In the presence of ferritin, visible light induces lipid peroxidation of the porcine photoreceptor outer segment

Iron Accumulation and Lipid Peroxidation in the Aging Retina: Implication of Ferroptosis in Age-Related Macular Degeneration

Iron is an essential component in many biological processes in the human body. It is critical for the visual phototransduction cascade in the retina. However, excess iron can be toxic. Iron accumulation and reduced efficiency of intracellular antioxidative defense systems predispose the aging retina to oxidative stress-induced cell death. Age-related macular degeneration (AMD) is characterized by retinal iron accumulation and lipid peroxidation. The mechanisms underlying AMD include oxidative stress-mediated death of retinal pigment epithelium (RPE) cells and subsequent death of retinal photoreceptors. Understanding the mechanism of the disruption of iron and redox homeostasis in the aging retina and AMD is crucial to decipher these mechanisms of cell death and AMD pathogenesis. The mechanisms of retinal cell death in AMD are an area of active investigation; previous studies have proposed several types of cell death as major mechanisms. Ferroptosis, a newly discovered programmed cell death pathway, has been associated with the pathogenesis of several neurodegenerative diseases. Ferroptosis is initiated by lipid peroxidation and is characterized by iron-dependent accumulation. In this review, we provide an overview of the mechanisms of iron accumulation and lipid peroxidation in the aging retina and AMD, with an emphasis on ferroptosis.

Cholesterol homeostasis in the vertebrate retina: biology and pathobiology

Cholesterol is a quantitatively and biologically significant constituent of all mammalian cell membrane, including those that comprise the retina. Retinal cholesterol homeostasis entails the interplay between de novo synthesis, uptake, intraretinal sterol transport, metabolism, and efflux. Defects in these complex processes are associated with several congenital and age-related disorders of the visual system. Herein, we provide an overview of the following topics: (a) cholesterol synthesis in the neural retina; (b) lipoprotein uptake and intraretinal sterol transport in the neural retina and the retinal pigment epithelium (RPE); (c) cholesterol efflux from the neural retina and the RPE; and (d) biology and pathobiology of defects in sterol synthesis and sterol oxidation in the neural retina and the RPE. We focus, in particular, on studies involving animal models of monogenic disorders pertinent to the above topics, as well as in vitro models using biochemical, metabolic, and omic approaches. We also identify current knowledge gaps and opportunities in the field that beg further research in this topic area.

Pretreatment with pyridoxamine mitigates isolevuglandin-associated retinal effects in mice exposed to bright light

The benefits of antioxidant therapy for treating age-related macular degeneration, a devastating retinal disease, are limited. Perhaps species other than reactive oxygen intermediates should be considered as therapeutic targets. These could be lipid peroxidation products, including isolevuglandins (isoLGs), prototypical and extraordinarily reactive γ-ketoaldehydes that avidly bind to proteins, phospholipids, and DNA and modulate the properties of these biomolecules. We found isoLG adducts in aged human retina but not in the retina of mice kept under dim lighting. Hence, to test whether scavenging of isoLGs could complement or supplant antioxidant therapy, we exposed mice to bright light and found that this insult leads to retinal isoLG-adduct formation. We then pretreated mice with pyridoxamine, a B6 vitamer and efficient scavenger of γ-ketoaldehydes, and found that the levels of retinal isoLG adducts are decreased, and morphological changes in photoreceptor mitochondria are not as pronounced as in untreated animals. Our study demonstrates that preventing the damage to biomolecules by lipid peroxidation products, a novel concept in vision research, is a viable strategy to combat oxidative stress in the retina.

Genome-wide microarrray analysis reveals roles for the REF-1 family member HLH-29 in ferritin synthesis and peroxide stress response

In Caenorhabditis elegans, the six proteins that make up the REF-1 family have been identified as functional homologs of the Hairy/Enhancer of Split (HES) proteins. These transcription factors act in both Notch dependent and Notch-independent pathways to regulate embryonic events during development; however, their post-embryonic functions are not well defined. As a first step toward understanding how the REF-1 family works together to coordinate post-embryonic events, we used gene expression microarray analysis to identify transcriptional targets of HLH-29 in L4/young adult stage animals. Here we show that HLH-29 targets are genes needed for the regulation of growth and lifespan, including genes required for oxidative stress response and fatty acid metabolism, and the ferritin genes, ftn-1 and ftn-2. We show that HLH-29 regulates ftn-1 expression via promoter sequences upstream of the iron-dependent element that is recognized by the hypoxia inducible factor, HIF-1. Additionally, hlh-29 mutants are more resistant to peroxide stress than wild-type animals and ftn-1(RNAi) animals, even in the presence of excess iron. Finally we show that HLH-29 acts parallel to DAF-16 but upstream of the microphthalmia transcription factor ortholog, HLH-30, to regulate ftn-1 expression under normal growth conditions.

