Relating Lipoprotein(a) Concentrations to Cardiovascular Event Risk After Acute Coronary Syndrome: A Comparison of 3 Tests

BACKGROUND

Lipoprotein(a) is a risk factor for cardiovascular events and modifies the benefit of PCSK9 (proprotein convertase subtilisin/kexin type 9) inhibitors. Lipoprotein(a) concentration can be measured with immunoassays reporting mass or molar concentration or a reference measurement system using mass spectrometry. Whether the relationships between lipoprotein(a) concentrations and cardiovascular events in a high-risk cohort differ across lipoprotein(a) methods is unknown. We compared the prognostic and predictive value of these types of lipoprotein(a) tests for major adverse cardiovascular events (MACE).

METHODS

The ODYSSEY OUTCOMES trial (Evaluation of Cardiovascular Outcomes After an Acute Coronary Syndrome During Treatment With Alirocumab) compared the PCSK9 inhibitor alirocumab with placebo in patients with recent acute coronary syndrome. We compared risk of a MACE in the placebo group and MACE risk reduction with alirocumab according to baseline lipoprotein(a) concentration measured by Siemens N-latex nephelometric immunoassay (IA-mass; mg/dL), Roche Tina-Quant turbidimetric immunoassay (IA-molar; nmol/L), and a noncommercial mass spectrometry-based test (MS; nmol/L). Lipoprotein(a) values were transformed into percentiles for comparative modeling. Natural cubic splines estimated continuous relationships between baseline lipoprotein(a) and outcomes in each treatment group. Event rates were also determined across baseline lipoprotein(a) quartiles defined by each assay.

RESULTS

Among 11 970 trial participants with results from all 3 tests, baseline median (Q1, Q3) lipoprotein(a) concentrations were 21.8 (6.9, 60.0) mg/dL, 45.0 (13.2, 153.8) nmol/L, and 42.2 (14.3, 143.1) nmol/L for IA-mass, IA-molar, and MS, respectively. The strongest correlation was between IA-molar and MS (r=0.990), with nominally weaker correlations between IA-mass and MS (r=0.967) and IA-mass and IA-molar (r=0.972). Relationships of lipoprotein(a) with MACE risk in the placebo group were nearly identical with each test, with estimated cumulative incidences differing by ≤0.4% across lipoprotein(a) percentiles, and all were incrementally prognostic after accounting for low-density lipoprotein cholesterol levels (all spline P≤0.0003). Predicted alirocumab treatment effects were also nearly identical for each of the 3 tests, with estimated treatment hazard ratios differing by ≤0.07 between tests across percentiles and nominally less relative risk reduction by alirocumab at lower percentiles for all 3 tests. Absolute risk reduction with alirocumab increased with increasing lipoprotein(a) measured by each test, with significant linear trends across quartiles.

CONCLUSIONS

In patients with recent acute coronary syndrome, 3 lipoprotein(a) tests were similarly prognostic for MACE in the placebo group and predictive of MACE reductions with alirocumab at the cohort level.

REGISTRATION

URL: https://www.clinicaltrials.gov; Unique identifier: NCT01663402.

Assessment of Apolipoprotein(a) Isoform Size Using Phenotypic and Genotypic Methods

Apolipoprotein(a) (apo(a)) is the protein component that defines lipoprotein(a) (Lp(a)) particles and is encoded by the LPA gene. The apo(a) is extremely heterogeneous in size due to the copy number variations in the kringle-IV type 2 (KIV2) domains. In this review, we aim to discuss the role of genetics in establishing Lp(a) as a risk factor for coronary heart disease (CHD) by examining a series of molecular biology techniques aimed at identifying the best strategy for a possible application in clinical research and practice, according to the current gold standard.

The kringle IV type 2 domain variant 4925G>A causes the elusive association signal of the LPA pentanucleotide repeat

