Growth arrest in G1 protects against oxygen-induced DNA damage and cell death

Elevated level of PLRG1 is critical for the proliferation and maintenance of genome stability of tumor cells

Pleiotropic regulator 1 (PLRG1), a highly conserved element in the spliceosome, can form a NineTeen Complex (NTC) with Prp19, SPF27, and CDC5L. This complex plays crucial roles in both pre-mRNA splicing and DNA repair processes. Here, we provide evidence that PLRG1 has a multifaceted impact on cancer cell proliferation. Comparing its expression levels in cancer and normal cells, we observed that PLRG1 was upregulated in various tumor tissues and cell lines. Knockdown of PLRG1 resulted in tumor-specific cell death. Depletion of PLRG1 had notable effects, including mitotic arrest, microtubule instability, endoplasmic reticulum (ER) stress, and accumulation of autophagy, ultimately culminating in apoptosis. Our results also demonstrated that PLRG1 downregulation contributed to DNA damage in cancer cells, which we confirmed through experimental validation as DNA repair impairment. Interestingly, when PLRG1 was decreased in normal cells, it induced G1 arrest as a self-protective mechanism, distinguishing it from effects observed in cancer cells. These results highlight multifaceted impacts of PLRG1 in cancer and underscore its potential as a novel anti-cancer strategy by selectively targeting cancer cells. [BMB Reports 2023; 56(11): 612-617].

Antioxidant Effect of Chrysanthemum morifolium (Chuju) Extract on H2O2-Treated L-O2 Cells as Revealed by LC/MS-Based Metabolic Profiling

Chrysanthemum has a long history of being used to attenuate various oxidative stress-related discomforts and diseases; however, its mechanisms remain unclear. In this study, the antioxidant effect of chrysanthemum aqueous extract was investigated, and the potential mechanisms were explored via a metabolomics study. Chrysanthemum extract could significantly inhibit hydrogen peroxide (H₂O₂)-mediated cell death in L-O2 hepatocytes. Propidium iodide staining and annexin V-PI dual staining revealed that the antioxidant effect of chrysanthemum extract was related to the relief of cell cycle arrest and inhibition of non-apoptotic cell damage. The activities of antioxidant enzymes including superoxide dismutase (SOD), catalase (CAT) and glutathione peroxidase (GSH-Px) were also upregulated by chrysanthemum extract. Through metabolomics studies, it was found that chrysanthemum extract mainly targeted the arginine synthesis pathway and purine metabolism pathway, in which antioxidation-related endogenous substrates including L-arginosuccinate, citrulline and inositol monophosphate were significantly upregulated by chrysanthemum extract. These results indicated that chrysanthemum extract can antagonize oxidative stress through multiple pathways and have potential therapeutic applications.

Methanolic Extract from Sea Cucumber, Holothuria scabra, Induces Apoptosis and Suppresses Metastasis of PC3 Prostate Cancer Cells Modulated by MAPK Signaling Pathway

Sea cucumber, Holothuria scabra, is a well-known traditional Asian medicine that has been used for suppressing inflammation, promoting wound healing, and improving immunity. Moreover, previous studies demonstrated that the extract from H. scabra contains many bioactive compounds with potent inhibitory effect on tumor cell survival and progression. However, the effect of the methanolic extract from the body wall of H. scabra (BWMT) on human prostate cancer cells has not yet been investigated. In this study, we aimed to investigate the effects and underlying mechanism of BWMT on prostate cancer cell viability and metastasis. BWMT was obtained by maceration with methanol. The effect of BWMT on cell viability was assessed by MTT and colony formation assays. The intracellular ROS accumulation was evaluated using a DCFH-DA fluorescence probe. Hoechst 33342 staining and Annexin V-FITC/PI staining were used to examine the apoptotic-inducing effect of the extract. A transwell migration assay was performed to determine the anti-metastasis effect. BWMT significantly reduced cell viability and triggered cellular apoptosis by accumulating intracellular ROS resulting in the upregulation of JNK and p38 signaling pathways. In addition, BWMT also inhibited the invasion of PC3 cells by downregulating MMP-2/-9 expression via the ERK pathway. Consequently, our study provides BWMT from H. scabra as a putative therapeutic agent that could be applicable against prostate cancer progression.

Hyperoxia-induced Cellular Senescence in Fetal Airway Smooth Muscle Cells

Supplemental O₂ (hyperoxia; 30-90% O₂) is a necessary intervention for premature infants, but it contributes to development of neonatal and pediatric asthma, necessitating better understanding of contributory mechanisms in hyperoxia-induced changes to airway structure and function. In adults, environmental stressors promote formation of senescent cells that secrete factors (senescence-associated secretory phenotype), which can be inflammatory and have paracrine effects that enhance chronic lung diseases. Hyperoxia-induced changes in airway structure and function are mediated in part by effects on airway smooth muscle (ASM). In the present study, using human fetal ASM cells as a model of prematurity, we ascertained the effects of clinically relevant moderate hyperoxia (40% O₂) on cellular senescence. Fetal ASM exposed to 40% O₂ for 7 days exhibited elevated concentrations of senescence-associated markers, including β-galactosidase; cell cycle checkpoint proteins p16, p21, and p-p53; and the DNA damage marker p-γH2A.X (phosphorylated γ-histone family member X). The combination of dasatinib and quercetin, compounds known to eliminate senescent cells (senolytics), reduced the number of hyperoxia-exposed β-galactosidase-, p21-, p16-, and p-γH2A.X-positive ASM cells. The senescence-associated secretory phenotype profile of hyperoxia-exposed cells included both profibrotic and proinflammatory mediators. Naive ASM exposed to media from hyperoxia-exposed senescent cells exhibited increased collagen and fibronectin and higher contractility. Our data show that induction of cellular senescence by hyperoxia leads to secretion of inflammatory factors and has a functional effect on naive ASM. Cellular senescence in the airway may thus contribute to pediatric airway disease in the context of sequelae of preterm birth.

