Platelet-Activating Factor Activates Two Distinct Effector Pathways in Human Eosinophils

Abstract

In granulocytes, platelet-activating factor (PAF) shares many of its biological effects with other chemotactic factors, such as FMLP, complement fragments, and lipid mediators. Two unique effects are that PAF is relatively resistant to pertussis toxin (PTX) and that PAF activates the inflammatory functions of eosinophils more strongly than it activates those of neutrophils. To investigate the molecular mechanisms of the responses of eosinophils to PAF, we analyzed superoxide anion production by a chemiluminescence method that provides real-time kinetic data for the cellular responses. We found that PAF induced bimodal superoxide anion production in human eosinophils, consisting of an intense, but transient, first phase and a larger and sustained second phase. In contrast, PAF induced essentially a transient unimodal response in human neutrophils. The two phases of eosinophil response were mediated by distinct cellular mechanisms: the second phase was highly dependent on cellular adhesion and β2 integrins, but the first phase was independent of both adhesion and β2 integrins. The upstream signaling mechanisms were also different: the second phase was mediated by PTX-resistant G-protein(s) and through activation of phosphatidylinositol 3-kinase, while the first phase was mediated by PTX-sensitive G-protein(s). Furthermore, the second-phase response was ∼100-fold more resistant to inhibition by a competitive PAF receptor antagonist than the first phase. Thus, eosinophils and neutrophils react differently to PAF, and PAF activates two separate and distinct effector pathways in human eosinophils. These two activation pathways may explain the eosinophils’ strong and diverse biological responses to PAF.

Platelet-Activating Factor Receptor and Innate Immunity: Uptake of Gram-Positive Bacterial Cell Wall into Host Cells and Cell-Specific Pathophysiology

“…In this regard, we established that cell wall causes PAFr-dependent cardiotoxicity and that this effect is cell specific. It is well known that different cell types exhibit unique physiological responses to ligation of PAFr based on the activation of distinct signaling pathways (32,33,58). Consistent with this fact, cell wall uptake invoked G protein-independent signaling in neurons, cardiomyocytes, and endothelial cells, whereas lung epithelial cells demonstrated G protein-dependent signaling and lack of cell wall uptake.…”

“…4G). PAFr can activate multiple signaling cascades with and without G protein signaling but, characteristically, ␤-arrestin-mediated signaling through PAFr leads to the activation of ERK1/2 and p38 MAP kinases without G protein participation (27,29,30,32,33,57,58). Consistent with this finding, rBCEC 6 cells, primary neurons, and HL-1 cardiomyocytes challenged with cell wall in the presence of the G protein inhibitor pertussis toxin did not show a decrease in the amount of uptake of cell wall (data not shown) or a change in phosphorylation of ERK1/2 (Fig.…”

“…Ligation of PAFr induces pleiotropic effector responses that are highly dependent on the cell context, in part due to coupling to a variety of pathways (26,27). Two signal transduction cascades that have been studied extensively are the G protein-coupled phosphorylation of protein kinases, which leads to chemotaxis, and the G protein-independent PAFr interaction with the scaffold protein ␤-arrestin-1, which results in receptor recycling by endocytosis (27)(28)(29)(30)(31)(32)(33). The ␤-arrestin-dependent pathway is responsible for pneumococcal uptake via PAFr (21,34), but no direct PAF-like G protein-stimulating activity has been described for the bacteria (35).…”

Platelet activating factor in allergies

“…PAF increases the production of leukotriene C4 (LTC 4 ), oxygen-derived radicals, and degranulation of eosinophils. 8 Very interesting observations by Kato et al 9 have shown that PAF can activate eosinophils in two stages. Phase one -fast activation of eosinophils is carried out by G proteins that, with the participation of phospholipase Cβ (PLCβ), lead to the rapid but short release of oxygen-derived free radicals, and the second phase, which also occurs with the participation of PAF, but eosinophils must be additionally stimulated by cell adhesion involving β 2 integrin.…”

Activation of Platelet-activating Factor Receptor-coupled Gαq Leads to Stimulation of Src and Focal Adhesion Kinase via Two Separate Pathways in Human Umbilical Vein Endothelial Cells

“…Depending on the cell types, multiple G proteins interact with the PAF receptor resulting in a myriad of distinct signaling pathways. In leukocytes, chemotactic responses to PAF use the pertussis toxin-resistant G proteins (8), whereas in eosinophils, PAF signals through both pertussis toxin-sensitive and -resistant G proteins (9). Activation of the p38 MAPK by PAF in Chinese hamster ovary (CHO) cells occurs through the pertussis toxin-insensitive G␣ q protein, whereas the activation of extracellular signal-regulated kinases 1 and 2 upon PAF stimulation in these cells signals through the ␣ subunit of pertussis toxin-sensitive G o but not G i protein (10,11).…”