Adenosine 5'-diphosphate as a factor in platelet aggregation induced by human plasma remnant lipoproteins

Impaired blood rheology and elevated remnant-like lipoprotein particle cholesterol in hypercholesterolaemic subjects

Blood rheology, fasting serum concentrations of remnant-like lipoprotein particle cholesterol (RLP-C) and concentrations of other lipids were compared in 23 hypercholesterolaemic and 69 normocholesterolaemic subjects, and the relationship between red blood cell (RBC) deformability and RLP-C concentrations were studied in a different set of six hypercholesterolaemic and six normocholesterolaemic subjects. Passage time of whole blood and concentrations of total cholesterol (TC), triglyceride (TG), low-density lipoprotein cholesterol (LDL-C) and RLP-C were significantly higher in hypercholesterolaemic than in normocholesterolaemic subjects. Passage time of whole blood correlated positively with TC, TG, LDL-C and RLP-C and negatively with high-density lipoprotein cholesterol. Furthermore, the passage time of 10% haematocrit-adjusted RBCs in phosphate-buffered saline, which reflects RBC deformability, correlated positively with the passage time of whole blood and RLP-C. Thus, hypercholesterolaemic subjects had impaired blood rheology and elevated RLP-C concentrations, which may be associated with the pathophysiology of atherosclerosis in hypercholesterolaemic subjects. Impaired RBC deformability may contribute to impaired blood rheology associated with elevated RLP-C in hypercholesterolaemic subjects.

Effects of VLDL and Remnant Particles on Platelets

There is a considerable body of evidence supporting an association between hypertriglyceridaemia, a hypercoagulable state and atherothrombosis. A disorder of triglyceride metabolism is a key feature of the metabolic syndrome that increases risk of both ischaemic heart disease and type 2 diabetes approximately 3-fold. An increasing prevalence of obesity and metabolic syndrome is likely to contribute markedly to the prevalent ischaemic heart in the foreseeable future, and therefore it is crucial to understand mechanisms linking hypertriglyceridaemia and a hypercoagulable state. Activation of platelets and the coagulation cascade are intertwined. VLDL and remnant lipoprotein concentrations are often increased with the metabolic syndrome. These lipoproteins have the capacity to activate platelets and the coagulation pathway, and to support the assembly of the prothrombinase complex. VLDL also upregulates expression of the plasminogen activator inhibitor-1 gene and plasminogen activator inhibitor-1 antigen and activity, a process accompanied by platelet aggregation and clot formation. The surface membrane of activated platelets also supports the assembly and activity of the prothrombinase complex, resulting in further thrombin generation and amplification of the coagulation cascade. Fibrinolysis is also less efficient when thrombin is generated. Thrombin induces thrombin activatable fibrinolysis inhibitor. Thrombin activatable fibrinolysis inhibitor is a carboxypeptidase that cleaves the carboxylic lysine residues on fibrin, thereby abolishing the critical binding site for tPA-plasminogen decreasing plasmin formation. Thus the evidence is supportive of dysregulated coagulation, and impaired fibrinolysis with a predisposition to atherothrombosis, in conditions such as the metabolic syndrome, in which there are increased concentrations of VLDL and remnant lipoproteins. The purpose of this review is to describe the current evidence supporting a procoagulant state induced by VLDL and remnant lipoproteins. The role of these lipoprotein classes in (1) platelet activation; (2) the intrinsic coagulation cascade, and (3) clot formation and fibrinolysis is discussed.

