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

Apolipoprotein(a), an enigmatic anti-angiogenic glycoprotein in human plasma: A curse or cure?

Angiogenesis is a finely co-ordinated, multi-step developmental process of the new vascular structure. Even though angiogenesis is regularly occurring in physiological events such as embryogenesis, in adults, it is restricted to specific tissue sites where rapid cell-turnover and membrane synthesis occurs. Both excessive and insufficient angiogenesis lead to vascular disorders such as cancer, ocular diseases, diabetic retinopathy, atherosclerosis, intra-uterine growth restriction, ischemic heart disease, stroke etc. Occurrence of altered lipid profile and vascular lipid deposition along with vascular disorders is a hallmark of impaired angiogenesis. Among lipoproteins, lipoprotein(a) needs special attention due to the presence of a multi-kringle protein subunit, apolipoprotein(a) [apo(a)], which is structurally homologous to many naturally occurring anti-angiogenic proteins such as plasminogen and angiostatin. Researchers have constructed different recombinant forms of apo(a) (rhLK68, rhLK8, RHACK2, KV-11, and AU-6) and successfully exploited its potential to inhibit unwanted angiogenesis during tumor metastasis and retinal neovascularization. Similar to naturally occurring anti-angiogenic proteins, apo(a) can directly interfere with angiogenic signaling pathways. Besides this, apo(a) can also exert its anti-angiogenic effect indirectly by inducing endothelial cell apoptosis, by inhibiting endothelial progenitor cell functions or by upregulating nuclear factors in endothelial cells via apo(a)-bound oxPLs. However, the impact of the anti-angiogenic potential of native apo(a) during physiological angiogenesis in embryos and wounded tissues is not yet explored. In this context, we review the studies so far done to demonstrate the anti-angiogenic activity of apo(a) and the recent developments in using apo(a) as a therapeutic agent to treat impaired angiogenesis during vascular disorders, with emphasis on the gaps in the literature.

Lipoprotein(a) metabolism: potential sites for therapeutic targets

Lipoprotein(a) [Lp(a)] resembles low-density lipoprotein (LDL), with an LDL lipid core and apolipoprotein B (apoB), but contains a unique apolipoprotein, apo(a). Elevated Lp(a) is an independent risk factor for coronary and peripheral vascular diseases. The size and concentration of plasma Lp(a) are related to the synthetic rate, not the catabolic rate, and are highly variable with small isoforms associated with high concentrations and pathogenic risk. Apo(a) is synthesized in the liver, although assembly of apo(a) and LDL may occur in the hepatocytes or plasma. While the uptake and clearance site of Lp(a) is poorly delineated, the kidney is the site of apo(a) fragment excretion. The structure of apo(a) has high homology to plasminogen, the zymogen for plasmin and the primary clot lysis enzyme. Apo(a) interferes with plasminogen binding to C-terminal lysines of cell surface and extracellular matrix proteins. Lp(a) and apo(a) inhibit fibrinolysis and accumulate in the vascular wall in atherosclerotic lesions. The pathogenic role of Lp(a) is not known. Small isoforms and high concentrations of Lp(a) are found in healthy octogenarians that suggest Lp(a) may also have a physiological role. Studies of Lp(a) function have been limited since it is not found in commonly studied small mammals. An important aspect of Lp(a) metabolism is the modification of circulating Lp(a), which has the potential to alter the functions of Lp(a). There are no therapeutic drugs that selectively target elevated Lp(a), but a number of possible agents are being considered. Recently, new modifiers of apo(a) synthesis have been identified. This review reports the regulation of Lp(a) metabolism and potential sites for therapeutic targets.

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.

Lp(a) particles mold fibrin-binding properties of apo(a) in size-dependent manner: a study with different-length recombinant apo(a), native Lp(a), and monoclonal antibody

OBJECTIVE

Small-sized apolipoprotein(a) [apo(a)] isoforms with high antifibrinolytic activity are frequently found in cardiovascular diseases, suggesting a role for apo(a) size in atherothrombosis. To test this hypothesis, we sought to characterize the lysine (fibrin)-binding function of isolated apo(a) of variable sizes.

