8-Hydroxyeicosapentaenoic Acid Decreases Plasma and Hepatic Triglycerides via Activation of Peroxisome Proliferator-Activated Receptor Alpha in High-Fat Diet-Induced Obese Mice

Hindawi Publishing Corporation Journal of Lipids Volume 2016, Article ID 7498508, 9 pages http://dx.doi.org/10.1155/2016/7498508 Research Article 8-Hydroxyeicosapentaenoic Acid Decreases Plasma and Hepatic Triglycerides via Activation of Peroxisome Proliferator-Activated Receptor Alpha in High-Fat Diet-Induced Obese Mice Hidetoshi Yamada,1 Sayaka Kikuchi,1 Mayuka Hakozaki,1 Kaori Motodate,1 Nozomi Nagahora,1 and Masamichi Hirose2 1 Iwate Biotechnology Research Center, 22-174-4 Narita, Kitakami, Iwate 024-0003, Japan Department of Molecular and Cellular Pharmacology, Iwate Medical University School of Pharmaceutical Sciences, Shiwa, Iwate 028-3694, Japan 2 Correspondence should be addressed to Hidetoshi Yamada; Received 28 October 2015; Revised 16 March 2016; Accepted 29 March 2016 Academic Editor: Rosemary Lee Walzem Copyright © 2016 Hidetoshi Yamada et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. PPARs regulate the expression of genes involved in lipid homeostasis. PPARs serve as molecular sensors of fatty acids, and their activation can act against obesity and metabolic syndromes. 8-Hydroxyeicosapentaenoic acid (8-HEPE) acts as a PPAR ligand and has higher activity than EPA. However, to date, the PPAR ligand activity of 8-HEPE has only been demonstrated in vitro. Here, we investigated its ligand activity in vivo by examining the effect of 8-HEPE treatment on high fat diet-induced obesity in mice. After the 4-week treatment period, the levels of plasma and hepatic triglycerides in the 8-HEPE-fed mice were significantly lower than those in the HFD-fed mice. The expression of genes regulated by PPAR𝛼 was significantly increased in 8-HEPE-fed mice compared to those that received only HFD. Additionally, the level of hepatic palmitic acid in 8-HEPE-fed mice was significantly lower than in HFD-fed mice. These results suggested that intake of 8-HEPE induced PPAR𝛼 activation and increased catabolism of lipids in the liver. We found no significant differences between EPA-fed mice and HFD-fed mice. We demonstrated that 8-HEPE has a larger positive effect on metabolic syndrome than EPA and that 8-HEPE acts by inducing PPAR𝛼 activation in the liver. 1. Background An imbalance between energy intake and expenditure can result in the accumulation of excess triglycerides in adipose tissues. Long-term continuation of this imbalance can result in chronic obesity, which is associated with hyperlipidemia, fatty liver, and adipocyte hypertrophy. Hyperlipidemia is a risk factor for atherosclerosis [1] and is also a predisposing factor for cardiovascular diseases. Fatty liver is another risk factor for cardiovascular disease and is thought to have a negative influence on the regulation of insulin signaling [2]. Adipocyte hypertrophy contributes to dysfunction in the adipose tissues and is associated with insulin resistance and increased risk of developing diabetes [3]. As a chronic high level of triglycerides is a common factor for hyperlipidemia, fatty liver, and adipocyte hypertrophy, any increase in triglyceride expenditure should potentially alleviate the risk of cardiovascular diseases and diabetes. PPARs are members of a nuclear receptor superfamily and play critical roles in the regulation of storage and catabolism of lipids [4]. They contribute to these regulation processes by activating gene expression in a liganddependent manner, which involves recognition of and binding to peroxisome proliferator response elements (PPREs) that are composed of TGACCT-related direct repeats separated by one nucleotide [5, 6]. PPARs form heterodimers on PPREs via the retinoid-X receptor, the receptor for 9cis-retinoic acid [7, 8]. Three types of PPAR have been 2 identified, namely, 𝛼, 𝛾, and 𝛿. PPAR𝛼 is expressed at high levels in the liver where it promotes fatty acid oxidation, ketogenesis, lipid transport, and gluconeogenesis [9, 10]. PPAR𝛼 responds to the concentration of fatty acids in the liver and enhances fatty acid breakdown by upregulating genes encoding 𝛽-oxidation enzymes [11–13]. Phenoxyalkylcarboxylic acid derivatives (fibrates) have been used to treat hypertriglyceridemia through activation of PPAR𝛼 [14–16]. Activation of PPAR by fibrates results in a substantially reduced level of serum triglycerides [16]. However, these drugs have the adverse side effects of hepatic toxicity, myopathy, and cholelithiasis. Thus, other PPAR activators are being investigated to determine whether they show fewer adverse effects than fibrates. The identification of unsaturated fatty acids as PPAR ligands provided firm evidence that the direct interaction of nuclear receptors with these fatty acids is required for some PPAR-dependent transcription activity [7, 11, 17–20]. Unsaturated fatty acids can bind to all three types of PPAR, with PPAR𝛼 exhibiting the highest affinity for concentrations equivalent to circulating blood levels [11, 21]. In contrast, the long-chain fatty acid erucic acid (C22:1) is a weak ligand that appears to have more affinity for PPAR𝛿 [22]. Overall, saturated fatty acids are poor PPAR ligands compared to unsaturated fatty acids [7, 11, 19]. Hydroxyeicosapentaenoic acids (HEPEs) are unsaturated fatty acids and are the oxylipin products of the lipoxygenase pathway. In a previous study, we showed that dried pacific krill is a source of HEPEs (5-HEPE, 8-HEPE, 9-HEPE, 12-HEPE, and 18-HEPE) and that 8-HEPE has high ligand activity for PPARs [23]. 8-HEPE increases the levels of expression of genes regulated by PPARs in Fao (rat hepatoma cell line), 3T3-F442A (mouse preadipocyte cell line), and C2C12 (mouse myoblast cell line) cells. Furthermore, 8-HEPE enhances adipogenesis and glucose uptake. By contrast, at the same concentrations, eicosapentaenoic acid (EPA) shows only a weak effect, indicating that 8-HEPE is a more potent inducer of physiological effects. 8-HEPE has a greater affinity for PPAR activation than EPA in vitro; it is possible that 8-HEPE might be of value in the treatment of obesity and metabolic syndrome. Pacific krill contain about 20 mg of 8-HEPE per 100 g [23] and therefore could potentially be used as a food supplement. However, no animal experiments on the in vivo effectiveness of 8HEPE have been reported. Here, we treated high-fat dietinduced obese mice with 8-HEPE to investigate its antiobesity effects. 2. Material and Methods 2.1. 8-HEPE and EPA Purification from Pacific Krill. 8HEPE and EPA were purified as described previously [23]. Dried krill (Kawashu, Iwate, Japan) were powdered and then extracted with methanol. The extract was subjected to column chromatography using Diaion HP-20 (Mitsubishi Chemical, Tokyo, Japan). The 8-HEPE- and EPA-containing fraction was eluted with methanol from the HP-20. 8-HEPE and EPA were separated on an InertSustain ODS-3 column (20.0 mm dia. × 250 (...truncated)