How efficient is resveratrol as an antioxidant of the Mediterranean diet, towards alterations during the aging process?

Abstract

Mediterranean diet (MD) is the most relevant nutritional aspect of the multisecular Mediterranean civilisation which includes wine as an element of health and wellbeing when consumed with moderation. Mediterranean meals provide food micronutrients which include polyphenols, especially resveratrol from grape and red wine. MD, also called Cretan diet, has been proven to prevent diseases including cardiovascular pathologies, cancer, and to prevent aging. Interestingly, the grape and more precisely in grape skin contains the highest concentration of RSV. In consequence, red wine is the most concentrated food source of RSV found in the human diet. This review topic deals to how efficient is RSV towards alterations during the aging process; obtained from recent data of clinical trials, preclinical studies, and cell culture approach; especially RSV protecting effect on brain aging of elderly; its role on the microglial cells playing a central role in the neuro-inflammation; and in its anti-inflammatory effects on ocular diseases.

Keywords:

• Aging
• antioxidant
• Mediterranean diet
• resveratrol

Introduction

Mediterranean diet (MD) is the most relevant nutritional aspect of the multisecular Mediterranean civilisation which includes wine as an element of health and wellbeing when consumed with moderation. Indeed, besides macronutrients (protein, carbohydrates, and lipids), Mediterranean meals provide food micronutrients which include polyphenols, vitamins, fibres, polyunsaturated fatty acids, and oligo-elements brought by fruits, vegetables, olive oil, fish, infusions, wine, and others. Interestingly, grape and olives provide unique polyphenols with antioxidant properties, especially resveratrol from grapes and red wine. MD, also called Cretan diet, has been proven to prevent diseases including cardiovascular pathologies [1], cancer [2,3], and to slow aging [4].

Resveratrol (or 3,4′,5 trihydroxy-trans-stilbene) (RSV) is a natural polyphenol firstly identified in 1940 in the root of the white hellebore [5]. Later, RSV was also found in large quantities in the root of the Japanese knotweed (Polygonum cuspidatum) [6]. In 1976, Langcake and Pryce [7] discovered RSV in vine and showed to be produced in a huge amount by the vine plant in response to biotic infections (for instance Botrytis cinerea) but also synthesised by reacting to abiotic stresses such as ultraviolet irradiation, phytochemicals, or exposure to ozone. RSV is also produced by a variety of plants which are edible, or not, by humans. Such as the butterfly tree or pine tree, in rhubarb, in peanuts as well as in numerous berries: grapes, blackberries, blackcurrants, blueberries, and cranberries. Interestingly, the grape and more precisely the grape skin contains the highest concentration of RSV. Consequentially, red wine seems to be the most concentrated food source of RSV found in human diet.

RSV is one of the best-known phytophenol exhibiting pleiotropic properties. Indeed, besides to be a vine phytoalexin that stimulate the natural defence of plants, RSV provides numerous beneficial effects to animals and to humans by maintaining homeostasis of a wide range of organs and tissues, including the prevention of cardiovascular diseases, cancer, neurodegenerescence, and low-grade inflammation inhibition [8]. Interestingly, RSV improves mice longevity and physical activity through the sirtuin pathway [9].

This review topic deals with recent data on RSV efficiency against alteration processes during aging; obtained from clinical trials, preclinical studies and cell culture approaches. Particular attention was given in this review to RSV protecting effect on brain aging of elderly, regarding its role on the microglial cells, which play a central role in the neuroinflammation and in anti-inflammatory RSV effects on ocular diseases.

A review of preclinical and clinical studies aimed to study the effects of RSV in the prevention of neurodegenerative diseases linked with aging on elderly is presented.

Clinical trials linking resveratrol with aging

Many experimental studies in cell cultures and animal models have shown that RSV has potential benefits against inflammation, oxidative stress, cancer or cardiovascular risk and increases lifespan, among others. Although the available literature on these compounds is very abundant, clinical trials, especially those related to older people and aging, are still scarce (Table 1).

Table 1. Synopsis of results from clinical trials about RSV in aged people.

