Modulators of intraplatelet calcium concentration affect the binding of thrombospondin to blood platelets in healthy donors and patients with type 2 diabetes mellitus

Pravastatin and simvastatin improves acetylsalicylic acid-mediated in vitro blood platelet inhibition

BACKGROUND

Insight into the pathophysiology of atherothrombosis indicates that an integrated risk factor approach, focusing particularly on the management of dyslipidaemia (with statins) and thrombosis (with ASA), may constitute an optimal therapeutic approach. We investigated whether pravastatin, simvastatin and atorvastatin may directly modulate under in vitro conditions the reactivity of blood platelets originating from healthy volunteers. In addition, we analysed the influence of statins on the platelet sensitivity to ASA under such conditions.

MATERIALS AND METHODS

We monitored collagen- or ADP-induced platelet aggregation, CD36, PAC-1 and CD62 expression on platelet surface and thromboxane generation after incubation with pravastatin, simvastatin, atorvastatin and/or ASA.

RESULTS

The incubation of whole blood with simvastatin and pravastatin significantly decreased CD36 expression. In the presence of 50 μM ASA, simvastatin and pravastatin significantly reduced the PAC-1 expression (30% reduction for simvastatin, P < 0·01, and 15% reduction for pravastatin, P < 0·01), platelet aggregation (20% reduction for both statins, P < 0·01) and thromboxane generation (35% reduction for simvastatin, P < 0·001, and 30% reduction for pravastatin, P < 0·001) compared to ASA alone. Atorvastatin changed neither baseline platelet aggregation nor ASA-mediated platelet inhibition.

CONCLUSIONS

Our results suggested that statins may directly interact with platelet membranes or may modulate a signalling pathway in platelets (the pleiotropic effects of statins). It is possible that the statin effect on CD36 and ASA-mediated protein acetylation can be reached by the modulation of a distribution or a function of membrane-associated proteins. Further studies are certainly needed to better elucidate the mechanism(s) underlying the statins' effects on platelet sensitivity to ASA.

Calcium mobilization by the plant estrogen ferutinin does not induce blood platelet aggregation

Platelet activation is closely associated with an increase in intracellular Ca(2+) concentration. Various compounds including Ca(2+) ionophores are able to trigger platelet aggregation by increasing intracellular Ca(2+) concentration in platelets. In the present study, we monitored the effect of the phytoestrogen ferutinin, which acts as a Ca(2+) ionophore in human blood platelets; its ionophore-like properties include upregulation of [Ca(2+)](in), activation of fibrinogen receptors and increased fibrinogen binding. Using spectrofluorometry and triple-color flow cytometry, we demonstrate that ferutinin increases [Ca(2+)](in) in both isolated platelets and platelets in whole blood from humans. This effect was almost completely blocked by the Ca(2+) chelator EGTA and was not sensitive to either Gd(3+) or econazole, which inhibit VOC and SOC channels, respectively. Nor was the effect sensitive to thapsigargin, an inhibitor of endoplasmic reticulum Ca(2+) ATPases. Ferutinin stimulated the expression of the active form of the GPIIb-IIIa complex and whole blood platelet aggregation only weakly and had no statistically significant effect on the binding of fibrinogen. These results demonstrate apparently inconsistent effects of ferutinin, which raises intraplatelet Ca(2+) concentration but fails to have an effect on spontaneous blood platelet aggregation. This pattern of responses may be caused by the combination of ferutinin's Ca(2+) ionophoric and estrogenic properties.

A novel role for CD36 in VLDL-enhanced platelet activation

Type 2 diabetes is characterized by increased plasma triglyceride levels and a fourfold increase in ischemic heart disease, but the mechanism is unclear. CD36 is a receptor/transporter that binds fatty acids of lipoproteins. CD36 deficiency has been linked with insulin resistance. There is strong evidence of in vivo interaction between platelets and atherogenic lipoproteins suggesting that atherogenic triglyceride-rich lipoproteins, such as VLDL, that are increased in diabetic dyslipidemia are important in this process. This study demonstrates that VLDL binds to the platelet receptor CD36, enhances platelet thromboxane A2 production, and causes increased collagen-mediated platelet aggregation. VLDL enhanced collagen-induced platelet aggregation by 1) shortening the time taken for aggregation to begin (lag time) to 70% of control (P = 0.001); 2) increasing maximum aggregation to 170% of control (P = 0.008); and 3) increasing thromboxane production to 3,318% of control (P = 0.004), where control represents platelets stimulated with collagen (100%). A monoclonal antibody against CD36 attenuated VLDL-enhanced collagen-induced platelet aggregation by 1) inhibiting binding of VLDL to platelets by 75% (P = 0.041); 2) lengthening lag time to 190% (P < 0.001); and 3) decreasing thromboxane production to 8% of control (P < 0.001). In support of this finding, platelets from Cd36-deficient rats showed no increase in aggregation, thromboxane production, and VLDL binding in contrast to platelets from rats expressing CD36. These data suggest that platelet Cd36 has a key role in VLDL-induced collagen-mediated platelet aggregation, possibly contributing to atherothrombosis associated with increased VLDL levels.

Mechanisms involved in platelet hyperactivation and platelet-endothelium interrelationships in diabetes mellitus

This article discusses the recent findings about the mechanisms involved in platelet hyperactivation in diabetes mellitus. Particular attention is focused on the advances pertaining to 1) new features of platelet activation; 2) the role of calcium fluxes; 3) the role of glucose, insulin, and insulin resistance; 4) the role of nitric oxide; and 5) the role of endothelium.