Activation of c-Jun N-terminal kinase and apoptosis in endothelial cells mediated by endogenous generation of hydrogen peroxide

Distinct Effects of Beta-Amyloid, Its Isomerized and Phosphorylated Forms on the Redox Status and Mitochondrial Functioning of the Blood-Brain Barrier Endothelium

The Alzheimer's disease (AD)-associated breakdown of the blood-brain barrier (BBB) promotes the accumulation of beta-amyloid peptide (Aβ) in the brain as the BBB cells provide Aβ transport from the brain parenchyma to the blood, and vice versa. The breakdown of the BBB during AD may be caused by the emergence of blood-borne Aβ pathogenic forms, such as structurally and chemically modified Aβ species; their effect on the BBB cells has not yet been studied. Here, we report that the effects of Aβ₄₂, Aβ₄₂, containing isomerized Asp7 residue (iso-Aβ₄₂) or phosphorylated Ser8 residue (p-Aβ₄₂) on the mitochondrial potential and respiration are closely related to the redox status changes in the mouse brain endothelial cells bEnd.3. Aβ₄₂ and iso-Aβ₄₂ cause a significant increase in nitric oxide, reactive oxygen species, glutathione, cytosolic calcium and the mitochondrial potential after 4 h of incubation. P-Aβ₄₂ either does not affect or its effect develops after 24 h of incubation. Aβ₄₂ and iso-Aβ₄₂ activate mitochondrial respiration compared to p-Aβ₄₂. The isomerized form promotes a greater cytotoxicity and mitochondrial dysfunction, causing maximum oxidative stress. Thus, Aβ₄₂, p-Aβ₄₂ and iso-Aβ₄₂ isoforms differently affect the BBBs' cell redox parameters, significantly modulating the functioning of the mitochondria. The changes in the level of modified Aβ forms can contribute to the BBBs' breakdown during AD.

Hydrogen peroxide signaling in vascular endothelial cells

Redox signaling is implicated in different physiological and pathological events in the vasculature. Among the different reactive oxygen species, hydrogen peroxide (H₂O₂) is a very good candidate to perform functions as an intracellular messenger in the regulation of several biological events.

In this review, we summarize the main physiological sources of H₂O₂ in the endothelium and the molecular mechanisms by which it is able to act as a signaling mediator in the vasculature.

Antiapoptotic and proapoptotic signaling of cyclophilin A in endothelial cells

Endothelial cell (EC) dysfunction is a key event in the onset and progression of atherosclerosis. Apoptosis may lead to endothelial dysfunction and contribute to vascular complications. Cyclophilin A (CyPA) is the main reactive oxygen species-induced factor that enhances the inflammatory activity of vascular cells in atherosclerotic plaques. However, the mechanism by which CyPA induces EC apoptosis is not entirely clear. Through Western blot, it demonstrated that extracellular CyPA activated the Akt and NF-κB pathway, followed by the upregulation of antiapoptotic protein Bcl-2 expression in ECs. When blocking intracellular CyPA by small interfering RNA in ECs, the effects of TNF-α-induced EC apoptosis and proapoptotic protein caspase-3 expression were significantly inhibited. This study shows that CyPA may initiate antiapoptotic and proapoptotic signaling in ECs, especially in response to reactive oxygen species stimulation. It serves as a potential target for atherosclerosis therapy.

Dose-dependent regulation of superoxide anion on the proliferation, differentiation, apoptosis and necrosis of human hepatoma cells: the role of intracellular Ca2+

