Probucol protects lipoprotein (a) against oxidative modification

Lipoprotein(a) the Insurgent: A New Insight into the Structure, Function, Metabolism, Pathogenicity, and Medications Affecting Lipoprotein(a) Molecule

Lipoprotein(a) [Lp(a)], aka "Lp little a", was discovered in the 1960s in the lab of the Norwegian physician Kåre Berg. Since then, we have greatly improved our knowledge of lipids and cardiovascular disease (CVD). Lp(a) is an enigmatic class of lipoprotein that is exclusively formed in the liver and comprises two main components, a single copy of apolipoprotein (apo) B-100 (apo-B100) tethered to a single copy of a protein denoted as apolipoprotein(a) apo(a). Plasma levels of Lp(a) increase soon after birth to a steady concentration within a few months of life. In adults, Lp(a) levels range widely from <2 to 2500 mg/L. Evidence that elevated Lp(a) levels >300 mg/L contribute to CVD is significant. The improvement of isoform-independent assays, together with the insight from epidemiologic studies, meta-analyses, genome-wide association studies, and Mendelian randomization studies, has established Lp(a) as the single most common independent genetically inherited causal risk factor for CVD. This breakthrough elevated Lp(a) from a biomarker of atherosclerotic risk to a target of therapy. With the emergence of promising second-generation antisense therapy, we hope that we can answer the question of whether Lp(a) is ready for prime-time clinic use. In this review, we present an update on the metabolism, pathophysiology, and current/future medical interventions for high levels of Lp(a).

Lipoprotein(a) in cardiovascular diseases

Lipoprotein(a) (Lp(a)) is an LDL-like molecule consisting of an apolipoprotein B-100 (apo(B-100)) particle attached by a disulphide bridge to apo(a). Many observations have pointed out that Lp(a) levels may be a risk factor for cardiovascular diseases. Lp(a) inhibits the activation of transforming growth factor (TGF) and contributes to the growth of arterial atherosclerotic lesions by promoting the proliferation of vascular smooth muscle cells and the migration of smooth muscle cells to endothelial cells. Moreover Lp(a) inhibits plasminogen binding to the surfaces of endothelial cells and decreases the activity of fibrin-dependent tissue-type plasminogen activator. Lp(a) may act as a proinflammatory mediator that augments the lesion formation in atherosclerotic plaques. Elevated serum Lp(a) is an independent predictor of coronary artery disease and myocardial infarction. Furthermore, Lp(a) levels should be a marker of restenosis after percutaneous transluminal coronary angioplasty, saphenous vein bypass graft atherosclerosis, and accelerated coronary atherosclerosis of cardiac transplantation. Finally, the possibility that Lp(a) may be a risk factor for ischemic stroke has been assessed in several studies. Recent findings suggest that Lp(a)-lowering therapy might be beneficial in patients with high Lp(a) levels. A future therapeutic approach could include apheresis in high-risk patients in order to reduce major coronary events.

Probucol promotes reverse cholesterol transport in heterozygous familial hypercholesterolemia. Effects on apolipoprotein AI-containing lipoprotein particles

In order to investigate the effect of Probucol therapy on reverse cholesterol transport, apo AI-containing lipoprotein particles were isolated and characterized, and their cholesterol effluxing capacity and LCAT activity were assayed in four familial hypercholesterolemia patients before and after 12 weeks of Probucol therapy. Four major subpopulations of apo A-containing lipoprotein particles are separated before and after drug treatment; LpAI, LpAI:AII, LpAIV, LpAI:AIV:AII. Probucol reduces both total plasma and LDL-cholesterol (-17 and -14%, respectively). Apo B decreases slightly (-7.6%). Plasma HDL-cholesterol and apo AI decrease by 36.6 and 34.7%. LpA-I showed a marked decrease (-46%). Moreover, plasma LCAT and CETP activities were markedly increased under Probucol treatment. Analysis of lipoprotein particles showed that Probucol induces a decrease of protein content and an increase of cholesterol and triglycerides contents. Interestingly, Probucol induces an enhancement of LCAT activity in LpAI (4.5-fold). This drug induces a trend toward greater cholesterol efflux from cholesterol-preloaded adipose cells promoted by Lp AI and Lp AIV but not by Lp AI:AII. This study confirms the hypothesis, in addition to the lowering LDL-cholesterol levels and antioxidant effects of Probucol, that HDL reduction was not an atherogenic change in HDL system but may cause an antiatherogenic action by accelerating cholesterol transport through HDL system, promoting reverse cholesterol transport from peripheral tissues.

