Catabolism Of Lp Research Articles

Genetic and Mechanistic Insights into the Modulation of Circulating Lipoprotein (a) Concentration by Apolipoprotein E Isoforms.

Lipoprotein (a) [Lp(a)] is a highly atherogenic lipoprotein species. A unique feature of Lp(a) is the strong genetic determination of its concentration. The LPA gene is responsible for up to 90% of the variance in Lp(a), but other genes also have an impact. Genome-wide associations studies indicate that the APOE gene, encoding apolipoprotein E (apoE), is the second most important locus modulating Lp(a) concentrations. Population studies clearly show that carriers of the apoE2 variant (ε2) display reduced Lp(a) levels, the lowest concentrations being observed in ε2/ε2 homozygotes. This genotype can lead predisposed adults to develop dysbetalipoproteinemia, a lipid disorder characterized by sharp elevations in cholesterol and triglycerides. However, dysbetalipoproteinemia does not significantly modulate circulating Lp(a). Mechanistically, apoE appears to impair the production but not the catabolism of Lp(a). These observations underline the complexity of Lp(a) metabolism and provide key insights into the pathways governing Lp(a) synthesis and secretion.

Lp(a) and the Risk for Cardiovascular Disease: Focus on the Lp(a) Paradox in Diabetes Mellitus.

Lipoprotein(a) (Lp(a)) is one of the strongest causal risk factors of atherosclerotic disease. It is rich in cholesteryl ester and composed of apolipoprotein B and apo(a). Plasma Lp(a) levels are determined by apo(a) transcriptional activity driven by a direct repeat (DR) response element in the apo(a) promoter under the control of (HNF)4α Farnesoid-X receptor (FXR) ligands play a key role in the downregulation of APOA expression. In vitro studies on the catabolism of Lp(a) have revealed that Lp(a) binds to several specific lipoprotein receptors; however, their in vivo role remains elusive. There are more than 1000 publications on the role of diabetes mellitus (DM) in Lp(a) metabolism; however, the data is often inconsistent and confusing. In patients suffering from Type-I diabetes mellitus (T1DM), provided they are metabolically well-controlled, Lp(a) plasma concentrations are directly comparable to healthy individuals. In contrast, there exists a paradox in T2DM patients, as many of these patients have reduced Lp(a) levels; however, they are still at an increased cardiovascular risk. The Lp(a) lowering mechanism observed in T2DM patients is most probably caused by mutations in the mature-onset diabetes of the young (MODY) gene and possibly other polymorphisms in key transcription factors of the apolipoprotein (a) gene (APOA).

Galactose-specific asialoglycoprotein receptor is involved in lipoprotein (a) catabolism.

Lp(a) [lipoprotein (a)] is a highly atherogenic plasma lipoprotein assembled from low-density lipoprotein and the glycoprotein apolipoprotein (a). The rate of Lp(a) biosynthesis correlates significantly with plasma Lp(a) concentrations, whereas the fractional catabolic rate does not have much influence. So far, little is known about Lp(a) catabolism. To study the site and mode of Lp(a) catabolism, native or sialidase-treated Lp(a) was injected into hedgehogs or ASGPR (asialoglycoprotein receptor)-knockout (ASGPR-) mice or wild-type (ASGPR+) mice, and the decay of the plasma Lp(a) concentration was followed. COS-7 cells were transfected with high- (HL-1) and low-molecular-mass ASGPR subunits (HL-2), and binding and degradation of intact or desialylated Lp(a) were measured. In hedgehogs, one of the few species that synthesize Lp(a), most of the Lp(a) was taken up by the liver, followed by kidney and spleen. Lp(a) and asialo-Lp(a) were catabolized with apparent half-lives of 13.8 and 0.55 h respectively. Asialo-orosomucoide increased both half-lives significantly. In mice, the apparent half-life of Lp(a) was 4-6 h. Catabolism of native Lp(a) by wild-type mice was significantly faster compared with ASGPR- mice and there was a significantly greater accumulation of Lp(a) in the liver of ASGPR+ mice compared with ASGPR- mice. The catabolism of asialo-Lp(a) in ASGPR- mice was 8-fold faster when compared with native Lp(a) in wild-type mice. Transfected COS-7 cells expressing functional ASGPR showed approx. 5-fold greater binding and 2-fold faster degradation of native Lp(a) compared with control cells. Our results for the first time demonstrate a physiological function of ASGPR in the catabolism of Lp(a).

