Modulation of Chronic Inflammation by Quercetin: The Beneficial Effects on Obesity.

Obesity has become one of the most prevalent health problems globally and represents a major risk factor for the development of chronic diseases such as insulin resistance, type 2 diabetes mellitus, and cardiovascular disease. 1 Moreover, obesity induces chronic inflammation in adipose tissue, the liver, skeletal muscle, and the vascular system. Chronic inflammation induces release of proinflammatory cytokines such as tumor necrosis factor-α (TNF-α), monocyte chemoattractant protein-1 (MCP-1), and interleukin-6 (IL-6), and immune cell infiltration is closely associated with the development of insulin resistance through interactions with the insulin signaling pathway in adipose tissue and skeletal muscle 2 , 3 and is closely linked to the pathogenesis of atherosclerosis in vessel walls. 4 Quercetin, a flavonoid compound, is found in vegetables and plants such as onions, apples, and green tea. For instance, the dominant onion flavonoids have been determined to be quercetin, quercetin-3-O-β-glucoside (Q3G), quercetin-4ʹ-O-β-glucoside (Q4ʹG), and quercetin-3,4ʹ-di- O-β-glucoside (Q3,4ʹG). 5 Interestingly, cooking methods affect the final flavonoid content; total quercetin abundance is increased 1.5-fold by microwave heating for 1 min, whereas the levels of Q4ʹG are decreased by boiling. 6 To date, researchers have focused more attention on the beneficial physiological roles of quercetin, which has anti-oxidative, anti-inflammatory, and anti-fibrotic effects, 7 – 9 although quercetin has been reported to have its potential pro-oxidative property in addition to its antioxidative property. 10 , 11 More interestingly, there is increasing evidence that quercetin is associated with epigenetic changes in cancer 12 and in maternal under-nutrition during gestation and lactation. 13 Therefore, studying the physiological roles of quercetin on chronic inflammation would contribute to preventive and therapeutic applications in obesity-related diseases. In this review, we focused on chemical properties, dietary sources, and anti-inflammatory effects of quercetin on insulin resistance, atherosclerosis, intestinal microbiota, and maternal under-nutrition with epigenetic activity.

Quercetin is the major representative of the flavonoid subclass of flavonols and is ubiquitously present in plants, fruits, and vegetables. The characteristic structure of flavonoids comprises a basic backbone of flavan, which has a C6-C3-C6 structure in which 2 benzene rings (C6) are bonded by 3 carbons (C3). Flavonols are present in plants as flavonoid-sugar compounds, which are referred to as glycosides in general. Figure 1 illustrates the structures of quercetin aglycone, which lacks a sugar moiety, and of quercetin-β-glucoside, which has been reported to be contained mainly in onions. 14 , 15 Figure 1 Structure of quercetin aglycon ( A ), and quercetin-4ʹ-O-β-D-glucoside ( B ). Structure of quercetin aglycon ( A ), and quercetin-4ʹ-O-β-D-glucoside ( B ). The types and amounts of flavonoids are extremely varied among plants. To date, the levels of quercetin in various foodstuffs have been reported in numerous studies. 16 – 20 When foods containing quercetin are consumed, the rate of intestinal absorption is higher for quercetin glycosides than aglycon. 21 Hollman et al, who evaluated the bioavailability of quercetin, reported detecting increased concentrations of quercetin in plasma immediately after oral administration of onion and apple supplements. 15 Absorbed quercetin is rapidly metabolized in the liver, and circulates as methyl, glucuronide, and sulfate metabolites. 22 In human healthy adults, 163 diverse metabolites and quercetin conjugates such as quercetin-3-glucuronide, isorhamnetin-3-glucuronide, quercetin diglucuronide, and quercetin-30-sulphate were measured in the plasma after long-term quercetin-containing supplements; especially the concentrations of conjugates increased at the 1000 mg/day dose for 90 days. 23 Therefore, circulating quercetin and its metabolites in peripheral tissues are expected to exhibit bioavailability, resulting in anti-inflammatory effects.