Lipoprotein(a) oxidation and autoantibodies: a new path in atherothrombosis

Lipoprotein(a) (Lp(a)) is considered a vascular pathogen of outstanding importance. High plasma levels of this lipoprotein are associated with premature arterial disease; however, the mechanisms involved have not been clarified. The atherosclerotic process is increasingly regarded as a chronic inflammatory reaction in the arterial wall where oxidation-mediated endothelial injury involving modified forms of low-density lipoprotein (LDL) seems to be a key event. Autoimmune pathways are involved in the progression of atherosclerosis and humoral response to oxidatively modified LDL can be considered among these pathways. A number of factors can be encountered in the pathogenesis of the accelerated arterial disease seen in patients with antiphospholipid (Hughes) syndrome (APS) and systemic lupus erythematosus (SLE). Among these, high levels of Lp(a) have been described in both and increasing evidence indicates that patients with antiphospholipid antibodies (aPL) are under oxidative stress. Recent studies suggest that the so-called 'oxidation theory of atherosclerosis' may also be applied to Lp(a). This fact makes this lipoprotein potentially suitable as a target of the immune system and antibodies reacting against oxidatively-modified Lp(a) by malondialdehyde have been recently described in APS and SLE. It is therefore likely that an immune response to the oxidized moiety of Lp(a) might be influential in the pathogenicity of this lipoprotein and, subsequently, of atherosclerosis.

Clinical evaluation of probucol in hypercholesteremia: individual lipoprotein responses and inhibitory effect on carotid atherosclerosis progression

Probucol treatment has been evaluated in 140 patients with hypercholesterolemia attending a single Lipid Clinic, in an attempt to identify the relations between lipid/lipoprotein responses and patient characteristics. Probucol was administered as a single drug at the standard dose (0.5 g tablets b.i.d.) for at least 6 months. One-hundred (71%) patients displayed a reduction of low-density lipoprotein cholesterol (LDL-C), which was significantly correlated with the baseline LDL-C level (r = 0.64; p < 0.0001). Most of the patients (90%) also responded with a reduction of high-density lipoprotein cholesterol (HDL-C); the HDL-C reduction was also directly related to baseline HDL cholesterolemia (r = 0.67, p < 0.0001). A highly significant correlation was found between the individual LDL-C and HDL-C responses. Eleven patients who continued with probucol treatment had a B-mode ultrasonographic investigation performed at baseline and after 24 months. No changes in carotid mean and maximal intimal-medial thickness were recorded, in contrast to an increase (i.e., indicative of atherosclerosis progression) in matched patients with hypercholesterolemia receiving other lipid-lowering regimens. Our report underlines that probucol can still provide a valuable option for the treatment for hypercholesterolemia, being particularly effective in patients with a combined increase of LDL-C and HDL-C levels.

Lipoprotein(a): structural implications for pathophysiology

The assembly between a low-density lipoprotein particle and apolipoprotein(a), a highly carbohydrate-rich protein, gives origin to a peculiar class of lipoproteins, only found in the hedgehog, primates, and humans, termed lipoprotein(a). Apolipoprotein(a), which shares a high degree of sequence homology with the fibrinolytic proenzyme plasminogen, is linked to the apolipoprotein B-100 component of low-density lipoprotein via a disulfide bond and confers distinct biochemical and metabolic properties to lipoprotein(a). Because of its peculiar structural features and the observed correlation between high lipoprotein(a) levels and the development of a variety of atherosclerotic disorders, this lipoprotein has become the focus of an intense research effort. Although accumulation of lipoprotein(a) in the vessel wall at sites of vascular injury has been clearly evidenced, the mechanism(s) by which lipoprotein(a) exerts its pathogenic effect in this milieu remain largely unknown. It has been hypothesized that the pathological effect of lipoprotein(a) is related either to its similarity to low-density lipoprotein (i.e., a pro-atherogenic effect) or to the apolipoprotein(a) similarity to plasminogen (i.e., a pro-thrombotic/anti-fibrinolytic effect). However, it is probable that both components contribute to the pathogenicity of lipoprotein(a). The fact that lipoprotein(a) levels are largely genetically determined, varying widely among individuals and racial groups, adds additional elements to the scientific interest that surrounds this lipoprotein. Both clinical and biochemical studies of lipoprotein(a) have been complicated by the high degree of structural heterogeneity of apolipoprotein(a), which is considered the most polymorphic protein in human plasma. Our aim in this paper is to provide an overview of the most salient structural features of lipoprotein(a) and their possible pathophysiological implications.

