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

Effect of Omega-3 Fatty Acid Supplementation on the Postprandial Metabolism of Apolipoprotein(a) in Familial Hypercholesterolemia

AIM

Lipoprotein(a) (Lp(a)) is a low-density lipoprotein-like particle containing apolipoprotein(a) (apo(a)) that increases the risk of atherosclerotic cardiovascular disease (ASCVD) in familial hypercholesterolemia (FH). Postprandial redistribution of apo(a) protein from Lp(a) to triglyceride-rich lipoproteins (TRLs) may also increase the atherogenicity of TRL particles. Omega-3 fatty acid (ω3FA) supplementation improves postprandial TRL metabolism in FH subjects. However, its effect on postprandial apo(a) metabolism has yet to be investigated.

METHODS

We carried out an 8-week open-label, randomized, crossover trial to test the effect of ω3FA supplementation (4 g/day) on postprandial apo(a) responses in FH patients following ingestion of an oral fat load. Postprandial plasma total and TRL-apo(a) concentrations were measured by liquid chromatography with tandem mass spectrometry, and the corresponding areas under the curve (AUCs) (0-10h) were determined using the trapezium rule.

RESULTS

Compared with no ω3FA treatment, ω3FA supplementation significantly lowered the concentrations of postprandial TRL-apo(a) at 0.5 (-17.9%), 1 (-18.7%), 2 (-32.6%), and 3 h (-19.2%) (P＜0.05 for all). Postprandial TRL-apo(a) AUC was significantly reduced with ω3FA by 14.8% (P＜0.05). By contrast, ω3FA had no significant effect on the total AUCs of apo(a), apoC-III, and apoE (P＞0.05 for all). The decrease in postprandial TRL-apo(a) AUC was significantly associated with changes in the AUC of triglycerides (r=0.600; P＜0.01) and apoB-48 (r=0.616; P＜0.01).

CONCLUSIONS

Supplementation with ω3FA reduces postprandial TRL-apo(a) response to a fat meal in FH patients; this novel metabolic effect of ω3FA may have implications on decreasing the risk of ASCVD in patients with FH, especially in those with elevated plasma triglyceride and Lp(a) concentrations. However, the clinical implications of these metabolic findings require further evaluation in outcome or surrogate endpoint trials.

Lipoprotein(a) and ischemic heart disease--a causal association? A review

The aim of this review is to summarize present evidence of a causal association of lipoprotein(a) with risk of ischemic heart disease (IHD). Evidence for causality includes reproducible associations of a proposed risk factor with risk of disease in epidemiological studies, evidence from in vitro and animal studies in support of pathophysiological effects of the risk factor, and preferably evidence from randomized clinical trials documenting reduced morbidity in response to interventions targeting the risk factor. Elevated and in particular extreme lipoprotein(a) levels have in prospective studies repeatedly been associated with increased risk of IHD, although results from early studies are inconsistent. Data from in vitro and animal studies implicate lipoprotein(a), consisting of a low density lipoprotein particle covalently bound to the plasminogen-like glycoprotein apolipoprotein(a), in both atherosclerosis and thrombosis, including accumulation of lipoprotein(a) in atherosclerotic plaques and attenuation of t-PA mediated plasminogen activation. No randomized clinical trial of the effect of lowering lipoprotein(a) levels on IHD prevention has ever been conducted. Lacking evidence from randomized clinical trials, genetic studies, such as Mendelian randomization studies, can also support claims of causality. Levels of lipoprotein(a) are primarily determined by variation in the LPA gene coding for the apolipoprotein(a) moiety of lipoprotein(a), and genetic epidemiologic studies have documented association of LPA copy number variants, influencing levels of lipoprotein(a), with risk of IHD. In conclusion, results from epidemiologic, in vitro, animal, and genetic epidemiologic studies support a causal association of lipoprotein(a) with risk of IHD, while results from randomized clinical trials are presently lacking.

Effects of simvastatin and carnitine versus simvastatin on lipoprotein(a) and apoprotein(a) in type 2 diabetes mellitus

AIM

The aim of the present study was to compare the effects of simvastatin and L-carnitine coadministration versus simvastatin monotherapy on lipid profile, lipoprotein(a) (Lp(a)) and apoprotein(a) (Apo(a)) levels in type II diabetic patients.