Cell-cell contacts protect against t-BuOOH-induced cellular damage and ferroptosis in vitro

Ferroptosis is a recently discovered pathway of regulated necrosis dependent on iron and lipid peroxidation. It has gained broad attention since it is a promising approach to overcome resistance to apoptosis in cancer chemotherapy. We have recently identified tertiary-butyl hydroperoxide (t-BuOOH) as a novel inducer of ferroptosis. t-BuOOH is a widely used compound to induce oxidative stress in vitro. t-BuOOH induces lipid peroxidation and consequently ferroptosis in murine and human cell lines. t-BuOOH additionally results in a loss of mitochondrial membrane potential, formation of DNA double-strand breaks, and replication block. Here, we specifically address the question whether cell-cell contacts regulate t-BuOOH-induced ferroptosis and cellular damage. To this end, murine NIH3T3 or human HaCaT cells were seeded to confluence, but below their saturation density to allow the establishment of cell-cell contacts without inducing quiescence. Cells were then treated with t-BuOOH (50 or 200 µM, respectively). We revealed that cell-cell contacts reduce basal and t-BuOOH-triggered lipid peroxidation and consequently block ferroptosis. Similar results were obtained with the specific ferroptosis inducer erastin. Cell-cell contacts further protect against t-BuOOH-induced loss of mitochondrial membrane potential, and formation of DNA double-strand breaks. Interestingly, cell-cell contacts failed to prevent t-BuOOH-mediated replication block or formation of the oxidative base lesion 8-oxo-dG. Since evidence of protection against cell death was both (i) observed after treatment with hydrogen peroxide, methyl methanesulfonate or UV-C, and (ii) seen in several cell lines, we conclude that protection by cell-cell contacts is a widespread phenomenon. The impact of cell-cell contacts on toxicity might have important implications in cancer chemotherapy.

SMG-1 kinase attenuates mitochondrial ROS production but not cell respiration deficits during hyperoxia

PURPOSE

Supplemental oxygen (hyperoxia) used to treat individuals in respiratory distress causes cell injury by enhancing the production of toxic reactive oxygen species (ROS) and inhibiting mitochondrial respiration. The suppressor of morphogenesis of genitalia (SMG-1) kinase is activated during hyperoxia and promotes cell survival by phosphorylating the tumor suppressor p53 on serine 15. Here, we investigate whether SMG-1 and p53 blunt this vicious cycle of progressive ROS production and decline in mitochondrial respiration seen during hyperoxia.

MATERIALS AND METHODS

Human lung adenocarcinoma A549 and H1299 or colon carcinoma HCT116 cells were depleted of SMG-1, UPF-1, or p53 using RNA interference, and then exposed to room air (21% oxygen) or hyperoxia (95% oxygen). Immunoblotting was used to evaluate protein expression; a Seahorse Bioanalyzer was used to assess cellular respiration; and flow cytometry was used to evaluate fluorescence intensity of cells stained with mitochondrial or redox sensitive dyes.

RESULTS

Hyperoxia increased mitochondrial and cytoplasmic ROS and suppressed mitochondrial respiration without changing mitochondrial mass or membrane potential. Depletion of SMG-1 or its cofactor, UPF1, significantly enhanced hyperoxia-induced mitochondrial but not cytosolic ROS abundance. They did not affect mitochondrial mass, membrane potential, or hyperoxia-induced deficits in mitochondrial respiration. Genetic depletion of p53 in A549 cells and ablation of the p53 gene in H1299 or HCT116 cells revealed that SMG-1 influences mitochondrial ROS through activation of p53.

CONCLUSIONS

Our findings show that hyperoxia does not promote a vicious cycle of progressive mitochondrial ROS and dysfunction because SMG-1-p53 signaling attenuates production of mitochondrial ROS without preserving respiration. This suggests antioxidant therapies that blunt ROS production during hyperoxia may not suffice to restore cellular respiration.

Role of FEN1 S187 phosphorylation in counteracting oxygen-induced stress and regulating postnatal heart development

Flap endonuclease 1 (FEN1) phosphorylation is proposed to regulate the action of FEN1 in DNA repair as well as Okazaki fragment maturation. However, the biologic significance of FEN1 phosphorylation in response to DNA damage remains unknown. Here, we report an in vivo role for FEN1 phosphorylation, using a mouse line carrying S187A FEN1, which abolishes FEN1 phosphorylation. Although S187A mouse embryonic fibroblast cells showed normal proliferation under low oxygen levels (2%), the mutant cells accumulated oxidative DNA damage, activated DNA damage checkpoints, and showed G₁-phase arrest at atmospheric oxygen levels (21%). This suggests an essential role for FEN1 phosphorylation in repairing oxygen-induced DNA damage and maintaining proper cell cycle progression. Consistently, the mutant cardiomyocytes showed G₁-phase arrest due to activation of the p53-mediated DNA damage response at the neonatal stage, which reduces the proliferation potential of the cardiomyocytes and impairs heart development. Nearly 50% of newborns with the S187A mutant died in the first week due to failure to undergo the peroxisome proliferator-activated receptor signaling-dependent switch from glycolysis to fatty acid oxidation. The adult mutant mice developed dilated hearts and showed significantly shorter life spans. Altogether, our results reveal an important role of FEN1 phosphorylation to counteract oxygen-induced stress in the heart during the fetal-to-neonatal transition.-Zhou, L., Dai, H., Wu, J., Zhou, M., Yuan, H., Du, J., Yang, L., Wu, X., Xu, H., Hua, Y., Xu, J., Zheng, L., Shen, B. Role of FEN1 S187 phosphorylation in counteracting oxygen-induced stress and regulating postnatal heart development.