Dose-dependent regulation of cellular processes is one important characteristic of signaling molecules. Although recent studies suggest that reactive oxygen species (ROS) may act as in vivo signaling molecules, the dose-dependent regulation of ROS on cellular processes together in one system needs to be evaluated. After treating cells with gradually increased O(2)(-), generated by the hypoxanthine-xanthine oxidase system, it was found that: (i) the proliferation of hepatoma cells firstly increased at 1-2 microM O(2)(-), then decreased markedly as the concentration increased; (2) at 8 or 16 microM O(2)(-), re-differentiation of hepatoma cells was induced, as indicated by the indices relating to cell malignancy or differentiation, such as cell surface charge, alpha-fetoprotein, gamma-glutamyltranspeptidase, tyrosine-alpha-ketoglutarate transaminase, cAMP, and the tumor's clonogenic potential; (iii) at 16 microM O(2)(-), accompanied by the re-differentiation of cells, cell apoptosis was also simultaneously induced as indicated by the appearance of apoptotic bodies, detached cells, and other apoptotic morphological features, as well as specific DNA fragmentation; (iv) at the highest concentration of O(2)(-) (32 microM) in this study, cell necrosis was dramatically induced as shown by Trypan blue exclusion; (v), an increase of intracellular Ca(2+) ([Ca(2+)](i)) was observed at all concentrations of O(2)(-) treatment, and this [Ca(2+)](i) increase was found to be involved in the regulation of O(2)(-) on the cellular processes. In conclusion, these results indicate that O(2)(-) could dose-dependently regulate the processes of cells, where Ca(2+) is one of its molecular targets, and hence provide a direct support for the hypothesis that ROS themselves are important signaling molecules.

High natural gene expression variation in the reef-building coral Acropora millepora: potential for acclimative and adaptive plasticity

BACKGROUND

Ecosystems worldwide are suffering the consequences of anthropogenic impact. The diverse ecosystem of coral reefs, for example, are globally threatened by increases in sea surface temperatures due to global warming. Studies to date have focused on determining genetic diversity, the sequence variability of genes in a species, as a proxy to estimate and predict the potential adaptive response of coral populations to environmental changes linked to climate changes. However, the examination of natural gene expression variation has received less attention. This variation has been implicated as an important factor in evolutionary processes, upon which natural selection can act.

RESULTS

We acclimatized coral nubbins from six colonies of the reef-building coral Acropora millepora to a common garden in Heron Island (Great Barrier Reef, GBR) for a period of four weeks to remove any site-specific environmental effects on the physiology of the coral nubbins. By using a cDNA microarray platform, we detected a high level of gene expression variation, with 17% (488) of the unigenes differentially expressed across coral nubbins of the six colonies (jsFDR-corrected, p < 0.01). Among the main categories of biological processes found differentially expressed were transport, translation, response to stimulus, oxidation-reduction processes, and apoptosis. We found that the transcriptional profiles did not correspond to the genotype of the colony characterized using either an intron of the carbonic anhydrase gene or microsatellite loci markers.

CONCLUSION

Our results provide evidence of the high inter-colony variation in A. millepora at the transcriptomic level grown under a common garden and without a correspondence with genotypic identity. This finding brings to our attention the importance of taking into account natural variation between reef corals when assessing experimental gene expression differences. The high transcriptional variation detected in this study is interpreted and discussed within the context of adaptive potential and phenotypic plasticity of reef corals. Whether this variation will allow coral reefs to survive to current challenges remains unknown.

Assessing bioenergetic function in response to oxidative stress by metabolic profiling

It is now clear that mitochondria are an important target for oxidative stress in a broad range of pathologies, including cardiovascular disease, diabetes, neurodegeneration, and cancer. Methods for assessing the impact of reactive species on isolated mitochondria are well established but constrained by the need for large amounts of material to prepare intact mitochondria for polarographic measurements. With the availability of high-resolution polarography and fluorescence techniques for the measurement of oxygen concentration in solution, measurements of mitochondrial function in intact cells can be made. Recently, the development of extracellular flux methods to monitor changes in oxygen concentration and pH in cultures of adherent cells in multiple-sample wells simultaneously has greatly enhanced the ability to measure bioenergetic function in response to oxidative stress. Here we describe these methods in detail using representative cell types from renal, cardiovascular, nervous, and tumorigenic model systems while illustrating the application of three protocols to analyze the bioenergetic response of cells to oxidative stress.

Over-expression of mitogen-activated protein kinase phosphatase-2 enhances adhesion molecule expression and protects against apoptosis in human endothelial cells

BACKGROUND AND PURPOSE

We assessed the effects of over-expressing the dual-specific phosphatase, mitogen-activated protein (MAP) kinase phosphatase-2 (MKP-2), in human umbilical vein endothelial cells (HUVECs) on inflammatory protein expression and apoptosis, two key features of endothelial dysfunction in disease.

