Cigarette smoking, oxidative stress, the anti-oxidant response through Nrf2 signaling, and Age-related Macular Degeneration

Mitochondrial oxidative stress in the retinal pigment epithelium leads to localized retinal degeneration

PURPOSE

Oxidative stress in the RPE is widely accepted as a contributing factor to AMD. We have previously shown that ribozyme-mediated reduction in the antioxidant enzyme manganese superoxide dismutase (MnSOD) leads to some of the features of geographic atrophy in mice. To develop a mouse model independent of viral injection, we used a conditional knockout of the Sod2 gene in the RPE to elevate mitochondrial oxidative stress in that cell layer.

METHODS

Experimental mice in which exon 3 of Sod2 was flanked by loxP sites were also transgenic for PVMD2-rtTA and tetO-PhCMV cre, so that cre recombinase was expressed only in the RPE. Pups of this genotype (Sod2(flox/flox)VMD2cre) were induced to express cre recombinase by feeding doxycycline-laced chow to nursing dams. Controls included mice of this genotype not treated with doxycycline and doxycycline-treated Sod2(flox/flox) mice lacking the cre transgene. Expression of cre in the RPE was verified by immunohistochemistry, and deletion of Sod2 exon 3 in the RPE was confirmed by PCR. Mice were followed up over a period of 9 months by spectral-domain optical coherence tomography (SD-OCT), digital fundus imaging, and full-field ERG. Following euthanasia, retinas were examined by light and electron microscopy or by immunohistochemistry. Contour length of rod outer segments and thickness of the RPE layer were measured by unbiased stereology.

RESULTS

Following doxycycline induction of cre, Sod2(flox/flox) cre mice demonstrated increased signs of oxidative stress in the RPE and accumulation of autofluorescent material by age 2 months. They showed a gradual decline in the ERG response and thinning of the outer nuclear layer (by SD-OCT), which were statistically significant by 6 months. In addition, OCT and electron microscopy revealed increased porosity of the choroid. At the same interval, hypopigmented foci appeared in fundus micrographs, and vascular abnormalities were detected by fluorescein angiography. By 9 months, the RPE layer in Sod2(flox/flox) cre mice was thicker than in nontransgenic littermates, and the rod outer segments were significantly longer over most of the retina, although localized atrophy of photoreceptors was also obvious in some eyes.

CONCLUSIONS

Conditional tissue-specific reduction in MnSOD induced oxidative stress in mouse RPE, leading to RPE dysfunction, damage to the choroid, and death of photoreceptor cells. The RPE oxidative stress did not cause drusen-like deposits, but the model recapitulated certain key aspects of the pathology of dry AMD and may be useful in testing therapies.

p62 provides dual cytoprotection against oxidative stress in the retinal pigment epithelium

As a signaling hub, p62/sequestosome plays important roles in cell signaling and degradation of misfolded proteins. p62 has been implicated as an adaptor protein to mediate autophagic clearance of insoluble protein aggregates in age-related diseases, including age-related macular degeneration (AMD), which is characterized by dysfunction of the retinal pigment epithelium (RPE). Our previous studies have shown that cigarette smoke (CS) induces oxidative stress and inhibits the proteasome pathway in cultured human RPE cells, suggesting that p62-mediated autophagy may become the major route to remove impaired proteins under such circumstances. In the present studies, we found that all p62 mRNA variants are abundantly expressed and upregulated by CS induced stress in cultured human RPE cells, yet isoform1 is the major translated form. We also show that p62 silencing exacerbated the CS induced accumulation of damaged proteins, both by suppressing autophagy and by inhibiting the Nrf2 antioxidant response, which in turn, increased protein oxidation. These effects of CS and p62 reduction were further confirmed in mice exposed to CS. We found that over-expression of p62 isoform1, but not its S403A mutant, which lacks affinity for ubiquitinated proteins, reduced misfolded proteins, yet simultaneously promoted an Nrf2-mediated antioxidant response. Thus, p62 provides dual, reciprocal enhancing protection to RPE cells from environmental stress induced protein misfolding and aggregation, by facilitating autophagy and the Nrf2 mediated antioxidant response, which might be a potential therapeutic target against AMD.

Nicotine accelerates diabetes-induced retinal changes

PURPOSE

To investigate the effects of nicotine on retinal alterations in early-stage diabetes in an established rodent model.

MATERIALS AND METHODS

Sprague-Dawley rats were examined using a combination of confocal scanning laser ophthalmoscopy and spectral domain optical coherence tomography to determine changes in retinal structure in response to nicotine exposure, diabetes and the combined effects of nicotine and diabetes. Diabetes was induced by a single injection of 65 mg/kg streptozotocin and nicotine injections were administered subcutaneously daily. Retinal thickness in the superior, inferior, nasal and temporal quadrants were determined based on the spectral domain optical coherence tomography (SD-OCT) volume scans (20° × 20°) centered on the optic disc. Segmentation of discrete retinal layers was performed on a subset of SD-OCT cross-sections to further examine changes in each treatment group. Survival of neurons within the ganglion cell layer (GCL) was assessed by confocal morphometric imaging.

RESULTS

The control group did not experience any significant change throughout the study. The nicotine treatment group experienced an average decrease in total retinal thickness (TRT) of 9.4 µm with the majority of the loss localized within the outer nuclear layer (ONL) as determined by segmentation analysis (p < 0.05). The diabetic group exhibited a trend toward decreased TRT while segmentation analysis of the diabetic retinopathy (DR) group revealed significant thinning within the ONL (p < 0.05). The combination of nicotine and diabetes revealed a significant increase of 8.9 µm in the TRT (p < 0.05) accompanied by a decrease in the number of GCL neurons.

CONCLUSIONS

We demonstrated significant temporal changes in retinal morphology in response to nicotine exposure, diabetes and with the combined effects of nicotine and diabetes. These findings may have implications in determining treatment strategies for diabetic patients using products containing nicotine, such as cigarettes, smokeless tobacco, electronic cigarettes or smoking cessation products.

