Interactions of low density lipoprotein2 and other apolipoprotein B-containing lipoproteins with lipoprotein(a)

High-level lipoprotein [a] expression in transgenic mice: evidence for oxidized phospholipids in lipoprotein [a] but not in low density lipoproteins

Efforts to elucidate the role of lipoprotein [a] (Lp[a]) in atherogenesis have been hampered by the lack of an animal model with high plasma Lp[a] levels. We produced two lines of transgenic mice expressing apolipoprotein [a] (apo[a]) in the liver and crossed them with mice expressing human apolipoprotein B-100 (apoB-100), generating two lines of Lp[a] mice. One had Lp[a] levels of approximately 700 mg/dl, well above the 30 mg/dl threshold associated with increased risk of atherosclerosis in humans; the other had levels of approximately 35 mg/dl. Most of the LDL in mice with high-level apo[a] expression was covalently bound to apo[a], but most of the LDL in the low-expressing line was free. Using an enzyme-linked sandwich assay with monoclonal antibody EO6, we found high levels of oxidized phospholipids in Lp[a] from high-expressing mice but not in LDL from low-expressing mice or in LDL from human apoB-100 transgenic mice (P <0.00001), even though all mice had similar plasma levels of human apoB-100. The increase in oxidized lipids specific to Lp[a] in high-level apo[a]-expressing mice suggests a mechanism by which increased circulating levels of Lp[a] could contribute to atherogenesis.

The effect of aggressive and moderate lowering of LDL-cholesterol and low dose anticoagulation on plasma lipids, apolipoproteins and lipoprotein families in post coronary artery bypass graft trial

The reported results (The Post Coronary Artery Bypass Graft Trial Investigators. The effect of aggressive lowering of low-density lipoprotein cholesterol levels and low-dose anticoagulation on obstructive changes in saphenous-vein coronary-artery bypass grafts. New Engl J Med 1997;336:153-162) of the Post Coronary Artery Bypass Graft (Post CABG) trial have shown that aggressive lowering was more effective than moderate lowering of low density lipoprotein (LDL) cholesterol in reducing the progression of atherosclerosis in saphenous-vein grafts (27 vs. 39%; P < 0.001); low dose warfarin had no effect on the progression of atherosclerosis. The present report describes the effect of long-term (an average of 4.3 years) aggressive treatment with high (40-80 mg/day) and moderate treatment with low (2.5-5 mg/day) doses of lovastatin on lipids, apolipoproteins (apo) and apoA- and apoB-containing lipoprotein families. To achieve the target LDL-cholesterol levels (60-85 mg/dl for aggressive group and 134-140 mg/dl for moderate group), cholestyramine (8 g/day) was given to 25% of subjects on aggressive and 5% of subjects on moderate treatment. Although with both treatment strategies there were significant decreases (P<0.001) in the levels of total cholesterol, LDL-cholesterol, apoB, LDL-apoB and cholesterol-rich Lp-B family, percent changes in the levels of these variables were greater in the aggressive- than in the moderate-treatment groups. These treatments had only marginal effects in increasing the levels of high density lipoprotein cholesterol, apoA-I and Lp-A-I and Lp-A-I:A-II families. The long-term aggressive treatment exerted no effect on the concentrations of triglycerides, apoC-IlI, apoC-III in VLDL + LDL and triglyceride-rich Lp-Bc families. Neither treatment affected the levels of Lp(a). The potentially modifying influence of warfarin and apoE phenotypes on lovastatin-induced changes in lipoprotein variables was found to be of little significance. It is likely that the beneficial effect of lovastatin in reducing the progression of atherosclerosis in grafts is mediated through its specific lowering effect on cholesterol-rich Lp-B particles.

Determination of free apolipoprotein(a) in serum by immunoassay and its significance for risk assessment in patients with coronary artery disease

This paper describes a new enzyme-linked ligand sorbent assay (ELLSA) to quantify free apolipoprotein(a) (apo(a)). The new test immobilizes free apo(a) utilizing a specific peptide that carries the amino acid sequence of a non-covalent apo(a) binding site on apoB3375-3405 (ligand-peptide). The ligand-peptide coupled to Sepharose was used in affinity chromatography to separate free apo(a) from whole serum. Isolated free apo(a) consisted of full length apo(a) and smaller apo(a). Additionally, free apo(a) levels determined by ELLSA as well as by electroimmunodiffusion correlated moderately well. Significantly increased serum concentrations of free apo(a) were found in coronary artery disease. The mean value of free apo(a) was three times higher in patients than in controls while the lipoprotein(a) (Lpla)) concentration was doubled. Utilizing receiver operating characteristic diagrams, it was shown that the free apo(a)-ELLSA had a better diagnostic test performance in atherosclerotic risk assessment than the Lp(a)-test: specificity free apo(a)-ELLSA 0.77, Lp(a)-test 0.81 [with (a:a)-enzyme immunoassay (EIA)] to 0.83 [with (a:B)-EIA]; sensitivity free apo(a)-ELLSA 0.57, Lp(a)-test 0.36 to 0.40. In conclusion, the new free apo(a)-ELLSA allows for the specific quantification of free apo(a). This provides an interesting indicator for atherosclerotic risk assessment.

