Hedgehog lipoprotein(a) is a modulator of activation of plasminogen at the fibrin surface. An in vitro study

Identification and characterization of novel lysine-independent apolipoprotein(a)-binding sites in fibrin(ogen) alphaC-domains

Accumulation of lipoprotein(a) (Lp(a)) in atherosclerotic plaques is mediated through interaction of fibrin-(ogen) deposits with the apolipoprotein(a) (apo(a)) moiety of Lp(a). It was suggested that because apo(a) competes with plasminogen for binding to fibrin, causing inhibition of fibrinolysis, it could also promote atherothrombosis. Because the fibrin(ogen) alphaC-domains bind plasminogen and tissue-type plasminogen activator with high affinity in a Lys-dependent manner, we hypothesized that they could also bind apo(a). To test this hypothesis, we studied the interaction between the recombinant apo(a) A10 isoform and the recombinant alphaC-fragment (Aalpha-(221-610)) corresponding to the alphaC-domain by enzyme-linked immunosorbent assay and surface plasmon resonance. Both methods revealed a high affinity interaction (Kd = 19-21 nm) between the immobilized alphaC-fragment and apo(a), indicating that the former contains an apo(a)-binding site. This affinity was comparable to that of apo(a) for fibrin. At the same time, no interaction was observed between soluble fibrinogen and immobilized apo(a), suggesting that, in the former, this and other apo(a)-binding sites are cryptic. Further experiments with truncated recombinant variants of the alphaC-fragment allowed localization of the apo(a)-binding site to the Aalpha-(392-610) region. The presence of epsilon-aminocaproic acid only slightly inhibited binding of apo(a) to the alphaC-fragment, indicating the Lys-independent nature of their interaction. In agreement, the influence of plasminogen or tissue-type plasminogen activator on binding of apo(a) to the alphaC-fragment was minimal. These results indicate that the alphaC-domains contain novel high affinity apo(a)-binding sites that may provide a Lys-independent mechanism for bringing Lp(a) to places of fibrin deposition such as injured vessels or atherosclerotic lesions.

Lipoprotein Lp(a) and atherothrombotic disease

High plasma concentrations of lipoprotein (a) [Lp(a)] are now considered a major risk factor for atherosclerosis and cardiovascular disease. This effect of Lp(a) may be related to its composite structure, a plasminogen-like inactive serine-proteinase, apoprotein (a) [apo(a)], which is disulfide-linked to the apoprotein B100 of an atherogenic low-density lipoprotein (LDL) particle. Apo(a) contains, in addition to the protease region and a copy of kringle 5 of plasminogen, a variable number of copies of plasminogen-like kringle 4, giving rise to a series of isoforms. This structural homology endows Lp(a) with the capacity to bind to fibrin and to membrane proteins of endothelial cells and monocytes, and thereby inhibits binding of plasminogen and plasmin formation. This mechanism favors fibrin and cholesterol deposition at sites of vascular injury and impairs activation of transforming growth factor-beta (TGF-beta) that may result in migration and proliferation of smooth muscle cells into the vascular intima. It is currently accepted that this effect of Lp(a) is linked to its concentration in plasma, and an inverse relationship between apo(a) isoform size and Lp(a) concentrations that is under genetic control has been documented. Recently, it has been shown that inhibition of plasminogen binding to fibrin by apo(a) from homozygous subjects is also inversely associated with isoform size. These findings suggest that the structural polymorphism of apo(a) is not only inversely related to the plasma concentration of Lp(a), but also to a functional heterogeneity of apo(a) isoforms. Based on these pathophysiological findings, it can be proposed that the predictive value of Lp(a) as a risk factor for vascular occlusive disease in heterozygous subjects would depend on the relative concentration of the isoform with the highest affinity for fibrin.

The lysine-binding function of Lp(a)

The atherogenicity of Lp(a) is attributable to the binding of its apolipoprotein(a) component to fibrin and other plasminogen substrates. It can attenuate the activation of plasminogen, diminishing plasmin-dependent fibrinolysis and transforming growth factor-beta activation. Apolipoprotein(a) contains a major lysine-binding site in one of its kringle domains. Destroying this site by site-directed mutagenesis greatly reduces the binding of apolipoprotein(a) to lysine and fibrin. Transgenic mice expressing wild-type apolipoprotein(a) have a 5-fold increase in the development of lipid lesions, as well as a large increase in the focal deposition of apolipoprotein(a) in the aorta, compared to the lysine-binding site mutant strain and to non-transgenic litter mates. Although the adaptive function of apolipoprotein(a) remains obscure, a gene with similar structure has evolved by independent remodeling of the plasminogen twice during the course of mammalian evolution.

