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Bioactive Constituents, Metabolites, and Functions

Nutshell extracts of Xanthoceras sorbifolia: a new potential source of bioactive phenolic compounds as natural antioxidant and immunomodulator Li Zhao, Xing Li, Fei Zhang, Juan-Juan Han, Ting Yang, Ze-Qing Ye, Zhe-Zhi Wang, and Yuan Zhang J. Agric. Food Chem., Just Accepted Manuscript • DOI: 10.1021/acs.jafc.7b05590 • Publication Date (Web): 03 Apr 2018 Downloaded from http://pubs.acs.org on April 3, 2018

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Journal of Agricultural and Food Chemistry

Nutshell extracts of Xanthoceras sorbifolia: a new potential source of bioactive phenolic compounds as natural antioxidant and immunomodulator

Li Zhao†, Xing Li†, Ze-Qing Ye, Fei Zhang, Juan-Juan Han, Ting Yang, Zhe-Zhi Wang*, and Yuan Zhang*

1

National Engineering Laboratory for Resource Development of Endangered Crude Drugs in Northwest China, Key Laboratory of the Ministry of Education for

Medicinal Resources and Natural Pharmaceutical Chemistry, College of Life Sciences, Shaanxi Normal University, Xi’an 710119, P. R. China

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ABSTRACT

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Nutshell of Xanthoceras sorbifolia, a waste product in the production of edible oil, is

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rich in health-promoting phenolic acids. However, the individual constituents,

4

bioactivities, and mechanism of action are largely unknown. In this study, 20 phenolic

5

compounds were characterized in nutshell extracts (NE) of X. sorbifolia by GC-MS.

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Four established in vitro studies showed that NE has significant antioxidant potential.

7

Results in vivo indicated that oral administration of NE effectively ameliorated

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clinical disease severity of experimental autoimmune encephalomyelitis (EAE) and

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reduced the neuroinflammation and the central nervous system (CNS) demyelination.

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The underlying mechanism of NE-induced effects involved decreased penetration of

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pathogenic immunocyte into the CNS, a reduced production of proinflammatory

12

cytokines and factors, and suppressed differentiation of Type 1 T helper (Th1) and

13

Th17 cells through the JAK/STAT pathway. Taken together, our studies showed that

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X. sorbifolia nutshell, considered a waste material in the food industries, is a novel

15

source of natural antioxidants and immunomodulator.

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KEYWORDS:

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Xanthoceras sorbifolia, nutshell extracts, phenolic constituents, antioxidant activities,

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experimental autoimmune encephalomyelitis, JAK/STAT pathway

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INTRODUCTION

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Multiple sclerosis (MS) is known to be a neuroinflammatory demyelinating disease

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(1). Experimental autoimmune encephalomyelitis (EAE), also called Experimental

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allergic encephalomyelitis, is a well-accepted animal model to study MS (1, 2).

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Demyelination and inflammation of the CNS in MS/EAE impair physical and

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cognitive abilities, eventually resulting in death (3). Till now the pathogenesis of MS

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has not been fully elucidated, however, it is generally accepted that, in MS and EAE,

27

auto-reactive CD4+ T cells crossed the broken blood-brain-barriers (BBB), induced

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abnormal inflammatory reactions, and resulted in the tissue damage of the CNS (4).

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Among these CD4 positive T cells, Th17 cells that secreting IL-17 and GM-CSF are

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considered to be one of the most pro-inflammatory and encephalitogenic subsets

31

resulting in autoimmune and inflammatory diseases (4, 5). In addition, in both MS and

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EAE models, there is a close correlation between oxidative stress and progression of

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neuroinflammation (6-11). Accumulating evidence strongly supports the idea that the

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CNS damage was induced by the over-reactive oxygen species (ROS) in MS (6, 12).

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Therefore, therapeutic strategy that focuses on improving antioxidant potential and

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modulation of the immune response would provide a new option for the prevention

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and treatment of this chronic disease.

