cell damage following repeated exposure to radiation and hyperoxia. Non-tumorigenic murine alveolar type II epithelial cells (C10) were exposed to >95% O₂ for 8 h only (O₂), 0.25 Gy ionizing γ-radiation (IR) only, or a double-hit combination of both challenges (O₂ + IR) followed by 16 h of normoxia (ambient air containing 21% O₂ and 5% CO₂) (1 cycle = 24 h, 2 cycles = 48 h). Cell survival, DNA damage, apoptosis, and indicators of oxidative stress were evaluated after 1 and 2 cycles of exposure. We observed a significant (p < 0.05) decrease in cell survival across all challenge conditions along with an increase in DNA damage, determined by Comet analysis and H2AX phosphorylation, and apoptosis, determined by Annexin-V staining, relative to cells unexposed to hyperoxia or radiation. DNA damage (GADD45α and cleaved-PARP), apoptotic (cleaved caspase-3 and BAX), and antioxidant (HO-1 and Nqo1) proteins were increased following radiation and hyperoxia exposure after 1 and 2 cycles of exposure. Importantly, exposure to combination challenge O₂ + IR exacerbated cell death and DNA damage compared to individual exposures O₂ or IR alone. Additionally levels of cell cycle proteins phospho-p53 and p21 were significantly increased, while levels of CDK1 and Cyclin B1 were decreased at both time points for all exposure groups. Similarly, proteins involved in cell cycle arrest was more profoundly changed with the combination challenges as compared to each stressor alone. These results correlate with a significant 4- to 6-fold increase in the ratio of cells in G2/G1 after 2 cycles of exposure to hyperoxic conditions. We have characterized a novel in vitro model of double-hit, low-level radiation and hyperoxia exposure that leads to oxidative lung cell injury, DNA damage, apoptosis, and cell cycle arrest.

Hyperoxia promotes polarization of the immune response in ovalbumin-induced airway inflammation, leading to a TH17 cell phenotype

Previous studies have demonstrated that hyperoxia-induced stress and oxidative damage to the lungs of mice lead to an increase in IL-6, TNF-α, and TGF-β expression. Together, IL-6 and TGF-β have been known to direct T cell differentiation toward the TH17 phenotype. In the current study, we tested the hypothesis that hyperoxia promotes the polarization of T cells to the TH17 cell phenotype in response to ovalbumin-induced acute airway inflammation. Airway inflammation was induced in female BALB/c mice by intraperitoneal sensitization and intranasal introduction of ovalbumin, followed by challenge methacholine. After the methacholine challenge, animals were exposed to hyperoxic conditions in an inhalation chamber for 24 h. The controls were subjected to normoxia or aluminum hydroxide dissolved in phosphate buffered saline. After 24 h of hyperoxia, the number of macrophages and lymphocytes decreased in animals with ovalbumin-induced airway inflammation, whereas the number of neutrophils increased after ovalbumin-induced airway inflammation. The results showed that expression of Nrf2, iNOS, T-bet and IL-17 increased after 24 of hyperoxia in both alveolar macrophages and in lung epithelial cells, compared with both animals that remained in room air, and animals with ovalbumin-induced airway inflammation. Hyperoxia alone without the induction of airway inflammation lead to increased levels of TNF-α and CCL5, whereas hyperoxia after inflammation lead to decreased CCL2 levels. Histological evidence of extravasation of inflammatory cells into the perivascular and peribronchial regions of the lungs was observed after pulmonary inflammation and hyperoxia. Hyperoxia promotes polarization of the immune response toward the TH17 phenotype, resulting in tissue damage associated with oxidative stress, and the migration of neutrophils to the lung and airways. Elucidating the effect of hyperoxia on ovalbumin-induced acute airway inflammation is relevant to preventing or treating asthmatic patients that require oxygen supplementation to reverse the hypoxemia.

Activation of neutral-sphingomyelinase, MAPKs, and p75 NTR-mediating caffeic acid phenethyl ester-induced apoptosis in C6 glioma cells

Background

Caffeic acid phenethyl ester (CAPE), a component of propolis, is reported to possess anti-inflammatory, anti-bacterial, anti-viral, and anti-tumor activities. Previously, our laboratory demonstrated the in vitro and in vivo bioactivity of CAPE and addressed the role of p53 and the p38 mitogen-activated protein kinase (MAPK) pathway in regulating CAPE-induced apoptosis in C6 glioma cells.

Results

C6 cancer cell lines were exposed to doses of CAPE; DNA fragmentation and MAPKs and NGF/P75NTR levels were then determined. SMase activity and ceramide content measurement as well as western blotting analyses were performed to clarify molecular changes. The present study showed that CAPE activated neutral sphingomyelinase (N-SMase), which led to the ceramide-mediated activation of MAPKs, including extracellular signal-regulated kinase (ERK), Jun N-terminus kinase (JNK), and p38 MAPK. In addition, CAPE increased the expression of nerve growth factor (NGF) and p75 neurotrophin receptor (p75NTR). The addition of an N-SMase inhibitor, GW4869, established that NGF/p75NTR was the downstream target of N-SMase/ceramide. Pretreatment with MAPK inhibitors demonstrated that MEK/ERK and JNK acted upstream and downstream, respectively, of NGF/p75NTR. Additionally, CAPE-induced caspase 3 activation and poly [ADP-ribose] polymerase cleavage were reduced by pretreatment with MAPK inhibitors, a p75NTR peptide antagonist, or GW4869.

