Extracellular Inosine is Modulated by H2O2 and Protects Sertoli Cells against Lipoperoxidation and Cellular Injury

Extracellular purines are involved in the regulation of a wide range of physiological processes, including cytoprotection, ischemic preconditioning, and cell death. These actions are usually mediated via triggering of membrane purinergic receptors, which may activate antioxidant enzymes, conferring cytoprotection. Recently, it was demonstrated that the oxidative stress induced by cisplatin up-regulated A¹ receptor expression in rat testes, suggesting an involvement of purinergic signaling in the response of testicular cells to oxidant injury. In this article, we report the effect of hydrogen peroxide on purinergic agonist release by cultured Sertoli cells. Extracellular inosine levels are strongly increased in the presence of H²O², suggesting an involvement of this nucleoside on Sertoli cells response to oxidant treatment. Inosine was observed to decrease H²O²-induced lipoperoxidaton and cellular injury, and it also preserved cellular ATP content during H²O² exposure. These effects were abolished in the presence of nucleoside uptake inhibitors, indicating that nucleoside internalisation is essential for its action in preventing cell damage.     

Extracellular inosine modulates ERK 1/2 and p38 phosphorylation in cultured Sertoli cells: possible participation in TNF-alpha modulation of ERK 1/2

Extracellular ATP and adenosine modulation of MAPKs is well described in different cells types, but few studies have addressed the effects of extracellular inosine on these kinases. Previous results showed that hydrogen peroxide and TNF-alpha increase extracellular inosine concentration in cultured Sertoli cells and this nucleoside protects Sertoli cells against hydrogen peroxide induced damage and participates in TNF-alpha induced nitric oxide production. In view of the fact that MAPKs are key mediators of the cellular response to a large variety of stimuli, we investigated the effect of extracellular inosine on the phosphorylation of ERK 1/2 and p38 MAPKs in cultured Sertoli cells. The involvement of this nucleoside in the activation of ERK 1/2 by TNF-alpha was also investigated. Inosine and the selective A1 adenosine receptor agonist R-PIA increases the phosphorylation of ERK 1/2 and p38, and this was blocked by the selective A1 adenosine receptors antagonists, CPT and DPCPX. These antagonists also inhibited TNF-alpha increase in the phosphorylation of ERK 1/2. TNF-alpha also rapidly augmented extracellular inosine concentration in cultured Sertoli cells. These results show that extracellular inosine modulates ERK 1/2 and p38 in cultured Sertoli cells, possible trough A1 adenosine receptor activation. This nucleoside also participates in TNF-alpha modulation of ERK 1/2.     

Protection of renal epithelial cells against oxidative injury by endoplasmic reticulum stress preconditioning is mediated by ERK1/2 activation

We investigated the role of the endoplasmic reticulum (ER) stress response in intracellular Ca2+ regulation, MAPK activation, and cytoprotection in LLC-PK1 renal epithelial cells in an attempt to identify the mechanisms of protection afforded by ER stress. Cells preconditioned with trans-4,5-dihydroxy-1,2-dithiane, tunicamycin, thapsigargin, or A23187 expressed ER stress proteins and were resistant to subsequent H2O2-induced cell injury. In addition, ER stress preconditioning prevented the increase in intracellular Ca2+ concentration that normally follows H2O2 exposure. Stable transfection of cells with antisense RNA targeted against GRP78 (pkASgrp78 cells) prevented GRP78 induction, disabled the ER stress response, sensitized cells to H2O2-induced injury, and prevented the development of tolerance to H2O2 that normally occurs with preconditioning. ERK and JNK were transiently (30-60 min) phosphorylated in response to H2O2. ER stress-preconditioned cells had more ERK and less JNK phosphorylation than control cells in response to H2O2 exposure. Preincubation with a specific inhibitor of JNK activation or adenoviral infection with a construct that encodes constitutively active MEK1, the upstream activator of ERKs, also protected cells against H2O2 toxicity. In contrast, the pkASgrp78 cells had less ERK and more JNK phosphorylation upon H2O2 exposure. Expression of constitutively active ERK also conferred protection on native as well as pkAS-grp78 cells. These results indicate that GRP78 plays an important role in the ER stress response and cytoprotection. ER stress preconditioning attenuates H2O2-induced cell injury in LLC-PK1 cells by preventing an increase in intracellular Ca2+ concentration, potentiating ERK activation, and decreasing JNK activation. Thus, the ER stress response modulates the balance between ERK and JNK signaling pathways to prevent cell death after oxidative injury. Furthermore, ERK activation is an important downstream effector mechanism for cellular protection by ER stress.     

