Ameliorative effect of myricetin on insulin resistance in mice fed a high-fat, high-sucrose diet

The prevalence of type 2 diabetes mellitus has increased at an epidemic rate, and it has now become a serious global health problem [1]. Obesity-associated insulin resistance is a strong risk factor for type 2 diabetes, the predominant form of diabetes [2]. In addition, insulin resistance is associated with metabolic syndrome, a cluster of metabolic abnormalities, including glucose intolerance, hypertension, hyperlipidemia, and a non-infective inflammatory state [3]. Metabolic syndrome can increase the risk of cardiovascular diseases (CVD) [4]. CVD is a major complication of type 2 diabetes, and a primary cause of immature death in patients with type 2 diabetes [5]. Thus, it is strongly recommended that the treatment strategy for type 2 diabetes focus on improving insulin resistance [6].

Accumulating evidence suggests that adipocytokines play an important role in regulating insulin sensitivity, which links obesity and insulin resistance [7, 8]. Adipocytes increase the secretion of leptin, a hormone involved in energy expenditure [9], and proinflammatory cytokines such as tumor necrosis factor-α (TNF-α) [10] and interleukin-6 (IL-6) [11], but decrease adiponectin secretion in obesity [12]. Adiponectin is an adipocyte-derived hormone involved in glucose metabolism and increases insulin sensitivity [13], whereas TNF-α and IL-6 play a crucial role in the development of insulin resistance [14]. An obese condition elevates TNF-α and IL-6 to stimulate expression of monocyte chemoattractant protein-1 (MCP-1), a proinflammatory chemokine that induces insulin resistance [15, 16]. Thus, suppression of obesity-induced inflammation could be useful for improving insulin resistance.

Myricetin is a bioflavonoid abundant in tea, berries, fruits, and vegetables [17]. The estimated daily intake of myricetin from food sources was about 0.98-1.1 mg per day [18]. Myricetin has been reported to have anti-inflammatory activity in vitro [19, 20, 21]. Myricetin reduces secretion of TNF-α and IL-1β in lipopolysaccharide (LPS)-stimulated RAW264.7 cells [19]. Myricetin inhibits LPS-induced production of IL-12 by downregulating transcription factor nuclear factor kappa-B (NF-κB) binding activity in RAW264.7 cells [20]. In addition, myricetin decreases IL-1β-induced production of IL-6 in synovial cells [21]. Therefore, myricetin could have the potential to alleviate obesity-induced insulin resistance through its anti-inflammatory activity.

Myricetin could be a promising therapeutic agent for treatment of type 2 diabetes. Liu et al. reported that injection of myricetin enhances insulin action in rats receiving fructose-rich chow [22]. Chang et al. found that dietary myricetin decreases body weight and improves the blood lipid profile in rats fed a high-fat diet [23]. However, the effect of myricetin on insulin resistance and inflammation in an animal model of type 2 diabetes is not fully understood. Thus, the purpose of this study was to investigate the effect of myricetin on adiposity, insulin resistance, and inflammatory markers in a diet-induced obese, insulin-resistant animal model. It has been reported that C57BL/6J mice fed a high-fat, high-sucrose (HFHS) diet develop obesity and type 2 diabetes [24, 25, 26]. In this study the effect of chronic consumption of myricetin on adiposity, blood lipid profile, glucose, insulin, adipocyte-derived hormones, and proinflammatory cytokines was determined in these animals fed HFHS diet.