Conclusions

Taken together, N-SMase activation played a pivotal role in CAPE-induced apoptosis by activation of the p38 MAPK pathway and NGF/p75NTR may explain a new role of CAPE induced apoptosis in C6 glioma.

Apoptosis Induction by the Total Flavonoids from Arachniodes exilis in HepG2 Cells through Reactive Oxygen Species-Mediated Mitochondrial Dysfunction Involving MAPK Activation

Arachniodes exilis is used as a folk medicine in China and proved to have antibacterial, anti-inflammatory, and sedative activities. In the present study, the antitumor effect of the total flavonoids of A. exilis (TFAE) against HepG2 cells was evaluated. The results showed that TFAE inhibited the growth of HepG2 cells in a dosage- and time-dependent manner. Flow cytometry and Hoechst 33342 fluorescence staining results showed that TFAE could significantly increase the apoptosis ratio of HepG2 cells, which is accompanied with increased intracellular reactive oxygen species (ROS) production and decreased mitochondrial membrane potential (ΔΨm). Western blotting indicated that TFAE downregulated the ratio of Bcl-2/Bax, increased cytochrome c release, and activated the caspases-3 and -9. Further analysis showed that TFAE stimulated the mitogen-activated protein kinase (MAPK). However, treatment with NAC (reactive oxygen species scavenger) and MAPK-specific inhibitors (SP600125 and SB203580) could reverse the changes of these apoptotic-related proteins. These results suggested that TFAE possessed potential anticancer activity in HepG2 cells through ROS-mediated mitochondrial dysfunction involving MAPK pathway.

Static mechanical stress induces apoptosis in rat endplate chondrocytes through MAPK and mitochondria-dependent caspase activation signaling pathways

Mechanical stress has detrimental effects on cartilaginous endplate chondrocytes due to apoptosis in vivo and in vitro. In this study, we investigated the possible apoptosis signaling pathways induced by mechanical stress in cultured rat cervical endplate chondrocytes. Static mechanical load significantly reduced cell viability in a time- and load-dependent manner, as demonstrated by the Cell Counting Kit-8 (CCK-8) assay. Chondrocyte apoptosis induced by mechanical stress was confirmed by annexin V/propidium iodide (PI) staining and terminal deoxynucleotidyl transferase dUTP nick-end labeling (TUNEL). Western blot analysis revealed that static load-induced chondrocyte apoptosis was accompanied by increased phosphorylation of c-Jun N-terminal kinase (JNK), extracellular signal-regulated kinase 1/2 (ERK1/2), and p38 mitogen-activated protein kinase (MAPK). The loss of mitochondrial membrane potential (ΔΨm), increased Cytochrome c release, and activated Caspase-9 and Caspase-3, indicating that the mitochondrial pathway is involved in mechanical stress-induced chondrocyte apoptosis. Treatment with inhibitors of JNK (SP600125), p38 MAPK (SB203580), and ERK (PD98059) prior to mechanical stimulation reversed both the static load-induced chondrocyte apoptosis and the activation of JNK, p38 MAPK, and ERK. Taken together, the data presented in this study demonstrate that mechanical stress induces apoptosis in rat cervical endplate chondrocytes through the MAPK-mediated mitochondrial apoptotic pathway.

Nrf2 and cardiovascular defense

The cardiovascular system is susceptible to a group of diseases that are responsible for a larger proportion of morbidity and mortality than any other disease. Many cardiovascular diseases are associated with a failure of defenses against oxidative stress-induced cellular damage and/or death, leading to organ dysfunction. The pleiotropic transcription factor, nuclear factor-erythroid (NF-E) 2-related factor 2 (Nrf2), regulates the expression of antioxidant enzymes and proteins through the antioxidant response element. Nrf2 is an important component in antioxidant defenses in cardiovascular diseases such as atherosclerosis, hypertension, and heart failure. Nrf2 is also involved in protection against oxidant stress during the processes of ischemia-reperfusion injury and aging. However, evidence suggests that Nrf2 activity does not always lead to a positive outcome and may accelerate the pathogenesis of some cardiovascular diseases (e.g., atherosclerosis). The precise conditions under which Nrf2 acts to attenuate or stimulate cardiovascular disease processes are unclear. Further studies on the cellular environments related to cardiovascular diseases that influence Nrf2 pathways are required before Nrf2 can be considered a therapeutic target for the treatment of cardiovascular diseases.