References	Sample and design	Treatment	HBOT outcomes
Magyar et al. [10]	N = 40. Postinfarction Caucasian patients. Double-blind, placebo-controlled trial.	–Ten milligrams RSV capsule daily for 3 months vs. 10-mg placebo.	Improvement in left ventricular diastolic function and endothelial function measured by flow-mediated vasodilation. Significant decrease in LDL. Ameliorated red blood cell deformability and reduced platelet aggregation.
Tomé-Carneiro et al. [12]	N = 75. Statin-treated patients in primary cardiovascular disease prevention. Triple-blind, placebo-controlled trial.	Capsule (350 mg) daily for 6 months containing RSV-enriched grape extract (8 mg RSV) vs. grape extract without RSV and a placebo.	Reduction in oxidised LDL and ApoB. Increased non-HDL-C (total atherogenic cholesterol load)/ApoB ratio. No changes in hepatic, thyroid, and renal function were found.
Zamora-Ros et al. [16]	N = 1000. High CV risk patients. PREDIMED (PREvención con DIeta MEDiterránea) study. Large, parallel-group, multicenter, controlled, randomised clinical trial	Evaluation of wine or RSV consumption through 137-item validated food frequency questionnaire.	Positive association with fasting blood glucose (only RSV) and heart rate. Better plasma lipid profile: higher HDL and lower triglyceride concentrations.
Carrizzo et al. [11]	N = 59. Hypertension and dyslipidemia patients. Prospective, randomised controlled clinical trial.	Superior thyroid artery rings ex vivo treatment with RSV (12.5–100 µmol/L).	Induction of vasorelaxation and improvement of endothelial dysfunction mediated by nitric oxygen metabolism.
Gliemann et al. [22]	N = 27. Healthy physically inactive aged men. Randomised double-blind placebo-controlled trial.	Exercise training and 250 mg trans-RSV daily for 8 weeks vs. exercise training and placebo.	Amelioration of the positive effects of exercise on LDL, total cholesterol/HDL ratio and triglyceride levels. No effects on VCAM-1 and SIRT1.
Tomé-Carneiro et al. [13]	N = 75. Statin-treated patients in primary cardiovascular disease prevention. Triple-blind, placebo-controlled trial.	Capsule (350 mg) daily for 6 months containing RSV-enriched grape extract (8 mg RSV) and a double dose for the following 6 months vs. grape extract without RSV and a placebo.	Increase in the anti-inflammatory serum adiponectin and a decrease of the thrombogenic plasminogen activator inhibitor type 1. Reduction in inflammatory-related genes expression in peripheral blood mononuclear cells.
Semba et al. [18]	N = 783. Persons aged 65 years or more randomly selected from a population registry. InCHIANTI (Invecchiare in Chianti) study. Prospective cohort study.	Measurements of RSV and RSV derivates in urine.	No relation with all-cause mortality for 9 years follow-up. No association with serum CRP, IL-6, IL-1β, TNF-α, prevalent or incident cardiovascular disease or cancer.
Faghihzadeh et al. [15]	N = 50. Non-alcoholic fatty liver disease patients. Randomised, double-blind, placebo-controlled study.	–Five hundred milligrams RSV capsule daily for 12 weeks vs. a placebo capsule.	Reduction ALT and hepatic steatosis (FibroScan score). No effects on blood pressure, insulin resistance markers, and triglycerides.
Rabassa et al. [17]	N = 769. Persons aged 65 years or more randomly selected from a population registry. In CHIANTI study. Prospective cohort study.	Estimation of total dietary RSV intake at baseline with a validated food-frequency questionnaire and total urinary RSV analysis.	Inverse relation with frailty syndrome over 3-y of follow-up but not after 6- and 9-y of follow-up.
Alway et al. [21]	N = 30. Healthy subjects. Randomised, placebo-controlled study.	Exercise training and 500 mg trans-RSV daily for 12 weeks vs. exercise training and placebo.	No effects on cardiovascular risk. Improvement in the indices of mitochondrial density, and muscle fatigue resistance. Hypertrophy of type I and IIA fibre sizes, and an increase in total muscle nuclei.
Kjaer et al. [19]	N = 66. Metabolic syndrome patients but otherwise healthy. Randomised, placebo-controlled, double-blind, parallel group clinical trial.	75 mg RSV, 500 mg or placebo tablets twice daily for 16 weeks.	No effects on CRP, IL-6 and soluble urokinase plasminogen activator receptor in plasma and inflammatory gene expression in adipose and muscle tissues. No effects on glucose homeostasis, blood pressure, or hepatic lipid content.
Pollack et al. [14]	N = 30. Glucose-intolerant patients. Randomised, double-blind placebo-controlled crossover study.	RSV (2–3 g/daily) or placebo capsules each for 6 weeks with a 3-week washout period.	No modifications in glucose tolerance, insulin sensitivity, weight, blood pressure, or lipid profile. Improvement in fasting reactive hyperaemia index. Increased mitochondrial number and modulation of pathways involved in oxidative phosphorylation and inflammation in skeletal muscle.

