Abstract: BACKGROUND: Transforming microglia from the M1 phenotype to the M2 phenotype is considered to be a promising strategy for the treatment of neurodegenerative diseases. Many studies have shown that triptolide can inhibit neuroinflammation and alleviate a variety of neurodegenerative diseases, but its mechanism is still unclear.  
OBJECTIVE: To investigate the protective effect of triptolide on SH-SY5Y cell injury induced by lipopolysaccharide-activated microglia and its mechanism.
METHODS: CCK8 assay was used to detect the viability of BV2 cells and screen the best concentration of triptolide. The BV2 cells were divided into three groups: control group, model group (1 μg/mL lipopolysaccharide), and triptolide group (1 nmol/L triptolide + 1 μg/mL lipopolysaccharide). The supernatant was collected and the content of nitric oxide was detected by Griess assay. The levels of proinflammatory cytokines interleukin 6, tumor necrosis factor α, interleukin-1β and anti-inflammatory factor interleukin 10 were measured by ELISA. The expression levels of inducible nitric oxide synthase and arginase 1 in BV2 cells were determined by immunofluorescence staining and western blot assay. Three groups of microglia-conditioned medium were collected and treated on SH-SY5Y cells respectively: control-microglia-conditioned medium group, model-microglia-conditioned medium group and triptolide-microglia-conditioned medium group. TUNEL staining was utilized to detect the cell apoptosis rate in SH-SY5Y cells. Immunofluorescence staining and western blot assay were used to examine the expression of caspase 3, Bax and Bcl-2.  
RESULTS AND CONCLUSION: (1) Compared with the control group, the levels of nitric oxide, interleukin-6, tumor necrosis factor α and interleukin-1β were significantly increased, and the level of interleukin-10 was significantly decreased, and the expression of inducible nitric oxide synthase protein increased, the expression of arginase 1 protein decreased in the supernatant of BV2 cells in the model group. Compared with the model group, the levels of nitric oxide, interleukin-6, tumor necrosis factor α and interleukin-1β were significantly decreased, and the level of interleukin-10 was significantly increased, and the expression of inducible nitric oxide synthase protein was decreased; the expression of arginase 1 protein was increased in the supernatant of BV2 cells in the triptolide group. (2) Compared with the control-microglia-conditioned medium group, the apoptosis rate increased, the expression of caspase 3 and Bax increased, and the expression of Bcl-2 decreased in SH-SY5Y cells of the model-microglia-conditioned medium group. Compared with the model-microglia-conditioned medium group, the cell apoptosis rate decreased, the expression of caspase 3 and Bax decreased, and the expression of Bcl-2 increased in the triptolide-microglia-conditioned medium group. (3) The results indicate that triptolide can reduce the neurotoxicity induced by lipopolysaccharide-activated microglia, and its mechanism may be related to promoting the polarization of microglia to M2 type.

Key words: triptolide, lipopolysaccharide, microglia, inflammatory factor, polarization, neuron, apoptosis