Perspectives on pharmacological and clinical benefits from sirtuin 1 activators in oxidative damage

Overview

Sirtuins, named after their homology to the Saccharomyces cerevisiae Silent Information Regulator Two, constitute a family of highly conserved nicotinamide adenine dinucleotide-dependent enzymes that deacetylate histones and residues of acetylated lysine. The main aim of this article is to put forward the pharmacological importance of major sirtuin 1 activators of natural or synthetic origin tested in case of oxidative damage during the last years. The related bioactivity of these activators as “lead” compounds in the search for new drugs and medicines is also described. With recent development about the cross talks between sirtuin 1 and its modulators (e.g. resveratrol), pharmacological and clinical research on this topic is getting new horizon.

Key words:
antioxidants; caloric restriction; longevity; metabolic diseases; nutraceuticals; oxidative stress; polyphenols; resveratrol; sirtuins; SIRT1 activators.

BACKGROUND

Oxidative damage is known to be associated with the aging process and various chronic diseases as diabetes mellitus, atherosclerosis, cataract, macular degeneration, and Alzheimer disease. Oxidative damage may be either a cause or an effect; therefore, its accurate evaluation using specific biomarkers is needed. When oxidative damage acts as a causative factor of disease, its biomarkers can be a useful tool to better understand the pathogenic mechanisms involved, find novel drug targets, and evaluate effective defense strategies such as pharmaceuticals, nutraceuticals, or food and derivatives. Generally, oxidative stress arises when the production of reactive oxygen species overwhelms the intrinsic anti-oxidant defenses. Reactive oxygen species play important roles as second messengers in many intracellular signaling cascades aimed at maintaining the cell in homeostasis with its immediate environment. At higher levels, however, they can cause indiscriminate damage to biological molecules, leading to loss of function and even cell death.

There are several thousands of published reports that deal with antioxidants in health and disease with promising, sometimes conflicting results which may lead to a kind of confusion about applications of antioxidants (1). Nevertheless, there is growing scientific agreement that antioxidants, particularly the polyphenolic forms, may help lower the incidence of disease, such as certain cancers, cardiovascular and neurodegenerative diseases, DNA damage, or even have anti-aging properties. On the other hand, questions remain as to whether some antioxidants or phytochemicals potentially could do more harm than good, as an increase in glycation-mediated protein damage (carbonyl stress) and some risk has been reported. Nevertheless, the quest for healthy aging has led to the use of antioxidants as a means to disrupt age-associated deterioration in physiological function, dysregulated metabolic processes or prevention of many age-related diseases. Although a diet rich in polyphenolic forms of antioxidants does seem to offer hope in delaying the onset of age-related disorders, it is still too early to define their exact clinical benefit for treating age-related disease.

In fact, the natural products from plants and microorganisms traditionally have provided the pharmaceutical industry with important sources of "lead" compounds in the search for new drugs and medicines where the diversity of polyphenol structure is huge. The most important polyphenols in our food are being phenolic acids and flavonoids.  Several health beneficial activities have been attributed to these naturally occurring plant flavonols, including antioxidative, anticarcinogenic, vasoprotective, anti-inflammatory, neuroregenerative, antidiabetic, antiplatelet activities, among which antioxidant properties remain the main topic investigated in recent years (2). The flavonoid silymarin and silibinin are substances with documented cytoprotective properties. Their mechanisms of action are still poorly understood and various explanations were provided. Other naturally occurring substances of plant origin have been claimed to possess cytoprotective actions include curcumin, tetrahydrocurcumin, catechin, quercetin and beta-carotene. Other naturally occurring cytoprotective agent is resveratrol (trans-3,4’,5-trihydroxystilbene) which is also a polyphenolic compound found in plant sources belonging to the stilbene class. Resveratrol has been detected in trees, in a few flowering plants, in peanuts, and in grapevines but the major dietary sources of resveratrol include grapes, wine, peanuts, and peanut products. The protective role of resveratrol in a number of tissues was reported (3-6). For resveratrol, we have found that among the mechanisms responsible for the an effective hepatoprotective effect  in the experimental fulminant hepatic failure are reduction in nitric oxide production, downregulation of inducible nitric oxide synthase, modification of other oxidative stress parameters and modulation of heme oxygenase-1 which led to overall improvement in hepatotoxic markers and morphology after the hepatic insult (7). One of the suggested mechanisms of the beneficial effects of resveratrol is due to the activation of sirtuin 1 (silent information regulator T1 or SIRT1).

