Visible Light-Induced Lipid Peroxidation of Unsaturated Fatty Acids

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Visible Light-Induced Lipid Peroxidation of Unsaturated Fatty Acids in Retina and the Inhibitory Effects of Blueberry Polyphenols Yixiang Liu, Di Zhang, Jimei Hu, Guang-Ming Liu, Jun Chen, Lechang Sun, Zedong Jiang, Xichun Zhang, Qingchou Chen, and Baoping Ji J. Agric. Food Chem., Just Accepted Manuscript • DOI: 10.1021/acs.jafc.5b04341 • Publication Date (Web): 12 Oct 2015 Downloaded from http://pubs.acs.org on October 12, 2015

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Journal of Agricultural and Food Chemistry

Visible Light-Induced Lipid Peroxidation of Unsaturated Fatty Acids in Retina and the Inhibitory Effects of Blueberry Polyphenols Yixiang Liu,† Di Zhang,§ Jimei Hu,※ Guangming Liu,† Jun Chen,† Lechang Sun,† ※ Zedong Jiang,† Xichun Zhang,† Qingchou Chen,† and Baoping Ji*,※ †

College of Food and Biological Engineering, Jimei University, Xiamen, Fujian, PR

China. §

School of Food and Biological Engineering, Jiangsu University, Zhenjiang, Jiangsu,

PR China. ※

College of Food Science & Nutritional Engineering, China Agricultural University,

Beijing, PR China.

Running title: blueberry polyphenols protecting against visible light-induced retinal injury

*

Corresponding author: Email, [email protected]; Fax, +86-010-62347334.

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ABSTRACT: The lipid peroxidation of unsaturated fatty acids (UFAs) in retina not

2

only threatens visual cells but also affects the physiological health of retina. In this

3

work, the potential damages caused by daily visible light exposure on retinal UFAs

4

were evaluated via a simulated in vitro model. At the same time, benefits of dietary

5

supplementation of blueberries to the eyes were also assessed. After prolonged light

6

exposure, lipid peroxidation occurred for both docosahexaenoic and arachidonic acids

7

(DHA and AA, respectively). The oxidized UFAs presented obvious cytotoxicity and

8

significantly inhibited cell growth in retinal pigment epithelium cells. Among the

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different blueberry polyphenol fractions, the flavonoid-rich fraction, in which

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quercetin was discovered as the main component, was considerably better in

11

preventing visible light-induced DHA lipid peroxidation than the anthocyanin- and

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phenolic acid-rich fractions. Then the retinal protective activity of blueberry

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polyphenols against light-induced retinal injury was confirmed in vivo. Based on the

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above results, inhibiting lipid peroxidation of UFAs in retina is proposed to be another

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important function mechanism for antioxidants to nourish eyes.

16

KEYWORDS: Unsaturated fatty acids, retina, lipid peroxidation, blueberries,

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polyphenols

18 19 20 21 22 2

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INTRODUCTION

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Unsaturated fatty acids (UFAs), which are abundant in the outer photoreceptor

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segments of the retina, perform important functions in retinal structure and functions.

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In human retina, the major UFAs in the outer photoreceptor segments include

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docosahexaenoic acid (DHA, 22:6n–3), oleic acid (OA, 18:1) and arachidonic acid

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(AA, 20:4n–6), and the levels of the three UFAs reached about 50%, 10%, and 8% of

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the total fatty acids, respectively.1-3 Increasing evidences have shown that both DHA

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and AA affect photoreceptor membrane function though altering permeability, fluidity,

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thickness,

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oxidation-induced photoreceptor apoptosis and could promote rod and cone

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development. DHA may also be involved in phototransduction and rhodopsin

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regeneration.4-6

and

lipid

properties.4

Moreover,

DHA

alone

could

prevent

35

However, the negative effects of UFAs in vision health are beginning to attract

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researcher attention, because an increasing number of people are currently suffering

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from certain eye diseases. These eye diseases are due to overexposure to the light of

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video terminals, such as computers, widescreen mobile phones and televisions, and

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are attributed to the lipid peroxidation of UFAs in the retina induced by excessive

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light radiation.7-9 This kind of photo-induced retinal injury presents physiological

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inevitability. Besides being rich in UFAs, the retina is where light is focused.