In randomized clinical trials, participants who were overweight-to-obese patients with pre- and stage 1 hypertension were randomized to receive a dose of 162 mg/day quercetin from onion skin extract for 6 weeks. As a result, no significant effects were observed in terms of serum C-reactive protein and TNF-α, as well as the levels of glucose, insulin, and HOMA-IR compared to a placebo group. 74 When healthy (pre) hypertensive men and women received quercetin-3-glucoside (160 mg/day) for 4 weeks, the levels of soluble endothelial selectin (p = 0.03) and IL-1β (p = 0.009), and the z score for inflammation (p = 0.02) were lower compared to placebo. 75 No effects of supplementation of four quercetin capsules per day containing 100 mg quercetin dihydrate (100 mg/day for 10 weeks) were observed for the inflammatory markers, IL-6 and soluble vascular cell adhesion molecule-1, in healthy male smokers, although quercetin reduced the serum levels of total cholesterol and LDL-cholesterol compared to a placebo group. 76 Women with type 2 diabetes receiving quercetin (500 mg/day for 10 weeks) exhibited reduced systolic blood pressure. However, there were no effects on serum levels of IL-6, TNF-α, and C-reactive protein in comparing the quercetin and placebo groups. 77 In addition, women with polycystic ovary syndrome were assigned to 2 groups of quercetin treatment: 1 g/day (two 500 mg capsules) daily for 12 weeks, and placebo. Quercetin reduced HOMA-IR levels (p < 0.001) and slightly increased serum adiponectin compared to a placebo group, the authors suggesting that quercetin supplementation may improve adiponectin-mediated insulin resistance. 78 Furthermore, women with rheumatoid arthritis (RA) were assigned into quercetin (500 mg/day) or placebo groups for 8 weeks. The study’s results indicated that there were no effects of quercetin on plasma oxidative and inflammatory status, or systolic and diastolic blood pressure in patients with RA. 79 On the other hand, Javadi et al demonstrated that plasma TNFα levels were significantly decreased in a quercetin group compared to placebo in women with RA allocated into a quercetin (500 mg/day) or placebo group for 8 weeks. 80 Based on previous clinical studies, the effects of quercetin remain unclear. Further studies with various design and sample sizes, and with different quercetin doses, are needed, considering the beneficial effects of quercetin observed in previous animal and cell investigations.

Epigenetics is referred to as heritable phenotypic alterations in gene expression that are independent of DNA sequence changes. 95 , 96 Common epigenetic modifications in mammalian cells include changes in DNA methylation, histone modification, and expression of various non-coding microRNAs (miRNAs). DNA methylation is catalyzed by DNA methyltransferases (DNMTs) and is thought to act at promoters so as to induce gene silencing. Histone modifications to specific amino acid residues modulate chromatin structure and gene expression. For example, the histone acetylation state is thought to be adjusted by histone acetyltransferases (HATs) and histone deacetylases (HDACs). miRNAs are known to regulate gene expression post-transcriptionally and function in RNA-silencing. 97 Studies of epigenetic mechanisms have allowed advances in the understanding of cancer. 98 , 99 Interestingly, polyphenolic compounds derived from plants seem to exert anti-tumor effects through epigenetic activities. 100 , 101 Treatment with quercetin upregulates miR-503-5p and miR-6867-5p expression and exhibits the potential for anti-proliferative and anti-inflammatory actions in endometriosis implanted mouse models. 102 Treatment with quercetin decreases global DNA methylation levels and the activity of DNMTs, HDACs, and histone methyltransferases (HMTs) in quercetin-treated HeLa cells, a human cervical cancer cell line. 12 A combination of quercetin and butyrate with chemopreventive activity suppresses human esophageal cancer cell growth and downregulates the expression of DNMT1, NF-κBp65, HDAC1, and Cyclin D1. 103 In addition, quercetin enhances apoptosis by increasing the expression level of Fas ligand through the upregulation of HAT activity in human leukemia HL-60 cells. 104 Thus, quercetin is expected to act as a candidate natural therapeutic agent to prevent cancer through epigenetics activity. The HFD-fed mice showed hypermethylation in the peroxisome proliferator activated-receptor gamma coactivator 1 alpha (PGC-1α) promoter and Pgc-1α mRNA expression, which is a transcriptional coactivator, in skeletal muscle. Conversely, quercetin supplementation reduced the increases in DNA methylation and PGC-1α expression, 105 suggesting that quercetin may regulate PGC-1α expression through DNA methylation in obesity. Moreover, the treatment of quercetin inhibited inflammation in livers of nickel-treated mice by modulating nuclear factor-E2 related factor 2 (Nrf2) nuclear translocation and HO-1 activity and decreased DNMTs activity and DNA methylation level of the Nrf2 DNA. 106 Therefore, quercetin may epigenetically regulate the obesity and inflammation. There is growing evidence that nutrients may modify epigenetic programs, thus regulating gene expression. For example, maternal under-nutrition or restriction of dietary protein during pregnancy leads to many diseases, including obesity, diabetes, and renal disease in adult offspring. 