A structural assessment of the apo[a] protein of human lipoprotein[a]

Optimization of plasma-based BioID identifies plasminogen as a ligand of ADAMTS13

ADAMTS13, a disintegrin and metalloprotease with a thrombospondin type 1 motif, member 13, regulates the length of Von Willebrand factor (VWF) multimers and their platelet-binding activity. ADAMTS13 is constitutively secreted as an active protease and is not inhibited by circulating protease inhibitors. Therefore, the mechanisms that regulate ADAMTS13 protease activity are unknown. We performed an unbiased proteomics screen to identify ligands of ADAMTS13 by optimizing the application of BioID to plasma. Plasma BioID identified 5 plasma proteins significantly labeled by the ADAMTS13-birA* fusion, including VWF and plasminogen. Glu-plasminogen, Lys-plasminogen, mini-plasminogen, and apo(a) bound ADAMTS13 with high affinity, whereas micro-plasminogen did not. None of the plasminogen variants or apo(a) bound to a C-terminal truncation variant of ADAMTS13 (MDTCS). The binding of plasminogen to ADAMTS13 was attenuated by tranexamic acid or ε-aminocaproic acid, and tranexamic acid protected ADAMTS13 from plasmin degradation. These data demonstrate that plasminogen is an important ligand of ADAMTS13 in plasma by binding to the C-terminus of ADAMTS13. Plasmin proteolytically degrades ADAMTS13 in a lysine-dependent manner, which may contribute to its regulation. Adapting BioID to identify protein-interaction networks in plasma provides a powerful new tool to study protease regulation in the cardiovascular system.

Analysis of 61 SNPs from the CAD specific genomic loci reveals unique set of SNPs as significant markers in the Southern Indian population of Hyderabad

Background

The present study is a part of the major project on coronary artery disease (CAD) carried out at Indian Statistical Institute, Hyderabad to investigate the pattern of association of SNPs selected from the CAD specific genomic loci. The study is expected to portray the genetic susceptibility profile of CAD specifically in the Southern Indian population of Hyderabad.

Methods

The study was conducted in a cohort of 830 subjects comprising 350 CAD cases and 480 controls from Hyderabad. A prioritized set of 61 SNPs selected from the NHGRI GWAS catalogue were genotyped using FluidigmNanofluidic SNP Genotyping System and appropriate statistical analyses were used in interpreting the results.

Results

After data pruning, out of 45 SNPs qualified for the association analysis, four SNPs were found to be highly significantly associated with increased risk for CAD even after Bonferroni correction for multiple testing (p < 0.001). These results were also replicated in the random subsets of the pooled cohort (70, 50 and 30%) suggesting internal consistency. The ROC analysis of the risk scores of the significant SNPs suggested highly significant area under curve (AUC = 0.749; p < 0.0001) implying predictive utility of these risk variants.

Conclusions

The rs10455872 of LP(A) gene in particular showed profound risk for CAD (OR 35.9; CI 16.7–77.2) in this regional Indian population. The other significant SNP associations observed with respect to the pooled CAD cohort and in different anatomical and phenotypic severity categories reflected on the role of genetic heterogeneity in the clinical heterogeneity of CAD. The SNP rs7582720 of WDR12 gene, albeit not individually associated with CAD, was found to be conferring significant risk through epistatic interaction with two SNPs (rs6589566, rs1263163 in ZPR1, APOA5-APOA4 genes) of the 11q23.3 region.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12872-022-02562-4.

Lipoprotein(a): Pathophysiology, measurement, indication and treatment in cardiovascular disease. A consensus statement from the Nouvelle Société Francophone d'Athérosclérose (NSFA)

Lipoprotein(a) is an apolipoprotein B100-containing low-density lipoprotein-like particle that is rich in cholesterol, and is associated with a second major protein, apolipoprotein(a). Apolipoprotein(a) possesses structural similarity to plasminogen but lacks fibrinolytic activity. As a consequence of its composite structure, lipoprotein(a) may: (1) elicit a prothrombotic/antifibrinolytic action favouring clot stability; and (2) enhance atherosclerosis progression via its propensity for retention in the arterial intima, with deposition of its cholesterol load at sites of plaque formation. Equally, lipoprotein(a) may induce inflammation and calcification in the aortic leaflet valve interstitium, leading to calcific aortic valve stenosis. Experimental, epidemiological and genetic evidence support the contention that elevated concentrations of lipoprotein(a) are causally related to atherothrombotic risk and equally to calcific aortic valve stenosis. The plasma concentration of lipoprotein(a) is principally determined by genetic factors, is not influenced by dietary habits, remains essentially constant over the lifetime of a given individual and is the most powerful variable for prediction of lipoprotein(a)-associated cardiovascular risk. However, major interindividual variations (up to 1000-fold) are characteristic of lipoprotein(a) concentrations. In this context, lipoprotein(a) assays, although currently insufficiently standardized, are of considerable interest, not only in stratifying cardiovascular risk, but equally in the clinical follow-up of patients treated with novel lipid-lowering therapies targeted at lipoprotein(a) (e.g. antiapolipoprotein(a) antisense oligonucleotides and small interfering ribonucleic acids) that markedly reduce circulating lipoprotein(a) concentrations. We recommend that lipoprotein(a) be measured once in subjects at high cardiovascular risk with premature coronary heart disease, in familial hypercholesterolaemia, in those with a family history of coronary heart disease and in those with recurrent coronary heart disease despite lipid-lowering treatment. Because of its clinical relevance, the cost of lipoprotein(a) testing should be covered by social security and health authorities.

Frequent LPA KIV-2 Variants Lower Lipoprotein(a) Concentrations and Protect Against Coronary Artery Disease

Background

Lipoprotein(a) (Lp(a)) concentrations are a major independent risk factor for coronary artery disease (CAD) and are mainly determined by variation in LPA. Up to 70% of the LPA coding sequence is located in the hyper-variable kringle IV type 2 (KIV-2) region. It is hardly accessible by conventional technologies, but may contain functional variants.

Objectives

This study sought to investigate the new, very frequent splicing variant KIV-2 4733G>A on Lp(a) and CAD.

Methods

We genotyped 4733G>A in the GCKD (German Chronic Kidney Disease) study (n = 4,673) by allele-specific polymerase chain reaction, performed minigene assays, identified proxy single nucleotide polymorphisms and used them to characterize its effect on CAD by survival analysis in UK Biobank (n = 440,234). Frequencies in ethnic groups were assessed in the 1000 Genomes Project.

Results

The 4733G>A variant (38.2% carrier frequency) was found in most isoform sizes. It reduces allelic expression without abolishing protein production, lowers Lp(a) by 13.6 mg/dL (95% CI: 12.5-14.7; P < 0.0001) and is the strongest variance-explaining factor after the smaller isoform. Splicing of minigenes was modified. Compound heterozygosity (4.6% of the population) for 4733G>A and 4925G>A, another KIV-2 splicing mutation, reduces Lp(a) by 31.8 mg/dL and most importantly narrows the interquartile range by 9-fold (from 42.1 to 4.6 mg/dL) when compared to the wild type. In UK Biobank 4733G>A alone and compound heterozygosity with 4925G>A reduced HR for CAD by 9% (95% CI: 7%-11%) and 12% (95% CI: 7%-16%) (both P < 0.001). Frequencies in ethnicities differ notably.

Conclusions

Functional variants in the previously inaccessible LPA KIV-2 region cooperate in determining Lp(a) variance and CAD risk. Even a moderate but lifelong genetic Lp(a) reduction translates to a noticeable CAD risk reduction. (J Am Coll Cardiol 2021;78:437–49)

Lipoprotein (a): When to Measure and How to Treat?

