Grape pomace extract supplementation activates FNDC5/irisin in muscle and promotes white adipose browning in rats fed a high-fat diet.

Irisin is a myokine regulated by peroxisome proliferator-activated receptor gamma co-activator-1α (PGC-1α) in the exercising skeletal muscle and released into the bloodstream after cleavage of FNDC5. Circulating irisin can up-regulate UCP-1 expression in white adipose tissue (WAT) promoting the formation of brown-like adipocytes. The aim of this study was to evaluate if supplementation with a grape pomace extract (GPE) could activate the FNDC5/irisin pathway via PGC-1α in rats fed a high fat diet (HFD). For this purpose we characterized the activation of: i. the FNDC5/irisin pathway and AMPK in skeletal muscle and ii. proteins involved in the formation of brown-like cells in epididymal WAT (eWAT). Consumption of the GPE activated the FNDC5/irisin pathway, increased AMPK phosphorylation in skeletal muscle and enhanced irisin plasma levels. In eWAT, the GPE increased the level of proteins involved in WAT browning, i.e. PGC-1α, PPARγ, PRDM16 and UCP-1. The GPE also prevented HFD-induced adipocyte hypertrophy and systemic insulin resistance. Consistently, in L6 myotubes, (-)-epicatechin (EC), a flavonoid abundant in the GPE, prevented palmitate-mediated downregulation of FNDC5/irisin protein expression and secretion, in part via PGC-1α activation. Consumption of the GPE, a winemaking residue rich in bioactive compounds, could be a beneficial strategy to counteract the adverse effects of Western style diets through the promotion of WAT browning.

Phenolic metabolites in plasma and tissues of rats fed with a grape pomace extract as assessed by liquid chromatography-tandem mass spectrometry.

Grape pomace extract (GPE) is a rich and relatively low-cost source of phenolic compounds. However, little is known about the main GPE metabolites in mammals, which could help explain the observed health-promoting effects. This study investigated the presence of parent compounds from flavanol, flavonol and stilbene families and their metabolites in rat plasma and tissues after an acute intake of GPE in doses of 300 and 600 mg kg/body weight. The measurement of free compounds and their metabolites was performed by ultra-high performance liquid chromatography-tandem mass spectrometry (UHPLC-MS/MS). Results showed the presence of epicatechin, epicatechin methyl-glucuronide, epicatechin methyl-sulphate, catechin, catechin-glucuronide, quercetin methyl-glucuronide, resveratrol-3-glucuronide, resveratrol-4-glucuronide and resveratrol-3-sulphate in plasma, which was dose dependent. The most abundant measured compound in plasma was epicatechin-glucuronide. The presence of glucuronidated and methyl-glucuronidated forms of catechin were observed in the liver at both doses, while epicatechin-glucuronide and methyl-glucuronide were detected only upon intake of 600 mg GPE/kg body weight. At this dose epicatechin-glucuronide and methyl-glucuronide were also detected in muscle, and catechin methyl-glucuronide in adipose tissue. Results show the main GPE metabolites present in rat tissues after oral consumption, contributing to better understand the health benefits of GPE and its potential utilization as a functional ingredient.

Grape pomace reduced reperfusion arrhythmias in rats with a high-fat-fructose diet.

Metabolic syndrome (MetS) is a risk factor for sudden cardiac death in humans, but animal models are needed for the study of this association. Grape pomace (GP), obtained from the winemaking process, contains phenolic compounds with potential cardioprotective effects. The aim of this study was to evaluate if a high-fat-fructose (HFF) diet facilitates the occurrence of arrhythmias during the reperfusion, and if a GP supplementation could counteract these effects. Wistar rats were fed with control (Ctrl), HFF diet and HFF plus GP (1 g kg-1 day-1) for six weeks. The HFF diet induces characteristic features of MetS (higher systolic blood pressure, dyslipidemia and insulin resistance) which was attenuated by GP supplementation. In addition, HFF induced increased reperfusion arrhythmias that were reduced upon GP supplementation. GP also reduced the non-phosphorylated form of connexin-43 (Cx43) while enhancing heart p-AKT and p-eNOS protein levels and reducing Nox4 levels enhanced by the HFF diet, indicating that GP may increase NO bioavailability in the heart. We found a murine model of MetS with increased arrhythmogenesis and translational value. Furthermore, GP prevents diet-induced heart dysfunction and metabolic alterations. These results highlight the potential utilization of winemaking by-products containing significant amounts of bioactive compounds to prevent/attenuate MetS-associated cardiovascular pathologies.

