Lp(a) lipoprotein enters cultured fibroblasts independently of the plasma membrane low density lipoprotein receptor

Lp(a) lipoprotein shares the apoB antigen with low density lipoprotein (LDL). The Lp(a) antigen is unique for Lp(a) lipoprotein. Fibroblast association (i.e. plasma membrane binding plus intracellular accumulation), plasma membrane binding, intracellular accumulation and degradation of 125I-Lp(a) lipoprotein were studied in strains from subjects with or without autosomal dominant hypercholesterolemia (HC). Subjects without HC (non-HCs) have cell surface receptors for low density lipoprotein (LDL receptors). On the average, HC heterozygotes have half-normal LDL receptor activity and "receptor-negative" HC homozygous cell strains lack functional receptors. Fibroblast processing of 125I-Lp(a) lipoprotein was compared to fibroblast processing of 125I-LDL. LDL receptor-dependent processing of 125I-LDL was saturated at about 50 microgram apo 125I-LDL.ml-1 in non-HC fibroblasts. 125I-Lp(a) lipoprotein was, however, largely processed independently of receptor mechanisms by non-HC cells (highest concentration examined 150 microgram apo 125I-Lp(a) lipoprotein . ml-1). Lp(a) lipoprotein did not interfere with 125I-LDL for fibroblast association, but inhibited 125I-LDL degradation. The interference with 125I-LDL degradation was time dependent. Only slightly higher 125I-Lp(a) lipoprotein processing values were found in non-HC and HC heterozygous strains than in "receptor-negative" HC homozygous strains. However, non-HC cells had more than tenfold higher 125I-LDL processing values than "receptor-negative" HC homozygous cells.     

The lysine-binding function of Lp(a)

The atherogenicity of Lp(a) is attributable to the binding of its apolipoprotein(a) component to fibrin and other plasminogen substrates. It can attenuate the activation of plasminogen, diminishing plasmin-dependent fibrinolysis and transforming growth factor-β activation. Apolipoprotein(a) contains a major lysine-binding site in one of its kringle domains. Destroying this site by site-directed mutagenesis greatly reduces the binding of apolipoprotein(a) to lysine and fibrin. Transgenic mice expressing wild-type apolipoprotein(a) have a 5-fold increase in the development of lipid lesions, as well as a large increase in the focal deposition of apolipoprotein(a) in the aorta, compared to the lysine-binding site mutant strain and to non-transgenic litter mates. Although the adaptive function of apolipoprotein(a) remains obscure, a gene with similar structure has evolved by independent remodeling of the plasminogen twice during the course of mammalian evolution.     

In vitro studies of the interaction of calcium ions and other divalent cations with the Lp(a) lipoprotein and other isolated serum lipoproteins

The interaction of isolated Lp(a) lipoprotein with different divalent cations was studied and compared to that of other isolated lipoprotein classes.
Purified Lp(a) lipoprotein was found to be most sensitive to the metal ions tested, and the Lp(a) lipoprotein was the only lipoprotein which was precipitated by calcium ions alone. The precipitation apparently depends on the ionic radii of the cations used as well as on the lipoprotein class tested. The precipitation reaction between calcium ions and the Lp(a) lipoprotein, and the interaction between calcium ions and LDL (without precipitation) seem to follow the known rules for small ion - macromolecule interaction reasonably well. The calcium ion - Lp(a) lipoprotein interaction results in a small aggregate. The binding is of ionic type and the precipitation reaction is initially reversible. It was estimated that LDL particles have a mean of 290 equivalent and non-interacting binding sites for calcium ions.
The above observations concerning the Lp(a) lipoprotein may be of interest in view of the significantly higher frequency of early coronary heart disease in Lp(a+) than in Lp(a-) individuals, and in view of the previously reported biochemical differences between individuals of different Lp phenotype.     