4WS24-5 The catabolism of Lp(a)

4WS24-5 The catabolism of Lp(a)

Defects of the LDL receptor in WHHL transgenic rabbits lead to a marked accumulation of plasma lipoprotein[a

In this study, we created LDL receptor (LDLr) defective (WHHL) transgenic rabbits expressing human apo[a] to examine whether LDLr mediates the Lp[a] clearance from the plasma. By crossbreeding WHHL rabbits with human apo[a] transgenic rabbits, we obtained two groups of human apo[a] transgenic rabbits with defective LDLr functions: apo[a]+/0 WHHL heterozygous (LDLr+/−) and apo[a]+/0 WHHL homozygous (LDLr−/−) rabbits. The lipid and lipoprotein levels of human apo[a] WHHL rabbits were compared to those of human apo[a] transgenic rabbits with normal LDLr functions (LDLr+/+). The apo[a] production rate was evaluated by analyzing apo[a] mRNA expression in the liver, the major site for apo[a] synthesis in transgenic rabbits. We found that pre-β lipoproteins were markedly increased accompanied by a 2-fold increase in the plasma Lp[a] in apo[a]+/0/LDLr+/− rabbits and a 4.2-fold increase in apo[a]+/0/LDLr−/− rabbits compared with that in apo[a]+/0 rabbits with normal LDLr function. In apo[a]+/0/LDLr−/− rabbits, there was a marked increase in plasma total cholesterol and triglycerides, as was found in their counterpart non-transgenic WHHL rabbits. Northern blot analysis revealed that hepatic apo[a] expression in WHHL transgenic rabbits was similar to that in LDLr+/+ transgenic rabbits, suggesting the accumulation of plasma Lp[a] in WHHL transgenic rabbits was not due to increased apo[a] synthesis.In conclusion, absence of a functional LDLr leads to a marked accumulation of plasma Lp[a] in human apo[a] transgenic WHHL rabbits and LDLr may participate in the catabolism of Lp[a] in rabbits.

Truncated apo B-70.5-containing lipoproteins bind to megalin but not the LDL receptor.

Apo B-100 of LDL can bind to both the LDL receptor and megalin, but the molecular interactions of apo B-100 with these 2 receptors are not completely understood. Naturally occurring mutant forms of apo B may be a source of valuable information on these interactions. Apo B-70.5 is uniquely useful because it contains the NH2-terminal portion of apo B-100, that includes only one of the two putative LDL receptor-binding sites (site A). The lipoprotein containing apo B-70. 5 (Lp B-70.5) was purified from apo B-100/apo B-70.5 heterozygotes by sequential ultracentrifugation combined with immunoaffinity chromatography. Cell culture experiments, ligand blot analysis, and in vivo studies all consistently showed that Lp B-70.5 is not recognized by the LDL receptor. The kidney was identified as a major organ in catabolism of Lp B-70.5 in New Zealand white rabbits. Autoradiographic analysis revealed that renal proximal tubular cells selectively removed Lp B-70.5. On ligand blotting of renal cortical membranes, Lp B-70.5 bound only to megalin. The ability of megalin to mediate cellular endocytosis of Lp B-70.5 was confirmed using retinoic acid/dibutyryl cAMP-treated F9 cells. This study suggests that the putative LDL receptor-binding site A on apo B-100 might not by itself be a functional binding domain and that the apo B-binding sites recognized by the LDL receptor and by megalin may be different. Moreover, megalin may play an important role in renal catabolism of apo B truncations, including apo B-70.5.

Lipoprotein (a) metabolism estimated by nonsteady-state kinetics.

Lipoprotein (a) [Lp(a)] is a low-density lipoprotein (LDL) particle with an additional apolipoprotein named apo(a). The concentration of Lp(a) in plasma is determined to a large extent by the size of the apo(a) isoform. Because elevated Lp(a) concentrations in plasma are associated with risk for premature coronary heart disease it is important to determine whether variations in production or catabolism mediate differences in Lp(a) concentration. We determined metabolic parameters of Lp(a) in 17 patients with heterozygous familial hypercholesterolemia or severe mixed hyperlipidemia by fitting a monoexponential function to the rebound of Lp(a) plasma concentration following LDL-apheresis. In 8 of those 17 patients this was done twice following two different aphereses. Although this approach allows one to estimate metabolic parameters without the use of a tracer, it requires several major assumptions such as that apheresis itself does not change production or catabolism of Lp(a) and that Lp(a) metabolism can be described by a single compartment. One apheresis decreased Lp(a) concentration by 59.1+/-8.3%. The fractional catabolic rate (FCR) was 0.16+/-0.12 d(-1) and production rate 6.27+/-5.26 mg x kg(-1) x d(-1). However, observed (concentration before first apheresis) and predicted steady-state concentrations differed considerably (more than 20%) in 9 of 17 patients, indicating that not all assumptions were fulfilled in all patients. Production rate but not FCR was correlated with Lp(a) plasma concentration (r2 = 0.43, P = 0.004) and molecular weight of apo(a) (r2 = 0.48, P = 0.011), which confirms radiotracer experiments showing that variations in Lp(a) plasma concentrations are due to differences in production not catabolism. When parameters were estimated twice in a subgroup of eight patients, satisfactory reproducibility was observed in six patients. Although parameters determined on two occasions correlated well, only FCR was concordant (intraclass correlation coefficient). Thus, despite the limitations arising from the assumptions implicit to this method, metabolic parameters of Lp(a) can be estimated from the rebound of plasma concentration following apheresis.

