Inhibition of plasminogen activation by lipoprotein(a): critical domains in apolipoprotein(a) and mechanism of inhibition on fibrin and degraded fibrin surfaces

Apolipoprotein(a) inhibits in vitro tube formation in endothelial cells: identification of roles for Kringle V and the plasminogen activation system

Elevated plasma concentrations of lipoprotein(a) are associated with increased risk for atherothrombotic diseases. Apolipoprotein(a), the unique glycoprotein component of lipoprotein(a), is characterized by the presence of multiple kringle domains, and shares a high degree of sequence homology with the serine protease zymogen plasminogen. It has been shown that angiostatin, a proteolytic fragment of plasminogen containing kringles 1–4, can effectively inhibit angiogenesis. Moreover, proteolytic fragments of plasminogen containing kringle 5 are even more potent inhibitors of angiogenesis than angiostatin. Despite its strong similarity with plasminogen, the role of apolipoprotein(a) in angiogenesis remains controversial, with both pro- and anti-angiogenic effects reported. In the current study, we evaluated the ability of apolipoprotein(a) to inhibit VEGF- and angiopoietin-induced tube formation in human umbilical cord endothelial cells. A 17 kringle-containing form of recombinant apo(a) (17K), corresponding to a well-characterized, physiologically-relevant form of the molecule, effectively inhibited tube formation induced by either VEGF or angiopoietin-1. Using additional recombinant apolipoprotein(a) (r-apo(a)) variants, we demonstrated that this effect was dependent on the presence of an intact lysine-binding site in kringle V domain of apo(a), but not on the presence of the functional lysine-binding site in apo(a) kringle IV type 10; sequences within in the amino-terminal half of the molecule were also not required for the inhibitory effects of apo(a). We also showed that the apo(a)-mediated inhibition tube formation could be reversed, in part by the addition of plasmin or urokinase plasminogen activator, or by removal of plasminogen from the system. Further, we demonstrated that apo(a) treated with glycosidases to remove sialic acid was significantly less effective in inhibiting tube formation. This is the first report of a functional role for the glycosylation of apo(a) although the mechanisms underlying this observation remain to be determined in the context of angiogenesis.

Lipoprotein(a): Cellular Effects and Molecular Mechanisms

Lipoprotein(a) (Lp(a)) is an independent risk factor for the development of cardiovascular disease (CVD). Indeed, individuals with plasma concentrations >20 mg/dL carry a 2-fold increased risk of developing CVD, accounting for ~25% of the population. Circulating levels of Lp(a) are remarkably resistant to common lipid lowering therapies, and there are currently no robust treatments available for reduction of Lp(a) apart from plasma apheresis, which is costly and labour intensive. The Lp(a) molecule is composed of two parts, an LDL/apoB-100 core and a unique glycoprotein, apolipoprotein(a) (apo(a)), both of which can interact with components of the coagulation cascade, inflammatory pathways, and cells of the blood vessel wall (smooth muscle cells (SMC) and endothelial cells (EC)). Therefore, it is of key importance to determine the molecular pathways by which Lp(a) exerts its influence on the vascular system in order to design therapeutics to target its cellular effects. This paper will summarise the role of Lp(a) in modulating cell behaviour in all aspects of the vascular system including platelets, monocytes, SMC, and EC.

Oxidized phospholipids are present on plasminogen, affect fibrinolysis, and increase following acute myocardial infarction

OBJECTIVES

This study sought to assess whether plasminogen, which is homologous to lipoprotein (a) [Lp(a)], contains proinflammatory oxidized phospholipids (OxPL) and whether this has clinical relevance.

BACKGROUND

OxPL measured on apolipoprotein B-100 (OxPL/apoB), primarily reflecting OxPL on Lp(a), independently predict cardiovascular disease (CVD) events.

METHODS

The authors examined plasminogen from commercially available preparations and plasma from chimpanzees; gorillas; bonobos; cynomolgus monkeys; wild-type, apoE(-/-), LDLR(-/-), and Lp(a)-transgenic mice; healthy humans; and patients with familial hypercholesterolemia, stable CVD, and acute myocardial infarction (AMI). Phosphocholine (PC)-containing OxPL (OxPC) present on plasminogen were detected directly with liquid chromatography-mass spectrometry (LC-MS/MS) and immunologically with monoclonal antibody E06. In vitro clot lysis assays were performed to assess the effect of the OxPL on plasminogen on fibrinolysis.

RESULTS

LC-MS/MS revealed that OxPC fragments were covalently bound to mouse plasminogen. Immunoblot, immunoprecipitation, density gradient ultracentrifugation, and enzyme-linked immunosorbent assay analyses demonstrated that all human and animal plasma samples tested contained OxPL covalently bound to plasminogen. In plasma samples subjected to density gradient fractionation, OxPL were present on plasminogen (OxPL/plasminogen) in non-lipoprotein fractions but on Lp(a) in lipoprotein fractions. Plasma levels of OxPL/apoB and OxPL/apo(a) varied significantly (>25×) among subjects and also strongly correlated with Lp(a) levels. In contrast, OxPL/plasminogen levels were distributed across a relatively narrow range and did not correlate with Lp(a). Enzymatic removal of OxPL from plasminogen resulted in a longer lysis time for fibrin clots (16.25 vs. 11.96 min, p = 0.007). In serial measurements over 7 months, OxPL/plasminogen levels did not vary in normal subjects or in patients with stable CVD, but increased acutely over the first month and then slowly decreased to baseline in patients following AMI.