Importance of Lp(a) lipoprotein and HLA genotypes in atherosclerosis and diabetes

Lp(a) lipoprotein [Lp(a)] was found in previous studies to be independently associated with early atherosclerosis and its sequelae. Lp(a) in vitro bound to glucosaminoglycans and was easily aggregated at physiological Ca²⁺ concentration, and small Lp(a) aggregates were phagocytosed by macrophages. Lp(a) was also found to be related to carbohydrate metabolism, and increased Lp(a) levels have been described in diabetic patients with clinical complications and were recently found in rheumathoid arthritis patients. In this study of nondiabetic male patients with documented CAD before 50 years of age and controls, a significant correlation was found between Lp(a) and IGF-1 levels. HLA class II DR13 (DR6) was more frequent and DR15 (DR2) was less frequent in patients than in controls. The calculated relative risk for CAD was 4.0 for DR17 (DR3), but the difference was not significant. These differences seem to be related to high Lp(a) levels. It is suggested that phagocytosis of preferably Lp(a) aggregates can induce an immunological tissue response that may contribute in the pathogenesis of Lp(a)-associated diseases and may be more prominent in combination with some inherited HLA class II haplotypes. Probably due to sex hormone effects, the association may be most pronounced in young males and in older females.     

Analysis of the mechanism of lipoprotein(a) assembly

We have assessed the ability of a battery of purified recombinant apolipoprotein(a) (r-apo(a)) derivatives to bind to immobilized low-density lipoprotein (LDL) by ELISA. Removal of the apo(a) kringle IV type 8 and type 9 sequences dramatically reduced apo(a) binding to LDL. The binding of apo(a) to LDL was effectively inhibited by arginine, lysine, the lysine analogue ε-aminocaproic acid and proline; comparable inhibition was observed using the 17K and KIV_5–8 r-apo(a) derivatives, suggesting a direct role for sequences contained in the latter species in mediating the initial non-covalent interactions which precede specific disulfide bond formation. We also determined that r-apo(a) binds directly to a synthetic apoB peptide spanning amino acid residues 3732–3745; this interaction appeared to be mediated by sequences present in apo(a) kringle IV types 8 and 9, and could be inhibited by arginine, lysine and proline. The results of this study indicate that the efficiency of Lp(a) assembly is a direct function of the initial non-covalent interactions between apo(a) and LDL; in addition, these studies suggest that Cys3734 in apoB mediates covalent linkage with apo(a) by virtue of the ability of the apoB sequences surrounding this residue to directly interact with apo(a) KIV type 9.     

脂蛋白（a）及低密度脂蛋白与成纤维细胞表面结合特性的比较

脂蛋白（a)[Lipoprotein(a),Lp(a)]的代谢途径尚有争议。本文比较研究了Lp(a)和低密度脂蛋白（Low Density Lipoprotein,LDL)与成纤维细胞表面的结合特性,结果显示,Lp(a)与成纤维细胞呈特异性,高亲和力,可饱和性结合,但是Lp(a)与细胞的亲和力,最大结合容量均比LDL低,竞争性结合试验显示LDL只能部分抑制Lp(a)与成纤维细胞表面特异性的结合;细胞表面LDL受体活性的下调,可使LDL与细胞的结合量下降58％,而Lp(a)的结合量仅下降12．3％,这些结果表明LDL受体只能参与部分Lp(a)与细胞表面的结合及代谢。     

Restriction site polymorphism at the LPA (Lp(a) apoliprotein; apoliprotein(a)) locus

A restriction site polymorphism in the Lp(a) apolipoprotein gene (the LPA gene) is reported. The basis for the polymorphism is presence or absence of an MspI restriction site that appears to be 3' to the last kringle IV structure of the gene. The "1" gene (presence of the restriction site) has a frequency of 0.316 and the "2" gene (absence of the restriction site) has a frequency of 0.684. Both members of each of 67 monozygotic (MZ) twin pairs had the same genotype and there was Mendelian segregation of the DNA variants in 40 families with a total of 75 children. There was a lower proportion of people with genotype 1-1 in the top quartile than in the 3 bottom quartiles of the population distribution of Lp(a) lipoprotein levels but the difference did not reach statistical significance.     

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.     

Changes in Lp(a) lipoprotein and other plasma proteins during acute myocardial infarction

The sequential changes of Lp(a) lipoprotein concentrations in patients (n=59) suffering acute myocardial infarction (AMI) were examined and compared with other plasma proteins. The temporal and quantitative characteristics of the responses in concentration of acute phase reactants (CRP, haptoglobin, α₁-antitrypsin, α-acid glycoprotein), lipids (total cholesterol, triglycerides, HDL cholesterol, LDL cholesterol) and apolipoproteins AI and B were similar to previous reports. Lp(a) lipoprotein showed transient changes with an initial decrease of 10–25% compared to the 3-month control value, followed by rebound on day 7–11 above admission level, before again declining. We were able to demonstrate a quantitative relationship between infarct size and alterations in plasma levels of acute phase reactants. However, in addition to rather unusual significant fluctuations during AMI, Lp(a) lipoprotein changes seemed unrelated to infarct size. These findings do not support the view that Lp(a) lipoprotein acts as an acute phase reactant.     

