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

Oxygen toxicity: cellular mechanisms in normobaric hyperoxia

In clinical settings, oxygen therapy is administered to preterm neonates and to adults with acute and chronic conditions such as COVID-19, pulmonary fibrosis, sepsis, cardiac arrest, carbon monoxide poisoning, and acute heart failure. In non-clinical settings, divers and astronauts may also receive supplemental oxygen. In addition, under current standard cell culture practices, cells are maintained in atmospheric oxygen, which is several times higher than what most cells experience in vivo. In all the above scenarios, the elevated oxygen levels (hyperoxia) can lead to increased production of reactive oxygen species from mitochondria, NADPH oxidases, and other sources. This can cause cell dysfunction or death. Acute hyperoxia injury impairs various cellular functions, manifesting ultimately as physiological deficits. Chronic hyperoxia, particularly in the neonate, can disrupt development, leading to permanent deficiencies. In this review, we discuss the cellular activities and pathways affected by hyperoxia, as well as strategies that have been developed to ameliorate injury.

Cell Division Cycle 2 Protects Neonatal Rats Against Hyperoxia-Induced Bronchopulmonary Dysplasia

PURPOSE

Hyperoxia-induced bronchopulmonary dysplasia (BPD) is a lung disease in preterm infants. We aimed to explore the role of cell division cycle 2 (CDC2) on histopathologic changes of lung tissues, as well as the viability, apoptosis, and inflammation of lung cells in rats with hyperoxia-induced BPD.

MATERIALS AND METHODS

Hyperoxia-induced BPD in neonatal rats and hyperoxia-induced A549 cells were constructed. The mRNA expression of CDC2 was detected by qRT-PCR. The fibrosis score of lung tissues was evaluated by hematoxylin-eosin staining. The viability and apoptosis of A549 cells were detected by cell counting kit-8 assay and flow cytometry. The protein expressions of bcl-2, bax, and caspase-3 were measured by western blot. The levels of tumor necrosis factor-α (TNF-α), interleukin (IL)-6, and IL-1β in A549 cells were detected by enzyme-linked immunosorbent assay. The pcDNA3.1-CDC2 was injected into rats to determine the role of CDC2 in hyperoxia-induced BPD in vivo.

RESULTS

The expression of CDC2 was decreased in lung tissues of neonatal rats with hyperoxia-induced BPD and hyperoxia-induced A549 cells. The fibrosis score was increased in the lung tissues of neonatal rats with hyperoxia-induced BPD. Overexpression of CDC2 increased the viability and protein expression of bcl-2; and inhibited the apoptosis, inflammation, and protein expression of bax and caspase-3 in hyperoxia-induced A549 cells. Up-regulation of CDC2 alleviated the histopathologic changes in lung tissues of neonatal rats with hyperoxia-induced BPD.

CONCLUSION

Overexpression of CDC2 promoted the viability and inhibited the apoptosis and inflammation of hyperoxia-induced cells, and alleviated the histopathologic changes of lung tissues in neonatal rats with hyperoxia-induced BPD.

Lactobacillus paracasei subsp. paracasei M5L induces cell cycle arrest and calreticulin translocation via the generation of reactive oxygen species in HT-29 cell apoptosis

Lactobacillus paracasei subsp. paracasei M5L (L. paracasei M5L) was isolated and co-cultured with HT-29 colon cancer cells to study its anti-colorectal cancer effects and mechanism. Using the MTT assay we found that L. paracasei M5L significantly inhibits HT-29 cell proliferation. Morphological and biochemical characteristics of apoptosis were observed and confirmed by hematoxylin and eosin (HE) staining and transmission electron microscopy (TEM). Lactobacilli could change the cell cycle distribution and induce calreticulin (CRT) translocation from the endoplasmic reticulum to the surface of the cytomembrane. We also determine that vast reactive oxygen species (ROS) were generated, while the activities of superoxide dismutase (SOD) and catalase (CAT) were noticeably diminished following L. paracasei M5L treatment. This study reveals that L. paracasei M5L induces apoptosis in HT-29 cells through ROS generation followed by CRT accompanied endoplasmic reticulum (ER) stress and S phase arrest. These results provide new insights into the possible molecular mechanism of L. paracasei M5L as a novel probiotic with the potential for further application.

Oxygen regulates molecular mechanisms of cancer progression and metastasis

Oxygen is the basic molecule which supports life and it truly is "god's gift to life." Despite its immense importance, research on "oxygen biology" has never received the light of the day and has been limited to physiological and biochemical studies. It seems that in modern day biology, oxygen research is summarized in one word "hypoxia." Scientists have focused on hypoxia-induced transcriptomics and molecular-cellular alterations exclusively in disease models. Interestingly, the potential of oxygen to control the basic principles of biology like homeostatic maintenance, transcription, replication, and protein folding among many others, at the molecular level, has been completely ignored. Here, we present a perspective on the crucial role played by oxygen in regulation of basic biological phenomena. Our conclusion highlights the importance of establishing novel research areas like oxygen biology, as there is great potential in this field for basic science discoveries and clinical benefits to the society.

