4.2. Flavanones

Naringenin and hesperidin, the two major flavanones that are abundant in citrus fruits such as grape, tomatoes, and oranges, have been reported to possess antioxidant, anti-diabetic, lipid-lowering, anti-atherogenic, and anti-inflammatory activities [61,134,135,136]. Both naringin and naringenin (the aglycone form of naringin) have been extensively studied and have been found to possess anti-obesity and anti-diabetic properties [137,138]. The anti-obesity effect of naringenin was dependent on the reduction in adipose tissue mass and inhibition of preadipocyte proliferation [60]. Naringenin suppressed the proliferation of preadipocytes without showing detrimental effects on subsequent adipogenesis. Moreover, naringenin increased fatty acid oxidation in hepatocytes by enhancing peroxisomal β-oxidation in mice [139]. This compound also remarkably increased the activity of various enzymes required for fatty acid oxidation in hepatocytes, such as acetyl-coenzyme A acetyltransferases (ACAT, also known as thiolase), acyl-coenzyme A oxidase, carnitine O-octanoyl transferase (COT, also known as medium-chain/long-chain carnitine acyltransferase), and 3-ketoacyl-coenzyme A [139]. Naringenin included as a 0.1% dietary supplement in rats fed a high-cholesterol diet reduced the cholesterol levels of plasma and triacylglycerol and the cholesterol levels in hepatocytes by decreasing the activity of 3-hydroxy-3-methylglutaryl-coenzyme (HMG-CoA) reductase and ACAT [59]. In 3T3-L1 adipocytes, naringenin repressed glucose uptake [60] and suppressed PI3K and Akt phosphorylation normally induced by insulin, thus regulating insulin-induced GLUT4 translocation [140]. Moreover, naringenin prevented dyslipidemia and improved glucose metabolism by modulating the decrease in blood glucose and lipids independently of fibroblast growth factor 21 (FGR 21) [141]. Taken together, the findings indicate that routine consumption of naringenin impairs glucose uptake in the adipose tissue by exacerbating insulin resistance in susceptible individuals. These antagonistic actions of naringenin on the homeostasis of glucose may depend on an individual’s capacity to absorb and metabolize this flavonoid [60]. Naringin, a flavanone-7-O-glycoside between the flavanone naringenin and the disaccharide neohesperidose, also shows biological and pharmacological properties, such as antioxidant, anti-inflammatory, anti-carcinogenic, lipid-lowering, and anti-diabetic effects [62,64,142]. Several studies have demonstrated that in db/db mice or rats, naringin regulated the plasma lipids in hypercholesterolemic animals fed a HFD [64,138,143]. Diet supplementation with 0.02% naringenin in rats fed a high-fat and high-cholesterol diet for three weeks had no hypolipidemic effect [45,47]. In the livers of db/db mice, naringin modified the activities of hepatic lipid-metabolizing enzymes and improved plasma lipid metabolism [64]. Furthermore, in T2DM-affected mice, naringin may upregulate hepatic and adipocyte PPARγ and GLUT4 to regulate the expression of hepatic enzymes involved in glycolysis and gluconeogenesis, thereby improving hyperglycemia [64,144]. Daily consumption of naringin consumption decreased plasma low-density lipoprotein (LDL)-cholesterol in hypercholesterolemic individuals [138], suppressed the biosynthesis of hepatic cholesterol, and decreased the levels of plasma lipids and glucose [138,143], supporting that naringin plays a vital role in obesity prevention.

Regarding the lipid-lowering tendency, another flavanone, hesperetin (the aglycone form of hesperidin), lowered the plasma levels of cholesterol and triacylglyceride and the action of the cholesterol biosynthesis rate-limiting enzyme, HMG-CoA reductase, when fed at a 0.02% dietary level to high cholesterol and high-fat-fed rats [145]. Hesperetin also lowered the activity of another key cholesterol-regulating enzyme, ACAT, which is involved in the esterification and absorption of cholesterol. Moreover, hesperetin obstructed cholesterol biosynthesis, resulting in a lower intracellular supply of cholesterol and over-expression of hepatic LDL receptors, as well as increased the clearance of circulating LDL particles [146].