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Moreover, the retinal area is always under high oxygen tension because of its visual

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imaging function and active metabolism.10 Numerous present studies have revealed

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that when lipid peroxidation of retinal UFAs occurs, lipid peroxidation products, such

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as malondialdehyde (MDA), hydroxynonenal (HNE), and hydroxyhexenal (HHE), in

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turn react with cellular macromolecules (DNA, proteins, and lipids) in the retina. This

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reaction consequently leads to retinal pigment epithelial dysfunction and

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photoreceptor cell damage.11-13 As a result, new approaches for protecting retina from

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light-induced injury should be the focus of research. Studies should concentrate on

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both preventing lipid peroxidation of UFAs in retina and rescuing retinal pigment

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epithelium (RPE) and photoreceptor cells that have been damaged.

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As a preventive measure, specific antioxidants in the diet are receiving an

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increasing amount of attention as potential agents for improving retinal defense

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against oxidative stress. The Age-Related Eye Disease Study recently confirmed that

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increasing the body’s defenses against oxidative stress with specific antioxidants, such

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as lutein, vitamin A, and lycopene, can preserve vision in patients with macular

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degeneration and reduce the rate of disease progression.14 At present, certain

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polyphenolic compounds that are present in high concentrations in fruits, vegetables

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and other plant-derived foods have been highlighted to present eye benefits. Specific

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dietary flavonoids, such as fisetin, luteolin, quercetin, eriodictyol, baicalein, galangin,

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curcumin, and epigallocatechin gallate, are reportedly capable of protecting retinal

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cells from light- and oxidant stress-induced cell death by blocking the accumulation

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of reactive oxygen species.15,16 In recent years, the unique physiological activity of

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these antioxidants, especially of anthocyanins, in protecting or improving one’s vision

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has been noticed both by nutritionists and food scientists. As for conclusive evidences

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that confirm the visual benefits of anthocyanins, one milestone involves the detection 4

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of anthocyanins in the eye organ after animals, such as rabbits, rats, and pigs, were

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fed with anthocyanin-rich forage.17,18 The possible routes of action may be associated

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with the accelerated resynthesis of rhodopsin,8,19,20 modulation of retinal enzymatic

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activity,19,21 protection of retinal cells by antioxidation,22,23 and improved

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microcirculation.19 In our previous studies, we discovered that polyphenol-rich

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blueberries were efficient in ameliorating light-induced retinal damage in vivo.24 One

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of the protective routes that were clarified is that blueberry anthocyanins exhibit

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protective activities against aging and light-induced damage in RPE cells.9 However,

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as seen in the abovementioned analysis, blocking the lipid peroxidation of UFAs in

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the retina may be another important pathway for blueberries to prevent photo-induced

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retinal damage.

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Therefore, this work aims to further verify whether blueberry polyphenols can

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preserve the outer segments of photoreceptors when the retina is under an

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environment of excessive illumination. Three main fatty acids (DHA, AA, and OA) in

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the outer photoreceptor segments were selected to simulate visible light-induced lipid

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peroxidation in the retina. Then, the differences in cytotoxicity of the three oxidized

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fatty acids on RPE cells were assessed. Different fractions of blueberry polyphenols

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were subsequently evaluated for their potential to defend against lipid peroxidation in

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the retina. Finally, the protective effect of blueberry polyphenols on visible

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light-induced retinal damage was checked in vivo.

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MATERIALS AND METHODS

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Plant Materials and Sample Preparation. Fresh wild blueberries (Vacciniun spp.),

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grown from the Greater Hinggan Mountains in Northeast China, were supplied by the 5

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Science and Technology Bureau of Greater Hinggan Mountains district. For the

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long-term preservation, these fresh berries were freeze-dried and stored at -20 °C until

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use.

