High Plasma Levels Of Lipoprotein Research Articles

Elevated Lipoprotein(a) and Risk of Ischemic Stroke

BackgroundHigh lipoprotein(a) is associated with increased risk of myocardial infarction and aortic valve stenosis. Previous studies have examined the association of lipoprotein(a) and risk of stroke; however, the results are conflicting. ObjectivesThe purpose of this study was to test if high lipoprotein(a) is associated with high risk of ischemic stroke observationally and causally from human genetics. MethodsThe study included 49,699 individuals from the Copenhagen General Population Study and 10,813 individuals from the Copenhagen City Heart Study with measurements of plasma lipoprotein(a), LPA kringle-IV type 2 number of repeats, and LPA rs10455872. The endpoint of ischemic stroke was ascertained from Danish national health registries and validated by medical doctors. ResultsCompared with individuals with lipoprotein(a) levels <10 mg/dl (<18 nmol/l: first to 50th percentile), the multivariable-adjusted hazard ratio for ischemic stroke was 1.60 (95% confidence interval [CI]:1.24 to 2.05) for individuals with lipoprotein(a) levels >93mg/dl (>199 nmol/L: 96th to 100th percentile). In observational analyses for a 50 mg/dl (105 nmol/l) higher lipoprotein(a) level the age- and sex-adjusted hazard ratio for ischemic stroke was 1.20 (95% CI: 1.13 to 1.28), while the corresponding age- and sex-adjusted genetic causal risk ratio for KIV-2 number of repeats was 1.20 (95% CI: 1.02 to 1.43) and for rs10455872 was 1.27 (95% CI: 1.06 to 1.51). The highest absolute 10-year risk of ischemic stroke was 17% in active smoking individuals >70 years of age with hypertension and lipoprotein(a) levels >93 mg/dl (>199 nmol/l: 96th to 100th percentile). In the Copenhagen City Heart Study, risk estimates for high levels of lipoprotein(a) were in the same direction but did not reach statistical significance. ConclusionsIn a large contemporary general population study, high plasma levels of lipoprotein(a) were associated with increased risk of ischemic stroke both observationally and causally from human genetics.

Abstract 390: Human Aortic Valve Interstitial Cells Exhibit Decreased Cholesterol Efflux Associated with Hypo- α1-nascent HDL Particles

Objectives: High plasma levels of lipoprotein (a) (Lp(a)) are associated with increased risk of aortic valve stenosis. The purpose of this study was to determine ability of human aortic valve interstitial cells (HAVICs) to generate HDL particles from plasma of patients with high Lp(a), and understand the mechanism involved. Methods: HAVICs were isolated from consented participants and cultured in DMEM, 10% FBS, 5% glucose. ABCA1 proteins expression was tested in HAVICs lyses after detection by polyclonal anti-ABCA1 antibody. Cholesterol efflux was determined in HAVICs against HDL isolated by PEG from patients with aortic valve stenosis exhibiting mild or severe plasma Lp(a), compared to normolipidemic controls. FPLC and 2D-PAGGE were used to illustrate HDL profiles from these patients after incubation with HAVICs. Results: Our data showed abundant expression of ABCA1, and a significant decrease in HAVICs cholesterol efflux (5%, for 8h) in response to HDL from plasma of patients with high Lp(a) and aortic valve stenosis (mild 16±0.01%cpm, severe 23.4±0.21%cpm) compared to normolipidemic HDL (29.57cpm; P&lt;0.01). This was consistent with FPLC profiles indicating a similar reduction in in cholesterol within nascent HDL (nHDL) when compared to nHDL generated from HDL isolated from plasma of normal controls. Importantly, we noticed a generation of smaller population of HDL particles formed following cholesterol efflux. Plasma from patient with aortic valve stenosis showed a distinct decrease in preβ- and α1-migrating HDL, but no in other α-migrating HDL particles, when compared to plasma from normolipidemic controls. We also observed a defect in α1-HDL in the nHDL generated from patient with high Lp(a) and aortic valve stenosis compared to nHDL from plasma of normal controls. Conclusion: These findings demonstrate a defect in HAVICs ABCA1 cholesterol removal associated a loss of α1 species of nHDL in patient with high Lp(a) and aortic valve stenosis. Therefore, the HAVICs cholesterol efflux defect associated with the elevated plasma Lp(a) may explain the pathological changes associated aortic valve calcification.

