Actions of quercetin on gluconeogenesis and glycolysis in rat liver

Aqueous leaf extract of Clinacanthus nutans improved metabolic indices and sorbitol-related complications in type II diabetic rats (T2D)

Clinacanthus nutans (Burm. f.) Lindau (C. nutans) has been reported to lower blood glucose level; however, evidence on its efficacy in lowering diabetic complications is limited. The antidiabetic properties of C. nutans aqueous leaf extract on serum metabolic indices, sorbitol production, and aldose reductase enzyme activities in the kidneys, lens, and sciatic nerve of type II diabetic (T2D) rats were evaluated. All rats except normal control rats were fed with a high-fat diet for 8 weeks to induce obesity and subsequently injected with 35 mg/kg streptozotocin to induce type II diabetes. Aqueous leaf extract of C. nutans (100 and 200 mg kg-1 day-1) and quercetin (10 mg kg-1 day-1) were fed orally for 4 weeks. Diabetic rats administered with C. nutans at 100, 200 mg kg-1 day-1 and quercetin had significantly (p < 0.05) lower fasting blood glucose levels post-intervention: 14.2, 14.0, and 19.9 mm, respectively, compared with the untreated group (22.1 mm). Total cholesterol was significantly (p < 0.05) lower in the C. nutans groups in comparison with the diabetic control group. Levels of F2-isoprostane, a marker of oxidative stress, were attenuated in the presence of the extract. Aldose reductase enzyme activity increased by 64, 99, and 0% and total antioxidant activities by 22, 29, and 126%, respectively. Sorbitol levels in the kidney, lens, and nerve were reduced in diabetic rats administered with C. nutans and quercetin group (by 8, 16, and 3%, respectively). The protective effect of the extract to the liver and kidney was confirmed through liver and kidney enzyme markers and histological analyses. The C. nutans has the potential to attenuate T2D-induced metabolic perturbations and complications related to sorbitol accumulation.

Bioavailability of Quercetin from Onion Extracts after Intraruminal Application in Cows

The aim of the present study was to investigate the bioavailability of quercetin from onion bulb (OB) and onion skin (OS) extracts in ruminants. Three non-lactating cows equipped with a permanent rumen fistula intraruminally received equimolar amounts of quercetin as either aglycone, rutin, or OB or OS extract, respectively, at a dose of 50 mg of quercetin equivalents/kg of body weight. Blood samples were drawn before and frequently within the 24 h period after application of the respective substance. Quercetin and quercetin metabolites with an intact flavonol structure (kaempferol, isorhamnetin, and tamarixetin) were analyzed in plasma samples by high-performance liquid chromatography with fluorescence detection. All quercetin sources administered resulted in a fast increase of the plasma concentrations of quercetin and total flavonols (sum of quercetin and its metabolites), followed by a rapid decline, whereby significant higher concentrations occurred with OB extract and rutin compared to quercetin aglycone and OS extract, respectively. The results clearly demonstrate a higher systemic availability of quercetin from OB extract and rutin. Taken together, OB extract with a high content of quercetin glucosides is an interesting source for the application of quercetin to ruminants.

Chimonanthus nitens Oliv. leaf extract exerting anti-hyperglycemic activity by modulating GLUT4 and GLUT1 in the skeletal muscle of a diabetic mouse model

The present study aimed to explore the potent molecular mechanisms behind the hypoglycemic effect of Chimonanthus nitens Oliv. leaf extract (COE) in combination with a high-glucose-fat diet-fed and streptozotocin-induced diabetic mouse model. COE (50 and 200 mg per kg body weight per day) was given to the diabetic-model mice by intragastric administration for 4 weeks. It was found that the fasting blood glucose level (FBG), serum insulin level (FINS), and insulin sensitivity index (ISI) were significantly improved in the COE-treated diabetic-model mice. Glucose metabolism genes expression analysis of the skeletal muscle showed that COE exerted a glucose-lowering effect through the following two ways: on the one hand, COE enhanced insulin sensitivity by upregulating the transcription level of GLUT4, and in addition, it enhanced the insulin signaling pathway to promote the translocation of GLUT4 and upregulated thermogenesis genes expression, including PGC-1α and UCP-1; while on the other hand, GLUT1 expression was also increased in both the transcription and translation levels in the presence of COE. These two ways may result in promoting glucose uptake in skeletal muscle, thus leading to the reduction of the blood glucose level. The results suggested that COE ameliorated hyperglycemia in the diabetic-model mice through regulating glucose transporters, and then was likely to increase glucose uptake, which provided more evidence for applying COE to treat anti-hyperglycemia.