Hesperidin is a flavone glycoside (bound to the disaccharide rutinose) abundant in citrus fruits such as lemons and limes that shows lipid-lowering effects [147]. Hesperidin supplementation to the regular diet regulated the activities of glycolytic and gluconeogenesis enzymes of hepatic glucose metabolism and improved hyperglycemia in db/db, C57BL6 mice [134,148]. The flavonoid was reported to be beneficial for lowering blood glucose levels by upregulating hepatic glucokinase, PPARγ, and adipocyte GLUT4 [64,149]. This compound is also very effective for advancing the lipid metabolism in db/db mice by increasing fecal triglyceride excretion and impeding lipid-metabolizing enzymes including glucose-6-phosphate dehydrogenase (G6PDH) and fatty acid synthase [64]. In STZ-induced diabetic rats, hesperidin supplementation also decreased glucose-6-phosphatase (G6Pase), which is a glucose-6-phosphate (G6P)-hydrolyzing enzyme, and increased glucokinase (GK), which is a G6P-generating enzyme, collectively diminishing glucose export via glucose transporter membrane proteins [63]. Hesperidin also reduced plasma and hepatocyte cholesterol levels partially by suppressing hepatic HMG-CoA reductase and ACAT activities, resulting in decreased hypercholesterolemia and atherosclerosis [150,151]. It also increased fecal cholesterol excretion [64]. These coordinated responses of hesperidin supplementation play a significant function in controlling glucose and lipid metabolism in db/db mice [64]. Taken together, hesperetin and hesperidin have diverse effects on glucose and lipid metabolism and exhibit lipid-lowering activity both in vitro and in vivo.

Eriodictyol, another flavanone abundant in lemons, also significantly controlled obesity and diabetes [65]. This flavonoid inhibited the adipocyte-specific fatty acid binding protein in 3T3-L1 adipocytes by suppressing PPARγ and increasing the glucose uptake, improving insulin resistance [65]. Eriodictyol also impeded diabetes-related lipid peroxidation by decreasing the levels of TNFα, intercellular adhesion molecule 1 (ICAM-1), and vascular endothelial growth factor (VEGF) [66].

4.3. Isoflavones

Isoflavones are another class of flavonoids commonly found in leguminous plants, including soybean and soy products; the major dietary isoflavones are daidzein and genistein, which are present primarily in soy foods [85]. Numerous studies have suggested that isoflavones favorably affect adiposity, glucose homeostasis, insulin secretion, and lipid metabolism [152]. Isoflavones have beneficial effects on major risk factors of cardiovascular disease such as excess body weight, hyperinsulinemia, and hyperlipidemia, which are commonly associated with obesity. A mixture of synthetic daidzein and genistein fed at 23.6 mg/kg body weight per day to Sprague-Dawley rats reduced plasma triglycerides more significantly than in casein-fed rats [67]. Additionally, hamsters fed pure synthetic daidzein (16 mg/kg body weight/day) considerably lowered blood glucose and plasma total cholesterol levels compared to casein-fed rats [153,154]. Moreover, supplementation of 500–1500 ppm genistein with a serum equivalent of approximately 2 μM in the diet showed hypolipidemic effects by decreasing fat-pad weights by 50% in C57/BL6 mice [155]. In C57/BL6 ovariectomized mice, subcutaneous injections of genistein (8–200 mg/kg/day) for 21 days decreased adipose tissue gain [155]. Isoflavones reduced adipose tissue deposition, and in vitro studies showed that genistein and daidzein enhanced lipolysis by suppressing 3′,5′-cyclic-AMP phosphodiesterase (cAMP-specific PDE) [156,157]. Furthermore, genistein substantially activated AMPK and acetyl-CoA carboxylase (ACC) in cultures of 3T3-L1 adipocytes and suppressed adipocyte differentiation [71]. Genistein induced intracellular reactive oxygen species (ROS) release, which quickly triggered AMPK and led to apoptosis. Adipocytes treated with genistein readily decreased the protein expressions of PPARs and C/EBP. Genistein supplementation also repressed the incorporation of glucose into lipids and increased the output of fatty acids into the medium in an isolated perfused liver preparation [158]. Hence, in the liver and the adipose tissues, genistein may affect lipid metabolism by disrupting both lipolysis and lipogenesis. In type 1 diabetes mellitus (T1DM) animals, dietary supplementation of genistein led to modulation of glucose metabolism and insulin levels [159,160]. A previous study revealed that genistein had anti-diabetic effects by improving plasma lipids [161], thereby increasing insulin sensitivity [69]. A recent study demonstrated that mice given a soy-supplemented diet (containing approximately 198 ppm daidzein and 286 ppm genistein) from conception through adulthood exhibited an improved lipid profile and glucose metabolism [162]. Soy intake also led to increased phosphorylation of AMPK and favorable metabolic changes, including enhanced mitochondrial biogenesis and glucose uptake in the skeletal muscle [162], with decreased blood glucose, TGFβ 1, and HbA1C levels [70,163]. Indeed, recent findings indicated that isoflavone administration lowered plasma glucose, although insulin sensitivity or the plasma lipid profile was unaffected in obese and diabetic animals [164]. Taken together, the metabolism could be differentially modulated by a mixture of isoflavones, soy protein, or genistein. Supplementation of daidzein or genistein in diet at a dose of 0.02% can suppress the onset of diabetes and enhance glucose homeostasis through stabilization of pancreatic β-cell function in non-obese diabetic (NOD) mice, [165]. Isoflavone supplementation was also associated with suppression of the activities of gluconeogenic enzymes such as phosphoenolpyruvate carboxykinase (PEPCK) and G6Pase, as well as β-oxidation of fatty acids and increased lipogenesis in the liver [165]. It was recently reported that genistein reduced fasting glucose in non-genetic diabetes mice [69]. Consistent with this observation, genistein improved glucose tolerance and hyperglycemia and significantly enhanced islet β-cell proliferation and survival in STZ-induced diabetic mice [166]. Genistein was found to exert its effect on β-cells by modulating multiple signaling pathways, including activation of calmodulin kinase II and Ca²⁺ signaling [167] and suppression of the NF-κB, ERK-1/2, and JAK/STAT pathways [168]. In pancreatic β-cells, genistein-induced stimulation of cAMP/PKA signaling was important for its insulinotropic and mitogenic properties [68,169]. In post-menopausal women with T2DM, daily isoflavone intake (100 mg of aglycones) for one year resulted in improved insulin sensitivity and blood lipid parameters [170]. However, in another study of postmenopausal women with T2DM, consumption of isoflavones (132 mg) for 3 months did not improve plasma A1C, blood glucose, and insulin levels [171]. Although the disparities were likely caused by differences in treatment dosage and duration, many in vitro or in vivo studies have revealed the anti-obesity and anti-diabetic effects of dietary isoflavones.

