Under chronic inflammatory conditions, monocytes/macrophages often exhibit a desensitized phenotype, which is characterized by attenuated reactive oxygen species (ROS) production in close association with depletion of protein kinase Cα (PKCα). This behavior has been observed in monocytes derived from septic blood although the stimulus responsible for initiating these alterations remained obscure. Using RAW264.7 macrophages, we provide evidence that components of neither gram-negative nor gram-positive bacteria deplete PKCα, whereas the TH1 cytokine interferon-γ (IFNγ) does. As shown by western blot analysis, lipopolysaccharide, as well as lipoteichoic acid, did not alter PKCα expression, but IFNγ dose-dependently decreased PKCα protein level. Taking into consideration that diacylglycerol and Ca2+ as established PKCα activators are released in response to phospholipase C activation, we pretreated cells with the phosphatidylcholine-specific phospholipase C (PC-PLC) inhibitor tricyclodecan-9-yl potassium xanthate (D609) and the phosphatidylinositol- specific phospholipase C inhibitor 1-(6-(17β-3-methoxyestra-1,3,5(10)-trien-17-yl)amino)hexyl)-1H-pyrrole- 2,5-dione (U73122). In cells preincubated with D609, IFNγ-mediated PKCα depletion was attenuated, whereas U73122 did not impair this process. Moreover, phorbol 12-myristate 13-acetate-initiated ROS formation, which was attenuated in macrophages pretreated with IFNγ, was restored in the presence of the PC-PLC inhibitor. These results suggest that IFNγ causes PC-PLC stimulation, diacylglycerol release, Ca2+ influx, and concomitant PKCα activation, which subsequently depletes PKCα. Strategies to antagonize IFNγ might be helpful to prevent monocyte/macrophage desensitization.
Antioxid. Redox Signal. 7, 1217–1222.