SIRTUIN 1 FUNCTIONS

Sirtuins, named after their homology to the Saccharomyces cerevisiae silent information regulation 2 (Sir2) protein, constitute a family of highly conserved NAD+-dependent enzymes that deacetylate histones and residues of acetylated lysine (8). A common feature of the activity of sirtuins is their dependence on intracellular levels of NAD (nicotinamide adenine dinucleotide) in its oxidized (NAD+) or reduced form (NADH). Sirtuins are reported to act as sensors that detect cellular energy availability (9). Calorie restriction extends lifespan in organisms ranging from yeast to mammals.  In yeast, the Sir2 gene mediates the life-extending effects of calorie restriction (10). It is shown that the mammalian Sir2 orthologue, SIRT1, activates a critical component of calorie restriction in mammals; that is, fat mobilization in white adipocytes (11). Recent studies suggest a key role for the mammalian SIRT1 in the adequate cellular response to metabolic stress events such as nutrient overload or nutrient deprivation and that SIRT1 and SIRT1 activators play role in the protection from detrimental effects of metabolic stressors (12). The key role in the regulation of adipogenesis is played by nuclear receptor PPAR-γ (peroxisome proliferator-activated receptor-γ). Indeed, upon food withdrawal SIRT1 protein binds to and represses genes controlled by the fat regulator PPAR-γ, including genes mediating fat storage (11). SIRT1 represses PPAR-γ by docking with its cofactors. Repression of PPAR-γ by SIRT1 is also evident in 3T3-L1 adipocytes, where overexpression of SIRT1 attenuates adipogenesis, and RNA interference of SIRT1 enhances it.  Mobilization of fatty acids from white adipocytes upon fasting is compromised in SIRT1 (+/-) mice. In differentiated fat cells, up-regulation of SIRT1 triggers lipolysis and loss of fat. As result, a reduction in fat is sufficient to extend murine lifespan (10). The above-mentioned results provide a possible molecular pathway connecting calorie restriction to life extension in mammals.

In a study using liver-specific SIRT1-knockout mice, Chen et al. (13) challenge the assumption that calorie restriction always activates SIRT1 in all tissue types by demonstrating that SIRT1 activity is reduced in the liver during calorie restriction yet activated when mice are fed a high calorie diet. The study determines that liver-specific SIRT1-knockout mice have at least some protection, compared with wild-type mice, from accumulating fat while on a high-calorie ‘western’ diet. In contrast, while under calorie restriction, the liver-specific knockout mice have the same phenotype as wild-type mice. These observations suggest that hepatic SIRT1 may be inactivated during calorie restriction in normal mice and activated while on a high-calorie diet, opposite to what occurs in the muscle and the white adipose tissue that can be explained by different redox status and NAD+/NADH ratio in the liver from other tissues during studied conditions. The authors raised a possibility that SIRT1 inhibitors specifically targeted to the liver may be of benefit in treating obesity. Indeed, SIRT1 acts as cellular energy sensor that links metabolic stressors with an adequate cellular response. In the liver, SIRT1 activates PGC-1α (PPAR-γ coactivator-1α) during fasting to induce gluconeogenesis (13).

Recent evidence demonstrates that there are similarities between AMPK (adenosine 5´-monophosphate-activated protein kinase) and SIRT1. They have similar effects on diverse processes such as cellular fuel metabolism, inflammation, and mitochondrial function. These similarities occur because AMPK and SIRT1 both regulate each other and share many common target molecules (14). These findings and the concurrent demonstration by many laboratories of common activators, actions, and target molecules of SIRT1 and AMPK led to an examination of a possible linkage between SIRT1 and the primary upstream AMPK kinase, liver kinase B1. For example, the evidence for a SIRT1/liver kinase B1/AMPK signaling mechanism was obtained by Hou et al. (15), who demonstrated that the ability of polyphenols (resveratrol, SI 17834) to activate AMPK in cultured HepG2 cells and mouse liver in vivo required the presence of both SIRT1 and liver kinase B1.

