Antioxidant effects of resveratrol in cardiovascular, cerebral and metabolic diseases

Highlights

• Increasing information about resveratrol metabolism.

• We focused on antioxidant effects of resveratrol.

• We highlight the molecular mechanisms involved in resveratrol antioxidant effects.

• We reported possible toxical effect of resveratrol depending on its dosage.

• Clinical evidences of antioxidant effects of resveratrol.

Abstract

Resveratrol—a natural polyphenolic compound—was first discovered in the 1940s. Although initially used for cancer therapy, it has shown beneficial effects against most cardiovascular and cerebrovascular diseases. A large part of these effects are related to its antioxidant properties. Here we review: (a) the sources, the metabolism, and the bioavailability of resveratrol; (b) the ability of resveratrol to modulate redox signalling and to interact with multiple molecular targets of diverse intracellular pathways; (c) its protective effects against oxidative damage in cardio-cerebro-vascular districts and metabolic disorders such as diabetes; and (d) the evidence for its efficacy and toxicity in humans. The overall aim of this review is to discuss the frontiers in the field of resveratrol’s mechanisms, bioactivity, biology, and healthrelated use.

Introduction

Resveratrol—3,4′,5-trihydroxy-trans-stilbene (MW: 228.2)—is a natural non-flavonoid polyphenol compound containing a stilbene structure similar to that of estrogen diethylstilbestrol (Fig. 1a). It is a fat-soluble compound existing in cis, trans-, and piceid isomeric forms (Fig. 1b). It was first isolated in 1940 from the roots of white hellebore (Veratum grandiflorum O. Loes) and later, in 1963, from the roots of Polygonum cupsidatum, a plant used in traditional Chinese and Japanese medicine (Nonomura et al., 1963). Resveratrol has been in use since ancient times as an Indian herbal preparation termed ‘Darakchasava’, which is derived from fermented grapes. Remarkably, the effects described for Darakchasava more than 4500 years ago (Singh et al., 2013) are the same described for resveratrol today. Today, Darakchasava is produced by several pharmaceutical companies and contains about 1.3–6.0 mg/L resveratrol (Paul et al., 1999). Despite its ancient discovery, the first real interest in resveratrol came in 1992 when it was postulated to explain some of the cardio-protective effects of red wine (Siemann and Creasy, 1992). It was suggested to be the solution to the “French Paradox”, a term used to describe the observation that the French population had a very low incidence of cardiovascular disease despite a high consumption of wine and saturated fat (Liu et al., 2007). In 1997, Jang and colleagues reported that resveratrol acts as a chemo-preventive agent, due to its ability to inhibit carcinogenesis at multiple stages (Jang et al., 1997). More recently, anti-inflammatory and antioxidant properties have been reported also (Baur and Sinclair, 2006, Vang et al., 2011), so today it has become a highly important natural active ingredient with potential therapeutic effects and market prospects.

Resveratrol is produced by various plants as a defense against stress, injury, excessive sunlight, ultraviolet radiation, infection, and invading fungi (Singh et al., 2013). For example, the roots of the plant P. cuspidatum, much cultivated in Asia, provides a rich source of resveratrol from which commercially available trans-resveratrol (98% pure) is isolated by high-speed counter-current chromatography (Yang et al., 2001). Resveratrol is also considered a nutraceutical present in grapes, peanuts, pine trees, cassia and other plants, and many food products (Ramprasath et al., 2010; Soleas et al., 1997). In wine, the concentration of resveratrol varies: red wines contain between 0.2 and 5.8 mg/L, depending upon the grape variety, whereas white wines contain ∼0.68 mg/L (Romero-Pérez et al., 1999, Sato et al., 1997). This variation derives from the fact that red wine is extracted with the grape skin intact, whereas white wine is fermented after removal of the skin. Red wine contains more trans-resveratrol than white wine, whereas white has a higher concentration of cis-resveratrol (Feijóo et al., 2008). Concentrations of resveratrol in some natural foods are given in Table 1.