PURPOSE OF REVIEW

The purpose of this article is to review current evidence for lipoprotein (a) (Lp(a)) as a risk factor for multiple cardiovascular (CV) disease phenotypes, provide a rationale for Lp(a) lowering to reduce CV risk, identify therapies that lower Lp(a) levels that are available clinically and under investigation, and discuss future directions.

RECENT FINDINGS

Mendelian randomization and epidemiological studies have shown that elevated Lp(a) is an independent and causal risk factor for atherosclerosis and major CV events. Lp(a) is also associated with non-atherosclerotic endpoints such as venous thromboembolism and calcific aortic valve disease. It contributes to residual CV risk in patients receiving standard-of-care LDL-lowering therapy. Plasma Lp(a) levels present a skewed distribution towards higher values and vary widely between individuals and according to ethnic background due to genetic variants in the LPA gene, but remain relatively constant throughout a person's life. Thus, elevated Lp(a) (≥50 mg/dL) is a prevalent condition affecting >20% of the population but is still underdiagnosed. Treatment guidelines have begun to advocate measurement of Lp(a) to identify patients with very high levels that have a family history of premature CVD or elevated Lp(a). Lipoprotein apheresis (LA) efficiently lowers Lp(a) and was recently associated with a reduction of incident CV events. Statins have neutral or detrimental effects on Lp(a), while PCSK9 inhibitors significantly reduce its level by up to 30%. Specific lowering of Lp(a) with antisense oligonucleotides (ASO) shows good safety and strong efficacy with up to 90% reductions. The ongoing CV outcomes study Lp(a)HORIZON will provide a first answer as to whether selective Lp(a) lowering with ASO reduces the risk of major CV events. Given the recently established association between Lp(a) level and CV risk, guidelines now recommend Lp(a) measurement in specific clinical conditions. Accordingly, Lp(a) is a current target for drug development to reduce CV risk in patients with elevated levels, and lowering Lp(a) with ASO represents a promising avenue.

The rs10455872-G allele of the LPA gene is associated with high lipoprotein(a) levels and increased aortic valve calcium in a Mexican adult population

Polymorphisms in the LPA gene have been associated with aortic valve calcification (AVC). There are wide differences in the allelic frequencies, Lp(a) levels, and the association with AVC among ethnic groups. The aim of this study was to determine the association of the LPA gene polymorphisms with Lp(a) levels and risk of developing AVC, in Mexican-Mestizos population. Six LPA polymorphisms (rs10455872, rs7765803, rs6907156, rs1321195, rs12212807 and rs6919346) were genotyped by TaqMan assays in 1,265 individuals without premature coronary artery disease. The presence of AVC was determined by computed tomography. The association of the LPA polymorphisms with AVC, Lp(a), and other cardiovascular risk factors (CVRF) was evaluated using logistic regression analysis. Compared to AA genotype, subjects with AG+GG genotypes had high prevalence of Lp(a) ≥ 30 mg/dL (7.1% vs. 23.7%, p<0.001) and AVC (19.0% vs. 29.4%, p=0.007). In a model adjusted for several CVRF, the LPA rs10455872-G allele was associated with high Lp(a) levels and AVC. Carriers of G allele had a high risk of Lp(a) ≥ 30 mg/dL (OR= 3.86, CI 95%: 2.2 - 6.7, p=0.001) and AVC (OR= 2.54, CI 95%: 1.56 - 4.14, p=0.001), independently of other CVRF. In this population, carriers of rs10455872-G allele had 3.86 and 2.54 higher risk of Lp(a) ≥ 30 mg/dL or presence of AVC, respectively.

Prognostic Value of Lipoprotein(a) Levels in Patients Undergoing Coronary Angiography for Premature Acute Coronary Syndromes

Little is known about the association between lipoprotein(a) [Lp(a)] levels and future ischemic cardiovascular events in patients with premature acute coronary syndrome (ACS). A total of 1464 consecutive patients who underwent coronary angiography for premature ACS (males <45 years and females <55 years) were enrolled in this study. Patients were divided into quartiles according to serum Lp(a) levels (Q1: ≤11.1 nmol/L; Q2: 11.1-27.7 nmol/L; Q3: 27.7-79.3 nmol/L; and Q4: >79.3 nmol/L). Major adverse cardiovascular events (MACEs) increased with Lp(a) quartiles after 2-year follow-up (among quartiles, respectively; P = .001). Kaplan-Meier curves revealed significant differences in event-free survival rates among Lp(a) quartile groups ( P = .001). Multivariate Cox proportional hazards regression analysis indicated that serum Lp(a) level was an independent predictor of MACE either as a continuous variable (hazard ratio [HR]: 1.002, 95% confidence interval [CI]: 1.001-1.004; P = .009) or as a categorical variable (HR: 1.443, 95% CI: 1.074-1.937; P = .015). Furthermore, Lp(a) levels (as a variable) significantly improved the prognostic value for MACE. These findings suggest that Lp(a) measurement has value for cardiovascular risk stratification in patients with premature ACS.

Lipoprotein (a) levels and outcomes in stable outpatients with symptomatic artery disease

BACKGROUND AND AIMS

Although genetic and epidemiological studies support that people with high lipoprotein (a) [Lp(a)] levels are at an increased risk for arterial disease, its prognostic value in patients with established artery disease has not been consistently evaluated.

METHODS

FRENA is a prospective registry of consecutive outpatients with coronary, cerebrovascular or peripheral artery disease. We assessed the risk for subsequent myocardial infarction, ischemic stroke or limb amputation according to Lp(a) levels at baseline.

RESULTS

As of December 2016, 1503 stable outpatients were recruited. Of these, 814 (54%) had levels <30 mg/dL, 319 (21%) had 30-50 mg/dL and 370 (25%) had ≥50 mg/dL. Over a mean follow-up of 36 months, 294 patients developed subsequent events (myocardial infarction 122, ischemic stroke 114, limb amputation 58) and 85 died. On multivariable analysis, patients with Lp(a) levels of 30-50 mg/dL were at a higher risk for myocardial infarction (hazard ratio [HR]: 4.67; 95%CI: 2.77-7.85), ischemic stroke (HR: 8.27; 95%CI: 4.14-16.5) or limb amputation (HR: 3.18; 95%CI: 1.36-7.44) than those with normal levels. Moreover, patients with levels ≥50 mg/dL were at increased risk for myocardial infarction (HR: 19.5; 95%CI: 10.5-36.1), ischemic stroke (HR: 54.5; 95%CI: 25.4-116.7) or limb amputation (HR: 22.7; 95%CI: 9.38-54.9).

CONCLUSIONS

Stable outpatients with symptomatic artery disease and Lp(a) levels >30 mg/dL were at a 5-fold higher risk for subsequent myocardial infarction, stroke or limb amputation. Those with levels >50 mg/dL were at an over 10-fold higher risk.

Lipoprotein(a) in clinical practice: New perspectives from basic and translational science

Elevated plasma concentrations of lipoprotein(a) (Lp(a)) are a causal risk factor for coronary heart disease (CHD) and calcific aortic valve stenosis (CAVS). Genetic, epidemiological and in vitro data provide strong evidence for a pathogenic role for Lp(a) in the progression of atherothrombotic disease. Despite these advancements and a race to develop new Lp(a) lowering therapies, there are still many unanswered and emerging questions about the metabolism and pathophysiology of Lp(a). New studies have drawn attention to Lp(a) as a contributor to novel pathogenic processes, yet the mechanisms underlying the contribution of Lp(a) to CVD remain enigmatic. New therapeutics show promise in lowering plasma Lp(a) levels, although the complete mechanisms of Lp(a) lowering are not fully understood. Specific agents targeted to apolipoprotein(a) (apo(a)), namely antisense oligonucleotide therapy, demonstrate potential to decrease Lp(a) to levels below the 30-50 mg/dL (75-150 nmol/L) CVD risk threshold. This therapeutic approach should aid in assessing the benefit of lowering Lp(a) in a clinical setting.

