The importance of lipoproteins in the etiology of atherosclerosis is well established. Evidence is now accumulating to implicate thrombin in the pathogenesis of atherosclerosis. We have investigated whether atherogenic lipoproteins can support thrombin generation. In the absence of platelets or endothelial cells, both very low-density lipoprotein (VLDL) and oxidized low-density lipoprotein (LDL) support assembly of the prothrombinase complex and generation of thrombin. Thrombin generation (per μg of apolipoprotein) supported by VLDL was 19.4-fold greater than that supported by high-density lipoprotein (HDL), P < .00001, and 11.7-fold greater than that supported by LDL, P < .00001. Oxidation of LDL increased lipoprotein-supported thrombin generation 12-fold compared to unmodified LDL, P < .0001. We have shown that the phenomenon of lipoprotein-supported thrombin generation is mediated predominantly by specific phospholipids and is enhanced by oxidation of these phospholipids. The addition of vitamin E (α-tocopherol) markedly reduced the increase in thrombin generation observed after oxidation of LDL (822 ± 57 v 138 ± 47 nmol/L;P < .0001). These effects suggest that lipoproteins are important in the production of thrombin and that vitamin E may confer protection from the detrimental effects of lipoprotein oxidation by limiting thrombin formation. These results suggest that atherogenic lipoproteins are linked to the development of atherosclerosis in part by their capacity to support thrombin generation.

Normal human bone marrow from patients undergoing heart surgery was analysed quantitatively for components of the fibrinolytic system, using functional and immunological assays. Marrow was found to contain considerable fibrinolytic activity, reflecting high levels of t-PA (tissue-type plasminogen activator). The t-PA was in an active form, despite the presence of the inhibitors PAI-1 and PAI-2. Plasminogen and alpha2-antiplasmin (alpha2-AP) were also present in marrow. The balance of proteases and inhibitors differed dramatically from that observed in plasma, with higher levels of t-PA, PAI-1 and PAI-2, and lower levels of u-PA (urokinase), plasminogen, alpha2-AP and t-PA-PAI-1 complex in bone marrow, and resulted in favourable conditions for fibrinolysis. The presence of plasmin-alpha2-AP complex at concentrations of the same order of magnitude as total plasminogen and alpha2-AP demonstrated that active generation of plasmin was indeed occurring. A role for the active fibrinolytic system in normal human bone marrow may be the removal of unnecessary fibrin deposits formed in the cavities of the marrow, in order to maintain flow through this tissue.

Plasma and platelet factor Va represent different substrates for activated protein C (APC). In this study, we have measured platelet-dependent APC resistance and the effect of aspirin and a platelet glycoprotein IIbIIIa antagonist (GR144053F) on this phenomenon. In platelet rich plasma (PRP), progressive APC resistance was observed with increasing platelet activation. APC sensitivity ratios of 1.8, 1.7, and 1.4 were observed after platelet activation with thrombin receptor activating peptide (TRAP), collagen, and A23187, respectively. Ultracentrifugation at 77,000g for 1 hour abolished APC resistance indicating that the phenotype is associated exclusively with the platelet membrane. APC resistance was not observed in the presence of phosphatidylcholine-phosphatidylserine (PCPS) vesicles or purified human plasma lipoproteins. APC resistance was observed in the presence of platelet-derived microparticles, but to a lesser degree than that in the presence of activated platelets. The platelet-dependent APC resistance phenotype was also observed when endogenous APC was generated by Protac (American Diagnostica, Inc, Greenwich, CT). In vitro inhibition of platelet activation with aspirin had no effect, but the fibrinogen receptor antagonist, GR144053F, inhibited platelet-dependent APC resistance. These results indicate that platelet activation results in an APC-resistant phenotype comparable to that observed in the plasma of patients with factor V gene mutations affecting critical APC cleavage sites. This suggests that platelet activation at the site of endothelial damage downregulates a critical natural anticoagulant mechanism. The antithrombotic effect of aspirin may be due to an indirect effect on platelet-dependent APC resistance with reduced platelet retention within a developing thrombus. The more potent antithrombotic effect of glycoprotein IIbIIIa antagonists may in addition be the result of reduced platelet factor Va expression and modulation of the platelet-dependent APC resistance phenotype.

Leukaemic and normal bone marrow samples were compared in terms of their content of the fibrinolytic agents, tissue plasminogen activator (t‐PA) and urokinase‐type plasminogen activator (u‐PA) and their inhibitors, plasminogen activator inhibitors 1 and 2 (PAI‐1 and PAI‐2). Normal marrow contained t‐PA as the principal plasminogen activator, whereas in leukaemic marrow samples u‐PA was the predominant activator. Both normal and leukaemic marrows contained PAI‐1 in similar amounts, but whereas normal marrow contained significant amounts of PAI‐2 the leukaemic marrows contained very little. Plasminogen activators were present in uncomplexed, active forms and plasmin–α2‐antiplasmin complexes were generated locally more prominently in leukaemic marrows. u‐PA associated with blast cells may contribute to the severe forms of haemorrhage sometimes occurring in myeloid types of leukaemia.