Resveratrol improves insulin signaling in a tissue-specific manner under insulin-resistant conditions only: in vitro and in vivo experiments in rodents

The pharmacology of resveratrol in animals and humans

In addition to thousands of research papers related to resveratrol (RSV), approximately 300 review articles have been published. Earlier research tended to focus on pharmacological activities of RSV related to cardiovascular systems, inflammation, and carcinogenesis/cancer development. More recently, the horizon has been broadened by exploring the potential effect of RSV on the aging process, diabetes, neurological dysfunction, etc. Herein, we primarily focus on the in vivo pharmacological effects of RSV reported over the past 5 years (2009-2014). In addition, recent clinical intervention studies performed with resveratrol are summarized. Some discrepancies exist between in vivo studies with animals and clinical studies, or between clinical studies, which are likely due to disparate doses of RSV, experimental settings, and subject variation. Nevertheless, many positive indications have been reported with mammals, so it is reasonable to advocate for the conduct of more definitive clinical studies. Since the safety profile is pristine, an added advantage is the use of RSV as a dietary supplement. This article is part of a Special Issue entitled: Resveratrol: Challenges in translating pre-clinical findings to improved patient outcomes.

Resveratrol mimics insulin activity in the adipogenic commitment of human bone marrow mesenchymal stromal cells

Bone marrow mesenchymal stromal cells (BM-MSCs) are multipotent cells capable of differentiating toward osteoblatic and adipocytic phenotypes. BM-MSCs play several key roles including bone remodeling, establishment of hematopoietic niche and immune tolerance induction. Here, we investigated the effect of resveratrol (RSV), a therapeutically promising natural polyphenol, on the commitment of human BM-MSCs primary cultures. Cell differentiation was evaluated by means of morphological analysis, specific staining and expression of osteogenic and adipocytic master genes (Runx-2, PPARγ). To maintain BM-MSC multipotency, all experiments were performed on cells at very early passages. At any concentration RSV, added to standard medium, did not affect the phenotype of confluent BM-MSCs, while, when added to osteogenic or adipogenic medium, 1 μM RSV enhances the differentiation toward osteoblasts or adipocytes, respectively. Conversely, the addition of higher RSV concentration (25 μM) to both differentiation media resulted exclusively in BM-MSCs adipogenesis. Surprisingly, the analysis of RSV molecular effects demonstrated that the compound completely substitutes insulin, a key component of adipogenic medium. We also observed that RSV treatment is associated to enhanced phosphorylation of CREB, a critical effector of insulin adipogenic activity. Finally, our observations contribute to the mechanistic elucidation of the well-known RSV positive effect on insulin sensitivity and type 2 diabetes mellitus.

Metabolic effects of resveratrol: addressing the controversies

Resveratrol, a polyphenol found in a number of plant-based foods such as red wine, has received a great deal of attention for its diverse array of healthful effects. Beneficial effects of resveratrol are diverse; they include improvement of mitochondrial function, protection against obesity and obesity-related diseases such as type-2 diabetes, suppression of inflammation and cancer cell growth and protection against cardiovascular dysfunction, just to name a few. Investigations into the metabolic effects of resveratrol are furthest along and now include a number of clinical trials, which have yielded mixed results. There are a number of controversies surrounding resveratrol that have not been resolved. Here, we will review these controversies with particular emphasis on its mechanism of metabolic action and how lessons from resveratrol may help develop therapies that harness the effects of resveratrol but without the undesirable properties of resveratrol.

Thoughts on the progression of type 2 diabetes drug discovery

INTRODUCTION

Type 2 diabetes is a growing epidemic in need of effective treatments. There has been research in recent years involving numerous drug therapies and targets. This article is a review of the progress thus far in type 2 diabetes drug discovery.

AREAS COVERED

This editorial reviews type 2 diabetes drug discovery mainly over the past decade through a literature search of PubMed. Furthermore, the author reviews several avenues of research, including the expansion of knowledge for possible drug therapies involving the β-cell, such as targeting its proliferation, function and apoptosis. This knowledge has led to possible drug therapies in clinical trials, particularly insulin secretagogues that are glucose-dependent. Other areas of research in type 2 diabetes drug discovery discussed by the author relate to the new frontier of genome-wide association studies (GWAS), the challenges of oral insulin development and drug targets of inflammation. The author also reviews sirtuin activators and resveratrol, especially its relationship to insulin resistance.

EXPERT OPINION

The progression of type 2 diabetes drug discovery holds much promise, but many agents are in the nascent stages of investigation and human trials proving efficacy are needed. Furthermore, basic science research into some of these agents may need to be further elucidated before clinical trials can be initiated.

Resveratrol and diabetes: from animal to human studies

Diabetes mellitus is a serious disease affecting about 5% of people worldwide. Diabetes is characterized by hyperglycemia and impairment in insulin secretion and/or action. Moreover, diabetes is associated with metabolic abnormalities and serious complications. Resveratrol is a natural, biologically active polyphenol present in different plant species and known to have numerous health-promoting effects in both animals and humans. Anti-diabetic action of resveratrol has been extensively studied in animal models and in diabetic humans. In animals with experimental diabetes, resveratrol has been demonstrated to induce beneficial effects that ameliorate diabetes. Resveratrol, among others, improves glucose homeostasis, decreases insulin resistance, protects pancreatic β-cells, improves insulin secretion and ameliorates metabolic disorders. Effects induced by resveratrol are strongly related to the capability of this compound to increase expression/activity of AMPK and SIRT1 in various tissues of diabetic subjects. Moreover, anti-oxidant and anti-inflammatory effects of resveratrol were shown to be also involved in its action in diabetic animals. Preliminary clinical trials show that resveratrol is also effective in type 2 diabetic patients. Resveratrol may, among others, improve glycemic control and decrease insulin resistance. These results show that resveratrol holds great potential to treat diabetes and would be useful to support conventional therapy. This article is part of a Special Issue entitled: Resveratrol: Challenges in translating pre-clincial findigns to improved patient outcomes.

