Platelets inhibit the activity of platelet-activating factor acetylhydrolase in monocyte-derived macrophages.

Blood platelets are capable of interacting with monocytes and macrophages and of enhancing various functions of these cells, which are believed to play a role in thrombosis and inflammation. An increase in the uptake of oxidised low density lipoprotein (LDL), in the synthesis of procoagulant tissue factor, thrombospondin and leukotrienes, as well as stimulation of oxygen radical production by platelets has been described (1-5). In circulating blood, a substantial proportion of monocytes was found to be associated with platelets, but the pathophysiological significance of such platelet-monocyte conjugates is not yet clear (6,7). Immigration of monocytes into the arterial intima and their differentiation into macrophages are initial steps in the development of an atherosclerotic lesion (8). During differentiation, there is a tremendous increase in the activity and secretion of the enzyme PAF acetylhydrolase (PAF = platelet-activating factor = 1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine) (9,10), and there is some evidence that this enzyme may contribute to the development of atherosclerosis. It cleaves PAF, and the remaining lyso-PAF is chemotactic for monocytes (11). Furthermore it also acts on oxidised low density lipoproteins and enhances their uptake into macrophages (12,13). We were therefore interested in investigating whether platelets may modulate the differentiation of monocytes into macrophages and the activity of PAF acetylhydrolase.

Generation of reactive oxygen species and activity of platelet-activating factor acetylhydrolase in human monocyte-derived macrophages.

Monocytes were prepared from healthy human volunteers and were allowed to differentiate into macrophages by adhesion to plastic surface and cultured over 7 days in presence of either 10% fetal calf serum (FCS), human control serum or serum from hyperlipaemic patients. Hyperlipaemic serum stimulated the differentiation (measured as an increase in cellular protein and DNA content) to a higher extent when compared to control serum and FCS. With all sera a marked increase of the cellular activity of the enzyme platelet-activating factor acetylhydrolase (PAF-AH) and a tremendous decrease in the capacity of cells to generate reactive oxygen species (ROS) was observed. After seven days of culture the increase in PAH-AH activity was about 19-fold with hyperlipaemic serum, 11-fold with control serum and 6-fold with FCS. During the same period of time ROS generation measured as zymosan-induced chemiluminescence decreased by about 98% and no significant differences between the three types of serum were found. The results indicate that the activity of PAF-AH and the capacity of ROS generation which are both assumed to play an important role in the oxidation of low-density lipoproteins (LDL) and thus in the development of atherosclerosis, change in opposite direction during the differentiation of blood monocytes into macrophages, and that hyperlipaemic serum stimulates PAF-AH activity but not ROS generation.