METHODS AND RESULTS

Recombinant apo(a) [r-apo(a)] preparations consisting of 10 to 34 kringles and a monoclonal antibody that neutralizes the lysine-binding function were produced and used in parallel with lipoprotein(a) [Lp(a)] particles isolated from plasma in fibrin-binding studies. All r-apo(a) preparations displayed similar affinity and specificity for lysine residues on fibrin regardless of size (K(d) 3.6+/-0.3 nmol/L) and inhibited the binding of plasminogen with a similar intensity (IC50 16.8+/-5.4 nmol/L). In contrast, native Lp(a) particles displayed fibrin affinities that were in inverse relationship with the apo(a) kringle number. Thus, a 15-kringle apo(a) separated from Lp(a) and a 34-kringle r-apo(a) displayed an affinity for fibrin that was higher than that in the corresponding particles (K(d) 2.5 versus 10.5 nmol/L and K(d) 3.8 versus 541 nmol/L, respectively). However, fibrin-binding specificity of the r-apo(a) preparations and the Lp(a) particles was efficiently neutralized (IC50 0.07 and 4 nmol/L) by a monoclonal antibody directed against the lysine-binding function of kringle IV-10.

CONCLUSIONS

Our data indicate that fibrin binding is an intrinsic property of apo(a) modulated by the composite structure of the Lp(a) particle.

Kringles of the plasminogen--prothrombin gene family share conformational epitopes with recombinant apolipoprotein (a): specificity of the fibrin-binding site

Monoclonal antibodies directed against recombinant apolipoprotein (a) (r-apo(a)) lacking plasminogen-like KIV-2 repeats were used to identify structurally related conformational epitopes in various members of the plasminogen-prothrombin gene family. A number of procedures including a fibrin-binding inhibition immunoassay and surface plasmon resonance studies were used. Two antibodies (A10.1 and A10.4) recognised common conformational structures in r-apo(a), prothrombin, factor XII, plasminogen and its tissue-type and urokinase-type activators. In contrast, two other antibodies recognised specifically an epitope comprising residues of the lysine-binding site (A10.2) or close to it (A10.5) and inhibited the fibrin-binding function of r-apo(a) (IC(50)=36 pmol/l and 9.76 nmol/l, respectively). Interestingly, these antibodies distinctly recognised the elastase-derived fragments of plasminogen K4 (A10.2) and K1+2+3 (A10.5) without affecting plasminogen binding to fibrin. These results suggest that highly conserved conformational regions are common to various proteins of the plasminogen-prothrombin gene family and are in agreement with the concept that these proteins constitute a monophyletic group derived from an ancestral gene.

A region in domain II of the urokinase receptor required for urokinase binding

The urokinase receptor is composed of three homologous domains based on disulfide spacing. The contribution of each domain to the binding and activation of single chain urokinase (scuPA) remains poorly understood. In the present paper we examined the role of domain II (DII) in these processes. Repositioning DII to the amino or carboxyl terminus of the molecule abolished binding of scuPA as did deleting the domain entirely. By using alanine-scanning mutagenesis, we identified a 9-amino acid continuous sequence in DII (Arg(137)-Arg(145)) required for both activities. Competition-inhibition and surface plasmon resonance studies demonstrated that mutation of Lys(139) and His(143) to alanine in soluble receptor (suPAR) reduced the affinity for scuPA approximately 5-fold due to an increase in the "off rate." Mutation of Arg(137), Arg(142), and Arg(145), each to alanine, leads to an approximately 100-fold decrease in affinity attributable to a 10-fold decrease in the apparent "on rate" and a 6-fold increase in off rate. These differences were confirmed on cells expressing variant urokinase receptor. suPAR-K139A/H143A displayed a 50% reduction in scuPA-mediated plasminogen activation activity, whereas the 3-arginine variant was unable to stimulate scuPA activity at all. Mutation of the three arginines did not affect binding of a decamer peptide antagonist of scuPA known to interact with DI and DIII. However, this mutation abolished both the binding of soluble DI to DII-III in the presence of scuPA and the synergistic activation of scuPA mediated by DI and wild type DII-DIII. These data show that DII is required for high affinity binding of scuPA and its activation. DII does not serve merely as a spacer function but appears to be required for interdomain cooperativity.

