Interaction of Triglyceride-rich Lipoproteins with Platelets and Vitamin K-dependent Coagulation Factors

Abstract: 1. During incubation of platelets with 3H-arachidonic acid (20:4, n-6) and 14C-cholesterol doubly labelled and colloidal gold labelled chylomicrons (CMs) and chylomicron remnants (CMRs) CMs were taken up more efficiently than CMRs. Addition of unlabelled CMs, VLDLs, LDLs and HDLs decreased the uptake of labelled CMs. Electron microscopic studies demonstrated an accumulation of CM-Au's both in the open canalicular system and cytoplasm of the platelets. There was no evidence for a lipoprotein receptor mediated breakdown of CMRs. 2. Plasma exposed CMs and CM-prothrombin complexes could induce platelet aggregation and enhance the platelet serotonin- and arachidonic acid release. Platelet aggregation induced by plasma exposed CMs could be inhibited in a dose dependent manner by an antiserum against prothrombin. Coagulation factor Xa inhibitor (TenStop) inhibited platelet aggregation and serotonin release that were induced by CM-prothrombin complexes in a dose-dependent manner. Native chyle CMs did not induce platelet aggregation, but decreased ADP and thrombin induced platelet aggregation and serotonin release. Neither did CMRs induce platelet aggregation, but they potentiated the aggregation and serotonin release induced by ADP and thrombin. 3. The ability of chyle CMs to bind human prothrombin was studied in vitro. The binding was Ca2+ and temperature dependent but could not be reversed with EDTA. The metabolism in vivo of CM-125I-prothrombin complexes was compared to that of free 125I-prothrombin injected in saline or together with CMs. The plasma decrease of 125I-prothrombin was faster in the group obtained CM-125I-prothrombin complexes than in the other groups. The radioactivity in the liver was higher in the group that obtained CM-125I-prothrombin complexes. 4. All vitamin K-dependent coagulation proteins (factors VII, IX, and X, prothrombin, proteins C and S) and C4b binding protein (C4BP) were found in TG-rich lipoproteins of human plasma. A relative increase of prothrombin, protein S and C4BP was seen in TG-rich lipoproteins after a fat meal compared to the fasting lipoproteins. There was no association of the coagulation proteins with LDLs and HDLs. Nor were coagulation factor V, serum amyloid P component or thrombomodulin associated with TG-rich lipoproteins in vivo.