Lipoprotein(a) [Lp(a)] concentrations are regulated by the LPA gene mainly via the large kringle IV-type 2 (KIV-2) copy number variation and multiple causal variants. Early studies suggested an effect of long pentanucleotide repeat (PNR) alleles (10 and 11 repeats, PNR10 and PNR11) in the LPA promoter on gene transcription and found an association with lower Lp(a). Subsequent in vitro studies showed no effects on mRNA transcription, but the association with strongly decreased Lp(a) remained consistent. We investigated the isolated and combined effect of PNR10, PNR11, and the frequent splice site variant KIV-2 4925G>A on Lp(a) concentrations in the Cooperative Health Research in the Region of Augsburg F4 study by multiple quantile regression in single-SNP and joint models. Data on Lp(a), apolipoprotein(a) Western blot isoforms, and variant genotypes were available for 2,858 individuals. We found a considerable linkage disequilibrium between KIV-2 4925G>A and the alleles PNR10 and PNR11. In single-variant analysis adjusted for age, sex, and the shorter apo(a) isoform, we determined that both PNR alleles were associated with a highly significant Lp(a) decrease (PNR10: β = -14.43 mg/dl, 95% CI: -15.84, -13.02, P = 3.33e-84; PNR11: β = -17.21 mg/dl, 95% CI: -20.19, -14.23, P = 4.01e-29). However, a joint model, adjusting the PNR alleles additionally for 4925G>A, abolished the effect on Lp(a) (PNR10: β = +0.44 mg/dl, 95% CI: -1.73, 2.60, P = 0.69; PNR11: β = -1.52 mg/dl, 95% CI: -6.05, 3.00, P = 0.51). Collectively, we conclude that the previously reported Lp(a) decrease observed in pentanucleotide alleles PNR10 or PNR11 carriers results from a linkage disequilibrium with the frequent splicing mutation KIV-2 4925G>A.

LPA disruption with AAV-CRISPR potently lowers plasma apo(a) in transgenic mouse model: A proof-of-concept study

Lipoprotein(a) (Lp(a)) represents a unique subclass of circulating lipoprotein particles and consists of an apolipoprotein(a) (apo(a)) molecule covalently bound to apolipoprotein B-100. The metabolism of Lp(a) particles is distinct from that of low-density lipoprotein (LDL) cholesterol, and currently approved lipid-lowering drugs do not provide substantial reductions in Lp(a), a causal risk factor for cardiovascular disease. Somatic genome editing has the potential to be a one-time therapy for individuals with extremely high Lp(a). We generated an LPA transgenic mouse model expressing apo(a) of physiologically relevant size. Adeno-associated virus (AAV) vector delivery of CRISPR-Cas9 was used to disrupt the LPA transgene in the liver. AAV-CRISPR nearly completely eliminated apo(a) from the circulation within a week. We performed genome-wide off-target assays to determine the specificity of CRISPR-Cas9 editing within the context of the human genome. Interestingly, we identified intrachromosomal rearrangements within the LPA cDNA in the transgenic mice as well as in the LPA gene in HEK293T cells, due to the repetitive sequences within LPA itself and neighboring pseudogenes. This proof-of-concept study establishes the feasibility of using CRISPR-Cas9 to disrupt LPA in vivo, and highlights the importance of examining the diverse consequences of CRISPR cutting within repetitive loci and in the genome globally.

PCSK9 inhibition with alirocumab decreases plasma lipoprotein(a) concentration by a dual mechanism of action in statin-treated patients with very high apolipoprotein(a) concentration

BACKGROUND

Inhibition of proprotein convertase subtilisin/kexin type 9 with alirocumab decreases plasma lipoprotein(a) [Lp(a)] levels. The kinetic mechanism for lowering Lp(a) by alirocumab may differ according to pre-treatment apolipoprotein(a) [apo(a)] levels.

METHODS

The effect of 12-week alirocumab (150 mg subcutaneously fortnightly) on the kinetics of apo(a) was compared in statin-treated patients with high (n = 10) and very high Lp(a) concentrations (n = 11).

RESULTS

In patients with high apo(a) concentrations, alirocumab lowered plasma apo(a) pool size (-17%, p < 0.01) chiefly by increasing the fractional catabolic rate (FCR) of apo(a) (+27%, p < 0.001). By contrast in patients with very high apo(a) concentrations, alirocumab significantly lowered plasma apo(a) pool size (-32%, p < 0.001) by both increasing apo(a) FCR (+30%, p < 0.001) and lowering production rate (-11%, p < 0.05).

CONCLUSIONS

In statin-treated patients with very high apo(a) concentrations, alirocumab lowers plasma Lp(a) concentration by a dual mode of action that increases the clearance and decreases the production of Lp(a) particles.

Development of an LC-MS/MS Proposed Candidate Reference Method for the Standardization of Analytical Methods to Measure Lipoprotein(a)

BACKGROUND

Use of lipoprotein(a) concentrations for identification of individuals at high risk of cardiovascular diseases is hampered by the size polymorphism of apolipoprotein(a), which strongly impacts immunochemical methods, resulting in discordant values. The availability of a reference method with accurate values expressed in SI units is essential for implementing a strategy for assay standardization.