Serum Lipoprotein (a) Levels in Black South African Type 2 Diabetes Mellitus Patients

Lipoprotein (a) (Lp(a)) which is a low-density lipoprotein-like particle containing apo(a) is considered as an emergent cardiovascular risk factor. Type 2 diabetes mellitus (T2DM) is associated with a two- to threefold increase in the risk of cardiovascular disease (CVD). The aim of this study was to investigate the levels of Lp(a) in Black South African T2DM patients and its association with other metabolic factors. 67 T2DM patients and 48 healthy control participants were recruited for the cross-sectional study. The Lp(a) level was determined by ELISA and the result was analyzed using SPSS. The Lp(a) level in diabetics was found to be significantly increased (P = 0.001) when compared to the normal healthy group. In the diabetic group, the Lp(a) levels correlated significantly with the duration of diabetes (P = 0.008) and oxidized LDL (ox-LDL) levels (P = 0.03) and decreased total antioxidant capacity (P = 0.001). The third tertile of Lp(a) was significantly correlated with increased ox-LDL, C-reactive protein, and triglycerides and decreased total antioxidant capacity.

Lipoprotein (a) measurements for clinical application

The high degree of size heterogeneity of apo(a), the distinct protein component of lipoprotein (a) [Lp(a)], renders the development and selection of specific antibodies directed to apo(a) more difficult and poses significant challenges to the development of immunoassays to measure its concentration in plasma or serum samples. Apo(a) is extremely variable in size not only between but also within individuals because of the presence of two different, genetically determined apo(a) isoform sizes. Therefore, the antigenic determinants per particle available to interact with the antibodies will vary in the samples and the calibrators, thus contributing to apo(a) size-dependent inaccuracy of different methods. The lack of rigorous validation of the immunoassays and common means of expressing Lp(a) concentrations hinder the harmonization of results obtained by different studies and contribute to the lack of common cut points for identification of individuals at risk for coronary artery disease or for interventions aimed at reducing Lp(a) levels. The aim of our review is to present and critically evaluate the issues surrounding the measurements of Lp(a), their impact on the clinical interpretation of the data, and the obstacles we need to overcome to achieve the standardization of Lp(a) measurements.

Lipoprotein (a): a Unique Independent Risk Factor for Coronary Artery Disease

The current epidemic affecting Indians is coronary artery disease (CAD), and is currently one of the most common causes of mortality and morbidity in developed and developing countries. The higher rate of CAD in Indians, as compared to people of other ethnic origin, may indicate a possible genetic susceptibility. Hence, Lp(a), an independent genetic risk marker for atherosclerosis and cardiovascular disease assumes great importance. Lp(a), an atherogenic lipoprotein, contains a cholesterol rich LDL particle, one molecule of apolipoprotein B-100 and a unique protein, apolipoprotein (a) which distinguishes it from LDL. Apo(a) is highly polymorphic and an inverse relationship between Lp(a) concentration and apo(a) isoform size has been observed. This is genetically controlled suggesting a functional diversity among the apo(a) isoforms. The LPA gene codes for apo(a) whose genetic heterogeneity is due to variations in its number of kringles. The exact pathogenic mechanism of Lp(a) is still not completely elucidated, but the structural homology of Lp(a) with LDL and plasmin is possibly responsible for its acting as a link between atherosclerosis and thrombosis. Upper limits of normal Lp(a) levels have not been defined for the Indian population. A cut off limit of 20 mg/dL has been suggested while for the Caucasian population it is 30 mg/dL. Though a variety of assays are available for its measurement, standardization of the analytical method is highly complicated as a majority of the methods are affected by the heterogeneity in apo(a) size. No therapeutic drug selectively targets Lp(a) but recently, new modifiers of apo(a) synthesis are being considered.

Immunoglobulin Fc domain fusion to apolipoprotein(a) kringle V significantly prolongs plasma half-life without affecting its anti-angiogenic activity

Angiogenesis is crucial for tumor growth and metastasis. Blocking this process is, therefore, a potentially powerful approach for the treatment of cancer. Human apolipoprotein(a) kringle V (rhLK8) is an angiogenesis inhibitor and is currently under development as an anti-cancer therapeutic. However, a relatively short in vivo half-life limits its widespread clinical use. This study was performed to evaluate whether fusion of an Fc domain to rhLK8 can extend plasma half-life. RhLK8-Fc fusion protein was expressed in CHO DG44 cells as a dimer and was readily purified by protein G affinity chromatography. The anti-angiogenic activity of rhLK8-Fc was similar to that of rhLK8, as determined by migration and tube formation assays with endothelial cells in vitro and a chorioallantoic membrane assay in vivo. Pharmacokinetic profiles in mice after single intravenous administration of rhLK8 or rhLK8-Fc showed that Fc fusion significantly increased the elimination half-life (t(½)) and the systemic exposure (AUC(inf)) of the protein, in parallel with a significant decrease in total clearance (CL). These data suggest that Fc fusion to rhLK8 is a powerful strategy for extending the plasma half-life of rhLK8 without affecting its anti-angiogenic activity, and could thus improve the clinical applicability of rhLK8.

Targeted antivascular therapy with the apolipoprotein(a) kringle V, rhLK8, inhibits the growth and metastasis of human prostate cancer in an orthotopic nude mouse model

Antivascular therapy has emerged as a rational strategy to improve the treatment of androgen-independent prostate cancer owing to the necessity of establishing a vascular network for the growth and progression of the primary and metastatic tumor. We determined whether recombinant human apolipoprotein(a) kringle V, rhLK8, produces therapeutic efficacy in an orthotopic human prostate cancer animal model. Fifty thousand androgen-independent human prostate cancer cells (PC-3MM2) were injected into the prostate of nude mice. After 3 days, these mice were randomized to receive the vehicle solution (intraperitoneally [i.p.], daily), paclitaxel (8 mg/kg i.p., weekly), rhLK8 (50 mg/kg i.p., daily), or a combination of paclitaxel and rhLK8 for 4 weeks. Treatment with paclitaxel or rhLK8 alone did not show significant therapeutic effects on tumor incidence or on tumor size compared with the control group. The combination of rhLK8 and paclitaxel significantly reduced tumor size and incidence of lymph node metastasis. Significant reduction in microvessel density and cellular proliferation and induction of apoptosis of tumor cells, and tumor-associated endothelial cells, were also achieved. Similarly, PC-3MM2 tumors growing in the tibia showed significant suppression of tumor growth and lymph node metastasis by the combination treatment with rhLK8 and paclitaxel. The integrity of the bone was significantly preserved, and apoptosis of tumor cells and tumor-associated endothelial cells was increased. In conclusion, these results suggest that targeting the tumor microenvironment with the antivascular effect of rhLK8 combined with conventional cytotoxic chemotherapy could be a new and effective approach in the treatment of androgen-independent prostate cancer and their metastases.