Aging leads to elevation of O-GlcNAcylation and disruption of mitochondrial homeostasis in retina

Retina is particularly susceptible to aging as oxidative damage accumulates within retina, leading to age-related retinal dysfunction or even visual loss. However, the underlying mechanisms still remain obscure and effective therapeutic strategy is urgently in need. Here, we quested for the answer particularly focusing on mitochondrial homeostasis and O-GlcNAcylation in rat retina. By comparing expression of electron transfer chain complexes and key factors in mitochondrial biogenesis and dynamics in retinas of aged and young Sprague-Dawley rats, we found that mitochondrial Complex I, II, IV and V were increased in aged retina with decreased mtTFA and Mfn2. Also, we noticed that p38 and JNK of MAPK signaling were substantially more activated in aged retina, suggesting stress induction. In addition, we found that pan-O-GlcNAcylation was remarkably stronger with lower OGA expression in aged retina. To further elucidate the roles of Mfn2 and O-GlcNAcylation, we employed ARPE-19 cells and found that ATP production, oxygen consumption, and mitochondrial membrane potential were reduced and ROS level was increased by Mfn2 knockdown, while treating with PUGNAc or UDP-GlcNAc heightened oxygen consumption and reduced ROS. Our results suggest disrupted mitochondrial homeostasis may increase oxidative stress; yet enhanced O-GlcNAcylation might defend against oxidative stress and promote mitochondrial respiration in aged retina.

Nrf2 signaling modulates cigarette smoke-induced complement activation in retinal pigmented epithelial cells

Whereas cigarette smoking (CS) and dysregulated complement are thought to play central roles in age-related macular degeneration (AMD), their exact roles are unknown. The aim of this study was to determine if CS activates complement and if the antioxidant transcription factor Nrf2 modulates this response. In AMD specimens, Nrf2 immunolabeling was strong in the cytoplasm, with scattered nuclear labeling of macular retinal pigmented epithelial (RPE) cells that appeared normal, but was decreased and without nuclear labeling in dysmorphic cells overlying drusen, a hallmark AMD lesion. Cigarette smoke extract (CSE) induced Nrf2 nuclear translocation in RPE cells with increased antioxidant and complement gene expression. Whereas CFH protein was not altered by CSE, the cell membrane regulator proteins CD46, CD55, and CD59 were decreased, and C3a and C3b, but not iC3b, were increased compared to controls. C5b-9 was increased by CSE, but at sublytic levels, only after addition of normal human serum. Nrf2 knockdown enhanced the increase in C3a and C3b from CSE, but not iC3b, C5a, or C5b-9. CSE also increased IL-1b expression and secretion after C3a generation and was reduced by a C3aR antagonist. In contrast, the Nrf2 activator CDDO-Im restored complement gene expression in RPE cells exposed to CSE. We provide evidence of altered Nrf2 in human AMD and that CSE induces a proinflammatory environment specifically by generating C3a and C3b, and Nrf2 deficiency magnifies this specific complement response.

Salvianolic acid A protects RPE cells against oxidative stress through activation of Nrf2/HO-1 signaling

Reactive oxygen species (ROS) impair the physiological functions of retinal pigment epithelial (RPE) cells, which is known as one major cause of age-related macular degeneration. Salvianolic acid A (Sal A) is the main effective aqueous extract of Salvia miltiorrhiza. The aim of this study was to test the potential role of Sal A against oxidative stress in cultured RPE cells and to investigate the underlying mechanistic signaling pathways. We observed that Sal A significantly inhibited hydrogen peroxide (H2O2)-induced primary and transformed RPE cell death and apoptosis. H2O2-stimulated mitogen-activated protein kinase activation, ROS production, and subsequent proapoptotic AMP-activated protein kinase activation were largely inhibited by Sal A. Further, Sal A stimulation resulted in a fast and dramatic activation of Akt/mammalian target of rapamycin complex 1 (mTORC1) signaling, followed by phosphorylation, accumulation, and nuclear translocation of the NF-E2-related factor 2 (Nrf2), along with increased expression of the antioxidant-response element-dependent gene heme oxygenase-1 (HO-1). Both Nrf2 and HO-1 were required for Sal A-mediated cytoprotective effect, as Nrf2/HO-1 inhibition abolished Sal A-induced beneficial effects against H2O2. Meanwhile, the PI3K/Akt/mTORC1 chemical inhibitors not only suppressed Sal A-induced Nrf2/HO-1 activation, but also eliminated its cytoprotective effect in RPE cells. These observations suggest that Sal A activates the Nrf2/HO-1 axis in RPE cells and protects against oxidative stress via activation of Akt/mTORC1 signaling.

An anti-interleukin-2 receptor drug attenuates T- helper 1 lymphocytes-mediated inflammation in an acute model of endotoxin-induced uveitis

The aim of the present study was to evaluate the anti-inflammatory efficacy of Daclizumab, an anti-interleukin-2 receptor drug, in an experimental uveitis model upon a subcutaneous injection of lipopolysaccharide into Lewis rats, a valuable model for ocular acute inflammatory processes. The integrity of the blood-aqueous barrier was assessed 24 h after endotoxin-induced uveitis by evaluating two parameters: cell count and protein concentration in aqueous humors. The histopathology of all the ocular structures (cornea, lens, sclera, choroid, retina, uvea, and anterior and posterior chambers) was also considered. Enzyme-linked immunosorbent assays of the aqueous humor samples were performed to quantify the levels of the different chemokine and cytokine proteins. Similarly, a biochemical analysis of oxidative stress-related markers was also assessed. The inflammation observed in the anterior chamber of the eyes when Daclizumab was administered with endotoxin was largely prevented since the aqueous humor protein concentration substantially lowered concomitantly with a significant reduction in the uveal and vitreous histopathological grading. Th1 lymphocytes-related cytokines, such as Interleukin-2 and Interferon-γ, also significantly reduced with related anti-oxidant systems recovery. Daclizumab treatment in endotoxin-induced uveitis reduced Th1 lymphocytes-related cytokines, such as Interleukin-2 and Interferon gamma, by about 60–70% and presented a preventive role in endotoxin-induced oxidative stress. This antioxidant protective effect of Daclizumab may be related to several of the observed Daclizumab effects in our study, including IL-6 cytokine regulatory properties and a substantial concomitant drop in INFγ. Concurrently, Daclizumab treatment triggered a significant reduction in both the uveal histopathological grading and protein concentration in aqueous humors, but not in cellular infiltration.

