Polymorphonuclear cells isolated from human peripheral blood cleave lipoprotein(a) and apolipoprotein(a) at multiple interkringle sites via the enzyme elastase. Generation of mini-Lp(a) particles and apo(a) fragments

Genetic and Pharmacologic Inhibition of the Neutrophil Elastase Inhibits Experimental Atherosclerosis

Background

To investigate whether neutrophil elastase (NE) plays a causal role in atherosclerosis, and the molecular mechanisms involved.

Methods and Results

NE genetic–deficient mice (Apolipoprotein E^−/−/NE^−/− mice), bone marrow transplantation, and a specific NE inhibitor (GW311616A) were employed in this study to establish the causal role of NE in atherosclerosis. Aortic expression of NE mRNA and plasma NE activity was significantly increased in high‐fat diet (HFD)–fed wild‐type (WT) (Apolipoprotein E^−/−) mice but, as expected, not in NE‐deficient mice. Selective NE knockout markedly reduced HFD‐induced atherosclerosis and significantly increased indicators of atherosclerotic plaque stability. While plasma lipid profiles were not affected by NE deficiency, decreased levels of circulating proinflammatory cytokines and inflammatory monocytes (Ly6C^hi/CD11b⁺) were observed in NE‐deficient mice fed with an HFD for 12 weeks as compared with WT. Bone marrow reconstitution of WT mice with NE^−/− bone marrow cells significantly reduced HFD‐induced atherosclerosis, while bone marrow reconstitution of NE^−/− mice with WT bone marrow cells restored the pathological features of atherosclerotic plaques induced by HFD in NE‐deficient mice. In line with these findings, pharmacological inhibition of NE in WT mice through oral administration of NE inhibitor GW311616A also significantly reduced atherosclerosis. Mechanistically, we demonstrated that NE promotes foam cell formation by increasing ATP‐binding cassette transporter ABCA1 protein degradation and inhibiting macrophage cholesterol efflux.

Conclusions

We outlined a pathogenic role for NE in foam cell formation and atherosclerosis development. Consequently, inhibition of NE may represent a potential therapeutic approach to treating cardiovascular disease.

Lipoprotein(a), cardiovascular disease, and contemporary management

Elevated lipoprotein(a) (Lp[a]) is a causal genetic risk factor for cardiovascular disease. To determine if current evidence supports both screening and treatment for elevated Lp(a) in high-risk patients, an English-language search of PubMed and MEDLINE was conducted. In population studies, there is a continuous association between Lp(a) concentrations and cardiovascular risk, with synergistic effects when low-density lipoprotein (LDL) is also elevated. Candidates for Lp(a) screening include patients with a personal or family history of premature cardiovascular disease, familial hypercholesterolemia, recurrent cardiovascular events, or inadequate LDL cholesterol (LDL-C) responses to statins. Given the comparative strength of clinical evidence, reducing LDL-C to the lowest attainable value with a high-potency statin should be the primary focus of lipid-modifying therapies. If the Lp(a) level is 30 mg/dL or higher in a patient who has the aforementioned characteristics plus residual LDL-C elevations (≥70-100 mg/dL) despite maximum-potency statins or combination statin therapy, the clinician may consider adding niacin (up to 2 g/d). If, after these interventions, the patient has progressive coronary heart disease (CHD) or LDL-C levels of 160-200 mg/dL or higher, LDL apheresis should be contemplated. Although Lp(a) is a major causal risk factor for CHD, no currently available controlled studies have suggested that lowering it through either pharmacotherapy or LDL apheresis specifically and significantly reduces coronary risk. Further research is needed to (1) optimize management in order to reduce CHD risk associated with elevated Lp(a) and (2) determine what other intermediate- or high-risk groups might benefit from Lp(a) screening.

Lipoprotein(a): a promising marker for residual cardiovascular risk assessment

Atherosclerotic cardiovascular diseases (CVD) are still the leading cause of morbidity and mortality worldwide, although optimal medical therapy has been prescribed for primary and secondary preventions. Residual cardiovascular risk for some population groups is still considerably high although target low density lipoprotein-cholesterol (LDL-C) level has been achieved. During the past few decades, compelling pieces of evidence from clinical trials and meta-analyses consistently illustrate that lipoprotein(a) (Lp(a)) is a significant risk factor for atherosclerosis and CVD due to its proatherogenic and prothrombotic features. However, the lack of effective medication for Lp(a) reduction significantly hampers randomized, prospective, and controlled trials conducting. Based on previous findings, for patients with LDL-C in normal range, Lp(a) may be a useful marker for identifying and evaluating the residual cardiovascular risk, and aggressively lowering LDL-C level than current guidelines' recommendation may be reasonable for patients with particularly high Lp(a) level.

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.

Lipoprotein (a) in patients with psoriasis: associations with lipid profiles and disease severity

BACKGROUND

Lipoprotein (a) [Lp(a)] is a genetically determined molecule whose role has been implied in cardiovascular pathology, and whose levels have been reported to be elevated in patients with psoriasis. Aim To assess the serum levels of Lp(a) in patients with psoriasis, and to investigate the associations of Lp(a) with other lipids and with psoriasis severity.

METHODS

Thirty-four patients with psoriasis and 26 healthy control subjects took part in the study. Serum levels of Lp(a) and total, high density lipoprotein (HDL), low density lipoprotein (LDL), and very low density lipoprotein (VLDL) cholesterol fractions were measured in all participants. The levels of triglycerides and total cholesterol were measured using enzymatic colorimetric tests; HDL and LDL cholesterol concentrations were determined by precipitation methods; the VLDL concentration was calculated according to the formula: VLDL cholesterol = triglycerides/5.

RESULTS

Patients with psoriasis showed significantly higher serum levels of Lp(a) relative to controls. Even when controlling for normolipidemic vs. hyperlipidemic status, abnormal levels of Lp(a) (> 30 mg/dL) were observed significantly more often in patients than in controls. In both patients and controls, Lp(a) levels correlated positively with total and HDL cholesterol levels. In patients, Lp(a) levels correlated positively with psoriasis severity.

CONCLUSIONS

Lp(a) may be a factor contributing to an increased cardiovascular risk in patients with psoriasis. A pathogenetic link may exist between this lipoprotein and psoriatic pathophysiology.

A polymorphism in the protease-like domain of apolipoprotein(a) is associated with severe coronary artery disease

OBJECTIVES

The purpose of this study was to identify genetic variants associated with severe coronary artery disease (CAD).