Association of Lp(a) rather than integrally-bound apo(a) with triglyceride-rich lipoproteins of human subjects

The majority of apolipoprotein (a) [apo(a)] in plasma is characteristically associated with Lipoprotein (a) [Lp(a)], having a buoyant density (1.05-1.08 g/ml) intermediate between low density lipoproteins (LDL) and high density lipoproteins (HDL). In the fed (postprandial) state or in the presence of fasting (endogenous) hypertriglyceridemia, a small proportion of plasma apo(a) is found in the density < 1.006 g/ml fraction of plasma, associated with larger and less dense triglyceride-rich lipoproteins (TRL). In order to further characterize the presence of apo(a) in ultracentrifugally-separated TRL (UTC-TRL), this lipoprotein fraction was isolated from plasma obtained in the fed state (three hours after an oral fat load) from healthy normolipidemic subjects (Lp(a): 38 +/- 8 mg/dl (mean +/- S.E.), n = 4) and also from plasma obtained after an overnight fast from hypertriglyceridemic patients (plasma TG: 8.16 +/- 2.00 mmol/l, Lp(a): 41 +/- 3 mg/dl, n = 18). Apo(a) in 3 h-postprandial UTC-TRL (5 +/- 2% of total plasma apo(a)) and in hypertriglyceridemic UTC-TRL (8 +/- 2% total apo(a)) was separable by electrophoresis and/or gel chromatography (FPLC) from the majority of UTC-TRL lipid. Apo(a) in UTC-TRL fractions had slow pre-beta electrophoretic mobility and was isolated in a lipoprotein size-range smaller than VLDL and larger than LDL, consistent with it being Lp(a). Recentrifugation of UTC-TRL resulted in the majority of apo(a) being recovered in the density > 1.006 g/ml fraction. Addition of proline to plasma samples before ultracentrifugation (final concentration: 0.1 M) substantially reduced the amount of Lp(a) in UTC-TRL. TRL separated from plasma by FPLC contained less apo(a) (2-5% of total plasma apo(a)), but this apo(a) was also readily dissociable from TRL lipid, had slow pre-beta electrophoretic mobility, and was associated with a lipoprotein with the size of Lp(a). Our data suggest that apo(a) in the TRL fraction of subjects with postprandial triglyceridemia or endogenous hypertriglyceridemia is not an integral component of plasma VLDL or chylomicrons, but represents the presence of non-covalently bound Lp(a).

Lipoprotein(a) assembly. Quantitative assessment of the role of apo(a) kringle IV types 2-10 in particle formation

We have developed a system for the quantitative assessment of the efficiency of lipoprotein(a) [Lp(a)] formation in vitro. Amino-terminally truncated derivatives of a 17-kringle form of recombinant apo(a) [r-apo(a)] were transiently expressed in human embryonic kidney cells. Equimolar amounts of r-apo(a) derivatives were incubated with a fourfold molar excess of purified human low density lipoprotein, and r-Lp(a) formation was assessed by densitometric analysis of Western blots. Although r-Lp(a) formation was observed with each r-apo(a) derivative, both the rate and extent of particle formation were greatly lower on removal of kringle IV type 7. Additional substantial decreases in these parameters were observed on removal of kringle IV type 8, thereby suggesting a major role for these two kringles in Lp(a) assembly. We directly demonstrated that the lysine-binding sites (LBSs) within kringle IV types 5-9 are "masked" in the context of the Lp(a) particle and are consequently unavailable for interaction with lysine-Sepharose. Using site-directed mutagenesis, we also demonstrated that the previously described LBS in kringle IV type 10 is not required for r-Lp(a) formation: r-Lp(a) formation using a mutated form of apo(a) that lacks this LBS is comparable in efficiency to that of wild-type r-apo(a) and can be inhibited to a similar extent by epsilon-amino-n-caproic acid. In summary, the results of our study indicate that apo(a) kringle IV types 7 and 8 are required for maximal efficiency of Lp(a) formation, likely by virtue of their ability to mediate lysine-dependent non-covalent interactions with apoB-100 that precede disulfide bond formation.

Lipoprotein(a) induces cell growth in rat peritoneal macrophages through inhibition of transforming growth factor-beta activation

To elucidate the atherogenicity of lipoprotein(a) (Lp(a)), we examined its growth-stimulating activity in rat resident peritoneal macrophages. When macrophages were incubated with Lp(a), cell numbers were increased 1.5-fold as compared with control macrophages. Furthermore, apolipoprotein(a) (apo(a)), a plasminogen-like glycoprotein which is covalently attached to a low density lipoprotein-like particle (Lp(a)), also induced macrophage growth, while the growth-stimulating effect of Lp(a-) was negligible. These results suggest that apo(a) plays an active role in the mitogenic activity of Lp(a). Lp(a)-induced macrophage growth was inhibited by exogenously added active transforming growth factor-beta (TGF-beta) dose-dependently, and also by the addition of plasmin, which converts latent TGF-beta to an active form. Moreover, the amounts of endogenous active TGF-beta in the medium were significantly reduced by the incubation with Lp(a). It is evident from these results that Lp(a) induces macrophage growth by inhibiting TGF-beta activation. The capacity of Lp(a) to stimulate macrophage growth shown here could be novel atherogenic function of Lp(a).