Regarding clinical trials with RSV and elderly people, most studies focus on assessing the effects on cardiovascular risk. In this sense, the daily intake of 10 mg RSV for 3 months was evaluated in patients after myocardial infarction with a mean age of 66 [10]. The supplementation improved left ventricle diastolic function, protected against unfavourable haemorheological changes, enhanced endothelial function and lowered plasma low-density lipoprotein (LDL) when compared with control patients. The effects of RSV were also investigated on superior thyroid artery function from patients with hypertension and dyslipidemia [11]. The results evidenced an improvement of endothelial dysfunction through the modulation of nitric oxide (NO) metabolism.

The consumption of grape extract capsules (350 mg) containing 8 mg RSV for 6 months were investigated in aged patients treated with statins [12]. The supplement reduced oxidised LDL and apolipoprotein-B (ApoB) levels improving lipid profile. The same authors, using a similar procedure for 6 months followed by another 6 months doubling the doses, evidenced an increase in adiponectin levels and a downregulation in inflammatory-related genes in peripheral blood mononuclear cells [13]. In a randomised, double-blind crossover study, RSV (between 1000 and 1500 mg twice daily) was administered to glucose-intolerant adults with a mean age of 67 for 6 weeks [14]. RSV significantly improved vascular function measured by reactive hyperaemia peripheral arterial tonometry, increased mitochondrial number in vastus lateralis skeletal muscle but had no effects on glucose metabolism and insulin sensitivity. Another study investigated the effects of RSV supplementation (500 mg for 12 weeks) on non-alcoholic fatty liver disease subjects [15]. The supplementation significantly improved alanine aminotransferase levels and hepatic steatosis, although no significant changes were found in blood pressure, lipid profile, and insulin resistance.

The associations between levels of RSV consumed through the diet, measured as total urinary RSV metabolites and the prevalence of cardiovascular risk factors were investigated in 1000 aged high-risk patients from the PREDIMED study [16]. Multiple linear regression reported beneficial effects of RSV in fasting blood glucose, lipids profile, and heart rate. In another study using subjects from the PREDIMED study, multiple logistic regression analysis evidenced that moderate red wine drinkers (≥1 drink/day) presented a reduced risk of prevalent metabolic syndrome, abnormal waist circumference, and low high-density lipoprotein (HDL) levels. The relationship between habitual dietary RSV consumption and the development of frailty syndrome was investigated in subjects aged ≥65 [17]. The results evidenced an inverse association between high levels of RSV and the risk of developing frailty syndrome during the first 3 years of follow-up, although no significant effects were found after 6 and 9 years.

However, not all studies evidenced positive effects of RSV consumption through the diet or specific supplementation. In this sense, total urinary RSV metabolites determined in old people (≥65 years) were not related to all-cause mortality for 9 years follow-up or cardiovascular disease, cancer, and inflammation [18]. In another randomised placebo-controlled clinical trial, supplementation of patients with metabolic syndrome with 150 or 1000 mg RSV for 16 weeks did not improve the inflammatory status [19]. In addition, no effects of RSV were reported in hepatic lipid content, glucose homeostasis, and blood pressure but significantly increased cholesterol levels.