An examination of resveratrol's mechanisms of action in human tissue: impact of a single dose in vivo and dose responses in skeletal muscle ex vivo

The current study tested the hypothesis that a single, moderate dose of RSV would activate the AMPK/SIRT1 axis in human skeletal muscle and adipose tissue. Additionally, the effects of RSV on mitochondrial respiration in PmFBs were examined. Eight sedentary men (23.8±2.4 yrs; BMI: 32.7±7.1) reported to the lab on two occasions where they were provided a meal supplemented with 300 mg of RSV or a placebo. Blood samples, and a muscle biopsy were obtained in the fasted state and again, with the addition of an adipose tissue biopsy, two hours post-prandial. The effect of RSV on mitochondrial respiration was examined in PmFBs taken from muscle biopsies from an additional eight men (23.4±5.4 yrs; BMI: 24.4±2.8). No effect of RSV was observed on nuclear SIRT1 activity, acetylation of p53, or phosphorylation of AMPK, ACC or PKA in either skeletal muscle or adipose tissue. A decrease in post absorptive insulin levels was accompanied by elevated skeletal muscle phosphorylation of p38 MAPK, but no change in either skeletal muscle or adipose tissue insulin signalling. Mitochondrial respiration in PmFBs was rapidly inhibited by RSV at 100–300 uM depending on the substrate examined. These results question the efficacy of a single dose of RSV at altering skeletal muscle and adipose tissue AMPK/SIRT1 activity in humans and suggest that RSV mechanisms of action in humans may be associated with altered cellular energetics resulting from impaired mitochondrial ATP production.

Resveratrol protects against polychlorinated biphenyl-mediated impairment of glucose homeostasis in adipocytes

Resveratrol (RSV) is a plant polyphenol that exhibits several favorable effects on glucose homeostasis in adipocytes. Recent studies from our laboratory demonstrated that coplanar polychlorinated biphenyls (PCBs) that are ligands of the aryl hydrocarbon receptor impair glucose homeostasis in mice. PCB-induced impairment of glucose homeostasis was associated with augmented expression of inflammatory cytokines in adipose tissue, a site for accumulation of lipophilic PCBs. This study determined if RSV protects against PCB-77 induced impairment of glucose disposal in vitro and in vivo and if these beneficial effects are associated with enhanced nuclear factor erythoid 2-related factor 2 (Nrf2) signaling in adipose tissue. PCB-77 increased oxidative stress and abolished insulin stimulated 2-deoxy-d-glucose uptake in 3 T3-L1 adipocytes. These effects were restored by RSV, which resulted in a concentration-dependent increase in NAD(P)H:quinone oxidoreductase 1 (NQO1), the downstream target of Nrf2 signaling. We quantified glucose and insulin tolerance and components of Nrf2 and insulin signaling cascades in adipose tissue of male C57BL/6 mice administered vehicle or PCB-77 (50 mg/kg) and fed a diet with or without resVida (0.1%, or 160 mg/kg per day). PCB-77 impaired glucose and insulin tolerance, and these effects were reversed by RSV. PCB-77 induced reductions in insulin signaling in adipose tissue were also abolished by RSV, which increased NQO1 expression. These results demonstrate that coplanar PCB-induced impairment of glucose homeostasis in mice can be prevented by RSV, potentially through stimulation of Nrf2 signaling and enhanced insulin stimulated glucose disposal in adipose tissue.

Resveratrol and cancer: focus on in vivo evidence

Resveratrol is a naturally occurring polyphenol that provides a number of anti-aging health benefits including improved metabolism, cardioprotection, and cancer prevention. Much of the work on resveratrol and cancer comes from in vitro studies looking at resveratrol actions on cancer cells and pathways. There are, however, comparatively fewer studies that have investigated resveratrol treatment and cancer outcomes in vivo, perhaps limited by its poor bioavailability when taken orally. Although research in cell culture has shown promising and positive effects of resveratrol, evidence from rodents and humans is inconsistent. This review highlights the in vivo effects of resveratrol treatment on breast, colorectal, liver, pancreatic, and prostate cancers. Resveratrol supplementation in animal models of cancer has shown positive, neutral as well as negative outcomes depending on resveratrol route of administration, dose, tumor model, species, and other factors. Within a specific cancer type, there is variability between studies with respect to strain, age, and sex of animal used, timing and method of resveratrol supplementation, and dose of resveratrol used to study cancer endpoints. Together, the data suggest that many factors need to be considered before resveratrol can be used for human cancer prevention or therapy.

Antidiabetic effects of Mukia maderaspatana and its phenolics: an in vitro study on gluconeogenesis and glucose uptake in rat tissues

CONTEXT

Traditional medicine is used by over 60% of the world's population for health care. Mukia maderaspatana (L.) M. Roem. (Cucurbitaceae) (Mukia) is extensively used in folklore medicine as an antidiabetic plant. It is rich in phenolics that contribute to its medicinal properties.

OBJECTIVE

Mukia extract and phenolics such as quercetin and phloroglucinol are investigated for their in vitro antidiabetic activity.

MATERIALS AND METHODS

Quercetin, phloroglucinol, and methanol extract of the dried whole plant (0.25 and 0.5 mg/ml) were studied for the inhibition of gluconeogenesis in rat liver slices and glucose uptake in isolated rat hemi-diaphragm (50 and 100 µg/ml). Phenolics of Mukia were analyzed by HPLC.