Antiangiogenic kringles derived from human plasminogen and apolipoprotein(a) inhibit fibrinolysis through a mechanism that requires a functional lysine-binding site

Many proteins in the fibrinolysis pathway contain antiangiogenic kringle domains. Owing to the high degree of homology between kringle domains, there has been a safety concern that antiangiogenic kringles could interact with common kringle proteins during fibrinolysis leading to adverse effects in vivo. To address this issue, we investigated the effects of several antiangiogenic kringle proteins including angiostatin, apolipoprotein(a) kringles IV(9)-IV(10)-V (LK68), apolipoprotein(a) kringle V (rhLK8) and a derivative of rhLK8 mutated to produce a functional lysine-binding site (Lys-rhLK8) on the entire fibrinolytic process in vitro and analyzed the role of lysine binding. Angiostatin, LK68 and Lys-rhLK8 increased clot lysis time in a dose-dependent manner, inhibited tissue-type plasminogen activator-mediated plasminogen activation on a thrombin-modified fibrinogen (TMF) surface, showed binding to TMF and significantly decreased the amount of plasminogen bound to TMF. The inhibition of fibrinolysis by these proteins appears to be dependent on their functional lysine-binding sites. However, rhLK8 had no effect on these processes owing to an inability to bind lysine. Collectively, these results indicate that antiangiogenic kringles without lysine binding sites might be safer with respect to physiological fibrinolysis than lysine-binding antiangiogenic kringles. However, the clinical significance of these findings will require further validation in vivo.

Apoprotein(A) Isoforms and Plasma LP(A) Concentration in Members of Four Families

Apoprotein(a) is a multikringle protein which shows a genetically inherited size polymorphism. The APO(a) gene is located at the telomeric region of chromosome 6q2.6-q 2.7. Apo(a) size polymorphism is a major determinant of Lp(a) levels. The aim of this study is to describe the influence of apo(a) size polymorphism on the plasma Lp(a) levels in the members of four families. K3EDTA plasma was obtained from every subject after over-night fast. Apo(a) isoforms were determined by 3-15% SDS-PAGE followed by Western immunoblot technique. Plasma Lp(a) level was de - termined with immunonephelometric method. Every child inherited one isoform from its mother and the other from its father. The children from the first family had Lp(a) levels similar to those measured in their parents. The daughters from the second and fourth family inherited the dominant S3 apo(a) isoform from their mothers and also mother's high Lp(a) levels (0.365 g/L - daughter from the second, and 0.465 g/L and 0.446 g/L - daughter from the fourth family respectively). The elder daughter from the third family, carrier of double banded S4S1 apo(a) isoform, had the highest Lp(a) level among the children from all four families. We found out a generation decrease of the Lp(a) level in two families. On the basis of our findings we concluded that the inheritance of the apo(a) isoforms in the members of all four families is in accordance with the simple Mendelian's model and that the apo(a) size polymorphism influences the Lp(a) level in the blood of the examined subjects.

Arginyl-glycyl-aspartyl (RGD) epitope of human apolipoprotein (a) inhibits platelet aggregation by antagonizing the IIb subunit of the fibrinogen (GPIIb/IIIa) receptor

An unknown epitope of apolipoprotein (a) antagonizes fibrinogen binding to agonist-stimulated platelet's fibrinogen (GPIIb/IIIa) receptor yielding lipoprotein (a) mediated decreased platelet aggregation. The purpose of this study was to test the hypothesis that human apolipoprotein (a)'s single arginyl-glycyl-aspartyl (RGD) epitope, unique to apolipoprotein (a) in lipoprotein (a) binds to the RGD binding motif on the IIb subunit of the GPIIb/IIIa receptor thus reducing platelet-bound fibrinogen and consequently decreasing agonist-stimulated platelet aggregation. Platelets (N=30 subjects) were prepared from fresh plasma, washed three times in Tyrode's buffer and stimulated using 10 microM ADP or 2 microg/ml collagen. Lipoprotein (a) was isolated from plasma using lectin affinity chromatography followed by ultracentrifugation. The peptide RGDS inhibited (125)I-labelled lipoprotein (a) binding to autologous platelets with IC-50's of 25.1+/-2.2 (mean+/-SEM) and 15.4+/-1.3 microM for collagen- and ADP-stimulation respectively. Further, RGDS reduced platelet binding of (125)I-labelled fibrinogen IC-50's of 35.5+/-3.2 (mean+/-SEM) and 20.7+/-2.2 microM for collagen- and ADP-stimulation respectively. The monoclonal antibody PAC-1, uniquely directed at the RGD binding motif on the IIb subunit on collagen- and ADP-stimulated platelets, inhibited binding of (125)I-labelled lipoprotein (a) with IC-50's of 6.4+/-0.7 and 2.5+/-2.2 microg/10(8) platelets for collagen- and ADP-stimulation respectively. Additionally, PAC-1 reduced platelet bound of (125)I-labelled fibrinogen with IC-50's of 9.0+/-1.4 and 4.1+/-2.2 microg/10(8) platelets for collagen- and ADP-stimulation respectively. In a dose-related fashion, a polyclonal antibody, specific for the RGD epitope on apolipoprotein (a), restored platelet aggregation to control levels, inhibited (125)I-labelled lipoprotein (a) binding, and increased (125)I-labelled fibrinogen by displacing lipoprotein (a) from the GPIIb/IIIa receptor. Thus a never before demonstrated aspect of the mechanism of lipoprotein (a)'s suggested novel role as an endogenous regulator of fibrinogen binding to collagen- and ADP-stimulated platelets has been shown. In conclusion, lipoprotein (a), via apolipoprotein (a)'s RGD epitope, binds to the RGD binding motif on the IIb protein of the GPIIb/IIIa receptor consequently reducing platelet-bound fibrinogen which results in decreased platelet aggregation.

Adeno-associated virus-mediated expression of apolipoprotein (a) kringles suppresses hepatocellular carcinoma growth in mice

Hepatocellular carcinoma (HCC) constitutes more than 90% of all primary liver cancers. HCC is a hypervascular tumor that develops from dedifferentiation of small avascular HCC and is therefore a good target for anti-angiogenic gene therapy. Recent studies have identified apolipoprotein(a) [apo(a)] kringles LK68 and LK8 (LKs) as having a potential antiangiogenic and anti-tumor activity, and the current study evaluates the therapeutic potential of gene therapy with recombinant adeno-associated virus carrying genes encoding LKs (rAAV-LK) in the treatment of hypervascular HCC. We generated rAAV-LK to obtain persistent transgene expression in vivo, which is essential for anti-angiogenic therapy. The rAAV-produced LKs substantially inhibited proliferation and migration of human umbilical vein endothelial cells (HUVECs) in vitro, validating their anti-angiogenic potential. Intramuscular administration of rAAV-LK gave 60% to 84% suppression (P < .05) of tumor growth in mice bearing subcutaneously transplanted HCC derived from Huh-7 and Hep3B cells, respectively. Histological and immunohistochemical analyses of HCC tumor sections showed that a single administration of rAAV-LK gave rise to persistent expression of LKs that inhibited tumor angiogenesis and triggered tumor apoptosis, and, thus, significantly suppressed tumor growth. The administration of rAAV-LK provided a significant survival benefit (P < .05), and 3 of 10 rAAV-LK-treated mice were still alive without visible tumors and without clinical symptoms 188 days after treatment. In conclusion, rAAV-LK is a potential candidate for anti-angiogenic gene therapy in the treatment of HCC.

Expression of human apolipoprotein(a) kringles in colon cancer cells suppresses angiogenesis-dependent tumor growth and peritoneal dissemination

BACKGROUND

Anti-angiogenesis therapy has been regarded as a promising treatment of cancer based on the fact that most tumors and their metastasis are angiogenesis-dependent. Gene therapy can potentially expand the horizons of tumor angiogenesis therapy by virtue of its ability to produce high concentrations of therapeutic agents in a local area for a sustained period. The present study was performed to evaluate the therapeutic potential of gene therapy for the treatment of cancer and metastasis.

