p38β Mitogen-Activated Protein Kinase Signaling Mediates Exenatide-Stimulated Microglial β-Endorphin Expression

Recent discoveries established that activation of glucagon-like peptide-1 receptors (GLP-1 Rs) mediates neuroprotection and antinociception through microglial beta-endorphin expression. This study aimed to explore the underlying signaling mechanisms of microglial beta-endorphin. GLP-1Rs and beta-endorphin were coexpressed in primary cultures of microglia. Treatment with the GLP-1 R agonist exenatide concentration-dependently stimulated microglial expression of the beta-endorphin precursor gene proopiomelanocortin (POMC) and peptides, with EC50 values of 4.1 and 7.5 nM, respectively. Exenatide also significantly increased intracellular cAMP levels and expression of p-protein kinase A (PKA), p-p38, and p-cAMP response element binding protein (CREB) in cultured primary microglia. Furthermore, exenatide-induced microglial expression of POMC was completely blocked by reagents that specifically inhibit adenylyl cyclase and activation of PKA, p38, and CREB. In addition, knockdown of p38 beta (but not p38 alpha) using short interfering RNA (siRNA) eliminated exenatide-induced microglial p38 phosphorylation and POMC expression. In contrast, lipopolysaccharide increased microglial activation of p38, and knockdown of p38 alpha (but not p38 beta) partially suppressed expression of proinflammatory factors (including tumor necrosis factor-alpha, interleukin-1 beta, and interleukin-6). Exenatide-induced phosphorylation of p38 and CREB was also totally blocked by the PKA inhibitor and siRNA/p38 beta, but not by siRNA/p38 alpha. Seven-day intrathecal injections of siRNA/p38 beta (but not siRNA/p38 alpha) completely blocked exenatide-induced spinal p38 activation, beta-endorphin expression, and mechanical antiallodynia in rats with established neuropathy, although siRNA/p38 beta and siRNA/p38 alpha were not antiallodynic. To our knowledge, our results are the first to show a causal relationship between the PKA-dependent p38 beta mitogen-activated protein kinase/CREB signal cascade and GLP-1R agonism-mediated microglial b-endorphin expression. The differential role of p38 alpha and p38 beta activation in inflammation and nociception was also highlighted.