RESULTS AND DISCUSSION

Glucose (1.2 mg/g/h) was synthesized from pyruvate and the synthesis was completely inhibited by insulin (1 U/ml). Quercetin at 0.25 and 0.5 mg/ml caused 65% and 89% inhibition (0.42 mg/g/h and 0.13 mg/g/h glucose). Addition of insulin did not increase inhibition. Phloroglucinol inhibited 100% glucose production with or without insulin. Mukia (0.25 mg/ml) inhibited gluconeogenesis (0.65 mg/g/h) by 45%, and with insulin, inhibition increased to 50% (0.59 mg/g/h). At 0.5 mg/ml, glucose production was stimulated by1.2-fold, but with insulin it was inhibited by 89% (0.13 mg/g/h glucose). Mukia had no effect on glucose uptake, but potentiated the action of insulin mediated glucose uptake (152.82 ± 13.30 mg/dl/g/30 min) compared with insulin control (112.41 ± 9.14 mg/dl/g/30 min) (p < 0.05). HPLC analysis revealed the presence of phenolics.

CONCLUSION

Results provide scientific rationale for the use of Mukia in folk medicine as an antidiabetic nutraceutical.

Resveratrol ameliorates hepatic metaflammation and inhibits NLRP3 inflammasome activation

OBJECTIVE

Resveratrol (RSV) regulates NAD bioavailability and sirtuin-related metabolism, which relates to aging, metabolic syndrome and non-alcoholic fatty liver disease. The purpose of this study was to investigate the effects of resveratrol on hepatic metaflammation in a rodent model of high-fat (HF) diet-induced obesity (DIO).

MATERIALS/METHODS

DIO was induced in a subset of mice given an HF diet (45% kcal fat). After 6weeks of HF diet feeding, RSV was delivered via an osmotic pump for 4weeks. The experimental groups were as follows: 1) lean control fed with a standard diet, 2) HF diet-induced obese control, and 3) HF_RSV (8mg/kg/day). After 4weeks of each treatment, blood and liver tissues were collected and the indices of glucose control, serum and liver triglyceride (TG), sirtuin pathway, inflammation, and NOD-like receptor family, pryin domain containing 3 (NLRP3) inflammasome were analyzed.

RESULTS

Body weight and food intake were not altered by administering resveratrol. Glucose control was impaired, and serum and liver TG levels were increased by the HF diet. Hepatic inflammation was aggravated in mice fed with the HF diet, as shown by the increased levels of the pro-inflammatory markers interleukin-1 (IL-1), IL-6 and tumor necrosis factor-alpha in the liver. However, resveratrol administration significantly improved glucose control, and serum and liver TG contents. Also, resveratrol treatment reduced the levels of the pro-inflammatory markers. These improvements were accompanied by alterations in sirtuin pathway and NLRP3 inflammasome activation.

CONCLUSION

These results demonstrate that resveratrol ameliorates hepatic metaflammation, accompanied by alterations in NLRP3 inflammasome.

Effects of resveratrol in experimental and clinical non-alcoholic fatty liver disease

The prevalence of obesity and related conditions like non-alcoholic fatty liver disease (NAFLD) is increasing worldwide and therapeutic options are limited. Alternative treatment options are therefore intensively sought after. An interesting candidate is the natural polyphenol resveratrol (RSV) that activates adenosinmonophosphate-activated protein kinase (AMPK) and silent information regulation-2 homolog 1 (SIRT1). In addition, RSV has known anti-oxidant and anti-inflammatory effects. Here, we review the current evidence for RSV-mediated effects on NAFLD and address the different aspects of NAFLD and non-alcoholic steatohepatitis (NASH) pathogenesis with respect to free fatty acid (FFA) flux from adipose tissue, hepatic de novo lipogenesis, inadequate FFA β-oxidation and additional intra- and extrahepatic inflammatory and oxidant hits. We review the in vivo evidence from animal studies and clinical trials. The abundance of animal studies reports a decrease in hepatic triglyceride accumulation, liver weight and a general improvement in histological fatty liver changes, along with a reduction in circulating insulin, glucose and lipid levels. Some studies document AMPK or SIRT1 activation, and modulation of relevant markers of hepatic lipogenesis, inflammation and oxidation status. However, AMPK/SIRT1-independent actions are also likely. Clinical trials are scarce and have primarily been performed with a focus on overweight/obese participants without a focus on NAFLD/NASH and histological liver changes. Future clinical studies with appropriate design are needed to clarify the true impact of RSV treatment in NAFLD/NASH patients.

Inhibition of age-related cytokines production by ATGL: a mechanism linked to the anti-inflammatory effect of resveratrol

Ageing is characterized by the expansion and the decreased vascularization of visceral adipose tissue (vAT), disruption of metabolic activities, and decline of the function of the immune system, leading to chronic inflammatory states. We previously demonstrated that, in vAT of mice at early state of ageing, adipocytes mount a stress resistance response consisting in the upregulation of ATGL, which is functional in restraining the production of inflammatory cytokines. Here, we found that, in the late phase of ageing, such an adaptive response is impaired. In particular, 24-months-old mice and aged 3T3-L1 adipocytes display affected expression of ATGL and its downstream PPARα-mediated lipid signalling pathway, leading to upregulation of TNFα and IL-6 production. We show that the natural polyphenol compound resveratrol (RSV) efficiently suppresses the expression of TNFα and IL-6 in an ATGL/PPARα dependent manner. Actually, adipocytes downregulating ATGL do not show a restored PPARα expression and display elevated cytokines production. Overall the results obtained highlight a crucial function of ATGL in inhibiting age-related inflammation and reinforce the idea that RSV could represent a valid natural compound to limit the onset and/or the exacerbation of the age-related inflammatory states.

