Regulation of hydrogen peroxide generation in cultured endothelial cells.

Endogenous hydrogen peroxide (H2O2) release from aortic endothelial cells was studied in the presence of antioxidant enzyme inhibitors, mitochondrial inhibitors, a microsomal cytochrome P-450 inhibitor, and after oxidative stress induced with H2O2 or menadione. Extracellular H2O2 generation was determined spectrofluorometrically using 3-methoxy-4-hydroxy phenylacetic acid, and intracellular H2O2 production (in or near peroxisomes) was measured indirectly using aminotriazole, which inactivates catalase in the presence of H2O2. Extracellular H2O2 release was 0.079 +/- 0.005 nmol/min/mg protein in Hanks' balanced salt solution, was constant during a 120-min incubation period, and was not affected by the cell passage number. The half-life for catalase inactivation with aminotriazole was 23 min. Inhibition of catalase, glutathione reductase, or gamma-glutamylcysteine synthetase did not change the rate of extracellular release of H2O2. Furthermore, inhibition of the mitochondrial respiratory chain (rotenone, antimycin A) or microsomal cytochrome P-450 (8-methoxypsoralen) did not change extracellular H2O2 release or intracellular H2O2 production (at peroxisomes) by endothelial cells or cells in which glutathione reductase was inactivated. When the cells were exposed to exogenous H2O2 (30 microM), extracellular H2O2 was scavenged primarily by the glutathione redox pathway. Exogenously added H2O2 (100 microM) changed intracellular H2O2 production (in or near peroxisomes) only when the glutathione redox cycle was inactivated. Menadione (20 microM), which undergoes intracellular redox cycling, increased extracellular H2O2 release almost 4-fold to 0.3 nmol/min/mg protein. Furthermore, menadione increased peroxisomal H2O2 levels and decreased the half-life for catalase inactivation in the presence of aminotriazole to 13 min. Catalase inhibition increased extracellular H2O2 release during menadione treatment, indicating that H2O2 can diffuse across the plasma membrane during oxidant stress.(ABSTRACT TRUNCATED AT 250 WORDS)     

Protective effects of catalpol against H2O2-induced oxidative stress in astrocytes primary cultures

It has been proposed that ROS production, including H(2)O(2), may lead to neurodegenerative disorders such as Parkinson's disease and Alzheimer's disease. Catalpol, an iridoid glycoside, presents in the root of Rehmannia glutinosa, protects cells and mice from damage caused by a variety of toxic stimuli. In this study, we investigated whether catalpol could protect astrocytes from oxidant stress induced by H(2)O(2) because of the critical role of astrocytes in the brain and found the possible mechanism of protection. The results showed that catalpol could significantly increase the cell viability and reduce the intracellular ROS formation. Furthermore, catalpol attenuated H(2)O(2)-induced oxidative stress via preventing the decrease in the activities of antioxidant enzymes in glutathione redox cycling such as glutathione peroxidase, glutathione reductase and glutathione content. However, the catalase activity did not appear to be elevated by catalpol adequately. Together, the main mechanism underlying the protective effects of catalpol in H(2)O(2)-injured astrocytes might be related to the maintenance of glutathione metabolism balance and the decrease of ROS formation. Therefore, catalpol may be developed as a potential preventive or therapeutic drug for neurodegenerative diseases associated with oxidative stress.     

醛糖还原酶:心肌缺血损伤干预的新靶点

醛糖还原酶是糖代谢多元醇通路的限速酶。最新研究表明：心肌缺血时醛糖还原酶活性增强，抑制醛糖还原酶可以通过保护糖酵解途径和抑制氧化应激，减轻心肌缺血损伤，改善缺血再灌注后心肌功能。醛糖还原酶抑制剂作为潜在的治疗心肌梗死的有效措施引起了研究者的关注。     

The effect of N-acetyl-l-cysteine and ascorbic acid on visible-light-irradiated camphorquinone/N,N-dimethyl-p-toluidine-induced oxidative stress in two immortalized cell lines

