Lp(a): An Interloper into the Fibrinolytic System?

Elevated lipoprotein(a) levels: A crucial determinant of cardiovascular disease risk and target for emerging therapies

Cardiovascular disease (CVD) remains a significant global health challenge despite advancements in prevention and treatment. Elevated Lipoprotein(a) [Lp(a)] levels have emerged as a crucial risk factor for CVD and aortic stenosis, affecting approximately 20 of the global population. Research over the last decade has established Lp(a) as an independent genetic contributor to CVD and aortic stenosis, beginning with Kare Berg's discovery in 1963. This has led to extensive exploration of its molecular structure and pathogenic roles. Despite the unknown physiological function of Lp(a), studies have shed light on its metabolism, genetics, and involvement in atherosclerosis, inflammation, and thrombosis. Epidemiological evidence highlights the link between high Lp(a) levels and increased cardiovascular morbidity and mortality. Newly emerging therapies, including pelacarsen, zerlasiran, olpasiran, muvalaplin, and lepodisiran, show promise in significantly lowering Lp(a) levels, potentially transforming the management of cardiovascular disease. However, further research is essential to assess these novel therapies' long-term efficacy and safety, heralding a new era in cardiovascular disease prevention and treatment and providing hope for at-risk patients.

Lipoprotein(a) as a Risk Factor for Cardiovascular Diseases: Pathophysiology and Treatment Perspectives

Cardiovascular disease (CVD) is still a leading cause of morbidity and mortality, despite all the progress achieved as regards to both prevention and treatment. Having high levels of lipoprotein(a) [Lp(a)] is a risk factor for cardiovascular disease that operates independently. It can increase the risk of developing cardiovascular disease even when LDL cholesterol (LDL-C) levels are within the recommended range, which is referred to as residual cardiovascular risk. Lp(a) is an LDL-like particle present in human plasma, in which a large plasminogen-like glycoprotein, apolipoprotein(a) [Apo(a)], is covalently bound to Apo B100 via one disulfide bridge. Apo(a) contains one plasminogen-like kringle V structure, a variable number of plasminogen-like kringle IV structures (types 1-10), and one inactive protease region. There is a large inter-individual variation of plasma concentrations of Lp(a), mainly ascribable to genetic variants in the Lp(a) gene: in the general po-pulation, Lp(a) levels can range from <1 mg/dL to >1000 mg/dL. Concentrations also vary between different ethnicities. Lp(a) has been established as one of the risk factors that play an important role in the development of atherosclerotic plaque. Indeed, high concentrations of Lp(a) have been related to a greater risk of ischemic CVD, aortic valve stenosis, and heart failure. The threshold value has been set at 50 mg/dL, but the risk may increase already at levels above 30 mg/dL. Although there is a well-established and strong link between high Lp(a) levels and coronary as well as cerebrovascular disease, the evidence regarding incident peripheral arterial disease and carotid atherosclerosis is not as conclusive. Because lifestyle changes and standard lipid-lowering treatments, such as statins, niacin, and cholesteryl ester transfer protein inhibitors, are not highly effective in reducing Lp(a) levels, there is increased interest in developing new drugs that can address this issue. PCSK9 inhibitors seem to be capable of reducing Lp(a) levels by 25-30%. Mipomersen decreases Lp(a) levels by 25-40%, but its use is burdened with important side effects. At the current time, the most effective and tolerated treatment for patients with a high Lp(a) plasma level is apheresis, while antisense oligonucleotides, small interfering RNAs, and microRNAs, which reduce Lp(a) levels by targeting RNA molecules and regulating gene expression as well as protein production levels, are the most widely explored and promising perspectives. The aim of this review is to provide an update on the current state of the art with regard to Lp(a) pathophysiological mechanisms, focusing on the most effective strategies for lowering Lp(a), including new emerging alternative therapies. The purpose of this manuscript is to improve the management of hyperlipoproteinemia(a) in order to achieve better control of the residual cardiovascular risk, which remains unacceptably high.

Inverse Association of Lipoprotein(a) on Long-Term Bleeding Risk in Patients with Coronary Heart Disease: Insight from a Multicenter Cohort in Asia

BACKGROUND

 Lipoprotein(a), or Lp(a), has been recognized as a strong risk factor for atherosclerotic cardiovascular disease. However, the relationship between Lp(a) and bleeding remains indistinct, especially in the secondary prevention population of coronary artery disease (CAD). This investigation aimed to evaluate the association of Lp(a) with long-term bleeding among patients with CAD.

METHODS

 Based on a prospective multicenter cohort of patients with CAD consecutively enrolled from January 2015 to May 2019 in China, the current analysis included 16,150 participants. Thus, according to Lp(a) quintiles, all subjects were divided into five groups. The primary endpoint was bleeding at 2-year follow-up, and the secondary endpoint was major bleeding at 2-year follow-up.

RESULTS

 A total of 2,747 (17.0%) bleeding and 525 (3.3%) major bleeding were recorded during a median follow-up of 2.0 years. Kaplan-Meier survival analysis showed the highest bleeding incidence in Lp(a) quintile 1, compared with patients in Lp(a) quintiles 2 to 5 (p < 0.001), while the incidence of major bleeding seemed similar between the two groups. Moreover, restricted cubic spline analysis suggested that there was an L-shaped association between Lp(a) and 2-year bleeding after adjustment for potential confounding factors, whereas there was no significant association between Lp(a) and 2-year major bleeding.

CONCLUSION

 There was an inverse and L-shaped association of Lp(a) with bleeding at 2-year follow-up in patients with CAD. More attention and effort should be made to increase the clinician awareness of Lp(a)'s role, as a novel marker for bleeding risk to better guide shared-decision making in clinical practice.

Diet and Lp(a): Does Dietary Change Modify Residual Cardiovascular Risk Conferred by Lp(a)?

