One of the most common

One of the most common dietary approaches against obesity-associated diseases is the increase in the consumption of ω3 polyunsaturated fatty acids [18], [19]. Since seminal paper by Bang and Dyeberg in which the low prevalence of coronary heart disease among the Inuit was associated with the high marine ω3 intake, many experimental research and clinical trials focused on the cardioprotective effects of fish oils [20], [21], [22], [23]. In addition, nutrigenomics has shown cardioprotective mechanisms of ω3 fatty acids through reduction of inflammatory markers, synthesis of proresolving lipid mediators and modulation of genes involved in fatty deoxycorticosterone receptor synthesis and oxidation [19], [24], [25], [26]. Other prominent mechanism mediating EPA (C20:5) and DHA (C22:6) anti-inflammatory and insulin-sensitizing effects include binding and activation of GPR120, a lipid-sensing GPCR, which plays an important role in the control of inflammation through JNK and NF-κB inhibition [26]. Importantly, not only EPA and DHA, but also alpha-linolenic acid (ALA – C18:3), a source of ω3 fatty acid common in vegetable oils, has shown to have affinity for GPR120 [26]. The advantages of ALA sources, such as flaxseed oil (FS), over EPA and DHA is that the food sources are less expensive and is free of heavy metal contamination (e.g., mercury) [27]. Recently, our group demonstrated that FS-rich diets reduced inflammation in central nervous system and peripheral tissues such as liver, skeletal muscle and white adipose tissue. Interestingly, these responses were abolished under GPR120 inhibition [25], [28].
Materials and methods
Results
Discussion Recently, it has been suggested that short- and long-chain fatty acids can activate several GPCRs, which has been linked to metabolic and immune regulation [34], [35]. GPR120 has emerged as a main mediator of anti-inflammatory and insulin-sensitizing effects of ω3-unsaturated fatty acids [26] in cells and tissues such as hypothalamus [28], liver, adipose tissue and muscle [25]. Herein, we hypothesized that partial replacement of lard by flaxseed oil, a natural source of ω3 fatty acid, would reduce the content of inflammatory and ER stress proteins in the aortas of Swiss and LDLr knockout mice. Swiss mice become obese and diabetic when exposed to HF diet and exhibit chronic low-grade inflammation [21]. In the vascular tissue, Swiss mice developed the pro-inflammatory status that precedes atherogenesis, but without plaque development. On the other hand, LDLr knockout mice are prone to develop dyslipidemia and atherosclerotic lesions when exposed to environmental stimuli. Therefore, we investigated the role of ALA-mediated GPR120 activation and β-arrestin 2 recruitment in the inhibition of TLR4 and TNF-α downstream signaling in aorta. Moreover, we have chosen flaxseed oil because is more affordable and safe in comparison to other ω3 sources [27]. As the impairment of glucose metabolism can contribute to several vascular complications [36], [37], we first evaluated whether the fatty acid substitution would affect the glucose homeostasis. An improvement in glucose homeostasis was observed in Swiss mice regardless of body weight and energy intake; however, this effect was not detected in the LDLr-KO model. Insulin sensitivity was not improved in both experimental models. Although ITT results were not affected by the FS diet, the effects of these unsaturated fatty acids on glucose homeostasis probably occurred due to following mechanisms: (1) reduction of inflammatory cytokines and stress kinases, as proposed by Zhang and Leung (2014) [38]; (2) activation of GPR120 in the gut and subsequent release of the incretin glucagon-like peptide-1 (GLP-1) [39]; (3) direct activation of PI3K by Gαq/11, an alpha subunit of G protein [40]. In order to confirm whether ALA, as the main fatty acid present in FS oil/diet, was incorporated into the vascular tissue, we performed a lipidomic assay in the aortas of Swiss mice. Interestingly, FS diet increased not only the relative amount of ALA but also oleic acid (C18:1), while reducing stearic acid (C18:0) content in the aorta. However, this alteration was not accompanied by changes in Scd1 gene expression. It is noteworthy that the bioavailability of ω3 fatty acid may be difficult to access due to aspects such as limitations of measurement techniques, time of fat intake, and the quality of fat [41], [42]. In 2004, Morise et al. [43] treated hamsters with different doses of ALA fatty acid and found proportional incorporation of this fat in different tissues. Similar to our findings, the authors found out that the supplementation failed to convert ALA to DHA, although a conversion of ALA to EPA was successfully detected. To investigate the effects of FS diet on lipoproteins levels, we measured cholesterol fractions and, as expected, LDL-c significantly decreased in response to FS diet in both experimental models. FS diet increased HDL-c levels in Swiss mice, but not in LDLr-KO mice. However, the TG levels were decreased in LDLr-KO mice. Mechanisms involved in the positive effect of dietary fatty acid manipulation (i.e. ω3 enrichment) on lipid profile include the hypotriglyceridemic effects of ω3 [44], reduction of SREBP-1c levels [45], upregulation of fatty acid oxidation by PPAR-α activation [46] and inhibition of lipogenic enzymes such as ACC and FAS by modulation of LXRα [47].