Lipid and apolipoprotein predictors of atherosclerosis in youth: apolipoprotein concentrations do not materially improve prediction of arterial lesions in PDAY subjects. The PDAY Research Group

We compared serum lipid and apolipoprotein predictors of atherosclerosis in cases from the multicenter study, Pathobiological Determinants of Atherosclerosis in Youth (PDAY). The lipid measures included HDL cholesterol (HDL-C) and non-HDL-C, and the apolipoprotein measures included concentrations of apoA1, apoB, and Lp(a), and sizes of the apo(a) proteins. We tested whether the apolipoprotein measures predicted atherosclerotic lesions as well as the more traditional lipid measures. We estimated extent of lesions as fatty streaks or raised lesions (fibrous plaques, complicated or calcified lesions) in 3 sites: thoracic aorta, abdominal aorta, and right coronary artery. Neither apoA1 nor apoB measures were as strongly or consistently correlated with extent of lesions as the corresponding lipid measure (HDL-C and non-HDL-C, respectively). Beyond the basic model that included sex, age, race, smoking status, hypertension, and the lipid measures, apoA1 and apoB added only an average 1.3% increased explanatory ability to the model, whereas HDL-C plus non-HDL-C added an average 2.5%. The results suggest that the traditional lipid measures are more useful than apolipoprotein measures for detecting young persons at high risk of precocious atherosclerosis. Because of large racial differences, the two Lp(a)-related measures, Lp(a) concentrations and apo(a) size, were evaluated in blacks and whites separately. Under these circumstances, neither of the Lp(a)-related measures was strongly or consistently correlated with extent of lesions.

Atherothrombogenicity of lipoprotein(a): the debate

Although lipoprotein(a) (Lp[a]) has been recognized as an atherothrombogenic factor, the underlying mechanisms for this pathogenicity have not been clearly defined. Plasma levels have received most of the attention in this regard; however, discrepancies among population studies have surfaced. Particularly limited is the information on the fate of Lp(a) that enters the arterial wall, in terms of mechanisms of endothelial transport and interactions with cells and macromolecules of the extracellular matrix. A typical Lp(a) represents a low-density lipoprotein (LDL)-like particle having as a protein moiety apo B-100 linked by a single interchain disulfide bond to a unique multikringle glycoprotein, called apolipoprotein(a) (apo[a]). In vitro studies have shown that Lp(a) can be dissected into its constituents, LDL and apo(a). In turn, the latter can be cleaved by enzymes of the elastase and metalloproteinase families into fragments that exhibit a differential behavior in terms of binding to macromolecules of the extracellular matrix: fibrinogen, fibronectin, and proteoglycans. By immunochemical criteria, apo(a) predominantly localizes in areas of human arteries affected by the atherosclerotic process, where elastase and metalloproteinase enzymes operate and where apo(a) fragments are potentially generated. The accumulation of these fragments in the vessel wall is likely to depend on their affinity for the constituents of the extracellular matrix. Thus, factors that modulate inflammation and inflammation-mediated fragmentation of Lp(a)/apo(a) may play an important role in the cardiovascular pathogenicity of Lp(a). This pathogenicity may be attenuated by measures directed at preventing the activation of those vascular cells that secrete enzymes with a proteolytic potential for Lp(a)/apo(a), namely, leukocytes, macrophages, and T cells.

Enzymatic and chemical modifications of lipoprotein(a) selectively alter its lysine-binding functions

The pathogenicity of lipoprotein(a) [Lp(a)] as a risk factor for cardiovascular disease may depend upon its lysine binding sites (LBS) which impart unique functions to Lp(a) not shared with low density lipoprotein. Biologically relevant modifications of Lp(a) were tested for alterations of LBS activity using two previously described functional assays, a LBS-Lp(a) immunoassay and a lysine-Sepharose bead assay. In the LBS-Lp(a) immunoassay, minimal changes in the LBS activity of Lp(a) were observed after modification with lipoprotein lipase, sphingomyelinase, or phospholipase C. In contrast, a significant (p<0.003) increase in the LBS activity of Lp(a) occurred after phospholipase A2 (PLA2) treatment, and this increase was confirmed using the lysine-Sepharose bead assay. The increase depended upon the release of fatty acids from Lp(a) by PLA2. A decrease in the LBS activity of Lp(a) occurred after oxidation of Lp(a) with 2,2'-azobis(2-amidinopropane) dihydrochloride (AAPH) (44% decrease), but CuSO4 oxidation increased LBS activity (210%). N-acetylcysteine (NAC) treatment of Lp(a) decreased (48%) LBS activity while homocysteine treatment had no (89%) effect. Thus, modification of phospholipids and protein moieties can alter the LBS-activity of Lp(a). Such enzymatic and chemical modifications may contribute to the variability in LBS function of Lp(a) seen within the population.

