Induction of oxidative and nitrosative stresses in human retinal pigment epithelial cells by all-trans-retinal

Reveal the mechanism of hepatic oxidative stress in mice induced by photo-oxidation milk using multi-omics analysis techniques

INTRODUCTION

Photo-oxidation is recognized as a contributor to the deterioration of milk quality, posing potential safety hazards to human health. However, there has been limited investigation into the impact of consuming photo-oxidized milk on health.

OBJECTIVES

This study employs multi-omics analysis techniques to elucidate the mechanisms by which photo-oxidized milk induces oxidative stress in the liver.

METHODS

Mouse model was used to determine the effect of the gavage administration of milk with varying degrees of photo-oxidation on the mouse liver. The damage degree was established by measuring serum markers indicative of oxidative stress, and with a subsequent histopathological examination of liver tissues. In addition, comprehensive metabolome, lipidome, and transcriptome analyses were conducted to elucidate the underlying molecular mechanisms of hepatic damage caused by photo-oxidized milk.

RESULTS

A significant elevation in the oxidative stress levels and the presence of hepatocellular swelling and inflammation subsequent to the gavage administration of photo-oxidized milk to mice. Significant alterations in the levels of metabolites such as lumichrome, all-trans-retinal, L-valine, phosphatidylglycerol, and phosphatidylcholine within the hepatic tissue of mice. Moreover, photo-oxidized milk exerted a pronounced detrimental impact on the glycerophospholipid metabolism of mice liver. The peroxisome proliferator-activated receptors (PPAR) signaling pathway enrichment appreciated in the animals that consumed photo-oxidized milk further supports the substantial negative influence of photo-oxidized milk on hepatic lipid metabolism. Gene set enrichment and interaction analyses revealed that photo-oxidized milk inhibited the cytochrome P450 pathway in mice, while also affecting other pathways associated with cellular stress response and lipid biosynthesis.

CONCLUSION

This comprehensive study provides significant evidence regarding the potential health risks associated with photo-oxidized milk, particularly in terms of hepatic oxidative damage. It establishes a scientific foundation for assessing the safety of such milk and ensuring the quality of dairy products.

The endoplasmic reticulum: Homeostasis and crosstalk in retinal health and disease

The endoplasmic reticulum (ER) is the largest intracellular organelle carrying out a broad range of important cellular functions including protein biosynthesis, folding, and trafficking, lipid and sterol biosynthesis, carbohydrate metabolism, and calcium storage and gated release. In addition, the ER makes close contact with multiple intracellular organelles such as mitochondria and the plasma membrane to actively regulate the biogenesis, remodeling, and function of these organelles. Therefore, maintaining a homeostatic and functional ER is critical for the survival and function of cells. This vital process is implemented through well-orchestrated signaling pathways of the unfolded protein response (UPR). The UPR is activated when misfolded or unfolded proteins accumulate in the ER, a condition known as ER stress, and functions to restore ER homeostasis thus promoting cell survival. However, prolonged activation or dysregulation of the UPR can lead to cell death and other detrimental events such as inflammation and oxidative stress; these processes are implicated in the pathogenesis of many human diseases including retinal disorders. In this review manuscript, we discuss the unique features of the ER and ER stress signaling in the retina and retinal neurons and describe recent advances in the research to uncover the role of ER stress signaling in neurodegenerative retinal diseases including age-related macular degeneration, inherited retinal degeneration, achromatopsia and cone diseases, and diabetic retinopathy. In some chapters, we highlight the complex interactions between the ER and other intracellular organelles focusing on mitochondria and illustrate how ER stress signaling regulates common cellular stress pathways such as autophagy. We also touch upon the integrated stress response in retinal degeneration and diabetic retinopathy. Finally, we provide an update on the current development of pharmacological agents targeting the UPR response and discuss some unresolved questions and knowledge gaps to be addressed by future research.

Ginkgo biloba extracts protect human retinal Müller glial cells from t-BHP induced oxidative damage by activating the AMPK-Nrf2-NQO-1 axis

OBJECTIVES

Retinal Müller glial cell loss is almost involved in all retinal diseases, especially diabetic retinopathy (DR). Oxidative stress significantly contributes to the development of Müller glial cell loss. Ginkgo biloba extracts (GBE) have been reported to possess antioxidant property, beneficial in treating human retinal diseases. However, little is known about its role in Müller glial cells. This study investigated the protective effect of GBE (prepared from ginkgo biloba dropping pills) in human Müller glial cells against tert-butyl hydroperoxide (t-BHP)-induced oxidative stress and its underlying molecular mechanism.

