Insoluble complex formation of lipoprotein (a) with low density lipoprotein in the presence of calcium ions

Activity of MUC1 cancer antigen-binding plasma anti-α-galactoside antibody correlates inversely with size of autologous lipoprotein(a)

Variation in ligand-binding affinity of natural plasma anti-α-galactoside antibody (anti-Gal) is a plausible reason for differing anti-cancer defense among individuals since serine- and threonine-rich peptide sequences (STPS) in the cancer-specific MUC-1 antigen are surrogate ligands for this antibody. As affinity of a natural antibody could be modulated by systemic antigens by processes including affinity maturation, we examined the contribution of the size of lipoprotein(a) [Lp(a)], an efficient autologous anti-Gal-binding macromolecule that possesses variable numbers of STPS due to genetically determined size polymorphism, towards the specific activity (activity per unit mass) of anti-Gal. Binding of purified Lp(a) to FITC-labeled anti-Gal, measured in terms of increase in fluorescence of the latter, was inhibited by LDL in proportion to Lp(a) size presumably because LDL molecules also bind noncovalently and in proportion to Lp(a) size at the O-glycosylated and STPS-rich region of Lp(a). For the same reason, circulating forms of smaller Lp(a) which carried fewer or no noncovalently attached LDL molecules were more efficient ligands for the antibody than the same number of larger ones ( P < 0.0001). Result suggested that smaller Lp(a), with their STPS ligands less obstructed by adhering LDL, would be more effective systemic antigens for anti-Gal. In confirmation of this, the specific activity of anti-Gal decreased with Lp(a) size (r − 0.5443; P < 0.0001) but increased with Lp(a) concentration (r 0.6202; P < 0.0001) among 73 normal plasma samples. IgG to IgM ratio, an index of immunoglobulin class switching characteristic of affinity maturation, was decidedly higher for anti-Gal in small Lp(a) individuals than in their large Lp(a) counterparts ( P = 0.0014). Results indicated that modulation of activity of anti-Gal by Lp(a) size may account for the lower incidence of cancer reported in people carrying more plasma Lp(a) which are generally smaller as well.

Hemodialysis Vascular Access Thrombosis and Lipoprotein(A)

Background

Vascular access remains the Achilles's heel of successful hemodialysis and thrombosis is the leading cause of vascular access failure. Elevated lipoprotein(a) (Lp(a)) levels in hemodialysis patients were reported, and in some studies were also associated with hemodialysis vascular access thrombosis.

Patients and Methods

In our study 84 hemodialysis patients with native arteriovenous fistula were included. Two groups of patients were defined: group A including 61 patients with their vascular access either never or only once thrombosed, and group B including 23 patients with two or more thromboses of their vascular access. We determined serum concentrations of Lp(a) in all our patients.

Results

Average serum Lp(a) concentration for all the patients included in the study was 0.273 ± 0.31 g/l. No relationship was found between serum Lp(a) concentrations and age, gender and duration of dialysis treatment. Serum Lp(a) concentrations were higher in group A than in group B patients (0.301 g/l versus 0.198 g/l), but the difference was not statistically significant. There was also no statistically significant difference between group A and group B regarding age, gender and duration of hemodialysis treatment. The use of a cut-off value for Lp(a) of 0.3 g/l and 0.57 g/l also failed to provide a significant difference between group A and B patients.

Conclusion

We found no significant differences in Lp(a) concentrations between group A (thrombosis-non-prone) and group B (thrombosis-prone) patients. Our results suggest that Lp(a) is not an independent risk factor for vascular access occlusion in hemodialysis patients.