Interrelationship of triglyceride-rich lipoproteins, serum lipoprotein (a) concentration and apolipoprotein(a) size

At present, the biochemical mechanisms underlying lipoprotein(a) (Lp(a)) metabolism are not fully understood. We analysed sera from 202 patients with atherosclerotic disease and 109 healthy subjects as a control group to investigate the possible relationship between triglyceride-rich lipoproteins (TRL) and serum lipoprotein(a) levels. To assess the influence of apolipoprotein (apo) (a) isoforms on the Lp(a)-TRL association, the apo(a) phenotypes of 177 patients and 95 controls were included in the analysis. Patients with atherosclerotic disease showed triglyceride levels almost within the normal range. There was no significant correlation between serum Lp(a) levels and triglyceride concentrations, or between Lp(a) and TRL levels in either group. When a subset of subjects from each group with serum triglycerides above 1.7 mmol l-1 was considered, a significant negative correlation between lipid concentration of very low density lipoproteins (VLDL) and serum Lp(a) levels was found only in patients. Control subjects with triglyceride levels under or over 1.7 mmol l-1 showed similar median Lp(a) levels (0.06 gl-1), in contrast to atherosclerotic patients, in whom median Lp(a) concentration was higher in the subset with serum triglycerides under 1.7 mmol l-1 than in those with triglyceride concentration above this value (0.16 vs. 0.13 gl-1). When patients with triglyceride concentrations above 1.7 mmol l-1 were classified into quartiles according to VLDL lipid concentration, subjects with the highest quartiles showed the lowest Lp(a) median levels. Despite the dependence of the Lp(a) concentration on apo(a) size isoforms, we found no effect of apo(a) genetic polymorphism on triglyceride levels or on TRL concentrations. We conclude that the variation in TRL metabolism may constitute a source of variation in serum Lp(a) concentrations that is independent of the genetically determined apo(a) molecule size.

Familial hypobetalipoproteinemia is not associated with low levels of lipoprotein(a)

To assess whether very low concentrations of LDL affected lipoprotein(a) [Lp(a)] concentrations and apo(a) associations with lipoproteins, we studied Lp(a) levels and associations in heterozygous subjects with familial hypobeta-lipoproteinemia FHBL) associated with several truncated forms of apoB-100, ranging from apoB-31 to apoB-89. Distributions of apo(a) isotypes were assessed by a combined electrophoresis-immunoblotting procedure that detects 34 isoforms. Lp(a) concentrations were quantified by two ELISAs, one detecting total apo(a) and the other apoB-bound apo(a) in plasma. Associations of apo(a) with plasma lipoproteins were evaluated by gel permeation chromatography (FPLC) and DGUC followed by analyses of elution and gradient fractions by apo(a) ELISA. In addition, associations were examined by nondenaturing electrophoresis or immunoprecipitation of whole plasma and examination of contents by immunoblotting. Finally, interactions between r-apo(a) and LDLs were evaluated in reconstitution experiments. The distributions of apo(a) isotypes did not differ between FHBL-affected and unaffected members of the same kindreds, and concentrations of Lp(a) were similar even when subjects were matched for isotypes both within and across kindreds. In subjects heterozygous for apo(a) isoforms, the smaller isoforms were inversely related to Lp(a) concentrations, the larger isoforms were not. The regression lines between Lp(a) concentrations and the smaller apo(a) isoforms were significant and negative in slope for both FHBL-affected and unaffected subjects, but the slopes of the lines did not differ. In multiple regression analyses, only the sizes of the smaller apo(a) isoforms contributed to the prediction of Lp(a) concentrations. ApoB-size made no difference. In simple apoB-100/apoB-truncation heterozygotes, virtually all apo(a) was complexed with apoB-100-containing particles but not apoB-truncation particles, and r-apo(a) recombined with apoB-100-containing LDLs but not with apoB-89-containing LDLs. Thus, (1) low apoB levels do not affect the plasma concentrations of Lp(a), (2) apo(a) binds apoB-100 to form Lp(a) particles of usual sizes and densities, and (3) apoB truncations even as large as apoB-89 do not form covalent bands with apo(a), although noncovalent associations with apoB-89 may be present in plasma.

Human Lp(a): regions in sequences of apoproteins similar to domains in signal transduction proteins

The major apoproteins of Lp(a)--apo(a) and apo B-100--are linked by only one intermolecular disulfide bond. This linkage has been suggested to be located between apo(a) Cys4057 and apo B-100 Cys3734. Several studies, however, have suggested other noncovalent interactions between different regions of apo(a) and apo B-100. One possible mechanism for these interactions may involve the apo(a) proline-rich interkringle regions that share sequence similarities with the proline-rich regions of Src homology 3 (SH3) domain-binding proteins such as 3BP-1. SH3 and SH2 domains, and their respective ligands, proline-rich regions, and phosphotyrosine motifs, are noncatalytic segments common to signal transduction proteins. Therefore, we used sequence comparison algorithms and molecular modeling programs to identify corresponding SH3 and SH2 candidate regions as well as potential phosphotyrosine sites in the apo B-100 sequence. Six SH2 and 16 SH3 candidate regions, along with 21 potential phosphotyrosine sites, are contained in the apo B-100 sequence. In Lp(a), these regions of apo B-100 may be involved in the noncovalent, protein-protein interactions between apo(a) and apo B-100. The presence of candidate SH3 and SH2 regions in apo B-100, and potential phosphotyrosine sites in apo B-100, apo(a), and apo A-I, suggests an alternative signaling pathway unrelated to the known B/E receptor.