We investigated the beneficial effect of chronic consumption of myricetin on the development of obesity and insulin resistance as well as the modulation of inflammatory markers in C57BL/6J mice fed an HFHS diet. Chronic consumption of HFHS diet induces obesity, hyperinsulinemia, and hyperglycemia in these animals [24, 25, 26, 28]. In this study, consuming the HFHS diet for 12 weeks resulted in obese C57BL/6J mice, whereas 0.12% dietary myricetin reduced body weight, weight gain, and epidydimal WAT weights in mice fed the HFHS diet (Table 2). However, 0.06% myricetin did not significantly affect these values. Myricetin (0.12%), but not 0.06% myricetin, also effectively reduced serum triglycerides and cholesterol compared to those in HFHS diet fed mice (Table 2). The average daily intake of myricetin by the 0.12 MTN and 0.06 MTN groups was calculated to be 120 ± 7 and 61 ± 3 mg/kg, respectively, based on food intake and average body weight. Previous studies have reported that dietary myricetin administered at 75 mg/kg decreases body weight and epidydimal fat pad weight without a beneficial effect on the blood lipid profile, whereas 150 mg/kg myricetin is effective at exerting anti-obesity effect and improving atherogenic index in rats fed a high fat diet [23]. In our study, 120 mg/kg myricetin inhibited the development of HFHS diet-related obesity and hyperlipidemia in mice. Myricetin induces the expression of hepatic peroxisome-activated receptor-α (PPARα) protein and reduces sterol regulatory element binding protein (SREBP) expression, resulting in increased fatty oxidation in high-fat diet-fed rats [23]. Modifying the expression of these proteins could contribute to improving obesity and hyperlipidemia in mice fed the HFHS diet. Weight management has been suggested to be the most important method of reducing risk factors for cardiovascular complications such as hyperglycemia, hyperlipidemia, and hypertension in obese patients with type 2 diabetes [29]. Thus, myricetin could be beneficial in the management of type 2 diabetes.

Consuming the HFHS diet elevated serum insulin and glucose levels, as well as HOMA-IR relative to the control group (Table 3), suggesting that the HFHS diet induced insulin resistance. It was reported that the injection of myricetin (1 mg/kg per injection, thrice daily) improved insulin resistance in rat fed a high-fructose diet [22]. Yang et al. [28] reported that HFHS diet-induced hyperinsulinemia and hyperglycemia are related to the downregulation of genes involved in insulin signaling. In the present study, 0.12% myricetin was effective for decreasing insulin and HOMA-IR in mice fed the HFHS diet and reduced serum glucose levels near to those of the control group, suggesting that myricetin improved insulin resistance and hyperglycemia.

Although consuming the HFHS diet did not significantly influence serum adiponectin, it elevated serum leptin levels, which was consistent with the results of a previous study [30]. Myricetin (0.12%) decreased leptin compared with that in the HFHS group without significantly influencing adiponectin levels (Fig. 1). Leptin, a hormone produced by adipocytes, reduces food intake and body weight [31]. In an obese condition, leptin signaling is attenuated leading to leptin resistance and hyperleptinemia [32]. Leptin resistance in the brain results in excess triglyceride accumulation in adipose tissue, muscle, liver, and the pancreas, leading to insulin resistance [33]. Myricetin could improve HFHS diet-induced hyperleptinemia by attenuating obesity; thus, contributing to improving insulin sensitivity and hyperglycemia in these animals.

Myricetin (0.12%) effectively suppressed the HFHS diet-induced elevation in serum TNF-α and IL-6, suggesting an alleviation of obesity-induced low-grade inflammation (Fig. 2). Myricetin at this dose tended to reduce MCP-1 levels in mice fed the HFHS diet, although the difference was not significant. TNF-α and IL-6 modulate insulin signaling, leading to impaired insulin action [10, 34]. MCP-1 plays an important role in macrophage infiltration into adipose tissue, which then induces inflammation and insulin resistance [16]. Thus, the reduction of TNF-α and IL-6 levels by myricetin could have contributed to improving insulin sensitivity in mice fed the HFHS diet. In addition, myricetin induces insulin action on insulin receptor substrate-1 (IRS-1)-associated phosphatidylinositol 3-kinase (PI 3-kinase) and translocation of glucose transporter subtype 4 (GLUT4) in high fructose-fed rats [22]. Elevated TNF-α and IL-6 levels are associated with reduced production and availability of endothelial nitric oxide synthase resulting in endothelial dysfunction [35]. Since obesity is an independent risk factor for CVD, reduction in these proinflammatory cytokines by supplementation of myricetin could contribute to decreased risk for CVD. Further study of the effect of myricetin on expression of proinflammatory cytokines and proteins involved in the insulin signaling pathway could be necessary to further understand the mechanisms underlying the improvement in insulin resistance by myricetin in these animals.

In conclusion, myricetin may have a protective effect against diet-induced of obesity and insulin resistance in mice fed HFHS diet, and that alleviation of insulin resistance could partly occur by improving obesity and reducing serum proinflammatory cytokine levels.