METHODS AND RESULTS

We used 3 case-control studies of white subjects whose severity of CAD was assessed by angiography. The first 2 studies were used to generate hypotheses that were then tested in the third study. We tested 12,077 putative functional single nucleotide polymorphisms (SNPs) in Study 1 (781 cases, 603 controls) and identified 302 SNPs nominally associated with severe CAD. Testing these 302 SNPs in Study 2 (471 cases, 298 controls), we found 5 (in LPA, CALM1, HAP1, AP3B1, and ABCG2) were nominally associated with severe CAD and had the same risk alleles in both studies. We then tested these 5 SNPs in Study 3 (554 cases, 373 controls). We found 1 SNP that was associated with severe CAD: LPA I4399M (rs3798220). LPA encodes apolipoprotein(a), a component of lipoprotein(a). I4399M is located in the protease-like domain of apolipoprotein(a). Compared with noncarriers, carriers of the 4399M risk allele (2.7% of controls) had an adjusted odds ratio for severe CAD of 3.14 (confidence interval 1.51 to 6.56), and had 5-fold higher median plasma lipoprotein(a) levels (P=0.003).

CONCLUSIONS

The LPA I4399M SNP is associated with severe CAD and plasma lipoprotein(a) levels.

Elements in the C terminus of apolipoprotein [a] responsible for the binding to the tenth type III module of human fibronectin

In previous studies, we showed that the C-terminal domain, F2, but not the N-terminal domain, F1, is responsible for the binding of apolipoprotein [a] (apo[a]) to human fibronectin (Fn). To pursue those observations, we prepared, by both elastase digestion and recombinant technology, subsets of F2 of a different length containing either kringle (K) V or the protease domain (PD). We also studied rhesus monkey apo[a], which is known to contain PD but not KV. In the case of Fn, we used both an intact product and its tenth type III module (10FN-III) expressed in Escherichia coli. The binding studies carried out on microtiter plates showed that the affinity of F2 for immobilized 10FN-III was approximately 6-fold higher than that for Fn (dissociation constants = 1.75 +/- 0.31 nM and 10.25 +/- 1.62 nM, respectively). The binding was also exhibited by rhesus apo[a] and by an F2 subset containing the PD linked to an upstream microdomain comprising KIV-8 to KIV-10 and KV, inactive by itself. Competition experiments on microtiter plates showed that both Fn and 10FN-III, when in solution, are incompetent to bind F2. Together, our results indicate that F2 binds to immobilized 10FN-III more efficiently than whole Fn and that the binding can be sustained by truncated forms of F2 that contain the catalytically inactive PD linked to an upstream four K microdomain.

In vitro inhibition of fibrinolysis by apolipoprotein(a) and lipoprotein(a) is size- and concentration-dependent

Lipoprotein(a) (Lp(a)) is considered an independent risk factor for atherosclerotic heart and circulatory diseases. The unique, polymorphic character of Lp(a) is based on its apolipoprotein(a) (apo(a)), which has remarkable structural analogies with plasminogen, an important protein for fibrinolysis. The formation of plasmin from plasminogen is a fundamental step in the dissolution of fibrin. Repression of this step may lead to a deceleration of fibrinolysis. It has been suggested that Lp(a) has antifibrinolytic properties through apo(a) and that the apo(a)-size polymorphism has a distinct influence on the prothrombotic properties of Lp(a). However, the results on this topic are controversial. Therefore we used a standardized in vitro fibrinolysis model to provide further information on the influence of Lp(a) on plasmin formation. Monitoring the time-course of plasmin formation, we investigated the inhibition of plasmin formation through dependence on Lp(a), respectively, free apo(a) concentration. Furthermore, we investigated the influence of three Lp(a)/apo(a) phenotypes ((22K)Lp(a), 22 kringle-4 repeats; (30K)Lp(a), 30 kringle-4 repeats; (35K)Lp(a), 35 kringle-4 repeats). Adding varying amounts of Lp(a) to our model, we observed that the rate of plasmin formation was inversely related to the Lp(a) concentration. At 0.1 micromol/l (30K)Lp(a), for example, the plasmin formation was reduced by 12.7% and decreased further by 40.7% at 0.25 micromol/l Lp(a). A similar but more distinct effect was observed when free (30K)apo(a) was added to the model (25.3% at 0.1 micromol/l vs. 59.3% at 0.25 micromol/l). Comparing the antifibrinolytic influence of different apo(a) phenotypes we found that the reduction of plasmin generation advanced with the size of apo(a). At 0.1 micromol/l Lp(a) the reduction of the plasmin formation increased in the order (22K)Lp(a), (30K)Lp(a) and (35K)Lp(a) from 3.7% to 10.7% and 22.3%, respectively. Experiments with different phenotypes of free apo(a) showed similar results (0.5 micromol/l: (22K)apo(a), 56.4% vs. (30K)Lp(a), 80.4%). Summarizing these results, our study indicates a distinct interrelation of Lp(a)/apo(a) phenotype and concentration with the formation of plasmin. From the antifibrinolytic Lp(a)/apo(a) effect in vitro it may be hypothesized that Lp(a)/apo(a) also has an inhibitory influence on in vivo fibrinolysis.

Lipoprotein(a): new insights into mechanisms of atherogenesis and thrombosis

Lipoprotein(a) (Lp[a]) continues to be a controversial molecule regarding its role in human vascular disease. Although the physiologic role of this molecule is still unclear, novel discoveries within the last few years have suggested numerous mechanisms whereby Lp(a) may contribute to atherosclerosis and its complications in human subjects. These effects may differentially occur in vascular tissue and circulating blood compartments. A complex interplay between tissue-specific effects is probably more relevant to the pathogenicity of this molecule than one single effect alone. This review briefly describes the structure of Lp(a) in relation to its biochemical function, summarizing the current literature on various pathophysiologic mechanisms of Lp(a)-induced vascular disease and the role of cell and tissue-specific effects in promoting atherogenesis and thrombosis.

Structure-function relationships in apolipoprotein(a): insights into lipoprotein(a) assembly and pathogenicity

PURPOSE OF REVIEW

Lipoprotein(a) is a structurally and functionally unique lipoprotein consisting of the glycoprotein apolipoprotein(a) covalently linked to LDL. Lipoprotein(a) is assembled extracellularly by a two-step mechanism, still incompletely understood, in which initial non-covalent interactions between apolipoprotein(a) and apolipoprotein B precede specific disulfide bond formation. Elevated concentrations of plasma lipoprotein(a) are a risk factor for a variety of vascular diseases, including coronary heart disease, ischaemic stroke and venous thrombosis. Whereas many pathogenic mechanisms have been proposed for lipoprotein(a), it remains to be conclusively demonstrated which mechanisms are relevant to human disease.