The Relationship of Various Factors in the Pathogenesis of Atherosclerosis

In this study we investigated the levels of lipid parameters, fibronectin, tissue-type plasminogen activator and plasminogen activator inhibitor (t-PA-PAI-1) complex and si alidase in patients with coronary heart disease and a control group. Total cholesterol, triglyceride, low-density lipoprotein (LDL), and very-low-density lipoprotein (VLDL) cholesterol and lipoprotein Lp(a), levels in patients with coronary heart disease were found to be significantly higher than in the control group (p < .001). High-density lipoprotein (HDL) cholesterol levels in patient group were significantly lower than control group (p < .001). Plasma fibronectin and t-PA-PAI-1 complex levels in patients with coronary heart disease were found to be significantly higher than control group (p < .05 and p < .001, respectively). In addition, we found that serum sialidase levels in patients with coronary heart disease were significantly higher than in the control group (p < .001). The electrophoretic mobility of lipoproteins from patients with coronary heart dis ease was found to be greater than those from the control group. As a result Lp(a) may play an important role in the pathogen esis of atherosclerosis by causing foam cell formation because of interacting with LDL or fibronectin and by interfering with the fibrinolytic system because of binding to plasminogen re ceptors. In addition, modifications of Lp(a) (including desi alylation) may effect these events. Key words: Coronary heart disease—tPA-PAI-1 complex-Fibronectin-sialidase-Lipid parameters.

Lipoprotein[a] and cancer: Anti-neoplastic effect besides its cardiovascular potency

While the death rate from cancer has substantially decreased over the past decade, the search for effective and tolerable therapies is a great challenge as yet. The evidence that malignant cells cannot grow to a clinically detectable tumor mass and spread in the absence of an adequate vascular support, has opened a new area of research towards the selective inhibition or even destruction of tumor vessels. Angiostatin and angiostatin-related proteins are a family of specific angiogenesis inhibitors produced by tumors from a family of naturally occurring proteins, which also includes plasminogen and lipoprotein[a]. The anti-angiogenic activity of these proteins resides in cryptic and highly-repetitive molecular domains hidden within the protein moiety, called kringles. Lipoprotein[a] is an intriguing molecule consisting of a low-density lipoprotein core in addition to the covalently bound apolipoprotein[a]. Apolipoprotein[a] is characterized by an inactive protease domain, a single copy of the plasminogen kringle V and multiple repeats of domains homologous to the plasminogen kringle IV. Reliable studies on animal models indicate that the proteolytic break-down products of apolipoprotein[a] would posses anti-angiogenic and anti-tumoral properties both in vitro and in vivo, a premise to develop novel therapeutic modalities which may efficiently suppress tumor growth and metastasis. This review is focused on the biochemical structure, metabolism and the anti-angiogenic activity of this unique and elusive kringle-containing lipoprotein.

Lipoprotein(a): an emerging cardiovascular risk factor

Lipoprotein(a) is a cholesterol-enriched lipoprotein, consisting of a covalent linkage joining the unique and highly polymorphic apolipoprotein(a) to apolipoprotein B100, the main protein moiety of low-density lipoproteins. Although the concentration of lipoprotein(a) in humans is mostly genetically determined, acquired disorders might influence synthesis and catabolism of the particle. Raised concentration of lipoprotein(a) has been acknowledged as a leading inherited risk factor for both premature and advanced atherosclerosis at different vascular sites. The strong structural homologies with plasminogen and low-density lipoproteins suggest that lipoprotein(a) might represent the ideal bridge between the fields of atherosclerosis and thrombosis in the pathogenesis of vascular occlusive disorders. Unfortunately, the exact mechanisms by which lipoprotein(a) promotes, accelerates, and complicates atherosclerosis are only partially understood. In some clinical settings, such as in patients at exceptionally low risk for cardiovascular disease, the potential regenerative and antineoplastic properties of lipoprotein(a) might paradoxically counterbalance its athero-thrombogenicity, as attested by the compatibility between raised plasma lipoprotein(a) levels and longevity.