An Apolipoprotein(a) Peptide Delays Chylomicron Remnant Clearance and Increases Plasma Remnant Lipoproteins and Atherosclerosis In Vivo

Humans with high expression of apolipoprotein(a) [apo(a)] and high plasma levels of lipoprotein(a) [Lp(a)] are at increased risk for atherosclerosis, but the mechanism is not known. We have previously shown that the KIV(5-8) domain of apo(a) has unique cell-surface binding properties, and naturally occurring fragments of apo(a) encompassing this domain are thought to be atherogenic in humans. To investigate the effect of KIV(5-8) on lipoprotein metabolism and atherosclerosis in vivo, we created several independent lines of liver-targeted KIV(5-8) transgenic mice. The transgenic mice have plasma apo(a) peptide concentrations that are similar to Lp(a) concentrations in humans at risk for coronary artery disease. Remarkably, the transgenic mice had a 2- to 4-fold increase in cholesterol-rich remnant lipoproteins (RLPs) when fed a cholesterol-rich diet, and a 5- to 20-fold increase in atherosclerosis lesion area in the aortic root. Using an in vivo clearance study, we found only slight differences in the triglyceride and apolipoprotein B secretion rates between the 2 groups of mice, suggesting an RLP clearance defect. Using an isolated perfused mouse liver system, we showed that transgenic livers had a slower rate of RLP removal, which was retarded further when KIV(5-8), full-length apo(a), or Lp(a) were added to the perfusate. An apo(a) peptide that does not interact with cells, K(IV2)3, did not retard RLP removal, and low-density lipoprotein (LDL) had a much smaller effect than Lp(a). We propose that high levels of apo(a)/Lp(a), perhaps acting via a specific cell-surface binding domain, inhibit hepatic clearance of remnants, leading to high plasma levels of RLPs and markedly enhanced atherosclerosis. We speculate that the KIV(5-8) region of apo(a) competes with one or more receptors for remnant clearance in the liver and that this process may represent one mechanism accounting for increased atherosclerosis in humans with high secretion levels of apo(a).

Multiple Liver-Specific Factors Bind to a 64-bp Element and Activate Apo(a) Gene

The high plasma levels of lipoprotein(a) [Lp(a)] are associated with atherosclerosis. The apo(a) gene is responsible for the variance of Lp(a) concentration and its expression is liver-specific. By 5′-deletion analysis, we, in a luciferase gene reporter assay, have identified a 64-bp AT-rich region of upstream apo(a) gene (−703 to −640) that binds to multiple liver-specific factors. The 64 bp cis-element contained three dyad symmetry elements (DSEs) that are crucial for synergistic binding to the factors. We have demonstrated that both DSE-2 and -3 together are responsible for factor binding in vitro, and for gene activation in liver cells. Further, we have purified one of the UV cross-linked DNA–protein complexes to homogeneity by streptavidin magnetic bead chromatography. The identification of a further upstream negative regulatory region (−1432 to −704) led us to predict that as yet unidentified transcriptional repressor(s) might also repress apo(a) gene transcription.

High Plasma Levels of Lipoprotein(a) in Uremic Patients are Related to Markers of Inflammation, and to Activated, not Impaired, Fibrinolysis

High Plasma Levels of Lipoprotein(a) in Uremic Patients are Related to Markers of Inflammation, and to Activated, not Impaired, Fibrinolysis -

Relationship between lipoprotein(a) levels, oxidative stress, and blood pressure levels in patients with essential hypertension.

High plasma levels of lipoprotein(a) [Lp(a)] are considered a risk factor for the development of coronary artery disease. In vitro experiments have shown that oxidized Lp(a) is able to impair the arterial endothelium-dependent dilation, thus suggesting a possible role of Lp(a) in the genesis of essential hypertension. The aim of our work was to investigate the correlation of blood pressure levels with plasma Lp(a) concentration, apo(a) isoform size, and peroxidative stress in patients with essential hypertension. The study was performed in 54 untreated hypertensive patients whose blood pressure was monitored for 24 h by ambulatory blood pressure monitoring. Lp(a) concentration was measured by a double monoclonal antibody-based enzyme immunoassay demonstrated to be insensitive to apo(a) size heterogeneity. Apo(a) isoforms were determined by a high-resolution SDS-agarose gel electrophoresis followed by immunoblotting. A significant correlation was found between Lp(a) levels and the night-time systolic and diastolic pressures (r=0.32, P<0.05 and r=0.30, P<0.05, respectively), as well as with the mean night-time fall in systolic and diastolic blood pressures (r=-0.28, P<0.05 and r=-0.29, P<0.05, respectively). These relationships were further potentiated when peroxidative stress data were taken into consideration (r=0.37 and r=0.40, P<0.01 for the night-time systolic and diastolic pressures, respectively and r=-0.34 and r=-0.38, P<0.01 for the night-time fall in systolic and diastolic blood pressures, respectively). Apo(a) isoform size did not affect these relationships. Our data suggest that Lp(a) and peroxidative stress may be involved as cofactors in essential hypertension, with a mechanism that remains to be elucidated.