Convergent evolution of apolipoprotein(a) in primates and hedgehog

Apolipoprotein(a) [apo(a)] is the distinguishing protein component of lipoprotein(a), a major inherited risk factor for atherosclerosis. Human apo(a) is homologous to plasminogen. It contains from 15 to 50 repeated domains closely related to plasminogen kringle four, plus single kringle five-like and inactive protease-like domains. This expressed gene is confined to a subset of primates. Although most mammals lack apo(a), hedgehogs produce an apo(a)-like protein composed of highly repeated copies of a plasminogen kringle three-like domain, with complete absence of protease domain sequences. Both human and hedgehog apo(a)-like proteins form covalently linked lipoprotein particles that can bind to fibrin and other substrates shared with plasminogen. DNA sequence comparisons and phylogenetic analysis indicate that the human type of apo(a) evolved from a duplicated plasminogen gene during recent primate evolution. In contrast, the kringle three-based type of apo(a) evolved from an independent duplication of the plasminogen gene approximately 80 million years ago. In a type of convergent evolution, the plasminogen gene has been independently remodeled twice during mammalian evolution to produce similar forms of apo(a) in two widely divergent groups of species.

How often has Lp(a) evolved?

The lipoprotein Lp(a) is associated with increased risk of atherosclerosis and myocardial infarction in humans. Lp(a) is mostly confined to primate species, due to the limited phylogenetic distribution of its distinguishing protein component, apolipoprotein(a) which is a close homolog of plasminogen. The known properties of Lp(a) are reviewed here. Many of these derive from the ability of Lp(a) to bind to the same substrates as plasminogen. A possible new animal model of Lp(a) is the hedgehog, which contains an Lp(a)-like particle that is the apparent product of independent evolution of a multi-kringle, apolipoprotein(a)-like protein by duplication and modification of portions of the hedgehog plasminogen gene.

The recurring evolution of lipoprotein(a). Insights from cloning of hedgehog apolipoprotein(a)

The lipoprotein Lp(a), a major inherited risk factor for atherosclerosis, consists of a low density lipoprotein-like particle containing apolipoprotein B-100 plus the distinguishing component apolipoprotein(a) (apo(a)). Human apo(a) contains highly repeated domains related to plasminogen kringle four plus single kringle five and protease-like domains. Apo(a) is virtually confined to primates, and the gene may have arisen during primate evolution. One exception is the occurrence of an Lp(a)-like particle in the hedgehog. Cloning of the hedgehog apo(a)-like gene shows that it is distinctive in form and evolutionary history from human apo(a), but that it has acquired several common features. It appears that the primate and hedgehog apo(a) genes evolved independently by duplication and modification of different domains of the plasminogen gene, providing a novel type of "convergent" molecular evolution.

Association of lipoprotein(a) with atherothrombotic events and fibrinolytic variables. A case-control study

Elevated plasma levels of lipoprotein(a) [Lp(a)] have been associated with an increased risk of cardiovascular disease. The aim of the present study was to investigate whether Lp(a) plasma levels were associated with subsequent ischemic events and with fibrinolytic variables in patients with established atherosclerotic disease enrolled in the prospective PLAT study. Lp(a) levels and fibrinolytic variables in 37 atherosclerotic patients who subsequently developed an atherothrombotic event during the first year of follow-up (cases) were compared with those in paired controls, matched for age, sex, diagnosis at enrollment and lipid pattern, who remained free from vascular events during the same time frame. Median and mean Lp(a) levels were similar in cases (6.05 mg/dl; 13.8 +/- 19.4 mg/dl) and controls (6.05 mg/dl; 17.1 +/- 21.6 mg/dl). In the whole group plasma Lp(a) levels correlated significantly with the increase of t-PA antigen (r = 0.368; p = 0.002) and fibrinolytic activity (r = 0.410; p = 0.001) induced by venous stasis but not with baseline fibrinolytic variables. These findings indicate that in patients with established atherosclerotic disease Lp(a) may interfere in vivo with the fibrinolytic process but is not predictive of subsequent ischemic events.

Effects of lipoprotein(a) on the binding of plasminogen to fibrin and its activation by fibrin-bound tissue-type plasminogen activator

