Antifibrinolytic Effect of Recombinant Apolipoprotein(a) in Vitro Is Primarily Due to Attenuation of tPA-Mediated Glu-Plasminogen Activation

Abstract

The effect of a 17-kringle form of recombinant apo(a) [r-apo(a)] on in vitro fibrin clot lysis was studied. In these assays, fibrin clots were formed in the wells of microtiter plates, and lysis of the clots was monitored by measurement of the turbidity at 405 nm. The results indicate that r-apo(a) produces a dose-dependent antifibrinolytic effect in clots formed using either purified components or barium-adsorbed plasma. This effect was found to be independent of clot structure, since lysis of clots formed using both high and low concentrations of thrombin was prolonged by r-apo(a) to the same extent. The two components of the antifibrinolytic effect of r-apo(a) were determined to be (i) attenuation of tPA-mediated plasminogen activation (the major component) and (ii) inhibition of plasmin degradation of fibrin, although r-apo(a) did not directly attenuate plasmin activity, as measured by S-2251 hydrolysis. r-Apo(a) interfered most substantially with tPA-mediated activation of Glu-plasminogen and less substantially with tPA-mediated Lys-plasminogen activation and urokinase-mediated activation of plasminogen. In summary, we have demonstrated that apo(a) is able to attenuate fibrin clot lysis in vitro, primarily as a consequence of the interference by apo(a) with tPA-mediated Glu-plasminogen activation. These studies illuminate possible mechanisms by which Lp(a) may contribute to the development of vascular disease in vivo.

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

“…This last result is noteworthy in that it had been previously found that apo(a) was unable to inhibit fibrinolysis in a system of purified components in which the Glu-to Lys-plasminogen step was bypassed by the addition of only Lys-plasminogen (46). In addition, apo(a) was not able to influence the activity of plasmin itself in degrading fibrin (46). These findings are supported by studies of thrombolysis in rabbit jugular vein (50) and transgenic mice expressing apo(a) (51).…”

Inhibition of Plasminogen Activation by Lipoprotein(a)

“…Several in vitro and in vivo studies have shown that apo(a)/Lp(a) can inhibit fibrinolysis (15)(16)(17). Moreover, binding interactions have been demonstrated between apo(a)/Lp(a) and fibrin(ogen), plasmin-modified fibrin(ogen), plasminogen, and tPA (13, 18 -22).…”

Apolipoprotein(a) Enhances Platelet Responses to the Thrombin Receptor–Activating Peptide SFLLRN

“…10 -12 The similarity between apo(a) and plasminogen has been interpreted as suggesting a potential prothrombotic/antifibrinolytic effect for Lp(a) that may underlie thromboembolic complications associated with elevated Lp(a) levels in vivo. Several studies have demonstrated that Lp(a) can compete with plasminogen for binding to fibrin surfaces 10,11 and that both Lp(a) 11 and apo(a) 13 inhibit plasminogen activation mediated by tissue plasminogen activator. It has also been demonstrated that Lp(a) binds to isolated platelets via a lysine-dependent interaction 14 ; there are conflicting reports as to whether the glycoprotein (GP) IIb-IIIa complex is involved in Lp(a) binding to platelets.…”