4.4. Flavones

Flavones are another class of flavonoids found mainly in celery, parsley, and many different herbs. The major dietary flavones include apigenin and luteolin [85]. Plants containing apigenin, such as passionflower and chamomile, have been used as traditional medicines for hundreds of years to treat a variety of diseases. Oral administration of apigenin (0.78 mg/kg body weight) for 10 days was reported to reverse the reduction in hepatic antioxidants in alloxan-induced insulin-dependent diabetic mice, confirming the free-radical scavenging activity [172]. In STZ-induced diabetic rats, intraperitoneal administration of apigenin had a significant anti-hyperglycemic effect [72]. In clonal β-cells, apigenin treatment attenuated 2-deoxy-d-ribose-induced apoptosis through its antioxidant effect by controlling the mitochondrial membrane potential [73]. In human THP-1 monotypic cells, apigenin suppressed TNF-α- and IL-1β-induced activation of NF-κB [173] and, in HepG2 hepatocytes, the flavonoid improved AMPK phosphorylation [56]. Apigenin was 200-fold more potent than metformin, a well-known activator of AMPK. In HepG2 cells exposed to high glucose, apigenin was found to decrease ACC phosphorylation and impede lipid accumulation [56], supporting that apigenin has beneficial effects on dyslipidemia and diabetes by regulating AMPK-dependent energy metabolism.

Another anti-obesity and anti-diabetic flavone, luteolin, is abundant in vegetables and fruits such as onion leaves, cabbage, broccoli, celery, parsley, carrots, peppers, apple skins, and chrysanthemum flowers [174,175,176]. In primary mouse adipose cells and 3T3-L1 adipocytes, luteolin was reported to potentiate insulin action and enhance the expression and transcriptional activation of PPARγ target genes [74]. Luteolin also mediated the beneficial effects on metabolic pathways in insulin resistance and DM pathophysiology by repressing the circulating levels of inflammatory molecules such as MCP-1 and resistin [177]. Additionally, luteolin enhanced insulin release in uric acid-damaged pancreatic β-cells by suppressing the reduction of MAFA, principally via the NF-κB and inducible nitric oxide synthase–nitric oxide (iNOS–NO) signaling pathways [75].