The atherogenic lipoprotein Lp(a) is internalized and degraded in a process mediated by the VLDL receptor.

Lp(a) is a major inherited risk factor associated with premature heart disease and stroke. The mechanism of Lp(a) atherogenicity has not been elucidated, but likely involves both its ability to influence plasminogen activation as well as its atherogenic potential as a lipoprotein particle after receptor-mediated uptake. We demonstrate that fibroblasts expressing the human VLDL receptor can mediate endocytosis of Lp(a), leading to its degradation within lysosomes. In contrast, fibroblasts deficient in this receptor are not effective in catabolizing Lp(a). Lp(a) degradation was prevented by antibodies against the VLDL receptor, and by RAP, an antagonist of ligand binding to the VLDL receptor. Catabolism of Lp(a) was inhibited by apolipoprotein(a), but not by LDL or by monoclonal antibodies against apoB100 that block LDL binding to the LDL receptor, indicating that apolipoprotein(a) mediates Lp(a) binding to this receptor. Removal of Lp(a) antigen from the mouse circulation was delayed in mice deficient in the VLDL receptor when compared with control mice, indicating that the VLDL receptor may play an important role in Lp(a) catabolism in vivo. We also demonstrate the expression of the VLDL receptor in macrophages present in human atherosclerotic lesions. The ability of the VLDL receptor to mediate endocytosis of Lp(a) could lead to cellular accumulation of lipid within macrophages, and may represent a molecular basis for the atherogenic effects of Lp(a).

Cellular uptake of lipoprotein[a] by mouse embryonic fibroblasts via the LDL receptor and the LDL receptor-related protein

The sites and precise mechanisms of the catabolism of the atherogenic lipoprotein[a] (Lp[a]) are unknown. It has been proposed that the low density lipoprotein receptor (LDL-R) and the low density lipoprotein receptor-related protein (LRP) are involved in the catabolism of Lp[a]. To address the question whether and to what extent the LDL-R and/or LRP are involved in the catabolism of Lp[a], we studied the cellular uptake of Lp[a] via those two receptors using mouse embryonic fibroblast (MEF) cell lines lacking either the LDL-R, the LRP, or both receptors due to disruption of the respective mouse genes. 125I-labeled LDL and 125I-labeled Lp[a] uptake by wild-type fibroblasts (MEF1) was compared with that by fibroblasts homozygous for the disrupted LRP allele (MEF2), fibroblasts with two defective alleles for the LDL-R (MEF3), and fibroblasts homozygous for defects both in the LDL-R and LRP gene (MEF4). Compared with MEF1, 125I-labeled LDL uptake by MEF2 was 77%, by MEF3 30%, and by MEF4 24% of that by MEF1. However, no significant differences in the specific 125I-labeled Lp[a] uptake by the four mouse embryonic cell lines was observed. In comparison with MEF1, the 125I-labeled Lp[a] uptake by MEF2 was 98%, by MEF3 111%, and 73% by MEF4. Approximately 50% of the total cellular uptake of 125I-labeled Lp[a] was nonspecific. In conclusion, our results suggest that Lp[a] is a poor ligand for the LDL receptor and the LRP. The data of the displacement studies, however, indicated that the nonspecific uptake of Lp[a] constitutes a major route for the cellular Lp[a] catabolism in this study.