TGF-β1 accelerates the DNA damage response in epithelial cells via Smad signaling

The evidence suggests that transforming growth factor-beta (TGF-β) regulates the DNA-damage response (DDR) upon irradiation, and we previously reported that TGF-β1 induced DNA ligase IV (Lig4) expression and enhanced the nonhomologous end-joining repair pathway in irradiated cells. In the present study, we investigated the effects of TGF-β1 on the irradiation-induced DDRs of A431 and HaCaT cells. Cells were pretreated with or without TGF-β1 and irradiated. At 30 min post-irradiation, DDRs were detected by immunoblotting of phospho-ATM, phospho-Chk2, and the presence of histone foci (γH2AX). The levels of all three factors were similar right after irradiation regardless of TGF-β1 pretreatment. However, they soon thereafter exhibited downregulation in TGF-β1-pretreated cells, indicating the acceleration of the DDR. Treatment with a TGF-β type I receptor inhibitor (SB431542) or transfections with siRNAs against Smad2/3 or DNA ligase IV (Lig4) reversed this acceleration of the DDR. Furthermore, the frequency of irradiation-induced apoptosis was decreased by TGF-β1 pretreatment in vivo, but this effect was abrogated by SB431542. These results collectively suggest that TGF-β1 could enhance cell survival by accelerating the DDR via Smad signaling and Lig4 expression.

Impact of breathing 100% oxygen on radiation-induced cognitive impairment

Future space missions are expected to include increased extravehicular activities (EVAs) during which astronauts are exposed to high-energy space radiation while breathing 100% oxygen. Given that brain irradiation can lead to cognitive impairment, and that oxygen is a potent radiosensitizer, there is a concern that astronauts may be at greater risk of developing cognitive impairment when exposed to space radiation while breathing 100% O(2) during an EVA. To address this concern, unanesthetized, unrestrained, young adult male Fischer 344 × Brown Norway rats were allowed to breathe 100% O(2) for 30 min prior to, during and 2 h after whole-body irradiation with 0, 1, 3, 5 or 7 Gy doses of 18 MV X rays delivered from a medical linear accelerator at a dose rate of ~425 mGy/min. Irradiated and unirradiated rats breathing air (~21% O(2)) served as controls. Cognitive function was assessed 9 months postirradiation using the perirhinal cortex-dependent novel object recognition task. Cognitive function was not impaired until the rats breathing either air or 100% O(2) received a whole-body dose of 7 Gy. However, at all doses, cognitive function of the irradiated rats breathing 100% O(2) was improved over that of the irradiated rats breathing air. These data suggest that astronauts are not at greater risk of developing cognitive impairment when exposed to space radiation while breathing 100% O(2) during an EVA.

TGFβ1 protects cells from γ-IR by enhancing the activity of the NHEJ repair pathway

Several groups have reported that TGFβ1 regulates cellular responses to γ-irradiation; however, the exact mechanism has not been fully elucidated. In the current study, the role of TGFβ1 in cellular responses to γ-irradiation was investigated in detail. The data indicate that TGFβ1 pretreatment decreased the aftermath of ionizing radiation (IR)-induced DNA damage in a SMAD-dependent manner. To determine the underlying mechanism for these effects, the extent of IR-induced DNA repair activity in the presence or absence of TGFβ1 was examined. Studies reveal that TGFβ1 upregulated DNA ligase IV (Lig4), augmented IR-induced nuclear retention of the DNA ligase, and enhanced nonhomologous end-joining (NHEJ) repair activity. In addition, knockdown of Lig4 reduced the TGFβ1-induced protection against IR. Overall, these data indicate that TGFβ1 facilitates the NHEJ repair process upon γ-irradiation and thereby enhances long-term survival.

IMPLICATIONS

These findings provide new insight and a possible approach to controlling genotoxic stress by the TGFβ signaling pathway.

Neonatal hyperoxia stimulates the expansion of alveolar epithelial type II cells

Supplemental oxygen used to treat infants born prematurely disrupts angiogenesis and is a risk factor for persistent pulmonary disease later in life. Although it is unclear how neonatal oxygen affects development of the respiratory epithelium, alveolar simplification and depletion of type II cells has been observed in adult mice exposed to hyperoxia between postnatal Days 0 and 4. Because hyperoxia inhibits cell proliferation, we hypothesized that it depleted the adult lung of type II cells by inhibiting their proliferation at birth. Newborn mice were exposed to room air (RA) or hyperoxia, and the oxygen-exposed mice were recovered in RA. Hyperoxia stimulated mRNA expressed by type II (Sftpc, Abca3) and type I (T1α, Aquaporin 5) cells and inhibited Pecam expressed by endothelial cells. 5-Bromo-2'-deoxyuridine labeling and fate mapping with enhanced green fluorescence protein controlled statically by the Sftpc promoter or conditionally by the Scgb1a1 promoter revealed increased Sftpc and Abca3 mRNA seen on Day 4 reflected an increase in expansion of type II cells shortly after birth. When mice were returned to RA, this expanded population of type II cells was slowly depleted until few were detected by 8 weeks. These findings reveal that hyperoxia stimulates alveolar epithelial cell expansion when it disrupts angiogenesis. The loss of type II cells during recovery in RA may contribute to persistent pulmonary diseases such as those reported in children born preterm who were exposed to supplemental oxygen.

Lipid-based signaling modulates DNA repair response and survival against Klebsiella pneumoniae infection in host cells and in mice

Klebsiella pneumoniae causes serious infections in the urinary tract, respiratory tract, and blood. Lipid rafts, also known as membrane microdomains, have been linked to the pathogenesis of bacterial infection. However, whether lipid rafts affect K. pneumoniae internalization into host cells remains unknown. Here, we show for the first time that K. pneumoniae was internalized into lung cells by activating lipid rafts. Disrupting lipid rafts by methyl-β-cyclodextrin inhibited pathogen internalization, impairing host defense. A deficient mutant of capsule polysaccharide (CPS) showed a higher internalization rate than a wild-type strain, indicating that CPS may inhibit bacterial entry to host cells. Furthermore, lipid rafts may affect the function of extracellular regulated kinase (ERK)-1/2, and knocking down ERK1/2 via short, interfering RNA increased apoptosis in both alveolar macrophages and epithelial cells after infection. To gain insights into bacterial pathogenesis, we evaluated the impact of lipid rafts on DNA integrity, and showed that raft aggregates also affect DNA damage and DNA repair responses (i.e., 8-oxoguanine DNA glycosylase [Ogg1]) through the regulation of reactive oxygen species. Importantly, cells overexpressing Ogg1 demonstrated reduced cytotoxicity during bacterial infection. Taken together, these results suggest that lipid rafts may modulate bacterial internalization, thereby affecting DNA damage and repair, which is critical to host defense against K. pneumoniae.