METHODS

MIO-M1 cells were pretreated with or without GBE prior to the exposure to t-BHP-induced oxidative stress. Cell viability, cell death profile and lipid peroxidation were subsequently assessed. Protein expression of the key anti-oxidative signalling factors were investigated.

KEY FINDINGS

We showed that GBE can effectively protect human MIO-M1 cells from t-BHP-induced oxidative injury by improving cell viability, reducing intracellular ROS accumulation and suppressing lipid peroxidation, which effect is likely mediated through activating AMPK-Nrf2-NQO-1 antioxidant respondent axis.

CONCLUSIONS

Our study is the first to reveal the great potentials of GBE in protecting human retinal Müller glial cell loss against oxidative stress. GBE might be used to prevent human retinal diseases particularly DR.

Repressing c-Jun N-terminal kinase signaling mitigates retinal pigment epithelium degeneration in mice with failure to clear all-trans-retinal

Retinal pigment epithelium (RPE) cell apoptosis arising from all-trans-retinal (atRAL) is in close contact with the etiology of dry age-related macular degeneration (AMD) and autosomal recessive Stargardt's disease (STGD1), but its underlying mechanisms remain elusive. In this study, we reported that c-Jun N-terminal kinase (JNK) activation facilitated atRAL-induced apoptosis of RPE cells. Reactive oxygen species production and endoplasmic reticulum stress were identified as two of major upstream events responsible for activating JNK signaling in atRAL-loaded RPE cells. Inhibiting JNK signaling rescued RPE cells from apoptosis induced by atRAL through attenuating caspase-3 activation leading to poly-ADP-ribose polymerase (PARP) cleavage, and DNA damage response. Abca4^-/-Rdh8^-/- mice upon light exposure exhibit rapidly increased accumulation of atRAL in the retina, and display severe RPE degeneration, a primary attribute of dry AMD and STGD1. Reducing JNK signaling by intraperitoneally injected JNK-IN-8 was highly effective in preventing RPE atrophy and apoptosis in light-exposed Abca4^-/-Rdh8^-/- mice. These findings afford a further understanding for contribution of JNK activation by atRAL to retinal damage.

Involvement of Oxidative and Endoplasmic Reticulum Stress in RDH12-Related Retinopathies

Retinol dehydrogenase 12 (RDH12) is expressed in photoreceptor inner segments and catalyses the reduction of all-trans retinal (atRAL) to all-trans retinol (atROL), as part of the visual cycle. Mutations in RDH12 are primarily associated with autosomal recessive Leber congenital amaurosis. To further our understanding of the disease mechanisms, HEK-293 cell lines expressing wildtype (WT) and mutant RDH12 were created. The WT cells afforded protection from atRAL-induced toxicity and oxidative stress. Mutant RDH12 cells displayed reduced protein expression and activity, with an inability to protect cells from atRAL toxicity, inducing oxidative and endoplasmic reticulum (ER) stress, with upregulation of sXBP1, CHOP, and ATF4. Pregabalin, a retinal scavenger, attenuated atRAL-induced ER stress in the mutant RDH12 cell lines. A zebrafish rdh12 mutant model (rdh12^u533 c.17_23del; p.(Val6AlafsTer5)) was generated through CRISPR-Cas9 gene editing. Mutant fish showed disrupted phagocytosis through transmission electron microscopy, with increased phagosome size at 12 months post-fertilisation. Rhodopsin mislocalisation and reduced expression of atg12 and sod2 indicated early signs of a rod-predominant degeneration. A lack of functional RDH12 results in ER and oxidative stress representing key pathways to be targeted for potential therapeutics.