In rats and humans, resveratrol is a molecule involved in the enterohepatic cycle of metabolism. In particular, after resveratrol is taken up rapidly by enterocytes, it is metabolized to glucuronide- (3-O-glucuronide and 4′-O-glucuronide) and sulfate-conjugates (3-O-sulfate), which are secreted back into the intestine where they may be deconjugated and reabsorbed or excreted in the feces (Walle et al., 2004, Marier et al., 2002). The enterohepatic cycle thus reduces the concentration of the free compound reaching target tissues. So, the low concentration of resveratrol found in blood is likely explained by this enterohepatic cycle and its rapid metabolism in the liver. Apart from dihydroresveratrol, the major metabolites formed are the glucuronide- and sulfate-conjugates, including disulfates and mixed sulfate-glucuronides (Wang et al., 2005). Concentrations of these metabolites are reported to be higher than resveratrol post-absorption and to have longer half-lives (Andres-Lacueva et al., 2009, Polycarpou et al., 2013). In fact, the majority of orally dosed resveratrol is found in urine as sulfate- or glucuronic acid-conjugates (Singh et al., 2013). In particular, the proportion of glucuronide- and sulfate-metabolites are reported to change depending on the tissue and species considered (Juan et al., 2010, Azorín-Ortuño et al., 2011): glucuronide conjugates are reported to be the main metabolites in rodents, whereas primarily sulfates are found in humans (Walle, 2011); moreover, the quantity of glucuronides is higher than sulfates in rat testes and liver, but not in lung (Juan et al., 2010). As a result, many authors are beginning to investigate the effects of resveratrol metabolites on in vitro and in vivo models: for example, resveratrol 3-O-D-sulfate, as well as resveratrol 3-O-D-glucuronide and resveratrol 4-O-D-glucuronide, was found to inhibit cycloxygenase (COX1 and COX2) (Calamini et al., 2010), whereas 3-O-D-sulfate and resveratrol 4-O-D-glucuronide were reported to reduce triacylglycerol content in 3T3-L1 adipocytes (Lasa et al., 2012). Recently, Polycarpou et al. demonstrated that resveratrol glucoronides are able to arrest the growth of different human colon cancer cells (Polycarpou et al., 2013). Thus, many of the effects reported for resveratrol may be due to the action of resveratrol’s metabolites.

Many studies have shown that resveratrol, like other polyphenols, has very low bioavailability (Goldberg et al., 2003). The bioavailability and pharmacokinetics of resveratrol have been studied in humans and in animal models. In humans, resveratrol is rapidly taken up after oral consumption of a low dose, with the plasma resveratrol concentration peaking about 30 min after consumption (Goldberg et al., 2003); in rats, the plasma half-life of resveratrol was reported to be 12–15 min after oral administration (Gescher and Steward, 2003). A study performed by Walle et al. using ¹⁴C-trans-resveratrol (25 mg orally) in humans showed that 70% of the resveratrol dose was absorbed by the body (Walle et al., 2004); a similar finding (∼50%) was reported for rats (Marier et al., 2002). The glucuronide- and sulfate-conjugated metabolites of resveratrol peaked in plasma at 30–60 min post-administration, with a plasma half-life of 9.2 h. (Walle et al., 2004). In contrast, only small amounts of unmodified resveratrol (<5 ng/mL) were detected in plasma in a similar timeframe (Singh et al., 2013). In another study conducted on mice, rats, and humans, it was shown that within 24 h after administration of 0.03 mg/kg body weight (BW) resveratrol, nearly 50% of the resveratrol was excreted in the urine. However, because <25% of the resveratrol was found in the urine with a dose of 1 mg/kg BW, these results suggest that resveratrol undergoes rapid gastrointestinal absorption in all the three species studied (Meng et al., 2004).

The amount of resveratrol ingested from dietary sources, such as red wine and juices, rarely exceeds 5 mg/L and often results in plasma levels that are either not detectable or several orders of magnitude below the micromolar concentrations that are employed in experimentation in vitro, i.e., ∼32 nM to 100 μM (Smoliga et al., 2011). For example, administration of about 25 mg resveratrol resulted in plasma concentrations of the free form that ranged from 1 to 5 ng/mL (Almeida et al., 2009), and administration of higher doses (up to 5 g) increased the plasma resveratrol concentration to about 500 ng/mL (Boocock et al., 2007). The low doses of resveratrol observed in the plasma after ingestion are very low, as the concentrations used in vitro are not reached. However, due to its lipophilic character, tissue levels of resveratrol may be higher than those found in plasma (Timmers et al., 2012). Nonetheless, some of the biological effects of resveratrol are observed at very low concentrations (Waite et al., 2005, Pearce et al., 2008), bringing forward the idea that resveratrol exerts its major effects on intestinal tissue, affecting the rest of the body through secondary effects that are independent of the plasma levels reached by the compound (Baur et al., 2006). In rodent models, the doses employed normally range from as low as 0.1 mg/kg BW to up to 1000 mg/kg BW, with even higher or lower doses occasionally being used (Baur et al., 2006). Interestingly, studies show that the bioavailability of resveratrol can be enhanced by using more potent resveratrol analogs (i.e. SRT501) (Howells et al., 2011), by enhancing delivery methods, such as liposomal encapsulation (Narayanan et al., 2009), or by combining it with piperine, a natural product from black pepper (Piper spp.) (Johnson et al., 2011).

Studies performed in vivo and in vitro have shown that resveratrol exerts pleiotropic effects and can prevent or slow the progression of several pathological conditions, including cardiovascular and metabolic diseases, ischemic brain injuries, and cancer (Jang et al., 1997, Inoue et al., 2003), as well as extend lifespan in different organisms and enhance stress resistance (Yang et al., 2013). The aim of the present review is to highlight the antioxidant effects of resveratrol, focusing our attention on cardiovascular, cerebral, and metabolic disorders, such as diabetes, and reporting also the results of the main clinical trials.