Interleukin-1beta stimulates platelet-activating factor production in U-937 cells modulating both its biosynthetic and catabolic enzymes

Interleukin-1beta (IL-1 beta) is a potent agonist of platelet-activating factor (PAF) synthesis. The monocyte-derived PAF may amplify the inflammatory and thrombotic processes. The IL-1 beta-induced enzymatic alterations leading to increased PAF synthesis are ill-defined. In the present study the last enzymatic activities of the remodeling (acetyl-CoA:lyso-PAF acetyltransferase) and de novo (DTT-insensitive CDP-choline:1-alkyl-2-acetyl-sn-glycerol cholinephosphotransferase) biosynthetic routes of PAF and its main catabolic enzyme, PAF acetylhydrolase, along with the intracellular and extracellular PAF levels were determined in homogenates and medium of U-937 after their stimulation with recombinant IL-1 beta. IL-1 beta at 2.5 ng/mL induced an early (0.5-3 h) and a late (12 h) elevation of intracellular PAF levels (2-fold). Only a small portion of intracellular PAF (similar to 10%) was released to the extracellular medium. IL-1 beta increased lyso-PAF acetyltrasnferase activity which was peaked at 3 h and kept elevated till 12 h. A rapid 1.5-fold increase of cholinephosphotransferase activity was observed in IL-1 beta stimulated cells. Finally, a transient stimulation of intracellular PAF-AH was induced by IL-1 beta at 3 h while incubation of U-937 with the PAF acetylhydrolase inhibitor pefabloc in the presence or absence of IL-1 beta led to a strong sustained increase of intracellular PAF levels. In conclusion, both biosynthetic routes of PAF, along with its degradation can be modulated by IL-1 beta in a time-specific manner. The inhibition of PAF acetylhydrolase strongly augments PAF's intracellular levels implying its crucial role for the regulation of cellular PAF. The regulation of PAF's enzymatic machinery under inflammatory conditions is more complicated than we thought to be. (c) 2013 Elsevier Ltd. All rights reserved.