Common variant in VEGFA and response to anti-VEGF therapy for neovascular age-related macular degeneration

Age-related macular degeneration (AMD) is a leading cause of visual impairment in aging populations in industrialized countries. Here we investigated whether the genotype of vascular endothelial growth factor A (VEGFA) gene is associated with response to anti-VEGF therapy. 223 eyes with neovascular AMD were treated with intravitreal anti-VEGF therapy. Responders were defined as patients who had an improvement in best corrected visual acuity (BCVA) of at least 5 letters or one line on the EDTRS visual acuity chart along with resolution of intraretinal or subretinal fluid over 12 months. Patients who did not meet the definition of responders were classified as poor-responders. The vision of responders (n = 148) improved while the vision of poor-responders (n = 75) worsened (P<0.001). Responders on average had a decrease in central foveal thickness (CFT), while poor-responders had an increase in CFT (P <0.001). Compared with the responder group, the poor-responder group had a higher frequency of the risk (T) allele (Allelic P = 0.019) and TT genotype (P = 0.002 under a recessive model) for the VEGFA-rs943080 polymorphism. VEGFA expression was 1.8-fold higher in cells with the VEGFA rs943080 TT genotype than in cells with the VEGFA rs943080 CC genotype (P = 0.012). Age, gender, smoking, diabetes mellitus, and hypertension did not play a significant role in treatment response, but BMI was found to be significantly different between responders and poorresponders (P = 0.033). In conclusion, we demonstrated a potential pharmacogenetic relationship between the VEGFA gene and treatment response to anti-VEGF therapy.The studies are registered at ClinicalTrials.gov under the identifiers NCT00474695 (http://clinicaltrials. gov/ct2/show/NCT00474695) and NCT01464723 (http://clinicaltrials.gov/ct2/show/NCT01464723).

Acetylcholinesterase function in apoptotic retina pigment epithelial cells induced by H2O2

AIM

To investigate the acetylcholinesterase (AChE) expression involved in retina pigment epithelial (RPE) apoptosis induced by higher concentrations H2O2.

METHODS

The human retinal pigment epithelium cell line ARPE-19 was from ATCC (Rockville, MD). Cultured ARPE-19 cells were treated with H2O2 at 0, 250, 500, 1 000, 2 000µmol/L and cell viability was measured with MTT assay. AChE expression and DNA fragments were analyzed by immunocytochemistry, TUNEL and PARP-1 Western blotting.

RESULTS

Immunofluorescence detected AChE exist in the normal human retinal tissue. When H2O2 >500µmol/L, AChE expression showed an increase after 2h, and this concentration was selected for the present study. RPE cell was induced with 1 000µmol/L H2O2 for 2h, compared to the control group, cell activity decline detected by MTT, AChE and PARP-1 protein expression was significantly increased detected by Western blotting. AChE immunofluorescence staining was positive in RPE cell after H2O2 incubate 2h. In addition, pretreatment with 100µmol/L epigallocatechin gallate (EGCG), cell viability increased from 31.20%±3.90% to 70.23%±12.96%.

CONCLUSION

AChE is weakly expressed in normal human RPE cells. Stimulation with H2O2 caused the stable increase of AChE expression in RPE cells, which may indicate that AChE may be an important role in AMD.

Dry age-related macular degeneration: mechanisms, therapeutic targets, and imaging

Age-related macular degeneration is the leading cause of irreversible visual dysfunction in individuals over 65 in Western Society. Patients with AMD are classified as having early stage disease (early AMD), in which visual function is affected, or late AMD (generally characterized as either "wet" neovascular AMD, "dry" atrophic AMD or both), in which central vision is severely compromised or lost. Until recently, there have been no therapies available to treat the disorder(s). Now, the most common wet form of late-stage AMD, choroidal neovascularization, generally responds to treatment with anti-vascular endothelial growth factor therapies. Nevertheless, there are no current therapies to restore lost vision in eyes with advanced atrophic AMD. Oral supplementation with the Age-Related Eye Disease Study (AREDS) or AREDS2 formulation (antioxidant vitamins C and E, lutein, zeaxanthin, and zinc) has been shown to reduce the risk of progression to advanced AMD, although the impact was in neovascular rather than atrophic AMD. Recent findings, however, have demonstrated several features of early AMD that are likely to be druggable targets for treatment. Studies have established that much of the genetic risk for AMD is associated with complement genes. Consequently, several complement-based therapeutic treatment approaches are being pursued. Potential treatment strategies against AMD deposit formation and protein and/or lipid deposition will be discussed, including anti-amyloid therapies. In addition, the role of autophagy in AMD and prevention of oxidative stress through modulation of the antioxidant system will be explored. Finally, the success of these new therapies in clinical trials and beyond relies on early detection, disease typing, and predicting disease progression, areas that are currently being rapidly transformed by improving imaging modalities and functional assays.