Resveratrol and clinical trials: the crossroad from in vitro studies to human evidence

Resveratrol (3,5,4’-trihydroxy-trans-stilbene) is a non-flavonoid polyphenol that may be present in a limited number of food-stuffs such as grapes and red wine. Resveratrol has been reported to exert a plethora of health benefits through many different mechanisms of action. This versatility and presence in the human diet have drawn the worldwide attention of many research groups over the past twenty years, which has resulted in a huge output of in vitro and animal (preclinical) studies. In line with this expectation, many resveratrol-based nutraceuticals are consumed all over the world with questionable clinical/scientific support. In fact, the confirmation of these benefits in humans through randomized clinical trials is still very limited. The vast majority of preclinical studies have been performed using assay conditions with a questionable extrapolation to humans, i.e. too high concentrations with potential safety concerns (adverse effects and drug interactions), short-term exposures, in vitro tests carried out with non-physiological metabolites and/or concentrations, etc. Unfortunately, all these hypothesis-generating studies have contributed to increased the number of ‘potential’ benefits and mechanisms of resveratrol but confirmation in humans is very limited. Therefore, there are many issues that should be addressed to avoid an apparent endless loop in resveratrol research. The so-called ‘Resveratrol Paradox’, i.e., low bioavailability but high bioactivity, is a conundrum not yet solved in which the final responsible actor (if any) for the exerted effects has not yet been unequivocally identified. It is becoming evident that resveratrol exerts cardioprotective benefits through the improvement of inflammatory markers, atherogenic profile, glucose metabolism and endothelial function. However, safety concerns remain unsolved regarding chronic consumption of high RES doses, specially in medicated people. This review will focus on the currently available evidence regarding resveratrol’s effects on humans obtained from randomized clinical trials. In addition, we will provide a critical outlook for further research on this molecule that is evolving from a minor dietary compound to a possible multi-target therapeutic drug.

Resveratrol reduces chronic inflammation and improves insulin action in the myocardium of high-fat diet-induced obeserats

OBJECTIVE: To evaluate the effects of resveratrol on insulin signaling and inflammation pathway in the myocardium of high-fat diet-induced obese rats. METHODS: Thirty Wistar rats were divided into a control group (n=10, standard diet), obese group (n=10, high-fat diet), and obese supplemented with resveratrol group (n=10, 20 mg/kg/day) for eight weeks. An insulin tolerance test was performed at the end of the study period "0" (without insulin), 5, 10, 15, 20, 25, and 30 minutes after an intraperitoneal injection of insulin (2 U/kg). Body and epididymal adipose tissue were weighed. Fragments of the myocardium were extracted for Western blot analyses of insulin pathway and proinflammatory molecules. RESULTS: Resveratrol increased the rate of glucose disappearance, phosphorylation of the insulin receptor, insulin receptor substrate 1, and protein kinase B; and reduced expression of tumor necrosis factor alpha and of the molecules involved in proinflammatory signal transduction, namely Ikappa B kinase and nuclear factor kappa B complex. The results also suggest that higher insulin sensitivity and lower levels of proinflammatory molecules occurred regardless of weight and epididymal adipose tissue loss. CONCLUSION: Resveratrol increases insulin action and reduces inflammatory molecules in the myocardium.

Effects of resveratrol on the insulin signaling pathway of obese mice

The present study was conducted to investigate the effects of resveratrol on the insulin signaling pathway in the liver of obese mice. To accomplish this, we administered resveratrol to high fat diet-induced obese mice and examined the levels of protein phosphorylation in the liver using an antibody array. The phosphorylation levels of 10 proteins were decreased in the high fat diet and resveratrol (HFR) fed group relative to the levels in the high fat diet (HF) fed group. In contrast, the phosphorylation levels of more than 20 proteins were increased in the HFR group when compared with the levels of proteins in the HF group. Specifically, the phosphorylation levels of Akt (The308, Tyr326, Ser473) were restored to normal by resveratrol when compared with the levels in the HF group. In addition, the phosphorylation levels of IRS-1 (Ser636/Ser639), PI-3K p85-subunit α/γ(Tyr467/Tyr199), PDK1 (Ser241), GSK-3α (S21) and GSK-3 (Ser9), which are involved in the insulin signaling pathway, were decreased in the HF group, whereas the levels were restored to normal in the HFR group. Overall, the results show that resveratrol restores the phosphorylation levels of proteins involved in the insulin signaling pathway, which were decreased by a high fat diet.

Combination of low dose of the anti-adipogenic agents resveratrol and phenelzine in drinking water is not sufficient to prevent obesity in very-high-fat diet-fed mice

PURPOSE

Resveratrol inhibits lipid accumulation but suffers from limited bioavailability. The anti-depressive agent phenelzine limits adipogenesis in various models of cultured preadipocytes, and this hydrazine derivative also inhibits de novo lipogenesis in mature adipocytes. It was therefore tested whether resveratrol effects on adiposity reduction and glucose tolerance improvement could be reinforced by co-administration with phenelzine.

METHODS

Mice fed a very-high-fat diet (VHFD, 60% calories as fat) were subjected to drinking solution containing low dose of resveratrol (0.003%) and/or 0.02% phenelzine for 12 weeks. Body fat content, glucose tolerance, food and water consumption were checked during treatment while fat depot mass was determined at the end of supplementation. Direct influence of the agents on lipogenesis and glucose uptake was tested in adipocytes.

RESULTS

Epididymal fat depots were reduced in mice drinking phenelzine alone or with resveratrol. No limitation of body weight gain or body fat content was observed in the groups drinking resveratrol or phenelzine, separately or in combination. The altered glucose tolerance and the increased fat body composition of VHFD-fed mice were not reversed by resveratrol and/or phenelzine. Such lack of potentiation between resveratrol and phenelzine prompted us to verify in vitro their direct effects on mouse adipocytes. Both molecules inhibited de novo lipogenesis, but did not potentiate each other at 10 or 100 μM. Only resveratrol inhibited hexose uptake in a manner that was not improved by phenelzine.