CONCLUSIONS

These data demonstrate that plasminogen contains covalently bound OxPL that influence fibrinolysis. OxPL/plasminogen represent a second major plasma pool of OxPL, in addition to those present on Lp(a). OxPL present on plasminogen may have pathophysiological implications in AMI and atherothrombosis.

FGF19 signaling cascade suppresses APOA gene expression

OBJECTIVE

Lipoprotein(a) is a highly atherogenic lipoprotein, whose metabolism is poorly understood. Currently no safe drugs exists that lower elevated plasma lipoprotein(a) concentrations. We therefore focused on molecular mechanisms that influence apolipoprotein(a) (APOA) biosynthesis.

METHODS AND RESULTS

Transgenic human APOA mice (tg-APO mice) were injected with 1 mg/kg of recombinant human fibroblast growth factor 19 (FGF19). This led to a significant reduction of plasma APOA and hepatic expression of APOA. Incubation of primary hepatocytes of tg-APOA mice with FGF19 induced ERK1/2 phosphorylation and, in turn, downregulated APOA expression. Repression of APOA by FGF19 was abrogated by specific ERK1/2 phosphorylation inhibitors. The FGF19 effect on APOA was attenuated by transfection of primary hepatocytes with siRNA against the FGF19 receptor 4 (FGFR4). Using promoter reporter assays, mutation analysis, gel shift, and chromatin immune-precipitation assays, an Ets-1 binding element was identified at -1630/-1615bp region in the human APOA promoter. This element functions as an Elk-1 binding site that mediates repression of APOA transcription by FGF19.

CONCLUSIONS

These findings provide mechanistic insights into the transcriptional regulation of human APOA by FGF19. Further studies in the human system are required to substantiate our findings and to design therapeutics for hyper lipoprotein(a).

Proteomics of Lipoprotein(a) identifies a protein complement associated with response to wounding

Lipoprotein(a) [Lp(a)] is a major independent risk factor for cardiovascular disease. Twenty percent of the general population exhibit levels above the risk threshold highlighting the importance for clinical and basic research. Comprehensive proteomics of human Lp(a) will provide significant insights into Lp(a) physiology and pathogenicity. Using liquid chromatography-coupled mass spectrometry, we established a high confidence Lp(a) proteome of 35 proteins from highly purified particles. Protein interaction network analysis and functional clustering revealed proteins assigned to the two major biological processes of lipid metabolism and response to wounding. The latter includes the processes of coagulation, complement activation and inflammatory response. Furthermore, absolute protein quantification of apoB-100, apo(a), apoA1, complement C3 and PON1 gave insights into the compositional stoichiometry of associated proteins per particle. Our proteomics study has identified Lp(a)-associated proteins that support a suggested role of Lp(a) in response to wounding which points to mechanisms of Lp(a) pathogenicity at sites of vascular injury and atherosclerotic lesions. This study has identified a high confidence Lp(a) proteome and provides an important basis for further comparative and quantitative analyses of Lp(a) isolated from greater numbers of plasma samples to investigate the significance of associated proteins and their dynamics for Lp(a) pathogenicity.

Do We Know When and How to Lower Lipoprotein(a)?

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OPINION STATEMENT

Currently, there are significant data to support a link between lipoprotein(a) [Lp(a)] levels and cardiovascular risk. However, there has not been a clinical trial examining the effects of Lp(a) reduction on cardiovascular risk in a primary prevention population. Until such a trial is conducted, current consensus supports using an Lp(a) percentile greater than 75% for race and gender as a risk stratification tool to target more aggressive low-density lipoprotein cholesterol (LDL-C) or apolipoprotein B (apoB) goals. Therefore, Lp(a) measurements should be considered in the following patients: individuals with early-onset vascular disease determined by clinical presentation or subclinical imaging, intermediate and high Framingham risk patients with a family history of premature coronary disease, and low Framingham risk patients with a family history and low high-density lipoprotein cholesterol (HDL-C) levels. Once LDL-C goals are met, Lp(a) levels may be taken into account in selecting secondary agents to reach more aggressive secondary goals, including non-HDL-C and apoB. To achieve Lp(a) reduction, one evidence-based approach is to initiate therapy with low-dose aspirin and extended-release niacin, titrated from 0.5 g up to 2 g over several weeks. If higher doses of niacin are desired, crystalline niacin allows for titration to a dosage as high as 2 g three times a day; however, the flushing side effect usually is quite prominent. Although hormone replacement therapy (HRT) has been shown to lower Lp(a), there are no indications for using HRT for primary or secondary prevention; therefore, we do not advocate initiating it solely for Lp(a) reduction. LDL apheresis is an option to lower LDL-C levels in patients with homozygous familial hypercholesterolemia who are not responsive to medical therapy. Although it does lower Lp(a), there is no treatment indication for this. A recent study supports the cholesterol absorption inhibitor ezetimibe's ability to lower Lp(a), a finding that deserves further investigation as it has not been previously reported in multiple ezetimibe trials. Additionally, the apoB messenger RNA antisense therapy mipomersen currently is in phase 3 trials and may serve as a potential inhibitor of Lp(a) production. Ultimately, more trial evidence is needed to determine whether lowering Lp(a) actually reduces cardiovascular risk, although this may be difficult to isolate without a specific Lp(a)-lowering therapy.

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.