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Diets accumulated plentiful phenolic compounds have shown a variety of

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bioactive properties including antioxidant, neuroprotective, and anti-inflammatory

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effects (13). Accordingly, dietary intake of naturally occurring phenolic compounds

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with specific antioxidant and immunomodulation activity could not only enhance the 3

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nutraceutical value and health benefits of the food, but also protect against chronic

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diseases like MS. It would therefore be of great interest to the modern-day food

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industry in its search for novel and economical sources rich in such bioactive

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compounds, which can be used either as nutraceuticals or in functional foods to fight

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against and prevent these diseases.

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Xanthoceras sorbifolia, an unusual tree species belonging to the family of

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Sapindaceae, is distributed widely throughout the north of China and has a lifespan of

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more than 200 years (14). This species is an economically and pharmaceutically

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important energy crop with more than 50% oil in its seeds, which not only serve as

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nutritious nuts but are also used to produce edible oil and biofuels (14). Currently,

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interest is mainly focused on producing oil from the seeds, a process that generates a

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large amount of residue including nutshells. Chemical studies of X. sorbifolia fruits

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and seeds have shown that they contain a variety of compounds, including flavonoids,

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triterpenoids, and sterols (15, 16). Pharmacological research has shown that these

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components have neuroprotective, anti-HIV, anti-oxidant, anti-inflammatory,

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anti-tumour, and anti-Alzheimer effects (17-19), indicating the potential of X.

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sorbifolia as a treatment for autoimmune diseases. Given that by-products represent

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approximately 50% of total biomass of the seeds, and that they also contain bioactive

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constituents with diverse pharmacologic properties, these wastes are a valuable source

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of natural compounds that have potential as candidate drugs with novel mechanisms

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of action in the treatment of autoimmune diseases.

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The main goal of our work was to provide complete scientific information on X. 4

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Sorbifolia seeds and to prevent wasting this resource after the extraction process in

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the oil industry. As part of our ongoing search for antioxidant and immunomodulation

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agents derived from medicinal and food plants(3, 5), our study provides for the first

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time information on the phenolic profile, in vitro antioxidant activities, and in vivo

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anti-inflammatory activities of the residue of X. Sorbifolia seeds, which, after

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extraction of the oil, could serve as an alternative new source of bioactive components

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in the food and pharmaceutical industries.

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MATERIALS AND METHODS

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Reagents. Chemicals are supplied from MilliporeSigma (St. Saint Louis, MO,

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USA), unless stated otherwise. Reagents of Silylation [TMCS (trimethylchlorosilane)

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and BSTFA (Ν,Ο-bis(trimethylsilyl)trifluoroacetamide)] were manufactured by

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Merck KGaA (Darmstadt, Germany). Standards [Benzoic acid, Isoeugenol,

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tran-Cinnamic acid, p-Hydroxybenzoic acid, Gentisic acid, Vanillic acid, Gallic acid,

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p-Coumaric acid, o-Hydroxycinnamic acid, o-Phthalic acid, Ferulic acid, Cinnamic

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acid, Caffeic acid, Sinapic acid, Quercetin, (+)-Catechin, Hydroxytyrosol,

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p-Hydroxyphenylacetic acid, and (-)-Epicatechin] were ordered from MilliporeSigma.

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The standard stock solution were prepared in methanol and kept at -18 °C in dark.

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Test extracts were dissolved in 0.9% saline and prepared immediately before

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administration to the animals.

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Plant material. Mature seeds of Xanthoceras sorbifolia Bunge (Figure 1A) were

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collected from Xi’an Botanical Garden of Shannxi Province at the harvest stage (Sep.,

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2007). The plant material was authenticated by Dr. Yi Ren at the National 5

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Engineering Laboratory for Resource Development of Endangered Crude Drugs in

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Northwest China, Xi’an, China. The voucher specimen No. XS070918 was deposited

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thereby.

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Nutshell extract (NE) preparation and derivatization. Air-dried nutshells of X.