EFFECT OF EXERCISE ON THE RELATIONSHIP BETWEEN APOLIPOPROTEIN(a) GENOTYPE AND LIPOPROTEIN(a) CONCENTRATION IN YOUNG ATHLETES 734

High plasma levels of Lipoprotein(a) (Lp(a)) are associated with increased risks of coronary heart disease. In sedentary adults, Lackner, C. et al. (1991) reported that the size of the apolipoprotein(a) gene(Apo(a)) is inversely related to the level of plasma Lp(a). The purpose of this study was to assess the influence of regular intensive training on the Apo(a) genotype / Lp(a) level in adolescents. Thirty elite young athletes (girls, n=8; boys, n=22) aged 15-18 years participated in this study. The population was divided into 2 groups: endurance athletes(n=16) and power athletes (n=14). Blood Lp(a) concentration was ascertained using an electroimmunodiffusion technique in agarose gel. Apo(a) genotypes were determined by means of pulsed field electrophoresis. No significant differences in Lp(a) levels were found, neither between boys and girls, nor between the two training groups. In the present study, genotype analysis revealed an inverse relationship between the size of theApo(a) allele and the Lp(a) plasma level (r =0.7, P = 0.0008). These data suggest that Lp(a) level is largely genetically determined, and rather unaffected by lifestyle variables such as regular physical activity.

Gemfibrozil and mediterranean diet for patients with high plasma levels of lipoprotein [Lp(a)] and cholesterol?Pilot study

Plasma levels of lipoprotein-a [Lp(a)] > 30 mg/dl represent an independent risk factor for cardiovascular diseases with both proatherosclerotic and prothrombotic activity. The results of dietary or pharmacological treatment are not encouraging and are often controversial. We have evaluated a combination of medical treatment with Gemfibrozil (600 mg bid) and a Mediterranean diet for 2 months in 15 patients with both hypercholesterolemia (> 240 mg/dl) and high levels of Lp(a) (> 30 mg/dl). Three patients dropped out within the first 2 weeks, complaining of epigastric pain and burning; the remainder (5 females and 7 males, mean age 70 years) completed the treatment without any side effects. The median values of Lp(a) decreased from 36.5 to 8.4 mg/dl (p < 0.0002) and total cholesterol from 254.5 to 208.0 mg/dl (p < 0.0001). The small number of patients does not permit any definitive conclusion on effectiveness to be drawn, but the results indicate further randomized studies might prove worthwhile.

Identification of Two Functionally Distinct Lysine-binding Sites in Kringle 37 and in Kringles 32−36 of Human Apolipoprotein(a)

The well documented association between high plasma levels of lipoprotein(a) (Lp(a)) and cardiovascular disease might be mediated by the lysine binding of apolipoprotein(a) (apo(a)), the plasminogen-like, multikringle glycoprotein in Lp(a). We employed a mutational analysis to localize the lysine-binding domains within human apo(a). Recombinant apo(a) (r-apo(a)) with 17 plasminogen kringle IV-like domains, one plasminogen kringle V-like domain, and a protease domain or mutants thereof were expressed in the human hepatocarcinoma cell line HepG2. The lysine binding of plasma Lp(a) and r-apo(a) in the culture supernatants of transfected HepG2 cells was analyzed by lysine-Sepharose affinity chromatography. Wild type recombinant Lp(a) (r-Lp(a)) revealed lysine binding in the range observed for human plasma Lp(a). A single accessible lysine binding site in Lp(a) is indicated by a complete loss of lysine binding observed for r-Lp(a) species that contain either a truncated r-apo(a) lacking kringle IV-37, kringle V, and the protease or a point-mutated r-apo(a) with a Trp-4174-->Arg substitution in the putative lysine-binding pocket of kringle IV-37. Evidence is also presented for additional lysine-binding sites within kringles 32-36 of apo(a) that are masked in Lp(a) as indicated by an increased lysine binding for the point mutant (Cys-4057-->Ser), which is unable to assemble into particles. An important role of these lysine-binding site(s) for Lp(a) assembly is suggested by a decreased assembly efficiency for deletion mutants lacking either kringle 32 or kringles 32-35.