The acute effects of citrus flavanones on the metabolism of glycogen and monosaccharides in the isolated perfused rat liver

Citrus flavanones are often linked to their antihyperglycemic properties. This effect may be in part due to the inhibition of hepatic gluconeogenesis through different mechanisms. One of the possible mechanisms appears to be impairment of oxidative phosphorylation, which may also interfere with glycogen metabolism. Based on these facts, the purpose of the present study was to investigate the effects of three citrus flavanones on glycogenolysis in the isolated perfused rat liver. Hesperidin, hesperetin, and naringenin stimulated glycogenolysis and glycolysis from glycogen with concomitant changes in oxygen uptake. At higher concentrations (300 μM), hesperetin and naringenin clearly altered fructose and glucose metabolism, whereas hesperidin exerted little to no effects. In subcellular fractions hesperetin and naringenin inhibited the activity of glucose 6-phosphatase and glucokinase and the mitochondrial respiration linked to ADP phosphorylation. Hesperetin and naringenin also inhibited the transport of glucose into the cell. At a concentration of 300 μM, the glucose influx rate inhibition was 83% and 43% for hesperetin and naringenin, respectively. Hesperidin was the less active among the assayed citrus flavanones, indicating that the rutinoside moiety noticeably decrease the activity of these compounds. The effects on glycogenolysis and fructolysis were mainly consequence of an impairment on mitochondrial energy metabolism. The increased glucose release, due to the higher glycogenolysis, together with glucose transport inhibition is the opposite of what is expected for antihyperglycemic agents.

Natural products, an important resource for discovery of multitarget drugs and functional food for regulation of hepatic glucose metabolism

Imbalanced hepatic glucose homeostasis is one of the critical pathologic events in the development of metabolic syndromes (MSs). Therefore, regulation of imbalanced hepatic glucose homeostasis is important in drug development for MS treatment. In this review, we discuss the major targets that regulate hepatic glucose homeostasis in human physiologic and pathophysiologic processes, involving hepatic glucose uptake, glycolysis and glycogen synthesis, and summarize their changes in MSs. Recent literature suggests the necessity of multitarget drugs in the management of MS disorder for regulation of imbalanced glucose homeostasis in both experimental models and MS patients. Here, we highlight the potential bioactive compounds from natural products with medicinal or health care values, and focus on polypharmacologic and multitarget natural products with effects on various signaling pathways in hepatic glucose metabolism. This review shows the advantage and feasibility of discovering multicompound-multitarget drugs from natural products, and providing a new perspective of ways on drug and functional food development for MSs.

Chimonanthus nitens Oliv. leaf extract exerting anti-hyperglycemic activity by modulating GLUT4 and GLUT1 in the skeletal muscle of a diabetic mouse model

COE supplementation ameliorated hyperglycemia via modulating glucose transporters of the skeletal muscle.

The Effects of Oral Quercetin Supplementation on Splanchnic Glucose Metabolism in 1-Week-Old Calves Depend on Diet after Birth

BACKGROUND

Inadequate colostrum supply results in insufficient intake of macronutrients and bioactive factors, thereby impairing gastrointestinal development and the maturation of glucose metabolism in neonatal calves. The flavonoid quercetin has been shown to have health-promoting properties, including effects in diabetic animals. However, quercetin interacts with intestinal glucose absorption and might therefore exert negative effects in neonates.

OBJECTIVE

We evaluated the interaction between neonatal diet and quercetin feeding on splanchnic glucose metabolism in neonatal calves.