RECENT FINDINGS

Structural and functional studies have verified that apolipoprotein(a) kringle 4 types 6-8 contain lysine binding sites of a weaker affinity for lysine analogues than kringle 4 type 10. Recent evidence has conclusively shown a role for kringle 4 types 7 and 8 in lipoprotein(a) assembly. Moreover, apolipoprotein(a) has been shown to undergo a conformational change, from a closed to an open form, which accelerates the rate of covalent lipoprotein(a) assembly. Functional studies in vitro have identified the domains in apolipoprotein(a) that mediate its inhibitory effects on fibrin clot lysis, binding to fibrin and other biological substrates, and pro-inflammatory and anti-angiogenic properties.

SUMMARY

Extensive structure-function studies of apolipoprotein(a) have begun to yield important insights into the domains in apolipoprotein(a) that mediate lipoprotein(a) assembly and the pathogenic effects of this lipoprotein. Continued investigations of these relationships will contribute critically to unravelling the many outstanding questions about lipoprotein(a) metabolism and pathophysiology.

Lysine-phosphatidylcholine adducts in kringle V impart unique immunological and potential pro-inflammatory properties to human apolipoprotein(a)

Lipoprotein(a), Lp(a), an athero-thrombotic risk factor, reacts with EO6, a natural monoclonal autoantibody that recognizes the phophorylcholine (PC) group of oxidized phosphatidylcholine (oxPtdPC) either as a lipid or linked by a Schiff base to lysine residues of peptides/proteins. Here we show that EO6 reacts with free apolipoprotein(a) apo(a), its C-terminal domain, F2 (but not the N-terminal F1), kringle V-containing fragments obtained by the enzymatic digestion of apo(a) and also kringle V-containing apo(a) recombinants. The evidence that kringle V is critical for EO6 reactivity is supported by the finding that apo(a) of rhesus monkeys lacking kringle V did not react with EO6. Based on the previously established EO6 specificity requirements, we hypothesized that all or some of the six lysines in human kringle V are involved in Schiff base linkage with oxPtdPC. To test this hypothesis, we made use of a recombinant lysine-containing apo(a) fragment, rIII, containing kringle V but not the protease domain. EO6 reacted with rIII before and after reduction to stabilize the Schiff base and also after extensive ethanol/ether extraction that yielded no lipids. On the other hand, delipidation of the saponified product yielded an average of two mol of phospholipids/mol of protein consistent with direct analysis of inorganic phosphorous on the non-saponified rIII. Moreover, only two of the six theoretical free lysine amino groups per mol of rIII were unavailable to chemical modification by 2,4,6-trinitrobenzene sulfonic acid. Finally, rIII, like human apo(a), stimulated the production of interleukin 8 in THP-1 macrophages in culture. Together, our studies provide evidence that in human apo(a), kringle V is the site that reacts with EO6 via lysine-oxPtdPC adducts that may also be involved in the previously reported pro-inflammatory effect of apo(a) in cultured human macrophages.

Inhibition of angiogenesis and angiogenesis-dependent tumor growth by the cryptic kringle fragments of human apolipoprotein(a)

Apolipoprotein(a) (apo(a)) contains tandemly repeated kringle domains that are closely related to plasminogen kringle 4, followed by a single kringle 5-like domain and an inactive protease-like domain. Recently, the anti-angiogenic activities of apo(a) have been demonstrated both in vitro and in vivo. However, its effects on tumor angiogenesis and the underlying mechanisms involved have not been fully elucidated. To evaluate the anti-angiogenic and anti-tumor activities of the apo(a) kringle domains and to elucidate their mechanism of action, we expressed the last three kringle domains of apo(a), KIV-9, KIV-10, and KV, in Escherichia coli. The resultant recombinant protein, termed rhLK68, exhibited a dose-dependent inhibition of basic fibroblast growth factor-stimulated human umbilical vein endothelial cell proliferation and migration in vitro and inhibited the neovascularization in chick chorioallantoic membranes in vivo. The ability of rhLK68 to abrogate the activation of extracellular signal-regulated kinases appears to be responsible for rhLK68-mediated anti-angiogenesis. Furthermore, systemic administration of rhLK68 suppressed human lung (A549) and colon (HCT-15) tumor growth in nude mice. Immunohistochemical examination and in situ hybridization analysis of the tumors showed a significant decrease in the number of blood vessels and the reduced expression of vascular endothelial growth factor, basic fibroblast growth factor, and angiogenin, indicating that suppression of angiogenesis may have played a significant role in the inhibition of tumor growth. Collectively, these results suggest that a truncated apo(a), rhLK68, is a potent anti-angiogenic and anti-tumor molecule.

Lipoprotein(a) and the atherothrombotic process: mechanistic insights and clinical implications

Although many epidemiologic studies have pointed at an association between plasma levels of lipoprotein(a) (Lp(a)) and cardiovascular risk, the data obtained have been conflicting because of a number of factors, particularly those dealing with plasma storage, lack of assay standardization, population sample size, age, gender, ethnic variations, and variable disease endpoints. Moreover, the attention has been primarily focused on whole Lp(a), with relatively less emphasis on its constituent apolipoprotein(a) and on the apolipoprotein B100-containing lipoprotein, mainly low-density lipoprotein (LDL), to which apolipoprotein(a) is linked. According to recent studies, small-size apolipoprotein(a) isoforms may represent a cardiovascular risk factor either by themselves or synergistically with plasma Lp(a) concentration. Moreover, the density properties of the LDL moiety may have an impact on Lp(a) pathogenicity. It has also become apparent that Lp(a) can be modified by oxidative events and by the action of lipolytic and proteolytic enzymes with the generation of products that exhibit atherothrombogenic potential. The role of the O-glycans linked to the inter-kringle linkers of apolipoprotein(a) is also emerging. This information is raising the awareness of the pleiotropic functions of Lp(a) and is opening new vistas on pathogenetic mechanisms whose knowledge is essential for developing rational therapies against this complex cardiovascular pathogen.