METHOD

A targeted LC-MS/MS method for the quantification of apolipoprotein(a) was developed based on selected proteotypic peptides quantified by isotope dilution. To achieve accurate measurements, a reference material constituted of a human recombinant apolipoprotein(a) was used for calibration. Its concentration was assigned using an amino acid analysis reference method directly traceable to SI units through an unbroken traceability chain. Digestion time-course, repeatability, intermediate precision, parallelism, and comparability to the designated gold standard method for lipoprotein(a) quantification, a monoclonal antibody-based ELISA, were assessed.

RESULTS

A digestion protocol providing comparable kinetics of digestion was established, robust quantification peptides were selected, and their stability was ascertained. Method intermediate imprecision was below 10% and linearity was validated in the 20-400 nmol/L range. Parallelism of responses and equivalency between the recombinant and endogenous apo(a) were established. Deming regression analysis comparing the results obtained by the LC-MS/MS method and those obtained by the gold standard ELISA yielded y = 0.98*ELISA +3.18 (n = 64).

CONCLUSIONS

Our method for the absolute quantification of lipoprotein(a) in plasma has the required attributes to be proposed as a candidate reference method with the potential to be used for the standardization of lipoprotein(a) assays.

Lipoprotein(a), Immunity, and Inflammation in Polyvascular Atherosclerotic Disease

Background and aims: lipoprotein(a) (Lp(a)) is a genetically determined risk factor for coronary artery disease and its complications, although data on the association with other vascular beds and the severity of atherosclerosis is limited. The aim of this study was to evaluate the association of atherosclerosis of various vascular beds with Lp(a), as well as its autoantibodies and generalized inflammatory markers. Material and methods: this study included 1288 adult patients with clinical and imaging examination of three vascular beds (coronary, carotid, and lower limb arteries). Patients were categorized according to the number of affected vascular beds (with at least one atherosclerotic stenosis ≥50%): 0 (n = 339), 1 (n = 470), 2 (n = 315), 3 (n = 164). We assessed blood cell count, lipid profile, C-reactive protein, circulating immune complexes, Lp(a), and its autoantibodies. Results: the number of affected vascular beds was associated with an increasing level of Lp(a) and a lower level of IgM autoantibodies to Lp(a). Hyperlipoproteinemia(a) (Lp(a) ≥ 30 mg/dL) was detected more frequently in patients with atherosclerosis. In logistic regression analysis adjusted for age, sex, hypertension, type 2 diabetes, and smoking, an elevated Lp(a) level was independently associated with stenotic atherosclerosis and lesion severity. There was a positive association of the number of affected vascular beds with C-reactive protein (r = 0.21, p < 0.01) and a negative association with circulating immune complexes (r = −0.29, p < 0.01). The neutrophil-to-lymphocyte ratio was significantly higher and the lymphocyte-to-monocyte ratio was significantly lower in patients with atherosclerosis compared to the controls (p < 0.01). Conclusion: Lp(a), C-reactive protein, circulating immune complexes, and neutrophil-to-lymphocyte ratio are associated with the stenotic atherosclerosis of different vascular beds. Lp(a) levels increase and IgM autoantibodies to Lp(a) decrease with the number of affected vascular beds.

Developing Electron Microscopy Tools for Profiling Plasma Lipoproteins Using Methyl Cellulose Embedment, Machine Learning and Immunodetection of Apolipoprotein B and Apolipoprotein(a)

Plasma lipoproteins are important carriers of cholesterol and have been linked strongly to cardiovascular disease (CVD). Our study aimed to achieve fine-grained measurements of lipoprotein subpopulations such as low-density lipoprotein (LDL), lipoprotein(a) (Lp(a), or remnant lipoproteins (RLP) using electron microscopy combined with machine learning tools from microliter samples of human plasma. In the reported method, lipoproteins were absorbed onto electron microscopy (EM) support films from diluted plasma and embedded in thin films of methyl cellulose (MC) containing mixed metal stains, providing intense edge contrast. The results show that LPs have a continuous frequency distribution of sizes, extending from LDL (> 15 nm) to intermediate density lipoprotein (IDL) and very low-density lipoproteins (VLDL). Furthermore, mixed metal staining produces striking “positive” contrast of specific antibodies attached to lipoproteins providing quantitative data on apolipoprotein(a)-positive Lp(a) or apolipoprotein B (ApoB)-positive particles. To enable automatic particle characterization, we also demonstrated efficient segmentation of lipoprotein particles using deep learning software characterized by a Mask Region-based Convolutional Neural Networks (R-CNN) architecture with transfer learning. In future, EM and machine learning could be combined with microarray deposition and automated imaging for higher throughput quantitation of lipoproteins associated with CVD risk.