Thioredoxin-1 redox signaling regulates cell survival in response to hyperoxia

The most common form of newborn chronic lung disease, bronchopulmonary dysplasia (BPD), is thought to be caused by oxidative disruption of lung morphogenesis, which results in decreased pulmonary vasculature and alveolar simplification. Although cellular redox status is known to regulate cellular proliferation and differentiation, redox-sensitive pathways associated with these processes in developing pulmonary epithelium are unknown. Redox-sensitive pathways are commonly regulated by cysteine thiol modifications. Therefore two thiol oxidoreductase systems, thioredoxin and glutathione, were chosen to elucidate the roles of these pathways on cell death. Studies herein indicate that thiol oxidation contributes to cell death through impaired activity of glutathione-dependent and thioredoxin (Trx) systems and altered signaling through redox-sensitive pathways. Free thiol content decreased by 71% with hyperoxic (95% oxygen) exposure. Increased cell death was observed during oxygen exposure when either the Trx or the glutathione-dependent system was pharmacologically inhibited with aurothioglucose (ATG) or buthionine sulfoximine, respectively. However, inhibition of the Trx system yielded the smallest decrease in free thiol content (1.44% with ATG treatment vs 21.33% with BSO treatment). Although Trx1 protein levels were unchanged, Trx1 function was impaired during hyperoxic treatment as indicated by progressive cysteine oxidation. Overexpression of Trx1 in H1299 cells utilizing an inducible construct increased cell survival during hyperoxia, whereas siRNA knockdown of Trx1 during oxygen treatment reduced cell viability. Overall, this indicated that a comparatively small pool of proteins relies on Trx redox functions to mediate cell survival in hyperoxia, and the protective functions of Trx1 are progressively lost by its oxidative inhibition. To further elucidate the role of Trx1, potential Trx1 redox protein-protein interactions mediating cytoprotection and cell survival pathways were determined by utilizing a substrate trap (mass action trapping) proteomics approach. With this method, known Trx1 targets were detected, including peroxiredoxin-1as well as novel targets, including two HSP90 isoforms (HSP90AA1 and HSP90AB1). Reactive cysteines within the structure of HSP90 are known to modulate its ATPase-dependent chaperone activity through disulfide formation and S-nitrosylation. Whereas HSP90 expression is unchanged at the protein level during hyperoxic exposure, siRNA knockdown significantly increased hyperoxic cell death by 2.5-fold, indicating cellular dependence on HSP90 chaperone functions in response to hyperoxic exposure. These data support the hypothesis that hyperoxic impairment of Trx1 has a negative impact on HSP90-oxidative responses critical to cell survival, with potential implications for pathways implicated in lung development and the pathogenesis of BPD.

Differential concentration-specific effects of caffeine on cell viability, oxidative stress, and cell cycle in pulmonary oxygen toxicity in vitro

Caffeine is used to prevent bronchopulmonary dysplasia (BPD) in premature neonates. Hyperoxia contributes to the development of BPD, inhibits cell proliferation and decreases cell survival. The mechanisms responsible for the protective effect of caffeine in pulmonary oxygen toxicity remain largely unknown. A549 and MLE 12 pulmonary epithelial cells were exposed to hyperoxia or maintained in room air, in the presence of different concentrations (0, 0.05, 0.1 and 1mM) of caffeine. Caffeine had a differential concentration-specific effect on cell cycle progression, oxidative stress and viability, with 1mM concentration being deleterious and 0.05 mM being protective. Reactive oxygen species (ROS) generation during hyperoxia was modulated by caffeine in a similar concentration-specific manner. Caffeine at 1mM, but not at the 0.05 mM concentration decreased the G2 arrest in these cells. Taken together this study shows the novel funding that caffeine has a concentration-specific effect on cell cycle regulation, ROS generation, and cell survival in hyperoxic conditions.

Transcriptome profiling of the newborn mouse lung after hypoxia and reoxygenation: hyperoxic reoxygenation affects mTOR signaling pathway, DNA repair, and JNK-pathway regulation

BACKGROUND

The use of oxygen in acute treatment of asphyxiated term newborns is associated with increased mortality. It is unclear how hyperoxic reoxygenation after hypoxia affects transcriptional changes in the newborn lung.

METHODS

On postnatal day 7, C57BL/6 mice (n = 62) were randomized to 120-min hypoxia (fraction of inspired oxygen (FiO2) 0.08) or normoxia. The hypoxia group was further randomized to reoxygenation for 30 min with FiO2 0.21, 0.40, 0.60, or 1.00, and the normoxia group to FiO2 0.21 or 1.00. Transcriptome profiling was performed on homogenized lung tissue using the Affymetrix 750k expression array, and validation was carried out by real-time polymerase chain reaction and enzyme-linked immunosorbent assay.

RESULTS

The hypoxia-reoxygenation model induced hypoxia-inducible factor 1 (HIF-1) targets like Vegfc, Adm, and Aqp1. In total, ~70% of the significantly differentially expressed genes were detected in the two high hyperoxic groups (FiO2 0.60 and 1.00). Reoxygenation with 100% oxygen after hypoxia uniquely upregulated Gadd45g, Dusp1, Peg3, and Tgm2. Pathway analysis identified mammalian target of rapamycin (mTOR) signaling pathway, DNA repair, c-jun N-terminal kinase (JNK)-pathway regulation, and cell cycle after hyperoxic reoxygenation was applied.

CONCLUSION

Acute hypoxia induces HIF-1 targets independent of the reoxygenation regime applied. Hyperoxic reoxygenation affects pathways regulating cell growth and survival. DNA-damage-responsive genes are restricted to reoxygenation with 100% oxygen.