Association between serum total and lipid-bound sialic acid concentration and the severity of coronary atherosclerosis

Serum total sialic acid has recently been shown to be a cardiovascular risk factor. Increased levels of this substance are associated with higher cardiovascular mortality and with cerebrovascular disease. It has also been shown that serum concentrations of total and lipid-associated sialic acid are significantly increased in hypertriglyceridemia. On the other hand, several circulating lipoproteins have been suggested to be related to the severity of coronary atherosclerosis, but contradictory results have been reported in the possible relationship between the concentrations of sialic acid and the severity of coronary lesions in patients with coronary heart disease. The purpose of this study was to investigate the possible relationship between serum total sialic acid concentration, recently shown to be a cardiovascular risk factor, and serum lipid-bound sialic acid concentration and the severity of coronary lesions. The study comprised 90 subjects, divided into three subgroups according to angiography results: 30 patients with no vessel disease, 30 patients with single-vessel disease, and 30 patients with double/triple-vessel disease. Serum total sialic acid determination was carried out with the thiobarbituric acid method of Warren; lipid-associated sialic acid was assayed with the method of Katopodis. Mean serum total sialic acid levels in patients with single-vessel disease (P <.05) and patients with double/triple-vessel disease (P <.001) were found to be significantly increased compared with that in patients with no vessel disease, whereas mean serum lipid-bound sialic acid levels were found to be significantly different between patients with double- or triple-vessel disease and patients with no vessel disease (P <.001). We also noted a significant difference between the levels of serum total sialic acid (P <.001) and lipid-bound sialic acid (P <.001) in patients with single-vessel disease and patients with double/triple-vessel disease. We found a significant correlation only between serum lipid-bound sialic acid and coronary angiographic score in patients with double/triple-vessel disease (r = 0.425, P <.05). Although the concentration of serum total sialic acid is increased proportionally with the number of diseased coronary arteries, only the concentration of serum lipid-bound sialic acid is related to the severity of coronary atherosclerosis, especially in patients with double/triple-vessel disease.

Serum total and lipid-bound sialic acid levels following acute myocardial infarction

Although serum total sialic acid has been shown to be a cardiovascular risk factor, with elevated levels associated with increased cardiovascular mortality and also with cerebrovascular disease, the reason for the elevation in serum sialic acid content remains obscure. It has been shown that an increased output of serum proteins by the liver due to some type of acute phase reaction may be one of the possible sources of an increased serum sialic acid concentration in patients with myocardial infarction. An increase in the activity of sialidase, which cleaves the terminal sialic acid residues from oligosaccharides, glycoproteins and gangliosides, may also play an important role in the elevation of serum total sialic acid in myocardial infarction. Elevated serum total sialic acid in the blood might result either from the shedding or secreting of sialic acid from the cell membrane surface, or releasing of cellular sialic acid from the cell into the bloodstream due to cell damage after myocardial infarction. The purpose of the present study is to investigate serum total and lipid-bound sialic acid and the enzymes serum lactate dehydrogenase, creatine kinase and aspartate aminotransferase in patients with acute myocardial infarction, at 24 h post-infarction (day 1), 48 h post-infarction (day 2) and 72 h post-infarction (day 3). A possible role of cell damage in the elevation of serum total and lipid-bound sialic acid levels in these patients was also evaluated. In this study, 40 patients with myocardial infarction ranging in age from 42 to 68 years, and 26 healthy volunteers ranging in age from 45 to 71 years were included. Serum total sialic acid determination was carried out by the thiobarbituric acid method of Warren and lipid-bound sialic acfd by the method of Katopodis. Our data shows that a) there is a gradual increase in the levels of serum total sialic acid and lipid-bound sialic acid during the first three days after the acute myocardial infarction and b) the elevation in serum total sialic acid levels correlates with the elevation in lactate dehydrogenase activity only on day 1 following infarction. Therefore, either the shedding or secreting of sialic acid from the cell or cell membrane surface may be partly responsible for an increased serum sialic acid concentration especially on day 1 following myocardial infarction.