CONCLUSIONS

Phenelzine has no interest to be combined with low doses of resveratrol for treating/preventing obesity, when considering the VHFD mouse model.

Synergistic effects of polyphenols and methylxanthines with Leucine on AMPK/Sirtuin-mediated metabolism in muscle cells and adipocytes

The AMPK-Sirt1 pathway is an important regulator of energy metabolism and therefore a potential target for prevention and therapy of metabolic diseases. We recently demonstrated leucine and its metabolite β-hydroxy-β-methylbutyrate (HMB) to synergize with low-dose resveratrol (200 nM) to activate sirtuin signaling and stimulate energy metabolism. Here we show that leucine exerts a direct effect on Sirt1 kinetics, reducing its Km for NAD⁺ by >50% and enabling low doses of resveratrol to further activate the enzyme (p = 0.012). To test which structure elements of resveratrol are necessary for synergy, we assessed potential synergy of structurally similar and dissimilar polyphenols as well as other compounds converging on the same pathways with leucine using fatty acid oxidation (FAO) as screening tool. Dose-response curves for FAO were constructed and the highest non-effective dose (typically 1–10 nM) was used with either leucine (0.5 mM) or HMB (5 µM) to treat adipocytes and myotubes for 24 h. Significant synergy was detected for stilbenes with FAO increase in adipocytes by 60–70% (p<0.05) and in myotubes >2000% (p<0.01). Sirt1 and AMPK activities were stimulated by ∼65% (p<0.001) and ∼50% (p<0.03), respectively. Similarly, hydroxycinnamic acids and derivatives (chlorogenic, cinnamic, and ferulic acids) combined with leucine/HMB increased FAO (300–1300%, p<0.01), AMPK activity (50–150%, p<0.01), and Sirt1 activity (∼70%, p<0.001). In contrast, more complex polyphenol structures, such as ellagic acid and epigallocatechin gallate required higher concentrations (>1 µM) and exhibited little or no synergy. Thus, the six-carbon ring structure bound to a carboxylic group seems to be a necessary element for leucine/HMB synergy with other stilbenes and hydroxycinnamic acids to stimulate AMPK/Sirt1 dependent FAO; these effects occur at concentrations that produce no independent effects and are readily achievable via oral administration.

Regulation of neurons in the dorsal motor nucleus of the vagus by SIRT1

Neurons in the dorsal motor nucleus of the vagus (DMV) play a critical role in the regulation of autonomic functions. Previous studies indicated that central activation of sirtuin 1 (SIRT1) has beneficial effects on homeostasis, most likely via modulation of the autonomic output. Sirtuins are NAD⁺-dependent deacetylases and have been associated with longevity. SIRT1 is one of the best-characterized sirtuins expressed in mammals, and may be involved in the regulation of metabolism. Resveratrol, a SIRT1 activator reduced hyperglycemia likely through activation of vagal output; however, the cellular mechanisms of action have not been determined. In this study, whole-cell patch-clamp electrophysiology on acute brainstem slices was used to test the hypothesis that activation of SIRT1 with resveratrol enhances neurotransmission in DMV neurons. Application of resveratrol increased the frequency of spontaneous excitatory postsynaptic currents (sEPSC). This effect was K_ATP channel-dependent and was prevented with pre-application of SIRT1 inhibitor, EX527. Resveratrol also increased miniature EPSC (mEPSC) frequency without change in amplitude. Furthermore, our data demonstrated that resveratrol regulates excitatory neurotransmission in a PI3 kinase-dependent manner, since wortmannin, a PI3K inhibitor prevented the increase of mEPSC frequency caused by resveratrol. In conclusion, our data demonstrate that resveratrol via SIRT1 increases excitatory neurotransmission to DMV neurons. These observations suggest that activation of SIRT1 may regulate the function of subdiaphragmatic organs through controlling the activity of parasympathetic DMV neurons.

Chemoprevention of nonalcoholic fatty liver disease by dietary natural compounds

Nonalcoholic fatty liver disease (NAFLD) refers to a wide spectrum of liver disease that is not from excess alcohol consumption, but is often associated with obesity, type 2 diabetes, and metabolic syndrome. NAFLD pathogenesis is complicated and involves oxidative stress, lipotoxicity, mitochondrial damage, insulin resistance, inflammation, and excessive dietary fat intake, which increase hepatic lipid influx and de novo lipogenesis and impair insulin signaling, thus promoting hepatic triglyceride accumulation and ultimately NAFLD. Overproduction of proinflammatory adipokines from adipose tissue also affects hepatic metabolic function. Current NAFLD therapies are limited; thus, much attention has been focused on identification of potential dietary substances from fruits, vegetables, and edible plants to provide a new strategy for NAFLD treatment. Dietary natural compounds, such as carotenoids, omega-3-PUFAs, flavonoids, isothiocyanates, terpenoids, curcumin, and resveratrol, act through a variety of mechanisms to prevent and improve NAFLD. Here, we summarize and briefly discuss the currently known targets and signaling pathways as well as the role of dietary natural compounds that interfere with NAFLD pathogenesis.