Leukocyte Integrin Antagonists as a Novel Option to Treat Dry Age-Related Macular Degeneration

Age-related macular degeneration (AMD) is a complex multifactorial degenerative disease that leads to irreversible blindness. AMD affects the macula, the central part of the retina responsible for sharp central vision. Retinal pigment epithelium (RPE) is the main cellular type affected in dry AMD. RPE cells form a monolayer between the choroid and the neuroretina and are in close functional relationship with photoreceptors; moreover, RPE cells are part of the blood retina barrier that is disrupted in ocular diseases such as AMD. During ocular inflammation lymphocytes and macrophages are recruited, contact RPE and produce pro-inflammatory cytokines, which play an important role in AMD pathogenesis. The interaction between RPE and immune cells is mediated by leukocyte integrins, heterodimeric transmembrane receptors, and adhesion molecules, including VCAM-1 and ICAM-1. Within this frame, this study aimed to characterize RPE-leukocytes interaction and to investigate any potentially beneficial effects induced by integrin antagonists (DS-70, MN27 and SR714), developed in previous studies. ARPE-19 cells were co-cultured for different incubation times with Jurkat cells and apoptosis and necrosis levels were analyzed by flow cytometry. Moreover, we measured the mRNA levels of the pro-inflammatory cytokine IL-1β and the expression of adhesion molecules VCAM-1 and ICAM-1. We found that RPE-lymphocyte interaction increased apoptosis and necrosis levels in RPE cells and the expression of IL-1β. This interaction was mediated by the binding of α₄β₁ and α_Lβ₂ integrins to VCAM-1 and ICAM-1, respectively. The blockade of RPE-lymphocyte interaction with blocking antibodies highlighted the pivotal role played by integrins. Therefore, α₄β₁ and α_Lβ₂ integrin antagonists were employed to disrupt RPE-lymphocyte crosstalk. Small molecule integrin antagonists proved to be effective in reducing RPE cell death and expression of IL-1β, demonstrating that integrin antagonists could protect RPE cells from detrimental effects induced by the interaction with immune cells recruited to the retina. Overall, the leukocyte integrin antagonists employed in the present study may represent a novel opportunity to develop new drugs to fight dry AMD.

All-trans-Retinaldehyde Contributes to Retinal Vascular Permeability in Ischemia Reperfusion

Purpose

Extracellular accumulation of all-trans-retinaldehyde (atRAL), a highly reactive visual cycle intermediate, is toxic to cells of the outer retina and contributes to retinal and macular degenerations. However, the contribution of atRAL to retinal capillary function has not been studied. We hypothesized that atRAL released from the outer retina can contribute to retinal vascular permeability. We, therefore, tested the contribution of atRAL to retinal ischemia-reperfusion (IR)-induced vascular permeability.

Methods

IR was induced in mice by transient increase in intraocular pressure followed by natural reperfusion. The visual cycle was ablated in the Lrat-/- mice, reduced by dark adaptation or the use of the RPE65 inhibitor and atRAL scavenger emixustat. Accumulation of FITC-BSA was used to assess vascular permeability and DNA fragmentation quantified cell death after IR. Primary bovine retinal endothelial cell (BREC) culture was used to measure the direct effects of atRAL on endothelial permeability and cell death.

Results

Inhibition of the visual cycle by Lrat-/-, dark adaptation, or with emixustat, all reduced approximately half of IR induced vascular permeability at 48 hours. An increase in BREC permeability with atRAL coincided with lactate dehydrogenase (LDH) release, a measure of cell death. Both permeability and toxicity were blocked by emixustat.

Conclusions

Outer retinal pathology may contribute to vascular permeability by release of atRAL, which can act directly on vascular endothelial cells to alter barrier properties and induce cell death. These studies may have implications for a variety of blinding eye diseases that include outer retinal damage and retinal vascular permeability.

Microglia Contribution to the Regulation of the Retinal and Choroidal Vasculature in Age-Related Macular Degeneration

The retina is a highly metabolically active tissue with high-level consumption of nutrients and oxygen. This high metabolic demand requires a properly developed and maintained vascular system. The retina is nourished by two systems: the central retinal artery that supplies the inner retina and the choriocapillaris that supplies the outer retina and retinal pigment epithelium (RPE). Pathological neovascularization, characterized by endothelial cell proliferation and new vessel formation, is a common hallmark in several retinal degenerative diseases, including age-related macular degeneration (AMD). A limited number of studies have suggested that microglia, the resident immune cells of the retina, have an important role not only in the pathology but also in the formation and physiology of the retinal vascular system. Here, we review the current knowledge on microglial interaction with the retinal vascular system under physiological and pathological conditions. To do so, we first highlight the role of microglial cells in the formation and maintenance of the retinal vasculature system. Thereafter, we discuss the molecular signaling mechanisms through which microglial cells contribute to the alterations in retinal and choroidal vasculatures and to the neovascularization in AMD.