Resveratrol protects RPE cells from sodium iodate by modulating PPARα and PPARδ

Selective killing of RPE cells in vivo by sodium iodate develops cardinal phenotypes of atrophic age-related macular degeneration. However, the molecular mechanisms are elusive. We tried to search for small cyto-protective molecules against sodium iodate and explore their mechanisms of action. Sodium iodate-mediated RPE cell death was associated with increased levels of reactive oxygen species (ROS) and IL-8. Resveratrol, a natural occurring polyphenol compound, was found to strongly protect RPE cells from sodium iodate with inhibition of production of ROS and IL-8. Resveratrol activated all isoforms of PPARs. Treatment with PPARα and PPARδ agonists inhibited sodium iodate-induced ROS production and protected RPE cells from sodium iodate. A PPARα antagonist significantly reduced resveratrol's protection of RPE cells from sodium iodate. Paradoxically, knocking down PPARδ also rendered RPE cells resistant to sodium iodate. Moreover, PPAR agonists reversed sodium iodate-induced production of IL-8. However, neutralizing extracellular IL-8 failed to protect RPE cells from sodium iodate. Taken together, these observations show that resveratrol protects RPE cells from sodium iodate injury through the activation of PPARα and alteration of PPARδ conformation. PPARα and δ modulators might ameliorate stress-induced RPE degeneration in vivo.

Mechanism of inflammation in age-related macular degeneration: an up-to-date on genetic landmarks

Age-related macular degeneration (AMD) is the most common cause of irreversible visual impairment among people over 50 years of age, accounting for up to 50% of all cases of legal blindness in Western countries. Although the aging represents the main determinant of AMD, it must be considered a multifaceted disease caused by interactions among environmental risk factors and genetic backgrounds. Mounting evidence and/or arguments document the crucial role of inflammation and immune-mediated processes in the pathogenesis of AMD. Proinflammatory effects secondary to chronic inflammation (e.g., alternative complement activation) and heterogeneous types of oxidative stress (e.g., impaired cholesterol homeostasis) can result in degenerative damages at the level of crucial macular structures, that is photoreceptors, retinal pigment epithelium, and Bruch's membrane. In the most recent years, the association of AMD with genes, directly or indirectly, involved in immunoinflammatory pathways is increasingly becoming an essential core for AMD knowledge. Starting from the key basic-research notions detectable at the root of AMD pathogenesis, the present up-to-date paper reviews the best-known and/or the most attractive genetic findings linked to the mechanisms of inflammation of this complex disease.

Nrf2 signaling is impaired in the aging RPE given an oxidative insult

Age-related macular degeneration (AMD) represents the leading cause of blindness in the elderly, yet no definitive therapy exists for early, dry disease. Several lines of evidence have implicated oxidative stress-induced damage to the retinal pigment epithelium (RPE) in the pathogenesis of AMD, suggesting that the aging RPE may exhibit increased susceptibility to cell damage induced by exogenous stressors. The transcription factor Nrf2 serves as the master regulator of a highly coordinated antioxidant response in virtually all cell types. We compared Nrf2 signaling in the RPE of young (2 months) and old (15 months) mice under unstressed and stressed (sodium iodate) conditions. The aging RPE expressed higher levels of the Nrf2 target genes NQO1, GCLM, and HO1 compared with the RPE of younger mice under unstressed conditions, suggesting an age-related increase in basal oxidative stress. Moreover, the RPE of older mice demonstrated impaired induction of the protective Nrf2 pathway following oxidative stress induced with sodium iodate. The RPE of old mice exposed to sodium iodate also exhibited higher levels of superoxide anion and malondialdehyde than young mice, suggesting inadequate protection against oxidative damage. Induction of Nrf2 signaling in response to sodium iodate was partially restored in the RPE of aging mice with genetic rescue, using conditional knockdown of the Nrf2 negative regulator Keap1 (Tam-Cre; Keap1loxP) compared to Keap1loxP mice. These data indicate that the aging RPE is vulnerable to oxidative damage due to impaired Nrf2 signaling, and that Nrf2 signaling is a promising target for novel pharmacologic or genetic therapeutic strategies.

Inflammatory cytokines protect retinal pigment epithelial cells from oxidative stress-induced death

Purpose

To investigate the effects of inflammatory factors and oxidative stress on cell survival of the human retinal pigment epithelial (RPE) cell line, ARPE-19.

Methods

Confluent RPE cells were treated with peripheral blood mononuclear cells-conditioned medium (PCM), H₂O₂, NaIO₃, interferon (IFN)-γ, tumor necrosis factor (TNF)-α, or combinations of these. Cell viability was determined by viability assays and by light microscopy. Effector molecules of cell death were investigated by immunofluorescence microscopy and flow cytometry. Microarrays were performed to screen for differential expression of anti-oxidative enzymes, and protein expression was validated by immunoblotting.

Results

Viability of RPE cells was reduced by exposure to inflammatory agents (PCM, IFNγ+/-TNFα) or to oxidative agents (H₂O₂ or NaIO₃). Unexpectedly, cells treated with either H₂O₂ or NaIO₃ were partially protected from cell death by the addition of PCM. This protection was conferred, at least in part, by IFNγ and TNFα. Cell death induced by H₂O₂ or NaIO₃ was preceded by mitochondrial dysfunction and by p62 upregulation, both of which were attenuated by PCM and/or by IFNγ+TNFα. RPE cells co-cultured with activated T cells, or treated with cytokines showed increased expression of anti-oxidative genes, with upregulation of superoxide dismutase 2 protein following PCM treatment.

Conclusion

Oxidative stress-induced cell death was reduced by concomitant inflammatory stress. This is likely due to the cytokine-mediated induction of the anti-oxidative stress response, upregulating protective anti-oxidant pathway(s). These findings suggest caution for the clinical use of anti-inflammatory agents in the management of immune-associated eye diseases such as age-related macular degeneration.

Smoking and age-related macular degeneration: biochemical mechanisms and patient support

A small percentage of the population associates smoking with ocular disease. Most optometrists do not stress the importance of smoking cessation to their patients, and the centrality of smoking regarding the risk for ocular disease is not emphasized in optometric education. Age-related macular degeneration has strong epidemiological associations with smoking, and so serves as an appropriate model for the adverse effects of cigarette smoke on the eye. This article aims to provide basic scientific information to optometrists and optometry students so that they can better understand the pathogenesis of age-related macular degeneration and provide education and support to their patients wishing to stop smoking.