PATIENTS/METHODS

In this double-blind, randomized clinical trial, 75 patients were assigned to one of two treatment groups for 4 months. Group A received simvastatin monotherapy; group B received L-carnitine and simvastatin. The following variables were assessed at baseline, after washout and at 1, 2, 3 and 4 months of treatment: body mass index, fasting plasma glucose, glycated hemoglobin, total cholesterol, low-density lipoprotein (LDL) cholesterol, high-density lipoprotein (HDL) cholesterol, triglycerides, Apolipoprotein A1, Apo B, lipoprotein(a) and apoprotein(a).

RESULTS

At the end of treatment in the carnitine and simvastatin combined group compared with the simvastatin alone group, we observed a significant decrease in glycemia (p < 0.001), triglycerides (p < 0.001), Apo B (p < 0.05), Lp(a) (p < 0.05), apo(a) (p < 0.05), while HDL significantly increased (p < 0.05).

CONCLUSIONS

The coadministration of carnitine and simvastatin resulted in a significant reduction in Lp(a) and apo(a) and may represent a new therapeutic option in reducing plasma Lp(a) levels, LDL cholesterol and Apo B100.

Antisense oligonucleotide directed to human apolipoprotein B-100 reduces lipoprotein(a) levels and oxidized phospholipids on human apolipoprotein B-100 particles in lipoprotein(a) transgenic mice

BACKGROUND

Lipoprotein (a) [Lp(a)] is a genetic cardiovascular risk factor that preferentially binds oxidized phospholipids (OxPL) in plasma. There is a lack of therapeutic agents that reduce plasma Lp(a) levels.

METHODS AND RESULTS

Transgenic mice overexpressing human apolipoprotein B-100 (h-apoB-100 [h-apoB mice]) or h-apoB-100 plus human apo(a) to generate genuine Lp(a) particles [Lp(a) mice] were treated with the antisense oligonucleotide mipomersen directed to h-apoB-100 mRNA or control antisense oligonucleotide for 11 weeks by intraperitoneal injection. Mice were then followed up for an additional 10 weeks off therapy. Lp(a) levels [apo(a) bound to apoB-100] and apo(a) levels ["free" apo(a) plus apo(a) bound to apoB-100] were measured by chemiluminescent enzyme-linked immunoassay and commercial assays, respectively. The content of OxPL on h-apoB-100 particles (OxPL/h-apoB) was measured by capturing h-apoB-100 in microtiter wells and detecting OxPL by antibody E06. As expected, mipomersen significantly reduced plasma h-apoB-100 levels in both groups of mice. In Lp(a) mice, mipomersen significantly reduced Lp(a) levels by approximately 75% compared with baseline (P<0.0001) but had no effect on apo(a) levels or hepatic apo(a) mRNA expression. OxPL/h-apoB levels were much higher at baseline in Lp(a) mice compared with h-ApoB mice (P<0.0001) but decreased in a time-dependent fashion with mipomersen. There was no effect of the control antisense oligonucleotide on lipoprotein levels or oxidative parameters.

CONCLUSIONS

Mipomersen significantly reduced Lp(a) and OxPL/apoB levels in Lp(a) mice. The present study demonstrates that h-apoB-100 is a limiting factor in Lp(a) particle synthesis in this Lp(a) transgenic model. If applicable to humans, mipomersen may represent a novel therapeutic approach to reducing Lp(a) levels and their associated OxPL.

A novel function of lipoprotein [a] as a preferential carrier of oxidized phospholipids in human plasma

Oxidized phospholipids (OxPLs) on apolipoprotein B-100 (apoB-100) particles are strongly associated with lipoprotein [a] (Lp[a]). In this study, we evaluated whether Lp[a] is preferentially the carrier of OxPL in human plasma. The content of OxPL on apoB-100 particles was measured with monoclonal antibody E06, which recognizes the phosphocholine (PC) headgroup of oxidized but not native phospholipids. To assess whether OxPLs were preferentially bound by Lp[a] as opposed to other lipoproteins, immunoprecipitation and ultracentrifugation experiments, in vitro transfer studies, and chemiluminescent ELISAs were performed. Immunoprecipitation of Lp[a] from human plasma with an apolipoprotein [a] (apo[a])-specific antibody demonstrated that more than 85% of E06 reactivity (i.e., OxPL) coimmunoprecipitated with Lp[a]. Ultracentrifugation experiments showed that nearly all OxPLs were found in fractions containing apo[a], as opposed to other apolipoproteins. In vitro transfer studies showed that oxidized LDL preferentially donates OxPLs to Lp[a], as opposed to LDL, in a time- and temperature-dependent manner, even in aqueous buffer. Approximately 50% of E06 immunoreactivity could be extracted from isolated Lp[a] following exposure of plasma to various lipid solvents. These data demonstrate that Lp[a] is the preferential carrier of PC-containing OxPL in human plasma. This unique property of Lp[a] suggests novel insights into its physiological function and mechanisms of atherogenicity.

