Key Points
Alzheimer’s disease (AD) is the most common and multifactorial neurodegenerative diseases. One of the pathological features of AD is the deposition of amyloid beta (Aβ), a protein involved in oxidative stress, neuronal apoptosis and inflammatory responses. The accumulation of Aβ is accompanied by high levels of pro-inflammatory cytokines produced by both glial cells and neurons. Suppression of inflammatory process in glial cells has been proposed as a therapeutic strategy for neurodegenerative disease. In this study, the anti-inflammatory properties of ALA in Aβ25-35- treated C6 glial cell and the molecular mechanisms underlying regulation of inflammatory response and Aβ degradation were examined. The study showed that the treatment of C6 glial cells with Aβ25-35 induced toxicity, resulting into cell death. However, ALA significantly reversed the decrease in cell viability as confirmed by MTT assays. ALA prominently inhibited the production of NO, TNF-α, and IL-6 cytokines as well as decreased protein levels of TNF-α and IL-6 in Aβ25-35-stimulated C6 glial cells. Increased ROS production plays an important role in the activation of iNOS and COX-2 by inflammatory stimuli such as Aβ. ALA significantly suppressed the over-production of ROS induced by Aβ. The data shows that the treatment with ALA increased Nrf-2 activation and induced HO-1 protein expression in a concentration- and time-dependent manner. Treatment with Aβ25-35 resulted in a significant decrease in the expression of IDE and NEP. ALA treatment significantly increased protein expression levels of IDE and NEP, which are capable of degrading Aβ. The results show that ALA increased cell viability and decreased ROS production in Aβ25-35-treated glial cells. ALA also up-regulated the expression of Nrf-2 and HO-1, demonstrated as protective against oxidative stress. The mechanisms underlying anti-inflammatory effect of ALA involve interference with NO production and protein/mRNA expressions of iNOS, COX-2, TNF-α, and IL-6. The induction of antioxidant activity and inhibition of inflammatory cytokine release by the treatment with ALA has a potential to induce degradation of Aβ as shown by promotion of IDE and NEP protein expressions. This study suggests that intake of ALA should be considered as an attractive alternative to that of fish oil in the treatment/prevention of neurodegenerative disorders such as AD.
ABSTRACT
Therapeutic approaches for neurodegeneration, such as Alzheimer’s disease (AD), have been widely studied. One of the critical hallmarks of AD is accumulation of amyloid beta (Aβ). Aβ induces neurotoxicity and releases inflammatory mediators or cytokines through activation of glial cell, and these pathological features are observed in AD patient’s brain. The purpose of this study is to investigate the protective effect of alpha-linolenic acid (ALA) on Aβ25-35-induced neurotoxicity in C6 glial cells. Exposure of C6 glial cells to 50 μM 25-35 caused cell death, over-production of nitric oxide (NO), and pro-inflammatory cytokines release [interleukin (IL)-6 and tumor necrosis factor-α], while treatment of ALA increased cell viability and markedly attenuated Aβ25-35-induced excessive production of NO and those inflammatory cytokines. Inhibitory effect of ALA on generation of NO and cytokines was mediated by down-regulation of inducible nitric oxide synthase and cyclooxygenase-2 protein and mRNA expressions. In addition, ALA treatment inhibited reactive oxygen species generation induced by Aβ25-35 through the enhancement of the nuclear factor-erythroid 2-related factor-2 (Nrf-2) protein levels and subsequent induction of heme-oxygenase-1 (HO-1) expression in C6 glial cells dose- and time-dependently. Furthermore, the levels of neprilysin and insulin-degrading enzyme protein expressions, which contribute to degradation of Aβ, were also increased by treatment of ALA compared to Aβ25-35-treated control group. In conclusion, effects of ALA on Aβ degradation were shown to be mediated trough inhibition of inflammatory responses and activation of antioxidative system, Nrf-2/HO-1 signaling pathway, in C6 glial cells. Our findings suggest that ALA might have the potential for therapeutics of AD.
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