EXPERIMENTAL APPROACHES

We infected HUVECs for 40 h with an adenoviral version of MKP-2 (Adv.MKP-2). Tumour necrosis factor (TNF)-α-stimulated phosphorylation of MAP kinase and protein expression was measured by Western blotting. Cellular apoptosis was assayed by FACS.

KEY RESULTS

Infection with Adv.MKP-2 selectively abolished TNF-α-mediated c-Jun-N-terminal kinase (JNK) activation and had little effect upon extracellular signal-regulated kinase or p38 MAP kinase. Adv.MKP-2 abolished COX-2 expression, while induction of the endothelial cell adhesion molecules, intercellular adhesion molecule (ICAM) and vascular cell adhesion molecule (VCAM), two NFκB-dependent proteins, was not affected. However, when ICAM and VCAM expression was partly reduced by blockade of the NFκB pathway, Adv.MKP-2 was able to reverse this inhibition. This correlated with enhanced TNF-α-induced loss of the inhibitor of κB (IκB)α loss, a marker of NFκB activation. TNF-α in combination with NFκB blockade also increased HUVEC apoptosis; this was significantly reversed by Adv.MKP-2. Protein markers of cellular damage and apoptosis, H2AX phosphorylation and caspase-3 cleavage, were also reversed by MKP-2 over-expression.

CONCLUSIONS AND IMPLICATIONS

Over-expression of MKP-2 had different effects upon the expression of inflammatory proteins due to a reciprocal effect upon JNK and NFκB signalling, and also prevented TNF-α-mediated endothelial cell death. These properties may make Adv.MKP-2 a potentially useful future therapy in cardiovascular diseases where endothelial dysfunction is a feature.

Thiol redox systems and protein kinases in hepatic stellate cell regulatory processes

Hepatic stellate cells (HSC) are the major producers of collagen in the liver and their conversion from resting cells to a proliferating, contractile and fibrogenic phenotype ('activation') is a critical step, leading to liver fibrosis characterized by deposition of excessive extracellular matrix. Cytokines, growth factors, reactive oxygen and nitrogen species (ROS/RNS), lipid peroxides and their products deriving from hepatocytes, Kupffer cells and other cells converge on HSC and influence their activation. This review focuses on glutathione and thioredoxin pathways, with particular emphasis on their role in HSC. These two systems have been shown to act in the metabolism of hydrogen peroxide, control of thiol redox balance and regulation of signalling pathways. Particular attention is paid to mitochondria and NADPH oxidase. Detailed knowledge of specific signalling, redox conditions and apoptotic processes will be of help in devising proper pharmacological treatments for liver fibrosis.

Mitochondrial reserve capacity in endothelial cells: The impact of nitric oxide and reactive oxygen species

The endothelium is not considered to be a major energy-requiring organ, but nevertheless endothelial cells have an extensive mitochondrial network. This suggests that mitochondrial function may be important in response to stress and signaling in these cells. In this study, we used extracellular flux analysis to measure mitochondrial function in adherent bovine aortic endothelial cells (BAEC). Under basal conditions, BAEC use only approximately 35% of their maximal respiratory capacity. We calculate that this represents an intermediate respiratory state between States 3 and 4, which we define as State(apparent) equal to 3.64. Interestingly, the apparent respiratory control ratio (maximal mitochondrial oxygen consumption/non-ADP-linked respiration) in these cells is on the order of 23, which is substantially higher than that which is frequently obtained with isolated mitochondria. These results suggest that mitochondria in endothelial cells are highly coupled and possess a considerable bioenergetic reserve. Because endothelial cells are exposed to both reactive oxygen (ROS) and reactive nitrogen species in the course of vascular disease, we hypothesized that this reserve capacity is important in responding to oxidative stress. To test this, we exposed BAEC to NO or ROS alone or in combination. We found that exposure to nontoxic concentrations of NO or low levels of hydrogen peroxide generated from 2,3-dimethoxy-1,4-napthoquinone (DMNQ) had little impact on basal mitochondrial function but both treatments reversibly decreased mitochondrial reserve capacity. However, combined NO and DMNQ treatment resulted in an irreversible loss of reserve capacity and was associated with cell death. These data are consistent with a critical role for the mitochondrial reserve capacity in endothelial cells in responding to oxidative stress.