The beneficial effects of RSV associated with the practice of physical exercise are not clear either. Although there is some evidence of beneficial effects, RSV as an antioxidant can affect the same pathways induced by physical exercise and counteract its positive effects [20]. The effects of RSV (500 mg/day) combined with 12 weeks of regular aerobic exercise were investigated in 65–80-year-old subjects [21]. The combined therapies did not reduce cardiovascular risk respect to exercise alone. However, supplementation with RSV augmented the improvement of mitochondrial function, muscle fatigue resistance, and the increased mean fibre area and muscle nuclei associated with exercise, which may contribute to reducing/reversing sarcopenia in elderly people. In another randomised clinical trial, the supplementation with RSV (250 mg trans-RSV for 8 weeks) to physically inactive 65 ± 1-year-old men blocked the beneficial effects of exercise training on cholesterol and triglyceride concentrations [22]. On the contrary, RSV did not modify the effects of exercise on vascular cell adhesion molecule 1 (VCAM-1) and sirtuin 1 (SIRT1) levels in vastus lateralis.

Resveratrol as a possible protective agent of brain aging: evidences from preclinical and clinical trials

The world population is progressively getting older. As people age impairment of the nervous system in different regions of the brain occurs. The specific cause of this impairment is not clear, however, several cellular and molecular events are involved in its progression, like mitochondria function impairment, oxidative stress increase, neuronal apoptosis activation, aggregated proteins deposition, and excite-toxicity [23].

Most of the preclinical studies reporting RSV neuroprotection effects used animal models of neurogenerative diseases. In this section, our goal is to understand if RSV helps to maintain brain function and to prevent neurodegenerative disorders during normal aging process, therefore we only include the results obtained with healthy animal models.

Regulation of microRNA-CREB (CREB, cAMP response element binding) pathway is among the mechanisms that could explain RSV protecting potential against neurodegenerative diseases in aging. MicroRNAs (miRNAs, 21–23 nucleotides) are very important regulators of the long-term memory formation and their deregulation may, therefore, contribute to memory impairment associated with neurodegenerative diseases. MiRNAs modifies translational (post-transcriptional) and even transcriptional processes [24]. MiRNA-134 is a brain-specific miRNA that regulates CREB and brain-derived neurotrophic factor (BDNF) expression [25]. Intraventricular injections of RSV in 8–9-month-old mice, for 1 week, enhances SIRT1 activity, which promotes a decrease in miRNA-134 and an increase in CREB and BDNF in mice hippocampus. These changes lead to an improvement of synaptic plasticity, long-term memory, and long-term potentiation in the hippocampus [26]. BDNF modulates synaptic plasticity and is very important in cognition, learning, and memory formation. Expression of BDNF mRNA is also induced in the hippocampus of male rat (Sprague-Dawley, weighing 250 ± 20 g) by RSV (2.5, 10, or 20 mg/kg total body weight) orally administrated by 10 and 30 days [27].

Decline in cognitive capacity after middle age is seen in most humans as well as in animal models. Chronic administration of RSV (10 mg/kg) for 8 months helps to maintain the integrity of rat hippocampus and to improve cognitive performance [28]. Cognitive function (in both executive function and spatial memory) of adult grey mouse lemurs (Microcebus murinus) improves after 18-month treatment of RSV treatment (supplementation of 200 mg/kg/day, ad libitum). Chronic caloric restriction (CR) and RSV treatment induce similar benefits on cognitive functions by probably activating similar brain structures (striatum-prefrontal circuits and hippocampus) and have similar effects on locomotion activity [29]. Continuing hippocampal neurogenesis sustains human-specific cognitive function in healthy older people. Cognitive-emotional resilience may be reduced by a decline in hippocampal neurogenesis, therefore, it is important to maintain neurogenesis in adult hippocampus [30]. Treatment with RSV (40 mg/kg) for 2 weeks induces the production of new neurons in 6-month-old Balb/C mice, which is a model with medium to low levels of baseline adult hippocampal neurogenesis but high relative numbers of surviving new-born cells [31]. In this model, RSV also promotes a decrease in cell apoptosis and an increase in doublecortin (DCX)-expressing intermediate cells. Performance in behavioural tests related to memory formation improves with RSV treatment [31]. The level of phospho-Akt (Akt, protein kinase B) increases with RSV treatment which suggests its involvement in the apoptosis decrease, since phosphorylated Akt directly inhibits the apoptotic machinery [32]. Phospho-protein kinase C (phospho-PKC) is another possible signalling pathway involved since the quantity of phospho-PKC (78 kDa) in the hippocampal lysates is higher in mice treated with RSV (40 mg/kg) [31]. This is in accordance with the serine/threonine PKC family involvement in long-term processes such as cell proliferation, synaptic remodelling and gene expression [33]. RSV also promotes neurogenesis in male F344 rats in late middle age having similar learning and memory abilities, and treated with RSV for 4 weeks. In this model, RSV also promotes an increase in microvasculature and a decline in astrocyte hypertrophy and microglial activation in the hippocampus [34]. An increase in microvascular density in the hippocampus is also observed in male C57BL/6 mice (18 months of age) fed for 6 months with food containing 150 µg RSV [35]. RSV also promotes a decrease in the number of vacuolar abnormalities in both hippocampal and cortical microvascular endothelial cells, therefore it is probably that RSV attenuates the age-related decline in vascular density and maintains normal levels of angiogenesis [35]. RSV could prevent age-related decline in vascular density and occurrence of microvascular abnormalities by: preventing oxidation of membrane lipids, promoting a reduction of platelet aggregation, enabling endothelium-dependent vascular relaxation and activating biochemical pathways that involve SIRT1 which induces processes that promote cellular functioning and control cell death and exerts vasoprotective effects against oxidative stress and inflammatory factors [35].