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Chemicals and Reagents. Amberlite XAD-7 used for purifying blueberry

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polyphenols was obtained from Sigma (Sydney, Australia). The Sephadex LH20 and

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Oasis HLB cartridges used for isolating and purifying anthocyanins were purchased

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from Amersham Biosciences AB (Uppsala, Sweden) and Waters (Milford, MA),

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respectively. Deionized water was produced using a Milli-Q unit (Millipore, Bedford,

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MA). The acetonitrile from Mallinckrodt Baker (Phillipsburg, NJ) was of

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high-performance liquid chromatography (HPLC) grade.

The ethanol and

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hydrochloric acid were purchased from China National Pharmaceutical Industry

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Corporation Ltd. (Shanghai, China). Analytical reagent-grade solvents were used

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during extraction.

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DHA,

AA,

OA,

methylsulfonic

acid,

1-Methyl-2-phenylindole,

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2′,7′-dichlorofluorescin diacetate (DCFH-DA), 1,1,3,3-Tetramethoxypropan (TMOP)

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and 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT) were

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purchased from Sigma Chemicals Co. (St. Louis, MO). Standard flavonoids such as

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resveratrol, myricetin, quercetin, rutin, and quercitrin were obtained from Chengdu

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Mansite Co. (Sichuan, China) Fetal bovine serum and Dulbecco’s modified

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Eagle’s/Ham’s F12 media were provided by Invitrogen Co. (Carlsbad, CA). Ferric

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chloride was purchased from China National Pharmaceutical Industry Corporation

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Ltd. Penicillin and streptomycin were obtained from Gibco Life Technologies (Grand 6

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Island, NY). Bovine serum albumin (BSA) was purchased from EMD Biosciences

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(La Jolla, CA). The lactic dehydrogenase (LDH) kit was purchased from Beyotime

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Institute of Biotechnology (Jiangsu, China).

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Extraction and Fractionation. Polyphenol separation and purification from

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blueberries were performed as previously described with several modifications.

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Ten grams of dry blueberries were immersed by 140 mL absolute methanol for 1 h in

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a 250–mL round–bottomed flask, and then homogenized using a homogenizer

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(XHF–D, Ningbo Science & Biotechnology Co., Ningbo, Zhejiang, China). The

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homogenized sample was further centrifuged for 10 min at 4,000 ×g, and the

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supernate was filtered through a moderate speed 102 qualitative filter paper

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(Hangzhou Special Paper Industry Co. Ltd., Hangzhou, Zhejiang, China). The above

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procedure was repeated to re-extract the residue. The two filtrates were combined and

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evaporated using a rotary evaporator at 40 °C and a vacuum pressure of 0.1 MPa. Part

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of the concentrated solution was loaded onto an Amberlite XAD-7 column, and the

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remaining part was lyophilized (crude extract) for further analysis. After 1 h, the XAD

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was washed with about 800 mL 1% (v/v) formic acid aqueous solution to remove

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non-polyphenolic compounds, after which the polyphenolics were eluted with about

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600 mL absolute methanol with 1% (v/v) formic acid. The eluent was concentrated at

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40 °C and lyophilized in vacuum using a freeze dryer (Four–ring Science Instrument

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Plant Beijing Co., Ltd., Beijing, China). Forty-eight hours later, a friable dark red

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powder was obtained. A 50–mg lyophilized sample (polyphenol mixture) was

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resolubilized in pH 7 phosphate buffer and applied to a Sephadex LH20 column. The 7

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column was first washed with a pH 7 phosphate buffer to remove phenolic acids, and

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then with 70% (v/v) methanol acidified with 10% (v/v) formic acid to elute

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anthocyanins and flavonoids. The phenolic acids removed, also called the phenolic

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acid-rich fraction, were collected and lyophilized for later use. The anthocyanin and

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flavonoid fraction was freeze–dried, resolubilized in 5% (v/v) formic acid in water,

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and then applied to an Oasis HLB cartridge. The cartridge was washed with 5% (v/v)

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formic acid, followed by ethyl acetate, and then with 10% (v/v) formic acid in

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methanol. The ethyl acetate eluted flavonoids, and this fraction was called the

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flavonoid-rich fraction after drying in a vacuum at 40 °C using a DZF-6021 vacuum

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drying oven (Hangzhou Lihui Environmental Testing Equipment Co. Ltd., Hangzhou,

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China). The anthocyanins were eluted with acidified methanol, and the eluents were

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freeze-dried and called anthocyanin-rich fraction. These extracts or fractions were

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kept at -20 °C until use.

