A novel role of glia maturation factor: induction of granulocyte‐macrophage colony‐stimulating factor and pro‐inflammatory cytokines

The glia maturation factor (GMF), which was discovered in our laboratory, is a highly conserved protein predominantly localized in astrocytes. GMF is an intracellular regulator of stress‐related signal transduction. We now report that the overexpression of GMF in astrocytes leads to the destruction of primary oligodendrocytes by interactions between highly purified cultures of astrocytes, microglia, and oligodendrocytes. We infected astrocytes with a replication‐defective adenovirus carrying the GMF cDNA. The overexpression of GMF caused the activation of p38 MAP kinase and transcription factor NF‐κB, as well as the induction of granulocyte‐macrophage colony‐stimulating factor (GM‐CSF) mRNA and protein in astrocytes. Small interfering RNA‐mediated GMF knockdown completely blocked the GMF‐dependent activation of p38 mitogen‐activated protein kinase (MAPK), NF‐κB, and enhanced expression of GM‐CSF by astrocytes. Inhibition of p38 MAPK or NF‐κB by specific inhibitors prevented GM‐CSF production. The cell‐free conditioned medium from overexpressing GMF astrocytes contained 320 ± 33 pg/mL of GM‐CSF, which was responsible for enhanced production and secretion of TNF‐α, IL‐1β, IL‐6, and IP‐10 by microglia. Presence of these inflammatory cytokines in the conditioned medium from microglia efficiently destroyed oligodendrocytes in culture. These results suggest that GMF‐induced production of GM‐CSF in astrocytes is depending on p38 MAPK and NF‐κB activation. The GM‐CSF‐dependent expression and secretion of inflammatory cytokine/chemokine, TNF‐α, IL‐1β, IL‐6, and IP‐10, is cytotoxic to oligodendrocytes, the myelin‐forming cells in the central nervous system, and as well as neurons. Our results suggest a novel pathway of GMF‐initiated cytotoxicity of brain cells, and implicate its involvement in inflammatory diseases such as multiple sclerosis.