H(2)O(2)-induced O(2) production by a non-phagocytic NAD(P)H oxidase causes oxidant injury

Non-phagocytic NAD(P)H oxidases have been implicated as major sources of reactive oxygen species in blood vessels. These oxidases can be activated by cytokines, thereby generating O(2), which is subsequently converted to H(2)O(2) and other oxidant species. The oxidants, in turn, act as important second messengers in cell signaling cascades. We hypothesized that reactive oxygen species, themselves, can activate the non-phagocytic NAD(P)H oxidases in vascular cells to induce oxidant production and, consequently, cellular injury. The current report demonstrates that exogenous exposure of non-phagocytic cell types of vascular origin (smooth muscle cells and fibroblasts) to H(2)O(2) activates these cell types to produce O(2) via an NAD(P)H oxidase. The ensuing endogenous production of O(2) contributes significantly to vascular cell injury following exposure to H(2)O(2). These results suggest the existence of a feed-forward mechanism, whereby reactive oxygen species such as H(2)O(2) can activate NAD(P)H oxidases in non-phagocytic cells to produce additional oxidant species, thereby amplifying the vascular injury process. Moreover, these findings implicate the non-phagocytic NAD(P)H oxidase as a novel therapeutic target for the amelioration of the biological effects of chronic oxidant stress.     

Lipid peroxidation, protein thiol oxidation and DNA damage in hydrogen peroxide-induced injury to endothelial cells: role of activation of poly(ADP-ribose)polymerase

These experiments are a continuation of work investigating the mechanism of oxidant-induced damage to cultured bovine pulmonary artery endothelial cells (BPEC). Earlier experiments implicated DNA strand breakage and activation of poly(ADP-ribose)polymerase as critical steps in cell injury. In the current report, a better defined model of oxidant stress was used to investigate DNA damage, lipid peroxidation and protein thiol oxidation in BPEC following oxidant stress. The dose and time response of LDH release following exposure to H2O2 were established. H2O2 was metabolized rapidly by BPEC (t1/2 = 20 min). Hydrogen peroxide-induced increases in thiobarbituric acid (TBA) reactive material were prevented by pretreatment with the lipophilic antioxidant diphenylphenylinediamine (DPPD). However, DPPD did not decrease LDH release. Conversely, pretreatment with 5 mM 3-aminobenzamide (3AB), a competitive inhibitor of poly(ADP-ribose)polymerase, prevented LDH release from BPEC following H2O2 treatment. Dithiothreitol (DTT), a sulfhydryl reducing agent, also prevented LDH release. The effects of 3AB and DTT on H2O2-induced changes in DNA strand breaks and NAD+ and ATP levels were investigated as well as the effect of H2O2 on soluble and protein-bound thiols. As DPPD inhibited peroxidation without preventing LDH release, lipid peroxidation does not appear to play a role in the loss of BPEC viability in response to oxidant stress. As protein thiol oxidation was not caused by H2O2, it does not appear to play a causative role in cytotoxicity, although DTT may protect via maintenance of soluble thiols. H2O2 induces DNA strand breaks, which activate poly(ADP-ribose)polymerase, leading to depletion of cellular NAD+ and ATP and loss in cell viability. This supports earlier studies implicating the activation of poly(ADP-ribose)polymerase in oxidant injury to cultured endothelial cells.     

Liposome-mediated augmentation of catalase in alveolar type II cells protects against H2O2 injury