Oxidative stress and lipid peroxidation products in cancer progression and therapy

The generation of reactive oxygen species (ROS) and an altered redox status are common biochemical aspects in cancer cells. ROS can react with the polyunsaturated fatty acids of lipid membranes and induce lipid peroxidation. The end products of lipid peroxidation, 4-hydroxynonenal (HNE), have been considered to be a second messenger of oxidative stress. Beyond ROS involvement in carcinogenesis, increased ROS level can inhibit tumor cell growth. Indeed, in tumors in advanced stages, a further increase of oxidative stress, such as that occurs when using several anticancer drugs and radiation therapy, can overcome the antioxidant defenses of cancer cells and drive them to apoptosis. High concentrations of HNE can also induce apoptosis in cancer cells. However, some cells escape the apoptosis induced by chemical or radiation therapy through the adaptation to intrinsic oxidative stress which confers drug resistance. This paper is focused on recent advances in the studies of the relation between oxidative stress, lipid peroxidation products, and cancer progression with particular attention to the pro-oxidant anticancer agents and the drug-resistant mechanisms, which could be modulated to obtain a better response to cancer therapy.

Prolonged mechanical ventilation induces cell cycle arrest in newborn rat lung

RATIONALE

The molecular mechanism(s) by which mechanical ventilation disrupts alveolar development, a hallmark of bronchopulmonary dysplasia, is unknown.

OBJECTIVE

To determine the effect of 24 h of mechanical ventilation on lung cell cycle regulators, cell proliferation and alveolar formation in newborn rats.

METHODS

Seven-day old rats were ventilated with room air for 8, 12 and 24 h using relatively moderate tidal volumes (8.5 mL.kg⁻¹).

MEASUREMENT AND MAIN RESULTS

Ventilation for 24 h (h) decreased the number of elastin-positive secondary crests and increased the mean linear intercept, indicating arrest of alveolar development. Proliferation (assessed by BrdU incorporation) was halved after 12 h of ventilation and completely arrested after 24 h. Cyclin D1 and E1 mRNA and protein levels were decreased after 8-24 h of ventilation, while that of p27(Kip1) was significantly increased. Mechanical ventilation for 24 h also increased levels of p57(Kip2), decreased that of p16(INK4a), while the levels of p21(Waf/Cip1) and p15(INK4b) were unchanged. Increased p27(Kip1) expression coincided with reduced phosphorylation of p27(Kip1) at Thr¹⁵⁷, Thr¹⁸⁷ and Thr¹⁹⁸ (p<0.05), thereby promoting its nuclear localization. Similar -but more rapid- changes in cell cycle regulators were noted when 7-day rats were ventilated with high tidal volume (40 mL.kg⁻¹) and when fetal lung epithelial cells were subjected to a continuous (17% elongation) cyclic stretch.

CONCLUSION

This is the first demonstration that prolonged (24 h) of mechanical ventilation causes cell cycle arrest in newborn rat lungs; the arrest occurs in G₁ and is caused by increased expression and nuclear localization of Cdk inhibitor proteins (p27(Kip1), p57(Kip2)) from the Kip family.

Bcl-X(L) is the primary mediator of p21 protection against hyperoxia-induced cell death

A tight balance between anti- and proapoptotic members of the Bcl-2 family controls cell survival and death. Exposure to hyperoxia shifts this balance towards a prodeath state that ultimately activates Bak- and Bax-dependent cell death. Mechanisms underlying this shift are undefined; however, the cell cycle inhibitor p21 delays the loss of antiapoptotic Mcl-1 and Bcl-X(L), and protects against hyperoxia. Here, H1299 human lung adenocarcinoma cells are used to investigate how these and other members of the Bcl-2 family cooperate with p21 to protect against hyperoxia. Expression of antiapoptotic Mcl-1 and Bcl-X(L), but not Bcl-2 or A1, declined during hyperoxia, whereas proapoptotic Bak, but not Bax, increased. Conditional overexpression of p21 selectively delayed the loss of Mcl-1 and Bcl-X(L), without affecting expression of the other members. siRNA knockdown of Mcl-1 and Bcl-X(L) sensitized cells to hyperoxia, but only the loss of Bcl-X(L) ablated the protective effects of p21. Conversely, overexpression of Mcl-1 and Bcl-X(L) protected against hyperoxia, but only Bcl-X(L) bound Bak and Bax. Altogether, these data suggest that Bcl-X(L) is the primary mediator by which p21 protects against hyperoxia-induced Bak/Bax-dependent cell death.

Effects of hyperoxia on transdifferentiation of primary cultured typeII alveolar epithelial cells from premature rats

Hyperoxia exposure is a significant risk factor for the impaired alveolarization characteristic of bronchopulmonary dysplasia. Type II alveolar epithelial cells (AECIIs) may serve as "alveolar stem cells" to transdifferentiate into type I alveolar epithelial cells (AECIs). Here, we show that hyperoxia is capable of inducing transdifferentiation of AECIIs in premature rats in vitro. Hyperoxia-induced transdifferentiation was characterized by typical morphological changes, inhibition of cellular proliferation, decline in expression rate of Ki67, accumulation of cells in the G(1) phase of the cell cycle, increased expression of AECI-specific protein aquaporin 5, and decreased expression of AECII-associated protein surfactant protein C. These results suggest that hyperoxia may induce transdifferentiation of AECIIs into AECIs and the transdifferentiation may be responsible for repairing early lung injury.

E2F4 and ribonucleotide reductase mediate S-phase arrest in colon cancer cells treated with chlorophyllin

Chlorophyllin (CHL) is a water-soluble derivative of chlorophyll that exhibits cancer chemopreventive properties, but which also has been studied for its possible cancer therapeutic effects. We report here that human colon cancer cells treated with CHL accumulate in S-phase of the cell cycle, and this is associated with reduced expression levels of p53, p21, and other G(1)/S checkpoint controls. At the same time, E2F1 and E2F4 transcription factors become elevated and exhibit increased DNA binding activity. In CHL-treated colon cancer cells, bromodeoxyuridine pulse-chase experiments provided evidence for the inhibition of DNA synthesis. Ribonucleotide reductase (RR), a pivotal enzyme for DNA synthesis and repair, was reduced at the mRNA and protein level after CHL treatment, and the enzymatic activity was inhibited in a concentration-dependent manner both in vitro and in vivo. Immunoblotting revealed that expression levels of RR subunits R1, R2, and p53R2 were reduced by CHL treatment in HCT116 (p53(+/+)) and HCT116 (p53(-/-)) cells, supporting a p53-independent mechanism. Prior studies have shown that reduced levels of RR small subunits can increase the sensitivity of colon cancer cells to clinically used DNA-damaging agents and RR inhibitors. We conclude that by inhibiting R1, R2, and p53R2, CHL has the potential to be effective in the clinical setting, when used alone or in combination with currently available cancer therapeutic agents.