Elevated lipoprotein(a) levels and small apo(a) isoforms are compatible with longevity: evidence from a large population of French centenarians

Epidemiological studies have shown lipoprotein(a) (Lp(a)) to be an independent risk factor for cardiovascular disease. Lp(a) is a cholesterol-rich, low-density lipoprotein (LDL)-like particle to which a large glycoprotein, apolipoprotein(a) (apo(a)) is attached. Plasma Lp(a) levels are highly genetically determined and influenced to a minor degree by environmental factors. In an effort to determine whether Lp(a) might be associated with longevity, we have evaluated Lp(a) levels and apo(a) isoform sizes in a population of French centenarians (n = 109) compared to a control group (n = 227). The mean age of centenarians was 101.5 +/- 2.4 years while the control group was 39.4 +/- 7.2 years. Plasma levels of total cholesterol and triglyceride were within the normal range in both centenarian and control subjects. Lp(a) levels were higher in centenarians (both male and female) than in the normolipidemic control group (mean Lp(a) level = 0.33 +/- 0.42 and 0.22 +/- 0.27 mg/ml, respectively, P < 0.005). The distribution of apo(a) isoforms was significantly shifted towards small isoform size in the centenarian population as compared to the controls (54.4 and 41.4% of isoforms < or = 27 kringles (kr), respectively, P = 0.04). Nonetheless, the apo(a) size distribution in centenarians did not entirely explain the high Lp(a) levels observed in this population. Factors other than apo(a) size, and which may be either genetic or environmental in nature, appear to contribute to the elevated plasma Lp(a) levels of our centenarian population. We conclude therefore that high plasma Lp(a) levels are compatible with longevity.

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.

Oxidation of apolipoprotein(a) inhibits kringle-associated lysine binding: the loss of intrinsic protein fluorescence suggests a role for tryptophan residues in the lysine binding site

Lipoprotein(a) [Lp(a)] is a low-density lipoprotein complex consisting of apolipoprotein(a) [apo(a)] disulfide-linked to apolipoprotein B-100. Lp(a) has been implicated in atherogenesis and thrombosis through the lysine binding site (LBS) affinity of its kringle domains. We have examined the oxidative effect of 2,2'-azobis-(amidinopropane) HCl (AAPH), a mild hydrophilic free radical initiator, upon the ability of Lp(a) and recombinant apo(a), r-apo(a), to bind through their LBS domains. AAPH treatment caused a time-dependent decrease in the number of functional Lp(a) or r-apo(a) molecules capable of binding to fibrin or lysine-Sepharose and in the intrinsic protein fluorescence of both Lp(a) and r-apo(a). The presence of a lysine analogue during the reaction prevented the loss of lysine binding and provided a partial protection from the loss of tryptophan fluorescence. The partial protection of fluorescence by lysine analogues was observed in other kringle-containing proteins, but not in proteins lacking kringles. No significant aggregation, fragmentation, or change in conformation of Lp(a) or r-apo(a) was observed as assessed by native or SDS-PAGE, light scattering, retention of antigenicity, and protein fluorescence emission spectra. Our results suggest that AAPH destroys amino acids in the kringles of apo(a) that are essential for lysine binding, including one or more tryptophan residues. The present study, therefore, raises the possibility that the biological roles of Lp(a) may be mediated by its state of oxidation, especially in light of our previous study showing that the reductive properties of sulfhydryl-containing compounds increase the LBS affinity of Lp(a) for fibrin.

Lipoprotein(a) isoforms display differences in affinity for plasminogen-like binding to human mononuclear cells