Tangeretin, which is prevalent in citrus fruits, including mandarins and oranges, also showed anti-obesity and anti-diabetic effects. In HFD-induced obese mice, administration of tangeretin (200 mg/kg) led to decreased total cholesterol and blood glucose and regulation of adipocytokines, such as adiponectin, IL-6, leptin, MCP-1, and resistin [76]. In diabetic rats, tangeretin treatment (100 mg/kg) for 30 days significantly reduced plasma glucose levels. In diabetic rats, tangeretin treatment enhanced glycolytic enzymes, leading to control of glucose metabolism in the hepatic tissues [77]. In 3T3-L1adipocytes, tangeretin was found to improve the secretion of insulin-sensitizing factor adiponectin while suppressing the secretion of the insulin receptor substrate factor MCP-1 [178].

4.5. Flavan-3-ols

Flavan-3-ols are also referred to as flavanols and are present in various teas, fruits, cocoa, and chocolates [179]. In fruits and cocoa, the most common falavan-3-ols are catechin and epicatechin, while in grapes, teas, and seeds of certain leguminous plants, the main falavan-3-ols are epicatechingallate (ECG), gallocatechin, epigallocatechin (EGC), and epigallocatechin gallate (EGCG). Tea and tea components have anti-obesity or anti-diabetic effects [180,181,182,183]. Regular administration of green tea or EGCG has been shown to be effective for preventing cardiovascular and metabolic diseases [184]. Catechin-enriched green tea enhanced energy expenditure and suppressed dietary lipid absorption [185]. In animal models, catechin and EGCG were studied for their effect to minimize diet-induced obesity by increasing fat oxidation and decreasing leptin levels and energy absorption [179,186]. Overweight or obese men aged 40–65 years who consumed 400 mg capsules of EGCG with the diet twice daily for eight weeks revealed the potential anti-obesity effects of EGCG [187]. In cases of T2DM, the beneficial effects of (−)-catechin in the treatment of obesity-related diseases were also observed, with enhanced insulin-dependent glucose uptake in differentiated adipocytes [79]. The molecular mechanism responsible for stimulating the effect of (−)-catechin on adiponectin expression involved the repression of Kruppel-like factor 7 (KLF7) expression, which regulates the expression of adiponectin and other adipogenesis-related genes, such as PPARγ, leptin, CEBPβ, and aP2 in adipocytes [79]. In mice fed a HFD, treatment with EGCG attenuated hyperlipidemia and fatty liver [80] and in human HepG2 cells, ECG and EGCG diminished the accumulation of hepatic lipids and suppressed fatty acid synthase and acetyl Co-A carboxylase 1 (ACC1) in vitro and in vivo [186,188]. ACC1 is located in the cytosol and slows the β-oxidation of fatty acids through malonyl-CoA formation to inhibit fatty acid transport, which is mediated by mitochondrial carnitine palmitoyl transferase (CPT1, also known as carnitine acyltransferase I) [189]. ACC1, caspase 3, cyclin-dependent kinase 2 (Cdk2), and AMPK were reported to be involved in flavan-3-ol-mediated modulation of obesity- and diabetes-related apoptosis and ROS generation [190]. Numerous studies reported the anti-diabetic effects of flavan-3-ols in animal and cell culture studies. In rat insulinoma-m5F cells under glucose-induced toxicity, treatment of flavan-3-ols led to improvement in the insulin secretory function and viability of β-cells through increased expression of insulin receptor substrate 2 (IRS2), AKT, forkhead box protein O1 (FOXO1), and pancreatic duodenal homeobox-1 (PDX-1) [191]. Additionally, flavan-3-ols enhanced mitochondrial action by increasing the quantity and entire efficiency of mitochondria [191]. Insulin function and the insulin-mediated signaling pathway was significantly modulated by IRS2 and deletion of the IRS protein led to T2DM [192]. The important factors in IRS signaling pathway were AKT, PDX-1, and FOXO1 [193]. PDX-1 could regulate the pancreas development and function and FOXO1 was reported to induce NeuroD and MAFA expression, an important function in modulation of β-cell proliferation and apoptosis [194].

This work was supported by Konkuk University in 2014.