Abstract 410: Lipoprotein(a) And Apolipoprotein(a) Inhibit Lysis Of Human Thrombi Formed From Whole Blood Under Conditions Of Flow

The apolipoprotein(a) (apo(a)) component of Lp(a) is homologous to the proenzyme plasminogen, containing domains which are highly similar to plasminogen kringle (K) 4, followed by sequences that closely resemble the K5 and protease domains of plasminogen. Due to key amino acid substitutions/deletions, the protease (P) domain of apo(a) is catalytically inactive. It has been hypothesized that by competing for binding to fibrin, apo(a) can inhibit plasminogen activation and hence fibrinolysis, thereby leading to a prothrombotic role for Lp(a) in the vasculature. Although previous studies have shown inhibition by apo(a), but not Lp(a), of fibrinolysis in an ex vivo clot lysis assay, the ability of Lp(a) to inhibit lysis of clots formed from whole blood under conditions of flow that more closely resemble arterial thrombi in vivo has not been assessed. In the current study, whole blood was collected from a human donor with low Lp(a) levels, recalcified, and rotated in a Chandler loop device at 30 rpm for 90 minutes. Blood in the closed loops was supplemented with fluorescently labelled fibrinogen, and 0 or 250 nmol/L of purified plasma-derived Lp(a), 17-kringle (17K) recombinant apo(a) or a 17KΔP apo(a) variant lacking the protease-like domain. Interestingly, this protease-like domain has been shown to mediate lysine-dependent binding of apo(a) to plasminogen. Resultant thrombi were exposed to tPA and clot lysis was measured as release of fluorescent fibrinogen over time in a fluorescent plate reader. The addition of 250 nmol/L Lp(a) significantly decreased clot lysis by up to 15% compared to control (p<0.05). On the other hand, 250 nmol/L 17K apo(a) markedly reduced clot lysis by up to 48% relative to control (p<0.01). 17KΔP apo(a), however, did not affect clot lysis in this system. Our findings show that Lp(a) impedes lysis of human clots formed from whole blood under flow conditions, albeit with a much lower potency than 17K apo(a). Interestingly, our results indicate that the antifibrinolytic effect of apo(a) is dependent on the presence of the inactive protease domain, revealing the importance of the apo(a)-plasminogen interaction. These findings suggest that markedly elevated plasma Lp(a) levels might predispose thrombotic events in vivo .