METHODS

Calves (n = 28) were assigned to 4 groups and fed either colostrum or a milk-based formula on days 1 and 2 and supplemented daily with 148 μmol quercetin aglycone/kg body weight [colostrum with quercetin (CQ+)/formula with quercetin (FQ+)] or without this substance [colostrum without quercetin (CQ-)/formula with quercetin (FQ-)] from days 2-8. From day 3 onward, all calves received milk replacer. A xylose absorption test was performed on day 3, and on day 7, blood samples were collected to study glucose first-pass uptake after [(13)C6]-glucose feeding and intravenous [6,6-(2)H2]-glucose bolus injection. Plasma concentrations of metabolites and hormones were measured by taking additional blood samples. A biopsy specimen of the liver was harvested on day 8 to measure the mRNA expression of gluconeogenic enzymes.

RESULTS

Higher postprandial plasma concentrations of glucose, lactate, urea, adrenaline, noradrenaline, insulin, and glucagon on day 7 in colostrum-fed calves indicate that metabolic processes were stimulated. Postabsorptive xylose and glucose plasma concentrations each increased by an additional 26%, and splanchnic glucose turnover decreased by 35% in colostrum-fed calves, suggesting improved glucose absorption and lower splanchnic glucose utilization in colostrum-fed calves. Quercetin supplementation resulted in higher noradrenaline concentrations and enhanced peak absorption and oxidation of [(13)C6]-glucose by 10%. Liver mitochondrial phosphoenolpyruvate carboxykinase mRNA abundance was reduced by 34% in colostrum-deprived calves.

CONCLUSIONS

Feeding colostrum during the first 2 d of life is crucial for maturation of splanchnic glucose metabolism in calves. Supplementing quercetin improves gastrointestinal absorption capacity, particularly in colostrum-deprived calves.

Effects of eugenol-reduced clove extract on glycogen phosphorylase b and the development of diabetes in db/db mice

We found that the 50% aqueous EtOH extract of clove (Syzygium aromaticum) had potent dose-dependent inhibitory activity toward glycogen phosphorylase b and glucagon-stimulated glucose production in primary rat hepatocytes. Among the components, eugeniin inhibited glycogen phosphorylase b and glucagon-stimulated glucose production in primary rat hepatocytes, with IC50 values of 0.14 and 4.7 μM, respectively. In sharp contrast, eugenol showed no significant inhibition toward glycogen phosphorylase b, even at a concentration of 400 μM. Eugenol-reduced clove extracts (erCE) were prepared and when fed to a db/db mouse they clearly suppressed the blood glucose and HbA1c levels. Furthermore, plasma triglyceride and non-esterified fatty acid levels in 5% and 10% erCE-fed db/db mice were significantly lowered, compared with control db/db mice without erCE supplementation. These results suggested that dietary supplementation with the erCE could beneficially modify glucose and lipid metabolism and contribute to the prevention of the progress of hyperglycemia and metabolic syndrome.

Citrus flavanones affect hepatic fatty acid oxidation in rats by acting as prooxidant agents

Citrus flavonoids have a wide range of biological activities and positive health effects on mammalian cells because of their antioxidant properties. However, they also act as prooxidants and thus may interfere with metabolic pathways. The purpose of this work was to evaluate the effects of three citrus flavanones, hesperidin, hesperetin, and naringenin, on several parameters linked to fatty acid oxidation in mitochondria, peroxisomes, and perfused livers of rats. When exogenous octanoate was used as substrate, hesperetin and naringenin reduced the mitochondrial NADH/NAD⁺ ratio and stimulated the citric acid cycle without significant changes on oxygen uptake or ketogenesis. When fatty acid oxidation from endogenous sources was evaluated, hesperetin and naringenin strongly reduced the mitochondrial NADH/NAD⁺ ratio. They also inhibited both oxygen uptake and ketogenesis and stimulated the citric acid cycle. Hesperidin, on the other hand, had little to no effect on these parameters. These results confirm the hypothesis that citrus flavanones are able to induce a more oxidised state in liver cells, altering parameters related to hepatic fatty acid oxidation. The prooxidant effect is most likely a consequence of the ability of these substances to oxidise NADH upon production of phenoxyl radicals in the presence of peroxidases and hydrogen peroxide.