METHODS

The murine colon carcinoma cell line CT26 was manipulated ex vivo to express an anti-angiogenic molecule, LK68, consisting of human apolipoprotein(a) kringle domains, KIV(9)-KIV(10)-KV, using retrovirus-mediated gene transfer. Its effects on colon tumor growth and metastasis were evaluated in experimental animal models established by injecting LK68-expressing and control CT26 cells subcutaneously or into the peritoneal cavity of BALB/c mice, respectively.

RESULTS

Expression of LK68 significantly suppressed colon tumor growth in mice, but did not influence the growth of tumor cells in vitro. Immunohistochemical analysis of tumor tissues revealed a significant reduction in microvessel density in LK68-expressing tumors. Thus, the suppression of tumor growth appears to result mainly from inhibition of tumor angiogenesis. This decrease in vessel density is correlated with a notable increase in tumor cell apoptosis in vivo, but has no influence on proliferation. Moreover, expression of LK68 prevents peritoneal dissemination, and consequently improves overall host survival.

CONCLUSIONS

These results collectively indicate that a gene therapy strategy using LK68 cDNA is useful for the treatment for both colon tumor growth and peritoneal dissemination.

Suppression of Colorectal Cancer Liver Metastasis and Extension of Survival by Expression of Apolipoprotein(a) Kringles

The formation of hepatic metastases in colorectal cancer is the main cause of patient death. Current therapies directed at hepatic metastasis of colorectal cancer have had minimal impact on outcome. Therefore, alternative treatment strategies for liver metastasis require development. The present study was performed to evaluate the application of cDNA of LK68 encoding apolipoprotein(a) kringles IV-9, IV-10, and V as possible candidates for gene therapy treatment of this life-threatening disease. The murine colorectal cancer cell line CT26 was transduced ex vivo with LK68 cDNA via retroviral gene transfer, and an experimental model of hepatic metastasis was established by injecting LK68-expressing and control cells into the spleens of BALB/c mice. Expression of LK68 did not affect the growth characteristics and viability of transduced CT26 cells in vitro. LK68 produced from CT26 cells substantially inhibited the migration of endothelial cells in vitro. In vivo, substantial suppression of liver metastasis and prolonged survival were observed in mice bearing LK68-expressing CT26 cells, compared with controls. LK68-expressing liver metastases were restricted to smaller sizes and displayed decreased microvessel density and increased tumor cell apoptosis. Our data collectively indicate that LK68 suppresses angiogenesis-dependent progression of prevascular micrometastases to macroscopic tumors and their growth, which are clinically accessible and biologically relevant therapeutic targets. We propose that antiangiogenic gene therapy with LK68 is a promising strategy for the treatment of colorectal cancer liver metastasis.

Apoprotein(a) phenotypes and plasma lipoprotein(a) concentration in patients with diabetes mellitus

OBJECTIVES

To determine whether apo(a) isoforms and plasma Lp(a) concentrations in association with some lipid parameters increase the relative risk for the development of atherosclerosis in patients with diabetes mellitus (IDDM and NIDDM).

DESIGN AND METHODS

Apo (a) isoforms, Lp(a) and plasma lipids were determined in 40 IDDM and 65 NIDDM patients and in 182 healthy individuals. Apo(a) isoforms were separated by 3 to 15% gradient SDS-PAGE followed by immunoblotting.

RESULTS

Logistical analysis showed that: Lp(a) levels >30 mg/dL (RR = 0.25, p < 0.000001; RR = 0.18, p < 0.00002), HTA (RR = 0.212, p < 0.00001; RR = 0.30, p < 0.00001), LMW-S1 apo(a) isoform (RR = 6.86, p < 0.0131; RR = 7.04, p < 0.0057) play a significant role in aterogenecity in both groups of patients with DM (IDDM and NIDDM). The 6.50-fold increase in risk was found in NIDDM patients with high Lp(a) levels (>30 mg/dL) and plasma total/HDL cholesterol ratio (4.5-5.8).

CONCLUSION

Elevated Lp(a) levels, LMW S1 apo(a) isoform, HTA and combination of increased Lp(a) levels and total/HDL cholesterol ratio increase the risk for the development of atherosclerosis in patients with DM (IDDM and NIDDM).

A Truncated Kringle Domain of Human Apolipoprotein(a) Inhibits the Activation of Extracellular Signal-regulated Kinase 1 and 2 through a Tyrosine Phosphatase-dependent Pathway

Most proangiogenic factors exert their biological effects primarily by activating extracellular signal-regulated kinase (ERK) and phosphoinositide 3-kinase (PI3-K)/Akt signaling pathways. These pathways appear to play a critical role in endothelial cell migration, because selective inhibition of either ERK or PI3-K/Akt signaling almost completely prevented endothelial cell migration. Recently, we demonstrated that a truncated kringle domain of human apolipoprotein(a), termed rhLK68, inhibits endothelial cell migration in vitro. However, its mechanism of action was not well defined. In this study, we determined the effects of rhLK68 on ERK1/2 and PI3-K/Akt signaling pathways to explore the molecular mechanism of rhLK68-mediated inhibition of endothelial cell migration. Treatment with rhLK68 inhibited ERK1/2 phosphorylation but did not influence Akt activation. Interestingly, an inhibitor of protein-tyrosine phosphatase, sodium orthovanadate, dose-dependently reversed both rhLK68-induced dephosphorylation of ERK1/2 and decreased migration of endothelial cells, whereas rhLK68 showed no significant effects on MEKs phosphorylation. In conclusion, these results indicate that inhibition of endothelial cell migration by rhLK68 may be achieved by interfering with ERK1/2 activation via a protein-tyrosine phosphatase-dependent pathway.

Human apolipoprotein(a) kringle V inhibits angiogenesis in vitro and in vivo by interfering with the activation of focal adhesion kinases

Apolipoprotein(a) [apo(a)] contains the largest numbers of kringle domains identified to date. Of these, apo(a) kringle V shows significant sequence homology with plasminogen kringle 5, which is reported to be a potent angiogenesis inhibitor. To determine the effects of apo(a) kringle V on angiogenesis, it was expressed as a soluble protein (termed rhLK8) in Pichia pastoris and its in vitro and in vivo anti-angiogenic properties were examined. rhLK8 inhibited the migration of human umbilical vein endothelial cells in vitro in a dose-dependent manner. This function was associated with the down-regulation of the activation of focal adhesion kinase and the inhibition of the consequent formation of actin stress fibers/focal adhesions. rhLK8 also inhibited new capillary formation in vivo, as assessed by the chick chorioallantoic membrane assay and the Matrigel plug assay. These results indicate that rhLK8 may be an effective angiogenesis inhibitor both in vitro and in vivo.

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.

Evaluation of a fully-automated particle-enhanced turbidimetric immunoassay for the measurement of plasma lipoprotein(a). population-based reference values in an area with low incidence of cardiovascular disease

OBJECTIVES

Lipoprotein(a) has been proposed as an independent risk factor for cardiovascular disease. This lipoprotein possesses a marked size polymorphism that makes difficult to measure accurately its concentration in plasma. The International Federation of Clinical Chemistry recently recommended to carefully evaluate new commercial methods for lipoprotein(a) measurement to discard the possible influence of lipoprotein(a) isoforms on immunoreactivity. They also recommended to perform population-based studies for different ethnic and geographic groups. Therefore, in the evaluation of a fully automated, particle-enhanced turbidimetric immunoassay for the measurement of lipoprotein(a) we have determined its reference interval in the Spanish population, an area with the lowest incidence of cardiovascular disease in Europe.