Brain monoaminergic systems impairment occurs with aging affecting cognitive processes such as memory and learning. Administration of RSV (20 mg/kg/day for 4 weeks) in old male rats (Wistar, 20 months) increases the activity of both hydroxylase (TPH) in pineal, hippocampus and striatum, and tyrosine hydroxylase (TH) in hippocampus and striatum. This increase in activity of TPH and TH enzymes endorsed an enhancement in serotonin (5-HT) levels in pineal gland, in hippocampus, and in striatum, in noradrenaline (NA) in hippocampus and in dopamine (DA) in striatum. In summary, RSV can reverse the neurochemical parameters decline observed during aging [36].

Cognitive impairment and decline observed in older people are associated with an increase in inflammatory markers, such as tumour necrosis factor-α (TNF-α) and interleukin-1β (IL-β) [37]. RSV treatment (50 mg/kg/day orally administered for 12 weeks) prevents elevations in plasma TNF-α and IL-1β levels in male Wistar albino rats with 24 months of age. RSV is also effective in preventing cognitive deficit in aged rats [38].

Brain is likely to have oxidative damage since requires high oxygen levels to its normal function, has a high amount of polyunsaturated fatty acids and inefficient antioxidant defence mechanisms [39,40]. Aging cognitive impairment is associated with an oxidative stress increase. Antioxidant properties of RSV are well known [41]. Senescence-accelerated mice treated with RSV (25, 50, 100 RSV mg/kg/day for 8 weeks) have higher level brain activity of superoxide dismutase (SOD) and glutathione peroxidase (GSH-Px), and lower lipid peroxidation [42].

A recent study shows that SOX2 (Sex-determining region Y Box 2) expression declines with aging [43]. In the same study, it was also observed that SOX2 mRNA expression increases in aged mice fed with RSV for 6 months. This is in accordance with the studies that show RSV extends lifespan and delays ageing, like the one carried out by Yu and Li [44] with Nothobranchius guentheri, a vertebrate model for aging studies, where an increase in animal lifespan, an improvement in cognitive ability, and locomotor activity were observed after RSV treatment. In N. guentheri, RSV also delays lipofuscin formation and beta-galactosidase activity [44].

Concerning RSV nutritional interventions and their effects on brain functions even in adults, the cerebral blood flow of healthy adults increases with RSV administration (250 and 500 mg) in a dose-dependent way without affecting their cognitive function [45]. No improvement in verbal memory is observed in elderly people (60–79 years) after receiving RSV (200 mg/day) for 26 weeks [46]. Daily consumption of a capsule with 75 mg RSV for 6 weeks is well tolerated and has the potential to maintain healthy circulatory function in obese but otherwise healthy adults [47]. Consumption of two of those capsules (150 mg) for 14 weeks enhances both cerebrovascular function and cognition in postmenopausal women aged 45–85 years [48]. In contrast, recent systematic reviews and meta-analysis, which included only clinical studies, RSV has no significant effect on memory improvement or cognitive performance as well as mood state [49,50]. A summary of the RSV preventive effects during elderly is shown in Figure 1.