Lipoprotein(a): an emerging cardiovascular risk factor

Lipoprotein(a) is a cholesterol-enriched lipoprotein, consisting of a covalent linkage joining the unique and highly polymorphic apolipoprotein(a) to apolipoprotein B100, the main protein moiety of low-density lipoproteins. Although the concentration of lipoprotein(a) in humans is mostly genetically determined, acquired disorders might influence synthesis and catabolism of the particle. Raised concentration of lipoprotein(a) has been acknowledged as a leading inherited risk factor for both premature and advanced atherosclerosis at different vascular sites. The strong structural homologies with plasminogen and low-density lipoproteins suggest that lipoprotein(a) might represent the ideal bridge between the fields of atherosclerosis and thrombosis in the pathogenesis of vascular occlusive disorders. Unfortunately, the exact mechanisms by which lipoprotein(a) promotes, accelerates, and complicates atherosclerosis are only partially understood. In some clinical settings, such as in patients at exceptionally low risk for cardiovascular disease, the potential regenerative and antineoplastic properties of lipoprotein(a) might paradoxically counterbalance its athero-thrombogenicity, as attested by the compatibility between raised plasma lipoprotein(a) levels and longevity.

Stimulation of interleukin-8 production in human THP-1 macrophages by apolipoprotein(a). Evidence for a critical involvement of elements in its C-terminal domain

In the vessel wall, macrophages are among the cells that upon activation contribute to the atherosclerotic process. Low density lipoproteins (LDL) can mediate this activation but only after enzymatic or oxidative modification. Lipoprotein(a) (Lp(a)) is an LDL variant that has been shown to have an atherogenic potential by no clearly established mechanisms. In the present study we examined whether native Lp(a) can activate macrophages and, if so, identify the structural elements involved in this action. For this purpose, we utilized human THP-1 macrophages, prepared by treating THP-1 monocytes with phorbol ester, and we exposed them to Lp(a) and its two derivatives, apo(a)-free LDL (Lp(a-)) and free apo(a). We also studied apo(a) fragments, F1 (N terminus) and F2 (C terminus) and subfragments thereof, obtained by leukocyte elastase digestion. By Northern blot analyses, Lp(a), but not Lp(a-), caused up to a 12-fold increase in interleukin 8 (IL-8) mRNA as compared with untreated cells. Free apo(a) also induced the production of IL-8 mRNA; however, the effect was 3-4-fold higher than that of Lp(a). The increase in mRNA was associated with the accumulation of IL-8 protein in the culture medium. F1 had only a minimal effect, whereas F2 was 1.5-2-fold more potent than apo(a), an activity mostly contained in the Kringle V-protease region. A monoclonal antibody specific for Kringle V inhibited the apo(a)-mediated effect on IL-8. We conclude that Lp(a) via elements contained in the C-terminal domain of apo(a) causes in THP-1 macrophages an increased production of IL-8, a chemokine with pro-inflammatory properties, an event that may be relevant to the process of atherosclerosis.

Plasma apolipoprotein(a) co-deposits with fibrin in inflammatory arthritic joints

Extravascular coagulation and diminished fibrinolysis are processes that contribute to the pathology of both inflammatory arthritis and atherosclerosis. We hypothesized that, given its homology with plasminogen, apolipoprotein (apo) (a), the distinctive glycoprotein of the atherogenic lipoprotein (Lp) (a), may be equally implicated in inflammatory arthritis. We detected the presence of apo(a) as part of Lp(a) in human arthritic synovial fluid. The abundance of apo(a) in synovial fluid rose in proportion to plasma apo(a) levels and was higher in inflammatory arthritides than in osteoarthritis. In addition, apo(a) immunoreactive material, but not apo(a) transcripts, was detected in inflammatory arthritic synovial tissues. These data indicated that synovial fluid apo(a) originates from circulating Lp(a) and that diffusion of Lp(a) through synovial tissue is facilitated in inflammatory types of arthritis. In synovial tissues, apo(a) co-localized with fibrin. These observations could be reproduced in a model of antigen-induced arthritis, using transgenic mice expressing human Lp(a). Although in this mouse model the presence of apo(a) did not change the severity of arthritis, the co-localization of apo(a) with fibrin in synovial tissue suggests that, in humans, apo(a) may modulate locally the fibrinolytic activity and may thus contribute to the persistence of intra-articular fibrin in inflammatory arthritis.

Instability of Lipoprotein(a) in Plasma Stored at −70 °C

Background: There is considerable evidence to suggest that plasma lipoprotein(a) [Lp(a)] concentration is a cardiovascular risk factor. Confusing results in epidemiologic studies, however, suggest that the effects of storage should be further investigated. The influence of the assay method, the initial plasma Lp(a) concentration, and the apolipoprotein(a) [apo(a)] genotype are all factors that should be considered.

Methods: Blood was obtained from 65 survivors of premature myocardial infarction and 95 age-matched controls. The plasma samples were stored in sterile conditions at −70 °C for 5 years in the presence of antioxidant and antiproteolytic substances. Plasma Lp(a) was measured by immunoturbidimetry, and apo(a) alleles were determined by pulsed-field gel electrophoresis and Southern blotting.

Results: Plasma Lp(a) was significantly higher in patients. The mean kringle number for the smallest isoform was also lower in patients than in controls, but no differences were found in the distribution of the largest isoform. All patients and controls were heterozygotes. During storage, mean Lp(a) decreased significantly in samples from patients (−23%; P <0.001) but not in samples from controls (−9%; P, not significant). This was not related to the kringle number and was limited to samples with initial plasma Lp(a) concentrations between 41 and 345 mg/L.

Conclusions: Plasma Lp(a) from patients is less stable than Lp(a) from controls, and the difference is not related to distribution of apo(a) genotypes but may be concentration-dependent. Differential sample stability may complicate the interpretation of several studies.

Renal handling of human apolipoprotein(a) and its fragments in the rat

The sites and mechanisms of the catabolism of atherogenic lipoprotein(a) (Lp(a)) are not well understood. Lp(a) is increased in patients with end-stage renal disease, suggesting a renal catabolism of Lp(a). To gain a better insight into renal handling of Lp(a), we established a heterologous rat model to study the renal catabolism of human Lp(a). Pure human Lp(a) was injected into Wistar rats, and animals were sacrificed at different time points (30 minutes to 24 hours). Intact Lp(a) was cleared from the circulation of injected rats with a half-life time of 14.5 hours. Strong intracellular immunostaining for apolipoprotein(a) (apo(a)) was observed in the cytoplasm of proximal tubular cells after 4, 8, and 24 hours. Apolipoprotein B (apoB) was colocalized with glomerular apo(a) 1 to 8 hours after Lp(a) injection, but renal capillaries and tubules remained negative. No relevant amounts of apo(a) fragments were found in the plasma of rats after injection of Lp(a). During all urine collection periods, apo(a) fragments with molecular weights of 50 to 160 kd were detected in the urine, however. Our results show that human Lp(a) injected into rats accumulates intracellularly in the rat kidney, and apo(a) fragments are excreted in the urine. The kidney apparently plays a major role in fragmentation of Lp(a). Despite the fact that rodents lack endogenous Lp(a), rats injected with human Lp(a) may provide a useful heterologous animal model to study the renal metabolism of Lp(a) further.

