Modulation of multiple neuroinflammatory signalling pathways by the dietary glycosidic flavonoid tiliroside

Hyperactivated microglia plays a key role in regulating neuroinflammatory responses which propagate damage to neurons. In recent years, substantial attention has been paid in identifying new strategies to abrogate neuroinflammation. Tiliroside, a dietary glycosidic flavonoid found in several medicinal and dietary plants is known to possess anti-inflammatory and antioxidant activities. This study is aimed at investigating the molecular mechanisms involved in the inhibition of neuroinflammation by the tiliroside.

Neuroinflammation inhibitory effects of tiliroside (2-6 μM) were investigated in BV2 microglia stimulated with a combination of LPS (100 ng/ml) and IFN (5 ng/ml). Results show that tiliroside significantly reduced the production of pro-inflammatory cytokines IL-6, TNFα, IL-1β while increasing the production of anti-inflammatory cytokine IL-10 in LPS/IFN-stimulated BV2 microglia. The compound reduced NO production in LPS/IFN-stimulated BV2 cells through inhibition of iNOS protein expression. Tiliroside also suppressed COX-2 protein expression and inhibited PGE2 production in activated microglia. Western blotting and functional experiments revealed that inhibition of neuroinflammation by tiliroside was shown to be mediated through inhibition of NF-B and p38 MAPK signalling pathways. Also, the compound activated SIRT1 and inhibited the expression of acetylated-NF-B-p65 in LPS/IFN-activated BV2 microglia. Further experiments revealed that inhibition of neuroinflammation by tiliroside is not dependent on SIRT1. Tiliroside increased the levels of Nrf2, HO-1 and NQO1 antioxidant proteins, indicating an activation of the Nrf2 protective mechanisms in the microglia. Furthermore, transfection of BV2 cells with Nrf2 siRNA resulted in the loss of anti-inflammatory activities of tiliroside. Results of neurotoxicity experiments showed that neuroinflammation-induced neurodegeneration, DNA fragmentation, ROS generation and calcium accumulation were significantly reduced in HT22 neurons when exposed to conditioned medium from BV2 microglia that were pre-treated with tiliroside prior to stimulation with LPS/IFN.

Results from this study suggest that tiliroside inhibits neuroinflammation in LPS/IFN-activated BV2 microglia by targeting NF-B and p38 MAPK signalling pathways. Furthermore, the compound activated Nrf2 antioxidant mechanisms in the microglia, which appears to contribute to its anti-inflammatory activity. The study also established that tiliroside protects HT22 neurons from neuroinflammation-induced toxicity.