Oxidation of lipoprotein(a) and low density lipoprotein containing density gradient ultracentrifugation fractions

Increased plasma concentrations of lipoprotein(a) (Lp(a)) are associated with an increased risk for atherosclerotic cardiovascular disease. It is thought that the atherogenicity of Lp(a) is mediated both through its LDL-like properties and its plasminogen-like properties. In this study we have investigated the LDL-like atherogenic properties of Lp(a) by comparing the susceptibility to in vitro oxidation of Lp(a) and LDL isolated from the same subject. The subjects studied varied widely in plasma Lp(a) concentration (331-1829 mg/l) and Lp(a) phenotype (from B to S4). Lipoproteins are notoriously unstable in vitro, consequently differences in in vitro handling could influence oxidizability. Therefore, the isolation and handling of Lp(a) and LDL were performed in an identical fashion. Lp(a) and LDL containing fractions were obtained by density gradient ultracentrifugation. Separate fractions containing various amounts of Lp(a) and LDL, quantitated by measuring both Lp(a) and apo B-100, were subsequently oxidized on equimolar apo B-100 basis. Despite large differences in the Lp(a)/apo B-100 ratio of the various fractions (ranging from 5.3 +/- 1.7 to 0.2 +/- 0.1) they showed quite similar oxidation characteristics. The most dense Lp(a) containing fractions showed an aberrant susceptibility to oxidation. Subsequent gel filtration and reconstitution experiments showed that this was due to protein (i.e., albumin) contamination. Removal of excess protein revealed an oxidation pattern similar to that of LDL. It is concluded that the susceptibility of Lp(a) to lipid-peroxidation is similar to that of LDL when isolated simultaneously and in the same way from the same subject. Thus, lipid-peroxidation of Lp(a) is not influenced by the presence of its distinguishing apolipoprotein(a).

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.

Oxidatively modified LDL and atherosclerosis: an evolving plausible scenario

Much evidence has accumulated that implicates the oxidative modification of low-density lipoprotein (LDL) in the early stages of atherogenesis. The antioxidant nutrients alpha-tocopherol, ascorbic acid, and betacarotene have been shown to inhibit in vitro LDL oxidation. In addition, they have been shown to increase the resistance of LDL to oxidation when given to animals and humans. Because plasma levels of these nutrients can be increased by dietary supplementation with minimal side effects, they may show promise in the prevention of coronary artery disease.

Lipoprotein (a)

Lipoprotein (a) is similar to low-density lipoprotein but is unique in having an additional apolipoprotein called apolipoprotein (a) (apo(a)) covalently linked to it. apo(a), which is a member of the plasminogen gene superfamily, has a protease domain which cannot be activated to cause fibrinolysis. Its sequence of kringles is much longer than that of plasminogen and there is remarkable genetic variation in its length. The consequent inherited differences in apo(a) molecular mass are largely responsible for the wide range of serum Lp(a) concentrations in different individuals with low levels predominating in Europid populations. Physiologically Lp(a) may participate in haemocoagulation or in wound-healing. Epidemiological evidence that it is a risk factor for atherosclerosis, particularly in populations with high serum LDL levels, has led to research to uncover its role in atherogenesis and thrombosis. Diseases such as renal disease, and probably atherogenesis and thrombosis. Diseases such as renal disease, and probably atherosclerosis itself, are associated with an increase in Lp(a) above its genetically determined level and it remains a subject of speculation as to whether such increases are as closely involved in atherothrombosis as are spontaneously high levels resulting from low-molecular-mass apo(a) variants.

Lipoprotein(A): physiologic function, association with atherosclerosis, and effects of lipid-lowering drug therapy

OBJECTIVE

To review the structure and physiologic function of lipoprotein(a) [Lp(a)], review the association of Lp(a) with the development of atherosclerosis, and to critically evaluate the current literature regarding the effects of lipid-lowering drug therapy on Lp(a) serum concentrations.

DATA SOURCES

English language clinical and animal studies, abstracts, and review articles pertaining to Lp(a).

STUDY SELECTION AND DATA EXTRACTION

Relevant human and animal studies examining Lp(a)'s role in atherosclerosis and the effect of drug therapy on Lp(a) serum concentrations.