Figure 1. Possible preventive effect of resveratrol (RSV) of neurodegenerative diseases during aging.

Role of resveratrol on the microglial cells, playing a role in the neuro-inflammation in aging and age-related diseases

Among age-related neurodegenerative diseases, Alzheimer’s disease, Parkinson’s disease, and multiple sclerosis are the most common. In such disorders, oxidative stress and neuro-inflammation associated with microglia activation play a central role in the physiopathogenesis by producing proinflammatory molecules, including TNF-α, IL-1 and IL-6, and NO among others [51–53]. Accordingly, increased glial activation and inflammatory mediators have been also reported during aging [54]. This chronic neuroinflammation leads to neuron death and finally to a general neurological dysfunction [55]. However, in the absence of satisfactory pharmacological therapies, prevention based on healthy food and lifestyle would be considered. Correspondingly, collected epidemiological and nutritional data regarding lifestyle and habits revealed that the MD is fairly correlated to the reduced risk of development of neurodegenerative disorders [56–61]. Preventive effect of MD is related to its richness in natural compounds issued from the high consumption of vegetable and virgin olive oil and a moderate intake of red wine [62,63]. Such a diet provides substantial amounts of polyphenolic compounds, including RSV. Increasing interest on such polyphenols leans on their antioxidative and anti-inflammatory activities, particularly against activated microglia. In numerous studies, activation of microglia was accomplished in vivo or in vitro by the exposure to Aβ1-42 peptide (Aβ, β-amyloid) or lipopolysaccharides (LPS), which bind each to the plasma membrane toll-like receptor (TLR). RSV has been shown to prevent TLR4 oligomerisation, then inhibiting its activation by Aβ1-42 peptide or LPS [64]. Activated TLR4 leads to the formation of IKK-IκB complex (I kappa B kinase), phosphorylation of IκB that releases the sequestrated NF-κB (factor nuclear kappa B) dimer (p50/p65), which translocate to the nucleus for the transactivation of several target genes (nitric oxide synthase iNOS, IL-1β, IL-6 and TNF-α, among others). Interestingly, RSV can inhibit all the steps described just before and involved in the NF-κB signalling pathway [64–66]. RSV reduced the LPS-induced synthesis of prostaglandin E2(PGE2) [67,68]. However, RSV did not change the expression of cyclooxygenase-2 (COX-2) but reduces the expression of cyclooxygenase-1 (COX-1) [68]. In addition to reducing the expression of cytokines, RSV suppresses also the NF-κB-stimulated production of NO [69–71]. Furthermore, it has been reported that RSV abrogates activator protein 1 (AP-1) dimer (c-Jun/FOS) activation through the inhibitory effect on extracellular signal-regulated kinase 1/2 (ERK1/2), c-Jun N-terminal protein kinase (JNK), and p38 mitogen-activated protein kinase (p38 MAPK) pathways [66]. In this study, authors showed that RSV increased phosphatase and tensin homologue gene (PTEN), Akt and mammalian target of rapamycin (mTOR) phosphorylation, leading to the inhibition of the LPS-induced proinflammatory cytokines via a down-regulation of phosphorylation of NF-κB and MAPKs family [66]. On the other hand, enhanced the expression of SIRT1 through the activation of AMP-activated protein kinase (AMPK). SIRT1 mitigates LPS-mediated proinflammatory cytokines release in microglia [72,73]. Moreover, RSV diminished NADPH oxidase-mediated generation of reactive oxygen species and such effect has been suggested to be dependent on SIRT1 [73,74]. Benefit of RSV is also illustrated by the induction of anti-inflammatory cytokine IL-10 through dependent up-regulation of suppressor of cytokine signalling-3 (SOCS3), which requires the Janus kinase/signal transducers and activators of transcription (JAK-STAT) signalling pathway [75]. A possible mechanism is presented in Figure 2.

Figure 2. Effect of resveratrol (RSV) on signalling pathways involved in LPS (or Ab)-induced inflammation in microglial cells.