Guidelines for the primary prevention of stroke: a guideline for healthcare professionals from the American Heart Association/American Stroke Association

BACKGROUND AND PURPOSE

This guideline provides an overview of the evidence on established and emerging risk factors for stroke to provide evidence-based recommendations for the reduction of risk of a first stroke.

METHODS

Writing group members were nominated by the committee chair on the basis of their previous work in relevant topic areas and were approved by the American Heart Association (AHA) Stroke Council Scientific Statement Oversight Committee and the AHA Manuscript Oversight Committee. The writing group used systematic literature reviews (covering the time since the last review was published in 2006 up to April 2009), reference to previously published guidelines, personal files, and expert opinion to summarize existing evidence, indicate gaps in current knowledge, and when appropriate, formulate recommendations using standard AHA criteria (Tables 1 and 2). All members of the writing group had the opportunity to comment on the recommendations and approved the final version of this document. The guideline underwent extensive peer review by the Stroke Council leadership and the AHA scientific statements oversight committees before consideration and approval by the AHA Science Advisory and Coordinating Committee.

RESULTS

Schemes for assessing a person's risk of a first stroke were evaluated. Risk factors or risk markers for a first stroke were classified according to potential for modification (nonmodifiable, modifiable, or potentially modifiable) and strength of evidence (well documented or less well documented). Nonmodifiable risk factors include age, sex, low birth weight, race/ethnicity, and genetic predisposition. Well-documented and modifiable risk factors include hypertension, exposure to cigarette smoke, diabetes, atrial fibrillation and certain other cardiac conditions, dyslipidemia, carotid artery stenosis, sickle cell disease, postmenopausal hormone therapy, poor diet, physical inactivity, and obesity and body fat distribution. Less well-documented or potentially modifiable risk factors include the metabolic syndrome, excessive alcohol consumption, drug abuse, use of oral contraceptives, sleep-disordered breathing, migraine, hyperhomocysteinemia, elevated lipoprotein(a), hypercoagulability, inflammation, and infection. Data on the use of aspirin for primary stroke prevention are reviewed.

CONCLUSIONS

Extensive evidence identifies a variety of specific factors that increase the risk of a first stroke and that provide strategies for reducing that risk.

Apolipoprotein(a) stimulates vascular endothelial cell growth and migration and signals through integrin alphaVbeta3

Elevated plasma concentrations of Lp(a) [lipoprotein(a)] are an emerging risk factor for atherothrombotic disease. Apo(a) [apolipoprotein(a)], the unique glycoprotein component of Lp(a), contains tandem repeats of a plasminogen kringle (K) IV-like domain. In the light of recent studies suggesting that apo(a)/Lp(a) affects endothelial function, we evaluated the effects of apo(a)/Lp(a) on growth and migration of cultured HUVECs (human umbilical-vein endothelial cells). Two full-length r-apo(a) [recombinant apo(a)] variants (12K and 17K), as well as Lp(a), were able to stimulate HUVEC growth and migration to a comparable extent; 17K r-apo(a) also decreased the levels of total and active transforming growth factor-beta secreted by these cells. Using additional r-apo(a) variants corresponding to deletions and/or site-directed mutants of various kringle domains in the molecule, we were able to determine that the observed effects of full-length r-apo(a) on HUVECs were dependent on the presence of a functional lysine-binding site(s) in the apo(a) molecule. With respect to signalling events elicited by apo(a) in HUVECs, we found that 17K treatment of the cells increased the phosphorylation level of FAK (focal adhesion kinase) and MAPKs (mitogen-activated protein kinases), including ERK (extracellular-signal-regulated kinase), p38 and JNK (c-Jun N-terminal kinase). In addition, we showed that LM609, the function-blocking antibody to integrin alphaVbeta3, abrogated the effects of 17K r-apo(a) and Lp(a) on HUVECs. Taken together, the results of the present study suggest that the apo(a) component of Lp(a) signals through integrin alphaVbeta3 to activate endothelial cells.

Apolipoprotein(a) inhibits the conversion of Glu-plasminogen to Lys-plasminogen: a novel mechanism for lipoprotein(a)-mediated inhibition of plasminogen activation

BACKGROUND

Elevated plasma concentrations of lipoprotein(a) [Lp(a)] are associated with an increased risk for thrombotic disorders. Lp(a) is a unique lipoprotein consisting of a low-density lipoprotein-like moiety covalently linked to apolipoprotein(a) [apo(a)], a homologue of the fibrinolytic proenzyme plasminogen. Several in vitro and in vivo studies have shown that Lp(a)/apo(a) can inhibit tissue-type plasminogen activator-mediated plasminogen activation on fibrin surfaces, although the mechanism of inhibition by apo(a) remains controversial. Essential to fibrin clot lysis are a number of plasmin-dependent positive feedback reactions that enhance the efficiency of plasminogen activation, including the plasmin-mediated conversion of Glu-plasminogen to Lys-plasminogen.

OBJECTIVE

Using acid-urea gel electrophoresis to resolve the two forms of radiolabeled plasminogen, we determined whether apo(a) is able to inhibit Glu-plasminogen to Lys-plasminogen conversion.

METHODS

The assays were performed in the absence or presence of different recombinant apo(a) species, including point mutants, deletion mutants and variants that represent greater than 90% of the known apo(a) isoform sizes.

