Sildenafil attenuates LPS-induced pro-inflammatory responses through down-regulation of intracellular ROS-related MAPK/NF-κB signaling pathways in N9 microglia

Excessive activation of microglial cells has been implicated in various neuroinflammation. The present study showed that sildenafil, a PDE5 inhibitor, significantly suppressed NO, interleukin 1β (IL-1β) and tumor necrosis factor α (TNF-α) production induced by LPS in microglial cells through decreasing the protein and/or mRNA expressions of inducible NO synthase (iNOS), IL-1β and TNF-α in a concentration-dependent manner. Sildenafil also blocked IκBα phosphorylation and degradation, inhibited the phosphorylation of mitogen-activated protein kinases (MAPKs), extracellular signal-regulated kinases 1 and 2 (ERK1/2), p38 MAPK, and c-Jun N-terminal kinase (JNK). Moreover, the increase of the expression of gp91phox, a critical and catalytic subunit of NADPH oxidase, and the levels of intracellular reactive oxygen species (iROS) induced by LPS were markedly inhibited by sildenafil. In summary, these data suggest that sildenafil exerts its in vitro anti-inflammatory effect in LPS-activated N9 microglial cells by blocking nuclear factor-κB (NF-κB) and MAPKs activation, which may be partly due to its potent down-regulation of the NADPH-derived iROS production.