Binding of Recombinant Apolipoprotein(a) to Human Platelets and Effect on Platelet Aggregation

Abstract

SummaryThe interaction of lipoprotein(a) [Lp(a)] with platelets is not well defined, particularly with regards to the individual contribution of the protein components of Lp(a), the apo B-100 and the apolipoprotein apo(a). This study investigated the binding of different recombinant apo(a) [r-apo(a)] isoforms, to human platelets and its effect on platelet aggregation. Scatchard analysis of saturation binding experiments demonstrated that human platelets display a single class of high affinity r-apo(a) binding sites (71 ± 46 molec./platelet, Kd = 5.6 ± 2.0 nmol/L). Platelet activation with strong agonists (thrombin, arachidonic acid) increased 2- to 10-fold the r-apo(a) binding, without affecting the affinity. Competition assays showed that the binding sites are highly specific for r-apo(a) and Lp(a). At high concentration t-PA could also bind to the r-apo(a) binding sites. By contrast, neither fibrinogen nor plasminogen inhibited to the r-apo(a) binding. The lysine analogue EACA inhibits the binding of r-apo(a) to platelets, thus suggesting the involvement of lysine residues in that interaction. Moreover, the r-apo(a) binding to platelets is unlikely mediated by GPIIb/IIIa-attached fibrin since it is not affected by platelet treatment with either LJ-CP8, a monoclonal antibody that specifically blocks fibrinogen binding to GPIIb/IIIa, nor GPRP, an inhibitor of fibrin polymerisation. Finally, we show that the distinct recombinant apo(a) proteins, as well as native Lp(a), promote an aggregation response of platelets to otherwise subaggregant doses of arachidonic acid. This proaggregant effect of r-apo(a) is dependent on its binding to platelets since it requires a minimum incubation time, and it is prevented by EACA at concentration inhibiting the r-apo(a)-platelet interaction.These results suggest that the prothrombotic action of Lp(a) may be in part mediated by modulating the platelet function through the interaction of its apo(a) subunit with a specific receptor at the platelet surface.

Lipoprotein (a): truly a direct prothrombotic factor in cardiovascular disease?

“…3) (67). Another study found that apo(a) and Lp(a) evoke platelet aggregation in response to doses of arachidonic acid that would normally be subaggregant (68). These effects were the apparent result of specific binding of apo(a) to lysinecontaining receptors on the platelet (68).…”

Determination of lipoprotein(a) kringle repeat number from genomic DNA: copy number variation genotyping using qPCR

“…Lp(a) is unique in its potential to be involved via either one or both of these pathways. Lp(a) could interfere with plasminogen activation (8) or platelet function (9), or it could contribute to inflammation (10) or endothelial dysfunction (11). Nevertheless, the lack of association in some populations has led to controversy regarding the use of Lp(a) as a marker of risk in common clinical practice.…”

The Mechanism by Which Flaxseed Oil Consumption Increases Bleeding Time in Patients with Type 2 Diabetes in Cape Breton, Nova Scotia, Canada is Independent of Lipoprotein(a) Concentration

“…However, previous studies (34)(35)(36)(37)(38) have suggested that there is a negative correlation between platelet reactivity in vitro and Lp(a) concentrations based on competitive displacement of fibrinogen from the IIb protein on the fibrinogen (GPIIb/IIIa) receptor on agonist-stimulated platelets and manipulation of platelet c-AMP concentrations by Lp(a). On the contrary, a number of authors have shown an increase (39,40) or no change (41) in in vitro platelet reactivity upon incubation with increasing concentration of Lp(a). It was hypothesized that while elevated Lp(a) concentrations would present an enhanced risk of atherosclerosis in terms of increased lipid influx into the arterial wall and potential interference with plasmin activity, they also may modify platelet reactivity in vivo in patients with type 2 diabetes.…”