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sorbifolia were ground into a fine powder in a mechanical grinder and a mesh of

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2mm-diameter were used. One gram powder was defatted with petroleum ether, and

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extracted with 70% aqueous methanol with BHT (40 ml, 1.0 g/l). Then, 6 M HCL (10

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ml) was added carefully, kept at 35 °C for 16 h, and stirring frequently. The cooling

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supernatant was collected, filtered, and then extracted with 10 ml ethyl acetate for 3

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times. This extract was evaporated to dryness on a rotovap. The resulted dried

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nutshell extracts (NE) were kept at 4 °C in dark for future investigation.

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For component analysis, 100 µg of the dried NE was dissolved in pyridine (1ml).

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Then, a mixture of 100 µl of TMCS and 200 µl of BSTFA were added to the screw

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cap glass tubes containg10 µl of the pyridine solution. The mixture solution were

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incubated at 80 °C for 45 min for silylation. Because the hypersensitivity of

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trimethylsilyl (TMS) derivatives to moisture, all the procedure mentioned above

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should be kept under the anhydrous conditions. The silylated mixture was directly

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analyzed

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chromatography-flame ionization detection (GC-FID).

by

gas

chromatography-mass

spectrometry

(GC-MS)

and

gas

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Determination of total phenolics and phenolic compounds of NE. Colorimetric

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Folin-Ciocalteu method was used to determinate the total phenolic contents (20).

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Qualitative and semi-quantification analysis of silylated phenolic compounds in NE 6

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were tested by using the GC-FID (Agilent 6890N) and GC-MS system (SHIMADZU

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QP2010) according to our previous study (21). In the splitless mode, 2 µl of silylated

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mixture was injected into the column. The oven temperature was programmed as

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follows: (1) 80 °C for 1 minute; (2) 80 °C to 120 °C, at a rate of 5 °C per minute; (3)

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120 °C to 240 °C, at a rate of 10 °C per minute; (4) 240 °C to 280 °C, at a rate of

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20 °C/minute; (5) 280 °C final hold for 5 minute.

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The percentage of the silylated phenolic compounds was analyzed using the

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normalization method from the areas of FID (semi-quantification). GC-MS was

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performed with the same operating conditions as GC-FID analysis. Components were

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identified by comparing the retention times (RT) with those of standards, as well as

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matching their recorded mass spectra with the libraries of

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Standards and Technology (NIST05.LIB and NIST05s.LIB) .

National Institute of

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In vitro determination of antioxidant activity. Antioxidant activity of NE of X.

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sorbifolia was evaluated using four antioxidant test systems (DPPH radical

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scavenging activity, •O2- scavenging activities, β-carotene bleaching assay, and ferric

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ion reducing power) as has been described in our previous study (21).

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In vivo assessment of the effects of NE on EAE mice. EAE was induced as

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described in our previous studies (3, 4). EAE mice were randomly divided into two

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treatment groups: PBS-treated control group and NE-treated group. Different dose of

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NE (50, 100, and 150 mg/kg/d) was oral gavaged daily starting from the first day of

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post-immunization (disease prevention), 10 days post-immunization (onset), as well

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as 15 days post-immunization (peak). Dose optimization study was performed and the 7

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suitable dosage was used for the further experiment (Figure 3A). Histopathological

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analysis, mononuclear cell (MNC) preparation, ELISA, flow cytometry, western blot

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and quantitative real-time PCR analysis were preformed according to our previous

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studies (3, 4).

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Statistical analysis. All data are presented as mean ± standard deviation (SD). For

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the in vitro data, EC50 values were calculated by regression analysis. Data analyses

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were performed with GraphPad Prism 6 (GraphPad, La Jolla, CA). , data were

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analyzed by Analysis of variance (ANOVA) with Tukey’s multiple comparisons test

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were used when comparing multiple groups. The value of p 0.05 was considered

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statistically significant.