Identification and analyses of inhibitors targeting apolipoprotein(a) kringle domains KIV-7, KIV-10, and KV provide insight into kringle domain function

Increased plasma concentrations of lipoprotein(a) (Lp(a)) are associated with an increased risk for cardiovascular disease. Lp(a) is composed of apolipoprotein(a) (apo(a)) covalently bound to apolipoprotein B of low-density lipoprotein (LDL). Many of apo(a)'s potential pathological properties, such as inhibition of plasmin generation, have been attributed to its main structural domains, the kringles, and have been proposed to be mediated by their lysine-binding sites. However, available small-molecule inhibitors, such as lysine analogs, bind unselectively to kringle domains and are therefore unsuitable for functional characterization of specific kringle domains. Here, we discovered small molecules that specifically bind to the apo(a) kringle domains KIV-7, KIV-10, and KV. Chemical synthesis yielded compound AZ-05, which bound to KIV-10 with a K_d of 0.8 μm and exhibited more than 100-fold selectivity for KIV-10, compared with the other kringle domains tested, including plasminogen kringle 1. To better understand and further improve ligand selectivity, we determined the crystal structures of KIV-7, KIV-10, and KV in complex with small-molecule ligands at 1.6-2.1 Å resolutions. Furthermore, we used these small molecules as chemical probes to characterize the roles of the different apo(a) kringle domains in in vitro assays. These assays revealed the assembly of Lp(a) from apo(a) and LDL, as well as potential pathophysiological mechanisms of Lp(a), including (i) binding to fibrin, (ii) stimulation of smooth-muscle cell proliferation, and (iii) stimulation of LDL uptake into differentiated monocytes. Our results indicate that a small-molecule inhibitor targeting the lysine-binding site of KIV-10 can combat the pathophysiological effects of Lp(a).

Lipoprotein(a) Levels and the Risk of Myocardial Infarction Among 7 Ethnic Groups

BACKGROUND

Lipoprotein(a) [Lp(a)] levels predict the risk of myocardial infarction (MI) in populations of European ancestry; however, few data are available for other ethnic groups. Furthermore, differences in isoform size distribution and the associated Lp(a) concentrations have not fully been characterized between ethnic groups.

METHODS

We studied 6086 cases of first MI and 6857 controls from the INTERHEART study that were stratified by ethnicity and adjusted for age and sex. A total of 775 Africans, 4443 Chinese, 1352 Arabs, 1856 Europeans, 1469 Latin Americans, 1829 South Asians, and 1221 Southeast Asians were included in the study. Lp(a) concentration was measured in each participant using an assay that was insensitive to isoform size, with isoform size being assessed by Western blot in a subset of 4219 participants.

RESULTS

Variations in Lp(a) concentrations and isoform size distributions were observed between populations, with Africans having the highest Lp(a) concentration (median=27.2 mg/dL) and smallest isoform size (median=24 kringle IV repeats). Chinese samples had the lowest concentration (median=7.8 mg/dL) and largest isoform sizes (median=28). Overall, high Lp(a) concentrations (>50 mg/dL) were associated with an increased risk of MI (odds ratio, 1.48; 95% CI, 1.32-1.67; P<0.001). The association was independent of established MI risk factors, including diabetes mellitus, smoking, high blood pressure, and apolipoprotein B and A ratio. An inverse association was observed between isoform size and Lp(a) concentration, which was consistent across ethnic groups. Larger isoforms tended to be associated with a lower risk of MI, but this relationship was not present after adjustment for concentration. Consistent with variations in Lp(a) concentration across populations, the population-attributable risk of high Lp(a) for MI varied from 0% in Africans to 9.5% in South Asians.

CONCLUSIONS

Lp(a) concentration and isoform size varied markedly between ethnic groups. Higher Lp(a) concentrations were associated with an increased risk of MI and carried an especially high population burden in South Asians and Latin Americans. Isoform size was inversely associated with Lp(a) concentration, but did not significantly contribute to risk.

