Hydrogen peroxide stimulates apoptosis in cultured human retinal pigment epithelial cells

Activation of mitogen-activated protein kinases is essential for hydrogen peroxide -induced apoptosis in retinal pigment epithelial cells

Retinal pigment epithelial (RPE) cells are constantly exposed to oxidative injury while clearing byproducts of photoreceptor turnover, a circumstance thought to be responsible for degenerative retinal diseases. The mechanisms of hydrogen peroxide (H(2)O(2))-induced apoptosis in RPE cells are not fully understood. We studied signal transduction mechanisms of H(2)O(2)-induced apoptosis in the human RPE cell line ARPE-19. Activation of two stress kinases (JNK and p38) occurs during H(2)O(2) stimulation, and H(2)O(2)-mediated cell death was significantly reduced by their specific inhibition. Exposure to a lethal dose of H(2)O(2) elicited Bax translocation to the mitochondria and release of apoptosis-inducing factor (AIF) from the mitochondria, both of which were abolished by either JNK- or p38-specific inhibitors. Both H(2)O(2)-induced cell death and JNK/p38 phosphorylation were partially inhibited by C. difficile toxin B, inhibitor of Rho, Rac, and cdc42. Use of pull-down assays revealed that the small GTPase activated by H(2)O(2) is Rac1. This study is the first to demonstrate that H(2)O(2) induces a Rac1/JNK1/p38 signaling cascade, and that JNK and p38 activation is important for H(2)O(2)-induced apoptosis as well as AIF/Bax translocation of RPE cells.

Pigment epithelium-derived factor inhibits oxidative stress-induced cell death by activation of extracellular signal-regulated kinases in cultured retinal pigment epithelial cells

Oxidative stress-induced retinal pigment epithelial (RPE) cell death is involved in the pathogenesis of age-related macular degeneration (AMD). Pigment epithelium-derived factor (PEDF) is an anti-angiogenic/neurotropic dual functional factor, and recently it was also shown to mediate anti-oxidative action. In the present study, the influence of PEDF in hydrogen peroxide (H(2)O(2))-induced RPE cell death was investigated using nontransformed human RPE cell line (ARPE-19). The recombinant PEDF was purified from E. coli. The MTT cell viability assay showed that PEDF rescued ARPE-19 from H(2)O(2)-induced cell death in a dose- and time-dependent manner. Western blot analysis revealed that PEDF stimulated the extracellular signal-regulated kinases (ERK1/2) phosphorylation. The PEDF cytoprotective effect was significantly attenuated by the ERK1/2 inhibitor PD98059. In this study, we demonstrate that PEDF induces ERK1/2 phosphorylation and we further suggest that the ERK signal cascade contributes to RPE cell's cytoprotection against oxidative stress.

Different death stimuli evoke apoptosis via multiple pathways in retinal pigment epithelial cells

Loss of retinal pigment epithelial (RPE) cells via apoptosis plays a prominent role in several retinal degenerative diseases, such as age-related macular degeneration, and with light damage. Strategies for preservation of vision that would interrupt the apoptotic cascade require understanding the molecular events associated with apoptosis. This study investigated the susceptibility of RPE to caspase-dependent and -independent apoptotic pathways when challenged with different stimuli, including oxidants, anti-Fas antibody, and activated cytotoxic T lymphocytes (CTLs). These experiments used novel RPE cell lines developed from wildtype and heterozygous mice with reduced levels of either Mn superoxide dismutatse (SOD) or CuZnSOD. Peroxide and 4-hydroxynonenal induced apoptosis through both caspase-independent and -dependent pathways, respectively. With both oxidants, translocation of apoptosis inducing factor into the nucleus was observed. Cells containing reduced levels of CuZnSOD were the most susceptible to oxidant-induced cell death. Targeted killing by CTLs and activation of the Fas death receptor induced caspase-dependent apoptosis. These results show stimulus-specific activation of either the caspase-dependent or -independent pathway. Since cultured RPE express the protein components required for different apoptotic pathways, they provide a good model system for studying molecular events associated with multiple signals that lead to cell death.

