Triptolide Inhibits Preformed Fibril-Induced Microglial Activation by Targeting the MicroRNA155-5p/SHIP1 Pathway

Abstract

Evidence suggests that various forms of <i>α</i>-synuclein- (<i>α</i>Syn-) mediated microglial activation are associated with the progression of Parkinson's disease. MicroRNA-155-5p (miR155-5p) is one of the most important microRNAs and enables a robust inflammatory response. Triptolide (T10) is a natural anti-inflammatory component, isolated from a traditional Chinese herb. The objective of the current study was to identify the role and potential regulatory mechanism of T10 in <i>α</i>Syn-induced microglial activation via the miR155-5p mediated SHIP1 signaling pathway. Mouse primary microglia were exposed to monomers, oligomers, and preformed fibrils (PFFs) of human wild-type <i>α</i>Syn, respectively. The expressions of TNF<i>α</i> and IL-1<i>β</i>, measured by enzyme-linked immunosorbent assay (ELISA) and qPCR, demonstrated that PFFs initiated the strongest immunogenicity in microglia. Application of inhibitors of toll-like receptor (TLR) 1/2, TLR4, and TLR9 indicated that PFFs activated microglia mainly via the NF-<i>κ</i>B pathway by binding TLR1/2 and TLR4. Treatment with T10 significantly suppressed PFF-induced microglial activation and attenuated the release of proinflammatory cytokines including TNF<i>α</i> and IL-1<i>β</i>. Levels of IRAK1, TRAF6, IKK<i>α</i>/<i>β</i>, p-IKK<i>α</i>/<i>β</i>, NF-<i>κ</i>B, p-NF-<i>κ</i>B, PI3K, p-PI3K, t-Akt, p-Akt and SHIP1 were measured via Western blot. Levels of miR155-5p were measured by qPCR. The results demonstrated that SHIP1 acted as a downstream target molecule of miR155-5p. Treatment with T10 did not alter the expression of IRAK1 and TRAF6, but significantly decreased the expression of miR155-5p, resulting in upregulation of SHIP1 and repression of NF-<i>κ</i>B activity, suggesting inhibition of inflammation and microglial activation. The protective effects of T10 were abolished by the use of SHIP1 siRNA and its inhibitor, 3AC, and miR155-5p mimics. In conclusion, our results demonstrated that treatment with T10 suppressed microglial activation and attenuated the release of proinflammatory cytokines by suppressing NF-<i>κ</i>B activity via targeting the miR155-5p/SHIP1 pathway in PFFs-induced microglial activation.