Potent reduction of plasma lipoprotein (a) with an antisense oligonucleotide in human subjects does not affect ex vivo fibrinolysis

It is postulated that lipoprotein (a) [Lp(a)] inhibits fibrinolysis, but this hypothesis has not been tested in humans due to the lack of specific Lp(a) lowering agents. Patients with elevated Lp(a) were randomized to antisense oligonucleotide [IONIS-APO(a)_Rx] directed to apo(a) (n = 7) or placebo (n = 10). Ex vivo plasma lysis times and antigen concentrations of plasminogen, factor XI, plasminogen activator inhibitor 1, thrombin activatable fibrinolysis inhibitor, and fibrinogen at baseline, day 85/92/99 (peak drug effect), and day 190 (3 months off drug) were measured. The mean ± SD baseline Lp(a) levels were 477.3 ± 55.9 nmol/l in IONIS-APO(a)_Rx and 362.1 ± 89.9 nmol/l in placebo. The mean± SD percentage change in Lp(a) for IONIS-APO(a)_Rx was -69.3 ± 12.2% versus -5.4 ± 6.9% placebo (P < 0.0010) at day 85/92/99 and -15.6 ± 8.9% versus 3.2 ± 12.2% (P = 0.003) at day 190. Clot lysis times and coagulation/fibrinolysis-related biomarkers showed no significant differences between IONIS-APO(a)_Rx and placebo at all time points. Clot lysis times were not affected by exogenously added Lp(a) at concentrations up to 200 nmol/l to plasma with very low (12.5 nmol/l) Lp(a) levels, whereas recombinant apo(a) had a potent antifibrinolytic effect. In conclusion, potent reductions of Lp(a) in patients with highly elevated Lp(a) levels do not affect ex vivo measures of fibrinolysis; the relevance of any putative antifibrinolytic effects of Lp(a) in vivo needs further study.

Impact of Apolipoprotein(a) Isoform Size on Lipoprotein(a) Lowering in the HPS2-THRIVE Study

Supplemental Digital Content is available in the text.

Background:

Genetic studies have shown lipoprotein(a) (Lp[a]) to be an important causal risk factor for coronary disease. Apolipoprotein(a) isoform size is the chief determinant of Lp(a) levels, but its impact on the benefits of therapies that lower Lp(a) remains unclear.

Methods:

HPS2-THRIVE (Heart Protection Study 2–Treatment of HDL to Reduce the Incidence of Vascular Events) is a randomized trial of niacin–laropiprant versus placebo on a background of simvastatin therapy. Plasma Lp(a) levels at baseline and 1 year post-randomization were measured in 3978 participants from the United Kingdom and China. Apolipoprotein(a) isoform size, estimated by the number of kringle IV domains, was measured by agarose gel electrophoresis and the predominantly expressed isoform identified.

Results:

Allocation to niacin–laropiprant reduced mean Lp(a) by 12 (SE, 1) nmol/L overall and 34 (6) nmol/L in the top quintile by baseline Lp(a) level (Lp[a] ≥128 nmol/L). The mean proportional reduction in Lp(a) with niacin–laropiprant was 31% but varied strongly with predominant apolipoprotein(a) isoform size (P_Trend=4×10⁻²⁹) and was only 18% in the quintile with the highest baseline Lp(a) level and low isoform size. Estimates from genetic studies suggest that these Lp(a) reductions during the short term of the trial might yield proportional reductions in coronary risk of ≈2% overall and 6% in the top quintile by Lp(a) levels.

Conclusions:

Proportional reductions in Lp(a) were dependent on apolipoprotein(a) isoform size. Taking this into account, the likely benefits of niacin–laropiprant on coronary risk through Lp(a) lowering are small. Novel therapies that reduce high Lp(a) levels by at least 80 nmol/L (≈40%) may be needed to produce worthwhile benefits in people at the highest risk because of Lp(a).

Clinical Trial Registration:

URL: https://clinicaltrials.gov. Unique identifier: NCT00461630.

Apolipoprotein(a) is the Product of a Pseudogene: Implications for the Pathophysiology of Lipoprotein(a)

Apolipoprotein(a) [apo(a)] is an apolipoprotein unique to lipoprotein(a) [Lp(a)]. Although it has no known function, Lp(a) is a risk factor for accelerated atherothrombosis. We hypothesize that LPA, the gene which encodes apo(a), is a heretofore unrecognized unprocessed pseudogene created by duplication of PLG, the gene which encodes plasminogen. Unprocessed pseudogenes are genes which were created by duplication of functional genes and subsequently lost function after acquiring various mutations. This hypothesis explains many of the unusual features of Lp(a) and apo(a). Also, this hypothesis has implications for the therapy of elevated Lp(a) and atherothrombosis theory. Because apo(a) is functionless, the diseases associated with elevated levels of Lp(a) are due to its impact on blood viscosity.