Sigma receptor ligands protect human retinal cells against oxidative stress

This study was undertaken to investigate the role of sigma receptors during the oxidative damage on human retinal pigment epithelial cells, and to assess whether sigma receptor ligands enhance survival and protect DNA of cells challenged by oxidative stress. Pretreatment with PRE-084, a sigma1 receptor agonist, resulted in significant increased viability in a dose-related manner. DNA damage induced by oxidative insult was significantly lower with PRE-084. The effects of PRE-084 were antagonized by pretreatment with sigma1 receptor antagonists (NE-100 and BD1047), but interestingly were synergized by cotreatment with BD1047 that also presented an affinity for the sigma2 receptor. The results suggest that sigma1 receptors play an important role against retinal damage, even though sigma2 receptor involvement cannot be excluded.

Oxidative stress in ARPE-19 cultures: do melanosomes confer cytoprotection?

The pigment melanin has antioxidant properties that could theoretically reduce oxidative damage to the retinal pigment epithelium (RPE), perhaps protecting against retinal diseases with an oxidative stress component like age-related macular degeneration. To determine whether melanin confers cytoprotection on RPE cells, melanosomes or control particles were introduced by phagocytosis into the human cell line ARPE-19 and oxidative stress was induced chemically (H2O2 or tert-butyl hydroperoxide) or with visible light. Since the iron-binding capacity of melanin is important for its antioxidant function, experiments were performed to confirm that the melanosomes were not iron saturated. Cytotoxicity was assessed by measures of plasma or lysosomal membrane integrity, mitochondrial function, and cell-substrate reattachment. Oxidative stress protocols were critically evaluated to produce modest cytotoxicity, which might allow detection of a small cytoprotective effect as expected for melanosomes. Particle internalization alone had no effect on baseline metabolic activity or on major RPE antioxidants. Particles were tested in multiple oxidative stress experiments in which culture conditions known to affect stress-induced cytotoxicity, notably culture density, were varied. No testing condition or outcome measure revealed a consistent protective (or cytotoxic) effect of melanosomes, indicating that measures of lysosome stability or whole cell viability do not demonstrate an antioxidant role for RPE melanosomes. If the melanosome, an insoluble particle, performs a cytoprotective function within cells, its effects may be limited to the local environment of the organelle and undetectable by conventional methods.

A new mitochondrial RNA deletion fragment accelerated by oxidative stress in rat L6 cells

RNA deletions may be easier to detect and more extensive than DNA deletions. Two large deletion fragments (1120 and 7811 bp) of mitochondrial RNA were observed in rat L6 muscle cells. At the site of the 1120 bp deletion, the remaining RNA fragment was re-linked by a short additional section (GGTATGAAGCT). These kinds of deletions were accelerated by oxidative stress and were not observed in mitochondrial DNA.

Pharmacological consequences of oxidative stress in ocular tissues

The eye is a unique organ because of its constant exposure to radiation, atmospheric oxygen, environmental chemicals and physical abrasion. That oxidative stress mechanisms in ocular tissues have been hypothesized to play a role in diseases such as glaucoma, cataract, uveitis, retrolental fibroplasias, age-related macular degeneration and various forms of retinopathy provides an opportunity for new approaches to their prevention and treatment, In the anterior uvea, both H2O2 and synthetic peroxides exert pharmacological/toxicological actions tissues of the anterior uvea especially on the sympathetic nerves and smooth muscles of the iris-ciliary bodies of several mammalian species. Effects produced by peroxides require the presence of trace amounts of extracellular calcium and the functional integrity of mitochondrial calcium stores. Arachidonic acid metabolites appear to be involved in both the excitatory action of peroxides on sympathetic neurotransmission and their inhibitory effect on contractility of the iris smooth muscle to muscarinic receptor activation. In addition to the peroxides, isoprostanes (products of free radical catalyzed peroxidation of arachidonic acid independent of the cyclo-oxygenase enzyme) can also alter sympathetic neurotransmission in anterior uveal tissues. In the retina, both H2O2 and synthetic peroxides produced an inhibitory action on potassium depolarization induced release of [3H] D-aspartate, in vitro and on the endogenous glutamate and glycine concentrations in vivo. Effects caused by peroxides in the retina are mediated, at least in part, by second messengers such as nitric oxide, prostaglandins and isoprostanes. The ability of H2O2 to alter the integrity of neurotransmitter pools from sympathetic nerves in the anterior uvea and glutaminergic nerves in the retina could underlie its role in the etiology of glaucoma.