Renovascular arteriovenous differences in Lp[a] plasma concentrations suggest removal of Lp[a] from the renal circulation

High plasma concentrations of lipoprotein[a] (Lp[a]) are considered a genetically determined risk factor for atherosclerosis. Lp[a] is produced by the liver. The site(s) and mechanism(s) of catabolism are presently unclear. Lp[a] is elevated secondary to end-stage renal disease which suggests a direct or indirect role of the kidney in the metabolism of Lp[a]. We therefore investigated, by a simple in vivo approach, whether Lp[a] is removed by the human kidney. Lp[a] plasma concentrations were measured simultaneously by various methods in the ascending aorta and renal vein of 100 patients undergoing coronary angiography or coronary angioplasty. Lp[a] levels differed significantly between the two vessels even after correcting for hemoconcentration (20.1 +/- 21.6 mg/dL versus 18.7 +/- 20.3 mg/dL, P < 0.001). This corresponds to a mean arteriovenous difference of -1.4 mg/ dL or -9% of the arterial concentration. No Lp[a] or intact apo[a] could be detected in urine from healthy probands. Although we cannot assign the kidney a regulatory role for Lp[a] plasma levels in humans with normal renal function, we conclude from our data that substantial amounts of this atherogenic lipoprotein are taken up by the kidney. The underlying mechanisms are unknown at the moment. This study therefore demonstrates for the first time that the human kidney plays an active role in the catabolism of Lp[a]. This may explain the elevated Lp[a] concentrations found in patients with chronic renal insufficiency.

Lipoprotein(a) levels in liver diseases and changes of lipoprotein(a) levels before and after interferon therapy in patients with chronic hepatitis-C

Lipoprotein(a), (Lp(a)) level was measured in Japanese hospitalized controls and various liver disease groups, and lowest in liver cirrhosis (LC) and highest in acute hepatitis (AH). Lp(a) level was higher in AH rather than in controls. Difference of Lp(a) levels was statistically significant between AH and LC, and fatty liver (FL) and AH. It is suggested that catabolism of Lp(a) is more altered in AH than in LC, and Lp(a) reacts as an acute phase protein much more in AH than in LC. Statistically significant increase of Lp(a) levels was recognized in responders to Interferon therapy for chronic hepatitis-C, whereas not significant in non-responders. This indicates that increased Lp(a) levels represent an improved liver function. The results clearly suggest that liver diseases modulate the Lp(a) metabolism.

Urinary excretion of apo(a) fragments. Role in apo(a) catabolism.

The biosynthesis and assembly of lipoprotein(a) [Lp(a)], a marker for atherosclerotic disease, appears to be well understood. However, information is lacking concerning the mode and site of Lp(a) catabolism. Apo(a) is reported to be excreted into the urine. To study the effect of this pathway on the overall catabolism of Lp(a), urinary apo(a) was characterized by immunoblotting. More than 10 distinct apo(a) bands with molecular masses between 30 and 160 kD were observed. Apo(a) fragments were not complexed to apoB. In more than 30 individuals the size of apo(a) bands was comparable irrespective of their apo(a) phenotype, although marked differences in the relative intensities of the bands were observed. Eight batches of 24-hour urine collections collected from one proband at 2-week intervals exhibited a significant correlation between creatinine and apo(a) concentrations as measured by DELFIA (r = .93; P < .01). In 193 healthy volunteers a highly significant correlation was found between urinary apo(a) concentrations normalized to creatinine levels and plasma Lp(a) values (p = 0.659; P < .0001). Of the total plasma apo(a), 0.073%, i.e., 121 micrograms apo(a), was excreted in the form of apo(a) fragments in 24-hour urine samples from 12 healthy volunteers. We conclude that the catabolism of Lp(a) via excretion of apo(a) fragments accounts for < 1% of the daily Lp(a) catabolism.

The relationship between lipoprotein(a) and low density lipoprotein receptors during the treatment of hyperthyroidism.

To determine whether the low density lipoprotein (LDL) receptor pathway is involved in the catabolism of plasma lipoprotein (a) [Lp(a)], serum lipids, Lp(a), and LDL receptor activity were measured in seven patients with hyperthyroidism before and after methimazole treatment given hyperthyroidism is associated with enhanced LDL receptor activity. LDL receptor activity in patients was estimated by the equation using the serum concentrations of apolipoprotein (apo) B and C-II. When euthyroidism was achieved after treatment, not only did serum total cholesterol, high density lipoprotein-cholesterol, apo B, and LDL-cholesterol (LDL-Ch) levels rise, but Lp(a) significantly increased and calculated LDL receptor activity significantly decreased. The changes in LDL receptor activity were significantly correlated with the changes in LDL-Ch as expected, but not with changes in Lp(a). These results suggest that the serum concentration of Lp(a) is lowered in hyperthyroidism, probably by a mechanism other than the enhanced activity of the LDL receptor, and that the LDL receptor pathway is involved in the catabolism of Lp(a) to a limited extent.

