Astrocytes, but not microglia, rapidly sense H 2O 2 via STAT6 phosphorylation, resulting in cyclooxygenase-2 expression and prostaglandin release

Abstract

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©2012 by The American Association of Immunologists, Inc.