Association of superoxide anions with retinal pigment epithelial cell apoptosis induced by mononuclear phagocytes

PURPOSE

Oxidative stress of the retinal pigment epithelium by reactive oxygen species and monocytic infiltration have been implicated in age-related macular degeneration. The purpose of this study was to determine the role of superoxide anions (O(2)(-)) in mononuclear phagocyte-induced RPE apoptosis.

METHODS

Mouse RPE cell cultures were established from wild-type and heterozygous superoxide dismutase 2-knockout (Sod2(+/-)) mice. The intracellular reactive oxygen species, O(2)(-) and hydrogen peroxide, were measured by using dihydroethidium assay and 5-(and 6)-chloromethyl-2',7'-dichlorodihydrofluorescence diacetate, acetyl ester assay, respectively. RPE apoptosis was evaluated by Hoechst staining and terminal deoxynucleotidyltransferase dUTP nick-end labeling assay. Changes in mitochondrial membrane potential were detected by 5,5',6,6'-tetrachloro-1,1',3,3'-tetraethylbenzimidazolylcarbocyanine iodide dye. Activated caspases and caspase-3 were detected in situ by FITC-VAD-fmk staining and caspase-3 substrate, respectively.

RESULTS

Mononuclear phagocytes and interferon-gamma-activated mononuclear phagocytes induced the production of intracellular RPE O(2)(-), a decrease in RPE mitochondrial membrane potential, caspase activation, and apoptosis of mouse RPE cells. All theses changes were significantly enhanced in the Sod2(+/-) RPE cells. Activated mononuclear phagocytes induced more of these oxidative and apoptotic changes in RPE cells than did unstimulated mononuclear phagocytes.

CONCLUSIONS

The authors have shown that the decreased expression of SOD2 and increased superoxide production correlate with RPE apoptosis induced by unstimulated and activated mononuclear phagocytes. The authors suggest that elevated O(2)(-) levels due to genetic abnormalities of SOD2 or immunologic activation of mononuclear phagocytes lead to greater levels of RPE apoptosis. The present study could serve as a useful model to characterize RPE/phagocyte interaction in AMD and other retinal diseases.

Oxidative stress-induced regulation of the methionine metabolic pathway in human lung epithelial-like (A549) cells

The effects of low, moderate and severe oxidative stress on the steady-state levels of the metabolites involved in the transmethylation/transsulfuration pathway were studied in lung epithelial (A549) cells. When cells were exposed to low (0.1 mM) or moderate (1.0 mM) concentrations of hydrogen peroxide (H(2)O(2)) or tert-butylhydroperoxide (t-butOOH), intracellular levels of S-adenosylmethionine (SAM) and S-adenosylhomocysteine (SAH) were significantly decreased, while the SAM/SAH ratio remained the same or elevated. Likewise, extracellular levels of SAM and SAH metabolites remained steady or elevated. Both intracellular and extracellular levels of homocyst(e)ine and cyst(e)ine were decreased. Cell contents of serine, cystathionine and methionine were also decreased. Total intracellular glutathione content was decreased only by moderate t-butOOH exposure. When cells were exposed to high concentrations (10mM) of either of the peroxides, extracellular levels of methionine, cystathionine, and total cyst(e)ine were depleted, mostly due to direct oxidation of sulfur amino acids by peroxides, as indicated by oxidative treatment of culture media alone. Similar to low and moderate oxidative conditions, the levels of SAM, SAH, and sulfur amino acids were decreased, while cell SAM/SAH ratio increased. Paradoxically, under high peroxide exposure, extracellular concentrations of SAM, SAH, and cyst(e)ine were increased, indicating cellular release, despite the severe methionine depletion. Intracellular total glutathione was also decreased. The results indicate that lung epithelial cells release high levels of SAM, probably as an adaptive response to increased oxidative stress, even when the substrate for SAM formation, methionine, is critically depleted.