Effect of remnant-like particle on shear-induced platelet activation and its inhibition by antiplatelet agents

Remnant-like particles (RLPs) have been reported to promote atherosclerosis and to have effects on platelet function. We studied the effects of RLP on shear-induced platelet activation and their inhibition by antiplatelet agents in vitro. RLP were separated using two monoclonal antibodies, anti apo B-100 and anti apo A-I. These RLP fractions were added to whole blood (WB) or platelet-rich plasma (PRP) in serial dilution of 1, 10 or 100 microg RLP triglyceride (TG) per ml of total sample volume. These samples were incubated, and then stimulated with a high shear stress of 108 dyn/cm(2). Shear-induced platelet aggregation (SIPA) was calculated from the percentage of single platelet loss. P-selectin expression on platelet surface and platelet-derived microparticle (PMP) generation were measured before and after stimulation with shear stress using flow cytometer. SIPA was significantly enhanced by RLP in WB but not in PRP. This enhancing effect was not dose-dependent and was greatest at 10 microg TG/ml. P-selectin expression induced by shear stress was only enhanced by RLP at a concentration of 100 microg TG/ml in both WB and PRP, while generation of PMP induced by shear stress was only enhanced by RLP at a concentration of 100 microg TG/ml in WB. Aspirin inhibited only the enhancement of SIPA by RLPs, while cilostazol inhibited the enhancement of not only SIPA but also p-selectin expression and PMP generation by RLPs.

Apolipoprotein (a) mediates the lipoprotein (a)-induced biphasic shift in human platelet cyclic AMP

Lipoproteins are known to influence platelet cyclic adenosine monophosphate (c-AMP) levels. Lipoprotein (a) (Lp(a))'s impact on platelet c-AMP levels has never been assessed. Increasing levels of purified human Lp(a) (1-100) mg/dl were incubated with washed human platelets. Lp(a) concentrations of 1-25 mg/dl resulted an initial statistically significant increase of platelet c-AMP above basal levels and decreased collagen-stimulated platelet aggregation levels. Higher concentrations progressively returned the platelet c-AMP concentrations to basal levels accompanied by further decreases in platelet aggregation. Increasing concentrations of purified apolipoprotein (a) (apo(a)) also resulted in a similar biphasic c-AMP response while Lp(a) without apo(a) was without impact. One antibody directed against apo(a) in intact Lp(a) removed the biphasic c-AMP pattern and eliminated Lp(a) platelet aggregation. Antibodies directed against apo B in intact Lp(a) gave results similar to intact Lp(a) in terms of the biphasic response of c-AMP upon platelet exposure to increasing levels of Lp(a). It is concluded that apo(a) mediates the Lp(a)-induced biphasic response in platelet c-AMP as the result of platelet exposure to increasing levels of Lp(a). The biphasic response in c-AMP assists in platelet aggregation decreases up to a concentration of 25 mg/dl Lp(a), such assistance being lost at higher Lp(a) concentrations.

Inhibitory effects of total flavones of Hippophae Rhamnoides L on thrombosis in mouse femoral artery and in vitro platelet aggregation

Total flavones of Hippophae Rhamnoides L (TFH) are extracted from Sea buckthorn, a Chinese herbal medicine. Sea buckthorn has antioxidant, anti-ulcerogenic and hepato-protective actions, and its berry oil is reported to suppress platelet aggregation. Though it is frequently used for patients with thrombosis, the likely mechanism(s) and effects of TFH on thrombogenesis remain unclear. Thus, we have investigated the effect in-vivo of TFH on thrombogenesis and in vitro on platelet aggregation, comparing them to those of aspirin. We measured thrombotic occlusion time in a mouse femoral artery thrombosis model by the photochemical reaction between intravenously injected rose bengal and green light irradiation. In vitro platelet aggregation in whole blood was measured by single platelet counting. Thrombotic occlusion time was 8.5 +/- 0.6 min in the control group. TFH at a dose of 300 micro g/kg, intravenously administered 15 min before the rose bengal injection, significantly prolonged it to 11.6 +/- 1.0 min (P < 0.05), a similar effect on in-vivo thrombogenesis to that of aspirin. TFH at a concentration of 3.0 micro g/ml significantly (P < 0.01) inhibited in vitro platelet aggregation induced by collagen (2 micro g/ml) in a concentration dependent manner, in contrast TFH did not affect aggregation induced by arachidonic acid (80 micro M) and ADP (0.3 micro M). The results of the present study, in which TFH prevented in-vivo thrombogenesis, probably due to inhibition of platelet aggregation, suggest a possible clinical approach for the prevention of thrombosis.