Hyperoxia resensitizes chemoresistant human glioblastoma cells to temozolomide

Temozolomide (TMZ) is standard chemotherapy for glioblastoma multiforme (GBM). Intratumoral hypoxia is common in GBM and may be associated with the development of TMZ resistance. Oxygen therapy has previously been reported to potentiate the effect of chemotherapy in cancer. In this study, we investigated whether hyperoxia can enhance the TMZ-induced cytotoxicity of human GBM cells, and whether and how it would resensitize TMZ-resistant GBM cells to TMZ. TMZ-sensitive human GBM cells (D54-S and U87-S) were treated with TMZ to develop isogenic subclones of TMZ-resistant cells (D54-R and U87-R). All cell lines were then exposed to different oxygen levels (1, 21, 40, or 80 %), with or without concomitant TMZ treatment, before assessment of cell cytotoxicity and morphology. Cell death and survival pathways elicited by TMZ and/or hyperoxia were elucidated by western blotting. Our results showed that TMZ sensitivity of both chemo-sensitive and resistant cells was enhanced significantly under hyperoxia. At the cell line-specific optimum oxygen concentration (D54-R, 80 %; U87-R, 40 %), resistant cells had the same response to TMZ as the parent chemosensitive cells under normoxia via the caspase-dependent pathway. Both TMZ and hyperoxia were associated with increased phosphorylation of ERK p44/42 MAPK (Erk1/2), but to a lesser extent in D54-R cells, suggesting that Erk1/2 activity may be involved in regulation of hyperoxia and TMZ-mediated cell death. Overall, hyperoxia enhanced TMZ toxicity in GBM cells by induction of apoptosis, possibly via MAPK-related pathways. Induced hyperoxia is a potentially promising approach for treatment of TMZ-resistant GBM.

Hyperoxia-induced LC3B interacts with the Fas apoptotic pathway in epithelial cell death

Epithelial cell death plays a critical role in hyperoxia-induced lung injury. We investigated the involvement of the autophagic marker microtubule-associated protein-1 light chain-3B (LC3B) in epithelial cell apoptosis after hyperoxia. Prolonged hyperoxia (>95% O(2)), which causes characteristic lung injury in mice, activated morphological and biochemical markers of autophagy. Hyperoxia induced the time-dependent expression and conversion of LC3B-I to LC3B-II in mouse lung in vivo and in cultured epithelial cells (Beas-2B, human bronchial epithelial cells) in vitro. Hyperoxia increased autophagosome formation in Beas-2B cells, as evidenced by electron microscopy and increased GFP-LC3 puncta. The augmented LC3B level after hyperoxia was transcriptionally regulated and dependent in part on the c-Jun N-terminal kinase pathway. We hypothesized that LC3B plays a regulatory role in hyperoxia-induced epithelial apoptosis. LC3B siRNA promoted hyperoxia-induced cell death in epithelial cells, whereas overexpression of LC3B conferred cytoprotection after hyperoxia. The autophagic protein LC3B cross-regulated the Fas apoptotic pathway by physically interacting with the components of death-inducing signaling complex. This interaction was mediated by caveolin-1 tyrosine 14, which is a known target of phosphorylation induced by hyperoxia. Taken together, hyperoxia-induced LC3B activation regulates the Fas apoptotic pathway and thus confers cytoprotection in lung epithelial cells. The interaction of LC3B and Fas pathways requires cav-1.

Caveolin-1 mediates Fas-BID signaling in hyperoxia-induced apoptosis

Fas-mediated apoptosis is a crucial cellular event. Fas, the Fas-associated death domain, and caspase 8 form the death-inducing signaling complex (DISC). Activated caspase 8 mediates the extrinsic pathways and cleaves cytosolic BID. Truncated BID (tBID) translocates to the mitochondria, facilitates the release of cytochrome c, and activates the intrinsic pathways. However, the mechanism causing these DISC components to aggregate and form the complex remains unclear. We found that Cav-1 regulated Fas signaling and mediated the communication between extrinsic and intrinsic pathways. Shortly after hyperoxia (4 h), the colocalization and interaction of Cav-1 and Fas increased, followed by Fas multimer and DISC formation. Deletion of Cav-1 (Cav-1-/-) disrupted DISC formation. Further, Cav-1 interacted with BID. Mutation of Cav-1 Y14 tyrosine to phenylalanine (Y14F) disrupted the hyperoxia-induced interaction between BID and Cav-1 and subsequently yielded a decreased level of tBID and resistance to hyperoxia-induced apoptosis. The reactive oxygen species (ROS) scavenger N-acetylcysteine decreased the Cav-1-Fas interaction. Deletion of glutathione peroxidase-2 using siRNA aggravated the BID-Cav-1 interaction and tBID formation. Taken together, these results indicate that Cav-1 regulates hyperoxia/ROS-induced apoptosis through interactions with Fas and BID, probably via Fas palmitoylation and Cav-1 Y14 phosphorylation, respectively.

