Potent SIRT1-activating compounds (STACs) were identified for the first time using recombinant human SIRT1 to screen compound libraries (Howitz et al. 2003). The potential effects of small molecule sirtuin activators have been studied in mimicking CR in extending the life span and delaying multiple diseases of aging, including T2D (Milne et al. 2007, Smith et al.
2009).
2.4.1 Polyphenols
SIRT1 activation by polyphenols has several health benefits. It confers protection against IR, inflammation, and oxidative stress, playing a potential beneficial role in mitochondrial function, glucose homeostasis, energy metabolism, cell cycle regulation, autophagy and metabolic health.
Howitz et al. initially identified plant polyphenols as STACs and studied their potential to mimic CR in vivo and in vitro (Howitz et al. 2003). These polyphenols belonged to several classes such as stilbenes, flavones, chalcones and anthocyanidins but had structural similarities with planar multi-phenyl rings and several hydroxyl groups. Resveratrol was found to be the most potent among these polyphenol SIRT1 activators inducing almost ten-fold activation of SIRT1 (Howitz et al. 2003). Resveratrol is also the most studied polyphenol due to its diverse positive metabolic effects (Chung et al. 2010). Other polyphenol SIRT1 activators include quercetin, butein, piceatannol, isoliquiritigenin, fisetin and catechins (Howitz et al. 2003, de Boer et al. 2006, Davis et al. 2009, Kawakami et al. 2014, Kim et al. 2015). Polyphenols can induce their effect either by increasing SIRT1 activity or its expression (Chung et al. 2010).
2.4.1.1 Resveratrol
Resveratrol is a polyphenolic phytoalexin isolated for the first time from white hellebore (Veratrum grandiflorum O. Loes) roots in 1940 and later from the roots of a traditional Japanese and Chinese medicinal plant, Polygonum cuspidatum (Baur and Sinclair 2006). Resveratrol was studied initially in the context of its cardio-protective effects (Siemann and Creasy 1992) and anti-cancer properties (Jang et al. 1997). Howitz et al. were the first to demonstrate the potential of resveratrol as a SIRT1 activator and a CR mimetic (Howitz et al. 2003). Since then, several publications have reported therapeutic effects of resveratrol on cancer, cardioprotection, diabetes, stroke, brain damage, inflammation and several aging disorders.
Resveratrol is classified as a stilbenoid, hydroxyl derivative of stilbene, which is produced naturally in plants by the enzyme stilbene synthase, often as a response to injury. It is found in several plant species such as grapes, apples, blueberries, raspberries, plums, peanuts, mulberries and pistachios. Resveratrol exists in two isomeric forms, cis- and trans- resveratrol (Figure 7)
.
Although the biological and antioxidant activites of both cis- and trans- forms are similar, trans- resveratrol has been studied extensively as cis-form is highly unstable and usually not found in foods (Orallo 2006).Figure 7. Chemical structures of trans- and cis –resveratrol.
Resveratrol was first identified as a SIRT1 activator using an assay screen with an acetylated peptide substrate labeled with a fluorophore (Howitz et al. 2003). However, the activation of SIRT1 by resveratrol became debatable, as the activation was reported to depend on the fluorophore moiety attached to the peptide substrate in the activity assay (Borra et al. 2005, Kaeberlein et al. 2005). Pacholec et al. also suggested that resveratrol is not a direct SIRT1 activator by using a native peptide substrate in several biochemical and biophysical studies (Pacholec et al. 2010). Yet finally, several groups showed that resveratrol can activate SIRT1 towards natural substrates through an allosteric mechanism, confirming resveratrol as a direct activator of SIRT1 (Gertz et al. 2012, Hubbard et al. 2013, Lakshminarasimhan et al. 2013).
Resveratrol enhances mitochondrial function and confers metabolic benefits protecting against diet-induced obesity and IR by inducing PGC-1α activity via SIRT1 (Lagouge et al. 2006).
Resveratrol has also activated AMPK in several studies (Baur et al. 2006, Dasgupta and Milbrandt 2007, Hou et al. 2008), which lead to the conclusions that resveratrol activates SIRT1 indirectly via AMPK activation. Resveratrol failed to enhance insulin sensitivity, mitochondrial biogenesis and metabolic rate in AMPK-deficient mice, which supports indirect activation of SIRT1 by resveratrol (Um et al. 2010). In another study, cAMP effectively induced the indirect activation of SIRT1 via the inhibition of phosphodiesterases by resveratrol (Park et al. 2012). In contrast, a conditional SIRT1 knockout demonstrated the necessity of SIRT1 in the activation of AMPK induced by resveratrol and the ability of resveratrol to stimulate SIRT1 and AMPK dose-dependently, simultaneously enhancing mitochondrial function (Price et al. 2012).
Resveratrol was initially reported to inhibit carcinogenesis (Jang et al. 1997), which generated more interest towards its therapeutic potential. The role of resveratrol in extending life span via CR has been documented in several organisms (Howitz et al. 2003, Wood et al. 2004, Rascón et al.
2012) and mammals (Baur and Sinclair 2006, Barger et al. 2008). Resveratrol extends lifespan in mice given a highly calorific diet and standard chow on alternate days (Baur et al. 2006, Pearson et al. 2008), although the observations with mice provided a standard chow ad libitum showed opposite results (Pearson et al. 2008, Strong et al. 2013). Obese humans with no endocrine disorder supplemented with resveratrol (5 g per day for 28 days) showed effects similar to CR with improvement of the metabolic profile and energy metabolism and enhanced AMPK, SIRT1 and PGC-1α protein levels (Timmers et al. 2011). The prominent role of resveratrol as a CR mimetic in improving metabolic health has been supported by studies ranging from rodents to humans (Lam et al. 2013, Kulkarni and Canto 2015).