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Determination of Total Phenolics. The total phenolic content was determined using

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the Folin–Ciocalteu assay as described by Singleton and Rossi,27 with some

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modifications. Briefly, 1 mL of diluted samples (100 µg/mL) was mixed with 0.5 mL

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of 0.2 mol/L Folin–Ciocalteu reagent in 10 mL volumetric flasks. After 5 min, 1.5 mL

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of a 20% sodium carbonate solution was added. The volume was then increased to 10

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mL with distilled water and mixed thoroughly. After 30 min at room temperature, the

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absorbance was measured at 760 nm using a spectrophotometer (722S, Shanghai

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Precision & Science Instrument Co., Ltd., Shanghai, China), and the total phenolic

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content was calculated using the standard curve of gallic acid. Results were expressed 8

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as weight percentages, which were calculated as follows: gallic acid equivalent

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(GAE)/wet weight of extracts or fractions × 100.

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Determination of Total Anthocyanins. Total anthocyanin content was measured

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using a pH differential method described previously. 28,29 Two dilutions of the extracts

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were prepared, one for pH 1.0 using potassium chloride buffer and the other for pH

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4.5 using sodium acetate buffer. The samples were diluted to a final concentration of

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50 µg/mL. The absorbance was then measured at 515 nm with distilled water as a

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blank. The samples showed no haze or sediment; thus, correction at 700 nm was

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omitted. The total anthocyanin content was then determined using Lambert-Beer’s law,

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which was calculated as follows:

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Total anthocyanin content (mg/L) = (A × MW × DF × 103)/(ε × L)

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where A is the difference in the absorbance at 515 nm between pH 1.0 and 4.5, MW is

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449.2 [the molecular weight of cyanidin-3-glucoside (g/mol)], DF represents a

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dilution factor, and ε denotes the extinction coefficient of cyanidin-3-glucoside

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[26,900 L × mol-1 × cm-1, where L (path length) = 1 cm]. Results were expressed as

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weight percentages, which were calculated as follows: (cyanidin-3-glucoside

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equivalent/wet weight of extracts or fractions) × 100.

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HPLC Analysis. A Shimadzu LC-10 A Series high performance liquid

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chromatography (HPLC) system was used to analyze flavonoids in this experiment.

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The analytical column used was a 150 mm×4.6 mm i.d. Shimadzu Inertsil ODS-3 C18

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column (Shimadzu, Tokyo, Japan) maintained at 35 °C. The injection volume was 20

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µL, and elution solvent--(A) H2O with 1.0% acetic acid and (B) methyl cyanides with 9

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1.0% acetic acid were applied as follows: flow rate 1 mL/min, isocratic 15% B for 1

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min, from 15%–40% B over 30 min, from 40%–60% B over 5 min, from 60%–80% B

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over 2 min, isocratic 80% B for 3 min and from 80%–15% B over 5 min.

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Simulating Light-Induced Lipid Peroxidation of UFAs in Retina. UFAs was

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dissolved in ethanol before being added into serum-free F12 mediums and the final

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concentration of ethanol in the mediums was 0.1% (v:v). The UFAs-rich mediums

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were transferred into a 24-well plate with the volume of 2 mL every well and then the

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plate was covered with its lid. To simulate the in vivo environment, the plate was

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placed into a cell culture incubator at 37 °C in a humidified 5% CO2 atmosphere.

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Next, the mediums were subjected to white light (420–800 nm) irradiation by an

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integrated light-emitting diode (LED) lamp system designed by the authors.9 After the

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light illumination, the level of lipid peroxidation and cytotoxicity of the oxidized

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UFA-rich mediums were analyzed.

