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The Oxysterol 27-Hydroxycholesterol Increases B-Amyloid and Oxidative

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The Oxysterol 27-Hydroxycholesterol Increases B-Amyloid and Oxidative Dasari et al. BMC Ophthalmology 2010, 10:22 http://www.biomedcentral.com/1471-2415/10/22 RESEARCH ARTICLE Open Access The oxysterol 27-hydroxycholesterol increases b-amyloid and oxidative stress in retinal pigment epithelial cells Bhanu Dasari1, Jaya RP Prasanthi1, Gurdeep Marwarha1, Brij B Singh2, Othman Ghribi1* Abstract Background: Alzheimer’s disease (AD) and age-related macular degeneration (AMD) share several pathological features including b-amyloid (Ab) peptide accumulation, oxidative damage, and cell death. The causes of AD and AMD are not known but several studies suggest disturbances in cholesterol metabolism as a culprit of these diseases. We have recently shown that the cholesterol oxidation metabolite 27-hydroxycholesterol (27-OHC) causes AD-like pathology in human neuroblastoma SH-SY5Y cells and in organotypic hippocampal slices. However, the extent to which and the mechanisms by which 27-OHC may also cause pathological hallmarks related to AMD are ill-defined. In this study, the effects of 27-OHC on AMD-related pathology were determined in ARPE-19 cells. These cells have structural and functional properties relevant to retinal pigmented epithelial cells, a target in the course of AMD. Methods: ARPE-19 cells were treated with 0, 10 or 25 μM 27-OHC for 24 hours. Levels of Ab peptide, mitochondrial and endoplasmic reticulum (ER) stress markers, Ca2+ homeostasis, glutathione depletion, reactive oxygen species (ROS) generation, inflammation and cell death were assessed using ELISA, Western blot, immunocytochemistry, and specific assays. Results: 27-OHC dose-dependently increased Ab peptide production, increased levels of ER stress specific markers caspase 12 and gadd153 (also called CHOP), reduced mitochondrial membrane potential, triggered Ca2+ dyshomeostasis, increased levels of the nuclear factor B (NFB) and heme-oxygenase 1 (HO-1), two proteins activated by oxidative stress. Additionally, 27-OHC caused glutathione depletion, ROS generation, inflammation and apoptotic-mediated cell death. Conclusions: The cholesterol metabolite 27-OHC is toxic to RPE cells. The deleterious effects of this oxysterol ranged from Ab accumulation to oxidative cell damage. Our results suggest that high levels of 27-OHC may represent a common pathogenic factor for both AMD and AD. Background are composed of acute phase proteins, complement com- Age-related macular degeneration (AMD) is the most ponents, apolipoproteins, lipids, polysaccharides along common cause of irreversible vision loss in elderly popula- with various other molecules [3-5]. Intriguingly, AMD has tion [1]. This disease is characterized by a progressive cell many pathological features that are common to Alzhei- damage that targets the choroid, retinal pigment epithe- mer’s disease (AD), including the deposition of b-amyloid lium (RPE) and retina. Accumulation of drusen in the (Ab) peptide [6]. Ab is suggested to play a key role in AD extracellular compartment between the choroid and the pathogenesis by triggering oxidative stress, inflammation RPE is an early event in the course of AMD [2]. Drusen and cell death [7]. Ab accumulation has also been demon- strated to be associated with drusen in eyes from AMD * Correspondence: [email protected] patients [8-10], mice models for AMD [11] and in RPE 1Department of Pharmacology, Physiology and Therapeutics, University of cells [12]. Recent studies from our laboratory have shown North Dakota School of Medicine and Health Sciences, 501 North Columbia that the oxysterol 27-hydroxycholesterol (27-OHC) causes Road, Grand Forks, North Dakota, 58202, USA Full list of author information is available at the end of the article AD-like pathology by increasing Ab production and © 2010 Dasari et al; licensee BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Dasari et al. BMC Ophthalmology 2010, 10:22 Page 2 of 12 http://www.biomedcentral.com/1471-2415/10/22 triggers apoptotic cell death in human neuroblastoma SH- Methods SY5Y cells [13,14] and in organotypic slices from rabbit Cells and Treatments hippocampus [15,16]. However, the extent to which and ARPE-19 cells (American Type Culture Collection, the mechanisms by which 27-OHC may also cause Ab Manassas, VA) were grown in DMEM/F12 Glutamax accumulation and cell death in in vitro model that is rele- media with 10% FBS and standard antibiotics (100 IU/ vant to retinal pigment epithelial cells and AMD studies mL penicillin, and 100 μg/mL streptomycin, Sigma, are lacking. St. Louis, MO) in a 5% CO2, 37°C incubator. Cells were Similar to AD, the causes of AMD are not fully under- seeded in 75-cm2 flasks, 6-well plates or 96-well plates. stood. Several lines of evidence suggest that genetic pre- All cell culture reagents were obtained from Invitrogen, disposition and environmental as well as dietary factors Carlsbad, CA. 27-OHC was obtained from Medical Iso- may contribute