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RESULTS AND DISCUSSION

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Total phenolic and individual phenolic compound analysis of NE. According to

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our extraction procedure, NE of X. sorbifolia gave brick-red powder in yields of 9.16

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± 0.31 % (w/w) on the basis of the dry weight of crush nutshell (Figure 1B). The odor

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was very mild. The amount of total phenolic of NE was 69.52 ± 2.66 mg GAE/g

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extract. We next investigated the compound composition and quantitation of NE using

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GC-MS after silylation. The total ion chromatogram (TIC) for this analysis is shown

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in Figure 1C. Retention time (RT) and mass spectra of the major phenolic compounds

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are listed in Table 1. A total of 20 constituents were determined by GC-FID and

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GC-MS, separately, representing 84.62 % of NE (Table 1). Major phenolic

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compounds in NE are caffeic acid (20.05%), p-hydroxyphenylacetic acid (9.32%), 8

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p-coumaric acid (8.84%), and o-hydroxycinnamic acid (8.29%). So far as could be

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ascertained from our literature survey, data on the phenolic composition of X.

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sorbifolia are lacking. Only one paper (16) characterized 23 bioactive phenols from

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the leaves of X. sorbifolia as the potential neuroinflammation inhibitors. Our data fill

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the gap in the literature deficiency and describe for the first time the phenolic profile

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of X. sorbifolia seed extracts. Prior reports indicated that phenolic content and

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antioxidant potential have a positive correlation in the plant crude extracts (21).

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Therefore, the high contents of total phenolics in NE indicated its strong antioxidant

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properties, which could be exploited as phenolic products in the food and

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pharmaceutical field in the future.

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In vitro antioxidant activity of NE. Given that ROS play a critical role in the MS

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pathology, several studies have focused on antioxidant therapies that are beneficial in

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animal models for MS (6, 12). Of the phenolic compounds identified in NE of X.

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sorbifolia, caffeic acid, coumaric acid, and their derivatives have been previously

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shown to exhibit obviously antioxidant activities (22). Because of the synergistic

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effect of various compounds, crude extracts usually have a more powerful antioxidant

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potential and a greater number of biological activities than the individual

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substances(23, 24),. Therefore, crude extracts are currently used for protection against

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spoilage and oxidation in both the pharmacological and food industry (25). In this

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study, both the lipophilic and hydrophilic antioxidant experiments were applied to

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fully display the antioxidant effects of NE (Figure 2, Table 2), and NE showed 9

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significant antioxidant potential in the four assays.

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As shown in Figure 2A, NE and the positive standards (VC, VE and BHT)

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demonstrated a strong ability to scavenge DPPH radicals in a dose-dependent manner.

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With regard to scavenging capacity, IC50 of NE (6.22 ± 0.16 µg/mL) was similar to

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VC (6.12 ± 0.21 µg/mL) and BHT (6.82 ± 0.46 µg/mL) (p = 0.066), which was

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2.1-times greater than VE (12.51 ± 0.65 µg/mL) (Table 2).

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The scavenging effect of NE toward •O2- was dose related (Figure 2B). At low

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concentrations (0.02-0.05 mg/mL), VC exhibited a better scavenging rate than NE.

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However, the maximum •O2- scavenging rate of VC and NE reached nearly 100% and

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was comparable (p > 0.05), suggesting a similar scavenging capacity. The IC50 of

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•O2- s scavenging activity of NE was found to be 46.33 ± 3.50 µg/mL, whereas that of

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VC was 38.69 ± 4.22 µg/mL (Table 2).

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As shown in Figure 2C, NE demonstrated significantly linoleate-derived free

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radicals scavenging effect and decreased the β-carotene bleaching. In the control

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group, the absorption value at 470 nm was reduced to a minimal value of 0.153±

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0.011 after 120 min, while NE was still stayed at 0.401± 0.029. These results indicate

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that NE significantly inhibited oxidation of linoleic acid. The antioxidant activities of

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the test samples in this system were decreased as follows: BHT > VE > NE (Table 2).

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In a reducing power assay, all samples shown their antioxidant activates in a

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dose-dependent manner (Figure 2D). According to the results shown in Table 2, the

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reducing power of NE was as strong as that of BHT (p > 0.05), a widely used

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commercial antioxidant, indicated that NE have good electron donating capacities, 10

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although it was still slightly less effective than VC.