Lipoprotein (a) in patients with hyperlipidaemia

Lipoprotein (a) [Lp(a)] is an atherogenic lipoprotein which is similar in structure to, but metabolically distinct from, LDL. Factors modulating plasma Lp(a) concentrations are poorly understood. We hypothesized that patients with hyperlipidaemia have elevated Lp(a) levels and determined the phenotype, concentration and distribution of Lp(a) in a group of hyperlipidaemic patients (n = 107) compared with a control group (n = 128). Lp(a) concentrations were significantly increased in the hyperlipidaemic patients (mean, 34 +/- 4 mg dL-1; median, 19 mg dL-1) as compared with the controls (20 +/- 3 mg dL-1; 9 mg dL-1) (P < 0.01). Interestingly, after dividing the patients into one group with elevated cholesterol (> 200 mg dL-1) (n = 44) and another group with elevated triglycerides (> 200 mg dL-1) (n = 51) we found that Lp(a) concentrations were 2.3-fold higher in the high cholesterol patients (mean, 45 +/- 5; median, 41 mg dL-1) compared to the high triglyceride subjects (20 +/- 4; 8 mg dL-1) (P < 0.01). Furthermore, a negative correlation between triglyceride and Lp(a) plasma concentrations was found in patients exhibiting triglyceride levels > 300 mg dL-1 (r = -0.41, P = 0.04, n = 36) and with triglycerides > 400 mg dL-1 (r = -0.52, P = 0.03, n = 17). These data indicate that plasma Lp(a) concentrations are elevated in hyperlipidaemia if the patients have high cholesterol levels, whereas Lp(a) is normal to low in patients with elevated triglycerides.(ABSTRACT TRUNCATED AT 250 WORDS)

A two-step model for lipoprotein(a) formation

Lipoprotein(a) (Lp(a)), a risk factor for coronary artery disease, is a LDL-like particle with apolipoprotein(a) (apo(a)) covalently linked to apolipoprotein B (apoB), the main protein component of LDL. Apo(a) is highly homologous to plasminogen and its gene probably arose by duplication of the plasminogen gene. It has many repeats of kringle-4-like domain, classified as type 1 through type 10 (T1-T10). T9 is responsible for the covalent linkage between apo(a) and LDL. However, we found that T9 has no affinity for LDL. Therefore, an initial noncovalent interaction between apo(a) and LDL is necessary to bring T9 and LDL together. T6 and possibly T7 of apo(a) were identified as the kringles which mediate this initial interaction. With these findings, a two-step model for Lp(a) formation is proposed. This model should be useful in the design of Lp(a) formation inhibitors. These inhibitors are potential antihyperlipoprotein(a) drugs.

Measurement of Lp(a) with a two-step monoclonal competitive sandwich ELISA method

OBJECTIVE

To evaluate the results of Lipoprotein (a)[Lp(a)] measurements by a competitive two-step monoclonal enzyme-linked immuno sorbent assay method comparing them with those by a conventional ELISA.

METHODS

Serum having various isoforms of Lp(a) and purified Lp(a) were assayed using the method described here and commercially available kits. The reference range was determined with the use of 324 normal subjects by means of calculation from Lp(a) results of logarithmic transformation.

RESULTS

Our method takes advantage of a competitive reaction between fixed antibody and free antibody to Lp(a), having the detection range up to 1000 mg/L with the lowest detection limit of 2 mg/L. The anti-Lp(a) monoclonal antibody employed in the assay system reacts uniformly with all phenotypes of Lp(a) but showing very low cross-reactivity for plasminogen and LDL. Within-run and between-run precisions were excellent, giving CVs of 2.9 and 4.0% with mean values of 145 and 635 mg/L, respectively. In comparison of the results by our method with those by a polyclonal method (Biopool) or a monoclonal antibody method (Terumo), they correlated well; Y (our method) = 0.99 x (polyclonal method, Biopool) - 1.9, r = 0.994 (n = 60), and Y = 0.94 X(monoclonal method, Terumo) -9.8, r = 0.97 (n = 60), respectively. The reference range was 105.9 +/- 25.4 mg/L, the difference between the sexes was not significant.

CONCLUSION

Our method has proven highly accurate and specific. It is applicable with auto analyzer because it does not require such a pre-dilution step as is necessary for Lp(a) determination by conventional ELISA assay. Accordingly, we can conclude that our test method is workable for both clinical laboratories and mass screening.