Apolipoprotein(a) Antisense Oligonucleotides: A New Treatment Option for Lowering Elevated Lipoprotein(a)?

BACKGROUND

Lipoprotein(a) [Lp(a)] is a particle similar to LDL that contains an additional protein called apolipoprotein(a) [apo(a)]. Recent epidemiologic and Mendelian randomization studies have provided evidence that Lp(a) may be causally related to the pathogenesis of atherosclerosis and cardiovascular disease (CVD). While the risk association between Lp(a) concentrations and CVD is weak it seems to be continuous in shape and without an obvious threshold for Lp(a) levels.

METHODS

Circulating concentrations of Lp(a) are genetically determined and desirable levels are < 50 mg/dl. A plasma concentration of 60 mg/dl is associated with an odds ratio for coronary heart disease of about 1.5 after adjustment for other cardiovascular risk factors.

RESULTS

Extended-release niacin is the pharmacologic means of choice for decreasing elevated Lp(a) levels by ~20-30% but it is often poorly tolerated due to adverse reactions. Diet, exercise and lipid-lowering drugs such as statins, fibrates and ezetimibe are without effect. In patients with severe progressive CVD and very high Lp(a) levels, lipoprotein apheresis may be used to decrease Lp(a) concentrations. However, it is an expensive and impractical treatment for most patients and its feasibility depends on the healthcare reimbursement system of the respective country. Since no established treatment reduces Lp(a) without influencing other lipoproteins, there has been no trial examining whether decreasing Lp(a) concentrations translates to clinical benefits.

CONCLUSION

Recently, an antisense oligonucleotide against apo(a), IONIS-APO(a)Rx, has been shown to selectively decrease Lp(a) by ~80%. A phase 2 study with this drug has been completed in late 2015 and results are expected to be published soon.

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

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

Lipoprotein(a) and HIV: Allele-Specific Apolipoprotein(a) Levels Predict Carotid Intima-Media Thickness in HIV-Infected Young Women in the Women's Interagency HIV Study

OBJECTIVE

In the general population, lipoprotein(a) [Lp(a)] has been established as an independent causal risk factor for cardiovascular disease. Lp(a) levels are to a major extent regulated by a size polymorphism in the apolipoprotein(a) [apo(a)] gene. The roles of Lp(a)/apo(a) in human immunodeficiency virus (HIV)-related elevated cardiovascular disease risk remain unclear.

APPROACH AND RESULTS

The associations between total plasma Lp(a) level, allele-specific apo(a) level, an Lp(a) level carried by individual apo(a) alleles, and common carotid artery intima-media thickness were assessed in 150 HIV-infected and 100 HIV-uninfected women in the WIHS (Women's Interagency HIV Study). Linear regression analyses with and without adjustments were used. The cohort was young (mean age, ≈31 years), with the majority being Blacks (≈70%). The prevalence of a small size apo(a) (≤22 Kringle repeats) or a high Lp(a) level (≥30 mg/dL) was similar by HIV status. Total plasma Lp(a) level (P=0.029) and allele-specific apo(a) level carried by the smaller apo(a) sizes (P=0.022) were significantly associated with carotid artery intima-media thickness in the HIV-infected women only. After accounting for confounders (age, race, smoking, body mass index, blood pressure, hepatitis C virus coinfection, menopause, plasma lipids, treatment status, CD4⁺ T cell count, and HIV/RNA viral load), the association remained significant for both Lp(a) (P=0.035) and allele-specific apo(a) level carried by the smaller apo(a) sizes (P=0.010) in the HIV-infected women. Notably, none of the other lipids/lipoproteins was associated with carotid artery intima-media thickness.

CONCLUSIONS

Lp(a) and allele-specific apo(a) levels predict carotid artery intima-media thickness in HIV-infected young women. Further research is needed to identify underlying mechanisms of an increased Lp(a) atherogenicity in HIV infection.

Impact of lipoprotein(a) levels and apolipoprotein(a) isoform size on risk of coronary heart disease

OBJECTIVES

Observational and genetic studies have shown that lipoprotein(a) [Lp(a)] levels and apolipoprotein(a) [apo(a)] isoform size are both associated with coronary heart disease (CHD) risk, but the relative independence of these risk factors remains unclear. Clarification of this uncertainty is relevant to the potential of future Lp(a)-lowering therapies for the prevention of CHD.