TGF-β signaling plays an important role in resisting γ-irradiation

Transforming growth factor-β1 (TGF-β1) regulates various biological processes, including differentiation, bone remodeling and angiogenesis, and is particularly important as a regulator of homeostasis and cell growth in normal tissue. Interestingly, some studies have reported that TGF-β1 induces apoptosis through induction of specific genes, whereas others suggest that TGF-β1 inhibits apoptosis and facilitates cell survival. Resolving these discrepancies, which may reflect differences in cellular context, is an important research priority. Here, using the parental mink lung epithelial cell line, Mv1Lu, and its derivatives, R1B and DR26, lacking TGF-β receptors, we investigated the involvement of TGF-β signaling in the effects of γ-irradiation. We found that canonical TGF-β signaling played an important role in protecting cells from γ-irradiation. Introduction of functional TGF-β receptors or constitutively active Smads into R1B and DR26 cell lines reduced DNA fragmentation, Caspase-3 cleavage and γ-H2AX foci formation in γ-irradiated cells. Notably, we also found that de novo protein synthesis was required for the radio-resistant effects of TGF-β1. Our data thus indicate that TGF-β1 protected against γ-irradiation, decreasing DNA damage and reducing apoptosis, and thereby enhanced cell survival.

The RNA surveillance protein SMG1 activates p53 in response to DNA double-strand breaks but not exogenously oxidized mRNA

DNA damage, stalled replication forks, errors in mRNA splicing, and availability of nutrients activate specific phosphatidylinositiol-3 kinase-like kinases (PIKKs) that in turn phosphorylate downstream targets such as p53 on serine 15. While the PIKK proteins ATM and ATR respond to specific DNA lesions, SMG1 responds to errors in mRNA splicing and when cells are exposed to genotoxic stress. Yet, whether genotoxic stress activates SMG1 through specific types of DNA lesions or RNA damage remains poorly understood. Here, we demonstrate that siRNA oligonucleotides targeting the mRNA surveillance proteins SMG1, Upf1, Upf2, or the PIKK protein ATM attenuated p53 (ser15) phosphorylation in cells damaged by high oxygen (hyperoxia), a model of persistent oxidative stress that damages nucleotides. In contrast, loss of SMG1 or ATM, but not Upf1 or Upf2 reduced p53 (ser15) phosphorylation in response to DNA double strand breaks produced by expression of the endonuclease I-PpoI. To determine whether SMG1-dependent activation of p53 was in response to oxidative mRNA damage, mRNA encoding green fluorescence protein (GFP) transcribed in vitro was oxidized by Fenton chemistry and transfected into cells. Although oxidation of GFP mRNA resulted in dose-dependent fragmentation of the mRNA and reduced expression of GFP, it did not stimulate p53 or the p53-target gene p21. These findings establish SMG1 activates p53 in response to DNA double-strand breaks independent of the RNA surveillance proteins Upf1 or Upf2; however, these proteins can stimulate p53 in response to oxidative stress but not necessarily oxidized RNA.

The depletion of DNA methyltransferase-1 and the epigenetic effects of 5-aza-2'deoxycytidine (decitabine) are differentially regulated by cell cycle progression

5-Aza-2'-deoxycytidine (decitabine) is a drug targeting the epigenetic abnormalities of tumors. The basis for its limited efficacy in solid tumors is unresolved, but may relate to their indolent growth, their p53 genotype or both. We report that the primary molecular mechanism of decitabine-depletion of DNA methyltransferase-1 following its "suicide" inactivation-is not absolutely associated with cell cycle progression in HCT 116 colon cancer cells, but is associated with their p53 genotype. Control experiments affirmed that the secondary molecular effects of decitabine on global and promoter-specific CpG methylation and MAGE-A1 mRNA expression were S-phase dependent, as expected. Secondary changes in CpG methylation occurred only in growing cells ~24-48 h after decitabine treatment; these epigenetic changes coincided with p53 accumulation, an index of DNA damage. Conversely, primary depletion of DNA methyltransferase-1 began immediately after a single exposure to 300 nM decitabine and it progressed to completion within ~8 h, even in confluent cells arrested in G 1 and G 2/M. Our results suggest that DNA repair and remodeling activity in arrested, confluent cells may be sufficient to support the primary molecular action of decitabine, while its secondary, epigenetic effects require cell cycle progression through S-phase.

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 oxygen, growth state, and senescence on the antioxidant responses of WI-38 fibroblasts

Mitotically active, growth-arrested cells and proliferatively senescent cultures of human fetal lung fibroblasts (WI-38) were exposed to six different oxygen tensions for various lengths of time and then analyzed to determine the responses of their antioxidant defense system. Glutathione (GSH) concentration increased as a function of ambient oxygen tension in early passage cultures; the effect was larger in exponentially growing cultures than in those in a state of contact-inhibited growth arrest, but was absent in senescent cells. Conversely, the activity of glutathione disulfide reductase was greater in growth-arrested cultures than in mitotically active cells irrespective of oxygen tension. Glucose-6-phosphate dehydrogenase was lowest in log-phase cells exposed to different oxygen tensions for 24 h and in senescent cells. Both hypoxia and hyperoxia depressed selenium-dependent glutathione peroxidase activity in early passage cultures, while the activity of the enzyme progressively declined with increasing oxygen in senescent cells. The GSH S-transferase activity was unresponsive to changes in ambient oxygen tension in either young or senescent cultures. Manganese-containing superoxide dismutase (MnSOD) activity was unaffected by oxygen tension, but was elevated in young confluent cultures as compared with cultures in log-phase growth. MnSOD activity was significantly higher in senescent cultures than in early passage cultures and was also responsive to increased oxygen tension in senescent cultures. Copper-zinc-containing superoxide dismutases activity was not affected by oxygen tension or the passage of time, but it declined in senescent cultures.

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.