Bioavailability of the flavonol quercetin in cows after intraruminal application of quercetin aglycone and rutin

The bioavailability of quercetin has been intensively investigated in monogastric species, but knowledge about its bioavailability in ruminants does not exist. Thus, the aim of the present study was to determine the bioavailability of quercetin in nonlactating cows equipped with indwelling catheters placed in one jugular vein after intraruminal and additionally after i.v. application, respectively. Quercetin was administered intraruminally in equimolar amounts, either in the aglycone form or as its glucorhamnoside rutin, each at 2 dosages [10 and 50 mg of quercetin/kg of body weight (BW)]. In a second trial, 0.8 mg of quercetin aglycone/kg of BW was applied i.v. Blood samples were drawn 0.5, 1, 1.5, 2, 2.5, 3, 4, 6, 8, 12, and 24 h after intraruminal application and every 5 min (first hour), every 10 min(second hour), and at 3 and 6h after i.v. bolus application, respectively. Quercetin and quercetin metabolites with an intact flavonol structure (isorhamnetin, tamarixetin, and kaempferol) in plasma samples were analyzed by HPLC with fluorescence detection. After intraruminal application of quercetin and rutin, respectively, quercetin and its methylated (isorhamnetin, tamarixetin) and dehydroxylated (kaempferol) derivatives were present in plasma mainly as conjugated forms, whereas free quercetin and its derivatives were scarcely detected. For rutin, the relative bioavailability of total flavonols (sum of conjugated and nonconjugated quercetin and its conjugated and nonconjugated derivatives after intake of 50 mg/kg of BW) was 767.3% compared with quercetin aglycone (100%). Absolute bioavailability of total flavonols was only 0.1 and 0.5% after quercetin aglycone and rutin applications, respectively. Our data demonstrate that bioavailability of quercetin from rutin is substantially higher compared with that from quercetin aglycone in cows after intraruminal (or oral) application, unlike in monogastric species.

Transcriptomic and metabonomic profiling reveal synergistic effects of quercetin and resveratrol supplementation in high fat diet fed mice

Dietary quercetin and resveratrol have been frequently used in treating various diseases, but the underlying mechanisms are not entirely clear. Here, we report combined transcriptomic and metabonomic profiling that showed that the combined supplementation with quercetin and resveratrol produced synergistic effects on a high-fat diet-induced metabolic phenotype in mice. Histological and phenotypic improvements in serum and hepatic total cholesterol, insulin, fasting blood glucose, and HbA1c were also observed in mice receiving combined quercetin and resveratrol supplementation. This combined quercetin and resveratrol supplementation resulted in significant restoration of gene sets in functional pathways of glucose/lipid metabolism, liver function, cardiovascular system, and inflammation/immunity, which were altered by high fat diet feeding. The integration of transcriptomic and metabonomic data indicated quercetin and resveratrol supplementation enhanced processes of glycolysis and fatty acid oxidation, as well as suppressed gluconeogenesis. These alterations discovered at both the transcriptional and metabolic levels highlight the significance of combined "omics" platforms for elucidating mechanistic pathways altered by dietary polyphenols, such as quercetin and resveratrol, in a synergistic manner.

Quercetin treatment changes fluxes in the primary metabolism and increases culture longevity and recombinant α₁-antitrypsin production in human AGE1.HN cells

Addition of the flavonoid quercetin to cultivations of the α(1)-antitrypsin (A1AT) producing human AGE1.HN.AAT cell line resulted in alterations of the cellular physiology and a remarkable improvement of the overall performance of these cells. In a first screening in 96-well plate format, toxicity and the effect of quercetin on the lactate/glucose ratio was analyzed. It was found that quercetin treatment reduced the lactate/glucose ratio dose dependently. An increase in culture longevity, viable cell density (160% of control), and A1AT concentration (from 0.39 g/L in the control to 0.76 g/L with quercetin, i.e., 195% of the control) was observed in batch cultivation with 10 μM quercetin compared to the control. A detailed analysis of quercetin effects on primary metabolism revealed dose-dependent alterations in metabolic fluxes. Quercetin addition resulted in an improved channeling of pyruvate into the mitochondria accompanied by reduced waste product formation and stimulation of TCA cycle activity. The observed changes in cellular physiology can be explained by different properties of quercetin and its metabolites, e.g., inhibition of specific enzymes, stimulation of oxidation of cytoplasmic, and mitochondrial NADH resulting in reduced NADH/NAD(+) ratio, and cytoprotective activity. The present study shows that the addition of specific effectors to the culture medium represents a promising strategy to improve the cellular metabolic phenotype and the production of biopharmaceuticals. The provided results contribute, additionally, to an improved understanding of quercetin action on the metabolism of human cells in a general physiological context.