Lipoprotein(a) [Lp(a)] is an independent, causal, genetically determined risk factor for cardiovascular disease (CVD). We provide an overview of current knowledge on Lp(a) and CVD risk, and the effect of pharmacological agents on Lp(a). Since evidence is accumulating that diet modulates Lp(a), the focus of this paper is on the effect of dietary intervention on Lp(a). We identified seven trials with 15 comparisons of the effect of saturated fat (SFA) replacement on Lp(a). While replacement of SFA with carbohydrate, monounsaturated fat (MUFA), or polyunsaturated fat (PUFA) consistently lowered low-density lipoprotein cholesterol (LDL-C), heterogeneity in the Lp(a) response was observed. In two trials, Lp(a) increased with carbohydrate replacement; one trial showed no effect and another showed Lp(a) lowering. MUFA replacement increased Lp(a) in three trials; three trials showed no effect and one showed lowering. PUFA or PUFA + MUFA inconsistently affected Lp(a) in four trials. Seven trials of diets with differing macronutrient compositions showed similar divergence in the effect on LDL-C and Lp(a). The identified clinical trials show diet modestly affects Lp(a) and often in the opposing direction to LDL-C. Further research is needed to understand how diet affects Lp(a) and its properties, and the lack of concordance between diet-induced LDL-C and Lp(a) changes.

Elevated Llpoprotein(A) and Fibrinogen Serum Levels Increase the Cardiovascular Risk in Continuous Ambulatory Peritoneal Dialysis Patients

Objective

To analyze the relationship between lipoprotein(a) [Lp(a)] and fibrinogen as potential cardiovascular risk factors in patients on continuous ambulatory peritoneal dialysis (CAPD).

Patients

A total of 47 uremic patients receiving CAPD, 21 with coronary artery disease (CAD), 26 without CAD.

Measurements

Lp(a) levels were determined by an immunoradiometric assay. Since Lp(a) serum concentrations vary depending on the size, apoprotein(a) [apo(a)] isoforms were determined (Westernblot). Fibrinogen was quantified according to Clauss.

Results

The mean Lp(a) serum concentration was 43 ± 5 mg/dL (SEM) (median 33 mg/dL) in CAPD patients and 21 ± 2 mg/dL (8 mg/dL) in controls (p < 0.01). Patients with low molecular weight apo(a) isoforms exhibited substantially elevated Lp(a) levels when compared with patients with high molecular isoforms (p < 0.01). In addition, we found elevated fibrinogen levels in the CAPD patients (538 ± 61 mg/dL) compared with healthy controls (288 ± 46 mg/dL). Twenty-one CAPD patients (45%) were suffering from CAD. Patients with CAD had higher Lp(a) levels (54 ± 5 mg/dL vs 34 ± 4 mg/dL) as well as higher fibrinogen concentrations (628 ± 59 mg/dL vs 459 ± 46 mg/dL). Furthermore, a positive correlation between the fibrinogen levels and the Lp(a) serum concentration was observed (r = 0.45, p = 0.01).

Conclusion

We suggest that elevated Lp(a) levels are influenced by the allelic variation of the apo(a) isoform. In addition to the typical dyslipidemia found in CAPD patients, high levels of Lp(a) and fibrinogen may contribute to the elevated risk of coronary artery disease and other cardiovascular complications.

Association of lipoprotein(a) with long-term mortality following coronary angiography or percutaneous coronary intervention

BACKGROUND

There is no consistent evidence to suggest the association of plasma lipoprotein(a) (Lp[a]) with long-term mortality in patients undergoing coronary angiography (CAG) or percutaneous coronary intervention (PCI).

HYPOTHESIS

Level of Lp(a) is associated with long-term mortality following CAG or PCI.

METHODS

We enrolled 1684 patients with plasma Lp(a) data undergoing CAG or PCI between April 2009 and December 2013. The patients were divided into 2 groups: a low-Lp(a) group (Lp[a] <16.0 mg/dL; n = 842) and a high-Lp(a) group (Lp[a] ≥16.0 mg/dL; n = 842).

RESULTS

In-hospital mortality was not significantly different between the high and low Lp(a) groups (0.8% vs 0.5%, respectively; P = 0.364). During the median follow-up period of 1.95 years, the high-Lp(a) group had a higher long-term mortality than did the low-Lp(a) group (5.8% vs 2.5%, respectively; P = 0.003). After adjustment of confounders, multivariate Cox regression analysis revealed that a higher Lp(a) level was an independent predictor of long-term mortality (hazard ratio: 1.96, 95% confidence interval: 1.07-3.59, P = 0.029).

CONCLUSIONS

Our data suggested that an elevated Lp(a) level was significantly associated with long-term mortality following CAG or PCI. However, additional larger multicenter studies will be required to investigate the predictive value of Lp(a) levels and evaluate the benefit of controlling Lp(a) levels for patients undergoing CAG or PCI.

Structure, function, and genetics of lipoprotein (a)

Lipoprotein (a) [Lp(a)] has attracted the interest of researchers and physicians due to its intriguing properties, including an intragenic multiallelic copy number variation in the LPA gene and the strong association with coronary heart disease (CHD). This review summarizes present knowledge of the structure, function, and genetics of Lp(a) with emphasis on the molecular and population genetics of the Lp(a)/LPA trait, as well as aspects of genetic epidemiology. It highlights the role of genetics in establishing Lp(a) as a risk factor for CHD, but also discusses uncertainties, controversies, and lack of knowledge on several aspects of the genetic Lp(a) trait, not least its function.

Monocyte subset distribution in patients with stable atherosclerosis and elevated levels of lipoprotein(a)

Background

Lipoprotein(a) (Lp(a)) is a proatherogenic plasma lipoprotein currently established as an independent risk factor for the development of atherosclerotic disease and as a predictor for acute thrombotic complications. In addition, Lp(a) is the major carrier of proinflammatory oxidized phospholipids (OxPL). Today, atherosclerosis is considered to be an inflammatory disease of the vessel wall in which monocytes and monocyte-derived macrophages are crucially involved. Circulating monocytes can be divided according to their surface expression pattern of CD14 and CD16 into at least 3 subsets with distinct inflammatory and atherogenic potential.

Objective

The aim of this study was to examine whether elevated levels of Lp(a) and OxPL on apolipoprotein B-100–containing lipoproteins (OxPL/apoB) are associated with changes in monocyte subset distribution.

Methods

We included 90 patients with stable coronary artery disease. Lp(a) and OxPL/apoB were measured, and monocyte subsets were identified as classical monocytes (CMs; CD14++CD16−), intermediate monocytes (IMs; CD14++CD16+), and nonclassical monocytes (NCMs; CD14+CD16++) by flow cytometry.

Results

In patients with elevated levels of Lp(a) (>50 mg/dL), monocyte subset distribution was skewed toward an increase in the proportion of IM (7.0 ± 3.8% vs 5.2 ± 3.0%; P = .026), whereas CM (82.6 ± 6.5% vs 82.0 ± 6.8%; P = .73) and NCM (10.5 ± 5.3 vs 12.8 ± 6.0; P = .10) were not significantly different. This association was independent of clinical risk factors, choice of statin treatment regime, and inflammatory markers. In addition, OxPL/apoB was higher in patients with elevated Lp(a) and correlated with IM but not CM and NCM.