Anthrax lethal factor activates K(+) channels to induce IL-1β secretion in macrophages

Anthrax lethal toxin (LeTx) is a virulence factor of Bacilillus anthracis that is a bivalent toxin, containing lethal factor (LF) and protective Ag proteins, which causes cytotoxicity and altered macrophage function. LeTx exposure results in early K(+) efflux from macrophages associated with caspase-1 activation and increased IL-1β release. The mechanism of this toxin-induced K(+) efflux is unknown. The goals of the current study were to determine whether LeTx-induced K(+) efflux from macrophages is mediated by toxin effects on specific K(+) channels and whether altered K(+)-channel activity is involved in LeTx-induced IL-1β release. Exposure of macrophages to LeTx induced a significant increase in the activities of two types of K(+) channels that have been identified in mouse macrophages: Ba(2+)-sensitive inwardly rectifying K(+) (Kir) channels and 4-aminopyridine-sensitive outwardly rectifying voltage-gated K(+) (Kv) channels. LeTx enhancement of both Kir and Kv required the proteolytic activity of LF, because exposure of macrophages to a mutant LF-protein (LF(E687C)) combined with protective Ag protein had no effect on the currents. Furthermore, blocking Kir and Kv channels significantly decreased LeTx-induced release of IL-1β. In addition, retroviral transduction of macrophages with wild-type Kir enhanced LeTx-induced release of IL-1β, whereas transduction of dominant-negative Kir blocked LeTx-induced release of IL-1β. Activation of caspase-1 was not required for LeTx-induced activation of either of the K(+) channels. These data indicate that a major mechanism through which LeTx stimulates macrophages to release IL-1β involves an LF-protease effect that enhances Kir and Kv channel function during toxin stimulation.

How ERK1/2 activation controls cell proliferation and cell death: Is subcellular localization the answer?

Extracellular signal-regulated protein kinases 1 and 2 (ERK1/2) are members of the mitogen-activated protein kinase super family that can mediate cell proliferation and apoptosis. The Ras-Raf-MEK-ERK signaling cascade controlling cell proliferation has been well studied but the mechanisms involved in ERK1/2-mediated cell death are largely unknown. This review focuses on recent papers that define ERK1/2 translocation to the nucleus and the proteins involved in the cytosolic retention of activated ERK1/2. Cytosolic retention of ERK1/2 denies access to the transcription factor substrates that are responsible for the mitogenic response. In addition, cytosolic ERK1/2, besides inhibiting survival and proliferative signals in the nucleus, potentiates the catalytic activity of some proapoptotic proteins such as DAP kinase in the cytoplasm. Studies that further define the function of cytosolic ERK1/2 and its cytosolic substrates that enhance cell death will be essential to harness this pathway for developing effective treatments for cancer and chronic inflammatory diseases.

Reactive oxygen species mediate oridonin-induced HepG2 apoptosis through p53, MAPK, and mitochondrial signaling pathways

Oridonin, a diterpenoid isolated from Rabdosia rubescences, could induce apoptosis through the generation of reactive oxygen species (ROS) in human hepatoma HepG2 cells. p53, a specific inhibitor of pifithrin alpha (PFT alpha), markedly inhibited ROS generation and apoptosis, showing that p53 was responsible for the cytotoxity of oridonin through mediation by ROS. Moreover, the ROS activated the p38 kinase, which in turn promoted the activation of p53, as verified by evidence showing that the ROS scavenger N-acetyl-cysteine (NAC) not only blocked the phosphorylation of p38 but also partially inhibited the activation of p53, and the p38 inhibitor SB203580 reduced the activation of p53 as well. Mitochondria were either the sources or the targets of ROS. This study showed that oridonin stimulated mitochondrial transmembrane permeabilization in a ROS-dependent manner because NAC almost thoroughly reversed the drop of mitochondrial transmembrane potential (Deltapsim) and the release of cytochrome c from the mitochondrial inter-membrane space into cytosol. Furthermore, as a result of mitochondrial permeability transition, procaspases-9 and -3 were cleaved into 37- and 17-kDa proteolytic products, respectively, which acted as executors of oridonin-induced apoptosis.

Activation of phospholipase C gamma 1 protects renal arteriolar VSMCs from H2O2-induced cell death

BACKGROUND

We evaluated the effect of hydrogen peroxide (H2O2) on viability of vascular smooth muscle cells (VSMCs) of renal resistance arterioles and determined whether responses are modulated by activation of PLCgamma1.

METHODS

Phospholipase C (PLC)-isozyme protein levels and activity were measured using Western blot analysis and enzymatic production of phosphoinositol 1,4,5-trisphosphate (IP3), respectively. Stimulation of PLCgamma1 was assessed by immunoblots of tyrosine phosphorylation.

RESULTS

Cytotoxicity of H2O2 exposure was concentration-dependent (30% death with 250 microM; 87% with 500 microM at 8 h) and time-dependent (7% at 1 h; 30% at 8 h with 250 microM H2O2. Catalase abolished such relations. H2O2 increased PLCgamma1 expression more than that of PLCdelta1 and almost doubled total PLC enzymatic activity between 2 and 8 h, changes prevented by catalase. The PLC inhibitor U73112 (3 microM) enhanced the cytotoxic concentration and time effects of H2O2. In acute studies, H2O2 rapidly caused tyrosine phosphorylation of PLCgamma1.