Recent studies have revealed that visible-light (VL)-irradiated camphorquinone (CQ), in the presence of a tertiary amine (e.g., N,N-dimethyl-p-toluidine, DMT), generates initiating radicals that may indiscriminately react with molecular oxygen forming reactive oxygen species (ROS). In this study, the ability of the antioxidants N-acetyl-l-cysteine (NAC) and ascorbic acid (AA) to reduce intracellular oxidative stress induced by VL-irradiated CQ/DMT or VL-irradiated hydrogen peroxide (H(2)O(2)) was assessed in an immortalized Murine cementoblast cell line (OCCM.30) and an immortalized Murine fibroblast cell line, 3T3-Swiss albino (3T3). Intracellular oxidative stress was measured with the membrane permeable dye, 2',7'-dichlorodihydrofluorescein diacetate (H(2)DCF-DA). VL-irradiated CQ/DMT and VL-irradiated H(2)O(2) each produced significantly (p<0.001) elevated intracellular oxidative levels in both cell types compared to intracellular ROS levels in VL-irradiated untreated cells. OCCM.30 cementoblasts were found to be almost twice as sensitive to VL-irradiated CQ/DMT and VL-irradiated H(2)O(2) treatment compared to 3T3 fibroblasts. Furthermore, 10mm NAC and 10mm AA each eliminated oxidative stress induced by VL-irradiated CQ/DMT and VL-irradiated H(2)O(2) in both cell types. Our results suggest that NAC and AA may effectively reduce or eliminate oxidative stress in cells exposed to VL-irradiated CQ/DMT following polymerization.     

Hydrogen peroxide-induced activation of SAPK/JNK regulated by phosphatidylinositol 3-kinase in Chinese hamster V79 cells

To clarify activation mechanisms of stress-activated protein kinase/C-Jun N-terminal kinase (SAPK/JNK) during oxidative stress, the roles of phosphatidylinositol 3-kinase (PI 3-kinase), concentration of intracellular calcium ([Ca2+]i), and cyclic AMP-dependent kinase (PKA) in hydrogen peroxide (H2O2)-induced SAPK/JNK activation were examined in Chinese hamster V79 cells. SAPK/JNK was dose-dependently activated after H2O2 treatment (from 10 microM to 1 mM), and a PI 3-kinase inhibitor (wortmaninn), intracellular calcium chelator (BAPTA-AM), and PKA activator (dibutyl cyclic AMP and forskolin) inhibited this activation. An increase in [Ca2+], was observed after treatment with H2O2. Immunoprecipitation revealed that a PI 3-kinase regulatory subunit, p85alpha, was associated with insulin receptor substance 1 (IRS-1) phosphorylated by H2O2 treatment. Furthermore, the formation of this complex of p85alpha and phospho-IRS-1 was abolished by the presence of BAPTA-AM but not forskolin. These results indicated that the PI 3-kinase activated through phosphorylation of IRS-1 upstream of SAPK/JNK after H2O2 treatment of V79 cells and that [Ca2+]i was a regulation factor for phosphorylation of IRS-1.     

Resistance of rat pulmonary alveolar epithelial cells to neutrophil- and oxidant-induced injury

We have previously reported that rat pulmonary alveolar epithelial cells are resistant to neutrophil-generated oxidants in contrast to the situation described for endothelial cells. In the present study, we investigated the roles of intracellular catalase and glutathione-dependent reactions in providing protection against cytotoxic concentrations of H2O2 and stimulated neutrophils. Catalase was found to be instrumental in protecting epithelial cells because when inhibited by either azide or 3-amino-1,2,4-triazole, there was an increase in the cytotoxic effect of exogenous H2O2 and stimulated neutrophils. Associated with this potentiation of injury was a reduction in epithelial cell clearance of H2O2. Partial inhibition of glutathione-dependent reactions by depleting intracellular glutathione with buthionine sulfoximine or by inhibiting the enzyme glutathione reductase with 1,3-bis(2-chloroethyl)-1-nitrosourea also augmented the cytotoxic effect of both H2O2 and stimulated neutrophils. This increase in neutrophil-induced cytotoxicity was caused by the addition of an oxidant-dependent mechanism of killing on top of the previously described oxidant-independent pathway. Importantly, the increased susceptibility to injury caused by inhibition of glutathione-dependent reactions was not associated with a reduction in epithelial cell consumption of exogenous H2O2, contrary to the case with catalase. This suggests that there are glutathione-dependent reactions that protect epithelial cells in ways separate from reducing the total burden of exogenous H2O2 on the cells.     