Conclusions

In conclusion, we provide a potential link between elevated levels of Lp(a) and a proatherogenic distribution of monocyte subtypes in patients with stable atherosclerotic disease.

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Lipoprotein(a) serves as an independent risk factor in atherosclerotic disease.

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Monocyte subsets exhibit distinct inflammatory and atherogenic properties.

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Patients with elevated levels of Lp(a) show a shift towards intermediate monocytes.

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This association was independent of clinical properties and inflammatory markers.

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Those patients also exhibited higher OxPL/apoB concentrations.

LPA rs10455872 polymorphism is associated with coronary lesions in Brazilian patients submitted to coronary angiography

BACKGROUND

Polymorphisms in the LPA gene were associated with coronary artery disease (CAD). However, there are differences in the allelic frequencies, Lp(a) levels, and significant association with CAD according to ethnic groups. In this scenario, the main aim of this study was to assess the influence of the LPA polymorphisms on coronary lesions in Brazilian patients.

METHODS

1,394 consecutive patients submitted to coronary angiography to study suggestive CAD and twenty coronary segments were scored. Genotyping for the LPA rs10455872 and rs3798220 polymorphisms were performed by high resolution melting analysis.

RESULTS

The frequencies of the rs10455872 G and rs3798220 C variant alleles were 6.4% and 6.2%, respectively. LPA rs10455872 G variant allele was associated with higher odds ratio of having coronary lesions in an adjusted model (OR = 2.02, 95% CI = 1.10-3.72, p = 0.02). Scores of coronary lesions (extension, severity, and Gensini scores) were significantly different among rs10455872 genotype groups. Coronary lesions was not associated with LPA rs3798220 (OR = 1.09, 95% CI = 0.67-1.76, p = 0.73) and scores of coronary lesions were not different among rs3798220 genotypes.

CONCLUSIONS

We confirmed the association of the LPA rs10455872 with CAD in a large sample of Brazilian patients. For the LPA rs3798220, our finding is consistent with studies which showed the lack of this genetic association.

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

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

Lipoprotein(a) metabolism: potential sites for therapeutic targets

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

Lipoprotein(a): resurrected by genetics

Plasma lipoprotein(a) [Lp(a)] is a quantitative genetic trait with a very broad and skewed distribution, which is largely controlled by genetic variants at the LPA locus on chromosome 6q27. Based on genetic evidence provided by studies conducted over the last two decades, Lp(a) is currently considered to be the strongest genetic risk factor for coronary heart disease (CHD). The copy number variation of kringle IV in the LPA gene has been strongly associated with both Lp(a) levels in plasma and risk of CHD, thereby fulfilling the main criterion for causality in a Mendelian randomization approach. Alleles with a low kringle IV copy number that together have a population frequency of 25-35% are associated with a doubling of the relative risk for outcomes, which is exceptional in the field of complex genetic phenotypes. The recently identified binding of oxidized phospholipids to Lp(a) is considered as one of the possible mechanisms that may explain the pathogenicity of Lp(a). Drugs that have been shown to lower Lp(a) have pleiotropic effects on other CHD risk factors, and an improvement of cardiovascular endpoints is up to now lacking. However, it has been established in a proof of principle study that lowering of very high Lp(a) by apheresis in high-risk patients with already maximally reduced low-density lipoprotein cholesterol levels can dramatically reduce major coronary events.

Enigmatic role of lipoprotein(a) in cardiovascular disease

Lipoprotein (a), [Lp(a)] has many properties in common with low-density lipoprotein, (LDL) but contains a unique protein apolipoprotein(a), linked to apolipoprotein B-100 by a single disulfide bond. There is a substantial size heterogeneity of apo(a), and generally smaller apo(a) sizes tend to correspond to higher plasma Lp(a) levels, but this relation is far from linear, underscoring the importance to assess allele-specific apo(a) levels. The presence of apo(a), a highly charged, carbohydrate-rich, hydrophilic protein may obscure key features of the LDL moiety and offer opportunities for binding to vessel wall elements. Recently, interest in Lp(a) has increased because studies over the past decade have confirmed and more robustly demonstrated a risk factor role of Lp(a) for cardiovascular disease. In particular, levels of Lp(a) carried in particles with smaller size apo(a) isoforms are associated with coronary artery disease (CAD). Other studies suggest that proinflammatory conditions may modulate risk factor properties of Lp(a). Further, Lp(a) may act as a preferential acceptor for proinflammatory oxidized phospholipids transferred from tissues or from other lipoproteins. However, at present only a limited number of agents (e.g., nicotinic acid and estrogen) has proven efficacy in lowering Lp(a) levels. Although Lp(a) has not been definitely established as a cardiovascular risk factor and no guidelines presently recommend intervention, Lp(a)-lowering therapy might offer benefits in subgroups of patients with high Lp(a) levels.

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.

Lipoprotein(a): A Unique Risk Factor for Cardiovascular Disease

Lipoprotein(a) (Lp(a)) is present in humans and primates. It has many properties in common with low-density lipoprotein, but contains a unique protein moiety designated apo(a), which is linked to apolipoprotein B-100 by a single disulfide bond. International standards for Lp(a) measurement and optimized Lp(a) assays insensitive to isoform size are not yet widely available. Lp(a) is a risk factor for coronary artery disease, and smaller size apo(a) is associated with coronary artery disease. The physiologic role of Lp(a) is unknown.

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.

Lipoprotein(a): an elusive cardiovascular risk factor

Lipoprotein (a) [Lp(a)], is present only in humans, Old World nonhuman primates, and the European hedgehog. Lp(a) has many properties in common with low-density lipoprotein (LDL) but contains a unique protein, apo(a), which is structurally different from other apolipoproteins. The size of the apo(a) gene is highly variable, resulting in the protein molecular weight ranging from 300 to 800 kDa; this large variation may be caused by neutral evolution in the absence of any selection advantage. Apo(a) influences to a major extent metabolic and physicochemical properties of Lp(a), and the size polymorphism of the apo(a) gene contributes to the pronounced heterogeneity of Lp(a). There is an inverse relationship between apo(a) size and Lp(a) levels; however, this pattern is complex. For a given apo(a) size, there is a considerable variation in Lp(a) levels across individuals, underscoring the importance to assess allele-specific Lp(a) levels. Further, Lp(a) levels differ between populations, and blacks have generally higher levels than Asians and whites, adjusting for apo(a) sizes. In addition to the apo(a) size polymorphism, an upstream pentanucleotide repeat (TTTTA(n)) affects Lp(a) levels. Several meta-analyses have provided support for an association between Lp(a) and coronary artery disease, and the levels of Lp(a) carried in particles with smaller size apo(a) isoforms are associated with cardiovascular disease or with preclinical vascular changes. Further, there is an interaction between Lp(a) and other risk factors for cardiovascular disease. The physiological role of Lp(a) is unknown, although a majority of studies implicate Lp(a) as a risk factor.