Decreased plasma levels of lipoprotein(a) in patients with hypertriglyceridemia

Lipoprotein(a) (Lp(a)) is an atherogenic lipoprotein which is similar in structure to, but metabolically distinct from, LDL. Factors modulating plasma Lp(a) concentrations are poorly understood. To investigate the possible interaction of Lp(a) with triglycerides, we determined the apo(a) phenotype, Lp(a) concentration, and distribution of Lp(a) in a group of patients with triglycerides > 400 mg/dl (n = 60) compared with a control group (n = 128). Lp(a) concentrations were significantly lower in hypertriglyceridemic patients (mean +/- S.E., 13 +/- 4 mg/dl; median, 6 mg/dl; 25/75 percentile, 2-13 mg/dl) as compared with the controls (mean, 22 +/- 2 mg/dl; median, 10 mg/dl; 25/75 percentile, 7-30 mg/dl). Plasma Lp(a) concentrations in the hypertriglyceridemic patients correlated negatively with triglyceride levels (r = -0.69, P = 0.03). The difference in Lp(a) levels between patients and controls was maintained when subjects were stratified by apo(a) phenotype and type of hyperlipidemia. After subdividing the hypertriglyceridemic patients into one group with apo(a) isoforms < or = S2 and one group with apo(a) isoforms > or = S3, we found that the differences in plasma Lp(a) concentrations between patients and controls were more pronounced in the group with the lower molecular weight apo(a) isoforms. These data indicate that hypertriglyceridemia is associated with lower plasma Lp(a) concentrations and suggest that increased levels of triglyceride-rich lipoproteins may influence the metabolism of Lp(a).

Characterization of monoclonal antibodies to apolipoprotein (a) and development of a chemiluminescent assay for phenotyping apolipoprotein (a) isomorphs

Apo(a) is linked to Lp(a) through non-covalent interactions and disulfide bond with apo B. Monoclonal antibodies were raised to reduced and carboxymethylated apo(a) in order to study apo(a) interaction with apo B and to develop a sensitive immunoassay for apo(a) and Lp(a). Nine antibodies were characterized for overlapping epitopes and for single or multiple binding sites on native Lp(a) or denatured apo(a). All monoclonal antibodies bound to Lp(a) and denatured apo(a) when these preparations were absorbed on polystyrene. In contrast, three antibodies (3D1, 4B4 and 6H9) failed to react with Lp(a) in solution, in a competitive displacement assay. This observation indicates that these epitopes are masked in native Lp(a). Cross-reactivity with plasminogen was noted for only one monoclonal antibody (4B4). An assay of competitive binding to immobilized Lp(a) or apo(a) revealed that four distinct groups of epitopes were recognized by the monoclonal antibodies: (A) 1G7, 3A5 partially overlapping with 8B6, (B) 5C4, 5B10 partially overlapping with 7C1, (C) 3D1 overlapping with 6H9, and (D) 4B4. A double antibody sandwich assay, using homologous and heterologous combinations of monoclonal antibodies, showed that monoclonal antibodies 1G7, 3A5 and 8B6 of group A, and 5C4 and 5B10 of group B recognized multiple epitopes on Lp(a) while all other antibodies (3D1, 6H9, 4B4) recognized single epitopes. Based on reports of others for the sequence of apo(a), deduced from the cDNA of the human apo(a) gene, it is proposed that monoclonal antibodies which recognize multiple epitopes are directed toward the repetitive kringle 4-like domains of apo(a) while those recognizing single epitopes are probably directed to the kringle 5 or the protease-like domain of apo(a). Monoclonal antibodies which recognized repetitive epitopes were used for the development of a highly sensitive chemiluminescent immunoblotting system for detection of apo(a) isomorphs after resolving plasma protein by polyacrylamide (4%) gel electrophoresis in the presence of sodium dodecyl sulfate. Seven relatively common isomorphs were identified and readily resolved as a mixture. The detection limit was 5-10 pg for each apo(a) isomorph. The high sensitivity allowed for the detection of isomorphs present in over 99% of plasma samples despite a wide range of ratios of apo(a) isomorphs.

Proteolytic degradation of low-density lipoprotein by lipoprotein(a) and by recombinant apo(a)

The plasma concentration of lipoprotein(a) (Lp(a)) is correlated with the risk of atherosclerosis, and both Lp(a) and LDL are present in atherosclerotic lesions. Lp(a) is similar in structure to LDL, its distinguishing feature from LDL being the presence of one additional glycoprotein, apo(a), that is linked to apoB-100. Upon incubation of 125I-LDL with isolated Lp(a), we found a dose and time-dependent increase in the proportion of TCA-soluble radioactive material, demonstrating degradation of LDL. The addition of unlabelled LDL decreased the degradation of 125I-LDL, while HDL or albumin had no such effect. Recombinant DNA-derived apo(a), R-apo(a), which itself expressed no amidolytic activity, displayed an increase in amidolytic activity after pre-incubation with LDL. Furthermore, activated R-apo(a) caused degradation of 125I-LDL. Treatment of R-apo(a) with phenylmethanesulfonyl fluoride inhibited LDL apoB-100 degradation, indicating that R-apo(a) has serine esterase type proteolytic activity. The results show that apo(a) is activated in the presence of LDL, and that this activation leads to proteolytic modification of LDL. The induction of apo(a) proteolytic activity by LDL suggests a novel mechanism whereby Lp(a) may be atherogenic.