Protection against hydrogen peroxide-induced cell death in cultured human retinal pigment epithelial cells by 17β-estradiol: A differential gene expression profile

It has been demonstrated that estrogen receptors are present in the retinal pigment epithelium (RPE)-choroids complex regardless of sex. This suggests that estrogen could play a functional role in the outer retina, especially the RPE. To gain further insights on the molecular mechanisms differentially activated by 17beta-estradiol (betaE2) in RPE cells, we investigated gene expression changes in response to betaE2 in cultured RPE cells using cDNA microarray technology. A total of 47 genes among 21,329 human genes are significantly altered in response to betaE2 treatment in RPE cells. Among these 47 altered genes, 34 are up-regulated and 13 are down-regulated by betaE2. The products of 34 genes have a known or suspected function. These functions belong to various categories, including caspases; extracellular matrix proteins; metabolism pathway components; GTP/GDP exchangers and G-protein GTPase activity modulators; transcription activators and repressors. Six genes which may contribute to the unique functions of the RPE cells have been validated by both quantitative real-time reverse transcription (RT)-PCR and semi-quantitative RT-PCR. In addition, we also demonstrated that betaE2 quenches H2O2-induced up-regulation of apoptosis-related protein, and protects RPE cell degeneration. These results indicate that estrogen regulates functions of RPE cells and is involved in the maintaining and survival of RPE cells during oxidative stress, and its deficiency during menopause period may be a factor contributing to the development of age-related macular degeneration in elderly women.

Neuroactive steroids protect retinal pigment epithelium against oxidative stress

This study was undertaken to assess whether neuroactive steroids, 17beta-estradiol and dehydroepiandrosterone-sulfate, enhance survival and protect DNA of human retinal pigment epithelial cells challenged by oxidative stress, and to investigate the role of sigma1 receptors in the effects of neuroactive steroids. Retinal pigment epithelial cells were treated with various concentrations of neuroactive steroids and then exposed to hydrogen peroxide. Pretreatment with steroids resulted in significant increased viability in a dose-related manner. DNA damage induced by oxidative insult was significantly lower with steroid pretreatment. The effects of 17beta-estradiol and dehydroepiandrosterone-sulfate were antagonized by pretreatment with a sigma1 receptor antagonist. The results suggest that neuroactive steroids protect retinal cells from oxidative stress, and that this effect is mediated by sigma1 receptors.

Blue Light Induces Mitochondrial DNA Damage and Free Radical Production in Epithelial Cells

Exposure of biological chromophores to ultraviolet radiation can lead to photochemical damage. However, the role of visible light, particularly in the blue region of the spectrum, has been largely ignored. To test the hypothesis that blue light is toxic to non-pigmented epithelial cells, confluent cultures of human primary retinal epithelial cells were exposed to visible light (390-550 nm at 2.8 milliwatts/cm2) for up to 6 h. A small loss of mitochondrial respiratory activity was observed at 6 h compared with dark-maintained cells, and this loss became greater with increasing time. To investigate the mechanism of cell loss, the damage to mitochondrial and nuclear genes was assessed using the quantitative PCR. Light exposure significantly damaged mitochondrial DNA at 3 h (0.7 lesion/10 kb DNA) compared with dark-maintained controls. However, by 6 h of light exposure, the number of lesions was decreased in the surviving cells, indicating DNA repair. Isolated mitochondria exposed to light generated singlet oxygen, superoxide anion, and the hydroxyl radical. Antioxidants confirmed the superoxide anion to be the primary species responsible for the mitochondrial DNA lesions. The effect of lipofuscin, a photoinducible intracellular generator of reactive oxygen intermediates, was investigated for comparison. Exposure of lipofuscin-containing cells to visible light caused an increase in both mitochondrial and nuclear DNA lesions compared with non-pigmented cells. We conclude that visible light can cause cell dysfunction through the action of reactive oxygen species on DNA and that this may contribute to cellular aging, age-related pathologies, and tumorigenesis.

