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Solanum nigrum polyphenol extracts inhibit hepatic inflammation, oxidative stress, and lipogenesis in high-fat-diet-treated mice Jia-Jen Chang, Dai-Jung Chung, Yi-Ju Li, Bo-Han Wen, and Chau-Jong Wang J. Agric. Food Chem., Just Accepted Manuscript • DOI: 10.1021/acs.jafc.7b03578 • Publication Date (Web): 05 Oct 2017 Downloaded from http://pubs.acs.org on October 6, 2017

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Journal of Agricultural and Food Chemistry is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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Solanum nigrum polyphenol extracts inhibit hepatic inflammation, oxidative

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stress, and lipogenesis in high-fat-diet-treated mice

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Ja-Jen Chang†, Dai-Jung Chung†#, Yi-Ju Lee‡, Bo-Han Wen†, Hsing-Yu Jao†,

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Chau-Jong Wang†§*

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University, Number 110, Section 1, Jianguo North Road, Taichung 402, Taiwan.

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Jianguo North Road, Taichung 402, Taiwan.

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§

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Institute of Biochemistry, Microbiology and Immunology, Chung-Shan Medical

Institute of Medicine, Chung-Shan Medical University, Number 110, Section 1,

Department of Medical Research, Chung-Shan Medical University Hospital, Number

110, Section 1, Jianguo North Road Taichung 402, Taiwan

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#Coauthor (These authors contributed equally to this work thus sharing the first

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authorship)

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*Correspondence:

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Microbiology and Immunology, Chung-Shan Medical University, Number 110,

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Section 1, Jianguo North Road, Taichung 402, Taiwan.

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E-mail: [email protected]

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Tel: +866-4-24730022 ext 11670.

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Fax: +866-4-23248167.

Professor

Chau-Jong

Wang,

Institute

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Biochemistry,

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Abstract

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Patients with diabetes, obesity, and hyperlipidemia are all high-risk groups for

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fatty liver; however, the mechanism of fatty liver formation is not completely

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understood. Studies have indicated that abnormal fat metabolism, oxidative stress, and

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insulin resistance are positively correlated with peroxidation and abnormal cytokine

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production. Recent studies have revealed that Solanum nigrum extracts (SNE) possess

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anti-inflammatory, antioxidation, antihyperlipidemia, and liver protection abilities.

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Therefore, the present study investigated the in vivo and in vitro effects of an SNE on

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nonalcoholic fatty liver (NAFL)-induced hepatitis. In vivo data demonstrated that the

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SNE reduced blood triglyceride, sugar, and cholesterol levels, as well as fat

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accumulation, oxidative stress, and lipid peroxidation in high-fat-diet-treated mice.

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The results indicated that the SNE downregulated the expression of fatty acid

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synthase, 3-hydroxy-3-methylglutaryl-coenzyme A reductase (HMG-CoA reductase),

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and sterol regulatory element-binding proteins (SREBPs) through the AMP-activated

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protein kinase (AMPK) pathway and upregulated the expression of carnitine

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palmitoyltransferase 1 (CPT1) and peroxisome proliferator-activated receptor alpha.

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Furthermore, we prepared an Solanum nigrum polyphenol extract (SNPE) from the

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SNE; the SNPE reduced hepatic lipid (oleic acid) accumulation. Therefore, SNE have

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the potential to alleviate NAFL-induced hepatitis, and polyphenolic compounds are

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the main components of SNE. Moreover, SNE can be used to develop health food

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products for preventing NAFL disease.

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Keywords:

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Solanum nigrum polyphenol extract, nonalcoholic fatty liver, anti-inflammatory,

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antioxidation, antihyperlipidemia

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Introduction

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Overweight and obesity are associated with several chronic diseases including

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fatty liver, metabolic syndrome, and cardiovascular disease1. Fatty liver disease is

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related to increased abdominal or visceral fat accumulation; high triglyceride (TG),

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low high-density lipoprotein (HDL) cholesterol, and high low-density lipoprotein

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(LDL) cholesterol levels; and insulin resistance. All of these lead to impaired

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metabolic regulation in adipose tissue2-3.