Molecular assembly of plasminogen and tissue-type plasminogen activator (t-PA) at the surface of fibrin results in the generation of fibrin-bound plasmin and thereby in the dissolution of a clot. This mechanism is triggered by specific interactions of intra-chain surface lysine residues in fibrin with the kringle domains of plasminogen, and is further amplified via the interaction of plasminogen kringles with the carboxy-terminal lysine residues of fibrin that are exposed by plasmin cleavage. By virtue of its marked homology with plasminogen, apo(a), the specific apolipoprotein component of Lp(a), may bind to the lysine sites available for plasminogen on the surface of fibrin and thereby interfere with the fibrinolytic process. A sensitive solid-phase fibrin system, which allows the study of plasminogen activation at the plasma fibrin interface and makes feasible the analysis of products bound to fibrin, has been used to investigate the effects of Lp(a) on the binding of plasminogen and its activation by fibrin-bound t-PA. Plasma samples from human subjects with high levels of Lp(a) were studied. We have established that Lp(a) binds to the fibrin surface and thereby competes with plasminogen (Ki = 44 nM) so as to inhibit its activation. We have further shown that Lp(a) blocks specifically carboxy-terminal lysine residues on the surface of fibrin. To further explore the role of apo(a) on the Lp(a) fibrin interactions, we have performed ligand-binding studies using a recombinant form of apo(a) that contains 17 kringle 4-like units. We have shown that recombinant apo(a) binds specifically to fibrin (Kd = 26 +/- 8 nM, Bmax = 26 +/- 2 fmol/well) and that this binding increases upon treatment of the fibrin surface with plasmin (Kd = 8 +/- 4 nM, Bmax = 115 +/- 14 fmol/well). Altogether, our results indicate clearly that binding of native Lp(a) through this mechanism may impair clot lysis and may favor the accumulation of cholesterol in thrombi at sites of vascular injury.

A 2-year study on the beneficial effects of 17 beta-oestradiol-dydrogesterone therapy on serum lipoproteins and Lp(a) in postmenopausal women: no additional unfavourable effects of dydrogesterone

INTRODUCTION

Postmenopausal hormone replacement therapy (HRT) has been described to reduce the risk of developing cardiovascular disease (CVD), which can be attributed at least in part to beneficial effects of oestrogens on serum lipoproteins. Little is known about a possible counteracting effect by the progestogen integrated in modern HRT regimens.

OBJECTIVE

To study the possible changes in serum lipids, lipoproteins and apolipoproteins during HRT with special emphasis on the possible progestational effect.

STUDY DESIGN

In an open-label longitudinal non-comparative study 23 healthy non-hysterectomized postmenopausal women were treated with continuous micronized 17 beta-oestradiol, 2 mg daily, in combination with cyclic dydrogesterone, 10 mg daily, the first 14 days of each 28-day treatment cycle. The women were followed for up to 2 years.

RESULTS

After 2 years serum total cholesterol and low-density lipoprotein cholesterol had decreased by 9.0% and 18%, respectively (P < 0.01), while high-density lipoprotein cholesterol had increased by 13% (P < 0.01). The latter change was accompanied with similar increases in apolipoprotein A-I (+16%; P < 0.01) and A-II (+13%; P < 0.01), while apolipoprotein B remained unchanged. Serum very low-density lipoprotein (VLDL) cholesterol and VLDL-triglycerides increased by 28% and 21%, respectively, the latter reflecting the slight increase in serum triglycerides by 21%. These values, however, remained within the normal range. Serum lipoprotein(a) decreased by 16% (P < 0.01). All calculated atherogenic indices decreased (P < 0.01) during the study period. Serum lipids and (apo)lipoproteins did not change after withdrawal of dydrogesterone for 14 days during the combination therapy in the last cycle studied. Serum fibrinogen decreased by 8.4% (P < 0.01) in the first 12 cycles, after which it increased to 13% above baseline value (P < 0.01 vs. baseline). Antithrombin III did not change and serum glucose decreased by 5.7%.

CONCLUSIONS

This HRT regimen induces (and also when given for a longer period) beneficial changes in the lipid profile, without affecting important indicators of thrombosis. Also, the glucose metabolism does not seem to be interfered with. Cyclic administration of dydrogesterone does not unfavourably affect serum lipids and (apo)lipoproteins when combined with 17 beta-oestradiol supplementation. Therefore, this combination hormone regimen can be recommended for use in HRT.

Rhesus monkey lipoprotein(a) binds to lysine Sepharose and U937 monocytoid cells less efficiently than human lipoprotein(a). Evidence for the dominant role of kringle 4(37)

Rhesus lipoprotein(a) (Lp[a]) binds less efficiently than human Lp(a) to lysine-Sepharose and to cultured U937 cells. Studies using elastase-derived plasminogen fragments indicated that neither kringle 5 nor the protease domain of Lp(a) are required in these interactions pointing at an involvement of the K4 region. Comparative structural analyses of both the human and simian apo(a) K4 domain, together with molecular modeling studies, supported the conclusion that K4(37) plays a dominant role in the lysine binding function of apo(a) and that the presence of arginine 72 rather than tryptophan in this kringle can account for the functional deficiency observed with rhesus Lp(a). These in vitro results suggest that rhesus Lp(a) may be less thrombogenic than human Lp(a).