Low erythrocyte catalase enzyme activity is correlated with high serum total homocysteine levels in Tunisian patients with acute myocardial infarction

Background

An imbalance between pro-oxidants and antioxidant systems has been suggested to be implicated in the physiopathology of acute myocardial infarction (AMI). We aimed to evaluate the antioxidant capacity in Tunisian patients and to assess the possible relationship between erythrocyte catalase enzyme activity and hyperhomocysteinaemia.

Methods

108 patients with AMI and 81 healthy subjects were enrolled in this study. Catalase erythrocyte enzyme activity was determined spectrophotometrically whereas “total antioxidant status” (TAS) concentration was measured by a commercially available method. Serum total homocysteine (tHcy) level was determined by a fluorescence polarization immunoassay (FPIA). Lipid peroxidation was measured with a fluorimetric method as “thiobarbituric acid reactive substances” (TBARS).

Results

Compared with healthy subjects, patients with AMI had significantly lower catalase activity (P<0.001), TAS concentrations (P<0.001), and significantly higher serum tHcy (P<0.001) and TBARS levels (P<0.001). Erythrocyte catalase enzyme activity was negatively correlated with serum tHcy and TBARS while serum tHcy and TBARS were in positive correlation. Furthermore, the unbalance between pro-oxidants and antioxidants seems to be more aggravated in patients with Q wave AMI compared to patients with non-Q wave AMI.

Conclusion

Our results suggest the involvement of hyperhomocysteinaemia in the drop of erythrocyte catalase activity related to myocardial ischemia reperfusion. Hyperhomocysteinaemia may increase the myocardial wall dysfunction under ischemia reperfusion by excessive production of reactive oxygen species which is made evident by increased lipid peroxidation.

Virtual slides

The virtual slide(s) for this article can be found here: http://www.diagnosticpathology.diagnomx.eu/vs/1623509866881834

Phytochemicals in ocular health: Therapeutic potential and delivery challenges

Diabetic retinopathy (DR) and age-related macular degeneration (AMD) are the leading causes of blindness in adults. The impact of these conditions on the quality of life is increasing in significance with a rise in life expectancy. The role of hyperglycemia, oxidative stress and inflammatory responses in the development and/or progression of DR and AMD, and several other sight threatening ocular diseases, is well established. In proliferative retinopathy, signals sent by the retina for nourishment, triggers the growth of fragile and abnormal blood vessels. Changes in ocular pressure may lead to rupture of these blood vessels causing severe vision problems. Recent in vitro and preclinical studies demonstrate that certain phytochemicals possessing potent antioxidant and anti-inflammatory activity and ocular blood flow enhancing properties may be very useful in the treatment of, or as a prophylactic measure for, DR and AMD. Apart from these properties they have also been investigated for their anti-bacterial, hormonal, enzyme stimulation, and anti-angiogenic activities. The attractive aspect of these potential therapeutic candidates is that they can act on multiple pathways identified in the etiology of DR, AMD, cataract and other ocular diseases. However, results from clinical trials have been somewhat ambiguous, raising questions about the concentrations of these bioflavonoids achieved in the neural retina following oral administration. Unfortunately, as of date, an efficient noninvasive means to deliver therapeutic agents/candidates to the back-of-the eye is still not available. This review examines some of these promising natural agents and discusses the challenges encountered in delivering them to the posterior segment ocular tissues through the oral route.

Genotype-phenotype associations in neovascular age-related macular degeneration

PURPOSE

To examine associations between recognized genetic susceptibility loci and angiographic subphenotypes of the neovascular variant of age-related macular degeneration (nvAMD).

METHODS

Participants (247 nvAMD, 52 early age-related macular degeneration [AMD], and 103 controls) were genotyped (complement factor H and ARMS2/HTRA1). nvAMD participants were assigned to one of two subcategories: mainly classic or mainly occult (based on the proportions of classic and occult choroidal neovascularization). nvAMD and early AMD were reassigned to two groups based on the extent and severity of drusen (retinal pigment epithelium dysfunction or not). Univariate and multivariate analysis were used to examine for associations between participant characteristics and genetic loci after adjusting for age, smoking status, and history of cardiovascular disease.

RESULTS

Univariate analysis confirmed the known significant associations between AMD stage and age, hypertension, and a history of cardiovascular disease. Those with retinal pigment epithelium dysfunction (F = 5.46; P = 0.02) or a positive smoking history (F = 3.89; P = 0.05) were more likely to have been classified as having mainly an occult rather than a mainly classic lesion. Multivariate analysis showed that significant associations were noted with the number of ARMS2/HTRA1 risk alleles (P < 0.001), smoking (ever vs. never) (P = 0.03), and cardiovascular disease (P = 0.01). With early AMD as the reference category, the mainly classic group exhibited significant associations with the number of ARMS2/HTRA1 risk alleles present (P < 0.001) and cardiovascular disease (P = 0.02). When mainly classic was compared with mainly occult, the latter was associated with the ARMS2/HTRA1 locus (P = 0.02).

CONCLUSION

ARMS2/HTRA1 risk genotype may play a role in determining neovascular subphenotype, whereas genetics/demographics, smoking, and systemic health factors contribute to the development of advanced AMD in the presence of early AMD.