RESULTS

Apo(a) substantially suppressed Glu-plasminogen conversion. Critical roles were identified for the kringle IV types 5-9 and kringle V; contributory roles for sequences within the amino-terminal half of the molecule were also observed. Additionally, with the exception of the smallest naturally-occurring isoform of apo(a), isoform size was found not to contribute to the inhibitory capacity of apo(a).

CONCLUSION

These findings underscore a novel contribution to the understanding of Lp(a)/apo(a)-mediated inhibition of plasminogen activation: the ability of the apo(a) component of Lp(a) to inhibit the key positive feedback step of plasmin-mediated Glu-plasminogen to Lys-plasminogen conversion.

Apolipoprotein(a), through its strong lysine-binding site in KIV(10'), mediates increased endothelial cell contraction and permeability via a Rho/Rho kinase/MYPT1-dependent pathway

Substantial evidence indicates that endothelial dysfunction plays a critical role in atherogenesis. We previously demonstrated that apolipoprotein(a) (apo(a); the distinguishing protein component of the atherothrombotic risk factor lipoprotein(a)) elicits rearrangement of the actin cytoskeleton in human umbilical vein endothelial cells, characterized by increased myosin light chain (MLC) phosphorylation via a Rho/Rho kinase-dependent signaling pathway. Apo(a) contains kringle (K)IV and KV domains similar to those in plasminogen: apo(a) contains 10 types of plasminogen KIV-like sequences, followed by sequences homologous to the plasminogen KV and protease domains. Several of the apo(a) kringles contain lysine-binding sites (LBS) that have been proposed to contribute to the pathogenicity of Lp(a). Here we demonstrate that apo(a)-induced endothelial barrier dysfunction is mediated via a Rho/Rho kinase-dependent signaling pathway that results in increased MYPT1 phosphorylation and hence decreased MLC phosphatase activity, thus leading to an increase in MLC phosphorylation, stress fiber formation, cell contraction, and permeability. In addition, studies using recombinant apo(a) variants indicated that these effects of apo(a) are dependent on sequences within the C-terminal half of the apo(a) molecule, specifically, the strong LBS in KIV(10). In parallel experiments, the apo(a)-induced effects were completely abolished by treatment of the cells with the lysine analogue epsilon-aminocaproic acid and the Rho kinase inhibitor Y27632. Taken together, our findings indicate that the strong LBS in apo(a) KIV(10) mediates all of our observed effects of apo(a) on human umbilical vein endothelial cell barrier dysfunction. Studies are ongoing to further dissect the molecular basis of these findings.

Polymorphism in the apolipoprotein(a) gene, plasma lipoprotein(a), cardiovascular disease, and low-dose aspirin therapy

OBJECTIVE

A minor allele variant (rs3798220) of apolipoprotein(a) has been reported to be associated with elevated plasma lipoprotein(a) [Lp(a)] and increased cardiovascular risk. We investigated whether this allele was associated with elevated Lp(a) and cardiovascular risk in the Women's Health Study, a randomized trial of low-dose aspirin, and whether aspirin reduced cardiovascular risk in minor allele carriers.

METHODS AND RESULTS

Genotypes of rs3798220 were determined for 25,131 initially healthy Caucasian participants. Median Lp(a) levels at baseline were 10.0, 79.5, and 153.9mg/dL for major allele homozygotes, heterozygotes, and minor allele homozygotes, respectively (P<0.0001). During the 9.9 years of follow-up, minor allele carriers (3.7%) in the placebo group had twofold higher risk of major cardiovascular events than non-carriers (age-adjusted hazard ratio (HR)=2.21, 95% CI: 1.39-3.52). Among carriers, risk was reduced more than twofold by aspirin: for aspirin compared with placebo the age-adjusted HR was 0.44 (95% CI: 0.20-0.94); risk was not significantly reduced among non-carriers (age-adjusted HR=0.91, 95% CI: 0.77-1.08). This interaction between carrier status and aspirin allocation was significant (P=0.048).

CONCLUSIONS

In the Women's Health Study, carriers of an apolipoprotein(a) variant had elevated Lp(a), doubled cardiovascular risk, and appeared to benefit more from aspirin than non-carriers.

Effects of non-oral postmenopausal hormone therapy on markers of cardiovascular risk: a systematic review

OBJECTIVE

To review the effects of non-oral administration of postmenopausal hormone therapy (HT) on risk markers for atherosclerotic and venous thromboembolic disease.Non-oral postmenopausal HT appears not to increase venous thromboembolic risk, whereas the effect on coronary heart disease risk is less clear.

DESIGN

Systematic review of literature obtained from MEDLINE, EMBASE, and CENTRAL databases from 1980 until and including April 2006. Terms for "postmenopausal hormone therapy" and for "non-oral administration" were combined in the search.

SETTING

Randomized clinical trials.

PATIENT(S)

Postmenopausal women, both healthy and with established cardiovascular disease or specified cardiovascular risk factors

INTERVENTION(S)

Non-oral HT (e.g., transdermal or intranasal) compared with oral HT or no treatment/placebo.

MAIN OUTCOME MEASURE(S)

Lipoprotein(a), homocysteine, C-reactive protein (CRP), cell adhesion molecules, markers of endothelial dysfunction, coagulation, and fibrinolysis.

RESULT(S)

Seventy-two studies investigating either transdermal or intranasal administration were included. For non-oral HT, decreases in lipoprotein(a), cell adhesion molecules, and factor VII generally were significant, resistance to activated protein C (APCr) was slightly increased, and other markers including CRP and homocysteine did not change. Compared with oral HT, changes in CRP and APCr were smaller, changes in cell adhesion molecules and some fibrinolytic parameters tended to be smaller, whereas changes in other factors including lipoprotein(a) and homocysteine did not differ.