CONCLUSION

H2O2 increased PLCgamma1 expression and almost doubled total PLC activity, changes abolished by catalase. We conclude that H2O2 is cytotoxic to cultured VSMCs of renal preglomerular arterioles, a process that is attenuated by compensatory increases in PLCgamma1 protein level, tyrosine phosphorylation of PLCgamma1 and PLC enzymatic activity to generate IP3.

Antioxidants improve antibacterial function in hyperoxia-exposed macrophages

Hyperoxia and pulmonary infections are well known to increase the risk of acute and chronic lung injury in newborn infants, but it is not clear whether hyperoxia directly increases the risk of pneumonia. The purpose of this study was to examine: (1) the effects of hyperoxia and antioxidant enzymes on inflammation and bacterial clearance in mononuclear cells and (2) developmental differences between adult and neonatal mononuclear cells in response to hyperoxia. Mouse macrophages were exposed to either room air or 95% O2 for 24 h and then incubated with Pseudomonas aeruginosa. After 1 h, bacterial adherence, phagocytosis, and macrophage inflammatory protein (MIP)-1alpha production were analyzed. Bacterial adherence increased 5.8-fold (p < 0.0001), phagocytosis decreased 60% (p < 0.05), and MIP-1alpha production increased 49% (p < 0.05) in response to hyperoxia. Overexpression of MnSOD or catalase significantly decreased bacterial adherence by 30.5%, but only MnSOD significantly improved bacterial phagocytosis and attenuated MIP-1alpha production. When monocytes from newborns and adults were exposed to hyperoxia, phagocytosis was impaired in both groups. However, adult monocytes were significantly more impaired than neonatal monocytes. Data indicate that hyperoxia significantly increases bacterial adherence while impairing function of mononuclear cells, with adult cells being more impaired than neonatal cells. MnSOD reduces bacterial adherence and inflammation and improves bacterial phagocytosis in mononuclear cells in response to hyperoxia, which should minimize the development of oxidant-induced lung injury as well as reducing nosocomial infections.

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.

Antioxidants preserve macrophage phagocytosis of Pseudomonas aeruginosa during hyperoxia

Pseudomonas. aeruginosa (PA) is a leading cause of nosocomial pneumonia in patients receiving mechanical ventilation with hyperoxia. Exposure to supraphysiological concentrations of reactive oxygen species during hyperoxia may result in macrophage damage that reduces their ability to phagocytose PA. We tested this hypothesis in cultured macrophage-like RAW 264.7 cells and alveolar macrophages from mice exposed to hyperoxia. Exposure to hyperoxia induced a similarly impaired phagocytosis of both the mucoid and the nonmucoid forms of PA in alveolar macrophages and RAW cells. Compromised PA phagocytosis was associated with cytoskeleton disorganization and actin oxidation in hyperoxic macrophages. To test whether moderate concentrations of O(2) limit the loss of phagocytic function induced by > or =95% O(2), mice and RAW cells were exposed to 65% O(2). Interestingly, although the resulting lung injury/cell proliferation was not significant, exposure to 65% O(2) resulted in a marked reduction in PA phagocytosis that was comparable to that of > or =95% O(2). Treatment with antioxidants, even post hyperoxic exposure, preserved actin cytoskeleton organization and phagocytosis of PA. These data suggest that hyperoxia reduces macrophage phagocytosis through effects on actin functions which can be preserved by antioxidant treatment. In addition, administration of moderate rather than higher concentrations of O2 does not improve macrophage phagocytosis of PA.

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.

Fibroblast growth factor-2 suppresses oridonin-induced L929 apoptosis through extracellular signal-regulated kinase-dependent and phosphatidylinositol 3-kinase-independent pathway

Oridonin, isolated from Rabdosia rubescences, has been reported to exert cytotoxic effects on L929 cells. In this study, we investigated the mechanisms of FGF-2 protection of L929 cells from oridonin-induced apoptosis. Phosphatidylinositol 3-kinase (PI3K)/protein kinase B (PKB) signal did not mediate this effect because the PI3K inhibitor wortmannin failed to reverse this protection and PKB activation was not observed in this process. In contrast, the extracellular signal-regulated kinase (ERK) was responsible for this rescue because its inhibition abolished the protective effect of fibroblast growth factor (FGF)-2. ERK had dual regulatory functions: mediating cell apoptosis or preventing cells from initiating the apoptotic response by phosphorylation or promoting expression of Bcl-2 in dependence of different stimuli. In L929 cells treated with oridonin alone, the activated ERK decreased the ratio of Bcl-2/Bax by mediating the phosphorylation of Bcl-2, resulting in apoptosis; the Ras inhibitor manumycin A and Raf inhibitor GW5074 failed to inhibit this apoptosis, indicating that there is a signal other than Ras/Raf pathway activated ERK. However, in the presence of FGF-2, Bcl-2 phosphorylation was blocked, and the Ras/Raf/ERK signal pathway was activated and protected against the oridonin-induced apoptosis by the alternative function of promoting of Bcl-2 expression.