Oxidative stress differentially modulates phosphorylation of ERK, p38 and CREB induced by NGF or EGF in PC12 cells

This study assessed if oxidative stress induced by treatment of PC12 cells with H2O2 modulated signaling cascades induced by nerve growth factor (NGF) or epidermal growth factor (EGF) because oxidative stress and impaired growth factor function are associated with aging and aging-associated diseases such as Alzheimer's disease. Phosphorylation of extracellular signal-regulated kinases 1 and 2 (ERK 1/2) and of p38 kinase was rapidly increased after treatment with NGF, EGF, or H2O2, with NGF causing more prolonged increases than the other agents. Pretreatment with H2O2 did not alter phosphorylation of ERK1/2 induced by either growth factor, but increased the phosphorylation of p38 kinase induced by treatment with NGF or EGF alone. CREB phosphorylation at SER 133 was rapidly increased by treatment with either NGF or EGF. Pretreatment with H2O2 reduced CREB phosphorylation induced by either growth factor. This seemed to be a direct effect because H2O2 also inhibited CREB phosphorylation induced by the adenylyl cyclase stimulator forskolin. These results demonstrate that oxidative stress can differentially modulate growth factor-initiated signaling cascades. Furthermore, because CREB is an evolutionarily preserved protein involved in the formation of long term memory, these results indicate a new target of oxidative stress that may be important in disorders involving impaired memory, such as Alzheimer's disease.     

D-Xylose metabolism by Candida intermedia: isolation and characterisation of two forms of aldose reductase with different coenzyme specificities

To study individual enzyme components responsible for the initial step of D-xylose utilisation by the yeast Candida intermedia, a two-step protocol has been developed that enables clear-cut separation and isolation of two structurally similar but functionally different aldose reductases (ALRs) in high yield. In the first step, the yeast cell extract is fractionated efficiently by biomimetic chromatography using the dye HE-3B (reactive Red 120) as pseudoaffinity ligand coupled to Sepharose CL-4B. In the second step, optimised high-resolution anion-exchange chromatography using Mono Q yields purified ALR1 and ALR2 in overall yields of 63 and 62%, respectively. ALR1 is strictly specific for NADPH (2.4 x 10(5) M(-1) s(-1)) whereas ALR2 utilises NADH and NADPH with similar specificity constants of approximately 2-4 x 10(5) M(-1) s(-1). Both enzymes are dimers with a subunit molecular mass of 36000 but they differ in pI and the number of titratable sulphydryl groups in the native protein. The chromatographic procedure identifies microheterogeneity in recombinant aldose reductase from Candida tenuis overexpressed in Escherichia coli.     

In vivo pharmacokinetics of aldose reductase inhibitors: 3-fluoro-3-deoxy-D-glucose NMR studies in rat brains

Direct quantification of the inhibitory effects of orally administered drugs (sorbinil, cyclandelate and sulindac) on aldose reductase activities in rat brains was performed non-invasively using the 3-fluoro-3-deoxy-D-glucose (3-FDG) 19F NMR spectroscopic technique. Quantitative data obtained directly from the target organ (brain) allowed for unprecedentedly accurate analysis of drug effects in the brain in vivo. Sorbinil, a potent aldose reductase inhibitor, exhibited a classic monophasic organ response, whereas cyclandelate and sulindac showed biphasic suppression patterns. The latter indicate that there are metabolites of these drugs which possess aldose reductase inhibitory activities. The estimated potency of aldose reductase inhibition for each of the three drugs in this study was significantly discrepant from the in vitro studies indicating the complicated nature of the bioavailability of a pharmaceutical agent in vivo, especially where pharmacologically active metabolites of a given drug are involved. Our method allows for a direct quantitative assay and hence the most reliable technique for evaluating aldose reductase inhibitory activities in the target organ.     

Antioxidant enzyme activities of iron-saturated bovine lactoferrin (Fe-bLf) in human gut epithelial cells under oxidative stress