Proteoglycans contribution to association of Lp(a) and LDL with smooth muscle cell extracellular matrix

Lp(a) interference with fibrinolysis could contribute to atherothrombosis. Additionally, accumulation of Lp(a) and LDLs, could lead to cholesterol deposition and foam cell formation in atherogenesis. The interactions between Lp(a) and LDL could cause their entrapment in the extracellular matrix of lesions. We found that association of Lp(a) with matrix secreted by cultured human arterial smooth muscle cells increased 2 to 3 times the subsequent specific binding of radioactive LDL. Chondroitin sulfate proteoglycans seem responsible for formation of the specific matrix-Lp(a) and matrix-LDL aggregates. The proteoglycans appeared also to participate in a cooperative increase of radioactive LDL binding to matrix pretreated with Lp(a). In the matrix preincubated with LDL, approximately 50% of the additional lipoprotein was bound by ionic interactions. In the matrix preincubated with Lp(a), 20% of the additional LDL was held by ionic bonds, and the rest was held by strong nonionic associations. Binding analysis in physiological solutions confirmed that chondroitin sulfate-rich proteoglycans from the smooth muscle cell matrix have a high affinity for Lp(a) and LDL. The results provide an explanation to the observed localization of Lp(a) and LDL in the extracellular matrix of arterial lesions and suggest a mechanism for their cooperative accumulation there.

Enhanced macrophage uptake of lipoprotein(a) after Ca2+-induced aggregate-formation

We tested the hypothesis that aggregated lipoprotein(a) [Lp(a)] is avidly taken up by macrophages. Lp(a) was isolated by sequential centrifugations and gel chromatography from a patient with high plasma levels of Lp(a) who was being treated with low density lipoprotein (LDL)-apheresis. Aggregated Lp(a) was prepared by mixing native Lp(a) with 2.5 mmol/L CaCl2, and 54% of the 125I-Lp(a) aggregated after interacting with CaCl2. The binding and degradation of aggregated Lp(a) in macrophages were 4.6- and 4.7-fold higher than those of native Lp(a), respectively. An excess amount of LDL did not inhibit either increase. Cholesterol esterification in macrophages was markedly stimulated by aggregated Lp(a), and macrophages were transformed into foam cells. Cytochalasin B, a phagocytosis inhibitor, strongly inhibited the degradation and cholesterol esterification (78 and 83%, respectively). These findings suggested that aggregation may be partially involved in Lp(a) accumulation, thereby contributing to the acceleration of atherosclerosis.

Metabolic effects of the menopause and oestrogen replacement

There is little doubt that the metabolic disturbances seen following the loss of ovarian function are most important in the development of cardiovascular disease in women. The loss of hormones at the menopause appears to reduce both insulin secretion and elimination, but increasing insulin resistance thereafter brings about an increase in circulating insulin concentrations. Changes in lipids and lipoproteins are in an adverse direction, as are changes in body fat distribution, and changes in haemostatic factors would tend to favour coagulation rather than fibrinolysis. HRT with oestrogen appears to improve most of the metabolic abnormalities related to the menopause, but this is in part dependent on the type of oestrogen used and the route of administration. The addition of progestogen may influence the metabolic changes induced by oestrogens, and this will vary according to the type of the progestogen. Overall, the metabolic effects of any of the current HRT regimens would seem likely to be beneficial for CHD. Nevertheless, future HRT regimens should ideally be tailored to produce the most favourable changes in CHD metabolic risk factors, particularly in the case of the regimens which attempt to avoid cyclical bleeding.

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).