Oxidant injury to the alveolar epithelium can be mediated by exposure to oxidant gases such as O2 at high concentrations and O3, inflammatory cell-derived reactive O2 species, and the intracellular metabolism of xenobiotics such as paraquat. An in vitro model of alveolar epithelial oxidant injury was developed based on exposure of cultured rat type II pneumocytes to superoxide and hydrogen peroxide (H2O2) enzymatically generated in the culture medium. Cytotoxicity was assessed by the release of lactate dehydrogenase (LDH) into the culture medium, which was a more reliable indicator of damage than release of 51Cr by prelabeled cells. Incubation of cells for 6-8 h with xanthine plus xanthine oxidase and glucose plus glucose oxidase induced the release of greater than 50% of total intracellular LDH. Oxidant exposure also resulted in significant detachment of cells from culture dishes. Modulation of oxidant damage was accomplished using liposomes as vectors for the delivery of catalase. Treatment of cells with catalase liposomes for 2 h resulted in augmentation of cellular catalase specific activities up to 631% of controls. Catalase was partitioned into intracellular and surface-associated compartments in catalase liposome-treated cells. Partial and complete protection against oxidant injury, induced by xanthine plus xanthine oxidase and glucose plus glucose oxidase, respectively, was achieved by pretreatment of cells with catalase liposomes. LDH release during oxidant exposure was inversely related to augmentation of cellular catalase activities. Catalase liposome-treated cells also exhibited an enhanced ability to scavenge enzymatically generated H2O2 from the culture medium. These observations suggest a useful approach to modulation of alveolar injury induced by reactive O2 species.     

表皮生长因子对培养的气道上皮细胞抗臭氧损伤的保护作用

本研究观察到臭氧（Ｏ３）对体外培养经３Ｈ－ＵｄＲ标记的免气道上皮细胞有明显细胞毒性作用,且损伤程度与Ｏ３作用时间呈正相关。Ｏ３暴露组细胞内丙二醛（ＭＤＡ）产生增多（Ｐ＜０．０１）,提示Ｏ３损伤细胞的机制与胞膜脂质过氧化有关。表皮生长因子（ＥＧＦ）可明显降低Ｏ３所致的３Ｈ释放率（Ｐ＜０．０１）、降低Ｏ３的细胞毒指数及细胞内ＭＤＡ含量（Ｐ＜０．０１）,证明ＥＧＦ对气道上皮细胞有保护作用。进一步还观察到浓度为５ｎｇ／ｍｌ的ＥＯＦ可以取消Ｏ３所引起的细胞内还原型谷胱甘肽（ＧＳＨ）含量降低（Ｐ＜０．０１）,并增加细胞内谷胱甘肽总含量（Ｐ＜０．０５）,但不能改变Ｏ３所致的氧化型谷胱甘肽（ＧＳＳＧ）含量的增加（Ｐ＞０．０５）,对ＧＳＨ／ＧＳＳＧ比值也无明显提高,这些都提示ＥＧＦ的细胞保护机理可能与其促进细胞内谷胱甘肽合成有关,而对ＧＳＳＧ转化为ＧＳＨ的还原过程影响不明显。     

Roles of mitogen-activated protein kinase signal-integrating kinases 1 and 2 in oxidant-mediated eIF4E phosphorylation

Oxidative stress alters cellular metabolic processes including protein synthesis. The eukaryotic initiation factor, eIF4E, acts in the rate-limiting steps of initiation and promotes nuclear export. Phosphorylation of eIF4E by mitogen activated protein kinase signal-integrating kinases 1 and 2 (Mnk) influences the affinity of eIF4E for the 5'-mRNA cap and fosters nuclear export activity. Although phosphorylation of eIF4E on Ser209 is observed following oxidant exposure, the contribution of Mnk isoforms and the significance of phosphorylation remain elusive. Using a Mnk inhibitor and fibroblasts derived from Mnk knockout mice, we demonstrate that that H2O2 enhances eIF4E phosphorylation in cells containing Mnk1. In contrast, cells containing only Mnk2 show little change or a decrease in eIF4E phosphorylation in response to H2O2. H2O2 also shifted eIF4GI protein from the nucleus to the cytoplasm suggesting that the increases in eIF4E phosphorylation may reflect enhanced substrate availability to cytoplasmic Mnk1. In Mnk1(+/+) cells, H2O2 also enhanced eIF4E phosphorylation in the nucleus to a greater degree than in the cytoplasm, an effect not observed in cells containing Mnk2. In response to H2O2, all MEFs showed increased eIF4E:4E-BP1 and 4E-BP2:eIF4E binding and reduced eIF4E:eIF4GI binding. We also observed a dramatic increase in the amount of Mnk1 associated with eIF4E following affinity chromatography. These changes coincided with a smaller reduction in global protein synthesis in response to H2O2 in the DKO cells. These findings suggest that changes in eIF4GI distribution may enhance eIF4E phosphorylation and that the presence of either Mnk1 or 2 or any degree of eIF4E phosphorylation negatively regulates global protein synthesis in response to oxidant stress.     