Anti-inflammatory effects of RSV on ocular diseases

Studies reported anti-inflammatory properties of RSV towards low-grade inflammation. One pathway is the inhibition of the expression of proinflammatory genes in human monocytes by synthesis stimulation of anti-inflammatory miRNAs [76].

Age-related macular degeneration (AMD) is one of the main causes of the alteration of vision in adults and often elderly people. Most studies indicate an early role of the retinal pigment epithelium (RPE) in the onset of the disease. At the early stage of AMD, the initiation is suspected to be related with cigarette smoke [77].

RSV has been used to determine its ability to protect cells of the retinal pigment epithelium from oxidation and thus estimate the potential value of this molecule as a preventive and curative treatment of AMD. Sheu et al. studied the toxic effects of acrolein associated with hydrogen peroxide (H₂O₂). They showed a beneficial effect of RSV at relatively low concentrations (10 µM) on the RPE cells [78], associated with the inhibition of phagocytosis induced by acrolein and protection against the toxicity of the acrolein/H₂O₂ mix.

Pintea et al. working on RPE cells in culture reported a protective effect of polyphenols, including RSV against the cytotoxicity induced by H₂O₂ [79]. RSV can induce an increase in the enzyme activity of the antioxidant defence system: SOD, GSH-Px and catalase, and to increase the level of reduced glutathione.

The lens tissue is especially sensitive to oxidative damage since the constitutive fibre cells are not renewable and show a limited lifespan. The accumulation of damages to these cells throughout the life of the individual causes the degradation of proteins and eventually leads to cataracts. Using human lens epithelial cells [80] reported that RSV reduced cell death as well as the accumulation of ROS, when subjected to the oxidative attack by H₂O₂. This cell protection appears to involve the increased expression of defence enzymes such as SOD-1, catalase, and haem oxygenase-1 (HO-1).

Kubota. et al. reported a study in mice on ocular inflammation during uveitis induced by endotoxins (EIU) [81]. A 5 days of prevention by oral supplementation with RSV was able to inhibit the production of two key proteins involved in the inflammatory process: intercellular adhesion molecule 1 (ICAM-1) and monocyte chemoattractant protein 1 (MCP-1). The MCP-1 protein expressed by endothelial cells lining the vasculature.

In diabetes, the maintained hyperglycaemia lead to chronic inflammation with a slow but progressive alteration of the RPE cells leading to the impairment and then to the loss of central vision. In conditions of inflammation triggered by a hyperglycaemia condition on retinal pigment cells [82], reported that RSV can inhibit the stress-dependent proinflammatory secretion of IL-6 and IL-8. At the same conditions, RSV inhibits the COX-2 activity involved in the production of proinflammatory prostaglandins. The cellular cohesion is maintained, and RSV prevents the degradation of the blood-retina barrier as shown by preservation of connexin 43 expression and Gap-junction.

On the other hand, glaucoma is an eyes alteration which leads to a lack of circulation of aqueous humour and finally to ocular hypertension. Concerning initiation of glaucoma promoted by hyperoxygenation, Luna. et al. [83] observed in the trabecular meshwork cells that RSV inhibits the production of ROS when cells were subjected to oxidative stress. The expression of inflammation markers such as messengers of interleukin-1α, interleukin-6, interleukin-8, and E-selectin is reduced. RSV was shown to decrease the adhesion of leukocytes into the retinal vessels through the inhibition of ICAM-1 and MCP-1 in an endotoxin-induced uveitis [81]. In oxysterol-treated human retinal cells (ARPE-19), RSV exhibits a cytoprotective effect toward the enhancement of IL-8, MCP-1, and vascular endothelial growth factor (VEGF) secretion [84]. From the literature, it appears that RSV may play preventive effects on the pathogenesis of wet AMD (*) by decreasing the associated low-grade inflammation, especially as shown by the decrease of IL-6 secretion, and consequently by the maintenance of a low level of neutrophil chemoattraction that prevents VEGF-A dependent retina neovascularisation [85]. A summary is shown in Figure 3.

Figure 3. Possible preventive effect of resveratrol (RSV) on the pathogenesis of wet AMD.

(*) Wet macular degeneration is an exudative and neovascularised form in contrast with the dry form where there are no exudation which is the most frequent form.

