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Human macrophages secret platelet-activating factor acetylhydrolase
D M Stafforini1, M R Elstad, T M McIntyre
1Nora Ecctes Harrison Cardiovascular Research and Training Institute, University of Utah, Salt Lake City, Utah 84112.
Insights
Macrophages secrete platelet-activating factor acetylhydrolase (PAF-AH), an enzyme that inactivates inflammatory lipids. This finding suggests macrophages limit inflammation by releasing PAF-AH into plasma during monocyte maturation.
Area of Science:
- Biochemistry
- Immunology
- Cell Biology
Background:
- Monocyte maturation to macrophages significantly reduces pro-inflammatory lipid platelet-activating factor (PAF) accumulation.
- This decrease correlates with a substantial increase in intracellular PAF acetylhydrolase (PAF-AH) activity.
Purpose of the Study:
- To investigate whether macrophages secrete PAF-AH.
- To characterize the secreted PAF-AH and compare it to plasma PAF-AH.
Main Methods:
- Biochemical assays to compare enzyme activity and inhibitor sensitivity.
- Electrophoretic analysis to assess mobility.
- Lipoprotein association and transfer studies.
- Hydrolysis assays for phospholipids and PAF.
Main Results:
- Macrophages secrete PAF-AH, which is biochemically and immunologically identical to human plasma PAF-AH.
- The secreted enzyme shares sensitivity to inhibitors, electrophoretic mobility, and lipoprotein association characteristics with plasma PAF-AH.
- Both intracellular and secreted PAF-AH hydrolyze oxidatively fragmented phospholipids and PAF.
Conclusions:
- Macrophages are a significant cellular source of plasma PAF-AH.
- Macrophage-secreted PAF-AH plays a role in inactivating lipid mediators in plasma and at inflammatory sites.
- These secretory changes during monocyte-macrophage differentiation may help regulate acute inflammatory responses.
Abstract:
When monocytes mature to macrophages, their ability to accumulate the pro-inflammatory lipid autacoid, platelet-activating factor (PAF), is markedly decreased (Elstad, M. R. Stafforini, D. M., McIntyre, T. M., Prescott, S. M., and Zimmerman, G. A. (1989) J. Biol. Chem. 264, 8467-8470) in conjunction with a 260-fold increase in the activity of intracellular PAF acetylhydrolase (PAF-AH). We now demonstrate that macrophages also secrete PAF-AH and that the secreted enzyme is biochemically and immunologically identical to the human plasma PAF-AH. It is sensitive to the same active-site-directed inhibitors, has the same electrophoretic mobility, is associated with lipoprotein particles, and transfers between low density lipoprotein and high density lipoprotein in a pH-dependent manner like the plasma PAF-AH. In addition, both activities hydrolyze oxidatively fragmented phospholipids and PAF. These data indicate that macrophages are a cellular source of the plasma PAF-AH. Thus, macrophages secrete an enzyme that inactivates lipid mediators at sites of inflammation and in plasma. These changes during the maturation of monocytes to macrophages may serve to limit the acute inflammatory response.