Essential role of STAT1 in caspase-independent cell death of activated macrophages through the p38 mitogen-activated protein kinase/STAT1/reactive oxygen species pathway.
Unlike other immune cells, activation of macrophages by stimulating agents, such as lipopolysaccharide (LPS), confers significant resistance to many apoptotic stimuli, but the underlying mechanism of this phenomenon remains largely unknown. Here, we demonstrate that LPS-induced early caspase activation is essential for macrophage survival because blocking caspase activation with a pancaspase inhibitor (zVAD [benzyloxycarbonyl-Val-Ala-Asp]) rapidly induced death of activated macrophages. This type of death process by zVAD/LPS was principally mediated by intracellular generation of superoxide. STAT1 knockout macrophages demonstrated profoundly decreased superoxide production and were resistant to treatment with zVAD/LPS, indicating the crucial involvement of STAT1 in macrophage death by zVAD/LPS. STAT1 level and activity were reciprocally regulated by caspase activation and were associated with cell death. Activation of STAT1 was critically dependent upon serine phosphorylation induced by p38 mitogen-activated protein kinase (MAPK) because a p38 MAPK inhibitor nullified STAT1 serine phosphorylation, reactive oxygen species (ROS) production, and macrophage death by zVAD/LPS. Conversely, p38 MAPK activation was dependent upon superoxide and was also nullified in STAT1 knockout macrophages, probably due to impaired generation of superoxide. Our findings collectively indicate that STAT1 signaling modulates intracellular oxidative stress in activated macrophages through a positive-feedback mechanism involving the p38 MAPK/STAT1/ROS pathway, which is interrupted by caspase activation. Furthermore, our study may provide significant insights in regards to the unanticipated critical role of STAT1 in the caspase-independent death pathway.
Kim
Hun Sik
HS
Department of Medicine, Samsung Medical Center, Sungkyunkwan University School of Medicine, 50 Irwon-dong, Kangnam-ku, Seoul 135-710, South Korea.
Lee
Myung-Shik
MS
D000818:Animals; D017209:Apoptosis / Q000502:physiology*; D061945:Caspase Inhibitors; D020169:Caspases / Q000502:physiology*; D004268:DNA-Binding Proteins / Q000502:physiology*; D025461:Feedback, Physiological / Q000502:physiology; D008070:Lipopolysaccharides / Q000276:immunology; D008262:Macrophage Activation / Q000276:immunology; D017737:Macrophages, Peritoneal / Q000201:enzymology / Q000276:immunology / Q000378:metabolism*; D051379:Mice; D008809:Mice, Inbred C3H; D008810:Mice, Inbred C57BL; D018345:Mice, Knockout; D017382:Reactive Oxygen Species / Q000378:metabolism*; D050794:STAT1 Transcription Factor; D013481:Superoxides / Q000378:metabolism; D015534:Trans-Activators / Q000502:physiology*; D015854:Up-Regulation; D048051:p38 Mitogen-Activated Protein Kinases / Q000502:physiology*
D061945:Caspase Inhibitors; D004268:DNA-Binding Proteins; D008070:Lipopolysaccharides; D017382:Reactive Oxygen Species; D050794:STAT1 Transcription Factor; C494080:Stat1 protein, mouse; D015534:Trans-Activators; D013481:Superoxides; D048051:p38 Mitogen-Activated Protein Kinases; D020169:Caspases