Binding of lipoprotein(a) (Lp(a)) to membrane proteins of the monocyte-macrophage cell lineage may be an important event in atheroma formation. Since Lp(a) with distinct apolipoprotein(a) (apo(a)) isoforms may show differences in their affinity with regard to fibrin binding, the existence of such a functional behavior in the interaction of apo(a) in Lp(a) with these cells was explored using the monocytic cell line THP-1. Lp(a) preparations containing small size apo(a) isoforms (M(r) = 450,000 to 550,000) and high molecular mass isoforms (M(r) > or = 700,000) were purified from plasmas containing > 0.35 g/L of Lp(a) obtained from subjects (n = 14) with cardiovascular atherosclerotic disease. Binding of plasminogen to THP-1 cells was performed using the method of radioisotopic dilution. For binding of Lp(a) to cells, the THP-1 monocytic cells were incubated with varying concentrations of the different Lp(a) preparations; cells were then washed and the amount of Lp(a) bound was detected with a radiolabeled polyclonal antibody directed against apo(a). Binding due to kringle interactions with lysine residues was calculated by subtracting from the total bound the amount of Lp(a) bound (approximately 10%) in the presence of 6-aminohexanoic acid. Analysis of data with the Langmuir equation indicated identical and independent (non-interacting) sites and allowed evaluation of the Kd. Binding isotherms of small size isoforms showed saturation and a high affinity (Kd = 25.8 +/- 19 nmol/L) relative to that of plasminogen (Kd = 1750 +/- 760 nmol/L). A similar difference (Kd = 17.5 +/- 7.9 nmol/L versus Kd = 600 +/- 220 nmol/L) was found when binding experiments were performed with a fibrin surface. In contrast, binding isotherms of the high molecular mass isoforms did not show saturation at the highest Lp(a) concentrations used, thus indicating a lower affinity. In conclusion, these results show that apo(a) isoforms may display polymorphism-linked functional heterogeneity with regard to cell binding, which may explain the higher association with cardiovascular risk of small size isoforms. These qualitative differences in the binding of apo(a) isoforms to fibrin or cells may modulate the cardiovascular risk associated with high levels of Lp(a).

Lipoprotein lipase-enhanced binding of lipoprotein(a) [Lp(a)] to heparan sulfate is improved by apolipoprotein E (apoE) saturation: secretion-capture process of apoE is a possible route for the catabolism of Lp(a)

Recently, it has been recognized that cell-bound heparan sulfate (HS) proteoglycans (HSPG) are able to bind and subsequently initiate degradation of lipoproteins. Two mediators of lipoprotein catabolism, both with HS binding capacity, lipoprotein lipase (LPL) and apolipoprotein E (apoE), are involved in this process. This mechanism is known as the secretion-capture process of apoE. Lipoprotein(a) [Lp(a)] was shown to have a strong binding capacity to cell-associated HSPG. This binding capacity was increased by LPL addition. We investigated the effects of recombinant apoE (r-apoE) enrichment of Lp(a) on the binding to HS. Lp(a), isolated by ultracentrifugation and gel filtration, was incubated with r-apoE and reisolated by ultracentrifugation, resulting in r-apoE-enriched Lp(a). ApoE-enriched Lp(a) and control Lp(a) were coated to microtiter plates. The capacity to bind biotin-conjugated HS (b-HS) in the presence or absence of inactivated bovine LPL was studied. R-apoE-enriched Lp(a) showed increased b-HS binding capacity versus control Lp(a). Addition of LPL resulted in an increased b-HS binding capacity of both control and r-apoE-enriched Lp(a). To investigate whether binding of Lp(a) to endothelial cell HSPG occurred in vivo, 39 volunteers were injected with heparin (50 U/kg) and plasma lipid and Lp(a) levels were determined before and 20 minutes after heparin injection. No significant increase in plasma Lp(a) concentrations was found. The results showed that Lp(a) can be enriched with apoE and that this resulted in increased LPL-enhanced binding to HSPG. From the in vitro studies, it can be concluded that the secretion-capture process of apoE is a possible catabolic route for Lp(a). However, whether this also occurs in vivo remains to be confirmed.

Lipoprotein(a): structural implications for pathophysiology

The assembly between a low-density lipoprotein particle and apolipoprotein(a), a highly carbohydrate-rich protein, gives origin to a peculiar class of lipoproteins, only found in the hedgehog, primates, and humans, termed lipoprotein(a). Apolipoprotein(a), which shares a high degree of sequence homology with the fibrinolytic proenzyme plasminogen, is linked to the apolipoprotein B-100 component of low-density lipoprotein via a disulfide bond and confers distinct biochemical and metabolic properties to lipoprotein(a). Because of its peculiar structural features and the observed correlation between high lipoprotein(a) levels and the development of a variety of atherosclerotic disorders, this lipoprotein has become the focus of an intense research effort. Although accumulation of lipoprotein(a) in the vessel wall at sites of vascular injury has been clearly evidenced, the mechanism(s) by which lipoprotein(a) exerts its pathogenic effect in this milieu remain largely unknown. It has been hypothesized that the pathological effect of lipoprotein(a) is related either to its similarity to low-density lipoprotein (i.e., a pro-atherogenic effect) or to the apolipoprotein(a) similarity to plasminogen (i.e., a pro-thrombotic/anti-fibrinolytic effect). However, it is probable that both components contribute to the pathogenicity of lipoprotein(a). The fact that lipoprotein(a) levels are largely genetically determined, varying widely among individuals and racial groups, adds additional elements to the scientific interest that surrounds this lipoprotein. Both clinical and biochemical studies of lipoprotein(a) have been complicated by the high degree of structural heterogeneity of apolipoprotein(a), which is considered the most polymorphic protein in human plasma. Our aim in this paper is to provide an overview of the most salient structural features of lipoprotein(a) and their possible pathophysiological implications.