Interaction of LDL and Lp[a] with human skin fibroblasts

We have studied the interaction of LDL and Lp[a] with fibroblasts. Our studies suggest that Lp[a] does not effectively compete with LDL for binding to the LDL receptor, and does not efficiently suppress the activity of the intracellular enzyme HMG-CoA reductase. However, Lp[a-], formed by reduction of the disulfide bond between apo[a] and apoB, behaves much like homologous LDL, whether or not apo[a] is removed from the mixture, and in spite of the fact that one or more apoB disulfides may also have been cleaved. In our studies we also noted that Lp[a] often enhanced binding of 125I-LDL by fibroblasts. Further investigation has suggested that this interaction is time-dependent. Experiments in receptor-negative fibroblasts indicate that the enhancement is not related to the presence of the LDL receptor; however, it is inhibited by the removal of calcium from the medium. The presence of sialic acid at millimolar concentrations in the medium inhibits much of the Lp[a]-enhanced binding of 125I-LDL to the cells. These studies suggest that Lp[] may in some way enhance LDL binding to cells, perhaps via interaction with cell surface glycosaminoglycans or proteoglycans or with collagen.

Triglyceride-rich lipoprotein interactions with Lp(a)

We found a significantly reduced incidence of increased lipoprotein(a) (Lp(a)) levels in subjects with triglycerides (TG) greater than 150 mg/dl compared with those with TG levels lower than 150 mg/dl. This was the case in patients with angiographically documented coronary artery disease (CAD) and in subjects with no CAD. We explored the potential role of lipoprotein lipase (LPL) in mediating this relationship. Lp(a) and LDL2 exhibited a minimal effect on the rate constant for degradation of VLDL-TG by LPL (13% inhibition). Binding analyses indicated no differences between VLDL and LDL with respect to Lp(a) binding, and lipolysis only reduced binding by 30% at 75% degradation of VLDL-TG. Our study indicates that the inverse relationship between elevated plasma TG and Lp(a) levels is not caused by activation of LPL by Lp(a) either due to failure of Lp(a) to bind to VLDL or its lipolytic remnants. It is hypothesized that this relationship could stem from the enhanced clearance of TG-rich lipoproteins in individuals with higher levels of Lp(a) by receptor-mediated events.

Comparison of ligand-binding sites of modeled apo[a] kringle-like sequences in human lipoprotein[a]

Human lipoprotein[a] contains at least two high-molecular-weight, disulfide-linked apolipoproteins, apo[a] and apo B-100. Apo[a] is a highly glycosylated, hydrophilic apoprotein that somewhat resembles plasminogen by containing an extended kringle domain and a carboxyl-terminal serine protease domain. The apo[a] kringle domain is composed of 11 distinct kringle types. Ten of these display high sequence homology to plasminogen kringle 4 (PGK4). The crystallographic coordinates for PGK4 were used to generate three-dimensional molecular models of the apo[a] kringle types, and the lysine-binding region of PGK4 was used to compare the different potential receptor-ligand and ligand-binding sites contained in each different PGK4-like kringle of apo[a]. A receptor-ligand site can be proposed for each kringle type. Potential serine protease cleavage sites, containing arginine-threonine and threonine-arginine, are located on the surface of the kringles. The ligand-binding site of one apo[a] kringle model is almost identical to that of PGK4 and may be a lysine-binding site of apo[a]. Four other apo[a] kringle models appear to have structurally similar lysine-binding sites, but with differences that may influence ligand-polypeptide specificity. Five apo[a] kringle models have ligand-binding sites that probably do not bind lysine; one of these is the highly repeated kringle in the known apo[a] polymorph.

Apolipoprotein(a) is present in the triglyceride-rich fraction in type IV hypertriglyceridemia

This study was designed to quantify the levels of apo(a) and assess their distribution among lipoprotein fractions in type IV hypertriglyceridemia. Plasma density < 1.006 g/mL fraction (VLDL) was obtained by preparative and zonal ultracentrifugation. Apo(a) was detected by immunoblotting with anti-apo(a) antibodies of agarose gel electrophoresed lipoproteins. Apo(a) was consistently found in VLDL in 30 hypertriglyceridemic subjects, but not in 10 normolipidemic or 10 hypercholesterolemic subjects with normal triglyceride levels. Apo(a)B particle concentrations were then measured using a selective 'sandwich' ELISA with anti-apo(a) as the capture antibody and anti-apoB as the detecting antibody. The mean apo(a)B level of VLDL in six hypertriglyceridemic patients was 30% (range 1.3-50%) of the total plasma concentration. Apo(a)-containing particles of both plasma, d < 1.006 g/mL and d > 1.006 g/mL, had the same pre-beta mobility on agarose gel electrophoresis and a similar apparent molecular weight on non-denaturing 2-16% gradient polyacrylamide gel electrophoresis. Fractionation of plasma by zonal ultracentrifugation confirmed the presence of a heterogeneous distribution of apo(a) in hypertriglyceridemic subjects. Apo(a) was present throughout the density range from VLDL to Lp(a). After normalization of plasma triglyceride levels with dietary fish oil, apo(a) was no longer detected in VLDL suggesting that detection of apo(a) in the plasma, d < 1.006 g/mL, density fraction is related to type IV hypertriglyceridemia.