Isopropyl-phloroglucinol-DHA protects outer retinal cells against lethal dose of all-trans-retinal

All‐trans‐retinal (atRAL) is a highly reactive carbonyl specie, known for its reactivity on cellular phosphatidylethanolamine in photoreceptor. It is generated by photoisomerization of 11‐cis‐retinal chromophore linked to opsin by the Schiff's base reaction. In ABCA4‐associated autosomal recessive Stargardt macular dystrophy, atRAL results in carbonyl and oxidative stress, which leads to bisretinoid A2E, accumulation in the retinal pigment epithelium (RPE). This A2E‐accumulation presents as lipofuscin fluorescent pigment, and its photooxidation causes subsequent damage. Here we describe protection against a lethal dose of atRAL in both photoreceptors and RPE in primary cultures by a lipidic polyphenol derivative, an isopropyl‐phloroglucinol linked to DHA, referred to as IP‐DHA. Next, we addressed the cellular and molecular defence mechanisms in commonly used human ARPE‐19 cells. We determined that both polyunsaturated fatty acid and isopropyl substituents bond to phloroglucinol are essential to confer the highest protection. IP‐DHA responds rapidly against the toxicity of atRAL and its protective effect persists. This healthy effect of IP‐DHA applies to the mitochondrial respiration. IP‐DHA also rescues RPE cells subjected to the toxic effects of A2E after blue light exposure. Together, our findings suggest that the beneficial role of IP‐DHA in retinal cells involves both anti‐carbonyl and anti‐oxidative capacities.

All-trans-retinal induces autophagic cell death via oxidative stress and the endoplasmic reticulum stress pathway in human retinal pigment epithelial cells

Failure of all-trans-retinal (atRAL) clearance contributes to retina degeneration. However, whether autophagy can be activated by excess atRAL accumulation in retinal pigment epithelial (RPE) cells is not known. This study showed that atRAL provoked mitochondria-associated reactive oxygen species (ROS) production, activated the nuclear factor (erythroid-derived 2)-like 2 and apoptosis in a human RPE cell line, ARPE-19 cells. Moreover, we found that autophagic flux was functionally activated after atRAL treatment. The antioxidant N-acetylcysteine attenuated the expression of autophagy markers, suggesting that ROS triggered atRAL-activated autophagy. In addition, autophagic cell death was observed in atRAL-treated RPE cells, while inhibition of autophagy with 3-methyladenine or LC3, Beclin1, p62 silencing ameliorated atRAL-induced cytotoxicity. Suppression of autophagy quenched mitochondrial ROS and inhibited HO-1 and γ-GCSh expression, indicating that atRAL-activated autophagy enhances intracellular oxidative stress, thereby promoting RPE cell apoptosis. Furthermore, we found that inhibiting endoplasmic reticulum (ER) stress suppressed atRAL-induced mitochondrial ROS generation, subsequently attenuated autophagy and apoptosis in RPE cells. Taken together, these results suggest that atRAL-induced oxidative stress and ER stress modulate autophagy, which may contribute to RPE degeneration. There may be positive feedback regulatory mechanisms between atRAL-induced oxidative stress and autophagy or ER stress.

Bis-allylic Deuterated DHA Alleviates Oxidative Stress in Retinal Epithelial Cells

Oxidative stress plays a crucial role in developing and accelerating retinal diseases including age-related macular degeneration (AMD). Docosahexaenoic acid (DHA, C22:6, n-3), the main lipid constituent of retinal epithelial cell membranes, is highly prone to radical and enzymatic oxidation leading to deleterious or beneficial metabolites for retinal tissue. To inhibit radical oxidation while preserving enzymatic metabolism, deuterium was incorporated at specific positions of DHA, resulting in D₂-DHA when incorporated at position 6 and D₄-DHA when incorporated at the 6,9 bis-allylic positions. Both derivatives were able to decrease DHAs’ toxicity and free radical processes involved in lipid peroxidation, in ARPE-19 cells (Adult Retinal Pigment Epithelial cell line), under pro-oxidant conditions. Our positive results encouraged us to prepare lipophenolic-deuterated-DHA conjugates as possible drug candidates for AMD treatment. These novel derivatives proved efficient in limiting lipid peroxidation in ARPE-19 cells. Finally, we evaluated the underlying mechanisms and the enzymatic conversion of both deuterated DHA. While radical abstraction was affected at the deuterium incorporation sites, enzymatic conversion by the lipoxygenase 15s-LOX was not impacted. Our results suggest that site-specifically deuterated DHA could be used in the development of DHA conjugates for treatment of oxidative stress driven diseases, or as biological tools to study the roles, activities and mechanisms of DHA metabolites.

