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* Department of Pediatrics, Gunma University School of Medicine, and
Gunma Prefectural Institute of Public Health and Environmental Sciences, Maebashi, Gunma, Japan;
Department of Reproductive and Developmental Medicine, Hokkaido University Graduate School of Medicine, Sapporo, Hokkaido, Japan; and
Departments of Medicine and Immunology, Mayo Graduate and Medical Schools, Mayo Clinic, Rochester, MN 55905
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
2 integrins, but the first phase was independent of both
adhesion and 2 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.
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