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Many complicated biochemical reactions occur in the liver, including the

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metabolic activities of multiple nutrients such as TG oxidation and the synthesis of

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lipoproteins, cholesterol, and phospholipids. Abnormal lipid and lipoprotein

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metabolism can result in dyslipidemia, including elevated plasma cholesterol and/or

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TG levels and reduced HDL levels, which contribute to the increased prevalence of

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morbid obesity. Several proteins associated with dyslipidemia have been reported to

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play crucial roles in lipogenesis, including fatty acid synthase (FAS)4-5, sterol

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regulatory element-binding proteins (SREBPs), and 3-hydroxy-3-methylglutaryl

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-coenzyme A reductase (HMG-CoA reductase)6-8, and the expression levels of these

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enzymes are dynamic in response to cellular energy status. Acetyl-CoA carboxylase

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(ACC) catalyzes the carboxylation of acetyl-CoA to produce malonyl-CoA in a

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biotin-dependent manner9. Malonyl-CoA is a substrate of FAS for fatty acid synthesis

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and also a potent inhibitor of carnitine palmitoyltransferase 1 (CPT1) for fatty acid

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β-oxidation. FAS is mainly involved in catalyzing the transformation of malonyl-CoA

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into palmitate during the long-term process of long-chain fatty acid catalysis, which

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catalyzes the synthesis of palmitate from acetyl-CoA and malonyl-CoA. FAS

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regulated by multiple transcription factors. SREBPs, a transcription factor family,

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regulate the expression of enzymes for the synthesis of fatty acids, cholesterol, TG,

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and phospholipids, suggesting that this family plays a central role in energy

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homeostasis through the modulation of glycolysis, lipogenesis, and adipogenesis. In

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humans, the SREBP gene family mainly consists of SREBP-1 and SREBP-2,

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specifically responding to lipogenic and cholesterogenic metabolism, respectively10-14.

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In addition, this increase in lipid synthesis is caused by the increased activities of

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HMG-CoA reductase, a key enzyme of cholesterol synthesis. HMG-CoA reductase is

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a membrane protein located on the endoplasmic reticulum and plays a role in

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cholesterol synthesis and mevalonate production from HMG-CoA15. In mammals,

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hepatic lipolysis is mainly controlled by several enzymes, including peroxisome

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proliferator-activated receptors (PPARs), CPT1, and AMP-activated protein kinase

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(AMPK), in response to nutritional and hormonal conditions. PPARs are members of

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the steroid hormone nuclear receptor superfamily, and they have three subtypes:

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PPARα, PPARβ (PPARδ), and PPARγ; their respective tissues and regulators play

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different roles. PPARα is highly expressed in the liver and promotes lipid uptake, fatty

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acid β-oxidation, ketogenesis, and gluconeogenesis by regulating the expression of

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genes involved in fatty acid catabolism. PPARα is thus a potential target for

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alleviating lipid disorders, diabetes, and obesity16. CPT1 is localized in the

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mitochondrial outer membrane and synthesizes acylcarnitine for fatty acid transport

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across the inner mitochondrial membrane. AMPK is widely present in mammalian

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tissues. It is a serine/threonine kinase comprising a heterotrimeric complex and is

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regarded as an energy sensor in most tissues. AMPK regulates cellular lipid

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metabolism through increased fatty acid oxidation and constitutive lipoprotein

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exocytosis. In general, AMPK is a very important factor in intracellular energy

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regulation, regulating metabolism and energy in the body17-18.

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Solanum nigrum is traditionally used as food and has some pharmacological

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effects such as fever reduction, antifever diuretics, liver protection, eyesight

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improvement, blood pressure reduction, and glucose tolerance reduction19-20. Solanum

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nigrum extract (SNE) contain high amounts of polyphenols exhibiting antioxidant and

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antitumor properties. Many herbally derived polyphenolic compounds can prevent

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obesity by inhibiting hypolipidemia and reducing adipose tissue mass, thus

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suppressing the occurrence of metabolic, hepatic, and cardiovascular alterations

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associated with obesity21. Sohrabipour et al demonstrated that administration of

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Solanum nigrum reduced the blood glucose, cholesterol, and triglycerides to normal

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level in streptozotocin-induced diabetic rats22.

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On the basis of previous reports that an SNE has anti-atherogenic properties and

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hepatoprotectivee effects, the present study was conducted to examine whether an

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SNE can reduce hepatic lipid accumulation induced by a high-fat diet (HFD). In

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addition, the hepatic regulation of lipid synthesis, oxidation, inflammation, and

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clearance was investigated.