Helicobacter pylori accelerates hepatic fibrosis by sensitizing transforming growth factor-β1-induced inflammatory signaling

Our earlier report has shown that Helicobacter pylori promoted hepatic fibrosis in a murine model. Herein, in order to elucidate the mechanism by which H. pylori accelerate liver fibrosis, the authors investigated the changes in expression levels of mitogen-activated protein kinases (MAPKs), p53-related proteins, antioxidants, and proinflammatory cytokines in liver samples. H. pylori infection enhanced CCl4-induced MAP kinase activation and p53 signaling pathway as well as Bax- and proliferating-cell nuclear antigen expressions, whereas H. pylori alone induced neither of these expressions nor hepatic fibrosis. Moreover, mRNA expressions of inflammatory cytokines, glutathione peroxidase expression, and the proliferative index were strongly augmented in livers of the H. pylori with CCl4 treatment group compared with those of the CCl4-alone treatment group, whereas there was no difference in apoptotic index between the two groups. Interestingly, H. pylori treatment increased the number of α-fetoprotein-expressing hepatocytes independently of CCl4 intoxication. In vitro analyses, using an immortalized rat hepatic stellate cell (HSC) line, revealed that H. pylori lysates increased the proliferation of HSCs, which was boosted by the addition of transforming growth factor-beta1 (TGF-β1). Furthermore, the treatment of H. pylori lysates promoted the translocation of nuclear factor kappa-light-chain enhancer of activated B cells (NF-κB) into the nucleus based on an increase in the degradation of NF-κB inhibitor alpha, in the presence of TGF-β1, as did H2O2 treatment. In conclusion, H. pylori infection along with an elevated TGF-β1 may accelerate hepatic fibrosis through increased TGF-β1-induced pro-inflammatory signaling pathways in HSCs. Moreover, H. pylori infection might increase the risk of TGF-β1-mediated tumorigenesis by disturbing the balance between apoptosis and proliferation of hepatocytes.

Opuntia humifusa partitioned extracts inhibit the growth of U87MG human glioblastoma cells

Opuntia humifusa, a member of the Cactaceae family widely distributed in the southern regions of the Korean peninsula, has potential bioactive functions and medicinal benefits. In the present study, we investigated the effect of hexane, ethyl acetate extracts and water partitioned fraction of O. humifusa on proliferation, G1 arrest and apoptosis in U87MG human glioblastoma cells. Glioblastoma cellular proliferation was evaluated using the MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) assay, and the effects of O. humifusa partitioned extracts on cell cycle and apoptosis were analyzed by flow cytometry. Our results revealed that when U87MG cells were treated with hexane extracts and water partitioned fraction of O. humifusa, the number of viable cells decreased in a concentration-dependent manner. In addition, water partitioned fractions of O. humifusa induced G1 arrest and non-apoptotic cell death as well as significant increases in ROS production in U87MG cells. In conclusion, water partitioned fractions of O. humifusa induce G1 arrest and inhibit U87MG human glioblastoma cell proliferation.

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.

TGF-beta signaling promotes survival and repair in rat alveolar epithelial type 2 cells during recovery after hyperoxic injury

Hyperoxic rats treated with inosine during oxygen exposure have increased levels of active transforming growth factor (TGF)-beta in the bronchoalveolar lavage (BAL), yet alveolar epithelial type 2 cells (AEC2) isolated from these animals demonstrate less hyperoxia-induced DNA damage and increased expression of active Smad2. To determine whether TGF-beta1 signaling per se protected AEC2 against hyperoxic damage, freshly isolated AEC2 from hyperoxic rats were incubated with TGF-beta1 for 24 h and assayed for DNA damage by fluorescein-activated cell sorter analysis of TdT-mediated dUTP nick end labeling. TGF-beta1 was protective over a concentration range similar to that in BAL of inosine-treated hyperoxic animals (50-5,000 pg/ml). TGF-beta1 also augmented hyperoxia-induced DNA repair activity and cell migration, stimulated autocrine secretion of fibronectin, accelerated closure of a monolayer scratch wound, and restored hyperoxia-depleted VEGF secretion by AEC2 to normoxic levels. The TGF-beta receptor type I activin-like kinase-4, -5, and -7 inhibitor peptide SB-505124 abolished the protective effect of TGF-beta on hyperoxic DNA damage and increased TdT-mediated dUTP nick end labeling in normoxic cells. These data suggest that endogenous TGF-beta-mediated Smad signaling is required for AEC2 homeostasis in vitro, while exogenous TGF-beta1 treatment of hyperoxia-damaged AEC2 results in a cell that is equipped to survive, repair, migrate, secrete matrix, and induce new blood vessel formation more efficiently than AEC2 primed by hyperoxia alone.

The insulin-like growth factor-I-mTOR signaling pathway induces the mitochondrial pyrimidine nucleotide carrier to promote cell growth

The insulin/insulin-like growth factor (IGF) signaling pathway to mTOR is essential for the survival and growth of normal cells and also contributes to the genesis and progression of cancer. This signaling pathway is linked with regulation of mitochondrial function, but how is incompletely understood. Here we show that IGF-I and insulin induce rapid transcription of the mitochondrial pyrimidine nucleotide carrier PNC1, which shares significant identity with the essential yeast mitochondrial carrier Rim2p. PNC1 expression is dependent on PI-3 kinase and mTOR activity and is higher in transformed fibroblasts, cancer cell lines, and primary prostate cancers than in normal tissues. Overexpression of PNC1 enhances cell size, whereas suppression of PNC1 expression causes reduced cell size and retarded cell cycle progression and proliferation. Cells with reduced PNC1 expression have reduced mitochondrial UTP levels, but while mitochondrial membrane potential and cellular ATP are not altered, cellular ROS levels are increased. Overall the data indicate that PNC1 is a target of the IGF-I/mTOR pathway that is essential for mitochondrial activity in regulating cell growth and proliferation.

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.

