Effect of lipoprotein (a) on platelet activation induced by platelet-activating factor: role of apolipoprotein (a) and endogenous PAF-acetylhydrolase.

Abstract

Lipoprotein (a) [Lp(a)] is considered an atherogenic lipoprotein, which is also implicated in thrombosis. Lp(a) binds to platelets and modulates the effects of various platelet agonists. Platelet activating factor (PAF) is a potent platelet agonist degraded and inactivated by PAF-acetylhydrolase (PAF-AH), which in plasma is associated with lipoproteins. Lp(a) is enriched in PAF-AH, thus a functional characteristic of this lipoprotein is its capability to hydrolyze and inactivate PAF. In the present study, we investigated the effect of Lp(a) on PAF-induced platelet activation. The potential roles of the apo(a) moiety and especially of the PAF-AH content of Lp(a) on the above effect were also addressed. Lp(a) was isolated by affinity chromatography from plasma of apparently healthy fasting donors with serum Lp(a) concentrations >/=20 mg/dl. Reduced Lp(a) [Lp(a-)] was prepared by incubation of Lp(a) with dithiothreitol (DTT), whereas inactivation of Lp(a)-associated PAF-AH was performed by incubation of Lp(a) with pefabloc [pefa-Lp(a)]. Platelet-rich plasma (PRP) or washed platelets were prepared from peripheral venous blood samples of normolipidemic, apparently healthy fasting donors with their serum Lp(a) levels lower than 0.8 mg/dl. The surface expression of the platelet integrin-receptor alpha(IIb)beta3 and the fibrinogen binding to the activated alpha(IIb)beta3 was studied by flow cytometry. Lp(a), at doses higher than 20 microg/ml, inhibits PAF-induced platelet activation in a dose-dependent manner. Pefa-Lp(a), lacking PAF-AH activity, exhibited a similar to Lp(a) inhibitory effect. Importantly, the Lp(a) inhibitory effect was not influenced by the apo(a) isoform size, whereas Lp(a-) was a more potent inhibitor compared to Lp(a). Similarly to PAF, Lp(a) inhibits platelet aggregation induced by ADP or Calcium ionophore A23187. Lp(a), pefa-Lp(a) or Lp(a-) effectively inhibited PAF- or ADP-induced surface expression of alphaIIbbeta3, the Lp(a-) being more potent compared to Lp(a) or to pefa-Lp(a). Finally, Lp(a) significantly inhibited fibrinogen binding to platelets activated with PAF. Lp(a) inhibits PAF-induced platelet activation in a non-specific manner. The Lp(a)-associated PAF-AH does not play any important role in this effect, whereas the apo(a) moiety of Lp(a) significantly reduces its inhibitory effect. The inhibition of alpha(IIb)beta3 activation and fibrinogen binding to the activated platelets may represent the major mechanism by which Lp(a) inhibits PAF-induced platelet aggregation.