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Experimental method

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Preparation of SNE and Solanum nigrum polyphenol extract

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In brief, Solanum nigrum was collected from the central part of Taiwan. Whole plants

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were cut into small pieces, vacuum freeze-dried, and stored at room temperature until

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further use. Dried Solanum nigrum was mixed with water for 30 min and subjected to

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continuous heat extraction at 100°C for 40 min. The resulting water extract was

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filtered and subsequently concentrated in a water bath at 90°C until it became creamy

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and was dried in an oven at 70°C. For the preparation of the Solanum nigrum

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polyphenol extract (SNPE), we mixed 100 g of dried Solanum nigrum powders in 300

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mL of ethanol, followed by heating at 50°C for 3 h. The extracts were filtered,

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followed by lyophilization under reduced pressure at room temperature. The powders

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were resuspended in 500 mL distilled water, extracted with 180 mL of ethyl acetate

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three times, resolved in 250 mL of distilled water, stored at -70°C overnight, and

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lyophilized. Furthermore, the identities of the 18 components were established from

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recorded mass spectra. The powders were resuspended in distilled water and passed

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through a 0.22-μm filter for use in cell culture.

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HPLC-UV analysis

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The SNE was separated through HPLC-UV; this process was using a Waters system

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(the in-line 4-channel Degasser AF, Waters 600 HPLC Controller and pump, 2998

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PDA detector). Separating column was by a Mightysil RP-18 GP (5 μm particle size,

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250 x 4.6 mm I.D) at room temperature. The mobile phase contained of formic acid in

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water (solution A, pH = 2.5), acetonitrile (solution B) and methanol (solution C).

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HPLC method was flowing with gradient elution program: 0 min, 100% A; 5 min,

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90% A, 0% B; 15 min, 85% A, 3% B; 20 min, 85% A, 5% B; 30 min, 85% A, 8% B;

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50 min, 85% A, 15% B; 65 min, 75% A, 25% B; 70 min, 70% A, 30% B; 80 min,

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50% A, 50% B; 85 min, 100% B; 100 min, 100% B. Flow rate was 1.0 mL/min and

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injection volume was 20 μL. The UV detection range was on 210 ~ 400 nm. Methanol

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and acetonitrile (liquid chromatography grade, 99.9% and 99+ %, respectively) were

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purchased from Merck Millipore (Germany). Formic acid (reagent grade, 96%) was

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purchased from Tedia (USA). The standers of various phenolic compounds included

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peltatoside, isoquercitrin and miquelianin (liquid chromatography grade, 99%, 95%

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and 98%, respectively). These were obtained from Extrasynthese (French).

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Animal model design

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Male C57BL/6 mice (25–30 g; age, 42 days) were purchased from Lasco Laboratory

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Animal Co. (Yilian County, Taiwan), housed in an environmentally controlled

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laboratory, and acclimatized for 7 days. All mice housed under standard laboratory

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conditions (18–23C, humidity 55–60%, and 12 h light/dark cycle) in agreement with

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animal protocols approved by the Institutional Animal Care and Use Committee of the

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Chung Shan Medical University (IACUC approval No. 740), Taichung, Taiwan. We

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used an HFD to induce nonalcoholic fatty liver (NAFL) in the animal model. The feed

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was pulverized to prepare high-lipid feed by using lard and cholesterol in the

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preparation of high-lipid formulations. In the HFD group, the contents of lard and

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cholesterol were 20% and 2%, respectively. Lard and cholesterol were mixed

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uniformly after the addition of water. The formulation for the high-lipid feed was

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based on a reference. The animals were divided into five dietary groups (n =

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12/group) : (A) control diet group, (B) HFD group, and(C, D, E) diet containing HFD

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group (0.5% SNE ;1% SNE, and 2% SNE group). After the completion of the

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experiment, the mice were killed, and liver and blood samples were collected for

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further examination. Plasma was separated by centrifugation at 5000 g for 10 min at

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4°C and was then maintained at −20°C for additional analyses (Commission Permit

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No. 1235).

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Histological examination of the liver

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After the mice were killed, the liver was immediately fixed in 10% buffered

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formaldehyde for 1 day and then maintained in a decalcification solution. After 3 days,

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the specimens were reimmersed in formaldehyde, and hematoxylin and eosin (H&E)

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staining was performed for histological examination. The liver section was observed

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under high-power fields (× 200).

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Immunohistochemical staining

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A formalin-fixed paraffin-embedded liver section was analyzed to determine the

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expression of inflammation markers in the liver. The bone section was incubated in 10

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mmol/L citrate-Tween buffer at 100°C and assessed using the Thermo-HRP system.