Enhanced expression of glutathione-S-transferase A1-1 protects against oxidative stress in human retinal pigment epithelial cells

Glutathione-S-transferases (GSTs) play an important role in protection mechanisms against oxidative stress. We sought to determine whether over-expression of human GSTA1-1 in RPE cells is able to attenuate H(2)O(2)-induced oxidative stress. SV40-transformed human fetal RPE cells were stably transfected with pRC/hGSTA1-1 vector which carries a full-length of human GSTA1-1 cDNA. The control RPE cells were either non-transfected or transfected with control vector pRC. Expression of hGSTA1-1 protein in these cells was confirmed by Western blot and immunocytochemical analyses. The protective effects of hGSTA1-1 on cell viability and mitochondrial DNA (mtDNA) damage caused by H(2)O(2) were examined with MTT assay and quantitative PCR (QPCR), respectively. The hGSTA1-1 transfected RPE cells exhibited a similar morphology and growth rate as control RPE cells. Immunocytochemical analysis showed robust expression hGSTA1-1 in hGSTA1-1 transfected cells versus background staining in control cells. Western blotting of protein extracts from cells transfected with hGSTA1-1 revealed a 26 kDa protein band which corresponds to the size of recombinant mature hGSTA1-1. The active GST present in the hGSTA1-1 transfected cells was approximately three times higher than in control cells. The MTT assay showed a significantly greater viability of hGSTA1-1 cells in response to H(2)O(2) (100 and 200 microm) compared to control cells (p<0.05). QPCR indicated that mtDNA damage was significantly decreased in hGSTA1-1 cells than in control cells (p<0.05). Human GSTA1-1 transfection protect against RPE cell death and mtDNA damage caused by H(2)O(2), suggesting an important role of GST in protection against oxidative stress in RPE cells.

Melatonin protects human retinal pigment epithelial (RPE) cells against oxidative stress

Oxidative stress is involved in the pathogenesis of age-related macular degeneration (AMD). Administration of conventional antioxidants has been shown to slow the progression of AMD and vision loss. Melatonin, an endogenous neurohormone produced by the pineal gland and retina, has been reported to be a potent antioxidant and free radical scavenger. In this study we tested whether melatonin can protect retinal pigment epithelial (RPE) cells against hydrogen peroxide (H(2)O(2))-induced cell death. Since mitochondrial DNA (mtDNA) is preferentially susceptible to oxidative damage, we tested whether melatonin can reduce H(2)O(2)-induced mtDNA lesions. A human RPE cell line (ARPE-19) was cultured and exposed to H(2)O(2) (100 and 200 microm) for 1 hr to induce cell death. Prior to H(2)O(2) treatment, cells were treated with various concentrations (0.1-200 microm) of melatonin for 2, 24 or 72 hr. Control cells received either melatonin or ethanol alone. Cell viability, as determined by MTT assay, showed no significant (P>0.05) protection against H(2)O(2) toxicity in cells receiving 2- and 24-hr pretreatment of melatonin at either concentration. However, when melatonin was administered diurnally for 3 consecutive days, this prolonged treatment markedly reduced H(2)O(2)-induced cell death (P>0.05) MtDNA damage, as assessed with quantitative PCR, was significantly decreased (P<0.05) in RPE cells pretreated with melatonin as compared to those without melatonin treatment. These results suggest that melatonin may play a role in protecting RPE cells from oxidative stress.