Molecular analysis of apo(a) fragmentation in polygenic hypercholesterolemia: characterization of a new plasma fragment pattern

Hypercholesterolemia is frequently associated with elevated Lp(a) levels, an independent risk factor for coronary, cerebrovascular, and peripheral vascular disease. A portion of apolipoprotein(a) [apo(a)] circulates as a series of fragments derived from the N-terminal region of apo(a). The relationship of elevated lipoprotein(a) [Lp(a)] levels to those of circulating apo(a) fragments in polygenic hypercholesterolemia is indeterminate. Therefore, plasma Lp(a) and plasma and urinary apo(a) fragment levels were measured by ELISA in 82 patients with polygenic type IIa hypercholesterolemia (low density lipoprotein cholesterol >/=4.13 mmol/L and triglycerides <2.24 mmol/L) and in 90 normolipidemic subjects. Lp(a) levels were significantly elevated in patients compared with control subjects (0.35+/-0.4 and 0.24+/-0.31 mg/mL, respectively; median 0.13 and 0.11 mg/mL, respectively; P=0.039), although apo(a) isoform distribution did not differ. Patients displayed significantly higher plasma and urinary apo(a) fragment levels than did control subjects (respective values were as follows: 4.97+/-5.51 and 2.15+/-2.57 [median 2.85 and 1.17] microg/mL in plasma, P<0.0001; 75+/-86 and 40+/-57 [median 38 and 17] ng/mg urinary creatinine in urine, P<0.0001). The ratio of plasma apo(a) fragments to Lp(a) levels was also significantly higher in patients than in control subjects (1.93+/-1.5% and 1.75+/-2.36%, respectively; P<0.0001). We conclude that increased plasma Lp(a) levels in polygenic hypercholesterolemia are associated with elevated circulating levels of apo(a) fragments but that this increase is not due to decreased renal clearance of apo(a) fragments. Furthermore, we identified a new pattern of apo(a) fragmentation characterized by the predominance of a fragment band whose size was related to that of the parent apo(a) isoform and that was superimposed on the series of fragments described previously by Mooser et al (J Clin Invest. 1996; 98:2414-2424). This new pattern was associated with small apo(a) isoforms and did not discriminate between hypercholesterolemic and normal subjects. However, this new apo(a) fragment pattern may constitute a novel marker for cardiovascular risk.

Structural elucidation of the N- and O-glycans of human apolipoprotein(a): role of o-glycans in conferring protease resistance

Apolipoprotein(a) (apo(a)) is a multikringle domain glycoprotein that exists covalently linked to apolipoprotein B100 of low density lipoprotein, to form the lipoprotein(a) (Lp(a)) particle, or as proteolytic fragments. Elevated plasma concentrations of apo(a) and its fragments may promote atherosclerosis, but the underlying mechanisms are incompletely understood. The factors influencing apo(a) proteolysis are also uncertain. Here we have used exoglycosidase digestion and mass spectrometry to sequence the Asn (N)-linked and Ser/Thr (O)-linked oligosaccharides of human apo(a). We also assessed the potential role of apo(a) O-glycans in protecting thermolysin-sensitive regions of the polypeptide. Apo(a) contained two major N-glycans that accounted for 17% of the total oligosaccharide structures. The N-glycans were complex biantennary structures present in either a mono- or disialylated state. The O-glycans were mostly (80%) represented by the monosialylated core type 1 structure, NeuNAcalpha2-3Galbeta1-3GalNAc, with smaller amounts of disialylated and non-sialylated O-glycans also detected. Removal of apo(a) O-glycans by sialidase and O-glycosidase treatment dramatically increased the sensitivity of the polypeptide to thermolysin digestion. These studies provide the first direct sequencing data for apo(a) glycans and indicate a novel function for apo(a) O-glycans that is potentially related to the atherogenicity of Lp(a).

Properties of human free apolipoprotein(a) and lipoprotein(a) after either freezing or lyophilization in the presence and absence of cryopreservatives

Apolipoprotein(a), apo(a), the specific multikringle glycoprotein constituent of lipoprotein(a), Lp(a), occurs in the plasma mostly bound to apoB100-containing lipoproteins but also in a free form. Often the properties of these products are determined after storage in the cold; yet limited information is available on their stability at low temperatures. To shed light on this subject, we examined the effect of two parameters, freezing and lyophilization, in either the absence or the presence of cryopreservatives. Lp(a)s each having a single apo(a) size isoform containing either 14 or 17 kringle (K) IVs were isolated from the plasma of healthy donors by combining density gradient ultracentrifugation and lysine-Sepharose column chromatography using solutions containing both antioxidants and proteolytic inhibitors. Apo(a) was obtained from parent Lp(a) by a mild limited reductive procedure. Either freezing at -20 degrees C or lyophilization in the presence of 5% sucrose did not change the electrophoretic, immunochemical, and lysine-binding properties of Lp(a) including its ability to generate free apo(a). Irrespective of source, apo(a) remained stable when either frozen at -20 and -80 degrees C or lyophilized in the presence of 125 mM trehalose. In all cases, the absence of cryopreservatives caused the samples to aggregate irreversibly. Thawed or reconstituted samples of both free and bound apo(a) kept at 4 degrees C under sterile conditions in the presence of antioxidants, proteolytic inhibitors, and cryopreservative exhibited no significant changes in properties within the time of observation. Both apo(a) isoforms gave comparable results. We conclude that apo(a), either free or bound, can be kept stable at low temperatures in the presence of appropriate cryopreservatives.

Apolipoprotein (a) fragments in relation to human carotid plaque instability

PURPOSE

An elevated plasma level of lipoprotein (a) is an independent risk factor for atherothrombotic cardiovascular disease by yet undefined mechanisms. We have previously reported that matrix metalloproteinases cleave apolipoprotein (a) into 2 main fragments, F1 and F2, the latter (the C-terminal domain) exhibiting in vitro a high-affinity binding to extracellular matrix components, including fibrin(ogen). We therefore tested the hypothesis that the lipoprotein (a) matrix metalloproteinase-derived F2 is localized in potentially or morphologically unstable human carotid plaque at regions of increased matrix metalloproteinase activity.