Studies on the structure and function of the apolipoprotein(a) gene

Lp(a) is an LDL-like lipoprotein that is a major inherited risk factor for atherosclerosis. It is distinguished from Lp(a) by the addition of apolipoprotein(a). The gene structure of apolipoprotein(a) is homologous to plasminogen, and competition with plasminogen activity may account for some of the pathophysiology associated with Lp(a). Six highly related genes have now been identified, and at least four are found in close proximity in overlapping genomic clones. Studies have begun on the regulation of apolipoprotein (a) gene expression, and the human apolipoprotein(a) gene has been inserted into transgenic mice, where it leads to the development of arterial lesions.     

氧化修饰引起脂蛋白（a）结构和生化特性变化

本文观察了体外丙二醛（ＭＤＡ），铜离子（Ｃｕ２＋氧化修饰的脂蛋白（ａ）［Ｌｐ（ａ）］结构和生物学性质的变化。氧化修饰Ｌｐ（ａ）过氧化程度增高，负电荷增加，易被巨噬细胞—清道夫受体识别和摄取。ＭＤＡ修饰Ｌｐ（ａ）出现新的ＭＤＡ－ＬＤＬ位点；同纤维蛋白溶酶原（Ｐｇ）竞争抑制试验显示氧化、修饰Ｌｐ（ａ）同Ｐｇ同源性增加。提示载脂蛋白（ａ）状态同动脉粥样硬化的病理过程有关。     

The effect of percutaneous oestrogen replacement therapy on Lp(a) and other lipoproteins

Objectives: To determine the effect of percutaneous oestrogen replacement therapy on lipoprotein (a) and other plasma lipoproteins. Methods: Open longitudinal prospective study conducted at the hormone replacement clinic of the Prince of Wales Hospital, New Territories, Hong Kong. Thirty women who had undergone a total abdominal hysterectomy and bilateral salpingo-oophorectomy for benign gynaecological conditions were treated with 1.5 mg of percutaneous 17β-oestradiol gel applied daily for a period of 12 consecutive months. Measurements of plasma lipoproteins were made before the commencement of treatment and repeated at 6- and 12-month intervals. Results: There was a significant reduction in the concentrations of Lp(a) during the first 6 months of treatment, with median values falling from 7.87 mg dL⁻¹ to 6.16 mg dL⁻¹ (P = 0.004, 0–6 months). During the second 6 months, the median concentration increased to 9.38 mg dL⁻¹, (P = 0.072, 66-12 months), which did not significantly differ from the baseline level (P = 0.545, 0–12 months). Significant reductions in the concentrations of apoprotein A-I (apo A-I), apoprotein B (apo B), high density lipoprotein cholesterol (HDL-C), and HDL₃-C were also present after 6 months (P = 0.043, 0.049, 0.028, 0.013, respectively), but there were no differences between the baseline values of these lipoproteins and those at the completion of the study (P = 0.948, 0.244, 0.839, 0.117 respectively). Drug compliance was maintained throughout the study, with similar mean oestradiol concentrations at 6 and 12 months. Conclusions: The percutaneous administration of 17β-oestradiol has variable short term effects on plasma lipoproteins which are not maintained over a longer duration of treatment. By avoiding the ‘first pass’ effect on the liver, this method of delivery does not appear to produce the sustained changes in lipoproteins seen with oral treatment.     

Lp(a) phenotypes, other lipoprotein parameters, and a family history of coronary heart disease in middle-aged males

In a study of 95 presumably healthy, 40-42-year old males from Northen Sweden, the Lp(a) phenotype distribution differed between those who had, and those who did not have one or more close relatives (parent or sib) with coronary heart disease. In the former group, 60% of the males were Lp(a+), as opposed to 28% in the latter group. Thus, in the homogeneous population sample studied, analysis of the normal inherited Lp(a) variation permitted the identification of distinct subpopulations, with respect to familial occurrence of coronary heart disease. None of a series of other parameters distinguished such sub-populations. The results reported are in agreement with our previous finding of a close association between phenotype Lp(a+) and risk of contracting coronary heart disease.     

