Anti-inflammatory mechanism of ginseng saponins in activated microglia
Introduction
Microglia are resident immune cells of the central nervous system that are activated in response to brain injury. Upon activation, microglia release various neurotrophic factors such as BDNF, NT-3 and NGF, which support neuronal cell survival, or neurotoxic factors including nitric oxide (NO) and pro-inflammatory cytokines (Block and Hong, 2005, Kim and de Vellis, 2005). While microglial activation is necessary and critical for host defense, overactivation of microglia is neurotoxic (Streit et al., 2005). Thus, microglial activation plays an important role in the progression of neurodegenerative diseases such as Alzheimer's disease (AD) and Parkinson's diseases (PD) (Wyss-Coray and Mucke, 2002).
Most neurodegenerative diseases are diagnosed when approximately 70–80% of neuronal loss has already occurred; this loss is usually accompanied by microglial activation (Hornykiewicz and Kish, 1987, Wyss-Coray and Mucke, 2002, Block et al., 2007). The severe neuronal loss is found in substantia nigra region in PD patients, while in hippocampus and cerebral cortex area in AD patients (Block et al., 2007, Feldman, 2007). Therefore, early detection of microglial activation and anti-inflammatory therapy to reduce microglial activation may slow or halt disease progression before irreversible damage and clinical symptoms occur. Even after diagnosis, anti-inflammatory therapy may attenuate the disease progression (Block et al., 2007).
For the last decade, efforts have been made to develop anti-inflammatory agents that are able to inhibit microglial activation and prevent neuronal cell death. Such anti-inflammatory agents are represented by: 1) NSAIDS (non-steroidal anti-inflammatory drugs) (McGeer and McGeer, 2007); 2) the broad spectrum tetracycline antibiotic minocycline (Fan et al., 2005); 3) cholesterol-lowering statins (e.g., simvastatin, atorvastatin) (Lindberg et al., 2005); 4) ligands of estrogen and cannabinoid receptors (Tripanichkul et al., 2006, Elijaschewitsch et al., 2006); 5) PPAR-γ agonists (e.g., rosiglitazone, pioglitazone) (Chaturvedi and Beal, 2008); and 6) orally available glial-specific inhibitor MW01-5-188WH (Ranaivo et al., 2006). However, the long-term use of anti-inflammatory drugs is limited due to their side effects. Therefore, novel anti-inflammatory agents with fewer side effects are needed.
Ginseng, the root of Panax ginseng C.A. Meyer (Araliaceae), is frequently used as a crude substance and is taken orally as a traditional medicine in Asian countries. Now, ginseng is considered the most widely taken herbal medicine in the world (Blumenthal, 2001). Ginseng has been used to prevent and treat various diseases such as diabetes, cancer, allergy and hypertension (Radad et al., 2006). Its main constituents are saponins, polysaccharides, alkaloids, and polyacetylenes. In particular, saponins, namely ginsenosides, are believed to be responsible for most of the actions of ginseng.
Several papers have reported the neuroprotective effects of ginseng in CNS disorders. Ginsenoside Rb1 and Rg1 showed neurotrophic and neuroprotective actions in cholinergic and dopaminergic neurons (Benishin, 1992, Rudakewich et al., 2001). In addition, anxiolytic effects and improvement of learning and memory by ginseng have been reported (Cha et al., 2005, Wang et al., 2006). Recently, ginsenosides Rg3 and Rh2 were reported to protect ischemic brain injury (Park et al., 2004, Tian et al., 2005).
Despite the neuroprotective effects of ginseng, the mechanisms underlying neuroprotection at the molecular and cellular level remain elusive. Moreover, the effects of ginseng on microglial activation, which plays a key role in neurodegeneration, have been sparsely reported. Therefore, this study examined whether ginseng extract and total saponins suppress microglial activation. Furthermore, the detailed molecular mechanisms underlying the anti-inflammatory effects of ginseng were analyzed.
Section snippets
Reagents
KRG (Korea red ginseng water extract) prepared from the root of Panax ginseng C.A. Meyer was donated from KGC (Korea Ginseng Corporation, Seoul, Korea). The ginsenoside contents were analyzed by HPLC as previously reported (Kim et al., 2000). To prepare saponin fraction, KRG was extracted with BuOH (21% yield). KRG contains ginsenosides Rb1 (16.3%), Rf (7.4%), Rg3 (4.6%) and Rh2 (0.1%) as main constituents. To isolate ginsenosides and their metabolites, KRG was fermented by Bifidobacterium
Effect of KRG and GTS on NO and TNF-α release in LPS-stimulated microglial cells
BV2 microglial cells were stimulated with LPS (100 ng/ml) in the presence or absence of Korean red ginseng extract (KRG) or ginseng total saponins (GTS) to determine if either reduces the levels of NO and TNF-α, which are critical mediators of neuroinflammation and neurotoxicity. As shown in Fig. 1A, pre-treatment of BV2 cells with KRG or GTS significantly inhibited the LPS-induced NO and TNF-α production in a dose-dependent manner. The inhibition was more evident in cells treated with GTS than
Discussion
In the present study, we report that ginseng saponins suppress microglial activation and subsequent neuronal cell death. The anti-inflammatory and neuroprotective effects of ginsenosides were generally better than those of ginseng extract, suggesting that the saponins are the main ingredients responsible for the anti-inflammatory effects of ginseng extract. Ginseng saponins (GTS) significantly suppressed the LPS-induced production of NO and TNF-α in BV2 microglial cells and primary microglia.
Acknowledgement
This work was supported by a grant (2007) from the Korean Society of Ginseng funded by the Korea Ginseng Cooperation.
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