Betulinic acid derivatives can protect human Müller cells from glutamate-induced oxidative stress

Müller cells are the predominant retinal glial cells. One of the key roles of Müller cells is in the uptake of the neurotransmitter glutamate and in its conversion to glutamine. Müller cell dysfunction due to oxidative stress elicited by high glutamate concentrations can lead to toxicity, which promote the pathogenesis of retinal diseases like diabetic retinopathy and glaucoma. This study investigated the anti-oxidant activity and mechanisms of betulinic acid (BA) and its derivatives in human Müller cells. Human MIO-M1 Müller cells were pre-treated in the presence or absence of BA, BE as well as their derivatives (named H3-H20) followed by incubation with glutamate. Cell viability was evaluated with the MTT and calcein-AM assays. Reactive oxygen species (ROS) production in MIO-M1 cells was measured using CM-H₂DCFDA and flow cytometry. The activation of cellular apoptosis and necrosis was analyzed with annexin V/PI staining and flow cytometry. The modulation of signaling pathways involved in glutamate-mediated cytotoxicity and ROS production was evaluated by immunoblotting. The BA derivatives H3, H5 and H7 exhibited minimal cytotoxicity and significant anti-oxidant activity. These compounds significantly suppressed ROS production and attenuated cellular necrosis elicited by glutamate-induced oxidative stress. The protective effects of H3, H5 and H7 in MIO-M1 cells were associated with the attenuation of Akt, Erk, and JNK signaling. The BA analogues H3, H5 and H7 are protective against glutamate-induced oxidative stress in human Müller cells, and elicit their actions by modulation of the Erk, Akt and JNK signaling pathways. These agents are potential candidate molecules for the prevention or treatment of human retinal diseases.

Interplay between IDO1 and iNOS in human retinal pigment epithelial cells

Human retinal pigment epithelial (hRPE) cells form a selectively permeable monolayer between the neural retina and the highly permeable choroidal vessels. Thus, hRPE cells bear important regulatory functions and are potential targets of pathogens in vivo. Endogenous bacterial endophthalmitis (EBE) is frequently caused by infections with the Gram-positive bacterium Staphylococcus aureus (S. aureus). Upon microbial infection, interferon gamma (IFN-γ), a major cytokine of the adaptive immune response, induces a broad spectrum of effector molecules, such as the tryptophan-degrading enzyme indoleamine 2,3-dioxygenase-1 (IDO1). We stimulated human RPE (hRPE) cells in vitro with proinflammatory cytokines and analyzed the expression levels and enzymatic activities of IDO1 and inducible nitric oxide synthase (iNOS), another antimicrobial effector molecule. The antimicrobial capacity was analyzed in infection experiments using S. aureus and Toxoplasma gondii (T. gondii). Our aim was to characterize the particular importance of IDO1 and iNOS during EBE. We found that an IFN-γ stimulation of hPRE cells induced the expression of IDO1, which inhibited the growth of T. gondii and S. aureus. A co-stimulation with IFN-γ, interleukin-1 beta, and tumor necrosis factor alpha induced a strong expression of iNOS. The iNOS-derived nitric oxide production was dependent on cell-culture conditions; however, it could not cause antimicrobial effects. iNOS did not act synergistically with IDO1. Instead, iNOS activity inhibited IDO1-mediated tryptophan degradation and bacteriostasis. This effect was reversible by the addition of the iNOS inhibitor N^G-monomethyl-l-arginine. In conclusion, iNOS mediates anti-inflammatory effects in hRPE cells stimulated with high amounts of IFN-γ together with tumor necrosis factor alpha and Interleukin-1 beta and prevents potential IDO1-dependent tissue damage.