Synthetic oleanane triterpenoids: multifunctional drugs with a broad range of applications for prevention and treatment of chronic disease

We review the rationale for the use of synthetic oleanane triterpenoids (SOs) for prevention and treatment of disease, as well as extensive biological data on this topic resulting from both cell culture and in vivo studies. Emphasis is placed on understanding mechanisms of action. SOs are noncytotoxic drugs with an excellent safety profile. Several hundred SOs have now been synthesized and in vitro have been shown to: 1) suppress inflammation and oxidative stress and therefore be cytoprotective, especially at low nanomolar doses, 2) induce differentiation, and 3) block cell proliferation and induce apoptosis at higher micromolar doses. Animal data on the use of SOs in neurodegenerative diseases and in diseases of the eye, lung, cardiovascular system, liver, gastrointestinal tract, and kidney, as well as in cancer and in metabolic and inflammatory/autoimmune disorders, are reviewed. The importance of the cytoprotective Kelch-like erythroid cell-derived protein with CNC homology-associated protein 1/nuclear factor (erythroid-derived 2)-like 2/antioxidant response element (Keap1/Nrf2/ARE) pathway as a mechanism of action is explained, but interactions with peroxisome proliferator-activated receptor γ (PARPγ), inhibitor of nuclear factor-κB kinase complex (IKK), janus tyrosine kinase/signal transducer and activator of transcription (JAK/STAT), human epidermal growth factor receptor 2 (HER2)/ErbB2/neu, phosphatase and tensin homolog (PTEN), the phosphatidylinositol 3-kinase/protein kinase B (PI3K/Akt) pathway, mammalian target of rapamycin (mTOR), and the thiol proteome are also described. In these interactions, Michael addition of SOs to reactive cysteine residues in specific molecular targets triggers biological activity. Ultimately, SOs are multifunctional drugs that regulate the activity of entire networks. Recent progress in the earliest clinical trials with 2-cyano-3,12-dioxooleana-1,9(11)-dien-28-oic acid (CDDO) methyl ester (bardoxolone methyl) is also summarized.

Roles for the ubiquitin-proteasome pathway in protein quality control and signaling in the retina: implications in the pathogenesis of age-related macular degeneration

The accumulation of damaged or postsynthetically modified proteins and dysregulation of inflammatory responses and angiogenesis in the retina/RPE are thought be etiologically related to formation of drusen and choroidal neovascularization (CNV), hallmarks of age-related macular degeneration (AMD). The ubiquitin-proteasome pathway (UPP) plays crucial roles in protein quality control, cell cycle control and signal transduction. Selective degradation of aberrant proteins by the UPP is essential for timely removal of potentially cytotoxic damaged or otherwise abnormal proteins. Proper function of the UPP is thought to be required for cellular function. In contrast, age--or stress induced--impairment the UPP or insufficient UPP capacity may contribute to the accumulation of abnormal proteins, cytotoxicity in the retina, and AMD. Crucial roles for the UPP in eye development, regulation of signal transduction, and antioxidant responses are also established. Insufficient UPP capacity in retina and RPE can result in dysregulation of signal transduction, abnormal inflammatory responses and CNV. There are also interactions between the UPP and lysosomal proteolytic pathways (LPPs). Means that modulate the proteolytic capacity are making their way into new generation of pharmacotherapies for delaying age-related diseases and may augment the benefits of adequate nutrition, with regard to diminishing the burden of AMD.

Systemic administration of the iron chelator deferiprone protects against light-induced photoreceptor degeneration in the mouse retina

Oxidative stress plays a key role in a light-damage (LD) model of retinal degeneration as well as in age-related macular degeneration (AMD). Since iron can promote oxidative stress, the iron chelator deferiprone (DFP) was tested for protection against light-induced retinal degeneration. To accomplish this, A/J mice were treated with or without oral DFP and then were placed in constant bright white fluorescent light (10,000 lx) for 20 h. Retinas were evaluated at several time points after light exposure. Photoreceptor apoptosis was assessed using the TUNEL assay. Retinal degeneration was assessed by histology 10 days after exposure to damaging white light. Two genes upregulated by oxidative stress, heme oxygenase 1 (Hmox1) and ceruloplasmin (Cp), as well as complement component 3 (C3) were quantified by RT-qPCR. Cryosections were immunolabeled for an oxidative stress marker (nitrotyrosine), a microglial marker (Iba1), as well as both heavy (H) and light (L) ferritin. Light exposure resulted in substantial photoreceptor-specific cell death. Dosing with DFP protected photoreceptors, decreasing the numbers of TUNEL-positive photoreceptors and increasing the number of surviving photoreceptors. The retinal mRNA levels of oxidative stress-related genes and C3 were upregulated following light exposure and diminished by DFP treatment. Immunostaining for nitrotyrosine indicated that DFP reduced the nitrative stress caused by light exposure. Robust H/L-ferritin-containing microglial activation and migration to the outer retina occurred after light exposure and DFP treatment reduced microglial invasion. DFP is protective against light-induced retinal degeneration and has the potential to diminish oxidative stress in the retina.

Pathological consequences of long-term mitochondrial oxidative stress in the mouse retinal pigment epithelium

Oxidative stress in the retinal pigment epithelium (RPE) is hypothesized to be a major contributor to the development of age-related macular degeneration (AMD). Mitochondrial manganese superoxide dismutase (MnSOD) is a critical antioxidant protein that scavenges the highly reactive superoxide radical. We speculated that specific reduction of MnSOD in the RPE will increase the level of reactive oxygen species in the retina/RPE/choroid complex leading to pathogenesis similar to geographic atrophy. To test this hypothesis, an Sod2-specific hammerhead ribozyme (Rz), delivered by AAV2/1 and driven by the human VMD2 promoter was injected subretinally into C57BL/6J mice. Dark-adapted full field electroretinogram (ERG) detected a decrease in the response to light. We investigated the age-dependent phenotypic and morphological changes of the outer retina using digital fundus imaging and SD-OCT measurement of ONL thickness. Fundus microscopy revealed pigmentary abnormalities in the retina and these corresponded to sub-retinal and sub-RPE deposits seen in SD-OCT B-scans. Light and electron microscopy documented the localization of apical deposits and thickening of the RPE. In RPE flat-mounts we observed abnormally displaced nuclei and regions of apparent fibrosis in the central retina of the oldest mice. This region was surrounded by enlarged and irregular RPE cells that have been observed in eyes donated by AMD patients and in other mouse models of AMD.

