Research ArticleOleuropein attenuates hepatic steatosis induced by high-fat diet in mice
Introduction
Non-alcoholic fatty liver disease (NAFLD) can be considered as a spectrum of liver pathologies, with, on the one hand, simple steatosis with absence of necrosis or signs of inflammation and, on the other hand, severe signs of inflammation with fibrosis or cirrhosis [1]. An excessive and inappropriate dietary-fat intake, combined with peripheral insulin resistance, continued triglyceride (TG) hydrolysis via lipoprotein lipase, and other genetic alterations in the key lipid metabolic pathways, results in increased blood free-fatty-acid (FFA) concentration [2], leading to increased TG concentration in the liver. The in vivo activation of liver X receptor (LXR), sterol-regulatory-element-binding protein 1c (SREBP1c), and peroxisome-proliferator-activated receptors gamma (PPARγ) affects the lipid accumulation in the liver induced by a high-fat diet (HFD) [3], [4]. LXR has been shown to activate SREBP1c [5], which stimulates the key lipogenic genes, including those encoding the acetyl-CoA carboxylase and the fatty-acid synthase [6], [7]. In animal models with fatty liver, PPARγ is transcriptionally upregulated and consequently activates the lipogenic target genes, thus exacerbating hepatic steatosis [8].
Oleuropein is a nontoxic secoiridoid derived from olives and olive oil, which influences their sensory organoleptic properties and is responsible for their typically bitter and pungent aroma [9]. This secoiridoid compound has a variety of biochemical roles, including antimicrobial [10], antioxidative [11], and anticancer [12] activities. Several in vitro studies have demonstrated that oleuropein has a high antioxidant activity comparable to a hydrosoluble analog of tocopherol [11], and inhibits the proliferation and migration of various cancer cell lines, such as leukemia, melanoma, colon, breast, and kidney cancer cells, in a dose-responsive manner [12]. More recently, it has been reported that oleuroepin significantly decreased the body weight, body fat accumulation, and plasma TG concentrations in rats with diet-induced obesity. Moreover, uncoupling-protein-1 contents of the interscapular brown adipose tissue and rates of urinary noradrenaline and adrenaline excretions were significantly decreased in rats fed oleuropein as opposed to HFD-fed control animals [13]. Although a number of studies have been carried out to investigate the biochemical roles of oleuropein, the protective activity of oleuropein against NAFLD has never been reported. Therefore, the purpose of the present study was to determine whether oleuropein has a protective effect against hepatic steatosis induced by a HFD in mice. The regulatory effect of oleuropein, on the expression of several key transcription factors and their target genes involved in adipocyte differentiation, and toll-like-receptor-(TLR)-mediated signaling molecules involved in metabolically triggered inflammation, were also investigated in a mouse model of HFD-induced hepatic steatosis.
Section snippets
Animal care and experimental protocol
Male C57BL/6N mice (five-week-old) were purchased from Orient Bio (Gyeonggi-do, South Korea) and were housed in standard cages placed in a room at 21 ± 2.0 °C temperature, 50 ± 5% relative humidity, and a 12 h-light/12 h-dark cycle. All mice consumed a commercial diet and tap water ad libitum for 1 week prior to their division into three weight-matched groups (n = 8 per group): the normal diet (ND), HFD, and oleuropein-supplemented diet (OSD) groups. ND was a purified diet based on the AIN-76 rodent diet
Body and liver weights
After 10-week feeding, HFD-fed mice showed significantly higher final body weight and cumulative body weight gain compared to ND-fed mice. Oleuropein supplemented to HFD significantly reduced final body weight (ND, 42 ± 2.4 g vs. HFD, 30 ± 3.2 g vs. OSD, 28 ± 2.4 g) and body weight gain (ND, 8 ± 0.7 g vs. HFD, 22 ± 1.9 g vs. OSD, 10 ± 2.2 g) in mice (Fig. 1A and B). Daily food intake (2.7–2.9 g/day) did not differ among experimental groups. Since the HFD is calorically denser than the ND (4616 vs. 3929 kcal/kg),
Discussion
Macroscopic and microscopic results demonstrated that, in HFD-fed mice, lipid accumulation in the liver was observed as early as 8 weeks up to 16 weeks, establishing a novel mouse model of diet-induced hepatic steatosis [15], [16]. In a previous study, the plasma TC, TG, and FFA concentrations of rats fed a 0.2%-(18.4 mg/kg body weight/day)-oleuropein-supplemented diet for 28 days were found to be significantly lower than those of HFD-fed rats [13]. Furthermore, the 6-week treatment of
Conflict of interest
The authors declared that they do not have anything to disclose regarding funding from industry or conflict of interest with respect to this manuscript.
Acknowledgements
This work was supported by a grant of the Korea Health 21 R&D Project, Ministry of Health & Welfare (#090282), Republic of Korea and by the SRC program of the Korea Science and Engineering Foundation (KOSEF) grant funded by the Korea government (#2009-0063409).
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