Astrocytes, but Not Microglia, Rapidly Sense H2O2 via STAT6 Phosphorylation, Resulting in Cyclooxygenase-2 Expression and Prostaglandin Release

Emerging evidence has established that astrocytes, once considered passive supporting cells that maintained extracellular ion levels and served as a component of the blood brain barrier, play active regulatory roles during neurogenesis and in brain pathology. In the current study, we demonstrated that astrocytes sense H2O2 by rapidly phosphorylating the transcription factor STAT6, a response not observed in microglia. STAT6 phosphorylation was induced by generators of other reactive oxygen species (ROS) and reactive nitrogen species, as well as in the reoxygenation phase of hypoxia/reoxygenation, during which ROS are generated. Src JAK pathways mediated STAT6 phosphorylation upstream. Experiments using lipid raft disruptors and analyses of detergent-fractionated cells demonstrated that H2O2-induced STAT6 phosphorylation occurred in lipid rafts. Under experimental conditions in which H2O2 did not affect astrocyte viability, H2O2-induced STAT6 phosphorylation resulted in STAT6-dependent cyclooxygenase-2 expression and subsequent release of PGE(2) and prostacyclin, an effect also observed in hypoxia/reoxygenation. Finally, PGs released from H2O2-stimulated astrocytes inhibited microglial TNF-alpha expression. Accordingly, our results indicate that ROS-induced STAT6 phosphorylation in astrocytes can modulate the functions of neighboring cells, including microglia, through cyclooxygenase-2 induction and subsequent release of PGs. Differences in the sensitivity of STAT6 in astrocytes (highly sensitive) and microglia (insensitive) to phosphorylation following brief exposure to H2O2 suggest that astrocytes can act as sentinels for certain stimuli, including H2O2 and ROS, refining the canonical notion that microglia are the first line of defense against external stimuli. The Journal of Immunology, 2012, 188: 5132-5141.