The Effect of Metformin Treatment on CRBP-I Level and Cancer Development in the Liver of HBx Transgenic Mice

Retinoids regulate not only various cell functions including proliferation and differentiation but also glucose and lipid metabolism. After we observed a marked up-regulation of cellular retinol-binding protein-I (CRBP-I) in the liver of hepatitis B virus x antigen (HBx)-transgenic (HBx Tg) mice which are prone to hepatocellular carcinoma (HCC) and fatty liver, we aimed to evaluate retinoid pathway, including genes for the retinoid physiology, CRBP-I protein expression, and retinoid levels, in the liver of HBx Tg mice. We also assessed the effect of chronic metformin treatment on HCC development in the mice. Many genes involved in hepatic retinoid physiology, including CRBP-I, were altered and the tissue levels of retinol and all-trans retinoic acid (ATRA) were elevated in the liver of HBx Tg mice compared to those of wild type (WT) control mice. CRBP-I protein expression in liver, but not in white adipose tissue, of HBx Tg mice was significantly elevated compared to WT control mice while CRBP-I protein expressions in the liver and WAT of high-fat fed obese and db/db mice were comparable to WT control mice. Chronic treatment of HBx Tg mice with metformin did not affect the incidence of HCC, but slightly increased hepatic CRBP-I level. In conclusion, hepatic CRBP-I level was markedly up-regulated in HCC-prone HBx Tg mice and neither hepatic CRBP-I nor the development of HCC was suppressed by metformin treatment.

Resveratrol improves adipose insulin signaling and reduces the inflammatory response in adipose tissue of rhesus monkeys on high-fat, high-sugar diet

Obesity is associated with a chronic, low-grade, systemic inflammation that may contribute to the development of insulin resistance and type 2 diabetes. Resveratrol, a natural compound with anti-inflammatory properties, is shown to improve glucose tolerance and insulin sensitivity in obese mice and humans. Here, we tested the effect of a 2-year resveratrol administration on proinflammatory profile and insulin resistance caused by a high-fat, high-sugar (HFS) diet in white adipose tissue (WAT) from rhesus monkeys. Resveratrol supplementation (80 and 480 mg/day for the first and second year, respectively) decreased adipocyte size, increased sirtuin 1 expression, decreased NF-κB activation, and improved insulin sensitivity in visceral, but not subcutaneous, WAT from HFS-fed animals. These effects were reproduced in 3T3-L1 adipocytes cultured in media supplemented with serum from monkeys fed HFS ± resveratrol diets. In conclusion, chronic administration of resveratrol exerts beneficial metabolic and inflammatory adaptations in visceral WAT from diet-induced obese monkeys.

Exercise- and resveratrol-mediated alterations in adipose tissue metabolism

Owing to its obligatory role in locomotion and the fact that it accounts for the vast majority of whole-body glucose and lipid oxidation, much work has focused on studying the biochemical adaptations that occur in skeletal muscle in response to exercise. However, over the past several years there has been a growing appreciation that adipose tissue is an important player in regulating systemic carbohydrate and lipid homeostasis. Despite this, the examination of how exercise alters adipose tissue function and metabolism is, when compared with skeletal muscle, in its infancy. The purpose of the current review is to highlight some of the recent findings from our laboratory and others that focus on the emerging area of adipose tissue exercise biochemistry. Specifically, the role of exercise on the induction of mitochondrial and glyceroneogenic enzymes will be examined and will be compared with the well-characterized effects of thiazolidinediones, which are insulin-sensitizing drugs. A particular emphasis will be placed on the role of interleukin-6 in mediating the effects of exercise. Finally, we will discuss recent data from our laboratory demonstrating beneficial effects of resveratrol supplementation on adipose tissue metabolism.

Does resveratrol improve insulin signaling in chronically ischemic myocardium?

BACKGROUND

Resveratrol is a naturally occurring polyphenol believed to be cardioprotective. We previously demonstrated that resveratrol improves insulin signaling and glucose metabolism in liver and skeletal muscle of swine with metabolic syndrome. Although resveratrol has metabolic benefits in peripheral tissues, its effect on insulin signaling in ischemic myocardium (IM) is unclear. Therefore, we developed a clinically relevant swine model of metabolic syndrome and chronic myocardial ischemia to investigate the effects of resveratrol on insulin signaling in cardiac tissue.

MATERIALS AND METHODS

Thirteen male Yorkshire swine were fed a high-cholesterol diet for 4 wk then underwent surgical placement of an ameroid constrictor to their circumflex artery to induce chronic myocardial ischemia. The high-cholesterol control group was given no drug (n = 7). The experimental group was provided the same diet and received supplemental resveratrol (100 mg/kg/d) (n = 6). Tissue was harvested 7 wk after ameroid placement for western blot and histological analyses.

RESULTS

In IM, there was no significant difference between the two groups in the insulin signaling markers studied. In nonischemic myocardium, there was a significant decrease in phosphorylated AMP-activated protein kinase α (P = 0.021) in the group treated with resveratrol; otherwise, there were no significant differences between the groups. Immunostaining for glucose transporter 4 and Periodic acid-Schiff staining for myocardial glycogen stores was similar between the groups.

CONCLUSIONS

Resveratrol has complex effects on glucose metabolism. Although prior studies demonstrated that resveratrol supplementation improves insulin sensitivity in peripheral tissues, in chronically IM, there are no significant alterations.

High-dose resveratrol supplementation in obese men: an investigator-initiated, randomized, placebo-controlled clinical trial of substrate metabolism, insulin sensitivity, and body composition

Obesity, diabetes, hypertension, and hyperlipidemia constitute risk factors for morbidity and premature mortality. Based on animal and in vitro studies, resveratrol reverts these risk factors via stimulation of silent mating type information regulation 2 homolog 1 (SIRT1), but data in human subjects are scarce. The objective of this study was to examine the metabolic effects of high-dose resveratrol in obese human subjects. In a randomized, placebo-controlled, double-blinded, and parallel-group design, 24 obese but otherwise healthy men were randomly assigned to 4 weeks of resveratrol or placebo treatment. Extensive metabolic examinations including assessment of glucose turnover and insulin sensitivity (hyperinsulinemic euglycemic clamp) were performed before and after the treatment. Insulin sensitivity, the primary outcome measure, deteriorated insignificantly in both groups. Endogenous glucose production and the turnover and oxidation rates of glucose remained unchanged. Resveratrol supplementation also had no effect on blood pressure; resting energy expenditure; oxidation rates of lipid; ectopic or visceral fat content; or inflammatory and metabolic biomarkers. The lack of effect disagrees with persuasive data obtained from rodent models and raises doubt about the justification of resveratrol as a human nutritional supplement in metabolic disorders.