Wound healing essentials: let there be oxygen

The state of wound oxygenation is a key determinant of healing outcomes. From a diagnostic standpoint, measurements of wound oxygenation are commonly used to guide treatment planning such as amputation decision. In preventive applications, optimizing wound perfusion and providing supplemental O(2) in the perioperative period reduces the incidence of postoperative infections. Correction of wound pO(2) may, by itself, trigger some healing responses. Importantly, approaches to correct wound pO(2) favorably influence outcomes of other therapies such as responsiveness to growth factors and acceptance of grafts. Chronic ischemic wounds are essentially hypoxic. Primarily based on the tumor literature, hypoxia is generally viewed as being angiogenic. This is true with the condition that hypoxia be acute and mild to modest in magnitude. Extreme near-anoxic hypoxia, as commonly noted in problem wounds, is not compatible with tissue repair. Adequate wound tissue oxygenation is required but may not be sufficient to favorably influence healing outcomes. Success in wound care may be improved by a personalized health care approach. The key lies in our ability to specifically identify the key limitations of a given wound and in developing a multifaceted strategy to specifically address those limitations. In considering approaches to oxygenate the wound tissue it is important to recognize that both too little as well as too much may impede the healing process. Oxygen dosing based on the specific need of a wound therefore seems prudent. Therapeutic approaches targeting the oxygen sensing and redox signaling pathways are promising.

Neutrophil elastase inhibition of cell cycle progression in airway epithelial cells in vitro is mediated by p27kip1

Neutrophil elastase (NE), a serine protease present in high concentrations in the airways of cystic fibrosis patients, injures the airway epithelium. We examined the epithelial response to NE-mediated proteolytic injury. We have previously reported that NE treatment of airway epithelial cells causes a marked decrease in epithelial DNA synthesis and proliferation. We hypothesized that NE inhibits DNA synthesis by arresting cell cycle progression. Progression through the cell cycle is positively regulated by cyclin complexes and negatively regulated by cyclin-dependent kinase inhibitors (CKI). To test whether NE arrests cell cycle progression, we treated normal human bronchial epithelial (NHBE) cells with NE (50 nM) or control vehicle for 24 h and assessed the effect of treatment on the cell cycle by flow cytometry. NE treatment resulted in G(1) arrest. Arrest in G(1) phase may be the result of CKI inhibition of the cyclin E complex; therefore, we evaluated whether NE upregulated CKI expression and/or affected the interaction of CKIs with the cyclin E complex. Following NE or control vehicle treatment, expression of p27(Kip1), a member of the Cip/Kip family, was evaluated. NE increased p27(Kip1) gene and protein expression. NE increased the coimmunoprecipitation of p27(Kip1) with cyclin E complex, suggesting that p27(Kip1) inhibited cyclin E complex activity. Our results demonstrate that p27 is regulated by NE and is critical for NE-induced cell cycle arrest.

Downregulation of PCNA potentiates p21-mediated growth inhibition in response to hyperoxia

Prolonged exposure to hyperoxia inhibits cell proliferation in G1 via increased expression of p21. While p21 inhibits proliferating cell nuclear antigen (PCNA)-dependent DNA synthesis, it can also directly lower PCNA abundance; however, it is unclear whether loss of PCNA contributes to growth arrest. Here, we investigate how PCNA loss affects ability of p21 to exert G1 growth arrest of lung epithelial cells exposed to hyperoxia. In A549 cells that express p21 and growth arrest in G1 during hyperoxia, small interfering RNA (siRNA) knockdown of p21 led to G1 checkpoint bypass, increased cell death, and restoration of PCNA expression. Conditional overexpression of the PCNA binding domain of p21 in H1299 cells that do not normally express p21, or exposure to hyperoxia, caused a time-dependent loss of PCNA. Titrating PCNA levels using siRNA to approximate the low amount observed in cells expressing p21 resulted in S phase arrest. While lowering PCNA by itself caused S phase arrest, the combination of hyperoxia and siRNA against PCNA dramatically reduced PCNA abundance resulting in G1 arrest. G1 growth arrest was markedly enhanced upon the addition of p21 to these cells. Our findings suggest a model in which reducing expression of the abundant protein PCNA allows the less abundant protein p21 to be more effective at suppressing the processivity functions of remaining PCNA, thereby fully exerting the G1 checkpoint. Given that high p21 expression is often associated with lower PCNA abundance, our findings are suggestive of a global growth inhibitory mechanism involving p21-mediated PCNA suppression.

p21Cip1 protection against hyperoxia requires Bcl-XL and is uncoupled from its ability to suppress growth