The effects of oral RSV based nutritional supplement were administrated to octogenarians suffering from age-related macular degeneration. The daily oral intake of the supplement for 12 weeks providing 100 mg micronised/microencapsulated trans-RSV significantly restored the retinal structure and visual function [86]. Long-term treatment (two to three years) in the same three patients maintained the clinical benefits in ocular structure and function without side effects [87].

RSV is thus a molecule capable of acting in various cell types of the eye by increasing the level of natural antioxidant defences and to limit the generation of free radicals and the onset of irreversible damage, initiators of diseases such as AMD, cataracts, or glaucoma.

Discussions and perspectives

This review topic deals with how efficient is RSV towards alterations during the aging process, as protective agent of brain aging on the elderly. MD, also called Cretan diet, has been proven to prevent diseases including cardiovascular pathologies and to prevent aging. The highest level of centenarian is found in countries where they adopt MD and a good life style i.e. in Crete, Sardinia, and in other places of Mediterranean coast. By the way, this diet can be compared to the Okinawa diet which privileges omega-3 fatty acid, polyphenols from seaweed and from tea.

Impairment of the nervous system in different regions of the brain occurs mostly in aging people leading to neurodegenerative diseases, such as Alzheimer’s disease, Parkinson’s disease (PD), and multiple sclerosis, which are the most common. Aging cognitive impairment is considered to be associated with increased oxidative stress and the well-known antioxidant properties of dietary RSV appear interesting to explore. The published data are often surprising. For instance, consumption of RSV capsules enhances both cerebrovascular function and cognition in postmenopausal women [48]. In contrast, for some reviews and meta-analysis, RSV has no significant effect on memory improvement or cognitive performance [49]. The contradictory results obtained with different antioxidants, and in our case with resveratrol derives from their capability to exert pro-oxidant and antioxidant activities of resveratrol depending on its concentration and the target cell type [88].

The beneficial effects of RSV associated with the practice of physical exercise are not clear either. Although there is some evidence of beneficial effects, RSV counteracts its positive effects [20]. At the opposite, supplementation with RSV augmented the improvement of mitochondrial function, muscle fatigue resistance, and the increased mean fibre area and muscle nuclei associated with exercise, which may contribute to reducing/reversing sarcopenia in elderly people [21]. In the PREDIMED study, the authors also described the inverse association between high levels of RSV and the risk of developing frailty syndrome during the first 3 years of follow-up, but no significant effects were found after 6 and 9 years [17].

The apparent inconsistent results in cognitive improvement or not are summarised here: RSV nutritional interventions [45] lead to an increase in brain functions, cerebral blood flow of healthy adults without affecting their cognitive function. While no improvement in verbal memory was observed in elderly people after receiving RSV [46], the consumption of RSV enhances both cerebrovascular function and cognition in postmenopausal women [48]. In addition, meta-analysis [50] considers that RSV has no significant effect on memory improvement or cognitive performance. The possible reasons for these discrepancies could be due to variations in the protocol of RSV administration and the patient status.

Moreover, in some aging disorders, oxidative stress and neuro-inflammation, associated with microglia activation play a central role in the physiopathogenesis by producing proinflammatory molecules. Anti-inflammatory diet components such as RSV exhibit basic mechanisms leading to protecting effects, at least in biological models.

RSV is thus a molecule capable of acting in various cell types of the eye by increasing the level of natural antioxidant defences and to limit the generation of free radicals and the onset of irreversible damage, initiators of diseases such as AMD, cataracts, or glaucoma.

In conclusion, as reported by Huhn et al. [46], only a few controlled studies on RSV were done so far. The evidence is based on animal research and first interventional human trials warrant further investigations. The MD concept of food synergy should benefit from future trials that evaluate the basic knowledge of lifestyle patterns to prevent declining cognitive functions with the aging process.

Acknowledgements

The authors thank COST action NutRedOx, UNESCO Chair Culture and Traditions of Wine for their support.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

The NutRedOx Network [COST Action CA16112] is supported by the European Cooperation in Science and Technology (COST). AS and JAT are funded by Instituto de Salud Carlos III [CIBERobn CB12/03/30038], which is co-funded by the European Regional Development Fund.