Poly(ADP-ribose)polymerase Activation Mediates Lung Epithelial Cell DeathIn Vitrobut Is Not Essential in Hyperoxia-Induced Lung Injury

Hyperoxia induces extensive DNA damage and lung cell death by apoptotic and nonapoptotic pathways. We analyzed the regulation of Poly(ADP-ribose)polymerase-1 (PARP-1), a nuclear enzyme activated by DNA damage, and its relation to cell death during hyperoxia in vitro and in vivo. In lung epithelial-derived A549 cells, which are known to die by necrosis when exposed to oxygen, a minimal amount of PARP-1 was cleaved, correlating with the absence of active caspase-3. Conversely, in primary lung fibroblasts, which die mainly by apoptosis, the complete cleavage of PARP-1 was concomitant to the induction of active caspase-3, as assessed by Western blot and caspase activity. Blockade of caspase activity by Z-VAD reduced the amount of cleaved PARP-1 in fibroblasts. Hyperoxia induced PARP activity in both cell types, as revealed by poly-ADP-ribose accumulation. In A549 cells, the final outcome of necrosis was dependent on PARP activity because it was prevented by the PARP inhibitor 3-aminobenzamide. In contrast, apoptosis of lung fibroblasts was not sensitive to 3-aminobenzamide and was not affected by PARP-1 deletion. In vivo, despite evidence of PARP activation in hyperoxia-exposed mouse lungs, absence of PARP-1 did not change the extent of lung damage, arguing for redundant oxidative stress-induced cell death 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.

Selective inhibitors of MEK1/ERK44/42 and p38 mitogen-activated protein kinases potentiate apoptosis induction by sulindac sulfide in human colon carcinoma cells

The nonsteroidal anti-inflammatory drug (NSAID) sulindac prevents experimental colon cancer and can regress precancerous polyps in humans. Sulindac sulfide inhibits cyclooxygenase (COX)-mediated prostaglandin synthesis and retards the growth of cultured colon cell lines primarily by inducing apoptosis. Given the known role of mitogen-activated protein kinase (MAPK) in signal transduction and the regulation of cell survival and death, we determined the effect of sulindac sulfide on MAPK activation, COX-2 expression, and apoptosis induction in HCA-7 human colon cancer cells. Sulindac sulfide treatment was associated with activation of ERKp44/42 and p38 MAPK in a dosage- and time-dependent manner, and also activated upstream MEK. Similar results were seen in HCT-15 cells and also with the selective COX-2 inhibitor NS398. ERKp44/42 and p38 activation were accompanied by an induction of COX-2 protein expression. Selective inhibitors of sulindac sulfide–induced ERKp44/42 (PD98059) and p38 MAPK (SB203580) activation also suppressed the induction of COX-2 by this NSAID. Furthermore, both MAPK inhibitors significantly augmented sulindac sulfide–induced apoptosis, as did suppression of constitutive COX-2 using antisense oligonucleotides. In conclusion, MEK/ERK and p38 MAPK activation mediate COX-2 induction by sulindac sulfide. Selective inhibitors of these MAPKs potentiate apoptosis induction by this NSAID, suggesting a novel strategy for the prevention or treatment of colorectal cancer.

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.

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.

Deficiency in the c-Jun NH2-terminal kinase signaling pathway confers susceptibility to hyperoxic lung injury in mice

Hyperoxia generates an oxidative stress in the mouse lung, which activates the major stress-inducible kinase pathways, including c-Jun NH2-terminal kinase (JNK). We examined the effect of Jnk1 gene deletion on in vivo responses to hyperoxia in mice. The survival of Jnk1-/- mice was reduced relative to wild-type mice after exposure to continuous hyperoxia. Jnk1-/- mice displayed higher protein concentration in bronchoalveolar lavage (BAL) fluid and increased expression of heme oxygenase-1, a stress-inducible gene, after 65 h of hyperoxia. Contrary to other markers of injury, the leukocyte count in BAL fluid of Jnk1-/- mice was markedly diminished relative to that of wild-type mice. The decrease in BAL leukocyte count was not associated with any decrease in lung myeloperoxidase activity at baseline or after hyperoxia treatment. Pretreatment with inhaled lipopolysaccharide increased BAL neutrophil content and extended hyperoxia survival time to a similar extent in Jnk1-/- and wild-type mice. Associated with increased mortality, Jnk1-/- mice had increased pulmonary epithelial cell apoptosis after exposure to hyperoxia compared with wild-type mice. These results indicate that JNK pathways participate in adaptive responses to hyperoxia in mice.