Effect of atorvastatin on different fibrinolyis mechanisms in hypercholesterolemic subjects

BACKGROUND

Hydroxymethyl-glutaryl-CoA-reductase inhibitors (statins) reduce cardiovascular events by cholesterol lowering as well as non-lipid related actions. Among them, the modulation of fibrinolysis could play a relevant role in vascular protection. Atorvastatin is able of reducing platelet activity and thrombin generation before low-density lipoprotein cholesterol (LDL-C) decrease in hypercholesterolemic subjects in which coagulation and fibrinolysis are linked by the activation of thrombin activable fibrinolysis inhibitor (TAFI). The aim of our study was to evaluate whether atorvastatin could modulate fibrinolysis by interactions with endothelial mechanisms and thrombin generation.

METHODS

Forty-four pure hypercholesterolemic subjects (26 M, 18 F, mean age 52.7+/-13.7, LDL-C 194.8+/-9.3t mg/dl) were evaluated for plasmin-antiplasmin complexes (PAP), tissue-plasminogen acivator (t-PA) and its inhibitor (PAI-1) (ELISA), TAFI activity (HPLC), platelet P-selectin (P-sel) (cytofluorymetric detection), platelet-dependent thrombin generation (PDTG, coagulative-chromogenic method) and lipid profile at baseline and after 7, 14, 28 and 90 days of atorvastatin (10 mg/die) treatment.

RESULTS

PAP were significantly reduced at baseline in hypercholesterolemic versus control subjects (P<0.05) and were related to P-sel (P<0.01), PDTG (P<0.01) and its inhibitor (PAI-1) after venous occlusion (VO) (P<0.05). Atorvastatin induced a significant increase of PAP at T(2) related to modifications of P-sel (P<0.01) and PDTG (P<0.01) before significant LDL-C reduction (P=0.132). PAI-1 was significantly changed at T(3) with relation to LDL-C (P<0.01), Von Willebrand factor (VWF) (P<0.01) and sE-sel (P<0.05).

CONCLUSIONS

The profibrinolytic activity of atorvastatin in hypercholesterolemic subjects is related, initially, to the positive effects exerted on platelet function and thrombin generation which can modulate fibrinolysis by TAFI activity.

Plasma lipoprotein Lp(a), markers of haemostasis and inflammation, and risk and severity of coronary heart disease

BACKGROUND

Elevated plasma (Lp(a)) levels may represent an independent risk factor for atherothrombotic complications but the relation between Lp(a) levels and the extent of coronary artery disease (CHD) has been discussed controversially. Little is known about potential atherothrombogenic mechanisms of Lp(a).

DESIGN

Case-control study.

METHODS

We assessed the relationship between plasma Lp(a) and angiographically defined CHD, evaluating the severity of coronary atherosclerosis by three different scores. A total of 312 patients with stable angina aged 40-68 years with at least one coronary stenosis > 50% were studied. A group of 479 voluntary blood donors matched for age and sex served as controls. A complete lipid profile and a large number of markers of coagulation, fibrinolysis and inflammation were measured.

RESULTS

Plasma levels of Lp(a) were significantly higher in patients (14.8 mg/dl; 5.4-47.1 mg/dl; median/interquartile range) than in controls (9.7 mg/dl; 3.5-25.3) (P<0.0001). In a logistic regression model, the fully adjusted Odds Ratio for CHD was 3.3 (95% confidence interval (CI) 1.8-5.6, P<0.0001) for patients in the upper quartile of the Lp(a) distribution compared to the bottom quartile. There was no appreciable association between Lp(a) and apolipoproteins, markers of haemostasis, fibrinolysis and inflammation and the severity of CHD.

CONCLUSIONS

These results indicate that elevated plasma Lp(a) levels may be an independent risk factor for CHD but unrelated to the severity and extension of CHD. Furthermore, there is no good evidence that the presumed link between Lp(a) and CHD is mediated by increased levels of markers of inflammation, or interference with markers of fibrinolysis or coagulation.

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

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

Lipoprotein(a): still an enigma?

PURPOSE OF REVIEW

Lipoprotein(a) belongs to the class of the most atherogenic lipoproteins. Despite intensive research - in the last year more than 80 papers have been published on this topic - information is still lacking on the physiological function of lipoprotein(a) and the site of its catabolism. Important advances have been made in the knowledge of these points, which may have some therapeutic implications.

RECENT FINDINGS

The association of high lipoprotein(a) values with an increase in risk for coronary events has been documented in further prospective studies. This increased risk may relate to recent findings that apolipoprotein(a) is produced in situ within the vessel wall. In addition, lipoprotein(a) binds and inactivates the tissue factor pathway inhibitor and induces plasminogen activator inhibitor type 2 expression in monocytes. A new antisense oligonucleotide strategy has been proposed which efficiently inhibits apolipoprotein(a) expression in vitro and in vivo. Apolipoprotein(a), however, suppresses angiogenesis and thus may interfere with the infiltration of tumor cells. Finally, the enzymatic activity leading to the formation of apolipoprotein(a) fragments in plasma and their catabolism have been further elucidated.

SUMMARY

We are still far away from understanding the pathways involved in lipoprotein(a) catabolism, and the physiological function of this lipoprotein. Recent findings, however, provide new insight into pathomechanisms in patients with increased lipoprotein(a) related to hemostasis, which may serve as a basis for designing new treatment strategies.