CONCLUSION(S)

Potentially unfavorable changes seen with oral HT on two important markers, CRP and APCr, are substantially smaller with non-oral HT. Non-oral HT has minor effects on the other cardiovascular risk markers studied. Therefore, compared with oral HT, non-oral HT appears be safer with respect to atherosclerotic and venous thromboembolic disease risk.

Elevated Lipoprotein(a) does not promote early atherosclerotic changes of the carotid arteries in young, healthy adults

BACKGROUND

Elevated levels of Lipoprotein(a) [Lp(a)] have been linked to an increased risk of ischemic cardiovascular events. Yet the mechanism by which Lp(a) might contribute to this increased risk is not clear.

METHODS

To elucidate whether high plasma levels of Lp(a) contribute to the development of early atherosclerotic vessel wall changes, the intima-media thickness of the common carotid arteries [CCA-IMT] of 151 healthy young volunteers without additional relevant cardiovascular risk factors was measured by high-resolution ultrasound. Plasma concentrations of Lp(a) were quantified and other established risk factors, such as body mass index [BMI], plasma levels of cholesterol, triglycerides and homocysteine, were determined. Furthermore, the carotid arteries were examined for the presence of plaques and stenoses.

RESULTS

Univariate analysis showed a significantly negative correlation of CCA-IMT with HDL cholesterol and positive correlations with age, BMI, total and LDL cholesterol, triglycerides and even with homocysteine, but not with Lp(a). When the study population was dichotomized according to Lp(a) levels, no statistically significant differences in CCA-IMT could be detected between persons with plasma Lp(a)<300mg/l or >or=300mg/l, respectively.

CONCLUSION

Our data suggest that elevated Lp(a) levels alone do not contribute to increased cardiovascular risk by promoting early atherogenesis in vivo.

Lipoprotein(a) and Thrombocytes: Potential Mechanisms Underlying Cardiovascular Risk

Plasma levels of lipoprotein(a), Lp(a), is an independent risk factor for cardiovascular disease. Lp(a) has many properties in common with low-density lipoprotein (LDL), including a cholesteryl ester-rich lipid core and the presence of one copy of apolipoprotein B-100; both apoB-100 and the lipid core are pro-atherogenic. In addition, Lp(a) contains a unique hydrophilic, carbohydrate-rich protein, apo(a), linked to apoB through a single disulfide bond connecting the C-terminal regions of the two proteins. The similarities between apolipoprotein(a), apo(a), and plasminogen has initiated numerous studies on the possible role of Lp(a) as a prothrombotic agent. Studies to date suggest that Lp(a) has antifibrinolytic and procoagulant properties. In this review, we summarize recent studies focused on the interaction between Lp(a) and platelets. Collectively, results to date illustrate that thrombogenicity associated with Lp(a) could be due to risk associated with the LDL moiety, with the apo(a) moiety, or from the combination of those in Lp(a). Present findings suggest that the various components of Lp(a) may impact to a varying degree on different underlying pathways involved in platelet activation and aggregation. On balance, results indicate an effect by Lp(a) on platelet function and future studies focused on specific Lp(a) components, such as the role of apo(a) and of the LDL-like lipid moiety, are needed.

Catalysis of Covalent Lp(a) Assembly: Evidence for an Extracellular Enzyme Activity that Enhances Disulfide Bond Formation

The assembly of lipoprotein(a) (Lp(a)) particles occurs via a two-step mechanism in which noncovalent interactions between apolipoprotein(a) (apo(a)) and the apolipoproteinB-100 component of low density lipoprotein precede the formation of a single disulfide bond. Although we have previously demonstrated that the rate constant for the covalent step of Lp(a) assembly can be enhanced by altering the conformational status of apo(a), the resultant rates of covalent Lp(a) particle formation measured in vitro are relatively slow. The large excess of Lp(a) (over apo(a)) observed in vivo can be accounted for by a preferential clearance of apo(a) over Lp(a) and/or a sufficiently high rate of covalent Lp(a) assembly. In the present study, we report that cultured human hepatoma cells secrete an oxidase activity that dramatically enhances the rate of covalent Lp(a) assembly. This activity is likely possessed by a protein because it is heat-sensitive and is retained in the concentrate following ultrafiltration through a 5 kDa cutoff filter. However, a small molecule cofactor for the activity is suggested by the observation that the activity is lost upon dialysis. Plots of Lp(a) assembly rate versus input apo(a) concentration gave rectangular hyperbolae; the reaction displayed an unusual dependence on the concentration of apoB-100, with increasing concentrations of apoB-100 resulting in slower rates of Lp(a) assembly at low concentrations of apo(a), an effect that was alleviated by higher apo(a) concentrations. Interestingly, V(max(app))/K(m(app)) ratios were insensitive to apoB-100 concentration, which is diagnostic of a ping-pong reaction mechanism. In this way, the putative Lp(a) oxidase may be functionally analogous to protein disulfide isomerase, which exhibits a similar mechanism during the catalysis of disulfide bond formation during protein folding, although we have ruled out a role for this enzyme in Lp(a) assembly.