Hydrogen peroxide induced down-regulation of CD28 expression of Jurkat cells is associated with a change of site α-specific nuclear factor binding activity and the activation of caspase-3

CD28 is the requisite co-stimulatory molecule in the activation of T cells and in the generation of immune responses. But expression of CD28 declined and oxidants accumulated in the elderly. Although accumulation of reactive oxygen species (ROS) during senescence has been reported extensively, the effect of oxidants on CD28-expression remains totally unknown. In this study, we tried to address the molecular mechanism underlying the decrease in CD28-expression of Jurkat T cells cultured in H2O2. Our results indicate that H2O2 could partially block the expression of CD28. This correlates well with a change of nuclear protein binding activity to the motif of site alpha of the CD28 gene, while the site beta-binding activity remained unaltered. On the other hand, since caspase-3 is activated by H2O2, inhibitors of caspase-3 should increase the expression of CD28. What is more interesting is the fact that the site alpha-binding activity was mostly restored after caspase-3 inhibitors had being added. However, caspase-3 is not activated by caspase-8. Maybe it is activated by caspase-9, which is triggered by cytochrome c. We believe that the procaspase-3 is activated by ROS, and the active caspase-3 can induce the change of the site alpha-binding activity, causing a decrease in CD28 expression.

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.

Antioxidants in tears and plasma: Inter-relationships and effect of vitamin C supplementation

PURPOSE

To investigate inter-relationships between total antioxidant capacity and ascorbate concentration in plasma and tears, and the effect of antioxidant supplementation with reference to these variables.

METHODS

Twenty-one subjects were studied in this placebo-controlled, cross-over intervention trial. Fasting plasma and tear ascorbate concentrations and total antioxidant capacity (as Ferric Reducing/Antioxidant Power (FRAP)) were measured pre- and post-supplementation with vitamin C (1 g/day).

RESULTS

Mean +/- SD ascorbate in tears and plasma at entry were 17 +/- 6 and 52 +/- 13 micro M, respectively; FRAP values were, respectively, 273 +/- 94 and 1101 +/- 168 micro M. There was no significant correlation between tear and plasma levels (r = -0.068; P = 0.771 for ascorbate; r = 0.418; P = 0.059 for FRAP). Neither was significant correlation seen between the two variables in plasma (r = 0.162; P = 0.483) or tears (r = 0.353; P = 0.117). Acute responses (up to 3 hours) showed a similar pattern of increase in both fluids, however, peak response in tears (33 +/- 4 micro M) was much smaller and slightly later than in plasma (125 +/- 13 micro M). After 4 weeks' supplementation, ascorbate increased (P < 0.001) in both fluids, however, the increase in tear ascorbate was small (5 micro M), compared to plasma (38 micro M). The increase in tear ascorbate appeared to plateau after 2 days' supplementation; plasma levels were still increasing. Higher tear ascorbate at entry was associated (P < 0.05) with smaller supplementation-related response. No significant changes in FRAP were seen in either fluid (P > 0.05).

CONCLUSIONS

Ascorbate concentration in both plasma and tears increased with vitamin C supplementation, but the total antioxidant capacity of these fluids did not. Furthermore, the increase in tear ascorbate was modest in comparison to that in plasma, and is suggestive of a "ceiling" for tear ascorbate of under 40 micro M. Results support the concept of a control mechanism for an integrated antioxidant defense system, and suggest that the amount of ascorbate in tears is both actively controlled and purposefully limited.

Estrogen attenuates oxidative stress-induced apoptosis in C6 glial cells

We examined the mechanism of 17beta-estradiol (estrogen)-mediated inhibition of apoptosis in C6 (rat glioma) cells following exposure to hydrogen peroxide (H(2)O(2)). Cells were preincubated with 4 microM estrogen for 2 h and then exposed to 100 microM H(2)O(2) for 24 h. Exposure to H(2)O(2) caused significant increases in intracellular calcium (Ca(2+)), as determined by fura-2, which was attenuated by preincubation with estrogen. H(2)O(2) and ionomycin caused cell death in a dose-dependent manner, as measured by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT) assay. Preincubation with estrogen restored viability in cells exposed to H(2)O(2) but not in cells exposed to ionomycin. Western blot analysis showed an increase in Bax/Bcl-2 ratio, calpain activity, and caspase-3 activity following treatment with H(2)O(2), and estrogen pretreatment decreased levels of all three. Cell morphology, as evaluated by Wright staining, indicated apoptosis in cells treated with H(2)O(2), and pretreatment with estrogen reduced apoptosis. Results from MTT and Wright staining were further supported by the terminal deoxyribonucleotidyl transferase (TdT)-mediated dUTP Nick End Labeling (TUNEL) assay. These results indicate a role for estrogen in preventing apoptosis in C6 glial cells exposed to H(2)O(2). Our results suggest that estrogen may have a protective role in minimizing glial cell apoptosis in neurological diseases such as demyelinating disease or central nervous system trauma.