The role of lipoprotein(a) in the pathogenesis of atherosclerotic cardiovascular disease and its utility as predictor of coronary heart disease events

Lipoprotein(a), is a highly heterogeneous lipoprotein, due to variations in the size of apolipoprotein(a), and the density of the apoB100-containing particles to which apo(a) is linked. Although high plasma levels of Lp(a) have been associated with an increased risk for atherosclerotic cardiovascular disease, the mechanism underlying this association is still largely undetermined, as is the potential role played by the particle's heterogeneity. Lp(a) pathogenicity may also be influenced by the action of environmental factors and post-translational events relating to oxidative processes, and the action of lipolytic and proteolytic enzymes. Complicating the study of Lp(a) are the competing methods for its quantification due to its complex structure, and the lack of standardized methodologies. The recognition that Lp(a) particles may not all be alike in atherogenic potential should encourage studies to identify genetic and nongenetic factors underlying its heterogeneity, in order to reach a better understanding of its actual impact on atherosclerotic cardiovascular disease.

Lipoprotein (a) in Behçet's disease as an indicator of disease activity and in thrombotic complications

PURPOSE

To evaluate the utility of plasma concentrations of lipoprotein (a) (Lp(a)) as an indicator of disease activity in Behçet's disease and to investigate its role in thrombotic complications of this disease.

METHODS

30 patients (19 male, 11 female) with Behçet's disease (8 active, 22 inactive) were enrolled in the study group and 30 healthy individuals (16 male, 14 female) in the control group. Seven of the inactive Behçet's disease patients had a history of thrombotic complications. The disease activity was evaluated by clinical manifestations (oral aphthous lesions, genital ulcerations, uveitis and vasculitis) and laboratory investigations (leucocyte count, lipoprotein (a), C-reactive protein (CRP), complement 3 (C3) and complement 4 (C4) concentrations).

RESULTS

Plasma Lp(a) and other acute phase reactant concentrations were significantly higher in the study group than in the controls (p < 0.01). These concentrations were also higher during the active period of the disease than during the inactive phase (p < 0.01). Lp(a) concentrations were significantly correlated with concentrations of other acute phase reactants. There was no difference between the groups with and without thrombotic complications for any of these measurements. CONCLUSIONS. Plasma levels of Lp(a) might be an indicator of disease activity in Behçet's disease. There is no correlation between Lp(a) levels and thrombotic sequela in inactive Behçet's disease. However, further studies are needed on the thrombogenic role of Lp(a) during the active phase of thrombophlebitis, and in larger series.

Lipoprotein(a), atherosclerosis, and apolipoprotein(a) gene polymorphism

High plasma lipoprotein(a) [Lp(a)] levels have been implicated as an independent risk factor for coronary artery disease in Caucasians, Chinese, Africans, and Indians. Apo(a) that evolved from a duplicated plasminogen gene during recent primate evolution is responsible for the concentration of Lp(a) in the artery wall leading to atherosclerosis, by virtue of its ability to bind to the extracellular matrix and its role in stimulating the proliferation and migration of human smooth muscle cells. Several types of polymorphisms, size as well as sequence changes both in the coding and regulatory sequences, have been reported to influence the variability of Lp(a) concentration. Apo(a) exhibits genetic size polymorphism varying between 300 and 800 kDa that could be attributed to the number of k-4 VNTR (variable number of transcribed kringle-4 repeats). An inverse relationship between Lp(a) level and apo(a) allele sizes is a general trend in all ethnic populations although apo(a) allele size distribution could be significantly variable in ethnic types. A negative correlation between the number of pentanucleotide TTTTA(n) repeat (PNR) sequences in the regulatory region of the apo(a) gene and Lp(a) level has also been observed in Caucasians and Indians, but not in African Americans. However, a significant linkage disequilibrium was noted between the PNR number and k-4 VNTR. In order to correlate the role of apo(a) gene polymorphisms to apo(a) gene regulation, we have proposed that liver-specific transcriptional activators and repressors might contribute to the differential expression of apo(a) gene, in an individual-specific manner.

Lack of association of lipoprotein(a) levels with coronary calcium deposits in asymptomatic postmenopausal women

OBJECTIVES

This study sought to determine the relationship of lipoprotein(a) (Lp(a)) and other cardiac risk factors to coronary atherosclerosis as measured by calcification of coronary arteries in asymptomatic postmenopausal women.

BACKGROUND

Lipoprotein(a) is considered a risk factor for coronary heart disease. Coronary calcium deposition is believed to be a useful noninvasive marker of coronary atherosclerosis in women. However, to our knowledge, there are no reports of the relationship of Lp(a) to coronary calcium in postmenopausal women.

METHODS

In 178 asymptomatic postmenopausal women (64 +/- 8 years), we measured Lp(a) and other cardiac risk factors: age, hypertension, diabetes, low-density lipoprotein cholesterol, smoking status, body mass index, physical activity level and duration of hormone replacement therapy. Electron-beam computed tomography was done to measure coronary calcium (calcium score). We analyzed the relationship between calcium score and cardiac risk factors using multivariate analysis.

RESULTS

Although calcium score correlated with traditional risk factors of age, diabetes, hypertension and smoking, it did not correlate with Lp(a) in the asymptomatic postmenopausal women. Similar multivariate analyses were done in the subjects age >60 years and in the subjects with significant coronary calcium deposit (calcium score > or =50). These analyses also have failed to show an association of levels of Lp(a) with coronary calcium deposits.

CONCLUSIONS

We conclude that in asymptomatic postmenopausal women, Lp(a) levels do not correlate with coronary atherosclerosis as measured by coronary calcium deposits.

Atherothrombogenicity of lipoprotein(a): the debate

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

Fibrinogen deficiency reduces vascular accumulation of apolipoprotein(a) and development of atherosclerosis in apolipoprotein(a) transgenic mice

To test directly whether fibrin(ogen) is a key binding site for apolipoprotein(a) [apo(a)] in vessel walls, apo(a) transgenic mice and fibrinogen knockout mice were crossed to generate fibrin(ogen)-deficient apo(a) transgenic mice and control mice. In the vessel wall of apo(a) transgenic mice, fibrin(ogen) deposition was found to be essentially colocalized with focal apo(a) deposition and fatty-streak type atherosclerotic lesions. Fibrinogen deficiency in apo(a) transgenic mice decreased the average accumulation of apo(a) in vessel walls by 78% and the average lesion (fatty streak type) development by 81%. Fibrinogen deficiency in wild-type mice did not significantly reduce lesion development. Our results suggest that fibrin(ogen) provides one of the major sites to which apo(a) binds to the vessel wall and participates in the generation of atherosclerosis.