Isolation and characterization of two sub-species of Lp(a), one containing apo E and one free of apo E

Lipoprotein Lp(a) was isolated by immunoaffinity chromatography using anti apolipoprotein B and anti apolipoprotein (a) immunosorbents. Besides apolipoproteins (a) and B, this fraction was shown to contain apolipoproteins C and E. Therefore, it was decided to further purify this crude Lp(a) into particles containing apolipoprotein E and particles free of apo E, using chromatography with an anti apolipoprotein E immunosorbent. Lp(a), free of apolipoprotein E was cholesterol ester rich and triacylglycerol poor and was found mainly in the LDL size range. In contrast, Lp(a) containing apolipoprotein E was triacylglycerol rich and was distributed mainly in the VLDL and IDL size range. Binding of these two fractions, one containing apo E and one free of it, to the apo B/E receptor of HeLa cells was studied. Both fractions bound to the receptor but the one containing apo E had a better affinity than the one free of apo E. Further studies are needed to identify the clinical importance of these two different entities.

Genetic basis and pathophysiological implications of high plasma Lp(a) levels

Lipoprotein(a) or Lp(a) is a genetic variant of plasma low density lipoproteins (LDL) containing apoB100 covalently linked to apolipoprotein(a) or apo(a), the specific marker of Lp(a). Lp(a) is heterogeneous in size and density, accounting in part for the marked size polymorphism of apo(a), 300 to 800 kDa. The apo(a) size polymorphism is related to the different number of kringle repeats which are structurally similar although not identical to the kringle 4 of plasminogen. Recent studies on a genomic level have indicated that the apo(a) gene contains at least 19 different alleles varying in length between 48 and 190 kb, partially impacting on the plasma levels of Lp(a). High plasma levels of Lp(a) have been found to be associated with an increased prevalence of premature atherosclerotic cardiovascular disease by mechanism(s) yet to be established. Both atherogenic and thrombogenic potentials have been postulated and have been related to the LDL-like and plasminogen-like properties of Lp(a), respectively.

Three types of naturally occurring modified lipoproteins induce intracellular lipid accumulation in human aortic intimal cells--the role of lipoprotein aggregation

Blood monocytes or intimal smooth muscle cells from normal aorta were incubated with low density lipoprotein (LDL) from patients with coronary atherosclerosis, or with LDL from diabetic patients, or with lipoprotein(a) (Lp(a)). In each case there was a 2- to 4-fold rise in the intracellular cholesteryl ester content. LDL from healthy subjects failed to induce intracellular lipid accumulation in these cells. LDL from patients with coronary atherosclerosis, LDL from diabetic patients, and Lp(a) form aggregates under cell culture conditions. The ability of these lipoproteins to increase the cholesteryl ester content of cultured cells is directly correlated to the degree of lipoprotein aggregation. When aggregates were removed from the lipoprotein preparations by filtration, the latter became less effective in promoting intracellular lipid accumulation. Incubation of cells with lipoprotein aggregates, isolated by gel filtration, induced a 3- to 5-fold elevation of the cellular cholesteryl ester content. These results suggest that LDL from atherosclerotic patients, or LDL from diabetic patients, or Lp(a) have a tendency to form aggregates and that these aggregates are avidly taken up by intimal smooth muscle cells followed by lipid accumulation. This aggregation tendency may play a role in atherogenesis.

Lipoprotein(a) and apolipoprotein(a) in a New World monkey, the common marmoset (Callithrix jacchus). Association of variable plasma lipoprotein(a) levels with a single apolipoprotein(a) isoform

In an earlier report (Chapman et al, Biochemistry 1979;18:5096-5108), we suggested that the common marmoset may represent an important model for the study of human plasma lipoprotein metabolism. We now extend the interest of this monkey model to the study of lipoprotein(a) (Lp[a]) and apolipoprotein(a) (apo[a]). Density gradient ultracentrifugal fractionation of marmoset plasma revealed a bimodal distribution of Lp(a), with one peak of concentration occurring in association with very low density lipoproteins (VLDLs) and a second in the density range 1.040-1.080 g/ml. The dense Lp(a) subspecies displayed physicochemical properties (chemical composition, particle size, and electrophoretic mobility) that closely resembled those of its counterpart in humans and baboons but that were distinct from those of low density lipoprotein (LDL). Furthermore, the particle size of marmoset Lp(a) was invariant (31 nm) over the density interval 1.040-1.080 g/ml, whereas that of LDL decreased progressively with an increase in density (approximately 26-25.2 nm). Use of polyclonal and monoclonal antibodies to human apo(a) and of a polyclonal antibody to marmoset Lp(a) allowed immunologic identification of a single apo(a) isoform in the marmoset whose size was similar to that of apo B-100; apo(a) and apo B-100 were associated in Lp(a) particles by a disulfide linkage. The total protein mass of apo-Lp(a) was estimated to be 800,000 or more by electrophoresis in sodium dodecyl sulfate-polyacrylamide-agarose gels. The amino acid compositions of marmoset and human apo(a) resembled each other but were distinct from those of the corresponding forms of apo B-100. Immunologic evidence is provided for a high degree of cross reactivity between apo(a) in marmosets, baboons, and humans, supporting the idea of the existence of a marked degree of structural homology between these proteins. In addition, electroimmunoblotting of marmoset apo(a) and marmoset plasminogen showed that these proteins shared certain epitopes in common, suggesting that marmoset apo(a) may possess kringle-like structural features. Finally, despite possession of a single apo(a) isoform, marmoset Lp(a) levels varied over a 100-fold range (0.5-49 mg/dl plasma). Considered together, our present findings suggest that the common marmoset monkey constitutes a unique model in which to study the regulation of apo(a) gene expression and the posttranslational processing of apo(a), as well as factors that modulate the synthesis, intravascular metabolism, and cellular catabolism of Lp(a).