Endoplasmic Reticulum Stress-Induced Autophagy Provides Cytoprotection from Chemical Hypoxia and Oxidant Injury and Ameliorates Renal Ischemia-Reperfusion Injury

We examined whether endoplasmic reticulum (ER) stress-induced autophagy provides cytoprotection from renal tubular epithelial cell injury due to oxidants and chemical hypoxia in vitro, as well as from ischemia-reperfusion (IR) injury in vivo. We demonstrate that the ER stress inducer tunicamycin triggers an unfolded protein response, upregulates ER chaperone Grp78, and activates the autophagy pathway in renal tubular epithelial cells in culture. Inhibition of ER stress-induced autophagy accelerated caspase–3 activation and cell death suggesting a pro-survival role of ER stress-induced autophagy. Compared to wild-type cells, autophagy-deficient MEFs subjected to ER stress had enhanced caspase–3 activation and cell death, a finding that further supports the cytoprotective role of ER stress-induced autophagy. Induction of autophagy by ER stress markedly afforded cytoprotection from oxidants H₂O₂ and tert-Butyl hydroperoxide and from chemical hypoxia induced by antimycin A. In contrast, inhibition of ER stress-induced autophagy or autophagy-deficient cells markedly enhanced cell death in response to oxidant injury and chemical hypoxia. In mouse kidney, similarly to renal epithelial cells in culture, tunicamycin triggered ER stress, markedly upregulated Grp78, and activated autophagy without impairing the autophagic flux. In addition, ER stress-induced autophagy markedly ameliorated renal IR injury as evident from significant improvement in renal function and histology. Inhibition of autophagy by chloroquine markedly increased renal IR injury. These studies highlight beneficial impact of ER stress-induced autophagy in renal ischemia-reperfusion injury both in vitro and in vivo.     

Schisandrin B enhances the glutathione redox cycling and protects against oxidant injury in different types of cultured cells

Tert-butylhydroperoxide (tBHP) challenge caused an initial depletion of cellular reduced glutathione (GSH), which was followed by a gradual restoration of cellular GSH in AML12, H9c2, and differentiated PC12 cells. The time-dependent changes in cellular GSH induced by tBHP were monitored as a measure of GSH recovery capacity (GRC), of which glutathione reductase (GR)-mediated glutathione redox cycling and γ-glutamate cysteine ligase (GCL)-mediated GSH synthesis were found to play an essential role. While glutathione redox cycling sustained the GSH level during the initial tBHP-induced depletion, GSH synthesis restores the GSH level thereafter. The effects of (-)schisandrin B [(-)Sch B] and its analogs (Sch A and Sch C) on GRC were also examined in the cells. (-)Sch B and Sch C, but not Sch A, ameliorated the extent of tBHP-induced GSH depletion, indicative of enhanced glutathione redox cycling. However, the degree of restoration of GSH post-tBHP challenge was not affected or even decreased. Pretreatment with (-)Sch B and Sch C, but not Sch A, protected against oxidant injury in the cells. The (-)Sch B afforded cytoprotection was abolished by N,N'-bis(chloroethyl)-N-nitrosourea pretreatment suggesting the enhancement of glutathione redox cycling is crucially involved in the cytoprotection afforded by (-)Sch B against oxidative stress-induced cell injury.     

Oxygen toxicity in control and H2O2-resistant Chinese hamster fibroblast cell lines

Following exposure to 95% oxygen, clonogenic cell survival was assayed and qualitative morphologic changes were observed in a Chinese hamster fibroblast cell line (HA-1). The time in 95% O2 necessary to clonogenically inactivate 90% of the cells was inversely related to the cell density of the cultures at the beginning of hyperoxic exposure (from 1 to 6 X 10(4) cells/cm2). The O2-induced loss in clonogenicity and evidence of morphologic injury were shown to be significantly delayed (17-22 h) in an H2O2-resistant variant of the parental HA-1 cell line. After the delay in onset of clonogenic cell killing or morphologic injury, the process of injury proceeded in a similar fashion in both cell lines. The H2O2-resistant cell line demonstrated significantly greater catalase activity (20-fold), CuZn superoxide dismutase activity (2-fold), and Se-dependent glutathione peroxidase activity (1.5-fold). The greater activities of CuZn superoxide dismutase and catalase were accompanied by similarly greater quantities of immunoreactive protein as determined by immunoblotting. These data demonstrate that the cells adapted and/or selected for growth in a highly peroxidative environment also became refractory to O2-induced toxicity, which may be related to increased expression of antioxidant enzymes. However, the magnitude of this cross-resistance to O2 toxicity was less than the magnitude of the cellular resistance to the toxicity of exogenous H2O2, suggesting that in this system the toxicity of 95% oxygen is not identical to H2O2-mediated cytotoxicity.     