Prooxidant activity of fisetin: effects on energy metabolism in the rat liver

Flavonols, which possess the B-catechol ring, as quercetin, are capable of producing o-hemiquinones and to oxidize NADH in a variety of mammalian cells. The purpose of this study was to investigate whether fisetin affects the liver energy metabolism and the mitochondrial NADH to NAD+ ratio. The action of fisetin on hepatic energy metabolism was investigated in the perfused rat liver and isolated mitochondria. In isolated mitochondria, fisetin decreased the respiratory control and ADP/O ratios with the substrates α-ketoglutarate and succinate. In the presence of ADP, respiration of isolated mitochondria was inhibited with both substrates, indicating an inhibitory action on the ATP-synthase. The stimulation of the ATPase activity of coupled mitochondria and the inhibition of NADH-oxidase activity pointed toward a possible uncoupling action and the interference of fisetin with mitochondrial energy transduction mechanisms. In livers from fasted rats, fisetin inhibited ketogenesis from endogenous sources. The β-hydroxybutyrate/ acetoacetate ratio, which reflects the mitochondrial NADH/NAD+ redox ratio, was also decreased. In addition, fisetin (200 μM) increased the production of (14)CO2 from exogenous oleate. The results of this investigation suggest that fisetin causes a shift in the mitochondrial redox potential toward a more oxidized state with a clear predominance of its prooxidant activity.

Structure-activity relationships of flavonoids as potential inhibitors of glycogen phosphorylase

Flavonoids are ubiquitous components in vegetables, fruits, tea, and wine. Therefore, they are often consumed in large quantities in our daily diet. Several flavonoids have been shown to have potential as antidiabetic agents. In the present study, we focused on inhibition of glycogen phosphorylase (GP) by flavonoids. 6-Hydroxyluteolin, hypolaetin, and quercetagetin were identified as good inhibitors of dephosphorylated GP (GPb), with IC 50 values of 11.6, 15.7, and 9.7 microM, respectively. Furthermore, a structure-activity relationship study revealed that the presence of the 3' and 4' OH groups in the B-ring and double bonds between C2 and C3 in flavones and flavonols are important factors for enzyme recognition and binding. Quercetagetin inhibited GPb in a noncompetitive manner, with a K i value of 3.5 microM. Multiple inhibition studies by Dixon plots suggested that quercetagetin binds to the allosteric site. In primary cultured rat hepatocytes, quercetagetin and quercetin suppressed glucagon-stimulated glycogenolysis, with IC 50 values of 66.2 and 68.7 microM, respectively. These results suggested that as a group of novel GP inhibitors, flavonoids have potential to contribute to the protection or improvement of control of diabetes type II.

Antidiabetic effect of Phlomis anisodonta: effects on hepatic cells lipid peroxidation and antioxidant enzymes in experimental diabetes

The present study investigated the effects of aerial parts of Phlomis anisodonta methanolic extract (PAE) on streptozotocin (STZ)-induced diabetic rats by measuring fasting blood glucose, serum insulin, change in body weight, ferric reducing antioxidant power (FRAP), lipid peroxidation (LPO), and liver antioxidant enzymes including superoxide dismutase (SOD), catalase (CAT) and glutathione peroxidase (GPx). Male Wistar rats were randomly divided into six groups of six animals. Treatment of diabetic rats with oral administration of PAE at doses of 100, 200 and 400 mg kg(-1) for 10 days resulted in a significant reduction in fasting blood glucose, and an increase in serum insulin levels in comparison with diabetic control group. PAE also protected rats from STZ-induced loss in body weight. Hepatic FRAP increased and LPO in diabetic rats decreased after treatment by PAE at doses of 200 and 400 mg kg(-1). PAE-treated diabetic rats at three doses indicated a significant increase in hepatic SOD, CAT, and GPx activities. These results suggest that PAE is beneficial in the control of diabetes by reduction of blood glucose and increasing insulin levels and combating oxidative stress by activation of hepatic antioxidant enzymes.