Chemoprevention by dietary constituents in the form of functional food has emerged as a novel approach to control inflammatory diseases and cancers. Recently we reported for the first time that iron content is a critical determinant in the anti-tumour activity of bovine milk lactoferrin (bLf). We therefore wanted to evaluate the chemo-preventative efficacy of Apo-bLF and 100% iron-saturated bLF (Fe-bLF) on hydrogen peroxide (H2O2)-induced colon carcinogenesis, and their influence on antioxidant enzyme activities within colon carcinogenesis. This was undertaken through observing how oxidative stress induced by H2O2 alters antioxidant enzyme activity within HT29 colon cancer cells, and then observing changes in this activity by treatments with the different antioxidants ascorbic acid (AA), Apo-bLF and Fe-bLF. All antioxidant enzymes (catalase, glutathione peroxidase (GPx), glutathione reductase (GR), glutathione-s-transferase (GsT) and superoxide dismutase (SOD)) appeared to be increased within HT29 cells, even prior to H2O2 exposure, and all enzymes showed significant decreased activity when cells were treated with the antioxidants AA, Apo-bLF or Fe-bLF, with or without H2O2 exposure. The results indicate that all three antioxidants have the ability to scavenge ROS, lower antioxidant enzyme activities within already excited states, and possibly allow colon cancer cells to be overcome by oxidative stress that would normally be prevented, perhaps leading to damage and potential apoptosis of the cancer cells. In conclusion, the anti-oxidative effects of Apo-bLF and Fe-bLf studied for the first time, show dynamic changes that may allow for necessary protection from imbalanced oxidative conditions, and potential at reducing the ability of cancer cells to protect themselves from oxidative stress states.     

Oxidative stress impairs heme detoxification in the midgut of the cattle tick, Rhipicephalus (Boophilus) microplus

In the cattle tick Rhipicephalus (Boophilus) microplus digestion of blood is intracellular, accomplished by the so-called digest cells that fill the midgut lumen. Hydrolysis of hemoglobin in the digestive vesicles of these cells results in the release of large amounts of heme, a pro-oxidant compound, whose iron atom, together with H(2)O(2), may participate in the Fenton reaction and lead to the production of hydroxyl radicals. Here, we investigated the role of catalase, an enzyme responsible for H(2)O(2) detoxification. Fully engorged female ticks injected with 3-amino-1,2,4-triazole (AT), a catalase inhibitor, showed increased H(2)O(2) in the gut, together with diminished life span and lower egg-laying rates. Increased mortality observed upon AT injection was reversed by further injection of exogenous catalase, 2 days after AT treatment, confirming that increased death was due to inhibition of this enzyme by AT. In primary cultures of digest cells, intracellular H(2)O(2) is limited to specific organelles, while treatment with AT in vitro resulted in increased H(2)O(2) spreading all over the cell, confirming the role of catalase in regulating H(2)O(2) levels. Ticks fed on a calf that had been injected with AT showed marked inhibition of catalase activity in the gut and diminished life span, oviposition and engorgement. Digest cells of these ticks had an altered morphology, showing heme spread all over the cytosol, instead of being limited to the hemosomes. The amount of aggregated heme found in isolated hemosome was also strongly decreased in AT-treated cattle. All together, our results indicate that catalase performs an important role in the control of redox balance in R. microplus, which dramatically affects hemosome formation and stability. This enzyme may be a target in the development of new methods for tick control.     

Bovine retinal pericytes are resistant to glucose-induced oxidative stress in vitro

Diabetic retinopathy is a sight-threatening complication of diabetes, and loss of pericytes represents early signs of its development. We tested the hypothesis that high glucose levels may induce signs of oxidative stress in cultured bovine retinal pericytes. Pericytes were exposed to either normal (5.5 mM) or high (22 mM) glucose levels for 1, 3, and 5 days. Signs of oxidative stress were measured by expression of copper/zinc superoxide dismutase, manganese superoxide dismutase, catalase, and glutathione peroxidase using real-time RTPCR. To elucidate the role of oxidative stress, we also measured glutathione (GSH) concentration in the cells and investigated the impact of thiol-reactive metal ions and hydrogen peroxide (H(2)O(2)) on intracellular GSH. Despite the stimulation with high glucose, thiol-reactive metal ions, or H(2)O(2), there was no clear increased expression of antioxidant enzymes or influence of GSH levels. Lipid peroxidation (malondialdehyde level) was increased in bovine aortic smooth muscle cells, but not in bovine retinal pericytes. The data indicate that pericytes do not develop oxidative stress in response to hyperglycemia. However, it is not definitively excluded that oxidative stress may occur after longer time periods of glucose stimulation.     