Multi-modal expression of apolipoprotein (a) gene in vivo

The apolipoprotein (a) [apo(a)] gene encodes a protein component of lipoprotein (a) [Lp(a)] whose plasma levels vary among individuals. To study the implications of Lp(a), we examined plasma Lp(a) levels and molecular weights of apo(a) in patients with cerebrovascular disease (CVD) or diabetes mellitus (DN). Mean Lp(a) concentrations were higher in the CVD cases with atherothrombotic brain infarction than in those with brain hemorrhage and lacunar infarction. Lp(a) levels were lower in the DM cases on diet therapy alone than in those treated with insulin or oral hypoglycemic agents. These results suggest that Lp(a) is thrombogenic and atherogenic, and that insulin may modulate Lp(a) levels. We subclassified the apo(a) gene into four types (A-D) by polymorphisms in the 5'-flanking region. We also measured plasma Lp(a) concentrations and examined expression of the gene by an in vitro assay. Homozygotes of type C had higher Lp(a) levels than those of type D, and the relative expression of type C was higher than that of type D in vitro. Lp(a) levels, however, varied even within the same 5'-allele having similar apo(a) isoforms. Thus, Lp(a) concentrations are genetically determined and may be modified by some hormones and cytokines. When we examined transcript levels for apo(a) by RT-PCR in various normal tissues, apo(a) was strongly expressed in liver while not in thyroid or leukocytes. Small amounts of apo(a) transcript were observed in all other organs and tissues. Apo(a) in these tissues may also play a role in inframmation, tissue remodeling, cell migration, and other physiological functions.

Plasma lipoprotein(a) levels are high in patients with central retinal artery occlusion

High plasma lipoprotein(a) (Lp[a]) concentration is an independent risk factor for atherosclerosis and thrombosis. To study the implications of Lp(a) in central retinal artery occlusion (CRAO), we examined Lp(a) levels and molecular weights (MWs) of apolipoprotein(a) (apo(a)). Mean Lp(a) concentration was significantly higher in the cases with CRAO than in the controls. Lp(a) levels higher than 30 mg/dl were also more frequent in the CRAO cases than in the controls. Lp(a) concentrations correlated significantly with low-MW isoforms of apo(a). Impaired fibrinolysis and atherogenesis induced by Lp(a) may play a role in the pathophysiology of CRAO. Since high Lp(a) levels were reported in CRVO by other investigators, patients with central retinal vein occlusion (CRVO) were also examined for Lp(a). Although Lp(a) levels were higher in the CRVO cases than in the controls, the difference was not significant. Therefore, high Lp(a) levels may not be associated with venous thrombosis and/or embolism.

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

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

LP(a) phenotypes and levels in angiographically proven coronary heart disease patients and controls

Lipoprotein Lp(a) excess has been identified as a powerful predictor of premature atherosclerotic vascular diseases. To evaluate this in a North-Indian population, 130 CAD patients and 130 controls were analyzed. The size of the apo(a) phenotypic isoforms was inversely proportional to Lp(a) concentrations. The mean concentration of Lp(a) in the CAD patients was 42±34 mg/dl whereas in the normal subjects it was much lower, 27±27 mg/dl. 157 subjects out of the total 260 subjects showed plasma levels of >20mg/dl. The frequency of high Lp(a) levels was much higher in patients(73%) than controls (43%). These data suggest (1) that there is heterogeneity of the Lp(a) polymorphism, (2) Higher Lp(a) levels were found in patients than in the controls, (3) Patients showed 1.5 fold increase in Lp(a) levels as compared to the controls. We conclude that low molecular weight apo(a) isoforms are significantly associated with increased risk of CAD in the North-Indian population.

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

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

Convergent evolution of apolipoprotein(a) in primates and hedgehog

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

Lipoprotein(a) concentration and molecular weight of apolipoprotein(a) in patients with cerebrovascular disease and diabetes mellitus

Plasma lipoprotein(a) [Lp(a)] concentrations are genetically determined, and hyper-Lp(a)-emia is an independent risk factor for atherosclerosis and thrombosis. To study the implications of Lp(a) in cerebrovascular disease (CVD) and diabetes mellitus (DM), we examined plasma Lp(a) levels and molecular weights of apolipoprotein(a) [apo(a)] in 118 patients with CVD, and 125 cases with DM. Although mean Lp(a) concentrations were higher in those cases with atherothrombotic brain infarction than in those with brain hemorrhage and lacunar infarction, the difference was not statistically significant. Lp(a) levels were significantly higher in the DM cases treated with insulin and in those treated with oral hypoglycemic agents than in those on diet therapy alone, suggesting that insulin and oral agents modulate apo(a) expression. Lp(a) concentrations correlated significantly with the low-molecular-weight isoforms of apo(a) in all CVD and DM groups.

Novel Interaction of Apolipoprotein(a) With β-2 Glycoprotein I Mediated by the Kringle IV Domain

Lipoprotein(a) [Lp(a)], which has been shown to interact with fibrin(ogen) and other components of the blood clotting cascade, 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. Identification of further potential apo(a)-protein ligands may be crucial to illuminate apo(a)'s function(s) and pathophysiological properties. We used the repetitive apo(a) kringle IV type 2, which is variable in number in apo(a), to screen a human liver cDNA library by the yeast two-hybrid interaction trap system. Among 11 positive clones that emerged from the screen, eight clones were identified as β-2 glycoprotein I and one as fibronectin. Coimmunoprecipitation experiments confirmed that β-2 glycoprotein I and apo(a)/Lp(a) interact in human plasma and in cell culture supernatants of COS-1 cells, which ectopically expressed apo(a). The apo(a)-β2-glycoprotein I interaction indicates new potential roles for Lp(a) in fibrinolysis and autoimmunity.

Novel Interaction of Apolipoprotein(a) With β-2 Glycoprotein I Mediated by the Kringle IV Domain

Lipoprotein(a) [Lp(a)], which has been shown to interact with fibrin(ogen) and other components of the blood clotting cascade, 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. Identification of further potential apo(a)-protein ligands may be crucial to illuminate apo(a)'s function(s) and pathophysiological properties. We used the repetitive apo(a) kringle IV type 2, which is variable in number in apo(a), to screen a human liver cDNA library by the yeast two-hybrid interaction trap system. Among 11 positive clones that emerged from the screen, eight clones were identified as β-2 glycoprotein I and one as fibronectin. Coimmunoprecipitation experiments confirmed that β-2 glycoprotein I and apo(a)/Lp(a) interact in human plasma and in cell culture supernatants of COS-1 cells, which ectopically expressed apo(a). The apo(a)-β2-glycoprotein I interaction indicates new potential roles for Lp(a) in fibrinolysis and autoimmunity.

