The antioxidant mechanisms underlying the aged garlic extract- and S-allylcysteine-induced protection

Abstract

Aged garlic extract (AGE) is an odorless garlic preparation containing S-allylcysteine (SAC) as its most abundant compound. A large number of studies have demonstrated the antioxidant activity of AGE and SAC in both in vivo--in diverse experimental animal models associated to oxidative stress--and in vitro conditions--using several methods to scavenge reactive oxygen species or to induce oxidative damage. Derived from these experiments, the protective effects of AGE and SAC have been associated with the prevention or amelioration of oxidative stress. In this work, we reviewed different antioxidant mechanisms (scavenging of free radicals and prooxidant species, induction of antioxidant enzymes, activation of Nrf2 factor, inhibition of prooxidant enzymes, and chelating effects) involved in the protective actions of AGE and SAC, thereby emphasizing their potential use as therapeutic agents. In addition, we highlight the ability of SAC to activate Nrf2 factor--a master regulator of the cellular redox state. Here, we include original data showing the ability of SAC to activate Nrf2 factor in cerebral cortex. Therefore, we conclude that the therapeutic properties of these molecules comprise cellular and molecular mechanisms at different levels.

Figure 1

During the process of aging γ-glutamyl-S-allylcysteine is converted to S-allylcysteine (SAC) by a γ-glutamyltransferase.

Figure 2

Antioxidant mechanism associated to S-allylcysteine (SAC). SAC can scavenge superoxide anion (O₂ ^•−), hydrogen peroxide (H₂O₂), hydroxyl radical (OH^•), peroxynitrite radical (ONOO⁻), and peroxyl radical (LOO^•) produced in neuronal cells, as well as hypochlorous acid (HOCl) and singlet oxygen (¹O₂) produced in microglial cells (blue lines). Moreover, SAC also exhibits chelating properties on Fe²⁺ and Cu²⁺ ions (red line), hence avoiding Fenton reaction. SAC also inhibits NF-kB translocation into the nucleus (green line), thus preventing apoptotic signaling. COX-2: cyclooxygenase-2, NOX: NADPH oxidase, nNOS: neuronal nitric oxide synthase, SOD: superoxide dismutase, XO: xanthine oxidase.

Figure 3

S-Allylcysteine (SAC) induces the activation of the transcription factor nuclear factor-E2-related factor 2 (Nrf2) in cerebral cortex. Animals received SAC 100 mg/kg every day for 5 days. Quantification was made by ELISA at 450 nm in nuclear extracts from frontal cortex of rats at 24 h after the last administration of SAC. Values are expressed as mean ± SEM. n = 4-5. ^a P < 0.0244 versus control group. Student's t-test. OD: optical density.

Figure 4

Effect of aged garlic extract (AGE) or S-allylcysteine (SAC) on Nrf2/Keap1 complex. Left panel: Upon unstressed conditions, this complex is dissociated and Nrf2 can either suffer proteosomal degradation or respond to stimuli typical of basal cell metabolism. In the later, Nrf2 is phosphorylated and translocated to the nucleus forming heterodimers with Maf and acting on antioxidant response element (ARE). Right panel: Under stress oxidative conditions, or in the presence of inducers, several cysteine residues suffer changes inducing its Nrf2 dissociation and further translocation of this factor to nucleus, where it will induce phase 2 genes transcription. SAC could modify cysteine residues on Keap1 domain, hence releasing Nrf2 and allowing its transactivation. Nrf2: transcription factor nuclear factor-E2-related factor 2, Keap1: kelch-related erythroid cell-derived protein with CNC homology (ECH) protein 1, UBQ: ubiquitin, ROS: reactive oxygen species, NQO1: NAD(P)H:quinone oxidoreductase 1, GST: glutathione-S-transferase, HO-1: heme oxygenase-1, GCL: glutamate cysteine ligase.

Figure 5

The known ways in which iron is directly involved in the generation of reactive oxygen species. The production of these species is potentially harmful for several cell types and tissues. Adapted from [66].

Figure 6

S-allylcysteine (SAC) prevents the progression of Alzheimer's disease (AD) by multiple mechanisms: (1) antioxidant, SAC scavenges free radicals and oxidant specie (direct antioxidant) and restores glutathione peroxidase, glutathione reductase, and superoxide dismutase levels (indirect antioxidant). Consequently, SAC diminishes lipid peroxidation, DNA fragmentation, protein oxidation, and endoplasmic reticulum (ER) stress. The decrease in endoplasmic reticulum stress attenuates Ca²⁺ release and the subsequent activation of calpain and the caspase-12-dependent pathway, which altogether decrease the cell death; (2) antiamyloidogenic, SAC decreases Aβ formation and/or increases Aβ clearance. SAC lowers amyloid precursor protein (APP) mRNA expression, BACE (β-site APP cleavage enzyme 1) expression and activity and restores PKC activity under AD-like condition, which benefits APP cleavage and decreases the available APP for Aβ. In addition, SAC can bind to Aβ-inhibiting Aβ fibrillation and destabilizing preformed Aβ-peptide fibrils; (3) anti-inflammatory, SAC decreases IL-1β and TNF-α levels and IL-1β-positive plaque-associated microglia; (4) antitangle, SAC reduces tau2 reactivity and its phosphorylation; this reduction in tau appears to involve GSK-3β protein; (5) anti-glycative; SAC declines both activity and mRNA expression of aldose reductase (AR), which subsequently decreases the production of sorbitol and prevents advanced glycation end products (AGEs) formation, such as carboxymethyllysine (CML) and pentosidine, decreasing glycative stress.