107 – 110 Such diseases are closely associated with the development of chronic inflammation. Maternal over-nutrition in utero results in developmental programming of genes involved in obesity, inflammation, and pro-fibrogenic genes in the liver of the offspring. 111 The unfolding pattern of histone H3 lysine 4 trimethylation in children and mothers was associated with human undernutrition. 112 Although quercetin exerts anti-inflammatory action with a wide range of mechanisms of actions, few reports have addressed mechanisms by which quercetin modulates inflammation by regulating epigenetic pathways, which is linked to maternal under-nutrition. We previously reported that the feeding of quercetin to protein-restricted dams during lactation upregulates AMPK activation in the liver of 23 week-old adult offspring. 113 Significant increases in AMPK-associated phosphorylated ACC and endothelial nitric oxide synthase (eNOS) levels are found in the liver of such adult offspring. Quercetin treatment during lactation may lead to long-term alterations to the AMPK pathway in the liver of adult offspring of protein-restricted dams. However, whether quercetin treatment during lactation directly activated AMPK in adult offspring remain unclear. The decreased levels of histone acetylation and increased levels of promoter methylation of PGC-1α promoter methylation were correlated with the activity of AMPK in human placenta of diabetic mothers. When diabetic placental explant was treated with metformin, an anti-diabetic drug, AMPK was activated, concomitant with increased H3K27 acetylation and decreased PGC-1α promoter methylation. 114 Thus, because metformin as well as quercetin upregulate the AMPK activation, 51 we hypothesized that quercetin treatment during lactation may activate AMPK via epigenetic regulation. More interestingly, maternal quercetin intake during lactation may cause long-term alterations to inflammation and autophagy flux in the kidneys of high-fructose-diet fed adult female rat offspring. 13 Maternal quercetin intake during lactation decreases the number of infiltrating macrophages and depresses IL-6 mRNA levels in the kidneys of adult female offspring fed a high-fructose diet after birth. After inducing obesity in female rats fed a HFD, maternal quercetin treatment improved glucose metabolism, insulin sensitivity, hepatic inflammation, and adipose tissue deposition in the adult offspring of obese dams. 115 On the other hand, maternal quercetin treatment, starting from 3 days before conception until the end of gestation, resulted in increased iron storage and decreased 8-oxo-dG levels in the liver of 12-week old adult murine offspring. 116 In addition, the authors indicated that maternal quercetin treatment increases IL-1β, IL-6, and IL-10 levels in the liver of the adult offspring. Importantly, maternal quercetin intake during gestation and/or lactation may modulate long-term alterations, including inflammatory responses in adult offspring. However, further experiments are required to clarify whether quercetin treatment during gestation and/or lactation contributes to the regulation of epigenetic pathways after birth.

This review provided recent evidence of the anti-inflammatory effects of quercetin. First, quercetin is involved in the attenuation of insulin resistance and atherosclerosis in obesity-related diseases. Insulin resistance is closely associated with the development of chronic low-grade inflammation. Quercetin may improve insulin resistance through inhibiting the production and expression of proinflammatory cytokines and/or enzymes. Importantly, quercetin is associated with inhibition of the TLR4/NF-κB signaling pathway. Based on previous cell culture studies and animal experiments, quercetin treatment is clearly required for preventative and therapeutic applications. However, different studies may also show the effects of quercetin in clinical studies. In this regard, the bioavailability of quercetin in organisms may be variable. It is necessary to establish more comprehensive studies that help to guide clinical studies. Second, some studies have shown the anti-inflammatory effects of quercetin on intestinal microbiota in obesity. Prebiotics alter the intestinal microbiota and reduce serum levels of IL-6 in children with overweight or obesity. 117 In addition, quercetin treatment may mitigate the onset and/or development of atherosclerosis. 94 Not only quercetin but also a combination of quercetin and prebiotic treatment may contribute to improve chronic inflammation in obesity-related diseases. Third, we summarized that maternal quercetin intake during lactation may exert long-term alterations in inflammation. Several studies have investigated epigenetic modulation mediated by quercetin. Quercetin is thought to act as a candidate therapeutic agent to prevent cancer through epigenetic activities. On the other hand, few reports have addressed suggestions that maternal quercetin exerts anti-inflammatory effects in adult offspring programmed by maternal under-nutrition and over-nutrition through modulation of epigenetic pathways. Further investigations are required to advance our understanding of the effects of maternal quercetin intake during gestation and/or lactation on anti-inflammatory activity in obesity- and age-related diseases.

