Astrocytes, but not microglia, rapidly sense H ₂O ₂ 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 H ₂O ₂ 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 H ₂O ₂-induced STAT6 phosphorylation occurred in lipid rafts. Under experimental conditions in which H ₂O ₂ did not affect astrocyte viability, H ₂O ₂-induced STAT6 phosphorylation resulted in STAT6- dependent cyclooxygenase-2 expression and subsequent release of PGE2 and prostacyclin, an effect also observed in hypoxia/ reoxygenation. Finally, PGs released from H ₂O ₂-stimulated astrocytes inhibited microglial TNF-α 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 H ₂O ₂ suggest that astrocytes can act as sentinels for certain stimuli, including H ₂O ₂ and ROS, refining the canonical notion that microglia are the first line of defense against external stimuli. Copyright