Retinal pigment epithelium response to oxidant injury in the pathogenesis of early age-related macular degeneration

Age-related macular degeneration (AMD) represents the leading cause of vision loss in the elderly. Accumulation of lipid- and protein-rich deposits under the retinal pigment epithelium (RPE) heralds the onset of early AMD, but the pathogenesis of subretinal deposit formation is poorly understood. Numerous hypothetical models of deposit formation have been proposed, including hypotheses for a genetic basis, choroidal hypoperfusion, abnormal barrier formation, and lysosomal failure. This review explore the RPE injury hypothesis, characterized by three distinct stages (1) Initial RPE oxidant injury, caused by any number of endogenous or exogenous oxidants, results in extrusion of cell membrane "blebs," together with decreased activity of matrix metalloproteinases (MMPs), promoting bleb accumulation under the RPE as basal laminar deposits (BLD). (2) RPE cells are subsequently stimulated to increase synthesis of MMPs and other molecules responsible for extracellular matrix turnover (i.e., producing decreased collagen), affecting both RPE basement membrane and Bruchs membrane (BrM). This process leads to progression of BLD into basal linear deposits (BLinD) and drusen by admixture of blebs into BrM, followed by the formation of new basement membrane under the RPE to trap these deposits within BrM. We postulate that various hormones and other plasma-derived molecules related to systemic health cofactors are implicated in this second stage. (3) Finally, macrophages are recruited to sites of RPE injury and deposit formation. The recruitment of nonactivated or scavenging macrophages may remove deposits without further injury, while the recruitment of activated or reparative macrophages, through the release of inflammatory mediators, growth factors, or other substances, may promote complications and progression to the late forms of the disease.

Mechanisms for countering oxidative stress and damage in retinal pigment epithelium

Clinical and experimental evidence supports that chronic oxidative stress is a primary contributing factor to numerous retinal degenerative diseases, such as age-related macular degeneration (AMD). Eyes obtained postmortem from AMD patients have extensive free radical damage to the proteins, lipids, DNA, and mitochondria of their retinal pigment epithelial (RPE) cells. In addition, several mouse models of chronic oxidative stress develop many of the pathological hallmarks of AMD. However, the extent to which oxidative stress is an etiologic component versus its involvement in disease progression remains a major unanswered question. Further, whether the primary target of oxidative stress and damage is photoreceptors or RPE cells, or both, is still unclear. In this review, we discuss the major functions of RPE cells with an emphasis on the oxidative challenges these cells encounter and the endogenous antioxidant mechanisms employed to neutralize the deleterious effects that such stresses can elicit if left unchecked.

Role of the ubiquitin-proteasome in protein quality control and signaling: implication in the pathogenesis of eye diseases

The ubiquitin-proteasome pathway (UPP) plays important roles in many cellular functions, such as protein quality control, cell cycle control, and signal transduction. The selective degradation of aberrant proteins by the UPP is essential for the timely removal of potential cytotoxic damaged or otherwise abnormal proteins. Conversely, accumulation of the cytotoxic abnormal proteins in eye tissues is etiologically associated with many age-related eye diseases such as retina degeneration, cataract, and certain types of glaucoma. Age- or stress-induced impairment or overburdening of the UPP appears to contribute to the accumulation of abnormal proteins in eye tissues. Cell cycle and signal transduction are regulated by the conditional UPP-dependent degradation of the regulators of these processes. Impairment or overburdening of the UPP could also result in dysregulation of cell cycle control and signal transduction. The consequences of the improper cell cycle and signal transduction include defects in ocular development, wound healing, angiogenesis, or inflammatory responses. Methods that enhance or preserve UPP function or reduce its burden may be useful strategies for preventing age-related eye diseases.

Proteomic analysis shows synthetic oleanane triterpenoid binds to mTOR

New multifunctional drugs that target multiple disease-relevant networks offer a novel approach to the prevention and treatment of many diseases. New synthetic oleanane triterpenoids (SO), such as CDDO (2-cyano-3,12-dioxooleana-1,9-dien-28-oic acid) and its derivatives, are multifunctional compounds originally developed for the prevention and treatment of inflammation and oxidative stress. However, the protein binding partners and mechanisms of action of these SO are not yet fully understood. Here we characterize the putative target profile of one SO, CDDO-Imidazolide (CDDO-Im), by combining affinity purification with mass spectroscopic proteomic analysis to identify 577 candidate binding proteins in whole cells. This SO pharmaco-interactome consists of a diverse but interconnected set of signaling networks; bioinformatic analysis of the protein interactome identified canonical signaling pathways targeted by the SO, including retinoic acid receptor (RAR), estrogen receptor (ER), insulin receptor (IR), janus kinase/signal transducers and activators of transcription (JAK/STAT), and phosphatase and tensin homolog (PTEN). Pull-down studies then further validated a subset of the putative targets. In addition, we now show for the first time that the mammalian target of rapamycin (mTOR) is a direct target of CDDO-Im. We also show that CDDO-Im blocks insulin-induced activation of this pathway by binding to mTOR and inhibiting its kinase activity. Our basic studies confirm that the SO, CDDO-Im, acts on a protein network to elicit its pharmacological activity.

Light-emitting diodes (LED) for domestic lighting: any risks for the eye?

Light-emitting diodes (LEDs) are taking an increasing place in the market of domestic lighting because they produce light with low energy consumption. In the EU, by 2016, no traditional incandescent light sources will be available and LEDs may become the major domestic light sources. Due to specific spectral and energetic characteristics of white LEDs as compared to other domestic light sources, some concerns have been raised regarding their safety for human health and particularly potential harmful risks for the eye. To conduct a health risk assessment on systems using LEDs, the French Agency for Food, Environmental and Occupational Health & Safety (ANSES), a public body reporting to the French Ministers for ecology, for health and for employment, has organized a task group. This group consisted physicists, lighting and metrology specialists, retinal biologist and ophthalmologist who have worked together for a year. Part of this work has comprised the evaluation of group risks of different white LEDs commercialized on the French market, according to the standards and found that some of these lights belonged to the group risk 1 or 2. This paper gives a comprehensive analysis of the potential risks of white LEDs, taking into account pre-clinical knowledge as well as epidemiologic studies and reports the French Agency's recommendations to avoid potential retinal hazards.

