Beyond
cholesterol, inflammatory
ether phospholipids such as
platelet-activating factor (paf) may play a role in
atherogenesis. (1) We detected a paf-like compound ('LA-paf') associated with human serum
lipoproteins, mainly in
LDL but not with the
lipoprotein-poor fraction. (2) LA-paf was also found in washed human platelets, from where it was partially released during platelet aggregation in response to paf (50 nM) or
thrombin (1 U). In addition, resident monocyte/macrophage-like U937 cells carried huge amounts of LA-paf (41 ng per 10(7) cells) and metabolized added [3H]paf to a labelled compound co-eluting with the retention time of LA-paf in standard HPLC. (3) Functionally, LA-paf had a comparable potency to synthetic paf, because LA-paf aggregated washed
aspirin-treated platelets in a concentration-dependent manner. The specific paf receptor antagonist
WEB2086 inhibited the platelet aggregation induced by three distinct LA-paf preparations as compared with synthetic paf with similar inhibitory concentrations (IC50: 35.6 +/- 12.8, 24.0 +/- 4.0, 38.0 +/- 15.8 nM for LA-paf, and 43.6 +/- 6.5 nM for synthetic paf), indicating that LA-paf interacted with
paf receptors. (4) However, LA-paf had a distinct retention time using high-pressure liquid chromatography (HPLC) as compared with synthetic paf. LA-paf eluted at 9-15 min and synthetic paf at 21-24 min. In addition, total and non-specific [3H]paf binding to intact washed human platelets was affected differently by the two unlabelled agonists: while LA-paf increased total and non-specific (but not specific) binding in a significant manner (P < 0.002 and P < 0.007) as
LDL did (P < 0.006 and P < 0.03), synthetic paf decreased total binding (P < 0.03). Similarly,
low-density lipoproteins (
LDL) increased significantly the total [3H]paf binding. In contrast, paf did not affect specific [125I]
LDL binding to human fibroblasts. Our results show the presence of LA-paf in
lipoproteins, washed human platelets and monocyte/macrophage-like cells. As
LDL and LA-paf purified from the same
LDL particles increased significantly the total [3H]paf binding to intact human platelets, it might modulate platelet adherence to vascular endothelial cells.