Another important target for SIRT1 is NF-κB (nuclear factor-kappa B), a regulator of many processes, including the cell cycle, apoptosis, and inflammation. SIRT1 downregulates NF-κB-mediated proinflammatory effects by deacetylating the RelA/p65 subunit of NF-κB (16,17). It is well-documented that chronic overfeeding, by increasing circulating fatty acids, might lead to inflammation, insulin resistance and injury in the liver.  SIRT1-overexpressing transgenic mice have decreased hepatic NF-κB activity, which protects from high-fat diet/lipid-induced hepatic inflammation, glucose intolerance, and nonalcoholic fatty liver disease (18). Another study finds that SIRT1-mediated attenuation of NF-κB signaling prevents cytokine-induced pancreatic β-cell damage (19).

SIRTUIN 1 ACTIVATION

Milne et al. (20) identified novel small-molecule SIRT1 activators with 1000-fold higher pharmacologic potency compared with the structurally unrelated polyphenolic SIRT1 activator, resveratrol. Obese rodent models treated with these SIRT1 activators show improved whole-body glucose homeostasis and insulin sensitivity in liver, skeletal muscle, and adipose tissue, as well as an increased mitochondrial capacity in skeletal muscle. A very potent SIRT1 agonist, SRT1720, further increases endurance and enhances oxidative metabolism in muscle, liver, and brown adipose tissue, which might contribute to the observed protection from diet-induced obesity (12). Such increased metabolic performance might be induced by concerted deacetylation of the SIRT1 substrates PGC-1α, FOXO-1 (forkhead box-containing protein-1), and p53. SIRT1-mediated, indirect activation of AMPK and acetyl-CoA carboxylase might also contribute to the observed protection from nonalcoholic fatty liver disease  in SRT1720-treated mice fed a high-fat diet (20). These interesting reports on small-molecule SIRT1 activators also suggest promising new therapeutic approaches to treat metabolic diseases such as type 2 diabetes or nonalcoholic fatty liver disease (12,20).

Resveratrol is the most widely investigated activator of SIRT1 as a small polyphenol that improves insulin sensitivity and vascular function, boosts endurance, inhibits tumor formation, and ameliorates the early mortality associated with obesity in mice. Most of these effects are consistent with the modulation of SIRT1 targets, such as PGC-1α and NF-κB. Moreover, it can also activate AMPK, inhibit cyclooxygenases, and influence a variety of other enzymes (8). The novel activator, SRT1720, as well as various methods to manipulate NAD+ metabolism (e.g. resveratrol-induced AMPK's elevation of NAD levels in recovering and spared cells), are emerging as alternative methods to increase SIRT1 activity, and in many cases recapitulate effects of resveratrol.

CURRENT AND FUTURE RESEARCH ON SIRTUIN 1 ACTIVATION AND ITS CLINICAL POTENTIAL

This article is focusing on the pharmacological importance of major SIRT1 activators of natural or synthetic origin tested during the last years and their related bioactivity as lead compounds. The bioactivity of these compounds depends on various molecular biological effects and on their pharmacokinetic profiles. Nowadays, an emphasis is put on the importance of oxidative damage on the electron transport chain for the rational use of mitochondria-targeted antioxidants e.g. polyphenols. Moreover, molecules of interest of flavonoid type are thoroughly studied as highly potential lead compounds in type 2 diabetes mellitus (20). Therefore, it will be important to fully elucidate the role of SIRT1 in the effects of polyphenols, such as resveratrol, which appears to reduce oxidative stress even in the brain (21). More work is needed to address these questions.

These efforts are especially important in light of the fact that SIRT1 activators are entering clinical trials in humans, and “nutraceutical” formulations containing resveratrol are already widely available. With recent development about the cross talk between SIRT1 and its modulators, pharmacological and clinical research on this topic is getting new horizon.

Abbreviations

• AMPK - adenosine 5´-monophosphate-activated protein kinase
• NAD⁺ - nicotinamide adenine dinucleotide - oxidized form
• NF-κB - nuclear factor-kappa B
• PGC-1α - peroxisome proliferator-activated receptor-gamma coactivator-1alpha
• PPAR-γ - peroxisome proliferator-activated receptor-gamma
• Sir2 - silent information regulator 2 from Saccharomyces cerevisiae
• SIRT1 - silent information regulator T1 or sirtuin 1

This work was supported by the research grants: VZ MSM 0021620807, GACR 305/09/0004, GACR 305/07/0061, PRVOUK-P25/LF1/2.

Corresponding address:

MUDr. Nikolina Kutinová Canová, Ph.D.

Charles University in Prague, First Faculty of Medicine, Institute of Pharmacology

Albertov 4, 120 00 Prague 2

e-mail: ncano@lf1.cuni.cz

phone/fax: +420 224968104