Thiol chemistry in peroxidase catalysis and redox signaling

The oxidation chemistry of thiols and disulfides of biologic relevance is described. The review focuses on the interaction and kinetics of hydrogen peroxide with low-molecular-weight thiols and protein thiols and, in particular, on sulfenic acid groups, which are recognized as key intermediates in several thiol oxidation processes. In particular, sulfenic and selenenic acids are formed during the catalytic cycle of peroxiredoxins and glutathione peroxidases, respectively. In turn, these enzymes are in close redox communication with the thioredoxin and glutathione systems, which are the major controllers of the thiol redox state. Oxidants formed in the cell originate from several different sources, but the major producers are NADPH oxidases and mitochondria. However, a different role of the oxygen species produced by these sources is apparent as oxidants derived from NADPH oxidase are involved mainly in signaling processes, whereas those produced by mitochondria induce cell death in pathways including also the thioredoxin system, presently considered an important target for cancer chemotherapy.

Identification of a protective role for protein phosphatase 1cgamma1 against oxidative stress-induced vascular smooth muscle cell apoptosis

The development of therapeutic strategies to inhibit reactive oxygen species (ROS)-mediated damage in blood vessels has been limited by a lack of specific targets for intervention. Targeting ROS-mediated events in the vessel wall is of interest, because ROS play important roles throughout atherogenesis. In early atherosclerosis, ROS stimulate vascular smooth muscle cell (VSMC) growth, whereas in late stages of lesion development, ROS induce VSMC apoptosis, causing atherosclerotic plaque instability. To identify putative protective genes against oxidative stress, mouse aortic VSMC were infected with a retroviral human heart cDNA expression library, and apoptosis was induced in virus-infected cells by 2,3-dimethoxy-1,4-naphthoquinone (DMNQ) treatment. A total of 17 different, complete cDNAs were identified from the DMNQ-resistant VSMC clones by PCR amplification and sequencing. The cDNA encoding PP1cgamma1 (catalytic subunit of protein phosphatase 1) was present in several independent DMNQ-resistant VSMC clones. DMNQ increased mitochondrial ROS production, caspase-3/7 activity, DNA fragmentation, and decreased mitochondrial transmembrane potential in VSMC while decreasing PP1cgamma1 activity and expression. Depletion of PP1cgamma1 expression by short hairpin RNA significantly enhanced basal as well as DMNQ-induced VSMC apoptosis. PP1cgamma1 overexpression abrogated DMNQ-induced JNK1 activity, p53 Ser(15) phosphorylation, and Bax expression and protected VSMC against DMNQ-induced apoptosis. In addition, PP1cgamma1 overexpression attenuated DMNQ-induced caspase-3/7 activation and DNA fragmentation. Inhibition of p53 protein expression using small interfering RNA abrogated DMNQ-induced Bax expression and significantly attenuated VSMC apoptosis. Together, these data indicate that PP1cgamma1 overexpression promotes VSMC survival by interfering with JNK1 and p53 phosphorylation cascades involved in apoptosis.

Response of mitochondrial reactive oxygen species generation to steady-state oxygen tension: implications for hypoxic cell signaling

Mitochondria are proposed to play an important role in hypoxic cell signaling. One currently accepted signaling paradigm is that the mitochondrial generation of reactive oxygen species (ROS) increases in hypoxia. This is paradoxical, because oxygen is a substrate for ROS generation. Although the response of isolated mitochondrial ROS generation to [O(2)] has been examined previously, such investigations did not apply rigorous control over [O(2)] within the hypoxic signaling range. With the use of open-flow respirometry and fluorimetry, the current study determined the response of isolated rat liver mitochondrial ROS generation to defined steady-state [O(2)] as low as 0.1 microM. In mitochondria respiring under state 4 (quiescent) or state 3 (ATP turnover) conditions, decreased ROS generation was always observed at low [O(2)]. It is concluded that the biochemical mechanism to facilitate increased ROS generation in response to hypoxia in cells is not intrinsic to the mitochondrial respiratory chain alone but may involve other factors. The implications for hypoxic cell signaling are discussed.