METHODS

Carotid plaques removed after endarterectomy (n = 18) were evaluated for structural features indicative of instability (thin fibrous cap, inflammation, and proximity of the necrotic core to the lumen); each plaque was classified as unstable (n = 10) or stable (n = 8). Western blot analysis was performed to quantitate apolipoprotein (a) and its fragments F1 and F2 in plaque extracts. Immunohistochemical staining was used to localize apolipoprotein (a) and its fragments within the atherosclerotic plaque. In situ zymography was used to determine regions of gelatinase (matrix metalloproteinase 2 and matrix metalloproteinase 9) activity.

RESULTS

Western blot analyses demonstrated a 2.5-fold higher density of F2 in unstable plaques than in stable plaques (3.07 +/- 1.9 vs 1.18 +/- 0.8; P <.05). In morphologically unstable plaques, there was preferential distribution of F2 within regions of fibrous cap inflammation and/or foam cell accumulation and within abluminal necrotic cores. In morphologically stable plaques, however, localization was predominantly found in the medial smooth muscle cells. Regions of enhanced matrix metalloproteinase 2 and matrix metalloproteinase 9 activity co-localized with the transmural distribution of F2 within the plaque.

CONCLUSIONS

These findings suggest that F2 in regions of increased matrix metalloproteinase activity is a potential mechanism for superimposed thrombotic events in morphologically unstable human carotid plaques. The relationship between plasma lipoprotein (a) levels and accumulation of F2 and the potential correlation of F2 to human plaque disruption and thrombosis warrant further study.

The rate of plasmin formation after in vitro clotting is inversely related to lipoprotein(a) plasma levels

Lipoprotein(a) levels are largely genetically determined and are linked to increased risk of coronary artery disease. The hypothesis that elevated lipoprotein(a) levels lead to decreased fibrinolysis, due to the close structural homology with plasminogen, could in part explain the genesis of this risk, although contrasting results have been obtained in different studies. The aim of our study was to evaluate whether the rate of plasmin formation, enhanced in vitro by a fixed amount of human tissue plasminogen activator after clotting, was related to plasma lipoprotein(a) levels in 45 healthy subjects. Aliquots of human plasma were clotted with calcium chloride and thrombin followed by addition of tissue plasminogen activator. We then measured the time course of plasmin formation, determined as hydrolysis of H-D-valyl-L-leucyl-L-lysine-p-nitroanilide dihydrocortide (S-2251). The log of lipoprotein(a) level was negatively related to the rate of plasmin formation (r(s)=-0.46, P=0. 002), and multiple regression analysis indicated that this relationship was not influenced by the amount of plasminogen, fibrinogen, plasminogen activator inhibitor-1, tissue plasminogen activator, or by the size of apo(a) isoforms. These data support the concept that lipoprotein(a) can inhibit plasminogen activation and plasmin formation and can thereby play an important role in the genesis of atherosclerosis as an antifibrinolytic agent.

Dominant role of the C-terminal domain in the binding of apolipoprotein(a) to the protein core of proteoglycans and other members of the vascular matrix

The C-terminal domain of apolipoprotein(a) binds in vitro to the protein core of proteoglycans as well as fibrinogen and fibronectin, suggesting that this domain plays a role in anchoring lipoprotein(a) or free apolipoprotein(a) to the vascular subendothelial matrix.

Lipoprotein(a) and coronary heart disease risk

Although retrospective case-control studies continue to indicate that plasma lipoprotein(a) concentrations are associated with coronary heart disease (CHD), several large population-based prospective studies have failed to confirm that Lp(a) is an independent risk factor. However, evidence exists from several studies to suggest that elevated plasma Lp(a) increases the CHD risk associated with more traditional risk factors. Although identification of the functional role of Lp(a) in atherogenesis has been thwarted by the physical, chemical, and genetic complexity of Lp(a), the structural similarity of Lp(a) to both the fibrinolytic proenzyme plasminogen and low-density lipoprotein (LDL) has suggested a prothrombotic or atherogenic role (or both) for this lipoprotein. Because the clinical determination and application of plasma Lp(a) concentration poses several challenges, we cannot recommend its routine measurement at this time. Rather, plasma Lp(a) determinations should be limited to either patients at high risk for the development of CHD or patients at borderline risk for the development of CHD in whom uncertainty may exist about how aggressively to treat modifiable risk factors such as elevated LDL cholesterol.

Macrophage metalloelastase, MMP-12, cleaves human apolipoprotein(a) in the linker region between kringles IV-4 and IV-5. Potential relevance to lipoprotein(a) biology

In this study we found that macrophage metalloelastase, MMP-12 cleaves, in vitro, apolipoprotein(a) (apo(a)) in the Asn3518-Val3519 bond located in the linker region between kringles IV-4 and IV-5, a bond immediately upstream of the Ile3520-Leu3521 bond, shown previously to be the site of action by neutrophil elastase (NE). We have also shown that human apo(a) injected into the tail vein of control mice undergoes degradation as reflected by the appearance of immunoreactive fragments in the plasma and in the urine of these animals. To define whether either or both of these enzymes may be responsible for the in vivo apo(a) cleavage, we injected intravenously MMP-12(-/-), NE -/- mice and litter mates, all of the same strain, with either lipoprotein(a) (Lp(a)), full-length free apo(a), or its N-terminal fragment, F1, obtained by the in vitro cleavage of apo(a) by NE. In the plasma of Lp(a)/apo(a)-injected mice, F1 was detected in control and NE -/- mice but was virtually absent in the MMP-12(-/-) mice. Moreover, fragments of the F1 type were present in the urine of the animals except for the MMP-12(-/-) mice. These fragments were significantly smaller in size than those observed in the plasma. All of the animals injected with F1 exhibited small sized fragments in their urine. These observations provide evidence that, in the mouse strain used, MMP-12 plays an important role in the generation of F1 from injected human Lp(a)/apo(a) and that this fragment undergoes further cleavage during renal transit via a mechanism that is neither NE- nor MMP-12-dependent. Thus, factors influencing the expression of MMP-12 may have a modulating action on the biology of Lp(a).

Effect of cardiopulmonary bypass and heparin on plasma levels of Lp(a) and Apo(a) fragments

Fragments of apolipoprotein(a) [apo(a)], the distinctive glycoprotein of lipoprotein(a) [Lp(a)], are present in human plasma and urine and have been implicated in the development of atherosclerosis. The mechanism responsible for the generation of apo(a) fragments in vivo is poorly understood. In this study, we examined the plasma levels of Lp(a) and apo(a) fragments [or free apo(a)] and urinary apo(a) in 15 subjects who underwent cardiac surgery necessitating cardiopulmonary bypass. We also measured the plasma concentration and activity of polymorphonuclear elastase, an Lp(a)-cleaving enzyme in vitro, and plasma levels of C-reactive protein. Despite a marked activation of polymorphonuclear cells and a pronounced inflammatory response, as documented by an 8-fold and a 35-fold increase in plasma levels of polymorphonuclear elastase and C-reactive protein, respectively, the proportion of plasma free apo(a) to Lp(a) and urinary excretion of apo(a) remained unchanged over a 7-day period after surgery, and polymorphonuclear elastase activity remained undetectable in plasma. No fragmentation of apo(a) was observed ex vivo in plasma samples collected before and after surgery. These data indicate that in this model, apo(a) is not fragmented in plasma and are consistent with the hypothesis that apo(a) fragments result from a constitutively active tissue mechanism that is not modified by cardiac surgery with cardiopulmonary bypass.