Mechanisms of H2O2-induced oxidative stress in endothelial cells exposed to physiologic shear stress

Hydrogen peroxide (H2O2) is produced by inflammatory and vascular cells and induces oxidative stress, which may contribute to vascular disease and endothelial cell dysfunction. In smooth muscle cells, H2O2 induces production of O2 by activating NADPH oxidase. However, the mechanisms whereby H2O2 induces oxidative stress in endothelial cells are not well understood, although O2 may play a role. Recent studies have documented increased O2 in endothelial cells exposed to H2O2 via uncoupled nitric oxide synthase (NOS) and NADPH oxidase under static conditions. To assess responses to H2O2 in porcine aortic endothelial cells (PAEC) under shearing conditions, a constant flow rate of 24. 4 ml/min was applied to produce physiologically relevant shear stress (8. 2 dynes/cm). Here we demonstrate that treatment with 100 muM H2O2 increases intracellular O2 levels in PAEC. In addition, we demonstrate that l-NAME, an inhibitor of NOS, and apocynin, an inhibitor of NADPH oxidase, reduced O2 levels in PAEC treated with H2O2 under physiologic shear suggesting that both NOS and NADPH oxidase contribute to H2O2-induced O2 in PAEC. Co-inhibition of NOS and NADPH oxidase also reduced intracellular O2 levels under shear. We conclude that H2O2-induced oxidative stress in endothelial cells exhibits increased intracellular O2 levels through NOS and NADPH oxidase under shear. The inhibition of NOS and NADPH with H2O2 exposure is nonlinear, suggesting some interdependent or compensating system within endothelial cells. These findings suggest a complex interaction between H2O2 and oxidant-generating enzymes that may contribute to endothelial dysfunction in cardiovascular diseases.     

Role of protein-tyrosine kinase syk in oxidative stress signaling in B cells

Oxidative stress induces the activation of multiple signaling pathways related to various cellular responses. In B cells, Syk has a crucial role in intracellular signal transduction induced by oxidative stress as well as antigen receptor engagement. Treatment of B cells with hydrogen peroxide (H(2)O(2)) induces enzymatic activation of Syk. Syk is essential for Ca(2+) release from intracellular pools through phospholipase C-gamma2 and the activation of c-Jun N-terminal kinase, p38 mitogen-activated protein kinase, and phosphatidylinositol 3-kinase-Akt survival pathway following H(2)O(2) stimulation. Oxidative stress-induced cellular responses in B cells follow different patterns, such as necrosis, apoptosis, and mitotic arrest, according to the intensity of H(2)O(2) stimulation. Syk is involved in the protection of cells from apoptosis and induction of G2/M arrest. Syk leads to the activation of the phosphatidylinositol 3-kinase-Akt survival pathway, thereby enhancing cellular resistance to oxidative stress-induced apoptosis. On the other hand, Syk-dependent phospholipase C-gamma2 activation is required for acceleration toward apoptosis following oxidative stress. These findings suggest that oxidative stress-induced Syk activation triggers the activation of several pathways, such as proapoptotic and survival pathways, and the balance among these various pathways is a key factor in determining the fate of a cell exposed to oxidative stress.     

The NADPH quinone reductase MdaB confers oxidative stress resistance to Helicobacter hepaticus

An mdaB mutant strain in a quinone reductase (MdaB) of Helicobacter hepaticus type strain ATCC51449 was constructed by insertional mutagenesis, and the MdaB protein was purified and compared to the Helicobacter pylori enzyme. While wild type H. hepaticus cells could tolerate 6% O(2) for growth, the mdaB strain was clearly inhibited at this oxygen level. Disruption of the gene downstream of mdaB (HH1473) did not affect the oxidative stress phenotype of the strain. The mdaB mutant was also more sensitive to oxidative stress reagents such as H(2)O(2), cumene hydroperoxide, t-butyl hydroperoxide, and paraquat. All H. hepaticus mdaB strains isolated constitutively up-expressed another oxidative stress-combating enzyme, superoxide dismutase; this is in contrast to H. pylori mdaB strains. H. hepaticus MdaB is a flavoprotein catalyzing quinone reduction using a two-electron transfer mechanism from NAD(P)H to quinone. The H. hepaticus enzyme specific activity was far less than for the H. pylori enzyme purified in the same manner.     