Hyperoxia impairs antibacterial function of macrophages through effects on actin

Oxidative stress may impair alveolar macrophage function in patients with inflammatory lung diseases or those exposed to high concentrations of oxygen. We investigated putative mechanisms of injury to macrophages by oxidative stress, using RAW 264.7 cells exposed to 95% oxygen for 48 h. Hyperoxia-exposed macrophages were less able to phagocytose and kill Klebsiella pneumoniae than normoxic controls, despite increased production of nitric oxide, a free radical important in pathogen killing. Exposure of macrophages to hyperoxia had marked effects on the actin cytoskeleton, including increased actin polymerization, loss of cortical actin, formation of stress fibers, de novo synthesis of actin, and actin oxidation. Hyperoxia induced changes in cell morphology, with increased cell size and pseudopod formation. Exposure of macrophages to jasplakinolide, an agent that increases actin polymerization, also impaired their ability to phagocytose Klebsiella. Alveolar macrophages isolated from mice exposed to 100% oxygen for 84 h also demonstrated impaired phagocytic function, as well as similar effects on the actin cytoskeleton and cell morphology to macrophages exposed to hyperoxia in vitro. We conclude that oxidative stress in vitro and in vivo impairs macrophage antibacterial function through effects on actin.

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

Therapy with high oxygen concentrations (hyperoxia) is often necessary to treat patients with respiratory failure. However, hyperoxia may exacerbate the development of acute lung injury, perhaps by increasing lung epithelial cell death. Therefore, interrupting lung epithelial cell death is an important protective and therapeutic strategy. In the present study, hyperoxia (95% O(2)) results in murine lung epithelium cell death by DNA-laddering, terminal deoxynucleotidyltransferase dUTP nick end labeling, and Annexin V-fluorescein isothiocyanate flow cytometry assay. We show that hyperoxia increases superoxide production, as assessed by nicotinamide adenine dinucleotide phosphate reduced (NADPH) oxidase activity and flow cytometric assay, and increases phospho-extracellular signal-regulated kinase (ERK)1/2 by Western blot analysis. These processes are inhibited by a reactive oxygen species inhibitor, diphenylene iodonium (DPI), and by an inhibitor of the mitogen-activated protein (MAP) or ERK kinase (MEK)/ERK1/2 pathway, PD98059. ERK1/2 activation in hyperoxia is also inhibited by DPI. Hyperoxia-induced cell death is associated with cytochrome c release, subsequent caspase 9 and 3 activation, and poly (ADP-ribosyl) polymerase cleavage, which can all be suppressed by DPI and PD98059. However, the broad caspase inhibitor z-VAD-FMK protects cells from death without affecting superoxide generation and ERK1/2 activation. Taken together, our data suggest that hyperoxia, by virtue of activating NADPH oxidase, generates reactive oxygen species (ROS), which mediates cell death of lung epithelium via ERK1/2 MAPK activation, and functions upstream of caspase activation in lung epithelial cells.

Activation of the G2 cell cycle checkpoint enhances survival of epithelial cells exposed to hyperoxia

Reactive oxygen species produced during hyperoxia damage DNA, inhibit proliferation in G1- through p53-dependent activation of p21(Cip1/WAF1/Sdi1), and kill cells. Because checkpoint activation protects cells from genotoxic stress, we investigated cell proliferation and survival of the murine type II epithelial cell line MLE15 during hyperoxia. These cells were chosen for study because they express Simian large and small-T antigens, which transform cells in part by disrupting the p53-dependent G1 checkpoint. Cell counts, 5-bromo-2'-deoxyuridine labeling, and flow cytometry revealed that hyperoxia slowed cell cycle progression after one replication, resulting in a pronounced G2 arrest by 72 h. Addition of caffeine, which inactivates the G2 checkpoint, diminished the percentage of hyperoxic cells in G2 and increased the percentage in sub-G1 and G1. Abrogation of the G2 checkpoint was associated with enhanced oxygen-induced DNA strand breaks and cell death. Caffeine did not affect DNA integrity or viability of cells exposed to room air. Similarly, caffeine abrogated the G2 checkpoint in hyperoxic A549 epithelial cells and enhanced oxygen-induced toxicity. These data indicate that hyperoxia rapidly inhibits proliferation after one cell cycle and that the G2 checkpoint is critical for limiting DNA damage and cell death.

Mitogen-activated protein kinases regulate HO-1 gene transcription after ischemia-reperfusion lung injury

Lung ischemia-reperfusion (I-R) is an important model of oxidant-mediated acute lung and vascular injury. Heme oxygenase-1 (HO-1) is a cytoprotective gene that is markedly induced by lung I-R injury. HO-1 mRNA is increased in mouse lung after 30 min of lung hilar clamping (ischemia) followed by 2-6 h of unclamping (reperfusion) compared with control mice. In a variety of vascular cell types, HO-1 mRNA is induced after 24 h of anoxia followed by 30 min-1 h of reoxygenation (A-R). Transfection studies reveal that the promoter and 5'-distal enhancer E1 are necessary and sufficient for increased HO-1 gene transcription after A-R. Immunoblotting studies show all three subfamilies of MAPKs (ERK, JNK, and p38) are activated by 15 min of reperfusion. We also demonstrate that HO-1 gene transcription after A-R involves ERK, JNK, and p38 MAPK pathways. Together, our data show that I-R not only induces HO-1 gene expression in mouse lungs and vascular cells but that gene transcription occurs via the promoter and E1 enhancer and involves upstream MAPK pathways.