Lipoprotein(a) assembly. Quantitative assessment of the role of apo(a) kringle IV types 2-10 in particle formation

We have developed a system for the quantitative assessment of the efficiency of lipoprotein(a) [Lp(a)] formation in vitro. Amino-terminally truncated derivatives of a 17-kringle form of recombinant apo(a) [r-apo(a)] were transiently expressed in human embryonic kidney cells. Equimolar amounts of r-apo(a) derivatives were incubated with a fourfold molar excess of purified human low density lipoprotein, and r-Lp(a) formation was assessed by densitometric analysis of Western blots. Although r-Lp(a) formation was observed with each r-apo(a) derivative, both the rate and extent of particle formation were greatly lower on removal of kringle IV type 7. Additional substantial decreases in these parameters were observed on removal of kringle IV type 8, thereby suggesting a major role for these two kringles in Lp(a) assembly. We directly demonstrated that the lysine-binding sites (LBSs) within kringle IV types 5-9 are "masked" in the context of the Lp(a) particle and are consequently unavailable for interaction with lysine-Sepharose. Using site-directed mutagenesis, we also demonstrated that the previously described LBS in kringle IV type 10 is not required for r-Lp(a) formation: r-Lp(a) formation using a mutated form of apo(a) that lacks this LBS is comparable in efficiency to that of wild-type r-apo(a) and can be inhibited to a similar extent by epsilon-amino-n-caproic acid. In summary, the results of our study indicate that apo(a) kringle IV types 7 and 8 are required for maximal efficiency of Lp(a) formation, likely by virtue of their ability to mediate lysine-dependent non-covalent interactions with apoB-100 that precede disulfide bond formation.

Simple and rapid procedure for the purification of lipoprotein(a)

Lipoprotein(a) [Lp(a)] is a low-density lipoprotein-like particle displaying strong athero-thrombotic properties. Highly purified Lp(a) is increasingly requested for standardization of Lp(a) measurements and for biological studies. Several procedures have been described for Lp(a) separation and purification but none of them appear completely suitable. We present here a procedure for Lp(a) purification based on sequential elutions after lysine-Sepharose affinity chromatography. We were able to identify four distinct subspecies of Lp(a) showing different affinity to epsilon-amino groups of lysine-Sepharose, simply by modifying molarity and pH of the eluents; the fraction obtained in highly purified state represented the major form and could be eluted with 0.5 M sodium phosphate buffer (pH 4.4). Advantages of this procedure are represented by simplicity, rapidity and final yield.

A quantitative immunoassay for the lysine-binding function of lipoprotein(a). Application to recombinant apo(a) and lipoprotein(a) in plasma

Apo(a), the unique apoprotein of lipoprotein(a) (Lp[a]), can express lysine-binding sites(s) (LBS). However, the LBS activity of Lp(a) is variable, and this heterogeneity may influence its pathogenetic properties. An LBS-Lp(a) immunoassay has been developed to quantitatively assess the LBS function of Lp(a). Lp(a) within a sample is captured with an immobilized monoclonal antibody specific for apo(a), and the captured Lp(a) is reacted with an antibody specific for functional LBS. The binding of this LBS-specific antibody is then quantified by using an alkaline phosphatase-conjugated disclosing antibody. The critical LBS-specific antibody was raised to kringle 4 of plasminogen. When applied to plasma samples, the LBS activity of Lp(a) ranged from 0% to 100% of an isolated reference Lp(a); the signal corresponded to the percent retention of Lp(a) on a lysine-Sepharose but did not correlate well with total Lp(a) levels in plasma. Mutation of residues in the putative LBS in the carboxy-terminal kringle 4 repeat (K4-37) in an eight-kringle apo(a) construct resulted in marked but not complete loss of activity in the LBS-Lp(a) immunoassay. These data suggest that this kringle is the major but not the sole source of LBS activity in apo(a). The LBS-Lp(a) immunoassay should prove to be a useful tool in establishing the role of the LBS in the pathogenicity of Lp(a).