Genistein, an isoflavone included in soy, inhibits thrombotic vessel occlusion in the mouse femoral artery and in vitro platelet aggregation

Diet can be the most important factor that influences risks for cardiovascular diseases. Genistein included in soy is one candidate that may benefit the cardiovascular system. Here, we investigated the inhibitory effects of genistein on thrombotic vessel occlusion in the mouse femoral artery using a photochemical reaction, and in vitro platelet aggregation in whole blood measured by single platelet counting. Genistein (10 mg/kg), intravenously administered 10 min before the rose bengal injection, significantly prolonged the thrombotic occlusion time from 6.1+/-0.4 to 8.4+/-0.8 min (P<0.05). Genistein at doses higher than 30 microM significantly (P<0.01) inhibited in vitro platelet aggregation induced by collagen (1 and 3 microg/ml). When 10 mg/kg genistein was intravenously administered, ex vivo platelet aggregation induced by collagen (1 and 3 microg/ml) was significantly suppressed (P<0.01). In conclusion, genistein prevented in vivo thrombogenesis and suppressed in vitro platelet aggregation. These results suggest that dietary supplementation of soy may prevent the progression of thrombosis and atherosclerosis.

Remnant lipoproteins induce endothelial plasminogen activator inhibitor-1

Remnant lipoproteins (RLPs) accumulate in type III hyperlipoproteinemia, a condition associated with significant cardiovascular morbidity. The effect of RLPs on fibrinolysis is unknown. Our aim was to study the effect of RLPs on endothelial expression of plasminogen activator inhibitor-1 (PAI-1). After 24-h culture of human aortic endothelial cells with RLPs at concentrations of 0 (control), 0.038, or 0.076 mg triglyceride/mL, postculture PAI-1 antigen concentrations were: 870 +/- 80, 1963 +/- 183 (P = 0.005), and 3551 +/- 177 ng/mL (P < 0.001), respectively. Furthermore, after 24-h incubation of endothelial cells with RLPs (0 or 0.076 mg triglyceride/mL), PAI-1 activity increased from 0.667 +/- 0.144 to 1.268 +/- 0.198 U/mL, respectively (P = 0.008) and endothelial PAI-1 mRNA increased to 2.7 +/- 0.66 that of control (P = 0.048). In conclusion, RLPs from patients with type III hyperlipoproteinemia induce endothelial cell PAI-1 expression, which may contribute to a prothrombotic state.

Effects of etofibrate upon the metabolism of chylomicron-like emulsions in patients with coronary artery disease

Slow chylomicron intravascular catabolism has been associated with coronary artery disease and screening for drugs that can speed-up this process can be important. In this study, the effects of etofibrate upon chylomicron metabolism was tested by determination of the plasma kinetics of a chylomicron-like emulsion model in 12 patients with coronary artery disease, aged 59+/-11 years, (total cholesterol: 240+/-41 mg/dl; triglycerides: 188+/-42 mg/dl) submitted to a randomized, crossover, double-blind, placebo-controlled study with administration of 1 g per day etofibrate or placebo for 1-month. A 1-month washout period was inserted between the treatment periods. Patients were intravenously injected a chylomicron-like emulsion doubly labeled with 14C-cholesteryl oleate and 3H-triolein at baseline and after treatments. After etofibrate treatment, there was decrease of total cholesterol and triglyceride plasma levels and a trend to increase high-density lipoprotein cholesterol plasma levels. Etofibrate elicited 62% enhancement of post-heparin lipolytic activity and 100% increase of 3H-triglyceride fractional clearance rate compared with placebo treatment. 14C-cholesterol ester fractional clearance rate was 260% greater after etofibrate than after placebo. Therefore, a potent effect of etofibrate on both chylomicron lipolysis and remnant removal was achieved, indicating that this drug can be used to improve this metabolism in future prospective studies.