Recombinant kringle IV-10 modules of human apolipoprotein(a): structure, ligand binding modes, and biological relevance

The kringle modules of apolipoprotein(a) [apo(a)] of lipoprotein(a) [Lp(a)] are highly homologous with kringle 4 of plasminogen (75-94%) and like the latter are autonomous structural and functional units. Apo(a) contains 14-37 kringle 4 (KIV) repeats distributed into 10 classes (1-10). Lp(a) binds lysine-Sepharose via a lysine binding site (LBS) located in KIV-10 (88% homology with plasminogen K4). However, the W72R substitution that occurs in rhesus monkeys and occasionally in humans leads to impaired lysine binding capacity of KIV-10 and Lp(a). The foregoing has been investigated by determining the structures of KIV-10/M66 (M66 variant) in its unliganded and ligand [epsilon-aminocaproic acid (EACA)] bound modes and the structure of recombinant KIV-10/M66R72 (the W72R mutant). In addition, the EACA liganded structure of a sequence polymorph (M66T in about 42-50% of the human population) was reexamined (KIV-10/T66/EACA). The KIV-10/M66, KIV-10/M66/EACA, and KIV-10/T66/EACA molecular structures are highly isostructural, indicating that the LBS of the kringles is preformed anticipating ligand binding. A displacement of three water molecules from the EACA binding groove and a movement of R35 bringing the guanidinium group close to the carboxylate of EACA to assist R71 in stabilizing the anionic group of the ligand are the only changes accompanying ligand binding. Both EACA structures were in the embedded binding mode utilizing all three binding centers (anionic, hydrophobic, cationic) like plasminogen kringles 1 and 4. The KIV-10/T66/EACA structure determined in this work differs from one previously reported [Mikol, V., Lo Grasso, P. V. and, Boettcher, B. R. (1996) J. Mol. Biol. 256, 751-761], which crystallized in a different crystal system and displayed an unbound binding mode, where only the amino group of EACA interacted with the anionic center of the LBS. The remainder of the ligand extended into solvent perpendicular to the kringle surface, leaving the hydrophobic pocket and the cationic center of the LBS unoccupied. The structure of recombinant KIV-10/M66R72 shows that R72 extends along the ligand binding groove parallel to the expected position of EACA toward the anionic center (D55/D57) and makes a salt bridge with D57. Thus, the R72 side chain mimics ligand binding, and loss of binding ability is the result of steric blockage of the LBS by R72 physically occupying part of the site. The rhesus monkey lysine binding impairment is compared with that of chimpanzee where KIV-10 has been shown to have a D57N mutation instead.

Lipoprotein(a) as a risk factor for coronary artery disease

Since its identification by Kåre Berg in 1963, lipoprotein(a) [Lp(a)] has become a focus of research interest owing to the results of case-control and prospective studies linking elevated plasma levels of this lipoprotein with the development of coronary artery disease. Lp(a) contains a low-density lipoprotein (LDL)-like moiety, in which the apolipoprotein B-100 component is covalently linked to the unique glycoprotein apolipoprotein(a) [apo(a)]. Apo(a) is composed of repeated loop-shaped units called kringles, the sequences of which are highly similar to a kringle motif present in the fibrinolytic proenzyme plasminogen. Variability in the number of repeated kringle units in the apo(a) molecule gives rise to different-sized Lp(a) isoforms in the population. Based on the similarity of Lp(a) to both LDL and plasminogen, it has been hypothesized that the function of this unique lipoprotein may represent a link between the fields of atherosclerosis and thrombosis. However, determination of the function of Lp(a) in vivo remains elusive. Although Lp(a) has been shown to accumulate in atherosclerotic lesions, its contribution to the development of atheromas is unclear. This uncertainty is related in part to the structural complexity of the apo(a) component of Lp(a) (particularly apo(a) isoform size heterogeneity), which also poses a challenge for standardization of the measurement of Lp(a) in plasma. The fact that plasma Lp(a) levels are largely genetically determined and vary widely among different ethnic groups adds scientific interest to the ongoing study of this enigmatic particle. Most recently, the identification of proteolytic fragments of apo(a) in both plasma and urine has fueled speculation about the origin of these fragments and their possible function in the atherosclerotic process.

Lipoprotein(a) and inflammation in human coronary atheroma: association with the severity of clinical presentation

OBJECTIVES

The purpose of this study was the investigation of the in vivo role of lipoprotein(a) [Lp(a)] and inflammatory infiltrates in the human coronary atherosclerotic plaque and their correlation with the clinical syndrome of presentation.

BACKGROUND

Lipoprotein(a) is an atherogenic and thrombogenic lipoprotein, and has been implicated in the pathogenesis of acute coronary syndromes. Lipoprotein(a) induces monocyte chemoattraction and smooth muscle cell activation in vitro. Macrophage infiltration is considered one of the mechanisms of plaque rupture.

METHODS

This study of atherectomy specimens investigated the in vivo role of Lp(a) at different stages of the atherogenic process, and its relationship with macrophage infiltration. We examined coronary atheroma removed from 72 patients with stable or unstable angina. Specimens were stained with antibodies specific for Lp(a), macrophages (KP-1), and smooth muscle cells (alpha-actin). Morphometric analysis was used to quantify the plaque areas occupied by each of the three antigens, and their colocalization.

RESULTS

All specimens had localized Lp(a) staining; the mean fractional area was 58.2%. Ninety percent of the macrophage areas colocalized with Lp(a) positive areas, whereas 31.3% of the smooth muscle cell areas colocalized with Lp(a) positive areas. Patients with unstable angina (n = 46) had specimens with larger mean plaque Lp(a) areas than specimens from stable angina patients (n = 26): 64.4% versus 47.7% (p = 0.004). Unstable angina patients with rest pain (n = 28) had greater mean plaque Lp(a) area than unstable angina patients with crescendo exertional pain (n = 18): 71.1% versus 52.4% (p < 0.001). Mean KP-1 area was 31.2% in unstable rest angina versus 18.3% in stable angina (p = 0.05); alpha-actin area was greater in stable (48.5%) and crescendo exertional angina (48.8%) than in rest angina (30.4%). The strongest correlation between plaque KP-1 and Lp(a) area was in unstable rest angina (r = 0.88, p < 0.001), and between alpha-actin and Lp(a) areas in the crescendo exertional angina (r = 0.62, p < 0.01).