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To evaluate the potential protective activities of blueberry polyphenols against

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light-induced lipid peroxidation in retina, different fractions of blueberry polyphenols

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were added into the UFAs-rich mediums. The changes of both the lipid peroxidation

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free radicals and aldehydic products were observed until the light exposure ended.

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Measurement of Lipid Hydroperoxide (LOOH) level. Lipid peroxidation free

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radicals produced by UFAs were measured with a DCFH fluorescent probe.30 DCFH

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was prepared from DCFH–DA by basic hydrolysis according to the previous method.

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A 5 mg portion of DCFH–DA was dissolved in 10 mL absolute methanol as the stock

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solution, which was then stored under nitrogen at −20 °C and used within 2 months. A 10

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volume of 500 µL of DCFH–DA stock solution was mixed with 2 mL of 0.01 mol/L

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NaOH aqueous solution at 4 °C and then stored at 4 °C for 30 min in the dark. The

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reaction was neutralized with 2 mL of 0.01 mol/L HCl and diluted with pH 6.0

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phosphate-buffered saline (PBS). The final concentration of the working solution was

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5.0 µmol/L. After irradiation by visible light, cell culture media containing UFAs were

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transferred to a standard 96-well plate. In the plate, 150 µL of different culture

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medium per well was added, and 10 µL of the working solution was added as quickly

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as possible. To prevent air disturbance, we covered the plate with the lid and sealed

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the plate with adhesive tape. After incubation of the media for 1 h at 37 °C, the

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fluorescence from each well was recorded by a multifunctional microplate reader

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(Molecular Devices Co.) at 522 nm emission and 498 nm excitation.

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Measurement of Active Carbonyl Compounds. The 1-methyl-2-phenylindole

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colorimetric method was employed to assay the levels of active carbonyl compounds

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of UFA rich media after light radiation.31,32 As for omega-3-polyunsaturated fatty

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acids, 4-hydroxy-2-hexenal (4-HHE) is the major product in the peroxidative

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decomposition of DHA. AA is the omega-6-polyunsaturated fatty acids, and its major

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lipid peroxidation products are malondialdehyde (MDA) and 4-hydroxy-2-nonenal

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(4-HNE). In the methanesulfonic acid reaction system, 4-HHE, 4-HNE, and MDA can

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react with 1-methyl-2-phenylindole. The reaction molar ratio for the three aldehydic

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products is 1:2, and the reaction end-products present the same characteristic

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absorption spectrum at 586 nm. Therefore, MDA has been used as the standard

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material in calculating the contents of aldehydic products in DHA, AA, and OA 11

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systems. First, Reagent 1, Reagent 2, and MDA stock solution were prepared in

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advance.

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acetonitrile–methanol (v:v, 3:1) solution to a final concentration of 10 mmol/L.

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Reagent 2: Ferric chloride was dissolved in 37% methanesulfonic acid to a final

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concentration of 60 µmol/L. 10 mmol/L MDA stock solution: 1 mmol TMOP was

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dissolved in 100 mL of 1% (v:v) sulfuric acid aqueous solution, and the mixture was

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reacted for 2 h. To construct the MDA standard curve, the MDA stock solution was

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diluted to 0.5, 1.0, 5.0, 10.0, 20, 30, 50.0 µmol/L with serum-free F12 medium, and

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then 200 µL of the media were mixed with 650 µL of Reagent 1 and 150 µL of

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Reagent 2. The mixture was incubated for 40 min at 37 °C and detected at 586 nm.

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Based on the different absorbance values in different MDA concentrations, the MDA

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standard curve was constructed. For sample analysis, the MDA standard solution was

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replaced with oxidized UFA rich medium, and the level of aldehydic products was

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calculated according to the MDA standard curve.

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Cell Culture. A human RPE cell line, ARPE-19 (ATCC CRL–2302) (American Type

237

Culture Collection, Manassas, Virginia, USA), was used in the present study and

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cultured as previously described.9 Cell cultures were grown in Dulbecco’s modified

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Eagle’s/Ham’s F12 media (Invitrogen) supplemented with 10% fetal bovine serum

240

(Sigma–Aldrich), containing 1% antibiotic mixture of penicillin (100 U/mL) and

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streptomycin (100 mg/mL) (Invitrogen) at 37

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atmosphere.

