Silymarin protects dopaminergic neurons against lipopolysaccharide‐induced neurotoxicity by inhibiting microglia activation

Abstract

An inflammatory response in the central nervous system mediated by activation of microglia is a key event in the early stages of the development of neurodegenerative diseases. Silymarin is a polyphenolic flavanoid derived from milk thistle that has anti-inflammatory, cytoprotective and anticarcinogenic effects. In this study, we first investigated the neuroprotective effect of silymarin against lipopolysaccharide (LPS)-induced neurotoxicity in mesencephalic mixed neuron-glia cultures. The results showed that silymarin significantly inhibited the LPS-induced activation of microglia and the production of inflammatory mediators, such as tumour necrosis factor-alpha and nitric oxide (NO), and reduced the damage to dopaminergic neurons. Therefore, the inhibitory mechanisms of silymarin on microglia activation were studied further. The production of inducible nitric oxide synthase (iNOS) was studied in LPS-stimulated BV-2 cells as a model of microglia activation. Silymarin significantly reduced the LPS-induced nitrite, iNOS mRNA and protein levels in a dose-dependent manner. Moreover, LPS could induce the activation of p38 mitogen-activated protein kinase (MAPK) and c-jun N-terminal kinase but not extracellular signal-regulated kinase. The LPS-induced production of NO was inhibited by the selective p38 MAPK inhibitor SB203580. These results indicated that the p38 MAPK signalling pathway was involved in the LPS-induced NO production. However, the activation of p38 MAPK was not inhibited by silymarin. Nevertheless, silymarin could effectively reduce LPS-induced superoxide generation and nuclear factor kappaB (NF-kappaB) activation. It suggests that the inhibitory effect of silymarin on microglia activation is mediated through the inhibition of NF-kappaB activation.

Protective effect of 1,2,4-benzenetriol on LPS-induced NO production by BV2 microglial cells

“…We previously showed that LPS activated p38 and JNK, but not ERK MAPK. The phosphorylation of p38 reached a maximum between 15 and 20 min and JNK remained high for 120 min [15]. These results were consistent with observations that only p38 MAPK inhibitor (SB203580) could inhibit LPSinduced NO production [15].…”

“…The phosphorylation of p38 reached a maximum between 15 and 20 min and JNK remained high for 120 min [15]. These results were consistent with observations that only p38 MAPK inhibitor (SB203580) could inhibit LPSinduced NO production [15]. Similar to sesame antioxidants [20], BT at a concentration of 50-100 lM, significantly decreased (50-70%) LPSactivated p38 MAPK ( Figure 6).…”

“…Excessive NO and ROS production in the brain are believed to contribute to neurodegenerative processes [24][25][26]. Various dietary-derived polyphenolic compounds that inhibit the LPSinduced NO production may have neuroprotective potential [15,20,27]. We found that BT at concentrations of 50-100 lM inhibited 50-70% of LPS-stimulated iNOS mRNA expression and 70% of NO production in BV-2 microglia.…”

“…Specific inhibitors of p38 MAPK have been proven to reduce inflammation, slow down microglia activation and provide neuroprotective effects [15,[30][31][32]. Recently, studies have shown that compounds that inhibit p38 MAPK activation in microglia represent potential anti-inflammatory effects and protect neurons against excitotoxicity or LPS-induced neurotoxicity [9,20,33].…”

“…They also increase the sensitivity of the cells to both granulocytemacrophage colony-stimulating factor (GM-CSF) and macrophage colony-stimulating factor (M-CSF) in the enhancement of NO production [14]. NO and inflammatory mediators, such as tumor necrosis factor-a (TNF-a) released from LPSactivated microglia can damage dopaminergic neurons [15]. The long-term supplementation of BT (1.2%, w/w) increases mouse lipid peroxidation and type IV and I allergy responses [16].…”