Redox-dependent expression of cyclin D1 and cell proliferation by Nox1 in mouse lung epithelial cells

NADPH oxidases produce reactive oxygen species (ROS) that serve as co-stimulatory signals for cell proliferation. In mouse lung epithelial cells that express Nox1, Nox2, Nox4, p22(phox), p47(phox), p67(phox), and Noxo1, overexpression of Nox1 delayed cell cycle withdrawal by maintaining AP-1-dependent expression of cyclin D1 in low serum conditions. In cycling cells, the effects of Nox1 were dose dependent: levels of Nox1 that induced 3- to 10-fold increases in ROS promoted phosphorylation of ERK1/2 and expression of cyclin D1, whereas expression of Nox1 with Noxo1 and Noxa1 (or expression of Nox4 alone) that induced substantial increases in intracellular ROS inhibited cyclin D1 and proliferation. Catalase reversed the effects of Nox1 on cyclin D1 and cell proliferation. Diphenylene iodonium, an inhibitor of NADPH oxidase activity, did not affect dosedependent responses of ERK1/2 or Akt to serum, but markedly inhibited the sequential expression of c-Fos and Fra-1 required for induction of cyclin D1 during cell cycle re-entry. These results indicate that Nox1 stimulates cell proliferation in actively cycling cells by reducing the requirement for growth factors to maintain expression of cyclin D1, whereas during cell cycle re-entry, NADPH oxidase activity is required for transcriptional activation of Fos family genes during the immediate early gene response.

Lipid rafts mediate H2O2 prosurvival effects in cultured endothelial cells

Reactive oxygen species (ROS) generated during pathological events, such as inflammation and ischemia-reperfusion, activates both proapoptotic and antiapoptotic signaling programs in endothelial cells. Because cholesterol-rich, plasma membrane rafts serve as platforms for organizing and integrating signaling transduction processes, we asked whether these membrane regions play a mechanistic role in H2O2-induced responses. Bovine aortic endothelial cell cultures exposed to a 500-microM bolus of H2O2 showed progressive activation of caspase 3 and an increase in the number of TUNEL-positive cells. Pretreatment with either wortmannin or PD 098059 heightened these apoptotic responses, demonstrating that both PI3 kinase/Akt and ERK1/2 serve as signaling mediators to alleviate H2O2 cytotoxic effects. To investigate the role of lipid rafts in these signaling processes, endothelial cells were pretreated with methyl-beta-cyclodextrin (CD) or filipin to ablate raft structures. H2O2-induced phosphorylation of Akt and ERK 1/2 was attenuated, while caspase 3 and the number of TUNEL positive cells was enhanced in CD-pretreated cells exposed to H2O2. Reconstitution of raft domains restored H2O2-induced Akt and ERK1/2 phosphorylation, which was concomitant with reduction of caspase 3 activation and DNA fragmentation. Taken together, our findings suggest that plasma membrane compartments rich in cholesterol participate in signal transduction pathways activated by oxidative stress.

Antihypertrophic effect of Na+/H+ exchanger isoform 1 inhibition is mediated by reduced mitogen-activated protein kinase activation secondary to improved mitochondrial integrity and decreased generation of mitochondrial-derived reactive oxygen species

Although inhibition of Na+/H+ exchanger isoform 1 (NHE-1) reduces cardiomyocyte hypertrophy, the mechanisms underlying this effect are not known. Recent evidence suggests that this may be associated with improved mitochondrial function. To understand the mechanistic bases for mitochondrial involvement in the antihypertrophic effect of NHE-1 inhibition, we examined the effect of the NHE-1-specific inhibitor N-[2-methyl-4,5-bis(methylsulphonyl)-benzoyl]-guanidine, hydrochloride (EMD, EMD87580; 5 microM) on the hypertrophic phenotype, mitogen-activated protein kinase (MAPK) activity, mitochondrial membrane potential (Deltapsim), permeability transition (MPT) pore opening, and superoxide generation in phenylephrine (PE)-treated neonatal rat cardiomyocytes. EMD significantly suppressed markers of cell hypertrophy, including cell surface area and gene expression of atrial natriuretic peptide and alpha-skeletal actin. EMD inhibited the PE-induced MPT pore opening, prevented the loss in Deltapsim, and attenuated superoxide generation induced by PE. Moreover, the activation of p38 MAPK (p38) and extracellular signal-regulated kinase (ERK) 1/2 MAPKs induced by PE was significantly attenuated in the presence of EMD as well as the antioxidant catalase. To examine the role of MPT and mitochondrial Ca2+ uniport in parallel with EMD, the effects of cyclosporin A (0.2 microM) and ruthenium red (10 microM) were evaluated. Both agents significantly attenuated PE-induced hypertrophy and inhibited both mitochondrial dysfunction and p38 and ERK1/2 MAPK activation. Our results suggest a novel mechanism for attenuation of the hypertrophic phenotype by NHE-1 inhibition that is mediated by a reduction in PE-induced MAPK activation and superoxide production secondary to improved mitochondrial integrity.

