S100A8/A9 Induces Microglia Activation and Promotes the Apoptosis of Oligodendrocyte Precursor Cells by Activating the NF-κB Signaling Pathway

S100A8/A9, a heterodimer complex composed of calcium-binding proteins S100A8 and S100A9, is significantly increased in the serum of multiple sclerosis (MS) patients. Relevant reports have revealed that MS pathology is commonly associated with the activation of microglial cells and the damage of oligodendrocyte precursor cells (OPCs). Moreover, microglia activation following stimulation increases the expression of pro-inflammatory cytokines, such as interleukin-1β (IL-1β) and tumor necrosis factor-α (TNF-α), which further exacerbate the damage to OPCs. In this study, we were the first to confirm that S100A8/A9 treatment induced the activation, proliferation and migration of the murine microglia cell line BV-2; moreover, this treatment caused the cells to switch from an anti-inflammatory activated (M2) phenotype to a pro-inflammatory activated (M1) phenotype. Meanwhile, the level of the phosphorylated nuclear factor-κB (p-NF-κB) P65 protein was remarkably elevated, and the production of pro-inflammatory factors (IL-1β, TNF-α, MMP-9) and chemokines (CCL2, CCL3, CXCL10) was also increased in the S100A8/A9-treated BV-2 microglial cells. Inhibition of NF-κB P65 phosphorylation reversed the effects of S100A8/A9 on the production of pro-inflammatory factors and chemokines. We also explored the effects of S100A8/A9 and S100A8/A9-activated BV-2 microglial cells on the viability of OPCs. The results showed that both the S100A8/A9 complex and the conditioned medium (CM) of the S100A8/A9-activated BV-2 microglial cells resulted in OPC apoptosis, which was more pronounced in the case of the CM treatment. However, OPC apoptosis in the CM group was obviously decreased through the inhibition of NF-κB p65 phosphorylation. This study indicates that S100A8/A9 induces the activation of BV-2 microglial cells and promotes the production of pro-inflammatory factors by activating the NF-κB signaling pathway, which further exacerbates OPC damage.