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

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

Norepinephrine-induced oxidative stress causes PC-12 cell apoptosis by both endoplasmic reticulum stress and mitochondrial intrinsic pathway: inhibition of phosphatidylinositol 3-kinase survival pathway

Norepinephrine (NE) induces endoplasmic reticulum (ER) unfolded protein response and reduces maturation and translocation of NE transporter to cell membrane via enhanced formation of reactive oxygen species in PC-12 cells. In the present study, we investigated whether ER stress is also implicated in the proapoptotic effect of NE. We found that the apoptotic effect of NE was associated with increased processing of ER-resident pro-caspase-12, cleavage of caspase-9 and -3, and mitochondrial release of cytochrome c. ER stress was evidenced by upregulation of ER chaperone GRP78 and transcription factor CHOP and the translocation of XBP-1 from the ER to the nucleus by NE. NE also reduced phospho-Akt (Ser473), indicating suppression of the phosphatidylinositol 3-kinase (PI3-kinase)/Akt survival pathway. Similar results were produced by thapsigargin. NGF, which promotes the PI3-kinase/Akt activity, reduced the effects of NE and thapsigargin on apoptosis and activation of caspase-12 and -3. However, the effects of NE, but not of thapsigargin, were abolished by pretreatment with SOD and catalase. In contrast, the PI3-kinase inhibitors LY-294002 and wortmannin abolished the protective effects of both SOD/catalase and NGF on NE-induced apoptosis. The functional importance of caspase-12 activation was supported by the use of Z-ATAD-FMK, which reduced the NE-induced processing of caspase-12 and cell apoptosis, but the caspase-12, -9, and -3 inhibitors had no effects on the increase in cytosolic cytochrome c produced by NE. In contrast, the release of mitochondrial cytochrome c was abolished by SOD/catalase and NGF. These results indicate that NE induced cell apoptosis by both ER stress and a mitochondrial death pathway and that the effects of NE were mediated via oxidative stress and inhibition of the PI3-kinase/Akt survival pathway.

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.

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.

Cell-cycle-dependent regulation of oxidative stress responses and Ca2+ permeable channels NtTPC1A/B in tobacco BY-2 cells

Plants are always exposed to the menace of oxidative stress and protect themselves by activating a variety of defense responses. However, molecular mechanisms for oxidative stress-induced gene expression are largely unknown. We here studied the roles of the oxidative stress-responsive putative voltage-dependent Ca(2+) permeable channels, NtTPC1A and NtTPC1B, and cell cycle in H(2)O(2)-induced expression of antioxidant enzymes, glutathione peroxidase (GPX) and ascorbate peroxidase (APX), in tobacco BY-2 cells. H(2)O(2)-induced [Ca(2+)](cyt) rise and expression of GPX and APX were inhibited by the cosuppression of NtTPC1A/B as well as Al ion, a specific blocker for NtTPC1s, and enhanced by overexpression of AtTPC1, suggesting that NtTPC1s are the major Ca(2+)-permeable channels activated by H(2)O(2) and that Ca(2+) influx via NtTPC1s is involved in induction of H(2)O(2)-triggered gene expression. Oxidative stress-induced signal transduction mechanisms were highly dependent on the phases of the cell cycle; H(2)O(2)-induced [Ca(2+)](cyt) rise and expression of GPX and APX as well as the level of NtTPC1s transcripts correlated with each other and were maximal at G1 phase. In contrast, the cell cycle-dependence of hypoosmotic shock-induced [Ca(2+)](cyt) rise that is known to be independent of NtTPC1s was almost reverse and maximal at S phase. These results suggest that the cell cycle-dependent regulation of oxidative stress-induced [Ca(2+)](cyt) rise and expression of NtTPC1s contribute to the cell cycle dependence of H(2)O(2)-induced expression of peroxidases. Various Ca(2+)-mediated signal transduction pathways are differentially regulated by the cell cycle.

p21(Cip1/WAF1/Sdi1) does not affect expression of base excision DNA repair enzymes during chronic oxidative stress

Exposure to chronic oxidative stress during elevated oxygen (hyperoxia) damages DNA and inhibits cell proliferation in G(1) through induction of the cyclin-dependent kinase inhibitor p21. Cells that fail to express p21 growth-arrest in S phase. The observation that growth arrest in G(1) is associated with reduced DNA damage and enhanced survival suggests that p21 may affect expression of base excision repair (BER) enzymes used to repair oxidized DNA. This hypothesis was tested in p21 wild-type and p21-deficient mice and human lung adenocarcinoma H1299 cells with tetracycline-on regulated expression of p21. The mRNA levels of Ogg1, Tdg, Udg, Mpg, Nth1, and Mgmt remained constant during 3 days of hyperoxia. The expression of Ogg1, Nth1, and APE protein also remained unchanged. Although hyperoxia increased p21, its absence did not significantly affect expression of these repair enzymes. These findings reveal that hyperoxia induces p21 without significantly altering BER enzyme expression. This suggests that p21 may protect oxidized cells by affecting the activity of BER enzymes and/or through other mechanisms, such as apoptosis.

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.

50-Hz extremely low frequency electromagnetic fields enhance cell proliferation and DNA damage: possible involvement of a redox mechanism

HL-60 leukemia cells, Rat-1 fibroblasts and WI-38 diploid fibroblasts were exposed for 24-72 h to 0.5-1.0-mT 50-Hz extremely low frequency electromagnetic field (ELF-EMF). This treatment induced a dose-dependent increase in the proliferation rate of all cell types, namely about 30% increase of cell proliferation after 72-h exposure to 1.0 mT. This was accompanied by increased percentage of cells in the S-phase after 12- and 48-h exposure. The ability of ELF-EMF to induce DNA damage was also investigated by measuring DNA strand breaks. A dose-dependent increase in DNA damage was observed in all cell lines, with two peaks occurring at 24 and 72 h. A similar pattern of DNA damage was observed by measuring formation of 8-OHdG adducts. The effects of ELF-EMF on cell proliferation and DNA damage were prevented by pretreatment of cells with an antioxidant like alpha-tocopherol, suggesting that redox reactions were involved. Accordingly, Rat-1 fibroblasts that had been exposed to ELF-EMF for 3 or 24 h exhibited a significant increase in dichlorofluorescein-detectable reactive oxygen species, which was blunted by alpha-tocopherol pretreatment. Cells exposed to ELF-EMF and examined as early as 6 h after treatment initiation also exhibited modifications of NF kappa B-related proteins (p65-p50 and I kappa B alpha), which were suggestive of increased formation of p65-p50 or p65-p65 active forms, a process usually attributed to redox reactions. These results suggest that ELF-EMF influence proliferation and DNA damage in both normal and tumor cells through the action of free radical species. This information may be of value for appraising the pathophysiologic consequences of an exposure to ELF-EMF.

