EP2-PKA signaling is suppressed by triptolide in lipopolysaccharide-induced microglia activation

Microglia are key players for the inflammatory responses in the central nervous system. Suppression of microglial activation and the resulting production of proinflammatory molecules are considered a promising strategy to alleviate the progression of neurodegenerative disorders. Triptolide was demonstrated as a potent anti-inflammatory compound both in vitro and in vivo. The present study explored potential signal pathways of triptolide in the lipopolysaccharide (LPS)-induced inflammatory response using primary rat microglial cells.

Microglial cells were pretreated with triptolide and stimulated with LPS. To investigate the anti-inflammatory effect of triptolide, we used Griess reagent and Western blot for NO release and iNOS expression, respectively. Moreover, we applied microglia-conditioned medium to neuronal cells and used the MTS assay to test cell viability. We found that triptolide inhibited LPS-induced NO and iNOS synthesis in microglial cells, which in turn protected neurons. To evaluate the involvement of the EP2 pathway, we used real-time PCR and Western blot to determine EP2 expression. We found that LPS induced a large increase in EP2 expression in microglia, and triptolide almost completely inhibited LPS-induced EP2 expression. Using the selective EP2 agonist butaprost and the EP2 antagonist AH6809, we determined that triptolide inhibited LPS-stimulated NO production in microglia mainly through the EP2 pathway. Additionally, by further treating triptolide-treated microglia with the downstream PKA-specific activator 6-Bnz-cAMP or the Epac-specific activator 8-pCPT-2-O-Me-cAMP, we found that 6-Bnz-cAMP but not 8-pCPT-2-O-Me-cAMP increased NO production in triptolide-LPS treated microglia. These results indicate that the EP2-PKA pathway is very important for triptolide's effects.

Triptolide inhibits LPS-stimulated NO production in microglia via a signaling mechanism involving EP2 and PKA. This finding may help establish the pharmacological function of triptolide in neurodegenerative disorders. Moreover, the observation of inflammatory EP2 signaling in primary microglia provides important evidence that EP2 regulates innate immunity in the central nervous system.