Intestinal microbiota, which exist with a certain diversity in the human gastrointestinal lumen, are also involved in obesity. For instance, it has been revealed in humans and in animal models that obesity is associated with changes in the relative abundance of the two dominant bacterial divisions, the Bacteroidetes and the Firmicutes, with increased levels of Actinobacteria. 81 , 82 Turnbaugh et al demonstrated that colonization of germ-free mice with microbiota from obese animals resulted in significantly greater increases in total body fat than colonization with a non-obese “lean” microbiota. 83 Although there are some studies indicating that the proportions of intestinal microbiota are different among populations 84 or are not involved in obesity, 85 imbalances and alterations in composition and/or function of intestinal microbiota, so-called “dysbiosis”, have been identified to be related to onset and/or development of obesity. In addition, the gut microbiota may be associated with the onset and/or development of atherosclerosis including inflammation and lipid metabolism. 86 For example, berberine isolated from various medicinal plants showed the anti-atherosclerotic effect with the changes in composition and functions of gut microbiota, which is associated with anti-inflammatory and glucose and lipid metabolisms. 87 In recent years, attention has been focused on the effects of food components such as quercetin on intestinal microbiota in obesity. Etxeberria et al found that quercetin administration in rats effectively alleviates intestinal dysbiosis induced by a high-fat sucrose diet. 88 Quercetin supplementation attenuates the Firmicutes/Bacteroidetes ratio and inhibits the growth of bacterial species associated with diet-induced obesity. In another study, a combination of quercetin and resveratrol was able to ameliorate obesity and reverse the gut microbiota dysbiosis in HFD-fed rats. 89 Moreover, there are recent reports indicating that the relationship between quercetin and gut microbiota is associated with an anti-inflammatory status. Citrobacter rodentium -induced colitis in mice is well documented as an animal model of inflammatory bowel disease (IBD). Interestingly, pre-administered quercetin could alleviate Citrobacter rodentium -induced colitis, due to quercetin’s ability to suppress pro-inflammatory cytokines such as IL-6 and TNF-α, and/or to modify gut microbiota. That is, pre-administration quercetin may enhance population numbers of Bifidobacterium, Bacteroidetes , and Lactobacillus , and may reduce those of Fusobacterium and Enterococcus . 90 In in vitro studies, quercetin reduces the levels of inflammatory mediators in LPS-stimulated macrophages by enhancing secretion of anti-inflammatory substances by Bifidobacterium adolescentis . 91 Stearic acid is tentatively identified as the anti-inflammatory molecule from B. adolescentis stimulated by quercetin. 92 From these findings of the relationships between quercetin and microbiota, quercetin is expected to play a role in moderating intestinal inflammation in obesity. Indeed, a recent study using a mouse model of nonalcoholic fatty liver disease (NAFLD) associated with obesity demonstrates that quercetin can revert gut dysbiosis and related endotoxemia-mediated TLR4 pathway induction, with subsequent inhibition of inflammasome responses. 93 In addition, it is reported that the atherosclerotic lesions and size of plaques were reduced in mice fed HFD diets with oral quercetin treatment, by the alternation of the composition of the gut microbiota. 94 Quercetin treatment may contribute to mitigate the onset and/or development of atherosclerosis by modulating intestinal microbiota balance. In order to elucidate the anti-inflammatory effects of quercetin in obesity, including atherosclerosis, further investigations regarding intestinal microbiota are needed.