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Finally, the results were developed by reacting the liver section with 3,3ʹ-

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diaminobenzidine (DakoCytomation, Carpinteria, CA, USA) and subjecting it to H&E

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staining and subsequent microscopic examination. Furthermore, the section was

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conjugated with a mouse monoclonal antibody against interleukin (IL)-6 (1:200;

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Abcam Co, Cambridge Science Park, Cambridge, UK) and tumor necrosis

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factor-alpha (TNF-α; 1:200). The intensity of IL-6- and TNF-α-positive cells in a

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cross section was observed by examining five randomly selected areas of the section

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under high-power fields (×200). Image Pro Plus 4.0 was used to quantify the

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integrated optical density within the positive areas.

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Cell Line and Cell Culture

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Human hepatoma (HepG2) cells were obtained from the American Type Culture

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Collection and grown in Dulbecco’s modified Eagle’s medium supplemented with

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10% fetal bovine serum, 100 μg/mL penicillin, 100 μg/mL streptomycin, and 2 mM

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L-glutamine (HyClone R, Thermo Scientific, Logan, UT, USA). The cells were

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cultured at 37°C in a humidified atmosphere of 95% air to 5% CO2.

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Cytotoxicity assay

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HepG2 cells were seeded at a density of 1 × 10 5 cells/well into a 24-well plate and

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treated with oleic acid (OA) and the SNPE at various concentrations for 24 h. The

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MTT reagent (0.5 mg/mL) was added to each well and incubated for 4 h. The medium

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was removed, and isopropanol was added to dissolve the MTT–formazan complex.

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The absorbance was measured at 563 nm with a spectrophotometer (Hatachi 3210,

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Hitachi, Tokyo, Japan).

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Oil red staining

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HepG2 cells were seeded in a six-well plate (3 × 106 cells/well) and treated with 500

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mM OA and indicated concentrations of the SNPE for 24 h. The cells were washed

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two times with phosphate-buffered saline (PBS), fixed with 4% formaldehyde in PBS

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for 1 h, and then stained with 1 μg/mL oil red for 30 min at room temperature. After

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the staining process, the lipid distribution in cells was immediately analyzed using a

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FACScan flow cytometer (Becton Dickinson, Mountain View, CA, USA). In addition,

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lipid-bound Nile red fluorescence was detected through inverted fluorescence

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microscopy.

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Western blot analysis

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HepG2 cells were seeded in a 10-cm dish (3 × 106 cells/well) and treated with 500

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mM OA at indicated concentrations of the SNPE for 24 h. The proteins were

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harvested from the cells in a cold radioimmunoprecipitation assay buffer (1% NP-40,

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50 mM Tris-base, 0.1% sodium dodecyl sulfate [SDS], 0.5% deoxycholic acid, 150

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mM NaCl, pH 7.5) containing leupeptin (1.7 μg/mL) and sodium orthovanadate (10

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μg/mL). The cell mixture was vortexed at 4°C for 4 h. All mixtures were then

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centrifuged at 12000 rpm and 4°C for 10 min, and the protein contents of the

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supernatants were determined with the Coomassie Brilliant Blue total protein reagent

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(Kenlor Industries, Costa Mesa, CA, USA) by using bovine serum albumin (BSA) as

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the standard. Equal amounts of protein samples were subjected to SDS–

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polyacrylamide gel electrophoresis and electrotransferred to nitrocellulose membranes

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(Millipore, Bedford, MA, USA). The membranes were blocked with 5% nonfat milk

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powder with 0.1% Tween-20 in tris-buffered saline and then incubated with the first

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antibody at 4°C overnight. Thereafter, the membranes were washed three times with

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0.1% Tween-20 in PBS and incubated with the secondary antibody to antimouse

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horseradish peroxidase (GE Healthcare, Little Chalfont, Buckinghamshire, UK).

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Antibodies against AMPK and phospho-AMPK (AMPK-P) were purchased from Cell

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Signaling Technology (Beverly, MA, USA). FAS, SREBP-1c, and CPT1 antibodies

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were obtained from Santa Cruz Biotechnology (Santa Cruz, CA, USA). The antibody

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against

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(Charlottesville, VA, USA). Band detection was revealed through enhanced

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chemiluminescence (ECL) by using ECL Western blotting detection reagents and

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exposed ECL hyper film in FUJIFILM LAS-3000 (Tokyo, Japan). The proteins were

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quantified through densitometry by using FUJIFILM-Multi Gauge V2.2 software

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(FUJIFILM, Stockholm, Sweden).

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Statistical analysis

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Data are reported as the mean ± standard deviation of three independent experiments

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and were evaluated using the Student t test. A P value of