The oral iron chelator deferiprone protects against iron overload-induced retinal degeneration

PURPOSE

Iron-induced oxidative stress may exacerbate age-related macular degeneration (AMD). Ceruloplasmin/Hephaestin double-knockout (DKO) mice with age-dependent retinal iron accumulation and some features of AMD were used to test retinal protection by the oral iron chelator deferiprone (DFP).

METHODS

Cultured retinal pigment epithelial (ARPE-19) cells and mice were treated with DFP. Transferrin receptor mRNA (Tfrc), an indicator of iron levels, was quantified by qPCR. In mice, retinal oxidative stress was assessed by mass spectrometry, and degeneration by histology and electroretinography.

RESULTS

DFP at 60 μM decreased labile iron in ARPE-19 cells, increasing Tfrc and protecting 70% of cells against a lethal dose of H(2)O(2). DFP 1 mg/mL in drinking water increased retinal Tfrc mRNA 2.7-fold after 11 days and also increased transferrin receptor protein. In DKOs, DFP over 8 months decreased retinal iron levels to 72% of untreated mice, diminished retinal oxidative stress to 70% of the untreated level, and markedly ameliorated retinal degeneration. DFP was not retina toxic in wild-type (WT) or DKO mice, as assessed by histology and electroretinography.

CONCLUSIONS

Oral DFP was not toxic to the mouse retina. It diminished retinal iron levels and oxidative stress and protected DKO mice against iron overload-induced retinal degeneration. Further testing of DFP for retinal disease involving oxidative stress is warranted.

Age-related macular degeneration: genetic and environmental factors of disease

Age-related macular degeneration (AMD) is the most common cause of visual impairment among the elderly in developed countries, and its prevalence is thus increasing as the population ages; however, treatment options remain limited because the etiology and pathogenesis of AMD are incompletely defined. Recently, much progress has been made in gene discovery and mechanistic studies, which clearly indicate that AMD involves the interaction of multiple genetic and environmental factors. The identification of genes that have a substantial impact on the risk for AMD is not only facilitating the diagnosis and screening of populations at risk but is also elucidating key molecular pathways of pathogenesis. Pharmacogenetic studies of treatment responsiveness among patients with the "wet" form of AMD are increasingly proving to be clinically relevant; pharmacogenetic approaches hold great promise for both identifying patients with the best chance for vision recovery as well as tailoring individualized therapies.

Increased erythrocyte antioxidant status protects against smoking induced hemolysis in moderate smokers

Cigarette smoking is common in societies worldwide and has been identified as injurious to human health. Human red blood cells are important targets for electrophilic and oxidant foreign compounds. In the present study, the possible role of antioxidant status on smoking-induced erythrocyte hemolysis of smokers was studied. Erythrocyte superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GPx) activities, reduced glutathione (GSH) level, erythrocyte membrane lipid peroxidation, total cholesterol and phospholipids were determined. Further, nitrite/nitrate levels (NO2/NO3) in both plasma and erythrocyte lysate were measured. Results showed increased plasma and erythrocyte membrane lipid peroxidation and nitrite/nitrate levels in smokers. The activities of SOD, CAT and GPx were also increased with reduced glutathione (GSH) level in smokers. No significant change was observed in smokers red cell hemolysis and cholesterol/phospholipid (C/P) ratio compared to controls. Erythrocyte membrane lipid peroxidation was positively correlated with SOD ( r = 0.482, p < 0.01) and GPx ( r = 0.368, p < 0.018) in smokers. Increased levels of nitrite/nitrate and antioxidant status of erythrocytes might be playing a crucial role in protecting red cell from free radical damage induced by cigarette smoke.

Age-dependent retinal iron accumulation and degeneration in hepcidin knockout mice

PURPOSE

Iron dysregulation can cause retinal disease, yet retinal iron regulatory mechanisms are incompletely understood. The peptide hormone hepcidin (Hepc) limits iron uptake from the intestine by triggering degradation of the iron transporter ferroportin (Fpn). Given that Hepc is expressed in the retina and Fpn is expressed in cells constituting the blood-retinal barrier, the authors tested whether the retina may produce Hepc to limit retinal iron import.

METHODS

Retinas of Hepc(-/-) mice were analyzed by histology, autofluorescence spectral analysis, atomic absorption spectrophotometry, Perls' iron stain, and immunofluorescence to assess iron-handling proteins. Retinal Hepc mRNA was evaluated through qPCR after intravitreal iron injection. Mechanisms of retinal Hepc upregulation were tested by Western blot analysis. A retinal capillary endothelial cell culture system was used to assess the effect of exogenous Hepc on Fpn.

RESULTS

Hepc(-/-) mice experienced age-dependent increases in retinal iron followed by retinal degeneration with autofluorescent RPE, photoreceptor death, and subretinal neovascularization. Hepc(-/-) mice had increased Fpn immunoreactivity in vascular endothelial cells. Conversely, in cultured retinal capillary endothelial cells, exogenous Hepc decreased both Fpn levels and iron transport. The retina can sense increased iron levels, upregulating Hepc after phosphorylation of extracellular signal regulated kinases.

CONCLUSIONS

These findings indicate that Hepc is essential for retinal iron regulation. In the absence of Hepc, retinal degeneration occurs. Increases in Hepc mRNA levels after intravitreal iron injection combined with Hepc-mediated decreases in iron export from cultured retinal capillary endothelial cells suggest that the retina may use Hepc for its tissue-specific iron regulation.