Lipoprotein aggregation as an essential condition of intracellular lipid accumulation caused by modified low density lipoproteins

We have tested a hypothesis that aggregates of modified low density lipoproteins (LDL) play the key role in the accumulation of lipids by cells of unaffected aortic intima. It was demonstrated using analysis of relative dispersion of light transmission fluctuations as well as gel filtration on Sepharose CL-2B that LDL modified by oxidation, glycosylation, desialylation and malondialdehyde treatment form aggregates under the conditions of culture. Native LDL failed to aggregate under the same conditions. It was demonstrated that modified LDL, unlike native LDL, bring about a 2- to 3-fold rise in cholesteryl ester levels of cultured cells. Moreover, direct and strong correlation (r = 0.86) was observed between the degree of lipoprotein aggregation and the amount of cholesteryl esters accumulated. Removal of modified LDL aggregates by filtration through a 0.1 micron filter or gel filtration completely prevented the intracellular accumulation of cholesteryl esters. These findings indicate that LDL aggregates play an essential, if not the decisive, role in the intracellular accumulation of lipids in vitro.

Plasminogen polymorphism in swine

1. Plasminogen polymorphism in swine (Sus scrofa) plasma was demonstrated by immunoblotting. 2. Eleven plasminogen phenotypic patterns, including a null pattern, were detected. 3. The null pattern was associated with extremely low plasma triglyceride and increased unesterified cholesterol levels. 4. Changes in plasminogen polymorphic patterns from the fetal to neonate status were observed after nursing commenced.

Molecular modeling of protein-glycosaminoglycan interactions

Forty-nine regions in 21 proteins were identified as potential heparin-binding sites based on the sequence organizations of their basic and nonbasic residues. Twelve known heparin-binding sequences in vitronectin, apolipoproteins E and B-100, and platelet factor 4 were used to formulate two search strings for identifying potential heparin-binding regions in other proteins. Consensus sequences for glycosaminoglycan recognition were determined as [-X-B-B-X-B-X-] and [-X-B-B-B-X-X-B-X-] where B is the probability of a basic residue and X is a hydropathic residue. Predictions were then made as to the heparin-binding domains in endothelial cell growth factor, purpurin, and antithrombin-III. Many of the natural sequences conforming to these consensus motifs show prominent amphipathic periodicities having both alpha-helical and beta-strand conformations as determined by predictive algorithms and circular dichroism studies. The heparin-binding domain of vitronectin was modeled and formed a hydrophilic pocket that wrapped around and folded over a heparin octasaccharide, yielding a complementary structure. We suggest that these consensus sequence elements form potential nucleation sites for the recognition of polyanions in proteins and may provide a useful guide in identifying heparin-binding regions in other proteins. The possible relevance of protein-glycosaminoglycans interactions in atherosclerosis is discussed.

Tissue distribution of [3H]cholesteryl linoleyl ether-labeled human Lp(a) in different rat organs

The sites of tissue uptake of human lipoprotein(a) (Lp(a] were studied in rats using [3H]cholesteryl linoleyl ether [( 3H]CLE) as a marker. Since rat plasma has no cholesteryl ester transfer activity, the amount of label in various tissues should reflect the quantitative uptake of Lp(a). Isolated Lp(a) was labeled with [3H]CLE by incubation overnight of Lp(a), a source of cholesteryl ester transfer activity (1.23 g/ml infranate of human plasma), and [3H]CLE-labeled Intralipid. Following labeling, the homogeneity and integrity of Lp(a) was shown by agarose electrophoresis and immunoblotting. Intact Lp(a) was injected via the tail vein of rats (120-170 g, n = 4 at each time point), and tissues were collected at various times thereafter (4-48 h). The disappearance curve of [3H]CLE-labeled Lp(a) from rat plasma was bimodal and had an initial rapid t1/2 of 1.8 h followed by a slower component, t1/2 = 13.3 h. Tissue uptake at all sampling times was greatest in liver (28.5% at 48 h of total dpm injected), followed by the intestine (9-12%), with less than 3% uptake by spleen. The small intestine was divided into four segments, and while the 3H radioactivity was similar in the proximal segments, a time-related increase in [3H]CLE was seen in its most distal portion. These studies indicate that the tissue sites of degradation in the rat of human Lp(a) are similar to human low-density lipoproteins (LDL); the increase in label in the distal portion of the small intestine with time may represent [3H]CLE excreted through the bile and absorbed by the mucosal cells.