The cyclin-dependent kinase inhibitor p21Cip1/Waf1/Sdi1 protects the lung against hyperoxia, but the mechanism of protection remains unclear because loss of p21 does not lead to aberrant cell proliferation. Because some members of the Bcl-2 gene family have been implicated in hyperoxia-induced cell death, the current study investigated their expression as well as p21-dependent growth suppression and cytoprotection. Conditional overexpression of full-length p21, its amino-terminal cyclin-binding (p211-82NLS) domain or its carboxy-terminal PCNA-binding (p2176-164) domain inhibited growth of human lung adenocarcinoma H1299 cells, but only the full-length protein was cytoprotective. Low levels of p21 inhibited cell proliferation, whereas higher levels were required for protection. Expression of the anti-apoptotic protein Bcl-XL declined during hyperoxia but was maintained in cells expressing p21. RNA interference (RNAi) knockdown of Bcl-XL enhanced hyperoxic death of cells expressing p21, whereas overexpression of Bcl-XL increased cell survival. Consistent with growth suppression and cytoprotection requiring different levels of p21, hyperoxia inhibited PCNA expression in p21+/+ and p21+/- mice but not in p21-/- mice. In contrast, p21 was haplo-insufficient for maintaining expression of Bcl-XL and protection against hyperoxia. Taken together, these data show that p21-mediated cytoprotection against hyperoxia involves regulation of Bcl-XL and is uncoupled from its ability to inhibit proliferation.

3,3'-Diindolylmethane is a novel mitochondrial H(+)-ATP synthase inhibitor that can induce p21(Cip1/Waf1) expression by induction of oxidative stress in human breast cancer cells

Epidemiologic evidence suggests that high dietary intake of Brassica vegetables, such as broccoli, cabbage, and Brussels sprouts, protects against tumorigenesis in multiple organs. 3,3'-Diindolylmethane, one of the active products derived from Brassica vegetables, is a promising antitumor agent. Previous studies in our laboratory showed that 3,3'-diindolylmethane induced a G(1) cell cycle arrest in human breast cancer MCF-7 cells by a mechanism that included increased expression of p21. In the present study, the upstream events leading to p21 overexpression were further investigated. We show for the first time that 3,3'-diindolylmethane is a strong mitochondrial H(+)-ATPase inhibitor (IC(50) approximately 20 micromol/L). 3,3'-Diindolylmethane treatment induced hyperpolarization of mitochondrial inner membrane, decreased cellular ATP level, and significantly stimulated mitochondrial reactive oxygen species (ROS) production. ROS production, in turn, led to the activation of stress-activated pathways involving p38 and c-Jun NH(2)-terminal kinase. Using specific kinase inhibitors (SB203580 and SP600125), we showed the central role of p38 and c-Jun NH(2)-terminal kinase (JNK) pathways in 3,3'-diindolylmethane-induced p21 mRNA transcription. In addition, antioxidants significantly attenuated 3,3'-diindolylmethane-induced activation of p38 and JNK and induction of p21, indicating that oxidative stress is the major trigger of these events. To further support the role of ROS in 3,3'-diindolylmethane-induced p21 overexpression, we showed that 3,3'-diindolylmethane failed to induce p21 overexpression in mitochondrial respiratory chain deficient rho(0) MCF-7 cells, in which 3,3'-diindolylmethane did not stimulate ROS production. Thus, we have established the critical role of enhanced mitochondrial ROS release in 3,3'-diindolylmethane-induced p21 up-regulation in human breast cancer cells.

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

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

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.

Hyperoxic ventilated premature baboons have increased p53, oxidant DNA damage and decreased VEGF expression

Hyperoxia is implicated in the pathogenesis of bronchopulmonary dysplasia (BPD), a chronic lung disease of premature infants. High levels of supplemental oxygen can result in microvascular endothelial cell death and may disrupt lung development. In postnatal animals, hyperoxia inhibits expression of vascular endothelial growth factor (VEGF), which is required for normal vascular development. A potential mechanism of oxygen effects on VEGF is induction of p53, a transcription factor that represses VEGF gene transcription. Oxidant DNA damage can increase p53. We used a moderately premature baboon model of hyperoxia to examine p53, oxidant DNA damage, and VEGF expression. Fetal baboons delivered at 140 d of gestation (75% of term) were ventilated with 100% oxygen or oxygen as needed for 6 or 10 d. Lungs from the 10-d 100% oxygen animals had increased nuclear p53, compared with the oxygen as needed animals. The mechanism of increased p53 was probably related to oxidant DNA damage, which was documented by increased oxidized guanine. Dual fluorescent confocal microscopy found increased oxidized guanine in mitochondrial DNA of distal lung epithelial cells. Distal epithelial cell VEGF expression was decreased and p21, another downstream target of p53, was increased in the distal epithelium of the hyperoxic animals. These data show that p53 is induced in hyperoxic fetal lung epithelium and are consistent with p53 repression of VEGF expression in these cells. The findings suggest that oxidant DNA damage may be a mechanism of increased p53 in hyperoxic fetal lung.

Hyperoxia alters the expression and phosphorylation of multiple factors regulating translation initiation

Hyperoxia is cytotoxic and depresses many cellular metabolic functions including protein synthesis. Translational control is exerted primarily during initiation by two mechanisms: 1) through inhibition of translation initiation complex formation via sequestration of the cap-binding protein, eukaryotic initiation factor (eIF) 4E, with inhibitory 4E-binding proteins (4E-BP); and 2) by prevention of eIF2-GTP-tRNA(i)(Met) formation and eIF2B activity by phosphorylated eIF2alpha. In this report, exposure of human lung fibroblasts to 95% O2 decreased the incorporation of thymidine into DNA at 6 h and the incorporation of leucine into protein beginning at 12 h. The reductions in DNA and protein synthesis were accompanied by increased phosphorylation of eIF4E protein and reduced phosphorylation of 4E-BP1. At 24 h, hyperoxia shifted 4E-BP1 phosphorylation to lesser-phosphorylated isoforms, increased eIF4E expression, and increased the association of eIF4E with 4E-BP1. Although hyperoxia did not change eIF2alpha expression, it increased its phosphorylation at Ser51, but not until 48 h. In addition, the activation of eIF2alpha was not accompanied by the formation of stress granules. These findings suggest that hyperoxia diminishes protein synthesis by increasing eIF4E phosphorylation and enhancing the affinity of 4E-BP1 for eIF4E.