METHODS

Plasma Lp(a) levels and apo(a) isoform size, estimated by the number of kringle IV (KIV) repeats, were measured in 995 patients with CHD and 998 control subjects. The associations between CHD risk and fifths of Lp(a) levels were assessed before and after adjustment for KIV repeats and, conversely, the associations between CHD risk and fifths of KIV repeats were assessed before and after adjustment for Lp(a) levels.

RESULTS

Individuals in the top fifth of Lp(a) levels had more than a twofold higher risk of CHD compared with those in the bottom fifth, and this association was materially unaltered after adjustment for KIV repeats [odds ratio (OR) 2.05, 95% confidence interval (CI) 1.38-3.04, P < 0.001]. Furthermore, almost all of the excess risk was restricted to the two-fifths of the population with the highest Lp(a) levels. Individuals in the bottom fifth of KIV repeats had about a twofold higher risk of CHD compared with those in the top fifth, but this association was no longer significant after adjustment for Lp(a) levels (OR 1.13, 95% CI 0.77-1.66, P = 0.94).

CONCLUSIONS

The effect of KIV repeats on CHD risk is mediated through their impact on Lp(a) levels, suggesting that absolute levels of Lp(a), rather than apo(a) isoform size, are the main determinant of CHD risk.

Inhibition of plasminogen activation by apo(a): role of carboxyl-terminal lysines and identification of inhibitory domains in apo(a)

Apo(a), the distinguishing protein component of lipoprotein(a) [Lp(a)], exhibits sequence similarity to plasminogen and can inhibit binding of plasminogen to cell surfaces. Plasmin generated on the surface of vascular cells plays a role in cell migration and proliferation, two of the fibroproliferative inflammatory events that underlie atherosclerosis. The ability of apo(a) to inhibit pericellular plasminogen activation on vascular cells was therefore evaluated. Two isoforms of apo(a), 12K and 17K, were found to significantly decrease tissue-type plasminogen activator-mediated plasminogen activation on human umbilical vein endothelial cells (HUVECs) and THP-1 monocytes and macrophages. Lp(a) purified from human plasma decreased plasminogen activation on THP-1 monocytes and HUVECs but not on THP-1 macrophages. Removal of kringle V or the strong lysine binding site in kringle IV10 completely abolished the inhibitory effect of apo(a). Treatment with carboxypeptidase B to assess the roles of carboxyl-terminal lysines in cellular receptors leads in most cases to decreases in plasminogen activation as well as plasminogen and apo(a) binding; however, inhibition of plasminogen activation by apo(a) was unaffected. Our findings directly demonstrate that apo(a) inhibits pericellular plasminogen activation in all three cell types, although binding of apo(a) to cell-surface receptors containing carboxyl-terminal lysines does not appear to play a major role in the inhibition mechanism.

Estrogen modulates plasminogen promoter activity

Postmenopausal women treated with estrogen hormone replacement therapy and female patients with hypoplasminogenemia receiving oral contraceptives show increasing plasminogen (PLG) concentrations. The elevated PLG levels are in contrast to the estrogen dependent decline of lipoptrotein(a) [Lp(a)], whose main protein component apolipoprotein(a) [APO(a)] is highly homologous to PLG in protein and gene structure and is also located in its immediate vicinity on chromosome 6q26. The intergenic region between both genes comprises several transcription-regulatory regions with enhancer sequences that increase the basal activity of the PLG core promoter. Using luciferase reporter assays we demonstrate that the minimal PLG promoter is insensitive to estrogen. However, an estrogen response element located 11.5 kb upstream of the PLG transcription start site is able to convey a dramatic estrogen-dependent elevation of PLG-minimal promoter driven reporter gene expression. In contrast, the activating effect of two additional enhancer elements, among them an DNase I hypersensitivity region that has been shown to regulate the APO(a) minimal promoter activity, is abrogated by estrogen. Thus, the identified estrogen-responsive elements provide a gene and tissue specific framework by which PLG expression is regulated and whose activity is orchestrated by yet unknown accessory factors.