Apolipoprotein E polymorphism is associated with both carotid and coronary atherosclerosis in patients with coronary artery disease

BACKGROUND AND AIMS

Apolipoprotein E (apoE) polymorphism plays a significant role in the development of atherosclerosis and cardiovascular disease. Therefore, the aim of the present study was to examine the association between apoE polymorphism and carotid intima-media thickness (IMT), and severity and extent of coronary artery disease (CAD).

METHODS AND RESULTS

B-mode ultrasound and quantitative coronary angiography (QCA) were used to assess carotid, and coronary artery atherosclerosis in 91 patients with clinically suspected CAD referred for cardiac catheterization. Two apoE phenotype groups were defined: apoE3 (E3/E3) and apoE4 (including E4/E3, E4/E4 phenotypes). Maximum IMT was higher in the apoE4 group than in the apoE3 group (p=0.022). The global atheroma burden index was similarly higher in the apoE4 group than in the apoE3 group (p=0.033). ApoE4 subjects had higher levels of apolipoprotein B (apoB) (p=0.008), triglycerides (p=0.006), remnant lipoprotein-cholesterol (RLP-C) (p=0.023), and lipoprotein(a) [(Lp(a)] (p=0.041) than apoE3 subjects. The mean LDL particle size was smaller in the apoE4 group than in the apoE3 group (p=0.041).

CONCLUSIONS

ApoE polymorphism was associated with both carotid and coronary atherosclerosis. Patients with the apoE4 isoform had an increased carotid IMT and a more severe and extensive CAD than patients with the apoE3 isoform.

Increased plasma concentrations of lipoprotein(a) during a low-fat, high-carbohydrate diet are associated with increased plasma concentrations of apolipoprotein C-III bound to apolipoprotein B-containing lipoproteins

BACKGROUND

Low-fat, high-carbohydrate (LFHC) diets have been shown to increase plasma concentrations of lipoprotein(a) [Lp(a)] and of triacylglycerol- rich lipoproteins (TRLs).

OBJECTIVE

We tested whether increases in plasma Lp(a) induced by an LFHC diet are related to changes in TRLs.

DESIGN

Healthy men (study 1; n = 140) consumed for 4 wk each a high-fat, low-carbohydrate diet (HFLC; 40% fat, 45% carbohydrate) and an LFHC diet (20% fat, 65% carbohydrate). Plasma lipids; lipoproteins; apolipoprotein (apo) B, A-I, and C-III; and Lp(a) were measured at the end of each diet. In a second group of men following a similar dietary protocol (study 2; n = 33), we isolated apo(a)-containing particles by immunoaffinity chromatography and determined the concentrations of apo C-III in ultracentrifugally isolated subfractions of apo B-containing lipoproteins.

RESULTS

In study 1, plasma concentrations of Lp(a) (P < 0.001), triacylglycerol (P < 0.001), apo B (P < 0.005), apo C-III (P < 0.005), and apo C-III in apo B-containing lipoproteins (non-HDL apo C-III) (P < 0.001) were significantly higher with the LFHC diet than with the HFLC diet. Stepwise multiple linear regression analysis showed that the association of changes in Lp(a) with changes in non-HDL apo C-III was independent of changes in body mass index, apo B, LDL cholesterol, and HDL cholesterol. Plasma lipid and lipoprotein changes were similar in study 2, and we found that both total apo C-III and the apo C-III content of apo(a)-containing particles were increased in a TRL fraction consisting predominantly of large VLDL particles [TRL-apo(a)].

CONCLUSIONS

The increase in plasma Lp(a) with an LFHC diet is significantly associated with an increase in non-HDL apo C-III. Enrichment of TRL-apo(a) with apo C-III may contribute to this dietary effect on Lp(a) concentrations.