Resveratrol has beneficial effects against T2D by various mechanisms, shown both in animal models and humans (Szkudelski and Szkudelska 2015, Bhatt et al. 2012, Kumar et al. 2013, Movahed et al. 2013). Resveratrol (5g per day for 28 days) reduced insulin levels and fasting and postprandial serum glucose in T2D patients (Elliot et al. 2009). The administration of low doses of resveratrol (2x5 mg per day for 4 weeks) to overweight T2D patients decreased IR (Brasnyό et al. 2011). SIRT1 and AMPK expression in skeletal muscle was increased in T2D patients given resveratrol (3 g per day for 12 weeks), which regulated EE (Goh et al. 2014). Preclinical studies have reported promising findings on the utility of resveratrol or its analogues in the treatment of T2D in humans (Oyenihi et al. 2016). A meta-analysis of eleven randomized controlled trials showed improved glucose control and insulin sensitivity in patients with diabetes (Liu et al.
2014).
Resveratrol has also been shown to provide significant health benefits and may protect from cancer (Carter et al. 2014), cardiovascular diseases (Bonnefont-Rousselot 2016), inflammation (Poulsen et al. 2015), obesity (Fernández-Quintela et al. 2017), neurodegenerative diseases (Pallas et al. 2013), renal diseases (Kitada et al. 2013), ophthalmic diseases (Abu-Amero et al. 2016), and several other diseases. The limited bioavailability of resveratrol, its short initial half-life
(approximately 8-14 minutes for the primary molecule) and rapid clearance in human body is challenging in studies of its clinical effects in humans, although resveratrol offers many potential health benefits (Baur and Sinclair 2006, Timmers et al. 2012, Novelle et al. 2015). The dosing and efficacy of resveratrol needs to be proven in clinical studies (Poulsen et al. 2015).
2.4.1.2 Pinosylvin (3,5-dihydroxy-trans-stilbene)
Pinosylvin is a stilbenoid polyphenol found in the heartwood of Pinus species and a component of pine leaf. Pinosylvin acts as a phytoalexin, produced by the plants in response to environmental stress and mechanical damage (Jorgensen 1961). Pinosylvin has structural resemblance with resveratrol (Figure 8)
.
Lee et al. were the first to report antifungal and antibacterial properties of pinosylvin (Lee et al. 2005). Pinosylvin and its analogues have shown anti-inflammatory effects by inhibiting the activity of prostaglandin E2 production in lipopolysaccharide-induced RAW 264.7 cells (Park et al. 2004). Pinosylvin protects from inflammation by inhibiting pro-inflammatory mediators via inhibition of the NF-κB pathway (Lee et al. 2006). The beneficial effects of pinosylvin on adjuvant arthritis (Macickova 2010) and inflammation (Moilanen et al. 2016) have been demonstrated in vivo. The chemopreventive properties of pinosylvin against cancer have been supported by in vitro studies (Park et al. 2012, Park et al. 2013). Pinosylvin showed an anti-apoptotic effect on endothelial cells and enhanced cell survival (Jeong et al. 2013), and it confers protection against oxidative stress-induced cell death (Koskela et al. 2014). Pinosylvin was shown to inhibit necrosis in bovine aortic endothelial cells. In this study, they also observed AMPK activation by pinosylvin and subsequent induction of autophagy (Park et al. 2014).Figure 8. Chemical structure of pinosylvin.
2.4.2 Other SIRT1 activators
Milne et al. identified several potent small molecule SIRT1 activators, such as SRT1460, SRT1720 and SRT2183, through a high-throughput in vitro fluorescence polarization assay and mass spectrometry. Their investigation lead to the discovery of novel compounds that are structurally unrelated to resveratrol but act through a similar enzymatic mechanism and possess 1000-fold more potency than resveratrol. These compounds demonstrated significant beneficial effects on whole-body glucose homeostasis, insulin sensitivity and mitochondrial function in skeletal muscle, adipose tissue and liver in hyperinsulinemic-euglycemic clamp
studies in Zucker fa/fa rats. These compounds mimic the favorable effects of CR and could be potential therapies in the treatment of T2D (Milne et al. 2007). SRT501, a proprietary formulation of resveratrol and SRT1720, demonstrated a marked improvement in the metabolic profile similar to CR by improving insulin sensitivity and glucose homeostasis, enhancing mitochondrial biogenesis and suppressing inflammatory pathways (Smith et al. 2009). Pacholec et al. demonstrated that SRT1460, SRT1720 and SRT2183 are not direct activators of SIRT1. They also provided evidence that these compounds directly interact with fluorophore-containing peptide substrates but do not activate SIRT1 with native full-length substrates. In addition, they observed that SRT1720 did not improve mitochondrial capacity and failed to lower plasma glucose in high fat diet mice (Pacholec et al. 2010). The reports of a common allosteric mechanism of direct activation of SIRT1 has somewhat alleviated the controversies surrounding the specificity of SRT1720 (Hubbard et al. 2013). In another study, SRT1720 was found to extend life span and provide health benefits in mice on a standard diet (Mitchell 2014). A report by Park et al. (Park et al. 2017) showed that SRT1720 requires SIRT1-independent activation of AMPK to exert its benefits on glucose homeostasis.