Abstract
Human umbilical vein endothelial cells in culture synthesize platelet-activating factor (PAF) in response to the stimulation with thrombin. The most part of newly-synthesized PAF remains cell-associated even in the presence of physiological concentration of extracellular albumin which strongly binds PAF. Therefore, PAF may be incorporated into the surface membrane of endothelial cells as a part of the compositional phospholipids and thereby may lower the antithrombogenic property of these cells; and endothelial PAF generation may result in the activation of effector cells such as leukocytes and thrombocytes contacting with the surface. PAF is known to be inactivated by a specific enzyme called PAF acetylhydrolase, and previous studies have revealed the existence of this enzyme in HDL and LDL. This study was intended to clarify the effects of plasma lipoproteins on thrombin-induced production and extracellular release of PAF by endothelial cells.
Confluent monolayers of endothelial cells were incubated with [3H]acetate and thrombin in the presence or absence of either LDL or HDL (500μg protein/ml) of which PAF acetylhydrolase had been inactivated completely by the pretreatment with diisopropylfluorophosphate. [3H]PAF was separated by thin-layer chromatography and quantified by scintillation spectrometry.
Thrombin stimulated endothelial cells to produce 1, 200±616.4 dpm/106 cells PAF (n=3). Approximately 2-fold enhancement of PAF production was observed in the presence of LDL (2, 507±125.9 dpm/106 cells, n=3). About 7% of the total [3H]PAF was detected in extracellular medium in the presence of LDL, whereas the release in its absence was estimated to be 3%. HDL displayed no effect on endothelial PAF production and the extracellular release.
In conclusion, the atherogenicity of LDL may be mediated, in part, by its enhancing effect on endothelial PAF production. PAF may exert its biological activities being retained by the cells.