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UFAs

Reagent

Cytotoxicity.

1:

1-methyl-2-phenylindole

Cell

viability

o

was

dissolved

in

a

C under a humidified 5% CO2

was

measured

12

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[3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide] (Sigma–Aldrich)

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assay as previously described.9 RPE cells were seeded in 96-well plates

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(Corning–Costar, Corning, NY, USA) at a concentration of 5 × 105 cells/mL, and then

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allowed to attach after 48 h. The medium was then replaced with serum–free F12

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medium containing 0.01, 0.025, 0.05, 0.075, 0.1, and 0.5 mmol/L UFAs, respectively.

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Considering the hydrophobic nature of UFAs, they were firstly dissolved with slight

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ethanol before being added into the medium. Twenty–four hours later, the cell

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supernate was removed for lactate dehydrogenase (LDH) assay. About 200 µL of 0.5

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mg/mL MTT serum–free F12 medium was added into each well of plates, after which

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incubation was performed for 4 h. After removal of the MTT solution, 150 µL of

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dimethyl sulfoxide (Sigma–Aldrich) was added, and the absorbance was measured at

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570 nm using a plate reader (Molecular Devices Co., CA, USA). Results were

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expressed as the percentage of viable cells with respect to untreated control cells. Cell

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viability (%) was calculated as follows: [(mean absorbance of the sample – reference

258

absorbance)/mean absorbance of the control] ×100. The cellular release of LDH

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following UFAs exposure was used as a measure of cellular damage/integrity.

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Enzymatic activity was determined using an LDH kit (Beyotime Institute of

261

Biotechnology) according to the manufacturer’s instructions.

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Cell Viability Assay. After subjecting to light irradiation, the cell supernatants of RPE

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cells with about 80% confluence were replaced by the oxidized UFAs-rich mediums.

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RPE cells were continuously incubated in this medium for 24 h under the same

265

normal condition described above and cell viability was finally analyzed using an 13

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MTT assay.

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Verification of the Retinal Benefits of Blueberry Polyphenols in vivo. The

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protective effects of blueberry polyphenols on light-induced retinal damage were

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further estimated in a rabbit model. Animal administration and light exposure were

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adopted according to our earlier study.24 All animals were handled according to the

271

Association for Research in Vision and Ophthalmology (ARVO) statement for Use of

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Animals in Ophthalmic and Vision Research. After 1 week adaptation period, the

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rabbits were randomly divided into the following four groups (n = 5 per group):

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control (no light exposure and vehicle administration), Group 1 (light exposure and

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vehicle administration), Group 2 (light exposure and administration of whole

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blueberries, 4.8 g/kg/day), and Group 3 (light exposure and administration of

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blueberry polyphenols, 31.2 mg/kg/day, amounting to 4.8 g of whole blueberries).

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Whole blueberries and blueberry polyphenols were fed to animals by intragastric

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administration for 4 consecutive weeks prior to light exposure. After light exposure,

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morphological changes in retinas were observed.

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Histological Analysis. After isolation of eyes and removal of cornea and lens, the

282

remaining ocular tissue was immersed for 2 h in 4% paraformaldehyde, followed by

283

cryoprotection in 30% sucrose in PBS (pH 7.4) at 4 °C. The eyes were then embedded

284

in paraffin and sliced into 12 µm-thick sections. Then the hematoxylin-eosin staining

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was employed and the retinal sections were analyzed under a light microscope

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(Chongqing Optical and Electrical Instrument Co. Ltd., Chongqing, China).

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Statistical Analysis. The statistical significance of the differences between the control 14

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and treatment groups was analyzed by one-way ANOVA using Origin version 8.0,

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followed by Tukey tests. A normality test showed that all the raw data had a normal

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distribution, and all groups were determined to have equal variance by a variance test.

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Data were expressed as the means ± SD of at least three individual experiments, each

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run in triplicate. p