Epigallocatechin-3-gallate induces mitochondrial membrane depolarization and caspase-dependent apoptosis in pancreatic cancer cells

Polyphenols such as epigallocatechin-3-gallate (EGCG) from green tea extract can exert a growth-suppressive effect on human pancreatic cancer cells in vitro. In pursuit of our investigations to dissect the molecular mechanism of EGCG action on pancreatic cancer, we observed that the antiproliferative action of EGCG on pancreatic carcinoma is mediated through programmed cell death or apoptosis as evident from nuclear condensation, caspase-3 activation and poly-ADP ribose polymerase (PARP) cleavage. EGCG-induced apoptosis of pancreatic cancer cells is accompanied by growth arrest at an earlier phase of the cell cycle. In addition, EGCG invokes Bax oligomerization and depolarization of mitochondrial membranes to facilitate cytochrome c release into cytosol. EGCG-induced downregulation of IAP family member X chromosome linked inhibitor of apoptosis protein (XIAP) might be helpful to facilitate cytochrome c mediated downstream caspase activation. On the other end, EGCG elicited the production of intracellular reactive oxygen species (ROS), as well as the c-Jun N-terminal kinase (JNK) activation in pancreatic carcinoma cells. Interestingly, inhibitor of JNK signaling pathway as well as antioxidant N-acetyl-L-cysteine (NAC) blocked EGCG-induced apoptosis. To summarize, our studies suggest that EGCG induces stress signals by damaging mitochondria and ROS-mediated JNK activation in MIA PaCa-2 pancreatic carcinoma cells.

Calcium, ATP, and ROS: a mitochondrial love-hate triangle

The mitochondrion is at the core of cellular energy metabolism, being the site of most ATP generation. Calcium is a key regulator of mitochondrial function and acts at several levels within the organelle to stimulate ATP synthesis. However, the dysregulation of mitochondrial Ca2+homeostasis is now recognized to play a key role in several pathologies. For example, mitochondrial matrix Ca2+overload can lead to enhanced generation of reactive oxygen species, triggering of the permeability transition pore, and cytochrome c release, leading to apoptosis. Despite progress regarding the independent roles of both Ca2+and mitochondrial dysfunction in disease, the molecular mechanisms by which Ca2+can elicit mitochondrial dysfunction remain elusive. This review highlights the delicate balance between the positive and negative effects of Ca2+and the signaling events that perturb this balance. Overall, a “two-hit” hypothesis is developed, in which Ca2+plus another pathological stimulus can bring about mitochondrial dysfunction.

H2O2-dependent activation of GCLC-ARE4 reporter occurs by mitogen-activated protein kinase pathways without oxidation of cellular glutathione or thioredoxin-1

The gp91phox homologue Nox1 produces H2O2, which induces cell growth, transformation, and tumorigenicity. However, it has not been clear whether H2O2 effects are mediated indirectly via a generally oxidizing cellular environment or whether H2O2 more directly targets specific signaling pathways. Here, we investigated signaling by H2O2 induced by Nox1 overexpression using a luciferase reporter regulated by the antioxidant response element ARE4. Surprisingly, Nox1-derived H2O2 activated the reporter gene 15-fold with no effect on the redox state of the major thiol antioxidant substances, glutathione and thioredoxin. H2O2 signaling to ARE4 was mediated by activation of both the c-Jun N-terminal kinase and ERK1/2 pathways modulated by Ras. Thus, "redox signaling" resulting in kinase signaling pathways is distinct from "oxidative stress," and is mediated by discrete, localized redox circuitry.