The influence of age on in vitro plasmin generation in the presence of fibrin monomer

BACKGROUND AND OBJECTIVES

The components of the fibrinolytic system interact to generate plasmin from its zymogen form, plasminogen. At birth, all the components of the fibrinolytic system are present but with differing plasma concentrations. The present study was undertaken to explore the effect of physiological, age-dependent factors of the fibrinolytic system during childhood on the capacity to generate plasmin.

DESIGN AND METHODS

Total plasmin generation was measured in venous plasma from umbilical cords and adults, on plastic and cell surfaces, in the presence of fibrin monomer, Desafib. Plasminogen, its inhibitors alpha2-antiplasmin and plasminogen activator inhibitor type 1, and plasmin-alpha2-antiplasmin complex in the time samples were assayed by enzyme-linked immunosorbent assay. The effect of addition of plasminogen on the plasmin generation in cord plasma and the effect of lipoprotein on adult and cord plasmin generation were measured.

RESULTS

On the surface of human umbilical vein endothelial cells, onset of plasmin generation was earlier (40 min) compared to plastic (60 min) but total plasmin generation was similar on both surfaces. The addition of plasminogen to cord plasma increased plasmin generation. Supplementation of lipoprotein in adult plasma had an inhibitory effect, but there was no significant effect in cord plasma.

INTERPRETATIONS AND CONCLUSIONS

Plasmin generation is reduced in newborn compared to adult plasma. Decreased plasmin generation in cord plasma is likely due to decreased plasminogen concentration. The antifibrinolytic effect of lipoprotein is more pronounced in adults as compared to newborns due to the presence of higher plasminogen concentration.

Lipoprotein(a) and atherosclerosis: new perspectives on the mechanism of action of an enigmatic lipoprotein

Although elevated plasma concentrations of lipoprotein(a) (Lp(a)) have been identified as a risk factor for coronary heart disease, the pathophysiologic and physiologic roles of Lp(a) continue to elude basic researchers and clinicians alike. Lp(a) is a challenging lipoprotein to study because it has a complex structure consisting of a low-density lipoprotein-like moiety to which is covalently attached the unique glycoprotein apolipoprotein(a) (apo(a)). Apo(a) contains multiply repeated kringle domains that are similar to a sequence found in the fibrinolytic proenzyme plasminogen; differing numbers of kringle sequences in apo(a) give rise to Lp(a) isoform size heterogeneity. In addition to elevated plasma concentrations of Lp(a), apo(a) isoform size has been identified as a risk factor for coronary heart disease, although studies addressing this relationship have been limited. The similarity of Lp(a) to low-density lipoprotein and plasminogen provides an enticing link between the processes of atherosclerosis and thrombosis, although a clear demonstration of this association in vivo has not been provided. Clearly, Lp(a) is a risk factor for both atherothrombotic and purely thrombotic events; a plethora of mechanisms to explain these clinical findings has been provided by both in vitro studies as well as animal models for Lp(a).

New risk factors for atherosclerosis in hypertension: focus on the prothrombotic state and lipoprotein(a)

Although adequate control of blood pressure is of basic importance in cardiovascular prevention in hypertensive patients, correction of additional risk factors is an integral part of their management. In addition to classical risk factors, epidemiological research has identified a number of other conditions that might significantly contribute to cardiovascular risk in the general population and might achieve specific relevance in patients with high blood pressure. In fact, more than 20% of patients with premature cardiovascular events do not have any of the traditional risk factors and, although effective intervention on blood pressure and additional risk factors has significantly reduced cardiovascular morbidity and mortality, the contribution to stroke, coronary artery disease and renal failure is still unacceptably high. Evaluation of new risk factors may further expand our capacity to predict atherothrombotic events when these factors are included along with the traditional ones in the assessment of global cardiovascular risk in hypertensive patients. Because it could be anticipated that the role of these novel factors will become increasingly evident in the future, researchers with an interest in hypertension and physicians dealing with problems related to cardiovascular prevention should give them appropriate consideration. This review summarizes the basic biology and clinical evidence of two emerging risk factors that are reciprocally related and contribute to the development and progression of organ damage in hypertension: the prothrombotic state and lipoprotein(a).

The apolipoprotein(a) component of lipoprotein(a) mediates binding to laminin: contribution to selective retention of lipoprotein(a) in atherosclerotic lesions

Lipoprotein(a) [Lp(a)] entrapment by vascular extracellular matrix may be important in atherogenesis. We sought to determine whether laminin, a major component of the basal membrane, may contribute to Lp(a) retention in the arterial wall. First, immunohistochemistry experiments were performed to examine the relative distribution of Lp(a) and laminin in human carotid artery specimens. There was a high degree of co-localization of Lp(a) and laminin in atherosclerotic specimens, but not in non-atherosclerotic sections. We then studied the binding interaction between Lp(a) and laminin in vitro. ELISA experiments showed that native Lp(a) particles and 17K and 12K recombinant apolipoprotein(a) [r-apo(a)] variants interacted strongly with laminin whereas LDL, apoB-100, and the truncated KIV(6-P), KIV(8-P), and KIV(9-P) r-apo(a) variants did not. Overall, the ELISA data demonstrated that Lp(a) binding to laminin is mediated by apo(a) and a combination of the lysine analogue epsilon-aminocaproic acid and salt effectively decreases apo(a) binding to laminin. Secondary binding analyses with 125I-labeled r-apo(a) revealed equilibrium dissociation constants (K(d)) of 180 and 360 nM for the 17K and 12K variants binding to laminin, respectively. Such similar K(d) values between these two r-apo(a) variants suggest that isoform size does not appear to influence apo(a) binding to laminin. In summary, our data suggest that laminin may bind to apo(a) in the atherosclerotic intima, thus contributing to the selective retention of Lp(a) in this milieu.