Conversion of 7-ketocholesterol to oxysterol metabolites by recombinant CYP27A1 and retinal pigment epithelial cells

Of the different oxygenated cholesterol metabolites, 7-ketocholesterol (7KCh) is considered a noxious oxy-sterol implicated in the development of certain pathologies, including those found in the eye. Here we elucidated whether sterol 27-hydroxylase cytochrome P450 27A1 (CYP27A1) is involved in elimination of 7KCh from the posterior part of the eye: the neural retina and underlying retinal pigment epithelium (RPE). We first established that the affinities of purified recombinant CYP27A1 for 7KCh and its endogenous substrate cholesterol are similar, yet 7KCh is metabolized at a 4-fold higher rate than cholesterol in the reconstituted system in vitro. Lipid extracts from bovine neural retina and RPE were then analyzed by isotope dilution GC-MS for the presence of the 7KCh-derived oxysterols. Two metabolites, 3β,27-dihydroxy-5-cholesten-7-one (7KCh-27OH) and 3β-hydroxy-5-cholesten-7-one-26-oic acid (7KCh-27COOH), were detected in the RPE but not in the neural retina. 7KCh-27OH was also formed when RPE homogenates were supplemented with NADPH and the mitochondrial redox system. Quantifications in human RPE showed that CYP27A1 is indeed expressed in the RPE at 2-4-fold higher levels than in the neural retina. The data obtained represent evidence for the role of CYP27A1 in retinal metabolism of 7KCh and suggest that, in addition to cholesterol removal, the functions of this enzyme could also include elimination of toxic endogenous compounds.

Cholesterol oxidation in the retina: implications of 7KCh formation in chronic inflammation and age-related macular degeneration

This review will discuss the formation and potential implications of 7-ketocholesterol (7KCh) in the retina. 7KCh is a proinflammatory oxysterol known to be present in high amounts in oxidized LDL deposits associated with atheromatous plaques. 7KCh is generated in situ in these lipoprotein deposits where it can accumulate and reach very high concentrations. In normal primate retina, 7KCh has been found associated with lipoprotein deposits in the choriocapillaris, Bruch's membrane, and the retinal pigment epithelium (RPE). In photodamaged rats, 7KCh has been found in the neural retina in areas of high mitochondrial content, ganglion cells, photoreceptor inner segments and synapses, and the RPE. Intermediates found by LCMS indicate 7KCh is formed via a free radical-mediated mechanism catalyzed by iron. 7KCh seems to activate several kinase signaling pathways that work via nuclear factor κB and cause the induction of vascular endothelial growth factor, interleukin (IL)-6, and IL-8. There seems to be little evidence of 7KCh metabolism in the retina, although some form of efflux mechanism may be active. The chronic mode of formation and the potent inflammatory properties of 7KCh indicate it may be an "age-related" risk factor in aging diseases such as atherosclerosis, Alzheimer's, and age-related macular degeneration.

Photodamage generates 7-keto- and 7-hydroxycholesterol in the rat retina via a free radical-mediated mechanism

Albino Sprague-Dawley rats are known to undergo photoreceptor degeneration after exposure to constant light, but the molecular mechanism(s) by which the photoreceptors degenerate is not fully understood. We hypothesized that cytotoxic oxysterols are generated in situ in the retina under such conditions and may be involved in the degenerative mechanism. Thus, photodamaged and control rat retinas were analyzed for oxysterols by liquid chromatography mass spectroscopy. Elevated levels of two known cytotoxic oxysterols, 7-ketocholesterol (7KCh) and 7alphabeta-hydroxycholesterol (7HCh), were found in the photodamaged retinas, at levels six-fold and 50-fold greater, respectively, than those found in non photodamaged controls. Notably, two key intermediates, 5,6alpha,beta-epoxycholesterol (5,6-epoxyCh) and 7alphabeta-hydroperoxy-cholesterol, were also identified, indicating that the formation of 7KCh and 7HCh is mediated by a free radical mechanism. By immunohistochemistry, 7KCh was localized to the ganglion cell layer, photoreceptor inner segments and retinal pigment epithelium (RPE), which coincides with the localization of ferritin in the retina. Exposure of a mixture of ferritin and low-density lipoprotein to intense white light in vitro produced similar oxysterol species as seen in vivo. We propose that the increased levels of 7KCh and 7HCh, especially in photoreceptor inner segments and RPE, may arise due to ferritin-catalyzed reactions and may be important contributors to the photoreceptor degeneration observed in photodamaged rats.