Lysine-Binding Heterogeneity of Lp(a): Consequences for Fibrin Binding and Inhibition of Plasminogen Activation

Abstract

SummaryLipoprotein(a) [Lp(a)] is recognized as an independent risk factor for atherosclerosis. Lp(a) consists of a LDL-like moiety with an additional glycoprotein, apo(a), linked to apolipoprotein B-100. Apo(a) has a high homology with plasminogen (Pg). In vivo, Pg is activated on a fibrin surface by tissue Pg activator (tPA). We prepared Lp(a) from plasma by sequential ultracentrifugation followed by lysine-sepharose affinity chromatography. We found that a changing (donor dependent) fraction of the Lp(a) did not bind to lysine-sepharose. This fraction, designated Lp(a)lys–, was further purified using gel filtration. Bound Lp(a) [Lp(a)lys+] was eluted with 0.2 M EACA. Apo(a) isoforms in both fractions were identical. In contrast Lp(a)lys+ inhibited Pg activation by tPA in vitro (IC50% 20 mg/1), whereas Lp(a)lys– did not. In addition Lp(a)lys– did not bind to CNBr-digested fibrinogen whereas Lp(a)lys+ did (K d, app = 0.2 nM). Therefore we conclude that a changing donor dependent fraction of human plasma Lp(a) does not inhibit Pg activation in vitro and does not bind to CNBr-digested fibrinogen.

Modification of apolipoprotein(a) lysine binding site reduces atherosclerosis in transgenic mice.

“…This group has also provided the cogent warning that one cannot presume that measurement of lysine binding of Lp(a) variants will faithfully translate to relative binding affinity of Lp(a) to fibrin itself (36, 47). Leerink and colleagues isolated lysine binding and nonbinding Lp(a) fractions from several human donors (14). Although the molecular causes of this binding difference were not determined, they reported that the lysine binding fraction of Lp(a) bound fibrin and inhibited plasminogen activation, whereas the lysine nonbinding fraction did neither (14).…”

“…Leerink and colleagues isolated lysine binding and nonbinding Lp(a) fractions from several human donors (14). Although the molecular causes of this binding difference were not determined, they reported that the lysine binding fraction of Lp(a) bound fibrin and inhibited plasminogen activation, whereas the lysine nonbinding fraction did neither (14). Since it is not yet practical to conduct human clinical trials with naturally occurring sequence variants that are of a scope to achieve statistically significant conclusions, we have used a transgenic mouse model of fatty streak development to address the in vivo effect of the LBS of apo(a) kringle 4-37.…”

“…After 18 wk on the high fat diet, there is both a significantly greater area of oil red O staining, and of focal regions of intense apo(a) staining in the vessel wall of wildtype versus mutant apo(a) mice. Although a few cases of naturally occurring mutations in the LBS of kringle 4-37 of apo(a) have been documented to date (33), it is commonly observed that individual isolates of human Lp(a) differ in their degree of binding to lysine-Sepharose (14,55). The results in this transgenic mouse model suggest the utility of further studies in animal models and in humans to determine whether such isoforms of the highly polymorphic Lp(a) are more benign to their human hosts.…”

“…In vitro studies show that apo(a) competes for the binding of plasminogen to fibrin and other substrates and reduces the generation of active plasmin (7)(8)(9)(10)(11)(12)(13)(14). A resulting decrease in fibrinolysis can prolong the persistence of mural thrombi which stimulate repair processes that lead to the local thickening of the vessel wall.…”

Lipoprotein(a) Oxidation, Autoimmune and Atherosclerosis

“…Pepin et al [8] found that Lp(a) in atherosclerotic lesions was oxidized, and the oxidized Lp(a) could be a more tightly bound fraction than apoB. Two studies reported large variations in the lysine binding capacity of Lp(a) purified from different individuals [26,27]. This heterogeneity appeared not to be associated with apo(a) isoforms.…”