Interaction between low density lipoprotein receptor (LDLR) and apolipoprotein E (apoE) alleles contributes to normal variation in lipid level

Subjects drawn from a population-based register were studied with respect to lipid level association. The association of isoforms of apolipoprotein E (apoE) with lipid level in the general population was found to be limited to people with one particular genotype at the low density lipoprotein receptor (LDLR) locus. The results presented in this paper suggest that functional LDLR variants enhance or limit the effect of isoforms of apoE. The association between apoE4 and serum total and LDL cholesterol level may be mediated through the LDL (apoB100, apoE) receptor to a greater extent than previously thought.     

In vitro studies of the interaction of isolated Lp(a) lipoprotein and other serum lipoproteins with glycosaminoglycans

The interaction of isolated Lp(a) lipoprotein or other lipoprotein classes with different glycosaminoglycans (GAG) bound to activated Sepharose was studied. In contrast to LDL, the Lp(a) lipoprotein did not bind to the GAG tested if sodium was used as a buffer cation. In the presence of Ca++, however, even the Lp(a) lipoprotein was bound to GAG. This type of binding, probably mediated by divalent cation bridges, is apparently not a simple function of the GAG used. Addition of GAG in solution revealed that this binding may be the only one existing under physiological conditions, and it appears possible that the Lp(a) lipoprotein is bound more firmly to GAG than is LDL under such conditions.     

The inheritance of sinking-pre-beta lipo-protein and its relation to the Lp(a) antigen*

We have investigated the genetics of plasma sinking-pre-beta lipoprotein (spβ) as determined by the method of Breckenridge and Maguire, using several approaches: (i) a population study, (ii) a twin study and (iii) the use of family data. In addition, by the use of split samples, the spβ level as determined by us was correlated with Lp(a) typing carried out in Oslo by Dr. Kåre Berg. Although the spβ level is a continuous character, the results clearly showed it to be to a considerable extent under the control of the major autosomal gene pair constituted by the alleles Lp^a and Lp, which mainly control the production of the Lp(a) antigen, Lp^a being dominant. In our data the boundary between the LpLp and Lp^a Lp genotypes appeared to fall between the spβ 2 and 3 mg% levels, while that between Lp^a Lp and Lp^a Lp^a was in the 15 mg% region. These boundaries, which were inferred from both typing and population statistics, received good confirmation from the family data. It appears that some 88% of the variation in spp level is directly ascribed to segregation of Lp^a and Lp. On the basis of the twin study and other data, we conclude that the residual observed variation in spβ is almost entirely ascribed to analytical error of determination and polygenic effects, the influence of environment being negligible. The heritability is close to 100%.     

Severe coronary disease in an adult considered at low cardiovascular disease risk with a healthy lifestyle

Lipoprotein(a) [Lp(a)] is a lipoprotein subclass well-known among the lipid community to accelerate atherosclerosis and promote thrombosis through incompletely understood mechanism. We report a case of a young man with a healthy lifestyle and no major coronary or vascular risk factors who presented to the emergency department with an acute coronary syndrome and was ultimately found to have severe coronary artery disease. A diagnostic workup revealed elevated Lp(a). He was treated with consequent reduction in Lp(a) concentration. This case highlights the need to better understand atypical lipoproteins, how they relate to cardiovascular disease, the implications for screening family members, and the need to standardize patient management guidelines for the purpose of mortality risk reduction.     

Normal DNA polymorphism at the low density lipoprotein receptor (LDLR) locus associated with serum cholesterol level

A restriction fragment length polymorphism (RFLP) at the low density lipoprotein receptor (LDLR) locus detectable with the restriction enzyme PvuII exhibits association with total serum cholesterol level. People who are homozygous for absence of the PvuII restriction site have a significantly higher total cholesterol level than heterozygotes (the number of homozygotes for presence of the restriction site was too small to permit meaningful comparison). This difference is significant at the 2% level. Thus, this study of sex- and age-adjusted cholesterol levels in a sample of healthy people yields additional evidence and sustains our previous proposal that normal alleles at the LDLR locus contribute to the population variation in total cholesterol levels. Absence of the PvuII site appears to confer an odds ratio of approximately 2.7 for having a cholesterol level in the top quartile of the population distribution.