(-)-Epicatechin reduces blood pressure increase in high-fructose-fed rats: effects on the determinants of nitric oxide bioavailability.

This work investigated the blood pressure (BP)-lowering effect of the flavanol (-)-epicatechin in a model of metabolic syndrome. Rats were fed a regular chow diet without (Control) or with 10% (w/v) fructose in the drinking water (high fructose, HF) for 8 weeks. A subgroup of the HF-fed rats was supplemented with (-)-epicatechin 20 mg/kg body weight (HF-EC). Dietary (-)-epicatechin reverted the increase in BP caused by the fructose treatment. In aorta, superoxide anion production and the expression of the NADPH oxidase (NOX) subunits p47(phox) and p22(phox) were enhanced in the HF-fed rats. The increase was prevented by (-)-epicatechin. Similar profile was observed for NOX4 expression. The activity of aorta nitric oxide synthase (NOS) was increased in the HF group and was even higher in the HF-EC rats. These effects were paralleled by increased endothelial NOS phosphorylation at the activation site Ser1177. Among the more relevant mitogen-activated protein kinase pathways in vascular tissue, c-Jun-N-terminal kinase was shown to be activated in the aorta of the HF-fed rats, and (-)-epicatechin supplementation mitigated this activation. Thus, the results suggest that dietary (-)-epicatechin supplementation prevented hypertension in HF-fed rats, decreasing superoxide anion production and elevating NOS activity, favoring an increase in NO bioavailability.

(-)-Epicatechin mitigates high-fructose-associated insulin resistance by modulating redox signaling and endoplasmic reticulum stress.

We investigated the capacity of dietary (-)-epicatechin (EC) to mitigate insulin resistance through the modulation of redox-regulated mechanisms in a rat model of metabolic syndrome. Adolescent rats were fed a regular chow diet without or with high fructose (HFr; 10% w/v) in drinking water for 8 weeks, and a group of HFr-fed rats was supplemented with EC in the diet. HFr-fed rats developed insulin resistance, which was mitigated by EC supplementation. Accordingly, the activation of components of the insulin signaling cascade (insulin receptor, IRS1, Akt, and ERK1/2) was impaired, whereas negative regulators (PKC, IKK, JNK, and PTP1B) were upregulated in the liver and adipose tissue of HFr rats. These alterations were partially or totally prevented by EC supplementation. In addition, EC inhibited events that contribute to insulin resistance: HFr-associated increased expression and activity of NADPH oxidase, activation of redox-sensitive signals, expression of NF-κB-regulated proinflammatory cytokines and chemokines, and some sub-arms of endoplasmic reticulum stress signaling. Collectively, these findings indicate that EC supplementation can mitigate HFr-induced insulin resistance and are relevant for defining interventions that can prevent/mitigate MetS-associated insulin resistance.

Flavonoids and metabolic syndrome.

Increasing evidence indicates that several mechanisms, associated or not with antioxidant actions, are involved in the effects of flavonoids on health. Flavonoid-rich beverages, foods, and extracts, as well as pure flavonoids are studied for the prevention and/or amelioration of metabolic syndrome (MS) and MS-associated diseases. We summarize evidence linking flavonoid consumption with the risk factors defining MS: obesity, hypertriglyceridemia, hypercholesterolemia, hypertension, and insulin resistance. Nevertheless, a number of molecular mechanisms have been identified; the effects of flavonoids modifying major endpoints of MS are still inconclusive. These difficulties are explained by the complex relationships among the risk factors defining MS, the multiple biological targets controlling these risk factors, and the high number of flavonoids (including their metabolites) present in the diet and potentially responsible for the in vivo effects. Consequently, extensive basic and clinical research is warranted to assess the final relevance of flavonoids for MS.