Molecular basis for “null” lipoprotein(a) phenotypes and the influence of apolipoprotein(a) size on plasma lipoprotein(a) level in the baboon.

High plasma levels of lipoprotein(a) (Lp(a)) and its unique apolipoprotein, apo(a), are an independent risk factor for cardiovascular disease. Plasma Lp(a) levels vary over a 1000-fold range and are determined by the apo(a) locus, which has at least 34 alleles expressing apo(a) isoforms with molecular weights from < 300,000 to > 800,000. In addition, "null" apo(a) alleles produce no detectable plasma apo(a). We used primary cultures of baboon hepatocytes to investigate the molecular basis for null apo(a) phenotypes. Immunoprecipitation of apo(a) after radiolabeling of hepatocytes revealed that some null alleles gave rise to intracellular protein products that were not secreted. Pulse-chase analysis and endoglycosidase digests demonstrated that these proteins were retained in the endoplasmic reticulum. We also examined the molecular basis for the documented inverse correlation between apo(a) size and plasma Lp(a) concentration. Steady-state labeling and pulse-chase analysis of hepatocytes from animals expressing two isoforms of apo(a) revealed that the endoplasmic reticulum residence time of secreted apo(a) isoforms was determined by their size. This accounted for the inverse relationship between isoform size and level of secretion. We conclude that the efficiency of post-translational processing of apo(a) is a major determinant of plasma Lp(a) concentration.

Does apolipoprotein(a) heterogeneity influence lipoprotein(a) effects on fibrinolysis?

High plasma levels of lipoprotein(a) [Lp(a)] are considered to be an independent risk factor for premature cardiovascular disease and are inversely associated with apolipoprotein(a) [apo(a)] isoform sizes. The contribution of apo(a) polymorphism to the inhibition of fibrinolysis, a mechanism that may favor thrombus development, was therefore evaluated by measuring the ability of Lp(a) particles of distinct apo(a) isoform content to compete with plasminogen for fibrin binding during plasminogen activation by fibrin-bound tissue-type plasminogen activator. The rate of plasmin generation was most efficiently inhibited by an isoform with a molecular weight (M(r)) of approximately 540 Kd. An isoform with M(r) approximately 590 Kd produced a less pronounced effect, whereas the isoform with M(r) approximately 610 Kd failed to inhibit plasminogen activation. These effects were directly proportional to the amount of Lp(a) bound to the carboxy-terminal lysine residues of degraded fibrin. The relative affinity of the binding (kd range, 16 to 180 nmol/L) reflected the ability of individual Lp(a) isoforms to inhibit the binding of plasminogen (kd, 600 nmol/L). The question of the influence of kringle sequence variability on the binding to fibrin was not addressed by the present work. These data suggest that apo(a) isoform types with high affinity for fibrin may influence the ability of Lp(a) to interfere with fibrinolysis and contribute thereby to the association of elevated levels of Lp(a) with atherosclerotic and thrombotic risks.

Lipoprotein(a). Link between structure and pathology.

It is now established that high plasma levels of lipoprotein(a) are associated with an increased risk of atherosclerotic cardiovascular disease. However, the mechanisms underlying this increased risk have not been elucidated. Lipoprotein(a) represents a class of lipoprotein particles having a cholesteryl ester-rich low-density lipoprotein (LDL)-like structure with a protein moiety represented by apolipoprotein B100 covalently linked to apolipoprotein(a), the specific marker of lipoprotein(a). Lipoprotein(a) particles with a triglyceride-rich core have also been described. Apolipoprotein(a) is a glycoprotein containing about 30% carbohydrates by weight, with a polypeptide chain highly polymorphic in size (300-700 kDa) and structurally similar to plasminogen. There appears to be a relation between apolipoprotein(a) size and lipoprotein(a) species. From a number of studies, it is becoming apparent that lipoprotein(a) can transverse the endothelium and accumulate in the arterial initima either extra- or intracellularly. Immunochemical evidence has also indicated that apolipoprotein(a) in the artery wall is colocalized with fibrin(ogen), suggesting that this complexation may have an atherogenic potential by promoting the transformation of resident macrophages into foam cells. This might also occur by the chemical modification of lipoprotein(a) by the action of either oxygen-free radicals, malondialdehyde, or interactions with matrix components. These findings invite the speculation that much of the apolipoprotein B detected in atherosclerotic lesions is contributed by lipoprotein(a). The role that lipoprotein(a) size and density heterogeneity and apolipoprotein(a) polymorphism might play in the intima accumulation of apolipoprotein B is not established.