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Our study demonstrated that, for the first time, the phenolic composition as well

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as the antioxidant capacities of NE, the crude extracts of the defatted residue of X.

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Sorbifolia seeds. And the results indicated that the investigated extracts could be

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utilizing as the potential phenolic antioxidants that should be specifically studied for

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their effects on human health.

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Oral NE effectively enhanced clinical recovery from MOG-induced EAE.

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Given that the important role of oxidative damage mediated CNS lesion, antioxidant

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therapy represents an attractive treatment for neuroinflammation and/or inflammatory

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autoimmune diseases (6, 12). To test the immunoregulation activity of NE for

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treatment of neuroinflammatory disorders, we used EAE mouse model that

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recapitulates human MS.

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To determine the effects of NE administration on disease prevention or relapse,

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oral treatment was started from day -5 post immunization (p.i.). Compared to

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PBS-treated control mice, the clinical score records reflected a significantly delay of

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disease onset and obviously inhibition of disease incidence (P < 0.01; Figure 3A). In

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Figure 3A, 100% of mice in the PBS-treated group developed EAE around day 11 p.i.,

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while only 39.6% of mice treated with NE (50 mg/kg/d) showed delayed disease onset

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(~day 14 p.i.) and mild clinical symptoms. Furthermore, the loss of body weight of

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EAE mice were also prevented by oral treatment of NE, in consistent with the

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decreased clinical signs (Figure 3B). For dose optimization, 100 mg/kg/d were chosen 11

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and used for the following experiments.

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To further test whether oral administration of NE has beneficial effects on

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ongoing EAE, in the therapeutic regimen, mice were fed with NE daily at the day 10

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p.i. (onset)or at the day 15 p.i. (peak) of clinical EAE. Generally, severe clinical

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symptoms were observed in the PBS-treated group, such as limp tail, wadding gait,

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and/or paralysis of limbs. From Figure 3C we can see that the maximum clinical score

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at ~ 18 days after EAE induction was 3.1 ± 0.56, while NE-treated mice showed 2.1 ±

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0.52 of maximum EAE score accompanied by a reduced accumulative score.

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Furthermore, NE treatment at the peak (day 16 p.i.) of clinical EAE also effectively

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reduced disease severity and suppressed EAE progression (Figure 3D). Taken

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together, these data indicated that NE has a significant therapeutic effect in EAE.

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As we known, the safety, effective, and ideal selection in clinical treatment of

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MS should prevent disease aggravation induced by neuroinflammation at the

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induction and effector stages, halt the neurodegeneration and the related disability

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mediated by axonal injury, as well as prevent disease relapse already underway. In

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this regard, oral treatment of NE presents an important therapeutic agent, as it

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effectively inhibited disease progression at the onset (induction phase), peak (effector

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phase), and stable (chronic phase) of clinical EAE, and represents a promising

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alternative to current therapies for MS treatment.

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Oral NE reduced neuroinflammation and demyelination in EAE. As is well

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known, EAE is an ideal experimental model that recapitulates several features of 12

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human MS (26). MS/EAE is induced by peripherally over-activated myelin-reactive

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CD4+ T cells, as well as other immune cells, migrating into the CNS, and then

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activating resident CNS immune cells (astrocytes and microglia cells) to produce

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plentiful pro-inflammatory cytokines and chemokines. The immunological attacks in

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the CNS of MS/EAE focus on the myelin sheaths, leading to the death of

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oligodendrocytes, myelin loss, axons functionally compromised, and/or CNS lesion

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(27). At the day of 30 post-immunization, to evaluate the therapeutic ability of NE on

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the pathology of CNS, NE-treated and control mice were sacrificed. Lumbar spinal

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cords were isolated for histological analysis. As shown in Figure 4A&B, NE-treated

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mice showed significantly decreased inflammation (P= 0.006) and reduced

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demyelination (P= 0.035) compared with the control group,. The total number of

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MNCs infiltrating in the CNS also had a significantly reduced in the NE-treated group

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(P = 0.0069; Figure 4C). To clarifying the underlying mechanism of NE function, the

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expression level of cytokine and chemokine from the spinal cords of NE-treated and

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control mice were assayed using Cytokines & Chemokines PCR Array (QIAGEN Inc).