Lipoprotein(a) and atherosclerosis: new perspectives on the mechanism of action of an enigmatic lipoprotein

Although elevated plasma concentrations of lipoprotein(a) (Lp(a)) have been identified as a risk factor for coronary heart disease, the pathophysiologic and physiologic roles of Lp(a) continue to elude basic researchers and clinicians alike. Lp(a) is a challenging lipoprotein to study because it has a complex structure consisting of a low-density lipoprotein-like moiety to which is covalently attached the unique glycoprotein apolipoprotein(a) (apo(a)). Apo(a) contains multiply repeated kringle domains that are similar to a sequence found in the fibrinolytic proenzyme plasminogen; differing numbers of kringle sequences in apo(a) give rise to Lp(a) isoform size heterogeneity. In addition to elevated plasma concentrations of Lp(a), apo(a) isoform size has been identified as a risk factor for coronary heart disease, although studies addressing this relationship have been limited. The similarity of Lp(a) to low-density lipoprotein and plasminogen provides an enticing link between the processes of atherosclerosis and thrombosis, although a clear demonstration of this association in vivo has not been provided. Clearly, Lp(a) is a risk factor for both atherothrombotic and purely thrombotic events; a plethora of mechanisms to explain these clinical findings has been provided by both in vitro studies as well as animal models for Lp(a).

Lipoprotein [a] is cleared from the plasma primarily by the liver in a process mediated by apolipoprotein [a]

The cellular and molecular mechanisms responsible for lipoprotein [a] (Lp[a]) catabolism are unknown. We examined the plasma clearance of Lp[a] and LDL in mice using lipoproteins isolated from human plasma coupled to radiolabeled tyramine cellobiose. Lipoproteins were injected into wild-type, LDL receptor-deficient (Ldlr-/-), and apolipoprotein E-deficient (Apoe-/-) mice. The fractional catabolic rate of LDL was greatly slowed in Ldlr-/- mice and greatly accelerated in Apoe-/- mice compared with wild-type mice. In contrast, the plasma clearance of Lp[a] in Ldlr-/- mice was similar to that in wild-type mice and was only slightly accelerated in Apoe-/- mice. Hepatic uptake of Lp[a] in wild-type mice was 34.6% of the injected dose over a 24 h period. The kidney accounted for only a small fraction of tissue uptake (1.3%). To test whether apolipoprotein [a] (apo[a]) mediates the clearance of Lp[a] from plasma, we coinjected excess apo[a] with labeled Lp[a]. Apo[a] acted as a potent inhibitor of Lp[a] plasma clearance. Asialofetuin, a ligand of the asialoglycoprotein receptor, did not inhibit Lp[a] clearance. In summary, the liver is the major organ accounting for the clearance of Lp[a] in mice, with the LDL receptor and apolipoprotein E having no major roles. Our studies indicate that apo[a] is the primary ligand that mediates Lp[a] uptake and plasma clearance.

Elements in the C terminus of apolipoprotein [a] responsible for the binding to the tenth type III module of human fibronectin

In previous studies, we showed that the C-terminal domain, F2, but not the N-terminal domain, F1, is responsible for the binding of apolipoprotein [a] (apo[a]) to human fibronectin (Fn). To pursue those observations, we prepared, by both elastase digestion and recombinant technology, subsets of F2 of a different length containing either kringle (K) V or the protease domain (PD). We also studied rhesus monkey apo[a], which is known to contain PD but not KV. In the case of Fn, we used both an intact product and its tenth type III module (10FN-III) expressed in Escherichia coli. The binding studies carried out on microtiter plates showed that the affinity of F2 for immobilized 10FN-III was approximately 6-fold higher than that for Fn (dissociation constants = 1.75 +/- 0.31 nM and 10.25 +/- 1.62 nM, respectively). The binding was also exhibited by rhesus apo[a] and by an F2 subset containing the PD linked to an upstream microdomain comprising KIV-8 to KIV-10 and KV, inactive by itself. Competition experiments on microtiter plates showed that both Fn and 10FN-III, when in solution, are incompetent to bind F2. Together, our results indicate that F2 binds to immobilized 10FN-III more efficiently than whole Fn and that the binding can be sustained by truncated forms of F2 that contain the catalytically inactive PD linked to an upstream four K microdomain.