Redox activation of p21Cip1/WAF1/Sdi1: a multifunctional regulator of cell survival and death

Cell division requires the coordinated assembly of cyclins and cyclin-dependent kinases that promote cell-cycle progression through S phase and mitosis. Two families of cyclin-dependent kinase inhibitors prevent abnormal or premature proliferation by blocking cyclin kinase activity. Expression of the cyclin-dependent kinase inhibitor p21, a member of the Cip/Kip family, increases when cells are damaged. In addition to controlling cell-cycle progression, p21 participates in DNA repair and apoptotic processes. The recent appreciation that p21 regulates cell survival and death implies that it is a master regulator of cell fate. This review discusses how p21 can affect the cellular response to oxidative stress.

In vivo inosine protects alveolar epithelial type 2 cells against hyperoxia-induced DNA damage through MAP kinase signaling

Inosine, a naturally occurring purine with anti-inflammatory properties, was assessed as a possible modulator of hyperoxic damage to the pulmonary alveolar epithelium. Rats were treated with inosine, 200 mg/kg ip, twice daily during 48-h exposure to >90% oxygen. The alveolar epithelial type 2 cells (AEC2) were then isolated and cultured. AEC2 isolated from inosine-treated hyperoxic rats had less DNA damage and had increased antioxidant status compared with AEC2 from hyperoxic rats. Inosine treatment during hyperoxia also reduced the proportion of AEC2 in S and G2/M phases of the cell cycle and increased levels of the DNA repair enzyme 8-oxoguanine DNA glycosylase. Bronchoalveolar lavage (BAL) recovered from hyperoxic, inosine-treated rats contained threefold higher levels of active transforming growth factor-beta than BAL from rats exposed to hyperoxia alone, and Smad2 was activated in AEC2 isolated from these animals. ERK1/2 was activated both in freshly isolated and 24-h-cultured AEC2 by in vivo inosine treatment, whereas blockade of the MAPK pathway in vitro reduced the protective effect of in the vivo inosine treatment. Together, the data suggest that inosine treatment during hyperoxic exposure results in protective signaling mediated through pathways downstream of MEK. Thus inosine may deserve further evaluation for its potential to reduce hyperoxic damage to the pulmonary alveolar epithelium.

Ataxia telangiectasia mutated (ATM) and ATM and Rad3-related protein exhibit selective target specificities in response to different forms of DNA damage

The ataxia telangiectasia mutated (ATM) and ATR (ATM and Rad3-related) protein kinases exert cell cycle delay, in part, by phosphorylating Checkpoint kinase (Chk) 1, Chk2, and p53. It is well established that ATR is activated following UV light-induced DNA damage such as pyrimidine dimers and the 6-(1,2)-dihydro-2-oxo-4-pyrimidinyl-5-methyl-2,4-(1H,3H)-pyrimidinediones, whereas ATM is activated in response to double strand DNA breaks. Here we clarify the activation of these kinases in cells exposed to IR, UV, and hyperoxia, a condition of chronic oxidative stress resulting in clastogenic DNA damage. Phosphorylation on Chk1(Ser-345), Chk2(Thr-68), and p53(Ser-15) following oxidative damage by IR involved both ATM and ATR. In response to ultraviolet radiation-induced stalled replication forks, phosphorylation on Chk1 and p53 required ATR, whereas Chk2 required ATM. Cells exposed to hyperoxia exhibited growth delay in G1, S, and G2 that was disrupted by wortmannin. Consistent with ATM or ATR activation, hyperoxia induced wortmannin-sensitive phosphorylation of Chk1, Chk2, and p53. By using ATM- and ATR-defective cells, phosphorylation on Chk1, Chk2, and p53 was found to be ATM-dependent, whereas ATR also contributed to Chk1 phosphorylation. These data reveal activated ATM and ATR exhibit selective substrate specificity in response to different genotoxic agents.

Molecular events associated with reactive oxygen species and cell cycle progression in mammalian cells

Cell cycle progression is regulated by a wide variety of external factors, amongst them are growth factors and extracellular matrix factors. During the last decades evidence has been obtained that reactive oxygen species (ROS) may also play an important role in cell cycle progression. ROS may be generated by external and internal factors. In this overview we describe briefly the generation of ROS and their effects on processes that have been demonstrated to play an essential role in cell cycle progression, including such systems as signal transduction cascades, protein ubiquitination and degradation, and the cytoskeleton. These different effects of ROS influence cell cycle progression dependent upon the amount and duration of ROS exposure. Activation of growth factor stimulated signaling cascades by low levels of ROS result in increased cell cycle progression, or, in case of prolonged exposure, to a differentiation like growth arrest. From many studies it seems clear that the cyclin kinase inhibitor protein p21 plays a prominent role, leading to cell cycle arrest at higher but not directly lethal levels of ROS. Dependent upon the nature of p21 induction, the cell cycle arrest may be transient, coupled to repair processes, or permanent. At high concentrations of ROS all of the above processes are activated, in combination with enhanced damage to the building blocks of the cell, leading to apoptosis or even necrosis.

BCR/ABL oncogenic kinase promotes unfaithful repair of the reactive oxygen species-dependent DNA double-strand breaks

The oncogenic BCR/ABL tyrosine kinase induces constitutive DNA damage in Philadelphia chromosome (Ph)-positive leukemia cells. We find that BCR/ABL-induced reactive oxygen species (ROSs) cause chronic oxidative DNA damage resulting in double-strand breaks (DSBs) in S and G(2)/M cell cycle phases. These lesions are repaired by BCR/ABL-stimulated homologous recombination repair (HRR) and nonhomologous end-joining (NHEJ) mechanisms. A high mutation rate is detected in HRR products in BCR/ABL-positive cells, but not in the normal counterparts. In addition, large deletions are found in NHEJ products exclusively in BCR/ABL cells. We propose that the following series of events may contribute to genomic instability of Ph-positive leukemias: BCR/ABL --> ROSs --> oxidative DNA damage --> DSBs in proliferating cells --> unfaithful HRR and NHEJ repair.