Redox regulation of PI3K/Akt and p53 in bovine aortic endothelial cells exposed to hydrogen peroxide

To clarify the apoptotic and survival signal transduction pathways in activated vascular endothelial cells exposed to oxidative stress, the effects of inhibitors of signal transduction on hydrogen peroxide (H(2)O(2))-induced apoptosis in bovine aortic vascular endothelial cells (BAEC) were examined. Treatment of BAEC with 1 mM H(2)O(2) caused increases of DNA fragmentation, p53 expression, Bax/Bcl-2 ratio, and the activities of caspases 3 and 9. The increases of DNA fragmentation, Bax/Bcl-2 ratio, and caspase activities were abrogated by BAPTA-AM (an intracellular Ca(2+) chelator) and N-acetyl-L-cysteine (an antioxidant), and augmented by wortmannin [a phosphatidylinositol 3-kinase (PI3K) inhibitor]. The increase of the intracellular Ca(2+) concentration ([Ca(2+)](i)) observed in H(2)O(2)-stimulated cells was unaffected by wortmannin, suggesting that the potentiating effect of wortmannin on the apoptosis was not due to an alteration of [Ca(2+)](i). H(2)O(2) increased the levels of PI3K activity and Akt phosphorylation. Both were attenuated by wortmannin and, to a lesser extent, by genistein (a tyrosine kinase inhibitor) and suramin (a growth factor receptor inhibitor), but not affected by BAPTA-AM. These results suggest that H(2)O(2) induces Ca(2+)-dependent apoptosis and Ca(2+)-independent survival signals such as redox-regulated activation of PI3K/Akt, which is partly mediated by the activation of growth factor receptors in BAEC.     

Effect of high-risk human papillomavirus oncoproteins on p53R2 gene expression after DNA damage

The p53R2 protein is a p53-inducible small subunit of ribonucleotide reductase. It plays a crucial role in p53-dependent cellular response to DNA damage and oxidative stress by providing deoxyribonucleotides (dNTPs) to the DNA repair machinery and by scavenging reactive oxygen species (ROS). To investigate the effects of high-risk human papillomavirus (HPV) oncoproteins on p53R2 expression after DNA damage, we analyzed the p53R2 protein levels in human cells ectopically expressing the HPV-16 E6 and E7 genes, and in the HPV-positive cancer cell lines SiHa, CaSki and HeLa, exposed to adriamycin or to H(2)O(2). We found that in normal cells, p53R2 expression is efficiently induced by both H(2)O(2) and adriamycin, supporting the role of p53R2 in cellular response to oxidative stress. Ectopic expression of E6 impaired p53 and p53R2 induction after DNA damage in human fibroblasts. Moreover, SiHa, CaSki and HeLa cells were unresponsive to H(2)O(2) exposure, and adriamycin induced p53R2 levels only in SiHa cells. Our results imply that high-risk HPV infection may suppress the p53R2-dependent dNTPs supply to the DNA repair system and the ROS scavenging activity; they also suggest that an altered p53R2 response to genotoxins and to oxidative stress may contribute to HPV-induced genetic instability and carcinogenesis.     

Syk is required for p38 activation and G2/M arrest in B cells exposed to oxidative stress

Syk has been demonstrated to play a crucial role in oxidative stress signaling in B cells. In this study, we have investigated the role of Syk in p38 activation and the regulation of cell-cycle progression upon oxidative stress. In B cells, p38 is activated by hydrogen peroxide (H(2)O(2)) stimulation. Syk is required for p38 activation following stimulation with 10-100 microM H(2)O(2), but not with 1 mM H(2)O(2). H(2)O(2)-induced p38 activation is abrogated in phospholipase C-gamma2 (PLC-gamma2)-deficient as well as Syk-deficient cells, suggesting that Syk activates p38 through PLC-gamma2 upon H(2)O(2) stimulation. Although stimulation with 20-100 microM H(2)O(2) induces cellular apoptosis in B cells, pretreatment with SB203580, a p38-specific inhibitor, has no effect on H(2)O(2)-induced apoptosis. Flow cytometric analysis reveals that B cells exposed to 10-20 microM H(2)O(2) exhibit cell-cycle profile of G2/M arrest, and pretreatment with SB203580 inhibits only a little H(2)O(2)-induced G2/M arrest. On the other hand, Syk-deficient cells show no induction of G2/M arrest following H(2)O(2) stimulation. These findings indicate that Syk plays a role in the regulation of cell-cycle progression in G2/M phase via p38-dependent and -independent pathways after oxidative stress.