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Receptor-dependent metabolism of platelet-activating factor in murine macrophages
Noriyasu Ohshima1, Satoshi Ishii, Takashi Izumi
1Department of Biochemistry and Molecular Biology, Faculty of Medicine, The University of Tokyo, Hongo 7-3-1, Bunkyo, Tokyo 113-0033 Japan.
Abstract:
Degradation of platelet-activating factor (PAF) was examined by incubating PAF with macrophages from PAF receptor-deficient mice. The degradation rate was halved as compared with wild-type mice. The reduction of the rate was comparable with the presence of a PAF antagonist WEB 2086 in wild-type cells. PAF was internalized rapidly (t(12) approximately 1 min) into wild-type macrophages. The PAF internalization was inhibited by the treatment of 0.45 m sucrose but was not affected by phorbol 12-myristate 13-acetate, suggesting that PAF internalizes into macrophages with its receptor in a clathrin-dependent manner. Internalized PAF was degraded into lyso-PAF with a half-life of 20 min. Treatment of concanavalin A inhibited the conversion of PAF into lyso-PAF, suggesting that uptake of PAF enhances PAF degradation. Lyso-PAF was subsequently metabolized into 1-alkyl-2-acyl-phosphatidylcholine. In addition, release of PAF acetylhydrolase from macrophages was enhanced when wild-type macrophages were stimulated with PAF but not from macrophages of PAF receptor-deficient mice. Thus, the PAF stimulation of macrophages leads to its degradation through both intracellular and extracellular mechanisms.
Insights
Platelet-activating factor (PAF) degradation in macrophages is primarily receptor-mediated and occurs via clathrin-dependent internalization. This process enhances PAF breakdown through both intracellular and extracellular pathways.
Area of Science:
- Immunology
- Cell Biology
- Biochemistry
Background:
- Platelet-activating factor (PAF) is a potent lipid mediator involved in inflammation and allergic responses.
- Understanding the mechanisms of PAF degradation is crucial for developing targeted therapies.
Purpose of the Study:
- To investigate the role of the PAF receptor in PAF degradation by macrophages.
- To elucidate the mechanisms of PAF internalization and subsequent breakdown.
Main Methods:
- Incubation of PAF with macrophages from wild-type and PAF receptor-deficient mice.
- Assessment of PAF degradation rates and internalization.
- Inhibition studies using PAF antagonists, sucrose, and phorbol 12-myristate 13-acetate.
- Analysis of PAF metabolites and PAF acetylhydrolase release.
Main Results:
- Degradation rate of PAF was significantly reduced in macrophages from PAF receptor-deficient mice.
- PAF internalization into wild-type macrophages occurred rapidly and was clathrin-dependent.
- Internalized PAF was degraded to lyso-PAF, and this conversion was enhanced by PAF uptake.
- PAF stimulation increased extracellular PAF acetylhydrolase release from wild-type macrophages.
Conclusions:
- Macrophage-mediated PAF degradation is largely dependent on the PAF receptor.
- PAF is internalized via clathrin-dependent endocytosis, leading to intracellular degradation.
- Both intracellular and extracellular mechanisms contribute to PAF clearance upon stimulation.