Redox survival signalling in retina-derived 661W cells

Reactive oxygen species have been implicated in processes involving cellular damage and subsequent cell death, especially in organs such as the eye that are constantly exposed to excitatory signals. However, recent studies have shown that oxidant species can also act as intracellular signalling molecules promoting cell survival, but little is known about this mechanism in the retina. The present study demonstrates for the first time that hydrogen peroxide (H2O2) is generated rapidly and acts as a pro-survival signal in response to a variety of apoptotic stimuli in retina-derived 661W cells and in the retinal ganglion cell line RGC-5. Focussing on 661Ws and serum deprivation, we systematically investigated pro-survival and pro-death pathways and discovered that the rapid and transient burst of H2O2 activates the AKT survival pathway. Activation of the apoptotic machinery takes place following the decline of H2O2 to basal levels. To substantiate this proposed pro-survival role of H2O2, we inhibited the oxidant burst, which exacerbated cell death. Conversely, maintenance of the oxidant signal using exogenous H2O2 enhanced cell survival. Overall, the results presented in this study provide evidence for a novel role of H2O2 in mediating survival of retinal cells in response to apoptotic stimuli.     

518A2 melanoma cells are protected by G3139 and other antineoplastic agents against the cytotoxic effects of DTIC

G3139 is an antisense Bcl-2 phosphorothioate oligonucleotide that has been combined with DTIC in a phase III clinical trial in melanoma. However, its actual mechanism of action in melanoma is controversial. Treatment of 518A2 melanoma cells with either G3139 or G4126 (a two-base mismatch) and then with light-activated DTIC caused these cells (but not SK-Mel-30 or 346.1 cells) to be protected against the cytotoxic effects of DTIC. This cytoprotection was not recapitulated with a phosphodiester congener of G3139 nor with a small interfering RNA (siRNA) also targeted to the Bcl-2 mRNA. Administering the drugs in reverse order also did not produce cytoprotection, and an 18- mer phosphorothioate homopolymer of thymidine was also inactive. Subsequently, it was discovered that gemcitibine and cis-platinum also induced cytoprotection to DTIC in this cell line, suggesting that the cytoprotection is a stress response to chemical proapoptotic stress. Cytoprotection was completely inhibited by O(6)-benzylguanine, an inhibitor of O(6)-guanosine alkyltransferase (OGAT) activity. However, a direct assay of OGAT activity demonstrated that 518A2 melanoma cells are essentially completely devoid of it, either basally or induced. The cytoprotection may thus be caused by a chemical stress-induced increase in mismatch repair activity.     

Upregulation of endogenous glutathione system by 3H-1,2-dithiole-3-thione in pancreatic RINm5F beta-cells as a novel strategy for protecting against oxidative beta-cell injury

This study was undertaken to investigate the inducibility of glutathione (GSH), glutathione reductase (GR) and glutathione peroxidase (GPx) by 3H-1,2-dithiole-3-thione (D3T) in beta-cells, and the resultant cytoprotection against oxidant injury. Incubation of the insulin-secreting RINm5F cells with D3T led to significant induction of GSH, GR and GPx. D3T-mediated induction of GSH was abolished by buthionine sulfoximine (BSO), suggesting a critical involvement of γ-glutamylcysteine ligase (γGCL). Consistently, incubation of RINm5F cells with D3T resulted in increased expression of γGCL protein and mRNA. Pretreatment of RINm5F cells with D3T provided remarkable protection against oxidant-elicited cytotoxicity. On the other hand, depletion of cellular GSH by BSO sensitized RINm5F cells to oxidant injury. Furthermore, cotreatment of RINm5F cells with BSO to reverse D3T-mediated GSH induction abolished the cytoprotective effects of D3T on oxidant injury. Taken together, this study demonstrates that upregulation of glutathione system by D3T is effective for protecting against oxidative beta-cell injury.     