The LDL receptor is not required for normal catabolism of Lp(a) in humans

The LDL receptor is not required for normal catabolism of Lp(a) in humans

A PvuII polymorphism of the low density lipoprotein receptor gene is not associated with plasma concentrations of low density lipoproteins including LP(a).

Lipoprotein(a) [Lp(a)] is a low density lipoprotein (LDL), in which apolipoprotein B-100 (apo B-100) is attached to apolipoprotein(a) [apo(a)], a glycoprotein of variable size. Lp(a) may be as atherogenic as LDL. In normal populations, Lp(a) concentrations in plasma are largely determined by the apo(a) gene locus on chromosome 6, but regulation of synthesis and catabolism of Lp(a) is poorly understood. In some studies, a PvuII restriction fragment length polymorphism (RFLP) in the LDL receptor gene seems to affect concentrations of LDL in plasma, and other studies have indicated that Lp(a) catabolism could be mediated by the LDL receptor. We therefore expected that the PvuII polymorphism in the LDL receptor gene might be associated with Lp(a) levels in 170 Caucasian men aged 40 years, selected to have a high representation of low molecular weight apo(a) phenotypes. However, plasma concentrations of cholesterol, LDL-cholesterol, HDL-cholesterol, triglycerides and Lp(a) were all unrelated to the LDL receptor gene PvuII polymorphism both in the group as a whole and when it was subgrouped by apo(a) phenotype. Therefore our data do not support the concept that this particular LDL receptor gene polymorphism is associated with LDL receptor function, and our data therefore neither support nor rule out a role for the LDL receptor in Lp(a) catabolism.

Reduction of Serum Lipoprotein(a) Using Estrogen in a Man With Familial Hypercholesterolemia

To the Editor. —The significance of lipoprotein(a) [Lp(a)] as a genetic risk factor for atherosclerotic vascular disease has been well documented.1-3However, many aspects of its metabolism remain unclear.1,2There have been reports that Lp(a) is removed from the circulation by low-density lipoprotein (LDL) receptors,4but the role of this receptor in the catabolism of Lp(a) is still controversial. We recently observed interesting changes in serum lipids, including Lp(a), in a patient with familial hypercholesterolemia following estrogen therapy for prostatic cancer, which may help our understanding of Lp(a) catabolism. A 67-year-old man with familial hypercholesterolemia (which consists of a marked elevation of LDL-cholesterol to more than 6.45 mmol/L and thickening of the Achilles tendon to 18 mm) and non-insulin-dependent diabetes mellitus was admitted to the hospital for the treatment of prostatic cancer. He had been treated for a long time with cholestyramine (8 g/d) and probucol (1

Catabolism of lipoprotein(a) in familial hypercholesterolaemic subjects

The in vivo turnover of autologous lipoprotein(a) (Lp(a)) was studied in four heterozygous familial hypercholesterolaemic (FH) subjects and four subjects who were hyperlipidaemic but not FH. Each of the FH subjects exhibited a much lower fractional catabolic rate (FCR) for LDL than each of the non-FH subjects. Lp(a) was purified by sequential density gradient centrifugations and was radio-iodinated. The labelled Lp(a) ran as a single band on electrophoresis in gradient polyacrylamide gels. Less than 5% of the label was in lipid, with about 40% of the remainder on apolipoprotein B (apo B) and 60% on apo(a). Labelled and unlabelled Lp(a) competed equally poorly with LDL for binding to LDL receptors on cultured fibroblasts. The FCR of Lp(a), calculated from the decay of the specific radioactivity of the Lp(a) isolated from the daily blood samples, was the same in FH subjects as in non-FH subjects. There was no consistent relationship between Lp(a) FCR and the plasma Lp(a) concentration or between FCR and the Lp(a) phenotype, at least within this sample of subjects. There was a strong association between Lp(a) concentration and production rate, with values for non-FH and FH subjects falling on the same line. The rate of decline of radioactivity in whole plasma was consistently slower than the fall in specific radioactivity of the isolated Lp(a). This difference was more marked in FH subjects than in non-FH subjects and resulted from the accumulation of radioactivity derived from the injected Lp(a) at a lower density than Lp(a), in the fractions containing LDL. The amount of radioactivity in this fraction increased for the first few days after injection and then fell, the fall being more rapid in non-FH than in FH subjects. These results provide no evidence for the involvement of LDL receptors in the catabolism of Lp(a) itself but suggest that they could be responsible for some of the clearance of the lipid and apo B components after removal of apo(a) in the circulation.