Lipoprotein(a) and Apolipoprotein(a) Isoforms

Background— Elevated plasma levels of lipoprotein(a) [Lp(a)] are an independent risk factor for cardiovascular disease in whites. Blacks have 2- to 3-fold higher plasma levels of Lp(a) than whites and yet do not have a correspondingly higher rate of coronary events. It remains unclear whether elevated plasma levels of Lp(a) are an independent risk factor for coronary atherosclerosis in individuals of African descent.

Methods and Results— The relationship between plasma levels of Lp(a), apolipoprotein(a) isoform sizes, and the presence of coronary calcium was examined in 761 blacks and 527 whites (men aged >40 years, women aged >45 years) from a population-based sample. No relationship was found between plasma levels of Lp(a), apolipoprotein(a) isoform size, or a combination of these 2 variables and coronary artery calcium (CAC) in whites or blacks. No correlation was observed between plasma levels of Lp(a) and coronary calcium scores in any group, although all black men with very high plasma levels of Lp(a) (>300 μmol/L; n=7) were CAC-positive. Whites with high plasma levels of Lp(a) plus elevated plasma levels of LDL cholesterol (men) or reduced levels of HDL cholesterol (men and women) or who smoked (women) had a higher prevalence of CAC. In contrast, no joint effects between plasma levels of Lp(a) and other cardiovascular risk factors on coronary calcium were found in blacks.

Conclusions— No consistent independent relationship between plasma levels of Lp(a) or apolipoprotein(a) isoform size and coronary calcium was found in whites or blacks.

Baboon lipoprotein(a) binds very weakly to lysine-agarose and fibrin despite the presence of a strong lysine-binding site in apolipoprotein(a) kringle IV type 10

Human apolipoprotein(a) kringle IV type 10 [apo(a) KIV(10)] contains a strong lysine-binding site (LBS) that mediates the interaction of Lp(a) with biological substrates such as fibrin. Mutations in the KIV(10) LBS have been reported in both the rhesus monkey and chimpanzee, and have been proposed to explain the lack of ability of the corresponding Lp(a) species to bind to lysine and fibrin. To further the comparative analyses with other primate species, we sequenced a segment of baboon liver apo(a) cDNA spanning KIV(9) through the protease domain. Like rhesus monkey apo(a), baboon apo(a) lacks a kringle V (KV)-like domain. Interestingly, we found that the baboon apo(a) KIV(10) sequence contains all of the canonical LBS residues. We sequenced the apo(a) KIV(10) sequence from an additional 10 unrelated baboons; 17 of 20 alleles encoded Trp at position 70, whereas only two alleles encoded Arg at this position and thus a defective LBS. Despite the apparent presence of a functional KIV(10) LBS in most of the baboons, none of the Lp(a) in the plasma of the corresponding baboons bound specifically to lysine-Sepharose (agarose) even upon partial purification. Moreover, baboon Lp(a) bound very poorly to plasmin-modified fibrinogen. Expression of baboon and human KIV(10) in bacteria allowed us to verify that these domains bind comparably to lysine and lysine analogues. We conclude that presentation of KIV(10) in the context of apo(a) lacking KV may interfere with the ability of KIV(10) to bind to substrates such as fibrin; this paradigm may also be present in other non-human primates.

Stimulation of Vascular Smooth Muscle Cell Proliferation and Migration by Apolipoprotein(a) Is Dependent on Inhibition of Transforming Growth Factor-β Activation and on the Presence of Kringle IV Type 9

Elevated plasma concentrations of lipoprotein(a) are a risk factor for the development of a variety of atherosclerotic disorders. Despite intensive study, the mechanisms by which lipoprotein(a) promotes these disorders remain to be unequivocally defined. It has been demonstrated that lipoprotein(a), through its unique constituent apolipoprotein(a) (apo(a)), stimulates vascular smooth muscle cell (SMC) migration and proliferation. These effects arise from the ability of apo(a) to inhibit the formation of active transforming growth factor beta (TGF-beta) from its latent precursor, which in turn is caused by the ability of apo(a) to decrease the formation of plasmin from its precursor plasminogen. We utilized a battery of recombinant apo(a) variants that represent systematic deletions of the various domains in the molecule to further probe the mechanism underlying the effect of apo(a) on SMC responses. All recombinant apo(a) variants that contained kringle IV type 9 were able to stimulate SMC proliferation and migration and to decrease the formation of active TGF-beta; conversely all recombinant apo(a) variants lacking kringle IV type 9 had no effect on these parameters. The kringle IV type 9-dependent effects of apo(a) on SMC proliferation required the presence of plasminogen, suggesting for the first time that this kringle mediates the ability of apo(a) to inhibit pericellular plasmin formation.