A derivative of betulinic acid protects human Retinal Pigment Epithelial (RPE) cells from cobalt chloride-induced acute hypoxic stress

The Retinal Pigment Epithelium (RPE) is a monolayer of cells located above the choroid. It mediates human visual cycle and nourishes photoreceptors. Hypoxia-induced oxidative stress to RPE is a vital cause of retinal degeneration such as the Age-related Macular Degeneration. Most of these retinal diseases are irreversible with no efficient treatment, therefore protecting RPE cells from hypoxia stress is an important way to prevent or slow down the progression of retinal degeneration. Betulinic acid (BA) and betulin (BE) are pentacyclic triterpenoids with anti-oxidative property, but little is known about their effect on RPE cells. We investigated the protective effect of BA, BE and their derivatives against cobalt chloride-induced hypoxia stress in RPE cells. Human ARPE-19 cells were exposed to BA, BE and their eighteen derivatives (named as H3H20) that we customized through replacing moieties at C3 and C28 positions. We found that cobalt chloride reduced cell viability, increased Reactive Oxygen Species (ROS) production as well as induced apoptosis and necrosis in ARPE-19 cells. Interestingly, the pretreatment of 3-O-acetyl-glycyl- 28-O-glycyl-betulinic acid effectively protected cells from acute hypoxia stress induced by cobalt chloride. Our immunoblotting results suggested that this derivative attenuated the cobalt chloride-induced activation of Akt, Erk and JNK pathways. All findings were further validated in human primary RPE cells. In summary, this BA derivate has protective effect against the acute hypoxic stress in human RPE cells and may be developed into a candidate agent effective in the prevention of prevalent retinal diseases.

The double stranded RNA analog poly-IC elicits both robust IFN-λ production and oncolytic activity in human gastrointestinal cancer cells

Purpose

Type III IFN (IFN-λ) is the dominant frontline response over type I IFN in human normal intestinal epithelial cells upon viral infection, this response being mimicked by the dsRNA analog poly-IC. Poly-IC also induces cell death in murine intestinal crypts ex vivo. Here we examined whether these innate defense functions of normal intestinal epithelial cells are recapitulated in gastrointestinal carcinoma cells so that they could be harnessed to exert both immunoadjuvant and oncolytic functions, an unknown issue yet.

Experimental design

Four human gastrointestinal carcinoma cell lines versus the Jurkat lymphoma cell line were used to assess the effects of intracellular poly-IC on i) IFN-λ secretion and cell proliferation and ii) role of NFκB signaling using the NFκB inhibitory peptide SN50 as a screening probe and a siRNA approach.

Results

Poly-IC induced in all cell lines except Jurkat both a robust IFN-λ secretion and a cytoreductive effect on adherent cells, restricted to proliferating cells and associated with cellular shedding and reduced clonogenicity of the shed cells. Collectively these findings demonstrate the oncolytic activity of poly-IC. Inhibiting NFκB in T84 cells using a siRNA approach decreased IFN-λ production without protecting the cells from the poly-IC oncolytic effects. In line with these findings IFN-λ, that upregulated the anti-viral protein MxA, was unable per se to alter T84 cell proliferation.

Conclusion

Our demonstration that poly-IC-induced concomitant recapitulation of two innate functions of normal intestine, i.e. IFN-λ production and cell death, by human gastrointestinal cancer cells opens new perspectives in gastrointestinal cancer treatment.

Paeoniflorin attenuates atRAL-induced oxidative stress, mitochondrial dysfunction and endoplasmic reticulum stress in retinal pigment epithelial cells via triggering Ca2+/CaMKII-dependent activation of AMPK

Abnormal accumulation of the free-form all-trans-retinal (atRAL), a major intermediate of human visual cycle, is considered to be a key cause of retinal pigment epithelial (RPE) dysfunction in the pathogenesis of retinal degenerative diseases such as age-related macular degeneration (AMD). Paeoniflorin (PF), a monoterpene glucoside isolated from Paeonia lactiflora Pall., has been used in clinical treatment of retinal degenerative diseases in China for several years; however, the underlying mechanism remains unclear. The aim of this study is to investigate the protective effect of PF against atRAL toxicity in human ARPE-19 cells and its molecular mechanism. The results of our study showed that the pre-treatment of PF dose-dependently attenuated atRAL-induced cell injury by the reduction of Nox1/ROS-associated oxidative stress, mitochondrial dysfunction and GRP78-PERK-eIF2α-ATF4-CHOP-regulated endoplasmic reticulum (ER) stress in ARPE-19 cells. Additionally, our data showed that PF mainly exerted its activity via triggering calcium-calmodulin dependent protein kinase II (CaMKII)-mediated activation of AMP-activated protein kinase (AMPK). AMPK inhibition significantly reversed the protective effect of PF against atRAL toxicity in ARPE-19 cells. Overall, our findings provided the novel mechanism of PF protecting human RPE cells, which may prevent the progression of retinal degenerative diseases.