Extracellular norepinephrine reduces neuronal uptake of norepinephrine by oxidative stress in PC12 cells

Cardiac norepinephrine (NE) uptake activity is reduced in congestive heart failure. Our studies in intact animals suggest that this effect on the cardiac sympathetic nerve endings is caused by oxidative stress and/or NE toxic metabolites derived from NE. In this study, we investigated the direct effects of NE on neuronal NE uptake activity and NE transporter (NET), using undifferentiated PC12 cells. Cells were incubated with NE (1–500 μM) either alone or in combination of Cu2+ sulfate (1 μM), which promotes free radical formation by Fenton reaction for 24 h. NE uptake activity was measured using [3H]NE. Cell viability was determined with the use of Trypan blue exclusion and 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl-tetrazolium bromide assay, and cellular oxidative stress by dichlorodihydrofluorescein fluorescence and the GSH/GSSG ratio. Cell viability was reduced by NE >100 μM. At lower doses, NE produced oxidative stress and a dose-dependent reduction of NE uptake activity without affecting cell viability significantly. Cu2+, which has no direct effect on NE uptake activity, potentiated oxidative stress and reduction of NE uptake activity produced by NE. This decrease of NE uptake activity was associated with reductions of NE uptake binding sites and NET protein expression by using the radioligand assay and Western blot analysis, but no changes in NET gene expression. In addition, the free-radical scavenger mannitol, and antioxidant enzymes superoxide dismutase and catalase, reduced oxidative stress and attenuated the reductions of NE uptake activity and NET protein produced by NE/Cu. Thus our results support a functional role of oxidative stress in mediating the neuronal NE uptake reducing effect of NE and that this effect of NE on NET is a posttranscriptional event.

Reactive oxygen species-independent G1 arrest induced by evening primrose extract in Ehrlich ascites tumor cells

We previously demonstrated that evening primrose extract (EPE) induced apoptosis in Ehrlich ascites tumor cells (EATC), and this effect was specific on tumor cells. Furthermore, our results demonstrated that EPE exposure elicited a rapid increase in the activity of superoxide dismutase and intracellular peroxides levels. These changes caused translocation of Bax to mitochondria and a subsequent release of mitochondrial cytochrome c. However, no activation of caspase-3 was observed in EPE-treated EATC. On the other hand, apoptosis-inducing factor (AIF) was translocated from mitochondria to nuclei. The EPE-induced translocation of AIF was suppressed with the addition of catalase, suggesting that the rapid intracellular peroxide levels after addition of EPE triggers off induction of apoptosis, which is AIF-mediated and caspase-independent. In this study, we have shown that EPE elicited a dose-dependent accumulation of cells in the G1 phase and inhibited DNA synthesis. Our results also demonstrated that cell cycle arrest and inhibition of proliferation in EATC by EPE are associated with decreased Rb phosphorylation. Furthermore, inhibitions of Rb phosphorylation and DNA synthesis by EPE were not suppressed with the addition of catalase. The present study suggests that intracellular peroxides, which trigger off induction of apoptosis, are not the trigger of EPE-induced G1 arrest in cell cycle.

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.

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.

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.

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.

Hyperoxia induces macrophage cell cycle arrest by adhesion-dependent induction of p21Cip1 and activation of the retinoblastoma protein

Hyperoxia induces growth arrest, apoptosis, necrosis, and morphological changes (spreading and adhesion) in various types of cells. The mechanism of hyperoxia-induced cell growth arrest has not been well elucidated, especially in macrophages. One possible mechanism is a role of cell adhesion in hyperoxia-induced cell cycle arrest. To evaluate this finding, macrophages were cultured in normoxia (21% O2) or hyperoxia (95% O2) in adhesion or low adhesion conditions. Incubation of macrophages in hyperoxia induced cell cycle arrest. The hyperoxia-induced cell cycle arrest was prevented by low adhesion conditions. To evaluate pathways potentially involved in hyperoxia-induced growth arrest, we measured extracellular regulated kinase and retinoblastoma protein activation and p21Cip1 and p53 accumulation. Hyperoxia strongly induced activation of extracellular regulated kinase and retinoblastoma protein as well as up-regulation of p21Cip1. These effects of hyperoxia were attenuated under low adhesion conditions, suggesting a role for integrin-dependent signaling. The induction of p21Cip1 and activation of retinoblastoma protein occurred via a p53-independent mechanism. These results suggest that adhesion-dependent pathways are required for hyperoxia-induced cell cycle arrest in macrophages.

Pathways of cell signaling in hyperoxia

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

The Pxmp2 and PoleI genes are linked by a bidirectional promoter in an evolutionary conserved fashion

Pxmp2 is the most abundant peroxisomal membrane protein in higher eukaryotes. Its expression is tissue-specific with highest levels of expression in liver, kidney and heart tissue. We have analysed the 5'-flanking genomic region of the murine Pxmp2 gene and we found, that the first exon of the gene encoding the DNA polymerase epsilon (PoleI) was localized adjacent to the first exon of the Pxmp2 gene in head to head orientation. Both genes were separated by only 393 bp containing a CpG island with numerous binding sites for Sp1. A TATA box, however, was lacking. Northern blot analysis revealed that both genes were expressed differently, indicating that their expression was regulated independently. We have analysed the promoter activity of the small genomic fragment separating the Pxmp2 and PoleI genes using luciferase as a reporter molecule in transient transfection assays. The small genomic fragment was a functional promoter, controlling gene expression regardless of its orientation. Promoter activity was 60-70% compared with the activity of the strong CMV promoter. The Pxmp2 and PoleI genes were also linked on the human and rat genome. Furthermore, the sequence of the intergenic fragment was highly conserved among these species. Thus, the small intergenic fragment is probably the common basic element of two independently regulated promoters.