CONCLUSIONS

Lipoprotein(a) is ubiquitous in human coronary atheroma. It is detected in larger amounts in tissue from culprit lesions in patients with unstable compared to stable syndromes, and has significant colocalization with plaque macrophages. A correlation of plaque alpha-actin and Lp(a) area suggests a role of Lp(a) in plaque growth.

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.

Apolipoprotein(a) binds via its C-terminal domain to the protein core of the proteoglycan decorin. Implications for the retention of lipoprotein(a) in atherosclerotic lesions

Although it is known that lipoprotein(a) (Lp(a)) binds to proteoglycans, the mechanism for this binding has not been fully elucidated. In order to shed light on this subject, we examined the interactions of decorin, a proteoglycan with a well defined protein core and a single glycosaminoglycan (GAG) chain, with Lp(a) and derivatives, namely Lp(a) deprived of apo(a), or Lp(a-), free apo(a), and the two main proteolytic fragments, F1 and F2. By circular dichroism criteria, the decorin preparations used had the same secondary structure as that previously reported for native decorin. Authentic low density lipoprotein from the same human donor was used as a control. In a solid phase system, Lp(a-)and low density lipoprotein bound to decorin in a comparable manner. This binding required Ca2+/Mg2+ ions, was lysine-mediated, and was markedly decreased in the presence of GAG-depleted decorin, suggesting the ionic nature of the interaction likely involving apoB100 and the GAG component of decorin. Free apo(a) also bound to decorin; however, the binding was neither cation-dependent nor lysine-mediated, unaffected by sialic acid depletion of apo(a), and markedly decreased when either reduced and alkylated apo(a) or reduced and alkylated decorin was used in the assay. Of note, the binding of apo(a) was unaffected when it was incubated with a spectrally native decorin that had been renatured from either 4 M guanidine hydrochloride by extensive dialysis or cooled from 65 to 25 degrees C. On the other hand, the binding significantly increased when decorin was depleted of GAGs, which by themselves had no affinity for apo(a). The binding of apo(a) to the decorin protein core was also elicited by the C-terminal domain of apo(a), and it was favored by high NaCl concentrations, 1 to 2 M. No binding was exhibited by the N-terminal domain accounting for the lack of effect of apo(a) size polymorphism on the binding. In the case of whole Lp(a), the binding to immobilized decorin was mostly GAG-dependent and ionic in nature. A minor contribution by apo(a) was detected when GAG-depleted decorin was used in the assay. Our results indicate that the binding of Lp(a) to decorin involves interactions both electrostatic (apoB100-GAG) and hydrophobic (apo(a)-decorin protein core), and that the binding of apo(a) requires decorin protein core to be in its native state.

International Federation of Clinical Chemistry standardization project for the measurement of lipoprotein(a). Phase I. Evaluation of the analytical performance of lipoprotein(a) assay systems and commercial calibrators

A secondary reference material for lipoprotein(a) is required to standardize the measurement of lipoprotein(a) in clinical laboratories worldwide. Towards this aim, the International Federation of Clinical Chemistry Working Group for the Standardization of Lipoprotein(a) Assays has initiated a standardization project involving a total of 33 diagnostic company and clinical chemistry laboratories from 12 countries. In Phase 1, the analytical performance of 40 lipoprotein(a) assay systems was evaluated by testing sera and manufactured lipoprotein(a) calibrator materials for precision, linearity, and parallelism. Twenty test systems were nonoptimized according to the results for a pooled serum, which tested nonlinear in 16 systems and imprecise in 4. Acceptable analytical properties and harmonization of lipoprotein(a) values were shown by some commercial calibrators, suggesting their possible use as reference materials. This study highlights the problems that currently occur for lipoprotein(a) measurement in existing assay systems.

Apolipoprotein(a) binds to low-density lipoprotein at two distant sites in lipoprotein(a)

Lipoprotein(a) [Lp(a)] consists of low-density lipoprotein (LDL) and apolipoprotein(a) [apo(a)] linked with a disulfide bond. Scanning force microscopy (SFM) of Lp(a) showed, for the first time, a belt-like structure of apo(a) with both ends attached to a spherical LDL. The two ends of apo(a) were bound to the LDL sphere at two distant sites. Occasionally, the ends were attached to two touching spheres. Under the same imaging conditions, LDL appeared as individual spheres. Electron microscopy (EM) studies of Lp(a) by several groups over the past decade failed to reveal this belt-like structure of apo(a). Images of isolated apo(a) in air or in phosphate buffer showed apo(a) as individual belts, and these belts tended to crowd together. Lp(a) formed leaf-like aggregates; apo(a) aggregates were fishnet-like, whereas LDL aggregates were less characteristic. Quantitative analysis of Lp(a) showed the diameter of the LDL to be 24.8 +/- 8.7 nm (n = 46), which is close to the reported value of 24.2 +/- 4.2 nm found with EM. The length of the belts attached to the spheres was measured to be 173.5 +/- 6.6 nm (n = 15). I also found, by using a functionalized tip, that the interaction force between apo(a) and its ligand, lysine, was related to the ionic strength of the bulk solution. This force can be reduced by the presence of epsilon-aminocaproic acid.

Lipoprotein(a): identification of subjects with a superbinding capacity for fibrinogen

We have previously shown that the binding of lipoprotein(a) [Lp(a)] to immobilized fibrinogen involves the domain located in kringles IV-5 to IV-8, but not kringle IV-10. In extending those studies to subjects living in Chicago and in the island of Sardinia, we found that about 6% of them had an Lp(a) with Bmax values of 27.7+/-6.0 fmol, which were about 5-8-fold higher than those of controls (3.4+/-2.8 fmol) and in the range of those observed for free apo(a) derived from the Lp(a) of controls (36.6+/-2.9 fmol). This superbinding phenotype was unaffected by age, sex, type of lipid disorder and hypolipidemic agents, and also had a familial incidence. We are currently exploring the hypothesis that this fibrinogen superbinding phenotype is due to conformational changes of apolipoprotein(a) [apo(a)] resulting from the lipid content and composition of the Lp(a) particle and/or sequence anomalies in the kringle domain IV-5 to IV-8.