PGC-1α Protects RPE Cells of the Aging Retina against Oxidative Stress-Induced Degeneration through the Regulation of Senescence and Mitochondrial Quality Control. The Significance for AMD Pathogenesis

PGC-1α (peroxisome proliferator-activated receptor gamma coactivator 1-alpha) is a transcriptional coactivator of many genes involved in energy management and mitochondrial biogenesis. PGC-1α expression is associated with cellular senescence, organismal aging, and many age-related diseases, including AMD (age-related macular degeneration), an important global issue concerning vision loss. We and others have developed a model of AMD pathogenesis, in which stress-induced senescence of retinal pigment epithelium (RPE) cells leads to AMD-related pathological changes. PGC-1α can decrease oxidative stress, a key factor of AMD pathogenesis related to senescence, through upregulation of antioxidant enzymes and DNA damage response. PGC-1α is an important regulator of VEGF (vascular endothelial growth factor), which is targeted in the therapy of wet AMD, the most devastating form of AMD. Dysfunction of mitochondria induces cellular senescence associated with AMD pathogenesis. PGC-1α can improve mitochondrial biogenesis and negatively regulate senescence, although this function of PGC-1α in AMD needs further studies. Post-translational modifications of PGC-1α by AMPK (AMP kinase) and SIRT1 (sirtuin 1) are crucial for its activation and important in AMD pathogenesis.

Taurine Attenuates Calpain-2 Induction and a Series of Cell Damage via Suppression of NOX-Derived ROS in ARPE-19 Cells

Nicotinamide adenine dinucleotide phosphate (NADPH) oxidases (NOXs) are key transmembrane proteins leading to reactive oxygen species (ROS) overproduction. However, the detailed roles of NOXs in retinal pigment epithelial (RPE) cell metabolic stress induced by Earle's balanced salt solution (EBSS) through starvation remain unclear. In this study, we investigated what roles NOXs play in regard to calpain activity, endoplasmic stress (ER), autophagy, and apoptosis during metabolic stress in ARPE-19 cells. We first found that EBSS induced an increase in NOX2, NOX4, p22phox, and NOX5 compared to NOX1. Secondly, suppression of NOXs resulted in reduced ER stress and autophagy, decreased ROS generation, and alleviated cell apoptosis. Thirdly, silencing of NOX4, NOX5, and p22phox resulted in reduced levels of cell damage. However, silencing of NOX1 was unaffected. Finally, taurine critically mediated NOXs in response to EBSS stress. In conclusion, this study demonstrated for the first time that NOX oxidases are the upstream regulators of calpain-2, ER stress, autophagy, and apoptosis. Furthermore, the protective effect of taurine is mediated by the reduction of NOX-derived ROS, leading to sequential suppression of calpain induction, ER stress, autophagy, and apoptosis.

Ultrastructural alterations in the retinal pigment epithelium and photoreceptors of a Stargardt patient and three Stargardt mouse models: indication for the central role of RPE melanin in oxidative stress

Background

Stargardt disease (SD) is characterized by the accumulation of the age-pigment lipofuscin in the retinal pigment epithelium (RPE) and subsequent neuroretinal degeneration. The disease leads to vision loss early in life. Here, we investigate age-dependent ultrastructural changes in three SD mouse models: albino Abca4^-/- and pigmented Abca4^-/- and Abca4^-/-.Rdh8^-/- mice. Since we found indications for oxidative stress primarily in albino SD mice, we tested RPE melanin for its antioxidative capabilities.

Methods

SD mouse eyes were investigated by light, fluorescence and electron microscopy and were compared to the respective albino and pigmented wild type mice and to a human donor SD eye. To confirm the role of RPE melanin in scavenging oxidative stress, melanin from S. officinalis as a standard and porcine RPE were tested for their capability to quench superoxide anions.