The Cdk and PCNA domains on p21Cip1 both function to inhibit G1/S progression during hyperoxia

This study investigates molecular mechanisms underlying cell cycle arrest when cells are exposed to high levels of oxygen (hyperoxia). Hyperoxia has previously been shown to increase expression of the cell cycle regulators p53 and p21. In the current study, we found that p53-deficient human lung adenocarcinoma H1299 cells failed to induce p21 or growth arrest in G(1) when exposed to 95% oxygen. Instead, cells arrested in S and G(2). Stable expression of p53 restored induction of p21 and G(1) arrest without affecting mRNA expression of the other Cip or INK4 G(1) kinase inhibitors. To confirm the role of p21 in G(1) arrest, we created H1299 cells with tetracycline-inducible expression of enhanced green fluorescent protein (EGFP), EGFP fused to p21 (EGFp21), or EGFP fused to p27 (EGFp27), a related cell cycle inhibitor. The amino terminus of p21 and p27 bind cyclin-dependent kinases (Cdk), whereas the carboxy terminus of p21 binds the sliding clamp proliferating cell nuclear antigen (PCNA). EGFp21 or EGFp27, but not EGFP by itself, restored G(1) arrest during hyperoxia. When separately overexpressed, the amino-terminal Cdk and carboxy-terminal PCNA binding domains of p21 each prevented cells from exiting G(1) during exposure. These findings demonstrate that exposure in vitro to hyperoxia exerts G(1) arrest through p53-dependent induction of p21 that suppresses Cdk and PCNA activity. Because PCNA also participates in DNA repair, these results raise the possibility that p21 also affects repair of oxidized DNA.

Increased apoptosis and expression of p21 and p53 in premature infant baboon model of bronchopulmonary dysplasia

Bronchopulmonary dysplasia (BPD) is a major complication of premature infants who receive prolonged ventilatory support. The pathophysiology of BPD involves oxidant injury, baro/volutrauma, and disordered lung repair. Exposure of premature lung that is poorly adapted for air breathing (>3% oxygen in fetal lung) to a higher concentration of oxygen can cause significant oxidant injury. Cell growth and differentiation of the developing lung require selective and ordered cell division. As hyperoxia can increase the expression of cell-cycle checkpoints that can cause growth arrest of lung cells, in this report we examined the expression of checkpoint proteins p53 and p21 in a premature infant the baboon model of BPD. Additionally, we also determined whether enhanced apoptosis occurs in baboon BPD model. We have shown that p53 and p21 expression are increased in 125-day as well as 140-day premature baboons with BPD. We also demonstrate increased apoptosis in lung tissue of premature baboons with BPD. These results demonstrate that cell growth inhibition is a likely factor in the evolution of BPD. Additionally, lung cells may undergo increased apoptosis that can impair the repair process in the postventilatory recovery period.

Hyperoxia-induced DNA damage causes decreased DNA methylation in human lung epithelial-like A549 cells

The effect of hyperoxia on levels of DNA damage and global DNA methylation was examined in lung epithelial-like A549 cells. DNA damage was assessed by the single-cell gel electrophoresis (comet assay) and DNA methylation status by the cytosine extension assays. Cells exposed to ionizing radiation (0, 1, 2, 4, or 8 Gy) showed increasing rates of percentage of DNA in the tail and tail length with increasing radiation dose. When cells were exposed to room air (normoxia) for 1 day and 95% O2 (hyperoxia) for 1, 2, 3, 4, and 5 days, data indicated that hyperoxia caused time-dependent increases in levels of (a) single strand breaks, (b) double strand breaks, and (c) 8-oxoguanine. Decreased DNA methylation also was observed at day 5 of hyperoxic exposure, suggesting that hyperoxia-induced DNA damage can influence patterns of DNA methylation in a lung-derived cell line.

Hyperoxia activates the ATR-Chk1 pathway and phosphorylates p53 at multiple sites

Hyperoxia has been shown to cause DNA damage resulting in growth arrest of cells in p53-dependent, as well as p53-independent, pathways. Although H2O2 and other peroxides have been shown to induce ataxia telangiectasia-mutated (ATM)-dependent p53 phosphorylation in response to DNA damage, the signal transduction mechanisms in response to hyperoxia are currently unknown. Here we demonstrate that hyperoxia phosphorylates the Ser15 residue of p53 independently of ATM. Hyperoxia phosphorylated p53 (Ser15) in DNA-dependent protein kinase null (DNA-PK-/-) cells, indicating that it may not depend on DNA-PK for phosphorylation of p53 (Ser15). We show that Ser37 and Ser392 residues of p53 are also phosphorylated in an ATM-independent manner in hyperoxia. In contrast, H2O2 did not phosphorylate Ser37 in either ATM+/+ or ATM-/- cells. Furthermore, H2O2 failed to phosphorylate Ser15 in ATM-/- cells. Additionally, overexpression of kinase-inactive ATM-and-Rad3-related (ATR) in HEK293T cells diminished Ser15, Ser37, and Ser392 phosphorylation compared with vector-only transfected cells. In contrast, wild-type ATR overexpression did not diminish Ser15, Ser37, or Ser392 phosphorylation. We also show that checkpoint kinase 1 (Chk1) is phosphorylated on Ser345 in response to hyperoxia, which could be inhibited by caffeine or wortmannin, potent inhibitors of phosphoinositide 3-kinase-related kinases. Hyperoxia also phosphorylated Chk1 in ATM+/+ as well as in ATM-/- cells, demonstrating an ATM-independent mechanism in Chk1 phosphorylation. Together, our data suggest that hyperoxia activates the ATR-Chk1 pathway and phosphorylates p53 at multiple sites in an ATM-independent manner, which is different from other forms of oxidative stress such as H2O2 or UV light.