Age-related macular degeneration: etiology, pathogenesis, and therapeutic strategies

Age-related macular degeneration is the principal cause of registered legal blindness among those aged over 65 in the United States, western Europe, Australia, and Japan. Despite intensive research, the precise etiology of molecular events that underlie age-related macular degeneration is poorly understood. However, investigations on parallel fronts are addressing this prevalent public health problem. Sophisticated biochemical and biophysical techniques have refined our understanding of the pathobiology of drusen, geographic atrophy, and retinal pigment epithelial detachments. Epidemiological identification of risk factors has facilitated an intelligent search for underlying mechanisms and fueled clinical investigation of behavior modification. Gene searches have not only brought us to the cusp of identifying the culpable gene loci in age-related macular degeneration, but also localized genes responsible for other macular dystrophies. Recent and ongoing investigations, often cued by tumor biology, have revealed an important role for various growth factors, particularly in the neovascular form of the condition. Transgenic and knockout studies have provided important mechanistic insights into the development of choroidal neovascularization, the principal cause of vision loss in age-related macular degeneration. This in turn has culminated in preclinical and clinical trials of directed molecular interventions.

Oxidative stress-induced mitochondrial DNA damage in human retinal pigment epithelial cells: a possible mechanism for RPE aging and age-related macular degeneration

Oxidative stress is believed to contribute to the pathogenesis of many diseases, including age-related macular degeneration (AMD). Although the vision loss of AMD results from photoreceptor damage in the central retina, the initial pathogenesis involves degeneration of RPE cells. Evidence from a variety of studies suggests that RPE cells are susceptible to oxidative damage. Mitochondrial DNA (mtDNA) is particularly prone to oxidative damage compared to nuclear DNA (nDNA). Using the quantitative PCR assay, a powerful tool to measure oxidative DNA damage and repair, we have shown that human RPE cells treated with H(2)O(2) or rod outer segments resulted in preferential damage to mtDNA, but not nDNA; and damaged mtDNA is not efficiently repaired, leading to compromised mitochondrial redox function as indicated by the MTT assay. Thus, the susceptibility of mtDNA to oxidative damage in human RPE cells, together with the age-related decrease of cellular anti-oxidant system, provides the rationale for a mitochondria-based model of AMD.

Oxidative stress causes ERK phosphorylation and cell death in cultured retinal pigment epithelium: prevention of cell death by AG126 and 15-deoxy-delta 12, 14-PGJ2

BACKGROUND

The retina, which is exposed to both sunlight and very high levels of oxygen, is exceptionally rich in polyunsaturated fatty acids, which makes it a favorable environment for the generation of reactive oxygen species. The cytotoxic effects of hydrogen peroxide (H2O2) induced oxidative stress on retinal pigment epithelium were characterized in this study.

METHODS

The MTT cell viability assay, Texas-Red phalloidin staining, immunohistochemistry and Western blot analysis were used to assess the effects of oxidative stress on primary human retinal pigment epithelial cell cultures and the ARPE-19 cell line.

RESULTS

The treatment of retinal pigment epithelial cells with H2O2 caused a dose-dependent decrease of cellular viability, which was preceded by a significant cytoskeletal rearrangement, activation of the Extracellular signal-Regulated Kinase, lipid peroxidation and nuclear condensation. This cell death was prevented partially by the prostaglandin derivative, 15d-PGJ2 and by the protein kinase inhibitor, AG126.