Results

Histological alterations indicative of oxidative stress and/or lysosomal dysfunction were present in albino Abca4^-/- and Abca4^-/-.Rdh8^-/- mice. Retinal damage, such as inner segment rupture and pyknotic or free photoreceptor nuclei in the subretinal space and RPE vacuolization were exclusively found in albino Abca4^-/- mice. Shortened and disorganized photoreceptor outer segments and dead RPE cells were found in albino Abca4^-/- and Abca4^-/-.Rdh8^-/- mice, with earlier onset in albino Abca4^-/- mice. Undegraded phagosomes and lipofuscin accumulation were present in the RPE of all three SD strains, but numbers were highest in Abca4^-/-.Rdh8^-/- mice. Lipofuscin morphology differed between SD strains: (melano-)lipofuscin granules in pigmented Abca4^-/- mice had a homogenous electron density and sharp demarcations, while lipofuscin in albino Abca4^-/- mice had a flocculent electron density and often lacked a surrounding membrane, indicating loss of lysosomal integrity. Young Abca4^-/-.Rdh8^-/- mice showed (melano-)lipofuscin granules with homogenous electron density, while in aged animals granules with flocculent electron density predominated. Both strains of pigmented SD mice had melanolipofuscin clusters as found in the human SD eye. Like melanin from S. officinalis, porcine RPE melanin can also quench superoxide anions.

Discussion

The presented pathologies in albino Abca4^-/- and Abca4^-/-.Rdh8^-/- mice suggest oxidative stress and/or lysosomal dysfunction within the RPE. Since albino Abca4^-/- mice have the earliest onset and severest damage and as absence of melanin and also melanin turnover with age are known to diminish RPEs anti-oxidative properties, we assume that RPE melanin plays a role in SD related damages. A lack of pathology in pigmented Abca4^-/- mice due to lower stress levels as compared to the Abca4^-/-.Rdh8^-/- mice underlines this hypothesis. It is also supported by the finding that RPE melanin can quench superoxide anions. We therefore suppose that RPE melanin is important in retinal health and we discuss its role as an oxidative stress scavenger.

Neuroprotective effect of tetramethylpyrazine against all-trans-retinal toxicity in the differentiated Y-79 cells via upregulation of IRBP expression

It is estimated that abnormal accumulation of all-trans-retinal (atRAL) is a leading cause of photoreceptor degeneration in retinal degenerative diseases. Deficiency of interphotoreceptor retinoid-binding protein (IRBP), a retinoid transporter in the visual cycle, is responsible for the impaired clearance of atRAL and results in atRAL toxicity in retina. Therefore, IRBP has been proposed to be a potent target in preventing atRAL-induced photoreceptor degeneration. In this study, the neuroprotective effect of tetramethylpyrazine (TMP) against atRAL toxicity in the differentiated Y-79 cells, a in vitro model of photoreceptor, was first investigated. Our findings showed that atRAL could induce cytotoxicity, oxidative/nitrosative stresses, apoptosis and leukostasis in the differentiated Y-79 cells; however, the pre-treatment of TMP significantly attenuated such effects in a dose-dependent manner. Furthermore, our results indicated that TMP exerted its neuroprotective effect mainly through upregulating IRBP expression. The present study significantly contributes to better understanding the important role of IRBP in retinal degenerative diseases and forms the basis of the therapeutic development of TMP in such diseases in the future.

Sirt1 Inhibits Oxidative Stress in Vascular Endothelial Cells

The vascular endothelium is a layer of cells lining the inner surface of vessels, serving as a barrier that mediates microenvironment homeostasis. Deterioration of either the structure or function of endothelial cells (ECs) results in a variety of cardiovascular diseases. Previous studies have shown that reactive oxygen species (ROS) is a key factor that contributes to the impairment of ECs and the subsequent endothelial dysfunction. The longevity regulator Sirt1 is a NAD⁺-dependent deacetylase that has a potential antioxidative stress activity in vascular ECs. The mechanisms underlying the protective effects involve Sirt1/FOXOs, Sirt1/NF-κB, Sirt1/NOX, Sirt1/SOD, and Sirt1/eNOs pathways. In this review, we summarize the most recent reports in this field to recapitulate the potent mechanisms involving the protective role of Sirt1 in oxidative stress and to highlight the beneficial effects of Sirt1 on cardiovascular functions.