Induced p21Cip1 in premature baboons with CLD: implications for alveolar hypoplasia

Aberrant pulmonary epithelial and mesenchymal cell proliferation occurs when newborns are treated with oxygen and ventilation to mitigate chronic lung disease. Because the cyclin-dependent kinase inhibitor p21 inhibits proliferation of oxygen-exposed cells, its expression was investigated in premature baboons delivered at 125 days (67% of term) and treated with oxygen and ventilation pro re nata (PRN) for 2, 6, 14, and 21 days. Approximately 5% of all cells expressed p21 during normal lung development of which <1% of these cells were pro-surfactant protein (SP)-B-positive epithelial cells. The percentage of cells expressing p21 increased threefold in all PRN-treated animals, but different cell populations expressed it during disease progression. Between 2 and 6 days of treatment, p21 was detected in 30-40% of pro-SP-B cells. In contrast, only 12% of pro-SP-B cells expressed p21 by 14 and 21 days of treatment, by which time p21 was also detected in mesenchymal cells. Even though increased epithelial and mesenchymal cell proliferation occurs during disease progression, those cells expressing p21 did not also express the proliferative marker Ki67. Thus two populations of epithelial and mesenchymal cells can be identified that are either expressing Ki67 and proliferating or expressing p21 and not proliferating. These data suggest that p21 may play a role in disorganized proliferation and alveolar hypoplasia seen in newborn chronic lung disease.

Signaling through Cdk2, importin-alpha and NuMA is required for H2O2-induced mitosis in primary type II pneumocytes

Proliferation of alveolar type II pneumocytes, the multipotent stem cells of the alveoli, has been implicated in the development of lung adenocarcinoma. Hydrogen peroxide (H(2)O(2)), a potent promoter of signaling cascades, can mediate the transmission of many intracellular signals including those involved in cell proliferation. In this study using rat primary type II pneumocytes, we demonstrate that H(2)O(2) significantly increases mitosis through a pathway that includes cyclin-dependent kinase 2 (Cdk2); importin-alpha, a nuclear trafficking regulator; and nuclear mitotic apparatus protein (NuMA), an essential component in mitotic spindle pole formation. Upon H(2)O(2) treatment, Cdk2 is phosphorylated at position thr-160 leading to increases in importin-alpha and NuMA protein levels and resulting in a significant increase of G(2)/M phase in a roscovatine-dependent manner. Type II pneumocytes transfected with NuMA cDNA also show significant increases in G(2)/M phase, NuMA, Cdk2 thr-160 and importin-alpha expression. These effects were prevented by catalase. These results demonstrate that H(2)O(2) orchestrates a complex signaling network regulating S phase entry, nuclear trafficking and spindle pole formation through activation of Cdk2, importin-alpha, and NuMA. This pathway is essential for H(2)O(2)-induced mitosis in type II pneumocytes.

Hyperoxia alters phorbol ester-induced phospholipase D activation in bovine lung microvascular endothelial cells

We investigated the effect of hyperoxia on phospholipase D (PLD) activation in bovine lung microvascular endothelial cells (BLMVECs). Generation of intracellular reactive oxygen species in BLMVECs exposed to hyperoxia for 2 or 24 h was three-fold higher compared with normoxic cells as measured by dichlorodihydrofluorescein di(acetoxymethyl ester) fluorescence. Exposure of BLMVECs to hyperoxia for 2 or 24 h attenuated 12-O-tetradecanoylphorbol 13-acetate (TPA)-mediated PLD activation compared with normoxic cells, however, hyperoxia did not alter basal PLD activity. Antioxidants, such as propyl gallate and pyrrolidine dithiocarbamate, reversed the effect of hyperoxia on TPA-induced PLD activity. Furthermore, the TPA-induced PLD activation was inhibited not only by the protein kinase C inhibitor, Go6976, but also by the tyrosine kinase inhibitor, genistein, and by the Src kinase specific inhibitor, PP-2, suggesting the involvement of protein kinase C and also tyrosine kinases in TPA-induced PLD activation. Western blot analysis of cell lysates from the hyperoxic (2 or 24 h) BLMVECs stimulated with TPA with anti-phosphotyrosine antibody showed an attenuation in overall tyrosine phosphorylation of proteins. In conclusion, we have demonstrated that hyperoxia enhanced the generation of reactive oxygen species in lung microvascular endothelial cells and attenuated TPA-induced protein tyrosine phosphorylation and PLD activation. As protein tyrosine phosphorylation and PLD play important roles in inflammatory responses, this could provide a mechanism for the regulation of endothelial barrier function during hyperoxic lung injury.

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.