Activated monocytes induce human retinal pigment epithelial cell apoptosis through caspase-3 activation

Dysfunction and loss of human retinal pigment epithelial (HRPE) cells is a significant component of many ocular diseases, in which mononuclear phagocyte infiltration at the HRPE-related interface is also observed. In this study, we investigated whether HRPE cell apoptosis may be induced by overlay of IFN-gamma-activated monocytes. Human monocytes primed with IFN-gamma overlaid directly onto HRPE cells elicited significant increases in terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling (TUNEL)-positive HRPE cells (p < 0.0001) and decreases of proliferating cell nuclear antigen-positive (p < 0.0001) HRPE cells. The activated monocytes also induced HRPE cell caspase-3 activation, which was inhibited by the caspase-3 inhibitor, Z-DEVD-fmk. However, co-incubations in which activated monocytes were prevented from direct contact with HRPE cells or in which the monocytes were separated from the HRPE cells after 30 minutes of direct contact, did not induce significant HRPE cell apoptosis. Function-blocking anti-CD18 and anti-intercellular adhesion molecule-1 (ICAM-1) antibodies significantly reduced activated monocyte-induced TUNEL-positive HRPE cells by 48% (p = 0.0051) and 38% (p = 0.046), respectively. Anti-CD18 and anti-ICAM-1 antibodies significantly inhibited caspase-3 activity by 56% (p < 0.0001) and 45% (p < 0.0001), respectively. However, antibodies to vascular cell adhesion molecule-1, TNF-alpha, IL-1beta, or TNF-related apoptosis-inducing ligand did not inhibit apoptosis or caspase-3 activation. Direct overlay of monocytes also induced reactive oxygen metabolites (ROM) within HRPE cells. The intracellular HRPE cell ROM production was inhibited by the anti-CD18 and anti-ICAM-1 antibodies, but not by superoxide dismutase, presumably due to its failure to penetrate into HRPE cells. Accordingly, neither superoxide dismutase nor N(G)-monomethyl-L-arginine had significant effects on HRPE cell apoptosis or caspase-3 activation. Our results suggest that activated monocytes may induce ROM in HRPE cells through cell-to-cell contact, in part via CD18 and ICAM-1, and promote HRPE cell apoptosis. These mechanisms may compromise HRPE cell function and survival in a variety of retinal diseases.

2-Methoxyestradiol induces apoptosis in Ewing sarcoma cells through mitochondrial hydrogen peroxide production

The Ewing sarcoma is the second most common bone tumor in children and young adults. Despite the advances in therapy, the 5-year survival rate for patients with metastatic disease is poor, indicating the need for alternative treatments. Here, we report that 2-methoxy-estradiol (2-Me), a natural estrogen metabolite, induced a caspase-dependent apoptosis of Ewing sarcoma-derived cells independently of their p53 status. 2-Me-induced apoptosis occurred through the mitochondrial death pathway as evidenced by reduction of the mitochondrial transmembrane potential, cytochrome c release and caspase-9 activation. Treatment of cells with 2-Me resulted in generation of intracellular H(2)O(2), which occurred earlier than caspase-9 activation. The H(2)O(2)-reducing agent Ebselen and the lipid peroxidation inhibitor vitamin E decreased both 2-Me-induced caspase-9 activation and cell death, thus providing evidence for a role of H(2)O(2) and lipid peroxides in the initiation of this process. Rotenone, an inhibitor of the mitochondrial respiratory chain, abolished both apoptosis and H(2)O(2) production, thereby identifying mitochondria as the source of H(2)O(2). Moreover, we observed that treatment of cells with 2-Me or H(2)O(2) induced activation of the c-Jun N-terminal kinase (JNK). Overexpression of a dominant-negative mutant of JNK1 reduced 2-Me-induced apoptosis indicating that JNK participates in this process. Altogether, our results provide evidence that 2-Me triggers apoptosis of Ewing sarcoma cells through induction of a mitochondria redox-dependent mechanism and suggest that this compound or other agents that selectively increase the level of reactive oxygen species may prove useful to the development of novel strategies for treatment of Ewing tumors.