Background: Cannabinoid receptor type 2 (CBR2) inhibits microglial reactivity through a molecular mechanism yet to be elucidated. We hypothesized that CBR2 activation induces an anti-inflammatory phenotype in microglia by inhibiting extracellular signal-regulated kinase (ERK) pathway, via mitogen-activated protein kinase-phosphatase (MKP) induction. MKPs regulate mitogen activated protein kinases, but their role in the modulation of microglial phenotype is not fully understood.
Results: JWH015 (a CBR2 agonist) increased MKP-1 and MKP-3 expression, which in turn reduced p-ERK1/2 in LPS-stimulated primary microglia. These effects resulted in a significant reduction of tumor necrosis factor-alpha (TNF) expression and microglial migration. We confirmed the causative link of these findings by using MKP inhibitors. We found that the selective inhibition of MKP-1 by Ro-31-8220 and PSI2106, did not affect p-ERK expression in LPS+JWH015-treated microglia. However, the inhibition of both MKP-1 and MKP-3 by triptolide induced an increase in p-ERK expression and in microglial migration using LPS+JWH015-treated microglia.
Conclusion: Our results uncover a cellular microglial pathway triggered by CBR2 activation. These data suggest that the reduction of pro-inflammatory factors and microglial migration via MKP-3 induction is part of the mechanism of action of CBR2 agonists. These findings may have clinical implications for further drug development.