Antifibrinolytic effect of recombinant apolipoprotein(a) in vitro is primarily due to attenuation of tPA-mediated Glu-plasminogen activation

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.

Identification of two functionally distinct lysine-binding sites in kringle 37 and in kringles 32-36 of human apolipoprotein(a)

The well documented association between high plasma levels of lipoprotein(a) (Lp(a)) and cardiovascular disease might be mediated by the lysine binding of apolipoprotein(a) (apo(a)), the plasminogen-like, multikringle glycoprotein in Lp(a). We employed a mutational analysis to localize the lysine-binding domains within human apo(a). Recombinant apo(a) (r-apo(a)) with 17 plasminogen kringle IV-like domains, one plasminogen kringle V-like domain, and a protease domain or mutants thereof were expressed in the human hepatocarcinoma cell line HepG2. The lysine binding of plasma Lp(a) and r-apo(a) in the culture supernatants of transfected HepG2 cells was analyzed by lysine-Sepharose affinity chromatography. Wild type recombinant Lp(a) (r-Lp(a)) revealed lysine binding in the range observed for human plasma Lp(a). A single accessible lysine binding site in Lp(a) is indicated by a complete loss of lysine binding observed for r-Lp(a) species that contain either a truncated r-apo(a) lacking kringle IV-37, kringle V, and the protease or a point-mutated r-apo(a) with a Trp-4174-->Arg substitution in the putative lysine-binding pocket of kringle IV-37. Evidence is also presented for additional lysine-binding sites within kringles 32-36 of apo(a) that are masked in Lp(a) as indicated by an increased lysine binding for the point mutant (Cys-4057-->Ser), which is unable to assemble into particles. An important role of these lysine-binding site(s) for Lp(a) assembly is suggested by a decreased assembly efficiency for deletion mutants lacking either kringle 32 or kringles 32-35.

Antifibrinolytic activity of apolipoprotein(a) in vivo: human apolipoprotein(a) transgenic mice are resistant to tissue plasminogen activator-mediated thrombolysis

The extensive homology between apolipoprotein(a) and plasminogen has led to the hypothesis that the increased risk for atherosclerosis, cardiac disease and stroke associated with elevated levels of apolipoprotein(a) may reflect modulation of fibrinolysis. We have investigated the role of apolipoprotein(a) on clot lysis in transgenic mice expressing the human apolipoprotein(a) gene. These mice develop fatty streak lesions resembling early lesions of human atherosclerosis. Pulmonary emboli were generated in mice by injection, through the right jugular vein, of a human platelet-rich plasma clot radiolabelled with technetium-99m-labelled antifibrin antibodies. Tissue plasminogen activator was introduced continuously via the right jugular vein. Clot lysis, determined by ex vivo imaging, was depressed in mice carrying the apolipoprotein(a) transgene relative to their sex-matched normal littermates. These results directly demonstrate an in vivo effect of apolipoprotein(a) on fibrinolysis, an effect that may contribute to the pathology associated with elevated levels of this protein.

A single point mutation (Trp72-->Arg) in human apo(a) kringle 4-37 associated with a lysine binding defect in Lp(a)

Human lipoprotein(a) or Lp(a) binds, like plasminogen, to lysine Sepharose. However, contrary to plasminogen in which kringles 1 and 4 have been implicated, the binding site or sites on apo(a), the specific glycoprotein of Lp(a), have not been determined. For the first time we now report the occurrence of a human Lp(a) that has a mutant form of apo(a) where Arg has replaced Trp in position 72 of kringle 4-37 and is unable to bind to lysine Sepharose. This observation suggests that Trp72 of apo(a) kringle 4-37 may play a dominant role in lysine binding. Lysine binding has been associated with the thrombogenic potential of Lp(a). Thus, the Trp72-->Arg mutation may render Lp(a) 'benign' from the cardiovascular viewpoint.

A prospective investigation of elevated lipoprotein (a) detected by electrophoresis and cardiovascular disease in women. The Framingham Heart Study

BACKGROUND

Sinking prebeta lipoprotein is a putative marker for elevated levels of lipoprotein (a). Although prospective data suggest that increased plasma lipoprotein (a) is an independent risk factor for coronary heart disease in men, no prospective studies are available in women.