Urinary apo(a) discriminates coronary artery disease patients from controls

Increased plasma lipoprotein (a) (Lp(a)) levels are associated with premature cardiovascular diseases and stroke. Since Lp(a) immune reactivity is found in urine we compared urinary apolipoprotein (a) (apo(a)) with plasma Lp(a) levels in 116 patients suffering from angiographically proven coronary artery diseases with that of 109 controls. Urinary apo(a) investigated by immuno blotting, revealed a distinct apo(a) fragmentation pattern with molecular weights between 50 and 160 kDa. Apolipoprotein B however was not secreted into urine. Lp(a) and apo(a) were measured by a fluorescence immuno assay. Within single individuals, urinary apo(a) levels correlated significantly with creatinine (Rho, 0.98; P < 0.0005). Medians and 25/75 percentiles of urinary apo(a) in coronary artery disease (CAD) patients were 5.70, 3.25 and 10.35 microg/dl and in controls 2.64, 1.43 and 3.50 microg/dl respectively. At cut-off levels of 30 mg/dl for plasma Lp(a) and 10 microg/dl of urinary apo(a) respectively, both paramenters showed comparable sensitivities (33.8% vs. 26.7%), yet the specificity (76.1% vs. 91.7%) and the positive predictive value (60.0% vs.76.4%) of urinary apo(a) were much higher. In receiver-operating characteristic plots, urinary apo(a) was much more sensitive at high specificities i.e. greater than 60% as compared to Lp(a). Urinary secretion of apo(a) fragments normalized to creatinine is stable in a given individual and significantly associated with coronary artery disease.

Factors affecting fibrinolytic potential: cardiovascular fitness, body composition, and lipoprotein(a)

The purpose of the study was to determine the factors that affect basal (resting) and poststressor fibrinolytic activity or potential. Variables of interest included cardiovascular fitness (maximal oxygen consumption [Vo2max]), body fat, body mass index (BMI), and lipids/lipoproteins, including lipoprotein(a) [Lp(a)]. Blood was collected from 46 middle-aged men before and after a maximal exercise test. Pearson and Spearman correlation coefficients were calculated to determine associations between the variables of interest and tissue plasminogen activator (t-PA) and plasminogen activator inhibitor-1 (PAI-1) activities in the basal state and after stimulation with maximal exercise. Multiple regression analyses were also conducted to determine independent predictors of the fibrinolytic variables. Maximal exercise produced significant increases in t-PA activity and decreases in PAI-1 activity. Postexercise t-PA activity was inversely related to basal PAI-1 activity (r = -.34). Vo2max was positively correlated with t-PA activity (basal, r = .39; postexercise, r = .67) and inversely related to PAI-1 activity (basal, r = -.41; postexercise, r = -.42). Body fat was correlated with postexercise t-PA activity (r = -.60) and both basal and postexercise PAI-1 activity (r = .42), but the correlation with basal t-PA activity was not significant (P = .058). Postexercise t-PA activity was positively correlated (r = .37) with high-density lipoprotein cholesterol (HDL-C) and negatively correlated (r = -.42) with low-density lipoprotein cholesterol (LDL-C). Basal PAI-1 activity was negatively correlated with HDL-C (r = -.37), Lp(a) was not correlated with any fibrinolytic variable or fitness. Multiple regression analyses showed that Vo2max was an independent predictor of both basal and postexercise t-PA activity (R2 = .16 and .34, respectively). Triglyceride (TG) levels and Vo2max were significant independent predictors of PAI-1 activity (R2 = .31). In conclusion, cardiovascular fitness was a strong independent predictor of fibrinolytic potential. In addition, poststressor measures of fibrinolytic potential may provide more information about the fibrinolytic system than basal values.

The complete cDNA sequence encoding plasminogen from the European hedgehog (Erinaceus europaeus)

Using a PCR-based strategy, we have determined the complete cDNA sequence encoding hedgehog plasminogen (Plg). The 2700-nucleotide cDNA (corresponding to a 2.9-kb liver-derived transcript) encodes an open reading frame of 811 amino acids which shares 74-76% identity with Plg characterized from mouse, human and rhesus monkey. Residues corresponding to the catalytic triad, tPA-cleavage site, as well as seven of the eight lysine-binding residues in kringle IV are conserved in the hedgehog. However, potential N-linked glycosylation sites which have been reported in human and rhesus Plg are not present in analogous positions in the hedgehog Plg sequence.

Oxidized lipoprotein (a) induces cell adhesion molecule Mac-1 (CD 11b) and enhances adhesion of the monocytic cell line U937 to cultured endothelial cells

Because of structural similarities between low density lipoproteins (LDL) and lipoprotein (a) (Lp(a)), we have investigated the properties and the functional activities of oxidized Lp(a) and focused on whether oxidized Lp(a), like oxidized LDL, can induce monocyte differentiation and adhesion of monocytic cells to endothelial cells grown in culture. Oxidized Lp(a), prepared in vitro by cupric ion oxidation, gave absorption curves of conjugated dienes with a lag-phase of 61.7 +/- 6.6 min (mean +/- S.D.) as compared to 85.2 +/- 7.2 min (n = 6, P < 0.01) for oxidized LDL from the same donors and at equimolar concentrations. Degradation of oxidized 125I Lp(a) by the monocytic cell line U937 at 37 degrees C was 1.6 +/- 0.3 nmol/g of cell protein, significantly (P < 0.01) greater than the degradation of oxidized 125I-LDL, which was 1.15 +/- 0.2 nmol/g of cell protein. Equimolar concentrations of oxidized Lp(a) and LDL inhibited the growth of U937 by 82 +/- 8.2% and 64 +/- 7.1%, respectively, when compared with the effect (negligible) produced by native Lp(a) and LDL. In addition, equimolar concentrations of oxidized Lp(a) and LDL induced adhesion molecule, Mac-1 (CD 11b), expression in U937 by 64 +/- 7.1% and 58 +/- 6.1% (P > 0.05), respectively, of the effect produced by phorbol esters (PMA) (P < 0.01). U937 cells incubated with oxidized Lp(a) and LDL, showed an adherence to cultured endothelial cells at 42 +/- 5.2% and 34 +/- 4.8%, respectively (P < 0.05), of the adherence shown by the same cells activated by PMA (P < 0.01). Our results suggest that oxidized Lp(a) like oxidized LDL plays an important role in the development of atherogenesis by inducing adhesion of monocytes to the arterial intimal and by stimulating intimal monocytes to differentiate into macrophages.