Resveratrol potentiates rapamycin to prevent hyperinsulinemia and obesity in male mice on high fat diet

High doses of rapamycin, an antiaging agent, can prevent obesity in mice on high fat diet (HFD). Obesity is usually associated with hyperinsulinemia. Here, we showed that rapamycin given orally, at doses that did not affect weight gain in male mice on HFD, tended to decrease fasting insulin levels. Addition of resveratrol, which alone did not affect insulin levels, potentiated the effect of rapamycin, so that the combination decreased obesity and prevented hyperinsulinemia. Neither rapamycin nor resveratrol, and their combination affected fasting levels of glucose (despite lowering insulin levels), implying that the combination might prevent insulin resistance. We and others previously reported that resveratrol at high doses inhibited the mTOR (Target of Rapamycin) pathway in cell culture. Yet, as we confirmed here, this effect was observed only at super-pharmacological concentrations. At pharmacological concentrations, resveratrol did not exert ‘rapamycin-like effects' on cellular senescence and did not inhibit the mTOR pathway in vitro, indicating nonoverlapping therapeutic mechanisms of actions of rapamycin and resveratrol in vivo. Although, like rapamycin, resveratrol decreased insulin-induced HIF-1-dependent transcription in cell culture, resveratrol did not inhibit mTOR at the same concentrations. Given distinct mechanisms of action of rapamycin and resveratrol at clinically relevant doses, their combination warrants further investigation as a potential antiaging, antiobesity and antidiabetic modality.

Resveratrol directly affects in vitro lipolysis and glucose transport in human fat cells

Resveratrol is a naturally occurring polyphenol found in many dietary sources and red wine. Recognized as a cancer chemoprevention agent, an anti-inflammatory factor and an antioxidant molecule, resveratrol has been proposed as a potential anti-obesity compound and to be beneficial in diabetes. Most of the studies demonstrating the anti-adipogenic action of resveratrol were performed as long-term treatments on cultured preadipocytes. The aim of this study was to analyse the acute effects of resveratrol on glucose uptake and lipolysis in human mature adipocytes. Samples of subcutaneous abdominal adipose tissue were obtained from overweight humans and immediately digested by liberase. Fat cells were incubated (from 45 min to 4 h) with resveratrol 1 μM-1 mM. Then, glycerol release or hexose uptake was determined. Regarding lipolysis, the significant effects of resveratrol were found at 100 μM, consisting in a facilitation of isoprenaline stimulation and an impairment of insulin antilipolytic action. At 1 and 10 μM, resveratrol only tended to limit glucose uptake. Resveratrol 100 μM did not change basal glucose uptake but impaired its activation by insulin or by benzylamine. This inhibition was not found with other antioxidants. Such impairment of glucose uptake activation in fat cells may led to a reduced availability of glycerol phosphate and then to a decreased triacylglycerol assembly. Therefore, resveratrol increased triacylglycerol breakdown triggered by β-adrenergic activation and impaired lipogenesis. Consequently, our data indicate that resveratrol can be considered as limiting fat accumulation in human fat cells and further support its use for the mitigation of obesity.

Acetylation and deacetylation--novel factors in muscle wasting

We review recent evidence that acetylation and deacetylation of cellular proteins, including transcription factors and nuclear cofactors, may be involved in the regulation of muscle mass. The level of protein acetylation is balanced by histone acetyltransferases (HATs) and histone deacetylases (HDACs) and studies suggest that this balance is perturbed in muscle wasting. Hyperacetylation of transcription factors and nuclear cofactors regulating gene transcription in muscle wasting may influence muscle mass. In addition, hyperacetylation may render proteins susceptible to degradation by different mechanisms, including intrinsic ubiquitin ligase activity exerted by HATs and by dissociation of proteins from cellular chaperones. In recent studies, inhibition of p300/HAT expression and activity and stimulation of SIRT1-dependent HDAC activity reduced glucocorticoid-induced catabolic response in skeletal muscle, providing further evidence that hyperacetylation plays a role in muscle wasting. It should be noted, however, that although several studies advocate a role of hyperacetylation in muscle wasting, apparently contradictory results have also been reported. For example, muscle atrophy caused by denervation or immobilization may be associated with reduced, rather than increased, protein acetylation. In addition, whereas hyperacetylation results in increased degradation of certain proteins, other proteins may be stabilized by increased acetylation. Thus, the role of acetylation and deacetylation in the regulation of muscle mass may be both condition- and protein-specific. The influence of HATs and HDACs on the regulation of muscle mass, as well as methods to modulate protein acetylation, is an important area for continued research aimed at preventing and treating muscle wasting.