DESIGN AND METHOD

We evaluated a commercial kit of reagents calibrated against the Proposed Reference Material and determined the effect of lipoprotein(a) size polymorphism on the measurements. A population-based study was carried out in two different villages on the Mediterranean coast of Spain.

RESULTS

Imprecision at different lipoprotein(a) concentrations ranged between 3.0 and 15.4%. Recovery was 98.5 +/- 2.1. Detection limit was 4.8 nmol/L. There were no significant interferences from lipemia, jaundice, hemolysis, paraproteinemia, apolipoprotein B or plasminogen. We did not observe any effect of the lipoprotein(a) size polymorphism on the measurements. Mean (and SD) values for plasma lipoprotein(a) (n = 369) were 53.6 (65.3) nmol/L, the median was = 25.3 nmol/L and range varied between <4.8 and 356.0 nmol/L.

CONCLUSION

The present article presents an accurate and practical assay for measuring plasma lipoprotein(a) concentrations and describes its reference values in a population of Spanish Caucasians. Our results are similar to those obtained in other Caucasian populations (between 10 and 25% higher than in participants of the CARDIA study).

Functional approach to investigate Lp(a) in ischaemic heart and cerebral diseases

BACKGROUND

Lp(a), a major cardiovascular risk factor, contains a specific apolipoprotein, apo(a), which by virtue of structural homology with plasminogen inhibits the formation of plasmin, the fibrinolytic enzyme. A number of clinical reports support the role of Lp(a) as a cardiovascular or cerebral risk factor, and experimental data suggest that it may contribute to atherothrombosis by inhibiting fibrinolysis.

DESIGN

A well-characterized model of a fibrin surface and an apo(a)-specific monoclonal antibody were used to develop a functional approach to detect pathogenic Lp(a). The assay is based on the competitive binding of Lp(a) and plasminogen for fibrin, and quantifies fibrin-bound Lp(a). High Lp(a) binding to fibrin is correlated with decreased plasmin formation. In a transversal case-control study we studied 248 individuals: 105 had a history of ischaemic cardiopathy (IC), 52 had cerebro-vascular disease (CVD) of thrombotic origin, and 91 were controls.

RESULTS

The remarkably high apo(a) fibrin-binding in CVD (0.268 +/- 0.15 nmol L-1) compared with IC (0.155 +/- 0.12 nmol L-1) suggests the existence of peculiar and poorly understood differences in pro- or anti-thrombotic mechanisms in either cerebral and/or coronary arteries.

CONCLUSIONS

Our results demonstrated that Lp(a) fibrin-binding and small Apo(a) isoforms are associated with athero-thrombotic disease.

Differences between Genotyping and Phenotyping Methods for Assessing Apolipoprotein(a) Size Polymorphisms

The aim of the present study was to analyze, on a double-blind basis, the relationships between the apolipoprotein(a) (apo(a)) gene and protein size polymorphisms in healthy volunteers (n = 99) and patients with premature myocardial infarction (n = 91). Apo(a) genotypes were determined by pulse-field electrophoresis and phenotypes were separated by sodium dodecyl sulfate-agarose gel electrophoresis. Results showed that phenotyping overestimated apo(a) size with respect to genotyping (mean (SD) = 3.7 (3.4) kringle units; p < 0.001) in subjects with a double-band genotype, although both measurements were highly correlated (r = 0.83; p < 0.001). We also observed that the protein band in subjects with a single-band phenotype was related more closely to the smallest allele than to the largest allele band. The correlation of plasma lipoprotein(a) (Lp(a)) concentration was stronger with the phenotype than with the genotype. We hypothesize that post-translational modifications in the apo(a) molecule may be the most plausible explanation for the discrepancies observed. In conclusion, the present study highlights the dissimilarities between phenotyping and genotyping methods for the measurement of apo(a) size and suggests that laboratories should carefully consider these relationships and the transfer of results between such methodologies.

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.

An ELISA procedure for human Lp(a) quantitation using monoclonal antibodies

The present study describes a monoclonal antibody-based enzyme immunoassay (ELISA) for the quantitation of lipoprotein(a), Lp(a), in human plasma. Two antibodies to Lp(a), 2F4E7 and 8G12G7, were produced and characterized as specific and high affinity antibodies against Lp(a). A reference control serum was utilized to prepare the standard curve in a Lp(a) concentration range from 0.015 to 0.5 ug/ml. A biotinylated monoclonal antibody against apoB-LDL was used as the second antibody. The comparison of the standardized ELISA using mAb 2F4E7 with an ELISA using a characterized mAb against Lp(a) (clone KO9) as capture antibody showed that the Lp(a) concentration of two standard sera was similar with both assays. Furthermore, when compared with an electroimmunoassay kit, similar Lp(a) concentrations for the standard were also obtained.

Apolipoprotein(a): structure-function relationship at the lysine-binding site and plasminogen activator cleavage site

Apolipoprotein(a) [apo(a)] is the distinctive glycoprotein of lipoprotein Lp(a), which is disulfide linked to the apo B100 of a low density lipoprotein particle. Apo(a) possesses a high degree of sequence homology with plasminogen, the precursor of plasmin, a fibrinolytic and pericellular proteolytic enzyme. Apo(a) exists in several isoforms defined by a variable number of copies of plasminogen-like kringle 4 and single copies of kringle 5, and the protease region including the backbone positions for the catalytic triad (Ser, His, Asp). A lysine-binding site that is similar to that of plasminogen kringle 4 is present in apo(a) kringle IV type 10. These kringle motifs share some amino acid residues (Asp55, Asp57, Phe64, Tyr62, Trp72, Arg71) that are key components of their lysine-binding site. The spatial conformation and the function of this site in plasminogen kringle 4 and in apo(a) kringle IV-10 seem to be identical as indicated by (i) the ability of apo(a) to compete with plasminogen for binding to fibrin, and (ii) the neutralisation of the lysine-binding function of these kringles by a monoclonal antibody that recognises key components of the lysine-binding site. In contrast, the lysine-binding site of plasminogen kringle 1 contains a Tyr residue at positions 64 and 72 and is not recognised by this antibody. Plasminogen bound to fibrin is specifically recognised and cleaved by the tissue-type plasminogen activator at Arg561-Val562, and is thereby transformed into plasmin. A Ser-Ile substitution at the activation cleavage site is present in apo(a). Reinstallation of the Arg-Val peptide bond does not ensure cleavage of apo(a) by plasminogen activators. These data suggest that the stringent specificity of tissue-type plasminogen activator for plasminogen requires molecular interactions with structures located remotely from the activation disulfide loop. These structures ensure second site interactions that are most probably absent in apo(a).