DATA SYNTHESIS

Possible physiologic functions and potential atherogenic mechanisms of Lp(a) are discussed. Evidence supporting the association of Lp(a) with atherosclerosis is presented. Studies evaluating the effects of lipid-lowering drug therapy on Lp(a) concentrations are reviewed and critiqued.

CONCLUSIONS

Lp(a) concentrations are correlated with the risk of atherosclerotic vascular disease (AVD) in both animals models and human studies. Drug therapies that have produced a consistent reduction in Lp(a) concentration include niacin alone or in combination with a bile acid sequestrant or neomycin. However, additional, larger studies are needed to evaluate the ability of drug therapies to specifically reduce elevated Lp(a) concentrations. Preliminary information suggests that reduction in Lp(a) concentrations may be associated with atherosclerotic plaque regression. Although drugs are available to lower Lp(a), one cannot conclude that lowering of Lp(a) is warranted until clinical trials demonstrating beneficial effects have been published.

Lipoprotein (a) displays increased accumulation compared with low-density lipoprotein in the murine arterial wall

Lipoprotein (a) (Lp(a)) is known to be an independent risk factor for cardiovascular disease, but the mechanisms by which it contributes to this disease remain unclear. Current evidence indicates that the closely related plasma particle, low-density lipoprotein (LDL), may initiate atherosclerosis through deposition in the arterial wall. This study has compared the ability of both lipoproteins to enter and accumulate within the arterial wall. Experiments were conducted in vivo with animals from two strains of mice: C57BL/6 mice, which develop fatty streak lesions upon challenge by a high-fat diet, and C3H/HeJ mice, which are resistant to lesion formation. Animals from both strains were maintained up to 16 weeks either on chow or high-fat diet. The mice were intravenously injected with 125I-labeled human Lp(a) or 125I-labeled human LDL in equimolar amounts and the lipoprotein allowed to circulate in vivo for 2 or 24 h. Transverse sections of the aortic root including sites of predilection for lesion formation at the commissures of the valve were prepared and examined after autoradiography. The autoradiographic grains over lesions and histologically uninvolved areas were enumerated and compared after normalization. Both Lp(a) and LDL demonstrated nearly ten times greater accumulation in lesions compared with histologically uninvolved areas from C57BL/6 mice. Analyses of histologically uninvolved areas from both strains of mice showed a significantly higher accumulation of Lp(a) than LDL. Finally, significantly higher accumulations of both Lp(a) and LDL occurred in the histologically uninvolved intima and subintima of lesion-prone C57BL/6 mice as compared with lesion-resistant C3H/HeJ mice after 5 weeks on the diets. We propose that enhanced accumulation of Lp(a) in the arterial wall accounts, in part, for the increased risk of cardiovascular disease.

The relation of lipoprotein[a] concentrations and apolipoprotein[a] phenotypes with asymptomatic atherosclerosis in subjects of the Atherosclerosis Risk in Communities (ARIC) Study

Plasma levels of lipoprotein[a] (Lp[a]) are associated with increased risk of coronary artery disease and show an inverse correlation with apolipoprotein[a] (apo[a]) molecular weight. We determined Lp[a] levels and apo[a] phenotypes in 171 cases with preclinical extracranial carotid atherosclerosis as ascertained by B-mode ultrasound and in 274 control subjects free of carotid atherosclerosis. Lp[a] protein levels measured by enzyme-linked immunosorbent assay ranged from 4 to 361 micrograms/mL in cases and from 2 to 392 micrograms/mL in controls, but median levels of Lp[a] were higher in cases than in controls (51 micrograms/mL versus 33 micrograms/mL, P < .003). In both groups, all 11 apo[a] polymorphs that are resolved by the procedure used were present, resulting in 43 and 39 different apo[a] phenotypes in cases and controls, respectively. An inverse relation between apo[a] polymorph size and Lp[a] level was observed in both cases (r = -0.49, P < .001) and controls (r = -0.34, P < .001). Apo[a] phenotype distributions were similar in cases and controls. However, in 17 phenotypes with three or more subjects per group, the difference of mean Lp[a] concentrations between cases and controls was 32 +/- 36 micrograms/mL (mean +/- SD). Thus, the higher Lp[a] levels in cases were not associated with a greater prevalence of small apo[a] polymorphs. Stepwise logistic regression analyses of known risk factors for coronary heart disease showed that plasma Lp[a] concentration was an independent predictor of case-control status, while Lp[a] phenotype was not, irrespective of the presence or absence of Lp[a] concentration in the model.(ABSTRACT TRUNCATED AT 250 WORDS)