Plasma Lp(a) and t-PA-PAI-1 complex levels in coronary heart disease

In this study we investigated serum Lp(a) and plasma t-PA-PAI-1 complex levels in patients with coronary heart disease (CHD). Serum total cholesterol, triglyceride, LDL and VDL cholesterol levels (p < 0.001) and HDL cholesterol levels (p < 0.01) in patients group were found to be significantly different from those in control group. The mean Lp(a) and t-PA-PAI-1 complex levels in patients with coronary heart disease were significantly higher as compared to control group (p < 0.001). This data indicate that the elevated levels of serum Lp(a) and plasma t-PA-PAI-1 complex may play an important role in the pathogenesis of coronary atherosclerosis.

Lipoprotein(a) stimulates the proliferation of cultured human arterial smooth muscle cells through two pathways

We investigated the effect of lipoprotein(a) (Lp(a)) on proliferation of human arterial smooth muscle cells (SMCs) and its mechanisms of action. Low density lipoprotein (LDL), Lp(a) and apolipoprotein(a) (apo(a)) significantly stimulated the proliferation of SMCs. Lp(a) and apo(a) reduced the amount of active transforming growth factor-beta (TGF-beta) with the mink lung epithelial cell bioassay, however LDL had no effect. Lp(a), but not apo(a), significantly stimulated the proliferation of SMCs even in the presence of a neutralizing antibody for TGF-beta. Our results suggest that Lp(a) stimulates the proliferation of SMCs via apo(a)-induced inhibition of TGF-beta activation and stimulation of SMCs by the LDL-particle of Lp(a).

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 rheumatoid 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.

Immunochemically detectable lipid-free apo(a) in plasma and in human atherosclerotic lesions

Although Lp(a) is an independent risk factor for cardiovascular diseases in humans, the precise pathogenetic mechanisms are still unknown. We have shown that Lp(a) accumulates in human atherosclerotic lesions, and some particles undergo oxidation. Since, following agarose electrophoresis of both plaque extracts and plasma, a region close to the origin immunostained intensely for apo(a) but was lipid-free, we sought to identify whether such samples contained lipid-free apo(a), as previously reported to occur in plaque extracts. Immunochemically identifiable apo(a) was found following density-gradient ultracentrifugation both in the 1.05 < d < 1.09 and the d > 1.21 density fraction from both plasma and plaque extracts. However, because in a competitive binding RIA, displacement curves of apo(a) in plasma and the d > 1.21 were not parallel, it is premature to ascribe a relative amount of total apo(a) to this fraction. Whereas apo(a) immunoblots of SDS-PAGE under reducing conditions of the d > 1.21 fraction of a plaque extract with high apo(a) content showed high molecular weight bands consistent with apo(a) isoforms, the corresponding d > 1.21 fraction showed multiple low molecular weight bands characteristic of fragmentation. Since the d > 1.21 of arterial extracts contained all the material immunostaining for apo(a) migrating towards the cathode, characteristic of immunoglobulins (IgG), we asked whether fragments of apo(a) might have associated with human IgG both in plasma and tissue extracts, or whether our anti-apo(a) reacted with epitopes on human IgG. Immunoblotting with our anti-apo(a) of samples of plasma and plaque extracts run on agarose electrophoresis or SDS-PAGE further demonstrated intense staining of multiple bands in the molecular weight range of human IgG. Furthermore, a fraction of plasma and tissue extracts that bound to a protein G affinity column demonstrated immunostaining for apo(a) and was in the size range of IgG. Although one polyclonal anti-apo(a) provided by another laboratory showed the same findings as our antibody, two other polyclonal anti-apo(a) failed to demonstrate immunostaining of human IgG, either on agarose electrophoresis or SDS-PAGE. We speculate that the Lp(a) immunogen used to prepare our anti-apo(a) may have undergone modest oxidation, thus exposing epitopes not normally expressed on apo(a) in native Lp(a). Either antibodies to these epitopes could be recognizing apo(a) fragments, possibly released during oxidation, which are then covalently bound to IgG, or oxidation of apo(a) creates epitopes on apo(a) that are homologous with IgG, thereby leading to cross-reactivity with IgG.(ABSTRACT TRUNCATED AT 400 WORDS)