Inhibition of fibrinolysis by lipoprotein(a)

A high plasma concentration of lipoprotein Lp(a) is now considered to be a major and independent risk factor for cerebro- and cardiovascular atherothrombosis. The mechanism by which Lp(a) may favour this pathological state may be related to its particular structure, a plasminogen-like glycoprotein, apo(a), that is disulfide linked to the apo B100 of an atherogenic LDL-like particle. Apo(a) exists in several isoforms defined by a variable number of copies of plasminogen-like kringle 4 and single copies of kringle 5 and the catalytic region. At least one of the plasminogen-like kringle 4 copies present in apo(a) (kringle IV type 10) contains a lysine binding site (LBS) that is similar to that of plasminogen. This structure allows binding of these proteins to fibrin and cell membranes. Plasminogen thus bound is cleaved at Arg561-Val562 by plasminogen activators and transformed into plasmin. This mechanism ensures fibrinolysis and pericellular proteolysis. In apo(a) a Ser-Ile substitution at the Arg-Val plasminogen activation cleavage site prevents its transformation into a plasmin-like enzyme. Because of this structural/functional homology and enzymatic difference, Lp(a) may compete with plasminogen for binding to lysine residues and impair, thereby, fibrinolysis and pericellular proteolysis. High concentrations of Lp(a) in plasma may, therefore, represent a potential source of antifibrinolytic activity. Indeed, we have recently shown that during the course of the nephrotic syndrome the amount of plasminogen bound and plasmin formed at the surface of fibrin are directly related to in vivo variations in the circulating concentration of Lp(a) (Arterioscler. Thromb. Vasc. Biol., 2000, 20: 575-584; Thromb. Haemost., 1999, 82: 121-127). This antifibrinolytic effect is primarily defined by the size of the apo(a) polymorphs, which show heterogeneity in their fibrin-binding activity--only small size isoforms display high affinity binding to fibrin (Biochemistry, 1995, 34: 13353-13358). Thus, in heterozygous subjects the amount of Lp(a) or plasminogen bound to fibrin is a function of the affinity of each of the apo(a) isoforms and of their concentration relative to each other and to plasminogen. The real risk factor is, therefore, the Lp(a) subpopulation with high affinity for fibrin. According to this concept, some Lp(a) phenotypes may not be related to atherothrombosis and, therefore, high Lp(a) in some individuals might not represent a risk factor for cardiovascular disease. In agreement with these data, it has been recently reported that Lp(a) particles containing low molecular mass apo(a) emerged as one of the leading risk conditions in advanced stenotic atherosclerosis (Circulation, 1999, 100: 1154-1160). The predictive value of high Lp(a) as a risk factor, therefore, depends on the relative concentration of Lp(a) particles containing small apo(a) isoforms with the highest affinity for fibrin. Within this context, the development of agents able to selectively neutralise the antifibrinolytic activity of Lp(a), offers new perspectives in the prevention and treatment of the cardiovascular risk associated with high concentrations of thrombogenic Lp(a).

Mapping of a minimal apolipoprotein(a) interaction motif conserved in fibrin(ogen) beta - and gamma -chains

Lipoprotein(a) (Lp(a)) is a major independent risk factor for atherothrombotic disease in humans. The physiological function(s) of Lp(a) as well as the precise mechanism(s) by which high plasma levels of Lp(a) increase risk are unknown. Binding of apolipoprotein(a) (apo(a)) to fibrin(ogen) and other components of the blood clotting cascade has been demonstrated in vitro, but the domains in fibrin(ogen) critical for interaction are undefined. We used apo(a) kringle IV subtypes to screen a human liver cDNA library by the yeast GAL4 two-hybrid interaction trap system. Among positive clones that emerged from the screen, clones were identified as fibrinogen beta- and gamma-chains. Peptide-based pull-down experiments confirmed that the emerging peptide motif, conserved in the carboxyl-terminal globular domains of the fibrinogen beta and gamma modules specifically interacts with apo(a)/Lp(a) in human plasma as well as in cell culture supernatants of HepG2 and Chinese hamster ovary cells, ectopically expressing apo(a)/Lp(a). The influence of lysine in the fibrinogen peptides and of lysine binding sites in apo(a) for the interaction was evaluated by binding experiments with apo(a) mutants and a mutated fibrin(ogen) peptid. This confirmed the lysine binding sites in kringle IV type 10 of apo(a) as the major fibrin(ogen) binding site but also demonstrated lysine-independent interactions.

Lipoprotein Lp(a) and atherothrombotic disease

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

The role of lipoprotein[a] in atherosclerosis

Recent studies confirm and extend previous evidence that lipoprotein (Lp) plays a significant role in atherosclerosis and is one of the top five or six risk factors for cardiovascular disease. In Japanese patients, Lp levels and apo phenotypes are significant predictors for myocardial infarction. Lp levels are significantly higher in ischemic stroke patients than in controls. However, plasma concentrations of Lp are not predictive of ischemic cerebral infarction in either men or women. Serum Lp levels are significantly higher in patients with carotid plaques or measurable intima-media thickness than in controls without. Despite these associations, there is no significant relationship between Lp level and arterial endothelial function, smooth muscle response, or carotid wall thickness, even though other lipid risk factors like low-density lipoprotein cholesterol (LDL-C) and LDL-C/high-density lipoprotein cholesterol (HDL-C) ratio are correlated with abnormal arterial function and structure. There is new evidence that the association of Lp with extracellular matrix (ECM) secreted by arterial smooth muscle cells increases two- to threefold the subsequent specific binding of LDL. Alpha-defensins released from activated or senescent neutrophils stimulate the binding of Lp to ECM of endothelial cells. Several factors that affect the accumulation of Lp and oxidized LDL in the arterial intima have been identified. Several recent studies have provided new insights into the physiologic role that Lp might play in compromising fibrinolysis. The interaction of Lp with cells is clearly distinct from that with ECM and with fibrinogen; the regulation sites within Lp and plasminogen for these regulatory molecules are not identical. These recent advances bring us significantly closer to understanding how Lp exerts its atherogenic and thrombogenic properties.

Structural basis for the pathophysiology of lipoprotein(a) in the athero-thrombotic process

Lipoprotein Lp(a) is a major and independent genetic risk factor for atherosclerosis and cardiovascular disease. The essential difference between Lp(a) and low density lipoproteins (LDL) is apolipoprotein apo(a), a glycoprotein structurally similar to plasminogen, the precursor of plasmin, the fibrinolytic enzyme. This structural homology endows Lp(a) with the capacity to bind to fibrin and to membrane proteins of endothelial cells and monocytes, and thereby to inhibit plasminogen binding and plasmin generation. The inhibition of plasmin generation and the accumulation of Lp(a) on the surface of fibrin and cell membranes favor fibrin and cholesterol deposition at sites of vascular injury. Moreover, insufficient activation of TGF-beta due to low plasmin activity may result in migration and proliferation of smooth muscle cells into the vascular intima. These mechanisms may constitute the basis of the athero-thrombogenic mode of action of Lp(a). It is currently accepted that this effect of Lp(a) is linked to its concentration in plasma. An inverse relationship between Lp(a) concentration and apo(a) isoform size, which is under genetic control, has been documented. Recently, it has been shown that inhibition of plasminogen binding to fibrin by apo(a) is also inversely associated with isoform size. Specific point mutations may also affect the lysine-binding function of apo(a). These results support the existence of functional heterogeneity in apolipoprotein(a) isoforms and suggest that the predictive value of Lp(a) as a risk factor for vascular occlusive disease would depend on the relative concentration of the isoform with the highest affinity for fibrin.

Pathophysiological implication of the structural domains of lipoprotein(a)

Numerous epidemiological studies have shown that lipoprotein(a) (Lp(a)) is an independent risk factor for the premature development of cardiovascular disease. In spite of such evidence, the structural and functional features of this atherogenic, cholesterol-rich particle are not clearly understood. We have demonstrated the presence of two distinct structural domains in apolipoprotein(a) (apo(a)), which are linked by a flexible and accessible region located between kringles 4-4 and 4-5. We have isolated the Lp(a) particle following removal of the N-terminal domain by proteolytic cleavage; the residual particle, containing the C-terminal domain (comprising the region from Kr 4-5 to the protease domain), is linked to apo B-100 by disulphide linkage, and is termed 'mini-Lp(a)'. Mini-Lp(a) exhibited the same binding affinity to fibrin as the corresponding Lp(a). This finding indicated that the kringles responsible for fibrin binding are restricted to Kr 4-5 to Kr 4-10, an observation consistent with the failure of the N-terminal domain to bind to fibrin. N-terminal fragments of apo(a) have been detected in the urine of normal subjects, thereby indicating that part of the catabolism of Lp(a), which is largely indeterminate, could occur via the renal route.