Resveratrol Is Not as Effective as Physical Exercise for Improving Reproductive and Metabolic Functions in Rats with Dihydrotestosterone-Induced Polycystic Ovary Syndrome

Polycystic ovary syndrome (PCOS) is a reproductive and metabolic disorder associated with obesity and insulin resistance that often precedes the development of type-2 diabetes. Rats continuously exposed to dihydrotestosterone from prepuberty display typical reproductive and metabolic PCOS characteristics including anovulation, polycystic ovaries, insulin resistance, and obesity. Our aim was to investigate if resveratrol improves reproductive and metabolic functions in PCOS rats. The effect was compared to exercise. Control and PCOS rats were treated with vehicle or resveratrol (400 mg · kg−1 · day−1) for 5-6 weeks. Another group of PCOS rats received vehicle treatment and exercised for 5-6 weeks. Insulin sensitivity was determined by euglycemic-hyperinsulinemic clamp. The glucose infusion rate was lower in the PCOS-vehicle group compared to control-vehicle rats (P<0.05). Exercise increased insulin sensitivity compared with PCOS-vehicle rats (P<0.05), but resveratrol did not. Resveratrol treatment and exercise resulted in smaller adipocytes, upregulated estrogen-related receptorαgene expression in subcutaneous fat, and improved estrus cyclicity in the previously acyclic PCOS rats. Although resveratrol had positive effects on adiposity and cyclicity in a similar manner to exercise, resveratrol does not seem to be a good candidate for treating insulin resistance associated with PCOS because no improvement in insulin sensitivity was observed in PCOS rats on normal chow.

Synergistic effects of leucine and resveratrol on insulin sensitivity and fat metabolism in adipocytes and mice

BACKGROUND

Sirtuins are important regulators of glucose and fat metabolism, and sirtuin activation has been proposed as a therapeutic target for insulin resistance and diabetes. We have shown leucine to increase mitochondrial biogenesis and fat oxidation via Sirt1 dependent pathways. Resveratrol is a widely recognized activator of Sirt; however, the biologically-effective high concentrations used in cell and animal studies are generally impractical or difficult to achieve in humans. Accordingly, we sought to determine whether leucine would exhibit synergy with low levels of resveratrol on sirtuin-dependent outcomes in adipocytes and in diet-induced obese (DIO) mice.

METHODS

3T3-L1 mouse adipocytes were treated with Leucine (0.5 mM), β-hydroxy-β-methyl butyrate (HMB) (5 μM) or Resveratrol (200 nM) alone or in combination. In addition, diet-induced obese mice were treated for 6-weeks with low (2 g/kg diet) or high (10 g/kg diet) dose HMB, Leucine (24 g/kg diet; 200% of normal level) or low (12.5 mg/kg diet) or high (225 mg/kg diet) dose resveratrol, alone or as combination with leucine-resveratrol or HMB-resveratrol.

RESULTS

Fatty acid oxidation, AMPK, Sirt1 and Sirt3 activity in 3T3-L1 adipocytes and in muscle cells, were significantly increased by the combinations compared to the individual treatments. Similarly, 6-week feeding of low-dose resveratrol combined with either leucine or its metabolite HMB to DIO mice increased adipose Sirt1 activity, muscle glucose and palmitate uptake (measured via PET/CT), insulin sensitivity (HOMAIR), improved inflammatory stress biomarkers (CRP, IL-6, MCP-1, adiponectin) and reduced adiposity comparable to the effects of high dose resveratrol, while low-dose resveratrol exerted no independent effect.

CONCLUSION

These data demonstrate that either leucine or its metabolite HMB may be combined with a low concentration of resveratrol to exert synergistic effects on Sirt1-dependent outcomes; this may result in more practical dosing of resveratrol in the management of obesity, insulin-resistance and diabetes.

Resveratrol ameliorates diabetes-related metabolic changes via activation of AMP-activated protein kinase and its downstream targets in db/db mice

SCOPE

This study investigated the effects of resveratrol (RV) on diabetes-related metabolic changes in a spontaneous model of type 2 diabetes, as well as activation of AMP-activated protein kinase (AMPK) and downstream targets.

METHODS AND RESULTS

C57BL/KsJ-db/db mice were fed a normal diet with RV (0.005% and 0.02%, w/w) or rosiglitazone (RG, 0.001%, w/w) for 6 weeks. Both doses of RV significantly decreased blood glucose, plasma free fatty acid, triglyceride, apo B/apo AІ levels and increased plasma adiponectin levels. RV activated AMPK and downstream targets leading to decreased blood HbA1c levels, hepatic gluconeogenic enzyme activity, and hepatic glycogen, while plasma insulin levels, pancreatic insulin protein, and skeletal muscle GLUT4 protein were higher after RV supplementation. The high RV dose also significantly increased hepatic glycolytic gene expression and enzyme activity, along with skeletal muscle glycogen synthase protein expression, similar to RG. Furthermore, RV dose dependently decreased hepatic triglyceride content and phosphorylated I kappa B kinase (p-IKK) protein expression, while hepatic uncoupling protein (UCP) and skeletal muscle UCP expression were increased.

CONCLUSION

RV potentiates improving glycemic control, glucose uptake, and dyslipidemia, as well as protecting against pancreatic β-cell failure in a spontaneous type 2 diabetes model. Dietary RV has potential as an antidiabetic agent via activation of AMPK and its downstream targets.

Resveratrol prevents dexamethasone-induced expression of the muscle atrophy-related ubiquitin ligases atrogin-1 and MuRF1 in cultured myotubes through a SIRT1-dependent mechanism

Resveratrol (3,5,4'-trihydroxystilbene) has been ascribed multiple beneficial biological effects but the influence of resveratrol on glucocorticoid-induced muscle atrophy is not known. We examined the effects of resveratrol on dexamethasone-induced atrogin-1 and MuRF1 expression, FOXO1 acetylation, protein degradation and atrophy in cultured L6 myotubes. In addition, the role of the deacetylase SIRT1 in the effects of resveratrol was determined by transfecting myotubes with SIRT1 siRNA. The catabolic effects of dexamethasone were prevented by resveratrol and the protective effects of resveratrol on dexamethasone-induced atrogin-1 and MuRF1 expression were abolished in myotubes transfected with SIRT1 siRNA. Results suggest that resveratrol can prevent glucocorticoid-induced muscle wasting and that this effect is at least in part SIRT1-dependent.