Scavenger receptor-BI is a receptor for lipoprotein(a)

Scavenger receptor class B type I (SR-BI) is a multi-ligand receptor that binds a variety of lipoproteins, including high density lipoprotein (HDL) and low density lipoprotein (LDL), but lipoprotein(a) [Lp(a)] has not been investigated as a possible ligand. Stable cell lines (HEK293 and HeLa) expressing human SR-BI were incubated with protein- or lipid-labeled Lp(a) to investigate SR-BI-dependent Lp(a) cell association. SR-BI expression enhanced the association of both (125)I- and Alexa Fluor-labeled protein from Lp(a). By confocal microscopy, SR-BI was also found to promote the internalization of fluorescent lipids (BODIPY-cholesteryl ester (CE)- and DiI-labeled) from Lp(a), and by immunocytochemistry the cellular internalization of apolipoprotein(a) and apolipoprotein B. When dual-labeled ((3)H-cholesteryl ether,(125)I-protein) Lp(a) was added to cells expressing SR-BI, there was a greater relative increase in lipid uptake over protein, indicating that SR-BI mediates selective lipid uptake from Lp(a). Compared with C57BL/6 control mice, transgenic mice overexpressing human SR-BI in liver were found to have increased plasma clearance of (3)H-CE-Lp(a), whereas mouse scavenger receptor class B type I knockout (Sr-b1-KO) mice had decreased plasma clearance (fractional catabolic rate: 0.63 ± 0.08/day, 1.64 ± 0.62/day, and 4.64 ± 0.40/day for Sr-b1-KO, C57BL/6, and human scavenger receptor class B type I transgenic mice, respectively). We conclude that Lp(a) is a novel ligand for SR-BI and that SR-BI mediates selective uptake of Lp(a)-associated lipids.

Apolipoprotein(a) acts as a chemorepellent to human vascular smooth muscle cells via integrin αVβ3 and RhoA/ROCK-mediated mechanisms

Lipoprotein(a) (Lp(a)) is an independent risk factor for the development of cardiovascular disease. Vascular smooth muscle cell (SMC) motility and plasticity, functions that are influenced by environmental cues, are vital to adaptation and remodelling in vascular physiology and pathophysiology. Lp(a) is reportedly damaging to SMC function via unknown molecular mechanisms. Apolipoprotein(a) (apo(a)), a unique glycoprotein moiety of Lp(a), has been demonstrated as its active component. The aims of this study were to determine functional effects of recombinant apo(a) on human vascular SMC motility and explore the underlying mechanism(s). Exposure of SMC to apo(a) in migration assays induced a potent, concentration-dependent chemorepulsion that was RhoA and integrin αVβ3-dependent, but transforming growth factor β-independent. SMC manipulation through RhoA gene silencing, Rho kinase inhibition, statin pre-treatment, αVβ3 neutralising antibody and tyrosine kinase inhibition all markedly inhibited apo(a)-mediated SMC migration. Our data reveal unique and potent activities of apo(a) that may negatively influence SMC remodelling in cardiovascular disease. Circulating levels of Lp(a) are resistant to lipid-lowering strategies and hence a greater understanding of the mechanisms underlying its functional effects on SMC may provide alternative therapeutic targets.

Polymorphism in apolipoprotein(a) kringle IV 37(Met/Thr): frequency in a London population and its association with coronary artery disease

BACKGROUND

A raised concentration of lipoprotein(a) [Lp(a)] in human plasma has been considered as a risk factor for coronary artery disease (CAD). Apolipoprotein(a) and plasminogen genes are exceptionally similar to a variable number of plasminogen-like kringle IV repeats in the apo(a) gene. Polymorphisms have been previously identified in the apolipoprotein(a) kringle IV 37.

HYPOTHESIS

In order to determine the frequency of the apolipoprotein(a) kringle IV 37 Met66-->Thr polymorphism in a London-based population and to assess the relationship of this polymorphism with CAD in Caucasian patients, we genotyped two groups of people of different ethnic origin (Caucasian and Afro-Caribbean) for the mutation using standard polymerase chain reaction (PCR) techniques.

METHODS

The first group consisted of 182 unrelated Caucasian patients (107 men and 75 women, mean age 59.7 +/- 10.2 years) recruited at St. George's Hospital. They were defined as patients with 0, 1 or > or = 2 vessel disease patients depending on the degree of stenosis in none, one, or several major epicardial arteries. The second group comprised 64 unrelated patients of Afro-Caribbean origin attending a hypertension clinic at St. George's Hospital.

RESULTS

It was shown that the prevalence of the Met66-->Thr mutation is markedly higher in Caucasians than in Afro-Caribbeans and that this mutation is not associated with either Lp(a) levels or severity of CAD.