Increased epithelial cell proliferation in very premature baboons with chronic lung disease

Coordinated proliferation of lung cells is required for normal lung growth and differentiation. Chronic injury to developing lung may disrupt normal patterns of cell proliferation. To examine patterns of cell proliferation in injured developing lungs, we investigated premature baboons delivered at 125 days gestation (approximately 67% of term) and treated with oxygen and ventilation for 6, 14, or 21 days (PRN). Each PRN treatment group contained 3 or 4 animals. During normal in utero lung development, the proportion of proliferating lung cells declined as measured by the cell-cycle marker Ki67. In the PRN group, the proportion of proliferating lung cells was 2.5-8.5-fold greater than in corresponding gestational controls. By 14 days of treatment, the proportion of cells that expressed pro-surfactant protein B (proSP-B) was ~2.5-fold greater than in gestational controls. In the PRN group, 41% of proliferating cells expressed proSP-B compared with 5.8% in the gestational controls. By 21 days of treatment, proliferation of proSP-B-expressing epithelial cells declined substantially, but the proportion of proliferating non-proSP-B-expressing cells increased approximately sevenfold. These data show that the development of chronic lung disease is associated with major alterations in normal patterns of lung-cell proliferation.

Cigarette smoke-induced oxidative stress and TGF-beta1 increase p21waf1/cip1 expression in alveolar epithelial cells

Sustained oxidative stress caused by cigarette smoking induces a chronic inflammatory response, resulting in the destruction of the alveolar cell wall characteristic of emphysema. The loss of tissue may involve the progressive depletion of epithelial cells through inhibition of proliferation leading to cell death. The cell cycle regulator p21(waf1/cip1) acts as a G(1)/S and G(2)/M phase checkpoint regulator. We hypothesize that cigarette smoke-induced oxidative stress and transforming growth factor beta 1 (TGF-beta(1)) may inhibit cellular proliferation by p21(waf1/cip1) in type II alveolar epithelial cells (A549). A significant increase was observed in p21(waf1/cip1) mRNA expression in A549 cells by cigarette smoke condensate, H(2)O(2), and TGF-beta(1). In conclusion, cigarette smoke-induced oxidative stress and TGF-beta(1) modulate expression of the cell cycle inhibitor p21(waf1/cip1). This may be important in the growth arrest and cell survival of alveolar type II cells in the G(1) phase.

Normal remodeling of the oxygen-injured lung requires the cyclin-dependent kinase inhibitor p21(Cip1/WAF1/Sdi1)

Alveolar cells of the lung are injured and killed when exposed to elevated levels of inspired oxygen. Damaged tissue architecture and pulmonary function is restored during recovery in room air as endothelial and type II epithelial cells proliferate. Although excessive fibroblast proliferation and inflammation occur when abnormal remodeling occurs, genes that regulate repair remain unknown. Our recent observation that hyperoxia inhibits proliferation through induction of the cyclin-dependent kinase inhibitor p21(Cip1/WAF1/Sdi1), which also facilitates DNA repair, suggested that p21 may participate in remodeling. This hypothesis was tested in p21-wild-type and -deficient mice exposed to 100% FiO(2) and recovered in room air. p21 increased during hyperoxia, remained elevated after 1 day of recovery before returning to unexposed levels. Increased proliferation occurred when p21 expression decreased. In contrast, higher and sustained levels of proliferation, resulting in myofibroblast hyperplasia and monocytic inflammation, occurred in recovered p21-deficient lungs. Cells with DNA strand breaks and expressing p53 were observed in hyperplastic regions suggesting that DNA integrity had not been restored. Normal recovery of endothelial and type II epithelial cells, as assessed by expression of cell-type-specific genes was also delayed in p21-deficient lungs. These results reveal that p21 is required for remodeling the oxygen-injured lung and suggest that failure to limit replication of damaged DNA may lead to cell death, inflammation, and abnormal remodeling. This observation has important implications for therapeutic strategies designed to attenuate long-term chronic lung disease after oxidant injury.

Regulatory roles of p21 and apurinic/apyrimidinic endonuclease 1 in base excision repair

Many types of DNA damage induce a cellular response that inhibits replication but allows repair by up-regulating the p53 pathway and inducing p21(Cip1, Waf1, Sdi1). The p21 regulatory protein can bind proliferating cell nuclear antigen (PCNA) and prohibit DNA replication. We show here that p21 also inhibits PCNA stimulation of long patch base excision repair (BER) in vitro. p21 disrupts PCNA-directed stimulation of flap endonuclease 1 (FEN1), DNA ligase I, and DNA polymerase delta. The dilemma is to understand how p21 prevents DNA replication but allows BER in vivo. Differential regulation by p21 is likely to relate to the utilization of DNA polymerase beta, which is not sensitive to p21, in the repair pathway. We have also found that apurinic/apyrimidinic endonuclease 1 (APE1) stimulates long patch BER. Furthermore, neither APE1 activity nor its ability to stimulate long patch BER is significantly affected by p21 in vitro. We propose that APE1 serves as an assembly and coordination factor for long patch BER proteins. APE1 initially cleaves the DNA and then facilitates the sequential binding and catalysis by DNA polymerase beta, DNA polymerase delta, FEN1, and DNA ligase I. This model implies that BER can be regulated differentially, based upon the assembly of relevant proteins around APE1 in the presence or absence of PCNA.

Role of cyclins in epithelial response to oxidants

Oxidants are involved in a large variety of pulmonary diseases. Among the various cell types that compose the respiratory system, the epithelial cells appear to be a major target for oxidative stress. When cells are exposed to DNA-damaging agents such as oxidants, a feedback control is activated that acts as a brake on the cell cycle to inhibit entry into the S phase until DNA repair is completed. Progression through the G1 phase and the G1-S transition involves sequential assembly and activation of key regulators of the cell cycle machinery, the cyclin-dependent kinases (CDKs). Activity of the CDKs is regulated by several mechanisms, which include the CDK inhibitors (CKIs). The CKI p21(CIP1) appears to play an important role in the response of epithelial cells to oxidants.

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.