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As shown in Figure 4D, NE treatment substantially reduced expression of several

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pro-inflammatory factors, including Il17a, Il17f, Il1a, Il1b, Il12b, Il21, Il23a, Il25

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(Il17e), Il27, Il6, Ifng, Csf2 (GM-CSF), Csf3 (G-CSF), Tgfb1, and Tnf, while it

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induced expression of some anti-inflammatory cytokines/neurotrophins such as Il10

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and Lif. Consistent with these findings, expression of most chemokines we tested,

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including Ccl1, Ccl2, Ccl20, Ccl7, Cx3cl1, Cxcl1, Cxcl10, Cxcl11, Cxcl12, and Cxcl5,

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was significantly decreased (Figure 4D). Among them, the most robustly inhibited 13

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was the IL-17 family (Figure 4D), the essential cytokine produced by T helper cells

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(Th17 cells), which have strong pro-inflammation effects in the pathogenesis of EAE

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and MS (28).

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NE inhibited Th1 and Th17 cell subsets though the Janus kinase/STAT (Signal

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transducer and activator of transcription) signaling pathway. To study the effect

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of orally administered NE on the immune response, expression of surface markers

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and/or cytokines in mononuclear cells from the peripheral and CNS of different EAE

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groups were investigated by flow cytometry. The percentages and absolute numbers

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of CD4 and CD8 positive T cells, both in the spleen and CNS of NE treatment group,

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were decreased significantly (P < 0.05). Compared with PBS-treated control,

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percentages of MOG-reactive Th1 (CD4+IFN-γ+) and Th17 (CD4+IL-17+) cells were

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remarkably reduced by NE treatment in both peripheral and CNS (Figure 5A&B).

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Nevertheless, the portions of Th2 (CD4+IL4+) and Treg (CD4+Foxp3+) cells were not

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significantly changed (data not shown). When stimulated ex vivo, splenocytes of

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NE-treated EAE mice produced less MOG-induced IFN-γ, IL-17 and GM-CSF in the

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supernatant of splenic culture, while there was no effect on Th2/Treg cytokines IL-5

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and IL-10 (Figure 5C). These results indicated that NE ameliorates clinical symptom

280

by suppressing Th1 and Th17 cells development, and perhaps by inducing

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immunoregulatory cytokines production.

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Typically, an important limitation in the use of small molecule compounds for

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therapy is that their mechanism of action is not fully understood, a factor that adds to 14

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our misgivings about their use in clinical treatment. While our data suggest that NE

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suppresses development of EAE by inhibiting pro-inflammatory factor secretion by

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Th1 and Th17 cells (Figure 4D), how NE action in the differentiation of T cell subsets

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has not yet been clarified. As the JAK/STAT signaling plays an important role in this

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process (29), here we hypothesized that NE targeting JAK/STAT pathway to exert its

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regulatory effects on Th1/Th17 development. Therefore, protein samples were

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obtained from splenic CD4+ T cells purified from NE-treated or control EAE mice,

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and key signaling molecules of the Janus Kinase/STAT pathway were evaluated by

292

immunoblot. STAT1

293

related to the Th1/Th17 cells polarization (28, 30). As shown in Figure 5D, p-STAT1

294

and p-STAT3 were significantly decreased in NE-treated group. Conversely, the level

295

of STAT5 and STAT6, which are associated to Treg and Th2 cell differentiation (31,

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32), were not changed (data not shown). To further access whether the NE were

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functional directed toward Th1 or Th17 cell differentiation, CD4+ T cells were

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purified from NE- or PBS-treated EAE mice, then assayed by JAK/STAT signaling

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pathway array (QIAGEN Inc). Among all the genes significantly influenced by NE

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treatment, 5 were upregulated and 25 down-regulated significantly (p