CONCLUSION

15d-PGJ2 and AG126 may be useful pharmacological tools in the future capable of preventing oxidative stress induced RPE cell death in human ocular diseases.

Oxidant and antioxidant modulation of chloride channels expressed in human retinal pigment epithelium

Retinal pigment epithelium (RPE) possesses regulated chloride channels that are crucial for transepithelial fluid and ion transport. At present, little is known about the molecular nature of chloride channels in human adult RPE (haRPE) or the effects of oxidative stress on membrane conductance properties. In the present study, we assessed ClC channel and cystic fibrosis transmembrane conductance regulator (CFTR) expression and membrane chloride conductance properties in haRPE cells. ClC-5, ClC-3, ClC-2, and CFTR mRNA expression was confirmed with RT-PCR analysis, and protein expression was detected with Western blot analysis and immunofluorescence microscopy. Whole cell recordings of primary cultures of haRPE showed an outwardly rectifying chloride current that was inhibited by the oxidant H(2)O(2). The inhibitory effects of H(2)O(2) were reduced in cultured human RPE cells that were incubated with precursors of glutathione synthesis or that were stably transfected to overexpress glutathione S-transferase. These findings indicate a possible role for ClC channels in haRPE cells and suggest possible redox modulation of human RPE chloride conductances.

The effects of pneumolysin and hydrogen peroxide, alone and in combination, on human ciliated epithelium in vitro

We have investigated the effects of pneumolysin and H2O2, putative virulence factors of Streptococcus pneumoniae, on the ciliary beat frequency and structural integrity of human ciliated epithelium in vitro. Human ciliated epithelium was obtained by brushing the inferior nasal turbinate of healthy human volunteers. Ciliary slowing (CS) was measured using a photo-transistor technique and epithelial damage (ED) was documented using a visual scoring index. Effects of recombinant pneumolysin (100 ng/ml), a mutant pneumolysin preparation with markedly reduced haemolytic activity (100 ng/ml) and reagent H2O2 (100 microM) were measured alone and in combination, in the absence and presence of catalase (1000 units/ml). When used individually, both recombinant pneumolysin and H2O2 caused significant (P < 0.05) CS and ED. The effects of H2O2 but not those of pneumolysin were almost completely attenuated by catalase, while the mutant pneumolysin preparation did not cause significant CS or ED. When used in combination, the effects of pneumolysin and H2O2 on CS and ED were additive as opposed to synergistic. These actions of pneumolysin and H2O2 may contribute to the pathogenesis of respiratory tract infections caused by the pneumococcus.

Bcl-2 overexpression increases survival in human retinal pigment epithelial cells exposed to H(2)O(2)

The integrity of the retinal pigment epithelium, especially that of the macula is essential for the preservation of vision into old age. The chronic exposure to sunlight and peroxidized lipids from phagocytized photoreceptor outer segments imposes a high level of oxidative stress on the retinal tissues, which increases with age as antioxidant protection declines and therefore could accelerate apoptosis. Bcl-2 known to facilitate mitochondrial DNA repair and cellular survival in other tissues was overexpressed in a single clone of human retinal pigment epithelium cells after stable transfection with humanbcl-2 in rhoSFV-neoexpression factor. Near confluent cells (2nd-4th generation permanently bcl-2 transfected) were protected from mitochondrial dysfunction after exposure to H(2)O(2) up to 150 microM. With 200 microM H(2)O(2), function in transfected cells declined by only 25% control activity as determined by MTT reduction assays, compared to wild type and vector only transfected cells expressing normal bcl-2 levels. Similarly the bcl-2 -transfected cells were more resistant to mitochondrial DNA damage after H(2)O(2) treatment than the other groups and suffered 50% less damage after exposure to 200 microM H(2)O(2), as assayed by quantitative polymerase chain reaction assays. These data suggest that bcl-2 overexpression protects human RPE cells from mitochondrial respiratory dysfuction, mitochondrial DNA damage and promotes cellular survival in response to oxidative stress induced by H(2)O(2).

Role of cyclins in epithelial response to oxidants

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