Apoptosis by pan-caspase inhibitors in lipopolysaccharide-activated macrophages

Although apoptosis has been observed in macrophages during the course of infections, the mechanism of apoptosis in activated macrophages is not fully understood. This study shows that pan-caspase inhibitor benzyloxycarbonyl-Val-Ala-Asp-fluoromethylketone (ZVAD) or t-butyloxycarbonyl-Asp-fluoromethylketone (Boc-D) caused the death of lipopolysaccharide (LPS)-activated macrophages and RAW 264.7 cells with apoptotic features. The apoptosis was also observed in lipoprotein-treated bacteria but not in CpG oligonucleotide- or flagellin-treated macrophages, indicating a difference of cellular responses downstream of different Toll-like receptors. Consistent with the induction of cell death by pan-caspase inhibitors, no activation of known caspases was detected in LPS-ZVAD-treated cells, suggesting an involvement of unknown proapoptotic caspases in the cell death. ZVAD inhibited the activation of extracellular signal-regulated kinase (ERK) and p38 but not of nuclear factor (NF)-kappa B induced by LPS, suggesting that the ZVAD-sensitive molecule lies upstream of the ERK and p38 pathways but downstream of the divergent site of NF-kappa B and mitogen-activated protein kinases. Our results demonstrate that apoptosis of macrophages induced by LPS+ZVAD is independent from the known proapoptotic caspases and suggest that activity of an unidentified ZVAD-sensitive molecule(s) is involved in the survival of LPS-activated macrophages.

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.

Different accumulation of activated extracellular signal-regulated kinases (ERK 1/2) and role in cell-cycle alterations by epidermal growth factor, hydrogen peroxide, or asbestos in pulmonary epithelial cells

The extracellular signal-regulated kinase (ERK) pathway is induced by cytokines and oxidative stress. In this study we examined the patterns of localization of phosphorylated ERK proteins in relationship to subsequent phenotypic responses by the mitogenic agent epidermal growth factor (EGF) (5 ng/ ml); hydrogen peroxide (H(2)O(2)) (100 to 300 microM), an inducer of apoptosis; and crocidolite asbestos (5 microg/cm(2) dish) in a nontransformed murine alveolar type II epithelial cell line (C10). Laser scanning cytometry and flow cytometry were used to determine: (1) whether expression of phosphorylated ERKs was cell cycle-related; and (2) whether cell-cycle alterations by agents could be modified after addition of the mitogen-activated protein kinase/ERK kinase (MEK) 1 inhibitor PD98059. In contrast to other stimuli which induced transient increases in phosphorylated ERKs, asbestos caused fiber-associated localization of phosphorylated ERKs that were elevated from 1 to 24 h (P < or = 0.05), and striking apoptosis followed by increased numbers of cells in the S phase at 72 h. In both control and asbestos-exposed cells, the percentage of phosphorylated ERK-positive cells was greatest in cells in the G(2)/M and S phases of the cell cycle. All stimuli caused increased proportions of cells in G(2)/M at 24 h that were inhibited by PD98059 (30 microM). Increases in G(2)/M cells by H(2)O(2) and asbestos also were decreased at 48 h by the MEK1 inhibitor. In addition, PD98059 abrogated elevations in S-phase cells by EGF and H(2)O(2) at 24 h and by asbestos at 72 h. Our results suggest that ERKs mediate cell-cycle alterations during the development of epithelial cell apoptosis or proliferation by diverse ERK stimuli.

AP-1 and STAT mediate hyperoxia-induced gene transcription of heme oxygenase-1

We have previously shown marked induction of the stress-inducible gene heme oxygenase-1 (HO-1) in vivo and in vitro after hyperoxia. In RAW 264.7 cells, HO-1 induction is transcriptionally regulated and dependent on cooperation between the HO-1 gene promoter and the 5' distal enhancer element SX2. In our present study, further deletional and mutational analyses demonstrate that signal transducer and activator of transcription (STAT) DNA binding sites located in the promoter of HO-1 and activator protein (AP)-1 DNA binding sites in the distal enhancer element SX2 are necessary for optimal HO-1 gene activation after hyperoxia. Interestingly, a second 5' distal enhancer element, AB1, located 10 kb upstream from the HO-1 promoter, alone is activated after hyperoxia but cannot confer maximal hyperoxia-induced HO-1 gene transcription. Mutational analysis of the AB1 enhancer shows that AP-1 is essential for AB1-mediated HO-1 gene transcription after hyperoxia. Electromobility shift assays show increased STAT1, STAT3, STAT5, and AP-1 DNA binding activity in RAW 264.7 cells after hyperoxia. Taken together, our data suggest that the 5' distal enhancer elements of the HO-1 gene in concert with the promoter regulate HO-1 gene induction and highlight the complexity of HO-1 gene transcription in response to hyperoxia.