Arginyl-glycyl-aspartyl (RGD) epitope of human apolipoprotein (a) inhibits platelet aggregation by antagonizing the IIb subunit of the fibrinogen (GPIIb/IIIa) receptor

An unknown epitope of apolipoprotein (a) antagonizes fibrinogen binding to agonist-stimulated platelet's fibrinogen (GPIIb/IIIa) receptor yielding lipoprotein (a) mediated decreased platelet aggregation. The purpose of this study was to test the hypothesis that human apolipoprotein (a)'s single arginyl-glycyl-aspartyl (RGD) epitope, unique to apolipoprotein (a) in lipoprotein (a) binds to the RGD binding motif on the IIb subunit of the GPIIb/IIIa receptor thus reducing platelet-bound fibrinogen and consequently decreasing agonist-stimulated platelet aggregation. Platelets (N = 30 subjects) were prepared from fresh plasma, washed three times in Tyrode's buffer and stimulated using 10 μM ADP or 2 μg/ml collagen. Lipoprotein (a) was isolated from plasma using lectin affinity chromatography followed by ultracentrifugation. The peptide RGDS inhibited 125I-labelled lipoprotein (a) binding to autologous platelets with IC-50's of 25.1 ± 2.2 (mean ± SEM) and 15.4 ± 1.3 μM for collagen- and ADP-stimulation respectively. Further, RGDS reduced platelet binding of 125I-labelled fibrinogen IC-50's of 35.5 ± 3.2 (mean ± SEM) and 20.7 ± 2.2 μM for collagen- and ADP-stimulation respectively. The monoclonal antibody PAC-1, uniquely directed at the RGD binding motif on the IIb subunit on collagen- and ADP-stimulated platelets, inhibited binding of 125I-labelled lipoprotein (a) with IC-50's of 6.4 ± 0.7 and 2.5 ± 2.2 μg/108 platelets for collagen- and ADP-stimulation respectively. Additionally, PAC-1 reduced platelet bound of 125I-labelled fibrinogen with IC-50's of 9.0 ± 1.4 and 4.1 ± 2.2 μg/108 platelets for collagen- and ADP-stimulation respectively. In a dose-related fashion, a polyclonal antibody, specific for the RGD epitope on apolipoprotein (a), restored platelet aggregation to control levels, inhibited 125I-labelled lipoprotein (a) binding, and increased 125I-labelled fibrinogen by displacing lipoprotein (a) from the GPIIb/IIIa receptor. Thus a never before demonstrated aspect of the mechanism of lipoprotein (a)'s suggested novel role as an endogenous regulator of fibrinogen binding to collagen- and ADP-stimulated platelets has been shown. In conclusion, lipoprotein (a), via apolipoprotein (a)'s RGD epitope, binds to the RGD binding motif on the IIb protein of the GPIIb/IIIa receptor consequently reducing platelet-bound fibrinogen which results in decreased platelet aggregation.