PDCD4-MAPKs-NF-κB positive loop simultaneously promotes microglia activation and neuron apoptosis during neuroinflammation

Abstract Neuroinflammation and neuron injury are common features of the central nervous system (CNS) diseases. It will be of great significance to identify their shared regulatory mechanisms and explore the potential therapeutic targets. Programmed cell death factor 4 (PDCD4), an apoptosis-related molecule, extensively participates in tumorigenesis and inflammatory diseases, but its expression and biological function during CNS neuroinflammation remain unclear. In the present study, utilizing the lipopolysaccharide (LPS)-induced neuroinflammation model on mice, we first reported an elevated expression of PDCD4 in both injured neurons and activated microglia of the inflamed mice brain. A similar change of PDCD4 expression was observed in the in vitro microglial activation model. Silencing PDCD4 by shRNA significantly inhibited the phosphorylation of MAPKs (p38, ERK, and JNK), prevented the phosphorylation and nuclear transportation of NF-κB p65, and thus attenuated the LPS-induced microglial inflammatory activation. Interestingly, LPS also required the MAPKs/NF-κB signaling activation to boost PDCD4 expression in microglia, and thus form a positive loop. Moreover, a persistent elevation of PDCD4 expression was detected in the H2O2-induced neuronal oxidative damage model. Knocking down PDCD4 significantly inhibited the expression of pro-apoptotic protein BAX, suggesting the pro-apoptotic activity of PDCD4 in the neuron. Taken together, our data indicated that PDCD4 may serve as a hub regulatory molecule that simultaneously promotes both the microglial inflammatory activation and the oxidative stress-induced neuronal apoptosis within CNS. The microglial PDCD4/MAPKs/NF-κB positive feedback loop may exaggerate the vicious cycle of neuroinflammation and neuronal injury, and thus may become potential therapeutic targets for neuroinflammatory diseases.