METHODS AND RESULTS

From 1968 through 1975, sinking prebeta lipoprotein was determined by paper electrophoresis in 3103 women Framingham Heart Study participants who were free of prevalent cardiovascular disease. A sinking prebeta lipoprotein band was detectable in 434 of the women (14%) studied. The median follow-up interval was approximately 12 years. Incident cardiovascular disease was associated with band presence using a proportional hazards model that included age, smoking, body mass index, systolic blood pressure, glucose intolerance, low- and high-density lipoprotein cholesterol, and ECG left ventricular hypertrophy. Multivariable adjusted relative risk estimates (with 95% confidence intervals) for outcomes in the band present versus absent groups were as follows: myocardial infarction (82 events), 2.37 (1.48 to 3.81); intermittent claudication (62 events), 1.94 (1.07 to 3.50); cerebrovascular disease (83 events), 1.88 (1.12 to 3.15); total coronary heart disease (174 events), 1.61 (1.13 to 2.29); and total cardiovascular disease (305 events), 1.44 (1.09 to 1.91). A subset analysis indicated that band presence was 50.9% sensitive and 95.4% specific for detecting plasma lipoprotein (a) levels of > 30 mg/dL, the threshold value linked to increased cardiovascular disease risk in men.

CONCLUSIONS

Sinking prebeta lipoprotein was a valid surrogate for elevated lipoprotein (a) levels in Framingham Heart Study women. Band presence and, equivalently, elevated plasma lipoprotein (a), was a strong, independent predictor of myocardial infarction, intermittent claudication, and cerebrovascular disease. Confirmation of these findings in other longitudinal studies of women is needed.

Lysine-binding species of lipoprotein(a) in coronary artery disease

Elevated levels of plasma lipoprotein(a) [Lp(a)] have frequently been associated with coronary artery disease (CAD). Recently Lp(a) was fractionated into two species with different affinities for Lysine-Sepharose. The influence of lysine-binding heterogeneity of Lp(a) on its cardiovascular pathogenicity has not previously been studied. The authors have determined plasma levels of total Lp(a), its lysine-binding [lys+] and unretained [lys-] species in 67 male CAD patients undergoing cardiac catheterization. Forty-three patients have severe CAD (two- or three-vessel disease) and 24 patients have less pronounced CAD (one-vessel disease or less than 50% narrowing of coronary vessels). All patients were ranked in order of their Lp(a) levels and then grouped into quartiles. The prevalence of severe CAD was significantly higher in the upper Lp(a) quartile as compared with the other three quartiles (odds ratio 10-5; chi-square 11.2; P = 0.0008). Similar results were obtained when the same analysis was carried out for [lys+] and [lys-] species of Lp(a) (odds ratio 11.52 and 3.3, respectively; chi-square 12.3 and 4.34, respectively; P = 0.0004 and 0.037, respectively). Thus, measurement of either species of Lp(a) does not provide any additional improvement in the prediction of CAD as compared to the estimation of total Lp(a) levels.

Sulfhydryl compounds influence immunoreactivity, structure and functional aspects of lipoprotein(a)

Human plasma Lp(a) is susceptible to various sulfhydryl compounds. In this study we present evidence indicating that after treatment of Lp(a) with sulfhydryl compounds, immunoreactivity is changed, structural changes occur and functional characteristics regarding the numerous kringle structures in apo(a) disappear. Purified Lp(a) was subjected to variable concentrations (0.01-10 mM) of various sulfhydryl compounds: DTT, 2-mercapto-ethanol (BME), N-acetylcysteine (NAC) and homocysteine (HCys). Free SH groups were blocked by iodoacetamide. Reduced and alkylated Lp(a) was tested in two ELISAs, one detecting apo(a) alone and one detecting apo(a)-apoB complexes. In both ELISAs polyclonal antibodies were used. For comparison a commercial apo(a) IRMA utilizing two monoclonal antibodies was used. The results indicate that a similar decrease in response of both ELISAs is observed, whereas the IRMA response is less affected. Western blotting of "DTT treated" Lp(a) after SDS-PAGE under nonreducing conditions showed that separate apo(a) and apoB-100 bands became detectable at 1 mM DTT. Native PAGE (2.5-16%) indicated structural changes of Lp(a) beginning to occur at 0.03 mM DTT. Epsilon-aminocaproic acid-inhibitable binding of "DTT-treated" Lp(a) to Desafib-X decreased with increasing DTT concentrations in concert with a loss of the capacity of Lp(a) to inhibit plasminogen activation upon treatment with DTT. The observed immunological and functional changes of Lp(a) indicate that apo(a) kringle function is severely affected by sulfhydryl compounds.