to the pathogenesis of these two pro- topes Inc, Pelham, NH. Stock solutions of 27-OHC were gressive degenerative disorders. Recent epidemiological prepared in 100% ethanol and stored at -70°C. Working studies have demonstrated that high plasma cholesterol concentrations were prepared by dissolving 27-OHC levels are associated with a high risk for AD [17]. Like- stock solution in media. When cells reached confluency, wise, high intake of cholesterol and saturated fat is asso- they were incubated with 0, 10 or 25 μM27-OHCin ciated with increased AMD [18]. Cholesterol (free and culture media for 24 hrs, time by which 27-OHC esterified) is highly distributed in the human drusen increases Ab levels in SH-SY5Y cells [13]. Experiments [5,19,20]. The source of the cholesterol that accumulates were carried out in triplicate. The concentrations of the in the retina is suggested to derive from both local cells 27-OHCusedinthepresentstudyarethesameas and plasma origins [4,21-23]. Currently, the mechanisms thosewedemonstratedtocauseAD-likepathologyin by which cholesterol may increase the incidence of AD human neuroblastoma cells and in organotypic slices or AMD are not clear. Several lines of evidence suggest [13,15]. Joffre and colleagues have used concentrations that oxidized cholesterol metabolites (oxysterols) may be up to 50 μM of 7-ketocholesterol, 24-hydroxycholes- the link by which cholesterol contributes to the patho- terol, and 25-hydroxycholesterol [30]. genesis of AD [24]. The oxysterol pathway has also been proposed as a unifying hypothesis for the cause of AMD Quantification of Ab by Enzyme-Linked Immunosorbent [25-27]. Assay (ELISA) Oxysterols are oxidation products of cholesterol that Cells were treated with 27-OHC for 24 hrs. Spent media result from either autoxidation or enzymatic oxidation. was collected, protease and phosphatase inhibitors cock- While 7-ketocholesterol is the major oxysterol generated tail (Thermo Scientific, Rockford, IL) was added. The by autoxidation on the B hydrocarbon ring of choles- media was centrifuged at 16,000 × g for 5 min at 4°C. terol, 24-hydroxycholesterol, 25-hydroxycholesterol and 100 μL of supernatant was used for Ab1-42 and Ab1-40 27-hydroxycholesterol are major oxysterols produced by quantification by colorimetric sandwich ELISA (Cov- enzymatic oxidation on the lateral chain of the choles- ance, Denver, PA) according to the manufacturer’s terol structure. Oxysterols have diverse physiological instructions and as we have previously described [13]. and biochemical functions ranging from regulation of Ab1-42 and Ab1-40 levels were expressed in pg/mL. cholesterol homeostasis to regulation of nuclear recep- tors [26-28]. However, abnormal oxysterol levels can Western Blot Analysis cause oxidative stress, inflammation and apoptotic cell After 24 hrs treatment with 27-OHC, cells were har- death [25,28-32]. vested on ice and homogenized in a mammalian protein In this study, we determined in the human RPE cell extraction reagent (M-PER, Thermo Scientific, Rockford, line ARPE-19 the effects of 27-OHC on pathological IL) supplemented with protease and phosphatase inhibi- hallmarks that are common to both AMD and AD. We tors. Nuclear extracts for NFB were prepared by using have specifically determined levels of Ab and b-site of NE-PER extraction kit (Thermo Scientific, Rockford, IL). APP cleaving enzyme (BACE-1), which initiates Ab pro- Protein concentrations were determined with bicincho- duction, caspase 12 and gadd153 as markers of ER ninic acid (BCA) protein assay reagent. Proteins (10 μg) stress, mitochondrial membrane potential as a marker of were separated on SDS-PAGE gels followed by transfer mitochondrial stress, the nuclear factor B(NFB) and onto a polyvinylidene difluoride (PVDF) membrane heme-oxygenase 1 (HO-1) two proteins activated by oxi- (Biorad, Herculus, CA) and incubation with antibodies dative stress, ROS generation and glutathione depletion to BACE-1 (Mouse, 1:1000, Millipore, Bedford, MD), and TNF-a as marker of inflammation. Additionally, caspase 12 (Rat, 1:1000, Abcam, Cambridge, MA), calcium homeostasis, cytotoxicity and cell death assays gadd153 (Mouse, 1:1000, Abcam, Cambridge, MA), and were also carried out. HO-1(Mouse,1:500,AssayDesigns,AnnArbor,MI). Dasari et al. BMC Ophthalmology 2010, 10:22 Page 3 of 12 http://www.biomedcentral.com/1471-2415/10/22 b-actin (Mouse, 1:5000, Santa Cruz Biotechnology, Santa fluorescence was monitored at 510 nm with an Orka Cruz,CA)wasusedasagelloadingcontrol.5μgof imaging camera (Hamamatsu). The images of multiple nuclear extracts were used for NFB(NFBp65; cells collected at each excitation wavelength were pro- Mouse, 1:100, Santa Cruz Biotechnology, Santa Cruz, cessed using the Ca2+ imaging, PCI software (Compix CA) and lamin (Rabbit, 1:500, Cell Signaling Technol- Inc., Cranbery, PA) to provide ratios of Fura-2 fluores- ogy, Inc. Danvers, MA) was used as loading control. For cence from excitation at 340 nm to that of excitation at antibodies of mouse origin, goat anti-mouse secondary 380 nm (F340/F380).
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