Thrombotic vascular risk factors in inflammatory bowel disease

BACKGROUND

Thrombosis may be an important effector mechanism in the pathogenesis of Crohn's disease.

METHODS

This study therefore investigated the prevalence of independent thrombotic risk factors (factor VII coagulant activity, lipoprotein (a), fibrinogen, plasma triglycerides, and smoking) in patients with Crohn's disease, ulcerative colitis, and normal controls.

RESULTS

In Crohn's disease (n = 75), the mean plasma VII:C, lipoprotein (a) and fibrinogen concentrations were significantly greater than in the normal population (n = 85). In ulcerative colitis (n = 35), only the mean factor VII:C concentration was significantly higher than normal. Ninety three per cent of patients with Crohn's disease and 86% of those with ulcerative colitis had at least one risk factor for thrombotic vascular disease, compared with 61% of the normal population (p < 0.001).

CONCLUSIONS

In many young patients with inflammatory bowel disease, plasma concentrations of these prothrombotic factors were in excess of the limits that are regarded as posing an increased risk for the development of occlusive vascular disease.

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).

How often has Lp(a) evolved?

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

N-acetylcysteine treatment lowers plasma homocysteine but not serum lipoprotein(a) levels

High levels of lipoprotein(a) (Lp(a)) or homocysteine in plasma have both been associated with an increased risk for premature cardiovascular disease. For both components, the plasma levels are primarily genetically determined, and they have been very restintant to therapeutic approaches. It has been suggested that N-acetylcysteine (NAC) breaks disulphide bonds in Lp(a) as well as between homocysteine and plasma proteins. In the present study we analyze if this mechanism, in vivo, could be used to lower plasma concentrations of Lp(a) and homocysteine. Treatment with NAC and placebo was performed in a double blind cross over design with 2 weeks wash-out between treatments. Eleven subjects with high plasma Lp(a) (> 0.3 milligram) were recruited from the Lipid Clinic at Sahlgren's Hospital, Göteborg, Sweden. Main outcome measures were treatment effects on plasma Lp(a) and plasma amino thiols (homocysteine, cysteine and cysteinyl glycine). There was no significant effect on plasma Lp(a) levels. Plasma thiols were significantly reduced during treatment with NAC: homocysteine by 45% (P < 0.0001), cysteinyl glycine by 24% (P < 0.0001) and cysteine by 11% (P = 0.0002). The high dose of NAC was well tolerated. In conclusion NAC has no effect on plasma Lp(a) levels while the reduction in homocysteine is considerable and might be of clinical significance in cases with high plasma homocysteine levels.

Levels of factor VIIc associated with decreased tissue factor pathway inhibitor and increased plasminogen activator inhibitor-1 in dyslipidemias

Tissue factor pathway inhibitor (TFPI), a kunitztype inhibitor of the extrinsic coagulation pathway, factor VII coagulant (FVIIc), FVIIa, and the fibrinolytic factors plasminogen activator inhibitor-1 (PA1-1) and tissue plasminogen activator (TPA) have been studied in various hyperlipidemias. Compared with a normal lipidic group, mean TFPI activity was 70% higher (P < .001) and 36% higher (P < .001) in type IIa and IIb hyperlipidemias, respectively, and was lower by 13% in type IV hyperlipidemia (P = .05). TFPI was correlated with LDL cholesterol (P < .001), total cholesterol (P < .001), HDL cholesterol (P < .01), apolipoproteins (apo) AI (P < .001) and B (P < .001) and lipoprotein a (P < .01). TFPI was negatively correlated with the triglyceride level (P < .05); the correlation was dependent on LDL cholesterol and HDL cholesterol levels, which were decreased in type IV hyperlipidemia. FVIIc activity (P < .001) was increased by 30% in both type IV and type IIb hyperlipidemia and was correlated with triglyceride levels. FVIIa was not significantly increased in any group compared with control group. FVIIc was correlated with triglyceride level (P < .001), while FVIIa was not. Interestingly, FVIIa was correlated with FVIIc (r = .5, P < .001) in the control group as well as in the hyperlipidemic groups (r = .32, P < .01). These results favor the hypothesis that higher FVIIc concentrations in hyperlipidemic patients are likely due to enhancement of synthesis of FVII and that a part of this FVII circulates in an activated chemical form. Compared with the control group, PAI-1 activity was twofold higher (P < .08) in type IIa hyperlipidemia, threefold higher (P < .001) in type IIb hyperlipidemia, and fourfold higher in type IV hyperlipidemia (P < .001). PAI-1 activity correlated with triglyceride levels (P < .001), apoB levels (P < .001) and total cholesterol levels (P < .05). These correlations were dependent on apoB and probably reflect the correlation between PAI-1 and VLDL. In contrast, TPA level was normal in the different hyperlipidemias. No correlation was found between TFPI, FVIIc, and PAI-1. Variation of TFPI activity appears to be related to the variations of its main lipoprotein carriers: LDL, HDL, and Lp (a). The association in hypertriglycemic patients of hypercoagulability (increased FVIIc and decreased TFPI) and hypofibrinolysis (increased PAI-1) may explain thrombosis predisposition of some of these patients. However, it would be interesting to study the increased levels of endothelium-derived TFPI in plasma induced by the injection of heparin.

Little association of lipid parameters and large sensory nerve fiber function in diabetes mellitus

The natural history of diabetic neuropathy and its risk factors are not well understood. The potential association of various lipids [e.g., high-density lipoprotein (HDL) and low-density lipoprotein (LDL) cholesterol, triglycerides], and lipoprotein(a) [Lp(a)] concentrations, with large sensory nerve fiber function as assessed by vibratory thresholds was examined in a group of 91 individuals with diabetes mellitus. In multivariate analyses, no independent relationships of any of the lipid or lipoprotein parameters measured in this study were found with vibratory thresholds (i.e., dependent variable). Independent associations of age, duration of diabetes, height, and medications that lower blood pressure with vibratory thresholds were shown and explained 51% of the overall variability of the model. In gender-specific models, age, height, and medications that lower blood pressure were statistically significant independent determinates (i.e., males R2 = 0.61, females R2 = 0.39). These cross-sectional data suggest that lipid and lipoprotein parameters measured in this study have little association with large sensory nerve fiber dysfunction. The interesting association with the use of medications that lower blood pressure and vibratory thresholds warrants further investigation.