The number of identical kringle IV repeats in apolipoprotein(a) affects its processing and secretion by HepG2 cells

A variable number of 5.6-kilobase kringle IV repeats in the human apolipoprotein(a) (apo(a)) gene results in a size polymorphism of the protein and correlates inversely with the plasma levels of the atherogenic lipoprotein(a) (Lp(a)). In order to analyze whether this association reflects a direct effect of kringle IV repeat number on Lp(a) plasma concentration, we have studied the expression of recombinant apo(a) (r-apo(a)) isoforms in the human hepatocarcinoma cell line HepG2. Following transient transfection of apo(a) cDNA expression plasmids that differed only in the number of kringle IV repeats, we observed a gradual decrease of Lp(a) in the medium of the cells with an increasing number of kringle IV repeats, mimicking the relationship present in humans in vivo. The analysis of apo(a) protein in the lysate and in the medium of cells that were transfected with a plasmid encoding an apo(a) isoform with 22 kringles revealed a predominant intracellular precursor with little secretion of the mature apo(a) protein. In contrast, transfection of a plasmid encoding an isoform with 11 kringles led to effective secretion of the mature peptide into the medium, indicating differential processing rates of apo(a) isoforms in the secretory path way. The intracellular accumulation of an apo(a) precursor in the endoplasmic reticulum was demonstrated by cell fractionation and [35S]Met metabolic labeling/temperature block experiments using HepG2 cells stably transfected with recombinant apo(a). The direct and causal effect of kringle IV repeat number on the expression of recombinant apo(a) in HepG2 cells, and presumably liver cells, provides a novel mechanism for the genetic regulation of the concentration of a protein.

Effect of sample storage on quantitation of lipoprotein(a) by an enzyme-linked immunosorbent assay

This study evaluated the effect of storage on the quantitation of lipoprotein (Lp)(a) in 25 serum samples. Aliquots of serum were stored for up to three years at either -20 degrees C or -70 degrees C and Lp(a) subsequently analyzed using an enzyme-linked immunosorbent assay kit. Concentrations of Lp(a) declined during storage, and the temperatures employed elicited significantly different (P < 0.05) values within 12 mon which further diverged during three years of storage. Compared to baseline values, significant decreases (P < 0.05) in Lp(a) levels were evident after six months of storage at -20 degrees C with apparent losses (geometric mean) reaching 36.9% (95% confidence interval: 30.9%, 42.9%) after three years. Similarly, significantly lower (P < 0.05) Lp(a) values were recorded after six months of storage at -70 degrees C and at three years the decrease (geometric mean) was 19.1% (95% confidence interval: 14.3%, 24.0%). The losses, after three years, in terms of the arithmetic mean were 53.5 and 26.2% at -20 and -70 degrees C, respectively. Phenotype analysis suggested that large isoforms are more susceptible to degradation than smaller moieties. This may be related to the observation that apparent losses are reduced in samples containing over 8 mg/dL Lp(a). Nevertheless, Lp(a) levels in stored samples retained a strong correlation with the baseline values. These results must be considered specific for the storage conditions and analytical procedures employed.

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).

Human Lp(a): regions in sequences of apoproteins similar to domains in signal transduction proteins

The major apoproteins of Lp(a)--apo(a) and apo B-100--are linked by only one intermolecular disulfide bond. This linkage has been suggested to be located between apo(a) Cys4057 and apo B-100 Cys3734. Several studies, however, have suggested other noncovalent interactions between different regions of apo(a) and apo B-100. One possible mechanism for these interactions may involve the apo(a) proline-rich interkringle regions that share sequence similarities with the proline-rich regions of Src homology 3 (SH3) domain-binding proteins such as 3BP-1. SH3 and SH2 domains, and their respective ligands, proline-rich regions, and phosphotyrosine motifs, are noncatalytic segments common to signal transduction proteins. Therefore, we used sequence comparison algorithms and molecular modeling programs to identify corresponding SH3 and SH2 candidate regions as well as potential phosphotyrosine sites in the apo B-100 sequence. Six SH2 and 16 SH3 candidate regions, along with 21 potential phosphotyrosine sites, are contained in the apo B-100 sequence. In Lp(a), these regions of apo B-100 may be involved in the noncovalent, protein-protein interactions between apo(a) and apo B-100. The presence of candidate SH3 and SH2 regions in apo B-100, and potential phosphotyrosine sites in apo B-100, apo(a), and apo A-I, suggests an alternative signaling pathway unrelated to the known B/E receptor.

A sandwich ELISA based on anti-apo(a) and anti-apo B monoclonal antibodies for lipoprotein(a) measurement

Lipoprotein(a) (Lp(a)) is one of the most important independent risk factors for the prediction of premature atherosclerosis. Lp(a) is a low-density lipoprotein (LDL)-like particle which contains a glycoprotein (apoprotein(a)) disulfide linked to apo B-100. We describe a sandwich ELISA based on an anti-apo(a) monoclonal antibody (MAb) and an anti-apo B MAb for the quantitative determination of Lp(a) in human serum. The assay is sensitive, precise and specific. Samples with different apo(a) isoforms had a linear response in a range of 3-70 mg/dl of Lp(a). Correlations between the ELISA and a commercial ELISA, an immunoradiometric assay and electroimmunodiffusion were 0.92, 0.96 and 0.98, respectively. The frequency distribution of Lp(a) concentration in blood donors showed the skew toward the right reported in other populations. Patients with angiographically assessed coronary atherosclerosis had three times higher levels of Lp(a) than those with no signs of coronary atherosclerosis.

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.

Macrophage foam cell lipoprotein(a)/apoprotein(a) receptor. Cell-surface localization, dependence of induction on new protein synthesis, and ligand specificity

Understanding the interaction of the atherogenic lipoprotein, lipoprotein(a) [Lp(a)], with macrophages may provide important insight into the physiology and pathophysiology of this lipoprotein. We have recently shown that cholesterol loading of macrophages, such as occurs in atheroma foam cells, leads to marked upregulation of a novel receptor activity for native Lp(a) and its plasminogen-like protein component, apoprotein(a) [apo(a)]. We show here that the Lp(a)/apo(a) receptor activity on cholesterol-loaded macrophages is trypsin sensitive, indicating that a cell-surface protein is involved and that the upregulation by cholesterol loading requires new protein synthesis. Ligand studies revealed that the foam cell receptor activity recognizes Lp(a) containing both small and large isoforms of apo(a) as well as rhesus monkey Lp(a), which contains an inactive kringle-4(37) (K4(37) lysine-binding domain. Elastase degradation products of plasminogen did not compete for 125I-labeled recombinant apo(a) [125I-r-apo(a)] internalization and degradation by foam cells, indicating that the K4(37) sequence, as well as the K5 and "protease" domains of apo(a), are not sufficient for receptor interaction. Consistent with these data, the degradation of 125I-r-apo(a) was completely blocked by an anti-Lp(a) polyclonal antibody that does not cross-react with plasminogen. Furthermore, the multiple sialic residues of apo(a) are also not involved in receptor interaction, since desialylated r-apo(a) interacted with foam cells as well as native r-apo(a). In contrast, reduced and denatured r-apo(a) was degraded by foam cells only slightly better than by control cells [28% increased degradation by foam cells versus 450% for native r-apo(a)], suggesting that the upregulated receptor activity recognizes certain secondary and tertiary structural features of apo(a).(ABSTRACT TRUNCATED AT 250 WORDS)