Upregulation of endogenous glutathione system by 3H-1,2-dithiole-3-thione in pancreatic RINm5F beta-cells as a novel strategy for protecting against oxidative beta-cell injury

This study was undertaken to investigate the inducibility of glutathione (GSH), glutathione reductase (GR) and glutathione peroxidase (GPx) by 3H-1,2-dithiole-3-thione (D3T) in beta-cells, and the resultant cytoprotection against oxidant injury. Incubation of the insulin-secreting RINm5F cells with D3T led to significant induction of GSH, GR and GPx. D3T-mediated induction of GSH was abolished by buthionine sulfoximine (BSO), suggesting a critical involvement of γ-glutamylcysteine ligase (γGCL). Consistently, incubation of RINm5F cells with D3T resulted in increased expression of γGCL protein and mRNA. Pretreatment of RINm5F cells with D3T provided remarkable protection against oxidant-elicited cytotoxicity. On the other hand, depletion of cellular GSH by BSO sensitized RINm5F cells to oxidant injury. Furthermore, cotreatment of RINm5F cells with BSO to reverse D3T-mediated GSH induction abolished the cytoprotective effects of D3T on oxidant injury. Taken together, this study demonstrates that upregulation of glutathione system by D3T is effective for protecting against oxidative beta-cell injury.     

Inhibition of organic anion transport in endothelial cells by hydrogen peroxide.

ATP loss is a prominent feature of cellular injury induced by oxidants or ischemia. How reduction of cellular ATP levels contributes to lethal injury is still poorly understood. In this study we examined the ability of H2O2 to inhibit in a dose-dependent manner the extrusion of fluorescent organic anions from bovine pulmonary artery endothelial cells. Extrusion of fluorescent organic anions was inhibited by probenecid, suggesting an organic anion transporter was involved. In experiments in which ATP levels in endothelial cells were varied by treatment with different degrees of metabolic inhibition, it was determined that organic anion transport was ATP-dependent. H2O2-induced inhibition of organic anion transport correlated well with the oxidant's effect on cellular ATP levels. Thus H2O2-mediated inhibition of organic anion transport appears to be via depletion of ATP, a required substrate for the transport reaction. Inhibition of organic anion transport directly by probenecid or indirectly by metabolic inhibition with reduction of cellular ATP levels was correlated with similar reductions of short term viability. This supports the hypothesis that inhibition of organic anion transport after oxidant exposure or during ischemia results from depletion of ATP and may significantly contribute to cytotoxicity.     

Radiation-induced cyclooxygenase 2 up-regulation is dependent on redox status in prostate cancer cells

Cyclooxygenase 2 (COX2) is the inducible isozyme of COX, a key enzyme in arachidonate metabolism and the conversion of arachidonic acid (AA) to prostaglandins (PGs) and other eicosanoids. Previous studies have demonstrated that the COX2 protein is up-regulated in prostate cancer cells after irradiation and that this results in elevated levels of PGE(2). In the present study, we further investigated whether radiation-induced COX2 up-regulation is dependent on the redox status of cells from the prostate cancer cell line PC-3. l-Buthionine sulfoximine (BSO), which inhibits gamma glutamyl cysteine synthetase (gammaGCS), and the antioxidants alpha-lipoic acid and N-acetyl-l-cysteine (NAC) were used to modulate the cellular redox status. BSO decreased the cellular GSH level and increased cellular reactive oxygen species (ROS) in PC-3 cells, whereas alpha-lipoic acid and NAC increased the GSH level and decreased cellular ROS. Both radiation and the oxidant H(2)O(2) had similar effects on COX2 up-regulation and PGE(2) production in PC-3 cells, suggesting that radiation-induced COX2 up-regulation is secondary to the production of ROS. The relative increases in COX2 expression and PGE(2) production induced by radiation and H(2)O(2) were even greater when PC-3 cells were pretreated with BSO. When the cells were pretreated with alpha-lipoic acid or NAC for 24 h, both radiation- and H(2)O(2)-induced COX2 up-regulation and PGE(2) production were markedly inhibited. These results demonstrate that radiation-induced COX2 up-regulation in prostate cancer cells is modulated by the cellular redox status. Radiation-induced increases in ROS levels contribute to the adaptive response of PC-3 cells, resulting in elevated levels of COX2.