Atherogenesis and vascular calcification in mice expressing the human LPA gene

Background: Lp(a) lipoprotein (Lp(a)) contains polymorphic glycoprotein, apolipoprotein(a) (apo(a)) and low density lipoprotein (LDL). The extensive homology between apo(a) and plasminogen is believed to contribute to the pathogenicity of apo(a), but the precise mechanisms by which Lp(a) participates in atherogenesis is still unknown. We used LPA-yeast artificial chromosome (LPA-YAC) transgenic mice with or without the human APOB (hAPOB) gene to study pathogenicity of apo(a)/Lp(a) and illucidate its role in regulation of serum lipid levels. Methods: Middle-aged (1-year-old) mice were fed a control (AIN-76), a high-cholesterol (HC) or a high-cholesterol/high-fat (HCHF) diet for 7 weeks. For the study of serum total apo(a) and lipid levels, mice were sampled prior to the experiment, at 2 weeks and at 7 weeks when the animals were sacrificed. Hearts with ascending aorta were fixed in formalin, embedded in gelatine and prepared for sections on a cryostat. Livers were washed in ice cold saline and submerged in RNAlater trade mark buffer and stored at -70 degrees C until mRNA analysis. Results: Wild type mice fed the control diet did not develop aortic lesions. Presence of the LPA gene was sufficient to induce development of aortic lesions, but neither coexpression of the hAPOB gene nor feeding the HC diet or the HCHF diet augmented the development of aortic lesions in LPA-YAC transgenic mice. On the control diet transgenic females had larger aortic lesion size than transgenic males. Furthermore, aortic lesions in transgenic females were associated with calcification more often than in transgenic males. Serum total cholesterol levels were higher both in wild type and LPA-YAC transgenic males than in females mainly because of higher serum high-density lipoprotein cholesterol levels. HC and HCHF feeding had more pronounced effect on total cholesterol levels in LPA-YAC/hAPOB transgenic mice than in either wild type or LPA-YAC transgenic mice, due to increased low density lipoprotein cholesterol levels. Furthermore, these diets reduced serum total apo(a) levels in both transgenic mouse lines. Conclusion: Expression of the human LPA gene in mice is sufficient to trigger development of aortic lesions. Similar frequency of calcified lesions in LPA-YAC transgenic mice with or without hAPOB gene may suggest that apo(a) is the part of the Lp(a) molecule that causes aortic calcification. The basis for reduced serum total apo(a) level in response to cholesterol feeding is not clear, but interplay between LPA and factors involved in cholesterol or bile acid homeostasis is worth of future studies.

Identification of sequences in apolipoprotein(a) that maintain its closed conformation: a novel role for apo(a) isoform size in determining the efficiency of covalent Lp(a) formation

We have previously demonstrated that, in the presence of the lysine analogue epsilon-aminocaproic acid, apolipoprotein(a) [apo(a)] undergoes a conformational change from a closed to an open structure that is characterized by a change in tryptophan fluorescence, an increase in the radius of gyration, an alteration of domain stability, and an enhancement in the efficiency of covalent lipoprotein(a) [Lp(a)] formation. In the present study, to identify sequences within apo(a) that maintain its closed conformation, we used epsilon-aminocaproic acid to probe the conformational status of a variety of recombinant apo(a) isoforms using analytical ultracentrifugation, differential scanning calorimetry, intrinsic fluorescence, and in vitro covalent Lp(a) formation assays. We observed that the closed conformation of apo(a) is maintained by intramolecular interaction(s) between sequences within the amino- and carboxyl-terminal halves of the molecule. Using site-directed mutagenesis, we have identified the strong lysine-binding site present within apo(a) kringle IV type 10 as an important site within the C-terminal half of the molecule, which is involved in maintaining the closed conformation of apo(a). Apo(a) exhibits marked isoform size heterogeneity because of the presence of varying numbers of copies of the kringle IV type-2 domain located within the amino-terminal half of the molecule. Using recombinant apo(a) species containing either 1, 3, or 8 copies of kringle IV type 2, we observed that, while apo(a) isoform size does not alter the affinity of apo(a) for low-density lipoprotein, it affects the conformational status of the protein and therefore influences the efficiency of covalent Lp(a) assembly. The inverse relationship between apo(a) isoform size and the efficiency of covalent Lp(a) formation that we report in vitro may contribute to the inverse relationship between apo(a) isoform size and plasma Lp(a) concentrations that has been observed in vivo.

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.

Evaluation of lipoprotein(a) as a prothrombotic factor: progress from bench to bedside

PURPOSE OF REVIEW

Since the homology between apolipoprotein(a) (apo(a)) and plasminogen was discovered in 1987, the role of lipoprotein(a) (Lp(a)) as an inhibitor of the normal fibrinolytic role of plasmin(ogen) has been a major research focus. In this review we summarize recent basic research aimed at identifying mechanisms by which Lp(a) can either inhibit fibrinolysis or promote coagulation, as well as recent clinical studies of Lp(a) as a risk factor for thrombosis either in the presence or absence of atherosclerosis.

RECENT FINDINGS

It has recently been reported that the inhibition of plasminogen activation by apo(a) results from the interaction of apo(a) with the ternary complex of tissue-type plasminogen activator, plasminogen and fibrin, rather than competition of apo(a) and plasminogen for binding sites on fibrin. Lp(a) species containing smaller apo(a) isoforms bind more avidly to fibrin and are better inhibitors of plasminogen activation. Recent clinical studies have provided strong evidence that Lp(a), either alone or in synergy with other thrombotic risk factors, significantly increases the risk of venous thromboembolism and ischemic stroke.

SUMMARY

Lp(a) both attenuates fibrinolysis, through inhibition of plasminogen activation, and promotes coagulation, through alleviation of extrinsic pathway inhibition. Further basic and clinical studies are required to more clearly define the role of Lp(a) in thrombotic disorders, and to determine the extent to which thrombotic risk is dependent on apo(a) isoform size.